Polymer composite materials based on coconut fibres

For basic mechanical properties, flax, hemp, bamboo and jute can contribute their high strength and stiffness to composites, while coir with high elongation to failure can ameliorate the

POLYMER COMPOSITE MATERIALS

BASED ON COCONUT FIBRES

SUBTITLE OF THE PHD

Le Quan Ngoc TRAN

Dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Engineering

December 2012

Members of the Examination

Committee:

Prof Paul Sas, Chair

Prof Ignace Verpoest, Promoter

Dr Aart Willem Van Vuure, Promoter

Prof Christine Dupont-Gillain

Prof Jin Won Seo

Prof Bart Blanpain

Prof Peter Van Puyvelde

Prof Stepan Lomov

© 2009 Katholieke Universiteit Leuven, Groep Wetenschap & Technologie, Arenberg Doctoraatsschool, W de Croylaan

6, 3001 Heverlee, België

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All rights reserved No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other means without written permission from the publisher

ISBN 978-94-6018-615-8

D/2013/7515/3

Cover image: SEM image of fracture surface of coir epoxy composite, showing defibrillation of the coir fibres

Acknowledgements

My career in composite materials started from a great occasion when I met Prof Ignace Verpoest in Vietnam in 2001 He opened a wide door for me to enter into the interesting field of composite materials by receiving me into the Master program EUPOCO Since that time, I have learned from him not only valuable knowledge and experience but also his kindness in supporting people In the past four years of my PhD,

as my promoter, Prof Ignace Verpoest has provided me patient guidance, enthusiastic encouragement and useful inputs for this research work I would like to take this opportunity to express my deepest gratitude to him for all his supports

I would like to express my great appreciation to my co-promoter Dr Aart W Van Vuure for his knowledge, advice and available time for guiding me Working in the natural fibre composites group, I enjoy very much both his leadership and friendship I also thank for his patience in correcting my papers and the first draft of this dissertation

I would like to offer my special thanks to Prof Christine Dupont-Gillain for her kind help in building up the method for wetting measurement of natural fibres, and giving useful comments and inputs for papers and the thesis manuscript

My grateful thanks are extended to Prof Stepan Lomov, Prof Bart Blanpain and Prof Peter Van Puyvelde, as members of advisory committee and examination committee, for their advice on my research, reading and providing valuable remarks for the manuscript I would like to thank other members of the jury, Prof Jin Won Seo, for her effort to read my thesis and evaluate my work and Prof Paul Sas for being the chairman

of my thesis defense

The four years research consisting of many experiments and testing would have never been successful without the technical assistance of Kris Van de Staey, Bart Pelgrims, Manuël Adams, Danny Winant, Sylvie Derclaye, Yasmine Adriaensen and Michel Genet I greatly appreciate their help Additional thanks to Gregory Pyka for his training on using SEM-CT I would also like to thank Aniko Lantos, Huberte Cloosen and other MTM secretaries for their kind help in important administrative work

My warmest thanks to CMG friends who are always willing to offer me their help, especially, my officemates Carlos Fuentes, Lina Osorio, Eduardo Trujillo, Ichiro Taketa and Yasmine Mosleh for sharing nice time (in and out the office) during these years The thesis work of Linde De Vriese, Elisa Melcon Miguel, Delphine Depuydt and Laurena Van Oproy is contributed to this work They have had high motivation in working with coir fibre composites, and obtained good initial results which help to have further studies in this thesis Thank you very much

My acknowledgements are addressed to KU Leuven for providing I.R.O Scholarships and Belgian Science Policy Department (BelSPO) for supporting our research I also wish to thank the staff involved in the BelSPO-MOST project Prof Bui Chuong, Dr Truong Chi Thanh for their advice and providing the fibres for the research

My family and I would have never had such a nice life in Leuven without the care and support of Belgian and Vietnamese friends I am really thankful to Mr Jo Mariën and

his wife Claire Mariën Con cảm ơn Chú Thiếm Kim rất nhiều về sự quan tâm giúp đỡ con và gia đình trong suốt thời gian ở Bỉ Cám ơn các chiến hữu Cần Thơ đã giúp đỡ

và chia sẻ vui buồn những lúc xa quê

Finally, I want to express my deepest thank to my parents for their support and encouragement throughout the years My special thanks go to my wife Loan and my little daughter Au Lam who was born in Leuven, for their love and support They are the driving force in my life This thesis is dedicated to them

Abstract

The interest in using natural fibres in composite materials has greatly increased over the past decades thanks to their good mechanical properties in combination with environment-friendly characteristics In this research, Vietnamese coir fibres are studied and modified for use in composite materials To be efficiently used in composite materials, the microstructure and the mechanical properties of coir fibres are first characterised Secondly, the surface of natural fibres has a complex morphology with chemical heterogeneity and relatively high roughness, which strongly influences the fibre-matrix interfacial adhesion Therefore, it is important to acquire a systematic understanding of the fibre-matrix interfacial interactions in composites Lastly, unidirectional (UD) composites of coir fibre in both thermoplastic and thermoset matrices are examined to evaluate the possible value of coir fibre for composites

The microstructure of technical coir fibres is examined using SEM and SEM-CT The results show that technical coir fibres comprise plenty of elementary fibres and

a lacuna at the centre The elementary fibre is built up by two main cell walls which consist of bundles of microfibrils aligned in a high angle to the fibre axis Coir fibre appears to have high porosity at 22 to 30% The mechanical properties of coir fibre are determined in tensile tests including single fibre tensile testing with optical strain mapping and single fibre tensile testing using different test lengths The results of both methods indicate that coir fibres are not very strong and stiff, but have high strain to failure

