Technology of Polymer Packaging

Arabinda Ghosh Technology of Polymer Packaging

Technology of Polymer Packaging

Arabinda Ghosh

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ISBN: 978-1-56990-576-0 E-Book ISBN: 978-1-56990-577-7

The Author:

Prof. Dr.-Ing. Arabinda Ghosh, University for Applied Sciences, Stuttgart (HdM), Nobelstr. 10, 70569 Stuttgart, Germany

In Memory of

My parents

and

The legendary

Professor Dr.-Ing. habil. h. c. Rudolf Heiss

1903–2009(Director of the Fraunhofer Institute for Food and Packaging Technology, Munich

1936–1975)

Since the first production of polyethylene on a large scale by ICI (Imperial Chemi-cal Industries) in the 1930s, polymer materials, or as they are simply called, plas-tics, have been inevitable as successful packaging materials. Plastics protect all kinds of products like food, pharmaceuticals, cosmetics, medical products, and other nonfoods against deterioration. Although the amount of tissue material such as paper, paper board, and corrugated board used for packaging is a bit higher than polymers, polymers are inevitable for primary packaging. They fulfill all of the legislative regulations worldwide for direct contact with the product, particularly with food.

No other packaging material shows such a continuous and rapid development as does polymer packaging material. Scientists, experts, and technologists of the pack-aging sector are responsible for the development and application of tailor-made solutions. This book will contribute to the practical knowledge of specialists.

Besides basic and applied knowledge on technology, a number of valuable sugges-tions on critical cases are given in this book.

Finally, I hope this book will be a valuable help for the reader to solve technical problems and be a contribution to successful packaging development.

Plastics

Glass

Metal

Paper

Others

PE – Film, BM, IM, Roto …

PP – Film, TF, BM, IM, …

PS – TF, Sheets, …

PET – BM, Film, …

PVC – Film, Sheet, …

Global Packaging Materials – a Breakdown

Plastics Have the Highest Growth Rate among All Materials in the Packaging Sector

BM: blow molding, IM: injection molding, TF: thermoforming, Roto: roto moldingRaj Datta, Haldia Petrochemicals, National Conference, IIP, Kolkata 2012

Preface

I want to express my acknowledgement to a number of people in the packaging industries of Germany, Switzerland, and Belgium for their valuable information and kind permission to publish relevant information or figures.

For valuable information:Mrs. Elisabeth Mersteiner, RPC Kutenholz, Germany

Dr. Alfred Koblischek, Alcan-Tscheulin, Germany

Dr. Karl-Heinz Hausmann, DuPont, Switzerland

Mr. Helmut Meyer, Reifenhauser, Germany

Mr. Herbert Stotkiewitz, Bosch Packaging, Germany

Mr. Harald Braun, Rovema, Germany

Mr. Matthias Huter, Solvay, Germany

Mr. Matthias Schraegle, Huhtamaki, Germany

Mr. Peter Ludwig, EK-Pack, Germany

Mr. Raj Datta, Haldia Petrochemicals, India

Dr. Christof Herschbach, Windmoeller & Hoelscher, Germany

Dr. Sven Fischer, Krones, Germany

Dr. Georg Kinzelmann, Henkel, Germany

For figures:Dr. Michael Roth, Battenfeld, Germany

Mr. William Reay, Kuraray, Belgium

Mr. Herbert Stotkiewitz, Bosch Packaging, Germany

Mr. Matthias Huter, Solvay, Germany

Otto Hofstetter AG, CH – Uznach, Switzerland

Acknowledgements

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .VII

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IX

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XV

1 Basics of Polymer Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Definition of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Manufacturing of Polymer Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Classification of Plastics: Molecular Structure . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Plastics Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4.1 Usual Additives in the Packaging Sector . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4.1.2 Light Stabilizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4.1.3 PVC Stabilizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4.1.4 Antiblock Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1.5 Antifog Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1.6 Nucleating Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1.7 Lubricants as Processing Aids . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1.8 Slip Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1.9 Antistatic Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.1.10 Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.1.11 Optical Brighteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.1.12 Chemical Blowing Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.1.13 Antimicrobial Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 Required Performance of Polymer Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.6 Different Types of Polymers Used for Packaging . . . . . . . . . . . . . . . . . . . . . . . 91.6.1 Polyurethanes as Adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

XII Contents

1.7 Short Description of Some Polymers for Packaging Applications . . . . . . . . . 12

1.8 Major Polymers Used in Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.8.1 Important Points for the Technologist . . . . . . . . . . . . . . . . . . . . . . . . . . 16

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 Manufacturing of Polymer Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.1 Extrusion of Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1.1 Technology of Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.1.2 Continuous Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.1.2.1 Manufacturing of Blown Film . . . . . . . . . . . . . . . . . . . . . . . . . . 262.1.2.2 Manufacturing of Cast Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.1.2.3 Collapsible Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.1.2.4 Flexible Films for Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.1.3 Important Features for the Technologist . . . . . . . . . . . . . . . . . . . . . . . . 482.1.4 Discontinuous Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

2.1.4.1 Injection Molding (IM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.1.4.2 Injection Blow Molding (IBM) . . . . . . . . . . . . . . . . . . . . . . . . . . 522.1.4.3 Extrusion Blow Molding (EBM) . . . . . . . . . . . . . . . . . . . . . . . . . 542.1.4.4 Stretch Blow Molding (SBM) . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.1.4.5 Different Types of PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602.1.4.6 Thermoforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.2 Sealing of Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712.2.2 Principles of Heat Generation for Sealing of Packaging Materials . . . 732.2.3 Technology of Sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.2.3.1 Direct Heating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752.2.3.2 Indirect Heating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3 Converting of Polymer Packaging (Composite Packaging) . . . . . . . . 83

3.1 Technology of Converting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 833.1.1 Modes of Converting Packaging Material . . . . . . . . . . . . . . . . . . . . . . . 843.1.2 Technology of Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

3.1.2.1 Extrusion and Coextrusion Coating . . . . . . . . . . . . . . . . . . . . . 853.1.2.2 Coating with Lacquer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863.1.2.3 Coating with Polymer Dispersion . . . . . . . . . . . . . . . . . . . . . . . 88

3.1.3 Technology of Lamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.1.3.1 Extrusion and Coextrusion Lamination . . . . . . . . . . . . . . . . . . 903.1.3.2 Dry Lamination, Solvent Based . . . . . . . . . . . . . . . . . . . . . . . . . 913.1.3.3 Dry Lamination, Solvent-Free Adhesive . . . . . . . . . . . . . . . . . . 95

XIII

3.1.3.4 Glue or Water-Based Lamination . . . . . . . . . . . . . . . . . . . . . . . . 973.1.3.5 Wax or Hot-Melt Lamination . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.1.4 Important Features for the Technologist . . . . . . . . . . . . . . . . . . . . . . . . 99

3.2 Vacuum Deposition of Ultrathin Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003.2.1 Physical Vapor Deposition (PVD) Process . . . . . . . . . . . . . . . . . . . . . . 1013.2.2 Chemical Vapor Deposition (CVD) Process . . . . . . . . . . . . . . . . . . . . . 102

3.3 Radiation Upgrading of Packaging Material . . . . . . . . . . . . . . . . . . . . . . . . . 1043.3.1 Effect of Radiation on Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

3.4 Extended (Foamed) Packaging Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.4.1 Physical Foaming with Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053.4.2 Chemical Nucleating Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1063.4.3 Foam Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1063.4.4 Foam Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1063.4.5 Foam Thermoforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

3.5 Special Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1073.5.1 Sealing through Liquid and Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1073.5.2 Transverse Sealing of Side-Folded Pouches . . . . . . . . . . . . . . . . . . . . . 1093.5.3 Weak Points of a Collapsible Polymer Tube . . . . . . . . . . . . . . . . . . . . . 1113.5.4 Pinholes in Packs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1123.5.5 Complaint Management for New Technologists . . . . . . . . . . . . . . . . . 113

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

μm micrometer

ABL aluminum-barrier laminate

AC alternating current

Adh adhesive

AFA antifog agents

Al aluminum

Al2O3 aluminum oxide

AlO aluminum monoxide

AlOx mixture of AlO and Al2O3

Ba barium

bar unit for pressure (105 Pa)

BN boron nitride

BOPA / BONy) biaxial-oriented flexible nylon

BOPET biaxial-oriented flexible PET

BOPP biaxial-oriented flexible polypropylene

Ca calcium

CBL ceramic-barrier laminate

CH3 methyl group

Cl2 chlorine

CO2 carbon dioxide

COC cycloolefin copolymer

Coex coextrusion

COF coefficient of friction

CPP cast polypropylene

CVD chemical vapor deposition

D diameter

EAA ethylene–acrylic acid copolymer

Abbreviations

XVI Abbreviations

EBA ethylene–butyl acrylate copolymer

EB-Gun electron beam gun

EBM extrusion blow molding

EMA ethylene–methyl acrylate copolymer

EMAA ethylene–methacrylic acid copolymer

EPS extended polystyrene

ESCR environmental stress crack resistance

EVA ethylene–vinyl acetate-copolymer

EVOH ethylene–vinyl alcohol-copolymer

Fe2O3 iron oxide (ferric oxide)

H2O water vapor / moisture

HALS hindered amine light stabilizer

HCl hydrochloric acid

HDPE high density polyethylene

HDT heat distortion temperature

HFFS horizontal-form-fill-seal machine

HIPS high impact polystyrene

HSL heat seal lacquer

IBM injection blow molding

IM injection molding

IML in-mold labeling

IR infrared

K potassium

LDPE low density polyethylene

LLDPE linear low density polyethylene

m/c machine

MDPE medium density polyethylene

mLLDPE metallocene LLDPE

MXD6 meta-xylene diamine

N nitrogen

Na sodium

NaOH sodium hydroxide

OPA / ONy monoaxially oriented nylon

OPLA mono- or biaxially oriented polylactic acid copolymers

OPS oriented polystyrene

PA polyamide / nylon

PAN polyacrylonitrile

PBL polymer barrier laminate

XVII

PE polyethylene

PEN polyethylene naphthalate

PET-A amorphous polyester

PET-C crystalline polyester

PET-G glycol modified polyester (cyclohexanedimethanol)

PET polyester / polyethylene terephthalate

PLA polylactic acid

PO polyolefin

PP polypropylene

PS polystyrene

PTFE polytetrafluoroethylene

PTMT polytetramethylene terephthalate

PU polyurethane

PVC polyvinyl chloride

PVC-P plasticized polyvinyl chloride

PVC-U unplasticized polyvinyl chloride

PVD physical vapor deposition

PVdC polyvinylidene chloride

PVOH polyvinyl alcohol

RPM revolutions per minute

SB solvent based

SBM stretch blow molding

SF solvent free

SiO silicon monoxide

SiO2 silicon dioxide

SiOx mixture of SiO and SiO2

SPPF solid phase pressure forming

SSE single screw extruder

TE melt temperature

TG glass transition temperature

TiO2 titanium dioxide

TSE twin screw extruder

ULDPE ultra low density polyethylene

UV ultraviolet

VAC vinyl acetate

VFFS vertical-form-fill-seal machine

VLDPE very low density polyethylene

Zn zinc

�� 1.1� Definition of Polymers

Polymers, commonly called plastics, are artificial products that are not available in nature. They are produced artificially from basic organic materials, crude oil, natu-ral gas, or even biomass. The initial products, so-called monomers, are low molecu-lar weight gases or liquids. High molecular weight macromolecules with solid con-sistency are synthesized through chain reactions and sometimes through cross-linking. Also, inorganic elements are used to manufacture polymers like PVC, where chlorine is used. There are, however, many examples in nature where high molecular weight products are made through biosynthesis from low molecu-lar weight substances in plants, animals, or insects. Examples are resins or rubber in plants and carbohydrates, fats, or proteins in animals or insects.

�� 1.2� Manufacturing of Polymer Resins

Low molecular weight monomers are produced through fractional distillation and cracking of petroleum or natural gas. There are three different reactions in the synthesis of plastics from the monomers: polymerization, polycondensation, and polyaddition. In polymerization, the unsaturated double bonds of the monomers are cracked and then the radicals polymerize in a random manner into high molec-ular weight plastics. Examples are PE (Fig. 1.1), PP, and PVC (see abbreviations list on pages XV–XVII). Polyaddition polymerization can take place in an autoclave ei-ther at low or high pressure or in a tubular reactor. In polycondensation, the mono-mers react with each other through the loss of one low molecular weight product like water and create high molecular weight plastics through a chain reaction. Ex-amples are PA and PET. Finally, in polyaddition, the molecular structure of the monomers is rearranged and linked with each other through cross-linking to high

1 Basics of Polymer Packaging

2 1 Basics of Polymer Packaging

molecular weight plastics. A typical characteristic of cross-linking is that it is a chemical bonding through the main valency and is not a physical bonding like van der Waals forces or hydrogen bonding, which has an ionic nature. Polyurethane is an example of this type of plastic (see Fig. 1.2). Macromolecules of the first two types have a linear structure with some side branches, but the characteristic of polyurethanes is a typical cross-linking structure. Also, elastomers (rubbers) have cross-linking structures.

Polyethylene

= Energy in the form of heat, γ-rays, UV rays, and catalysts or peroxide radicals

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

………

E

H

H

C

H

H

C

E

+

H

H

C

H

H

C

E E

H

H

C

H

H

C +

H

H

C

H

H

C +

H

H

C

H

H

C

………

—[ CH2 —— CH2 ]—n

E

Figure 1.1 Polymerization of polyethylene (PE)

Amorphous

Rubber/ ElastomerThermoplasticsThermoset Plastics

Semicrystalline

Figure 1.2 Molecular structure of different polymers

31.3�Classification of Plastics: Molecular Structure

�� 1.3� Classification of Plastics: Molecular Structure

Looking at molecular structures, plastics are classified into three groups:

1. Thermoplastic polymers2. Thermoset polymers3. Elastomers

Thermoplastics have a linear structure with no or a very low level of branching and can go through repeated melting and solidification cycles. During heating, the Brownian movement of the macromolecules increases, resulting in a reduction of stiffness. At elevated temperature they are so soft that they can be deformed with low force. At higher temperatures they ultimately melt and can be extruded for different production processes.

There are two types of thermoplastics. In the first type, the macromolecules build a random cluster. Different chain segments have hydrogen-bridge bonding when they come close to one another. Through this phenomenon they have a stiff struc-ture. Moreover, they form an inhomogeneous molecular structure. They are called amorphous thermoplastics. Examples are polystyrene (PS) or polyvinylchloride (PVC). They are transparent. The transparency can be enhanced through molecu-lar orientation during manufacture.

In the second type, the macromolecule chains are partly arranged as amorphous structures and partly in a parallel structure like a packet, the so-called crystallites. These crystallites arise through high physical bonding between the chain seg-ments, which run parallel to one another. Besides chain packets, other geometrical structures like spherulites are also possible. The molecular structures of crystal-lites are so congested that light can pass only partially through these structures. These types of thermoplastics are called semicrystalline polymers. Examples are polyethylene (PE), polypropylene (PP), or nylon (PA). Semicrystalline polymers with a high amount of crystallites are opaque, for example HDPE (high density polyethylene). Due to their inhomogeneous structure, semicrystalline polymers show inhomogeneous characteristics during thermoforming. Amorphous thermo-plastics are much stiffer than semicrystalline plastics. The level of crystallinity in a semicrystalline plastic will determine the melting temperature. A highly crystal-line polymer such as HDPE, PA, and often PTFE (with very high crystallinity) will have a higher melting temperature than a less crystalline polymer such as LDPE.


Here are a few examples:

HDPE: 130°C; LLDPE: 110°C; Ionomers: 90°C; PP: 165°C; PET: 250°C; PA-12: 175°C; PA 6 12: 215°C; PA-6: 220°C; PA-66: 254°C

The macromolecules of elastomers are cross-linked, but the knots of cross-linking are widespread. A typical characteristic is their low stiffness (elasticity), but be-cause of cross-linking they are unable to melt. At elevated temperatures a thermal destruction takes place, but they are still unable to melt.

Thermoset plastics (duroplastics) have a structure similar to elastomers, but the cross-linking is by far more congested. The macromolecules are completely unable to move and show almost no Brownian motion. They are very stiff and brittle. Like elastomers (rubber), here also a thermal destruction takes place at elevated tem-peratures, for example in Araldite. Usually the monomers are low molecular weight liquids that react to form a thermoset network that is irrevocably cross-linked and does not allow any thermoplastic deformation thereafter.

Thermoplastics are the only polymers that are used to make polymer packaging. Elastomers and duroplastics are used only occasionally as a supporting material.

�� 1.4� Plastics Additives

The plastics to make packaging materials are supplied as granules (resins) or pow-der. Although they could be extruded to different products like film or other pack-aging materials, they do not fulfill the different requirements that packaging should possess, or sometimes the production speed or machinability of a film is poor. In order to achieve an optimal characteristic, the plastic resins are mixed with additives. These are auxiliary substances for plastic resins that will optimize different characteristics. The most common terms used in the processing of addi-tives are next:

CompoundingResins are mixed with a very low amount (∼1%) of additives. A twin screw extruder (TSE) or sometimes a planetary extruder (for PVC manufacturing) is necessary to mix the components homogeneously.

BlendTwo or three different resins (not additives) are mixed at different ratios in a single screw extruder (SSE) ideally equipped with a mixing head or even a twin screw extruder. The goal is to get better properties in the packaging material.

51.4�Plastics Additives

MasterbatchWhen a virgin resin is mixed with a compound, which is mostly the same resin with some particular additive, then this compound is called the masterbatch. The amount of compound could be 5–10%.

Blends are made with resins of similar granule sizes and densities, so that’s why it can be processed by a single screw extruder. The manufacturing of the master-batch and the compounding of different components with different granule sizes and densities or powder, pastes, and sometimes even liquids (slip agent) are done with twin screw extruders. In twin screw extruders the grade of mixing is high, and hence the resins made are homogeneous.

1.4.1� Usual Additives in the Packaging Sector

1.4.1.1� AntioxidantsThrough the presence of residual monomers, dirt, or residuals of catalysts, oxida-tion takes place in a resin with the presence of oxygen. Heat and light catalyze this process. Chain scission of the polymer produces radicals that cause further degra-dation of the polymer. This type of reaction is called autoxidation. The polymer loses its brightness and stiffness and ultimately ages. The chain scission of a linear polymer with 10,000 monomer units and a contamination level of 100 ppm is suf-ficient to halve the molecular weight of the polymer. Antioxidants like aromatic amines or phenols scavenge the radicals, hinder degradation, and stabilize the polymer, particularly in outdoor use.

1.4.1.2� Light StabilizerLight, particularly the ultraviolet spectrum, also induces degradation in polymers, deteriorating the optical and mechanical properties. In particular, polymers con-taining impurities or chromophores are more sensitive to light. Not all polymers are similarly sensitive; PE is more resistant to light than PP. The stabilizers are mostly HALS (hindered amine light stabilizer) of different structures.

1.4.1.3� PVC StabilizerPVC, when processed at high temperature, loses hydrochloric acid (HCl), which scissions the macromolecules and causes cross-linking. This results in discolor-ation and changes the physical and chemical properties. Stabilizers are mostly car-boxylated metals like K, Ca, or Ba, which scavenge HCl. Other stabilizers are alkyl phosphites and fatty acid esters.


1.4.1.4� Antiblock AgentsThin films tend to stick together through surface forces, which cause blocking of a roll during unrolling. The film unrolls inhomogeneously, and it can even tear. Anti-block agents are made of inorganic particles that keep some distance between the film layers, enabling air to get in between them. The film can then be unrolled smoothly. Typical antiblocking agents are silica, talc, or limestone.

1.4.1.5� Antifog AgentsMoist food packed at ambient temperature when cooled creates fog through con-densation of water vapor. The humidity deposits on the bottom side of the top film and makes it hazy. Because of the high difference in surface tension between poly-olefin films and water droplets, there is no homogeneous layer of water on the film. In order to gain a clear view into the pack, antifogging agents (AFAs) are extruded in the film. The antifogging agents migrate to the film surface and reduce the sur-face tension of water droplets, creating a homogeneous water layer. The view is then clear. Typical AFAs are glycerol or sorbitan esters.

1.4.1.6� Nucleating AgentsIn order to increase the speed of crystallization in semicrystalline polymers such as HDPE, PP or PA (nylon), and polyesters, in particular, nucleating agents are added. The mechanical strength of the film increases, so a thinner film can be produced. This procedure is used for high-speed production or to produce thinner films of expensive resins like nylon. The nucleating agents are generally resins of a higher melting point (butane) or inorganic salts of alkali metals (sodium-2-chlorobenoate).

1.4.1.7� Lubricants as Processing AidsLubricants are polymer processing aids that enable smooth production of the poly-mer melt without tearing the melt flow or producing a melt flow surface like shark skin. Also, the production speed is increased. These are mostly hydrocarbon waxes, fluoroelastomers, fluoropolymers, or silicone-based additives. These are extruded first to get a layer on the inner wall before the main resin is extruded. This retards the adhesion of burned particles on the inner wall of the barrel.

1.4.1.8� Slip AgentsSlip agents reduce the coefficient of friction (COF) of a film during its machining. It is particularly important for films in high-speed packaging lines, as in a vertical-form-fill-seal machine (VFFS). The agents are mixed with the resin and are not com-

71.4�Plastics Additives

patible with the resin. After production, they migrate to the film surface and behave as a surface lubricant. Two very well-known slip agents are erucamide and oleamide.

1.4.1.9� Antistatic AgentsPolymers, particularly nonpolar polyolefins, are bad conductors of electricity, and hence, polymer products generate very high electrical charges locally through fric-tion. This can cause an unwanted discharge of electrical current and dust absorp-tion from the air, particularly when the air is dry. Antistatic agents are of two types, external and internal. External agents are sprayed or coated on the surface, like in tubes. They can act at once but may get lost through abrasion. The internal agents behave like the slip agents—they are nonsoluble in the polymer matrix and migrate to the surface. They are used to absorb water vapor and make the polymer surface able to conduct electricity. The local charge can be distributed or can be removed from the film surface. Examples are fatty acid esters or alkyl phosphates.

There are also permanent, nonmigrating antistatic agents, which essentially are polymers that form a second phase in the matrix polymer film in the form of a more or less continuous network, and in this way, create antistatic properties. In this case, no migration will take place, and the antistatic properties will be imme-diately effective and be permanent over time.

1.4.1.10� ColorantsColorants give a polymer matrix a particular color. There are two types of colo-rants. Dyes are soluble in a polymer matrix and give a transparent look. Pigments, on the other hand, are insoluble. If the pigments have a size smaller than 0.2 μm, then visible light can pass through the polymer and it will appear transparent. If the particles are bigger than 0.2 μm, then light cannot pass and the polymer ma-trix appears opaque. The dyes are of organic origin. Pigments are inorganic mate-rials, mostly oxides like TiO2 or Fe3O4.

1.4.1.11� Optical BrightenersWhite-colored polymers and also transparent polymer films often degrade under low-wavelength light, in particular UV rays. A white-colored polymer changes to yellowish as the absorption spectrum of the material changes. This phenomenon is also known in white textiles, paper, lacquers, or dyes. Color wavelengths in the region of violet, indigo, and blue are absorbed more, so the intensity of the yellow color increases. The polymer materials look dull. In order to get a brightening ef-fect, blue–color–additives are used to compensate for the lost wavelengths. These possess a fluorescing effect.


1.4.1.12� Chemical Blowing AgentsThese are mostly organic chemical agents that when heated evolve gases. The gases increase the volume of a polymer, thus reducing the mass and the cost. Fur-thermore, their foam structure gives a particular touch effect (soft touch effect) and is interesting for marketing. Through the foamed structure the layer diffracts light and appears white, which reduces or saves on white printing ink. This also reduces the cost of garbage in countries like Germany. These may be carbonates, azo com-pounds, semicarbazides, or similar products. A sound knowledge of chemistry is advantageous in working with these agents.

1.4.1.13� Antimicrobial AgentsIn order to kill microorganisms or at least stop their growth, antimicrobial agents can be used in polymer films. There are different killing systems: some inhibit the metabolism of microorganisms through contact, some dehydrate them, and so on. If the agent is heat stable, then it can be extruded. If not, then coating with a lower drying temperature is the better way. Some agents could also be sprayed. Silver metal, quaternary ammonium compounds, or N-halamine-based antimicrobial ad-ditives are known.

For all types of additives, the legislative aspects for food, cosmetic, and other appli-cations must be considered.

�� 1.5� Required Performance of Polymer Packaging

Plastic packaging must fulfill a number of requirements or performance properties in order to be used as primary packaging. They are mainly the mechanical proper-ties, the barrier properties, the sealing properties, and sufficient chemical resis-tance (ESCR) against the environment and also against the product packed in it.

The mechanical properties are the tensile strength, puncture resistance, tearing strength, stiffness, and so on. Barrier properties mean low permeation by light, oxygen, moisture, CO2, aroma, or fat. Every packaging material must be properly sealed so that the seal strength and also the sealing integrity are sufficient high for the expected shelf life and mode of handling of the package. Finally, the chemical resistance guarantees the integrity of the whole package for the shelf life. Migra-tion of product components into the packaging material or vice versa must be re-duced to a minimum so that stress cracking or damage of the packaging material or auxiliary parts like printing ink, lacquer, or adhesives either does not arise at all

91.6�Different Types of Polymers Used for Packaging

or is kept to a minimum during its shelf life. Not only the legislative requirements but also the responsibility of a producer to its customers must be fulfilled.

�� 1.6� Different Types of Polymers Used for Packaging

To fulfill different requirements for a packaging application, different polymers are used. In the following chapter the usual polymers employed in packaging are dis-cussed. These can be divided into three groups: structural polymers, which are used to make the body of the packaging; functional polymers, which are used in a lesser amount but to achieve particular properties, mostly high barrier properties and for environmental stress crack resistance (ESCR); and special polymers that are used in a much lesser amount to achieve very sophisticated properties, such as tie or adhesive layers, blends for good sealing, blends for peel sealing, or special touch effects.

Group 1: Structural PolymersThese groups of polymers are used to make the packaging body.

PE Polyethylene

LDPE Low density PE

MDPE Medium density PE

HDPE High density PE

LLDPE Linear low density PE

EVA Ethylene–vinylacetate

PP Polypropylene

� Homopolymer � Block copolymer � Random copolymer � Graft copolymer � BOPP: biaxially oriented flexible PP

PS Polystyrene

HIPS High impact polystyrene

OPS Monoaxially oriented polystyrene


PET Polyester

PET-A Amorphous polyester

PET-C Crystalline polyester

PET-G Glycol modified polyester (cyclohexane dimethanol)

BOPET Biaxially oriented flexible PET

PA-6 Polyamide / nylon

OPA / BOPA (ONy / BONy) mono- or biaxially oriented nylon

PVC Polyvinylchloride

PLA Polylactic acid

OPLA Mono- or biaxially oriented polylactic acid copolymers

Group 2: Functional PolymersThese groups of polymers offer functional effects like a high or very high barrier to oxygen, moisture, CO2, aroma, or fat. Some offer high puncture resistance.

