Foaming of Wood Flour/Polyolefin/Layered Silicate Composites

by

Yoon Hwan Lee (Kevin)

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Faculty of Forestry University of Toronto

©Copyright by Y. H. Lee 2008

ii

FOAMING OF WOOD FLOUR/POLYOLEFIN/LAYERED SILICATE COMPOSITES

Kevin Y. H. Lee Degree of Doctor of Philosophy, 2008

Faculty of Forestry University of Toronto

ABSTRACT

This research provides a new insight on various properties, such as rheological,

mechanical, and flame-retarding properties, as well as the foaming behaviors of wood flour

/plastic composites (WPCs) through the addition of a small amount of nanosized clay particles.

Although WPCs have replaced natural wood in many applications, their industrial usage has

been limited because of their weak modulus, low impact strength, low screwing-ability/nailing-

ability, high density compared to natural wood, as well as their flammability compared to

plastics. In this context, the incorporation of nanoclay and foam structure into WPC has been

studied to dramatically alleviate these drawbacks.

The melt blending method was used to prepare different types of clay-filled wood flour

composites such as intercalated and exfoliated clay nanocomposites. The effects of key

processing variables such as the mixing time, mixing temperature and screw speed on clay

dispersion were investigated from the thermodynamic and kinetic point of view. Their

nanostructure was determined by using X-ray diffraction (XRD) and transmission electron

microscopy (TEM). Accordingly, effective strategies for controlling intercalation and exfoliation

of polyolefin/clay nanocomposites were proposed and evaluated.

iii

Wood flour composites with high levels of clay dispersion were synthesized successfully

using a general new route (i.e., maleated-polyolefin-based clay masterbatch and dilution). The

effects of nanoclay particles on the rheological, thermal, and mechanical properties were

identified. In addition, it was demonstrated that a small amount of well-dispersed nanoclay in

WPC significantly improved flame retardancy of WPCs. The mechanism of improved flame-

retarding effects on nanoparticles was elucidated as well. The relationship between the clay

dispersion and the material properties were also clarified. Furthermore, the foaming behaviors of

HDPE-based and PP-based wood flour/nanoclay composites were investigated using N2 as the

blowing agent in an extrusion process. The cell nucleation and growth behaviors of wood

flour/polyolefin/clay composite foams were elucidated while varying the temperature, pressure,

wood flour content, clay content and dispersion degrees.

iv

Acknowledgements

First, I would like to express my sincere gratitude to my supervisor Professor

Chul B. Park and Professor M. Sain for providing his invaluable guidance, discussion,

support throughout my research..

I would also like to acknowledge Prof. D.N. Roy and Prof. N. Yan from the

Faculty of Forestry, Prof. J.K. Spelt from the Department of Mechanical and Industrial

Engineering, and Prof. A.K. Mohanty from the University of Guelph for the willingness

to serve on my dissertation committee and for their invaluable comments and suggestions.

I also greatly appreciate Dr. K.H. Wang, Dr. Y.S. Kim, Dr. T. Kuboki, and Dr. W. Zheng

for their valuable discussions and suggestions in various aspects of my experiments.

I would also like to thank all the colleagues in the Microcellular Plastics

Manufacturing Laboratory as well as the Forestry lab for their help and friendship.

Also, I wish to acknowledge the professional technical support from Dr. N.

Coombs, and Dr. S. Petrov in the Department of Chemistry, and from Teresa, and Sheila

in the purchasing department of Mechanical and Industrial Engineering. I would like to

thank P. Deborah who helped me finishing up my thesis and provided her kind guidance

while I have been in the Faculty of Forestry

Last but not least I want to thank my parents, father-in-law, mother-in-law and

beloved brother and sister for their love and support. I would like to thank sincerely my

wife and my daughter, Young-Mi for her understanding, patience, and enouragement.

v

TABLE OF CONTENTS

Abstract ii

Acknowlegements iv

Table of Contents v

Nomenclature x

List of Figures xiv

List of Tables xix

CHAPTER 1 INTRODUCTION 1

1.1 Polymeric Composites 1

1.2 Wood Fiber/Plastic Composites 1

1.3 Foaming of WPC 2

1.4 Polymer/Clay Nanocomposites 3

1.5 Objectives of Thesis 4

1.6 Organization of Thesis 5

CHAPTER 2 THEORETICAL BACKGROUND AND LITERATURE REVIEW 8

2.1 Wood and Wood Fiber 8

2.2 Wood Fiber/Plastic Composites 10

2.3 Thermoplastic Foams and Microcellular Foams 13

2.3.1 Blowing Agents 13

2.3.2 Microcellular Foam Processing 14

2.3.3 Overall Process for Microcellular Foam Processing 15

2.3.3.1 Formation of Polymer/Gas Solution 15

2.3.3.2 Cell Nucleation 18

2.3.3.3 Cell Growth 23

2.3.4 Effects of Processing Conditions in a Continuous Foaming

Process 24

2.4 Wood Fiber/Plastic Composite Foams 25

vi

2.4.1 Volatiles from Wood Fiber 26

2.4.2 Batch Processing 26

2.4.3 Injection Molding Processing 27

2.4.4 Extrusion Processing 28

2.4.4.1 Extrusion Processing Conditions of WPC Foams 29

2.4.4.2 Extrusion Foaming of WPC with CBA 29

2.4.4.3 Extrusion Foaming of WPC with PBA 32

2.5 Polymer/Layered Silicate Nanocomposites 33

2.5.1 Structure and Properties of Layered Silicates 34

2.5.2 Types of Polymer/Clay Nanocomposites 36

2.5.3 Techniques Used for the Characterization of Nanocomposites 36

2.5.4 Preparative Methods 39

2.5.5 Nanocomposite Properties 40

2.5.6 Foaming Processes of Polymer/Clay Nanocomposites 40

2.6 Wood Fiber/Plastic/Nanoclay Composites 41

2.7 Structural Characterization of WPC Foams 42

CHAPTER 3 EXPERIMENTATION 56

3.1 Materials 56

3.1.2 PP Resins 56

3.1.3 Clay Used for Nanocomposites 56

3.1.4 Wood Flours 56

3.1.5 Functionalized Polyolefins Used for Nanocomposites 57

3.1.6 PBA Used for WPCs 57

3.2 Analytical Instruments 57

3.2.1 XRD 57

3.2.2 TEM 58

3.2.3 Rotational Rheometer 58

3.2.4 Differential Scanning Calorimeter (DSC) 58

3.2.5 Scanning Electronic Microscopy (SEM) 59

vii

3.2.6 Mechanical Testing 60

3.2.7 Flame Retardancy Testing 60

3.3 Experimental Equipment and Procedure 61

3.3.1 Experimental Setup and Procedure for Preparing

Wood Flour/Polyolefin/Nanoclay Composites 61

3.3.1.1 Wood Flour/HDPE/Nanoclay Composites 61

3.3.1.2 Wood Flour/PP/Nanoclay Composites 62

3.3.2 Experimental Setup and Procedure for Continuous

Extrusion Foaming Process 62

3.3.2.1 Wood Flour/HDPE/Nanoclay Composite Foams 62

3.3.2.2 Wood Flour/PP/Nanoclay Composite Foams 63

CHAPTER 4 STRATEGIES FOR INTERCALATATION AND EXFOLIATION

OF POLYOLEFIN/CLAY NANOCOMPOSITES 72

4.1 Introduction 72

4.2 Experimental 74

4.3 Results and Discussion 74

4.3.1 HDPE/Clay Nanocomposites 74

4.3.1.1 Effect of Clay Content on the Clay Dispersion 74

4.3.1.2 Effects of Processing Parameters on the

Clay Dispersion 75

4.3.1.3 Effects of Mixing Method on the Clay Dispersion 76

4.3.1.4 Summary 78

4.3.2 PP/Clay Nanocomposites 78

4.3.2.1 Effect of Molecular Weight of Base Materials

on the Clay Dispersion 78

4.3.2.2 Effects of Clay Content and Processing

Parameters on the Clay Dispersion 79

4.3.2.3 Effect of Mixing Method on the Clay Dispersion 80

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4.3.2.4 Effect of scCO2 on the Clay Dispersion 81

4.3.2.5 Summary 82

4.4 Conclusions 82

CHAPTER 5 SYNTHESIS, PHYSICAL, MECHANICAL, AND FLAME

RETARDING PROPERTIES OF WOOD FLOUR/HDPE/NANOCLAY

COMPOSITES 100

5.1 Introduction 100

5.2 Morphology Characterization 102

5.3 Rheological Behavior 102

5.4 Crystallization Behavior 105

5.5 Mechanical Properties 105

5.6 Flame Retarding Properties 107

5.7 Conclusions 109

CHAPTER 6 CONTINUOUS EXTRUSION FOAMING OF WOOD

FLOUR/POLYOLEFIN/NANOLAY COMPOSITES 124

6.1 Introduction 124

6.2 Wood Flour/HDPE/Nanoclay Composites 127

6.2.1 Die Pressure and Foam Density 127

6.2.2 Composite Foam Morphology 128

6.2.3 Summary 131

6.3 Wood Flour/PP/Nanoclay Composites 131

6.3.1 Clay Dispersion 131

6.3.2 Foam Density and Morphology 132

6.3.3 Summary 133

6.4 PP/Clay Nanocomposites without Wood Flour 133

6.4.1 Clay Dispersion 133

6.3.2 Volume Expansion Ratio and Cell Morpholgy 133

6.3.3 Summary 136

ix

6.5 Conclusions 137 CHAPTER 7 SUMMARY OF CONTRIBUTION AND RECOMMENDATIONS

FOR FUTURE WORK 157

REFERENCES 163

APPENDICES A-1

x

NOMENCLATURE

WPC = Wood fiber/plastic composites

CBA = Chemical blowing agents

PBA = Physical blowing agents

HDPE = High density polyethylene

PP = Polypropylene

PVC = Poly(vinyl chloride)

