polymer composite materials based on bamboo fibres · 2017-11-08 · polymer composite materials...

POLYMER COMPOSITE MATERIALS BASED ON BAMBOO FIBRES

Eduardo TRUJILLO DE LOS RÍOS

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

September 2014

Supervisors: Prof. Jan Ivens Dr. Aart W. Van Vuure Members of the Examination Committee: Prof. Adhemar Bultheel, Chairman Prof. Jan Ivens, Promoter Dr. Aart W. Van Vuure, Promoter Prof. Ignace Verpoest Prof. Peter Van Puyvelde Prof. Joris Van Acker Prof. Adriaan Beukers Prof. Ton Peijs

Prof.

© 2014 KU Leuven, Science, Engineering & Technology Uitgegeven in eigen beheer, Eduardo Trujillo De Los Ríos, Belgium Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotokopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaandelijke schriftelijke toestemming van de uitgever. All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other means without written permission from the publisher. ISBN 978-94-6018-883-1 D/2014/7515/107

XV

Table of contents

Acknowledgements……………………………………………………………………….. I

Abstract…………………………………………………………………………………… III

Samenvatting……………………………………………………………………………… V

List of abbreviations…………………………………………………………………….... IX

List of symbols…………………………………………………………………………..... XI

Table of contents………………………………………………………………………...... XV

Chapter 1: General introduction………………………………………………………... 1

Chapter 2: Problem statement and objectives……………...…………………………... 5

Chapter 3: Natural fibres and their composites: A literature review……………….... 11

3.1 Natural fibres…………………………………………………………………….... 11

3.1.1 Fibre microstructure………………………………………………………….. 12

3.1.2 Chemical composition of natural fibres…………………………………….... 13

3.2 Mechanical properties of plant fibres……………………………………………... 14

3.3 Natural fibre composites and their advantages……………………………………. 16

3.3.1 Environmental impact……………………………………………………….. 18

3.3.1.1 Bio-composites………………………………………………………... 19

3.3.2 Coupling agents…………………………………………………………….... 20

3.3.2.1 Alkali treatment……………………………………………………….. 21

3.3.2.2 Maleic anhydride polypropylene…………………………………….... 23

3.4 Limitations in the use of natural fibre composites……………………………….... 24

3.4.1 Moisture absorption………………………………………………………….. 24

3.4.2 Thermal degradation…………………………………………………………. 25

3.4.2.1 Thermal degradation in natural fibres……………………………….... 25

3.4.2.2 Thermal degradation in natural fibre composites……………………... 30

3.5 Bamboo plants…………………………………………………………………….. 32

3.5.1 Morphology of bamboo Guadua angustifolia culm…………………………. 33

3.5.1.1 Diameter, wall thickness and internode length of the culm…………... 34

3.5.2 Anatomy of the bamboo culm……………………………………………….. 35

3.5.3 Bamboo fibres………………………………………………………………... 36

3.5.3.1 Microstructure and chemical composition of bamboo fibres……...….. 37

3.5.3.2 Mechanical properties of bamboo technical fibres………………….... 38

3.6 Extraction of bamboo technical fibre………………..……………………………. 40

3.6.1 Steam explosion…………………………………………………………….... 42

XVI

3.6.2 Mechanical extraction………………………………………………………... 44

3.6.3 Chemical extraction………………………………………………………….. 45

3.7 Bamboo fibre composites…………………………………………………………. 46

3.8 Applications of natural fibre composites………………………………………….. 48

3.9 Conclusions………..…………………………………………………………….... 50

References…………………………………………………………………………….. 51

Chapter 4: Bamboo technical fibre characterization…………………………………. 59

4.1 Introduction……………………………………………………………………….. 59

4.1.1 The modified Weibull distribution…………………………………………... 60

4.1.2 Effect of defect density distribution…………………………………………. 64

4.1.3 Effect of within-fibre diameter variation…………………………………….. 66

4.2 Dry fibre bundle test………………………………………………………………. 66

4.2.1 Theoretical background…………………………………………………….... 67

4.3 Materials…………………………………………………………………………... 69

4.4 Methods………………………………………………………………………….... 70

4.4.1 Single fibre test………………………………………………………………. 70

4.4.1.1 Measurement of the cross sectional area…………………………….... 70

4.4.1.2 Measurement of the fibre perimeter…………………………………... 70

4.4.1.3 Tensile test set up……………………………………………………... 71

4.4.1.4 Statistical calculations...………………………………………………. 73

4.4.1.5 Scanning electron microscopy (SEM) observations.……...………….. 72

4.4.2 Dry fibre bundle test (DFT)………………………………………………….. 73

4.4.2.1 Reference methodologies……………………………………….…….. 73

4.4.2.2 Preparation of DFB samples………………………………………….. 74

4.4.2.3 Testing of the bamboo DFB samples…………………………………. 76

4.5 Results and discussion………………………………………………………….…. 78

4.5.1 Fibre extraction………………………………………………………………. 78

4.5.2 Fibre cross-sectional area and perimeter…………………………………….. 80

4.5.3 Mechanical properties of bamboo technical fibres…………………………... 82

4.5.3.1 Dependency of fibre strength on fibre length………………………..... 82

4.5.3.2 Dependency of fibre strength on fibre diameter, fibre volume

and fibre surface……………………………………………..………... 84

4.5.3.3 Estimation of the modified Weibull parameters ……………………... 86

4.5.3.4 Correlation with within-fibre diameter variations …………….…….... 88

4.5.3.5 Benchmarking of single fibre mechanical properties...……………….. 89

4.5.4 Dry bamboo fibre bundle…………………………………………………….. 91

4.6 Conclusions……………………………………………………………………….. 95

References…………………………………………………………………………….. 97

XVII

Chapter 5: Unidirectional continuous and discontinuous bamboo fibre

epoxy composites...………………………………………………………….. 101

5.1 Introduction……………………………………………………………………….. 101

5.2 Overview of different models for the prediction of mechanical properties………. 103

5.2.1 Prediction of longitudinal tensile stiffness…………………………………... 103

5.2.2 Prediction of longitudinal tensile strength………………………………….... 107

5.3 Materials…………………………………………………………………………... 109

5.3.1 Bamboo fibres………………………………………………………………... 109

5.3.2 Epoxy resin…………………………………………………………………... 110

5.4 Methods………………………………………………………………………….... 110

5.4.1 Fibre patterns……………………………………………………………….... 110

5.4.2 Elaboration of the prepregs…………………………………………………... 113

5.4.3 Composite production………………………………………………………... 115

5.4.3.1 Manufacturing of the samples for tensile testing……………………... 115

5.4.3.2 Sample production for 3-point bending test…………………………... 116

5.4.4 Sample testing……………………………………………………………….. 117

5.4.4.1 Tensile test……………………………………………………………. 117

5.4.4.2 Flexural test………………………………………………………….... 119

5.4.5 Scanning electron microscopy (SEM) observations…………………………. 119

5.5 Results and discussion…………………………………………………………….. 119

5.5.1 Continuous fibre composites (UD-C)……...……………………………….... 121

5.5.2 UD-D with fixed fibre bundle overlapping ength……………………………. 123

5.5.3 UD-D with random fibre bundle overlapping length…...………...…………. 124

5.5.4 UD-D with individual technical fibres with random fibre ends…..…………. 125

5.5.5 Tensile fracture characteristics for UD-D samples…………………...…….... 125

5.5.6 Experimental stiffness compared with predicting models………………….... 127

5.5.7 Experimental strength compared with predicting models………………….... 129

5.5.8 Properties evaluated in 3-point bending test…………………………………. 134

5.6 Conclusions……………………………………………………………………….. 137

References…………………………………………………………………………….. 138

Chapter 6: Thermal degradation in bamboo fibres and bamboo fibre

polypropylene composites………....………………………………………... 141

6.1 Introduction……………………………………………………………………….. 141

6.2 Materials…………………………………………………………………………... 143

6.2.1 Bamboo fibres………………………………………………………………... 143

6.2.2 Polypropylene (PP) and maleic anhydride polypropylene (MAPP)…………. 143

6.3 Methods………………………………………………………………………….... 143

6.3.1 Thermal treatment of single technical bamboo fibres……………………….. 143

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6.3.2 Thermogravimetric analysis (TGA)....…………...………………………….. 144

6.3.3 Single fibre test………………………………………………………………. 145

6.3.4 Scanning electron microscopy (SEM) observations…………………………. 146

6.3.5 Bamboo fibre PP/MAPP composites……………………………………….... 146

6.3.5.1 UD bamboo prepregs………………………………………………..... 146

6.3.5.2 Composite production……………………………………………...…. 147

6.3.5.3 Composite production under inert atmosphere……………………….. 148

6.3.6 Sample preparation and testing………………………………………………. 149

6.4 Results and discussion…………………………………………………………….. 150

6.4.1 Thermogravimetric analysis (TGA)…………………………………………. 150

6.4.1.1 Isothermal TGA experiments…………………………………………. 152

6.4.2 Tensile properties for single fibre after thermal treatment…..………………. 154

6.4.2.1 Fibre strength…………………………………………………………. 154

6.4.2.2 Fibre stiffness and strain to failure……………………………………. 158

6.4.2.3 Influence of an inert atmosphere in mechanical properties…………… 160

6.4.2.4 Thermally treated bamboo fibres under SEM……………….………... 162

6.4.3 Bamboo fibre polypropylene composites (BFPP)………………………….... 164

6.4.3.1 Fibre degradation during composite processing…………………….... 164

6.4.3.2 Matrix degradation……………………………………………………. 166

6.4.3.3 TGA analysis for bamboo fibre polypropylene composites…………... 166

6.4.3.4 Flexural strength of bamboo fibre polypropylene composites………... 168

6.4.3.5 Bamboo fibre MAPP composites……………………………………... 174

6.5 Conclusions……………………………………………………………………….. 176

References…………………………………………………………………………….. 178

Chapter 7: Technology assessment and application potential of bamboo fibre

composites…………………………………………………………………....

181

7.1 Introduction……………………………………………………………………….. 181

7.2 Availability of the resources………………………………………………………. 182

7.3 Environmental benefits……………………………………………………………. 184

7.4 The extraction process and the effect on the ecological impact and fibre cost…… 185

7.5 Manufacturing and performance of the final composites; potential applications…. 188

7.6 Durability and recyclability……………………………………………………….. 191

7.7 Conclusions……………………………………………………………………….. 192

References…………………………………………………………………………….. 192

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Chapter 8: General conclusions……….……………………...…………………………. 195

8.1 Bamboo technical fibre……………………………………………………………. 196

8.2 Unidirectional continuous and discontinuous bamboo fibre composites………..... 197

8.3 Thermal degradation in single technical fibres and bamboo fibre

polypropylene composites……………………………………………………….... 197

8.4 Potential use of bamboo fibre composites………………………………………… 198

8.5 Suggestions for future research...………………………………………………..... 199

Appendices………………………………………………………………………………... 201

Appendix 1……………………………………………………………………………. 202

Appendix 2……………………………………………………………………………. 207

Curriculum Vitae

List of publications

General introduction 1

Chapter 1 General introduction

_______________________________________________________

Most likely the reader has already noticed that the climate is changing as some

phenomenon like extreme weather, droughts, cyclones, melting of the Artic sea ice,

etc., can be experienced in daily life and others are reported in different regions of

the globe. The world's leading climate scientists agree that there is at the moment an

increase of the global temperature by 0.85 ºC since the late 19th century1 and the last

three decades have been warmer than any preceding decade. One of the main causes

of this “global warming” is the emissions of greenhouse gases, like carbon dioxide

(CO2), from different human activities. One of the main sources of CO2 in the

atmosphere is the combustion of fossil fuels - coal, oil and gas, responsible for some

63% of the total man-made global warming1.

Lately, climate experts have pointed out that if little or no action is taken to reduce

global greenhouse gas emissions by the end of the 21st century, global warming is

likely to reach a 2-5°C increase above the average temperature2 measured in pre-

industrial times. An increase beyond this range would lead to higher risk of

catastrophic changes in the global environment. To stay within this margin, the

greenhouse gas emission increase must be stopped by 2020 at the latest, to then

reduce the emissions by at least half of 1990 levels by 20503. Climate change is not

anymore a warning but a reality. Unfortunately, it is becoming more tangible, so

much so that at least 20% of the entire European Union budget for 2014-2020 will

be spent on climate-related projects and policies.

1 European Commission Climate Action website: ec.europa.eu/clima/policies/brief/causes/index_en.htm

2 European Environment Agency climate change website: eea.europa.eu/themes/climate/intro

3 UNFCCC/Kyoto Protocol website: unfccc.int

2 Chapter 1 b

This action marks a major step forward in transmuting Europe into a greener and

competitive low carbon economy. Being one of the front runners, the EU also

vouched to help developing countries to adapt to the impacts of climate change.

Nevertheless, these kinds of policies have not been adopted by all industrialized

countries.

As an example, transportation by road contributes in average about one-fifth of the

EU's total emissions of carbon dioxide (CO2) which rose by nearly 23% between

1990 and 20104. To reduce these emissions, European Union legislation requires the

greenhouse gas intensity of vehicle fuels to be reduced up to 10% by 2020, with a

reduction in CO2/km emissions of 30% in comparison with the current average5.

Another regulation that involves the transportation sector is the End of Life Vehicles

(ELV) directive6

. It aims at making vehicle dismantling and recycling more

environmentally friendly, setting clear quantified targets for reuse and recycling of

vehicles and their components. In order to achieve these various targets, innovative

research is nowadays carried out in order to replace traditional materials by lighter

component parts with recycling potential, like fibre reinforced polymer composites

using synthetic and natural fibres.

Glass and carbon are the most common man-made fibres to reinforce polymer

composites with a great acceptance due to their good mechanical properties.

Nevertheless, they are non-renewable materials and their production requires high

energy consumption. Natural fibres such as bamboo, flax, hemp, jute and coconut

(coir) are renewable resources emerging as good alternatives for reinforcing

materials because of their bio-based character, high specific mechanical properties

that can compete with glass fibres, reasonable cost and sustainable supply. Several

studies involving life cycle assessment (LCA), agree that natural fibres are likely to

be environmentally superior to synthetic fibres in each phase of the entire life cycle

of the composite materials. Moreover at the end of life in the case of thermoplastic

matrices, they allow mechanical downcycling (recycling) of the composite (e.g. for

injection moulding).

Nowadays, several projects using natural fibre composites are being carried out with

the aim to make them commercially available in new high level applications for

different sectors. For example, flax thermoset composites are being investigated to

be used in a car trunk load floor by PSA Peugeot Citroën. Boeing is working on

sandwich panels with skins made of flax and thermoset resin matrix, while other

4 European Commission Climate Action website: ec.europa.eu/clima/policies/transport/vehicles_en.htm

5 European Environment Agency climate change website: eea.europa.eu/themes/climate/policy-context

6 European Commission Climate Action website: ec.europa.eu/environment/waste/elv_index.htm

General introduction 3

partners in the same project are investigating thermoplastic matrices such as PP and

PLA. Also, the European project “BioBuild” aims to use biocomposite materials,

including flax and hemp fibres, to bring about a step-change in the reduction of

embodied energy in building-façade and internal-partition systems, whilst being

commercially competitive. Those are a few examples of the initiatives that are now

implemented in the growing sector of green composites.

The present research “polymer composite materials based on bamboo fibres”

presents the development of an environmentally friendly material as an alternative

among other natural and synthetic fibre reinforced composites. It also seeks to

preserve and to stimulate the growth of a worldwide natural resource, bamboo, with

the advantage that it absorbs and stores large amounts of CO2. One hectare of

bamboo can sequester 62 tons of CO2/year, four times more in comparison to a

young forest and generates up to 35% more oxygen than an equivalent stand of

trees7. It has been found that carbon sequestration by growing forests is a cost-

effective solution to mitigating climate change.

In recent years, there has been an increasing interest to scientifically study the

potential of bamboo fibre as reinforcing material for polymer matrix composites.

Nevertheless, few efforts have been carried out to extract long bamboo fibres from

the culm, particularly because of difficulties in its extraction. In this state of mind,

this large bottleneck for the availability of long high quality technical bamboo fibres

has encouraged the development of a new extraction technique presented in this

thesis. This new process is expected to have low environmental impact because of a

low overall energy consumption and avoiding the use of chemicals. Bamboo

technical fibres are extracted from the Guadua angustifolia species, one of the three

largest bamboos in the world and the most important tropical bamboo in America.

The fibres are studied to be used as reinforcement in composite materials in

combination with thermoset and thermoplastic matrices.

In this thesis, the whole spectrum of the development of bamboo fibre reinforced

composites will be presented. The first step deals with the characterization of the

extracted technical fibre. The second one consists of the preparation techniques and

the manufacturing of high quality composites and their testing. The results are

benchmarked with other studies of natural fibres and natural fibre composites in

order to demonstrate the potential of this new material amongst well established

composite materials. As of today, particular challenges must be overcome in order to

implement the bamboo fibre composites in real high-tech applications. Some of

7 Environmental bamboo foundation: www.bamboocentral.org

4 Chapter 1 b

them, such as variability in fibre mechanical properties, the thermal degradation of

fibres during and after composite manufacturing and the optimum manufacturing

parameters for the best performance are investigated in the next chapters of the

thesis.

Problem statement and objectives 5

Chapter 2 Problem statement and objectives

_______________________________________________________

The main goal of this research is to develop a bamboo fibre composite with high

mechanical properties using technical fibres extracted from the Colombian bamboo

Guadua angustifolia. For that, two main challenges need to be tackled. The first one

consists in the extraction of long bamboo technical fibres that can meet the

requirements established in the field of reinforcements for composite materials,

while conserving the good intrinsic properties of the technical fibres at a competitive

cost. The second aspect consists in optimizing the transfer of the good mechanical

properties of the fibres into the composites performance, with both thermoplastic

and thermoset matrices. Therefore, the main objectives stated for this study are:

To develop operating prototype machines incorporating environmentally

friendly principles for extraction, cleaning and alignment of technical fibres

that allow scale-up in industrial production;

To perform a complete characterization of the bamboo technical fibre after

extraction and cleaning;

To explore the processing feasibility of bamboo fibre composites by different

techniques and to study the resulting composite properties;

To carry out a systematic study of the influence of processing parameters on

the mechanical properties of bamboo fibres and bamboo fibre composites.

In order to achieve these objectives, several activities need to be carried out through

the different chapters of this study as is schematically outlined in Figure 2-1.

6 Chapter 2 b

Figure 2-1. Schematic overview of the activities carried out in this thesis.


Before the start of this study, the principles of a new environmentally friendly

continuous process for extracting bamboo fibres were established in a previous

study in Colombia. The fibres at that stage were extracted using a manual process. In

order to obtain higher quantities of fibres needed for this research, these principles

were translated into a prototype machine for the continuous (in-line) production of

long fibres. It has to be noticed that the implemented mechanical principles can be

scaled-up for future large-scale industrial production. The technology has been

implemented for the fibre extraction of Colombian bamboo (Guadua angustifolia),

but it is expected to be equally applicable to other bamboo species due to its

common characteristics. Due to the proprietary nature, the extraction principles

cannot be disclosed in this thesis.

The cleaning-combing operation needed to remove the soft tissue present on the

fibre surface after the extraction process, was also carried out manually. To increase

the yield of clean fibres in a standardized process, a prototype machine was

developed and constructed at KU Leuven after applying a design of experiments

methodology (DOE), considering the most important process parameters for an

optimum balance between the fibre cleanness and the fibre strength. The cleaning

process is suitable to be put in line with the extraction process. These lab-scale

prototype machines allow the production of long bamboo fibres in a fast and

reproducible way, ready to be incorporated into the composites. Because of the

protection of intellectual property, both the extraction and cleaning-combing process

principles are kept secret. This also applies to a new lab-scale equipment to obtain a

unidirectional tape with randomized bamboo technical fibre ends and a business

plan for the potential commercialization of the UD bamboo fibre prepregs. For this

reason, even though some of these activities were developed in the framework of

this research, the schematics and principles of the prototypes and operations are not

presented in this thesis.

A comprehensive literature review about the key concepts of natural fibres and the

state of the art of natural fibres and natural fibre composites with emphasis on

bamboo is presented in Chapter 3. To evaluate the robustness of the new extraction

process, an extensive characterization of the mechanically extracted technical fibres

is carried out in Chapter 4. The single technical fibre (SF) test generates their most

important mechanical properties such as maximum strength, Young’s modulus and

strain to failure. Through this test, it is not only possible to obtain the fibre strength

distribution and the corresponding parameters for the modified Weibull distribution,

but also the effect on properties after exposing the bamboo fibres to relatively high

temperatures, related to polymer processing.

8 Chapter 2 b

For the thermal characterization, the effect of typical composite manufacturing

conditions in two different environments (air and inert atmosphere) is investigated,

by treating the fibres at different times and temperatures. This study is performed to

understand the mechanisms of thermal degradation of the bamboo technical fibres.

With the obtained results, it is possible to estimate the properties of the bamboo

fibre composites after manufacturing, especially with thermoplastic matrices, where

high temperatures are used during the consolidation phase. This topic is also

evaluated and complemented in Chapter 6. In parallel, SEM and XPS are applied to

give better details of the fibre surface when exposed to these high temperatures.

Complementary to the SF characterization, a dry fibre bundle test (DFB) was

developed and implemented for the first time for natural fibres, to correlate with the

results from individual fibre tests and to evaluate the advantages and disadvantages

of this technique. SF tensile test results are used later to evaluate the properties of

the bamboo fibre composites in both thermoset and thermoplastic composites.

The next step in the research includes the production and characterization of high

quality bamboo fibre composites. In order to compare with existing composite

systems, epoxy resin and polypropylene (PP) matrices were selected due to their

extensive use in industry. Special attention was paid to evaluate the feasibility of the

composite manufacturing using these matrices. Bamboo epoxy composites are

presented in Chapter 5. First, the properties of unidirectional bamboo fibre

composites have been determined, without the presence of fibre ends inside the

specimens. Then, the effect of fibre ends was introduced, and the effect of the

distribution of these fibre ends on the UD properties was investigated and

benchmarked with a fully unidirectional fibre composite. This is important for the

development of continuous tape (prepreg) that inevitably will contain fibre ends.

Tensile tests and 3-point bending tests were performed for the evaluation of the

properties.

In Chapter 6, bamboo fibre thermoplastic (TP) matrix composites using PP and

maleic anhydride grafted polypropylene (MAPP) were produced by compression

moulding at different temperatures. The aim of this chapter is to maximise the

composite properties, taking into account the effect of the processing conditions.

Due to the high temperatures during the TP manufacturing process, a reduction in

composite properties is expected due to thermal degradation of the fibres. In order to

minimise fibre damage, two different actions are applied. The first is to find the

most effective process parameters (temperature and time of exposure) to obtain good

mechanical properties. The second action consists of a more ambitious procedure,

investigating for the first time in natural composites, the use of inert atmosphere (i.e.

argon) for the delay of fibre degradation. The goal is to explore the possibility of


using higher processing temperatures, thus expanding the processing window for the

use of other TP matrices, currently restricted to low melting temperature matrix

systems. The mechanical properties of the unidirectional bamboo fibre composites

are obtained from 3-point bending tests. The transverse three point bending test

(T3PB) is used as an indirect method to determine the fibre-matrix interface quality.

The technical assessments, the potential application of bamboo fibre and bamboo

fibre composites as well as their competitive advantages over other natural fibres

and glass fibres, are discussed in Chapter 7. Also, important aspects such as the

availability of the raw material, ecological and environmental impacts of the

resource and the composite material, manufacturing and performance of the final

material and its recyclability are presented. Finally, the potential use of bamboo

technical fibres for the reinforcement of polymeric matrices in high performance

applications is summarized, together with the future work, in the general

conclusions in Chapter 8.

10 Chapter 2 b

10

Literature review 11

Chapter 3 Natural fibres and their composites:

A literature review

_______________________________________________________

3.1 Natural fibres

Commonly, natural fibres are classified in three categories according to their origin:

plant fibres, animal fibres and mineral fibres. Plant fibres are further divided in

several main groups depending on the place they are extracted (stem, leaves, fruit,

seeds, etc.). Figure 3-1 shows a schematic representation of these categories with a

number of examples for each group.

Figure 3-1. Classification of natural fibres according to their origin and the location within the plant [1].

12 Chapter 3 b

The term “fibre” covers materials whose length (L) is several times their diameter

(d), and therefore, they have a high L/d or aspect ratio. For example, hemp and jute

fibres have this ratio between 100 to 1000, cotton and wool have it between 1000

and 300 [2-4]. Synthetic fibres can reach any value of L/d ratio due to their

continuous manufacturing process.

3.1.1 Fibre microstructure

With natural fibres, a distinction between elementary fibre and technical fibre has to

be made. The first one refers to the fibres of a few millimetres long that can be

extracted for example with chemical treatments. The technical fibres correspond to

the fibres obtained after some standard extraction process (e.g. mechanically), and

they are composed itself of elementary fibres held together by means of an interface

of pectin and lignin. Technical fibres are the ones used in this study for single fibre

characterization and composite manufacturing. On a microscopic scale, elementary

fibres have two types of cell walls consisting in a thin outer primary wall and an

inner layered secondary wall arranged as concentric ellipsoids with a small channel

in the middle called lumen [5] (see Figure 3-2). Each elementary fibre has a complex

layered structure where the primary wall is encircling the secondary wall with

random orientation of the cellulose microfibrils.

Figure 3-2. Structural architecture of a natural fibre. a) sisal fibre rope, b) sisal technical fibre, c) mesoscopic

scale: technical fibre, compose of elementary fibres and d) microscopic scale: elementary fibre consisting in a

thin outer primary wall and an inner layered secondary wall arranged as concentric cylinders with a small

channel in the middle called lumen. Adapted from [6].

a

b c

d


The secondary wall is divided in multiple layers (S1, S2 and S3) based on the

orientations of the microfibrils. S1 is made up of a cross-fibrillar network, whereas

S3 has a more transverse orientation. Both sections have fibrils oriented at large

angles to the fibre axis. The middle part of the secondary wall (S2) is characterized

by helicoidal wound cellular microfibrils formed from long chain cellulose

molecules that are approximately parallel and longitudinally directed. The

mechanical properties of the fibre are determined by this section, because it makes

up ~80% of the total thickness and thus acts as the main load bearing component [7,

8]. The angle between the fibre axis and the micro fibrils is called microfibrillar

angle [9]. Vegetable fibre is in itself as a composite material, consisting mainly of

cellulose microfibrils embedded in a matrix that has lignin and hemicellulose as

primary constituents [3].

3.1.2 Chemical composition of natural fibres

According to Bledzki et al. [10] and McKendry [11], hemicelluose, cellulose and

lignin are the three main components of biomass. In general, they cover respectively

20-40, 40-60 and 10-25 wt % for all natural fibres. The amount of these three

constituents varies considerably among the plant fibres (Table 3-1). Other

components, usually regarded as surface impurities, are both the pectin, which

provides flexibility to the plant, and waxes which consist in different types of

alcohols [12]. Climatic conditions, age and the maturation process influence not

only the structure of fibres, but also, their chemical composition.

Table 3-1. Chemical composition of some natural fibres [10, 13-15].

Cellulose is a natural polymer whose elementary macromoleule unit is the anhydro-

d-glucose. It contains three hydroxyl groups (-OH) (see Figure 3-3). Two of these

hydroxyls groups form intermolecular bonds, while the third one forms

Fibre Cellulose

(%)

Hemicellulose

(%)

Lignin

(%)

Banana 65 - 5

Coir 43 0.15–0.25 45

Cotton 82.7 5,7 -

Flax 71 16,7 2

Hemp 59-78 18–22 6

Henequen 60 28 8

Jute 45-63 18-21 21-26

Kenaf 50.5 21 17

Ramie 68.6 13,1 7

Sisal 70 12 12

Bamboo 60.8 24-28 32.2

14 Chapter 3 b

intramolecular hydrogen bonds [10, 12, 16]. Due to the presence of these hydroxyl

groups, the natural fibres, in general, have hydrophilic properties. The fibre moisture

level can reach 3-13% and can lead poor interface properties when used to reinforce

hydrophobic matrices [17].

Figure 3-3. Cellulose structure [7].

Several studies on various plant species predict that the tensile strength and Young’s

modulus increase with increasing cellulose content and decreasing microfibrillar

angle [12, 17]. This is because cellulose is an oriented long-chained, linear polymer

which is packed into a crystalline lattice over much of its length. Thus, it possesses

two of the basic strengthening elements: high degree of crystallinity and high

molecular weight with a degree of polymerization of around 10,000. Cellulose has

been estimated to have a theoretical modulus of 140 GPa [10, 12, 18].

The second main component, hemicellulose, comprises several sugar units and

exhibit a high degree of chain branching. It is an amorphous polymer with

significantly lower molecular weight than cellulose [19]. It is strongly bound to

cellulose fibrils presumably by hydrogen bonds and it forms the supportive matrix

for cellulose microfibrils. Hemicellulose is very hydrophilic and mainly responsible

for the moisture sorption behaviour of the fibres, soluble in alkali, and easily

hydrolyzed in acids [16].

The third main constituent is lignin, with a complex three-dimensional copolymer of

aliphatic and aromatic constituents. It is totally amorphous and hydrophobic in

nature. It is considered to be a thermoplastic polymer with very high molecular

weight that gives rigidity to the plants and trees [12].

3.2 Mechanical properties of plant fibres

Table 3-2 presents some characteristics and results in mechanical properties of

natural fibres compared to glass and carbon fibres. The strength and stiffness of

plant fibres mainly depend on several factors such as chemical constitution of the


fibre, the microfibril angle, the plant age, the environmental growing conditions and

the defects introduced during extraction. When the density of the material is taken

into account, the specific properties of some natural fibres are comparable with those

of glass fibres. [20]. More information about mechanical properties of natural fibres

can be found in Summerscales et al. [21, 22] and Bledzki et al. [10].

Fibre Density

(gr/cm3)

Diameter

(µm)

Microfibril

angle

(deg.)

Elongation

at failure

(%)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Bagasse - 490 - - 70 -

Coir 1.2 - 30 - 49 30 175 4 - 6

Cotton 1.6 20 - 7.0 – 8.0 287 - 597 5 - 12

Curaua 1.3 66 - 3.9 913 30

Flax 1.5 50 - 100 10 2.7 – 3.2 345 - 1035 50 - 70

Hemp 1.1 120 6 1.6 389 - 900 35

Henequen - 180 - 3.7 – 5.9 430 - 570 10 - 16

Jute 1.3 260 8 1.5 –1.8 393 - 773 26

Kenaf 1.3 106 - 1.8 427 - 519 23 - 27

Pineapple 1.3 - - 2.4 608 - 700 24 - 29

Ramie 1.5 34 7 3.6 – 3.8 400 - 938 24 - 32

Sisal 1.5 50 - 80 20 - 25 2.0 – 2.5 337 - 413 8 - 10

Bamboo 0.8– 1.1 100 - 200 10 - 391-713 18 - 55

E-Glass 2.5 9 -15 - 2.5 1200 – 1500* 70

Carbon (PAN) 1.4 5 - 9 - 1.4 – 1.8 4000 230 – 240

Table 3-2. Mechanical properties of some plant technical fibres compared to glass and carbon fibres [10, 23-

26]. *The strength of glass fibres highly depends on its previous handling history: a recent extruded fibre can

exhibit a strength of around 3500 MPa. However, prior to composite production, the strength in some cases

can be reduced approximately to 1250 MPa (due to all manipulations) and furthermore, the strength may

become as low as 900 MPa during the composite fabrication [27].

The industrial adoption of natural resources for reinforcing composites is an active

subject of research. The acceptance of natural fibre reinforced plastics in technical

applications depends on the availability of material data and specific design

information. Establishing the reliability needed for final product applications

requires extensive testing and a substantial amount of research [28]. In fact, one of

the main concerns for massive industrial application of natural fibres, is their

variability in mechanical properties [29]. In comparison with synthetic fibres,

natural fibres have a significantly higher variation in diameter between technical

fibres and along of individual fibres [29-32]. In addition, the fibre strength was

found to be negatively correlated to fibre diameter and gauge length [32]. This

variation in fibre properties can be characterized and predicted when quality

management is used, but also controlled when this information serves as a feedback

for optimizing the extraction and further fibre preparation.

16 Chapter 3 b

3.3 Natural fibre composites and their advantages

Cellulosic materials are the most abundant form of biomass and most likely to be

used as reinforcement fibres [33]. Moreover environmental friendliness, natural

abundancy and renewability make natural fibres a possible alternative to synthetic

reinforcing fibre materials, while offering good specific properties such as strength

and stiffness [34]. Additionally, unlike the traditional engineering fibres (e.g. glass

and carbon fibres), lignocellulosic fibres cause no health hazards due to fibre cutting

and low machine wear during post-processing and composite finishing. In composite

use, the thermal conductivity of the natural fibre composite is low; therefore they

make a good thermal barrier [35]. Also, the hollow nature of vegetable fibres leads

to a better acoustic insulation and damping properties once combined to a certain

types of matrices [36].

Nowadays, a substantial amount of research is carried out to analyse the possibility

of using natural fibres as reinforcement in polymer matrices, often shortened to

“composites”. Most composites have strong, stiff fibres embedded in a thermoset

(e.g. epoxy or unsaturated polyester resin) or thermoplastic (e.g. polypropylene)

matrix. These polymers are weaker in comparison to the fibres, but when the two

phases are combined, it is possible to generate a light and strong composite material.

In this structure, the fibres will bear the majority of the load and the polymers are

mainly used as fibre binder, as load transfer medium between the fibres, to protect

the fibres against e.g. abrasion and to prevent the fibres from buckling.

The composite material performance in the final application, depends on several

aspects such as: fibre volume fraction (Vf), mechanical properties of fibre and matrix,

impregnation, adhesion between fibre and matrix, fibre length (L) and fibre

orientation [2]. Regarding this last aspect, the unidirectional (UD) fibre

configuration achieves the maximum performance in the final composite, since the

fibres are aligned and placed in specific directions according to the required working

loads in the use phase. Structural and semi-structural composite parts are generally

manufactured with continuous aligned long fibres. Tables 3-3 to 3-6 show relevant

studies of (UD) natural fibres composites with thermoset and thermoplastic matrices

evaluated in flexural and tensile tests.


Table 3-3. Flexural properties of natural fibres composites with thermoset matrices. *Flexural strength / Flexural modulus.

Table 3-4. Tensile properties of natural fibres composites with thermoset matrices. *Tensile strength / Tensile modulus.

System Alkali treatment applied Vf

(%)

Before treatment After treatment Increase*

(%) Ref. Flex. strength

(MPa)

Flex. modulus

(GPa)

Flex. strength

(MPa)

Flex. modulus

(GPa)

UD Flax + epoxy

3% NaOH, 20 min at 20 °C 40 218 18 283 22 30/22 [37]

UD Jute + epoxy 26% NaOH, 20 min at 20 °C

40 178 9

260

17

46/89 [38]

UD Jute + vinylester 5% NaOH, 4h at 30 °C

35 199 12

239

15

20/25 [39]

UD Hemp + epoxy 22% NaOH, 1h at RT 33 148 6 225

12

52/100 [17]

UD Hemp + polyester 6% NaOH, 48h at 19 °C 60 77 7 100

9 30/27 [40]

UD Kenaf + polyester

6% NaOH, 48h at 19 °C 60 30 4

120

13

300/225 [40]

UD sisal + epoxy

2% NaOH, 4h at 60°C 37 168 12 225 16 34/33 [41]

UD sisal + uns. polyester

- 40 65 1.9 - - - [42]

System Alkali treatment applied Vf

(%)

Before treatment After treatment

Increase*

(%) Ref. Tensile

strength

(MPa)

Tensile

modulus

(GPa)

Tensile

strength

(MPa)

Tensile

modulus

(GPa)

UD Flax + epoxy -

20

127

17

-

-

-

[43]

UD sisal + epoxy 2% NaOH, 4h at 60°C 37 200 8

238

7 19/-17 [41]

UD sisal + uns. polyester - 40 99 3 - - - [42]

UD Jute + epoxy 26% NaOH, 20 min at 20°C 40 159 15

215

23 35/53 [38]

18 Chapter 3 b

System Process parameters Vf (%) % MA Flexural properties Ref.

σ (MPa) E (GPa) ε (%)

UD Jute – PP

(coated* yarns)

140, 150 and 160°C

20 Bar 15 min

49

-

101

11

-

[44]

UD Jute yarns-PP

(MB** + coated*

yarns)

180°C 10 min

20 Bar

21 - 122 9 - [45]

UD Flax-PP

(retted fibres)

12 min pre-heating,

220 °C (2.5 min)

17 - 90 ±25 11±5 1.8 [46]

UD Flax-PP

(boiled fibres)

12 min pre-heating,

220 °C (2.5 min)

27 - 129±19 16± 2 1.2 [46]

UD Flax-MAPP

(retted fibres)

12 min pre-heating,

205°C 2.5 min

28 0.6 164±56 14± 6 2.0 [46]

UD Flax-MAPP

(boiled fibres)

12 min pre-heating,

205 °C (2.5 min)

32 0.6 212±14 23± 2 1.4 [46]

UD Glass-MAPP

200°C 45min

25 Bar

58

10

1037

32

3.3

[47]

UD Glass-PP 200°C 45min

25 Bar

58 - 774

38 2.1 [47]

Table 3-5. Flexural properties for UD natural and glass fibre composites with thermoplastic matrices.

*PVA/PP (polyvinyl alcohol/polypropylene), **MB (micro braiding): continuous jute yarns were used as a

core and PP fibres were braided around the reinforcing yarn. MA= maleic anhydride (functional group grafted

onto one end of the PP chain that acts as a compatibilizer).

Table 3-6. Tensile properties for UD natural fibres composites with thermoplastic matrices. MA= maleic

anhydride (functional group grafted onto one end of the PP chain that acts as a compatibilizer).

3.3.1 Environmental impact

Traditionally glass and carbon fibres are the most common fibres to reinforce

polymer composites. They have a great acceptance due to their good mechanical and

thermal properties and have been used for several years in many applications, which

System Process parameters Vf (%) % MA Tensile properties Ref.

σ (MPa) E (GPa) ε (%)

UD Hemp-MAPP

180 °C 3min

3 Bar

30

10

37

12

0.5

[48]

UD Hemp-PP

Film stacking 35 - 125 - - [17]

UD Hemp-MAPP

Film stacking

35

0.5

122

-

-

[17]


can vary from aerospace components to sporting goods. Nevertheless, the end of life

disposal of these composites is not that evident.

Natural fibres are environmentally friendly, not only during their growth stage but

also after their lifetime because they are biodegradable. A comparative study

including life cycle assessment (LCA) studies of natural fibres and glass composites

was carried out by Joshi et al. [49]. His study reveals that natural fibre composites

are likely to be environmentally superior to glass fibre in most cases because:

Carbon dioxide fixation of natural fibres is an important issue in the reduction of

the greenhouse effect;

In many bulk composites applications, natural fibre composites have higher fibre

content for equivalent performance, reducing more polluting base polymer

content;

The light-weight natural fibre composite in comparison with classical materials,

e.g. steel, improves fuel efficiency and reduces emissions in the use phase of the

component, especially in automotive applications;

The end of life incineration of natural fibres results in recovered energy and

carbon credits; being basically carbon neutral;

Finally, their production has a low environmental impact as compared to glass

fibre production, particularly because of low energy utilisation., during the

production of natural fibres, the energy consumption required to produce a

natural fibre mat, including cultivation, harvesting and fibre extraction, amounts

to just less than one-fifth of the energy required to produce glass fibre mat [49,

50].

The environmental and ecological advantages of bamboo plantations and their use as

reinforcement in composite materials for their beneficial properties in comparison to

glass fibres, e.g. CO2 and energy consumption, will be further discussed in Chapter

7.

3.3.1.1 Bio-composites

The new worldwide environmental legislations and the general tendency towards an

appropriate use of the renewable resources have produced a great interest in the use

of biomaterials that can be environmentally sustainable and compatible with the

environment. In response to this necessity, many countries have been stimulating

“green chemistry” and the production and use of “green products” derived from

nature. Composites are not the exception to this new paradigm and there is, therefore,

considerable interest from manufacturers in developing new “green” composites.

20 Chapter 3 b

The development of bio-composites based on biopolymers and natural fibres as

reinforcements, offers totally or partially biodegradable composites with mechanical

properties that can compete in some applications with composites based on

traditional polymers such as PP. It has been reported by Oksman et al. [51], that the

composite strength of polylactic acid (PLA) matrix and flax fibres are about 50%

better compared to similar PP/flax fibre composites. Nevertheless, the developments

of these materials are still in their infancy with most of the related process or

product research initiated since the late 1980s. Nowadays, only a few fully

commercial applications have been released in the market [52].

As stated by Nishino [33] and Oksman et al. [51], most sustainable plastics cannot

compete economically with conventional petroleum-derived plastics in their present

state. Economically favourable composites, therefore, are expected to be made from

costly sustainable plastics in combination with relatively inexpensive natural

reinforcement fibres. Until now, many studies have been made on the potential of

natural fibres, but just a moderate number of investigations have been made on the

possibilities to use bio-based polymers as matrix for such fibres. One of the

challenges in replacing conventional composite materials with bio-composites is to

design materials that exhibit structural and functional stability during storage and

usage. Then, these materials might be able to degrade once disposed of after their

intended lifetime [53]. A more detail overview regarding this topic is given by

Plackett [52] and Averous et al. [54].

3.3.2 Coupling agents

Studies on composite materials have shown that the properties of composite

materials depend on their individual components and their interfacial compatibility.

Bonding between the reinforcing fibre and the matrix has a significant effect on the

properties of the composite; it means that the stress transfer and load distribution

efficiency at the interface is determined by the degree of adhesion between the

components. Generally, coupling agents facilitate the optimum stress transfer at the

interface between fibres and matrix [2, 35].