EVOH Ethylene–vinyl alco-hol-copolymer

High barrier against gases, aromas

PVdC Polyvinylidene chloride High barrier against gases, moisture, aromas

MXD6 meta-xylene diamine High barrier against gases, moisture, aromas

Amorphous PA High barrier against gases, moisture, aromas, as additive to PA-6

PVOH Polyvinyl alcohol High barrier against gases

PEN Polyethylene naphthalate Better barrier against gases

COC Cycloolefin copolymer Performance polymer

PAN Polyacrylonitrile Properties similar to PVC, but a lesser amount

PA-6 Polyamide/nylon For better puncture resistance

Ionomers For oil fat resistance, high hot tack, low seal initiation temperature, high stiffness, and high puncture resistance (although lower than PA)

111.6�Different Types of Polymers Used for Packaging

Group 3: Special PolymersThese groups of polymers offer special effects, like providing very good sealing or adhesion performance (for example, EMA to PET, EMAA to aluminum foil, and so on).

mLLDPE Metallocene LLDPE

ULDPE Ultra low density PE

VLDPE Very low density PE

EVA Ethylene–vinyl acetate copolymer

EAA Ethylene–acrylic acid copolymer

EMAA Ethylene–methacrylic acid copolymer

EBA Ethylene–butyl acrylate copolymer

EMA Ethylene–methyl acrylate copolymer

Ionomers Metal ion modified EAA or EMA

1.6.1� Polyurethanes as Adhesives

Modified polymers or polymer compounds like maleic acid anhydride are used as tie layers during coextrusion. Elastomers or modified elastomers are used as plas-ticizers for a soft touch.

Different polymers show different properties regarding their barrier characteris-tics. A perfect packaging material is always a combination of different polymers or of polymers with paper or metal. Mostly aluminum is used to make composites, because it is a soft metal, is easy to convert, and fulfills almost all requirements to make a very high barrier film or sheet. Because aluminum is not resistant against a number of chemicals that are present in a lot of foods or other products, it is coated with a suitable lacquer. Paper is used to increase the stiffness of packaging and also as a very good printing substrate.


�� 1.7� Short Description of Some Polymers for Packaging Applications

PE PE or polyethylene is the most common polymer for packaging purposes. Based on the density and molecular structure, a number of PEs or modified PEs are used.

LDPE Low density PE is polymerized from the gaseous monomer ethylene at high pressure. The molecular structure is highly branched. The molecular volume is relatively high, and the density is low, generally between 0.910 to 0.940 g/cc. Its melting point (melting region) is around 100°C. Because of the low melting point, it is used as a sealing polymer. Moreover it is neutral against different food, cos-metics, and chemicals. It offers a good barrier against moisture but less against oxygen. LDPE is generally used as a sealing layer, as a blend in collapsible tubes or pouches.

MDPE MDPE is not an official abbreviation but it is used by the polymer technol-ogists to mean a PE in the density range around 0.94 g/cc produced at low pres-sure using a Ziegler–Natta (ZN) catalyst system. The physical properties are simi-lar to LDPE or HDPE. It has a lower sealing temperature than HDPE, and its organoleptic property is pretty good. An MDPE sealing layer is often neutral against delicate foods like water or fresh milk.

HDPE The macromolecules are almost branchless, linear, and show higher den-sity and are produced at high pressure, generally more than 0.950 g/cc. Because of the high linearity, the amount of crystallites in HDPE is much higher than in LDPE, it can be up to 80%. HDPE has a much higher tensile strength and stiffness than LDPE. Its melting point is 130°C. HDPE pouches can also be used for moder-ate retorting purposes. HDPE has a high moisture barrier and is generally used as caps, bottles, carrying bags, heavy-duty sacks, or as a blend in collapsible tubes.

LLDPE It is usually a copolymer of ethylene and butane, hexane, or octane pro-duced with a ZN (Ziegler-Natta) catalyst system at low pressure. Ethylene makes the main chain; the other monomers make side chains like a comb. LLDPE has a higher melting point than LDPE and can also be used for partial retorting pur-poses. The melting point is approximately 120°C. It offers better sealing strength than LDPE. Also, it is used as a blend with LDPE or HDPE for fine tuning of differ-ent structures or as a sealing layer.

mLLDPE The letter “m” means metallocene. The distribution of polymer molecu-lar weight in a metallocene variation is pretty narrow because of special catalysts containing the metallocene configuration, in comparison to polymers that are po-lymerized through a standard procedure using Ziegler-Natta (ZN) catalysts. Films made of such a polymer show higher transparency than standard ethylene copoly-

131.7�Short Description of Some Polymers for Packaging Applications

mers despite a composition similar to conventional LLDPE or HDPE. Their sealing window is also narrow because of the narrow distribution of molecular weight. Therefore trouble in sealing seams may arise if the temperature distribution at a sealing jaw is broad, particularly in old packaging machines.

PE Copolymers PE has a number of copolymers, particularly acid copolymers, to achieve special properties.

EVA It is a copolymer of ethylene and vinyl acetate (VAC). The softness, transpar-ency, and elasticity of a film increases with a higher content of VAC. The comono-mers reduce the crystalline percentage in a polymer. Also, the sealing integrity of EVA is higher than LDPE when fatty foods are packed. However, the VAC content is kept generally below 6.5% for food packaging because an acidic odor is perceptible if the VAC content is higher. In particular, if films or pouches made of EVA are to be upgraded with the use of beta radiation, then the odor may be disturbing although not unhealthy.

EMA EMA is a polymer similar to EVA, but ethylene is copolymerized with methyl acrylate instead of with vinyl acetate. Advantages over EVA copolymers are the higher heat resistance (thermal stability of more than 300°C versus 220°C for EVA copolymers) and the superior adhesion to PET substrates in cases where these polymers are made on a tubular reactor.

EAA/EMAA The comonomer here is acrylic acid or a methyl ester of it. This is highly tacky and is used, depending on the target, as a sealing layer or as a tie layer. The acrylic acid content in EAA or EMAA is generally 5 to 15% for Al compos-ites.

Ionomer This polymer is created when EMAA is partially neutralized with NaOH or other base (e.g., Zn). Because there are almost no crystallites left or because they are extremely small (<100 nm), this is highly transparent. It can be very tough, and some modifications are used as a fat-resistant sealing layer with high hot tack. Because of the cross-linking introduced by the metal ions, it also has ex-cellent thermoforming, stiffness, hot tack, and shrink performance and is used for example in shrink bags for meat packaging.

PP Propylene gas is the monomer of PP. This a semicrystalline polymer with a melting point at 160°C (homo-polypropylene). Based on the position of the CH3 side group, there are three different types of PP: isotactic, atactic, and syndiotactic. The crystalline content is different in the different types. It is stiffer than PE and can be used for retort applications because of its higher melting point. It has versa-tile applications. Filled PP can be thermoformed to containers whether transparent or filled with suitable additives. After biaxial stretching it is called BOPP, and is used widely as a good printing substrate for flexible packaging. It is a very good moisture barrier but a poor oxygen barrier. If necessary, the smooth surface of BOPP is deposited with Al, SiOx, or AlOx to achieve a high oxygen barrier.


Random-PP This is a copolymer of propylene with ethylene, where the ethylene clusters are distributed in a random manner—hence the name random-PP. The eth-ylene content is generally around 5%. This is softer than homo-PP.

Block-PP This is a copolymer of propylene with ethylene, where the ethylene clusters are distributed in blocks—hence the name block-PP. The ethylene content is generally around 5%. Transparency is higher, and the elasticity at low tempera-ture is higher than that of other PP types. It is suitable for deep-frozen food pack-ages, which could be heated in a microwave.

PA Polyamide or nylon is a semicrystalline polymer, and mostly the PA-6 or PA-6-type copolymers are used for packaging purposes. Nylon has a very good oxygen barrier property and high puncture resistance. For packing meat or meat products, fish, or different medical products, a composite like PA/PE, eventually with a high barrier layer of PVdC or EVOH, is the best solution. PA-6 is a polycondensate of ε-caprolactam. Other polyamides used for special packaging purposes are PA-66 and PA-12. PA-66 is a condensation product of hexamethylenediamine and adipic acid. The monomer of PA-12 is laurolactam. Laurolactam is tough and has a high dimensional stability. It is used as a blend with other PAs to extrude multilayer composites for thermoforming or blow molding. Amorphous PA freezes out of the melt without crystallites. It has higher stiffness and transparency than the other types. Economical use is only possible with shrink films. PA films are also oriented monoaxially or biaxially to make dimension-fixed film with higher mechanical properties and a highly smooth surface suitable for high-quality printing or depo-sition of Al, SiOx, or AlOx for a very high barrier.

Amorphous PA usually has excellent oxygen barrier properties, in particular at high humidity levels, and can be used to reduce the severity of retort shock. In ad-dition, it can be used to modify EVOH for better thermoformability.

PET Although called polyester, we understand that PET is almost always polyeth-ylene terephthalate, which is a condensation product of terephthalic acid and eth-ylene glycol. It is a semicrystalline polymer. Although both nylon and PET split a molecule of H2O during polycondensation, PET is much more sensitive to H2O during extrusion than is nylon. Other than nylon, PET resin has to be dried prop-erly before extrusion. Otherwise water vapor in the melt creates acetaldehyde through hydrolysis, which has a pungent odor, making it unsuitable for packing foodstuffs. PET has different variations:

PET-A is an amorphous variation of PET and is extruded as a sheet to be thermo-formed into containers. It is also injection molded to test-tube-like preforms to be stretch-blow-molded in a second step to bottles.

PET-C containers are thermoformed from a PET-A sheet and then crystallized after thermoforming to PET-C. These containers are very hard and dual ovenable, meaning both in microwaves and baking ovens.

151.7�Short Description of Some Polymers for Packaging Applications

PET-G is a glycol-modified variety of PET that is amorphous and highly transpar-ent. This is both injection molded and extruded to sheets. The alcohol part, gly-col, is replaced partially by cyclohexane dimethanol. Oriented PET-G films are also used for label manufacturing.

BOPET is the biaxially oriented variation of PET, which is highly dimensionally stable. It is the best polymer substrate for printing, and it is used for high-quality packaging, particularly for retort packing.

PVC-U PVC-U (unplasticized) is an amorphous polymer and is polymerized from vinyl chloride. PVC is a sensitive polymer: it degrades under heat. Extrusion of PVC needs a number of additives like a stabilizer, optical modifier, and Cl2 ab-sorber beforehand. Extrusion is complicated and takes place in multiple steps. PVC has versatile uses, like sheets for thermoforming as food containers and pharma-ceutical blisters. Because it is an amorphous polymer, it can be thermoformed eas-ily with a very good wall thickness distribution. Bottles are seldom made. It is used in monoaxially oriented form as shrink labels. PVC can be modified with plasticiz-ers like citric acid derivatives or phthalates to form soft PVC, which is used as a stretch film for wrapping purposes.

PS Polystyrene is an amorphous polymer and is polymerized from the monomer styrene. For packaging applications mostly homopolymer is used. It is highly trans-parent, glossy, and brittle. For thermoforming applications, it is grafted with plas-ticizer to enhance the impact properties. This is called HIPS: high impact polysty-rene. It can be thermoformed easily like PVC-U in a broader temperature window than semicrystallines like PP and is used for thermoformed containers or cups. Also, as an oriented film, it is used for labels. PS is a cheap thermoplastic and is used in vast amounts as sheet with or without high barrier sheets with EVOH.

PVdC PVdC is very sensitive polymer. The dispersion grade for coating is usually copolymerized with vinyl chloride. The extrusion grade is copolymerized with eth-ylene for higher thermal stability. It is a unique polymer with a very high barrier against oxygen, moisture, and aroma. Particularly for pharmaceutical packaging, the PVC-U sheets are coated with a PVdC dispersion.

EVOH EVOH is a copolymer of ethylene and vinyl alcohol. The vinyl alcohol part is brittle and offers very high oxygen and aroma barrier properties. The ethylene component offers flexibility. The higher the vinyl alcohol content, the higher the barrier effect but also with brittleness and poorer thermoformability in multilayer rigid packaging structures. A high vinyl alcohol grade with less ethylene (27 mol%) offers indeed a high barrier, but it is not suitable for flexible packs like collapsible tubes or thermoforming applications. These types can be used as the top film on containers. Grades like 38 mol% or 44 mol% ethylene are suitable for thermoform-ing or for collapsible tubes. Another problem of EVOH is its sensitivity to moisture. Particularly during retorting at high temperature under water vapor, the EVOH


layer in a composite gets wet and loses its high barrier property against oxygen. It takes weeks to recover the barrier property.

In order to overcome these shortcomings, EVOH suppliers are continuously devel-oping modified grades to improve the thermoformability. Also, amorphous polyam-ides or ionomers can be used for this purpose.

�� 1.8� Major Polymers Used in Packaging

Table 1.1 Polymer Attributes and Applications

Polymer Key Attributes ApplicationsLDPE Transparency, sealability Flexible food and pharma packagingLLDPE Puncture resistance, ESCR, hot tack Laminates, rotomolded containersHDPE High strength & toughness Laminates, woven sacks, blow-molded

containers, crates, capsPP Chemical resistance, hot fillable, steril-

izable, good impact & clarity, hingeLaminates, woven sacks, blow- and injection-molded containers, caps

PET Gas barrier, clarity Laminates, bottlesNylon Gas barrier, oil resistant, impact Multilayer flexible and rigid packageEVA ESCR, toughness Multilayer flexible and rigid packagePVC Clarity, extremely versatile Blister packaging, filmsStyrenics Stiffness, expandable Thermoformed trays & containers

Source: Raj Datta, Haldia Petrochemicals, National Conference, IIP, Kolkata 2012

1.8.1� Important Points for the Technologist

1. In order to manufacture a tailor-made polymer packaging material for a particu-lar application, the proper resin has to be found. Generally, the sealing layer is decisive for products like food or cosmetics that contain fat, spices, or other components that may be corrosive for this layer. The pack must function through the shelf life. Proper resin, for example, LDPE-grade, from a particular supplier helps a lot. Some other LDPE types are not suitable. In most cases a correct blend will help. It is a question of patience and also a matter of luck how quickly the proper solution can be found. Experienced specialists at the resin manufac-turers and also additive suppliers are good resources.

17References

2. Critical trials with new specifications should not be made on a production ex-truder, particularly with new additives. Preliminary trials should be done ei-ther on a trial extruder in one’s own company or at an extruder of some univer-sity or institute or at the resin or additive supplier. Otherwise there is a risk that the production extruder will have to be cleaned, which could take days. Only the final trials should be done on the production extruder or line to be sure that the specification will function or at least will not cause a big problem with the extruder.

3. One should have a confidential relationship with a supplier before one discloses the specification or even a part of the specification to a supplier. Even tempera-ture adjustment at the extruder may give a specialist key knowledge on the probable resin.

�� References

Saechtling Hans-Juergen, Kunststoff Taschenbuch, 28th ed., Hanser, Munich (2001)Stoeckhert K., Kunststoff Lexikon, 8th ed., Hanser, Munich (1992)Johannaber Friedrich, Kunststoff Maschinenführer, 3rd ed., Hanser, Munich (1992)de Mink Paul, Borealis, Neue Innovationen von extrudiertem Polypropylen für den Verpackungsmarkt, 4th

Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (1997)Schoene Werner, BASF, Polypropylen für Hartverpackungen, 4th Stuttgarter Verpackungstage (Stuttgart

Packaging Symposium), (1997)de Mink Paul, Borealis, Einsatzgerechte Compoundierung von Polypropylen Granulate, 8th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium), (2001)Hornbach Heinz, Ciba, Additivsysteme für den Verpackungsbereich - von der Stabilisierung zum Effekt,

12th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2005)Hausmann Karl-Heinz, DuPont, Leistungsfähige Rohstoffe - Beispiel Surlyn, Nucrel, 19th Stuttgarter Ver-

packungstage (Stuttgart Packaging Symposium), (2012)Volk Aschulmann, Einsatz von Additiven bei Verpackungsfolien, 18th Stuttgarter Verpackungstage

(Stuttgart Packaging Symposium), (2011)Hausmann Karl-Heinz, DuPont, Personal InformationBASF, Ludwigshafen, Kunststoffverarbeitung im Gespräch, 5th ed., (1987)BASF, Ludwigshafen, Kunststoffwerkstoffe im Gespräch, 2nd ed., (1987)BASF, Ludwigshafen, Kunststoff-Physik im Gespräch, 7th ed., (1987)Masterbatch Verband, Farb- und Additivmasterbatches in der Praxis, VM Verlag, (2006)Herschbach Christoph, W&H, Schlauchfolienextrusion für anspruchsvolle Verpackungslösungen, 18th

Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2012)Weyers Gerd, Nippon-Gohsei, EVOH - High Barrier - in der Lebensmittelverpackung, 15th Stuttgarter Ver-

packungstage (Stuttgart Packaging Symposium), (2008)Hutter Matthias, Solvin, PVdC - ein bewährtes, universelles Barrierematerial mit Innovationspotential,

15th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2008)


Schambony Simon, BASF, UV-Schutz in transparenten Verpackungen, 15th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2008)

Proksch Karl-Heinz, Polyone, Nutzen und Risiken der Additive in Packstoffen, 14th Stuttgarter Verpa-ckungstage (Stuttgart Packaging Symposium), (2007)

�� 2.1� Extrusion of Resins

The first step in making polymer packaging is to melt the thermoplastic resins and then form them into films or sheets or into an end product like a cap, bottle, or cup. The machines with which this is done are called extruders. An extruder has two main components, a barrel and a screw. Both are made of alloy steel and are very robustly constructed because a pressure of several hundred bars arises during melting. The resins are furnished sometimes with additives that are abrasive; the extruder must be able to work such a product without getting damaged. Besides hard alloy steel, the inside of a barrel and the screw are treated through a series of physical processes like heating and quenching in water to produce high hardness. Whereas the barrel is a plain pipe, the screw is constructed in a very complicated manner. Around a shaft, helical threads are cut with different channel depths. The barrel has a number of heaters and the same number of fans on the outside. The heaters heat the barrel from the outside, and the fans cool it when necessary. Gen-erally, the extruders for melting resins have one screw, a so-called single screw extruder (SSE). If a resin mixture is to be melted where mixing is very tough, then a twin screw extruder is used. The additive manufacturers have such twin screw extruders. The manufacturers of general packaging materials like film, sheet, and tube use single screw extruders. An extruder has three functions: convey the resin, melt it completely, and homogenize the melt, which is then processed into the tar-get material.

An extruder is described according to “Euromap” with three numbers, for exam-ple, 1-25-30. The first number stands for the number of screws, the second one the length of the extruder in terms of its cylinder diameter (D), and the third figure is the diameter in millimeters. It is usual to describe the different lengths of different parts of an extruder in terms of its diameter. So the length of the conveying zone in a 25 D may be 10 D, the compression zone 7 D, and the homogenizing zone 8 D. Generally, the extruders have a length of 20 to 30 D, depending upon what parts are integrated into the screw. For thermally stable polymers like PE or PP, a longer

2 Manufacturing of Polymer Packaging

20 2 Manufacturing of Polymer Packaging

extruder can be used because the resin can be kept hot longer without any mate-rial damage. For PVC or PVdC, which are very unstable under heat, the length of an extruder is less.

2.1.1� Technology of Extrusion

As already mentioned, the functions of an extruder are to convey, melt, and homog-enize a resin before purging.

The screw has a solid shaft of constant or increasing diameter toward its end. Heli-cal flights divide the screw in pitches. The screw has generally three zones (Fig. 2.1). In the first zone, the channel volume in the pitches is constant, and hence the resin is simply conveyed in this zone. In the second zone, the channel volume is reduced by reducing the channel depth, so the resin is compressed here. At the same time, the barrel is heated from the outside, and the resin melts completely. In the third zone, the metering zone, the channel volume is again constant so the melt can be mixed and homogenized properly to keep a constant temperature and viscosity. High-quality packaging materials can only be manufactured if the melt is free from specks (unmolten resin) or dirt particles and is homogeneous. At the end of most extruders, an adapter is placed to create a certain back pressure, so the melt stays a bit longer in the extruder to get better homogenized in the metering zone. The applied back pressure depends upon the resin type and is generally 50 to 100 bar. A film from an inhomogeneous melt shows streaks, which arise through inhomogeneous refraction of light through the film.

Figure 2.1 Standard three-zone extruder

Standard Three-Zone Extruder

Although only one end of the heavy screw is mounted with the gear system or di-rectly with the motor and the other end is free, it does not rub the barrel during rotating. The melt inside the extruder stabilizes the screw position. Once mounted, the extruder is fed with a resin, and it is always kept full with melt, even if it is

212.1�Extrusion of Resins

turned off. During restarting the barrel is heated until all of the resin melts, and it can then turn. To accelerate the process, the extruder is always fed with a so-called cleaning resin, which is a low-melting resin type, before stopping production. When restarting an extruder, a lot of melt is always lost, until all of the parameters are optimum for the next production. To avoid the loss of material and time, extrud-ers generally run around the clock.

Important facts in producing an ideal melt for a packaging application are an ex-truder with the proper screw, the choice of the proper resin, the proper additive packet, and finally the proper working parameters. In particular, it is very import-ant to not overheat the extruder. If the extruder is overheated, then resin may burn and produce brown to black particles. Some of them stick to the barrel inner wall. Even if later, good melt quality is produced, the burned particles leave the barrel wall from time to time, which leads to a dirty melt. Spots or even holes in the films or pipes are produced. It may take a whole day to clean the extruder.

The extruder is mounted on a robust base. The screw is turned by a powerful elec-tromotor via a suitable gear system to achieve different RPMs for the screw. The resin is poured into the extruder through a funnel at the beginning of the extruder. To avoid premature melting of the resin, the barrel is cooled at this position. Pre-mature melting of the resin may jam the extruder and then no resin can be fed.

The nozzle of the extruder is connected with subsequent tools to produce the de-sired material, for example, an annular ring for blown film, a coat hanger die for chill roll films, or injection-molding tools.

Types and Technology of Different Screw TypesA typical screw is shown in Fig. 2.2. The drive shank is mounted on the gear or directly on the motor. It has helical flights and three sections, the feed, compres-sion, and metering sections. The channel depth in the feed section is high and re-duces gradually at the compression section as simultaneously the root diameter increases. The resin melts through heat from friction and from external heat sup-plied outside the barrel. Lastly, the melt is homogenized in the metering section before it is purged out. The pitch is constant throughout the screw. Compression is also possible by reducing the pitch rather than reducing the channel depth. The leading edge of the flight pushes the resin and melt forward. Both pressure and friction are high in this section. The trailing edge has, in contrast, no pressure function. The screw tip can have different forms, depending on use. The screw is the main part of an extruder because through proper construction it is possible to process a particular resin in a particular way. First it must fulfill the proper com-pression that is necessary to melt the resin. The screw may have a constant shaft diameter and reducing pitch length at the compression zone, or the shaft could have an increasing gradient with the pitch length constant.


Figure 2.2 Standard three-zone extruder screw, courtesy of William Reay, Kuraray, EVAL Division

It may have two zones, in which compression takes place in the first zone and ho-mogenizing in the second zone. Most often there are three zones: transport, com-pression, and homogenizing zones. The pitch is constant throughout, but the shaft diameter is low in the first zone and increases gradually to a highest value in the second zone. In the third zone it is once more constant. The compression zone may be short with high gradient, which is suitable for thermally robust resins like PE or PP, or it may be a bit long for thermally sensitive resins. Screws may have a decom-pression zone with a higher channel depth just after the compression zone. This enables the gas bubbles, which arise particularly when working with recycled res-ins, to move up and leave the extruder through a valve. In some high-quality screws there is a shearing part, for example, a Maddock element. The diameter of the screw shaft is highest in this part so that the melt has the narrowest possible route through the extruder. This causes the specks to press against the barrel inner wall and ultimately melt.

Finally, there may two different types of barrier screws. The barrier screws have a transition flight after the compression and shearing parts. The objective of the transition flight is to separate the specks from the melt, press them into this part, and ultimately melt them completely. There are two types of barrier screws. One is named after Maillefer, this one having a variable pitch for the transition flight but constant shaft diameter, and the other after Barr, this one with variable channel depth but constant pitch for the transition flight.

Types of BarrelsThere are two types of barrels: a barrel with a constant inner diameter and a barrel that has a number of slant grooves in the feed zone. The grooves are placed sym-metrically along the perimeter of the barrel with the highest depth at the begin-ning, which reduces gradually and vanishes after some 3 to 4 D with the inner di-ameter of the barrel. Through the first type of barrel, a constant amount of resin can always be processed. In a three-zone extruder with such a barrel, the highest melt pressure, around 200 bar, is always at the end of the compression zone.


In a grooved barrel, in contrast, more resin can be transported at the beginning through the grooves than in a normal barrel, and the highest pressure of the melt is just after the position where the grooves end. The highest melt pressure in a grooved barrel is around 1000 bar and is much higher than in a nongrooved one. It is therefore easier to furnish a screw with different parts like shearing or barrier parts, which need more power to drive. Not all types of resin are suitable to be ex-truded in a grooved cylinder. Resin suppliers advise the manufacturers whether it is suitable for a grooved feed zone or not.

Universal ExtruderA universal extruder is a machine where different resins may be processed. It has no barrier part and only sometimes a shearing part. It is suitable to produce an acceptable quality of melt with different resins. The extruder is optimized for a broad spectrum of applications, such as LDPE, HDPE, LLDPE, EVA, and ionomers with different additives.

High-Performance ExtruderA high-performance extruder is one that has to produce a very high quality melt out of resins to produce a high-quality product. The extruder screw is specially constructed to match a particular polymer like nylon (PA), PET, or PS. Other types of polymers or even resins with some additives cannot be worked with these ex-truders at sufficiently high quality. The objective of a high-performance extruder is to produce very high quality product, mostly films for a particular application. High-performance extruders always have a shear part, if not barrier parts.

CoextrusionPackaging performance can, in most cases, not be fulfilled by a single layer of ma-terial. Properties of different resins or other packaging materials like paper or alu-minum are combined into a multilayer packaging material called a composite. These are the ideal packaging material to fulfill different requirements.

Different layers of composites are written in a line with a slash between the layers, like PET/Al/PE 12/8/60, or the layers can be written one on top of another, like below.

PET 12 Al 8 PE 60

The first version takes less space and is easier to write. The layer thicknesses are written next to the composite in micrometers (μm). In this book the first version will be used. The left side represents the outer layer and the right side the inner


layer. The inner layer has most of the sealing function: it is in direct contact with the product, for example, food. This layer must fulfill all of the legislative requirements.

For a pure polymer composite, all of the resins of the different layers are melted in different extruders, and the melts are combined in a suitable series to make the final packaging material (Fig. 2.3).

Figure 2.3 Coextrusion line with three extruders

Because the bonding forces between the layers of most resins are not high enough, special bonding resins are necessary, which are called tie resins. For each type of resin, main layer, or tie layer, one extruder is necessary. The suitable layer struc-ture is built in a feed block, where the different melt streams from the extruders are fed. The feed block is the heart of a coextrusion line (Fig. 2.4).