PS = Polystyrene

CTMP = Chemithermomechanical pulp

ATH = Aluminum trihydrate

Mg(OH)2 = Magnesium hydroxide

cs = Solubility of gas in the polymer (cm3/g or ggas/gpolymer)

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

ps = Saturation pressure

R = Gas constant (J/K)

Ho = Solubility coefficient constant (cm3 [STP]/g-Pa)

ΔHs = Molar heat of sorption (J)

D = Diffusivity

Do = Diffusivity coefficient constant (cm2/s)

Ed = Activation energy (J)

W = Required work to generate a bubble

pbγ = Surface tension

bA = Surface area,

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Vb = Bubble of volume

= Gibbs free energy in homogeneous nucleation

= Homogeneous nucleation rate

Co = Concentration of gas molecules in solution

fo = Frequency factor of gas molecules joining the nucleus

k = Boltzman constant

r = Critical radius

σ = Surface tension

pΔ = Pressure difference between the bubble and the melt.

= Heterogeneous nucleation rate

= Gibbs free energy in heterogeneous nucleation

3bpγ = Surface energy of the polymer-bubble interface

PΔ = Gas pressure used to diffuse the gas into the polymer

θ = Wetting angle of the polymer-additive gas interface.

HIPS = High impact polystyrene

= Pressure drop rate

Psat = Saturation pressure

TGA = Thermogravimetric analyzer

MMT = Montmorillonite

CEC = Cation exchange capacity

XRD = Wide-angle X-ray diffraction

TEM = Transmission electron microscopy

SAXS = Small angle X-ray scattering

NMR = Nuclear magnetic resonance

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AFM = Atomic force microscopy

L = Mean crystallite size (thickness)

K = Crystallite shape constant (=0.89)

λ = Wavelength of Cu Kα radiation (1.540598 Å )

FWMH = Full width middle height of the peak

bhkl = FWMH of the peak for the (hkl) reflection

b0 = Instrumental resolution

N = Number of clay platelets

= Thickness of one single platelet (=1 nm)

= Clay interlayer spacing, i.e. distance between two adjacent clay platelets

λ = Wavelength of the radiation

2θ = Diffraction angle.

PLA = Polylactide

PP-g-MAn = PP grafted with maleic anhydride

PE-g-MAn = HDPE grafted with maleic anhydride

ρf = Foam density , g/cm3

ρ = Density of unfoamed sample, g/cm3

M = Mass of foam sample, g

V = Volume of foam sample, cm3

Φ = Volume expansion ratio,

Vf = Void fraction

N0 = Cell density

N = Number of bubbles in the micrograph

a = Area of the micrograph

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M = Magnification factor of the micrograph

MFI = Melt flow index

G´ = Storage modulus

G″ = Loss modulus

η* = Complex viscosity

DSC = Differential Scanning Calorimeter

Tm = Melting temperature

Tc = Crystallization temperature

ΔHexp = Experimental heat of fusion heat of fusion

χc = Degree of crystallinity

ΔH* = Heat of fusion of the fully crystalline polymer

Wf = Weight fraction of pure polymer in the composites

B = Burning rate (mm/min)

LDPE = Linear low density polyethylene

PE-g-AA = Acrylic acid grafted PE

xiv

Figure 1.1 Cellulosic fiber classification 7Figure 2.1 A bubble nucleated on (a) a smooth planar surface and (b) in a

conical cavity 48

Figure 2.2 Why the pressure drop rate (dP/dt) is important 49Figure 2.3 Structure of 2:1 layered silicates 50Figure 2.4 Schematic illustrations of the clay dispersion in polymer/clay

nanocomposites 51

Figure 2.5 Effect of fine particle size (thickness) on X-ray diffraction curve 52Figure 2.6 Schematic illustration of changes in silicate interlayer spacing

based on Bragg’s law 52

Figure 2.7 XRD patterns of different types of clay nanocomposites 53Figure 2.8 SEM micrographs: (a) microcellular and (b) WPC foam with 50

wt% wood fiber 54

Figure 2.9 Cumulative cell size distribution curve 55Figure 3.1 A structure of organic modifier of Cloisite 20A: HT is

hydrogenated tallow (~65% C18; ~30% C16; ~5% C14), Anion: chloride

65

Figure 3.2 Schematic illustration of flame retardant test fixture 69Figure 3.3 Procedure for wood fiber/polyolefin/clay nanocomposite

preparation 70

Figure 3.4 Schematic illustration of single screw extrusion setup for foaming 71Figure 4.1 XRD patterns of PE-g-MAn/clay nanocomposites at various

concentrations of clay 85

Figure 4.2 Density versus pressure isotherms for liquid and supercritical CO2.

86

Figure 4.3 XRD patterns of PE-g-MAn/clay nanocomposites at various concentrations of clay

87

Figure 4.4 XRD patterns of PE-g-MAn/clay nanocomposites at various mixing speeds

87

Figure 4.5 XRD patterns of PE-g-MAn/clay nanocomposites at various mixing temperatures

88

Figure 4.6 XRD patterns of PE-g-MAn/clay nanocomposites at various mixing times

88

Figure 4.7 XRD patterns of PE-g-MAn/clay nanocomposites at different mol. wt. of PE-g-MAn: (a) lower MFI (2.0) and (b) higher MFI (12.5)

89

LIST OF FIGURES

xv

Figure 4.8 XRD patterns of PE-g-MAn/clay (15%) nanocomposites at

different mixing temperature, mixing speed, and mixing time: (a) 150°C, (b) 180°C, and (c) 210°C

90

Figure 4.9 XRD patterns of PE-g-MAn/clay (10%) nanocomposites at different mixing temperature, mixing speed, and mixing time: (a) 150°C, (b) 180°C, and (c) 210°C

91

Figure 4.10 XRD patterns of PE-g-MAn/clay (7%) nanocomposites at different mixing temperature, mixing speed, and mixing time: (a) 150°C, (b) 180°C, and (c) 210°C

92

Figure 4.11 XRD patterns of HDPE/clay (1%) nanocomposites prepared from (a) masterbatch and (b) direct melt blending

93

Figure 4.12 XRD patterns of HDPE/PE-g-MAn/clay nanocomposites diluted from masterbatch at different clay concentrations: (a) 10%, (b) 12%, and (c) 15%

94

Figure 4.13 X-ray diffraction patterns for all 5 wt% clay/PP-g-MAn nanocomposites (mixing time = 10 min, temperature = 170°C, screw speed =110 rpm)

95

Figure 4.14 X-ray diffraction patterns for clay/MFI-24 nanocomposites as a function of clay content

95

Figure 4.15 X-ray diffraction patterns for 5 wt% clay/MFI-24 nanocomposite at various mixing speeds (mixing time = 10 min, temperature = 170 °C)

96

Figure 4.16 X-ray diffraction patterns for 5 wt% clay/MFI-24 nanocomposites at various processing temperatures (mixing time = 10 min, screw speed = 110 rpm)

96

Figure 4.17 X-ray diffraction patterns for 5 wt% clay/MFI-24 nanocomposite at various mixing times

97

Figure 4.18 X-ray diffraction patterns for 10 wt%/MFI-24 masterbatch and its diluted nanocomposites

97

Figure 4.19 X-ray diffraction patterns for 10wt% clay/MFI-24 masterbatch based nanocomposites and nanocomposites made by direct melt blending

98

Figure 4.20 TEM micrographs for (a) 10 wt% clay/MFI-24 masterbatch based nanocomposite and (b) nanocomposite made by direct melt blending

98

Figure 4.21 XRD pattern of clay Cloisite 20A and PP/clay (1 wt%) nanocomposites with and without 5% CO2

99

xvi

Figure 5.1 XRD patterns for wood fiber/HDPE/nanoclay composites with respect to the preparation method of the composites and clay contents (weight ratio of PE-g-MAn to clay = 9.0)

112

Figure 5.2 TEM micrographs of (a) 3% and (b) 5% clay melt-blended by masterbatch method, and (c) 5% clay/wood fiber (30%)/HDPE nanocomposites by direct blending.