According to Gassan and Bledzki [7, 15], to improve the properties of natural

composites, the structure and surface of the natural reinforcing fibres can be

modified in two different ways. In one hand, the physical treatments change

structural and surface properties of the fibres, without changing their chemical

composition, and thereby influence the mechanical bonding with the matrix. In this

case, surface cross-links can be created, the surface energy can be changed and

reactive groups can be generated. Electric discharge methods, such as corona and


cold plasma [55], thermo-treatment [56] and mercerization [57] are examples of

these treatments.

On the other hand, when two materials are incompatible, it is often possible to

increase compatibility by introducing a third material that has intermediate

properties between those of the other two by means of chemical methods. They can

be divided in two subgroups: “compatibilization” and “coupling” methods. The first

one can be polymers with functional groups grafted onto the chain of the polymer.

Graft copolymerization (e.g. maleic anhydride (MA)), it is common treatment used

in thermoplastic matrices [12]. The second method, mostly used in thermoset

polymers, typically utilises a difunctional chemical reagent to enable a chemical

reaction between the fibre surface and the matrix polymer. Isocyanates, silane and

organosilanes coupling agents, are some examples of this treatment [7, 58].

3.3.2.1 Alkali treatment

Alkali treatment in single fibres

Alkalization cannot be considered as an authentic physical modification since the

chemical composition of the plant fibre is altered after treatment, nor can it be

classified under the pure chemical methods, because no additional coupling agent is

introduced into the composite [57]. The alkalisation process (mercerization) is one

of the most commonly used chemical for bleaching and/or cleaning the surface of

plant fibres [16], where the fibres are immersed in a NaOH solution at specific

concentration for a specific period of time. This chemical treatment produces

modifications on the fibres related with the surface morphology and fibre internal

structure that will be reflected in the mechanical properties of the composite material

[59, 60]. The following reaction takes place as result of alkali treatment [12, 19]:

Fibre – OH + NaOH Fibre – O- – Na

+ + H2O

The important modification is the removal of hydroxyl groups in the network

structure. As a result, the penetration of sodium hydroxide into crystalline regions

(cellulose I), cellulose II is formed. This is an irreversible exothermic process

resulting in the modification (rearrangement) of micro-fibrils that specially happens

at high concentrations of the solution [61]. In addition, it has been reported that the

mercerization also removes part of the hemicellulose, certain amount of lignin, wax

and oils covering the external surface of the technical fiber [7, 12]. This creates a

rough fibre surface topography promoting a good mechanical interlocking between

fibre and matrix [16, 40, 62]. It is important to select the right concentration of alkali

22 Chapter 3 b

solution, the temperature and the time of exposure according to each fibre, in order

to avoid weakening or damage of the fibre because of the excessive delignification

[12, 19]. The hemicellulose and lignin in the technical fibre can be considered as the

“glue” of the elementary fibres. This surface cleaning makes the interfibrillar region

less dense and less rigid allowing the fibrils to re-arrange along the fibre major axis,

resulting in a better load sharing between them with further improvement in the

technical fibre strength [58, 63, 64]. Generally, this benefit was observed together

with an improvement of the aspect ratio (L/d) of the technical fibres [65, 66], and

can be seen in Table 3-7 for some cases when comparing the fibre strength before

and after alkali treatment. For technical bamboo fibres, it was also reported that the

increase of the fibre strength and stiffness due to the alkali treatment could be

caused specifically by the removal of waxes and some amount of lignin from the

fibre surface, resulting in a slight reduction of the fibre diameter but more

importantly, a diminution of weak non-cellulosic materials [67].

Table 3-7. Strength and stiffness values after single fibre test before and after alkali treatment.

Effect of the alkali treated fibres on natural fibre composite properties

Several studies show the benefits of alkali treatment on the composite performance

in flax [37, 43], jute [7, 39], hemp [17, 40], kenaf [40] and sisal [41] fibres. Tables

3-3 and 3-4 illustrate the positive effect of this treatment on the tensile and flexural

properties in a number of natural fibres and thermoset matrix composites. Besides of

the mechanical interlock, this improvement can be explained not only by the

eventual improvement of the intrinsic properties of the technical fibres, but also, by

their increased aspect ratio after treatment [38, 64, 72]. Moreover, it is a result of an

improved adhesion due to the activation of the hydroxyl groups on the fibre surface,

thus increasing the number of possible reaction points for better chemical bonding

[64].

Fibre

Fibre

Diam.

(µm)

Before treatment After treatment Alkali treatment

(% NaOH, time

and temperature)

Ref. Tensile

Strength

(MPa)

Tensile

Modulus

(GPa)

Tensile

Strength

(MPa)

Tensile

Modulus

(GPa)

Sisal 150 391 15 496 13 2%, 4h at 60°C [41]

Sisal - 500 16 810 27 16%, 48h at RT [18]

Sisal - 365 - 481 - 4%, 24h at RT [68]

Hemp 26 514 25 347 20 10%, at RT [69]

Curauá 94 913 30.3 736 27 5%, 1h at RT [70]

Palm 180 - 208 233 7 366 5 6%, 3h at 95 °C [71]


According to some studies [10, 57], low concentrations of NaOH (<3%) are not

strong enough to cause significant swelling of the cell wall but it is sufficient to

remove non-cellulose (impurities) on the surface of the fibre. Moreover, it was

found that the mechanical properties reach an optimum for a certain NaOH

concentration followed by a drastic decrease. Van de Weyenberg [57] treated flax

fibres with NaOH at several concentrations. For the longitudinal strength, the

highest improvement was achieved with a 3% NaOH concentration. For the

longitudinal modulus, the transverse strength, and the transverse modulus, the 1%

NaOH concentration presented the best results. Jacob et al. [73] examined the effect

of NaOH concentration (0.5, 1, 2, 4 and 10%) for treating sisal fiber-reinforced

composites and found that the maximum tensile strength was obtained with the 4%

NaOH treatment at RT. A similar result was found by Mishra et al. [74], who

reported that 5% NaOH treated sisal fiber-reinforced polyester composites gave

better tensile strength than the 10% treated composites.

3.3.2.2 Maleic anhydride

Maleic anhydride (MA) grafted polypropylene (MAPP) is a coupling agent that also

acts as a compatibilizer. It consists of long polymer chains with a MA functional

group grafted onto one end of the PP chain. MAPP acts as a bridge between the

nonpolar polypropylene matrix and the polar fibres by chemically bonding with the

cellulose fibres through the MA groups, and bonding to the matrix by means of

polymer chain entanglement [69].

The mechanism of reaction of maleic anhydride with PP and fiber can be explained

as the activation of the copolymer by heating (170 °C) before fiber treatment (Figure

3-4a) and then the esterification of cellulose fiber (Figure 3-4b) [58]. The coupling

occurs at one side through covalent bonds and hydrogen bonds between the

hydroxyl groups of the cellulosic fibres and the acidic anhydride groups of MA,

while on the other side a co-crystallisation and entanglement occurs with the

polymer [75].

Compatibilization with MA in sisal [75], flax [46] and jute [76, 77] fibre composites,

results in composites with enhanced mechanical properties and reduced water

absorption due to an increased interaction between the anhydride groups and the

hydroxyl groups on cellulose. In this case, a better wettability and higher fibre-

matrix interfacial adhesion is promoted, as a result of the chemical bonding.

24 Chapter 3 b

Figure 3-4. Schematic representation of the interphase between MAPP and the hydroxyl groups of a cellulose

fibre in two steps: a) activation of the copolymer by heating (T= 170 °C) (before fibre treatment) and b)

esterification of the cellulose [10, 19, 58].

According to Chattopadhyay et al. [78], short bamboo fibre reinforced

polypropylene containing 5 wt% MAPP has a 37% increase in impact strength, 81%

increase in the flexural strength, 150% increase in the flexural modulus, 105%

increase in the tensile strength, and 191% increase in the tensile modulus. Chen et

al. [79], found that the tensile modulus, the tensile strength, and the impact strength

of short bamboo fibres and PP increased significantly incorporating 0.5 wt % maleic

anhydride in the PP matrix. In Tables 3-5 and 3-6 show flexural and tensile UD

natural fibre thermoplastic composites with different concentrations of MA.

3.4 Limitations in the use of natural fibre composites

3.4.1 Moisture absorption

The effect of moisture on the mechanical properties of natural fibre composites

during long term service in outdoor or exposed to a moist environment, is nowadays

an active field of research [65, 80]. According to Jindal [81] and Stamboulis et al

[5], the intrinsic hydrophilicity character of technical natural fibres is a major

obstacle which prevents the extensive application of natural fibres in composites.

This originates moisture absorption and may cause dimensional instabilities

(swelling) in the produced parts, creating some stresses at the interface and

preventing the fibre properties to be transferred into the composite [57].

By reducing the moisture sensitivity with a treatment, the technical fibre becomes

more hydrophobic [82] by the reduction of hemicellulose, largely responsible for the

water absorption behaviour exhibited by the fibres [83]. Hence, the compatibility

with hydrophobic polymeric matrices such as PP increases beneficing the interfacial

adhesion, and therefore the void content in the composite can be reduced, giving less

chance to moisture to penetrate into the material.

a.

b.


3.4.2 Thermal degradation

Natural fibres are lignocellulosic polymers subjected to thermal degradation at

elevated temperatures, e.g. during composite processing [7, 84]. The majority of

natural fibres have low degradation temperatures (~185 °C), which make them

inadequate for processing temperatures above 200 °C [85]. For this reason natural

fibres limit the choice of the polymers as a matrix [76]. Both thermoset resins (e.g.

epoxy and unsaturated polyester resins with curing temperatures below 180 °C), and

some thermoplastic polymers (e.g. PP and MAPP) can be used with all range of

natural fibres [57]. It is thus of practical significance to understand and control the

thermal decomposition process of natural fibres to keep the thermal stability of the

components during the manufacturing process [86]. The most important aspects to

consider regarding this topic will be explained in the next paragraphs.

3.4.2.1 Thermal degradation in natural fibres

It has been established that no fibre damage occurs at temperatures below to 160°

[87] independent of the exposure time. Li et al. [19] stated that thermal treatments

between 170°C and 180°C will not affect the tensile properties if such temperatures

are maintained for less than one hour. According to Wielage et al. [88], Rachini et

al. [89] and Placet [30], the first degradation typically occurs at temperatures above

180°C and more severe damage occurred within a temperature range between 215

and 310°C [84]. For natural fibres, the thermal degradation is strongly influenced by

its chemical composition [90]. A typical thermogravimetric analysis (TGA) and its

first derivative (DTG) curves for the pure main components of natural fibres, such as

cellulose, hemicellulose and lignin, are shown in Figure 3-5.

Figure 3-5. Thermal analysis for pure hemicellulose, cellulose and lignin determined by TGA and DTG [91].

Lignin Hemi-cellulose

Cellulose

26 Chapter 3 b

According to Yang et al. [91], these three components behave significantly different

behaviour during pyrolysis in air environment. Hemicellulose starts with an early

decomposition between 220 and 315 °C, with a maximum mass loss rate (0.95

wt.%/°C) at 268 °C. Cellulose begins at a higher temperature range (315 - 400 °C)

with the maximum weight loss rate (2.84 wt.%/ °C) attained at 355 °C. Lignin was

found to be the most difficult one to decompose, doing it slowly under the whole

temperature range from ambient to 900 °C. Similar results for those three

constituents were found by Martin et al. [92] and Rachini et al. [89]. Generally, the

contribution of hemicellulose and cellulose on the thermal degradation is more

important than the one of lignin [89]. A typical TGA (and its corresponding DTG)

thermal decomposition process for a natural fibre is shown in Figure 3-6.

Figure 3-6. Typical thermogravimetric decomposition process of a natural fibre [1].

For a typical natural technical fibre it has been found that the thermal degradation

registered during TGA, with a heating rate of 10 °C/min, can be divided in three

individual stages according to several studies, see Table 3-8. In this table typically

the first peak occurs at temperatures below of 100 °C and corresponds to the initial

moisture evaporation. The second peak is attributed to the hemicellulose

decomposition that occurs between 220 and 290 °C. Cellulose degradation shows

the third peak between 310 and 400 °C. For flax fibres it was found that 2nd

and 3rd

peaks occurred at around 344 and 459 °C respectively. Significant lignin

decomposition can be expected at around 425 °C, nevertheless, this 4th

peak is not

always visible [93].


Table 3-8. Initial weight loss and initial temperature degradation for some natural fibres and PP determined by

DTG in air environment.

Fibre thermal decomposition mechanism

At relatively high temperatures, the thermal degradation is a complex process that

mainly depends on the structure and the chemical composition of the natural fibre

and the intensity of the treatment, e.g. temperature and time of exposure. In air, the

thermal degradation mechanism of cellulosic fibres can be divided in two stages (see

Figure 3-7):

At low temperatures, between 100 °C and 250 °C, some of the changes in

physical properties of the fibres can be explained in terms of alterations in the

structure such as depolymerisation, hydrolysis, dehydration and

decarboxylation and recristallysation [87]. Besides that, the formation of free

radicals is also noticed. They can contribute to the formation of

hydroperoxide groups, responsible to a large extent to the depolymerisation

of cellulose (by bond scission). Besides depolymerization, a variety of

oxidation and decomposition reactions take place, leading to the formation of

carbonyl, carboxyl, lactone and aldehyde functional groups [93]. All these

reactions, specially the oxidative degradation of cellulose, takes place

primarily in the amorphous region of the cellulose [88] giving a drastic

decrease in molecular weight and consequently in mechanical performance.

At higher temperatures (~300 °C), the cellulose thermal degradation is caused

by the destruction of hydrogen bridges, changes in crystallinity, and some

processes mentioned before, i.e. the formation of free radicals, carbonyl

groups, and carboxyl groups but in a higher scale, especially in air [93]. The

decomposition under inert environment is mainly due to the presence of free

radicals [89, 95].

Temperature (°C)

Fibre 1st

peak

2nd

peak

3th

peak

Techni-

que

Flow rate

(mL/min)

Heating

rate

(°C/min)

Ref.

Sisal 75 222 310 DTG 20 10 [92]

Curauá 86 225 363 DTG 50 10 [85]

Flax 90 344 459 DTG 20 10 [94]

Hemp 87 253 395 DTG - 10 [89]

Jute <100 287 353 DTG - 10 [77]

28 Chapter 3 b

Figure 3-7. Schematic stages for the thermal degradation of a cellulosic fibre [61, 87, 93, 96].

Single fibre properties after thermal treatment

Kohler et al. [97] evaluated the tensile strength of flax treated at 200°C during

different exposure times. A drop in tensile strength was observed after 30 minutes.

Mieck et al. [98] reported major damage to flax fibres after four minutes of exposure

at 240°C. Additionally, in a study carried out by Van de Velde et al. [94], flax

technical fibres were treated at different temperature-time of exposure couples (120

and 180°C during 15, 30, 60 and 120 minutes). As a result, no significant decrease

was observed in tensile strength after 120°C treatment, independent of the time of

exposure. However, at higher temperatures and longer exposure times the general

tendency is a remarkable decrease of the mechanical properties. Moreover, it has

been reported that relatively low temperatures (150 °C) and longer exposure times

(up to 4h), significantly reduced the tenacity of natural fibres [87]. It was also

noticed that alkali treated natural fibres are more thermally stable compared to the

untreated fibres due to the partial removal of amorphous cellulose, known to have

low thermal resistance [1, 99].

For bamboo technical fibres, a study of Ochi et al. [100] revealed that for thermal

treatments below 140°C there is no significant decrease in mechanical properties.

Around temperatures of 160°C, a gradual decrease of the tensile strength is noticed

after 30 min of exposure. At temperatures between 180°C and 200°C, the tensile


strength drops by around 25% during the first 10 min and tends to stabilize after 30

min. The Young’s modulus for the bamboo fibres was found to be independent of

heat treatment in the mentioned ranges of temperature and time. Figure 3-8 shows

the decrease of the technical fibre tensile strength when exposed at different

temperature-time of exposure couples for bamboo and flax fibres. Another

consequence of the heat treatment is that lignin migrates to the fibre surface due to

its softening, that begins at around 214°C [89, 101]. This phenomenon was

examined by AFM observations in the bamboo technical fibres surface, before and

after they were treated in an autoclave (3 bar at 150 °C) for around 1 hour [102].

When this occurs, the hydrophobicity of the fibre surface increases [55] leading for a

better compatibility with thermoplastic matrices and hence reduced moisture

sensitivity in the final composite material.

.

Figure 3-8. Effect on the fibre strength of thermally treated flax and bamboo technical fibres, when exposed at

different temperature-time of exposure couples [87, 88, 97, 98, 103].

Effect of air and inert atmosphere during thermal treatment

In general, the degradation reactions are much less significant in an inert atmosphere

than in air. Thermal resistance of flax [94], jute [86] and hemp [56, 89] fibres was

found to be higher under inert atmosphere since the process of decomposition of

cellulose occurs much quicker in air environment. According to Prasad et al. [56],

after giving a heat treatment of 30 minutes at 180 °C in air and nitrogen environment,

an undesirable general degradation of the mechanical properties, in some cases of

around 15%, was noticed especially in air. Nevertheless, technical fibre

30 Chapter 3 b

defibrillation occurs in both cases. By comparing untreated hemp fibres with the

inert thermally treated, no remarkable degradation in terms of mechanical properties

was noticed, with a maximum strength difference of around 7%. In another study,

the same author [101] noticed under optical and SEM observations, a more severe

damage on the primary wall when the technical fibre was treated in air, compared

with the effects under nitrogen environment when the thermal treatment was carried

out at 220 °C for 30 minutes.

3.4.2.2 Thermal degradation in natural fibre composites

The main decomposition range of various natural fibres overlaps with the processing

temperatures of some thermoplastics. As mentioned before, the thermal

decomposition for lignocellulosic fibres is estimated to start around 200 °C and

strongly depends of the exposure time. Classically, during the manufacturing of

natural fibre composites, the process temperature is chosen in accordance with the

rheological properties of the polymer matrix, e.g. viscosity [30, 92]. This

temperature must not lead to a loss of the fibres’ integrity. Table 3-9 gives an

overview of commonly used thermoplastics with their melting temperature. Until

now, thermoplastics with a low melting and shaping temperature have been used as

matrix in natural fibre composites (e.g. PP, PE, PVDF).

Table 3-9. Common used thermoplastics and their melting and shaping temperature [104]. *Common used

temperature to produce parts in manufacturing, usually between 25 to 30 °C above Tm.

During production of natural fibre reinforced plastics, shorts periods of exposure to

high temperatures are allowed without a significant drop in the properties of the

fibre [94, 98]. This is a positive aspect when considering industrial processes such as

pre-impregnating fibres with thermoplastics (“prepregs”). For that, the fibres are

stabilized by a thermoplastic in molten state exposing the fibres at high temperature

for a relatively short time. Thermoplastic prepregs are matrix-fibre combination with

the exact amount of polymer necessary to achieve the desired fibre volume fraction.

Thermoplastic Melting

Temperature [Tm] (°C)

Shaping

Temperature* (°C)

Polyethylene (PE) 120-145 150-200

Polypropylene (PP) 165 185-200

Polyvinylidene Fluoride (PVDF) 170 190-230

Polyamide 6 (PA6) 220 240

Polybutylene Terephthalate (PBT) 225 250-280

Polyethylene Terephthalate (PET) 255 280-300

Polyetheretheketone (PEEK) 343 365-380


They are ready to be used for the manufacturing of laminated composite structures

with well defined fibre orientations.

The use of inert atmosphere can be adapted for the manufacturing of natural fibre

composites in a well controlled process at relatively low cost [56]. According to

Wielage et al. [88], the consolidation process under a non-oxidative environment

reduces the decomposition not only of the natural fibres, but also, of the

thermoplastic matrix itself, e.g. PP and MAPP. Nevertheless, the latter statements

had been based on single technical fibre analysis. To the best author’s knowledge,

no studies evaluating the mechanical behaviour of natural fibre thermoplastic

composites manufactured under inert atmosphere have been made.

Thermal oxidative degradation of polypropylene

At high temperatures the components of the chain back bone of the polymer start to

separate (chain scission), resulting in a reduction of the molecular weight and

mechanical properties over time. In addition to thermal degradation, the chains can

be broken mechanically by shear stresses induced during the process [69]. The main

mechanism of degradation of PP is random-chain scission via free-radical transfer

process (auto-oxidation) [105]. This process involves several steps such as initiation,

propagation, chain branching and termination [106]. The thermal degradation of PP

is highly dependent of the environment [88]. In nitrogen, the process starts around

300 °C, with a maximum weight loss at 431 °C. However in air, the degradation

process shifts towards lower temperatures (235 °C) [86].

Antioxidants and stabilisers are usually added to the formulation of the PP to

prevent thermal oxidative degradation during processing and service (special care

must be taken for food or medical applications) [105]. In the case of the stabilizers,

they are used to keep the polymer chains and the original molecular structure intact,

thus enabling properties such as strength, stiffness and toughness to be retained over

a longer period of time. Most stabilizers slow down polymer degradation by reacting

rapidly with available peroxide radicals to produce another less active radical. Other

stabilizers interrupt the thermal oxidative degradation cycle at the hydroperoxide

propagation step, to slow down or prevent the cycle from completing [69]. Primary

antioxidants inhibit the oxidation reaction by combining with the formed free

radicals. Hindered phenolics (e.g. butylated hydroxytoluene (BHT)) are commonly

used as primary antioxidants. Secondary antioxidants, also called peroxide

decomposers, inhibit oxidation of PP by decomposing hydroperoxides. Phosphites

and thioesters are commonly used as secondary antioxidants, and are usually

combined with primary antioxidants to produce a synergistic protection against

oxidation [104].

32 Chapter 3 b

3.5 Bamboo plants

Bamboo plants belong to the Gramineae family which essentially means that they

are grasses (not trees), with more than 90 genera and over 1200 different species

around the world. Bamboo plants are naturally distributed in tropical and subtropical

areas, between 46° North and 47° South latitude and are thus commonly found in

Asia, Africa and Latin America. However, it has been introduced to the north,

including Central America [107, 108], see Figure 3-9.

Figure 3-9. Distribution of woody bamboo in the world. Source: www.eeob.iastate.edu. According to

Lobovikov et al. [107] the percentage world contribution from the different continents is: 65% Asia, 28%

America and 7% Africa.

The bamboo species Guadua angustifolia belongs to the woody bamboos and is the

largest and economically most important bamboo in the Western Hemisphere [109],

especially in South America, where Colombia has more than 51.500 Ha of

plantations [110]. According to Londoño [108], bamboo guadua is not only one of

the three largest species, but also one of the most important bamboo in the world due

its excellent mechanical properties with several advantages:

- High growth rate (11 to 21 cm per day). Culms reach their final height in the

first 6 months of growth, and come to maturity when they are 4 to 6 years old

[111]. In comparison with a wood tree, bamboos have a very short growth

cycle and major productivity [112]. Moreover, bamboo forests control

erosion and regulate the level of rivers;

- This plant plays and important economic role in the places where it grows. In

South America, including Colombia, it has been used specifically for building

and handicraft purposes, but unfortunately its exploitation has not had an

additional value and only a few technological developments have been

associated with this natural resource;


- Guadua angustifolia is one of the tropical species that have been identified as

having great potential to fix atmospheric carbon dioxide. The carbon fixation

estimated for 400 clumps/ha of Colombian bamboo, for a growth period of 6

years is 54.3 tonnes [110]. Therefore, bamboo is an effective plant in terms

of global warming management in comparison with young forests. According

to Lybeer et al. [113], bamboo is one of the most important non-timber forest

products and one of the more important agricultural non-annual plants in the

world.

3.5.1 Morphology of bamboo Guadua angustifolia culm

In general terms, the bamboo culm is a thin and hollow cylinder divided in to several

internodes each one with its own internal diaphragm that separates the internodes.

The fibres in the nodes are not unidirectional but randomly oriented in an entangled

manner giving isotropic properties to the nodes and providing additional

reinforcement to the culm [114] (see Figure 3-10). The function of these nodes is the

prevention of buckling due to bending and they may also play the role of axial crack

arresters due to the fact that the fibres in the internodes are oriented along the

bamboo's culm. Among plants, bamboo has a unique structurally smart structure that

helps it to withstand extreme natural environmental conditions and it is a unique

example of a natural unidirectional fibre reinforced composite [34].

Figure 3-10. a) Different segments of bamboo G. angustifolia plant, b) parts in which is divided the bamboo

culm and c) discontinuity of the bamboo fibre within the node (discontinuity drawing according to [115]).

34 Chapter 3 b

3.5.1.1 Diameter, wall thickness and internode length of the bamboo culm

As shown in Figure 3-11a and b, the bottom segment has the largest diameter and

wall thickness, but the average internode length is lower in comparison with the

other segments. These characteristics make the base of the culm suitable to carry

compressive loads. The diameter and thickness of the wall decrease from the bottom

to the top [115-117]. It was found for Moso bamboo (Phyllostachys pubescens) that

the length of the internodes reaches the maximum length value of 32 cm in the

middle section of the bamboo plant [116], see Figure 3-11c.

Figure 3-11. a) Internodal length for bamboo G. angustifolia, adapted from [118], b) Wall thickness variation

(G. angustifolia) across the culm, adapted from [117] and c) intermodal length vs intermodal number for

Moso bamboo (Phyllostachys pubescens) [114].

For bamboo G. angustifolia the maximum average culm diameter is found on the

bottom part of the culm. The average biggest wall thickness and longer internode

distance, are found in the bottom and middle part of the culm respectively [117], see

Table 3-10. For this species, the total high of the bamboo plant range between 20

and 23 m, whereas the commercial height is in average 15 m [117, 118], see Figure

3-10a.


Table 3-10. Average values for the main anatomical variables in bamboo G. angustifolia in three different

segments [117]. *See Figure 3-10a.

3.5.2 Anatomy of the bamboo culm

Figure 3-12. Bamboo G. angustifolia microstructure [67]: (a) bamboo culm, (b) cross-section of the culm

showing the fibre distribution through the wall thickness, (c) vascular bundle, the main anatomical constituent

of the plant, composed of vesselsI, floem

II, protoxilem

III, parenchyma tissue

IV, and fibre bundles

V, (d) bamboo

technical fibre composed by several elementary fibres, (e) elementary fibres with pentagonal or hexagonal

shape, and (f) model of polylamellae structure of a thick-walled elementary fibre proposed by Liese [115],

where, in the thick lamellae (L1–L4), the cellulose fibrils are oriented at a small angle to the fibre axis,

whereas the thin ones (N1–N3) show mostly a more transverse orientation, P, primary wall, O, external sheet

of secondary wall.

The gross anatomical structure of a transverse section of any culm internode is

determined by the shape, size, arrangement and number of the vascular bundles.

They are the basis to understand the mechanical behaviour in the different sections

of the culm because they are closely related to the fibres [115]. As can be seen in

Figure 3-12b and c, the vascular bundles can be detected easily because of their dark

colour. These bundles are distributed densely in the outer region of the wall and

sparsely in the inner region, which contains mainly parenchyma tissue with fewer,

Segment of the

culm*

Average

diameter

(cm)

Average wall

thickness

(cm)

Average internode

length

(cm)

Bottom 11.5 1.8 20.2

Middle 11.1 1.3 34.9

Top 5.9 0.9 33.4

36 Chapter 3 b

large vascular bundles [116]. Through the height, they are concentrated in the upper

part of the culm compared with the base. For bamboo Guadua, in the periphery zone

of the culm (2.5 mm of thickness) the number of vascular bundles per cm2

is in

average between 346 and 530, whereas in the interior layer, for the same thickness,

there are 52 to 96 vascular bundles [115, 117].

Around the vascular bundles there is a spongy tissue called parenchyma, see Figure

3-12c, composed of large cells that are mixed with some short cubic cells. Inside

these cells, some nutrients in the form of starch grains are stored filling 50 to 70% of

the tissue [119]. The amount of parenchyma tissue decreases along the culm length

and across the cross section (decreasing from the inside to the outside of the culm

wall) [114, 115, 120].

3.5.3 Bamboo fibres

A typical cross section of a technical bamboo G. angustifolia fibre shows an

irregular “beam” shape (Figure 3-12c and d), having a size variation depending on

their position across the surface culm section. Each fibre, irrespective of its position,

contains several elementary fibres [67]. The cross-section of these primary fibres is

either pentagonal or hexagonal and they are arranged in a honeycomb pattern,

separated by a thin wall of matrix (Figure 3-12e) [34, 67, 121].

According to Amada et al. [116], the average density of bamboo is 0.8 g/cm3 and the

average fibre volume percentage of fibres in a piece of bamboo is approximately

32% taking into account the reduction of the diameter and wall thickness over the

height. Considering the gradient of the fibres on the wall thickness, the fibre volume

fraction (Vf) is about 15 – 20% and 60 – 65% at the inner and outer surface

respectively. Fibres constitute about 40-50% of the weight of the total tissue and

between 60 - 70% of the weight of the total culm tissue [115]. As mentioned before,

the bamboo culm resembles a unidirectional fibre reinforced composite as is shown

in Figure 3-13.

3


Figure 3-13. Cut bamboo (G. angustifolia) culm in the field, showing some bamboo technical fibres vertically

oriented. The bamboo culm resembles a unidirectional fibre reinforced composite.

3.5.3.1 Microstructure and chemical composition of bamboo fibres

According to Liese [122], the elementary fibre size decreases from the bottom to the

top along the culm length. Across the culm wall, the fibre length often increases

from the periphery, reaching its maximum at about the middle and decreases toward

the inner part. The fibres in the inner part of the culms are always much shorter (20 -

40%). The elementary fibres are long and tapered at both ends with a small hole in

the centre called lumen, see Figure 3-12e [123]. Longitudinally within the internode,

the shortest elementary fibres are always near the nodes and the longest in the

middle part, this a variation of more than 100%. The elementary fibre length is

positively and strongly correlated with the average fibre diameter, cell wall

thickness (layers) and internode diameter. For bamboo G. angustifolia the mean

fibre elementary length is 2.1 mm and its diameter varies from 10 to 25 µm [124].

According to several studies [115, 120, 125, 126], bamboo fibres have a specific

layered structure that is unique amongst the vegetable fibres. This polylamellate

structure is depicted in Figure 3-12f. It consists of an alternation of thick and thin

layers. In the thick layers, the fibrils are oriented at a small angle towards the length

axis of the fibre, corresponding with a microfibril angle between 2 and 10°. The thin

layers show a more transverse orientation of the fibres. The amount of layers

depends on the position and maturity of the fibre. The lamellar structure leads to the

high mechanical characteristics of bamboo fibres.

According to Londoño et al. [117], the composition of the culm of the Guadua

angustifolia, on average, is 51% parenchyma tissue, 40% fibre and 9% conducting

cells. Compared to other tropical and subtropical species, this species exhibits a

typical percentage of fibres. Bamboo consists of about 50 - 70% of cellulose, 30%

38 Chapter 3 b

pentose and 20 - 25% lignin, and typically has a silica content ranging from 0.5 to

4 % (mostly is deposited in the skin zone). There are some differences in these main

constituents according to the species’ conditions of growth [120]. After cellulose,

lignin represents the second most abundant constituent in the bamboo. Bamboo

lignin is a typical grass lignin, which is built up from three phenyl-propane units, p-

coumaryl, coniferyl and sinapyl alcohol, interconnected through biosynthetic

pathways [122, 127]. The lignification and the thickening of the cell walls are the

main changes during maturation, both increase downward from top to bottom,

whereas lignification increases from inside to outside. Full lignification of bamboo

culm is completed within one growing season [128].

3.5.3.2 Mechanical properties of bamboo technical fibres

Bamboo fibres are often called “natural glass fibres” because of their properties [25,

67]. Among the well-known natural fibres, bamboo has a favourable combination of

low density and high mechanical properties giving good specific properties (i.e.

stiffness and strength). Several studies have been carried out to characterise single

technical bamboo fibres. A summary of these results, with the corresponding

extraction method, is presented in Table 3-11. Nevertheless, these results should be

taken as indicative values because in many studies the authors do not specify

important details such as: the bamboo species, the age and growing conditions of the

plant, part of the culm where the fibres were extracted from, etc. Also, some missing

information about the methodology for single fibre testing was omitted, including:

the testing gauge length, the loading rate, the applied technique for the determination

of the fibre’s cross section, etc. All of this makes impossible a direct comparison

between the different studies in single bamboo fibre properties.

Numerous investigations on strength properties in relation to the culm age have not

shown consistent results [129]. Whereas some reports show higher strength values

for one year old bamboo culm compared to older ones, other studies revealed a

general increase with age up to 6 to 8 years old, followed by a decrease in all

strength properties in culms of ten years old [130]. It is commonly accepted that a

three-year-old bamboo is ready to be used for industrial purposes, but a study carried

out by Lybeer et al. [113] concludes that younger bamboos would be also

appropriate for industrial purposes.


Table 3-11. Physical and mechanical properties of bamboo fibres according to the extraction method [13, 32,

81, 100, 131-138]. HC: highest fibre diameter concentration.

Moreno et al. [139], performed a mechanical characterization of bamboo G.

angustifolia fibres, taking into account three different variables: age, height and wall

thickness of the culms. With respect to the positioning of the fibre on the wall

thickness, fibres extracted from the outer third of the wall present the lowest values

of tensile strength (Figure 3-14). Considering the age of the culms, young and

mature culms present similar strength values (655 and 663 MPa respectively). The

best condition for obtaining the highest strength values can be reached by extracting

the fibres at 2/3 of the wall thickness from the inside of the wall thickness, from a

Extraction

method

Fibre physical properties

Mechanical properties

Diameter

(µm)

Length

(mm)

Density

(gr/cm3)

Strength

(MPa)

Young’s

Modulus

(GPa)

Strain to

failure

(%)

Mechanical

(beating) 100 - 200 - 0.8 - 1.1 391 - 713 18 - 55 -

Mechanical

(rolling mill) 100-600

220 –

270 - 270 - -

Mechanical

(grinding) - - 1.4 450-800 18 - 30 -

Mechanical

(pin roller)

262 ± 160 - - 420 ± 170 38 ± 16 9.8

Steam

explosion - - - 516 17 -

Steam

explosion 366 ± 74 ~250 1.3 726 33 2 ± 0.6

Steam

explosion 15 - 210 - - 441 220 36 13 1.3

Steam

explosion 195 ± 150 - - 308 ± 185 26 ± 14 2.5

Manually

stripped 300-900 “Long” - 106-203 - -

Chemical 90- 400 HC:

200-350 - 0.9 341 19 1.7

Chemical 270 10 1.3 450 18 -

Chemical 230 - - 395 ± 155 26 ± 14 2.8

Chemical +

compression

50-400

HC: 150-250 >10 0.8 - 0.9 645 - -

Chemical +

roller mill HC: 50-100 120-170 - 370 - -

No mentioned 150-350 - - 200 - 800 - -

40 Chapter 3 b

culm of 3 years old (769 MPa). The total average strength was found to be 643 ± 12

MPa (100 mm gauge length). The tensile apparent Young’s modulus did not present

significant differences across the height of the culm with an average value of 27 GPa.

Nevertheless, this result contrasts with the study of Bangarshetti et al. [140], who

found a significant variation in strength properties of bamboo along the length and

also across the cross section.

Figure 3-14. Both a) tensile strength and b) tensile modulus properties of bamboo (G. angustifolia) fibres

depending on the age of the culm and their position in the wall thickness [139]. * From the outside towards

the inner side.

In general, the fibre extraction process and further fibre manipulation significantly

determine the fibre quality and mechanical properties [8, 30, 141]. An adequate

procedure results in a much better and easier extraction of the fibres from the plant,

and therefore, minimizes the mechanical loading and subsequently damage to the

fibres. Mechanical overstraining of the fibres during the extraction process can result

in the formation of defects [8]. Different methods for extracting bamboo fibres are

explained in the next paragraphs.

3.6 Extraction of bamboo technical fibre

It is technically difficult to obtain large bamboo technical fibre quantities with a

consistently uniform shape (i.e. long fibres). Tung et al. [142] reported that it is very

difficult to extract very thin, but long fibre bundles without causing serious damage

to them by the processing. Little effort has been devoted to the extraction of long

bamboo fibres because of the difficulty of extracting good quality (technical)

bamboo fibres from the culm. As a consequence, there is a limited availability of the

fibre and only a few research groups have investigated this material as a potential

UD reinforcement in polymer matrix composites [13, 34, 81, 134, 136, 143, 144].


To practically apply the benefits of bamboo fibres, it would be necessary to develop

a process to extract long high quality fibres from bamboo plants. This extraction

process might be carried out in a standardized way, and should be able to compete

with existing methods for other natural fibres in terms of production volume, cost,

environmental impact (e.g. waste water and chemicals) and uniform consistency in

fibre properties. Nowadays, there are several extraction methods used for extracting

bamboo fibres such as: steam explosion, chemical extraction, retting, mechanical

methods using hammer mills, toothed rollers, compression (crushing), shearing, etc.

Table 3-11 shows various extraction processes and the obtained fibre characteristics.

From this comparison, it is clear that the fibre separation process determines the

morphology and mechanical properties of the extracted fibres. Figure 3-15 shows

bamboo fibres used in different studies as reinforcement in polymeric matrices.

Figure 3-15. Bamboo fibres used in other studies extracted by several methods. Chemical extraction (alkali

treatment): a) [145]*, b) [146]*, c) [147]*, d) 1% concentration and 70° for 10 hours [137]. Mechanical

extraction: e) milled bamboo [144], f) [148]*, g) pin rollers [137]. Steam explosion plus mechanical process:

h) 100 min 170°C, 0.8 MPa, 8 times [13], i) 175 °C and 0.8 MPa for 60 minutes. The process war repeated 9

times [137]. *Details about the process are not specified.

b. a. c.

d. e. f.

g. h. i.

42 Chapter 3 b

3.6.1 Steam explosion

This technique is a very common method to extract natural fibres in general (e.g.

flax [149] and hemp [150] fibres). It has been known as an effective method to

separate the lignin from the woody materials and is also applied in the pulp industry.

A schematic view of the process is shown in Figure 3-16. A typical procedure to

obtain the fibres consists first in removing both the nodes and the skin from the culm

followed by the cutting of the internodes in strips. Then, the bamboo strips are

placed into an autoclave with over-heated steam at 175°C and 0.8 MPa for around

60 minutes and suddenly depressurizing the autoclave to atmospheric pressure. This

operation causes micro steam explosions inside the parenchyma, facilitating the

extraction of the fibres of a few hundreds of microns diameter. The process needs to

be repeated several times (9-12 cycles) in order to obtain fine technical fibres [142].

Figure 3-16. Schematic process for the extraction of bamboo fibres by steam explosion.

It has been stated that after using this extraction process, a further mechanical

process is necessary to improve the quality of the bamboo fibres [133]. The need of

this additional step, was also reported by Okubo et al. [13], because the technical

fibres were not effectively separated and a large amount of impurities and

parenchyma remained on the surface, see Figure 3-17a. Therefore an additional

mechanical process consisting in rubbing the technical fibres was applied for further

separation, see Figure 3-17b. Ashimori et al. [20] used steam explosion repeating the

cycle 10 times. They also complemented the extraction with a mechanical process,

where the bamboo strips were scratched to refine the fibres, in order to reach

diameters between 125 and 425 µm. A second attempt from the same author with

the aim to obtain finer bamboo fibres, consisted in freezing the steamed-exploded

fibres and then crush them mechanically. They found that freezing fibres did not

cause splitting in fibre bundles and it is not very effective method to reduce the fibre

diameter, even at cryogenic temperatures.


.

Figure 3-17. SEM micrographs of a) bamboo fibres extracted by steam explosion and b), same fibres after

additional mechanical process (rubbing) in order to clean the fibre. After this process the fibres appeared as a

“cotton” fibres obtaining diameters between 10 – 30 µm [13].

A fibre quality comparison between steam exploded and two types of mechanical

extraction (i.e. crushing and shearing) methods for bamboo fibres was made by

Ogawa et al. [144]. They founded a great difficulty in controlling the diameter,

length and aspect ratio of the bamboo fibres by those three methods. The crushing

method (Figure 3-18a) had a lower efficiency due to the necessity of a screening

process (by sifting) to obtain uniform diameter. In steamed fibres (Figure 3-18b), the

heat may damage the fibres, decreasing the mechanical properties. A reduction in

strength of about 20% due to the heat exposure during the steam explosion

technique was reported by Okubo et al [25]. Finally, fibre dimensions obtained by

the shearing (rubbing) method varied widely (Figure 3-18c). According to Phong et

al. [137], the steam-explosion technique offered lower moisture contents than

untreated fibres at all relative humidity levels (i.e 50, 60, 70, 80 and 90% RH). This

can be attributed to the chemical changes on the fibre during the thermal treatment.

Liu et al. [151] compared the fibre diameter distribution of bamboo (D.

membranaceus) of two batches of fibres obtained by steam explosion and

mechanical extraction (rolling mill). In both cases, before the extraction, the bamboo

strips were treated with alkali to facilitate the extraction process of the fibres. In

general, under visual inspection, the steamed fibres had a dark color and rough

surfaces. In contrast, the ones extracted mechanically presented brighter color and

smoother surfaces. The fibre diameter distribution of these fibres is shown in Figure

3-19. In this study the mechanical process gives a normal distribution with a peak

value between 120 and 140 μm. On the other side, no regularity in fibre diameter

distribution can be found for the steam exploded fibres, giving higher diameters.

Elementary fibre Soft tissue

a. b.

44 Chapter 3 b

Figure 3-18. Bamboo fibre characteristics after three different extraction processes [144].

Figure 3-19. Fibre diameter distribution of bamboo (D. membranaceus) fibres extracted by mechanical

extraction and steam explosion techniques. A picture of mechanically extracted fibres (above) and steam

exploded fibres (down side) can be seen. The fibre diameter determination was based on the average apparent

density of the fibres, and individual fibre mass and length [151].