An integrated physical-chemical-micromechanical approach is implemented to investigate the fibre-matrix interfacial compatibility and adhesion of the coir fibre composites In this study, the interface between untreated and alkali treated coir fibres and various thermoplastics is characterised The differences of fibre surface chemistry and properties of the matrices in terms of surface energy and potential chemical reactions are considered Wetting measurements of the fibres and the matrices are carried out to obtain their static equilibrium contact angles in various liquids, and these are used to estimate the surface energies comprising of different components The work of adhesion is calculated for each composite system, accordingly Also, fibre surface chemistry is examined by X-ray photoelectron spectroscopy (XPS) to have more information about functional groups at the fibre surface, which assists in a deeper understanding of the interactions at the composite interfaces To determine the quality of the composite interfaces, single fibre pull-out tests and transverse three point bending tests are performed on UD composites to

measure interfacial shear strength and interfacial strength (mode I) respectively The results suggest that the higher interfacial adhesion of coir fibres with polyvinylidene fluoride compared with polypropylene can be attributed to higher fibre-matrix physico-chemical interaction corresponding with the work of adhesion Whilst the improvement of interfacial adhesion for coir fibres with maleic anhydride grafted polypropylene compared with polypropylene can probably be attributed to a chemical adhesion mechanism In addition to the specific results for coir fibre composites, the integrated physical-chemical-micromechanical approach to investigate and improve fibre-matrix interface has been developed This knowledge can be applied to study the interface of other natural fibre composite systems

Mechanical properties of UD coir fibre composites with both thermoplastic and thermoset matrices are assessed by tensile tests in fibre direction, flexural tests and unnotched Izod impact tests In agreement with the interface evaluation, higher flexural strength and stiffness are found in the alkali treated fibre composites, probably thanks to the better interfacial adhesion The impact strength of coir polypropylene composite is not significantly different from that of neat polymer, while the coir fibres can improve the toughness of epoxy by minimum a factor of three, when the impact strength is considered as toughness indicator

An initial study on coir-bamboo fibre hybrid composites is carried out to investigate the hybrid effect of tough coir fibre and brittle bamboo fibre in composites With a low bamboo fibre fraction, a hybrid effect with an increase of composite strain to failure is obtained, which can be attributed to the high strain to failure of the coir fibres Meanwhile, the bamboo fibres provide high stiffness and strength to the composites The results show a potential for coir-bamboo hybrid composites, which justifies further study on this topic

Samenvatting

In de afgelopen decennia is de interesse in het gebruik van natuurlijke vezels voor gebruik in composietmaterialen sterk toegenomen, vanwege hun goede mechanische eigenschappen in combinatie met mileuvriendelijke karakteristieken In dit onderzoek worden Vietnamese cocosvezels onderzocht en gemodificeerd voor gebruik in composietmaterialen Om een efficiënt gebruik van de vezels toe te laten

in composietmaterialen, worden eerst de microstructuur en de mechanische eigenschappen van de cocosvezels gekarakteriseerd In de tweede plaats heeft het oppervlak van natuurvezels een complexe morfologie met chemische heterogeniteit

en een relatief grote ruwheid Deze factoren beïnvloeden sterk de vezel-matrix interfase adhesie Daarom is het belangrijk om een systematisch begrip te verwerven van de vezel-matrix interfase interacties in composieten Tenslotte worden unidirectionele (UD) composieten van cocosvezel in zowel thermoplastische als thermohardende matrices onderzocht, om een beoordeling te maken van de mogelijke waarde van cocosvezels voor gebruik in composieten

De microstructuur van technische cocosvezels is onderzocht met SEM en SEM-CT

De resultaten laten zien dat de technische cocosvezels bestaan uit een reeks van elementaire vezels met een lacuna in het centrum De elementaire vezels zijn voornamelijk opgebouwd uit twee celwanden die bestaan uit bundels van micro-fibrillen die een grote hoek maken met de vezelas Cocosvezels blijken een hoge porositeit te hebben van 22 tot 30% De mechanische eigenschappen van cocosvezel worden bepaald met behulp van trekproeven, zowel met trekproeven op enkelvoudige technische vezels met behulp van optische rekmetingen, als met trekproeven op technische vezels met een reeks van testlengtes De resultaten van beide methoden geven aan dat cocosvezels niet zozeer sterk en stijf zijn, maar wel een hoge breukrek hebben

Een geïntegreerde fysisch-chemische-micromechanische aanpak werd gebruikt om

de vezel-matrix compatibiliteit en adhesie te onderzoeken in cocosvezel composieten In deze studie werd de interfase gekarakteriseerd van zowel onbehandelde als met alkali behandelde cocosvezels in een reeks van thermoplastische matrices Verschillen in oppervlaktechemie van de vezels en eigenschappen van de matrices in termen van oppervlakte-energie en mogelijke chemische reacties werden beschouwd Bevochtigings experimenten van de vezels

en de matrices werden uitgevoerd om hun statische evenwichts contacthoeken te bepalen in verscheidene vloeistoffen Met deze contacthoeken werden de oppervlakte-energieën en de verschillende componenten hiervan bepaald, voor

zowel vezels als matrices Vervolgens wordt hiermee de theoretische adhesie arbeid bepaald voor elk composiet systeem Verder wordt de oppervlakte-chemie van de vezels bepaald met behulp van Röntgen fotoelectron spectroscopy (XPS), om meer informatie te verkrijgen over functionele groepen aan het vezeloppervlak Hiermee kunnen interacties in de composiet interfase beter begrepen worden

