1 Carbon Particulate Assisted Extrusion Foaming of ...

1

Carbon Particulate Assisted Extrusion Foaming of Polyethylene Terephthalate (PET) by

Controlled-Hydrolysis for Thermal Insulation Applications

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

the Graduate School of The Ohio State University

By

Junjie Pan

Graduate Program in Chemical Engineering

The Ohio State University

2018

Thesis Committee

Dr. L. James Lee, Advisor

Dr. Jose Castro

2

Copyrighted by

Junjie Pan

2018

i

ii

Abstract

IR-absorbing foams as roofing and external wall materials have gained

considerable interest for their enhanced insulation and sustainability. However, the most

widely used polystyrene (PS) foam in the insulation industry is not feasible for IR-

absorbing application due to its low thermal stability. As a semi-crystalline polymer,

polyethylene terephthalate (PET) is a desirable substitute which has excellent thermal

stability and mechanical strength. Also, PET’s recyclability is very important due to the

severe plastic pollution. However, obtaining low-density semi-crystalline PET foam is a

great challenge. Chemical modification of the PET resins such as chain extension and

branching is the most widely used way to enhance its foamability. However, it inevitably

reduces the crystallinity and thus leads to relatively poor mechanical and thermal properties.

Herein, we developed a simple and affordable controlled-hydrolysis approach to prepare

low-density PET foam with high crystallinity. The effect of the water content and type of

hydrolysis agents on the foam expansion ratio, cell morphology, extent of degradation and

crystallinity were investigated. Since controlled-hydrolysis kept the linear chain structure

and decreased the molecular weight to an acceptable level, our PET foam has high

crystallinity and thus excellent tensile strength and high thermal stability (>200℃). Based

on the optimized hydrolysis conditions, both activated carbon (AC) and micrographite

(mGr) were selected as the infrared attenuation agent for IR absorbing. We also

investigated the influence of AC and mGr as a nucleation agent on foam density and cell

iii

morphology. By simulating the housing and vehicle roofing conditions, we successfully

demonstrated the superiority of the carbon particulate containing PET foam over the PS

foam for future IR-absorption roofing application.

iv

Acknowledgments

First, I would like to express my sincerest gratitude to my advisor Dr. L. James Lee

for his guidance and support on my thesis. Without his patient and detailed instruction, I

couldn’t became well trained in polymer processing research within only one year.

Furthermore, I benefit a lot from his constructional suggestion and encouragement on my

future career plan.

I would like to express my special appreciation to my co-advisor Dr. Jose Castro.

He helped developed the methodology of this thesis and his knowledge on rheology always

impressed me. Also, this thesis couldn’t have progressed smoothly without his help on

sample testing in his lab.

I also would like to thank Dr. Feng Chen and Dr. Xiangmin Han. Their previous

research provided a basis for my work. Also, thank you Dr. Eusebio Duarte Cabrera, Mr.

Dan Zhang and Mr. Min Wu for instructing me on the use of various devices and help

testing some of the samples. And it’s really helpful when discussing with you about

technical details in this thesis.

Finally, thanks to Mr. Leigh Evrard, Mr. Michael Wilson and Joshua Hassenzahl

from both CBE and ISE machine shops. Without their help, I could not have finished this

thesis smoothly.

v

Vita

September 1991 ...........................................Shanghai, China

2010..............................................................Shanghai Qibao High School

2014..............................................................B.E., East China University of Science and

Technology

2016..............................................................ThermoFisher Scientific (China)

2018..............................................................Graduate Student, The Ohio State University

Fields of Study

Major Field: Chemical Engineering

vi

Table of Contents

Abstract ........................................................................................................................... ii

Acknowledgments.............................................................................................................. iv

Vita ........................................................................................................................... v

Table of Contents ............................................................................................................... vi

List of Tables ..................................................................................................................... ix

List of Figures ..................................................................................................................... x

Chapter 1. Background and Literature Review .................................................................. 1

1.1 Introduction .............................................................................................................. 1

1.2 Extrusion Foaming ................................................................................................... 3

1.3 Polyethylene Terephthalate ...................................................................................... 6

1.4 Challenges of PET foaming ..................................................................................... 7

1.5 Conventional Approaches ........................................................................................ 8

1.5.1 Copolyester ....................................................................................................... 9

1.5.2 Resin Chain Extension ...................................................................................... 9

1.6 Non-chemical Modification Approach ................................................................... 11

1.6.1 Optimization of Foaming conditions .............................................................. 11

1.6.2 Controlled-Hydrolysis..................................................................................... 12

1.6.3 Selection of Hydrolysis Agent ........................................................................ 13

1.6.4 Mircographite as IAA and Nucleation Agent ................................................. 14

1.7 Objectives ............................................................................................................... 15

Chapter 2. Materials and Experiments ............................................................................. 16

2.1 Materials and Processing Equipment ..................................................................... 16

2.2 Foam Extrusion of Crystalline PET (CPET) Resins by Controlled-Hydrolysis .... 17

2.2.1 CPET Foaming with Moisture ........................................................................ 17

2.2.2 CPET Foaming with Wet Activated Carbon .................................................. 17

2.2.3 Extrusion Foaming and Condition Optimization ............................................ 18

2.3 Sample Characterization ......................................................................................... 20

vii

2.3.1 Foam Density .................................................................................................. 20

2.3.2 Water Content ................................................................................................. 20

2.3.3 Cell Morphology ............................................................................................. 21

2.3.4 Molecular Weight ........................................................................................... 22

2.3.5 Shear Viscosity ............................................................................................... 23

2.3.6 Crystallinity..................................................................................................... 23

2.4 Functional Properties Measurement ....................................................................... 24

2.4.1 Tensile Test ..................................................................................................... 24

2.4.2 Thermal Stability ............................................................................................ 24

2.4.3 Insulation Measurements ................................................................................ 25

2.5 Strand Die Design and Experiments ...................................................................... 26

Chapter 3. Results and Discussion ................................................................................... 29

3.1 Optimization of Processing Conditions .................................................................. 29

3.2 Neat PET Foams ..................................................................................................... 30

3.3 Foaming CPET by Controlled-hydrolysis .............................................................. 32

3.3.1 Foaming with Moisture ................................................................................... 33

3.3.2 Foaming with Activated Carbon (AC) ............................................................ 34

3.3.3 Comparison between Moisture and Wet AC .................................................. 37

3.3.4 Chain Extension vs. Controlled-hydrolysis .................................................... 43

3.4 Adding micrographite (mGr) as IAA and nucleation agent ................................... 44

3.5 Mechanical and Thermal Properties of CPET Foams ............................................ 47

3.5.1 Tensile Properties............................................................................................ 47

3.5.2 Thermal Stability ............................................................................................ 48

3.5.3 IR-absorbing and Insulation Properties .......................................................... 49

3.6 Strand Die Experiment ........................................................................................... 52

Chapter 4. Conclusion and Future Work .......................................................................... 53

Bibliography ..................................................................................................................... 54

Appendix A. Experimental Design ................................................................................ 63

Appendix B. Optimization of Foaming Conditions ....................................................... 65

Appendix C. Water Content Measurement .................................................................... 68

Appendix D. Shear Viscosity of CPET and XPET Resins ............................................ 70

Appendix E. Cell Morphology of Dry CPET/Carbon Particles Foam (less than 0.05

wt. % water) .................................................................................................................. 71

viii

Appendix F. Crystallinity Calculation from DSC Thermograms .................................. 72

ix

List of Tables

Table 1.1 Foam applications 1 ............................................................................................. 1

Table 2.1 Material description and pre-treatments ........................................................... 16

Table 2.2 Theoretical design of the strand die .................................................................. 27

Table 3.1 Density, cell morphology and crystallinity of neat PET foams ........................ 30

Table 3.2 Crystallization behavior of PET foams ............................................................. 43

Table A.1 Neat PET foams ............................................................................................... 63

Table A.2 Foaming CPET resins with moisture ............................................................... 63

Table A.3 CPET/AC with water foams ............................................................................ 64

Table A.4 Using mGr as IAA and to further manipulate the foams morphology ............ 64

Table B.1 Optimized processing condition of neat CPET foaming.................................. 65

Table B.2 Optimized processing condition of XPET foaming ......................................... 66

Table B.3 Optimized processing condition of foaming CPET with moisture .................. 66

Table B.4 Optimized processing condition of foaming CPET using AC as water carrier 67

x

List of Figures

Figure 1.1 Foaming mechanism 14 ..................................................................................... 4

Figure 1.2 Polyethylene Terephthalate (PET) .................................................................... 6

Figure 1.3 Chain extension and branching of PET by PMDA 22 .................................... 10

Figure 2.1 Leistritz twin screw extruder ........................................................................... 17

Figure 2.2 Preparation of wet AC ..................................................................................... 18

Figure 2.3 Foam extrusion diagram .................................................................................. 19

Figure 2.4 Capillary die and its geometry ......................................................................... 20

Figure 2.5 Viscometer ....................................................................................................... 22

Figure 2.6 Thermal stability test within the temperature ranging from 25℃ to 220℃ .... 24

Figure 2.7 Diagram of Mimicking the IR-absorbing roof application .............................. 25

Figure 2.8 Experiment setting of the IR-absorbing roof application ................................ 26

Figure 2.9 Schematic of the strand die.............................................................................. 26

Figure 2.10 Optimized strand die geometry ..................................................................... 28

Figure 3.1 SEM picture of neat CPET foam and the cell size distribution ....................... 31

Figure 3.2 SEM picture of crosslink PET foam and the cell size distribution .................. 31

Figure 3.3 SEM picture of commercial PET foam and the cell size distribution ............. 31

Figure 3.4 CPET foams with moisture or wet AC as hydrolysis agent ............................ 32

Figure 3.5 Effect of water content on foam density .......................................................... 32

Figure 3.6 SEM picture of neat CPET foam with 0.09 wt. % moisture ........................... 33

xi



Figure 3.9 Water content change of filtrated AC under ambient condition...................... 35

Figure 3.10 SEM picture of CPET/0.5 wt. % AC /0.05 wt. % water foam ...................... 36

Figure 3.11 SEM picture of CPET/0.5 wt. % AC/0.13 wt. % water foam ....................... 36

Figure 3.12 SEM picture of CPET/0.5 wt. % AC/0.2 wt. % water foam ......................... 37

Figure 3.13 SEM picture of CPET/1.0 wt. % AC/0.40 wt. % water foam ....................... 37

Figure 3.14 Cell density change with increasing extent of hydrolysis ............................. 38

Figure 3.15 Cell size change with increasing extent of hydrolysis................................... 38

Figure 3.16 Die pressure under different water contents .................................................. 39

Figure 3.17 Intrinsic viscosity of CPET foams ................................................................. 40

