Scale-Up of Extrusion Foaming Process for Manufacture of Polystyrene Foams Using Carbon Dioxide

Hongtao Zhang

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Mechanical and Industrial Engineering University of Toronto

Hongtao Zhang

Master of Applied Science

Department of Mechanical and Industrial Engineering University of Toronto

2010 Abstract

An initial evaluation of the scalability of extrusion foaming technology is conducted in this thesis. Both lab- and pilot-scale foam extrusion systems along with annular dies and flat dies were used to investigate the effects of extrusion system scale on the foam expansion. The effects of the processing conditions including die temperature and blowing agent content on the volume expansion of extruded polystyrene foams blown with carbon dioxide are presented. A systematic comparison of the effects of extrusion system scale on the expansion behavior of polystyrene foams blown with carbon dioxide at the consistent pressure-drop rate, demonstrated that the scale of the foam extrusion system does not affect the principles of the foaming process, and the effects of extrusion system size on the scale-up of foam techniques, such as shear rate and temperature uniformity, could be suppressed by tailoring the processing conditions and experimental parameters.

Acknowledgments

Throughout my graduate study at the University of Toronto, there are a multitude of people that have helped, supported, and encouraged me to make the experience a wealth.

I would like to express my deep and sincere gratitude to my supervisor, Professor Chul B.

Park. His valued supervision, guidance and support have made me go this far. The knowledge I have learned from him will have a great impact on my future career.

I would also like to thank Professor Hani Naguib and Professor Lidan You for agreeing to join my thesis committee.

My gratitude is extended to the Department of Mechanical and Industrial Engineering at the University of Toronto, for providing financial support through the University of Toronto

Fellowship.

I would like to thank my colleagues and fellow researchers in the Microcellular Plastics

Manufacturing Laboratory for their help and friendship over the past years. I thank all present and past MPML members, including Dr. Jing Wang, Dr. Zhenjin Zhu, Dr. Changwei Zhu, Dr.

Saleh Amani, Dr. Sunny Leung, Dr. Takashi Kuboki, Dr. Gary Li, Dr. Gangjian Guo, Dr.

Qingping Guo, Dr. Wenli Zhu, Dr. Wentao Zhai, Dr. Richard Lee, Dr. Yongrak Moon, Nan

Chen, Lilac Wang, Raymond Chu, Anson Wong, Yanting Guo, Peter Jung, Esther Lee, Marilyn

Law, Qingfeng Wu, Jingjing Zhang, Mingyi Wang, Weidan Ding, Wei Wang, Mohammad

Hasan, Ryohei Koyama, Davoud Alizadeh, Kamlesh Majithiya, and Reza Nofar.

iii

Also, I would like to acknowledge the professional technical support from Ryan Mendell,

Jeff Sansome in the Machine Shop for their quality work and professional advice.

My special thanks go to many staff members, including Brenda Fung, Donna Liu, Sheila

Baker, and Teresa Lai.

Finally, I would like to thank my family for their deep love and consistent support under any circumstance.

Table of Contents

Abstract ...... ii

Acknowledgments...... iii

Table of Contents...... v

List of Tables...... ix

List of Figures ...... x

List of Symbols...... xiv

Chapter 1 Introduction

1.1 Preamble ...... 1 1.2 Thermoplastic Foams and Their Processing ...... 2 1.3 Polystyrene Foams and Its Applications ...... 4 1.4 Foam Extrusion...... 5 1.5 Motivations of the Study...... 6 1.6 General Objectives...... 6 1.7 Overview of the Thesis ...... 7

Chapter 2 Theoretical Background and Literature Review

2.1 Theoretical Background on Foam Processing...... 11 2.1.1 Formation of Polymer/Gas Solution...... 11 2.1.2 Nucleation ...... 16 2.1.3 Cell Growth and Stabilization ...... 21 2.2 Blowing Agents...... 26 2.3 Foam Extrusion...... 27 2.3.1 Foam Extrusion Process ...... 28 2.3.2 Conventional Foam Extrusion...... 28 2.3.3 Microcellular Foam Extrusion...... 29 2.4 Volume Expansion Mechanism in Foam Extrusion...... 30 2.5 Characterization of Polymer Foams...... 33 v

2.5.1 Volume Expansion Ratio...... 33 2.5.2 Cell Density...... 34 2.5.3 Cellular Morphology ...... 34 2.6 Issues of Scale-Up in Foam Extrusion ...... 35 2.7 Objectives of the Thesis...... 36

Chapter 3 Processing Parameters and Strategies for PS foaming

3.1 System and Die Pressure...... 45 3.2 Pressure-Drop Rate ...... 46 3.3 Die Temperature ...... 46 3.4 Contents of Blowing Agents ...... 47

3.5 Strategies for Achieving Low-density PS Foams Using CO2 ...... 47

Chapter 4 Design and Construction of Experimental Equipment

4.1 Materials ...... 51 4.1.1 Plastics...... 51 4.1.2 Blowing Agent ...... 52 4.2 Lab-Scale Extrusion System ...... 52 4.3 Set-Up of Pilot-Scale Extrusion System ...... 52 4.3.1 Overview of the System ...... 52 4.3.2 Primary Extruder ...... 53 4.3.3 Blowing Agent Delivery System...... 53 4.3.4 Secondary Extruder ...... 54 4.3.5 Dryer...... 54 4.3.6 Feeder...... 54 4.3.7 Connector ...... 55 4.3.8 Flow Restrictor...... 55 4.3.9 Die...... 56 4.3.10 Downstream...... 57 4.4 Axiomatic Design of Flat Die ...... 57 4.4.1 Background and Problem Description...... 57 4.4.2 Coupling Analysis of Existing Design ...... 58 4.4.3 Decomposition of First Level FRs and DPs ...... 59

4.4.4 Coupling Analysis of Detailed Design ...... 60 4.4.5 Addition of Cost Constraint...... 61 4.4.6 Conclusion...... 62

Chapter 5 Investigation of Effects of Extrusion System Size on the Expansion of PS Foams Blown with CO2

5.1 Introduction...... 70 5.2 Estimation of Pressure-Drop Rate...... 71 5.2.1 Pressure-Drop Rate in Annular Die...... 71 5.2.2 Pressure-Drop Rate in Flat Die...... 73 5.3 Fabrication of Foams Using PS 685D and Annular Die ...... 74 5.3.1 Experimental Design and Procedure ...... 74 5.3.2 Results and Discussion...... 76

5.3.2.1 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Maximum Achievable Expansion Ratio ...... 76

5.3.2.2 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Cell Density ...... 78

5.3.2.3 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Cell Uniformity...... 81 5.3.3 Conclusions ...... 82 5.4 Fabrication of Foams Using PS 523W and Flat Die ...... 82 5.4.1 Experimental Design and Procedure ...... 83 5.4.2 Results and Discussion...... 84

5.4.2.1 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Maximum Achievable Expansion Ratio ...... 84

5.4.2.2 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Cell Density ...... 85

5.4.2.3 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences

on Cell Uniformity...... 85 5.4.3 Conclusions ...... 86

Chapter 6 Summary and Conclusions ...... 102

Chapter 7 Recommendations and Future Work...... 106

vii

References ...... 107

Appendix ...... 116

viii

List of Tables

Table 1.1 Classification of plastic foams………………………………………………… 8

Table 1.2 Common foam extrusion systems ……………………………………………… 8

Table 4.1 Rheological properties of two types of polystyrene…………………………….. 63

Table 5.1 Set-up of parameters on both lab- and pilot-scale system using annular die…… 87

Table 5.2 Set-up of parameters on both lab- and pilot-scale system using flat die ……….. 87

Table 5.3 Best fitting parameters for the master plot of PS 685D …………….………….. 88

List of Figures

Figure 1.1 Typical plastic foaming process using a physical blowing agent …………… 9

Figure 1.2 Flowchart of scale-up of extrusion foaming technology …………………….. 10

Figure 2.1 Sorption isotherm for general gas/polymer system …………………………… 38

Figure 2.2 Diffusivity of CO2 in polystyrene vs. temperature …………………………… 38

Figure 2.3 The free energy, ΔG, vs. radius of bubble, r, associated with the homogenous nucleation ………………………………………………………………………. 39

Figure 2.4 Heterogeneous nucleation schematic …………………………………………... 39

Figure 2.5 Comparing the energy needed for homogenous and heterogeneous nucleation…40

Figure 2.6 Model of a nucleated cell inside a polymer matrix ……………………………. 40

Figure 2.7 Cell coalescence caused by rupture of thin cell wall separating two cells …….. 41

Figure 2.8 Fundamental mechanism of gas loss in polymer foaming …………………….. 41

Figure 2.9 Foaming extrusion units and mechanisms ………………………………….. 42

Figure 2.10 Schematic of a tandem foam extrusion system …………………………….. 42

Figure 2.11 Schematic of a microcellular continuous processing system ……………….. 43

Figure 2.12 Effect of initial hump on volume expansion …………………………………. 43

Figure 2.13 Fundamental mechanism of volume expansion of extruded PP foams ……….. 44

Figure 3.1 Effect of initial hump on volume expansion …………………………………. 50

Figure 4.1 Schematic of lab-scale tandem foaming extrusion system …………………. 64

Figure 4.2 Schematic of pilot-scale tandem extrusion system with downstream …….... 64

Figure 4.3 Schematic of connector between two extruders ………………………………. 65

Figure 4.4 Schematic of resistance calibration system ……………………………………. 66

Figure 4.5 (a) Gas injection pressure vs. gas flow rate; (b) Barrel pressure vs. gas flow rate; (c) Pressure difference vs. gas flow rate ……………………………………… 66

Figure 4.6 Existing flat die design …………………………………………………….. 68

Figure 4.7 Decomposition of first level FRs and DPs …………………………………….. 68

Figure 4.8 3-D general view of coupler ………………………………………………….. 69

Figure 5.1 FRs and DPs of axiomatic design in foam processing ………………………… 89

Figure 5.2 Schematic of an annular die ………………………………………………….. 90

Figure 5.3 Volume expansion ratios using annular die & lab-scale tandem extrusion system ……………………………………………………………………….. 91

Figure 5.4 Volume expansion ratios using annular die & pilot-scale tandem extrusion system ………………………………………………………………………. 91

Figure 5.5 Cell densities using annular die & lab-scale tandem extrusion system … 92

Figure 5.6 Cell densities using annular die & pilot-scale tandem extrusion system … 92

Figure 5.7 Die pressure profile using annular die & lab-scale tandem extrusion system …………………………………………………………………………………. 93

Figure 5.8 Die pressure profile using annular die & pilot-scale tandem extrusion system …………………………………………………………………………………. 93

Figure 5.9 Density vs. dp/dt using annular die & lab-scale tandem extrusion system …………………………………………………………………………………. 94

Figure 5.10 Density vs. dp/dt using annular die & pilot-scale tandem extrusion system …………………………………………………………………………………. 94

Figure 5.11 Cell structures of PS (685D) foams at 130°C & 3wt% CO2 using annular die & (a) lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system ………………………………………………………………………… 95

Figure 5.12 Cell structures of PS (685D) foams at 130°C & 5wt% CO2 using annular die & (a) lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system ………………………………………………………………………….. 95

Figure 5.13 Cell structures of PS (685D) foams at 130°C & 7wt% CO2 using annular die & (a) lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system ………………………………………………………………………… 96

Figure 5.14 Schematic of a flat die ……………………………………………………….. 97

Figure 5.15 Volume expansion ratios using flat die & lab-scale tandem extrusion system ………………………………………………………………………………….. 98

Figure 5.16 Volume expansion ratios using flat die & pilot-scale tandem extrusion system

………………………………………………………………………….. 98

Figure 5.17 Cell densities using flat die & lab-scale tandem extrusion system ……….. 99

Figure 5.18 Cell densities using flat die & pilot-scale tandem extrusion system ……… 99

Figure 5.19 Cell structures of PS (523W) foams at 130°C & 3wt% CO2 using flat die & (a) lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system …………………………………………………………………………………. 100

xii

Figure 5.20 Cell structures of PS (523W) foams at 130°C & 5wt% CO2 using flat die & (a) lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system ………………………………………………………………………………… 100

Figure 5.21 Cell structures of PS (523W) foams at 130°C & 7wt% CO2 using flat die & (a)

lab-scale tandem extrusion system; (b) pilot-scale tandem extrusion system

…………………………………………………………………………………. 101

xiii

List of Symbols

Ab Surface area

B f Width of the flat die gap, m

Cinjected Amount of injected gas

Cs Solubility of gas in the polymer, cm3/g

3 C0 Concentration of gas molecules in solution, #/m

D Diffusivity, cm2/s

Da Outside diameter of the annular gap, m

Dlab Diameter of primary screw on lab-scale system

Dpilot Diameter of primary screw on pilot-scale system

2 D0 Diffusivity coefficient constant, cm /s

Ed Activation energy for diffusion, J

f0 Frequency factor of gas molecules joining the nucleus, 1/s

* ΔG hom The amount of free energy

H Henry's law constant, cm3 [STP]/g-Pa

xiv

Ha Width of the annular die gap, m

Hf Height of the flat die gap, m

3 H0 Solubility coefficient constant, cm [STP]/g-Pa

ΔHs Molar heat of sorption, J h Sheet thickness, cm k Boltzman constant, J/K

n kp Power-law index, Pa·S

M premature Amount of premature cell growth

Mt Mass uptake at time t, g

M0 Undissolved gas amount per unit volume

M∞ Equilibrium mass uptake after an infinite time, g

N Nucleation rate n Non-Newtonian index in power law

ps Saturation pressure, Pa

Psolubility Solubility pressure

ΔP Difference between the gas pressures inside the bubble and surrounding matrix

Q Volumetric flow rate, m3/s

R Gas constant, J/K

RPM lab Speed of primary screw on lab-scale system

RPM pilot Speed of primary screw on pilot-scale system

T Temperature, K t Elapsed time, s

Vb Expansion of gas inside a bubble of volume

VER Volume expansion ratio

η 0 Zero-shear-rate viscosity, Pa·s

λ Relaxation time, s

γ pb Surface tension

xvi 1

Chapter 1 Introduction

1.1 Preamble

Natural Polymers have been with human beings since the beginning of time and began to be chemically modified during the 1800s to produce a number of materials. The manufacture of synthetic polymers has started since the early twentieth century. During World War II, the polymer industry has continued to grow increasingly and since then it has evolved into one of the fastest growing industries in the world. For the year 2000, nearly 200 million tons of synthetic polymeric materials, or plastics, were produced throughout the world to satisfy the ever- increasing market demands [1, 2].

Compared to other materials, polymers have plenty of advantages such as light weight, corrosion-resistance, and ease of manufacture. Polymers, however, can be processed by foaming with blowing agents and thereby turn into much lighter forms, which are called polymer foams.

Polymer foams have an increasing impact on our life since they first came into use during the

1940s. Most people around the world will encounter polymer foams every day no matter where they live. Today, substantial needs for polymer foams with improved cushioning, insulating, energy absorbing, structural performances, and other characteristics have made the plastic foaming industry to be one of the most promising industries [3].

