Nonwoven Flax Fibre Reinforced Pla Biodegradable Composites

NONWOVEN FLAX FIBRE REINFORCED PLA BIODEGRADABLE COMPOSITES

A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences

2013

SHAH ALIMUZZAMAN

SCHOOL OF MATERIALS

LIST OF CONTENTS

LIST OF CONTENTS…………………………………………………………………...2 LIST OF TABLES……………………………………………………………………….7 LIST OF FIGURES……………………………………………………………………...8 LIST OF EQUATIONS………………………………………………………………...12 LIST OF ABBREVIATIONS ………………………………………………………….14 ABSTRACT……………………………………………………………………………16 DECLARATION……………………………………………………………………….17 COPYRIGHT STATEMENT ………………………………………………………….18 DEDICATION………………………………………………………………………….19 ACKNOWLEDGEMENTS…………………………………………………………….20 PREFACE………………………………………………………………………………21

CHAPTER 1 INTRODUCTION……………………………………………………...23 1.1 Preamble………………………………………………………………………...23 1.2 Research Background…………………………………………………………...24 1.3 Research Aim and Objectives…………………………………………………...26 1.4 Thesis Layout……………………………………………………………………27

CHAPTER 2 LITERATURE REVIEW………………………………………………29 2.1 Introduction……………………………………………………………………...29 2.2 Composite Material……………………………………………………………...29 2.2.1 Classification of Composites……………………………………………….30 2.2.1.1 Classification on the basis of matrix material…………………………..30 2.2.1.2 Classification on the basis of reinforcing material……………………..31 2.2.2 Prepreg……………………………………………………………………..33 2.3 Manufacturing Process of Fibre Reinforced Composites……………………….34 2.3.1 Formation of prepreg……………………………………………………….34 2.3.1.1 Nonwovens for flat prepreg manufacturing…………………………….35 2.3.1.2 Three dimensional (3D) nonwovens for 3D prepreg manufacturing…...44 2.3.2 Composite fabrication……………………………………………………...46 2.3.2.1 Compression moulding…………………………………………………49 2.4 Mechanical Properties of Composites…………………………………………..53 2

2.4.1 Tensile properties…………………………………………………………..54 2.4.2 Flexural properties…………………………………………………………55 2.4.3 Impact properties…………………………………………………………...55 2.4.4 Crushing properties………………………………………………………...56 2.5 Biodegradable Composites……………………………………………………...56 2.5.1 Materials used in biocomposites…………………………………………...60 2.5.1.1 Reinforcing material……………………………………………………60 2.5.1.1.1 Flax fibre……………………………………………………………62 2.5.1.2 Matrix materials………………………………………………………..67 2.5.1.2.1 Thermoset resins……………………………………………………67 2.5.1.2.2 Thermoplastic resins………………………………………………..67 2.5.1.2.2.1 PLA fibre………………………………………………………..68 2.5.2 Process used in biocomposites……………………………………………..70 2.6 Summary………………………………………………………………………...70

CHAPTER 3 RESEARCH METHODOLOGY………………………………………73 3.1 Introduction……………………………………………………………………...73 3.2 Materials………………………………………………………………………...73 3.2.1 Material selection…………………………………………………………..73 3.2.2 Material testing……………………………………………………………..74 3.3 Prepreg Formation………………………………………………………………75 3.3.1 Evaluation of prepreg………………………………………………………76 3.4 Flat Composite Panels…………………………………………………………...76 3.4.1 Method of composite fabrication…………………………………………..76 3.4.2 Processing parameters…………………………………………………...…77 3.4.3 Fabrication procedure……………………………………………………...78 3.4.4 Analysis of composite properties…………………………………………..78 3.5 3D Composite Panels……………………………………………………………80

CHAPTER 4 PREPREG AND COMPOSITE MANUFACTURING PROCESS……81 4.1 Introduction……………………………………………………………………...81 4.2 Prepreg Manufacturing ………………………………………………………….81 4.2.1 Air laying web forming machine…………………………………………..81 4.2.2 Hot pressing instrument……………………………………………………83 4.2.3 Experimental method of prepreg manufacturing…………………………..84 3

4.3 Composite Manufacturing………………………………………………………86 4.3.1 Compression moulding machine…………………………………………...86 4.3.2 Production parameters for composite fabrication………………………….88 4.3.3 Experimental procedure……………………………………………………88 4.3.4 Composite sample coding………………………………………………….90 4.4 Manufacturing of 3D Nonwoven Prepreg……………………………………….92 4.4.1 3D nonwoven machine……………………………………………………..92 4.4.2 Development of 3D mould unit…………………………………………….93 4.4.3 Experimental method of 3D prepreg manufacturing………………………94 4.5 Manufacturing of 3D Composite………………………………………………..95 4.5.1 Aluminium mould for 3D composite production…………………………..95 4.5.2 Experimental Procedure of 3D composite manufacturing…………………96 4.6 Summary………………………………………………………………………...97

CHAPTER 5 CHARACTERISATION EQUIPMENT AND PROCEDURES……...98 5.1 Introduction……………………………………………………………………...98 5.2 Properties of Material Testing…………………………………………………..98 5.3 Area Density and Thickness of the Prepreg……………………………………100 5.4 Tensile Strength of the Prepreg………………………………………………..101 5.5 Physical Properties of the Composites…………………………………………103 5.5.1 Density measurement……………………………………………………..103 5.5.2 Constituents of the composite samples…………………………………...104 5.5.2.1 Methods for determining the constituents…………………………...104 5.5.2.2 Calculating the amount of constituents……………………………...105 5.6 Mechanical Property Testing…………………………………………………..107 5.6.1 Tensile testing……………………………………………………………107 5.6.2 Flexural testing…………………………………………………………..110 5.6.3 Impact testing…………………………………………………………….114 5.6.4 Crushing test……………………………………………………………..118 5.7 Thermal Property Testing……………………………………………………...120 5.7.1 Differential scanning calorimetry (DSC)…………………………………121 5.7.2 Thermogravimetric analysis (TGA)………………………………………124 5.8 Water Absorption Testing……………………………………………………...124 5.9 Biodegradability Testing……………………………………………………….126

5.9.1 Soil burial test…………………………………………………………….126 5.9.2 Mechanical property and weight loss tests after biodegradation…………127 5.10 Surface Morphology Testing…………………………………………………128 5.11 Summary……………………………………………………………………...130

CHAPTER 6 PHYSICAL, MECHANICAL AND THERMAL PROPERTIES……131 6.1 Introduction…………………………………………………………………….131 6.2 Flax and PLA Fibre Evaluation………………………………………………..131 6.3 Evaluation of Prepreg………………………………………………………….132 6.3.1 Area density and thickness of the prepreg………………………………..132 6.3.2 Tensile strength of prepreg………………………………………………..133 6.4 Evaluation of Physical Properties of the Composites………………………….134 6.4.1 Density of the composite………………………………………………….134 6.4.2 Constituent of the composite……………………………………………...135 6.5 Mechanical Properties of the Composites……………………………………...139 6.5.1 Tensile properties…………………………………………………………139 6.5.2 Flexural properties………………………………………………………..142 6.5.3 Impact properties…………………………………………………………145 6.6 Thermal Properties of Composites…………………………………………….148 6.6.1 Differential scanning calorimetry………………………………………...148 6.6.2 Thermogravimetric analysis………………………………………………150 6.7 Evaluation of the 3D Composite……………………………………………….152 6.7.1 Density of the 3D composite……………………………………………...152 6.7.2 Constituent of the 3D composite………………………………………….154 6.7.3 Mechanical properties of 3D composite………………………………….154 6.7.3.1 Crashworthiness……………………………………………………..154 6.8 Summary……………………………………………………………………….161

CHAPTER 7 WATER ABSORPTION AND BIODEGRADABILITY OF COMPOSITES………………………………………………………..162 7.1 Introduction…………………………………………………………………….162 7.2 Water Absorption of the Composite…………………………………………...162 7.3 Biodegradability of the Composite…………………………………………….165 7.3.1 Weight loss after biodegradation…………………………………………165 7.3.2 Mechanical properties after biodegradation………………………………166 5

7.3.2.1 Residual flexural properties………………………………………….166 7.3.2.2 Residual impact properties…………………………………………..168 7.3.3 Surface morphology before and after biodegradation…………………….170 7.4 Relationship between Water Absorption and Biodegradability of Composites 172 7.5 Summary……………………………………………………………………….173

CHAPTER 8 CONCLUSIONS AND FUTURE WORK…………………………...174 8.1 Conclusions…………………………………………………………………….174 8.2 Recommendations for Further Research Work………………………………...178

REFERENCES………………………………………………………………………..180

APPENDICES………………………………………………………………………...200 APPENDIX A…………………………………………………………………………201 APPENDIX B…………………………………………………………………………207

WORD COUNT: 50,511 Words

LIST OF TABLES

Table 2.1 Variable factors and levels of the test ………………………………….52 Table 2.2 Composition of biocomposites ………………………………………...53 Table 2.3 Comparison of glass fibre with some natural fibre properties………….62 Table 2.4 Chemical composition of flax fibre…………………………………….65 Table 4.1 Production parameters………………………………………………….88 Table 4.2 Sample details…………………………………………………………..91 Table 6.1 Properties of Raw materials…………………………………………...131 Table 6.2 Area density of the prepregs…………………………………………..133 Table 6.3 Thermal properties of Neat PLA and PLA/Flax biocomposites………148 Table 6.4 TGA Characterization of Neat PLA, Flax fibre and PLA/Flax biocomposites…………………………………………………………151 Table 6.5 Crashworthiness parameters of the 3D biocomposite samples……….157 Table 7.1 Flexural strength of the 100PLA and its biocomposites before and after biodegradation……………………………………………...167 Table 7.2 Flexural modulus of the 100PLA and its biocomposites before and after biodegradation……………………………………………...168 Table 7.3 Notched Izod impact strength of the 100PLA and its biocomposites before and after biodegradation………………………169

LIST OF FIGURES

Figure 2.1 Natural fibre chopped form (left side) and mat form (right side)………33 Figure 2.2 Basic principles of aerodynamic web formation ………………………38 Figure 2.3 Rotary through-air bonding system with restraining wire……………...43 Figure 2.4 Carding machine used for the web formation of 3D shell structure……45 Figure 2.5 Schematic diagram of the film stacking process……………………….51 Figure 2.6 Procedures for preparation of PLA nonwoven flax composites………..52 Figure 2.7 Carding process for manufacturing biocomposites…………………….53 Figure 2.8 Classification of biocomposites ………………………………………..57 Figure 2.9 Schematic representation of a flax fibre from stem to microfibril……..64 Figure 4.1 Schematic diagram of air laying web forming process………………...82 Figure 4.2 Complete air laying web forming machine (a), feeding unit (b), carding and stripping unit (c), and transporting unit (d)……… …...... 83 Figure 4.3 Two aluminium platens (a) and Hot pressing instrument (b)…………..84 Figure 4.4 Schematic diagram of prepreg fabrication process route………………84 Figure 4.5 Prepreg fabrication process route in the Laboratory…………………...85 Figure 4.6 Compression moulding machine……………………………………….87 Figure 4.7 Metal platens used as mould unit to produce flat composite…………..88 Figure 4.8 Composite manufacturing process route in the Laboratory……………89 Figure 4.9 A line diagram of a typical processing cycle…………………………...89 Figure 4.10 Typical samples of the composite material…………………………….90 Figure 4.11 Principle of 3D web forming system…………………………………..92 Figure 4.12 Principle of 3D web consolidation……………………………………..93 Figure 4.13 Mould unit (b) with dome shaped mould (a)…………………………...94 Figure 4.14 3D Prepreg fabrication process route in the Laboratory………………..95 Figure 4.15 Aluminium Mould used for 3D biocomposite………………………….96 Figure 4.16 3D Composite manufacturing steps in the Laboratory…………………96 Figure 4.17 Dome shaped 3D biocomposite samples……………………………….97 Figure 5.1 Schematised testing protocol of a fibre……………………………….100 Figure 5.2 Nonwoven prepreg undergoing breaking strength testing…………….102 Figure 5.3 Length-wise direction of prepreg samples for tensile strength test…...102 Figure 5.4 Equipment used in digestion method: (a) Oven, (b) Desiccator 8

with crucibles having samples, (c) Digestion and washing apparatus, and (d) weighing balance………………………………….105 Figure 5.5 Unacceptable failures (a) and Acceptable failures (b)………………...108 Figure 5.6 Vertical band saw……………………………………………………..108 Figure 5.7 Composite specimen undergoing tensile testing……………………...109 Figure 5.8 Flexure testing assembly……………………………………………...111 Figure 5.9 Failure of composite samples in different modes under bending load..112 Figure 5.10 Composite specimen undergoing flexure testing……………………...113 Figure 5.11 Schematic representation of the Charpy (a) and Izod (b) impact equipment……………………………………………………………..115 Figure 5.12 Scheme of designations describing the direction of blow…………….116 Figure 5.13 Composite specimen undergoing impact testing……………………...117 Figure 5.14 Notched samples for Izod impact test…………………………………117 Figure 5.15 Top and bottom platens used for crush test…………………………...118 Figure 5.16 3D Composite specimen undergoing crush testing…………………...118 Figure 5.17 Typical load-displacement curve of the composite crush test………...119 Figure 5.18 Schematic of Differential Scanning Calorimetry……………………..121 Figure 5.19 Schematic DSC curve demonstrating the appearance of several common features……………………………………………...122 Figure 5.20 Differential scanning calorimetry – DSC Q100 used in measuring thermal characteristics of sample……………………………………..123 Figure 5.21 A typical DSC curve of first heating, cooling and second heating cycle…………………………………………………..123 Figure 5.22 Thermogravimetric analyser – TGA Q500 used in measuring thermal characteristics of sample……………………………………..124 Figure 5.23 Water absorption measurements………………………………………125 Figure 5.24 Soil burial test for biodegradation…………………………………….126 Figure 5.25 Edwards coating system, E306A, USA (a), Scanning Electron Microscope, Philips XL30 (b)………………………………………...128 Figure 5.26 Prepared samples for SEM……………………………………………129 Figure 5.27 Projectina Micro Macro Projection Microscope (MMP-1000)……….129 Figure 6.1 SEM micrograph of the diameter of the flax fibres…………………...132 Figure 6.2 Effect of different axis on the tensile strength of prepreg…………….134 Figure 6.3 Fibre orientations in the prepreg………………………………………134 9

Figure 6.4 Effect of process variables on the density of the biocomposites……...135 Figure 6.5 Effect of process variables on the constituents of the biocomposites...136 Figure 6.6 Effect of process variables on the void content of the biocomposites...137 Figure 6.7 SEM micrograph of the fracture surface of the biocomposite. (a) shows the fibre fracture and fibre pull-out. (b) shows the lumen in the flax fibre………………………………………………..137 Figure 6.8 SEM micrographs of fracture surface of 40% (a), 50% (b) and 60% (c) flax fibre reinforced composites………………………...138 Figure 6.9 SEM micrographs of the biocomposite……………………………….138 Figure 6.10 Effect of moulding pressure on tensile strength………………………139 Figure 6.11 Tensile stress-strain curves of PLA and PLA/Flax biocomposites…...140 Figure 6.12 Effect of process variables on tensile strength………………………..141 Figure 6.13 Effect of process variables on tensile modulus……………………….141 Figure 6.14 Effect of process variables on flexural strength………………………143 Figure 6.15 Effect of process variables on flexural modulus……………………...143 Figure 6.16 SEM micrographs of the longitudinal section of flexural tested samples…………………………………………………………144 Figure 6.17 Effect of process variables on notched Izod impact strength…………145 Figure 6.18 SEM micrographs of fracture surface of 40% (a), 50% (b) and 60% (c) flax fibre reinforced composites………………………...146 Figure 6.19 Photograph of the impact tested biocomposite samples with 60% flax fibre…………………………………………………………146 Figure 6.20 DSC curves of first heating cycle as a function of temperature of neat PLA and PLA/Flax biocomposites……………………………149 Figure 6.21 DSC curves of second heating cycle as a function of temperature of neat PLA and PLA/Flax biocomposites…………………………....149 Figure 6.22 Thermogravimetric curves as a function of temperature of neat PLA, neat flax and their biocomposites……………………….150 Figure 6.23 Derivative thermogravimetry curves as a function of temperature of neat PLA, neat flax and their biocomposites……………………….152 Figure 6.24 The density of 2D and 3D biocomposites at different fibre composition and moulding temperature………………………………153 Figure 6.25 The density of 2D and 3D biocomposites at different moulding time with fixed fibre composition and moulding temperature……….153 10

Figure 6.26 The void content of 2D and 3D biocomposites at different fibre composition and moulding temperature………………………………154 Figure 6.27 Load-displacement curves of 3D biocomposites with different fibre content…………………………………………………………..155 Figure 6.28(a) Progress of crashing test of 60P/40F 3D biocomposite……………….156 Figure 6.28(b) Progress of crashing test of 50P/50F 3D biocomposite……………….156 Figure 6.28(c) Progress of crashing test of 40P/60F 3D biocomposite……………….156 Figure 6.29 Load-displacement curves of 3D biocomposites with different moulding temperature……………………………………….157 Figure 6.30 Load-displacement curves of 3D biocomposites with different moulding time……………………………………………….158 Figure 6.31 Effect of process parameters on the total energy absorption………….159 Figure 6.32 Effect of process parameters on the specific energy absorption……...160 Figure 6.33 Effect of process parameters on the crush force efficiency…………...160 Figure 7.1 Effect of fibre composition on water absorption of the biocomposites……………………………………………………..162 Figure 7.2 SEM micrographs of the biocomposite surfaces with different fibre content…………………………………………………163 Figure 7.3 Effect of process variables on water absorption of the biocomposites after 45 days…………………………………………..163 Figure 7.4 SEM micrographs of the biocomposite surfaces with different moulding temperature and time: a) 180 °C and 5 minutes, b) 200 °C and 15 minutes……………………………………………..164 Figure 7.5 Weight loss percentages of PLA, flax fibre and their biocomposites...166 Figure 7.6 Effect of the burial time on the residual flexural strength……………167 Figure 7.7 Effect of the burial time on the residual flexural modulus…………...168 Figure 7.8 Effect of the burial time on the residual notched Izod impact strength…………………………………………………..169 Figure 7.9 SEM images show the morphology of neat PLA (a – e), 60 PLA/40 flax (f – j), 50 PLA/50 flax (k – o), and 40 PLA/60 flax (p – t) biocomposites at different burial time………..171 Figure 7.10 Comparison between water absorption and biodegradation………….172

LIST OF EQUATIONS

Equation 4.1 1 Ton pressure on Ram = × 154.44 Bar pressure…………..86

( ) 2 Equation 4.2 Am = 2πr …………………………………………………………..94

4 Equation 5.1 = × 10 …………………………………………………..99

Equation 5.2 = ………………………………………………………….100

Equation 5.3 M = …………………………………………………….101

Equation 5.4 M = …………………………………………………..101

Equation 5.5 = ……………………………………………………103

Equation 5.6 = × 100 ……………………………………………….105

Equation 5.7 = 100 - ………………………………………………… ...106

Equation 5.8 = × ……………………………………………………..106

Equation 5.9 = (100 - ) × ……………………………………………...106

Equation 5.10 = 100 - [ ( ) ] ……………………….107

Equation 5.11 …………………………………………………………..109

Equation 5.12 ( ) ……………………………………………..110

Equation 5.13 …………………………………………………..110

Equation 5.14 ………………………………………………………113

Equation 5.15 { ( ) ( )} ………………………………...113

Equation 5.16 …………………………………………………………...113

Equation 5.17 { ( ) ( ) } ……………………..114

Equation 5.18 ( ) …………………………………………………..114

Equation 5.19 = × 10³ ……………………………………………….117

Equation 5.20 SEA = ………………………………………………………...120

( ) ∫ Equation 5.21 = ……………………………………………...120

Equation 5.22 ηc = × 100 % ………………………………………………120

Equation 5.23 Xc = ………………………………………………………124

Equation 5.24 C = × 100 % …………………………………………….125

Equation 5.25 Weight loss (%) = × 100 ………………………………...127

Equation 5.26 Residual mechanical property (%) = × 100 …………………..128

Equation 6.1 Vo % = 0.96 F + 0.005 Te – 0.28 Ti – 29.3 …..…………………..139

Equation 6.2 = 71.52 + 0.39 F – 0.4 Te – 0.44 Ti ………..……………….147 Equation 7.1 W% = 1.59 F + 0.32 Te + 0.75 Ti – 106.32 ……….…………….164

LIST OF ABBREVIATIONS

PLA Polylactic acid ELV End of life vehicle 2D Two dimentional 3D Three dimentional DSC Differential Scanning Calorimeter TGA Thermogravimetric Analysis DTG Derivative Thermogravimetry MPa Megapascal GPa Gegapascal ABS Acrylonitrile butadiene-styrene PET Polyethylene terepthalate

Tg Glass transition temperature

Tm Melting temperature

Tc Crystallization temperature LOI Limiting oxygen index DCM Dichloromethane MMC Metal Matrix Composites CMC Ceramic Matrix Composites PMC Polymer Matrix Composites RTM Resin transfer moulding SMC Sheet moulding compound NFCs Natural fibre composites RFACS Robotic fibre assembly and control system PP Polypropylene

∆Hm Melting enthalpy SEM Scanning Electron Microscope FEG Field emission gun DFL Dry film lubricant RH Relative humidity MD Machine direction CD Cross-machine direction 14

100 PLA Neat Polylactic acid 60P/40F A blend of 60% of PLA fibre and 40% of flax fibre by weight 50P/50F A blend of 50% of PLA fibre and 50% of flax fibre by weight 40P/60F A blend of 40% of PLA fibre and 60% of flax fibre by weight

ABSTRACT

The awareness of environmental sustainability drives the composite industry to utilize natural fibres. Natural fibres are a readily available resource with a relatively low price. In this study natural fibre flax reinforced polylactic acid (PLA) biocomposites were made using a new technique incorporating an air-laying nonwoven process. Flax and PLA fibres were blended and converted to fibre webs in the air-laying process. Composite prepregs were then made from the fibre webs. The prepregs were finally converted to composites by compression moulding. The relationship between the main process variables and the properties of the biocomposite was investigated. It was found that with increasing flax content, the mechanical properties increased. As the moulding temperature and moulding time increased, the mechanical properties decreased. The physical, thermal and morphological properties of the biocomposites were also studied. The appropriate processing parameters for the biocomposites were established for different fibre contents.

The biodegradability and water absorption properties of the composites were evaluated. The composites were incubated in compost under controlled conditions. The percentage weight loss and the reduction in mechanical properties of PLA and biocomposites were determined at different time intervals. It was found that with increasing flax content, the mechanical properties of the biocomposites decreased more rapidly during the burial trial. The increasing of flax content led to the acceleration of weight loss due to preferential degradation of flax. This was further confirmed by the surface morphology of the biodegraded composites from Scanning Electron Microscope (SEM) image analysis.

This study also investigated the manufacturing of 3D PLA/Flax nonwoven prepregs by using a new system of 3D nonwoven web formation, and 3D biocomposite was made using these prepregs. A new mould unit for web and a new aluminium mould for biocomposite were developed. The physical properties of 3D biocomposites were investigated and it was found that there is no significant difference between 2D and 3D biocomposites in density and void content. The effects of fibre content and processing variables on the crushing behaviour, energy absorption and failure mode of 3D shell biocomposites were experimentally studied.

DECLARATION

No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.

Shah Alimuzzaman

i. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the “Intellectual Property”) and any reproductions of copyright works in the thesis, for example graphs and tables (“Reproductions”), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see http://documents.manchester.ac.uk/DocuInfo.aspx?DocID=487), in any relevant Thesis restriction declarations deposited in the University Library, The University Library’s regulations (see http://www.manchester.ac.uk/library/ aboutus/regulations) and in The University’s policy on presentation of Theses.

This thesis is dedicated to my (late) father (Mr. Shah Shamsul Islam), mother (Mrs. Hamida Begum), father in law (Mr. M.H. Samad), mother in law (Mrs. Rina Samad), my three daughters (Eusha Zaman Saima, Sumaia Bentay Belal, Hafsa Bentay Belal) and my beloved wife (Mrs. Afrina Parvin Sumona).

ACKNOWLEDGEMENTS

First and foremost, praises and thanks to Allah S.W.T. who bestowed upon us all the blessings and the faculties of thinking, learning and searching.

I would like to thank my respected supervisor Dr. R. Hugh Gong for accepting me as a PhD student under his supervision, which has created the opportunity for me to carry out this research project at The University of Manchester. At the same time I am sincerely wishes to thank him for his supervision, continuous guidance, advices and encouragement through the period of this study.

Special thank goes to The Government of the People’s Republic of Bangladesh, for offering the Bangabandhu Fellowship. This study would not have been possible without the financial support of my sponsor, Bangladesh Government funded through the “Bangabandhu Fellowship on Science and Information & Communication Technology” project.

My thanks also go to Dr. Mahmudul Akando, Tilsatec Advanced Materials, Tilsatec Ltd., UK for the valuable discussions, advices and cooperation during performing the work by supplying raw materials. I also would like to thank Peter Moroz, Technician (spinning); Andrij Zadoroshnyj, Technician (polymer processing and testing); Mark Harris, Workshop Supervisor; Adrian Handley, Technician (Textile Testing); and Dr. Christopher Wilkins, Senior Experimental Officer (SEM) for the constant laboratory work support.

I also wish to express my appreciation to all staff who assisted me at one time or another and to my colleagues and friends within Textiles and Paper of this University for their help and encouragement during my study.

Finally I would like to thanks my parents, my wife, my three daughters and other family members for providing me moral support during my studies.

BIOGRAPHICAL

Author

Shah Alimuzzaman

Professional

The author is working as Associate Professor in the Fabric Manufacturing Engineering Department, Bangladesh University of Textiles (BUTex), Dhaka, Bangladesh since October 2009. Previous teaching experience includes, working as Assistant Professor and Lecturer in the same University since 1993-2009. Industrial experience includes, working as Assistant Dyeing Master in R.A. Knit Dyeing and Finishing Mills (Pvt.) Ltd. Narayangonj, Bangladesh since 1992 – 1993.

Professional Membership

CText FTI, The Textile Institute, Manchester, UK. Life Fellow, Institute of Engineers of Bangladesh (IEB). Life Member, Institute of Textile Engineers and Technologists (ITET), Bangladesh.

Education

Bachelor of Science in Textile Technology, College of Textile Technology (present Bangladesh University of Textiles), University of Dhaka, Bangladesh, 1986 – 1989 (Held in 1992).

Master of Engineering in Textile Technology, University of Gent, Belgium, 1994 – 1996.

Master of Philosophy (MPhil) in Textile Technology, University of Manchester Institute of Science and Technology (UMIST), Manchester, UK, 2001 – 2003.

Student Conferences

Nonwoven Flax Reinforced PLA Biodegradable Textile Composites, Materials Post Graduate Conference, 16th and 17th May 2013, Renold Building, University of Manchester, Manchester, UK.

Development of Nonwoven Biodegradable Textile Composites, Materials Post Graduate th th Conference, 17 and 18 May 2012, Material Science Building, University of Manchester, Manchester, UK.

3D Nonwoven Biocomposites, Materials Post Graduate Conference, 19th and 20th May 2011, Material Science Building, University of Manchester, Manchester, UK.

International Conference Publication

Nonwoven Flax Fibre Reinforced PLA Biocomposites, 1st International Conference on Natural Fibres – Sustainable Materials for Advanced Applications, University of Minho, Guimaraes, Portugal, 9th, 10th and 11th June 2013.

Journal Publication

Shah Alimuzzaman, R. H. Gong, and M. H. Akonda, “Nonwoven Polylactic Acid and Flax Biocomposites”, Polymer Composites, 2013, Volume 34, Issue 10, Page (1611 – 1619).

Shah Alimuzzaman, R. H. Gong, and M. H. Akonda, “Impact Property of PLA/Flax Nonwoven Biocomposite”, Hindawi Publishing Corporation, Journal of Conference Papers in Materials Science, Volume 2013, Article ID 136861.

Shah Alimuzzaman, R. H. Gong, and M. H. Akonda, “Three-Dimensional Nonwoven Flax Fibre Reinforced Polylactic Acid Biocomposites”, Polymer Composites, 2014, Volume 35.

Shah Alimuzzaman, R. H. Gong, and M. H. Akonda, “Biodegradability of Nonwoven Flax Fibre Reinforced Polylactic Acid Biocomposites”, Polymer Composites, 2014, Volume 35.

CHAPTER 1

INTRODUCTION

1.1 Preamble

Composite material is a mixture of two or more distinct constituents. The constituents retain their identities in the composite. Namely, they do not dissolve or otherwise merge completely into each other although they act in concert. Normally, the constituent components can be physically identified and exhibit an interface between one another. The properties of a composite can be significantly different from those of the constituents alone but are considerably altered by the constituent structures and contents. From a structural point of view, if the composites are anisotropic, their mechanical properties are different in different directions. Most of the living tissues such as bone, dentin, collagen, cartilage, and skin are essentially composites. These are natural composites. Synthetic composites are usually a combination of two constituent phases, i.e. a reinforcing phase such as fibre or particle and a continuous phase called matrix.

The normal view is that it is the properties of the matrix that are improved on incorporating another constituent to produce a composite. A composite may have a ceramic, metallic or polymeric matrix. The mechanical properties of these three classes of material differ considerably. The reinforcing phase or reinforcement enhances or reinforces the mechanical properties of the matrix. In most cases the reinforcement is harder, stronger and stiffer than the matrix, although there are some exceptions. Reinforcing fibre may be carbon fibre, glass fibre, aramid fibre, natural fibre (flax, hemp, jute, sisal, bamboo etc.). There are two types of polymer based matrix materials, one is synthetic polymer based called synthetic matrix (polyester, polypropylene, polyethylene, epoxy, etc.) and the other is natural or biopolymer based called natural matrix also called biodegradable matrix. Reinforcing fibres can be used in any suitable form such as yarn, woven fabric, knitted fabric or nonwoven fabric etc. Matrix materials can also be used in any suitable form such as fibre (continuous filament or staple fibre), liquid, film or powder etc. [Matthews and Rawlings, 2006].

Biocomposites are the combination of natural fibres (biofibres) such as wood fibres (hardwood and softwood) or nonwood fibres (wheat, kenaf, hemp, jute, sisal, and flax) with polymer matrices from both of renewable and non-renewable resources. Biofibres are one of the major components of biocomposites. The fibrous material derived from tree, plant, or shrub sources is defined as biofibre [Sharma et al., 2011]. The degree of biodegradability in biobased polymers depends on their structure and their service environment. Natural or biofibre composites are emerging as a viable alternative to glass fibre composites, particularly in automotive, packaging, building, and consumer product industries, and becoming one of the fastest growing additives for thermoplastics [Mohanty et al., 2005].

1.2 Research Background

Regular polymer composites are non-biodegradable and pollute the environment. The ever increasing environmental awareness has brought much attention to the development of recyclable and environmentally–sustainable composite materials. Environmental legislation as well as consumer demand in many countries is putting more pressure on manufacturers of materials and end-products to consider the environmental impact of their final products at all stages of their life-cycle, including recycling and ultimate disposal. Consequently, there are an increasing number of scientists and engineers who are dedicated to minimizing the environmental impact of composite production. Using natural fibres with polymers based on renewable resources will allow many environmental issues to be solved. The so called green composites are being produced by embedding biofibres with a variety of renewable resource-based biopolymers [Sharma et al., 2011]. Bledzki et al. [2006] pointed out the importance of biocomposites from biofibres in the automotive industry. The end of life vehicle (ELV) directive in Europe states that by 2015, vehicles must be constructed of 95% recyclable materials, with 85% recoverable through reuse or mechanical recycling and 10% through energy recovery or thermal recycling [Peijs, 2003]. This will lead to an increased use of biofibres. The diverse range of products utilizing biofibres and biobased resins is giving life to a new generation of composites for a number of applications like hurricane-resistant housing and structures [Wool, 2002]. Biocomposites are becoming more popular because of the increase in oil price, recycling and environmental concerns. In biocomposites, the petrochemical resin is replaced by a vegetable or animal resin and the reinforcement (glass fibre, carbon fibre, 24

etc.) is replaced by natural fibres. PLA (Polylactic acid) is frequently used as the matrix in biocomposites [Fowler et al., 2006; John and Thomas, 2008 and Avella et al., 2009]. It is a synthetic aliphatic polyester from renewable agriculture products, it is biodegradable and with properties comparable to some fossil-oil-based polymers [Oksman et al., 2003].