Figure 3.18 Shear viscosity of CPET foams ..................................................................... 41

Figure 3.19 Water content change of dried resin and AC................................................ 42

Figure 3.20 DSC Thermograms of PET foams ................................................................. 43

Figure 3.21 Cell morphology of CPET with 0.12 wt. % moisture /0.2 wt. % mGr foam 45

Figure 3.22 Cell morphology of CPET with 0.12 wt. % moisture /0.5 wt. % mGr foam 46

Figure 3.23 Cell density change with increasing mGr content ......................................... 46

Figure 3.24 Cell size change with increasing mGr content .............................................. 46

Figure 3.25 Mechanical properties of PET foams prepared by controlled hydrolysis ..... 47

Figure 3.26 Welding extruded rods into thin sheets. ....................................................... 48

Figure 3.27 Thermal stability of different foams from 25℃ to 200 ℃ ............................ 49

Figure 3.28 Temperature of the air at 2 inches above the roof ......................................... 51

Figure 3.29 Roof upper surface temperature change using different roofing materials ... 51

xii

Figure 3.30 Indoor temperature change using different roofing materials ....................... 51

Figure 3.31 Foam samples prepared by strand die ........................................................... 52

Figure C.1 TGA and water content of wet activated carbon put in the atmosphere after

water filtration (wet AC = dried for around 20 hours in this thesis) ................................ 68

Figure C.2 TGA and water content of different types of activated carbon ....................... 68

Figure C.3 Water content of neat CPET resin with moisture (measured by HR83 Halogen

Mettler Toledo and data points collected manually)......................................................... 69

Figure D.1 Shear viscosity of CPET and XPET Resins ................................................... 70

Figure E.1 SEM picture of CPET/0.2 wt. % mGr (both dried) foam ............................... 71

Figure E.2 SEM picture of CPET/0.5 wt. % CNT (both dried) foam ............................... 71

Figure F.1 DSC thermograms of Commercial PET foam ................................................ 72

Figure F.2 DSC thermograms of Crosslink PET (XPET) foam ...................................... 73

Figure F.3 DSC thermograms of Virgin CPET resin ........................................................ 73

Figure F.4 DSC thermograms of Neat CPET foam (moisture content ~ 0.05 wt. %) ..... 74

Figure F.5 DSC thermograms of Neat CPET foam (moisture content ~ 0.09 wt. %) ..... 74

Figure F.6 DSC thermograms of Neat CPET foam (moisture content ~ 0.12 wt. %) ...... 75

Figure F.7 DSC thermograms of Neat CPET foam (moisture content ~ 0.25 wt. %) ...... 75

Figure F.8 DSC thermograms of CPET/0.5 wt. % AC /0.05 wt. % water foam .............. 76

Figure F.9 DSC thermograms of PET/0.5 wt. % AC/0.13 wt. % water foam .................. 76

Figure F.10 DSC thermograms of PET/0.5 wt. % AC/0.20 wt. % water foam ................ 77

1

Chapter 1. Background and Literature Review

1.1 Introduction

Polymeric foam, an extension of polymer materials, is a two-phase structure with gas

voids surrounded by a continuous polymer phase. In the context of light-weight and eco-

friendly materials, polymeric foams gained considerable interest from the industry.

By selecting appropriate polymers and adjusting the foam density and morphology,

people can tailor the foam properties for a variety of applications 1 (Table 1.1). Polymer

foam is an ideal packaging material because of its light weight and excellent

chemical/electrical resistance. Also, due to the low thermal conductivity of the gas phase,

Table 1.1 Foam applications 1

Functions Markets Properties Typical polymers

Cushioning Furniture,

transportation,

construction

Energy

absorption,

flexibility

Flexible PU, PE,

ABS

Insulators Construction,

automotive

Low thermal

conductivity,

sound absorption

Rigid PU, PS, PE,

rigid PVC

Protection Packaging Soft and flat

surface cushioning

RIM PU, PS bead,

PE and PP sheet

Strength/weight Athletics, construction,

marine, medical,

decoration, household

Strength and

softness

RIM PU, x-linked

PE, PS, PVC,

flexible PU

phenolic, acrylics

Chemical/electrical Packaging, electrical Chemical and

electrical inertness

Flexible vinyl

epoxy, silicones,

rubber

low density foam is widely used as thermal insulator. Depending on the rigid/flexible

nature, foams can also serve as structural materials for construction and furniture. Currently,

2

both industry and academia have a wide range of research interest on polymeric foams,

including nanocellular foams with ultra-low thermal conductivity, nanocomposite foams,

biodegradable foams, etc. 2-8.

With increasing demand on energy conservation and environmental protection, the

insulation properties of the foam as roofing or external wall need to be further improved.

If more heat can be shielded, less energy (air conditioning) will be consumed to control the

indoor temperature. Basically, heat transfer through foam block can be divided into

conduction (in both gas and solid phase), convection (gas phase) and radiation (block). Due

to the closed cell structure, the convection is almost negligible. Also, it is difficult to change

the heat conduction in gas and solid phase (accounts for 75% of total heat transfer) when

the blowing agent and polymer matrix are selected 9. Radiation heat transfer, which

accounts for about 25% of total heat transfer, provides room for further improvement by

reducing the transmission of infrared radiation (IR). Infrared attenuation agent (IAA) is an

effective additive for absorbing IR. A wide range of IAA has been reported including

carbon particles, certain organic chemicals and conductive polymers 10-12.

Currently, polystyrene foam (PS) including expanded polystyrene (EPS) and

extruded polystyrene (XPS) is the major insulator material and it occupies around 75 % of

the external wall insulation markets 13. However, PS and IAA do not seem to be a feasible

combination. The foam roof made with IAA can easily reach around 100℃ which is

beyond the extreme temperature of PS. To find a suitable substitute for PS foam, we

attempted to prepare semi-crystalline polyethylene terephthalate (PET) foams with high

thermal stability and good IR-absorption performance for roofing and external wall

applications.

3

1.2 Extrusion Foaming

Among various mainstream foaming technologies (batch foaming, injection

molding foaming, reactive foaming for thermoset polymer, etc.) 14, extrusion foaming is a

continuous and large-scale processing method for thermoplastic foams.

Blowing agent is used to introduce gas voids into the polymer matrix. It can be

categorized into physical blowing agent (PBA) and chemical blowing agent (CBA).

Physical blowing agent includes inert gas such as nitrogen (N2), argon (Ar) or carbon

dioxide (CO2) and volatile hydrocarbon such as hydrochlorofluorocarbon (HCFC),

hydrofluorocarbon (HFC), pentane, etc. Chemical blowing agent generates gas bubbles

through chemical reactions or thermally induced deposition 14. Generally, physical blowing

agent is more widely used for low density foams 1.

Extrusion foaming is a complicated process and it covers a wide range of disciplines

such as thermodynamics, mass transfer, gas/polymer rheology, kinetics of crystallization,

and chemical reactions (for reactive foaming)14. The mechanism basically applies to almost

all foaming methods using either chemical or physical blowing agents (Fig 1.1).

4

Figure 1.1 Foaming mechanism 14

Step 1: Formation of gas/polymer homogeneous phase

With a blowing agent injected to extruder in the melting zone, a homogeneous

phase of polymer melt and gas blowing agent is formed. Adequate solubility of blowing

agent is required. Also, a specified ratio of gas/polymer melt flow rate is required to ensure

the formation of saturation blends. If the gas flow rate is too high, the undissolved gas

would shot out from the extruder and lead to a unstable processing condition. If the gas

flow rate is too low, the low concentration of dissolved gas would limit the foam expansion.

Step.2: Bubble nucleation

Under sudden pressure drop (e.g. extruded from the die during extrusion foaming),

the saturated gas/polymer mixture reaches over-saturated. The instability of the metastable

system leads to the formation of nuclei which become growing sites for the gas bubble in

later stages. The steady state homogeneous nucleation rate can be described by the classical

nucleation theory 4

𝑁1 = 𝐶0𝑓1 exp (−∆𝐺𝑐𝑟𝑖𝑡

𝑘𝐵𝑇)

5

∆𝐺𝑐𝑟𝑖𝑡 = 16𝜋𝜎3

3∆𝑃2

C0 - Number of gas molecules dissolved per unit volume of primary phase;

f1 - Coefficient weakly depending on temperature

𝐺𝑐𝑟𝑖𝑡 - Critical Gibbs free energy of nucleation

𝑘𝐵 - Boltzmann constant

T - Foaming temperature

∆𝑃 - Pressure drop

𝜎 – Surface tension of gas bubble/polymer interface

The nucleation rate is strongly influenced by the solubility of the blowing agent,

foaming temperature and rate of pressure drop. If the nucleation happens at the interface

of the two-phase (heterogeneous nucleation), the classical theory above can be modified

which will be discussed in Section 1.6.

Step.3: Bubble growth

When the gas nuclei form, the gas molecules in the polymer matrix start to diffuse

into the nuclei and the gas bubbles start to grow. The phase separation becomes more

significant and the metastable system gradually stabilizes. During this period, the gas

bubble may collapse or combine with one another. Depending on the interconnectivity of

the gas bubbles (resulting from cell wall breaking when bubbles grow bigger), open cell

or close cell foams can be obtained for completely different applications. Gas diffusivity,

polymer melt viscosity, the rate of polymer crystallization, etc. are closely related to

bubble growth. Basically, bubble growth and gas nucleation compete with each other,

significantly affecting the final cell morphology.

6

Step.4: Stabilization and solidification

Upon further cooling, the polymer melt viscosity increases and the gas bubbles

gradually stop growing when the melt solidifies. Due to the concentration difference of

the blowing agent inside and outside the foam, the air gradually replaces the blowing

agent in the gas bubble after foaming and eventually a new balance is obtained.

1.3 Polyethylene Terephthalate

Polyethylene terephthalate (PET) is a well-known synthetic polyester. The PET

molecular structure is shown in Fig 1.2. It has wide applications including water bottles,

containers, films (food packaging), fibers (clothing), electrical instruments, etc.15. As a

thermoplastic polymer, PET resins are mostly melt processed by extrusion and injection

molding into products of various geometries. Virgin PET resins can be either amorphous

PET (APET) or semi-crystalline PET (CPET) 1. The transparent APET is widely used as

water bottles while the opaque CPET is mainly used for the trays of oven-ready meals due

to its high thermal stability 16.

Figure 1.2 Polyethylene Terephthalate (PET)

However, PET is not biodegradable due to the difficulty of breaking up the ester

linkage and current technology requires complex procedures to decompose it biologically

17. Therefore, due to the tremendous demand of PET products, huge amount of PET bottles

7

end up in landfill, causing severe environmental concerns around the world. It is predicted

that ‘by 2050, there will be as much waste plastic in the ocean by mass as there are fish’ 18.