Plastic foaming technology has been developing for many decades; likewise, polymeric foams have evolved from scientific concepts to lab research, pilot line samples, and commercialization, since the 1930s [4]. Continuous advancements of foaming technology have greatly spurred commercial applications of plastics foams. Microcellular foaming technology

was introduced early in 1980s and has been further developed in an attempt to meet various

commercial needs. This technique enables engineers to make microcellular plastics by

controlling details of foam cellular structures [5].

Plastic foams have been produced by a variety of processes such as batch foaming,

extrusion foaming, and injection foam molding. Regardless of the methods, the foaming

mechanism always involves a series of kinematic events in polymer/blowing agent mixtures

which include dissolution of blowing agents, phase separation and hardening of cellular

structures. Extensive experimental and theoretical investigations have been conducted to

elucidate the plastic foaming behaviors and further develop foaming technologies. Microcellular

foaming technology has been applied to many polymer/gas systems and has been approved to be

very effective in reducing material consumption without compromising structural properties to a

large extent [6]. As we all know, the target of developing microcellular foaming technologies is

to implement it in industry and provide manufactures with tremendous advantages and benefits.

However, scaling up the foaming technology from lab-scale equipment to industry-scale

equipment can create so many unexpected problems and a lot of challenges. In this thesis, a

systematic investigation of scale-up of extrusion foaming technology will be presented; the

research will help with understanding the scaling up procedure and provide useful guidelines for

future works.

1.2 Thermoplastic Foams and Their Processing

Plastic materials can be classified into two general classes, thermosets and

thermoplastics. Unlike thermosets which can not be reprocessed once the product is formed,

thermoplastics can be reprocessed many times through certain approaches. Thermoplastics occupy more than 80% of the commercial polymer products in the world due to its advantage of

being reprocessed [7]. Likewise, thermoplastic foams have been considered as the ideal

candidates for fulfilling the increasing market needs.

Foam plastics are polymers with the present of voids or cells throughout the material. In

other words, plastic foams consist of two phases: dispersed gaseous voids and a continuous

polymer matrix. The cellular structure of plastic foams possesses excellent mechanical properties

and low-weight advantages compared to conventional structural materials. They are very

competitive in terms of the performances on impact-resistant ability, buoyancy, strength-to-

weight ratios, and cost efficiency. These unique properties enable plastic foams to be used

effectively for a diverse range of industrial applications such as insulation, packaging, light-

weighting, and many others [8, 9].

In general, plastic foams can be classified in different ways: by cell density or cell size as

conventional, fine-celled, and microcellular foams; by expansion ratio or bulk foam density as

high-, medium-, and low-density foams; by cell structure as open- and closed-cell foams [10].

The classification of plastic foams is summarized and referred to Table 1.1 [11-13].

There are some general principles and concepts that apply to plastic foam processing,

even though plastic foams are made from various materials and are produced in a lot of different

forms. Most plastic foams are fabricated by an expansion process, which consists of the

expansion of a gaseous phase dissolved or dispersed throughout the polymer melt. The gaseous

phase may be created by means of the separation of a dissolved gas, the release of gas from a chemical reaction, or the vaporization of a volatile liquid [15]. From a modeling point of view,

this foam expansion process typically comprises three fundamental stages: cell nucleation, cell

growth, and stabilization of foam structures [16]. Foam expansion process begins with cell

nucleating or forming of expandable bubbles within the polymer/blowing agent mixture by

reducing the system pressure or increasing the system temperature (either by applying external

heat or under the influence of heat of reaction). Phase separation between the dissolved gas and

the polymer matrix occur in this stage. Once nucleated, cells continue to grow as blowing agent

(or gas) diffuses into it. When a bubble reaches a critical size, the bubble continues to grow and gas diffuses into it rapidly. Then the bubble growth will be continuing until the cell stabilizes or

ruptures. Figure 1.1 illustrates a typical plastic foaming process using a physical blowing agent.

1.3 Polystyrene Foams and Its Applications

Thermoplastics include two kinds of polymers, amorphous polymers and crystalline

polymers. Polystyrene (PS), one of the amorphous polymers, is a rigid, transparent thermoplastic

in solid or glassy state at normal temperature. Once heated above its glass transition temperature,

PS turns into a viscous liquid form and can be an ideal plastic for extrusion and injection

molding. PS has been recognized and applied in a wide range of industries, and is one of the

most favorable plastics in the world. Especially, PS has been widely demonstrated to be an

excellent material for foaming process because of its amorphous structure and low glass

transition temperature around 105 oC [4].

As the first commercially produced foam, PS foams have been broadly used for a variety

of applications such as insulation, packaging, and furniture due to the desirable mechanical and

thermally insulating properties and the advantage of inexpensive cost. Basically, PS foams

exhibit favorable stretch-induced hardening, a typical characteristics of foam, due to the

amorphous molecular structure of PS [18]. Besides, PS foams display a better rigidity compared

to other thermoplastic foams, since PS has a higher flexural modulus at room temperature. PS

foams are often used as building insulation materials because they are perfect thermal insulators.

They also posses very good damping properties and therefore are often used in packaging. PS

foams can be used for non-weight-bearing architectural structures as well, due to its rigid nature.

In a word, PS foams can be found in thousands of places such as offices, home, grocery stores, restaurants and many others.

1.4 Foam Extrusion

Foam extrusion was developed from plastic extrusion process, and has been employed by

industries as a fundamental foaming approach since 1970s [19]. Extrusion process is an efficient

method to convert thermoplastics into common geometry products, considering the melting, molding, and forming nature of thermoplastics. After implemented by gas injection, mixing and

cooling functions, the extrusion process turns out to be an ideal approach for plastic foaming.

For a basic foam extrusion, several steps can be outlined to describe the foaming process.

A blowing agent (either physical or chemical) is mixed with a polymer melt under certain

pressure and dissolved in the melt inside a barrel. The saturated polymer/gas mixture is forced

downstream and then cooled by exchanger or cooling extruder. When the mixture approaches the

die exit, it will experience an abrupt pressure drop. Bubbles will be nucleated in the die once the

die pressure is below the solubility pressure. Nucleated bubbles continue to grow until cells

stabilize or rupture, resulting in foam expansion. These steps make plastic foam extrusion be a

complex procedure and therefore a delicate and professional extrusion foaming technology is

strongly needed.

In general, foam extrusion setup comprises three types: single extruder, tandem extruder,

and twin-screw extruder system. Each extrusion setup has its own strengths and weakness from

both technical and commercial perspectives. Table 1.2 shows the summary of common foam

extrusion systems.

1.5 Motivations of the Study

Many fundamental studies have been done and continuous efforts have been made to

improve foaming technologies by researchers [20, 21]. Microcellular foaming technology has

been verified to be an effective approach to make microcellular plastics, both on batch and

continuous foaming experimental systems [22-26]. Particularly, extrusion foaming processes

have attracted much attention because of its high productivity and variety of products compared

to batch foaming processes. However, those attempts can only be seen as the successes from

fundamental to lab-scale. The scale-up of foaming technologies from lab- to pilot-scale and from

pilot- to industry-scale, encounters more difficulties and involves various challenges. It is always

encouraging that researchers should not only focus on the incubation of novel technologies, but

consider the practical industry applications. Figure 1.2 illustrates a big picture of a long-term

plan of development and application of extrusion foaming technologies.

1.6 General Objectives

Regardless of the foaming system difference on screw size, residence time, flow rate,

melt temperature uniformity, and shear rate, the foaming process always experiences three basic

steps which are dissolution of gas in a polymer matrix, cell nucleation, and cell growth and

stabilization. Theoretically, we can get similar results using different size foaming equipment if

we adopt an identical foaming technique, because the foaming equipment are merely the devices

to implement their functions ensuring the stages of the foaming process. Of course, many

processing parameters are involved in foaming process, including system pressure, die pressure,

pressure-drop rate, processing temperature, and blowing agent content. The control of those

processing parameters is the key for fabricating desired foams. The general objective of this thesis is to carry on experiments of extrusion foaming using both lab- and pilot-scale systems

and identify the effects of equipment size on expansion behaviors of thermoplastics using a

physical blowing agent. Detailed objectives will be presented after a literature review in Chapter

1.7 Overview of the Thesis

Chapter 2 presents a literature review and theoretical background, leading to detailed

objectives of this thesis. It includes an in-depth review of polymer/gas solution formation, cell

nucleation, and cell growth and stabilization and an overview of foam extrusion.

Characterization of polymer foams and issues of scale-up are also included in this chapter.

Chapters 3 presents the processing strategies for PS foaming. System and die pressure, pressure-

drop rate, die temperature and blowing agent content are discussed as key processing parameters.

Chapter 4 describes the design and construction of experimental equipment. The set-up of pilot-

scale extrusion system is presented in detail. In Chapter 5, an investigation of effects of extrusion system size on the expansion of PS foams blown with CO2 is presented. The comparison of

effects of processing conditions on volume expansion and their influences on maximum

achievable expansion ratio, cell density, and cell uniformity is discussed systematically. Chapter

6 provides a summary and presents the conclusions of this study. Finally, recommendations and

future research directions are presented in Chapter 7.

Table 1.1 Classification of Plastic Foams

Table 1.2 Common Foam Extrusion Systems

Reproduced from [19]

Figure 1.1 Typical Plastic Foaming Process Using a Physical Blowing Agent

Figure 1.2 Flowchart of Scale-up of Extrusion Foaming Technology

Chapter 2 Theoretical Background and Literature Review

2.1 Theoretical Background on Foam Processing

There are three major steps in a foaming process: formation of polymer/gas solution, cell

nucleation, and cell growth. In order to better understand those steps, a review of theoretical backgrounds is absolutely necessary.

2.1.1 Formation of Polymer/Gas Solution

In continuous foam processing, formation of a uniform polymer/gas solution is needed

since the quality of solution formation strongly affects the number of bubbles nucleated

afterward. An accurate amount of blowing agent must be ensured to be injected into the barre1 to

mix and dissolve into the polymer melt in a continuous process. Extra gas that cannot be

dissolved into the polymer melt will form large voids, which are detrimental to foaming process.

Therefore, it is important that the amount of blowing agent that can be absorbed and dissolved

into the polymer (i.e., the solubility) be determined at different processing temperatures and

pressures. This information is necessary for the production of microcellular foam in order to

avoid the presence of large voids.

Solubility

Solubility limit, the maximum amount of gas that can be dissolved into the polymer

matrix, can be determined in a batch process over a limited range of temperature. In this process,

a sheet polymer sample is saturated with the blowing agent by placing it in a high-pressure

chamber connected to a blowing agent cylinder. Because of the high pressure, the blowing agent

12 starts to diffuse into the polymer matrix, and then diffusion continues until a certain limit of blowing agent concentration is reached (Figure 2.1). Theoretically, the solubility limit only could be obtained at time infinity. The instantaneous concentration of the blowing agent in the polymer, however, can be calculated using the following equation [27]:

M 8 ∞ 1 ⎡ D(2m +1)2 π 2t ⎤ t 1 exp = − 2 ∑ 2 ⎢− 2 ⎥ (2.1) M ∞ π m=0 (2m +1) ⎣ h ⎦ where D = diffusivity, cm2/s,

Mt = mass uptake at time t, g,

h = sheet thickness, cm,

M∞ = equilibrium mass uptake after an infinite time, g,

t = elapsed time, s.

If the absorption time is sufficient enough, the amount of mass uptake eventually tends to level off at a maximum achievable amount, M, which is related to the solubility limit of the gas in the polymer. The solubility limit can be calculated by dividing the mass uptake (M) by the mass of the polymer sample.

The maximum amount of gas that can be dissolved into the polymer matrix may vary depending on the system pressure and temperature, and can be estimated by Henry's law [28]:

Cs = Hps (2.2)

3 where Cs = solubility of gas in the polymer, cm /g or g(gas)/g(polymer),

H = Henry's law constant, cm3 [STP]/g-Pa,

ps = saturation pressure, Pa.

The constant H is a function of temperature can be described by:

ΔH H = H exp(− s ) (2.3) 0 RT

where R = gas constant, J/K,

T = temperature, K,

3 H0 = solubility coefficient constant, cm [STP]/g-Pa,

ΔHs = molar heat of sorption, J.

ΔHs can be either a negative or positive value, depending on the polymer-gas system. It should

be noted that ΔHs is negative for most polymers with CO2 as the penetrant [28].

Combining Equations (2.1) and (2.2), the solubility of a blowing agent in the polymer at a

certain processing pressure and temperature can be determined. The estimation of the solubility

of CO2 in some polymers has been investigated in References [l7, 29]. For example, the

solubility of CO2 in PS at a temperature of 200°C and a pressure of 27.6 MPa (4000 psi) is

around 11 wt%. If the flow rate of the polymer in an extrusion process is known, the gas must be

continuously injected with a proportional flow rate so that the ratio of gas-to-polymer weight can

be maintained below the solubility limit during the process.

Diffusivity

The initial slope of the curve in Figure 2.1 corresponds to the diffusivity of the blowing agent into the polymer matrix. This slope can be used to calculate the diffusivity using the following equation [27]:

0.04919 D ≅ t (2.4) ( ) h 2 1/ 2

2 2 where (t/h )1/2 is the value of (t/h ) at Mt/M∞ = 1/2. The time required for the completion of absorption can be estimated as the following equation [30]:

πh 2 t ≅ (2.5) D 16D i.e., the time of absorption is governed by the diffusivity (D) and the diffusion distance which is h/2.

The diffusivity D is basically a function of temperature. The relationship between the temperature and the diffusivity can be referred to as follows [27, 28]:

Ed D = D exp(− ) (2.6) 0 RT

2 where D0 is diffusivity coefficient constant, cm /s,

Ed = activation energy for diffusion, J.

2 For example, in a polystyrene and CO2 system, the value of Do is 0.128 cm /s and Ed/R is

3 4.35x10 K [28]. Figure 2.2 shows a plot of the diffusivity vs. temperature for the PS-CO2 system. As we can see from the plot, the values of diffusivities at 25oC (batch foaming

temperature) and 200oC (typical extrusion temperature) are 6x10-8 cm2/s and 1x10-5 cm2/s,

respectively. Apparently, the diffusivity at 200oC is more than two orders higher than that at

25°C, which means that diffusion rate is much faster in an extrusion foaming process than in a

batch foaming process.

Dissolution

In the batch process, the polymer sample continues to absorb the blowing agent and stops

as the concentrations of the blowing agent inside the polymer and on its surface are at

equilibrium. Because extra gas can barely be absorbed by the polymer above its solubility limit,

large voids due to the undissolved gas would not be generated in the batch process. In contrast, it

is possible to generate an amount of undissolved gas in the polymer matrix in a continuous

process, if an excess amount of gas is injected. Therefore, it should be ensured that the amount of

gas injected is below the solubility limit at the processing conditions. An advantage of the

extrusion foaming process, however, is that the dissolution time is significantly reduced due to

higher gas diffusivity at the high processing temperatures. Thus an extrusion foaming process is

relatively more cost-effective.