Om de kwaliteit van de composiet interfase te bepalen worden pull-out testen uitgevoerd op enkelvoudige technische vezels, alsmede transversale buigproeven uitgevoerd op unidirectionele composieten Dit om respectievelijk de afschuifsterkte van het grensvlak te bepalen en de mode I interfase sterkte De resultaten suggereren dat de hogere interfase sterkte van cocosvezel met polyvinylidene fluoride vergeleken met polypropyleen kunnen worden toegeschreven aan sterkere vezel-matrix fysisch-chemische interactie, in overeenstemming met de theoretische adhesie-energie Tegelijkertijd wordt de verbetering in interfase adhesie voor cocosvezel met maleinezuur anhydride gemodificeerde polypropeen toegeschreven aan een chemisch adhesie mechanisme

Naast specifieke resultaten voor cocosvezel composieten, werd in deze studie de geïntegreerde fysisch-chemische-micromechanische aanpak ontwikkeld om de vezel-matrix interfase te onderzoeken en te verbeteren Deze kennis kan gebruikt worden om de interfase te onderzoeken in andere (natuurvezel) composieten

De mechanische eigenschappen van unidirectionele cocosvezel composieten met zowel thermoplastische als thermohardende matrix werden onderzocht door trekproeven in vezelrichting, buigtesten en Izod impacttesten zonder kerf In overeenstemming met de interfase evaluatie, worden hogere buigsterkte en stijfheid gevonden in alkali behandelde composieten, waarschijnlijk door betere interfase adhesie De impactsterkte van cocosvezel polypropeen composiet is niet significant verschillend van die van onversterkte polypropeen, terwijl cocosvezel de taaiheid van epoxy kan verbeteren met minimaal een factor drie (indien impactsterkte wordt gebruikt als indicator van taaiheid)

Een initiële studie werd uitgevoerd op cocosvezel-bamboevezel hybride composieten, om het hybride effect te onderzoeken in composiet van taaie cocosvezels en sterke maar brosse bamboevezels Met een lage bamboevezel fractie wordt een positief hybride effect gevonden voor de composiet breukrek, wat kan worden toegeschreven aan de hoge breukrek van de cocosvezels Tegelijkertijd geven de bamboevezels hoge stijfheid en sterkte aan de composieten De resultaten

laten het potentieel zien van cocos-bamboe hybride composieten, wat een verdere studie van dit onderwerp ondersteunt

List of abbreviations

CTE Coefficient of Thermal Expansion

E-modulus of fibre calculated for fibre solid material

E-modulus of matrix Debonding force

Displacement frequency Fibre volume fraction Volume fraction of fibre solid material

Matrix volume fraction Work of adhesion

Work of adhesion following geometric mean approach Fibre embedded length

Longitudinal coefficient of thermal expansion of fibre (or ) Liquid surface tension

Surface energy base component (or ) Solid surface energy

Surface energy acid component

Surface energy Lifshitz – van de Waals component

Surface energy dispersive component Surface energy polar component Static/ equilibrium contact angle