Moreover, researchers also reported the health concern due to the migration antimony (Sb)

from landfill PET products, an important catalyst during polycondensation 19.

Attempts of developing approaches for treating plastics are being pursued. Some

bacteria were discovered and engineered for biodegrading PET with high efficiency 20.

However, it is a long way to go for those trials to be realized at industrial scale. So far,

post-consumer recycling is still the most practical and economic treatment. The

conventional mechanical recycling process includes washing of PET flakes (removing the

contamination), drying and melt processing into new resins again for further new product

manufacturing 21.

Foaming of Recycled PET, as a possible recycling application, has been gaining

great interest from the industry. In addition to protecting the environment, foaming PET

has huge potential for high temperature and high mechanical strength applications due to

its semi-crystalline nature, which can further expand the application spectrum of the current

market dominating foams such as polystyrene (PS), polyurethane (PU) or poly (vinyl

chloride) (PVC).

1.4 Challenges of PET foaming

It’s challenging to foam virgin PET resins into low density by extrusion foaming

since linear PET has low melt strength 1, 21, 22. The melt strength is closely related to the

rheological behavior. Due to the low melt viscosity/elasticity, the gas bubble cannot be

stably retained during foaming. The weak cell walls cannot hold the growth of gas bubble,

8

leading to the collapse and coalescence of the fine cell structures. The low melt strength

also results from the slow rate of crystallization of neat PET 1, 23, 24. Therefore, it takes a

relatively long time period to stabilize and harden the foam from melt state, during which

collapse and shrinkage may inevitably take place. Also, the high processing temperature of

PET (Tm ~250℃) and its crystallization behavior further narrow the processing window

during foaming. Another difficulty of getting low density foams is that the density of PET

is 1.38 g/cm3, higher than many other polymers 21. It’s much more challenging to make the

density of PET foam as low as the commercial PS foam whose resin density is merely 1.04

g/cm3.

Other concerns are the hydrolysis and thermal degradation of PET under high

temperatures25-28. When the materials are exposed to oxygen, thermal oxidation of PET

also happens 29. As a result, chain scission and the corresponding loss of molecular weight

will lower the melt viscosity, which further worsens the foaming behavior. The mechanical

property of the final products could be severely reduced as well due to the low molecular

weight. Therefore, thoroughly removing moisture before melt processing is the key to

obtain high quality foams, which is adopted by almost all of the research and industrial

studies mentioned below 33-42.

1.5 Conventional Approaches

To improve the foamability and avoid the problems mentioned above, chemical

modification of PET resins is currently the most common approach.

9

1.5.1 Copolyester

Copolyester of PET is synthesized by adding comonomer during the

polycondensation reaction (monomer: terephthalic acid and ethylene glycol). The

properties of PET are changed significantly due to the comonomer introduced. Glycol-

modified PET (PETG) is a typical example developed by Eastman 1. By adding

cyclohexanedimethanol as a comonomer, the melt strength of PETG is much higher than

that of normal PET. Also, its glass transition temperature (Tg) and melt temperature (Tm)

become much lower. PETG shows no crystallization and changes the resin from semi-

crystalline to amorphous. Therefore, it can be more easily foamed and allows for wider

processing window and easier processing conditions 30. Handa, C.P. et al successfully

produced PETG foam with a density of 0.04 g/cm3 by batch foaming using CO2 as a

blowing agent 31. Park, C.P. et al investigated the foaming behavior of PETG under

extrusion foaming using a mixed blowing agent of HCFC-142b and ethyl chloride (EtCl)

32. The secondary foaming and moldability were also investigated. Despite the improved

foamability, PETG suffers from low thermal stability (Tg ~ 80℃) and weak mechanical

strength due to its amorphous structure. Therefore, its application is largely limited.

1.5.2 Resin Chain Extension

Besides starting from polymerization, resin modification by melt modification (in

a batch mixer or extruder) or solid-state polycondensation has been widely reported. The

basic principle of chain extension is to increase the molecular weight of linear PET and the

corresponding intrinsic viscosity ([η]). The chain extenders are basically chemicals with

multifunctional groups that can react with hydroxyls and carboxyl end groups of PET

10

molecules. Common chain extenders contain functionalities such as cyclic anhydride,

epoxide, oxazoline, isocyanate or carbodiimide, etc.33. The reaction of PET with

pyromellitic dianhydride (PMDA), one of the most widely used chain extender, is shown

in Fig 1.322. Depending on the extent of the reaction, the branching or even crosslinking or

gel-like structures can be formed. The long-chain, branching, or crosslink PET has higher

melt strength and improved foamability compared to the linear PET.

Figure 1.3 Chain extension and branching of PET by PMDA 22

Xanthos, M et al investigated the extrusion foaming of different PET resins by both

chemical and physical blowing agents 34. Resins modified by branching agent or reactive

processing showed significantly higher expansion than virgin resins. Foams with density

of around 0.10 g/cm3 were obtained by using CO2 as a blowing agent. They also compared

the foamability of both virgin resins and modified resins using different physical blowing

agents (N2, Ar, CO2) 35. Also, the rheological behavior of the resins was investigated and

11

correlated to the foamability. Xanthos group also investigated the effect of a variety of

chain extension and branching agents on PET rheology and foaming behavior such as

diepoxide, ethylene-glycidyl methacrylate copolymer 36, triglycidyl isocyanurate 37,

dianhydride 38 and some commercial oligomers 39. To modify the resin with these additives,

batch mixing, reactive extrusion and their processing conditions were researched 36-38. In-

situ Polymerization-modification and solid state polycondensation were also applied as a

supplemental approach for chain extension reaction 40, 41. The modified PET resins are also

widely used in batch foaming process foaming 39-42.

However, resin modification by chain extension or branching tends to reduce the

relatively high crystallinity of the final products. The loss of the linear chains inhibits the

crystallization and leads to relatively poor thermal and mechanical properties.

1.6 Non-chemical Modification Approach

To the best of our knowledge, very few non-chemical method of PET foaming has

been reported. In this thesis, we try to develop a non-resin modification approach to foam

CPET for thermal insulation applications. Without changing the PET linear structure, the

high crystallinity can be kept and thus high thermal stability and mechanical strength.

1.6.1 Optimization of Foaming conditions

It’s highly challenging to significantly improve the foaming behavior of PET by

simply controlling the processing conditions. Xanthos, M et al investigated the relation

between foam expansion and screw rotation speed, resin intrinsic viscosity, and ratio of

chemical blowing agent 33. Barzegari, M. R. et al compared die pressure influence on the

foam expansion and cell density of different PET resins 43. Fan, C. et al systematically

12

investigated the amount of physical blowing agent and die temperature during PET

extrusion foaming 44. Both higher gas flow rate and lower die temperature were

advantageous to lower the foam density. Although optimization of processing condition is

advantageous for foaming modified resins, but this approach does not work well for

crystalline resins because of the very narrow processing window. In this study, we take all

the mentioned parameters into consideration for the experimental design.

1.6.2 Controlled-Hydrolysis

Although degradation or hydrolysis of PET during melt processing is not favorable

in most cases and should be avoided (Section 1.4), controlled-degradation can allow for

better foam expansion of PET 1. Very few research was reported on controlled-degradation

for extrusion foam, but the effect of molecular weight (or intrinsic viscosity) on PET foam

was investigated in many cases.

Guo, H. et al compared the foam expansion of virgin PET resins with different

molecular weight 45. They found that lower IV resins might be more favorable for low

density foams. Zheng, W.G. et al reported that a high IV PET (1.19 dL/g) led to

microcellular foams but with poor expansion ratio in extrusion foaming, while a higher

expansion ratio was obtained by foaming a lower IV resin (0.8 dL/g) 46. Wet and dry PET

resins were compared by Barzegari, M. R. et al for extrusion foaming, and they found that

the expansion ratio of a wet PET resin was 3 times higher than that of a dry resin 43.

However, they didn’t try to optimize the water content so that excessive degradation

happened and the molecular weight became too low, which led to low melt strength and

cell collapse. All these studies indicated that lowering the molecular weight is likely to

13

obtain low-density crystalline PET foam if we can carefully control the extent of

degradation reaction.

Herein, we define the controlled-hydrolysis as lowering the molecular weight to an

acceptable level for better foam expansion (not too low in case of poor mechanical

property). In this thesis, we would like to systematically investigate the effect of water

content and the choice of hydrolysis agent (or water carrier) to balance the foam density,

cell morphology and crystallinity for low-density foam with desirable mechanical and

thermal properties.

1.6.3 Selection of Hydrolysis Agent

Water is widely used in extrusion foaming mainly as a co-blowing agent to

manipulate the cell morphology because it is low-cost, benign and environmentally-

friendly. The way to introduce water may make a great difference to the foam morphology

47. For hydrophilic materials, using moisture is the simplest approach 48, 49. PET resins

absorb water under ambient temperature and pressure. The moisture not only attaches to

the surface of the resins but also diffuses into the core of the resins which cannot be easily

removed due to the hydrogen bond. Activated carbon (AC) is another choice of water

carrier. Due to its complicated porous structure, it has great potential to trap a large quantity

of water. The water held by the micro-scale porous structure is quite stable under high

pressure during extrusion while gasifies under sudden pressure drop. Therefore, we expect

water in AC can act as a co-blowing agent as well. Research on using water as a co-blowing

agent to manipulate the cell morphology was reported by our group in the past years. Yeh,

S.K. et al applied CO2 and water (carried by AC) as a dual blowing agent to prepare PS

14

foams which had large cell size and lower density for improved the insulation property 47.

Zhang, C.L. et al formed the bimodal cell structures when using the dual blowing agent to

foam PS 50 and they further investigated the effect of carbon particles on the cell

morphology 51.

1.6.4 Mircographite as IAA and Nucleation Agent

We also use micrographite as IAA for IR-absorption application. Micrographite has

high IR-absorption efficiency among the carbon particle family, which was demonstrated

by our lab before 10. Also, carbon micro/nano-scale particles are widely used as nucleation

agents in extrusion foaming. The heterogeneous nucleation rate can be simply obtained by

modifying the free energy term in classical nucleation theory (Section 1.2) which can be

described by

∆𝐺𝑐𝑟𝑖𝑡 = 16𝜋𝜎3

3∆𝑃2

𝑓(𝑚, 𝑤)

2

𝑓(𝑚, 𝑤) is the energy reduction factor (≤ 1 ) which is a function of the contact angle

between gas, polymer and nanoparticle, and the relative curvature of the nucleant surface

to the critical radias of the nucleated phace 4. The cell morphology of the composite foam

is affected by aspect ratio, dispersion and surface chemistry of nanoparticles. The effect of

carbon nanotube, nanoclay, carbon nanofiber, etc. on foaming PS, PMMA and PP were

reported 52-56. So far very few research has been reported on using carbon particles to

manipulate the PET foaming behavior (several batch foaming of PET/clay nanocomposite

foam was reported 57, 58). Therefore, this will be another highlight of this thesis.