On the other hand, injection of the proper amount of gas does not necessarily guarantee a

formation of a one-phase polymer/gas solution. The time required for complete gas diffusion into

the polymer has to be always considered in the formation of the polymer/gas solution; if the

required diffusion time is longer than the melt residence time inside the foaming system, a one-

phase solution would not be achieved. Park et al. [17, 29] studied the diffusion behavior in an

16 extrusion process involving a mixing screw. It was indicated that a shear mixing promotes convective diffusion by the screw rotation, i.e., a high gas concentration region (gas bubble) has more chances to be brought into contact with a low gas concentration region (polymer melt).

Moreover, an increase of the interfacial area due to the stretching of gas bubbles in the shear field, which is generated by the motion of screw, enhances the diffusion process. It is also observed that introducing a dissolution enhancing device containing static mixers is beneficial to the dissolution process, because mixing elements which reorient along the flow direction generate shear fields to promote the solution formation.

In summary, the following two approaches should be taken to produce a one-phase solution in the extrusion foaming process: i) injecting the blowing agent below the solubility limit; ii) enhancing diffusion by installing a dissolution enhancing device containing mixing elements.

2.1.2 Nucleation

A transformation of a large number of gas molecules to small cells of micron size, which is called nucleation, must be generated in a microcellular foaming process, and thus it is a critical step in this process. The nucleation of bubbles in a liquid has been investigated using the classical nucleation theory [31, 32]. This classical nucleation theory was originally developed for a single-component system, in which the second phase is created by evaporation of the liquid when superheated. Blander and Katz [31] further applied the theory to a diffusion system where one of the components is volatile as a blowing agent. Here, the bubble nucleation is governed by a diffusion process of the dissolved component into the nucleation sites.

In order to create bubbles in polymer melts, a minimum amount of energy must be given to the system for breaking the free energy barrier. This energy can be provided by either heating

or pressure drop. One of the two nucleation types is called homogenous nucleation, where cells

can be nucleated randomly throughout the polymer matrix. The other is heterogeneous

nucleation, which is often generated at certain preferable sites including phase boundary and

sites in additives where less energy is required for nucleation compared to homogenous

nucleation.

Homogenous Nucleation

In the case of homogeneous nucleation, Colton and Suh [33, 34] described the nucleation

behavior in microcellular foaming using the classical nucleation theory. In the foaming process,

the thermodynamic system is the polymer melt and gas dissolved in it. According to this theory,

the work required to generate a bubble of radius r in a liquid can be given by [32]:

W = γ pb Ab − ΔPVb (2.7)

where the first term, γ pb Ab , is the work required to create a bubble with a surface area, Ab , and a

surface tension, γ pb ; the second term , ΔPVb , is the work done by the expansion of gas inside a

bubble of volume Vb ; the difference between the gas pressures inside the bubble and the

surrounding matrix, ΔP, is estimated to be a saturation pressure. By substituting Ab and Vb with the surface area and the volume of a sphere, Equation (2.7) can be rewritten as follows:

2 4 3 W = 4πr γ − πr ΔP (2.8) pb 3

Figure 2.3 depicts the variation of energy (W) with radius (r). It is seen that there exists a

maximum energy which must be overcome for the bubble to grow spontaneously. If the energy

induced in the system is lower than this maximum energy (or free energy barrier), the bubbles

* with radius (r

16πγ ΔG* = pb hom 3ΔP2 (2.9)

The nucleation rate is described by:

. ΔG* N = C f exp(− ) (2.10) 0 0 kT

3 where C0 = concentration of gas molecules in solution, #/m ,

f0 = frequency factor of gas molecules joining the nucleus, 1/s,

k = Boltzman constant, J/K.

The classical nucleation theory for a homogenous system indicates that the higher the

saturation pressure (ΔP), the greater the number of cells nucleated. This prediction has been

practically verified in the batch process [33, 34]. The saturation pressure in a batch process can

be estimated as the gas concentration in the polymer as given by Henry's law (Equation (2.2)).

When the amount of gas increases, the chance of nucleation is increased and thus a larger

number of nucleated cells are achieved.

Pressure-drop rate is an important parameter that influences the number of cells

nucleated, but the effect of the pressure-drop rate on cell nucleation was not predicted by this

nucleation theory. Park et al. [24] investigated the effect of the pressure-drop rate in an extrusion

foaming process. Various pressure-drop rates were induced by using diverse dies and the final

foam structures were examined. Their study verified the prediction that the higher the pressure-

drop rate, the greater the number of cells nucleated. This effect can be explained by the

mechanism of cell nucleation/growth competition [29]. If the pressure-drop rate is high, the

polymer-gas system experiences a certain pressure drop in a shorter time period and some cells are nucleated in the die due to the thermodynamic instability. The gas in the polymer/gas system tends to diffuse to the existing cells because it lowers the free energy of the system. During this shorter time period, the pre-nucleated cells grow less because the time available for gas diffusion to the cells is very short. Because less gas is consumed for the cell growth, more gas is available for the chances of nucleation in the polymer matrix, i.e. Co is higher (Equation 2.10) and thus a

nucleation rate increases. A final foam structure with a high cell density can be achieved as a result. Therefore, a higher pressure-drop rate is necessary for achieving a microcellular structure in the foaming process.

From Equation (2.10), it is predicted that the nucleation rate increases with temperature increase. However, this effect has not been extensively studied for an extrusion process. Ramesh et al. [35] indicated that increasing the temperature increases the cell density in a PS-CO2 system. Goel and Beckman [36] studied the nucleation behavior of a PMMA-CO2 system and stated that the increase of the foaming temperature decreases the number of nucleated cells.

Matuana et al. [38] found that the foaming temperature does not have a significant effect on the number of nucleated cells in a PVC-CO2 system. Baldwin et al. [37] demonstrated that

increasing the foaming temperature increases the cell density in the case of amorphous PET and

CPET below l00oC, but not strongly affect the cell density in the case of semi-crystalline PET

and CPET.

In summary, the following two principles is recommended in a microcellular extrusion foaming process: i) A higher pressure is needed to maintain a large amount of gas dissolved in the polymer melt according to Henry's law; ii) A high pressure-drop rate needs to be induced to generate a larger number of cells from a polymer/gas solution. High pressure-drop rate dies must be employed so that a rapid pressure drop can be promoted.

Heterogeneous Nucleation

The other type of nucleation is heterogeneous nucleation, which is initiated at some preferred sites. Heterogeneous nucleation occurs at the interface of a liquid and a neat surface accounting for the surface energy (Figure 2.4) [39]. This can be promoted by mixing the polymer with an additive, which is called a nucleating agent. Nucleation then tends to occur at the boundary of the matrix and additive rather than inside the polymer matrix; this phenomenon is not often observed in homogenous nucleation. At these boundaries, the free energy barrier for nucleation is lower than that in homogenous nucleation (Figure 2.5). As a result, heterogeneous nucleation is more likely to occur rather than homogenous nucleation.

By precisely controlling the amount of additive, cell nucleation can be promoted to a desired extent [25, 40-42]. Xu et al. [25] identified that the cell density of extruded PS foams increases with adding talc as the nucleating agent. Moreover, the effects of talc on cell nucleation were found to be different with various geometry dies; i.e. a relatively significant effect was observed with a lower pressure-drop rate die. Using additives consisting of very small particles of less than a micron size as a nucleating agent, which is well dispersed in the polymer matrix, may help promote cell nucleation in a microcellular foaming process. Han et al. [41] indicated that adding a small amount of intercalated or exfoliated nano-clay significantly reduces cell size and increases cell density. However, it is difficult to generate a large number of cells of micron

size using additives due to poor dispersion and agglomeration of the additive particles [40, 42].

And also, cell density would not be sensitive to the amount of the nucleating agent if it exceeds

some critical value.

Effect of Shear Stress on Cell Nucleation

In addition to the amount of nucleating agent, shear stress also has significant effect on

the cell nucleation density during the foaming process. Chen et al. [44] studied the effect of shear

stress on cell nucleation density. It was found that the effect of shear stress becomes more

important if the saturation pressure, or the amount of gas in the polymer, is getting lower. In

other words, the shear stress is more critical while the driving force for cell nucleation is

insufficient. In a dynamic system such as extrusion foaming, the shear force also affects the

heterogeneous nucleation rate [43, 45-46]; the number of nucleated bubbles increases as the

shear force increases. Lee [45, 46] developed a lump cavity nucleation model for studying this

phenomenon. According to this model, the cavities on the rough surfaces of the tiny nucleating

particles, which are not completely wetted by the polymer melt, can form potential sites for

bubble nucleation. Once the gas phase in the cavity grows and matures due to diffusion of the

dissolved blowing agent into the cavity or a pressure drop, the applied shear force helps enhance

the chance of detaching it from the cavity, which is beneficial to generating a bubble.

2.1.3 Cell Growth and Stabilization

Once cells are nucleated, they continue to grow resulting from the gas diffusion from the

matrix into the nucleated cells, because the solubility of gas in the polymer is dropped with the pressure decrease (Henry's law). Moreover, the cells tend to grow to minimize this difference

because the pressure inside the cells is greater than the pressure in the surrounding matrix [8].

The cell growth mechanism is affected by a number of system parameters such as viscosity,

diffusion coefficient, gas concentration, and the number of nucleated bubbles. For example, the

cell growth rate increases due to the decrease in resistance against the cell growth, as the

polymer viscosity decreases with a temperature increase. The cell growth may stop if all the gas

dissolved in the polymer matrix is depleted or the matrix is too stiff to allow the further cell

growth.

Figure 2.6 shows a nucleated cell inside a polymer matrix charged with a gaseous

blowing agent. When the cell is nucleated, the gas concentration around the cell decreases. This

movement generates a gradient of gas concentration around the cell, which induces further cell growth. Basically, the cell growth is governed by the time allowed for the cells to grow, the system temperature, the hydrostatic pressure or stress applied to the polymer matrix, the viscoelastic property of the polymer/gas solution, and the degree of super-saturation [47].

During the cell growth, cell coarsening, cell coalescence and cell collapse are three critical issues should be taken into consideration in order to prevent cellular structure degradation. Any of these issues is detrimental to the cell-population density and may deteriorate the mechanical properties of foams. Therefore, proper strategies should be implemented in the continuous foaming process.

Cell Coarsening

For a given foam volume, the system is more stable with fewer large cells than with more small cells [8]. Since the gas concentration in a small bubble is higher than that in a large bubble, the gas concentration gradient will drive the gas to diffuse from the smaller bubble to the larger one. As a result, the smaller bubbles tend to get smaller, and eventually disappear; the larger bubbles tend to get larger, and finally the two bubbles become one large bubble. This

23 phenomenon is called cell coarsening. When the cell coarsening occurs, the cell-population density is deteriorated.

Cell Coalescence

Cell coalescence is a mechanism where two growing contiguous cells in a polymer melt combine with each other because of cell wall rupture. When a number of small cells are nucleated in a continuous process, they start to grow very quickly due to the high diffusion of gas from the matrix into the cells especially at high temperature. On the other hand, the polymer matrix is softened at high temperature, and thus has less resistance against the growth of cells by holding bubbles. When the nucleated cells grow and then come in contact with each other, contiguous cells tend to coalesce since the total free energy will be reduced through the coalescence of cells [24, 48-49]. Figure 2.7 is a schematic of two growing contiguous cells in the polymer melt. Cell rupture is promoted when the stretched thin cell wall separating two cells is not strong enough to sustain the tension developed during cell growth. Moreover, it should be noted that the shear field generated during the shaping process tends to stretch nucleated bubbles, and this will further accelerate cell coalescence [49]. When cells are coalesced, the initial cell- population density will deteriorate as a result. In other words, although a large nuclei density is achieved by the independent control of cell nucleation, the final cell-population density of the foam produced might not be fine-celled because of the effect of cell coalescence.

As we can see, cell coalescence occurs mainly resulting from the weak melt strength of the molten polymer. The melt strength may be considered the degree of resistance to the extensional flow [50] of the cell wall during the drainage of polymer in the cell wall when the volume expansion takes place. Namely, the cell wall stability increases when the melt strength increases [26]. Since the melt strength increases with the decrease of the temperature [51], the

processing temperature should be controlled to maintain as low as possible during the foaming process in order to minimize the degree of cell coalescence.

The approach of lowering the temperature to prevent cell coalescence at the die orifice has been extensively used in conventional foam extrusions, especially in low-density foaming process [39, 52]. Controlling the die temperature alone, however, results in a non-uniform foam

structure. In order to solve this problem, Behravesh et al. [39, 53-54] proposed a new method to

suppress cell coalescence by increasing the polymer melt strength via temperature control in

extrusion foaming. The melt temperature was controlled independently to achieve uniform

cooling before the melt reaches the die, by using a heating exchanger with static mixers. By this

means, microcellular HIPS foams with a cell density of 1010 cells/cm3 and fine-celled HDPE

8 3 foams with a cell density of 10 cells/cm were successfully produced using CO2 as a blowing

agent.

The other approach to suppress cell coalescence is the employment of high-melt-strength

(HMS) materials. Naguib et al. [55, 56] obtained foams with a very large volume expansion ratio

(up to 90) using branched polypropylene material. This kind of material has a property of high

melt strength and melt elasticity, which is beneficial to solving the problem of bubble stability.

In contrast, the expansion ratio of foams obtained using linear polypropylene materials was much

lower than that achieved using branched polypropylene material because of severe cell

coalescence. Park et al. [57] also produced fine-celled biodegradable polyester foams with a high

cell density, which is on the order of 108 cells/cm3, by using branched materials.

Cell Collapse

Cell collapse is basically caused by the escape of the blowing agent (especially a gaseous blowing agent) into the atmosphere as the polymer melt is at a high temperature. Namely, most of the dissolved gas attempts to diffuse into the atmosphere rather than diffuse into the nucleated cells. Moreover, even the diffused gas in the cells eventually diffuses into the atmosphere, because the ultimate phase separation of the polymer and the gas has the minimum free energy of the polymer and gas system [49]. In the end, little gas remains in the matrix and the cells, and thus the cells collapse until stabilizing, resulting in the very low final expansion [55, 56]. Figure

2.8 shows a schematic of the gas escape phenomenon in foam processing. Therefore, gas loss has to be prevented as much as possible by proper cooling if foams with a large volume expansion ratio are to be produced. Besides, it should be noted that CO2 is very prone to escape because its diffusivity is much grater than that of high molecular-weight blowing agents such as isopentane or butane.

The rate of gas escape can be greatly decreased by lowering the die temperature, since the diffusivity of gas decreases as the temperature drops [1, 58-61]. Park et al. [39, 53-57, 90] developed an effective strategy for the promotion of larger volume expansion in extrusion foaming, i.e., to block gas escape by freezing the foam skin. By this means, Naguib [55, 56] also produced low-density polypropylene foams with an extremely high volume expansion ratio of up to 90 by tailoring the processing conditions; Behravesh et al. [39, 53, 54, 90] achieved low- density microcellular HIPS foams with a cell density of 1010 cells/cm3 and a volume expansion ratio of over 20, and also produced low-density, fine-celled HDPE foams with a cell density of

108 cells/cm3, an average cell size on the order of 50µm, and volume expansion ratio in the range of 1.5-20. In addition, filamentary biodegradable polyester foams with a cell density on the order of 108 cells/cm3 and a volume expansion ratio of over 40 were successfully achieved by Park et al. [57].