113

Figure 5.3 The effect of clay dispersion on G’ and η* of HDPE and wood fiber/HDPE nanocomposites with (a) pure HDPE, (b) 1 wt% clay, (c) 3 wt% clay, and (d) 5 wt% clay (weight ratio of PE-g-MAn to clay = 9.0)

114

Figure 5.4 Ratio of complex viscosity of intercalated and exfoliated clay-filled composites η* to that of their respective base composites η*B at a frequency ω of 0.1 rad/s

115

Figure 5.5 Crystallinity values of pure HDPE, wood fiber/HDPE/5% clay composites, and base composite without clay

116

Figure 5.6 Effects of clay dispersion on (a) tensile modulus and (b) flexural modulus of wood fiber/HDPE/clay nanocomposites (weight ratio of PE-g-MAn to clay = 9.0)

117

Figure 5.7 Increments in tensile modulus and flexural modulus of wood fiber/HDPE/clay nanocomposites with respect to their base composites with increasing levels of clay dispersion and clay content

118

Figure 5.8 Effects of clay dispersion on (a) tensile strength and (b) flexural strength of wood fiber/HDPE/clay nanocomposites (weight ratio of PE-g-MAn to clay = 9.0)

119

Figure 5.9 Effect of clay dispersion on the notched Izod impact strength of wood fiber/ HDPE/clay nanocomposites (weight ratio of PE-g-MAn to clay = 9.0)

120

Figure 5.10 Effect of clay dispersion on the burning rate for wood fiber/HDPE/clay (weight ratio of PE-g-MAn to clay = 9.0)

121

Figure 5.11 Decrements in burning rate wood fiber/HDPE/clay nanocomposites with respect to their respective base composites with the increase of levels of clay dispersion and clay content

122

Figure 5.12 Residues after pyrolysis of wood fiber/HDPE composites at 500°C: (a) without clay, (b) with 5% intercalated clay, (c) with 5% exfoliated clay

123

xvii

Figure 6.1 Schematic of gas foaming process of WPCs 138Figure 6.2 Die pressure as a function of die temperature (a) with different

wood fiber content and (b) with different clay content (30% wood fiber)

139

Figure 6.3 Foam density as a function of die temperature with respect to wood fiber content (a) 0%, (b) 10%, (c) 20%, and (d) 30%

140

Figure 6.4 Cumulative cell size distribution as a function of wood fiber content (a) 0%, (b) 10%, (c) 20%, and (d) 30%

141

Figure 6.5 Cumulative cell size distribution as a function of clay content and dispersion (a) 1%, (b) 3%, and (c) 5% for composite foams with 30% wood fiber

142

Figure 6.6 SEM micrographs of composite foams with 30% wood fiber: (a) no clay (b) 5% intercalated clay, and (c) 5% exfoliated clay

143

Figure 6.7 Reduction of nucleation energy by function F(θc,β) on (a) a smooth planar surface and (b) in a conical cavity

144

Figure 6.8 XRD patterns for wood fiber/PP/nanoclay composites 145Figure 6.9 Foam density of 25% wood fiber/PP/nanoclay composites as a

function of die temperature with respect to clay content 146

Figure 6.10 Cumulative cell size distribution of 25% wood fiber/PP/nanoclay composite foams as a function of clay content at a die temperature of 140°C

147

Figure 6.11 SEM micrographs of 25% wood fiber/PP composite foams with: (a) no clay (b) 3% clay, and (c) 5% clay at 140°C

148

Figure 6.12 The effect of clay content on η* of PP composites with wood fiber of 25% and 50%

149

Figure 6.13 Cumulative cell size distribution of 50% wood fiber/PP/nanoclay composite foams as a function of clay content at a die temperature of 150°C

150

Figure 6.14 SEM micrographs of PP foam at different die temperatures: a) 160°C; b) 150°C; c) 140°C; and d) 135°C. The scale bar is 100μm. CO2 content injected to barrel

151

Figure 6.15 SEM micrographs of PP/clay (1 wt%) nanocomposite foam at different die temperature: a) 160°C; b) 150°C; and c) 145°C. The scale bar is 100μm. CO2 content injected to barrel is fixed to 5wt%.

152

xviii

Figure 6.16 SEM micrographs of LPP/clay nanocomposite foams with different clay content: a) 0; b) 0.2 wt%; c) 0.5 wt%; d) 1.0 wt% and e) 5.0 wt%. The scale bar is 100μm. CO2 content injected to barrel is fixed to 5wt%.

153

Figure 6.17 Effect of die temperature on foam volume expansion ratio of neat PP and PP/clay nanocomposites with different clay content

154

Figure 6.18 Effect of die temperature on foam cell density of PP and PP/clay nanocomposites with different clay content.

155

Figure 6.19 Effect of clay content on foam cell density of PP/clay nanocomposites at a die temperature of 150°C

156

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LIST OF TABLES

Table 1.1 Foam classification 6

Table 2.1 Typical wood composition 46

Table 2.2 Comparison of wood species 47

Table 2.3 Chemical formula of commonly used layered silicates 50

Table 3.1 Physical properties of Cloisite 20A 65

Table 3.2 Typical particle sizes of Cloisite 20A 65

Table 3.3 Typical properties of PE-g-MAn used for experimentation 66

Table 3.4 Typical properties of PP-g-MAn used for experimentation 66

Table 3.5 Formulation of the composites used (wt%) 67

Table 3.6 Formulation of the composites used (wt%) 68

Table 4.1 Qualitative overview of the effects of processing parameters on the

XRD patterns

84

Table 5.1 Crystallization and melting parameters of selected HDPE and

composites

111

Table 7.1 Relevant material cost/performance chart

162

1

CHAPTER 1

INTRODUCTION

1.1 Polymeric Composites

During the last few decades, thermoplastics have gained ever-increasing acceptance as an

important family of engineering materials and are steadily replacing metals in a wide variety of

applications. The commercial consumption of the thermoplastics has steadily increased and this

trend is expected to continue despite the increase in their prices. This situation has created an

impetus for cost reduction via composites by employing fillers in thermoplastics [1]. Presently,

the fiber-reinforced plastic composite market is dominated by inorganic fillers such as glass fiber,

talc, mica, CaCO3, and so on [2]. These high density conventional fillers offer wide property

changes in the composites, but on a volume basis, their use is not cost-effective [3].

1.2 Wood Fiber/Plastic Composites

In the last decade, natural organic reinforcements such as cellulosic fibers have

penetrated slowly into this market because they offer many advantages over most common

inorganic fillers. Cellulosic fibers originate from wood and other plant materials, as shown in

Figure 1.1 [4]. They are abundantly available and have lower costs and density. They lead to a

reduced wear of processing equipment and are renewable, recyclable, non-hazardous and

biodegradable. The replacement of inorganic fillers with comparable cellulosic fibers provides

weight savings and decreases the cost without reducing the rigidity of the composites [5]. Wood

2

fibers are used most extensively among the cellulosic fibers used as fillers [2].

Wood fiber/plastic composites (WPCs) can be a cost-effective alternative to many plastic

composites or metals in terms of bending stiffness or weight [6]. The wood fibers are non-

abrasive so that relatively large concentrations can be incorporated into plastics without causing

serious machine wear during blending and processing. WPCs are becoming increasingly

acceptable to consumers as replacement of natural wood due to advantages like durability,

permanent color, and reduced maintenance in spite of their high price [7]. The main applications

of WPCs are in building products, such as fencing, rails, decking, door and window profiles,

decorative trims, and so on. These composites are also gaining acceptance in automotive,

industrial and marine applications [2,7]. However, the shortcomings of WPCs as an artificial

wood, such as high density, low ductility and low impact strength compared to natural wood,

have limited their utility in many applications [5].

1.3 Foaming of WPC

These shortcomings can be effectively compensated through the foaming of WPCs.

Foams can be classified based on their cell density and cell size or on expansion ratio, as shown

in Table 1.1 [8]. Microcellular plastics typically exhibit high impact strength (up to a five-fold

increase over unfoamed plastic [9,10]), high toughness (up to a five-fold increase over unfoamed

plastic [11]), high fatigue life (up to a fourteen-fold increase over unfoamed plastic [12]), high

heat-insulation property [13], high sound-insulation property [14], and high thermal stability [15].

In addition, foaming of WPC results in material weight and cost reduction, better surface and

sharper contours and corners than unfoamed profiles [16]. For these reasons, research efforts

have been made to decrease the cell size and to increase the cell uniformity and the cell

3

(population) density [11,17,18]. During production, the foamed composites run at lower

temperatures and at faster speeds than their unfoamed counterparts due to the plasticizing effects

of gas, thus the production cost is reduced as well [16].

Foamed WPC can be manufactured using either chemical blowing agents (CBA) or

physical blowing agents (PBA) such as CO2 and N2. Generally, CBA are mixed with

thermoplastic polymers before the introduction of the blended material into an extruder inlet,

while PBA can be injected into the polymer melt for intimate mixing prior to foaming. The

gases released during the decomposition of CBA include; CO2, N2, NH3, H2O, and their

combinations. Various types of CBA are available including endothermic, exothermic and a

combination of the two. The trend is to replace CBA with PBA, since PBA are more

economical, environmentally safe and allow for higher cell density and larger volume

expansion ratio.

