Title: Polymer Composite Materials based on Coconut Fibres (Polymeercomposietmaterialen op basis van kokosvezels)
Other Titles: Polymer composite materials based on coconut fibres
Authors: Tran, Le Quan Ngoc
Issue Date: 10-Jan-2013
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.
Table of Contents: 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 Fibre wetted perimeter 80 Advancing dynamic contac angles 82 Effect of liquid absorption on the contact angles 86 Advancing static contac angles approximation using the MKT 86 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 Load-displacement curves and apparent IFSS 118 Two interfacial parameters fitting theoretical Fmax to the experimental data
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 Longitudinal properties 143 Transverse properties 148
5.3.2 Tensile properties of UD composites 151
5.3.3 Impac strength of UD composites 156 Impact strength of UD coir/PP and UD coir/epoxy composites 156 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
ISBN: 978-94-6018-615-8
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Technologiecluster Materialentechnologie
Materials Technology TC @ Groep T
Metallurgy and Materials Engineering - miscellaneous
Structural Composites and Alloys, Integrity and Nondestructive Testing

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