2.2 Blowing Agents

Polymer foaming process involves the introduction of foaming agents, which play a

significant role in both the manufacturing and performance of polymer foams. There are two

general types of foaming agents, physical foaming agent and chemical foaming agent. A

chemical foaming agent is a pure chemical which reacts to produce blowing gas by thermal

decomposition. Chemical foaming agents are extensively used to produce relatively higher

density foams. A physical foaming agent is metered directly into the polymer melt in either

liquid or gas phase at a specified temperature and pressure, without any chemical changes.

Physical foaming agents are the main sources of blowing agents used for low-density foam

production [7].

Physical blowing agent (PBA) is involved in physical foaming process and has a large

effect on foam density. PBAs are classified as either volatile liquids or pressurized gases. PBA

can provide gas for the expansion of polymers by the change of physical state, which may

involve volatilization of a liquid or release of a pressurized gas at a specified temperature and

pressure.

Conventional liquid PBAs are volatile liquids such as chlorofluorocarbons (CFCs), and

include short-chain (C5 to C7) aliphatic hydrocarbons and halogenated C1 to C4 aliphatic hydrocarbons. For instance, CFCs had been the sole physical blowing agents for styrenic and olefinic foams for more than forty years, because of their advantages such as low thermal conductivity, non-flammability, and high solubility in styrenics and olefinics materials. Since these blowing agents have low diffusivities due to larger molecular size, the loss of gas from the extrudate during expansion is quite less and thus the volume expansion of the foam is high.

However, these blowing agents were found to be hazardous to the ozone layer of atmosphere,

thereby being banned to be used by the Montreal Protocol in 1987. Common gaseous PBAs

include carbon dioxide (CO2), nitrogen (N2), short-chain (C2 to C4) aliphatic hydrocarbons and

halogenated C1 to C4 aliphatic hydrocarbons, which are usually used in the production of low-

density (less than 50kg/m3) foams. They are relatively inexpensive but may require special

equipments for use in some cases [9]. Due to the increasing concerns in environmental impact

and the safety issue of using the volatile liquids such as butane and pentane that have high

flammability, these PBAs have been gradually replaced by inert gases such as carbon dioxide

and nitrogen [9, 62]. These inert gases dissolve as vapors in the polymer melt and diffuse out of

the solution as vapors upon pressure reduction to expand the polymer melt.

Particularly, CO2 is an inexpensive inert gas which is safe to use as a blowing agent. Due

to its volatile characteristics, CO2 is essential for producing high cell-density foams or

microcellular foams. The volatility of the gas creates a larger thermodynamic instability upon a sudden pressure drop; this generates higher cell nucleation rate, which leads to a higher possibility of obtaining microcellular structures. Since CO2 has high diffusivity and low

solubility in the polymer matrix, it is difficult to achieve a high volume expansion ratio with CO2 as a blowing agent in high temperature processing.

2.3 Foam Extrusion

Foaming approaches can be implemented in processes such as batch foaming, foam

extrusion, and injection foam molding. It is worth noting that the foam extrusion dominates the commercial low-density thermoplastic foam production.

2.3.1 Foam Extrusion Process

A typical PBA-based foam extrusion generally comprises polymer melting, polymer and gas mixing, cooling, and shaping. Figure 2.9 shows a schematic of the necessary processes for foam extrusion along with its relevant mechanisms [4].

A schematic of a tandem foam extrusion system is outlined in Figure 2.10. Firstly, polymer resins are plasticized and blowing agents are injected directly into the polymer melt or pre-compounded polymer materials in the first extruder. Inside the barrel the pumping action of the first extruder generates a very high pressure, which is essential to the dissolution and saturation of the blowing agents in the polymer melt. Secondly, the second extruder provides mixing and initial cooling for the polymer melt and the gear pump controls the melt flow rate, independent of temperature and pressure changes. And then the single-phase polymer/gas solution is fed into the heat exchanger which provides further cooling to suppress cell coalescence. Lastly, the gas-saturated polymer melt enters the extrusion die, and foaming occurs due to pressure drops while the melt exits through the die [7, 15].

2.3.2 Conventional Foam Extrusion

Detailed reviews of conventional polymeric foams and their processes have been given in many references [51, 63]. Foam products with various densities can be obtained in a conventional foam process. The state-of-the-art foams, however, possess characteristics of a fully-grown cell size greater than 100 µm (usually in the mm range), a cell population density lower than 106 cells/cm3, and a non-uniform cell size distribution [64, 65]. As a result, the mechanical properties of the conventional foams are relatively poor.

The most commonly used blowing agents in conventional foam processing include fluorocarbons (FC), hydro-chlorofluorocarbons (HCFC), chlorofluorocarbons (CFC), n-pentane,

and n-butane [48]. These agents can dissolve into the polymer melt in large quantities due to

their high solubility [51]. Therefore, a foam product with a high void fraction can be achieved at

a relatively low pressure in the foaming system. In addition, the loss of blowing agent from the

extrudate during expansion is small because the diffusivities of these agents are low due to their

large molecular size [8, 66-68]. These characteristics of the blowing agents allow the extrudate to

expand significantly, and thus the final product has a low foam density. Despite the favorable

properties of the conventional blowing agents, there are some serious environmental and safety

concerns in utilizing them. CFCs and HCFCs are known to deplete the ozone layer and their use

has been phased out [69]; other long chain blowing agents such as n-pentane and n-butane are

hazardous because of their high flammability. Moreover, due to the lower volatility of these

blowing agents compared to CO2 and N2, the thermodynamic instability during foaming is relatively low, resulting in the production of low cell density foams.

2.3.3 Microcellular Foam Extrusion

The concept of microcellular plastics was created by Suh [70] in response to industrial

needs of reducing material costs for certain plastic foams without a major compromise to mechanical properties. Microcellular plastics are characterized by a cell density of higher than

109 cells/cm3 and a cell size on the order of 10 microns. Because of the high cell density and the

uniform cell size distribution, microcellular foams have many advantages over conventional

foams. In addition, microcellular plastics processing technology uses environmentally benign

gases as the blowing agents [10, 71].

Microcellular plastics processing technology was first studied in a batch process [22] and

a product with a small cell size on the order of 10 µm was successfully achieved. However, the

batch process has a big disadvantage; a very long time is required for saturation of gas in the

polymer. This disadvantage is caused by a low rate of gas diffusion into the polymer at room

temperature. In order to overcome the shortcomings of the batch process, a cost-effective,

continuous microcellular process has been developed in extrusion system [17, 24, 73]. In this process, a much shorter time is needed for the saturation of the polymer with gas. A schematic of the microcellular extrusion process is shown in Figure 2.11. A polymer is melted in an extrusion

barrel, and a metered amount of gas is delivered to the polymer melt stream under a high

pressure. The injected gas diffuses into the polymer matrix at a rapid rate because of the

convective diffusion induced by the high shear rate in the extruder barrel and the increased diffusivity at an elevated temperature [72]. This results in the formation of a single-phase polymer/gas solution. The polymer/gas solution is then subjected to a thermodynamic instability, thereby nucleating microcells. This is usually achieved by rapidly dropping the solubility of gas in the polymer melt by controlling the temperature and pressure at the nucleation die [17, 24].

Compared to the batch foaming, a much shorter processing time is required in this continuous microcellular process due to a significant reduction in the diffusion time.

2.4 Volume Expansion Mechanism in Foam Extrusion

Foaming is dynamic in nature, and pressure, temperature, and volume vary throughout

the process. Since the purpose of doing foaming is to produce foams with a desirable expansion

ratio, it is critical to be able to control volume expansion. Besides, the effective control of the

volume expansion is very important to increase the efficiency of costly blowing agents.

Continuous efforts have been made to identify the mechanism of volume expansion in foam

extrusion.

Behravesh et al. [54, 90] claimed that an initial hump at the die exit promotes gas loss

during volume expansion. As gas escape from foams occurs through cell-to-cell diffusion, the

31 thickness and temperature of the cell walls separating the cells are critically important in determining the rate of gas escape. While the cells grow, the thickness of cell walls decreases. At the die exit, the temperature of the cell walls decreases due to the cooling through convection at the die orifice and isentropic expansion of gas. If the cells grow too quickly when the temperature of the cell walls is still high, cell walls become quite thin and thus the gas will escape very fast through the hot thin cell walls. In other words, the rapid initial expansion of extruded foam is detrimental to blocking gas loss, as shown in Figure 2.12. Their study indicated that the volume expansion of extruded foams is very sensitive to die temperature.

Naguib [56] adopted a CCD system to monitor the expansion phenomena of extruded foams at various processing conditions. His study identified the fundamental mechanisms governing the volume expansion of polypropylene foams. It was concluded that the volume expansion of extruded foams blown with a physical blowing agent is governed by either gas loss or crystallization of the polymer. Figure 2.13 illustrates the fundamental mechanism with the typical “mountain shape” curve of expansion versus die temperature. If the processing temperature is too high resulting in the too long solidification time, the gas that has diffused into the nucleated cells from the plastic melt may escape out of the foam. Consequently, foam contraction will occur and the volume expansion ratio will be decreased as a result. On the other hand, if the processing temperature is too low and close to the crystallization temperature, the polymer melt will be solidified too quickly before the foam is fully expanded. As a result, the volume expansion will be limited to a low extent. It was concluded that there is an optimum processing temperature for achieving maximum expansion. When the melt temperature (i.e. the processing temperature) is very high, the maximum volume expansion is governed by gas loss; the volume expansion ratio will increase with the decrease of the processing temperature. When the processing temperature is relatively low, the volume expansion ratio is governed by the

32 solidification (i.e. the crystallization) of the polymer; the volume expansion ratio will increase as the temperature increases.

Xu et al. [25, 74-76] indicated that the unavoidable and undesirable premature cell growth inside a die has a noticeable effect on volume expansion ratio. A great amount of premature cell growth results in big size cells at the die exit. The big size cells will cause instantaneous expansion (i.e. an initial hump) at the die exit due to the pressure drop, thereby accelerating the gas loss. The amount of premature cell growth is determined by cell density, premature cell growth time, and premature cell growth rate, which are directly influenced by the die geometry. In their study, Xu at el. revealed that when the premature cell growth is severe, the volume expansion of extruded foams will be significantly getting worse. Equation (2.11) was

proposed to estimate the amount of premature cell growth, M premature , in a straight filamentary die.

4 3 M ≈ πNC ⋅ (Dt ) 2 + M premature 3 s Pr emature 0 (2.11) where N = cell density,

Cs = dissolved gas concentration per unit volume,

D = diffusivity,

tPr emature = premature cell growth time,

M0 = undissolved gas amount per unit volume.

Clearly, the premature cell growth amount is a function of cell density, gas concentration, gas

diffusivity, and premature cell growth time. If P < Psolubility, all the injected gas cannot dissolve

into the polymer melt and the term (M0) in Equation (2.11) will be described as follows:

Psolubility − P M 0 ≥ ×Cinjected (2.12) Psolubility

where Psolubility = solubility pressure,

Cinjected = amount of injected gas.

If the mixing of gas and polymer is fairly good and the residence time is long enough for the gas

dissolving [51], the value of M0 will be minimized, but P should be greater than Psolubility in order

to remove M0. It was also predicted that the volume expansion ratio would be deteriorated if the amount of premature cell growth exceeds some critical value.

2.5 Characterization of Polymer Foams

The volume expansion ratio and the cell density are often applied to describe the

processing-to-structure relationships, because these tow parameters indicate the degree of the cell nucleation and the expansion which have been controlled during the foam processing.

2.5.1 Volume Expansion Ratio

In general, the relative density of the foam is the reciprocal of its volume expansion ratio.

The relative density of a foam sample is often employed to evaluate the volume expansion of the

foam. The volume expansion ratio (VER) of a foam sample can be calculated as the ratio of the

bulk density of pure material to the bulk density of the foam sample as follows:

V ρ VER = foam ≈ polymer Vpolymer ρ foam (2.13)

The weight and volume of the foam samples can be measured by using a measuring apparatus. Water immersion and displacement is a common technique for measuring the bulk density of foam specimens [7].

2.5.2 Cell Density

Cell population density is defined as the number of cells per unit volume with respect to the unfoamed polymer. The cell density of the foam structure can be calculated using the following equation:

3 ⎛⎞# of cells 2 Cell density=⋅⎜⎟ VER (2.14) ⎝⎠area where # of cells = total number of cells in the area,

area = defined area, cm2,

VER = volume expansion ratio.

The number of cells in a defined area can be measured from the micrographs taken by a scanning electron microscope with the aid of image utility software.

2.5.3 Cellular Morphology

A scanning electron microscope (SEM) is typically used to examine the cell morphology of a foam sample. The cell morphology of a foam can be characterized by its cell size, cell density, and cell size distribution. The cell size of the cells in the foam can be measured from the

SEM micrographs. Cell size distribution of a foam specimen can be either estimated from the

SEM micrographs or measured with the mercury porosimetry [79].

2.6 Issues of Scale-Up in Foam Extrusion

Research experiment is normally performed on small laboratory extruders due to either

the concern about costly experimental resin, additives or other ingredients, or to minimize the

pre-experimental samples produced under equilibrium extrusion conditions. For instance, an

experiment running on a 2.5-inch extruder with a nominal throughput of 68 kg/hour requires

extruding 22.5-32.5 kilograms before the extruder reaches equilibrium processing conditions

producing a representative sample [77, 91]. Consequently, most research experiments have been done on smaller extruders.

If a process is being developed on a small extruder (1.5-inch diameter) to save material and facilitate development work, and the process has to be scaled up to a production extruder, this could be best accomplished first by developing the process on a pilot extruder (2.5-inch diameter) with a process of DOE (Design of Experiment) [78] to determine the critical processing parameters and interactions. When approaching the scale-up of a process with the mindset of DOE, the first concern is to develop model of the process behavior, from which true development of the process could be possible. The DOE defines the processing window, the

critical process parameters, and any process-product interactions. The validation of a series of

experimental running on a 2.5-inch extruder can establish the process viability and the process

criteria required for scale-up to a larger extruder, and provide confidence that scale-up is

feasible.

Scaling up foaming technology from a small extruder to a larger extruder can create

unexpected problems and many challenges. A minor processing problem on a small extruder can

become a major problem on a large extruder. As an example of a scale-up issue, degradation that

results from shear heating on a small extruder may be a minor or nonexistent problem. However,

36 if you scale up to a larger extruder, additional shear heat will magnify the polymer degradation.

In addition, other extruder issues can take place during scale-up to larger diameter extruders.

Extruder areas that might be affected include melt pressure uniformity, absolute melt pressure, melt temperature uniformity, absolute melt temperature, degree of mixing, shear heat, and die design.

During a scale-up from lab-scale extrusion to pilot-scale extrusion, any limiting factor, whether it is product or process related, needs to be identified and monitored, and their limits should be established. The product limiting factor could be property deterioration due to higher melt temperature resulting form higher processing rates. The ability to transfer heat into or from the resin to obtain the correct melt temperature is one of the process limiting factors. For instance, with low viscosity resins, heat transfer may be the limiting factor, due to lower conductive heat transfer in a large extruder compared to the viscous heat generated in a smaller extruder. Other potential process limiting factors in the extrusion process scale-up include volume capacity of the extruder, downstream cooling capacity.