Fibre strength calculated for the fibre solid material

Matrix stress at fibre failure strength Apparent interfacial shear strength

Table of Contents

Acknowledgement i

Abstract iii

Samenvatting v

List of Abbreviations ix

List of Symbols xi

Table of Contents xiii

Chapter 1 Introduction 1

1.1 General introduction 2

1.2 Literature review 4

1.2.1 Natural fibres 4

1.2.2 Coir fibres 11

1.2.3 Coir fibre composites 19

1.2.4 Interface of natural fibre composites 21

1.2.5 Concluding remarks 29

1.3 Problem statement and the goal of thesis 29

Thesis structure 32

References 32

Chapter 2 Microstructure and mechanical properties of coir fibres 37

2.1 Introduction 38

2.2 Materials and methods 38

2.2.1 Coir fibres 38

2.2.2 Investigation of fibre microstructure using SEM and SEM-CT 41

2.2.3 Measurement of fibre density 43

2.2.4 Single fibre tensile tests 45

2.3 Results and discussion 48

2.3.1 Fibre surface and fibre internal microstructure 48

2.3.2 Density of coir fibres 57

2.3.3 Tensile mechanical properties of coir fibres 58

2.4 Conclusions 63

References 64

Chapter 3 Wetting analysis and surface characterisation of coir fibres 65

3.1 Introduction 66

3.2 Materials and methods 68

3.2.1 Materials 69

3.2.2 Dynamic contact angle measurement 71

3.2.3 Static equilibrium contact angle approximation 73

3.2.4 Fibre surface energy estimation 76

3.2.5 Fibre surface characterisation using X-ray photoelectron spectroscopy 78

3.3 Results and discussion 80

3.3.1 Contact angle measurements 80

3.3.1.1 Fibre wetted perimeter 80

3.3.1.2 Advancing dynamic contac angles 82

3.3.1.3 Effect of liquid absorption on the contact angles 86

3.3.1.4 Advancing static contac angles approximation using the MKT 86

3.3.1.5 Static contac angles from relaxation experiments 87

3.3.2 Surface energy of coir fibre 90

3.3.3 Surface chemical analysis of coir fibre 93

3.4 Conclusions 94

References 94

Chapter 4 Interfacial adhesion and compatibility of coir fibre composites 97

4.1 Introduction 98

4.2 Materials and methods 100

4.2.1 Materials 100

4.2.2 Wetting analysis 101

4.2.3 Single fibre pull-out test 105

4.2.4 Three point-bending test of UD composites 110

4.3 Results and discussion 112

4.3.1 Surface enegies and the work of adhesion 112

4.3.2 Fibre surface chemistry 116

4.3.3 Fibre-matrix interfacial adhesion with pull-out test 118

4.3.3.1 Load-displacement curves and apparent IFSS 118

4.3.3.2 Two interfacial parameters fitting theoretical F max to the experimental data

122

4.3.4 Transverse strength and interface properties of composites 126

4.3.5 IFSS verus transverse bending strength 127

4.3.6 Work of adhesion in relation with practical adhesion 128

4.4 Conclusions 129

References 131

Chapter 5 Mechanical properties of unidirectional coir fibre composites 133

5.1 Introduction 134

5.2 Materials and methods 134

5.2.1 Materials 134

5.2.2 Production of composite samples 135

5.2.3 Test methods 139

5.2.4 Determination of coir fibre volume fraction 141

5.2.5 Coir/bamboo hybrid composites 142

5.3 Results and discussion 143

5.3.1 Flexural properties of UD composites 143

5.3.1.1 Longitudinal properties 143

5.3.1.2 Transverse properties 148

5.3.2 Tensile properties of UD composites 151

5.3.3 Impac strength of UD composites 156

5.3.3.1 Impact strength of UD coir/PP and UD coir/epoxy composites 156

5.3.3.2 Effect of fibre volume fraction and fibre treatment on the impact strength of UD coir fibre epoxy composites 158

5.3.4 Tensile properties of UD coir/bamboo hybrid composites 159

5.4 Conclusions 163

References 164

Chapter 6 Conclusions 165

6.1 General conclusions 166

6.1.1 Microstructure and mechanical properties of technical coir fibres 166

6.1.2 Wetting measurements and surface energy estimation of the fibres 167

6.1.3 Fibre-matrix interfacial compatibility and adhesion 168

6.1.4 Performance of coir fibre composites 168

6.2 Future work 169

Apendix A 171

Apendix B 173 Curriculum Vitae

List of publication

Chapter 1 Introduction

1.1 General introduction

Composite materials, by which is usually meant fibre reinforced polymers, are used

in a wide range of applications from aerospace, automotive and construction to leisure and sporting goods, where high mechanical properties in combination with light weight make them greatly attractive materials Moreover, these materials excel

in chemical resistance, durability and design flexibility Generally, a typical composite material consists of a continuous phase, known as matrix, and a reinforcement phase, typically in the form of fibres, distributed within it As a rule, the reinforcement fibres ensure the strength and rigidity of the material, whereas the matrix keeps the fibres in desired orientation and maintains the shape of the part The matrix is also a medium for stress transfer between the fibres, and protects them from environmental impacts such as chemicals, humidity and temperature In this, the fibre-matrix interface is an important element, where stress transfer from the fibre to the matrix and vice versa takes place

Besides using synthetic fibres, particularly carbon, glass and aramid fibres, natural fibres such as flax, jute, coconut fibre (coir), hemp and bamboo have received a growing interest for application in polymer composites during the last decades These fibres are available in large amounts, at low cost, have low energy utilisation and are renewable and biodegradable In most cases the specific properties of natural fibre composites have been found to compare favourably with these of glass fibre composites [1, 2] In this research, the focus will be on coir fibres

Generally, coir fibres are considered as a low-value product which is mainly used to make mattresses, doormats or brushes Other applications are coir nettings and geotextiles for soil protection and erosion control, and rubberised coir mats used

in upholstery padding for automobiles Nowadays, there are three good reasons to use natural fibres, namely: economy, ecology and society; hence, coir fibres can be a good candidate as reinforcement for composite materials They are cheaper in cost than other natural fibres, easily extracted, and available in large amounts For basic mechanical properties, flax, hemp, bamboo and jute can contribute their high strength and stiffness to composites, while coir with high elongation to failure can ameliorate the composite toughness [3]

As mentioned above, the interfacial adhesion between fibre and matrix plays an important role in the final composite mechanical properties The knowledge of the interface has been developed for the existing synthesized fibre composites, but research has not really focused yet on natural fibre composites Natural fibres are usually extracted from different parts of the plant, which typically have different surface chemical compositions, leading to different properties in terms of surface energy and potential for chemical reactions Apparently, the fibre surface is rough and chemically heterogeneous, which affects the interfacial properties when the fibres are used in composite materials Therefore, a fundamental understanding of the fibre-matrix interfacial compatibility and adhesion is necessary The first important concern is wetting between fibre and matrix to create a good fibre-matrix contact This strongly depends on the surface energies of the fibre and the matrix Subsequently, the fibre-matrix adhesion comprising different levels of interfacial interactions, from molecular scale to bulk composite level, is an essential element to

be studied for natural fibre composites

Terminology

The terms and definitions, which are used frequently in the thesis, are described in the following glossary:

Elementary fibre is the structural unit of the plant, composing of cell walls and

formed out of cellulose crystalline microfibrils connected by amorphous lignin and

hemicellulose

Technical fibre is the extracted fibre after a standard extraction process, which is

used as reinforcement for composites A technical fibre consists of numerous elementary fibres, and its configuration mainly depends on the biological structure

of the plant Figure 1-1 displays the technical fibre in various plants In case of coconut, the technical coir fibre naturally presents as such as in the husk and it is surrounded by organic tissues, while in case of flax or bamboo, its technical fibre has a configuration depending on the fibre extraction method which separates a bundle of elementary fibres to form a technical fibre

Single fibre, in this thesis, is referred to as one technical fibre as it is used in some

tests

Figure 1-1 Representative images of a technical fibre (circles) (a) coir from coconut shell (b) flax

fibre from the stem [4] (c) bamboo fibre from the culm [5]

1.2.1 Natural fibres

Figure 1-2 Overview of natural fibres [2, 6, 7]

Natural fibres generally are fibres which are not synthesised but obtained from nature using different fibre extraction processes Natural fibres can be divided in subgroups based on their origins as plant fibres, animal or mineral fibres Figure 1-2 shows the three subgroups highlighting some common fibres used in composite materials

Figure 1-3 Worldwide production of natural fibres, in million ton (Sources: FAOSTAT, 2009 and

FAO, 2009) [6].