15

1.7 Objectives

• Proposed formulation: crystalline PET (CPET)/ moisture or activated carbon

(AC) (hydrolysis agent) / micrographite (mGr) (IAA);

• Foaming CPET resin by controlled-hydrolysis to produce low density foams

(density < 0.20 g/cm3), comparable to the foam made by chemical-modified

resins;

• Investigating the effect of water content and the type of hydrolysis agent on foam

density/morphology, extent of degradation and crystallinity;

• Keeping high crystallinity of PET and the corresponding high thermal stability

and mechanical strength;

• Using mGr as IAA and nucleation agent to manipulate the foam morphology;

• Conducting lab demo of applying our PET foam for IR-absorbing roofing and

external wall applications.

16

Chapter 2. Materials and Experiments

2.1 Materials and Processing Equipment

A crystalline PET resin (Laser+® C E60A) with crystallinity > 50% and a

crosslink PET resin (Array 3962) were supplied by DAK Americas. The blowing agent

hydrofluorocarbon (HFC, R134a) was purchased from DuPont. The boiling temperature

is -26.3°C. Coconut shell activated carbon (AC) from Carbon Resources was chosen as

water carrier. Micro-graphite (mGr) from Qingdao Yanhai Carbon Materials Inc. was

chosen as nucleation and IAA agents.

Table 2.1 Material description and pre-treatments

Material Specification Description Treatments

before use

Crystalline PET

(CPET)

Laser+ E60A, DAK

America

I.V.= 0.81 dL/g

Tm = 245 ℃

Dried at 130℃

overnight

Crosslink PET

(XPET)

Array 3962, DAK America I.V.= 0.67 dL/g

Tm = 230 ℃

Dried at 120℃

overnight

Activated carbon

(AC)

Carbon Resources

Company, USA

Made from

coconut shell,

diameter 7 µm

Varied from

different purposes

Micro-Graphite

(mGr)

Qingdao Yanhai Carbon

Materials Inc., China

Thickness: ~0.5

µm

Stored in a

desiccator

A Leistritz (German) twin screw extruder (Fig 2.1) was used for foaming. The

diameter of the screws is 27 mm and the length to diameter ratio (L/D) is 40:1. There are

11 temperature zones under control (9 in the extruder and 2 in the die). The materials are

17

fed into the extruder by a feeder and the speed can be controlled automatically. The

physical blowing agent is injected into the extruder in zone 4 by a syringe pump.

Figure 2.1 Leistritz twin screw extruder

2.2 Foam Extrusion of Crystalline PET (CPET) Resins by Controlled-Hydrolysis

2.2.1 CPET Foaming with Moisture

To control the water content, we dried the resin for different time periods under

120℃ (conditions listed in Appendix A). To note, the drying condition varied with the

moisture content which depends on the storage time and conditions. Therefore, it was

necessary to change the drying condition from time to time. The reins with different water

contents were for further water content test and extrusion foaming.

2.2.2 CPET Foaming with Wet Activated Carbon

Activated carbon (AC) was used as a water carrier. Fig 2.2 shows how to fill the

porous structure of AC with water. First, AC particles were completely immersed in water,

18

during which stirring was necessary to make a uniform suspension. Second, the AC/water

suspension was placed overnight to allow water diffusion into the porous structure, which

was then vacuum filtrated and left under ambient condition for 24 hours to evaporate the

excessive water. Third, the filtrated cake was grinded into wet AC particles for extrusion

foaming. The wet AC was dried for 5 min at 100℃ and we also compared AC without

treatment (contain certain amount of water under storage condition)

AC was premixed with dried PET resins by handshaking in a plastic bag for foam

extrusion (formulations listed in Appendix A).

Figure 2.2 Preparation of wet AC

2.2.3 Extrusion Foaming and Condition Optimization

3 kg of all the ingredients for each formulation mentioned in Sections 2.2.1 and

2.2.2 were prepared for extrusion foaming. The previous residue inside the extruder was

purged out and the steady processing condition was reached before each run.

Fig 2.3 shows the procedures of extrusion foaming. PET resins with

micro/nanofillers or water carriers were continuously fed into the hopper by a single screw

feeder. The HFC was first filled in a syringe pump and then injected into the polymer melt

(zone 4). Then the blowing agent was mixed with and dissolved into the PET melt in the

extruder. The temperature setting of the first nine zones were fixed for all formulations.

19

Temperatures of zones 1 to 3 increased gradually from 230℃ to 260℃ for solid conveying.

Temperatures of zones 3 to 10 changed from 260℃ to 250℃ for melting, mixing and

pressurizing. When the gas/polymer mixture was extruded from the capillary die (Fig 2.4),

the foam was obtained under the pressure drop. The strand-shaped samples were collected

manually 15 ~ 20 cm below the outlet of the die by a putty knife when the whole system

reached steady state (stable die pressure, no blowing agent shooting out).For foaming with

water, all the foaming conditions with different water content were kept the same as

foaming dry PET resins (Appendix C.1). Therefore, the foaming behaviors under different

water contents were mainly attributed to the water content, while any difference caused by

processing conditions was excluded.

Figure 2.3 Foam extrusion diagram

20

Figure 2.4 Capillary die and its geometry

2.3 Sample Characterization

2.3.1 Foam Density

According to ASTM D792, a homemade device is used to measure the bulk density

of the foam samples. It can be expressed as

ρ𝑓𝑜𝑎𝑚 = 𝑚𝑓𝑜𝑎𝑚

𝑚𝑓𝑜𝑎𝑚 + 𝑤 − 𝑏 ρ𝑤𝑎𝑡𝑒𝑟

𝑚𝑓𝑜𝑎𝑚 is the true mass of the foam sample in air. 𝑤 is the weight measured when

the sinker is immersed completely in water without the foam sample. To note, the water

bath is on a shelf without any contact with the balance. 𝑏 is the weight measured when the

sinker bundled with the foam is immersed completely in water.

2.3.2 Water Content

The water content in AC was measured by thermogravimetric analysis (TGA). The

weight loss in the range from 50 to 200℃ was determined as the water percentage.

The water content of PET resins was measure by a HR83 Halogen Mettler Toledo

instrument. The water content was determined by the total weight loss at 180℃ for 8 hours

21

in which we assume all water can be removed from the resins under this condition. The

weight loss under resin drying condition (120℃) was also measured. Based on these two

tests, the water content of the resins could be obtained.

2.3.3 Cell Morphology

A FEI Nova NanoSEM 400 scanning electron microscope (SEM) system was used

to characterize the cell density and cell size of the foam. Foam samples were first immersed

in the liquid nitrogen for 5 minutes before cracking. The sample was then stick to a SEM

platform with the fracture surface upwards. A 1 to 2 nm thin layer of platinum was coated

on the sample for imaging.

The cell density (N0) is the number of cells per cubic centimeter and can be

expressed by

𝑁0 = (𝑛

𝐴 )

32

ρ𝑢𝑛𝑓𝑜𝑎𝑚𝑒𝑑

ρ𝑓𝑜𝑎𝑚

Where n is the number of the cells in a single SEM picture and the A is the area of

this SEM picture. ρ𝑓𝑜𝑎𝑚 is the foam density and ρ𝑢𝑛𝑓𝑜𝑎𝑚𝑒𝑑 is the density of the sample

following the same formulation and processing condition but not adding blowing agent.

By analyzing the SEM picture using ImagePro Plus software, the average cell

diameter (D) can be obtained, which can be expressed by:

𝐷 = ∑ 𝑛𝑖𝑑𝑖

∑ 𝑛𝑖

Where 𝑛𝑖 is the number of cells with the diameter 𝑑𝑖 in the SEM pictures.

22

2.3.4 Molecular Weight

To quantify the extent of hydrolysis during extrusion, the molecular weight of the

foams was determined by measuring the intrinsic viscosity according ASTM D4603-18.

The solvent was a mixture of phenol and 1,1,2,2-tetrachloroethane with a mass ratio of

60:40. PET foams were dissolved in the solvent with a concentration of 0.5 g/dL at 110°C

for 15 min. The time of the PET solution flowing through the two marks of the ubbelohde

type viscometer was recorded (Fig 2.5).

Figure 2.5 Viscometer

The intrinsic viscosity [𝜂] of PET was defined by the Billmeyer relationship:

[𝜂] = 0.25𝜂𝑟 − 1 + 3ln (𝜂𝑟)

𝐶

𝜂𝑟 − 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 𝑒𝑞𝑢𝑎𝑙𝑙𝑖𝑛𝑔 𝑡1

𝑡0;

𝑡0 − average solvent flow time, s;

𝑡1 − average solution flow time, s;

23

C – concentration of the PET solution, 0.5 dL/g

The viscosity-average molecular weight (Mv) can be calculated by the Mark–

Houwink equation: [η] = KMv𝑎 . For PET, constant K and a equals 7.44 × 106 mL/g and

0.648 respectively 59.

2.3.5 Shear Viscosity

Flowability of the polymer melt is determined by measuring the shear viscosity.

The resin or pellets samples were dried under 120℃ overnight and then hot-pressed into

disks with at least 1 mm thickness. The foam samples were first grinded into small pieces

and dried under 120℃ overnight before hot press. Then the shear viscosities were

measured by a parallel plate rheometer (ARES II, TA instrument) at 260℃ for the shear

rate ranging from 0.1 to 1000 rad/s. In addition, the polymer chain length is closely related

to the shear viscosity, from which we can also qualitatively compare the molecular weight

of PET foams.

2.3.6 Crystallinity

Crystallinity of the foam was characterized by Differential Scanning Calorimetry

(DSC, Q200, TA Instrument). The heat flow of the foam samples was recorded by heating

the samples from 30℃ to 300℃ at a rate of 10℃ per minute. To obtain the crystallinity,

the area of the melt peak and cold crystallization peak can be integrated from the heat flow

versus temperature graph. The crystallinity can be calculated by 60:

Crystallinity X𝑐 % = ∆𝐻𝑚−∆𝐻𝑐

∆𝐻𝑚0 × 100 %

24

Where ∆𝐻𝑚 is the melt enthalpy; ∆𝐻𝑐 is the cold crystallization enthalpy; ∆𝐻𝑚0 is the 100%

crystallization enthalpy of PET and can be found in the polymer handbook 61.

2.4 Functional Properties Measurement

2.4.1 Tensile Test

The tensile behavior of the foams were measured by an RSA3 instrument (TA

Instruments, USA). The cylindrical-shaped foams with diameter ranging from 1.90 mm to

2.5 mm depending on the expansion ratio were collected during extrusion and cut into 30

mm length. During the test, the samples were fixed tightly to avoid slippery between two

clamps with a gap of 10 mm. The stress and stain were recorded when the sample were

stretched under a constant strain rate (0.008 s-1).

