Handbook of Plastic Films

Handbook of Plastic Films

Editor: Elsayed M. Abdel-Bary

Rapra Technology Limited

Handbook ofPlastic Films

Editor: E.M. Abdel-Bary


Shawbury, Shrewsbury, Shropshire, SY4 4NR, United KingdomTelephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.rapra.net

TECHNOLOGYrapra

First Published in 2003 by

Rapra Technology LimitedShawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2003, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no partof this publication may be photocopied, reproduced or distributed in anyform or by any means or stored in a database or retrieval system, without

the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any materialreproduced within the text and the authors and publishers apologise if

any have been overlooked.

Typeset by Rapra Technology LimitedCover printed by The Printing House, Crewe, UK

Printed and bound by Rapra Technology Limited, Shrewsbury, UK

ISBN: 1-85957-338-X

iii

Contents

1. Technology of Polyolefin Film Production ...................................................... 5

1.1 Introduction ........................................................................................... 5

1.2 Structures of the Polyolefins................................................................... 7

1.2.1 Low-Density Polyethylene (LDPE) ............................................. 7

1.2.2 High-Density Polyethylene (HDPE, MDPE, UHMWPE) ........... 8

1.2.3 Linear Low-Density Polyethylene (LLDPE)................................ 8

1.2.4 Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE) ... 8

1.2.5 Polypropylene (PP)..................................................................... 9

1.2.6 Polypropylene Copolymers ........................................................ 9

1.3 Morphology of Polyolefin Films ............................................................ 9

1.4 Rheological Characterisation of the Polyolefins ................................... 10

1.4.1 High-Density Polyethylene ....................................................... 10

1.4.2 Linear Low-Density Polyethylene ............................................ 11

1.4.3 Very- and Ultra-Low-Density Polyethylene .............................. 11

1.4.4 Low-Density Polyethylene, Long Branches .............................. 11

1.4.5 Polypropylene .......................................................................... 12

1.5 Blown Film Production (Tubular Extrusion) ........................................ 13

1.5.1 Extruder Characteristics .......................................................... 14

1.5.2 Screw Design ........................................................................... 15

1.5.3 Frost-line and Blow Ratio ........................................................ 15

1.6 Cast Film Production ........................................................................... 16

1.6.1 Extrusion Conditions ............................................................... 16

1.6.2 Calendering Finishing .............................................................. 17

1.6.3 Extrusion Coating.................................................................... 17


iv

1.7 Orientation of the Film ........................................................................ 18

1.7.1 Orientation During Blowing .................................................... 18

1.7.2 Orientation by Drawing........................................................... 18

1.7.3 Biaxial Orientation (Biaxially Oriented PP, BOPP) .................. 18

1.8 Surface Properties ................................................................................ 19

1.8.1 Gloss ........................................................................................ 19

1.8.2 Haze ........................................................................................ 20

1.8.3 Surface Energy ......................................................................... 20

1.8.4 Slip........................................................................................... 21

1.8.5 Blocking ................................................................................... 21

1.9 Surface Modification ........................................................................... 21

1.9.1 Corona Discharge .................................................................... 21

1.9.2 Antiblocking ............................................................................ 22

1.9.3 Slip Additives ........................................................................... 23

1.9.4 Lubricants ................................................................................ 24

1.9.5 Antistatic Agents ...................................................................... 24

1.10 Internal Additives ................................................................................ 24

1.10.1 Antioxidants ............................................................................ 24

1.10.2 Ultraviolet Absorbers ............................................................... 24

1.11 Mechanical Properties .......................................................................... 25

1.11.1 Tensile Properties ..................................................................... 26

1.11.2 Impact Properties ..................................................................... 28

1.11.3 Dynamic Mechanical Properties .............................................. 29

1.11.4 Dielectric Properties ................................................................. 30

1.12 Microscopic Examination .................................................................... 31

1.12.1 Optical – Polarised Light Effect with Strain ............................. 31

1.12.2 Scanning Electron Microscopy (SEM) – Etching ...................... 31

1.12.3 Atomic Force Microscopy (AFM) ............................................ 31

1.13 Thermal Analysis ................................................................................. 31

1.13.1 Differential Scanning Calorimetry (DSC) ................................. 31

v

Contents

1.13.2 Temperature-Modulated DSC (TMDSC) ................................. 32

1.14 Infrared Spectroscopy .......................................................................... 32

1.14.1 Characterisation ...................................................................... 32

1.14.2 Composition Analysis of Blends and Laminates....................... 33

1.14.3 Surface Analysis ....................................................................... 33

1.14.4 Other Properties ...................................................................... 34

1.15 Applications ......................................................................................... 35

1.15.1 Packaging ................................................................................ 35

1.15.2 Laminated Films ...................................................................... 36

1.15.3 Coextruded Films .................................................................... 37

1.15.4 Heat Sealing ............................................................................. 38

1.15.5 Agriculture ............................................................................... 38

1.16 Conclusion ........................................................................................... 38

2. Processing of Polyethylene Films ................................................................... 41

2.1 Introduction ......................................................................................... 41

2.2 Parameters Influencing Resin Basic Properties ..................................... 42

2.2.1 Molecular Weight (Molar Mass) and Dispersity Index ............ 42

2.2.2 Melt Index (Flow Properties) ................................................... 42

2.2.3 Density .................................................................................... 44

2.2.4 Chain Branching ...................................................................... 45

2.2.5 Intrinsic Viscosity .................................................................... 46

2.2.6 Melting Point and Heat of Fusion............................................ 47

2.2.7 Melt Properties – Rheology ..................................................... 48

2.2.8 Elongational Viscosity ............................................................. 49

2.2.9 Elasticity .................................................................................. 49

2.3 Blown Film Extrusion (Tubular Film) .................................................. 50

2.3.1 Introduction ............................................................................. 50

2.3.2 Description of the Blown Film Process ..................................... 50

2.3.3 Various Ways of Cooling the Film ........................................... 51


vi

2.3.4 Extruder Size ........................................................................... 54

2.3.5 Horsepower ............................................................................. 55

2.3.6 Selection of Extrusion Equipment ............................................ 55

2.4 Cast Film Extrusion ............................................................................. 57

2.4.1 Description of the Cast Film Process ........................................ 57

2.4.2 Effects of Extrusion Variables on Film Characteristics ............. 58

2.4.3 Effect of Blow-up Ratio on Film Properties ............................. 61

2.5 Processing Troubleshooting Guidelines ................................................ 62

2.6 Shrink Film .......................................................................................... 62

2.6.1 Shrink Film Types .................................................................... 65

2.6.2 Shrink Film Properties ............................................................. 66

2.6.3 The Manufacture of Shrink Film ............................................. 67

2.6.4 Shrink Tunnels and Ovens ....................................................... 70

3. Processing Conditions and Durability of Polypropylene Films ...................... 73

3.1 Introduction ......................................................................................... 73

3.2 Structures and Synthesis ....................................................................... 78

3.3 Film Processing .................................................................................... 85

3.4 Additives .............................................................................................. 85

3.5 Ultraviolet Degradation of Polypropylene............................................ 86

3.5.1 UV Degradation Mechanisms .................................................. 86

3.5.2 Effect of UV Degradation on Molecular Structureand Properties of PP................................................................. 87

3.5.3 Stabilisation of PP by Additives ............................................... 88

3.6 Case Studies ......................................................................................... 90

3.6.1 Materials and Experimental Procedures ................................... 90

3.6.2 Durability-Microstructure Relationship ................................... 91

3.6.3 Durability-Processing Condition Relationship ......................... 94

3.6.4 Durability-Additive Property Relationship............................... 97

3.7 Concluding Remarks ......................................................................... 101

vii

Contents

4. Solubility of Additives in Polymers.............................................................. 109

4.1 Introduction ....................................................................................... 109

4.2 Nonuniform Polymer Structure.......................................................... 109

4.3 Additive Sorption ............................................................................... 110

4.4 Quantitative Data on Additive Solubility in Polymers ....................... 114

4.5 Factors Affecting Additive Solubility ................................................. 118

4.5.1 Crystallinity and Supermolecular Structure............................ 118

4.5.2 Effect of Polymer Orientation ................................................ 119

4.5.3 Role of Polymer Polar Groups ............................................... 120

4.5.4 Effect of the Second Compound ............................................ 121

4.5.5 Features of Dissolution of High Molecular Weight Additives .. 122

4.5.6 Effect of Polymer Oxidation .................................................. 124

4.6 Solubility of Additives and Their Loss ............................................... 125

5. Polyvinyl Chloride: Degradation and Stabilisation ...................................... 131

5.1 Introduction ....................................................................................... 131

5.2 Some Factors Affecting the Low Stability of PVC .............................. 132

5.3 Identification of Carbonylallyl Groups .............................................. 136

5.4 Principal Ways to Stabilise PVC ......................................................... 138

5.5 Light Stabilisation of PVC ................................................................. 144

5.6 Effect of Plasticisers on PVC Degradation in Solution ....................... 145

5.7 ‘Echo’ Stabilisation of PVC ................................................................ 151

5.8 Tasks for the Future ........................................................................... 153

6. Ecological Issues of Polymer Flame Retardants ........................................... 159

6.1 Introduction ....................................................................................... 159

6.2 Mechanisms of Action ....................................................................... 160

6.3 Halogenated Diphenyl Ethers – Dioxins ............................................ 162


viii

6.4 Flame Retardant Systems ................................................................... 166

6.5 Intumescent Additives ........................................................................ 168

6.6 Polymer Organic Char-Former........................................................... 175

6.7 Polymer Nanocomposites .................................................................. 180

7. Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres . 187

7.1 Introduction ....................................................................................... 187

7.2 Interaction of Nitrogen Dioxide with Polymers ................................. 188

7.2.1 Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF ...... 188

7.2.2 Non-Saturated Polymers ........................................................ 191

7.2.3 Polyamides, Polyurethanes, Polyamidoimides ........................ 196

7.3 Reaction of Nitric Oxide with Polymers ............................................ 202

7.4 Conclusion ......................................................................................... 209

8. Modifications of Plastic Films ..................................................................... 213

8.1 Introduction ....................................................................................... 213

8.2 Modification of Mechanical Properties .............................................. 213

8.2.1 Orientation ............................................................................ 214

8.2.2 Crystallisation ........................................................................ 214

8.2.3 Crosslinking ........................................................................... 214

8.3 Chemical Modifications ..................................................................... 215

8.3.1 Fluorination ........................................................................... 215

8.3.2 Chlorination .......................................................................... 217

8.3.3 Bromination ........................................................................... 217

8.3.4 Sulfonation ............................................................................ 218

8.3.5 Chemical Etching ................................................................... 218

8.3.6 Grafting ................................................................................. 220

8.4 Physical Methods Used for Surface Modification............................... 222

8.4.1 Plasma Treatment .................................................................. 222

8.4.2 Corona Treatment ................................................................. 223

ix

Contents

8.5 Characterisation ................................................................................ 224

8.5.1 Gravimetric Method .............................................................. 224

8.5.2 Thermal Analyses .................................................................. 225

8.5.3 Scanning Electron Microscopy ............................................... 225

8.5.4 Swelling Measurements .......................................................... 226

8.5.5 Molecular Weight and Molecular Weight Distribution .......... 226

8.5.6 Dielectric Relaxation ............................................................. 226

8.5.7 Surface Properties .................................................................. 227

8.5.8 Spectroscopic Analysis ........................................................... 227

8.5.9 Electron Spectroscopy for Chemical Analysis (ESCA)or X-Ray Photoelectron Spectroscopy (XPS) ......................... 228

8.6 Applications ....................................................................................... 228

9. Applications of Plastic Films in Packaging .................................................. 235

9.1 Introduction ....................................................................................... 235

9.2 Packaging Functions .......................................................................... 235

9.3 Flexible Package Forms...................................................................... 236

9.3.1 Wraps .................................................................................... 237

9.3.2 Bags, Sacks and Pouches ........................................................ 238

9.3.3 Pouch Production .................................................................. 239

9.3.4 Dispensing and Reclosure Features ........................................ 239

9.4 Heat-Sealing ...................................................................................... 240

9.5 Other Uses of Packaging Films........................................................... 241

9.6 Major Packaging Films ...................................................................... 241

9.6.1 Low-Density Polyethylene (LDPE) and LinearLow-Density Polyethylene (LLDPE) ....................................... 242

9.6.2 High-Density Polyethylene (HDPE) ....................................... 243

9.6.3 Polypropylene (PP)................................................................. 244

9.6.4 Polyvinyl Chloride (PVC)....................................................... 245

9.6.5 Polyethylene Terephthalate (PET) .......................................... 245


x

9.6.6 Polyvinylidene Chloride (PVDC) ........................................... 246

9.6.7 Polychlorotrifluoroethylene (PCTFE) ..................................... 247

9.6.8 Polyvinyl Alcohol (PVOH) .................................................... 248

9.6.9 Ethylene-Vinyl Alcohol (EVOH) ............................................ 248

9.6.10 Polyamide (Nylon) ................................................................. 249

9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films ..... 250

9.6.12 Ionomers ................................................................................ 251

9.6.13 Other Plastics ......................................................................... 251

9.7 Multilayer Plastic Films ..................................................................... 252

9.7.1 Coating .................................................................................. 252

9.7.2 Lamination ............................................................................ 253

9.7.3 Coextrusion ........................................................................... 253

9.7.4 Metallisation .......................................................................... 253

9.7.5 Silicon Oxide Coating ............................................................ 254

9.7.6 Other Inorganic Barrier Coatings .......................................... 255

9.8 Surface Treatment .............................................................................. 255

9.9 Static Discharge ................................................................................. 256

9.10 Printing .............................................................................................. 256

9.11 Barriers and Permeation..................................................................... 257

9.12 Environmental Issues ......................................................................... 261

10. Applications of Plastic Films in Agriculture ................................................ 263

10.1 Introduction ....................................................................................... 263

10.2 Production of Plastic Films ................................................................ 263

10.3 Characteristics of Plastic Films Used in Agriculture ........................... 264

10.4 Stability of Greenhouse Films to Solar Irradiation ............................. 265

10.4.1 Ultraviolet Stabilisers ............................................................. 265

10.4.2 Requirements for Stabiliser Efficiency.................................... 268

10.4.3 Evaluation of Laboratory and Outdoor Photooxidation........ 271

xi

10.5 Other Factors Affecting the Stability of Greenhouse Films .............. 272

10.5.1 Temperature ..................................................................... 272

10.5.2 Humidity .......................................................................... 273

10.5.3 Wind ................................................................................ 273

10.5.4 Fog Formation ................................................................. 273

10.5.5 Environmental Pollution .................................................. 274

10.5.6 Effects of Pesticides .......................................................... 274

10.6 Ageing Resistance of Greenhouse Films .......................................... 275

10.6.1 Measurement of Ageing Factors ....................................... 275

10.6.2 Changes in Chemical Structure......................................... 276

10.7 Recycling of Plastic Films in Agriculture ......................................... 277

10.7.1 Introduction ..................................................................... 277

10.7.2 Contamination by the Environment ................................. 277

11. Physicochemical Criteria for Estimating the Efficiency of Burn Dressings ... 285

11.1 Introduction .................................................................................... 285

11.2 Modern Surgical Burn Dressings ..................................................... 286

11.2.1 Dressings Based on Materials of Animal Origin ............... 286

11.2.2 Dressings Based on Synthetic Materials ............................ 286

11.2.3 Dressings Based on Materials of Vegetable Origin ........... 290

11.3 Selection of the Properties of Tested Burn Dressings ....................... 290

11.3.1 Sorption-Diffusion Properties ........................................... 291

11.3.2 Adhesive Properties .......................................................... 292

11.3.3 Mechanical Properties ...................................................... 292

11.4 Methods of Investigation of Physicochemical Properties ofBurn Dressings ................................................................................ 292

11.4.1 Determination of Material Porosity ................................. 292

11.4.2 Determination of Size and Number of Pores .................... 293

11.4.3 Estimation of Surface Energy at Material-MediumInterface ........................................................................... 294

11.4.4 Determination of Sorptional Ability of Materials ............. 294

Contents


xii

11.4.5 Determination of Air Penetrability of Burn Dressings ...... 295

11.4.6 Determination of Adhesion of Burn Dressings ................. 296

11.4.7 Determination of Vapour Penetrability of Burn Dressings .. 296

11.5 Results and Discussion .................................................................... 297

11.5.1 Determination of Sorption Ability of Burn Dressings ....... 297

11.5.2 Kinetics of the Sorption of Liquid Media byBurn Dressings ................................................................. 303

11.5.3 Determination of Vapour Penetrability of Burn Dressings .. 305

11.5.4 Determination of the Air Penetrability of Burn Dressings .. 308

11.5.5 Determination of Adhesion of Burn Dressings ................... 315

11.6 The Model of Action of a Burn Dressing ........................................ 318

11.6.1 Evaporation of Water from the Dressing Surface ............. 318

11.6.2 Sorption of Fluid by Burn Dressing from BulkContaining a Definite Amount of Fluid ............................ 320

11.6.3 Mass Transfer of Water from Wound to Surroundings ..... 321

11.7 Criteria for the Efficiency of First-Aid Burn Dressings .................... 322

11.7.1 Requirements of a First-Aid Burn Dressing ...................... 322

11.7.2 Characteristics of First-Aid Burn Dressings ...................... 322

11.8 Conclusion ...................................................................................... 324

12. Testing of Plastic Films ................................................................................ 329

12.1 Introduction .................................................................................... 329

12.2 Requirements for Test Methods ...................................................... 330

12.2.1 List of Requirements ........................................................ 330

12.2.2 Interpretation of Test Results ........................................... 330

12.3 Some Properties of Plastic Films ...................................................... 332

12.3.1 Dimensions ...................................................................... 332

12.3.2 Conditioning the Samples................................................. 332

12.4 Mechanical Tests ............................................................................. 333

12.4.1 Tensile Testing (Static) ...................................................... 333

12.4.2 Impact Resistance ............................................................. 336

xiii

12.4.3 Tear Resistance................................................................. 337

12.4.4 Bending Stiffness (Flexural Modulus) ............................... 339

12.4.5 Dynamic Mechanical Properties ....................................... 339

12.5 Some Physical, Chemical and Physicochemical Tests ....................... 340

12.5.1 Density of Plastics ............................................................ 340

12.5.2 Indices of Refraction and Yellowness ............................... 340

12.5.3 Transparency .................................................................... 341

12.5.4 Resistance to Chemicals ................................................... 341

12.5.5 Haze and Luminous Transmittance .................................. 341

12.5.6 Ignition, Rate of Burning Characteristics andOxygen Index (OI) ........................................................... 342

12.5.7 Static and Kinetic Coefficients of Friction ........................ 342

12.5.8 Specular Gloss of Plastic Films and Solid Plastics ............. 343

12.5.9 Wetting Tension of PE and PP Films................................. 344

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films .. 345

12.5.11 Shrink Tension and Orientation Release Stress ................. 345

12.5.12 Rigidity ............................................................................ 345

12.5.13 Blocking Load by Parallel-Plate Method .......................... 346

12.5.14 Determination of LLDPE Composition by 13C NMR ..... 346

12.5.15 Creep and Creep Rupture................................................. 346

12.5.16 Outdoor Weathering/Weatherability ................................ 347

12.5.17 Abrasion Resistance ......................................................... 347

12.5.18 Mar Resistance ................................................................. 348

12.5.19 Environmental Stress Cracking......................................... 348

12.5.20 Water Vapour Permeability .............................................. 348

12.5.21 Oxygen Gas Transmission ................................................ 349

12.6 Standard Specifications for Some Plastic Films................................ 349

12.6.1 Standard Specification for PET Films ............................... 350

12.6.2 Standard Specification for LDPE Films (for GeneralUse and Packaging Applications) ..................................... 350

12.6.3 Standard Specification for MDPE and General GradePE Films (for General Use and Packaging Applications) ... 350

Contents


xiv

12.6.4 Standard Specification for OPP Films............................... 351

12.6.5 Standard Specification for Crosslinkable Ethylene Plastics . 351

13. Recycling of Plastic Waste ........................................................................... 357

13.1 Introduction .................................................................................... 357

13.2 Main Approaches to Plastic Recycling ............................................ 358

13.2.1 Primary Recycling ............................................................ 358

13.2.2 Secondary Recycling......................................................... 358

13.2.3 Tertiary Recycling ............................................................ 359

13.2.4 Quaternary Recycling....................................................... 360

13.2.5 Conclusion ....................................................................... 362

13.3 Collection and Sorting .................................................................... 362

13.3.1 Resin Identification .......................................................... 362

13.3.2 General Aspects of Resin Separation ................................ 363

13.3.3 Resin Separation Based on Density .................................. 364

13.3.4 Resin Separation Based on Colour ................................... 365

13.3.5 Resin Separation Based on Physicochemical Properties .... 365

13.4 Recycling of Separated PET Waste .................................................. 367

13.5 Recycling of Separated PVC Waste ................................................. 368

13.5.1 Chemical Recycling of Mixed Plastic Waste ..................... 369

13.5.2 Chemical Recycling of PVC-Rich Waste ........................... 370

13.6 Recycling of Separated PE Waste .................................................... 371

13.6.1 Contamination of PE Waste by Additives ......................... 372

13.6.2 Contamination of PE Waste by Reprocessing ................... 372

13.7 Recycling of HDPE ......................................................................... 373

13.7.1 Applications for Recycled HDPE ..................................... 373

13.7.2 Rubber-Modified Products ............................................... 373

13.8 Recycling Using Radiation Technology ........................................... 373

13.9 Biodegradable Polymers .................................................................. 374

1

Preface

The plastic industry continues to grow very rapidly and plays an important role inmany fields such as engineering, medical, agriculture and domestic. It is now verydifficult to find the point at which plastic cannot be considered as an essentialcomponent. The understanding of the nature of plastic films, their productiontechniques, applications and their characterisation is essential for producing newtypes of plastic films. This handbook has been written to discuss the production andmain uses of plastic films.

Chapter 1 deals with the various types of polyolefins and their suitability for filmmanufacture. The rheology, structure and properties of the polymers are discussed inrelation to the type of film manufacturing processes that are most applicable to thetypes of polymer. Post-extrusion modifications of the films such as orientation, surfacechemistry and additives are discussed. Characterisation methods used to measurefilm mechanical properties; structure and additives are described, as well as othermore specific properties. Finally some particularly important applications that requirespecial structures or modifications are given.

In Chapter 2, the main parameters influencing resin basic properties are described.The methods of processing of polyethylene films such as cast film extrusion, blowextrusion of tubular films are discussed. Effects of extrusion variables on filmcharacteristics and effect of blow ratio on film properties are considered.

Chapter 3 details the structure, synthesis and film processing of polypropylene. Theeffects of some additives and UV stabilisers are discussed.

The solubility of additives plays an important role in determining the efficiency andthe properties of the films as well. For this reason Chapter 4 deals with differentaspects of additives solubility in polymers in relation to the polymer degradationand stabilisation.

The topic covered in Chapter 5 is the stability of polyvinyl chloride (PVC) filmsduring procesing and service. The possibility of increasing the intrinsic stability ofPVC during processing with the minimal contents or in total absence of stabilisersand other additives is discussed.

2


Chapter 6 discusses flame retardants, which as special additives have an importantrole in saving lives. These flame retardant system basically inhibit or even suppressthe combustion process by chemical or physical action in the gas or condensed phase.Conventional flame retardants have a number of negative attributes and the ecologicalissues surrounding their applications are driving the search for new polymer flameretardant systems forward.

Chapter 7 covers thermal and photochemical oxidation of polymers under theinfluence of the aggressive, polluting atmospheric gases. Among pollutants, sulfurdioxide, ozone, nitrogen oxides stand out as the most deleterious impurities ofatmosphere. Thus, this chapter is devoted to consideration of the results obtained instudies of interactions of nitrogen oxides with polymers.

Chapter 8 discusses the modifications of plastic films to improve their mechanical orphysical properties to meet the requirements of certain applications. This can beachieved by subjecting the films to mechanical or chemical treatments. A number ofsurface modification techniques such as plasma, corona discharge and chemicaltreatments have been used.

Chapter 9 deals with applications of plastic films in packaging. A description of theproperties of the most common films used in packaging such as low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), high-densitypolyethylene (HDPE), polypropylene (PP), PVC, polyvinylidene chloride (PVDC),polyamide (Nylon), and other plastics are given in this chapter.

Chapter 10 deals with the application of plastic films in agriculture. The mechanicalproperties suitable that make these films suitable for use in agriculture are discussed.Stability of these plastic films under the effect of different environmental conditionsis reported. Types of UV stabilisers and their compatibility with plastic are given.Also, recycling of plastic films used in agriculture is of great importance and finally,a case study of their reuse as agriculture films is given.

Chapter 11 deals with the principal medical treatment of burns using dressings madewith a polymeric layer or layers. It is difficult to estimate the effectiveness of the newburn dressings, as their physicochemical properties are not usually presented inliterature. Thus, chapter 11 discusses this subject for the first time. Thephysicochemical criteria for estimating the efficiency of burn dressings and thepossibility of using plastic films is given.

Chapter 12 covers the most common test methods generally used for plastic films.The requirements necessary for the test methods are summarised.

3

The problem of plastic films recycling is touched on in Chapter 13. The majority ofplastic films are made from polyethylene (LDPE, LLDPE or HDPE) which compriseapproximately 68% of the total film production. Non-polyethylene resins constitutethe remainder of the plastic film. Different types of recycling are given and recyclingof some selected types of films are discussed.

This handbook represents the efforts of many experts in different aspects of plasticfilms. Their efforts in preparing contributions to the volume are to be noted and Itake the opportunity to express my heartfelt gratitude for their time and effort. Mygratitude extends also to many colleagues for their kind comments in many aspects.A special thanks is extended to the staff of Rapra Technology, for the fine productionof this Handbook, particularly Claire Griffiths, Editorial Assistant, Steve Barnfieldwho typeset the book and designed the cover and Frances Powers who commissionedthe book and oversaw the whole project.

Elsayed M. Abdel-Bary

January 2003

Preface

4


5

1 Technology of Polyolefin Film Production

Robert Shanks

1.1 Introduction

A film is a two-dimensional form of a polymer. A film is typified by a large surface areato volume ratio. Films are required to exhibit barrier properties to any contaminatingsubstances that may try to enter, or any desirable substances that may try to leave, acrossthe film. This property is resistance to diffusion. Since a film is very thin, it must havehigh mechanical properties such as tensile strength, impact resistance and tear strength.The mechanical properties usually depend on molecular structure, molar mass and molarmass distribution. Visibility through a film is often important, so low haze will be required.These are the bulk properties of the film [1].

The film will often be required to improve the appearance of an item contained within it,so surface properties such as gloss and printability are important. The latter property,printability, is related to a relatively high surface energy to achieve wetting and goodwork of adhesion. Suitable surface energy may be achieved through modification.Protection may also be improved if the friction is low; this property is called slip. Whena film is used to enclose and protect items, it may need to provide adhesion to itself or tothe contents. The immediate form of adhesion is called tack. Subsequently the polymermust flow to provide complete adhesion.

Manufacture of a film will usually be through an extrusion of the melt, so the meltrheology must be suited to the manufacturing process. Rheology is controlled by chemicalstructure, molar mass and long branches. The way in which the film is extruded, extendedand solidified by cooling will control the microstructure and hence many of the properties.A summary of the various polyolefins used in film manufacture is provided in Table 1.1.

In this chapter, polyolefin films are reviewed. First, the various types of polyolefins andtheir suitability for film manufacture are considered. The rheology, structure and propertiesof the polymers are discussed in relation to the type of film manufacturing processes thatare most applicable to the types of polymer. Post-extrusion modifications of the films,such as orientation, surface chemistry and additives, are discussed. Characterisationmethods used to measure film mechanical properties, structure and additives are described,as well as other more specific properties. Finally, some important particular applicationsthat require special structures or modifications are described.

6


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7

Technology of Polyolefin Film Production

1.2 Structures of the Polyolefins

1.2.1 Low-Density Polyethylene (LDPE)

Molecular structures of some example polyethylenes are shown in Figure 1.1. LDPEhas rheological properties that are suitable for production of film by the blown filmprocess [2]. LDPE has some long branches and many short branches. Typically, theremay be three long branches and 30 short branches per molecule. The molar mass isrelatively low, and it has a broad molar mass distribution. The melt strength, orzero-shear viscosity, and the shear-thinning nature of LDPE enhance processing. Thefilm has relatively low tensile strength but good impact strength. LDPE films showgood clarity (i.e., low haze) and gloss. The good clarity and gloss result from relativelylow crystallinity. LDPE is polymerised by the high-pressure radical process. Thereare two main reactor types, the autoclave and the tubular reactor. The autoclavetends to provide more branching and broader molar mass distribution. LDPE has abroad melting range, with a peak melting temperature of 110 °C. The density mayvary from 0.915 to 0.930 g/cm3 for LDPE.

Figure 1.1 Molecular structures for linear and branched polyethylenes (LPE and BPE)with 100 monomers, four or eight short branches, and one long branch of 40 carbons:

(a) LPE (100); (b) BPE (4, 100); (c) BPE (8, 100); and (d) BPE (8, L1(40), 100)

8


1.2.2 High-Density Polyethylene (HDPE, MDPE, UHMWPE)

HDPE has a linear structure, with little or no branching. HDPE is typically formed bythe Ziegler-Natta, Phillips or Unipol processes. Each process involves relatively lowpressure and is catalysed by an organometallic complex with a transition metal.Polymerisation is usually performed in slurry with a liquid such as heptane, or in the gasphase with the catalyst in a fluidised bed form. Variations of HDPE are ultra-high molarmass polyethylene (ultra-high molecular weight polyethylene, UHMWPE), where themolar mass is of the order of 1,000,000 g/mol, and medium-density polyethylene (MDPE),where some short branches are introduced by copolymerisation with a 1-alkene, such as1-butene. HDPE has higher crystallinity and therefore shows higher tensile strength thanLDPE, though its impact strength is deficient for many applications. UHMWPE providesincreased tensile strength due to the longer molecules providing more tie molecules betweencrystals. MDPE provides better impact strength because of its reduced crystallinity. HDPEshows a more Newtonian rheology than LDPE, and so is less suitable for extrusionprocessing, by either the blown film or cast film processes [3].

1.2.3 Linear Low-Density Polyethylene (LLDPE)

LLDPE is a copolymer of ethylene and a 1-alkene, typically 1-butene, 1-hexene or 1-octene, though branched alkenes such as 4-methyl-1-pentene are also used. These polymershave densities in the range 0.915-0.930 g/cm3 and they contain 2-7% (w/w) or about 1-2% mol/mol of the 1-alkene. They are polymerised using multisite catalysts such asZiegler-Natta with either a gas-phase or slurry process. Since the boiling temperature of1-octene is too high for the gas-phase process, the slurry process must be used. Thecomonomer composition has a broad distribution, so that some molecules, or segmentsof molecules, have few branches while others have many branches. This distribution isreflected in the broad melting temperature range of the LLDPE. The properties of LLDPEtend to be in between those of LDPE and HDPE. They have short branches but not longbranches, so that crystallisation-dependent mechanical properties are improved, butprocessing rheological properties are inferior to those of LDPE [4].

1.2.4 Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE)

Very- (density 0.89-0.915 g/cm3) (VLDPE) and ultra- (density <0.89 g/cm3) (ULDPE) low-density polyethylenes have higher copolymer content. ULDPE are also called polyolefinelastomers (POE) because of their properties. These polyethylenes have recently beencommercialised as a result of the new metallocene catalyst technology that allows highercomonomer levels and provides a narrower distribution of comonomer composition as

9

well as of molar mass. These polymers have lower melting temperatures, less crystallinity,greater toughness and elasticity, but lower tensile strength than other polyethylenes. Theymainly only have short branches, but some varieties also have some long branches [5].

1.2.5 Polypropylene (PP)

Isotactic polypropylene (iPP) is useful for temperature-resistant and glossy film production.iPP has greater strength and higher melting temperature than any of the other polyolefins.The crystal size can be made small by rapid cooling and/or nucleating agents, so thathighly transparent, high-gloss films can be produced. The rheological properties are notideal for the blown film process, but such processing is used in a two-stage extrusion andblowing process. Syndiotactic PP (sPP) is now becoming available commercially as a resultof metallocene catalyst polymerisations. sPP provides more elastic films than iPP. iPP hasmany advantages over polyethylenes because of its strength, thermal resistance, gloss andclarity. It is particularly suitable for more durable products [6].

1.2.6 Polypropylene Copolymers

Copolymers of propylene with small amounts of ethylene (0-5% w/w) provide increasedtoughness, at the expense of tensile strength. Random copolymers show the greatest propertychanges, such as increased elasticity and a decrease in melting temperature. Copolymers withmore block-like structure, where the ethylene is distributed in some of the molecules ormolecular segments, provide a good compromise in properties between toughness and strength.

1.3 Morphology of Polyolefin Films

All the polyolefins are semicrystalline polymers. The crystallinity provides the tensile strengthbut reduces the transparency. Larger crystals scatter transmitted light, producing anopalescent appearance, known as haze. Crystals on the surface reduce the surface smoothnessand cause surface scattering of incident light and reduce the gloss. An example of themorphology of a polypropylene film is provided in the optical microscope picture in Figure1.2. Processing conditions can modify the natural tendency of each polyolefin to providethese crystallisation-dependent properties. Rapid cooling will give smaller crystals. So theuse of cold rollers in the cast film process usually gives smaller crystals and in particulargreater surface smoothness. In the blown film process, the use of a refrigerated air streamincreases the crystallisation rate. Crystallisation is evident as a fogging of the film a shortdistance from the extrusion die; this is called the frost-line height. Orientation can bemeasured using wide-angle X-ray scattering (WAXS) [7].


10


Orientation of crystals will direct the axes of the crystals, and correspondingly the crystal-dependent properties, along the orientation or draw direction. Usually films are oriented,or drawn, in two orthogonal directions, called biaxial drawing, first parallel to theextrusion direction, then laterally. Drawing in the extrusion direction involves coolingthe melt until crystallisation takes place, then passing the film between rollers withincreasing differential speed. The lateral drawing depends on the method of manufacture.When the blown film process is used, orientation is provided during the blowing process.The cast film process requires a lateral drawing frame called a tenter. The edges of thefilm are grasped and the frame moves apart as the film moves forward. Orientationprovides enhanced physical properties in the drawn directions. When the film is biaxiallydrawn, the properties are greater in the direction that was drawn last [8].

1.4 Rheological Characterisation of the Polyolefins

1.4.1 High-Density Polyethylene

HDPE consists of linear molecules. The shear stress versus shear rate curve will beapproximately linear except for very high molar mass. The linear relationship isNewtonian. This means that at high shear rates, as experienced in processing, the viscosity

Figure 1.2 Polarised optical microscope picture of polypropylene blended with 30%poly(ethylene-co-propylene). The copolymer is a dispersed phase shown by the dark

regions mainly at the edges of the polypropylene spherulites

11

is high and so the force required for extrusion will be high. Another problem is that theviscosity at low shear rates is not increased. This zero-shear viscosity is related to themelt strength of the polymer. If the melt strength is low, the molten film may rupture asit emerges from the extruder as a tube that is then rapidly expanded by a gas pressure.High melt strength is required to resist rupture and create a dimensionally stable bubble.The melt strength is less critical in the cast film process, although the film must remainstable until it reaches the cooling rollers. The force required for extrusion will still be aproblem, since more energy will be needed to extrude a particular mass of polymer, andthis will require more electricity and a more powerful extruder motor. HDPE has hightensile strength, but low impact and tear strengths, so damage during processing bytearing is a potential problem. Processing of HDPE can be improved by blending withother polyethylenes, in particular LDPE.

1.4.2 Linear Low-Density Polyethylene

LLDPE typically have a broad molar mass distribution and a broad distribution of the 1-alkene comonomer, or branches. The tensile strength is lower than that of HDPE buthigher than for LDPE. They have improved toughness compared with HDPE. Thoughthey have short branching comparable with LDPE, they do not have long branches. Thelack of long branches decreases their shear-thinning rheological characteristics comparedwith LDPE and so processing is not as efficient. They are often blended with LDPE sincethe long branches enhance processing. They have greater tensile strength than LDPE,but, with their higher crystallinity, they are less transparent.

1.4.3 Very- and Ultra-Low-Density Polyethylene

VLDPE and ULDPE have many short branches distributed along the main chains moreevenly than for LLDPE where single-site catalysts have been used in the polymerisation.Short branches are not important in the rheology. These polymers will have essentiallyNewtonian behaviour. Polymers with very high molar mass have more pronounced shearthinning, though entanglements between only the main chains are not as effective asbetween several long chains such as when long branches are present.

1.4.4 Low-Density Polyethylene, Long Branches

LDPE has long branches that are known to provide non-Newtonian rheological response.LDPE is shear thinning, so that the power required for extrusion at typical high shearrates is less than proportional to the shear rate. This makes extrusion of LDPE more


12


economical than for other polyethylenes, and the extruder motor does not need to be aspowerful. At low shear rates, the viscosity rises significantly, so the zero-shear viscosity,or melt strength, is high. A typical rheology curve for LDPE is shown in Figure 1.3.LDPE has better bubble strength in the blown film process, so that resistance to burstingand bubble stability are greater prior to solidification. In the cast film process, the filmwill be stable in the molten state between the extrusion die and the cold rollers. The longbranches provide more intermolecular entanglements when the shear rate is low. As theshear rate increases, the long branches break free of entanglements and the viscositydecreases markedly. These rheological characteristics are of prime importance duringprocessing [9].

1.4.5 Polypropylene

Polypropylene has a methyl branch on each monomer unit and so a pseudo-asymmetriccarbon is present. This introduces tacticity, and the isotactic form of polypropylene isthe only one that is suitable for film formation. Polymerisation is performed using Ziegler-Natta and several other newer proprietary catalysts. The catalysts have been developed

Figure 1.3 Parallel plate, continuous shear rheology curve for low-densitypolyethylene at 200 °C

13

to provide maximum isotactic structure (>99%), so that atactic polypropylene does notneed to be extracted and the mechanical properties are maximised. High catalytic activityis desired so that residual catalyst does not need to be extracted from the polymer.

Polypropylene has a lower density than most of the polyethylenes (0.905 g/cm3) and ahigher strength. Its melting temperature at 162 °C is significantly higher than that ofHDPE, making it suitable to form retortable and microwave-resistant products. Theglass transition temperature is high, (e.g., 10 °C, but this varies with the crystallinity andmethod of measurement) so impact resistance is poor. Impact resistance is improved bycopolymerisation with ethylene. Usually about 5% ethylene is used, and blockycopolymers are formed, so that the ethylene-containing molecules form an immiscibledispersed phase in a matrix of homopolymer polypropylene. The toughness is increasedwithout decreasing the overall melting temperature significantly. Other randomcopolymers provide increased toughness and elasticity with decrease in tensile strengthand melting temperature.

Single-site or metallocene-catalysed polypropylenes have narrower molar massdistribution, though the isotacticity may not be greater. The copolymers with ethylenehave a more even distribution of ethylene, and so a very small proportion of ethylenewill provide a large decrease in melting temperature compared with the traditionalpolypropylene copolymers.

1.5 Blown Film Production (Tubular Extrusion)

Formation of film is by extrusion. The extrusion process involves a series of events thateach affect the stability and consistency of the extrudate and hence the film. The processesin the extruder include feed, melting, mixing, metering and filtration. The die is an annularshape that produces a tube of polymer. The tube is inflated by air pressure injected insideat the die. Inflation of the tube makes the film dimensions greater and provides orientationof the polymer. The tube passes through zones of cooled air, which solidifies the polymerand controls the crystallisation [10]. A diagram showing the essential features of theblown film process is shown in Figure 1.4.

In the formation of polypropylene, a two-step tubular orientation process is required.This is because of the poor melt strength of polypropylene. The film must first be cooledto enable crystallisation. The film is reheated to be just at the melting temperature andthe tube is blown again before passing through a cooling ring. A comparison of filmorientations in the transverse direction (TD) and machine direction (MD) shows theproperties to be similar if the stretching occurs simultaneously in each direction. Insequential stretching, the last stretching step predominates, so TD is usually stronger,


14


except for tear strength. Oriented PP (OPP) stretch ratios of 6 x 6 are common. Shrinkfilms can be prepared from LLDPE and copolymers of ethylene and propylene, butradiation modification is necessary to partially crosslink the polymer.

1.5.1 Extruder Characteristics

The extruder must be able to process a wide variety of polyethylenes with varyingmolar mass, molar mass distribution, comonomer content and comonomer distribution.Less powerful extruders may only be suitable for LDPE production, since its shear-thinning characteristics assist high throughput at lower power. Additives such asantioxidants, ultraviolet stabilisers, lubricants, slip agents and tackifiers may need tobe included at the extrusion stage, so a facility for separate injection or dry blending ofthese additives may be required. The extruder must provide the means to melt andconvey the molten polymer through a die that will produce the film. Typically, a single-screw extruder will be suitable. There are many types of single-screw extruders, but,generally, they are best suited to distributive mixing. Distributive mixing is where thecomponents only need sufficient mixing to provide a uniform melt. Twin-screw extrudersprovide more intensive mixing, and so are used when dispersive mixing is required.Dispersive mixing is where high shear is needed to subdivide a dispersed phase intosmaller particles, where the dispersed phase may be another polymer or a filler withaggregated particles [11].

Figure 1.4 Diagram showing the extrusion blown film process

15

1.5.2 Screw Design

The extruder screw is usually divided into three zones: feed zone, compression zone andmetering zone. The feed zone conveys the polymer pellets, filler and additives from thehopper into the main part of the extruder. In the compression zone, the polymer is melted,mixed with any other components and compressed into a continuous stream of moltenpolymer compound. The metering zone provides a uniform flow rate to convey the polymerto the die. Polyethylenes are semicrystalline polymers with a broad melting range,particularly if they are copolymers or have random branching, such as LDPE or LLDPE.The melting or compression zones of the screw must be broad. This is the region wherethe depth of flight is decreased to provide the compression. Polyethylenes have a highermolar mass than other polymers used for extrusion, so the melt viscosity is reasonablyhigh. Polyolefins have weak intermolecular forces, so the mechanical properties are derivedfrom a high molar mass and regularity of the chains for close packing. In addition to theforce required for extrusion, the strength of the molten films is important in successfulfilm formation. Of the polyolefins, polypropylene is the most difficult for film productionbecause it has relatively low melt strength. Very high molar mass will improve the filmformation, but make the extrusion part of the process more energy-consuming [10].

1.5.3 Frost-line and Blow Ratio

The molten film exits from the extruder through an annular die so that a tube of polymeris formed. The tube is sealed at the top as it passes between pinch rollers. The tube isexpanded using air pressure. The tube will only expand significantly when the polymeris molten. The rate at which the polymer exits from the die, the air pressure and theimpingement of external chilled air determine the blow ratio. The blow ratio is the ratioof the final tube diameter to the diameter of the annulus in the die. This ratio, togetherwith the width of the slot in the die, determines the film thickness and the transverseorientation of the film. The film is also oriented in the direction parallel to the die by adifferential between the speed of the polymer exiting the die and the speed of the pinchrollers pressing the tube flat and feeding it to the auxiliary equipment. The transverseorientation occurs up until the polymer solidifies and is often the dominant orientationfor properties. The blow ratio able to be used is limited by the melt strength of thepolymer. Linear polymers are more likely to exhibit film rupture in the melted region ofthe tube. Polymers with long branches have higher melt strength and so are much betterfor production of blown film. The rheology of the polymer is important for other aspects,since an unstable bubble may be formed [12]. The bubble should be symmetrical aboutthe centre-line of the die to the pinch rollers. If the film is uneven in thickness or insolidification, then symmetry will be difficult to control. A thicker portion of the filmwill be stronger and will resist blowing and so remain thicker. A thinner portion of the


16


film will expand easier and will become thinner, so this part of the film will bulge outwardseven more. The thinner part of the bubble may even rupture. Differential heater bandscan be placed around the film near the exit from the extruder to provide fine adjustmentof the film temperature as the film is expanded.

The frost-line is the point at which the polymer solidifies by crystallisation. Thetransparency of the polymer is decreased on crystallisation, and this is observed as asharp transition in the film not very far above the die. The frost-line depends on theextrusion speed and the temperature, as well as on the cooling air that is directed on tothe polymer tube from the outside. The cooling air is usually refrigerated, and itstemperature, velocity and angle of impingement on to the film may all be varied. Rapidcrystallisation will provide smaller crystals, and so the film will be clearer, apparent in alow haze, and have a smoother surface, apparent in a high gloss [13].

1.6 Cast Film Production

1.6.1 Extrusion Conditions

Cast film is extruded through a very thin horizontal slit die. The film is drawn from theextruder by calender rolls. This process does not expand the width nor decrease thethickness of the film, though the calendering occurs immediately after extrusion. Theextrusion process is the same as for other extrusions. The melted polymer must bedistributed evenly along a slit die, usually using channels in the die. The die is referredto as a coat hanger or fish tail die. The calender rolls are chilled so that they provide amelt quenching, giving smaller crystals than the blown film process. The film has avery smooth surface due to the calendering process [8]. The smooth surface can causeself-adhesion of the film, called blocking. An antiblocking agent may be added to reducethe blocking. Cast films will usually have superior gloss and low haze compared withblown films. Orientation of polypropylene flat film uses a tenter frame (chain) withclamps in the transverse direction, a quench roll, then reheating rolls followed by tenterand wind-up roll for the machine direction. The tenter frame is enclosed in an oventhat is used to heat and relax the film [6]. Figure 1.5 provides a schematic illustrationof the cast film process.

Coextrusion is used to make multilayer films by extruding several polymers at the onetime through a single complex die. Each individual polymer will have its own extruderfeeding into a central die. An individual polymer may be included into more than onelayer, yet it only need come from one extruder. Multilayer films are common despite thecomplexity of the equipment required for their manufacture. Each layer has a special

17

purpose in the film. The requirements for mechanical protection, diffusion barrierproperties, substrate and interlayer adhesion, and heat shrinkage cannot all be met througha single polymer. The most suitable polymer for each purpose can be chosen and assembledinto the multilayer structure.

1.6.2 Calendering Finishing

The melted film is usually cooled and pressed to provide a high-quality surface by passingit through a set of chilled calender rollers immediately after extrusion. The rollers will beof highly polished steel to provide a smooth glossy surface to the film. Rapid coolingalso assists formation of a glossy surface on the film, since the crystals will be kept smalland crystallisation may be minimised. A series of rollers may be used to provide orientationby stretching the film in a longitudinal direction.

1.6.3 Extrusion Coating

Extrusion coating is when the melted polymer film is extruded on to an existing filmbefore passing through the calender rollers. The existing film will be another polymer,metallic foil or paper. Multiple layers may be formed by extrusion coating both sides ofthe primary film or building a multilayer structure by introducing several extrusion-coated layers. Coextrusion can only be used for polymers with similar processingconditions. Where the processing conditions are different, particularly in the case ofsubstrates that cannot be melted with the polymer, such as metallic foils and paper, thenextrusion coating is the only choice [6].

Figure 1.5 Diagram showing the extrusion cast film process with calendering andextrusion coating


18


1.7 Orientation of the Film

1.7.1 Orientation During Blowing

Extrusion blow moulded film usually receives biaxial orientation. During blowing, the diameterof the extruded tube is increased, and this causes the structure of the film to be orientedperpendicular to the extrusion direction. Orientation should take place below the meltingtemperature when the polymer is crystalline so that the crystals are oriented. The expansionof the extruded tube will take place while the polymer is entirely melted, so that the effect ofblowing will not provide a level of orientation equivalent to the diameter expansion [14].

At the same time as the film is being expanded by blowing, it is being drawn by pulling alongthe axis of extrusion. This provides a parallel orientation. Again, most of the parallel drawingoccurs on the polymer melt between when it leaves the extruder and when it crystallises. Thecrystallisation region is called the frost-line. At the frost-line the film will have its maximumdiameter and resist further expansion or drawing compared with the region immediatelybefore the frost-line. At the frost-line the completely transparent melt becomes foggy due tocrystallisation. The change in opacity depends on the crystallinity of the particular polymer.

Sometimes additional orientation is imparted on the film after the blowing process. This isthe case if the film is to be a shrinkable film. Shrinkable films will contract upon heating. Thisis useful for providing tightly fitting wrapping.

1.7.2 Orientation by Drawing

Orientation of a polymer must be performed at a temperature between the glass transition andmelting temperatures. Polyolefins, particularly polypropylene, are normally moderately heated.The enhancement of tensile properties is directly related to the draw ratio. After drawing, thefilm should be further heated to relax or set the structure. This will provide dimensional stability.If the film is to be heat-shrinkable, then the relaxation is not performed. Some crosslinkingfrom radiation treatment prior to drawing may be used to increase the plastic memory effect inthe film. Excessive drawing can cause strain hardening because of the introduction of extendedchain crystals at the expense of chain-folded crystals. The strain-hardened film will have astiffer or more leathery feel; it will lose elasticity and may have a rougher surface.

1.7.3 Biaxial Orientation (Biaxially Oriented PP, BOPP)

It is desirable to orient films in planar directions, parallel and perpendicular to the flow.The parallel orientation can be provided by a draw-off faster than the extrusion speed.

19

The perpendicular direction has been described for blown film production. In extrudedsheet, a frame that attaches to the edges of the film and moves apart must provide theperpendicular orientation. This is called a tenter frame. The film is often drawn to aboutthree times its original width. As well as adding strength to the film, a thinner gauge offilm can be produced. The drawing process overcomes the effect of die swell that occursas the film leaves the die. Orientation of cast film is illustrated in Figure 1.6.

1.8 Surface Properties

1.8.1 Gloss

Gloss is the reflection of light from a surface. The nature and origin of gloss and haze areillustrated in Figure 1.7. A high gloss requires a smooth surface. Surface imperfections maybe introduced by the processing. Excessive drawing into the strain-hardening region willusually reduce the gloss. Blown film usually has a lower gloss, since crystallisation of thefilm at the frost-line introduces surface roughness due to the crystals. Rapid crystallisationof the film by the use of chilled air impinging on the bubble reduces the size of crystals andimproves the gloss. Extrusion cast film passes through chilled rollers after leaving the

Figure 1.6 Schematic diagram for biaxial orientation of cast film


20


extruder. The rapid cooling and the polished surface of the rollers provide a high-glosssurface. Extrusion cast films have the higher gloss, but the extrusion blown process producesfilm at a lower cost. The rheology of the polymer will contribute to the surface of the film.Shark skin is the term applied to a rheological problem in the processing [15].

1.8.2 Haze

Haze is an internal bulk property, but, because of the importance of appearance, it hasbeen considered along with surface properties due to its relation to gloss (see Figure 1.7).Crystallinity, optical defects, ‘fish eyes’, phase separation of blends, contaminants, gelparticles and dispersion of pigments (carbon black) are structures that increase haze.Haze is the internal scattering of light. Haze makes it difficult to clearly see an objectthrough a film as a result of the interference from randomly scattered light reaching theviewer in addition to light coming straight from the object. Smaller crystals provided bya nucleating agent will decrease haze. The other phenomena described above can also bereduced by nucleating agents, better formulation and processing.

1.8.3 Surface Energy

The surface energy of polyolefin films is very low. It is difficult to find other substancesthat will adhere to polyolefins. Suitable adhesion can be obtained by melt adhesion ofpolyolefins to each other, but only when the polyolefins are very similar. For instance,polyethylenes have good mutual adhesion. The branched polyethylenes with lower meltingtemperature are most used because they can be melted more rapidly and they have suitable

Figure 1.7 Diagram showing the origin of gloss and haze in polymer films

21

rheology to flow on to the adherend. Melt adhesion of films will require higher-meltinglayers other than the surface layer, since if more than the surface layer melts then thestructural integrity of the film will be destroyed.

Copolymers of ethylene with vinyl acetate, methyl acrylate, acrylic acid, maleic acid andmany other polar monomers are used to increase the surface energy of polyethylenes tomake them more readily wettable. Similarly, polypropylene can be grafted with maleicanhydride to increase the adhesion of other substances to it. The surface energy ofpolyolefins is also increased through corona discharge treatment.

1.8.4 Slip

Polyolefin films generally have a smooth surface, with the exception of defects and surfacecrystal structures. They have a low surface energy and so frictional forces are low. Relativeto their strength, the frictional forces can cause damage to the films. Slip additives candecrease the frictional forces. The factors that cause poor slip are often desired for otherattributes such as adhesion of printing and adhesion to other surfaces in packaging. Additivesthat increase self-adhesion of packaging films will decrease the slip, so that desirableproperties are not universal – they depend on the intended application of the film.

1.8.5 Blocking

The low surface energy and softness of polyethylenes make them self-adhere if pressedtogether under a load for a considerable time. This self-adhesion is called blocking. Thepolyolefins can flow, or creep, under load, and so mutual adhesion can occur if thepressure is sufficient or the time of contact is long. This is a significant problem in largefilm rolls or when film is stacked in large quantity. Blocking is reduced when the surfaceis less smooth, such as when crystallisation or processing conditions causemicrotopographies. A smooth surface with high gloss and clarity is generally preferred,so that blocking will be a serious problem.

1.9 Surface Modification

1.9.1 Corona Discharge

Processes for modifying the surface properties of plastic films are important. Surfaceadhesion can be increased by oxidative treatments such as corona, flame, priming orsubcoating. Corona discharge is most widely used, and surface oxidation of the film


22


occurs, giving polar functional groups [16]. A corona discharge treatment facility isshown in Figure 1.8. Increased polarity will increase the surface energy and enablewetting by inks or adhesives. Printing is carried out by the flexographic technique,whereby a rubber roller with raised imprints imparts the ink, or by the rotogravureprocess, whereby an engraved steel roller imparts the ink. Film coating, film wetting,dispersion coating (by an emulsion), solvent coating, barrier and heat seal coatingsare all improved after oxidative surface treatment. Extrusion coating is the processwhereby a film is extruded on to an existing film. Film lamination occurs when existingfilms are bonded together with an adhesion layer applied by any of the previouslymentioned methods. Typical are the use of poly(vinylidene dichloride), aluminiumfoil and ionomer films, and metallisation by vapour deposition of aluminium.Coextrusion is the process in which two or more films are extruded and broughttogether in the die. Each layer will contribute to specific properties, such as barrierand adhesive layers.

1.9.2 Antiblocking

Blocking can be reduced by decreasing the surface contact area of the films. Smallparticles at the surface can decrease the contact. Particles such as silica, diatomaceousearth and talc are useful antiblocking agents [17]. To provide the protection efficiently,they must be included in the polymer during processing. Many of the particles willbe in the bulk of the film, so they will not contribute to antiblocking characteristics.Those particles that exist at the surface will be active. The particles must be smallenough not to introduce haze when in the interior or decrease gloss when at thesurface. Films that have a rougher surface due to surface crystallites or otherimperfections will have a natural protection against blocking. The mechanism ofantiblock additives is shown in Figure 1.9.

Figure 1.8 Corona discharge surface treatment system

23

1.9.3 Slip Additives

The coefficient of friction can be reduced by adding slip agents. The requirement for a slipagent is that it is miscible with the polymer melt, but separates as the polymer is crystallising.Sometimes the separation will take several hours or days to become complete. The slip agentshould form a very thin layer on the surface. The layer is more effective if it is randomlystructured. The mutual miscibility and separation are important, and long-chain amideshave been found to be best when the chain length is about 22 carbon atoms, whereas 18carbons is less effective. An amide with a cis double bond is better than the fully saturatedanalogue because the cis double bond prevents surface crystallisation of the amide comparedwith the saturated amide. Erucamide has thus been found to be the most effective slip agentfor polyolefins [18]. Other fatty amides such as ethylene bis-stearamide, oleamide andstearamide have been used. The function of a typical slip additive is shown in Figure 1.10.

Figure 1.9 Mineral particles (silica or talc) prevent blocking, since some particlesbecome located at the surface and so limit contact of the films

Figure 1.10 Structure of erucamide (C21H41CONH2, cis-13-docosenamide) slip additive, andits mechanism for increasing slip. The diagram shows an array of erucamide molecules along

the surface of a film, with the bent cis structure ensuring that they stay irregular


24


1.9.4 Lubricants

Lubricants are added to assist with processing. The polymer can be extruded more readilywhen there is a layer of lubricant between the polymer and the surfaces within the extruder.This is particularly important at the die lips, where the lubricant can decrease surfaceirregularities such as ‘shark skin’ effects. The melt can maintain laminar flow at higherextrusion rates with lubricants present. Common lubricants are stearic acid, stearatesalts, paraffin wax and chlorinated paraffins [19]. A fluoropolymer has been reported asa processing additive for polypropylene [20].

1.9.5 Antistatic Agents

Polyolefins do not absorb water. The dry surface causes a build-up of static electricity.Antistatic agents are polar substances that are mixed with the polymer and migrate tothe surface after the polymer emerges from the extruder. Polyoxyethylenes are one typeof antistatic agent. When on the surface, they can absorb sufficient water to preventstatic electricity build-up. A problem is that they can attract dust and other materialsthat would not normally adhere to the polyolefins. Another problem is that they can beeasily removed by friction or extraction with polar liquids [19].

1.10 Internal Additives

1.10.1 Antioxidants

Immediately the polyolefin is made, it requires protection from oxidation. This isparticularly the case during the processing stages when high temperatures are used.Hindered phenols and triaryl phosphites are typically used. During the extrusion process,the polyolefin is then protected from the heat and oxygen. During the lifetime of thefilm, other thermal processes may be encountered during printing, packaging of foodand heat sealing. The antioxidants act by removal of radicals from the polymer – byhydrogen donation in the case of hindered phenols, and by removal of oxygen fromperoxy groups in the case of phosphites [18]. Figure 1.11 shows a typical hindered phenolantioxidant and a triphenyl phosphite secondary antioxidant.

1.10.2 Ultraviolet Absorbers

Films that are to be used in the sunlight require protection. While the polyolefins do notabsorb ultraviolet light, they do contain many other anomalous functional groups,additives and impurities that do absorb. Hydroxybenzophenones, hydroxybenzotriazoles

25

and tetramethylpiperidines (hindered amine light stabilisers, HALS) are typical compoundsused for ultraviolet stabilisation. Factors such as compatibility with the polyolefin, lowvolatility and absence of any colour are just some of the many stringent requirements ofthese additives. Hydroxybenzophenones and hydroxybenzotriazoles absorb ultravioletlight and form an excited electron state that can undergo a radiationless transfer of theenergy to heat, to regenerate the ground state. HALS act by a cyclic mechanism in whichthey form a nitroxide that can couple with a radical and then transfer the radical toterminate another radical and release the nitroxide. HALS are the most effective stabilisersfor polypropylene, but the type of HALS used depends on the application of thepolypropylene and the other components in the composition [17]. Figure 1.11 shows atypical hindered amine light stabiliser and a hydroxybenzophenone ultraviolet absorber.

1.11 Mechanical Properties

There are many standard test methods to define the performance requirements of polyolefinfilms. These are specified in the relevant ASTM, DIN and ISO standards. The materialmechanical properties are used for polymer specification for a particular purpose. Thereare many tests available, and each measures a narrowly defined property. Often it is difficultto predict performance by a material property, and so a product-specific test is designed.

Figure 1.11 Structures of common heat and light stabilisers: (a) hindered phenolantioxidant: (b) tris(2,4-di-tert-butylphenyl) phosphite secondary antioxidant; (c)

hydroxybenzophenone ultraviolet absorber: (d) hindered amine light stabiliser


26


The mechanical properties can be separated into tensile and impact tests, though there aremany other tests, such as tear testing, abrasion resistance and adhesion tests. The morphologyof the film is of major importance in controlling the mechanical properties [21]. Themorphology of a film is strongly connected with the key processing variables [8].

1.11.1 Tensile Properties

1.11.1.1 Strain Rate and Tensile Properties

The strain rate is important in measuring the tensile properties of films. During packagingoperations, films are often subjected to extremely high strain rates in the machinery.These processes must be duplicated in the testing procedures where possible. Slow strainrates may be preferred when material properties are measured to distinguish betweenvarious polyolefin structures. Typical parameters that are obtained from a tensile test arethe modulus, yield stress, break stress and elongation at break. The area under the stress-strain curve is used as a measure of the energy to break.

1.11.1.2 Strain Hardening

When a polyolefin film is considerably extended, the crystal structure orientation will besignificant and the polymer will become harder. Often a strain-hardened film is describedas leathery. The morphology may change from a chain-folded to an extended-chainconfiguration. The tie molecules become fully extended so that further strain is limitedbefore film breakage occurs. Strain hardening is a limit to the useful elongationalperformance of a film because the elasticity is lost. A stress-strain curve for linear low-density polyethylene showing the yield and strain-hardening regions is shown in Figure 1.12.

1.11.1.3 Stress Relaxation

Stress relaxation is defined as a change in stress when the material is under constant strain.When a packaging film is stretched around an article, or a large number of articles, theelasticity of the film will keep the article(s) under constant tension to provide protection andease of transport and handling. When stress relaxation occurs, the tension of the packagingis lost, and the contents are no longer held together. Stress relaxation can be measured witha standard tensile testing instrument, where the stress is measured over time while the specimenis held under a constant strain. Polyolefins with high molar mass and high crystallinity willbe the most resistant to stress relaxation. An illustration of the processes involved in stressrelaxation is shown in Figure 1.13; first the amorphous molecules are elongated, then onrelaxation they slide past each other to return to a random-coil conformation.

27

Figure 1.12 Stress-strain curve for linear low-density polyethylene showing the yield stressafter the initial linear elastic region, then the strain-hardening region after 350% strain

Figure 1.13 Schematic for the mechanism of creep and stress relaxation

1.11.1.4 Creep

Creep is the change in strain when the specimen is subjected to a constant stress. Polyolefinfilms that are exposed to pressure for extended periods will gradually elongate. Pressures


28


are generated inside many packages, and polymer will creep, causing the package tobecome larger. Creep is a complementary property to stress relaxation, and so the samemolecular characteristics resist creep. Creep is illustrated in Figure 1.13; under constantload the molecules gradually slide past each other, resulting in elongation.

1.11.1.5 Burst Strength

When a film experiences a high pressure over a short time, it may burst. This is similar toelongation at break, except that the film will be under a biaxial tension. A test of burststrength may be more suitable for a film under pressure than a tensile test. The short-term nature of the test is in contrast to the long time for creep.

1.11.2 Impact Properties

1.11.2.1 Dart – Puncture Resistance

An impact test is a short-term test; the stress is applied very quickly. The dart test caninvolve either a dart falling through a constant distance or a dart propelled by gas pressure.The shape of the end of the dart is an important factor in the test. A rounded end isgenerally used, but the test can be modified to measure the resistance of a film to anypuncture by impact with any particular object. The falling dart test will result in a pass/fail type result compared with a specification for the load. The instrument can be designedto measure the deceleration of the dart as it passes through the film. The energy requiredto break the film is then calculated from the energy lost by the dart. In this latter situation,the dart is always required to break the film.

1.11.2.2 Tensile – Tear Strength

Films often break in shear instead of tension. A film can be tested in a tensile instrumentso that the strain is applied under shearing conditions. A cut may be made in the film todirect the tear. The geometry and conditions of the test are defined so that standardconditions are used. The Elmendorf tear strength test is used to measure the performanceof films, and it has been related to processing conditions and dart impact strength [22].

1.11.2.3 Tensile Impact

This is a short-term impact test where the specimen is mounted in a pendulum testinstrument so that it receives a rapid tensile force as the pendulum strikes the specimen

29

holder. The tensile impact test is applicable to films, whereas other forms of pendulumimpact tests, such as Izod and Charpy tests, require more rigid specimens. The tensileimpact test can apply a greater strain rate than a typical tensile test instrument.

1.11.3 Dynamic Mechanical Properties

Dynamic mechanical analysis (DMA) measures the properties when an oscillatingstress is applied to the material. The stress and strain are often out of phase, and thissituation can be used to obtain the viscoelastic properties. The viscous or time-dependent properties are out of phase with the stress, while the elastic or instantaneousproperties are in phase with the stress. The in-phase property is called the storagemodulus, in that the elastic energy is stored and can be subsequently released whenthe stress is removed. The out-of-phase property is called the loss modulus, in thatenergy is lost to heat during viscous flow. The properties are usually measured withtemperature and/or frequency. Temperature and frequency can be combined to providea time-temperature transposition, so that long-term or very short-term propertiescan be measured within real-time limitations [23]. Figure 1.14 shows DMA curvesfor polypropylene.

Figure 1.14 DMA curve for polypropylene, showing the storage modulus, E′, lossmodulus, E″, and damping factor, tan δ


30


1.11.4 Dielectric Properties

Polar olefin copolymer films, such as poly(ethylene-co-vinyl acetate), poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate), and blends or laminates canbe characterised using dielectric analysis. The dielectric properties of all types ofpolyolefins are important because of the electrical applications of these polymers.However, dielectric analysis can also be performed in an analogous manner to DMA,in that the mechanical force is replaced by a voltage and the permittivity is measuredwhile the temperature and/or frequency is varied. The viscoelastic properties of thepolymer can be measured by the dielectric response over a wider range of frequenciesthan the mechanical tests. The dielectric test is more sensitive than the mechanical testwhen the polymer has more polar groups. Figure 1.15 provides a schematic of theprocesses involved in a typical dielectric analysis.

Figure 1.15 Dielectric analysis of polar ethylene copolymers involves measurementof the storage (permittivity) and loss (loss factor) components with temperature

)C(ecnaticapaCI tnerruc=V egatlov=f ycneuqerf=δ elgnaesahp=

(ecnatcudnoC R 1- )

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(rotcafssoL ′′ε )ω ycneuqerfralugna=σ ytivitcudnoc=

CIV f

= sinδπ2

RI

V− =1 cosδ

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⎞⎠⎟⎛⎝⎜

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′′ = ′′ +ε ε σωεdipole

0

31

1.12 Microscopic Examination

1.12.1 Optical – Polarised Light Effect with Strain

The crystallinity of a polyolefin in a film can be viewed with an optical microscope usingpolarised light. The film must be very thin, although reflected light can be used forthicker opaque films. The microscope can be combined with a hot stage to viewcrystallisation and melting events.

1.12.2 Scanning Electron Microscopy (SEM) – Etching

Scanning electron microscopy (SEM) can reveal morphology, though usually the samplemust be etched so that amorphous regions are modified more than crystalline, or acomponent in a blend is eroded more rapidly. The etching creates a new surface topographythat can be viewed with the SEM. Care must be taken so that the electron beam does notdamage or create artefacts on the surface.

1.12.3 Atomic Force Microscopy (AFM)

The surface of polyolefin films can reveal information about the bulk. Crystal growth at thesurface and other irregularities that may arise from processing or treatments such as coronadischarge can be studied using atomic force microscopy. Variations in the hardness or frictionacross the surface as found in blends can be studied to reveal the distribution of componentsacross the surface. Figure 1.16 shows an atomic force microscope picture of the surface oflinear low-density polyethylene formed by the extrusion blow moulding process.

1.13 Thermal Analysis

1.13.1 Differential Scanning Calorimetry (DSC)

DSC is used to measure the crystallisation and melting temperatures of polyolefins aswell as the enthalpies of crystallisation and melting. The results shown in Figure 1.17 areused to identify, characterise and measure the crystallinity [24]. The thermal history andmechanical stresses of a film can be investigated through the melting and crystallisationresponse measured by DSC [25]. The crystallisation temperature increases with nucleation,so that the efficiency of added nucleation agents can be measured. The crystal structurevaries with processing conditions and other treatments such as orientation, and thesecan be measured by analysis of the melting of the polyolefin on heating.


32


1.13.2 Temperature-Modulated DSC (TMDSC)

TMDSC is analogous to DMA in that an oscillating force, in this case a temperatureprogramme, is applied to the sample. The response can be resolved into reversing and non-reversing specific heat capacities. Recrystallisation, rearrangement of the crystals and meltingcan be studied simultaneously. This is important for an understanding of the equilibration ofthe morphology of the polyolefins after various processing or other thermal treatments [26].

1.14 Infrared Spectroscopy

1.14.1 Characterisation

Infrared spectroscopy is a convenient method to identify polyolefin films. The main classesof polyolefins can be easily identified. A detailed interpretation of the infrared spectrum

Figure 1.16 Atomic force microscope picture of the surface of linear low-densitypolyethylene film prepared by the extrusion blown film process

33

will enable even very similar structures to be distinguished. The extent of branching canbe measured and crystallinity can be indirectly measured [27].

1.14.2 Composition Analysis of Blends and Laminates

Blends of polyolefins can be identified, and, when the components are known, quantitativeanalysis can be performed. The component layers of laminates can be identified afterseparation of the layers, or by analysis of the edge of the film using an infrared microscope.

1.14.3 Surface Analysis

Surface additives, such as glyceryl monooleate, polyisobutylene and slip agents, and coronatreatments can be measured using surface infrared spectroscopic analysis. Multiple internalreflection is the common method, but specular reflectance and grazing angle reflectanceare other useful techniques by which the infrared spectrometer can be used to studysurface chemistry. Figure 1.18 shows a schematic for the surface analysis of a polymer

Figure 1.17 DSC curves for melting of linear low-density polyethylene and low-densitypolyethylene (for clarity, this curve has been shifted upwards by five units)


34


film by multiple internal reflection. The angle of the infrared beam to the surface of theinternal reflectance element (typically zinc selenide or germanium) along with thewavelength of the infrared beam determine the depth of penetration into the surface ofthe film. A low depth of penetration will provide spectra more sensitive to the surfaceadditives or modifications.

1.14.4 Other Properties

1.14.4.1 Thickness

Thickness is known as gauge. Uniformity must be achieved by the manufacturing process,Generally films are considered to be of ≤250 μm; greater thicknesses are called sheet.Some films may be 10-20 μm in thickness, and individual layers in multilayer films areoften only 5 μm thick.

1.14.4.2 Moisture Resistance

Polyolefins are nonpolar, so they are particularly efficient at resisting moisture. Theirresistance to liquid water is not necessarily carried on to their resistance to water vapouror humidity. High-density polyethylene film is the most resistant to water vapour becausegas molecules have difficulty diffusing through the crystalline structure.

Figure 1.18 Apparatus for multiple internal reflectance (attenuated total reflectance)infrared spectroscopy for surface analysis of polymer films

35

1.14.4.3 Gas Permeation

Films are used to encase and protect other items. An important property is to resist gaspermeation, in particular, the permeation of oxygen, carbon dioxide and water vapour.Polyolefins provide very poor barrier layers. Usually a multilayer film that includes anotherpolymer or other material is required to provide suitable barrier properties. The barrierproperties are increased when higher-crystallinity polymers are used. The density of theamorphous phase of the polymer has been found to be a guide as to the permeationresistance [28].

1.14.4.4 Orientation

Orientation of a film may be measured by annealing the film at temperatures below themelting temperature. Oriented films will shrink more than others. The shrinkage is oftencontrolled by partial crosslinking of the film. There are many applications in the foodindustry for shrink-wrapping of produce. Shrink-wrapping of polyethylene films can beapplied over other packaging materials to group containers into specific quantities.

1.14.4.5 Dimensional Stability

Dimensional stability is usually a consequence of orientation. Under heat treatmentssuch as may be experienced during packaging of hot foods, sterilisation processes forfoods and heat sealing, the dimensional stability of the film must be maintained. This isthe opposite requirement to shrink-wrap films. Humidity can contribute to changes indimension, due to absorption of moisture by other components of a multilayer film orby the contents of the film package.

1.15 Applications

1.15.1 Packaging

The main application of films is for packaging (Figure 1.19). The functions of a packagecan be summarised as: containment, dispensing, preservation, protection, communicationand display. Packaging machinery is diverse, (e.g., vertical fill, shrink-wrap, sleeve-wrap,stretch-wrap and blister packaging machines), and thus requires many properties of thefilm, (e.g., stiffness, stretchability, heat sealability), which in addition must be suitablefor various applications, (e.g., in side weld bags, in bottom seal bags and to hold liquid


36


products). Many of the films are required to be multilayer films to achieve all of thedesired properties [14]. Polyethylenes with new improved properties due to high molarmass are being used in heavy-duty applications [29].

1.15.2 Laminated Films

The performance requirements for packaging are increasing, with new demands forprotection of the product, speed and ease of packaging and sealing, together with printing,handling and storage requirements. An individual polymer cannot meet all theserequirements, so multilayer films are necessary. These can be prepared by extrusion offurther layers on to existing films or adhering existing films together. This process iscalled lamination. Most of the layers are polymer, but a metal foil (usually aluminium)may be used. Paper or paperboard is frequently used as substrates for the lamination. Atextile layer may be used, but these are mainly classified separately from laminated films.

Figure 1.19 Examples of typical packaging applications for polyolefin films

37

A basic requirement for a laminated film is good adhesion between the layers. Thematerials in the layer will often be chemically different, to provide the diversity ofproperties, and so adhesion may not be suitable. In such cases, a separate adhesive layermust be included between the functional layers. The adhesive layer functions in the sameway as a compatibiliser in polymer blends. Often a copolymer will be used, where eachof the component monomers will contribute to the adhesion with one of the adjacentlayers. Some multilayer films are produced with the adhesion component included as ablend with a functional layer. In this way the number of individual layers is reduced,simplifying the lamination process.

The separate layers of the film may consist of the same polymer but in different forms. Alayer could be mineral-filled, pigmented, foamed, oriented, radiation or chemicallycrosslinked, include antioxidant or ultraviolet stabilisers, be printed or otherwise modified.A further protective layer may then be placed over the modified layer, or the modifiedlayer may be the protective layer.

Lamination of films is often preferred to application of a polymer layer as a lacquer sincethe latter includes solvent or an emulsion is used, so that drying and removal of volatilesis required. The laminated film will often have superior gloss such as when polypropyleneis laminated on to printed paper substrates. Surface spreading by the coating layer andfilm thickness will be easier to control by laminating than by solution coating. Ovensand solvent removal systems can be replaced by calender cooling rollers.

1.15.3 Coextruded Films

Coextruded films contain most of the characteristics of laminated films, except that allof the layers are formed by extrusion at the same time. This precludes aluminium foil,paper and textile layers, which must be included by lamination. The coextrusion requiresseveral extruders with a common die. The die has a complex construction and may be ofeither a tubular film type or a cast film linear slit type. The polymer for each layer is fedinto the die by a separate extruder. One extruder per layer may be used or, if more thanone layer contains the same polymer, then its extruder can feed into more than one layer.The latter connection uses the minimum number of extruders, but is less flexible thanthe ‘one extruder per layer’ configuration.

After coextrusion, all of the layers must receive any treatments together. For instance, ifa layer is to be biaxially oriented, then all layers must be oriented. If a layer is to beradiation-crosslinked, then all of the layers must receive the radiation treatment. Individuallayers cannot be printed, then covered with another protective layer. An advantage isthat the complete multilayer film is produced in one process. The process is not suitablefor polymers with significantly different processing temperatures. Shrinkage stresses mayoccur on cooling and so layers may separate.


38


1.15.4 Heat Sealing

Polyolefin films are joined by heat sealing, a process whereby the film is partially meltedwhile pressing against the other component for a well-defined short time. The strengthof adhesion is characterised by the time-temperature-pressure relationship and theproportion of the polymer melted. Melting will destroy the original crystal structure andany orientation in the film, so the mechanical properties will be changed. A multilayerfilm is preferred so that only the surface adhesive layer is partially melted, while thestructural layer is unaffected. This requires that the surface layer have a lower meltingtemperature than the structural layer [30].

Typically, if the structural layer is polypropylene, then the surface layer can be an ethylene-propylene random copolymer, with a small proportion of ethylene. The adhesive layermay also be grafted with maleic anhydride to provide better adhesion to polar substrates.If the structural layer is a polyethylene, such as LLDPE, then the adhesive layer can be anethylene copolymer, such as poly(ethylene-co-vinyl acetate). Recently VLDPE have beenused as adhesive layers where low surface energy for better wetting is important.

1.15.5 Agriculture

Films for agriculture have become important to protect crops and to protect and bindproducts. The very thin films can provide sufficient tensile strength to contain the product,and with self-adhesion of the film the wrapping process is very efficient. The films maybe pigmented black to protect the contents from sunlight. Pigmented film may be spreadon the ground to control weeds by removal of sunlight and reduce evaporation of water[31].

1.16 Conclusion

Polyolefin films are complex in their manufacture by the blown film extrusion andextrusion cast film processes. The films are typically strengthened by biaxial orientationand can be made very thin. The inert polyolefin surface is often modified by oxidation toprovide polar groups for adhesion, and by the use of additives to provide slip or toprevent blocking. Specialised properties such as gas barrier, printability, heat sealing andshrinkability are achieved by coextrusion or lamination of films with different chemicalstructures. The diversity of films available may seem simple, but when the details areconsidered even a common packaging film involves a complex range of technologies.Further advances will enable films to possess even more specialised functionality whilebeing thinner and stronger.

39

References

1. K.J. McKenzie in Kirk-Othmer, Concise Encyclopedia of Chemical Technology,Wiley-Interscience, New York, NY, USA, 1999, 845.

2. J.A. Brydson, Plastics Materials, 4th Edition, Butterworths, London, UK, 1982, 187.

3. H. Saechtling, International Plastics Handbook, Carl Hanser, Munich, Germany,1983, 347.

4. D.Y. Chiu, G.E. Ealer, F.H. Moy and J.O. Buhler-Vidal, Journal of Plastic Film andSheeting, 1999, 15, 2, 153.

5. A.K.C. Mehta, M.C. Chen and C.Y. Lin in Metallocene-Based Polyolefins –Preparation, Properties and Technology, Eds., J.K. Scheirs and W. Kaminsky, JohnWiley, Chichester, UK, 2000, 463.

6. E.P. Moore, Polypropylene Handbook, Carl Hanser, Munich, Germany, 1996, 334.

7. H. Fruitwala, P. Shirodkar, P.J. Nelson and S.D. Schregenberger, Journal of PlasticFilm and Sheeting, 1995, 11, 4, 298.

8. J.A. Degroot, A.T. Doughty, K.B. Stewart and R.M. Patel, Journal of AppliedPolymer Science, 1994, 52, 3, 365.

9. F.J. Velisek, Journal of Plastic Film and Sheeting, 1991, 7, 4, 332.

10. F. Rodriguez, Principles of Polymer Systems, 4th Edition, Taylor and Francis,London, UK, 1996, 451.

11. S.W. Shang and R.D. Kamla, Journal of Plastic Film and Sheeting, 1995, 11, 1, 21.

12. P.J. Carreau, A. Ghaneh-Fard and P.G. Lafleur, Proceedings of ANTECH ‘98,Atlanta, GA, USA, 1998, Volume 3, 3598.

13. A. Ghaneh-Fard, Journal of Plastic Film and Sheeting, 1999, 15, 3, 194.

14. K.R.J. Osborn and W.A. Jenkins, Plastic Films: Technology and PackagingApplications, Technomic, Lancaster, PA, USA, 1992, 141.

15. V. Firdaus and P.P. Tong, Journal of Plastic Film and Sheeting, 1992, 8, 4, 333.

16. E. Foldes, A. Toth, E. Kalman, E. Fekete and A. Tomasovszky-Bobak, Journal ofApplied Polymer Science, 2000, 76, 10, 1529.


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17. R. Gachter and H. Muller, Plastics Additives Handbook, Carl Hanser, Munich,Germany, 1985, 97.

18. G. Pritchard, Plastics Additives: An A-Z Reference, Chapman and Hall, London,UK, 1998, 633.

19. J.D. Stepek and H. Daoust, Additives for Plastics, Springer-Verlag, Berlin,Germany, 1983, 243.

20. S. Amos, Modern Plastics, 2000, 77, 10, 131.

21. D. Ferrer-Balas, M.L. Maspoch, A.B. Martinez and O.O. Santana, Polymer,2000, 42, 4, 1697.

22. W.D. Harris, C.A.A. Van Kerckhoven and L.K. Cantu, Proceedings of ANTEC‘91, Montreal, Canada, 1991, 178.

23. K.P. Menard, Dynamic Mechanical Analysis, CRC Press, Boca Raton, FL, USA,1998, 61.

24. M.M. Jaffe, J.D. Menczel and W.E. Bessey, in Thermal Characterisation ofPolymeric Materials, Ed., E.I. Turi, Academic Press, New York, NY, USA,1997, 1956.

25. V.B.F. Mathot in Calorimetry and Thermal Analysis of Polymers, Ed., V.B.F.Mathot, Carl Hanser, Munich, Germany, 1993, 231.

26. J.J. Janimak and G.C. Stevens, Thermochimica Acta, 1999, 332, 2, 125.

27. W. Klopffer, Introduction to Polymer Spectroscopy, Springer-Verlag, Berlin,Germany, 1984, 53.

28. G. Loeber, Kunststoffe Plaste Europe, 1999, 89, 12, 33.

29. J.J. Wooster and B.A. Cobler, Tappi Journal, 1994, 77, 12, 155.

30. F. Martinez and N. Barrera, Tappi Journal, 1991, 74, 10, 165.

31. O.J. Sweeting, The Science and Technology of Polymer Films, Wiley-Interscience,New York, NY, USA, 1971.

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2 Processing of Polyethylene Films

Amin Al-Robaidi

2.1 Introduction

A plastic is solid in its unprocessed and processed states. It is softened enough throughthe application of a combination of heat, pressure and mechanical working to be formedinto a variety of shapes such as car bumpers, containers and plastic films. Mostthermoplastic polymers are linear polymers. However, these chains twist and turn aroundto form a tangled structure [1].