Figure 2.4 Coextrusion with three extruders, feed block, and coat hanger die, courtesy of Battenfeld


One melt stream may be split in two more streams and combine with other streams to get the required structure. Not only the positions of different layers but also their thicknesses are adjusted in the adapter. The fine adjustment of layer thick-nesses is done at the extruders by changing the screw speed. Next are a few possi-ble examples.

LDPE/Tie/PVdC/Tie/LDPE 30/5/10/5/50 μm

This has a symmetrical structure with five layers and three different resins. Three extruders are necessary to melt the three different resins. The layers of LDPE and Tie are split. This is a high barrier flexible film for pouches.

PS/Scrap/Tie-1/EVOH/Tie-2/LDPE 50/500/20/25/15/100 μm

This has six layers with six different materials. Six extruders are necessary. It is a high barrier rigid sheet for thermoformed trays. Because after thermoforming there is a lot of scrap sheet, it is cut and called “regrind” or “scrap.” PS is the lion’s share of the scrap. Therefore, no tie layer is necessary to bond this layer with the outside virgin PS. The thickness distribution shows that the scrap extruder is the biggest one.

Another interesting fact is the position of the scrap layer in the composite. It could be theoretically placed at any position in the structure. For greater legislative secu-rity, however, it is wise to place it outside the high barrier layer of EVOH, far away from food contact.

There are two types of feed blocks. In the first type, the melt channels are fixed, which means one type of coextrusion is possible. The second type is an adjustable feed block, where the channels can be changed to produce a number of different coextruded structures. The die for coextrusion is the same one as for a single melt extrusion. Because the melts in coextrusion have to move a longer distance until they come out from the die, static mixtures are constructed in the pipes, where a melt is mixed continuously to avoid a demixing.

The purpose of coextrusion is to develop different functional layers in a film, sheet, or pipe. These are generally a high barrier layer, a color layer, a regenerate layer, and so on. The position of a certain layer in this structure requires sophisticated thought processes. A regenerated layer is always placed at the middle of a coex-truded structure so that the filled product does not come in contact with it, and it should not be on the outside because it may have some impurities. If a high barrier layer is used simultaneously, then the regenerate layer is put on the outer side of the barrier layer so that any type of unwanted migration between the filled product and the regenerated layer may be avoided or at least retarded. A high barrier or puncture strength effect of a packaging material is improved if the functional lay-ers in the structure are split into two different layers instead of being in one layer


with the same thickness. An important solution during coextrusion on calenders is to avoid the sticking of tie materials by encapsulating all of the layers with the sealing layer at the sheet border.

The most important fact during coextrusion is the compatibility of the tie layer with both of the neighboring polymer layers. With the exception of nylon-6 and EVOH, all other polymers need suitable tie layers to become bonded. Other factors are the melt temperature and the melt viscosity of the different resins. After the adapter, all of the layers flow at a single temperature. The viscosity difference be-tween the layers should not be high. For different melts there is usually a viscosity window in which the melts flow well and result in a very good composite.

2.1.2� Continuous Processes

Continuous processes are characterized by the production of polymer materials continuously and not stepwise. Typical products are the so-called endless materi-als like film or sheet or pipe for tubes. It is much cheaper than processes with a stop-and-go character because in a continuous process there is no time loss through stop and restart, less wear on the machine parts, and the precision of processing parameters is high.

2.1.2.1� Manufacturing of Blown FilmBlown films are flexible films and are manufactured with semicrystalline thermo-plastics like PE, PP, or PA-6 and their copolymers. Sometimes they are a modified PE like ionomers. The chill roll films are flexible films, and the same resin types as in blown film can be worked here. The film thickness is generally between 10 and 300 μm. Amorphous thermoplastics like PVC or PS are too stiff to make a blown film.

At first the resin is molten in an extruder, at the end of which there is a screen system to get a clean melt that is free of nonmolten parts and dirt particles. Gener-ally there are two screen systems connected in such a way that when one screen is exhausted, it is replaced by a second one without stopping the production process. The melt is now driven mostly by an adapter to ensure good homogenization, and it then passes through a bent pipe with a 90° angle to the ring die (Fig. 2.5). The annular gap of the ring die is ∼1 mm. The function of the ring die is to turn a pipe flow of melt into an annular flow with wall thickness of around 1 mm and the shape is like macaroni. This ring is then blown up with air like a big bubble to the production diameter. In most cases the bubble is blown upward. In small produc-tion lines the bubbles are also blown downward. The air pressure inside the bubble is kept constant to keep a constant bubble diameter. The bubble cools down gener-ally with air from both outside and from inside.


Figure 2.5 Manufacturing of blown film

The air inside is circulated to also create a constant cooling effect from the inside. The bottom of the bubble is a viscous liquid, and it solidifies through cooling as it moves upward. At a certain height it is a completely solid film. This line is called the frost line. Beyond this level no change in the bubble diameter or film thickness takes place. An important characteristic of a blown film is the blow-up ratio. It is the ratio of the final bubble diameter to the diameter of the ring die. The value is usually between 2 and 5.

The height of the bubble is generally several meters. It passes through a cage to stabilize its position, which is called the calibrating cage. In modern systems the rods of the cage are perforated: an air cushion avoids any mechanical friction be-tween the bubble wall and the rods. This avoids scratches on the film. Next the bubble is diverted through a series of slant rolls to the nip rolls where the bubble flattens. The flat bubble after the nip rolls is completely free from air. To get an ex-act pressure along the whole length of the flattened bubble, one of the nip rolls is always made of highly polished steel and the other one is coated with a rubber layer of suitable hardness. During all processes of film manufacturing, where a constant pressure between two films is necessary, like flattening of blown film or lamination of two films, one roll is made out of steel and the other has a rubber coating. In addition to pressing the bubble, the nip rolls turn at a constant speed to


haul off the bubble upward. In most cases the flattened bubble is slit with sharp knives to get two single webs and is wound up to two mother rolls. The borders are cut into fine pieces, eventually pressed and fed back in the extruder. In modern production lines the flattened bubble can be slit exactly at the outermost position. There is no border for recycling. Sometimes the films are already slit on the mother roll at the customer’s roll width before winding up. In some cases the flat tube is directly used for further converting; for example, it can be printed in-line through flexo-printing, cut, and sealed on one side for making bags.

The production speed of a line must be adjusted by the screw speed of the extruder, the haul-off speed of the nip rolls, and the wind-up speed of the core of the mother roll. Because the production speed is constant, the revolution speed of the core during winding is high at a lower roll diameter, and it reduces constantly with the increase of the roll diameter.

The film thickness of the blown film depends upon a number of influencing factors. First there is the blow-up ratio, which depends upon the amount of air pressed into the bubble. The higher the blow-up ratio, the lower the film thickness. The higher the haul-off speed, the lower the thickness. The film thickness is higher when the screw speed of the extruder is high because more melt is produced. Finally, the thickness depends on the cooling intensity, which determines the height of the frost line. The height is low if it is cooled intensively, and high if cooled less inten-sively. The lower the cooling intensity, the higher the frost line, and the thinner the film, and vice versa. Generally the thickness of the film from a particular melt is regulated by changing the screw speed. All of the other parameters are kept un-touched because this is the least complicated method.

Important factors are the cooling of the bubble and the transparency of the blown film. Because blown films must be flexible, the polymer is always of a semicrystal-line type. The transparency depends on the polymer type, the thickness of the film, and how much of it is crystalline. The melt is 100% amorphous: the growth of crys-tallites takes place as the melt cools down. Every semicrystalline resin has a charac-teristic temperature window where the speed of crystallite generation is the high-est. For LDPE it is around 90°C. In order to get a transparent film, the melt must be cooled down quickly through this window. As already mentioned, the bubble is cooled both inside and outside in modern blown film lines. At the outside of the ring die there are adjustable iris rings, and also the air speed and temperature can be adjusted. The inner cooling takes place through a double wall ring. Cooled air from the outer ring flows along the bubble inside, whereas the warm air is diverted through the core pipe, keeping the pressure inside the bubble always constant.

Another important factor is the thickness distribution of the film. In older lines the thickness is measured after slitting at the single webs with beta-rays. In mod-


ern lines the thickness is measured along the bubble with an infrared or capaci-tive device, which turns along the bubble circumference before the stabilizing cage. If the film thickness at a position is different from the standard value, then electronic feedback is sent to the corresponding position at the die. The necessary correction takes place by narrowing or widening of the die gap. This is done with bolts, the lengths of which can be changed by electrical heating or by the piezo-electric effect.

In spite of such steps, the film thickness is always inhomogeneous. If a thick posi-tion of a film is wound several hundred times at the same position on the roll, then a bulge is created at this position, which creates trouble during converting. This is avoided in different ways: reversing the ring die, eventually along with the ex-truder at around 300°, or the bubble is turned in different directions with a num-ber of diverting rolls before it is flattened at the nip rolls.

Surface Treatment of Polyolefin MaterialsThe surface energy or surface tension of polyolefin films or tubes is low because of their covalent nature—too low for proper adhesion with printing inks, adhesives, or lacquers during converting. The film after slitting is treated with a corona (electri-cal discharge), a gas flame, or with ozone to increase the surface tension from some 33 dyne/cm to 40–42 dyne/cm to ensure high adhesion during the converting pro-cesses. Although the polyolefin films are always treated with corona or the like on the converting machine, it is very important to undertake the treatment just after their production. This is necessary because the high surface energy dissipates during the storage time. If the surface treatment is not done during the production, then a single treatment during converting does not result in good adhesion.

The effects of surface treatment are manifold (Fig. 2.6). First, it burns or oxidizes the foreign materials like additive residue or fatty substances. Second, it makes the surface rough. The surface area increases to a very high extent because there are dense craters on the films. Third, many of the covalent bonds on the surface are polarized to acid, ketone, or aldehyde groups to polar radicals, which provide high bonding with printing ink, lacquer, or adhesives. Because polyolefin films are mostly used as a sealing layer in composites, only the side for printing, future coat-ing, or laminating is surface treated. The other side is designated for sealing. The corona-treated side is not suitable for sealing. Through surface treatment the abil-ity of the macromolecules to diffuse is hindered. It is therefore very important to recognize the side that has been surface treated: a mistake in recognizing this will definitely cause problems. First, the bonding during converting is low, because this side was designated for sealing, and second, the corona-treated side does not seal properly because the diffusion of the macromolecules is insufficient.


α

α

Low wettability before coronatreatment – contact angle α high

High wettability after coronatreatment – contact angle α low

Figure 2.6 Effects of corona treatment

Although corona treatment is the most common surface treatment, care must be taken to apply a proper dose. If the corona discharge is too high, then holes may be created on the film, and the other side of the film, the sealing side, becomes blunt and cannot seal properly. To avoid such problems, some manufacturers of collaps-ible tubes treat the cylindrical surface with corona only up to the sealing border. The sealing border is treated with a flame such that the treatment does not pene-trate the tube wall to blunt the inner sealing layer. Moreover, a too-high corona dose, particularly for thin films, may result in blocking of the film layers because the adhesion force is too high.

Winding of FilmThe film is wound on mother rolls after the flat bubble has been slit. Generally the film is wound on a paperboard core of 76 or 152 mm diameter. Because the wind-ing speed must be the same as the speed of film production, the revolution of the core is high at the beginning and reduces ultimately with increasing diameter. The winding tension must be kept at an optimum value so that the film in the roll is not too hard or too soft. Too-hard winding may cause an unwanted reduction of the film thickness, and the probability of blocking is also high. Too-soft winding causes a telescoping of the film layers, which is fatal for further work with the roll.

Previously, a mother roll was produced that was cut to the customer’s width in a later step. Nowadays the blown film can be cut to the final width just after production. This mode of production is applicable only on high-performance lines, where the same quality of film is produced in large amounts. In automatic roll exchangers, the re-serve core is ready to take up the film when one roll has reached its final diameter.

2.1.2.2� Manufacturing of Cast FilmFor manufacturing cast film, the melt from the extruder is pushed through a screen system to get a clean melt free from nonmolten parts and dirt particles. Thereafter the melt is pressed through a coat hanger or fish tail die. The die has two parts. The lower part of the die is massive, and the upper part is flexible to adjust the gap


between the lips when necessary. There are a number of bolts along the flexible upper lip to adjust the die gap. The melt comes out of the flat die as a flat film or sheet, which freezes on rolls to a film. To manufacture thin and flexible films like in blown films, the melt from the die is guided on a rotating chilled roll. These films are called “chill roll films.” To manufacture rigid sheets, the melt from the die passes through a number of rolls (in general three), called a calender. The sheets produced are used for thermoforming.

2.1.2.2.1� Chill Roll FilmAs already mentioned, a thin layer of melt from the die is diverted to the chill roll (Fig. 2.7). The surface of the chill roll is highly polished and is chromium plated. The chill roll temperature is around 10°C. The distance between the die lip and the chill roll is kept as low as possible so that the melt cools down immediately after the die on the roll. There are several reasons why this is done. First, the melt will oxidize a minimal amount in air at an elevated temperature, which build chemicals like aldehydes or ketones that have a bad odor. These types of films are generally used for sealing purposes for direct contact with food or other products, where the sealing layer should have no odd odor or taste. Second, the film is cooled down much more quickly than are blown films with air. There is less crystallinity in the films, so the films show higher transparency. Third, the loss of the film border through a neck-in effect is less. The film thickness at the border region is inhomo-geneous because of the neck-in effect and has to be trimmed off. That’s why the effective width of the film is higher if neck-in is less.

Chill roll

Extruder

Funnel

Air knife:Pressingmelt onchill roll

Coat hanger die

Evacuation (air + moisture)

Guide roll

Figure 2.7 Extrusion of chill roll flexible film


In order to get the melt properly onto the chill roll, two more devices are used. An air knife, just in front of the die, presses the melt and sets it quickly on the chill roll. Behind the die there is a vacuum lip to suck air and condensed water so that no air bubbles or condensed water can get between the melt and the chill roll, which would otherwise create an isolation.

Because the film thickness is less, 10–250 μm, not only the side with direct contact with the chill roll but also the other side cools down to some extent. Because of the intensive cooling, the chill roll films of semicrystalline polymers have less crystal-linity than in blown films. The transparency is higher. Intense cooling not only causes higher transparency, but it also creates to some extent a thermal tension in the film.

To get a high-quality film, the melt is kept in contact with the chill roll for as long as possible. The thickness of the film is regulated through the RPM of the chill roll. The film thickness is measured by a beta-ray device in the cross section to the ma-chine direction. Any intolerable deviation of film thickness is reported to an adjust-ing device of the flex lip die by a computer. There are several ways to adjust the gap between the die lips. Sometimes there is an electronically regulated wrench to turn the bolts; sometimes there are thermal bolts or piezoelectric translators. The film is then trimmed at its ends to get a core part with an exact thickness, and it is then wound up. The trimmed parts are cut and pressed in a special extruder to make regranules. These can be fed to the main extruder.

2.1.2.2.2� Calender SheetRigid films or sheets are made principally in the same way as cast film, but the thickness of a sheet is much higher, and there is an assembly of generally three calender rolls instead of the chill roll. For the manufacture of PVC sheets there is a special arrangement, which will be discussed later. The arrangement of the rolls depends on the polymer type, particularly its viscosity, and on the experience of the manufacturer. Generally they are vertical or horizontal systems. Sometimes the arrangement is slanted, particularly for PET-A (Fig. 2.8).Sheets need a higher amount of melt than do thin films. A melt pump is often used between the extruder and the die to get a constant speed and pressure of the melt stream. For coextruded sheets, each extruder has its melt pump, after which the melt stream passes an adapter to get the required structure of the sheet. The die is of the same type as a chill roll film, but the die gap is higher. In a vertical calender with three rolls, the melt passes, for example, through the lower and middle rolls first. It turns around the middle one, passes through the gap between the middle and upper roll, and is then ready to be wound up. Usually there is an in-line thermoforming system switched behind the rolls. Because the core of the sheet is still hot, it has to be heated only on the outside to have an ideal thermoforming temperature.


A) Vertical B) Horizontal C) Slant

Figure 2.8 Extrusion of calender sheets with different roller arrangements: a) vertical; b) hori-zontal; c) slant

The sheet thickness is regulated at the gap between the first pair of rolls. This gap between the die lips has almost no influence on the final thickness of the sheet. A small bulge of melt is kept at the entrance of this gap. This ensures a constant melt stream through the gap and hence the proper thickness of the sheet. The tempera-tures of the rolls are not very low as in a chill roll: it is around 30°C. Because the sheet, with a thickness of 250–2000 μm, is much thicker than a chill roll film, it is cooled only at the inside of the middle roll. The outer part is cooled down by air. The sheets produced by a three-roll calendering have one glossy side, which has been in contact with the middle roll, and one hazy side. This phenomenon is much more intense with thin aluminum foil. For economical production the thinner alu-minum foils are rolled pairwise. The side that is in touch with the roller is glossier than the side in contact with the second aluminum foil.

The calender sheets for packaging purposes are generally thermoformed to cups, bowls, trays, pharmaceutical blisters, and the like.

2.1.2.2.3� Water Bath SheetsCast sheets are also manufactured by guiding the polymer melt from a coat hanger die into a water bath. The sheet is diverted through guide rolls in the bath, cooled down intensively, and then guided out of the bath. It is then squeezed between rolls to remove surplus water and dried with hot air. The edges are trimmed and then it is wound on rolls. If necessary, the sheet is heat treated to achieve certain special properties before rolling. Because the sheet cools down quickly in a water bath, the sheet has less crystallinity. The sheet, although of semicrystalline mate-rial, behaves like an amorphous sheet, and hence the thermoforming window is broader. During thermoforming, water bath PP sheets behave almost like PVC or PS and can be thermoformed in a simple thermoforming machine also under a broad temperature range. The sheet thickness is between 100 and 1500 μm. The technology is indeed obsolete.


2.1.2.2.4� Manufacturing of PVC SheetsPVC needs different types of additives to get suitable properties for different appli-cations. In the European countries it is almost no longer used for food packaging. The main applications are pharmaceutical blisters. Sometimes the amount of ac-tual polyvinylchloride is less than 50% in the mixture. The manufacturing of PVC sheets is therefore very special and complicated. The ingredients of PVC are mixed and molten in a planetary roller extruder with a central spindle and some 9 to 12 planetary spindles around it. This extruder is a typical mixing extruder used to mix and homogenize the ingredients of different particle sizes and densities. The molten mixture looks like sausages and is passed through a vacuum line to suck out different gases and thermally cracked substances. Next it is fed into a single screw extruder, where the mixture is molten and homogenized. The die at the ex-truder head has multiple holes to produce doughy pellets, which are fed into a four-calender system that is arranged mostly in an L-form (Fig. 2.9). The sheet thickness is adjusted at the gap between the last pair of calender rolls. The first two rolls are horizontal and at around 200°C. The diverted sheet passes next through a number of different tempered stripper rolls, until the PVC sheet is cooled down to room temperature and is wound up. The sheet thickness, depending on the appli-cation, is between 60 and 1000 μm.

Masterrolls

Stripper rolls Additional equipmentCalender train

Roller train

Conveyor beltEmbosser

Deflector rolls

Cooling drums

Figure 2.9 PVC sheet calendering: four-roll calender (L-form)

2.1.2.3� Collapsible TubesTubes or collapsible tubes are important packaging materials for liquid products of different viscosity. They are soft, have a round or oval cross section, and can be used for almost all types of liquids, from condensed milk up to highly viscous sili-cone adhesives (Fig. 2.10). A tube has a cylindrical body, a shoulder, and an orifice,


usually with an outer thread to close it with a cap. Sometimes the cap is not threaded and instead is pressed onto the orifice. The tube is filled from the back side and closed suitably after filling. Aluminum tubes are closed by multiple dead folds. Other tubes of polymer origin are heat sealed. The biggest utility of tubes is that they can be made into a multiple-portion pack, where the rest of the product in the tube does not come in contact with the environment after a portion has been removed. Because of this characteristic, few other packaging is capable of keeping the product longer with high quality than in a collapsible tube. The first generation of tubes were made of lead and tin because they are soft metals and can be easily collapsed, which is the typical characteristic of a tube. Nowadays only aluminum is used for metal tubes with the very little exception of tin tubes. The first polymer tubes were made in the 1950s out of LDPE and PVC. Nowadays there are mainly three types of tubes for packaging purposes: aluminum tubes, laminate tubes, and extruded or coextruded polymer tubes. There are also a few flexible plastic tubes.

The general structures of various kinds of collapsible tubes are:

Pure aluminum tubes Al/Lacquer

Al-laminated tubes PE/Adh/Al/Adh/PE

Plastic-laminated tubes PE/Tie/EVOH/Tie/PE

Laminated tubes with PET-SiOx film PE/Adh/PET-SiOx/Adh/PE

Laminated tubes with PE coating Laminated tube + outer PE

Mono- and multilayer plastic tubes PE or PE/Tie/EVOH/Tie/PE

Flexible tubes PET/PE

Descriptions of these different types of tubes:

1. Pure aluminum tubes offer the highest barrier (see Fig. 2.10). This kind of tube is used to pack extremely sophisticated liquids like pharmaceuticals, artists’ paints, or volatile chemicals.

2. The next type of tube is the laminate tube, which offers a pretty good barrier and has a characteristic longitudinal seam, as the cylinder part is made out of a flat rectangular web through longitudinal sealing. An advantage of this kind of tube is that the probability of cracking during squeezing the product is less. There are three kinds of laminate tubes: Al-barrier, ceramic-barrier, and polymer-barrier.

3. There is a special kind of laminate tube, particularly with an Al-barrier, where the cylindrical part is coated on the outside with a LDPE layer. The idea is to make the outer longitudinal seam invisible.


4. Another type of tube is the polymer tube; either a single web (LDPE) or a high barrier tube LDPE with EVOH as the high barrier polymer (see Fig. 2.10). Char-acteristic of these kinds of tubes is their seamlessness. They are extruded through a ring die and have no longitudinal seam; this type of tube is popular for cosmetics.

5. The last kind of tube is actually a flexible pouch (see Fig. 2.10).

Figure 2.10 Left to right: pure Al tube, polymer tube, laminated tube with PE coating, and flexible tube (pouch) of the structure PET-SiOx/PE

Aluminum TubesThe highest product protection against environmental deterioration is provided by Al tubes because Al offers an absolute barrier. It has a dead-fold character, that is, after a portion is dispensed, no suck-back effect takes place. The rest of the product does not come in contact with air. Aluminum tubes are made by pressing an Al disk to get the Al rolling. The cylinder part, the shoulder, and the neck are made. The rolling is cut at the neck and at the back end to get an exact shape. The outer thread is rolled on the neck. Even the protecting Al membrane at the orifice can also be made in this process. To enable smooth production, the Al disks are lubricated with zinc stearate or similar material. The cylinder end is then trimmed to get the exact length of the tube. Traces of lubricating material are glowed out at around 500°C for 3 min to get a fat-free surface on which the lacquers can adhere much better than on a fatty surface. Also, the Al crystallites rearrange in such a way that the tube becomes soft and is now collapsible. The tube is then lacquered on the inside with an epoxy phenol lacquer, seldom with a polyamide-imide lacquer, to ensure sufficient chemical resistance. Polyamide-imide lacquer is, however, not easy to


handle. The lacquer is then dried at 270°C for 6 min. Next the primer lacquer is coated on the outside. The tube is then heated at around 100°C to dry the outer lacquer partially and printed by the dry offset method. The final drying takes place at 170°C for 6 min. If very high tightness is necessary, the inside at the end of the tube is coated with rubber, where it is folded after filling. This is necessary to pack either sensitive products like some pharmaceuticals or products with very low sur-face tension, which tends to create leakage. Aluminum tubes have a wall thickness of 80 to 130 μm, depending on the tube size.

Laminated TubesThese types of tubes are made with laminates of suitable structures. Depending on the high barrier material chosen, the laminate type may be ABL (Al barrier lami-nate), CBL (ceramic barrier laminate), or PBL (polymer barrier laminate). The ce-ramic barrier material is SiOx, and the polymer barrier is mostly EVOH. The lami-nate is printed mostly by high-quality gravure printing. The cylindrical part of the tube is made by longitudinal sealing of a rectangular piece of laminate. Every lam-inate tube has a longitudinal seam. It may be a lap seal, or both ends may be fixed with two thin PET tapes sealed at the inside and outside. An injection-molded shoulder is sealed at one end of the cylinder. If necessary, a security film mem-brane is sealed on the orifice. A cap is brought onto the shoulder, either threaded or pressed. The laminate tube is now ready to be filled. The sealing of the shoulder with the cylindrical part must be of high quality because it is the weakest point of such a tube. During use, a tube is always squeezed at this place. Another important point is the inner side of the longitudinal seam, where the product comes in con-stant touch with the cross sectional layers. Aggressive products can delaminate the tube wall. Proper sealing technique is necessary. Laminate tubes have a wall thickness of 150 μm and more.

Aluminum Laminate TubesAluminum laminate tubes are made by lamination of three components: an Al foil of generally 20 μm and two LDPE sheets of different thicknesses as required. The adhesive is mostly a two-component polyurethane type. Previously, the layer thickness of Al foil was around 40 μm, which has been successively reduced even to 12 μm in some Asian countries. For some critical products, an extrusion lami-nation of Al with PE is chosen. Some product components from toothpastes may migrate through the sealing layer (PE) and dissolve the adhesive layer between the Al and PE layers. This often results in delamination of the Al foil. In such cases an extrusion lamination is preferred. Also, a suitable primer is necessary to achieve a good bond between the Al foil and the extruded layer. This type of tube is used for toothpaste.


Laminated Tubes on EVOH BaseIn this type of laminated tube, the Al layer is replaced with a polymer high-barrier material, mostly EVOH. EVOH offers a high barrier against oxygen transmission, and the PE layers offer a good barrier against moisture. The cylinder of the tube is made of a coextruded sheet of the type PE/Tie/EVOH/Tie/PE. For a better moisture barrier, HDPE or a blend of LDPE and HDPE can be used. All of the other manufac-turing steps are the same as for an Al laminated tube.

Laminated Tubes on SiOx BaseAnother type of Al-free laminated tube is made by replacing the Al foil with a SiOx deposited PET film. The biaxially oriented PET film is usually 12 μm thick, but other thicknesses are possible. The SiOx deposited PET film offers a high barrier against both oxygen and moisture transmission. This layer must be laminated with a sealing layer like PE. An extrusion coating of a PE layer may hamper the adhe-sion of the SiOx layer due to high temperature and deteriorate the barrier property. Solvent-based lamination offers higher adhesion force.

Laminated Tubes with an Outer CoatingAs already mentioned, every laminated tube has a longitudinal seam, the edge of which can be seen or can be felt with the fingers. Although technically quite okay, it does not look nice. In order to utilize the high barrier of an Al layer with the si-multaneously better appearance of an edge-free tube surface, the tubes are coated on the outside with a PE layer. To produce this type of tube, the laminate roll is unrolled, sealed longitudinally as a lap seal continuously, and passed through a vertical ring die, where the outside of the tube is coated with a PE layer. The longi-tudinal seam is now hidden and almost invisible.