The production volumes of natural fibres are shown in Figure 1-3 It can be seen that cotton is the most important natural fibre with a high quantity in the market Besides this, a high market share is found for the other plant fibres such as jute, flax, coir, hemp and sisal, which have been used in composite materials Used as reinforcement, the mechanical properties of the fibres are the main concern, which are decided by the structure of the fibres and their chemical compositions These characteristics of common natural plant fibres will be described in the following sections

1.2.1.1 Chemical composition

Figure 1-4 Schematic presentation of the hierarchy of a typical cell wall, from a simplified model

of a primary cell wall down to the microfibril structure of crystalline cellulose, to the cellulose

molecule with its monomer units (After Akin [6])

Natural plant fibres of the stem, leaf, fruit or seed of the plant, typically have a cell wall structure and comprise of cellulose, hemicelluloses, lignins and aromatics, waxes and other lipids, pectin, ash and water-soluble compounds Figure 1-4 presents a typical cell wall with main components and a schematic representation of their organisation Climatic conditions and age not only influence the structure of the fibres but also the chemical composition [6, 8] To have efficient processing and quality improvement of the fibres, a good understanding of the fibre chemistry is

necessary In Table 1-1, the major chemical components of common natural fibres are presented.

Table 1-1 Chemical composition of common natural plant fibres [6, 8-14]

Cellulose is the essential component of plant fibres It is a linear condensation

polymer of glucose consisting of a linear carbohydrate polymer of β-1,4-linked

glucose units (d-anhydroglucopyranose units) The basic repeating unit of cellulose

is the dimer cellobiose, which comprises of two glucose units bound by the β-1,4

linkage as well as intermolecular hydrogen bonds Figure 1-5 shows a typical structure of cellulose The properties of cellulose are decided by how glucose is bound in the linear polymer The cellulose structure consists of thousands of glucose units, which can stack together to form crystal with intramolecular hydrogen bonds providing a stable polymer with high tensile strength Cellulose occurs in plant cell walls as microfibrils (e.g 2–20 nm diameter and 100–40000 nm long) providing a linear and structurally strong framework The mechanical properties of natural fibres depend on its cellulose type, because each type of cellulose has its own crystalline unit cell geometry and the geometrical conditions determine the mechanical properties [6, 8]

Figure 1-5 Schematic presentation of cellulose, showing the linear nature of the polymer made of glucose units: (A) cellulose unit; (B) structure of the dimer cellobiose; (C) cellulose molecule with

β-1,4 linkage between C atoms 1 and 4 (After Akin [6])

Hemicellulose is reported to be the second most abundant carbohydrate of plant cell

walls after cellulose It comprises a heterogeneous group of polysaccharides which remains associated with the cellulose after lignin has been removed, and differs from cellulose in both composition and structure Firstly, hemicelluloses contain several different sugar units whereas cellulose contains only 1,4- -d glucopyranose units They exhibit a considerable degree of chain branching, whereas cellulose is a strictly linear polymer Moreover, the degree of polymerization of native cellulose is ten to one hundred times higher than that of hemicellulose Hence, hemicelluloses are generally in amorphous form with lower molecular weight than cellulose They are quite hydrophilic and mainly responsible for the moisture sorption behaviour of the fibres [6, 8] Figure 1-6 shows a schematic illustration of hemicelluloses and celluloses together in a cell wall

Figure 1-6 A schematic cell wall, in which cellulose and hemicellulose are arranged into layers in a

matrix of pectin polymers [6]

Lignin is a compound of complex hydrocarbon polymers with both aliphatic and

aromatic constituents, and has an amorphous structure These compounds are very diverse and present in many forms within plants and plant cell walls In the structure

of a cell wall, lignin and hemicellulose are linked by covalent bonds, and celluloses are often bonded by lignin or the lignin/hemicellulose complex [6, 15]

Pectin consists mainly of heteropolysaccharides, which consist essentially of

polygalacturon acid Pectin amounts are often low in natural plant fibres, but they are strategically located within the plant tissues as a matrix to hold tissues, including fibres, together [6] (Figure 1-6)

Waxes consist of long chain alcohols which are insoluble in water as well as in

several acids They are usually located on the cuticle of the plant or on the fibre surface as a protective barrier which prevents drying and microbial entry inside the plant However, the waxy layers influence the processing and quality of natural fibres, and are normally removed to obtain good quality cellulose fibres

1.2.1.2 Physical structure and mechanical properties of natural fibres

A technical natural fibre commonly consists of several cells (referred to as elementary fibres) The cell is mainly formed out of crystalline microfibrils based on cellulose (major load-bearing components in plant cell walls), which are connected into a cell wall layer, by amorphous lignin and hemicellulose Hemicelluloses are

assumed to be the mediators between cellulose and lignin, as they can bind to cellulose via hydrogen bonds and even covalently to lignin [16] Multiples of such cellulose–lignin/hemicellulose layers stick together to build up the cell wall (Figure 1-6) This structure can be considered as a composite, in which the cellulose crystals play a role as reinforcement in a matrix of lignin/hemicellulose compound

The cell wall layers can be of different thickness, chemical organisation and orientation of the cellulose microfibrils (microfibril angle – MFA) Figure 1-7 presents schematics of the fibre cell (elementary fibre) consisting of several layers with different MFA The thickness of the cell wall layers and their cellulose MFA play a dominant role in the mechanical properties of plant fibres

Figure 1-7 Schematics of possible cell wall organisation in (A) wood fibres, (B) bast fibres, (C) monocotyledonous plant fibres and (D) seed fibres Black lines indicate orientation of cellulose microfibrils; stress-strain curves of fibre with different density (E) and MFA (F) [6].

