RHEOLOGICAL CHARACTERISTICS OF PULP-

FIBRE–REINFORCED POLYAMIDE COMPOSITE

by

Robenson Cherizol

A thesis submitted in conformity with the requirements

for the degree of Doctor Philosophy

Department of Chemical Engineering & Applied Chemistry

University of Toronto

© Copyright by Robenson Cherizol 2016

ii

RHEOLOGICAL CHARACTERISTICS OF PULP-FIBRE–REINFORCED

POLYAMIDE COMPOSITE

Robenson Cherizol

Doctor of Philosophy

Department of Chemical Engineering & Applied Chemistry

University of Toronto

2016

iii

Abstract

Recently, there has been increasing interest in utilizing pulp-fibre–reinforced, higher-

melting–temperature engineering thermoplastics, such as polyamide 11 and polyamide 6

in the automotive, aerospace and construction industries. Moreover, the rheological

characteristics of those composites were not fully investigated in relation to processing

approaches and pulp-fibre aspect ratio. Two processing approaches were used in this

thesis: the extrusion compounding process and the Brabender mixer technique using

inorganic salt lithium chloride (LiCl). The fibre-length distribution and content, and the

densities of the PA11 and modified bio-based PA11 after compounding, were

investigated and found to coincide with the final properties of the resultant composites.

The effects of fibre content, fibre aspect ratio, and fibre length on rheological properties

were studied. The rheological properties of high-yield–pulp (HYP) –reinforced bio-based

Polyamide 11 (PA11) composite (HYP/PA11) were experimentally investigated using a

capillary rheometer. Experimental test results showed a steep decrease in shear viscosity

with increasing shear rate; this melt-flow characteristic corresponds to shear-thinning

behavior in HYP/PA11. The morphological properties of HYP/PA11 composite were

examined using SEM: no fibre pullout was observed. This was due to the presence of the

hydrogen bond, which created excellent compatibility between high-yield pulp fibre and

bio-based Nylon 11. The viscoelastic characteristics of biocomposites derived from

natural-fibre–reinforced thermoplastic polymers and of predictive models were reviewed

to understand their rheological behavior. Novel predicted multiphase rheological-model–

based polymer, fibre, and interphasial phases were developed. Rheological characteristics

iv

of the composite components influenced the development of resultant microstructures;

this in turn affected the mechanical characteristics of a multiphase composite.

Experimental and theoretical test results of HYP/PA11 showed a steep decrease in

apparent viscosity with increasing shear rate; this melt-flow characteristic corresponds to

shear-thinning behavior in HYP/PA11.The nonlinear mathematical model to predict the

rheological behavior of HYP/PA11was validated experimentally at 200˚C and 5000S-1

shear rate.

v

Acknowledgments

I would like to thank my advisors, Prof. Mohini Sain and Jimi Trong Ph.D., for their

supervision and support. They have inspired me with their valuable qualities as

professionals and scientists. I am also very grateful to my committee members, Profs.

Christopher Yip, Hani Naguib and Saed Sayad, for their scientific and constructive input,

criticisms, advice, and time.

I genuinely appreciate the financial support of MITAC, Automotive Partnership Canada

(APC), the University of Toronto, and Ford Motor Company that allowed me to pursue

my three-year internship at Ford facilities at Windsor.

I wish to express my warm and sincere thanks to my fellow students and other

department staff at the Faculty of Forestry, and Pauline Martini and Joan at the

Department of Chemical Engineering and Applied Chemistry. Special thanks to Mr. S.

Law, Mr. Lynn, MS. Birat KC, MSc. Ahmed Sobh, Dr. S. Kunar, and Dr. Suhara

Panthapulakkal, and the staff of “PERDC” of the Ford Essex engine plant at Windsor for

their help and support during this thesis work.

I am permanently indebted to my beloved friends and relatives for their love and support.

I would like to thank my parents, Applys and Lucilda Cherizol, for their constant

encouragement, inspiration, and valuable guidance throughout my life.

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Finally, words alone cannot express the thanks I owe to my wife Ernette, to my kids

Alyssa and Daniel. They not only gave me their unconditional love, but also supported

me unconditionally through the ups and downs of my doctoral studies.

vii

Table of Contents

Abstract .............................................................................................................................. iii

Acknowledgments............................................................................................................... v

Table of Contents .............................................................................................................. vii

List of Tables ................................................................................................................... xiv

List of Figures ................................................................................................................... xv

Chapter 1 Introduction ....................................................................................................... 1

1.1 Background and Motivation ..................................................................................... 1

1.2 Hypotheses ................................................................................................................ 3

1.3 Objectives ................................................................................................................. 4

1.4 Outline of the Thesis ................................................................................................. 4

Chapter 2 Literature Review ............................................................................................ 10

2.1 High-Yield Pulp fibre ............................................................................................. 10

2.2 Polymeric matrix ..................................................................................................... 10

2.2.1 Bio-based Nylon 11 (PA11)............................................................................. 11

2.2.2 Surface modification of natural fibres ............................................................. 13

2.3 Extrusion Process .................................................................................................... 15

2.3.1 Effects of processing parameters on properties of composites ........................ 16

viii

2.3.2 Low-temperature compounding ....................................................................... 19

2.4 Rheology ................................................................................................................. 20

2.5 Modeling of Fluid Flows ........................................................................................ 23

2.5.1 Classification of fluid flow modeling .............................................................. 23

2.5.1.1 Continuum hypothesis .............................................................................. 24

2.5.1.2 Molecular theory ....................................................................................... 25

2.5.2 Continuum models of fluid flows and constitutive equations ......................... 26

2.6 Viscoelasticity of composite materials ................................................................... 28

2.7 Problem Statement .................................................................................................. 30

Chapter 3 Evaluation of the Influence of Fibre Aspect Ratio and Fibre Content on the

Rheological Characteristic of High-Yield-Pulp Fibre–Reinforced Polyamide 11

(HYP/PA11) Green Composite......................................................................................... 33

3.1 Abstract ................................................................................................................... 33

3.2 Introduction ............................................................................................................. 34

3.3 Materials and Methods ............................................................................................ 36

3.3.1 Materials .......................................................................................................... 36

3.3.2 Methods............................................................................................................ 37

3.3.2.1 Composites preparation ............................................................................ 37

3.3.2.2 Effect of processing conditions ................................................................. 37

ix

3.3.2.3 Fibre content analysis and determination of fibre length distribution

(original fibre length) ............................................................................................ 38

3.3.2.4 Rheological properties measurement ........................................................ 39

3.3.2.5 Scanning electron microscopy .................................................................. 39

3.4 Results and Discussion ........................................................................................... 40

3.4.1 Effect of fibre content on the length and shape distribution on HYP-fibre–

reinforced bio-based nylon composite ...................................................................... 40

3.4.2 Rheological characteristics of HYP–reinforced bio-based polyamide ............ 41

3.4.3 Effect of the temperature on the shear viscosity versus shear rate of HYP-

reinforced PA11 ........................................................................................................ 43

3.4.4. Effect of HYP fibre content on the rheological behavior of HYP/PA11 ........ 45

3.4.5 Effect of high-yield–pulp fibre aspect ratio on rheological properties ............ 47

3.4.6 Scanning electron microscopy ......................................................................... 48

3.5 Conclusions ............................................................................................................. 49

CHAPTER 4 Effect of Lithium Chloride on the Fibre Length Distribution, Processing

Temperature and the Rheological Properties of High-Yield-Pulp-Fibre–Reinforced

Modified Bio-Based Polyamide 11 Composite ................................................................ 50

4.1 Abstract ................................................................................................................... 50

4.2 Introduction ............................................................................................................. 51

4.3 Material and Methods ............................................................................................. 53

x

4.3.1 Materials .......................................................................................................... 53

4.3.2 Methods............................................................................................................ 54

4.3.2.1 Composites preparation ............................................................................ 54

4.3.2.2. Effect of processing conditions ................................................................ 55

4.3.2.3 Fibre content and length distribution analysis after compounding ........... 56

4.3.2.4 Actual density measurement ..................................................................... 56

4.3.2.5 Differential scanning calorimetry (DSC) .................................................. 57

4.3.2.6 Rheological properties measurement ........................................................ 57

4.4 Results and Discussion ........................................................................................... 58

4.4.1 Effect of the lowering the processing temperature on the pulp fibre distribution

and the bio-based polyamide density after processing ............................................. 58

4.4.2 Densities and actual fibre contents .................................................................. 60

4.4.3 Effect of fibre content on the length and shape distribution on HYP-reinforced

bio-based modified PA11 composite ........................................................................ 62

4.4.4 Rheological characteristics of HYP–reinforced bio-based polyamide ............ 65

4.4.5 Effect of the processing parameters on the rheological property .................... 67

4.4.6 Effect of the inorganic salt lithium chloride on the rheological properties of

HYP fibre–reinforced bio-based polyamide composite ............................................ 68

4.4.7 Effect of HYP fibre content on the rheological characteristics of modified bio-

base (PA11 + 3%LiCl) composite ............................................................................ 70

xi

4.5 Conclusions ............................................................................................................. 72

CHAPTER 5 Review of Non-Newtonian Mathematical Models for Rheological

Characteristics of Viscoelastic Composites ...................................................................... 73

5.1 Abstract ................................................................................................................... 73

5.2 Introduction ............................................................................................................. 74

5.3 Viscoelastic characteristics of materials ................................................................. 75

5.4 Rheological modelling of viscoelastic composites ................................................. 78

5.5 Governing Equations .............................................................................................. 80

5.6 Constitutive equations ............................................................................................. 81

5.6.1 K-BKZ model .................................................................................................. 81

5.6.2 Upper-Convected Maxwell model (UCM) ...................................................... 83

5.6.3 White-Metzner Model ...................................................................................... 84

5.6.4 Phan-Thien-Tanner model (PTT) .................................................................... 85

5.6.5 Giesekus-Leonov model .................................................................................. 88

5.6.6 Oldroyd-B Model ............................................................................................. 89

5.7 Conclusion .............................................................................................................. 89

CHAPTER 6 Modeling the Rheological Characteristics of Flexible High Yield Pulp-

Fibre–Reinforced Bio-Based Nylon 11 Bio-Composite ................................................... 91

6.1 Abstract ................................................................................................................... 91

6.2 Introduction ............................................................................................................. 92

xii

6.3 Mathematical model................................................................................................ 94

6.3.1 Governing Equations ....................................................................................... 94

6.3.2 Assumptions and boundary conditions ............................................................ 95

6.3.3 Model development ......................................................................................... 96

6.4 Materials and Methods ............................................................................................ 99

6.4.1 Materials .......................................................................................................... 99

6.4.2 Experiment ....................................................................................................... 99

6.4.3 Rheological Measurements .............................................................................. 99

6.5 Results and discussion .......................................................................................... 100

6.5.1 Experimental Results ..................................................................................... 100

6.5.2 Variation of the viscosity with a function of shear rate of HYP reinforced

PA11 at various temperatures ................................................................................. 103

6.5.3 Effect of fibre content on the rheological behavior of HYP/PA11 ................ 104

6.5.4 Effect of the fibre aspect ratio on the rheological property ........................... 105

6.5.5 Predicted results ............................................................................................. 107

6.5.6 Modeling results versus experimental results ................................................ 109

6.6 Conclusions ........................................................................................................... 111

CHAPTER 7 Conclusions and Recommendations ......................................................... 112

7.1 Conclusions ........................................................................................................... 112

7.2 Scientific and engineering contribution of this thesis ........................................... 113

xiii

7.3 Study limitations ................................................................................................... 113

7.4 Recommendations ................................................................................................. 114

7.5 Publications and Conferences ............................................................................... 115

References ....................................................................................................................... 117

xiv

List of Tables

Table 3.1 ............................................................................................................. 39

Table 3.2. ............................................................................................................ 41

Table 4.1 ............................................................................................................. 56

Table 4.2 ............................................................................................................. 60

Table 4.3 ............................................................................................................. 61

Table 4.4 ............................................................................................................. 62

Table 4.5. ......................................................................................................................... 63

Table 4.6. ......................................................................................................................... 64

xv

List of Figures

Figure 2.1. Nylon 11

Figure 2.2. Structure of Nylon 11

Figure 2.3. Molecular and continuum flow models (Gad-el-Hak, 1999)

Figure 2.4. Process development and optimization

Figure 3.1. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 3.2. Variation of the viscosity with a function of shear rate of HYP/PA11 at

various temperatures.

Figure 3.3. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 3.4. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 3.5. An SEM micrograph of pulp-fibre–reinforced polyamide composite fracture

surface showing partially melted nylon fibre.

Figure 4.1. Schematic figure of twin-screw extruder

Figure 4.2. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 4.3 Variation of the viscosity with a function of shear rate of HYP/PA11 at

various temperatures.

Figure 4.4. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

xvi

Figure 4.5. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 4.6. An SEM micrograph of pulp-fibre–reinforced polyamide composite fracture

surface showing partially melted nylon fibre.

Figure 6.1. High yield pulp fibre of length L before load is applied

Figure 6.2. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C

Figure 6.3. Variation of the viscosity with a function of shear rate of HYP/PA11 at

various temperatures.

Figure 6.4. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 6.5. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Figure 6.6. Prediction of shear viscosity vs. shear rate of HYP/PA11at 200˚C

Figure 6.7. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

1

Chapter 1

Introduction

1.1 Background and Motivation

Pulp-fibre–reinforced thermoplastic composites are in high demand in the automotive,

aerospace, and construction industries. Pulp fibres are biodegradable and renewable, they

consume less energy than glass fibres, and consequently they generate less pollution

[Krigstin & Sain., 2006; Ananinedjiwala et al., 2011; Mohanty et al., 2002]. High-yield

pulps (HYP) are increasingly gaining importance as a partial replacement for hardwood

bleached Kraft pulp (HBKP) in uncoated and coated wood-free paper grades. Although

HYP was chiefly used as a cheaper alternative to HBKP throughout the 1990s, the pulp

and paper industry has come to value it for its ability to confer certain physical properties

on paper in order to meet customer or equipment demands. For example, replacing birch

or eucalyptus BKP with aspen-bleached chemithermomechanical pulp (BCTMP), a type

of HYP, has been found to be the most cost-effective method of increasing paper bulk

while maintaining tensile strength and brightness [Ananinedjiwala et al., 2011]. Other

reasons for the high demand for the utilization of green high-yield pulp fibres are their

low density and good thermal and acoustic properties. Pulp fibres, moreover, do not

abrade processing tools [Ananinedjiwala et al., 2011; Mohanty et al., 2002].

Materials from biological sources regroup natural polymers, so they can be expected to

exhibit viscoelastic behavior. Appropriate processing and production conditions of

2

polymer products are determined by their rheological characteristics. The high yield pulp

“HYP” fibres derived from hardwood that were used in this study are short crystalline

fibres [Saheb & Jog, 1999; Awal et al., 2010]. Short-fibre reinforced-polymer composites

are extensively used in manufacturing industries due to their light weight and improved

mechanical properties [Awal et al., 2010]. Hence, HYP has been used not only for its low

lignin content, but also for its potential thermal stability and its strong adhesion when it is

bonded with high-temperature–engineering thermoplastic polymers [Saheb & Jog, 1999;

Awal et al., 2010; Awal et al., 2009].

The study of the rheological behavior of viscoelastic polymer composites is mostly

limited to a two-phase fibre-polymer. The so-called interphase zone appears in the

viscoelastic damping of polymer composite processing [Van Rijswijk & Bersee, 2007].

This deformation significantly affects the predicted overall rheological characteristics of

natural-fibre–reinforced thermoplastic composites [Ho et al., 2012]. During processing,

fibre-reinforced polymers are subjected to rigorous deformations that cause fibres to

translate, agglomerate, bend, and rotate with the flow of the fibre matrix [Van Rijswijk &

Bersee, 2007; Ho et al., 2012; Huq et al., 2006]. This strongly influences the rheological

and mechanical properties in different parts of the final product because of the close

dependence of these properties on the orientation state of the fibres. Likewise, rheological

properties that are a function of the flow-induced fibre configuration in the matrix also

influence the physical properties of fibre-reinforced polymer composite [Ho et al., 2012;

Huq et al., 2006; Agarwal, 2009].

The rheological characteristics of natural-fibre–reinforced thermoplastic composites are

vital to their final mechanical properties. Although natural-fibre–reinforced polymer

3

composites and their processing have been partially addressed in several papers, models

of their rheological behavior and analysis of the rheology–processing parameter

relationships have been neither developed nor reported. This study reports on the state-of-

the-art technology in the rheology of lightweight composites from wood-fibre

thermoplastic composites, including their viscoelasticity and complex rheological

behaviors as influenced by different conditions. Hence, an overview of the viscoelastic

properties of biomaterials derived from natural-fibre–reinforced thermoplastic polymers

is presented in order to analyze their rheological behavior as part of predicting the

apparent shear viscosity of polymer melts.

Modeling the rheological properties of natural fibre reinforced bio-based thermoplastic

polymer composite is very challenging and poorly considered in the research. In addition,

study of the effect of the pulp aspect ratio and interphase element on the rheological

properties of green composite is absent from the literature. The purpose of this work is

therefore to present the experimental and predicted results of the rheological

characteristics of high-yield pulp-fibre–reinforced bio-based polyamide (HYPP/PA11)

composite as a function of fibre aspect ratio and their interphasial effect.

1.2 Hypotheses

The main hypotheses of this work are:

The apparent viscosity of bio-fibre–reinforced thermoplastic melt increases with

increase of the fibre aspect ratio.

4

The processing temperature of bio-based polyamide 11 decreases with the

addition of the inorganic salt lithium chloride (LiCl).

1.3 Objectives

The objectives of this work are:

To investigate the effect of the fibre content and aspect ratio on the rheological

behavior of high-yield pulp-fibre–reinforced bio-based polyamide composite.

To study the processability and the rheological characteristics of high-yield pulp-

fibre–reinforced bio-based polyamide composites for different compounding

process approaches.

To develop a mathematical model for prediction of the rheological characteristics

of bio-fibre–reinforced polyamide composite.

1.4 Outline of the Thesis

This thesis is divided into seven chapters: A general introduction, a general literature

review, four chapters that include the main findings of this thesis, and a final chapter

containing the main conclusions and recommendations.

Chapters 1 and 2 offer a general literature review and background on the scientific

concepts discussed in the context of this thesis. They review thermoplastic polymer, pulp

fibre, the extrusion process, and viscoelasticity. In more detail, they then survey the

rheological characteristics of polymeric material in order to identify the ones contributing

5

to the composite’s final properties. The modeling and rheology approaches are introduced

since they are the main scientific focus of this study.