2.7 Objectives of the Thesis

The objectives of the thesis are as follows:

i) To study the expansion behaviors of PS foams blown with 100% CO2 using both lab-

scale and pilot-scale extrusion foaming systems; volume expansion ratio, cell density,

and cell morphology of foams will be characterized for both systems and the results will

be compared in order to validate the foaming techniques.

ii) To investigate the effects of system size on the expansion behaviors of PS foams

blown with 100% CO2; annular die and flat die will be adopted in the experiments; based

37 on the experiments, the governing factors will be determined and general conclusions will be attempted.

Figure 2.1 Sorption isotherm for general gas/polymer system

Figure 2.2 Diffusivity of CO2 in polystyrene vs. temperature [90]

Figure 2.3 The free energy, ΔG, vs. radius of bubble, r, associated with the homogenous nucleation

Figure 2.4 Heterogeneous nucleation schematic [39]

Figure 2.5 Comparing the energy needed for homogenous and heterogeneous nucleation

Figure 2.6 Model of a nucleated cell inside a polymer matrix

Figure 2.7 Cell coalescence caused by rupture of thin cell wall separating two cells

Figure 2.8 Fundamental mechanism of gas loss in polymer foaming

Figure 2.9 Foaming extrusion units and mechanisms [4]

Figure 2.10 Schematic of a tandem foam extrusion system

Figure 2.11 Schematic of a microcellular continuous processing system [90]

Figure 2.12 Effect of initial hump on volume expansion

Figure 2.13 Fundamental mechanism of volume expansion of extruded PP foams [56]

Chapter 3 Processing Parameters and Strategies for PS foaming

Plastic properties, system parameters, processing setup, foaming, and post-handling make

up the essential elements of a professional foam extrusion technology. Processing parameters are the most critical factors to achieving desired foam expansion. These processing parameters such

as system pressure, die pressure, pressure-drop rate, die temperature, and blowing agent content should be considered very carefully. Moreover, foaming strategies need to be identified and then

implemented in foaming process.

3.1 System and Die Pressure

Proper system and die pressure profiles are prerequisites for microcellular foaming.

System and die pressures should be high enough to permit the dissolving of all the injected

blowing agents into the polymer matrix, and to form a one-phase polymer/gas solution. In other

words, the system and die pressures must be higher than the solubility pressure, which is the

minimum pressure required for the injected blowing agent to be completely dissolved into the

polymer matrix [17, 25]. Undissolved gas in the polymer matrix forms undesirable big pockets in

the resultant foams, which leads to the decrease of cell density resulting in the deterioration of

the mechanical properties. These big pockets are detrimental to the volume expansion ratio since

they can further promote gas loss via the reduced surface area, which gas must pass through so as

to escape from the foam.

3.2 Pressure-Drop Rate

The pressure-drop rate at the die exit plays an important role in cell nucleation. It is

understandable that the more rapidly the pressure drops, the greater the number of cells that

would be nucleated, because a greater thermodynamic instability would be induced. Park et al.

[17, 80] demonstrated that a higher pressure-drop rate results in a higher cell density. Moreover,

Xu et al. found that a high pressure-drop rate is favorable not only for achieving high cell density

[25], but also for obtaining high expansion ratio [83]. If the die pressure is maintained higher

than the solubility pressure, unlike the pressure-drop rate, the amount of pressure drop has

insignificant effects on cell density [25].

3.3 Die Temperature

Precise monitoring and controlling of the die temperature is necessary because the gas

loss via the hot skin layer of the extrudate is promoted due to the increased diffusivity of the

blowing agent when the die temperature is too high [21]. If the die temperatures are too high, the

gas tends to escape through the polymer skin layer rather than to expand the foam; therefore, it is

very difficult to produce low density foams. Besides, at high temperatures cell coalescence is also accelerated very seriously throughout the cell structure. High die temperatures not only

deteriorate the cell density and morphology, but decrease the volume expansion due to the

enhance gas loss by coalesced cells [82]. Therefore, by decreasing the melt temperature, gas loss

can be reduced, since the diffusivity of the blowing agent is decreased and the melt strength of

the polymer is increased. However, if the die temperature is too low, the polymer melt becomes

too stiff to be expanded by gas [21]. As a result, optimum die temperature needs to be

determined in order to achieve high expansion ratio and high cell density, by balancing these factors such as gas loss, cell coalescence and polymer stiffening.

3.4 Contents of Blowing Agents

It was indicated that the volume expansion ratio and cell density are sensitive functions of the blowing agent amount by many studies [21, 81, 83]. As mentioned earlier, the cell density increases with the increase of the blowing agent content [21, 81]. However, high cell density due to increased content of blowing agents may result in the decrease of the expansion ratio, since the expansion ratio is significantly affected by gas loss. The effect of cell density of extruded foams on the expansion ratio has been extensively studied by Xu et al. [83]. Because of the increased number of cells, the cell-to-cell distance decreases and thus the distance for gas diffusion from the matrix into the cells decreases. In other words, it would take much less time for a cell to grow. Consequently, the cell growth rate would be considerably increased. If the cell growth rate is too high at the initial foaming stage, gas loss could be much worse since the melt is still quite hot and the melt strength is too weak [21]. As a result, the optimum blowing agent content needs to be identified for each foaming process so as to balance a large volume expansion ratio and high cell density.

3.5 Strategies for Achieving Low-density PS Foams Using CO2

To resolve the deficiencies of PS foams, such as rigidity, and a lack of toughness and resilience, technologies need to be further developed to improve the foam structure, which fundamentally determines the properties of foams. The cellular structure is basically determined by the cell density, the volume expansion ratio, the average cell size [25]. Extensive research has been done to decrease the cell size with a large cell density by increasing the volume expansion ratio. Therefore, cell-growth control strategies need to be achieved in order to obtain a desired expansion ratio.

Cell nucleation usually occurs inside the die during extrusion foaming process. The nucleation rate is generally determined by the pressure drop profile and the flow rate [24, 25].

The unavoidable cell growth that occurs inside the die is called premature cell growth, which affects the final expansion ratio of extruded foam [84]. The severe premature cell growth inside a die can develop an initial hump by big second-phase bubbles [76, 85]. The initial hump is significantly detrimental to achieving a large volume expansion ratio. Figure 3.1 shows how the initial hump badly affects volume expansion. Xiang Xu et al. analyzed the premature cell growth along the die [83]. Pressure-drop rate was found to have a higher impact on premature cell growth than on the cell density. Since the amount of premature cell growth decreases as the pressure-drop rate increases, less premature cell growth is expected for the die with a higher pressure-drop rate. It was identified that the premature cell-growth time is strongly dependent on the die geometry [83].

The processing temperature was also identified to be a critical processing parameter in determining the volume expansion [54]. Naguib et al. identified the expansion mechanisms of extruded foam at various temperatures [21]. It was also observed that the initial hump is detrimental to the expansion ratio [81]. In other words, the volume expansion ratio can be greatly decreased by a large amount of premature cell growth at certain processing temperatures, because a large amount of premature cell growth causes the initial hump.

As a result, it can be understood that both processing temperature and die geometry are two critical parameters for the expansion of extruded PS foams [85]. Therefore, the die should be carefully designed to maintain the premature cell growth amount below a critical value and the processing temperature needs to be properly monitored and controlled to achieve a high expansion ratio [75].

On the other hand, carbon dioxide (CO2) has been extensively used as the blowing agent in PS foaming, because it is environmentally benign and inexpensive. However, the technical difficulties of controlling the cell growth with CO2 have limited its use due to the high diffusivity

and the low solubility of CO2. Xiang Xu et al. investigated effects of CO2 content on the

expansion behaviors of PS foams [75]. It was claimed that the amount of blowing agent affects

the final expansion ratio through the changes in the maximum achievable expansion ratio, cell

density, and the premature cell growth amount. Firstly, the achievable expansion ratio increases

proportionally with an increase in CO2 content. However, more gas losses due to the plasticizing

effect and thus the blowing-agent efficiency is decreased. Secondly, the cell density increases

with an increase in the CO2 content. It is also known that the cell density affects the expansion

ratio [46, 83]. Therefore, the expansion ratio will be increased with a high cell density due to a

high CO2 content. However, an increased cell density will tend to increase the amount of

premature cell growth. Thirdly, an increase in CO2 content significantly increases the amount of

premature cell growth because cell nucleation will occur earlier in the die due to the increase of

solubility pressure, and thus the premature cell-growth time is increased accordingly. The

increased amount of premature cell growth may cause the formation of an initial hump, which

affects the final expansion ratio negatively, despite the increase of the maximum achievable

expansion ratio due to the gas content increase.

In summary, the basic approaches for the promotion of low-density extruded PS foams

using CO2 include: 1) to monitor and control the processing temperatures for reducing gas loss during expansion; 2) to optimize the die design for obtaining a high pressure-drop rate in order to achieve a large expansion ratio; 3) to increase gas content for increasing the maximum achievable expansion ratio with a proper die maintaining the premature cell growth amount below the critical level.

Figure 3.1 Effect of initial hump on volume expansion [84]

Chapter 4 Design and Construction of Experimental Equipment

Bothe lab-scale and pilot-scale foaming extrusion lines have been used to study the

expansion behaviors of PS foams blown with CO2, in order to compare the experimental results

and analyze the effect of equipment size on foaming techniques. The lab-scale tandem extrusion

system was ready to implement the experiments, but the customized pilot-scale extrusion system needed to be constructed with separate pieces of used equipments, and the designs of critical parts such as flow restrictor, flat die, die adaptor, and connector between extruders have to be accomplished precisely and systematically.

4.1 Materials

4.1.1 Plastics

The plastic materials used in the study were two types of polystyrene from different

companies. One is STYRON 685D from Dow Chemical Inc. This material is a high heat

resistance, and high stiffness, general purpose polystyrene which has a melt flow index of 1.5

dg/min, a density of 1.04 g/cm3, and a glass transition temperature of 108 oC. The other is

Polystyrene 523W supplied by TOTAL Petrochemicals Inc. This material has a melt flow index of 11.0 dg/min, a density of 1.04 g/cm3, and a glass transition temperature of 106 oC. Obviously,

the melt flow indexes (MFI) of these two materials are very different from each other, representing their different viscosities. The comparison of the rheological properties of these two materials is shown in Table 4.1.

4.1.2 Blowing Agent

The physical blowing agent used in the experiments was a commercial grade carbon

dioxide (CO2) with a minimum of 99.5% purity from Linde Gas. The amount of CO2 varied at 3,

5, and 7wt% and no nucleation agent was used.

4.2 Lab-Scale Extrusion System

A schematic drawing of the lab-scale tandem foam extrusion system used in this study is

shown in Figure 4.1. It consists of two single-screw extruders, a continuous gas injection pump,

and a foaming die. The first extruder is a 1.5” extruder with a mixing screw of 32:1 L/D ratio

(length to diameter ratio) used for plasticating and metering the polymer resin. The blowing

agent is injected and dispersed in the polymer melt in this first extruder. The second extruder is a

2.5” extruder that provides enough residence time both for mixing the blowing agent with the

polymer homogeneously and for initial cooling of the melt. An annular die was attached on the

second extruder by a die adaptor on this lab-scale tandem.

4.3 Set-Up of Pilot-Scale Extrusion System

4.3.1 Overview of the System

Figure 4.2 shows a schematic of the pilot-scale tandem foam extrusion system used in

this study. This extrusion system is a 2.5”-3.5” tandem foam extrusion line whose flow rate is up

to 80 kg/hr. The primary extruder has a screw of 2.5” diameter and 36:1 L/D ratio and the screw

has a twisted Maddock in the position after injection ports. There are two injection ports for

blowing agents or other liquids on the barrel and a total of 8 temperature controllers used for the

8-zones of the barrel. The secondary extruder is qualified for PS and PE foaming by using the

physical gas foaming method with several additives, and by providing an easy control system

with automatic and pneumatic systems equipped on each part. This extruder has a screw of 3.5”

diameter and is mainly used for cooling down the melt. The blowing agent delivery system has

an American Lewa diaphragm pump with a variable frequency drive, capacity of 35 ml/min and

7000 psi, and is feedback controlled with a mass-rate flow meter and backpressure regulator. For

the feeding system, there are two AEC Whitlock dual desiccant-bed driers, both of which have

point monitors, 7-day programmable timers and a temperature range of 80-200°C. Comet four-

station volumetric feeders along with optional powder feed hopers is used for the mixing. All the

four hoppers have low-lever sensors and alarms. Two of them are for plastic pellets and another

two are for plastic additives. Another big size annular die was used on this pilot-scale extrusion

system. Customized downstream equipment was used during the post-handling process.

4.3.2 Primary Extruder

The first extruder has a screw of 2.5-inch diameter and 36:1 L/D ratio with a twisted

Maddock after injection ports. The extruder’s drive uses a 100 Hp DC Baldor motor with a

Eurotherm Model 590-DRV motor controller (100 Hp rating) set to shut down at 36 Hp (the

screw would snap at about 42Hp), through an NRM PM-70 (1.5 SF) heavy-duty NRM gearbox

having an overall gear ratio of 20.62:1, and top screw speed is 85 RPM. Feed throat is water-

cooled. There are two zones of cast mag alloy electric heaters with oil cooling lines through them

(that start the devolatilization section). Approximate maximum delivery is about 80 kg/hr depending on materials. A total of thirteen temperature controllers are available, eight of which are used for the barrel, plus a melt temp readout, melt pressure readout and melt pressure transducer.

4.3.3 Blowing Agent Delivery System

The gas injection system is American Lewa Model EK-1/M210/5 mm diaphragm pump

with variable frequency drive (Reliance 1Hp 1800RPM), capacity of 0.56 gph and 7000 psi,

feedback controlled with a mass-rate flow meter, and back pressure regulator (Tescom 6000).

For facilitating CO2 delivery, a Filtine Model PCP-25-2AL-XP mechanical refrigeration chiller

rated at 1000 Btu/hr is included. A Haskel pneumatic booster pump was added to take care of the

cavitations nicely, since chilled CO2 was found to be cavitating in the suction side of the pump.

4.3.4 Secondary Extruder

The second extruder consists of a 3.5-inch screw with a built-in variable speed drive unit,

and maximum screw speed is 26 RPM. The L/D ratio is 30:1 in order to have a good cooling

ability with minimum heat generated by shearing action of screw motion. Extra flights on the

screw were made to generate enough thrust force to push the polymer melt forward, thereby

preventing the melt leakage backward into the motor assembly. Solenoid valve cooling with city

water is available for this secondary extruder.

4.3.5 Dryer

There are two AEC Whitlock dual desiccant-bed dryers; one is Model WD-25MR having

a capacity of 3.0 cu.ft., and the other is Model WD-50MR having a capacity of 6.0 cu.ft. Both of

them are mounted on wheeled carts, and have dew point monitors, 7-day programmable timers

and a temperature range of 82-205°C. The large unit has an aftercooler/precooler for drying

above 82 °C.

4.3.6 Feeder

Comet four-station volumetric (spinning disks with holes) feeder along with optional

powder feed hopers is used for the mixing. The Model 154 with CT-100 microprocessor has two

Model 150 MG (P/N 15473.28) hoppers for 5-80 lb/hr plastic pellets, and two Model 150 MG

(P/N 15431.46) hoppers for 0.5-5 lb/hr plastic additive pellets. All the four hoppers have low-

lever sensors and alarms, and the two larger volume hoppers have Model HL-1 hopper loaders,

which are mounted on the two Whitlock dries. The blender is mounted on a steel mezzanine,

measuring 9’ high x 7’3” wide x 4’ wide.

4.3.7 Connector

An elbow shape connector is required to precisely connect the first and second extruder,

in terms of the parallel position of these two extruders. Many key issues were involved in the

design of the connecting parts including the interfaces of the two extruders, sealing surfaces, and feasibility of smooth melt flowing. A flexible design of the connector was also necessary because the relative position of the extruders may change 90 degree in case of the floor space limitation. Therefore, the T join design was applied in the part named “connection_box”. A schematic of the connector design can be referred to Figure 4.3. The connector includes six parts such as connecting_part_1, flange_1, connection_box, plug, flange_2, and connecting_part_2, and their detailed drawings are shown in Appendix.

4.3.8 Flow Restrictor

Precise control of the flow of blowing agent is significantly important for foaming

process. During the foaming process, if the polymer melts flow backward into an injection port

due to the severe pressure fluctuation inside the barrel, it will block the tiny holes on the bottom

of the injection port. Therefore, maintaining a pressure difference of at least 1000 psi between

the positions before and after the injection port is advised to prevent the polymer melt from

flowing backward into the injection port. In order to build up this pressure difference, a proper

restrictor needs to be designed to get the desirable amount of resistances. Since the resistance is

built up between the restrictor and the inner wall of injection port, a series of restrictors have

been made in order to get the tight fittings to different extents. A detailed description of a proper

restrictor selection is given in the following.

CO2 was used as the blowing agent in this study. It was injected at a supercritical state,

because the injection pressure and temperature at the injection port were very high. In other words, the injected CO2 was in a state where there was no distinction between gas and liquid

states. Therefore, the restrictors had to be experimentally examined to find out some with an

appropriate resistance. A set of device was designed for a calibration purpose, and a schematic of

the calibration device is shown in Figure 4.4. In this system, a set of continuous syringe pump

was used to inject gas into the injection port. Pumps were setup working in the mode of constant

pressure, and the restrictor within the injection port was heated up to 230 oC by band heaters with

a temperature controller. There were two pressure transducers right before and after the injection

port. When the system was stabilized, the values of pressure were recorded from the digital

pressure readout, and the amount of gas flow rate could be read from the pump display. After a

series of experimental examinations of various flow restrictors, one appropriate restrictor was

selected to be used for the experiments. A group of the curves of flow rate vs. various pressures for this restrictor were plotted in Figure 4.5.

4.3.9 Die

In order to understand the foaming expansion of polymer melt solution, two types of dies, annular die and flat die, were used in this study to induce various pressure-drop rates and achieve desired foam structures. Compared to the annular die, the flat die is more difficult to produce foams with uniform shape due to the long narrow die orifice. For each foaming extrusion system, both annular die and flat die were employed to study and compare the foam expansion behaviors.

The detailed axiomatic design of the flat die used on pilot-scale extrusion system, will be discussed in Section 4.4.

4.3.10 Downstream

In a professional foam extrusion line, downstream is a necessary element, which has

effect on the shape and quality of the final product. For the annular die, typical downstream

equipment for producing sheet foam consists of calibrator, take-off unit, and cutting machine.

Basic machine elements associated with the annular die include mandrel, cooling unit, take-off unit, and winder. Different kinds of customized downstream equipment have been developed and utilized in this study in order to satisfy the specific experimental requirements.

4.4 Axiomatic Design of Flat Die

4.4.1 Background and Problem Description

When studying the extrusion of polymer materials, one desirable type of die is a flat die.

As the polymer is passed through this die, a long sheet, with the cross section of the die outlet is

created. A flat die generally is able to generate sheets with a fixed width, but often with a

variable thickness, which is achieved through the use of a movable piece on the die outlet. There

is an existing flat die designed to operate with a relatively small extruder, which can produce a

sheet with a relatively small width – only 3.25”. A need for a flat die producing a sheet with a width of 4”, which can be used with the pilot-scale extruders, was proposed.

In order to minimize material costs, it has been proposed that an existing flat-die be

modified to produce a product with the desired width of 4”. This die also has to be modified in

order to fit the existing die adaptor that is available with the pilot-scale extruders. This die adaptor is of a much larger diameter than the inlet of the existing flat die. Again as a means of

minimizing cost, it is important that a new adaptor not be needed. This requires a number of

modifications to be made to the existing die. An axiomatic method [86, 87] has to be applied for

securing the die, a new die outlet has to be designed and the die cavity also has to be modified.

4.4.2 Coupling Analysis of Existing Design

The existing flat die is a succinct design comprising five major parts (see Figure 4.6).

There are the upper and lower cavities, the upper and lower die tips and the adjusting screw holder. The adjusting screw is used to vary the thickness of the sheet extrusion. The two main functional requirements were likely to have been the product quality and the compatibility (or interchangeablility) with the extruder. The product quality requirement includes the product dimensions, finish, uniformity, etc. The compatibility with the extruder simply refers to the requirement that it be easy for the die to be mounted to the desired extruder.

Considering these two factors to represent the primary functional requirements, the design is deemed to be a decoupled design. This is because the area of the die influencing the product quality can be designed without necessarily requiring a change to the mounting of the die to the extruder. This however, does not work in reverse. A change to the mounting system, especially where the die and die adaptor are in contact is likely to require the die cavity to be redesigned. This will have an effect on the flow of the molten polymer, and, thus will affect the quality of the final product. In a mathematical form, this can be viewed as follows:

⎡ FR1(int erchangeability)⎤ ⎡X O⎤⎡ DP1(mounting _ design)⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.1) ⎣ FR2( product _ quality) ⎦ ⎣X X ⎦⎣ DP2(die _ design) ⎦

This representation makes it clearer that the design is, in fact decoupled. Looking at the matrix multiplication, it is shown that the interchangeability is only affected by the mounting design, since the die design is finalized within the constraints posed by the mounting design. The product quality is affected by both the mounting design, which will dictate many of the constraints for the die design, and the die design, which is what will dictate the flow of polymer and will ultimately control whether or not the final product is acceptable.

4.4.3 Decomposition of First Level FRs and DPs

In this case, 4 second level functional requirements were determined. The interchangeability requirement is decomposed into two second-level functional requirements.

One is the mating to the die adaptor, which refers to the way in which the die and die adaptor are in contact in order to prevent any leakage. The other is the fastening to the die adaptor, which refers to the way in which the die is held in place. The corresponding design parameters are, respectively, the use of a machined collar on the rear face of the die and the use of four bolts through the die body. The product quality requirement has been decomposed into 2 functions as well. The first is the smooth flow of polymer to the die tip, and the second is the product dimensions, in particular, the cross-sectional dimensions. The respective design parameters are the design of the die cavity to optimize the flow from the inlet to the die tip, and the widening of the die tips. The flowchart in Figure 4.7 outlines the decomposition of the functional requirements and design parameters.

The coupling of the second level functional requirements and design parameters can be illustrated in matrix form as follows:

⎡ Mating _ with _ die _ adaptor ⎤ ⎡X O⎤⎡Collar⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.2) ⎣ Fastening _ to _ die _ adaptor⎦ ⎣O X ⎦⎣ Bolts ⎦

⎡Smooth _ flow _ within _ die⎤ ⎡X O⎤⎡ die _ cavity ⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.3) ⎣ product _ dimensions ⎦ ⎣O X ⎦⎣die − tip _ dimensions⎦

It was mentioned above that the first concept was to have bolts going through the die tips as well as the die body. This would cause the fastening to be affected by the die-tip dimensions as in order to change the thickness of the sheet, it would be necessary to loosen the fastening bolts.

This would result in a coupled design, which would not be acceptable. To revise this, and make

60 the second level acceptable, this idea was quickly changed such that the fastening bolts are beneath the die tips. This may result in marginally longer setup times, but reduces the system to an uncoupled system at the second level.

4.4.4 Coupling Analysis of Detailed Design

The second level, upon which this design is based, is uncoupled. Upon reviewing the design, one of the second level functional requirements may be revised to more accurately represent the functions being satisfied. The “mating with the die adaptor” functional requirement may be changed to “smooth transition across die adaptor/die interface.” This is more accurate, as the mating between the two components is not the critical part, but rather that the flow is not disturbed while crossing between the die adaptor and the die. The collar ensures that the die is in the correct position to limit the disturbance to the polymer. The remaining 2nd level functional requirements and design parameters can remain the same, and remain uncoupled.

⎡smooth _ transition _ between _ die − adaptor _ and _ die⎤ ⎡X O⎤⎡Collar⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.4) ⎣ Fastening _ to _ die _ adaptor ⎦ ⎣O X ⎦⎣ Bolts ⎦

⎡Smooth _ flow _ within _ die⎤ ⎡X O⎤⎡ die _ cavity ⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.5) ⎣ product _ dim ensions ⎦ ⎣O X ⎦⎣die − tip _ dimensions⎦

The overall design remains decoupled. The interchangeability requirement from earlier, will now be changed to compatibility, as it is only being designed to fit one extruder, not to be interchanged between multiple machines.

⎡ FR1(compatibility) ⎤ ⎡X O⎤⎡ DP1(mounting _ design)⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.6) ⎣ FR2( product _ quality)⎦ ⎣X X ⎦⎣ DP2(die _ design) ⎦

4.4.5 Addition of Cost Constraint

It is obvious that a limit to the cost would be an additional constraint. As is stated in Suh

[86], 1990, the precise value of the cost is not important, as long as it doesn’t exceed some limit.

Thus, the cost is a constraint and not a functional requirement. In order for a design to even be

considered as practical, the cost must be less than $1000, preferably less than $600.

The addition of this constraint requires a revision of the functional requirements and the

design parameters. The primary functional requirements do not change. Where the changes take

place is in the second level design parameters. It was decided that the width of the sheet will be

maintained at 3.25 inches, eliminating the need for a new lower die tip. The only change to the

die-tips basically involves increasing the height of the angled portion of the die tip. This allows a thicker extrusion due to the increased volume before the outlet. The angle of the die tips must be

maintained, however, in order to provide smooth flow to the die tips. The bolt pattern remains

unchanged from the design previously presented.

The other modification to the design would be that, since the die cavity is not changing, it

is necessary to provide a coupler between the die adaptor outlet and the die inlet. This is

essentially an adaptor between the large diameter outlet of the die adaptor and the small inlet

diameter on the die. The rear face requires the same tolerance as was intended for the original

concept’s collar. This ensures that the coupler will fit tightly inside the die adaptor, and thus

prevent a leakage of the polymer. A 3-D general view of coupler is shown in Figure 4.8.

Therefore, the coupling analysis of the second level functional requirements and design

parameters can be revised as follows:

⎡smooth _ transition _ between _ die _ adaptor _ and _ die⎤ ⎡X O⎤⎡Coupler⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.7) ⎣ Fastening _ to _ die _ adaptor ⎦ ⎣O X ⎦⎣ Bolts ⎦

⎡Smooth _ flow _ within _ die⎤ ⎡X O⎤⎡ die _ tip _ slope ⎤ ⎢ ⎥ = ⎢ ⎥⎢ ⎥ (4.8) ⎣ product _ thickness ⎦ ⎣O X ⎦⎣die − tip _ depth⎦

It is seen that the design is still uncoupled at the second level. The first level is the same as

before; the overall design remains a decoupled system.

4.4.6 Conclusion

Based on a perceived need for a die producing a sheet with a width of 4 inches, the

design of the modified polymer sheet extrusion die has been completed using the axiomatic

design approach. After the coupling analysis was performed and the detailed design developed,

the system was determined to be decoupled, so the design appeared ready to be fabricated.

However, it was necessary to apply a cost constraint to ensure that the manufacturing of the die

would not exceed a reasonable amount considering the use the die is being designed for. This

new constraint required a revision of all of the previous design work. In the end, a sufficient,

decoupled design was developed, and manufactured at a greatly reduced cost than the original

concept.

Table 4.1 Rheological properties of two types of polystyrene

Figure 4.1 Schematic of lab-scale tandem foaming extrusion system

Figure 4.2 Schematic of pilot-scale tandem extrusion system with downstream

Figure 4.3 Schematic of connector between two extruders

Figure 4.4 Schematic of resistance calibration system

Figure 4.5 (a) Gas injection pressure vs. gas flow rate

Figure 4.5 (b) Barrel pressure vs. gas flow rate

Figure 4.5 (c) Pressure difference vs. gas flow rate

Figure 4.6 Existing flat die design

Figure 4.7 Decomposition of first level FRs and DPs

Figure 4.8 3-D general view of coupler

Chapter 5 Investigation of Effects of Extrusion System Size on the Expansion of PS Foams Blown with CO2

5.1 Introduction

In engineering applications, pilot-scale foam extrusion systems are more often used to

provide quantitative proof that the design or technique has potential to succeed on the industry-

scale. Pilot-scale foam extrusion systems have not been used in academic study before, even

though they are more practical to check the production feasibility of the foaming technology.

Compared to lab-scale extrusion systems, pilot-scale extrusion systems have a big difference in

residence time, flow rate, and temperature uniformity. In this research, with the help of both lab-

scale and pilot-scale tandem extrusion system, experiments of PS foaming using 100% CO2 have

been conducted on different size extrusion systems by maintaining the consistent die pressure-

drop rate at the same processing conditions of die temperature and CO2 content. In particular, the

effects of different size extrusion equipment on the expansion behavior of polystyrene foams,

which were fabricated by the means of scaling-up the foaming technology, have been

systematically investigated.

Two sets of experiments were carried out to investigate the effects of extrusion system

size on the expansion of PS foams using CO2. The material of PS 685D and annular dies of two different sizes were used in the first set of experiments, using CO2 as the blowing agent. The

second set of experiments employed PS 523W as the material, flat dies of two different sizes as

the shaping dies and CO2 as the blowing agent.

5.2 Estimation of Pressure-Drop Rate

As mentioned earlier, maintaining the consistent die pressure-drop rate at the same

processing conditions of die temperature and CO2 content, is the prerequisite of investigating the effects of different size extrusion systems on foam expansion. Pressure-drop rates in the die are basically determined by many factors including die geometry, flow rate, die temperature, and characteristics of the polymer. The estimation of pressure-drop rate in annular die and flat die will be discussed in the following content.

5.2.1 Pressure-Drop Rate in Annular Die

In the foaming process, an estimate of pressure losses for the flow of Non-Newtonian

materials through annular die channels can be calculated by using the equation outlined in [88]:

1 dp ⎡2m+1 (m + 2)Q ⎤ m = ⎢ m+2 ⎥ (5.1) dx ⎣ φπDa H a ⎦

3 where Q is the volumetric flow rate (m /s), Ha is the width of the annular die gap (m), Da is the outside diameter of the annular gap (m), m and Ø are the parameters in the power law expressed by this equation in [88]:

1 1 − −1 m m η=φ ⋅γ& (5.2)

Moreover, the power law is usually described by this equation [89]:

n−1 η = k p ⋅γ& (5.3)

n where n is the power-law index (Non-Newtonian), kp is the power-law index (Pa·S ). Therefore,

the equation (5.1) can be transformed into:

n n+1 ⎛ 1 ⎞ n 2 ⋅ k ⋅ ⎜2 + ⎟ ⋅Q dp p n = ⎝ ⎠ n n 2n+1 (5.4) dx π ⋅ Da ⋅ H a

The velocity of polymer/gas solution in an annular die can be calculated in terms of volumetric

flow rate and parameters of die geometry:

dx Q = (5.5) dt π ⋅ Da ⋅ H a

The die pressure-drop rate can be expressed as:

dp dp dx = ⋅ (5.6) dt dx dt

After combining equation (5.4), (5.5), and (5.6), the pressure-drop rate in an annular die can be

estimated by:

n n+1 ⎛ 1 ⎞ n+1 2 ⋅ k p ⋅⎜2 + ⎟ ⋅Q dp ⎝ n ⎠ = n+1 n+1 2n+2 (5.7) dt π ⋅ Da ⋅ H a

Equation (5.7) indicates that if at the same processing conditions of temperature and gas content,

only the volumetric flow rate of the polymer/gas solution and parameters of annular die

geometry (Da, Ha) affect the die pressure-drop rate, as long as the die pressure is higher than the solubility pressure. Therefore, by the means of adjusting the parameters Da (outside diameter of

annular gap), Ha (width of annular die gap), and Q (volumetric flow rate), the same die pressure-

drop rates on different scale extrusion systems can be obtained when both the die temperatures

and the gas contents are identical.

5.2.2 Pressure-Drop Rate in Flat Die

Likewise, the pressure-drop rate in a flat die can be calculated in the following part.

An estimate of pressure losses for the flow of Non-Newtonian materials through flat die

channels can be calculated by using the equation stated in [88]:

1 dp ⎡2m+1 (m + 2)Q ⎤ m = ⎢ ⎥ (5.8) dx m+2 ⎣⎢ φB f ⋅ H f ⎦⎥

3 where Q is the volumetric flow rate (m /s), Bf is the width of the flat die gap (m), Hf is the height of the flat die gap (m), m and Ø are the parameters in the power law. By equation (5.2) and (5.3), the equation (5.8) can be transformed into:

n n+1 ⎛ 1 ⎞ n 2 ⋅ k p ⋅ ⎜2 + ⎟ ⋅Q dp ⎝ n ⎠ = n 2n+1 (5.9) dx B f ⋅ H f

The velocity of melt flow in a flat die can be described by volumetric flow rate and parameters of

die geometry:

dx Q = (5.10) dt B f ⋅ H f

By combining equation (5.6), (5.9), and (5.10), the pressure-drop rate in a flat die can be

estimated as follows:

n n+1 ⎛ 1 ⎞ n+1 2 ⋅ k p ⋅ ⎜2 + ⎟ ⋅Q dp ⎝ n ⎠ = n+1 2n+2 (5.11) dt B f ⋅ H f

Therefore, if at the same processing conditions of temperature and gas content, the identical pressure-drop rate of flat dies on different scale extrusion systems can be successfully achieved by adjusting the parameters Q (volumetric flow rate), Bf (width of flat die gap), and Hf (height of

flat die gap).

5.3 Fabrication of Foams Using PS 685D and Annular Die

The first set of experiments was carried out on both lab- and pilot-scale foaming

extrusion systems to investigate the effect of extrusion system size on the expansion of PS foams

blown with CO2. The material of PS 685D and annular dies of two different sizes were used in the experiments.

5.3.1 Experimental Design and Procedure

By the means of axiomatic design, in order to compare the results of expansion ratio and

cell density of the foams fabricated on different extrusion systems, which can be seen as

functional requirements, design parameters need to be identified. As shown in Figure 5.1, the

design parameters include blowing agent, system pressure, die temperature, and die pressure-

drop rate. The interest of this study is to find out the extrusion system effect on expansion of PS

foams, therefore, except for system pressure, other design parameters should be identical to

facilitate the comparison of different systems. Among these design parameters, blowing agent

and die temperature are relatively easy to keep consistent, but die pressure-drop rate needs to be

considered very carefully. In other words, the parameters of annular die geometry and the flow

rate should be calculated using Equation (5.7) and adjusted to meet the critical requirements.

Two annular dies, which have same structures but different dimensions, were used on the

lab-scale and pilot-scale extrusion systems, respectively. The die gap is adjustable as shown in a

schematic of annular die (Figure 5.2). The outside diameter of the die gap of the annular die used

75 on the lab-scale system is 21.4mm. The die gap was fixed on 0.45mm while the flow rate was

100g/min. The other annular die used on the pilot-scale system has an outside diameter of 33mm of the die gap. Accordingly, after calculation using Equation (5.7), the die gap of this big annular die was fixed on 0.58mm with the flow rate of 260g/min to satisfy the requirement of consistent pressure-drop rate in two extrusion foaming systems. The set up of parameters on both lab- and pilot-scale system is shown in Table 5.1.

The power-law index in Equation (5.7), kp, can be described as:

n−1 K p =η0 ⋅λ (5.12)

Where η 0 is zero-shear-rate viscosity, Pa·s, and λ is relaxation time, s. Since kp is also a function of temperature and gas content, the values can be estimated by a means of shift factors with the reference temperature and gas content [92, 93]. Therefore, the values of pressure-drop rate at each gas content and different die temperatures can be estimated by Equation (5.7), using the best-fitting parameters for the master plot of PS 685D from Table 5.3 [93].

PS resin (685D) was processed using two foaming experimental setup (Figure 4.1, 4.2) respectively, and a metered amount of CO2 was injected into the polymer melt. Using the annular dies, experiments were carried out at various levels of CO2 content, and die temperature. The die temperature was carefully varied from 170 °C to 110 °C with a decrement of 5 °C. The pressure changes with the decrease of temperature were recorded at the steady state of each processing condition. The amount of CO2 varied at 3, 5, and 7wt% and no nucleation agent was used. The foam samples were randomly selected at each processing condition and were characterized in terms of the cell density and expansion ratio.

5.3.2 Results and Discussion

All the experiments results are summarized to show how the die temperature and gas content affect the volume expansion. Effects of processing conditions on volume expansion and their influence on maximum achievable expansion ratio, cell density, and cell uniformity are herein compared and discussed respectively.

5.3.2.1 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Maximum Achievable Expansion Ratio

The curves of die temperature versus volume expansion ratio using the lab-scale system at three gas contents are shown in Figure 5.3. It was observed that the volume expansion ratio was a strong function of the die temperature and gas content.

When the die temperature was as high as 170°C, the achieved volume expansion ratio was only around 10 folds, regardless of the gas content. That means when the die temperature is too high, most of the gas will escape through the hot skin layer of the foam during expansion.

Namely, if the die temperature is very high, the gas diffusivity is high and the melt strength of the polymer is weak, therefore, the loss of the blowing agent governs the foam expansion. At 5%

CO2 content, in the die temperature range of 140°C to 170°C, the expansion ratio increased as the die temperature decreased. In other words, the gas diffusion was blocked at the surface and more gas remained in the foam to contribute to the volume expansion as the die temperate was lowered. Very similar results were obtained when the CO2 content was at 3% and 7%. When the die temperature was further decreased from 135°C to 110°C, the volume expansion ratio decreased. Even though it was expected that more gas was preserved in the foam at this lower die temperature (110°C) than at 135°C, the increased stiffness of the frozen skin layer adversely

affected volume expansion and limited the achieved expansion ratio of extruded foam. At 3%

CO2 content, the slight increase of the volume expansion ratio with the decrease in the die

temperature around 110°C seems to be due to the melt fracture that occurred on the extrudate.

In brief, the experimental results show that at each of three gas contents the volume

expansion ratio increased initially with the increase of the die temperature, and decreased with decreasing the die temperature after exceeding an optimum temperature. In other words, these expansion ratio graphs clearly showed a “mountain shape” [56], confirming that the expansion ratio was affected by gas loss and melt stiffening to a large extent.

In addition, the achieved maximum expansion ratios at 3%, 5%, and 7% CO2 content were 21.5, 24.1, and 28.3, respectively. It is clear that the amount of blowing agent affected the final expansion ratios through the changes in the maximum achievable expansion ratio. Since more gas was available for the cell growth resulting in the promoted cell nucleation, the maximum achievable expansion ratio increased with an increase of CO2 content. Moreover, the

viscosity of polymer/gas mixture decreased as more gas was injected. As a result, more gas lost

due to the plasticizing effect, and thus the blowing-agent efficiency was consequently decreased

with the increase of gas content.

On the other hand, the expansion ratios plotted against the die pressures for three gas

contents using the pilot-scale system are shown in Figure 5.4. The typical “mountain shape” of

the curves was also observed in expansion ratio versus die pressure graphs. The achieved

maximum expansion ratios at 3%, 5%, and 7% CO2 content were 17.2, 21.7, and 28.4,

respectively. As shown in Figure 5.3 and 5.4, the maximum expansion ratios at 5% and 7% CO2 content using lab- and pilot-scale systems didn’t have much difference. However, at 3% CO2 content, the maximum expansion ratio using pilot-scale system was much lower than that using

lab-scale system. This may be due to the extra cooling caused by the cooling mandrel in the

downstream of the pilot-scale system. The extra cooling cooled down the extrudate too quickly

to further enhance bubble growth.

Compared to the results of the lab-scale system, the same trend of the curves indicates

that the expansion ratios fluctuated with the die temperatures in a same “mountain shape”; the

maximum achieved expansion ratios were observed in the similar optimum temperature ranges.

5.3.2.2 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Cell Density

It was claimed that high gas content is favorable to increase the cell density [75]. The

results of cell densities at the three gas contents of 3%, 5%, and 7% using the lab-scale system

are shown in Figure 5.5. These curves also show a similar tendency that the cell density

increased with the increase of CO2 content. Moreover, Figure 5.5 shows that the cell density was

almost unchanged at the each gas content. Since the same annular die was used in this set of

experiments using lab-scale system, the back pressure built by the resistance of die was varied as

a function of only the polymer melt and die temperature. For instance, in the experiment at 7%

CO2 content, although the processing pressure was varied with die temperature, the actual

dissolved gas amount did not change because the injected gas amount was maintained at same level (7%) in the experiment. Therefore, the cell densities didn’t change at 7% CO2 content.

Similar results were obtained when the CO2 content was at 3% and 5%. Figure 5.7 shows the

plot of die pressure profile in the experiments using the lab-scale system. Since at the each gas

content all the die pressures were higher than the solubility pressure, all the injected gas was

supposed to have dissolved into the polymer matrix in all the experiments. The amount of gas dissolved into the polymer is the most significant factor in determining the nuclei density in

plastics processing [38]. Therefore, it was not surprising that the same cell densities were achieved at same gas content in the experiments regardless of the processing pressure. The consistent cell density in all the experiments indicates that the effect of cooling was mainly on the cell size and volume expansion ratio but not on the cell density. This implies that cell growth can be independently regulated by the processing and die temperatures, without a significant influence on nucleation.

Moreover, in Figure 5.3 another tendency was observed, which was related to cell density. The optimum processing temperature tended to be lower with the increase of the CO2 content, and this is because cell density affected the maximum expansion ratio to some extent.

Basically the expansion ratio will be increased with the increase of the cell density due to a high

CO2 content [13-15]. An increased cell density, however, will tend to increase the amount of

premature cell growth which is always detrimental to foam expansion. In other words, an

increase in CO2 content significantly increases the amount of premature cell growth, because the

cell nucleation occurs earlier in the die due to the increase of solubility pressure and the

premature cell-growth time is also consequently increased. As a result, the more the gas content

is increased, the more difficult it is to control the optimum processing temperature.

Figure 5.6 shows the results of cell density in the experiments using the pilot-scale

system. The plot exhibited similar tendency as in the lab-scale system: the cell density increased

with the increase of CO2 content; the cell densities didn’t change in the experiment with the same

gas content. In addition, at 7% gas content, the cell density using pilot-scale system was on the

same order as that using lab-scale system. When the CO2 content was at 3%, only slightly

difference in the results of the cell density was obtained on both extrusion systems. The big

difference was that the cell density at 5% CO2 content using the pilot-scale system was much

lower than that using the lab-scale system. This phenomenon was caused by the difference in

shear stress in these two foaming extrusion systems. Namely, the shear stress in lab-scale

extrusion systems was larger than that in pilot-scale extrusion systems.

For these two different scale extrusion systems, their primary extruders have the same

outer-to-inner diameter ratio. If the primary extruders were operating at the same screw tip

speed, we will have the same average shear rate, translating to similar shear heating [77]. In

order to get the same screw tip speed, the relationship between primary screw speed and primary

screw diameter should be:

RPM lab Dpilot = (5.1) RPM pilot Dlab

where, RPM lab = speed of primary screw on lab-scale system,

RPM pilot = speed of primary screw on pilot-scale system,

Dlab = diameter of primary screw on lab-scale system,

Dpilot = diameter of primary screw on pilot-scale system.

By this equation, the optimum RPM ratio ( RPM lab / RPM pilot ) in this experiment should be

2.5/1.5 (=1.67). Because the actual RPM ratio in this set of experiments was 35/16 (=2.18), the screw tip speed on lab-scale system was higher than that on pilot-scale system. As a result, higher average shear rate was generated on lab-scale system, which led to the higher cell density.

The pressure-drop rates at different temperatures for three gas contents using both systems have been estimated. Figure 5.9 and 5.10 show the plots of cell density vs. dp/dt using

lab- and pilot-scale extrusion system, respectively. It was observed that the cell density increased

with the increase of die pressure-drop rate at each of three gas contents on both extrusion

systems, confirming that a higher pressure-drop rate is necessary for achieving a higher cell

density.

5.3.2.3 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Cell Uniformity

The cell morphologies of the two series of experimental results were obtained by using a

Scanning Electron Microscope (SEM). Two sets of the SEM micrographs of foam samples

fabricated by both lab- and pilot-scale extrusion systems at the each same condition were shown

in Figure 5.11-5.13.

The cellular properties of the foam samples achieved by these two extrusion systems at

the die temperature, 130°C, and gas content varied at 3%, 5%, and 7%, were herein observed and

compared. Figure 5.11 shows the microstructures of the foams extruded at 130°C and 3% CO2 using the two extrusion systems. Both cell structures were likely uniform and closed-cell. The cell size in the foams produced using pilot-scale system was bigger than that using lab-scale one, because the cell density in pilot-scale system was lower. The microstructures of the foams extruded at 130°C and 5% CO2 using the two extrusion systems were shown in Figure 5.12. Due

to the big difference in cell density resulting from the different shear rates in these two size

extrusion systems, the cell size therefore presented significant differences. Although both cell structures were still closed-cell, the cell size was more uniform in the case of lab-scale system.

Figure 5.13 shows the microstructures of the foams extruded at 130°C and 7% CO2 using the two

extrusion systems. The two cell structures were very similar in terms of cell size, cell density and

cell uniformity. It was observed that the effect of shear rate on cell density became less at higher

gas content.