This type of tube is the most expensive of all of the tube types. These are applied generally for premium cosmetics.

Extruded or Coextruded Plastic TubesMono- or multilayer plastic tubes are made in two steps. In the first step, a pipe is extruded or coextruded continuously. A constant wall thickness is achieved through proper positioning of ring dies. The hot pipe is then calibrated in a wa-ter-cooled vacuum calibrator to an exact diameter. The pipe is then cut to the re-quired tube length. In the second step, the cylinder is mounted on a mandrel and pressed against a molten HDPE on a suitable die to produce the shoulder of the tube. Simultaneously the cylinder is sealed with the shoulder. Other types of clos-ing systems can also be made other than an outer thread. It is easier to get a high-quality shoulder seam on this type of tube than on laminated tubes. The resin


type for the tube cylinder and the shoulder for extruded tubes are the same, but they are different in the case of laminated tubes.

The wall thickness of a mono tube is adjusted at the die head and also by the screw speed of the extruder. The total wall thickness of coex-tubes are adjusted at the die. The wall thickness of a single layer in a coex-tube can only be adjusted by the screw speed. Plastic tubes are very smooth when touched and have a high suck-back effect, which is, the tube wall goes back to its original shape when the tube is released after squeezing. There is no dead fold effect. The tube always looks like a new one, even at the end of use. That is why plastic tubes are used mainly for cos-metic creams.

After the tube is made, it is then printed with the dry offset method. Practically all of the hollow packaging materials, like tubes or aerosol cans, are printed with this method. The plastic tubes are always lacquered after printing because otherwise they would have a high electrostatic charge on their surface, which absorbs dust. It is an important factor for all types of plastic packaging materials, particularly plas-tic tubes, to avoid this phenomenon. Generally, during the winter when the air is pretty dry, dust is absorbed on the outside and causes a dirty appearance.

The printing ink may be solvent based, and then the printed tubes must be dried. Nowadays UV printing is also usual, where the curing of ink or lacquer is done by UV ovens. After the tubes have been printed and lacquered, the closure is mounted on it, either by turning it on threads or by pressing it on the shoulder. Two factors are important to ensure sufficient tightness after closing: the first one is the choice of raw material of the tube and the closure, and the second one is the geometry of the closure inside, which presses against the tube opening. The tubes can then be sent to the fillers.

Flexible TubesFlexible tubes could actually be defined as flexible bags with an injection-molded port. A rectangular laminated and printed film cut is folded and sealed at both lon-gitudinal sides. It now has the shape of a flat long bag. The folded end is then cut and sealed with a port. The port is injection molded. The market share of these tubes is very low.

2.1.2.4� Flexible Films for PackagingMono- or coextruded, blown, or chill roll flexible films are thin films and have ver-satile uses for packaging purposes. Under flexible packs are all types of packaging that can be squeezed or folded without getting cracks at folding lines or corners. Mostly they are pouches. Their handling is easy, and they do not crack if dropped on the floor. Another unique application of flexible packs is in vacuum packs.


Flexible films can also be thermoformed, filled, and evacuated if necessary. Exam-ples are different kinds of food, particularly meat, meat products, or cheese. Also, a number of medical products like implant pieces of metal or ceramic are vacuum packed. Coextruded or sometimes laminated films of the type PA/LDPE have versa-tile uses in the packaging sector. Sometimes these films can be furnished with a high barrier layer of EVOH, PA-6/EVOH/Tie/LDPE. LDPE can be replaced by LL-DPE, mLLDPE, PP, EVA, ionomer or other suitable sealing layer depending on the application. Vacuum packs for food in flexible films are the most economical pack-ages in terms of cost efficiency for a particular shelf life. Vacuum medical packs can have a shelf life of years without getting a leak.

In comparison to flexible packs, trays, cups, or other rigid containers cannot be evacuated. Either these packs have air inside or are modified atmosphere packs (MAP), where air is replaced by a suitable gas mixture after evacuation. Also, air from a pouch or tray can be replaced by flushing with a particular gas mixture without prior evacuation.

Depending on the target of the packaging, it may be used as single-layer packag-ing, mostly for nonfoods and general purposes. In many cases, particularly for so-phisticated packaging purposes like food, pharmaceuticals, cosmetics, or house-hold chemicals, multilayer films, manufactured through coextrusion, coating, or lamination, are used. In some cases, pouches are made out of films and supplied to the filler. These are used on a pouch-filling machine, like a vacuum chamber ma-chine. In most cases, however, flexible film rolls, single or multilayer, are sent to the filler, who uses them on a form-fill-seal machine for filling purposes, like a vertical-form-fill-seal machine.

In some cases unprinted pouches are labeled. For sophisticated or superior appli-cations, high-quality printing is necessary. Furthermore, high-speed packaging machines need films with high dimensional stability and tensile strength. For such purposes, monoaxially or biaxially oriented films like BO-PET, BOPP, or BOPA with superior mechanical and thermal properties are selected as the basic film. Accord-ing to necessity, they can be printed and coated with a suitable layer or laminated with suitable films to make the ultimate composite.

There are some other types of flexible films, discussed next, which are used for stretching or shrink purposes.

2.1.2.4.1� Stretch FilmsStretch films are thin, transparent, and soft films with very high tearing length, meaning, they can be lengthened to a great extent without tearing. Films like this are used to wrap a product. It may be a small piece of product like a tray with food, or it may also be a pallet with voluminous packets. Films for food applications must fulfill legislative requirements and the packets on a pallet should not be very heavy.


Besides stretch wrapping, the film must be able to cling to itself. Such films are al-most always coextruded nowadays to get the ideal cling properties. Although the film can be stretched, it should be able to maintain a good stretch tension and should relax less. Unnecessary relaxation reduces the wrap tension. Stretch films for outdoor applications, such as to secure shipping pallets, should have UV protec-tion to reduce UV degradation. Any UV degradation reduces the mechanical prop-erties quickly; the wrapping tension may be reduced or the film relaxes unneces-sarily and loosens the wrapping. Another important characteristic is the capability for low-temperature use. In middle Europe the film should be suitable for use at –20°C to 40°C.

Earlier such films were made mostly with plasticized PVC. Nowadays they are made with different types of PE and its copolymers as multilayer film with a core layer of EVA or elastomers and outer layers of metallocene polyethylene. These films can be manufactured either as blown film or as chill roll film. Very often with a vinyl ace-tate (VA) content of around 15%. The film thickness varies depending on the appli-cation and is generally between 15 and 50 μm. The resin composition is chosen so that the glass temperature is generally below –25°C. Attention must be paid to avoid unnecessary sticking of the stretch film on packets on a pallet. If necessary, the cardboard of the packets should have a suitable lacquer to avoid such sticking.

2.1.2.4.2� Shrink FilmsShrink films are defined as films that shrink when they are heated to elevated tem-peratures. Through this characteristic, pouches or bags of such films can wrap a product with pretty high tension. There are two types of applications: vacuum-shrink pouches for foodstuffs, and shrink films for outdoor use to secure pallets, particu-larly with heavy goods like bricks. Films for food applications must comply with the legislative rules and regulations.

If blown or chill roll films are stretched in the machine or cross direction or in both directions, then internal tensions arise. If they are then chilled quickly, then the tension freezes under ambient temperature so that such films or flat bubbles can be wound onto roll stocks just like general-purpose films. When such films or pouches, however, are heated, then the internal tension is released and the pouch tries to return to its original size before stretching, i.e., it shrinks. This phenome-non is called the “memory effect.” The grade of the shrink tension depends on the material used, its thickness, the process parameters, and the speed of cooling after stretching. Generally it is sufficient to cool such films at ambient temperature to create a memory effect.

The shrinkability, i.e., the memory effect, is utilized for pouches to pack foodstuffs like meat, meat products, or hard cheeses. The thicknesses of such pouches are generally 50 to 100 μm. Special-size large pouches to pack fresh beef for meat


ripening have a higher wall thickness. For packaging purposes, a vacuum pack is made with the flexible pouches at first. Then the filled pouch is either passed through a shrink tunnel with hot air or an IR heater, or the pouch is quenched for a few seconds in a hot water bath at around 90°C to make it shrink. The uneven areas, particularly the corners of the pouches, become smooth. It is not only a question of good appearance, but also the space between foodstuffs and the inner wall of the pouch reduces. This prohibits unnecessary loss of meat juice for red fresh meat and the growth of aerobic microorganisms like fungus.

In order to get high barrier properties against oxygen or water vapor, suitable coextrusion structures are chosen. A structure like PE/Tie/PVdC/Tie/PE gives the foodstuff the highest possible protection against the said two components. A chloride-free high barrier structure can be obtained by replacing the PVdC layer with EVOH. However, an EVOH coex-structure does not offer as high an oxygen barrier as PVdC. Moreover, care must be taken to avoid long contact of the EVOH coex-structure with hot water during shrinking, because the humidity that gets dissolved in the EVOH layer deteriorates the oxygen barrier of the pouch. Care must be taken during extruding PVdC because it can dissociate if kept for a long time at high temperature, which causes a lot of trouble with the extruder. Because a chloride radical is freed, which can easily react with iron, the cylinder and the screw for PVdC extrusion are properly coated with chromium and nickel.

Double-Bubble ProcessShrink bags for food packaging are generally manufactured through the dou-ble-bubble process (Figs. 2.11 and 2.12). First, the primary bubble is extruded and moves downward to a quenching bath at around 90°C to temper it to the orienta-tion temperature. This bubble then passes through two pairs of nip rolls. After the first pair of nip rolls, air is blown into the bubble to stretch it in the transverse di-rection. This is the second bubble. The stretching in the machine direction takes place through rotation of the second pair of nip rolls faster than the first pair. After the second pair of nip rolls, the bubble is flat and is rolled around a core to roll stock. In order to avoid unwanted sticking of the bubble walls during pressing at the first pair of nip rolls, sterilized starch powder is blown into the primary bubble just after extrusion. Thus sticking can be avoided and also the pouches can be opened easily during filling. Depending on the grade of stretching, the tubes and also the final pouches can have a high memory effect. If the storage temperature of the flat bubble is high, for example on warm days, the bubble may shrink a bit and reduces its width. During manufacture of pouches from such a flat tube, it is sim-ply cut and heat sealed at one end. During sealing, the seam should be cooled un-der pressure, such as with water-cooled electrical impulse sealing, to get a flat


seam. If the hot seam is not cooled down, than a shrink process takes place through the memory effect and the border is highly uneven, which may cause trouble during handling of such pouches. The free shrinking of such pouches is around 60%.

Extruder

1st bubble

2nd bubble

Water bath

Figure 2.11 Double-bubble production of shrink tube

Figure 2.12 Cross section of five-layer, high barrier, double-bubble shrink film on base of PVdC, courtesy of Peter Ludwig, EK-Pack


Triple-Bubble ProcessShrink pouches for food packaging can also be manufactured by the triple-bubble process (Fig. 2.13). The tube is generally coextruded in eight or nine layers. All of the layers directly enter the die without any adapter. This means that all extruders must run and all of the channels to the die must be fed with melt. Otherwise trou-ble may arise in the coex-melt flow.

Figure 2.13 Cross section of nine-layer, high barrier triple-bubble shrink film of basis EVOH and nylon. The PE layer originates from three extruders. Courtesy of Peter Ludwig, EK-Pack

The first bubble is produced and cooled on the outside with water. Simultaneously, the outer diameter of the bubble is adjusted through water calibration. Because of the sudden cooling, the amount of crystallites are less, and hence the tube is more transparent.

The tube is heated in a water bath or by IR heating before it is stretched multiaxi-ally to the second bubble. This bubble is captured between two pairs of nip rolls. Pressed air is inserted into the bubble to stretch it in the transverse direction. The second pair of rolls turns quicker than the first pair and stretches the tube in the machine direction. The grade of transverse stretching can be controlled by the bub-ble diameter, which can be adjusted by the air pressure in this bubble between the nip rolls.

Before getting to the third bubble, the tube is heated in a water bath to some 60°C and can be either thermofixed or relaxed with hot air. If the choice is a shrink tube, then the shrink value is adjusted according to relaxation. In contrast, a biaxially oriented film with good dimensional stability can be manufactured by thermofix-ing at elevated temperature.

Shrink tubes and hence shrink pouches manufactured by the triple-bubble process generally have a high barrier layer of EVOH instead of PVdC. PVdC is the usual high barrier layer in the double-bubble process. To avoid a premature shrinkage of


a triple-bubble tube without PVdC at room temperature, special treatment at the third bubble is necessary (see also Section 3.3, “Radiation Upgrading of Packaging Material”).

Shrink Films for Other ApplicationsShrink films for wrapping goods on pallets can be manufactured both as blown film and as chill roll film. In the first version it is a double-bubble tube. The base film is a blown film, which is then heated in a steam canal to an elevated tempera-ture to be blown for the second time. The transverse orientation is given through the higher diameter of the cross section. The higher speed of the second pair of nip rolls gives the orientation in the machine direction. The film is cooled very quickly to create a high memory effect.

Shrink film in the second version is a chill roll film, which is heated to the orienta-tion temperature through a number of heated rolls. This is then guided through a number of very closely mounted rolls. The rotation speeds of these rolls increase successively so that the film is stretched in the machine direction. The extent of stretching can be regulated by the regulation of roll speeds. The film can then be cooled down quickly to give a monoaxial memory effect. It shrinks only in the ma-chine direction. To create a memory effect also in the transverse direction, the film is heated once more and then tentered in the transverse direction to increase the width of the film. Once more the film is cooled down quickly. Now it has a memory effect in both directions and hence shrinks accordingly.

An important field of application of shrink films is as shrink sleeves and shrink labels (Fig. 2.14). Shrinkable flat tubes are printed and used as shrink sleeves or shrink labels. Particularly for extrusion, blow-molded or stretch blow-molded bot-tles are a common application. Naturally it can also be used for other containers.

Figure 2.14 Monoaxial shrink labels for blow-molded bottles


Stretch films are cheaper for applications on pallets than shrink films. In the case of shrink films, a heater is always necessary for the shrink process.

2.1.2.4.3� Oriented FilmsOrientation in polymer technology means an arrangement of the polymer macro-molecules in a particular direction, in which it is stretched. The distribution of the macromolecules in a melt is always isotropic. Isotropic means any property of a system that is distributed in all three axial directions homogeneously. This is also the case for the macromolecules in all blown or cast films. They are distributed in a random manner and are in a relaxed condition. Thermodynamically it is said to have a low energy level.

If, however, a film is stretched at an elevated temperature in a certain direction, then the macromolecules tend to follow that direction just like an elastic material. They are now under tension. If then the film is cooled down to room temperature, the internal tension and also the position of the macromolecules freeze. If such a stretched film, however, is not cooled down but is heated to an elevated tempera-ture for some time, then the mechanical tensions and also the “memory effect” in the macromolecules are almost fully lost. The macromolecules of the film are now fixed in their new dimension. That is why the heating process is also called “ther-mal fixing.” For semicrystalline polymers, a part of the macromolecules gets crys-tallized through orientation crystallization and gives the film high mechanical properties like tensile strength. The dimension of such a film is almost constant, even if it is heated for a long time at elevated temperature, for example in retort-ing. Through the reduction of its thickness the film gets more transparent than before stretching. The surface of biaxially oriented films increases to some 20-fold after stretching in the machine and transverse directions. Also, the surface of the film becomes very smooth. These films are very good substrates for printing or coating. They are high-quality films and are widely used in the premium packag-ing sector. Due to dimensional stability they can be printed and laminated with other polymer films, paper, or aluminum foils. Pouches of such films are suitable for retorting at around 125°C for 30 minutes without deteriorating the dimen-sions or the print quality.

The three types of polymers used to make such oriented films are PET, nylon (PA), and PP. The films are called BOPET, BON (BOPA), and BOPP. The letters “BO” mean “biaxially oriented.” BOPET has the highest dimensional stability among the three and is used particularly for high-quality retorting film. The surface of this type of film is very even and glossy. The usual thicknesses of the films are PET 12 μm, nylon 15 μm, and PP 20 μm. There are also some standards like 19 μm PET, but the other thicknesses are mostly used. All of these types of films have extraordinary suitability to be printed, coated, or vacuum deposited with aluminum or SiOx. For general purpose uses the films can be exchanged, depending on the cost. But for


special applications with sophisticated requirements, the proper one must be cho-sen. For the highest mechanical strength, PET is chosen. Also, the chemical prop-erties of the surface may be important. Some labels for low-temperature applica-tions (fresh meat) may stick very well on a nylon surface, but problems may occur if nylon is replaced by PET. Nowadays the manufacturers of such films supply a variety of coated or coextruded qualities. Different properties like a barrier or a foamy core are made. In particular, a foamy core of BOPP or BOPET is used to re-duce the weight of the film. Because of their foamy character, they are opaque white and have a soft touch, which is important for marketing. Due to the white color, white ink can be saved during printing.

The manufacturing of biaxially oriented films takes place in two steps (Fig. 2.15). In the first step, the chill roll film is heated to the orientation temperature and ori-ented in the machine direction by stretching it with a number of tempered rolls. The length may be some four-fold greater after orientation. This film is then once more heated and stretched in the transverse direction (tentering). The final width is some five-fold. Great care is taken to guarantee a constant temperature and ten-sion during tentering in the transverse direction.

Figure 2.15 Production line for mono- and biaxially oriented film


2.1.3� Important Features for the Technologist

1. Patience is most important during trials at an extruder. After every temperature adjustment or change of additive or blend percentage, sufficient time must pass until a steady state has been reached. Then one can take a sample and analyze to get some authentic results. Simply through sufficient patience and time during a trial some technologists can get some usable results that competitors missed with similar trials due to impatience.

2. Before starting with a critical customer’s orders, like for transparent film or tube, one should run the extruder with cleaning agents like fluoroelastomers or fluoropolymers long enough to get a good coating inside the extruder. Also, during production the main resin has to be mixed with a suitable percentage of these agents to ensure continuous trouble-free production. Food laws have to be considered to not exceed the highest amount of these agents.

3. Lack of proper cleaning can result in burned particles in an extruder, which is very annoying. These black or deep-brown burned particles come out of the ex-truder irregularly and can cause trouble for hours.

4. In many cases the contact layer of a few micrometers of a high-performance polymer (more expensive than standard polymer) is decisive for having an eco-nomical and tight seal. It is not necessary to make the complete sealing layer with the premium polymer. The cost of the sealing layer could be reduced through proper coextrusion of a mass of LDPE and a thin front layer of high-per-formance polymer.

5. Streaks in films or tubes are also annoying because they are bad optical proper-ties. Temperature regulation and proper cleaning of tools help generally; blends of compatible polymers of lower melting point can also help.

6. It is generally known that nylon-6 and EVOH have similar chemical affinities and do not need a tie layer during coextrusion. A structure of PA-6/EVOH/Tie/LDPE works with only one tie layer between EVOH and LDPE. On the other hand, different grades of EVOH with a mol% of ethylene between 27 and 48% are used even more for different applications. The vinyl alcohol content influences the barrier effect and the ethylene content the softness of the copolymer. For a high barrier lidding film, 27 mol% of ethylene can be used because the lidding film is never squeezed. For a collapsible tube or for thermoforming cups with high depth, this grade is too brittle. A higher mol% of ethylene is necessary. The chemical nature of EVOH of different grades varies with the variation of mol% of ethylene. Trials are necessary to check whether a tie layer even between nylon-6 and EVOH is the better solution for a particular coextrusion.


2.1.4� Discontinuous Processes

Discontinuous processes are characterized by the production of polymer materials discontinuously in cycles. The production process consists of a number of cycles that are performed one after another. The machine cannot run continuously and has to be stopped after a cycle is over; and then it begins a new cycle. Discontinu-ous processes are applied to the manufacturing of single articles like caps, bottles, dispensers, cups, or trays or the production of collapsible tubes (such as tubes for cosmetics).

2.1.4.1� Injection Molding (IM)Injection molding is a very important process for manufacturing different package parts like cups, trays, caps, preforms for bottles, dispensers, and the like. Products manufactured by injection molding show very high precision in their geometry. This is simply because the polymer melt is pressed and frozen in a cavity of exact geom-etry. The polymer melt from an extruder is injected into a cavity between tools. For production with thermoplastic polymers the tools are water cooled. The melt freezes to the ultimate product. It is a primary molding process where the product is manu-factured directly from the melt. For injection molding of thermoset polymers or elas-tomers, the tools are heated to achieve the ultimate form. Due to their cross-linked structures, the thermoset polymer or elastomer molecules show almost no Brown-ian motion, which is necessary for a melt to flow. The application form of these polymers (powder or resin) for injection molding is the pre-stage. The final molecu-lar structure is achieved after injection molding through heat into the cavities.

For manufacturing packaging parts, thermoplastic polymers are almost always used.

Products like cups or trays with a simple design or where a high precision is not necessary are manufactured by another discontinuous process called thermoform-ing because it is cheaper. Products with complicated designs or precision geometry like fittings are injection molded. Injection molding is characterized by less waste in the manufacturing process.

The resin can be processed 100% into the product.

The extruder for injection molding has some special features (Fig. 2.16). Unlike extruding in continuous processes, where the extruder is used only to produce a melt, in injection molding it has to fulfill two functions. In one stage it converts the resin to a proper melt, and in a next step it functions like a piston to inject the melt into the cavities.

In the first step, the screw turns to produce the melt. The nozzle in front of the ex-truder is closed so that the melt gathers at the nozzle. The water-cooled tools are brought to the other side of the nozzle. The screw moves backward through the pressure of the melt. There are different technologies like a ring gasket to hinder


the melt from passing through the screw channel backward. After sufficient melt has been produced as is necessary for the ultimate products, the screw stops rotat-ing. The nozzle opens.

Tool parts

Break point

Nozzle of barrel

Funnel

HeaterScrew and piston

FEED ZONE

Die cavity DieFeed channel

BarrelMETERING ZONE COMPRESSION ZONE

Figure 2.16 Injection molding

In the second step, the screw moves forward to inject the melt with high pressure (injection pressure) through the feed channel into the water-cooled tools. After the complete melt has been injected into the tools, the screw presses the melt for a further few seconds (after pressure). This ensures the complete filling of the cavity with the melt without contraction. The contraction of hot plastic during cooling is compensated for with after-pressure. The time after which the maximum amount of melt has been pressed into the cavity is called the sealing point (Fig. 2.17).

0.72

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0 1 2 3 4 5 5.5 6

Wei

ght

in g

Duration after Pressure (s)

Figure 2.17 The sealing point is at 5 s. The product weight is constant beyond this time


In the third step, the nozzle closes (mostly through a needle). The screw starts ro-tating to produce a melt, and simultaneously the tools open and the products fall down. The screw is then pushed backward through the melt pressure, and thus the cycle is complete.

The tools must be closed with sufficient force that they do not open even under the highest possible injection pressure or after-pressure.

Sealing PointThe important parameters of the sealing point are the melt temperature, injection pressure, after-pressure, injecting time, sealing time, and the closing force of the tools. The closing system is done by hydraulic devices. The injection-molding ma-chines are characterized based on the closing force, which is given in tons.

The number of cavities in a tool depends upon the size and complexity of the product shape. For very sophisticated forms, the closing power and hence the in-jection pressure is kept low, otherwise the tool can get damaged under high pres-sure, particularly when the melt viscosity is high. The number of cavities is gen-erally from 4 to 100 (small caps). All injection-molded objects have a typical mark from the feed channel after the nozzle. In modern extruders with needle closures, the mark is very fine, but it is still a typical sign for objects produced through injection molding.

In many cases two-stage injection molding is done, in which the product from the first stage of IM is placed in a second cavity. The product now serves as a part of the second cavity in which the second IM takes place. To get a good bond, both melts must be compatible. Generally the melts are of the same type of resin but have different colors.

Besides two or more stage IM, coinjection molding is also possible (Fig. 2.18). The process of coinjection is very sophisticated. Melts from two different extruders are injected into the cavity one after another through a valve. In most cases it is ap-plied for preforms for stretch-blow-molded bottles of a structure PET/PA/PET or PET/EVOH/PET. It is actually a three-step process. First, PET is injected to fill up about one-third of the cavity volume. In the second stage, the valve is turned to al-low the barrier material PA or EVOH to flow through the PET layer, and this fills up about two-thirds of the cavity. In the third step, the valve is closed, and once more PET is injected to fill up the cavity. Through regulating the melt flow and the speed of the melt front, it is possible to create three layers, where the barrier layer is placed between the inner and outer PET layer. The technology is much more com-plicated than coextrusion. Coextrusion is a continuous process, where different layers flow simultaneously side by side in the adapter. Coinjection needs special fluid mechanical regulation to allow three layers side by side (see Fig. 2.18).


1st step: PET 2nd step: PET/Barrier

3rd step: Sealing off

Figure 2.18 Coinjection molding

Generally the number of tool parts in IM is two, which are always conically con-structed so that after injection the tools may be separated easily and the product will fall down through the force of gravity. In some cases there are pins or rings that push the product to ensure an empty cavity after each cycle. In some complex cases there may be three tool parts. The parts are mounted on a rod in such a way that all of the parts move away from one another when the tool walls are separated.

At least two tool parts and very often three are necessary to make the outer thread on the neck of a preform. The product can be easily taken out after the parts move away. For an inner thread, however, the product must be turned out from the tool or it is pushed out at a still elevated temperature from the tools by rings. This is the case for manufacturing caps. The IM process ends and the tool walls are opened when the caps are at a higher temperature than room temperature and hence still in an elastic stage. The threads can be pushed over the tools without deteriorating their ultimate form and function.

In-mold labeling is possible during IM. The label is placed in the cavity by a device and held on the cavity inner wall through electrostatic force. The melt flows and get fixed on the label. The label inner layer and the molten polymer must be com-patible to achieve a good bond. Even paper-based labels may be used after they have been coated with a suitable polymer.

Materials used for packaging-purpose IM are generally different types of PE and PP, PA-6, PET, and PVC.

2.1.4.2� Injection Blow Molding (IBM)With IBM, small bottles or jars for liquid pharmaceuticals or drinks are manufac-tured. The rigidity of the container depends upon the polymer used, the bottle ge-ometry, and the distribution of wall thickness. The geometry of the polymer preform has to be tested accurately before the final tool is constructed. Therefore injection blow molding is an expensive process and is applied for the long term and a high number of products. In order to increase the bottle stiffness, mechanical stretching can also be integrated. Mechanical stretching can be integrated in the step where the tool-rod is stretched mechanically like a telescope before the bottle is blown.


1. Injection stretch blow-molded bottles show a higher stiffness than without stretching. IBM parts are generally soft; the size is between 5 to 100 ml. The manufacturing process may have two, three, or four steps.

2. Two-step process: In the first step, a preform is injection-molded on the core tool. The neck of the future bottle or jar including the outer thread is finished in this step. Then the outer tools open and the core with the hot preform is turned 180° and placed in a second pair of outer tools that has the shape of the final bottle. After closing the outer tools, air is introduced through an annular gap around the core tool, and the preform is blown to the ultimate container. The production speed is increased by installing a higher number of tools, for exam-ple, eight tools. The outer tools are water cooled. The bottles immediately ac-quire their final form and are removed after opening the outer tools.