3.6.2 Mechanical extraction

Different mechanical extraction processes for bamboo fibres were explored by

Deshpande et al. [136]. They applied a compression technique (CT) and a roller mill

technique (RM) in combination with alkali treatment (0.1 N NaOH for 72 h) in order


to extract bamboo fibres. The CT method, consisting in compressing the strips with

a constant load of 10 tons during 10 s, yielded fibres with larger diameters and larger

deviations compared with fibres from RM. The average tensile strength and its

correspondent standard deviation were higher with fibres obtained from CT.

Additionally, they found that bamboo fibres with larger diameters had a higher

strength compared to fibres with lower diameters. This behaviour is different from

what is generally observed in natural fibres and must be attributed to increased fibre

damage upon extracting finer fibres.

Ogawa et al. [144] obtained short bamboo fibres by end-milling and found that the

fibre shape can be controlled by adjusting the end-mill parameters and using a spiral

path (see Figure 3-20). As mentioned before in section 3.6, other methods include

beating, shearing, rolling, gridding and the use of tooth rollers and pin rollers to

extract the fibres [81, 133, 134, 137]. More details about the fibre characteristics and

mechanical properties obtained by the mechanical extraction technique are shown in

Table 3-11.

Figure 3-20. End-mill technique to extract short bamboo fibres [144].

3.6.3 Chemical extraction

Typically in chemical extraction, bamboo culm pieces are soaked in a chemical

solution (e.g. NaOH) with the purpose of dissolving the soft tissue that binds the

technical fibres. Rao et al. [132] examined a chemical process for extracting bamboo

fibres combined with retting. The results of the investigation show that the fibres

after chemical extraction show a large number of major defects along the length.

In literature, the further industrial adoption of this method has been found

contradictory. Some drawbacks have been pointed out by Prasad et al. [56]. On the

one hand, it is an expensive process that will not be able to compete with the

46 Chapter 3 b

existing ones (e.g. mechanical and steam explosion methods); and on the other hand,

it was found that after treatment, the chemical nature of the fibres can change and

this might not be desired.

From another point of view, Phong et al. [137] found that in comparison with steam

explosion and mechanical extraction techniques, the chemical technique has some

advantages such as cheaper equipment, higher aspect ratio of the fibre, relative low

energy consumption and better control of fibre properties. They treated bamboo

strips of around 2-3 mm thickness, in 1, 2 and 3% NaOH solution at 70°C during 10

hours and obtaining the higher technical fibre mechanical properties at 1%

concentration. Comparisons between chemical extraction, mechanical extraction and

steam explosion are shown in Table 3-12.

Table 3-12. Comparison of different extraction methods for bamboo fibres [137].

3.7 Bamboo fibre composites

Bamboo fibres are suitable to be used as reinforcement in polymeric composites in

order to manufacture light and strong composites due to their excellent mechanical

properties and they could be appropriate to be used for structural and semi-structural

composites applications [67]. Several studies on bamboo fibre reinforced composites,

mainly with random technical fibre configuration, have been published using

thermoset [14, 42, 81, 136], thermoplastic [13, 78, 133-135, 152, 153] and

biodegradable [78, 80, 100, 135, 154, 155] matrices. A summary of these studies is

shown in Tables 3-13 and 3-14, discriminated by tensile and flexural tests properties.

Nevertheless, the results of these studies are in general rather poor and suggest that

the intrinsic good properties of the bamboo technical fibres were not being fully

transferred to the composite. Apparently improvements in fibre quality and better

interface fibre-matrix material are still required.

Method Cost of

equipment

Energy

consumption

Installing

area

Environ-

mental

impact

Fibre

l/d

Fibre

diameter

(µm)

Control of

fibre

property

Steam

explosion Expensive High Large No High 195 ± 150 Good

Mechanical

Reasonably

expensive

Relatively

low

Small No Low - Acceptable

Chemical

(alkali

treatment)

Cheap

(excluding

waste water

treatment)

Medium Very large Yes High 230 ± 180 Good


Table 3-13. Overview of the tensile mechanical properties of bamboo technical fibre composites. BF*:bamboo technical fibres. ** Lysine-based diisocyanate.

System

Fibre characteristics Fibre

extraction

method

Treatment

applied

Vf

(%)

Before treatment After treatment

Ref. F. diam

(µm)

F. length

(mm)

Tensile

strength

(MPa)

Tensile

modulus

(GPa)

Tensile

strength

(MPa)

Tensile

modulus

(GPa)

UD

Unsat. polyester + BF*

Hand-lay-up

90 - 400 - Retting + mechanical - 40 126 2.5 - - [42]

UD

Araldite matrix + BF*

Hand-lay-up

100-

600 220-270

Milling machine +

razor blade -

43

37

25

425

381

320

20.3 - - [81]

Random

Starch-based resin + BF

Hot press

200 25 Steam

explosion

Alkali

treatment 50 45 - 60 -

[154]

Random

PP + BF*, Injection - 0.2-0.5

Grinding bamboo chips

MAPP 20 20 2.1 22 2.1 [134]

Random

PP + BF*, Hot press 150 6-12

Grinding bamboo chips MAPP 30 15 2.5 17 2.6 [153]

Random

PP + BF*, Hot press

125-

210 -

Steam

explosion MAPP 41 - - 30 3.6

[13]

Random

PP + BF*, Hot press

10-

30 -

Steam explosion +

mechanical MAPP 41 - - 35 4.7 [133]

Random

PLA + BF*, Comp.mould 70 0,5 Not mentioned LDI** 30 29 2.6 46 3.9 [80]

Random

PBS+ BF*, Comp.mould

70 0,5 Not mentioned LDI** 30 21 23 38 25 [80]

Random

MAPP + BF*, Comp. mould 90- 125 1 - 6

NaOH (20%) +

pressure, 1 h.

MAPP 50 29 1.7 50 1.5 [78]

48 Chapter 3 b

Table 3-14. Overview of the flexural mechanical properties of bamboo technical fibre composites.

BF*:bamboo technical fibres.

Glass/bamboo fibre hybrid composite has been also developed by Thwe et al. [134,

153] and Dieu et al. [156]. They conclude that hybridisation could enhance the

durability of natural fibre composite under environmental aging and also, that it is

possible to include 25% of bamboo fibres in weight without sacrificing the

mechanical properties, while the density is reduced about 13%.

3.8 Application of natural fibre composites

Natural fibres have been used nowadays as substitution of synthetic fibres in several

applications and different sectors. Moreover, they exhibit a favourable nonbrittle

System

Fibre

characteristics Fibre

extraction

method

Vf

(%)

Bending properties

Ref. F.

diam.

(µm)

F.

length

(mm)

Flexural

Strength

(MPa)

Flexural

Modulus

(GPa)

UD

Polyester + BF*

Hand lay-up

15

250

400

-

NaOH (0,1 N)

during 72 h +

Compression

15

15

20

144

162

100

8.6

8.0

1.2

[136]

UD

Polyester +BF*

Hand lay-up

150

250 -

NaOH (0,1 N)

during 72 h +

Roller mill

15

15

158

143

3,5

3,2 [136]

UD

Unsat. polyester + BF*

Hand-lay-up

900 -

400 -

Retting +

mechanical 40 128 3.7 [42]

Orthogonal plies

PP + BF*

Compression moulding

-

10-

70

Chemical

treatment

(Alkali)

no specified

50

- 2.3 [135]

Random

PP + BF*


90 -

125 1 - 6

NaOH (20%)

under pressure

during 1 h

50 49 2.8 [78]

Random

MAPP + BF*


90 -

125 1 - 6

NaOH (20%)

under pressure

during 1 h

50 68 4.1 [78]

Random

PLA + BF*


- 1.1

Steam explosion

+ mech. Process

50 107 - [155]

Random

Biodegrad. Resin (CP-

300) + BF*


-

10-

70

Chemical

treatment

(Alkali)

no specified

50

60 -

4.3

5.1 [135]


fracture on impact, which is an important requirement in some applications (e.g.

passenger compartments) [53], good material damping [157], good specific

properties and an advantegeous “green image”. For an overview of different

applications of natural fibres, see Figure 3-21. According to the fibre length, their

use can range from high-end products such as surf boards, fishing rods, bicycle

frames to short fibre reinforcement in different applications [158]. Current

applications for natural-fibre-reinforced composites mainly focus on the

construction and automotive industries, although there are a number of niche

applications where low volumes of material are used. Figure 3-22 shows some

commercial applications of natural fibre components in automotive components,

part of bicycle frames and sports goods.

Figure 3-21. Different applications for natural fibres according to the fibre length [158].

The automotive industry is now looking for “green” composites. The two most

important factors now driving the use of natural fibres by the European automotive

industry, are cost and weight; with the consequent improvements in fuel efficiency.

Some of the produced parts are interior panels, bumpers, spoilers and body parts

[158]. DaimerCrysler already commercialised the first flax fibre reinforced polyester

composites in engine and drive train covers in buses and car passengers satisfying

the cost, weight and durability (after 50.000 km of road tests) requirements [50].

50 Chapter 3 b

Figure 3-22. Applications in natural fibre components in automotive parts (source: http://www.ircomas.org),

and recent application of natural fibres in bicycle frames and tennis rackets for increased material damping

(source: http://www.lineo.eu/).

3.9 Conclusions

There is in general an increasing interest in the use of natural fibres in several

sectors for new high-end applications driven by cost, good mechanical properties,

“green” image and governmental environmental regulations.

Bamboo technical fibres can be an attractive alternative to other natural fibres and

glass fibres in composite applications. Bamboo fibre can be used for a variety of

structural and semi-structural applications due to its good specific properties,

renewability, fast growing and other environmental benefits, which include a large

CO2 capture and low energy consumption per kg of fibres. Moreover they offer a

continuous supply along the year. These advantages are becoming very important

selection criteria for the materials of the future.

Despite showing a good potential, long high quality bamboo technical fibres have

not been used as a unidirectional reinforcement in polymeric composites materials

due to the non availability of the fibres. Unidirectional distribution of the fibres is

the most suitable configuration for high end composite applications in order to

obtain the maximum efficiency of the technical fibres into the composite, according

to the load conditions in each particular case.

Mechanical extraction methods (e.g. rubbing, crushing and compression) are used as

complementary operations for steam explosion or chemical extraction and vice versa.

Each of these extraction methods has their own advantages and disadvantages in

http://www.ircomas.org/


terms of cost, energy consumption environmental pollution and fibre quality. “Pure”

mechanical extraction is only found when obtaining short bamboo fibres (e.g.

milling, grinding). To practically apply the benefit of bamboo fibres, it is necessary

to develop a process to extract long high quality fibres from the bamboo culm in

industrial scale, with low environmental impact (e.g. low water consumption, no use

of chemicals or high temperature) and low cost (e.g. in line process).

Several limitations have been found for the use of the natural fibres in composite

materials, such as the variability in mechanical properties, thermal degradation

during composite manufacturing and their hydrophilic behaviour. Furthermore,

moisture absorption causes swelling of the fibre and thus reduces the interface

between fibres and composites. Deep understanding of each of these phenomena is

needed in order to minimize their impact in the mechanical properties. By doing so,

it will be possible to establish the correspondent limits, and the conditions that need

to be taken into account for an appropriate characterization manufacturing and use

of the natural fibre composites. When these limitations are understood, the use of

natural fibres will widen and will be better accepted by various high-performance

composites industry.

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Bamboo technical fibre characterization 59

Chapter 4 Bamboo technical fibre characterization

_______________________________________________________

4.1 Introduction

The cylindrical shape of the bamboo culm (see Figure 4-1) limits its direct use in

several engineering applications [1]. A more flexible alternative is the extraction of

the bamboo fibres from the culm, to be used as reinforcement of polymeric matrices.

Figure 4-1. Bamboo (Guadua angustifolia) culm in the plantation.

The industrial adoption of natural resources for reinforcing composites is an active

subject of research. The acceptance of natural fibre reinforced plastics in technical

applications depends on the availability of material data and specific design

information. Establishing reliable properties of the fibres to be safely used in final

product applications, requires extensive testing and a substantial amount of research

[2]. In fact, the variability in mechanical properties of natural fibres is one of the

60 Chapter 4b

main concerns for their industrial applicability [3-5]. In comparison with synthetic

fibres, natural fibers have a significantly higher variation in diameter between the

fibres and within a fibre [4-6]. Nevertheless, this variation in fibre properties can not

only be characterized and predicted when quality management is used [7], but also

controlled when this information serves as a feedback for optimizing the extraction

and further fibre preparation.

In spite of their benefits, studies on bamboo fibre reinforced plastics are relatively

scarce because fibres are not readily available [8-10]. A step forward in the

extraction of high quality, long bamboo fibres has been achieved in earlier work at

KU Leuven. The complete characterization of the fibre strength is the subject of this

chapter, with the purpose to explore the possibility to use bamboo fibres as

reinforcement in polymeric matrices. Two methods were used to estimate the fibre

properties; the single technical fibre (SF) test and the dry fibre bundle (DFB) test.

For the first method, geometrical and strength data were obtained for a large

population of single bamboo technical fibres. The fibres were individually tested in

tension using various gauge lengths for further statistical analysis. To determine the

strength distribution of the fibres, the fibre data were analyzed using the modified

Weibull model [11]. To use the model it must be determined if the defect density is

to be presented as function of the fibre length, fibre volume or fibre surface area to

obtain the best fitting. The within-fibre diameter variation parameter λ was also

determined. These results provide a practical, quantitative predictive model for the

strength of the currently studied bamboo technical fibres. On the other hand,

mechanical properties and Weibull parameters were also obtained from the DFB test

in order to estimate the fibre properties of a larger population of bamboo technical

fibres and to be compared with the SF measurements.

The tests results should provide a better understanding of the intrinsic properties of

the material and a more accurate appreciation of its potential. The generated data can

further be used for the design and modelling of bamboo fibre composites, in order to

fulfil the correspondent safety demands for end products. Also, the presented

methodology is sufficiently generic to be applied to other natural fibres.

4.1.1 The modified Weibull distribution

The Weibull model [12] is a widely used statistical approach for describing the

tensile strength of brittle materials such as carbon and glass fibre [13-15]. As shown

in Table 4-1, it was also applied to a wide range of natural fibres such as jute, hemp,

sisal, flax, coconut and bamboo. Two assumptions that underlie the theory are that


the material is brittle and that the strength is governed by the most serious flaw [16,

17]. The brittleness assumption is satisfied if the material has elastic behaviour up to

failure. The weakest link theory can be applied to a technical bamboo fibre by

assuming the fibre as a chain consisting of several segments with certain strength.

These segments can be regarded not only as a concatenation of elementary fibres,

where the fibre ends are considered as “weak points”, but also, as the minimum

length or volume where any type of defect can be found. Strength distribution of

technical natural fibres is usually described by means of a two parameter Weibull

distribution [18]. To improve the accuracy of the prediction, a modified Weibull

model had been introduced by the implementation of a third parameter [11, 19]:

m

V

VP

00

exp1

where P is the probability of failure of a fibre of volume V at a stress less than or

equal to σ. The parameters of the distribution are the scale parameter σ0, the shape

parameter m and the volume sensitivity β. The scale parameter σ0 is a measure of the

characteristic strength of the fibre. It corresponds to the fracture stress with a failure

probability of 63.2% of a fibre with reference volume V0. The shape parameter m

mainly defines the variability of the distribution. The higher m, the more narrow and

right-skewed the distribution, and thus the more consistent the quality of the fibres.

For natural fibres this value ranges between 1 and 6 and synthetic fibres usually

have shape factors between 5 and 15 [20, 21]. Andersons et al. [14] found a shape

factor between 5.1 and 5.4 for glass fibres. The strength variability was caused by

the inherent flaw distribution along the fibre and by the fibre-to-fibre strength

variability within a batch. This was found mainly due to variations in the production

process and the damage introduced during the handling of the fibres.

The parameter β is a measure of the sensitivity of the strength with respect to the

tested volume V. It is an empirical parameter that was introduced to improve the

predictive power of the Weibull model with respect to experimental data [11, 19].

The implementation of this parameter was found in several studies [6, 11, 20, 22]

with positive results. If β=1, the conventional Weibull distribution is obtained, in

which each part of volume V contributes equally to the failure probability. Choosing

β between 1 and 0 weakens the volume-dependency: the lower β, the lower the

decrease of strength for increasing volume V. The volume dependency entirely

disappears if β=0 [23]. A final parameter is the reference volume V0. V0 is

independent of the other parameters and therefore it has to be selected arbitrarily.

Some studies based the selection on the dimensions of an elementary fibre [21, 24].

More commonly, V0 is chosen to correspond to a standard unit. The latter convention

is also adopted in this work, which uses a reference volume V0 of 1 mm3.

(4-1)

62 Chapter 4b

Fibre Extraction

method

Fibre cross

sectional area

determination

Loading

rate

(mm/min)

Average

diameter

(µm)

Gauge

length

(mm)

Shape

parameter

(m)

Scale

parameter

σo (MPa)

Average

strength

(MPa)

Scale

variable

Weibull

estimation

Ref.

Flax

Enzyme

retted -

manual

Digital images 0.5 19 5

5

3.0

3.5

3041

253

1597

1489

L

V MLE

* [4]

Flax Green Optical

microscope 1

-

-

-

5

8

10

4.2

4.0

3.3

1459

1359

1356

-

-

-

L LR**

[17]

Flax Dew retted Optical

microscope -

-

-

-

5

8

10

3.6

3.3

2.2

1168

1093

1543

-

-

-

L LR**

[17]

Flax

Enzyme

retted,

manual

selection

Optical

microscope 0.5

-

-

-

5

10

20

SW: 5.2

MW: 2.8

SW: 1430

MW: 1400

960

870

740

L MLE

*

[25]

Bamboo Alkali

Optical

microscope

0.8 140 10 2.7 412 367 L

LR**

[26]

Bamboo

Manually

stripped

off

Profile projector 1.3

350

550

850

80

80

80

2.9

4.0

2.7

228

159

119

203

144

106

-

Weib.

analys.

program

[1]

Bamboo Steam

explosion

By weight and

density 0.1, 1.5

-

-

15

35

9.3

3.5

855

683

813

639 V

LR**

[21]

Brown

coir - By weight and

density 0.1, 1.5

-

-

15

35

9.3

3.7

360

206

343

186

V LR**

[21]


Table 4-1. Weibull distribution parameters for some natural and synthetic fibres. *MLE: maximum likelihood method. **LR: linear regression method. L: length.

V: volume. SW: standard Weibull distribution. MW: modified Weibull distribution.

Coir Retting Optical

microscope 20 250

1

8

35

50

6.3

3.0

4.5

4.9

-

-

-

-

428

384

211

162

L LR**

[27]

Jute

Retting

and further

alkali

treatment

Digital

microscope -

44

54

47

48

5

10

15

20

2.2

1.4

1.3

1.2

436

415

410

377

384

365

363

340

L LR**

[28]

Hemp

Manual

Extraction

Optical

measurement

0.01

40

10

2.7

-

285

L

LR**

[29]

Hemp Water

retted

Optical

microscope 0.5

-

-

1.5

10

3.4

4.2

876

745

786

677 L

Weib.

analys.

program

[20]

Sisal

Mech.

process

(decorticati

ons)

SEM 0.1 200

10

20

30

40

4.6

3.7

3.6

3.0

-

-

-

-

391

392

385

400

- MLE

*

[30]

Agave

Americ.

Traditional

method

Image analysis

software

20

3.1

7

5.0

205

126

L

LR**

[31]

E-glass -

Provided by the

manufacturer 1.5 23

20

40

80

5.5

5.5

5.1

3810

3880

4440

-

-

-

-

L MLE

*

[14]

Carbon

(T700S)

- Provided by the

manufacturer 1 20

20

40

4.0

2.9

6300

5600

5700

5000 L LR [32]

Carbon

(G34-

700)

- Provided by the

manufacturer 1 20

20

40

5.1

6.1

4000

3600

3600

3400 L LR

64 Chapter 4b

From Equation 4-1 the average strength (σv), the standard deviation (σsd) and the

modus (Ms), the value with higher repetition into the data, can be calculated

respectively with Equations 4-2 to 4-4 [21]. Γ corresponds to the gamma function.

00

11

m

v

V

V m

22

0

21

vsd m

1

0

1 mm

s

m

mVM

According to Equation 4-1, for a constant fibre diameter the average strength (σ2) at

certain volume (V2), can be estimated from a known strength (σ1) at its

corresponding volume (V1) and shape parameter (m) as follows [11]:

2

2 1

1

mVV

This equation has been useful for predicting fibre strength [22], especially in

analyses of the micromechanical fibre fragmentation test, commonly used to

evaluate the interfacial shear strength at extremely short fibre lengths [33].

4.1.2 Effect of defect density distribution

Equation 4-1 assumes that the defect density is homogeneously distributed over the

volume of the material. Weibull [12] noted, however, that other defect density

distributions are also plausible. In the fibre literature (see Table 4-1) it is commonly

assumed that the defect distribution is only function of the length of the fibre. The

resulting modified Weibull distribution is identical to Equation 4-1 except that

volumes are substituted by lengths (L):

m

L

LP

00

exp1

The derived statistics (Equations 4-2 to 4-5) can similarly be adapted by substituting

volume by length. While some studies use as reference length L0 the length of an

elementary fibre [21, 24], this study uses 1 mm, as it is more practical.

(4-6)

(4-5)

(4-4)

(4-2)

(4-3)


For fibres with different diameters, the volume-based model and the length-based

model lead to different strength predictions, as is illustrated in Figure 4-2. The figure

divides fibres into segments with equal failure probability for length-based defect

densities (left) and volume-based defect densities (right). If fibres have the same

length, a length-based defect density predicts a failure probability that is

independent of the diameter, while a volume-based defect density gives rise to more

defects for thicker fibres and thus to a higher probability of failure. If the fibres have

the same volume, wider fibres are also shorter, leading for the length-based model to

less defects and thus to a higher strength.

Figure 4-2. Probability of failure for a single (technical) bamboo fibre under tension when it is

considered as a concatenation of small pieces taking into account the a) length (L) and b) volume

(V) as a chain link.

The defect distribution within bamboo fibres can be affected by their morphology

and by the extraction process. The length and diameter of elementary fibres are

homogeneously distributed within the fibre, and the elementary fibre ends are also

distributed homogeneously (see section 4.5.3.2). Since the fibre ends form “weak

links” between the elementary fibres, this fibre architecture suggests a volume-based

defect distribution [21, 24, 34]. The extraction process, however, may add defects

that are distributed differently. For example, certain compression, rubbing or

combing-based extraction steps may systematically introduce damage every certain

distance along the fibre length, thus favouring a length-based defect distribution. In

combing processes involving needles, thicker fibres may have a higher probability

LL

0

a) Considering:

Sa

me

le

ngth

an

d

diffe

ren

t vo

lum

e

Sa

me

vo

lum

e a

nd

diffe

ren

t le

ng

th

VV

0

b) Considering:

66 Chapter 4b

of needle-attack and undergo higher tensile stresses during the combing process.

Some chemical processes may introduce damage on the fibre surface. The latter two

processes thus favour yet another defect distribution: a surface-based one. Given that

the bamboo fibre diameter can range between 150 and 250 µm for the same batch

[35], it is thus meaningful to consider how defects are distributed within the fibres

and to relate this to the fibre extraction method.

4.1.3 Effect of within-fibre diameter variation

Some studies [6, 36] argue that the reduction in mechanical properties in wool fibres

with increasing fibre length is not only caused by the accumulation of defects along

the fibre but also by diameter variations along the length of a fibre. Therefore,

Zhang et al. [6] and Xia et al. [28] incorporate in their studies on respectively wool

and jute fibres a within-fibre diameter variation parameter λ that replaces β in

Equation 4-6 and obtaining:

0 0

1 exp

m

LP

L

Where λ is the slope of the line obtained when plotting the logarithm of the

coefficient of variation of the fibre diameter (CVFD) versus the logarithm of the

measured fibre length. This parameter represents the exponential parameter of the

change of the within-fibre diameter variation over the fibre length. Zhang et al. [6]

obtained the CVFD after measuring the fibre diameter variation of a number of fibres

every 40 μm intervals along the fiber length (10, 20, 50 and 100 mm). The

parameter λ is positive, which reflects that there is an increase of the within-fibre

diameter variation (CVFD) with increased fibre length. This parameter must not be

confused with β, mentioned in section 4.1.1 and which is determined by curve fitting

using equation 4-6. λ is an empirical parameter that was introduced to improve the

predictions of the Weibull model with respect to the fibre strength at different gauge

lengths.

4.2 Dry fibre bundle test

An alternative for the estimation of the Weibull parameters and the evaluation of the

mechanical properties of the technical fibres, is the dry fibre bundle (DFB) test [37,

38]. This method is based on the study of the rupture of a bundle (cluster) of fibres,

assuming spatial random distribution of failure of the fibres. The methodology for

the dry fibre bundle test was initially developed by Daniels [39] and Coleman [40]

(4-7)


and later on applied in several studies, particularly on synthetic fibres as for example

carbon [38], E-glass [41-43], Kevlar® [44] and ceramic filaments [45].

According to the mentioned studies, the DFB test helps to overcome some of the

drawbacks found when testing individual (synthetic) fibres. Some of these

disadvantages are a tedious selection of very small diameter fibres from a tow

inducing damage to the samples, preparation and gluing samples onto a paper frame

as well as the large amount of required samples to obtain reliable results

(normally >30 is recommended) [38, 44]. All this preparation becomes very time-

consuming for the SF testing. For these reasons, the DFB test has gained increased

support in recent years, since the statistics concerning the fibre strength are more

conveniently obtained using fibre bundles and are more relevant to the situation

which might prevail in the finished fibre reinforced composite material [44].

To the best knowledge of the author, Weibull parameters and mechanical properties

for natural fibres have not been reported in literature using DFB testing methods,

even though the SF test is known to be a time consuming preparation technique.

4.2.1 Theoretical background

For the implementation of the DFB test several conditions have to be taken into

account [38, 41, 44]. The strength distribution of a technical single bamboo fibre

under tension is assumed to follow the two-parameter Weibull distribution; also that

the relationship between the applied stress (σf) and the strain (ε) for single fibres

follows Hooke’s law up to fracture as shown in Equation 4-8.

From equations 4-6 and 4-8 it is possible to obtain Equation 4-9.

0( ) 1 exp ( / )mP L

Where P(ε) is the failure probability of a single fibre under strain no greater than ε.

ε0 is the scale parameter of the Weibull distribution for the strain. ε0 can be obtained

from Equation 4-10 [44]:

1/

max

m

o Lm

(4-8)

(4-9)

(4-10)

f fE

68 Chapter 4b

εmax corresponds to the maximum strain at maximum load (Fmax) reached by the

bundle. At an applied strain, the number of surviving fibres (N) in a bundle which

initially consist of No fibres is shown in Equation 4-11 [38]. N can be related to the

applied load (F) on the bundle by Equation 4-12.

01 ( ) exp ( / )m

o oN N P N L

This latest expression (Equation 4-12) is the load-strain (F-ε) relationship for a

bundle of fibres under tension, where A is the average cross-sectional area of a

single fibre and Ef is the Young's modulus of the fibre. The Weibull shape parameter

(m) can be obtained via two methods; the first one [38, 44] is through the maximum

load using Equation 4-13, where So corresponds to the initial slope of the load-strain

(F-ε) curve.

1

max maxln( / )om S F

The second method to determine m consists in applying a graphical method with the

F-ε curve as is shown schematically in Figure 4-3 using So, and S* [37]. This last

value (S*) corresponds to the slope of the straight line defined by the origin of the F-

ε curve connected to the maximum load. Then, the Weibull modulus is obtained

with Equation 4-14. This method gave similar values for the m parameter in

comparison with those obtained by other studies using the maximum load method

described above [44]. The maximum load method (Equation 4-13) will be preferred

in case the maximum strain is not clear to determine from the graphic method.

Figure 4-3. Graphic method to determine the fibre scale parameter (m) from the load-strain (F-ε) curve [37].

(4-13)

1/ ln( / )om S S

(4-11)

(4-12) 0( ) exp ( / )m

o fF AN AN E L

1/ ln( / )om S S (4-14)

F(N)

ε(%)

εmax


The second Weibull scale parameter (σo), can be obtained from equation 4-15 as

follows [38, 41]:

1/

max / m

o m

The bundle strength (σb) corresponds to the Fmax divided by the area of the individual

fibres multiplied by the total number of fibres present in the bundle (Equation 4-16).

In this study, the bundle strength does not represent the individual technical fibre

strength (σf). Coleman [40] established his theory on the basis of Weibull theory and

Daniels’ theory [39]. The relationship between the strength of the fibre bundle and

strength of the individual fibers is indicated in Equation 4-17 [46], where e is the

base of the natural logarithm. Finally, the fibre Young’s modulus can be obtained

from Equation 4-18 [44].

max /b oF N A

/f o oE S AN

4.3 Materials

Bamboo culms (Guadua angustifolia) were collected from a typical bamboo

plantation in Colombia, specifically from the Coffee Region, at 1.300 meters above

sea level, annual average temperature of 23 °C, annual average precipitation of

2.200 mm and relative humidity of 80% according to the environmental authorities

of the region. Technical fibres were extracted by the author from the bamboo culms

using a proprietary purely mechanical extraction process that neither uses chemicals

nor high temperature. The maximum length of the extracted fibres was the internode

length, which for 48 month culms is reported to be between 20 and 35 cm [47].

(4-17)

(4-16)

(4-15)

(4-18)

1

( ) (1 1/ )mf b me m

70 Chapter 4b

4.4. Methods

4.4.1 Single fibre test

4.4.1.1 Measurement of the cross sectional area

A first important source of uncertainties when obtaining the strength properties of

the fibres is the methodology for determining the cross sectional area, due to the non

circular cross-section, that can lead to a large difference in the reported tensile

strength measurements [48]. For this reason, special care was taken for the

measurement of the cross sectional area of the individual technical bamboo fibres.

This included not only strict standard conditioning of the fibres before

measurements, but also the comparison of two different techniques implemented for

15 randomly selected technical fibres, to verify the accuracy of the results. The first

method (Aw) comprises of weighing the fibres and then dividing the weight by the

fibre length and apparent density (1.4 gr/cm3

[35]), obtained by a gas pycnometer

Beckman 930 at ~3 mm fibre length and following the methodology described by

Tran L. [49]. The diameter can be estimated assuming that the fibres have a constant

cross-section and circular shape.

The second method (AS), is considered more accurate, consisting in the direct

measurement of the cross sectional area with image analysis of the tested fibres. For

this, each fibre is collected at the end of the tensile test and cut carefully near the

breaking point. The fibre is then vertically encapsulated in soft resin, polished and

imaged by Scanning Electron Microscopy (SEM XL30 FEG). The images are

processed using the image analysis software Digimizer 3.0 to measure the cross-

sectional area. The area of all other fibers, used in single fibre test (420 fibres), was

only measured using the first method (Aw).

4.4.1.2 Measurement of the fibre perimeter

The variation in perimeter was determined along the length of 12 randomly selected

individual technical bamboo fibres. For this, wetting measurements in n-Hexane

were performed with a Krüss K100SF tensiometer using the Wilhelmy technique

according to the methodology described by [50]. The perimeter measurement was

done every 100 µm along the technical bamboo fibre length of 20 mm at a speed of

1 mm/min with a detection speed of 3 mm/min (approaching speed) and a detection

sensitivity of 1µg. With the obtained data, the cumulative coefficient of variation of

the fibre perimeter (CVFp) was calculated for 5 different lengths along the fibre (3.8,

7.8, 11.6, 15.5 and 20 mm), see a schematic representation in Figure 4-4. In


principle this technique allows measuring the fibre perimeter continuously along the

fibre with a reasonable accuracy of less than 8% relative error [51]. From the

perimeter, the fibre diameter can be estimated, assuming circular shape.

As mentioned before in section 4.1.3, according to Zhang et al. [6], the slope of the

line obtained when plotting the logarithm of the coefficient of variation of the fibre

diameter (CVFD) versus the logarithm of the measured fibre length, corresponds to

the within-fibre diameter variation parameter (λ) along the technical fibre length.

Figure 4-4. Schematic representation for the calculation of the CVFP at 5 different fibre lengths (for a total

length of 20 mm). The measurements were performed on 12 randomly selected bamboo technical fibres using

the Wilhelmy technique. The arrows represent the technical fibre perimeter measurement at this point.

4.4.1.3 Tensile test set up

Before tensile testing, randomly selected fibres were visually inspected. This

selection consisted in making a manual loop with the technical fibre to verify the

absence of major damage along the length of the technical fibre, see Figure 4-5. This

fibre selection was carried out with the purpose of testing only technical fibres in

good condition without pre-damage due to external reasons such as packaging,

100 µm

Perimeter 1

Perimeter 2

Technical fibre #1

Technical fibre #2

Average CVFP1

Average CVFP2

Perimeter n

Total measured distance = 20 mm

Perimeter n+1

Technical fibre # n

Length 1

Length 2

Length n

Average CVFPn

72 Chapter 4b

transportation or manipulation of the fibres after the extraction. The good condition

of the fibres will assure more reliable results. The fibres considered suitable to be

tested were pre-conditioned at 21°C ± 2°C and 50 ± 2 %RH for at least 24 hours

before the experiment in order to assure they reached a moisture equilibrium.

The single fibre tensile test was carried out using a mini-Instron 5943 under standard

environmental conditions (21°C ± 2°C and 50 ± 2 %RH). The fibres were

pneumatically clamped at 5 bar of pressure using rubber-faced clamps of 10 x 30

mm. These clamps allowed reducing fibre manipulations and avoiding the use of a

paper frame. A vertical visual reference was used during placement of the samples

on the grips in order to avoid fibre misalignment. The latter can lead to bending

stresses at the grips, and thus causing premature failure. The crosshead speed was

0.85 mm/min with a 1kN load cell. The load was registered during the entire test.

Fibres of gauge length 1, 2, 5, 10, 20, 30 and 40 mm were tested. The number of

samples and average fibre diameter per batch are given in Table 4-3 in the results

section.

Figure 4-5. Visual inspection (applying manual loop) of the bamboo technical fibres before tensile testing, a)

technical fibre suitable to be tested and b), a rejected technical fibre due to major damage (indicated by the

arrows).

The strain rates on the fibres are different for each span length considering that the

displacement rate used in this study was always constant (0.85 mm/min). The strain

rate varied between 0.00035 s-1

and 0.014 s-1

for 1 mm and 40 mm fibre gauge

lengths respectively. These values can be considered to be sufficiently low in order

to expect no significant variations in the fibre strength due to the differences of the

strain rate, especially for a brittle bamboo technical fibre.

a. b.


4.4.1.4 Statistical calculations

All statistical calculations were performed using MATLAB 2009b (The MathWorks

Inc., Natick, Massachusetts, US). The statistical inferences assume a confidence

level of 95% (α= 0.05).

4.4.1.5 Scanning electron microscopy (SEM) observations

Micrographs of fibres and composites were made by scanning electron microscopy

(SEM30 XL FEG). The samples were sputter coated with gold for further

observations using secondary electrons using a voltage between 10 and 15 kV.

4.4.2 Dry fibre bundle test

4.4.2.1 Reference methodologies

Through scientific literature regarding the dry fibre bundle (DFB) testing, no

international standard or norm was found to perform the DFB test applied to

synthetic fibres. This is even though the methodology described in section 4.2.1, has

been developed and implemented by several authors exclusively for this type of test.

The bundle characteristics and testing parameters for glass and carbon fibres ranged

from one study to another in terms of the number of filaments in the roving

(between 194 [41] and 4000 [42]), the gauge length (between 30 and 100 mm [38,

41-44]) and the displacement rate (between 0.02 [37] and 0.05 [41] mm/min). It

might be considered that these DFB characteristics differ significantly in comparison

with the DFB samples made of bamboo fibres. These differences are related to the

amount of fibres in the samples and the regular “perfect” cross sectional area found

in individual synthetic filaments, instead of an irregular geometry and the larger

diameter of technical bamboo fibres.

For natural fibres, standard test methods for wool (ASTM D1294 [52] and D2524

[53]) and cotton fibres (ASTM D1445 [54]) were found. These ASTM standards

specify the methodology to calculate the bundle tensile strength (σb), the Young’s

modulus (Ef) and the strain at maximum load (εmax). The basic differences between

the procedures employed in method D1294 and D2524 are in the gauge lengths

(25.4 and 3.2 mm respectively) and the methods for clamping the bundle sample. In

test method D2524, specific clamps are required whereas in D1294, conventional

clamps may be used. The ASTM D1445 standard also uses 3.2 mm of gauge length

(flat bundle) using a standard clamping system. Nevertheless, the preparation

methodology for the DFB samples in these standards was found to be poor.

74 Chapter 4b

The technique to uniformly load the fibres consists in aligning them by hand and to

stretch the fibres by holding them with the fingers and keeping them together with

masking tape. In the next section, the importance of an appropriate DFB preparation

in order to obtain reliable data will be explained.

4.4.2.2 Preparation of the bamboo DFB samples

The first aspect to be taken into account during the DFB preparation is the alignment

of the fibres in order to avoid a non-uniform loading. The theoretical background

shown before in section 4.2.1 is valid only if assuming that the fibres in the dry

bundle, undergo the same strain when loading (see Equation 4-11 and 4-12) among

the N surviving fibres before breakage. An inadequate alignment causes an uneven

distribution of the load throughout the fibres in the bundle during tensile testing

infringing the theoretical assumption and leading to a premature failure of the

bundle and thus to a sub-estimation of the fibre parameters. The influence of a non-

uniform loading on the bundle is shown schematically in Figure 4-6a. It might be

also noted that an accurate measurement of the Young’s modulus (Ef) requires that

the fibres in the bundle are perfectly aligned and uniformly strained [37].

Figure 4-6. a) Schematic sequential representation of a non-uniform loading during the DFB test due to the

misalignment of the fibres in the sample. b) effect of misalignment in the grips during the tensile testing [55].

For the preparation of the samples, the bamboo fibres where carefully stretched,

aligned and evenly spread by hand reaching an average areal density of 232 ± 24

g/m2 and thickness of 0.47 ± 0.03 mm as shown in Figure 4-7a. The bundles were

glued with cyanoacrylate glue at the ends between aluminium foils (griping zone), to

load as uniformly as possible all the fibres during the tensile test. The distance

between these aluminium foils corresponded to the desired gauge length. The tested

flat bundle configuration (see Figure 4-7b), allows an appropriate and uniform

holding of the bundle to be used with conventional tensile test grips, without a

special set-up to perform the tensile test In this study, the influence of the gauge

a. b.


length and the amount of fibres in the bundle was evaluated. For that, four types of

bamboo DFB samples were prepared varying the gauge length (40 and 100 mm) and

the width (10 and 250 mm), as presented in Table 4-2.

Figure 4-7. a) Front and b) lateral view of the bamboo dry fibre bundle.

Table 4-2. Geometry and general characteristics of the bamboo DFB test specimens. *Calculated from the

correspondent fibre length, the total mass of the fibres present in the bundle (after testing, the fibres were

carefully cut corresponding to the gauge length and weighed), and the average fibre cross sectional area

(calculated independently from the 420 samples in SF test).

Type of bamboo dry fibre bundle

Properties

Gauge length (mm) 100 40 100 40

Width (mm) 25 ± 0.5 25 ± 0.7 10 ± 0.6 10 ± 0.6

Average thickness (mm) 0.44 ± 0.10 0.51 ± 0.10 0.49 ± 0.10 0.43 ± 0.10

Average areal density (g/m2) 215 ± 25 256 ± 23 223 ± 22 236 ± 26

Average number of fibres* 195 ± 15 211 ± 17 64 ± 15 76 ± 13

Number of successfully tested samples 6 5 8 8

a. b.

TkL TkS ThL ThS

76 Chapter 4b

4.4.2.3 Testing of the bamboo DFB samples

The accurate measurement of the load-strain (F-ε) curve is one of the most

important aspects for a reliable evaluation of the fibre Weibull parameters and the

mechanical properties for individual technical bamboo fibres through DFB testing.

This condition states the necessity of measuring the true strain during the DFB

tensile test, which is generally calculated using the displacement of the cross-head

measurement of the testing machine [37]. In this research, an optical extensometer

was used to obtain the true strain values of the bamboo DFB at different places on

the sample. This was achieved using correlation of digital images taken of the

sample during loading (4 images per second) by the Vic2D software (LIMESS

Messtechnik und Software GmbH). To achieve better image recognition by the

software, the samples were first painted with black random speckles to give a unique

pattern to the surface, following the methodology described by [56]. For optimal

results, it is necessary to obtain an adequate speckle pattern where it must have a

considerable quantity of black speckles with different shapes and sizes. Such

patterns can then be identified by the strain mapping software program, see Figure

4-8.

Figure 4-8. a) Set up for the DFB tensile test including the camera for the optical extensometer in order to

register the deformation of the sample and b), speckles pattern on the sample.

As the tensile test proceeds, the system takes subsequent images and analyses them

in comparison to either the initial image or the previous image. The correlation

between the images allows the determination of local displacement and strain values.

The difference of using the displacement of the grips and the optical extensometer to

determine the bundle strain (ε), for the same bamboo DFB sample, is clearly shown

in Figure 4-9. The measured gauge length was 50 mm for TkL and ThL specimens,

and 20 mm for TkS and ThS samples. An Instron 4467 testing machine with a load

a.

b.

Dry fibre bundle

Camera

Speckles on the bamboo dry fibre bundle surface

5 mm


cell of 30 kN was used to perform the tensile test with a crosshead displacement rate

of 0.3 mm/s with a correspondent strain rate of 0.0075 s-1

and 0.003 s-1

for the short

(40 mm) and long (100 mm) DFB samples, respectively.

Figure 4-9. Comparison of the load-strain (F-ε) curves for the same DFB demo sample (TkS), using optical

extensometer and the displacement of the grips for the determination of the strain (ε).

Figure 4-10. Comparison of the strain measurements with an optical extensometer in different zones (strains 1,

2 and 3) of the DFB sample during the tensile test to verify the homogeneous deformation of the bundle to

obtain reliable results.