Conventional technologies for processing fine-celled or microcellular plastic foams using

PBA are essentially based on subjecting single-phase, polymer/gas solution systems to

thermodynamic instability. This is done in order to nucleate micro-cells by rapidly decreasing the

solubility of the gas in the polymer, which is usually accomplished via a temperature and/or

pressure drop rate control. Three major process steps must be performed in order to make use of

such a thermodynamic instability: (a) formation of a polymer/gas solution, (b) microcell

nucleation, and (c) cell growth and density reduction. Producing a uniform fine-celled or

microcellular structure in WPC has been demonstrated to be extremely important for improving

their mechanical properties [9].

1.4 Polymer/Clay Nanocomposites

4

However, expanding markets call for high performance WPC with superior and unique

properties (e.g. flame retardance) to meet individual application requirements. Also, due to low

stiffness of commodity plastics, such as high density polyethylene (HDPE) and polypropylene

(PP), the flexural and tensile moduli of the WPC are significantly lower than those of natural

wood.

The use of nanosized, layered, silicate particles (i.e., clay), which have a much larger

surface area (~750 m2/g) and a much higher aspect ratio (>100) than conventional, macro-sized

fillers to reinforce polymers, has drawn a great deal of attention in recent years. Adding a small

amount of nanoclay can dramatically improve a number of properties such as stiffness and

strength, thermal and dimensional stability, flame retardance, barrier properties, and so on [19-

22]. Thus, the introduction of nanoclay particles into WPC can be interesting from the

perspective of improving their mechanical properties and flame retardance, which are desirable

effects particularly with respect to automotive and construction applications.

Furthermore, adding a small amount of nanosized clay into WPC may be able to lead to

a better cellular structure in foaming of the composites and thereby synergistically produce high

performance WPC foams with superior properties.

1.5 Objectives of Thesis

The main objective of this research is to demonstrate the feasibility of creating uniformly,

fine-celled, wood fiber/plastic/nanoclay composite foams as a strategy to reduce the weight

while improving mechanical properties and flame retarding properties. The specific objectives of

this research are as follows; (a) To achieve well-dispersed (complete exfoliated)

nanoclay/cellulosic fiber/polyolefin composites; (b) To establish structure-property relationships

for wood fiber/polyolefin (HDPE, PP)/nanoclay composites; (c) To gain a understanding of

5

fundamental mechanisms governing flame retarding properties for wood

fiber/polyolefin/nanoclay systems; (d) To explore the effects of nanoclay particles and their

dispersion on cell morphology and foam density in the continuous foaming of wood

fiber/polyolefin/nanoclay composites.

1.6 Organization of Thesis

A general introduction about the dissertation is given in Chapter 1 together with its

significance and objectives. This is followed by a literature review of WPCs, WPC foams and

nanocomposites based on clay (layered silicate) in Chapter 2. Chapter 3 discusses in detail the

setup for the synthesis of WPC composites with controlled clay dispersion using the method of

melt blending and WPC nanocomposite foams and their analytical instruments. Effective

strategies for controlling intercalation and exfoliation of polyolefin/clay nanocomposites are

proposed and evaluated in Chapter 4. In Chapter 5, the effects of clay dispersion on the

rheological, thermal, mechanical properties of WPC composites are discussed. Flame retarding

property effects of clay particles and their dispersion are studied for WPC and the mechanism is

elucidated using pyrolysis. In Chapter 6, detailed discussion on the effects of clay content and

dispersion on foamability was presented using the continuous foaming process. In Chapter 7, the

major contributions based on conducted work are summarized and several suggestions are made

on the direction of future research.

6

Foam type Cell size Cell density

Conventional > 300 μm < 106 cell/cm3

Fine-celled 10-300 μm 106-109 cell/cm3

Microcellular 109 cell/cm3

Table 1.1 Foam classification

7

Cellulosic fibers

Wood fibers Fruit fibersBast fibers Seed fibers Leaf fibers

Hard/soft wood

Flax, hemp, jute, kenaf

Coconut Cotton, coir

Sisal, banana

Figure 1.1 Cellulosic fiber classification [4]

8

CHAPTER 2

THEORETICAL BACKGROUND AND LITERATURE REVIEW 2.1 Wood and Wood Fiber

Compared to other inorganic structure fillers, wood’s low cost per unit volume, lower

abrasion effects on processing equipment, lack of health hazards, low density, ability for surface

modifications, and its abundance in nature make it a suitable alternative in both filler and fiber

forms. Sources include wood shavings, newsprint, paper, and waste wood, such as demolition

wood, wooden pallets, shipping containers, and scrap from construction sites.

However, its properties can vary between and within trees of the same species

depending on source, species, and growth cycle [23]. Its poor compatibility with hydrophobic

thermoplastics, low thermal stability above 190°C, dimensional instability due to excessive

moisture uptake, and tendency for phase separation at freezing temperatures has limited its usage

in thermoplastics composites, especially in the areas of PP, PE, poly(vinyl chloride) (PVC), and

polystyrene (PS).

The chemical makeup of wood is complex. Wood is made up of cellulose, hemicelluloses,

and lignin and is infiltrated with many other compounds that are extractives as shown in Table

2.1 [4]. The type and amount of the extractives are often distinctive with certain species of trees

and can have a pronounced effect on properties. The ratio of these components differs between

hardwood (deciduous) and softwood (coniferous) species.

9

Cellulose functions as the primary structural component within the wood fiber cell walls.

It is a long-chained, unbranched, condensation polymer of β-D-glucose units, with a degree of

polymerization as high as 30,000. Cellulose molecules are completely linear and have a strong

tendency to form intra- and intermolecular hydrogen bonds. This bonding between adjacent

cellulose, forms tightly packed crystalline structures called microfibrils. Micofibrils build up

fibrils and finally cellulose fibers. The fibrous structure along with strong hydrogen bonds makes

cellulose insoluble in most solvents while rendering it with high tensile properties, equivalent to

steel [23]. Cellulose retains its properties up to 190°C and loses about 10% of its strength when

exposed for 10 min at 200°C. In thermoplastic wood fiber composites, cellulose is primarily used

for reinforcement. The hydroxyl groups on the fiber surface are usually either blocked or

modified to be more reactive with thermoplastics.

Hemicellulose is a short chain with a degree of polymerization in the low hundreds and

hence is a low-molecular-weight polymer. It primarily serves as a connecting agent that links or

bonds the microfibrils providing additional structural reinforcement to the wood fiber cell wall. It

varies from relatively unbranched, alkali-resistant species to relatively nonlinear, soluble types.

The nature and proportions of hemicellulose that are found in different wood does vary but tends

to follow broad consistent patterns.

Lignin, amorphous polymeric material, acts as cement in bonding the cellulose

hardwood lignin. Its degradation temperature is at a low (110°C) but its rate of degradation is

very slow. Its precise structure is complex and is still unknown. The thermoplastic characteristics

of lignin enable the manufacture of hardboard-type wood products.

Extractives are often low-molecular-weight oleophillic (organic-loving) compounds.

They can be removed by organic solvents or water. These components are formed from the

10

accumulation and degradation of sugars within the tree (physiological) or to protect the tree

against biological damage (pathological). Extractives are comprised of numerous types of

compounds, some of them volatile. The volatiles can be divided into three main subgroups: the

first; aliphatic compounds, the second subgroup; terpenes, and terpenoids, and the third

subgroup; phenolic compounds consisting of gums, fats, resins, sugars, oils, starches, alkaloids,

and tannins. The composition varies greatly between and within species. Softwoods contain up to

10% extractives. Extractives experience the greatest weight loss between 100°C and 200°C.

Extractives can be removed from the structural (cellulose, hemicellulose, and lignin) portions of

a tree. The influence of extractives is significant especially during foaming of thermoplastic

wood fiber composites.

In WPC applications, softwoods are generally preferred since they have higher aspect

ratios in addition to providing a regular structure of lumens. Table 2.2 lists the characteristics

found in wood for both softwoods and hardwoods. Softwood anatomy is less complex than that

of hardwood. When using sawdust or other particles of wood for filling or reinforcement, the

exposed surface could be a random mixture of all the cells found in softwoods. Maldas et al. [24]

studied the effect of the nature of cellulosic fibers in a polymer matrix. The cellulosic fibers used

were derived from cotton, softwood, and hardwood in the forms of different pulps

(chemithermomechanical pulp (CTMP), kraft, and sawdust).

2.2 Wood Fiber/Plastic Composites

The essential advantage of combining cellulosic fibers, such as wood, rice hull, kenaf or

flax, with plastics is to combine desirable properties of both components, hence enhancing or

extending their usefulness [25-27]. The available literature indicates that among many forms of

11

available cellulosic fibers, wood fibers are the most favored form of fibers in commercial usage.

Because of their high specific stiffness and strength, WPCs are a cost-effective alternative to

many plastic composites or metals [6]. Wood fiber is a non-abrasive substance, which means that

relatively large concentrations of this material can be incorporated into plastics without causing

serious machine wear during blending and processing. In spite of their higher price, WPCs are

becoming increasingly acceptable to consumers as a replacement for natural wood due to such

advantages as durability, color permanence, resistance to degradation and fungal attacks, and

reduced maintenance. Furthermore, adding wood fibers to plastic products makes good use of

waste wood. WPCs are mainly employed in building products, such as decking, fencing, rails,

door and window profiles, and decorative trims. Moreover, these composites are also gaining

acceptance in automotive and other industrial applications.