The mechanical properties of plant fibres depend on the organisation of cell walls in terms of cell wall/lumen ratio and the cellulose MFA in the dominant cell wall layers In relation with fibre cross-section, higher density fibres are stiffer and

stronger than the lower density ones The elastic modulus and strain at failure of fibres are also dependent on the MFA A small MFA, in which cellulose fibrils are oriented almost parallel to the axial direction, leads to a high modulus of elasticity, whereas the stiffness is considerably reduced for higher MFA In Figure 1-7, it can

be seen that the stress-strain curve shows a stiff and elastic response with a brittle fracture at low MFA For large cellulose MFA the interaction of the cellulose fibrils with the matrix becomes more crucial for the overall mechanical behaviour of the cell wall Typically, the stress–strain curves of tissues and fibres with high microfibril angles show a biphasic or triphasic behaviour [17, 18], as shown in Figure 1-7E and 1-7F

Table 1-2 shows the physical and mechanical properties of selected natural plant fibres, which reflects the influence of the fibre structure on their mechanical properties For instance, the high MFA in coir fibres results at low stiffness and high strain at failure The high elongation at failure of coir fibres assists their relatively high impact strength It shows that nature is very smart, since the coconut fibres need to prevent the coconut from breaking when it falls out of the tree

Table 1-2 Physical characteristics and mechanical properties of common natural fibres (given

values from random single fibre or bundle tests) [6, 8-14]

Diameter (  m) 100-460 12-20 200-400 16-50 30-150 11-20 50-200 Density (g/cm3) 1.1-1.3 1.5-1.6 1.4-1.5 1.4-1.6 1.3-1.5 1.4-1.5 1.0-1.5

MFA (o) 30-49 20-30 85-90 2-6.2 7-10 5-10 10-25 E-modulus (GPa), range

(most frequently published)

2.8-6 (5)

4.5-12.6 (8)

11-89 (30)

3-90 (65)

3-64 (30)

8-100 (70)

9-38 (12) Tensile strength (MPa), range

(most frequently published)

95-270 (200)

220-840 (450)

140-1000 (500)

310-1110 (800)

190-800 (500)

343-1500 (700)

80-855 (600) Elongation at break (%), range

(most frequently published)

15-50 (30)

2-10 (8) 2-3

1.3-6 (3)

0.2-3.1 (1.8)

1.2-4 (3)

1.9-14 (3)

1.2.2 Coir fibres

Coconut fibres are usually known under the name ‘coir’ fibres in literature, and are

obtained from the fruit of the coconut palm (Cocos nucifera L.) growing extensively

in tropical countries Coconut palm is the most economically important cultivated plant in over 93 countries situated in the tropical coastal ecosystem of the world, providing more than 200 products or byproducts for human use It occupies an area

of approximately 12 million hectares globally, with an annual production of around

57 billion nuts [19] The palms are mainly grown for the oil-rich copra (‘meat’) contained inside the coconuts (Figure 1-8) In a mature coconut, the white meat (28 wt.%) is surrounded by a hard protective shell (12 wt.%) and a thick husk (35 wt.%) This husk surrounding the large seed constitutes of 30 wt.% fibre and 70 wt.% pith material (waste material from coir fibre industry, with high content of lignin) [20, 21] Figure 1-8 shows the cross-section of a coconut consisting of the copra, the core shell and the husk shell

Figure 1-8 Coconuts from the palm and cross section of a coconut (adapted from [21]).

Traditionally, coir was extracted from husks that had been soaked for 6–9 months (retted) in sea water or lagoon water and then beaten with a wooden mallet The fibres were used for production of ropes, yarns, mats, brushes and padding of mattresses Nowadays, the coir extraction processes have significantly improved, the quality coir fibre being extracted either by wet processing (following retting procedures) or mechanical decortications without soaking The colour and properties

of coir fibre are not only dependent on the type of coconut palm, but also on harvest time White fibres are obtained from green coconuts which are harvested after about 6-7 months on the plant (the green coconuts have thin copra and mainly provide coconut water for drinking) While brown fibre is obtained by harvesting fully mature coconuts of 11-12 months when the nutritious layer in the seed is ready to be processed into copra and desiccated coconut The brown fibres are stronger but less flexible than the white ones Coir fibres are available in high quantity, and considered as commodity in the world market Their production is estimated at around 1 million ton per year (FAOSTAT, 2009) at prices of order 30 to 40 Eurocents per kilo

1.2.2.1 Extraction of coir fibres

Extraction of coir fibres from coconut husk shells is mainly carried out in the following steps: retting (the pre-treatment process in the traditional procedure), extraction of coir fibre bundles, cleaning the coir bundles (removal of pith from coir) and drying

Retting is a microbial separation process, which consists essentially of soaking the

husk in water for a period Depending on the condition of the husks and the nature

of the water, retting duration can vary from 2 to 9 months in the traditional process When the husks are mature and dry, the retting process takes nearly 6–9 months, while it requires 2–3 months for green husks Currently, the retting time is reduced

to 2-3 weeks thanks to an improved retting process, in which the husks are crushed before soaking in water Crushing the husks can help to increase the surface area in contact with the water, and this accelerates the action of bacteria separating the fibre bundles from pith tissues [6]

Extraction process

Following retting, the extraction process involves the breakdown and the separation

of the coir fibre bundles from the connecting tissues or pith in between the fibre bundles and also from the outer exocarp (outer layer) Mechanical extraction of the retted husks is often applied using various designed machines The three following types of machines are usually used for the extraction of various kinds of coir fibres [6]

A decorticator is the first common machine for extracting the fibres from fresh husks or husks that have been soaked for a few hours, and enables extraction of pith tissues The husks are mechanically beaten against a cylindrical cage made out of tor steel bars The rotary shaft fixed several plates consisting of sharp blades, facilitates the holding and hammering of the husks (Figure 1-9) The disadvantage of this machine is that the long coir fibres cannot be produced Only mixed-grade coir is produced by this machine

Figure 1-9 Coir fibre decorticator [22]