Using optical extensometer

Using the displacement from the grips

DFB sample

Grip

Strain 1

Strain 2

Strain 3

Grip

Strain 1 Strain 2 Strain 3

78 Chapter 4b

To avoid misalignment of the bundle sample in the grips which could cause a

premature failure of the fibres in the bundle, as is shown in Figure 4-6b, a visual

reference served as a guide to assure proper positioning of the DFB samples in the

grips. To verify the uniform loading of the DFB during the tensile test, the sample

strain was measured in at least three different zones as is shown in Figure 4-10.

These measurements should be close to each other to assure reliable results.

4.5 Results and discussion

4.5.1 Fibre extraction

An innovative method for extraction of long technical bamboo fibres was co-invented

by the author and a prototype is currently operating at pilot scale. This machine was

designed with mechanical principles that can be scaled-up for future large volume

industrial production. The technology has been implemented for Colombian bamboo

(Guadua angustifolia), but it is equally applicable to other bamboo species. This

novel fibre extraction technique presents several advantages compared to the other

methods described in section 3.7.1 (e.g. steam explosion, chemical and mechanical

extraction), such as:

Production of long bamboo technical fibres. The length of the extracted fibres is

the internode length, which for 48 month old culms is reported to be between

20 and 35 cm [47].

During the extraction process the use of high temperature, high pressure or

chemicals is not needed, reducing both the damage introduced to the fibres and

the amount of energy required for the extraction of bamboo fibres. These

aspects make this new process a more environmentally friendly method

It is a standardized in-line process that avoids storage or batches during the

process and assures a continuous production. See also Chapter 7 for more

details about the and technical and environmental assessments of the extraction

method.

A second operation required to remove the soft tissue (parenchyma) present on the

fibre surface of the extracted technical fibres (see Figure 4-11), is the cleaning

process. For this procedure, a proven lab-scale prototype machine, which can also be

scaled up for industrial purposes, was developed and constructed at KU Leuven.

Figure 4-12 shows cleaned bamboo technical fibres after the extraction and cleaning-

combing process. The extracted technical fibres described in this section, were used in


this study not only for SF and DFB characterization, but also for the production of

composites with thermoset and thermoplastic matrices (Chapters 5 and 6).

Elementary fibres for Guadua angustifolia have an average length of 2.1 mm and an

average diameter of 17 µm [57]. The single technical fibre can be regarded as a

concatenation of elementary fibres whose pentagonal or hexagonal cross-sections

are arranged in a honeycomb pattern, see Figure 4-13. The mechanical properties of

the mechanically extracted fibres as well as some fibre morphology aspects will be

discussed in the next sections.

Figure 4-11. a) Mechanically extracted technical bamboo fibre with soft tissue (parenchyma) and b), bamboo

technical fibre surface after cleaning.

Figure 4-12. Bamboo technical fibres after mechanical extraction (maximum length between 20 and 35 cm).

a.

b.

a.

b.

80 Chapter 4b

Figure 4-13. Technical bamboo fibre composed of elementary fibres.

4.5.2 Fibre cross-sectional area and perimeter

The cross sectional area of 15 fibres was determined using two methods. The first

one (Aw), was based on the weight of individual fibres and the apparent average

density of bamboo fibres. The second one (AS), was based on analysis of SEM

images (Figure 4-14a). The average relative error between both methods was 3.8 ±

1.5%. For the area determination for all tested fibres (420 specimens), only method

Aw was used because of its practical simplicity. Fibre diameters were found to range

from 100 to 220 µm, as shown in Figure 4-14b. The fibre diameter distribution of all

tested fibres is shown in Figure 4-15 (diameter obtained from area by assuming

circular shape), also distinguishing between a thick and a thin fraction. The

coefficient of variation was 20%. Figure 4-16 presents a typical perimeter variation

profile along the length of a single technical bamboo fibre using the Wilhelmy

method. The measured fibres with this technique (12 samples) show a low perimeter

variation with an average of 0.68 mm ± 0.1 and a average within perimeter variation

along the measured length (20 mm) of 0.078 mm.

20 µm


Figure 4-14. a) SEM image of a bamboo fibre cross-section with contour perimeter line and b) different fibre

diameters found in this batch of fibres.

Figure 4-15. Fibre diameter distribution for all single bamboo technical samples indicating two halves of the

fibre population (Nt=420) (diameter obtained from cross-sectional area by assuming circular shape).

Figure 4-16. Typical variation of fibre perimeter along the length of a technical bamboo fibre, measured

every 100 µm using the Wilhelmy technique.

a. b.

Nt= 210

Nu

mb

er

of

fib

res

Fibre diameter range (µm)

82 Chapter 4b

4.5.3 Mechanical properties of bamboo technical fibres

4.5.3.1 Dependency of fibre strength on fibre length

For each gauge length, individual fibre strength values are plotted in Figure 4-17 and

average results are shown in Table 4-3. These data show a decrease in fibre strength

with increasing gauge length. ANOVA revealed statistically significant strength

differences between the groups with gauge length 1 and 2 mm, 5 and 10 mm and 20,

30 and 40 mm. The fibres with gauge length 1 mm have not only a higher strength,

but also a lower variance. Because the gauge length is shorter than the length of an

elementary fibre (2.1 mm), it is likely that entire elementary fibres are clamped

between top and bottom. This is illustrated in Figure 4-18. In this case the crack has

to run through the cell wall breaking the elementary fibres (Figure 4-18a), instead of

causing debonding between them (Figure 4-18a). As a consequence, the strength

increases and a different fracture type can occur, especially at very short gauge

lengths (i.e. 1 mm). The same may also occur for gauge length 2 mm, but to a much

lesser extent.

Table 4-3. Fibre diameter and strength for bamboo fibres tested at different gauge lengths. *Each group is

divided in ‘thick’ fibres, having diameter above the median, and ‘thin’ fibres for strength comparisons. **Two

sided t-test (for p≥0.05, there is no significant difference). ***Linear regression between fibre diameter and

strength.

For larger gauge lengths, the number of elementary fibre ends increases. The

lamellar interphase that bonds the elementary fibres is typically weaker than the

elementary fibre. The interphase, which is rich in lignin [51] may become the critical

site through which fibre failure takes place in tension, as has been observed for flax

fibres [58]. The larger the gauge length, the larger the number of critical sites, and

thus the higher is the probability of failure. Recall that critical flaws can also have

other origins such as imperfections developed during growth and defects introduced

during extraction [27, 59].

Tested

gauge

length

(mm)

Average

fibre

diameter

(µm)

N°

samples

Average

strength

all fibres

(MPa)

*Average

strength

“thin” fibres

(MPa)

*Average

strength

“thick”

fibres*

(MPa)

**p value (t-

test) for “thin”

and “thick”

fibres (α=0.05)

***R2

(linear

regression)

1 132 ± 33 37 943 ± 94 942 ± 100 945 ± 90 0.935 0.006

2 137 ± 36 34 898 ± 124 960 ± 118 837 ± 100 0.002 0.180

5 161 ± 27 50 833 ± 113 802 ± 120 863 ± 98 0.055 0.026

10 156 ± 27 43 821 ± 125 839 ± 143 802 ± 101 0.338 0.023

20 164 ± 32 54 754 ± 72 738 ± 74 770 ± 66 0.099 0.001

30 112 ± 8 159 733 ± 121 718 ± 131 749 ± 109 0.117 0.038

40 156 ± 23 43 748 ± 115 787 ± 111 707 ± 107 0.021 0.055

All 146 ± 26 420 790 ± 132 790 ± 144 791 ± 118 0.949 0.007


Figure 4-17. Bamboo fibre strength at different gauge lengths and their correspondent PDF’s (Probability

distribution functions).

Figure 4-18. Schematic view of a single technical bamboo fibre tested at two different gauge lengths with a

typical type of fracture depending of the gauge length. For a) the crack has to run through the cell walls

breaking the elementary fibres and b), the crack runs through the lignin layer surrounding the elementary

fibres due to a weaker interphase.

a. b.

84 Chapter 4b

It was possible to carry out this Weibull analysis without fractioning the analysis in

“short” (i.e. 1 and 2 mm) and “long” (>2 mm) gauge lengths, with no concern about

the critical discontinuities at short lengths. The technical bamboo fibres are

composed by hundreds of elementary fibres, for this reason, even at short gauge

lengths, the typical discontinuities of the elementary fibres at certain points will be

bridged by many other elementary fibres placed around, as seen in Figures 3-11 and

4-13. This situation contrasts, for example, with the flax fibre, where the technical

fibres are assembled only by 10–40 elementary fibres of 33 mm average length [60].

In this case, the use of Weibull statistics at short gauge lengths and the effect of

these discontinuities need to be verified first to ensure reliable results.

To obtain more information about the origin and mechanisms of fibre failure, SEM

micrographs were made of the post-mortem fibre fracture surface. As shown in

Figure 4-19, mainly three types of fracture were distinguished: (a) a straight crack

right through the elementary fibres, (b) a crack proceeding along the primary wall

layer that surrounds the elementary fibres and (c), a combination of the two. For

fracture type (a) and (b), observed also by [58], the stress-strain curve is linear until

failure. For fracture type (c) the stress-strain curve showed, for a number of samples,

small drops in the load during the test before reaching ultimate failure. It was

analyzed whether there was a correlation between type of failure, fibre diameter and

gauge length. The latter two parameters appeared not to influence the occurrence of

the failure types except for very short fibre gauge lengths.

Figure 4-19. Fracture micrographs showing a) clear transversal fracture (b) crack propagation through the

primary layer of the elementary fibres and (c) a combination of the two cases for bamboo fibres after single

fibre tensile test.

4.5.3.2 Dependency of fibre strength on fibre diameter, fibre volume and fibre

surface

Figure 4-20 shows the strength of individual technical fibres versus their

corresponding fibre diameter for all samples included in this study. Table 4-3 reports

the average fibre diameter for all fibres and for each set of different gauge length.


ANOVA (α=0.05) revealed no statistically significant differences in strength as

function of fibre diameter, even when evaluated at different gauge lengths. In

addition, the fibres were divided into a group of ‘thin’ fibres and a group of ‘thick’

fibres, based on whether the fibre diameter was respectively lower or higher than the

median fibre diameter. If the fibre diameter would influence the strength, then a

systematic difference in strength should be found for each gauge length. Table 4-3

shows only a statistically significant difference, according to the Student’s t-test, for

the group with gauge lengths 2 and 40 mm. When all ‘thin’ and ‘thick’ fibres

(Figure 4-20) are compared together, almost no difference in strength is found. The

‘thick’ fibres do, however, have a lower variance than the thin fibres. When the

largest group (30 mm) is further divided into four groups with increasing diameter,

ANOVA does not show a significant difference between these groups. These results

indicate that, notwithstanding a factor 3 difference between the lowest and the

highest tested fibre diameter, there is hardly any effect of the average fibre diameter

on strength.

Figure 4-20. Single technical fibre strength versus fibre diameter where the median for fibre strength and fibre

diameter are shown.

For the length-based model, predicted average strength and modus, Figure 4-17

presents the probability density functions and 95% confidence intervals for the

experimentally obtained data. For the surface-based and volume-based models,

Figure 4-21 plots the predicted average strength, modus and 95% confidence

intervals. It seems in both cases a similar trend as for fibre length is followed. As no

effect of fibre diameter was found, the effects of fibre surface area and fibre volume

on fibre strength must be attributed to the effect of fibre length, as both area and

volume are linearly related to length.

86 Chapter 4b

Figure 4-21. Predictions of fibre strength for all fibres as function of a) fibre surface area and b) fibre volume.

4.5.3.3 Estimation of the modified Weibull parameters

The modified Weibull distribution was used to quantify the strength of bamboo

fibres. It must first be established whether the defect density distribution should be

described as a function of fibre volume (Equation 4-1), length (Equation 4-6) or

fibre surface area. Subsequently three parameters must be estimated. These

parameters were estimated for each of the three defect density distributions based on

the entire dataset using maximum likelihood estimation (MLE). The obtained

parameter values are shown in Table 4-4.

Scale

variable

Scale

parameter

σ0

(MPa)

Shape

parameter

m

Sensitivity

parameter β

Length

982 ± 29 7.6 ± 0.5 0.48 ± 0.09

Surface area 761 ± 39 6.7 ± 0.5 0.93 ± 0.44

Volume

637 ± 85 6.7 ± 0.5 0.46 ± 0.22

Table 4-4. Maximum likelihood estimates of the modified Weibull parameters for all fibres, assuming that the

defect density distribution is function of fibre length, fibre surface area or fibre volume. The reported

variations are 95% confidence intervals.

It can be seen that the parameter values are physically acceptable for all three defect

density distributions. The 95% confidence intervals on the parameters are overall the

smallest for the length-based model. This indicates that the length-based model

yields the most precise prediction. The wider confidence intervals for volume and

surface area also support the finding in the previous section that the fibre diameter is

poorly correlated with strength. For the length-based model, the predicted average


strength, modus, probability density functions and 95% confidence intervals (for

experimental points) were shown in Figure 4-17.

It was noticed earlier that for gauge length L=1 mm, the failure behaviour may be

positively biased, because it is shorter than the elementary fibre length. To

investigate this influence, the MLE analysis was repeated for gauge lengths L ≥ 2

mm. For the three models, the results were similar, with the lowest differences

occurring for the length-based model (σ0= 987 ± 42 MPa, m=7.5 ± 0.6, β=0.49 ±

0.11). From this insensitivity it can be concluded that the length-based model

reproduces the strength increase towards lower gauge lengths. It is thus a practical

predictor of bamboo fibre strength over the entire range of measured gauge lengths.

Figure 4-22. Comparison between experimental strength and predictions of the modified and non-modified

Weibull model. Predictions are based on the results of all fibres. Error bars 95% confidence intervals on the

average strength.

The modified Weibull parameters obtained for the length-based model are

physically meaningful. The scale parameter σ0 of 982 MPa is somewhat higher than

the average strength for gauge length L= 1 mm, as it corresponds to a failure

probability of 63.2% at that gauge length. The shape parameter m of 7.6 is relatively

high in comparison with other natural fibres and some glass fibres. This indicates

that the bamboo fibres obtained by the current extraction process have relatively low

strength variability. The sensitivity exponent β of 0.48 ± 0.09 indicates that bamboo

fibre strength decreases significantly less with length than predicted by Weibull’s

original model, which has β=1. As shown in Figure 4-22, when using Weibull’s

original model (σ0=1148 and m=8.6, β=1) it had a stronger length-dependency than

Mod. Weibull (length-based model, Eq. 4-6)

Weibull (original length-based model)

Experimental data

88 Chapter 4b

the experimental data and the modified Weibull length-based model. It also

mispredicted the average strength for short (≤5 mm) and long (≥30 mm) gauge

lengths. The modified Weibull distribution predicts more accurately the

experimental values in comparison to the original model. Repeating the Weibull

fibre strength predictions taking into account fibre volume yielded no improvement.

A potential explanation in terms of within-fibre diameter variations is further

explored in the following section. The Weibull parameters calculated by linear

regression (LR) are shown in Appendix 1.

4.5.3.4 Correlation with within-fibre diameter variations

Variations in cross-sectional area along the length of a fibre induce variations in

stress that may lead to a different overall fibre strength compared to a constant fibre

cross-section with the same average area. As a measure of the within-fibre diameter

variation with gauge length, the parameter λ was shown in section 4.1.3. This

parameter is graphically determined as the slope of the plot of the logarithm of the

coefficient of variation of the fibre diameter CVFD versus the logarithm of the

measured length along the fibre. Because the fibre perimeter was more readily

measured than the fibre diameter, this study uses fibre perimeter. The obtained

coefficient of variation of the fibre perimeter CVFP is closely related to the CVFD,

since perimeter and diameter are proportional if the cross-sectional shape does not

vary.

Figure 4-23. Average CVFP along the fibre length.

Figure 4-23 shows that CVFP increased with fibre length and the exponent λ=0.47

(R2= 90%). This value was very close to parameter β (0.48) obtained earlier,

although this may be coincidental, as there is to our knowledge no direct rigorous


relation between both parameters. In literature it was found that for wool [6] and jute

[28] λ was found to be 0.18 and 0.33 respectively. According to these studies, an

improved strength prediction of the original Weibull’s model is found if the fibre

diameter variation λ is taken into account. In this study however, the difference

between β and λ was very small (0.48 vs 0.47), and hence the suggestion from

literature to use λ could not be confirmed.

4.5.3.5 Benchmarking of SF mechanical properties

A correlation between the mechanical properties (i.e. fibre strength and Young’s

modulus) and various corresponding extraction processes can be seen in Figure 4-

24. In this figure, starting at the lowest values for the strength, one finds the

manually stripped fibres and the mechanical extraction carried out with a rolling mill

machine. For the first method, even though the fibres do not experience any

extraction, the low strength can be explained because the fibres are not completely

cleaned. Very often the cross sectional area of the fibres is overestimated due to

impurities and soft tissue attached to the surface that do not contribute in carrying

the load and give an underestimation of the mechanical properties. For the second

case, milling clearly shows to be an aggressive extraction process that severely

damages the technical fibre with an additional disadvantage of producing relatively

short fibres (<5 mm) [61].

The next group concerns chemical extraction processes giving strength values

ranging from 340 to 450 MPa and a Young’s modulus close to 19 GPa. With these

methods, in some cases, it is necessary to add a mechanical operation for removing

the soft tissue and impurities still present on the fibres after the chemical treatment

for the final refinement of the fibres. Steam explosion gives a higher modulus than

the previous methods, with a rather constant value of 35 GPa with a few exceptions

and with a strength ranging from a moderate value of around 500 MPa to a good

value of 720 MPa reported by Defoirdt et al. [21]. Some mechanical processes,

using different principles (e.g. grinding) in combination with an additional mild

chemical treatment, reach strength values of 625 MPa and a Young’s modulus of 36

GPa at best. The mechanical extraction process carried out in this study introduced

little damage to the fibres and preserved the intrinsic good properties of the fibres as

is shown in the upper right corner in Figure 4-22. Figure 4-25 shows the variability

of technical bamboo fibre strength and Young’s modulus reported in literature

represented by a “box area” whose sides represent the values for the standard

deviation of the data. For the case when the authors did not report the standard

deviation, a single point in the graph indicated the average values of their results.

90 Chapter 4b

Figure 4-24. Overview of extraction processes and corresponding fibre mechanical properties; the strength

values for the currently studied bamboo technical fibres were determined at 5 mm gauge length (see Table 4-3)

[21, 62-69].

Figure 4-25. Strength vs Young’s modulus from some studies on technical bamboo fibres [8, 21, 62-65, 67, 70]

*Mechanical properties measured at 5 mm gauge length (see Table 4-3). As a reference, the E-glass fibre has

an effective tensile strength of around 1500 MPa and a Young’s modulus of 73 GPa.

Fib

re s

tren

gth

(M

Pa)

Y

ou

ng’s m

od

ulu

s (GP

a)

This study*


The results show that the bamboo fibres extracted in this research with the author’s

proprietary mechanical method, have high values of strength and modulus, being at

the top right of the graph. The specific properties of the bamboo fibres (normalized

to the material’s density) characterized in this study are 595 kN∙m/kg and 31x103

kN∙m/kg for the strength and Young’s modulus respectively. These values are

comparable to the specific properties of E-glass, one of the most used synthetic

fibres used nowadays to reinforce polymer composites. This means that bamboo

fibres can potentially replace this synthetic fibre in several applications.

4.5.4 Dry bamboo fibre bundle

A typical load-strain (F-ε) curve for a TkL (Thick-Long) sample is shown in Figure

4-26 in comparison with a theoretical curve obtained with Equation 4-12. In general,

the experimental curves exhibited a regular increase of the force until a well defined

maximum force value indicated by point A. This characteristic is the main indication

of an appropriate failure of the bundles.

Figure 4-26. Comparison of the (typical) experimental and theoretical load – strain (F-ε) curves for a TkL

sample. The parameters to obtain the theoretical curve using Equation 4-12 were taken from Table 4-5.

Moreover, the experimental curve showed a deviation from the initial linear

behaviour at a certain load value, as seen at point B in Figure 4-26. This behaviour

was also observed in similar studies concerning DFB tests on carbon [37], E-glass

and Kevlar [44]. A gradual change in the slope but with a continuous curve (without

slacks) is an indication that the fibres effectively fail in an individual and random

way [37, 44]. Figure 4-26 shows that the Weibull parameters (i.e. m and εo) and the

A

B

C

92 Chapter 4b

test conditions, especially the alignment of the fibres and the determination of the

true deformation of the bundle, were correctly chosen or determined. This is proved

by the good correspondence between the experimental and theoretical curves for

most cases. It must be noted that failures of technical fibres near the grips were

rarely observed, which confirms the absence of stress concentrations on the bundle

due to the clamping system. Figure 4-27 shows a bamboo DFB specimen after

failure.

Figure 4-27. Bamboo DFB specimen after failure a) TkS and b) TkL sample.

During the tensile test, after the maximum load point (Fmax), see Figure 4-26 point A,

the fibres abruptly moved due to multiple fibre fractures occurring at the same time

as can be seen in Figure 4-28. These alterations made it difficult to continue tracking

the real displacement (ε) of the bundle by the camera, and generated some erroneous

data (noise) as shown in Figure 4-26 point C. This aspect did not affect the

calculation of the data because this deviation occurred beyond Fmax. The results of

the DFB test including bundle and technical fibre strength and Weibull parameters

for single technical fibres are shown in Table 4-5.

Figure 4-28. Tensile test sequence on bamboo DFB test at different times (TkS sample). Some of the visible

damages, which normally include movement of the broken fibres, are shown with the arrows.

a. b.


According to the equation, (4-17), it can be seen that the fibre strength only depends

on the Weibull shape parameter m. For this reason it is possible to determine this

property (σf) derived from the DFB test, without the necessity to perform single fibre

test to find the Weibull parameters (e.g. the shape parameter m). For comparisons,

the fibre strength was calculated for each type of bundle using two m values: a) the

corresponding m value calculated from the DFB test according to the sample type

and b), using the average m value from the single fibre test calculated at different

gauge lengths (m=7.6). Table 4-5 shows that the fibre strengths calculated with those

two criterions are different. The first case gives around 20% and 15% higher average

values for the long (100 mm) and short gauge lengths (40 mm) respectively. As the

m parameter calculated form selected fibres is normally higher, indicating low

strength variability, it can be understood that it does not represent the non selected

batch of fibres present in the DFB sample. For this reason, the m value calculated

from the DFB test will be preferred for the technical fibre strength calculation, as is

more representative for the conditions of the experiment.

Table 4-5. Dry fibre bundle strength and properties of individual technical bamboo fibres derived from the dry

fibre bundle test. aEquation 4-16,

bEquation 4-17 using the corresponding m value obtained from the DFB test

according to the case, cEquation 4-17 using the average m value obtained from the SF test at different gauge

lengths (7.6), dEquation 4-18,

eEquation 4-14 (graphic method),

fEquation 4-13 (maximum load),

gEquation 4-

15, hEquation 4-10.

Type of bamboo dry fibre bundle

Properties

Bundle strength [MPa] (σb)a 223 ± 38 335 ± 38 262 ± 34 336 ± 49

Fibre strength [MPa] (σf)b

379 ± 41 536 ± 43 448 ± 56 539 ± 78

Fibre strength [MPa] (σf)c

312 ± 22 468 ± 32 366 ± 38 470 ± 32

Fibre Young’s modulus [GPa] (Ef)d

38 ± 4 45 ± 6 37 ± 5 37 ± 5

Shape parameter (m)e

3.6 ± 0.9 4.4 ± 1.3 3.5 ± 1 4.4 ± 1.1

Shape parameter (m)f

3.7 ± 1.2 4.6 ± 1.1 3.6 ± 1 4.7 ± 0.9

Scale parameter [MPa] (σo)g

319 ± 38 480 ± 37 375 ± 49 482 ± 65

Shape parameter for the strain

[mm/mm *10-3

] (εo)h

8.5 11.1 8.7 11.2

TkL TkS ThL ThS

94 Chapter 4b

The fibre strength (σf) calculated from the DFB test was consistently lower (~30 %)

than the values obtained in single fibre tests at the same gauge length (see Figure 4-

29). It was the same case when comparing the shape parameter m. This was an

expected result taking into account that the strength variation of the technical fibre is

averaged out in the DFB test, obtaining smaller standard deviations. In addition, no

pre-selection of the fibres was done, in contrast to what was done for the SF test.

This means that a bigger amount of fibres was tested from a specific batch and in

their original conditions, giving more “realistic” strength values. The Young’s

modulus of the technical fibre was easily determined from the DFB test with a few

samples for each case. The average value for all bundle geometries was 39 ± 4 GPa,

showing a good correspondence with the value of 43 ± 2 GPa calculated from single

fibre tests after the machine compliance correction applied by Osorio et al [35] for

the same type of bamboo technical fibres.

Figure 4-29. Fibre strength comparison between values obtained by single technical bamboo fibre (SF) test

and dry fibre bundle (DFB) test at different gauge lengths. The SF strength reported at 100 mm gauge length

was extrapolated from the experimental results at 40 mm gauge length using Equation 4-5 with β=0.48 and

using the fibre length (L) instead of the fibre volume (V). The fibre strength from the DFB test corresponds to

the average value of the ThL and ThS shown in Table 4-5.

A drop in strength properties was also reported in literature when comparing results

from DFB and single fibre properties [44]. This can be explained mainly by two

reasons. A cooperative fibre failure denominated as “doublets” or “multiplets”,

where one fibre failure can trigger the next one, has been reported in literature when

testing synthetic DFB specimens [44]. This phenomenon can reduce the bundle

strength (σb), and hence affect the calculation of the Weibull parameters and the

Values from the SF test

Values from the DFB test

40 mm gauge length

100 mm gauge length


strength properties of the individual fibres. Nevertheless, in the present study, this

effect was expected to be much less significant due to the significantly lower fibre

density present in the dry bundle samples in comparison with a roving of synthetic

fibres. Also, because of the reduced neighbouring fibres associated with the flat

bundle geometry instead of the traditional round DFB shape.

On the other hand, as was reported by Coleman [40], the ratio of the tensile strength

of a bundle to the mean tensile strength of the constituent filaments decreases in case

of an increasing strength dispersion of the constituent filaments. According to the

same author, the tensile strength of a dry bundle has the same order of magnitude,

but is less than the mean strength of the fibres. For this reason, it cannot be expected

that the value of the bundle strength (σb) gives reliable values for the individual

technical fibre strength (σf). This last value needs to be determined using the fibre

Weibull parameters combined to Equation 4-17.

4.6 Conclusions

In this chapter, mechanically extracted bamboo single technical fibres were

characterized with two different approaches for a wider overview of the mechanical

properties of this fibre. The first method consisted of SF (single fibre) testing carried

out to evaluate the effect of defects introduced by the extraction process as function

of different scale variables: fibre length, fibre surface area and fibre volume

applying the Weibull defect distribution analysis. It was found that Weibull’s

original model (β = 1) does not predict accurately the fibre strength distribution of

the technical fibres leading to an overestimation of the fibre strength at short gauge

lengths. The necessity to incorporate a length sensitivity exponent (β = 0.48) giving

as a result the modified Weibull distribution, indicates that bamboo fibre strength

decreases significantly less with length than predicted by Weibull’s original model.

The same distribution (length-based model) gives a more accurate prediction of the

strength distribution, and it can also be used for predicting fibre strength at different

gauge lengths. For this reason the length (L) is the recommended scale parameter to

be used for the Weibull strength distribution. The average technical fibre strength

was found to decrease with increasing gauge length, from 943 MPa at L=1 mm to

733 MPa at L=40 mm. Furthermore, the fibre strength was found to be nearly

independent of the mean fibre diameter. Diameter variations within the fibre may be

better correlated with strength, but further investigation is required to establish this

relation. Finally, the Weibull shape parameter (m) was found to be 7.6 for all tested

fibres (after visual selection), which means that the strength distribution is narrower

(having lower scatter) than that of most natural fibres and some synthetic fibres.

96 Chapter 4b

These results confirm that the quality of the bamboo fibres produced by the current

mechanical extraction method is high.

The second method for the technical bamboo fibre characterization was the dry fibre

bundle (DFB) test. The developed methodology is generic enough to be used for

other natural fibres but certain conditions need to be fulfilled such as:

- The straightness of the technical fibres in the DFB;

- The clamping of all technical fibres present in the sample;

- The accurate mounting of the specimen relative to the vertical axis of the

universal machine, in order to avoid misalignment in the grips and consequently

inhomogeneous loading during the tensile test;

- The accurate measurement of the strain during the test.

This procedure allowed to characterize a bundle of fibres under tension, avoiding

intensive preparation of individual samples for the SF test. The results from this

experiment gave systematically lower strength in comparison with SF tests due to

the fact that no particular selection of the fibres was carried out. Also, less scatter is

obtained and the results are calculated from the average behaviour of a large number

of fibres, which is statistically more representative. In general, this procedure

maximized the reliability of the results with minimal preparation time and amount of

material. Nevertheless, the preparation of the samples (i.e alignment of the fibres)

and the methodology for the testing (i.e accurate determination of the bundle strain)

play a critical role in the results. Moreover, it is necessary to previously have

determined the Weibull parameters for the individual technical fibres. The

sensitivity of the methodology to the bundle geometry was assessed by testing four

combinations of cross sectional area and length. The results indicated that the

amount of fibres is not critical but the length of the fibres was the most dominant

variable for the fibre strength calculations.

The strength properties and the morphology of bamboo technical fibres are highly

dependent on the extraction process, since defects introduced by the extraction

method reduce the tensile strength of the fibres. Also, the fibre’s aspect ratio

influences the final properties of bamboo fibre reinforced polymers. For this reason,

fibre properties cannot be generalized and should be linked with the correspondent

extraction method and the tested gauge length in order to avoid unfair statements

and comparisons. After an extensive literature review, no major progress was found

in the extraction of long bamboo fibres, even though this research topic had been

studied since the 80’s. This could be evidence that it has been a relatively short

period of time to develop a fully standardized extraction process, which could be


improved through time, as it is the case for other traditional natural fibres such as

flax, hemp or jute.

The novel mechanical extraction applied proved to be an effective method to

produce long high-quality bamboo fibres suitable to reinforce polymer composites.

The extracted bamboo fibres exhibit good mechanical properties among other

natural fibres and comparable specific properties (normalized to the materials

density) to glass fibres under tensile loading. Parallel research to this work suggests

that bamboo fibres are relatively insensitive to humidity [57]. Apart from this, there

are some distinctive aspects that present bamboo fibres as a good alternative for the

composite industry: the aesthetic properties of the final composite (the “bamboo

look”), environmental benefits due to the environmentally friendly extraction

process and the possibility of downcycling (recycling) of the composites (e.g. for

injection moulding), a marketable “green” image, and potentially low cost.

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Unidirectional continuous and discontinuous bamboo fibre - epoxy composites 101

Chapter 5 Unidirectional continuous and discontinuous bamboo

fibre – epoxy composites

_______________________________________________________

5.1 Introduction

Figure 5-1. Fibre bundle discontinuity into the bamboo culm. Schematic of the node (left) given by [1].

A large bottleneck that impeded the introduction of bamboo fibres as composite

reinforcing materials for many years has been the extraction of undamaged long

fibres [2-6]. Recently, a new environmentally friendly mechanical process was

developed producing high quality long bamboo technical fibres suitable to be used

as reinforcement in polymeric matrices. Now, with the availability of the fibres, a

second obstacle to be overcome towards the large-scale industrialization of bamboo

fibre composites, apart from the challenges mentioned in section 3.4, is to produce

continuously a unidirectional (UD) prepreg (preform) to be used as reinforcement in

the composite industry.

Discontinuity of the fibre in the nodes [1]

Internode length (± 25 – 35 cm)

102 Chapter 5b

This step is necessary due to the inherent nodal constitution of the bamboo culm,

where the reinforcing fibres entangle in the nodes and contribute as such to the

buckling strength of the culm [1, 7]. This structural morphology, limits the extracted

technical fibres to the internode length (between 25 and 35 cm for the species

Guadua angustifolia [8]), see Figure 5-1.

To overcome this situation, the production of a continuous bamboo yarn cannot be

considered because the fibres have a high bending stiffness that impedes twisting

without fibre damage. Moreover, the twisting itself not only damages the fibre, but

also reduces drastically the stiffness of the yarn in the composite due to the fact that

the fibre will be off-axis with respect to the load [9], see Figure 5-2. As an

alternative to produce an endless yarn, certain mixed yarn technologies could be

considered, where the bamboo fibres would constitute the core and a thermoplastic

matrix yarn would be wrapped around them. But, because of the considerable

thickness of bamboo fibres (between 90 and 250 µm), the resulting yarn will be

quite coarse and prepregs or textiles based on these yarns (e.g. woven or braided)

would result in thick composite plies with low fibre volume fraction.

Figure 5-2. a) Schematic representation of a twisted and non-twisted yarn [10], b) experimental ( ) and

simulated stiffness results from different models for flax fibre bundles when loaded at different twist angles

[9].

The aim of this chapter is to translate the intrinsic good bamboo technical fibre

properties, shown in Chapter 4, into good composites. Therefore, a novel approach

is intended in this study to use the discontinuous bamboo technical fibres for large-

scale composite applications by the development of continuous unidirectional

preform, in which the bamboo fibre ends are distributed in a defined overlap pattern.

For this, a systematic study was conducted on the effect on mechanical properties of

different UD preforms with discontinuous fibre patterns, ranging from technical

a. b.


fibre bundles with different overlapping lengths, to individual technical fibres with

randomly placed fibre ends. The use of a continuous preform or prepreg tape will

allow the use of existing technologies to produce high performance composite parts

and to overcome the actual restriction of having discontinuous fibres. The results

will be benchmarked with the mechanical behaviour of a fully continuous

unidirectional bamboo fibre-epoxy composite (UD-C), to show the feasibility of the

use of technical bamboo fibres in continuous preforms or prepregs for high-end

composite applications. The research will be complemented with impregnated fibre

bundle tests and 3-point bending tests (3PBT).

5.2 Overview of different models for the prediction of composite mechanical

properties

The experimental stiffness and tensile strength results of aligned discontinuous

bamboo fibre-epoxy composites (UD-D) and continuous bamboo fibre-epoxy

composites (UD-C) will be compared with the predictions from several models. In

general, they have common assumptions such as the linear elastic behaviour of

fibres and matrix, the matrix is isotropic, and the fibres are either isotropic or

transversely isotropic. Also, the fibres have regular cross sectional area and the same

length, the fibres are well bonded to the matrix and fibre-matrix debonding is not

considered; there is a regular packing geometry of the fibres and perfect fibre

alignment. It should be noticed that the last assumptions, do not represent the real

conditions present in the studied composite samples. The next sections give an

overview of the models used to predict the properties of the bamboo fibre-epoxy

composites with different fibre configurations.

5.2.1 Prediction of longitudinal tensile stiffness

Rule of mixtures (ROM)

The rule of mixtures is the simplest mechanical model to estimate the properties of a

multiple component system. It estimates the composite material properties by taking

a volume weighed average of the corresponding properties of the individual

constituents. Concerning the prediction of the longitudinal stiffness of a composite,

this model assumes the presence of continuous aligned fibres through-out the entire

length of the composite and perfect bonding between fibre and matrix. Furthermore,

it assumes iso-strain behaviour for all the components during loading conditions.

Applying Hooke’s law and the previous assumptions yields Equation 5-1 that is used

to estimate the longitudinal stiffness of a composite:

104 Chapter 5b

c f f m mE V E V E

Shear-lag theory

The shear-lag theory [11] considers a single cylindrical linear elastic and isotropic

fibre of finite length L and radius rf that is encased in a concentric cylindrical shell

of a linear elastic, isotropic matrix with radius Rm. A one-dimensional stress-state

situation is applied in which the matrix tensile strain becomes equal to the applied

strain (εappl) at a radial distance Rm from the fibre axis. Furthermore, the shear

transfer between matrix and fibre is thought to occur by interfacial shear stresses and

the tensile stress at the fibre ends is assumed to be zero. As a result, the maximum

tensile stress occurs in the middle of the fibre whereas the maximum shear stress

occurs in the interface at the ends of the fibre [11, 12]. Additional assumptions are

that the fibres are aligned and packed in an orderly manner, as it is also presumed

for the ROM. Also, both the fibre and the matrix are assumed to be perfectly linearly

elastic and isotropic, and the stress is transferred between the two constituents

without yielding (perfect bonding). In general, the shear-lag theory is a modified

ROM which incorporates a fibre-length correction factor ηl, that depends on the fibre

aspect ratio, a fibre packing factor (see Table 5-1), the matrix shear modulus and the

fibre’s Young’s modulus (Equations 5-2 to 5-5).

m R

f f

R K

r V

There are some discussions in literature about the exact value of the radius of the

cylindrical matrix shell Rm. This value can be obtained as shown in Equation 5-5

and it will depend on the packing state of the fibres (KR). Table 5-1 gives an

overview of the different values of the KR constant according to different authors.

(5-2)

(5-3)

(5-5)

(5-1)

(5-4)

(1 )c l f f f mE V E V E

tanh( / 2)1

/ 2l

nL

nL

2 2

ln

m

mf

f

Gn

RE

r


Table 5-1. Overview of the different values found in literature for the packing dependent constant KR.

Halpin-Tsai equations

The Halpin-Tsai equations are a series of semi-empirical equations to predict the

longitudinal tensile Young´s modulus of composite materials. This model assumes

that the fibres are transversely isotropic and linear elastic, also that they are

percectly parallel and well bonded to their interface. A review of the entire

derivation of the model is given by Halpin and Kardos [16]. The Halpin-Tsai

equations were originally developed for continuous fibre composites and were

derived from the work of Hermans [17] and Hill [18]. Halpin and Tsai found that

three of Herman’s equations can be written in a common form as seen in Equations

5-6 and 5-7.

The parameter η is a function of the ratio of the fibre and matrix stiffness (Ef/Em) and

of the shape of the reinforcement. It was found in literature that the significance of

the ξ parameter for these equations had two interpretations. The first one considers it

as a fitting parameter, to adjust the Halpin-Tsai equations to the experimental data,

considering the packing arrangement and the geometry of the reinforcing fibres [19].

As a second approach, it has been suggested that ξ should vary from small values to

infinity as a function of the fibre aspect ratio L/d. By comparing model predictions

with available 2-D finite element results, it was found this parameter can be

determined by two times the aspect ratio of the fibres [13, 20], as shown in Equation

5-8. This was the approach followed in the present study.

Fibre packing Value of KR Reference

Hexagonal, center-to-center distance 3.62 [11]

Hexagonal, half of the nearest neighbour distance 1.81 [13]

Composite cylinder 1 [14]

Square 0.78 [15]

(5-7)

(5-6)

(5-8) ξ = 2(L/d)

1

1

f

c m

f

VE E

V

( / ) 1

( / )

f m

f m

E E

E E

106 Chapter 5b

Mori-Tanaka model

The Mori-Tanaka (MT) model [21] is a commonly used micromechanics-based

model used to predict the elastic properties of a wide range of composites

(unidirectional, short fibre and particulate) [22]. The MT model defines explicit

expressions for the elastic constants of unidirectional anisotropic ellipsoidal short-

fibre-reinforced composites, which will be summarized next [23]:

The elastic stiffness matrix of composite, C-, can be expressed as in tensor notation

as:

( ) :m f m

fC C V C C B

where Vf is the inclusion volume fraction, which is considered as the fibre volume

fraction for the fibre composite material system, Cm

and Cf are the elastic stiffness

matrix of the matrix and fibre material, respectively. The fourth-order tensor B is

defined as:

1

(1 ) : : ( )m f m

fB I V E S C C

where I is a fourth-order unit tensor, the compliance matrix of the matrix material

Sm

= (Cm

)-1

, and E stand for Eshelby’s tensor, for which the analytical expressions

are listed clearly by Mura [24]. In this formulation, both fibre and matrix can be

generally anisotropic elastic, however, most commonly, only the reinforcing

inclusions is of anisotropic behavior. It is remarked that this model additionally

assumes that fibre and matrix are linearly elastic and perfectly bonded through the

entire deformation state, which might not be realistic, except at the earlier stage of

deformation.

The matrix is usually considered isotropic, whereas the fibres are modelled as

transversely isotropic. The Mori-Tanaka model does not depend on either the size of

the inclusion, nor on their positional coordinates, but it does depend on shape and

orientation of the inclusions, as well as their volume fraction [25]. The MT model

was implemented to estimate the elastic moduli of unidirectional kenaf epoxy

composites, where the prediction was 20% higher than the experimental results [23].

(5-9)

(5-10)


5.2.2 Prediction of longitudinal tensile strength

Rule of mixtures (ROM)

The rule of mixtures applies the iso-strain assumption, indicating that the component

with the lowest breakage strain will fail first. Since bamboo fibres, used in this

research, are more brittle than the epoxy matrix and since the produced composites

contain relatively high fibre volume fractions, it is expected that the composite will

break at the breakage strain of the fibres (see Equation 5-11). In this equation σ*f

indicates the longitudinal fibre tensile strength, σ´m the longitudinal tensile stress in

the matrix at fibre failure and Vf and Vm the volume fractions of fibre and matrix

respectively. This method further assumes that the stress concentration around one

broken fibre triggers the failure of other fibre failures (at the same strain) and that no

mechanisms of crack propagation deceleration are present.

σc= Vf σ*f + Vm σ´m

Kelly-Tyson model

For the modelling of the strength of discontinuous fibre composites Kelly and Tyson

[26] extended the ROM for strength prediction of composites reinforced with fibres

aligned in loading direction. This model, it is assumed that no voids are present in

the composite. This model takes into account the interaction between axial tensile

stresses in the fibre and shear stresses at the interface of the fibre. The Kelly-Tyson

model further assumes interface failure to occur first and models the shear stress at

the interface as a constant in the debonded or yielded area [12]. By introducing the

critical aspect ratio Sc, defined as the smallest fibre aspect ratio at which the axial

fibre tensile stress can just reach the fibre strength and by assuming that the fracture

occurs when the axial fibre tensile stress reaches the fibre strength, the composite

strength is determined as stated in Equation 5-12.

where rf is the radius of the fibre, Sc is the critical aspect ratio, L is the fibre length

and τrz is the interfacial shear stress around the debonded fibre.