2.4.2 Thermal Stability

The strand-shaped foams were first welded into sheets and cut into the same size (1

cm × 3 cm). As shown in Fig 2.6, they were set at the margin of the plate with one side

pressed by a weight. The plate was heated up in an oven from room temperature to 220℃.

The thermal stability was observed at different temperatures when the samples softened,

collapsed or decomposed.

Figure 2.6 Thermal stability test within the temperature ranging from 25℃ to 220℃

25

2.4.3 Insulation Measurements

The heat transfer of the foams under Infra-red radiation was measured by

mimicking the roof of the house or vehicles (Fig 2.7 and 2.8). Different foam samples (cut

into 2.5 cm × 2.5 cm) were used as roofing materials for comparing the insulation

performance. The side wall and the bottom wall with a cylindrical dimension (diameter: 2

cm, height: 2.5 cm) were made from polystyrene foam with a thickness of 2 mm. The infra-

red lamp was placed 25 cm top right above the foam roof. To better mimic the real

condition, convection was caused by a fan at the side of the infrared lamp. Accordingly,

the temperature above the roof would not be too high with more heat removed by air

convection. Otherwise, the heat of the roof would mostly transfer to the indoor region,

which is not likely to happen in the real condition.

During the experiment, the roof and indoor temperatures were measured

simultaneously by two thermocouples once the IR lamp was switched on. One more

thermocouple was used to monitor the temperature two inches above the foam roof.

Figure 2.7 Diagram of Mimicking the IR-absorbing roof application

26

Figure 2.8 Experiment setting of the IR-absorbing roof application

2.5 Strand Die Design and Experiments

A strand die was designed to verify the applicability of our approach to the potential

industrial manufacturing. Due to the foam expansion at the outlet of the strand die, the

extrudate can be welded together and the plate-shaped foam can be obtained (Fig 2.9),

which is very widely applied to the industrial-scale processing.

Figure 2.9 Schematic of the strand die

We tried to repeat the optimum foaming results under the same processing condition by

simply dividing the 1 mm diameter hole into multiple holes (making total hole area the

27

same). We can estimate the strand die geometry by using the capillary rheometer function

which can be expressed by 62:

𝑄 = (∆𝑃𝑅

2𝑚𝐿)

1𝑛

(𝑛𝜋𝑅3

1 + 3𝑛)

Q – flow rate;

∆𝑃 – pressure drop;

𝐿 – length of the capplilary;

R – radius of the capplilary;

n – power law parameter

Here, we assume the same total flow rate and set n = 1 by considering the polymer melt as

Newtonian fluid. Then the equation above can be simplified to 𝑃 ∝ 𝐿

𝑑4 . Therefore, we

can obtain the die geometry under several holes (Table 2.2), which are supposed to achieve

the equivalent function of the capillary die.C

Table 2.2 Theoretical design of the strand die

Hole number Diameter(mm) Length(mm)

Capillary die (one-hole) 1 12

3 0.58 4

4 0.5 3

5 0.45 2.5

6 0.41 2

The theoretical calculation can provide a good starting point while further modification by

trial and error is always necessary. In our experiment, we started from 5 holes, 0.5 mm

diameter and 2.5 mm length. However, such design causes unexpectedly high die pressure

under the same foaming condition and thus we put two more holes to reduce the die

pressure. In the end, we obtained the optimized die geometry (7 holes, 0.5 mm diameter

28

and 2.5 mm length) shown in Fig 2.10. The deviation of the optimized condition from the

theoretical value is probably because of the Newtonian fluid assumption. Also, the same

total flow rate assumption may not be perfectly valid due to the geometry change of the die

(comparing Fig 2.4 and 2.9).

Figure 2.10 Optimized strand die geometry

29

Chapter 3. Results and Discussion

3.1 Optimization of Processing Conditions

The optimized foaming conditions are summarized in Appendix B.

The melting zone setting for CPET is ranged from 260℃ to 245℃ (melt

temperature around 245℃). Due to the narrow processing window, the die temperature

was not able to be lower than 235℃ (heat transfer issue). Under each die temperature from

255℃ to 235℃, the influence of screw revolution rate, feeding rate and gas flow rate were

also investigated. They were adjusted and coordinated under each die temperature from

255℃ to 235℃. The optimized processing conditions is shown in Table B.1. To note, we

didn’t find any significant effect of die temperature difference on the expansion ratio of

CPET foams.

The crosslink PET (XPET) has a lower melting point and a much wider processing

window than CPET. The processing temperatures were thus much lower and the die

temperature could be lowered than 215℃. In this case, the lower die temperature

significantly increased the foam expansion ratio, which coincided with what was reported

by Barzegari, M.R. et al 43. Same procedure as mentioned above was adopted to optimize

the processing conditions which are shown in Table B.2.

When foaming PET by controlled hydrolysis approach, we adopted the same

optimized processing conditions of neat CPET (Table B.4&5). For both moisture and AC

based formulations, the foams were extruded at 15 rpm (screw revolution rate), 130 rpm

30

(feeding rate) and 2 ml/min (blowing agent flow rate) to compare the results. Die pressure

for using AC as hydrolysis agent dropped to 600~1000 psi (depending on the water content)

while it dropped to 200 ~ 800 psi by adding moisture.

3.2 Neat PET Foams

The density and cell morphology of CPET and XPET foams prepared under the

optimized conditions are shown in Table 3.1. A commercial PET foam was also tested for

comparison. Both XPET foam and commercial PET foam has a much lower density than

the CPET foam. XPET foam prepared in our lab has a higher density than the commercial

one. This is probably because XPET resin has intrinsic viscosity around 0.66 dL/g, not

desirable enough for foam application. Most of modified resins showing excellent

foamability reported in the literature have a much higher IV (> 0.8 dL/g). The non-uniform

cell size and relatively low cell density can also be attributed to the low IV and the

corresponding low melt strength. In addition, our lab-scale extruder cannot reach high

pressure (maximum die pressure ~1600 psi), which limited the rate of pressure drop for

desirable nucleation efficiency.

Table 3.1 Density, cell morphology and crystallinity of neat PET foams

Sample Density

(g/cm3)

Cell density

(cells/cm3)

Average cell size

(µm)

Crystallinity

Xc (%)

CPET foam 0.6 4.43×106 44 8.6%

XPET foam 0.18 5.39×105 170 1.6%

Commercial PET foam 0.06 1.31×106 165 5.9%

CPET with IV around 0.80 dL/g showed a much lower expansion ratio than XPET

when using HFC 134-a as a blowing agent. Zheng, W.G. et al reported similar results of

31

the foam made by a 0.80 dL/g non-modified virgin resin 46. Figs 3.1 to 3.3 show that CPET

foams has a much higher cell density (4.43×106 cells/cm3) and smaller cell size (44 µm)

than XPET foam and commercial PET foam. The crystallinity of CPET foam is much

higher than XPET due to its linear chain structure.

Figure 3.1 SEM picture of neat CPET foam and the cell size distribution

Figure 3.2 SEM picture of crosslink PET foam and the cell size distribution

Figure 3.3 SEM picture of commercial PET foam and the cell size distribution

32

3.3 Foaming CPET by Controlled-hydrolysis

Using either moisture or wet activated carbon as hydrolysis agent, the expansion

ratio of CPET resin was significantly improved (Fig 3.4). The relation between foam

density and water content was plotted in Fig 3.5.

Figure 3.4 CPET foams with moisture or wet AC as hydrolysis agent

Figure 3.5 Effect of water content on foam density depending on different types of

hydrolysis agent (1) moisture (2) 0.5 wt. % AC (3) 1.0 wt. % AC

33

3.3.1 Foaming with Moisture

The relation between foam density and water content was plotted in Fig 3.5. Under

the ordinary drying condition (Section 3.2), CPET resins with less than 0.05 wt. % moisture

showed a very low expansion ratio (density around 0.5~0.6 g/cm3). When increasing the

moistrue content, the foam density was significantly lowered. From Fig 3.6 to Fig 3.7, the

cell size became larger and more uniform with increasing the water content while the cell

density decreased to some extent. The cell morphology became more like that of the

commercial PET foam. The optimum water content was 0.12 % for this CPET resin and

the foam density was lowered to about 0.16 g/cm3.

Figure 3.6 SEM picture of neat CPET foam with 0.09 wt. % moisture


34


However, excessive degradation was observed when the water content of the resins

reached higher than 0.2 wt. %. We observed that the extrudate could not hold the gas bubble

growth due to its low melt strength (molecular weight drop too much). Also, the foam could

hardly be shaped. The foam density became higher (Fig 3.5) due to the cell collapse. Low

cell density and large & nun-uniform cell size were also observed (Fig 3.8).

3.3.2 Foaming with Activated Carbon (AC)

The filter cake of activated carbon (AC) was dried under the ambient condition

before grinding into powder for foaming. The water content of AC particles was measured

by testing the sample of different time periods using TGA (Appendix C), which is plotted

in Fig 3.9. After 48 hours, the water content of AC finally reached a balance around 27

wt. %. To avoid particle aggregation due to excessive water, a minimum about 20 hour

drying was required for a 30 g filter cake under 25℃. Otherwise, the AC particles would

not disperse uniformly during premixing with PET resins and then aggregation during

premixing would cause periodical instability during processing. Therefore, in this thesis,

we used wet AC for extrusion foaming right after we grinded it into powder around 24

hours after filtration.

35

Figure 3.9 Water content change of filtrated AC under ambient condition

Three types of AC with water content of 10 %, 30 % and 40% (Appendix C) were

prepared as mentioned in Section 2.2.2. A series of parallel experiments using 0.5 and 1.0

wt. % AC as water carrier were conducted to compare the effect of water content and carrier

loading on foam expansion. Due to the particle aggregation mentioned above, using 0.5

wt. % AC could get at most 0.20 wt. % water. Therefore, we also used 1.0 wt. % AC for

testing higher water content conditions.

Similar to the trend of foaming PET with moisture, the foam density decreased

with increasing the water content because of hydrolysis (Fig 3.5). The lowest foam density

was obtained with 0.2 wt. % water content when using 0.5 wt. % AC. Fine cell structure

and uniform cell size were achieved (Figs 3.10 ~ 3.12) with no cell collapse being observed

(no excessive hydrolysis). Although more water can be carried by 1.0 wt. % AC, the foam

36

expansion was not as effective as 0.5 wt. % AC loading. This is probably because the slow

screw revolution rate (15 rpm) was not high enough for good dispersion of the AC under

higher filler loading. Also, the excessive hydrolysis was observed when the water content

was 0.40 wt. % (Fig 3.13). Judging from cell morphology, we cannot tell that water in AC

can act as a co-blowing agent as we expected before since the cell geometries obtained by

the two approaches are very much similar. Significant changes of the cell morphology such

as bimodal cell structure were reported when co-blowing agent was applied 47, 50, 51 .

Therefore, we assume that the water trapped by AC mainly served as a hydrolysis gent

while the co-blowing agent effect might be negligible.

Figure 3.10 SEM picture of CPET/0.5 wt. % AC /0.05 wt. % water foam

`

Figure 3.11 SEM picture of CPET/0.5 wt. % AC/0.13 wt. % water foam

37



3.3.3 Comparison between Moisture and Wet AC

The cell morphology of CPET foams prepared by controlled hydrolysis is shown

in Fig 3.14 & 3.15. For both formulations, the cell size became more uniform, the cell

density decreased, and the averaged cell size increased with increasing water content,

particularly at the optimal condition (e.g. 0.12 wt.% moisture or 0.5 wt.% AC/0.20 wt.%

water) with the highest expansion ratio. Compared to the XPET foam at almost the same

expansion ratio, CPET foams show a more uniform cell size.

38

Figure 3.14 Cell density change with increasing extent of hydrolysis

Figure 3.15 Cell size change with increasing extent of hydrolysis

The extent of the hydrolysis can be compared by both die pressure during extrusion

and the intrinsic viscosity of the foams (Figs. 3.16 & 3.17). When using moisture as the

39

hydrolysis agent, the die pressure decreased from ~7.6 MPa (1100 psi) to ~ 4.2 MPa (600

psi) when the water content increased from 0.05 to 0.12 wt.%. The die pressure dropped to

~ 1.4 MPa (200 psi) when the resin contained 0.25 wt.% moisture, indicating the excessive

resin degradation and the melt viscosity became too low to be favourable for foaming.

When using wet AC as the hydrolysis agent, a different trend was observed where a much

higher die pressure (~ 6.2 MPa (900 psi)) was obtained even with a water content as high

as 0.20 wt.%. This is because the water was trapped in the AC during extrusion until the

pressure was released near the outlet of the die during foaming.

Figure 3.16 Die pressure under different water contents

However, the molecular weight of foams prepared by both moisture or wet AC

based formulations still dropped to a similar range judging from the intrinsic viscosity and

melt viscosity results. Although water in AC could inhibit hydrolysis during extrusion, the

molecular weight of the CPET/AC foam still decreased after foaming. At the lowest foam

40

density (~0.15 g/cm3), the intrinsic viscosity of the CPET foams dropped from ~0.81 to

~0.52 dL/g for both formulations. The melt viscosity of the foam extrudate shown in Fig.

1(e) also reveals the same trend, i.e. a lower shear viscosity would lead to a higher

expansion ratio of the non-modified CPET resins, and the maximal expansion ratio was

reached when the resin shear viscosity was ~50 Pa•s for both formulations. When the shear

viscosity dropped to ~20 Pa•s, the foam lost its mechanical strength and could not be

shaped. This demonstrates that the controlled resin degradation is the key to achieve the

desirable foamability in PET foaming, and water/moisture acts more as a hydrolysis agent,

not a co-blowing agent as in PS foaming.

Figure 3.17 Intrinsic viscosity of CPET foams

41

Figure 3.18 Shear viscosity of CPET foams

The water/moisture content needs to be tightly controlled in PET foaming as the

intrinsic viscosity would drop to ~0.3 dL/g when the moisture content reached 0.2 wt.%, a

typical moisture content in PET in ambient condition, and the molecular weight and melt

strength were too low for foaming. Even though we were able to control the moisture

content in PET during lab scale extrusion foaming, it would be much better to use the water

in AC based formulation in large scale production because the water content in AC could

remain stable for an extended time period, while the moisture content in resin kept

changing in a TGA experiment showing in Fig 3.19. Furthermore, less resin degradation

during extrusion in the CPET/AC case would allow a more stable operation condition.

42

Figure 3.19 Water content change of dried resin and AC (after removal of

excessive water) under ambient condition

From the DSC thermograms, CPET foams showed much lower crystallinity (~ 10%)

than the virgin CPET resin (~36%). Even so, the crystallinity was still much higher than

that of the XPET foams (~1.6%). The CPET foams with wet AC showed a higher

crystallinity than neat PET foams with moisture. This is because micro-scale particles can

enhance the nucleation efficiency of crystallization63~66. The extent of controlled

hydrolysis did not affect the crystallinity much. However, when the molecular dropped too

much (e.g. 0.24 wt.% moisture or 1% AC/0.4 wt.% water), the foams showed a much

higher crystallinity. This is probably because the shorter chain and the wider molecular

weight distribution promoted crystallization when the molecular weight decreased

significantly 67,68.

43

Figure 3.20 DSC Thermograms of PET foams

Table 3.2 Crystallization behavior of PET foams

Samples Tc (℃) ∆Hc (J/g) Tm (℃) ∆Hm (J/g) Xc %

Commercial PET foam none 0.0 243.5 6.7 5.9%

XPET foam 137.7 18.3 217.3 19.8 1.3%

Virgin CPET resin none 0.0 243.1 41.4 36.6%

CPET/0.05 moisture foam 123.7 21.5 247.7 31.2 8.6%



CPET/0.25 moisture foam none 0.0 250.2 31.1 27.5%

CPET/0.5 AC/0.05 water 122.7 18.6 249.1 28.3 8.6%

CPET/0.5 AC/0.13 water 118.8 18.3 248.3 34.1 14.0%

CPET/0.5 AC/0.20 water 121.7 19.9 249.9 31.3 10.1%

CPET/1.0 AC/0.40 water none 0.0 249.1 20.0 17.7%

3.3.4 Chain Extension vs. Controlled-hydrolysis

Chain extension is a kind of bottom-up strategy which is applicable to all kinds of

linear resins for improving foamability. It’s especially suitable for low IV resins that is

44

almost useless for good-quality products. Also, it’s an important procedure during

mechanical recycling of PET to offset the molecular weight decrease under high

temperature melt reprocessing 69. However, it’s not a good choice for high crystalline

foams due to its damage to the crystallinity.

Linear PET resins (virgin or recycled resins) are a better choice for high crystalline

PET foams. However, it’s very challenging to be foamed without chemical modification.

The molecular weight is closely related to the foam expansion ratio for linear resins. To

achieve low-density foams, a relatively low molecular weight resin is preferred. Based on

our results (together with all the related work reported before 43, 45, 46), a IV lower than 0.8

dL/g but higher than a lower limit (to avoid severe cell collapse and poor foam quality) is

the best choice for low density PET foams. The exact value will be quantified by testing

IV in the future.

For both virgin and recycled PET resins with relatively high IV (IV > 0.8), the

controlled-hydrolysis, instead of chain extension, approach is a desirable choice for high-

crystallinity and low-density foams. We have to pay more attention to the extent of

hydrolysis to avoid decreasing the molecular weight too much.

3.4 Adding micrographite (mGr) as IAA and nucleation agent

Apart from AC, we designed another micrographite-based route to prepare IR-

absorbing foams. Because of its 2D planar structure, micrographite has high IR-absorption

efficiency. With a thickness at around 500 nm, it also can act as a gas nucleation agent to

munapulate the cell morphology. We started from the optimized formulation CPET/0.12

moisture. Compared to neat PET foam with moisture, the cell size distribution of the

45

composite foam turned to be more uniform (Fig 3.21 & Fig 3.22). Also, the gas bubble

became smaller and the bubble density increased significantly due to the high efficiency of

heterogeneous nucleation (Fig 3.23 and Fig 3.24). With increasing the mGr content, the

foam density slightly increased. Similar results for nanocomposite foams were widely

reported. This can be explained by the competition between gas nucleation and bubble

growth. When the nucleation rate becomes faster, more gas bubbles nuclei are generated,

which inhibits the bubble growth in the confined space. Therefore, the bulk density of the

foam becomes higher. In our experiment, we didn’t try to add more mGr (> 0.5 wt. %).

According to our previous research, particle aggregation might happen due to the poor

dispersion under slow extrusion rate so that the gas nucleation can hardly be further

improved. Also, merely around 0.2 wt. % carbon IAA is already enough for IR absorbing

applications.

Figure 3.21 Cell morphology of CPET with 0.12 wt. % moisture /0.2 wt. % mGr foam

46

Figure 3.22 Cell morphology of CPET with 0.12 wt. % moisture /0.5 wt. % mGr foam

Figure 3.23 Cell density change with increasing mGr content

Figure 3.24 Cell size change with increasing mGr content

47

3.5 Mechanical and Thermal Properties of CPET Foams

3.5.1 Tensile Properties

The CPET foams prepared by controlled hydrolysis exhibited excellent mechanical

strength from the tensile test (Fig 3.25). The XPET foam showed the highest stress but less

than 20 % strain. The poor elongation ratio is probably because of the nonuniform cell

structure and large bubble size. Also, low intrinsic viscosity of the XPET resin might be

another reason. The CPET foam showed a slightly lower stress but a much better elongation

ratio than both XPET and commercial PET foams. Although the comparison between

foams with different densities may not be fair, it still demonstrates that even after a certain

extent of degradation the CPET foam quality could be decent enough. The foams prepared

by moisture-based and wet AC-based formulations show comparable mechanical

properties. This further verifies the similarity of the two approaches. To note, the wet AC-

based foam shows a lower elongation ratio but a higher stain. This is probably because of

the higher crystallinity of the foam with AC. The higher intermolecular attraction due to

the higher crystallinity limits the free motion of polymer chains, leading to a higher

stiffness (higher stress) but a lower extension ratio under the same external force.

Figure 3.25 Mechanical properties of PET foams prepared by controlled hydrolysis

48

3.5.2 Thermal Stability

For the thermal and insulation property test, planar foam specimens were prepared

by welding the foam rods and cutting them into the same size (Fig 3.26). A PS foam with

the same geometry was cut from a commercial PS foam as a bench mark.

The thermal stability is compared by heating the samples from 25 to 200℃ (Fig

3.27). PS foam collapsed at <100℃ and gradually decomposed with increasing

temperatures, indicating its weakness for high temperature applications. XPET foam

softened at around 140℃, which shows comparable extreme application temperatures as

commercial PET foams . When adding micrographite, its thermal stability almost didn’t

improve. The CPET foam prepared by controlled-hydrolysis showed excellent thermal

stability even over 200℃. This is because the chain scission reaction of linear PET still

produces linear molecules and thus keeps much higher crystallinity than XPET, which

further demonstrates one of theadvantages of ourapproach over conventional chain

extension modification.