Commodity polymers are low in cost and high in volume. They are used to a greatextent in our day-to-day lives [2]. Worldwide usage is enormous and is increasing yearby year. Rates of increase up to 5% per year are not unusual. Of these commoditypolymers, polyolefins play the largest role, and therefore their production is of greatimportance. Polyolefins represent about 45% of total plastic usage: linear low-densitypolyethylene (LLDPE) and low-density polyethylene (LDPE) make up 51% of totalpolyolefin use, the rest being 23% polypropylene (PP) and 26% high-densitypolyethylene (HDPE). In this chapter, some of the important properties, productiontechniques, and chemical and physical phenomena that are of interest for polyolefinswill be explained.

Polyethylenes are made via free-radical and Ziegler-Natta (ZN) processes. The free-radical method, usually at high pressures (up to 3500 atm) and high temperatures (upto 300 °C), is the older of the two methods. The ZN processes usually have muchmilder conditions. Recent developments use metallocene catalysts to produce tailor-made polyolefins. The common means of distinction between the various types ofpolyethylene is by density. LDPE and LLDPE have a low density below 0.94 g/cm3,whereas that of HDPE is above 0.94 g/cm3. The density is influenced by polymerstructure. The properties of each type are given in Chapter 1.

LLDPE for film application is characterised by the different processes used to producethe film and also by end-use application. The processes used to produce the film aredetermined by the molecular weight (molar mass) of the resin. For example, typicalmelt index ranges of 0.5-2 g/10 min are suitable for blown film, 2-6 g/10 min for slotcast film, and 5-12 g/10 min for extrusion coating.

42


A disadvantage of LLDPE resins is the need to modify existing and available extruders atthe processor site to convert from LDPE to LLDPE products, because of the higher shearviscosity of LLDPE with narrow molecular weight (molar mass) distribution (MWD).Owing to its different shear and extensional rheology, LLDPE extrusion and processingconditions differ from those for LDPE. To optimise its processing, different screw designshave been developed and air rings have been modified according to the differentextensional rheology.

2.2 Parameters Influencing Resin Basic Properties

Polyethylene resins are generally characterised by three parameters:

(1) Molecular weight distribution (measure of processing ease and product properties),

(2) Melt index (measure of molecular weight) and

(3) Density (measure of branching or rigidity).

These three parameters are considered to be intrinsic properties of polyethylene. Thefollowing section discusses each of the basic parameters influencing resin properties.

2.2.1 Molecular Weight (Molar Mass) and Dispersity Index

In any polymer, there is a distribution of chain lengths or molecular weights. Accordingly,the molecular weight of polyethylene is not a uniquely defined quantity, but insteaddepends on what averaging formula is used. The reader is asked to consult any textbookon polymer science regarding this item [3, 4].

2.2.2 Melt Index (Flow Properties)

The melt index (MI) is an inverse measure of the length or average size of polyethylenechains. For a given class of polyethylene, MI can be used to estimate the molecularweight. By ASTM definition, the melt index is the weight of molten material (in grams)extruded in 10 min through an orifice at 190 °C under an applied stress of 2.16 kg,which means a stress of 303 kPa. Only under these conditions is the measurement definedas the melt index. Measurement under different loads is possible in connection withinternational standards, sometimes reported as I2 (flow rate under specific load). Flowrates under defined loads and other conditions are referred to as flow indices. The commonflow index measured at 190 °C and 690 kPa or I5 (flow rate under a load of 5 kg) is used

43

Processing of Polyethylene Films

for higher molecular weight resin, usually HDPE grades. The common flow index for allgrades is the I21 (HLMI = high-load melt index) measured at 190 °C and 3034 MPa.

In practical terms, the melt index is a guide to the relative level of properties of a materialand the relative ease of flow that can be expected from the resin during fabrication [5,6]. The melt index is inversely related to the molecular weight. As the molecular weightincreases, the melt index decreases, and vice versa. Since the strength characteristics ofpolymers are related to the molecular weight, then melt index can be used as an indicatorof polymer strength. With an increase in melt index, the tensile strength, tear strength,stress cracking resistance, heat resistance, weatherability, impact strength and shrinkage/warpage decrease. The modulus of rigidity remains relatively unaffected with melt indexincrease For HDPE, the increase in melt index improves the gloss but has relatively littleeffect on transparency [4, 7]. With an increase in melt index, the ease of processing alsoincreases if all other parameters (such as molecular weight distribution) are held constant.

2.2.2.1 Melt Flow Blend Relationship

It is often of interest to calculate the melt flow (MF) of a blend of resins whose individualMF values are all somewhat different. The literature contains a number of equations forcalculating the viscosity of blends. When blending polyethylene resin, the most frequentlyused equation is the arithmetic average of the logarithms (ln or log) of the melt indices,which is obtained as follows.

Given two polymers, A and B, of the same type and with weight-average molecularweights Mw(A) and Mw(B), respectively, the weight-average molecular weight of themixture is defined as:

Mw(mix) = xAMw(A) + xBMw(B) (2.1)

where x = weight fraction and so xA + xB = 1. Then the melt index equation for a blendof two samples A and B is given by:

ln MI = xA ln MIA + xB ln MIB (2.2)

The melt flow ratio can either be I21/I2 or I21/I5, where the former is typical for injectionmoulding and low-density film grades and the latter is typical for higher molecular weightHDPE film, blow moulding and pipe grades. The melt flow ratio is a rough indicator ofthe molecular weight distribution and the shear-thinning characteristic of the polymer.The higher the melt flow ratio, the broader the expected molecular weight distribution,with the accompanying increases in shear-thinning behaviour. The melt flow ratio has acorrelation with the molecular weight distribution, but it is not a purely linear relationshipover a broad range [7-9].

44


2.2.3 Density

Density can be taken as a measure of the crystallinity of polyethylene. Since branching ofthe macromolecular chain affects the solid-state structure or crystallinity of polyethylene,density is also an indicator of chain branching. In this relation, we have to differentiatebetween short-chain branching (SCB) and long-chain branching (LCB). Long-chainbranches are mainly present in LDPE, whereby short-chain branches predominate inHDPE and LLDPE. Polyethylene (PE) in its solid state is a semicrystalline material. Tofirst approximation, its structure may be represented as a two-phase composite: a mixtureof a hard crystalline phase (density ~1 g/cm3) and a soft noncrystalline amorphous phase(density ~0.84 g/cm3). The actual composite density depends on the relative amounts ofcrystalline and amorphous phases. Because short-chain branching disturbs the regularityof the PE chain, the crystallisation process is hindered and, accordingly, the crystallinityof the solid-state structure is reduced [9]. Thus, with increasing frequency of chainbranching, the crystallinity and density decrease. HDPE, with little or no chain branching,has a density of approximately 0.94-0.97 g/cm3 and a crystallinity by weight in the range60-85%. LDPE and LLDPE, with more and longer-chain branches, have a density of0.9-0.93 g/cm3 and crystallinity by weight of 40-60%. The density range can by classifiedas shown below into four categories in accordance with ASTM D-1248 [10]:

(1) Type I (low density) 0.910-0.925 g/cm3

(2) Type II (medium density) 0.926-0.940 g/cm3

(3) Type III (high density) 0.941-0.959 g/cm3

(4) Type IV (very high density) 0.960-0.995 g/cm3

Density is typically measured using a density gradient column. The specimen could be apellet or a piece of a plaque produced under controlled cooling conditions. The rate ofcooling affects the crystallinity, and this will affect the density. A plaque quenched in ice-water will have a lower density than a slowly cooled plaque. Boiling a slowly cooledplaque will increase the measured density. As a result of the faster cooling employed, afabricated pellet will typically have a density (if unmodified by inorganic additives) lowerthan the ASTM density [9, 11].

The density measured is strongly affected by the addition of inorganic additives withhigher density, such as antiblock agents for film-grade resins and other fillers. Whenblending materials, the specific volume should be arithmetically averaged:

1/ρ = x1(1/ρ1) + x2 (1/ρ2) (2.3)

where x is the weight fraction, ρ is density, and 1 and 2 represent samples 1 and 2.

45

For PE blends, since the densities are to some extent similar, arithmetically averaging thedensities themselves is usually a good approximation. Increasing the density, with allother parameters held constant, will increase the shrinkage, modulus of elasticity, yieldstrength, heat resistance, gloss for HDPE, permeability resistance and hardness. Anincrease in density will increase the impact strength, stress cracking resistance, transparencyand tear resistance. For HDPE resin, an increase in density will increase the tensile strength,and with increase in SCB, the tensile strength will be lower. Weatherability will remainrelatively unaffected.

2.2.4 Chain Branching

Polyethylene produced with the high-pressure technology contains both long-chainbranches (100-200 or more carbon atoms) and short-chain branches. Polyethyleneproduced with low-pressure technology has only short-chain branches (1-20 or so carbonatoms). The two types of branching have different effects. For example, LCB has asignificant effect on melt rheological (flow) properties, whereas short-chain branching(SCB) has no measurable effects. These differences lead to the definitions of ‘branched’PE, which contain LCB, and ‘linear’ PE, which essentially do not have any LCB. LCBaffects the following melt flow properties:

(1) Extensional viscosity (‘strain hardening’),

(2) Shear viscosity (‘shear thinning’) and

(3) Elasticity (first normal stress differences).

LCB also affects the solid-state properties due to its influence on the melt flow properties.Owing to its tendency to strain-harden, the presence of LCB can lead to orientationeffects, and those remain in the resultant solidified PE. This is seen in the blown filmarea. SCB, in contrast, has little effect on melt flow properties but plays a major role inthe solid-state properties. SCB will affect the density, modulus of elasticity, heat of fusion,optical properties, impact resistance, tear resistance and melting point. While branchfrequency (BF) is undoubtedly the most important SCB parameter, branch length andbranch distribution also have important effects. It is well known that, at constant SCBfrequency, the longer the branch, the lower the density. The density difference decreasesas the density of PE approaches the HDPE region.

Homogeneous SCB in the situation in which the branch frequency is the same for all PEchains and, moreover, the branches are randomly distributed along the main chain.

Heterogeneous branching refers either to branch frequency varying from molecule tomolecule or to a non-random distribution of branches along a given PE chain. At constant


46


overall branch frequency, the more heterogeneous the branching, the higher the density.In high-pressure PE, SCB is homogeneous while LCB increases as the molecular weightincreases [9, 12].

This distribution can be measured by separating the polymer into fractions usingchromatographic techniques. Temperature-rising elution fractionation (TREF) is a methodwidely used nowadays to determine the comonomer and branching distribution inpolyolefins. The technique used to measure the distribution is carbon-13 nuclear magneticresonance (13C NMR). It gives the sequence distribution along the chain, but averagesthe distribution among chains.

SCB can be detected using Fourier transform infrared spectroscopy (FTIR) or 13C NMR.SCB can be measured using three different units: frequency per 1000 backbone carbonatoms, weight per cent (wt%) or mole per cent (mol%). If SCB is reported in frequencyper 1000 backbone carbon atoms, it can be converted using the fact that each monomeror comonomer contributes two carbon atoms to the backbone. Thus, there are 500monomer/comonomer units making up the 1000 backbone carbon atoms.

2.2.5 Intrinsic Viscosity

The viscosity of a polymer solution or polymer melt, in general, is a measure of thefluid’s resistance to flow. Intrinsic viscosity is a measure of the hydrodynamic volume ofa polymer molecule in solution. It is sensitive to molecular weight conformation as wellas molecular size [9, 12-16].

For a given set of polyethylene resins produced with the same process, intrinsic viscositycan be used as an indicator of the average molecular weight of the polymer. Becauseintrinsic viscosity is affected by molecular conformation, LDPE (with their long-chainbranching) will assume a hydrodynamic volume in solution smaller than that of a linearPE of equivalent molecular weight. PE produced by different processes will have differentlinear relationships between ln(melt index) and intrinsic viscosity. The molecular weightdistribution may be a factor as well. The dilute solution viscosity test defines what isknown as the viscosity-average molecular weight.

Intrinsic viscosity (dl/g) measurements are normally made by using a three-point zero-concentration extrapolation. Because it is a zero-concentration measurement, it isindependent of concentration; the intrinsic viscosity is dependent on the solvent used inthe test. Measurements of PE dilute solution viscosity have been made at 140 °C and atconcentrations of 0.1, 0.2 and 0.3 g polymer/100 cm3 decalin.

47

More recent measurements have been made at temperatures higher than 140 °C withtrichlorobenzene (TCB) at concentrations ranging from 0.025 up to 0.25 g polymer/100 ml solvent. The latter set of conditions (TCB, 140 °C) corresponds to the moretypical size exclusion chromatography (SEC) conditions used to determine the molecularweight of PE.

2.2.6 Melting Point and Heat of Fusion

As the temperature rises from ambient temperature, the properties of a PE resinchange from solid to viscous-like material. Some of these changes are due to facilitatedplastic deformation of the crystalline phase. The most part, however, is due to meltingof the crystalline phase.

The melting temperature of PE lamellar crystallites decreases as lamellar thicknessdecreases, or as crystallite internal perfection decreases (e.g., due to the partialincorporation of short branches). Since lamellae vary in thickness and perfection,the melting of PE occurs over a range of temperatures. The melting point of PE resinis to some extent a matter of definition. The most popular definition is in terms of aspecified point on a differential scanning calorimeter (DSC) trace. The apparentmelting point will vary with the method of measurement: mechanical response,dilatometer, X-ray scattering, light scattering or calorimetry. DSC is the mostfrequently used method for PE.

The apparent melting point decreases with increasing short-chain branch frequency(decreasing density), increasing short-chain branch homogeneity, decreasingcrystallisation temperature (faster cooling) or decreasing short-chain branch length.In the first three cases, the effect is due to decreasing lamellar thickness. In the lastcase, the effect is due to decreasing internal perfection of the crystallites. In practicalterms, the melting point can be used as an indicator of the density for a given PEtype. While LDPE may have a melting point of ~120-127 °C, HDPE resins havemelting points in the range of 129-135 °C.

The heat of fusion, ΔHf, is the amount of energy required to accomplish the solid-to-melt phase transition. The heat of fusion is linearly proportional to the density. Intheory, if the heat of fusion of 100% crystalline PE is known, the heat of fusion ofthe crystalline fraction of the PE sample can be determined by taking a ratio of thesample to 100% crystalline. In calculating the per cent crystallinity of a sample, avalue (average of five literature values) of 68.4-69.2 cal/g (286.6-299.0 J/g) is usedas the heat of fusion of 100% crystalline PE.


48


2.2.7 Melt Properties – Rheology

For convenience, flow can be classified into shear flow or extensional flow. Shear flowsare those in which the velocity component varies only in a plane normal to its flowdirection. Shear rheology plays an important role in PE extrusion. Extensional flow, incontrast, is characterised by a velocity component that varies only in its own flow direction.Elongational viscosity is important in film blowing, for example. It is generally not possibleto predict the extensional rheology of polyethylene from its shear flow results or viceversa [9, 10]. The viscosity is reported versus either the shear rate or the extension rate.

2.2.7.1 Relation of Viscosity/Shear Rheology

In the melt of a high molecular weight polymer like PE, under equilibrium conditions, thechain molecules are not stretched out from chain end to chain end, but instead we assume theso-called ‘random-coil’ configuration, i.e., the contour of a chain resembles a three-dimensionalrandom walk. The random-coil molecules are mutually interpenetrating, so that theenvironment of a given molecule consists mostly of segments of other molecules. Thesesegments of other molecules collectively form a kind of ‘tube’ in which the molecule ofinterest is ensheathed. Flow of the polymer melt involves both the axial motion of moleculesthrough their respective tubes (‘reptation’) and the lateral translation of the tubes themselves[7-9]. These processes obviously require the cooperative motion of large numbers of chainsegments, which helps to explain why the viscosity (or resistance to flow) of polymer melts isorders of magnitude higher than that of small liquid molecules of similar chemistry.

Another difference between a polymer melt and a small liquid molecule is that the viscosityof PE melts depends on the flow rate [5, 6]. In particular, the shear viscosity of PE meltsdecreases with increasing shear rate, a phenomenon known as ‘shear thinning’. Generallyspeaking, PE with a broad MWD shear-thins to a greater extent than PE with a narrowMWD. Shear thinning is of particular importance in the various extrusion processes(film, sheet, pipe, tubing, profile, blow moulding) as it permits easy flow through theextruder and die together with shape retention or sag resistance outside the die. Twopolyethylenes that have the same melt index may be processed differently at the highershear rates encountered in fabrication steps depending on their level of shear thinning.

PE viscosity decreases with increasing temperature or decreasing molecular weight. Forlinear PE over a wide range of MWD, the zero-shear viscosity (i.e., the shear viscosity atthe limit of zero shear rate) is proportional to:

exp(E/RT) Mw3.4

where E is the flow activation energy (5-6 kcal/mol), R is the gas constant, T is absolutetemperature and Mw is the weight-average molecular weight.

49

At constant MWD, the effect of increasing temperature or decreasing molecular weightis to shift the logarithmic viscosity versus shear rate vertically towards lower viscosities,and also horizontally towards higher shear rates [13-15]. The vertical shift is the same asfor the zero-shear viscosities. For a temperature change, the horizontal shift has the samemagnitude as the vertical shift. For a molecular weight change, the horizontal shift isusually smaller in magnitude than the vertical shift.

The zero-shear viscosity of branched PE is higher than that of linear PE at constant ‘coilvolume’ (i.e., the spherical volume ‘pervaded’ by a random-coil molecule). This is easilyunderstood in the light of the discussion of melt structure.

In the case of a branched PE molecule, the axial reptation of the main chain through itstube is necessarily coupled to the lateral translation of the side chains and their tubes[12]. The effect of this added constraint is to increase the viscosity over that of linearpolymer. The viscosity of branched PE is also more sensitive to temperature than that oflinear PE (apparent flow activation energy 10 kcal/mol versus 6-7 kcal/mol).

2.2.8 Elongational Viscosity

The rheological properties of PE in an elongational type of flow are very different fromthose in shear flow. This viscosity is measured under more of a ‘tensile’ mode. At particularelongation rates, the apparent elongational viscosity does not even reach a steady-statevalue within the time required to impose the desire total strain.

Furthermore, while the viscosity of narrow MWD linear PE resin may begin to level offwithin this time period, the viscosities of branched or very broad MWD linear PE resinsoften take a sharp upturn with increasing strain, similar to a rubber band, which becomesstiffer the more it is stretched [9, 17]. This phenomenon is known as ‘strain hardening’.The rubber band analogy is a good one, because strain hardening is more an elasticphenomenon than a viscous one.

Strain hardening is observed more often for PE resin containing long-chain branching. Itplays an important role in processes that involve highly elongational flows. In filmextrusion, strain hardening limits the draw-down capability of branched or very broadMWD linear PE; however, it compensates by providing enhanced aerodynamic bubblestability in blown film. In extrusion coating and slot cast extrusion, strain hardeningprovides resistance to neck-in edge waver and draw resonance.

2.2.9 Elasticity

Polymer melts are, in general, viscoelastic. That is, regardless of the mode of deformation(shear or stretching) imposed on them, they exhibit both viscous and elastic properties.


50


The degree of viscous or elastic behaviour depends on the rate of imposed deformationand the temperature.

The molecules in a polyethylene melt can be considered as ‘springs’ that prefer a randomconformation, but which, under an applied stress, can become uncoiled and extended.Thus, in a flowing melt, the molecules are partially uncoiled and extended in the directionof flow, thereby achieving a balance between the external stress driving the flow on theone hand and the retractive spring force on the other. Upon removal of the externalstress, the melt will rebound as the molecules relax to their equilibrium random-coilconformation. The magnitude of this rebound or recoverable strain characterises theelasticity of the melt.

Melt elasticity increases with increasing flow stress, with increasing elongationalcomponent of the flow, with broadening MWD and with increasing long-chain branchcontent. With regard to the last two factors, elasticity is especially sensitive to the presenceof a few large and/or highly branched molecules in the system.

The die swell percentage (ratio of diameter of extrudate/capillary die x 100%) is anindicator of the elasticity of the polymer melt after having been extruded through thecapillary die. It is not the most accurate measurement, however, since it involves thephysical measurement of the diameter of the extrudate. Elasticity determines the parisonswell characteristics in blow moulding, and the warping characteristics in injectionmoulding. In PE film, heat shrinkability, optical clarity and mechanical anisotropy aredirectly or indirectly affected by elasticity.

2.3 Blown Film Extrusion (Tubular Film)

2.3.1 Introduction

The most widely used method for film extrusion is the tubular or blown film technique,which accounts for about 85% of all film production. Cast film extrusion is the othermajor process and accounts for about 10-12% of all polyethylene film extrusion [17].These two methods (blown film and slot cast extrusion) are described in this sectionand the next.

2.3.2 Description of the Blown Film Process

To make polyethylene film, solid pellets are first dropped from a feed hopper into theextruder barrel, melted by subjecting them to heat and pressure, and the melt conveyed by

51

a rotating screw to the die. After having travelled all along the screw channel, the meltpasses through a screen pack and supporting breaker plate and adapter into the die. Thescreen pack filters all contamination and foreign matter and removes them, before finallythe melt is forced through the narrow slit of a die. The screen pack and breaker plate alsohelp to increase the back-pressure in the barrel to improve mixing of the melt [18, 19]. Thedie might be straight or ring-shaped. The resulting thin film has the form of a tube or‘bubble’. Coming out of the extruder, the film is cooled and is finally rolled up on a core.

As the variety of polymers and blends has steadily increased, and film requirements havebecome more demanding, extruder and screw technologies have evolved to the pointwhere screws are tailor-made for a specific system and application [19-22]. Figure 2.1depicts a typical decreasing-pitch screw for LLDPE.

In standard tubular blown film extrusion, the hopper feeds the resin to the screw, whichconveys, compresses, shears, melts and pumps the material to the die. In tubular filmextrusion, dies are circular in shape and are bottom or side fed (Figure 2.2).

2.3.3 Various Ways of Cooling the Film

The resin enters the die, flows around the core pin and exits as a thick-walled tube.While the resin is still in a molten state, air is introduced into the tube through a port in

Figure 2.1 Typical decreasing-pitch polyethylene screw


52


the centre of the die to expand the bubble to the desired diameter or lay-flat width. Noadditional air is required once the bubble diameter has been reached. By introducing airthrough the die-torpedo into the tube of film, the tube can be expanded to two or threetimes the diameter of the die. Thus, within limits, many different widths of film can beobtained from the same die. By varying the speed of the rollers closing the end of thebubble (the nip rollers), the amount of longitudinal stretching can be varied, and this isnormally used to adjust the thickness of the film to the required value. This process hasseveral advantages: With only one die, a range of film widths and thicknesses can beproduced as well [23]. A bag can be produced by only one heat seal and two cuts, andvery wide film can be produced (by slitting the tube) with equipment wide enough tohandle only half this width. On the other hand, a drawback of the process is that the rateof cooling of the film is rather low, particularly if air cooling is used [23, 24]. At highoutput rates there are difficulties of controlling bubble movements sufficiently to keepfilm thickness between close limits. Further, because the film is nipped between tworollers at one end of the bubble, the film temperature at this point must be sufficientlylow to prevent ‘blocking’. The use of additives to prevent blocking can allow greaterproduction speeds to be attained, and improvements in haul-off techniques have givenbetter control over the swaying and shaking of the bubble.

The cooling rate is becoming the limiting factor on the whole process [25-27]. In thisrespect the process compares unfavourably with the slot-die process; also, with air cooling

Figure 2.2 Blown film die in different arrangements

53

it is not possible to use shock-cooling and high extrusion temperatures to effect a reductionin haze, as is possible with other processes.

The bubble might be externally cooled by means of an air ring encircling the base of thebubble. Cooling air is uniformly distributed and solidifies or quenches the tube. Thecollapsing frame serves to collapse the tube into a lay-flat (Figure 2.3), whereupon itenters the nip rolls for final flattening. The nip rolls seal the air in the bubble and drawthe film upward from the die.

For given extruder output rate and blow-up ratio, the film gauge is controlled by the niproll speed. It is desirable to have nip rolls of adjustable height to allow a lower die-to-nipdistance than is normally used for LDPE extrusion [27]. The reduced height providesgreater bubble support and stability, and allows the film to enter the nips when warm,minimising wrinkles from improper bubble geometry and/or melt temperature and gaugenonuniformity. Idler rolls guide the lay-flat from that point to the wind-up rolls, whichwind the film on to cores.

Figure 2.3 Air ring cooling the blown film


54


2.3.4 Extruder Size

In size, extruders are available from bench-top models for laboratory work up to 500mm diameter screws. Screw sections and compression ratio have important functions.Commonly, the length-to-diameter (L:D) ratios of screws range from 15:1 up to 33:1.Recent developments consider a mixer at the screw end in the length range of 3D [27,28]. A number of different mixers are available. The compression ratio is the ratio of thechannel volume of one screw flight in the feed section to that in the metering section. Acompression ratio of about 4:1 is recommended for film extrusion. High compressionforces result in high internal heating, in good mixing of the melt and in pushing back anytraces of air carried forward with the melt. Barrel diameter is determined primarily bydesired output. Knowing the drive motor speed and the reducer ratio of a given extruder,maximum rpm can be determined. Therefore, the throughput T for any given size extrudercan be calculated, based on the following formulae (where D is the extruder diameterand h is the depth of the metering section):

(1) Neutral screw

• when D and h are in inches

T = 1.15D2h lb/h rpm (2.4)

• when D and h are in millimetres

T = 7 × 10–5D2h kg/h rpm (2.5)

(2) Controlled screw

• when D and h are in inches

T = 1.4D2h lb/h rpm (2.6)

• when D and h are in millimetres

T = 4.6 × 10–5D2h kg/h rpm (2.7)

Then it is easy to match dies and extruder capacities [28, 29]. Maximum extruder rpmcan be calculated by dividing gear reduction into rated motor speed.

Some estimates of extruder size and die sizes based on throughput and cooling capacityare shown in Table 2.1.

55

tuphguorhtnodesabseziseiddnaezisredurtxefosetamitseemoS1.2elbaTyticapacgniloocdna

ezisredurtxE retemaideiD

hcni mc hcni mc

5.1 18.3 4otpu 61.01otpu

5.2 53.6 21ot6 5.03ot2.51

5.3 98.8 81ot21 7.54ot5.03

5.4 34.11 82ot81 1.17ot7.54

2.3.5 Horsepower

The horsepower needed to drive the extruder motor is important. Based on a givenpound per hour per horsepower (lb/h hp) relationship, the die, extruder and motor canbe matched to give an optimum equipment arrangement. The examples in Section 2.3.6give the pound per hour per horsepower values generally expected for 0.2 and 2.0 meltindex resins extruded with neutral and water-cooled screws. Table 2.2 gives the normallysupplied drive motor horsepower ratings for several extruder sizes.

ezisredurtxE2.2elbaT susrev spihsnoitalerrewopesroh

ezisredurtxE gnitarrotomevirD

hcni mc ph Wk

5.1 18.3 5.7ot5 95.5ot87.3

5.2 53.6 04ot02 88.92ot19.41

5.3 98.8 001ot06 75.47ot17.44

5.4 34.11 051ot57 68.111ot39.55

2.3.6 Selection of Extrusion Equipment

Selection of blown film equipment should be made in a logical sequence [28]. Beforedeciding on specific equipment, the type, size and end-use of the film to be producedshould be determined. Knowing these variables, selection of proper equipment for agiven application is fairly simple. Blow ratio is the tool by which the proper die sizecan be selected (Figure 2.4). In general, blow-up ratios of 1.5:1 to 3.5:1 are best suitedfor film production.


56


Die size and cooling capacity are the major considerations in the selection of an extruder.The general ‘rule of thumb’ in the industry is to expect a throughput of 5-7 lb/h inch(0.9-1.2 kg/h cm) of die circumference. Die capacity and drive motor horsepower, whichdepend on several extruder size factors, can then be calculated. Some typical values aregiven in Table 2.3 and Table 2.4, respectively.

Figure 2.4 Schematic drawing of the blow ratio of film and the blown film width afterthe nip rolls have flattened the bubble to a double layer of film

57

2.4 Cast Film Extrusion

2.4.1 Description of the Cast Film Process

In cast film extrusion, the melt is forced through a flat or slotted die opening, either directlyinto a cooling water-bath, or tangentially contacting a highly polished, water-cooled chillroll (cf. Chapter 1). The flat film is cooled by two or more of these rolls and is carried byidlers to conventional treating and winding equipment. The film is stretched longitudinallybetween the die and the cooling-bath or rollers to the required thickness. Chilled steelrollers are preferred to a water-bath if the film contains hydrophilic materials such asantistatic agents, which cause some wetting of the film, with subsequent drying difficulties.Chilled rolls also allow greater production speeds than a water-bath [22, 25-27].

Flat film dies are often very long and heavy to install or to change [18]. Unfortunately,there is no close relation between die opening and film thickness. Generally, high gauges

sezisredurtxelarevesnognidnepedrewopesrohrotomevirD4.2elbaT

xednitleM wercSetaR

phh/bl Wkh/gk

0.2 lartueN 01ot8 4.61ot1.31

0.2 delooC 6ot5 8.9ot2.8

2.0 lartueN 6ot5 8.9ot2.8

2.0 delooC 4ot3 6.6ot9.9

wercsfoepytdnaxednitlemnognidnepedyticapaceiddetaluclaC3.2elbaT

retemaideiD tuptuO

hcni mc h/bl h/gk

4 61.01 57 0.43

6 42.51 011 9.94

8 23.02 051 0.86

01 04.52 091 2.68

21 84.03 022 8.99

61 46.04 003 1.631


58


require large openings. To produce a film of 25-27 μm thickness, the opening is normallyaround 0.5 mm. Usually one of the jaws is adjustable by means of screws so that the dieopening can be reset, using a brass feeler gauge of known thickness and a torque wrench.

Die temperature and resin temperature at the die lands are usually higher than in blownfilm dies, ranging up to 300 °C. It is important to keep melt temperature uniform. Thedie always has a number of heating zones. To minimise temperature variation andfluctuations in film quality, the temperature along the die should be kept within verystrict tolerances of 1 °C. To avoid film faults and imperfections, the inside die surface aswell as the polishing rolls must be kept well polished. Slightly surface irregularities willresult in gauge variations and die lines (lengthwise parallel groves).

A regular die cleaning schedule will usually prevent faults in the inner surface. For theproduction of very thin films, an air knife is very commonly used. When the hot film isdrawn down on to the first cooled chill roll, it may ‘neck-in’ (shrink) at the edges. Neck-in is the difference between the hot melt width at the die lips and the film width on thechill roll. When film with beading is wound up, the roll will sag in the middle, making itdifficult to use it later for packaging or bag-making. Therefore, such a film must betrimmed at both edges.

Neck-in will not occur in blown film production, since the bubble has no edges that mayshrink. Another cast film defect, which has something to do with cooling, is ‘puckering’(a slight bulging across the film recurring at regular intervals). Running the first chill rollhot may reduce puckering. If the roll runs cold, the film may later warm up in storageand expand, and the roll may become loose. If the melt flows well, there is little dangerof severe puckering.

The production of packaging bags from flat film requires special machinery to effectsealing of the sides of the bag as well as the bottom.

A considerable reduction in haze can be obtained by shock-cooling. This is not possiblein the tubular film process except by friction contact with water-cooled metal, with theconsequent marking of the film.

2.4.2 Effects of Extrusion Variables on Film Characteristics

2.4.2.1 Optical

Generally, with increased stock temperatures, higher gloss and lower haze will be obtainedfor low-density polyethylene. As the melt becomes hotter or more fluid, molecules will

59

have more time to align themselves and give a smooth film, which is a prerequisite forgood gloss and low haze. Other problems such as bubble stability may result, however.In tubular extrusion of high-density polyethylene, the melt temperature increase has aminimal effect on the optical properties of the inherently hazy film [28-31]. When theblow-up ratio (BUR) is increased from 1.5:1 to 3:1, the gloss increases and the hazedecreases in low-density polyethylene film, but the effect is negligible in high-densitypolyethylene film.

In slot casting and as is observed in tubular film extrusion, increasing the melt temperaturegenerally improves optical properties, although the degree varies with various high-densitypolyethylene resins. Increasing film speed generally results in poorer optical properties –increased haze and lower gloss.

2.4.2.2 Haze

Haze may be of two kinds: surface roughness caused by melt flow phenomena, andsurface roughness and internal optical irregularities caused by crystallisation. The fasterthe film cools between the extrusion die and the freezing point, the less is the haze resultingfrom crystallisation, and the greater is the haze resulting from flow. In high-densitypolyethylene, in which the haze caused by crystallinity is dominant, shock-cooling canbe used to produce almost haze-free film.

Haze in film was formerly accepted as an unavoidable property of polyethylene, but ithas been shown that commercially available polymers of slightly higher density than thenormal 0.918 g/cm3 give film of lower haze. The higher density of these materials resultsin a greater susceptibility to ‘brittle’ tearing, other things being equal, and in greatergloss, clarity and stiffness.

In soft-goods packaging, haze in the film dulls the colour of the packaged article as seenthrough the film, and much of the sales appeal of using a transparent package is lost.Provided adequate toughness is retained, film of the lowest possible haze is required forthis market.

2.4.2.3 Gloss

Since it has been found that small increases in the density of polymers could result inbetter gloss, the packaging market has demanded this property in order to add sparkle tothe package. However, it has been suggested that too much gloss may destroy some ofthe appeal of a package by reflecting too much of the shop’s lighting and of the variously


60


coloured goods in the vicinity. A reduction in chill roll temperature generally improvesboth haze and gloss in high-density polyethylene films.

2.4.2.4 Impact Strength

To have good impact strength, a balance of orientation between machine direction (MD)and transverse direction (TD) molecular structure is needed. Therefore, an increase inblow-up ratio tends to balance this orientation in both directions. As the stock temperatureis increased, impact strength increases and a better balance of orientation (machinedirection versus transverse direction) results. High-density polyethylene has a greatertendency than low-density polyethylene to orient and show large machine direction versustransverse direction differences, i.e., splittiness.

The toughness of film is its most important feature in the packaging of agriculturalproducts (potatoes, carrots, etc.) and building applications. For these uses, the hazinessof the film is not of great importance, and, as toughness and haze are to a certain extentmutually exclusive, hazy film has been accepted.

The tensile strength of the film is, as expected, mainly dependent on melt flow index, butthe apparent brittleness depends mainly on the density of the polymer: the higher thedensity, the more brittle is the film. Brittleness also depends partly on the extrusionconditions of the film.

Brittleness of film should not be mistaken for the kind of brittleness encountered inmouldings. Examination of a ‘brittle’ tear does not show a sharp fracture but reveals anarrow edge of stretched film. A ‘brittle’ film requires much less energy to tear thandoes a ‘non-brittle’ one because a smaller area of film is stretched before the tearoccurs. If the stretching is highly localised, a ‘brittle’ tear occurs; if the stretching isdistributed over an area, a ‘non-brittle’ tear results. ‘Brittle’ tears are more likely tooccur if the tearing stress is applied at high speed, such as in an impact, and film isnormally tested under such conditions.

As the density of polyethylene increases, so also does its rigidity. Film made from HDPEis stiffer in both handling and appearance. In over-wrapping machines designed to handlestiff ‘Cellophane’ or paper, stiffness exceeding some minimum value may be essential.

Increasing the melt temperature in the slot casting process decreases tensile strength inthe machine direction, but produces film with more balanced orientation and a resultingincrease in impact strength. Impact strength is increased by a reduction in chill rolltemperature in the slot casting process.

61

2.4.2.5 Blocking

Although blocking is not strictly a property of the film itself, it is one of the most seriouslimitations on the high-speed production of film by the blown extrusion process. For higher-density polymers, blocking is a less serious limitation, because the increased rigidity of the filmprevents the intimate contact obtained between flexible films. In contrast, in low-densitypolyethylene extrusion, increased stock temperature can sometimes cause blocking. Excessivefilm temperature during the collapsing of the tube will cause the inside surfaces to stick together.

2.4.2.6 Bubble Stability

As the blow-up ratio increases, the blown tube becomes larger and nonuniformities in polymerflow from the die and cooling air are magnified. With increasing blow-up ratio, the increasedsurface area that results is more susceptible to draughts. These external forces tend to makethe bubble waver and cause wrinkling and poor gauge.

2.4.3 Effect of Blow-up Ratio on Film Properties

2.4.3.1 Optical

Up to a certain point as the frost-line is increased, gloss increases and haze decreases. However,beyond this point, the gloss decreases and the haze increases. The increase in gloss anddecrease in haze from increased frost-line height occur because the molecules tend to becomebetter oriented and do not allow molecule ‘ends’ to protrude, which would give a ‘rough’surface. Too long cooling time gives rise to internal haze inherent in polyethylene.

2.4.3.2 Impact Strength

Impact strength is associated with molecular orientation. Thus, on increasing frost-line heightwithout increasing the blow-up ratio, the orientation in the machine direction increases. This‘one-direction’ orientation allows the molecules to line up or orient, thereby producing a filmthat is ‘splitty’ and poor in impact strength.

2.4.3.3 Bubble Stability

Bubble stability can decrease as stock temperature is increased. Instability results when thebubble (hot melt) lacks stiffness to resist the forces exerted by the cooling air and draughts.


62


Increased frost-line height produces a tube with a longer length of molten or hot material.This soft material is very susceptible to wavering because of lack of strength, draughts orvariables such as nonuniform cooling. In order to maintain the stable bubble necessaryfor good gauge, low frost-lines are desirable.

2.4.3.4 Puckering

Puckering is an expression used in the slot casting process. Puckering is caused by anonuniform frost-line on the first chill roll, which produces density variations in themachine direction. These density differences cause length variations that appear assmall bags, or puckers. Increasing the melt temperature decreases the tendency forpuckering, while reduction in chill roll temperature increases the puckering tendency[32, 33].

The draw distance (distance between die and chill roll) affects the film properties.Optimum draw distance for good film production varies with equipment size andproduction rates, and must be found experimentally. In general, optimum draw distancesmay range from 1-2 inch (2.54-5.08 cm) in laboratory equipment to over 12 inch(30.48 cm) in commercial equipment.

2.5 Processing Troubleshooting Guidelines

Economical film-making means the production of high-quality film in long trouble-freeruns at the highest possible production rate. Since certain changes in machine conditionsmay improve quality while decreasing output, or vice versa, it is frequently necessary tofind some kind of compromise between the two goals – high output and superior quality.In Table 2.5 some guidelines are given for machine operators to check their machine(s)and product(s) periodically to prevent unnecessary trouble during film production.

2.6 Shrink Film

Shrink-wrapping use is growing very fast worldwide and especially in Europe, both forlight/small articles and for heavy/huge pallets. Low-cost polyethylene shrink film isproduced on conventional equipment, by the blown extrusion process. No additionalmachinery is needed. Only processing conditions and resin characteristics need to beproperly selected, according to the film’s application. The film shrinks because a highdegree of molecular orientation or internal stress was introduced into it during itsmanufacture. These stresses are ‘frozen’ in the film by the air cooling. When the film is

63

senilediuggnitoohselbuortgnissecorP5.2elbaTmelborP noitulosro/dnaesuacelbissoP

eguagrooP .1 ngisedgnirriaro/dnaeidrooP

.2 tnemtsujdadeengnirriaro/dnaeiD

.3 sthguardriA

.4 gnirrianitriD

.5 egnahcniserroerutarepmeT

scitporooP .1 derisedytreporprofniserreporpmI

.2 serutarepmeteiddnanoisurtxeesiaR

.3 thgiehenil-tsorfesiaR

.4 nihtiwdehsilpmoccaebylnonac4,3,2(oitarpu-wolbesaercnI)gnikcolbdnaytilibatselbbubfostimil

selknirW .1 lortnoceguagrooP

.2 tnetgnispallocrosllordengilasiM

.3 sthguardriA

.4 taergoothtgnelmlifdetroppusnU

.5 srotanimilecitatsesu;mlifnipu-dliubcitatshcumooT

.6 wolootsipilsmliF

.7 ffitsootmliF

.8 enil-tsorflevelapeeK

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.4 elbissopfillorhcnipoteidmorfecnatsidesaercnI

.5 sgnirgnilooclanoitiddaesU

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.01 sllorpinfoaeraninoitalucricriaevorpmI

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.5 gninepoeidecudeR

.6 senildlewdnaeidetanimilE


64


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tcapmiwoL .7 htgnertsesaerckaewnistlusererusserpllorpinhgihooT

.8 stopsrepilacwolrofseguagkcehC

.9 tohoottegyamsllorpiN

.01 ,emarfgnispallocmorfsecafrusmlifnosehctarcsetanimilEsrellordnastessug

snoitcefrepmimliF,sffo-raet,seloh(

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.1 eidro/dnasneercsytriD

.2 nisernislegevissecxE

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.5 nwod-warddevorpmirofgninepoeidesaerceD

.6 wercsoterusserp-kcabretaergybnoitasinegomohevorpmI

.7 noitasinegomohniserdevorpmirofwercsfogniloocretawesU

.8 lortnoceguagevorpmI

.9 naelcparcsdnaniserpeeK

.01 snoitcefrepmirofsecafrusdnaleidkcehC

gnicnuoB .1 wolootrohgihootenil-tsorF

.2 wolootrohgihooterutarepmettleM

.3 wolsootffo-ekaT

.4 gnigrusredurtxE

.5 gnirriamorfwolfriafotnemtsujdareporpmI

.6 thgitootsrabediuG

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.1 desolcyletelpmoctonsllorpiN

.2 ylnevenunrowsllorpiN

.3 gnidlohtonpinoterusserpriA

.4 mlifniselohnipllamS

.5 egnahcerutarepmetriaro/dnastlemoteudegnahcthgiehenil-tsorF

.6 gnirriamorfwolfrianiegnahcoteudegnahcthgiehenil-tsorF

.7 evlavgnikaelfoesuacebelbbubotniegakaelriA

.8 lortnocretemaidelbbubrofeidotnignidaelsenilrianiegakaelriA

.9 gnigrusredurtxE

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.5 lortnoceguagrooP

.6 serocdehsurcnignitluser,tohootpu-dniwtaerutarepmetmliF

65

reheated to a temperature above its softening point (such as in a shrink-tunnel oroven), the molecules tend to revert to their entangled unstrained state. Thus, the internalstresses are released, causing the film to shrink.

2.6.1 Shrink Film Types

Two shrink film types are mostly used today (Table 2.6). At present, bi-oriented shrinkfilm is more popular than mono-oriented film. In the future, mono-oriented film is expectedto make major gains in the total pallet-wrapping and sleeve-wrapping business. Bi-orientedfilm will remain for full overwrap of small and medium-sized packages. The most widelyused film thicknesses are:

• 25-50 μm (thin shrink film) and

• 57-150 μm (heavy-duty shrink film).

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.2 rewolgnittegerutarepmetretawwercS

.3 retaergwercsotretawfowolF

.4 egdirbotgninnigebreppohdeeF

.5 desaercnisahdeepsffo-ekaT

.6 wercsfonoitcesdeefniremylopdettam-erP

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.1 gnittesretaertroftaergootdeepsffo-ekaT

.2 wolootgnittesretaerT

.3 taergootllorretaertdnaedortceleneewtebgnittespaG

.4 kcihtootsiroselohnipsah,).cte,nolapyH,ralyM(llorcirtceleiD

.5 tinugnitaertehtenuT

.6 egdeprahsootsahrabretaerT

.7 pagkrapsreporprofkcehC

.8 desugniebsitiwohdnahctabknikcehC

.9 pilshcumootevahyamniseR

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.3 )ralyM(esoolsicirtceleiD

.4 llamsootsllorgnitaertdnaedortceleneewtebgnittespaG


66


2.6.2 Shrink Film Properties

The properties that fully describe a shrink film are the following:

(1) Thickness and thickness uniformity,

(2) Percent shrinkage (MD, TD),

(3) Shrink strength (MD, TD),

(4) Tear resistance (Elmendorf),

(5) Impact strength (dart drop),

(6) Puncture resistance,

(7) Clarity (haze, gloss),

(8) Slip and antiblock, and

(9) UV resistance.

Normally a given application needs only some of these properties to be checked. Twoexamples are shown in Table 2.7.

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epyT DM DT

detneiro-onoM 08-06 02-01

detneiro-iB 06-05 05-03

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htiw,selcitrathgilfognigakcapyalpsiDsrenrocprahs

derotsebot,stellapyvaehfognipparWrianepoehtni

ssenkcihT ytimrofinussenkcihtdnassenkcihT

egaknirhstnecreP egaknirhstnecreP

ytiralC htgnertsknirhS

ecnatsisererutcnuP htgnertstcapmI

ecnatsiserraeT ecnatsiserVU

67

2.6.3 The Manufacture of Shrink Film

To produce film shrinkable in a given direction, molecular chains must be oriented in thesame direction by processing. This orientation is obtained by stretching the film in therequired direction. The greater the film stretching, the higher the molecular orientationand hence the shrinkage.

Every blown polyethylene film is shrinkable in the machine direction (MD), because it isnormally stretched much more in this direction. In fact, from Figure 2.5 it can be seenthat a film tube having diameter D1, length L1 and thickness T1, in the molten state, isblown into a film tube having diameter D2, length L2 and thickness T2, in the solid state,at room temperature.

These two film tubes must have the same mass. This condition may be written as

ρ2πD2T2L2 = ρ1πD1T1L1 (2.8)

where ρ1 and ρ2 are the film densities at the relative points. These densities are differentdue to the different film temperatures and pressures.

Figure 2.5 Frost-line and blow-up ratio (BUR) in shrink film


68


A normal thin blown film can be stretched to about 9:1 in the machine direction and toonly 2:1 in the transverse direction. For this reason such a film will shrink very much inMD and almost nothing in TD. This means that this film is a mono-oriented type shrinkfilm. Usually a thin shrink film is required to be bi-oriented, i.e., with more balancedshrinkage. This condition is achieved by properly regulating the processing conditionsshown in Table 2.8.

knirhsehtnosnoitidnocgnissecorpehtfoecneulfnI8.2elbaTEPDLfoseitreporp

egaknirhS ecrofknirhS

DM DT DM DT

ehtninoitatneiroregnortSnoitceridenihcam

● ❍ ● ❍

RUBregraL ❍ ● ❍ ●

deepsffo-ekatrehgiH ●●● ❍❍❍ ●●● ❍❍❍

epahselbbubepyt-kceN ● ●●●

enil-tsorfrehgiH ❍❍❍ ● ●●●

deepsnoitcudorP ●●● ❍❍❍ ●●● ❍❍❍

deepspageidregraL ●●● ●●●

pageidfohtgnelregraL ❍❍❍ ❍❍❍

mlifrekcihT ❍❍❍ ❍❍❍

erutarepmettlemrehgiH ❍❍❍ ❍❍❍ ❍❍❍ ❍❍❍

xednitlemrehgiH ❍❍❍ ❍❍❍ ❍❍❍ ❍❍❍

●●● esaercnignortS● esaercnielttiL❍ esaercedriaF

❍❍❍ esaercedgnortS

To obtain a balanced shrink film, the same amount of molecular orientation must beproduced in both MD and TD. First, the stretching ratio in the machine direction (MDSR)and the blow-up ratio (BUR), in other words, the stretching ratio in the transversedirection, should be made equal. This corresponds to putting MDSR = BUR.

In practice, the stretching in the machine direction (MD) predominates, due to otherfactors (shearing suffered by the melt during its passage through the die, bubble shape,

69

etc.). Thus, the BUR needed to obtain balanced shrinkage is somewhat higher than thetheoretical value. This corresponds to assuming a slightly higher coefficient (1.1 to 1.2)in the practical formula for balanced shrinkage.

2.6.3.1 Bubble Shape and Frost-Line

Bubble shape and frost-line are important parameters controlling the shrinkages in TDand MD. Referring to Figure 2.5, two types of bubble shape can be seen: one with a longneck (continuous line), corresponding to a higher frost-line; the other with no neck (dottedline), corresponding to a lower frost-line. The shape with a long neck gives more balancedshrinkages. This is easy to explain. In fact, for bubbles of this shape, transverse stretchingpredominates just below the frost-line, where the film is frozen, and no further relaxationcan take place. Thus, a high level of molecular orientation in TD is retained in the film.

2.6.3.2 Resin Melt Index

Melt index has a remarkable effect on shrink strength and a slight effect on per cent TDshrinkage and on BUR for balanced shrinkages. In particular, the lower the MI, thehigher the shrink strength. This means that for heavy-duty shrink film a low MI ispreferred. Also, a low MI corresponds to lower BUR for balanced shrinkage and tohigher TD shrinkage.

The MWD has only a slight effect on shrink temperature. In particular, a film obtainedfrom a resin with narrow MWD (or low swell) shrinks at lower tunnel temperatures.The presence of slip and antiblock additives has no effect on film shrinkage.

Ethylene-vinyl acetate copolymers (EVA) with low vinyl acetate (VA) content give shrinkfilms having the following properties:

(1) Faster shrinkage (higher tunnel production),

(2) Lower shrink temperature,

(3) Higher impact resistance (at low temperature),

(4) Higher puncture resistance,

(5) Lower slip and antiblock, and

(6) Higher gas/moisture permeability.

These films find application both for light shrink-wrapping and for heavy pallet wrapping.


70


UV stabilisers are necessary for applications in which the wrapped item (usually a pallet)is stored in the open air. Stabilisers have no effect on shrinkage. They only affect shrinkfilm prices.

2.6.4 Shrink Tunnels and Ovens

The shrink-wrapping technique consists of wrapping and heat-sealing the article looselyin the film. The loose film perimeter should exceed the article’s perimeter by no morethan 7-10%. The package is then conveyed through a shrink tunnel or into an oven.Many types of heating system are used, the best being that using hot circulating air,because it gives more uniform heating. The most important properties of a shrink filmfrom the point of view of an end-user are the following:

(1) Percent shrinkage (MD, TD),

(2) Shrink strength (MD, TD),

(3) Shrink speed and

(4) Shrink temperature.

The last two depend on film thickness and on the nature of item to be packed. In fact,both these factors affect the amount of heat required to reach the softening point of thefilm. Finally, it is worth pointing out that the maximum shrink strength is reached whenthe film cools outside the oven and not inside. In fact, inside the oven the film shrinkswith a small shrink force, because it is hot and soft. When it cools rapidly to roomtemperature outside the oven, the film shrinks tightly around the article, with a muchhigher shrink force.

References

1. Kunststoff-Handbuch IV: Polyolefine, Carl Hanser, Munich, Germany, 1969.

2. Kunststoff-Handbuch II: Polyvinylchloride, Carl Hanser, Munich, Germany, 1963.

3. Plastics Engineering Handbook of the Society of the Plastic Industry, 5th Edition,Ed., M. L. Berins, Chapman and Hall, London, UK, 1991.

4. P.J. Lucchesi, S.J. Kurtz and E.H. Roberts, inventors; Union CarbideCorporation, assignee; US Patent 4,486,377, 1984.

71

5. J.C. Miller, R. Wu and G.S. Cielozyk, Tappi Extrusion Conference, Hilton HeadIsland, SC, USA, 1985.

6. J.C. Miller, Tappi Journal, 1984, 67, 6.

7. A.V. Ramamurthy, Journal of Rheology, 1986, 30, 2, 337.

8. A.V. Ramamurthy, Proceedings of the 2nd Annual Meeting of the PolymerProcessing Society International, Montreal, Canada, 1986.

9. J.C. Miller and S.J. Kurtz, Proceedings of the IXth International Congress inRheology, 1984.

10. ASTM D1248, Standard Specification for Polyethylene Plastics ExtrusionMaterials for Wire and Cable, 2002.

11. W.A. Fraser and G.S. Cieloszyk, inventors; Union Carbide Corporation, assignee;US Patent 4,243,619, 1981.

12. S.J. Kurtz, T.R. Blakeslee, III and L.S. Scarola, inventors; Union CarbideCorporation, assignee; US Patent 4,282,177, 1981.

13. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent4,552,712, 1985.



16. D.N. Jones in Proceedings of the 1984 Polymers, Laminations and CoatingConference, Tappi Press, 1984.

17. W.H. Darnell and E.A.J. Mohl, SPE Journal, 1956, 12, 20.

18. W.D. Mohr, R.L. Saxton and C.H. Jepson, Industrial Engineering and Chemistry,1957, 49, 1857.

19. S. Eccher and A. Valentinotti, Industrial Engineering and Chemistry, IndustrialEngineering and Chemistry, 1958, 50, 829.

20. B.H. Maddock, SPE Journal, 1959, 15, 383.


72


21. W.D. Mohr, J.P. Clapp and F.C. Starr, SPE Technical Papers, 1961, VII, January.

22. L.F. Street, International Plastics Engineering, 1961, 1, 289.



25. B.H. Maddock, Proceedings of Pressure Development in Extruder ScrewsInternational Congress, Amsterdam, The Netherlands, 1960, 139.

26. C. Maillefer, Modern Plastics, 1963, 40, 132.


28. W.A. Fraser, L.S. Scarola and M. Concha, Tappi Journal, 1981, 64, 4.

29. J.C. Miller and S.J. Kurtz, Advances in Rheology, 1984, 3, 629.

30. S.J. Kurtz, L.S. Scarola and J.C. Miller, Plastics Engineering, 1982, 38, 6, 45.

31. J.C. Miller, Tappi Journal, 1984, 67, 6.

32. Film Converting Techniques for Linear Low Density Polyethylene, UnionCarbide Corporation, 1985.

33. S.J. Kurtz, Advances in Rheology, 1984, 3, 399.

73

3 Processing Conditions and Durability ofPolypropylene Films

H. Aglan and Y.X. Gan

3.1 Introduction

The durability of polypropylene (PP) films under tensile loadings and ultraviolet (UV)irradiation is a very important end-use property. In this chapter, an overview of thestructures, synthesis, processing and applications of semicrystalline PP films isintroduced. The UV degradation mechanisms and the effect of UV degradation on thedurability of PP films are then presented. The functions of different additives in PPfilms are described. Research findings based on a case study of the durability of severalgroups of PP films with additives such as UV stabilisers, antioxidants and colouringpigments, (e.g. calcium carbonate), are summarised. In the case study, typical PP filmspecimens taken from different processing stages are tested to establish the effect ofcomposition and processing conditions on the durability of PP films. Microstructuralfeatures of the films are identified and correlated with their durability. It has beenfound that a lack of proper addition of UV stabiliser and antioxidant agent severelydegrades the durability of PP products. UV-degraded PP woven fabrics made fromstretched PP films totally lost their load-bearing capability and displayed severelydamaged structure with extensive microcracks, voids and dispersed secondary particles.It has also been found that unstretched PP materials have very good durability understatic tensile loading. The stress-strain behaviour shows several distinct deformationstages: elastic deformation, yielding and cold flow followed by strain strengthening.The stress-strain relationship of stretched PP films reveals an elastic deformation stagefollowed by limited plastic deformation from which no obvious cold flow was found.However, stretching results in the drastic decrease in durability of these films. Thedurability of the stretched films is less than 70%, while before stretching it was about600%. Calcium carbonate pigment causes a decrease in tensile strength of stretched PPtape, while UV stabiliser does not change the strength and durability of PP filmsappreciably. A study of the surface morphology of these PP film samples revealed asimilar smooth surface with unidirectional texture. Defects in the form of crevices,grooves and intruded particles were found on the surface of PP films with calciumcarbonate colouring pigment.

PP is a thermoplastic polyolefin polymer [1]. The structure of PP is stereoregular [2-5].Crystalline PP was invented in the early 1950s by independent groups in the USA and

74


Europe. It entered the stage of large-scale production in 1957. Prior to 1950,polypropylene polymer was a branched low molecular weight (molar mass) oil, whichhad no significant use. After the discovery of polypropylene, obtained from the TiCl3-based first-generation catalyst, at the Polytechnic of Milan in 1954, nothingrevolutionary happened until the discovery of the active MgCl2-supported high-yieldZiegler-Natta catalysts at the Ferrara Giulio Natta Research Center in 1968. Thatevent was the beginning of the revolution that brought about the creation of the third-and fourth-generation catalysts. The Ziegler-Natta achievements made the stereoregularpolymerisation of PP possible. The fourth-generation catalysts are super-active, andare introducing an innovative and revolutionary new dimension to heterogeneouscatalysis. Because of the specific tailored architecture of the catalyst, it is possible togive the catalyst the capability of determining the physical shape of the polymergenerated and its external and internal morphology. Thus, the type of specificdistribution within a single PP granule can be precisely controlled. This represented areal breakthrough for PP synthesis technology. It was possible to design new, versatile,clean and economical processes to create a new family of materials.

PP is a very useful material for various applications because of its good properties andprocessability in large-scale production by extrusion, injection moulding and casting.Various products can be manufactured from several types of PP, including (1) isotactic,crystalline PP homopolymers, (2) random copolymers and (3) impact or heterophasiccopolymers. The advantages of PP are that it is lightweight, unaffected by moisture, fire-proof, acid-resistant and possesses high stiffness. Structural PP products can maintainexcellent impact capability, high strength, high toughness and good dimensional stabilityunder service conditions. In addition, PP is cost-effective. The principal forms of PP inapplications include film and sheet, filament and fibres, pipes, profiles and wire coating.

PP has been accepted as a versatile piping material for a considerably long time. Theadvantages in this application include resistance to chemicals and corrosive media,ease and economy of handling and installation, low friction losses, low thermal andelectrical conductivity, high temperature resistance, minimum build-up of soil deposits,and good outdoor durability in all weather. PP piping is used in industrial drainagesystems, in the chemical processing industry and in the oil industry for handling saltwater and crude oil. PP is also used for internal lining of metal pipes and tanks. PPpipes can withstand temperatures up to 105 °C. Even under pressure, the servicetemperature can be as high as 90 °C.

PP rods are used for the fabrication of prototype and production parts such as gears,spools, pulleys, casters, etc. Various structural elements possessing high strength,toughness and surface hardness have been produced from PP for applications requiringa heat-resistant, noncorrosive material. PP wire and cable coatings possess desirable

75

Processing Conditions and Durability of Polypropylene Films

properties such as surface hardness, crush resistance, high softening temperature, lowdielectric constant and low environmental stress cracking. PP wire coating can be appliedin solid or foamed forms. PP has also found applications in the medical and biochemicalfields. The majority of moulded PP used in medical applications is for implantations[6, 7], repairs [8-11], membranes [12] and disposable devices such as syringes [13].Non-woven fabrics are used in items such as surgical masks and gowns. PP can alsoserve as a matrix polymer for fabrication of fibre-reinforced composite materials [14-20]. In addition, PP can form copolymers and polymer blends [21-25].

PP films are widely used in industry and daily life. Food packaging applications arevery good examples. From the early 1960s to now, PP has been the dominant filmpackaging material in the snack food, bakery and candy (sweets) industries. PP filmsare heat-sealable and they have the advantages of being grease- and oil-resistant forthe protection of the contents. Snack food packaging is the largest single use of PPfilm. It is used because of its excellent moisture barrier properties, stiffness, gloss,printability and crispness. When needed, special coatings provide excellent oxygenbarrier properties as well. Snack foods that are potato-based are adversely affected byUV light, a condition that requires an opaque package. Opaque PP films and/or clearfilms that are metallised address this need. Snack food packaging consists of more thanone layer. At the very least, a heat-seal layer is required to seal the package. Otherlayers include slip films to facilitate processing through the converting line, oxygenbarrier layers for content protection, and adhesive layers to hold it all together. Theintroduction of highly flavoured snack foods adds still another requirement to thepackage: fragrance retention. The packaging of food products today requires asophisticated array of specially engineered films designed specifically for the productsthey are chosen to protect.

Opaque PP films are also used to label soft drinks and other beverages. The filmsproduce an attractive package, do not rip, provide some abrasion resistance, and donot come off when the bottle is chilled, for example, in a refrigerator or by ice-water.Cables, wires and capacitors, widely used in the electronics industry, have made use ofspecial PP films to provide the necessary insulation. Moisture absorption negativelyaffects the ability of a plastic to provide insulation. Since PP offers low moistureabsorption and has inherently good insulating properties, it is ideal for this application.

Commercial applications of cast films are growing rapidly. The film is mostly used forpackaging purposes [26], e.g., bread wrap, bakery products, bag liners, grocery bags,textiles and miscellaneous wrapping and packaging applications. Other applicationsinclude electrical cable wrapping and laminations with substrates such as paper,cellophane and aluminium foil. Oriented PP film, with high mechanical strength in thestretching direction, has found some other important applications. The film is slit,

76


stretched in the machine direction and knifed into strands or fibrils. Such types of PPfilm have been used for weaving and manufacturing of heavy-duty bags, and manyother industrial and commercial products [27].

Various tapes and pressure-sensitive labels are also made from PP cast films or orientedfilms. Cast PP films are also often used as the outer protective layer in diaper (nappy)construction. The purpose of using PP films is to keep the moisture inside the diaper.Recently, cast PP films have been used for manufacturing stationary products, includingclear overlays, dividers, photo albums, baseball cards and protector pages. Since PPfilms can be easily captured and recycled, the environmental concerns surroundingpolyvinyl chloride (PVC) have accelerated the use of cast PP films in these applications,many of which were previously served by PVC films.

Because of the outstanding combination of cost, performance, excellent physicalproperties, strong and continuous expansion of process versatility, and environmentalfriendly processes and materials during manufacturing, use and recycling stages, PP sawan explosive growth in the amount of production in the worldwide market in its earlystage. The world market for PP has grown from around 1.5 million tons in the 1970s toabout 13 million tons in the early 1990s. The production in 1995 was about 22 milliontons and up to 30 million tons in 2000. Following its explosive early growth, the PPbusiness has maintained surprising vigour. Its growth rate for US production has remainedabove 7% for the past two decades [28]. The recent production of all major plastics andtheir growth rates are shown in Tables 3.1 and 3.2, respectively.

]92[3991niatipacrepsmargolikninoitcudorpcitsalP1.3elbaT

noigerdlroW PP EPDL EPDLL EPDH SP CVP latoT

aciremA.N 2.01 8.9 5.7 6.31 2.7 6.21 9.06

eporuE.W 7.9 2.11 78.1 8.7 3.5 1.21 79.74

napaJ 7.51 0.7 30.5 1.8 9.01 7.61 34.36

aisAfotseR 70.1 56.0 26.0 18.0 45.0 7.1 4.5

dlroWfotseR 49.0 59.1 78.0 51.1 29.0 36.2 64.8

latoT 52.3 91.3 55.1 80.3 40.2 93.4 5.71

enelyporpyloP:PPenelyhteylopytisned-woL:EPDL

enelyhteylopytisned-wolraeniL:EPDLLenelyporpylopytisned-hgiH:EPDH

enerytsyloP:SP

77

From the results shown in Table 3.1, it can be seen that the production in weightper capita for PP is only slightly lower than that for PVC. The three developedregions, i.e., North America, Western Europe and Japan, consume much more PPthan elsewhere. The other areas of the world consume very little of any plastic.Table 3.2 shows that linear low-density polyethylene (LLDPE) has the highest growthrate. PP ranks second and it is one of the fastest-growing plastics in the periodfrom 1993 to 2000.

The increase in worldwide PP capacity is shown in Table 3.3 by a comparison of1994 and 1998 figures [28]. It can be seen that the growth in the developing regionswas even more dramatic than that in the three developed regions. Capacity in the‘Rest of Asia’ region excluding Japan is expected to be the largest producing regionin the world.

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napaJ 1.3 8.2 8.7 8.3 3.3 6.2

aisAfotseR 2.11 9.5 2.11 2.21 8.9 8

dlroWfotseR 1.61 3.7 4.02 9.01 8.11 3.8

latoT 9.6 5.3 7.9 1.6 6.5 6.4

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)raeyrep%(snotolik latotfo% snotolik latotfo%

aciremA.N 4335 62 3136 32 3.4

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78


3.2 Structures and Synthesis

The propylene molecule is the monomer unit of polypropylene. There are a number ofdifferent ways to link the monomer together, depending on the stereo arrangement. Threemajor factors control the stereoregularity of PP [30, 31]:

(1) The first factor is the degree of branching. The molecular chain of PP will be straight(or linear) if the next monomer unit always attaches to the chain end as shown inFigure 3.1(a). If the next monomer may add on to the backbone, this results in theformation of branches, as seen in Figure 3.1(b).

(2) The pendant methyl sequence can also change the stereoregularity of PP. The addition ofpropylene to the growing PP chain can be regiospecific or non-regiospecific as illustratedin Figure 3.2. We can see that the addition of monomer can be in a head-to-tail manner(Figure 3.2(a)) or in other ways such as head-to-head or tail-to-tail (Figure 3.2(b)).

Figure 3.1 BranchingReprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.

Copyright 1996, Hanser Publishers.

Figure 3.2 RegiospecificityReprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.


79

(3) Still another way to control the stereoregularity is the position of the tertiary hydrogen.As shown in Figure 3.3, there exist two possibilities for the arrangement of the tertiaryhydrogen. If the propylene monomer is always added in the same stereo arrangement,the alignment of the tertiary hydrogen will be in a same hand way, either right-handed orleft-handed (Figure 3.3(a)). Any change in the stereo arrangement of the adding monomercan result in an opposite hand distribution of the tertiary hydrogen (Figure 3.3(b)).

Figure 3.3 ChiralityReprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.


PP as a commercially used material and in its most widely used form is made with catalyststhat produce crystallisable polymer chains. These give rise to a product that is asemicrystalline solid with good physical, mechanical and thermal properties. Another formof PP, produced in much lower volumes as a by-product of semicrystalline PP productionand having very poor mechanical and thermal properties, is a soft, tacky material used inadhesive, sealants and caulk products. The first product is often referred to as crystallisableor iPP, while the second type is called noncrystallisable or ‘atactic’ polypropylene (aPP).

In addition to the two commonly defined PPs, isotactic and atactic polypropylene, thereis an intermediate state, which is defined as syndiotactic polypropylene (sPP). Themolecular chain of isotactic PP is linear. The pendant methyl sequence is regiospecific.That is, the addition of propylene to the growing chain is head-to-tail. In addition, thesame hand arrangement of the tertiary hydrogen can be found in iPP. The regularity ofthe isotactic polypropylene allows it to crystallise. The arrangement of carbon atoms inthe main chain of crystallised isotactic PP is in the shape of a helix, when it is viewedobliquely from one end. Unlike iPP, sPP results from the consistent insertion of thepropylene monomer in the opposite hand from the previous monomer unit. This is adifferent type of stereoregularity from that of isotactic PP. Syndiotactic PP is notcommercially significant. Atactic PP can be produced from any one or more of: theinconsistency in the degree of branching, the change in the pendant sequence, and thenon-stereospecificity. The three types of structure of PP are shown in Figure 3.4.


80


Figure 3.4 Three PP structuresReprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.


Isotactic PP can crystallise in several forms, which have different density of the structural(unit) cell. The α form is dominant. Other forms include the β, γ and mesomorphic(smectic) forms [32]. All of these forms are composed of molecular chains in a helicalconformation with a common repeat distance of 6.5 Å. They differ in unit cell symmetry,inter-chain packing and structural order [33, 34]. The structure, conditions forformation, melting behaviour and morphological characterisation of these forms arediscussed by Philips and Wolkowicz [32]. The α-form of isotactic polypropylenehomopolymer is semicrystalline in nature. As with any semicrystalline polymer, themorphology of α-form iPP exhibits a hierarchy of characteristic scales as shown inFigure 3.5. The macromorphology of PP can be seen on a visual scale in the range ofmillimetres. The morphology of a gross reactor particle in the as-supplied state is shownin Figure 3.6 at a magnification of x40. The spherical shape of this pure granular PP, asreported in earlier work by other researchers [35-38], can be seen from the top part ofthe micrograph. However, the skin-core structure [39-43] cannot be so easily identifiedwithout using any cross-section slicing. Under carefully controlled optical conditions,such as small-angle light scattering, a spherulite texture is revealed in a finer scale onthe order of 1-50 μm. The spherulite structure is built up by smaller blocks and lamellae.At a higher magnification of x500, the lamellar structure of pure PP can be seen, asillustrated in Figure 3.7. These lamellae are composed of crystallographically orderedregions. The molecular chains in the crystalline regions are arranged with specificsymmetry, which has been described elsewhere [44-47]. The unit cell of isotactic PP ismonoclinic with a monoclinic angle of about β = 99°. The lattice dimensions of the cellare a = 6.6 Å and b = 20.8 Å [46,47].

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Figure 3.6 SEM micrograph of PP granule


Figure 3.5 PP morphology at various scalesReprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p114.


82


The synthesis of isotactic PP has been commercialised through several processes, namelythe Spheripol [48], Exxon (Sumitomo) [49], Mitsui Hypol [50], Unipol [51] and Amoco[52] processes. Such processes have been summarised by Lieberman and LeNoir [53]. Aflowchart for each process is shown in Figures 3.8-3.12. Because of the application offourth-generation catalysts, the removal of catalyst and atactic polymer is not necessary.The use of hydrocarbon diluent in liquid or gaseous form is prevented. Thus the yield ofthe PP products is remarkably increased and the cost of the synthesis of iPP homopolymer

Figure 3.7 Micrograph of PP showing lamellar structure

Figure 3.8 Flowchart for the Spheripol processReproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.

Copyright 1995, Gulf Publishing Company.

83

Figure 3.9 Flowchart for the Exxon (Sumitomo) processReproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.


Figure 3.10 Flowchart for the Mitsui Hypol processReproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.



and/or copolymer is reduced. Among the synthesis processes, the Spheripol process for theproduction of PP homopolymers or copolymers (as shown in Figure 3.8) has found thewidest application. In this process, catalyst components and monomer are fed to a loopreactor for homopolymerisation. The high heat removal capability of the loop reactors

84


Figure 3.11 Flowchart for the Unipol processReproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 142.


Figure 3.12 Flowchart for the Amoco process [52]

allows very large outputs. The operating pressure in the synthesis reactors does not requireexcessive wall thickness or special fabrication techniques because of the small diameter ofthe loop reactors. Therefore, it is possible to build a large-capacity plant economically.

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3.3 Film Processing

Polypropylene has been processed into films since the beginning of its production. Developmentof new catalysts and innovation in film processing technology have been a tremendous helpin the expansion of this area. Depending on the type of film and the process by which it ismade, the resulting film products can be used for various purposes and applications [54].The melt stability for processing film products is especially important as compared withprocessing moulded bulk products. In film processing, the melt stability is crucial in preventingrheological changes and maintaining film strength. Additives that control film slippage andantiblocking properties are also critical in the final stage of film processing.

The most common process used to produce PP films is the chill roll cast method [55].Another important process is the tenter frame. The chill roll cast method is for non-oriented film production, and the tenter frame method is for production of orientedfilms. Blown film processes [56] may also be used for both oriented and non-orientedfilm production, but they are not widely used.

3.4 Additives

Stabilisation agents consisting of phenolic and phosphite antioxidants are usually used toobtain processing stability. The PP polymer for film production also contains otherfunctionalising additives and groups besides the stabilisers. The two main functionalisingadditives are antiblocking and slip agents. These materials are combined to provide therelease of one film from another or from take-off equipment. Since films have large surfaceareas and may be wound under tension and at high speeds, there can be substantial staticcharges and compressive forces present between film layers. Thus, an antiblock agent,which is an inert inorganic material, is added to solve these problems. Diatomaceous silica,calcium carbonate, talc, or glass spheres are commonly used antiblock agents [57-62].

Particle size and dispersability are two important characteristics for antiblocks to providethe surface separation effect [63, 64]. Depending on the film thickness, antiblock averageparticle size can range from less than 1 μm up to 15 μm. Particles or agglomerates largerthan 25 μm can appear as defects. In addition, the presence of agglomerates indicatesthat some of the antiblocking agent was not well dispersed, and the normal concentrationmay be ineffective. Therefore, the presence of these inorganic antiblock materials at thefilm surface can affect the film-handling characteristics.

Slip agents are generally materials that tend to separate from PP and have some inherentlubricating properties. Since slip agents eventually end up on the film surface, they mayincrease the haze of the film. The most widely used slip agents for PP films are fattyamides, such as erucamide and oleamide.


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3.5 Ultraviolet Degradation of Polypropylene

3.5.1 UV Degradation Mechanisms

Virgin PP obtained directly from a commercial process is very susceptible to UV irradiationand air oxidation. If stored unstabilised at room temperature, the durability, strength andphysical properties of the PP product deteriorate rapidly over a period of weeks or monthsdepending on the physical form, temperature, available oxygen, intensity of UV radiationand other conditions. At elevated temperatures, such as during summer storage, thedegradation process can be accelerated. This uncontrolled degradation is exothermic, andthe released heat and gases can lead to a further increase in the degradation rate [65, 66].

The oxidation processes of PP are considerably complex and depend on a variety offactors, including oxygen availability, impurities, residual catalyst, crystallinity (or thecontent of crystallised portion), storage temperature, air pollutants, radiation exposuretime, chemical exposure, film thickness, loading or stress conditions in the part,comonomer concentration, and additive type and content. Earlier studies have proventhat the degradation of PP can be divided into three steps: initiation, propagation andtermination [67]. These steps are briefly described in the following three subsections.

3.5.1.1 Initiation of PP degradation

If the PP product is exposed to air, the following reaction proposed by Tudos [68] occurs:

RH + O2 → R• + HOO• (3.1)

where RH stands for the polypropylene molecules and R• is alkyl radical.

According to Hawkins [69], UV radiation can assist the initiation of the degradation ofPP. In most cases, the products resulting from the oxidation as described in equation(3.1) can remain separate under steady UV irradiation and exposed to air. However, theymay also recombine to form a hydroperoxide as shown in the following equation:

R• + HOO• → ROOH (3.2)

3.5.1.2 Propagation of PP degradation

The propagation of the degradation, as indicated by Becker and co-workers [67], occursby a series autoxidation scheme as depicted by the following three reactions:

87

R• + O2 → ROO• (3.3)

ROO• + RH → R• + ROOH (3.4)

R• + R′H → RH + R′• (3.5)

Since small free-radical fragments are very active, they may contribute to the propagationof degradation by the following two reactions:

RH + HOO• → R• + H2O2 (3.6)

RH + HO• → R• + H2O (3.7)

It can be seen from the above reactions that the propagation steps cause the radical sitesto move, but there is no overall increase in the number of radicals.

3.5.1.3 Termination of PP degradation

The last step of the degradation process is termination, where quenching of radicalsoccurs. The number of radicals can be reduced by combining two radical sites to form anon-radical product. Several reactions may lead to the termination, as described in thefollowing equations [70]:

R• + •R′ → R–R′ (3.8)

RO• + •R → ROR (3.9)

2ROO• → ROOR + O2 (3.10)

2RO• → ROOR (3.11)

3.5.2 Effect of UV Degradation on Molecular Structure and Properties of PP

The degradation of PP, initiated either by UV irradiation or through thermal activation,causes change in crystallisation and melting behaviours of PP [71, 72]. Degradation alsoleads to chain scission or cleavage, leading to a decrease in the durability of the films.Loss in molecular weight (molar mass) also occurs [73]. There are several types of chainscission found in PP [74]. The most common is a unimolecular scission of carbon- andoxygen-centred radicals. This cleavage produces several products. The products fromthe carbon-centred radicals are an olefin and a new carbon radical. These products canre-enter the oxidation cycle as PP•, which can be depicted as [67]:


88


–CH(CH3)–CH2–CH(CH3)–CH2–CH(CH3)– + R•

→ RH + –CH(CH3)–CH2–C(CH3)–CH2–CH(CH3)– (3.12)

Accordingly, the chain scission can be expressed as:

–CH(CH3)–CH2–•C(CH3)–CH2–CH(CH3)–→ –CH(CH3)–CH2–(CH3)C=CH2 + •CH(CH3)– (3.13)

The resulting olefin is even more susceptible to oxidation than the original PP with asaturated hydrocarbon structure. If the degradation is initiated by an alkoxy radical, acarbonyl-containing molecule in the form of –CH2–C(CH3)=O, and another carbon-centred radical, can be formed. All of these processes lead to an appreciable loss inmolecular weight of PP.

The reduction in the molecular weight of the PP polymer leads to a change in many of itscorresponding properties. One of the most detrimental is the loss of durability and ductility,thus a drastic decrease in toughness of the polymer. In addition, the chain scission willproduce products that will tend to cause an increase in the colour of the polymer and thegeneration of oxygenated compounds, which will adversely affect the durability, strengthand physical properties of the final PP products.

UV light can accelerate the chain scission processes. In addition, the availability of oxygenand heat are also key factors in the determination of the degradation kinetics. At PPprocessing temperatures, the degradation reaction rate is extremely rapid. The succeedingextrusion or injection moulding procedure can also result in severe degradation of the PPpolymer. In the solid form, PP is a semicrystalline polymer with a crystalline content thatis normally between 40% and 60%. The crystalline regions are essentially impervious tooxygen, so the oxidation only occurs in the amorphous region. It has been reported byMita [75] that the diffusion rate of oxygen is much slower than the reaction rate, so thatthe oxidation process is basically a surface phenomenon [76]. In most cases, the surfacecan become dull, crazed, or even powdery. Obviously, unstabilised PP is very prone tooxidation and degradation in the presence of air. Therefore, adding appropriate stabilisersis necessary to convert PP into a durable, useful material.

3.5.3 Stabilisation of PP by Additives

Stabilised PP can be obtained by using appropriate additives which can control radicalproducts or potential radicals. There are many stabilisers and UV antioxidants available,and they can be classified into two types, i.e., primary and secondary. Some secondaryUV stabilisers in fact also have primary characteristics. The detailed stabilisationmechanisms are still unknown due to the complicated oxidation intermediates.

89

Primary antioxidants are those additives that interfere with the oxidation cycle by reactingwith the formed radicals and interrupting the cycle. Primary antioxidants are also calledradical scavengers. Both hindered phenols (HP) and hindered amines (HA) are effectiveprimary antioxidants. An HP can react with the radical species generated in the initiationand propagation stages of degradation. Specifically, the HP is able to transfer its phenolichydrogen to the generated radical, causing a non-radical product to be formed. In transferringthe hydrogen, the HP itself becomes a radical known as a hindered phenoxy. This is astable radical that will not abstract a hydrogen from the matrix PP polymer. Hinderedphenol enables the radicals to be managed in two different ways. On the one hand, theinitial radical species are effectively removed from participation in the propagation steps;on the other hand, the abstraction of hydrogen from the HP prevents another initiatingevent from occurring on the PP polymer backbone. This step immediately results in at leastone less reactive radical being formed. Regardless of the radical being quenched, the overalleffect of the HP is to delay the oxidation and, eventually, the degradation of the PP. Hinderedphenols are able to terminate more than one radical per phenol moiety. The structure ofHP allows the oxygen-based radical to be delocalised to the carbon atom bearing asubstituent, forming a quinone-like structure [77]. This is why even a very small amount ofHP antioxidant addition can achieve the stabilisation of the degradation for PP.

The shortcoming of the phenolic stabiliser is the development of colour. Some of thequinone-like structures that are active in the stabilisation process are also intense colourbodies or colour centres, giving a distinct yellow colour to PP. Even at very lowconcentration, the matrix polymer of PP demonstrates an obvious colour change. Inaddition, the interaction between HP and catalyst residues can intensify the developmentof colour. Further reaction with air pollutants such as nitrogen oxides and sulphur oxidesat room temperature results in increase of colour centres.

Hindered amines have been used as stabilisers against the oxidative degradation initiated byUV light. More recently, high molecular weight compounds have been shown to be effectiveas thermal stabilisers [78]. This class of compounds plays an important role in the commercialapplications and success of PP. Hindered amines act as radical scavengers through the nitroxylradical from amines [79, 80]. The stabilisation mechanism of HA can be depicted as:

R′–NH (oxidisation) → R′–NO• (3.14)

R′–NO• + •R → R′–NO–R (3.15)

From equations (3.14) and (3.15), it can be seen that the active species is not the aminefunctionality. The species that is active is the nitroxyl radical. The oxidation of the amineleads to the production of nitroxyl groups [80]. The nitroxyl group is regenerative. Theprocess can be ended by cyclic regeneration. Some of the regenerated products may beinefficient as radical scavengers; thus, some of the HA stabiliser is lost during exposure [81].


90


The secondary stabilisers decompose hydroperoxides and prevent new oxidation cyclesfrom beginning. This class of compounds is called secondary, because their bestperformance occurs in the presence of primary antioxidants. When used in PP bythemselves, secondary antioxidants do not exhibit any appreciable activity. The value ofthese compounds comes when they are combined with the correct primary antioxidants.When the appropriate combination is made, a strong synergistic effect results. Thecommonly used secondary antioxidants can be classified into two categories: one isphosphites, the other is thio compounds. Both the phosphites and thio compounds aresynergistic with the hindered phenols because they attack a source of free radicals, thehydroperoxides. They can reduce a hydroperoxide to an alcohol. Consequently, thehomolytic cleavage of the ROOH into two radicals can be prevented. Combined withhindered amines, secondary antioxidants are effective in suppressing UV degradation[82]. By absorbing the UV radiation before it has a chance to energise a chromophore,the formation of a radical is prevented. Obviously, it is impossible for the antioxidants toabsorb all the UV light, and thus some radicals are still formed. These radicals aresubsequently neutralised by the primary antioxidant of hindered amine.

3.6 Case Studies

In this work, the UV degradation behaviour of PP films was investigated with emphasison their durability, strength and surface morphology. Both unaged and UV-degradedwoven PP fabrics from stretched, knifed and relaxed film tapes were studied to reveal thedegradation effects. Pure PP film and several groups of PP film materials with additivessuch as UV stabiliser, antioxidant and colouring pigment (calcium carbonate) werecharacterised. Typical PP material specimens taken from different processing stages weretested to identify the effect of processing conditions on the durability of PP films. Scanningelectron microscopy (SEM) was used to study the microstructural features of the filmsand correlate them with the durability.

3.6.1 Materials and Experimental Procedures

3.6.1.1 Materials and Processes

All the PP films used in the current study were first formed using a chill roll film process.Then, several procedures including water-bathing, air-knifing and stretching were appliedto obtain PP tapes. The fabric was prepared by weaving. Classification of the test samplestaken during processing and for two PP woven fabrics after been exposed to UVdegradation for two weeks are given in Table 3.4.

91

3.6.1.2 Static Tensile Tests

Static tensile tests were performed using a Materials Testing System (MTS 810) equippedwith a 2100 kN load cell. The specimens were gripped between two hydraulic wedgegrips (type 647.10A-01). Static tests were carried out under displacement controlcondition at a crosshead speed of 1.5 mm/min. The gauge length was 15 mm. All thetests were conducted at room temperature of approximately 25 °C. At least three sampleswere tested for each material, and the stress-strain behaviour was established based onthe average values. The stress was calculated based on the initial cross-sectional areabefore testing.

3.6.1.3 Microscopic Examination

The surface morphology of each film material was examined using a Hitachi S-2150scanning electron microscope operated at an acceleration voltage of 20 kV. Themicrographs were recorded on Polaroid 55 instant films, and the images were capturedsimultaneously with Quartz PCI Version 3.01 image processing software, and stored forfurther editing and printing.

3.6.2 Durability-Microstructure Relationship

In order to examine the effect of UV degradation on both the durability and surfacemorphology, comparative studies on unaged and UV-degraded woven PP fabrics weremade. The typical stress-strain behaviour of the unaged PP woven fabrics is shownin Figure 3.13. It can be seen from this curve that the relationship between stress and

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1 1PP gnihtab-retawdnanoisurtxeretfA

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92


strain displays almost elastic behaviour in the strain range up to 30%. The calculatedmodulus based on this linear portion is about 700 MPa. In the strain range from30% to 40%, the stress-strain relationship is nonlinear. This indicates that plasticdeformation dominates the behaviour of the PP woven fabric material in this range.The ultimate tensile strength reached about 200 MPa. After this range, the stressdropped and the specimen failed. A multi-step fracture behaviour was observed dueto the mechanical interlocking of the woven fabrics. The durability for this wovenfabric is about 63%.

The typical surface morphology of woven PP fabrics shows a striped texture, asillustrated in Figure 3.14. Parallel lines along the stretching direction can be seen, inaddition to surface defects in the form of longitudinal cracks along the stretchinglines. These cracks were only localised in the area where severely deformed materialexists. The PP materials raised due to the extrusion and stretching processes exhibitsurface crazes.

Unlike the unaged PP woven fabrics, the UV-degraded PP woven fabric sample, A1,does not possess any durability and load-bearing capability. It was found that, afteraging under intense UV irradiation, the woven fabrics disintegrated. The tensile testspecimens prepared from A1 were so brittle that they could not be tested.

Figure 3.13 Stress-strain behaviour for unaged woven PP fabric material

93

Microscopic examination of the surface of the UV-degraded PP woven fabric specimen,A1, shows fraying edge strips, broken pieces and fine particles. Such morphologicalfeatures are shown in Figure 3.15. Numerous microcracks can be easily deposited on thesurface. Obviously, severe UV degradation resulted in the formation of these through-thickness cracks. The through-thickness nature of UV-degradation-induced cracks canexplain why the fabric material loses its load-bearing capability totally when such acondition of degradation is reached.

Figure 3.14 Typical SEM morphology of woven PP fabric material

Figure 3.15 Typical SEM morphology of UV-degraded woven PP fabric material


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3.6.3 Durability-Processing Condition Relationship

The properties of PP films are dependent on the processing conditions. The effect of crystallinityand orientation of PP crystals on the durability and strength of PP fibres have been studied[83]. Generally, intensive drawing and stretching result in the orientation of PP crystals anddecrease the crystallinity of PP. Thus the content of the amorphous portion increases. Asindicated by Galanti and Mantell [83], well-oriented PP has a higher durability than that ofpoorly oriented PP, and stretched amorphous PP displays a higher strength than that ofregularly crystallised PP. From the view of engineering design and application, comprehensivestudies in this field still need to be carried out. The following section will present the resultsof the current investigation on the effect of processing conditions on the durability of PP film.

Two types of PP film, PP1 and PP7, as defined in Table 3.4, were chosen to investigate theeffects of processing conditions. As indicated in Table 3.4, PP1 is the starting film productand PP7 is the tape ready for weaving. This means that these two films were obtained fromtwo extreme conditions. Thus, the change in structure and properties from PP1 to PP7 canreasonably reflect the effect of the entire processing procedure on the durability of PP films.

The stress-strain curve for the PP1 sample, extruded and water-bathed from the pure PPmaterial, is shown in Figure 3.16(a). The ultimate tensile strength for this sample was about37 MPa. The durability was about 500%. The stress-strain relationship displayed a highdegree of nonlinear behaviour after an elastic region. The deformation and failure of thismaterial could be divided into five stages. The first stage is elastic deformation correspondingto a strain of approximately 10%. The calculated Young’s modulus for the PP1 sample isabout 320 MPa. The second stage is the nonlinear deformation in which both plastic andelastic deformation can be observed. This region corresponds to a strain range from 10% to20% approximately. The third stage represents yielding. The maximum yield strength wasabout 31 MPa. Considerably large plastic deformation can be observed after the yield point.The strain at the end of this stage reached 50% with a drop in strength to about 27 MPa. Thefourth stage was the cold flow of the PP, which corresponds to the strain range from 50% to200%. The fifth stage is the strain hardening of the PP material. With further increase instrain, the strength of the material increased about 25%. Following the strain-hardeningstage, the material failed catastrophically.

The PP7 tape material underwent a series of processing procedures such as knifing, stretchingand relaxation. From the typical stress-strain behaviour of the PP7, as shown in Figure 3.16(b),it can be seen that the relationship between stress and strain displays nonlinearity in thestrain range up to 20%. The calculated modulus based on the first linear portion of the curveis about 900 MPa. In the strain range from 20% to 33%, the stress-strain relationship ismore nonlinear. This indicates that plastic deformation dominates the behaviour of the PP7tape material in this stage. The ultimate tensile strength reached about 220 MPa. After thisrange, the stress dropped and the specimen broke. The durability is about 33%.

95

Figure 3.16 Stress-strain behaviour of (a) PP1 and (b) PP7

The difference in the tensile behaviour of these two kinds of PP film materials can beseen clearly by comparison of Figures 3.16(a) and (b). The unstretched PP tape materialshowed much higher durability than the stretched products. However, its strength ismuch lower than that of the other. This indicates that stretching followed by knifing canincrease the load capability of the PP material and considerably decrease its durability,based on the strain to failure on the stress-strain curve.


96


A micrograph taken from the PP1 sample (Figure 3.17) shows raised lines along theextrusion direction. A typical knifed and stretched sample cut from the tape (PP7) wasexamined. The microstructure is shown in Figure 3.18. Comparing Figures 3.17 and3.18, it can be seen that the strip size changed from the original width of about 1 mm toabout 0.3 mm. In some areas in particular, cracking along the strip lines can be seen onthe surface of stretched PP materials.

Figure 3.17 Microstructure of PP1 sample

Figure 3.18 Microstructure of PP7 sample

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3.6.4 Durability-Additive Property Relationship

In order to examine the effect of additive on the durability and load-carrying ability ofPP films, four different materials were manufactured using four different processingconditions. These PP film materials contain either no additives, a UV stabiliser, a whitecolouring pigment (calcium carbonate) or a mixed additive (Amoco 100/03). The fourprocessing conditions were designated PP1, PP3, PP4 and PP7 are described in Table 3.4.

The stress-strain curves for the four PP1 samples, extruded and water-bathed pure PPmaterial, are shown in Figure 3.19(a). The PP film with Amoco 100/03 mixed additivehas the highest tensile strength of 60 MPa, while the specimen of PP1 film with UVstabiliser has the lowest strength of about 35 MPa. The durability for all four materialsexceeds 450%. For the Amoco product, this value reached nearly 700%. The stress-strain relationship displayed a high degree of nonlinear behaviour after an elastic region.The deformation and failure of these materials could be divided into five stages. Thefirst stage is elastic deformation corresponding to a strain of approximately 10%. Thesecond stage is the nonlinear deformation in which both plastic and elastic deformationcan be observed. This region corresponds to a strain range from 10% to 20%approximately. After this stage, the third stage can be seen. The typical feature in thethird stage is yielding. The yield strength for these materials is in the range from 28 to35 MPa. Considerably large plastic deformation can be observed after the yield point.The strain at the end of this stage reached 50%, with a drop in the strength to about25 MPa. The fourth stage was the cold flow of the PP, which corresponds to the strainrange from 50% to 200%. Again a considerable amount of deformation occurred inthis stage. The total strain due to the plastic deformation in the form of cold flow islarger than 150%. The fifth stage is the strain strengthening of the PP1 materials. Withfurther increase in strain, the strength of the materials increased about 25-40%.Following the strain-hardening stage, the material failed catastrophically, and the stress-strain curves displayed a sharp drop.

The additive-loaded or unloaded PP3 showed the same stress-strain behaviour as foundfor the PP1 materials. As shown in Figure 3.19(b), the four PP3 materials alsodemonstrated several distinct stages of deformation, including elastic deformation,yielding, cold flow followed by strain strengthening. However, both the yield strengthand ultimate tensile strength of PP3 materials are a little bit larger than those for the PP1materials. The typical specimen of PP3 without any additive has a higher yield strengthof about 38 MPa, and its ultimate tensile strength is about 62 MPa. The Young’s modulusis approximately 350 MPa. Differences in the durability were found from the stress-strain curves for these PP3 materials with different additives. The durability for all thesematerials exceeded 400%. The material supplied by Amoco displayed the maximumdurability of 700%. The PP3 with white filler also displayed the same durability as that


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Figure 3.19 Stress-strain behaviour of (a) PP1, (b) PP3, (c) PP4 and (d) PP7 filmmaterials with various additives

(a)

(b)

for the Amoco product. The specimens with UV stabiliser kept the same durability. It hasthe same strain to failure as that for the PP3 material without additive. However, theultimate tensile strength of the PP3 with added UV stabiliser showed a considerabledecrease as compared with the samples without any additives.

99

(c)

(d)

Figure 3.19 Continued

The PP4 and PP7 materials underwent a series of processing procedures such as knifing,stretching and relaxation. The four PP4 films were from the stretched zone, while thePP7 films were in the form of tape, and were ready for weaving. From the typical stress-strain behaviour of the PP4 film materials, as shown in Figure 3.19(c), it can be seen that


100


three materials – the PP4 without any additive, with UV stabiliser and with white filler –have a similar relationship between stress and strain, while the Amoco product displayeda decrease in both strength and modulus. However, the durability of this film materialreached 68%, which is larger than that of the other three films. All the stress-straincurves of the four PP4 materials display considerable nonlinearity in the strain range upto 30%. Beyond this strain range, the stress-strain relationship is more nonlinear. Thisindicates that plastic deformation dominates the behaviour of the PP4 films. The ultimatetensile strength for these PP4 films is in the range from 275 to 375 MPa. The sharp dropon the stress-strain curves indicated the final rupture of these specimens.

The stretched tape materials, PP7, showed a similar nonlinear deformation behaviour understatic overloading similar to the PP4 film materials. Nevertheless, both the ultimate tensilestrength and the durability of the PP7 materials are smaller than those of the PP4 materials.The stress-strain curves for the four PP7 materials are shown in Figure 3.19(d). The ultimatetensile strength is in the range from about 150 to 275 MPa. The durability was in the rangefrom 20% to about 33%. Moreover, the PP7 samples with calcium carbonate filler show atendency in decrease in tensile strength. The ultimate tensile strength for this kind of materialis about 150 MPa, which is only 60% of that of the other three PP7 materials.

The surface morphology of the four groups of PP films with and without additives wasexamined. Samples from the PP1 group have a unidirectional texture. Parallel lines alongthe extrusion direction can be seen. These lines are raised materials. The surfacemorphology for the four PP3 materials was found to be very similar to that of the PP1materials. Comparing the morphology of PP1 and PP3 with that of PP4 and PP7, it was

Figure 3.20 Micrograph of PP1 containing calcium carbonate pigment

101

found that the strip size changed from the original width of about 1 mm for PP1 and PP3to less than 0.4 mm for PP4 and PP7. The PP films containing colouring pigment showcrevices and scratches due to the movement of calcium carbonate particles along theextrusion direction. This is clearly shown in Figure 3.20, a micrograph taken from thesurface of the PP1 film with white colouring pigment, calcium carbonate particles.

3.7 Concluding Remarks

The durability of PP is highly sensitive to ageing under ultraviolet irradiation. UnagedPP woven fabrics have unidirectional structure and possess good durability and load-bearing capability. The durability is about 60%. The ultimate tensile strength is about200 MPa. The Young’s modulus reaches 700 MPa. UV degradation causes severe damageto the structure and drastic decrease in durability and strength of PP film materials.The load-bearing capability for typical aged PP woven fabrics was totally lost aftertwo weeks of UV exposure. The surface morphology of the aged PP fabrics wasdominated by numerous microcracks.

The durability of PP film materials is also sensitive to processing conditions. Withoutstretching, the films with striped structure demonstrated cold flow followed by strainstrengthening. The durability for this material exceeds 490%. The typical specimen ofPP1 has a yield strength of 31 MPa, and its ultimate tensile strength is about 37 MPa.The Young’s modulus is approximately 320 MPa. After extensive stretching, knifing andrelaxation, the distance between the parallel raised lines decreased, the durability is reducedto about 33%, while the load-carrying capability increased remarkably. Limited plasticdeformation without cold flow was found. The sample of the PP7 has a tensile strengthof about 220 MPa, and the Young’s modulus is about 900 MPa.

The durability of the PP films is not sensitive to the addition of additives. UV stabilisersand antioxidants do not change the durability and morphology of the PP films appreciably.All the PP film materials have a very smooth surface and possess a similar structure withunidirectional raised lines. Calcium carbonate can produce crevices during extrusion,while the durability of the films containing calcium carbonate is almost the same asthose without any whitening additives.

Acknowledgements

This work was supported by the Egyptian Foreign Relations Coordination Unit (FRCU)of the Supreme Council of Universities/USAID under University Linkage Project IIGrant #93-02-16.


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109

4 Solubility of Additives in Polymers

Alexander Mar’in

4.1 Introduction

Polymer-based materials usually contain, besides the polymer, various low molecularweight (low molar mass) compounds such as stabilisers, plasticisers, dyes, dissolvedgases, accidental and technological impurities. During exploitation, polymer materialscan come into contact with water, organic liquids, solid substances and foodstuffs,which could result in the transfer of additives and impurities dissolved in the polymerto the surroundings, polluting them and decreasing the lifetime of the polymer. Onthe other hand, low molecular weight compounds from the surroundings can passinto the polymer. The distribution of additives between polymers and surroundingsis controlled by processes based on sorption (dissolution) and diffusion. This topiccovers different aspects of additive solubility in polymers in light of polymerdegradation and stabilisation.

4.2 Nonuniform Polymer Structure

A polymeric substance is nonregular. This irregularity may display itself at the molecular,topological and morphological levels. The molecular irregularity is due to the chain-likestructure of the polymer molecule and the existence of non-equivalency (anisotropy)along and across the polymer chain. Topological irregularity is due to the existence ofpolymer chain ends and various polymer chain entanglements surrounded by relativelyordered substance in which short-range order is obeyed. Morphological irregularity isbased on the existence of relatively large zones markedly differing in the character of thearrangement of segments of macromolecules forming these zones and in their physicalproperties. In crystalline polymers, this irregularity gives rise to the formation of crystallineand amorphous regions, fibrils and spherulites.

Gases and additives dissolved in a polymer are mainly present in the amorphous regionsin zones around knots, folds and various chain entanglements where there is a free volumelarge enough to hold the molecule [1-8]. The degree of topological irregularity (disorder)depends on the conditions of polymer synthesis as well as on the conditions of polymersample preparation, (i.e., on crystallisation) [9-12].

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4.3 Additive Sorption

The most important characteristic of the sorption (that is, dissolution) of an additive in apolymer is its sorption isotherm, i.e., the relationship between the concentration or vapourpressure of the compound around the polymer and its concentration in the polymer. Thesimplest isotherm corresponds to the case of an ideal solution. This isotherm is describedby Henry’s law: the concentration of compound A in the polymer ([A]p) is directlyproportional to its concentration in the surrounding medium ([A]m) or to its pressure Pa:

[A]p = γ*[A]m (4.1)

or [A]p = (γ*/RT)Pa = γ*pPa (4.1a)

where γ* and γ*p are solubility coefficients.

In the case of an ideal solution, d2[A]p/d[A]m2 = 0. That is, dissolution of any compound

A does not change the properties of the polymer medium. In practice, linear isotherms(4.1) and (4.1a) are observed only at low concentration of a dissolved compound.

Positive and negative deviations from the law (4.1) are possible. In the first case, d2[A]p/d[A]m

2 > 0; and in the second, d2[A]p/d[A]m2 < 0. A positive deviation means that the

sorption of any molecule of A facilitates the sorption of the next one. Two mechanismsfor the positive deviations are possible: (i) an increase in the mobility of themacromolecules caused by a dissolved compound (for example, plasticising the polymer)and (ii) the formation of aggregates (clusters) of several A molecules dissolved in thepolymer. Positive deviations may be described by the following equation obtained fromthe theory of regular solutions:

μ/RT = ln(P/P0) = ln φ1 + φ2 + χφ22 (4.2)

where P is the vapour pressure of the additive in the system, P0 is the pressure of its saturatedvapour, and χ is the Flory-Huggins parameter (solvent-solute interaction parameter).Equation (4.2) connects the chemical potential of the solvent (μ) with the volume fractionsof the additive (φ1) and the polymer (φ2). The value of χ can be determined in terms of thesolubility parameters of the additive and of the polymer, δ1 and δ2, respectively:

χ = V1(δ2 – δ2)2/RT (4.3)

where V1 is the molar volume of sorbate. Equations (4.2) and (4.3) are widely used inpractice and allow one to predict the additive sorption. The values δ1 and δ2 can beobtained from independent experiments or by simulation. In crystalline polymers, it isnecessary to take into account the volume accessible for the molecules of additive, whichdoes not always coincidence with the total volume of amorphous phase of the polymer.

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Solubility of Additives in Polymers

A negative deviation from equation (4.1) corresponds to the case when the polymer possessesa limited number (concentration) of centres that can sorb one A molecule each. In thiscase, with increasing A concentration in the polymer, the number of non-occupied centresdecreases. In many cases the negative deviations can be described by a law analogous tothe Langmuir equation (4.4) or its combination with Henry’s law (4.5) [2-7, 11]:

AA

1 Apm

m

[ ] =[ ]

+ [ ]a

b (4.4)

AA

1 Apm

m

[ ] = [ ] +[ ]

+ [ ]γ Am

a

b (4.5)

where a and b are constants; the ratio a/b corresponds to limit of the A concentrationin polymer.

The nature of sorption centres may be different. Polymer polar groups interacting withan additive (for example, due to the formation of hydrogen bonds), as well regions witha lower density of a polymer substance (the elements of free volume) in the polymer, maybe regarded as such centres. The latter have either a relaxation or a topological nature.Some authors [3-5] consider sorption centres as microvoids and unrelaxed volume in thepolymer below the glass transition temperature that disappear at high temperature. Incontrast, the centres arising around knots and other chain entanglements are more stableand can also exist in the polymer melt [7, 8, 12].

Suppose that in a polymer a certain concentration of the same centres Zi is present thatcan interact with compound A. Let us also suppose that the sorption of additive proceedsin two steps. First, the additive forms a true solution, the concentration of A in thissolution being related to its concentration around the polymer by Henry’s law, that is,[A] = γ[A]m. Then this truly dissolved additive is reversibly sorbed by centres Zi:

A Z AZa+ ← →⎯⎯iK

i(4.6)

The total concentration of A in the polymer is [A]p = [A] + ∑[AZi] = [A] + [AZa], where[A] and [AZa] are the concentrations of true dissolved (mobile) and immobile molecules,respectively. If the additive concentration outside the centres is neglected ([A] << [AZa]),the sorption isotherm of A will be:

[A]p = [AZa] = γKa[Za][A]m(1 + γKa[A]m)–1 (4.7)

or 1/[A]p = 1/γKa[Za][A]m + 1/[Za] (4.7a)

112


where γ is the coefficient of true (outside the centres) solubility of A ([A] = γ[A]m).Formula (4.7) is equivalent to the Langmuir type isotherm (4.4) assuming a = γKa[Za]and b = γKa.

Formula (4.7) is observed in the case of the sorption of different additives includingantioxidants by polyolefins. The concentration of sorption centres ([Za]) depends onthe type of additive and polymer used, and in most cases remains constant over a widetemperature range from solid polymer to polymer melt [6, 7, 9, 11-13], indicating theexistence of stable disorder.

The precipitation of polymer from different solvents has been used as a method tochange the concentration of chain entanglements [12]. During precipitation,macromolecules have to overcome the interaction with molecules of the solvent, whichis more difficult in the case of a ‘good’ solvent, and a polymer sample obtained afterprecipitation from a ‘good’ solvent has to possess a less perfect structure. Figure 4.1shows that the sorption isotherms of 2,6-di-tert-butyl-4-methylphenol (BMP) bypolyethylene (PE) obey equation (4.7a); PE samples precipitated from decane have ahigher concentration of sorption centres ([Za]) than samples precipitated fromchlorobenzene [12].

In some cases, at high concentration of additive, the sorption isotherms change theirshape and a strong increase in sorption is observed. This dependence can be explainedby means of polymer swelling resulting in changes in the polymer properties andmechanism of sorption [7-13].

Polymers usually contain different additives present together. The existence in thepolymer of centres capable of sorbing two additives, (i.e. A and B), should result in adecrease in the concentration of one compound in the presence of another one due tocompetition for sorption centres. Formally it may be presented as:

B AZ BZ+ ← →⎯i i(4.8)

If ∑[BZi] ≈ [B]p, there should be a linear dependence between the concentrations of thetwo compounds: the dissolution of B results in an equivalent decrease of theconcentration of A in the polymer. However, it is not possible to have the completereplacement of one additive by another one. Substitution is observed only in a limitedrange of concentration of both additives, which shows that there are some centres thatdo not take part in the substitution process (Figure 4.2) [7, 9, 13].

113

Figure 4.1 Sorption isotherms of BMP by PE samples precipitated from chlorobenzene(1) and decane (2) solutions containing of 1.0% of PE. T = 180 °C

Figure 4.2 Replacement of dibenzoylmethane (DBM) by phenyl benzoate (PB) inpolypropylene from ethanol solution; [DBM]0 = 0.2 mol/l, T = 40 °C


114


4.4 Quantitative Data on Additive Solubility in Polymers

The solubility of an additive corresponds to the concentration at which the additive inthe polymer is present in equilibrium with the additive outside the polymer or with itssaturated vapour. Formally, the solubility of A (SA) corresponds to the point on the sorptionisotherm ([A]p = SA) where the concentration of A in the surroundings is equal to theconcentration of A in its saturated vapour. According to equation (4.7), SA is less than[Za] and reaches [Za] with increasing temperature [8].

Various methods of measuring additive solubility in polymers have been proposed. Thedirect method includes the study of the kinetics of additive dissolution in a polymerwhen the additive is present in equilibrium with its saturated vapour or with an additiveintroduced on the surface of polymer film [7, 14-17]. For this purpose polymer film withan additive is kept in a closed vacuum tube or in an inert medium for different periods oftime. Usually, the solubility value corresponds to some plateau on the curve of theconcentration of additive in the polymer versus time. At high temperatures, dissolutioncan be accompanied by change in the polymer structure and the solubility will changewith time [8, 16, 17].

To measure additive solubility, the ‘sandwich’ method is also used [18-21]. Polymer filmis placed between films oversaturated with additive. The solubility measured in this waymay be higher than in the case of the free additive method. This is probably due to thefact that the additive concentration in the oversaturated film does not correspond to thetrue equilibrium.

There are some indirect methods of measuring additive solubility. One of them is based onthe measurement of the concentration profile of the additive inside the film [18-23]. Thismethod makes possible the simultaneous determination of the additive diffusion coefficient.

Another method includes the determination of the temperature dependence of the vapourpressure of the additive above the pure additive and above the polymer containing adefinite concentration of the additive [24]. The intersection point of the two curves (inthe coordinates lg Pa versus 1/T) corresponds to the temperature at which the additiveconcentration in the sample is equal to its solubility. The temperature dependence of thetransparency of polymer films with various additive concentrations [25] allowsmeasurement of additive solubility. If the additive concentration in the polymer exceedsits solubility at the given temperature, the excess additive emerges to form crystals ordrops, which sharply decrease the sample transparency. This method is not precise.

Billingham and coworkers [15] considered the solubility of additives in polymers basedon regular solution theory: solubility is defined by the condition that the (negative) freeenergy of mixing of the liquid additive with the polymer is equal to the (positive) free

115

energy required to convert the crystalline additive into a liquid at the same temperature.In this case the solubility of the additive in the polymer is represented by:

–ln SA = (ΔHf/RT)(1 – T/Tf) + (1 – V1/V2) + χ (4.9)

where ΔHf is the heat of fusion of the additive, Tf is its melting temperature, V1 and V2

are the molar volumes of the additive and of the solvent, and χ is the interactionparameter. According to equation (4.9) a crystal with a higher heat of fusion is expectedto be less soluble in a polymer than one with a lower heat of fusion. The additivesolubility in the polymer can be predicted from data on its solubility in a homologousset of solvents by extrapolation of the solubility data in the coordinates ln SA versus 1/V2

to the point 1/V2 = 0. This approach does not take into account the features of thepolymer structure.

For the description of the temperature dependence of additive solubility in a polymer,the van’t Hoff equation:

Ss = Ss0 exp(–ΔH/RT) (4.10)

is used, where ΔH is the heat of solution. Equation (4.10) is correct in only narrowtemperature ranges. Among the reasons for the violation of this dependence are phasetransitions in the polymer, (i.e., near its melting point), and the dissolved additive. Anotherreason is the existence of stable sorption centres whose concentration in the polymerdoes not depend on temperature.

The data published on antioxidant solubility in polymers refer mainly to polyolefins,and markedly differ from one another. These differences are apparently due to differencesin the methods of measuring their solubility and to differences in the structures of thesamples studied. Table 4.1 shows data on the solubility of different stabilisers in polymers.The solubility of stabilisers decreases with their molecular weight, but there is no simpledependence between these characteristics. The solubility of phenolic-type stabilisers inpolyolefins and in rubbers is greater than that of aromatic amines with the same molecularweight. Sulfides are highly soluble in polyolefins, probably due to the presence of aliphaticgroups in their molecules [21]. There is a difference in the solubility in PE of two stericallyhindered amines with close molecular weight (396-423 and 481) [22]; nitroxides are lesssoluble than the corresponding amines [23].

As seen from Table 4.1, the solubility of many stabilisers at room temperature is markedlylower than the concentrations at which the additives are usually added to polymers (0.1–0.5% by weight). Thus, an excess of a stabiliser added to a polymer often emerges (sweatsout or blooms) from it.


116


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117

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118


4.5 Factors Affecting Additive Solubility

4.5.1 Crystallinity and Supermolecular Structure

Additive solubility in nonpolar rubber is greater than that in crystalline polyolefins (seeTable 4.1) because the crystalline regions of polyolefins are not available for additives, andcrystals decrease the plasticising action of dissolved compounds. There is no simplecorrelation between polymer crystallinity and additive solubility. The solubility of additivesdepends not only on the volume of amorphous fraction but also on its structure. It wasshown [30] that the solubility of diphenylamine and phenyl-β-naphthylamine in solidpolyethylene with different crystallinity is practically constant and only slightly decreasesat high crystallinity of the polymer. The authors attribute this to the irregularity of theamorphous regions of the polymer, the density of which decreases with increasing polymercrystallinity. Moisan [31] showed that the solubility of Irganox 1076 in polyethylene at 60°C only changes weakly with polymer crystallinity in the range from 43 to 57% (densityrange 0.92–0.94 g/cm3), but at higher temperatures (70 and 80 °C) the solubility decreaseswith polymer crystallinity. It should be noted that crystallinity measured at room temperaturecan change considerably with temperature, especially in the polymer premelting region.

The role of the polymer supermolecular structure and polymer prehistory on antioxidantsolubility has been studied [32-37]. It has been shown that the solubilities ofdiphenylamine, of the methyl ester of 3,5-di-tert-butyl-β-hydroxypropionic acid and of2,2′-methylenebis(4-methyl-6-tert-butylphenol) in polyolefins prepared by rapidly coolingthe polymer melt (structure with small spherulites) are higher than in the samples preparedby slow crystallisation near the polymer melting temperature (the structure with largespherulites) [32, 33]. The difference in solubility can reach a factor of 2, while thecrystallinity measured by the IR method is practically the same [33].

The precipitation of polymer from different solvents has been used as a method to changethe polymer structure and antioxidant efficiency [34-36]. It was shown that additivesolubility in polypropylene (PP) precipitated from decane (a ‘good’ solvent for PP) washigher than that in chlorobenzene.

The solubility of phenyl-β-naphthylamine in PP/PE blends and ethylene-propylenecopolymers was studied in solid film and in the melt [37]. It was shown that the solubilityof the antioxidant at 60 °C is practically independent of the composition of the polymermixture, whilst the solubility in copolymers has a minimum at a propylene content in therange near 2% and a wide maximum at 40%. The solubility of different stabilisers inLDPE, in LDPE/LLDPE blends and in ethylene-vinyl acetate copolymer has been studied[18]. The additive solubilities in LDPE and in the blend are close, while in ethylene-vinylacetate copolymer it is higher, especially in the case of 2,6-di-tert-butyl-4-methylphenol.

119

4.5.2 Effect of Polymer Orientation

Orientation drawing of polyolefins results in considerable change in the polymer structureand additive behaviour: spherulites transform into fibrils; in amorphous zones, the amountof regular conformers increases and that of irregular conformers decreases [38-42]. Thesolubility and diffusion coefficient of additives usually decrease with drawing, butsometimes these relationships are more complicated [40, 43, 44]. Figure 4.3 shows theeffect of elongation of PE on the solubility of various antioxidants at 60 °C. Thecrystallinity of PE determined by differential thermal analysis does not change withdrawing, while the crystallinity determined by IR spectroscopy increases from 36 to48% for (λ = 0-5.5), showing the change in the conformation set of macromolecules[44]. Because the orientation drawing can result in deformation and the disappearanceof chain entanglements due to pull-out of macromolecules from knots and othertopological irregularities, one may expect that after further melting the additive solubilityof oriented samples should tend to decrease compared with that of non–oriented ones.The chain entanglements cannot recover quickly, otherwise a memory of the change inpolymer structure should remain after polymer melting. Figure 4.4 shows the effect oforientation drawing on the solubility of phenyl benzoate [45]: curve 1 corresponds to

Figure 4.3 Dependence of the solubility of the methyl ether of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid (1), of 2,2′-methylenebis[4-methyl-6-(1-methylcyclohexyl)phenol] (2) and the ester of 3,5-di-tert-butyl-4-

hydroxyphenylpropionic acid and ethylene glycol (3) in polyethylene on thedegree of drawing (l) at 60 °C


120


samples which after drawing were heated in vacuum to 140 °C followed by fast cooling;curve 2 corresponds to samples without additional treatment. As can been seen fromFigure 4.4, orientation of the polymer affects the solubility of the additive even if sampleswere heated above the polymer melting point.

4.5.3 Role of Polymer Polar Groups

In a polymer containing polar groups, the mechanism of sorption may also include theinteraction of polar groups of the polymer (X) with polar groups of the dissolved additive(A), as represented [8, 46, 47] by:

A X AX+ ↔ (4.11)

In going from nonpolar to polar polymers, e.g., from polyolefins to aliphatic polyamidescontaining –CONH– groups, the polymer density increases as a result of the formationof hydrogen bonds between these groups. It was shown [8] that, for polymers such aspolyamide-12 (PA-12), polyamide-6,10 and polyamide-6,6 (or polyamide-6; PA-6), thesolubility of phenyl-β-naphthylamine increases in the order polyethylene to polyamide–12 and decreases with higher concentrations of these groups, whereas the solubilities of

Figure 4.4 Solubility of phenyl benzoate in polyethylene at 60 °C as a function of degreeof drawing: (1) samples after additional heating, (2) samples without additional heating

121

the less polar additives phenyl benzoate and 2,6-di-tert-butyl-4-methylphenol decreaseover the whole range studied (Figure 4.5).

4.5.4 Effect of the Second Compound

The decrease in the solubility of one compound in the presence of another compounddue to competition for sorption centres was mentioned above. In some cases a morecomplicated situation is observed. The effect of octamethylcyclotetrasiloxane (OMTS)introduced into a PE melt on the solubility of phenyl-β-naphthylamine (PNA) in solid PEis shown in Figure 4.6: the solubility decreases and then passes through a maximum withOMTS concentration. The content of trans and gauche conformations in the polymer isalso changed, which can be attributed to rearrangement of the sorption centres. OMTSalso affects the polymer melt: the vapour pressure of PNA (the concentration in gasphase over the polymer) depends on the OMTS concentration in the melt [14].

Plasticisers present in polymers change the polymer structure owing to the increase inmobility of polymer chains, which affects the solubility and diffusion of additives in thepolymer. The solubility of antioxidants in nonplasticised PVB is low compared with thatin pure plasticiser, i.e., for the ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid

Figure 4.5 Solubility of phenyl-β-naphthylamine (1) and of phenyl benzoate (2) inpolyethylene and polyamides at 40 °C as a function of the concentration of amide groups


122


and ethyleneglycol: 0.014 and 0.84 mol/kg (0.9% and 53% by weight) respectively [28].If the solubility in the polymer (SPVB) and in the plasticiser (SPLA), i.e., of antioxidant inplasticised PVB (Sadd), is considered as a simple sum of the solubilities

Sadd = SPVB + x(SPLA – SPVB) (4.12)

where x is the percentage of plasticiser in the polymer, one can expect a linear growth ofantioxidant solubility in the polymer with plasticiser concentration (Figure 4.7, dottedline). Experiment shows that, at low concentrations of the plasticiser (1–5% by weight),the solubility is higher than what it should be according to equation (4.12); but at higherconcentrations of plasticiser (10–40%), it is less than Sadd (Figure 4.7, solid line) [28].The effect observed is due to the fact that, at small concentrations, the plasticiser stronglyaffects the mobility of macromolecules and, as a result, increases the antioxidant solubility;at high concentrations, there is solution of the polymer in the plasticiser with strongpolymer–plasticiser interactions, which disturb the antioxidant dissolution.

4.5.5 Features of Dissolution of High Molecular Weight Additives

For additive dissolution, sorption centres should contain an excess volume large enoughto locate the additive molecule. If this volume is less than that necessary for sorption,

Figure 4.6 Solubility of phenyl-β-naphthylamine in polyethylene at 60 °C as a functionof OMTS concentration

123

dissolution of A can occur only when the rearrangement of this centre results in a changeto the polymer structure. The process of centre rearrangement may be represented byequation (4.13) followed by polymer swelling:

A Z AZ A Z+ → ↔ +i i*

i* (4.13)

This could be important when large additive molecules are considered. The solubilityof sterically hindered amines with molecular weights from 1364 to 2758 inpolypropylene has been studied [16, 17]. It was shown that the solubility of the stabilisersin the polymer at 100 °C passes through a maximum with time and depends on themolecular weight of the stabiliser: the higher the molecular weight of the stabiliser, thehigher its maximum concentration in PP. It was assumed that, at high temperatures,molecules of larger size are able to change the polymer structure to a greater extentthan those of smaller size, so the apparent solubility may increase with the molecularweight of the additive as observed experimentally. Thus, the process of dissolution ofhigh molecular weight additives gives rise to a certain ‘destruction’ of the initial polymerstructure. The decrease in additive solubility with time is probably due to annealing ofthe polymer in the presence of additives. Additive dissolution is accompanied by achange in the polymer crystallinity and in the concentration of irregular conformationsin the amorphous zones of the polymer [16, 17].

Figure 4.7 Solubility of the methyl ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionicacid as a function of dihexyl adipate content in PVB at 60 °C


124


4.5.6 Effect of Polymer Oxidation

The oxidation reaction first involves the zones with lower polymer packing densitycontaining polymer chain entanglements and can result in the disappearance of some ofthem. This process may be represented as:

Z1 + nO2 → εZ2 (4.14)

where Z1 and Z2 are different types of sorption centres, and ε < 1.

Aliphatic chain scission proceeds in the oxidation reaction according to:

CH2 CH

O

CH2 CH2 C H

O

CH2+(4.15)

As seen from equation (4.15), polar aldehyde groups are formed in this process at theends of broken chains. Thus, the newly formed centre Z1 may contain polar groups. Inthis case the solubility of polar additives may increase with oxidation. The solubilities ofdiphenylamine and phenyl benzoate were studied in polyethylene and in several aliphaticpolyamides after oxidation [47]. Figure 4.8 shows the solubility of phenyl benzoate as afunction of oxidation degree. At low oxidation degrees, up to 0.2–0.3 mol/kg of oxygen,

Figure 4.8 Solubility of phenyl benzoate at 60 °C as a function of amount of oxygenabsorbed during oxidation of polymers: PE, PA-12 and PA-6

125

the solubilities of both additives in all polymers studied decrease; at deeper stages, thesolubility in polyamides still decreases, whilst in PE it increases. To explain theexperimental data, we should assume that oxidation results in the decomposition of onetype of sorption centre and the simultaneous formation of other ones. In polyamides theconcentration of polar amide groups is higher compared to those of new ones formed inoxidation. For this reason we only observe the decrease in additive solubility caused bypolymer oxidation. In nonpolar polyethylene, the effects of both processes are comparable,and we observe a more pronounced and complicated variation of additive solubilities.

4.6 Solubility of Additives and Their Loss

An additive dissolved in a polymer can transfer from the polymer into the surroundingmedium. This process includes the stages of diffusion of the additive to the surface andits removal from the surface, (i.e., by sweating out, evaporation or washing out). Thesolubility of an additive in the polymer can affect all these stages [48-57].

Any excess of an additive in a polymer (above its solubility) may exude on the polymersurface, forming either drops or powder; it blooms or sweats out. A quantitativedescription of the process of sweating out is difficult, because at high additiveconcentrations exceeding its solubility the diffusion coefficient is not constant and theresidual additive concentration after sweating out is greater, the greater its initialconcentration in the polymer.

The evaporation of an additive, A, from a polymer depends on its solubility in the polymer.In some cases, the rate of additive evaporation, We, is connected with its surfaceconcentration, [A]sf, and its solubility in the polymer, [A]s, by:

We = Wa[A]sf/[A]s (4.16)

where Wa is the rate of evaporation of the individual additive. A detailed description ofadditive loss due to evaporation may be found elsewhere [51].

High molecular weight additives are not volatile and their diffusion in a polymer atelevated temperature is very slow, which is why washing out is the principal cause ofundesired loss of stabilisers and other additives from polymeric material used outdoorsor for the flow of liquids in tubes and containers. There are some factors that mayinfluence the washing out of stabiliser from a polymer, i.e., the additive solubilityand the solvent solubility in the polymer [55]. Owing to low additive solubility, partof the additive may be present in a polymer in a metastable state or form a separatephase and it can be lost quickly. On the other hand, the solvent facilitates the migrationof stabilisers passing into the polymer and increasing the segmental mobility of


126


macromolecules. The ability of the solvent to escape the additive is connected withthe solvent solubility in the polymer: the higher the solvent solubility, the higher thewashing-out effect.

The diffusion coefficient of an additive in many cases increases with the additiveconcentration in the polymer. The dependence of the diffusion coefficient on diffusantconcentration may be due either to plasticisation of the polymer or to the presence ofsorption centres that bind a part of the diffusing molecules.

Assuming that only mobile molecules of A, outside the sorption centres, participatein the diffusion, their concentration is represented [9] by:

AA

Z A

p

a a p

[ ] =[ ]

[ ] [ ] ⎞⎠

⎛⎝K – (4.17)

It is possible to find the theoretical dependence of the diffusion coefficient (D) on thetotal concentration of A in the polymer:

Φ = –Dt(d[A]/dx)

DD

= [ ][ ] [ ]⎛

⎝⎞⎠

t

a a p

Z

K Z A–2 (4.18)

where Dt is the diffusion coefficient of truly dissolved molecules. Figure 4.9 showsthat D of phenyl-β-naphthylamine in PP depends on the antioxidant concentrationand on the concentration of sorption centres according to equation (4.18).

The results obtained show that features of additive dissolution and diffusion couldbe explained by taking into account non-unique additive distributions in the polymerand the existence of sorption centres around polymer chain entanglements which areable to hold the additive molecule. The concentration of these entanglements may bechanged either during polymer synthesis or during polymer treatment (orientation,crystallisation, etc.). The influence of polymer disorder on additive behaviour andpolymer properties should be considered as an important factor in the physicalchemistry and technology of polymeric materials.

127

Figure 4.9 Dependence of the diffusion coefficient of phenyl-b-naphthylamine on itsconcentration in PP at 60 °C

References

1. Y.A. Shlyapnikov, European Polymer Journal, 1998, 34, 1177.

2. R.M. Barrer, J.A. Barrie and J. Slater, Journal of Polymer Science, 1958, 27, 177.

3. W.R. Vieth, R.M. Tam and A.S. Michaels, Journal of Colloid and InterfaceScience, 1966, 22, 360.

4. D.R. Paul and W.J. Koros, Journal of Polymer Science, Polymer Physics Edition,1976, 14, 675.

5. R.J. Pace and A. Datyner, Journal of Polymer Science, Polymer Physics Edition,1980, 18, 1103.

6. A.P. Mar’in and Y.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B,1974, 16B, 471.

7. Y.A. Shlyapnikov and A.P. Mar’in, European Polymer Journal, 1987, 23, 623.

8. Y.A. Shlyapnikov and A.P. Mar’in, European Polymer Journal, 1987, 23, 629.


128


9. P-G. de Gennes, Macromolecules, 1984, 17, 703.

10. A.P. Mar’in, Y.A. Shlyapnikov, A.Z. Makhamov and A.T. Dzhalilov, OxidationCommunications, 1997, 20, 57.

11. P. Bruni, C. Conti, A. Mar’in, Y.A. Shlyapnikov and G. Tosi, European PolymerJournal, 1997, 33, 1665.

12. Y.A. Shlyapnikov, S.G. Kiryushkin and A.P. Mar’in, Antioxidative Stabilisation ofPolymers, Taylor and Francis, London, 1996.

13. Y.A. Shlyapnikov and A.P. Mar’in, Acta Chimica Hungarica, 1987, 124, 531.

14. A.P. Mar’in, E.A Sviridova and Y.A. Shlyapnikov, Polymer Degradation andStability, 1995, 47, 349.

15. N.C. Billingham, P.D. Calvert and A.S. Manke, Journal of Applied PolymerScience, 1981, 26, 3453.

16. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of MacromolecularScience A, 1998, 35, 1299.

17. A.P. Mar’in, V. Borzatta, M. Bonora and L. Greci, Journal of Applied PolymerScience, 2000, 75, 7, 883.

18. E. Foldes, Polymer Degradation and Stability, 1995, 49, 57.

19. E. Foldes and B. Turscanyi, Journal of Applied Polymer Science, 1992, 46, 507.

20. E. Foldes, Journal of Applied Polymer Science, 1993, 48, 1905.

21. J.Y. Moisan, European Polymer Journal, 1980, 16, 979.

22. J. Malik, A. Hrivik and E. Tomova, Polymer Degradation and Stability, 1992,35, 61.

23. V. Dudler, Polymer Degradation and Stability, 1993, 42, 205.

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25. N.P. Frank and R. Frenzel, European Polymer Journal, 1980, 16, 647.

26. I.A. Shlyapnikova, A.P. Mar’in, G.E. Zaikov and Y.A. Shlyapnikov,Vysokomolekulyarnye Soedineriya Series B, 1985, 27A, 1737.

129

27. L.S. Feldshtein and A.S. Kuzminsky, Kautchuk i Rezina, 1970, 10, 16.

28. A. Mar’in, L.A. Tatarenko and Y.A. Shlyapnikov, Polymer Degradation andStability, 1998, 62, 507.

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30. B.A. Gromov, N.E. Korduner, V.B. Miller and Yu.A. Shlyapnikov, DokladyAkademii Nauk SSSR, Chemistry, 1970, 190, 1381.


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34. Y.A. Shlyapnikov, T.V. Monakhova and T.A. Bogaevskaya, Polymer Degradationand Stability, 1994, 46, 247.

35. P. Bruni, A.P. Mar’in, E. Maurelli and G. Tosi, Polymer Degradation andStability, 1994, 46, 151.

36. P. Bruni, C. Conti, A.P. Mar’in, Y.A. Shlyapnikov and G. Tosi, AmericanChemical Society, Polymer Preprints, 1996, 37, 1, 101.

37. I.G. Kalinina, I..I. Barashkova, G.P. Belov, K.Z. Gumargalieva, A.P. Mar’in andY.A. Shlyapnikov, Vysokomolekulyarnye Soedineriya Series B, 1994, 36B, 1028.

38. I.M. Ward and D.W. Hadley, An Introduction to the Mechanical Properties ofSolid Polymers, John Wiley, Chichester, UK, 1993.

39. V.A. Marikchin and L.P. Myasnikova, Supermolecular Structure of Polymers,Nauka, Leningrad, Russia, 1977, 86.

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41. L.S. Shibryaeva, A.P. Mar’in and Y.A. Shlyapnikov, VysokomolekulyarnyeSoedineriya Series B, 1995, 37B, 696.

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46. A.P. Mar’in, I.A. Shlyapnikova, G.E. Zaikov and Y.A. Shlyapnikov, PolymerDegradation and Stability, 1991, 31, 61.

47. T.V. Monakhova, A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation andStability, 1993, 40, 365.

48. A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1991,31, 181.

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131

5 Polyvinyl Chloride: Degradationand Stabilisation

K.S. Minsker, G.E. Zaikov and V.G. Zaikov

5.1 Introduction

Some aspects of the manufacture of polyvinyl chloride (PVC) that does not contain labilegroups in the backbone are considered. This will provide a drastic increase in the intrinsicstability of polymeric products, and the possibility of PVC processing with a minimalcontent or total absence of stabilisers and other additives. The data presented allow thecreation of rigid, semi-rigid (semi-flexible) and flexible (plasticised) materials and productswith minimal content of chemical additives and increased service lifetime for exploitationin natural and special conditions.

PVC is one of the most well-known multi-tonnage and practically important polymericproducts. Thousands of rigid, semi-flexible and flexible (plasticised) materials and productsbased on PVC are widely used in all spheres of national economies and everyday life.PVC was first synthesised by E. Baumann in 1872, but its industrial manufacture beganmuch later – in 1935 in Germany according to the literature data, and in 1930 in theUSA according to the data of the DuPont Company.

Global PVC production is impressive: 220 thousand tons in 1950, about 1.5 milliontons in 1960, more than 3 million tons in 1965, more than 5 million tons in 1970, and itscurrent production (mid-2002) is estimated to be more than 23 million tons.

A basic problem with PVC is its low stability. Under the action of heat, ultraviolet (UV)light, oxygen, radiation, etc., it easily disintegrates according to the law of transformationof adjacent groups with the elimination of hydrogen chloride and the formation ofsequential carbon-carbon double bonds in the macromolecules and the appearance ofundesirable coloration (from yellow to black). Therefore, it is necessary to apply a set ofmethods that will lead to the increased stability of PVC itself, and of materials andproducts based on it, when exposed to the various factors that occur during synthesis,storage, processing and use.

It is logical to assume that, among the many aspects causing the low stability of PVC andthe rather short lifetime of materials and products based on it, the most important pointis to understand the reasons for its abnormally high rates of disintegration compared tolow molecular weight models. Researchers in the fields of synthesis and processing of

132


PVC appear to have found this problem rather complex, for, in essence, it is still underdiscussion. So far, such workers in industrial research centres in various countries havenot been able to agree upon the identification of the weak site in the structure of the PVCmacromolecule responsible for causing its abnormally low stability.

5.2 Some Factors Affecting the Low Stability of PVC

It used to be thought that the low stability of PVC was connected to the possible presenceof labile groups in the macromolecular structure, which activate polymer disintegration.These labile groups are distinct from sequences of regular vinyl chloride repeat units:

~CH2CHCl–CH2–CHCl–CH2–CHCl~

The overwhelming majority of researchers believe that such groupings are [1-8]:

(1) Chlorine atoms bonded to tertiary carbon atoms C–Cl (At);

(2) Vicinal chlorine atoms in the macromolecular structure:~CH2–CHCl–CHCl–CH2~ (Av);

(3) Unsaturated end-groups such as ~CH=CH2 and/or ~CCl=CH2;

(4) β-Chloroallyl groups ~CH2–CH=CH–CHCl~ (Ac);

(5) Oxygen-containing hydroxy and peroxy groups (A0).

However, even after just a brief consideration of the process of PVC disintegration, it isobvious that there are far fewer labile groups (which can be considered to be the cause oflow PVC stability) in the macromolecules. This is because, on PVC dehydrochlorination,tertiary chlorine (At) and vicinal (Av) groups turn into β-chloroallyl groups and thehydroperoxide groups transform into carbonyl groups, as shown in Scheme 5.1.

In addition, PVC research throughout the world has shown that the initial (freshlysynthesised) PVC macromolecules (which are processed in materials and products) do notcontain di- (A2), tri- (A3) and/or polyene (Ap) groups [2, 3, 9-14]. Internal peroxide groups

~CH2–CHCl–O–O–CH2–CHCl~

are not found either, since if they were formed during PVC synthesis they would quicklydisappear as a result of hydrolysis and/or homolytic cleavage of the O–O bond. Thereare reliable experimental results, including those obtained during the study of the thermal

133

Achievements for Polyvinyl Chloride: Degradation and Stabilisation

destruction of fractionated PVC, showing that, although unsaturated end-groups arepresent in the structure of polymeric molecules, they do not affect the disintegration rateof PVC [10, 13-15].

Thus, the process of gross dehydrochlorination of PVC (overall rate constant VHCl) canbe described with sufficient accuracy by Scheme 5.2, where: α0 represents the regularvinyl chloride ~CH2–CHCl~ groups; KCl, Kt, Kv, Kc and Kp are the rate constants for theappropriate dehydrochlorination reactions of PVC; and Ktr is the rate constant for thereaction that terminates polyene growth.

~H2C C

Cl

CH3

CH2~

~H2C CH CH CH2~

Cl Cl

~HC CH~

ClOOH

– H2O~H2C C CH CH2~

O

Cl

~H2C CH

CH

CHCl~– HCl

~H2C CH

CH

CH

CH

CHCl~

At

Av

A0

Ax Ap

Scheme 5.1

Ac A2

Kc

Av

At

α0

Ap

A*

Kp

KtrKt

KCl

Kv

Scheme 5.2

134


Following from Scheme 5.2, we have:

VHCl = KCl[a0] + Kc[Ac] + Kp[Ap]

with real values of: KCl = 10–8 to 10–7 s–1 and [α0] = 1 mol/mol PVC; Kt = 10–4 s–1 and[At] = 10–3 mol/mol PVC; Kc = 10–4 to 10–5 s–1 and [Ac] = 10–4 mol/mol PVC; Kv = 10–3 to10–4 s–1 and [Av] = 10–5 mol/mol PVC; and Kp = 10–2 s–1 (448 K).

It is obvious that Scheme 5.2 assumes the concept of β-chloroallyl-activated disintegrationof PVC accepted by the majority of researchers, but without real proof [1-5]. However,this postulate contradicts many experimental facts [16, 17], in particular the following:

(1) Calculated values of VHCl differ greatly from the experimental ones.

(2) The β-chloroallyl activation of PVC disintegration assumes an autoacceleration ofthe PVC gross dehydrochlorination process with time [16-18], whereas a lineardependence is observed experimentally (Figure 5.1). The gross rate constant of PVCdisintegration, according to experimental data and shown in Figure 5.1 at Kc = 10–4

to 10–5 s–1, should contain the term Kp ≅ 10–2 s–1 (at 448 K) from the very beginning

Figure 5.1 Kinetic curves for PVC dehydrochlorination: (1) calculated data forβ-chloroallyl activation; (2) experimental data (448 K, 10–2 Pa)

135

of PVC thermal destruction. However, according to data obtained on the thermaldisintegration of low molecular weight model compounds [19-21], this is observedonly as a result of the destruction of model compounds containing a chlorine atom ina β-position to conjugated (C=C)n bonds (at n ≥ 2), i.e., due to the effect of theadjacent group of the long-range order (Table 5.1). Thus, the concept of β-chloroallylactivation of PVC dehydrochlorination does not satisfy even a preliminary analysisof the experimental results, is therefore erroneous and should no longer be considereda viable theory.

On the basis of theoretical considerations of PVC thermal degradation, and in view ofall the available experimental data, it can be concluded that: even if internal β-chloroallylgroups (as well as tertiary and vicinal chlorides) are present in the macromolecularstructure, they do not contribute to the process of PVC gross dehydrochlorination as aresult of their sufficient relative stability. It was assumed, and then proved, that thegroup that is responsible for the low stability of PVC is an oxovinylene (carbonylallyl)conjugated dienophile group:

–C(O)–CH=CHCl–CH2–

the double bond of which is activated by the adjacent electrophilic C=O group.Apparently, this group is present in PVC macromolecules in rather small amounts,γ ≅ 10–4 mol/mol PVC, but disintegrates at a rather high rate (Kp ≅ 10–2 s–1) with HClelimination [14, 17, 22-24].

wolfonoitcurtsedlamrehtroftnatsnocetarnoitanirolhcordyheD1.5elbaTsdnuopmocledomthgiewralucelom

.oN dnuopmoC

egnarerutarepmeTsdnuopmocerehw)K(

taedargedottratsetarelbaeciton

puorGxedni

noitisopmoceDtnatsnocetar

sK( 1– )

1 enatneporolhciD-4,2 395-365 α0 6.2 × 01 9–

2 osem enatneporolhciD-4,2- 395-365 α0 9.1 × 01 9–

3 enatneporolhc-3-lyhtE-3 355-884 At 9.7 × 01 6–

4 2-enexehorolhC-4 364-334 Ac 1.5 × 01 4–

5 2-enecedorolhC-4 864-834 Ac 0.5 × 01 5–

6 5,3-eneidanonorolhC-7 963-343 Ap 4.3 × 01 2–

7 4,2-eneidatcoorolhC-6 683-063 Ap 6.2 × 01 2–


136


It is extremely important to emphasise that the concept of oxovinylene activation ofPVC disintegration does not contradict any currently known experimental facts. Inaddition, new proofs (including original ones) of the existence of basic groups in thestructure of PVC macromolecules have been obtained recently. In particular, oxovinylenegroups in the PVC macromolecule are easily split (under mild conditions) with alkalinehydrolysis (5% aqueous KOH solution, and 5% solution of PVC in cyclohexanone) [13,14], which is a characteristic reaction for α,β-unsaturated ketones, as shown in thefollowing reaction [25]:

~CH CH C(O)~ C

O

H

+ H3C C~

O

H2O

KOH(5.1)

Using this reaction, it is easy to estimate the content of labile oxovinylene groups (γ0) inthe macromolecular structure by the decrease of viscosity-average molecular weight ofPVC [13-17].

5.3 Identification of Carbonylallyl Groups

It is important to remark that both β-chloroallyl and polyene groups are inert to alkalinehydrolysis, but easily decomposed on oxidation (in the presence of hydrogen peroxide)and by ozonolysis [13]. The ozonolysis method allows estimation of the total amount ofinternal unsaturated (β-chloroallyl, chloropolyenyl and oxovinylene) groups in the PVCstructure by the decrease of PVC molecular weight. Thus, it is experimentally shownthat practically all the internal unsaturated groups in PVC macromolecules are oxovinyleneones, and that the PVC dehydrochlorination rate is linearly connected to the content ofinternal labile oxovinylene groups in polymeric molecules [14, 26], determined by usingalkaline hydrolysis (Figure 5.2). It is known that PVC synthesised in the absence ofoxygen is always more stable than PVC manufactured industrially. This is due to thepresence of sufficiently stable internal β-chloroallyl (not oxovinylene) groups (oxidativeozonolysis) in the first type of PVC structure. As a whole, the real process of HClelimination during PVC disintegration in the transformation reaction of adjacent groupsis complex, since generally this or that contribution is brought in by all abnormal groupscontained in the PVC structure. However, apparently, the contribution of differentreactions to this process varies and in a number of cases some of them can be neglected.

The kinetic analysis takes into account the real contents of characteristic (includingabnormal) groups in PVC. Also, the rate constants of their disintegration (Table 5.2)

137

have precisely shown [14, 17, 24, 27, 28] that the ratio of the appropriate reaction rateconstants is:

KCl : Kc : Kt : Kp ≅ 1 : 100 : 100 : 100,000

and, for this reason, the thermal stability of PVC is determined by the effect of theadjacent group of the long-range order (conjugation effect). So the total elimination rateof HCl from PVC is described with sufficient accuracy by the simple equation:

Vt

K K V VHCl Cl p Cl p

d HCl

d= [ ] = [ ] + [ ] = +α γ0 0 (5.2)

Even taking into account the participation of tertiary chloride (At) and β-chloroallyl(Ac) groups in PVC disintegration, the contribution of the expression Vp = Kp[γ0]comprises about 90% or more of the total gross rate of PVC dehydrochlorination.This confirms oxovinylene (not β-chloroallyl) activation as the major process in PVCthermal disintegration.

The development of the concept of oxovinylene activation of PVC thermal destructionappears to be an important point in the theory and practice of PVC chemistry and

Figure 5.2 Dependence of PVC dehydrochlorination rate on the content ofcarbonylallyl groups in the polymer molecules (448 K, 10–2 Pa)


138


objectively defines the necessity for a new specific approach for studying various aspectsof the destruction and stabilisation of PVC. In particular, studies are needed of the newcharacteristic reactions with unsaturated ketones, confirming the presence of oxovinylenegroups in the PVC structure, or the interaction of ~C(O)–CH=CH–CHCl~ groups withorganic phosphites P(OR)3 [29-33] and dienes [34, 35].

5.4 Principal Ways to Stabilise PVC

Organic phosphites react easily in mild conditions (290-330 K) with oxovinylene groupsin the presence of proton donors to yield the stable ketophosphonates:

dnasgnipuorgcitsiretcarahcfonoitanirolhcordyhedfostnatsnocetaR2.5elbaTerutcurtsCVPlaitiniehtnistnetnocrieht

puorG

CVPnistnetnoC K844tanoitadargedfotnatsnocetaR

xednItnuomAlom/lom(

)CVPsrohtuA xednI

eulaVs( 1- )

srohtuA

~lCHC–HC=HC–OC~

γ0 01~ 4-

8791reksniM.K4891kivroS.E

4891namremmiZ.GKp 01 1- 01- 2- 7791reksniM.K

5891senratS.W

HC~( 2 HC–lCC) 2 HC– 2 ~lC

A lC0 0

8791reksniM.K4891namremmiZ.G

K lC 01 5- 01- 4-

1791reyeM.Z3791ykstiorT.B3891senratS.W

HC–lCC~ 2 HC 2 lC

At0 01~ 3-

4891kivroS.E1891ukalucaraC.A

5891namlegeZ.VKt 01 4- 3891senratS.W

1791reyeM.Z

HC~ 2 )HC=HC(– 1>n ~lCHC–

Ap 0 6791reksniM.K Kp 01~ 2- 1791reyeM.Z4891reksniM.K

HC~ 2 HC–lCHC– 2 ~lCHC–

α0 1 - K lC 01 7- 01- 8- 1791reyeM.Z2791reksniM.K

139

The reaction kinetics for the interaction of organic phosphites with oxovinylene groupsare shown in Figure 5.3. The formation of ketophosphonate structures according to reaction(5.3) results in the disappearance of internal C=C bonds in the PVC structure. As a result,neither ozonolysis of a polymeric product nor especially alkaline hydrolysis leads todegradation of macromolecules and, consequently, decrease of PVC molecular weight.

~C CH

O

CH

CH~

Cl

+ P(OR)3

O P-(OR)3

~C CHCl~

C CH2

O

CH CHCl~

P(OR)3O (5.3)

Figure 5.3 The changes in the ~C(O)–CH=CH~ group content in PVC during interactionwith tri(2-ethylhexyl) phosphite (C0 = 10–2 mol/mol PVC): (1) 289 K; (2) 298 K; (3) 448 K


140


It is important to note that organic phosphites do not react with β-chloroallyl groups,as has been confirmed by the method of competing reactions of organic phosphites(trialkyl-, arylalkyl- and triarylphosphites) with a mixture (1:1 mol/mol) of methylvinyl ketone (model of an oxovinylene group) and 4-chloropentene-2 (model of a β-chloroallyl group) at 353 K. Practically, the organic phosphite selectively reactsquantitatively (with regarding to proton donor) with methyl vinyl ketone, while 4-chloropentene-2 is quantitatively allocated after realisation of the reaction, excludingthe small amount (less than 7 wt%) of products of its dehydrochlorination. The mainreaction product (up to 75 wt%) is:

CH3–C(O)–CH2–CH2–P(OR)2

In this reaction, trialkyl- and alkylarylphosphites are more active than triarylphosphites.

Dienophilic oxovinylene groups react with conjugated dienes according to the followingDiels-Alder reaction:

The reactions of PVC with cyclopentadiene, piperylene, isoprene, 5-methylheptatriene-1,3,6, etc., proceed in mild conditions (353 K) and result in destruction of internalunsaturated C=C groups in PVC chains. These reactions are new and have not beenreported before, and are similar to the reaction of PVC with organic phosphites, asshown in reaction (5.4).

The collection of methods used to increase PVC stability to the action of various factors(such as heat, light, oxygen, etc.), in terms of storage, processing and use is closelyconnected to the level of theoretical development of PVC degradation. Therefore, it isclear that the significant advances in theoretical developments of the reasons for thethermal instability of PVC (the presence of oxovinylene groups in the backbone), themechanism of the process (the fundamental influence of adjacent groups of the long-range order) and the kinetics of their disintegration were necessary, and have enableda new look at the determination of effective methods of PVC stabilisation under thermaland other influences.

~C CH

O

CH

CHCl~ +

RHC

CH

CH

CHR– CHCH

RHC

CH

CH

CHR–

~C

O

CHCl~

(5.4)

141

According to Scheme 5.2 it is impossible (and unnecessary) to increase the stability ofPVC by the reduction of rate VHCl, since this process is rather slow. According to theexperimental data, the rate of PVC statistical dehydrochlorination, VHCl, is constant(law of randomness) and does not depend on how the polymer was synthesised or itsmolecular weight. Hence, it is a fundamental characteristic of PVC, showing that allparts in clusters ~BXBXBX~ participate similarly in the process of HCl elimination underthe law of randomness. On the other hand, the rate of formation of the conjugatedsystems, Vp, differs markedly, since it increases linearly with the content of oxovinylenegroups in the initial PVC macromolecules (γ0) (Figure 5.2).

Thus, the basis of effective PVC stabilisation, which determines processing propertiesand the durability of rigid materials and products, is due mainly to the increased self-stability of PVC [17, 36-39]. This can be achieved by chemical stabilisation of the labileoxovinylene groups present in the initial PVC macromolecules, first of all by studyingspecific polymer analogous reactions with either of the reaction centres 1-3:

~C CH

CH

CH~

ClO1

2

3 (5.5)

The conjugation ~C(O)CH=CH~ has to be destroyed and/or the labile chlorine atom hasto be replaced with a more stable adjacent group by interaction with the appropriateadditives (stabilisers). This principle underlies the stabilisation of PVC in real formulationsduring manufacture of rigid materials and products, which is called ‘chemical stabilisation’of PVC [17, 36, 37]. The reactions on centres 1-3 mentioned above are as follows:

(1) Polymer analogous reactions on C O fragments of oxovinylene chloride groups:

~C CH

O

CH CHCl~ R' CH CH

O

R"

R' CH CH R"

OH OH

R3SiH

R3GeH R3 Si

CH CH CH CHCl~~

R' CH CH R"

O O

~C CH C CHCl~

(5.6)

[5]

[40]


142


(2) Polymer analogous reactions on C C fragments of oxovinylene groups:

(3) Polymer analogous reactions on labile C Cl groups:

R2Sn(COOR)2

Cd(COOR)2

Zn(COOR)2, etc.

CH CH

O

R' R"

(ZnCl2)

~C(O) CH CH CH CH2–

OC(O)R

~C(O) CH CH CH

O—CHR'—CHClR" (5.8)

P(OR)3

R' CH CH CH CH R"

CHHC

C

O

C

OO

~C CH2

O

CH CHCl~

P(OR)3

O

CH~~C

O

P

CH

ORO

CHCl~

CH

HC CH

CH

CHCH

CHCl~

O

~C

R"R'

CH

O

~HC

CH

CH

CH

C

OC

CHCl~

O

O

and/or

(5.7)

[31-35]

[41]

[21]

[16, 17]

[43]

143

The concept of oxovinylene activation of the disintegration of PVC has allowed revealingnew unexpected possibilities for effective stabilisation – not only thermal, but also lightstabilisation – of this polymer. This also allows previously unknown classes of chemicalcompounds to be used for its stabilisation, in particular, conjugated diene hydrocarbons,Diels-Alder reaction adducts, protonic acids, α,β-dicarbonic compounds, etc. [34, 35, 40-46]. It has also enabled new real reactions for PVC stabilisation to be revealed, including theapplication of known additives that have been used for a long time for PVC stabilisation (forinstance, organic phosphites, epoxy compounds, proton-donating compounds, etc.). So, onthis basis it is possible to manage the PVC ageing process more effectively (Scheme 5.3). Therelation between the chemical structure of additives and their efficiency as stabilisers for


Scheme 5.3

144


PVC gives an opportunity for the scientifically based and economically expedient selectionof the appropriate stabilisers and their synergistic combinations for producing rigid materialsbased on PVC.

5.5 Light Stabilisation of PVC

Polymer analogous transformations of the oxovinylene groups in PVC macromoleculeson chemical stabilisation with the appropriate chemical additives lead not only to increasedself-stability of PVC and inhibition of macromolecular crosslinking, but also to a noticeableincrease in the colour stability of PVC.

The transformation of oxovinylene groups as a result of polymer analogoustransformations with chemical additives in the ketophosphonate, cyclohexane, dioxolane,dihydropyran, etc., structural groups in PVC and the ‘curing’ of labile oxovinylene chloridegroups result in an increase in the optical density of PVC in the UV region of the spectrum.As a result, these groups act as internal light stabilisers and result in the phenomenon ofself-photostabilisation of PVC [47] (Figure 5.4).

Figure 5.4 Dependence of whiteness retention coefficient Kw in PVC films on exposuretime: (1) unstabilised PVC; and polymer treated with: (2) 2-tris(2-ethylhexyl)phosphite; (3) 2-ethylhexyl-9,10-epoxy stearate with ZnCl2; (4) piperylene;

(5) cyclopentadiene (295 K, λ = 254 nm, 1 to 1.5 × 1015 photon/s cm2)

145

Thus, the determining factor causing the high rate of PVC disintegration and its need forstabilisation is the presence of abnormal groups, mainly oxovinylene ones, in the structureof its macromolecules.

5.6 Effect of Plasticisers on PVC Degradation in Solution

In both plasticised (semi-rigid and flexible) PVC materials as well as PVC in solution, therates of thermal destruction and effective stabilisation are caused by essentially differentfundamental phenomena in comparison to those involved in the ageing of PVC in theabsence of solvent. The following aspects of both structure and macromolecular dynamicshave a significant influence on the stability of PVC: the chemical nature of the solvent,its basicity, specific and nonspecific solvation, the concentration of PVC in the solution,the segmental mobility of macromolecules, the thermodynamic properties of the solvent,the formation of associates, aggregates, etc. The chemical stabilisation of PVC plays aless significant role.

As regards PVC destruction in solution, one of the basic reasons for a change in thekinetic parameters is the nucleophilic activation of the PVC dehydrochlorinationreaction. The process is described by an E2 mechanism. Thus, there is a linear dependencebetween PVC thermal dehydrochlorination rate and the relative basicity of the solvent,B cm–1 (Figure 5.5) [48-50]. The value B cm–1 is evaluated by measuring the shift of acharacteristic band (phenolic OH) at λ = 3600 cm–1 in the IR spectrum due to interactionwith the solvent [51]. It is very important that, in solvents with relative basicity B >50 cm–1, the rate of PVC dehydrochlorination is always above the rate of PVCdehydrochlorination without the solvent; while, when B < 50 cm–1, PVC disintegrationrate is always less than that without the solvent. The revealed dependence VHCl = f(B)is described by the equation:

V V k BHCl HCl* ( )= + − 50 (5.9)

Inhibition of PVC disintegration in solvents with basicity B < 50 cm–1 is a very interestingand practically important phenomenon. It has been given the name ‘solvationalstabilisation’ of PVC. However, ignoring the fact that, even at low concentration (2wt%), PVC solutions should be represented not as solutions with isolated macromolecules,but rather as structured systems, results in a number of cases of deviation from a lineardependence of PVC dehydrochlorination rate on the solvent basicity B cm–1. In particular,an abnormal destruction behaviour of PVC is observed in certain ester-type solvents(plasticisers) (Figure 5.5, points 25-28), apparently caused by structural changes of themacromolecules. This has never before been taken into account when working withPVC solutions.


146


Figure 5.5 Influence of the solvent’s basicity on the rate of thermal dehydrochlorinationin solution: (1) n-dichlorobenzene, (2) o-dichlorobenzene, (3) naphthalene,(4) nitrobenzene, (5) acetophenone, (6) benzonitrile, (7) di-(n-chlorophenyl-

chloropropyl) phosphate, (8) triphenyl phosphite, (9) phenyl-bis(β-chloroethyl)phosphate, (10) tri-(n-chlorophenyl) phosphate, (11) 2-ethylhexylphenyl phosphate,

(12) tricresyl phosphate, (13) cyclohexanone, (14) phenyl-bis(β-chloropropyl)phosphate, (15) tri-β-chloroethyl phosphate, (16) tri-β-chloropropyl phosphate,(17) di-(2-ethylhexyl) phosphate, (18) 2-ethylhexylnonyl phosphate, (19) tri-(2-

ethylhexyl) phosphate, (20) tributyl phosphate, (21,25) dibutyl phthalate,(22,26) di-(2-ethylhexyl) adipate, (23,27) dioctyl phthalate, (24,28) dibutyl sebacate.

Concentration of PVC in solution: (1-24) 0.2 wt%, (25-28) 2 wt%; 423 K, under nitrogen

It was revealed quite unexpectedly that not only ‘polymer-solvent’ interactions, but also‘polymer-polymer’ interactions, have a significant influence on the rate of PVCdisintegration in solution. It is known that the structure and properties of the appropriatestructural levels depend on the conformational and configurational nature of themacromolecules, including the supermolecular structure of the polymer, which in turndetermines all the basic (both physical and chemical) characteristics of the polymer.‘Polymer-polymer’ interaction leads to the formation of structures on the supermolecularlevel. In particular, on going to a more concentrated solution, the PVC-solvent systemconsistently passes through a number of stages, from isolated PVC macromolecules in

147

solution (infinitely dilute solution) to associates and aggregates of macromolecules insolution. On further increase of PVC concentration, the formation of a spatial fluctuationalnet with a structure similar to that of the bulk polymer occurs.

When the polymer concentration in solution increases, the rate of the PVCdehydrochlorination reaction changes as well, and various types of effect of the solventon the PVC disintegration rate in solution are observed depending on the numericalvalue of the basicity B cm–1 [52-57]. If the relative basicity of the solvents used is B > 50cm–1, the polymer degradation rate decreases when its concentration increases. If thebasicity of the employed solvents is B < 50 cm–1, the polymer degradation rate increaseswith increasing polymer concentration. In all cases the rate of HCl elimination from thepolymer tends to approach the values of PVC dehydrochlorination rate usually observedin the absence of solvent VHCl

PVC = 5 x 10–8 (mol HCl/mol PVC)/s (Figure 5.6).

Figure 5.6 The change in PVC dehydrochlorination rate as a function of itsconcentration in solution: (1) cyclohexanol, (2) cyclohexanone, (3) benzyl alcohol, (4)1,2,3-trichloropropane, (5) o-dichlorobenzene, (6) no solvent; 423 K, under nitrogen


148


Equation (5.9) turns into equation (5.10) if one takes into account that the PVCdegradation rate is determined not only by the relative basicity of the solvent, B, but alsoby its concentration in solution, C (mol PVC/litre). Also, the degree of ‘polymer-polymer’interaction (degree of macromolecule structurisation in a solution is given by ΔC = C –C0, where C0 is the concentration at the beginning of PVC macromolecule association insolution) is considered:

V VA B

C dHCl HCl= +−( )+

0 1

1

50

Δ (5.10)

where A1 = (0.8 ± 0.2) × 10–9 (mol HCl/mol PVC)/s; and d1 is a dimensionless factorreflecting the ‘polymer-solvent’ interaction (d1 = 0.5 ± 0.25).

The deviation from the onset of macromolecule association in a solution is taken as anabsolute value, since it can be changed in both directions to more concentrated or moredilute polymer solution.

Equation (5.10) well describes the change of PVC thermal dehydrochlorination rate as afunction of its concentration in a solution of relative solvent basicity B, irrespective ofthe chosen solvent (Figure 5.7).

The observable fundamental effect has significant importance in the production ofplasticised materials and products made from PVC, in particular when esters are used.Despite the very high basicity of ester-type plasticisers (B = 150 cm–1) in the range ofPVC concentration in solutions above 2%, a noticeable reduction in the degradationrate of PVC is observed (Figure 5.5, points 25-28), and stabilisation of PVC occurs. Thiseffect is caused by the formation of dense globules, associates, etc., in the PVC-plasticisersystem. Practically, this allows economic formulations of plasticised materials to be createdfrom PVC with very low content of metal-containing stabilisers, used as HCl acceptors,or without their use at all.

Temperature is very important in the formation of heterophase systems. Even at lowconcentrations of PVC in ester-type plasticisers (for example, in dioctyl phthalate at C >0.1 mol/l), true solutions are formed only at temperatures above 400 K. The globularstructure of PVC suspension and the formation of associates are retained at temperaturesup to 430-445 K. In other words, plasticised PVC is able to keep its structural individualityon a supermolecular level, which is formed during polymer synthesis. Specifically, underthese conditions an ester-type plasticiser behaves not as a highly basic solvent, but as astabiliser during PVC thermal degradation due to formation of associates, etc. This leadsto a reduction of the amount of stabiliser, extension of useful lifetime of materials andproducts, etc.

149

It is necessary to note that the change in the degradation rate of PVC brought about byassociation of macromolecules is a general phenomenon and does not depend on howit was achieved. In particular, a change of character of the dehydrochlorination rate ofPVC in solution is observed, similar to concentrated PVC solutions (Figures 5.6 and5.7), if a change of PVC structure in solution occurs upon addition of even chemicallyinert nonsolvents such as hexane, decane, undecane, polyolefins, polyethylene wax,etc. [53, 56-59] (Figure 5.8). It is interesting to observe that the degree of relativechange of PVC disintegration rate under the action of a second inert nonsolvent ismuch higher than for concentrated PVC solution. This is especially true in the case ofusing low-basicity solvents (trichloropropane and dichlorobenzene); it is the result ofmore dense formations on the supermolecular level, corresponding associates andaggregates, and, accordingly, a significant change of PVC destruction rate. The higherthe content of nonsolvent (including inert polymer) in a blend and the lower thethermodynamic compatibility of the components in a solution, the more structural

Figure 5.7 The change in PVC dehydrochlorination rate as a function of itsconcentration in solution: (1,2) 1,2,3-trichloropropane, (3,4) cyclohexanol; (1,3)experimental data, (2,4) data calculated using equation (5.10) at A1 = 10–9 and

d1 = 0.8 and 0.7, respectively, at 423 K, under nitrogen


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formation takes place in a solution, including that in the presence of polymer blends(associates, aggregates). Formation of a fluctuational net with participation ofmacromolecules is the probable explanation.

The reason for the change in PVC thermal dehydrochlorination rate in the case of itsblends with chemically inert, thermodynamically incompatible polymers is the same. Itis due to the fact that in concentrated PVC solution (structural chemical changes ofpolymer in solution), the parameters determining the rate of PVC disintegration willobviously be similar. Therefore, for PVC thermal destruction, the concentration of thesecond polymer blended with PVC and its degree of thermodynamic affinity to PVC, inaddition to the influence of polymer concentration in the solution, the basicity of thesolvent, B cm–1, and ‘polymer-solvent’ interaction forces, have to be taken into account.In view of these factors equation (5.10) turns into:

V VA B

C dA

BCHCl HCln

n

C= +

−( )+ + +

+0 1

1

1 250

Δ αα α

(5.11)

Figure 5.8 The change in PVC thermodegradation rate on the content of the secondinert polymer in solution of trichloropropane (1,3), dichlorobenzene (2) and

cyclohexanol (4-6) for blends of PVC and polyethylene (1,4), polypropylene (2,5)and polyisobutylene (3,6); 423 K, under nitrogen

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where α is the fraction of the second polymer, varying from 0 to 0.99; n is adimensionless parameter describing the degree of thermodynamic affinity of PVC tothe second polymer and varying from 0 (for complete thermodynamic compatibilityof the components) up to a value of ~10 (for complete thermodynamic incompatibilityof the polymers); and d2 is a dimensionless coefficient reflecting the interaction ofthe second polymer with the solvent, which equals 2.5 ± 0.1 for the destruction ofPVC blended with polyethylene in dichlorobenzene, trichloropropane andcyclohexanol.

Observable changes in PVC thermal disintegration rate under the action of a solventthat is thermodynamically incompatible with PVC or for a concentrated solution ofPVC are caused by the transformation of the solvent from macromolecular globulesof PVC to the structure that existed in the absence of the solvent. This evokes theunexpected effect of ‘solvent action’, either retardation or acceleration of PVC thermaldisintegration depending on the solvent basicity, B cm–1. A solvent transformationthat accelerates PVC disintegration (B > 50 cm–1) results in a decrease of its interactionwith PVC and leads to a delay in the HCl elimination process, i.e., to stabilisation.This occurs in the case of both concentrated PVC solutions as well as the addition ofanother polymer that is thermodynamically incompatible with PVC. In solvents thatslow down PVC disintegration (B < 50 cm–1) by virtue of low nucleophilicity, theeffect of solvent transformation and the weakening of its interaction with PVC hasthe opposite result. In this case an increase of HCl elimination rate from PVC uponincrease of its concentration in solution or by using a chemically inert nonsolventoccurs. It is obvious that, irrespective of how the changes to the PVC structure insolution are made, either by increase of its concentration in solution or by additionof another thermodynamically incompatible inert nonsolvent, the varying structural-physical condition of the polymer results in a noticeable change of its thermaldehydrochlorination rate in solution. These effects are caused by structural-physicalchanges in the polymer-solvent system, and the previously unknown phenomena canbe classified as ‘structural-physical stabilisation’ (in the case of a reduction in thegross rate of PVC disintegration in highly basic solvents at B > 50 cm–1) or ‘structural-physical antistabilisation’ (in the case of an increase in the gross rate of PVCdisintegration in low-basicity solvents with B < 50 cm–1), respectively.

5.7 ‘Echo’ Stabilisation of PVC

Finally, it is necessary to describe one more appreciable achievement in the field of ageingand stabilisation of PVC in solution. In real conditions the basic reason for the sharpaccelerated ageing of plasticised materials and products is oxidation of the solvent by theoxygen of the air (Figure 5.9, curve 3).