(5-11)

(5-12) * 1

2

f c mc f f m

f

r S EV V

L E

*

2

f

c

rz

S

with

108 Chapter 5b

Global load sharing model (GLS)

The global load sharing model (GLS) introduced by Curtin [27, 28] assumes that the

stress from a broken fibre is redistributed globally across all the remaining intact

fibres in the cross section of the composite. An important characteristic of Curtin’s

model is that it accounts for load contributions of broken fibres and the matrix shear

loading recovers away from the fibre breakage. Moreover, it incorporates Weibull

statistics [29, 30] in the estimation of composite tensile strength (for more

information about Weibull’s model see paragraph 4.1.1). The ineffective fibre length

(δ) is calculated using the Kelly-Tyson approximations, leading to Equation 5-13 for

the longitudinal tensile strength of the composite.

1

1

1

2 1

2 2

m

c f

mV

m m

where m and σ0 are the Weibull shape and scale parameters respectively and Lo

corresponds to the scale variable for the fibre length. More information about these

parameters can be found in section 4.1.1.

Local load sharing (LLS) model

The local load sharing (LLS) model [31-33] emerges from the intention to predict

the composite tensile strength in an analytical approach. This method is able to

predict the tensile strength of the discontinuous fibre composites with reasonably

good accuracy and low computational cost. An important difference in comparison

with the other models, is that it incorporates matrix shear loading to re-distribute the

applied load with only the adjacent fibres and therefore can account more accurately

for fibre breakage. A comprehensive description of this model is given by Taketa

[34] and is summarized as follows.

First, a spring element unit cell is designed using only dominant parameters for

composite strength. As shown in Figure 5-3a, one fibre is surrounded by six other

fibres. The unit cell considers the fibre represented by axial springs in the

longitudinal direction and the matrix by shear springs in transverse direction. Each

of the spring elements is assigned a stiffness matrix. Fibre elongation and matrix

shear are the mechanisms of load transfer through unidirectional composites. The

stiffness matrices of fibre spring element KL and matrix shear spring element KT are

defined in Equation 5-14, where l is the fibre spring length, Ef is fibre Young’s

modulus and G is the effective matrix shear modulus.

(5-13)

1

10 0 ,

1

m mrz co

f

L

r

with


Figure 5-3. a) Schematic of spring element unit cell and b) graphical visualization of the construction of a 3D-

volume by the stacking of hexagonal unit cells [31].

2

1 1

1 1

f f

L

E rK

l

A number of the unit cells are built up as three dimensionally illustrated in Figure 5-

3b. This enables a large enough simulation size (number of fibres and fibre length)

according to the case to ensure that the model is equivalent to the sample size for the

calculation of the tensile strength.

Strength variation is included through a Weibull distribution of the reinforcing

material. At each strain increment, the stresses in the springs are compared to the

fibre strength. If the fibre strength is exceeded, the spring element stiffness is

removed and a stress re-distribution is performed. At each step the normalized

composite stress is also calculated. This iterative scheme is repeated until the

relative difference in two subsequent composite stress values is larger than a pre-set

value. Monte Carlo simulation is then performed and the composite strength is taken

as the average of the simulated values. The LLS model code used in this PhD thesis

was developed at the University of Tokyo by Dr. T. Okabe, Dr. Nishikawa and Mr.

K. Ishii. The model was modified in order to incorporate all parameters and

restrictions needed for the present study, as it will be detailed further in section 5.5.7.

5.3. Materials

5.3.1 Bamboo fibres

Untreated bamboo fibres were used in this study. The location for the extraction of

the culms and the characteristics of the fibres and their extraction are described in

section 4.3.

Unit cell (hexagonal array)

a. b.

1 1

1 13

m f

T

G r lK

d

and (5-14)

110 Chapter 5b

5.3.2 Epoxy resin

Three different epoxy resins were used in this study. The resin used for the bamboo

fibre composites and which will be used for tensile testing is a commercial available

epoxy resin Epikote 828 LVEL and a DYTEK® DCH-99 Amine (1,2-

diaminocyclohexane) hardener. After mixing the two components in a mass ratio of

100/15.2 w/w the system is degassed in a vacuum oven at 1 bar for 15 minutes.

After the composite production the resin requires a curing temperature of 70°C for

an hour and an additional post-curing step at 150°C for 1 hour.

For the manufacturing of the impregnated bundles (IB), according to the standard

ISO 10618, it is recommended that the matrix has at least twice or triple the failure

strain of the reinforcement. For this reason epoxy Araldite® LY564 was chosen

together with amine Aradur® 3486 as a hardener with a mixing ratio of 100/34 w/w.

The same procedure for preparation and curing as described above was followed.

For 3-point bending test characterization, Epoxy HM 533 was used as the resin

component, supplied by Hexcel Composites S.A, with an areal weight of 250 g/m2

and a curing temperature of 125 °C. It is a film type of epoxy resin used to produce

prepregs, which makes it interesting to explore for composite manufacturing and to

determine the mechanical properties obtained with this type of resin. Table 5-2

shows the main properties of the epoxy resins used for the different test samples.

Table 5-2. Properties of the epoxy resins used as a matrix in this study. All data are obtained from data sheets.

*UD-C: unidirectional continuous bamboo fibre-epoxy composites. **UD-D: unidirectional discontinuous

bamboo fibre-epoxy composites.

5.4 Methods

5.4.1 Fibre patterns

In this study, different types of unidirectional discontinuous technical fibre patterns

(preforms) with a fibre length (Ls) of 50 mm were produced to manufacture UD-D’s,

in order to be compared with the mechanical behaviour of UD-C samples. A

schematic overview of the studied patterns vperarying a) the overlapping length of

the technical fibre ends (Lv) and b), the width of the fibre bundles is shown in Figure

Epoxy resin Tensile

strength

(MPa)

Tensile

modulus

(GPa)

Elongation

at fracture

(%)

Density

(g/cm3)

Sample manufacturing

Epikote 828 70 2.7 4.1 1.1 UD-C* and UD-D**

Araldite LY564 70 -74 2.8 – 3.0 4.6 – 5.0 1.2 Impregnated fibre bundle test

HM 533 50 2.9 4.0 1.2 3PBT


5-4. The fibre bundle in this case, can be defined as a “cluster” of technical fibres

that are together and have the same Ls. The mentioned preforms were made to fit

into the cavity mould that had the final dimensions of the testing sample (25 mm

width (w) and 260 mm length). In general, the created patterns can be classified in

four different categories as follows:

1. Composites with fibre bundles, width half of w, Ls=50 mm and whose fibre

ends overlap (Lv)10, 30 and 50% of the adjacent fibre (see an schematic view

in Figure 5-4a, b and c);

2. Thinner fibre bundles, 1/5 of the sample width (w), Ls=50 mm and random Lv,

see Figure 5-4d;

3. Individual UD bamboo technical fibres with Ls= 50 mm whose fibre ends are

placed randomly along the composite (Lv= random overlap length between

individual fibres), see Figure 5-4e;

4. Finally, the fourth category belongs to the UD continuous disposition of the

fibres (UD-C and impregnated bundle (IB)) where the technical fibres have

the same length as the sample composite as is shown in Figure 5-4f.

Each composite, contained four layers of the preforms with the corresponding

patterns according to the case, having an inversion of symmetry between adjacent

layers (brick wall pattern), see Figure 5-5. No fibre ends were inserted in the

clamping area in order to reduce stress concentrations and to avoid premature failure

close to the grips. All the fibre bundles were weighed beforehand, after being

conditioned at room temperature (20° and 50% relative humidity for at least 72 h,

and dried again at 60 °C 24 h before they are used for composite manufacturing. The

fibre volume fraction (Vf) was recalculated through weight measurements after

composite production and normalized at 40%.

Finally, two different approaches can be considered for the determination of the

bamboo fibre’s aspect ratio (L/d), as input variable in some of the applied models. In

the first case, “d” represents the diameter of the entire fibre bundle which is defined

above as a “cluster” of technical fibres that are packed together and have the same

length Ls, as seen Figure 5-4 and 5-7. The second one proposes a more traditional

approach in which “d” represents the diameter of individual technical fibres. In this

case, an average fibre diameter of 160 µm was taken from 420 measurements. The

second approach was applied to calculate the fibre aspect ratio, taking into account

that every technical fibre will be surrounded by the resin and thus will acts as an

individual unit within the composite if a homogeneous fibre distribution is achieved

during the production of the composite plates.

112 Chapter 5b

Figure 5-4. Schematic general view of the different fibre pattern configurations. Unidirectional discontinuous

composites (UD-D): a) 10%, b) 30% and c) 50% of fibre bundle overlapping, d) random fibre bundles and

e) full fibre randomization. Continuous unidirectional composites (UD-C and IB): f) continuous fibres. All

composites were normalized at 40% Vf. The fibre length (Ls) is 50 mm for all samples except UCD (f) and the

width (w) was 25 mm. The technical fibre ends (or discontinuities) can be seen in Figure 5-6.

Figure 5-5. Schematic pattern for UD-D samples with fixed fibre bundle overlapping length. a) the upper view

showing the inversed symmetry that exists between adjacent layers of fibres and b), the side view presenting a

“brick wall” pattern assembly.

Composite length

Thic

kne

ss

b.

Lv Ls

First and third preform (layer)

Second and fourth preform (layer)

a.

Fibre bundle

Fibre

disconti-

nuities

w w w w w w

a. f. b. c. d. e.

Lv=

random

Lv=

random

w

Category 1 Category 2 Category 3 Category 4

Ls Lv

Th

ick

ness


5.4.2 Elaboration of the prepregs

UD-D with fixed fibre bundle overlapping length

This configuration comprises Lv= 10, 30 and 50%, Ls= 50 mm and width ½ w. The

fibre bundles were manually introduced in series into the cavity mould in which

adjacent bundles were shifted 5, 15 and 25 mm, giving as a result three different

overlapping lengths (Lv) of 10, 30 and 50% respectively. The width of the fibre

bundles corresponded to half the width of the mould (½ w), see Figures 5-4 and 5-6.

The patterns were schematically presented in Figure 5-4a-c.

Figure 5-6. Manual placement of the fibre bundles in the multicavity mould (260 x 25 mm). 1= placement of

the first layer of fibre bundle into to the mould at fixed overlapping length, 2= UD fibre bundle, 3= stacking

of several UD layers of fibre bundles, 4= upper mould. The fibre length (Ls) was set at 50 mm.

UD-D with random fibre bundle overlapping length

This fibre preform incorporates Lv= random, Ls= 50 mm and 1/5 w. In this case, the

effect of the fibre discontinuities was diminished by reducing the width of the

bundle to 5 mm, and by randomizing the overlapping length between them (see

Figure 5-4d). For the placement of the fibre bundles in order to make the preforms,

the next steps were followed:

- The cavity mould was virtually divided in five rows along the length (1/5 w),

in order to position fibre bundles of the same width in each row;

- Then, with the help of a random number generator (RNG), discrete numbers

for each row (between 0 and 260) were generated in order to position the

b.

w=25 mm

a.

2

4

3 1 Fibre

discontinuities

114 Chapter 5b

starting edge of the bundle according to this number. An example is given in

Figure 5-7a for rows 1 and 2 where the edges were positioned at distances

Xn+A and Xn+B respectively. In this way, the first bundles of each row are

positioned, see Figure 5-7a;

Figure 5-7. Schematic representation of the procedure for the placement of the fibre bundles for the UD-D

samples with random fibre bundle overlapping length. a) Placement of the first fibre bundles in each row

(from 1 to 5) with the help of a random generation number (RNG). b) the positioning of the rest of the fibre

bundles.

- Then, the “free” space in each row was filled in series with more fibre

bundles. At the end the first layer is completed as depicted in Figure 5-7b

with Lv= random;

- The same operation is repeated 3 times more in order to have 4 layers in total

per sample. If by chance, a bundle discontinuity needed to be placed into the

gripping zone, this discontinuity was avoided. This was with the aim to

reduce stress concentrations and to avoid premature failure close to the grips.

UD-D with random single fibre overlapping length

Individual technical fibres (Lv= random, Ls= 50 mm) were placed manually directly

into the metal mould, in order to produce a UD discontinuous preform with

randomized fibre ends, as depicted in Figure 5-4e. A random number generator

b.

a.

Fibre bundle (Ls= 50 mm)


helped for the positioning of the fibres (discrete numbers), where the maximum Lv is

50 mm. This operation gives as a result a different position of the fibre ends and

filling completely the length and width of the cavity mould (260 x 25 mm). This

operation is done for four preforms (contained in one sample), each one randomized

with different patterns.

Unidirectional continuous bamboo fibre-epoxy composites (UD-C and IB)

Figure 5-8. Unidirectional bamboo fibre preforms ready to be used for composite production.

For the unidirectional preform (Figure 5-4f) continuous bamboo fibres were used; as

for all the cases explained before, the fibres were weighed (being in room conditions

for at least 72 hours), carefully aligned and evenly spread by hand in order to have a

homogeneous thin layer of fibres (Figure 5-8). The fibre volume fraction of the final

composites was recalculated through weight measurements after composite

production and normalized at 40% for further comparisons. The same procedure was

followed for the preparation of the fibre preforms for the impregnated bundles (IB).

5.4.3 Composite production

5.4.3.1 Manufacturing of the samples for tensile testing

Light RTM was used to manufacture composite samples, see Figure 5-9. This

technique allowed keeping good alignment of the fibres and also a good surface

quality at both sides of the composite. The fibre preforms, described in the previous

sections, were placed into a mould with 5 cavities with a size of 260 x 25 mm, and

for the IB, another multicavity mould with smaller dimensions (250 x 10 mm) was

used. Both moulds had the corresponding upper moulds with final dimensions of the

samples for tensile testing, avoiding additional cutting of the composite plates. The

116 Chapter 5b

average dimensions of the samples are shown in Table 5-3. A fibre volume fraction

of 40% was targeted and verified by weight measurements after manufacturing.

Figure 5-9. Light RTM process for the production of bamboo fibre-epoxy composites.

5.4.3.2 Sample production for 3-point bending test

Flexural three point bending tests (3PBT) were conducted with two different fibre

orientations, longitudinal and transverse, to evaluate the flexural properties of

bamboo fibre composites with a thermoset matrix (epoxy resin). The transverse fibre

orientation is chosen to evaluate the adhesion strength between fibre and matrix, as

the transverse mechanical properties of the composite are matrix and interface

dominated.

Figure 5-10. Schematic view of the mould used to produce 3PBT samples of bamboo fibre-epoxy composites.

Resin inlet Resin oulet

Mould

Sealant tape

Vacuum bag

Upper

mould

Bottom plate

Hot plate

Bamboo fibres

a.

b.


For the composite manufacturing, several layers of UD fibres and epoxy film (HM

533) were alternated and laid up into the mould cavities of 60 x 40 mm, see Figure

5-10. Final consolidation is achieved by compression moulding applying pressure (3

bar) plus vacuum to minimize void content due to air entrapments in the material.

Samples were later on cut from the cured plates using a diamond saw and polished

appropriately to avoid side flaws that could act as crack initiators. Sample

dimensions are given in Table 5-3. The fibre volume fraction was measured through

weight measurements and the test results were normalized to 40%.

5.4.4 Sample testing

5.4.4.1 Tensile test

Tensile test samples were prepared according to the standard ASTM D3039. An

Instron 4467 machine with a load cell of 30 kN was used for the tests and a

crosshead speed of 2 mm/min was applied. The gauge length between the two

clamps was set at 150 mm and an extensometer with gauge length of 25 mm was

employed for measuring accurately the elongation of the composites. The use of end

tabs was not necessary because there was no failure close to the clamps, but instead

the samples were mechanically clamped using sand paper in the grips to prevent

slippage. The tensile test set up is shown in Figure 5-11. Before testing, all

specimens were conditioned at room conditions (21°C ± 2°C and 50 ± 2 %RH) for

at least 24 hours, see all specimen dimensions in Table 5-3.

Figure 5-11. Set-up for tensile test of bamboo fibre-epoxy composites.

118 Chapter 5b

Table 5-3. Dimensions of bamboo fibre – epoxy composites for tensile and 3PBT tests. For UD-D and UD-C

Ls= 50 mm.

Tensile test set up for the impregnated bundle test

Impregnated fibre bundle tests were performed according to ISO-10618. The same

set-up described above for the UD-C and UD-D samples was used, except for the

crosshead speed (1mm/min) and the extensometer (50 mm gauge length), as seen in

Figure 5-12. The fibre volume fraction was found to be 38 ± 4 % after weight

measurements. For all type of tensile samples the Young’s modulus was measured

between 0.1 and 0.3% of strain.

Figure 5-12. a) fibre bundles ready to be tested, b) flexibility of the bundle due to its small thickness and c)

bundle during tensile test.

Type of composite Length

(mm)

Width

(mm)

Thickness

(mm)

Span

length

(mm)

Tested

samples

Tensile test

UD-D fix fib. bundle overlap. length (Lv= 10%) 250 25 1.96 ± 0.11 150 5



UD-D random fibre bundle overlapping length 260 25 1.92 ± 0.09 150 5

UD-D random technical fibre overlapping length 260 25 1.84 ± 0.11 150 5

UD-C impregnated fibre bundle (IB) 250 10 0.76 ± 0.12 150 16

UD-C Continuous fibres 250 25 1.98 ± 0.3 150 5

3 point bending test

3PBT longitudinal 60 11 ±0.3 1.68 ± 0.03 35 6

3PBT transversal 60 5 ± 0.2 1.64 ± 0.04 35 5

a. b.

c.


5.4.4.2 Flexural test

Flexural three-point bending tests (3PBT) were conducted with two types of fibre

disposition (longitudinal and transverse) and performed on a universal testing

machine (Instron 4426) according to ASTM D790M (see Figure 5-13). The bending

modulus of each sample was determined by calculating the slope of the stress-strain

curve between 0.1 and 0.3% of the strain. The crosshead speed was set at 1mm/min,

and a 1kN loadcell was used during the test. The load and the flexural displacement

are registered during the complete test. At least five samples were tested in each

configuration. See Table 5-3 for the sample dimensions.

Figure 5-13. Set-up for three-point bending test (3PBT) of bamboo fibre-epoxy composites.

5.4.5 Scanning electron microscopy (SEM) observations

Micrographs of fibres and composites were made by scanning electron microscopy


observations using secondary electrons using a voltage between 10 and 15 kV.


Due to some variations in the fibre volume fraction of the composites, an efficiency

factor and normalised values at 40% Vf for strength and Young’s modulus will be

used to allow comparison between the different specimens. Two values for the

efficiency factors are calculated; the first one corresponds to the ratio between the

experimental result and the calculation of the property using the rule of mixtures

(ROM), which assumes a perfect composite. For this, a fibre strength of 600 MPa

and Young’s modulus of 43 GPa were used for the calculation.

120 Chapter 5b

Table 5-4. Tensile test results for unidirectional continuous (UD-C) and discontinuous patterns for bamboo fibre-epoxy composites (UD-D). The efficiency factor is

calculated as the ratio between the experimental result of the property and the calculated value using the rule of mixtures at the same fibre volume fraction. aThe first

value (up) corresponds to the efficiency factor calculated with the ROM, the second one (down) was obtained having as a reference the maximum composite strength

obtained with continuous fibres (254 MPa) that corresponds to the impregnated bundle test. bThe results are normalized at a fibre volume fraction of 40%. Lv =

overlapping length. For all UD-D samples the length of the fibres (Ls) was 50 mm.

UD-D with fixed

fibre bundle

overlapping length

(Lv = 10%)

UD-D with fixed

fibre bundle

overlapping length

(Lv = 30%)

UD-D with fixed

fibre bundle

overlapping length

(Lv = 50%)

UD-D with

random fibre

bundle overlapping

length

(Lv = random)

UD-D with random

fibre overlapping

length

( Lv = random)

UD-C

(impregnated

fibre bundle)

UD-C

(continuous

fibres)

Property

aEff.

Factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

aEff.

factor

(%)

bAverage

value*

Tensile

stiffness

(GPa)

86

84

16 ± 1

90

89 17 ± 1

91

84 16 ± 2

89

89 17 ± 1

90

89 17 ± 1

94

- 19 ± 1

92

94 18 ± 1

Tensile

strength

(MPa)

30

33 85 ± 18

28

31 80 ± 12

29

31 81 ± 13

35

39 100 ± 18

63

75 191 ± 21

90

- 254 ± 18

79

87 222 ± 13

Strain at

breakage

(%)

- 0.6 ± 0.11 - 0.5 ± 0.14 - 0.5 ± 0.1 - 0.7 ± 0.1 - 0.8 ± 0.03 - 1.3 ± 0.1 - 1.4 ± 0.2

Lv Lv Lv Lv=

random

Lv=

random

120 Chapter 5


The second efficiency factor is obtained as the ratio between the experimental result

and the maximum experimental composite strength value obtained with continuous

fibres, corresponding to the impregnated bundle test. All results of the tensile tests

performed on UD-D and UD-C samples, are presented in Table 5-4 and a typical

stress-strain curve is presented in Figure 5-14.

Figure 5-14: Typical tensile stress-strain curves for continuous (UD-C) and discontinuous (UD-D) bamboo

fibre epoxy composites.

5.5.1 Continuous fibre composites (UD-C)

Table 5-4 reveals that the experimental values for longitudinal tensile stiffness and

strength for UD-C respectively reach 92% and 79% of theoretical values found with

the ROM. This points out that a strong fibre-matrix interface is present between

fibres and matrix and that the resin impregnates the fibres very well and the good

alignment of the fibres. A visual inspection of the samples after failure indicated, in

general, a brittle fracture with a crack mostly propagating in one plane as shown in

Figure 5-15a. The fracture surfaces were also examined under SEM, showing good

resin impregnation and a quite clear fracture of the sample with relatively low

presence (~25-30 %) of fibre pull out (see also Figure 5-15b). This estimation is

based on the number of pulled out fibres relative to the total number of bamboo

fibres in the micrographs. Also, it was possible to observe a good dispersion of the

fibres because the layer-wise initial configuration of the composite is not visible in

the final material.

122 Chapter 5b

Figure 5-15. a) Composite fracture after testing and b) SEM observations of the fracture plane of the UD

continuous fibre composites after tensile testing.

Impregnated bundles

For UD-C impregnated bundles, the efficiency factor values (with respect to the

ROM) for longitudinal tensile stiffness and strength are 94% and 90% respectively.

The stiffness value (~19 GPa), is improved by ~5% in comparison with the standard

UD-C samples. This improvement can be attributed to the fact of having less fibres

in the sample; in practice, it is much easier to control the alignment of the fibres,

positively affecting the stiffness and strength of the composite material.

The tensile strength for this type of specimens was around 13% higher than for the

“standard” UD-C samples. This result can be expected if the volume of the sample is

considerably smaller, meaning less probability of presence of defects and improving

the strength properties. When the technical fibre strength was back calculated from

the impregnated bundle test, the values were found to be close to the strength

properties given by the dry fibre bundle test (DFB) presented in section 4.5.4 (see

Figure 5-16). Even though, the parameters vary between both tests, this can be an

indication that DFB test values are representative for the mechanical behaviour of

the fibre in the final composite material.

a. b.

25 mm


Figure 5-16. Strength comparison between single fibre and dry fibre bundle test for technical bamboo fibres at

different gauge lengths. The single fibre strength data reported at 100 mm gauge length was extrapolated from

the experimental results at 40mm gauge length using Equation 4-5 (m=7.6), using the length of the technical

fibre (L) as a scale variable.

5.5.2 UD-D with fixed fibre bundle overlapping length

The results of the tensile tests, performed on UD-D samples (Lv= 10, 30 and 50%)

and Ls= 50 mm and ½ w) are given in Table 5-4. The introduction of discontinuities

at the technical fibre ends significantly reduces the strength of the composite

samples. The strength reduces to around one third with the insertion of these weak

points and the efficiency factor drops to approximately 30%. The strain to failure

also reduces from approximately 1.3% to only 0.5%. These discontinuities result in

the presence of a matrix rich area in the composite (Figure 5-17), that under tensile

load produces a strain magnification zone that creates shear stresses with the

adjacent fibre bundle, as seen in a schematic representation in Figure 5-18. These

shear stresses in turn, trigger a crack initiation that easily propagates due to the

limited toughness of the epoxy resin and causes the failure of the sample. The

Young’s modulus did not show a significant decrease and remained at ~17 GPa for

all the UD-D configurations, which represents a 6% of reduction in comparison with

the full UD continuous fibre composite (UD-C).

40 mm gauge length

100 mm gauge length

Single fibre test

Dry fibre bundle test Back calculated (ROM) fibre strength from the impregnated bundle test

124 Chapter 5b

Figure 5-17. As an example, a UD-D sample with LO=50%, where the introduction of the fibre ends

(discontinuities) results in the presence of resin-rich zones.

5.5.3. UD-D with random fibre bundle overlapping length

The results of the tensile tests, performed on this type of composites (Lv= random,

Ls= 50 mm and 1/5 of the w) are given in Table 5-4. As compared to the composites

with fixed overlapping length, the reduction of the fibre bundle width and their

overlap randomization allowed a minor increase in the efficiency factor of ~5% for

the longitudinal tensile strength. SEM observations however pointed out, in some

cases, a similar fracture phenomenon as present in the UD-D samples with fixed

overlapping length. This indicates that the width of the fibre discontinuity is still

high and acts as a crack initiator leading to premature failure of the samples.

Resin-reach zone

Shear stress

UD-D

Strain magnifi-cation zone

Tensile sample

Fibre

ends

Tensile

sample

Crack initiation

Technical bamboo fibres

Fibre disconti-nuities

Resin-reach zone

Figure 5-18. Schematic illustration of the fracture mechanism for UD-D samples.


In industry short fibre tapes are emerging with a relatively new technology called

“fibre patch preforming” [35], which can be a suitable way to tackle the

discontinuity of the fibres. This technology allows short fibres to be positioned

according to nearly any specification, with a robot providing a full range of

possibilities in the fibres’ position and orientation. These preforms are cut from

continuous UD preforms of carbon fibre (in 60 x 20 mm patches) and then placed by

a robot directly into the mould in a defined pattern, being ready to use in standard

infiltration processes, and can be combined with other preform types. The range of

applications includes production of small and complex geometric parts including

medical prosthetics or high-performance sport equipment. This technology can be

potentially used for the current discontinuous bamboo fibres.

5.5.4. UD-D with individual (technical) fibres with random fibre ends

In this fibre configuration (Ls= 50 mm, Lv= random), the crack initiator effect of the

fibre discontinuities was removed due to the randomization of the fibre ends,

resulting in properties close to the UD-C properties, as seen in tensile tests (Table 5-

4). The introduction of randomized fibre discontinuities (individual fibres with Ls=

50 mm) leads to a preservation of 85% of the longitudinal tensile strength in

comparison with the UD-C. These results clearly show that randomization of the

discontinuities on individual bamboo technical fibres inside the composite, is

necessary to take advantage of the good mechanical properties of the fibres and to

translate them to the composite. By varying the fibre ends over the length of the

sample, the overall stress fields surrounding the discontinuities are expected to be

minimized, slowing down the initiation of cracks. After tensile testing, the

specimens exhibited a mixed mode of fracture, where in most of the cases the

fracture was scattered randomly along the cross-sectional plane of the samples,

indicating less contribution of the additive stress concentrations (due to the in plane

bundle discontinuities), and minimized by the presence of the random fibre ends.

5.5.5 Tensile fracture characteristics for UD-D samples

In general, for all UD-D samples with different overlapping lengths (Lv), the

discontinuities resulted in two different types of tensile fracture, as shown in Figure

5-19. The first consists of staggered fracture following the fibre bundle “brick wall”

construction inside the composite. The shear stresses created by the discontinuity of

the fibres initiate the crack and then it runs along the rich-resin zone created by the

fibre ends (discontinuities). Then, the crack continues growing preferably through

the fibre length parallel to the fibre bundles due to delamination between the

126 Chapter 5b

initiated crack and the adjacent fibre bundle which is “bridging” the fibre ends until

it connects to with another bundle discontinuity and causes the final failure of the

composite. This type of fracture occurs especially in composites with Lv=10%

(Figure 5-19a), due to the proximity of the discontinuities; nevertheless this case is

also observed in some samples of the other configurations.

Figure 5-19. Tensile fracture of UD-D samples. Specimens with a) Lv=10%, b) Lv=30%, c) Lv=50%, d)

random Lv with slits of one fifth of the composite width, e) UD-D with single technical fibres in random

disposition and f) their typical tensile failure (highlighted by a black line). For all samples Ls=50mm.

a. b.

d. c.

e. f.


The second case consists of a rather straight failure across the UD-D sample. Here,

the crack starts as explained before due to a strain magnification, but instead of

being deflected by delamination along the fibre length to the next fibre discontinuity

(fibre bundle end), it continues all along the composite width. This situation is

presented in all fibre patterns but especially in configurations with Lv=30 and 50%

(Figures 5-19b and c).

In UD-D samples with randomized individual fibre ends, the failure mode can be

described as a long splitting along the gauge length mostly in the middle of the

specimen (SGM), see Figure 5-19f. The crack is initiated in the most critical defect

along the sample and running preferably in longitudinal direction of the fibres. All

different fibre patterns described above (Figure 5-4), did not affect the linear elastic

behaviour of the material as is shown in Figure 5-14.

5.5.6. Experimental stiffness compared with predicting models in UD bamboo

fibre-epoxy composites

Composite stiffness estimations from different models are compared in Table 5-5.

For this comparison the rule of mixtures (ROM), the shear lag theory, the Mori-

Tanaka model and the Halpin-Tsai equations were chosen. These models were

already described in the introductory part of this chapter in paragraph 5.2 and can be

compared with the experimental results in Table 5-4 and Figure 5-20.

Table 5-5. Results for the longitudinal stiffness of bamboo fibre-epoxy composites estimated with several

models and compared with the experimental results (40% Vf).

The Young’s modulus of the bamboo technical fibre used in the models was 43 GPa

[36]. When comparing the experimental and theoretical results for the Young’s

modulus of the studied composites, the simplest model, the rule of mixtures (ROM),

shows a relatively large over-estimation for this property. This is due to the large

simplifications accounted for in this model, by assuming perfect bonding between

fibres and matrix giving an iso-strain condition under load, and by assuming

infinitely long and perfectly aligned fibres. Not only large simplifications, but also,

assumptions can lead to an inaccuracy for a model prediction as is the case for the

shear-lag theory and Mori-Tanaka model in this specific case of bamboo fibre-epoxy

Property Rule of

Mixtures

Shear- lag

theory

Mori-

Tanaka

model

Halpin-

Tsai

equation

Experimental

results

UD-C UD-D

Longitudinal tensile

stiffness (GPa) 18.8 18.6 18.9 17.2 18-19 16-17

128 Chapter 5b

composites. The results for the shear-lag theory, in which a square array was chosen

for the present study, show a slightly better approximation of the longitudinal

stiffness. It assumes, however, that the fibres can be modelled as perfect cylinders

and uses a hexagonal fibre packing. Bamboo fibres and their distribution inside the

composite do not satisfy these conditions.

Figure 5-20. Experimental results for the longitudinal Young’s modulus for bamboo fibre-epoxy composites

with different UD fibre configurations versus the estimation from different models.

The application of the Mori-Tanaka model requires knowledge of the different

engineering constants that set up the stiffness or compliance matrix. Considering the

anisotropic nature of the natural fibres, the required transverse stiffness and Poisson

ratio’s for bamboo fibres are still unknown. For these, a strong assumption needs to

be made, assuming that the fibre is an isotropic material with a fixed Poisson’s ratio

(0.33). This assumption was also made by Herrera-Franco et al. [37] when

modelling the tensile strength properties of UD henequen fibre and HDPE

composites, and by Sabeel et al. [38] with a value of 0.38 for the evaluation of jute-

glass fibre hybrid composites predicted by using the classical laminate theory.

Finally, the semi-empirical Halpin-Tsai equation gave a reasonably good estimation

of the longitudinal tensile stiffness for the bamboo fibre reinforced epoxy

composites. A possible reason is that this model not only takes into account the

geometry of the reinforcement, represented in the fibre aspect ratio (L/d) but also,

Experimental results


the fibre packing efficiency, instead of geometrical arrangement of the

reinforcement as is considered for the shear-lag model [39]. These considerations,

together with the good efficiency factor for the composite modulus reported

previously, suit better the “real” conditions of the composites and give better results.

In general, when comparing experimental results and theoretical predictions for the

Young’s modulus in natural fibre composites, controversial results were found in

literature. This poor agreement is attributed to the fact that the models do not take

into account the bonding between fibres and matrix [39]. However, for

unidirectional jute fibre reinforced composites with different fibre volume fraction,

the Halpin-Tsai model was successfully applied [40]. On the other hand, natural

fibre polymer-based composites analyzed through the same model, revealed that the

theoretical values for the Young’s modulus were higher than the experimentally

obtained values [41]. This can be again attributed to the poor interface between fibre

and matrix. It should be noted that this model is one of the most used material

property models, particularly in engineering design applications, due to its simple

universal form of expressions, and its wide applicability to a number of different

materials [42]. Nevertheless, Halpin–Tsai equations have been found to be

inaccurate at high Vf, because they do not take into account the limit in maximum

packing fraction in a real system [39] and give relatively low values for moderate to

high aspect ratios [13].

5.5.7 Experimental strength compared with predicting models in UD bamboo

fibre-epoxy composites

In this section, several models are applied to predict the tensile strength of the UD

discontinuous bamboo fibre-epoxy composites. The results of these predictions are

presented in Table 5-6 and Figure 5-21. The results of the continuous fibre

composites (UD-C) are given as a reference.

130 Chapter 5b

Table 5-6. Overview of the different modelling attempts to predict the tensile strength of unidirectional continuous (UD-C) and discontinuous (UD-D) bamboo-epoxy

composites. The experimental results are also included as a comparison. The values of the experimental results were normalized at a fibre volume fraction of 40%. For

all UD-D samples the length of the fibres (Ls) was 50 mm. Lv= overlapping length.

UD-D with

fixed fibre

bundle

overlapping

length

(Lv = 10%)

UD-D with

fixed fibre

bundle

overlapping

length

(Lv = 30%)

UD-D with

fixed fibre

bundle

overlapping

length

(Lv = 50%)

UD-D with

random fibre

bundle overlapping

length

(Lv = random)

UD-D with

random fibre

overlapping

length

( Lv = random)

UD-C

(impregnated

fibre bundle)

UD-C

(continuous

fibres)

Model Strength

(GPa)

Strength

(GPa)

Strength

(GPa)

Strength

(GPa)

Strength

(GPa)

Strength

(GPa)

Strength

(GPa)

Experimental* 85 ± 18 80 ± 12 81 ± 13 100 ± 18 191 ± 21 254 ± 18 222 ± 13

ROM 263 263 263 263 263 263 263

Kelly-Tyson 248 248 248 248 248 250 250

GLS 132 132 132 132 132 132 132

LLS 96 ± 12 98 ± 14 96 ± 14 126 ± 20 213 ± 16 219 ± 14 219 ± 14

Lv Lv Lv

Lv=

random

Lo=

random

Lv=

random

130 Chapter 5


Figure 5-21. Experimental results for the tensile strength for composites with different UD fibre

configurations versus the estimation from different models.

Local load sharing model LLS

The LLS model implements interactions between adjacent fibres that lead to a stress

redistribution through the application of matrix shear spring elements. With each

strain increment, the resulting tensile stresses in the axial fibre spring elements are

compared to the fibre strength (according to the probability of failure from the

Weibull distribution). If the fibre strength is exceeded, the fibre spring element is

removed from the system, simulating a broken fibre. After that, the stress

redistribution of the resultant load is recalculated [31, 32]. The original LLS model

model only allows the prediction of the tensile strength of continuous fibre

composites. After a modification, a UD-C can be modelled as a UD-D when

multiple axial fibre spring elements are removed from the system before starting the

simulation. This procedure allowed to implement the discontinuities already present

in the UD-D samples, as broken axial fibre spring elements under unloaded

conditions.

In order to implement the LLS model for this particular study with discontinuous

bamboo fibre-epoxy composites, several parameters needed to be set beforehand. As

132 Chapter 5b

it was stated before, the value of the matrix shear spring element stiffness (ז) was

experimentally determined (32 GPa) by applying the model to the experimental

results of the unidirectional composites (UD-C). This parameter has no real physical

meaning and represents the behavior of the matrix and locally affected interface

when the fibre is broken. The same value was used to calculate the strength

predictions for the other UD-D samples. The numerical experiments were repeated

ten times in order to obtain reliable statistical variations. The list of introduced

parameters and their values required for the LLS model are presented in Table 5-7.

Table 5-7. Overview of the different initial parameters set for the calculation of the tensile strength using the

local load sharing (LLS) model.

The modified LLS model was able to predict the UD-D composites tensile strengths

for the different pattern configurations tested. Since the model takes into account the

neighbour fibres which are surrounding the discontinuous fibres through the use of

shear spring elements, good and reliable predictions can be achieved. As explained

previously, the interactions between adjacent fibres allow the stress redistribution

with the application of matrix shear spring elements which are able to detect the

amount of continuous fibres around the fibre discontinuities that were introduced in

the model. A larger amount of continuous fibres surrounding the discontinuities

gives better bridging to these weak points, avoiding the formation of stress

concentrations and hence giving better strength properties. This effect can be clearly

seen in Figure 5-21 which shows that when the fibre discontinuities become

narrower, the strength of the composite is higher as is well predicted by the modified

LLS model (shown in the same figure). Figure 5-22 schematically represents this

situation for a typical transversal planes in the UD-D studied patterns (categories 1,

2 and 3 presented in section 5.4.1). This Figure shows the discontinuous fibres (▬)

and their neighbouring continuous fibres (▬) connected by the shear spring

elements, simulating the fibre distribution in the real composites according to each

case.

Parameter Value Parameter Value

Fibre Young’s modulus [Ef] (GPa) 43 Young’s modulus of the matrix [Em] (GPa) 2.7

Fibre radius [rf] (μm) 180 Fibre volume fraction [Vf] (%) 40

Weibull characteristic strength of

life [σo] (MPa)

850 Composite length [Le] (mm) 150

Weibull shape parameter [m] 7.6 Effective matrix shear yield stress [ז] (GPa) 32

Weibull reference length for the

fibre [Lo] (mm) 1


Figure 5-22. Schematic representation of the modified LLS model applied in this thesis (see section 5.2.2)

showing, in a transversal plane, the neighbouring fibres close to the fibre discontinuities according to the

different studied patterns: a) category 1, b) category 2 and c) category 3. For additional information about the

fibre patterns and categories, see section 5.4.1.

The difference between experimental and predicted values by the modified LLS

model were within a margin error of around 15%, with predictions systematically

higher than the experimental results. A possible explanation is that the strain field of

full continuous fibre composites (UD-C) under axial load is homogeneous, while

this is not the case for discontinuous composites (UD-D’s). When the fibre

discontinuities are introduced in the composite, the stress field becomes

inhomogeneous near the fibre edges, generating stress concentrations in the regions

adjacent to the pre-existing fibre discontinuities when loading the sample, and

producing premature failure. This situation cannot be accounted for in the LLS

model, preventing not only the incorporation of local stress concentrations around

the fibre discontinuities, but also the decrease in load transfer between adjacent

fibres upon interface debonding.

a.

b.

c.

Fibre discontinuity

UD-D sample

UD-D sample

UD-D sample

Cut fibre Continuos fibre

Large fibre discontinuity

134 Chapter 5b

Other models

Although most of the studied analytical models manage to predict the longitudinal

tensile strength of continuous bamboo fibre epoxy composite within a reasonable

degree of accuracy, they fail to accurately predict the longitudinal tensile strength of

discontinuous samples. The application of the rule of mixtures gave an

overestimation of the composite strength of approximately 216%. The Kelly-Tyson

model gives only a slight improvement in the prediction of the composite tensile

strength by accounting for stress redistribution in the ineffective length of the fibre.

However, this model still presumes perfect plastic yielding behaviour of the matrix

and constant frictional shear stress at the interface to bring forth a linear axial fibre

stress in the ineffective length. Furthermore, it does not incorporate the statistical

influence of the fibre strength. The Kelly-Tyson model did not predict the tensile

strength of discontinuous composites with sufficient accuracy.

The GLS model does take into account the Weibull distribution of the fibre strength.

It treats the composite as a chain of bundles, and accounts for load contributions of

broken fibres. Stress recovery occurs in the ineffective length due to the presence of

a presumed constant interfacial shear stress. However, this model does not consider

the stress redistribution from the broken fibre to the adjacent fibres which has an

important effect on the final mechanical characteristics of the composites.

Furthermore, this model neglects the contribution of the matrix to the tensile

strength of the composite.

5.5.8 Properties evaluated in 3-point bending

Table 5-8 shows flexural properties for unidirectional bamboo fibre-epoxy

composites with longitudinal and transverse distribution of the fibres. For

longitudinal flexural strength, an efficiency factor of 82% was reached after

comparing the experimental value (265 MPa) with the theoretical value (323 MPa).

This indicates a good fibre/matrix adhesion, possibly enhanced due to the surface

fibre roughness that promotes mechanical interlocking. The composite stiffness,

with the same fibre disposition, reaches 95% of efficiency factor, with an

experimental value of 19 GPa, indicating a good alignment of the technical fibre in

UD disposition.


Table 5-8. Flexural properties with longitudinal and transversal disposition of the fibres. The fibre volume

fraction was normalized at 40%.

The good adhesion between fibre and matrix mentioned above is confirmed by the

experimental results in the transverse 3PBT. This test provides a direct estimation of

interface tensile strength. In the present study, the bamboo fibre-epoxy composites

reached a transverse strength of 33 MPa and a stiffness of 2.7 GPa with untreated

fibres (Table 5-8). Figure 5-23 shows the fracture surface after transversal 3PBT

with fibres covered by the polymer and good dispersion (wetting) of the resin

around the technical bamboo fibres. From these results, it is possible to affirm that

the interfacial fibre-matrix strength is reasonably good. For flax-epoxy composites

(Vf = 40 %), it was reported that the best value in transverse 3PB was 20 MPa after

1% of alkali treatment (20 min at RT) [43]. In this case, the treatment removed

impurities and waxy substances from the fibre surface and promoted the creation of

a rougher topography and enhanced the interface quality by mechanical interlocking.

Also, pineapple leaf fibres and PHBV resin composites (Vf = 28 %), achieved 32

MPa with untreated fibres [44].

Figure 5-23. SEM images for untreated bamboo fibre – epoxy composites: (a) fibre surface covered by epoxy

resin after transversal 3PBT and b), a polished cross sectional area of the composite.

It was also noticed that the bending properties (strength and stiffness) are very close

to the values obtained with the impregnated bundle test when tested in tension as can

be seen by comparing Tables 5-4 and 5-8. This shows that the flexural strength is a

bit higher, which is a consistent result due to the fact that in 3PBT only the bottom

Fibre orientation Flexural strength

(MPa)

Flexural stiffness

(GPa)

Strain to failure

(%)

Longitudinal 265 ± 12 19.3 ± 0.8 2.4

Transversal 33 ± 2 2.7 ± 0.2 1.3

200 µm

136 Chapter 5b

part of the sample is under tension. In the case of the impregnated bundle tensile test,

the whole sample volume is loaded and there is a higher probability of defects

present in the sample; hence, a lower strength can be expected.

Figure 5-24. Comparisons between a) flexural strength and b) flexural modulus for unidirectional natural

fibre-thermoset resin composites found in similar studies [2, 45-50]. This comparison is only indicative due to

the significant difference in Vf of the reported samples. *Normalized results from this study with untreated

bamboo fibres.

Figure 5-24 shows a comparison for flexural strength and flexural stiffness between

different natural fibres and thermoset matrices in 3-point bending testing with

longitudinal disposition of the reinforcement. In literature, a significant

improvement in properties for several natural fibre composites with thermoset

matrices was found after fibre alkali (NaOH) treatments at different concentrations,

temperature and time of exposure (see section 3.3.2, Table 3-3 and 3-4). For flexural

a.

1. Bamboo + Epoxy (Vf: 40%)*

2. Flax + Epoxy (Vf: 40%)

3. Jute + Epoxy (Vf: 40%)

4. Jute + Vinylester (Vf: 35%)

5. Hemp + Epoxy (Vf: 35%)

6. Sisal + Epoxy (Vf: 37%)

7. Kenaf + Cashew nut Shell (Vf: 64%)

8. Bamboo + Polyester (Vf: 15%)

9. Kenaf + Polyester (Vf: 60%)

10. Hemp + Polyester (Vf: 60%)

11. Hemp + Cashew nut Shell (Vf: 65%)

b.

1. Bamboo + Epoxy (Vf: 40%)*

2. Flax + Epoxy (Vf: 40%)

3. Jute + Epoxy (Vf: 40%)

4. Jute + Vinylester (Vf: 35%)

5. Hemp + Epoxy (Vf: 35%)

6. Sisal + Epoxy (Vf: 37%)

7. Kenaf + Cashew nut Shell (Vf: 64%)

8. Kenaf + Polyester (Vf: 60%)

9. Hemp + Polyester (Vf: 60%)

10. Hemp + Cashew nut Shell (Vf: 65%)


properties of bamboo fibre-epoxy composites with the same type of fibres used in

this study as well as the same manufacturing process, no improvement in flexural

strength after different fibre alkali treatments was observed (1,3, and 5% for 20 min

at RT) [36]. The fact that the flexural strength is the highest for the non-treated

fibres is an important advantage for future industrial uses, reducing both cost and

fibre preparation, as well as having less environmental impact for the entire

manufacturing process.

5.6 Conclusions

Unidirectional continuous (UD-C) and discontinuous (UD-D) bamboo fibre-epoxy

composites were successfully manufactured to evaluate the effect of different

unidirectional fibre patterns on their composite tensile properties and results were

compared with predictive models. Not only a UD fibre alignment procedure was

developed allowing the production of fibre-epoxy samples, but also a novel

discontinuous UD fibre randomization technique has been proposed at lab scale

(proof of concept), to produce continuous tape from discontinuous fibres.

In general, a high strength is found for the unidirectional continuous samples (UD-C)

tested in tension and bending. The results are close to what could be expected based

on single technical fibre properties. A reason for this good behaviour is the high

quality bamboo fibre obtained after the mechanical extraction process that conserves

their intrinsic good characteristics with a high fibre surface roughness that promotes

a good mechanical interlocking with the epoxy matrix. Moreover a good bonding is

present of the epoxy with the chemical groups present on the fibre surface,

according to the good results of the transverse flexural strength with untreated fibres.

It was found, in comparison with similar systems, that alkali treatment is not

necessary to enhance the strength of the composite. This fact reduces the production

cost and strengthens the environmental advantages of this natural fibre.

In this study, the full randomization of technical fibres of 50 mm length to make

composites was approached with positive results, reducing the longitudinal tensile

strength by only 15% in comparison with the UD-C (impregnated bundle). This

aspect opens the possibility for a number of industrial applications in which an

endless prepreg is needed. Also, in a real production process, the technical bamboo

fibres, will be longer, in its majority at least twice of the used fibre length,

generating less presence of discontinuities and diminishing their effect on the

composite strength. In this case, together with a standardized production process for

the discontinuous bamboo preform, it is expected that the difference between

138 Chapter 5b

UD-D’s and UD-C’s will be closer. The composite stiffness variations were not

significant for any of the studied UD fibre patterns.

For the strength predictions the best approximation was achieved by the local load

sharing model (LLS). This model was applied in a novel approach incorporating the

effect of the fibre discontinuities inside the bamboo fibre epoxy composites, and it

was able to predict the experimental tensile strength within an error margin of

around 14%. This positive result is based on a more realistic approach of the failure

of the material, represented in the probability of failure of the bamboo fibres,

Weibull distribution parameters found specifically for the studied fibres, and the

load redistribution to the neighbour fibres after fibre breakage.

Impregnated bamboo fibre bundle tests can be used as a good alternative to evaluate

the composite tensile properties. They present several advantages in comparison

with the “standard” composite such as less material needed and less preparation for

the specimen manufacturing. Also, because they use rovings (tow) or yarns, that do

not need to be woven into a fabric, stitched or delivered in a UD configuration in

order to have a preform (prepreg) to proceed with the composite manufacturing,

saving time and costs. Additionally, higher mechanical properties were obtained for

the impregnated bundles in comparison with the UD-C. This is due to the easier

control of the fibre alignment for the case of the stiffness, and reduced sample

volume (with the corresponding less probability of defects) that improves the

strength. These data can be interpreted as target values that can be achieved for the

composite material.

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Thermal degradation in bamboo fibres and bamboo fibre polypropylene composites 141

Chapter 6 Thermal degradation in bamboo fibres and

bamboo fibre polypropylene composites

_______________________________________________________

6.1 Introduction

Thermoplastic matrices are a good alternative in comparison with thermosets to

produce bamboo fibre composites, they allow clean and fast processing, lower cost,

provide more impact resistant composites and allow mechanical downcycling of the

composite at the end-of-life (e.g. for injection moulding). For these reasons, recent

developments have been shifting to thermoplastic matrix composites [1]. However,

the degradation or thermal instability of natural fibres occur typically at

temperatures above 180°C due to the inherent organic composition of the natural

fibres, as shown previously in section 3.4.2. The overall advantages of bamboo

fibres have not yet been fully exploited for high-tech thermoplastic composite

applications because of the limitation of the low required melting temperature of the

polymer. This sets a serious limit to the processing conditions which narrows down

the thermoplastic choices [2].

High temperatures are applied mainly during the manufacturing (e.g. compression

molding), where the thermoplastic matrix needs to have reduced viscosity to

impregnate the fibre bundles and to shape the part. The exposure of the fibres to

high temperature, often in an oxidative environment (air), results in a decrease of the

mechanical properties accompanied with side effects such as discolouration and

unpleasant odour of the composites [3, 4], which means a significant disadvantage

for applications such as car interior applications.

142 Chapter 6b

Only polymers with a sufficiently low processing temperature, mostly polyolefines

such as polyethylene and polypropylene (PP), are considered suitable for natural

fibre reinforced parts [5]. Polypropylene is used as matrix for a number of reasons.

Firstly, it is easy to process and it is one of the cheapest polymers on the market, as

well as having a low processing temperature. On the other hand, a drawback is its

hydrophobic character (and this accounts for many thermoplastics) which is

incompatible with the typically more hydrophilic nature of plant fibres, creating the

need to modify either the surface of the fibre or the matrix. In the case of PP, maleic

anhydride modified polypropylene (MAPP) is commonly used to enhance chemical

adhesion with natural fibres, proving to be very effective in enhancing the

mechanical properties of the composite [6-8].

Several studies using TGA and DSC techniques on the thermal behaviour of

lignocellulosic (technical) fibres and their composites, such as hemp [9-11], sisal [12,

13], flax [14-16], kenaf [17, 18], jute [3, 14, 19] and bamboo fibres [7, 20, 21] are

available in literature. These studies have shown that the use of an inert atmosphere

during the thermal treatment can delay the thermal degradation in single natural

fibres [22, 23]. In spite of the available data, to the best of the author’s knowledge,

there are not studies that make a reliable connection between single fibre thermal

degradation, and its influence on the mechanical properties of fibres and composites

through mechanical testing. Also, there is a lack of information on the potential

benefit of using inert gas in thermal degradation of individual technical fibres or

during the manufacturing of the natural fibre composites. This is in spite of the good

TGA results under inert environment mentioned above.

It is therefore of practical importance to characterize the effect of high temperature

and exposure time during the processing on natural fibres and their composites. The

results will help to establish and/or adapt manufacturing parameters to prevent

excessive fibre degradation that can reduce the performance of the bamboo fibre-

thermoplastic composite. Also, this will open new options for thermoplastic matrices

and new industrial applications.

In this chapter, the results of the research on the thermal degradation of bamboo

technical fibres and bamboo fibre polypropylene composites (BFPP) are presented,

with the aim to characterize the loss in mechanical properties due to thermal

degradation. Individual technical fibres were thermally treated in different

environments (i.e. air and argon) at different temperature-time couples and

subsequently characterized mechanically using the single fibre tensile test. TGA

analysis (including isothermal TGA), was carried out at the same temperature-time

couples to establish a relationship between the mass loss and single technical fibre

strength.


Bamboo fibre polypropylene composites were produced at different consolidation

temperatures and in different environments (air and argon), in order to determine the

mechanical properties of the material. The samples were characterized in flexural 3

point bending tests (3PBT), with two fibre orientations (longitudinal and transverse).

The relation between the degradation due to thermal treatment and mechanical

properties of single technical fibres in final bamboo fibre PP composites (BFPP) will

be discussed in combination with the TGA results.

6.2 Materials

6.2.1 Bamboo fibres

Mechanically extracted technical bamboo fibres were used in this study. The

characteristics of the fibre and the extraction method are described in sections 4.3

and 4.5.1.

6.2.2 Polypropylene (PP) and maleic anhydride polypropylene (MAPP)

Polypropylene (PP) film with a density of 900 kg/m³ and 20 µm thickness was

supplied by Propex GmbH (Germany) and 0.3% maleic anhydride (MA) grafted

polypropylene (MAPP Bynel 50) was provided by Dupont (Switzerland). Table 6-1

shows the thermal and mechanical properties of these matrices.

Table 6-1. Thermal and mechanical properties of polypropylene (PP) and maleic anhydride polypropylene

(MAPP) used as a matrix, from manufacturer’s data sheets. DSC scans to determine Tm were conducted in a

temperature range from 40 to 600 °C, at a constant heating rate of 10 °C/min.

6.3 Methods

6.3.1 Thermal treatment of single bamboo fibres

The characterization of the thermal degradation of technical bamboo fibres was

carried out by measuring their tensile strength and stiffness after the treatment, at


Matrix Tc

(°C)

Tm

(°C)

Density

(g/cm3)

CTE

(10-6

/K)

Young’s

modulus

(GPa)

Strength

(MPa)

Strain to

failure (%)

PP 115.7 160.6 0.9 62.7 – 73.2 1.6 – 1.8 55 - 65 >300

MAPP 100.3 147 0.89 112.2 – 175.8 0.3 18 475

144 Chapter 6b

different temperature-time couples in different environments (i.e. air and argon). The

starting temperature in the oven was 20 °C (time = 0); technical fibres were heated

at a rate of 5 °C/min until the target temperature (maximum temperature) and then

kept constant until the desired time. Before and after the thermal treatment, the

fibres were conditioned under standard environmental conditions (21°C± 2°C and

50±2 %RH) for 48 hours. Table 6-2 shows all temperature and time of exposure

combinations for the thermal treatment on individual technical fibres.

Time (min)

Max. temperature (°C) 25 50 60 70 80 100 120

180

200

220

250

Table 6-2. Overview of the temperature-time couples for thermal treatment of bamboo technical fibres in air

( ) and argon ( ) atmospheres. The treatment started at RT with a heating rate of 5 °C/min. These

combinations (round points) are also shown in Figures 6-9 and 6-10.

6.3.2 Thermogravimetric analysis (TGA)

This technique measures the mass change of a polymer as a function of temperature

in a controlled atmosphere (e.g. air or argon) as a function of increasing temperature

(with constant heating rate), or as a function of time (with constant temperature

and/or constant mass loss) [24]. TGA can characterize weight loss or gain due to

decomposition, oxidation or dehydratation, to determine the thermal stability and

oxidative stability of a material at temperatures up to 1000 °C. Usually the first

derivative of the TGA curve (DTG) is also calculated in order to identify the points

of greatest rate of change on the weight loss curve.

Thermogravimetric analysis (TGA) for bamboo technical fibres and PP samples

were carried out on a SDT Q600 T.A. Instruments, starting at room temperature

conditions. The experiments were carried out under air and inert atmosphere at a

flow rate of 20mL/min and a heating rate of 5 °C/min. For the TGA measurements,

samples between 11 and 16 mg were tested. For the TGA of bamboo fibres, it must

be mentioned that the fibres were conditioned at 50% HR for 72 hours before the

respective test. This is an important aspect because the moisture content of the fibre

directly depends on the environment. For the graphs and the analysis of the

information, TA Universal Analysis software (TA Instruments, USA) was used.


6.3.3 Single fibre tensile test

The specifications of the machine and the methodology followed for single fibre

tensile tests were described in section 4.4.1.3. For the determination of the technical

fibre’s cross sectional area, the samples were weighed before and after the thermal

treatment. The average diameter for all tested bamboo technical fibres was 181 ± 39

µm. For each batch at least 20 successful tests were carried out to measure the fibre

strength for each thermal treatment with a gauge length of 30 mm, and 10 single

fibre tests were conducted at a gauge length of 70 mm for the determination of the

Young’s modulus.

The fibre Young’s modulus was determined from the slope of the stress-strain curve

(between 0.1 and 0.3% of deformation) of single fibres tested at 70 mm gauge length.

With this long gauge length, it is expected that the machine compliance (e.g.

slippage at the grips) becomes negligible, giving an accurate measurement of the

elongation of the samples during the test, and thus, a reliable calculation for the

Young’s modulus. This is done for practical reasons, taking into account the

numerous tests to be performed for this study. To corroborate the reliability of this

methodology, the Young’s modulus was measured at different span lengths (5, 10,

25, 40 and 70 mm; 20 fibres for each length), and then compared with the values

obtained by Osorio et al [25] (43 GPa) for the same type of fibres. The results

showed that fibres tested at “long” span length (i.e. 70 mm) did not show a

significant difference with the mentioned reference value, see Figure 6-1.

Figure 6-1. Results from this study for the estimation of the Young’s modulus of bamboo fibres from the

stress-strain curve at different gauge lengths (5, 10, 25, 40 and 70 mm) and compared with the value found

by Osorio et al [25] after machine compliance correction (----).

Young’s modulus after machine compliance correction measured by Osorio et al. [25]

5 10 25 40 70

146 Chapter 6b

In Osorio’s study [25], a theoretical correction for the machine compliance,

developed by Defoirdt et al [26], was applied for the same type of bamboo technical

fibres in order to determine the real elongation of the specimens. This methodology

consists of plotting the modulus versus 1/span length. The extrapolation to 1/span =

0 (infinite fibre length), provides the material modulus for which slip and machine

compliance may be ignored. Thus, when the real material modulus is known, an

estimation can be made for the machine compliance. Knowing this compliance,

strain values can be corrected and this calculation must be done for every single

experiment.

6.3.4 Scanning electron microscopy (SEM) observations

Micrographs of fibres and composites were made by Scanning Electron Microscopy


observations using a voltage between 10 and 15 kV.

6.3.5 Bamboo fibre – PP/MAPP composites

6.3.5.1 UD bamboo prepregs

For the preparation of bamboo fibre prepregs, special care was taken to accurately

align and evenly distribute the fibres in a unidirectional (UD) array. The bamboo

fibres were subsequently stabilized by mechanical clamping, as seen in Figure 6-2a.

Two layers of polymer film, polypropylene or maleic anhydride grafted

polypropylene (MAPP), were attached to the fibres on both sides using a hot iron as

seen in Figures 6-2b-c. The hot plate of the iron had a temperature of 172 °C and

was in contact with the fibres for 6 seconds on each side, enough time to pre-

impregnate the bamboo technical fibres with the polymer (Figure 6-2d). A Teflon®

sheet was placed in between the polymer and the hot plate during this operation to

avoid adhesion of the film to the iron. The average areal density of the fibres used in

the prepregs was 210 g/m2 ± 20 g/m

2.

Figure 6-2. Preparation of the UD bamboo fibre prepregs with PP and MAPP thermoplastic polymers.

a. b. c. d.


6.3.5.2 Composite production

The prepregs were dried in an oven at 65°C for at least 72 h and subsequently placed

in a desiccator to avoid moisture absorption, prior to composite production.

Figure 6-3. Preparation of bamboo fibre-PP composites: a) bamboo fibre – PP prepreg, b) preparation of the

stacking sequence of prepregs and PP films to target 45% Vf, c) schematic view of compression moulding and

d), composite samples in longitudinal and transversal fibre direction ready to be tested in 3PBT.

Bamboo fibre thermoplastic composites with polypropylene (BFPP) and maleic

anhydride polypropylene (BFMA) were prepared by compression moulding (Pinette

hot press). Unidirectional bamboo prepregs (6 layers) and thermoplastic films of PP

or MAPP, where intercalated and placed into a cavity mould of 100 x 50 mm

(Figure 6-3a and b). Fibre volume fraction (Vf) was targeted at 45% by weight

measurements. Several temperatures were used for the consolidation of the

composites in air (CAI) and argon (CAR) environments, see Table 6-3.

Composite

Atmosphere

during

manufacturing

Pre-heating

temperature (°C)

(5 min)

Maximum

consolidation

temperature (°C)

(5 min)

CAI-175 Air 155 175

CAI-185 Air 165 185

CAI-200 Air 180 200

CAI-220 Air 200 220

CAI-230 Air 210 230

CAR-200 Argon 180 200

CAR-220 Argon 200 220

CAR-230 Argon 210 230

Table 6-3. Bamboo fibre – polypropylene/MAPP composites consolidated at different temperatures and

environments. The pre-heating temperature was chosen 20 °C below the consolidation temperature (see also

Figure 6-4), in order to evenly heat the mould.

The temperature profile of the consolidation experiments is shown in Figure 6-4.

The manufacturing procedure proceeded as follows:

- Starting at ambient temperature (20 °C)

- Heating rate of 5 °C/min up to the preheating temperature

- Dwell at preheating temperature for 5 minutes

a. b. c. d.

148 Chapter 6b

- Heating rate of 5 °C/min up to the consolidation temperature

- Dwelling for 5 minutes

- Cooling down to room temperature

- A constant pressure of 15 bar was maintained through the whole process.

Figure 6-4. Temperature profile for the consolidation of bamboo-PP/MAPP composites. The pressure was

kept at 15 bar during the whole process

6.3.5.3 Composite production under inert atmosphere

Bamboo fibre-polypropylene composites (BFPP) were manufactured in argon inert

gas, to produce CAR composites (see Table 6-3). They followed the same

methodology for the preparation of BFPP in air, described in the previous section,

except for the final consolidation step. During the compression moulding, the inert

gas atmosphere was maintained by a bagging system, in order to maintain a

controlled atmosphere around the mould during the consolidation phase.

Thermalimide®

film with a high working temperature (427 °C), supplied by Airtech

was used as bagging material. A schematic view of the set up is presented in Figure

6-5, and a general view plus details are shown in Figure 6-6.

Figure 6-5. Schematic representation of the set-up for the production of bamboo fibre-PP composites under

inert atmosphere.

Valves

Argon


A copper tube is connected on one side the bag (Figure 6-5 and 6-6c) with two

bypass valves giving the possibility to control the atmosphere inside of the bag (see

Figure 6-6c). The first valve is connected to a vacuum pump in order to evacuate the

air from the mould; for this purpose, vacuum (1 bar) was applied for at least 5

minutes to the mould. The second valve is used to regulate the inert gas flow inside

of the bag (see Figure 6-6d). The gas flow was maintained at least for 10 minutes

before the start of the compression moulding (Figure 6-6e), when the system started

to be compressed, the excess of argon was evacuated by the vacuum exit. A small

positive internal pressure of argon is maintained during all the process, controlled by

visual inspection, to avoid air into the bagging. The temperature was monitored

using a thermocouple attached directly to the mould, and a constant pressure of 15

bar was applied during the consolidation process. The temperature profiles followed

in these experiments are shown in Table 6-3.

Figure 6-6. Set-up for the production of bamboo BFPP under inert atmosphere. a) General view of the set-up

in the hot press, b) bagging system to contain the inert gas during the compression moulding, c) vacuum

applied to the mould to evacuate the air, d) injection of the inert gas (Ar) into the bagging system and e)

compression moulding under inert gas atmosphere.

6.3.6 Sample preparation and testing

After the production of the BFPP and BFMA composite plates (10 x 5 cm), samples

with average dimensions of 50, 14.4±0.2 and 1.85±0.1 mm, were cut using a low

speed bending saw machine (see Figure 6-7a). All specimens were kept at standard

a. b.

c. d. e.

a. b.

Input

inert gas

Vacuum Bag

Mould

Metal

tape

Mould

Valves

Copper tube

150 Chapter 6b

room conditions (21°C ± 2°C and 50 ± 2 %RH) for at least 24 hours before testing.

Flexural three point bending tests (3PBTs) were performed according to the ASTM

D790-03 standard on a universal testing machine (Instron 4426), with a span length

of 32 mm (pure bending regime).

Figure 6-7. a) Three-point-bending test composite samples with longitudinal and transverse disposition of the

fibres; these orientations are represented schematically in b) and c) respectively.

The BFPP and BFMA samples had 0° and 90° fibre orientation (see Figure 6-7b and

c respectively), to evaluate longitudinal and transverse properties. The longitudinal

disposition was evaluated with the aim to determine the flexural properties such as

flexural strength and flexural stiffness. The transversal direction was carried out to

estimate the interface strength between the polymer and the thermoplastic matrix.

The Young’s modulus was calculated from the slope at the beginning of the stress-

strain curve, between 0.1 and 0.3% of deformation.


6.4.1 Thermogravimetric analysis (TGA)

TGA/DTG curves for bamboo fibres in air and argon environments are shown in

Figure 6-8. It is noticed that the first drop in mass loss, around 5.2%, at temperatures

around 100 °C is due to the moisture evaporation. The values found in literature for

natural fibres due to humidity release for the same range of temperature, ranged in

between 3 and 7 % [23, 27].

From 100 °C up to 193 °C there is no significant mass change. After this, the first

signs of thermal degradation started to take place with a slightly observable peak at

275 °C in air, and at 288 °C when treated in inert atmosphere. This mass loss is

attributed to the degradation of hemicellulose as reported by several authors [28, 29].

They reported that the decomposition of pure hemicellulose starts between 220 and

315 °C, with a maximum mass loss rate (0.95 wt.%/°C) at 268 °C [29]. For natural

a.

b.

c.


fibres (e.g. sisal), this value started at 234 °C with a maximum at 297 °C [28]. The

hemicelluloses, are the most reactive component in the fibre (degrading at lower

temperature), due to their relatively low molecular weight [30].

Figure 6-8. TGA/DTG analysis for single bamboo fibres under different environments (air and argon).

At temperatures between 100 and 250 °C, some of the changes in physical properties

of the fibres can be explained in terms of alterations in either physical or chemical

structures such as depolymerization, hydrolysis, dehydration, decarboxylation and

recrystallization. Besides this, the formation of free radicals has been noticed [14].

They can contribute to accelerate the degradation process due to the formation of

hydroperoxide groups, responsible to a large extent for the depolymerization of

cellulose (bond scission). At higher temperatures, the cellulose thermal degradation

observed in the TGA analysis (Figure 6-8), for the bamboo technical fibres (310 °C),

is likely caused by the destruction of hydrogen bridges, the loss of water,

depolymerisation, glass transitions, changes in crystallinity, and some processes

mentioned before (i.e. the formation of free radicals, carbonyl groups, and carboxyl

groups) but at a larger scale (especially in air) [16]. The cellulose decomposition

under inert environment is shifted upwards by around 20-30 °C.

In air environment, the degradation peaks for hemicellulose (275 °C), cellulose

(310 °C) and lignin (444°C), were identified very clearly from the DTG analysis.

The same result was not found, for example for sisal fibres, where there were no

distinct peaks for each constituent, due to the presence of oxygen that accelerates the

152 Chapter 6b

degradation process, making not possible the identification of the peaks [28]. The

thermogravimetric behaviour of bamboo fibres has a direct correlation with their

chemical constituents: hemicellulose, cellulose and lignin. In fact, the TG/DTG

curves of common lignocellulosic fibres such as jute, sisal, wood and cotton display

similar aspects that could be correlated to the thermal decomposition of their main

constituents [31].

The lignin degradation is only visible for the fibres treated in air at around 444 °C.

Pure lignin was found to be the most difficult one to decompose, doing it slowly

over the whole temperature range from ambient to 900 °C [29]. Generally, the

contribution of hemicellulose and cellulose to the thermal degradation is of much

higher importance [23]. In general in literature, different peaks were found

depending on the type of environment where the analysis was carried out.

Decomposition in air was faster, more complete and proceeded at lower temperature

than in inert environment. A clear mass loss delay was observed when the fibre was

treated under inert atmosphere. This is because in air environment, the process of

decomposition of the cellulose occurs much quicker as a result of the reaction of free

radicals (R∙) with oxygen (oxidation process), during thermal degradation [32]. This

reaction produces a peroxy radical (ROO∙) which in turn removes an hydrogen atom

from another polymer molecule to form a hydroperoxide (ROOH) and so

regenerates another free radical through which the process can continue [33]. With

the presence of an inert atmosphere during the thermal treatment the reaction with

oxygen is limited to the minimum, attenuating the above reaction sequence.

In Figure 6-8, the onset temperature defined by the “shoulder” of the curve, between

100 and 190 °C remains almost invariable. But even if the mass loss is not evident in

this range, the tensile properties can be affected much earlier [28]. According to

Aziz et al [17], during this shoulder region there is still a gradual degradation,

including: depolymerisation, hydrolysis, oxidation, dehydration and decarboxylation,

mainly in the cellulose, and leading to a reduction of the fibre strength.

6.4.1.1 Isothermal TGA experiments

Isothermal TGA measurements for bamboo technical fibres were carried out over a

period of 170 minutes at different temperatures and in two different environments

(air and argon), see Figures 6-9 and 6-10. In both figures, an arrow indicates the

moment when the targeted temperature is reached and round points for air ( ) and

argon ( ) environments, show different combinations of temperature – exposure

time. These variables were used to accomplish thermal treatment on several batches

of single bamboo fibres for further tensile characterization.


Figure 6-9. Isothermal TGA in air atmosphere. The arrow ( ) indicates the moment when the target

temperature is reached. The round points ( ) show different temperature-time- couples (see Table 6-2) for

further single (technical) fibre thermal characterization (see also Figure 6-11 and Table 6-4).

further single (technical) fibre thermal characterization (Figure 6-11).

Figure 6-10. Isothermal TGA in inert (argon) atmosphere. The arrow ( ) indicates the moment when the

target temperature is reached. The round points ( ) show selected different temperature-time couples (see

Table 6-2) for further single (technical) fibre thermal characterization (see also Figure 6-16 and Table 6-4).

180 °C

200 °C

220 °C 250 °C

180 °C 200 °C 220 °C 250 °C

160 °C 180 °C

200 °C

250 °C

220 °C

180 °C

200 °C

220 °C

250 °C

160 °C 180 °C 200 °C 220 °C 250 °C

160 °C 180 °C

200 °C

220 °C

250 °C

154 Chapter 6b

The results show no apparent degradation at 160 °C at least after 140 min of

exposure (Figures 6-9 and 6-10). Above this temperature, the thermal stability of the

bamboo fibres starts to gradually decrease and becomes more critical when the

exposure time is longer as it can be seen in Figure 6-9. According to several authors

[23, 34, 35], first degradation typically occurs at temperatures above 180°C and

more severe damage due to thermal decomposition of most natural fibres occurs

within a temperature range between 215 and 310°C.

An initial mass loss due to moisture evaporation, reaching a “plateau”, at around 4-

5% was found. The total moisture content of a bamboo technical fibre at 47% RH, is

6% ± 0.5 measured by [36] in a previous study with the same type of fibres. The

moisture content of the fibres will be discussed in more detail in section 6.4.2.1. It is

clearly noticed that the mass loss in fibres exposed to an inert environment (Figure

6-10), is significantly delayed in comparison with air treatments; a difference in

mass loss of 3, 14 and 15 % can be observed after 150 minutes of exposure at 200,

220 and 250 °C respectively. These results indicate that an inert atmosphere protects

the fibre against thermal degradation.

6.4.2 Tensile properties for single fibre after thermal treatment

6.4.2.1 Fibre strength

For a better understanding of the mechanical behaviour of the composites, tensile

tests on single bamboo technical fibres were carried out after exposure at different

temperature-time couples. The results are presented in Figure 6-11 and Table 6-4;

they show that the fibre strength is evidently affected by the temperature and

decreases with increased exposure time.

This dependency has been also observed in other studies on natural fibres [16, 35],

where higher temperatures and longer exposure times result in higher decreases of

the mechanical properties. The decrease of the mechanical properties was not only

attributed to the thermal degradation. Studies on flax [37] and sisal [38] fibres at

elevated temperatures, attributed the decay in mechanical properties to differences in

coefficients of thermal expansion between cellulose, hemicellulose, lignin and

pectin. As a consequence, at elevated temperatures the mismatch in expansion

between structure forming components of the fibre creates internal stresses, resulting

in weakening of the fibre. Moreover, the thermal degradation of the fibre

components will intensify this problem.


Figure 6-11.Single bamboo technical fibre strength after thermal treatment at different temperature-time

couples in air atmosphere.

Table 6-4. Tensile mechanical properties for thermally treated bamboo technical fibres. The results for argon

treated fibres are also shown in Figures 6-15 and 6-16.


Temperature

(°C)

Time of

exposure (min)

Strength

(MPa)

Young’s

modulus (GPa)

Strain to

failure (%)

Air

Room temp. 0 733 ± 120 44 ± 2 1.8 ± 0.22

180 25 735 ± 100 39 ± 3 1.6 ± 0.16

180 60 564 ± 120 46 ± 4. 1.3 ± 0.12

180 120 462 ± 90 45 ± 2 1.1 ± 0.18

200 25 709 ± 120 45 ± 3 1.3 ± 0.31

200 50 487 ± 80 43 ± 4 0.67 ± 0.25

200 70 330 ± 50 41 ± 2 0.74 ± 0.16

200 90 312 ± 70 42 ± 4 0.50 ± 0.11

200 100 277 ± 80 41 ± 2 0.74 ± 0.10

220 25 591 ± 100 43 ± 3 1.4 ± 0.16

220 60 290 ± 70 36 ± 3 0.62 ± 0.10

220 70 226 ± 70 44 ± 3 0.52 ± 0.06

220 80 181 ± 70 37 ± 2 0.57 ± 0.18

250 50 149 ± 50 38 ± 2 0.37 ± 0.08

250 60 99 ± 60 37 ± 4 0.42 ± 0.13

Argon

200 25 680 ± 140 44 ±2 1.6 ± 0.17

200 50 591 ± 70 41 ± 2 1.1 ± 0.26

200 70 530 ± 90 45 ± 3 1.2 ± 0.25

250 50 271 ± 70 38 ± 2 0.8 ± 0.15

156 Chapter 6b

The reduction in the technical fibre strength after thermal exposure found in this

research, is in agreement with the study of Ochi et al [39], where temperatures

below 140°C do not significantly affect the mechanical properties of bamboo fibres.

However, in his study faster fibre degradation was found where at temperatures

around 160 °C, a gradual decrease of the tensile strength was observed after 30 min

of exposure, contrary to the present results. Moreover, at temperatures between

180°C and 200°C, the tensile strength dropped around 25% in the first 10 minutes

and tended to stabilize after 30 min of exposure.

Correlation between mass loss and fibre strength

In order to establish a correlation between the fibre mass loss and fibre strength after

thermal treatment, specimens for tensile testing were selected based on the

isothermal TGA results, specifically chosen from the round points ( ) shown in

Figure 6-9. Two criteria in the selection of these round points, based on literature

and previous experiences, were: a maximum 18% fibre mass loss and an exposure

time less than 120 minutes. This was done in order to establish the fibre strength

after certain time of exposure as shown in Figure 6-11, but also to determine the

fibre strength after certain mass loss.

A correlation was found between mass loss and strength properties of the technical

fibres in air atmosphere (Figure 6-12a). A distinction between mass loss due to both

moisture evaporation and thermal degradation is pointed out. The point P1 is the

reference corresponding to the fibre strength at room conditions (21°C± 2°C and

50±2 %RH), meaning zero mass loss. P2 shows fibre strength measurements after 25

minutes of thermal exposure that corresponds to the “plateau” region previously

shown in Figure 6-9. ANOVA (α= 0.05) revealed no statistically significant

differences between the average fibre strength corresponding to P1 and P2. The point

P3 marks the starting point of mass loss due to thermal degradation.

Figure 6-12b shows the P1, P2 and P3 points in an isothermal TGA in air at 200 °C.

This curve is representative for the other four temperatures analyzed with the same

technique (180, 220 and 250 °C). P2 is situated in the “plateau” region 25 minutes

after the beginning of the TGA experiment, at ~143 °C and a mass loss of around 4-

5%. At this point, the fibres have been exposed to a temperature above 100 °C

during 8.5 minutes. These conditions are often considered sufficient to release all

moisture present in the fibre, but this is not correct. To explain this, two concepts

need to be defined; “free water” and “bound water”. The first one is defined as the

“unbound” water in polymers and the second one, is the water chemically bonded

(strongly attached) to the hydroxyl groups of cellulose molecules [40].


Figure 6-12. a) Correlation between the fibre strength and mass loss in air environment. P1 corresponds to the

fibre strength at starting room conditions. P2 refers to strength values, 25 minutes after starting the isothermal

treatment. This point corresponds to the TGA “plateau” region previously shown in Figure 6-9. P3

corresponds to the zone where the technical bamboo fibres start to lose mass due to thermal degradation (>

6% mass loss). b) Isothermal TGA at 200 °C (taken as an example for all isothermal experiments), showing

the same corresponding P1, P2 and P3 points explained above.

Mass loss due to thermal degradation

P1

P2

Mass loss due to

moisture evaporation

a.

P3

b.

P3

P1

P2

Mass loss due to thermal degradation Mass loss due

to moisture

evaporation

b.

Mas

s (%

)

Tem

per

atu

re (

°C)

Temperature vs time

Mass loss vs time

158 Chapter 6b

The plateau reached by the mass loss in Figures 6-9 and 6-12b in P2 is a consequence

of the evaporation of the “free water”. After a standard drying procedure, it is

expected that a natural fibre still has constitutional tightly bound water remaining in

its structure [37]. For the same type of bamboo technical fibres, the moisture content

was reported as 6% ± 0.5 [36] at 47% RH. At this point, it is expected that the free

water is released because even at prolonged heat exposure (100 °C for 16 h), no

further mass loss was observed. In Figure 6-12b, the mentioned 6% of mass loss is

reached at around 200 °C (where the fibres are already ~20 min above 100 °C).

After this, the mass loss started a quasi-linear decrease. Fibre thermal treatments of

manufacturing processes that gives more than ~7% of mass loss, start to

significantly affect the mechanical properties of the fibres (~33% of fibre strength

reduction), and hence the performance of the composites.

Figure 6-12a, shows a strong influence of the mass loss of the fibres due to thermal

degradation, on their strength properties. A simple quadratic curve fit (Equation 6-1)

describes this dependency with reasonable correlation (R2= 88 %), providing an

empirical model to estimate the fibre strength at certain mass loss of dry bamboo

technical fibres.

y= 3.1x2 – 106 x +1056 (6-1)

6.4.2.2 Fibre stiffness and strain to failure

A slight decrease in Young’s modulus of the technical fibre is observed after

thermal treatment, see Figure 6-13. The values ranged in between 36 and 45 GPa,

showing a rather constant tendency for all temperature-time couple combinations,

and remaining stable for low temperature treatments. Similar results are reported by

Ochi et al [39], where the Young’s modulus for the bamboo fibres was found to be

almost independent of the applied thermal treatment. The TGA results (Figure 6-9)

and the Young’s modulus measured on thermally treated fibres at specific

temperature-time couples in air (Table 6-2) were used to establish a correlation

between mass loss and the stiffness of the fibre (Figure 6-14). This relationship can

be described with a regression of the obtained experimental data, by a simple

quadratic equation (Equation 6-2) with reasonable correlation of the effect of the

thermal treatment (R2 = 0.81).

y= 0,0408x2 – 1.58x + 53.22 (6-2)


Figure 6-13. Young’s modulus for single bamboo technical fibres at different time-temperature couples in air

environment.

Figure 6-14. Correlation between Young’s modulus and mass loss in single bamboo technical fibres in air.

This relationship given by equation 6-2 (R2 = 0.81), becomes also an empirical model to describe the stiffness

in function of the mass loss for technical bamboo fibres.


Mass loss due to moisture evaporation

160 Chapter 6b

Thermal treatment shows a significant effect on the strain to failure of the bamboo

technical fibre. Exposure times of 50 minutes or more at temperatures of 200 °C or

higher, show a dramatic drop in failure strain to less than 50% of the original value,

see Table 6-4. Changes taking place in molecular structure of the fibre due to

decomposition of the long polymers cause a decrease in elasticity of the fibre and

the fibre becomes more fragile [30]. Furthermore, Gassan and Bledzky [14] reported

a significant reduction of the tenacity in natural fibres when treated at low

temperatures (i.e. 150 °C) and longer exposure times (up to 4h). Other studies report

up to a factor or 3 in reduction of the failure strain after thermal treatment [15]. The

strain at maximum strength vs mass loss is shown in Figure 6-15, showing the high

sensitivity of this property due to a thermal treatment.

Figure 6-15. Correlation between strain at maximum strength and mass loss in single bamboo technical fibres

after thermal treatment in air (R2=0.68).

6.4.2.3 Influence of an inert atmosphere on the mechanical properties of

thermally treated bamboo technical fibres

A clear positive effect on fibre strength is observed when the thermal treatment is

carried out under inert atmosphere, showing less fibre strength reduction amounting

up to 37% and 45% for treatments at 200 °C for 70 minutes and 250 °C for 50

minutes respectively (see Figure 6-16). Higher thermal resistance in flax [15], jute [3]

and hemp [22] technical fibres, had been found when treating these fibres under

inert atmosphere.

Mass loss due to moisture evaporation



Fibre stiffness did not show a significant response when thermally treating technical

bamboo fibres at 200 °C under different environments (i.e. air and argon) and

several times of exposure (25, 50 and 75 min), see Figure 6-17. For the same

treatment combinations, the strain at maximum strength showed less reduction under

inert atmosphere (Figure 6-17).

Figure 6-16. Fibre strength comparison for thermally treated bamboo technical fibres in air and argon

atmospheres at different temperatures.

Figure 6-17. Young’s modulus comparison for thermally treated bamboo technical fibres in air and argon

atmospheres at 200°C.

18%

45%

37%

162 Chapter 6b

6.4.2.4 Thermally treated bamboo fibres under SEM

SEM micrographs were made of the fibre fracture surface after tensile testing. For

untreated fibres, mainly three types of fracture were identified: (a) a straight crack

right through the elementary fibres, (b) a crack proceeding along the primary layer

that surrounds the elementary fibres and (c), a combination of the two. There was

not found a regular pattern of failure types (see also section 4.3.1 for more

information about the type of failure in individual untreated fibres).

For thermally treated fibres, two main characteristics were observed in the type of

fracture. The first one corresponds to a significant structural damage of the technical

fibre with a limited presence of pull-out of elementary fibres. This is largely

observed for treatments up to 200 °C and exposure times between 30 and 50 minutes

(see Figure 6-18a). This phenomenon can be attributed to the degradation of the

hemicellulose which tends to degrade first at moderate temperatures. The second

characteristic corresponds to a relatively clean fracture of the fibres, caused by the

damage on the structure and resulting in a higher brittleness of the fibres. This is

observed mostly for severe thermal treatments (i.e. 220 and 250 °C for exposure

times longer than 60 minutes), as shown in Figure 6-18b.

In parallel to these phenomena, a smoothening of the fibre surface was observed for

some of the fibres (see Figure 6-18c). The more regular surface can be attributed to

the softening of the lignin layer considered to be a thermoplastic polymer [22, 41],

exhibiting a glass transition temperature of around 90° and a melting temperature of

around 170 -200 °C [42, 43]. Studies on bamboo technical fibres [44], indicated that

the surface is homogeneously covered with a lignin layer instead of cellulose.

Furthermore, AFM observations reported by Fuentes et al. [45], show that autoclave

treatment (3 bar at 150 °C) applied to bamboo technical fibres, reduced the surface

irregularities by smoothening of the lignin layer present on the fibres.

XPS analyses were carried out on thermally treated fibres in presence of argon and

air. The results indicated that the thermal treatment of technical bamboo fibres in

any of the environments did not change the surface chemical components of the

fibres, in comparison with the untreated samples (see Appendix 2). Also, after SEM

observations for the different batches, there was no evidence for any of the time-

temperature couples including air and inert atmospheres, for splitting or visible

damage to the fibre surface as a consequence of exposure to the high temperatures.

These two aspects where reported by Prasad et al. [22], where the splitting of hemp

fibres was present in both air and nitrogen environment at 220 °C, but fibre surface

damage happened only in air.


Figure 6-18. SEM images of the failure modes after tensile testing of thermally treated bamboo technical

fibres showing a) structural damage of the bamboo technical fibre with a limited presence of pull-out, b) a

relatively clear fracture and c) a smoothening of the fibre surface observed for some of the fibres.

a.

b.

c.

164 Chapter 6b

6.4.3 Bamboo fibre polypropylene composites (BFPP)

6.4.3.1 Fibre degradation during composite processing

The fibre mass loss after following the same temperature profile used to

manufacture the BFPP composites (CAI and CAR profiles) was characterized by

TGA. Three different manufacturing patterns were analyzed in air (CAI-200, CAI-

220 and CAI-230) and argon (CAR-200, CAR- 220 and CAR-230) atmospheres.

Figure 6-19 shows a TGA profile for the CAI-200 composite, following all the steps

that were applied for the manufacturing of the BFPP (i.e. starting temperature

conditions, heating rate of 5 °C/min, dwell and maximum consolidation

temperatures). In this analysis, the total mass loss of the fibre at the end of the

process was 6.7% in air conditions and 5.8% when the treatment was performed in

inert atmosphere. The same difference in mass loss (~1%) between these two

environments was observed for the other two studied patterns (CAI/CAR-220 and

CAI/CAR-230), as can be seen in Table 6-5. The mass loss measured by the TGA

for technical bamboo fibres after the corresponding manufacturing process (as it was

done for CAI/CAR-200 in Figure 6-19), was used in Equations 6-1 and 6-2, to

obtain the estimated fibre strength and stiffness (see Table 6-6). These values were

used to recalculate the properties of the BFPP and will be shown as the “theoretical

composite strength” in section 6.4.3.4.

The estimated fibre strength after thermal treatments CAR-200, CAR-220 and CAR-

230 (Table 6-5), was calculated to be respectively 12, 17 and 18% higher than for

fibres treated in air environment with the same temperature profile (CAI’s). This

improvement, theoretically, will give an improvement in strength properties in the

final composites between 10 and 13%.

These results suggest not only that it is advantageous to use an inert atmosphere for

the manufacturing of BFPP, but also reveal the magnitude of the fibre strength

reduction, as a consequence of the composite manufacturing. It is common to

consider the original fibre strength in virgin state as a reference for the estimation of

the final composite properties. In the case of untreated bamboo technical fibres, this

value is around 733 MPa, and the theoretical strength of the unidirectional BFPP

(with the rule of mixtures) will be ~360 MPa (45% Vf). However, the theoretical

composite strength taking into account the strength reduction of the fibre after

processing will be evidently lower (e.g. almost a reduction by half for the composite

strength for CAI-230).


Figure 6-19. TGA analysis for bamboo fibres carried out in air and argon, following the same temperature

profile for the manufacturing of the composite (CAI-200 and CAR-200).

Table 6-5. Fibre mass loss, and estimated technical fibre properties after different thermal treatments

following the same temperature profiles used in the manufacturing of BFPP composites. The theoretical

composite strength according to the estimated fibre strength for each manufacturing process is also shown.

*Starting at room conditions. ** Estimated after TGA measurements following the composite temperature

profile. ***Estimated properties obtained by Equations 6-1 and 6-2 respectively.

Estimated properties after

thermal treatment

Composite

Maximum

processing

temperature

(°C)

Process

duration

(min)*

Mass

loss**

(%)

Fibre

strength***

(MPa)

Fibre

Young’s

modulus***

(GPa)

Air

CAI-200 200 45.2 6.7 485 44

CAI-220 220 49.6 9.0 351 42

CAI-230 230 51.1 10.9 269 41

Inert (argon)

CAR-200 200 45.2 5.8 545 44

CAR-220 220 49.6 7.9 412 43

CAR-230 230 51.1 9.7 319 41

166 Chapter 6b

6.4.3.2 Matrix degradation

The matrix is also exposed to thermal degradation during composite manufacturing.

Figure 6-20 shows the TGA analysis for the polypropylene used in this study in air

and argon environments. According to this test, the onset degradation temperature in

air is taking place at around 250 °C while in inert atmosphere (argon) it is around

293 °C. This result shows that the maximum consolidation temperature used in this

research (230 °C, see Table 6-3), is low enough not to expect a drop in properties of

the PP matrix.

These results are in agreement with similar studies [3], where it was also found that

the degradation of polypropylene occurs about 65 °C lower in air atmosphere than in

inert atmosphere. It was found that in argon, PP starts to degrade at around 300°C

with a maximum weight loss at 431 °C. This degradation is initiated primarily by

thermal scissions of C–C chain bonds accompanied by a transfer of hydrogen at the

site of scission [46]. In air environment, the thermal stability of PP is significantly

reduced by oxidative dehydrogenation accompanied by hydrogen abstraction [47].

6.4.3.3 TGA analysis for bamboo fibre-polypropylene composites

A TGA comparison between bamboo fibres, neat PP and BFPP is shown in Figure

6-20. The BFPP’s present better performance than the individual constituents (i.e.

neat PP and bamboo fibres), especially in air. Nevertheless, it was noticed that both

single technical fibres and BFPP, have equal starting mass loss at 193 °C as can be

seen in the detail in Figure 6-20. The main contribution of the initial mass loss in the

composite is caused by the fibre, nevertheless the degradation appears be slower in

the composite, meaning that the fibres are protected by the polymer.

The overall better performance of BFPP in comparison with the neat PP and the

single fibre is in accordance with the results of Doan et al. [3], attributing the better

thermal behaviour of the composite to the interfacial adhesion between fibre and

matrix. According to the results obtained by Martin et al. [28] on DSC and TGA

analyses in sisal fibres, the maximum temperatures to produce composites

reinforced by sisal fibre should be under 185 °C due to the degradation of

hemicellulose and cellulose components, that starts around 180 °C; which is lower

than what was observed in bamboo fibres. The starting temperature degradation for

the hemicellulose in bamboo technical fibres was found to be around 200 °C in

combination with a mass loss around 6% based on the TGA (section 6.4.1, Figure 6-

8). Martin et al [1] also reported a significant decrease in the strength properties after

6% mass loss as it was shown in this study in Figures 6-12a and b. Regarding the


degradation of the fibre in terms of mechanical properties, the results indicate that it

is more relevant to consider the mass loss instead of the temperature and time of

exposure, to assess the fibre strength degradation and hence the properties of the

composite.

Figure 6-20. TGA for technical bamboo fibres, neat PP and BFPP in air and argon environments; inset shows

DTG detail for single bamboo fibre and BFPP, showing similar onset of degradation at 193°C.

Influence of the fibre volume fraction on the thermal degradation of the BFPP

When the matrix and reinforcement are analyzed with the TGA technique (Figure 6-

20), it is observed that the thermal stability of the neat PP in air is considerably

lower than for the bamboo fibres (this is again assigned to the degradation of PP by

thermal scission of C–C chain bonds [3]), and the opposite is found when they are

analyzed under inert atmosphere. In a similar study, the neat PP had a higher thermal

resistance in comparison with jute fibres in nitrogen [48]. As a consequence, the

overall thermal resistance of PP-jute composites decreased with increasing fibre

content in nitrogen atmosphere, but increased in air environment due to the lower

thermal stability of PP compared to jute. George et al. [49] also reported a decrease

of the thermal stability of the whole composite when increasing the fibre volume

fraction of jute fibre in the system. Even though experiments with different fibre

Initial DTG curves for single bamboo fibre and BFPP samples

168 Chapter 6b

volume fractions were not performed in this study, a similar situation can be

expected for the BFPP’s in inert environment.

The degradation of the individual components, i.e. polymer and fibres, characterized

by TGA, can help to understand the degradation mechanism of the entire natural

fibre composite. Its threshold decomposition temperature indicates the limit for the

composite manufacturing temperature, but might be accompanied by other

complementary techniques such as isothermal TGA and thermal single fibre testing

to have a clearer picture of the degradation phenomena.

6.4.3.4 Flexural strength of bamboo fibre-polypropylene composites

The flexural strength of bamboo fibre-polypropylene composites (BFPP) and their

correspondent efficiency factors at different consolidation temperature profiles are

shown in Table 6-6 and Figure 6-21. The flexural strength of UD bamboo fibre

composites is affected by two factors. First, the fibre strength is reduced by thermal

degradation due to the exposure to relatively high temperatures during the

manufacturing process (see section 6.4.2). Second, the effective fibre/matrix

adhesion determines the ability to transfer the load between the two constituents.

Composite

Maximum

processing

temperature

(°C)

Experimental

flexural

strength

(MPa)

Strain at

maximum

strength

(%)

Efficiency

factor*

(%)

Theoretical

composite

strength

(MPa)**

Air

CAI-175 175 178 ± 15 2.4 ± 0.15 55 360

CAI-185 185 192 ± 8 1.9 ± 0.12 53 345

CAI-200 200 169 ± 2 2,1 ± 0,08 67 251

CAI-220 220 148 ± 3 1,1 ± 0,03 78 190

CAI-230 230 90 ± 15 0,8 ± 0,2 58 154

Argon

CAR-200 200 173 ± 5 2,3 ± 0,30 61 278

CAR-220 220 157 ± 4 1,5 ± 0,08 72 218

CAR-230 230 126 ± 9 0,9 ± 0,06 71 177

Table 6-6. Flexural properties of BFPP consolidated under different environments. *The efficiency factors

and the theoretical composite strength were calculated with the correspondent fibre strength after thermal

treatment according to Table 6-5, see also Figure 6-26. ** Calculated with the ROM, considering the

properties of the fibre after thermal degradation due to the manufacturing process and a Vf = 45%.


Figure 6-21. Longitudinal flexural strength for BFPP consolidated in air and argon at different temperatures,

the values were normalized at Vf= 45%. The theoretical composite strength (obtained with the ROM), was

calculated with the correspondent fibre strength after thermal treatment (according to Table 6-5) for air and

argon treatments. The efficiency factors are shown in Figure 6-26.

Experimental flexural strength results for BFPP show no statistically significant

difference according to the Student’s t-test (α= 0.05), between CAI-175 and CAI-

200 and between CAI-175 and CAI-185. This shows an unclear tendency between

these relatively low consolidation temperatures. Furthermore, they exhibited a

continuous deformation (plateau), without a clear fracture when tested in 3PBT (see

CAI-200 stress-strain curve in Figure 6-22). This continuous deformation is an

indication that either the processing temperature is not enough to melt and properly

consolidate the layers of fibres and polypropylene, creating bad impregnation (see

e.g. fracture surface of CAI-185 in Figure 6-23), or that the fibre-matrix interface is

weak. The bad or weak fibre/matrix interface generates an extensive pull-out during

the high strain of the sample. The higher the strain, the higher is the pull-out length

[50]. For higher consolidation temperatures, e.g. CAI-220, the matrix flows better

around and between the fibres and the composites showed a clearer fracture surface

(Figure 6-22). The reduced fibre strength will also play a role in this behaviour.

The good thermal resistance for single fibres under inert environment is effectively

translated to the flexural strength of the composites, where an improvement of 6 and

28% at 220 and 230 °C respectively is obtained in comparison with the composites

produced in air (Table 6-6). This shows that the composite production under inert

170 Chapter 6b

gas is protecting the fibre delaying the reduction in strength properties. The

composites are becoming more brittle with increasing processing temperature with a

better strain to failure behaviour in argon environment (e.g. ~25% more when

comparing CAI and CAR at 220 °C. In the SEM images, shown in Figure 6-24, it

can be observed that the pull-out length of the BFPP’s decreases with increasing

temperature, which is a sign of embrittlement and shift of interface failure to fibre

failure.

Figure 6-22. Typical flexural stress-strain curve for CAI-200 and CAI-220.

Figure 6-23. SEM images of fractured BFPP samples after consolidation in air at 185 °C (CAI-185) showing

bad impregnation and large scale pull-out.

CAI-200

CAI-220


Figure 6-24. SEM images of fractured BFPP samples after consolidation at different temperatures: a)

CAI/CAR-200, b) CAI/CAR-220 and c) CAI/CAR-230 after processing in air and inert conditions.

Figure 6-25 shows when comparing the strength - strain-to-failure results for all

produced BFPP’s, that there is a narrow process window to obtain a good balance

between good impregnation and low fibre degradation, where the inert atmosphere

can enlarge the window to obtain better properties. This is in agreement with what is

found in literature (based in TGA analysis), regarding the benefit of using inert

atmosphere with natural fibres to obtain better mechanical properties. Wielage et al

[35] stated that in air atmosphere, thermal decomposition of technical flax fibres

started significantly earlier than in inert atmosphere. The results, show that the use

Air environement Inert environment M

ax. C

on

soli

dati

on

tem

per

atu

re:

200 °

C

Max. C

on

soli

dati

on

tem

per

atu

re:

230 °

C

Max. C

on

soli

dati

on

tem

per

atu

re:

220 °

C

172 Chapter 6b

of inert gas increases the resistance of the fibres to a relatively high temperature.

This increases the possibility to use higher processing temperatures or longer cycle

times, opening new alternatives for thermoplastic matrices with better properties,

such as PVDF and PA6 with shaping temperatures of 190-230 and 240 °C

respectively.

Figure 6-2xx. Stress at maximum strain of BFPP consolidated in air and argon atmospheres.

Figure 6-25. Flexural strength vs strain at maximum strength of BFPP at different consolidation temperatures

in air and argon environments. 1= full impregnation, low fibre pull-out and high fibre degradation. 2= bad

impregnation and high fibre pull-out and 3= partial impregnation and still presence of fibre pull-out.

Finally, as it was demonstrated, the fibre damage after thermal exposure will vary

according to the severity of the thermal treatment. For this reason, an estimation of

realistic theoretical composite properties and efficiency factors (EF) will depend on

the fibre strength values that are chosen. Figure 6-26 shows significant differences

between the EF’s calculated with the fibre strength of an untreated fibre (730 MPa);

in comparison with EF’s obtained with the correspondent fibre strength after the

manufacturing process (Table 6-5), for CAI and CAR samples.

3

1

2


Figure 6-26. Efficiency factors calculated with two different values, the first one with the original fibre

strength of the bamboo technical fibre (~730 MPa) and the second one with the fibre strength after each

manufacturing process (see Table 6-6), in air and argon atmospheres.

In the first case (Figure 6-21), when EF is calculated with the strength of an

untreated fibre, the values present a constant decrease (Figure 6-26) because of the

fibre strength reduction due to the thermal exposure. For the CAR samples, the EF’s

are higher than the CAI because the fibre is protected by the argon as show in Table

6-5. For the second case, when the EF is calculated according to the fibre strength

corresponding to each manufacturing process, is possible to observe a peak value for

CAI-220. This EF improvement can be explained by the fact that there is a balance

between moderate fibre degradation and good wetting, for CAI-230 samples, the

fibre degradation is too high and the EF decreased. When the process is carried out

in an inert atmosphere, the EF’s present an improvement for the CAR-220/230. In

this case, there is good wetting, due to the relative high temperatures, and at the

same time the strength of the fibre is better conserved (reaching values close to

80%). These results proves that the loss in properties of the fibre using high-

temperature manufacturing have to be taken into account, because it points out the

importance of an efficient fibre/matrix consolidation manufacturing process as the

heat exposure can affect the intrinsic properties of the fibres.

174 Chapter 6b

6.4.3.5 Bamboo fibre-maleic anhydride polypropylene composites (BFMA)

BFMA samples in longitudinal direction CAI-175, CAI-185 and CAI-200 had a

maximum flexural strength of 163, 179 and 152 MPa respectively. These values did

not present an improvement in properties in comparison with the BFPP consolidated

at the same temperatures shown in Table 6-6. Although an improvement in interface

strength was found (see Table 6-7 further-on), this shows that the behaviour in the

UD composite is still largely determined by fibre strength. Although MAPP has a

better adhesion to natural fibres, it is also a weaker polymer than PP. In all, flexural

composite strength is not affected much.

Even if epoxy resins give good efficiency factors for bamboo fibre composites of 82

and 95 % for strength and stiffness respectively, the BFPP composite strength for PP

and MAPP remains relatively low. This reflects a combination of insufficient bond

strength and weak polymer mechanical properties. In Figure 6-27 flexural properties

of bamboo fibre thermoplastic composites are compared with similar natural fibre

composites. This graph is indicative because fibre volume fractions vary strongly.

Chattopadhyay et al. [7] reported a general increase of 81% in the flexural strength,

150% in the flexural modulus, 105% in tensile strength and 191% in tensile modulus

for short bamboo fibres (1-6 mm) reinforced MAPP compared to the PP benchmark.

Also Chen et al [8], demonstrated that a content that the MA in bamboo PP

composites (short fibre), resulted in higher tensile strength. This positive effect is

attributed to the stronger interaction between the fibre and matrix due to the reaction

between acid groups of the maleic anhydride groups and hydrophilic groups on the

fibre surface [51]. In the case of flax, good results have been obtained with MAPP

[52]. It is clear that compatibility is the key in these systems. For more data on

flexural and tensile properties of natural fibre thermoplastic composites, see Tables

3-5 and 3-6.

When the unidirectional composite is tested in transverse direction, the fibre-matrix

interface will often dominate the final composite properties, and the interface quality

of the composite can be characterized. Transverse properties of BFMA are shown in

Table 6-7. From these results a small improvement in interface strength for BFMA

is apparent, but transverse strength will be limited due to the relatively low strength

of MAPP (see Table 6-1). Furthermore, there are indications that the surface of the

current studied bamboo fibres is covered with lignin [45], reducing its capability to

form covalent bonds with MAPP. Similar results on transverse 3 point bending

values for BFPP and BFMA of 15 and 18 MPa respectively, are reported in

literature [45].


Figure 6-27. Flexural properties for unidirectional natural fibre/thermoplastic composites [52-54]. 1,2

Results

from this study.

Table 6-7. Results for transverse flexural strength of BFPP and BFMA. *Values from [45]. The

manufacturing procedure was the same as for CAI-175, CAI-185 and CAI-200 explained in section 6.3.5.1.

The low improvement of the MAPP on the mechanical properties in the present

study can be attributed mainly to several reasons. First, the adhesion to bamboo fibre

may not be optimal yet. It has been stated that the MAPP increased the reaction

between the anhydride functions and the hydroxyl functions on cellulose, and as a

result, the mechanical properties of the composites are enhanced [8, 52]. The surface

constituents for the analyzed bamboo technical fibre are close to the reference

materials for lignin (see Figure A2.2, Appendix 2). This indicates that the technical

fibre is still cover by a layer of lignin and possibly other molecules but not cellulose

after mechanical extraction reducing its capability to form covalent bonds with

MAPP. This finding is in agreement with the studies of Fuentes et al. [45], in the

same bamboo fibre samples used in this study. Secondly, in this study the MAPP

employed, with relatively poor mechanical properties as compared to examples in

literature [55], MA was added in a low percentage as compatibilizer. A third reason

Matrix Transverse flexural strength (MPa)

175 °C 185 °C 200 °C*

Polypropylene 18 ± 2.2 17 ± 1 15 ± 0.6

MAPP 18 ± 0.8 20 ± 0.3 18 ± 0.7

176 Chapter 6b

is that possibly not enough temperature was applied to the BFMA for the activation

of the copolymer by heating (170 °C) a is stated by Bledsky et al. [56]. Further

experiments varying the content of maleic anhydride and with higher consolidation

temperatures will be necessary in order to explore the possibility to obtain the

maximum performance of the material.

6.5 Conclusions

The aim of this study was to characterize the loss in mechanical properties of

individual technical fibres and BFPP and BFMA due to thermal degradation. The

fibres were exposed at different temperature-time couples and the composites were

manufactured at different maximum consolidation temperatures, both under two

environments i.e. air and argon.

TGA results show that technical bamboo fibres are not affected at temperatures

below 160 °C but show a significant weight loss upon exposure to temperatures

above 200 °C. The mass loss in the fibre was characterized mainly by two stages,

the first one, common to all treatments, corresponded to a rapid decrease of the mass

(~4-5 %), due to moisture evaporation present in the structure of the fibre and

followed by a plateau. In a second stage, a mass loss above ~6% is due to thermal

degradation of the fibre and becoming more severe according to the temperature and

the time of exposure.

The decrease of the technical fibre strength in air environment was in agreement

with what is found in other studies and the strength decreased according to the

severity of the treatment (temperature and time of exposure). The Young’s modulus

was not significantly influenced by the different thermal treatments applied in this

study. The results show that the inert atmosphere had a positive effect on the tensile

strength for individual fibres; this is observed especially in a temperature range

between 220 and 250 °C with improvements of 37% and 45% respectively in

comparison with the air treated samples.

Through the results it was possible to establish the maximum time and/or

temperature allowed during the processing of bamboo fibre thermoplastic

composites to avoid excessive loss in properties. Even if the thermal degradation in

fibres is a complex process, in this study it was possible to link the composite

properties after a specific manufacturing process, with the strength and the Young’s

modulus of the fibre at the end of that process. For this, an empirical model based on

the correlation between single fibre experimental results and TGA measurements

was developed allowing estimating the fibre properties as a function of the mass loss


(above 200 °C) after thermal treatment or after a composite manufacturing process.

This procedure can be also applied to other natural fibres, and is described as

follows:

- First, it is necessary to obtain the full correlation between mass loss and fibre

strength and modulus. For this, the procedure described in paragraphs 6.3.1,

6.4.2.1 and 6.4.2.2 can be followed, yielding equations like 6-1 and 6-2;

- Define the desired manufacturing temperature and time profile for the production

of the composite (including heating rate(s) and dwell time(s));

- Measure the mass loss of the fibres by TGA following the same steps of the

manufacturing profile defined above. The initial conditions of the fibres (e.g.

moisture content) should be the same as the ones for the composite production;

- The fibre strength or fibre stiffness after the fibre treatment or manufacturing

process can be found by filling in the mass loss (x value) in Equations 6-1 and 6-

2.

With this novel approach it is proposed to rather consider the link between fibre

properties and fibre mass loss as a key phenomenon, instead of the traditional

connection between the fibre properties and the time-temperature variables, as

usually employed in literature. Most of the encountered studies on the thermal

degradation of natural fibres in literature are exclusively based on TGA-DSC

analysis, excluding the measurement of the fibre properties. The proposed procedure

is applicable for other natural fibres, taking into account that the structure and

chemical composition between them are quite similar. But, all the corresponding

measurements such as isothermal TGA, and fibre strength measurements, should be

performed for each particular fibre.

Unidirectional BFPP consolidated at different temperatures were characterized by 3

point bending tests. To reach the potential of bamboo fibre thermoplastic composites,

further work is needed to improve the compatibility between fibre and matrix.

Experiments carried out with MAPP did not show improvement in properties in

comparison with PP matrix. This is attributed to a lack of interaction caused by a

lignin layer that is surrounding the fibres reducing its capability to form covalent

bonds and the low mechanical properties of the MAPP. Other contributing factors

may be the low content of MA and the need of a higher temperature for the

activation of the copolymer.

178 Chapter 6b

The flexural strength for bamboo fibre/ polypropylene composites consolidated

under inert atmosphere indicated a significant improvement (30%) on the flexural

strength, especially for relatively high processing temperatures, in comparison with

production in a normal air environment. This process can be easily adopted for the

processing of natural fibres and thermoplastic matrices; nevertheless it needs to be

combined with an optimized manufacturing process in order to avoid thermal over-

exposure of the fibres.

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Technology assessment and application potential of bamboo fibre composites 181

Chapter 7 Technology assessment and application potential of bamboo

fibre composites

_______________________________________________________

7.1 Introduction

Previous chapters were dedicated to the characterization of bamboo technical fibres

(BF) and bamboo fibre composites (BFC) using different testing methodologies and

conditions. The main goal of this chapter is to assess the potential industrialisation

possibilities of BFC from a technological and economical point of view, in

comparison to other natural fibres and glass fibres. A successful implementation of a

new technology such as bamboo fibre prepreg, its commercialization and its

potential extensive use in the composites industry depends on different factors such

as:

The availability of the resource;

Environmental benefits and sustainability of the resource;

The extraction process and the effect on the ecological impact and fibre cost;

Manufacturing and environmental impact of the prepreg production;

Competitive cost of the prepreg;

Performance of the final composites and potential applications (market);

Durability and recycling of the parts (end of life possibilities).

An overview of these factors will be presented in the following sections,

nevertheless, it has to be considered that it is a new technology and not all

information is available yet.

182 Chapter 7 b

7.2 Availability of the resources

Regarding the availability of the resource, approximately 36 million hectares is

covered worldwide with bamboo forest: 24 million hectares in Asia and Oceania, 10

million hectares in Latin America and 2.7 million hectares in Africa, distributed over

the tropical and subtropical climate zones. Around 1200 bamboo species have been

identified, of which 64% are in Asia, 33% in Latin American and the rest in Africa

and Oceania [1, 2].

Not all bamboo species are adequate to be used as a source of fibre due to the culm

size, specifically the height and thickness of the culm wall, and consequent low

volumes of fibre yield. For this reason, only “giant” bamboos will be considered for

this purpose. Table 7-1 shows a list of bamboo species that have been identified as a

potential source of fibre due to their anatomical characteristics and their common

use in traditional construction. In tropical zones the most prevalent ones are the

Bambusa, Dendrocalamus, Gigantochloa, Guadua and Phyllostachys. According to

their characteristics, it can be assumed that they are a suitable fibre source in relation

to the fibre yield and the extraction processing feasibility.

Type of bamboo Height

(m)

Diameter

(cm)

Origin

Bambusa balcooa 12 – 20 8 - 15 Southeast Asia

Bambusa chungii 10 - 15 6 China

Bambusa polymorpha 27 15 China

Bambusa vulgaris 18 10 Asia, Americas

Bambusa bambos (L.) Voss 30 15 - 18 Southeast Asia

Bambusa oldhamii 6 - 12 3 - 12 Taiwan

Dendrocalamus edulis 20 16 China

Dendrocalamus giganteus 25 - 35 30 India, Sri Lanka and Thailand

Dendrocalamus asper 25 20 Malaysia

Dendrocalamus latiflorus 20 20 Taiwan and Southern China

Dendrocalamus strictus 8 - 20 8 India

Gigantochloa atroviolacea 10 - 12 10 Indonesia

Gigantochloa atter 25 10 Indonesia, Malaysia

Gigantochloa apus 16 10 Malaysia and Indonesia

Gigantochloa levis 16 10 - 15 Philippines

Guadua aculeata 25 12 Mexico and Panamá

Guadua angustifolia Kunth 15 - 30 11 Colombia, Ecuador, Perú and Brazil

Guadua chacoensis 20 8 - 12 Argentina and Bolivia

Guadua magna 12 - 23 6 - 12 Brazil

Guadua lynnclarkiae 18 15 Perú

Guadua superb Huber 20 9 - 12 Bolivia

Phyllostachys heterocycla var.

pubenses (Moso)

12 17 China, introduced to United States

and Japan

Table 7-1. Some eligible “giant “bamboo species as a source of fibres [3-5].


In general, the total amount of bamboo plantations are well defined, but information

about the number of hectares for a specific type of species (e.g. the ones defined in

Table 7-1), is not well specified, outdated or even not reported in an official

inventory. This makes it difficult to have a global estimation of the potential of the

resource for fibre extraction in the world. Nevertheless, according to the collected

data, China has bamboo plantations of approximately 5 million hectares, in which

the bamboo species Phyllostachys hetrocycla var. pubenses (Moso) occupies around

70% [2]. In India the area of bamboo growths exceeds 11.4 million hectares of

which 67% corresponds to the species Dendrocalamus strictus and 13% corresponds

to the specie Bambusa bambos [1]. In Colombia, Guadua angustifolia covers

approximately 28000 ha [6]. For the same species, in a very conservative estimation,

Ecuador reports 15000 ha and Mexico 4000 ha. Brazil has a total surface close to 18

million hectares of bamboo plantations where the most dominant genius is Guadua

plus around 30000 ha of the species Bambusa vulgaris [2]. In total it can be

estimated that more than 15 million hectare of giant bamboo is available worldwide

and a rough estimation of more than millions of 480 million tons of raw fibre

available per year. For more information about the distribution of the bamboo

resources per country or by continent see Londoño [7] and Lobovikov et al.[2].

The sustainability of the bamboo resource is assured by several factors, one of the

most important is the fast growth and the possibility to be harvested annually

without depletion and deterioration of the soil. The species Guadua angustifolia

grows at its peak 21 cm per day and in one month reaches 80% of its maximum

height; it then completes in an additional 5 months reaching on average 23.6 meters

[8]. The productivity is between 1200 and 1350 culms per hectare per year [3].

Bamboo can grow on slopes and other areas where foresting of wood and agriculture

is not possible [9], and it does not need to use land used for food (as is the case of

sugar cane for bio-fuels). The total demand for flax and hemp fibres for material

users is expected to represent 190% of the current surface allocated for the growing

of such fibres in 2015 and 350 % in 2030 [10]. This means that the agricultural

surfaces dedicated to flax and hemp fibres for the material users will have to double

by 2015 to face the demand. This situation can lead to problems concerning the

allocation of fields for food crops and non food crops, making BFs a good

alternative to supply the future demand of high quality fibres.

184 Chapter 7 b

7.3 Environmental benefits

As mentioned in section 3.5, one of the most important environmental benefits of

the bamboo is the large capturing of CO2. It was stated by Cruz [11] that Guadua

angustifolia captures 150 tons of CO2 per hectare in the first 7 years after planting,

with an average of 21 tons/ha/year). A natural growth of guadua plantation (without

technical management), with a density of 5755 culms/hectare does capture a total of

133 tonnes of carbon. These values are rather consistent with Arango and Camargo

[12], in their study on bamboo plantations of 7-8 years old (7700 culms / hectare)

which captured 25 tons of CO2 /ha/year. After 6 – 7 years the carbon absorption

stabilizes due to the fact that all vegetative development is completed. With an

adequate management and harvesting, the self-regeneration of the bamboo

plantation provides a permanent CO2 absorption, which does not happen with other

species [3]. This high carbon rate sequestration can be translated into carbon credits1

in the voluntary carbon market, meaning that managed bamboo forests can generate

extra benefits for both farmers and the environment at the same time. For example,

several Chinese companies have already pre-ordered more than 8000 tonnes of

carbon credits. The money they pay supports the planting of new bamboo forests in

China. Now, companies in China that want to offset their carbon emissions can buy

bamboo carbon credits on the voluntary market through the China Green Carbon

Foundation (CGCF)2

[13]. In Colombia the accessibility to apply to carbon

voluntary market schemes is still not available but there are already good conditions

to meet REDD+3

initiatives, in terms of well established political and legal

frameworks that will facilitate actions in the near future [12].

Besides the CO2 capture, bamboo cultivation has other environmental benefits.

Thanks to the dense network of bamboo roots they help for the reduction of soil

erosion. Also, one hectare of Guadua angustifolia can retain 30000 liters of water

helping with the regulation of the hydraulic flow, conserving the water in the rainy

season and using it later in the dry season [3], and finally the number of employment

1A carbon credit for greenhouse gas reductions is achieved by one party that can be purchased and used to

compensate (offset) the emissions of another party. Carbon offsets are typically measured in tonnes of CO2

equivalents (or CO2e) and are bought and sold through a number of international brokers and trading

platforms.

2 CGCF is the first nation-wide non-profit public funding foundation dedicated to combating climate change

by increasing the carbon sink in China. It was recognized as a valid mechanism to mitigate climate change

during the Bali conference in 2007.

3 Reducing Emissions from Deforestation and Forest Degradation (REDD+) is a global initiative designed to

pay groups or countries for protecting their forests and reducing emissions of greenhouse gas pollutants,

especially CO₂.


that this activity generates in rural areas. Therefore, local authorities stimulate the

planting and harvesting of bamboo by giving subsidies to the farmers (e.g. 50% of

the establishment and maintenance costs of the plantation up to the 5th

year in

Colombia).

One bamboo hectare can produce between 495 and 600 tons of biomass after 6 years

of the maturation of the plantation, being the weight of the culm the part of the plant

that produce most biomass followed by the rizome [11, 14]. One hectare of adult

guadua bamboo can also produce 5.8 times more biomass compared to most other

forest species [15]. This biomass can be a source of renewable energy, according to

the UK Department of Energy and Climate Change4; switching to biomass from

fossil fuel alternatives reduces carbon emissions by 70%. Biomass also emits

significantly lower levels of ash, nitrogen, sulfur, mercury and other heavy metals

that are harmful to the environment. Part of this biomass (e.g. branches, leaves, etc.)

can be used for alternative uses aside of the fibre production, e.g. the extraction of

baseline chemicals for fertilizers, etc.

7.4 The extraction process and the effect on the ecological impact and fibre cost

The newly developed extraction process does not use pressure, chemicals or high

temperature, and all the operations involved were designed to be an in-line process

avoiding batches during the extraction and limited use of water which can be

recovered. For this reason it is expected that the price of the fibre extraction can be

competitive regarding other natural fibres and glass fibres. It might be noticed that

this good mechanical behaviour of BF or its composites reported in Chapters 4 and 5

were achieved without any fibre treatment (e.g. chemical treatment) being an

important environmental and cost aspect issues that strength the use of the BFC.

The relative cost advantage of natural fibres over glass fibres will disappear if

expensive chemical treatments are needed to improve the composite performance

[16]. These additional treatments are expensive operations that have to be done

during the fibre preparation (e.g. soaking in alkali, rinsing the fibres, drying, etc.)

and which can have negative environmental impact due to the use of chemicals and

water in the process.

An estimation of the environmental impact of such treatment process, e.g. the

calculation for the energy consumption, in this early stage (prototype machines) will

not be accurate due to the scaling effect of the production at industrial volumes. For

4 https://www.gov.uk. Press release, August 2013.

https://www.gov.uk/

186 Chapter 7 b

this reason, data from the flax industry will be used as reference for some

comparisons with glass fibres, assuming that certain parts of the flax extraction

process, e.g. scutching, can be similar in terms of volume of the machinery and

energy consumption. According to Le Duigou et al. [17], the energy consumption

increases from 4.4 MJ/Kg for flax scutched fibres to 11.6 MJ/kg for hackled fibres

due to the additional energy consumed by the hackling process (0.55 kWh/kg of

fibres hackled). The scutching step requires 0.116 kWh/kg of fibres. In general, the

production of flax fibres appears to be an environmentally attractive alternative to

glass fibres in terms of non-renewable energy consumption. The energy

consumption for the production of flax fibres and mats is around 73% and 82%

lower than used for the manufacturing of glass fibres and mats respectively (see

Table 7-2).

Table 7-2. Non renewable energy requirements for flax and glass fibres and their mats.

Most of the environmental aspects used are favourable to flax fibres, with the

exception of the eutrophication indicator. For flax fibres, this remains high, mainly

due to the use and production of fertilizers [17]. The usage of fertilizers, herbicides

and other pesticides plays an important role in the environmental impact values.

Their optimal use in the case of the bamboo plantations requires performing a soil

test to check deficiencies, but in general, the NPK fertilizers composed of three main

elements: Nitrogen, Phosphor and Potassium, are the most used ones. As an

alternative, natural fertilizers can be used as a complement to the chemical ones in

order to reduce the impact on the eutrophication indicator. They can be coffee pulp

residues (e.g. in the Coffee Region), already used extensively for this purpose in

agricultural soils. This last option is available thanks to the currently waste related

policies formulated by the Colombian government which have focused its effort on

the solid organic residues management from the organic municipal waste generated

by the population (70% of the total residues production5

), producing natural

fertilizers such as compost.

5 www.asocompost.org

Type Non renewable energy requirements

(MJ/kg fibres)

Reference

Flax fibre (scutched)

Flax fibre (hackled)

Flax fibre (hackled)

4.4

11.7

12.4

[17]

[17]

[18]

Flax fibre mat 9.5 [19]

Glass fibre 45 [17]

Glass fibre mat 54.7 [19]


During the operations for the extraction of long BFs (> 50 mm), several side stream

products are produced (see Figure 7-1) and can be valorized, they are:

Biomass (e.g. chemicals, fertilizers, coal, etc.);

Dust and particles;

Short fibres (>2mm);

Medium length fibres (5 – 50 mm).

Figure 7-1. Raw materials extracted from the bamboo after different operations and their corresponding

intermediate products that can be offered to the market. SMC= sheet moulding compound, BMC= bulk

moulding compound and LFT= long fibre reinforced thermoplastic.

A concise overview of each of these side products will be presented a continuation

to explain their potential market (main uses) and profit opportunity.

Op

erati

on

s R

aw

mate

rials

M

an

ufa

ctu

rin

g

Inte

rmed

iate

pro

du

cts

Forestry

Extraction of the fibres / cleaning

Harvesting

Cutting and preparation

Biomass

(branches

& leaves)

Dust

&

particles

Short fibres

(< 5 mm)

Medium

length fibres

(5-50 mm)

Long fibres

(>50 mm)

UD

discont.

randomiz.

Chemical

transfor-

mation

Separa-

tion

Extrusion

&

pelletizing

Random

positioning

UD

prepregs

Baseline

chemicals (e.g.

fertilizers)

Polymer

fillers

Pellets for

injection

moulding

Mats, SMC,

BMC, LFT

188 Chapter 7 b

7.5 Manufacturing and performance of the final composites; potential

applications

Long bamboo fibres

The manufacturing of high performance composites will be possible with the

unidirectional disposition of the fibres into the composite. This fibre configuration

can reach the highest mechanical performance (see Figure 7-2) due to the axial

alignment of the fibres to the loading direction and the absence of crimp optimizing

and strengthening the composite with the added advantage of reducing the amount

of resin, thereby minimizing the weight of the final composite.

Figure 7-2. Ashby-plot, tensile modulus vs. tensile strength, for various processing techniques used in the

processing of composite materials including natural fibre composites. In this plot the processing technique is

linked with the type of fibre reinforcement [20].

For this purpose, it is necessary to stabilize the extracted long bamboo fibres (>50

mm) in a continuous unidirectional pattern where the fibre ends are randomly

distributed. This is possible holding the fibres together with a binder or stitching to

have a preform that can be used in RTM and autoclaving. Another alternative is by

adding the exact amount of matrix (thermoset or thermoplastic) necessary to achieve

a suitable fibre volume fraction to obtain a pre-impregnated material (“prepreg”).

Tensile modulus (GPa)

Ten

sile

str

en

gth

(M

Pa)


A configuration of narrow strips makes possible to use different existing

technologies such as automated fibre placement and filament winding. Also, it opens

the possibility to apply both weaving and braiding to produce textile fibre

architectures ready to be used in composite production. A wider bamboo tape would

allow the use of manufacturing technologies such as: automated tape laying,

thermoforming and matched die moulding. These scenarios will open several

possibilities for future applications for the UD bamboo fibre prepreg (tape) or

preform, adding the highest value to the long bamboo fibres as an intermediate

product that will be offered to the composites industry.

In terms of the performance, the strength of the BFC (Vf=40%) with epoxy matrix

in tensile and flexural tests was found to be comparatively high with respect to other

natural fibres as shown in section 5.6 (see Tables 3-3 to 3-6). Furthermore, the BFC

are suitable to be used in applications where the stiffness is the most important

performance characteristic since it has been shown in section 5.4.1 that the

composite stiffness is not affected by the different fibre end patterns that were

investigated, benchmarked and compared to fully unidirectional fibre composite.

The specific properties of the bamboo fibres, when normalized to the material’s

density, are in average 595 MPa*cm3/g (σ/δ) and 31 GPa*cm

3/g (E/δ) for the

strength and Young’s modulus respectively. These values are comparable to the

specific properties of E-glass which are in average 610 MPa*cm3/g and 30 GPa

*cm3/g. The specific stiffness formula E/δ actually only applies to materials subject

to a tensile stress. For applications that are subject to bending stresses (“beams”), the

square root of the stiffness (E1/2

/δ) has to be introduced into the formula, which

increases the relative importance of the specific weight. And for materials loaded in

compressive stress or for plates in bending, the cubic root of the stiffness has to be

used (E1/3

/δ) for further comparisons. A study revealed also that the compressive

properties of BFC were 150 MPa for the strength and 16 GPa for the modulus.

These values correspond to the 67% and 82% of the maximum tensile strength and

tensile modulus respectively and being slightly better than flax-epoxy composites at

the same fibre volume fraction (Vf =40%) [21]. The specific stiffness (E/δ) and the

specific stiffness for bending (E1/3

/δ) in flax and hemp (as it is also the case for

bamboo fibres), are higher that the values of E-glass [22], which make them ideal

for stiff and light composites.

Medium and short bamboo fibres and particles

Medium and short bamboo fibres are obtained during the extraction and cleaning

processes. They can be easily separated to produce random mats or pellets for the

production of low cost materials, because they do not require intermediate

190 Chapter 7 b

operations such as fibre alignment, as is the case for the UD prepreg. Long fibre

reinforced thermoplastics (LFT), sheet moulding compound (SMC), bulk moulding

compound (BMC) and injection moulding are visible manufacturing processes to

improve the mechanical properties of the polymer and to improve dimensional

stability of the parts. The particles (e.g. dust) also obtained during the extraction

process, can be used as filler for polymers to cheapen and enhance properties of the

end products. In general bamboo fibres, similar to flax and hemp, are less abrasive

in comparison to glass fibres and present less fibre breakage (shortening) during

compounding and extrusion processes.

Potential market

In 2010, the global natural fibre composites market reached €1.6 Billion. Lucintel6

forecasts the natural fibre composite market to to have a high growth increasing to

€2.9 Billion in 2016 and to grow with a good CAGR (Compound Annual Growth

Rate) of 11.2%7 during the 2014-2019 period. The automotive market will remain

the largest segment in terms of both value and volume during this period. The major

drivers of using natural fibre composites are to take advantage of the natural fibres

properties such as: lightweight, possibilities of recycling, sound insulation, acoustics

and governmental regulations that encourage the protection of the human health and

the environment. Also, an increasing demand from the building and construction

industries as well as from the electrical and electronic industries is leading the

growth or the natural fibre composites market. According to these predictions a

range of different applications will continue having a big impact in the natural fibres

business7:

Automotive sector: door panels, seat backs, headliners, dash boards;

Electrical and electronics: mobile cases, laptop cases;

Sporting goods: tennis rackets, bicycles, frames, snowboards;

Building and construction: door panels, decking, railing, window frames.

6

Lucintel is a global management consulting and market research firm (http://www.lucintel.com).

Opportunities in natural fibre composites.

7 Lucintel (http://www.prnewswire.com/news-releases). Global natural fibre composites market 2014-2019:

Trends, forecast, and opportunity analysis.


7.6 Durability and recyclability

There is little knowledge about the durability and the BFC long term behaviour

(creep, fatigue, etc.) and they still need to be investigated. Furthermore, moisture

uptake trough time is a major concern for the industry players. Some studies on

tensile testing of individual fibres conditioned to different humidity levels show an

increase in failure strain with increasing humidity level. Damage analysis of broken

fibres showed that this is due because of the plasticization of the fibres. Moreover,

tests performed on unidirectional bamboo epoxy composite samples show a

statistically relevant increase in both failure strain and strength with increase in

atmospheric humidity level [23]. This good behaviour at high humidity levels is

expected because the bamboo fibre tends to be less moisture sensitive (e.g. in

comparison with flax and hemp), because of the higher lignin content on the surface

on the fibre as it was demonstrated in Appendix 2. Also, some studies in fatigue

behaviour with flax fibres reported promising results at high cycling fatigue loads

that may potentially be expected also for BFC. Shah [24], found that the fatigue

strength degradation rates are lower in natural fibres reinforced plastics than in glass

fibre reinforced plastics. Moreover, according to Bensadoun et al. [25], it was

observed that at normalized stress by density, the fatigue life of flax-epoxy plain

weave composite have comparable behaviour to glass plain weave epoxy composite,

which validates the possibility of replacing glass fibre by flax in the near future for

certain applications.

The recyclability is another important parameter that will affect the end-of-life

option for the BFC. The first aspect to consider is the type of matrix used, because

the alternatives for recycling will depend directly on it. There are nowadays several

techniques that can be applied to natural fibre composites. The most common one is

the mechanical recycling, especially for thermoplastic composites applying a

downgrade recyclability and giving as a result pieces with short fibres that later on

can be added to an extrusion machine to be compounded and pelletized [26]. Other

options are the incineration for energy recovery and the thermal recycling. For the

first, it has been shown that the calorific value of natural fibres are superior to

synthetic ones leading to an increased energy recovery once burned and leads to a

positive carbon credits and lower warming effect [19]. All natural fibre composites

can be incinerated for energy recovery where between 50% and 70% of the heat can

be recuperated as energy [20]. In this case, an advantage is that in the case of natural

fibre composites, not only the matrix material but also the fibres can be combusted;

the composite can thus be considered a delayed biomass, after having served a

lifetime as a light-weight component. In the case of glass fibre composites, only the

matrix burns and the fibres tend to remain as residue, although usable as raw

material for cement production [27]. The second mainly consist of the degradation

192 Chapter 7 b

of the matrix trough temperature in order to recover the reinforcing fibres, but due to

the low degradation temperature of natural fibres >200 oC, this is not an option.

However, it is possible for the thermoplastic composite structure to increase the

temperature close to the melting point of the polymer matrix in order to re-shape it,

and to obtain a new product. Several layers of new material can be added for

reinforcement and for better looking of the component. The concern with this

method is the degradation to the fibre due to the thermal treatment needed during the

process. Chemical recycling of natural fibre composites seems to be not feasible due

to unavoidable damage to the fibres when dissolving the matrix; moreover chemical

treatments are expensive and not environment-friendly.

7.7 Conclusions

The availability of the bamboo resource is assured due to the high amount of

available hectares of giant species around the world making it an ideal source of

fibre for reinforcing composites. Several governmental incentives (e.g. subsidies)

stimulate the planting and harvesting bamboo in a continued manner due to its

environmental and social benefits. After an overview of the ecological,

environmental, technological and performance issues concerning the production and

use of BFs and BFCs, they appear to be competitive with similar natural fibres

already implemented in the composite market. Also, they present superior

environmental advantages in comparison to glass fibres, and in terms of mechanical

properties they may replace them in certain applications. The market forecast for the

natural fibre composites seems to be very positive with a regular annual growth led

by more stringent environmental regulations, new recycling possibilities and product

design and innovation. Nevertheless there are several aspects for the bamboo fibres

and their composites that still need to be investigated in the future: Some are

summarized in the future research segment in section 8.4.

References

[1] Chaowana P. Bamboo: An Alternative Raw Material for Wood and Wood-Based Composites.

Journal of Materials Science Research. 2013;2(2):p90.

[2] Lobovikov M, Paudel M, Piazza M, Ren H, Wu J. World bamboo resources: a thematic study

prepared in the framework of the Global Forest Resources Assessment. Non wood fores products

18 -World bamboo resources, FAO, 2005. p. 1-18.

[3] Minke G. Building with Bamboo: Design and Technology of a Sustainable Architecture: De

Gruyter; 2012.

[4] Londoño X. A decade of observations of a "Guadua angustifolia" plantation in Colombia. The

Journal of the American Bamboo Society. 1998;12:37-42.


[5] Londoño X. Dos nuevas especies de guadua para el Perú (Poaceae: Bambusoidae: Bambuseae:

Guaduinae). Journal of the Botanical Research Institute of Texas. 2013;7(1).

[6] Kleinn C, Morales-Hidalgo D. An inventory of Guadua (Guadua angustifolia) bamboo in the

Coffee Region of Colombia. European Journal of Forest Research. 2006;125(4):361-8.

[7] Londoño X. La Guadua un bambú importante de América. First Bamboo Seminar. Guayaquil,

Ecuador 2001. p. 12-8 [Spanish].



[9] Vogtlander J, Van der Lugt P, Brezet H. The sustainability of bamboo products for local and

Western European applications. LCAs and land-use. J Clean Prod. 2010;18(13):1260-9.

[10] Bewa H. Fibres végétales: une disponibilité régulée de la ressource. JEC Composites

conferences. Paris2012.

[11] Cruz H. Bambú - Guadua. Bosques naturales en Colombia y plantaciones comerciales en

Mexico: Colmex; 2009.

[12] Arango A, Camargo J. Bosques de guadua del Eje Cafetero de Colombia: oportunidades para

su inclusión en el mercado voluntario de carbono y en el Programa REDD+. Recursos Naturales y

Ambiente. 2011;61:77-85.

[13] Evans K. Bamboo carbon credits now on sale in China: INBAR. Forest news A blog by the

Center for International Forestry Research (http://blogcifororg/13245/bamboo-carbon-credits-now-

on-sale-in-china#U8v4_fmSzng), 2012.

[14] Riaño N, Londoño X, López Y, J G. Plant growth and biomass distribution on Guadua

angustifolia Kunth in relation to ageing in the Valle del Cauca - Colombia. J Am Bamboo Soc.

2002;16(1):43-51.

[15] Environmental impact of bamboo. http://wwwguaduabamboocom/blog/environmental-impact-

of-bamboo.

[16] Thomason J. Why are natural fibres failing to deliver on composite performance? 17th

International Conference on Composite Materials, ICCM172009.

[17] Le Duigou A, Davies P, Baley C. Environmental impact analysis of the production of flax

fibres to be used as composite material reinforcement. Journal of biobased materials and bioenergy.

2011;5(1):153-65.

[18] González-García S, Hospido A, Feijoo G, Moreira M. Life cycle assessment of raw materials

for non-wood pulp mills: Hemp and flax. Resour, Conserv Recycl. 2010;54(11):923-30.

[19] Joshi SV, Drzal LT, Mohanty AK, Arora S. Are natural fiber composites environmentally

superior to glass fiber reinforced composites? Composite Part A: Appl Sci Manuf. 2004;35(3):371-6.

[20] Shah DU. Developing plant fibre composites for structural applications by optimising

composite parameters: a critical review. J Mater Sci. 2013;48(18):6083-107.

[21] Wouters K. Compressive properties of natural fibre reinforced composites. Master thesis:

Leuven University; 2014.

[22] Confédération européenne du lin et du chanvre. Book: Flax and Hemp Fibres: A Natural

Solution for the Composite Industry: JEC Composites, editor Reux F.; 2012.

[23] Verheyden S. Karakterisering van vochtabsorptie van bamboe vezel composiet [Master thesis].

Leuven: University of Leuven, Faculteit Ingenieurswetenchappen, 80 p.; 2010.

[24] Shah DU. Characterization and optimization of the mechanical performance of plant fibre

composites for structural applications: PhD thesis, Univerisity of Nottingham; 2013.

[25] Bensadoun F, Depuydt D, Baets J, Vuure V, Willem A, Verpoest I. Influence of fibre

architecture on impact and fatigue behaviour of flax fibre-based composites. The 19th International

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[26] Summerscales J, Dissanayake N, Virk A, Hall W. A review of bast fibres and their composites.

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http://blogcifororg/13245/bamboo-carbon-credits-now-on-sale-in-china#U8v4_fmSzng)

http://blogcifororg/13245/bamboo-carbon-credits-now-on-sale-in-china#U8v4_fmSzng)

http://wwwguaduabamboocom/blog/environmental-impact-of-bamboo

http://wwwguaduabamboocom/blog/environmental-impact-of-bamboo

194 Chapter 7 b

194

General conclusions 195

Chapter 8 General conclusions

____________________________________________________________________

This PhD thesis has presented a study on the mechanical properties of long bamboo

technical fibres and their composites with thermoset and thermoplastic matrices, and

includes recent developments in the fibre preparation and manufacturing techniques

in order to obtain high quality bamboo composites.

The main contribution of this research work to the state of the art is the development

of a unidirectional bamboo fibre composite with high mechanical properties. This

was possible thanks to a novel mechanical extraction process, applied during this

PhD research work that proved to be an effective method to produce long high-

quality bamboo fibres. These unique long high quality fibres are an innovative and

renewable alternative for other natural fibres and particularly for synthetic fibres (e.g.

glass) used in composites. The conclusions of this research are summarized as

follows:

8.1 Bamboo technical fibre

As a first stage, a new environment friendly process was developed to extract

bamboo fibres. The effectiveness of this process was verified by measurements of

tensile strength, Young’s modulus and strain to failure, all confirming the good fibre

quality. The modified length-based Weibull model, accurately describes the fibre

strength distribution of the bamboo technical fibres at various gauge lengths. The

Weibull shape parameter (m) revealed low variation for the fibre strength,

demonstrating the good condition of the fibres produced by the new mechanical

extraction method. It was possible to establish a close relation between the

196 Chapter 8 b

extraction process and the quality and morphology of the bamboo fibres. For this

reason, fibre properties cannot be generalized and should be linked with the

correspondent extraction method in order to state fair comparisons and statements.

Moreover, an adequate statistical treatment of the data is necessary to compare,

describe and predict the mechanical properties of the fibres and hence its composites.

Single technical fibre characterization requires sample pre-selection, where weaker

fibres can break during separation. Also, for statistical reasons a large number of

single fibre tests are needed. Therefore, as an alternative for fibre characterization,

the dry fibre bundle test was proposed to obtain reliable values for the mechanical

properties and Weibull parameters for a larger population of non-impregnated fibres,

with a relatively small amount of experiments. In comparison with the single fibre

test, the bundle test systematically gives lower fibre strength and higher variability

due to the fact that no pre-selection of the fibres is needed. The Young’ modulus,

however, remains almost unchanged. The developed methodology is generic enough

to be used for other natural fibres as well, avoiding intensive manipulation of

individual single fibre sampling and testing.

8.2 Unidirectional continuous and discontinuous bamboo fibre epoxy

composites

Unidirectional long bamboo fibre/epoxy composites were produced showing in

general the high potential of bamboo fibres for high-end composite applications.

Experiments show that the mechanical properties of bamboo fibres are transferred

to the composite properties, reaching 92% for stiffness and 79% for strength of the

ideal bamboo fibre composite properties. This points out the strong fibre-matrix

interface present between fibres and epoxy matrix, promoted by the high fibre

surface roughness, and confirmed by a relatively high transverse flexural strength

(33 MPa).

The limited (<25 cm) length of technical bamboo fibres inevitably results in fibre

ends and consequent stress discontinuities in real size composites. An innovative

approach for industrialization of bamboo fibres in high-performance composite

materials is the development of an up-scalable method to randomize the fibre ends

for the production of a continuous bamboo fibre prepreg (tape). This fibre

configuration will allow the future commercialization at industrial scale and the use

of existing technology for the composite manufacturers.


In order to simulate the effect of the inherent fibre ends on the composite

mechanical properties, six discrete patterns of fibre discontinuity distributions were

designed and tested. The results show that a highly randomized distribution is

necessary to preserve the composite properties: 85% of the longitudinal tensile

strength of a fully continuous bamboo fibre composite is obtained. The composite

stiffness variations were not significant for any of the studied fibre-end patterns. The

results clearly show that full randomization of the fibre ends in UD composites is

necessary to fully exploit the good mechanical properties of the fibres. The

experimental composite strength was predicted by a modified local load sharing

model (LLS) within a reasonable margin of around 14%.

It was found that the impregnated fibre bundle test is a good option to evaluate the

composite tensile properties, with some advantages over the “standard” composite

tensile test: less material is needed and easier specimen preparation saves time and

cost. It gives easier control of the fibre alignment for the measurement of the

modulus, and the reduced sample volume (with the corresponding lower probability

of defects) will improve the composite strength.

8.3 Thermal degradation in single technical fibres and bamboo fibre

polypropylene composites

In literature, the thermal degradation of natural fibres is only scarcely linked with

the loss in mechanical properties of single fibres (i.e. strength, stiffness and strain to

failure) and composites (e.g. fibre degradation during the manufacturing process).

This makes it difficult for a process engineer to design a manufacturing process with

natural fibres and thermoplastic matrices, where sufficient time at elevated

temperature is needed to achieve full impregnation and consolidation.

Single technical bamboo fibres were exposed at different temperatures and times of

exposure in presence of air and argon. The results show that the fibres start to be

affected at temperatures above 160 °C and that thermal treatments or manufacturing

processes that induce more than ~7% of mass loss, start to considerably affect the

mechanical properties of the fibres. The use of an inert argon atmosphere for thermal

treatment of technical fibres results in an improvement of the strength properties

compared to exposure to air, of 45 % at 250 °C respectively (for 50 min of

treatment). The strength results of single thermally treated fibres show that the mass

loss due to the thermal degradation is dependent on the temperature and becomes

more critical as the time of exposure increases. The Young’s modulus was not

significantly influenced by any of the applied thermal treatment cases. Furthermore,

198 Chapter 8 b

XPS analyses show no changes in the surface components for untreated or thermally

treated fibres.

Results show that a correlation exists between the mass loss and the fibre strength. If

the mass loss of the technical fibres for a given set of process parameters (i.e.

temperature and time of exposure) is known after TGA measurements, the

correlation graph allows the prediction of fibre and composite strength. This study

served not only to have a better understanding of the final composite behaviour, but

also to have an approach for the process parameters selection, assuring no damage

occurs in the internal structure of the fibre (caused by too high temperatures during

the production of bamboo fibre/thermoplastic composites).

Unidirectional bamboo fibre polypropylene composites consolidated at relatively

high temperatures in an inert atmosphere, exhibit 30% higher flexural strength

compared to composites produced in air. The use of inert atmosphere during

manufacturing of natural fibre thermoplastic composites is a promising method that

can easily be adapted. Nevertheless, it is necessary to consider the costs associated

to future mass production and nitrogen might be a cheaper alternative to argon.

Matrix modification using MAPP showed no significant improvement of the

composite properties in comparison with the unmodified PP samples.

8.4 Potential use of bamboo fibre composites

In the literature, it is well established that natural certain fibres are suitable for use as

reinforcement in polymeric matrices and they have the potential to replace synthetic

fibres such as glass in several applications due to a number of advantageous

characteristics. Some of them are good specific mechanical properties, good acoustic

insulation and damping, renewability, reduced environmental footprint as well as

giving better possibilities of recycling for the composites.

Bamboo fibres in particular present several ecological, environmental, technical and

mechanical performance aspects concerning the production and use of bamboo

technical fibres and their composites. These make them suitable to fulfil the

requirements of the new worldwide requirements toward the use of renewable and

sustainable materials at competitive price. Nevertheless, as important as the

technical and environmental aspects associated with the use of natural fibres, is their

relevance in the local economies and the societal impacts, especially in the

developing countries where the bamboo grows.


Bamboo grows massively in equatorial countries through four continents and this

aspect makes the bamboo fibres an excellent alternative to the already existing high

quality natural fibres used in composites such as flax and hemp. This is under the

premise that for low cost and commercial exploitation, the fibre production should

be close to where plants grow. They should not be seen as a competence itself,

conversely, they together become strong to face the challenge of developing and

maintain markets in which they can compete effectively with synthetic fibres (e.g.

glass fibres).

8.5 Suggestions for future research

Bamboo fibre epoxy composites have good mechanical properties, but epoxy is a

relatively expensive matrix and non-biodegradable fossil fuel-based resin. It is

suggested for future work to study various thermoset and thermoplastic matrix

systems, preferably bio-based, to meet the market expectations. The following

studies can be considered:

- Bio-based matrices like GreenPoxy 55®

resin (whose molecular structure

is originated from plants), polyamide 6 (PA 6) and Solanyl®

(made from

leftovers of potato starch), are very interesting options in terms of

properties and biodegradability;

- Regarding thermoplastic matrices, Polyoxymethylene (POM) presents

good mechanical properties and acceptable production temperature; it has

also shown good adhesion to other natural fibres.

The tensile strength of UD discontinuous bamboo fibre epoxy composites is 15%

lower than for the UD fully continuous composite. Additional work is needed to

explain and to improve this performance. It can be hypothesized that a tougher

thermoplastic matrix will be less sensitive to the discontinuities at the fibre ends.

Further improvement in the standardization of the manufacturing of the continuous

fibre preform (tape) is required by enhancing all variables involved in the

randomization process (proof of concept machine) to obtain the desired results.

To reach the full potential of bamboo fibre thermoplastic composites, further work is

needed to improve the compatibility. Experiments carried out in this thesis with

MAPP did not show improvement in properties in contrast with data on other natural

fibre composites presented in literature. The pure MAPP used in this study did have

low intrinsic mechanical properties. Further studies with blends of MAPP with

higher performance polymers and by varying and controlling the content of maleic

200 Chapter 8 b

anhydride or other compatibilizers, can improve the actual mechanical properties of

the bamboo fibre composites. Furthermore, treatments made on the dry fibres such

as cold plasma can also be considered.

To achieve better properties for thermoplastic matrices, further research to optimise

the manufacturing process to avoid fibre degradation by minimizing the exposure of

the fibre to high temperature is needed. The use of inert atmosphere can help to

improve the mechanical properties of the final composite.

In-service properties of bamboo fibre reinforced plastics (BFRP) must be evaluated

to determine the long term reliability:

- Fatigue behaviour;

- Creep behaviour;

- Interlaminar fracture toughness, impact behaviour and properties after

impact;

- Effects of moisture absorption, swelling and wet-dry cycling on the decay

in properties.

Bamboo fibre composites are expected to be applied in many high performance

industrial applications, such as sporting goods, building and construction and the

transportation sector, etc. The characterization of long term BFRP behaviour is very

important to achieve long service life and for a good performance in a LCA

assessment, especially in the use phase. The light-weight natural fiber composites

improve fuel efficiency and reduce emissions in the use phase of the composite

component, especially in transport applications1. This advantage will dominate the

results and favour the BFRP.

1Joshi et al. Composites Part A 35 (2004), 371-376.

Appendix 1 201

Appendices

201

202 Appendix 1

Appendix 1

A1.1 Calculation of the Weibull parameters by linear regression

Different approaches available in the literature can be used to estimate the σ0 and m

parameters of the Weibull distribution from a set of experimentally measured

fracture stresses. The most common methodologies are both the maximum

likelihood (ML) and linear regression (LR), known as the graphical method. It has

been stated that ML method leads to the most precise estimation of the Weibull

modulus (m) [1]. However, several studies [2, 3] concluded that this method yielded

similar values as LR for m and σ0. Also, it was shown that ML tends to overestimate

the Weibull modulus leading in a lower safety factor in reliability predictions. For

these reasons, the LR method is preferred from the engineering point of view [4, 5].

In natural fibres both methods are used as is show in Table 4-1.

To obtain the two parameter Weibull by LR it is necessary to rearrange the

distribution (Equation 4-6) by taking twice the natural logarithm, and rewriting it in

a linear form y= mx+C:

0

0

1ln(ln(1/1 )) ln( ) ln( ) ln 1

mLP m m

L m

A straight line should be found when ln(σ) is plotted against ln(ln(1/1-P)), the

Weibull modulus (m) corresponds to the slope of the line. The characteristic strength

(σ0) can be calculated based on the intersection with y-axis via linear regression and

m, as follows:

0

0

ln lnL

C mL

thus:

0

0

ln

exp

LC

L

m

The p-value (P) is estimated by a function known as the probability index. These are

the most commonly used probability estimators (Equations A1-4 to A1-7) found in

literature [6-12]:

(A1-1)

(A1-2)

(A1-3)

Appendix 1 203

Where n is the size of the sample and i is the rank of ith

data point. In order to

calculate P from the experimental values, the data are sorted in ascending order and

given number as a counter (i). Among this commonly used probability estimators, it

is reported that estimator A1-4 gives the least-biased estimation of the Weibull

modulus for n ≥20, followed by estimators A1-7 and A1-6, and it is considered the

best probability estimator by many authors [4, 13, 14]. Furthermore, it was found

that after statistical treatment of the data, the estimator A1-4 yielded systematically

the highest m value, while estimator A1-5 yielded the lowest number and highest σ0

[6]. Figure A1-1 shows the Weibull plot based on Equation A1-1 for all bamboo

fibres correspondent to the 30 mm span length (430 samples) in which the Weibull

parameter is determined by different probability estimators described above.

Figure A1-1. Weibull plot for fibres tested at 30 mm span length using LR method and different probability

estimators (P).

It is observed that there were not significant differences when using the different

estimators, nevertheless the estimator A1-4 gave slightly better fitting with a R2

○ Estimator A1-5 m = 8.0, σo = 790 MPa, R²= 0,973 Δ Estimator A1-6 m = 8.2, σo = 790 MPa, R²= 0,976 □ Estimator A1-7 m = 8.3, σo = 790 MPa, R²= 0,974 ◊ Estimator A1-4 m = 8.3, σo = 789 MPa, R²= 0,981

(A1-7)

(A1-4)

(A1-5)

(A1-6)

n

iP

5.0

1

n

iP

4.0

3.0

n

iP

25.0

)8/3(

n

iP

204 Appendix 1

equal to 0.98. This estimator was used to estimate the Weibull parameters for the

different gauge lengths as shown in Figure A1-2. The experimental data fit a straight

line reasonably well for each span length, which demonstrates good agreement with

the Weibull statistics. Deviations from Weibull distribution could be mainly

attributed to the weakest fibres, probably damaged during the preparation process

[15] or another external factor.

Figure A1-2. Weibull plots for the strength to failure of bamboo fibres at different gauge lengths: a) 1 mm, b)

2 mm, c) 5 mm, d) 10 mm, e) 20 mm, f) 40 mm. Fitting of the data to the straight line demonstrates good

agreement with the Weibull statistics.

a. b.

c. d.

e. f.

Appendix 1 205

As discussed in section 4.5.3 it was assumed that during the tensile test at these short

span lengths, some elementary fibres are clamped; but at the same time there is

presence of fibre ends randomly distributed along the gauge length. All this

contribute to have a “miscellany system” that when pulled at tension could have an

influence on the fibre fracture. An indication of that is when the data plot under

Weibull coordinates shows some deviations and sharp corners. These characteristics

suggest a presence of a mixture of failure modes [16], as it seems to be the case for

gauge lengths of 1 and 2 mm (see Figure A1-2a and b) where the linear regression

fits with lower R2. When the gauge length became longer, a more consistent type of

fibre failure occurred represented in the straightness of the data plot.

Table A1-1. Weibull parameters obtained by linear regression (LR) at different span lengths with the length

(L) as a scale variable.

References

[1] Khalili A, Kromp K. Statistical properties of Weibull estimators. J Mater Sci. 1991;26(24):6741-

52.

[2] Patankar SN. Weibull distribution as applied to ceramic fibres. Journal of Materials Science

Letters. 1991;10(20):1176-81.

[3] Doremus R. Fracture statistics: A comparison of the normal, Weibull, and Type I extreme value

distributions. Journal of applied Physics. 1983;54:193-9.

[4] Wu D, Zhou J, Li Y. Methods for estimating Weibull parameters for brittle materials. J Mater

Sci. 2006;41(17):5630-8.

[5] Wu D, Lu G, Jiang H, Li Y. Improved estimation of Weibull parameters with the linear

regression method. J Am Ceram Soc. 2004;87:1799-802.

[6] Zafeiropoulos NE, Baillie CA. A study of the effect of surface treatments on the tensile strength

of flax fibres: Part II. Application of Weibull statistics. Composite Part A: Appl Sci Manuf.

2007;38(2):629-38.

[7] Cao Y, Wu Y. Evaluation of statistical strength of bamboo fiber and mechanical properties of

fiber reinforced green composites. J Cent S Univ Technol. 2008;15(0):564-7.

[8] Defoirdt N, Biswas S, De Vrise L, Tran N, Van Acker J, Ahsan Q, et al. Assessment of the


2010;41(5):588-95.

[9] Kulkani A, Satyanarayana K, Rohatgi P. Weibull analysis of coir fibres. Fibre Sci Technol.

1983;19:59-76.

Tested gauge

length (mm)

Shape

parameter

(m)

Scale (σ0)

parameter

(MPa)

1 12.1 984

2 9.4 960

5 8.8 880

10 8.0 872

20 12.8 785

30 8.3 789

40 7.8 796

All 7.6 850

c.

206 Appendix 1

[10] Xia ZP, Yu JY, Cheng LD, Liu LF, Wang WM. Study on the breaking strength of jute fibres

using modified Weibull distribution. Composite Part A: Appl Sci Manuf. 2009;40(1):54-9.

[11] Placet V. Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the


2009;40(8):1111-8.




[13] Wu D, Zhou J, Li Y. Unbiased estimation of Weibull parameters with the linear regression

method. Journal of the European Ceramic Society. 2006;26(7):1099-105.

[14] Bergman B. On the estimation of the Weibull modulus. Journal of Materials Science Letters.

1984;3(8):689-92.

[15] Andersons J, Sparnins E, Joffe R, Wallstrom L. Strength distribution of elementary flax fibres.

Compos Sci Technol. 2005;65:693-702.

[16] Rinne H. The Weibull distribution: a handbook: CRC Press; 2010.

Appendix 2 207

Appendix 2

A2.1 X-ray photoelectron spectroscopy (XPS) methodology

The XPS analyses were carried out at the division of Bio- and Soft Matter (BSMA)

of the Institute of Condensed Matter and Nanosciences (IMCN) of the University of

Louvain (UCLouvain). Bamboo fibres were fixed overhanging on a flat stainless

steel trough with an insulating double sided adhesive tape. The analysed zone was

situated on the overhanging part of the fibre, which totally prevents any contribution

of the sample support to the recorded signal. The signal was carefully optimized in

X, Y and Z. Analyses were repeated in triplicate. XPS analyses were performed on a

Kratos Axis Ultra spectrometer (Kratos Analytical – Manchester – UK) equipped

with a monochromatized aluminium X-ray source (powered at 10 mA and 15 kV)

and an eight channeltrons detector. The spectrometer was interfaced with a HP PC

workstation. Instrument control and data acquisition were performed with the

Vision2 program.

The pressure in the analysis chamber was about 10-6

Pa. The angle between the

normal to the sample surface and the direction of photoelectrons collection was

about 0°. Analysis was performed in the hybrid lens mode (a combination of

magnetic and electrostatic lenses) with the slot aperture and the iris drive position

set at 0.5, the resulting analysed area was 700 µm x 300 µm. The pass energy of the

hemispherical analyser was set at 160 eV for the survey scan and 40 eV for narrow

scans. In the latter conditions, the full width at half maximum (FWHM) of the Ag

3d5/2 peak of a standard silver sample was about 0.9 eV. Charge stabilisation was

achieved by using an electron source mounted co-axially to the electrostatic lens

column and a charge balance plate used to reflect slow electrons back towards the

sample. The magnetic field of the immersion lens placed below the sample acts as a

guide path for the low energy electrons returning to the sample. The electron source

was operated at 0.16 A filament current and a bias of –1.2 eV. The charge balance

plate was set at 4 eV.

Powdered samples were deposited on a double side adhesive tape fixed on a piece of

polymer allowing complete insulation of the samples. The following sequence of

spectra was recorded: survey spectrum, C 1s, O 1s, N 1s, Ca 2p, Zn 2p, Si 2p, P 2p

and C 1s again to check for charge stability as a function of time and the absence of

degradation of the sample during the analyses. To set the binding energy scale, the

C-(C,H) component of the C1s peak of carbon has been fixed to 284.8 eV.Spectra

were decomposed with the CasaXPS program (Casa Software Ltd., UK) with a

208 Appendix 2

Gaussian/Lorentzian (70/30) product function after subtraction of a linear baseline.

Molar fractions (%) were calculated using peak areas normalised on the basis of

acquisition parameters, experimental sensitivity factors and transmission factors

(depending on kinetic energy, analyser pass energy and lens combination) provided

by the manufacturer.

A2.1.1 Thermal treatment for samples analyzed with XPS

Untreated and treated technical bamboo fibres in air, nitrogen and argon

environments were analyzed with the XPS technique. Thermal treatments were

performed in a horizontal tube furnace, starting at room standard conditions. When

using argon, and in order to assure the absence of air in the cavity, a constant flow of

gas was passed through the furnace for 30 minutes before starting from room

temperature up to 220°C, at a constant temperature ramp of 5 °C/min, where it is

held during 40 min. The temperature was monitored with a thermocouple placed

close to the sample holder.

A2.2 Results

A2.2.1 XPS analysis

The XPS technique was used to characterize the surface chemistry of bamboo of

untreated and thermally treated bamboo fibres in two different environments (i.e. air

and argon). Table A2-1 presents the surface components of the treated and untreated

fibres detected by XPS, consisting of relative atomic percentages of the elements,

oxygen-carbon ratio and the decomposition of the C 1s peak into four sub-peaks C1-

C4. According to the number of oxygen atoms bonded to C, those peaks represent

[1-3]: carbon solely linked to carbon involved in ester or hydrogen C-C or C-H (C1),

carbon singly bound to oxygen or nitrogen C-O or C-N (C2), carbon doubly bound

to oxygen O-C-O or C=O (C3) and carbon involved in ester or carboxylic acid

functions O=C-O (C4). The decomposed C1s spectra for untreated and thermally

treated bamboo technical fibres in three different environments are shown in Figure

A2.1.

In Figure A2-2 shows the results of the surface chemical constituents of untreated

and treated (air and argon environments) bamboo technical fibres obtained from the

decomposition of the high resolution carbon 1s spectrum for each fibre. The content

of lignin on the surface was analysed by the O/C (oxygen to carbon ratio) and C1

(the relative concentration of the C1 component). As noticed, the atomic ratios for

cellulose and lignin are different and also pure cellulose lacks of C1 bonds.

Appendix 2 209

Thermal

treatment

atmosphere

Number

of

samples

C O N Si O/C Binding energy (eV)

(%) (%) (%) (%) (%) 284.8 286.3 287.5 288.8

C1 (%) C2 (%) C3 (%) C4 (%)

(C-C/C-H) (C-O) C=O/O-C-O O-C=O

Untreated fibre* 10 74.3± 1.5 22.9 ± 0.3 1.8 ± 0.7 0.6 ± 0.4 0.31 ± 0.02 58.0 ± 3.07 28.8 ± 2.3 7.6 ± 1.3 5.6 ± 0.4

Bamboo (air) 3 79.1 ± 1.6 16.7 ± 0.8 2.1 ± 0.4 1.7 ± 0.7 0.21 ± 0.07 53.3 ± 3.7 31.2 ± 3.8 10.5 ± 1.9 4.9 ± 0.4

Bamboo (Ar) 3 79.3 ± 0.5 16.7 ± 0.6 2.5 ± 0.5 1.0 ± 0.8 0.21 ± 0.08 52.5 ± 3.5 34.3 ± 2.1 9.3 ± 0.9 3.7 ± 0.6

Table A2-1. Relative atomic percentages (excluding hydrogen), O/C ratio and decomposition of C 1s peaks for untreated and thermally treated bamboo fibres in air and

argon atmospheres obtained by XPS. *Values obtained by Fuentes et al [4].

Figure A2-1. Typical C1s spectra comprising of the decomposition into four components C1 – C4 for a) untreated bamboo fibre [5], b) thermally treated bamboo fibre

in air and c) in argon (Ar).

Untreated fibre Treated in air Treated in argon

1_B_Air_1 : C 1s start/6

10

20

30

40

50

60

70

80

90

Inten

sity (

102 co

unts/

s)

296 292 288 284 280Binding Energy (eV)


5

10

15

20

25

30

35

40

45

50

Inten

sity (

102 co

unts/

s)



5

10

15

20

25

30

35

40

45

Inten

sity (

102 co

unts/

s)


Appendix 2 209

1_B_Argon_1 : C 1s start/6

10

20

30

40

50

60

70

Inte

nsity

(102 co

unts/

s)



10

20

30

40

50

60

70

Inte

nsity

(102 co

unts/

s)



10

20

30

40

50

60

70

Inte

nsity

(102 co

unts/

s)


210 Appendix 2

The ratios between the total amount of oxygen and carbon (O/C) of 0.21 and 0.21

for fibres treated in air and argon respectively, are close to the ones reported

previously by Fuentes et al. [5], ratio O/C of 0.31, for untreated bamboo technical

fibres. These values, clearly lower than 1, could reveal the abundant presence of C-

O-C ether functions which are characteristic of the presence of lignin at the surface

of the fibers [6].

Hypothetically, if only cellulose and lignin are present on the fibre surface, the

results show that the surface constituents for the analyzed samples are close to the

reference materials for lignin (see Figure A2.2). This indicates that the technical

fibre is cover by lignin and possibly other molecules but not cellulose. This is in

agreement with the studies of Fuentes et al. [4, 5] in which is stated that the surface

of the technical bamboo fibres (same as used in this study), is composed by lignin

instead of cellulose or hemi-cellulose. This is supported by the value of the ratio

O/C of 0.21 for both treated fibres and the untreated fibre (0.31) (see Table A2.1).

This value is close to the value ranges reported for lignin (0.24 – 0.36) [6, 7] and far

different from the O/C ratio of pure cellulose 0.83 [8]. Finally, the results obtained

from this XPS study indicate that the thermal treatment for technical bamboo fibres

in air and argon does not change the superficial components of the fibres, in

comparison with the untreated samples.

Figure A2.2. A correlation graph between the percentage of C1/C ratio versus O/C ratio for chemical groups

at the surface of untreated and thermally treated bamboo technical fibres (in air and argon), lignin form Granit

and theoretical values for cellulose and lignin according to Shchukarev [8].

Appendix 2 211

References

[1] Inari GN, Petrissans M, Lambert J, Ehrhardt JJ, Gérardin P. XPS characterization of wood

chemical composition after heat-treatment. Surf Interface Anal. 2006;38(10):1336-42.

[2] Kazayawoko M, Balatinecz J, Sodhi R. X-ray photoelectron spectroscopy of maleated

polypropylene treated wood fibers in a high-intensity thermokinetic mixer. Wood Science and

Technology. 1999;33(5):359-72.

[3] Shen Q, Mikkola P, Rosenholm JB. Quantitative characterization of the subsurface acidâ€“base

properties of wood by XPS and Fowkes theory. Colloids and Surfaces A: Physicochemical and

Engineering Aspects. 1998;145(1):235-41.



and Bioenergy. 2012;6(4):456-62.

[5] Fuentes CA, Tran LQN, Dupont-Gillain C, Vanderlinden W, De Feyter S, Van Vuure AW, et al.

Wetting behaviour and surface properties of technical bamboo fibres. Colloids Surf A: Physicochem

Eng Aspects. 2011;380:89-99.

[6] Johansson L-S, Campbell J, Koljonen K, Stenius P. Evaluation of surface lignin on cellulose

fibers with XPS. Appl Surf Sci. 1999;144:92-5.

[7] Tran LQN, Fuentes CA, Dupont-Gillain C, Van Vuure AW, Verpoest I. Wetting analysis and

surface characterisation of coir fibres used as reinforcement for composites. Colloids Surf A:


[8] Shchukarev A, Sundberg Br, Mellerowicz E, Persson P. XPS study of living tree. Surf Interface

Anal. 2002;34(1):284-8.

Curriculum Vitae

Personal data

Name: Efraín Eduardo Trujillo De Los Ríos

Nationality Colombian

Address: Ijzerenmolenstraat 22/301

3001 Heverlee

Belgium

e-mail: [email protected]

[email protected]

Education

2009 - present PhD., Materials Engineering, Katholieke Universiteit Leuven,

Belgium.

Dissertation: Polymer composite materials based on bamboo fibres.

2005 - 2007 M.Sc. Materials Engineering: Polymers and Composites (EUPOCO

program), Katholieke Universiteit Leuven, Belgium.

Thesis: Bamboo fibre composites.

1999 - 2004 B.Sc., Industrial Engineering, Universidad Nacional de Colombia.

Thesis: Physical-chemical characterization of the fibre of Guadua

angustifolia Kunth.

Awards

2014 Colciencias, Locomotora de la innovación para el apoyo del

desarrollo tecnológico (call 642). Development of a bamboo fibre

reinforcement to be used in high-tech composite applications,

Colombia.

2011 “Winner of the award for the best exploitation plan”. Doctoral

school training course “Exploitation of Research – Technology &

Knowledge Transfer”, Belgium.

2010 "JEC - SAMPE Student Award" for the best student selected on

subject environmental aspects of composites, France.

2005 “Development and Co-operation Prize 2005”, financed by the

Belgian Development Cooperation for young scientists, based on a

scientific work of high relevance for development, Belgium.

2004 Laureate bachelor’s thesis of Industrial Engineering at Universidad

Nacional de Colombia, Manizales.

Publications

Journal papers

1. E. Trujillo, M. Moesen, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest. Bamboo fibres

for reinforcement in composite materials: Strength Weibull analysis. Compos Part A:

Appl Sci Manuf. 2014;61:115-25.

2. E. Trujillo, M. Moesen, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest. Statistical

analysis of the mechanical properties bamboo fibres obtained by the dry fibre bundle

test, (2014). (To be submitted).

3. L. Osorio, E. Trujillo, A.W. Van Vuure, F. Lens, J. Ivens, I. Verpoest. Composite

micromechanics explain the behaviour of bamboo fibres, (2014). (To be sumitted).

4. L. Osorio, E. Trujillo, A.W. Van Vuure, J. Ivens, I. Verpoest. Morphological aspects and

mechanical properties of single bamboo fibres and flexural characterization of

bamboo/epoxy composite. Journal of Reinforced Plastics and Composites 2011; 30 (5):

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Conference proceedings

1. E. Trujillo, D. Perremans L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest, (2014).

Characterization of unidirectional discontinuous bamboo fibre/epoxy composites. ECCM

16 European Conference on Composite Materials, Seville, Spain.

2. E. Trujillo, J. Vertommen, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest, (2013).

Investigating the flexural properties of bamboo fibre – PP composites consolidated under

inert atmosphere. ICCM-19 International Conference on Composite Materials. Montreal,

Canada.

3. E. Trujillo, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest, (2013). Assessment of the

properties of bamboo fibre and bamboo fibre composites. In Lomov,

S. (Ed.), Composites Week @ Leuven. Composites Week @ Leuven. Leuven, Belgium.

4. E. Trujillo, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest. Bamboo (Guadua

angustifolia) fibres for storng-light composite materials. 9th World Bamboo Congress.

Antwerp, Belgium.

5. E. Trujillo, M. Moesen, L. Osorio, A.W. Van Vuure, J. Ivens,

I. Verpoest, (2012). Weibull statistics of bamboo fibre bundles: methodology for tensile

testing of natural fibres. In Quaresimin, M. (Ed.), 15th European Conference on

Composite Materials. European Conference on Composite Materials. Venice, Italy.

6. E. Trujillo, L. Osorio, A.W. Van Vuure, J. Ivens, I. Verpoest, (2010). Characterisation

of polymer composite materials based on bamboo fibres. Proceedings ECCM-14, 14th

European Conference on Composite Materials. European Conference on Composite

Materials. Budapest, Hungary.

7. E. Trujillo, L. Osorio, C.A. Fuentes, A.W. Van Vuure, I. Verpoest, (2010). Bamboo fibre

thermoplastic composites for transport applications. Society for the Advancement of

Material and Process Engineering, SAMPE Europe. Paris. France.

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