A defibre-ing machine can be used to produce bristle fibres which are considered as long coir fibres In the defibre-ing machine, the husk segments are gripped at the edge of a large wheel, and then moved towards the picker drum The sharp pins of this drum remove the short fibres and pith, leaving the bristle coir (long fibres) The first drum defibres half of the husk segment, which is then transferred to a second wheel while the defibred part of the husk is held firmly by a conveyor chain The defibreing is completed by the second picker drum The extracted fibres need to pass through a cleaner drum or wash to remove the pith adhering to the bundles This type of machine is used for extracting the studied coir fibres in this thesis So, the details of extraction process will be presented in Chapter 2

The last machine is a modified decorticator, in which the defibre-ing and the decorticating processes are combined Firstly, the husk is introduced to a picker drum, in which the pith and exocarp of the husk are partly removed And it is then automatically transferred to a section similar to the decorticator for further removal

of pith by mechanical beating This machine can be used with green husks, retted brown husks or wetted husks, and produces mixed fibre bundles which have better quality than the fibre bundles extracted by the decorticator alone

Cleaning and drying coir fibres

The received fibres after the extraction process are cleaned to remove the pith and other attached tissues The bristle coir or long fibre bundles have a smaller amount

of residual pith Therefore, bristle fibre bundles are cleaned (hackled) by combing through a set of steel spikes While the mixed fibre bundles are fed into a cone-shaped rotating screen sifter By gravitational action, fibre bundles are separated from the pith tissues The coir fibre is then fed into a turbo cleaner, which consists of fixed steel rods rotating at a high speed, for further cleaning By centrifugal action, remaining pith tissues and other waste attached to the fibre bundles are removed by this mechanical process, and better-quality coir is obtained [6]

The cleaned fibre bundles are dried in a drying machine or under the sun to reduce the moisture content to about 15% For sun drying, it takes approximately 6 h, during which time the coir is turned over several times to ensure a uniformly dried product

1.2.2.2 Morphology and chemical composition

Figure 1-10 A typical technical coir fibre and its fracture surface [21, 23]

The technical coir fibre (also referred to as coir fibre) typically is relatively cylindrical consisting of numerous axially oriented elementary fibres which are joined together, and often contains a central hollow channel which is named lacuna Coir fibres have a diameter in the range of 130-390 m, and the longest fibre length

is around 22 cm

As a plant cell wall, the elementary fibre comprises several cell wall layers which are built up by cellulose microfibrils and compound of hemicellulose and lignin In

Figure 1-11b, highly lignified elementary fibres are observed by lignin staining of the coir fibre cross section The diameter of elementary fibres was determined to be

on average 18 m with a relatively large lumen width of 12.5m The length of these elementary fibres was measured to be in the range of 0.9 to 1.06 mm [21, 24, 25]

Figure 1-11 Cross section of coir fibre (a) showing lacuna and lumens (b) a high content of lignin is

observed by lignin staining (in red) (adapted from [25, 26])

Concerning chemical composition, coir fibre is composed of cellulose, lignin, hemicellulose and a small amount of other substances Table 1-3 shows the chemical composition of coir fibres as reported by various literature sources In comparison with the other natural fibres (Table 1-1), coir fibres have a relatively high lignin content This result is consistent with the staining analysis of the fibre structure shown in Figure 1-11b It is also observed that the content of lignin is high in the middle lamellae between elementary fibres

Table 1-3 Chemical composition of coir fibres reported in literature

cells firmly bonded in the fibre [31, 32] The fibre surface is also reported to contain

a high fraction of waxes

Figure 1-12 Coir fibre surface consisting of attached pith and rich in silica dots analysed by EDS

Coir fibres, as a natural fibre, absorb moisture from the surroundings when the dry fibres are exposed to the atmosphere, they will take up moisture and reach equilibrium The moisture absorption results in changing the properties of coir such

as tensile strength, elastic recovery, electrical resistance, rigidity, etc As a result of absorption of water, the fibres tend to swell, altering their dimensions, and thus leading to changes in the size, shape, stiffness Similarly, when the fibres in moisture equilibrium are exposed to a dry atmosphere, moisture is lost to the

surroundings to establish a new equilibrium [6] The amount of water in coir fibre can be expressed in terms of moisture content or moisture regain, in which moisture content indicates the amount of water present in a moist sample and moisture regain

is the amount of water that a completely dry fibre will absorb from the atmosphere

at a standard condition of 20 oC and a relative humidity of 65% (expressed as a percentage of the dry fibre weight) Nawaratne [33] found that the moisture content

of fresh-water-retted fibre samples was 10.20% with a moisture regain of 11.31%, while the moisture content in sea-water-retted coir was 7.92% with a moisture regain of 8.60% In comparison with the other common natural fibres (flax, jute, hemp), coir fibres show relatively less moisture absorption

Van Dam [24] also reported the thickness swelling of coir due to water absorption when the fibres were placed in water The result shows that the thickness swelling is

in the range from 22% to 34% after 15 minutes in water The longitudinal swelling, which increases the fibre length, was measured to be 0.9% after 15 minutes

Mechanical properties

For application as composite reinforcement, the mechanical properties of the coir fibres will strongly decide the final properties of the composite As shown in Table 1-1, in comparison with the other natural fibres, coir fibre has low tensile strength and E-modulus but high elongation at failure This is explained by low cellulose content in combination with a high MFA The MFA in a range of 30-49° of the S2 cell wall layer determines the characteristics of the fibre [23, 34] (the second secondary cell wall layer S2 is typically the thickest layer of the elementary fibre cell wall) The increasing MFA decreases the fibre stiffness but increases the strain

at failure This interrelation enables the fibres to adjust both stiffness and toughness

by shifting the cellulose fibril orientation in the cell wall [35, 36] Nature is smart since coir fibres have to prevent the nut from breaking when it falls out of the tree Thus, the strength of the fibres is not as important as the energy absorption at impact [3]