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RO RH ROOH R

ROOH RO HO

RO RO

2

2 2

2

3

6

• •

• •

• •

+ ⎯ →⎯⎯ +

⎯ →⎯⎯ +

+ ⎯ →⎯⎯

K

K

K inactive products

(5.12)

Peroxides, formed by oxidation of ester-type plasticisers, initiate the disintegration ofmacromolecules. In these conditions the rate of PVC destruction increases by two ormore orders of magnitude and is determined by the oxidation stability parameter of thesolvent to oxygen K K K Kef = −

2 30 5

60 5. . . Thus, a higher oxidation stability of the solvent (in

particular, an ester-type plasticiser) lowers the degradation rate of semi-rigid and flexiblePVC materials and increases its useful lifetime [60-63]. Inhibition of the oxidation processof the solvents (including plasticisers) due to the incorporation of stabilisers, antioxidantsor their synergistic compositions slows down the thermo-oxidative disintegration of PVCin solution (Figure 5.9, curve 5).

Effective inhibition of the oxidation of ester-type plasticisers by oxygen of the air causesthe rate of PVC thermo-oxidative destruction in concentrated solutions to become closer

Figure 5.9 ‘Echo’ stabilisation of PVC. Elimination of HCl during thermal (argon) (1,2)and thermo-oxidative (air) (3-5) destruction of PVC in solution of dioctyl sebacate: (1-4)

unstabilised PVC, (5) PVC stabilised with diphenylpropane (0.02 wt%) – ‘echo’stabilisation; (2,4) PVC with no solvent; 448 K

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to the rate of polymer disintegration. This behaviour is characteristic of the thermaldestruction of PVC in the presence of plasticisers acting as solvent. In other words, itbecomes slower than PVC disintegration in the absence of solvent. This occurs due to astructural-physical stabilisation. In these cases, inhibition of the solvent oxidation reactionby using ‘echo’-type antioxidant stabilisers improves PVC stabilisation (Figure 5.9,curve 5). This fundamental phenomenon of PVC stabilisation in solution and its thermo-oxidative destruction has been called ‘echo stabilisation’ of PVC [49, 62, 63].

5.8 Tasks for the Future

The creation of high-quality and economic semi-rigid and flexible materials and productsmade from PVC, including those where solvents are employed, requires specific approachesthat are essentially different from the principles of manufacture of rigid PVC materialsand products. In particular, consideration and use of the following fundamentalphenomena should be considered: solvational, structural-physical and ‘echo’ stabilisationof the polymer in solution.

As far as paramount tasks of fundamental and applied research in the field of PVCmanufacture and processing at the beginning of the 21st century are concerned, they areobviously the following:

(1) The manufacture of industrial PVC that does not contain labile groups in its backbone.This will provide a drastic increase in the intrinsic stability of polymeric PVC products,the possibility of processing with the minimal content or total absence of stabilisersand other chemical additives, and the opportunity to create PVC-based materialsand products with essentially increased useful service lifetime.

(2) Wide use of the latest achievements in the field of destruction and stabilisation ofPVC, in both the presence and the absence of solvents. The phenomena of chemical,solvational, structural-physical, self- and ‘echo’ stabilisation of PVC will allow thecreation of rigid, semi-rigid and flexible (plasticised) materials and products withminimal content of chemical additives, and will lead to increased useful service lifetimeunder natural and special conditions.

(3) The use of nontoxic and nonflammable products that do not emit toxic and otherpoisonous gaseous and liquid products at elevated temperature during the manufactureand processing of PVC materials and their products.

(4) Complete elimination of all toxic and even low-toxicity (particularly compoundsbased on barium, cadmium and lead, etc.), chemical additives from all formulations.


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(5) The search for nontoxic and highly effective inorganic chemical additives, primarily,stabilisers of zeolite type, modified clays, etc.

At the same time, new ‘surprises’ will undoubtedly be presented to us by this outstandingpolymer puzzle. Certainly, as we look for a plastic for use as a ‘work-horse’ for manydecades, studies on PVC will lead to new stimuli in the development of scientific ideasand practical development, and the opening-up of new pathways. These will result fromthe essential need to delay PVC ageing in natural and special conditions, and to reducethe amounts of the appropriate chemical additives, down to their complete elimination.

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6 Ecological Issues of Polymer Flame Retardants

G.E. Zaikov and S.M. Lomakin

6.1 Introduction

The use of polymer flame retardants has an important role in saving lives. The mainflame retardant systems for polymers currently in use are based on halogenated,phosphorus, nitrogen and inorganic compounds. All of these flame retardant systemsbasically inhibit or even suppress the combustion process by chemical or physical actionin the gas or condensed phase. Conventional flame retardants, such as halogenated,phosphorus or metallic additives, have a number of negative attributes. The ecologicalissue of their application demands the search for new polymer flame retardant systems.Among the new trends in flame retardancy, the following should be pointed out:intumescent systems, polymer nanocomposites, preceramic additives, low-meltingglasses, different types of char-formers and polymer morphology modificationprocessing. Brief explanations of the three major types of flame retardant systems(intumescent systems, polymer nanocomposites and polymer organic char-formers) arethe subject of this overview.

Our environment has a mostly polymeric nature, and all polymers, whether natural orsynthetic, will burn, so the use of polymer flame retardants has an important role insaving lives. There are four main families of flame retardant chemicals:

(1) Inorganic flame retardants including aluminium trioxide, magnesium hydroxide,ammonium polyphosphate and red phosphorus. This group represents about 50%by volume of global flame retardant production [1].

(2) Halogenated flame retardants, primarily based on chlorine and bromine. Thebrominated flame retardants (BFR) are included in this group. This group representsabout 25% by volume of global production [1].

(3) Organophosphorus flame retardants are primarily phosphate esters and representabout 20% by volume of global production [1]. Organophosphorus flame retardantsmay contain bromine or chlorine.

(4) Nitrogen-based organic flame retardants are used for a limited number of polymers.

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6.2 Mechanisms of Action

Depending on their nature, flame retardants can act chemically and/or physically in thesolid, liquid or gas phase. They interfere with combustion during a particular stage ofthis process, e.g., during heating, decomposition, ignition or flame spread.

Substitution of one type of flame retardant by another consequently means a changein the mechanism(s) of flame retardancy. Halogen-containing flame retardants actprimarily by a chemical interfering with the radical chain mechanism that takes placein the gas phase during combustion. High-energy OH and H radicals formed duringcombustion are removed by bromine released from the flame retardant. Althoughbrominated flame retardants are a highly diverse group of compounds, the flameretardancy mechanism is basically the same for all compounds. However, there aredifferences in the flame retardancy performance of brominated compounds, as thepresence of such compounds in the polymer will influence the physical properties ofthe polymer. In general, aliphatic bromine compounds are easier to break down andhence more effective at lower temperatures, but are also less temperature-resistantthan aromatic retardants.

Aluminium hydroxide and other hydroxides act in a combination of various processes.When heated, the hydroxides release water vapour, which cools the substrate to atemperature below that required to sustain the combustion processes. The water vapourliberated also has a diluting effect in the gas phase and forms an oxygen-displacingprotective layer. Additionally, together with the charring products, the oxide forms aninsulating protective layer.

Phosphorus compounds mainly influence the reactions taking place in the solid phase.By thermal decomposition, flame retardants are converted to phosphorous acid, whichin the condensed phase extracts water from the pyrolysing substrate, causing it to char.However, some phosphorus compounds may, similarly to halogens, act in the gas phaseas well by a radical trapping mechanism.

Interest in flame retarding polymers goes back to the 19th century with the discovery ofhighly flammable cellulose nitrate and celluloid. In more recent times a large volume ofconventional plastics such as phenolics, rigid polyvinyl chloride (PVC) and melamineresins possess adequate flame retardancy. By the 1970s the major flame retardant polymerswere the thermosets, namely, unsaturated polyesters and epoxy resins that utilised reactivehalogen compounds and alumina hydrate as an additive. There was also a large marketfor phosphate esters in plasticised PVC, cellulose acetate film, unsaturated polyestersand modified polyphenylene oxide. Alumina trihydrate (ATH) was the largest-volumeflame retardant in unsaturated plastics.

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Ecological Issues of Polymer Flame Retardancy

Consumption of halogen-containing flame retardant additives in the 1970s was muchless than that of other additives. The term ‘halogenated flame retardants’ covers a largenumber of different organic substances, all with chlorine or bromine in their molecularstructure. Bromine and chlorine have an inhibitory effect on the formation of fire inorganic materials. Flame retardants are added to plastics and textiles in order to complywith fire safety requirements. The halogenated flame retardant additives include:

(1) Dechlorane Plus,

(2) a chlorinated acyclic (for polyolefins),

(3) tris(dibromopropyl) phosphate,

(4) brominated aromatics,

(5) pentabromochlorocyclohexane and

(6) hexabromocyclododecane (for polystyrene).

A number of chlorinated flame retardant products were produced under the Dechloranetrade name. The products include:

(1) two moles of hexachlorocyclopentadiene and contained 78% chlorine,

(2) Dechlorane Plus,

(3) a Diels-Alder reaction product of cyclooctadiene and hexachlorocyclopentadiene with65% chlorine,

(4) a Diels-Alder product with furan and

(5) a product containing both bromine and chlorine with 77% halogen developed forpolystyrene and acrylonitrile-butadiene-styrene (ABS) materials [1].

In 1985-1986 a German study detected brominated dioxins and furans from pyrolysis ofa brominated diphenyl oxide in the laboratory at 510-630 °C [2]. The relevance of thesepyrolysis studies to the real hazard presented by these flame retardants under actualconditions of use has been questioned. Germany and Holland have considered a ban orcurtailed the use of brominated diphenyl oxide flame retardants because of the potentialformation of highly toxic and potentially carcinogenic brominated furans and dioxinsduring combustion [1, 2]. The issue has spread to other parts of Europe, where regulationshave been proposed to restrict their use.

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The chemical stability of the substances – particularly in the cases of polybrominatedbiphenyls (PBB) and polybrominated diphenyl ethers (PBDE) – is also the reason whybrominated flame retardants have been the focus of international environmental debatefor many years. PBDE and PBB, which are the most stable of the BFR described, arewidespread in the environment, are bioaccumulated and accumulate in sediments, wherethey are degraded only very slowly.

6.3 Halogenated Diphenyl Ethers – Dioxins

Chlorinated dibenzo-p-dioxins and related compounds (commonly known simply asdioxins) are contaminants present in a variety of environmental media. This class ofcompounds has caused great concern to the general public as well as intense interest inthe scientific community. Laboratory studies suggest the probability that exposure todioxin-like compounds may be associated with other serious health effects, includingcancer. Conventional laboratory studies have provided new insights into the mechanismsinvolved in the impact of dioxins on various cells and tissues and, ultimately, on toxicity[1]. Dioxins have been demonstrated to be potent modulators of cellular growth anddifferentiation, particularly in epithelial tissues. These data, together with the collectivebody of information from animal and human studies, when coupled with assumptionsand inferences regarding extrapolation from experimental animals to humans, and fromhigh doses to low doses, allow a characterisation of dioxin hazards.

Polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF) andpolychlorinated biphenyls (PCB) are chemically classified as halogenated aromatichydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclicaromatic compounds with similar physical and chemical properties, and the two classes arestructurally similar. Certain of the PCB (the so-called coplanar or mono-ortho coplanarcongeners) are also structurally and conformationally similar. The most widely studied ofthese compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, oftencalled simply dioxin, represents the reference compound for this class of compounds. Thestructures of TCDD and several related compounds are shown in Figure 6.1 [3].

These compounds are assigned individual toxicity equivalence factor (TEF) values asdefined by the international convention ‘Interim Procedures for Estimating RisksAssociated with Exposures to Mixtures of Chlorinated Dibenzo-p-Dioxins andDibenzofurans’ (US Environmental Protection Agency, USEPA, March 1989). Results ofin vitro and in vivo laboratory studies have contributed to the assignment of a relativetoxicity value. TEF are estimates of the toxicity of dioxin-like compounds relative to thetoxicity of TCDD, which is assigned a TEF of 1.0. All chlorinated dibenzodioxins (CDD)and chlorinated dibenzofurans (CDF) with chlorines substituted in the 2, 3, 7 and 8

163

positions are assigned TEF values [1]. Additionally, the analogous brominateddibenzodioxins (BDD) and brominated dibenzofurans (BDF) and certain polychlorinatedbiphenyls have recently been identified as having dioxin-like toxicity and thus are alsoincluded in the definition of dioxin-like compounds. Generally accepted TEF values forchlorinated dibenzodioxins and dibenzofurans are shown in Table 6.1 [4].

2,3,7,8-Tetrachlorodibenzo-p-dioxin 1,2,3,7,8-Pentachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzofuran 2,3,4,7,8-Pentachlorodibenzofuran

3,3′,4,4′,5,5′-Hexachlorobiphenyl 3,3′,4,4′,5′-Pentachlorobiphenyl

Figure 6.1 The structures of dioxin and similar compounds


164


A World Health Organization/International Program on Chemical Safety meeting heldin the Netherlands in December 1993 considered the need to derive internationallyacceptable interim TEF for the dioxin-like PCB. Recommendations arising from thatmeeting of experts suggest that in general only a few of the dioxin-like PCB are likely tobe significant contributors to general population exposures to dioxin-like compounds[5]. Dioxin-like PCB may be responsible for approximately one-quarter to one-half ofthe total toxicity equivalence associated with general population environmental exposuresto this class of related compounds.

]4[FDCdnaDDCrof)FET(srotcafecnelaviuqeyticixoT1.6elbaT

*dnuopmoC FET,srotcafecnelaviuqeyticixoT

DDC-irtdna-id,-onoM 0

DDCT-8,7,3,2 1

DDCTrehtO 0

DDCeP-8,7,3,2 5.0

DDCePrehtO 0

DDCxH-8,7,3,2 1.0

DDCxHrehtO 0

DDCpH-8,7,3,2 10.0

DDCPHrehtO 0

FDC-irtdna,-id,-onoM 0

FDCT-8,7,3,2 1.0

FDCTrehtO 0

FDCeP-8,7,3,2,1 50.0

FDCeP-8,7,4,3,2 5.0

FDCePrehtO 0

FDCxH-8,7,3,2 1.0

FDCxHrehtO 0

FDCpH-8,7,3,2 10.0

FDCPHrehtO 0

FDCO 100.0

.narufoznebiddetanirolhc,FDC;nixoidoznebiddetanirolhc,DDC*.Oatco,pHatpeh,xHaxeh,ePatnep,Tartet:sexiferP

165

There are 75 individual compounds comprising the CDD, depending on the positioning ofthe chlorine(s), and 135 different CDF. These are called individual congeners. Likewise, thereare 75 different positional congeners of the BDD and 135 different congeners of the BDF.Only seven of the 75 congeners of the CDD or the BDD are thought to have dioxin-liketoxicity; these are ones with chlorine/bromine substitutions in, at least, the 2, 3, 7 and 8positions. Only 10 of the 135 possible congeners of the CDF or the BDF are thought to havedioxin-like toxicity; these also are ones with substitutions in the 2, 3, 7 and 8 positions.While this suggests 34 individual CDD, CDF, BDD or BDF with dioxin-like toxicity, inclusionof the mixed chloro/bromo congeners substantially increases the number of possible congenerswith dioxin-like activity. There are 209 PCB congeners. Only 13 of these 209 congeners arethought to have dioxin-like toxicity; these are PCB with four or more chlorines with just oneor no substitution in the ortho position. These compounds are sometimes referred to ascoplanar, meaning that they can assume a flat configuration with rings in the same plane.

Similarly configured polybrominated biphenyls are likely to have similar properties;however, the database on these compounds with regard to dioxin-like activity has beenless extensively evaluated. Mixed chlorinated and brominated congeners also exist,increasing the number of compounds considered dioxin-like. The physical/chemicalproperties of each congener vary according to the degree and position of chlorine and/orbromine substitution. Very little is known about the occurrence and toxicity of the mixed(chlorinated and brominated) dioxin, furan and biphenyl congeners.

In general, these compounds have very low water solubility, high octanol-water partitioncoefficients and low vapour pressure, and they tend to bioaccumulate. Although thesecompounds are released from a variety of sources, the congener profiles of CDD andCDF found in sediments have been linked to combustion sources [1].

The Hazards Substance Ordinance in Germany specifies the maximum level of chlorinateddibenzodioxins and furans that can be present in materials marketed in Germany. Thishas been extended to the brominated compounds. The two largest-volume flameretardants, decabromodiphenyl oxide and tetrabromo-bisphenol A, are said to meet theserequirements [2].

The International Program for Chemical Safety (IPCS) of the World Health Organizationhas made several recommendations. Polybrominated diphenyls production (in France)and use should be limited because of the concern over high persistency, bioaccumulationand potential adverse effects at low levels. There are limited toxicity data on deca- andoctabromodiphenyls. Commercial use should cease unless safety is demonstrated. Forthe polybrominated diphenyl oxides, a Task Group felt that polybrominateddibenzofurans, and to a lesser extent the dioxins, may be formed. For decabromodiphenyloxide, appropriate industrial hygiene measures need to be taken, and environmentalexposure minimised by effluent and emission control. Controlled incineration proceduresshould be instituted. For octabromodiphenyl oxide, the hexa- and lower isomers should


166


be minimised. There is considerable concern over persistence in the environment andaccumulation in organisms, especially for pentabromodiphenyl oxide.

There are no regulations proposed or in effect anywhere around the world banning theuse of brominated flame retardants. The proposed EU Directive on the brominateddiphenyl oxides has been withdrawn. Deca- and tetrabromo-bisphenol A as well as otherbrominated flame retardants meet the requirements of the German Ordinance regulatingthe dioxin and furan content of products sold in Germany [6].

The European search for a replacement for decabromodiphenyl oxide in high-impactpolystyrene (HIPS) has led to consideration of other bromo-aromatics, such as Saytex8010 from Albemarle, and a heat-stable chlorinated paraffin from Atochem. The formerproduct is more costly, and the latter, if sufficiently heat-stable, lowers the heat distortionunder load (HDUL) significantly. Neither approach has been fully accepted. In September1994, the USEPA released a final draft of exposure and risk assessment of dioxins anddioxin-like compounds [5]. This reassessment finds the risks greater than previouslythought. Based on this reassessment, a picture emerges that tetrachlorodiphenyl dioxinsand related compounds are potent toxicants in animals, with the potential to produce aspectrum of effects. Some of these effects may occur in humans at very low levels, andsome may result in adverse impacts on human health. The USEPA also concluded thatdioxin should remain classified as a probable human carcinogen [5].

Polymer producers have been seeking non-halogen flame retardants, and the search has beensuccessful in several polymer systems. Non-halogen flame retardant polycarbonate/ABS blendsare now commercial. They contain triphenyl phosfate or resorcinol diphosfate (RDP) as theflame retardant. Modified polyphenylene oxide (GE’s Noryl) has used phosfate esters as theflame retardant for the past 15-20 years, and the industry recently switched from the alkylatedtriphenyl phosphate to RDP. Red phosphorus is used with glass-reinforced Polyamide-6,6(PA-6,6) in Europe, and melamine cyanurate is used in unfilled PA. Magnesium hydroxide isbeing used commercially in polyethylene wire and cable. The non-halogen solutions presentother problems, such as poor properties (plasticisers lower the heat distortion temperature),difficult processing (high loadings of ATH and magnesium hydroxide), corrosion (redphosphorus) and handling problems (red phosphorus).

In this chapter, we have tried to present the basic trends in the flame retardants hierarchy.

6.4 Flame Retardant Systems

The main flame retardant systems for polymers currently in use are based on halogenated,phosphorus, nitrogen and inorganic compounds (Figure 6.2). Basically, all these flameretardant systems inhibit or even suppress the combustion process by chemical or physical

167

action in the gas or condensed phase. To be effective, the flame retardants must be stableat processing temperatures yet decompose near the decomposition temperature of thepolymer in order for the appropriate chemistry to take place as the polymer decomposes.Conventional flame retardants, such as halogenated, phosphorus or metallic additives,have a number of negative attributes. The ecological issue of their application requiresthat new polymer flame retardant systems are sought. Among the new trends in flameretardancy, the use of intumescent systems, polymer nanocomposites, preceramic additives,low-melting glasses, different types of char-formers and polymer morphology modificationshould be noted [1]. However, the close interactions between the different flame retardanttypes should be considered in order to achieve synergistic behaviour. A block scheme ofpolymer flame retardant systems is given in Figure 6.2.

FLAME RETARDANTS (FR)

Mg HYDROXIDE, ALUMINATRIHYDRATE, BORON FR

ANTIMONY OXIDE

Ecologically friendly flame retardant systems

POLYMER ORGANIC CHARH FORMERS

Preceramics, Low-melting Glass

POLYMER MORPHOLOGY MODIFICATION

PHOSPHORUS FRHALOGENATED FR Nitrogen-containing FR

INTUMESCENT SYSTEMS POLYMER NANOCOMPOSITES

Figure 6.2 A block diagram of polymer flame retardant systems


168


Brief discussions of the three major types of flame retardant systems (intumescent systems,polymer nanocomposites and polymer organic char-formers) are presented next.

6.5 Intumescent Additives

Intumescent behaviour, resulting from the combination of charring and foaming of thesurface of burning polymers, is being widely developed for fire retardancy because it ischaracterised by a low environmental impact. Among alternative candidates,considerable attention has been paid to intumescent materials because they providefire protection with the minimum of overall fire hazard [7]. Since the first intumescentcoating material was patented in 1938 [8], the mechanism of intumescent flameretardancy has referred to the formation of a foam that acts as an insulating barrierbetween the fire and the substrate. In particular, such intumescence depends significantlyon the ratio of carbon, nitrogen and phosphorus atoms in the compound [7, 9]. Althoughintumescent coatings are capable of exhibiting good fire protection for the substrate,they have several disadvantages, such as water solubility, brushing problems andrelatively high cost [10].

The fire retardation of plastic materials is generally achieved by incorporating fire retardantadditives into the plastic during processing [11, 12]. Since the processing requires thatadditives can withstand temperatures up to about 200 °C or more, intumescent systemswith insufficient thermal stability cannot be incorporated into various plastics. Thephosphate-pentaerythritol system has been investigated and developed as an intumescentmaterial [7]. For example, a systematic study on a mixture of ammonium polyphosphateand pentaerythritol has shown that intumescence occurs on flaming [13, 14]. Thus, newintumescent materials with appropriate thermal stability have been synthesised for betterfire retardancy [15].

The most important inorganic nitrogen-phosphorus compound used as an intumescentflame retardant is ammonium polyphosphate, which is applied in intumescent coatingsand in rigid polyurethane foams. The most important organic nitrogen compounds usedas flame retardants are melamine and its derivatives, which are added to intumescentvarnishes or paints. Melamine is incorporated into flexible polyurethane cellular plastics,and melamine cyanurate is applied to unreinforced PA. Guanidine sulfamate is used as aflame retardant for PVC wall coverings in Japan. Guanidine phosphate is added as aflame retardant to textile fibres, and mixtures based on melamine phosphate are used asflame retardants for polyolefins or glass-reinforced PA.

All the above-mentioned compounds – ammonium polyphosphate, melamine, guanidineand their salts – are characterised by an apparently acceptable environmental impact.

169

Mechanistic studies in PA-6 with added ammonium polyphosphate (APP), ammoniumpentaborate (NH4B5O8; APB), melamine and its salts have been carried out using combustionand thermal decomposition approaches [16, 17]. It was shown that APP interacts with PA-6 to produce alkylpolyphosphoric ester, which is a precursor of the intumescent char. Onthe surface of a burning polymer, APB forms an inorganic glassy layer that protects thechar from oxidation and hinders the diffusion of combustible gases. Melamine and its saltsinduce scission of the H–C–C(O) bonds in PA-6, which leads to increased crosslinking andcharring of the polymer [17]. APP added at 10-30 wt% to PA-6 is ineffective in the lowmolecular weight (low molar mass) polymer since the limiting oxygen index (LOI) remainsat the level of 23-24 [18] corresponding to non-fire-retarded PA-6. However, APP becomesvery effective at loadings of 40 and 50%, where the LOI increases to 41 and 50, respectively.

A condensed-phase fire retardant mechanism is proposed for APP in PA-6 [18]. In fact,an intumescent layer is formed on the surface of burning PA-6/APP formulations, whichtends to increase the content of APP.

Thermal analysis has shown that APP destabilises PA-6, since thermal decomposition isobserved at a temperature 70 °C lower than that of pure PA-6 [18]. However, theintumescent layer effectively protects the underlying polymer from the heat flux. Therefore,in the conditions of the linear pyrolysis experiments, the formulation PA-6/APP (40%)decomposes more slowly than pure polymer [18]. These experiments prove the fireretardant action of the intumescent char. Mechanistic studies of thermal decompositionin the PA-6/APP system show that APP catalyses the degradation of the polymer andinteracts with it, forming essentially 5-amidopentyl polyphosphate (Scheme 6.1).

On further heating, 5-amidopentyl polyphosphate again liberates polyphosphoric acidand produces the char. The intumescent shielding layer on the surface of the polymer iscomposed of foamed polyphosphoric acid, which is reinforced with the char [18].

Scheme 6.1 Reaction of APP with PA-6


170


The effectiveness of APB in high molecular weight PA-6 (Mn = 35,000) is similar to that ofAPP as measured by oxygen index [19]. In contrast to APP, APB does not give an intumescentlayer. Instead, a brown-black glassy-like compact layer is formed. As thermal analysis hasshown, APB destabilises PA-6 since the latter decomposes at 50 °C lower. It is likely thatfreed boric acid catalyses the thermolysis of the Nylon. In contrast to APP, no other chemicalinteraction of PA-6 and APB was found. In fact, the residue obtained in thermogravimetryin a nitrogen atmosphere for PA-6/APB formulations corresponds to that calculated on thebasis of the individual contributions of PA-6 and APB to the residue [19]. It is likely that amolten glassy layer of boric acid/boric anhydride accumulates on the surface of burningpolymer, which protects the char from oxidation. This layer reinforced by the char createsa barrier against diffusion of the volatile fuel from the polymer to the flame, which decreasesthe combustibility of PA-6 [19].

A systematic mechanistic study of halogen-free fire retardant PA-6, via the combustionperformance and thermal decomposition behaviour of non-reinforced PA-6 with addedmelamine, melamine cyanurate, melamine oxalate, melamine phthalate, melaminepyrophosphate or dimelamine phosphate, has been reported [20]. Melamine, melaminecyanurate, melamine oxalate and melamine phthalate promote melt dripping of PA-6,which increases as the additive concentration increases. These formulations self-extinguishvery quickly in air, and their LOI increase with increasing concentration (Table 6.2) [20].The melt dripping effect is very strong in the case of melamine phthalate, where a smallamount of the additive (3-10%) leads to large increases in LOI (from 34 to 53).

The combustion behaviour of melamine pyrophosphate and dimelamine phosphate isdifferent from that of melamine itself and the other melamine salts (Table 6.2). The formerare ineffective at concentrations below 15% and become effective at a loading of 20-30%because an intumescent char is formed on the surface of burning specimens. The mechanism

enimalemdeddahtiw6-APthgiewralucelomhgihrofsecidninegyxO2.6elbaT]02[)42=IOL,6-APeruprof(stlasstiro

evitiddA)%tw(noitartnecnocevitiddA

3 5 01 51 02 03

enimaleM - 92 13 33 83 93

etahpsohpenimalemiD - 32 42 52 62 03

etahpsohporypenimaleM - 42 52 52 03 23

etalaxoenimaleM - 82 92 - 33 -

etarunaycenimaleM - 53 73 93 04 04

etalahthpenimaleM 43 84 35 - - -

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of the fire retardant action of both melamine pyrophosphate and dimelamine phosphate issimilar to that of APP, since, analogously with ammonia, melamine volatilises, whereas theremaining phosphoric acids produce esters with PA-6, which are precursors of the char[17]. Some part of the freed melamine condenses, probably forming the derivatives melemand melon [21]. Melamine partially evaporates from the composition PA-6/melamine (30%),whereas the other part condenses, giving 8% solid residue at 450 °C. However, similarbehaviour with a more thermostable residue is shown by melamine cyanurate. Melaminepyrophosphate, like dimelamine phosphate [17], gives about 15% of thermostable char.As mentioned before, it is likely that a glassy layer of molten boric acid and boric anhydrideaccumulates on the surface of the burning polymer and protects the char from oxidation.The glass reinforced by the char creates a barrier against diffusion of the volatile fuel fromthe polymer to the flame, which decreases the combustibility of PA-6 [19].

As infrared characterisation of solid residue and high-boiling products has shown [17],carbodiimide functionalities are formed on thermal decomposition of PA-6 with melamineand its salts. An unusual mechanism of chain scission of PA-6 through CH2–C(O) bonds[22] is likely to become operative in the presence of melamines (Scheme 6.2). The resultant

Scheme 6.2 Mechanism of thermal decomposition of PA-6 in the presence ofmelamine [22]


172


isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to N-alkylisocyanurate. Carbodiimide can also trimerise to N-alkylisotriazine. Thesesecondary reactions increase the thermal stability of the solid residue and increase theyield of the char.

In order to understand better the chemical reactions that are responsible for theintumescent behaviour of APP/pentaerythritol (PER) mixtures, as model examples, astudy of the thermal degradation of pentaerythritol diphosphate (PEDP) was undertaken[23]. PEDP is a model compound for structures identified in APP/PER mixtures heatedbelow 250 °C. Five major degradation steps between room temperature and 950 °Chave been identified using thermogravimetric analysis (TGA), and volatile productsare evolved in each step. The formation of the foam reaches a maximum at 325 °C,corresponding to the second step of degradation; foam formation decreases at highertemperatures. There are no differences in the TGA or differential scanning calorimetry(DSC) curves in nitrogen or air up to 500 °C. Above this temperature, thermal oxidationleads to almost complete volatilisation in a single step, which is essentially completedat 750 °C. The elucidation of the chemical reactions that occur upon degradation iseasier if each step is studied separately. The separation of the steps is accomplished byheating to a temperature at which one step goes to completion, and the followingreaction occurs at a negligible rate [23]. The chemical reactions that occur in the firsttwo steps lead to the initial formation of a char-like structure, which will undergosubsequent graphitisation.

The first reaction is the elimination of water, with the condensation of OH groups.This overlaps with the elimination of organics when as little as 28% of the possiblewater has been evolved. This involves essentially complete scission of the phosphateester bonds and results in a mixture of polyphosphates and a carbonaceous char. Threemechanisms have been proposed in the literature for this reaction [24, 25]: a free-radical mechanism, a carbonium ion mechanism, and a cyclic cis-elimination mechanism.The free-radical mechanism has been ruled out because of the lack of effect of free-radical inhibitors on the rate of pyrolysis [25]. The carbonium ion mechanism issupported by acid catalysis and kinetic behaviour, and may compete with the cis-elimination mechanism [24, 25].

The carbonium ion mechanism should occur exclusively if there is no hydrogen atomon the β-carbon atom, as in PEDP, which is necessary for the cyclic transition state ofthe elimination mechanism. The olefin is generated from the thermodynamically moststable carbonium ion. Hydride migration or skeletal rearrangement may take place togive a more stable carbonium ion of high reactivity. After ring opening in the ionicester pyrolysis mechanism, a second ester pyrolysis reaction occurs, which could alsotake place by the cis-elimination mechanism, as shown in Scheme 6.3.

173

The formation of char can occur by either free-radical or acid-catalysed polymerisationreactions from the compounds produced in the pyrolysis. For example, the Diels-Alderreaction followed by ester pyrolysis and sigmatropic (1,5) shifts leads to an aromatisedstructure as shown in Scheme 6.4 [24].

Repetition of these steps can eventually build up the carbonaceous char, which is observed.The reaction pattern shown in Schemes 6.4 and 6.5 should help to provide the structuresobserved by spectroscopy in the foamed char [24]. These reactions probably occur in anirregular sequence and in competition with other processes; the final products are obtainedby some random combination of polymerisation, Diels-Alder condensation, aromatisation,etc. Ester pyrolysis supplies the chemical structures, which build up the charred materialthrough relatively simple reactions [24].

In summary, intumescent behaviour resulting from a combination of charring and foamingof the surface of burning polymers is being widely developed for fire retardancy becauseit is characterised by a low environmental impact. However, the fire retardant effectivenessof intumescent systems is difficult to predict because the relationship between theoccurrence of the intumescence process and the fire protecting properties of the resultingfoamed char is not yet understood.

Scheme 6.3 Ester pyrolysis mechanism [24]


174


Scheme 6.4 Free-radical char formation [24]

175

6.6 Polymer Organic Char-Former

There is a strong correlation between char yield and fire resistance. This follows becausechar is formed at the expense of combustible gases and because the presence of a charinhibits further flame spread by acting as a thermal barrier around the unburned material.Polymeric additives – poly(vinyl alcohol) (PVOH), systems – that promote the formationof char in the PVOH/PA-6,6 system have been studied [26]. These polymeric additives

Scheme 6.5 Acid-catalysed char formation [24]


176


usually produce a highly conjugated system – aromatic structures that char during thermaldegradation and/or transform into crosslinking agents at high temperatures:

Scission of several carbon-carbon bonds leads to the formation of carbonyl end-groups.For example, aldehyde end-groups arise from the following reaction:

( CH CH2)n CH CH2

OHOH

( CH CH2)n CH CH2

OH

H2O+ (6.1)

The identification of a low concentration of benzene among the volatile products ofPVOH has been taken to indicate the onset of a crosslinking reaction proceeding by aDiels-Alder addition mechanism [27]. Clearly, benzenoid structures are ultimately formedin the solid residue, and the IR spectrum of the residue also indicated the development ofaromatic structures:

CH CH2 CH CH2 n CH

OHOH

CH2

OH

CH CH2 CH CH2 n CH

OH O

CH3+ CH

OH

(6.2)

CH2 CH CH CH

OH

CH CH + CH2 CH CH C CH2 CH

O

OH

CH

CH CH

CH

CHCH

CHCH2

OH

CH2 C CH2

O

CH

OH

(a)

(b)

(6.3)

177

Acid-catalysed dehydration promotes the formation of conjugated sequences of doublebonds (a), and Diels-Alder addition of conjugated and isolated double bonds in differentchains may result in intermolecular crosslinking, producing structures that form graphiteor carbonisation (b).

In contrast to PVOH, PA-6,6 subjected to temperatures above 300 °C in an inertatmosphere is completely decomposed. The wide range of degradation products, whichinclude several simple hydrocarbons, cyclopentanone, water, CO, CO2 and NH3, suggestthat the degradation mechanism is highly complex. Further research has led to the generallyaccepted degradation mechanism for aliphatic polyamides [28]:

C (CH2)x C

OO

NH (CH2)y NHn

H2OC (CH2)x C

OO

OH + NH2 (CH2)y NHn

C (CH2)x C

OO

*NH (*CH2)y *NHn

++ +

Hydrocarbons,cyclic ketones, esters,nitriles, carbon char C (*CH2)x CO2 + NH3 + *(CH2)y + *NH

O

+n

(a)

The idea of introducing PVOH into PA-6,6 was based on the possibility of high-temperature acid-catalysed dehydration [29]. This reaction can be provided by the acidproducts of PA-6,6 degradation hydrolysis, which would promote the formation ofintermolecular crosslinking and char. Such a system has been called ‘synergeticcarbonisation’ because the char yield and flame suppression parameters of the polymerblend of PVOH and PA-6,6 show significant improvement in comparison with those ofpure PVOH and PA-6,6 separately [30].

An additional improvement to the flame resistance properties of the PVOH/PA-6,6 systemwas suggested by means of substitution of pure PVOH by PVOH-ox [poly(vinyl alcohol)oxidised with potassium permanganate (KMnO4)] [30]. Earlier it was reported that theoxidation of PVOH in alkaline solutions occurs through the formation of two intermediate

(6.4)


178


complexes. The final step of this process was attributed to the formation of polyvinylketoneas a final product of oxidation of the substrate [31]. The fire retardancy approach wasmade on the basis of the fire behaviour of PVOH-ox samples. Using cone calorimetertests, a dramatic decrease in the rate of heat release and a significant increase in theignition time were shown experimentally for the oxidised PVOH in comparison with theoriginal PVOH (see Table 6.3). One reason for this phenomenon may be the ability ofPVOH oxidised by KMnO4 (polyvinylketone structures) to act as a neutral and/ormonobasic bidentate ligand [32]. Other experimental results (IR and electronic spectra)provide strong evidence of coordination of the ligand (some metal ions Cd2+, Co2+, Cu2+,Hg2+, Ni2+) through the monobasic bidentate mode [33]. Based on the above, the followingstructure can be proposed for the polymeric complexes (where M = metal):

HC

C

O

M

O

C

O

C

CH

C

O

n

Polymer complex scheme 6A

]03[HOVP/6,6-AProfatadretemirolacenoC3.6elbaT

lairetaMxulftaeH

m/Wk( 2)dleiyrahC

)%tw(noitingI)s(emit

RHRkaePm/Wk( 2)

taehlatoTesaeler

m/JM( 2)

HOVP 02 8.8 93 5.552 6.951

53 9.3 25 3.045 3.111

05 4.2 14 9.777 7.511

*xo-HOVP 02 8.03 7211 6.721 9.63

53 7.21 477 0.491 4.301

05 1.9 81 3.503 8.911

OnMK(etanagnamrepmuissatophtiwdesidixo)lohoclalyniv(yloP* 4 .)

179

Cone calorimeter combustion tests for PVOH and PVOH oxidised by KMnO4 (Table 6.3)clearly indicate the substantial improvement of fire resistance characteristics for PVOH-oxin comparison with PVOH. PVOH-ox gives about half the peak rate of heat release (peakRHR, kW/m2), when compared with pure PVOH. Even at 50 kW/m2, the yield of charresidue for PVOH oxidised by KMnO4 was 9.1% [30].

The result of elemental analysis of PVOH-ox indicates the presence of 1.5% of manganeseremaining in this polymeric structure [30]. It has been suggested that the catalytic amountof chelated manganese structure incorporated in the polymer can provide a rapid high-temperature process of carbonisation followed by formation of char [30].

The sample of PVOH-ox displayed even better flame retardant properties due to the catalyticeffect of the manganese-chelate fragments on the formation of char (Table 6.3). However,there is a less satisfactory correlation in the determination of total rate of heat release(Table 6.3) [30]. Although, the cone calorimeter measurements indicated a decrease oftotal heat release for PA-6,6/PVOH and PA-6,6/PVOH-ox in comparison with pure PVOH,the sample of PA-6,6 with PVOH-ox showed a higher value of total heat release than PA-6,6 with PVOH (Table 6.3). This fact has been qualitatively explained by the influence ofa catalytic amount of chelated manganese structure incorporated in the polymer on thesmouldering of the polymer samples.

The flame out time for PA-6,6/PVOH-ox is larger than the flame out times of PA-6,6/PVOHand PA-6,6 alone (Table 6.4). The values of average heat of combustion indicate the exothermalprocess of smouldering provided by chelated manganese structures (Table 6.4). Approximatelyequal amounts of char yield for PA-6,6/PVOH and PA-6,6/PVOH-ox have been found [30].

tuoemalfehtdnanoitsubmocfotaehehtrofatadretemirolacenoC4.6elbaTm/Wk05foxulftaehatasnoitisopmoc6,6-AProfemit 2

noitisopmoC )s(emittuoemalFfotaehegarevA

)gk/JM(noitsubmoc

6,6-AP 215 5.13

)%tw,02/08(HOVP/6,6-AP 924 1.52

)%tw,02/08(xo-HOVP/6,6-AP 747 5.92

The polymer organic char-former (PVOH system) incorporated in PA-6,6 reduced the peakrate of heat release from 1124.6 kW/m2 (for PA-6,6) and 777.9 kW/m2 (for PVOH) to476.7 kW/m2 and increased the char yield from 1.4% (for PA-6,6) to 8.7% due to a ‘synergistic’carbonisation effect. The cone calorimeter was operated at 50 kW/m2 incident flux.


180


Cone calorimeter data of PA-6,6 composition with PVOH oxidised by KMnO4 (manganesechelate complexes) show an improvement in the peak rate of heat release from 476.7kW/m2 (for PA-6,6/PVOH, 80/20 wt%) to 305.3 kW/m2 (for PA-6,6/PVOH-ox, 80/20wt%) [30]. On the other hand, the exothermal process of smouldering for PA-6,6/PVOH-ox compositions has been noted [30]. This reaction is evidently provided by chelatedmanganese structures, which increases the total heat release of PA-6,6/PVOH-ox blendin comparison with PA-6,6/PVOH blend.

6.7 Polymer Nanocomposites

Polymer layered silicate (clay) nanocomposites are materials with unique properties whencompared with conventional filled polymers. Polymer nanocomposites, especially polymer-layered silicates, represent a radical alternative to conventionally filled polymers.

Solventless, melt intercalation of high molecular weight polymers is a new approach tosynthesise polymer-layered silicate nanocomposites. This method is quite general and isbroadly applicable to a range of commodity polymers from nonpolar polystyrene tostrongly polar Nylon. Polymer nanocomposites are thus processable using currenttechnologies and easily scaled to manufacturing quantities. In general, two types ofstructures are possible: (1) intercalated and (2) disordered or delaminated with randomorientation throughout the polymer matrix. Owing to their nanometre size dispersion,the nanocomposites exhibit improved properties compared to the pure polymers orconventional composites. The improved properties include increased modulus, decreasedgas permeability, increased solvent resistance and decreased flammability. For example,the mechanical properties of a PA-6 layered-silicate nanocomposite with a silicate massfraction of only 5% show excellent improvement over those for pure PA-6 [34]. Thenanocomposite exhibits 40% higher tensile strength, 68% greater tensile modulus, 60%higher flexural strength and 126% increased flexural modulus [34].

In the polymer industry there is a need for new, more effective and environmentallyfriendly flame resistant polymers. Recent data on the combustion of polymernanocomposites indicate that they could be employed for this purpose [35].

There are several proposed mechanisms as to how the layered silicate affects the flameretardant properties of polymers [35]. The first is increased char layer that forms whennanocomposites are exposed to flame. This layer is thought to inhibit oxygen transportto the flame front, as well as gaseous-fuel transport from the polymer, and thereforereduces the heat release rate of the burning surface. At higher temperatures, the inorganicadditive has the ability to act as a radical scavenger due to adsorption on to Lewis acidsites. This may interrupt the burning cycle, as radical species are needed to break polymerchains into fuel fragments. The disordered nanocomposites also inhibit the availabilityof oxygen as a combustible ‘fuel’ species by increasing the path length of these species to

181

the flame front. The path length is dramatically increased due to the surface area of thesilicates (approximately 700 m2/g for Na+ montmorillonite). There is also a high possibilityof alumina-silicate solid-phase catalysis of polymer decomposition, which can dramaticallychange the overall scheme of the kinetics of the thermal degradation process.

Combustibility of some polymer nanocomposite materials was studied using a conecalorimeter [36, 37] under irradiation of 35 kW/m2, which is equivalent to that typical ofa small fire [38]. The RHR, which is one of the most important parameters associated withthe flammability and combustion of a material, such as those illustrated in Figure 6.3, canbe evaluated during this test [36, 37].

Figures 6.3-6.5 compare the results obtained for PA-6,6 as such and for intercalated PA-6,6 hybrid produced by using a Carver press to mix PA-6,6 with 5 wt% of Cloisite 15A(montmorillonite modified by ion exchange with dimethyl-ditallow ammonium, atetraalkylammonium salt from Southern Clay Products Inc.), in an inert nitrogenatmosphere at 260 °C for 30 minutes. It can be seen that the RHR displays a lowermaximum peak in the case of the nanocomposite (Figure 6.3), whereas the quantity ofheat released (the area under the RHR curve) is about the same for both products,suggesting that their thermal degradation mechanisms are the same [37]. The release ofheat by the nanocomposite over a longer period, however, points to its slower degradation.Figures 6.4 and 6.5 on mass loss and specific extinction area illustrate the advantages ofnanocomposite over initial PA-6,6 fire behaviour.

Figure 6.3 Rate of heat release versus time for PA-6,6 and PA-6,6 nanocomposite at aheat flux of 35 kW/m2


182


Figure 6.4 Mass loss rate versus time for PA-6,6 and PA-6,6 nanocomposite at a heatflux of 35 kW/m2

Figure 6.5 Specific extinction area (smoke) versus time for PA-6,6 and PA-6,6nanocomposite at a heat flux of 35 kW/m2

183

During the combustion test of the nanocomposite specimen, the carbon layer that formedon its surface from the start grew over time and resisted the heat. The formation of acarbonised layer on the surface of the polymer is a feature of all the nanocompositesstudied so far: the pattern illustrated in Figure 6.6 has been reported for othernanocomposites based on polystyrene, polyethylene and polypropylene [37]. Examinationsof this residue by X-ray diffraction and transmission electron microscopy (TEM) haverevealed an intercalated nanocomposite structure [37]. The TEM image [37] of the carbonresidue obtained by combustion of a PA-6,6 nanocomposite in Figure 6.6 shows theintercalation of silicate layers (dark zones) with ‘carbon’ layers (light zones). It should beemphasised that this intercalated structure was derived from the combustion of adelaminated hybrid. It is clear that the disordered structure collapsed during thecombustion and was replaced by a self-assembled, ordered structure.

Figure 6.6 TEM image of carbon residue obtained by combustion of PA-6,6nanocomposite [37]

(Reproduced with permission from J.W. Gilman, T. Kashiwagi, C.L. Jackson,E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire

Retardancy of Polymers: the Use of Intumescence, Eds., M. Le Bras, G. Camino,S. Bourbigot and R. Delobel, RSC, Cambridge, UK, 1998. Copyright 1998, RSC.)

References

1. S.M. Lomakin and G.E. Zaikov, Ecological Aspects of Flame Retardancy, VSPInternational Science Publishers, Utrecht, The Netherlands, 1999, 170.

2. H. Beck, A. Dross, M. Ende, R. Wolf and P. Trubiroha, Bundesgesundheitsblatt,1991, 34, 564.


184


3. R.M.C. Theelen in Biological Basis for Risk Assessment of Dioxin and RelatedCompounds, Eds., M. Gallo, R. Scheuplein and K. Van der Heijden, BanburyReport No. 35, Cold Spring Harbor Laboratory Press, Plainview, NY, USA, 1991.

4. U.G. Ahlborg, G.C. Becking, L.S. Birnbaum, A. Brouwer, H.J.G.M. Derks, M.Feeley, G. Golor, A. Hanberg, L.C. Larsen, A.K.D. Liam, S.H. Safe, C. Schlatter,F. Waern, M. Younes and E. Yrjanheikki, Chemosphere, 1994, 28, 6, 1049.

5. Office of Health and Environmental Assessment Office of Research andDevelopment, Estimating Exposure to Dioxin-Like Compounds, EPA/600/6-88/005Ca, Cb, Cc, USEPA, Cinncinnati, OH, USA, 1994.

6. J. Green, Journal of Fire Sciences, 1996, 14, 426.

7. C.E. Anderson Jr., J. Dziuk Jr., W.A. Mallow and J. Buckmaster, Journal of FireSciences, 1985, 3, 151.

8. H. Tramm, C. Clar, P. Kuhnel and W. Schuff, inventors; Ruhrchemie AG,assignee, US Patent 2,106,938, 1938.

9. M. Kay, A.F. Price and I. Lavery, Journal of Fire Retardant Chemistry, 1979, 6, 69.

10. D.E. Cagliostro, S.R. Riccitiello, K.J. Clark and A.B. Shimizu, Journal of Fire andFlammability, 1975, 6, 205.

11. R. Delobel, M. Le Bras, N. Ouassou and F. Alistiqsa, Journal of Fire Sciences,1990, 8, 85.

12. G. Camino, L. Costa and L. Trossarelli, Polymer Degradation and Stability,1984, 7, 25.

13. G. Camino, G. Martinasso, L. Costa and R. Gobetto, Polymer Degradation andStability, 1990, 28, 17.

14. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability,1992, 36, 31.

15. H. Heinrich, inventor; Chemie Linz (Deutschland) GmbH, assignee, GermanPatent, DE 4,015,490Al, 1991.

16. S.V. Levchik, G. Camino, L. Costa and G.F. Levchik, Fire and Materials, 1995,19, 1.

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17. S.V. Levchik, G.F. Levchik, A.I. Balabanovich, G. Camino and L. Costa, PolymerDegradation and Stability, 1996, 54, 217.

18. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability,1992, 36, 229.

19. S.V. Levchik, G.F. Levchik, A.F. Selevich and A.I. Leshnikovich, Vesti AkademiiNauk Belarusi, Seryya Khimichnykh, 1995, 3, 34.

20. S.V. Levchik, G.F. Levchik, G. Camino and L. Costa, Journal of Fire Sciences,1995, 13, 43.

21. L. Costa, G. Camino and M.P. Luda di Cortemiglia in Fire and Polymers:Hazards Identification and Prevention, ACS Symposium Series No.425, Ed., G.L.Nelson, American Chemical Society, Washington, DC, USA, 1990, 211.

22. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992,43, 43.

23. G. Camino, G. Martinasso and L. Costa, Polymer Degradation and Stability,1990, 27, 285.

24. G. Camino and S. Lomakin in Fire Retardant Materials, Eds., A.R. Horrocks andD. Price, CRC Press, Boca Raton, FL, 2001, USA.

25. P. Haake and C.E. Diebert, Journal of the American Chemical Society, 1971, 93,6931.

26. Y. Tsuchiya and K. Sumi, Journal of Polymer Science, 1969, A17, 3151.

27. Polyvinyl Alcohol. Properties and Applications, Ed., C.A. Finch, John Wiley,London, UK, 1973, 622.

28. B.G. Achhammer, F.W. Reinhard and G.M. Kline, Journal of Applied Chemistry,1951, 1, 301.

29. S.M. Lomakin and G.E. Zaikov, Khimicheskaia Fizika, 1995, 14, 39.

30. G.E. Zaikov and S.M. Lomakin, Plasticheskie Massy, 1996, 39, 211.

31. R.M. Hassan, Polymer International, 1993, 30, 5.

32. R.M. Hassan, S.A. El-Gaiar and A.M. El-Summan, Polymer International, 1993,32, 39.


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33. R.M. Hassan, M.A. El-Gahami and M.A. Abd-Alla, Journal of MaterialsChemistry, 1992, 2, 613.

34. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi andO. Kamigaito, Journal of Materials Research, 1993, 8, 1185.

35. E.P. Giannelis, Advanced Materials, 1996, 8, 1, 29.

36. J.W. Gilman, T. Kashiwagi, M. Nyden, J.E.T. Brown, C.L. Jackson, S. Lomakin,E.P. Giannelis and E. Manias in Chemistry and Technology of Polymer Additives,Eds., S. Al-Malaika, A. Golovoy and C.A. Wilkie, Blackwell Science, Oxford,UK, 1999, 249-265.

37. J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin,J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of

187

7 Interaction of Polymers with the NitrogenOxides in Polluted Atmospheres

G.B. Pariiskii, I.S. Gaponova and E.Y. Davydov

7.1 Introduction

In this chapter, the mechanisms of the reactions of nitrogen oxides with solid polymersare considered. Active participants in reactions with nitrogen oxides are double bonds,the amide groups of macromolecules, alkyl, alkoxy and peroxy radicals, as well ashydroperoxides. The structure of the reaction front during nitration of rubbers has beenstudied using the electron spin resonance (ESR) imaging technique. The reactions withnitrogen oxides provide a simple way of preparing spin-labelled polymers. The structural-physical effects on the kinetics and mechanism of reactions of nitrogen dioxide havebeen demonstrated by the example of filled polyvinylpyrrolidone (PVP).

Thermal and photochemical oxidation of polymers have been the subject of detailed andprolonged investigations, because these processes are of major importance for thestabilisation of polymeric materials. However, since the 1960s, the influence of aggressivegases in polluted atmospheres on polymer stability has attracted considerable attention[1]. Among such pollutants in the atmosphere, sulfur dioxide, ozone and the nitrogenoxides stand out as the most deleterious. However, the pursuance of this research hasrun into a number of problems. The interaction of pollutants with polymers involves thepenetration of gases into solids and thus results in a complex kinetic description of theprocess. Also, as a rule, these reactions are long term for the concentrations of pollutantsfound in the environment. Consequently, other aging processes occur in the actualconditions of use and storage of polymer materials.

To establish the effect of a given aggressive gas on a particular polymer, the reaction isgenerally studied at pollutant concentrations that are much higher than those actuallyexisting in polluted atmospheres. The results obtained by this means are then linearlyextrapolated to the concentrations of reactants found in the atmosphere. This expedientis, a priori, ambiguous in view of the fact that the role of the individual stages of auniform aging process is changed in conditions of accelerated testing.

The problem of non-equivalent kinetics is inherent to polymer reactions in solids [2]. Inthis case particles existing in different surroundings react with different rate constants.As a result, the most active particles will be removed from the reaction, and the overallrate constant will decrease with time. On the other hand, relaxation processes in polymersrestore the initial distribution of particles and so their reactivity. Thus the kinetics will

188


depend on the relation between the rate of the chemical reaction and the rate of therelaxation processes [3]. This fact also makes it necessary to reconsider critically thevalidity of extending the results of accelerated tests for polymer ageing.

This chapter is devoted to a consideration of the results obtained in studies of theinteractions of nitrogen oxides with polymers. There are eight nitrogen oxides, but onlyNO, NO2 and N2O4 are actually important as pollutants. Nitric oxide (NO) exists as afree radical, but it is reasonably stable in reactions with organic compounds. Theparamagnetic nitrogen dioxide (NO2) is more active compared with NO. This gas isuniversally present in equilibrium with its dimer molecule:

2NO2 N2O4

with Kp = 0.141 atm at 298 K [4]. Nitrogen dioxide absorbs light in the near-UV andvisible spectral range. Excited molecules are generated by light with λ > 400 nm. Thedissociation of NO2 into an oxygen atom and NO by light with λ < 365 nm takes placewith a quantum yield near to unity [5].

7.2 Interaction of Nitrogen Dioxide with Polymers

Detailed investigations of the reactions of NO2 with various polymers have been carriedout by Jellinek and co-workers [1, 6]. The degradation of polymer films has been studiedat different pressures of NO2, in mixtures of NO2 with air, under the combined action oflight (λ > 280 nm), O2 and NO2. Based on the data obtained, Jellinek classified all polymersinto three groups:

(1) vinyl polymers – polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethylmethacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl chloride (PVC) andpolyvinyl fluoride (PVF);

(2) polymers with non-saturation – primarily rubbers;

(3) polyamides, polyurethanes and polyamidoimides.

The presentation of the results in this section will be carried out according to this classification.

7.2.1 Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF

The linear extrapolation of the results of accelerated tests to NO2 concentrations likelyto be found in the atmosphere (1-5 ppm) predicts that polymer properties will be essentiallyconstant for a long time.

189

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres

The first investigations of the interaction of NO2 with PE and PP were performed byOgihara and co-workers [7, 8] at 298-383 K and NO2 pressure of 20 kPa. It wasestablished that NO2 reacts at room temperature with the >C=C< double bondsoriginally contained in PE with the formation of dinitro compounds and nitronitritesby the following reactions:

>C=C< + NO2 → >C•−C(NO2)< (= R1•) (7.1)

R1• + NO2 → >C(NO2)−C(NO2)< (7.2)

R1• + ONO → >C(ONO)−C(NO2)< (7.3)

Hydrogen atom abstraction does not take place at room temperature. The nitro, nitrite,nitrate, carbonyl and hydroxyl groups are formed at T > 373 K. The following mechanismwas postulated:

RH + NO2 → R• + HNO2 (7.4)

R• + NO2 → RNO2 (7.5)

R• + ONO → RONO (7.6)

RONO → RO• + NO (7.7)

The reactions of RO• radicals lead to the formation of macromolecular nitrates, alcoholsand carbonyl compounds. The activation energy of the NO2 addition to the double bondsof PE is 8-16 kJ/mol. The activation energies of H atom abstraction are 56-68 kJ/mol inPE and 60 kJ/mol in PP.

PE, PP, PAN and PMMA change their characteristics slightly at high concentrationsof NO2 (1.3-13 kPa) even under the joint action of pollutant, O2 and UV light [6].Nitrogen dioxide is capable of abstracting tertiary hydrogen atoms in PS with a lowrate (P = 20-80 kPa), with the formation of nitro and nitrite side groups [reactions(7.5) and (7.6)]. This process is accompanied by main-chain scission [9, 10]. Thecombined action of 0.3 kPa NO2 and light (λ > 280 nm) on PS does not lead to main-chain decomposition in the early stage (10 h), after which the degradation process isdeveloped with a constant rate. PVC and PVF show a minor loss of chlorine andfluorine atoms on exposure to NO2 [1, 6].

An attempt to investigate quantitatively the ageing of PS and poly-tert-butyl methacrylate(P-t-BuMA) has been taken by Huber [11]. The research was performed in a flow systemof air containing 60-900 ppm of NO2 and/or 60-900 ppm SO2 at 300 K under thesimultaneous action of light with λ > 290 nm. The degradation of P-t-BuMA films was

190


expressed in terms of the quantity of ruptures per 10,000 monomer units, α. The kineticdependence is represented by the equation:

α = (P/Q)[exp(Qt) – 1] (7.8)

where P and Q are constants. This equation describes an autoaccelerated process.As Q → 0, so α → Pt, that is, the degradation proceeds with a constant rate. The P andQ values decrease as the film thickness increases, and yet the P value diminishes morestrongly than Q. Therefore, the accelerated character of the degradation appears moreclearly for thin films. PS degradation in the same conditions proceeds much more slowlyand has a more pronounced autoacceleration (Table 7.1).

-ProfseulavQdnaPehT1.7elbaT t ehtrednunoitadargedmlifSPdnaAMuB-ONmpp001fonoitca 2 rianithgildna

epytremyloP mc/gm(ssenkcihtmliF 2) P × 01 4 h( 1– ) Q × 01 4 h( 1– )

-P t AMuB- 4.1 170.0 620.0

-P t AMuB- 6.2 050.0 -

-P t AMuB- 8.2 140.0 710.0

SP 4.1 430.0 630.0

The autoaccelerated character of P-t-BuMA degradation was linked to the ester groupdecomposition, with isobutylene formation, which gives free radicals in the reactionwith NO2 and thus promotes the degradation process.

The IR spectrum of PS shows peaks corresponding to carbonyl (1686 cm–1) and hydroxyl(3400 cm–1) groups after exposure to a mixture of NO2 (100 ppm) and air. No bandsconnected with the insertion of NO2 into the P-t-BuMA and PS macromolecules wereobserved. It is believed that the following sequence of reactions occurs in PS [11]:

RH + NO2 → R• + HNO2 (7.9)

R• + O2 → RO2• (7.10)

RO2• + RH → ROOH + R• (7.11)

R• + NO2 → RNO2 (7.12)

R• + NO2 → RONO (7.13)

191


RONO → RO• + NO (7.14)

ROOH + NO → RO• + •OH + NO (7.15)

ROOH + hν → RO• + •OH (7.16)

RO• → degradation + R• (7.17)

Hydroperoxide decomposition under the action of NO and light gives rise to acceleratedPS degradation.

7.2.2 Non-Saturated Polymers

Among these are primarily rubbers. These polymers are far more sensitive to NO2 actionthat the polyolefins. Appreciable degradation of macromolecules as well as moderatecrosslinking were observed for rubbers.

Comprehensive kinetic investigations of butyl rubber (a copolymer of isobutylene with1.75% isoprene) in an NO2 atmosphere (NO2 pressure 1.33-133 kPa), in a mixture ofNO2 and air, and under the combined action of NO2, O2 and UV light (λ > 280 nm) havebeen performed by Jellinek and co-workers [12, 13]. According to the proposedmechanism, the total number of chain ruptures is made up of three parts: (1) rupturesthat are due to the NO2 interaction only, (2) ruptures that result only from the action ofO2, and (3) ruptures that are caused by the combined action of NO2 and O2.

The kinetic dependence of the degree of degradation, α = (1/DPt − 1/DP0), is described bythe following equation:

α = kef′ t2 + kef″ [NO2][1 − exp(−k3t)] (7.18)

where DP0 and DPt are the number-average degrees of polymerisation in the original anddegraded macromolecule (at time t). The first term in equation (7.18) is connected withruptures of macromolecules due to photolysis of the reaction products (hydroperoxides,nitro and nitrite groups). The second term describes the degradation for the (NO2 + O2)system in the absence of light.

It should be noted that the assumed mechanism [12, 13] is very complex, involving awealth of elementary reactions, the rate constants of which are unknown in the solidphase. It is well known that the reaction products can be more active relative to thenitrogen oxides than the original polymer. In connection with this, the application ofvarious physical-chemical techniques is extremely important to investigate the

192


degradation process. The development of methods to study the movement of the reactionfront across the polymer sample is also required. The use of the ESR technique permitsone to draw additional conclusions on the mechanisms of the interaction of polymerswith nitrogen oxides from the structure of the resulting free radicals and the kinetics oftheir formation.

The interaction of polyisoprene (PI) with NO2 gives rise to di-tert-alkylnitroxyl radicals[14]. The ESR spectra of these radicals show a characteristic anisotropic triplet signal witha width of 2A||

N = 6.2 mT and g|| = 2.0028 ± 0.0005 in the solid polymer, and a triplet withaN = 1.53 ± 0.03 mT and g = 2.0057 ± 0.0005 in dilute solutions. These macroradicals arestable in the absence of NO2 during storage for many months in both inert atmosphere andair. The proposed scheme to explain the formation of these radicals involves three mainstages: (1) generation of N-containing alkyl radicals, (2) synthesis of tertiary macromolecularnitroso compounds, and (3) spin-trapping of the tertiary alkyl or allyl radicals:

~CH2-C(CH3)=CH-CH2~ + NO2 ~C•(CH3)-CH(ONO)-CH2~ (7.19)

~C•(CH3)-CH(ONO)-CH2~ + RH ~C(CH3)(NO)-CH(OH)-CH2~ (7.20)

~C(CH3)(NO)−CH(OH)−CH2∼ + •Rtert → Rtert−N(O•)−Rtert (7.21)

The reactions of NO2 with double bonds provide a very simple and rapid method for thesynthesis of spin-labelled macromolecules of rubbers. The temperature variation of therotational mobility of macromolecules in block PI has been studied using spin-labelledsamples [14]. The temperature dependence of the rotational correlation time τ is describedby τc = τ0 exp(E/RT). The τc values within the fast motion region (τc < 10–9 s) are welldescribed by the parameters E = 34.7 kJ/mol and log τ0 = −14.2.

The spatial distribution of these macromolecular nitroxyl radicals allows theestimation of the spatial distribution of the nitration reaction in bulk PI. Thepossibilities of the ESR imaging technique to determine the form of the reactionfront of PI nitration has been considered [15]. The ESR imaging spectra were registeredin an inhomogeneous magnetic field on cylindrical samples of 0.4 cm diameter and 1cm height at NO2 and O2 concentrations of 1 x 10–4 to 2 x10–3 mol/l and 2 x 10–3 to1.4 x 10–2 mol/l, respectively. The spatial distributions of R2NO• radicals at variousreaction times are shown in Figure 7.1. The width of the distribution varies over 20-30% for 740 h. The maximum concentration of nitroxyl radicals is observed in thesuperficial layer, and it progressively decreases towards the centre. The width of thislayer is ~1 mm, and radicals are unavailable in the sample centre. The nitroxyl radicalyield with respect to absorbed NO2 molecules is 0.01. The shape and variation of thedistribution in the presence of O2 are the same as in pure NO2, but the reaction frontis narrower. The rate of R2NO• formation in the presence of O2 is much lower than

193

in pure NO2 at the cost of a decay of alkyl radicals in the reactions with O2: W(NO2)/W(NO2+O2) = 102. The distribution at a fixed distance from the surface is likelydetermined by macrodefects in the sample volume, namely, the availability of cracksand porosity. The front form is determined by the ‘membranous’ regime of the nitrationprocess rather than by structural changes.

PMMA, which in itself is stable on exposure to NO2, enters into reactions after previousirradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation ofdouble bonds as a result of ester group decomposition. The ESR spectrum observed afterexposure of samples to NO2 is shown in Figure 7.2. The spectrum represents thesuperposition of the signals of two nitroxyl radicals at low frequencies of rotationalmobility (10–9 s < τc < 10–7 s):

• Dialkylnitroxyl radicals

~C(CH3)(COOCH3)–N(O•)–C(CH3)(CHO)–CH2~

give an anisotropic triplet signal with hyperfine interaction (HFI) constant A||N = 3.2

± 0.1 mT and g|| = 2.0026 ± 0.0005;

Figure 7.1 ESR-imagination of nitroxyl radicals distribution in cylindrical sample(l = 10 mm, d = 4 mm) of PI in course of interaction with nitrogen dioxide

([NO2]) = 8.8 × 10-4 mole/l; 30 min; 20 °C). The contour lines correspond to verticalsections with equal [R2NO*]. The concentrations are given in arbitrary units

([R2NO*]max = 0.125 au).


194


• Acylalkylnitroxyl radicals

~C(CH3)(COOCH3)−CH(OH)–C(CH3)[N(O•)COOCH3]–CH2~

give a triplet signal that is characterised by A||N = 2.1 ± 0.1 mT and g|| = 2.0027

± 0.0005.

The free-radical process of NO2 interaction with PMMA containing double bonds isrepresented by the scheme opposite.

The formation of nitroxyl radicals testifies to the fact that main-chain decomposition byreaction (7.24) and side-chain ester group cleavage by reaction (7.26) take place in thepolymer. Thus, the interaction of NO2 with double bonds is able to initiate free-radicalreactions of polymer degradation when hydrogen atom abstraction reactions from C–Hbonds are inefficient.

Figure 7.2 ESR spectrum of nitroxyl radicals generated by NO2 in PMMApre-irradiated by UV light at 298 K.

195

(7.22)

(7.23)

(7.24)

(7.25)

(7.26)

(7.27)


196


7.2.3 Polyamides, Polyurethanes, Polyamidoimides

Polymers with amide and urethane groups in the macromolecules represent a specialclass of materials that are sensitive to NO2. The action of NO2 at pressures of 0.5-2mm Hg on polyamide-6,6 films with various morphologies has been studied by Jellinekand co-workers [17, 18]. It was shown that a degradation process takes place. Thedegradation of polyamide is a diffusion-controlled reaction and depends on the degreeof crystallinity and the sizes of the crystallites. The process is inhibited by small quantitiesof benzaldehyde or benzoic acid. Increase of the degradation rate was observed duringthe combined action of NO2, air and UV light. The assumed mechanism of the processas follows:

~CO–NH~ + NO2 → ~CO–N•~ + HNO2 (7.28)

~CO–N•~ + NO2 → ~CO–N(NO2)~ (7.29)

~CO–N•–CH2~ [~CO–N=CH2 + •CH2~] → chain rupture (7.30)

There is reason to believe that only a small quantity of amide groups, not linked by thehydrogen bonds, enter into the reaction. These groups can be interlocked by benzoicacid with the formation of the following structure:

CO NH

HO O

C

Ph

• • • • • •

• • • • • •

Research into the effect of NO2 on polyamide textiles has been described [19]. The exposureof samples in an NO2 atmosphere of low concentration at room temperature for 100 hdoes not lead to a decrease in the whiteness and tensile strength. However, thesecharacteristics are decreased at higher temperatures. The availability of nitrogen oxides inthe air under the action of UV light results in the additional degradation of textiles.

The conversion of N−H bonds by nitrogen dioxide is also inherent to polycaproamide(PCA). The UV spectra of PCA films display features of absorption at 390-435 nm duringexposure to NO2 at concentrations of 10–4 to 10–3 mol/l [20]. The absorption bandswere assigned to nitrosamide groups resulting from N−H group conversion. This

197

conclusion is confirmed by IR spectroscopy. The intensity of the band with ν = 3293 cm–1,which is associated with stretching vibrations of the hydrogen-linked N−H groups,decreases sharply. The intensities of the amide I (ν = 1642 cm–1) and amide II (ν = 1563cm–1) bands, which are characteristic of PCA, also decrease. Instead of these bands,absorptions at ν = 1730 cm–1, which corresponds to the absorption of C=O groups, andat ν = 1504 and 1387 cm–1, which correspond to stretching vibrations of N=O groups ofnitrosamides, appear in PCA. Thus, nitrosation through the amide group is the mainprocess of PCA transformation in an NO2 atmosphere, which leads to disintegration ofthe system of hydrogen bonds. Taking into account the equilibrium:

NO+NO3− N2O4 2NO2 (7.31)

the formation of nitrosamides can be represented as follows:

∼CONHCH2∼ + N2O4 → ∼CON(NO)CH2∼ + HNO3 (7.32)

It was found that the initial rate of nitrosamide group accumulation is proportional to[NO2]n, where n ≈ 2.

As was shown by ESR, the reaction of NO2 with N−H bonds also producesacylalkylnitroxyl macroradicals:

ΝΟ2

∼CONHCH2∼ + NO2 →

∼CON•CH2∼ →

~CON(ONO)CH2~ (7.33)–HNO2

~CON(ONO)CH2~ ~CON(O•)CH2~ + NO (7.34)

As well as in PCA, the interaction of NO2 with PVP leads to UV bands characteristic ofthe nitrosamide group [20]. The formation of these groups in PVP is associated withsplitting of the side-chain cyclic fragments from the main chain:

N O

CHCH2 CH2–HNO2

PVP + NO2 (R1)

(7.35)


198

Handbook of Plastic Films Types of Hot Runner Systems

N O

R1 +CH2CHCH2 (7.36)

Thereafter, the reaction of NO2 with the cyclic double bond gives rise to thenitrosamide product:

N O

+ 2NO2

N OO2N

NO2

R1

N OO2N

NORO

OR +

N OO2N

NO

(7.37)

The ESR spectra observed when NO2 (10–4 to 10–3 mol/l) reacts with PVP represent thesuperposition of the signals of acylalkylnitroxyl radicals (A||

N = 1.94 mT, g|| = 2.003) andiminoxyl radicals (A||

N = 4.33 mT, A⊥N = 2.44 mT, g|| = 2.0029, g⊥ = 2.0053). The formation

of these iminoxyl radicals is initiated by the hydrogen atom abstraction reaction fromC−H bonds that are in the α-position with respect to the amide group by reaction (7.35)and the following reaction:

N O

CHCH2 CH2–HNO2

PVP + NO2 (R2)

(7.38)

Nitric acid is thought to be the source of nitrogen oxide in the given system:

2HNO2 → H2O + NO2 + NO (7.39)

199

The recombination of NO and R1• initiates the formation of iminoxyl radicals:

N O

CHCH2 CH2

–HNO2

R1 + NO

N O

CHCH2 CH2+NO2

N O

CHCH2 CH2

ON

(7.40)

The formation of NO explains the production of acylalkylnitroxyl radicals as follows:

N O

CCH2 CH2R2

N

CCH2 CH2

O

NO

N

CCH2 CH2

O

N O

R

+ R1 (R2)

(7.41)

An approach based on the analysis of the composition of nitrogen-containing radicals inPVP depending on the content of filler aerosil has been put forward to elucidate the


200


effect of polymer structural-physical organisation [21]. The influence of structuralorganisation may be manifest in the rates of iminoxyl and acylalkylnitroxyl radicalformation. Filling gives the possibility of changing the physical structure of the polymerin interface layers. The decrease in the molecular packing density as a result of filling canaccelerate the rate of reaction (7.41) involving breakage of the pyrrolidone cycle. Thepacking density decrease enhances the reaction rate through the promotion of mutualdiffusion of R• macroradicals and nitroso compounds. It is well established that thequantitative relation between iminoxyl and acylalkylnitroxyl radicals is changed withthe degree of filling.

Formation of a gel fraction has been detected on exposure of polyurethane films to NO2

[21]. Degradation of macromolecules simultaneously takes place in the sol fraction ofthe samples. The changes in the destruction degree and the gel-fraction yield with timeare complex to analyse. The gel fraction at 333 K and P(NO2) = 20 mm Hg initiallyincreases up to 20% and thereafter reduces to nearly zero. The number of scissions inthe sol fraction increases at the beginning, subsequently reduces, and then grows again.The exposure of films to NO2 is accompanied by the release of CO2 at all temperatures.The IR spectra in this case show N−H bond (3300 cm–1) consumption. The proposedmechanism includes the reaction of NO2 with the N−H groups of both the main chainand the side branches:

~OCO–NH–CH2~ + NO2 → ~OCO–N•–CH2~ + HNO2 (7.42)

~OCO–N(RH)–CH2~ + NO2 → ~OCO–N(R•)–CH2~ + HNO2 (7.43)

The recombination of ~OCO–N•–CH2~ (R1•) and ~OCO–N(R•)–CH2~ (R2

•) results inpolymer crosslinking. The conversion of R1

• causes macromolecule decomposition andCO2 release. The exposure of polyurethane films to an NO2 atmosphere or a mixture ofNO2 with air leads to the progressive reduction of the tensile strength limit [22].

The influence of NO2 on the mechanical properties of polyamidoimide films has beenconsidered at 323 K and P(NO2) = 13 kPa [23]. The temperature dependences of thestorage modulus E′ and loss modulus E″ have been obtained for various times of NO2

exposure. A nonmonotonic decrease of E′ was observed at 473 K, but the maximum ofthe E′ temperature dependence appears at approximately the same temperature. Samplesexposed to NO2 for eight days show an increase in E′ at the glass transition temperature(563 K). The phenomenon is associated with chain breakage and the recombination ofmacroradicals giving rise to crosslinking. Chain breakage is supported by results obtainedby the present authors. The ESR spectra of polyamidoimide exposed to an NO2

atmosphere show the formation of iminoxyl radicals with spectral parameters that areclose to those of PVP iminoxyl radicals. The possible mechanism of their formationincludes the main-chain decomposition step as follows:

201

NO2N COO N COO

O

ON

N COOO

NO

RHN COO

HO

NO

N COOHO

NOH

NO2

–HNO2

N COOHO

NO(7.45)

O NH CO CO

CO

N–HNO2

NO2

O N CO CO

CO

N

N CO CO

CO

N

R + O

(7.44)


202


7.3 Reaction of Nitric Oxide with Polymers

Nitric oxide is a low-activity free radical and can be used as a ‘counter’ of radicals in gasand liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitrosocompounds, which are spin traps. Thus, the initiation of free-radical reactions in solidpolymers in the presence of nitric oxide provides further information on their mechanism.It is well established that at room temperature NO is not able to remove allylic and tertiaryhydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinionson the capability of NO to react with low molecular weight (low molar mass) dienes andpolyenes. Some authors believe that NO is able to add to dienes and polyenes, for example,to substituted o-quinonedimethane, phorone and β-carotene, with the formation of freeradicals [27-29]. Another way of looking at these reactions lies in the fact that they can beinitiated by NO2 impurities [25, 26].

This section of the review is concerned with radical reactions in polymers, induced byphoto- and γ-irradiation, in the presence of nitric oxide. Irradiation of powdered PMMAin an NO atmosphere by the light of a mercury lamp results in the formation of three types-of macromolecular nitroxyl radicals [30]. The radical composition depends on temperatureand the wavelength of the light. If the photolysis of PMMA is performed at room temperatureusing unfiltered light from a high-pressure mercury lamp, acylalkylnitroxyl radicalsR1N(O•)C(=O)R2 are formed. The irradiation of samples at 383 K produces, in addition toacylalkylnitroxyl radicals, dialkylnitroxyl macroradicals R1N(O•)R2. Finally, if PMMAirradiation is carried out at room temperature using UV light with 260 nm < λ < 400 nm,the signal of iminoxyl radicals R1C(=NO•)R2 is also observed in the ESR spectrum.

Acetyl cellulose (AC) under action of light at room temperature gives rise to dialkyl- andacylalkylnitroxyl radicals [30]. The removal of NO from the samples leads to increasing ofcomponents of acylalkylnitroxyl radicals in the ESR spectrum. This phenomenon is probablyconnected with the formation of diamagnetic complexes of NO with acylalkylnitroxylradicals. Dialkylnitroxyl radicals do not form complexes of this type at 298 K.

The γ-irradiation of PMMA at room temperature, as a photolysis, brings about the formationof acylalkylnitroxyl radicals [30]. Iminoxyl radicals also arise, but their quantity is essentiallysmaller than in AC under γ-irradiation.

The formation of nitroxyl radicals during photolysis as well as in the course of radiolysisof PMMA and AC in the presence of NO is explained by the following scheme:

γ, hνpolymer → R1

• (R•2) (7.46)

R1• + NO → R1NO (7.47)

R2• + R1NO → R1N(•O)R2 (7.48)

203

The structure of nitroxyl radicals is determined by the nature of the free radicals that aregenerated by γ- and photo-irradiation of PMMA and AC. Photo-irradiation of PMMAand AC leads to the formation of •C(O)OCH3 radicals, which give in turn acylalkylnitroxylradicals by reactions (7.46)-(7.48). Dialkylnitroxyl radicals arise when two macroradicalsare involved in the reactions with NO.

The free-radical reactions in solid polymers in the presence of NO are of particularsignificance for the preparation of spin-labelled polymers. This method has becomeparticularly important for chemically inert, rigid and insoluble polymers, for instance,polytetrafluoroethylene (PTFE), because of the difficult problem of introducing spin labelsby chemical reactions of nitroso compounds, nitrons or nitroxyl biradicals [31]. OrientedPTFE films γ-irradiated at room temperature in air after prolonged NO exposure containnitroxyl radicals whose ESR spectra are displayed in Figure 7.3 [32].


Figure 7.3 ESR spectra of perfluoronitroxyl radicals in PTFE films stretched tofourfold increase in its length at parallel (a) and perpendicular (b) orientation of

magnetic field directions.

204


The rotation of the samples leads to changes in angle α between the magnetic field andstretching directions. At 298 K and α = 0°, the ESR spectrum is a triplet consisting ofquintets with splitting of AN = 0.46 mT and AF = 1.11 mT, and g|| = 2.0060. At α = 90°,the splittings increase to AN = 1.12 mT and AF = 1.61 mT, and g⊥ = 2.0071. Theradicals observed are nitroxyl radicals with the following structure: ~CF2–N(O•)–CF2~.A possible mechanism for nitroxyl macroradical synthesis has been suggested [32]. Inan oxygen-containing atmosphere, some of the middle alkyl radicals formed in thecourse of γ-irradiation are capable of decomposing with rupture of the main chain as aresult of the high energy transfer to these radicals:

~CF2–CF2–C•F–CF2~ → ~CF2• + CF2=CF–CF2~ (7.49)

In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to formterminal peroxy radicals:

~C•F2 + O2 → ~CF2OO• (7.50)

Under the action of NO on samples containing neighbouring terminal double bonds andperoxy radicals, the latter are converted into macromolecular nitrates and nitrites:

~CF2 CF2OO• + NO ~CF2OONO …

~CF2O• + NO2

~CF2ONO2

~CF2O• + NO2

~CF2O• + NO ~CF2ONO (7.53)

Decomposition of alkoxy radicals in an NO atmosphere causes the synthesis of terminalnitroso compounds:

NO~CF2–CF20• → ~C•F2 + CF2O → ~CF2NO (7.54)

The adjacent terminal double bonds and terminal nitroso compounds formed can enterinto a reaction to synthesise nitroxyl radicals:

~CF2N=O + CF2=CF–CF2~ + NO → ~CF2–N(O•)CF2–CF(NO)–CF2~ (7.55)

(7.51)

(7.52)

205

The advantage of the suggested method for the preparation of spin-labelled polymer isthat the nitroxyl free-radical fragment is incorporated in the basic macromolecular chainwithout disturbing its orientation.

An analogous investigation of the action of NO on γ-irradiated tetrafluoroethylene-hexafluoropropylene copolymer (TFE-HFP) containing 13 mol% of HFP units has beenperformed [33]. After exposure of powders and films of TFE-HFP to a dose of 105 Gy inair, there are three types of stable peroxide macroradicals:

(1) End radicals ~CF2–CF2O2• (denoted ReO2

•);

(2) Secondary mid-chain radicals ~CF2–CF(OO•)–CF2~ (denoted RcO2•);

(3) Tertiary mid-chain radicals ~CF2–C(CF3)(OO•)–CF2~ (denoted RtO2•).

Their total concentration is [RO2•] ≈ 3 x 10–3 mol/kg, of which (25 ± 5)% are tertiary

peroxy radicals. Under the action of NO on evacuated samples, the radicals decay toform peroxy radical conversion products and tertiary nitroso compounds:

~CF2–C(CF3)(NO)–CF2~

Heating these samples in vacuum up to 473 K leads to the formation of nitroxyl radicalsof the type:

~CF2–N(O•)–CF2~

The nitroxyl radicals appear in the temperature range where the tertiary nitrosocompounds decay in vacuum with the generation of tertiary alkyl radicals (Rt

•). The firststep of Rt

• formation is β-scission by the reaction:

~CF2−C•(CF3)−CF2−CF2~ → ~CF2−C(CF3)=CF2 + •CF2−CF2~ (7.56)

In the presence of NO formed upon decomposition of the tertiary nitroso compounds,the terminal alkyl radicals can be converted into terminal nitroso compounds, whichreact with the adjacent double bonds to form nitroxyl macroradicals:

NO + •CF2−CF2~ → ON−CF2−CF2~ (7.57)

~CF2–C(CF3)=CF2 + ON−CF2−CF2~ → ~CF2–C•(CF3)–CF2–N(O•)–CF2~ →+X→ ~CF2–C(CF3)(X)–CF2–N(O•)–CF2~ (7.58)

where X is NO or NO2. Nitrogen dioxide can be formed by the interaction of NO withRO2

• in reaction (7.51).