Polyolefins such as PE and PP are the most commonly used polymer matrix for WPCs

because of its relatively low processing temperature and good processability [7]. However,

hydrophobic polyolefins and hydrophilic wood fibers are naturally incompatible. The strength

and impact properties of WPCs are commonly even lower than those of pure polyethylene as a

result of the poor dispersion of fibers and weak interfacial interaction between the fibers and the

matrix material [28]. The surface modification of fibers or use of external processing aids can

facilitate the dispersion and adhesion of these fibers in the polymer matrix [29]. The most widely

used coupling agents (based on reactive groups) are derivatives of either maleic anhydride or

siloxanes. It has been shown that the mechanical properties of WPCs are improved significantly

when coupling agents are used in the composites [30-34]. Yuskova et al. [35] studied the

interaction energy between polymer and different forms of cellulosic fibers (lignin, cotton, and

wood fiber) and found that wood fiber contributed the most desirable strength to the composite

12

matrix due to its high adhesion interaction. The surface adhesion between the fiber and the

polymer plays an important role in the transfer of stress from the matrix to the fiber and thus

contributes toward the performance of the composite. However, due to the low stiffness of

HDPE, the flexural and tensile moduli of the WPCs are substantially lower than those of natural

wood.

Another critical drawback of WPCs is their high flammability. Improving their flame

retardancy will thus expand the range of their applications. Halogenated flame-retardants, such

as organic brominated compounds, are often used to improve the flame-retarding properties of

polymers; unfortunately, these also increase both the smoke and carbon monoxide yield rates due

to their inefficient combustion [36]. The other commonly used flame retardants are aluminum

trihydrate (ATH), magnesium hydroxide [Mg(OH)2], or intumescent systems. However, they all

exhibit some significant disadvantages. For example, the application of ATH and Mg(OH)2

requires a very high loading of the filler (40-60 wt%) within the polymer matrix to obtain

acceptable performance levels, which yields high-density products and also adversely influences

mechanical properties and processability [37].

The moisture that is present in wood fibers has many adverse effects during processing

and also on final products. It causes undesirable voids in solid WPCs, and dimensional variations.

It also results in poor adhesion due to its sorption to the fibers, and consequently lowers the

mechanical properties [38].

Another limitation imposed on processing when wood fibers are used as filler, is the

maximum processing temperature allowed to avoid its degradation due to higher temperatures.

Therefore, cellulosic fiber/thermoplastic composites have been limited to the use of low-melting

13

polymers such as PE, PP, PS, and PVC. Clemons [7] demonstrated that the processing

temperatures are limited to a maximum of about 200oC when wood fiber is used in the PS matrix.

2.3 Thermoplastic Foams and Microcellular Foams

2.3.1 Blowing Agents

All thermoplastic foams are blown using either CBA or PBA. CBA are substances that

decompose at processing temperatures, thus liberate gases like CO2 and/or N2. Solid organic and

inorganic substances (such as azodicarbonamide and sodium bicarbonate) are used as CBA. In

general, CBA are divided by their enthalpy of reaction into two groups including exothermic and

endothermic foaming agents. The reaction that produces the gas can either absorb energy

(endothermic) or release energy (exothermic). Nowadays, a combination of exothermic and

endothermic CBA is also used for foaming.

PBA are materials that are injected into the system in either a liquid or gas phase. Some

PBA such as, pentane or isopropyl alcohol, have a low boiling point and remain in a liquid state

in the polymer melt under pressure [8]. When the pressure is reduced, the phase change of the

foaming agent from liquid to vapor happens immediately and the vapor comes out of the solution

with the polymer, thereby expanding the melt. As the boiling point of a gas lowers, the volatility

of the gas increases. Higher volatility or vapor pressure requires more pressure to keep the gas in

its liquid phase in the polymer melt. Another type of PBA is inert gases, such as N2 or CO2.

Inertness represents the reactivity and corrosiveness of a gas to the polymer, any additive, the

machinery, or the surrounding environment. As a blowing agent becomes more inert, it is less

reactive (or corrosive) to its surroundings. These inert gases dissolve as vapors in the polymer

melt and diffuse out of the solution as vapors to expand the polymer melt. The solubility of the

14

gases affects the final density. Both CO2 and N2 have low solubility in polyolefin matrices. The

diffusivity of these gases is important in maintaining the cell structure and resulting density.

In conventional foam processing, the most commonly utilized blowing agents are FCs,

CFCs, n-pentane, and n-butane [39]. These blowing agents have a high solubility, and can thus

be dissolved in large quantities into the polymer matrix. For example, the solubility of FC-114 in

polystyrene is above 20 wt% at a pressure of 6.9 MPa and at a temperature of 200°C [40]. These

blowing agents allow for a foam structure that have high void fraction at low processing pressure

and high volume expansion due to small amounts of gas loss. Since these blowing agents have

low diffusivities due to their larger molecular size, the loss of gas from the extrudate during

expansion is small [39]. Therefore, the final foam product can have a low foam density. Despite

all the advantages of the conventional foaming blowing agents, some serious environmental and

safety concerns exist. The use of CFCs was stopped by the Montreal Protocol [41], signed by 24

countries in 1987. Other blowing agents, such as n-pentane, are hazardous because of their high

flammability. As a result, alternative blowing agents for polymer foam processing are being

researched and developed.

2.3.2 Microcellular Foam Processing

The generation of high cell density for polymeric foaming becomes possible by inducing

a sudden thermodynamic instability in a polymer/gas solution. After polymer/gas solution

formation, the cells should be preserved by controlling their growth until the gas bubbles are

stabilized [42]. The microcellular foaming system should have the following essential processing

mechanisms to successfully achieve these conditions: a mechanism for completely dissolving a

large and soluble amount of a blowing gas into a polymer under a high processing pressure; a

15

mechanism for inducing a thermodynamic instability in the homogeneous polymer/gas solution

formed earlier; and a mechanism for controlling the growth of bubbles while preventing them

from coalescing and collapsing. Based upon the successful implementation of microcellular

foaming, the system requirements for the microcellular foaming of wood fiber-reinforced

polymer composites can be established [42,43].

2.3.3 Overall Process for Microcellular Foam Processing

The three major steps in the foaming process are, the formation of polymer and gas

solution, cell nucleation, and cell growth processes. These steps are basic to microcellular

processing and are applied to both batch and continuous manufacturing processes.

2.3.3.1 Formation of Polymer/Gas Solution

In continuous foam processing, it is essential to obtain the formation of a uniform

solution since its quality significantly affects the number of bubbles nucleated later on in the

process. Above all, the amount of blowing agent injected should be below the solubility limit of

the processing pressure and temperature in order to ensure complete mixing and dissolving of

gas into the polymer. This must be carefully controlled because large voids will form if an excess

amount of blowing agent exists and cannot be dissolved into the polymer. For this reason, it is

crucial to determine the solubility (or the amount) of blowing agent that can be absorbed and

dissolved into the polymer 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.

16

Solubility

The solubility limit of gas dissolved into the polymer can vary, depending on the

system pressure and temperature and can be estimated by Henry's law [41]:

ss pHc = (2.1)

where cs is the solubility of gas in the polymer in cm3/g or ggas/gpolymer, H is Henry's law

constant (cm3 [STP]/g-Pa), and ps is the saturation pressure in Pa. The constant H is a function

of temperature described by:

)RTH

exp(HH soΔ

−= (2.2)

where R is a gas constant in J/K, T is the temperature in K, Ho is a solubility coefficient constant

(cm3 [STP]/g-Pa), ΔHs is the molar heat of sorption in J. The molar heat of sorption, ΔHs, can be

either a negative or positive value, depending on the polymer-gas system.

Equations (2.1) and (2.2) can be used to estimate the solubility of a blowing agent in the

polymer at a certain processing pressure and temperature. The estimation of the solubility of CO2

in some polymers can be found in some research papers [44,45]. Based on the polymer flow rate

in extrusion, the gas flow rate can be controlled so that the gas-to-polymer weight ratio may be

maintained below the solubility limit.

Diffusivity

The diffusivity D is mainly a function of temperature, and this can be represented as the

following equation [46,47]:

D DERTo

d= −exp( ) (2.3)

17

where Do is the diffusivity coefficient constant in cm2/s, and Ed is the activation energy for

diffusion in J. Thus, the diffusion rate can be increased by processing the plastic/gas mixture at a

higher temperature.

Dissolution

In a continuous process, retaining an undissolved gas in the polymer matrix is possible if

an excess amount of gas is injected. Therefore, it is critical to ensure that the amount of gas

injected is below the solubility limit in the processing conditions. However, one advantage of

using the extrusion foaming process is the reduction in dissolution time because of higher gas

diffusivity at the high processing temperature. This makes the extrusion process a more cost-

effective method.

In addition to the proper amount of gas injection, a sufficient amount of dissolution time

is required to generate a uniform solution. Although the appropriate amount of gas can be

injected, it does not necessarily guarantee the formation of a uniform solution. If the required

time of gas diffusion in the polymer matrix is longer than the melt residential time inside the

system between gas injection and nucleation, it is obvious that a uniform solution would not be

achieved.