Figure 1-13 Typical stress-strain curve of coir fibre in a tensile test, and fibre fracture surface showing the pull-out of elementary fibres, also giving indications of the high MFA [24]

Figure 1-13 presents a typical stress-strain curve of a coir fibre with a failure strain

of approximately 25% The fracture surface of the fibre shows pulled out elementary fibres, explaining the high plastic deformation of coir during a tensile test The mechanical properties of coir fibres are also dependent on fibre type (genetic variety and maturity of the nut) which may result in different diameter, cellulose content, cell wall thickness of the elementary fibres and MFA In Table 1-4, the mechanical properties of various types of coir fibres are shown

Table 1-4 Tensile properties of different types of coir fibres

Fibre species Tensile strength

1.2.3 Coir fibre composites

Natural fibre composites have been studied and used in industry, especially in automotive applications, thanks to their good mechanical properties combined with their light weight This results in specific mechanical properties comparable to those

of glass fibre composites From the above literature survey, coir fibres show a high potential for application in polymer composites which are applied in impact loading and do not require high strength and stiffness In literature, a variety of studies on coir fibre composites can be found, considering different matrices and fibre geometries Both traditional matrices (thermoplastics and thermosets) and

biodegradable polymers were used with different fibre forms such as short fibres, unidirectional fibres, woven mats, etc Table 1-5 presents the mechanical properties

of various coir fibre composites published in literature

Table 1-5 Mechanical properties of untreated coir fibre composites with different fibre geometries

(kJ/m²)

Reference

As seen in Table 1-5, in composites coir fibres are mostly used in the form of short fibres or random fibre mats Consequently, the composite strength and stiffness is relatively low Tensile strength of the composite is highly influenced by the orientation of the fibre layers The strength of fibres most effectively contributes to the composite strength when the fibres are perfectly aligned in unidirectional direction Obviously, there is a lack of published results on unidirectional (UD) coir fibre composites

Hill et al studied the impact properties of random coir fibre polyester composites, and presented the influence of fibre weight fraction on composite impact strength [40] The results showed that the impact strength of the composite increases with a factor of three compared to neat resin At low fibre loading (less than 20 wt%), no increase of impact strength is seen and there is an approximately linear increase thereafter, followed by a decrease at the highest fibre loading (45 wt%) (Figure 1-14) The impact strength of a composite is influenced by many factors including the toughness properties of the reinforcement, the nature of the interfacial region, and the frictional work involved in pulling the fibres from the matrix In this case, the tough coir fibres and the fibre-matrix interfacial interactions played a key role in the impact strength of the composite When the fibre loading (of tough fibres) increases,

typically toughness enhancement occurs in composites However, at high fibre loading, the resin is prevented to wet completely the fibre bundles, which changes the interfacial properties Less fibres can be loaded properly or participate in energy absorption by pull-out and the composite toughness goes down

Figure 1-14 The variation in the impact strength with fibre loading for composites reinforced with (line A) acetylated coir, (line B) unmodified coir, (line C) acetylated oil palm fibres, and (line D)

unmodified oil palm fibres [40] The nature of the interphase region is of high importance in determining the toughness of the composite If the fibre–matrix interfacial strength is too low, poor stress transfer occurs leading to a weak composite On the other hand, a strong interfacial adhesion allows efficient stress transfer, but produces a composite exhibiting poor toughness properties, because more localisation of damage occurs with less fibre pull-out

1.2.4 Interface of natural fibre composites

Natural fibres extracted from different plants and different parts of plants typically have different surface physico-chemical properties Most natural fibres are relatively hydrophilic, have a rough surface and are physico-chemically heterogeneous The fibre surface properties strongly influence the fibre-matrix interactions in the composite The first important concern is wetting between fibre and matrix to create

a good fibre-matrix contact Subsequently, a strong fibre-matrix adhesion ensures that high stresses can be transferred across the interface without disruption In composite materials, the interfacial adhesion between fibre and matrix plays an important role in the final mechanical properties

1.2.4.1 Interfacial adhesion and types of bonding

Generally, the adhesion at the interface can be described by the following main interactions: 1) physico-chemical interactions, related to wettability and compatibility of the fibre and the matrix plus physical adhesion (e.g Van der Waals forces); 2) chemical bonding (covalent bonds) and 3) mechanical interlocking created on rough fibre surfaces, and other interactions such as molecular entanglement, interdiffusion etc [44] Good interfacial adhesion initially requires a good wetting between the fibre and the matrix, to achieve an extensive and proper interfacial contact The wettability mainly depends on the surface energies of the two materials Essentially the fibre-matrix interactions are controlled by the functional groups on the surface of the fibre and the matrix in the interfacial contacting area

A general description of the fibre-matrix interfacial interactions is presented in following sections In addition, more details of the state of the art are also discussed

in Chapter 3 and 4

Physico-chemical interactions and wetting

In literature, the wetting of a solid by a liquid is described by the physical attraction between two materials depending on their surface energies Accordingly, bonding due to wetting involves short-range interactions of electrons on an atomic scale which develop only when the atoms of the constituents approach within a few atomic diameters or are in contact with each other (in equilibrium interatomic distance) [44] When a good contact between two materials is formed, the other bonding mechanisms will occur to create interfacial adhesion

Wetting can be quantitatively expressed in terms of the thermodynamic work of adhesion, , of a liquid to a solid using the Dupre equation

(1-1) where is the solid surface energy, is the liquid surface tension, and is the interfacial energy Here, represents a physical bond resulting from highly

localized intermolecular dispersion forces

Young was the first to describe the equilibrium contact angle when a sessile drop of liquid is in contact with a solid surface [45] The relation between surface energies

of the solid and the liquid through the contact angle is expressed as follows: