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

1

Copyrighted by

Junjie Pan

2018

2i 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 ii 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.

iii 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.

iv 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

v 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

vi 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

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

viii 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

ix 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 x Figure 3.7 SEM picture of neat CPET foam with 0.12 wt. % moisture ...... 33

Figure 3.8 SEM picture of neat CPET foam with 0.25 wt. % moisture ...... 34

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 xi 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

xii 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, Energy Flexible PU, PE, transportation, absorption, ABS construction flexibility Insulators Construction, Low thermal Rigid PU, PS, PE, automotive conductivity, rigid PVC sound absorption Protection Packaging Soft and flat RIM PU, PS bead, surface cushioning PE and PP sheet Strength/weight Athletics, construction, Strength and RIM PU, x-linked marine, medical, softness PE, PS, PVC, decoration, household flexible PU phenolic, acrylics Chemical/electrical Packaging, electrical Chemical and Flexible vinyl electrical inertness 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, 1 both industry and academia have a wide range of research interest on polymeric foams, including nanocellular foams with ultra-low thermal conductivity, nanocomposite foams,

2-8 biodegradable foams, etc. .

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. 2

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).

3

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

∆퐺푐푟푖푡 푁1 = 퐶0푓1 exp (− ) 푘퐵푇 4 16휋휎3 ∆퐺 = 푐푟푖푡 3∆푃2

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

f1 - Coefficient weakly depending on temperature

퐺푐푟푖푡 - Critical Gibbs free energy of nucleation

푘퐵 - Boltzmann constant

T - Foaming temperature

∆푃 - Pressure drop

휎 – 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.

5 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 6 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, 7 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.

8 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

3 produced PETG foam with a density of 0.04 g/cm 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 9 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

3 of around 0.10 g/cm 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

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

10 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 11 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 12 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 13 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

16휋휎3 푓(푚, 푤) ∆퐺 = 푐푟푖푡 3∆푃2 2

푓(푚, 푤) 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.

14 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.

15 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 Laser+ E60A, DAK I.V.= 0.81 dL/g Dried at 130℃ (CPET) America Tm = 245 ℃ overnight Crosslink PET Array 3962, DAK America I.V.= 0.67 dL/g Dried at 120℃ (XPET) Tm = 230 ℃ overnight Activated carbon Carbon Resources Made from Varied from (AC) Company, USA coconut shell, different purposes diameter 7 µm Micro-Graphite Qingdao Yanhai Carbon Thickness: ~0.5 Stored in a (mGr) Materials Inc., China µm 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

16 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,

17 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. 18 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

19

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

푚푓표푎푚 ρ푓표푎푚 = ρ푤푎푡푒푟 푚푓표푎푚 + 푤 − 푏

푚푓표푎푚 is the true mass of the foam sample in air. 푤 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. 푏 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 20 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

푛 3 ρ푢푛푓표푎푚푒푑 푁0 = ( )2 퐴 ρ푓표푎푚

Where n is the number of the cells in a single SEM picture and the A is the area of this SEM picture. ρ푓표푎푚 is the foam density and ρ푢푛푓표푎푚푒푑 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:

∑ 푛 푑 퐷 = 푖 푖 ∑ 푛푖

Where 푛푖 is the number of cells with the diameter 푑푖 in the SEM pictures.

21 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 [휂] of PET was defined by the Billmeyer relationship:

휂 − 1 + 3ln (휂 ) [휂] = 0.25 푟 푟 퐶

푡1 휂푟 − 푟푒푑푢푐푒푑 푣푖푠푐표푠푖푡푦 푒푞푢푎푙푙푖푛푔 ; 푡0

푡0 − average solvent flow time, s;

푡1 − average solution flow time, s; 22 C – concentration of the PET solution, 0.5 dL/g

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

푎 6 Houwink equation: [η] = KMv . For PET, constant K and a equals 7.44 × 10 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:

∆퐻푚−∆퐻푐 Crystallinity X푐 % = × 100 % ∆퐻푚0

23 Where ∆퐻푚 is the melt enthalpy; ∆퐻푐 is the cold crystallization enthalpy; ∆퐻푚0 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℃

24 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

25

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 26 same). We can estimate the strand die geometry by using the capillary rheometer function which can be expressed by 62:

1 ∆푃푅 푛 푛휋푅3 푄 = ( ) ( ) 2푚퐿 1 + 3푛 Q – flow rate;

∆푃 – pressure drop;

퐿 – 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

퐿 Newtonian fluid. Then the equation above can be simplified to 푃 ∝ . Therefore, we 푑4 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

27 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

28 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 29 (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 Cell density Average cell size Crystallinity 3 3 (g/cm ) (cells/cm ) (µm) 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

30 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

31 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

32 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

Figure 3.7 SEM picture of neat CPET foam with 0.12 wt. % moisture

33

Figure 3.8 SEM picture of neat CPET foam with 0.25 wt. % moisture

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.

34

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 35 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

36

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

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

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.

37

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

38 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

39 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

40

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.

41

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.

42

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/0.09 moisture foam 123.3 16.2 248.4 27.7 10.2% CPET/0.12 moisture foam 121.7 20.0 250.2 27.3 6.4% 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 43 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 44 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

45

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

46 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 47 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

48

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 49 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

50

Figure 3.29 Roof upper surface temperature change using different roofing materials

Figure 3.30 Indoor temperature change using different roofing materials

51 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

52 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

53

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62

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 (dried 2 h at 120℃) foam PET (dried 1 h at 120℃) foam PET (not dried) foam

63

Table A.3 CPET/AC with water foams

Sample Water content Density Molecular weight Crystallinity CPET/0.2% AC with moisture CPET/0.5% AC with moisture CPET/1.0% 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 ℃) CPET/0.5% wet AC (Dried for 5 min at 100 ℃) CPET/1.0% 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/0.5% wet AC/0.2% mGr CPET/0.5% wet AC/0.5% mGr CPET (0.12 wt. % water)/ 0.1% mGr CPET (0.12 wt. % water)/ 0.2% mGr CPET (0.12 wt. % water)/ 0.5% mGr

64

Appendix B. Optimization of Foaming Conditions

Table B.1 Optimized processing condition of neat CPET foaming

Extruder zone temperature (℃) Z4 die Z1 Z2 Z3 PBA Z5 Z6 Z7 Z8 Z9 Z10 Z11 Injection 230 250 260 260 260 260 250 245 245 245 245 P (PSI) P (PSI) Flux (mL/min) Screw Speed (rpm) Torque (%) Die PBA PBA 15 10-30 1000~1100 900-1000 2.0

65

Table B.2 Optimized processing condition of XPET foaming

Extruder zone temperature (℃) Z4 die Z1 Z2 Z3 PBA Z5 Z6 Z7 Z8 Z9 Z10 Z11 Injection 200 220 235 235 235 235 230 225 225 220 215 P (psi) P (psi) Flux (mL/min) Screw Speed (rpm) Torque (%) Die PBA PBA 10-30 <10 1200-1500 1000-1200 1.5-4.0

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

Extruder zone temperature (℃) Z4 die Z1 Z2 Z3 PBA Z5 Z6 Z7 Z8 Z9 Z10 Z11 Injection 230 250 260 260 260 260 250 245 245 245 245 P (psi) P (psi) Flux (mL/min) Screw Speed (rpm) Torque (%) Die PBA PBA 200-800 200-600 15 10-20 (depending on (depending on 2.0 water content) water content)

66

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

Extruder zone temperature (℃) Z4 die Z1 Z2 Z3 PBA Z5 Z6 Z7 Z8 Z9 Z10 Z11 Injection 230 250 260 260 260 260 255 245 245 245 245 P (psi) P (psi) Flux (mL/min) Screw Speed (rpm) Torque (%) Die PBA PBA 600-1000 500-800 15 20~30 (depending on (depending on 2.0 water content) water content)

67

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

68

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

Mettler Toledo and data points collected manually)

69

Appendix D. Shear Viscosity of CPET and XPET Resins

Figure D.1 Shear viscosity of CPET and XPET Resins

70

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

71

Appendix F. Crystallinity Calculation from DSC Thermograms

Figure F.1 DSC thermograms of Commercial PET foam

72

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

Figure F.3 DSC thermograms of Virgin CPET resin

73

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

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. %)

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

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