Park et al. [48] investigated the diffusion behavior in an extrusion process containing a

mixing screw. It was observed that shear mixing caused by the screw rotation promotes

convective diffusion. In convective diffusion, the screw motion creates the contact between a

high gas concentration region (gas bubble) and a low gas concentration region (polymer melt).

Furthermore, by stretching gas bubbles in the shear field generated by the motion of the screw

which enhances the diffusion process, the interfacial area will be increased, thereby improving

the diffusion mechanism. In addition, a dissolution enhancing device containing static mixers in

18

the extrusion system will enhance the dissolution process by generating shear fields as the

mixing elements are reorienting the melt along the flow direction, thus promoting solution

formation.

2.3.3.2 Cell Nucleation

Nucleation is a critical step in fine-celled or microcellular foaming processes.

Nucleation can be defined as the transformation of small clusters of gas molecules into

energetically stable groups or pockets. In order to create bubbles in liquids or polymer melts, a

minimum amount of energy must be given to the system so that it can break the free energy

barrier. This energy can be provided by heating or through a pressure drop. Two types of

nucleation mechanisms can be observed: homogeneous and heterogeneous nucleation.

Homogeneous nucleation is a type of nucleation where cells are nucleated randomly throughout

the liquid or polymer melt matrix. It requires higher nucleation energy than heterogeneous

nucleation. Heterogeneous nucleation is defined as nucleation at certain prefered sites, such as on

the phase boundary, or sites provided by the additive particles.

Homogeneous Nucleation

In case of homogeneous nucleation, the classical nucleation theory has been used to

describe the nucleation behavior in microcellular foaming by Colton and Suh [49]. In this theory,

the required work to generate a bubble of radius r in a liquid is given by

bbpb VPAW Δ−= γ (2.4)

where the first term, bpb Aγ , is the work required to create a bubble with surface tension, pbγ ,

and surface area, bA , and the second term, bVPΔ , is the work done by the expansion of gas inside

19

a bubble of volume Vb. The difference between the two terms is the actual work required to

generate a cell. After the substitution of geometric equations of a sphere for Ab and Vb, the

equation becomes as follows:

W r r Ppb= −443

2 3π γ π Δ (2.5)

In order for the bubble to grow spontaneously, the maximum energy barrier must be overcome.

If the induced energy in the system is lower than the maximum energy, the bubble, which is

smaller than the critical bubble size, collapses. The amount of free energy can be calculated by

differentiating W with respect to r from the previous equation.

2

3*hom 3

16P

G pbΔ

=Δπγ

(2.6)

The nucleation rate is as follows:

)exp(*

kTGfCN oo

Δ−=& (2.7)

where Co is the concentration of gas molecules in the solution, fo is the frequency factor of gas

molecules joining the nucleus, and k is the Boltzman constant.

According to the classical nucleation theory, a greater number of cells can be nucleated as

the saturation pressure, PΔ , increases. The saturation pressure can be estimated as the gas

concentration in the polymer according to Henry’s law (Equation 2.1). When the amount of gas

in the polymer increases, the chance to nucleate more cells also increases.

Even though the classical nucleation theory yields very valuable information about the

pressure drop and cell nucleation relationship, it does not predict the effect of the pressure drop

rate on cell nucleation. The effect of the pressure drop rate on nucleation is another important

parameter and should be carefully examined. In the classical nucleation theory, an instantaneous

20

pressure drop and instantaneous nucleation are assumed and thus, the nucleation rate, N& of

Equation (2.7) corresponds to the number of nucleated cells. However, in reality, the pressure

drop is not instantaneous and happens over a finite time period. The nucleation rate will be

affected based on how fast the pressure drops, or based on the pressure drop rate.

The effect of the pressure drop rate on cell nucleation is as follows: the faster the

pressure drop, the more that cells are nucleated [50]. Since the higher pressure drop rate requires

a shorter time period, the already nucleated cells do not have a chance to grow a great deal.

Therefore, more gas is utilized for cell nucleation and less is used for cell growth. As a result,

high cell density foams or microcellular foams can be produced with high pressure drop rate dies.

Heterogeneous Nucleation

Heterogeneous nucleation is the other type of nucleation which is promoted at some

preferred sites. The mechanism of heterogeneous nucleation in the polymer foaming processes

has not been thoroughly studied due to its complexity. For example, it is well known that cell

density in the foam structure can be improved significantly by adding certain fillers [51]. In

many cases, foams with some additives or fillers have higher cell density compared to those

without additives [52]. However, the fundamental studies on the nucleation enhancement

behavior with these additives have not been investigated in depth. Chen et al. [53] from Trexel

Inc., investigated the mechanism of heterogeneous nucleation with filled polymers. The

hypothesis of this process is that the undissolved gas between the polymer and the additives

creates cells when the system pressure drops during the foaming process. Within the

heterogeneous nucleation theory, certain sites or spots in the polymer matrix which contain

undissolved gas may become cells, provided that the size of the spots is larger than the following

critical value [53]:

21

p

rΔ

=σ2 (2.8)

where r is critical radius, σ is the surface tension and pΔ is the pressure difference between the

bubble and the melt.

Micropores can be cracks or defects on the polymer-additive interface. During the mixing

process, the polymer melt may not be able to fill these micro-pores and gaps between two phases

completely due to the surface tension. From the above equation, it is evident that the surface

tension force increases as the radius, r, becomes smaller. Therefore, the pores cannot be filled,

although the pressure difference between the polymer melt and micro-pores is larger. This

creates some space for the gas to accumulate, thereby yielding cell nucleation.

It was experimentally verified that a certain amount of gas accumulates at the polymer-

additive interface and that the spots where gas accumulates generate cells if the size of the spots

is larger than the critical value. This explains why heterogeneous nucleation generally requires

much less gas to produce fine-celled structures compared to homogeneous nucleation. The rate at

which the bubbles nucleate heterogeneously is given by the following equation [53]:

)exp(111 kTG

fCN het∗Δ−

= (2.9)

where 1C is the concentration of gas molecules, 1f is the frequency factor of gas molecules

joining the nucleus, k is the Boltzman’s constant, and T is the temperature in K. ∗Δ hetG is Gibbs

free energy and can be expressed for heterogeneous nucleation, which occurs at smooth planar

surfaces as follows (see Figure 2.1 (a)):

)(3

162

3

cbp

het FPG θ

πγΔ

=Δ ∗ (2.10)

where 3bpγ is the surface energy of the polymer-bubble interface, PΔ is the gas pressure used to

22

diffuse the gas into the polymer. )( cF θ , which is the reduction of energy due to the inclusion of

additives (nucleants), can be expressed as follows:

2)cos1)(cos2)(41()( θθθ −+=cF (2.11)

where θc is the contact angle of the polymer-additive gas interface.

In actual polymeric foaming processes, the geometry of the nucleating sites, which

depends on the nucleating agents themselves, the presence of unknown additives or impurities

and the nature of the internal walls of equipment, varies from one site to another. Therefore,

instead of assuming that all nucleating sites are either smooth planar surfaces, observable

nucleation rates can occur in conical cavities that exhibit geometries consistent with the image

presented in Fig. 2.1 (b) where the semiconical angles, β are randomly distributed between 0 and

90° at different nucleating sites. In this case, F(θc,β) is the reduction of energy, which can be

expressed as [54]

]

sin)(coscos

)sin(22[41),(

2

ββθθ

βθβθ−

+−−= ccccF (2.12)

Since the phase boundaries between the additive and the polymer matrix have a lower

free energy barrier for nucleation than homogeneous nucleation, nucleation is more likely to

occur at these sites. The number of cells nucleated can be controlled by the amount of additives

[55,56]. Also, if the additive particle size is fine graded (less than a micron) and well dispersed in

the polymer matrix, a uniformly distributed microcell structure can be produced [55].

Lee et al. [57] examined the gas absorption behavior of polymer systems to explain

heterogeneous nucleation in mineral filled polymers: HDPE with/without talc, and PVC

with/without CaCO3. It was suggested that the accumulated gas in the filler-polymer interface

helps to create cells in the foaming process. Ramesh et al. [58] developed a model for

23

heterogeneous nucleation in the blend of PS and high impact polystyrene (HIPS) based on the

presence of microvoids.

2.3.3.3 Cell Growth

After cells are nucleated, they continue to grow because of gas diffusion from the

polymer matrix. Since the pressure inside the cells is greater than the surrounding pressure, cells

tend to grow in order to decrease the pressure difference between the inside and the outside [59].

The cell growth mechanism is affected by the viscosity, diffusion coefficient, gas concentration,

and the number of nucleated cells. The temperature can control the amount of cell growth, which

then affects two important parameters: diffusivity and melt viscosity. For instance, if the

temperature decreases, the diffusivity of gas decreases and the melt viscosity of the matrix

increases, thus, decreasing the cell growth rate. In the foaming process, maintaining the gas in

the polymer matrix by close temperature control is essential for achieving good cell growth and

thus, high volume expansion. In microcellular foams, because the cell size is very small and the

cell density is very high, the cell wall thickness separating the two cells is smaller and the rate of

growth is faster than in conventional foams.