Natural fibres such as cellulose based fibres have been widely recognized as strong natural reinforcements for soft recyclable polymers to form relatively high strength biodegradable composites compared with traditional polymers. Since these fibres are fully extracted from the nature, their advantages in terms of biodegradability and re- productivity are outstanding. Many studies have been carried out to investigate the suitability of natural fibres such as flax, hemp, ramie, and sisal as reinforcement [Romhany et al., 2003 and Oksman et al., 2003]. Flax fibres exhibit some unique mechanical properties. Baley [2002] and Charlet et al. [2007] showed that flax fibres can have specific tensile properties greater than those of E-glass fibres. Other advantages such as production with low investment cost make natural fibres an interesting product for low-wage countries. Thermal recycling is also possible where glass causes problems in combustion furnaces. The low specific weight results in a higher specific strength and stiffness than glass. It provides better thermal and acoustic insulation properties, especially as an automotive interior or construction material part, due to the presence of lumen/void in the fibre [Taylor, 2002 amd Holbery and Houston, 2006]. There is no additional requirement for an extensive and costly recovery and separation infrastructure for recycling. These natural fibres do not cause any allergies or lung diseases if breathed in or contact with. However, they also have some disadvantages such as that the price of fibres can fluctuate due to harvest or agricultural politics; lower durability, although fibre treatments can improve this considerably; high moisture absorption, which causes swelling of the fibres; variability in physical and mechanical properties such as lower strength properties, particularly impact strength; decomposition in alkaline environments or in biological attack and photochemical degradation because of the ultraviolet radiations [Golbabaie, 2006].

The reinforcing fibres are either mixed with resin to manufacture composites like sheet moulding compounds, or they are first converted into some form of reinforcements such as fibre mats and then these mats can be impregnated with resin to manufacture

composites. Thermoplastic fibres can also be mixed with the natural fibres to form mats and subsequently the thermoplastic fibres are melted to act as a matrix for the composites. Woven, knitted, braided and nonwoven fabrics are widely used as preform or prepreg for composite production. Nonwovens have a high potential as reinforcement in relatively low cost composites because they involve fewer steps during manufacturing as compared to other types of textile reinforcements. In a nonwoven, the assembly of textile fibres is held together by mechanical interlocking, fusing of the fibres, as in the case of thermoplastic fibres, or by bonding with an adhesive medium such as starch, casein, rubber latex, a cellulose derivative or synthetic resin. Most nonwoven composites are based on needle-punched webs. The needle punching process can cause fibre damage. In recent years, extensive research has been done concerning the manufacture of 3D (three dimensional) nonwovens. The researchers [Farer et al., 2000 and Dong, 2002] opened up the possibility of manufacturing 3D nonwoven webs and using them as preforms for manufacturing composites. This can eliminate the need to convert flat nonwovens to 3D shapes for applications which require these types of products.

1.3 Research Aim and Objectives

The aim of the present research is to develop a completely biodegradable composite material based on flax fibre and PLA matrix. This research also aims to investigate the properties and processing of nonwoven flax fibre reinforced PLA biocomposites to establish the appropriate processing parameters. Another aim is to explore the possibility of producing 3D composite material directly from 3D shell structured nonwoven fabric (prepregs). In order to achieve these aims, the following tasks were planned:

 Studying the related literature in the field of biocomposites;  Selecting suitable raw material for processing in the available laboratory machine and selecting suitable blend ratios of reinforcing fibre to matrix fibre for making the nonwoven webs.  Manufacturing the flat nonwoven webs by using suitable techniques and investigate the quality of the webs.  Converting the flat nonwoven web to composite using hot-pressing.

 Investigating the effect of different processing variables on the physical and mechanical properties of these biocomposites to establish the optimum processing parameters.  Finally, exploring the possibilities of manufacturing 3D shell structured nonwoven webs and using these as prepreg for manufacturing 3D biocomposites, and investigating the effects of processing parameters on the quality of these biocomposites.

1.4 Thesis Layout

Chapter 1 gives a general introduction on composite and biocomposite materials, and describes the research background and objectives.

Chapter 2 presents a review of the literature in the areas of textile composites, technologies relevant to the project, and the properties of flax and PLA fibres.

Chapter 3 presents the methodology of this research.

Chapter 4 describes the equipment and techniques employed for the production of samples used in this study. This includes the study of air-laying nonwoven web formation, web bonding methods, and the fabrication of composite panels using compression moulding. This chapter also includes the fabrication of 3D mould units for 3D nonwoven web and 3D composite panel. 3D nonwoven web formation and 3D composite panel fabrication methods are also described in this chapter.

Chapter 5 presents the physical, mechanical and thermal property test methods and equipment used to characterise the materials, prepreg, 2D and 3D biocomposite panels. The scientific principles involved in the techniques and the standard methods followed by the techniques are described in this chapter.

Chapter 6 includes the results and discussion regarding the effect of process variables on the physical, mechanical and thermal properties of flat biocomposite panels based on the standard testing methods. This chapter also focus on the stability of biocomposite panels by evaluating the thermal properties analysis and the effect of process variables on the physical and mechanical properties of 3D biocomposite.

Chapter 7 focus on the biodegradability of flat composite panels by the standard soil- burial test.

Chapter 8 ends the thesis with conclusions and recommendations for future work.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The market demand for environment friendly materials is rapidly increasing. Biodegradable fibres and polymers, mainly extracted from renewable resources, are expected to be a major contribution to the production of new industrial high performance biodegradable composites, partially solving the problem of waste management. This project was concerned with developing flax fibre reinforced PLA biodegradable composite material using nonwoven air-laying web forming technique. The work was based on a combination of textile and composite technologies. This chapter includes a short introduction to composites and nonwoven technologies followed by a literature survey to give a brief overview of areas relevant to this project.

2.2 Composite Material

Two inherently different materials are mixed to form a new material called composite material which is different to both but better in properties. Composites are developed by emphasizing the good properties of different materials while avoiding their drawbacks [Bolton, 1998]. In the most commonly used composites, one constituent is known as the reinforcing phase (in the form of fibres, particles, or flakes) and the other serves as a medium which is known as the matrix (in continuous form) [Kaw, 2006]. The reinforcements are embedded in the matrix to improve its properties. Reinforcement fibres are usually of high strength/stiffness and are generally orthotropic (having different properties in different directions). Reinforcement is the main constituent of composite materials responsible for its mechanical properties. Normally, matrix or resin is a synthetic polymer with an objective to bind the reinforcement elements. The matrix material is ordinarily of a high performance type. Moreover, both fibres and matrix may be organic or inorganic in nature [Reinhart, 1998 and Peters, 1998].

Composite materials are increasingly replacing conventional metallic materials due to their high strength, light weight, long life, net shape manufacturing and design flexibility. The term ‘Textile Composite’ refers to a class of innovative composite 29

materials in which the reinforcement is produced from any of the textile processes, namely spinning, weaving, knitting, braiding or nonwoven [Bogdanovich and Pastore, 1996]. There are endless possibilities for forming composites by using different combinations of reinforcements, fillings and matrices.

2.2.1 Classification of Composites

There are two classification systems of composite materials. One of them is based on the matrix material and the other is based on the reinforcing material structure [Kopeliovich, 2010].

2.2.1.1 Classification on the basis of Matrix Material

On the basis of the matrix used, composite materials can be classified into three categories.

 Metal Matrix Composites (MMC)  Ceramic Matrix Composites (CMC)  Polymer Matrix Composites (PMC)

For this project the interest is in the field of polymer matrix composites where resins are used as polymer matrices. On the basis of resin used, composites can be classified into two categories, such as thermoset composites and thermoplastic composites. To manufacture thermoset composites the thermoset resins are used as a matrix material and thermoplastic resins are used to produce thermoplastic composites.

Polymer matrix composites are also formed by mixing thermoplastic fibres with reinforcing fibres and then the blend is subjected to a high temperature and pressure so that the thermoplastic fibres melt and act as a matrix material [Vaidya, 2002]. This system of manufacturing thermoplastic composite enhances the delamination resistance of composite materials.

An advantage of thermoplastic composites is their ability to be recycled by remelting and remoulding. However, after several recycling cycles, the mechanical performances of the matrix are affected and the composites need to be sent for disposal. In most cases, inorganic reinforcements such as glass fibres remain. In order to fully recycle the composite materials without creating any extra pollution, the use of natural

biodegradable reinforcements such as flax or hemp fibres combined to a biodegradable matrix is required. These composites can then be composted. The carbon dioxide formed during the composting step is balanced by the carbon dioxide fixed during the growing step of the flax plant [Pervaiz and Sain, 2003].

2.2.1.2 Classification on the basis of Reinforcing Material

On the basis of the reinforcing material and the structure, composite materials can be classified into three categories [Marquelte, 2011].

 Fibre Reinforced Composites  Particle Reinforced Composites  Dispersion Strengthened Composites

In fibre reinforced composites, fibres (natural or synthetic) are used as reinforcing materials in different forms and architectures. Textile composite is a kind of fibre reinforced composites, where textile fibres are used as reinforcing material. For this research the interest is in the field of fibre reinforced composites where textile fibres are used as reinforcement. The fibre element performs the main load-bearing component in fibre reinforced composites. These types of composites create lightweight yet durable and rigid materials. Fibre reinforced composites can be classified according to the form in which the reinforcement fibre material is used. These are short discontinuous, long discontinuous and continuous fibre reinforced composites. It can be further classified according to the structure of the reinforcement such as woven, nonwoven, braided, knitted etc.

The parameters of fibres i.e. length, orientation and volume content dominate the engineering properties of the composite. Among them, the length of the fibre is very important and continuous and long discontinuous fibre composites are better in terms of engineering properties [Reinhart, 1998]. Short or long staple fibre composite prepregs are discontinuous in nature. These short or long chopped fibres are either distributed randomly or in a preferred direction. Long fibre composite prepregs are straight continuous in nature. The orientation of the fibres is either unidirectional such as yarns or filaments or bidirectional such as in woven fabrics.

According to their structure composites may be classified as laminates and sandwich panels. To achieve a certain thickness of the product, composites are formed using 31

several layers in the prepreg. These composites are considered as multi-layer or laminate composites. In laminar composites the layers of reinforcement are stacked in a specific pattern to obtain required properties in the resulting composite piece [Miravete, 1999]. These layers are called plies or laminates. Laminates can be composed of reinforcement material which may be nonwoven, braided, fibre reinforced, matt, 2D- woven or uni-directional fibres. Advantages of laminated composites are relatively well defined position of fibres in final composite piece, higher strength, higher fibre to volume ratio; their disadvantages are relatively poor through-the-thickness properties [Tsai et al., 2000 and Potluri et al., 2006] and problems of process induced deformations.

The fibre reinforced composite, where fibres are distributed in random order and based on nonwoven fabric principle is called randomly oriented fibre reinforced composite. Random nonwoven mats consist of chopped fibre strands with a length commonly varying between 25 mm and 300 mm according to the type of mat [Li et al., 2009]. Random fibre composites do not necessarily belong to the short fibre types; chopped rovings may contain fibres of 50 mm in length and continuous filaments. This type of composites can be manufactured from short or long chopped strands as well as continuous strands.

Random fibre composites are mainly used in non-structural applications. Because of their random fibre orientation and the use of mostly discontinuous strands in this type of material, the mechanical properties such as strength and stiffness of these composites are inferior as compared to long and orientated fibre composites. Due to their handleability, isotropy, processability, and especially low cost this type of composite is widely used [Verpoest, 2000].

Random fibre composites can be classified in the following ways:

 Length of the fibres: finite length (chopped) or continuous length;  The bonding method: mechanical, chemical or thermal method;  The areal weight: fleeces or mats;  Introducing method of matrix material into the preform: dry or impregnated preforms.

Random nonwoven mats are also called chopped strand mats. These mats can be made from manmade fibre such as glass fibre as well as natural fibre such as flax or kenaf fibre. Chopped fibre strands are cut from the rovings or yarns. The fibre strands are randomly deposited on a conveyor belt with the help of either gravity, air stream or water dispersion to produce fibre mats. Then the fibres are bonded together with the help of chemical binders (either by a powder or an emulsion binder) sprayed on them or they are bonded mechanically. The mechanical bonding can be obtained by fibre entanglement, knitting or stitching. Figure 2.1 shows the natural fibre chopped form and mat form [Ren et al., 2009].

Figure 2.1 Natural fibre chopped form (left side) and mat form (right side) [Ren et al., 2009].

Fibre entanglement can be achieved by needle punching, or by using air-jet or water-jet action. The deposited fibre strands can also be bonded by heat treatment. This heat treatment can be selected to avoid fibre damage due to the mechanical bonding. The area density of chopped strand mat ranges from 250 to 1000 g/m2. The composites made from chopped strand mats generally have a fibre volume fraction of 25 to 40%. It is lower compared with woven or unidirectional composites [Verpoest, 2000].

2.2.2 Prepreg

A prepreg consists of a combination of a matrix and fibre reinforcement. When the assembly of the textile reinforcement structure and matrix material is made prior to composite manufacture, the structure is referred to as a prepreg. It is ready for use in the composite manufacturing process. Textile preforms are simply different structures of only reinforcing material (without matrix) made by using textile processes such as weaving, knitting, braiding, nonwoven, yarn, etc. Preform can be used to manufacture

both thermoset and thermoplastic composites although it has some disadvantages such as delamination etc., whereas prepreg is mainly used to manufacture thermoplastic composite materials.

The shape of the textile reinforcement for a particular component is realized by carefully placing tows, yarns, fabrics or a combination of them. According to Ogin [2000], prepreg for textile composites may be two dimensional (2D) or three dimensional (3D) and can be made in several ways: by producing the entire structure using regular textile processing techniques such as weaving, braiding, knitting, and nonwoven; or by joining together individual fabrics using techniques such as stitching or bonding; or by a combination of these.

2.3 Manufacturing Process of Fibre Reinforced Composites

The manufacturing system of fibre reinforced composite material involves the following steps

 Formation of prepreg or perform  Composite fabrication

2.3.1 Formation of prepreg

Prepreg or preform can be manufactured by using any type of textile processing mentioned in the previous Section 2.2.2 of this chapter. The nonwoven process is mostly used to produce prepreg for manufacturing randomly oriented fibre reinforced composite materials.

Nonwoven fabrics are structurally different from conventional woven and knitted fabrics. The manufacturing system of nonwoven fabrics is completely different from that of conventionally woven and knitted fabrics. All nonwoven fabrics are based on a fibrous web. Each nonwoven manufacturing system involves the following four generic steps [Turbak, 1993].

 Fibre or raw material selection,  Web formation,  Web consolidation or bonding, and  Web finishing and converting.

The properties of the nonwoven fabrics are largely dependent on fibre properties, fabric structural geometry, and particularly the type of fibre to fibre bonding. The quality of a final nonwoven fabric is closely associated with each of the four steps. Although various nonwoven fabrics are accomplished by selecting the different combinations of the four steps, to meet the requirements of different properties and end-use applications, fabrics are mainly classified on the basis of two steps: web formation technique or bonding technique.

Nonwoven fabrics are classified into four groups on the basis of the web formation technique:

 Dry laid nonwovens,  Wet laid nonwovens,  Spun bonded nonwovens, and  Melt-blown nonwovens.

Nonwoven fabrics can also be classified into three groups on the basis of the bonding technique:

 Mechanically bonded nonwovens,  Chemically bonded nonwovens, and  Thermally bonded nonwovens

Dry laid nonwovens are manufactured with long fibres as compared to their wet laid counterparts. The fibre length for the dry laid nonwovens usually ranges from 25 mm to 70 mm, while the fibre length for the wet laid nonwovens ranges from 2 mm to 20 mm. Since the flax is a long staple fibre with 65 mm fibre length on average, only dry laid method is discussed in detail here as this method is relevant to this work.

2.3.1.1 Nonwovens for flat prepreg manufacturing

Dry laid nonwovens are formed by laying the fibres in the form of a loosely held fibrous web using techniques such as carding and then bonding the fibres together by using any of the bonding techniques mentioned earlier. In a dry laid process, staple fibres are formed into a web following the opening actions by a card. Depending on how the fibres are stripped off the cylinder, this type of web forming technique can be further divided into three sub-categories: [Turbak, 1993]

 Conventional carded,  Centrifugal dynamic (or random) carded, and  Air laid.

In the conventional carded technique, the fibres are carded by a single or double cylinder card, and then the fibres are transferred to a doffer to form a web. A major problem for this technique is the very high ratio of the strength in the machine direction to that in the cross machine direction, usually up to 6-10:1. Although a cross-lapper can be used to increase the isotropy of the webs, the effect is limited. Nonwoven card manufacturers have redesigned the doffing section of the card with one doffer and two condensing rollers for higher production rate. The doffers can run at a higher speed to take as much of the fibre load from the cylinder as possible in order to improve the production rate. The condensing rollers are mounted with special wires and run at a slower speed to condense the web further. The web uniformity is improved by the condensation of the fibres in the web and the machine direction to the cross machine direction strength ratio of 4:1 could be achieved by this process [Australian Wool Innovation, 2007].

In the centrifugal dynamic (or random) carded technique, high-speed random rollers are used in the card. The combination of centrifugal force and an aerodynamic transferring action randomizes fibre orientation, resulting in a decreased ratio of the strength in the machine direction to that in the cross machine direction. This, however, leads to unfavourable compression and consequential buckling and bending of the fibres, and hence, decreased strength of the finished fabric [Fang, 1995].

In air laying technique, the nonwoven webs are formed with the help of air flow. The staple fibres are opened by an opening roller such as a liker-in or a unit based on roller card. The web is formed by passing the mixture of air and short or long staple fibres over a condenser or the fibres fall freely and are collected randomly on a perforated conveyor belt with partial vacuum. Due to the action of the high-velocity air-flow to the fibres, the air laid technique can produce a web with nearly random fibre distribution. Accordingly, the properties of the web are nearly isotropic, which is important for many end-use products. Compared with carded webs, air laid webs have the following benefits: [Wirth, 1981 and Kleppe, 1990]

- Fibres are randomly oriented in all the three dimensions of the webs i.e. isotropic web structure, - Fabric with better compression recovery and more voluminous webs can be produced, - Higher production, - Not restricted to a certain fibre type or genre as the majority of fibres can be processed including ceramic and metal fibres, - Absence of laminar structure (usually no layering) i.e. one stage production, - Economical advantages resulting from the investment volume and the operating cost for the installation, - The isotropic distribution gives a high degree of insulation properties, and - 3D shapes can be made using this air laying method.

In contrast, the main disadvantages are: [Fang, 1995]

- Quality of fibre opening has a strong impact on the uniformity of the web, - Air flow adjacent to the walls of the conduits is irregular and hence the inconsistency in the web structure, and - Possible entangling of fibres in air stream which leads to undesired web faults.

Some basic principles for aerodynamic web formation are discussed below and are shown in the Figure 2.2.

Free Fall: The fibres are opened by an opening roller, and then fall freely on the perforated conveyor belt from the top. The fallen fibres are sucked by air at the bottom to form the web. In this system, the opening roller rotates in the opposite direction to the feed roller.

Overpressure system: In this case, air containing the opened fibres is blown into the duct with pressure. The fibres are driven by the air between a perforated screen and delivery roller to form the web.

Sub-pressure system: The opened fibre and air mixture is passed over the perforated screen. Vacuum is created by the air suction which causes the fibres to form the web.

Closed air circuit: A closed air circuit is created by blowing the air from one side which drives the fibres forward, and simultaneously a vacuum is created by air suction which causes the fibres to form the web on the perforated screen.

Combined overpressure and sub-pressure system: It works similarly to the principle of closed circuit. Separate fans are used for blowing and suction of air in order to have better control on the fibres.

Figure 2.2 Basic principles of aerodynamic web formation [Albrecht et al., 2003].

The quality and uniformity of the web are dependent on the amount of fibres handled in the web formation region, and the aerodynamic force applied to them [Pourmohammadi and Russel, 2000; and Albrecht, 2003]. In this study, the machine used to produce air laid fibre web was working with the principle of sub-pressure system. Despite some detailed differences, an air laid system generally consists of feeding, carding, stripping, transporting and condensing units.

a) Feeding unit: The most essential requirement for a feeding unit is to feed a uniform layer of opened fibres into the carding unit. There are two main methods of feeding, hopper feed and lap feed. Usually, the lap feed can provide a better uniformity, while the hopper feed can give a higher production rate, and it may also lead to compression and consequential buckling and bending of the fibres, and, hence, decreased strength of the finished fabric, as aforementioned [Fang, 1995]. b) Carding unit: The main cylinder of a nonwoven card in an air laid system usually has smaller diameter and higher speed than those of a conventional card. Two main types of carding unit have been developed as follows:

1) Carding unit with a single carding point: carding the fed fibres from the feeding unit directly, for example, the Rando-webber of Curlator and the DOA machine of Austria [Harvey, 1975 and Krima, 1971]. 2) Carding unit with multiple carding points: carding the fibres by more than one toothed roller or flat. In such a case, the fibres are carded before they are stripped by the air flow. Examples of using this type of carding unit include the Fehrer web former K12 and the Proctor 735. c) Stripping unit: In the stripping unit, fibres are stripped from the cylinder by an air-flow coming from a slot adjacent to the cylinder. This stripping action is aided by a centrifugal force produced by the high-speed rotation of the cylinder. d) Transporting unit: This unit is mainly composed of an air duct and a fan that transport the stripped fibres to the condensing unit, the deposition screen. The design of the duct is to provide a turbulence-free air flow, and thus, a uniform distribution of fibres. The duct is generally made into a Venturi or an arched divergent shape so as to reduce the impulsive force of the air flow [Harvey, 1975], although other designs, such as an airfoil shape, have also been used. A distributor may also be used in the duct or above the fibre deposition screen for improving the uniformity of fibre distribution. e) Condensing unit: The deposition screen is the main part of this unit. Two main types of deposition screen are perforated drum and belt, where the laid fibres are

formed into a loosely held fibrous web structure. There is a suction box beneath the screen to produce a partial vacuum and condense the loosely held fibrous web structure to some extent. Owing to the lower centrifugal force, higher production speeds can be used for the belt than for the drum [Harvey, 1975].

Web consolidation is the step of bonding loosely held fibres in the web through some means after a web is formed. This step provides strength and integrity to the web. Bonding fibres together to form a fabric-like structure is critical in the success of nonwoven. There are many different methods of bonding the web to make the final nonwoven products and each has its own characteristics. In general, these methods and techniques are divided into three generic categories: mechanical, chemical and thermal bonding. In mechanical bonding, the nonwoven web is bonded by entangling the fibres through mechanical means, such as needle punching or spunlacing (hydro- entanglement). The needle punching method can cause the fibre damage and affects the mechanical properties of the final product [Lee and Kang, 2000 and Tejyan et al., 2012]. In chemical bonding, some binding polymer such as resin or emulsions is deposited in the web and then cured thermally to achieve bonding. In thermal bonding, the bonding is achieved by fusing the thermoplastic fibres in the web at the cross-over points. Bi- component fibres or a blend of inorganic and thermoplastic fibres are generally used in thermal bonding technique. The low melting point fibres in the blend are melted by heating and solidify on cooling to give strength to the web. Conventionally, the choice of a particular bonding technique is dictated mainly by the ultimate product applications and/or type of the web. Occasionally, a combination of two or more techniques is employed to achieve desired bonding. In this study, the thermal bonding technique is used for the web consolidation. Thus, only further details associated with thermal bonding are described.

The basic idea for thermal bonding was first introduced by Reed [1942]. Although this method involves the application of heat to a nonwoven web containing fusible fibres, it can be used where fibres are not thermoplastic by adding thermoplastic fibres, powders or granules to perform as a binder. Compared to other bonding processes, thermal bonding and the products thus obtained offer a number of advantages. This bonding process is much less energy intensive, kinder to the environment and more economical. It can be applicable to nearly every type of nonwoven webs, and address the demanding

quality requirements of the market place, providing strong bond points that are resistant to hostile environment and to many solvents too [http://www.apparelsearch.com/ education_research_nonwoven.htm; Watzi, 1994]. By varying the process parameters, product properties can vary from nonporous, thin, non-extensible, and non-absorbent to open, bulk, extensible and absorbent. The development of new raw materials, better web formation technologies and higher production speeds have made thermal bonding a viable process for the manufacturing of both durable and disposable nonwovens. It has been the major method preferred by most nonwoven manufacturers [Holiday, 1993].

Thermal bonding technique can be divided into point bonding and area bonding, based on the bonded sites in the web. The point bonding is accomplished with cohering of the filaments in small, discrete, and closely spaced area of the web, formed at certain temperature and pressure. Since point bonding can be accomplished with as little as 10% bonding area with 90% un-bonded area, such webs are considerably softer and more textile-like [Smorada, 1978]. The area bonding involves the use of all available bond sites in the web; however, every filament/fibre contact is not necessarily bonded, since not every contact necessarily is capable of forming a bond [Turbak, 1993; http://www.apparelsearch.com/education_research_nonwoven.htm]. The bonding is achieved by passing the web through a source of heat, usually steam or hot air, and pressure. The area-bonded web is stiffer and more paper-like in appearance than point- bonded web.

As aforementioned, thermal bonding process is carried out at a temperature at which the fusible fibres or fibre components change into a viscous state. For formed webs whose fibres are not thermoplastic, materials that are thermoplastics must be added to act as binders. A binder polymer generally has the following essential characteristics [http://www.apparelsearch.com/education_research_nonwoven.htm]:

- Efficient melt flow; - Good adhesion to the carrier fibre; - Lower melting point than the carrier fibre; - Appropriate stiffness/elasticity.

Materials that can be used as a binder for thermally bonded nonwovens include:

- Binding fibres;

- Binding powders; - Binding web.

In this study a blend of natural flax fibres and thermoplastic PLA fibres is used to produce nonwoven web. Thus, only further details associated with binding fibres is described.

Single-component and bi-component fibres, as binder fibres, are most widely used in thermal bonding of nonwovens [http://www.apparelsearch.com/education_research_ nonwoven.htm]. Single-component fibres are the least sophisticated, but the most economical. This is because these fibres are often already in existence and low in cost. There are several factors affecting the bonding, including fibre chemistry, morphology, linear density, staple length, crimp and processing conditions.

In the thermal bonding process, the fusion can be achieved by the direct action of heat and pressure via a calender, an oven, a radiant heat source, or an ultrasonic wave source. The degree of fusion determines many of the web qualities, most notably hand or softness. There are mainly four methods to achieve a thermal bond: thermal calendering, hot through-air, radiant heat and ultrasonic bonding. In the present study two types of web such as flat and 3D shell structured web are produced. Thermal calendering principle is used for flat web and through-air bonding is used for 3D web bonding. Thus, further details with these two methods are described.

a) Thermal calendering: In this thermal bonding method, heat and pressure are applied to the web simultaneously by heated rollers. Bonding is achieved by using an amorphous polymer binder fibre, a bi-component binder fibre, a film, or the outer surface of a homogeneous carrier fibre as the bonding agent [Turbak, 1993 and Porter, 1978]. This method uses contact heating, and, hence, has a high heat transfer efficiency. However, the temperature and pressure must be controlled precisely. As the fibrous web is a good barrier of heat transfer, thick nonwovens cannot be bonded by simple hot calendering because the required temperature can hardly be achieved across the thickness of the webs without over heating on the surface of the webs [Gibson and McGill, 2000 and Shimaala and Whitwell, 1976]. In this study two heated platens are used as compression moulding instead of heated roller for web bonding, because the prepreg is finally subjected to compression moulding for converting to composite material. 42

b) Hot through-air bonding: In this process, a loose web containing base and binder fibres [Randall and Thibodeau, 1984] is subjected to a heated gas, usually air, which is drawn through it by a vacuum [Jirsak and Wadsworth, 1999 and Randall and Thibodeau, 1984]. As a result, heat is transferred to the fibres by convection and conduction [Albrecht et al., 2003], which causes the binder fibres to either soften or melt and thermally bond at the crossover points after the subsequent cooling of the fibres. Generally there are two configurations for hot through-air bonding systems; the perforated drum (the rotary system) and the perforated conveyor (the flat bed system). Of the two systems, the rotary system is preferred due to its compact design, as it packs all the units into one insulated housing, which gives it lower energy consumption due to minimal thermal loss [Russel, 2007].

Figure 2.3 [Russel, 2007] illustrates a typical hot through air rotary bonding system in which a web is travelling around a porous cylinder supported by a woven wire or synthetic fabric mesh. A suction fan is then used to suck the heated air from the top side of the web, through it and then finally through the porous supporting cylinder. In some cases, a pressure or restraining wire may be used to hold the web as it revolves around the cylinder and to control the surface properties of the final web [Randall and Thibodeau, 1984].

Figure 2.3 Rotary through-air bonding system with restraining wire [Russel, 2007].

The tricky aspect in hot through-air bonding is to control the temperature and air flow in such a way that the web is heated quickly to the melting point of the binder fibre and then the air flow has to be reduced to minimize the variation in web thickness [Russel, 2007].

Fabrics with area densities ranging from 20 to 4000 g/m2 and thicknesses of up to 200 mm are successfully bonded by the hot through-air process [Albrecht et al., 2003] and the final products usually possess softness, drapability and high bulk [Russel, 2007].

2.3.1.2 Three dimensional (3D) nonwovens for 3D prepreg manufacturing

Extensive research has been carried out in this field in recent years. In the textile industry, nonwovens are manufactured in a 2D structure as flat fabrics and then these fabrics are converted to 3D structures for making different end products by sewing and fusing processes. If 3D nonwoven shell structures can be produced in a single process, directly from fibres, the packaging, freight and labour costs and the cost of wastage inevitably generated during panel cutting can be saved possibly up to 70% of the production cost [Gong et al., 2000 and Dong, 2002]. It can also shorten the process and save equipment investment, space and energy. This research is focussing on the area of producing 3D shapes in order to make seamless products. This innovation would help to avoid the process of conversion.

The manufacturing of 3D nonwovens will potentially be more productive, efficient and cost effective. It is one of the newer and least developed nonwoven processes and techniques. In the recent past, researchers have found some methods of converting fibres to 3D shell structures by the process of melt-blowing and also by combining the air laying process of web formation with through air thermal bonding. The following methods are used to manufacture 3D nonwovens:

 Robotic fibre assembly and control system (RFACS);  3D Nonwoven shell structures.

3D melt-blown nonwoven structures are produced at North Carolina State University by the integration of robotics with the melt-blown process [Farer et al., 2000; Farer et al., 2003; and Velu, 2003]. Since this research is concerned with natural flax fibre and chopped-strand PLA fibre nonwovens, the RFACS method is not suitable.

The 3D nonwoven process developed in the University of Manchester [Gong et al., 2000; Gong et al., 2001; and Wang, 2005] to produce 3D nonwoven shell structures is divided into the following two steps:

 Formation of 3D web in the shell structure, and  Consolidation of the web structure.

The staple fibres are opened by an opening unit based on a roller card. These opened fibres are stripped off the cylinder surface with the help of high velocity airflow and are then carried to perforated 3D moulds. The moulds carrying the fibres on their surfaces are moved out of the mould chamber with the help of guide tracks into a bonding chamber for consolidation.

Figure 2.4 Carding machine used for the web formation of 3D shell structure [Dong, 2002].

The systems of the web formation and consolidation are shown in the Figure 2.4. The web formation system is based on airflow which is introduced into the duct adjacent to the cylinder and the outlet of the duct is connected to the mould chamber. The size of the chamber is dependent on the type of the mould. The side walls of the duct are

parallel to each other and the width of the duct is equal to the working width of the card. The fibre distribution in the air as well as on the surface of the mould is highly dependent on the design of the duct which is vertically divergent at the end.

The consolidation is achieved by the process of through-air thermal bonding. 3D nonwoven shells formed on the mould surface are taken out of the mould chamber and are introduced into the thermal bonding section. In this section, hot air is drawn through the fibrous web which is supported by the original mould.

The hot air is introduced into the system with the help of flexible adiabatic pipes which are connected to the hot air source. The air outlet is connected to a suction fan. An air guide is provided in the system to improve the airflow distribution around the web.

The entire product is exposed to a uniform temperature with the help of the through air thermal bonding process. The web is subjected to a temperature near to the melting point of binding fibres in order to melt the fibres so that when they solidified, the web is bonded to the desired structure.

2.3.2 Composite fabrication

Most thermoplastic processing operations involve heating, forming into the desired shape, and then cooling. Processing techniques used on thermoplastics could also be used in the processing of short fibre reinforced plastic composites [Advani and Sozer, 2002]. These manufacturing methods are hand lay-up; spray lay-up; filament winding; pultrusion; extrusion and injection moulding; carding and hot pressing; film-stacking and hot pressing; compression moulding; resin transfer moulding (RTM); and sheet moulding compounding (SMC) and hot pressing. A brief description of some of the general composite manufacturing techniques is provided below:

Hand lay-up process is one of the oldest composite manufacturing techniques and is still widely used for prototype part manufacturing and in the marine industry. It is a labour intensive process in which the liquid resin is applied to the mould followed by the placement of the reinforcement. The process of application of resin and reinforcement layer continued until a suitable thickness is achieved. After fibre wet-out, the laminate is allowed to cure. The spray lay-up process is also used as an alternative to hand lay-up process in which the chopped fibres and resin are deposited on to the mould by means of a spray gun [Mazumdar, 2002 and Khan, 2010]. 46

Filament winding process is used for making tubular parts and specialised structures like pressure vessels. The process involves winding the resin impregnated fibres at the desired angle over a rotating mandrel. The fibres moving through the resin bath and after impregnation they move back and forth by means of the guide while the mandrel rotates at a specified speed. The desired angle is achieved by controlling the motion of the guide and the mandrel [Mazumdar, 2002].

Pultrusion is a low-cost and a high volume manufacturing process in which the fibre reinforcement after impregnation with resin is pulled through a heated die to make the part. Pultrusion is used for the fabrication of composite parts with constant cross-section profile e.g. rods, beams, channels, tubes, walkways and bridges, handrails, light poles, etc [Mazumdar, 2002].

Resin transfer moulding (RTM) is the major processing method to make randomly oriented natural fibre nonwoven mats reinforced polymer composites. Rouison et al. [2004] reported making this kind of composites by RTM. First the surfaces of the mould were cleaned and coated with mould release agent. Once these coatings were cured, layers of natural fibre mats having the mould size were placed in the mould cavity. Then the mould was tightly closed and a vacuum was applied. The resin was mixed with the initiator and placed in the injection pot. From there the resin was injected in the mould with compressed air at a constant gauge pressure and make sure that the mould was filled completely. Then the inlet ports were closed as well and hot water at constant temperature was circulated in the mould. The composite was cured under these conditions. The resin injection time was observed to increase dramatically at high fibre contents due to the low permeability of the mat. Keeping a constant mould temperature was the key to obtain fast and homogeneous curing of the part [Rouison et al., 2004].

The advantages associated with the RTM process are: lower investment and operating cost, dimensional accuracy, manufacturing of complex parts, good surface finish, low volatile emission due to closed moulding process. However, the limitations are complex tooling design and also substantial trial-and-error experimentation or flow simulation modelling is required for manufacturing the complex parts [Mazumdar, 2002].

Resin infusion process is an alteration to RTM in which only vacuum is used to drive the resin flow and the laminates are enclosed in a one sided mould covered with a bag. The resin is introduced inside the bag by means of one set of pipe work while the 47

second set allows the vacuum to be drawn from the bag. This technique is commonly known as vacuum bagging [Mazumdar, 2002 and Khan, 2010]. In the vacuum bagging process, the preform is covered by peel ply, resin infusion film and a resin infusion mesh. The whole system is covered by a plastic film and a vacuum bag is formed. On one side of the preform, the vacuum is applied so that extra air is removed and on the other side, a resin infusion tube is inserted. One end of the resin infusion tube is put in a cup or a glass containing resin and the other end is inserted inside the bag. When the vacuum is applied it causes the resin to flow over and through the preform and proper wetting of the fibres is achieved [Cripps et al., 2000].

Injection moulding can be used for the production of natural fibre composites (NFCs). Injection moulding can manufacture geometrically complex components with accurate dimensions and the process is automated. But there is limitation on fibre fraction and fibre length when using injection moulding to process fibre reinforced biocomposites [Advani and Sozer, 2002] because higher natural fibre fraction and longer fibre length will make moulding difficult. During injection moulding, the following takes place: a) the polymer material of powders or pellets form is continuously fed through a hopper into the screw zone and melted; b) then the material is forced under pressure into the mould by axial motion of the screw in order to shape a desired form with a fixed cross- section; c) once the material is in the mould, it is shaped and cooled; d) the mould is opened and the product is ejected, and e) then the mould is closed and it is ready to start the next cycle [Rosato et al., 2000].

Extrusion is needed for injection moulded composite products before injection moulding. This is because injection moulding machines and screws are much shorter than extruders and therefore, the ratio of length to diameter for injection moulding screws is lower than for extruders. The lower length to diameter ratio of the screw in injection machine makes it less efficient in mixing and non-homogenous melt comparison with extruders. For this reason, if the composite is processed by injection moulding, prior extrusion compounding is necessary for the materials. The extrusion process basically consists of continuously shaping a polymer through the orifice of a suitable mould (die), and subsequently solidifying it into a product [Henson, 1997]. The temperature in the extruder should be high enough to ensure the polymer fully melted and low enough to avoid burning the fibre.

In general, injection moulding is the preferred route to manufacturing more complicated shaped panels that are difficult, if not impossible, to make by single press-moulded methods; composite panel manufacturers expect that injection moulding will become increasingly important. At present, practically all natural fibre composites (NFCs) are manufactured using compression moulding (press moulded technology), thermoplastic or thermoset. However the trend is now towards thermoplastic matrix systems. The reason for this change lies in the easier processing and recycling possibilities of thermoplastics.

2.3.2.1 Compression Moulding

Compression moulding is a forming process in which a plastic material is placed directly into an open, heated metal mould cavity, then is softened by the heat, and forced to conform to the shape of the mould as the mould closes. The mould temperature is maintained using electric heaters, and the mould is held shut with a hydraulic cylinder, or toggle clamp. Material is placed in the mould, and it is closed under high pressure and high temperature [Strong, 2006]. Contact with the heated mould surface softens the material, allowing it to fill in the entire cavity and initiating a chemical reaction, which cures the part. Cure time is determined by the thickest cross section, mould temperature, material type and grade. After curing, the mould opens and the part is ejected. Hydraulic press is the common type of compression moulding machine. Unlike some of the other processes, the fact that materials are usually measured before moulding prevents the excess flash.

Although today compression moulding is used for thermosets in the most common applications, the process can also be employed with thermoplastics. There are various types of reinforcements which can be used in advanced composite thermoplastics. These reinforcements can be unidirectional tapes [Akonda and Havis, 2011], woven fabrics, randomly oriented fibre mat, or chopped strands. Thermoplastic resins may be loaded into the mould either in the form of pellets, film [Plackett et al., 2003; Ouagne et al., 2010; and Yuan et al., 2011], sheet, solution [Kumar et al., 2010], paste [Mallick, 2000; Ren et al., 2009; and Patel et al., 2010], or fibre [Bos et al., 2006; Shanks et al., 2006; Lee et al., 2009; and Zhang et al., 2009] or the mould may be loaded from a plasticizing extruder [Oksman et al., 2003]. Thermoplastic materials are heated above their melting

points, formed and cooled. For both thermosets and thermoplastics, the better the feed material is distributed over the mould surface during the compression step.

Compression moulding is a good method to make complex, and high-strength parts. It is also suitable for manufacturing high volume of objects. The process has short cycle time and as a result high production rate. Fully automated production lines are also possible for compression moulding process. It is widely used to produce larger flat or moderately curved parts of many structural automotive components such as hoods, fenders, scoops, spoilers, lift gates as well as smaller more intricate parts.

Compression moulded composite such as sheet moulding compound (SMC) is mainly used for preparing thin structural parts [Dumont et al., 2007]. For manufacturing large, thin, strong, stiff, and light-weight fibre reinforced composite, compression moulding process can be effectively used [Park et al., 2001]. It was found that during injection moulding and extrusion process; sometimes the fibres are damaged due to the rotating screw [Carneiro and Maia, 2000]. However during compression moulding no damage has been reported to the fibres, which preserves the isotropic properties of the composites and reduces the changes in physical properties. Moulding temperature and pressure play an important role in physical and mechanical properties of bicomposites. Higher temperature reduces the viscosity of the thermoplastic and provides better wetting of fibres, but higher temperature can damage the natural fibre. Moulding pressure is important in compressing the material and removing the air trapped inside which is responsible for voids in composites [Barboza, 1994)].

SMC is another composite material that, due to its mechanical and aesthetic properties, is being employed in many applications in the production of industrial components, ranging from sport and building constructions to transportation, electrical/electronics, chemical engineering, as aerospace and the marine industry [Baker, 1997]. SMC is conventionally composed of chopped glass fibre in a thermosetting resin matrix system. Nowadays it has been developed by using natural fibres such as flax, hemp and jute to replace glass fibre as reinforcement. [Ren et al., 2009 and Patel et al., 2010].

Film stacking technique and hot press method, used for thermoplastic composite manufacturing have also been used to manufacture biocomposites. In this technique the polymer resin is converted into a film form. Film-stacking process consists of heating and compressing a stack of alternated layers of polymer films and fibre mats for a 50

measured amount of time. The fibre mats are prepared using several techniques such as nonwovens. The polymer films are prepared by heating and compressing the polymer pellets. The reinforcing fibre mats are cut to a shape of similar dimensions. The composites are moulded from a stack of polymer films interleaved with fibre mats as required. The stack is put on a hydraulic press equipped with heating plates. The assembly is heated and pressed once the polymer has melted. Then the composite is cooled to room temperature under constant pressure. The ratio between the mass of the thermoplastic films and of the fibre mats determines the fibre volume fraction in the composite. Time, temperature and pressure are controlled during the process. The values of these parameters are dependent on the nature of the polymer, more particularly on its melting temperature and viscosity, but also on the reinforcing fibres [Bodros et al., 2007].

Ouagne et al. [2010] used the film stacking technique to produce flax reinforced thermoplastic composites. Figure 2.5 shows a schematic diagram of the film stacking process. The flax fibres were cut to a 10 mm length and the mats were prepared using a paper mill technique. The flax fibres have to be randomly scattered in order to have isotropic properties in the laminating 2D plane. The areal weight of flax fibre mat was 116 g/m2.

Figure 2.5 Schematic diagram of the film stacking process [Ouagne et al., 2010].

Yuan et al. [2011] also used this technique to produce flax fibre reinforced PLA biocomposites. The thickness of PLA film was approximately 0.2 mm. The surface of

flax fibre was treated by silane addition in different percentage. Lay-ups (150mm ×150mm) were prepared in which sections of flax fibre are stacked up with several PLA film layers on both sides. After pre-compression, the whole assembly was converted into composites using a laboratory press with a temperature and pressure control. The effects of flax fibre addition, silane addition, the hot-press temperature and the hot-press time were investigated with a fixed hot-press pressure of 3MPa. The details are shown in Table 2.1.

Table 2.1 Variable factors and levels of the test [Yuan et al., 2011]

Factors Levels Flax Silane Hot-press Hot-press Addition (%) Addition (%) Temperature (°C) Time (min) 1 30 1 190 3 2 40 3 200 5 3 50 5 210 7

Kumar et al. [2010] also used compression moulding for the production of composites of flax fibres as reinforcement in PLA. In this case flax fibres were used in nonwoven web form with a density of 200 ± 10 g/m2, prepared by needle punching technique and PLA was used in solution form. PLA pellets were dissolved in chloroform to make solution equivalent to 0.9, 0.8 and 0.7 weight fractions of the web. Then the PLA solution was poured over the nonwoven web inside a mould. Finally the composite material was made by hot pressing at 190 °C temperature and 50 bar pressure for 15 minute. The typical process is shown in Figure 2.6.

Figure 2.6 Procedures for preparation of PLA nonwoven flax composites [Kumar et al., 2010]. 52

The reinforcing fibre and the matrix fibre can be blended and converted into a nonwoven through carding. Carding provides a uniform blend of two fibres; this is followed by needle punching, then pre-pressing and finally hot-pressing to form the composite material. Figure 2.7 shows a simple schematic diagram of the manufacturing of composite material by using the carding process.

Figure 2.7 Carding process for manufacturing biocomposites [Lee et al, 2009].

Lee et al. [2009] used this technique to produce kenaf fibre reinforced PLA biocomposites. They converted the pre-pressed mat into composite by using hot pressing for 5 minutes at 200 °C temperature under 7 MPa pressure. The compositions of various PLA/Kenaf biocomposites investigated by them are listed in Table 2.2.

Table 2.2 Composition of biocomposites [Lee et al, 2009]

Fibre Percentage of composition (%) PLA fibre 100 90 70 50 30 Kenaf fibre 0 10 30 50 70

2.4 Mechanical Properties of Composites

Fibre reinforced composites especially long-fibre composites are generally required to function as load-bearing structures. It follows that elastic modulus, strength, impact and fracture toughness are particularly important properties. The polymeric nature of most matrix materials introduces viscoelastic characteristics into the mechanical behaviour of composites such that, depending on local circumstances, the concepts of elastic modulus,

strength and ductility may have to be expanded to embrace phenomena such as creep, stress relaxation, creep rupture and fatigue, and attributes such as impact resistance.

The mechanical properties depend on several variables of the composition:

 properties of the fibre  surface character of the fibre  properties of the matrix material  properties of any other phase  volume fraction of the second phase (and of any other phase)  spatial distribution and alignment of the second phase (including fabric structure)  nature of the interfaces.

Mechanical properties also depend on the many details of the processing stage, particularly those affecting the degree of adhesion between fibre and matrix and the physical integrity and overall quality of the final structure. If the mechanical properties of the fibre and the matrix are known, mathematical models enable the corresponding properties of samples with particular fibre volume fractions and fibre spatial arrangements to be calculated, but the models are imperfect. The fibre alignments in test coupons and service items generally deviate from the ideal states assumed for the models, and the properties deviate correspondingly from the calculated predictions. The effectiveness of the coupling between the phases in a composite is also an influential factor. It is neither fully quantified nor properly understood. Good coupling seems to be desirable where a composite with high moduli is the objective and also, in many cases, where high strengths are required. The lines of reasoning are less clear where toughness is the objective. Poor coupling is advantageous in that local decoupling between fibre and matrix can arrest, or deflect, a growing crack and extensive decoupling is an effective mechanism for energy absorption. On the other hand, a decoupled fibre may act as a stress concentrator and promote failure [Hodgkinson, 2000].

2.4.1 Tensile properties

When a composite sample is subjected to a tensile load or it is stretched axially, the applied force is transferred from the matrix to the fibre. The strength of the composite sample is dependent on the strength and stiffness of the reinforced fibre. The basic 54

purpose of tensile testing is to determine the tensile strength and modulus of the material. However closer observation provides more information about its behaviour under the applied load. A composite may split or delaminate, the nature of the failure may be brittle with no warning, or it may start with visible or audible signs. All this information is useful and knowledge of the failure mode is vital to establish the end use of the material [Godwin, 2000].

2.4.2 Flexural properties

The flexural properties of the composite samples are determined in order to measure their resistance when they are subjected to a bending load. When a bending force is applied on a composite sample, this force is transferred from the matrix to the fibres and ultimately causes the sample to break. Under the bending condition, a composite sample is subjected to compression at the point of application of the bending force and simultaneously the area which is bending is subjected to stretch or tension. The flexural test is used in industry to determine the mechanical properties of resins and fibre reinforced polymer composites because of the ease of the test method, instrumentation and the equipment required. The main purpose of flexural testing is to determine the flexural stress at break, strain at break and the modulus of the material.

2.4.3 Impact properties

The impact ‘resistance’ of a composite may refer to the ability of the composite to withstand a given blow without any damage (i.e. the resilience); the maximum force necessary to rupture or separate a composite structure, irrespective of the preceding level of damage (the impact strength); the amount of energy that is absorbed by a given mass of the composite (the crush resistance); or perhaps the level of damage that a composite can sustain during impact loading without suffering undue reduction to some primary structural function after the impact event (damage tolerance) [Hodgkinson, 2000]. Impact tests are used in studying the toughness of the composite materials. The toughness of a material is a factor of its ability to absorb energy during plastic deformation. Brittle materials have low toughness as a result of the small amount of plastic deformation that they can endure. The purpose of impact testing is to determine the energy absorbed by breaking the composite material and impact strength of the material.

2.4.4 Crushing properties

The energy absorption capability of a composite material is critical to developing improved human safety in an automotive crash. Energy absorption is dependent on many parameters like fibre type, matrix type, fibre architecture, specimen geometry, processing conditions, fibre volume fraction, and testing speed. In passenger vehicles the ability to absorb impact energy and be survivable for the occupant is called the “crashworthiness” of the structure. Crashworthiness is concerned with the absorption of energy through controlled failure mechanisms and modes that enable the maintenance of a gradual decay in the load profile during absorption.

2.5 Biodegradable Composites

The development of natural fibre reinforced biodegradable polymer composites promotes the use of environmentally friendly materials. The use of green materials provides alternative way to solve the problems associated with agriculture residues. Agricultural crop residues such as oil palm, pineapple leaf, banana, and sugar palm produced in billion of tons around the world. They can be obtained in abundance, low cost, and they are also renewable sources of biomass. Among this large amount of residues, only a small quantity of the residues was applied as household fuel or fertilizer and the rest which is the major portion of the residues is burned in the field. As a result, it gives a negative effect on the environment due to the air pollution [Abdul Khalil et al., 2008]. The vital alternative to solve this problem is to use the agriculture residues as reinforcement in the development of polymer composites [Mohanty et al., 2005]. A viable solution is to use the entire residues as natural fibres and combine them with polymer matrix derived from petroleum or renewable resources to produce a useful product for our daily applications as shown in Figure 2.8.

Due to concerns about disposal of plastics, polymer scientists have been strongly encouraged to develop new biodegradable polymer composite materials from renewable resources [Plackett and Sodergard, 2005]. However, research on biodegradable polymers as composite matrices are limited in comparison with research on thermoplastic and thermoset polymers because of the relatively poor availability and high price of biodegradable polymers. The research focused more on the influence of natural fibres on the mechanical properties of petroleum based polymers rather than

using biopolymer like PLA as a matrix. Many advantages are associated with the use of natural fibres, including low cost, abundance, low density, high specific properties and lack of residues upon incineration [Bledzki et al. 2002]. Thus, the combination of natural fibre with PLA offers an answer to maintaining the sustainable development of economical and ecological technology.

Figure 2.8 Classification of biocomposites [Mohanty et al., 2005].

A study on PLA composite was carried out by Ochi [2008] where in his investigation; the unidirectional biodegradable composite materials were fabricated from kenaf fibres and PLA. Oksman et al. [2003] reported the work on manufacture of PLA/Flax composites and compared them to more commonly used polypropylene (PP) flax fibre composites (PP/flax). Preliminary results show that the mechanical properties of PLA and flax fibre composites about 50% higher compared to PP/flax fibre composites; which are used today in automotive panels. Van den Oever et al. [2010] examined the different types of agro-fibres such as ramie, flax and cotton and they reinforced PLA to form composites. In their finding, flexural stiffness of composites increases linearly with fibre contents for all types of fibres (ramie, flax and cotton).

The development of biocomposites had been intensified and polymer matrices reinforced with natural fibres had gained more attention among the researchers [Vilaseca et al., 2007]. Biocomposite with starch used as a matrix is one of the most 57

popular biodegradable biocomposite and is highly investigated by researchers [Herrmann et al., 1998]. Biodegradable matrices were reinforced with natural fibres to improve the composites properties [Ma et al., 2005 and Tserki et al., 2006] and these composites provide positive environmental advantages, good mechanical properties and light weight [Rana and Jayachandran, 2000]. In the recent years, some researchers have studied the properties of biodegradable composites made from plasticized starch reinforced with natural fibres. Vilaseca et al. [2007] have developed composite materials from biodegradable starch and jute strands fabricated using injection moulding process. Mechanical properties of starch-based polymer and its composites with different percentages of untreated jute strands and alkali treated jute strands were determined. Averous and Boquillon [2004] studied thermal and mechanical behaviour of composites made from thermoplastic starch (TPS) reinforced with agro-materials (cellulose and lignocellulose fibres). It was observed that decreases of both storage and loss moduli against temperature for various composites and moduli increase with the increase in fibre content.

The technological development in the recent years has created the problem of global warming with carbon dioxide emission and caused the shortage of fossil fuels. Therefore, new materials derived from biodegradable renewable sources are considered as partial solution to these problems. New materials such as cellulose derivatives have been foreseen to use as the potential matrices in composites. In the past, limited studies have carried out use of cellulose esters as matrices in biocomposites [Toriz et al., 2003]. These cellulose esters are very well suited for use as matrices in natural fibre-based composites. The composites can be processed using extrusion and injection moulding to form structural components.

Due to the abundance of plastic wastes over last few decades, biodegradable green plastics are regarded as possible substitutes to petroleum-based plastics. Polyhydroxyalkanoates (PHAs) are a family of biodegradable polyesters which are synthesized via various bacteria and these polymers differ from petroleum based synthetic polymers with regard to their being renewable resource and biodegradability [Singh et al., 2008]. Therefore, PHAs have attracted the attention of many researchers and recently their composites with natural fibres have been studied extensively [Bledzki and Jaszkiewicz, 2010 and Coats et al., 2008]. Singh and Mohanty [2007] developed green composites using natural bamboo fibre and bacterial polyester i.e., 58

poly(hydroxybutyrate-co-valerate) (PHBV) which is member of the family of PHAs. The fabrication of the biocomposites was carried out using injection moulding following the extrusion compounding of PHBV and bamboo fibre with 30 or 40 wt.% fibre. The main objective of their study was to investigate the fundamental prospects of bamboo/PHBV biocomposites such as mechanical, thermo-mechanical and morphological properties.

The replacement of synthetic fibres with natural ones as reinforcements in polymer composites has received increasing attention because of their several advantages. In this context, natural bast fibres such as flax, hemp, jute, historically used as a cordage crop to produce twine, rope, and sackcloth, are now widely applied in even more fields, including thermoplastic-reinforced composites for noncritical applications in building areas as well as car panels in the automotive sector or new packaging solutions [Mathur, 2006 and Santos et al., 2007]. The characteristics of natural fibre reinforced composites have been assessed in many studies. The effect of the processing conditions on mechanical properties [Mano et al., 2010], morphology and fibres dimensions [Iannace et al., 2001], the fibre aspect ratio [Quijano-Solis et al., 2009], the orientation and distribution of fibres within the matrix [Herrera-Franco and Valadez-Gonz´alez, 2005], the inherent moisture absorption [Kim and Seo, 2006] as well as efficiency of interfacial adhesion of the fibres to the matrix [Demir et al., 2006; Seki, 2009; and Li et al., 2007] has been discussed. All these factors could contribute to the microcraking of the composite, degrading its performances, and limiting the use of natural fibres in plastics.

Although natural fibres are obtained from renewable sources and the polymer composites based on them are environmentally friendly as compared to the synthetic fibre reinforced polymer composites (SFRPCs), there are also some disadvantages, which are related to the utilization of unmodified/raw fibres in the preparation of the composites. These disadvantages are as quality variations, high moisture uptake and low thermal stability of the raw fibres [Lopez et al., 2006]. High moisture uptake is the major drawback of the natural fibres. This phenomenon weakens the interfacial bonding between the polymer matrix and fibre and causes deterioration of the mechanical properties. The high moisture sensitivity of some fibre such as lingo-cellulosic fibre causes even the dimensional instability [Carvalho, 1997] and limits the use of natural fibre as reinforcement in composite materials. In order to overcome this problem and ultimately to improve the fibre-matrix adhesion, in many cases, a pre-treatment of the 59

fibre surface or the incorporation of surface modifier is required during the composite preparation. Many investigations have been reported in the literature on the influence of various type of chemical treatment on the physical and mechanical properties of natural fibre reinforced polymer composites (NFRPCs) [Bledzki et al., 2010 and Chattopadhyay et al., 2011]. The moisture absorption can be reduced substantially by the chemical treatments of fibres. Various types of chemicals such as, alkali (sodium hydroxide), isocyanate, KMnO4 (permanganate), CTDIC (cardanol derivative of toluene diisocyanate), peroxide, enzyme etc. have been used for the treatment and a considerable change in the mechanical and physical properties of the composites have been obtained [Liu and Dai, 2007].

2.5.1 Materials used in biocomposites

Reinforcing and matrix materials are the two constituent of a biocomposite. These are explained in the following sections.

2.5.1.1 Reinforcing Material

The purpose of fibre as reinforcement is to provide integrity and strength to the structure by carrying the majority of the applied structural loads. Fibres are stronger because while having smaller diameter, they have fewer defects and have the possibility to align the crystal or molecular structure. Flaws or defect propagation usually cause failure of the material. However, due to the presence of many fibres in the composite structure, sudden damage does not usually occur. Most of the fibres have to rupture before the complete failure of the composite and hence usually warning signs are there before the collapse. Fibre reinforcement, which is the discontinuous phase, is responsible for the primary engineering properties of composites. The mechanical properties of composites increase by increasing the fibre volume content up to a level where enough matrix material is available to support the fibres and transfer the load within the composite [Reinhart, 1998]. The percentage of reinforcing fibre in the composite may be up to 70% by volume [Nawab and Yasir, 2009].

Natural fibres are used as reinforcing material in resin matrix composites in the early years of the composites industry including flax and hemp fibres for the bodywork of a Henry Ford car in 1941 [Lewington, 2003], but then fell from favour due to continuous glass fibres becoming commercially available and due to economic limitations at the

time when the vehicle was not mass produced. The company claimed that the composite could withstand 10 times more shock than steel. Environmental concerns have resulted in renewed interest in sustainable composites focusing on bio-based fibres and resins. The UK government publication “Securing the Future: Delivering UK Sustainable Development Strategy” emphasised sustainability in both industry and in agriculture with a revival of interest in materials from sustainable sources [Securing the Future, 2005].

The natural fibres are categorised into three groups: vegetable (cellulose), animal hair (protein) and mineral. Natural fibres, especially Bast (stem) fibres from plants such as flax, hemp, kenaf, jute, ramie and many others were investigated by some researchers as fibre reinforcement for composites [Singleton et al., 2003; Keller, 2003; Valadez- Gonzalez et al., 1999; and Oksman et al., 2003]. Fibres like flax and hemp are currently grown commercially in the United Kingdom or Europe. They are used in natural fibre composites for a wide range of automotive applications such as the interior panels of passenger cars and truck cabins, door panels and cabin linings as substitutes for glass fibre composites [Bledzki and Gassan, 1999].

The composite industry has grown rapidly over the past seventy years since the introduction of commercial continuous fibre reinforcements (glass in 1937, carbon in 1960 and aramid in 1971). The bio-based fibres and resins are perceived to be sustainable and greener. The types of natural fibres currently been investigated for use in plastics includes flax, hemp, jute, wood, rice husk, wheat, barley, oats, rye, cane (sugar and bamboo), grass, kenaf, ramie, oil palm empty fruit bunch, sisal, coir, pennywort, kapok, mulberry, raphia, banana fibre, pineapple leaf fibre and papyrus [Bledzki and Gassan, 1999]. The fibres most likely to be adopted as reinforcements are bast (stem) cellulose fibres from plants including flax and hemp (in temperate zones) or jute and kenaf (in tropical zones). A comparison of glass fibre with some natural fibre properties is shown in the Table 2.3 [Bavan and Kumar, 2010].

Table 2.3 Comparison of glass fibre with some natural fibre properties [Bavan and Kumar, 2010].

Fibre Density Tensile strength Tensile Elongation Moisture type (g/cm3) (MPa) modulus at break absorption (GPa) (%) (%) Glass 2.55 2400 73 3 - Flax 1.4 800 – 1500 60 – 80 1.2 – 3.2 7 Hemp 1.48 550 – 900 70 1.6 8 Jute 1.46 400 – 800 10 – 30 1.5 – 1.8 12 Sisal 1.33 600 – 700 38 2 – 3 11 Coir 1.25 220 6 15 – 25 10 Cotton 1.51 200 – 800 12 6 – 12 8 – 25

The density of natural fibres varies from 1.2 to 1.5 g/cm3, which is much less than that of glass fibre (2.55 g/cm3). Other properties of natural fibres are comparable or even superior to glass fibres (Table 2.3). Advantages of natural fibres over man-made fibres include low density, low cost, recyclability and biodegradability. These advantages make natural fibres potential replacement for glass fibres in composite materials. The bast fibres are advantageous over other cellulose based fibres (seed fibre, leaf fibre or fruit fibre) due to high modulus, tensile strength and low specific gravity i.e. stiffness and strength to weight ratios [Nabi and Jog, 1999]. Flax fibres and their products are widely used in the automotive and other industries. Flax is amongst the natural fibres now finding use in thermoplastic matrix composite panels for internal structures in the car industry including car door panels, car roof and boot linings, and parcel shelves [Bledzki and Gassan, 1999 and Brahim and Cheikh, 2007].

2.5.1.1.1 Flax fibre

Presently two types of flax are grown, fibre flax and seed flax. Fibre flax is optimised for the production of thin strong fibres. Seed flax gives coarser fibres, but far more linseed, since this plant does not have one straight stem, but the stem divides towards various flower heads [Dewilde, 1987]. The flax plant grows in moderate temperature and moist climates, and is presently cultivated among others in large parts of Western and Eastern Europe, in Canada and the USA and in Russia. World-wide approximately 5

million hectare flax is grown [Bos, 2004]. The flax fibre in the stem of the plant forms about 25% by weight although the amount of fibre recovered after isolation is much smaller than this (up to about 15%) [Peters, 1963].

Flax (grown for fibre) and linseed (grown for seed oil) are cultivars: varieties of the same plant species Linum Usitatissimum bred with an emphasis on the required product [Lewington, 2003 and Sharma and Van Sumere, 1992]. In the UK the flax plant is normally sown in March to May and may grow to one-metre high dependent on the variety. Generally, flax is cultivated where daily temperature remains below 30 °C [Sultana, 1992].

The term fibre is recognised in plant science as referring to a single cell with clearly defined microscopically visible features [Hearle and Peters, 1963]. The cell is long compared with its width, and both ends taper to points. The wall is thick and the pits are reduced and usually slit-mouthed. In both stem and leaves, the fibre cells, also called "elementary fibres", occur in bundles, also called "technical fibres". In actual practice fibre cells are isolated as bundles (technical fibres). The elementary flax fibres belong to a family of elongated cells found in plants. Individual types of cellulose fibres vary somewhat in their molecular constitution and arrangement [Morton and Hearle, 1962]. In all native cellulose (elementary) fibres, the cell-wall consists of long thin threads termed microfibrils. Unlike man-made fibres, natural fibres themselves could be considered as oriented short-fibre composites containing varying volume fractions of cellulosic microfibres of different lengths. For instance, the approximately one meter long so-called technical fibres are isolated from the flax plant for the use in textile industry. These technical fibres consist of elementary fibres, which are approximately 20 to 50mm long, diameters in between 10 and 25μm and an average aspect ratio of around 1500 (Figure 2.9). The elementary fibres overlap over considerable length and are glued together by a pectin interface. They are not circular but a polyhedron with 5 to 7 sides to improve the packing in the technical fibre [Bos et al., 2006].

The elementary fibres are single plant cells and cellulose (C6H10O5)n is a common material in plant cell walls. It occurs naturally in almost pure form in cotton fibre. Most of the elementary fibre consists of oriented, highly crystalline cellulose fibrils and amorphous hemicellulose. The crystalline cellulose fibrils in the cell wall are oriented at

an angle of about ±10 degrees with the fibre axis [Joshi et al., 2004 and Davies and Bruce, 1998] and give the fibre its high tensile strength.

Figure 2.9 Schematic representation of a flax fibre from stem to microfibril [Bos et al., 2006].

The principal constituent of flax fibres is cellulose, with smaller amounts of hemicellulose, lignin, pectins, oils, and waxes. The cellulose, hemicellulose, and pectins are found in the cell walls. The proportion of these components in a fibre depends on the age, source of the fibre, and the extraction conditions [Hearle and Peters, 1963]. The structural components of the fibres, i.e. cellulose, hemicellulose, and lignin, influence the mechanical properties and durability of fibres. Additional characteristics include fibre strength, fibre fineness, the polymerization of the cellulose, cleanness or purity, and homogeneity of the sample also affects the mechanical properties. Plant fibre properties directly influence the physical parameters of the fibre-reinforced composites [Jähn, 2002]. The chemical composition of flax fibre is shown in the Table 2.4 [Bismarck et al., 2002].

Commercial flax is in the form of bundles of individual fibre cells held together by a natural binding material. Scutching and hackling tend to break up the coarse bundles of fibre as they exist in the bast, but do not separate the fibre strands into their individual fibre cells. Flax is usually coloured yellowish-white, but the shade of the raw fibre varies considerably depending upon the conditions under which it has been retted. Dew- retted fibre is generally grey. Flax is usually soft and has a lustrous appearance. The lustre improves as the flax is cleaned, wax and other materials being removed.

Table 2.4 Chemical composition of flax fibre [Bismarck et al., 2002].

Fibre component Component percentage (%) Cellulose ≈ 65 Hemicellulose ≈ 16 Pectin 3.0 Lignin 2.5 Proteins 3.0 Fats and Waxes 1.5 Ash (minerals) 1.0 water 8.0

Fine structure and appearance: The flax plant has a single stem, reaching to a height of approximately 80 – 120 cm with few branches for fibre flax varieties. In oilseed varieties, the flax plant has a number of stems reaching to a height of approximately 40 – 60 cm with more branches. The diameter of the stem at the base varies between 1 and 2 mm. The bark (the outer layer of the stem) protects the material from external attacks, confers the rigidity of the stem, and also allows the water and other nutrients to penetrate to the centre of the stem. Both the bark and the xylem (the woody body or shive) are eliminated during the flax processing stages of retting and scutching.

The fibre bundles are made of several elementary fibres glued together by pectin cement. Initially, the technical fibres are extracted by partial separation of these fibre bundles (for example during hackling). The fibres can be as the same length as the flax stem. Elementary fibres have two types of cell walls; the outer primary and the inner secondary cell walls. These cell walls can be divided into three sections in terms of thickness and structure. The fibre cell also has a hollow space (lumen) running through the centre; the lumen is narrow but clearly defined and regular in width which is filled by cytoplasm during cell life and disappears when the plant dies (i.e. during retting). Each cell wall consists of concentric lamella in which the cellulose fibrils are embedded in an amorphous matrix composed of pectin and hemicelluloses [Charlet et al., 2010].

Strands of commercial flax may consist of many individual fibre cells; they vary in length from 6 – 65 mm with a mean diameter of about 0.02 mm. Seen under the

microscope, the fibre cells show up as long transparent, cylindrical tubes which may be smooth or striated lengthwise. They do not have the convolutions which are characteristic of cotton. The width of the fibre may vary several times along its length. There are swellings or nodes at many points, and the fibres show characteristic cross- markings. The cell walls of the flax fibre are thick and polygonal in cross-section. Immature flax fibres are more oval in cross-section, and the cell walls are thinner. The lumen is relatively much larger than in the mature fibre [Cook, 1984]. The key properties of flax are summarised below:

Tensile properties: Flax is a stronger fibre than cotton. It has an average tenacity of about 57.4 cN/tex (5.8 g/dtex) [Cook, 1984].

Elongation: Flax is a particularly inextensible fibre. It stretches only slightly as tension increases. The elongation at break is approximately 1.8% in dry and 2.2% in wet condition [Cook, 1984].

Elastic properties: Within its small degree of stretch, flax is an elastic fibre. It will tend to return to its original length when the tension is relaxed. It has a high degree of rigidity and resists bending.

Effect of moisture: Flax has a regain figure of about 12%. It is about 20% stronger when wet than dry, which helps it to withstand mechanical treatment in laundering [Cook, 1984].

Effect of heat: The flax fibre is highly resistant to decomposition up to about 120 °C, when the fibre begins to discolour. A number of authors have investigated the thermal degradation of flax fibres at temperatures around 200 °C [Garkhail, 2001; Gassan and Bledzki, 2001; Van de Velde and Kiekens, 2002; and Wielage et al., 1999]. They generally found that thermal degradation is not really significant in the first few minutes and at lower temperatures. Gassan and Bledzki [2001] showed that untreated flax retains its strength after exposure at 170 °C for 120 min, whilst at 210 °C the strength decreases by approximately 50% over the same time span. Mieck et al. [1994] report significant damage of the flax fibres after an exposure time of 4 minute at temperatures above 240 °C. Kohler and Wedler [1994] showed that flax fibres stored in a convection oven at 200 °C for 30 minute retains its strength. Generally, it can be concluded that when the processing parameters – especially the temperature and the time are well

under control, the production of flax fibre compounds should be possible without significant loss of stiffness and strength.

2.5.1.2 Matrix Materials

The matrix acts as a binder for the fibres because it has adhesion and cohesion characteristics. It helps in transferring of load to the fibres and between the fibres and also guards them from environmental impacts. Orientation and location of the fibres in the composite structure are maintained by the matrix. By distributing the load evenly among the fibres, it resists damage and crack propagation. The matrix contributes to the electrical and chemical properties of the composite [Reinhart, 1998 and Peters, 1998]. Most commercially produced composites use a polymer matrix material often called a resin which is classified into two types, namely thermoset and thermoplastic resins.

2.5.1.2.1 Thermoset resins

Thermoset resins are generally available in liquid form and after mixing with other ingredients they solidify. They form cross-linkages between the molecules during the curing process and thus once cured, they cannot be remoulded. Typical examples of thermoset resins include epoxy, unsaturated polyester, urethane, phenolics, vinyl-ester, etc. [Peters, 1998 and Varma and Gupta, 2000].

Thermoset resins are mainly selected for most structural composite materials. The main advantage of thermoset resin is that they have a very low viscosity and can thus be introduced into fibres at low pressures. Fibre impregnation is followed by chemical curing to give a solid structure, which can usually be carried out isothermally. Their major limitations are a longer curing time and poor performance in hot-wet environments [Penn and Wang, 1998 and Varma and Gupta, 2000].

2.5.1.2.2 Thermoplastic resins

Thermoplastic resins are usually cheaper for fabrication. They can be stored safely for long periods of time before moulding. This type of resin can be moulded i.e. when subjected to high temperature and pressure, the resin is softened and can be reshaped provided that it is not subjected to any permanent deformation. They are characterised by toughness and high impact strength. However, they suffer thermal degradation with repetitive temperature cycling [Reinhart, 1998]. An advantage of thermoplastic resin is

that the moulding can be carried out non-isothermally, i.e. a hot melt into a cold mould, in order to achieve fast cycle times i.e. their ability of being formed quickly in production lines. Typical examples of thermoplastic resins include polypropylene, polyethylene, polystyrene, PLA, ABS (acrylonitrile butadiene-styrene), Polyamide (Nylon), polycarbonate, thermoplastic polyester, polyphenylene sulfide, polysulfone, polyether-ether-ketone (PEEK) etc.

PLA is one of the most promising biobased polymers that has attracted the interest of many researchers. PLA, though discovered in the 1890s, is finding an edge in this new era of science and will soon replace many commodity plastics because of its biodegradability property and biobased nature. PLA is chosen as the matrix material for this study as it is a highly versatile, readily biodegradable, linear aliphatic thermoplastic polyester derived from 100% renewable resources, such as corn starch (in the United States), tapioca products (roots, chips or starch mostly in Asia) or sugarcanes (in the rest of the world) than the others. PLA is used in fibre form like as flax fibre in this project.

2.5.1.2.2.1 PLA fibre

PLA polymer is produced by the ring-opening polymerization of lactides, where the lactic acid monomers are obtained from the bacterial fermentation of the resources such as sugar feed stocks [Drumright et al., 2000]. Energy from the sun promotes photosynthesis within the plant cells; carbon dioxide and water from the atmosphere are converted into starch. This starch is readily extracted from plant matter and converted to a fermentable sugar (e.g. glucose) by enzymatic hydrolysis. The carbon and other elements in these natural sugars are then converted to lactic acid through fermentation [Lunt, 1998]. The cheapest and most abundant source of sugar is dextrose (glucose) from corn. The land mass necessary for feedstock production is minimal. Producing 500,000 tonnes of PLA requires less than 0.5% of the annual US corn crop [Gruber and O’Brien, 2001]; since corn is a cheap dextrose source, the current feedstock supply is more than adequate to meet foreseeable demand.

PLA can be prepared either by direct condensation of lactic acid or via the cyclic intermediate dimer (lactide), through a ring opening polymerization process. Because the direct condensation route is an equilibrium reaction, difficulties removing trace amounts of water in the late stages of polymerization generally limit the ultimate molecular weight achievable by this approach. Most work has focused on the ring 68

opening polymerization, although Mitsui Toatsu Chemicals has patented an azeotropic distillation process using a high-boiling solvent to drive the removal of water in the direct esterification process to obtain high molecular weight PLA [Drumright et al., 2000].

PLA fibre has a number of characteristics that are similar to many other thermoplastic fibres, such as controlled crimp, smooth surface and low moisture regain. The physical properties and structure have been studied by several researchers [Drumright et al., 2000], and these works confirmed that this polymer has significant commercial potential as a textile fibre. Its mechanical properties are considered to be broadly similar to those of conventional PET [Lunt and Bone, 2001], and probably due to its lower melting and softening temperatures, comparisons to polypropylene are also appropriate [Palade et al., 2001]. The related properties of PLA as a textile fibre are summarised below:

Appearance: Fibres are generally circular in cross-section and have a smooth surface.

Density: The specific gravity is 1.25 g/cm3, lower than natural fibres and PET (polyethyleneterepthalate).

Thermal properties: PLA is a stiff polymer at room temperature. The glass transition temperature (Tg) is typically between 55 – 65 °C. The melting temperature (Tm) of PLA is between 160 –170 °C [Farrington et al., 2005].

Fibre types and crimp: Both filament yarns and spun yarns can be made, as with PET. PLA can achieve good degree of crimp and good retention level through processing.

Tenacity: The tenacity at break is 32 – 36 cN/tex that is higher than for natural fibres although, of course, it can be varied according to the degree of drawing that is applied to the undrawn yarn. It is relatively unaffected by changes in humidity at ambient temperature, though as with other manufactured fibres there is a small but measurable increase in elongation. As the temperature is increased the tenacity does reduce quite quickly with a concomitant increase in fibre extension, a feature commonly found in synthetic fibres.

Moisture regain: At 0.4 – 0.6%, PLA has extremely low moisture regain, much lower than natural fibres and slightly higher than polyester [Farrington et al., 2005].

Flammability: Although PLA is not a non-flammable polymer, the fibre has good self- extinguishing characteristics; it burns for two minutes after a flame is removed, and burns with a white and a low smoke generation. PLA also has a higher LOI (limiting oxygen index) compared to most other fibres, meaning that it is more difficult to ignite as it requires a greater oxygen level [Farrington et al., 2005].

Moisture transport: PLA shows excellent wicking ability. This property and the additional properties of fast water spreading and rapid drying capability give the fibre a very positive inherent moisture management characteristic.

Identification: Bledzki et al. [2009] established a criterion for identification of PLA fibre. The analysis indicates that solubility testing can be used as a rapid screening tool because the samples under examination dissolved in dichloromethane (DCM) in few seconds at ambient temperature. PLA can be distinguished from other generic fibre types by using a routine fibre analysis protocol that includes solubility testing.

2.5.2 Process used in biocomposites

The techniques to produce biocomposites materials are mostly similar to the existing processing methods for composites or plastics [Advani and Sozer, 2002]. A general review of the manufacturing methods of composite material is described in the previous Section 2.3 of this chapter. The process used for composite manufacturing depending upon the type of the end product and the performance required. Generally, extrusion, injection, and compression moulding are the most common processing methods for a large number of biocomposites based on thermoplastics [Fowler et al., 2006].

2.6 Summary

The development of biocomposites from biodegradable polymers and natural fibres attracts interest because of similar reasons already described in Section 1.2 of Chapter 1 and also pressures on the world’s petroleum resources [Yussuf et al., 2010]. Another attraction to the biodegradable polymers is their natural degradation in soil without releasing toxic component to present the green environment.

From the literature survey it is seen that, longer natural fibres i.e. bast fibres provide better mechanical properties in composite structures. Different types of natural fibres such as flax [Massimo et al., 2004], jute [Cabral et al., 2005], kenaf [Feng et al., 2001],

sisal [Jayaraman, 2003] and hemp [Madsen et al., 2007] have been investigated for reinforcing both thermoplastic and thermoset matrices. Some of these fibres such as flax, jute (densities from 1.4 – 1.5 g/cm3) are becoming more popular, especially as replacement of high density glass fibres (density of glass fibre ≈ 2.6 g/cm3) in many structural and high performance applications to reduce the weight of the materials [Olesen, 1999]. From Table 2.3 it is seen that the modulus of flax fibre is higher and density is lower compared to the other natural fibres.

It is seen that a number of fully biodegradable polymers (natural and petroleum based) are used as matrix material. PLA is one of the promising biopolymers which can be synthesized from renewable resources such as sugar feedstock and corn. The polymer has a promising market in areas such as household plastic bags, sanitary and diapers barriers, and other important application fields. Even though the polymer requires modification to make it useful and is also considered expensive, it has some good mechanical properties, is biodegradable, and insoluble in water [Yussuf et al., 2010]. It is the only melt-processable fibre from annually renewable natural resources [Farrington et al., 2005].

Comparatively, nonwoven is the more cost effective process than the other textile manufacturing systems. The resins are mixed directly to the fibre state, thus the adhesion between the resin and reinforcing fibre is better than the other process.

Air laid nonwoven webs have isotropic fibre orientation distribution [Hearle and Stevenson, 1963], leading to isotropic composites. The mechanical properties of the product are similar in all direction due to the isotropic fibre distribution. The thermal bonding avoids any potential fibre damage caused by the needle punching method for nonwoven composites [Lee and Kang, 2000 and Tejyan et al., 2012].

Based on the above considerations, the present research was focused on developing a fully biodegradable composite based on thermally bonded air-laid nonwovens. It was also decided to use the compression moulding and hot pressing method to manufacture composites because compression moulding has the ability to mould large, fairly intricate parts and is cost effective.

In many applications such as in the automotive industry, three-dimensional (3D) shaped products are required. These 3D biodegradable composite materials are typically

produced by moulding from 2D nonwoven prepreg. Fabricating composites into components with a complex shape increases the cost because some fabrics and many prepreg tapes have poor drape. These materials are not easily moulded into complex shapes, and as a result some composite components need to be assembled from a large number of separate parts that must be joined by co-curing, adhesive bonding or mechanical fastening. Even fabricating composites into components with a simple dome shape by using 2D prepreg sheet develops creases on the surface of the final product. Therefore, another objective of this project is to develop 3D biodegradable composites directly using 3D nonwoven prepregs. The 3D nonwoven webs were made using a recently developed process in the University of Manchester. Using this system minimizes the investment of equipment used and shortens the process.

CHAPTER 3

RESEARCH METHODOLOGY

3.1 Introduction

The objectives of this research are to develop completely biodegradable flat and 3D nonwoven composite materials and to establish the optimum processing parameters by evaluating the properties of the composites. This chapter summarises the general approach to the study and provides detailed information on how the overall objectives of the research will be achieved. The selected fibres were blended by the roller card of an air laying web forming machine. The nonwoven webs, which were used as prepregs, were manufactured by the web forming machine. The webs were then pre-consolidated by using hot pressing. Finally the prepregs were consolidated by compression moulding to fabricate the composite materials. The composite materials were evaluated by analysing the physical, mechanical and thermal properties.

3.2 Materials

The quality and properties of a product mainly depend on the raw material. Raw material selection is an important and primary part of a research work. The reinforcing fibres and the matrix materials were the main raw materials of this project work.

3.2.1 Material Selection

Bast (stem) fibres from plants such as flax, hemp, kenaf, jute and ramie are more likely to be adopted as reinforcement in biodegradable composites. The advantages of these bast fibres over other cellulose based fibres are explained in Section 2.5.1.1. Among the bast fibres, flax is one of the lightest, and it has higher tensile strength, elastic modulus, specific modulus and elongation [Bavan and Kumar, 2010].

Matrix materials are usually used in liquid or film form to produce composite material. There are a number of natural based biodegradable polymers such as PLA, thermoplastic starch, cellulose, PHAs, etc. which are used as matrix material to produce fully biodegradable composites. To minimise delamination and achieve better blend effect with flax fibre for thermoplastic composite, staple PLA fibre is selected as matrix 73

material for this work. The properties of PLA are described in Section 2.5.1.2.2.1. It is the only melt-processable fibre from annually renewable natural resources [Farrington et al., 2005].

Flax and PLA fibres are both inexpensive and easily available commercially in the United Kingdom. These fibres are supplied by Tilsatec Advanced Materials, Tilsatec Ltd., Wakefield, UK, in commercial grade.

3.2.2 Material Testing

Properties of the raw materials play an important role in the final product. End-uses of the product depend on the properties of raw materials also. As reinforcing material, flax fibre plays an active role in the composite material. Fibre length, fibre diameter, fibre linear density, fibre strength are all important material parameters and are tested. In order to select the appropriate processing temperature in composite manufacture the melting point of PLA fibre is also tested.

To select the staple length of PLA fibre it is important to determine the mean length of flax fibre. PLA fibres were blended with flax fibre during the web forming process. Similar length of both fibres is a requirement of blending operation. The BS ISO 6989: 1981 standard method was followed to determine the flax fibre length by measurement of individual fibres.

Fibre diameter is important for the measurement of fibre volume fraction to determine the fibre and resin content. The average value of fibre diameter is calculated because, as natural fibre, flax fibre diameter differs along individual fibre axis and also between fibres. Fibre diameter is also important to calculate the cross-sectional area of the fibre and the tensile strength of a single fibre. Scanning electron microscope (SEM) was used to determine the fibre diameter. The BS ISO 11567: 1995 standard method was followed to determine the diameter and cross-sectional area of single fibre.

Linear density is an important property of textile fibres. As natural fibre, flax thickness varies between fibres. To select the linear density of PLA fibre it is important to determine the mean value of linear density of flax fibre. Both fibres are processed together for web formation and similar parameters are the requirements of uniform processing. The BS EN ISO 1973: 1996 standard method was followed to determine the linear density of the fibre by gravimetric method. 74

Single fibre strength test provides tensile properties of the flax and PLA fibres. A universal Instron testing machine was used to determine the tensile strength of single fibres. The BS ISO 11566: 1996 standard method was followed to determine the tensile properties of single fibre.

The melting point of PLA fibre was determined to select the processing i.e. moulding temperature for the composite manufacture because the moulding temperature depends on the melting temperature of PLA fibre. The consolidation temperature for composite manufacturing is usually required to be equal to or higher than the melting point of matrix, but below that of reinforcing fibre. Differential scanning calorimeter (DSC) is the ideal technique to assess the melting behaviour and is carried out to determine the Tg

(glass transition temperature), Tm (melting temperature) and crystallinity of PLA fibre.

3.3 Prepreg Formation

In order to develop the composite material there is a need of prepreg. From the literature survey it is found that around 50% to 60% matrix material is generally present in thermoplastic composite materials to achieve good fibre/matrix bonding. Voids are formed during the composite fabrication. Flax itself has voids due to the lumen. Fibre and matrix blend ratio by weight is selected by limiting the void content to 5 to 10% by volume [Oksman et al., 2003; Lee et al., 2009; Zhang and Li, 2009; Yuan et al., 2011; and Akonda and Havis, 2011]. The flax fibre content by weight is considered to examine the effect of different fibre content on the performance of composite material. The nominal flax fibre content was designed to be 40, 50, and 60%. According to the blend ratios by weight, the prepreg is divided into the following three types:

Type A (60P/40F): 60% PLA + 40% Flax

Type B (50P/50F): 50% PLA + 50% Flax

Type C (40P/60F): 40% PLA + 60% Flax

As stated in Section 2.2.2, prepreg can be made using different techniques. Nonwoven technique is used in this research for making prepreg from PLA/Flax fibre blend. This nonwoven technique consists of two steps: web formation and web consolidation.

The web was produced in the University of Manchester’s nonwoven laboratory by using air laying method which is discussed in detail in Section 4.2.1. 75

The nonwoven webs are pre-consolidated by using heat and pressure for easy handling. From the literature survey it is found that most nonwoven prepregs are made from stitch bonded mats. The stitch bonded nonwovens are based on needle-punching, which can cause fibre damage. The thermal bonding route used in this research does not suffer from this drawback. The matrix material is also used as binding material here. The web is consolidated in the University of Manchester’s nonwoven laboratory by using a hot pressing instrument. This is explained in Section 4.2.2. Prepreg fabrication procedures are discussed in detail in Section 4.2.3.

3.3.1 Evaluation of Prepreg

The area density (g/m2), thickness and tensile strength of the prepreg were tested for evaluating the prepreg and are discussed in Section 5.3 and 5.4. Area density of the prepreg was determined to analysis the web variation. The BS EN 29073-1: 1992 (ISO 9073-1:1989) standard was followed for the determination of mass per unit area. The thickness of the web was measured by Shirley thickness gauge to check the thickness variation of the prepreg. BS EN ISO 9073-2: 1997 standard method was found to determination of thickness. The tensile strength of the web was measured according to BS EN ISO 9073-18: 2008 by a universal Instron testing machine. This was done in four directions to determine the strength variation of the web in different axis and is explained in Section 5.4.

3.4 Flat Composite Panels

Flat composite samples were developed at first by using nonwoven fabric from flat nonwoven web for better understanding of different production variables and properties of composite materials.

3.4.1 Method of Composite Fabrication

From the literature review, the compression moulding method was found to be beneficial over the other method such as injection moulding, resin transfer moulding, film-stacking technique etc, for making thermoplastic biocomposite panels, as discussed in Section 2.3.2. Moreover, compression moulding is commonly used in industry and it is easy to handle.

3.4.2 Processing Parameters

The following processing parameters were considered in the manufacturing of PLA/Flax composite panels using the compression moulding method. Different settings are used for each variable in order to optimise the process. The settings are discussed below:

 Moulding temperature

The moulding temperature mainly depends on the melting point of matrix material. The melting point of PLA is between 160 °C to 170 °C. From the literature survey it was found that the processing temperature is normally selected to be 10 °C to 20 °C higher than the melting temperature of the matrix material [Zhang and Li, 2009; Yuan et al., 2011; and Kumar et al., 2010]. To examine the effect of different temperature on the performance of composite material three moulding temperatures 180, 190 and 200 °C are considered for this work. It was found that thermal degradation of flax fibre is not really significant in the first few minutes and between 180 to 200 °C temperatures i.e. flax is not affected by these moulding temperatures [Garkhail, 2001 and Gassan and Bledzki, 2001].

 Moulding time

From the literature survey it was found that the moulding or consolidation time for manufacturing thermoplastic composite material is around 5 minutes [Lee et al., 2009 and Zhang and Li, 2009] to 15 minutes [Kumar et al., 2010 and Li et al., 2009]. It was found that the moulding time depends on the moulding temperature and pressure. To find out the effect of different moulding time on the performance of composite material, three moulding times 5 minutes, 10 minutes, and 15 minutes are considered in this work. It was found that when the processing parameters- especially the temperature and time are well under control, the production of flax fibre composite should be possible without significant loss of stiffness and strength [Bos et al., 2006].

 Moulding pressure

From the literature survey it was found that a wide range of pressure could be used. For example, Yuan et al. [2011] used a fixed pressure of 30 bar, Kumar et al. [2010] used a fixed pressure of 50 bar, Bos et al. [2006] used a fixed pressure of 40 bar and Patel et al. [2010] used a fixed pressure of 100 bar. It was also found that the selection of pressure depended on the moulding temperature, time and thickness of the material. For this research, a range of pressures from 12.5 to 62.5 bar were selected to manufacture composite materials. The tensile properties were tested to determine the optimal pressure for the highest composite performance. This is reported in Section 6.5.1. The highest tensile strength was found to be from the composite material manufactured using 50 bar moulding pressure. Thus moulding pressure was fixed at 50 bar for subsequent work in this research.

3.4.3 Fabrication Procedure

The flat composite panels are manufactured by using compression moulding. The standard method was followed to manufacture the composite material and the manufacturing system is discussed in Section 4.3.

3.4.4 Analysis of Composite Properties

Composite materials based on natural plant fibres are increasingly regarded as an alternative to glass fibre reinforced parts. The proposed areas of end use of the product manufactured by this project include automotive industry- car interiors, interior floor panel, car door panels, car roof and boot linings, parcel shelves, automotive structural beam, vehicle body panel, exterior engine components; building industry- ceiling tiles, windows, doors, hard boards, particle board, panels for partition, false ceilings and in the construction of low cost, mobile or pre-fabricated buildings; house-hold products- furniture, cupboard, lamp shade, luggage, computer cases, electric fan blades, cover of electrical appliances; and biogas containers, railway coaches, food container, industrial packaging material. The composites need to have satisfactory mechanical properties for these applications.

The density of the composite is one of its important physical properties and is dependent on the amount of reinforcement present in the structure. The density is 78

calculated using the immersion method, BS EN ISO 1183-1: 2004 and discussed in Section 5.5.1. Generally, it is desirable to have a higher fibre volume fraction in a composite because the fibres are used as reinforcement to improve the properties of the matrix. It is also desirable to have a lower void content as the voids have a negative impact on the mechanical properties of the composites. The fibre reinforced composites generally contain the constituents of fibre content, resin content and void content. The constituents of the composite samples are measured volumetrically in percentage. The solubility test i.e. the digestion method [BS ISO 14127: 2008] was used to determine the constituents of the composite and discussed in details in Section 5.5.2.

A universal tester Instron 5569 was used to test tensile properties of the composite samples. The BS EN ISO 527-4: 1997 method was followed. The detailed results are presented in Section 5.6.1. In order to determine the flexural properties, the three-point flexure system described in the BS EN ISO 14125: 1998+A1:2011 standard method was used. The details of the test are discussed in Section 5.6.2. One of the major composite applications is structural components for the automotive industry, for example door panels or instrument panels. For such applications, the utmost impact strength is required in order to achieve maximum passenger safety. The Izod impact tests were conducted on notched samples according to BS EN ISO 180: 2000+A2:2013 using an Avery pendulum impact tester. The impact test results are discussed in Section 5.6.3.

Differential scanning calorimetry (DSC) technique determines the quantity of heat either absorbed or released when a substance undergoes a physical or chemical change. Several parameters can be estimated by performing a DSC scan. These parameters include the glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (∆Hm) and crystalline level. The DSC measurements are carried out using a calibrated TA Instrument DSC Q100. Thermal degradation characteristics of the biocomposites are determined by thermogravimetric analysis (TGA) using a TA instrument, TGA Q500. The thermal properties of the biocomposites are discussed in details in Section 5.7.

Water absorption behaviour indicates the stability of the composite and is an important property of the biocomposites. The water absorption of the composite samples was measured following the BS EN ISO 62: 2008 (method-1) and the results are discussed in Section 5.8.

Environmental legislation as well as consumer demand in many countries are putting more pressure on manufacturers of materials and end-products to consider the environmental impact of their final products at all stages of their life-cycle, including recycling and ultimate disposal. In order to evaluate the biodegradation, the compost soil burial test was used and the results are discussed in detail in Section 5.9.1. Biodegradation studies are based on the estimation of the loss in weight of the composites with degradation time and the weight loss is determined gravimetrically. The effect of biodegradation on the residual mechanical properties is also reported. The surface morphology was studied by using SEM before and after the burial test to assess the degradation of the materials.

3.5 3D Composite Panels

The composite materials used in automotive industry, house-hold products etc. are often in 3D form. Developing a composite material with 3D shell structure nonwoven fabric is an important part of this project work. 3D biodegradable composite materials are fabricated by using 3D nonwoven prepregs.

The 3D nonwoven fabric forming machine was used to fabricate 3D nonwoven shell structure which was used directly as prepreg for composite fabrication. This is discussed in details in Section 4.4.

A 3D shell structured aluminium mould was developed to fabricate 3D composite material. Similar to flat composite panel manufacture, the compression moulding method was used to manufacture 3D composite panels with the new 3D aluminium mould. The same processing variables as those used for making the flat composite panels (Section 3.4.2) were considered to manufacture 3D composites. The manufacturing method is discussed in detail in Section 4.5. The 3D composite panels were tested for physical and mechanical property evaluation.

CHAPTER 4

PREPREG AND COMPOSITE MANUFACTURING PROCESS

4.1 Introduction

One of the aims of this project was to eventually make 3D shell structures, but a better understanding of the behaviour of PLA/Flax fibre combination during the air laying web forming process and the optimum condition of process variables were required first. The air laying web forming machine available in the laboratory of the University of Manchester was utilized to manufacture flat PLA/Flax nonwoven webs.

This chapter includes a description of the experimental machine and procedure to manufacture flat and 3D shell structured PLA/Flax nonwoven webs which were used as prepreg to manufacture biodegradable composite samples. Development of 3D mould unit both for nonwoven webs and composites are described. This chapter also describes the equipment and techniques employed for the production of flat and 3D composite samples used in this study.

4.2 Prepreg Manufacturing

Nonwoven prepregs are manufactured in two steps. Web formation is the first. Both reinforcement and resin materials are used in staple fibre form to manufacture the webs in this project. The air laying web forming method was used to produce nonwoven webs. Web consolidation is the second step of the nonwoven prepreg fabrication. The thermal bonding method was used to consolidate the web in this project.

4.2.1 Air laying web forming machine

An air laying web forming machine generally consists of feeding, carding, stripping, transporting and condensing units. These were explained in detail in Section 2.3.1.1. Figure 4.1 shows a schematic diagram of the air laying web forming process [Gong et al., 2000]. The principle and the major constructional parts of this machine are similar to the machine explained in Section 2.3.1.1.

Figure 4.1 Schematic diagram of air laying web forming process [Gong et al., 2000].

Figure 4.2 shows the web forming machine used to produce 2D flat nonwoven web in this project. The feeding unit of this machine consist of a feed lattice which was operated by a separate motor. The fibres were fed manually to the feed lattice and taken by the feed roller to the carding zone. Figure 4.2(b) shows the feeding unit of the web forming machine. Carding unit of the machine consist of multiple carding points. The stripping unit was the same as described in Section 2.3.1.1. The carding and stripping unit of the web forming machine are shown in the Figure 4.2(c). Figure 4.2(d) shows the transporting unit of the machine which is described in detail in Section 2.3.1.1. There are two types of deposition screens which are discussed in Section 2.3.1.1. Belt type deposition screen is used in this machine. This is shown in Figure 4.2(a).

Figure 4.2 Complete air laying web forming machine (a), feeding unit (b), carding and stripping unit (c), and transporting unit (d).

4.2.2 Hot Pressing Instrument

A Clarke Strong-arm hot pressing instrument was used to prepare the prepregs. This is shown in the Figure 4.3 (b). It is electrically heated. Hydraulic pressure was used to press the nonwoven web sample. The machine mainly consists of two platens. There was a control device that controls the temperature of the platens. The bottom platen was fixed in its position and the top platen moves up and down, operated by the handle of the press. There were two additional aluminium platens of size 340 × 210 × 10 mm. These platens were used as mould unit to produce flat prepreg and shown in the Figure 4.3(a).

Figure 4.3 Two aluminium platens (a) and Hot pressing instrument (b).

The nonwoven webs were placed one over the other; the number of doubled webs depended on the thickness of the prepreg required. The webs were sandwiched between the two aluminium platens lined with two PET easy release sheets. The nonwoven web was preliminarily bonded by this instrument to strengthen the web for retaining the shape and size of the nonwoven web and also for ease of handling.

4.2.3 Experimental Method of Prepreg Manufacturing

PLA/Flax nonwoven prepregs were manufactured using the processing steps of opening, blending and carding for web formation and then bonding for stabilisation. Figure 4.4 shows a schematic diagram of the prepreg fabrication process route.

Figure 4.4 Schematic diagram of prepreg fabrication process route.

The following procedures were adopted in the laboratory to manufacture flat PLA/Flax prepreg samples and the steps are shown in the Figure 4.5(a – i):

a) Weigh both fibres according to the type of prepreg mentioned in Section 3.3. b) Open the fibres manually from the sliver form as much as possible. c) Spread the opened fibres evenly on the feed lattice of the web forming machine. Both PLA and flax fibres were placed one over the other. In this case PLA fibres were spread as bottom layer and flax fibres were spread as top layer. d) Forming the PLA/Flax fibre web. Carding and stripping action of the air laying web forming machine provides a uniform blend of the two fibres and produce a mixed fibre nonwoven web, which was collected from the deposition screen of the machine. The PLA/Flax nonwoven web produced after the carding process was pressed to reduce the thickness of the web, which was fluffy and voluminous. Because of the limited distance between the top and bottom platen of the hot pressing instrument. For pressing action the web was folded transversely in four parts on the deposition screen to minimise the thickness variation of the web. e) Cut the folded web longitudinally in two to fit the plate size of the pressing instrument. These cut parts were doubled to improve the uniformity of fibre distribution and reduce the irregularity.

Figure 4.5 Prepreg fabrication process route in the Laboratory. f) & g) Place two cut parts of the web one over the other and between the two aluminium platens lined with PET easy release sheets. h) In the final step of prepreg fabrication, the two aluminium platens with webs was placed between the two platens of the hot pressing instrument and cured at a pressure of about 4 kg/cm2 for 5 minute at 160 °C temperature. Figure

4.5(i) shows the prepreg. The final biocomposites were developed from the prepregs by using the compression moulding method.

4.3 Composite Manufacturing

Manufacturing of 2D or flat flax reinforced PLA biocomposites was carried out by using the compression moulding technique. Depending on the thickness of composite material required, a number of prepregs can be doubled to make the desired thickness. A range of composite materials were produced by using the PLA/Flax prepregs and the classification is based on the fibre/matrix mixing ratio and the process variables such as moulding temperature and moulding time.

4.3.1 Compression moulding machine

A compression moulding machine consists of the main frame unit, pump unit, heating and cooling unit. Figure 4.6 shows a compression moulding machine in the laboratory of the University of Manchester. The compression moulding machine mainly consists of the following units:

 Main Frame – This unit of the machine mainly consists of the top and bottom metal platens. The top platen is fixed in its position and the bottom platen is moved up and down by a hydraulic ram, operated by the pump. The top and bottom platens are supported by four vertical stands or frames.

 Hydraulic Pump – This unit consists of handle, pressure control valve and pressure indicator. The pressure is released by opening the control valve, and then the bottom platen goes down and makes a gap between two platens. The valve is tightened for increasing the pressure and upward movement of the bottom platen to close the gap. The pressure was expressed by Tons on a 4 inch diameter ram in the pressure indicator dial. From the conversion factors of different pressure unit the following relationship (Equation 4.1) was calculated for this machine. The pressure was expressed in bar for this research work. It was calculated by using the Equation 4.1 that 1 ton pressure on 4 inch diameter Ram is equivalent to 12.5 bar (appx.) pressure.

1 Ton pressure on Ram = × 154.44 Bar pressure (4.1)

( )

where, D is the diameter of the Ram; 154.44 is conversion factor for Ton/inch2 to bar. 1 Ton/inch2 (long) = 154.4426 bar [http://www.onlineconversion.com/pressure.htm].

Figure 4.6 Compression moulding machine.

 Heating and Cooling Unit – The top and bottom platens are connected to the electric heater for heating the mould unit and cold water supply for cooling the mould device. There are separate water supplies for the top and bottom platens. The temperature of the mould unit is controlled and fixed via the top and bottom platens by the electric heater. The temperature of the top and bottom platen is fixed separately by two separate heaters.

 Moulding device – Two separate rectangular metal platens were developed to manufacture 2D flat composite material. The size of each platen was 210 × 200 × 10 mm. Figure 4.7 shows two metal platens on a table.

Figure 4.7 Metal platens used as mould unit to produce flat composite.

4.3.2 Production parameters for composite fabrication

According to Section 3.4.2 the following production parameters were considered to manufacture PLA/Flax biocomposites using the compression moulding method for this project. Three levels were used for each variable to analysis the effect of the level on the performance of the composite materials. The parameters with their levels are shown in the Table 4.1.

Table 4.1 Production parameters

Levels Factors Fibre Composition (%) Mo ulding Moulding Moulding PLA / Flax Temperature (°C) Time (min.) Pressure (bar) 1 60PLA/40Flax 180 5 50 2 50PLA/50Flax 190 10 50 3 40PLA/60Flax 200 15 50

4.3.3 Experimental Procedure

The following procedure was adopted in the laboratory to manufacture PLA/Flax 2D flat composite samples:

1 A rectangular PET sheet was placed on a flat surface. The PET sheet was used as non-stick material. PLA/Flax nonwoven prepregs were placed on top of each other on the PET sheet. The number of prepregs depends on the thickness of the composite material required. Another rectangular PET sheet was placed on top of the prepregs, shown in the Figure 4.8(a). 88

2 The prepregs were sandwiched between two metal platens (mould unit) lined with two PET easy release sheets, shown in the Figure 4.8(b). 3 The heater was switched on and the hot-press i.e. moulding temperature in the machine was set. 4 The pressure control valve of the pump unit was opened to release the pressure when the temperature was raised to 100 °C, and then an open space between top and bottom platen was created by moving the bottom platen down. 5 The mould unit with prepregs was then placed between the top and bottom hot platens of the moulding machine at 100 °C temperature [Figure 4.8(c)]. The melting temperature of PLA is between 160 °C to 170 °C. The sample is fed to the machine before melting temperature is reached to avoid the instantaneous effect.

Figure 4.8 Composite manufacturing process route in the Laboratory.

6 The pressure control valve was closed and the pressure was increased by pushing the handle of the pump until the required moulding pressure is reached. 7 The sample was then moulded in the machine under the selected moulding temperature and pressure for the selected moulding time. 8 After completing the moulding, the heater is switched off and the two cold water inlet valves are opened to cool the material at room temperature. Figure 4.9 shows a line diagram of a typical processing cycle.

Figure 4.9 A line diagram of a typical processing cycle.

9 Take out the composite material with mould unit from the machine by releasing the pressure. The two metal platens of the mould unit were separated to obtain the composite material. Typical composite materials are shown in the Figure 4.10.

Figure 4.10 Typical samples of the composite material.

4.3.4 Composite Sample Coding

The composites are divided into categories based on the nominal fibre and resin fraction by weight, moulding temperature and moulding time used. For example, 60P200C15M means:

60P: 60% PLA fibre and 40% flax fibre by weight fraction; 200C: 200 °C moulding temperature; 15M: 15 minutes moulding time.

Twenty seven types of samples were developed in this project by using three levels and three factors described in Section 4.3.2. Table 4.2 shows the details of these samples. These sample codes are used subsequent graphs and tables to analyse the properties of the composite samples.

Table 4.2 Sample details

Sample Sample Fibre Moulding Moulding Moulding Nomenclature Code Composition (%) Temperature Time Pressure PLA + Flax (°C) (min) (bar)

60P180C05M 5 60P180C10M 180 10 60P180C15M 15 60P190C05M 5 60P190C10M 60P/40F 60 + 40 190 10 60P190C15M 15 60P200C05M 5 60P200C10M 200 10 60P200C15M 15 50P180C05M 5 50P180C10M 180 10 50P180C15M 15 50P190C05M 5 50P190C10M 50P/50F 50 + 50 190 10 50 50P190C15M 15 50P200C05M 5 50P200C10M 200 10 50P200C15M 15 40P180C05M 5 40P180C10M 180 10 40P180C15M 15 40P190C05M 5 40P190C10M 40P/60F 40 + 60 190 10 40P190C15M 15 40P200C05M 5 40P200C10M 200 10 40P200C15M 15

4.4 Manufacturing of 3D Nonwoven Prepreg

3D Nonwoven prepregs were manufactured as explained in the Section 2.3.1.2. The web formation on the mould unit and the consolidation of the webs were occurred in two steps on the same machine. In this machine the air laying method was used to produce 3D nonwoven webs and the through-air thermal bonding method was used for consolidation of the web. A 3D mould unit needs to be made according to the shape of the final composite material prior to the prepreg manufacturing.

4.4.1 3D Nonwoven Machine

Gong et al. [2000 and 2001] at University of Manchester developed a pilot process of making chef’s hats, directly on moulds positioned after the cylinder of a card. The whole equipment includes two sections: the web forming system and the bonding chamber. The principle of web forming system is schematically shown in the Figure 4.11. This web forming system consists of a roller card, an air duct (including upper stream duct and down stream duct), a mould chamber and a suction fan.

Figure 4.11 Principle of 3D web forming system.

Fibres are fed into the card and are opened by the taker-in. The opened fibres are carded between the cylinder and workers. The original doffer of the card was replaced by an air duct. The carded fibres on the cylinder are stripped by the airflow, which is generated by the suction fan, and transported in the duct and deposited on the perforated mesh moulds in the mould chamber. Each mesh mould was mounted on a mould unit and each unit was separated from the other unit. The mould units move one by one across the mould chamber along a pair of tracks and deliver the formed 3D webs to the bonding section continually. The weight of the web formed on the mould can be controlled by the weight of the fed lap and the speed of the feed roller.

The principle of web consolidation is schematically shown in the Figure 4.12. This web bonding system is made up with an air heating system which includes the temperature control and temperature distribution control; a suction system which includes air speed control and vacuum control; a mechanical structure which includes the web delivery and bonding chamber movement control and an electric control system.

Figure 4.12 Principle of 3D web consolidation.

To make the uniform temperature and air velocity distribution around the 3D nonwoven product, the bonding chamber was made to be axially symmetrical according to the shape of the product. The hot air inlet is connected with the hot air reservoir and duct heater through a flexible adiabatic pipe. The air outlet is connected to the suction fan, which pulls the hot air passing through the fibrous web that is supported on the mesh mould. An air guide is designed to improve the airflow distribution around the web. The position of the air guide can be adjusted along the central axis.

4.4.2 Development of 3D mould unit

A mould unit consists of a mould, a mould cylinder, a mould board, and two seal boards. The mould was fastened onto the mould cylinder and the symmetry axis of the mould was parallel to the direction of the upstream duct. The cylinder was mounted on the mould board. The mould board was fitted into a pair of tracks inside of the mould chamber, and was allowed to slide on the tracks. Two seal boards were fixed on one end of the mould board to separate the air pressure from the outside of the mould chamber. There were always two mould units in the mould chamber.

The shape of the mould depends on the end use of the composite material. Two simpler dome shaped moulds were designed and used for the initial investigation in this project. This mould was made from wire mesh. The diameter of the mould was 13cm and its height was 6.5cm. The porosity of the mesh was 49 holes per square cm. Figure 4.13 shows a dome shaped mould with a mould unit.

Figure 4.13 Mould unit (b) with dome shaped mould (a).

The surface area of the mould is equal to half of the surface area of a sphere and was calculated by using the Equation 4.2.

2 Am = 2πr (4.2) where,

Am is the surface area of the mould; r is the radius of the mould.

The surface area of the mould was calculated to be 265.5 cm2 by using the Equation 4.2.

4.4.3 Experimental method of 3D prepreg manufacturing

The 3D shell structured PLA/Flax nonwoven prepregs were manufactured using the processing steps of opening, blending and carding for 3D web formation on the mesh mould of the mould unit and then thermal bonding by through air process for stabilisation of the web. The following procedure was adopted in the laboratory to manufacture 3D PLA/Flax prepreg samples and the steps are shown in the Figure 4.14(a – f):

a) Weight flax and PLA fibres according to the required prepreg fibre mixture ratio. b) Fibres were pre-opened manually from sliver form. c) The opened PLA and flax fibres were spread evenly on the feed lattice of the web forming machine. d) The carding and stripping actions of the web forming machine provide a uniform blend of the two fibres and produce a mixed fibre nonwoven web, which was deposited on the mesh mould in the mould chamber of the machine by the airflow that was generated by the suction fan.

Figure 4.14 3D Prepreg fabrication process route in the Laboratory.

e) The air in the heat reservoir was heated up previously to 160 °C temperature and the temperature was kept constant by the temperature controller. The mould unit with 3D fibrous web was moved to the bonding position of the machine. f) The bonding chamber was moved forward by the pneumatic cylinder to cover the fibrous web and upper duct pushed the mould unit onto the lower duct to seal the ducts. The fan was run to draw out the hot air from the heat reservoir through the web to bond the web. Finally, the suction fan and duct heater were turned off; the pneumatic cylinder drives the bonding chamber back to enable the fabric to be removed from the mould. Figure 4.14(f) shows an example of the prepreg.

4.5 Manufacturing of 3D Composite

Manufacturing of 3D PLA/Flax biocomposites was carried out by using the same compression moulding technique as 2D PLA/Flax biocomposite. Depending on the thickness of 3D composite material required, a number of 3D prepregs can be doubled to make the desired thickness. A range of composite materials were produced by using the 3D prepregs and the classification is the same as 2D flat biocomposites. A simple dome shape was considered as 3D shell structure for the preliminary work of this project.

4.5.1 Aluminium mould for 3D composite production

A matching pair of dome (hemisphere) shaped moulds of aluminium were developed for 3D composite fabrication. These are shown in Figure 4.15.

Figure 4.15 Aluminium Mould used for 3D biocomposite.

The top mould (a) is a dome shaped and the bottom mould (b) has a matching dome shaped cavity. Two jacking screws were used to release the composite material by separating the top mould from the bottom one. The shape and size of the mould are the same as those of the 3D nonwoven prepreg. The 3D moulding device was placed into the moulding machine to fabricate the 3D composite material.

4.5.2 Experimental Procedure of 3D composite manufacturing

The 3D shell structured biodegradable composite material was developed by using the 3D aluminium mould. The 2D metal flat platens (used as mould to produce 2D biocomposites) were replaced by this 3D aluminium mould. The electrolube dry film lubricant (DFL 200D) was sprayed on the contact surfaces of the top and bottom moulds. This lubricant helps the removal of the 3D composite after compression moulding. The 3D shell PLA/Flax nonwoven prepregs were placed in the cavity of the bottom mould. The number of prepregs used depends on the thickness of the composite material required. The dome shaped top mould was placed on top of the prepregs, thus the prepregs were sandwiched between the two mould units, shown in the Figure 4.16(a).

Figure 4.16 3D Composite manufacturing steps in the Laboratory. 96

The remaining procedures and conditions were the same as for 2D composite fabrication described details in the Section 4.3.3. Examples of the fabricated 3D dome shaped biocomposite samples are shown in the Figure 4.17.

60P/40F biocomposites

50P/50F biocomposites

Figure 4.17 Dome shaped 3D biocomposite samples.

4.6 Summary

The experimental machine and equipment used for making 2D flat nonwoven webs, 2D flat composite materials, 3D shell structured nonwoven prepregs, and 3D shell structured biocomposite samples were explained in this chapter. The method was adapted from the standard procedure for making nonwoven webs. This chapter also described the mould units for making 3D nonwoven webs and the 3D aluminium mould for making 3D composite materials.

CHAPTER 5

CHARACTERISATION EQUIPMENT AND PROCEDURES

5.1 Introduction

This chapter describes the physical and mechanical test methods and equipment used to characterise the flax reinforced PLA biocomposites. The composite panels were developed utilising the compression moulding technique and after fabrication the specimens were cut using a vertical band saw machine according to the respective standard dimensions for physical and mechanical characterisation. For physical characterisation, the density was measured by the immersion method and then the fibre volume and void content of the biocomposites were determined by using the digestion method. The mechanical performance was measured using tensile testing, a three point bending test and notched izod impact strength tests. Before testing the composite samples, the physical and mechanical properties of the flax fibre, PLA fibre and the PLA/Flax nonwoven prepregs were also evaluated.

The thermal properties were measured by using DSC and TGA analysis. The performance of biodegradability was evaluated by using soil burial test and water absorption of the biocomposites was tested. Finally the surface morphology before and after the biodegradation and the post-fracture analysis was done by using scanning electron microscopy (SEM).

5.2 Properties of Material Testing

Flax fibre shows natural variation in properties in different aspects whereas PLA fibre has controlled properties according to requirements as a manufactured product. To select the required properties of the PLA fibres it was important to find out the relevant properties of the flax fibres. The following properties were determined in this project.

Fibre length was determined by measurement of individual fibres according to the method-A of the BS ISO 6989: 1981 standard. Fibre diameter was measured by using SEM. According to the BS ISO 11567: 1995 standard method 20 individual fibres/filaments were tested to calculate the mean value of the fibre diameter.

Linear density of the fibres was measured according to the BS EN ISO 1973: 1996 standard method. The gravimetric method of this standard was used to determine the linear density of both fibres. According to the standard 10 fibre tufts having a mass of several milligrams were taken from the sample and the fibres of each tuft were brought into parallel alignment by carefully combing them several times. The middle part of each combed tuft was cut to 50 mm length, under the minimum tension necessary to remove crimp, by means of the cutting device. The necessary precautions were taken so that there are no free fibre ends anywhere except at the two ends of the cut bundle. Five fibres were taken out from each of the ten bundles in turn, so as to form a bundle of 50 fibres and 10 of these bundles were made. These bundles were weighted individually, using the balance, to an accuracy of 0.1mg. The mean linear density of the fibre in each bundle was calculated by using the Equation 5.1.

4 = × 10 (5.1)

where,

is the mean linear density of the fibre in each bundle, in decitex (dtex);

mb is the mass of the fibre bundle, in milligrams (mg);

nf is the number of fibres in the bundle;

lf is the length of the individual fibres in the bundle, in millimetres (mm).

Tensile tests have been carried out on single flax and PLA fibres using a universal Instron 5500-type tensile testing machine equipped with a 2N capacity load cell. The tensile strength was determined according to the BS ISO 11566: 1996 standard method. A gauge length of 25 mm was used for this test. A schematic representation of the tensile testing protocol is given in Figure 5.1. Before the tensile test, the fibre was glued on a paper frame. The fibre diameter was estimated as the average of three measurements along the fibre. Then the top and bottom edges of the frame were clamped into the grips and its sides were cut. The fibre was tested in tension at a constant crosshead displacement rate of 2 mm/min until it failed. 20 single fibres were tested in these conditions. The fibres were conditioned before the test according to the standard BS EN ISO 291: 2008 at 23 ± 2 °C temperature and 50 ± 5 % RH for 24 hours.

Figure 5.1 Schematised testing protocol of a fibre.

The tensile strength of the fibre/filament was calculated for each test specimen using the Equation 5.2.

= (5.2)

where,

is the tensile strength, in megapascals (MPa);

is the maximum tensile force, in Newtons (N);

is the cross-sectional area of the fibre, in square millimetres (mm2).

Melting temperature of the PLA fibre was determined by analysing the DSC curve of the first and second heating cycle of the PLA fibre. The selection of moulding temperature of composite manufacturing was dependent on the melting temperature of the PLA fibre.

5.3 Area Density and Thickness of the Prepreg

The properties of composite materials are influenced by the properties of the prepreg. The area density and thickness of the prepreg were considered at this stage of the research. Area density of the 2D flat prepreg was measured according to the BS EN 29073-1: 1992 (ISO 9073-1:1989) standard method. An accurately graduated (mm) steel rule was used to measure the area and a razor blade was used to cut the prepreg in size 210mm × 170mm. The prepregs were weighted individually, using the balance, to an accuracy of 1mg in the atmospheric condition of 23 ± 2 °C temperature and R.H. 50 100

± 5%. Each value reported here is the average of ten test pieces. The area density of the 2D flat prepreg was calculated by using the Equation 5.3.

M = (5.3)

where, M is the area density in g/m2; m is the mass per sample area in gm; A is the area of the sample in mm2.

The area density i.e. weight per unit area of the 3D dome shaped prepregs was calculated by using the Equation 5.4.

M = (5.4)

where,

M is the area density in g/m2; W is the total weight of a single dome shaped prepreg in gm; r is the radius of the sample in mm.

Thickness of the prepreg was measured by using the Shirley thickness gauge under a pressure foot of 8.0 cm diameter and pressure of 250 gm. Ten measurements were taken for thickness test and the average value was reported according to the BS EN ISO 9073- 2: 1997 standard method.

5.4 Tensile Strength of the Prepreg

Tensile strength (grab test) of the prepreg was measured according to the BS EN ISO 9073-18:2008 standard method using a universal Instron 4411 testing machine. A rectangular specimen of 100 mm wide by 150 mm long was cut from the prepreg sample and the distance between the clamps (gauge length) was set at 75 mm. The specimen was gripped at both ends as shown in Figure 5.2 and the machine was set for an extension rate of 300 mm/min. The values of the load and displacement were recorded. Five specimens of each sample were tested in different axis such as machine (MD), cross-machine (CD) direction, 45° angle to MD (left to right) and 135° angle to MD (right to left). The main objective of the measurement of tensile strength of the prepreg in different direction of sample length axis was to select the sample length axis 101

for the tensile test of composite material. The results of tensile strength of prepregs in different direction was also indicated the fibre orientation in the webs. The breaking force was used as tensile strength in this work and the force was collected directly from the testing machine [Fedorova et al., 2007].

Figure 5.2 Nonwoven prepreg undergoing breaking strength testing.

Figure 5.3 shows the sampling directions of the prepregs for tensile strength test.

Figure 5.3 Length-wise direction of prepreg samples for tensile strength test.

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5.5 Physical Properties of the Composites

Since the mechanical properties of the biocomposites depend on their real fibre volume fraction and void content, they were measured after the consolidation operation, in order to confirm the fibre-matrix ratio. The density of the biocomposites is also important to calculate the void content. Thus the density and constituents of the composite materials are considered as physical properties for this research work.

5.5.1 Density Measurement

Density is one of the most important physical properties of a composite structure. The density is dependent on the amount of reinforcement present in the structure. The immersion method was used in this research as per standard BS EN ISO 1183-1:2004 to calculate the density of composite material. Three specimens were cut from each panel from different places and their weight was determined. A fine wire of maximum 0.5 mm diameter was used for hanging the composite pieces and its weight in air ( ) and water ( ) was measured. The weight in air of the specimens ( ) was measured by hanging them through a balance hook by means of the wire. Also the weight of the specimens in water ( ) was determined by immersing them in water in a 100 ml beaker while they were suspended stationary by the wire. Care was taken to avoid any air bubbles adhering to the specimen or found in the beaker otherwise the bubbles were removed by means of a fine wire. The weighing balance used for the measurement of weights had an accuracy of 0.1 mg.

The density of the specimen was calculated using the Equation 5.5.

= (5.5)

where,

is the density, in g/cm³, of the specimen;

is the apparent mass, in grams, of the specimen in air, (m₃ - m₁);

is the apparent mass, in grams, of the specimen in the immersion liquid, (m₄ - m₂);

is the density, in g/cm³, of the immersion liquid (water).

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5.5.2 Constituents of the Composite Samples

In textile composite manufacturing, fibres are used as reinforcement to increase the mechanical properties of the composite. It is desirable to have high fibre volume fractions. The constituents of the composite samples are measured volumetrically in percentage. The fibre reinforced composites generally contain the following constituents:

 Fibre  Resin  Void

Generally, it is desirable to have a higher fibre volume fraction in a composite sample because the fibres are used as reinforcement to improve the properties of the matrix. It is also desirable to have a lower void content as the voids have a negative impact on the mechanical properties of the composites.

The amount of fibre and void content in a composite sample is dependent on the selection of the composite manufacturing technique. It is also dependent on the type of reinforcement used; for example; a woven fabric, a chopped strand mat or a unidirectional tape etc.

To find the constituents of the composites, pieces of composite samples are cut from the panel and its constituents are determined.

5.5.2.1 Methods for determining the constituents

In order to calculate the constituents of the composite, it is necessary to separate the reinforcement and the matrix. For this purpose, the resin is either digested using the acid digestion method [BS ISO 14127: 2008] or it is burned out using the calcination method [BS EN ISO 1172: 1999]. The calcination method is used for glass fibre composites as these fibres can withstand at higher temperatures compared to other fibres. The method of acid digestion is normally used to dissolve the resin by nitric acid or sulphuric acid. It is commonly used for carbon and Kevlar fibres. Since the standard BS ISO 14127:2008 is for carbon fibres but due to the non-availability of standards on flax fibre, the same standard was used to calculate the fibre and resin weight fractions. Dichloromethane (DCM) was used in this experiment to digest the PLA matrix or resin at ambient temperature instead of nitric or sulphuric acid [Bledzki et al., 2009]. The desiccated dry

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mass of the specimen was determined before and after the resin digestion. The composite specimens were soaked in a bath at ambient temperature to digest the resin. Sintered glass crucibles, oven, desiccators, dichloromethane, conical flask and weighing balance were used in this experiment.

In order to determine the constituents of the composite samples, a small square specimen was cut from the composite sheet so that it could easily fit into a sintered glass crucible. The desiccated dry mass of the cleaned crucible was determined with an accuracy of ± 0.1mg. This mass was recorded as m₁. The sample was placed in the crucible, dried in the oven at 100 °C and then it was held in desiccators for half an hour. The mass of the crucible along with the sample was determined and recorded as m₂.

Then the sample was put in the conical flask along with dichloromethane to completely soak the sample. The sample was thoroughly stirred at ambient temperature to digest the resin in the dichloromethane. After resin digestion the sample was washed with water, filtered with sintered glass crucible, then dried in the oven and desiccated. Then the mass of the crucible along with residual fibres was determined and was recorded as m₃. The equipment used in digestion method is shown in Figure 5.4.

Figure 5.4 Equipment used in digestion method: (a) Oven, (b) Desiccator with crucibles having samples, (c) Digestion and washing apparatus, and (d) weighing balance.

5.5.2.2 Calculating the amount of constituents

In order to calculate the amount of constituents in the composite samples the Equations 5.6 to 5.10 were used.

Fibre content by weight ( ):

= × 100 (5.6)

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where,

is the fibre content as a percentage of the initial mass;

is the mass of the sintered glass crucible (g);

is the initial mass of the specimen and glass crucible (g);

is the final total mass of the crucible and the residue after digestion (g).

Resin content by weight ( ):

= 100 - (5.7)

where,

is the resin content as a percentage of the initial mass;

is the fibre content as a percentage of the initial mass.

Fibre content by volume ( ):

= × (5.8)

where,

is the fibre content as a percentage of the initial volume;

is the fibre content as a percentage of the initial mass;

is the density of the test specimen (g/cm³);

is the density of the reinforcing fibre (g/cm³).

Resin content by volume ( ):

= (100 - ) × (5.9)

where,

is the resin content as a percentage of the initial volume;

is the fibre content as a percentage of the initial mass;

is the density of the test specimen (g/cm³);

is the density of the resin (g/cm³).

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Void content by volume ( ):

= 100 - [ ( ) ] (5.10)

where,

is the void content as a percentage of the initial volume;

is the fibre content as a percentage of the initial mass;

is the density of the test specimen (g/cm³);

is the density of the reinforcing fibre (g/cm³);

is the density of the resin (g/cm³).

5.6 Mechanical Property Testing

Biocomposites are becoming more popular because of the increase in oil price, recycling and environmental concerns. They are lighter in weight and more cost effective than glass and carbon fibre composites. They can be used to replace synthetic composite materials in the automotive and building industries. They are widely used in high performance applications also. Satisfactory mechanical properties are vital in these types of applications. The tensile, flexural, impact and crashworthiness properties were considered for this research work to evaluate the mechanical performance of the composite materials.

5.6.1 Tensile Testing

During a tensile test, a rectangular strip which is cut from a panel having a longer length compared to its width is subjected to the tensile load or stresses in the length direction by holding one end of the strip and stretching the other end. Both the ends of the strip are held with the help of grips and if breakage occurs near the grip the sample is rejected. An example of acceptable and unacceptable failure under tensile load is shown in the Figure 5.5.

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Figure 5.5 Unacceptable failures (a) and Acceptable failures (b) [Baker et al., 2004].

The basic purpose of tensile testing was to determine the tensile strength and modulus of the material. There are varieties of specimen sizes, test piece specifications, and testing procedures described in a number of published standards to measure the tensile stress and strain at breaking point and the Young’s modulus. The standard followed for the determination of tensile properties in this study was BS EN ISO 527-4: 1997.

Figure 5.6 Vertical band saw.

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The tests were conducted on a universal Instron testing machine model 5569 with a 10 kN load cell at an extension rate of 2.0 mm/min. Five specimens of each type were cut in the machine directions using a vertical band saw (Figure 5.6) and the dimensions of all the specimens were measured before the test. A rectangular specimen of 180 by 25 mm was cut from the 2D flat composite sample and the initial distance between grips was fixed at 100 mm.

The distance between grips for the sample was lower than the recommended distance of 150 mm because it was not possible to obtain a longer piece from the present flat composite samples. The limitation of the moulding machine plate size was 200 by 200 mm. Extensometer was attached to the composite specimens during testing, as shown in Figure 5.7, to acquire data for establishing the values of the modulus of elasticity in megapascals.

Figure 5.7 Composite specimen undergoing tensile testing.

The tensile stress at the breaking load was calculated by using the Equation 5.11.

(5.11)

where,

is the tensile stress at break in megapascals (MPa); F is the force or load at break in Newtons (N);

Acs is the initial cross-sectional area of the specimen in square millimetres (mm2). 109

The tensile strain at the breaking load was calculated by using the Equation 5.12.

( ) (5.12)

where,

is the tensile strain at break expressed in percentage (%);

Lo is the gauge length of the test specimen expressed in millimetres (mm);

∆Lo is the increase in the specimen length between the gauge marks, expressed in millimetres (mm).

The stresses and strains were calculated at all the points from the load and displacement curve.

The tensile modulus or Young’s modulus of the specimen was calculated by using the Equation 5.13.

(5.13)

where,

Et is the Young’s modulus of elasticity, expressed in megapascals (MPa);

is the stress, in megapascals, measured at the strain value

= 0.05%;

is the stress, in megapascals, measured at the strain value

= 0.25%.

5.6.2 Flexural Testing

In order to determine the flexural properties i.e. flexural stress at break, strain at break and the modulus, the BS EN ISO 14125: 1998+A1:2011 standard method was followed. Two methods are usually used in this standard for the determination of flexural properties. One is called method A and other one is called method B. The three-point flexure system is used in method A and the four-point flexure system is used in method B. Thus the methods used for determination of flexure properties were three-point and four-point bending. Three-point bending is the method in which a bar of rectangular cross-section was loaded from the top while resting on the two supports whereas in 110

four-point bending two loads are symmetrically placed between the two supports as shown in Figure 5.8(a, b). The geometry of four-point loading provides a constant bending moment between the central loading members and this causes a reduction in contact stresses in the beams. Whereas in the three-point loading arrangement the stress concentrations exist at the loading point so, four-point bending method is more attractive if the state of stress is of concern but it is easier to perform the three-point bending test [Hodgkinson, 2000].

The test method A is chosen to limit shear deformation and to avoid an interlaminar shear failure. The three-point flexure system was chosen to determine the flexural properties of the sample in this research work. A flat rectangular specimen was simply supported close to its ends and centrally loaded (Figure 5.8(a)).

F-is the applied load, L-is the span length, l-is the specimen length, and h-is the thickness of the specimen

Figure 5.8 Flexure testing assembly [BS EN ISO 14125: 1998+A1:2011].

The flexure testing can result in a wide range of failure modes depending on the chosen method, type and layup of the materials being tested. The potential failure modes are tensile fracture, compressive fracture, tensile and compressive fracture accompanied by interlaminar shear and interlaminar shear fracture as shown in Figure 5.9. All failure modes are not acceptable especially those initiated by interlaminar shear. To avoid interlaminar shear failure, the specimen with large span-to-thickness ratio should be used. The standard for flexure testing BS EN ISO 14125: 1998+A1:2011recommends a minimum span-to-thickness ratio of 16:1.

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Figure 5.9 Failure of composite samples in different modes under bending load [BS EN ISO 14125: 1998+A1:2011].

This test method is not appropriate for the determination of design parameters, but used as a quality-control test. This is because the specimen is subjected to a combined stress state, and the flexure strength and modulus are combinations of the subsequent tensile and compressive properties of the material [BS EN ISO 14125: 1998+A1:2011 and Hodgkinson, 2000].

The specimen length and span length of the sample depends on the thickness of the sample. The standard suggests that the specimen length to thickness ratio of the sample for testing the flexural properties should be 20 and the span length to thickness ratio should be 16. The width of all samples should be 15 mm. Five specimens of each type were cut using a vertical band saw. The thickness of the specimen was 2 mm. Thus the dimensions of the specimens were 40 mm × 15 mm × 2 mm, and the span length was 32 mm. The testing was conducted on a universal Instron testing machine model 4411 with 5 kN load cell at a constant crosshead speed of 2.0 mm/min (Figure 5.10). The radii of loading nose and supports were selected as 5.0 mm and 2.0 mm respectively. All tests were performed at 23 ± 2 °C temperature and RH 50 ± 5%.

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Figure 5.10 Composite specimen undergoing flexure testing.

Flexural stress is defined as a material's ability to resist deformation under load. The flexural stress was calculated by using the Equation 5.14.

(5.14)

where,

is the flexural stress at break, in megapascals (MPa); F is the load at break, in Newtons (N); L is the span length, in millimetres (mm); b is the width of the specimen, in millimetres (mm); h is the thickness of the specimen, in millimetres (mm).

It was observed that most of the samples deflected more than 0.1 times the span length; therefore the flexural stress was calculated using the Equation 5.15.

{ ( ) ( )} (5.15)

where, s is the beam mid-point deflection, in millimetres (mm).

Flexural strain was calculated by using the Equation 5.16.

(5.16)

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where, is the flexural strain at break; s is the beam mid-point deflection at break in millimetres (mm); h is the thickness of the specimen, in millimetres (mm); L is the span length, in millimetres (mm).

Similarly it was observed that most of the samples deflected more than 0.1 times the span length and therefore the flexural strain was calculated using the Equation 5.17.

{ ( ) ( ) } (5.17)

For convenience, the strain values obtained by Equations 5.16 and 5.17 were multiplied by 100 to obtain the strain values in percentage.

After obtaining the load displacement curve, the slope of the curve was determined in the region where it was linear and the flexural modulus was determined using the Equation 5.18.

( ) (5.18)

where,

is the flexural modulus of elasticity, in megapascals (MPa); L is the span length, in millimetres (mm); b is the width of the specimen, in millimetres (mm); h is the thickness of the specimen, in millimetres (mm);

is the slope of the load displacement curve.

5.6.3 Impact Testing

The basic purpose of impact testing was to determine the energy absorbed by breaking the composite material, and the impact strength of the material. The Charpy and Izod impact tests are available to determine the impact strength of the composite materials. Both Charpy and Izod impact tests are very useful for isotropic materials for which they were developed [Hodgkinson, 2000]. The Izod specimen consists of a beam clamped 114

and struck as a cantilever, while the Charpy test has the beam simply supported and loaded in flexure by the pendulum striker. The Izod and Charpy tests both have provision for notched samples for use with tough specimens (Figure 5.11). The notched Izod impact test was used for this research because of the availability of the instrument in the laboratory of the School.

Figure 5.11 Schematic representation of the Charpy (a) and Izod (b) impact equipment [Hodgkinson, 2000].

The Izod test involved the striker, the testing specimen and the pendulum. The striker was fixed at the end of the pendulum. The test specimen was clamped vertically in Izod support anvils, fitted on the base of the machine and placed with the notch facing the striker. The support (fixed vice jaw) is provided with a machined vertical optional groove to suit the specimen size. The front clamp piece (movable vice jaw) and allen screw enable clamping of the test specimen in correct height with the help of Izod setting gauge supplied. The striker swings downward, hitting the test specimen above the notch at the bottom of its swing.

There are varieties of specimen sizes, test piece specifications, and testing procedures described in a number of published standards to measure the impact strength. The standard followed for the determination of impact strength in this study was BS EN ISO 180: 2000+A2:2013. The impact is divided into two groups, parallel impact – impact with the direction of blow parallel to the plane of reinforcement, and normal impact – impact with the direction of blow normal to the plane of reinforcement. Figure 5.12 shows a scheme of designations describing the direction of blow [BS EN ISO 180: 2000+A2:2013].

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Edgewise (e) and flatwise (f) indicate the direction of blow with respect to the specimen thickness h and width b. Normal (n) and parallel (p) indicate the direction of the blow with respect to the laminate plane.

Figure 5.12 Scheme of designations describing the direction of blow [BS EN ISO 180: 2000+A2:2013].

The tests were conducted on notched samples using an Avery pendulum impact tester with a pendulum of 4.2 J energy (Figure 5.13). At ambient laboratory environment, five specimens of each sample were used for the measurement of the impact properties and average results have been reported. The test specimens were cut to the required dimension according to the standard using a vertical band saw. The notches on the specimens were made by a specimen notch maker. The notched samples for impact testing are shown in Figure 5.14. The dimensions of the test specimen were 80 mm × 10 mm × 4 mm with 2 mm notched at the centre of the vertical edge. The direction of the blow in the Izod test was edgewise parallel (Figure 5.12) with a striking velocity of 2.44 m/sec.

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Figure 5.13 Composite specimen undergoing impact testing.

Figure 5.14 Notched samples for Izod impact test.

The Izod impact strength of notched specimens was calculated by using the Equation 5.19.

= × 10³ (5.19)

where,

is the notched Izod impact strength (KJ/m²);

is the energy absorbed by breaking the specimen (J); h is the thickness of the specimen, (mm);

is the remaining width of the test specimen (mm).

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5.6.4 Crushing Test

The crushing test was performed on 3D shell structured biocomposites in this study. The 3D composite samples were tested by a quasi-static axial compression test between two flat aluminium platens; the top one is static and the bottom one moving with constant speed [Hakim and Sultan, 2011; Palanivelu et al., 2010]. The platens were kept parallel to each other throughout the test. To hold the sample in place, a circular slot with the same diameter as the 3D sample was engraved in the bottom platen (Figure 5.15). The sample was placed in this circular slot.

Figure 5.15 Top and bottom platens used for crush test.

A Zwick/Roell Z050 digital testing machine (Figure 5.16) with full scale load range of 10 kN was used to perform the crush test. Three samples of each type of composite were tested. All composite samples were compressed between 45 to 60 mm crush length at a loading rate of 10 mm/min to the complete compaction of the samples being tested.

Figure 5.16 3D Composite specimen undergoing crush testing. 118

The automatic data acquisition system was used to obtain the load-displacement curve of the crush test. During the tests, the load-displacement data was recorded as a function of time at intervals of 0.1 s. A typical load-displacement curve of the composite crush test is shown in Figure 5.17.

Figure 5.17 Typical load-displacement curve of the composite crush test.

The energy absorbing capability can be estimated by the following parameters:

 Crush failure load – Initial failure load, or the first peak crush failure load, is the boundary between the pre-crushing zone (elastic) and the post- crushing zone (plastic). Before arriving at this point, the structure has the ability to return back to the original state. After the initial failure load, the structure deforms permanently. The value of initial crush and peak crush failure load can be found from the load-displacement curve of the crush test.

 Total energy absorption (EA) – It is the area under the load-displacement curve. It is a function of the specimen cross-sectional area and the material density. This energy can be obtained by numerical integration of the load displacement-curve.

 Specific energy absorption (SEA) – The specific energy is defined as the amount of energy absorbed per unit mass of the crushed material and the units of SEA are kJ/kg [Hakim and Sultan, 2011]. It provides a measure of the energy absorption ability of a structural component. To compare 119

different materials or different structural geometry of the samples, it is necessary to consider the specific energy. It was calculated by using the Equation 5.20.

SEA = (5.20)

where, SEA is the specific energy absorption; EA is the total energy absorption; m is the mass of a unit crush material.

 Crush force efficiency (CFE) – It is the ratio between average crush load and maximum crush load. The average or mean crush load was calculated by using the Equation 5.21. CFE is useful to measure the performance of the energy absorption of a structural component. Crush efficiency gives an idea about the ability of a structural component for energy absorption. The ideal value is 100% which means that after the initiation of crushing (peak crush load) the load will remain the same (mean load). It was calculated by using the Equation 5.22 [Palanivelu et al., 2010].

( ) ∫ = (5.21)

where,

is the mean crush load (kN); P(l) is the instantaneous crushing load corresponding to the instantaneous crushing deformation length dl;

is the maximum or total deformation length

ηc = × 100 % (5.22)

where,

ηc is the crush efficiency (%);

Pmax is the peak crush load (kN)

5.7 Thermal Property Testing

Generally, the melting point of fibre-reinforced biocomposites is in the range of the melting point of the matrix. To fix the processing temperature, it is important to know 120

the melting temperature of the matrix. Understanding the concepts of glass transition temperature (Tg), as well as crystallization temperature (Tc) and melting temperature

(Tm) is an important part in the study of polymers and their applications. Generally polymers behave in a significantly different manner when the temperature drops below

Tg (more brittle) or goes up to higher temperatures than Tg (more rubbery). Hence to select a material for a specific application it is essential to know the behaviour of composites under applied heat flow. Differential scanning calorimetry (DSC) analysis is wide used to determine the mentioned temperatures and crystallinity of the polymer and composite materials. Thermogravimetric analysis (TGA) is used to understand the degradation characteristics of the materials.

5.7.1 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry is the most widely used technique to study the changes occurred during heating a polymer. These changes are called “thermal transitions” of a polymer. Examples of the thermal transitions are glass transitions, crystallization, and melting of a polymer.

Generally, the Differential Scanning Calorimetry (DSC) system consists of two pans. It works on the basis of comparing the thermal properties of a sample against a standard reference material which has no transition in the temperature range of interest (alumina). The reference pan is held empty. Small holders contain either the sample or the reference material. The holders are placed in an adiabatic environment as shown in Figure 5.18. The temperatures of the holders are monitored using thermocouples, while the electrically supplied heat keeps the temperature of the two holders equal. The computer plots the difference in heat output of the two heaters against the average temperature. Using this graph one can find the thermal transition temperature such as glass transition, crystallization and melting point of the material. The enthalpies which correspond to the crystallization and melting peak can also be calculated.

Figure 5.18 Schematic of Differential Scanning Calorimetry.

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Differential scanning calorimetry (DSC) is a technique that measures the amount of energy required to establish a nearly zero temperature difference between the sample and a reference material [Walshaw et al., 1998]. DSC measurements provide quantitative and qualitative information about endothermic and exothermic reactions, as well as changes in heat capacity of a material, therefore it can be used to measure a number of characteristic properties of a sample. A schematic DSC curve demonstrating the appearance of several common features, e.g. glass transition, crystallization, and melting point of a material is given in Figure 5.19. Polymer absorbs heat when being melted and releases heat during degradation. Therefore, the melting point and degradation temperatures of polymers or composites can be observed from the DSC curve. After the glass transition, the molecules may gain enough energy to move and display a crystalline form. This point is called the crystallization temperature (Tc). The transition from amorphous solid to crystalline solid appears as an exothermic peak in the recorded DSC signal. Eventually, the sample gets to its melting temperature (Tm), as the temperature increases. Since melting of a material is an endothermic process, it shows itself as an endothermic peak in the DSC curve.

Figure 5.19 Schematic DSC curve demonstrating the appearance of several common features [Wang, 2004].

Differential Scanning Calorimetry (DSC) measurements were carried out using a calibrated TA Instrument DSC Q100 equipped with a cooling attachment, under a nitrogen atmosphere. Each sample were cut into tiny pieces (about 4 – 10 mg) and sealed in hermetic pans and lids (Figure 5.20). To build a reasonable reliability of the results, for each material three pieces of sample from different region were evaluated by DSC. The data were collected by repeated heating-cooling-heating cycles. The heating and cooling rate was 10 °C/min. The compression moulded sheets were used for DSC

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measurement. The samples were moulded with 180 °C moulding temperature and 5 minutes moulding time.

Figure 5.20 Differential scanning calorimetry – DSC Q100 used in measuring thermal characteristics of sample.

The samples were first scanned from –20 to 220 °C and then cooled to –20 °C and again heated up to 220 °C in a second scan. Glass transition (Tg) and melting temperatures

(Tm) were determined from the data of the first heating cycle (Figure 5.21). Because Tg of composite samples are not visible in the second heating cycles (Figure 5.21) although the Tm differs slightly from the first heating cycle. This is probably due to the relatively slow cooling rate used which led to crystallization of PLA. Tm was determined from the maximum region of endothermic melting peak, and Tg was an inflection temperature from baseline of the first heating cycle.

Cooling cycle

Second heating cycle

First heating cycle

Figure 5.21 A typical DSC curve of first heating, cooling and second heating cycle. 123

The degree of crystallinity (Xc) was calculated according to Equation 5.23 by using the DSC curve of the first heating cycle:

Xc = (5.23)

where,

is the heat of fusion of the neat PLA and its biocomposites;

is the heat of fusion for 100% crystalline PLA

( = 93.7 J/g) [Lee et al., 2009].

5.7.2 Thermogravimetric analysis (TGA)

Thermal degradation characteristics of the biocomposites were determined by thermogravimetric analysis (TGA) using a TA instrument, TGA Q500 on samples was having masses from 2 to 10 mg. The heating rate was 10 °C/min over the temperature range from 30 to 900 °C in a nitrogen environment. Figure 5.22 shows a TA instrument for TGA analysis of the materials. Thermogravimetric (TG) curves and derivative thermogravimetry (DTG) curves of the material are obtained from the TGA testing. The TG curves indicate the thermal stability of the materials, whereas the DTG curves shows the degradation temperature of the materials.

Figure 5.22 Thermogravimetric analyser – TGA Q500 used in measuring thermal characteristics of sample.

5.8 Water Absorption Testing

Water absorption is used to determine the amount of water absorbed by a biocomposite. The water absorption test followed BS EN ISO 62: 2008 (method-1) standard test 124

method. Samples were prepared for water absorption measurements by cutting into 60 mm × 60 mm × 1 mm strips. Before the measurement, the samples were dried in a vacuum oven at 50 ± 2 °C temperature for 24 hours, cooled to room temperature in a desiccator, and then immediately weighed to the nearest 0.1 mg which is then taken as the dry initial weight of the sample (m₁). Then the samples were immersed in a container of distilled water and maintained at a temperature of 23 ± 2 °C for a 45 day period as shown in Figure 5.23. At least 300 ml of water per test specimen should be used to prevent any extraction product from becoming excessively concentrated in the water during the test. Three samples of the same composition were placed together in the same container with required amount of water. In this case, care was taken to avoid the significant surface contact between test specimens or with the walls of the container.

Figure 5.23 Water absorption measurements.

During 45 days soaking time, the specimens were removed from the water at 5-day intervals, gently blotted with tissue paper to remove excess water from their surfaces, immediately weighed to the nearest 0.1 mg (m₂), and returned to the water. Each m₂ value was an average value obtained from three measurements. The percentage of weight increase due to water absorption (C) was calculated to the nearest 0.01% according to the Equation 5.24.

C = × 100 % (5.24)

where, C is the percentage by mass of water absorbed; m₁ is the mass of the test specimen after initial drying and before immersion (mg); m₂ is the mass of the test specimen after immersion (mg). 125

5.9 Biodegradability Testing

Biodegradability is one of the most important properties of the biocomposites, little was reported on this aspect of the composite. Soil burial test is the most widely used method to evaluate the biodegradability of the composite material [Cao et al., 2007; Wu, 2006; Wang et al., 2010; and Kumar et al., 2010]. Different types of soil and compost have been used for biodegradation test. Kim et al. [2006] showed that the biodegradability of the composite materials in compost soil is superior to that in a natural soil environment. The samples are buried at a depth of 8cm to 15cm from the surface in the soil [Cao et al., 2007; Wu, 2006; and Wang et al., 2010]. Biodegradability of the samples is assessed by measuring weight loss and mechanical properties of the specimen over time in soil environment. Soil burial test was followed to evaluate the biodegradability of the composite materials in this project.

5.9.1 Soil burial test

Biocomposite samples were examined for biodegradation using a soil burial test on a laboratory scale. In order to evaluate the mechanical properties after biodegradation, two sample dimensions were used as required by the relevant testing standards. For impact test, the samples were 80 mm × 10 mm × 4 mm and for flexural test, 40 mm × 15 mm × 2 mm. Burial test was carried out in a flower pot containing Miracle Gro. moisture control enriched compost [Kim et al., 2005 and Kim et al., 2006] and the specimens were buried in the compost soil with random pattern (Figure 5.24). This type of compost required 50% less watering. The flower pot containing the compost soil and samples were incubated at room temperature (25 ± 2 °C) for four months. The samples were buried at a depth of 12 – 15 cm from the surface in order to ensure the aerobic degradation [Wu, 2012].

Figure 5.24 Soil burial test for biodegradation. 126

The pots were covered with plastic film to avoid water evaporation from the compost surface. The pH of the compost soil was maintained at 7 and the moisture content was measured at regular intervals using a moisture meter and maintained at 40 – 50% by sprinkling water (Figure 5.24). Water was supplied at intervals of 2 days, and the soil was kept not to be dried. This humidity was optimal for microbial activity [Chandra and Rustgi, 1998]. Along with the various PLA/Flax biocomposites, the neat PLA and neat flax fibres were also buried as reference materials. PLA was buried as sheets of the same dimension as the biocomposites and flax was buried in fibre form.

Biodegradation was estimated by monitoring changes in weight and mechanical properties as a function of burial time. Tests were carried out after 10, 20, 30, 40, 60, 80, 100 and 120 days. The buried specimens were dug out of the soil after the required time. Obtained specimens were washed with water for removing the debris on the specimens and finally dried at 60 ± 2 °C temperature for 24 hours in an air-dried oven before undergoing weight loss and mechanical properties test. The flexural properties and notched Izod impact strength tests were selected to evaluate the residual mechanical properties after biodegradation. Tensile strength is an important property for the end products of the biocomposites. Large amount of samples were needed to evaluate the tensile property for biodegradation, because the specimen size of tensile property test is a number of times larger than the flexural and impact properties, as explained earlier in this chapter. Thus the flexural and impact properties were selected in this research work for the biodegradation due to the limitation of samples.

5.9.2 Mechanical Property and Weight Loss Tests after Biodegradation

Flexural tests were conducted according to BS EN ISO 14125: 1998+A1:2011 using an Instron 4411 in the three-point bending mode that is explained earlier in this chapter. The impact tests were conducted on notched samples according to BS EN ISO 180: 2000+A2:2013 using an Avery pendulum impact tester, which is explained in Section 5.6.3. Each value reported here is the average of five tests.

The percentage weight loss was measured using an electronic balance with a precision of 0.1 mg. The percentage weight loss was determined by using the Equation 5.25.

Weight loss (%) = × 100 (5.25)

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where,

Mo is the initial mass of the specimen (mg);

Md is the mass of the specimen after the designated burial day (mg).

The percentage weight loss was taken from the average of five samples.

The composite deterioration was also assessed by the mechanical properties. The residual mechanical properties were evaluated by using the Equation 5.26.

Residual mechanical property (%) = × 100 (5.26)

where, P is the selected property such as flexural strength, flexural modulus or impact strength measured after the designated burial day;

Po is the initial property before burial.

5.10 Surface Morphology Testing

Scanning electron microscope (SEM) was used to examine the surface of the matrix and biocomposite samples before and after soil burial test for evaluating the biodegradation. SEM was also used to examine the fracture surfaces of biocomposite samples after tensile, flexural and impact testing.

Figure 5.25 Edwards coating system, E306A, USA (a), Scanning Electron Microscope, Philips XL30 (b).

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The composite samples were inspected using a Philips model XL30 field emission gun (FEG) SEM microscope. Figure 5.25(b) shows the SEM equipment used in this work. For testing, small sections of the specimen were cut and stuck to the metal stubs using double-sided carbon tabs (Figure 5.26). The whole assembly was then coated with a very thin layer of carbon using an Edwards E306A sputter coater in order to improve the conductivity of the surface. Figure 5.25(a) shows the used sputter coater while operating and coating the samples with carbon.

Figure 5.26 Prepared samples for SEM.

The operation condition of the microscope was set at an accelerating voltage of 5.00 kV. For each specimen, images with 4 different magnifications of 50, 100, 200 and 500× were used. Smaller magnifications are more useful to understand the dispersion of the plant fibres into the polymer matrix, while increasing the magnification helps to observe the interface of fibre and matrix.

Figure 5.27 Projectina Micro Macro Projection Microscope (MMP-1000).

Along with the SEM, the prepreg samples were also examined by using the Projectina Micro Macro Projection Microscope (MMP-1000) with PIA 4000 software. The fibre orientation in the prepreg was examined using the projection microscope. Figure 5.27 129

shows the projection microscope used to examine the fibre distribution in the prepreg samples.

5.11 Summary

In this chapter, the experimental equipment and procedures used for testing physical and mechanical properties of the fibres, prepreg and composite materials were explained. The characterisation equipment and procedures for evaluating thermal and morphological properties of the biocomposite materials were described. The evaluation system of biodegradability of the composite materials was also explained. The standards followed by this research work to determine the different characteristics were indicated and equations used to calculate different features of the composite materials were showed.

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CHAPTER 6

PHYSICAL, MECHANICAL AND THERMAL PROPERTIES

6.1 Introduction

This chapter is concerned with physical, mechanical and thermal properties of PLA, flax fibre, prepreg, 2D flat and 3D biocomposite materials. The physical and mechanical properties of flax fibre were determined to select the matching PLA fibre. The melting point of PLA was determined to select the processing temperature. The physical, mechanical and thermal properties of the 2D flat biocomposite were determined to evaluate and compare with similar existing products. The physical and mechanical properties of 3D biocomposite were also evaluated to compare with 2D flat biocomposite.

6.2 Flax and PLA Fibre Evaluation

The properties of the raw materials of the biocomposite products are summarised in Table 6.1. Fibre length was determined by measurement of 500 individual fibres. The average length for flax fibre is 65 mm and for PLA fibre, it is 75 mm. Fibre diameter was measured using SEM and the mean values were calculated according to Table A-1, Appendix A. According to the standard method 20 individual fibres/filaments were tested to calculate the mean value of the fibre diameter and some examples are shown in the Figure 6.1.

Table 6.1 Properties of Raw materials

Features of the Raw materials Raw materials Flax fibre PLA fibre Fibre length (mm) 65 75 Fibre diameter (µm) 21.6 28.5 Linear density (dtex) 6.6 3.5 Tenacity (MPa) 754.4 104.2

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According to the standard method the mean linear density of the fibres in each bundle were calculated by using the Equation 5.1. The mean linear density of the fibres is 6.6 dtex for flax fibre and 3.5 dtex for PLA fibre. These were calculated by using the mean linear density of the fibre in each bundle (Table A-2, Appendix A).

Figure 6.1 SEM micrograph of the diameter of the flax fibres.

The tensile strength or tenacity of a single fibre or filament was calculated by using the Equation 5.2. The average tensile strength is 754.4 MPa for single flax fibre (Table A-3, Appendix A) and 104.2 MPa for single PLA fibre (Table A-4, Appendix A).

The melting point of PLA fibre was determined from the DSC curve of the first heating cycle of PLA fibre. The onset melting temperature is 163 °C and the peak melting temperature is 171 °C.

6.3 Evaluation of Prepreg

6.3.1 Area density and thickness of the prepreg

Table 6.2 shows the area density (mass per unit area) in g/m2 of different types of prepregs. The area density was calculated by using the Equation 5.3. It can be seen that the variations between the samples within the same types and between the types are very small. The ANOVA test (Table B-1, Appendix B) suggested that the difference in most of the results was non-significant. The overall mean area density of the prepreg was found to be 1065 g/m2. 132

Table 6.2 Area density of the prepregs

60 PLA/ 40 Flax 50 PLA/ 50 Flax 40 PLA/ 60 Flax No. of Mass per unit area Mass per unit area Mass per unit area Sample (g/m²) (g/m²) (g/m²)

1 1086.3 1050.4 1134.5 2 1058.0 1076.5 1103.9 3 1069.7 1146.2 1054.6 4 1072.8 1040.9 1057.7 5 1059.9 1033.6 1056.3 6 1065.3 1046.5 1051.0 7 1063.3 1031.7 1042.9 8 1019.6 1119.3 1034.2 9 1092.7 1049.3 1039.8 10 1102.0 1021.6 1068.3

Mean 1069.0 1061.6 1064.3 S.D. 22.7 40.7 31.4 C.V. 2.1 3.8 2.9

The average thickness of nonwoven mats was 7.04 mm with a CV% of 7.46 (Table A-5, Appendix A).

6.3.2 Tensile strength of prepreg

Figure 6.2 shows the effect of different axis of length-wise direction of the tensile test samples on the tensile strength of the prepreg. The tensile strength of the prepregs were found to be 323.6 N, 320.12 N, 313.52 N and 317.9 N for machine direction (MD), cross-machine direction (CD), 45° angle to MD and 135° angle to MD respectively. From the ANOVA test (Table B-2, Appendix B), it can be seen that there is no significant difference between the tensile strength obtained from the four different axes. This result indicates that the fibres are randomly oriented in the prepregs i.e. isotropic fibre distribution.

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390 370 350 330 310 290 270 Tensile Strength (N) Strength Tensile 250 Machine direction Cross-machine 45° angle to MD 135° angle to MD (MD) direction (CD)

Length-wise direction of the sample for Tensile test Figure 6.2 Effect of different axis on the tensile strength of prepreg.

Figure 6.3 shows the projection microscopic image of nonwoven web i.e. prepregs. It is clear that the fibres are randomly oriented. This is consistent with the quantitative analysis of the tensile strength of the prepregs.

Figure 6.3 Fibre orientations in the prepreg.

Due to the isotropic nature of the composites, the length-wise direction for tensile testing can be along any axis. However, the machine direction was selected as length- wise axis of during testing because of suitability of the sample size.

6.4 Evaluation of Physical Properties of the Composites

6.4.1 Density of the composite

The density of the biocomposites was determined by using the immersion method according to the British standard. The procedure was explained in detail in Section 5.5.1 of this thesis. The density was calculated using the Equation 5.5. Figure 6.4 shows the effect of process variables on the density of the composite materials. 134

Moulding Time-05min. 1.4 Moulding Time-10min. Moulding Time-15min. 1.2

1.0 0.8 0.6

0.4 Density (g/cm³) Density 0.2 0.0 180 190 200 180 190 200 180 190 200 60P/40F 50P/50F 40P/60F

Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.4 Effect of process variables on the density of the biocomposites.

It can be seen that the density of the biocomposites decreases with increasing flax fibre content. It may be because of an increasing trend of void formation with increasing flax fibre weight percentage. The fibre densities of PLA and flax are 1.25 g/cm3 and 1.40 g/cm3 respectively. The causes of void formation will be discussed in Section 6.4.2 of this chapter. There is no significant effect of moulding temperature and time on the density of biocomposites.

6.4.2 Constituent of the composite

The reinforcing fibre and resin content by weight and volume fraction and void content by volume were determined by digestion method according to British standard. The procedure was explained in detail in Section 5.5.2.1 of this thesis. The constituents were calculated using the Equations 5.6, 5.7, 5.8, 5.9 and 5.10. The detailed results are tabulated in Appendix A (Table A-6). The fibre and resin contents of the biocomposites are illustrated in the Figure 6.5. It can be seen that there is no significant change of fibre and resin content by weight fraction after composite manufacturing but there is significant change by volume fraction. Since the samples were taken randomly for determining the fibre weight fraction and the fibre/resin weight fractions are not significantly changed, it can be concluded that the flax and PLA fibres are uniformly distributed in the composite material. This result also indicates the voids are formed during the composite manufacturing.

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Fibre content by weight Fibre content by volume 80 Resin content by weight Resin content by volume 70

Constituents Constituents (%) 30

0 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 5 10 15 180°C 190°C 200°C 180°C 190°C 200°C 180°C 190°C 200°C 40% Flax + 60% PLA 50% Flax + 50% PLA 60% Flax + 40% PLA

Processing Parameters: Moulding time(min) at different Moulding temperature(°C) and Fibre content(%)

Figure 6.5 Effect of process variables on the constituents of the biocomposites.

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The void contents of the biocomposites are illustrated in the Figure 6.6. It is clear that the voids in the biocomposites increased as the nominal flax fibre fraction by weight increased. In general, the voids are closely related to the processing conditions because they can be formed by gases which may be generated in the thermal process. Since this study adopted thermoplastic PLA and there are several possible sources of gas. The voids may be formed due to the discontinuous resin matrix inside the composites, resulting from the uneven distribution of the PLA fibres or their failure to form a continuous phase in the biocomposites.

Moulding Time-05min. 45 Moulding Time-10min.

40 Moulding Time-15min.

35 30 25 20 15

Void Content (%) Content Void 10 5 0 180 190 200 180 190 200 180 190 200 60P/40F 50P/50F 40P/60F

Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.6 Effect of process variables on the void content of the biocomposites.

The elementary flax fibres consist of a primary cell wall, a secondary cell wall and a lumen. The lumen is an open channel in the centre of the fibre and it can be as small as 1.5% of the cross section of the fibres [Bos, 2004].

Figure 6.7 SEM micrograph of the fracture surface of the biocomposite. (a) shows the fibre fracture and fibre pull-out. (b) shows the lumen in the flax fibre. 137

The size of the lumen mainly depends on the maturity of the fibres. Generally the lumen size (cross-section of the lumen) is larger for immature fibre and smaller for matured fibres. The lumens can be observed in the fracture surface of the biocomposites and is shown in the Figure 6.7(b). These lumens also act as a void portion. Therefore, the flax fibres itself are carrying the voids naturally. This also explains why increased voids are formed with increasing flax fibre percentage. From Figure 6.6, it can also be seen that there is no significant trend of void content with changing processing (moulding) temperature and time. Figure 6.8(a-c) shows the SEM micrographs of the fracture surfaces of the biocomposites. It can be seen that the amount of voids increases with increasing flax fibre content. The amount of voids is consistent with the quantitative analysis of the void content (Figure 6.6).

Figure 6.8 SEM micrographs of fracture surface of 40% (a), 50% (b) and 60% (c) flax fibre reinforced composites. Figure 6.9(a-c) shows the SEM images of the surface of the PLA/Flax biocomposites with different flax fibre content. It can be seen that the number of pores gradually increases with increasing flax fibre content and a large number of pores are clearly visible in the biocomposite with 60% flax fibres (40P/60F) [Figure 6.9(c)].

Figure 6.9 SEM micrographs of the biocomposite.

A linear regression relationship between independent variables (fibre content, moulding temperature and moulding time) and dependent variable (void content, V%) was done 138

by Microsoft Excel (Table B-3 in Appendix B). The linear regression equation is shown below:

Vo % = 0.96 F + 0.005 Te – 0.28 Ti – 29.3 (6.1)

where,

Vo % is the void content (%); F is the fibre content (40 to 60% by weight);

Te is the moulding temperature (180 to 200 °C);

Ti is the moulding time (05 to 15 minute)

The R square value was found for this equation 0.73.

This shows that the fibre content has more significant influence on void content than moulding temperature, at the temperature range of 180 to 200 °C and moulding time, at the time range of 5 to 15 minutes.

6.5 Mechanical Properties of the Composites

6.5.1 Tensile Properties

The tensile properties of the biocomposites were determined using a universal Instron testing machine according to the British standard. The procedure was explained in detail in Section 5.6.1 of this thesis. The tensile properties were calculated using the Equation 5.11, 5.12, 5.13.

Moulding Temp. 180 °c Moulding Temp. 190 °c 90 Moulding Temp. 200 °c 80 70 60 50 40 30 20 Tensile Strength (MPa) Strength Tensile 10 0 12.5 25 37.5 50 62.5 12.5 25 37.5 50 62.5 12.5 25 37.5 50 62.5 60P / 40F 50P / 50F 40P / 60F Moulding Pressure (bar) at different Fibre Composition (%) Figure 6.10 Effect of moulding pressure on tensile strength. 139

Figure 6.10 shows the effect of moulding pressure on tensile strength at different fibre content. It can be seen that the tensile strength increased with increased moulding pressure up to 50 bar for all fibre compositions and the tensile strength decreased when the moulding pressure is higher than 50 bar. Thus the moulding pressure was fixed at 50 bar for reducing the number of sample and simplification of the work.

The typical tensile stress-strain curves of PLA and PLA/Flax biocomposites are shown in Figure 6.11. The neat PLA shows a more linear behaviour while the composites behave more nonlinearly as the strain increases. The linear phase corresponds to the linear deformation of the fibre and matrix while the nonlinear deformation of fibre composites has been explained as a three-phase mechanism by Panthapulakkal and Sain [2007]. Firstly, the microcrack initiates at the fibre-end/matrix interface and propagates along the fibre lengths; secondly, the matrix undergoes plastic deformation; and finally the microcracks in the matrix open and propagate through the deformed matrix. Due to the pulling-out of fibres from the matrix, catastrophic crack propagation also takes place through the matrix. The tensile properties of the biocomposite are higher than the neat PLA after the incorporation of flax fibre as anticipated. The tensile strength of biocomposites increases as the flax fibre content increases. It may also be observed that the failure strain of the biocomposites decreases with increasing flax fibre content. The tensile modulus also increases with increasing flax fibre content.

90 Neat PLA 60P/40F 80 50P/50F 70 40P/60F

60 50 40

30 Stress (MPa) Stress 20 10 0 0 0.5 1 1.5 2 2.5 3 Strain (%)

Figure 6.11 Tensile stress-strain curves of PLA and PLA/Flax biocomposites.

The tensile strength and modulus of the composites are shown in Figure 6.12 and Figure 6.13. The strength and modulus of the composites are higher than the neat PLA. The 140

tensile strength of the neat PLA and biocomposites generally decreases with increasing moulding time and temperature. As the temperature increases, the viscosity of the PLA decreases and it was observed that there was increasing PLA flowing over the edges of the composite during compression moulding. The loss of PLA increases the brittleness of the biocomposites, leading to lower strength. Similar findings were also reported by Shibata et al. [2006]. From the thermal analysis it was also found that degradation occurred with increasing moulding temperature.

Moulding Time-05min. Moulding Time-10min.

90 Moulding Time-15min. 80 70 60 50 40 30 20 10 Tensile Strength (MPa) Strength Tensile 0 180 190 200 180 190 200 180 190 200 180 190 200 Neat PLA 60P/40F 50P/50F 40P/60F Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.12 Effect of process variables on tensile strength.

From the Figure 6.13 it can be seen that the tensile modulus of the composite materials are not significantly changes with increasing moulding temperature and time. It can also be seen that there is no significant effect of moulding time and temperature on the tensile modulus of neat PLA.

Moulding Time-05min. Moulding Time-10min. 12 Moulding Time-15min. 10 8 6 4 2

Tensile Modulus (GPa) Modulus Tensile 0 180 190 200 180 190 200 180 190 200 180 190 200 Neat PLA 60P/40F 50P/50F 40 /60F Moulding Temperature (°C) at different Fibre Composition (%) Figure 6.13 Effect of process variables on tensile modulus. 141

The tensile strength and modulus of biocomposites increased as the flax fibre content increased from 40% to 50%. Further increase of flax content does not have significant effects. The maximum tensile strength and modulus, 80.3 MPa and 9.9 GPa respectively, are obtained with 50% flax composites at 180 °C moulding temperature with 5 minutes moulding time. These results are higher than what was reported by Oksman et al. [2003] who found that biocomposite tensile strength was highest, 53 MPa, at 30% flax content and decreased to 44 MPa at 40% flax content. Due to the extrusion process used in their study, the flax fibre length was likely to be shorter than what was used in the present study. Shorter flax fibres provide less interlocking [Cheung et al., 2008] and require greater matrix content to provide the bonding so the tensile strength peaks at a lower flax content and also decreases more quickly as the flax content increases. A shorter fibre length also leads to lower overall composite strength. Oksman et al. [2003] also found that the tensile modulus was higher at 30% flax content than 40% flax content, and the maximum modulus of 8.3 GPa is lower that what is obtained in the present study. Yuan et al. [2011] found that for a composite of 50% flax with 5% silane addition the tensile strength was 50.6 MPa and tensile modulus was 6.4 GPa. These values are much lower than what have been achieved in this study. The treatment of flax fibres by silane addition is probably one reason for the lower strength and modulus. It is possible to obtain higher tensile strength by using aligned fibres, as reported by Sawpan et al. [2011] and Plackett et al. [2003]. However, this high strength can only be obtained in the direction of the aligned fibres. The tensile strength of 80.3 MPa and tensile modulus of 9.9 GPa obtained in this project are better than any reported tensile properties for isotropic natural fibre/PLA biocomposites. The best processing conditions for the highest tensile properties are 50% flax, 180 °C moulding temperature and 5 minute moulding time.

6.5.2 Flexural Properties

The flexural properties of the biocomposites were determined using the three point bending method according to the British standard. Flexural properties were calculated using the Equations 5.15, 5.17, 5.18. The flexural test results are presented in Figure 6.14 and Figure 6.15. It can be seen that the flexural strength follows similar trends to the tensile strength and for similar reasons as discussed in the previous section. However, flexural strength is always higher than tensile strength. This effect has also been observed by Mieck et al. [2000] and Graupner and Mussig [2011]. 142

160 Moulding Time-05min. Moulding Time-10min. 140 Moulding Time-15min. 120 100 80 60 40

20 Flexural Strength (MPa) Strength Flexural 0 180 190 200 180 190 200 180 190 200 180 190 200 Neat PLA 60P/40F 50P/50F 40P/60F Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.14 Effect of process variables on flexural strength.

The maximum flexural strength of 138.5 MPa was obtained with 50% flax content at 180 °C moulding temperature and 5 minutes moulding time. The maximum flexural modulus of 7.93 GPa was also obtained with 50% flax content but at 190 °C moulding temperature and 5 minute moulding time. The flexural modulus with 50% flax content at 180 °C moulding temperature was only slightly lower at 7.87 GPa.

Moulding Time-05min. 10 Moulding Time-10min.

9 Moulding Time-15min.

8 7 6 5 4 3 2

Flexural Modulus (GPa) Modulus Flexural 1 0 180 190 200 180 190 200 180 190 200 180 190 200 Neat PLA 60P/40F 50P/50F 40P/60F Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.15 Effect of process variables on flexural modulus.

The flexural strength of 138.5 MPa and flexural modulus of 7.9 GPa found in this research are better than any previously reported flexural properties for isotropic natural fibre/PLA biocomposites [Yuan et al., 2011 and Sawpan et al., 2012]. The best

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processing conditions for the highest flexural properties are 50% flax, 180 °C moulding temperature and 5 minute moulding time.

Delamination (the separation of laminate layers) of composite materials under mechanical loading is a common failure mode. This is one of the advantages of the manufacturing method used in this project. Figure 6.16 shows the SEM images of the longitudinal section of samples after flexural test. Figure 6.16(a, b, c) shows the samples of different fibre composition with the same moulding temperature and time. Figure 6.16(d, e, f) shows the samples of different moulding temperature with the same moulding time and fibre content. Figure 6.16(b, d, g) shows the samples of different moulding time with the same moulding temperature and fibre content.

Figure 6.16 SEM micrographs of the longitudinal section of flexural tested samples.

It can be seen that no crack due to the delamination is visible in the longitudinal section of group of samples. The resin PLA and reinforcing material flax were both used as staple fibre form and they were randomly distributed in the web for the prepreg manufacturing. A number of prepregs were layered to the required thickness of the

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composite. During moulding, PLA was completely melted and evenly distributed among the flax fibres. Since the PLA was distributed in the web and between the webs i.e. prepregs, thus the molten PLA act as the binding material of flax fibres in the prepreg and between the prepregs.

6.5.3 Impact Properties

Impact strength of a composite is directly related to the toughness of the material. The fibres play an important role in the impact resistance of fibre reinforced biocomposites as they interact with the crack formation and act as stress-transferring medium. The impact strength was determined using the Izod method on notched samples according to British standard. The notched Izod impact strength was calculated using Equation 5.19. The notched Izod impact strength of the neat PLA and its biocomposites as the function of process variables is depicted in Figure 6.17.

Moulding Time - 05min. 35 Moulding Time -10min. Moulding Time -15min. 30 25 20 15 10 5 0 Notched IzodImpact Strength (KJ/m²) 180 190 200 180 190 200 180 190 200 180 190 200 100PLA 60P/40F 50P/50F 40P/60F Moulding Temperature (°C) at different Fibre Composition (%)

Figure 6.17 Effect of process variables on notched Izod impact strength.

As can be seen, impact strength increases with increased flax fibre content for all moulding temperatures and times. As the fibre content increases, more interfaces exist on the crack path (fracture surface), and more energy was consumed. In fact, the amount of flax fibres would have increased with increased fibre content, which could lead to increased pull-out (Figure 6.18-c) and also increased impact strength.

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Figure 6.18 SEM micrographs of fracture surface of 40% (a), 50% (b) and 60% (c) flax fibre reinforced composites.

From the thermal analysis it was found that the crystallinity of the biocomposites decreased with increased fibre content. This will be further discussed in Section 6.6.1 of this chapter. In principle, the lower the crystallinity is, the lower the brittleness becomes. At the fracture surface (Figure 6.18) it can be seen that flax fibres are broken in the case of more brittle sample (Figure 6.18-a) whereas fibres are pulled out from the surface in the cases of less brittle samples (Figure 6.18-c).

Figure 6.19 Photograph of the impact tested biocomposite samples with 60% flax fibre.

The image of the impact tested biocomposites is illustrated in Figure 6.19, which shows that the samples were not completely separated into two pieces but flax fibres bridged the gap. This mode of failure was associated with high energy absorption [Sawpan et al., 2011]. In addition, examination of the impact fracture surfaces showed fibre pull-out due to the fracture of flax fibre during impact loading (Figure 6.7(a)). Good impact strength is mainly due to the energy absorption when the fibres are pulled out of the matrix. It is assumed that the weaker bonding leads to better impact strength than very strong

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bonding which can cause a sudden failure [Ganster and Fink, 2006; and Graupner and Mussig, 2011]. This can also be explained by the voids in the biocomposites. Voids in the biocomposites increased with increased flax fibre content. Voids can cause weaker bonding.

It was found that the notched Izod impact strength of the 60P/40F biocomposite was 13.2 KJ/m2 at 180 °C moulding temperature and 5 minute moulding time. The maximum impact strength 28.3 KJ/m2 was obtained at 180 °C moulding temperature and 5 minute moulding time with 40P/60F biocomposite. The neat PLA (100PLA) shows very low impact strength (approximately 5 KJ/m2) and it is not significantly affected by the moulding temperature and time. So it can be seen that the addition of 40, 50 and 60% of flax fibre content increased the impact strength by about 170%, 216% and 477% respectively at moulding temperature 180 °C and moulding time 5 minute.

It was also observed that the impact strength decreased with increased moulding time and temperature. It might be that the longer period and higher temperature increased the crystallinity i.e. brittleness of the composites, resulting in reduced impact strength.

Based on the experimental data, the regression equation for notched Izod impact strength as a function of fibre content, moulding temperature and moulding time was developed (refer to Tables B-4 in Appendix B). The equation was determined using regression by Microsoft Excel. The equation is shown below:

= 71.52 + 0.39 F – 0.4 Te – 0.44 Ti (6.2)

Where,

is the notched Izod impact strength (KJ/m²); F is the fibre content (40 to 60% by weight);

Te is the moulding temperature (180 to 200 °C);

Ti is the moulding time (05 to 15 minute).

The above equation shows the relationship between notched Izod impact strength and the three variables (fibre content, moulding temperature and moulding time). It shows a satisfactory fit of the model of notched Izod impact strength (R² = 0.82) with experimental data. The above equation is only applicable to biocomposite containing 40 to 60 weight % flax fibre.

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6.6 Thermal Properties of Composites

6.6.1 Differential Scanning Calorimetry (DSC)

The DSC technique allows the determination of important material thermal behaviour parameters such as the glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (∆Hm) and crystalline level. Figure 6.20 and 6.21 shows the DSC curves of the first and second heating cycle respectively of neat PLA and PLA/Flax biocomposites. Tg, Tm and ∆Hm were calculated by analysing the DSC curve of the first heating cycle. It can be seen that Tg of the biocomposite samples are not visible in the

DSC curves of the second heating cycles. The possible causes of Tg being invisible in the DSC curve were explained earlier in the Section 5.7.1. The degree of crystallinity

(Xc) was calculated using the Equation 5.23. The results for all of the investigated biocomposites and neat PLA are summarized in Table 6.3.

Table 6.3 Thermal properties of Neat PLA and PLA/Flax biocomposites

° Sample Glass transition Melting temperature, Tm ( C) Melting Degree of PLA/Flax temperature, Onset melting Peak melting enthalpy, crystallinity, ° (%) Tg ( C) temperature temperature ∆Hm (J/g) Xc (%) Neat PLA 59 163 171 43 46 60P/40F 51 151 162 32 34 50P/50F 54 157 167 27 29 40P/60F 55 158 167 25 27

It can be seen that, the glass transition temperature and onset melting temperature of neat PLA are about 59 °C and 163 °C, respectively. These are higher than the values for the biocomposites. The introduction of flax fibres clearly reduces these temperatures. Interestingly, increasing the flax content appears to increase these temperatures. This increase is likely a result of decreasing space available for molecular motion with increasing flax fibre content [Wu, 2004].

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Figure 6.20 DSC curves of first heating cycle as a function of temperature of neat PLA and PLA/Flax biocomposites.

It can also be seen that the degree of crystallinity of neat PLA is about 46% and decreases with increasing flax content. This decrease in crystallinity can be explained by the fact that flax fibre has greater amorphousness.

Figure 6.21 DSC curves of second heating cycle as a function of temperature of neat PLA and PLA/Flax biocomposites. 149

6.6.2 Thermogravimetric analysis (TGA)

Thermogravimetric analysis was performed to understand the degradation characteristics of the PLA, flax and PLA/Flax biocomposites. From TGA testing, thermogravimetric (TG) curves and derivative thermogravimetry (DTG) curves were obtained and are shown in the Figures 6.22 and 6.23 respectively. The TG curves indicate the thermal stability of the materials, whereas the DTG curves show the degradation temperature of the materials. Similar to the decomposition behaviour of other plant fibres, such as kenaf, bamboo fibre and the recycled newspaper fibre [Kori et al., 2005 and Huda et al., 2005], flax fibre degrades in three stages. The first stage (30 – 150 °C) is due to the release of absorbed moisture in the fibres. The second transition (150 – 375 °C) is related to the degradation of cellulosic substances such as hemicelluloses and cellulose. In this stage, only small amount of flax fibre is degraded below 250 °C, and the degradation rate of flax fibre is more drastic after 250 °C. The third stage (375 – 600 °C) of the decomposition is due to the degradation of non- cellulosic materials in the fibre. From 600 °C onwards, the relationship between weight and temperature is almost linear.

Figure 6.22 Thermogravimetric curves as a function of temperature of neat PLA, neat flax and their biocomposites.

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From Figure 6.22 and Table 6.4 it can be seen that the thermal stability of neat PLA is better than that of flax fibre, and the onset temperature of thermal degradation of PLA is about 328 °C and flax is about 310 °C. The onset temperature of thermal degradation of the biocomposites is lower than neat PLA but higher than flax. It can be seen that the residual mass of ash content of the biocomposites increases with increasing flax fibre content. This indicates that, at higher temperatures, the biocomposites have better thermal stability than neat PLA and their thermal stability increases with increasing flax fibre content.

Table 6.4 TGA Characterization of Neat PLA, Flax fibre and PLA/Flax biocomposites

Sample Moulding Residue remaining Onset temperature of the PLA/Flax (%) temperature (°C) after 600 °C (%) thermal degradation (°C)

Neat PLA 0.08 328 60PLA/40Flax 180 0.64 320 60PLA/40Flax 190 0.48 317 60PLA/40Flax 200 0.22 314 50PLA/50Flax 180 4.92 318 50PLA/50Flax 190 3.12 315 50PLA/50Flax 200 2.15 313 40PLA/60Flax 180 7.69 316 40PLA/60Flax 190 5.71 314 40PLA/60Flax 200 4.23 311 Neat Flax 1.62 310

From Table 6.4, it can also be seen that for all fibre compositions the onset temperature of thermal degradation decreases with increasing moulding temperature. This is likely due to the thermal degradation of PLA matrix at higher moulding temperatures.

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Figure 6.23 Derivative thermogravimetry curves as a function of temperature of neat PLA, neat flax and their biocomposites.

From Figure 6.23 it can be seen that the area and peak of the derivative thermogravimetry curve of the neat PLA are higher and those for the neat flax are lower. It can also be seen that the peak and area both go down with increasing flax fibre content. This indicates that the weight loss decreases with increasing flax content.

6.7 Evaluation of the 3D Composite

6.7.1 Density of the 3D composite

Comparisons between the density of 2D and 3D biocomposites are illustrated in Figures 6.24 and 6.25. Figure 6.24 shows the effect of fibre composition on the density of the composites at different moulding temperatures. It can be seen that the density of both 2D and 3D composites decreases with increasing nominal flax fibre fraction by weight. However, the moulding temperature has no significant effect on the density of the biocomposites. The density reduction may be due to the increased void formation as the flax fibre content increases.

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2D Biocomposites 3D Biocomposites 1.4

1.2

1.0 0.8 0.6 0.4

Density (g/cm³) Density 0.2 0.0 60P/40F 50P/50F 40P/60F 60P/40F 50P/50F 40P/60F 60P/40F 50P/50F 40P/60F 180 190 200 Fibre Composition (%) at different Moulding Temperature (°C)

Figure 6.24 The density of 2D and 3D biocomposites at different fibre composition and moulding temperature.

The effect of moulding time on the density of the biocomposites at different fibre compositions and moulding temperature are shown in the Figure 6.25. It can be seen that there is no significant effect of moulding time on the density of both 2D and 3D biocomposites. It can also be seen that there is no significant differences between the densities of 2D and 3D biocomposites (Tables B-5 and B-6 in Appendix B).

2D Biocomposites 1.4 3D Biocomposites

1.2 1.0 0.8 0.6 0.4 Density (g/cm³) Density 0.2 0.0 05min 10min 15min 05min 10min 15min 60P/40F with 180 50P/50F with 190

Moulding time at different fibre composition (%) and moulding temperature (°C)

Figure 6.25 The density of 2D and 3D biocomposites at different moulding time with fixed fibre composition and moulding temperature.

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6.7.2 Constituent of the 3D composite

The effects of fibre composition on void content are shown in Figure 6.26. It can be seen that the voids of 2D and 3D biocomposites increased as the nominal flax fibre fraction by weight increased. The possible causes of void formation are described previously in Section 6.4.2 of this chapter. It can also be seen that, the processing temperature has no significant effect on void content.

45 2D Biocomposites

40 3D Biocomposites 35 30 25 20 15 10 Void Content (%) Content Void 5 0 60P/40F 50P/50F 40P/60F 60P/40F 50P/50F 40P/60F 60P/40F 50P/50F 40P/60F 180 190 200 Fibre Composition (%) at different Moulding Temperature (°C)

Figure 6.26. The void content of 2D and 3D biocomposites at different fibre composition and moulding temperature.

It can be seen that there is no significant differences between the void content of 2D and 3D biocomposites at different processing conditions (Tables B-7 and B-8 in Appendix B). Thus it can be concluded that the new 3D nonwoven fabric (prepreg) formation technique has no special effect on the void formation in the composite materials.

6.7.3 Mechanical properties of 3D composite

6.7.3.1 Crashworthiness

The progress and history of crushing test were investigated during the crushing operation. Figure 6.27 shows the load-displacement curves of three different composites. It can be seen that the mode of failure and the pattern of load-displacement curves of the various fibre content composites are similar. The load-displacement curves can be divided into two main regions, the first region starts from 0 kN until the initial crush failure load (Figure 5.17). This is the elastic region. It can be seen that the initial crush failure load increase with fibre content. After this point, the deformation enters the

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plastic zone where buckling occurs. Non-linearity was evident during the plastic deformation, where non-axisymmetric buckling was seen.

Figure 6.27 Load-displacement curves of 3D biocomposites with different fibre content.

Progressive buckling is characterized by deformation and structural damage of the material similar to the wrinkle formation of ductile fibre-reinforced materials. These wrinkles and buckles initiate and develop sequentially from top of the tested sample. The peak crush failure load is obtained within the plastic zone. The progress and history of crushing operation are shown in Figures 6.28(a – c). When the shape is close to being flattened, the load will increase rapidly again. This can be seen from the 40P/60F curve in Figure 6.27. This part of the load-displacement curve is not related to the load bearing performance of the shaped composite as the shape has failed completely by that stage. From Figures 6.27 and 6.28, it can be seen that although the composite with 60% flax has higher crush resistance in terms of load, it has lower displacement before being flattened. This is likely due to the higher stiffness provided by the higher flax content.

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Figure 6.28(a) Progress of crushing test of 60P/40F 3D biocomposite.

Figure 6.28(b) Progress of crushing test of 50P/50F 3D biocomposite.

Figure 6.28(c) Progress of crushing test of 40P/60F 3D biocomposite.

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Crashworthiness parameters such as crush failure load, total energy absorption, specific energy absorption and crush efficiency are illustrated in Table 6.5.

Table 6.5 Crashworthiness parameters of the 3D biocomposite samples

Crashworthiness parameters Mass of Initial Peak Mean Total Specific Crush 3D Sample a 3D Crush Crush Crush Energy Energy Force Sample Failure Failure Failure Absorption Absorption Efficiency Load Load Load (kg) (kN) (kN) (kN) (kJ) (kJ/kg) (%) 60P180C05M 0.047 0.91 6.04 2.75 0.16 3.32 45.49 50P180C05M 0.058 3.39 7.10 4.10 0.21 3.70 57.83 40P180C05M 0.058 4.80 9.69 5.91 0.27 4.66 61.00 60P190C05M 0.041 0.81 7.02 2.37 0.13 3.13 33.80 60P190C10M 0.043 0.91 5.97 2.32 0.13 2.95 38.95 60P190C15M 0.044 0.90 4.99 2.27 0.13 2.86 45.45 50P190C05M 0.054 1.14 4.09 2.27 0.12 2.31 55.54 50P200C05M 0.051 1.39 3.42 2.22 0.12 2.34 64.90

Crush failure load – Figure 6.27 shows the variation of the initial and peak crushing loads with the flax fibre content. It can be seen that the initial and peak crushing load increases with increasing flax fibre content. The 40P/60F composite material (60% flax fibre content) has the highest initial crushing load of 4.8 kN and the peak crushing load of 9.69 kN.

Figure 6.29 Load-displacement curves of 3D biocomposites with different moulding temperature. 157

Figure 6.29 shows the variation of the initial and peak crushing load with moulding temperature for composites with 50% fibre content and 5 minutes moulding time. It can be seen that the initial and peak crushing loads are higher at 180 °C moulding temperature. The crushing load decreases at higher moulding temperatures but the mean crush failure load is not affected significantly by increasing moulding temperature from 190 to 200 °C.

5 minute 8 10 minute 7 15 minute

5 4

Load (kN) Load 3 2 1 0 0 10 20 30 40 50 60 Displacement (mm)

Figure 6.30 Load-displacement curves of 3D biocomposites with different moulding time.

Figure 6.30 shows the variation of the crush failure load with moulding time. It can be seen that the peak crush failure load decreases with increasing moulding time although the initial and mean crush failure loads are not significantly affected by moulding time. It might be that the longer period of moulding increased the crystallinity i.e. brittleness of the composite materials [Shibata et al., 2006], resulting in reduced peak crush failure load.

Total energy absorption – The total energy absorption was calculated by the area under the load-displacement curve. The effect of process parameters on the total energy absorption is shown in Figure 6.31. It can be seen that the total energy absorption increases with increasing flax fibre content and the highest value of 0.27 kJ was obtained at 60% flax fibre content. From the thermal analysis it was found that the crystallinity of the biocomposites decreased with increased fibre content. In principle, the lower the crystallinity is the lower the brittleness becomes. It can also be seen that 158

the energy absorption is highest at 180 °C moulding temperature but are significantly lower at both 190 and 200 °C. It might be that the higher temperature of moulding increased the brittleness of the composites, resulting in reduced peak crush failure load. The total energy absorption of 3D biocomposites was not significantly affected by moulding time.

0.35

0.3 0.25 0.2 0.15 0.1 0.05 Total Energy Absorption (kJ) Absorption Energy Total 0 60P/40F 50P/50F 40P/60F 180 190 200 5 10 15 Fibre Content (%) Moulding Temperature (°C) Moulding Time (min) Processing Parameters

Figure 6.31 Effect of process parameters on the total energy absorption.

Specific energy absorption – The specific energy absorption (SEA) was calculated by using the Equation 5.20. Figure 6.32 shows that the effects of process parameters on the specific energy absorption of 3D biocomposites are similar to those on the total energy absorption. The specific energy absorption appears decreasing with increasing moulding time, but this is not significant. Therefore, 60% flax fibre content, 180 °C moulding temperature and 5 minute moulding time can be chosen as the best processing parameters for the PLA/Flax biocomposites.

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1 Specific Energy Absorption (kJ/kg) Absorption Energy Specific 0 60P/40F 50P/50F 40P/60F 180 190 200 5 10 15 Fibre Content (%) Moulding Temperature (°C) Moulding Time (min) Processing Parameters

Figure 6.32 Effect of process parameters on the specific energy absorption.

Crush force efficiency – The crush force efficiency (CFE) was calculated by using the Equation 5.22. Figure 6.33 shows the effect of process parameters on the crush force efficiency of 3D biocomposites. It can be seen that the crush efficiency increases with increasing flax fibre content, moulding temperature and moulding time.

70 60 50 40 30

20 Crush Force Efficiency (%) Efficiency Force Crush 10 0 60P/40F 50P/50F 40P/60F 180 190 200 5 10 15 Fibre Content (%) Moulding Temperature (°C) Moulding Time (min)

Processing Parameters

Figure 6.33 Effect of process parameters on the crush force efficiency. 160

6.8 Summary

This chapter presents the physical, mechanical and thermal properties of fibre/matrix, prepreg and biocomposites. Factors including flax fibre content, moulding temperature and moulding time were investigated. Flax fibre content is the most significant factor influencing the physical and mechanical properties of the biocomposites. The void content of the biocomposites also increases with increasing flax fibre content. The processing conditions for the highest tensile and flexural properties were 50% flax, 180 °C moulding temperature and 5 minute moulding time. The notched Izod impact strength increased with increased flax fibre content but it decreased with increasing moulding temperature and time.

The DSC results show that the melting temperature for all biocomposites is less than that of PLA but it increases with flax content. The TGA results show that the onset temperature of thermal degradation for all biocomposites is between the onset temperatures of PLA and flax fibre. Adding PLA matrix to the flax fibre raises the onset temperature of the biocomposites above that of flax fibre. PLA/Flax biocomposites are less thermally stable than the neat PLA matrix but more thermally stable than the flax fibre. However, the thermal stability of the biocomposites increases with increasing flax fibre content at higher temperatures. It is also found that the biocomposites manufactured by the method used in this project are free from delamination during strength tests.

The physical properties and crashworthiness of 3D biocomposites were tested and evaluated. The processing parameters for the highest crashworthiness properties were 60% flax fibre, 180 °C moulding temperature and 5 minute moulding time.

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CHAPTER 7

WATER ABSORPTION AND BIODEGRADABILITY OF COMPOSITES

7.1 Introduction

This chapter is focused on the biodegradation and stability (water absorption) of composites made from flax fibres and PLA matrix. Water absorption is generally considered to be a disadvantage in composites although it is a positive sign for the biodegradation. Biodegradation was examined by measuring the weight loss and comparing to SEM image analysis after the soil burial test. The residual mechanical properties are also evaluated after biodegradation.

7.2 Water Absorption of the Composite

The water absorption rates of neat PLA and its biocomposites are shown in Figure 7.1. In this figure the percentages of water absorbed, calculated using Equation 5.24, and are plotted against the soaking time. With the increase of flax fibre content, the water absorption of the biocomposites increases; and their values are very high in comparison to neat PLA.

Neat PLA 70 60P/40F (%) 50P/50F (%) 60

40P/60F (%) 50

20 Water Absorption (%) Absorption Water 10

0 0 5 10 15 20 25 30 35 40 45 50 Soaking Time (days)

Figure 7.1 Effect of fibre composition on water absorption of the biocomposites.

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Natural fibres are highly hydrophilic. Therefore, incorporation of natural fibres into polymeric matrices will generally increase the water sorption ability of the product [Sreekala et al., 2002]. The results are as expected given the hydrophobic nature of PLA and the hydrophilic nature of the flax fibre. The water absorption of the composite samples is very quick during the initial 5 days. Then the absorption rate decreases and become steady after 20 days. This is because water molecules penetrate easily through the pores of the composite samples during initial few days, but once the composite becomes saturated, the water absorption slows down.

a) 60P/40F b) 50P/50F c) 40P/60F Figure 7.2 SEM micrographs of the biocomposite surfaces with different fibre content.

Figure 7.2 shows the SEM images of the PLA/Flax biocomposite surfaces with different flax fibre content. It can be seen that the number of pores gradually increases with increasing flax fibre content in the composites and the large number of pores are clearly visible in the biocomposite with 60% flax fibres (40P/60F) [Figure 7.2(c)]. Since higher

Moulding Time-05min. Moulding Time-10min. Moulding Time-15min. 80 70 60 50 40 30 20 10 0 Water Absorption after 45 days (%) days 45 after Absorption Water 180 190 200 180 190 200 180 190 200 60P / 40F 50P / 50F 40P / 60F Moulding Temperature (°C) at different Fibre Composition (%)

Figure 7.3 Effect of process variables on water absorption of the biocomposites after 45 days. 163

fibre content is desired in biocomposites to achieve good mechanical properties, decreasing water absorption by controlling consolidation i.e. moulding temperature and time is important.

Figure 7.3 shows the effect of process variables on the water absorption of the biocomposites after 45 days soaking time. It can be seen that the water absorption of biocomposites increases with increasing moulding temperature and time as well as flax fibre content. For example, when the fibre content is 40% (60P/40F) and the moulding temperature and time is low (180 °C and 5 minute), the water absorption of biocomposite is about 21%; but at the same fibre content, when moulding temperature and time increase (200 °C and 15 minute), the water absorption of biocomposite increases to about 48%. Similarly, at 50 % flax content (50P/50F) the water absorption increases from 28% to 42% and 60% flax content (40P/60F) the water absorption increases from 58% to 63% (190 °C and 15 minute). This may be because at higher temperature and longer moulding period, fibre degradation occurs, thus more pores are formed between the fibre and matrix interface, which give paths for water to enter the biocomposites (Figure 7.4).

a) b)

Figure 7.4 SEM micrographs of the biocomposite surfaces with different moulding temperature and time: a) 180 °C and 5 minutes, b) 200 °C and 15 minutes.

A linear relationship was found (Table B-9 in Appendix B) between water absorption rate and the composite process parameters, namely the flax fibre content, moulding temperature and moulding time. The equation is shown as the following:

W% = 1.59 F + 0.32 Te + 0.75 Ti – 106.32 (7.1)

where, W% is the water absorption (%); F is the fibre content (40 to 60%); 164

Te is the moulding temperature (180 to 200 °C); Ti is the moulding time (5 to 15 minute).

The R square value was found for this equation 0.83.

The results indicate that it is very important to control the moulding temperature and time to decrease the water absorption of biocomposites when the fibre content is higher. The water absorption of the PLA/Flax biocomposites can be minimized if the moulding temperature and time are kept 180 °C and 5 minute respectively. These processing conditions are also recommended as the best for the tensile, flexural and impact properties, reported in Sections 6.5.1, 6.5.2 and 6.5.3.

7.3 Biodegradability of the Composite

Compost soil burial test was used to determine the biodegradability of the composites. Composition changes and mechanical behaviour of the composite after being exposed to natural microflora in the compost soil are evaluated. The composites were incubated in compost under controlled conditions. The percentage weight loss and the reduction in mechanical properties of the materials were determined at different time intervals.

7.3.1 Weight loss after biodegradation

Figure 7.5 shows the percentage of weight loss, calculated using Equation 5.25, as a function of burial time for PLA, flax fibres and the biocomposites. It can be seen that the percentage of weight loss increases with increasing burial period for all the samples. It is found that the weight loss of PLA and flax fibres after 120 days is 3.08% and 91.41% respectively. Clearly, flax has a much higher biodegradability than PLA.

It can also be found that the biodegradability of the composites increases with increasing flax content, which agrees with the results reported by Cao et al. [2007], Wu [2006] and Wang et al. [2010]. The weight loss of the 40P/60F composite (19.77 %) is about three times as much as the 60P/40F composite (6.03 %) after 120 days burial. The biodegradation of flax is the main reason for the biodegradation of PLA/Flax composites. This was also found by Teramoto et al. [2004]. Figure 7.5 also shows that flax fibres and its biocomposites degraded rapidly over the first 40 days, losing a mass equivalent to their approximate flax fibre content, and showed a gradual decrease in weight over the next 80 days. This is similar to the findings by Wu [2012].

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100PLA (%) 100Flax (%) 60P/40F (%) 50P/50F (%) 40P/60F (%) 100 90

80 70 60 50 40

Loss in Weight (%) Weight in Loss 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Burial Time (days)

Figure 7.5 Weight loss percentages of PLA, flax fibre and their biocomposites.

During soil degradation tests, flax is attacked by microorganisms and macro-organisms. Water penetrates from the edges of the composites in flax-based samples and degradation of flax fibre occurs. Biodegradation are primarily induced by the action of various microorganisms present in the soil. The activity of microorganisms is closely connected to the presence of water in soil. Macro-organisms also cause degradation of composites.

7.3.2 Mechanical properties after biodegradation

7.3.2.1 Residual flexural properties

The residual mechanical properties were evaluated using Equation 5.26. The results of flexural tests for the neat PLA (100PLA) and PLA/Flax biocomposites at different burial times are shown in Tables 7.1 and 7.2, and Figures 7.6 and 7.7.

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Table 7.1 Flexural strength of the 100PLA and its biocomposites before and after biodegradation

Types of Flexural strength (MPa) Materials Before After 10 days After 40 days After 120 days (%) biodegradation biodegradation biodegradation biodegradation

100PLA 70.52 39.74 24.05 19.52 60P/40F 116.73 67.42 35.31 30.15 50P/50F 138.46 65.31 29.24 25.80 40P/60F 102.53 26.31 16.14 14.12

Table 7.1 and Figure 7.6 show that the residual flexural strength for all the composites and neat PLA decreases rapidly after 10 days of burial and the decrease stabilizes after 40 days. It is also evident that the reduction of flexural strength is higher for composites with higher flax content. This is clearly caused by the faster degradation of composites with greater flax content. The result is also is in line with the weight loss data discussed earlier.

100PLA (%)

120 60P/40F (%) 50P/50F (%) 100 40P/60F (%)

Residual Flexural Strength (%) Strength Flexural Residual 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Burial Time (days)

Figure 7.6 Effect of the burial time on the residual flexural strength.

Table 7.2 and Figure 7.7 show the residual flexural modulus. The trend of flexural modulus is very similar to that observed for the flexural strength. Additionally, it is consistent with the results of water absorption.

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Table 7.2 Flexural modulus of the 100PLA and its biocomposites before and after biodegradation

Types of Flexural modulus (GPa) Materials Before After 10 days After 40 days After 120 days (%) biodegradation biodegradation biodegradation biodegradation

100PLA 3.56 3.31 2.81 2.46 60P/40F 7.15 3.27 2.17 2.24 50P/50F 7.87 3.23 1.76 1.17 40P/60F 7.66 1.60 1.02 0.84

The reduction in flexural properties of neat PLA, may be attributed to the preferential hydrolysis, whereas in the case of biocomposites this effect is combined with the fibre/matrix interface degradation. Fibre debonding and matrix degradation contribute to a lower adhesion at the interface and, consequently, to poorer mechanical properties.

100PLA (%) 120 60P/40F (%) 50P/50F (%) 100 40P/60F (%)

Residual Flexural Modulus (%) Modulus Flexural Residual 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Burial Time (days)

Figure 7.7 Effect of the burial time on the residual flexural modulus.

7.3.2.2 Residual impact properties

The results of notched Izod impact strength tests for the neat PLA (100PLA) and PLA/Flax biocomposites at different burial times are shown in Table 7.3 and Figure 7.8. It can be seen that the notched Izod impact strength of neat PLA and the biocomposites decreases significantly within the first 40 days, and thereafter remained stable.

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Table 7.3 Notched Izod impact strength of the 100PLA and its biocomposites before and after biodegradation

Types of Notched Izod impact strength (KJ/m2) Materials Before After 10 days After 40 days After 120 days (%) biodegradation biodegradation biodegradation biodegradation

100PLA 4.91 4.43 2.95 2.52 60P/40F 24.32 22.33 15.59 14.61 50P/50F 20.81 19.21 14.49 12.36 40P/60F 43.31 39.77 27.13 10.80

Hydrolysis is attributed to the breaking up of the polymer into smaller units. As the degradation proceeds, the microorganisms become very active and the biocomposites become very brittle [Kim et al., 2005].

100PLA (%) 120 60P/40F (%) 50P/50F (%) 100 40P/60F (%)

Residual Residual Notched IzodImpact Strength (%) 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Burial Time (days)

Figure 7.8 Effect of the burial time on the residual notched Izod impact strength.

Flax content shows no significant effect on residual Izod impact strength. However, the residual notched Izod impact strength of the 40P/60F composite (24.92 %) is significantly lower than the other composites after 120 days burial.

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7.3.3 Surface morphology before and after biodegradation

Figure 7.9 shows SEM images of neat PLA and PLA/Flax biocomposites before and after the biodegradation. The neat PLA and PLA/Flax composite samples exhibited a relatively smooth and clear surface before biodegradation. After 20 days, gradual erosion and cracking can be observed on the surface of the PLA and biocomposite samples [Figure 7.9(b, g, l, q)], indicating that the surfaces were attacked by the microorganism [Kim et al., 2005]. After 40 days, the degradation of the samples became more obvious [Figure 7.9(c, h, m, r)]. This degradation is confirmed by the increasing weight loss of the samples (Figure 7.5), which reached nearly 3.1% for neat PLA after 120 days. The SEM images in Figure 7.9 indicate that the PLA/Flax biocomposites are more readily degraded than neat PLA in the soil burial test. After 20 days burial time, the PLA/Flax biocomposites are coated with a film of bacteria [Figure 7.9(g, l, q)] indicating more bacteria growth than that observed on PLA at the same burial time [Wu, 2012]. Moreover, at 40, 80, and 120 days, gradually larger pores and more cracks are apparent in the biocomposites [Figure 7.9(h-j, m-o, r-t)], indicating a higher level of degradation. The rates of weight loss of the PLA/Flax biocomposites are also higher than that of PLA, exceeding nearly 20% for the 40P/60F biocomposite after 120 days (Figure 7.5). These results demonstrate that the addition of flax fibres to the PLA fibre enhanced the biodegradability of the composite.

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a) 100 PLA (0 days) f) 60P/40F (0 days) k) 50P/50F (0 days) p) 40P/60F (0 days)

b) 100 PLA (20 days) g) 60P/40F (20 days) l) 50P/50F (20 days) q) 40P/60F (20 days)

c) 100 PLA (40 days) h) 60P/40F (40 days) m) 50P/50F (40 days) r) 40P/60F (40 days)

d) 100 PLA (80 days) i) 60P/40F (80 days) n) 50P/50F (80 days) s) 40P/60F (80 days)

e) 100 PLA (120 days) j) 60P/40F (120 days) o) 50P/50F (120 days) t) 40P/60F (120 days) Figure 7.9 SEM images show the morphology of neat PLA (a-e), 60 PLA/40 flax (f-j), 50 PLA/50 flax (k-o), and 40 PLA/60 flax (p-t) biocomposites at different burial time.

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7.4 Relationship between water absorption and biodegradability of the composites

Figure 7.10 shows a comparison between water absorption and biodegradation of the neat PLA and PLA/Flax biocomposites. The solid lines represent the water absorption rate against soaking time and the dotted lines represent the weight loss due to the biodegradation against burial time.

70 25 Water Absorption

60 20 50

40 15 Loss in Weight 30 10

20 (%) Weight in Loss Water Absorption (%) Absorption Water 5 10

0 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Time (days)

Figure 7.10 Comparison between water absorption and biodegradation.

It can be seen that with the increase of flax fibre content, the water absorption of the biocomposites increased, and their values are very high in comparison to neat PLA. The causes of this effect are explained earlier in Section 7.2 of this chapter. A similar trend can be seen for weight loss due to the biodegradation of the materials. The water absorption of the composite samples is very quick during the initial 5 days, and then the absorption rate decreases and become steady after 20 days. In case of biodegradation, the samples degrade rapidly during the initial 40 days, and show a gradual decrease in weight loss after 40 days. It can be concluded that both water absorption and biodegradation increase with increasing flax content. The composite materials degrade more rapidly at the beginning of the burial than at the later stages. The cause may be that flax fibre has a much higher biodegradability than PLA during the initial burial days and after few weeks flax may be completely degraded. It is explained in Section 7.3.1. It can also be seen that the water absorption increases up to 20 days where as biodegradation increases up to 40 days. Microorganisms are mainly responsible for 172

biodegradation and they grow in presence of water. Microorganisms grow gradually and this may be the cause of the relatively longer degradation time.

7.5 Summary

This chapter presents the biodegradability, surface morphology and water absorption behaviour of flax fibre reinforced PLA biocomposites. Flax fibre content is the most significant factor influencing the biodegradation and water absorption. The flexural property, notched Izod impact strength and percentage weight loss of neat PLA and PLA/Flax biocomposites are significantly decreased after soil burial. These results were confirmed by SEM observations which showed the presence of many large holes and more cracks in the degraded surface of the biocomposites. Significant amounts of cavities are found on the surface of the composite. The addition of flax fibres into the composites causes acceleration of biodegradation due to preferential degradation of flax fibres.

The water absorption of the biocomposites also increases with increasing flax fibre content, moulding temperature and moulding time. The water absorption rate can indicate the biodegradation of biocomposites; higher rate of water absorption enhances the biodegradability of the composite material.

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CHAPTER 8

CONCLUSIONS AND FUTURE WORK

8.1 Conclusions

The aim of this project was to develop flax fibre reinforced PLA biocomposites based on air-laid nonwovens and to investigate their properties and processing to establish the appropriate processing parameters. To explore the possibility of producing 3D biocomposite material directly from 3D shell structured nonwoven prepregs was another objective of this work. PLA/Flax blended nonwovens with three blend ratios were produced. Thermal bonding method was used for nonwoven web consolidation and compression moulding method was used to produce 2D flat biocomposite by using 2D flat metal platens. 3D dome shaped mould for nonwoven web and biocomposites were developed. 3D biodegradable composite materials were fabricated by using 3D nonwoven prepregs. The 3D nonwoven webs were made using a recently developed process in the University of Manchester. In this process the 3D nonwoven webs were produced by using air-laying principle and webs were consolidated by through-air thermal bonding principle.

The main findings of this project are:

a) Development of a manufacturing procedure for fabricating 2D flat biocomposites: A new technique incorporating an air-laying nonwoven web forming process has been found for making flat nonwoven biocomposites based on flax and PLA fibres. Composite prepregs were made from the nonwoven webs by using thermal bonding, avoiding fibre damage by needle punching which is commonly used for nonwoven biodegradable composites. The prepregs were converted to composites by compression moulding.

b) Mechanical properties of 2D flat biocomposites: The tensile, flexural and impact properties were evaluated to establish the optimum processing parameters.

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 Tensile properties It was observed that the tensile properties were significantly affected by the process variables. Flax fibre content is the most significant factor influencing the tensile properties of the biocomposites. The best processing conditions for the highest tensile properties were 50% flax, 180 °C moulding temperature and 5 minute moulding time. The maximum tensile strength was 80.3 MPa and the maximum tensile modulus was 9.9 GPa. The results obtained in this research are better than any reported tensile properties for isotropic natural fibre/PLA biocomposites.

 Flexural properties The best processing conditions for the highest flexural properties were similar to those for tensile properties. The maximum flexural strength was 138.5 MPa and the maximum flexural modulus was 7.9 GPa. These results are also better than any previously reported flexural properties for isotropic natural fibre/PLA biocomposites. The biocomposite manufactured in this project is free from delamination during the flexural test. Between the layers of prepreg, flax and PLA fibres were randomly distributed that is one of the main features of the manufacturing method used in this research. During moulding the molten resin PLA was evenly distributed in the prepreg and between the layers of prepreg like one phase of prepreg. This molten PLA was used as binding material between the layers.

 Impact properties The impact strength increases with increasing flax content and the maximum notched Izod impact strength was 28.3 KJ/m2. The notched Izod impact strength increased with increased flax fibre content but it decreased with increased moulding temperature and moulding time. The best processing conditions for the highest PLA/Flax biocomposite performance in according to notched Izod impact strength was 60% flax fibre, 180 °C moulding temperature and 5 minute moulding time. c) Thermal properties of 2D flat biocomposites: The DSC results show that the melting temperature for all biocomposites is less than that of PLA but it increases with flax content. The crystallinity of neat PLA 175

was higher than the composites and the highest degree of crystallinity of neat PLA obtained was 46%. The degree of crystallinity of the composites decreased with increasing flax content. The TGA results show that the onset temperature of thermal degradation for all biocomposites is between the onset temperatures of PLA and flax fibre. Adding PLA matrix to the flax fibre raises the onset temperature of the biocomposites above that of flax fibre. PLA/Flax biocomposites are less stable than the neat PLA matrix but more thermally stable than the flax fibre. However, the thermal stability of the biocomposites increases with increasing flax fibre content at higher temperatures. d) Water absorption of 2D flat biocomposites: The water absorption of the biocomposites increases with increasing flax fibre content, moulding temperature and moulding time. With higher flax content, the water absorption of the biocomposites is more significantly influenced by moulding temperature and time. The composite with 60% flax content exhibits the maximum water absorption. The highest composite performance such as tensile, flexural and impact properties are achieved with a moulding temperature of 180 °C and moulding time of 5 minute. The water absorption rate can indicate the biodegradation of PLA/Flax biocomposites; higher rate of water absorption leads to greater biodegradability. It has been established that a linear relationship exists between water absorption rate and the other composite process parameters within the conditions tested in this project. e) Biodegradability of 2D flat composites Flax fibre content is the most significant factor influencing the biodegradation of composite material. The flexural property, notched Izod impact strength and percentage weight loss of neat PLA and PLA/Flax biocomposites decrease significantly with degradation. These results were confirmed by SEM observations which showed the presence of many large holes and cracks in the degraded surface of the biocomposites. Significant amounts of cavities are found on the surface of the composite. The addition of flax fibres into the composites causes acceleration of biodegradation due to preferential degradation of flax fibres. Also, the biodegradability of the composites increases with flax fibre content. It has been proven that the biodegradability of PLA/Flax composites is

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dependent on water absorption rate and it can be concluded that when the materials absorbed more water they degraded more easily. f) Development of a manufacturing procedure for fabricating 3D shell structured biocomposites: 3D shell structured PLA/Flax biocomposites were fabricated by using a novel method incorporating the 3D nonwoven web forming process. PLA and flax fibres were blended in the fibre opening stage and converted to webs on the 3D mould by using the air-laying principle. The 3D webs were then consolidated by through-air thermal bonding. The compression moulding technique was used to convert the 3D webs to the biocomposites. Fabricating 2D composites into components with a complex shape increases the cost because some fabrics and many prepreg tapes have poor drape. These materials are not easily moulded into complex shapes, and as a result some composite components need to be assembled from a large number of separate parts that must be joined by co- curing, adhesive bonding or mechanical fastening. Even fabricating composites into components with a simple dome shape by using 2D prepreg sheet develops creases on the surface of the final product. The new process of 3D nonwoven web forming system has been used successfully for manufacturing 3D nonwoven prepregs. This process makes 3D shaped composites that are free from joints. It also overcomes the problem of crease or wrinkle formation in forming 3D shaped prepregs by moulding. This technique can be employed to produce 3D composite materials, which may be used in a wide variety of automotive and structural application. It can also be used to produce novel biodegradable helmets. g) Evaluation of physical properties of 3D biocomposites and compare to 2D flat biocomposites: The density and void content of 3D biocomposites are significantly influenced by flax fibre content, same as for 2D flat biocomposites. The density decreases and void content increases with increasing flax fibre content for both 2D and 3D biocomposites. However, the moulding temperature and time have no significant effect on the density and void content of the biocomposites. There is no significant difference between the density and void content of 2D and 3D

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biocomposites at different processing conditions. It can be concluded that the 3D composites made using the new 3D nonwoven method can achieve the same physical properties as 2D composites of similar composition.

h) Crush test of 3D biocomposites: Flax fibre content is the most significant factor influencing the crushing properties of 3D biocomposites. Increasing the flax fibre content increases the crush failure load, total energy absorption, specific energy absorption and crush efficiency. The peak crush failure load is much higher than the initial crush failure load. The peak crush failure load is obtained during the plastic deformation zone. The highest crushing performance of 3D biocomposites was obtained with 60% flax fibre, 180 °C moulding temperature and 5 minute moulding time. These are similar to the optimum processing conditions for PLA/Flax 2D flat biocomposites.

8.2 Recommendations for Further Research Work

Future research can focus on enhancing the functionality and commercial viability of the work describe in this thesis.

The reinforcing fibre i.e. flax with only one fibre length and three weight percentage was used in this research. The use of different fibre length with more weight percentage would provide more precise optimal parameters. Moreover, the possibility of using other bast fibres like jute, hemp, etc. and the effect of processing parameters on the composite performance could be part of future work.

The void content of the composites was determined by using the digestion method in this research. This technique is non-visual, gives only total void fraction and cannot analyse individual void size, shape and distribution. The use of C-scan or X-ray Computed Tomography in future work would provide visual images of the void and fibre-matrix interface. This technique can be used for three dimensional analysis of internal structures. The inherent void content (lumen) of the flax fibre could also be determined in future work by analysing the sectional diagram using SEM.

The fibres could be chemically treated in future work to improve mechanical properties. The fibre-matrix interface could also be analysed by single fibre pull-out testing.

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The flexural and impact properties have been analysed for the evaluation of mechanical properties after biodegradation in this research. The effect of biodegradation on other important mechanical properties, especially tensile properties could also be analysed in future. It was not done in this research because of shortage of samples that is explained in Section 5.9.1. The fibre and matrix physiochemical changes could also be examined by thermo analytical testing after biodegradation.

Valuable data about the chemical structure of the biocomposites and thus degradation process can be obtained using FTIR analysis (for example in case of PLA analysis of C=O stretching vibration, etc.). Moreover, fire resistance of PLA/Flax biocomposites could also be analysed. This is because the biocomposites are extensively employed in a wide variety of automotive and structural applications, where biocomposites are usually exposed to fire following accidents.

In the present research, only dome shaped 3D biocomposites have been developed using the 3D nonwoven technique. Development of different shaped 3D structure could be carried out to optimise parameters so as to achieve practical commercial production. This technique can also be used to produce helmet shell structure. Only crashworthiness property was analysed in this research due to time limitations. The investigation of impact performance of 3D shell structure would indicate the usefulness as helmet shells in future applications.

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APPENDICES

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Appendix A

Table A-1 Diameter of the fibres

Number of Diameter of the fibre (µm) sample Flax fibre PLA fibre 1 22.4 27.3 2 20.2 31.9 3 23.4 30.0 4 15.3 30.3 5 19.4 24.8 6 24.5 30.9 7 21.8 28.6 8 23.2 26.2 9 20.2 27.3 10 21.8 25.0 11 21.8 29.6 12 22.8 25.2 13 20.8 31.9 14 23.2 31.4 15 21.5 27.0 16 25.2 26.1 17 23.9 27.2 18 19.7 31.9 19 19.2 29.2 20 21.3 27.3

Mean 21.6 28.5 S.D. 2.2 2.4 C.V. 10.4 8.5

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Table A-2 Linear density of the fibres

Number Number of Length of the Mass of the fibre Mean Linear density of fibres in individual bundle (mg) of the fibre in each bundle the bundle fibre in the bundle (dtex) bundle (mm) Flax PLA Flax PLA fibre fibre fibre fibre 1 50 50 1.7 0.9 6.8 3.4 2 50 50 1.6 0.9 6.4 3.7 3 50 50 1.6 0.9 6.3 3.7 4 50 50 1.7 0.9 7.0 3.5 5 50 50 1.7 0.9 6.9 3.6 6 50 50 1.6 0.9 6.3 3.5 7 50 50 1.6 0.8 6.5 3.2 8 50 50 1.6 0.9 6.4 3.6 9 50 50 1.8 0.9 7.2 3.5 10 50 50 1.7 0.8 6.6 3.4

Mean 6.6 3.5 S.D. 0.3 0.2 C.V. 4.7 4.6

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Table A-3 Single fibre strength of flax fibres

Number Maximum Fibre Tensile of load diameter strength sample (mN) (µm) (MPa) 1 246.9 22.4 624.3 2 223.9 20.2 698.0 3 275.0 23.4 638.9 4 163.0 15.3 881.9 5 205.3 19.4 693.8 6 439.9 24.5 933.1 7 230.9 21.8 618.6 8 323.9 23.2 766.2 9 191.2 20.2 596.6 10 207.0 21.8 554.6 11 353.8 21.8 947.9 12 261.3 22.8 639.4 13 243.0 20.8 715.1 14 376.6 23.2 890.9 15 270.2 21.5 741.5 16 459.8 25.2 921.9 17 247.5 23.9 551.7 18 278.6 19.7 913.1 19 212.0 19.2 729.9 20 367.8 21.3 1031.2

Mean 754.4 S.D. 147.1 C.V. 19.5

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Table A-4 Single fibre strength of PLA fibres

Number Maximum Fibre Tensile of load diameter strength sample (mN) (µm) (MPa) 1 50.6 27.3 86.4 2 69.1 31.9 86.5 3 64.6 30.0 91.4 4 78.8 30.3 109.0 5 63.3 24.8 131.1 6 72.8 30.9 97.0 7 70.4 28.6 109.5 8 65.6 26.2 121.8 9 66.0 27.3 112.8 10 62.2 25.0 126.8 11 54.4 29.6 79.1 12 58.9 25.2 118.1 13 63.1 31.9 79.0 14 71.3 31.4 92.1 15 63.8 27.0 111.5 16 69.1 26.1 129.2 17 73.4 27.2 126.3 18 68.9 31.9 86.2 19 65.8 29.2 98.2 20 53.7 27.3 91.7

Mean 104.2 S.D. 17.5 C.V. 16.8

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Table A-5 Thickness of the prepregs

No. of Thickness (mm) at 250 gm samples weight pressure 1 6.70 2 6.50 3 6.80 4 6.20 5 7.20 6 7.60 7 7.10 8 7.90 9 7.50 10 6.90

Mean 7.04 S.D. 0.53 C.V. 7.46

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Table A-6 Constituent of the composites

Fibre Fibre Resin Resin Void content Composite content by content by content by content by by volume Type mass (%) volume (%) mass (%) volume (%) (%) 60P200C15M 40.1 32.5 59.9 54.3 13.2 60P200C10M 40.4 32.6 59.6 53.8 13.6 60P200C05M 40.8 34.1 59.2 55.4 10.5 60P190C15M 37.1 33.4 62.9 63.3 3.3 60P190C10M 35.9 30.6 64.1 61.1 8.3 60P190C05M 34.9 29.2 65.1 61.1 9.7 60P180C15M 39.3 35.0 60.7 60.6 4.4 60P180C10M 38.2 30.5 61.8 55.2 14.3 60P180C05M 39.7 35.8 60.3 61.0 3.2 50P200C15M 46.1 35.4 53.9 46.3 18.3 50P200C10M 48.2 38.5 51.8 46.4 15.1 50P200C05M 47.4 40.6 52.6 50.4 9.0 50P190C15M 45.8 37.0 54.2 48.9 14.1 50P190C10M 46.7 37.8 53.3 48.3 13.9 50P190C05M 46.6 37.8 53.4 48.4 13.8 50P180C15M 46.2 40.7 53.8 53.0 6.3 50P180C10M 45.5 34.9 54.5 46.9 18.2 50P180C05M 44.3 35.7 55.7 50.2 14.1 40P200C15M 57.0 41.1 43.0 34.6 24.3 40P200C10M 58.4 39.9 41.6 31.8 28.3 40P200C05M 57.3 40.3 42.7 33.7 26.0 40P190C15M 56.5 42.9 43.5 37.0 20.1 40P190C10M 57.9 40.7 42.1 33.1 26.2 40P190C05M 58.2 37.7 41.8 30.2 32.1 40P180C15M 57.5 39.1 42.5 32.3 28.6 40P180C10M 56.6 38.4 43.4 32.8 28.8 40P180C05M 59.7 34.6 40.3 26.1 39.3

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Appendix B

Table B-1 ANOVA for area density of the prepregs

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 10 10689.64 1068.96 516.87 Column 2 10 10615.97 1061.60 1658.83 Column 3 10 10643.14 1064.31 983.91

ANOVA Degrees of Mean Source of Sum of freedom square F- value P-value F crit Variation squares (SS) (df) (MS) Between Groups 277.59 2 138.79 0.13 0.88 3.35 Within Groups 28436.54 27 1053.21

Total 28714.13 29

Table B-2 ANOVA for tensile strength of prepregs

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 5 1618.0 323.60 794.97 Column 2 5 1600.6 320.12 632.47 Column 3 5 1567.6 313.52 1274.31 Column 4 5 1589.5 317.90 1037.96

ANOVA Degrees of Mean Source of Sum of freedom square F- value P-value F crit Variation squares (SS) (df) (MS) Between Groups 267.35 3 89.12 0.09 0.96 3.24 Within Groups 14958.86 16 934.93

Total 15226.21 19 207

Table B-3 ANOVA table and Coefficient estimates of equation 6.1 Obtained by Excel regression model

Summary output

Regression Statistics Multiple R 0.86 R Square 0.73 Adjusted R Square 0.72 Standard Error 4.91 Observations 81

ANOVA Source of Degrees of Sum of Mean F - Significance Variation freedom (df) squares (SS) square (MS) value F Regression 3 5113.05 1704.35 70.58 0.00 Residual 77 1859.31 24.15 Total 80 6972.35

Standard Lower Upper Source of Variation Coefficients Error t Stat P-value 95% 95% Intercept -29.30 13.22 -2.22 0.03 -55.62 -2.98 Fibre Content (F) 0.96 0.07 14.40 0.00 0.83 1.10 Moulding Temperature (Te) 0.005 0.07 0.07 0.94 -0.13 0.14 Moulding Time (Ti) -0.28 0.13 -2.09 0.04 -0.55 -0.01

Dependent variable: Void content

Predictors: Intercept, F, Te, Ti.

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Table B-4 ANOVA table and Coefficient estimates of equation 6.2 Obtained by Excel regression model

Summary output

Regression Statistics Multiple R 0.91 R Square 0.82 Adjusted R Square 0.82 Standard Error 2.33 Observations 162

ANOVA Source of Degrees of Sum of Mean Significance Variation freedom (df) squares (SS) square (MS) F- value F Regression 3 3864.56 1288.19 237.80 0.0000 Residual 158 855.90 5.42 Total 161 4720.46

Standard Lower Upper Source of Variation Coefficients Error t Stat P-value 95% 95% Intercept 71.52 4.43 16.16 0.00 62.78 80.27 Fibre Content (F) 0.39 0.02 17.51 0.00 0.35 0.44 Moulding Temperature (Te) -0.40 0.02 -17.67 0.00 -0.44 -0.35 Moulding Time (Ti) -0.44 0.04 -9.73 0.00 -0.52 -0.35

Dependent variable: Izod impact strength

Predictors: Intercept, F, Te, Ti.

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Table B-5 ANOVA table for comparison between density of 2D and 3D biocomposites

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 9 9.77 1.08 0.023 Column 2 9 9.88 1.09 0.018

ANOVA Source of Sum of Degrees of Mean F - P - Variation squares (SS) freedom (df) square (MS) value value F crit Between Groups 0.001 1 0.001 0.038 0.848 4.494 Within Groups 0.323 16 0.020

Total 0.324 17

Table B-6 ANOVA table for comparison between density of 2D and 3D biocomposites

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 6 7.023 1.171 0.004 Column 2 6 7.015 1.169 0.005

ANOVA Source of Sum of Degrees of Mean F - P - Variation squares (SS) freedom (df) square (MS) value value F crit Between Groups 0.000 1 0.000 0.001 0.974 4.965 Within Groups 0.044 10 0.004

Total 0.044 11

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Table B-7 ANOVA table for comparison between void content of 2D and 3D biocomposites

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 9 157.88 17.54 146.77 Column 2 9 149.24 16.58 114.71

ANOVA Source of Sum of Degrees of Mean square F - P - Variation squares (SS) freedom (df) (MS) value value F crit Between Groups 4.15 1 4.15 0.03 0.86 4.49 Within Groups 2091.84 16 130.74

Total 2095.99 17

Table B-8 ANOVA table for comparison between void content of 2D and 3D biocomposites

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance Column 1 6 63.81 10.63 27.82 Column 2 6 64.78 10.79 28.59

ANOVA Source of Sum of Degrees of Mean square F - P - Variation squares (SS) freedom (df) (MS) value value F crit Between Groups 0.08 1 0.08 0.003 0.96 4.96 Within Groups 282.06 10 28.21

Total 282.14 11

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Table B-9 ANOVA table and Coefficient estimates of equation 7.1 Obtained by Excel regression model

Summary output

Regression Statistics Multiple R 0.91 R Square 0.83 Adjusted R Square 0.82 Standard Error 6.36 Observations 81

ANOVA Source of Degrees of Sum of Mean F - Significance Variation freedom (df) squares (SS) square (MS) value F Regression 3 14884.63 4961.54 122.72 0.00 Residual 77 3113.18 40.43 Total 80 17997.81

Standard Lower Upper Source of Variation Coefficients Error t Stat P-value 95% 95% Intercept -106.32 17.10 -6.22 0.0000 -140.37 -72.26 Fibre Content (F) 1.59 0.09 18.32 0.0000 1.41 1.76 Moulding Temperature (Te) 0.32 0.09 3.73 0.0004 0.15 0.50 Moulding Time (Ti) 0.75 0.17 4.32 0.0000 0.40 1.09

Dependent variable: Water absorption (%)

Predictors: Intercept, F, Te, Ti.

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