206


One more type of nitroxyl macroradical is observed if a powdered TFE-HFP, γ-irradiatedin air and exposed to NO with subsequent evacuation, is subjected to light irradiation atλ > 260 nm at 298 K [34]. In this case, a new type of nitroxyl macroradical with thestructure ∼CF2–N(O•)–CF3 was registered. The following scheme provides an explanationfor the radical formation in TFE-HFP under the action of light:

RcO2• + NO → [RcOONO] → RcONO2 (7.59)

ReO2• + NO → [ReOONO] → ReONO2 (7.60)

Rt• + NO → RtNO (7.61)

RcONO2 + hν → RcO• + NO2 (7.62)

ReONO2 + hν → ReO• + NO2 (7.63)

RtNO + hν → Rt• + NO (7.64)

RcO• → ~CF2–CFO + •CF2–CF2~ (7.65)

ReO• → ~CF2–CF2• + CF2O (7.66)

~CF2–CF2• + NO → ~CF2–CF2–NO (7.67)

~CF2−C•(CF3)−CF2~ + NO2 → ~CF2–C(CF3)(ONO)–CF2~ (7.68)

~CF2–C(CF3)(ONO)–CF2~ + hν → ~CF2–C(CF3)(O•)–CF2~ + NO (7.69)

~CF2–C(CF3)(O•)–CF2~ → •CF3 + ~CF2–C(=O)–CF2~ (7.70)

•CF3 + NO → CF3NO (7.71)

•CF3 + ON–CF2–CF2~ → CF3–N(O•)–CF2–CF2~ (7.72)

CF3NO + •CF2–CF2~ → CF3–N(O•)–CF2–CF2~ (7.73)

It is obvious that the simultaneous action of light and NO on TFE-HFP results inmacromolecular decomposition.

Polymer hydroperoxides are active participants in degradation processes. The reactionsof nitrogen oxides with these particles are of interest to understand the mechanism of theinfluence of pollutants on polymer stability in the course of the oxidation process. Thephenomenon of hydroperoxide decomposition under the action of NO was discussed

207

long ago using both macromolecular peroxides and their low molecular weight analogues[34]. Some authors assumed that the primary stage of peroxide decomposition can berepresented by the reaction [35]:

ROOH + NO → RO• + HONO (7.74)

Another mechanism [36] suggests that the reaction proceeds with the formation ofperoxide radicals:

ROOH + NO → ROO• + HNO (7.75)

The kinetics of hydroperoxide decomposition in PP at 298 K and various partial pressuresof NO has been studied in detail [34]. The decomposition kinetics are shown in Figure 7.4.

As can be seen, the hydroperoxide consumption rate is initially low and then sharplyincreases. The observed character of the kinetic curves cannot be explained by reactions(7.74) or (7.75). According to the ESR data, the decomposition of PP hydroperoxide in an


Figure 7.4 Kinetics of PP hydroperoxide decomposition in NO at variousconcentrations (1-3) and NO + NO2 mixture (4): (1) 1.61 × 10-3, (2) 3.22 × 10-3,

(3) 4.13 × 10-3, (4) 3.1 × 10-3 NO and 3.0 × 10-6 NO2 mol/l.

208


NO atmosphere gives dialkylnitroxyl radicals. It was shown that the induction periods forthe hydroperoxide decomposition and nitroxyl radical accumulation are very sensitive tothe presence of trace amounts of higher nitrogen oxides. This leads to the conclusion thatthe interaction of hydroperoxide with NO is more likely to proceed according to the scheme:

ROOH + N2O3 → [ROONO] + HNO2 (7.76)

ROONO RO• + NO2

RONO2

RO• + NO2

Alkoxy radicals may decompose or enter into substitution reactions with macromoleculesto form chain Rc

• and end Re• alkyl macroradicals, and low molecular weight alkyl

radicals r•, which with NO give nitroso compounds:

RO• → Rc• (Re

•, r•) (7.79)

Rc• (Re

•, r•) + NO → RcNO (ReNO, rNO) (7.80)

The increase in the rate of hydroperoxide decomposition with time can be related toreactions proceeding with participation of such nitroso compounds:

r′OOH + r″NO → r′O• + r′–N(O•)–OH (7.81)

r″–N(O•)–OH → r″• + HNO2 (7.82)

The alkyl radicals formed in the system may stimulate hydroperoxide decomposition [37]:

r• (Rc•, Re

•) + ROOH → rH (RcH, ReH) + RO2• (7.83)

RO2• + NO

RONO2

RO• + NO2

ROONO

Another process that can increase the hydroperoxide decomposition rate is the disproportionationof NO to N2 and NO3

• with the participation of nitroso compounds [24]:

(7.77)

(7.78)

(7.84)

(7.85)

209

RONO2 + N2

RNO• + 2NO R–N=N–ONO2 R• + N2 + •ONO2

R• + N2 + NO3

NO + NO3• → 2NO2

Reactions (7.83)-(7.88) may lead to an increasing NO2 concentration in the system and,consequently, result in the acceleration of reaction (7.76).

7.4 Conclusion

Nitrogen oxides are capable of influencing the free-radical stages of polymeric materialaging in polluted atmospheres. Nitric oxide is a comparatively low-activity free radical,and it cannot abstract even labile hydrogen atoms at ordinary temperatures to initiatethe radical degradation process. On the other hand, NO effectively recombines withfree radicals. This reaction is apparently controlled in solid polymers by the gas diffusionrate, and NO is capable of terminating the oxidation chain by reaction with peroxyand alkyl macroradicals. The reaction of NO with alkyl radicals gives nitrosocompounds, which are spin traps accepting free radicals. This process can slow downpolymer degradation in the presence of nitrogen oxides in subsequent conversions,which can break down into alkoxy radicals, effecting the degradation ofmacromolecules. In addition, nitric oxide initiates the decomposition of hydroperoxidesresulting from oxidation of polymers.

Nitrogen dioxide is a more active free radical as compared with NO, and is able tobreak off the labile hydrogen atoms at room temperature as well as to add to the C=Cbonds of macromolecules, inducing free-radical degradation of polymers. At the sametime, the NO2 radical can inhibit the free-radical reactions giving nitrogen-containingmolecules by the reactions with alkyl, alkoxy and peroxy radicals. The thermal andphotochemical conversions of these products also affect the aging process of polymericmaterials. Nitrogen dioxide is an initiator of the free-radical degradation of polyolefinsat elevated temperatures.

The low stability of polyamides to the action of NO2 is quite surprising, because theN–H bond of the amide group is rather strong. Therefore, the mechanism of polyamidedegradation connected with hydrogen atom abstraction by NO2 from N–H bonds isnot fully elucidated.

(7.86)

(7.87)


210


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19. H. Herzlinger, B. Kuster and H. Essig, Textile Praxis International, 1989, 44, 6,574, 655, 661.

20. I.S. Gaponova, E.Y. Davydov, G.G. Makarov, G.B. Pariiskii and V.P. Pustoshnyi,Polymer Science, Series A, 1998, 40, 4, 309.

21. H.H.G. Jellinek and T.J.Y. Wang, Journal of Polymer Science, Polymer ChemistryEdition, 1973, 11, 12, 3227.

22. H.H.G. Jellinek, F. Martin and J. Wegener, Journal of Applied Polymer Science,1974, 18, 6, 1773.

23. H. Kambe and R. Yokota, Proceedings of the 2nd International Symposiumon Degradation and Stabilisation of Polymers, Dubrovnik, Yugoslavia, 1978,Paper No.39.

24. J.F. Brown, Jr., Journal of the American Chemical Society, 1957, 79, 10, 2480.

25. A. Rockenbauer and L. Korecz, Chemical Communications, 1994, 145.

26. J.S.B. Park and J.C. Walton, Perkin Transactions 2, 1997, 12, 2579.

27. H.-G. Korth, R. Sustmann, P. Lommes, T. Paul, A. Ernst, H. de Groot, L. Hughesand K.U. Ingold, Journal of the American Chemical Society, 1994, 116, 7, 2767.

28. I. Gabr and M.C.R. Symons, Faraday Transactions, 1996, 92, 10, 1767.

29. I. Gabr, R.P. Patel, M.C.R. Symons and M.T. Wilson, Chemical Communications,1995, 9, 915.

30. I.S. Gaponova, G.B. Pariiskii and D.Ya. Toptygin, VysokomolekulyarnyeSoedineniya, Seriya A, 1988, 30, 2, 262.

31. A.M. Wasserman and A.L. Kovarskii, Spinovye Metki i Zondy v FizikokhimiiPolimerov (Spin Labels and Probes in Physical Chemistry of Polymers), Nauka,Moscow, Russia, 1986.


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32. I.S. Gaponova and G.B. Pariiskii, Chemical Physics Reports, 1997, 16, 10, 1795

33. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series B, 1998, 40, 11-12, 394.

34. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series A, 1995, 37, 11, 1133.

35. J.R. Shelton and R.F. Kopczewski, Journal of Organic Chemistry, 1967, 32, 9, 2908.

36. D.J. Carlsson, R. Brousseau, C. Zhang and D.M. Wiles, Polymer Degradationand Stability, 1987, 17, 4, 303.

37. K. Ingold and B. Roberts, Free-Radical Substitution Reactions, John Wiley, NewYork, NY, USA, 1972.

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8 Modifications of Plastic Films

E.M. Abdel-Bary

8.1 Introduction

Modifications of plastic films are generally used to improve mechanical or physicalproperties so that the films are suitable for certain applications. This can be achieved bysubjecting the films to mechanical or chemical treatments. Thus, surface treatments modifythe crystalline morphology and surface topography, increase the surface energy and removecontaminants. Removal of contaminants is necessary for good adhesion of the surface toother substrates. Other applications, such as printing, decorating, wetting and lamination,are improved by incorporation of a surfactant to change the surface tension of theadherents. Also, the presence of polar nitrogen-containing monomers on a polymer filmsurface allows one to obtain ionomers for versatile applications. Thus, such films can beused as anion-exchange membranes in electrodialysis processes, in water desalination[1], as a carrier for immobilisation of medical products [2], as a separator in alkalinebatteries [3] and in fuel cells, etc.

A number of surface modification techniques, such as plasma, corona discharge andchemical treatments, have been used to modify polymer surfaces, and the chemical methodsare of particular interest. In this case, adsorption on and adhesion to polymer surfaceshave been modified using many different methods, e.g., oxidation and other chemicalreactions, high-energy irradiation and plasma treatment. In the following sections, weshall discuss some of the parameters affecting the mechanical and/or physical orphysicochemical characteristics of such films.

8.2 Modification of Mechanical Properties

Improved mechanical properties of plastic films can be realised by changing the followingparameters: orientation, crystallisation and crosslinking. Regarding the orientationprocess, the properties of some polymer films can be improved by stretching a film aboveits glass transition temperature (Tg). Orientation may be in one direction only (uniaxialorientation) or in two directions, i.e., in both machine direction and transverse direction(biaxial orientation). Biaxially oriented film can be further categorised as balanced film,where orientation is roughly equal in both directions, or as unbalanced. This orientationof the molecules in thermoplastics is essentially a stretching process, which tends to align

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the molecules in the direction of the stretching force. Once the molecules have beenaligned, the ordered arrangement is frozen in, giving rise to a strained condition.

8.2.1 Orientation

Orientation of plastic films improves some of the physical properties such as tensilestrength, impact strength, clarity and stiffness. In many instances there is also improvementin gas and water vapour barrier properties. Particularly in the case of polypropylene(PP), the barrier properties are also improved.

Films that benefit appreciably from orientation include PP, polyethylene terephthalate(PET) and polyamide (Nylon). Polystyrene (PS) film is brittle material and becomes toughwhen biaxially oriented. Another aspect of film orientation is that of shrink-wrapping,where films such as low-density polyethylene (LDPE) and polyvinyl chloride (PVC) arestretched at a temperature above their softening points and then cooled to ‘freeze in’ theconsequent orientation of the molecules. When these films are reheated, the moleculestend to return to their unstretched dimensions. In contrast, heat-setting ‘annealing’ isused to prevent shrinkage when heating stretched films. If oriented polypropylene, forexample, is heated to about 100 °C immediately after being drawn, it shrinks considerablyunless it is restrained in some way. This can be prevented by heat-setting. The film isheated, under controlled conditions, and while held under restraint after cooling, thefilm will not shrink if heated to below the annealing temperature; the film is said to beheat-set. The physical and optical properties of the film remain unchanged.

8.2.2 Crystallisation

Crystallisation of polymers occurs as a result of the close approach of molecular chainsin ordered, crystalline areas, which leads to the formation of much stronger intermolecularforces than in the amorphous areas. The rate of cooling has a significant effect on thedegree of crystallinity and size of crystallites. Thus, rapid quenching of Nylon filmsduring the casting process produces an amorphous film, whereas slow cooling allows theformation of crystals. The properties of the final film are highly dependent on thecrystalline state of the polymer. Rapid quenching and consequent inhibition of crystalgrowth give a transparent film, which is more easily thermoformed.

8.2.3 Crosslinking

Crosslinking is used to improve the mechanical properties and to obtain an infusiblefilm. Crosslinked polyethylene film can be achieved by subjecting the film to high-energy

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Modifications of Plastic Films

radiation. When polyethylene film is irradiated, hydrogen and smaller amounts ofmethane, ethane and propane gases are liberated, and the polymer becomes increasinglyinsoluble as a result of crosslinking of the molecules via C–C bonds. This process slightlyimproves the gas and water vapour barrier properties, and the film has good clarity. Thetear strength of the film becomes good, and the resistance to tear initiation and to tearpropagation becomes high.

8.3 Chemical Modifications

Chemical modification of the surface of polymers is an attractive method of improvingthe barrier characteristics of polymers that are otherwise considered ideal materials forpackaging. With the exception of low gas barrier properties, polyolefins are extremelyattractive because of their low cost, toughness, processability and excellent water barrierproperties. Surface treatment is ideal for such polymers, because they are easily processedand made into better barriers by surface modification, either during processing orafterwards [4, 5].

Chemical reactions of the surface with gases are used to modify the surfaces of existingpolymers without changing bulk properties. This modification can be achieved by reactingthe polymer surface with gases. Modifications of the surface using fluorine, hydrogenfluoride, sulfur tetrafluoride, chlorine and bromine have been examined.

8.3.1 Fluorination

The manufacture of fluoromonomers and their subsequent polymerisations are hazardousand difficult. The fabrication of fluoropolymers is also difficult and expensive. Forexample, the processing of polytetrafluoroethylene (PTFE) involves costly compactionand a high-temperature (375 °C) sintering process [6]. Hence, the widespread use offluoropolymers is hindered by these considerations. Fluorination of polymers has beenshown to be a successful new route to fluoropolymers. Polymers are fluorinated eitherdirectly or indirectly [7]. In direct fluorination, highly active fluorinating agents such asfluorine, hydrogen fluoride, or sulfur tetrafluoride convert the polymeric materialcompletely to a fluorocarbon polymer.

8.3.1.1 Direct Fluorination

Fluorine is a highly active fluorinating agent because of its low dissociation energy. Itforms extremely stable bonds with carbon [8]. Fluorination of polymers by fluorine may

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be divided into two types: bulk fluorination and surface fluorination [9]. The surfacefluorination of polyethylene (PE) film by 10% F2 (diluted with N2) results in a depth offluorination ranging between 0 and 50 Å [10]. The extent and depth of fluorinationduring surface fluorination of polycarbonate (PC), PS and polymethyl methacrylate(PMMA) films with F2 diluted with He or N2 increase with reaction time, temperatureand F2 gas pressure [11]. The extents of fluorination for PS, PC and PMMA are as highas 64.3%, 55.3% and 20% respectively. The relation between depth of fluorination andreaction time is represented by:

d = Kt1/2 (8.1)

where d is the depth of fluorination and t is the reaction time. The proportionality constantK depends on the nature of the polymer; for example, the values for K are 13.2 and 5.6for PS and PC, respectively. Fluorination may be conducted using hydrogen fluoride [7]and sulfur tetrafluoride [12, 13].

8.3.1.2 Indirect Fluorination

In an effort to overcome the disadvantages of conventional fluorinating agents such asF2, HF, or SF4, nontoxic fluorocarbons, chlorofluorocarbons and sulfur hexafluoride areused. These gases cannot be used directly as fluorinating agents. However, when exposedto high-energy environments such as plasma, glow discharge, or γ-radiation, they generateactive fluorinating agents [14].

Another approach of considerable interest is to modify the surface of existing polymerswithout changing the bulk properties. Fluorine attaches to the polymer near the surfaceand, because of its bulkiness and polar nature, improves gas and nonpolar liquid barrierproperties [15].

Bulk fluorinated polymers (by F2 under controlled conditions to reduce crosslinking) canbe used for the same purpose in place of fluoropolymers with similar structures preparedfrom respective monomers [8]. However, to make it cost-effective, surface fluorinationrather than complete bulk fluorination of fabricated plastic items may be preferred. Suchsurface treatments could avoid problems encountered in moulding of fluoropolymers.Large fabricated plastic items can be given a surface coating of fluorinated polymer (0.1mm thickness) [16].

Fluorinated plastic surfaces are impervious to most solvents and have good chemical,solvent and water resistance [16, 17]. Various plastic bottles, containers and tanks arefluorinated to handle chemicals and solvents safely. Hence, fluorinated plastic containersare found in use as containers for gasoline (petrol), paint, turpentine, motor oil and

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varnish [18, 19]. However, fluorinated materials may not maintain their barrier propertiesafter repeated flexing.

Surface fluorination is used to improve the barrier properties of the inner surface ofpolyethylene during the blow moulding process for formation of bottles. Polyethylene isnonpolar and therefore a poor barrier to nonpolar hydrocarbons. Such treatments withhighly polar fluorine significantly improve its barrier properties. Surface-fluorinatedcontainers are commonly used for gasoline (petrol), herbicides, pesticides and otherproducts that normally penetrate polyethylene [20].

Mild surface treatment of polyethylene with low concentrations of fluorine can reducethe permeability of liquid penetrants such as pentane and hexane depending on thesolubility and size of the penetrant [21].

8.3.2 Chlorination

The chlorination reaction is too slow and not practicable, but it results in good barrierproperties with more resistance to flexing. The gas-phase chlorination of the surface ofLDPE has been studied under ambient light [22, 23] as well as in the presence of ultraviolet(UV) radiation [24]. The resultant surface was reported to consist of C–Cl and C–Cl2moieties [22, 23]. However, chlorination of the surface of PE leads also to the formationof allyl chloride and vinyl chloride moieties [24].

8.3.3 Bromination

The introduction of Br moieties on the polyolefin surface opens up a synthetic pathwayto introduce a wide range of specific functional groups on the surface under mild conditionsvia nucleophilic substitution of Br moieties by different nucleophiles [25]. The gas-phasebromination of PE, PP and PS film surfaces by a free-radical photochemical pathwayoccurs with high regioselectivity. The surface bromination was accompanied bysimultaneous dehydrobromination resulting in the formation of long sequences ofconjugated double bonds. Thus, the brominated polyolefin surface contains bromide(Br) moieties in different chemical environments.

As an example, we consider the free-radical mechanism for the bromination of the surfaceof PE film. The first step in this reaction is the homolytic bond cleavage of the brominemolecule into two bromine radicals upon exposure to radiation [26]:

UVBr2 → 2Br• (8.2)


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In the second step, the bromine radical abstracts a hydrogen atom from the methyleneunit of LDPE, which results in the formation of a radical centre on the LDPE chain:

–CH2–CH2– + Br• → –•CH–CH2– + HBr (8.3)

This radical centre further reacts with a bromine molecule to form the C–Br moiety anda bromine radical:

–•CH–CH2– + Br2 → –CHBr–CH2– + Br• (8.4)

This bromine radical then reacts with another –CH2– unit [equation (8.3)] and this chainreaction continues:

–CHBr–CH2– + Br• → –CHBr–•CH– + HBr (8.5)

The effects of the structure of the polymer on the mechanism of the bromination have beenstudied. Since the PS backbone contains 50% benzyl carbon atoms and 50% secondarycarbon atoms, an increased rate of bromination compared to that of PE is expected.

8.3.4 Sulfonation

Sulfonation involves exposure of the polymer surface to SO3/air followed by neutralisationwith NH4OH, NaOH, or LiOH. Chemical reduction of copper, tin, or silver counterionspresent from the neutralisation process following sulfonation is called ‘reductivemetallisation’. When combined with a thin protective overcoating of compatible barriercopolymer, dramatic permeation flux reductions of nearly 200-fold have been reported[5]. Sulfonation of polystyrene and aromatic polymers can be used to obtain proton-conducting polymer electrolytes for use in fuel cells [27]. The aromatic polymers areeasily sulfonated by concentrated sulfuric acid, by chlorosulfonic acid, by pure orcomplexed sulfur trioxide, or by acetyl sulfate. Sulfonation with chlorosulfonic acid orfuming sulfuric acid sometimes causes chemical degradation in these polymers [1].

Surface sulfonation yields excellent gas barrier properties under dry conditions, is relativelysimple and does not affect the mechanical stability of the polymer [5].

8.3.5 Chemical Etching

Chemical treatment is usually used for irregular and, in particular, large articles whenother treatment methods are not applicable. It involves immersion of the article [LDPEand high-density PE (HDPE)] in an etchant solution such as chromic acid [28],

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permanganate, sulfuric acid [29,30] or chlorosulfonic acid. Reflection infrared (IR)studies reveal extensive chemical changes on the surface in the case of LDPE but notHDPE or PP. New bands corresponding to the introduction of –OH, >C=O and SO3Hgroups were detected.

Oxidation of PE by sulfuric acid and potassium chlorate [29, 30] has been carriedout. In this case, the free energy of adhesion of the polymer is found to increaselinearly with the surface density of the hydrophilic sites created by oxidation.

The surface tension, polarity, wettability and bondability of fluoropolymer areimproved by sodium etching [31, 32]. The etching solution is the equimolar complexof sodium and naphthalene dissolved in tetrahydrofuran. X-ray photoelectronspectroscopy (XPS) shows the complete disappearance of the fluorine peak, theappearance of an intense oxygen peak, and broadening and shifting of the C 1s peakto lower binding energy. A significant number of functional groups, such as carbonyl,carboxyl and C=C unsaturation, are introduced.

The oxidation methods described up to now are heterogeneous in nature, since theyinvolve chemical reactions between substances located partly in an organic phaseand partly in an aqueous phase. Recently, a technique that is commonly referred toas phase transfer has come into prominence. This technique involves the use of phase-transferred permanganate (purple hydrocarbon) as an oxidant in a polar medium.Konar and co-workers [33, 34] have oxidised several polyolefins with the help oftetrabutylammonium permanganate in a hydrocarbon medium. Characterisation ofthe oxidised polyolefins confirmed the introduction of polar functional groups onthe polar surface [35, 36].

Other phase-transfer catalysts, such as tetrabutylammonium bromide,tetrapentylammonium iodide, dicyclohexyl-18-crown-6 (DC-18-C6) and benzyltriphenyl phosphonium chloride (BTPC), have been investigated [37]. The resultsobtained show that LDPE oxidised using DC-18-C6 and BTPC catalysts has arelatively greater polar contribution to the total surface free energies than when usingother catalysts. The carboxyl percentage attains 15.0% and 20.0%, respectively [38]while hydroperoxide attains 22.2% and 15.2%, respectively [36]. When a polymeris soaked in a heavily oxidative chemical liquid, such as chromic anhydride/tetrachloroethane, chromic acid/acetic acid or chromic acid/sulfuric acid, and treatedunder suitable conditions, polar groups are introduced on the polymer surface [39,40]. The surface of the polymer is heavily oxidised by nascent oxygen generatedduring the reaction as follows:

K2Cr2O7 + 4H2SO4 → Cr2(SO4)3 + K2SO4 + 4H2O + 3[O] (8.6)


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8.3.6 Grafting

Graft copolymerisation of vinyl monomers on to polymeric materials has been the subjectof extensive studies for about four decades. In spite of the huge number of publishedpapers and patents, and the interesting results obtained, there has been comparativelylittle commercialisation of the grafting process. The reasons for the lack of industrialisationon a large scale have been partly economic. Among the technical problems, which stillremain to a considerable extent, are the concurrent formation of homopolymer in mostcases and the lack of reproducibility in these largely heterogeneous reactions. In addition,there is the difficulty of controlling the grafted side chains in the molecular weight (molarmass) distribution.

There are now a considerable number of methods available for effecting graftcopolymerisation on to preformed polymers, each with its own particular advantagesand disadvantages. Graft copolymerisation is effected, generally, through an initiationreaction involving attack by a macroradical on the monomer to be grafted. The generationof the macroradical is accomplished by different means such as:

(1) Decomposition of a weak bond or liberation of an unstable group present in sidegroups in the chemical structure of the polymer;

(2) Chain transfer reactions;

(3) Redox reaction;

(4) Photochemical initiation; and

(5) Gamma-radiation-induced copolymerisation.

Grafting using γ-radiation is concentrated on polyolefins and some vinyl polymers andelastomers, which are usually difficult to graft by chemical means without prior chemicalmodification of the substrate.

8.3.6.1 Grafting Using High-Energy Radiation

The surface properties of commercial polymer thin films can be tailored under appropriateexperimental conditions of radiation-induced grafting. The growth in popularity ofradiation as the initiating system for grafting arises from the availability and cost ofionising radiation. This is due to the introduction of more powerful nuclear reactors.Apart from its cheapness, radiation is a very convenient method for graft initiation becauseit allows a considerable degree of control to be exercised over such structural factors as

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the number and length of the grafted chains by careful selection of the dose and doserate. Thus, the advantages of radiation-chemical methods are:

(1) Ease of preparation as compared to the conventional chemical method;

(2) General applicability to a wide range of polymer combinations (due to the relativelynonselective absorption of radiation in matter); and

(3) More efficient (and thus more economical) energy transfer provided by radiationcompared to chemical methods requiring heat.

The theory of radiation-induced grafting has received extensive treatment. The directeffect of ionising radiation in material is to produce active radical sites. The typical stepsinvolved in free-radical polymerisation are also applicable to graft copolymerisation,including initiation, propagation and chain transfer. However, the complicating role ofdiffusion prevents any simple correlation of individual rate constants to the overall reactionrate. Among the various methods of radiation grafting, four have received special attention:

(1) Direct radiation grafting of a vinyl monomer on to a polymer;

(2) Grafting on radiation-peroxidised polymer;

(3) Grafting initiated by trapped radicals; and

(4) The intercrosslinking of two different polymers.

Acrylic acid (AA) has been grafted on to PE films using γ-radiation [41, 42]. Gamma-radiation grafting of styrene on to PE films has been carried out [43]. The styrene-graftedfilms were then sulfonated to form cationic exchange membranes. Rieke and co-workersdescribed the properties obtained from grafting AA on to HDPE [44]. Their study pursuedthe concept of producing thermally sensitive crosslinks that could improve the propertiesof PE, (i.e., increase chemical reactivity). In 1977, Toi and co-workers determined thethermal properties for styrene-grafted HDPE by using γ-radiation [45]. No effect wasobserved on the crystallite size and the glass transition temperature after grafting. Ishigakiand co-workers reported the graft polymerisation of AA on to PE film by the pre-irradiation method [46, 47]. LDPE and HDPE were irradiated by electron beams of 2-50Mrad and then immersed in an AA aqueous solution. These products were tested assemipermeable membranes for water desalination under reverse osmosis conditions [48].

Hydrophilic monomers such as AA or vinylpyridine were grafted on to PE via 60Co γ-radiation. The hydrophilic monomer-grafted PE could be treated further forfunctionalisation, leading to the investigation of a few applications such as separationmembranes, polymeric catalysts and biosensors [49-53].


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8.3.6.2 Photografting

The surface photografting process is based on surface grafting reactions initiated byultraviolet radiation. These reactions are efficient and fast. They are limited to the surfaceof the polymer without affecting bulk properties, and they give very thin layers (less than10 nm) of grafted polymer [54]. The grafting of sheets of low-density and high-densitypolyethylene with acrylic acid by UV irradiation from a high-pressure mercury lampusing a batch process has been reported [55]. The surface of HDPE is more difficult tograft than that of LDPE because of the linear chain structure of HDPE, and consequentlyits higher degree of crystallinity, which gives it a rougher surface structure than LDPE.Surface grafting with acrylic acid, as expected, decreases the contact angle of water,approaching complete wetting for LDPE.

The molecular mechanism of bulk surface photografting has been given [56]. The primarygrafting given in this mechanism using benzophenone involves initiation and propagationof short linear chains which is terminated by the addition of ketyl radical. Benzophenoneacts as both initiator and terminator.

The main effects that are important for applications are increased wettability, as mentionedbefore, increased adhesion of inks and other substrates, and increased adsorption ofdyes. By grafting of reactive monomers like glycidyl acrylate, the polymer surface ismade reactive to stabilisers, hydrophilic polymers, heparin and other bioactive agents,which gives functional properties of great interest [57-59]. Biomedical applications areof particular interest [60]. Examples of other recent publications on surface photograftinginclude the preparation of polymeric catalyst [61, 62], polyethylene films for studyingelectrostatic interactions [63], and films for immobilisation of enzymes [64].

8.4 Physical Methods Used for Surface Modification

Modification techniques using physical methods have been designed to achieve increasedhydrophilicity, chemical modification and attachment of pharmacologically active agents.These physical methods include plasma treatment, corona treatment, ultraviolet andgamma radiation.

8.4.1 Plasma Treatment

The implantation process that occurs in plasma treatment is one of the most effectivemethods of surface modification of polymeric materials. The plasma activates gasmolecules, such as oxygen and nitrogen. The activated species interact with the polymer’ssurfaces, and then special functional groups, such as hydroxyl, carbonyl, carboxyl, amino

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and amido groups, are formed at the surface of the polymers. As a result, the implantationreactions lead to large changes in the surface properties of the polymer; for example, thepolymers change from hydrophobic to hydrophilic. ‘Plasma treatment’ is frequently usedfor the improvement of the adhesion and wettability of polymeric materials. A polyethylenefilm was treated with a nitrogen plasma, and its surface was inspected by XPS (C 1s andN 1s core levels) [65]. The original polyethylene film provides a sharp and symmetricalC 1s core-level spectrum whose peak appears at 285 eV with no N 1s core-level spectrum.However, the plasma-treated film gives an asymmetrical C 1s spectrum with a tail atmore than 285 eV, and a strong N 1s core-level spectrum. This comparison indicates thatsome nitrogen functionalities were generated at the polyethylene film surface throughnitrogen plasma treatment. Similarly, oxygen plasma treatment leads to the formation ofsome oxygen functionalities at the surface of polyethylene film [66]. It is clear that plasmatreatment implants atomic residues at the surface of polymeric materials. Carbonmonoxide, carbon dioxide, nitrogen monoxide, nitrogen dioxide and ammonia are usedas plasma gases for hydrophilic surface modification. Polypropylene, polyester,polystyrene, rubber and polytetrafluoroethylene, among others, but not polyethylene,have been successfully modified by plasma treatment. The details of the implantationprocess are reviewed in the literature [67].

8.4.2 Corona Treatment

In this technique, a sufficiently high-voltage electrical discharge is applied to the surfaceof a moving substrate (sheet or film). Pretreatment of films is usually carried out at thesame time as film extrusion, which is an advantage where antistatic and other additivesare present in the film. When film was extruded and stored prior to treatment, it wasfound that the additives had bloomed to the surface, and this made it difficult to achievean even treatment.

In one method, the film is passed between two electrodes, one of which is a metal bladeconnected to a high-voltage, high-frequency generator. The other is an earthed roller,which is separated from the high-voltage electrode by a narrow gap. The metal electrodeshould be slightly narrower in width than the film that is to be treated in order to preventdirect discharges to the roller. The electrical discharge is accompanied by the formationof ozone. This oxidises the film surface, rendering it polar. The level of treatment isgoverned by the generator output and the speed of throughput.

Both under- and overtreatment should be avoided – the latter causing surface powdering,brittleness and sealing difficulties. The effect of treatment diminishes with time, and thetreated surface is sensitive to handling and dust pickup. The corona treatment functionsat atmospheric pressure and relatively high temperature. In this case, very significantsurface oxidation occurs [6].


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One simple test for determining whether or not a film has been treated is to runwater over the surface. If the film is untreated, the water will be repelled, whereas atreated film will retain the liquid for several minutes. Between these two extremes, apartially treated film will tend to show areas of both good and bad adhesion, so thatthe test is only satisfactory for seeing whether or not the surface of the film has beentreated but not for detecting overtreatment.

An improvement on this primitive test method is the peel adhesion test. This is carriedout by applying a specified pressure-sensitive tape to the film surface, using a roller.The peel strength is then measured with the aid of a tensiometer. The higher the levelof treatment, the higher the peel strength.

The chemical changes occurring on the surface can be detected by using the XPStechnique. This technique enables one to identify the presence of hydroxyl, ether,ester, hydroperoxide, aldehyde, carbonyl or carboxylic groups in corona discharge-treated polyolefins.

8.5 Characterisation

Characterisation of modified films depends on the method of modification. For instance,the change in mechanical properties due to stretching can be evaluated by measuringthe changes in mechanical properties using tensile testing machines according to standardmethods. Characterisation of grafted films also differs somewhat from that of physicallytreated films. However, the selection of one or other measuring technique dependsgenerally on the extent of modification.

8.5.1 Gravimetric Method

Graft products are usually characterised by different methods. The first method is thecalculation of graft parameters known as the grafting percentage (GP), grafting efficiency(GE) and weight conversion percentage (WC). These parameters can be calculatedaccording to the following equations:

grafting percentage (GP) = A B

B− ×100 (8.7)

grafting efficiency (GE) = A B

C− × 100 (8.8)

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weight conversion percentage (WC) = AB

×100 (8.9)

where A, B and C are the weights of the extracted graft product, substrate and monomer,respectively.

This gravimetric method gives direct and rapid information about the graft reaction.Other characterisation methods are usually used to detect the changes in physicalproperties, which usually result from the changes in the morphology and structures ofthe substrates due to grafting.

8.5.2 Thermal Analyses

In polymers having a certain degree of crystallinity, differential scanning calorimetry(DSC) is used to determine the heat of fusion and, consequently, the changes in thedegree of crystallinity in grafted and ungrafted samples. The changes in the crystallinityof PE found after grafting include a small 2.5 °C drop in the location of the maximum inthe melting curve and a significant decrease in the area under the melting peak [69].Similar results were observed in the case of grafting PP and PE/ethylene-vinyl acetate(EVA) blends [70]. While the decrease in the melting temperature, represented by theshift in the melting curve, indicates that there is some change in the crystallinity causedby grafting, comparison of the areas before and after grafting indicates that this may bea small effect. By assuming that the difference in areas is due only to a difference in theamount of PE or PP present (in other words, no difference in the degree of crystallinity),the per cent graft can be calculated from:

%G PAN

PE= − × ×A A

A1 2

2100

ρρ (8.10)

where A1 is the area before grafting, A2 is the area after grafting, and ρ is the density.

8.5.3 Scanning Electron Microscopy

Scanning electron microscopy (SEM) is generally used to detect the topography of agrafted surface, which usually changes due to monomers grafted on to the surface. Inaddition, this method can also be used to detect the depth of grafting into the matrix. Ifa binary monomer mixture was used for grafting, scanning electron micrographs help todetect the grafted monomer distribution by comparison with micrographs of each graftedmonomer separately.


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8.5.4 Swelling Measurements

Equilibrium swelling of grafted samples in a proper solvent helps to detect the presenceof grafted monomer. For instance, polyethylene does not swell to any noticeable extentin water. However, if polyethylene is grafted with water-soluble polymers such aspolyacrylic acid or polyacrylamide, the equilibrium swelling of the product obtainedincreases markedly. Accordingly, the increase in swelling is evidence of grafting. In contrast,the swellability of natural rubber or styrene-butadiene-rubber vulcanisates in gasoline orbenzene decreases markedly due to grafting with polyacrylonitrile (PAN). This decreasein swelling, again, is evidence of grafting.

8.5.5 Molecular Weight and Molecular Weight Distribution

It is essential to know the molecular weight (molar mass) distribution of a graft in orderto design functional polymeric membranes precisely by application of radiation-inducedgraft polymerisation and to control the grafting process. For example, the length anddensity of the polymer chains grafted on to a cellulose triacetate microfiltration membranewill determine the permeability of liquid through and the adsorptivity of molecules onthe functionalised microfiltration membrane. Thus, the molecular weight distribution ofmethyl methacrylate grafted on to cellulose triacetate has been determined by acidhydrolysis of the substrate. From the gel-permeation chromatogram, the molecular weightdistribution was determined [71]. This method is valid only when it is possible to degradethe substrate. In the case of grafted natural rubber, for example, ozonolysis is a veryconvenient process to use to destroy the natural rubber segments, leaving the plastomerchains intact [72]. Alternatively, oxidation with perbenzoic acid can be used [73].Osmometry or solution viscosity may then be used to determine the molecular weight ofthe isolated non-rubber fraction.

8.5.6 Dielectric Relaxation

Dielectric relaxation measurements of polyethylene grafted with AA, 2-hydroxyethylmethacrylate (HEMA) and their binary mixture were carried out in a trial to explore themolecular dynamics of the grafted samples [74]. Such measurements enable informationto be obtained about their molecular packing and interaction. It was possible to predictthat the binary mixture used yields a random copolymer PE-g-P(AA/HEMA) that is greatlyenriched with HEMA. This method of characterisation is very interesting and is likely tobe developed in different polymer/monomer systems.

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8.5.7 Surface Properties

The surface properties of modified plastic films are very important in industrialapplications. A number of techniques are available for determining the composition of asolid surface. This is very important in many processes, such as oxidation discoloration,wear and adhesion. The technique used depends upon such important considerations assampling depth, surface information, analysis environment and surface suitability.

The most widely used techniques for surface analysis are Auger electron spectroscopy(AES), XPS, secondary ion - mass spectroscopy (SIMS), Raman and IR spectroscopy,and contact angle measurements.

8.5.8 Spectroscopic Analysis

8.5.8.1 Infrared (IR) Spectroscopy

Proof of chemical modification or changes in chemical structure due to physical treatmentssuch as corona discharge can be followed up by spectroscopic analysis using IR. Thus,the amount of acrylonitrile grafting on to PE using an electron beam was determinedfrom the absorbance of the nitrile group at 2240 cm–1 after extraction of homopolymer[69]. In order to minimise the effects of weighing error, an internal reference methodutilising the methylene absorbance of PE at 730 cm–1 was adopted. Thus, the mass ofPAN in a sample was correlated to the ratio of the absorbance A2240/A730, and the weightper cent graft defined before was computed from the mass of PAN.

8.5.8.2 X-Ray Fluorescence Spectroscopy (XFS)

This method can be used to detect and characterise the first several hundred nanometresof depth of a solid. It can be attached to a scanning electron microscope. The mainprinciple is that energetic electrons bombard the sample, where ionisation takes place.Ions with an electron vacancy in their atomic core rearrange to a lower energy state,resulting in the release of electromagnetic energy of a specific wavelength. Analysis ofthe wavelengths of the X-radiation emitted identifies the atomic species present.

8.5.8.3 Auger Electron Spectroscopy (AES)

This technique is used to characterise the chemical bonding state of the elements on thesurface. The maximal depth from which Auger electrons can escape is only about 0.3-0.6 μm. For most materials, AES uses a low-energy, 1-5 keV, electron beam gun forsurface bombardment to minimise surface heating.


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8.5.9 Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photo-electron Spectroscopy (XPS)

In this method the surface is bombarded with low-energy X-rays, which is less disruptivethan an electron beam. The energy is absorbed by ionisation, resulting in the directejection of a core-level electron, i.e., a photoelectron. Hence ESCA is also known as X-ray photoelectron spectroscopy. These electrons have an escape depth of less than ananometre. Although XPS is less sensitive than AES, it provides a direct measure of thebinding energy of core-level electrons through the relation:

binding energy of ionised core-level electron =energy of emitted photoelectron – incident X-ray energy

and it gives simpler spectral line shapes than AES.

This technique can be used to distinguish between different elements and differentchemical bonding configurations. It is the most popular surface analytical techniquefor providing structural, chemical bonding and composition data for polymericsystems. All elements, except hydrogen, are readily identified by XPS, since thedifferent core-level binding energies are highly characteristic. By measuring the relativepeak intensities and dividing them by the appropriate sensitivity factors, one mayfind the concentration of different elements on a surface. Moreover, small shifts inthe binding energy of a core level are corroborated by considering the presence ofdifferent functional groups. For example, when a carbon atom is bonded to differentgroups of atoms of increasing electronegativity, a systematic shift in the binding energyof the C 1s peak is observed. The higher the electronegativity of the group, the higherthe binding energy of the C 1s peak.

8.6 Applications

Since bringing about changes in physical properties is often the impetus for grafting, itis necessary to touch upon this briefly in this section. A number of general reviews ongrafting have also included some discussion on the changes in physical properties thatusually determine the field of applications. Grafting has often been employed to changethe moisture absorption and transport properties of plastic films when hydrophilicmonomers such as acrylamide, acrylic acid and methacrylic acid are grafted. Radiationgrafting of anionic and cationic monomers to impart ion-exchange properties to polymerfilms and other structures is rather promising. Thus, grafting of acrylamide and acrylicacid on to polyethylene and polyethylene/ethylene-vinyl acetate copolymer blend [70]allows a new product to be obtained with reasonable ion-exchange capacity.

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A number of possible uses of radiation grafting are being explored for microlithography,diazo printing, and various copying and printing processes.

Radiation grafting for various biomedical applications remains an extremely active fieldof development. The grafted side chains can contain functional groups to which bioactivematerials can be attached. These include amine, carboxylic and hydroxyl groups, whichcan be considered as centres for further modifications.

Photodegradation of polyethylene waste can be markedly accelerated via its graftingwith acrylamide [70]. In contrast, photostabilisation of polyethylene and polypropylenecan be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxypropoxy)benzophenone using γ-radiation [75]. In this case, the grafted compound,acting as a UV stabiliser, is chemically bound to the backbone chain of the polymer, andits evaporation from the surface can be avoided.

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9 Applications of Plastic Films in Packaging

Susan E. Selke

9.1 Introduction

Nearly all products are packaged at some point in their life-cycle. Plastic films are widelyused in packaging, and continue to grow in use as more and more applications switchfrom rigid to flexible packages. Flexible packages generally take up much less space thanthe rigid structures they replace, especially before they are filled with product. Theycommonly require less material, as well. Therefore, switching from rigid to flexiblepackaging can provide significant economic savings in warehouse space andtransportation, as well as in package cost. On the other hand, because flexible packagingdoes not usually have as much strength as rigid packaging, stronger distribution packagingmay be required. Opening and reclosing of flexible packaging may also be less user-friendly, and consumers may perceive some types of products in flexible packaging asbeing lower in quality than equivalent products in rigid or semi-rigid packages.

Common flexible packaging forms include wraps, bags and pouches. In these packages,plastic films may be used alone or combined with paper and/or metal to serve the basicpackaging functions of containment, protection, communication and utility in the deliveryof quality products to the consumer. While plastic films are most often found in flexiblepackage structures, they may also be used as a component in rigid or semi-rigid packagestructures, for example, as a liner inside a carton, or as lidding on a cup or tray.

The most common film used in packaging is low-density polyethylene (LDPE), definedbroadly to include linear low-density polyethylene (LLDPE). Appreciable amounts ofhigh-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC),polyvinylidene chloride (PVDC), polyamide (Nylon) and other plastics are also used

9.2 Packaging Functions

Before examining applications of plastic films in packaging, it is useful to take a momentto consider why we use packages at all, since that will help in evaluation of the advantagesand disadvantages of plastic films as packaging materials. The functions of packagingcan be described in many ways. One simple way of organising them is to consider thebasic packaging functions as containment, protection, communication and utility.

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The containment function of packaging is the most basic. Many types of goods cannotbe readily moved from one place to another unless they are contained in some manner.This is obvious for liquids and gases, but is also true for many small solid items, forexample, marbles, nails, laundry detergent powder and potato chips (crisps). The packageconfines these items in a way that makes it feasible to transport them.

Sometimes the containment function is considered part of the more inclusive protectionfunction of packaging. An important attribute of many forms of packaging is the abilityof the package to protect the product from some type of damage associated with itsinteraction with the environment. In the example above, the marbles and nails must beprotected against exposure to dust and dirt to remain in a condition that will permittheir sale. The laundry detergent needs to be protected from exposure to excess moisturethat could cause caking. The potato chips must be protected against light and oxygen,which can cause rancidity. In some cases, protection of the environment from the productis provided by the package. A water-soluble pouch for agricultural chemicals has, as itsprime function, protection of the user from exposure to the hazardous undiluted chemical.

Packages also serve as a vehicle for communication. In most cases, the package must insome way communicate what it contains. Sometimes this is as simple as being transparent,so that the user can see what is inside. Since the package is often the primary sales tool,however, communication needs are usually much more extensive. The package must notonly communicate what is inside, but also act to convince the potential consumer to purchasethe product. Often, there are a number of legally required communications, such as theamount of product, where and by whom the product is made, required warnings, etc.

Packages also provide utility, either for the end-user or for others who interact with thepackage along the supply chain. Utility includes attributes such as a tear strip for openingand tamper evidence, a zipper closure for resealing, and a hole for use in hanging thepackages on a display.

Individual packages or package elements often provide more than one function,simultaneously. For example, a stand-up pouch for snack foods provides: containment;protection of the product against oxygen and moisture; communication of identification,legally required, and sales messages; opening and reclosure features for consumer utility;and the ability to stand conveniently on the retailer’s shelf and present a reasonably flatfront panel to catch the consumer’s eye.

9.3 Flexible Package Forms

Flexible packages come in two basic forms: wraps, and bags or pouches. A wrap consists ofplastic film that has not been formed into a package shape. The film is simply wound around

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Applications of Plastic Films in Packaging

the product or products to be contained, and held in place in some fashion. In a bag orpouch, some shaping of the plastic is done, either before or at the same time as the product isadded. Most often, this shaping is done by heat-sealing the edges of the plastic together.

9.3.1 Wraps

9.3.1.1 Stretch-Wrap

One of the largest uses of plastic films in packaging is in stretch-wrap used for bundlingpallet-loads of products together, in order to unitise them for distribution. The plasticfilm, most often linear low-density polyethylene, is stretched as it is wound around theproducts and pallet, usually in a spiral fashion. When enough has been applied, the filmis cut, and the tail of the film is adhered to the load, usually by self-cling. When thestretching force is released, the film’s tendency to return to its unstretched dimensionscauses a restraining force to be exerted on the load, thus unitising it and keeping it fromshifting when the load is moved during distribution.

In addition to its unitising function, stretch-wrap also protects the load against moisture,dust and abrasion. Stretch-wrap can also be used to provide this protection to singleitems, or to unitise smaller than pallet-load quantities of goods.

While stretch-wrap is simple in conception, it may have a fairly complex structure. It isdesirable for each layer of stretch-wrap to stick to the layers below, but it is undesirablefor adjacent shrink-wrapped loads to stick to each other, or to other things with whichthey come in contact. Therefore, the stretch-wrap may have a multilayer structure, withtackifying agents added to the inside layer to enhance cling. Low-density polyethylene,polyvinyl chloride, ethylene-vinyl acetate and other polymers are used as stretch films, inaddition to LLDPE.

9.3.1.2 Shrink-Wrap

Shrink-wrap is an alternative to stretch-wrap for unitisation. When shrink-wrap is exposedto a source of heat, the previously aligned (oriented) molecules try to return to the lower-energy, unoriented, random-coil conformation. The product prevents the film fromreturning to its unstretched dimensions, and the force exerted by the material on theproduct acts to unitise the load.

For unitising pallet-loads of goods, stretch-wrap is much more common than shrink-wrap, since it generally requires less energy and is more economical. Shrink-wrap is

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more commonly used as a bundling wrap, unitising two or several products (either thesame or different), rather than for full pallet-loads of goods.

Often, shrink-wrap is used for product protection rather than unitisation, in applicationsranging from meat to toys. It can be designed to form a tight enclosure around the entireproduct, providing excellent protection against dirt, moisture and abrasion. Usually, thewrap is formed into a loose pouch before it is shrunk tightly around the product in ashrink tunnel, where the packaged product is exposed to hot air. LDPE and LLDPE arecommon materials for shrink films. PVC and PP are used in lesser quantities, as are somespecialty films.

9.3.2 Bags, Sacks and Pouches

To make a bag, sack or pouch, two or more edges of a plastic film are sealed together,forming a cavity in which the product can be placed. In most applications, the opening isthen closed so that the product is completely enclosed by the package. In some cases,such as merchandise sacks, one side remains open.

The terms ‘bag’, ‘sack’ and ‘pouch’ can be confusing. According to some authorities, sacksare larger than bags, and both refer to packages in which the top is open, while pouchesare smaller, and refer to packages that are totally sealed. However, these definitions do notconform to common use of these terms, which, in practice, are often used interchangeably.

Common styles of pouches include pillow pouches, three-side-seal pouches and four-side-seal pouches. Pillow pouches are produced by forming the plastic film into a cylinderand sealing the edges together in what will become the back seam in the finished package.The bottom of the cylinder is collapsed and sealed, the product introduced, and then thetop seam added. The shape of the filled package resembles a pillow – hence its name.

Three-side-seal pouches are formed, as the name indicates, by folding the film into arectangle and sealing the three non-fold sides. In some cases, the fourth side is sealed aswell, for additional strength. Four-side-seal pouches are formed from two pieces of materialthat are sealed together on all sides. Therefore, four-side-seal pouches need not berectangular in shape. In contrast to pillow and three-side-seal pouches, the front andback of a four-side-seal pouch may be made from different types of plastic film.

In any of these pouch styles, gussets may be added to expand the capacity of the pouchwithout increasing its width or height.

Pouches may be used alone, or may be combined with another package for productdistribution and/or sale. One very common package structure is bag-in-box packages,which consist of a pouch inside a folding carton or corrugated box.

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The pouch material may be plastic film alone, or a multilayer material containing paperand/or aluminium foil. Paper may be used to add strength, rigidity, printability or bulkto the flexible package. Foil may be incorporated to improve the barrier to permeantssuch as oxygen, water vapour, odours or flavours.

In the past several years, stand-up pouches have increasingly been used as substitutes forcartons or bottles. Stand-up pouches are designed to stand upright on the retail shelf.Their design involves gussets and special shaping of the bottom panel.

9.3.3 Pouch Production

There are two primary ways of using bags, sacks and pouches for packaging: as preformedpouches, or in form-fill-seal operations. In a form-fill-seal (FFS) operation, the web stock(usually preprinted, if applicable) is fed into either a horizontal or vertical FFS machine,in which it is formed into a pouch, the product added and the final seal formed. Ifpreformed pouches are used, the packages are formed and an opening left for productintroduction. The product is added to the package in a separate operation, and then thepackage is sealed.

Form-fill-seal operations are usually economically advantageous for large-scaleproduction. Buying of preformed pouches is generally more economical if productionquantities are small, or in cases where the material is difficult to seal and poses qualitycontrol problems.

9.3.4 Dispensing and Reclosure Features

One of the long-standing drawbacks of flexible packaging has been the difficulty ofproviding easy-to-use and effective dispensing and reclosure. In the past few years, severalinnovations have provided significant improvements in these package attributes.

The most common way to dispense products from flexible packages is to cut or tear thepackage open, or to peel open one of the seams. For some products, such as breakfastcereal in bag-in-box packages, this is a significant source of consumer complaints. Theseals often do not peel easily, and all too often the result is a bag with a split down theside, spilling cereal into the carton and making it nearly impossible to reclose the pouchto protect product freshness. Some flexible packages now incorporate zipper closures,often accompanied by a tear strip for initial opening. Other packages have resealableflaps, usually located along a seam.

For liquid products, some packages incorporate a threaded spout with a standard threadedcap. This may be located on the top of the pouch, or on the bottom, depending on the


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product and the package size. In bag-in-box packages, the outer carton may include aflap through which the spout can be extended for dispensing. For single-serve drinks, itis common to provide an attached straw (protected from dirt in its own pouch, which isglued to the side of the drink pouch), along with a designated spot on the package thathas been modified for easy puncturing.

9.4 Heat-Sealing

Heat-sealing is the usual method for producing seals and seams in flexible packaging.Occasionally, adhesive systems are used. There are a wide variety of types of heat-sealingsystems, but the most common, especially for films, are thermal or bar sealing, andimpulse sealing.

Thermal or bar sealing uses two heated bars that exert pressure on the materials to besealed and at the same time conduct heat to the interface, melting the materials. Thepressure ensures good contact between the materials, and assists in interpenetration ofthe melted viscous materials at the interface. When sufficient time has elapsed to producean initial seal, the materials are released. Therefore, the hot tack of the material is crucialin forming an adequate seal. The full strength of the seal forms as the material cools, butthe initial strength must be sufficient to maintain the seal integrity while cooling proceeds.

The sealing bars usually have rounded edges to avoid puncturing the material, and oftenone bar is fitted with a resilient surface to aid in achieving uniform pressure duringsealing. Usually, the heat-seal jaws are serrated rather than flat, and produce a patternedseal. In variants of thermal sealing, only one bar is heated and the other is not. Especiallyfor sealing lidding on containers, the bars may be shaped rather than rectangular,producing shaped seals. Another variant uses heated rollers rather than bars; the pouchis sealed as it travels through the rollers.

Impulse sealing also uses two jaws to produce the seal, but heat is generated by flow ofan impulse of electric current through a nichrome wire. The jaws do not remain hot, butcool down after each electrical impulse. The material being sealed is captured betweenthe jaws, the current flows to produce heating, and the material remains between thejaws for a cool-down period before it is released. Cooling may be aided by circulation ofcooling water through the jaws. With impulse sealing, materials do not require as goodhot tack as with thermal sealing. The seal will increase in strength during the coolingphase, before it is released from the heat-sealer, so it is not as subject to immediatefailure or distortion. On the other hand, the impulse seal is typically much narrowerthan the bar seal, and therefore is often not as strong. Impulse sealing is particularlyadvantageous for oriented materials, which have a tendency to wrinkle during sealing.As with bar sealing, the jaws can be shaped to produce shaped seals.

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Hot wire or knife sealing uses a heated wire or knife to cut and seal films simultaneously.It is often used to produce thin polyethylene bags for applications such as produce packaging.The seals are very narrow, often nearly invisible, and relatively weak. In band sealing,often used for the final seal on filled preformed pouches, the materials to be sealed aremoved through heated bands. Both heating and cooling phases can be provided. Othertypes of heat-sealing are used less frequently in production of flexible packaging.

9.5 Other Uses of Packaging Films

Plastics films are sometimes used as components in rigid or semi-rigid packaging structures.They can serve as liners inside closures for bottles and jars, as lidding on trays or cups, orcan be laminated on paperboard or other materials. While the plastic resins used ascoatings are not produced as stand-alone films, they are deposited on or in packages asfilms. Two common packaging applications of plastic films outside the flexible packagingcategory are skin packaging and bubble-wrap.

In skin packaging, a product is held tightly to a backing material by a plastic film. Usually,the backing material consists of a heat-seal coated paperboard. The product is placed onthe board, and then the heated plastic film lowered on to it. A vacuum is drawn throughthe backing material, causing the film to form tightly around the product and seal to theboard. Usually, the product is displayed in the retail environment by hanging the backingfrom a peg. Obviously, the product must be able to withstand momentary contact with thehot plastic without damage, and the plastic must not adhere to the product. The coatedbacking often requires perforations to permit adequate evacuation of trapped air. In somecases, films are used that permit sealing to uncoated board. Heavy-duty films sealed tocorrugated board can be used to provide protection to products during distribution, byphysically isolating the skin-packaged product from impacts to the outer container.

Bubble-wrap is a cushioning material produced by forming bubbles of air, of a definedsize, between two plastic films. The bubbles can be various sizes, depending on the end-use of the material. Generally, smaller bubbles are used to protect lighter-weight products,and larger bubbles are used for heavier products. Bubble-wrap does not provide suitableprotection for products that are very heavy, however.

9.6 Major Packaging Films

A variety of plastic resins are used to make packaging films. Sometimes they are usedalone, and often they are used in combinations that provide the benefits of multiplematerials. The most commonly used packaging resins will be described in this section.


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9.6.1 Low-Density Polyethylene (LDPE) and Linear Low-DensityPolyethylene (LLDPE)

Low-density and linear low-density polyethylene are the most commonly used packagingfilms. Low-density polyethylene is produced by a high-temperature, high-pressure processthat results in considerable short- and long-chain branching of the molecules. Linearlow-density polyethylene is produced at temperatures similar to those used for high-density polyethylene, resulting in linear molecules. The reduction of density comes aboutthrough the use of comonomers that put side groups on the main chain that act likebranching in decreasing crystallinity. In traditional Ziegler-Natta catalyst polymerisations,these comonomers are butene, hexene or octene. Some of the new family of polyethylenesusing metallocene catalysts incorporate higher alpha-olefins into the polymer structure,producing longer side groups, which act much like the long-chain branching in high-pressure LDPE.

LDPE and LLDPE are soft, flexible materials, with a hazy appearance. At equal densityand thickness, LLDPE has higher impact strength, tensile strength, puncture resistanceand elongation than LDPE. LLDPE based on octene generally has the highest strength,followed by hexene- and butene-based polymers, in that order. The cost per unit mass ofthe materials generally also follows the order octene > hexene > butene. LDPE has betterheat-seal properties than LLDPE. It seals at lower temperatures, seals over a widertemperature range, and has better hot tack, all of which result, to a great extent, from itslong-chain branching. Metallocene LLDPE containing higher alpha-olefins was designed,in part, to remedy this disadvantage of LLDPE. Another approach that has commonlybeen taken to producing the best mix of properties for a given application is to blendLLDPE and LDPE.

LDPE and LLDPE are good barriers to water vapour, but are poor barriers to oxygen,carbon dioxide and many odour and flavour compounds. They have good greaseresistance, and are quite inert. Low-temperature performance is good, as these materialsretain their flexibility at very low temperatures. They soften and melt at moderatelyelevated temperatures, so they are not suitable for applications involving significantexposure to heat.

Some characteristic LDPE and LLDPE properties are presented in Table 9.1. LDPE isgenerally the cheapest plastic film, on a per-unit-mass basis. Since LLDPE often permitsconsiderable down-gauging, it can be the lowest cost alternative on a per-use basis.

Very low-density polyethylene (VLDPE) is LLDPE with a higher concentration ofcomonomer, which reduces crystallinity, and consequently density, below the traditionalrange for LLDPE, to 0.905-0.915 g/cm3. These materials are very soft films, with excellentcling but reduced strength.

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9.6.2 High-Density Polyethylene (HDPE)

High-density polyethylene is a linear addition polymer of ethylene, produced attemperatures and pressures similar to those used for LLDPE, and with only very slightbranching. HDPE films are stiffer than LDPE films, though still flexible, and have poorertransparency. Their water vapour barrier is better, as is their gas barrier. However,permeability to oxygen and carbon dioxide is still much too high for HDPE to be suitableas a barrier for these permeants.

As is the case for LDPE, HDPE is very inert, and has good oil and grease resistance. Usinghigh molecular weight (high molar mass) resin, HMW-HDPE, which permits considerabledown-gauging, can reduce the cost of HDPE films on a per-use basis. This material ishigher in cost per unit mass, and is also somewhat more difficult to process than lowermolecular weight materials, due to its high viscosity. Another alternative for reducing thecost of HDPE film is the use of recycled material, often originating in milk cartons.

Because of the distinctly cloudy appearance of HDPE film, a small amount of whitepigment is commonly added to provide an attractive opaque white film. Typical HDPEproperties are shown in Table 9.1.

]2,1[smlifenelyhteylopfoseitreporplacipyT1.9elbaT

ytreporP remyloP

EPDL EPDLL EPDH

T(erutarepmetnoitisnartssalG g; ° )C 021– 021– 021–

T(erutarepmetgnitleM m; ° )C 511-501 421-221 831-821

ta,erutarepmetnoitrotsidtaeH(aPk554 ° )C

44-04 19-26

mc/g(ytisneD 3) 049.0-519.0 539.0-519.0 79.0-49.0

)aPG(suludomelisneT 5.0-2.0 1.1-6.0

)aPM(htgnertselisneT 13-8 54-02 54-71

)%(noitagnolE 569-001 058-053 0021-01

8.73ta*RTVW ° HR%09dnaCg( μ m/m 2 )d

005-573 521

O2 52ta,ytilibaemrep °C01( 3 mc 3 μ m/m 2 )mtad

012-061 37-04

)h42,yad=d(etarnoissimsnartruopavretaW:RTVW*ytidimuhevitaler:HR


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9.6.3 Polypropylene (PP)

Polypropylene is a linear addition polymer of propylene; resins used in packaging arepredominantly isotactic. PP has the lowest density of the commodity plastics, 0.89-0.91 g/cm3. PP films are suitable for high-speed packaging applications that demand arelatively stiff material, since they are considerably stiffer than HDPE, and also havemuch improved clarity. Clarity can be further improved by using copolymer resinscontaining some ethylene units, to reduce crystallinity. Another approach to improvingtransparency is the use of nucleating agents to reduce average crystallite size. Barrierproperties of PP are comparable to those of HDPE.

Unoriented PP film tends to be somewhat brittle, especially at low temperatures. Inmany applications, biaxially oriented film (BOPP) is preferred. Orientation also increasesthe stiffness of the film. PP, especially BOPP, does not heat-seal well. Therefore, it iscommonly coated or coextruded with sealants to make heat-sealable films. Typical PPproperties are shown in Table 9.2.

detneiroyllaixaib,)PP(enelyporpylopfoseitreporplacipyT2.9elbaT]4-1[smlif)CVP(edirolhclynivylopdna)PPOB(enelyporpylop

ytreporP remyloP

PP PPOB CVP

Tg (° )C 01– 01– 501-57

Tm (° )C 571-061 571-061 212


121-701 28-75

mc/g(ytisneD 3) 19.0-98.0 19.0-98.0 14.1-53.1

)aPG(suludomelisneT 5.1-1.1 4.2-7.1 1.4ot


)%(noitagnolE 056-005 051-03 054-41

8.73ta,RTVW ° HR%09dnaCg( μ m/m 2 )d 003-001 521-001 007,51-057


49-05 85-73 042-7.3

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9.6.4 Polyvinyl Chloride (PVC)

Polyvinyl chloride films are formed by combining PVC resin, produced by additionpolymerisation of vinyl chloride, with plasticisers and other additives to produce a flexiblefilm. Unmodified PVC is quite brittle and difficult to process because of its heat sensitivity.However, because of its polar nature, PVC has a high affinity for plasticisers, and hencecan be substantially modified. Plasticisers generally consist of high-boiling-point organicliquids, which serve a lubricating function in the resin. Some soft and flexible PVC filmsare approximately 50% plasticiser by weight.

For food packaging uses, plasticisers and other ingredients must be suitable for directfood contact. The major plasticisers used in such applications are adipates. Often,epoxidised soybean oil is added as a secondary plasticiser. For non-food use, a widerrange of plasticisers is available. Adipates and phthalates are most common. In additionto plasticisers, PVC films contain stabilisers, as the resin is heat-sensitive. Oil epoxideshave some stabilising functionality, and in food packaging uses supplement the activityof calcium, magnesium or zinc stearates. Phosphites may also be used. In non-foodapplications, organometallic salts of barium and zinc are commonly used.

The properties of PVC films are strongly influenced by the type and level of modifyingingredients, especially plasticisers, that have been added. In general, the films are quitesoft and flexible, easy to heat-seal, and have excellent self-cling, toughness, resilienceand clarity. Permeability is relatively high. Both oriented and unoriented films are available.Properties of PVC film are listed in Table 9.2.

Heavier gauge PVC, sheet rather than film, is often used in thermoformed packaging,such as in blister packaging.

9.6.5 Polyethylene Terephthalate (PET)

Polyethylene terephthalate is formed by condensation polymerisation of ethylene glycoland either terephthalic acid or dimethyl terephthalate. It is commonly used in biaxiallyoriented form, and has excellent transparency and mechanical properties. Heat-settingenables the film to be used for extended periods at temperatures ranging from –70 to +150°C. It can tolerate considerably higher temperatures for short periods, such as in dualovenable packaging for frozen foods. PET has good barrier properties, especially for odoursand flavours. The barrier properties can be enhanced by coating with PVDC, or bymetallising, as will be discussed in subsequent sections. Coating or coextrusion is oftenused to provide good heat-seal properties. Typical PET properties are listed in Table 9.3.


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9.6.6 Polyvinylidene Chloride (PVDC)

Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is an excellentbarrier to oxygen, water vapour, odours and flavours. However, its high crystallinityand sensitivity to heat-induced degradation make it extremely difficult to process.Therefore, homopolymer PVDC is not used commercially.

Copolymerisation of vinylidene chloride with various amounts and types of comonomers,usually vinyl chloride, acrylonitrile, methacrylonitrile, methacrylates or alkyl acrylates,produces a family of PVDC copolymer resins with improved processability, while maintainingdesired barrier properties. Vinylidene chloride content typically ranges from 72 to 94 wt%;molecular weights range from about 65,000 to 150,000 [6]. In general, the highest barrierresins are not melt-processable, but instead are applied by solvent or latex coating. Extrudableresins have undergone more modification, so consequently have somewhat decreased barrierproperties. PVDC films produced for household use are plasticised copolymers, and haveeven poorer barrier performance. However, they remain much better barriers than competitivepolyethylene films. Representative properties are shown in Table 9.4.

PVDC copolymer films can be heat-sealed. Therefore, in PVDC copolymer coatings orcoextrusions, the PVDC can serve as a combination barrier and heat-seal layer. However,the best barrier films generally do not provide the best heat-seal capability, and viceversa, so when both heat-sealability and barrier are desired, sometimes two differentlyformulated PVDC copolymer coatings are applied.

]5,2,1[smlif)TEP(etalahthperetenelyhteylopfoseitreporplacipyT3.9elbaT

ytreporP remylopTEP

detneironU detneirO

Tg (° )C 08-37 08-37

Tm (° )C 562-542 562-542

(aPk554ta,erutarepmetnoitrotsidtaeH ° )C 921-83

mc/g(ytisneD 3) 04.1-92.1 04.1

)aPG(suludomelisneT 1.4-8.2

)aPM(htgnertselisneT 27-84 072-022

)%(noitagnolE 000,3-03 011-07

8.73ta,RTVW ° g(HR%09dnaC μ m/m 2 )d 015-093 044

O2 52ta,ytilibaemrep ° 01(C 3 mc 3 μ m/m 2 )mtad 4.2-2.1 1.1

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Nearly all cellophane produced in North America is solvent-coated with PVDCcopolymers. Solvent and latex coatings are also used on plastic sheet for thermoformedcontainers, and on blow-moulded plastic bottles. Common substrates include polyolefins,polyesters, polyamides and styrenics. Coextrusions of PVDC copolymer with polyethyleneor polypropylene are used in shrinkable films for meat, cheese and other moisture- oroxygen-sensitive foods. Latex coatings of PVDC copolymers are used to provide moistureresistance, grease resistance and barrier to paper and paperboard packages.

9.6.7 Polychlorotrifluoroethylene (PCTFE)

Polychlorotrifluoroethylene (PCTFE) is another polymer with good barriercharacteristics, especially for water vapour. The homopolymer is very difficult to processbecause of its extremely high melt viscosity. A small amount of modification bycopolymerisation yields AlliedSignal Corporation’s trademarked Aclar films, whichcontain greater than 95% chlorotrifluoroethylene by weight. These films are consideredthe best available transparent moisture barriers for flexible packaging; however, theyare rather expensive.

Aclar films can be used alone, or can be laminated to paper, polyethylene, aluminiumfoil or other substrates. The film is heat-sealable, and can be thermoformed. Aclar blisterpackages are often used for unit packages for highly moisture-sensitive pharmaceuticals.

]6,2,1[smlif)CDVP(edirolhcenedilynivylopfoseitreporplacipyT4.9elbaT

ytreporPremylopCDVP

-lareneGesoprup

-hgiHreirrab

Tg (° )C 2+ot51– 2+ot51–

Tm (° )C 271-061 271-061

(aPk554ta,erutarepmetnoitrotsidtaeH ° )C

mc/g(ytisneD 3) 17.1-06.1 37.1

)aPG(suludomelisneT 7.0-3.0 1.1-9.0

)aPM(htgnertselisneT 001-84 841-38

)%(noitagnolE 001-04 001-05

8.73ta,RTVW ° g(HR%09dnaC μ m/m 2 )d 97 02

O2 52ta,ytilibaemrep ° 01(C 3 mc 3 μ m/m 2 )mtad 34.0-13.0 130.0


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9.6.8 Polyvinyl Alcohol (PVOH)

Polyvinyl alcohol films are unique in several respects. Polyvinyl alcohol polymers areproduced by hydrolysis (more correctly, alcoholysis) of polyvinyl acetate. If fullyhydrolysed, the polymer is readily soluble in water. Controlling the degree of hydrolysiscan produce films that are soluble in hot water but not in cold water. Because PVOHdegrades at temperatures well below melt, it cannot be processed by extrusion. Therefore,casting from a water solution is used to make film. As produced, the film is amorphous,but orientation induces some crystallinity.

The water-solubility of PVOH is the major reason for its use in niche markets where thisis a desired attribute. One application is as an inner pouch in packaging of agriculturalor other chemicals, to limit human exposure. The pouch with its contents can be placedinto the dilution and dispensing apparatus, without direct contact between the user andthe chemical. In the water, the pouch dissolves, releasing the chemical. The dissolvedpolymer does not clog spray nozzles, and is biodegradable.

Another application is in hospital laundry bags. Here, the hot-water-soluble variety isused. Soiled laundry is placed in the bags, and then bag and all can be placed into thewasher, so that no contact between the launderer and the potentially infectious linen isrequired. Since the polymer does not dissolve in cold water, it will not be affected byresidual liquid in the linens, but will dissolve readily in the hot wash water.

9.6.9 Ethylene-Vinyl Alcohol (EVOH)

Ethylene-vinyl alcohol resins are produced by hydrolysis (alcoholysis) of ethylene-vinylacetate random copolymer, analogous to the route for production of polyvinyl alcoholfrom polyvinyl acetate. Commercially available materials contain a substantialpercentage of ethylene, typically 27 to 48 mol%. The presence of ethylene renders theresins melt-processable.

The presence of –OH groups in the structure results in strong intermolecular hydrogenbonding. While EVOH is a random copolymer, CH2 and CHOH groups are isomorphous;they fit into the same crystalline structure. Therefore, the polymer crystallises readily.The combination of strong intermolecular forces and crystallinity makes it an excellentbarrier to gases, odours and flavours. However, the hydrogen bonds also make it amoisture-sensitive material, and high humidity decreases its barrier capability.

EVOH is most often used as an oxygen barrier. Since, in most applications, it is likely tobe exposed to moisture either from the environment or in the product, it is usually usedas a buried inner layer in a coextruded structure, where a good moisture barrier, often a

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polyolefin, protects it. Monolayer EVOH films, oriented or unoriented, are also available,which can be used alone but are usually combined with other materials by laminating, orcoating. Typical EVOH properties are listed in Table 9.5.

9.6.10 Polyamide (Nylon)

Polyamides, or Nylons, are a family of plastics containing characteristic amidefunctionality. They are commonly formed by condensation polymerisation of amino acids,or of carboxylic acids and amines.

Nylon films are used for specialty applications in packaging, where performancerequirements justify their relatively high cost. Nylons have excellent high-temperatureperformance, so can be used, for example, in boil-in-bag packages. Nylons also provideexcellent odour and flavour barrier, and reasonably good oxygen barrier. They are verypoor water vapour barriers, and generally have a tendency to lose some barrierperformance when exposed to large amounts of moisture. However, their performance isnot as water-sensitive as EVOH.

Most Nylons used in packaging have some crystallinity; the amount is heavily dependenton processing conditions, since Nylons have a narrow window for crystallisation. Filmsgenerally retain good flexibility at low temperatures, and have excellent strength properties.Owing to their relatively high cost, they are often coextruded with other plastics.

]7,2,1[smlif)HOVE(lohoclalyniv-enelyhtefoseitreporplacipyT5.9elbaT

ytreporPremylopHOVE

%lom23enelyhte

%lom44enelyhte

Tg (° )C 96 55

Tm (° )C 181 461

(aPk554ta,erutarepmetnoitrotsidtaeH ° )C

mc/g(ytisneD 3) 91.1 41.1

)aPG(suludomelisneT 6.2 1.2

)aPM(htgnertselisneT 77 95

)%(noitagnolE 032 083

04ta,RTVW ° g(HR%09dnaC μ m/m 2 )d 5351 427

O2 52ta,ytilibaemrep ° 01(C 3 mc 3 μ m/m 2 )mtad 8700.0 030.0


250


Polyamides manufactured from straight-chain amines and carboxylic acids are typicallynamed with numbers representing the number of carbons in each of the starting monomers.For example, Nylon-6,10 is made from a six-carbon amine and a ten-carbon carboxylicacid. Similarly, polyamides made from amino acids have a number designating the numberof carbons in the acid. When the carbons are not in a straight chain, more complex namesare necessary. Typical properties of some Nylon films are given in Table 9.6. Nylon-6 tendsto be the most-used Nylon packaging film in the USA, and Nylon-11 in Europe.

9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films

Ethylene-vinyl acetate is produced by addition copolymerisation of ethylene and vinylacetate. The acetate groups provide polar functionality that increases intermolecularforces in the film, and, because of the structural irregularity thus introduced, interferewith crystallisation. These films have excellent transparency, and provide very good heat-seal and adhesive properties, with excellent toughness at low temperatures. Typical film-grade EVA resins contain between 5 and 18% vinyl acetate. Resins designed for use asan adhesive layer in a multilayer structure are typically at the higher end, and stand-alone films at the lower end, of this concentration range.

]2,1[smlif)nolyN(edimaylopfoseitreporplacipyT6.9elbaT

ytreporPremyloP

6-nolyN 11-nolyN-DXMnolyN

6

Tg (° )C 06 46

Tm (° )C 022-012 091-081 342


mc/g(ytisneD 3) 61.1-31.1 50.1-30.1 52.1-02.1

)aPG(suludomelisneT 7.1-96.0 3.1 1.4-8.3


)%(noitagnolE 003 004-003 67-27

04ta,RTVW ° HR%09dnaCg( μ m/m 2 )d

003,4-009,3 000,2-000,1 036


20.1-74.0 5.21 62.0-60.0

ASU,YN,kroYweN,.cnI,aciremAslacimehCsaGihsibustiM:6-DXMnolyN

251

Common markets for EVA are poultry and meat wrap, stretch film and ice bags. Thefilms tend to be sticky, so may require the use of slip and antiblock additives.

Copolymers of ethylene with acrylic acid and with methacrylic acid are also available,and are commonly called acid copolymer resins. They are characterised by good clarity,strong adhesion to polar substances such as paper, and also to foil, and low melt andheat-seal temperatures.

9.6.12 Ionomers

Ionomers are formed by neutralisation of ethylene-acrylic acid or ethylene-methacrylicacid copolymers containing 7 to 30 wt% acid, to yield sodium or zinc salts. The resultingionic bonds function as reversible crosslinks in the polymer, readily disrupted by heat,but reforming on cooling. Therefore, these materials provide very strong bonding tonumerous substrates. Ionomers can be used for skin packaging to uncoated corrugatedboard, for example.

The heat-seal performance of ionomers is outstanding, even permitting sealing throughgrease contamination, which makes them ideal for packaging of processed meat. Theyhave superior hot tack, and excellent melt strength.

Ionomer films have excellent clarity, flexibility, strength and toughness. They can beused to package sharp objects, which break through many alternative materials whensubject to vibration during distribution. Ionomers have relatively poor gas barrier, andtend to absorb water readily. They also are relatively high cost compared to films such asethylene-vinyl acetate.

9.6.13 Other Plastics

Several other types of plastics are used in packaging films to some extent. Polycarbonatefilms have excellent transparency, toughness and heat resistance, but high cost. Theyhave some use in skin packaging, food packaging where exposure to high temperaturesfor in-bag preparation is required, and medical packaging.

Polystyrene is another film with excellent transparency, often used in window envelopesand window cartons. It has low gas barrier, so can be used for produce where a ‘breathable’film is required. In heavier gauges, polystyrene is widely used for transparentthermoformed trays. Expanded polystyrene is used for trays, egg cartons and otherapplications where its cushioning properties are desired. In general, these materials are


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classified as sheet, rather than film. Polystyrene film is generally biaxially oriented toimprove its properties, since the unmodified material is too brittle for most applications.Impact-modified polystyrene sheet that incorporates polybutadiene is often used wheretransparency can be sacrificed for impact resistance.

Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionateand copolymers are also used to a relatively small extent, most often as sheet rather thanfilm. Their high price and water sensitivity limits their usefulness.

A wide variety of copolymers are available. Some of these have been discussed already. Itis quite common to modify the chemical structure of a polymer to obtain a more desirablemix of properties. Another way to combine properties is to use blends of polymers.High-impact polystyrene (HIPS) is actually partially a copolymer and partially a blendof polybutadiene and polystyrene.

9.7 Multilayer Plastic Films

In many cases, the best combination of packaging attributes at the lowest cost is achievedby using a combination of materials. Therefore, plastic packaging films are often combinedwith one another or with other materials such as paper, aluminium or even glass, throughprocesses such as coating, lamination, coextrusion and metallisation.

9.7.1 Coating

Coating is commonly used to add a thin layer of a plastic on the surface of anotherplastic film or sheet, or, more commonly, on a non-plastic substrate such as paper,cellophane or foil. The coating may be applied as a solution, a suspension, or a melt.

Common reasons for using coating in flexible packaging are: to impart heat-sealabilityfor paper, cellophane, foil or plastics that are not themselves easily heat-sealed; to providemoisture protection for paper or cellophane; to improve barrier properties; and to provideprotection from direct contact of the base material with the product.

Coating with low-density polyethylene is often used on paper to give heat-sealability andmoisture protection, as well as to protect printing from abrasion. It is often used onaluminium foil to provide heat-sealability and abrasion resistance, and to preventinteraction between the foil and the product. PVDC copolymer coatings are often usedto improve barrier and heat-sealability.

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9.7.2 Lamination

Lamination is the process of combining two webs of film together. In flexible packagingapplications, lamination is often used to combine a plastic film with paper or foil, or tojoin paper and foil together. A variety of lamination methods are used. When plastic filmsare involved, either as a substrate or as an element in the finished structure, the laminatingadhesive is often low-density polyethylene, applied by extrusion, and the process is knownas extrusion laminating. When paper is contained in a flexible package, it is most oftenbeing used for its excellent printability, along with its ability to impart substance andstrength. When aluminium is used, it is most often employed for its excellent barrier tolight and to permeation. Occasionally, it is used primarily for its desirable appearance.

Another significant use of lamination is to produce a web with buried printing. In thesematerials, one web is reverse-printed, and is then laminated to a second web, eithermade from the same or a different polymer. The printing can be seen through thetransparent plastic, and is protected against abrasion so it maintains a fresh attractiveappearance much better than surface-printed materials.

9.7.3 Coextrusion

Coextrusion results in the production of a multilayer web without requiring initialproduction of individual webs and a separate combining step. The melted polymers arefed together carefully to produce a layered melt, which is then processed in conventionalways to produce a plastic film or sheet. When only plastics are being used in a flexiblepackaging structure, coextrusion is generally preferred to lamination, unless buriedprinting is involved. Obviously, coextrusion cannot be used to incorporate non-thermoplastic materials.

A major advantage of coextrusion over lamination is its ability to incorporate very thinlayers of a material, much thinner than those which can be produced as a single web.This is particularly important for expensive substrates, such as those often used to impartbarrier properties. The amount of the expensive barrier resin used need only be enoughto provide the desired performance. The thinness of the layer is not limited by the needto produce an unsupported film and handle it in a subsequent lamination step.

9.7.4 Metallisation

Metallisation is a way of applying a thin metal layer on a plastic film (or on paper), as analternative to using a lamination with aluminium foil. In commercial packaging practice,the metal being deposited is almost always aluminium. The process, known as vacuum


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metallising, involves evaporation of aluminium inside a vacuum chamber, and depositionof the aluminium vapour on a plastic film. The operation is usually done in a batchmode, with the substrate being metallised and an aluminium wire placed inside the vacuumchamber. The film is rolled past a chill roll, which removes the heat from the condensingaluminium, preventing melting of the film. Very high vacuums are needed, which hasretarded the development of continuous metallisation processes, although some areavailable. Vaporisation of the aluminium is most often achieved by resistance heating.Induction and electron-beam heating are used to a lesser extent.

Metallised films have significantly enhanced barrier characteristics, and are usually chosenfor this reason. Cost of metallised film is generally less than that of foil-containinglaminated materials. In snack packaging, for example, metallised film has almost totallyreplaced foil laminations.

The barrier performance of metallised film, as initially produced, is somewhat inferior tofoil, and is dependent on the thickness of the deposited metal layer. However, stressduring product distribution can lead to the development of flex cracks in foil, whichthen provide a route for gas transfer. Metallised foil, since it retains the flexibility andother mechanical characteristics of the film substrate, is not usually subject to flex cracking.Therefore, the barrier characteristics of metallised foil are sometimes superior to thoseof foil laminations, at later points in distribution. Also, many oxygen-sensitive productsrequire better barrier than can be attained with plastic alone, but can be successfullyprotected with metallised film. In addition to gas barrier, metallised film provides anessentially total light barrier.

Occasionally, metallised foil is used for its appearance, rather than for its barriercharacteristics. This is particularly the case when it is used for labels. In many suchapplications, however, paper, rather than film, is the metallisation substrate.

9.7.5 Silicon Oxide Coating

One of the disadvantages of metallised film is that the resultant material is opaque, andis not suitable for use in microwave ovens. The desire for transparent high-barrier coatingsled to the development of glass-like coatings based on silicon oxide, SiO2.

Silicon oxide coatings on film are usually applied in a manner analogous to vacuummetallising. The silicon oxide is evaporated, using electron-beam heating, and condensedon the film substrate in a vacuum chamber. The film is very thin, 400 to 1000 Å, anddoes not affect the mechanical properties of the material to any significant degree. Thechemical composition of the deposited film depends somewhat on conditions, and is

255

characterised as SiOx, where the value of x is between 1.0 and 2.0. At values close to 1,the layer imparts a distinct yellowish colour to the film. At values close to 2, it is nearlycolourless. The yellowing is a significant concern in some applications. The SiOx layergreatly increases barrier of the film, and it is transparent to microwave radiation.Therefore, it can be used in packages that will be heated in microwave ovens. The mostcommon substrate is PET, in thickness of 12.5-25 μm, although polypropylene, polystyreneand polyamides can also be used.

An alternative to evaporative deposition is chemical plasma deposition, in which a silicon-containing gas such as tetramethyldisiloxane or hexamethyldisiloxane is used as the silicasource. Little heat is required, and the degree of vacuum needed is lower. Therefore,plasma deposition can be used on heat-sensitive materials such as LDPE and oriented PP.The coating produced is thinner, and less yellow. Plasma deposition is the method ofchoice for SiOx coating of containers, and can also be used for film.

9.7.6 Other Inorganic Barrier Coatings

Processes have also been developed that deposit aluminium oxide coatings on plastic films,to increase barrier properties. Combinations of SiO and MgO have also been used.

Another type of inorganic barrier coating uses clay nanocomposites, which are deposited onthe film from a solution of PVOH/EVOH copolymer, in a mix of water and isopropyl alcohol,with nanodispersed 7 nm diameter silica and titanium dioxide particles. Microgravureequipment is used to coat the solution on to the film substrate. Barrier is reportedly comparableto that of films metallised with aluminium, but the coatings are transparent.

These materials are all still in relatively early phases of development.

9.8 Surface Treatment

In many packaging applications, it is necessary for something to stick to a plastic film.This may involve placing a label on a pouch or on a stretch-wrapped pallet, adheringtwo films together in a lamination, or, as is often the case, printing the film. Adequateadhesion requires that secondary bonding forces between the film and the object, such asthe ink, which is to be adhered, be sufficient to retain the material. Historically, this hasbeen a significant problem for plastic films, since the surface energy of the films is oftenlow, causing poor adhesion. Several techniques are commonly used to increase the surfaceenergy of polymers, hence improving adhesion. For films, the most common treatment iscorona discharge.


256


In corona discharge treatment, the film surface is exposed to a discharge between groundedand powered electrodes, at high voltage. The discharge of the electric current ionises theair in the gap between the electrodes. The ions produced initiate free-radical reactionswith the film surface, causing bond cleavage followed by oxidation. Oxidation of thesurface increases its ability to adhere to substances such as inks and adhesives. Theeffectiveness of corona discharge treatment dissipates with time, so ideally it should beapplied within a short time of the subsequent printing. In a roll of corona discharge-treated material that has been stored, the effectiveness of the treatment is likely to besignificantly higher on the inside layers than on the first few outside layers of film.

Other surface treatments that are sometimes applied to film also exist. However, coronadischarge is by far the most frequently encountered.

9.9 Static Discharge

Plastics, because they are nonconductive, are subject to build-up of electrostatic charges.When such charges build up, the result can range from the attraction of dust and lint tomaterial handling problems, shocks and sparks. Methods for controlling the build-up ofstatic charges include charge neutralisation through ionisation of the surrounding airand incorporation of conductive materials to dissipate the charge.

Antistatic agents can be incorporated into the film as additives, or can be used as asurface treatment. The agents commonly used include non-ionic ethoxylated alkylamines,anionic aliphatic sulfonates and phosphates, and cationic quaternary ammoniumcompounds. In some cases, humidifying the area can control static, so that a thin layer ofwater is absorbed on the film surface, which conducts the charge to ground.

Control of static discharge is especially important for packaging for sensitive electronicdevices. Film designed for such applications, usually polyethylene, is generally pigmentedpink to denote that it contains antistatic agents.

9.10 Printing

In many packaging applications, plastic films are printed to convey information to the user.When printing is desired, it is usually done on roll stock before packages, such as pouches,are formed. Printing on formed flexible packages is usually limited to date or lot coding.

Flexography is the printing method used most often for flexible packaging materials. Inthis process, a subcategory of relief printing, the printing plates are flexible elastomers,

257

with the images, or printing areas, raised above the nonprinting surrounding areas. Thin,highly fluid, rapid-drying inks are used. The ink is transferred by a system of rollers tothe top surface of the printing plates, which in turn transfer the ink to the film.

In lithography, the printing image and the background are on the same plane of the thinmetal printing plate. The plate is treated to attract water and repel ink in the non-imageareas, and the reverse in the image areas. A system of rollers is used to transfer both inkand water to the plate. The image on the plate is then transferred (offset) to a rubber-blanket-covered cylinder, and then to the film.

Rotogravure uses copper-plated printing cylinders, which have the image engraved intothe cylinder in the form of tiny cells. The cylinder rotates in an ink bath, filling the cellswith ink. Excess ink is wiped off by a doctor blade, and then the image is transferred to thefilm as it is pressed against the printing cylinder by an elastomer-covered impression cylinder.

For printing date and lot codes on formed packages, ink-jet printing is commonly used.In this process, electrically charged drops of ink are sprayed out of jets, and electrostaticallydirected to the desired printing location. This is an impactless form of printing, and isideal for printing rapidly changing information such as these codes.

Other types of printing, such as screen printing, as well as variations of the basic processesdescribed above, are used less frequently for plastic packaging films.

9.11 Barriers and Permeation

As has been discussed, in many packaging applications, protection of the product fromgain or loss of gases or vapours is important. The mechanism by which substances travelthrough an intact plastic film is known as permeation. It involves dissolution of thepenetrating substance, the permeant, in the plastic, followed by diffusion of the permeantthrough the film, and finally by evaporation of the permeant on the other side of thefilm, all driven by a partial pressure differential for the permeant between the two sidesof the film.

The barrier performance of the film is generally expressed in terms of its permeabilitycoefficient. For one-dimensional steady-state mass transfer, the permeability coefficientis related to the quantity of permeant, which is transferred through the film as representedby the equation:

PQl

At p=

Δ (9.1)


258


where P is the permeability coefficient, Q is the mass of permeant passing through thematerial, l is the thickness of the plastic film, A is the surface area available for masstransfer, t is time, and Δp is the change in permeant partial pressure across the film.

It can be shown that the permeability coefficient, as defined by equation (9.1), is equal tothe product of the Fick’s law diffusion coefficient, D, and the Henry’s law solubilitycoefficient, S, in situations where these laws adequately represent mass transfer (ideallydilute solutions, diffusion independent of concentration):

P DS= (9.2)

The permeability coefficient, under these circumstances, is a function of temperature,but is not a function of film thickness or permeant concentration.

While this is a very simplified approach to mass transfer, it is adequate for many packagingsituations. For example, with oxygen-sensitive products, reaction with oxygen iscommonly rapid compared to the rate of transfer, so the oxygen concentration withinthe package is relatively constant at nearly zero. Oxygen concentration in the surroundingair, measured as partial pressure, is constant at approximately 21 kPa. Regardless of theshape of the flexible package, mass transfer is essentially one-dimensional, through thethickness of the film. If temperature is constant and P is known, the amount of oxygentransported through the film in a given period can be easily calculated using equation(9.1). Conversely, if the sensitivity of the product is known in terms of the maximumamount of oxygen that can be taken up without resulting in unacceptable product quality,the time required for that amount of transfer (the product shelf-life) can be calculated.

A similar approach can often be taken for transfer of odour or flavour compounds.While the diffusivity, and hence the permeability coefficient, of such organic substancesis likely to be concentration-dependent, at the low levels associated with most packagingsituations, the dependence is slight.

Calculating shelf-life when water vapour transmission is involved is more problematic.In such cases, the partial pressure difference for water vapour between the inside and theoutside of the package is almost never constant. Simplifying assumptions generally usedconsider the time for moisture in the product itself and in the product headspace to reachequilibrium to be small compared to the time required for permeation, and ignore moisturechange in the headspace itself, calculating only moisture gain or loss in the product. Theresulting differential equation is:

ddQt l

PA p p= −( )12 1 (9.3)

259

where p1 is the partial pressure of water vapour outside the package, p2 is the partialpressure of water vapour inside the package, and p2 is a function of Q. Solution of thisequation requires knowledge of the moisture sorption isotherm for the product, whichrelates the moisture content of the product to the equilibrium relative humidity of the airin contact with the product, and thus to p2.

In the case where the sorption isotherm at the storage temperature can be approximatedas linear over the range of moisture contents of interest, it can be written as:

W = a + bM (9.4)

where W is the water activity of the air in equilibrium with the product with moisturecontent M (dry weight basis), and a and b are the best-fitting straight-line constants.Rewriting the basic permeability equation [equation (9.3)] in terms of water activity,and substituting:

Q = (M – Mi)w (9.5)

where Mi is the initial moisture content and w is the dry weight of the product gives:

wMt

PApl

W Wdd

s= −( )2 1 (9.6)

where W1 and W2 are the water activities at times 1 and 2, and ps is the saturation watervapour pressure at the storage temperature.

This equation can be integrated, giving the following relationship for moisture gainor loss:

-b

W W

W WPAp t

lw1 2 1

2 1 0

ln−[ ]−[ ]

⎛

⎝⎜

⎞

⎠⎟ =t s

(9.7)

Mass transfer characteristics for plastics are often expressed in terms of water vapourtransmission rates (WVTR), rather than permeability coefficients. WVTR reflect the rateof water vapour transfer under specific conditions, and must be translated to permeabilitycoefficients for application at different conditions. The relationship between Pwater andWVTR is the following:

Ppwater = WVTR

Δ (9.8)


260


where Δp is the difference in water vapour partial pressure under the conditions at whichthe WVTR was measured. In many cases, this was the ASTM standard condition of 32.2°C (90 °F) and 90% relative humidity (RH).

When permeability coefficients are not available at the temperature of interest, anArrhenius relationship can be used to determine the required value, from thepermeability coefficient at a nearby temperature and the activation energy. The equationused is the following:

P PER T T2 1

1 2

1 1= ⎛⎝⎜

⎞⎠⎟

−⎡

⎣⎢

⎤

⎦⎥

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪exp a

(9.9)

where T1 is the temperature at which P1 is known, T2 is the temperature at which P2 is tobe calculated, Ea is the activation energy, and R is the gas constant.

Care must be taken in applying equation (9.9). The permeability coefficient, as indicated,is a product of the diffusion coefficient and the Henry’s law solubility constant. Sincethese vary in different ways with temperature, equation (9.9) is valid only over reasonablysmall temperature ranges. A particular concern is that permeation rates are much higherabove the Tg than below this temperature, and the rate of change with temperaturediffers. Therefore, equation (9.9) should never be used to calculate the permeabilitycoefficient across a temperature range that spans Tg of the plastic.

Permeability coefficients for multilayer plastic film or sheet, either coextrusions orlaminations, can be calculated from the thickness and permeability coefficients of theindividual layers, as follows:

Ptt

i ii=

i=n

l

l P=

( )∑ /1

(9.10)

where the subscript t indicates the value for the total structure, i indicates the value foran individual layer, and there are n layers in the structure.

Special care must be taken when the barrier characteristics of a polymer are affected bythe presence of the permeant or of some other substance that may also be permeating.This situation is most often encountered with water-sensitive plastics, such as ethylene-vinyl alcohol, since co-permeation of water vapour and other components of interest,such as oxygen, may well occur during processing and storage. It may also arise in othersituations, such as co-permeation of organics involved in odour and taste.

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9.12 Environmental Issues

In recent years, consideration of the environmental effects of packaging decisions hasbecome more common. While thorough discussion of such issues is beyond the scope ofthis chapter, some general observations and conclusions will be made.

It is generally agreed that evaluation of the environmental impacts of a product or packagerequires consideration of the total life-cycle of the object. Such ‘cradle to grave’ analysisis commonly referred to as life-cycle assessment. Usually, when such analyses are carriedout, the most influential life-cycle stage is that of production of the raw materials andpackages, rather than transportation or disposal. Packages that minimise material useare therefore likely to have reduced environmental impact. Since flexible packaging systemsusually (although not always, since distribution packaging must be included) use lessoverall packaging material, they often have reduced environmental impact, compared tothe rigid packaging systems they replace.

In examining the impacts of waste disposal, two general conclusions can be drawn. Inmost cases, flexible packaging is less likely to be recovered for recycling than rigid orsemi-rigid packaging. Therefore, a higher proportion of flexible packaging is likely torequire disposal. On the other hand, flexible packaging, as discussed above, usuallymeans less total material requires handling. Unless recycling rates for the alternatives tothe flexible packages are very high, use of flexible packaging is likely to mean less materialrequiring disposal. Also, flexible packages containing plastics are sources of recoverableenergy in appropriate systems.

References

1. R.J. Hernandez, S.E.M. Selke and J.D. Culter, Plastics Packaging: Properties,Processing, Applications, and Regulations, Hanser, Munich, Germany, 2000.

2. S.E.M. Selke, Understanding Plastics Packaging Technology, Hanser, Munich,Germany, 1997.

3. D. Kong in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brodyand K.S Marsh, Wiley, New York, NY, USA, 1997, 407.

4. E. Mount and J. Wagner in The Wiley Encyclopedia of Packaging Technology,Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 415.

5. J. Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brodyand K.S. Marsh, Wiley, New York, NY, USA, 1997, 408.


262


6. P. DeLassus, W. Brown and B. Howell in The Wiley Encyclopedia of PackagingTechnology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA,1997, 958.

7. R. Foster Newton in The Wiley Encyclopedia of Packaging Technology, Eds.,A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 355.

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10 Applications of Plastic Films in Agriculture

E.M. Abdel-Bary, A.A. Yehia and A.A. Mansour

10.1 Introduction

The quantity of plastic materials used annually in the world in the agricultural sectoramounts to 2 million tons. About 50% of this is used in protected cultivation greenhouses,as mulching, for low tunnels, as temporary coverings of structures for fruit trees, etc. [1].Thin plastic film produced with low investment is economically and technically feasible,and provides the best cost/benefit ratio for use in greenhouses and low tunnels. The areacovered by both greenhouses and tunnels has been experiencing continual growth. Thisgrowth is expected to appear in many countries where protected cultivation replaces thetraditionally used more expensive glass-clad greenhouses.

Low-density polyethylene (LDPE), ethylene-vinyl acetate (EVA) and linear low-densitypolyethylene (LLDPE) films are the most common greenhouse covering films in agriculture.This chapter looks at the production of polyethylene-based plastic films for protectedcultivation. The mechanical properties that make these films suitable for the use in agricultureare discussed. The stability of these plastic films under the effects of different environmentalconditions is reported. These include solar irradiation, temperature, humidity, wind, fogformation and pesticides. Types of ultraviolet (UV) stabilisers and a determination of theircompatibility are given. Also, the recycling of plastic films used in agriculture is of greatimportance, and a case study of their recycling as agricultural films is given.

10.2 Production of Plastic Films

LDPE films dominate the market for protected cultivation in the countries of both theMediterranean region and worldwide. Most of these contain special additives, which areused either to enhance the performance of the film in the special conditions met in agreenhouse, or to prolong its lifetime by minimising the effects of the environment onthe structure of the film.

Advances in the formulation of the LDPE films in use today have led to an expectedlifetime of between one and five cultivating seasons [2]. The expected lifetime is, in fact,significantly affected by the environmental conditions that the film will face during itsuse. The climate of the region, the greenhouse design, the microclimate developing inside


the greenhouse, the use of agrochemicals and the environmental pollution of the areacan all severely affect the lifetime of the material by inducing ageing of the plastic film tovarious degrees. Thus, a film whose lifetime is estimated to be four seasons in North-Central Europe will only last two or three seasons in the Mediterranean [2].

The varying requirements for greenhouse systems between different regions because ofdifferent climatic conditions and differences in production methods has led, until veryrecently, to a significant variation in approaches, standards and practices adopted orimplemented by the interested National Research Institutions, Commercial Agencies andrelevant industries [3]. Some of the consequences of this differentiation and variabilityare reflected in the lack of standardisation concerning the testing methods for coveringmaterials of greenhouses. Usually, the testing methods used for plastics in general arealso applied to greenhouse covers, despite important functional differences. As aconsequence, quality control data provided by the producers of the covering materials ofgreenhouses are usually limited to only a few properties of these materials. In most cases,it is not possible to reproduce the relevant technical information provided, as this is notobtained systematically and is available in a somewhat confusing way [3]. Themanufacturing of plastic films used in agriculture is usually carried out by blown filmextrusion (tubular extrusion). Readers are asked to consult Chapters 1 and 2, where themanufacturing process is given in detail.

10.3 Characteristics of Plastic Films Used in Agriculture

The film products of interest here have been evaluated for their applied efficiency on thebasis of their characteristics and requirements related to mechanical resistance, totalpercentage light transmittance (T %) to visible solar and long-wavelength ultraviolet(UV-A) radiation, useful lifetime and energy-saving potential (greenhouse effect). Visiblesolar radiation regulates the nutrition of plants through the ‘chlorophyll function’. Long-wavelength ultraviolet radiation favours the formation of pigments and vitamins, whichis advantageous for the quality characteristics of the crop involved, with regard to flavour,intensity of colour, perfume or smell and good keeping of fruit or vegetables.

With reference to energy saving, values of the total thermal transmittance measured inW/m2 °C for the different covering materials and their possible combinations have beenestimated. The total thermal transmittance of a covering material indicates the generalheat loss (management, convection, radiation), estimated in watts, through a 1 m2 surfacereferred to a difference of 1 °C between the internal and external environmentaltemperatures of the prepared covering. These values enable estimation of the theoreticalthermal yield of a manufactured film related to the heating needs for a thermal differenceof 1 °C across the film.

265

Applications of Plastic Films in Agriculture

The incident heat calculation enables the agricultural operator to choose the propercovering material with respect to any thermal (°C) and luminous (flux) needs of thespecies to be established in the agricultural crop rotation desired. This can be done byusing the results showing the total thermal transmittance (W/m2 °C) and total lighttransmittance (T %) of some materials for greenhouse covering [4].

10.4 Stability of Greenhouse Films to Solar Irradiation

The performance and lifetime of the plastic films used as covering materials in protectedcultivation depend strongly on: (a) the original chemical structure of the materials, (b)the change in the properties of the material brought about by induced ageing, (c) thetype of physical structure used, (d) the climatic conditions of the area where the structureis installed, and (e) the use of agrochemicals, among other things. A brief description ofthe factors affecting the stability of polyethylene (PE) as a greenhouse covering under theeffect of different environmental conditions is given below.

It is well known that photodegradation of many plastic materials occurs on subjectingthese materials to solar radiation with wavelengths of 290-1400 nm [5, 6], the mostenergetic part of the solar spectrum. UV radiation in the range 290-400 nm can beabsorbed by the plastic, and this is followed by bond cleavage and depolymerisation,causing photodegradation.

The photodegradation process of the covering materials of a greenhouse is furthercomplicated by various interacting factors. The effect of UV radiation combined withvarying temperature, humidity, critical mechanical loads, friction, abrasion, exposure toagrochemicals, etc., accelerate ageing at various rates. Accordingly, it is difficult to predictthe lifetime of plastic films by laboratory testing of the photostability of films. For instance,high abrasion of the film by sand or soil particles carried by the wind leads to the formationof high concentrations of active centres giving rise to an increase in photodegradation.

10.4.1 Ultraviolet Stabilisers

Theoretically, LDPE should be stable under the effect of UV due to its stable structureand the absence of chromophores. However, during processing, it suffers partial oxidation,in which carbonyl and hydroxyl groups are formed. Also, it contains some impurities(photo-absorbing chromophores). Both impart photosensitivity to LDPE films [7, 8].

Special measures are therefore needed in order to protect greenhouse films against solarradiation and especially its most energetic and therefore harmful portion, namely, UV


radiation. Inhibition or at least retardation of the reaction responsible for degradationis, of course, a necessity for successful UV stabilisation. Retardation or protection againstphotodegradation can take place by using additives. These additives may retard thephotodegradation of the polymer in three ways, namely, ultraviolet screening, ultravioletabsorption and excited-state quenching. Thus, stabilisers are often included in the polymerto provide stability against photooxidation to protect the material from UV light damage.The effectiveness of a light stabiliser depends on many factors, including its solubilityand concentration in the polymer matrix [9].

10.4.1.1 Ultraviolet Screening

Ultraviolet screening compounds are based on inorganic or organic additives. In thistype of protection, the ultraviolet light is blocked before it can reach the polymer. Screeningis provided by pigments or by reflective coatings. Carbon black is also very effective andis used to stabilise many outdoor grades of polymers. In this case of UV screeners, anydamage is confined to surface regions because UV penetration is restricted to very shortdistances. However, many of the pigments, like chalk, talc, short glass fibres and carbonblack, impart an unattractive appearance, the grey, brown and black colours generallybeing unappealing. TiO2 is another common additive, which may act as a screener, but itmay occur in different forms, some of which are chemically active and can promotephotodegradation.

The first class of organic additives for improving the resistance to UV radiation is the UVabsorbers. They act by absorbing the harmful UV radiation above 290 nm, and thus donot allow it to reach the chromophores present in the chemical structure of LDPE as aresult of processing or as impurities.

Many organic compounds absorb light in the desired region but few act as stabilisers.Some have little or no effect when added to polymers and may actually increase the rateof degradation. For a UV absorber to be effective, it must be able to dispose of its excitationenergy without interacting with the polymer in harmful ways and without undergoingany photochemical reaction that would destroy its effectiveness. Accordingly, a stabilisermust have a structure that provides a rapid cascade back to the ground state throughthermally excited levels with a quantitative efficiency for return to the ground state notless than 0.999%, i.e., less than one molecule can be destroyed for every 100,000 moleculesthat are excited.

Derivatives of o-hydroxybenzophenone or benzotriazole are examples of UV absorbers.However, this class of stabilisers seems to perform better in thicker materials and notwell in the thin LDPE greenhouse films [8].

267

10.4.1.2 Excited-State Quenchers

The second class of UV stabilisers is the nickel excited-state quenchers. These quenchersact by deactivating the excited states of the chromophoric groups responsible for thephoto-initiation by energy transfer, instead of relying on direct absorption of the UVradiation [8]. With proper selection of the Ni quenchers, the results of the UV stabilisationare satisfactory. A typical example of a nickel excited-state quencher is nickeldibutyldithiocarbamate. However, formulations containing such Ni quenchers areprohibited because of the environmental impact of nickel compounds.

10.4.1.3 Hindered-Amine Light Stabilisers (HALS)

HALS, based on bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, are the most recent andinnovative class of light stabilisers. HALS do not absorb any light above 250 nm and socannot be regarded as UV absorbers or as excited-state quenchers. Through oxidation ofthe piperidyl group to the nitrosyl group, a radical becomes available that starts a veryefficient cycle of radical scavenging and peroxide decomposition. Thus, HALS areconverted to the corresponding nitroxyl radicals, which are the real species responsiblefor polymer stabilisation. Hindered nitroxyl radicals are effective chain breakingantioxidants that act by trapping alkyl radicals to give hydroxylamines and/oralkylhydroxylamines – the former regenerates nitroxyl. The overall high efficiency ofHALS as UV stabilisers in polyolefins is attributed to the regeneration of the nitroxylradical. The complementary nature of the chain breaking antioxidant mechanisms involved[10] are shown in Scheme 10.1. From these reactions, nitroxyl and alkoxyl radicals areformed according to equations (10.1) and (10.3)-(10.5). These radicals act as scavengersfor any radicals formed during UV irradiation [equation (10.2)].

This means that HALS operate as excellent antioxidants. Some of the HALS containfurther antioxidant groups; others are polymeric and less extractable. The main differencebetween UV absorbers and HALS is that the former absorb UV radiation and in turn aredestroyed by it, while the latter do not absorb UV radiation and are much more slowlyaltered by secondary side reactions.

Thus HALS act as radical traps for radicals produced from photochemical oxidation [8,9]. They offer an excellent approach to ultraviolet stabilisation and have replaced nickelquenchers and ultraviolet absorbers in many applications. Highly efficient chemicallyresistant light stabiliser systems have been developed. Market demands for extended-lifegreenhouses and thinner mulch films require even more powerful stabilisers. New non-interacting chemistries based on alkoxylamine HALS will offer a new generation ofstabilisers for agricultural polyethylene films.



N H Nhν, [O]

O•

ΔH(10.1)

N ON O• + P• P (10.2)

N O N O• + POOPP + POO• (10.3)

N• + H2O + PO•NH + POOH (10.4)

N• + POO• NO• + PO•(10.5)

Scheme 10.1

10.4.2 Requirements for Stabiliser Efficiency

The effectiveness of long-time stabilisation depends not only on the chemical nature butalso on the rate of additive loss, which in turn depends on the compatibility of theadditive with the polymer and is controlled by its volatility, solubility and diffusioncoefficient.

10.4.2.1 Compatibility of the Additive

Compatibility is the main problem for light stabilisers, because light stabilisers are generallyused in concentrations up to 2%. Ideally, stabiliser molecules should be disposed singlythroughout the polymer matrix. This will not generally happen, but compatibility withthe polymer should be sufficient to prevent gross phase separation.

269

Stabilisers are normally dissolved in the polymer melt at the processing temperature.However, their solubility limit may be exceeded on cooling and this may lead to visuallyobservable blooming. These processes depend on the nature of the chosen substrate aswell as on its morphology.

Additive diffusion is primarily a consequence of the thermal motion of the polymerchains above the glass transition temperature and of the related formation anddisappearance of free volumes. If the chains are flexible and move easily, only smallamounts of energy are necessary to move the polymer segments.

With increasing orientation of the polymer chains or on crosslinking and with increasingcrystallinity, the diffusion constant decreases. For this reason, the diffusion of additivesis faster in LDPE than in high-density polyethylene (HDPE). Numerous studies [11-14]confirm that the diffusion behaviour of UV absorbers depends mainly on polymer structureand morphology and to a minor extent on additive structure.

The solubility and compatibility of light stabilisers are particularly a problem when highlypolar light stabilisers are used for non-polar plastics such as polyolefin. However, evenin polyurethane, the compatibility of light stabilisers may become a problem.

10.4.2.2 Determination of Compatibility

Stabilisers such as antioxidants, metal deactivators and UV absorbers are added topolymers to reduce degradation during the manufacturing process and throughout thelifetime of the polymer products. In order to study the degradation of polymers or thecompatibility between additives and polymers, it is essential to have an analytical methodthat can provide both identification and a quantitative measure of additives in thepolymers. Fourier transform infrared (FTIR) spectroscopy [15, 16], UV spectroscopy[17], near-infrared reflectance gas chromatography, high-performance liquidchromatography (HPLC) and differential scanning calorimetry (DSC) can all be used asanalytical tools for identification and determination of the concentration of dissolvedstabilisers and their homogeneous distribution. FTIR and UV spectroscopy are the mostimportant techniques used, as they can be applied directly to the sample without disturbingthe morphology in the solid state. In addition, it is possible to detect any degradation orchanges taking place at earlier stages due to the sensitivity of these tools. Furthermore,the diffusion coefficient of additives can be estimated by using the disc-stacking technique[18], where a disc doped with the additives is placed in the middle of a stack of undopeddiscs. Diffusion is then allowed to take place at an appropriate temperature for anappropriate time. Then spectroscopic measurements can be done on different discs toevaluate the concentration of the additives in each disc. Accordingly, the diffusion



coefficient can be determined by knowing both the thickness of the discs and theconcentration of the additive by using a characteristic absorption band.

The principle behind the use of near-infrared reflectance analysis is the measurement ofthe light reflected by a sample when exposed to light in this region. The logarithm of theinverse of this reflected light can be related to the concentration of a particular componentfound in the sample. The obtained concentration value for different discs can also beused for determination of either the diffusion coefficient or the solubility of the additive.The same can be said for gas chromatography or HPLC. On the other hand, differentialscanning calorimetry can be used to determine the exothermic peak of the oxidation ofantioxidants, which allows direct determination of the concentration of the dissolvedantioxidants.

However, the commonly used methods for determination of compatibility do not giveadequate information about the molecular association of the stabilisers, but onlyinformation about the volume concentration of the stabilisers in the polymeric matrix.This means that it makes no difference whether the stabilisers are present as stacks ofmolecules or as single molecules. Such aggregation will lead to lower efficiency ofstabilisers. Of course, FTIR can offer some information from the change in theperturbation potential that results from polymer-stabiliser and stabiliser-stabilisermolecular interactions.

However, this needs a very careful study of the sample, and to have references formolecularly dispersed stabilisers as well as sophisticated calculation for the obtainedspectra. On the other hand, broad-band dielectric spectroscopy [19-22] can be appliedto investigate solubility and compatibility, as it offers an excellent possibility of detectingthe molecular reorientation of stabiliser molecules and segments simultaneously at thesame temperature. Accordingly, the degree of compatibility with most polymeric segmentscan be evaluated, where detailed investigation of the molecular dynamics have beencarried out [19-22] for various additives having different shapes, sizes and polarities indifferent polymeric matrices.

An empirical relation that determines the dependence of the relaxation frequencydifferences between the cooperative process of the additive and the glass process of thematrix (macro-Brownian cooperative reorientation of the segment associated with theglass temperature) and the additive length has been given [21]:

Δ log fm = 4 log[(L/d) – 1] (10.6)

where Δ log fm is the difference between log fm of the cooperative process of the additiveand log fm of the glass process of the matrix; L is the length of the additive; and d is the

271

polymer inter-chain distance. This equation was found to be valid not only for therelaxation process of an additive in a polymer but also for the δ relaxation process ofside-chain liquid-crystalline polymers and their additives. This equation implies that, ifthe molecules are molecularly dispersed, and have a length not longer than 1.8 nm, theymust relax cooperatively with the cooperatively reorienting segments of the glass processat the same relaxation frequency. However, in the case of Tinuvin P, a commerciallyavailable UV stabiliser (a benzotriazole derivative), the difference in the relaxationfrequency maxima of the stabiliser peak and the glass process of the matrix, Δ log fm, isgreater than three decades of frequency. These results indicate that the reorientations ofthe stabiliser are not coupled with the glass process of polystyrene segments, where thestabiliser molecules can relax locally at higher frequencies, (i.e., faster by a factor of1000). Furthermore, short additives can relax either cooperatively with the polymericsegments at the same relaxation frequency as the segments, or locally at higher frequencies.The ratio of the local contributions to the total relaxation strength (cooperative pluslocal) of the additives depends on the size of the stabiliser.

The biodegradation of representative samples of available commercial photo(bio)degradablepolyethylene films was examined with respect to the rate and extent of degradation,oxidation products and changes in molecular weight both during outdoor exposure and inlaboratory photo-ageing devices with different accelerating factors [23]. Although the rateof photooxidation was found to depend on the type of degradation system used, all thesamples showed a rapid rate of carbonyl formation, with concomitant reduction in molecularweight and mechanical properties on exposure to UV light. The photo-fragmented polymerswere shown to be much more hydrophilic in nature compared to the unoxidised analogues,and photo-fragments of all samples were found to contain high levels of low molecularweight (low molar mass) bioassimble carboxylic acids and esters.

The recycling behaviour of virgin polyolefins, both as homopolymers and as heterogeneouspolymer blends, which contained 10% of non-oxidised and photooxidisedphoto(bio)degradable plastics, has been examined. It was found that the initial mechanicalperformance of homogeneous blends was not greatly affected by the presence of non-oxidised degradable materials. However, blends containing degradable films that wereinitially partially photooxidised had a much more detrimental effect on the properties ofthe recycled blends during processing and weathering; the effect was minimal fordegradable polymers containing the iron-nickel dithiocarbamate system.

10.4.3 Evaluation of Laboratory and Outdoor Photooxidation

Laboratory and outdoor photooxidation of plastic films were evaluated using differenttechniques [24]. Thus melt blown, biaxially oriented, unstabilised and stabilised LDPEfilms with various thicknesses were exposed in two accelerated artificial weathering devices



with xenon (Xenotest) and UV-B fluorescent tube (QUV Weatherometer) sources undercontrolled temperature and humidity conditions. The structural changes during combinedphoto- and thermal degradation have been studied using tensile tensiometric, IRspectrophotometric and DSC methods. The effects of HALS and film thickness on thetime-dependent changes in elongation, carbonyl group concentration, crystallinity andthe onset temperature (Ton) of the post-fusion DSC oxidation exotherm have beenobserved. Photooxidation is accompanied by increased crystallinity, which maximises asmechanical properties start to deteriorate significantly and the rate of carbonyl groupformation increases. While there is a poor correlation between the reduction in mechanicalproperties and increased carbonyl index values, the former correlates well with the DSC-derived Ton values for unstabilised and stabilised films. This suggests that thermal analysismay be used to detect the physicochemical changes occurring in exposed films moreeffectively than other techniques such as IR.

However, many problems of premature film failure can occur during their use ingreenhouses, due to their interaction with the agrochemicals used. Both sulfur- andchlorine-containing agrochemicals inhibit the functioning of HALS, and can have a verydetrimental effect on the life of greenhouse films [25, 26].

The concentration of HALS in LDPE covering films before and after exposure to naturalweathering and accelerated photooxidation conditions has been determined [27]. It hasbeen found that photostabiliser disappearance above 0.4% up to 600 days is mostlyprobably the result of its physical loss during long photooxidation times under bothphotooxidative conditions. On the contrary, photostabiliser disappearance in the initialstage is due to chain scission and the consequent volatilisation and diffusion of thesefragments on the surface.

10.5 Other Factors Affecting the Stability of Greenhouse Films

10.5.1 Temperature

Cyclic temperature changes and the high temperatures developed at the metallic parts ofgreenhouse constructions during hot and sunny days lead to increased degradation. Onecan observe a lot of damage in the places where the plastic greenhouse films come intocontact with the metallic structural elements, especially when they are not painted. Thetemperature at these contact points may reach up to 70 °C and more depending on climaticconditions. In this case the diffusion of metal ions enhances the degradation process.

Metal particles, especially iron, may catalyse the decomposition of hydroperoxides formedas a result of oxidation, leading to unnecessarily high rates of degradation. The degradation

273

mechanism of PE films containing additives with metal ions at a simulated compostingtemperature has been studied. The hydroperoxide concentration [POOH] in the filmswas traced quantitatively by using iodometric potentiometric titration, and comparedwith Fourier transform infrared spectrometry (FTIR). The results show that [POOH]increases during the early stage of degradation, followed by a more or less flat maximum,before it starts to decrease. At the same time, similar results are obtained by FTIR analysis.It is also found that the rate laws for the carbonyl index and [POOH] increases seemmore complicated than an exponential-type increase in the early stage of oxidation [28].

Moreover, high temperatures lead to an increase in the rate of reaction for bothphotooxidation and chemical oxidation by agrochemicals, and thus to higher degradationrates. As mentioned before, HALS compounds act as radical traps, and consequentlythey also act as heat stabilisers and minimise the effects of high temperature [29].

10.5.2 Humidity

Lower resistance to oxidation and enhancement of degradation occur as a result ofincreased humidity as well as rainfall. This is due to the gradual washout of additivesthat may bloom on to the surface of plastic films. Besides, the degradation of plasticfilms may occur due to hydroxyl radicals or other reactive species generated as a resultof photolysis [30].

10.5.3 Wind

It has been suggested that tearing due to high winds can be a major problem in greenhouses.Another problem connected with windy areas is the wind load. This load can imposeincreased stress on plastic films and lead to premature failure of the film. Abrasion causedby soil and other particles, which are carried by the wind and impinge on the surface ofthe greenhouse film, may also be another problem.

10.5.4 Fog Formation

The term ‘fog’ is used to describe the condensation of water vapour in the form of smalldiscrete droplets on the surface of transparent plastic films. The physical conditions thatlead to this formation may be summarised as follows [31]:

(1) A fall in temperature of the inside surface of the film to below the dew point of theenclosed air/water vapour mixture;



(2) Cooling of the air near the film to a temperature at which it can no longer retain allthe water vapour, so that excess water condenses upon the film;

(3) The difference between the surface tension of condensed water and the critical wettingtension of the film surface, which causes the water to condense as discrete droplets,rather than as a continuous film.

A number of undesirable effects may result from fog formation in greenhouse films andleads to the following:

(1) Light transmission will be reduced where the total internal reflection of incident lightoccurs. Consequently, the rate of plant growth will reduce, crop maturity is delayedand the crop yield decreases;

(2) Light and heat transmission may be focused on delicate plant tissues owing to waterdroplets acting as lenses. This causes burning of the plants and crop spoilage.

To prevent fog formation, surface-active agents are usually added to PE during filmproduction. These compounds are incompatible with the polymer and subsequentlymigrate to the film surface where they increase the critical wetting tension. The result isreduction in contact angle between water and polymer surface, permitting the water tospread into a more uniform layer [31].

10.5.5 Environmental Pollution

Atmospheric pollution, such as nitrogen oxides, sulfur dioxides, hydrocarbons andparticulates, can enhance the degradation of polymers [32] and must also be taken intoconsideration. For instance, infrared studies have revealed that polyethylene reacts withNO2 at elevated temperature and that chemical attack is observed even at 25 °C. Similarly,SO2 is rather reactive, especially in the presence of UV radiation, which it readily absorbsand forms triplet excited sulfur dioxide. This species is capable of abstracting hydrogenfrom polymer chains, leading to the formation of macroradicals in the polymer structure,which in turn can undergo further depolymerisation [33].

10.5.6 Effects of Pesticides

The use of agrochemicals in greenhouses severely affects polymer films [29]. The pesticidesused for the protection of the crop influence the degradation and lifetime of the films.Usually pesticides have complicated formulations, and contain a number of compoundsbesides the active component. They contain sulfur and halogen in their chemical structure.It is a well-known fact that films are destroyed under the effect of pesticides.

275

Pesticides react with the stabilisers present in the film, decreasing its effect or completelydestroying it. Experimental results clearly show that pesticides with sulfur-containing activecompounds enter into antagonistic interaction with the stabilisers. One simple explanation[34] is that the interaction of the pesticide and the HALS compound prohibits the latter inexecuting its effect. Some sulfur-containing compounds and organic halogenides initiate theoxidation of PE and bring about rapid deterioration of its mechanical properties. The extentof this negative effect depends on the molecular weight, dispersion, allotropic modification,etc., of the elementary sulfur. The introduction of a UV absorber into film considerablyimproves the lifetime and light stability of the film. Also, HALS-stabilised greenhouse filmswere shown to last 33% longer than Ni-stabilised films in testing under real conditions.

However, the polymer materials used for greenhouse films are changing, and, in particular,the use of blends is continuously increasing, like the use of additives. These additives areused in relatively large amounts for different aims, like photooxidation resistance, anti-fogging, etc. Moreover, the films can absorb fertilisers and pesticides, which cancompromise the use of secondary materials coming from greenhouse covering films inmany applications. UV exposure gives rise to major modifications of the macromolecularchains, with chain breaking, formation of oxygenated groups, possible formation ofbranching and crosslinking, and so on [35-38]. Finally, the reprocessing operations caninduce further degradation due to the thermomechanical treatment in the melt [39-42].

10.6 Ageing Resistance of Greenhouse Films

10.6.1 Measurement of Ageing Factors

Evaluation of the stability and durability of greenhouse films is usually carried out usinglaboratory equipment. There is no standard testing scheme for evaluating the degradationof these properties when the plastic film is used as a greenhouse covering material. Thisis due to the fact that there are several interconnected factors that can lead to thedegradation of the mechanical properties. These factors are usually difficult to realise inthe laboratory. Following up the changes that occur in the mechanical properties ofplastic films as a result of ageing is very important in order to throw some light on theproblem and to identify the conditions of the film. However, other properties of the film,such as physical and chemical properties, are also affected by the degradation, e.g.,abrasion directly affects light transmittance and also other mechanical properties.

Many research groups have paid attention to this problem and concentrated their effortson measuring the effects of various ageing factors on the degradation of plastic materials[6, 43, 44]. Some of them are concerned with the very specific problem of ageing ofagricultural plastic film [24, 45].



In principle, all parameters should be covered in order to be able to predict accuratelythe performance and lifetime of the material under the particular circumstances thatwould occur when the material is used in a greenhouse. The accelerated ageing processconsists of simulating in an intensified manner the most critical parameters that lead tothe degradation of plastics. Several accelerated ageing tests for plastic films have becomecommercially available during the past decade. However, their relationship to real outdoorageing is extremely questionable. These tests are based on inducing artificial ageing inthe material through an intense UV source coupled with a day-night cycle and a waterspray cycle. Only a few studies have considered the use of pesticides or stress due towind loading combined with the effect of UV-induced ageing [25]. An empirical correlationbetween the lifetime of films under artificial weathering and in the greenhouse situationhas been given [46]. This standard defines three climatic zones depending on the level ofsolar radiation energy that they receive: 70-100, 100-130 or 130-160 kLy/yr, where thekilolangley is given by 1 kLy = 4.184 kJ/cm2.

However, no special conditions – such as the contact between the film and the metalparts of the greenhouse structure, and the application of pesticides that can stronglyinfluence the actual weathering of the plastic films – have been considered. Thus, theartificial ageing tests can only provide a rough estimate of the actual behaviour of plasticfilms when exposed to the real and complicated environmental factors that affect theplastic during its use [47]. For this reason several researchers have studied the degradationof plastic films under natural weathering conditions (outdoor tests) [48]. Only a limitednumber of tests were performed in greenhouses by this group [48].

10.6.2 Changes in Chemical Structure

Changes in chemical structure resulting from plastic film ageing have been followed usingspectroscopic methods. FTIR is the most frequently used technique [47, 49]. It providesinformation about the chemical structure of the macromolecules. For instance, when thechains are oxidised, carbonyl and OH groups are formed. The additive concentrations andtheir changes can also be detected by the same technique. The presence of parts of theagrochemicals in the film can also be detected spectroscopically. Electron spin resonance(ESR) is also used to detect the creation of free radicals during degradation [49].

Thermal analysis, such as DSC, is used to study the oxidation process as well as thechanges in the crystallinity of plastic films due to ageing [50].

Gel permeation chromatography (GPC) may be used to evaluate the changes in themolecular weight and molecular weight distribution of the films [8]. The integrated areabetween 1770 and 1690 cm–1 of the absorption band at 1734 cm–1 was used to determinethe concentration of Tinuvin 622 [poly(N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-

277

hydroxypiperidyl succinate)] before and after film exposure. The oxidation degree, i.e.,carbonyl index (CI), under different oxidation conditions was obtained by calculatingthe carbonyl absorption at 1713 cm–1 from the FTIR spectra at various oxidation timesusing the spectrum of the unoxidised starting material as a reference. All measuredabsorbances should be normalised by the film thickness using the equation:

CI = (A1713/d) x 100 (10.7)

where A1713 is the measured absorbance at 1713 cm–1 at a certain exposure time, and d isthe film thickness in micrometres.

10.7 Recycling of Plastic Films in Agriculture

10.7.1 Introduction

The amount of plastic materials used in agriculture has been continually increasing.Plastic materials are used for greenhouse covers, mulching, piping, packaging and otherapplications. Films used for greenhouses can be considered as an easy source of materialsfor recycling. Indeed, large amounts of film can be easily collected and, because of thehomogeneity of the polymers used for this application, the recycling operations can berelatively easy. However, UV exposure gives rise to major modifications of themacromolecular chains, with chain breaking, formation of oxygenated groups, possibleformation of branching and crosslinking, and so on [35, 36, 51, 52].

The recycling of post-consumer films for greenhouses is strongly dependent on the initialstructure of the plastic materials and on the processing conditions. Such films containsmall amounts of low molecular weight compounds probably coming from thephotooxidation of the PE molecules and from the absorption of fertiliser and pesticideresidues. The amount of these compounds is small, however, and does not prevent theuse of the recycled materials in many applications. The properties of the secondarymaterials deteriorate with the number of extrusion steps, but especially with the increasingextent of photooxidative degradation. However, the mechanical properties of the recycledpost-consumer film remain relatively good even after many extrusion passes, and suchfilm is useful for many applications [53].

10.7.2 Contamination by the Environment

Dow Chemicals have actively investigated the recycling of mulch film because the normalpractice of disposal by burning it on the fields is environmentally undesirable. The



contamination levels in mulch film make its recycling particularly challenging. Forinstance, soil contamination can be as high as 30-40%. Furthermore, the soil can containup to 3% iron, which is a polyethylene prodegradant [54]. In addition, it was found thatvegetable matter derived from harvested plants could not be removed during the washingoperations [54]. Other contaminants are fumigants, (e.g., methyl bromide), and theoxidised fractions of LDPE resulting from photodegradation of the mulch film.

Recently effort has been focused on the recycling of LDPE mulch film and greenhousefilm, both of which contain pesticide residues. This area poses special problems that aredifficult to overcome. It has been discovered that organochlorine and organosulfurpesticide residues can deactivate HALS. This deactivation is believed to occur as a resultof hydrolysis of the pesticides to acidic species that then react with the HALS. This hasimplications with respect to the long-term outdoor stability of the recycled product.

During in-service use, PE can become badly degraded and can form low molecular weightoxygenated products, (e.g., aldehydes, acids, ketones, waxes, etc.). These impurities canlead to embrittlement of the recycled polymer because low molecular weight oxidisedfractions are segregated from the melt during crystallisation and concentrate at thespherulite boundaries [55]. The resulting zone, rich in oxidised material, has very lowfracture toughness. Moreover, oxygenated degradation products of PE, such as carbonylgroups, are active chromophores and can sensitise the reprocessed polymer tophotodegradation.

Plastic waste management, in general, is a global environmental problem. The managementof such waste may be through the famous 4R approach:

• Reduction (of source material);

• Reuse;

• Recycling;

• Recovery.

The recycling and reuse of plastic waste films generated from greenhouses can share insolving the problem. The disposal of municipal solid waste has become an environmentalissue of growing concern [56]. It was determined that discarded plastics represent closeto 20% of municipal solid waste on a volume basis [57, 58]. This is due to the highvolume-to-weight ratio of polymeric materials.

The management of plastic waste follows the scheme:

• Source reduction;

• Recycling;

279

• Thermal reduction by incineration;

• Land-filling.

The most feasible methods for developing countries are source reduction and recycling.Source reduction is any measure that reduces the volume of plastic waste produced. Thisis accomplished through material efficiency, i.e., reducing the quantity of plastic materialused to produce a particular item.

Recycling generally involves the collection of waste plastic materials for reprocessing[59, 60]. Polyolefin blend technology is of critical importance to various applications,including greenhouse films. For instance, the LLDPE/LDPE blend is characterised byreduced haze and better bubble stability. One of the most common blends is LDPE/ethylene-propylene-diene terpolymer (EPDM) with improved low-temperature flexibility,rubbery properties, weathering resistance and high-temperature mechanical properties.The addition of EVA to LDPE has been commercially utilised to improve environmentalstress cracking resistance, toughness, film tearing resistance, flexibility and opticalproperties.

Both blending and coextrusion have been employed to deal with the problem ofagricultural plastic film waste. The main goal is to find a solution to the problem ofagricultural plastic waste from greenhouses by recycling and converting the waste intoproducts usable in the mulch and greenhouse film applications. The proposed solution isbased on the development of multilayer films consisting essentially of a top layer madefrom virgin resin and a bottom layer consisting of a blend of recycled PE waste filmmaterial in combination with virgin resin and other ingredients. Evaluation of greenhouseplastic wastes revealed that it is possible to obtain useful transparent plastic films to bereused with reduced cost [61].

Multilayer films for greenhouses are a current trend in the industry. LDPE films for thetop layer, stabilised with different concentrations (0.1, 1.0 and 2.5%) of UV quencher,have been produced in the laboratory by blow extrusion. The effect of natural weatheringon the film properties was investigated over a period of 12 months [62]. Significantchanges in the mechanical properties were observed in the later stages of degradation.Films stabilised with 0.1% stabiliser crumbled after 12 months of natural weathering,whereas films with higher concentrations retained their mechanical properties. It is believedthat the inclusion of a UV stabiliser interferes with the crystallisation process and thatthe stabiliser particles accumulate in the amorphous matrix. Degradation of the imperfectcrystalline region with its low oxygen permeability proceeds via crosslinking, whereaschain scission predominates in the amorphous region with excess of oxygen. Filmsstabilised with 2.5% UV quencher form a barrier against the transmission of UV radiationand the bottom layers are less affected by UV radiation. Recycled material can, therefore,be incorporated at high concentrations into these layers.



Optimisation of the top layer was based on a fixed concentration of three thermoplastics,i.e., 80% LDPE, 10% LLDPE and 5% EVA, the remaining 5% being a specially preparedmaster batch of LDPE containing 25% UV and heat stabilisers. Different types of UVstabilisers were taken in different concentrations. These are: Cyasorb 1084 [n-butylamine-nickel-2,2′-thio-bis(4-tert-octyl phenolate)] and Chimassorb 81 [2-hydroxy-4-n-octoxybenzophenone], acting as UV light absorbers; Chimassorb 944 LD [poly{6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl amino-hexamethylene-4-(2,2,2,6-tetramethylpiperidyl)imine}] and Tinuvin 622 LD, acting as radical scavengers; an energytransfer agent; and a peroxide decomposer. The data obtained show that the haziness ofall plastics films are within the range required for agricultural films.

It has also been found in the case study [62] that the utilisation of a single UV stabiliseris less efficient than the utilisation of a two- or three-component UV stabiliser. Thus,films containing three-component UV stabilisers in addition to a thermal stabiliser (Irganox1076) can retain at least 94% and 81% of tensile strength and elongation at break,respectively, after exposure to UV radiation for 600 h. The good resistance of theseplastic films can be attributed to the different mechanisms of action of the utilisedstabilisers. In other words, if the UV absorber Chimassorb 81 is added alone to theplastic blend, the films retain about 63% of the original elongation; whereas incombination with Cyasorb 1084, the retained elongation is increased to about 80%.Consequently, these results indicate the necessity of using a combination of UV absorbersand radical scavengers [62]. Furthermore, some plastic films of various compositionswere subjected to outdoor weathering tests in two different locations, Cairo and UpperEgypt. The results obtained indicate that the unprotected films deteriorate completelywithin three months, whereas the protected films can withstand almost one year withouta drastic decrease in mechanical properties.

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11 Physicochemical Criteria for Estimatingthe Efficiency of Burn Dressings

Klara Z. Gumargalieva and Gennady E. Zaikov

11.1 Introduction

The principal medical treatment of burns is the use of dressings, which often worsenthe effects of the injury. It is difficult to estimate the effectiveness of new burn dressings,as their physicochemical properties are not usually presented in the literature. Thischapter is devoted to a discussion of this subject for the first time. The authors addressthe complexity of physicochemical methods of analysis in order to create criteria forefficient dressings for a burn wound surface. The characteristics typical of burn woundsfor which dressings are required are shown in Table 11.1.

]3[sdnuownrubfoscitsiretcarahC1.11elbaT

eergednruB egamadfoegamI ssecorplacigoloisyhPhtpednruB

)mm(

IamedeodnassendeR

)amedeomuidem(yrotammalfnicitpesA

ssecorp0

II noitamrofcaSyrotammalfnicitpesA

ssecorp0

III,revocniksfoegamaDecafrusdnuowgniduxe

eussit,sisorcennikSsisorcen

2-1

VI ecafrusdnuowgniduxE,seussitfosisorcenlluFseussitfonoitasinobrac

5-2

Based on theoretical and experimental data, it was found that the maximal sorptionalability of a burn dressing is determined by the free volume of the dressing materialcalculated from the value of the material density. Kinetic parameters were determinedfrom the sorption curves. These parameters help in predicting the behaviour of burndressings. Criteria for estimating the efficiency of first-aid burn dressings are thenformulated.

286


11.2 Modern Surgical Burn Dressings

Dressings for wounds and burns must primarily be protective, sorptional and atraumatic. Incurrently used dressings, these properties are provided by a multilayer structure or structuralmodifications. Different classifications of dressings can be found in the literature: by material,by construction or by function [1-3].

The dressings applied in the modern treatment of wounds and burns are subdivided intothree groups according to the material of the layer sorbing the wound exudate. The materialmay be of animal origin, synthetic foamed polyurethane or of vegetable origin (Table 11.2).

11.2.1 Dressings Based on Materials of Animal Origin

Typical dressings in this group are collagen sponges. Besides hydrophilic properties,collagen sponges provide higher sorption of liquid (in the range of 40-90 g/g) [1, 4-9].The patent literature describes in detail the methods of obtaining collagen dressings forwounds and burns in the form of sponges and felt [10-13] based on materials of animalorigin. Also, the materials used include that made from biological artificial leathers basedon lyophilised bodies and swine cutis, produced as plates 0.5-0.7 mm thick. However,these materials possess lower sorptional capacity than collagen dressings.

Dressings called ‘cultivated cutis’ are also obtained from the epithelia of cells of thepatient himself [13]. The shortcoming of biological artificial leathers or bio-dressings istheir expense and, as a rule, their inability to retain their properties on storage.

11.2.2 Dressings Based on Synthetic Materials

The demands for inexpensive raw materials for the production of wound and burn dressingshas led to the production of materials based on synthetic polymers, particularly cellularpolyurethane [10, 14-18]. Cellular polyurethane intended for medical purposes is synthesisedusing toluene diisocyanate and polyoxypropyleneglycol [19].

Dressings based on polyurethanes have a pore distribution of about 200-300 pores/cm2,and allow the regulation of the number and size of pores in layers [20]. Dressings from thisgroup are prepared as a double layer; the density of the outside layer is high in order toprevent liquid evaporation and penetration of microorganisms. In rare cases, these dressingsare homogeneous through their thickness.

The influence of the pore size on the sorption properties of polyurethane sponges hasbeen reported [21], where macroporous sponge with a pore size from 200 up to 2000μm is completely nourished by exudate under pressure only. In this case, the size of thepores should be of the order of several micrometres.

287

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings

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288


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289

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reyalcitamuarta


290


Apart from polyurethanes, other polymeric materials (polyvinyl chloride, Nylon, etc.)have been used as the sorbing layer [22-25]. This group includes a compositional burndressing based on a silicon film, polyamide network and hydrophilic admixture, producedby Hall Woodroof Co., (USA) [13]. Polyurethane coverings with an atraumatic lowerlayer made from polyglycolic acid may be considered as a variety of compositionaldressings [26]. It is characteristic of dressings from this group that they preserve theirhigh strength properties even after absorption of wound exudate.

A two-component protective dressing ‘Hydron’ was recently applied in the treatment ofburns. It is a film formed on the wound, and consists of a powder of poly(2-hydroxyethylmethacrylate) dissolved in polyethyleneglycol 400 [2, 27]. Although they possess goodprotective properties, ‘Hydron’ dressings have low strength and sorptional capacity.

11.2.3 Dressings Based on Materials of Vegetable Origin

A large number of burn dressings, the so-called ‘cotton balling’, are based on cellulose,viscose or a combination of the two [28-32]. These dressings differ from each other bystructure and composition of the upper and lower layers. Most often, a sorption layerbased on cellulose is used in complex dressings. Such dressings are usually layered, withthe separate layers being produced from either the same or different materials; the layersmay be fixed mechanically or by using thermoplastic material. To decrease their adhesionto the wound surface, the lower layer is produced from various fabric and non-fabricmaterials (perforated Dacron, polypropylene, pressed paper, metallised fabric material,etc.). The total sorptional ability of these dressings is defined by the hydrophilicity andporosity of the basic material and is usually equal to 15-25 g/g.

Data on the action of wound and burn dressings based on another vegetable material –derivatives of alginic acid – have been reported [1, 33, 34]. Typical ‘Algipor’ specimensused are based on the mixed sodium-calcium salts of alginic acid as spongy plates ofabout 10 mm thickness with high absorption ability.

11.3 Selection of the Properties of Tested Burn Dressings

The data from the literature showed that burn dressings, particularly the first-aid ones,must perform three main functions [1, 2, 35, 36]:

(1) Absorb the wound exudate, which contains metabolic products and toxins;

(2) Provide optimum water, air and heat exchange between the wound and the atmosphere;

(3) Protect the wound from the penetration of microorganisms from the air.

291

Moreover, the burn dressing must be removable from the wound without further injuryto the patient. Therefore, the following properties of burn dressings have been studied todetermine their efficiency.

11.3.1 Sorption-Diffusion Properties

The sorption-diffusion properties of dressings are extremely important, because theydetermine the performance of the three main functions of dressings just mentioned.

11.3.1.1 Water absorption

Water is the main component of the exudate from wounds. At present, there is no opinionon how fast and to what degree the dressing must absorb the exudate in order to cleanthe wound from toxins and metabolic products while at the same time keeping the woundwet enough to prevent the removal of water from healthy tissue [1, 2, 35, 36].

11.3.1.2 Air penetrability

Sufficient air must be allowed to penetrate the dressing, since an increase of oxygenconcentration helps the healing process.

11.3.1.3 Vapour penetrability

Vapour penetrability of the skin of a healthy man may reach 0.5 mg cm–2 h–1 [37]. Waterloss by evaporation from burns is even higher (Table 11.3). In the absence of technicaldata, it may be concluded that high vapour penetrability will lead to ‘drying’ of thedressing, with a corresponding change in the surface energy of the dressing-wound

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epytecafruS mc(noitaropavE 3 mc/ 2 )h-

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nrubeergedtsriF 5.2-1

tcatnisretsilbhtiwnrubeergeddnoceS 8.2

reyalevitatnemreffoegamadonhtiwnrubeergeddnoceS 73

snrubeergedhtruofdnadrihT 13-02


292


interface. This will promote undesirable removal of water from the tissues, and maycause the dressing to come off the wound. Low vapour penetrability of the dressing willlead to the accumulation of liquid under the dressing, which may cause oedema.

11.3.1.4 Microorganism Penetrability

Penetration of microorganisms through the dressing must be blocked to prevent infection.

11.3.2 Adhesive Properties

The adhesive properties of dressings determine their ability to stay attached to the wound.Thus, the surface energy of the dressing surface facing the wound must always be lowerthan that of the wound surface.

11.3.3 Mechanical Properties

Two mechanical properties are important for dressings: (a) flexural rigidity and (b) strengthat break. The former defines the ability of the dressing to mould to the wound profile;the latter is important since it allows the dressing to be removed from the woundcompletely without breaking.

11.4 Methods of Investigation of Physicochemical Properties ofBurn Dressings

11.4.1 Determination of Material Porosity

The porosity of materials (the relation of pore space volume to total volume) is determinedby the following two methods.

(1) By measuring the density, and then using:

1P

l

l Pl

l P∑ ∑ ∑= +pores

pores

mat

mat(11.1)

where Q is the material porosity, ρ is the observable density, and ρ0 is the density ofthe material forming the porous medium. The value of ρ is determined by weighing

293

a sample of known geometrical size. The value of ρ0 is determined similarly forsamples pressed at 500 GPa.

(2) From photos obtained by a light microscope (MIN-10) we get:

QS

S=

⎡

⎣⎢⎢

⎤

⎦⎥⎥

pores

0

3 2/

(11.2)

where Spores and S0 are total surface area of pores and general surface area of thematerial in the field of vision of the microscope, respectively.

11.4.2 Determination of Size and Number of Pores

The number and size of the pores are determined with the help of the MIN-10 microscopein reflected light. The pore distribution curve (number of pores as a function of radius) iscalculated; typical results are given in Figure 11.1.

Figure 11.1 Typical curves of pore size distribution for various burn dressing materials.1: Farmexplant; 2: Syncrite; 3: Bayer brown collagen dressing; 4: Syspurderm


294


11.4.3 Estimation of Surface Energy at Material-Medium Interface

The surface energy of a material-medium interface is estimated using the wetting angleof the material surface by the medium. A drop of liquid is applied to the surface of thematerial, and the angle is measured between the tangent at the base of the drop and thematerial surface. The wetting angle is determined using a horizontal microscope. Theaccuracy of angle measurement does not exceed ±1°.

11.4.4 Determination of Sorptional Ability of Materials

The total amount of liquid sorbed by a ‘tiled’ material includes the liquid in macroporeswith size over 0.1 μm, that is micropores with size smaller than 0.1 μm, and that in thematerial matrix itself (dissolved liquid). The amount of dissolved liquid, and of liquidfilling the micropores, is calculated from the vapour pressure of the sorbed liquid overthe sample (sorption isotherms).

The sorption isotherm for a material with micropores possesses an S-type form (Figure 11.2).The first part of the curve is connected with the real dissolved liquid, and the second partwith the condensed liquid in micropores.

Figure 11.2 A typical sorption isotherm of a low molecular weight liquid by amicroporous material: part ➀ of the curve represents real dissolving of the liquid by

the material, and ➁ represents condensation of the liquid in the micropores within thematerial. Here Δm is the (change in) sorbed liquid mass; and P/P0 is the relative

pressure of liquid in the thermostated vessel (where P0 is the saturated vapour pressureof the liquid under the conditions used)

295

The maximal sorption (the amount of liquid, really dissolved and filling micro- andmacropores) is determined using the device shown in Figure 11.3. The device representsa vessel with liquid medium, in which a float of special perforated square construction isplaced. The float construction is calculated to prevent its sinking. This requires that theliquid medium does not penetrate through the perforations of the square, but insteadforms a meniscus on the side of the square facing the porous interlayer. The change ofthe mass of the porous material is determined from the immersion of the float with thesample. It is measured using a horizontal microscope.

11.4.5 Determination of Air Penetrability of Burn Dressings

Air penetrability (the volume of air that passes through a specific surface area during aspecific time) was determined using a device specially designed for this purpose. Thedevice is a cylindrical cell with perforated plate supporting the sample (Figure 11.4). Theair was passed through the cell with the help of an air compressor, equipped with amanometer and pressure controller. The time required to fill a polyethylene sack (45litres in volume) with air was measured.

A round form sample was prepared. The sample was then placed on the perforated plateof the cell. The compressed air passed through the cell pressed on the sample. The timetaken for the polyethylene sack to fill was measured. The method allows determinationof the air penetrability of dry or wet materials.

Figure 11.3 A schematic diagram of the device for determining the absorbtionability of porous materials. 1: sample; 2: perforated plate; 3: thermostated bath;

4: float


296


11.4.6 Determination of Adhesion of Burn Dressings

Adhesion of burn dressings was investigated on a modified form of a device previouslydescribed [38]. Thus, a 1 mm thick fibreglass plate, covered with three layers of medicalgauze, was placed into a fibreglass cell having a working surface of 3 × 10 mm2. The cellwas filled with 5 ml of whole blood and 1 ml of 2% thrombin. The dressing to be testedwas then placed on the plate surface for 1 min. The cell containing the sample wasplaced into a thermostat at 37 °C for 24 hours. Sample removal was performed at a 90°angle to the surface of the tested material.

11.4.7 Determination of Vapour Penetrability of Burn Dressings

Vapour penetrability (the mass of water that passes through a specific surface area duringa specific time) was determined using a device described elsewhere [39]. A glass vesselwas filled with a known amount of liquid, e.g., water or aqueous solution of sulfuricacid. This amount provided a known relative humidity. The investigated sample wasplaced on the vessel surface; and a metal ring was set and pressed to the vessel by aspecial clamp. The vessel with the contents was weighed and placed into desiccator with

Figure 11.4 A schematic diagram of the device for determining the air penetrability ofporous materials. 1: sample; 2: perforated plate; 3: polyethylene sack; 4: manometer;

5: pressure controller

297

dryer at 37 °C. After measured time periods the vessel was taken out from the desiccator,weighed and then put back in the desiccator. The amount of water that has passed throughthe sample was determined by the mass loss of the vessel contents. The vessel dimensionsused in the experiments were 40 mm diameter and 20 mm height.

11.5 Results and Discussion

11.5.1 Determination of Sorption Ability of Burn Dressings

On applying a dressing to a burn wound, first wetting of the surface layer of the materialoccurs, followed by sorption of the wound exudate into the dressing volume. Thus it isnecessary: (1) to know the components of the burn wound exudate, which need to besorbed by the material, and the way in which sorption occurs; and (2) to determine themaximum sorption of the separate components of the exudate by the dressing material.

With respect to the second item, the maximum water sorption of different materials hasbeen determined previously [40]. For this purpose, the sample was immersed in water,dried rapidly using filter paper and then weighed. However, this method did not allowthe sorption kinetics to be measured, and the accuracy of the maximum sorption waslow. That is why we have developed the device for continuous measurement of sorption.

The first item mentioned above has not yet been addressed in the published literature.The exudate from wounds contains water, salts, proteins, damaged cells and various lowand high molecular weight (low and high molar mass) substances in relatively loweramounts. Table 11.4 shows the approximate composition of oedema liquid in a burnwound. The composition of oedema liquid changes depending on the burn degree: theworse the burn, the higher the content of protein and the lower the albumin/globulinratio [3]. Similar data for blood plasma are also shown for comparison in Table 11.4.

mc/g(amsalpdoolbdnadiuqilamedeofonoitisopmoC4.11elbaT 3)

stnenopmoC diuqilamedeO amsalpdoolB

aerU 1.5 × 01 4- 5.5 × 01 4-

raguS 8.5 × 01 6- 0.11 × 01 6-

nietorP 4.3 × 01 2- 2.7 × 01 2-

stlaS 0.1 × 01 2- 0.1 × 01 2-

nilubolG/nimublA 9.3 5.1


298


Sorption of wound exudate may proceed via filling of micro- and macropores, or dissolvingin the material matrix. Let us consider the sorption of the various components of thewound exudate by the dressing material.

(1) Water fills pores and dissolves in the material matrix. Water solubility is defined bythe material hydrophilicity. The solubility of water, salts and other low molecularweight substances in polymers is subject to the following rules:

• In hydrophilic polymers, solubility is defined by the size and charge of the lowmolecular weight substance;

• In hydrophobic polymers, solubility is defined by vapour pressure (the higher thevapour pressure, the higher the solubility) [41].

(2) Protein fills pores up to 10–2 m in size and may dissolve only in hydrogels of ‘Hydron’type with water content over 30% by mass.

(3) Cells fill only open pores over 0.1-0.2 μm in size.

11.5.1.1 Solubility of water in polymers

As mentioned previously, modern burn dressings are heterogeneous materials, usuallyconsisting of several layers. The upper one exposed to the air is usually more hydrophobicand less porous than the others. The solubility of water in this layer will define its evaporationfrom the dressing surface and the heat exchange between the wound and the surroundings.Information about solubility of water in various polymers is reported in Table 11.5 [42].

The solubility of water was determined by the sorption method. Extreme values of sorptionat known water vapour pressures were calculated from the sorption curves, and then thesorption isotherms were constructed using the method described elsewhere [43].

Extreme values of solution φH O2

∞ at the saturation pressure were determined by extrapolationof φH O2

to P/Ps = 1. The value φH O2

∞ of equals the solubility of water in the polymer.

11.5.1.2 Maximum sorption ability of burn dressings

Modern burn dressings are heterogeneous materials that either have large pores or arefibrillar, and they possess a high free volume. In contact with a wound, the exudate willfill the free volume of the dressing. The degree of filling is defined by the hydrophilicityof the material, and the size and geometry of the free volume fraction.

299

11.5.1.3 Maximum sorption of water by burn dressings

Sorption of water by burn dressings is measured using a device developed for this purposeby the present authors. Experiments were performed in the following way. First, differentmasses were placed on the perforated plate of the device, and the relative immersion ofthe device into the water was measured, in units of the eyepiece graticule of the horizontalmicroscope. A calibration curve was then drawn using the coordinates ‘mass’ versus‘depth of immersion’. The slope coefficient of this calibration curve equals 0.70 ± 0.02 g/unit.

Then, a sample of a dressing was placed into the device, and the depth of immersionduring time h was measured. The mass of the medium sorbed by the material wascalculated from the correlation:

mc – 0.70h (11.3)

The extreme value of the mass of sorbed medium was determined at t → ∞.

sremylopsuoiravniretawfoytilibuloS5.11elbaT

remyloP 01(ytilibuloS 2 )g/g )K(T

enahpolleC 04 303

erbifesocsiV 64 303

nottoC 32 303

etatecaidesolulleC 81 303

etatecairtesolulleC 5.11 303

edimaorpacyloP 5.8 303

etalahthperetenelyhteyloP 3.0 303

enaxolislyhtemidyloP 70.0 803

)etalyrcahtemlyhteyxo-2(yloP *04 013

enelyporpyloP 700.0 892

enelyhteoroulfartetyloP 10.0 392

(enelyhteyloP ρ )329.0= 600.0 892

enahteruyloP *1 892

edirolhclynivyloP 5.1 703

.srohtuaehtybderusaeM*


300


Table 11.6 shows the experimental and theoretical [calculated from equation (11.3)]values of CH O2

∞ , and values of ρ0 determined experimentally and used for the theoreticalcalculations. A good correlation was observed between the experimental and theoreticalvalues of CH O2

∞ for the majority of dressings. This shows that practically the entire freevolume is filled by liquid medium for the contact of dressings with water.

fonoitprosmumixamehtfoatadlaciteroehtdnalatnemirepxE6.11elbaTsgnisserdnrubybretaw

)lairetam(emangnirevoC ρ0 mc/g( 3)latnemirepxE laciteroehT

)negalloc(xertileH 23 ± 2 030.0 ± 700.0 33 ± 2

)egnopsnegalloc(xertileH 85 ± 3 810.0 ± 500.0 55 ± 3

gnisserdnegalloC 8.1 ± 1.0 053.0 ± 70.0 8.2 ± 3.0

)negalloc(2-muiterroC 5.3 ± 3.0 003.0 ± 70.0 3.3 ± 3.0

)negalloc(3-muiterroC 1.2 ± 2.0 033.0 ± 50.0 0.3 ± 1.0

)negalloc(2-cetubmoC 0.77 ± 0.5 510.0 ± 500.0 0.66 ± 0.3

enahteruylopymaof(dragipE 0.01 ± 3.0 760.0 ± 500.0 0.51 ± 0.1

etisopmocnolyN-nociliS 5.7 ± 2.0 031.0 ± 30.0 7.7 ± 5.0

)enahteruylopymaof(mredrupsyS 2.6 ± 2.0 041.0 ± 30.0 1.7 ± 5.0

)enahteruylopymaof(etircnyS 0.02 ± 0.2 050.0 ± 10.0 0.22 ± 5.1

)enahteruylopymaof(tnalpxemraF 0.21 ± 5.0 460.0 ± 700.0 6.51 ± 0.3

)esolullec(nosnhoJ&nosnhoJ 4.11 ± 5.0 001.0 ± 30.0 0.01 ± 5.1

)esolullec(gnippots-doolB 7.51 ± 9.0 080.0 ± 600.0 5.21 ± 7.0

)esolullec(gnillennuT 3.4 ± 2.0 002.0 ± 500.0 0.5 ± 4.0

)esolullec(nitiwS 0.81 ± 0.2 050.0 ± 500.0 0.02 ± 0.1

)repap-esolullec(desillateM 4.21 ± 7.0 001.0 ± 40.0 0.01 ± 5.0

)esocsiv-esolullec(detarofrep-eldeeN 0.82 ± 5.2 330.0 ± 700.0 0.03 ± 0.2

esocsiV%001 0.52 ± 0.2 330.0 ± 700.0 0.03 ± 0.2

esocsiv%03+nottoc%07 0.13 ± 0.3 030.0 ± 700.0 3.33 ± 0.3

esocsiv%05+nottoc%05 0.52 ± 0.2 530.0 ± 700.0 5.82 ± 0.2

esocsiv%07+nottoc%03 0.82 ± 0.2 630.0 ± 700.0 7.72 ± 0.2

)elbategev(ropiglA 0.03 ± 0.3 110.0 ± 2000.0 0.09 ± 0.5

CH O2

∞

301

The exception is the ‘Algipor’ dressing, the large pores of which become denser on fillingwith water because of the collapse of the pore walls. At the end this leads to the decreaseof the total volume of the dressing. The liquid medium may not fill the whole volume ofthe dressing if the material is sufficiently hydrophobic and poorly wetted with water.

To test this assumption, seven collagen materials that differ in production method wereinvestigated for: density, maximal water sorption, wetting angle and heat of sorption ofwater by the material. The latter was determined using a microcalorimeter (LKB 2107) asfollows: A sample of known mass was exposed to vacuum in a thermostated Butch-type cell,and then an excess amount of water was introduced into the cell, causing the forced filling ofthe material volume. The results obtained are presented in Table 11.7 and Figure 11.5.

Figure 11.5 The dependence of the maximum sorption of water on the heat ofsorption for various collagen materials

noitprosfotaehdnaelgnagnittew,noitprosretawlamixam,ytisneD7.11elbaTsnegalloctnereffidybretawfo

ρ0mc/g( 3)

ø)ged(

ΔH)g/lac(latnemirepxE laciteroehT

110.0 47 19 071 6.43

610.0 35 5.26 07 4.52

310.0 94 77 09 2.03

310.0 74 77 011 9.13

310.0 8 77 021 2.13

410.0 4 4.17 011 8.92

410.0 03 4.17 05 2.72

CH O2(g/g)∞


302


The following conclusions can be derived from the data presented in Table 11.7:

(1) The experimental value of CH O2

∞ is lower than the ‘theoretical’ one. This may beexplained by two reasons: the decrease of the total volume (as in the case of ‘Algipor’),and the non-filling of a part of the material free volume by water.

(2) A satisfactory correlation exists between the theoretical values of CH O2

∞ and ΔH. Thus,the main reason for the difference between the experimental and theoretical values ofCH O2

∞ is evidently the non-filling of a part of the material free volume by water.

(3) The absence of a correlation between maximal water sorption and wetting angle, definedon the external surface of the material, shows that the values obtained as mentioned abovedo not reflect the real interaction of water with the internal surface of the collagen material.

Figure 11.6 The dependence of CH O2

∞ on the free volume of various burn dressingmaterials: 1, Helitrex collagen dressing; 2, Neutron collagen sponge; 3, Bayer browncollagen dressing; 4 and 5, Corretium-2 and -3 artificial leathers; 6, Combutec-2; 7,Epigard synthetic dressing; 8, Syspurderm foamy polyurethane dressing; 9, Syncrite

synthetic dressing; 10, Farmexplant foamy polyurethane dressing; 11, burn face mask;12, Biobrant compositional dressing; 13, Johnson & Johnson cellulose dressing; 14,Kendall cellulose dressing; 15, Torcatee non-adhesive cellulose dressing; 16, blood-stopping cellulose dressing; 17, Mesorb cotton balling dressing; 18, dressing with

tunnelling effect; 19, cellulose dressing with non-adhesive synthetic layer; 20, Switincotton balling dressing; 21 and 22, metallised dressings; 23 and 24, needle-perforated

fabric with atraumatic layer; 25, 100% viscose; 26 to 29, viscose-cotton balling

303

Thus, it may be concluded that, for the majority of hydrophilic burn dressings, themaximum sorption capacity with reference to water may be predicted satisfactorily. Forexample, the experimental values of CH O2

∞ correlate well with the free volume part of thematerials (Figure 11.6), the correlation coefficient being 0.96.

11.5.1.4 Maximum sorption of plasma by burn dressings

Sorption of blood plasma by burn dressings was determined by a similar method. Plasmawas obtained by centrifugation of conserved blood. The treatment of the experimentalresults was carried out similarly to the case of the investigation of the maximum sorptionof water. The value of Cplasma

∞ differs from CH O2

∞ . The difference is not higher than 10%,which is why data for Cplasma

∞ are not presented in Table 11.7.

11.5.2 Kinetics of the Sorption of Liquid Media by Burn Dressings

The study of the kinetics of the sorption of wound exudate by burn dressings is of greatimportance for the estimation of their efficiency. There are difficulties in the mathematicaldescription of the kinetics of the sorption process connected with the absence of a strictlyquantitative description of dressing structure.

11.5.2.1 Structure of burn dressings

Burn dressings are heterogeneous systems, consisting of several component phases. Asgeneral attention in dressings must be paid to the material possessing the maximumpenetrability with reference to the liquid medium, it is necessary to classify the types ofheterogeneous systems. For example, the penetrable parts of the material are placedunder a layer of another weakly penetrable material in such a way that diffusing flow isperpendicular to the surface layer. This is the case for double-layered dressings with adense external layer. The penetrable parts of the material can be dispersed in a continuousweakly penetrable phase.

Dressings based on collagen and cellulose possess fibrillar structure and the fibres arerandomly placed. In some cases, spatial orientation of fibres is present. The number ofopen pores in dressings of this type is large but the open pores possess irregular form andgreat tortuosity in the direction of mass transfer. Modern burn dressings are multilayerwith a denser external layer. Table 11.8 shows the mean radius of macropores and theirnumber per unit area for dressings based on polyurethane.


304


Detailed analysis of a number of mathematical models and results of experimentalinvestigations of heterogeneous systems has been performed by Zaikov [44]. It isknown that the calculation of diffusion coefficients in heterogeneous systems is verydifficult. According to ideas accepted at the present time, the penetration of liquidinto a porous body is ruled by the laws of capillarity. These ideas have been successfullyapplied to interpret the penetration of water into paper, leather, fabrics, etc. [45, 46].An equation that takes into account the real structure of porous bodies was obtainedby Deriagin [47].

11.5.2.2 Kinetics of sorption

The kinetics of sorption of water and blood plasma was investigated using the devicefor the maximal sorption of water. Figure 11.7 shows typical kinetic curves of sorptionof water and plasma by various dressings. All curves are satisfactorily described bythe equation:

mm

Dt

lt

∞= ⎡

⎣⎢

⎤⎦⎥2

2

1 2

π

/

(11.4)

The following conclusions can be made from the data obtained:

(1) Burn dressings differ significantly in their rates of sorption of liquid media;

(2) The rate of sorption is determined by the pore size and the material hydrophilicity.

suidarnaeM8.11elbaT R rebmunriehtdnaseroporcamfo N rofaeratinurepenahteruylopnodesabsgnisserd

emangnisserD R 01( 2– )mc N mc( 2– )

dragipE 2.2 ± 2.0 073 ± 01

mredrupsyS 8.1 ± 2.0 662 ± 5

etircnyS 8.2 ± 2.0 572 ± 5

tnalpxemraF 2.2 ± 2.0 003 ± 01

305

11.5.3 Determination of Vapour Penetrability of Burn Dressings

With multilayer dressings, the external layer, which regulates the mass transfer of waterfrom the wound into the surroundings, is denser than the inner ones. The process ofmass transfer of water through the material layers is often called aqua-, water or vapourpenetrability.

Penetrability and diffusion of water in polymers has been the subject of numerousinvestigations. The results given in some reviews and monographs [47, 48] are presentedin Table 11.9. The mass transfer of water molecules in polymers possesses a list of features.In hydrophobic matrices, the interaction between water molecules and the material matrixis weak (low solubility). Nevertheless, the interaction of water molecules with each otherstipulates a specific transfer mechanism.

In hydrophilic materials, the interaction between water molecules and the hydrophilicgroups of the material matrix stipulates high solubility of water in the matrix andincreased aqua-penetrability. Consequently, high aqua-penetrability may be a property

Figure 11.7 Curves for the sorption of water and blood plasma by various burndressings. 1: water, and 2: plasma, by needle-perforated material; 3: water, and

4: plasma, by Syspurderm polyurethane dressing


306


of hydrophobic as well as of hydrophilic materials; however, the causes will be different.For example, in hydrophilic polydimethylorganosiloxane, the high mobility of watermolecules is stipulated by the high mobility of the chain units in this polymer. That iswhy, despite the low solubility of water in polydimethylorganosiloxane, the coefficientof aqua-penetrability is significant.

In contrast, in regenerated cellulose, the diffusion coefficient is low because only thedissolved water molecules, which are not connected with the matrix of this polymer,participate in the mass transfer. In this case the high value of aqua-penetrability isstipulated by increasing the dissolved water content in the regenerated cellulose, whichincreases the fraction of the water molecules participating in mass transfer. This inturn leads to the increase of both the diffusion coefficient and the penetrabilitycoefficient.

The mass transfer of water through a porous body is practically equal to that of gasesin a polymer, provided there is no interaction between water molecules and the matrixof the polymeric material. Since hydrophilic materials, which actively interact withwater molecules, are commonly used for the production of dressings, diffusion shouldbe considered simultaneously with absorption.

]84[sremylopnisruopavretawfonoisuffiddnaytilibarteneP9.11elbaT

remyloP T )K(P × 01 51

m/mlom( 2 )aPsD × 01 21

(m2 )s/

esolulleC 892 0.1 0058 –

esolullecdetarenegeR 892 2.0 0075 1.0

etatecaesolulleC 303 0.1-5.0 0002 7.1

etatecaidesolulleC 892 0.1 7.51 –

etatecairtesolulleC 892 0.1 5.5 –

esolulleclyhtE 892 48.0 0597 81

enaxolisonagrolyhtemidyloP 803 2.0 00441 0007

(enelyhteyloP ρ )229.0= 892 1.0-0 03 32

etalahthperetenelyhteyloP 892 1.0-0 6.85 93.0

enelyporpyloP 892 1.0-0 71 42

edirolhclynivyloP 303 – – 3.2

edimaorpacyloP 892 5.0 431 790.0

pp0

307

As a rule, the rate of the absorption process is significantly higher than the diffusion rate.Therefore, it can be assumed that the absorption equilibrium is immediately reached,

and the concentration of water in the material CH O2 is obtained from the equation:

∂∂

∂

∂

∂∂

C

tD

C

x

C

tH OH O H O H O

a2 2 2= −

2

2

2 (11.5)

where DH O2 is the coefficient of water diffusion in the material, x is the diffusion

coordinate, and CH Oa

2 is the concentration of absorbed water.

The concentration of absorbed water can be calculated for particular cases. For example,if the concentration of functional groups able to link water molecules irreversibly islimited and equals Cf, we can assume that the bonded water molecules no longer participatein the diffusion process, but form domains in which fast absorption occurs.

For the case when the concentration of water at one of the surfaces (x = 0) is constantand equals CH O2

0 , the reaction zone reaches the second surface of the membrane, whichhas thickness l, during the time t [49]. Thus, during time t there will be no water flowthrough the surface x = l on the membrane exterior, and then steady-state flow will be setup immediately. The amount of water passing through the membrane is given by:

m DC

lStH O H O

H O2 2

2=Δ

(11.6)

where S is the area of the membrane and ΔC

lH O2 is the concentration gradient.

If the solubility of water in the material is ruled by Henry’s law:

CH O2 = σP (11.7)

where P is the water vapour pressure over the material, then substituting equation (11.7)into equation (11.6) gives:

m DPl

StH O H O H O2 2 2= σ Δ

(11.8)

Considering the diffusion coefficient DH2O being given by:

P DH O H O H O2 2 2= σ (11.9)


308


we obtain:

Pm

P S tH OH O

2

2=Δ (11.10)

The aqua-penetrability of burn dressings has been determined on the device described inSection 11.3. The values of the penetrability coefficients were calculated by equation(11.10). Table 11.10 shows the values of the coefficients of aqua-penetrability PH O2 forvarious burn dressings.

11.5.4 Determination of the Air Penetrability of Burn Dressings

As mentioned in Section 11.5.1, active sorption of wound exudate occurs for several minutesafter putting a dressing on a burn wound. Later, the evaporation of water from the external

73tasgnisserdnrubfostneiciffeocytilibartenep-auqafoseulaV01.11elbaT °C

)lairetam(emangnisserDm/mlom( 2 )aPs

)negalloc(xertileH 6.1 ± 1.0

)negalloc(egnopsxertileH 0.11 ± 0.1

)negalloc(gnisserdnworB 6.6 ± 6.0

)enahteruylopymaof(mredrupsyS 8.0 ± 2.0

)enahteruylopymaof(etircnyS 2.1 ± 2.0

)enahteruylopymaof(dragipE 3.4 ± 4.0

)enahteruylopymaof(tnalpxemraF 3.3 ± 3.0

)edimaylop-nocilis(tnarboiB 6.1 ± 61.0

)esolullec(nosnhoJ&nosnhoJ 0.2 ± 2.0

)esolullec(gnisserddesillatemdetarofreP 0.9 ± 7.0

)esolullec(gnisserdksamecaF 5.2 ± 2.0

)esolullec(lewotnruB 4.5 ± 5.0

esocsiv%05+nottoc%05 0.8 ± 7.0

esocsiv%03+nottoc%07 0.8 ± 7.0

esocsiv%001 0.7 ± 7.0

PH O9

210×

309

side of the dressing proceeds. This leads to a change in the state of the exudate in thematerial mass. On the whole, this changes the penetrability of the dressing with respect toair. In this case, in order for anaerobic conditions not to be created in the wound, it isnecessary to provide optimal air penetrability during the entire period of application.

Data on the penetrability of dressings to dry air are known in the literature. Thus, forexample, it is recommended [50] to determine air penetrability with the help of theindustrially produced VPTM-2 device. This device records automatically the amount ofair passing through a dressing of known area during time t under pressure oscillations ofabout 5 mm H2O. However, the application of such a device does not allow investigationof the air penetrability of dense materials such as foamy polyurethane compositions and,most importantly, of dressings in the wet state.

The construction and principle of action of a device, developed by the present authors,that allows thse determination of the air penetrability of any material in any state andunder any conditions were described in Section 11.3.

11.5.4.1 Penetrability of various materials to oxygen and nitrogen

The coefficient of gas penetrability (as well as the coefficient of vapour penetrability) iscalculated according to equation (11.10). Literature data on the penetrability of variouspolymers to oxygen and nitrogen are given in Table 11.11. As the data in this table show,

.sremylopnisesagfostneiciffeocnoitarapesdnaytilibarteneP11.11elbaT

remyloPtneiciffeocytilibarteneP × 01 51

m/mlom( 2 )aPsnoitarapeStneiciffeoc

O2 N2 O2 N/ 2

edimaorpacyloP 310.0 3300.0 8.3

edirolhclynivyloP 220.0 800.0 8.2

remotsaleenahteruyloP 230.0 01.0 2.3

(enelyhteyloP ρ )229.0= 53.0 31.0 7.2

enerytsyloP 31.3 37.0 9.2

nolfeT 70.2 76.0 1.3

esolulleclyhtE 2.3 39.0 4.3

enaxolislyhtemidyloP 861 0.38 0.2

rebburnociliS 002 0.78 3.2


310


the penetrability of polymers may differ by four orders of magnitude. Special attentionshould be paid to the high gas penetrability of polydimethylsiloxane and compositionsbased on it, which is the result of the increased solubility of gases in them at high rates ofdiffusion (Table 11.12) [51].

11.5.4.2 Penetrability of porous materials filled by a liquid medium

A short list of studies considering the investigation of the gas penetrability of polymericmembranes in contact with a liquid is given elsewhere [52]. It is observed that the sorptionof liquid by a polymer leads to a decrease in the gas penetrability coefficient in comparisonwith that of the liquid-free polymer.

11.5.4.3 Air permeability

Let us consider the mass transfer of air through a porous body in two cases: one in whichthe free volume of all the pores is filled by air, and the other with the free volume filled bya liquid medium. The porous body may be represented as consisting of two phases: thematerial forming the body’s matrix, and the free space.

We also assume that pores have cubic form and are disposed within the volume of thebody in such a way that they do not join up with each other. Such a model is sufficientfor porous burn dressings.

Let us determine the total thickness of the body in the direction of mass transfer, thetotal thickness of free space occupied by pores, and the total thickness of the layer occupiedby the material.

stneiciffeocytilibartenepfoseulaV21.11elbaT P m/mlom( 2 noisuffid,)aPsD m( 2 ytilibulosdna)s/ σ m/lom( 3 enaxolislyhtemidylopotnisesagfo)aP

02ta ° .]15[C

sesaG P × 01 51 D × 01 01 σ × 01 6

O2 38 3.32 63

N2 461 03 6.55

OC 2 027 – –

311

• The total thickness of the body in the direction of mass transfer is given by:

lΣ = V/S (11.11)

where V and S are the volume and surface area, respectively.

• The total thickness of the free space occupied by pores is given by:

Qpores = lΣQ1/3 = (V/S)Q1/3 (11.12)

where Qpores = Vpores/V is the porosity.

• The total thickness of the layer occupied by the material is given by:

lmat = lΣ – lpores = (V/S)(1 – Q1/3) (11.13)

Thus, air passing through a porous body will overcome the resistance of two layers, eachpossessing its own penetrability coefficient with respect to air.

The total penetrability coefficient PΣ of a porous body is thus given by:

1P

l

l Pl

l P∑ ∑ ∑= +pores

pores

mat

mat(11.14)

where Ppores and Pmat are the penetrability coefficients of the porous medium and thematerial forming the body’s matrix, respectively.

The following equation can be used to determine the ratio of the penetrability coefficientsof air for the porous body when its pores are filled with liquid and air:

P

PP P

P P∑( )∑( )

= +liq

air

mat air

mat liq

( / )( / )

1ξ (11.15)

where:

ξ = Q1/3/(1 – Q1/3) (11.16)

Values of Pmat are shown in Table 11.10. Values of Pair and Pliq can be estimated from thecoefficients of diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C(Table 11.13).


312


Values of the penetrability coefficient of oxygen in various media may be calculatedaccording to the following expression:

P = Dσ (11.17)

For any material Pair >> Pmat, so we obtain the simpler expression:

P

PPP

∑( )∑

≈ +air

liq

mat

liq( )

ξ 1 (11.18)

As, for the majority of dressings, ξ >> 1, and Pmat and Pliq are of the same order ofmagnitude, the decrease in air penetrability of a dressing when the pores fill with liquidmust be significant.

It has been shown by special experiments that air humidity (from 40 to 100%) does notpractically influence the rate of penetration. The experiments were performed accordingto the following scheme. First we determined the time t0 to fill a polyethylene sack, of 45litre volume, with air in conditions when the sample was not in the cell. This time t0 (aconstant of the device) depended on the pressure in the system (p):

log(1/t0) = –2.00 + 0.44 log p (11.19)

The time for polyethylene sack filling at p = 100 Pa was selected as the standard. At T =21 ± 1 °C, it is found that t0 = 16.0 ± 0.1 min.

Subsequently, the time tx to fill the polyethylene sack when the sample was placed into thecell was similarly determined. It was observed (Figure 11.8) that the dependence of tx on p

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)9.11elbaTnisasnoisnemid(

muideM P D σ

riA 5.2 × 01 9- 7.2 × 01 5- 4.9 × 01 5-

retaW 4.7 × 01 41- 0.3 × 01 9- * 5.2 × 01 5- *

amsalP – 0.2 × 01 9- * –

doolB 4.1 × 01 41- 4.1 × 01 9- * 0.1 × 01 5- *

]35[morfnekatseulaV*

313

has the same slope as in equation (11.19) for all investigated dressings in conditions of dryair penetration:

lg . lg1

0 44t

A px

x= − + (11.20)

where Ax is a constant depending on the structure and properties of the dressing material.

By bubbling humid air through a dressing saturated by water, the slope increased significantly.That is why it is necessary to perform several experiments for each dressing at different pressuresin order to extrapolate tx to the pressure of 100 Pa with the required accuracy (Figure 11.9).

The increase in the slope of log(1/tx) versus log p on bubbling air through a dressing saturatedwith water was attributed to the change in the material structure of the dressing resultingfrom the changes of form and size of the macropores. This is often accompanied by a decreaseof the total volume of the dressing.

The coefficient of air penetrability of the dressing (Px) was calculated according to:

Pml

S t t txx

x

=−( )0

(11.21)

where m is the polyethylene sack bulk (equal to 2 mol of air at 21 ± 1 °C), and S is the surfacearea in contact with the bubbling air (equal to 1.8 × 10–3 m2); and p = 100 Pa. Thus,

Figure 11.8 The dependence of 1/log t on the pressure in the system for dry air.1: needle-perforated material; 2: collagen sponge; 3: Syspurderm; 4: Syncrite; 5: Switin

cellulose dressing; 6: Farmexplant; 7: Epigard


314


Pl

t txx

x

=−( )11

0(11.22)

The values of air penetrability coefficients for dry dressings and dressings saturated withwater are shown in Table 11.14. From this table it can be seen that a significant decrease ofair penetrability takes place on saturation with water for all dressings except for ‘Biobrant’.

The penetrability coefficient for dry dressings can be calculated according to the equation:

1 11 3 1 3

PQP

Q

P∑( )= +

+( )air air mat

(11.23)

Pair is obtained from equation (11.17) using D = 2.7 × 10–5 m2/s and solubility at atmosphericpressure equal to 45 mol/m3. The value of Pair is 1.2 × 10–3 mol m/m2 s Pa. The values ofPΣ(air) were taken from Table 11.11.Values of Pmat were calculated from equation (11.18).

The values of PΣ(H2O) can be obtained from the equation:

1 11 3 1 3

PQP

Q

P∑( )= +

+( )H O H O mat2 2

(11.24)

Figure 11.9 The dependence of 1/log t on the pressure in the system for humid air.1: needle-perforated material; 2: Farmexplant; 3: Combutec-2; 4: Syspurderm;

5: Biobrant (silicon-polyamide) compositional dressing; 6: Epigard

315

The calculated values of PΣ( )H O2 fall close to 10–8 mol m/m2 s Pa for the majority of

dressings. This result reveals the extremely low air penetrability for the listed dressings.

For some dressings, the value of PΣ( )H O2 is significantly higher than 10–8 mol m/m2 s Pa.

This can be explained by two effects:

(1) The presence of air flow along the surface of pores (surface flow) [49];

(2) The pressure of channels in the materials that are free of water.

To test these suppositions, additional investigations are required.

11.5.5 Determination of Adhesion of Burn Dressings

Adhesion properties play a key role in dressing performance. The lower layer of a dressingmust be easily wetted, providing good adhesion of the dressing to the wound. Besides,the surface energy at the dressing-wound interface must be minimal to provide the smallesttrauma on its removal from the wound.

detarutas-retawdnayrdrofytilibartenepriafostneiciffeoC41.11elbaT12foerutarepmettasgnisserdnrub ± l ° .C

)lairetam(emangnisserDytilibartenepriafotneiciffeoC

m/mlom( 2 )aPs

yrD teW

)negalloc(xertileH 7.2 × 01 5- 0

)negalloc(2-cetubmoC 1.1 × 01 3- 0

)enahteruylopymaof(dragipE 3.1 × 01 4- 3.1 × 01 5-

)enahteruylopymaof(mredrupsyS 3.1 × 01 4- 0.1 × 01 6-

)enahteruylopymaof(etircnyS 1.1 × 01 3- 0.4 × 01 5-

)enahteruylopymaof(tnalpxemraF 5.4 × 01 5- 0

)nocilis+edimaylop(tnarboiB 8.1 × 01 4- 0.7 × 01 5-

)esolullec(nosnhoJ&nosnhoJ 6.1 × 01 4- 0.3 × 01 6-

)esolullec(lairetamdetarofrep-eldeeN 1.1 × 01 3- –


316


11.5.5.1 Adhesive strength: theory

Adhesive strength characterises the ability of an adhesive structure to preserve its integrity.Adhesive strength as well as the strength of homogeneous solids is of kinetic nature.That is why the rates of surface tension and temperature increase affect the adhesivestrength, and why the scale factors, (i.e., sample dimensions), are also of great importance.

Different theories of adhesion of polymers have previously been suggested [53, 54] as follows:

(1) Mechanical theory (MacBain), according to which the main role is devoted tomechanical filling of defects and pores of the surface (dressing) by the adhesive (blood);

(2) Adsorption theory (Mac-Loren), considering adhesion as a result of the performanceof molecular interaction forces between contacting phases – according to this theory,low adhesion, for example, may be reached between a substrate (dressing) withnonpolar groups and polar adhesive (blood);

(3) Electrical theory (Deriagin), based on the idea that the main factor controlling thestrength of adhesive compounds rests in the double electrical layer that is formed onthe adhesive-substrate interface;

(4) Diffusion theory (Vojytzky), considering the adhesion to be a result of interweavingof the polymer chains;

(5) Molecular-kinetic theory (Lavrentiev), which assumes that a continuous process ofrestoration and breakage of bonds proceeds in the zone of adhesive-substrate contact– thus, adhesive strength is defined by the difference between the activation energiesfor breakage and formation of bonds, and also depends on the correlation betweenthe amount of segments participating in the formation of bonds and the averagenumber of molecular bonds per unit contact area.

In recent years, the thermodynamic concept has received the most attention. Thus, themain role is devoted to the correlation of the surface energies of adhesive and substrate.The thermodynamic work of adhesion of a liquid to a solid (Wa) is described by theDupret-Jung equation:

Wa = γl(1 – cos θ) (11.25)

where gl is the surface tension of the liquid, and θ is the wetting angle. SubstitutingJung’s equation:

γs-l = γs – γs-l cos θ (11.26)

317

into equation (11.25), we obtain the correlation:

Wa = γs + γl – γs-l (11.27)

where γs and γs-l are the surface tension of the solid and of the solid-liquid interface,respectively.

It follows from equation (11.27) that, the higher Wa, the larger are the values of γs and γl

while γs-l are smaller. However, according to equation (11.27), the increase of γs mustlead to the growth of Wa and to an increase of γs-l. That is why the increase of the surfacetension of the substrate is accompanied by the action of two effects. The necessarycondition for adhesive strength is γl >> γs.

Values of γl and Ws−H O2 for different materials are shown in Table 11.15.

11.5.5.2 Adhesive strength of dressings

The adhesive strength of burn dressings was determined according to the method describedin Section 11.3. Table 11.16 shows the adhesive strength of various burn dressings andthe angle of wetting by water.


fokrowcimanydomrehtdnanoisnetecafrusehtfoseulaV51.11elbaT]72[slairetamsuoiravfonoisehda

lairetaM γs )m/Nm()m/Nm(

enelyhteoroulfartetyloP 5.81 38

rebburnociliS 0.12 87

enelyhteyloP 0.13 99

enerytsyloP 0.33 501

etalyrcahtemlyhtemyloP 0.93 301

edirolhclynivyloP 0.93 101

etalahthperetenelyhteyloP 0.34 401

edimaorpacyloP 0.64 701

ssalG 0.071 222

Ws−H O2

318


11.6 The Model of Action of a Burn Dressing

Three main processes proceed after the application of a dressing to a wound:

(1) Sorption of the wound exudate by the dressing;

(2) Water evaporation from the dressing surface;

(3) Mass transfer of gases through the dressing under conditions of ongoing sorptionand evaporation.

Processes (1) and (3) were analysed in detail in Section 11.4. It was found that sorptionof liquid media (water, plasma) proceeds rapidly and reaches a limiting value (maximalsorption ability) after several minutes for most dressings, i.e., a time that is significantlyshorter than the time for which the dressing acts (2-3 days).

The mass transfer of gases (oxygen and nitrogen) through the dressing is 2-4 orders ofmagnitude slower with wet samples than with the dry ones in similar conditions. Next,we consider water evaporation from the dressing surface.

11.6.1 Evaporation of Water from the Dressing Surface

Suppose that a dressing is saturated with water in air at 20 °C and 50% humidity. Thetemperature of the dressing surface is 32 °C. These conditions are chosen to take intoaccount the temperature gradient in the matrix of the dressing.

(htgnertsevisehdA61.11elbaT A (retawybgnittewfoelgnaehtdna) θ fo)sgnisserdnrubsuoirav

)lairetam(emangnisserD A )m/Nm( θ )ged(

)negalloc(muiterroC 022 ± 02 57 ± 2

)enahteruylopymaof(mredrupsyS 012 ± 02 –

)enahteruylopymaof(dragipE 053 ± 05 521 ± 3

)enahteruylopymaof(tnalpxemraF 002 ± 02 031 ± 2

)esolullec(kcap-nreB 071 ± 02 –

)edimaylop-nocilis(tnarboiB 07 ± 01 –

)esolullec(nosnhoJ&nosnhoJ 02 –

)esolullec(gnisserdevisehda-nongnippots-doolB 02 –

)esolullec(reyalrewoldesillatemhtiwgnisserD 071 ± 05 –

319

Let us determine the amount of water that evaporates from the surface of the dressingduring a given time period under stationery atmospheric pressure, and when the dressingsurface is completely saturated with water.

The partial pressure of air at 20 °C and 50% relative humidity equals:

PH O2 = 1.26 × 10–3 kg/cm2, Pair = 1.02 kg/cm2

For air at 32 °C in the saturated state:

PH O2 = 4.85 × 10–2 kg/cm2, Pair = 0.98 kg/cm2

The values of density, viscosity, heat conductivity and heat capacity of air at 26 °C equal:

ρ = 1.185 kg/cm3

μ = 1.861 × 10–6 g/m s

λ = 6.1 × 10–6 kcal/m s °CCp = 0.24 kcal/°C

After mathematical transformations using the method described elsewhere [55-58], thefollowing equation for the mass transfer of water in a dressing can be obtained:

W aP

RTp pm

av

= −( )ρ 1 2 (11.28)

where am is the coefficient of heat conductivity, ρav is the average value of the mixturedensity over and near the surface of the dressing, p1 and p2 are the partial pressurePH O2

at 37 °C and 20 °C, respectively, R is the universal gas constant, and P is thenormal pressure.

Substituting numerical values for a dressing of 1 m × 1 m size, we obtain:

W = 1.2 × 10–1 g/m2 s

If the dressing surface is not completely occupied by water, we should apply the equation:

WC

C= × ×( )

( )−surf H O

surf H O

22

2

g/m s0

11 2 10. (11.29)

where Csurf H O2( ) and Csurf H O2( )0 are the surface concentrations of water on the external

side of the dressing and on the free water surface, respectively.


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11.6.2 Sorption of Fluid by Burn Dressing from Bulk Containing a DefiniteAmount of Fluid

Let us consider the case where a burn dressing is applied to a wound containing a definiteamount of liquid. Assume that a dressing membrane of given size (thickness and surfacearea S) is in contact with the solution of restricted bulk volume V, which contains aconcentration C0(s-s) of diffusive substance. As the dressing becomes saturated by thissubstance, the concentration of the latter in the bulk will decrease.

The solution of the diffusion equation has the following form [55-58]:

m

m

a a

a a q

Dq t

l∞ = −−( )

+ +

⎡

⎣⎢⎢

⎤

⎦⎥⎥

12 1

1

42 2

2

2 exp (11.30)

where q is the positive solution of the characteristic equation:

tgq aq aVSl

= − =; ( )σ

where σ is the distribution coefficient of the substance between the membrane andthe solution.

When a sufficient part of the substance in solution is sorbed by the membrane, the valueof a is small and a simpler expression can be used:

m ma

Dt lt ≈ −

( )⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

∞ 14 2 1 2

π /(11.31)

From equations (11.30) and (11.31), two important correlations can be obtained. Thesorption ability of the dressing, i.e., the part of the substance sorbed from the solutionunder equilibrium conditions, equals:

m∞/m0 = 1/(1 + a) (11.32)

Thus, for the efficient action of the dressing, it is necessary that the concentration C beas high as possible in relation to the products of metabolism and toxins. Relating towater, CH2O ~ 1, it is desirable that the dressing volume (lS) should be close to the volumeof wound exudate (V).

321

The time to reach the degree 0.85 of maximum sorption of liquid media by the dressingequals:

ta l

DV

D S0 85

2 2 2

2 212 12. = =

π π σ(11.33)

It depends on many parameters, each being able to affect the time of completion of thesorption process.

11.6.3 Mass Transfer of Water from Wound to Surroundings

Generally, the change in the amount of water under the dressing in the wound ( mH O2) is

determined from the correlation derived from equations (11.29) and (11.31):

m VC ma

Dt l

C

CS tH O H O H O dressing

surf H O

surf H O2 2 2

2

2

= − −( )

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

− × ×( )( )

( )−0

2 1 2 011

41 2 10

π /. (11.34)

Let us consider the application of the correlation (11.34) for the following case. Thewound characteristics are:

S = 10–2 m–2, CH O2

0 = 106 g/m3, mH O2

0 = 50 g

V = 5 × 10–5 m3, l = 10–3 m, σH O2 = 1

C

C

surf H O

surf H O

2

2

( )

( )=

00 5.

Under these conditions:

t0 85

10

9 4212

25 10

10 109 5 10. .= ×

× ×= ×

−

− −π s# (or ~15 min)

m

m

∞

− − −=

+ × ×( ) =0 5 2 3

1

1 5 10 10 100 17

/. # (or 8.5 g)


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During the same time the following amount of water will evaporate from the dressingsurface:

mevap 2H O g( ) < × × × × =− −0 5 1 2 10 950 10 0 61 2. . .

i.e., the rate of evaporation is significantly (14 times) lower than that of water sorptionby the dressing. All the amount of water from the wound (wound exudate) will evaporateduring the time:

t =× × ×

= ×− −50

0 5 1 2 10 108 3 10

1 24

. .. s# (or ~23 h)

11.7 Criteria for the Efficiency of First-Aid Burn Dressings

11.7.1 Requirements of a First-Aid Burn Dressing

A first-aid burn dressing must meet the following criteria:

(1) Sorption of the wound exudate, containing products of metabolism and toxicsubstances, during the period of dressing action (24-48 h);

(2) Wound isolation from infection of the external medium;

(3) Optimum air and water transfer between wound and surroundings;

(4) Easy removal from the wound, causing no damage to the wound surface.

The characteristics of burn wound dressings, based on the approximate estimations discussedpreviously, are listed next. Note that no quantitative data have been reported in the literature.

11.7.2 Characteristics of First-Aid Burn Dressings

11.7.2.1 Sorption ability of dressings

A second- or third-degree burn wound releases on average 5 × 103 g/m2 of exudate. Asmay be seen from Table 11.11, the water amount is about 90%. The sorption of differentcomponents of the exudate proceeds at different rates. In this case, the free volume of thedressing material will be first filled with water. The diffusion of proteins and cells takesplace in space occupied by water.

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Modern burn dressings possess a porosity of 0.9 and almost the entire free volume can befilled with water (Figure 11.5). The maximum sorption ability for such dressings equals:

CH OH O

2

2≈ρ

ρ

and the amount of the liquid sorbed per unit area is:

m C l l lρ ρρ

ρρ≈ = ≈∞

H OH O 2

2

2 g/m106

because ρH O2 = 106 g/m3.

As a first-aid burn dressing must sorb 5 × 103 g/m3, it follows that:

5 × 103 ≈ 106l (11.35)

and therefore the thickness of a first-aid burn dressing equals:

l ≈ 5 × 103/106 ≈ 5 × 10–3 m (or 0.5 cm)

Thus, the first criterion for the efficiency of a first-aid burn dressing can be formulatedas follows:

A first-aid burn dressing must use its entire free volume for sorption. This volumemust be 0.9 or more of the total volume of the dressing. Dressing thickness must be0.5 cm or more. The majority of foreign, (i.e., non-Russian), first-aid dressings fulfilthis criterion.

11.7.2.2 Air penetrability of dressings

The air penetrability of most of the dry dressings ranges between 10–4 and 10–5 mol m/m2 sPa (Table 11.14). The air penetrability of the dressings saturated with water is muchlower and decreases to values between 10–6 and 10–5 mol m/m2 s Pa, that is, 0.2-2 dm3/m2 s. Thus, the second criterion for the efficiency of first-aid burn dressings can beformulated as follows:

A first-aid burn dressing must possess an air penetrability of 10–5 mol m/m2 s Pa orhigher after the sorption of water. For example, the Biobrant burn dressing fulfilsthis criterion.


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11.7.2.3 Adhesion of dressing to wound

The adhesion strength of dressings with respect to coagulated blood (Table 11.16) variesin a wide range, but it has the minimum value of ~20 N/m. This value should be acceptedas the optimal one, because it corresponds to the minimal pain and damage on removalfrom the surface of natural skin. Thus, the third criterion for the efficiency of first-aidburn dressing can be formulated as follows:

A first-aid burn dressing must possess an adhesive strength to the wound of 20 N/mor less after the end of its action. The following burn dressings, for example, fulfilthis criterion: Biobrant, blood-stopping remedy, Johnson & Johnson.

11.7.2.4 Isolation of wound from infection from external medium

It is known that microorganisms causing wound infection do not penetrate throughfilters possessing average pores size ~0.5 μm. So the fourth criterion for the efficiency offirst-aid burn dressings is as follows:

A first-aid burn dressing must possess no open pores with average diameter largerthan 5 × 10–7 m (0.5 μm). Moreover, it is implied that first-aid burn dressings possesssufficient mechanical strength and elasticity in both dry and humid conditions.

11.8 Conclusion

Experimental methods to estimate the main physicochemical properties of burn dressingswere worked out. Based on theoretical and experimental data we found the following:

(1) The maximal sorption ability of a burn dressing equals the free volume of the dressingmaterial, calculated from the value of the material density.

(2) Water can be used as a model liquid in the study of sorption ability instead of bloodplasma.

(3) Kinetic parameters were determined from the sorption curves. These parametersshowed that first-aid burn dressings markedly differ in the value of the rate of liquidmedia sorption at stages close to the sorption limits.

(4) The air penetrability parameter in the wet state decreases abruptly by 2-3 orders ofmagnitude for the majority of tested dressings. This is due to the filling of pore spaceby the liquid medium.

325

(5) Accordingly, it is recommended that the air penetrability parameter should bedetermined in the wet state, which represents the common condition of action forfirst-aid burn dressings.

(6) The value of the adhesive strength after the end of its action on the wound shouldnot exceed 20 N/m.

From the data obtained in this study, we formulated the following criteria to estimate theefficiency of first-aid burn dressings:

• Maximum sorption ability for water must be at least 10 g/g;

• Optimal thickness of dressings, fulfilling this value of sorptional capacity, must beabout 5 × 10–3 m (0.5 cm);

• Adhesive strength must not exceed 20 N/m;

• Average diameter of open (connected) pores must not exceed 5 × 10–7 m.

References

1. M.I. Fel’dshtein, V.S. Yakubovich, L.V. Raskina and T.T. Daurova, PolymerCoatings for Wound and Burn Treatment, Institute of Information, Moscow,Russia, 1981, 299 (in Russian).

2. G.B. Park, Biomaterials, Medical Devices and Artificial Organs, 1978, 6, 1.

3. V. Rudkovsky, V. Nezelovsky, V. Zitkevich and N. Zinkevich, Theory andPractice of Burn Treatment, Meditsina, Moscow, Russia, 1988, 200 (in Russian).

4. A. Robin and K.H. Stenzel in Biomaterials, Eds., L. Stark and G. Agarwal,Plenum Press, New York, NY, USA, 1969, 157.

5. A. Robin, R.R. Riggio and R.L. Nachman, Transactions of the American Societyof Artificial Internal Organs, 1968, 14, 1669.

6. H.C. Grillo and I. Gross, Surgical Research, 1962, 2, 69.

7. J. Oluwasanmi and M. Chapil, Journal of Trauma, 1976, 16, 348.

8. G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1.


326


9. J.I. Abbendhaus, R.A. McMahon, J.G. Rosenkranz and I.C. McNeil, SurgicalForum, 1965, 16, 477.

10. F.J. Richter and C.T. Riall, inventors; American Cyanamid Company, assignee;US Patent 3,566,871, 1971.

11. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,NY, USA, 1997.

12. G.E. Zaikov, Degradation and Stabilisation of Polymers, Nova SciencePublishers, New York, NY, USA, 1998.

13. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,NY, USA, 1995, 286.

14. J.H. Gardner and D.T. Rovee, inventors; Johnson & Johnson, assignee; US Patent3,521,631, 1970.

15. L.M. Wheeler, inventor; Parke Davis and Company, assignee; US Patent3,648,692, 1972.

16. No inventors; Johnson & Johnson, assignee; UK Patent 1,309,768, 1973.

17. G.L. Wilks and L.L.J. Samuels, Biomedical Materials Research, 1973, 7, 541.

18. I.A. Agureev, Voenno-Meditsinskii Zhurnal, 1963, 6, 74 (in Russian).

19. P. Lock, inventor; no assignee; French Patent 2,156,068A1, 1973.

20. K. Gorkisch, E. Vaubel and K. Hopf, Proceedings of the 2nd InternationalCongress on Plastics in Medicine, Amsterdam, The Netherlands, 1973, PaperNo.16.

21. A.L. Iordanskii, G.E. Zaikov and T.E. Rudakova in Transport, Kinetics,Mechanism, VSP Science Press, Utrecht, The Netherlands, 1993, 288.

22. USSR Certificate No. 245,281, 1969, Bulletin of Certificates, No. 19.

23. W.M. Chardack, M.M. Martin, T.C. Jewett and E.M. Pearce, Plastic andReconstructive Surgery, 1962, 30, 554.

24. C.W. Hall, D. Liotta, J.J. Chidoni, V.M.M. Lobo and A. Valente, Journal ofBiomedical Materials Research, 1972, 6, 571.

25. J.J. Guldarian. C. Jelenko, D. Calloway, L. Kalle and M.Lewin, Journal ofTrauma, 1973, 13, 32.

327

26. E.E. Schmitt and R.A. Polistina, inventors; American Cyanamid Company,assignee; US Patent 3,875,937, 1975.

27. S. Madou, Ed., Polymers for Medicine, Meditsina, Moscow, Russia, 1981, 350(in Russian).

28. USSR Certificate No. 267,010, Bulletin of Certificates, 1970, No.12.

29. G.E. Zaikov, A.L. Buchachenko and V.B. Ivanov, Ageing of Polymers, PolymerComposites and Polymer Blends, Nova Science Publishers, New York, NY, USA,2002.

30. F.C. Moore and L.A. Perkinson, inventors; Moore-Perk Corporation, assignee;US Patent 3,678,933, 1972.

31. M.G.M. Nilsson, R.G.A.B. Udden, P.E.C. Udden and B.A. Wennerblom,inventors; Svenska cellulos Aktiebolaget, assignee, US Patent 3,654,929, 1972.

32. H. Kinkel and S. Holzman, Chirurgie, 1965, 36, 535.

33. USSR Certificate No. 6,658,148, Bulletin of Certificates, 1979, No.15.

34. M.I. Kuzin, V.K. Sologub, V.V. Yudenich, Y.B. Monakov, K.S.Minsker and A.A.Berlin, Khirurgiya, 1979, 8, 86 (in Russian).

35. I.V. Yannas and J.F. Burke, Journal of Biomedical Materials Research, 1980, 14, 65.

36. S. Jacobson and U. Rothenaw, Journal of Plastic and Reconstructive Surgery,1976, 10, 65.

37. D. Spruit and K.E. Malten, Dermatology, 1966, 132, 115.

38. USSR Certificate No. 685,292, Bulletin of Certificates, 1979, No.34.

39. Textbook on Polymer Materials, Ed., N.A. Plate, Khimiya Publishers, Moscow,Russia, 1980, 255 (in Russian).

40. G.E. Zaikov, A.L. Iordanskii and V.S. Markin, Diffusion of Electrolytes inPolymers, VSP Science Press, Utrecht, The Netherlands, 1988, 328.

41. Y.V. Moiseev and G.E. Zaikov, Chemical Resistance of Polymers in ReactiveMedia, Plenum Press, New York, NY, USA, 1986, 586.

42. I.A. Barrie in Diffusion in Polymers, Eds., J. Crack and G.S. Park, AcademicPress, London, UK, 1968, 452.


328


43. S.I. Papkov and E.Z. Fainberg, Interaction of Cellulose and Cellulose Materialswith Water, Khimiya, Moscow, Russia, 1976, 231 (in Russian).

44. G.E. Zaikov, Chemical and Biochemical Kinetics, Nova Science Publishers, NewYork, NY, USA, 2002.

45. S.S. Voyutskii, Physico-Chemical Principles of Fiber Materials – Sorption byPolymer Dispersions, Khimiya, Leningrad, 1969, 336 (in Russian).

46. D.A. Fridrikhsberg, Course of Colloid Chemistry, Khimiya, Leningrad, 1974, 351(in Russian).

47. M.I. Al’tshuller and B.V. Deryagin, in Investigations in the Field of Surface Force,Nauka, Moscow, Russia, 1967, 235 (in Russian).

48. M.M. Mikhailov, Moisture Permeability of Organic Dielectrics, Gosenergoizdat,Moscow, Russia, 1960, 162 (in Russian).

49. N.I. Nikolaev, Diffusion in Membranes, Khimiya, Moscow, Russia, 1980, 232(in Russian).

50. Textbook on Textile Materials, Legkaya Industriya, Moscow, Russia, 1974, 342(in Russian).

51. I.M. Raigorodskii and V.A. Savin, Plasticheskie Massy, 1976, 1, 65 (in Russian).

52. V.N. Manin and A.N. Gromov, Physico-Chemical Resistance of Polymer MaterialsDuring Exploitation, Khimiya, Moscow, Russia, 1980, 247 (in Russian).

53. E. Laifut, Transfer Phenomena in Living Systems, Mir, Moscow, Russia, 1977,520 (in Russian).

54. V.E. Basin, Adhesion Durability, Khimiya, Moscow, Russia, 1981, 208 (in Russian).

55. L.M. Batuner and M.E. Pozin, Mathematical Methods in Chemical Technology,Khimiya, Leningrad, 1971, 822 (in Russian).

56. Polymer Analysis and Degradation, Eds., A. Jimenez and G. Zaikov, NovaScience Publishers, Huntington, NY, USA, 2000, 287.

57. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,NY, USA, 1998, 245.

58. A.Y. Polishchuk and G.E. Zaikov, Multicomponent Transport in Polymer Systemsfor Controlled Release, Gordon and Breach, New York, NY, USA, 1996, 231.

329

12 Testing of Plastic Films

E.M. Abdel-Bary and G. Akovali

12.1 Introduction

Plastics are a very important group of materials. They differ from most of the‘natural’ materials – such as metals, papers, ceramics, natural fibres – mainly as aresult of their ‘viscoelastic’ behaviour. The word ‘viscoelastic’ is used to describebehaviour that shows both viscous and elastic characteristics even at ambientconditions, when stressed. This behaviour is a direct result of the long-chain natureof the polymeric molecules that constitute the plastic material. Whereas the grossmechanical behaviour of most ‘natural’ materials under stress could be consideredas elastic or deformation flow, the response of all plastics to stress is a combinationof the two. The ratio of viscous and elastic components, termed ‘damping’, canvary greatly over quite a narrow temperature range for plastics and it also dependsmarkedly on the rate of stressing.

One of the most common forms of plastic material is the ‘film’. Test methods forplastic films have evolved not only from the techniques of the preceding technologies.The bigger manufacturers and users have also devised their own laboratoryprocedures to enable them to control film properties or determine the suitability ofa film for a particular process or application. In addition, research scientists havepublished the methods that they have used to study the theoretically interestingproperties of polymers. Standards organisations have attempted to devise standardtest methods acceptable to all branches of the industry. This chapter reports brieflyon the most common test methods generally used for plastic films, according to thefield of applications. Although most countries have their own standards andstandards organisation(s), consideration here will be restricted to tests publishedby the American Society for Testing and Materials (ASTM). Anyone interested inthe details of the ASTM tests can find them in ASTM D883 [1], which is one of themany parts of the ASTM Standards and is available in libraries or directly from theASTM or the US Government Printing Office.

330


12.2 Requirements for Test Methods

12.2.1 List of Requirements

There are several requirements necessary for a test method to be developed, some ofwhich are summarised below:

(1) The test method should be rapid, so that results can be used in quality control onhigh-output machinery without delaying producing or dispatch.

(2) Results must be reproducible and consistent between different testing stations andmachines. This means that the test should be insensitive to minor variations in specimenpreparation, to wear and to other small differences in test apparatus.

(3) The precision of the results should be no more than is required. The cost of extremeaccuracy is rarely justified in industry, and often a value that is accurate to within afew per cent will give all the information that is wanted.

(4) It is preferable that the results are scientifically significant. It is imperative that theyare of technological significance and give a meaningful indication of the real-lifeperformance of the film.

The main advantage of a standard method is that results obtained by its use in differentlaboratories can be compared.

12.2.2 Interpretation of Test Results

The main difficulties encountered both in deriving significant tests for polymers and ininterpreting the results are the (relatively rapid) changes in properties with rate ofdeformation and, particularly, with temperature.

The mechanical behaviour of conventional materials is fairly insensitive to temperaturein the normal range of ambient and packaging-processing temperature for the films usedin the packaging industry. However, a polymer, being viscoelastic, may change from aglassy solid through a leathery and then a rubbery stage to a sticky liquid in a temperaturerange of less than 100 °C.

This variation can be of practical importance not only for the manufacturer, who isprevented from using the high temperature sometimes demanded (for example, in printdrying), but also for the designer wishing to provide packages that can be used in

331

Testing of Plastic Films

environments ranging from cold storage at –30 °C to a window display in hot sunshine,where the temperature can exceed 60 °C.

Viscoelasticity is a complex subject, and all polymers exhibit a similar pattern ofbehaviour, the details of which are determined by the chemical nature of the polymer,its molecular weight (molar mass) and molecular weight distribution, degree ofcrystallinity and so on.

Taking polystyrene (PS) as an example of a simple amorphous polymer, one findsthat the elastic modulus is constant in the temperature range up to about 100 °C,which is the glassy region. Increasing the temperature above 100 °C leads to a drasticdecrease in the elastic modulus, as it exists in the leathery region. Further increase intemperature has no effect on the elastic modulus as PS falls in the rubbery region. Inall these three regions – glassy, leathery and rubbery – the moduli of commerciallyuseful polymers are independent of molecular chain length. In the last region, attemperatures exceeding about 170 °C, the polymer falls in the flow region. The basicmolecular phenomena causing these different types of behaviour are reasonably wellunderstood. In the glassy region, the long polymer molecule is frozen, with the atomsvibrating about fixed positions as in any rigid solid. In the leathery (transition) region,where the modulus changes rapidly with temperature, short-range diffusion ofsegments of the polymer chains takes place, but any movement is restricted toindividual atoms of two or three adjacent segments, and the molecule as a wholedoes not move. In the rubbery region, the modulus is fairly constant; here the short-range motions of polymer segments are very fast, and the cooperative movement ofadjacent segments takes place. Entanglements restrict the length of chain that canmove. In the rubbery flow region, the motion of molecules as a whole becomesimportant as a result of slippage of the entanglements; while in the region of flow,changes in the entire molecule take place quicker than the rate of testing, and there islittle elastic recovery at this time-scale. For the last two regions, the modulus dependson the chain length and its distribution.

The modulus versus temperature curve is also rate- (of testing or stressing) dependent,since the major changes in the modulus take place when a particular molecular activityis occurring at large magnitudes at rates faster than the test. This behaviour for themodulus holds true for any usual mechanical property such as yield strength, breakingstrength, breaking elongation, impact strength (or total breaking energy), etc.

Meanwhile, it is important to consider the parameters in the test, which are importantfor the proposed application, e.g., temperature, rate, humidity and geometry, so thatthey cover the range met in use. If not, the data of some other standard test should beable to provide the necessary, and probably the most important, information.

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12.3 Some Properties of Plastic Films

Several standard methods can be used to determine the properties of plastic films.Properties can be purely physical, physico-chemical, chemical or mechanical. For thefirst three, usually the form of the sample does not matter, and the same methods areused for film samples as well as samples with bar shape. Most of the mechanical tests arealso the same methods that are employed in testing plastics in any form, while some arespecific for the plastic films. Following the procedure described in a relevant standardfor tensile properties, it was shown that it is very important to define appropriately thewhole set of parameters involved in the test. In addition, special adaptation of theequipment used is required. Harmonisation of the testing methods for impact and initialtear resistance proved to be more readily obtained. However, some parameters enteringthe corresponding measuring procedures had to be adapted. In general, harmonisationhas been achieved regarding the measurement of the specific mechanical properties [2].Some of the characteristics of these films, usually taken into consideration, are first givenbelow, followed by the mechanical tests and then other tests.

12.3.1 Dimensions

Measurement of the average thickness of a film is straightforward, and no special problemsshould be encountered in their measurement. The accurate measurement of film thicknessis important because the values of some of the other properties – such as tensile strength,elongation at break, impact resistance, resistance to tear propagation – depend stronglyon the thickness of the material. In general, the thicknesses of plastic films are severaltens of micrometres, (e.g., low-density polyethylene (LDPE) agricultural films usuallyrange from 50 μm to more than 200 μm, the latter for greenhouse films). The trend is toreduce thickness to avoid the huge amount of waste at the end of their lifetime.

12.3.2 Conditioning the Samples

In general, the physical and electrical properties of plastics and electrical insulatingmaterials are strongly influenced by the temperature and stress history of the samples(used during their preparation) as well as the humidity. In order to make reliablecomparisons, it is necessary to standardise the temperature and humidity conditionsto which plastics are subjected prior to and during testing. Unless otherwise specifiedfor special polymers, the standard procedure recommended for conditioning samplesprior to testing is described by ASTM D618-61/90 Procedure A [3]. In this method,for specimens thinner or thicker than 7 mm, condition the specimens for a minimum40 h immediately prior to testing (or 88 h for the latter, over 7 mm thickness) in the

333


standard laboratory atmosphere at 23 °C and 50% relative humidity (RH), whilstproviding adequate air circulation on all sides. This can be achieved by placing thesamples in suitable racks, hanging them from metal clips or laying them on wide-mesh, wire screen frames with at least 25 mm between the screen and the surface ofthe bench.

12.4 Mechanical Tests

12.4.1 Tensile Testing (Static)

Tensile tests are mainly used to determine the tensile strength of a material. Such testingprovides data for research, development and engineering design as well as for qualitycontrol and specification. In tensile testing there are certain difficulties with thin films. Itis essential that the cut edges of the tensile specimen are free from nicks or flaws fromwhich premature failure could start.

For thinner films, grip surfaces are a problem. Both slippage in the grip and fracture ofthe sample at the grips must be avoided. Any technique, such as the use of a thin coatingof rubber on the faces or the use of emery cloth, that prevents slipping in the grips,prevents grip fractures and does not interfere with the portion of the sample under test,is acceptable.

From tensile tests, some material characteristics – such as the (tensile) modulus, percentelongation at break, yield stress and strain, tensile strength and tensile energy to breakvalues – can be obtained. Tensile properties (static) of plastics are covered in ASTMD638 (general) [4] and ASTM D882 (films) [5].

12.4.1.1 Tensile Strength

Tensile strength is calculated by dividing the maximum load by the initial cross-sectional area of the specimen, and is expressed as force per unit area (usually inmegapascals, MPa).

12.4.1.2 Yield Strength

Yield strength is the load at the yield point divided by the initial cross-sectional area, andis expressed as force per unit area (MPa), usually to three significant figures.

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12.4.1.3 Tensile Modulus of Elasticity

The tensile modulus of elasticity (or simply the elastic modulus, E) is an index of thestiffness, while the tensile energy to break (TEB, or toughness) is the total energy absorbedper unit volume of the specimen up to the point of rupture. The tensile modulus ofelasticity is calculated by drawing a tangent to the initial linear portion of the load versusextension curve, selecting any point on this tangent, and dividing the tensile force by thecorresponding strain. The results are expressed in MPa, and are usually reported to threesignificant figures. Secant modulus (used for cases where no initial linear proportionalityexists between stress and strain) is defined at a designated strain. TEB is calculated byintegrating the energy per unit volume under the stress-strain curve, or by integrating thetotal energy absorbed divided by the volume of the original gauge region of the specimen.TEB is expressed as energy per unit volume (in megajoules per cubic metre, MJ/m3),usually to two significant figures.

12.4.1.4 Tensile Strength at Break

Tensile strength at break is calculated in the same way as the tensile strength, except thatthe load at break is used in place of the maximum load. It should be noted that, in mostcases, tensile strength and tensile strength at break values are identical.

12.4.1.5 Percent Elongation at Break

Percent elongation at break is the extension at the point of rupture divided by the initialgauge length. It is usually reported to two significant figures.

12.4.1.6 Percent Elongation at Yield

Percent elongation at yield is the extension at the yield point divided by the initial gaugelength of the specimen, usually given to two significant figures.

12.4.1.7 Package Yield of a Plastic Film

A specific ASTM test method (ASTM D4321; [6]) exists for the determination of the‘package yield’ of plastic films, in terms of area per unit mass of the sample. In this test,values such as the nominal yield (the target value of the yield as agreed between the userand supplier), package yield (yield calculated by the standard), nominal thickness (the

335

target value of the film thickness as agreed between the user and supplier), nominaldensity and measured density are defined and obtained. The value of package yield isimportant for the manufacturer, because it determines the actual number of units orpackages that can be derived from a given mass of film in a particular application.

12.4.1.8 ASTM D882 Test for Thin Films

In tensile measurements, discrepancies can and usually do occur in the results, eitherbecause of the use of different specimen types with different geometries and/or becausedifferent test speeds are employed in the testing procedure. However, the data from suchtests cannot be considered appropriate for applications whose load time-scales differwidely from those actually used in the test employed. In fact, the shape of the specimenssuggested may be different depending on the film thickness. They are specified in differentstandards (such as ISO 527 for thick films [7-9], and ISO 1184 [9] and ASTM D882 forfilms less than 0.25 mm; [5]). A brief description of D882-95a is given next.

A load range is selected such that specimen failure occurs within its upper two-thirds,for which a few trial runs are recommended. The cross-sectional area, width (to anaccuracy of 0.25 mm) and thickness (to an accuracy of 0.025 mm for thin films withthicknesses less than 0.25 mm, and for thicker films to an accuracy of 1%) of thesample are measured at several points. The grip separation rate is set and the testspecimen is placed in the grips and tightened evenly. The machine is started, and loadversus extension values are recorded.

Some characteristic tensile values of different plastic films are presented in the tablegiven in ASTM D882-95a.

LDPE is one of the weakest films used as the covering of greenhouses, in terms of tensilestrength (11-37.9 MPa) [10]. As the density of polyethylene (PE) increases from LDPE tohigh-density polyethylene (HDPE), tensile strength at yield and stiffness values are seento increase, while elongation and flexibilities decrease [11]. This is because the crystallineregions significantly increase the modulus of elasticity and hence the ability of the plasticsto support loads at elevated temperature [12].

Another effect observed from the table in ASTM D882-95a is that of strengthening due tothe molecular orientation imparted during film blowing. This is because, on a molecularlevel, tensile properties are higher in the direction of the covalent C–C bond in the chain thanin the transverse direction, which is dominated by the much weaker van der Waals’ bonds.Since the crystals of LDPE films are preferentially oriented parallel to the machine direction,load applied in the machine direction may yield higher values of tensile strength than load


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applied perpendicular to that direction. In fact, not only the direction of the film, but also theprocess parameters – such as melt temperature, die parameters, blow-up ratio, draw ratio,frost-line height and cooling conditions – can lead to different mechanical properties betweentwo films with the same composition [13] (details are given in Chapter 2).

12.4.2 Impact Resistance

Impact values represent the total ability of the material to absorb impact energy, whichis composed of two parts: (a) the energy required to break the bonds, and (b) the workconsumed in deforming a certain volume of the material.

The impact resistance of plastics in general is specified by ASTM D256 [14] as the energyextracted from standardised pendulum-type hammers with one pendulum swing done eitherwith milled notched (Izod and Charpy tests) or unnotched samples, for relatively brittlesamples. The results are reported in terms of energy absorbed per unit specimen width.

For tough plastic films, on the other hand, the free-falling dart method is recommended.There is one specific ASTM standard given for the impact resistance of LDPE measuredby the free-falling dart method (ASTM D1709 [15] or ISO 7765-1 [16] and ISO 7765-2[17]), which is reported in two different cases, for 260 g and 881 g (for 0.20 mm thick)film. LDPE has good toughness, which decreases with the density of the material.

ASTM D1790 [17a] and D746 [18] are test methods for the routine determination of thespecific ‘brittleness’ temperature at which plastics exhibit brittle failure under specifiedimpact conditions. The first method is given for a thin (0.25 mm or less) plastic film, andthe second is for real loading conditions. Thus ways to predict the behaviour of thematerial at low temperatures can be made, which is important for plastic films that areused in variable temperature conditions. The test applies for similar conditions ofdeformation, and the brittleness temperature is estimated statistically in the test as thatat which 50% of the specimens would fail.

12.4.2.1 Impact Resistance by Free-Falling Dart Method

The test method ASTM D1709-91 [15] covers the determination of the energy that causesa plastic film to fail under specified conditions of impact of a free-falling dart. Thisenergy is expressed in terms of the weight (mass of the missile), falling from a specifiedheight, that would result in 50% failure of the specimens tested. The impact resistance ofa plastic film, while partly dependent on its thickness, has no simple correlation withsample thickness.

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The specimen for the test should be large enough to extend outside the specimen clampgaskets at all points. The specimens will be representative of the film under study, andshould be free from pinholes, wrinkles, folds or other obvious imperfections, unless suchimperfections constitute variables under study.

12.4.2.2 Pendulum Impact Resistance

Like other techniques to measure toughness, this test method (ASTM D256 [14]) provides ameans to determine the parameters of a material at strain rates close to those applicable insome enduse applications, and the results are more valid than those provided by low-speeduniaxial tensile tests. The dynamic tensile behaviour of a film is important, particularly whenthe film is used as a packaging material. The same uncertainties about correlations withthickness that apply to other impact tests (such as ASTM D1709 [15]) also apply to this test.

Several impact test methods are used for film samples. It is sometimes desirable to knowthe relationships among the test results derived by different methods. A study was conductedin which films made from two resins [polypropylene (PP) and linear low-density polyethylene(LLDPE)], with two film thicknesses for each resin, were impacted using ASTM test methodsD1709 [15], D3420 [19] and D4272 [20]. Differences in results between test methodsD1709 and D4272 may be expected, since test method D1709 represents failure-initiatedenergy while test method D4272 represents initiation plus completion energy.

12.4.2.3 Hail Resistance

Although impact resistance is a valuable property to measure, the complexity andmultiplicity of events occurring during impact make the value obtained applicable onlyunder narrow conditions and not suitable for general design purposes. Thus, service-related impact tests have been devised for large-volume applications as greenhousecoverings. According to this method, a complete half of a greenhouse roof is builthorizontally and is randomly shot with Nylon balls. The impact damage is registeredwith a camera. Single glass, 4 mm thick, is considered to be the reference material, andall other materials are compared to that.

12.4.3 Tear Resistance

The tear resistance of a plastic film is a complex function of its ultimate resistance torupture. There are different ASTM standards available for the tear resistance of films:ASTM D1004 [21] is designed to measure the force necessary to initiate tearing at very


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low rates of loading, while ASTM D1938 [22]covers the force necessary to propagate atear by a single tear. ASTM D1922 [23] is the determination of the average force to propagatetearing through a specified length of plastic film by use of an Elmendorf-type tearing tester.In ASTM D2582 [24], the puncture-propagation tear resistance of films is of interest.

In these tests, two different values are of interest and are measured:

(1) the force required to initiate the tear (ASTM D1004 and ISO 344 [25]);

(2) the force needed to propagate the tear (ASTM D1938, D1922 and ISO 6383-1; [26]).

ISO standards are specific to applications in greenhouses. The second value (the forceneeded to propagate the tear) can be considered to be of most interest, because, while itmight occasionally be impossible to prevent a film from tearing in greenhouse applications,(e.g., when the film is not fastened securely, flaps in the wind, and hits a protruding partof the structure), it is highly beneficial if the tear propagates with great difficulty. Resistanceto initiation of tear is also important and cannot be neglected in general.

The tear resistance of plastic films is very important with regard to their overall mechanicalbehaviour and common failure mechanisms, i.e., for agricultural plastic films. Theresistance to tear propagation for LDPE film is found to vary significantly. The reportedvalue of resistance to tear propagation is 5-20 N [27]. Possible sources of this variationare the anisotropy, elongation effects and variable thickness of the tested films, as well asthe use of different speeds during tearing.

12.4.3.1 Propagation Tear Resistance of Plastic Film and Thin Sheeting byPendulum Method

This test (ASTM D1922-94a; [23]) covers the determination of the average force topropagate tearing through a specified length of plastic film. It is widely used in packagingapplications. While it may not always be possible to correlate film tearing data withother mechanical or roughness properties, the apparatus for this test method provides acontrolled means for tearing specimens at straining rates approximating some of thosefound in actual packaging service. Owing to orientation during manufacture, plasticfilms and sheeting frequently show marked anisotropy in their resistance to tearing. Thisis further complicated by the fact that some films elongate greatly during tearing, even atthe relatively rapid rates of loading encountered in this test method. The degree of thiselongation is dependent in turn on film orientation and the inherent mechanical propertiesof the polymer from which it is made. There is no direct relationship between tearingforce and specimen thickness. The tearing force is usually expressed in milli newtons(mN) or gram-force.

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A comparison of propagation tear resistance (Elmendorf tear) in machine direction andtransverse direction of different types of plastic film is given in a table in ASTM D1922-94a. From the data given in this table,, it can be observed that LLDPE possesses thehighest value of tear resistance in both machine and transverse directions. PP, whichshows a low value of tear resistance in the machine direction, has a higher value in thetransverse direction. The difference between the two directions reflects the degree oforientation and anisotropy of the material. PS orientation during processing is notremarkable, and consequently its tear resistance does not differ in the two directions.

12.4.3.2 Puncture-Propagation Tear Resistance

This test method (ASTM D2582-93; [24]) covers the determination of the dynamic tearresistance of plastic film and film sheeting subjected to enduse snagging-type hazards.

The puncture-propagation tear test measures the resistance of a material to snagging, or,more precisely, to dynamic puncture and propagation of that puncture resulting in a tear.Failure due to snagging hazard occurs in a variety of enduses, including industrial bags,liners and tarpaulins. The tear resistance measured by the instrument in this test is innewtons (N).

Tear resistance can be measured using a standard drop height of 508 ± 2 mm or a non-standard drop height (or carriage weight).

12.4.4 Bending Stiffness (Flexural Modulus)

Test methods ASTM D747 [28] and D790 [29] cover the determination of the bendingstiffness of plastic sheets and films. In the test, specimens are subjected to three- or four-point bending loads, such as a cantilever beam, and the force and angle of bending areused to determine the apparent flexural modulus (or bending stiffness) and yield strength.

12.4.5 Dynamic Mechanical Properties

Tests by dynamic mechanical analysis (DMA) provide the elastic and loss moduli as wellas the loss tangent (damping) as functions of temperature, frequency and/or time. Theseplots are indicative of the viscoelastic characteristics of the plastic. As the modes ofmolecular motion in the specimen change with temperature (or frequency), acorresponding transition temperature occurs. The most significant transition temperaturesare the glass transition temperature (Tg) and the melting temperature (Tm). In addition,


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there may be a number of sub-glass transition temperatures, which can be very importantin determining the toughness of the material. In the temperature ranges where significantchanges are observed in the modes of molecular motion, a number of mechanicalproperties, e.g., elastic modulus, decrease rapidly with increasing temperature (at constantor near-constant frequency) or increase with increasing frequency (at constanttemperature). Hence DMA tests (provided by ASTM D4065; [30]) provide determinationof transition temperatures, elastic modulus and loss modulus over a range of temperatures(from –160 °C to degradation), frequencies (0.01 to 1000 Hz) and times, by free vibrationand resonant or nonresonant forced vibration techniques. DMA is usually applied formaterials with elastic modulus from 0.5 MPa to 100 GPa [31].

DMA tests have been shown to be useful to evaluate a number of properties, for example,(1) degree of phase separation (in multicomponent systems), (2) effects of a certainprocessing treatment, and (3) filler type and amount, among others. DMA is very usefulfor quality control in general, for specification acceptance and in research, and it canalso be used to determine, e.g., (1) stiffness and its change with temperature, (2) degreeof crystallinity, (3) magnitude of triaxial stress state in rubber phase for rubber-modifiedplastics, etc.

DMA tests incorporate laboratory practice for determining the dynamic mechanicalproperties of plastic films subjected to various oscillatory deformations on a variety ofinstruments (generally called dynamic mechanical analysers, thermomechanical analysers,mechanical spectrometers or even viscoelastometers).

12.5 Some Physical, Chemical and Physicochemical Tests

12.5.1 Density of Plastics

The density of solid plastics is a conveniently measurable property, which is useful tofollow the occurrence of physical changes, as well as to indicate uniformity among samples.ASTM D1505 [32] covers the method for density determination through observation ofthe level to which a test specimen sinks in a liquid column exhibiting a density gradient,in comparison with standards of known density.

12.5.2 Indices of Refraction and Yellowness

The refractive index test is useful for controlling the purity and composition of films oftransparent plastics for simple identification purposes, and it is done by use of arefractometer (ASTM D542; [33]), usually to four significant figures.

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For homogeneous, non-fluorescent, nearly colourless transparent and/or nearly whitetranslucent-opaque plastic films, the yellowness index test is recommended to determinethe degree of yellowness or degree of its change. Yellowness is defined as the deviation inchroma from whiteness in the dominant wavelength range from 570 to 580 nm relativeto magnesium oxide for CIE Source C. In the test, data are collected using a Hardy GEtype spectrophotometer or an equivalent system. A change in the yellowness index istaken as a measure of degradation (under exposure to heat, light or other environment)and has proved to be a very useful parameter for plastic films.

12.5.3 Transparency

The clarity of a film is measured by its ability to transmit light in the visible region.The regular transmittance of film and sheet materials (defined as the ratio of undiffusedtransmitted flux to the incident flux) can be obtained by following ASTM D1746 [34].

12.5.4 Resistance to Chemicals

Plastic films can be subjected to various chemicals and corrosive conditions, and theirresistance to these should be tested. ASTM D543 [35] covers a general test method forall type of plastic materials. The test follows the changes in weight, dimensions, appearanceand strength properties. As indicated in the test, the choice of type and concentration ofreagent, duration of immersion and temperature are all arbitrary, and this poses themain limitation of the method.

12.5.5 Haze and Luminous Transmittance

Light scattered from a film can produce a hazy or smoky field when viewed through thematerial. Haze is the cloudy or turbid appearance of an otherwise transparent materialas a result of light scattered within or from the surface of the specimen. ASTM D1003[36] provides a test method for the evaluation of specific light-transmitting and light-scattering properties of transparent plastic films. A hazemeter or a spectrometer [37] isused, which can provide very useful diagnostic data for the reason for the haze.

In the test, the intensity of the incident light (I1), the total light transmitted by the specimen(I2), the light scattered by the instrument (I3) and the light scattered by the instrumentand specimen (I4) are all measured. From these, the total transmittance (Tt) is calculatedas Tt = I2/I1; and the diffuse transmittance (Td) is calculated from:

TI I I I

Id = −4 3 2 1

1

( / )(12.1)


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From these, the per cent haze is calculated as:

haze d

t= ×T

T100 (12.2)

Materials having a haze value greater than 30% are considered diffusing and shouldbe tested.

12.5.6 Ignition, Rate of Burning Characteristics and Oxygen Index (OI)

Most plastic films are flammable. There are three different ASTM methods available totest the ignition and rate of burning characteristics and to evaluate the per cent oxygennecessary to initiate burning, namely the oxygen index. ASTM D635 [38] covers a small-scale laboratory screening procedure to compare the relative rates of burning of self-supporting plastic films tested in the horizontal position using a burner. ASTM D1929[39] is for determination of the self-ignition temperature (the lowest initial temperature ofair passing around the specimen at which, in the absence of an ignition source, the self-heating of the specimen leads to ignition) and flash ignition temperature (the lowest initialtemperature of air passing around the specimen at which a sufficient amount of combustiblegas is evolved to be ignited by a small external flame) of plastics by using a hot-air ignitionfurnace. The oxygen index test (ASTM D2863; [40]) covers tests that find the minimumoxygen concentration to support candle-like combustion of plastic film.

12.5.7 Static and Kinetic Coefficients of Friction

The frictional properties of film surfaces may contribute markedly to film behaviourin packaging machinery and to the stacking properties of sacks. Slip agents are frequentlyadded to film to improve its frictional behaviour. However, films containing additivesoften take considerable time to develop their full properties while the additives diffuseto the surface, and care must be taken in choosing the time after manufacture to carryout the test. The ASTM D1894-95 test method [41] covers determination of thecoefficients of starting and sliding friction of plastic film and sheeting, when relativesliding occurs between the film and other substances under specified test conditions.The procedure permits the use of a stationary sled with a moving plane film, or the useof a moving sled with a stationary plane film. The static or starting coefficient offriction (μs) is related to the force measured to begin movement of the surfaces relativeto each other. The kinetic or sliding coefficient of friction (μk) is related to the forcerequired to sustain this movement.

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Measurements of frictional properties may also be made on a film or sheet specimenwhen sliding over itself or over another substance. The coefficient of friction is related tothe slip properties of plastic films, which are of wide interest in packaging applications.These methods yield empirical data for control purposes in film production. For instance,slip properties are generated by additives in some plastic films, for example, polyethylene.These additives have varying degrees of compatibility with the film matrix. Some ofthem bloom, or extrude to the surface, lubricating it and making it more slippery. Becausethis blooming action may not always be uniform in all areas of the film surface, valuesfrom these tests may be limited in reproducibility. Besides, this blooming action of manyslip additives is time-dependent. For this reason, it is sometimes meaningless to comparethe slip and friction properties of films or sheets produced at different times, unless themethod is designed to study this effect.

Plastic films (not greater than 0.245 mm thick) and sheeting (greater than 0.245 mmthick) may exhibit different frictional properties in their respective principal directionsdue to anisotropy or extrusion effects. Specimens may be tested with their long dimensionsin either the machine direction or transverse direction of the sample, but it is more commonto test the specimen with its long direction parallel to the machine direction. The testsurface must be kept free of dust, lint, fingerprints, or any foreign matter that mightchange the surface characteristics of the specimen. The static and kinetic coefficients offriction (μs and μk, respectively), are calculated from:

μs = As/B (12.3)

μk = Ak/B (12.4)

where As is the initial scale reading (g) at which motion just begins, Ak is the average scalereading (g) obtained during uniform sliding of the film surface and B = sled weight (g).

12.5.8 Specular Gloss of Plastic Films and Solid Plastics

This test (ASTM D2457-90; [42]) covers the measurement of gloss of plastic films, bothopaque and transparent. Specular gloss is defined as the relative luminous reflectancefactor of a specimen in the mirror direction. Specular gloss is used primarily as a measureof the shiny appearance of film and surfaces. Precise comparisons of gloss values aremeaningful only when they refer to the same measurement procedure and the same generaltype of material. In particular, gloss values for transparent films should not be comparedwith those of opaque films, and vice versa.

Gloss is a complex attribute of a surface, which cannot be completely measured by any singlenumber. Specular gloss usually varies with surface smoothness and flatness. The instrument


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used consists of an incandescent light source to produce the incident beam, a means to locatethe surface of the specimen, and a receptor located to receive the required pyramid of raysreflected by the specimen. The receptor is a photosensitive device that is responsive to visibleradiation. The receptor measurement mechanism should give a numerical indication that isproportional to the light flux passing the receptor field stops within ±1% of full-scale readings.

Specimen surfaces should have good planarity, since surface warpage, waviness or curvaturemay seriously affect test results. The direction of machine marks, or similar texture effects,should be parallel to the plane of the axes of the two beams. Surface test areas must be keptfree of soiling and abrasion. Gloss is due chiefly to reflection at the surface; therefore,anything that changes the surface physically or chemically is likely to affect gloss.

12.5.9 Wetting Tension of PE and PP Films

In this test method (ASTM D2578-94 [43]) drops of a series of mixtures of formamideand cellosolve (ethyleneglycol monoethyl ether) of gradually increasing surface tensionare applied to the surface of the polyethylene or polypropylene film until a mixture isfound that just wets the film surface. The wetting tension of the PE or PP film surfacewill be approximated by the surface tension of this particular mixture. The ability of PEand PP films to retain inks, coating, adhesives, etc., is primarily dependent upon thecharacter of their surfaces, and can be improved by one of several surface-treatmenttechniques mentioned in Chapter 8.

The same treatment techniques have been found to increase the wetting tension of PE orPP film surfaces in contact with a mixture of formamide and ethyl cellosolve in the presenceof air. It is therefore possible to relate the wetting tension of a PE or PP film surface to itsability to accept and retain inks, coating, adhesives, etc. The measured wetting tension ofa specific film surface can only be related to acceptable ink, coating, or adhesive retentionthrough experience. Wetting tension in itself is not a completely reliable measure of ink orcoating retention, or adhesion. A wetting tension of 3.5 × 10-2 N/m or higher has generallybeen found to reveal a degree of treatment normally regarded as acceptable for tubularfilm made from PE and intended for commercial flexographic printing.

A table showing the measured wetting tension of PE and PP film as a function of theconcentration of a mixture of ethyl cellosolve and formamide is given in ASTM D2578-94 [43].

Note that a solution is considered to wet a test specimen when it remains intact as acontinuous film of liquid for at least 2 seconds. The reading of the liquid film behaviourshould be made in the centre of the liquid film. Shrinking of the liquid film about itsperiphery does not indicate lack of wetting. Breaking of the liquid film into droplets

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within 2 seconds does indicate lack of wetting. Too much liquid being placed upon thefilm surface may cause severe peripheral shrinkage.

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films

Unrestrained linear thermal shrinkage, expressed as the percentage of the original dimension,is defined as ‘the irreversible reduction in linear dimension at elevated temperatures whereno restraint to inhibit shrinkage is present’. During the manufacturing processes, internalstresses that occur might be locked into the film, which can be released afterwards byproper heating. The temperature at which shrinkage occurs is mainly related to the processingtechniques employed and may also be related to the phase transition in the base resin. Themagnitude of the shrinkage varies with the temperature. Shrinkage of a particular materialproduced by a process may be characterised by the ASTM D2732 test method [44], bymaking measurements at several temperatures through the shrinkage range of the material.The experiment is usually carried out in a constant-temperature liquid bath accurate to±0.5 °C. It is a prerequisite that the liquid of the bath should not plasticise or react with thespecimen. Polyethyleneglycol, glycerine and water have been found to have wide applicabilityfor this purpose. Immersion of the sample (100 × 100 mm2) for 10 s has been determinedto be generally adequate for most thermoplastics of up to 50 μm thickness.

Unrestrained linear shrinkage is calculated using:

unrestrained linear shrinkage (%) f= − ×L LL

0

0100 (12.5)

where L0 is the initial length of side (100 mm) and Lf is the length of side after shrinkage.

12.5.11 Shrink Tension and Orientation Release Stress

The ASTM D2838 test [45] measures the maximum force of a totally restrained specimenand the maximum force of a specimen permitted to shrink a predetermined amountprior to restraint in a liquid bath at selected temperatures. The results obtained areespecially important and useful for shrink-wrap films and shrink-wrap packaging design.

12.5.12 Rigidity

Rigidity affects the machinability of plastics. It depends mainly on the stiffness of the material,on its thickness, as well as on a number of other factors such as static electricity, frictional


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properties, etc. The standard test method ASTM D2923 [46] is specific for the rigidity ofpolyolefin films and sheeting. In the test, the resistance of the sample to flexure is measured(by a strain gauge affixed to the end of the sample) and by use of a microammeter connectedto the gauge and calibrated; rigidity is read directly as grams per centimetre of sample width.

12.5.13 Blocking Load by Parallel-Plate Method

Blocking (unwanted adhesion) is a problem with plastic films, which develops duringprocessing and/or storage, and happens when touching layers of films are in intimatecontact with almost complete exclusion of air between them. Blocking is induced byincrease of temperature and/or pressure. The standard test method provided by ASTMD3354 [47] simulates the operation of separating blocked films in some enduseapplications. The load (in grams) needed to separate blocked samples [five groups ofspecimens each cut to 100 × 180 mm2] is measured by a beam-balance system (similar toan analytical balance). The test, in summary, is as follows: One sheet of the blockedspecimen is secured to an aluminium block suspended from the end of the balance beam,while the other end is fixed to another aluminium block fastened to the balance base.Weight is then added equivalent to 90 ± 10 g/m to the other side of the beam until thefilms totally separate (or until they reach 1.905 cm separation). The film-to-film adhesionis expressed as grams, and the test is limited to maximum 200 g of load.

12.5.14 Determination of LLDPE Composition by 13C NMR

The performance properties of ethylene copolymer plastic films depend on the numberand type of short-chain branches. The ASTM D5017 method [48] allows one to measurethem for ethylene copolymers with propylene, 1-butene, 1-octene and 4-methyl-1-pentene.For this, the polymer sample (about 1.2 g) is dispersed in a solvent (1.5 ml) and a deuteratedsolvent (1.3 ml), put into a 10 mm nuclear magnetic resonance (NMR) tube, and analysedat high temperatures by using 13C NMR spectroscopy, usually a 13C pulsed Fouriertransform with a field strength of at least 2.35 T. Spectra are recorded under conditionssuch that the responses of each chemically different carbon are identical. The integratedresponses for carbons originating from different comonomers are used for calculation ofthe copolymer composition. Results are presented as mole per cent alkene and/or branchesper 1000 carbon atoms.

12.5.15 Creep and Creep Rupture

Creep is defined as the increasing strain over time in the presence of a constant stress,and is expressed as the per cent extension (creep strain per cent). The practical importance

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of creep is due to the need (a) to determine the limits of excessive deformation and (b) tounderstand creep rupture. The two mechanisms are especially important for LDPEgreenhouse covering materials, because LDPE, with 8.23% creep, has the second highestcreep value of all greenhouse covering films [11]. There is a significant variation in thecreep values of LDPE, which is attributed to the fact that the creep resistance of LDPEincreases with density and with the content of ethylene-vinyl acetate (EVA) in the material’scomposition. Creep is also strongly dependent on the service temperature of the coveringmaterial. ASTM D2990 [49] is a general test method for plastics to characterise creepand creep rupture. The test is applicable to different loading conditions, (e.g., tensile,flexural, compressive, etc.), and helps to determine the creep strength and modulus ofstandard specimens for use in comparing materials and in design.

12.5.16 Outdoor Weathering/Weatherability

The ASTM D1435 test [50] is used to evaluate the stability of plastic films when exposedoutdoors to the varied influences of the atmosphere and weather. The general climate,the season, the time of day, the presence or absence of industrial pollutants in theatmosphere, and annual variations in the weather are the most important factors, andthe results are taken as indicative only. Short-term accelerated exposure tests are alsoavailable by use of a special chamber equipped with a carbon-arc light (ASTM G152[51] and ASTM G153 [52]), which can indicate the relative outdoor performance, butcannot be used to predict the absolute long-term performance.

12.5.17 Abrasion Resistance

Abrasion is a surface phenomenon that occurs mechanically, and it is important in thesense that it can significantly degrade certain physical properties (light transmission,thermal effect through loss of thickness, etc.), as well as some mechanical properties,(e.g., impact resistance, tear resistance). As a result it has a direct impact on the functionalcharacteristics of covering materials. Abrasive damage to transparent plastic films isjudged by following the change of the optical properties (ASTM D1044; [53]) as well asby volume loss in general (by using abrasion testing machines, ASTM D1242; [54]).

The abrasion resistance of plastic films used in greenhouses is of utmost importance.Abrasion, in this case, occurs due to the effect of particles carried by the wind, which canbe significant in some areas where greenhouses are built. In this case, abrasion can leadto the loss of transparency and reduction in mechanical properties much earlier thanexpected. Abrasion in general is affected by the exact formulation of the film, and by theincorporation of (amount and type of) filler, additives and pigments, which can lead tovarying results. Another important factor is that rapid chemical oxidation of the surfacelayer may occur due to the buildup of localised high temperatures during abrasion [55].


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It is worth mentioning that abrasion can ultimately lead to increased degradation of thefilm, since more active centres for photooxidation are in general created by this procedure.There is a direct relationship between the density of a PE film and its abrasion resistance:increase of density increases the abrasion resistance.

12.5.18 Mar Resistance

Test ASTM D673 [56] covers the determination of the extent of resistance of plastic filmsurfaces to surface marring, mainly caused by falling abrasive particles. The test simulatesthe relatively mild airborne abrasive action that occurs in actual use, and different materialsare ranked according to their relative mar resistances.

12.5.19 Environmental Stress Cracking

The ability of a polymer surface to withstand an aggressive medium under load isknown as environmental stress-cracking resistance (ESCR). Environmental stresscracking is a characteristic that depends on the nature and level of stresses applied aswell as on the thermal history of the sample and the environment, and is also calledstress corrosion [57]. Under certain conditions of stress and in the presence of certainenvironments, environmental stress cracking occurs. For example, in the presence ofsoaps, wetting agents and detergents, ethylene plastics may exhibit mechanical failureby cracking. Typically, increased ESCR is obtained with increased polymer molecularweight. ASTM D1693 [58] is specific for the environmental stress cracking of ethyleneplastics. A stress crack is an external or internal rupture in the film caused by tensilestresses less than its short-time mechanical strength. The environment accelerates thedevelopment of stress cracks. The appearance of what seem to be cracks on the surfacesof transparent polymers develops under tensile stress, with the plane of the craze beingnormal to the stress direction. Crazes usually initiate at surfaces but can developinternally under special circumstances as well. They reflect light in a manner similar tocracks, and indeed often precede early fracture of the film. In the test, bent specimens,each having a controlled imperfection on one surface, are exposed to the action of asurface-active agent, and the proportion of the total number of specimens that crack ina given time is reported.

12.5.20 Water Vapour Permeability

In the packaging of hygroscopic materials, and particularly in packaging of food, thepermeability properties of the film to water vapour and other gases is very important.

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ASTM D3079 [59] and E96 [60]/F372 [61]/F1249 [62] cover standard test methods forplastic packaging films and plastics for water vapour permeability. These tests are especiallyimportant and are used for packaging plastic films. In the first of these, desiccant- orproduct-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constanttemperature, and weighings are repeated to constant rate of moisture gain. Water vapourpermeabilities are reported in grams per 30 days. In the second test method, desiccant- orproduct-filled packages are again exposed to a normal atmosphere of 90 ± 2% RH at twodifferent temperatures for 24 hours and 6 days, respectively. Hence cycling between coldand hot/moist atmospheres is achieved. In this test, weighings are repeated to constant rateof moisture gain, and water vapour permeability is reported in grams per cycle. In the thirdmethod mentioned, desiccant- or product-filled packages are exposed to a normalatmosphere of 90 ± 2% RH at constant temperature for at least 1 month; average rate ofwater gain is reported. In the last two methods, infrared (F372) and modulated infrared(F1249) detection of water vapour transmitted from a moist atmosphere to a dry air streamis made, which provides a measure of water vapour transmission rates.

12.5.21 Oxygen Gas Transmission

ASTM D1434 [63] and D3985 [64] cover standard test methods for packaging plasticfilms and sheeting materials for their oxygen gas transmission. Basically the methods usedcan be divided into three types, varying either pressure, volume or concentration. In variable-volume methods, gas is introduced at a high pressure on one side of the film, the chamberon the other side normally being at atmospheric pressure. The change in volume is followedas a function of time. A manometer is used to measure the pressure of oxygen transmitted,from which the rate of transmission at steady state can be calculated. In another method,a coulometric sensor is used, which measures the rate of oxygen transmitted through aspecimen exposed on one surface to oxygen and on the other to nitrogen.

Considerable experimental difficulties are normally encountered in achieving airtightseals and in the initial calibration of the instrument to allow for the deadspace in thefilter paper and discs used to support the film

12.6 Standard Specifications for Some Plastic Films

There are several standards available to specify plastic films, such as:

• ASTM D5047 [65] for polyethylene terephthalate (PET) films;

• ASTM D4635 [66] for LDPE films;


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• ASTM D3981 [67] for medium-density polyethylene (MDPE) films;

• ASTM D2103 [68] for PE films;

• ASTM D2673 [69] for oriented polypropylene (OPP) films;

• ASTM D2647 [70] for crosslinkable ethylene plastics.

12.6.1 Standard Specification for PET Films

Specification ASTM D5047 [65] covers biaxially oriented PET films in the range of1.5-35.5 μm, containing at least 90% PET homopolymer. The thicknesses should bewithin ±18% to ±14% of nominal for film tested in accordance with ASTM D374[71]; while the requirements for the width (within ±1.6 mm and ±3.2 mm of nominalfor rolls up to 1 m or larger, respectively), and weight (within ±10% and ±5% fororders up to 110 kg or over, respectively), are also given. The film will be testedappropriately to establish conformance to the critical requirements as agreed by thepurchaser and seller.

12.6.2 Standard Specification for LDPE Films (for General Use and PackagingApplications)

Specification ASTM D4635 [66] covers unpigmented, unsupported, tubular LDPE filmswith densities between 910 and 925 kg/m3 (0.910-0.925 gm/cm3), for general use andfor packaging applications. It is also applicable to polyethylene copolymer (low-pressurePE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylene-vinyl acetate copolymers. The thicknesses are 100 μm or less and the maximum widthsare 3 m. The specification covers dimensional tolerances (including thickness, width,length and yield), intrinsic quality requirements (density, workmanship, tensile strength,heat sealability, odour, impact strength, coefficient of friction, optical properties, surfacetreatment, etc.), and test methods.

12.6.3 Standard Specification for MDPE and General Grade PE Films (forGeneral Use and Packaging Applications)

Specification ASTM D3981 [67] is for unpigmented, unsupported, sheet or tubularMDPE films with densities between 926 and 938 kg/m3 (0.926-0.938 g/cm3), for generaluse and for packaging applications. It is also applicable to polyethylene copolymer

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(low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers,including ethylene-vinyl acetate copolymers. The thicknesses are 25-100 μm and themaximum widths are 3.05 m. The standard excludes heat-shrinkable films. Thespecification covers dimensional tolerances (including thickness, width, length andyield), intrinsic quality requirements (density, workmanship, tensile strength, heatsealability, odour, impact strength, coefficient of friction, optical properties, surfacetreatment, etc.) and test methods.

Specification ASTM D2103 [68] covers general specifications for polyethylene films.

12.6.4 Standard Specification for OPP Films

Specification ASTM D2673 [69] covers OPP films of 10-50 μm thickness with ±10%of the nominal value, composed of Group 1 or 2 propylene (ASTM D4101 [72]), or ablend of such Group 1 and/or Group 2 polypropylene with one or more other types ofpolymers where the polypropylene fraction is the main component. It must have normalappearance (be free of gel, streaks, pinholes, particulates, etc., as well as undispersedraw materials) and it should not block excessively. The average width will be within–3 to +19 mm of nominal.

If the film yields a minimum tensile strength of 103 MPa in at least one principal(machine or transverse) direction, it is termed oriented polypropylene (OPP). If thefilm is oriented in one (machine or transverse) direction and yields a minimum tensilestrength of 103 MPa in the orientation direction, it is called as uniaxially oriented PPfilm. If the film tensile strengths in both the machine and transverse directions exceed103 MPa, it is biaxially oriented PP. If the film tensile strengths in both the machineand transverse directions exceed 103 MPa, but do not differ by more than 55 MPa,and the machine and transverse elongations do not differ by more than 60%, it isbalanced oriented PP.

12.6.5 Standard Specification for Crosslinkable Ethylene Plastics

Specification ASTM D2647 [70] covers crosslinkable ethylene plastics and compounds.There are mainly two different types: mechanical types (type I) and electrical types (typeII). In the former, mechanical properties (strength, ultimate elongation, elongationretention after ageing, apparent modulus of rigidity, brittleness temperature) are themost important in applications.


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References

1. D833, Terminology Relating to Plastics, 2000.

2. D. Briassoulis and A. Aristopoulou, Polymer Testing, 2001, 20, 615.

3. ASTM D618-00, Standard Practice for Conditioning Plastics for Testing, 2000.

4. ASTM D638-02, Standard Test Method for Tensile Properties of Plastics, 2002.

5. ASTM D882-02, Standard Test Method for Tensile Properties of Thin PlasticSheeting, 2002.

6. ASTM D4321-99, Standard Test Method for Package Yield of Plastic Film, 1999.

7. ISO 527-1, Plastics - Determination of Tensile Properties - General Principles,1994.

8. ISO 527-2, Plastics - Determination of Tensile Properties - Test Conditions forMoulding and Extrusion Plastics, 1994.

9. ISO 527-3, Plastics - Determination of Tensile Properties - Part 3: TestConditions for Films and Sheets, 2001.

10. D. Briassoulis, D. Waayenberg, J. Gratraud and B.J. Von Elsner, Journal ofAgricultural Engineering Research, 1997, 67, 81.

11. P.C. Powell, Engineering Design Guides, 1979, 19, 1.

12. G. Gruenwald, Plastics: How Structure Determines Properties, Hanser, Munich,Germany, 1992.

13. R.M. Patel, Polymer Engineering Science, 1994, 34, 1506.

14. ASTM D256-00e1, Standard Test Methods for Determining the Izod PendulumImpact Resistance of Plastics, 2000.

15. ASTM D1709-01, Standard Test Methods for Impact Resistance of Plastic Filmby the Free-Falling Dart Method, 2001.

16. ISO 7765-1, Plastics Film and Sheeting - Determination of Impact Resistance bythe Free-Falling Dart Method - Part 1: Staircase Methods, 1999.

17. ISO 7765-2, Plastics Film and Sheeting — Determination of Impact Resistance bythe Free-Falling Dart Method — Part 2: Instrumented Puncture Test, 1999.

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17a. ASTM D1790-02, Standard Test Method for Brittleness Temperature of PlasticSheeting by Impact, 2002.

18. ASTM D746-98e1, Standard Test Method for Brittleness Temperature of Plasticsand Elastomers by Impact, 1998.

19. ASTM D3420-95, Standard Test Method for Pendulum Impact Resistance ofPlastic Film, 1995.

20. ASTM D4272-99, Standard Test Method for Total Energy Impact of PlasticFilms By Dart Drop, 1999.

21. ASTM D1004-94a, Standard Test Method for Initial Tear Resistance of PlasticFilm and Sheeting, 1994.

22. ASTM D1938-02, Standard Test Method for Tear-Propagation Resistance(Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method, 2002.

23. ASTM D1922-00a, Standard Test Method for Propagation Tear Resistance ofPlastic Film and Thin Sheeting by Pendulum Method, 2000.

24. ASTM D2582-00, Standard Test Method for Puncture-Propagation TearResistance of Plastic Film and Thin Sheeting, 2000.

25. ISO 344, Textile Machinery and Accessories - Spinning Machines - FlyerBobbins, 1981.

26. ISO 6383-1, Plastics - Film and Sheeting - Determination of Tear Resistance -Trouser Tear Method, 1983.

27. F. Henninger in Handbook of Polymer Degradation, Eds., S.H. Hamid, M.B.Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA, 1992, 411.

28. ASTM D747-02, Standard Test Method for Apparent Bending Modulus ofPlastics by Means of a Cantilever Beam, 2002.

29. ASTM D790-02, Standard Test Methods for Flexural Properties of Unreinforcedand Reinforced Plastics and Electrical Insulating Materials, 2002.

30. ASTM D4065-01, Standard Practice for Plastics: Dynamic MechanicalProperties: Determination and Report of Procedures, 2001.

31. J.D. Ferry, Viscoelastic Properties of Polymers, 2nd Edition, Wiley, New York,NY, USA, 1961.


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32. ASTM D1505-98e1, Standard Test Method for Density of Plastics by theDensity-Gradient Technique, 1998

33. ASTM D542-00, Standard Test Method for Index of Refraction of TransparentOrganic Plastics, 2000.

34. ASTM D1746-97, Standard Test Method for Transparency of Plastic Sheeting, 1997.

35. ASTM D543-95 (2001), Standard Practices for Evaluating the Resistance ofPlastics to Chemical Reagents, 2001.

36. ASTM D1003-00, Standard Test Method for Haze and Luminous Transmittanceof Transparent Plastics, 2000.

37. F.W. Billmeyer, Jr. and Y. Chen, Color Research and Application, 1985, 10, 219.

38. ASTM D635-98, Standard Test Method for Rate of Burning and/or Extent andTime of Burning of Plastics in a Horizontal Position, 1998.

39. ASTM D1929-96(2001)e1, Standard Test Method for Determining IgnitionTemperature of Plastics, 2001.

40. ASTM D2863-00, Standard Test Method for Measuring the Minimum OxygenConcentration to Support Candle-Like Combustion of Plastics (Oxygen Index), 2000.

41. ASTM D1894-01, Standard Test Method for Static and Kinetic Coefficients ofFriction of Plastic Film and Sheeting, 2001.

42. ASTM D2457-97, Standard Test Method for Specular Gloss of Plastic Films andSolid Plastics, 1997.

43. ASTM D2578-99a, Standard Test Method for Wetting Tension of Polyethyleneand Polypropylene Films, 1999.

44. ASTM D2732-01, Standard Specification for Polyethylene (PE) Plastic Tubing, 2001.

45. ASTM D2838-02, Standard Test Method for Shrink Tension and OrientationRelease Stress of Plastic Film and Thin Sheeting, 2002.

46. ASTM D2923-01, Standard Test Method for Rigidity of Polyolefin Film andSheeting, 2001.

47. ASTM D3354-96, Standard Test Method for Blocking Load of Plastic Film bythe Parallel Plate Method, 1996.

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48. ASTM D5017-96, Standard Test Method for Determination of Linear LowDensity Polyethylene (LLDPE) Composition by Carbon-13 Nuclear MagneticResonance, 1996.

49. ASTM D2990-01, Standard Test Methods for Tensile, Compressive, and FlexuralCreep and Creep-Rupture of Plastics, 2001.

50. ASTM D1435-99, Standard Practice for Outdoor Weathering of Plastics, 1999.

51. ASTM G152-00ae1, Standard Practice for Operating Open Flame Carbon ArcLight Apparatus for Exposure of Nonmetallic Materials, 2000.

52. ASTM G153-00ae1, Standard Practice for Operating Enclosed Carbon Arc LightApparatus for Exposure of Nonmetallic Materials, 2000.

53. ASTM D1044-99, Standard Test Method for Resistance of Transparent Plasticsto Surface Abrasion, 1999.

54. ASTM D1242-95a, Standard Test Methods for Resistance of Plastic Materials toAbrasion, 1995.

55. V. Shah, Handbook of Plastic Testing Technology, Wiley, New York, NY, USA, 1984.

56. ASTM D673 discontinued not replaced

57. R.P. Kambour and A.S. Holik, Journal of Polymer Science, A-2: Polymer Physics,1969, 7, 1393.

58. ASTM D1693-01, Standard Test Method for Environmental Stress-Cracking ofEthylene Plastics, 2001.

59. ASTM D3079-94 (1999), Standard Test Method for Water Vapor Transmissionof Flexible Heat-Sealed Packages for Dry Products, 1999.

60. ASTM E96-00e1, Standard Test Methods for Water Vapor Transmission ofMaterials, 2000.

61. ASTM F372-99, Standard Test Method for Water Vapor Transmission Rate ofFlexible Barrier Materials Using an Infrared Detection Technique, 1999.

62. ASTM F-1249-01, Standard Test Method for Water Vapor Transmission RateThrough Plastic Film and Sheeting Using a Modulated Infrared Sensor, 2001.

63. ASTM D1434-82(1998), Standard Test Method for Determining GasPermeability Characteristics of Plastic Film and Sheeting, 1998.


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64. ASTM D3985-02, Standard Test Method for Oxygen Gas Transmission RateThrough Plastic Film and Sheeting Using a Coulometric Sensor, 2002.

65. ASTM D5047-95, Standard Specification for Polyethylene Terephthalate Filmand Sheeting, 1995.

66. ASTM D4635-01, Standard Specification for Polyethylene Films Made fromLow-Density Polyethylene for General Use and Packaging Applications, 2001.

67. ASTM D3981-95, Standard Specification for Polyethylene Films Made fromMedium-Density Polyethylene for General Use and Packaging Applications,1995.

68. ASTM D2103-97 Standard Specification for Polyethylene Film and Sheeting, 1997.

69. ASTM D2673-99, Standard Specification for Oriented Polypropylene Film, 1999.

70. ASTM D2647-94 (2000) e1, Standard Specification for Crosslinkable EthylenePlastics, 2000.

71. ASTM D374, Standard Test Methods for Thickness of Solid ElectricalInsulation, 1999.

72. ASTM D4101, Standard Specification for Polypropylene Injection and ExtrusionMaterials, 2002.

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13 Recycling of Plastic Waste

E.M. Abdel-Bary

13.1 Introduction

Polymeric materials (plastics and rubbers) comprise a steadily increasing proportion ofthe municipal and industrial waste going into landfill. Owing to the huge amount ofplastic wastes and environmental pressures, recycling of plastics has become a predominantsubject in today’s plastics industry. Development of technologies for reducing polymericwaste, which are acceptable from the environmental standpoint and are cost-effective,has proven to be a difficult challenge, because of the complexities inherent in the reuse ofpolymers. Establishing optimal processes for the reuse/recycling of polymeric materialsthus remains a worldwide challenge in the new century.

Compared with other countries, there is a huge amount of plastic waste in the USA(taken as a reference), where five main types of polymers dominate in the plastics wastestream. The highest polymer waste results from low-density polyethylene (LDPE), at 5million tons per year; high-density polyethylene (HDPE) is second, at 4.1 million tons;then come polypropylene (PP) at 2.6 million tons, followed by polystyrene (PS) at 2million tons and polyethylene terephthalate (PET) at 1.7 million tons [1]. These fivepolymer types, together with polyvinyl chloride (PVC), also dominate the plastics wastestream in the European Community [2].

Plastic films find applications in agriculture as well as in plastic packaging, which is ahigh-volume market owing to the many advantages of plastics over other traditionalmaterials. However, such material is also the most visible in the waste stream, and hasreceived a great deal of public criticism as films have comparatively short life-cycles andusually are non-degradable [3].

The majority of plastic films are made from LDPE or linear low-density polyethylene(LLDPE), comprising approximately 68% of the total film production. In addition, HDPEresins are commonly used in film plastics. Non-polyethylene resins constitute the remainderof the film plastic types found in the market place. PP, PVC and Nylon resins comprisethe bulk of these other film types. Increasingly, certain multilayer or coextruded filmsare used in special applications that seek to combine the performance attributes of twoor more resins for such applications.

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Businesses can save money by reducing their disposal expenses, in the form of both tonnage-based tipping fees and container hauling fees. This is especially evident with plastic films,where a high volume-to-weight ratio can mean more container pulls per ton hauled.

13.2 Main Approaches to Plastic Recycling

There are four main approaches to recycling plastics (excluding, as not acceptable,dumping on land or at sea with or without prior treatment) [4]. These are primary,secondary, tertiary and quaternary recycling.

13.2.1 Primary Recycling

This is the recycling of clean, uncontaminated, single-type waste, and it remains themost popular as it ensures simplicity and low cost, especially when done ‘in-plant’ andfed with scrap of controlled history [5]. The recycled scrap or waste is either mixed withvirgin material to assure product quality or used as second-grade material [6]. Primaryrecycling is very simple without any precautions except the proper and clean collectionof the waste in the plant.

13.2.2 Secondary Recycling

13.2.2.1 Approaches to Secondary Recycling

There are two main approaches to secondary recycling. One approach is to separate theplastics from their contaminants and then segregate the plastics into generic types, one ormore of which is then recycled into products produced from virgin or primary recycledmaterial. The other approach is to separate the plastics from their associated contaminantsand remelt them as a mixture without segregation. The treatment of the plastics-containingwaste streams may include: size reduction by granulators, shredders or crumblers; separationof plastics from other waste materials and from one another; cleaning; drying; andcompounding [7, 8]. The actual order and number of operations in a particular treatmentsystem depends on the waste being processed and the desired quality of the final material [9].

13.2.2.2 Mechanical Recycling

Mechanical recycling is mainly related to secondary recycling. The main steps includeseparating, sorting and washing to get rid of contamination, especially for plastic films,

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Recycling of Plastic Waste

which possess a large surface area and consequently have a large degree of contamination.In a chemical recycling plant, one should have shredders, metal and mineral separators,prewashing and granulation, second washing stage, mechanical grinding and dirt removal,hydrocyclone separation, dewatering and melt processing.

The separation of plastic waste is one of the main factors restricting high performance inplastic recycling. The separation of plastics into desired categories as well as the eliminationof contaminants is an ongoing technological development process. The aim is to developautomatic and continuous separation technology to minimise the handling of waste andto achieve a more efficient recycling process. Probably the best alternative for pure plasticstreams is not to allow them to mix in the first place, neither among themselves nor withcontaminants. If separation starts at the consumer level and at the source point ofcollection, there will be fewer difficulties during the recycling.

13.2.3 Tertiary Recycling

Tertiary recycling includes chemical recycling. The terms ‘chemical recycling’ and‘feedstock recycling’ of plastics are sometimes collectively referred to as ‘advanced recyclingtechnologies’. In these processes, solid plastic materials are converted into smallermolecules as chemical intermediates through the use of heat. These chemical intermediates,usually liquids or gases, but sometimes solids or waxes, are suitable for use as feedstocksfor the production of new petrochemicals and plastics.

The technical and economic feasibility and overall commercial viability of advancedrecycling methods must be considered in each step of the recycling chain, consisting ofcollection, processing and marketing. All of them are critical to the success of chemicaland feedstock recycling. Today, most of these technologies remain developmental andhave not yet proven themselves sustainable in a competitive market. Nevertheless, theyremain of considerable interest in their longer-term potential.

The term ‘feedstock recycling’ encompasses chemical recycling but is often applied to thethermal depolymerisation of polyolefins and substituted polyolefins into a variety ofsmaller hydrocarbon intermediates. Fluidised bed pyrolysis investigations of LDPE haveprovided data on the suitability of the process and on the influence of the processconditions on the compatibility of the feedstock produced with the conventional petroleumfeedstock [10]. The gases produced from the pyrolysis of LDPE are mainly hydrogen,methane, ethane, ethylene, propane, propene, butane and butene. Also, it has been reportedthat the thermolysis products of HDPE consist of 80-90 wt% straight-chain alkanes and1-alkenes. Subsequent hydrogenation of the PE oil resulted in a diesel fuel with highcetane index and low sulfur and aromatic contents [11].

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In some cases, addition polymers such as polystyrene and polymethyl methacrylate canbe thermally depolymerised back to their corresponding monomers in reasonably highyield. The term ‘chemical recycling’ is often applied also to the depolymerisation ofcertain condensation polymers back to monomers. Examples of these types of plasticsare polyesters, polyamides and polyurethanes. Chemical recycling thus mainly includespyrolysis, gasification, hydrogenation, hydrolysis, glycolysis and depolymerisation.

A new reactor system was developed for the recovery of fuels from waste plastic mixturesin a steam atmosphere. The degradation mechanisms of two polyolefins (PE and PP),two polyamides (Nylon-6 and Nylon-6,6), polystyrene and three polyesters(polycarbonate, polybutylene terephthalate and polyethylene terephthalate) in bothnitrogen and steam as the carrier gas have been investigated [12]. The oil produced fromthe proposed reactor system was continuously upgraded to produce gasoline and keroseneover a Raney nickel catalyst in a steam atmosphere.

13.2.4 Quaternary Recycling

Quaternary recycling includes the recovery of the energy content of plastic wastes. Owingto a lack of other recycling possibilities, incineration (combustion) aimed at the recoveryof energy is currently the most effective way to reduce the volume of organic material.This may then be disposed of to landfill. Plastics, either thermoplastics or thermosets,are actually high-yielding energy sources. For example, one litre of heating oil has a netcalorific value of 10,200 kcal, whereas 1 kg of plastics releases 11,000 kcal worth ofenergy; for comparison, it should be added that 1 kg of briquettes (blocks of pressed coaldust) has a net calorific value of 4,800 kcal. It has been estimated that, by burning 1 tonof organic waste, approximately 250 litres of heating oil could be saved [13]. Cleanincineration of municipal solid waste (MSW) is widely accepted in countries like Swedenand Germany (50% of total MSW), Denmark (65%), Switzerland (80%) and Japan(70%) [14]. Although there are very stringent emissions regulations, more than 50 refuseincineration units are working in Germany.

The energy that can be recovered from the incineration of plastics depends on the type ofplastic. It has been estimated (in kcal/kg) as: 18,720 for PE; 18,343 for PP; 16,082 forPS; 13,179 for phenol-formaldehyde; 11,362 for foamed polyurethane (PU); 10,138 forNylon; 8,565 for polyvinyl acetate (PVAc); 7,516 for PVC; and 7,014 for PU. This energyis on average 10,000 kcal/kg. Each ton will release about 107 kcal.

However, plastics emit some objectionable gases and form some hazardous compounds.Thus, recovering energy from plastic waste is not cheap. The main goal must be to avoidthe formation of these hazardous compounds by the correct construction of incinerators

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and by considering all proper means to avoid pollution. The costs of operating,maintaining and monitoring an incinerator are quite substantial compared to those for aconventional power plant. Energy from waste (EFW) is a reliable and renewable sourceof energy, especially if the MSW is rich in organic matter. Furthermore, it reduces theamount of waste to be landfilled at the final stage. Costs involved in developing newlandfills can partly offset the high costs of energy recovery from an EFW facility.Incinerators with EFW installations are not considered just as power plants, as theirmain purpose is to reduce the amount of garbage being landfilled within the purpose ofan integrated waste management system.

Incineration plants should be designed and operated to produce the least amount ofpollution. The use of incineration plants is mandatory for plastic wastes from hospitalsand similar institutions, which is considered as a potential source of disease. Incineratorsdo not emit ethane gas, as this gas is completely combusted into CO2 and water, even atlow temperature. However, incinerators have often been associated with dioxin and furanemissions, which are avoided in modern ones by working at temperatures that are highenough to decompose such chemicals and prevent them from reaching the ecosystem.Although dioxin and furan are often perceived as two individual chemical products,there are in fact 75 congeners of polychlorinated dibenzodioxins (PCDD) and 135congeners of polychlorinated dibenzofurans (PCDF), each differing in its chemicalconfiguration and degree of toxicity. The most toxic of the dioxins is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). When assessing health risks, the total congenersare converted to the equivalent TCDD. Dioxins and furans are present in Nature and aregenerated by many sources, such as forest fires and as a by-product of a certain chemicalprocesses and the burning of wood in stoves and fireplaces, barbecues, diesel engines,power plants, ponds and so on. Scientists can account for about 60% of the dioxinsfound in Nature (referred to as ‘background level’), while the source of the remainder isstill unknown. Dioxins enter the human body through the food chain, inhalation andskin contact. As long as the quantities absorbed are very minute, however, they do notrepresent a health hazard.

The complete combustion of organic matter removes all the dioxins present in the garbageHowever, during cooling of the flue gases, traces of dioxins are formed. An energy-from-waste facility acts as a reliable and renewable source of energy. It is a safe method ofreducing the volume of waste dumped in landfills. A considerable reduction in the emissionof greenhouse gases compared to landfills can be achieved. However, further research isneeded to avoid completely this emission.

Recovery of energy from solid waste constitutes the fourth ‘R’ after reducing, reusingand recycling. Research as the fifth ‘R’ is the key element. Scientists and environmentalscientists have to work together to develop new methods for recycling more products.


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13.2.5 Conclusion

In conclusion, primary recycling is ideal for clean, uncontaminated single-type scrap,but degradation during service life or reprocessing should be taken under seriousconsideration.

Secondary recycling by type can be accomplished by various methods, but the costassociated with the separation and decontamination of the wastes undoubtedly poses aninherent obstacle. Dissolution-based techniques seem worth developing, but cannot yetbe considered to be the complete answer. Secondary recycling of plastics mixtures byremelting is intended to produce downgraded products as a result of incompatibilityproblems. Compatibilisation is effective only in specific cases of plastics mixtures.

Tertiary or ‘chemical’ recycling processes involve high levels of investment and succeedin recovering the chemical products, but negate the value added during the polymerisation.The latter comment is valid also for the last resort, quaternary recycling (energy recoveryof plastics waste), which can substitute other energy sources and solve disposal problems.However, it stands strongly accused of undesirable emissions.

The first and most important steps in plastic recycling are collection and sorting, afterwhich the recycling process depends on the type of plastic and the field of application.These issues will now be addressed.

13.3 Collection and Sorting

Collection involves gathering lightweight packaging films and other materials. Plasticpackaging from separate collection streams is separated from other lightweight packagingmaterial and sorted into fractions comprising film containers, mixed plastics and residues.Identification of the plastic type is one of the most important elements in recycling becausemost recycling processes prohibit certain types of plastics. For example, severe problemsappear during processing of recycled resin of unknown origin. Thus, extrusion and injectionmoulding require accurate identifications of plastic waste, otherwise a product with badappearance and impaired quality, especially poor mechanical properties, is obtained.

13.3.1 Resin Identification

Identification has become more complicated, not only due to the presence of plasticmaterials compounded with additives such as plasticisers, stabilisers, flame retardants,fillers and others, but also due to the presence of polymer blends in the waste. Some timeago, the Society of the Plastic Industry (SPI) introduced a labelling system for recyclable

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plastic material. It is now common for manufacturers to use this code system printed ormoulded on the product surface for easy plastic identification. The code is a three-sidedtriangular arrow with a number in the centre and letters underneath, indicating the typeof resin: 1 for PET; 2 for HDPE; 3 for vinyl polymers, especially PVC; 4 for LDPE; 5 forPP; 6 for PS; and 7 for others. It is thus easy to develop automated scanning systems thatcan read the SPI or other codes. This will help to identify the resin used [15]. Furthermore,separation of plastic containers has been proposed by printing bar codes on them [16].

Plastic films are more difficult to identify than plastic containers, because most films donot carry a code, and producers and recyclers need training on how to distinguish betweenfilm types. Sorting generally must occur early in the recovery process, near the initialpoint of generation, to be successful.

Optical systems for identification of mixed plastics have been used. A few technologiesoriginally designed and used in the film and packaging industry were considered earlier.Electromagnetic scanning equipment was used to recognise chlorine molecules and so to sortPVC from PET [17]. An X-ray fluorescence (XRF) analyser as a photoelectric sensor wasalso used to identify transparent PET, green PET, translucent or natural HDPE, pigmentedHDPE and PVC. The sensor system is connected to an automatic sorting line. Automation ofthe process reduces costs and improves the resale value of the separated plastics. Althoughdirt does not significantly influence the fluorescence intensity from bottles, paper labels doreduce the intensity but do not pose an obstacle in detecting vinyl bottles [17]. Paper labelsare virtually blind to X-rays.

Infrared and other spectral separation devices have been reported for the continuousexamination of waste products [18]. However, a satisfactory process for the identificationof plastic products for commercial purposes has yet to be developed.

13.3.2 General Aspects of Resin Separation

Resin separation from contaminants or from undesired materials to obtain the desiredstream can be achieved by a number of processes. These are: magnetic separation for theremoval of ferrous materials; an electrostatic method for nonferrous, mainly aluminium,separation; air separation via cyclones to separate paper; and flotation tanks orhydrocyclones used to separate various resins based upon specific gravity. After that theprocessed materials are shredded.

Automatic separation of shredded plastic waste is very difficult if the resins have similarspecific gravity. Fortunately, 85% by volume of world plastic consumption is of fourmain thermoplastic resins: PE, PP, PVC and PS [19]. In the next sections, some separationtechniques based on different properties will be discussed.


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13.3.3 Resin Separation Based on Density

Air classification can be used to separate plastics on the basis of their bulk densities; thusfilm and foamed plastics may be separated from heavier forms of plastic material orpaper [20-22].

The densities of the major thermoplastics give the potential to separate them into typesby a series of float-sink operations [23, 24]. Water may be used to separate PP, LDPEand HDPE from PS, PVC and PET. A liquid having a density of about 930 kg/m3 may beused to separate PP and LDPE from HDPE; the PP and LDPE may then be separatedusing a liquid having a density of about 910 kg/m3. The PS and PVC may be separatedusing a liquid with a density of about 1150 kg/m3 [20]. Blending or filling a plastic maychange its density to the point where it could cause difficulties in the float-sink operationprocesses. Labels, residual adhesives, metals and metallic-plastic composites cause similardifficulties, and therefore processes have been developed to remove these contaminantsbefore the mixed plastic materials enter a separation system [25]. As an example, asolvent washing stage, using either tetrachloroethylene- or hexane-related solvents, wasadded to the classic water-washing treatment. These solvents were believed to removenot only the glues but also any toxic organic chemicals that have been stored in beveragebottles by consumers or are present as additives in plastics and that inadvertently will bepresent in the end-products [26].

A different approach for separating mixed plastic wastes by density has been reported[27, 28]. The process uses the properties of a fluid near its critical point to allow fineseparations at mild temperatures and pressures. The density of the medium can be variedover a wide range and controlled to a sensitivity of ±0.01 g/cm3. Carbon dioxide is themost commonly used supercritical fluid and can be compressed to densities in the rangeof 1000 kg/m3. Since the separation of non-olefin thermoplastics will require fluid densitiesup to approximately 1400 kg/m3, mixtures of carbon dioxide and sulfur hexafluoride, avery dense supercritical fluid, may be used. By effecting small incremental changes ofpressure, pure CO2 efficiently separated LDPE, HDPE and PP. Separation of green PET,clear PET and PVC has also been demonstrated, and separation of light- and dark-colouredHDPE is possible. The different densities exhibited by PET in the neck and the body ofPET bottles can be separated by CO2/SF6. The possibility of separating various componentsof wire and cable scrap also exists.

The centrifugal field produced in a hydrocyclone has been extensively used for theseparation of plastics. In a hydrocyclone, the flow rate referred to the separation areais 100 times higher than in a static float-sink separator. The contamination of plasticsis of only minor relevance in this process compared to flotation. For the separation ofan n-component mixture, n – 1 separation stages (cyclone plants) are necessary.

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Furthermore, continuously operated plants must be equipped with a feeding deviceand screens for dehydration purposes [29]. Using hydrocyclones, PS, PET and PVCcan be separated from one another or from polyvinyl alcohol; polyolefins from municipalsolid wastes; PET from PE, PP and paper; and PP car bumpers from metals and othercontaminants [30-32].

13.3.4 Resin Separation Based on Colour

Photoelectric sensors are used for the separation of mixed, whole, or baled plasticcontainers. One of the systems uses mechanical means to reduce the baled plastic intoindividual bottles and to screen contaminants. After deballing and screening, the containersare manipulated into a single-line presentation to an optical sensor that performs a three-class identification: Class 1, dairy HDPE and PP; Class 2, PET and PVC; and Class 3,mixed colour HDPE containers. Another optical sensor can be used to discriminate greenand amber PET from clear PET containers, PP from dairy HDPE containers, and mixedcolour HDPE according to seven colour classifications. However, reliable identificationof post-consumer containers requires that measurements from much of the containersurface should be ignored. These areas include closures or necks, labels, edges, bottomsand areas with residue of dirt [33, 34].

13.3.5 Resin Separation Based on Physicochemical Properties

13.3.5.1 Electrification

The separation of mixed plastic wastes can be achieved using high-voltage drums,taking advantage of their different relative positions in the charging sequence. Theprocess involves first tribo-electrification of the shredded plastic particles of the mixtureby fluidisation. Subsequently, the electrified mixture is conveyed through an electrostaticfield that separates the individual particles according to the magnitude and the polarityof the electric charges acquired during the tribo-electrification. When fluidising a mixtureof two shredded plastics, the particles of the plastic with the lower work functiontransfer electronic charges to those with the higher work function. For example, thetribo-electrical contact between PVC and PET results in PVC having a negative chargeand PET a positive charge. In the case of PET/PS mixtures, PET has a negative chargeand PS a positive charge [35]. Mixtures containing more than two plastic species posea substantial problem with regard to their charging behaviour. Also, owing to thevarious additives contained in different types of resin, the respective positions of theplastic species are prone to change.


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13.3.5.2 Spectroscopy

Spectroscopy in the near-infrared (NIR) region of the spectrum could be a key to therapid identification of various plastics and their subsequent recovery. By illuminating thesample with light in the near-infrared and measuring the reflected light, a so-called NIRspectrum of the material is obtained, which contains information about the molecularvibrations excited by the light energy. For example, the IR vibrational spectra of plasticsshow characteristic absorption bands at wavenumbers ν of 1200, 1400, 1700 and 2200-2500 cm–1 for CH, and at 1300-1500 and 1900-2100 cm–1 for OH. The NH (1500 and2050 cm–1) and the CO (1730-1740 cm–1) vibrations contain the relevant spectralinformation for plastics. Plastic objects, such as beverage bottles, can be dropped througha vertical tube, and are identified and separated while falling. This also simulates othertransportation possibilities like conveyor belts. From the performance data, it was foundthat identification can be achieved within 0.2 s, although several measurements wereneeded to avoid mistakes due to dirt or labels. The problem of transparency for satisfactorymeasurements could be overcome by reflectance measurements [36].

Other spectroscopic methods are also possible. The identification of several thermoplastics– such as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PP, PS, high-impactPS (HIPS) and PVC – can be achieved by Fourier transform IR (FTIR), based on similarprinciples [37]. When exposed to γ-radiation, the high molecular weight (high molarmass) molecules of PVC containing chlorine atoms emit an X-ray return signature easilyvisible by an XRF analyser. Polyolefins, which have much lower molecular weights, emita lower backscattering signal that barely shows up on the XRF analyser, and so is easilyidentified and separated [16, 38]. Bayer have proposed a process for automaticidentification and sorting of post-consumer plastics, in which fluorescent dyes were addedto resins during compounding, a different one for each resin type. These dyes, havinghigh detection sensitivity, can be added in minute quantities, and so 5 g of dye per ton ofpolymer were sufficient for identification by a diode device [39].

13.3.5.3 Selective Dissolution of Polymer Mixtures

Finally, it must be emphasised that solvent recycling of a single-type plastic scrap servesas a model process providing fundamental knowledge for the development of a selectivedissolution process.

The principle of the selective dissolution of a single polymer in a polymer mixture can beused to separate the polymers. According to the concept of selective dissolution, one polymercould be dissolved at a time, and thus dissolution-based processes can deal with mixturesof polymers. This has an evident impact on the recycling of plastics in municipal solid

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waste. Mixtures of polyolefins, PS, PVC, thermosetting plastics and natural polymers(rubbers, fibres, paper, etc.) can be separated using certain solvent systems [40]. The mixtureis first treated with xylene at 5-50 °C to dissolve PS, and after separation the mixture isfurther heated at 90-150 °C to dissolve PE, leaving PP insoluble. The three mainthermoplastics – polyolefins, PS and PVC – may be separated by dissolving them in amixed solvent of xylene (85%) and cyclohexane (15%). From the dissolution, three separatephases could be obtained containing 99% each of the pure plastics, indicating that excellentseparation can be achieved. [41]. In relevant studies, toluene, xylene and kerosene havebeen proposed as suitable solvents for the selective dissolution of LDPE, but the informationgiven is very limited [42, 43]. Such selective dissolution is accomplished for each of thepolymers of the mixture by heating the waste dispersion at various temperatures.

13.4 Recycling of Separated PET Waste

The worldwide production of PET is above 1 × 106 tonnes per year. With such largeconsumption, the effective utilisation of PET waste is of considerable commercial andtechnological significance. PET waste may be converted into extruded or moulded articlesafter repelletising it. Recently, waste PET films or sheets have been cleaned, crushed,dried and mixed with LDPE waste [44]. The obtained mix was pelletised and blow-extruded into films. The maximum concentration of PET does not exceed 20%. Thefilms obtained were found to possess very good mechanical properties compared withLDPE only. Also, the films are expected to possess good printability due to the polarnature of PET.

PET may be depolymerised to yield raw materials for resin synthesis. Recycling ofsegregated waste may be possible by blending in small quantities with virgin monomer,bis(hydroxyethyl) terephthalate. However, it often lowers the quality of the final product[45]. It is therefore desirable to break down the polymer into smaller fragments oroligomers [46].

PET can also be fully depolymerised into dimethyl terephthalate (DMT). However, theregenerated DMT exhibits a significantly higher carboxyl content, adversely affectingproduct quality. It is more economic to convert PET into low molecular weight oligomersby glycolysis in the presence of a transesterification catalyst [47-49].

When glycolysis is carried out using ethyleneglycol, the oligomers may be directly recycledinto the polycondensation stage in PET manufacture, but this also lowers the productquality. Glycolysis can be carried out using different glycols, and the oligomers can beused in the synthesis of unsaturated polyester by reaction with an unsaturated anhydride[50] or used to synthesise other polymers [51].


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There are two distinct advantages of the process: (1) the PET waste is converted into acommercial value-added product; and (2) unsaturated polyester resins based onterephthalic acid (TPA) are obtained without the processing difficulties encountered withthe use of plain TPA.

Yang and Tsai [52] degraded PET fabric waste to glycolysed product by treating the PETwaste with sodium hydroxide using ethyleneglycol or glycerol as the solvent. Comparedwith the conventional aqueous alkaline hydrolysis, they found that the degradation ratein ethyleneglycol increases tenfold. They reported that the kinetics of the alkalineethyleneglycol treatment show that the weight loss is linear with respect to time. Theyconcluded that using ethyleneglycol can greatly shorten the treatment time to achieveresults similar to those with the conventional aqueous system.

A new chemical recycling process for PET using supercritical water has been developedby Yoshiyuki and co-workers [53]. In this method, the monomers obtained fromsupercritical water hydrolysis are the raw materials of each polymer. The purity of theterephthalic acid obtained from PET is about 99 wt%. It was confirmed that this processhas the advantage of reducing the reaction time and simplicity of the process whencompared with conventional methods such as methanolysis and glycolysis.

13.5 Recycling of Separated PVC Waste

As mentioned before, most of the technologies for the recycling of plastic wastes includedegradative extrusion, pyrolysis, hydrogenation, gasification, glycolysis, hydrolysis,methanolysis, incineration with HCl recovery, or input as a reducing agent into blast furnaces.

Most of these technologies are still in the research phase, or simply are not suitable for PVC-containing waste. The latter is particularly true for technologies such as glycolysis andhydrolysis, which play a role only for well-defined single-waste streams such as PET. Some ofthese technologies are currently generally regarded as the most feasible ones for realisationon a practical scale. However, one group of these technologies is not designed specifically forPVC waste, but deals with mixed plastic waste (MPW) in general. These technologies mainlyconcentrate on recovering the organic part of the MPW. They often have restrictions withregard to the maximum permissible chlorine (or PVC) input. Other technologies are designedto deal specifically with PVC waste (chlorine concentrations of well over 10%). They emphasiserecovery of the chlorine fraction in a useful form. Hence, together with the competingtechnologies for chemical recycling, three types of technologies have been discussed [54]:

(1) Technologies for chemical recycling of mixed plastic waste;

(2) Technologies for chemical recycling of PVC-rich waste;

(3) Alternatives to chemical recycling (incineration, mechanical recycling).

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13.5.1 Chemical Recycling of Mixed Plastic Waste

Regarding the chemical recycling of MPW with a PVC content of up to several percent, the process consists of two parts, a liquefaction step and an entrained bedgasifier. In the liquefaction step, the plastic waste is mildly thermally cracked(depolymerised) into synthetic heavy oil and some condensable and non-condensablegas fractions. The non-condensable gases are reused in the liquefaction as fuel togetherwith natural gas.

The heavy oil is filtered to remove large inorganic particles. The oil and condensed gasare then injected into the entrained gasifier. Also, chlorine-containing gases from theplastic waste are fed to the gasifier. The gasification is carried out with oxygen and steamat a temperature of 1200-1500 °C [55]. The products of the process are synthesis gas(predominantly H2/CO), pure sulfur and NH4Cl.

13.5.1.1 Polymer Cracking Process

In the polymer cracking process, some elementary preparation of the waste plastics feedis required, including size reduction and removal of most nonplastics. The reactor operatesat approximately 500 °C in the absence of air. The plastics crack thermally under theseconditions to hydrocarbons, which vaporise and leave the bed with the fluidising gas.The gas has a high content of monomers (ethylene and propylene) and other usefulhydrocarbons, with only some 15% being methane.

13.5.1.2 Conversion Process

The feedstock recycling process was designed to handle the recycling of mixed plasticwaste supplied by the collection system. The conversion of the pretreated mixed plasticinto petrochemical raw materials takes place in a multistage melting and reduction process.In the first stage the plastic is melted and dehalogenised to preserve the subsequent plantsegments from corrosion. The hydrogen chloride separated out in this process is absorbedand processed in the hydrochloric acid production plant. Hence, the major part of thechlorine present in the input, (e.g., from PVC), is converted into saleable HCl. Minoramounts become available as NaCl or CaCl2 effluent [56]. Gaseous organic products arecompressed and can be used as feedstock in a cracker.

In the subsequent stages the liquefied plastic waste is heated to over 400 °C and crackedinto components of different chain lengths. About 20-30% of gases and 60-70% of oilsare produced and subsequently separated in a distillation column.


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Naphtha produced by the feedstock process is treated in a steam cracker, and themonomers, (e.g., ethylene, propylene), are recovered. These raw materials are used forthe production of virgin plastic materials. The process is carried out under atmosphericpressure in a closed system and, therefore, no other residues or emissions are produced.

In sum, the products of the process are:

(1) HCl, which is neutralised or processed in a hydrochloric acid production plant;

(2) Naphtha, to be treated in a steam cracker;

(3) Monomers, (e.g., ethylene, propylene), which can be used for the production of virginplastic materials;

(4) High-boiling oils, which can be processed into synthesis gas or conversion coke andthen transferred for further use;

(5) Residues.

13.5.2 Chemical Recycling of PVC-Rich Waste

These processes aim to recover as much as possible of the chlorine present in PVC in ausable form (HCl or a saleable chloride salt). The two processes in question, which arediscussed below, are:

• Incineration process;

• Pyrolysis process.

13.5.2.1 Incineration Process

A plant for the processing of chlorine-containing fluid and solid waste streams is used.The goal is to process the waste by thermal treatment and to produce HCl using theenergy from the process itself. The plant is based on a rotary kiln and has a capacity of45 kilotonnes per year, (i.e., not only PVC waste), with a heat production capacity of 25MW at ca. 7500 production hours per year.

The waste is incinerated in the rotary kiln and a post-combustion chamber, directly after therotary kiln, at temperatures of 900-1200 °C. During this treatment HCl is released andrecovered. In this way a continuous production of high-quality HCl can be assured. Also, theformation of dioxins and furans can be diminished in this way, as the goal of the process is tooxidise the waste fully, so that no toxic chemicals (dioxins and furans) are formed.

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13.5.2.2 Pyrolysis Process

In this process the chemical and thermal degradation of the PVC waste takes place in areactor at low pressures (200-300 kPa) and moderate temperatures (maximum 375 °C).Chlorine generated from the PVC reacts with fillers, forming calcium chloride. Simultaneously,the metal stabilisers that may be present in PVC waste (lead, cadmium, zinc and/or barium)are converted to metal chlorides. This product consists of over 60% lead and may be purifiedand reused. After completion of the reactions, three main intermediate products are formed:a solid-phase product, a liquid product and a gas-phase product.

In sum the products of the process are:

(1) Calcium chloride (<1 ppm lead), which may be used as thaw salt or for other purposes;

(2) Coke (<0.1 wt% lead and <0.1 wt% chlorine), which may be used as fuel in acement kiln;

(3) Metal concentrate (up to 60 wt% lead);

(4) Organic condensate, which may be used as fuel for the process.

To treat the PVC waste, lime and water are needed to run the process. No dioxins, chlorine,metals or plasticisers are emitted from the process. Also, there are no liquid waste streamsin the process, since all streams are recycled within the system. There is a small volume ofcarbon dioxide gas formed by the reaction between lime/limestone and hydrogen chloride.

13.6 Recycling of Separated PE Waste

LDPE recycling is widespread, although not to the same extent as HDPE recycling. Themajority of LDPE that is recycled originates from post-industrial waste such as bundleshrink-wrap used to stabilise loads on pallets as well as greenhouse films and mulchfilms. There is only a limited proportion of recycled LDPE that can be classified as post-consumer recycle (PCR). Contamination in recycled PE can arise from a number of sources:

(1) From multicomponent systems that use dissimilar polymers such as PP closures, fromadhesive-backed paper labels, and even through the incorporation of additives suchas pigments;

(2) During use, (e.g., by the contents of the packaging);

(3) During collection, (e.g., owing to consumers mixing plastic types);

(4) By the environment, (e.g., soil in LDPE mulch film); and

(5) By reprocessing, (e.g., gels and black specks).


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13.6.1 Contamination of PE Waste by Additives

The pigments used in PE mouldings and films are often based on inexpensive metaloxides. For instance, the common brown, grey and orange pigments are based onvarious iron oxides and hydrates that act as prooxidants/prodegradants at the hightemperatures, (i.e., 220 °C), encountered during reprocessing of HDPE. Moreover,green pigments are usually based on chromium(III) oxide, which can readily catalysethe thermooxidative degradation of HDPE when present even in trace quantities.Virgin PE is usually adequately stabilised so that these catalytic compounds do notcause in-service degradation of the polymer; however, with reprocessing, theantioxidants are usually consumed and these pigments may then be able to exerttheir prodegradant effects [57].

The recycled polymer can also be contaminated by pigmented components in thefeedstock. In the recycling of HDPE bottles by melt processing, a major effort hasbeen directed toward producing a naturally coloured recycle stream [58]. The majorbarrier to overcome to reach this end is the removal of the coloured bottle caps.LDPE film is extensively used for packaging and for the production of shoppingbags. These films often contain a fatty acid amide lubricant (usually cis-docosenamide)that can be oxidised during thermal reprocessing of LDPE film. The lubricant readilyundergoes cleavage at the unsaturated site to give a series of aldehydes that havevery low odour thresholds. Such contamination imparts to the recycled material arancid odour that may restrict its application potential.

13.6.2 Contamination of PE Waste by Reprocessing

Recycling of HDPE PCR (usually milk bottles) by melt extrusion can lead tocrosslinking during the thermal reprocessing stage since the antioxidant added initially(during manufacture of the polymer) is consumed [59]. Loosely crosslinked regions(known as ‘gels’) can act as stress concentrations in film and cause ‘blow-outs’ inbottles made from recycled HDPE. Another common source of contamination inrecycled HDPE (as well as virgin HDPE) is ‘black specks’. These are small areas ofhighly degraded polymer that have been carbonised owing to excessive residencetime in an extruder. These ‘black specks’ typically occur in low-flow regions in theextruder where ‘hang-ups’ form. Often, such contamination may also appear yellow,brown, or amber depending on the extent of degradation. Black specks cause a majorproblem in the blow-moulding of natural or white bottles because they are aestheticallyundesirable.

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13.7 Recycling of HDPE

13.7.1 Applications for Recycled HDPE

New markets for recycled HDPE will also emerge as a result of advances in fabricationtechnology allowing recycled HDPE to be used in more demanding applications that arecurrently the domain of LLDPE and high-modulus HDPE. For instance, there is thepotential use of recycled HDPE in oriented laminates for applications such as pond linersand moisture barriers. High-performance sheets are obtained by laminating orientedHDPE webs to equalise cross-web and longitudinal properties [60]. Compared to thesame thickness of monolayer HDPE, the advantages are greatly improved tensile, tearand puncture strength, and doubling of the moisture barrier properties. Specialisedprocessing equipment has been developed to handle recycled material [61, 62].

The following are some of the commercial applications of the modified recycled HDPE:

(1) Large mouldings;

(2) Detergent bottles are another major outlet;

(3) Grocery sacks (‘check-out’ bags) are generally produced from high molecular weight(HMW) HDPE film and have a thickness in the range 15-18 μm, their most criticalperformance parameters being tear strength and dart impact strength.

13.7.2 Rubber-Modified Products

Recycled rubber crumb from used car tyres is another class of modifier that is findingwidespread potential for blending with HDPE PCR to yield truly 100% recycled polymericmaterials [44].

Profiles extruded from recycled HDPE are finding increased use in applications such asdecking, fence posts, boundary stakes, road posts, railroad sleepers and similar areas,replacing wood and concrete [63]. Such applications are generally thick sections and areideal outlets for consuming large volumes of HDPE recycle. Furthermore, they are resistantto rotting, not liable to insect attack, and do not splinter.

13.8 Recycling Using Radiation Technology

Owing to the ability of ionising radiation to alter the structure and properties of bulkpolymeric materials, and the fact that it is applicable to essentially all polymer types,


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irradiation holds promise for impacting the polymer waste problem. The three mainpossibilities for use of radiation in this application are:

(1) Enhancing the mechanical properties and performance of recovered materials ormaterial blends, principally through crosslinking, or through surface modification ofdifferent phases being combined;

(2) Treatment causing or enhancing the decomposition of polymers, particularly throughchain scission, leading to recovery of either low molecular weight mixtures, orpowders, for use as chemical feedstocks or additives;

(3) Production of advanced polymeric materials designed for environmental compatibility.

An overview of the polymer recycling problem describes the major technological obstaclesto the implementation of recycling technologies, and outlines some of the approachesbeing taken [64].

13.9 Biodegradable Polymers

The synthetic polymer industry has brought great benefits to modern society. For example,in the packaging and distribution of foodstuffs and other perishable commodities, thecommercial thermoplastic polymers are hydrophobic and biologically inert, and this hasmade them essential to modern retailing [65].

Similarly, in agriculture, plastics have largely replaced glass in greenhouses and cloches,and they have gained a unique position in the growing of soft fruits and vegetables oververy thin polymer films (mulching films) [66]. The major group of polymers used both inpackaging and in agriculture are the polyolefins, which, due to their resistance toperoxidation, water and microorganisms, are durable during use.

In the 1970s, it became evident that the very technical advantages which made polymersso useful were disadvantages when polymer-based products were discarded at the end oftheir useful life and in particular when they appeared as litter in the environment. Someitems of plastics packaging waste were found to have very damaging effects on wildlife[67], and this led to calls from the ‘Green’ movement to return to biologically based(renewable) polymers.

Materials made from naturally occurring or biologically produced polymers are the onlytruly biodegradable ‘plastics’ available. Since living things construct these materials, livingthings can metabolise them.

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In practice, a relatively small weight proportion of polymeric materials ends up as litter.In most developed societies, domestic organic waste, including plastics packaging, isdisposed of in sanitary landfill or by incineration. However, burying waste is no longeran ecologically acceptable way of disposing of consumer wastes [65, 68].

Another approach aimed at the solid waste problem is the development or evaluation ofbiodegradable polymers [69]. These are based on a variety of natural products (oftenstructurally modified to optimise properties), or are laboratory-made polymers withstructures designed to be susceptible to enzymatic attack. A rather large effort has grownup in this area. Polymer types being studied include cellulose derivatives (such as celluloseacetate) [70], polysaccharides such as chitin [71], starch and poly(3-hydroxybutrate)[72]. A related approach to materials that break down under natural environmentalconditions is the development of UV-degradable plastics, designed to decompose insunlight should they become ‘litter’ [73]. Examples include a copolymer of ethylene andcarbon monoxide, and modified PET.

Two different applications have emerged over the past two decades for degradablepolymers. The first is where biodegradability is part of the function of the product.Examples of this are temporary sutures in the body or in controlled release of drugs,where cost is relatively unimportant. Similarly, in agriculture, very thin films of photo-biodegradable polyethylene are used to ensure earlier cropping and to reduce weedformation [65, 66]. By increasing soil temperature, they also increase crop yields andensure earlier harvest. A major ecological benefit of mulching films is the reduction ofirrigation water and fertiliser utilisation [74]. No residues must persist in the soil insubsequent seasons to make the land less productive by interfering with root growth.

The technology of biodegradable polymers, as a solution to minimising the huge amountof plastic waste, is developing. There is an ever-widening range of polymers satisfyingthe requirements necessary for the numerous applications for which biodegradability ofthe materials is essential.

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47. S.C. Rustagi, D.P.A. Dabholkar, J.K. Niham, M.N. Marathe and K.B. Iyer,inventors; Indian Patent 145,323, 1977.

48. H.S. Ostrowski, inventor; Fiber Industries, Inc., assignee; US Patent 3,884,850, 1975.

49. H. Toshima, inventor; Japan Kokai 75:64,382, 1975.

50. S.N. Tong, D.S. Chen, C.C. Chen and L.Z. Chung, Polymer, 1983, 24, 469.

51. S-C. Lee, V-W. Sze and C.C. Lin, Journal of Applied Polymer Science, 1995,55, 1271.

52. M.C. Yang and H.Y. Tsai, Textile Research Journal, 1997, 67, 10, 760.

53. N. Yoshiyuki, Y. Masahiro and F. Ryuichi, R&D Kobe Steel EngineeringReport, 1997, 47, 3, 43.

54. D.A. Tukker, H. de Groot, L. Simons and S. Wiegersma, Chemical Recycling ofPlastic waste (PVC and other Resins), TNO Report STB-99-55 Final, TNO,Delft, The Netherlands, 1999.

55. H. Croezen and H. Sas, Evaluation of the Texaco Gasification Process forTreatment of Mixed Household Waste, Final Report of Phase I and II, Council ofEurope, Delft, The Netherlands, 1997.

56. M. Heyde and S. Kremer, LCA Packaging Plastic Waste, LCA Documents 2(5),Ecomed, Landsberg, Germany, 1999.

57. J. Scheirs and S.W. Bigger, Proceedings of the 35th International SymposiumMacromolecules (IUPAC), Akron, OH, USA, 1994, 606.

379

58. P.M. Phillips, Proceedings of Antec ’93, New Orleans, LA, USA, 1993, 998.

59. F.A. Sitek, Modern Plastics International, 1993, 23, 10, 74.

60. Uniloy article, ‘Use of post-consumer plastic in bottles’, Modern PlasticsInternational, 1991, 21, 6.

61. P. S. Blatz in Emerging Technologies in Plastics Recycling, Eds., G.D. Andrewsand P.M. Subramanian, ACS Symposium Series No.513, American ChemicalSociety, Washington, DC, USA, 1992, Chapter 20.

62. G.S. Bowes, Proceedings of Antec ’91, Montreal, Canada, 1991, 2556.

63. N. Humber, Proceedings of Recycle ’93, Davos, Switzerland, 1993, p.17/2-1.

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381

Abbreviations and Acronyms

AA Acrylic acid

ABS Acrylonitrile-butadiene-styrene

AC Acetyl cellulose

AES Auger electron spectroscopy

AFM Atomic force microscopy

APB Ammonium pentaborate

APP Ammonium polyphosphate

aPP Atactic polypropylene

ASTM American Society for Testing and Materials

ATH Alumina trihydrate

au Arbitrary units

BDD Brominated dibenzodioxin(s)

BDF Brominated dibenzofuran(s)

BF Branch frequency

BFR Brominated flame retardant

BMP 2,6-di-tert-butyl-4-methylphenol

BOPP Biaxially oriented polypropylene

BPE Branched polyethylene

BTPC Benzyl triphenyl phosphonium chloride

BUR Blow-up ratio

CDD Chlorinated dibenzodioxin(s)

CDF Chlorinated dibenzofuran(s)

CI Carbonyl index

DBM Dibenzoylmethane

DIN Deutsch Institut für Normung

DMA Dynamic mechanical analysis

382


DMT Dimethyl terephthalate

DSC Differential scanning calorimetry

EFW Energy from waste

EPDM Ethylene-propylene-diene terpolymer

ESCA Electron spectroscopy for chemical analysis

ESCR Environmental stress cracking

ESR Electron spin resonance

EU European Union

EVA Ethylene vinyl acetate

EVOH Ethylene-vinyl alcohol

FFS Form-fill-seal

FTIR Fourier transform infrared spectroscopy

GE Grafting efficiency

GP Grafting percentage

GPC Gel permeation chromatography

HA Hindered amines

HALS Hindered amine light stabilisers

HDPE High-density polyethylene

HDUL Heat distortion under load

HEMA 2-Hydroxyethyl methacrylate

HFI Hyperfine interaction

HIPS High impact polystyrene

HLMI High-load melt index

HMW High molecular weight

HP Hindered phenols

HPLC High-performance chromatography

IPCS International Program for Chemical Safety

iPP Isotactic polypropylene

iPP Isotactic polypropylene

IR Infra red

ISO International Standards Organisation

383


L:D Length-to-diameter ratio

LCB Long chain branching

LDPE Low-density polyethylene

LLDPE Linear low-density polyethylene

LOI Limiting oxygen index

LPE Linear polyethylene

MD Machine direction

MDPE Medium-density polyethylene

MDSR Machine direction stretching ratio

MF Melt flow

MI Melt index

MPW Mixed plastic waste

MSW Municipal solid waste

MWD Molecular weight distribution

NIR Near infra red spectroscopy

NMR Nuclear magnetic resonance

NO Nitric oxide

NO2 Nitrogen dioxide

OMTS Octamethylcyclotetrasiloxane

OPP Oriented polypropylene

PA-12 Polyamide-12

PA-6 Polyamide-6

PAN Polyacrylonitrile

PB Phenyl benzoate

PBB Polybrominated biphenyl(s)

PBDE Polybrominated diphenyl ether(s)

PC Polycarbonate

PCA Polycaproamide

PCB Polychlorinated biphenyl(s)

PCDD Polychlorinated dibenzodioxin(s)

PCDF Polychlorinated dibenzofuran(s)

384


PCR Post-consumer recyclable

PCTFE Polychlorotrifluoroethylene

PE Polyethylene

PER Pentaerythritol

PET Polyethylene terephthalate

PI Polyisoprene

PMMA Polymethyl methacrylate

PMP Poly(4-methyl-1-pentene)

PNA Phenyl-β-naphthylamine

PP Polypropylene

PS Polystyrene

P-t-BuMA Poly-tert-butyl methacrylate

PTFE Polytetrafluoroethylene

PU Polyurethane

PVAc Polyvinyl acetate

PVA-ox Oxidised polyvinyl alcohol

PVB Poly(vinyl butyral)

PVC Polyvinyl chloride

PVDC Polyvinylidene chloride

PVF Polyvinyl fluoride

PVOH Polyvinyl alcohol

PVP Polyvinylpyrrolidone

RDP Rescorcinol diphosphate

RH Relative humidity

RHR Rate of heat release

SCB Short chain branching

SEC Size exclusion chromatography

SEM Scanning electron microscopy

SIMS Secondary ion-mass spectroscopy

SPI Society of the Plastics Industry

sPP Syndiotactic polypropylene

385


sPP Syndiotactic polypropylene

TCB Trichlorobenzene

TCDD 2,3,7,8-Tetrachlorodibenzo-p-dioxin

TD Transverse direction

TEB Tensile energy to break

TEF Toxic equivalence factor

TEM Transmission electron microscopy

TFE-HFP Tetrafluoroethylene-hexafluoropropylene copolymer

Tg Glass transition temperature

TGA Thermogravimetric analysis

Tm Melting temperature

TMDSC Temperature modulated DSC

Ton Onset temperature

TPA Terephthalic acid

TREF Temperature-rising elution fractionation

UHMWPE Ultra high molecular weight polyethylene

ULDPE Ultra low-density polyethylene

USEPA United States Environmental Protection Agency

UV Ultra violet

VA Vinyl acetate

VLDPE Very low-density density polyethylene

WAXS Wide angle X-ray scattering

WC Weight conversion percentage

WVTR Water vapour transmission rate

XFS X-ray fluorescene spectroscopy

XPS X-ray photoelectron spectroscopy

XRF X-ray fluorescence analyser

ZN Ziegler-Natta

386


387

Contributors

Elsayed M Abdel-BaryDepartment of ChemistryFaculty of ScienceMansoura UniversityMansouraEgypt

Heshmat A AglanDepartment of Mechanical EngineeringTuskegee UniversityTuskegeeAL 36088USA

Guneri AkovaliMiddle-East Technical UniversityDepartment of ChemistryTR-06531 AnkaraTurkey

Amin Al-RobaidiJubeiha 11941PO Box 1628AmmanJordan

Evgenii Y DavydovNM Emanuel Institute of Biochemical PhysicsRussian Academy of Sciencesul. Kosygina 4Moscow 117977Russia

Yong X. GanDepartment of Mechanical Engineering

388


Tuskegee UniversityTuskegeeAL 36088USA

Irina S. Gaponova,Emanuel Institute of Biochemical PhysicsRussian Academy of Sciencesul. Kosygina 4Moscow 117977Russia

Klara Z GumargalievaN.N. Semenov Institute of Chemical PhysicsRussian Academy of SciencesMoscowRussia

S.M. LomakinInstitute of the Biochemical Physics of Russian Academy of Sciences 119991,Kosygin 4Russia

Ashraf A MansourDepartment of ChemistryFaculty of ScienceCairo UniversityCairoEgypt

Alexander Mar´inParallel Solutions, Inc.763 Concord AvenueCambridgeMA 02138USA

Karl S Minsker,Bashkirian State University32 Frunze Str.UfaBashkiriya 450074Russia

389

Contributors

Georgii B PariiskiiNM Emanuel Institute of Biochemical PhysicsRussian Academy of Sciencesul. Kosygina 4Moscow 117977Russia

Susan E SelkeSchool of PackagingMichigan State UniversityEast LansingMI 48824-1223USA

Robert A ShanksDepartment of Applied ChemistryFaculty of Applied ScienceRMIT UniversityGPO Box 2476VMelbourneVictoria 3001Australia

Abbas A YehiaDepartment of Polymers and PigementsNational Research CenterDokkiCairoEgypt

Gennady E ZaikovN.M. Emanuel Institute of Biochemical PhysicsRussian Academy of Sciences4 Kosygin Str.Moscow 119991Russia

Vadim G. ZaikovN. M. Emanuel Institute of Biochemical Physics4 Kosygin Str.Moscow 117334Russia

390


391

Index

Main Index

A

Aclar films 247Additive dissolution 122, 123, 126

kinetics of 114Additive solubility 115

additive loss 125crystallinity 118factors 118high molecular weight additives 122polymer orientation 119polymer oxidation 124polymer polar groups 120quantitative data 114sorption 110-112supermolecular structure 118

Additivesantioxidants 14lubricants 14slip agents 14solubility of 109tackifiers 14ultraviolet stabilisers 14

Adhesive strengthadsorption theory 316diffusion theory 316electrical theory 316mechanical theory 316molecular-kinetic theory 316theory 316

Agricultural films 263blown film extrusion 264characteristics of plastic films 264greenhouses 263light transmittance 264

polyethylene filmsstabilisers 267

Albemarle 166Algipor 290, 301, 302Aliphatic chain scission 124Allied Signal Corporation 247Amoco 82Amoco process 84Antiblocking agents 22, 85

mineral particles 23Antioxidants

dissolution 122hindered amines 89hindered phenols 24, 89phosphites 90polyolefins 112solubility 122thio compounds 90triphenyl phosphite 24

Antistatic agentspolyoxyethylenes 24

Ammonium polyphosphate/pentaerythritol mixturesintumescent behaviour 172

Applicationsagriculture 38, 263coextruded films 37heat sealing 38laminated films 36packaging 35

Aqua-penetrability 305Artificial weathering devices 271Atactic polypropylene 13, 79Atochem 166


392

B

BiaxialBiaxially oriented film 18, 213

drawing 10polypropylene 351

properties of 244Biobrant 314, 324

burn dressing 323Biodegradable polymers 374Blending

agricultural plastic film waste 279Blow-up ratio 59, 61Blown film

air ring cooling 53blow ratio 56die 52extrusion process 14extrusion (tubular film) 50process 50

cooling the film 51extruder size 54extrusion equipment 55horsepower 55Tenter frame 19

production 13Branched polyethylenes 7

molecular structures 7Burn dressings 285-286

adhesion of 296, 315adhesive properties 292adhesive strength 317-318air penetrability 291, 295, 308, 315,

323air permeability 310aqua-penetrability coefficients 308cellulose 303characteristics 322collagen 303compositional 290cotton balling 290degree of filling 298efficiency of 322

evaporation of water from 318hydrophilic 303kinetics of sorption 304materials

free volume of 303material porosity 292mechanical properties 292microorganism penetrability 292model of action 318nitrogen penetrability 309number of pores 293oxygen penetrability 309physicochemical properties 292pore size distribution 293properties of 290size of pores 293solubility of water in polymers 298sorption ability 297-298, 322sorption of fluid 320sorption of liquid media 303sorption of plasma 303sorption of water 299-300sorption-diffusion properties 291sorptional ability 285sorptional ability of materials 294structure of 303vapour penetrability 291-292, 296,

305water absorption 291

Butyl rubbereffect of nitrogen oxides on 191

C

Carbonium ion mechanism 172Carbonylallyl groups

identification of 136Cast film

biaxial orientation 19calendering 16extrusion 57packaging 75

393

Index

polypropylene filmsapplications 76

processchilled roll 57

productioncalendering finishing 17extrusion coating 17extrusion conditions 16

tenter frame 16Char formation

acid-catalysed 175free-radical 174

Characterisationdielectric relaxation 226electron spectroscopy for chemical

analysis 228gravimetric method 224molecular weight 226molecular weight distribution 226scanning electron microscopy 225spectroscopic analysis

Auger electron spectroscopy 227infrared spectroscopy 227x-ray fluorescence spectroscopy 227

surface properties 227swelling measurements 226thermal analyses 225x-ray photoelectron spectroscopy 228

Charpy tests 29, 336Chemical modification 215

bromination 217, 218chemical etching 218chlorination 217fluorination 215

bulk 216direct 215indirect 216surface 216

grafting 220high-energy radiation 220radiation-induced 221

photografting 222bulk surface 222

sulfonation 218Chemical recycling 359

depolymerisation 360Chimassorb 280Chromatographic techniques 46Coextrusion 16, 17, 22

agricultural plastic film waste 279Collagens 301

maximum sorption of water 301Corona discharge

surface treatment system 22Creep

mechanism of 26Crosslinkable ethylene plastics

standard specifications 351Cultivated cutis 286Cyasorb 1084 280

D

Dacron 290Dehydrochlorination 134

rate constant 135Density

gradient column 44Die capacity 57Die size 55-56Die swell percentage

polymer melt 50Dielectric properties 30Differential thermal analysis 119Dimensional stability 35Dioxin

structure of 163Dow Chemicals 277Dressings

adhesion of 324air penetrability 313animal origin 286characteristics of 287-289multilayer 305penetrability 309


394

polyurethane 304synthetic materials 286vegetable origin 290

DuPont Company 131Dynamic mechanical analysis 339Dynamic mechanical properties 29, 339

E

Elmendorf tear strength test 28Energy recovery 361Erucamide

structure of 23Ester pyrolysis mechanism 173Ethylene copolymers

dielctric analysis 30Ethylene-vinyl alcohol films

oxygen barrier properties 248typical properties 249

Extruderblow ratio 15characteristics 14compression zone 15dispersive mixing 14distributive mixing 14feed zone 15frost-line 15, 16metering zone 15screw 15

design 15size 55twin-screw 14

Extrusion 10cast film 19process

calendering 17extrusion coating 17

tubular extrusion 13film 51, 59

Exxon 82process 83

F

Feedstock recycling 359Film 5

brittleness 60drawing 18-19extrusion blow moulded 18extrusion coating 22extrusion of the melt 5film lamination 22gloss 5lamination 37manufacture 5melt adhesion 21pigmented 38printing 5, 22slip agents 23, 342surface properties 5tensile strength 60toughness 60

Film applicationlinear low-density polyethylene 41

Film blowing 48Film characteristics

blocking 61bubble stability 61extrusion variables 58gloss 59haze 59impact strength 60-61optical 58puckering 62

Film extrusionblown film 50slot cast extrusion 50

Film orientationshrink-wrapping 214

Film packaging 75Film properties

blow ratio 61gloss 61haze 61

395

Index

optical 61Film samples

impact test methods 337Film shrinkage 69Fire retardants

ammonium pentaborate 169ammonium polyphosphate 168-169brominated 166brominated diphenyl oxide 161char-formers 167chlorinated 161chlorinated dibenzo-p-dioxins 162diagram of 167dimelamine phosphate 170environmental impact 168guanidine sulfamate 168halogen-containing 159-161halogen-free 170halogenated diphenyl ethers - dioxins

162inorganic 159intumescent systems 167-168low-melting glasses 167mechanism of action 160

condensed-phase 169melamine pyrophosphate 170nitrogen-based organic 159organophosphorus 159phosphorus 160polybrominated biphenyls 162, 165polybrominated diphenyl ethers 162polymer morphology modification

167polymer nanocomposites 167polymer organic char-former 175preceramic additives 167

Flat filmdies 57packaging bags 58

Flexible packagingbags, sacks and pouches 238dispensing 239

forms 236heat-sealing 240pouch production 239reclosure 239wraps

shrink-wrap 237stretch-wrap 237

Fluidised bed pyrolysis 359Fluoropolymer

sodium etching 219Free-falling dart method 336Free-radical processes 41

G

Gas permeation 35General grade polyethylene films

standard specifications 350General Electric 166Greenhouse films

additive 268ageing factors 275ageing resistance 275changes in chemical structure 276compatibility 269effects of pesticides 274environmental pollution 274excited-state quenchers

nickel dibutyldithiocarbamate 267fog formation 273hindered-amine light stabilisers 267,

275humidity 273light stabilisers 269recycling 278solar irradiation 265solar radiation 265stabilisation 265, 268-269, 272temperature 272ultraviolet screening

carbon black 266chalk 266short glass fibres 266


396

stabilisation 265-266talc 266TiO2 266

wind 273

H

Hall Woodroof Co. 290Heat stabilisers

structures of 25High-density polyethylene 10

melt strength 11recycled 373applications 373

detergent bottles 373grocery sacks 373large mouldings 373

viscosity 11Hindered amine light stabilisers

hydroxybenzophenones 25hydroxybenzotriazoles 25tetramethylpiperidines 25

Hydron 290, 298Hydroperoxide decomposition 208

I

Impact propertiesdart-puncture resistance 28tensile impact 28tensile–tear strength resistance 28

Infrared spectroscopycharacteriastion 32composition analysis of blends and

laminates 33surface analysis 33

Inorganic barrier coatingsaluminium oxide 255clay nanocomposites 255

Internal additivesantioxidants 24ultraviolet absorbers 24

Intrinsic viscositypolyethylene resin 46

Isotactic polypropylene 9, 12, 79-80Izod tests 29, 366

L

Light stabilisersstructures of 25

Linear low-density polyethylene 11differential scanning calorimetry

curves 33film

atomic force microscopy of 32stress-strain curve 27

Linear polyethylene 7molecular structures 7

Linear low density polyethylene 8composition

by 13C nuclear magnetic resonance346

properties 8shrink films 14

Low-density polyethylene 7, 11continuous shear rheology curve 12covering films

hindered-amine ligt stabilising 272differential scanning calorimetry

curves 33films

standard specifications 350high-pressure radical process 7mulch film

recycling 278long branches 11resistance to tear propagation 338shrink properties 68

Lubricantschlorinated paraffins 24paraffin wax 24stearate salts 24stearic acid 24

397

Index

M

Mechanical propertiesabrasion resistance 26adhesion tests 26impact tests 26polyolefin films 25modification of 213tear testing 26tensile 26

Mechanical testsbending stiffness (flexural modulus)

339free-falling dart method 336hail resistance 337impact resistance 336impact test methods 337package yield of a plastic film 334pendulum impact resistance 337pendulum method 338percent elongation at break 334percent elongation at yield 334propagation tear resistance 338puncture-propagation tear resistance

339tear resistance 337tensile modulus of elasticity 334tensile strength 333tensile strength at break 334tensile testing (static) 333yield strength 333

Medium-density polyethylene filmsstandard specifications 350

Melt elasticity 50Microporous material

typical sorption isotherm 294Microscopic examination

atomic force microscopy 31optical–polarised light effect with

strain 31scanning electron microscopy–etching

31

Mitsui Hypol 82-83Mixed plastic waste

chemical recycling 369conversion process 369feedstock recycling process 369gasification 369identification of

electromagnetic scanning 363optical systems 363X-ray fluorescence 363

polymer cracking process 369Modulus 331Moisture resistance 34Mulch film 277

recycling 278Multilayer plastic films 16

coating 252coextrusion 253greenhouses 279lamination 253metallisation 253silicon oxide coating 254

N

Nitroxyl radicals 194Electron spin resonance spectrum 194structure of 203

Noryl 166Nylon-6

thermal decomposition 171

O

Orientation 35biaxial 18by drawing 18during blowing 18machine direction 60of film 18transverse direction 60

Oriented polypropylene 351


398

standard specifications 351Oxygen indices 170

P

Polyamide-6,6cone calorimeter data 179-180nanocomposite

carbon residue 183Polyamide-6,6/Polyvinyl alcohol

cone calorimeter data 178Packaging 235Packaging films

acid copolymer films 250biaxially oriented film 244cellophane 247ethylene-vinyl acetate 250ethylene-vinyl alcohol 248high-density polyethylene 243linear low-density polyethylene 242low-density polyethylene 242polyamide (Nylon) 249polychlorotrifluoroethylene 247polyethylene terephthalate 245polypropylene 244polyvinyl alcohol 248polyvinyl chloride 245polyvinylidene chloride 246uses of 241

Packaging materialsbarrier 257cellulose 252environmental issues 261high-impact polystyrene 252ionomers 251permeation 257plastics 251polystyrene 251printing

flexography 256ink-jet printing 257lithography 257

rotogravure 257screen printing 257

static discharge 256surface treatment

corona discharge 255Polyethylene

chlorination of 217films

wetting tension 344interaction of nitrogen dioxide with

189long-chain branching 45melt flow properties 45melts

shear viscosity 48oxidation of 219processing

troubleshooting 63-65resin

basic properties 42chain branching 45density 44, 45dispersity index 42elasticity 49elongational viscosity 49heat of fusion 47intrinsic viscosity 46melt flow blend relationship 43melt index 42melt properties 48melting point 47molecular weight 42rheology 48viscosity/shear rheology 48

short-chain branching 44-45waste

contamination by additives 372contamination by reprocessing 372

Perfluoronitroxyl radicalselectron spin resonance spectra of 203

Physical property modificationcorona treatment 223plasma treatment 222

399

Index

Physicochemical testsabrasion resistance 347blocking load

parallel-plate method 346creep 346creep rupture 346density of plastics 340environmental stress cracking 348haze transmittance 341ignition 342indices of refraction 340kinetic coefficients of friction 342luminous transmittance 341mar resistance 348orientation release stress 345outdoor weathering 347oxygen gas transmission 349oxygen index 342rate of burning characteristics 342resistance to chemicals 341rigidity 345shrink tension 345specular gloss 343static coefficients of friction 342transparency 341water vapour permeability 348weatherability 347yellowness 340

Piping materialpolypropylene 74

Plastic filmsabrasive damage 347applications 228artificial ageing tests 276contamination by the environment

277crosslinking 213-214crystallisation 213-214dimensions 332gloss 343grafting 228greenhouses 263-264

in packaging 235mechanical properties 213

modification of 213orientation 213-214photooxidation 271, 277physical properties 213premature failure 272production of 263properties of 332recycling in agriculture 277removal of contaminants 213resistance to tearing 338slip properties 343specular gloss 343stability 263, 347standard specifications 349tear resistance 338testing of 329-356thicknesses 332unrestrained linear thermal shrinkage

345Plastic materials

fire retardation 168photodegradation 265recycling 279

Plastic production 76growth rate 77

Plastic recyclingcollection 362mechanical recycling 358primary recycling 358, 362quaternary recycling 360secondary recycling 358, 362sorting 362tertiary recycling 359, 362

Plastic surfacesfluorinated 216

Plastic wastefilms

recycling 278reuse 278management 278


400

recycling 357resin identification 362

Plasticsseparation of 359

Polymethyl methacrylateexposure to nitrogen dioxide 193

Polluted atmospheres 187Polyamide films

typical properties 250Polyamides

interaction of nitrogen dioxide with196

Polyamidoimide filminfluence of nitrogen dioxide on 200

Polydimethylsiloxanediffusion of gases 310penetrability of gases 310solubility of gases 310

Polyethylene 7, 8decreasing-pitch screw 51density 60high-density 8high-pressure technology 45linear low-density 8low-density 7low-pressure technology 45self-adhesion 21solubility of phenyl benzoate in 120solubility of phenyl-b-naphthylamine

in 122surface treatment of 217ultra-low-density 8very-low-density 8

Polyethylene filmscrosslinked 214irradiated 215photo(bio)degradable 271processing 41properties of 243shrink-wrapping 35

Polyethylene resinintrinsic viscosity 46

Polyethylene terephthalate filmsproperties of 246standard specifications 350waste

chemical recycling process 368depolymerised 367glycolysis 367-368recycling of 367

Polyisopreneinteraction with nitrogen dioxide 192

Polymer filmsgloss 20haze 20surface analysis 34

Polymer mixturesselective dissolution 366

Polymer nanocompositescombustibility 181disordered 180intercalated 180

Polymer structuremorphological irregularity 109nonuniform 109

Polymeric materialsgraft copolymerisation 220

Polymersbarrier characteristics 215diffusion of water vapour 306flame retardancy 159-160interaction of nitrogen dioxides with

188interaction of nitrogen oxides with

187-188nitric oxide 188nitrogen oxide 188non-saturated 191penetrability 306, 310penetrability of gases 309photochemical oxidation 187reaction of nitric oxide with 202rheology 15separation coefficients of gases 309

401

Index

solubility of stabilisers 116-117solubility of water in 299thermal oxidation 187

Polyolefin elastomers 8Polyolefin films 9

crystallisation 9morphology of 9packaging applications 36production 5

Polyolefins 6-7, 10corona discharge treatment 21dielectric properties 30orientation drawing 119properties 6rheological characterisation 10structure of 6-7virgin

recycling behaviour of 271Polypropylene 9, 12

additives 88balanced oriented 351barrier properties 214branching 78calcium carbonate pigment 101chain scission 87-88chirality 79degradation 86durability-additive property 97durability-processing condition 94dynamic mechanical analysis curve 29films

durability 73processing conditions 73

hydroperoxide decomposition 207interaction of nitrogen dioxide with

189metallocene-catalysed 13micrograph 82microstructure 96morphology 81regiospecificity 78rods 74

stress-strain behaviour 94-95, 98-99stretched tape materials 100structures 80properties of 244two-step tubular orientation 13ultraviolet degradation 86uniaxially oriented 351wire coating 75worldwide capacity 77

Poly-tert-butyl methacrylate filmdegradation 190

Polytetrafluoroethyleneprocessing of 215

Polyurethanecellular 286films

exposure to nitrogen dioxide 200interaction of nitrogen dioxide with

196sponges

pore size 286Polyvinyl chloride

properties of 244Polyvinylidene chloride films

typical properties 247Porous materials

determining absorbtion ability 295determining air penetrability of 296penetrability 310

Post-consumer filmsrecycling 277

Polypropylene degradation 87Polypropylene fabric

scanning electron microscopemorphology 93

of ultraviolet light degraded 93stress-strain behaviour 92

Polypropylene films 75additives 85chill roll cast method 85durability-microstructure

relationship 91


402

film processing 85microscopic examination 91oriented 75opaque 75static tensile tests 91structures 78surface morphology 100synthesis 78ultraviolet degradation behaviour 90ultraviolet exposure 91wetting tension 344

Polypropylene granulesScanninmg electron micrograph 80

Polypropylene woven fabricsstress-strain behaviour 91

Polypropylene-polyethylene-co-polypropylene 10micrograph 10

Processingtroubleshooting 62

Polystyrenedegradation 191film

degradation 190Polyvinyl chloride 134

degradationeffect of plasticisers 145rate 148

dehydrochlorination 135, 137, 145,147-148

kinetic curves for 134rate constants 138thermal 150

disintegration 132, 134, 136thermal 151

‘echo’ stabilisation 151-152, 153global production 131light stabilisation 144low stability 132stabilisation 138, 140-141thermodegradation rate 150thermoformed packaging 245

Polyvinyl chloride filmsplasticisers 245properties 245

Polyvinyl chloride wastechemical recycling 368, 370incineration 368, 370

rotary kiln 370mechanical recycling 368pyrolysis process

chemical 371thermal degradation 371

recycling 368Polyvinyl alcohol

hospital laundry bags 248water-solubility 248

Polyvinylpyrrolidoneinteraction of nitrogen dioxide with

197

R

Recyclingdioxins 361energy recovery 360furans 361incineration 360incinerator 361radiation technology 373

Regenerated cellulosediffusion coefficient 306

Resin separationair separation 363colour 365density 364electrification 365float-sink operations 364flotation tanks 363fluidisation 365Fourier transform IR 366high-voltage drums 365hydrocyclone 364magnetic separation 363

403

Index

photoelectric sensors 365physicochemical properties 365spectroscopy 366supercritical fluid 364X-ray fluorescence analyser 366

Rheology 5Rubber-modified products 373

S

Saytex 8010 166Separated PE waste

recycling 371Shock-cooling 58Short-term tests

dart test 28impact test 28

Shrink film 18, 62, 65bi-oriented 65blow-up ratio 67bubble shape 69frost-line 67, 69manufacture 67mono-oriented 65ovens 70properties 66resin melt index 69shrinkage 66shrink-wrapping 62tunnels 70

Slip agentserucamide 23, 85ethylene bis-stearamide 23oleamide 23, 85stearamide 23

Slot casting 59process

melt temperature 60Sorption isotherm 110Spheripol 82

process 82-83Stabilisation agents

phenolic antioxidants 85phosphite antioxidants 85

Stress relaxationmechanism of 26

Surface additivescorona treatments 33glyceryl monooleate 33polyisobutylene 33slip agents 33

Surface modificationantiblocking 22antistatic agents 24chemical treatments 213chlorinated paraffins 24corona discharge 21, 213hydrophilic 223lubricants 24paraffin wax 24physical methods 222plasma 213slip additives 23stearate salts 24stearic acid 24

Surface propertiesblocking 21gloss 19haze 20slip 21surface energy 20

Syndiotactic polypropylene 79

T

Tensile propertiesburst strength 28creep 27, 28strain hardening 26strain rate 26stress relaxation 26

TestingASTM D882 335methods


404

requirements 330sample conditioning 332

resultsinterpretation of 330

thin films 335Tetrafluroroethylene-hexafluoropropylene

gamma-irradiatedaction of nitric oxide on 205

Thermal analysisdifferential scanning calorimetry 31

temperature-modulated 32Thermal dehydrochlorination 146Thickness 34Thin films

‘neck-in’ 58Tinuvin 622 LD 280Toxicity equivalence factors 162-164

CDD 164CDF 164

U

Ultra-low-density polyethylene 11Ultraviolet stabilisation 25Unipol 82Unipol process 84Ultraviolet degradation 87Ultraviolet stabilisers 70

V

Very-low-density polyethylene 11Vinyl polymers

interaction of nitrogen dioxide with188

Viscoelasticity 331

W

Wound exudatesorption of by dressings 298

Z

Ziegler-Natta processes 41

ISBN: 1-85957-338-X


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