3. Three-step process: The first step is similar to the two-step process and makes the preform through injection molding. The second step is also the same. In a third step, the bottle is taken away by a catcher device. The third step ensures a correct collecting of the final bottle so that no production disturbance arises during injection molding in the next cycle.

4. Four-step process: The four-step process has the same three steps as the three-step process, but there is a conditioning step either just before or just after the injection-molding step (Fig. 2.19). Its function is to temper the core tool properly so that the final blow-molding process can take place with higher precision.

Due to injection-molded preforms, the IBM bottles have a typical injection mark on the bottom. As already mentioned, in some cases a telescopic core tool is used that stretches the preform before blowing to enable a higher orientation effect. These containers show higher rigidity.

Injection molding

Conditioning +preblowing

Ejection

Final blowing

1)

2)

4)

3)

Figure 2.19 Injection blow molding; step 1; injection molding; step 2; conditioning and preblowing; step 3; final blowing; and step 4; ejection


2.1.4.3� Extrusion Blow Molding (EBM)First, a parison with a circular or other cross section is extruded continuously downward from the die (Fig. 2.20). After the parison is extruded to the desired length, which is controlled by an optical device, a pair of tools captures it. During closing, the bottom part of the parison is at once sealed and cut. Simultaneously a sharp and very hot knife or wire cuts the parison neck. The parison is open at the neck to blow it at the next station. The tools transfer at once to the next station, and the parison is extruded further.

A calibrating blow nozzle is then pressed at the neck opening of the parison at the second station. The outside of the neck is pressed against the tools to make either a thread or bayonet catch or another type of closing system. The parison is then blown to the final container: a bottle, jerry can, or whatever it may be. It cools down quickly at the water-cooled tool wall. The final container is tested for tightness be-fore the tools open to let the container fall down. The waste parts from the bottom and neck are ground and recycled.

A - Tools E - Bottom sealing + cuttingB - Calibration + Blow head F - BottleC - Parison G - Neck cutsD - Strainer/Torpedo head die H - Bottom cuts

1) 2) 3) 4)

AB

C

D

E H

H

F

GG

Figure 2.20 Extrusion blow molding: 1.) extrusion of tube, 2.) closing the tools and bottom sealing, 3.) introduction of calibrating rod and blow molding and cooling of con-tainer, 4.) opening of tools, removal of container, and neck and bottom cuts

Examples of EBM bottles are food (ketchup, sauce), cosmetics (liquid soap, cream), and household products (detergents). Beside bottles, numerous types of larger con-tainers like jerry cans are also manufactured by this method. A special feature of EBM is the construction of a handle, for which softer polymers like HDPE or PP are necessary. Only soft polymers can be blow formed to get a hollow grip on a bottle or jerry can. In comparison to IM, the wall thickness of EBM bottles is not exact. In IM the melt is compelled to take the exact dimension of the cavity. Expansion of


parison to bottle or container takes place more or less uncontrolled through the hollow space in the tools. The process parameters have to be optimized to get a good wall thickness distribution. This phenomenon is common for all blow-molded processes including thermoforming. All of the EBM objects have a typical linear seam at the bottom. The bottles are more rigid than IBM bottles. The container sizes may be bottles of a few hundred milliliters up to a 2000-liter oil tank.

For bottles with different diameters at different heights, such as bottles with a slim waist, the parison also has different thicknesses at different heights. This is done by changing the annular gap in between during extrusion. The parison will then have different thicknesses at different heights. After blow molding the thickness of the bottle wall is homogenous at all positions.

Special technology is necessary to blow a bottle with a handle. The cross section of the parison is oval-shaped like an ellipse. The tools have suitable elevated projec-tions inside that seal and cut the handle geometry during closing. During blowing, the air extends both the bottle and the handle up to the tool wall. There are then three types of waste: the bottom, the neck, and the handle part. The handle is hol-low so that after filling the liquid can flow through it.

For some dangerous products like pesticides, the legislative criteria do not allow such a flow of liquid through the handle. The handles of pesticide containers are blown in a separate process and sealed on the container top. Moreover, a special construction at the bottom of the containers is required to fulfill the regulations.

The bottles during blowing are always multiaxially oriented. The degree of orienta-tion is moderate. Processing parameters during EBM are the wall thickness of the parison, its temperature, the blowing pressure (generally 8–12 bar), and the tim-ing of pressure application to get a satisfactory distribution of wall thickness.

The parison can also be coextruded with one or more functional layers like a high barrier or colored layer so that the bottle or container can also be used for sophisti-cated products. EVOH, PVdC, nylon, or PEN can be used as barrier materials against oxygen. PP, HDPE, and PVdC are very good high barriers against water va-por, and suitable fillers may be used to get a high light barrier. Pigments may be used to get special effects.

EBM is suitable for in-mold labeling (IML). The label is placed inside the tools by a device and held on the inner wall by electrostatic force. The blown tube gets sealed onto the label. The label inner layer and the parison outer layer must be compatible to enable a good bond. Even paper labels may be used if they are coated with the proper resin or lacquer. Particularly for wet room applications like shampoo bottles, the label must be waterproof. Polymers used for EBM are differ-ent types of PE, its copolymers, PP, PA, PVC, and other special polymers as coex-trusion partners.


2.1.4.4� Stretch Blow Molding (SBM)SBM is a two-step process to make rigid bottles (Fig. 2.21). In the first step, a pre-form is made by injection molding, which looks like a test tube and has the final shape at its neck. Also, the outer thread is finished. The preform is heated in the second step until it is soft and elastic. Then it is stretched mechanically with a rod and finally blown into the bottle shape. Depending upon the material and design of the bottle and also its purpose, for cold fill or hot fill, the stretch and blow pro-cesses are differentiated.

Heating of injection molded

preform

Mechanical stretching + preblowing

Final blowing

Figure 2.21 Stretch blow molding

The typical characteristics of an SBM bottle are its high stiffness, elevated barrier properties, and higher transparency. The bottles are highly oriented by mechanical stretching in the axial direction by the rod and afterward multiaxially by blowing. During high stretching orientations, crystallization takes place in the material, re-sulting in huge number of tiny crystallites, which give the higher barrier and me-chanical properties. Also the transparency is increased. Typical applications of SBM bottles are for carbonated or noncarbonated soft drinks, alcoholic beverages, cosmetics, edibles, or other oils. The sizes of the bottles are generally 0.25–2.5 L. The materials used are mostly PET but also PP and PVC to some extent. For cost savings, the weight of the preform has been continuously reduced without draw-backs in usability. The high stiffness of the bottles is achieved through sophisti-cated design and shape.


The usual weight of preforms for a 500 cc bottle is as follows:

Noncarbonated water : 7–9 g Carbonated water: 13–15 g Carbonated soft drinks: 15–18 g Hot-fill: 20–22 g

Sometimes the wall thickness is extremely low: the empty bottle seems to be flexi-ble. The necessary rigidity of the bottle is supplied by the inner pressure of carbon-ated drinks. The bottom is either champagne type or in petalloid form to give a steady standing.

A higher barrier is achieved generally through blending PET and PEN. To achieve a still higher barrier, e.g., for beer, the bottle inside is coated with barrier materials like SiOx or amorphous carbon through plasma deposition. Special technology is necessary to construct bottles for hot-fill. The main problem here is the vacuum that arises when the bottle cools down after filling. Higher stiffness in the bottle is necessary to counteract the difference in air pressure from the outside. The weight of the preform for bottles of the same volume is the lowest for soft drinks, higher for carbonated drinks, and the highest for hot filling (see Table 2.1).

Table 2.1 Comparison of Containers Made by a Discontinuous Process

Process Extrusion blow molding Stretch blow molding Injection blow moldingBottle size 0.1–2000 L 0.5–2 L 10–250 mlMechanical properties

Semirigid, elastic Rigid, stiff, may crack under high pressure

Soft, elastic

Optical properties

Translucent Transparent Translucent

Degree of orientation

Moderate High Low

Typical mark Long seam at bottom Injection mark at bottom Injection mark at bottomBottle geometry Round, rectangular, oval Round to achieve

maximum rigidityRound bottles

Barrier properties High barrier possible through coextrusion

High barrier through con-verting or coinjection

Low barrier

Coextrusion modification

Usual Possible Not usual

Labeling IML usual Shrink sleeves, glue labeling

Shrink sleeves, glue labeling

Products filled Cosmetics, chemicals Soft drinks, beverages, cosmetics

Medicine, functional drinks, cosmetics


The function of stretch-blow molding process can be seen in Figs. 2.22 – 2.24. First, (Fig. 2.22) preforms are heated through a row of heating strips, whose radiating power can be adjusted. The neck of the preform is already finished during injection molding. The preform is placed with its neck in the socket, which protects it from heat. The cylindrical part of the preform is heated with a number of strips at differ-ent heights. The opposite wall of the heating strips reflects heat for more economi-cal production. The sockets with preforms rotate to distribute the heat homoge-nously around the preform. A number of sockets are mounted on a revolving chain. According to the shape of the future bottle, the preforms are heated at different levels with different intensities. The objective is to heat the preforms to optimum softness for forming.

Heating strips

Sockets for preforms

Figure 2.22 Preform heating

The heated preform can be seen in Fig. 2.23. The preform is taken out and put in the stretch-blow molder between the tools, Fig. 2.24, where it is mechanically stretched and then blown to the final bottle.


Figure 2.23 Hot preform coming out of the heat chamber

Figure 2.24 The preform between the tools just before stretch-blow molding


2.1.4.5� Different Types of PETThe importance of the polymer material PET has become significantly high over the last 10 years as it has substituted for glass bottles and even jars. PET or polyeth-ylene terephthalate is a semicrystalline polymer and is manufactured through a polycondensation reaction from terephthalic acid and ethylene glycol. By modifying the processing and application characteristics, different polyesters have been cre-ated. Three examples are PBT, PEN, and PET-G. The alcohol part may be replaced with 1,4-butanediol to get PBT or polybutylene terephthalate (also known as PTMT or poly-tetra-methylene terephthalate). PEN or polyethylene naphthalate is polymer-ized from NDC (dimethyl-2-6-naphthallene carboxylate). The crystallinity of PET can be reduced through the use of higher mole volume components like isophthalic acid to get highly transparent PET. Another example is PET-G, which is synthesized with terephthalic acid as the acid component and the two alcoholic components cy-clohexane dimethanol and ethylene glycol. PET-G is also called glycol modified.

PET is applied for packaging purposes in three different varieties: biaxially ori-ented film of 12 or 19 μm with high tensile strength and other mechanical and thermal properties, PET-A sheet for thermoforming, and PET containers or bottles, which are mostly stretch blow molded. Thermoforming of PET-A is modified by PET-G to get a better cutting result. PET-G is also used for injection molding.

PET is very sensitive to moisture during processing. Even traces of moisture may hydrolyze PET during extrusion and deteriorate its properties. Moreover, odd odor products like acetaldehyde may result. PET resins are always dried properly (130°C/10 h) before extrusion.

2.1.4.6� ThermoformingThermoforming means forming a two-dimensional flexible polymer film or rigid sheet under heat and pressure into three-dimensional cups, trays, blisters, or other molds for packaging purposes. This is the most common process for making such articles, although some of them could also be made through injection molding. Thermoformed containers are suitable for packing different types of products like pieces, granules, powders, solids, liquids, or a mixture of these. Principally, an ex-truded (monolayer) or coextruded (multilayer) polymer sheet is heated until it gets a soft consistency, and then it is formed in a cavity through pressure. The original sheet is elongated because the side walls of a cup are built new: the wall thickness reduces automatically. Thermoforming is actually an elongation process. To get an ideal elongation, the sheet has to be heated when the elongation is the highest. Depending on the thermoform temperature, part of the elongation is elastic (re-versible) and part is plastic (irreversible), where the reversible part is predominant at lower forming temperatures and the irreversible part at higher temperatures.


The forming temperature is between the glass transition temperature and the melt-ing point. Generally it is around 100°C or a little more for most packaging polymers except PP, which has a higher thermoforming temperature. Amorphous thermoplas-tics can be thermoformed easily because they have a homogeneous structure (amor-phous) and the thermoforming window is broad. The semicrystalline polymers on the other hand soften inhomogeneously because of their two different structures: amorphous and crystalline. In order to save time so-called SPPF (solid phase pres-sure forming) has been introduced by Shell to thermoform at a temperature around 10–20°C lower than usual. It is applied particularly for PP. Thermoforming takes place in a highly thermoelastic condition. Plug assistance is necessary to ensure a good thickness distribution in the container. Because of the lower forming tempera-ture, less time is necessary for thermoforming and the machine speed is higher. The mold temperature is around 10°C. Containers thermoformed in this way have a high inner tension due to the memory effect. Because such containers are used at ambient temperatures or below, mostly for dairy products, the memory effect can-not be activated and hence there is no effect on the packaging.

Containers, on the other hand, used for retorting purposes or for microwave heat-ing, are thermoformed at higher temperatures. Thermoforming takes place under thermoplastic conditions. The internal tension is very low (low memory effect). The containers do not shrink when heated at elevated temperature. PP is thermo-formed at around 155°C (310°F). Thermoforming takes place in the thermoplastic region rather than in the thermoelastic region. The ideal forming temperature de-pends upon a number of factors, like the polymer type, the applied additive packet, the geometry of the container, and the thermoforming ratio.

2.1.4.6.1� Theory of ThermoformingAt sufficiently low temperatures, like –50°C, all polymer macromolecules behave as frozen material. They show almost no Brownian movement. As the temperature increases, the atoms and also the macromolecules start swinging around their average position—the material gets softer. The modulus of elasticity or stiffness reduces accordingly. Depending on the polymer type, the modulus of elasticity falls at a particular temperature abruptly—the gradient is pretty high. This is a characteristic temperature for a polymer and is called the “glass transition tem-perature” (TG) (Fig. 2.25). Below this temperature, a polymer behaves like glass and is brittle. Above this temperature, the polymer begins to be soft and elastic. If the temperature is increased further, the modulus of elasticity falls further, until the polymer melts at the melt temperature (TE). In every polymer there is always a distribution of molecular weight. Lower molecular weight macromolecules melt earlier than the higher ones. That’s why it is better to say “melting region” for polymer materials rather than “melting point.”


Figure 2.25 Thermomechanical characteristics of polymers

The softening process of polymers from the glass transition temperature to the melting point depends upon the type of polymer. The distribution of the macromol-ecules in an amorphous polymer like PVC or PS is homogeneous. A semicrystalline polymer on the other hand has in addition to the amorphous part a crystalline part. To soften, the crystalline part needs more energy than the amorphous part be-cause the physical bonding forces, like the van der Waals forces of intercrystalline molecule chains, has to be overcome. The softening process in semicrystalline polymers like PP or PE is hence inhomogeneous. This makes the thermoforming of semicrystalline polymers complicated. Moreover, the gradient of softening with increasing temperature is for crystalline polymers much steeper than that of an amorphous polymer. Hence the thermoforming window of crystalline polymers, particularly for PP, is narrow. The heating of such polymers like PP is much more sophisticated, and hence the machines are more expensive than those for amor-phous polymers like PVC, PS, or PET-A. The heat stability and thermoforming abil-ity of a semicrystalline polymer can be increased by filling it with inorganic mate-rials like chalk, TiO2, or the like.

The Forming ProcessThe forming process can be assumed as elongation of the sheet to create a con-tainer. Biaxial orientation takes place during elongation. The wall thickness re-duces, and the container shows higher transparency, partly due to reduction of the wall thickness and partly due to orientation in crystallization. There are two types of deformations that take place during elongation: an elastic deformation and a plastic flow. The elastic deformation is reversible and shows a memory effect, as is usual for shrink films. This type of deformation prevails at lower forming tempera-


tures, which is called “SPPF,” meaning solid phase pressure forming. The memory effect however has no disadvantage in low-temperature packaging applications be-cause the shape of the container is always frozen. Had we reheated the container, then shrinking and deformation of the container would take place because of the memory effect.

Plastic flow, on the other hand, is an irreversible process. Also, after reheating, there is no shrinking or deformation. It dominates at higher forming temperatures. This type of thermoforming is necessary for food containers that are retorted at around 120°C or used for microwave heating. The forming temperature is as high as possible; see Fig. 2.26 below.

L0

L1

L1 - Reversible

L0 = Length before forming

L1 = Length after forming

L1 - Irreversible

Figure 2.26 Plastic flow is an irreversible process

2.1.4.6.2� Technology of Thermoforming

Step 1. HeatingThe first step of thermoforming is to heat the sheet to the forming temperature. There are two methods: the contact hot plate and IR radiation. In the case of a con-tact plate, a perforated aluminum plate is used that is coated with a heat-resistant lacquer. A vacuum through the hot plate enables the sheet to get a keen contact with the hot plate. Heat transfer is better. The polymer sheets are heated generally up to 140–165°C (284–329°F). Heat transmission takes place through conduction. Polymers generally have low heat conductivity, so that is why a keen contact is very important for the heat transfer. Any air bubble between the sheet and the plate means isolation. Heating may take place from one side or from both sides with sandwich plates (Fig. 2.27). One-sided heating is used for thermoforming flex-ible films, which are not thick, need a longer time, and the temperature distribu-tion is not very good. This is usual for a cheaper and simple machine. In the case of thermoforming PP, three pairs of sandwich plates are used to get the necessary temperature distribution. The sheet is heated only when in contact with the plates. There is generally no problem of overheating the sheet during machine stop.


One-sided heating + thermoforming at one station

Sandwich heating + thermoforming one after another

Threefold sandwich heating +thermoforming one after another (PP)

One-sided radiator, (IR, Quartz, Ceramic, Iron, Carbon;≥ 800 nm) afterward, thermoforming

Sandwich radiator + afterward, thermoforming

Preheating chamber + sandwich radiator (PP)

Preheating rolls + sandwich radiator (PP)

Figure 2.27 Different heating systems for thermoforming

With IR radiation, however, it is a bit different. Heat transfer takes place through absorption of IR radiation at a wavelength between 3000 to 3500 nm. The intensity of radiation is pretty high. The time of exposure of a sheet to the IR radiator de-pends upon the IR absorption coefficient, the specific heat capacity of the polymer, and the power of the IR device. During machine stop, the IR device must be re-moved or an isolation plate must be placed between the radiator and the sheet to avoid thermal damage of the sheet. For semicrystalline sheets like PP, the duration of the sheet under the radiator must be longer than for an amorphous polymer in order to get a homogeneous temperature distribution. In many cases there are other devices like heated rolls or a hot-air chamber to heat PP up to a temperature of around 120°C. The final heating to 160°C takes place through IR radiation. The IR radiation is generated by different devices like quartz lamps or ceramic panels.

The temperature of the sheet should be homogeneous, particularly when the form-ing takes place without a mechanical plug. In the case of inhomogeneous tempera-ture, the part of the sheet with higher temperature will form much more quickly because it is softer than the other parts. The thickness distribution of the container wall will be bad. Generally, amorphous polymers like PVC can tolerate a tempera-ture fluctuation of +/–8°C; semicrystallines like PP need a much narrower distri-bution like +/ –2–3°C. Semicrystallines with greater amount of amorphous struc-ture can tolerate 5–6°C.


The simplest heating system is one-sided heating with a hot plate. This is installed in simple thermoform-fill-seal machines, where the heating process and the ulti-mate forming take place at the same station. The machine speed is low as the said processes take place one after another. Machine speed is generally 5 to 6 cycles per min.

The machine speed is higher when sandwich heating is used, and forming can take place simultaneously at the next station. For forming PP sheets, particularly ho-mo-PP with a high forming temperature, three-fold sandwich heating is necessary.

Radiator heaters are usually used on high-speed thermoforming machines. These machines are constructed mainly for thermoforming and cutting. The cut pieces of container are stacked and supplied to fillers, like dairy farms. There are different technologies for proper heating of PP sheet. For high-speed machines, the sheet can be preheated on rolls or in a chamber.

Step 2. Forming ProcessAfter the sheet has reached the forming temperature, it is then formed into a con-tainer. As already mentioned, this can be done in two ways. There may be a cavity (female) with a number of vacuum holes at the bottom, where the sheet is drawn downward: so-called negative forming. Or there is a positive mold (male) with a number of vacuum holes around its bottom. The sheet is then draped around the male form in positive forming. The target is always to achieve the exact form of the mold and a homogenous distribution of wall thickness in the container.

Regarding forming technologies, we have to differentiate between two different ap-plications. In a TFFS machine, thermoforming, filling, sealing, and cutting take place one after another at successive stations. All of the above-mentioned steps take place simultaneously, when the machine stops. The step that needs the most time decides the machine cycle. In a simple machine with heating and forming at the same station, this is the slowest step and decides the machine cycle. In other machines, the filling step is generally the slowest one and decides the machine speed. The usual speed is 6 to 10 cycles per minute. The forming technology is shown in Fig. 2.28. The thermoformed cups are ejected from the cavity with air from the bottom.

TFFS machines where no filling but only forming and cutting take place and then the cups are stacked are much quicker. The forming technology is a bit different. The cavities have no narrow holes for evacuating from the bottom as in most TFFS machines. The cavities have slits or other systems with a higher cross-sectional area. Thermoforming takes place at high speed with plug and pressed air. The cups are ejected with mechanical devices.


Forming tool Vacuum holes

Sheet

Heater


Sheet

Heater

Vacuum

Heater

Vacuum + Press air


Sheet

Vacuum + Press air + Plug


Sheet

Plug

Figure 2.28 Different technologies of forming

There are three possibilities to form the sheet. It can be done with vacuum under-neath the sheet. The sheet is pressed in this case by the ambient air pressure. This is the case when the polymer is not too hard, like PA or PE, and the sheet thickness is not too high or the cavity is shallow, like in pharmaceutical blisters. For thicker sheets, the ambient air pressure is insufficient to form them quickly, and pressed air is applied from above to form the sheet. In both cases, whether simply vacuum or vacuum with pressed air, the distribution of wall thickness of the container is broad. The difference between the thinnest and the thickest position is pretty high. Particularly the bottom corners of rectangular containers may have only 10% of the sheet thickness. The third way, and the best solution is to use a plug.

The hot sheet is pushed at first with the plug in the cavity as deep as possible, and the ultimate forming takes place with air pressure. The thickness distribution of the wall can be optimized by regulating the speed of the plug and the timing of the pressed air. The sheet thickness can be reduced 10% or more through proper use of plugs instead of vacuum only. The container weight can also be reduced by design-ing vertical or horizontal ribs. Forming is difficult if the depth of the container is high. In such cases a plug is necessary. Thermoforming machines with plugs are always more expensive than machines without a plug. Generally, small machines with output that is not too high are made without plugs. For machines with high output, it is always advisable to use plugs.


The ratio of height to diameter of a container is a reliable figure in determining whether a plug is necessary for thermoforming a particular container.

For cylindrical containers, the ratio = H/D, where H = height of the container and D = diameter of the opening.

For rectangular containers, the diameter of a circle with the same cross-sectional perimeter is taken.

A useful instrument to estimate the approximate container stiffness after thermo-forming is the Vicat softening temperature (°C) for a polymer. It is defined as the temperature at which a needle penetrates 1 mm in a polymer probe under certain weight. A similar figure is the heat distortion temperature (HDT) (°C). It is defined as the temperature at which a polymer plate bends approximately 0.3 mm under a certain pressure.

Usually the Vicat-B method is selected for measuring the temperature, with param-eters as follows: force on needle = 50 N, cross section of needle top = 1 mm2, size of polymer probe in oil bath = 10 × 10 × 5 mm, temperature gradient of oil bath = 50°C/h, and depth of penetration = 1 mm.

The pressure at HDT-B = 0.46 N/mm2, probe size = 110 × 3.5 × 10 mm, bent depth = 0.21–0.33 mm, and temperature gradient in oil bath = 2°C/min.

A very important aspect is the shrink value for a polymer after thermoforming. It is defined as SV = (V1 – V2) / V1, where SV = shrink value, V1 = inner volume of cavity, and V2 = outer volume of container.

During thermoforming for general-purpose containers, the container is cooled quickly in the cavity to achieve a higher machine speed. Quick cooling causes shrinking. The cavity volume should be a bit bigger than the container. After shrinking, the volume of the container reaches its optimum value. Care must be taken if the polymer is changed or if even the same polymer is merely furnished with an additive. The shrink value changes and the volume and even the shape of the container may change. A virgin polymer has a higher shrink value than that filled with a whitening agent like TiO2, kaolin, or chalk. Among the polymers, PP has a higher shrink value than PVC or PS.

Average wall thicknesses of container:

Sheet form before forming: A × B × so

Container after forming: (A × B + 2A × H + 2B × H)s

where A = length, B = width, so = sheet thickness, H = depth (height) of container, and s = average thickness of the container wall.


Figure 2.29 below shows the thickness distribution of a container after different types of thermoforming.

400

300

200

100

0

Wal

l thi

ckne

ss(µ

m)

A B C F D E

withwithout plug

A

EDFCB

A

EDFCB

400

300

200

100

0

Wal

l thi

ckne

ss(µ

m)

A B C F D E

Figure 2.29 Distribution of wall thicknesses for different thermoforming ratios

2.1.4.6.3� Thermoforming MachinesThermoforming machines are differentiated into two groups: the thermo-form-fill-seal (TFFS) machine and the thermoform and die-cut machine. These ma-chines can be classified in the following groups:

In-line Thermoforming MachineThis type of thermoforming machine is installed in-line just after the calender rolls of the extrusion line for sheets. The sheet is still hot in the core and needs on its outside slight heat, mostly from an IR radiator, and then it is thermoformed. The thermoforming device with die-cut unit may be a synchronized moving one with the same speed as sheet production. Or the unit is fixed and it thermoforms and die-cuts the cups, which are then stacked.

Thermoform-Fill-Seal Machine (TFFS)Machines of this type are used widely for filling purposes (see Fig. 2.30). Typically, this type of machine has a heating and thermoforming station where the bottom sheet is heated and formed. Then the cups are filled and sealed with a top film. The containers can be evacuated if the bottom sheet is a flexible film like PA or PE or is gas flushed (modified atmosphere packaging). Thereafter the containers are cut into single packs. The bottom sheet is transported through the machine with two


chains at both sides. The flexible top film is transported along with it. The machine has four main functions: the heating of the bottom web, forming, filling, and seal-ing. These are done at different stations. The bottom sheet has to always be trans-ported through the machine at the same speed. The machine speed is determined by the station that takes the longest time. The transport of cups through the ma-chine is a secured process, as they are part of the transported sheet. Another im-portant point is the hygienic aspect of the cups for food packaging. The thermo-formed cups are automatically thermally retorted during thermoforming. Naturally, recontamination with microorganisms takes place at all standard machines. The filling station is mounted with hygienic devices in sophisticated machines. The highest security is offered on aseptic TFFS machines where abnormally high hy-gienic security is guaranteed.