Figure 3.26 Welding extruded rods into thin sheets. From left to right: (1) Commercial PS

foam; (2) XPET foam; (3) XPET/0.2 wt. % graphite foam (4) CPET (0.12 wt. % water)/

0.2 wt. % graphite foam (5) CPET/0.5 wt. % AC/ 0.2 wt. % water foam

49

Figure 3.27 Thermal stability of different foams from 25℃ to 200 ℃

3.5.3 IR-absorbing and Insulation Properties

Under the experiment settings, the real condition of IR absorbing was mimicked

successfully. This can be judged by the air temperature above the roof which was controlled

stably (within 26 to 35℃) and no hot spots existed due to the convection (Fig 3.28). Neat

PS or PET foam showed a lower roof temperature than foams with infra-red attenuation

agents (Figure 3.29). With merely less than 1 wt. % mGr or AC particles, the roof

temperature easily reached around 100℃, showing the high IR-absorbing efficiency of

carbon particles as IAA. Since most of the IR radiation was shielded by IAA in the foams,

the indoor temperature was about 10℃ lower than using neat PS or PET foams (Figure

3.30). Interestingly, the PET foam with mGr had a more stable roof and indoor temperature

50

than that with AC. This is probably because mGr has a planar structure that can block and

absorb the radiation more efficiently than sphere-shaped AC which allows for more

transmission and the light scattering between particles. Therefore, more temperature

fluctuation happened.

For enhanced roofing insulation by IR-absorbing, the roof temperature can easily

reach ~100 ℃ due to the addition of IIA. Therefore, compare to PS with relatively low

thermal stability, PET is a more suitable polymer matrix for this application based on the

results we generated above.

Figure 3.28 Temperature of the air at 2 inches above the roof

51

Figure 3.29 Roof upper surface temperature change using different roofing materials

Figure 3.30 Indoor temperature change using different roofing materials

52

3.6 Strand Die Experiment

The optimized fomulations mentioned above were repeated on the strand die. When

the foams were extruded, several strands were welded together, forming a thin plate-

shaped sample (Fig 3.31). Almost the same foaming condition to the capillary die was

obtained. This demonstrates the logic of our die design which is applicable to a pilot-

scale extrusion foaming equipment.

Figure 3.31 Foam samples prepared by strand die

53

Chapter 4. Conclusion and Future Work

A controlled-hydrolysis approach was developed to prepare the low-density and

high-crystallinity PET foams. The water content and two types of hydrolysis agent

(moisture and AC) were investigated to optimize and balance between the foam density,

extent of hydrolysis and cell morphology. Compared to conventional chain extension or

branch resin modification, controlled-hydrolysis may result in linear polymer chains and

thus high crystallinity. Due to the high crystallinity and acceptable molecular weight even

after degradation, the CPET foams show excellent mechanical strength and thermal

stability (>200℃). As an infrared attenuation agent, micrographite (mGr) further improves

the cell morphology. The CPET foam with mGr or AC shows excellent insulation

properties under IR radiation. With high thermal stability (stable even higher than 200℃),

CPET foam is more desirable than PS for high temperature applications (>100℃) such as

future IR-absorbing roofing and external wall applications.

Our future work will include the following:

• Further investigation on why liner PET of lower molecular weight is more

advantageous for low-density foams

• Strand die optimization and lab-scale production line build-up

54

Bibliography

1. Lee, S.-T. Foam extrusion: principles and practice. (CRC Press, 2014).

2. Okolieocha, C., Raps, D., Subramaniam, K. & Altstädt, V. Microcellular to

nanocellular polymer foams: Progress (2004–2015) and future directions – A

review. European Polymer Journal 73, 500–519 (2015).

3. Costeux, S. CO2 -blown nanocellular foams. Journal of Applied Polymer Science

131, 1-16 (2014).

4. Lee, L. et al. Polymer nanocomposite foams. Composites Science and Technology

65, 2344–2363 (2005).

5. Rezwan, K., Chen, Q. Z., Blaker, J. J. & Boccaccini, A. R. Biodegradable and

bioactive porous polymer/inorganic composite scaffolds for bone tissue

engineering. Biomaterials 27, 3413–3431 (2006).

6. Ibeh, C. C. & Bubacz, M. Current Trends in Nanocomposite Foams. Journal of

Cellular Plastics 44, 493–515 (2008).

7. Suh, K. W. et al. Lightweight Cellular Plastics. Advanced Materials 12, 1779–1789

(2000).

8. Sauceau, M., Fages, J., Common, A., Nikitine, C. & Rodier, E. New challenges in

polymer foaming: A review of extrusion processes assisted by supercritical carbon

dioxide. Progress in Polymer Science 36, 749–766 (2011).

55

9. Schuetz, M. A. & Glicksman, L. R. A Basic Study of Heat Transfer Through Foam

Insulation. Journal of Cellular Plastics 20, 114–121 (1984).

10. Loh, R. R., Polasky, M. E., Rynd, J.P., Lee, L.J., Han, X. & Koelling, K. W. (2009).

U.S. Patent No. US7605188 B2. Washington, DC: U.S. Patent and Trademark

Office.

11. Annan, N., Delaviz, Y., Han, X. & Loh, R. R.(2013). U.S. Patent No. US

2013/004.0122 A1. Washington, DC: U.S. Patent and Trademark Office.

12. Han, X., Annan, N. & Delaviz, Y. U.S. Patent No. US 2018/0112052 A1.

Washington, DC: U.S. Patent and Trademark Office.

13. Guo, S. R. The Application and Developmental Trend of Exterior Wall External

Insulation. Applied Mechanics and Materials 174–177, 1367–1371 (2012).

14. Lee, S.-T, Park, C.B. & Ramesh N.S. Polymeric foams - science and technology.

(CRC Press, 2006).

15. Olabisi, O. & Adewale, K. Handbook of Thermoplastics, Second Edition. (CRC

Press, 2015).

16. Kima, D. & Seo, J. A review: Breathable films for packaging applications. Trends

in Food Science & Technology 76, 15–27 (2018).

17. Jankauskait, V. & Macijauskas, G. Polyethylene Terephthalate Waste Recycling

and Application Possibilities: a Review. Material Science 14, 119-127 (2008).

18. Bard Wilkinson, C. Scientists hope new enzyme will 'eat' plastic

pollution. CNN(2018). at https://www.cnn.com/2018/04/16/world/plastic-

pollution-science-environment-enzyme-intl/index.html

https://www.cnn.com/2018/04/16/world/plastic-pollution-science-environment-enzyme-intl/index.html

https://www.cnn.com/2018/04/16/world/plastic-pollution-science-environment-enzyme-intl/index.html

56

19. Carneado, S., Hernández-Nataren, E., López-Sánchez, J. F. & Sahuquillo, A.

Migration of antimony from polyethylene terephthalate used in mineral water

bottles. Food Chemistry 166, 544–550 (2015).

20. Austin, H. P. et al. Characterization and engineering of a plastic-degrading aromatic

polyesterase. Proceedings of the National Academy of Sciences 115, E4350–E4357

(2018).

21. Khemani, K. C. Extruded Polyester Foams. in Polymeric Foams 669, 54–80

(American Chemical Society, 1997).

22. Scheirs, J. Modern polyesters: chemistry and technology of polyesters and

copolyesters. (John Wiley & Sons, 2003).

23. Chuah, H. H. Crystallization kinetics of poly(trimethylene terephthalate). Polymer

Engineering & Science 41, 308–313 (2001).

24. Lin, C. C. The rate of crystallization of poly(ethylene terephthalate) by differential

scanning calorimetry. Polymer Engineering and Science 23, 113–116 (1983).

25. Jabarin, S. A. & Lofgren, E. A. Thermal stability of polyethylene terephthalate.

Polymer Engineering and Science 24, 1056–1063 (1984).

26. Buxbaum, L. H. The Degradation of Poly(ethylene terephthalate). Angewandte

Chemie International Edition in English 7, 182–190 (1968).

27. Kao, C.-Y., Wan, B.-Z. & Cheng, W.-H. Kinetics of Hydrolytic Depolymerization

of Melt Poly(ethylene terephthalate). Industrial & Engineering Chemistry Research

37, 1228–1234 (1998).

57

28. Zimmerman, H. & Kim, N. T. Investigations on thermal and hydrolytic degradation

of poly(ethylene terephthalate). Polymer Engineering and Science 20, 680–683

(1980).

29. Seo, K. S. & Cloyd, J. D. Kinetics of hydrolysis and thermal degradation of

polyester melts. Journal of Applied Polymer Science 42, 845–850 (1991).

30. Park, C.P., Garcia, G.A. & Watson, R.G. U.S. Patent No. US 005475037A.

Washington, DC: U.S. Patent and Trademark Office.

31. Paul Handa, Y. et al. Some thermodynamic and kinetic properties of the system

PETG-CO2, and morphological characteristics of the CO2-blown PETG foams.

Polymer Engineering & Science 39, 55–61 (1999).

32. Park, C. P., Garcia, G. A. & Watson, R. G. Expandable Foam from Amorphous

Polyester Resin. Journal of Cellular Plastics 39, 229–238 (2003).

33. Xanthos, M., Wan, C., Dhavalikar, R., Karayannidis, G. & Bikiaris, D.

Identification of rheological and structural characteristics of foamable

poly(ethylene terephthalate) by reactive extrusion. Polymer International 53, 1161–

1168 (2004).

34. Xanthos, M., Zhang, Q., Dey, S. K., Li, Y., Yilmazer, U. & O'Shea, M. Effects of

Resin Rheology on the Extrusion Foaming Characteristics of PET. Journal of

Cellular Plastics 34, 498 – 510 (1998).

35. Xanthos, M., Dey, S. K., Zhang, Q. & Quintans, J. Parameters Affecting Extrusion

Foaming of PET by Gas Injection. Journal of Cellular Plastics 36, 102 – 111 (2000).

58

36. Xanthos, M., Young, M. , Karayanndis, G. P. and Bikiaris, D. N. Reactive

modification of polyethylene terephthalate with polyepoxides. Polymer

Engineering and Science 41, 643-655 (2001)

37. Dhavalikar, R., Yamaguchi, M. & Xanthos, M. Molecular and structural analysis

of a triepoxide-modified poly(ethylene terephthalate) from rheological data.

Journal of Polymer Science Part A: Polymer Chemistry 41, 958–969 (2003).

38. Xanthos, M., Wan, C., Dhavalikar, R., Karayannidis, G. & Bikiaris, D.

Identification of rheological and structural characteristics of foamable

poly(ethylene terephthalate) by reactive extrusion. Polymer International 53, 1161–

1168 (2004).

39. Yang, Z., Xin, C., Mughal, W., Li, X. & He, Y. High-melt-elasticity poly(ethylene

terephthalate) produced by reactive extrusion with a multi-functional epoxide for

foaming. Journal of Applied Polymer Science 135, 45805 (2018).