On the other hand, from Figure 5.11 through 5.13, it was found that the cell size

decreased with the increase of gas content in both lab- and pilot-scale systems. This phenomenon

also verified that high gas content is favorable to increase the cell density during the foaming

process.

5.3.3 Conclusions

An experimental study of PS foam extrusion using both lab- and pilot-scale tandem foam extrusion systems with CO2 as the blowing agent and PS 685D as the material had been

performed. Two adjustable annular dies, which have different geometries, were used on both the

lab-scale and pilot-scale extrusion systems, respectively. A comparison of the effects of

extrusion system size on the expansion behavior of PS foams blown with 100% CO2 at the

consistent pressure-drop rate, demonstrated that the scale of the foam extrusion system does not

affect the principles of foaming technology, but the effects of shear rate and temperature

uniformity on foam expansion should be considered carefully.

5.4 Fabrication of Foams Using PS 523W and Flat Die

The second set of experiments was carried out on both lab- and pilot-scale foaming

extrusion systems to investigate the effect of extrusion system size on the expansion of PS foams

blown with CO2. The materials of PS 523W and flat dies of two different sizes were used in the

experiments.

5.4.1 Experimental Design and Procedure

In order to conduct the experiment and compare the results of expansion ratio and cell density of the foams fabricated on different extrusion systems, pressure-drop rate needs to be kept consistent on both systems. Therefore, the parameters of flat die geometry and the flow rate should be considered and estimated using Equation (5.11) to satisfy the critical conditions.

Two different size flat dies, which have same structures, were used on the lab-scale and pilot-scale extrusion systems, respectively. A schematic of the flat die is shown in Figure 5.14. In this set of experiments, the gap width of the flat die used on the lab-scale system is 49mm. The height of the die gap was fixed on 0.32mm while the flow rate was 100g/min. The other flat die used on the pilot-scale system has the die gap width of 152mm. By using Equation (5.11), the die gap height of this big flat die was fixed on 0.3mm with the flow rate of 270g/min to satisfy the requirement of consistent pressure-drop rate on these two extrusion foaming systems. The set-up of parameters on both lab- and pilot-scale system is shown in Table 5.2.

PS resin (523W) was processed using two foaming experimental setup (Figure 4.1, 4.2) respectively, and a metered amount of CO2 was injected into the polymer melt. Using the flat dies, experiments were carried out at various levels of CO2 content, and die temperature. The die temperature was carefully varied from 150 °C to 110 °C with a decrement of 5 °C. The pressure changes with the decrease of temperature were recorded at the steady state of each processing condition. The amount of CO2 varied at 3, 5, and 7wt% and no nucleation agent was used. The foam samples were randomly selected at each processing condition and were characterized in terms of the cell density and expansion ratio.

5.4.2 Results and Discussion

Based on the experimental results, effects of processing conditions on volume expansion

and their influence on maximum achievable expansion ratio, cell density, and cell uniformity

will be compared and discussed respectively. The same phenomenon as observed in the first set

of experiments using the annular dies will not be re-explained in this section.

5.4.2.1 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Maximum Achievable Expansion Ratio

Figure 5.15 shows volume expansion ratios obtained using the lab-scale system with

respect to the die temperatures and CO2 contents. The experimental results show that at each of

3%, 5%, and 7% gas content, the volume expansion ratio increased initially with the increase of

the die temperature, and after passing an optimum temperature range (125-130°C) the volume

expansion ratio started to decrease with the die temperature drop. The “mountain shape” was

also observed indicating that the volume expansion ratio was a function of the die temperature.

On the other hand, it was noted that at the same die temperature, the volume expansion ratio

increased with the increase of CO2 content.

The graph of die temperature against volume expansion ratio using the pilot-scale system at three gas contents of 3%, 5%, and 7% is shown in Figure 5.16. The same “mountain shape” can also be recognized in the results. Interestingly, the curves at 5% and 7% were very close to each other. That means if the gas content is larger than 5%, the further increase of gas content

did not affect the volume expansion too much. Because PS 523W is a high MFI material and the

viscosity is relatively low, the further increase of gas content did not change the property of

polymer/gas mixture largely in terms of the plasticizing effect.

5.4.2.2 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Cell Density

Cell density curves at 3%, 5%, and 7% gas content using the lab-scale system are shown

in Figure 5.17. Clearly, at the same gas content, the cell densities at different die temperatures

were on the same level, which means that the processing pressure was always beyond the

solubility pressure during the process and all the amount of injected gas was supposed to have

dissolved in the polymer matrix.

Figure 5.18 shows the results of cell densities at 3%, 5%, and 7% gas content using pilot-

scale system. Each cell density curve is very similar to the one at same gas content using lab-

scale system. Especially, the consistent cell density in all the experiments indicates that the effect

of cooling was mainly on the cell size and volume expansion ratio but not on the cell density.

When comparing these cell density results, the big difference on cell density at 5% CO2 content which occurred in first set of experiments, was not found in this set of experiments. It can be explained in the point view of shear rate. In this set of experiments, the RPM ratio

( RPM lab / RPM pilot ) was 45/28 (=1.61), which is very close to the optimum RPM ratio, 1.67.

Therefore, the average shear rate in two different size extrusion systems was supposed to be

similar, and thus the shear heating and mixing can be seen at the same level. As a result, cell

density would not change with the difference of extrusion systems.

5.4.2.3 Comparison of Effects of Processing Conditions on Volume Expansion and their Influences on Cell Uniformity

SEM micrographs of the foams produced at 130°C, and gas content varied at 3%, 5%,

and 7%, using both lab- and pilot-scale extrusion systems are shown in Figure 5.19 through 5.21,

respectively. The cell size of the foam fabricated using the pilot-scale system was relatively bigger than the corresponding results using the lab-scale system, but the uniformity of the cell structure was better. It was also noted that using the same extrusion system, at same die pressure

as 130°C, the cell size decreased and cell density increased with the increase of CO2 content.

5.4.3 Conclusions

An experimental study of PS foam extrusion using both lab- and pilot-scale tandem foam extrusion systems with CO2 as the blowing agent and PS 523W as the material had been

performed. Two adjustable flat dies with different geometries, were used on both the lab-scale

and pilot-scale extrusion system, respectively. A comparison of the effects of extrusion system

scale on the expansion behavior of PS foams blown with 100% CO2 at the consistent pressure-

drop rate, indicated that the scale of the foam extrusion system does not affect the foaming

principles, and effects of extrusion system size on scale-up of foam techniques, such as shear rate

and temperature uniformity, could be suppressed by tailoring the processing conditions and

experimental parameters.

Table 5.1 Set-up of parameters on both lab- and pilot-scale system using annular die

Table 5.2 Set-up of parameters on both lab- and pilot-scale system using flat die

Table 5.3 Best fitting parameters for the master plot of PS 685D [92]

Figure 5.1 FRs and DPs of Axiomatic Design in Foam Processing

Figure 5.2 Schematic of an Annular Die

Figure 5.3 Volume Expansion Ratios Using Annular Die & Lab-Scale Tandem Extrusion System

Figure 5.4 Volume Expansion Ratios Using Annular Die & Pilot-Scale Tandem Extrusion System

Figure 5.5 Cell Densities Using Annular Die & Lab-Scale Tandem Extrusion System

Figure 5.6 Cell Densities Using Annular Die & Pilot-Scale Tandem Extrusion System

Figure 5.7 Die Pressure Profile Using Annular Die & Lab-Scale Tandem Extrusion System

Figure 5.8 Die Pressure Profile Using Annular Die & Pilot-Scale Tandem Extrusion System

Figure 5.9 Cell Density vs. dp/dt Using Annular Die & Lab-Scale Tandem Extrusion System

Figure 5.10 Cell Density vs. dp/dt Using Annular Die & Pilot-Scale Tandem Extrusion System

(a) (b)

Figure 5.11 Cell Structures of PS (685D) Foams at 130°C & 3wt% CO2 Using Annular Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

(a) (b)

Figure 5.12 Cell Structures of PS (685D) Foams at 130°C & 5wt% CO2 Using Annular Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

(a) (b)

Figure 5.13 Cell Structures of PS (685D) Foams at 130°C & 7wt% CO2 Using Annular Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

Bf : width of flat die gap

Hf : height of flat die gap (adjustable)

Figure 5.14 Schematic of a Flat Die

Figure 5.15 Volume Expansion Ratios Using Flat Die & Lab-Scale Tandem Extrusion System

Figure 5.16 Volume Expansion Ratios Using Flat Die & Pilot-Scale Tandem Extrusion System

Figure 5.17 Cell Densities Using Flat Die & Lab-Scale Tandem Extrusion System

Figure 5.18 Cell Densities Using Flat Die & Pilot-Scale Tandem Extrusion System

100

(a) (b)

Figure 5.19 Cell Structures of PS (523W) Foams at 130°C & 3wt% CO2 Using Flat Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

(a) (b)

Figure 5.20 Cell Structures of PS (523W) Foams at 130°C & 5wt% CO2 Using Flat Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

101

(a) (b)

Figure 5.21 Cell Structures of PS (523W) Foams at 130°C & 7wt% CO2 Using Flat Die & (a) Lab-Scale Tandem Extrusion System; (b) Pilot-Scale Tandem Extrusion System

102

Chapter 6 Summary and Conclusions

An experimental study was carried out to scale up the extrusion foaming process from a

lab-scale extrusion system to a pilot-scale extrusion system, for the manufacture of low-density polystyrene foams using CO2 as a lowing agent. The objective was to investigate the effects of extrusion system scale on the foaming techniques. A 1.5”-2.5” lab-scale tandem extrusion system and a 2.5”-3.5” pilot-scale tandem extrusion system along with annular dies and flat dies were used for the experiments. The basic experimental approach proposed is to keep consistent pressure-drop rates on both foam extrusion systems at the same conditions of the die temperature and the gas content, and compare the behaviors of foam expansion on these two extrusion systems. The effects of processing parameters, such as processing temperature and blowing agent content on the volume expansion ratio, cell density, and cell morphology were also investigated and analyzed.

The experimental study presented in this thesis leads to the following conclusions:

1. A pilot-scale tandem extrusion system along with an annular die and a flat die has been

successfully constructed and the downstream equipment has been properly customized.

This pilot-scale foam extrusion line has satisfied the functional requirements and been

ready for the study of scale-up of foaming techniques.

2. Low-density PS (685D) foams have been successfully produced using CO2 as a blowing

agent on a lab-scale extrusion system along with an annular die. The produced PS foams

103

have a cell density of 108 cells/cm3 and a controlled expansion ratio in the range of 7 to

28.

3. Low-density PS (685D) foams have also been successfully fabricated using CO2 as a

blowing agent on a pilot-scale extrusion system along with an annular die. The fabricated

PS foams have a cell density of 108 cells/cm3 and a controlled expansion ratio in the

range of 10 to 28.

4. The same foaming techniques were adopted in both experiments mentioned above and

scaled up from the lab- to the pilot-scale extrusion system. The basic approach for the

process development was to first develop a high nucleated-cell density and to promote a

large volume expansion ratio while suppressing cell coalescence and decreasing gas loss

during foam expansion.

5. The volume expansion curves for both extrusion systems showed a typical “mountain

shape”, confirming that the expansion behavior is a function of gas loss and polymer

stiffening. The achieved maximum expansion ratios using lab-scale system at 3%, 5%,

and 7% CO2 content were 21.5, 24.1, and 28.3, respectively; the achieved maximum

expansion ratios using pilot-scale at 3%, 5%, and 7% CO2 content were 17.2, 21.7, and

28.4, respectively. From these results, it is indicated that the scale-up of foaming

techniques was successfully attempted.

6. The amount of blowing agent affected the final expansion ratios through the changes in

the maximum achievable expansion ratio. The maximum achievable expansion ratio

increased with an increase of CO2 content. However, the extra cooling caused by the

cooling mandrel in the downstream of the pilot-scale system may lower the maximum

104

expansion ratio, since it cooled down the extrudate too quickly to further enhance the

bubble growth.

7. The optimum processing temperature tended to be lower with the increase of the CO2

content, because an increase in CO2 content significantly increased the amount of the

premature cell growth which is detrimental to the foam expansion.

8. The cell density increased with the increase of CO2 content on both extrusion systems. At

each gas content cell density was almost consistent, since all the die pressures were

higher than the solubility pressure for each gas content, and all the injected gas has

dissolved into the polymer matrix.

9. It was observed that the cell density increased with the increase of die pressure-drop rate

at each of three gas contents on both lab- and pilot-scale extrusion systems, confirming

that a higher pressure-drop rate is necessary for achieving a higher cell density.

10. The difference in the shear stress of these two foaming extrusion systems may cause cell

density difference through the influence of shear heating and mixing, since the shear

stress in lab-scale extrusion systems was larger than that in pilot-scale extrusion systems.

11. Low-density PS (523W) foams have been successfully produced using CO2 as a blowing

agent on a lab-scale extrusion system along with a flat die. The produced PS foams have

a cell density of 108 cells/cm3 and a controlled expansion ratio in the range of 10 to 23.

12. Low-density PS (523W) foams have also been successfully fabricated using CO2 as a

blowing agent on a pilot-scale extrusion system along with a flat die. The fabricated PS

105

foams have a cell density of 108 cells/cm3 and a controlled expansion ratio in the range of

8 to 22.

13. The same “mountain shape” was recognized on the volume expansion curves for both

extrusion systems as well. As the gas content was larger than 5%, the further increase of

gas content almost did not affect the volume expansion too much. Since PS 523W is a

high MFI material and the viscosity is relatively low, the further increase of gas content

did not change the viscosity property of polymer/gas mixture largely in terms of the

plasticizing effect.

14. A systematic comparison of the effects of extrusion system scale on the expansion

behavior of PS foams blown with 100% CO2 at the consistent pressure-drop rate,

indicated that the scale of the foam extrusion system does not affect the foaming

principles, and effects of extrusion system size on scale-up of foam techniques, such as

shear rate and temperature uniformity, could be suppressed by tailoring the processing

conditions and experimental parameters.

106

Chapter 7 Recommendations and Future Work

The following suggestions are made for directing the future research of investigating the

effects of extrusion system scale on the development of foaming technology:

1. Pressure fluctuation affected the foaming process during the experiments using pilot-

scale extrusion system. Since polymeric melt is heavily dependent on processing history,

downstream foaming cannot be viewed without paying close attention to upstream

kinetics. A gas pump may need to be located between the two extruders in order to

reduce the pressure fluctuation.

2. More reliable dies with cooling functions should be considered to improve the die

temperature control on die body and die lips. It would be very helpful for controlling

foam expansion and eliminating the errors.

3. Post-handling process should be taken care of carefully because it would also affect foam

expansion to some extent. Certainly, an adequate knowledge of extrusion system can

minimize unnecessary mistakes in selecting the right type and size of machinery during

lab-to-pilot scale-up.

107

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116

Appendix CAD Drawings of Connector Parts

1. Drawing of Connection_Part_1

117

2. Drawing of Flange_1

118

3. Drawing of Connection_Box

119

4. Drawing of Plug

120

5. Drawing of Flange_2

121

6. Drawing of Connection_Part_2