Chapters 1 and 2 present the theoretical basis of this work as well as investigating the

potential of pulp fibre to be used as a reinforced in composite manufacturing. More in-

depth literature review is also provided in Chapters 3 to 6 as necessary.

Chapter 3 is based on a paper published in the Open Journal of Polymer Chemistry, 2015,

5, 1-8. It presents the results of the evaluation of the influence of fibre aspect ratio and

fibre content on the rheological characteristic of high-yield pulp-fibre–reinforced

polyamide 11 (HYP/PA11) Green Composite. Moreover, this chapter includes detailed

results of the effects of processing conditions, fibre breakage, and agglomeration, and

fibre orientation on the rheology of HYP-fibre–reinforced nylon composite. The results

of the investigation of the flow properties of composite materials made with bio-based

polyamide 11 (PA11) and HYP fibre is presented in this chapter. The apparent viscosity

of HYP/PA11 composite pellets was investigated at medium and elevated shear rate

using a capillary rheometer. The experimental results showed that identically for fibre

content and aspect ratio, the shearing effects decreased as the temperature increased: that

is, the HYP/PA11 became more non-Newtonian in the higher temperature region, which

corresponds to the high pseudoplasticity of the HYP/PA11. At low pulp-fibre content, the

apparent viscosity was expected to increase rapidly with increasing concentrations of the

fibres because of the swiftly increasing interactions between particles as they became

more closely packed. Nevertheless, at elevated and very high pulp-fibre content, random

anisotropic structures of fibres were created in polymer melts. The increase in the

6

apparent viscosity was greater at lower shear rates, where pulp fibre and polymer

molecules were not completely oriented.

Further characterizations of HYP/PA11 in terms of the effect of pulp-fibre content and

processing conditions on the rheological properties of HYP-fibre–reinforced bio-based

nylon composite are presented in Chapter 4. The results presented in this chapter are

submitted as a paper in the Journal of Composite Materials. The aim of this work was to

investigate the effect of the inorganic salt lithium chloride on the melting temperature and

rheological characteristics of viscoelastic natural-fibre–reinforced bio-based polyamide

composite materials. Different lithium chloride concentrations were used to lower the

melting temperature of bio-based polyamide 11. The concentration of lithium chloride

decreases the melting temperature of bio-based polyamide 11, and increases its degree of

crystallinity and consequently its density. The increase in the concentration of lithium

chloride in the HYP/PA11 composite is inversely proportional to the melting temperature

of PA11, and proportional to the fibre content after compounding. In addition, the

reduction of the high-yield pulp-fibre length distribution after compounding is less

pronounced for the modified polyamide 11 using the Brabender mixer technique

compared to the values obtained using the extrusion process method.

Rheological testing was performed and compared for both compounding process

methods. The rheological properties of HYP-reinforced bio-based polyamide 11

(HYP/PA11) composite were investigated using a capillary rheometer. Rheological

characteristics of the composite components influenced the development of resultant

microstructures; this development in turn affected the mechanical characteristics of a

multiphase composite. The rheological tests were performed at steady state in function of

7

the shear rate. The low-temperature process compounding had higher apparent viscosity;

this is because during the Brabender mixing technique process the temperature was low,

and the mixing and melting were induced by the high shear rate created during

compounding process.

The Effect of Lithium Chloride on the Fibre Length Distribution, Processing Temperature

and the Rheological Properties of High-Yield-Pulp-Fibre–Reinforced Modified Bio-

Based Polyamide 11 Composite was investigated by keeping the fibre content constant at

30% while varying the salt concentration from 1 to 5%. In addition, the effect of the pulp-

fibre content on the rheological characteristics of high-yield pulp-fibre–reinforced

modified polyamide 11 composite were performed for 10%, 20%, and 30% pulp content

while the salt concentration was kept at 3% LiCl. Rheological tests were performed for

all the formulations and both compounding techniques. Experimental test results show a

steep decrease in shear viscosity with increasing shear rate, and this melt-flow

characteristic corresponds to shear-thinning behavior in (HYP)-fibre–reinforced modified

bio-based polyamide composite (HYP/PA11) material. The rheological behavior of the

modified bio-based polyamide 11 “PA11 + LiCl" presented higher shear viscosity

corresponding to shear thinning behavior at intermediate and high shear rate.

Chapter 5 is based on a paper published in the Journal of Green and Sustainable

Chemistry (GSC). This chapter is a review of the mathematical models for the prediction

of the rheological characteristic of wood-fibre–reinforced thermoplastic nylon

composites. An overview of the viscoelastic characteristics of biocomposites derived

from natural-fibre–reinforced thermoplastic polymers and of predictive models is

presented in order to explicate the composites' rheological behavior. In addition,

8

significant reviews of constitutive equations were carried out so as to gain a better

understanding of their applicability to pulp-fibre–reinforced bio-based polymer

composite in determining the viscosity. The principal models investigated in this chapter

are the Giesekus-Leonov model, the Upper Convected Maxwell (UCM) model, the

White-Metzner model, K-BKZ model, the Oldroyd-B model, and the Phan-Thien-Tanner

models. The aforementioned models are the most powerful for predicting the rheological

behavior of hybrid and green viscoelastic materials in the presence of high shear rate and

in all dimensions. However, the Phan-Thien Tanner model, The Oldroyd- B model, and

the Giesekus model were found to be usable in various modes to fit the relaxation

modulus accurately and to predict both shear-thinning and shear-thickening

characteristics. Moreover, the Phan-Thien Tanner, K-BKZ, Upper Convected Maxwell,

Oldroyd-B, and Giesekus models can predict the steady shear viscosity and the transient

first normal stress coefficient better than the White-Metzner model for pulp fibre

reinforced bio-based thermoplastic composites.

Chapter 6 details the results gained from the mathematical model for the rheological

characteristics of HYP–reinforced bio-based polyamide 11 composite. The results in

Chapters 6 were first published in the Journal of Encapsulation and Adsorption Sciences.

The main purpose of this work was to develop a mathematical model to investigate the

rheological characteristics of viscoelastic pulp fibre composite materials. This chapter is

divided into two parts: In the first part, the apparent viscosity in function of high shear

rate and steady state of the extrudate (HYP)-fibre–reinforced bio-based Polyamide 11

(PA11) composite (HYP/PA11) were investigated using a capillary rheometer. In the

second part, novel predicted multiphase rheological-model–based polymer, fibre, and

9

interphasial phases were developed. The influence of the rheological characteristics of the

composite components on the development of resultant microstructures was broadly

investigated; the effects of these in turn on the mechanical characteristics of a multiphase

composite, are presented in this chapter. The rheological testing parameter conditions

were similar to those described in the previous chapters. Experimental and theoretical test

results of HYP/PA11 showed a steep decrease in apparent viscosity with increasing shear

rate, and this melt-flow characteristic corresponded to shear thinning behavior in

HYP/PA11 composite. The results of the nonlinear mathematical model for predicting the

rheological behavior of HYP/PA11 were compared to those obtained experimentally at

220˚C and 5000S-1 shear rate. Predicted and experimental apparent viscosity results were

found to be in strong agreement.

Finally, Chapter 7 reviews the main conclusions of this thesis and offers

recommendations for further work on modeling the rheological properties of pulp fibre

reinforced bio-based polyamide composites.

10

Chapter 2

Literature Review

2.1 High-Yield Pulp fibre

Rising environmental awareness is driving companies' search for more environmentally

friendly materials for their products. Compared to synthetic fibres, natural fibres have a

strong set of advantages: they are lightweight, recyclable, biodegradable, and renewable

[Additives, 2009; Stewart, 2011; Bavan & Kumar, 2010; John & Thomas, 2008].

High-yield pulp is short fibre derived from a process that involves the chemical pre-

treatment of wood chips combined with mechanical fibre separation to enable production.

A recent survey reported that the integrated worldwide production of high-yield pulp is

approximately 4 million tonnes per year, of which 85% is obtained through the bleached

Chemithermomechanical pulp “BCTMP” process [Botha & Hunter, 2010]. The annual

production of BCTMP pulp in Canada is nearly 1.5 million tones of hardwood grades and

about 0.5 million tones per annum of softwood grades. Due to their higher content of

cellulose and hemicellulose and consequently lower percentage of lignin, hardwoods are

more desirable than softwoods.

2.2 Polymeric matrix

The matrix plays a critical role in the performance of composites. The polymeric matrix

transfers the load to the stiff fibres through shear stresses at the interface [Stamboulis et

11

al., n.d., Ho et al., 2012; Young & Montes-Moran, 2002]. Both thermoplastic and

thermoset polymers can be used as matrices in composites.

The thermoset composites are chemically reacted to a cross-linked three-dimensional

network component, which increases the composite performance [Talreja et al., 2001;

Adekunle et al., 2011; Aminabhavi et al., 1987; Van Rijswijk & Bersee, 2007].

Thermoplastics provide advantages such as low processing cost, design flexibility, ease

of molding complex structures, and recyclability. Thermoplastic polymers are a group of

polymeric materials with a wide range of flexibility, a medium range of elasticity, and a

wide range of upper temperature limits [Eder & Winkler, 2001; Bhatnagar et al., 2007;

Parlevliet et al., 2006; Ludvik et al., 2007]. For semicrystalline materials, maximum use

temperatures are limited by the melting point of the crystalline phase [Nahar et al., 2011;

Sahoo et al., 2011]. The properties of such materials are governed by the interplay of the

crystalline phase providing strength and temperature resistance and the amorphous phase

rendering the material tough and flexible [Nahar et al., 2011; Sahoo et al., 2011; Herrera-

Franco & Valadez-González, 2004]. Typical examples of semicrystalline polymers are

high-density polyethylene (HDPE), polyamide or nylon (PA) and polyvinylidene fluoride

(PVDF).

2.2.1 Bio-based Nylon 11 (PA11)

Polyamide 11 is a specialty nylon. It combines high ductility, excellent aging resistance,

and high barrier properties with mechanical strength and resistance to creep and fatigue

[Fornes & Paul, 2004; Xu et al., 2006]. It thus compares favorably to standard nylons

such as 6 and 66. Notably, its significantly lower water absorption results in better aging

12

resistance, higher chemical resistance, and less property fluctuation due to plasticization

by water [Fornes & Paul, 2004; Xu et al., 2006; Zhang et al., 2006; Liu et al., 2003].

Figure 2.1: Nylon 11

Although polyamide 11 is highly resistant to aging and chain breakdown, the reaction of

water with amide bonds creates a limit to the use of polyamide at higher temperatures and

in the presence of water [Xu et al., 2006]. The specific reaction induced by water, called

hydrolysis, can be accelerated in the presence of acids. At continuous service

temperatures of 65°C and below, however, the impact of hydrolysis on polyamide 11 in a

neutral medium such as water can be neglected. Under these conditions, the material can

have a service life of 20 years or more. Use at higher continuous service temperatures

depends on the performance requirements and more precise conditions.

Figure 2.2: Structure of Nylon 11

13

The excellent properties of polyamides and of polyamide 11 in particular are a result of

the amide linkages in the chain, which allow a strong interaction between the chains by

hydrogen bonds. Low creep, high abrasion resistance, good resistance to fatigue, and high

barrier properties are a direct result of these strong inter-chain links [Xu et al., 2006;

Zhang et al., 2006; Liu et al., 2003; Botelho, E. C., & Rezende, 2009; Barkoula et al.,

2008].

2.2.2 Surface modification of natural fibres

The final properties of composite materials depend on the properties of fibre and matrix,

fibre loading, and fibre-matrix adhesion. Compositing hydrophilic fibres with

hydrophobic polymer will negatively affect their mechanical properties due to poor

adhesion at interface [Herrera-Franco & Valadez-Gonzalez, 2004; Mohanty & Misra,

2008]. These properties may be improved by changing the fibre properties through

physical and chemical treatments.

There exist several techniques for modifying the surface energy of the fibres; these

include physical treatments (cold plasma treatment, corona treatment) and chemical

treatment (maleic anhydride, organosilanes, isocyanates, sodium hydroxide,

permanganate and peroxide) [Herrera-Franco & Valadez-Gonzalez, 2004; Mohanty &

Misra, 2008; Wang, B. & Sain, 2007; Dominkovics et al., 2007]. Physical treatments

such as electric discharge or calendaring modify the structural and surface properties of

fibres, and the mechanical bonding increases with the matrix [Mohanty & Misra, 2008].

However, these treatments do not modify the chemical composition of the fibres.

Chemical treatments, therefore, have been used extensively to change the surface

14

properties of natural fibres. Chemical modifications increase the compatibility of fibres

and dispersion in the matrix, which leads to better stress transfer at the fibre/matrix

interface [Dominkovics et al., 2007].

Various studies reported that the utilization of silanes-treated Jute-Epoxy composites

resulted in improved tensile and flexural strength and stiffness [Taylor, 2006; Severini et

al., 2002; Awal et al, 2012]. Similar studies reported that adhesion may be improved by

using coupling agents like maleic anhydride to incorporate hydroxyl groups on the matrix

through hydrophilization and consequently enhancing the wetting effect of the resin on

the fibres [Behzad, 2007; Brandl et al., 2004; Da Silva et al., 2012]. The hydroxyl groups

then interact with –OH molecules on the lignocellulosic fibres via hydrogen bonding,

thus producing a stronger bond [Severini et al., 2002; Awal et al, 2012; Brandl et al.,

2004]. The properties of fibre composite have also been found to improve after chemical

treatment of fibres with maleic anhydride, acetic anhydride, and silanes [Brandl et al.,

2004; Da Silva et al., 2012].

Acetylation is an important mechanism for improving the properties of natural fibres. The

reaction involves the generation of acetic acid as a by-product that must be removed from

the lignocellusic material before the fibre is used [Dittenber & GangaRao, 2012].

Chemical modification with acetic anhydride replaces the cell-wall polymer groups with

acetyl groups, modifying the properties of these polymers so that they become

hydrophobic [Dittenber & GangaRao, 2012; Sobczak et al., 2012]. Consequently, fibre-

matrix interfacial adhesion is improved. [Dittenber & GangaRao, 2012; Sobczak et al.,

2012].

15

Another important method for surface modification is graft copolymerization. For

instance, the treatment of cellulose fibres with hot polypropylene-maleic anhydride

copolymers provides covalent bonds across the interface [Tong et al., 2004]. The

mechanism of reaction can be divided into two steps: activation of the copolymer by

heating at 170˚C (before treating the fibre) and esterification of the fibres [Tong et al.,

2004; Ho et al., 2012]. The surface energy of the fibres becomes closer to the surface

energy of the matrix after the treatment. Accordingly, improved wettability and higher

interfacial adhesion can be expected [Ho et al., 2012].

2.3 Extrusion Process

There are two types of extruder: single-screw and twin-screw.

Twin-screw extruders are used as continuous mixers, devolatilizers, and reactors, with

pellets being the finished products. They perform the basic functions: feeding, melting,

mixing, venting, developing die pressure, and conveying [Conshohocken, 2012; Liu et

al., 2009; Ku & Lin, 2005]. A compounding extruder can either be co-rotating or counter-

rotating [Conshohocken, 2012; Liu et al., 2009; Ku & Lin, 2005]; however, the industry

prefers co-rotators. The motor inputs energy into the screw shafts, causing the rotating

screws to impart shear and energy into the process to melt the components, mix,

devolatilize, and pump as required [Liu et al., 2009].

Twin-screw extruders have modular barrels and screws [Ku & Lin, 2005]. Extruder

screws are assembled on high-torque splied shafts. The barrels can be configured as

feeding, plain, venting, side stuffing, and liquid addition. Each barrel section is

16

electrically heated, uses its own temperature controller, and is internally cored for high-

intensity cooling near the screw bores [Liu et al., 2009; Ku & Lin, 2005]. The modular

nature of screw extruders offers extreme process flexibility with respect to rearranging

barrels, making L/D longer or shorter, and for modifying screws.

The main functions of the various zones of the conical twin-screw extruder are as

follows:

(1) The feed section feeds the material through the hopper.

(2) The compression and melting zone: the polymer chips are compressed in this zone by

screw flights reducing their volume. The polymer starts melting in this zone, where the

first liquid film forms at the barrel wall, which is heated above melting temperature.

(3) The mixing zone: Here the feedstock is intensively mixed, becoming a homogenous

product.

(4) The metering zone: In this zone, the pressure builds up and the melt is transported to

the die head for exiting [Ku & Lin, 2005].

2.3.1 Effects of processing parameters on properties of composites

The final properties of fibre-reinforced polymer composites are strongly influenced by a

number of factors, including: interaction between the fibre and the polymer matrix, fibre

length, fibre dispersion, fibre degradation, and fibre orientation. The processing

parameters, such as screw speed, feeding rate, and temperature profiles, will also affect

17

the final properties of composites by influencing these factors. Various studies about the

effects of processing parameters on mechanical properties of fibre-reinforced

thermoplastics have been reported [Takase & Shiraishi, 1989; Lau et al., 2010].

Various authors have investigated the effects of processing conditions on the fibre length

distribution of fibre-reinforced Nylon 11. They found that fibre length was mainly

determined by shear rate and gave the major factors that affect the shear rate [Tadmor &

Gogos, 1979; Sperling, 2006; Selvakumar & Bhatnagar, 2009]. In the majority of cases,

the composites were extruded with a co-rotating conical twin-screw extruder. Effects of

different processing factors on fibre length distribution were reported [Tadmor & Gogos,

1979]. At high screw speed, temperature is increased because of the viscous heat

generation [Sperling, 2006]. Therefore, at higher screw speeds, higher temperature and

lower viscosity may result in lower shear stress acting between the glass fibre and the

polymer matrix at the interface; consequently, the fibre length may decrease less than

proportionally to the screw speed [Sperling, 2006; Selvakumar & Bhatnagar, 2009].

Similarly, the feed rate will also affect fibre length. Screw speed, feed rate, and feed ratio

will all affect fibre length [Selvakumar & Bhatnagar, 2009].

Natural fibre processing is a little different with glass fibre because natural fibre is more

flexible. Czarnecki et al. [Selvakumar & Bhatnagar, 2009] compared the rheological

behavior and fibre damage of glass and cellulose fibre reinforced polystyrene melts. They

found that glass fibres break down rapidly to very small aspect ratios while cellulose

fibres showed less damage [Mukhopadhyay et al., 2003]. They also mentioned that the

shear viscosity of the melt materials increases with fibre percentage increase [Taylor et

al., 2011; Deitzel et al., 2001; Barkoula et al., 2009].

18

The mixing time and temperature profiles are important factors for wood-fibre–reinforced

composites because of the thermal degradation of wood fibre. For pulp-fibre–reinforced

composites, sometimes these two factors are dominant and need to be considered first

during study [Mukhopadhyay et al., 2003; Taylor et al., 2011; Deitzel et al., 2001].

Recently, Stade investigated the temperature profile in an extruder [Segerholm et al.,

2007]. In his experiment, he inserted the kneading teeth with thermocouples arranged

around the screw to measure the actual temperature of the melting mixture. He observed

that inserting the fibres to the polymer melt initially decreases the melt temperature, but

then leads to a rapid rise in temperature due to the increased shear viscosity of the

compound [Segerholm et al., 2007].

Most recently, P.V. Joseph [Peltola et al., 2011] examined the effect of processing

variables on the mechanical properties of sisal-fibre reinforced polypropylene composites

using a Haake Rheocord mixer. He reported that mixing time is too short; the mechanical

properties are low because of the ineffective mixing and poor dispersion of the fibre in a

PP matrix [Peltola et al., 2011]. Consequently, the composite properties and the fibre

dispersion improve once the mixing time increases. However, as the mixing time

increases, fibre breakage becomes predominant so properties decrease. At low

temperature, the apparent viscosity of the mixture is very high, and this causes the

breakdown of the fibres during processing. The rotor speed, mixing time, and

temperature profiles affect the strength of the composite’s mechanical properties [Peltola

et al., 2011]. The decrease in strength at a mixing temperature above 170 ºC can be due to

the thermal degradation of the wood fibres. Moreover, the dispersion of fibre in PP will

be poor due to the decrease in viscosity at high temperature [Peltola et al., 2011; Ramani

19

et al., 1995]. At low rotor speed, the poor dispersion of fibre in PP causes low tensile

strength, and at too-high rotor speed, fibre breakage is dominant and also causes

reduction in strength [Ramani et al., 1995]. Other studies realized later on indicated that

numerous process parameters, such as screw speed, feeding rate, temperature profiles,

melting time, and so forth, will affect the mechanical properties of composites [Lodge,

1988; Le Moigne et al., 2011]. To optimize processing, accordingly, many parameters

and their interactions need to be considered and a mathematical model used to predict the

effects of those parameters on the final properties of the materials.

2.3.2 Low-temperature compounding

Even though the early research did not yield encouraging results, the automotive and

construction markets' needs for low-density reinforcement for engineering thermoplastics

inspired researchers to take a second look at wood-fibre–reinforced engineering

thermoplastics [Valenti et al., 1973].

Recently, USDA Forest Products Lab and Rayonier Inc. partnered to develop an

exceptional compounding method called low temperature compounding (LTC) to

produce pulp fibre reinforced nylon 6 composites [Ackley, D. E. & Rudder, 2014; Khor

et al., 2009]. In this new compounding approach, there are three phases: start-up

conditions, a transition phase, and steady-state conditions [Ackley, D. E. & Rudder,

2014; Khor et al., 2009]. During the start-up conditions, the process temperature is higher

and the temperature zones of the extruder are set at 232ºC, a little bit above the melt

temperature of nylon 6. The melt viscosity will increase in the presence of the natural

fibre. With the increase of melt viscosity from adding fibre, the melt temperature and

20

torque load on the extruder increase dramatically, which leads to the degradation of

natural fibre during the compounding process. Accordingly, the temperature of

intermediate zones is gradually reduced. This process is the ‘transition phase’. At steady-

state conditions, the twin-screw extruder is at equilibrium [Ackley, D. E. & Rudder,

2014; Khor et al., 2009].

2.4 Rheology

Rheology is the study of the mechanical principles of continuous mediums, whose

function is to determine the stresses and deformations at every point of these mediums

[Mason, 2006]. In general, polymers are viscoelastic liquids: that is, they exhibit both

viscous and elastic properties [Guo et al., 2005; Xie et al., 2012]. Viscoelastic materials

are most used in strategic industries because of their favourable mechanical properties

and typically high quality [Chopra & Larson, 2002; Rao, 2007; Wang et al., 2011;

Moutee et al., 2006]. These materials are rigid and have high resistance properties since

they consist of fibre levels and resin [Wang et al., 2011; Moutee et al., 2006].

The rheology of long-fibre polymers is quite complex because of various factors such as

fibre-matrix interaction, fibre-fibre interaction, fibre migration, and fibre breakage during

processing. Fibre flexibility is another factor that plays an important role in determining

the rheological behavior of long-fibre composites [Sin et al., 2010]. Because the fibre

length in these materials is greater than a critical value, they, unlike short fibres, do not

remain straight; they change their curvature under flow deformations. Typically, for glass

fibre, this critical value is assumed to be 1 mm. Fibre flexibility varies with the intrinsic

properties of the fibre, its aspect ratio, and strength of the flow field [Sin et al., 2010;

21

Eberle et al., 2009; Park et al., 2008]. If the fibres are flexible enough, bending forces

acting through the velocity field can influence their orientation state, which in turn can

influence the macroscopic properties of the fluid [Eberle et al., 2009]. As a result, fibre

flexibility has shown to be responsible for a considerable increase in polymer melt

viscosity [Eberle et al., 2009; Park et al., 2008].

Many experimental results have emphasized the role of flexibility on fluid viscosity.

Nawab and Mason, in their study of threadlike particles in castor oil, found that viscosity

becomes more and more shear-dependent with increasing fibre length [Eberle et al. 2008;

Sepehr et al., 2004c.]. They deduced that this behavior was due to elastic deformation of

the fibres. Blakeney showed that even a ‘slight curvature’ of fibres has a pronounced

effect on the viscosity [Sepehr et al., 2004a.]. Kitano et al. and Goto et.al. illustrated the

effect of fibre stiffness on suspension rheology [Ojo, A., & Akanbi, 2006; Qiao et al.,

2009]. They showed that fibres that are more flexible had a more pronounced effect on

the rheological properties [Eberle et al. 2008; Sepehr et al., 2004c.; Sepehr et al 2004a;

Ojo, A., & Akanbi, 2006; Qiao et al., 2009].

Recently, Keshtkar et al. performed a study on the effect of the fibre flexibility

parameters, stiffness, and aspect ratio, in semi-dilute and semi-concentrated regimes

[Kucharczyk et al., 2012]. They concluded that there exists a considerable increase in

steady-state shear viscosity in the semi-concentrated regime compared to the semi-dilute

regime. The viscosity of these suspensions also increased considerably with fibre

flexibility. They found that in both regimes, the addition of fibres results in significant

normal forces under shear flow, and as the fibre content increases, the first normal force

increases with it. Keshtkar and co-workers are also among the very few authors to

22

observe transient stress growth behavior for long-fibre composites [Kucharczyk et al.,

2012].

Physical properties such as elasticity modulus, strength, thermal expansion and thermal

conductivity, depend on the concentration, type, size, and orientation of reinforced fibres

in polymer composites. This greatly influences the mechanical properties in different

parts of the final product because of the close dependence of these properties on the

orientation state of the fibres. Similarly, rheological properties that are a function of the

flow-induced fibre configuration in the matrix also influence the physical properties of

the fibre composite [Müller et al., 2011].

Rheology of biomaterials is likewise very complex because various factors such as fibre-

matrix interaction, fibre-fibre interaction, moisture content, and fibre breakage and

migration affect flow-induced fibre orientation in these polymer melts [Ansari et al.,

2011; La Mantia & Morreale,2011; Abraham & George, 2007]. In high concentration

regimes, fibre-matrix and fibre-fibre interaction both become more important as the

conformation of the polymer chains is influenced by the orientation of neighbouring

fibres [La Mantia & Morreale, 2011; Abraham & George, 2007]. In addition, as fibres

flow in close proximity to each other, there is an increased possibility of encountering

enhanced hydrodynamic forces, friction and other mechanical interactions between fibres

[Sengupta et al., 2007]. For short-fibre composites (SFCs), flexibility is not important, as

their length is small enough to assume that the fibre remains straight and that their

curvature does not change during flow [Chen et al, 2007; Larson, 2005; Li & Renardy,

2000]. However, in the case of long fibres (average fibre length > 1mm), the curvature of

fibres cannot be neglected because it considerably affects the composite's properties and

23

has been shown to be responsible for large increases in melt viscosity [Larson, 2005; Li

& Renardy, 2000]. The flexibility of a fibre varies with the intrinsic properties of the

fibre, its aspect ratio, and the strength of the flow field. As a result, the rheological

properties of green-fibre composite are said to be affected by fibre properties, fibre

interactions, moisture content, melt fluid properties and the flow rate imposed [Soulages

et al., 2008; Switzer & Klingenberg, 2003; Keshtkar et al., 2009].

2.5 Modeling of Fluid Flows

2.5.1 Classification of fluid flow modeling

A fluid flow can be modeled based on two approaches: (i) continuum hypothesis and (ii)

molecular theory [Batchelor, 1967; Gad-el-Hak, 1999]. A classification of flow models is

displayed schematically in Figure 2.3 (Gadel-Hak, 1999).

24

Figure 2.3. Molecular and continuum flow models (Gad-el-Hak, 1999)

2.5.1.1 Continuum hypothesis

In the continuum hypothesis, a fluid is assumed to be a continuous medium describable in

terms of the spatial coordinate and time variations of macroscopic flow quantities such as

density, pressure, velocity, and temperature, and cannot be divisible; the molecular

structure and forces (e.g. intermolecular forces) of a fluid are ignored [Batchelor, 1967].

Principles of mass, momentum, and energy balance (also commonly called conservation

of mass, momentum, and energy) lead to a set of nonlinear partial differential equations

such as Navier-Stokes and Euler equations.

25

The continuum models are generally easier to solve mathematically at a relatively low

cost and short time and are more commonly used by engineers than the molecular models

[Gad-el-Hak, 1999]. Continuum models should therefore be used as long as they are

applicable. However, in cases where continuum models fail to accurately predict the fluid

flow at the microscale, high-cost molecular models are the only approach available to

determine the fluid flow in microchannels [Batchelor, 1967; Gad-el-Hak, 1999].

Conceptually, the continuum hypothesis is valid and leads to accurate predictions as long

as local properties of the fluid, such as density and velocity, can be defined as standards

over elements. These elements are large compared to the molecular structure, but small

enough compared to the macroscopic length scale that mathematical differentiation and

integration can be used to represent them. However, the flow condition must be near

thermodynamic equilibrium. The averaged-property condition is usually satisfied, but the

near-equilibrium condition mostly limits the validity of the continuum models [Gad-el-

Hak, 1999].

2.5.1.2 Molecular theory

The molecular theory characterizes fluid flow as a set of discrete particles to represent

molecules, atoms, ions, and electrons [Batchelor, 1967; Gad-el-Hak, 1999]. The

macroscopic state behaviors at any position in the flow are suitably averaged from a

sufficient number of the discrete particles within the smallest significant dimensions of a

flow [Gad-el-Hak, 1999]. The final objective is to determine the position, velocity, and

state of all particles at all times. Molecular-based flow models comprise molecular

dynamics (MD), direct simulation Monte Carlo (DSMC), and Boltzmann equations, as

26

shown in Figure 2.3. Molecular modeling of fluid flows, however, is not in the objective

of this thesis research.

2.5.2 Continuum models of fluid flows and constitutive equations

Continuum models are usually derived from the conservation of mass, momentum, and

energy, which as earlier noted are also called mass, momentum, and energy balances. The

conservation of mass, momentum, and energy can be expressed at every point in space (x,

y, z) and time (t) as a set of partial differential equations [Batchelor, 1967; Gad-el-Hak,

1999; Den, 1990; Landau & Lifshitz, 1987]. The general form of the momentum balance

equation is shown as follows:

g (2.1)

where ρ is the density of fluid, v is the velocity vector (vx, v

y, v

z), t is the time, p is the

pressure, and τ is the (extra) stress tensor.

In addition to the balance equations, which correspond to the governor equations,

constitutive equations that describe the response of the material to applied stress are also

required to solve problems of the rheological behavior of materials [Den, 1990].

Constitutive relations can be determined from numerous approaches, including

experimental observations and correlations, phenomenological theories, and molecular

theories [Landau & Lifshitz, 1987]. The mathematical description of a viscoelastic fluid

is much more complex than its Newtonian counterparts [Kundu, 1990; Panton, 1996]. In

addition to the conservation equations of mass and momentum an additional equation, the

constitutive equation or rheological equation of state, is required; this equation relates the

27

stress to the deformation [Sherman, 1990; Oliveira, 2009; Den, 1990; Argyris et al.,

1991; Gava , 2012; Likhtman & Graham, 2003]. For a viscoelastic liquid this relationship

is nonlinear and it has no standard form that is universally valid for each fluid in every

flow situation [Den, 1990; Argyris et al., 1991; Gava, 2012; Likhtman & Graham, 2003].

This situation is one of the reasons why the subject of viscoelasticity is so challenging.

The constitutive equation should not only describe the rheological characteristics of the

polymer melt, but also give the final fibre orientation of the composite [Banks et al.,

2006]. For this reason, it is fundamental to evaluate the role of the polymer rheology and

the fibre-polymer interaction. Enthusiasm for the utilization of a composite constitutive

equation arose due to the observation of the calculated experimental stresses of polymer

suspensions. It was reported that the total stress of the composite increases with the

addition of fibres [Kajiwara et al., 1995; Wang & Birgisson, 2007], and therefore a

satisfactory constitutive equation could be had by adding an extra stress term to an

already existing constitutive equation; this amended equation adequately describes the

polymer melt.

Note that assumptions and boundary conditions are very important in the

phenomenological description provided by mathematical models. A perfect model in this

case must use constitutive relations that accurately describe the material behavior under

the shear.

28

2.6 Viscoelasticity of composite materials

Materials from biological sources are viscoelastic [Banks et al., 2006]. A material is said

to be viscoelastic if it manifests both viscous and elastic properties under the same

conditions when it undergoes deformation. Viscous materials present resistance to shear

flow and strain linearly with time when a stress is applied [Zhou, 2011; Snijkers et al.,

2011; Assie, 2011; Wang & Birgisson, 2007]. The shear stress of elastic materials

depends on strain: when strain is applied and then released, they return to their initial

configuration [Zhou, 2011; Snijkers et al., 2011; Assie, 2011; Wang & Birgisson, 2007;

Doraiswamy,1998; Mason, 2006].

A fluid that does not behave in a Newtonian trend between shear stress and shear rate

when it undergoes deformation is commonly termed non-Newtonian [Chhabra, 2010;

Malkin & Isayev, 2006]. This means that the relation between shear stress and shear is

not a straight line but is non-linear [Malkin & Isayev, 2006]. High-molecular-weight

liquids, which include polymer melts and solutions of polymers, as well as liquids in

which fine particles are suspended, are usually non-Newtonian. In this case, the slope of

the shear-stress–versus–shear-rate plot will not be constant as we change the shear rate

[Han, 2007]. When viscosity decreases with increasing shear rate, the fluid is described

as shear-thinning [Goodwin & Hughes, 2008; Chhabra & Richardson, 2008]. In the

opposite case, where the viscosity increases as the fluid is subjected to a higher shear

rate, the fluid is described as shear-thickening [Goodwin & Hughes, 2008; Chhabra &

Richardson, 2008]. Shear-thinning fluids also are called pseudoplastic fluids, and shear-

29

thickening fluids are also called dilatants. Shear-thinning behavior is more common than

shear-thickening [Han, 2007; Goodwin & Hughes, 2008; Chhabra & Richardson, 2008].

Another type of non-Newtonian fluid is viscoplastic or “yield stress” fluid [Goodwin &

Hughes, 2008; Chhabra & Richardson, 2008; Graessley, 2004]. This is a fluid that will

not flow when only a small shear stress is applied [Goodwin & Hughes, 2008; Chhabra &

Richardson, 2008; Graessley, 2004]. The shear stress must exceed a critical value known

as the yield stress for the fluid to start flowing [Chhabra & Richardson, 2008; Graessley,

2004]. Hence, viscoplastic fluids behave like solids when the applied shear stress is less

than the yield stress. Once it exceeds the yield stress, the viscoplastic fluid will flow just

like an ordinary fluid [Graessley, 2004].

On the other hand, some classes of fluids exhibit time-dependent behavior [Goodwin &

Hughes, 2008; Chhabra & Richardson, 2008; Graessley, 2004]. This means that even at a

given constant shear rate, the viscosity may vary with time. This category of material

comprises thixotropic and rheopectic fluids, whose behaviors in this respect are opposites

[Goodwin & Hughes, 2008; Chhabra & Richardson, 2008; Graessley, 2004]. The

viscosity of a thixotropic liquid will decrease with time under a constant applied shear

stress [Graessley, 2004]. However, when the stress is removed, the viscosity will

gradually recover with time as well [Chhabra & Richardson, 2008; Graessley, 2004]. By

contrast, rheopectic fluid behavior can be observed when the fluid increases in viscosity

with time when a constant shear stress is applied [Chhabra & Richardson, 2008;

Graessley, 2004].

30

2.7 Problem Statement

Recently, there has been a large demand for the utilization of carbon fibre in industries

such as automotive, aerospace, and construction. Consequently, carbon-fibre waste

products will become an important environmental issue because such products are not

biodegradable. In addition, the cost of virgin carbon fibre is high, which means recovery

could provide considerable economic and environmental advantages. Thus, there is

strong and widespread interest in the development of composite pulp-fibre–reinforced

thermoplastic polymer composite. Materials from biological sources regroup bio-based

polymers, so polymer composites that contain them can be expected to exhibit

viscoelastic behavior. The preferred processing conditions for partially bio-based

composite products are determined by their rheological characteristics. This is despite the

fact that natural-fibre–reinforced bio-based polymer composites and their processing

conditions have not yet been even partially reviewed.

Modeling of the rheological characteristics of pulp-fibre–reinforced bio-based

thermoplastic polymer and the analysis of their rheological processing parameter

relationships has not hither to been investigated. This study reports on the state-of-the-art

technology in the rheology of composites, including viscoelasticity and complex

rheological behaviors as influenced by differing conditions, the pulp fibre aspect ratio,

the fibre content, and processing temperature effects. The present research focuses on the

processing of wood-fibre–reinforced polyamide composites, their rheological properties,

and the development of mathematical model to predict these properties.

31

In order to achieve the primary goals of this project, the processing parameters during the

extrusion process and the flow properties were both investigated. Also, the effects of the

fibre content and aspect ratio of high-yield pulp (HYP) were studied. An extrusion

process was adopted in order to develop the pulp-fibre–reinforced bio-based polyamide

composite. The biocomposites were processed under three different processing conditions

in order to gain a better understanding of the HYP fibre in the composite system in

relation to fibre breakage, burning, and agglomeration. Rheological tests were performed

using a capillary rheometer at steady state and at elevated shear rate. Finally, the

mathematical model was developed in order to predict and validate the apparent shear

viscosity in function of the pulp aspect ratio, the pulp fibre diameter, and the interphase

effect of the composite from high-yield pulp-fibre–reinforced bio-based polyamide. The

proposed project plan of the process development and optimization is as follows (See

Figure 2.4):

32

Yes

No

Figure 2.4. Process development and optimization

Fibres Reinforced Bio-

Based Polyamide

Experimental approach:

PA11 and Pulp: Aspect ratio;

Fibre length and content

Theoretical approach:

Rheological Model: Viscosity;

Shear rate and Shear stress

Structure Thermal and

Mechanical

Properties

Pro

cess

Op

tim

izati

on

Processing Parameters:

Temperature profiles; Screw

Speed; Feeding rate; Mixing

time

Natural Fibres

Renewable and biodegradable

Low energy consumption; less

pollution; low density and lightweight;

good thermal and acoustic properties

Validation

End

33

Chapter 3

Evaluation of the Influence of Fibre Aspect Ratio

and Fibre Content on the Rheological

Characteristic of High-Yield-Pulp Fibre–

Reinforced Polyamide 11 (HYP/PA11) Green

Composite

3.1 Abstract

The rheological behavior of composites made with low-density polyamide 11 (PA11) and

high yield pulp fibre (HYP) were evaluated. The rheological properties of high- yield

pulp-reinforced bio-based Nylon 11 HYP/PA11 composite were investigated using a

capillary rheometer. The rheological tests were performed in function of the shear rate for

different temperature conditions. The experimental results showed that identically for

fibre content and aspect ratio, the shearing effects decreased as the temperature increased;

that is, the HYP/PA11 became more non-Newtonian in the higher temperature region,

which corresponds to the high pseudoplasticity of the HYP/PA11. At low HYP content,

the shear viscosity was expected to increase rapidly with increasing concentrations of the

fibres because of the rapidly increasing interactions between particles as they become

more closely packed. However, at very high fibre content, random anisotropic structure

34

of the fibres in polymer melts was created. The increase in shear viscosity was greater at

lower shear rates, where fibre and polymer molecules are not completely oriented.

3.2 Introduction

Green-fibre–reinforced thermoplastic composites are in high demand in the automotive,

aerospace, and construction industries. Vegetable fibres are biodegradable and renewable,

they consume less energy than glass fibres, and consequently they generate less pollution

[Pervaiz & Sain, 2003; Bourmaud & Baley, 2009; Duc, 2011]. Other reasons for the high

demand for the utilization of green fibres are their low density and good thermal and

acoustic properties [Bourmaud & Baley, 2009]. Pulp fibres, moreover, do not abrade

processing tools [Bourmaud & Baley, 2009; Duc, 2011]. As noted in Chapter 2, materials

from biological sources regroup natural polymers, so they can be expected to exhibit

viscoelastic behavior [Bourmaud & Baley, 2009; Duc, 2011; Ayroud, 1983]. Appropriate

processing and production conditions for polymer products are determined by their

rheological characteristics [Ayroud, 1983; Thumm & Dickson, 2013]. Short-fibre

reinforced polymer composites are extensively used in manufacturing industries due to

their light weight and superior mechanical properties [Sadeghian & Golzar, 2008;

Thomen, 2001]. Hence, pulp fibre has been used not only for its low lignin content but

also for its potential thermal stability and for its strong adhesion when bonded with high-

temperature engineering thermoplastic polymers [Gu & Kokta, 2010; TAAPI, 2006;

Rijswijk & Bersee. H. E. N., 2007]. The high-yield pulp “HYP” fibres derived from

hardwood that were used in this study are short semicrystalline fibres.

35

Various experimental studies have emphasized the investigation of the role of flexibility

on fluid viscosity [Plackett et al., 2010; Kaw & Besterfield, 1998; Yeh, 1992]. These

studies concluded that the more flexible the fibres, the more pronounced their effect on

the rheological characteristics [Yeh, 1992; Gohil & Shaikh, 2010].

A novel study on the effect of fibre-length distribution on the rheological behavior of

castor oil composite showed that at high fibre length the shear viscosity becomes more

dependent on shear rate [Yeh, 1992; Kari et al., 2005]. This behavior is due to elastic

deformation of the fibres [Gohil & Shaikh, 2010].

Recently, several researchers have investigated the effect of fibre content on polymer

melt rheology [Lamnawar & Maazouz, 2008; Larache et al., 1994; George et al., 2001; -

Liu et al., 2000]. One such study showed an important increase in shear viscosity with

increased fibre loading at low shear rates, but only a small increase in viscosity at high

shear rates [Uhlherr et al., 2005]. Another similar study on polypropylene-based long

fibre observed an increase in shear viscosity with increased fibre content and fibre length;

however, this viscosity increase was very small, which the authors attributed to high

shear rates and fibre breakage during processing [Huq.& Azaiez, 2005; Sepehr et al.,

2004]. Non-Newtonian fluid characteristics such as shear thinning were also observed in

these studies.

That said, the literature devoted to experimental studies of the rheology of pulp fibres in

reinforced polymer composites is very restricted. This is likely due to the complex nature

of these materials, the difficulties encountered during their processing, and also the

36

difficulty of characterizing them rheologically [Eberle et al., 2009; Larson, 1999; Eberle,

2008].

Processing technique and conditions have a significant influence on the rheological and

overall properties of pulp-fibre–reinforced polymer composites because they dictate the

degree of dispersion and distribution of the fibre in the polymer matrix [Le Moine et al.,

2013; Guo et al., 2005; Le Moine et al., 2011]. Compared to other natural fibres, HYP is

more thermally stable in the presence of high-melting–temperature engineering

thermoplastics (under 180˚C) such as PA11, PA6, and PA66 [Guo et al., 2005; Le Moine

et al., 2011].

The aim of the study described in this chapter was to investigate the rheological

characteristics of high-yield-pulp–reinforced bio-based polyamide 11 composite.

Experiments were mainly carried out by varying the rheological testing temperatures, the

fibre content, and the aspect ratio.

3.3 Materials and Methods

3.3.1 Materials

The matrix biopolymer, bio-based Nylon 11, density 1.03, MFI 11, was supplied by

Arkema, France. Aspen HYP fibres were supplied by Tembec (Montreal, QC). The HYP

is the type used in wood-free printing and in writing-paper grades and multiple-coated

folding-board grades; pulp fibre length is 0.230 to 0.85mm. Finally, the pulp-fibre length

37

was reduced by using a mechanical crib in order to investigate the aspect ratio effect on

the rheological behavior of the HYP/PA11 composite.

3.3.2 Methods

3.3.2.1 Composites preparation

The experiment was processed using a conical twin-screw extruder. In the mixing

method, the HYP fibre was dried at 80˚C for 6 hours and then added to the corresponding

PA11 and well mixed before it was introduced to the extruder. The average temperature

of the barrel was 200˚C.

3.3.2.2 Effect of processing conditions

Many processing parameters affect the properties of final products. For extrusion,

temperature profiles affect the fibre degradation. In addition, screw speed and feeding

rate change fibre length, distribution, and orientation. The mechanical properties are the

combined result of all these changes. The processing parameters are optimized to obtain

the best properties. Table 3.1 represents the processing parameters for HYP/PA11 used in

this study.

38

Table 3.1. Extrusion temperature profile for 10%, 20%, and

30% HYP/PA11, 120 RPM (10 and 20%), 130 RPM (30%)

and 10 RPM feed rates.

Temperature (˚C)

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Zone 9 Zone 10

200 200 200 200 200 205 205 205 205 205

3.3.2.3 Fibre content analysis and determination of fibre length distribution

(original fibre length)

The samples of the fibre received from the provider were cut into small pieces and

immersed in formic acid for three days. The Nylon 11 was dissolved by the formic acid

and HYP was left. The wood fibre was filtered and washed with formic acid, then dried

in a vacuum oven for four hours. By measuring the weight of composite and fibre, we

could calculate the actual fibre content.

HYP fibre length was measured with the Fiber Quality Analyzer (FQA). The fibre was

diluted with D.I. water, and the diluted fibre entered a thin-planar channel. This channel

helped to gently orient the fibre 2-dimensionally, so that the fibre could be fully viewed

by the camera. The picture taken by the camera was then analyzed by the software to give

the fibre length distribution.

39

3.3.2.4 Rheological properties measurement

The rheological measurements of the composites’ melt-flow properties were carried out

in a twin-bore Rosand Capillary Rheometer model RH2000. (The standard RH2000 range

supports temperatures from -40˚C to 500˚C. The standard maximum force applied is

12kN.) The composite samples for testing were cut into very small pieces, then placed

inside the barrel and forced down into the capillary with the plunger attached to the

moving cross-head. Representative steady-shear viscosity versus high shear rate is

presented in the figures below for HYP/PA11, which was processed at an average

extrusion temperature of 200˚C. The viscosity of the sample was obtained from steady-

shear measurements for different temperature profiles, with the rate ranging from 50 to

5000 S-1. The rheological viscosity data presented in this paper thus represents an average

value of three measurements.

3.3.2.5 Scanning electron microscopy

Studies on the morphology of the composites were carried out using a scanning electron

microscope (SEM). The rheological test samples were ruptured by the rheometer and the

fracture surfaces were examined. A Hitachi S-800 Scanning Electron Microscope (SEM)

was used to study the fracture surface and dissolved HYP-reinforced PA11 composites.

The observation conditions were the following: pressure 0.3 mbar, acceleration voltage

15 keV.

40

3.4 Results and Discussion

3.4.1 Effect of fibre content on the length and shape distribution on

HYP-fibre–reinforced bio-based nylon composite

During the extrusion process, the shear stress applied by the screw broke the fibres. The

resulting fibre lengths affect the ultimate mechanical properties. In spite of the influence

of fibre damage and breakage during processing, the initial fibre length in the feedstock

determined the fibres’ final lengths. It was therefore important to analyze the initial fibre-

length distribution. Table 3.2 represents the effect of fibre content on the length and

shape distribution on HYP-fibre–reinforced bio-based polyamide composite.

Table 3.2. Fibre length distribution of pulp fibre (original fibre

length).

Fibre

Mean Length (measured)

Arithmetic (mm)

Length weight

(mm)

Weight weighted

(mm)

Only HYP 0.38 0.57 0.73

HYP (10%) 0.20 0.29 0.43

HYP (20%) 0.20 0.29 0.44

HYP (30%) 0.18 0.28 0.51

41

The arithmetical length-weighted and weight-weighted values of fibre length were found

to be 0.18 mm, 0.28 mm and 0.51 respectively for the fibres from 30% HYP/PA11. Most

of the fibre lengths lie within the range of 0.2–0.52 mm, since crushed pulp was used in

this study. To obtain these average fibre lengths, 1500 single fibres were examined. The

measured initial fibre length is shorter than the original length of the fibres the company

provided.

3.4.2 Rheological characteristics of HYP–reinforced bio-based

polyamide

Rheological characteristics of the polymer, fibre, and interphasial phases influenced the

final characteristics of the resultant microstructure of the composite materials; these

characteristics in turn affected the mechanical properties of a multiphase polymer-

composite system. As obtained from experiment, the shear viscosity as a function of the

steady-shear rate of HYP/PA11 at 200˚C is shown in Figure 3.1 (As noted, these results

are the average of three different experimental tests.)

Figure. 3.1. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

0

100

200

300

400

500

600

700

800

0 1000 2000 3000 4000 5000Sh

ea

r vis

co

sit

y (

Pa

.S)

Shear rate (s-1)

42

As also noted, the composite material used in the experimental study had an average fibre

length of 0.73 mm, and the experiment was conducted at 200˚C. The experimental results

showed that the viscosity of HYP/PA11 composite decreases with increasing shear rate.

This decrease in the shear viscosity with the increase of the shear rate corresponds to the

pseudoplastic fluid characteristic of the composites. This pseudoplastic behavior (also

referred to as shear-thinning behavior) as plotted in Figure 3.1 is mainly influenced by

the orientation of the polymer molecules, the agglomeration of the flexible pulp fibre, and

the entanglements within the polymer chains in the capillary rheometer. On the other

hand, the chain agglomerations are produced simultaneously as one chain is collapsed

into another chain. The entanglements that lead to agglomerations of the chains, as well

as the entanglements within the chains, are caused by the Brownian motions and low

relaxation of HYP fibre. The high shear-thinning behavior obtained for HYP/PA11 can

be also associated to the thermal degradation of HYP during the rheological testing and

the compounding process. The molten polymers tend to arrange themselves with their

major axes in the direction of shear, whereby points of entanglement are reduced. As a

result, the viscosity decreases. In other words, in this instance of non-Newtonian flow

behavior in polymer melts, the decrease in viscosity when the shear rate is increased by

applying load is associated with high shear-thinning behavior and with viscoelastic

characteristics of biocomposite materials. However, at very high shear rates (from 3,000

to 5,000 S-1); the molten HYP/PA11 showed a less restrained decrease in viscosity. This

non-Newtonian behavior is associated with the alignment and orientation of the fibre in

the polymer chains and the effect of the fibre aspect ratio. At both low and high shear

rates, the formation of agglomerates is evident; therefore the pulp molecules are

43

completely oriented due to the good dispersion in the bio-based PA11 matrix. This means

that the breakage of the fibre length allows the maintenance of an accurate fibre-aspect

ratio when the diameter of flexible HYP is kept unchangeable during the process. The

shorter length of the fibres also supports their alignment in the direction of the flow, thus

reducing fibre-fibre collisions and leading to a greater decrease in viscosity.

3.4.3 Effect of the temperature on the shear viscosity versus shear

rate of HYP-reinforced PA11

The variation in shear viscosity as a function of the shear rate of HYP/PA11 at various

temperatures was investigated; the rheological test results are presented in figure 3.2. The

rheological conditions were kept constant. The results are presented in the figure below.

44

Fig. 3.2. Variation of viscosity as a function of shear rate of HYP/PA11 at various temperatures.

The shear viscosity of the HYP/PA11 depended on the rate of shear at which it was

measured and presented. The shearing effects decreased as the temperature increased:

that is, the HYP/PA11 became more non-Newtonian in the higher-temperature region. At

higher temperatures the reduction of shear viscosity was more pronounced at

intermediate shear rate, while for 190˚C, this reduction was maximized at higher shear

rates (from 3000 to 5000 S-1). At low testing temperature (190˚C) and for low and

intermediate shear rate, the flow deformation was challenging. This was due to the fact

that the HYP-reinforced PA11 fibres were entangled and agglomerated. At this point,

such rheological behavior is called Near-Newtonian. At high shear rate, the shearing

effects increased while the effect of temperature was less pronounced, and flow

0

200

400

600

800

1000

0 1000 2000 3000 4000 5000

Sh

ear

vis

co

sit

y (

Pa.s

)

Shear rate (s-1)

190 degree

200 degree

210 degree

45

deformation was mostly dominated by the shearing effect. However, from 3000 to 5000

S-1 all the shear-viscosity variations in function shear rates followed the same rate of

deformation for different temperature profiles, which corresponds to shear-thinning

behavior.

3.4.4. Effect of HYP fibre content on the rheological behavior of

HYP/PA11

The effect of fibre content on the rheological characteristics of the composite was

investigated. Figure 3.3 shows the experimental results for 10%, 20%, and 30% HYP-

reinforced polyamide 11. These curves are typical of pseudoplastic materials, which

show a decrease in viscosity with increasing shear rate. At high fibre content, the material

offers higher shear viscosity even for high shear rate. In general, the incorporation of

fibres in polymer systems increases the viscosity, which rises further as fibre content is

increased.

46

Figure 3.3. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

The difference between 10% and 20% fibre content at intermediate and high shear rate is

not very significant. At low HYP content, shear viscosity was expected to rise rapidly

with increasing concentrations of fibre because of the rapidly increasing interactions

between particles as they became more closely packed. Nevertheless, at very high fibre

content, random anisotropic structures of the fibres in polymer melts were created. The

increase in shear viscosity was found to be more predominant at lower shear rates, where

fibre and polymer molecules were not completely oriented. In addition, higher pulp

content at high temperature, the effect of the pulp moisture content is more pronounced

by producing the hydrolysis reaction of polyamide 11 and consequently tends to decrease

the melt viscosity [Giles et al., 2005].

0

200

400

600

800

1000

0 1000 2000 3000 4000 5000

Sh

ea

r vis

co

sit

y (

Pa

.s)

Shear rate (s-1)

10%HYP/PA11

20%HYP/PA11

30%HYP/PA11

47

3.4.5 Effect of high-yield–pulp fibre aspect ratio on rheological

properties

The results from the study of the effect of the aspect ratio of flexible-pulp–fibre-

reinforced bio-based Nylon 11 are presented below in figure 3.4.

At low fibre aspect ratio, the decrease in viscosity as a function of the shear rate was

greater for both low and high shear rate. Contrarily, at higher fibre-aspect ratio the shear

viscosity showed a moderate decrease for low and intermediate shear rate.

Figure 3.5. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

At low and intermediate shear rate, the viscosity curves are slightly decreased and the

distance between each viscosity curve remains large. However, at high shear rate the

viscosity plots are tightly close. This is because fibre agglomeration and entanglement

0

200

400

600

800

1000

1200

1400

0 1000 2000 3000 4000 5000

Shear Rate (s-1)

Ar1 Ar3 Ar2

48

were not pronounced at high shear rate or frequency, due to the complete orientation of

the fibre and polymer molecules. Much as with high fibre content, the increase in shear

viscosity was found to be greater at lower shear rates, where fibre and polymer molecules

are not completely oriented.

3.4.6 Scanning electron microscopy

Fracture surfaces of the extrudate of HYP/PA11 composite were examined using SEM.

No fibre pullout was observed. The chemical structures of polyamide and wood fibre

both include a hydrogen bond, which makes for better compatibility between high-yield

pulp and polyamide. Figure 3.5 represents the SEM micrograph of HYP/PA11 composite.

Figure 3.6. An SEM micrograph of pulp-fibre–reinforced polyamide composite fracture surface showing

partially melted nylon fibre.

49

This micrograph indicated efficient bonding between the high-yield fibre and bio-based

nylon. The experimental and predicted data fit very well, which meant that there was

strong adhesion at their interface due to the presence of the third phase.

3.5 Conclusions

This study demonstrates that it is possible to extrude natural fibre with high-

thermoplastic-engineering bio-based nylon. Fibre distribution after processing has been

characterized by FQA for high-yield fibre in composites containing 10, 20 and 30%

fibres. The low-temperature–process compounding presents higher shear viscosity than

the high-temperature processing of HYP/PA11 for the same rheological parameters. This

is because during the compounding process the temperature was low, and the mixing and

melting processes were generated by the high shear rate created during extrusion.

Experimental test results of HYP/PA11 for both processing parameters show a steep

decrease in shear viscosity with increasing shear rate, and this melt-flow characteristic

corresponds to shear thinning behavior in HYP/PA11. Experimental results showed high

shear-thinning behavior in HYP/PA11 associated with a high degree of pseudoplasticity;

this was due to the good dispersion of high-yield pulp fibre into Nylon11 and the

orientation of the flexible pulp fibre in the direction of the molten PA11. Finally, the

morphological properties of HYP/PA11 composite were examined using scanning

electron microscopy (SEM). Due to the presence of the hydrogen bond, no fibre pullout

was observed and there was good adhesion between high-yield pulp fibre and polyamide.

50

CHAPTER 4

Effect of Lithium Chloride on the Fibre Length

Distribution, Processing Temperature and the

Rheological Properties of High-Yield-Pulp-Fibre–

Reinforced Modified Bio-Based Polyamide 11

Composite

4.1 Abstract

The aim of this work was to investigate the effect of lithium chloride (LiCl) on the fibre

length distribution, melting temperature and the rheological characteristics of high yield

pulp fibre reinforced polyamide biocomposite. The inorganic salt lithium chloride (LiCl)

was used to decrease the melting and processing temperature of bio-based polyamide 11.

The extrusion method and Brabender mixer approaches were used to carry out the

compounding process.

The densities and fibre content were found to be increased after processing using both

compounding methods. The HYP fibre length distribution analysis realized using the

FQA equipment showed an important fibre-length reduction after processing by both

techniques.

51

The rheological properties of HYP-reinforced net and modified bio-based polyamide 11

“PA11” (HYP/PA11) composite were investigated using a capillary rheometer. The

rheological tests were performed in function of the shear rate for different temperature

conditions. The low-temperature process compounding had higher shear viscosity; this

was because during the process the temperature was low and the mixing and melting

were induced by the high shear rate created during compounding process. Experimental

test results using the extrusion process showed a steep decrease in shear viscosity with

increasing shear rate, and this melt-flow characteristic corresponds to shear-thinning

behavior in HYP/PA11, this steep decrease in the melt viscosity can be associated to the

hydrolyse reaction of nylon for high pulp fibre moisture content at high temperature. In

addition to the low processing temperature, the melt viscosity of the biocomposite using

the Brabender mixer approach increases with increasing shear rate, and this stability in

the increase even at high shear rate for high pulp moisture content is associate to the

presence of inorganic salt lithium chloride which created the hydrogen bonds with pulp

during the compounding process.

4.2 Introduction

Short-fibre reinforced polymer composites are extensively used in manufacturing

industries due to their light weight and improved mechanical properties [Pervaiz & Sain,

2003; Bourmaud & Baley, 2009]. Hence, HYP has been used not only for its low lignin

content, but also for its potential thermal stability and its strong adhesion when it is

bonded with high-temperature engineering thermoplastic polymers [Sadeghian & Golzar,

2008; Thomen, 2001; Gu & Kokta, 2010; Bajpai, 2012].

52

Various experimental studies have investigated the effect of flexibility on fluid viscosity.

They concurred that the more flexible the fibres are, the more pronounced is their effect

on the rheological characteristics [Plackett et al., 2010; Kaw & Besterfield, 1998; Yeh,

1992]. A recent study on the effect of fibre-length distribution on the rheological

behavior of castor-oil composite showed that at high fibre length, the shear viscosity

becomes more dependent on shear rate [Gohil & Shaikh, 2010]. This behavior is due to

elastic deformation of the fibres [Gohil & Shaikh, 2010].

Recently, various authors have investigated the effect of fibre content on polymer melt

rheology [Kari et al., 2005; Lamnawar & Maazouz, 2008; Larache et al., 1994]. Their

study showed an important increase in shear viscosity with increased fibre loading at low

shear rates, but only a small increase in viscosity at high shear rates [Lamnawar &

Maazouz, 2008; Larache et al., 1994]. Another similar study on polypropylene-based

long fibre observed an increase in shear viscosity with increased fibre content and fibre

length [Larache et al., 1994]. However, this viscosity rise was very small, which the

authors attributed to high shear rates and fibre breakage during the processing [Larache et

al., 1994; George et al., 2001]. Non-Newtonian fluid characteristics such as shear

thinning were also observed in all the studies mentioned above.

There is only a very limited literature devoted to experimental studies of the rheology of

pulp-fibre–reinforced polymer composites due to the complex nature of these materials

and the difficulties encountered during their processing and their rheological

characterization [Larache et al., 1994; George et al., 2001; Liu et al., 2000; Uhlherr et al.,

2005; Huq & Azaiez, 2005].

53

The processing technique and conditions have a significant influence on the rheological

and overall properties of pulp-fibre–reinforced polymer composites because they dictate

the degree of dispersion and distribution of the fibre in the polymer matrix and the low

processing temperature required in order to avoid thermal degradation [Larson, 1999; -

Eberle, 2008; Le Moine et al., 2013; Guo et al., 2005]. Compared to other natural fibres,

HYP is more thermally stable in the presence of high- melting-temperature engineering

thermoplastics (under 180˚C) such as PA11, PA6, and PA66.

The principal objectives of the study described in this chapter were to determine the

effect of the addition of the inorganic salt lithium chloride (LiCl) to the bio-based

polyamide 11, and the characteristic of the modified bio-based polyamide 11 in the

presence of high yield pulp (HYP) fibre. The HYP fibre content, the length distribution,

and the density of the composites were measured and analyzed for both processing

techniques in order to investigate the effect of LiCl on the composite components.

Finally, the rheological results using the Brabender mixer technique and the conical twin

extruder respectively were determined and compared.

4.3 Material and Methods

4.3.1 Materials

The matrix biopolymer bio-based polyamide 11, density 1.03, MFI 11, was supplied by

Arkema (France). Aspen high yield pulp (HYP) fibres were supplied by Tembec

(Montreal, QC). The HYP is the type used in wood-free printing, in writing-paper grades

and in multiple-coated folding-board grades; fibre length is 0.230 to 0.85 mm.

54

4.3.2 Methods

4.3.2.1 Composites preparation

The experiment was processed using a conical twin screw extruder and a Brabender

mixer technique. In both mixing processes, the high yield pulp “HYP” fibre was dried at

80˚C for 6 hours and then added to the corresponding bio-based polyamide PA11 and

well mixed before the combination was introduced to the extruder. The average

temperature of the barrel was 200˚C. Figure 4.1 represents the different zones of the

conical twin-screw extruder. In addition, in the Brabender mixer technique process,

different lithium chloride content “LiCl” was added to the bio-based polyamide 11 at

corresponding process temperature prior to adding the pulp fibre.

Figure 4.1. Schematic figure of twin-screw extruder

55

4.3.2.2. Effect of processing conditions

Many processing parameters affect the properties of final products. For extrusion,

temperature profiles affect the fibre degradation. In addition, screw speed and feeding

rate change fibre length, distribution, and orientation. The mechanical properties reflect

all these changes, and the processing parameters are optimized to obtain the best

properties. Table 4.1 represents the processing parameters for HYP/PA11 used in this

study.

Table 4.1. Extrusion temperature profile for 10%, 20% and

30% HYP/PA11, 120 rpm (10% and 20%), 130 rpm (30%)

and 10 rpm feed rate.

Temperature (˚C)

Zone

1

Zone

2

Zone

3

Zone

4

Zone

5

Zone

6

Zone

7

Zone

8

Zone

9

Zone

10

200 200 200 200 200 205 205 205 205 205

However, the Brabender mixer technique was used as the principal compounding process

in this study. Inorganic salt lithium chloride (LiCl) was added to bio-based polyamide 11

in order to decrease it melting temperature, and consequently avoid fibre degradation and

burning. Different lithium chloride content was used to reduce the melting temperature of

polyamide 11 using the Brabender mixer technique.

56

4.3.2.3 Fibre content and length distribution analysis after compounding

The composite samples were cut into small pieces and immersed in formic acid for three

days. The bio-based PA11 was dissolved by the formic acid and HYP was left. The HYP

was filtered and washed with formic acid, then dried in a vacuum oven for four hours. By

measuring the weight of composite and pulp fibre, we could calculate the actual fibre

content.

HYP fibre length was measured with the Fiber Quality Analyzer (FQA). The HYP was

diluted with D.I. water. The diluted HYP fibre entered a thin-planar channel. This

channel helped to gently orient the fibre 2-dimensionally, so that the fibre could be fully

viewed by the camera. The picture taken by the camera was then analyzed by the

software to give the HYP fibre length distribution.

4.3.2.4 Actual density measurement

The density of polyamide 11 reinforced HYP fibre composites was determined by using

the ASTM D792 technique. The samples were first weighed both in water and air, and

then the density was calculated by:

ρ = a/[(a – w)ρwater] (4.1)

where ρ is the sample density in g/cm3, a is the sample weight in air in g, w is the sample

weight in water in g, and ρwater is the density of the water in g/cm3.

57

4.3.2.5 Differential scanning calorimetry (DSC)

The melting temperature and crystallization behavior of the yield pulp fibre reinforced

bio-based polyamide 11 composites were investigated using a TA instrument Q1000

differential scanning calorimeter (DSC) attached to a cooling system under a nitrogen

atmosphere. The DSC instrument was run from 45 °C to 250 °C with a heating rate of

10 °C /min. The sample weight was about 5 mg. The specimens were sealed in aluminum

pans by pressing and the prepared samples were placed in the furnace of DSC with an

empty reference pan. The heat flow rate as function of temperature was recorded

automatically. Melting temperature was identified on the peak point of the DSC curves.

The melting of polymer within a composite system would assist to select a suitable

temperature profile for the compounding process technique when the fibres and matrix

were compounded to produce green composite.

4.3.2.6 Rheological properties measurement

The rheological measurements of the composites’ melt-flow properties of were carried

out in a twin-bore Rosand Capillary Rheometer model RH2000. (The standard RH2000

range supports temperatures from -40˚C to 500˚C. The standard maximum force applied

is 12kN.) The composite samples for testing were cut into very small pieces, then placed

inside the barrel and forced down into the capillary with the plunger attached to the

moving cross-head. Representative steady-shear viscosity versus high shear rate is

presented in the figures below for HYP/PA11, which was processed at the average

extrusion temperature of 200˚C. The viscosity of the sample was obtained from steady-

shear measurements for different temperature profiles, with the rate ranging from 50 to

58

5000 S-1. The rheological viscosity data presented in this chapter thus represents an

average value of three measurements.

4.4 Results and Discussion

4.4.1 Effect of the lowering the processing temperature on the pulp

fibre distribution and the bio-based polyamide density after

processing

To avoid the degradation of the HYP fibre during the processing of the composite,

decreasing the melting temperature was attempted. The reduction of the melting point of

high-temperature–engineering polyamide was realizing by the utilization of inorganic salt

during the melt compounding processing in order to lower the melting temperature of

polyamide 11 (PA11). Lithium chloride (LiCl) was added to the polyamide 11 during the

extrusion process using the Brabender mixer technique. Next, the PA 11 and salt mixture

was used as a matrix and HYP fibre was incorporated into the matrix using a Brabender

mixer for the compounding.

59

Table 4.2. Melting temperatures of neat PA11 and modified

PA11 with varying LiCl content.

Composition Melting temperature (˚C)

Neat Polyamide (PA11) 188

PA11 + 1%LiCl 186

PA11 + 2%LiCl 182

PA11 + 3%LiCl 175

PA11 + 4%LiCl 172

PA11 + 5%LiCl 170

The melting temperatures of the PA 11/LiCl mixture are shown in Table 4.2. Table 4.3

shows the heat deflection temperatures of the PA11 polymer-reinforced HYP fibre

composites. 3% LiCl in PA11 was chosen in order to keep the concentration of LiCl low.

Figure 4.6 shows the rheological characteristics of Nylon 11 plastic-reinforced HYP

composites. From 3% to 5% LiCl in the PA11, the change in melting temperature is

insignificant. The stability of the melting point at high LiCl concentration is due to the

lowering of the crystallization temperature and the saturation of the degree of crystallinity

of the molecular chains.

60

Table 4.3. Heat deflection temperatures of polyamide 11

and HYP-fibre–reinforced bio-based PA11 composites.

Composition Heat deflection temperature (˚C)

Neat PA11 135

PA11 + 3%LiCl 52

PA11 + 3%LiCl + 10% HYP fibre 118

PA11 + 3%LiCl + 20% HYP fibre 126

PA11 + 3%LiCl + 30% HYP fibre 138

The heat deflection temperature was investigated for only 3% LiCl content. The addition

of LiCl to PA11 decreases the crystallization temperature and degree of crystallinity.

Consequently, the heat deflection temperature of PA11 decreased from 135ºC to 52ºC of

neat PA 11. However, with the addition of 10% HYP fibre, the heat deflection

temperature (HDT) increased to as high as 118ºC. For 30% HYP added to the modified

PA11, the heat deflection temperature rises up to 138 ºC. The higher is the content of the

pulp fibre, the higher is the heat deflection temperature of the composite. The increase in

the heat deflection temperature (HDT) of the resultant composite is proportional to the

concentration of high yield pulp fibre to the modified PA11 and LiCl.

4.4.2 Densities and actual fibre contents

The fibre contents of composites were controlled by the feeding rate of matrix and fibre.

However, the feeding rate cannot be calibrated precisely, especially the feeding rate of

61

HYP fibre. Table 4.4 shows the densities and actual fibre contents of composites

processed by the extrusion compounding method and the Brabender mixer technique.

Comparing the densities of bio-based polyamide 11 reinforced HYP composites made by

the two procedures, we can see that the composites made by the Brabender mixer

technique have higher density than composites made by the extrusion process at the same

fibre content. The 30% HYP/PA11 made via the Brabender mixer has a higher density

because its actual fibre content is 5% higher than the composite made by the extrusion

process. The different densities show that Brabender mixer technique gives samples with

fewer voids than the extrusion process.

Table 4.4. Densities and actual high-yield fibre contents.

Densities (g/cm3) Actual pulp fibre content

Polyamide 11 (PA11) reinforced HYP fibre composite : Extrusion compounding technique

PA11 1.03 0

PA11 + 10% HYP 1.05 12,5%

PA11 + 20% HYP 1.07 25%

PA11 + 30% HYP 1.12 33.2%

Modified Polyamide 11 (PA11) reinforced HYP fibre composite: Brabender mixer technique

PA11 + 3%LiCl + 10% HYP 1.07 14%

PA11 + 3%LiCl + 20% HYP 1.11 26%

PA11 + 3%LiCl + 30% HYP 1.21 38.3%

62

The modified bio-based PA11 reinforced HYP fibre composites have higher densities

than the regular bio-based PA11-reinforced HYP fibre composites at the same fibre

content even though the modified PA11 has a slightly higher density than regular PA11.

The reduction of the melting point, thus viscosity, could lead to a better fibre-matrix

wetting to eliminate free volume, holes/voids in the biocomposite with LiCl.

To minimize fibre thermal degradation, the processing temperature was set just below the

commercial melting temperature of polyamide. Table 4.4 shows that the densities and

actual fibre contents were proportional to the fibre content for both processing methods.

However, with the addition of LiCl to the Brabender mixer, the differences became more

pronounced.

4.4.3 Effect of fibre content on the length and shape distribution on

HYP-reinforced bio-based modified PA11 composite

During the extrusion process, the shear stress applied by the screw breaks the fibres. The

resulting fibre lengths affect the ultimate mechanical properties. In spite of the influence

of fibre damage and breakage during processing, the initial fibre length in the feedstock

determined the final fibre lengths. It was therefore important to analyze the initial fibre-

length distribution, which is one of the most significant parameters for natural fibre

reinforced polymer composites. After the polymer and fibre for the composite are

decided on, fibre length is the adjustable feature used to manage the ultimate properties

of bio-composite materials. Table 4.5 shows the results of HYP fibre length distribution

determined using a Fibre Quality Analyzer (FQA).

63

Table 4.5. Fibre length distribution of pulp fibre (extrusion

compounding process).

The arithmetical length and the length-weighted values of HYP fibre length were found

to be 0.20 mm, 0.27 mm, and 0.49 respectively for the pulp fibres from 30% HYP/PA11.

The arithmetic and length-weight measured values for 20% of pulp fibre were 0.24mm

and 0.36mm, and for 10% pulp fibre they were 0.29mm and 0.41 mm. Most of the fibre

lengths lie within the range of 0.20 mm to 0.57 mm, since HYP fibre was used in this

study. To obtain these average fibre lengths, 1,500 single fibres were examined. The

measured initial fibre length is greater than the actual length of the fibres in the

composites. The HYP fibre length decreases inversely with the increase of pulp fibre

content in the composite. This decrease of fibre length with the pulp fibre content in the

polymer melt concentration is due to fibre entanglement and agglomeration within the

polymer.

Fibre

Mean Length (measured)

Arithmetic (mm) Length weight

(mm)

Weight weighted

(mm)

Only HYP 0.38 0.57 0.73

HYP (10%) 0.29 0.41 0.63

HYP (20%) 0.24 0.36 0.54

HYP (30%) 0.20 0.27 0.49

64

Table 4.6. Fibre length distribution of pulp fibre (Brabender

mixer approach).

Table 4.6 shows that the HYP fibre length from the green composite produced using the

Brabender mixer technique did not decrease very much compared with the HYP fibre

length of the composite made using the conical twin extruder method. For modified bio-

based PA11-reinforced pulp fibre bio-composites processed using the Brabender mixer

technique, the mean fibre length did not decrease a great deal. In the normal bio-based

PA11 reinforced HYP fibre composites, the HYP fibre length is shorter than that of the

bio-based modified polyamide reinforced pulp fibre after extrusion, probably because the

higher temperature caused more thermal degradation of fibres, making them easier to

break. In addition, the use of LiCl to decrease the melting temperature of the bio-based

Fibre

Mean Length (measured)

Arithmetic (mm) Length weight

(mm)

Weight weighted

(mm)

Modified Bio-based PA11- reinforced HYP fibre composite: Brabender mixer

technique

Only HYP 0.38 0.57 0.73

HYP (10%) 0.33 0.46 0.68

HYP (20%) 0.28 0.38 0.60

HYP (30%) 0.26 0.35 0.56

65

PA11 may also have protected the pulp fibre from degradation and entanglement during

the slow and controlled process using the Brabender mixer technique, and consequently

kept the pulp fibre length constant after the compounding process.

4.4.4 Rheological characteristics of HYP–reinforced bio-based

polyamide

As already noted, rheological characteristics of the polymer, fibre, and interphasial

phases influence the final characteristics of the resultant microstructure of the composite

materials; these characteristics in turn affect the mechanical properties of a multiphase

polymer-composite system. As obtained from experiment, the shear viscosity as a

function of the steady-shear rate of HYP/PA11 at 200˚C is shown in Figure 4.2 (As

noted, these results are the average of three different experimental tests.)

Figure 4.2. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

0

100

200

300

400

500

600

700

800

0 1000 2000 3000 4000 5000

Sh

ea

r vis

co

sit

y (

Pa

.S)

Shear rate (s-1)

66

As noted, the composite material used in the experimental study had an average fibre

length of 0.73 mm, and the experiment was conducted at 200˚C. The experimental results

showed that the viscosity of HYP/PA11 composite decreases with increasing shear rate.

This decrease in the shear viscosity with the increase of the shear rate corresponds to the

pseudoplastic fluid characteristic of the composites. This pseudoplastic behavior (also

referred to as shear-thinning behavior) as plotted in Figure 4.2 is mainly influenced by

the orientation of the polymer molecules, the agglomeration of the flexible pulp fibre, and

the entanglements within the polymer chains in the capillary rheometer. The chain

agglomerations are produced simultaneously, as one chain is collapsed into another

chain. The entanglements that lead to agglomerations of the chains, as well as the

entanglements within the chains, are caused by the Brownian motions and low relaxation

of HYP fibre. The high shear-thinning behavior obtained for HYP/PA11 can be also

associated to the thermal degradation of HYP during the rheological testing and the

compounding process. The molten polymers tend to arrange themselves with their major

axes in the direction of shear, whereby points of entanglement are reduced. As a result,

the viscosity decreases. In other words, in this instance of non-Newtonian flow behavior

in polymer melts, the decrease in viscosity when the shear rate is increased by applying

load is associated with high shear-thinning behavior and with viscoelastic characteristics

of biocomposite materials. However, at very high shear rates (from 3000 to 5000 S-1) the

molten HYP/PA11 showed a less restrained decrease in viscosity. This non-Newtonian

behavior is associated with the alignment and orientation of the fibre in the polymer

chains and the effect of the fibre aspect ratio. Both at low and high shear rates the

formation of agglomerates is evident; therefore, the pulp molecules are completely

67

oriented due to the good dispersion in the bio-based PA11 matrix. This means that the

breakage of the fibre length allows the maintenance of an accurate fibre-aspect ratio

when the diameter of flexible HYP is kept unchangeable during the process. The shorter

length of the fibres also supports their alignment in the direction of the flow, thus

reducing the fibre-fibre collisions and leading to a greater decrease in apparent viscosity.

4.4.5 Effect of the processing parameters on the rheological

property

The rheological testing results for the different processing techniques (the extrusion

process for the high temperature process, and the Brabender mixer method for the low

temperature process) are presented below in Figure 4.3. The goal of decreasing the

process temperature was only realized for 30% HYP/PA11. The Brabender mixer

approach was used for the low-temperature compounding, and the process temperature

was below the melting point. The rheological properties of the high- and low-temperature

compounding are presented in Figure 4.3.

68

Figure 4.3. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

The low-temperature process compounding has higher shear viscosity compared to the

high temperature process; this is because a) during the low-temperature process the

polymer melting was generated by the high shear rate created during compounding and

also because the mixing process was incomplete.

4.4.6 Effect of the inorganic salt lithium chloride on the

rheological properties of HYP fibre–reinforced bio-based

polyamide composite

Variations in shear viscosity as a function of the shear rate of HYP/PA11 at various

processing temperatures using inorganic salt (LiCl) was realized in order to investigate

their effect on the melting point and the rheological properties HYP-fibre–reinforced bio-

based PA11 composite. The addition of inorganic salt lithium chloride to PA11 modified

it melting point and consequently modified the processing temperature. The rheological

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Sh

ea

r vis

co

sit

y (

Pa

.s)

Shear rate (S-1)

low temperature process

High temperature process

69

test results are presented in Figure 4.4 and were achieved using the Brabender mixing

technique. The processing conditions were 200˚C for the net polyamide without LiCl,

186˚C for 1% LiCl, 182˚C for 2% LiCl, 175˚C for 3%LiCl, 172˚C for 4%LiCl, and

170˚C for 5%LiCl content. In this chapter, LiCl + bio-based PA11 “LiCl + PA11” is

referred to as 'modified' polyamide 11 for brevity. The rheological test results presented

in the figure below are for constant fibre content (30%) and for various process

temperature and lithium chloride concentrations.

Figure 4.4 Variation of the viscosity with a function of shear rate of HYP/PA11 at various temperatures.

The variation in the shear viscosity of the HYP/PA11 in function of the rate of shear and

lithium chloride concentration was measured and presented. The shearing effects

decreased as the salt concentration increased: that is, the modified HYP/PA11 became

more non-Newtonian in the higher temperature region for low LiCl concentration. At

higher salt concentration the reduction of shear viscosity was more pronounced at

intermediate shear rate, while for 175˚C, this reduction is maximized at higher shear rates

(from 3000 to 5000 S-1). At low salt concentration and net processing temperature

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Shear rate (s-1

)

Sh

ea

r v

isc

os

ity

(P

a.s

)

5%LiCl(170˚C)

4%LiCl(172˚C)

3%LiCl (175˚C)

2%LiCl (182˚C)

1%LiCl (186˚C)

Net (200˚C)

70

(200˚C) and for low and intermediate shear rate, the flow deformation is challenging.

This is due to the fact that the HYP-reinforced PA11 fibres are entangled and

agglomerated. At this point, such rheological behavior is called Near-Newtonian. At high

shear rate, the shearing effects increased while the effect of temperature and the salt

concentration were less pronounced, and flow deformation was mostly dominated by the

shearing effect. However, from 3000 to 5000 S-1 all the shear-viscosity variations in

function of shear rates followed the same rate of deformation for different salt content

and temperature profiles, which corresponds to shear-thinning behavior.

4.4.7 Effect of HYP fibre content on the rheological characteristics

of modified bio-base (PA11 + 3%LiCl) composite

The effect of fibre content on the rheological characteristics of the HYP-fibre–reinforced

modified bio-based polyamide composite was investigated. Figure 4.5 shows the

experimental results for 10%, 20%, and 30% HYPP reinforced modified PA11 “PA11 +

LiCl”. These curves are typical of pseudoplastic materials, which show a decrease in

viscosity with increasing shear rate. At high fibre content, the material offers higher shear

viscosity even for high shear rate. In general, the incorporation of fibres in polymer

systems increases the viscosity, which rises further as fibre content is increased.

71

Figure 4.5. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

The difference between 10% and 20% fibre at intermediate and high shear rate is not very

significant. At low HYP content, shear viscosity is expected to rise rapidly with

increasing concentrations of fibre because of the rapidly increasing interactions between

particles as they become more closely packed and entangled. Nevertheless, at very high

pulp fibre concentration, random anisotropic structures of the fibres in polymer melts

were created, and they increased the shear viscosity. The increase in shear viscosity is

found to be more predominant at lower shear rates, where fibre and polymer molecules

are not completely oriented.

0

200

400

600

800

1000

1200

1400

1600

1800

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Shear rate (s-1)

Sh

ea

r V

isc

os

ity

(P

a.s

) 30%HYP/PA11+3%LiCl

20%HYPPA11+3%LiCl

10%HYP/PA11+3%LiCl

72

4.5 Conclusions

This study demonstrates that it is possible to process HYP fibre with high-thermoplastic-

engineering bio-based polyamide. For both processing methods and all formulations,

fibres showed a length reduction after compounding process. The observed fibre length

reduction using the extrusion process method was lower compared to the fibre length

reduction using the Brabender mixer technique. However, the highest fibre reduction was

observed for 30% pulp fibre in composite. The low-temperature compounding of

HYP/PA11 presents higher shear viscosity compared to the high-temperature

compounding for the same rheological parameters; this is because during the process the

temperature was low and the mixing and melting were produced by the high shear rate

created during compounding process. Experimental testing results from HYP/PA11 for

the extrusion processing technique showed a steep decrease in shear viscosity with

increasing shear rate at high temperature, and this melt-flow characteristic corresponds to

shear thinning behavior in HYP/PA11 and due to the high pulp moisture content which

tends to degrade polyamide 11. Results also showed high shear-thinning behavior in

modified HYP/PA11 associated with a high degree of crystallinity and pseudoplasticity;

this was due to the good dispersion of HYP into PA11 and the orientation of the flexible

fibre effects in the direction of the molten PA11.

73

CHAPTER 5

Review of Non-Newtonian Mathematical Models for

Rheological Characteristics of Viscoelastic

Composites

5.1 Abstract

This study presents an overview of viscoelastic characteristics of biocomposites derived

from natural-fibre–reinforced thermoplastic polymers and of predictive models used to

understand their rheological behavior. Various constitutive equations are reviewed for a

better understanding of their applicability to polymer melt in determining viscosity. The

models to be investigated are the Giesekus-Leonov model, the Upper Convected Maxwell

(UCM) model, the White-Metzner model, K-BKZ model, the Oldroyd-B model, and the

Phan-Thien-Tanner models. The aforementioned models are the most powerful for

predicting the rheological behavior of hybrid and green viscoelastic materials in the

presence of high shear rate and in all dimensions.

The Phan-Thien Tanner model, The Oldroyd- B model, and the Giesekus model can be

used in various modes to fit the relaxation modulus accurately and to predict both shear

thinning and shear thickening characteristics. The Phan-Thien Tanner, K-BKZ, Upper

convected Maxwell, Oldroyd-B, and Giesekus models predicted the steady shear

74

viscosity and the transient first normal stress coefficient better than the White-Metzner

model for green-fibre–reinforced thermoplastic composites.

5.2 Introduction

The rheological properties and extrudate behavior of polymer melts are of central

importance in the processing and fabrication of polymer products. The melt-rheological

behavior of short sisal, coir, and pineapple fibre-reinforced polymer systems has been

studied in various works [Anselm et al., 2014; Mohssine et al., 2007]. However, there is

limited research on the effects of parameters such as the fibre length, the fibre-matrix

interaction, and the aspect ratio of green-fibre–reinforced thermoplastic biopolymer

composites on nonlinear rheological behavior [Fengwei et al., 2012; Xiuyang, et al.,

2012].

The mathematical explanation of a viscoelastic fluid is much more complex than its

Newtonian counterparts. In addition to the conservation equations of mass and

momentum, the constitutive equation or rheological equation of state is required; this

relates stress to deformation. For a viscoelastic liquid this relationship is nonlinear, and it

has no standard form universally valid for each fluid in every flow situation. This reality

is one of the reasons why the subject of viscoelasticity is so challenging.

The constitutive equation should not only describe the rheological characteristic of the

polymer melt but also give the final fibre orientation of the composite. For this reason it

is fundamental to evaluate the role of the biocomposite’s rheology and the natural fibre–

polymer interaction. It has been observed that the total stress of the composite increases

75

as fibres are added; consequently, a satisfactory constitutive equation could be achieved

by adding an extra stress term to an already existing constitutive equation, which then

adequately describes the polymer melt [Zhang et al., 2009; Somwangthanaroj, 2010;

Marynowski, 2006].

Accordingly, constitutive equations found in the literature that adequately describe

polymer melt will be explored for their application in biocomposite processing. Particular

focus will be given to nonlinear rheological characteristics of viscoelastic materials. The

power-law, Cross WFL, Casson, Bird-Carreau and Hershel Bulkley models are among

the most preferred rheological models due to their ability to predict velocity and pressure

distributions in uniform flows in addition to their simple representation of shear thinning

behaviour [Marynowski, 2006; Dealy & Wissbrun, 1990; Mahmoud et al., 2012; Owens

& Phillips, 2002]. However, in the case of high shear stress of viscoelastic polymer melt,

the predictive power of these models is considerably reduced [Owens & Phillips, 2002].

In this study, a review of nonlinear rheological models for viscoelastic materials from

natural-fibre–reinforced thermoplastic polymers will be presented by a special review of

the Upper convected Maxwell, Phan-Thien-Tanner (PTT), K-BKZ, Oldroyd-B, Giesekus

and Whhite-Metzner constitutive models.

5.3 Viscoelastic characteristics of materials

Viscoelasticity is the property of a material to demonstrate both viscous and elastic

properties under the same conditions when it undergoes deformation [Han, 2007; Willett

et al., 1995; James, 2009]. Viscous materials present resistance to shear flow and strain

76

linearly with time when a stress is applied [Doraiswamy, 2002; Bird et al., 1987]. The

shear stress of these materials depends on strain: when strain is applied and then released,

they return to their initial configuration [Bird et al., 1987; Tucker & Advani, 1994]. Some

common and well-known viscoelastic materials include paint, blood, ketchup, honey,

mayonnaise, polymer melt, polymer solution and suspension, shampoo, and corn starch

[James, 2009; Jarecki & Ziabicki, 2011].

At constant temperature, water, air, ethanol, and benzene are represented as Newtonian

fluids [Higashitani & Pritchard, 1972; Tuna & Finlayson, 1984; Crochet & Legat, 1992].

This means that the rapport between the shear stress versus shear rate is a straight line

with a constant slope for a fixed temperature that is independent of the shear rate [Tuna &

Finlayson, 1984; Crochet & Legat, 1992; Dealy & Wissbrun, 1999; Doraiswamy, 2002].

In addition, the plot passes through the origin: that is, the shear rate is zero when the

shear stress is zero [Doraiswamy, 2002].

A fluid that does not behave in a Newtonian fashion between shear stress and shear rate

when it undergoes deformation is commonly termed non-Newtonian [Bird et al., 1987;

Tucker & Advani, 1994]. This means that the relation between shear stress and shear is

not a straight line but is non-linear. High-molecular-weight liquids, which include

polymer melts and solutions of polymers, as well as liquids in which fine particles are

suspended, are usually non-Newtonian [Tucker & Advani, 1994; Grafe & Graham, 2003;

Zhou & Kumar, 2010]. In this case, the slope of the shear stress versus shear rate plot will

not be constant as we change the shear rate [Grafe & Graham, 2003; Zhou & Kumar,

2010]. When viscosity decreases with increasing shear rate, the fluid is called shear-

thinning [Jayaraman et al., 2004; Letwimolnun et al., 2007; Collie et al., 2001]. In the

77

opposite case, where the viscosity increases as the fluid is subjected to a higher shear

rate, the fluid is called shear-thickening [Jayaraman et al., 2004; Letwimolnun et al.,

2007; Collie et al., 2001]. Shear-thinning fluids also are called pseudoplastic fluids and

shear-thickening fluids are called dilatants [Da Silva et al., 2012; Mitran & Yao, 2007].

Shear-thinning behavior is more common than shear thickening [Letwimolnun et al.,

2007; Collie et al., 2001].

Another type of non-Newtonian fluid is viscoplastic or “yield stress” fluid [Jayaraman et

al., 2004; Letwimolnun et al., 2007; Collie et al., 2001]. This is a fluid that will not flow

when only a small shear stress is applied. The shear stress must exceed a critical value

known as the yield stress for the fluid to start flowing. Hence, viscoplastic fluids behave

like solids when the applied shear stress is less than the yield stress. Once it exceeds the

yield stress, the viscoplastic fluid will flow just like an ordinary fluid [Letwimolnun et

al., 2007; Collie et al., 2001].

On the other hand, some classes of fluids exhibit time-dependent behavior [Denn, 2008a;

Denn, 2008b; Krutka et al., 2008]. This means that even at a given constant shear rate,

the viscosity may vary with time. This category of material comprises both thixotropic

and rheopectic fluids, whose behaviors in this respect are opposites [Denn, 2008a; Denn,

2008b; Krutka et al., 2008]. The viscosity of a thixotropic liquid will decrease with time

under a constant applied shear stress [Denn, 2008b; Krutka et al., 2008]. However, when

the stress is removed, the viscosity will gradually recover with time as well [Denn, 2008].

By contrast, rheopectic fluid behavior can be observed when the fluid increases in

viscosity with time when a constant shear stress is applied [Denn, 2008b; Krutka et al.,

2008].

78

5.4 Rheological modelling of viscoelastic composites

Mathematical models of viscoelasticity are mostly based on a differential or integral

representation [Tanner, 2000; Denn et al., 1975]. From a mathematical point of view, the

differential representation is easier to handle than the integral one. However, the integral

representation is capable of predicting the time dependence more generally [Tanner,

2000; Denn et al., 1975; Nonaka et al., 2009].

The characteristic feature of linear viscoelastic materials is that the stress is linearly

proportional to the strain history [Nonaka et al., 2009; Da Silva et al., 2012]. Linear

viscoelasticity is usually applicable only for small deformations, low rate, low stress, and

linear materials [Nonaka et al., 2009; Da Silva et al., 2012; Mitran & Yao, 2007].

However, in reality about 90% of fluids are nonlinear, with large deformations, and with

nonlinear response in the presence of such deformations. Nonlinear viscoelastic behavior

is usually exhibited when the deformation is large and most of the time when the material

changes its properties under deformations [Devereux & Denn, 1994; Ellison et al., 2007;

Fisher & Denn, 1977]. Consequently, nonlinear viscoelastic mathematical models are

needed [Fisher & Denn, 1977; Ellyin et al., 2007].

Existing nonlinear mathematical rheological models are often constructed through

modifications and extensions to higher-order stress or strain terms of the linear theory

[Galante, 1991]. As noted earlier, from a mathematical point of view, the integral

representation of a viscoelastic constitutive equation is more difficult to perform than the

differential form. Thus, several models characterized by elastic, viscous, and inertial

nonlinear contributions with various complexities have been developed for describing the

79

nonlinear behavior of these materials [Galante, 1991]. However, in these models, due to

the mathematical complications, only the elastic or viscous nonlinearity is often taken

into account and the inertial contribution is ignored. Moreover, there are only a few

theoretical models formulated with constant-value rheological material parameters.

For these reasons, nonlinear models with constant rheological coefficients are required.

The elastic and viscous nonlinearities are taken simultaneously into consideration through

a simple nonlinear generalized Maxwell fluid model consisting of a nonlinear spring

connected in series with a nonlinear dashpot obeying a power law with constant material

coefficients [Burghardt & Fuller, 1989; Kiriakidis et al., 1989]. According to a previous

study by Bauer (1984), suitable constitutive equations for viscoelastic materials must

relate stress, strain, and their higher time derivatives; or better said, they must take into

consideration the elastic, viscous, and inertial nonlinearities simultaneously [Tan et al.,

2010]. Moreover, various researchers have explored how polymer viscoelasticity affects

the diameter distribution of polymer melt-extrudate fibres and have demonstrated that

increasing the melt viscosity leads to an increase in fibre diameter but has little effect on

the diameter distribution [Tan et al., 2010]. The commonly used Phan-Thien-Tanner

(PTT) and Upper-Convected Maxwell (UCM) constitutive models assume constant shear

stress acting on the fibre surface and neglect the effects of heat transfer [Kiriakidis et al.,

1989; Tan et al., 2010; Jarecki & Lewandowski, 2009]. The K-BKZ type of constitutive

equation has been widely used in various studies on predicting the rheological behavior

of viscoelastic materials. For example, Galante used the constitutive equation to describe

viscoelastic effects in an integral equation of the K-BKZ type, suitable for polymer

solutions and melts [Altan et al., 1989]. The problem with the constitutive equation of K-

80

BKZ is that it is not fully applicable to predicting the nonlinear rheological behavior of

viscoelastic materials [Altan et al., 1989; Marders et al., 1992; Phan-Thien & Tanner,

1977].

The behavior and properties of a non-Newtonian fluid with incompressible flow are

provided by the conservation of mass and momentum equations, respectively.

5.5 Governing Equations

The governing equations for the annulus flows are presented as follows.

Continuity equation for incompressible fluids:

(5.1)

where v is the velocity vector.

Equation of momentum:

( + V. = - (5.2)

Where the density, p is the pressure, and is the stress tensor.

81

5.6 Constitutive equations

There are many numerical representations for viscoelastic models. The most common

models are Upper-Convected Maxwell (UCM), Phan-Thien-Tanner (PTT), Oldroyd-B,

Giesekus, K-BKZ and White-Metzner.

5.6.1 K-BKZ model

The K-BKZ integral constitutive equation with multiple relaxation times describes and

predicts the stresses within a fibrous suspension, solution, or molten polymer. Also, an

extra term is added to the constitutive equation to account for the extra stresses due to the

presence of fibres and to predict the orientation of a given fibre undergoing stresses

within the suspension or molten [Phan-Thien, 1978]. The motivation for developing such

a constitutive equation with these two considerations is to present an equation that can

describe the rheological behavior of polymeric fibrous solutions and moltens while also

to have a model, which can be used in numerical simulations:

τc = τf + τp (5.3)

The fibre equation is:

(γ) (5.4)

Where η(γ) is the viscosity of the polymer, γ is the shear rate, f is the fibre volume

fraction, D and L are the diameter and length of the fibre, n is the number of the

suspension, and h is the average distance from a given fibre to its nearest neighbor

82

[Wellington et al., 2007]. Dinh and Armstrong proposed the following expression to

calculate the distance between two fibres:

h = for aligned systems

h = (n )-1 for random systems

Orientation tensor:

= (5.5)

The polymer equation is:

(5.6)

Where τp is the shear stress for the polymer, λk and Gk are the relaxation times and

relaxation moduli, N is the number of relaxation modes, is the finger strain tensor,

and is its first and second invariants. H is the strain memory function, and the

following formula is used, proposed by Papanastasiou et al.:

H( (5.7)

83

For simple shear flow, the strain-memory function is given as:

H( (5.8)

( (5.9)

where are nonlinear model constants to be determined from shear and

elongation flow data, respectively. The strain-memory function in

simple shear flow is dependent on . This is expected since is viewed as

a shear-thinning parameter, while is viewed as an elongational-thinning parameter

[Phan-Thien, 1978; Wellington et al., 2007].

5.6.2 Upper-Convected Maxwell model (UCM)

The UCM model is a differential generalization of the Maxwell model for the case of

large deformations based on the upper-convected time derivative. The model can be

written as:

(5.10)

D = (5.11)

84

D is the tensor of the deformation rate

(5.12)

where λ is the relaxation time, η is zero shear viscosity, and is the upper-convected time

derivative of the stress tensor, which is expressed as:

= τ + V. τ – .τ – τ ( (5.13)

The UCM constitutive model incorporates memory effects of materials, but its viscosity

is constant at various shear rates [Giesekus, 1982; Giesekus, 1985; Mostafaiyan et al.,

2004].

5.6.3 White-Metzner Model

The White-Metzner model is derived from the network theory of polymers (White and

Metzner 1963). Modification of the viscosity and relaxation parameters as a function of

the shear rate, leads to the White-Metzner model. This model exhibits shear thinning,

not because of non affine motion, but because the relaxation is accelerated at high strain

rates, where the relaxation is faster than any deformation [Isaki et al., 1991]. The

viscoelastic differential constitutive model takes the form: τ1 + λ(γ) = 2η(γ)D, where

η(γ) can be obtained from the experimental shear viscosity curve and the function λ(γ)

can be obtained from the experimental first normal stress difference curve.

85

5.6.4 Phan-Thien-Tanner model (PTT)

The PTT model refers to a quasi-linear viscoelastic constitutive equation, which is widely

used in simulation of polymer solution flows. The original Phan-Thien-Tanner equation

was written using both of the following modifications simultaneously: the Gordon

Schowalter derivative and segment kinetics. It employs specific forms for the creation

and destruction rates of the network junctions in the network theory of Lodge and

Yamamoto [Yarin et al., 2010; Sebastien et al., 2011]. Although the Phan-Thien Tanner

model overpredicts both the shear viscosity at higher shear rates and the transient and

extensional properties, it accurately predicts the zero shear viscosity and seems suitable

for numerical simulations of polymer melts. It is worth noting that, compared to integral

models such as the Bird-Carreau and Wagner models, differential models such as the

PTT model provide robust numerical algorithms and exhibit good behavior in FEM

simulations [Yarin et al., 2010]. The interested reader is referred or more accurate

comparison between the K-BKZ (as integral model) and the PTT model.

In the PTT model, the extra stress tensor is considered as the sum of the viscoelastic

component , and the purely Newtonian component .

+

in which is given by:

86

= 2 D (5.14)

where D is the strain rate tensor.

The complete form of the PTT constitutive equation for the viscoelastic component is:

f(trτ) τ + λ + λ(γτ + τγ) = η (5.15)

or

+ ) + (5.16)

where the Oldroyd’s upper convective derivative is defined by:

= - . (5.17)

where V represents the velocity matrix, is the transpose of the velocity matrix, and

D(V)/Dt is the material derivative of the velocity matrix.

Analyzing the expression above, the first term represents the stress tensor transport and

the transient part of the flow [Sebastian et al., 2011]. In the second term, the slipping

between the fluid polymeric chains is computed. The third term includes the elastic

effects. Finally, the term on the right side of the equality represents the diffusive effects:

87

τ is the stress tensor, D the deformation-rate tensor, λ the fluid relaxation time, and G the

relaxation module. The parameter ξ controls the amount of movement between the fluid

polymeric chains [Oldroyd J.G, 1950]. For ξ = 0 the model is named PTT Affine and the

slipping between the polymeric chains is neglected. The function Y depends on the rate of

creation and destruction of the links between the chains. Moreover, the PTT model

incorporates the memory effect of materials and its viscosity can vary with the change of

the shear rate. When ξ is zero, PTT constitutive equation reduces to its simplified form

(SPTT) [Yarin et al., 2010; Sebastien et al., 2011]:

f(tr(τ))τ + λ = η (5.18)

Phan-Thien and Tanner assumed specific forms for the creation and destruction rates of

the network junctions and derived a constitutive equation containing two free parameters,

and ξ [Cirulis, 2010]. The exponential constitutive model takes the following form:

exp + = 2 (5.19)

and are the adjustable parameters of the model.

The parameters η and λ are the viscosity and relaxation time respectively, measured from

the equilibrium relaxation spectrum of the fluid. They are not considered as adjustable

parameters of the model. The PTT model can be solved using a single relaxation time or

multiple relaxation times, similar to the Giesekus model. The linear form of the PTT

88

model predicts shear thickening at high elongational rates, after which a plateau is

reached [Isaki et al., 1991; Sebastian et al., 2011].

5.6.5 Giesekus-Leonov model

Giesekus proposed a constitutive model based on a concept of configuration-dependent

molecular mobility. In this model, the viscoelastic component of the extra stress tensor is

represented with the following parameters . Owing to the highly nonlinear

nature of the model equations, all of the properties need to be obtained numerically.

+ - + = 0 (5.20)

The α parameter is the dimensionless Giesekus-model mobility factor and controls the

extensional viscosity and the ratio of the second normal stress difference to the first one.

The dimensionless Giesekus-model mobility factor used to evaluate the anisotropic drag

is represented by 0 < α < 1. For α = 0 the model becomes the isotropic UCM model,

while for α =1 the model is merely an anisotropic drag, and for α > 0 the model

represents shear-thinning behavior. The Giesekus model predicts the tension-thickening

region for elongational flow, after which a plateau is reached; but it shows the existence

of a tension-thinning region at high strain rates [Luo & Tanner, 1987; Chauvière &

Owens, 2002; Cirulis et al., 2910; Oldroyd, 1950].

89

5.6.6 Oldroyd-B Model

The Oldroyd-B model is a principal form of viscoelastic model:

- 2 (d - = 0 (5.21)

where d is the rate of deformation tensor, is the shear viscosity, is the relaxation

time, and is the second relaxation parameter [Oliveira, 2009; Byron et al., 2011;

Trebotich et al., 2005]. The Oldroyd-B model is mostly used to describe the rheological

characteristics of polymer liquids composed at low concentration and moderate shear

rates for high-viscosity Newtonian molecular-weight polymer.

1) At the model simplifies to a second-order fluid with a vanishing second normal

stress coefficient.

2) At the model reduces to the convected Maxwell model.

3) At the model reduces to a Newtonian fluid with viscosity

5.7 Conclusion

Understanding the complex behaviour of polymer materials and interpreting it as one

general equation requires a vast knowledge of the characteristics and formation of this

90

complex type of material. Since rheometers do not provide the necessary information for

all important rheological properties, constitutive equations are the best available tools for

effective process control.

The development of valuable models for composite behavior and the exploration of

appropriate constitutive equations to describe this complex behavior has been a high

priority for many researchers. However, understanding the rheological behavior of

viscoelastic composites is a redoubtable challenge.

The K-BKZ, PTT, Oldroyd-B, and Giesekus models was widely studied for future

implementation works. These models can be applied to the prediction and determination

of the shear viscosity of viscoelastic composites as a function of shear stress and shear

rate during extrusion and injection moulding processes.

91

CHAPTER 6

Modeling the Rheological Characteristics of Flexible

High Yield Pulp-Fibre–Reinforced Bio-Based Nylon

11 Bio-Composite

6.1 Abstract

The aim of this work was to develop a mathematical model to investigate the rheological

characteristics of viscoelastic pulp fibre composite materials. The rheological properties

of High Yield Pulp (HYP) reinforced bio-based Nylon 11 (Polyamide 11 or PA11)

composite (HYP/PA11) were investigated using a capillary rheometer. Novel predicted

multiphase rheological-model–based polymer, fibre, and interphasial phases were

developed. Rheological characteristics of the composite components influence the

development of resultant microstructures; this in turn affects mechanical characteristics

of a multiphase composite. The main rheological characteristics of polymer materials are

viscosity and shear rate. Experimental and theoretical test results of HYP/PA11 show a

steep decrease in apparent viscosity with increasing shear rate, and this melt-flow

characteristic corresponds to shear thinning behavior in HYP/PA11. The nonlinear

mathematical model to predict the rheological behavior of HYP/PA11 was validated

experimentally at 220˚C and 5000S-1 shear rate. Finally, predicted and experimental

viscosity results were compared and found to be in strong agreement.

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6.2 Introduction

The study of the rheological behavior of viscoelastic polymer composites is mostly

limited to a two-phase fibre-polymer. The so-called interphase zone appears in the

viscoelastic damping of polymer composite processing [Pervaiz & Sain, 2003]. This

deformation significantly affects the predicted overall rheological characteristics of

natural-fibre–reinforced thermoplastic composites [Bourmaud & Baley, 2009]. During

processing, fibre-reinforced polymers are subjected to rigorous deformations that cause

fibres to translate, agglomerate, bend, and rotate with the flow of the fibre matrix [George

et al., 2001; Gu & Kokta, 2010; Bajpai, 2010; Thomen, 2001; Plackett et al., 2010;

Uhlherr et al., 2005]. This strongly influences the rheological and mechanical properties

in different parts of the final product because of the close dependence of these properties

on the orientation state of the fibres. Likewise, rheological properties that are a function

of the flow-induced fibre configuration in the matrix also influence the physical

properties of fibre-reinforced polymer composite [Liu et al., 2000; Kaw & Besterfield.,

1998].

The effect of the interphasial zone has been already studied by Kaw and Besterfield

[1998] and Yeh [1992] as a third constituent of the predictive models with respect to the

elastic behavior of fibre-reinforced polymer composites. Gohil and Shaikh [2010] and

Kari et al. [2005] have investigated the interphasial effect in wood-fibre-reinforced

polymer composites. It was demonstrated that the interphase, considered as a

homogeneous and isotropic material, has a significant effect on the loss factor and on the

prediction of the elastic properties of three phases: fibre, interphase, and polymer

93

composite. Other work by Lamnawar and Maazouz [2008], and Larache, Agbossou, and

Pastor [1994] used theoretical and experimental approaches based on shear lag and shear

modulus to show the role of the interphase in the elastic properties of composite

materials. However, no studies have been found in the literature that consider these three

phases in investigating the effect of the interphase on prediction of the rheological

characteristics of viscoelastic pulp-fibre–reinforced thermoplastic polymer composites.

The rheological characteristics of such composites are vital to their final mechanical

properties. Although natural-fibre–reinforced polymer composites and their processing

have been partially reviewed in several papers [Lamnawar, & Maazouz, 2008; Larache et

al., 1994; Deshpande, 2004], models of their rheological behavior and analysis of the

rheology–processing parameter relationships have been neither investigated nor reported.

This study reports on the state-of-the-art technology in the rheology of biocomposites

from green-fibre polymers, including their viscoelasticity and complex rheological

behaviors as influenced by different conditions. Hence, an overview of the viscoelastic

properties of biomaterials derived from pulp-fibre-reinforced thermoplastic polymers is

presented in order to analyze their rheological behavior as part of predicting the viscosity

of polymer melts.

Giesekus proposed a constitutive model based on a concept of configuration-dependent

molecular mobility [Lamnawar, & Maazouz, 2008; Larache et al., 1994; Hosseinalipour

et al, 2012]. In this model, the viscoelastic component of the extra stress tensor is

represented with the parameters ; due to the highly nonlinear nature of the

94

model equations, all the properties need to be obtained numerically. Moreover, this

model is only able to predict low load and low shear rate in the presence of fibre.

The purpose of this study, therefore, is to present experimental and predicted results of

HYP/PA11 and to validate this novel rheological modeling approach. First, we fully

explored the viscoelastic-polymer-based Giesekus constitutive model. Second, we

introduced the fibre and interphasial phases in function of fibre diameter and aspect ratio

to the Giesekus model, considering the effect of fibre entanglement and agglomeration on

the variation of the viscosity with shear rate. Finally, the model was validated by

obtaining the experimental data needed to evaluate the model’s predictions.

6.3 Mathematical model

6.3.1 Governing Equations

The equations governing the flow are the mass and momentum conservation equations.

0 (1) (6.1)

(6.2)

where v is the velocity vector, ρ the polymer melt density, τ the polymeric extra stress

contribution, p the pressure, and g the gravitational velocity.

For polymer melts exhibiting Newtonian rheology, τ takes the form

g

95

= 2μD,(3) (6.3)

where μ is the melt viscosity and D = (∇u + (∇u)T)/2 is the rate-of- strain tensor. For

viscoelastic polymer melts, the stress tensor depends on the deformation history.

6.3.2 Assumptions and boundary conditions

The model was designed on the assumption that it should describe rheological behavior

as a function of the rate of deformation according to different conditions like fibre

flexibility, interphasial zone and aspect ratio, and anisotropic parameter. In addition, to

obtain a closed-form analytical expression for the velocity distribution as a function of

the viscoelastic parameters, the following assumptions and boundaries conditions were

introduced:

The flow of fibre-reinforced thermoplastic composites is assumed to be an

incompressible fluid during the extrusion and injection molding processes;

The velocity field is steady and fully developed, i.e., u = u(y), v = 0;

The isothermal viscosity laws are applied, i.e., the viscosity is only a function of shear

rate;

The components of the composite are anisotropic, then nonlinear viscoelastic models;

The flow is in a closed system and is driven only by applying load (zero pressure

gradient, i.e., ∇P = 0);

96

Given the high viscosity of the polymer melt, inertia is negligible.

6.3.3 Model development

A single Giesekus model derived from a Maxwell element would be sufficient to model

the observed relaxation-time behavior, the shear stress, and the viscosity of polymer

material. However, the strong nonlinear viscoelastic expression of shear viscosity in

function of high shear rate of HYP-fibre-reinforced polymer composite required the

extension of the model to include more parameters. Hence, in addition to the polymer

phase, the fibre and interphase phases have been formulated into the Giesekus base

model.

The literature offers various concepts for modeling change in the viscous properties. It

has been pointed out that only the viscous overstress and the strain rate are suitable

dependencies for formulating a viscosity function.

Figure 6.1. HYP fibre of length L before load is applied

Figure 6.1 shows the surface area of the flexible HYP at different points. Assuming that

the fibre is flexible, when shear is applied the fibre is agglomerated at the point of

diameter “D”.

L

97

The surface area of fibre before agglomeration is maximum, and is calculated as follows:

AMax = πDL (6.4)

At the agglomerated point, the surface area is minimum, and we obtain:

AMin = πD²∕4 (6.5)

Assuming that the composite material has three phases and that the fibre is

flexible, the total stress or the stress of the composite is represented as:

(6.6)

The shear stress of the polymer phase:

(6.7)

where α is a model parameter attributed to anisotropic Brownian motion or anisotropic

hydrodynamic drag on the constituent polymer molecules. It is required that 0 < α < 1 as

discussed by Giesekus [1982].

The shear stress of the fibre phase:

(6.8)

For

98

(6.9)

The shear stress at the interphasial phase:

(6.10)

By rearrangement, we have:

(6.11)

Substituting equations 7, 9 and 11 into (6) and rearranging it, we arrive at:

(6.12)

Predicting the shear viscosity during the extrusion process and using the capillary

rheometer involves a coupled analysis of flow, shear rate, aspect ratio, and interphasial

effect. The resultant equation is highly nonlinear due to the viscosity, which depends on

the shear rate, fibre aspect ratio, and process parameters; these nonlinearities have been

solved by the Newton-Rapson method of numerical approach.

99

6.4 Materials and Methods

6.4.1 Materials

The matrix biopolymer bio-based Nylon 11, density 1.03, MFI 11, was supplied by

Arkema, France. Aspen HYP fibres were supplied by Tembec (Montreal, QC). The HYP

is the type used in wood-free printing and in writing-paper grades and multiple-coated

folding-board grades; fibre length is 0.230 to 0.85mm. Finally, the pulp-fibre length was

reduced by using a mechanical crib in order to investigate the aspect-ratio effect on the

rheological behavior of the HYP/PA11.

6.4.2 Experiment

The experiment was processed using a conical twin extruder. In the mixing method, the

HYP fibre was dried at 80˚C for 6 hours and then added to the corresponding PA11 and

well mixed before it was introduced to the extruder. The average temperature of the

barrel was 200˚C.

6.4.3 Rheological Measurements

The rheological measurements of the composites’ melt-flow properties were carried out

in a twin-bore Rosand Capillary Rheometer model RH2000. (The standard RH2000 range

supports temperatures from -40˚C to 500˚C. The standard maximum force applied is

12kN.) The composite samples for testing were cut into very small pieces, then placed

inside the barrel and forced down into the capillary with the plunger attached to the

moving cross-head. Representative steady-shear viscosity versus high shear rate is

presented in the figures below for HYP/PA11, which was processed at the average

100

extrusion temperature of 200˚C. The apparent viscosity of the sample was obtained from

steady-shear measurements for different fibre aspect ratios, with the rate ranging from 50

to 5000 S-1. The rheology viscosity data presented in this paper represent an average

value of three measurements.

6.5 Results and discussion

The novel nonlinear viscoelastic material model was explored in order to simulate the

nonlinear rheological behavior of HYP/PA11. In this study, both experimental and

predictive results on HYP/PA11 were investigated in order to understand the power,

validity, and capabilities of the novel rheological model developed. The detailed

rheological properties are the major focus of this work.

6.5.1 Experimental Results

Rheological characteristics of the polymer, fibre, and interphasial phases influence the

final characteristics of the resultant microstructure of composite materials; these

characteristics in turn affect the mechanical properties of a multiphase polymer

composite system. Experimentally, the apparent viscosity as function of the steady-shear

rate of HYP/PA11 at 200˚C is shown in Figure 6.2. (As noted, these results are the

average of three different experimental tests.)

101

Figure 6.2. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

As noted, the composite material used in the experimental study had a fibre length of

0.73 mm, and the experiment was conducted at 200˚C. The experimental results showed

that the apparent viscosity of HYP/PA11 composite decreases with increasing shear rate.

This decrease in the shear viscosity with the increase in shear rate corresponds to the

pseudoplastic fluid characteristic of the composites. This pseudoplastic behavior (also

referred to as shear-thinning behavior) plotted in Figure 6.2 is mainly influenced by the

orientation of the polymer molecules, the agglomeration of the flexible fibre, and the

entanglements within the polymer chains in the capillary rheometer. Due to the flexibility

of HYP, when shear is applied to the material, the orientation of the fibre changes its

configuration from rectangular to spherical; such modifications are produced in the fibre

length and the surface area of the fibre once the orientation is completed at high shear

rate. On the other hand, the chain agglomerations are produced simultaneously with

collapsing one chain onto another chain. The entanglement of the chains followed by

0

100

200

300

400

500

600

700

800

0 1000 2000 3000 4000 5000

Sh

ea

r vis

co

sit

y (

Pa

.S)

Shear rate (s-1)

102

agglomerations, as well as the entanglements within the chains, are caused by the

Brownian motions and low relaxation of HYP. The high shear-thinning behavior obtained

for HYP/PA11 can be also associated to the thermal degradation of HYP during the

rheological testing. The molten polymers tend to arrange themselves with their major

axes in the direction of shear, and thereby points of entanglement are reduced. As a

result, the viscosity decreases. In other words, in this case of non-Newtonian flow

behavior of polymer melts, the decrease in viscosity when the shear rate is increased by

applying load is associated with high shear-thinning behavior and with viscoelastic

characteristics of biocomposite materials. However, at very high shear rates (from 3000

to 5000 S-1), the molten HYP/PA11 showed a less restrained decrease in apparent

viscosity. Such high decrease in the shear viscosity is associated with high shear-thinning

behavior. This non-Newtonian behavior is associated with the alignment and orientation

of the fibre in the polymer chains and the effect of the fibre aspect ratio. At low as at high

shear rates, the formation of agglomerates is evident; therefore, HYP molecules are

completely oriented due to the good green-HYP dispersion in the bio-based PA11 matrix.

This means that the breakage of the fibre length allows the maintenance of an accurate

fibre-aspect ratio when the diameter of flexible HYP is kept unchangeable during the

process. The shorter length of the fibres will also support their alignment in the direction

of the flow, thus reducing the fibre-fibre collisions and leading to a larger decrease in the

apparent viscosity.

103

6.5.2 Variation of the viscosity with a function of shear rate of

HYP reinforced PA11 at various temperatures

The variation of the apparent viscosity in function of the shear rate of HYP-reinforced

PA11 at various temperatures was investigated; the rheological test results are presented

in the figure 6.3. The rheological conditions were kept constant while different tests were

run for 190˚C, 200˚C, and 210˚C.

Figure 6.3. Variation of the viscosity with a function of shear rate of HYP/PA11 at various temperatures.

0

200

400

600

800

1000

0 1000 2000 3000 4000 5000

Sh

ear

vis

co

sit

y (

Pa.s

)

Shear rate (s-1)

190 degree

200 degree

210 degree

104

The apparent viscosity of the HYP/PA11 depended on the rate of shear at which it was

measured and presented. The shearing effects decreased as the temperature increased;

that is, the HYP/PA11 became more non-Newtonian in the higher temperature region. At

higher temperature the reduction of the shear viscosity was more pronounced at

intermediate shear rate, while for 190˚C, the reduction of the shear viscosity reached a

maximum at higher shear rates (from 3000 to 5000 S-1). This characteristic is due to the

fact that the HYP-reinforced PA11 fibres were agglomerated and entangled at low

temperature and low and intermediate shear rate; this made flow deformation difficult.

(At this point this rheological behavior is called near-Newtonian.) At high shear rate, the

shearing effects increased while the effect of temperature was less pronounced, and flow

deformation was mostly dominated by the shearing effect. However, from 3000 to 5000

S-1 all the apparent viscosity variations in function of shear rates followed the same rate

of deformation for different temperature profiles; this corresponds to shear-thinning

behavior.

6.5.3 Effect of fibre content on the rheological behavior of

HYP/PA11

The effect of the fibre content on the rheological characteristics of the composite was

investigated. Figure 6.4 shows the experimental results for 10%, 20%, and 30%

HYP/PA11. These curves are typical of pseudoplastic materials, which show a decrease

in viscosity with increasing shear rate. At high fibre content, the material offers higher

shear viscosity even for high shear rate. In general, the incorporation of fibres in polymer

systems increases the viscosity and increases further with fibre content.

105

Figure 6.5. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

The difference is not very significant for 10% and 20% fibre for intermediate and high

shear rate. At low HYP content, the shear viscosity was expected to increase rapidly with

increasing concentrations of the fibres because of the rapidly increasing interactions

between particles as they become packed more closely to each other. Nevertheless, at

very high fibre content, random anisotropic structures of fibres in the polymer melt were

created. The increase in shear viscosity was found to be more predominant at lower shear

rates where fibre and polymer molecules were not completely oriented.

6.5.4 Effect of the fibre aspect ratio on the rheological property

The results from the study of the effect of the aspect ratio of flexible-pulp–fibre-

reinforced bio-based Nylon 11 are presented below in Figure 6.5.

0

200

400

600

800

1000

0 1000 2000 3000 4000 5000

Sh

ea

r vis

co

sit

y (

Pa

.s)

Shear rate (s-1)

10%HYP/PA11

20%HYP/PA11

30%HYP/PA11

106

At low fibre aspect ratio, the decrease in viscosity as a function of the shear rate was

greater for both low and high shear rate. Contrarily, at higher fibre-aspect ratio the shear

viscosity shows a moderate decrease for low and intermediate shear rate.

Figure 6.6. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

At low and intermediate shear rate, the viscosity curves are slightly decreased and the

distance between each viscosity curve remains large. However, at high shear rate the

viscosity plots are tightly close. This is because fibre agglomeration and entanglement are

not pronounced at high shear rate or frequency, due to the complete orientation of the

fibre and polymer molecules. Much as with high fibre content, the increase in shear

viscosity is found to be greater at lower shear rates, where fibre and polymer molecules

are not completely oriented.

0

200

400

600

800

1000

1200

1400

0 1000 2000 3000 4000 5000

Shear Rate (s-1)

Ar1 Ar3 Ar2

107

6.5.5 Predicted results

In this section, the numerical plot below shows the results of the mathematical predictive

model for representing the rheological characteristics of the material and the influence of

processing conditions on the material’s resultant microstructure.

To verify the applicability of the formulated viscoelastic-material model, certain

rheological tests were simulated and compared with the experimental data. The

comparisons show that the newly developed rheological viscoelastic-material model is

capable of simulating not only the dependence of shear viscosity on the material’s shear

rate, but also the variation in the slope of the material’s rheological responses. The shear

viscosity curve observed for HYP/PA11 is depicted in Figure 6.6 for 30% HYP fibre.

Figure 6.6 shows that HYP/PA11 biocomposite acts as a pseudoplastic fluid and that the

shear viscosity plots have a tendency to decrease for high shear rates. The various

assumptions considered in these models, in fact, were well fitted to the fluid’s actual

behavior. The fibre-agglomeration effect considered when the model was first conceived

is a primary contribution to the originality of this study. Accordingly, both predicted and

experimental results showed how the flexibility and entanglement of HYP fibre

contribute to the change in the fibre’s total surface area and the resultant influence on the

rheological characteristics of HYP/PA11composite.

108

Figure 6.7. Prediction of shear viscosity vs. shear rate of HYP/PA11at 200˚C

The entanglement and agglomeration of the fibre are manifested at high shear rate, where

the decrease in the fibre’s surface area increases the material’s shear viscosity and

consequently shows a non-Newtonian behavior. Figure 6.6 demonstrates that the

flexibility and diameter of the pulp fibres affects the viscoelasticity of the resultant

composite. Another observation is that the interphase interaction also influenced the

viscosity behavior with the changing shear rate of HYP/PA11, together with the change

in the aspect ratio, which was mostly present in the interphase. The interphase interaction

in pseudoplastic fluid behavior is significant in the study of the shear viscosity with the

increase in shear rate of high-fibre–content reinforced polymer composite. The observed

decrease in the shear viscosity of composite material from the plot map of the predicted

model is associated with the high degree of pseudoplasticity due to perfect dispersion of

high-yield pulp fibre into the composites; this gives rise to the good reinforcing effect of

HYP on PA11.

0

100

200

300

400

500

600

700

800

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Shear rate (s-1

)

Sh

ea

r v

isc

os

ity

(P

a.s

)

109

6.5.6 Modeling results versus experimental results

In this section, the comparisons between the theoretical predictions and experimental

measurements are detailed. In the experiments, the shear viscosity was determined to be a

function of high shear rate while polymer flow rate and processing temperature were kept

constant. Experimental and predictive results from HYP/PA1 are shown in Figure 6.7.

The average shear viscosity at the initial shear rate was about 800 Pa. s for 200˚C for low

fibre aspect ratio and about 1000 Pa. s for intermediate and high fibre aspect ratio, and at

this point the melt exhibited both experimentally and predictively a shift from Newton to

non-Newtonian rheological characteristics. However, as the shear increased, HYP/PA11

showed highly non-Newtonian behavior, corresponding to high shear-thinning behavior.

The accuracy of the predictive model derived from the experimental testing results and

used for calculating the previous shear viscosity was high. Experimentally and

mathematically, the shear viscosity as function of steady-shear rate of HYP/PA11 at

200˚C is shown in Figure 6.7 (where Exp. = experiment and M. = modeling): Ar1, Exp.

Ar2, and Exp. Ar3 represent the fibre aspect ratio 1, 2, and 3; and Ar1 < Ar2 < Ar3.

110

Figure 6.8. Shear viscosity vs. shear rate of HYP/PA11 at 200˚C.

Despite the discrepancy between the values for viscosity obtained by the predictive

model and those from experiments from 2000 to 2500 S-1, the agreement between the

model’s predictions and experimental data is remarkable given the simplicity and

accuracy of the model, the exactitude of the material properties and parameters, and the

fitness of the proposed boundary conditions. At both low and high shear rate, good

agreement is evident from the results; hence, this novel mathematical predictive

rheological model is well fitted to the extrusion of viscoelastic biocomposite materials.

From 3000 to 5000 shear rate, both experimental and predictive results show a regular

decrease in the shear viscosity, which may be due to the fact that fibre distribution and

orientation are complete, and that the viscosity is mainly dependent on the mobility of the

polymer chains in the composite structure. The model’s plot of shear viscosity versus

shear rate is evidence that the model is applicable to mathematically representing the

0

200

400

600

800

1000

1200

0 1000 2000 3000 4000 5000

Shea

r vi

sco

sity

(P

a.s)

Shear rate ( S-1)

Exp. Ar1 Exp. Ar2 Exp. Ar3

M.Ar1 M. Ar2 M. Ar3

111

rheological behavior of a variety of natural-fibre–reinforced thermoplastic viscoelastic

composite materials.

6.6 Conclusions

In this study, a novel rheological model for viscoelastic materials was developed in order

to predict rheological properties and then compared with experimental results on

HYP/PA11. This nonlinear rheological model was developed for constant material

parameters, simultaneously considering viscous, elastic, and inertial nonlinearities and

interphasial phase variables and parameters. The predictive results indicated that the

developed model well supports the determination of rheological characteristics of the

investigated material, such as viscosity and shear stress. In addition, the rheological

model was plausibly demonstrated and validated based on the experimental results of

shear viscosity versus shear rate. Both experimental and predictive results showed high

shear-thinning behavior in HYP/PA11 associated with a high degree of pseudoplasticity

due to the good dispersion of HYP into PA11 and the orientation of the flexible fibre

effects in the direction of the molten PA11. The model was validated for different fibre

aspect ratios and high shear rate, up to 5000 S-1. Due to its consistency and its high

predictive ability, the model may be applied in rheological studies and investigations of

viscoelastic materials, particularly in the automotive, construction, and aerospace

industries.

112

CHAPTER 7

Conclusions and Recommendations

7.1 Conclusions

Stress and viscosity relation of high-yield-pulp fibre–reinforced bio-based PA11

composite by both the extrusion process and the batch mixing process has been

studied.

Fibre burning was observed as the fibre content increase from 10% to 20 and 30% using

the extrusion process.

A co-factor, 3% inorganic salt such as lithium chloride, was used to decrease the melting

temperature of PA11 from 188 ˚C to 175˚C.

Addition of the co-factor, LiCl, increased shear viscosity and enabled low temperature

processing.

The viscosity of HYP/PA11 at high shear rate have been investigated and found to be

increased for higher fibre content and aspect ratio.

The rheological tests were performed at very high shear rate (5000/S) and at steady

state. With both processing techniques used and with the different fibre parameters,

shear-thinning behavior was present in all formulations used during this thesis research

study.

A novel mathematical model was developed to predict the rheological characteristics of

viscoelastic materials. The model was validated experimentally for established boundary

conditions and assumption.

113

Shear-thinning behavior was observed experimentally and mathematically. This shear-

thinning behavior or pseudoplastic property, which corresponds to non-Newtonian fluid

behavior, was more pronounced at high shear rate.

7.2 Scientific and engineering contribution of this thesis

During the past decade, scientific computation has become increasingly important in

applied mathematics, providing insight into many problems that would preclude a simple

analytical description. Since biomaterials exhibit a complicated viscoelastic behavior that

cannot be predicted using a Newtonian approach, and since they show both atomistic

effects and macroscopic ones, they are ideally suited to being studied by means of non-

Newtonian models. At the most general level, this thesis has made use of applied

mathematical models from a variety of different perspectives to examine aspects of flow

characteristics and the effect of the pulp-fibre aspect ratio under shearing effects that

would be difficult to address either by pure theory or by experiment.

The final objectives of this work were to develop lightweight composites from HYP-

fibre–reinforced bio-based nylon11 for automotive applications in order to decrease the

weight of vehicles and consequently decrease their fuel consumption and emissions.

7.3 Study limitations

Processing HYP-fibre–reinforced bio-based PA11 composite through the extrusion

process is challenging, and there is not much in the literature about this problem.

This model can be only used for multiphase flexible fibre reinforced thermoplastic

polymer composites.

114

Numerical simulations of the rheological-characteristic studies of flexible multiphase

natural-fibre–reinforced bio-based thermoplastic composites using the Navier-Stokes

governor equation have not been developed to help in the conception of the

mathematical model.

7.4 Recommendations

1. Further attempts should be made to accelerate the production of green composite fibres

from pulp-reinforced bio-based polyamide composite materials.

2. A more detailed study is required in order to better understand the formation

mechanism of HYP-fibre–reinforced PA11 composites.

3. The mechanical performance of high-yield-pulp–reinforced bio-based PA11

composites should be studied to better understand their applicability as structured

composite materials for the automotive and aerospace sectors.

4. The model should be more refined and simple in order to widen exploration for the

prediction of viscoelastic properties of composite materials.

5. Finally, the novel predicted model should be used for the simulation and optimization

natural fibre reinforced bio-based polymer materials processing.

115

7.5 Publications and Conferences

Articles published

Cherizol, R., Sain, M. and Tjong, J. (2015) Modeling the Rheological Characteristics of

Flexible High-Yield Pulp-Fibre-Reinforced Bio-Based Nylon 11 Bio-Composite.

Journal of Encapsulation and Adsorption Sciences, 5, 1-10.

http://dx.doi.org/10.4236/jeas.2015.51001.

Cherizol, R., Sain, M. and Tjong, J. (2015) Review of Non-Newtonian Mathematical

Models for Rheological Characteristics of Viscoelastic Composites. Green and

Sustainable Chemistry, 5, 6-14.

http://dx.doi.org/10.4236/gsc.2015.51002.

Cherizol, R., Sain, M. and Tjong, J. (2015) Evaluation of the Influence of Fibre Aspect

Ratio and Fibre Content on the Rheological Characteristic of High-Yield Pulp-

Fibre–Reinforced Polyamide 11 “HYP/PA11” Green Composite. Open Journal of

Polymer Chemistry, 5, 1-8. http://dx.doi.org/10.4236/ojpchem.2015.51001.

Cherizol, R., Sain, M. and Tjong, J. (2016) Effect of Lithium Chloride on the Fibre

Length Distribution, Processing Temperature and the Rheological Properties of

High-Yield-Pulp-Fibre–Reinforced Modified Bio-Based Polyamide 11

Composite.

Conference: Oral presentation

13th International Symposium of Bioplastics, Biocomposites and Biorefining (ISBB)

Guelph 2014.

116

Predicting the Rheological Behavior of Flexible Bleached Chemithermomechanical Pulp

Reinforced Nylon 11 Green Composite.

117

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