1

2 3 45

67

8

9

1 - Bottom sheet or film 4 - Filling 7 - Cutting2 - Heating 5 - Lidding film 8 - Waste transport3 - Thermoforming with plug 6 - Sealing 9 - Final packs

Figure 2.30 Thermoform-fill-seal machine

Preform-Fill-Seal MachineThe third type of machine is almost the same as the TFFS machine, but no thermo-forming takes place. The thermoformed or injection-molded cups are stacked in a magazine. The cups are transported through the machine in a cradle to the filling station, where they are filled, sealed, and, if necessary, cut into single packs. The lidding material at these machines is always die-cut lids, which are either supplied by some converter or are die-cut at the filler from roll stocks. The transport of cups through the machines must be secured so that no cup falls down from the cradle.

An important point is the hygienic aspect of the cups for food packaging. Because the preformed cups in this type of machine are not thermally treated, the cups are always contaminated with microorganisms. Almost all machines of this type are aseptic machines with high hygienic security.

Types of containers:

All thermoformed containers undergo a certain amount of shrinkage after thermo-forming–depending upon the structure of the sheet, additives used, and extrusion


parameters. In some cases, a container reaches its ultimate shape about 24 hours after thermoforming. That’s why the thermoforming tools need, in some cases, to be of high precision.

Containers that are later sealed with a top film or a die-cut lid do not need a high precision tool for thermoforming because the sealing seam of the container has generally a width of 6 mm and more, so inhomogeneous shrinkage of the container does not interfere secure sealing. The sealing strength is sufficient high for the available seam width.

The shrink value of the bottom sheet may vary in a certain range.

Containers with a reclosable lid, on the other hand, must be thermoformed pre-cisely—both the cup and the lid. Their geometries must match one another to enable easy opening and closing. Problems arise if the shrink value of the sheet or the cavity geometry is not exact. Care must be taken when changing the packaging ma-terial, bottom sheet, or top film. Sufficient trials are necessary before a customer’s order is manufactured.

2.1.4.6.4� Technology and Manufacturing of Skin PackagingAs already mentioned, skin packaging is a positive thermoform packaging, i.e., it is a drape forming and takes place upward from the base. There is no mold like in thermoforming; the product itself is the positive mold around which the soft film is formed. The soft film is heated through a contact plate or a quartz or ceramic IR radiator to its softening point. The product is placed on a porous tray, usually a carton, which moves upward to the hot film. Then a vacuum is created between the film and the carton. The film is pressed under ambient pressure onto the product like a skin. The product packed in skin packaging is fixed on the base; sensitive technical parts can also be securely packed. Less damage can be expected during logistics and handling. Food, like pieces of meat, also looks attractive.

For nonfood products, the combination of carton and flexible polymer film is cho-sen. For food products a carton is not allowed direct food contact. In this case, the bottom material is also a polymer plate or tray—a vacuum is created sideways through suitable nozzles.

The product to be skinned should have no sharp corners, and the shape around which the flexible skin film has to be fixed should be without sharp curves. In ex-treme cases the film may tear through tension. For nonfoods this is only an optical problem, but for food it is critical due to leakage. Food deterioration can take place because of oxygen and microorganisms. If the film does not tear but just gets too thin during skinning, corrosion of the film may take place because of some aggressive components of food, like acids, low molecular weight fats, or ethereal oils. These can migrate into the polymer structure and make it brittle. Pinholes appear in a short time. In Fig. 2.31, the principle of skin packaging for food and nonfoods can be seen.

712.2�Sealing of Packages

Skin packaging of nonfood

Flexible film

Product

Table

Carton

IR-Heater

Product

TableCarton

IR-Heater

Skinned film

Vacuum

Step 1: Heating the skin film Step 2: Skinning through evacuation

Skin packaging of food

Flexible film

Product

Table

Polymer sheet

IR-Heater

Product

Table

IR-Heater

Skinned film

Vacuum

Step 1: Heating the skin film Step 2: Skinning through evacuation

Figure 2.31 Skin packaging for foods and nonfoods

�� 2.2� Sealing of Packages

2.2.1� Theory of Sealing

During manufacturing of polymer packaging materials like pouches or closing a pack after filling, they have to be sealed properly. Sealing means the combination of two packaging layers through melting and subsequent reunion into a common layer. When two neighboring layers of polymer melt, the macromolecules diffuse into one another. After cooling, the layers combine into one common layer. The bonding could be interpreted as physical bonding like a hook-and-loop fastener. This type of bonding takes place when closing pouches where both sealing sides are identical. In some cases, only one layer melts and adheres to the other nonmo-lten partner. For example, the lacquer of a die-cut lid melts and combines with the nonmolten bottom cup or tray. There must be a sufficiently high chemical affinity between the layers. This type of bonding could be interpreted as chemical bonding through adhesion.


The bond strength between the partners is called the sealing strength. The bond-ing type, whether physical or chemical, does not automatically determine the seal-ing strength. The sealing strength in both processes can be adjusted between tough sealing, where the pack has to be cut open with some device, and peel sealing, where it can be opened with the fingers.

The quality of sealing is defined by two characteristics: sealing strength and seal-ing integrity. Both are important for successful packaging. They depend on a num-ber of parameters:

� Sealing temperature � Sealing pressure � Sealing time � Profile of the sealing jaw � Type of sealing layer

The sealing temperature influences the melt viscosity that is suitable for proper sealing. If it is too low, the macromolecules of the neighboring layers cannot dif-fuse quickly enough into one another. If the sealing temperature and the sealing time are simultaneously too high, then too much melt may be pressed out of the sealing seam. This will result in a weak seal.

A sealing pressure is necessary to bring both layers close enough that quick diffu-sion of the macromolecules is possible. If, however, the pressure is too high, it may squeeze out melt from the sealing seam, particularly if the temperature and time are simultaneously high. This deteriorates both sealing strength and sealing integrity.

Sealing time renders sufficient heat for melting and diffusion of macromolecules between both layers. It should be as low as possible to increase the machine speed. On fast-moving packaging machines like vertical-form-fill-seal machines (VFFS), the transverse sealing jaws move toward one another at high speed. High-speed video has shown that the jaws in simple machines without electronic regulation repel for several milliseconds after hitting at high speed and then combine again. In such cases the actual sealing time is less than the calculated time. The problem can be eliminated by regulating the jaw speed. Another possibility is rotating seal-ing jaws, which do not repel.

The jaw profile allows intensive mechanical bonding of the layers. A plain profile does not allow high mechanical penetration of the layers into one another. A jaw profile with a staggered construction pushes the layers much deeper and increases the bonding. If patches of liquid product get between the sealing layers, they can be squeezed out through the convex profile of the sealing jaws. For powders, how-ever, a sealing jaw with a staggered profile is better. This enables sealing through the dust. The thickness of the sealing layers should be high.


Finally, the type of polymer determines the sealing strength and integrity. Higher sealing strength is achieved with HDPE or LLDPE because of their molecular struc-ture than with other polymers. Sealing layers of some acid copolymers or ionomers are better suited for sealing fatty foods than a layer of LDPE.

2.2.2� Principles of Heat Generation for Sealing of Packaging Materials

Good heat sealing is necessary both for manufacturing pouches and for closing a pouch, tray, or collapsible tube after filling. Depending on the packaging type or the packaging machine, different types of sealing are usual. The common sealing principles and technologies are described below.

There are four principles used to propagate or create heat at the contact surface of both partners to be sealed (Table 2.2). A metal is necessary to propagate heat through conduction, where heat is transferred quickly to the contact surface. De-pending on their construction, they may have different names. Four common sys-tems are described here: jaw sealing, impulse sealing, hot knife, and hot wire. Heat may also be transferred by convection in a gaseous phase like hot air or a gas flame. In both the conduction and convection principles, the packaging material is heated with a hot source.

Table 2.2 Heat Sealing

Conduction Convection Radiation FrictionJaw sealing Hot air sealing Laser sealing InductionImpulse sealing Gas flame sealing IR sealing High frequencyHot knife UltrasonicHot wire

Radiation enables heating through heat absorption. No physical contact is neces-sary between the source and packaging material. The wavelength of the ray and the absorption criteria of the polymer material determine the heat yield in the sealing layers. The radiation principle is not common in the packaging sector, al-though research has been done.

The fourth principle is friction, which may be an outer or inner friction. In outer friction, the sealing partners are rubbed against one another, and the temperature rises up to the melting point. In internal friction, the molecules are induced to a higher energy level until the layers melt at elevated temperature. It may also be possible that the induced material is a metal like aluminum, which gets hot and melts a neighboring layer of polymer.


2.2.3� Technology of Sealing

Before different sealing systems are described, three important criteria have to be discussed that determine the choice of sealing system. The first one is the art of heating: whether the heating is direct, like conduction or convection, or is indirect, where another type of energy is transformed to heat energy, like in ultrasonic, high frequency, or induction sealing. All systems with direct heating are simple systems and are robust. These are generally cheaper than the systems where another type of energy has to be transformed into heat energy.

The second one is place of heating: whether the heat is created directly at the con-tact layers or the heat has to be transmitted from outside the packaging material to inside. It is always advantageous to have the heat energy directly at the contact layer. Not only unnecessary heating of other parts of the packaging material can be avoided, but also the sealing seam has a better appearance without wrinkles. The temperature distributions in sealing layers are shown in Fig. 2.32.

Heating from two sides Heating from one side

T1 T3T2

Layer 2

Layer 1

T1 T3T2

Layer 2

Layer 1

T1T2

Layer 1

Layer 2

Heating at the sealing layer (one or both sides)

T1 = Room temperatureT2 = Temperature at the sealing layerT3 = Temperature of sealing jaw

Figure 2.32 Sealing layers, temperature distribution

The third criteria is the finishing of the sealing process through pressure: whether it takes place with cold jaws or with heated jaws. In an all-hot-jaw system, the sealed seam cannot be cooled down when the jaws are closed. This means that the seam, just after releasing the jaws, is still hot, the viscosity of the melt is low, and the sealing strength is also low. If the seam is under strain, e.g., through the weight of the product as in a VFFS machine, then it may be critical for sealing properties. Suitable solutions must be offered for this type of machine in order to achieve proper seams.


2.2.3.1� Direct Heating Systems

2.2.3.1.1� Hot-Jaw SealingThis is the most common sealing element in the packaging field (Fig. 2.33). They are very robust and highly durable and are considerably cheap. A bar of alloy metal is used that has both hardness and good heat conductivity. The bar is heated elec-trically with heating elements made of hollow copper or brass cylinders. At the core of the cylinders there is a ceramic stick with a coiled copper wire. The whole system is heated through the electrical resistance of the copper wire. These are also called electrical resistance sealing elements. The coil windings are distributed in a manner that enables a homogeneous temperature distribution along the bar. Sophisticated construction and also thermal isolation are necessary to get a nar-row temperature distribution. The sealing jaw can be furnished with a suitable profile. A VFFS machine has two jaws to heat the packaging material from both sides. A thermoforming machine has one-sided heating, where the heat is trans-mitted through the thin top film. Sometimes the sealing element is rubber that is stuck on an aluminum or brass base. The rubber sealing element does not strain the packaging material, so the seam surface is perfect.

Two drawbacks of this system are that the heat has to be transmitted only from outside to inside, that and the sealing strength of the seam is lowest just after the jaws are removed. The first drawback is solved through construction of a multi-ple-layer packaging material where the outer layer has a higher melting point. When the specialists speak of “Delta T,” they mean the difference in melting tem-perature between the outer and inner layers. A monolayer packaging material like PE is coated with heat-resistant lacquer. The second drawback is solved by avoid-ing any high tension on the fresh hot seam.

Unrolling offlexible web

Web transport

Dancingrolls

Form shoulder

Longitudinal seal(hot jaw)

Vacuumdrawing belt Transverse seal

(hot jaw)

Product filling

Form pipe

Figure 2.33 Vertical form-fill-seal machine with hot-jaw sealing elements, courtesy of Herbert Stotkiewitz, Bosch Packaging


2.2.3.1.2� Electrical Impulse SealingImpulse sealing is also heat element sealing, where heating takes place as an im-pulse only when an electrical current flows through a copper wire or tape. The wire is mounted on a robust bar of steel, which can be cooled with water. This sys-tem has the disadvantage of transmitting the heat from outside to inside. The big-gest advantage is in cooling the seam just after sealing, when the bar is still closed. The seam surface is a plane without wrinkles. In particular, these seams are ideal for shrink pouches. Most impulse sealing takes place through one-sided heating.

2.2.3.1.3� Hot-Wire and Hot-Knife SealingThe wire or knife or other geometry is heated continuously with electrical resis-tance heating. Sealing and cutting of pouches take place simultaneously. Both sides of the pouch along the cut mark are sealed. This type of sealing is usual for simple packaging solutions. This technology is widely used to simultaneously cut and seal small pouches of thin films for low-strain applications. Applications in-clude separating portions of vegetables at discounters.

2.2.3.1.4� Hot-Air SealingAir is heated in this system with a suitable device and diverted directly onto the sealing surface of the packaging material (Fig. 2.34). The most important example is the sealing of collapsible tubes with a polymer sealing layer after filling. The filled tube is introduced into a device to be heated with hot air. The air temperature is around 400°C and diverted through the device along the inner side of the tube.

Exhaust

Air In

Heating Element

Figure 2.34 Hot-air sealing


Afterward the air sweeps along the outside of the tube before it is exhausted. The hot tube is pressed at the next station with water-cooled jaws to seal it. Simultane-ously the outside of the tube can be embossed. The target is to heat the tube inside up to a temperature slightly higher than the necessary sealing temperature so that it is still hot at the next station for proper sealing. The outside of the tube is heated to some 90°C so that the outside can be embossed. The system works so precisely that even tubes with heat-sensitive products like cream or medicine can be sealed in this way.

The system requires strong precautions with a very sophisticated control process because the air temperature is more than 400°C.

2.2.3.1.5� Gas-Flame SealingComposite materials with a polymer sealing layer can also be sealed with a gas flame. Similar to the last process, the heat transfer takes place with the gas flame directly on the sealing layer, which is pressed with two clamps immediately after the heating. It is a high-speed process. In order to ensure that the packaging mate-rial does not burn during a machine stop, the nozzles of flame are automatically diverted from the packaging material. An example of such a system is the manu-facturing of composite carton boxes through longitudinal sealing.

2.2.3.2� Indirect Heating SystemsThe following sealing systems work with indirect heating. The necessary heat for sealing is created from another type of energy, which is then converted to heat en-ergy.

2.2.3.2.1� High-Frequency SealingThe packaging material behaves in this system as a dielectric in a high-frequency electrical field. The frequency of the field is generally 27 MHz. The packaging ma-terial must possess a sufficiently high dielectric dissipation factor (tan Δ > 0.1). Only PVC fulfills this requirement. Nonpolar polymers like polyolefins are not suit-able for this type of sealing. The sealing partners are placed between two elec-trodes under pressure, and then the high-frequency field is created (Fig. 2.35). It is a sophisticated sealing process and can be controlled very well. Examples are transparent boxes of PVC mostly for nonfoods articles. The sealing apparatus is pretty small and can be mounted easily in a production line.


Lower electrode

Upper electrode

Temperature profile

Sealed pouch withmelt bulge

Figure 2.35 High-frequency sealing

2.2.3.2.2� Ultrasonic SealingIn this type of sealing, the energy of ultrasonic vibration is converted into friction between the sealing partners. A generator converts the standard 230 Volt AC into a high-voltage and high-frequency (20 kHz) electrical current. The converter changes the electrical impulse through the piezoelectric effect into a longitudinal ultrasonic vibration of around 20 kHz. A booster adjusts the suitable amplitude transformation of the vibration and guides it to a sonotrode. The sonotrode acts as one sealing jaw and transfers the vibration through the upper sealing partner to the sealing seam (Fig. 2.36). The lower sealing partner is supported on a base called an anvil from the other side.

50 Hz230 V

20 – 40 kHz800 – 1500 V Generator

Booster

Sonotrode

AnvilCup + Lid

Converter

Figure 2.36 Ultrasonic sealing


In order to achieve high efficiency, the sealing partners must have an ideal geomet-ric construction such that the mechanical vibration is converted preferably at the contact position into heat energy. This heat energy melts the polymer and seals under suitable pressure. Soft materials like polyolefins can absorb the mechanical energy quickly and are suitable to be sealed through this method. Rigid packaging materials must have a sharp edge, where the vibration energy is converted quickly to heat. For flexible packaging material, the sonotrode and the supporting base must have a suitable geometrical profile such that the mechanical energy can quickly be converted into heat energy.

Ultrasonic sealing is a unique process that can seal through contaminants, particu-larly fatty products, because the sealing seam can be cleaned through vibration. Sealing by the ultrasonic method is differentiated into near-field and far-field seal-ing. For packaging applications where soft or thin layers are sealed, the distance between the sonotrode and the sealing seam is less than 6 mm. This type of sealing is called near-field sealing. The vibration can be transferred through harder material a longer distance—more than 6 mm. This kind of sealing is called far-field sealing.

2.2.3.2.3� Induction SealingThis type of sealing can be used if the packaging material contains a metal like aluminum. The principle is an alternating magnetic field that induces an electrical field in a metal. The metal gets hot. The polymer sealing layer adjacent to the metal melts (Fig. 2.37). The molten layers have to be pressed to create a sealing layer. This is an effective method of sealing laminates of aluminum or even Al-deposited composites. Particularly for tamper evidence purposes, flexible wads containing aluminum are sealed on glass or PET bottles or jars.

Generator

Inductor

Capwith wad

Entryfilled glass

Exitglass jar closed

Figure 2.37 Induction sealing of glass jars with injection sealing apparatus


Table 2.3 Different Sealing Applications for Packaging Purposes

Application field Usual sealing layer Usual jaw profileGeneral purpose HSL, PE, EVA, EAA LLDPE,

mLLDPEPlain, convex, profiled (vertical, horizontal, diagonal, cross line)

Sealing through liquid dirt: polarized medium nonpolarized medium

Plastomers Ionomers Convex or mixed profile

Sealing through fat Ionomers ProfiledHigh sealing strength HDPE, LLDPE, ionomers ProfiledRetorting application PP-, homo-, random-, and

block-copolymer, HDPE, LLDPEPlain, convex, profiled

Sealing through dust Thicker sealing layer, ionomer, EAA, plastomer

Profiled

�� References

Herschbach and Christof, Windmöller & Hölscher, Schlauchfolienextrusion für anspruchsvolle Verpa-ckungslösungen, 18th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2011)

Johannaber Friedich, Kunststoff Maschinenführer, 3rd ed., Hanser (1992) BASF Ludwigshafen, Kunststoffverarbeitung im Gespräch - Extrusion, 3rd ed. (1986) Personal information, AC-Folien, MuellheimPersonal information, EK-Pack, ErmengerstPersonal information, Constantia Haendler & Nattermann, Hann MündenSchulz Detlev, Huhtamaki, Laminate für spezielle Tubenanwendungen, 13th Stuttgarter Verpackungs-

tage (Stuttgart Packaging Symposium) (2006) Kneer Stephan, Gaplast, Airlessmotion: Delaminierende co-extrudierte Bag-in-Bottle Systeme, 18th Stutt-

garter Verpackungstage (Stuttgart Packaging Symposium) (2011) Schuster Klaus, Rommelag, Technologie mit Bottlepack - Blow-Fill-Seal, 18th Stuttgarter Verpackungstage

(Stuttgart Packaging Symposium) (2011) Personal information, Sven Fischer, KronesPersonal information, Elisabeth Mersteiner, RPC-KutenholzWahl, Marbach, Innovationen und Trends im Bereich Spritzgusswerkzeuge und IML-Technologie 18th

Stuttgarter Verpackungstage (Stuttgart Packaging Symposium) (2011) Geiger Andreas, Neo Pac, Die Tube: Eine hochwertige Lösung für Flüssigkeitsverpackung, 16th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium) (2009) von Carlsburg und Lars, KHS, Optimale Barriere für Premium PET-Flaschen, 16th Stuttgarter Verpa-

ckungstage (Stuttgart Packaging Symposium) (2009) Bisson Peter, Basell, Was ist ESCR? Ermittlung der ESCR bei geblasenen Holhkörpern, 14th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium) (2007) Oepen Sabine, BASF, Chemischbeständige Hohlkörper für Kosmetika, 15th Stuttgarter Verpackungstage

(Stuttgart Packaging Symposium) (2007)

81References

Kempin Lothar, Sara Lee, Migration, Spannungsrisse, Paneling: Praxisbeispiele aus dem Nonfood- Bereich,: Praxisbeispiele aus dem Nonfood-Bereich, 14th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium) (2007)

Siebert Hartmut, ABB Lummus, Chemische Beständigkeit Spritzgussteile, 14th Stuttgarter Verpackungs-tage (Stuttgart Packaging Symposium) (2007)

Heer Uwe, Kiefel, Neueste Trends im Hochleistungsthermoformen Anwendungsbeispiele aus der Verpa-ckung aus PP und PET, 13th Stuttgarter Verpackungstage (Stuttgart Packaging Symposi-um) (2006)

Stehle Gerd, Lebensmittel Verpacken, Milchwirtschaftlicher Fachverlag (1989) Kupfer Reinhard, Tetra Pak, Herstellung und Anforderungen an Siegelnähte bei aseptischen Verpackungs-

systemen, 13th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium) (2006) Ullrich Thomas, Teich, Peelbare Materialien für Kunststoffbecher: Aluminium und Kunststoff ,13th Stutt-

garter Verpackungstage (Stuttgart Packaging Symposium) (2006) Wilke Bernd, Bosch, Einsatz von Ultraschalltechnologie an Schlauch- Beutelmaschinen, 18th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium) (2011) Braun Harald, Rovema, Barrieredicht versiegelte Beutelpackungen mit VFFS-Maschinen, 15th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium) (2008)

�� 3.1� Technology of Converting

Converting means upgrading of a packaging material. A monolayer web (single layer) can be converted to a multilayer composite through a suitable procedure to get better performance. Better performance means better mechanical properties like sealing or puncture strength. It may also mean better barrier properties against light, oxygen, moisture, or aroma or better ESCR (environmental stress crack resistance). Converting may also fulfill marketing aspects like an attractive surface through special printing, lacquering, or a soft touch.

Better performance can already be achieved by blending resins or mixing additives into a basic resin. Also, radioactive treatment of some packaging films offers better mechanical properties, like tensile strength or sealing strength. But generally we understand converting to be the creation of one or more functional layers on a monolayer web to get a high-performance composite. In this chapter we shall deal with the usual converting processes for packaging webs, like coating, lamination, vacuum deposition, or treatment of packaging webs with radioactive rays.

3 Converting of Polymer Packaging (Composite Packaging)

84 3 Converting of Polymer Packaging (Composite Packaging)

3.1.1� Modes of Converting Packaging Material

1. Blends/Additives A mixture of different components that gives the optimum property. Additives are the simplest way of converting and can be done with an extruder, which is necessary to produce a monolayer (single) web. Trials are necessary to optimize the processing and application. A sound knowl-edge of the compatibility of different materials and the legislative regula-tions is necessary.

2. Coextrusion The target is to produce a multilayer web with superior functionality. This involves modification of a basic web with functional layers, like a high bar-rier, a colored layer, or better mechanical properties. In most cases the polymers are not compatible with each other, so suitable tie layers are nec-essary to bond the multilayer structure. The tie material is also supplied as a resin. Each polymer and tie material needs an extruder. The bonding of functional layers takes place with tie layers already in the melt phase. Coextrusion has versatile possibilities of application. Critical layers like a recycled layer should not be placed inside, to avoid direct contact with a product like food (see Section 2.1.1).

3. Coating A basic web called a substrate is coated with a functional layer. The sub-strate may be a polymer film like PET, PVC, or PE or a nonpolymer like paper, cardboard, or aluminum foil. The functional layer may be a sealing layer, a high barrier layer, or a layer with a releasing function. Sometimes two or more coating processes can take place simultaneously to reduce cost, but the machines are expensive.Extrusion coating Molten functional resin is coated on a substrate, mostly as a sealing layer. A primer is necessary for proper bonding of the polymer with the substrate.Lacquer coating The functional polymer is applied as a solution or as a dispersion (insolu-ble). The solvent or dispersion liquid has to be evaporated.

4. Lamination Lamination is an important upgrading process. It means combining two or more webs with a suitable adhesive for better functionality. The webs are also called substrates. They can be polymer film, paper, cardboard, or metal foil.Extrusion lamination The webs are bonded with a layer of molten polymer.Dry lamination The webs are bonded with a water-free (organic solvent) adhesive.Wet lamination The webs are bonded with water-based adhesive.

5. Vacuum deposition The functional material is evaporated under high vacuum for vacuum depo-sition on a suitable substrate. Materials are mostly aluminum or an inor-ganic material like SiOx or AlOx. These are also called target materials.

6. Radiation Upgrading of polymer packaging webs or flat tubes can also take place through radiation. Properties like tensile strength or sealing strength can be increased through tailor-made radiation with beta-rays.

7. Foaming Upgrading of packaging webs or injection-molded parts can take place through foaming. The weight of the pack can be reduced, stiffness can be increased, and in some cases a whitening effect is possible. There is cost reduction of waste through weight reduction.

853.1�Technology of Converting

3.1.2� Technology of Coating

A substrate is coated with one or more suitable functional solutions or dispersions. The substrate may be a polymer film like PET, paper, cardboard, or aluminum foil. A few typical terms or nomenclature that are used during the coating or lamina-tion process are defined below.

Adhesion Bonding between two different layers or websAir knife Air from a longish nozzle used to scrape off surplus lacquerBond strength Ultimate adhesion strength between coated layer and substrate or

between two substrates after coating or laminationCoating Create a layer on a substrateCoating amount Amount of lacquer or adhesive in g/m2 on a substrate.Cohesion Bond strength in a layer of adhesive or web (tensile strength)Corona treatment Adjustment of surface tension through electrical dischargeCuring Cross-linking reaction of prepolymers to an ultimate net-like structure like

polyurethane adhesives or UV curingCuring time Time for curing. Depends upon adhesive type and curing temperatureDoctor knife A blade with which surplus lacquer is scraped off from the substrateDrying Removing the solvent from adhesive or lacquer with hot airGreen tack Bond strength between substrates just after lamination before curingHot melt Mixture of molten wax and polymer of lower molecular weight for sealing

or adhesive purposesLaminate strength Bond strength between two substrates after curingPrimer Agent used for increasing adhesion between adhesive and substratePot life Time up to which the viscosity of a two-component lacquer or adhesive is

low enough for processing. Processing is not possible after this time as the viscosity gets too high through cross-linking

Residual solvent Traces of solvent in a composite packaging material after curing or drying. It should be as little as possible. There are legislative limits on it

Substrate Film, sheet, paper, board, or foil to be coated or laminated

Generally a substrate is treated with a corona to modify the surface tension so that the wettability of an adhesive or coating liquid is increased. This means that the adhesion force of adhesive or coating liquid on a substrate increases. If the adhe-sion is still poor, then a suitable primer is used on the corona-treated side. The bond strength of adhesive on substrate increases.