40. Zhong, H., Xi, Z., Liu, T. & Zhao, L. In-situ Polymerization-modification Process

and Foaming of Poly(ethylene terephthalate). Chinese Journal of Chemical

Engineering 21, 1410–1418 (2013).

41. Yan, H., Yuan, H., Gao, F., Zhao, L. & Liu, T. Modification of poly(ethylene

terephthalate) by combination of reactive extrusion and followed solid-state

polycondensation for melt foaming. Journal of Applied Polymer Science 132, 1-11

(2015).

42. Xia, T., Xi, Z., Yi, X., Liu, T. & Zhao, L. Melt Foamability of Poly(ethylene

terephthalate)/Clay Nanocomposites Prepared by Extrusion Blending in the

59

Presence of Pyromellitic Dianhydride. Industrial & Engineering Chemistry

Research 54, 6922–6931 (2015).

43. Barzegari, M. R., Hossieny, N., Keshkar, M., Chapleau, N. & Park, C. B. Extrusion

foaming behavior of PET. Annual Technical Conference - ANTEC, Conference

Proceedings 3, 2389–2393 (2012).

44. Fan, C. et al. Extrusion foaming of poly (ethylene terephthalate) with carbon

dioxide based on rheology analysis. Journal of Cellular Plastics 52, 277–298 (2016).

45. Guo, H., Nadella, K. & Kumar, V. Effect of intrinsic viscosity on solid-state

microcellular foaming of polyethylene terephthalate. Journal of Materials

Research 28, 2374–2379 (2013).

46. Zheng, W.G., Lee, P.C. & Park, C.B. Extrusion foaming behavior of PET with CO2.

Annual Technical Conference - ANTEC, Conference Proceedings, (2007).

47. Yeh, S.-K., Yang, J., Chiou, N.-R., Daniel, T. & Lee, L. J. Introducing water as a

coblowing agent in the carbon dioxide extrusion foaming process for polystyrene

thermal insulation foams. Polymer Engineering & Science 50, 1577–1584 (2010).

48. Rizvi, G., Matuana, L.M. & Park, C.B. Foaming of PS/wood fiber composites using

moisture as a blowing agent. Polymer Engineering and Science 40, 2124-2132

(1998).

49. Pallay, J., Kelemen, P., Berghmans, H. & Dommelen, D. V. Expansion of

polystyrene using water as the blowing agent Macromolecular Materials and

Engineering 275, 18-25 (2000).

50. Zhang, C., Zhu, B., Li, D. & Lee, L. J. Extruded polystyrene foams with bimodal

cell morphology. Polymer 53, 2435–2442 (2012).

60

51. Zhang, C., Zhu, B. & Lee, L. J. Extrusion foaming of polystyrene/carbon particles

using carbon dioxide and water as co-blowing agents. Polymer 52, 1847–1855

(2011).

52. Okamoto, M. et al. Biaxial Flow-Induced Alignment of Silicate Layers in

Polypropylene/Clay Nanocomposite Foam. Nano Letters 1, 503-505 (2001).

53. Han, X., Zeng, C., Lee, L. J., Koelling, K. W. & Tomasko, D. L. Extrusion of

polystyrene nanocomposite foams with supercritical CO2. Polymer Engineering &

Science 43, 1261–1275 (2003).

54. Zeng, C., Han, X., Lee, L. J., Koelling, K. W. & Tomasko, D. L. Polymer–Clay

Nanocomposite Foams Prepared Using Carbon Dioxide. Advanced Materials 15,

1743–1747 (2003).

55. Shen, J., Zeng, C. & Lee, L. J. Synthesis of polystyrene–carbon nanofibers

nanocomposite foams. Polymer 46, 5218–5224 (2005).

56. Chen, L., Ozisik, R. & Schadler, L. S. The influence of carbon nanotube aspect

ratio on the foam morphology of MWNT/PMMA nanocomposite foams. Polymer

51, 2368–2375 (2010).

57. Xia, T., Xi, Z., Yi, X., Liu, T. & Zhao, L. Melt Foamability of Poly(ethylene

terephthalate)/Clay Nanocomposites Prepared by Extrusion Blending in the

Presence of Pyromellitic Dianhydride. Industrial & Engineering Chemistry

Research 54, 6922–6931 (2015).

58. Scamardella, A. M., Vietri, U., Sorrentino, L., Lavorgna, M. & Amendola, E.

Foams based on poly(ethylene terephthalate) nanocomposites with enhanced

thermal stability. Journal of Cellular Plastics 48, 557–576 (2012).

61

59. Hosseini, S. S., Taheri, S., Zadhoush, A. & Mehrabani-Zeinabad, A. Hydrolytic

degradation of poly(ethylene terephthalate). Journal of Applied Polymer Science

103, 2304–2309 (2007).

60. Mandelkern, L. Crystallization of Polymers: Volume 2, Kinetics and Mechanisms.

(Cambridge University Press, 2004)

61. Blaine, R. L. Determination of polymer crystallinity by DSC. TA Instruments, New

Castle, DE (2013).

62. Macosko, C. W. Rheology: principles, measurements, and applications. (VCH,

1994).

63. K. A. Anand, U.S. Agarwal, R. Joseph, Carbon nanotubes induced crystallization

of poly(ethylene terephthalate), Polymer. 47 (2006) 3976–3980.

64. Y. Gao, Y. Wang, J. Shi, H. Bai, B. Song, Functionalized multi-walled carbon

nanotubes improve nonisothermal crystallization of poly(ethylene terephthalate),

Polymer Testing. 27 (2008) 179–188.

65. Y. Ke, C. Long, Z. Qi, Crystallization, properties, and crystal and nanoscale

morphology of PET-clay nanocomposites, Journal of Applied Polymer Science. 71

(1999) 1139 –1146.

66. C.F. Ou, M.T. Ho, J.R. Lin, The Nucleating Effect of Montmorillonite on

Crystallization of PET/Montmorillonite Nanocomposite, Journal of Polymer

Research. 10 (2003) 127–132.

67. S.A. Jabarin, Crystallization kinetics of polyethylene terephthalate. II. Dynamic

crystallization of PET, Journal of Applied Polymer Science. 34 (1987) 97–102.

62

68. S.A. Jabarin, Crystallization kinetics of poly(ethylene terephthalate). III. Effect of

moisture on the crystallization behavior of PET from the glassy state, Journal of

Applied Polymer Science. 34 (1987) 103–108.

69. Awaja, F. & Pavel, D. Recycling of PET. European Polymer Journal 41, 1453–

1477 (2005).

63

Appendix A. Experimental Design

Table A.1 Neat PET foams

Sample Density Crystallinity Molecular weight

Crystalline PET (CPET)

Crosslink PET (XPET)

Commercial PET foam

Table A.2 Foaming CPET resins with moisture

Sample Water content Density Molecular weight Crystallinity

Virgen PET resin (E60A+)

PET (dried 24 h at 120℃) foam



PET (not dried) foam

64

Table A.3 CPET/AC with water foams

Sample Water content Density Molecular weight Crystallinity

CPET/0.2% AC with moisture



CPET/0.2% wet AC

CPET/0.5% wet AC

CPET/1.0% wet AC

CPET/0.2% wet AC (Dried for 5 min at 100 ℃)



Table A.4 Using mGr as IAA and to further manipulate the foams morphology

Sample Density

CPET/0.5% wet AC/0.1% mGr



CPET (0.12 wt. % water)/ 0.1% mGr



65

Appendix B. Optimization of Foaming Conditions

Table B.1 Optimized processing condition of neat CPET foaming

Extruder zone temperature (℃)

Z1 Z2 Z3

Z4

PBA

Injection

Z5 Z6 Z7 Z8 Z9

die

Z10 Z11

230 250 260 260 260 260 250 245 245 245 245

Screw Speed (rpm) Torque (%) PDie

(PSI) PPBA

(PSI) FluxPBA

(mL/min)

15 10-30 1000~1100 900-1000 2.0

66

Table B.2 Optimized processing condition of XPET foaming


Z1 Z2 Z3

Z4

PBA

Injection

Z5 Z6 Z7 Z8 Z9

die

Z10 Z11

200 220 235 235 235 235 230 225 225 220 215


(psi) PPBA

(psi) FluxPBA

(mL/min)

10-30 <10 1200-1500 1000-1200 1.5-4.0

Table B.3 Optimized processing condition of foaming CPET with moisture


Z1 Z2 Z3

Z4

PBA

Injection

Z5 Z6 Z7 Z8 Z9

die

Z10 Z11

230 250 260 260 260 260 250 245 245 245 245


(psi) PPBA

(psi) FluxPBA

(mL/min)

15 10-20

200-800

(depending on

water content)

200-600

(depending on

water content)

2.0

67

Table B.4 Optimized processing condition of foaming CPET using AC as water carrier


Z1 Z2 Z3

Z4

PBA

Injection

Z5 Z6 Z7 Z8 Z9

die

Z10 Z11

230 250 260 260 260 260 255 245 245 245 245


(psi) PPBA

(psi) FluxPBA

(mL/min)

15 20~30

600-1000

(depending on

water content)

500-800

(depending on

water content)

2.0

68

Appendix C. Water Content Measurement

Figure C.1 TGA and water content of wet activated carbon put in the atmosphere after

water filtration (wet AC = dried for around 20 hours in this thesis)

Figure C.2 TGA and water content of different types of activated carbon

69

Figure C.3 Water content of neat CPET resin with moisture (measured by HR83 Halogen

Mettler Toledo and data points collected manually)

70

Appendix D. Shear Viscosity of CPET and XPET Resins

Figure D.1 Shear viscosity of CPET and XPET Resins

71

Appendix E. Cell Morphology of Dry CPET/Carbon Particles Foam (less than 0.05

wt. % water)

Figure E.1 SEM picture of CPET/0.2 wt. % mGr (both dried) foam

Figure E.2 SEM picture of CPET/0.5 wt. % CNT (both dried) foam

72

Appendix F. Crystallinity Calculation from DSC Thermograms

Figure F.1 DSC thermograms of Commercial PET foam

73

Figure F.2 DSC thermograms of Crosslink PET (XPET) foam

Figure F.3 DSC thermograms of Virgin CPET resin

74

Figure F.4 DSC thermograms of Neat CPET foam (moisture content ~ 0.05 wt. %)


75



76

Figure F.8 DSC thermograms of CPET/0.5 wt. % AC /0.05 wt. % water foam

Figure F.9 DSC thermograms of PET/0.5 wt. % AC/0.13 wt. % water foam

77

Figure F.10 DSC thermograms of PET/0.5 wt. % AC/0.20 wt. % water foam

1 Carbon Particulate Assisted Extrusion Foaming of ...

Documents