However, this may also cause cell coalescence, which is undesirable [60]. If the cells

coalesce during cell growth, the initial cell density will be deteriorated. As nucleated cells grow,

adjacent cells will begin to touch each other. These contiguous cells tend to coalesce because the

total free energy is lowered by reducing the surface area of cells via cell coalescence [61]. It may

be noted that the shear field generated during the shaping process tends to stretch nucleated

bubbles, which further accelerates cell coalescence [62]. When the cell density is deteriorated,

the mechanical and thermal properties are deteriorated as well.

24

Although Baldwin et al. [63] attempted to prevent cell coalescence in the die by

pressurizing the nucleated polymer solution during shaping, the extruded foam structure showed

that many adjacent cells were coalesced and that the cell density was deteriorated. Considering

the difficulty of maintaining a high backpressure in the shaping die in case of a large cross

section of extruded foam, it may not be realistic to prevent cell coalescence by controlling the

pressure alone in the shaping die.

Park et al. [62] suggested a means for suppressing cell coalescence by increasing the

melt strength of polymer via temperature control in microcellular extrusion processing. The melt

strength, by definition, may be treated as a degree of resistance to the extensional flow of the cell

wall during the drainage of polymer in the cell wall when volume expansion takes place.

Therefore, the cell wall stability will increase as the melt strength increases [64].

2.3.4 Effects of Processing Conditions in a Continuous Foaming Extrusion Process

In a continuous microcellular foaming process, a metered amount of gas (CO2 or N2) is

injected in the barrel of an extruder. The mixing of gas with a polymer melt depends on the shear

force generated by the screw rotation and other special mixing elements (e.g. static mixers). The

cells are nucleated by the rapid pressure drop that had been generated, when the mixture flows

through the die and when the cells keep growing until solidification. The three main differences

between a continuous extrusion foaming process and a batch foaming process are that in the

continuous process; (1) a metered amount of gas (instead of the saturation amount of gas) is

dissolved in the system, (2) the pressure gradient as the major nucleation driving force, is

determined by the flow instead of the saturation pressure, and (3) the foaming temperature is

25

always the extrusion die temperature (although the melt temperature before the die also has an

effect).

To determine the relationship between processing conditions and foam structure, the

effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, the

cell size and cell density, and cell morphology were investigated experimentally. To increase the

pressure drop in the die, one usually increases the screw rotation speed or reduces the die

temperature.

Figure 2.2 gives a qualitative explanation about the importance of the pressure drop rate

in determining the cell density [50]. Two processes with the same pressure drop across the die

are shown in the Figure 2.2. Here it is assumed that the pressure profile in the die is linear but the

pressure drop rate (dP/dt) of process 1 is higher than that of process 2. The amount of gas that is

dissolved in the system is indicated by the saturation pressure (Psat). Above the saturation

pressure, the mixture is a single phase and below it, nucleation begins. If the two processes have

an identical nucleation time (Δt), the pressure decrease (ΔP1 and ΔP2) during Δt is the real

thermodynamic instability that induces the cell nucleation or phase separation. Obviously, ΔP1 is

larger than ΔP2, which indicates that process 1 has a higher nucleation rate based on the classical

nucleation theory. This illustrates that the pressure drop and pressure drop rate presented thus far

are merely the operating conditions, not the real thermodynamic instability (ΔPi) in the classical

nucleation theory, although they correlate with each other. Consequently, a higher pressure drop

rate can always create a higher thermodynamic instability (ΔPi) and a higher nucleation rate.

2.4 Wood Fiber/Plastic Composite Foams

26

2.4.1 Volatiles from Wood Fiber

Wood fibers are composed of four basic constituents; cellulose, hemicelluloses, lignin

and extractives. Apart from these four constituents, wood also contains moisture. During the high

extrusion foam processing temperatures, wood fiber releases moisture and other volatiles, which

deteriorate the cell structure of WPC foams, by causing cell coalescence and cell collapse [65].

Rizvi et al. [66] investigated that even after wood fiber is oven-dried, it still releases about 3%

volatiles when the temperature is raised from 110oC (a typical drying temperature) to 200oC (a

typical processing temperature). Therefore, it can be deduced that in extrusion foam processing,

whenever the processing temperature is raised to a higher level in any zone of the barrel,

additional moisture and volatiles will be generated and affect the foaming process significantly.

In order to ensure fine-celled morphology, the moisture and the volatile contents of wood fiber

need to be reduced to a bare minimum, using any of the standard drying techniques, such as

online devolatilization [67], oven drying, hot air convective drying, drying in a K-mixer [68] and

the like.

2.4.2 Batch Processing

During the batch process, a polymer sample is first placed in a high-pressure chamber

where the sample is saturated with an inert gas (such as CO2 or N2) under high pressure at

ambient temperatures. A thermodynamic instability is then induced by rapidly lowering the

solubility of the gas in the polymer. This is accomplished by releasing the pressure and heating

the sample. This expansion drives the nucleation of a large number of microcells and the

nucleated cells grow to produce the foam expansion. Because of the low rate of gas diffusion

27

into the polymer at room temperature, an exceptionally long time is required for the saturation of

the polymer with gas, which is the major disadvantage of the batch process.

Matuana et al. [69-73] investigated the processing of microcellular-foamed structures in

PVC/wood fiber (silane treated) composites by a batch foaming process. They have established

the relationships between cell morphology and processing conditions, as well as between the cell

morphology and mechanical properties. It is seen that cell densities show a decreasing tendency

with the increase of foaming temperatures. The cell density decreased significantly after reaching

a foaming temperature above 90°C because of the activated cell coalescence by the lowered melt

strength at elevated temperatures. The cell size increased with the increase of foaming

temperature because of the increase of void fraction and cell coalescence.

Matuana et al. [74] also investigated microcellular foam of polymer blends of HDPE/PP

with wood fiber in a batch process. HDPE/wood composites had a reasonably high void fraction

at high foaming times compared to PP/wood composites and HDPE/PP blend-wood composites.

The batch foaming process used to generate cellular foamed structures in the composites

is not likely to be implemented in the industrial production of foams because it is not cost

effective. The microcellular batch foaming process is time-consuming because of the multiple

steps in the production of foamed samples [74]. In order to overcome the shortcomings of the

batch process, a cost-effective, continuous microcellular process (injection molding, extrusion,

and compression molding process) was developed based on the same concept of thermodynamic

instability that is found in the batch process.

2.4.3 Injection Molding Processing

Bledzki et al. [75] investigated the foaming of wood fiber/PP composites in an injection

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molding process. Wood fiber-reinforced PP composite foams were processed by an injection

molding process where several variables were considered when having operated an injection

molding machine. Some of these variables can affect the physical properties of the foam. It is

well established that the mold temperature and cooling time are important variables in this regard.

However, there are many other factors that can be adjusted, including such variables as front

flow speed and filling quantity, which might also have an effect on one or more foam properties.

The advantage of this injection molding process is the fact that cellular composites can be

prepared with a sandwich structure using a conventional injection molding machine using

different CBA. They also [76] demonstrated that exothermic foaming agents give better

performance when considering cell size, diameter, distance, and polydispersity compared to

endothermic and endo/exothermic CBAs for the cellular wood fiber reinforced PP composites. In

addition, they [77] reported that the melt flow index of PP and a variation of injection parameters

(mold temperature, front flow speed, and filling quantity) have a great influence on the properties

and structure of the wood fiber/PP composite foams. It is observed that because of the increase

of filling quantity, the specific flexural strength decreases gradually, which suggests that a

suitable injected mass should be selected. They [78] conducted a comparative study of cell

morphology, weight reduction, and mechanical properties between the box part and a panel

shape using soft wood fiber/PP composite foams by considering different processing

temperatures. The cell morphology of the injection molded box part differed in parts, with the

area near the injection point showing a finer cellular structure than that of the areas far from the

injection point area. As a result, the mechanical properties also differed in parts of the areas.

2.4.4 Extrusion Processing

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2.4.4.1 Extrusion Processing Conditions of WPC Foams

In order to minimize the volatile emissions from the wood fiber during extrusion

processing, the processing temperature should be kept to a minimum. However, lowering the

temperature causes an increase in the viscosity of the molten extrudate and thereby increases

processing difficulty. Therefore, the determination of an optimum processing temperature

becomes crucial to ensure the formation of acceptable cellular structure, while maintaining

satisfactory processing conditions. The highest processing temperature after the drying stage

primarily governs the emissions from wood fiber, which affects the foam morphology and has

been studied by Guo et al. [79]. The cell morphology above the optimum processing temperature

was visibly irregular and at lower temperatures, thus, the foaming effect was insignificant. Rizvi

[80] studied the control of residence time that contributes to foaming found in the devolitalizing

tandem extrusion system, based on the thermogravimetric analyzer (TGA) results. When the

wood fibers are exposed to a high temperature for a long time in the first extruder, the amount of

the volatile generated from wood fibers in the second extruder can be very small even at higher

temperatures, such as above 175oC in wood fiber/HDPE composite foams. Torres et al. [81]

studied the formation of bubbles in a single-screw extruder during extrusion processing inside

the barrel. They concluded that bubble formation depends heavily on the feeding behavior of the

polymer-fiber blend and less on the moisture contents of the fibers. If the dispersion of the fiber

is good in the feed section, the final extrudate exhibits finely dispersed bubbles, or else the

extrudate contains irregular and large bubbles.

2.4.4.2 Extrusion Foaming of WPC with CBA

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When WPC are foamed with CBA, the decomposition temperature of CBA and

processing temperatures dictate the processing conditions. To keep the processing temperatures

low so as to suppress the volatiles generated from wood fibers during processing, the CBA with

lower decomposition temperatures are preferable.

Rizvi et al. [82] identified the optimal processing window with various CBA

(decomposition temperatures are lower than 165oC) from wood fiber/HDPE composite foams.

As the extrudate surface temperature decreases below the crystallization temperature, the amount

of polymer being crystallized increases, which results in the increased stiffness of the polymer

matrix. As a result, the extrudate resists expansion and an increase in density follows. PS/wood

fiber foamed composites were investigated using moisture as a foaming agent. [81]

Li et al. [83] examined the influences of the contents, types (exothermic and

endothermic), and forms (pure and masterbatch) of CBAs, as well as the use of coupling agents

on the density reduction and cell morphology of extrusion-foamed neat HDPE and wood

fiber/HDPE composites.

Zhang et al. [84] have experimented with two system configurations (tandem extrusion

system vs. single extruder system) for wood fiber/polymer composites to demonstrate the system

effect on the cell morphology and foam properties. The system configuration had a strong effect

on the cell morphology, and the tandem extrusion system is highly effective for fine-celled

foaming of wood fiber/HDPE composites compared to a single screw extruder system.

Rigid PVC/wood fiber composites foamed in a continuous extrusion process were

investigated by Mengelogu et al. [85,86]. The effects of wood fiber moisture content, all-acrylic

foam modifier content, CBA content, and extruder die temperature on foamed composites

structures and properties were studied. Exothermic foaming agents produced smaller average cell

31

sizes compared to endothermic foaming agents regardless of the CBA content. This trend is

because of the lower solubility and higher diffusivity of N2 (exothermic) in the PVC matrix

compared to that of CO2 (endothermic).

Bledzki et al. [87] prepared wood fiber reinforced PP composite foams as a profile by a

continuous extrusion process. The effects of different CBA types and the variation of their

concentrations on the cell morphology and physio-mechanical properties of wood fiber/PP

composite foams were studied. The effects of fiber type (hard and soft wood fiber), fiber length

(finer soft wood fiber), and fiber contents on the properties were also investigated. It was

observed that the screw pressure should be constant to get a good foamed profile (optically in

surface) and that it may be improved with high pressure. The residence time of the materials in

the barrel plays a big role in microfoaming, especially in the extrusion process, as does the

inherent moisture content of the wood fibers.

Rodrigue et al. [88] investigated the effect of wood powder on the polymer foam

nucleation of wood-low density PE composites in an extrusion process and reported that the

wood particles act as nucleating agents to substantially reduce cell size and increase cell density.

Since the properties of foams are known to improve with decreasing cell size and

increasing uniformity [9], it is highly desirable to focus research efforts on developing foams

with morphologies that possess a reduced cell size and narrower cell-size distribution.

However, when a CBA is used for foaming, a higher processing temperature is needed in

order to decompose the blowing agent, which results in the release of a greater amount of

volatile gases from wood fibers. Ultimately, it is difficult to obtain a fine-celled structure from

the WPC extrusion process using CBA.

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2.4.4.3 Extrusion Foaming of WPC with PBA

Compared with CBA-based processing, the PBA-based processing (such as

environmentally friendly CO2 and N2) does not have decomposition temperature limitations and

as such, can be processed below the critical temperatures. Furthermore, it involves lower cost,

and in general, can produce better cell morphology.

There are several basic steps for continuous extrusion foaming of WPC with a PBA: (a)

dispersion of wood fiber in plastics, (b) moisture removal, (c) uniform formation of a

polymer/wood fiber/gas mixture, (d) cell nucleation, (e) cell growth, and (f) timely solidification

of the polymer matrix. Cell nucleation occurs in a rapid-pressure-drop nucleation die [50]. The

generation of a large number of bubble nuclei inside the polymer melt can be achieved by

subjecting the polymer/gas solution to a thermodynamic instability. The thermodynamic

instability can be induced by lowering the solubility of gas in the solution and by introducing a

rapid pressure drop, which results in the nucleation of a large quantity of micro-cells. Cell

nucleation is a very critical step, as it directly influences the number of cells created in the

polymer matrix. The nucleated cells continue to grow upon exiting the die and cell growth stops

either when all the gas dissolved in the plastic matrix is depleted or when the matrix becomes too

stiff due to cooling, to allow further growth. There are two critical issues involved in cell growth:

cell coalescence and cell collapse. Park and Behravesh [65] developed effective strategies that

can prevent cell coalescence and gas escape in the cell growth stage. Cell coalescence can be

suppressed by cooling the polymer/gas solution homogeneously, which increases the melt

strength. Whereas, gas escape can be controlled by cooling the surface of the extrudate to form a

solid skin layer, thereby, blocking the gas from escaping from the polymer.

When the polymer melt is extruded out of the die and its temperature decreases, it will

33

solidify through glassification or crystallization. Timely solidification is important, for a delayed

solidification may result in gas loss, whereas solidification that is too fast will not produce a

desired volume expansion ratio (or density reduction) [89].

Guo [90] studied the foaming behaviors with N2 and CO2 in the two different polymer-

based (i.e., mPE and HDPE) WPC systems. He discovered that N2 appears to be better than that

of CO2 for foaming of WPC, in terms of the cellular structure of WPC and the blowing agent

efficiency.

The expanding markets, however, call for high performance WPC with superior and

unique properties (e.g. flame retardance) to meet individual application requirements.

2.5 Polymer/Layered Silicate Nanocomposites

In recent years, polymer/layered silicate (i.e. clay) nanocomposites have attracted great

interest, both in industry and in academia, because they often exhibit remarkable improvement

in material properties when compared with virgin polymer or conventional micro and macro-

composites. These improvements can include high moduli [91,92], increased strength and heat

resistance [93], decreased gas permeability [94,95] and flammability [96,97], and increased

biodegradability [98]. On the other hand, there has been considerable interest in both theory

and simulations to address the preparation and properties of these materials [99,100]. They are

also considered to be unique model systems for the purpose of studying the structure and

dynamics of polymers in confined environments [101].

Although the intercalation chemistry of polymers when mixed with appropriately

modified layered silicate and synthetic layered silicates has long been known [102], the field of

polymer nanocomposites has gained momentum recently. Especially, there are two major

34

findings that have stimulated the revival of interest in these materials: first, the report from the

Toyota research group of a Nylon-6/montmorillonite (MMT) nanocomposite [103], in which

very small amounts of layered silicate loadings resulted in pronounced improvements of

thermal and mechanical properties; and second, the observation made by Vaia et al. [104] that

it is possible to melt-mix polymers with layered silicates, without the use of organic solvents.

Today, efforts are being conducted globally using almost all types of polymer matrices.

The key to yielding these improved properties rests in exfoliating and dispersing

completely individual platelets with high aspect ratios (over 100) in the polymer matrix.

Exfoliated nanocomposite preparation by conventional polymer processing, therefore requires

the occurrence of strong interfacial interactions between the polymer matrix and the clay in order

to make the entire surface of the clay layers available for the polymer. This is readily achieved

with high polar polymers such as polyamides. However, non-polar materials such as PE and PP

only interact weakly with mineral surfaces, making exfoliated nanocomposites by melt blending

considerably more difficult.

2.5.1 Structure and Properties of Layered Silicates

The layered silicates (i.e. clay) used in the nanocomposites belong to the same

structural family as the better-known minerals such as talc and mica [105] (i.e. 2:1

phyllosilicates). Their crystal lattice consists of two-dimensional, 1 nm thick layers which are

made up of two tetrahedral sheets of silica fused to an edge-shaped octahedral sheet of alumina

or magnesia. The lateral dimensions of these layers vary from 300 Å to several microns

depending on the particular silicate. Stacking of the layers leads to a regular Van der Waals gap

between them called the interlayer or gallery. Isomorphic substitution within the layer

generates negative charges that are normally counterbalanced by hydrated alkali or alkaline

35

earth cations residing in the interlayer. Because of the relatively weak forces between the

layers (due to the layered structure), the intercalation of various molecules and even polymers,

between the layers becomes facile. Pristine mica-type layered silicates usually contain hydrated

Na+ or K+ ions [106]. Ion exchange reactions with cationic surfactants including primary,

tertiary and quaternary ammonium or phosphonium ions render the normally hydrophilic

silicate surface to become organophilic, which makes possible intercalation of many polymers.

The role of alkyl ammonium cations in the organosilicates is to lower the surface energy of the

inorganic host and improve the wetting characteristics with the polymer. Additionally, the

alkyl ammonium cations could provide functional groups that can react with the polymer, or

initiate polymerization of monomers to improve the strength of the interface between the

inorganic particles and the polymer [99,107].

The most commonly used layered silicates are MMT, hectorite and saponite, which are

types of clay. Details on the structure and chemistry of these layered silica