3.1.2.1� Extrusion and Coextrusion CoatingFor extrusion coating, a polymer is melted in an extruder and squeezed from a coat hanger die on the substrate (Fig. 3.1). The layer thickness is generally 20 to 60 μm. With modern technology a layer thickness of 10 μm is possible. Such thin layers of polymers are an alternative to lacquer coating, which generally has a thickness


around 5 μm. The substrate is treated with corona or coated with primer to offer better adhesion for the coating material. The primer must be dried to get free from the solvent. The substrate is pressed with the coated layer between nip rolls. One nip roll is rubber coated, and the other roll is a highly polished, chromium-plated steel roll, and is cooled. The hardness of the rubber layer must be exact to ensure a constant pressure at every position on the substrate. The composite is then rolled up. The extrusion-coated layer is usually the sealing layer of the composite.

Winding upUnwindingSubstrate 1Paper, Carton, Al, Laminate, Polymer film

ExtruderPE, PP, PA, PET, Copolymers

Chill roll

Coating– Sealing function– Seal-peel-function– Protection layer– No curing

Figure 3.1 Extrusion coating

For coextrusion coating, the number of molten layers is more than one and the same for the number of extruders. Usually a tie layer is molten to ensure high ad-hesion of the main coating layer with the substrate. Generally no primer is neces-sary.

3.1.2.2� Coating with LacquerLacquer means a solution of polymer in a suitable organic solvent. The solvent ad-justs the viscosity of the solution for coating. After coating, the solvent is dried out so that a layer of the polymer material develops on the basic substrate (Fig. 3.2). This type of coating is usual in the packaging field and is used for versatile pur-poses. Typical applications are listed in Table 3.1.


Winding up

Unwinding

Coating unit

Cooling

Dryer

(80°C – 300°C Air)

Figure 3.2 In lacquer coating, the solvent has to be removed in a dryer

Table 3.1 Typical Lacquer Applications

Type of lacquer Examples of functionPrint protection lacquer Mechanical protection of printing inkPrimer lacquer Lacquer for better adhesionInner lacquer Lacquer in Al tubes or cans to avoid chemical reaction between

the product and packaging materialHeat seal lacquer For heat sealing between two partners. Generally on die-cut lid to

seal on cups or traysSpecial lacquer Special effects for sales promotion

Depending on the application there are different types of lacquer, including sol-vent based, solvent free, water based, dispersion, UV curing, electron beam curing, high solid, and low solid. The system chosen depends upon the application of the packaging material.

A lacquer is generally formulated with following ingredients:

� Polymer resin, also called binder � Solvent, mostly organic liquid but sometimes also water to dissolve or disperse the resin

� Pigments, or coloring agents of inorganic basis � Coloring agents of organic basis � Organic fillers for special effects, e.g., to adjust the viscosity � Different additives for different properties like quick drying

The binder of a lacquer may be of different types: a cellulose nitrate, acrylate, or a two-component polyurethane system is used in a primer or in a print protection lacquer. Heat-seal lacquers (HSL) are generally acrylate, PVC copolymer, and ter-polymers.


There are different roller arrangements on coating or lamination machines. De-pending on the application, there may be a double roller or reverse roller, with or without an air knife. Depending on the desired thickness of the coated layer, the number of rollers is different. For solvent-free adhesives with very thin layer thick-nesses of only 1 to 2 μm, generally four-roller systems are used to generate a homo-geneous layer.

3.1.2.3� Coating with Polymer DispersionDispersion lacquers are insoluble mixtures of a solid polymer in a liquid, most of-ten water. Fine polymer particles are dispersed with stabilizing agents in water. The stabilizing agents hinder gelation of polymer particles into big agglomerates. A common application of dispersion coating is to convert substrates like PET, poly-ethylene, or paper with PVdC into high barrier composites. In comparison to EVOH as a high barrier material, PVdC offers a high barrier both against oxygen and moisture. Because PVdC is not sensitive to moisture, it can be coated on the out-side of any substrate and can be used in any climate.

The dispersions are unstable systems and are homogenized with an emulsifier. At very low temperatures, gelation may take place, which destroys the dispersion. It is important to handle a PVdC dispersion at temperatures around 10°C or higher.

Another important factor is the drying process of dispersions. Because a lot of wa-ter has to be evaporated and the boiling point of water (100°C) is higher than the boiling points of common organic solvents like ethyl alcohol (78°C) or ethyl acetate (77°C), more energy is necessary for evaporation. Generally an infrared radiator dries the substrate just after coating and before it is diverted into the dryer. The temperature of the hot air in the dryer should not be too high. The substrate may shrink critically. Moreover, the shrinkage may be so great that the calculated num-ber of roll stocks cannot be cut out of the mother roll. Particularly for substrates like LDPE, EVA, or ionomers, it should not exceed 70°C. One should be cautious also for substrates like CPP or BOPP. An ideal PVdC dispersion coating arrange-ment can be seen in Fig. 3.3.

PVdC latex has to be stirred continuously and pumped through a suitable filter to free the dispersion from bigger agglomerates. Afterwards the dispersion is circu-lated through the coating pan to avoid any gelation in the system. A suitable de-foaming agent has to be applied to avoid foaming during coating, or otherwise the coated layer is not homogeneous. The concentration of defoaming agent should not be too high because it can disturb proper adhesion of the PVdC onto the primer layer.


Figure 3.3 PVdC dispersion coating, courtesy of Matthias Huter, Solvay

In all coating or lamination processes, the substrate has to be corona treated; in particular for polyolefin substrates it is very important. Polyolefin films must have already been corona treated during production on a blown or chill roll line. A sec-ond treatment of corona on the same side of the film at the coating line is neces-sary. Trouble in bond strength arises if these requirements are not fulfilled.

It is less critical to coat a substrate with PVdC dispersion to make a high barrier web than an extrusion or coextrusion coating with molten PVdC. Although the thermal stability of modified PVdC resin with a suitable comonomer is better, or a suitable tie-layer can reduce the thermal dissociation of PVdC, coating with a dis-persion can be considered to be a more secure process than extrusion.

3.1.3� Technology of Lamination

In order to fulfill all requirements in packaging a product, different webs are com-bined into a composite. One possibility is a coating, which has been discussed. Another procedure is lamination in which two or more substrates like paper, film, sheet, or foil are combined with a suitable adhesive.

The roll stocks of webs should be absolutely horizontal, without waves, the cut sides must be straight without telescoping, and the web tension should be tight and homogeneous. If a substrate is not polarized enough like PVC, it has to be co-rona treated. During corona treatment the same side of the web has to be treated that was already corona treated during manufacturing of the web, e.g., blown film.


If corona treatment had not been done during manufacturing of a polymer film, a treatment before coating or lamination does not help much. A critical fault is to treat the false side of the web with corona. First, the grade of corona treatment is not high enough, so the bond strength is insufficient, and second, the other side, which was selected as the sealing side, would not seal properly.

The choice of adhesive for a particular lamination depends mainly upon the pair of substrates to be laminated and the mode of use of the laminate. All of these criteria influence the laminate strength of a composite, their technical applicability, and also the question of migration of different components of the adhesive into the packaged food.

Another critical case is lamination on the printed side. Particularly in triplexes (three-layer composites) like PET/Al/PE 12/8/60 μm, the back side of the PET film is printed first, which is called reverse printing. The PET film itself gives some mechanical protection to the printed matter. The adhesive is fixed on both the PET film and also on the printing ink. Therefore, the laminate strength is not only de-termined by the compatibility of the adhesive between PET and Al, but also for its compatibility between ink and aluminum. Laminates for flexible pet food pouches of the type PET/Al/PP are manufactured in this way. The pouches are even retorted after filling at 125°C for 30 min. All of these criteria must be fulfilled for the suc-cessful manufacturing of such composites.

3.1.3.1� Extrusion and Coextrusion LaminationIn these cases the adhesive is a molten polymer layer from an extruder. In order to coat the substrates better with the molten polymer, the viscosity of the melt is kept as low as possible at higher temperatures than the usual extrusion temperature for film manufacturing. For example, the temperature of an LDPE melt during manu-facturing of a blown film is about 170°C. The temperature of molten LDPE for ex-trusion lamination is some 300°C—at the limit of its chemical stability. Moreover, the distance between the coat hanger die and the gap between two substrates on the rolls is as high as possible so that the melt can react with the oxygen of air to create polarized groups like aldehyde (–CHO), ketone (–C=O), or acid (–COOH) for better bonding. A better solution can be achieved with acid copolymers like EVA or EAA because their polarity is much higher than that of LDPE.

Still, in most cases a primer is necessary on one or both substrates for better adhe-sion. Substrates also need a corona treatment when necessary. Bond strength of extrusion or coextrusion laminated composites is indeed acceptable but lower than with a PU adhesive. For critical applications like retorting, the laminates are al-ways with a PU adhesive.

An important advantage of extrusion lamination is that no curing is necessary. The composite is finished as soon as the melt has cooled down to room temperature.


Further advantages of extrusion lamination are the freedom from the solvent of the composite and high-speed production. The processing speed of carton laminates is around 800 m/min.

A sketch of the principle of extrusion lamination can be seen in Fig. 3.4. Coextru-sion lamination can take place in the same way. The only difference is the number of melts and accordingly of extruders.

Winding upUnwindingSubstrate 1Paper, Carton, Al, Film

ExtruderPE, PP, copolymers

UnwindingSubstrate 2

Paper, Carton, Al, Film

Chill roll

Figure 3.4 Extrusion lamination

3.1.3.2� Dry Lamination, Solvent BasedThe word “dry” is used in many companies and means simply “free of water.” The solvents are organic compounds like ethanol or ethyl acetate. Depending on the number of substrates, the composites are called duplex (two substrates) or triplex (three substrates) and so on. Examples for duplex are Al/PE and paper/wax and for triplex PET/Al/PE and paper/Al/PE. The number of substrates may be higher de-pending on the application. In order to vacuum-pack sharp-edged products like a knee prosthesis, some seven layers of composite with one or more nylon layers are used. Nylon offers the highest puncture resistance among the packaging polymers. X-ray plates are packed in a seven- or eight-layer composite: one of the layers is


with Al for absolute light resistance. Also, composites for packing photographic film have multiple layers and are absolutely light resistant.

Adhesives for dry lamination are almost always polyurethane (PU)-based and may be one or two components. The component in a one-component adhesive is isocya-nate. It reacts with the “OH“ group of moisture from air and from a substrate like paper that contains sufficient moisture. The most usual however is two compo-nents (Fig. 3.5). The first component is the isocyanate, and the second component is “polyol” (multiply terminated alcohol). The components and their reaction mech-anisms are complicated, but they can be simplified as in the following two charts. The one-component adhesives actually have two components because moisture is necessary for the ultimate cross-linked polyurethane macromolecules (Fig. 3.6). Similarly, two-component adhesives have actually “three components” because be-sides polyol, the moisture (the –OH group) from the air reacts with isocyanates to undergo complex reactions. Thus, the three components are isocyanate, polyol, and moisture. In tropical countries the influence of moisture during the monsoon months is significant; the laminate strength during the monsoon months is signifi-cantly higher.

I

II

R OH + NCO NH C O R

O

HO R OH + OCN NCO OCN NH C O R O C NH NCO

Alcohol Isocyanate Urethane

Urethane, NCO-TerminatedOO

BifunctionalAlcohol

BifunctionalIsocyanate

Figure 3.5 Typical reaction mechanism of isocyanate-based, two-component adhesives; Reactions I and II show only the reaction mechanism. The reaction products have much less mol. wt. The above reactions must propagate further to create “prepolymers” of reasonable mol. wt. to be processed on a machine to produce very high mol. wt. cross-polyurethanes


H O H+ NCO NH2 + CO2

Water Isocyanate Amine

NH2+ NCO

Amine Isocyanate Urea Derivates

NH C NH

O

NH C NH

O

NCO+ N C NH

O

C O

NH BiuretUrea Derivates Isocyanate

Figure 3.6 Cross-linking reaction of a one-component PU adhesive. The reaction with OH-group takes place also during 2C adhesives in competition with other OH-groups. The reactivity of Isocyanate with other groups is as follows: aliphatic amine > NH3 > aromatic amines > aliphatic urea > primary alcohol > secondary alcohol > water > aromatic urea

The adhesive components are called “adhesive” and “activator” and are supplied as “prepolymers” with a viscosity that is suitable to be coated by rolls. Prepolymers are partly polymerized components from monomers and have higher molecular weight. After lamination, less time is necessary for them to get to the final cross-linked structure. Converters mix the prepolymers according to the recipe that the supplier suggests. For example, Liofol LA 3640 (previously UK 3640) adhesive and LA 6800 (previously UK 6800) activator from the company Henkel are mixed in a ratio of 50:1. As soon as the components are mixed, the cross-linking reaction starts and the viscosity of the adhesive increases. If necessary, the viscosity of the mixture is adjusted with organic solvents for proper machinability.

The supplied components (adhesive and activator) may be solvent-based (SB) or solvent-free. The molecular weight of prepolymers in solvent-based applications is higher, as is the viscosity, than in those that are solvent-free. The solvent dilutes the prepolymers to the optimum viscosity for machinability. However, the solvent has to be removed through drying, not only because it has no influence on bond strength, but also because traces of solvent may be unhealthy for humans when used to pack food. There are legislative limits on residual solvent that have to be maintained.


Prepolymers of solvent-free adhesives have lower viscosity and can be worked on the machine without solvent. There is no need to remove any solvent, and the ma-chines have no dryer. Due to the lower molecular weight and lower viscosity of the adhesive, the machines for solvent-free lamination are much more sophisticated than solvent-based machines. Moreover, solvent-free adhesives need a longer curing time because the prepolymers have a lower molecular weight than the solvent-based. In order to reduce the curing time of solvent-free adhesives, new generations are supplied using a higher molecular weight prepolymer. The suitable viscosity for machinability is adjusted by heating the adhesive. The application temperature may be as high as 90°C. Because isocyanates are harmful to health, an appropriate ex-haust system must be installed to maintain a proper working environment.

The ultimate adhesive after curing is cross-linked polyurethane with a high molec-ular weight, and it offers very high bond strength. The curing time is generally 7 to 14 days, depending on the humidity and temperature of the storage room.

Cross-Linking Reaction of Two-Component PU AdhesiveThe solid content of a solvent-based adhesive is between 30 and 60% and deter-mines how much solvent has to be evaporated. The layer thickness of the adhesive after drying is generally 3 to 5 μm. The lower value is for general-purpose packag-ing application. These are packages for which the application is not critical. The coating weight of the adhesive is higher if the application is critical, as in retorting or packing aggressive products with a long shelf life.

The advantages of solvent-based lamination are many, like higher “green tack” (bond strength just after lamination) due to higher molecular weight prepolymers. The handling of roll stock is easier than in solvent-free lamination. The fluctuation of web tension during winding does not cause any technical disadvantage. The curing process is quicker than in solvent-free processes. The bond strength of the laminate is the highest and hence is suitable for all applications. Full and quicker curing takes place at elevated temperatures (30 to 40°C) because the speed of the cross-linking reaction is higher. Particularly for critical laminates like retortable pet food, this is very important.

Disadvantages are, first, the higher amount of adhesive necessary for this process, and second, the removal of 100% of the solvent is pretty difficult. The removed sol-vent has to be handled properly according to legislative rules and regulations.

In processing (Fig. 3.7), the substrate with a higher mechanical or thermal prop-erty is selected for coating with the adhesive. The second substrate is laminated on the first one after the adhesive has been dried. Proper corona treatment for each substrate has to be done before lamination. A primer is used if necessary. The web tension and web propagation mode of the substrates through the machine have to be adjusted. The substrate should not float sideways, and its tension should not


fluctuate. The adhesive is coated on one substrate with rolls from the adhesive pan. The adhesive viscosity has to be checked regularly and is adjusted through dilution with solvent when necessary. The dryer temperature for solvent removal is around 70°C. The second substrate is pressed with the second substrate between nip rolls on the dried adhesive, which has high tackiness. The laminate is wound up on roll stock after cooling. All films must be cooled down before winding up, otherwise deterioration of the roll stock takes place through severe heat tension.

Winding up

UnwindingSubstrate 1 (Al, Film)

Coating unit2C-Adhesive (PU)1C-Adhesive (PU)

Dryer

UnwindingSubstrate 2(Al, Paper, Film)

Figure 3.7 In a dry-lamination system, the solvent has to be removed in a dryer

Examples of SB-laminated composites, particularly for heat application, like pasteur-izing, retorting, or microwave applications:

Tuna: PET/Al/OPA/CPP Rice: OA-PET/OPA/CPP (microwaveable)Pet food: PET/Al/OPA/CPP Soups: PET/OPA/CPP

3.1.3.3� Dry Lamination, Solvent-Free AdhesiveSolvent-free adhesives are 100% solid prepolymers. There is no solvent in the ad-hesive. The prepolymers have a lower molecular weight than in a solvent-based system. The viscosity cannot be adjusted for machinability with any solvent. That is why the components of the adhesive are dosed directly between the first two coating rolls through a nozzle. There is no adhesive pan in this system. The cur-ing time is generally longer than for solvent-based adhesive. Prepolymers with


moderate molecular weight are processed at elevated temperature (70–90°C). Machinability is better. The layer thickness is only 1–2 μm. The machines are much more sophisticated than SB machines. The green tack is lower than sol-vent-based adhesive due to lower molecular weight prepolymers. The handling of the mother roll is difficult because of the lower green tack.

Advantages are that less adhesive is necessary due to the lower thickness, they are environmentally friendly because of freedom from solvents, and there is no prob-lem with residual solvent in the laminate.

The disadvantages are high machine cost, low green tack, difficult handling of rolls, and the isocyanate vapor at 70–90°C must be removed quantitatively. The laminate is not suitable for critical applications like retorting because the bond strength is lower than in SB systems.

In processing (Fig. 3.8), a substrate with a higher mechanical or thermal property is selected for adhesive coating. In a composite with Al like PET/Al/PE, the adhe-sive is always coated on Al foil. The PET or PE is guided as the second substrate onto the Al-foil. Corona treatment of substrates must be done and if necessary also primer coating. As already mentioned, the adhesive is dosed from a nozzle directly between the heated rolls, and lamination occurs at heated nip rolls with a second substrate. The laminate has to be cooled down at a cooling roll before winding up.

Winding up


Cooling


Coating unit2C-Adhesive (PU)1C-Adhesive (PU)

Figure 3.8 In solvent-free lamination, no dryer is needed because there is no solvent


A few examples of SF-laminated composites are

Coffee, tea: PET/Al/PE Snacks: OPP/M-OPP white or M-PET/PECandies: OPP/OPP-A white/cold-seal Biscuits: acrylic lacquer/OPP-A white

Table 3.2 Comparison of Solvent-Based and Solvent-Free Lamination

Criteria Solvent-based lamination Solvent-free laminationMolecular weight of prepolymer high lowViscosity of adhesive high lowGreen tack high lowTackiness long shortSolid content 30–60% 100%

Coating amount, wet 5–8 g/m2 1–2 g/m2

Coating amount, dry 3–5 g/m2 1–2 g/m2

Cost of raw material high lowElasticity of adhesive layer high lowWinding tension of roll broad narrowCuring time 7–14 days (20°C) 7–14 days (20°C)Coating system Adhesive in pan + dip rolls Adhesive between rollsTemperature of adhesive Ambient temperature 70–90°CApplication of laminates All applications General-purpose only

3.1.3.4� Glue or Water-Based LaminationGlue or water-based lamination is used for composites where a lower bond strength is required (Fig. 3.9). It is applied generally for laminates of paper with polymer film or Al foil. The glue can be starch-based or a polymer dispersion. The system works like solvent-free lamination. Lamination of both substrates takes place just after coating with glue before the dryer. The moisture in the glue has to evaporate through one of the substrates. For this reason, paper is always one partner in such lamination. Examples are paper/Al for wrapping butter, tea, or cigarettes.


Winding up


Coating unitDispersionStarch glue

Cooling

Dryer


Moisture permeable

Figure 3.9 Glue or wet lamination

3.1.3.5� Wax or Hot-Melt LaminationHot melts are a mixture of wax with low molecular weight polymers. The process is comparable to extrusion lamination, but the bond strength is much lower. Like glue lamination it is used for packaging purposes, where high bond strength is not necessary. Similar to extrusion lamination, the laminate is ready for use just after the process. Applications include wrapping butter (print/Al/wax/parchment pa-per) or candies. An advantage is the flexibility of such laminates.

Examples of converted packaging materials1. Lidding film:Print Paper Adh. (SF) PET Adh. (SF) Al Ionomer

50 g 12 μm 9 μm 20 μm

SF = solvent free

2. Film for composite cansPaper PE melt Al Ionomer50 g 12 9 20 μm


3. Lidding for YogurtPrint Paper Adh. (SF) PET met* HSL

50 g 12 μm 7 μm

HSL= hot sealing lacquer* Metallized polyester

4. Stick pack for liquid pharmaceuticals:PET Print Adh. (SF) Al PE melt PET PE layer12 10 20 12 50 μm

5. Lidding for creamUV Print** Al HSL

39 6 μm

** Curing of ink by UV rays

6. Wrapper for champagne bottle***:Al PE melt AI9 20 12 μm

*** Feels during tearing like soft lead foil

3.1.4� Important Features for the Technologist

Technologists in a company in the packaging sector are not only responsible for production, quality management, R&D, or other topics, but they are also responsi-ble for the workers without whom a company cannot run. As soon as a technologist suggests some proposal for better quality or a better production process, he or she should go to the spot and learn about the employees’ working environment. A healthy working environment is very important for these persons because they may work there for tens of years.

A good technologist should realize personally the effect of a good proposal that he or she suggests. Only after that can one realize whether a proposal is feasible. One should discuss the feasibility with the workers in charge at that spot. A few exam-ples are the noise in the workplace, unhealthy vapors of some critical organic sol-vent, or working with some pungent-smelling chemicals. A lot of misunderstand-ing can be avoided and time and money can be saved by planning and working with a sure instinct.

A good atmosphere and a win-win situation between technologists and workers can only be created by avoiding critical situations.


�� 3.2� Vacuum Deposition of Ultrathin Layers

Aluminum foil has proved to offer a very high barrier in composites when lami-nated with other substrates like PET/Al/PE. Aluminum foils with a thickness be-low approximately 18 μm are however not completely pinhole free. Still, foils are applied in composites for high barrier packaging satisfactorily down to a low thick-ness of 6 μm. During handling of flexible packs with aluminum foil, pinholes are created at the folding lines or points. Depending on the numbers of pinholes and their size, the pack may no longer offer a high barrier. In order to solve this prob-lem, the idea of an ultrathin layer of vacuum-deposited Al film was born. Such a film also offers satisfactory barrier results after squeezing during handling.

A functional layer of Al with an unimaginably low thickness of 30–100 nm is de-posited on a flexible substrate under extremely high vacuum to get a high barrier composite. This metallized PET with 12 μm thickness is laminated with a suitable sealing layer like LDPE to make a perfectly flexible composite that can be squeezed during handling without getting pinholes. Basically any material can be deposited under high vacuum on a suitable substrate. Besides aluminum, AlOx and SiOx have also proved to be suitable depositing materials, called the target material for flexible laminates. AlOx or SiOx are oxides of aluminum and silicon with a mixture of two different oxides. For aluminum they are AlO and Al2O3. For silicon they are SiO and SiO2. AlOx and SiOx offer high barrier flexible films for packaging pur-poses, where freedom from metal is a big issue. Examples of packaging applica-tions are pouches for potato chips and coffee and in holographic effects.

Substrates that are deposited with suitable target materials under ultrahigh vac-uum are the usual polymer films for packaging, like PET, BOPP, BON, and PVC. Even paper can be vacuum deposited when necessary. Because the deposition pro-cess is not continuous but discontinous, the substrate should be as thin as possible so that a larger area can be deposited. Because of technical difficulties, the deposi-tion process is not a continuous process.

Besides a barrier effect, target materials also offer optical and susceptor effects in a microwave oven. Aluminum is by far the most-used target material. SiOx is used particularly for transparent packaging with a high barrier effect and also for appli-cations where freedom from metal is necessary. Under both oxides, SiO2 and SiO, SiO is responsible for the high barrier. But films deposited with SiO show a yellow-ish color. SiO2 offers high transparency but no barrier. An interesting property of SiOx is its barrier effect against UV rays, although it allows a large portion of visi-ble light through the film. This is a key feature for transparent packs for sensitive products like food or cosmetics.

The technology of deposition can be of a physical nature, called physical vapor deposition (PVD), where the target material is deposited by heating with electrical

1013.2�Vacuum Deposition of Ultrathin Layers

resistance. Alternatively it can be deposited by an electron beam gun or under a plasma. In physical processes the target material is evaporated or ejected out of a block through the electron beam and is deposited onto the substrate. Chemical depositions are also possible, so-called chemical vapor deposition (CVD), where the target material undergoes a chemical reaction before it is deposited onto the sub-strate.

3.2.1� Physical Vapor Deposition (PVD) Process

Thermal Deposition with Electrical Resistance HeatingThis is the most common process for vacuum deposition of thin layers. The depos-iting machine is a cylindrical horizontal container with two chambers one upon the other (Fig. 3.10). The winding chamber is at the top and has a bit lower vacuum (higher pressure) than the deposition chamber at the bottom. Aluminum wire of high purity (99.9%) is transported to an electrically heated crucible of high thermal stability at around 1500°C. Due to very high thermal strain, the boron nitride (BN) crucibles have an average duration of only 12 hours. The aluminum melts at once and is evaporated from the crucible under high vacuum. The substrate is then transported at high speed along a chill roll just over the crucible, on which alumi-num particles deposit like fish scale. If there are impurities in the aluminum, then it can splash out of the crucible and make a hole in the substrate. The layer thick-ness of the aluminum can be adjusted with different processing parameters. Sub-strates with a high water content, like paper, disturb the process through evapo-rated moisture, which can hinder proper deposition of aluminum onto the substrate. A cold trap in the winding chamber captures the water vapor just after unwinding through freezing. In order to get a higher barrier effect, deposition of the target material is also possible on both sides of the substrate.

4. Second depositing roll5. Second source of depositing material6. Wind up

1. Unwinding2. Depositing roll3. Source of depositing material

Figure 3.10 Vacuum deposition of ultrathin layers with Al, AlOx, or SiOx


Example of Process ParametersSubstrate: 12 μm PET, 2000 mm breadth, 20.000 m longLayer thickness: 100 nmWorking speed: 500 m/minPressure in winding chamber: 10–2 mbarPressure in deposition chamber: 10–4 mbarTemperature of chill roll: –10°CTemperature of cold trap: –120°CSpecific heat on substrate: 40 kW/m2 (condensation), 2.5 kW/m2 (radiation)

Electron Beam Gun DepositionThe machine looks similar to an electrical resistance heating machine. Here there is again a winding chamber and a deposition chamber. The vacuum is not high as in resistance heating. A block of the target material is placed in the deposition chamber. A curtain of high energy electron beam hits the block and ejects out tiny particles to get deposited onto the running substrate on the chill roll. Besides phys-ical deposition, chemical deposition is also possible after a suitable chemical reac-tion. The substrate is transported near the emerging material. The system is very robust (no damage of the crucible like in thermal deposition takes place), but it is very expensive.

Plasma DepositionThe third physical process for vacuum deposition of a thin layer is plasma deposi-tion. The energy necessary to evaporate aluminum is created by a plasma (ionized gas). Also here aluminum can be deposited on the substrate after it has been trans-ferred in vapor form. Not only aluminum but also AlOx or SiOx can be deposited on a substrate in all of the physical processes (electrical resistance heating, electron beam gun, or plasma).

3.2.2� Chemical Vapor Deposition (CVD) Process

The CVD process differs from the PVD process in that in this process the target material undergoes a chemical reaction before it is deposited onto the substrate. It has the advantage that the target material needs a much lower boiling point than in the physical processes, so less energy is necessary. Another advantage is the large number of chemical compositions of the target material after a chemical reaction.

The deposition of SiOx or AlOx from SiO or aluminum after oxidation is actually a CVD process. It is however not very easy to get the exact target material after a

1033.2�Vacuum Deposition of Ultrathin Layers

chemical reaction. The reactive components must have an exact composition, and disturbing reaction products must be removed.

Deposition of a thin layer is not only possible for web-like substrates like film or paper, but also bottles or cups can be vacuum deposited, e.g., high barrier PET bot-tles for beer or other drinks.

Quality Control of Deposited FilmsControl of layer thickness of the deposited layer is necessary to ensure a standard quality. It can be done both in-line and off-line. In-line methods measure the elec-trical resistance of the aluminum layer or the capacity of the aluminum layer or the optical density of the layer.

To measure electrical resistance, the deposited film is passed over two conducting rollers, and the resistance of a square field is measured. The unit is ohm per square. Because the resistance is directly proportional to the length and reciprocal to the breadth, it does not matter how big or small is the square. It is always con-stant and depends only on the thickness of the aluminum layer.

Similarly, the electrical capacity of the deposited aluminum layer is measured be-tween two conducting rollers. To measure the optical density, the intensities of light at the source and at the photocell behind the deposited film are measured. The calculated extinction is proportional to the thickness of the aluminum layer.

All of these three tests are also made off-line in laboratories. A Tesa test or Scotch-bond test is also made to test the adhesion of aluminum on the substrate, as is usual for printing ink.

Important for the quality of deposition are generally pinholes and adhesion of the target material on the substrate. Pinholes are of two kinds; sometimes there are gaps in the aluminum particles in the layer, which covers the substrate like fish scales. This is not so critical. For a very high barrier, double-sided deposition was introduced in the 1980s. If, however, the substrate is molten through ejection of hot drops of aluminum, then it is a bigger issue. Naturally the magnitude of quality deficiency depends upon the number of such pinholes.

Adhesion of the target layer on the substrate can be increased by suitable coating of the substrate by prior deposition.


�� 3.3� Radiation Upgrading of Packaging Material

The properties of polymers can be modified by ionizing radiation like beta-rays and gamma-rays. In packaging technology there are also some applications, particu-larly to enhance sealing strength and mechanical properties like tensile or punc-ture strength. When polymer materials are irradiated, chain scissoring takes place in a random manner because of the absorption of high energy. Radicals are grown. In some polymers like LDPE, the radicals undergo cross-linking and create a par-tially cross-linked rubber-like structure. There are, on the other hand, some poly-mers like PVC, where there is not much cross-linking; instead they degrade and the molecular weight is reduced. If LDPE is radiated up to the proper dose, then properties like sealing strength and tensile strength can be enhanced. Packaging solutions can be solved with thinner films than those not radiated.

3.3.1� Effect of Radiation on Plastics

When plastics are exposed to radiation, then the bonds break statistically through absorption of high-energy rays. This breakage is called degradation. A number of them reunite thereafter through cross-linking. A net-like structure is created. Three different effects can be recognized. In the first case, the cross-linking pre-dominates rather than degradation, which can be seen with LDPE and acid copoly-mers of PE and PS. Mechanical properties like tensile strength and some thermo-mechanical properties like sealing strength increase. Packaging films with lower thickness can be used successfully to pack goods for which nonradiated packaging material with higher thickness would be necessary. Because of the lower thick-ness, the stiffness of such radiated films is less, and hence they show a better flex-ible character.

Too high of a dose level, however, can change PE through extreme high cross-link-ing into an elastomer (rubber), which is no longer sealable. PET or PVOH is almost indifferent to radiation. PP, PVC, or PVdC degrades rather than cross-links.

Gamma radiation is not suitable for radiation of plastic films because only films on roll stocks can be radiated through gamma radiation. The dose of radiation is inho-mogeneous in a roll stock. The outer parts of the roll stock receive higher radiation than the inner part. Beta radiation is successfully used for radiation.

One typical application of beta radiation is on flattened shrink tubes from which shrink pouches are manufactured. The structure of the tube is PE/Tie/PVdC/Tie/PE. Both of the PE layers are generally tailor-made blends, where the outer PE (left

1053.4�Extended (Foamed) Packaging Materials

side) has a slightly higher melting point than the inner one. The application of ra-dioactive curing particularly for shrink pouches has another great benefit. Shrink tubes are generally preserved on roll stocks for a few weeks or even months before they are cut into pouches upon order from a customer. Nonradiated tubes can shrink to some extent if stored at elevated temperature, particularly during the summer months. Beta radiation in combination with a high barrier layer of PVdC hinders such premature shrinking.

In comparison to the above-mentioned five-layer shrink pouches, the EVOH-based and nonradiated high barrier shrink pouches have a sophisticated nine-layer structure:

PET or PA/Tie/PA/Tie/PA or EVOH/Tie/PE/PE/PE

with two or three different nylon layers. One important purpose is to avoid prema-ture shrinking of tubes during storage at elevated temperature.

�� 3.4� Extended (Foamed) Packaging Materials

Foamed packaging materials are dispersion systems of a polymer matrix with air or some other gas in it. There may also be a hollow space without air in a film—sim-ply a vacuum. Soft, flexible foams are manufactured with polyolefins (PO) or plasti-cized amorphous polymers like PVC-P. Hard foamed packaging materials result from nonplasticized amorphous polymers like EPS (extended polystyrene). Foams may have a closed structure, captured under a polymer skin, or an open structure, where the foamed segments can be seen from the outside.

Several targets are achieved through foaming. First, the weight of a package can be reduced and hence its cost. Second, it reduces the cost of packaging waste because of the lower weight, and third, the shock absorbing effect is better than in non-foamed material. Thermal isolation is better, and finally, foamed packaging film like BOPP has a certain whitening effect without the use of a coloring agent, so whitening agent or printing ink can be saved. The density of a foamed material can be as low as 0.4 g/cm3.

3.4.1� Physical Foaming with Gas

Single screw, double screw, or tandem extruders are used to manufacture foamed material through direct gas inlet. Gas with a higher pressure than the melt pres-sure is dissipated in the extruder; high dissipation can be achieved by enlarging the channel diameter. Foamed melt emerges generally through a horizontal annu-


lar die, and then it is cut to produce a flat sheet. The foams from direct foaming are coarse. Foamed sheets can be laminated with nonfoamed film through heat-seal lamination into the required laminates, for example, PP/PP-foam (one foamy side and other side is plain) or PP/PP-foam/PP (foamy layer is sandwiched between in-ner and outer plain layers).

Instead of air, low boiling point liquids like halogenized hydrocarbons such as pro-pane or butane can also be used to make foam. The cell size is lower than from di-rect propagation of gas.

3.4.2� Chemical Nucleating Agents

These are mixed homogeneously with the resins, which when molten, react chem-ically to produce tiny gas bubbles. For example, citric acid and hydrogen bicarbon-ate react to produce CO2. This type of foam has very small particle sizes. The amount of nucleating agent is generally between 0.1 to 1 percent. All thermoplas-tics like PE, PP, PVC, or PS can be foamed with this method.

3.4.3� Foam Extrusion

Generally, all types of extruders can be used to produce foamed material if the fol-lowing requirements are fulfilled:

� sufficiently high melt temperature to fully dissipate the nucleating agents, and � sufficiently high pressure distribution in the extruder to keep the reaction gas in a homogeneous dispersion in the melt.

3.4.4� Foam Injection Molding

Foamed products can also be manufactured through injection molding, but certain requirements must be fulfilled. The closing valve must be absolutely tight to pro-hibit any leak of foamed melt at the feeding channel. The injection pressure is generally lower than for normal injection molding. There is practically no af-ter-pressure in this process. The cooling intensity of tools must be high enough to avoid any after-swelling of the injection-molded piece.

1073.5�Special Topics

3.4.5� Foam Thermoforming

Thermoforming of foamed sheet has regulations similar to foam injection molding. Any type of crushing of the cup wall must be avoided during the application of plugs or pressure. Factors like forming temperature, timing, and speed of pressure application or evacuation are important.

�� 3.5� Special Topics

The packaging process is very complex and mistakes can take place at different stages. Some interesting topics are discussed here in detail.

3.5.1� Sealing through Liquid and Dust

Food or cosmetics packs are not always without fault and sometimes result in com-plaints. The quality of packaging films and the pouches thereof are of good quality. There are seldom mistakes like in thickness distribution, tensile strength, or bar-rier properties. Injection-molded articles are generally of high quality. Wall thick-ness distributions of thermoformed trays or cups are sometimes too low, particu-larly when some manufacturer tries to reduce the cost of production by reducing the wall thicknesses. Generally, two kinds of complaints for packages, particularly for food and cosmetics, arise often: printing mistakes and sealing problems. Print-ing mistakes are mostly of an optical nature and are seldom critical for packaging security. Mistakes in the sealing process, however, can create leakage in the pack, particularly during transport and handling. This means in most cases deterioration of the product. Care has to be taken in this most important step in a packaging process. Pouches are manufactured by sealing on one or more sides with the open side left open for filling. This side is sealed after filling. Cups, trays, or bottle mem-branes are sealed after filling. Closing the seal of a pack after filling is critical, particularly when liquids are packed. In many cases a drop of liquid falls down unintentionally after the dosing is finished and the pack starts moving to the next station on a packaging machine. This drop often falls at the future sealing position where the pack is closed (see Fig. 3.11).


Drop of liquidfat or sauce

Figure 3.11 Unwanted drop of liquid on future sealing position in a tube

When the sealing jaw with a temperature of some 180 to 200°C presses this drop through the top film or die-cut lid, temperature of the drop is raised to a tempera-ture of some 120°C. If we consider the drop as a water droplet, then it means an equivalent vapor pressure of some 2.5 bar. The sealing pressure on the usual pack-aging machine is around 6 bar. At the moment, the drop cannot cause any harm. As soon as the jaw is removed for the next cycle, the drop expands. Depending on its size and the breadth of the sealing seam, it may tear the seal or at least weaken it at this position. During transport or handling, the probability is high that the pack will leak at this position.

This type of staining or even washing of a future sealing seam by the liquid prod-uct is a general case in a vertical-form-fill-seal machine. The product filling tube ends after the longitudinal sealing jaw and just before the transverse sealing jaw. The liquid product is filled in the pouch after the transverse seal is finished. As the liquid is filled, it splashes inside the web tube and washes the tube completely on the inner side. It means that the transverse jaw always seals the wet tube, mostly contaminated with fat or fat-like food ingredients, which actually hinder proper sealing. Precautions have to be taken in the sealing process to still achieve an ac-ceptable sealing quality.

Similar trouble with sealing arises if stains of fat from sausage or cheese or the juice of fresh meat or fish is left on the future sealing area when putting the prod-uct in a tray (Fig. 3.12). Precautions have to be taken to avoid weak sealing at those


positions. It is best to avoid such stains, which is not easy on quick packing ma-chine. The sealing layer should be of a high performance polymer like LLDPE or ionomer of sufficiently high layer thickness. The profile of the sealing jaw must also be correct. The jaw profile on a VFFS machine for liquid packaging should be slightly convex. During pressing on films, such a profile presses out liquid drops from the sealing seam, and a secure seal is possible. Deeply profiled jaws are also helpful; they pierce through the liquid layer to combine the sealing polymers of both sides.

Drop of liquidfat or sauce

Figure 3.12 Unwanted liquid drop on future sealing position on a tray

It is unavoidable during the filling of bulk products to seal dust free. Bulk products are filled through volumetric dosing from the top in a pouch, whether on a VFFS machine or in sachets on a HFFS machine. The packaging process runs on a mod-ern machine at a speed up to 120 pouches per minute or even more. The complete packaging process has to be finished within one-half of a second. Particularly on the VFFS machine, one cannot wait until all of the bulk product from the cup or auger dosing falls into the pack. One could analyze the particle size distribution of a dosed amount during falling and find out the highest particle size at a particular time. The thickness of a sealing layer should be higher than the highest particle size to capture it securely in the sealing seam. Solid particles do not cause any trouble like liquid drops when captured in the seal.

3.5.2� Transverse Sealing of Side-Folded Pouches

Side-folded pouches offer a greater volume for products for comparatively less pouch breadth. During transverse sealing, at the top and bottom, three different layer combinations have to be sealed at a time; see Figs. 3.3 and 3.4. Not only for liquid filling but also for powder filling the transition positions A-B and B-A (Fig. 3.13) are critical. With the same sealing parameters, namely temperature, pres-sure, and time, two layers at B and four layers at A have to be sealed securely. The


layer thicknesses of the pouch OPA/Al/LLDPE in Fig. 3.14 are 15/9/75 μm. The thickness of the sealing layer for these pouches is selected a bit higher than in pillow pouches or four-sided sealed pouches without folding. This is necessary to distribute sufficient melt to cover the transition positions. A jaw with a horizontal profile with a number of hills and valleys also offers the necessary tightness during transport and handling. In order to avoid critical folding of the bottom seam, the filled pouches should not simply be dropped down after cutting but should be transported by a soft conveyor system to avoid any impact through its own weight and hence eventual leakage at the transition positions.

Transverse sealbottom and top

Longitudinal seal

A – four foldB – two foldC – three fold

Figure 3.13 Transverse sealing with three different layer combinations on VFFS machine


Figure 3.14 Transverse sealing of side-folded pouches of OPA/Al/LLDPE15/9/75 μm on a VFFS machine. Photographed by the author

3.5.3� Weak Points of a Collapsible Polymer Tube

Polymer tubes almost always have a circular cross section. When such a filled tube is closed through sealing, the circle is flattened. Depending on the stiffness of the tube wall, which depends upon the specification of the tube material, the diameter of the tube, and the wall thickness (generally 500 μm), there is a certain retention force on the seam. The tubes are heated in a first step with hot air (see Section 2.1.1) and then sealed with water-cooled jaws in the next step. Considering a speed of filling of 60 per minute, there is only some 250 milliseconds of time for sealing. As soon as the jaws move away after sealing, the sealing strength must be suffi-ciently high to counteract the retention force.

Just at this seam and at both sides near this seal there is a sharp fold, which stays continuously under tension. Depending upon the specification of the product, there may be a pretty high environmental stress crack probability. This is unavoid-able for tube filling. The specifications of the tube material and the tube geometry have to be selected to get a high ESCR so that no cracking takes place at this fold during shelf life and handling.


In order to dispense the product, a tube always has to be squeezed. The shoulder seam must also be sufficiently good to function after multiple pressings at this po-sition (Fig. 3.15).

Sharp fold

Edge pressing duringevery dosing

Sharp fold

Figure 3.15 Weak points on a collapsible tube

3.5.4� Pinholes in Packs

Pinholes in a pack hinder proper function of a barrier effect. Pinholes can arise first in the web, whether monolayer or composite, and second in the sealing seam. A third possibility is during severe handling through squeezing. In particular, Al foil in a composite can create pinholes at folding corners. As much as possible pinholes must be avoided, particularly during sealing. By choosing the proper sealing layer, sealing parameter, jaw profile, and breadth of seam, pinholes can be avoided. Pin-holes on packaging webs are not always possible to avoid completely, particularly in composites after lamination. Thin Al foils under 10 μm have a number of unavoid-able pinholes. In a composite like PET/Al/PE, the pinholes on Al foil can be identi-fied after lamination of Al with PET. Adhesive emerges through the pinholes be-cause of the pressure from nip rolls. These spots can be viewed with a loupe. They look like tiny craters. Bigger craters mean bigger pinholes and smaller mean small pinholes.

In order to judge the loss of barrier effect through pinholes, they have to be inves-tigated. Theoretically a pinhole may exist only in a single web, which could be Al, PET, or PE. The other two webs would offer a certain barrier effect, so it is not too


critical. A pinhole through two webs is not always critical. Even a pinhole through all three webs might not be critical. It depends upon the size of the pinhole. A rea-sonable method is to regularly measure the permeation value of a composite and store the sample. If trouble arises through abnormally high permeation, then the pinholes of that particular sample have to be compared with the standard sample.

Pinholes and the loss of barrier effect are also influenced by the packaging ma-chine, particularly by a VFFS machine. The condition of the shoulder, its rough-ness, and its surface have to be optimized.

In Table 3.3, the significance of the size of pinholes can be seen. For comparison sake, a human hair is 50 μm.

Table 3.3 Pinholes in Packages (Tightness Class VDMA 2006 / No. 13)

Pinhole size(μm) Significance<200 Bulk tight<100 Insect tight<30 Dust tight<25 Liquid tight<20 Moisture tight<1 Gas tight

3.5.5� Complaint Management for New Technologists

Although quality control concepts try to avoid faults in a production process, mis-takes happen in every production process. In spite of all efforts, it is unavoidable that the customer gets faulty products and complains about it. Generally, a com-plaint is directed to the sales department of the supplier from which the customer has bought the material. An experienced sales manager decides which complaint could be handled by the sales department itself and which ones have to be investi-gated thoroughly, and they are passed to the responsible department.

Good companies take care that the customer should not wait long with a complaint. Renowned companies assure their customers worldwide that within 24 hours after the supplier is informed about the complaint, a professional service person will arrive. Generally, an experienced person is sent to the customer to investigate and solve problems. Sometimes a new technologist is sent to the customer, as more ex-perienced persons are currently not available. This is a challenge and simultane-ously an opportunity for less experienced people to prove that they are able to deal alone with critical cases.


He or she must take time for preparation and should learn as many details as pos-sible about the case before visiting the customer. He or she should analyze the case with some experienced person and take note of the possible scenarios of how to investigate and hopefully solve it: at least recognize what has caused the trouble and what is necessary to repair or replace it. It is a mistake to arrive at a custom-er’s business without exact knowledge of the complaint.

In many cases the customer is frustrated, has had losses in production, and is not in an amused state. The attitude toward the service person may be harsh. One should not feel disappointed or even insulted but instead try to understand the situation of the customer as he is suffering from some mistake for which he is not to blame.

During analysis of the case, one will notice how good the preparation was for the problem. The more confidence one has can be important for troubleshooting. The customer must realize that the supplier’s technologist has reviewed the case and is doing his best.

Good complaint management keeps the business with the customer in good condi-tion, which is very important for the supplier. Well-known companies know that their products are sold mostly by reputation and not only by the sales department. Satisfied customers are the best references for new customers. Professional and successful technologists are often rewarded by management, as well.

�� References

Koblischek Alfred, Amcor, Lehrgang Kunststoffverpackungen, Stuttgart, (2012)Meckel-Jonas Claudia, Henkel, Henkel Technologies, 12th Stuttgarter Verpackungstage (Stuttgart Pa-

ckaging Symposium), (2005)Obermann Uwe, Amcor, Folienverbunde für kritische Füllgüter - Stabilität und Materialeigenschaften, 13th

Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2006)Schrägle Matthias, Huhtamaki, Innovative Entwicklungen bei flexiblen Verpackungen, 18th Stuttgarter

Verpackungstage (Stuttgart Packaging Symposium), (2011)Spaeter Helmut, Cavonic, 3D Coating, Barriere-Technologie, 18th Stuttgarter Verpackungstage (Stuttgart

Packaging Symposium), (2011)Koblischek Alfred, Alcan, Überblick Al-Verbunde für Flüssigkeitsver-packung, 16th Stuttgarter Verpa-

ckungstage (Stuttgart Packaging Symposium), (2009)Junge Stefan, Sika Chemie, Praxisbeispiele & Trends für starre & flexible Kunststoffverpackungen für

Haushalts- und Bauchemikalien, 16th Stuttgarter Verpackungstage (Stuttgart Packaging Sym-posium), (2009)

Vetter Oliver, Alcan, Transparent inorganic barrier films for packaging Applications, 15th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2008)

Aymar Felix, Henkel, Herausforderungen und Erfahrungen bei der Entwicklung eines Verpackungssys-tems für eine sehr reaktiven Polyurethan-Klebstoff, 14th Stuttgarter Verpackungstage (Stuttgart Packaging Symposium), (2007)

Index

A

acetaldehyde 14acrylic 97additives 5adhesive 89, 92air knife 32Al2O3 100Al barrier laminate 37AlOx 100aluminum 36amine light stabilizer XVIamorphous XVII, 3antiblock 6antifog agents 6antimicrobial 8antioxidants 5antistatic 7aromatic 5aseptic 69atactic 13auger dosing 109

B

bags 12barrel 19barrier 22, 25biaxial-oriented XVblends 5blocking 30blowing agents 8blown film 27blow-up ratio 28

BOPA / BONy) XVbottles 12, 14boxes 77branching 3brittle 4bubble 26

C

calendering 33caps 49cast films 46cast sheets 33catalysts 5cavity 49ceramic barrier laminate

37channels 44chemical resistance

(ESCR) 8chemical vapor deposition

102chill roll 31clarity 16closure 39coat hanger die 21coating 84coefficient of friction 6coextrusion 84COF XVcohesion 85coinjection 51

collapsible tube 112colorants 7composite cans 98composites 79compounding 4compression 19condensation 102conduction 63, 73converting 83copolymers 90core 28corona treatment 30, 85cosmetics VII, 107covalent bonds 29CPP 88craters 112cross-linking 93crystalline 3crystallites 3Curing 85curtain 102

D

deformation 4degradation 5density 9deposition 14die 24die-cut lid 70dielectric 77diffusion 29

116

dimensional stability 46dispersion 84, 85double-bubble 42drape forming 70dry lamination 84duroplastics 4

E

EAA 11elastic 46elasticity 61elastomer 104electron beam gun 101electrostatic 39elongation 60environmental stress

crack resistance (ESCR) 83, 111

epoxy phenol 36EPS XVIerucamide 7ESCR 83, 111ethylene–acrylic acid co-

polymer 11ethylene–butyl acrylate

copolymer 11ethylene–methacrylic acid

XVIethylene–vinyl acetate 11EVOH 14, 15extruder 17extrusion 19extrusion blow molding

54

F

fatty acid 5feed block 24female (cavity) 65fillers 65film 89filter 88

flexible 90, 100 – packaging 13

flow 26fluoroelastomers 48fluoropolymers 48foam extrusion 106foaming 84foam injection molding

106foam thermoforming 107fog 6foil 11, 37food packaging 42, 44friction 73frost line 27functional layers 55funnel 21

G

gamma radiation 104gas flame 29gelation 88glass transition tempera-

ture 61glue 97glycol XVIIgreen tack 85grooves 22

H

HALS XVIHCl XVIHDPE XVIHDT XVIhead 4heat seal lacquer 87HFFS 109HIPS XVIhot air 42hot fillable 16hot knife 54hot melt 85hot tack 10, 13

hydrogen bonding 2

I

impact 110induction sealing 74infrared XVI, 88injection blow molding

53injection molding 50in-line thermoforming 68in-mold labeling (IML) 55inner sealing layer 30ionomer 40, 73isotactic 13isotropic 46

K

kaolin 67

L

label 15lacquers 29lamination 37, 84lap seal 37latex 88LDPE XVIleakage 37, 70lips 31LLDPE XVIlongitudinal sealing 35lubricants 6

M

machine 6macromolecules 1, 12magnetic 79male (positive mold) 65mandrel 38MAP 40masterbatch 5MDPE 9

117

medical 14melt flow 6, 51melting point 61melt temperature 51, 61metallocene XVImetering zone 20migration 25mixing 34mLLDPE XVImodulus 61moisture 83molding 54molecular weight 61monomers 93MXD6 XVI

N

NaOH 13neck-in 31needle closures 51negative forming 65nip rolls 27, 86nonpolar 77nucleating agents 106

O

oleamide 7opaque 7OPLA XVIOPS XVIorientation 3, 42oxidation 102ozone 29

P

PA XVIPAN XVIpaneling 81parison 55peel sealing 72permeation 113PET XVII

PET-A XVIIPET-C XVIIPET-G XVIIpharmaceutical blisters

15phenols 5phosphites 5phthalates 15pigments 55pinhole 100plasma 101plasticized polyvinyl

chloride XVIIplasticizers 11plastomers 80plug assistance 61polarized 80polyaddition 1polyamide / nylon XVIpolycondensation 1, 60polyester 10polyethylene VIIpolyethylene naphthalate

XVIIpolyethylene terephthalate

XVIIpolylactic acid XVIIpolymer barrier laminate

37polymerization 1polyolefin XVIIpolystyrene XVIIpolytetrafluoroethylene

XVIIpolytetramethylene

terephthalate XVIIpolyurethane 2polyvinylchloride 3positive forming 65pot life 85preforms 14prepolymers 85pressure 86primer 86processing aids 6

propagation 94properties 48PVC-P 105PVD XVIIPVdC XVIIPVOH XVII

Q

quartz 64

R

radiation 64radiation upgrading 104radicals 104reactive 103recycling 28regulations 41relaxation 44responsibility of a

producer 9retention force 111retorting 46roll stocks 41RPM 21rubber 75

S

sachets 109sacks 12scrap 25screw 22, 25sealing 24, 26 – integrity 72 – jaw 72 – layer 72 – pressure 72 – strength 72 – through dust 80 – through liquid 80 – time 72

semicrystalline 3shearing 22

118

sheet 25shelf life 40shrink films 41shrinking 42side-folded pouches 109silicon dioxide XVIIsilicon monoxide XVIIsingle screw extruder 5SiOx XVIIskin packaging 70sleeves 45slip agent 5solid content 94solvent based 87solvent-free 87, 93specific heat 102spectrum 5spherulites 3SPPF (solid phase pres-

sure forming) 61squeezing 35stabilizer 5stiffness 5strain 74strength 74stress crack 111stretch 14stretch blow molding 56surface tension 29swelling 106syndiotactic 13

T

tandem 105tearing 40tensile 40tentering 47thermoform-fill-seal (TFFS)

65, 68thermoforming 48thermoforming window 61thermoset plastics 4tie layer 13, 24tightness 37

TiO2 62tools 21toughness 16transparency 28transverse direction 42transverse sealing 72triple-bubble 44tubes 49twin screw extruder 4

U

ultra low density PE 11unplasticized polyvinyl

chloride XVIIunsaturated 1UV 39, 41

V

vacuum deposition 83vacuum-shrink pouches

41van der Waals forces 2very low density PE 11VFFS (vertical-form-fill-

seal) 72, 109vinylacetate 9viscosity 20volumetric dosing 109

W

waste 49water bath 42water bath sheets 33wavelength 64wet lamination 84winding 94wrapping 97

Y

yield 73

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