ABSTRACT
In this study, coconut palm wastes were obtained, washed to remove sands and debris
and then dried at 95oC for 1 hr to remove surface moisture. Filler modifications through
carbonisation were done at varying temperatures of 300oC, 400oC, 500oC, 600oC and
700oC. Both raw fillers and carbonised fillers were ground to achieve 100 μm particle
sizes, after which they were characterised. Formulations were appropriately drawn for
the mixing and compounding process. Rheological determination and flow properties
were evaluated. Physico-mechanical measurement of hardness, abrasion resistance
index, compressive strength, tensile strength, flexural strength, modulus and elongation
at break to ascertain composites reinforcement levels were also carried out. All
measurements were according to American Society for Testing and Materials (ASTM)
standards. Furthermore, qualitative assessments of modification levels were estimated
using Fourier infrared spectroscopic studies (FTIR), X-ray diffraction analysis (XRD),
X-ray fluorescence (XRF) of elemental oxides presence, scanning electron microscopic
analysis (SEM), thermal gravimetric analysis (TGA) and chemical resistance
measurements through sorption inference and analysis. Carbonisation strengthened the
polymer-carbon bond and therefore increased reinforcements of the composite matrix
effectively at 500oC for the coconut shell and 600oC for the coconut fibre. Optimum
formulations of 500oC for the shell and 600oC for the fibre were technically utilised in
the engineering design and manufacture of vibration dampeners for motor cycle hub and
industrial oil seals for bambury mixers employed in the mixing and mastication of
rubbers. Qualitative comparisons in properties such as resilience/rebound study,
hysteresis, dynamic flex cracking, flex fatigue, weathering/ozone resistance and
chemical resistance showed a comparatively good products when functionally and
aesthetically compared to available commercial grades. The formulated products were
viii
of high performance quality. Modification through carbonisation therefore created a
positive effect and improvements on the morphology, degree of
crystallisation/crystallite formations, physico-mechanical properties, chemical
resistance, weathering/ozone resistance, and development in resilience/rebound
properties, thermal gravimetric degradation/stability, and improvement in active
elemental oxides and depletion of lignocelluloses of the coconut shell and fibre through
infrared spectroscopic study. The results of mechanical and chemical sorption properties
that gave the optimised formulation for the fibre and shell composites used in the
modeled products were further theoretically evaluated through predictive and statistical
analysis of variance (ANOVA). The new Duncan‟s multiple range test (DMRT) was
used to verify the significance differences between subject factors of mechanical
properties and samples (modification temperatures) at 95% probability and
deterministic levels. A great proportion of the properties and samples satisfied the
significant measurement levels and therefore positive agreements between theoretical
and experimental results were established as a contribution to reinforcements. All these
noticed improvements resulted in better filler-rubber adhesion and interactions and
specifically leading to the reinforcement of the resulting composites and therefore
present the composites as useful materials for predictive product development in
engineering designs and applications.
ix
TABLE OF CONTENTS
Title – – – – – – – – – – i
Declaration – – – – – – – – – ii
Certification – – – – – – – – – iii
Dedication – – – – – – – – – iv
Acknowledgments – – – – – – – – v
Abstract – – – – – – – – – vii
Table of Contents – – – – – – – – ix
List of Tables – – – – – – – – – xxiii
List of Figures – – – – – – – – xxv
List of Plates – – – – – – – – – xxix
List of Appendices – – – – – – – – xxxi
List of Symbols and Abbreviations – – – – – – xxxiii
CHAPTER ONE: INTRODUCTION
1.1 Preamble – – – – – – – – – 1
1.2 Polymer Modification – – – – – – – 2
1.2.1 Fillers – – – – – – – – – 4
1.2.2 Application of Fillers – – – – – – – 5
x
1.3 Agricultural By-Products – – – – – – – 8
1.4 Natural Fibres in Composites – – – – – – 8
1.5 Coconut Powder as Fillers in Composites – – – – 11
1.6 Natural Rubber – – – – – – – – 12
1.7 Chemistry and Development of Natural Rubber – – – – 13
1.8 Processing Techniques of Natural Rubber Latex – – – – 22
1.8.1 Preserved Field Latex – – – – – – – 23
1.8.2 Ribbed Smoked Sheet (RSS) – – – – – – 23
1.8.3 Coagulation – – – – – – – – 24
1.9 Types of Natural Rubber – – – – – – – 25
1.9.1 White and Pale Crepe – – – – – – – 25
1.9.2 Crepe Rubber – – – – – – – – 26
1.9.3 Thin Brown Rubber – – – – – – – 26
1.9.4 Technically Specified Rubber (TSR) – – – – – 26
1.9.5 Superior Processing Rubber – – – – – – 27
1.9.6 Hevea Crumb Rubber – – – – – – – 27
1.10 Modifications of Natural Rubber – – – – – 27
1.10.1 Filler Incorporation – – – – – – – 27
xi
1.10.2 Halogenation – – – – – – – 28
1.10.3 Cyclisation – – – – – – – – 28
1.10.4 Resinous Addition – – – – – – – 28
1.10.5 Epoxidisation – – – – – – – – 29
1.10.6 Grafting Process – – – – – – 29
1.10.7 Degraded Inclusion – – – – – – – 29
1.10.8 Blending – – – – – – – 29
1.11 Basic Properties of Natural Rubber – – – – – 29
1.11.1 Comparison between Raw Natural Rubber and Vulcanised Natural Rubber 30
1.11.2 Hardening of Natural Rubber – – – – – – 31
1.12 Application of Natural Rubber – – – – – – 32
1.13 Statement of Research Problem – – – – – – 32
1.14 Aims of the Research – – – – – – – 33
1.15 Research Objectives of the Study – – – – – 34
1.16 Scope/Limitations of the Study – – – – – – 34
1.17 Justification/Significance of the Study – – – – – 36
1.18 Potential Contributions to Knowledge – – – – – 37
CHAPTER TWO: LITERATURE REVIEW
2.1 Previous Works – – – – – – – – 40
xii
2.2 Classification of Fillers – – – – – – – 44
2.2.1 Black Fillers and Classification by Manufacturing Process – – 45
2.2.1 (a) Furnace Black Process – – – – – – 45
2.2.1 (b) Channel Process – – – – – – – 45
2.2.1 (c) Acetylene Black Process – – – – – – 46
2.2.1 (d) Lamp Black Process – – – – – – 46
2.2.2 Non-Black Fillers for Rubber – – – – – – 46
2.2.3 Principal Characteristics of Rubber Filler – – – 47
2.2.4 Stretch Resistance Characteristics – – – – – 48
2.2.5 Resilience and Hysteresis Characteristics – – – – 50
2.2.6 Abrasion Resistance Characteristics – – – – – 50
2.2.7 Tear Resistance Characteristics – – – – – 51
2.3 Lignocellulose Biomass Nature of Coconut Shell and Fibre – – 51
2.4 Filler-Polymer Interaction – – – – – – – 53
2.5 Filler Carbonisation – – – – – – – 54
2.6 Compounding and Mixing Process – – – – – 55
2.6.1 Compound Design – – – – – – – 57
2.6.2 Composites Formations Using Fibres, Fillers and Selective Additives – 57
2.6.3 Additives in Compounding – – – – – – 58
xiii
2.6.3 (a) Fillers – – – – – – – – 58
2.6.3 (b) Plasticisers – – – – – – – – 59
2.6.3 (c) Vulcanisation Chemicals – – – – – – 60
2.6.3 (d) Acceleration Agents – – – – – – 62
2.6.3 (e) Activators – – – – – – – – 62
2.6.3 (f) Anti-Degrading Agents – – – – – – 63
2.6.3 (g) Processing Aids – – – – – – – 64
2.6.3 (h) Pigments – – – – – – – – 64
2.6.4 Mechanism of Mixing – – – – – – – 64
2.6.5 Mastication Process of Rubber – – – – – – 66
2.6.6 Principle of Compounding – – – – – – 67
2.6.7 Rheological Determination – – – – – – 68
2.6.8 Rubber Degradation Processes – – – – – – 70
2.6.8 (a) Non-Chain-Scission Reactions – – – – – 70
2.6.8 (b) Chain-Scission Reactions – – – – – – 71
2.6.8 (c) Oxidative Degradation – – – – – – 72
2.6.8 (d) Ozone Cracking – – – – – – – 73
2.6.8 (e) Stages of Ozone Cracking – – – – – – 74
2.7 Physico-Mechanical Properties of Rubber – – – – 74
xiv
2.7.1 Hardness – – – – – – – – 74
2.7.2 Abrasion Resistance Index – – – – – – 75
2.7.3 Compression Set, Strength and Deflection – – – – 76
2.7.3 (a) Measuring Compression Set – – – – – 76
2.7.3 (b) Compression-Deflection – – – – – – 77
2.7.3 (c) Measuring Compression-Deflection – – – – 77
2.7.4 Tensile Strength and Tensile Set – – – – – 78
2.7.4 (a) Measuring Tensile Strength – – – – – – 78
2.7.4 (b) Tensile Set – – – – – – – – 78
2.7.4 (c) Measuring Tensile Set – – – – – – 79
2.7.5 Elongation at Break – – – – – – – 79
2.7.5 (a) Measuring Elongation – – – – – – 79
2.7.6 Young‟s Modulus – – – – – – – 79
2.7.6 (a) Measuring Young‟s Modulus – – – – – 80
2.7.6 (b) Yield Point – – – – – – – – 80
2.7.7 Tear Strength/Resistance – – – – – – 81
2.7.7 (a) Measuring Tear Resistance – – – – – – 81
2.7.8 Weathering/Ozone Resistance – – – – – – 82
2.7.8 (a) Measuring Weathering/Ozone Resistance – – – – 82
xv
2.7.9 Flexural Testing – – – – – – – – 82
2.8 Qualitative Parameters Study – – – – – – 83
2.8.1 Fourier Transforms Infrared Spectroscopy (FTIR) – – – 83
2.8.2 Scanning Electron Microscopy (SEM) – – – – – 85
2.8.2 (a) Application – – – – – – – – 86
2.8.2 (b) Sample Preparation – – – – – – – 87
2.8.3 X-ray Diffraction Study (XRD) – – – – – – 88
2.8.3 (a) X-ray Powder Diffraction – – – – – – 88
2.8.4 Thermal Gravimetric Analysis (TGA) – – – – – 90
2.8.4 (a) Mechanism of Weight Change in TGA – – – – 90
2.8.4 (b) Resolution Enhancement in TGA – – – – – 91
2.8.5 X-ray Fluorescence Analysis (XRF) – – – – – 92
2.8.5 (a) Underlying Physics – – – – – – – 92
2.8.5 (b) Characteristics Radiation – – – – – – 93
2.8.5 (c) Crystals – – – – – – – – 93
2.8.5 (d) Applications – – – – – – – – 94
2.9 Predicting the Performance of Products/Processes Using Mathematical
Models – – – – – – – – 95
2.9.1 Analysis of Variance (ANOVA) – – – – 97
2.9.2 T-Test – – – – – – – – – 98
xvi
CHAPTER THREE: MATERIALS AND METHODS
3.1 Collection, Treatment of Materials and Reagents- – – – 100
3.2 Characterisation of the Raw and Carbonised Fillers – – – 101
3.2.1 Determination of Loss on Ignition – – – – – 101
3.2.2 Measurement of pH – – – – – – – 102
3.2.3 Determination of Bulk Density – – – – – 102
3.2.4 Determination of Iodine Adsorption Number – – – – 102
3.2.5 Determination of Moisture Content – – – – – 103
3.2.6 Determination of Oil Absorption – – – – – 103
3.2.7 Determination of Ash Content – – – – – – 103
3.2.8 Determination of Particle Dimensions and Lumen – – – 104
3.3 Formulation Design, Mastication and Mixing – – – – 104
3.4 Determination of Cure Characteristics – – – – – 105
3.5 Physico-Mechanical Testing/Measurements – – – – 106
3.5.1 Hardness Test – – – – – – – – 106
3.5.2 Abrasion Resistance Index Test – – – – – 106
3.5.2 (a) Calculation of Abrasion Resistance Index (ARIA) – – – 107
3.5.3 Compressive Strength Measurements – – – – – 107
3.5.4 Tensile Strength/Modulus/Elongation at Break Measurements – 108
xvii
3.5.5 Flexural Strength/Modulus Measurements – – – – 108
3.5.5 (a) Test Sample – – – – – – – – 109
3.6 Qualitative/Quantitative Analysis – – – – – – 109
3.6.1 Fourier Transforms Infra-red Spectroscopy (FTIR) – – – 109
3.6.2 Scanning Electron Microscopy (SEM) – – – – – 110
3.6.3 X-ray Diffraction Analysis (XRD) – – – – – 110
3.6.3 (a) Setting of the XRD Equipment – – – – – 111
3.6.3 (b) Bulk Analysis – – – – – – – 112
3.6.3 (c) Generation of Raw Data – – – – – – 111
3.6.4 X-ray Fluorescence Analysis (XRF) – – – – – 112
3.6.4 (a) Pelletisation of Samples – – – – – – 112
3.6.4 (b) Setting of the XRF Equipment – – – – – 113
3.6.4 (c) Loading and Running of Sample – – – – – 113
3.6.4 (d) Generation of Raw Data – – – – – – 113
3.6.5 Thermal Gravimetric Analysis (TGA) – – – – – 113
3.6.6 Sorption Measurements – – – – – – – 113
3.7 Production of Vibration Dampener for Motor Cycle Hub and Industrial
Oil Seal for Bambury Mixer – – – – – 114
3.7.1 Mould Design for the Vibration Dampener – – – – 114
3.7.2 Compound Mixing Process for the Vibration Dampener – – 115
xviii
3.7.3 Moulding Process for the Vibration Dampener – – – 115
3.7.4 Product Evaluative Measurement (Vibration Dampener) – – 116
3.8 Production Process Condition for the Oil Seal – – – – 116
3.8.1 Mould Design for the Oil Seal – – – – – 116
3.8.2 Formulation Design for the Oil Seal – – – – – 117
3.8.3 Compounding for the Oil Seal – – – – – – 117
3.8.4 Evaluative Measurement for the Oil Seal – – – – 117
3.8.5 Chemical Resistance Evaluation for the Oil Seal – – – 118
3.8.6 Weathering/Ozone Resistance Measurement of the Oil Seal – – 118
3.9 Predicting the Performance of the Optimised Coconut Fibre and Shell
Filled Composites – – – – – – – 118
CHAPTER FOUR: RESULTS
4.1 Characterisation – – – – – – – – 120
4.2 Hardness Properties – – – – – – – 120
4.3 Abrasion Resistance Index – – – – – – 121
4.4 Compression Set Results – – – – – – – 121
4.5 Compressive Stress-Strain Relationship – – – – – 122
4.6 Results of Tensile Strength/Modulus/EAB – – – – 123
xix
4.7 Tensile Stress-Strain Relationship – – – – – 124
4.8 Results of Flexural Strength – – – – – – 125
4.9 Flexure Stress-Strain Relationship – – – – – 125
4.10 3-D Plots Presentation of Mechanical Analysis – – – – 126
4.11 Sorption Tests Results and Plots – – – – – – 129
4.12 Infrared Spectroscopic Analysis – – – – – – 130
4.13 Scanning Electron Microscopy Results and Plates – – – 136
4.14 X-Ray Diffractograms Results and Evaluation of the Degree of
Crystallisation – – – – – – – – 143
4.15 X-Ray Fluorescence Analysis of Elemental Oxides – – – 150
4.16 Thermal Gravimetric Results and Evaluation – – – – 151
4.17 Motor Cycle Vibration Dampener Evaluation Study – – – 153
4.17.1 Dynamic Fatigue Evaluation Test for the Vibration Dampener – 153
4.17.2 Resilience Evaluation by Vertical Rebound of the Vibration Dampener 153
4.17.3 Comparative Evaluation of the Produced Vibration Dampener with
Commercial Type – – – – – – – 154
4.18.1 Chemical Evaluative Measurement for the Oil Seal – – – 154
4.18.2 Flex Fatigue Evaluation for the Oil Seal – – – – 154
4.19 Statistical Modelling on Performance Prediction – – 155
4.20 Pictorial Presentation Stages of Experimental Sampling – – 155
xx
CHAPTER FIVE: DISCUSSION
5.1 Characterisation – – – – – – – – 162
5.2 Hardness Properties – – – – – – – 163
5.3 Abrasion Resistance – – – – – – – 164
5.4 Compressive Strength – – – – – – – 164
5.5 Compressive Stress-Strain Relationship – – – – – 164
5.6 Tensile Strength/Modulus/EAB – – – – – – 165
5.7 Tensile Stress-Strain Relationship – – – – – 167
5.8 Flexure Strength – – – – – – – – 167
5.9 Flexure Stress-Strain Relationship – – – – – 168
5.10 Mechanical Analysis Using 3-D Plots – – – – – 168
5.11 Sorption Evaluation – – – – – – – 169
5.12 Infrared Spectroscopic Evaluation – – – – – 169
5.13 Scanning Electron Microscope – – – – – – 170
5.14 X-Ray Diffractograms Evaluation – – – – – 171
5.15 Elemental Oxides Evaluation – – – – – – 173
5.16 Thermal Gravimetric Evaluation – – – – – 174
5.17 Dynamic Fatigue of Vibration Dampener – – – – 175
5.18 Resilience by Vertical Rebound Evaluation for Vibration Dampener – 175
xxi
5.19 Comparative Evaluation Study of Vibration Dampener – – 176
5.20 Chemical Evaluation of Oil Seal – – – – – 177
5.21 Flex Fatigue Evaluation of Oil Seal – – – – – 177
5.22 Performance Prediction Using Modelling via ANOVA – – – 178
CHAPTER SIX: SUMMARY, CONCLUSION AND RECOMMENDATIONS
6.1 Summary – – – – – – – – – 184
6.2 Conclusion – – – – – – – 186
6.3 Recommendations for Further Studies – – – – – 189
References – – – – – – – – – 190
Appendices – – – – – – – – – 217
Publications – – – – – – – – – 279
CHAPTER ONE
INTRODUCTION
1.1 Preamble
Product modification has become a current trend in the utilisation of modern materials
especially in achieving better vulcanisate properties as applied to rubber compounding.
Prominent additives for rubber product modification are a wide range of filler materials
– both natural and synthetic in origin. Carbon black filler has often taken the lead when
it comes to the use of fillers for improvement in reinforcement properties despite its
short comings such as its non-renewable petroleum origin, dark colour, contamination
and pollution, because of its proven reinforcive ability (Agunsoye et al., 2012).
Fillers have marked effect on the physical and chemical properties of vulcanisates.
Tensile and modulus properties could be modified using appropriate fillers (Ahmedna et
al., 1997; and Ahmedna et al., 2000).
Owing to environmental awareness and economic considerations, biological/agricultural
materials-reinforced polymer composites are seen to present a viable alternative to
synthetic material-reinforced polymer composites as finding natural filler that will bond
properly with the rubber is a challenge. However, merely substituting synthetic with
natural fillers only solves part of the problem. Therefore selecting a suitable material for
the matrix is a key consideration (Akay, 1993; Akinlabi et al., 2011).
Coconut palm waste is an abundant agro-by-product in southern part of Nigeria is
considered promising in this regard. Various works on the application of natural fillers
in composites like pineapple, sisal, jute, cotton, rice husks and bamboo as the
reinforcements in composites have been reported in the literatures. Using natural fillers
2
to reinforce the composed materials offers the following potential benefits in
comparison with mineral fillers: strong and rigid due to cellulosic nature, light weight,
environmentally friendly, economical, renewable and abundant resource (Sapuan et al.,
2003; Jain et al., 2012; Alok et al., 2013; Momoh et al., 2017a, b).
Coconut palm waste is a potential candidate for the development of new composites
because of its high strength and modulus properties (Michael and Wolcott, 1999;
Momoh et al., 2016a). Composites of high strength coconut filler can be used in a broad
range of applications as, building materials, marine cordage, fishnets, and other
household appliances. However, it should be noted that coconut palm waste filler like
all other natural fillers suffer the following disadvantages: (i) degradation by moisture
(ii) poor surface adhesion to hydrophobic polymers (iii) non-uniform filler sizes (iv)
unsuitable for high temperature applications due to low ignition point (v) susceptibility
to fungal and insect attack (Sapuan et al., 2003; Mishra and Shimpi, 2005; Megiatto et
al., 2008; Md. Rezaur et al., 2010).
This work therefore emphases the modification of coconut palm waste through
carbonisation in an attempt at combating these short comings. Extensive carbonised
powder characterisation and evaluation will be carried out in an attempt to establish the
filler properties under different carbonisation conditions.
1.2 Polymer Modification
Typically, commercial plastics are mixtures of one or more polymers and a variety of
additives such as plasticisers, flame retardants, processing lubricants, stabilizers, and
fillers. The exact formulation will depend upon the specific application or processing
requirement (Jae et al., 2010; Jacob et al., 2014). Additives are materials added
polymers to alter their properties in order to make them suitable for certain identified
3
functions. The exact character of these additives will depend on the method of
processing to be used and the desired properties required in the finished product
(Additives, 2004). Different additives can influence the same polymer to have such
varying characteristics and therefore applications. For example, poly(vinyl chloride) is a
thermally unstable polymer having high modulus of elasticity, or stiffness, typical of
other glassy polymers at room temperature. In order to obtain a flexible-graded resin for
use as packaging film or for wire insulation, the polymer must be blended with a
plasticiser to reduce its glass transition temperature (Tg) and with a small amount of an
additive to improve its thermal stability at the required processing temperature
(Ansarifar and Nijahwan, 2000; Ansarifar et al., 2004; Ansarifar et al., 2005; Andrzej
and Abdullah, 2010).
Polymeric composites are physical mixtures of a polymer (the matrix) and a reinforcing
filler (the dispersed phase) that serves to improve some mechanical properties such as
modulus of rigidity or abrasion resistance. Fillers may be inorganic (e.g. calcium
carbonate, graphite fibre) or organic (aromatic polyamide such as Kevlar) (Ansarifar et
al., 2005). Virtually, any material can be used as the composite matrix, including
ceramic, carbon, and polymeric materials. Typically, matrices for polymeric composites
are thermoset such as rubber or polyester resins; however, some engineering
thermoplastics with high Tg and good impact strength, such as thermoplastic
Polysulfones, have been used for composites. Principal applications for composites are
in construction and transportation. Similarly, by using suitable additives, a range of
products such as tyres, battery cases, elastic bands and erasers, can be obtained from
rubber. Different additives perform different functions and the major ones can be
grouped as follows: Fillers, plasticisers, blowing agents, processing aids/lubricants, anti4
ageing agents such as anti-degradants and anti-oxidant, flame retardants/inhibitors,
cross linking agents, activating agents and colourants (Ansarifar et al., 2004).
Generally, additives must be cheap and stable both under processing and service
conditions (Arayapranee et al., 2005; Arayapranee and Rempel, 2008; Araùjo et al.,
2008; Arayapranee and Rempel, 2009). The focus of this present work will be mainly
on fillers as the main additive of interest for the modification process.
1.2.1 Fillers
The original use of fillers was to reduce the volume cost of the polymer in which case
they are called extenders. Apart from cost reduction, fillers perform other functions such
as improvement in tensile properties, improvement in tear resistance, nerve strength,
impact resistance and improved resistance to solvent porosity. The physical properties
of polymers can be modified by incorporation of fillers, which comes in various forms
and types. Fillers in general can sometimes have adverse effects on properties e.g.
products with fillers usually have less glossy finish. Fillers can either be particulate,
rubbery or fibrous in nature, and each of these impact some characteristic to the
polymer. For instance particulate fillers in general affect many properties of the
polymer, such as:
(i) Substantial reduction in cost by volume
(ii) Corresponding increase in moduli properties and hardness
(iii)Reduction in extrudate die swell
(iv) Improved electrical insulation
(v) Corresponding adjustments in tensile strength, elongation at break and flexural
properties (Arroyo et al., 2003).
5
Fillers are particles added to material (plastics, rubbers, composite material, and
concrete) to lower the consumption of more expensive binder material or to better some
properties of the mixture material. Worldwide, more than 53 million tons of fillers with
a total sum of approximately EUR 16 billion are used every year in different application
areas, such as paper, plastics, rubber, paints, coatings, adhesives and sealants. As such,
fillers, produced by more than 700 companies, rank among the world‟s major raw
materials and are contained in a variety of goods for daily consumer needs (Arroyo et
al., 2003; Aribike et al., 2007; Awatefe et al., 2014; Awatefe et al., 2016).
1.2.2 Applications of fillers
Additives, fillers, and reinforcements are used to change and improve the physical and
mechanical properties of plastics and rubbers. In general, reinforcing fibres increase the
mechanical properties of polymer composites while particular fillers of various types
increase modulus. Examples are baron, carbon, fibrous mineral, glass and Kevlar
(Ayeni et al., 2013a, b). Electrical properties can be affected by many types of filler. For
example, by adding conductive fillers, an electromagnetic shielding property can be
built into plastics, which are normally poor electrical conductors (Ayeni et al., 2014).
Anti-static agents can be used to attract moisture, reducing the build-up of static charge.
Examples are aluminium powders and carbon fibre graphite (Ayeni et al., 2014).
Different fillers are employed widely as extenders to lower the cost of composite
materials e.g., are calcium carbonate, silica and clay (Ayeni et al., 2013a, b; Ayeni et
al., 2014).
Fillers also enhance properties of the products, for example, in composites. In such
cases, a beneficial chemical interaction develops between the host material and the
6
filler. As a result, a number of optimised types of fillers, nano-fillers or surface treated
goods have been developed (Ayo et al., 2011).
Low-aspect fillers could modify the properties and moulding of the compound to which
they are added. If the fillers are characterised with a low aspect ratio between the
longest and the shortest dimensions, the basic properties will be less changed from those
of the unfilled polymer (Ayo et al., 2011 and Momoh et al., 2016). Fillers at such level
will benefit the composites in the following ways:
(i) Shrinkage of parts will be less
(ii) Thermal resistance may be improved
(iii)Strength, especially compressive strength, will be improved
(iv) Impact resistance will often be lower than for the unfilled polymer
(v) Solvent resistance will often be improved (Arayapranee et al., 2005; Ayo et al.,
2011).
High-aspect fillers: fibres may modify the properties when the aspect ratio between the
longest and the shortest dimension of the filler is large, for example, greater than 25, the
filler can be characterised as a fibre. Fibre reinforcements will significantly affect the
properties of the compounds to which they are added. Example, the following
characteristic nature will be pronounced:
(i) Fibres Impact Strength: Assuming good bonding between the fibre and the
polymer matrix, the strength in the fibre direction will be significantly increased.
If many fibres are oriented in the same direction, large differences will be noted
between the modulus in the orientation direction and in the direction
7
perpendicular to the orientation. The latter will be very close to that for the
unfilled polymer (Ayo et al., 2011).
(ii) Fibres Affect Shrinkage: The fibres will also have a significant effect on the
shrinkage properties of the compounds: Shrinkage in the orientation direction
will be much less than the shrinkage in the cross direction (Brahma et al., 2005
and Momoh et al., 2016a, b).
(iii)Importance of Predicting Fibre Orientation: Because the fibre orientation varies
with the flow direction, in the thickness direction, and at weld line locations, it is
important to be able to predict these orientations, in order to predict the
properties of the moulded article (Belmars et al., 1983; Bledzki and Gassan,
1999; Brahma et al., 2005).
The typical fibre contents of a polymer composite may range from 20% to 80% of the
total weight. The most common form of fibre fillers is E-glass, typically used to
reinforce thermosets, such as polyester and epoxy resins. E-glass is a boron-aluminasilicate
glass having low alkali-metal content and containing small percentages of
calcium oxide (CaO) and magnesium oxide (MgO) (Hafsat et al., 2016). For special
application, such as in the manufacture of aerospace materials, fibres of boron, kevlar
and especially carbon or graphite, are preferred (Dhakal et al., 2007; Habibi et al., 2008;
Hafsat et al., 2016).
For highly demanding applications, microfibres or whiskers (synthetically-grown single
crystals) of alumina or silicon carbide may be used. Whiskers can have tensile strength
as high as 27.6 Gpa and modulus as high as 690 GPa (Han et al., 2012).Other composite
fillers recently being considered include Buckminsterfullerene, C60, that has been found
to increase both Tg and thermal stability (Han-Seung et al., 2006).
8
1.3 Agricultural By-Products
The various sources of filler available can either be natural or synthetic, that is, the filler
can occur naturally either as agricultural waste such as coconut palm which forms the
basis of this research study. Other sources include: Palm kernel shell, rice husk,
groundnut shell, palm kernel bunch, coconut palm frond, etc. Generally, agricultural byproducts
are major constituent of environmental menace. Although they replenish the
soil, but the environmental pollution they usually emanate over-rides the agricultural
advantages derived.
In rubber compounding, the choice of suitable compounding ingredients are always
determined by the properties required, the processing procedures and the cost of
production. The search for renewable fillers from natural source becomes imminent so
that they can act as substitute for the non-renewable fillers (Larbig et al., 1998; Lou et
al., 2007; Lee et al., 2009).
1.4 Natural Fibres in Composites
The increasing demand for greener and biodegradable materials leading to the
satisfaction of society requires a compelling towards the advancement of nano-materials
science. Natural fibres will take a major role in the emerging “green” economy based on
energy efficiency, the use of renewable materials in polymer products, industrial
processes that reduce carbon emissions and recyclable materials that minimise waste.
Natural fibres are a kind of renewable resources, which have been renewed by nature
and human ingenuity for thousands of years. They are also carbon neutral; they absorb
the equal amount of carbon dioxide they produce. These fibres are completely
renewable, environmentally friendly, high specific strength, non-abrasive, low cost, and
bio-degradability (Madhukiran et al., 2013). Due to these characteristics, natural fibres
9
have recently become attractive to researchers and scientists as an alternative method
for fibres reinforced composites (Lovely et al., 2006; Lopattananon et al., 2006;
Madhukiran et al., 2013).
Natural fibres now dominated the automotive, construction and sporting industries by its
superior mechanical properties. These natural fibres include flax, hemp, jute, sisal,
kenaf, coir and many others. The various advantages of natural fibres are low density,
low cost, low energy inputs and comparable mechanical properties and also better
elasticity of polymer composites reinforced with natural fibres, especially when
modified with crushed fibres, embroidered and 3-D weaved fibres. Glass fibre
reinforced polymer (GFRP) is a fibre reinforced polymer made of a plastic matrix
reinforced by fine fibres of glass (Mandal and Alam, 2012). Fibre glass is lightweight,
strong, and robust material used in different industries due to its excellent properties.
Although strength properties are somewhat lower than carbon fibre and it is less stiff,
the material is typically less brittle in nature, and the raw materials are much less
expensive. Its bulk strength and weight properties are very favourable when compared
to metals, and it can be easily formed using moulding processes (Mark, 1964; Lin et al.,
2002; Lin et al., 2006., Mandal and Alam, 2012).
In modern times, natural fibres such as sisal and jute fibre composite materials are
replacing the glass and carbon fibres owing to their easy availability and cost. The use
of natural fibres is improved remarkably due to the fact that field of application is
improved day by day especially in automotive industries (Mohanty et al., 2002). Natural
fibre composites have gained increasing interest due to their eco-friendly properties. A
lot of work has been done by researchers based on these natural fibres. Natural fibres
such as jute, sisal, silk and coir are inexpensive, abundant and renewable, lightweight,
with low density, high toughness, and biodegradable. Natural fibres such as jute have
10
the potential to be used as a replacement for traditional reinforcement materials in
composites for application which require high strength to weight ratio and further
weight reduction (Móczó and Pukάnszky, 2008; Mohanty et al., 2000; Mohanty et al.,
2002).
The performance of the natural fibre polymer composites is influenced by several
factors, such as fibres micro-fibrillar angle, defects, structure, physical properties,
chemical composition, cell dimensions, mechanical properties and the interaction of a
fibre with the polymer matrix. Thus, to understand the properties of natural fibrereinforced
composite materials, it is essential to recognise the mechanics, physical, and
chemical composition/properties of natural fibres. The most important matters in the
development of natural fibre reinforced composites are (Maulida et al., 2000):
i. Surface adhesion characteristics of the fibres,
ii. Thermal stability of the fibres, and
iii. Dispersion of the fibres in the case of thermoplastic composites (Mod et al.,
1981; Maulida et al., 2000; Morreale et al., 2008).
The polarity characteristic of the natural fibre process leads to incompatibility
difficulties with many polymers. Hydrophilic or polar characters of natural fibres
produced composites with weak interface. Several chemical modifications or
pretreatment of surface are being made to improve and enhance the adhesion or
interfacial bonding between polymers and natural fibres (Morton, 1987; Mueller and
Krobjilowski, 2003; Nurdina et al., 2009).
The pretreatments of the natural fibre are usually done to clean and remove pollution
from the fibre surface, to modify chemically the surface, decreases the rate of moisture
11
absorption tendency, and to increase the external unevenness (Momoh et al., 2016a, b).
The incorporation of natural fibres as filler or reinforcement produces significant
changes in thermal stability of polymeric matrix. The manufacturing and the processing
of these composites involves the collaboration of fibres and matrix at sufficiently high
temperatures, hence, can lead to degradation of the bio-material, which results in
unfavourable effects on the final properties (Park et al., 2003; Osabohien et al., 2006;
Onyeagoro, 2012a, b).
1.5 Coconut Powder as Fillers in Composites
Coconut shell is one of the most important natural fillers produced in tropical countries
like Malaysia, Nigeria, Thailand and India. Coconut shell filler is a potential candidate
for the development of new composites because of its high strength and modulus
properties. The coconut particles have remarkable interest in the automotive industry
owing to its hard-wearing quality and high hardness, good acoustic resistance, mothproof,
non-toxic, resistant to microbial and fungi degradation, and not easily
combustible (Pradhan et al., 2004; Pothan et al., 2006; Poletto et al., 2011;Onyeagoro,
2012a, b ).
Composites of high strength coconut filler can be used in broad range of application as:
building materials, marine cordage, fishnets, furniture and other house hold appliances.
Their availability, renewability, low density, and price as well as satisfactory
mechanical properties make them attractive and an ecological alternative to glass,
carbon and man-made fibres used for the manufacturing of composites (Premlal et al.,
2002; Pino et al., 2006; Park et al., 2008).
Coconut shells are available in abundance in Nigeria as a waste product after
consumption of coconut water and meat. Such abundance can fulfill the demand of filler
12
based composites while reducing waste procurement and processing of coconut shell
powder is cost effective than other artificial fillers.
The morphology and mechanical properties of coconut shell reinforced composites such
as polyethylene have been evaluated to establish the possibility of using it as new
material for engineering applications. The result showed that the hardness of the
composites increases with increase in coconut shell content though the tensile strength,
modulus of elasticity; impact energy and ductility of the composites decreased with
increase in the particle content (Poh et al., 2002; Poletto et al., 2010; Agunsoye et al.,
2012).
1.6 Natural Rubber
Natural rubber (NR) is a high molecular weight polymeric substance with viscoelastic
properties. Structurally it is Cis-1, 4 – polyisoprene. Isoprene is a diene and 1, 4
addition leaves a double bond in each of the isoprene unit in the polymer. Because of
this, natural rubber shows all the reactions of an unsaturated polymer. It gives addition
compound with halogens, ozone, hydrogen chloride and several other reactants that
react with olefins. An interesting reaction of natural rubber is its combination with
sulphur. This is known as vulcanisation. This reaction converts the plastic and viscous
nature of raw rubber into elastic. Vulcanised rubber will have very high tensile strength
and comparatively low elongation. Its hardness and abrasion resistance also will be high
when compared to raw rubber. Because of the unique combination of these properties,
natural rubber finds application in the manufacture of a variety of products (Patternman,
1986; Okieimen et al., 2003; Osabohien and Egboh, 2007).
The main use of natural rubber is in automobile. In developed countries nearly sixty
percent of all rubber consumed are for automobile tyres and tubes. In heavy duty tyres,
13
the major portion of the rubber used is NR (Osabohien and Egboh, 2007). In addition to
tyres a modern automobile has more than 300 components made out of rubber. Many of
these are processed from NR. Uses of NR in hoses, footwear, battery boxes, foam
mattresses, balloons, toys etc. are well known. In addition to this, NR now finds
extensive use in soil stabilisation, in vibration absorption and in road making. A variety
of NR based engineering products are developed for use in these fields ( Rivin, 1963;
Pandey et al., 2003; Prakash, 2009) Some basic characteristic of NR are represented in
Table 1.1
Table 1.1: Basic Characteristics of Natural Rubber
Property Value/Unit
Specific gravity 0.92
Refractive index 1.52
Coefficient of cubical expansion 0.00062/0C
Cohesive energy density 63.7 J/cc
Heat of combustion 10547.084 J/g
Thermal conductivity 0.00032 J/sec/cm/0C
Dielectric constant 2.37
Power factor (at 1000 cycles) 0.15 – 0.2
Volume resistivity 1015ῼ/cm
Dielectric strength 3937V/mm
Source: Professional Association of Natural Rubber in Africa, Standard African
Rubber (SAR) Manual, 1998
1.7 Chemistry and Development of Natural Rubber
Natural rubber of commerce is obtained from the latex of Hevea brasiliensis, a native of
Brazil but widely grown on plantations in tropical Africa (Nigeria) and Asia. The
composition of the Hevea latex varies between quite wide limits in composition
(Brydson, 1978).
14
The non-rubber components not only have a biological function but also influence both
the methods of coagulation to form dry rubber and also the techniques of latex
technology. The empirical formula for the natural rubber molecule appears to have been
first determined by Faraday who reported his findings in 1826. He calculated that
carbon and hydrogen were the only elements present and his results correspond to the
formula C5H8 (Blow and Hepburn, 1971; Boonstra, 1975; Boonstra, 1979).
The possibility showed that the NR molecules might contain a mixture of cis- and transgroups
was considered to be unlikely because such a mixed polymer would have an
irregular structure and be unable to crystallise in the manner of natural rubber. Infra-red
studies have subsequently confirmed that NR was the cis-polymer. Infra-red studies
have indeed shown for a long while that natural rubber was at least 95% cis – 1, 4 –
polyisoprene. The absence of any peak corresponding to a vinyl group precluded the
presence of measurable amounts of 1, 2-material but an infra-red band at 890cm-1 was at
one time thought to be due possibly to the products of a 3, 4 – structure (Blow and
Hepburn, 1971; Boyle et al., 2004; Matador, 2007).
Time-averaging techniques using high resolution nuclei magnetic resonance (NMR)
which are capable of detecting 3, 4 – groups at concentrations of less than 0.3% have
however failed to establish the existence of any such moiety and have also failed to
show up any trace of trans-material (Golub et al., 1962; Chen, 1966; Chen and Porter,
1994). The conclusion must therefore be that the molecule is more than 99% cis–1, 4-
polyisoprene. Since all the evidence points to the conclusion that the NR molecule is not
obtained in nature by the polymerisation of isoprene the absence of detectable pendant
groups as would be produced by 1, 2 – and 3, 4 – addition is hardly surprising (Brydson,
1978; Burfield etal., 1984; Matador, 2007).
15
Macromolecules of NR are long, regular, flexible and practically linear, thus it has very
good elastic properties (Tg -700C) and spontaneously crystallises (maximum
crystallisation rate is approximately at -250C) also under influence of deformation
forces already at relative prolongation of more than 80%. It has also excellent strength
characteristics and keeps them also in form of vulcanisates Matador, 2007). Tensile
strength of NR vulcanisates filled with active fillers may be also more than 30 MPa. Its
molecular weight Mw varies the most often in between 104 – 107 and polymolecularity
Mw/Mn approximately from 2.5 to 10. In non-vulcanised status it is reversibly prolonged
under high deformation rates already to (800 – 1000) %. It belongs to highly
deformation rates already to commercial types of NR rubber must be masticated prior to
compounding. NR types with regulated constant viscosity (CV) practically do not need
mastication and they have good processing properties (Da Dosta et al., 2002; Dailatos,
2009; Matador, 2007; Daniel et al., 2009).
NR belongs to highly non-saturated rubbers, because each of their structural unit
contains one double link. Also reactive α – methylene hydrogen are related with its
presence. Both types of these function groups may take part in different addition or
substitution Polymeranalogical (e.g. during hydrohalegenation). They are utilised for
chemical modification of the rubber itself as well as for its vulcanisation. In general, the
NR rubbers are vulcanised by means of sulphur systems, but also other vulcanising
agents can be used (Phenol formaldehyde resins, urethanes, peroxides and others).
Ozone and oxygen react very easily with NR function groups, which cause its very low
aging resistance (Colthup et al., 1990; Dick, 2001; Matador, 2007; Ciolacu et al., 2011.
Rubbers and elastomers are polymer materials that are characterised by ability of
reversible deformation under influence of external deformation forces. Extent of
deformation depends on the structure and molecular weight of deformed rubber and also
16
on external conditions of deformation; it can achieve some (100 up to 1000) % already
at low stress (Matador, 2007). This property, marked as elastic, eventually highly elastic
deformation, has entropy character. It rests in ability of the rubber macromolecules to
occupy more ordered forms under stress, and on removal of stress to return to their ideal
statistically random conformation, under ideal conditions without deformation of
chemical bond distances or their angles (only non-combinatorial entropy is changed).
The entropy reduction (ΔS) at unchanged free energy of the stressed system (ΔԌ ═ΔH
─TΔS ═ 0) must be connected with enthalpy reduction (ΔH), which becomes evident
externally by heat buildup of the deformed sample. In ideal case the macromolecules
may return to original position after elimination of stress and the stressed samples
cooled down to original temperature (Matador, 2007).
The rubbers have usually long and regular macromolecule chains without large
substituent, with partially oriented structural units. Thus their segments are moveable
and also at low temperatures they can freely rotate around simple chemical bonds. It is
related to their low glass transition temperature, Tg. Typical examples of such rubbers
are poly-cis-1, 4-butadiene and poly-cis-1, 4-isoprene. They have Tg around –110,
eventually –70oC. With increasing of the content of irregularities in polymer chain
(trans-1, 4; 1, 2; and 3, 4 structural units) or under presence of large substituent
(styrene-butadiene rubbers) their Tg is increasing. Under glass transition temperature or
crystallisation temperature (when rubber crystallises) the rubbers are solid polymers
similar to plastomers. During heating they changed from elastic, eventually high elastic
state to viscoelastic state above softening temperature they are plastic and they flow. It
is advantageous when rubbers at normal temperature crystallise only under stress and
their Tg is significantly lower than their usage temperature (Gilman et al., 2005;
Matador, 2007).
17
The rubbers gain optimum properties of engineering materials only in form of
vulcanisates. It is possible to transfer them into this form by means of vulcanisation.
Basis is in creating of chemical and physical cross-links among rubber macromolecules,
consequence of that three-dimensional network is created and material obtains unique
properties. In most cases this cannot be achieved only by cross-linking itself, but also
some other additives must be added to rubbers. Except of cross-linking agents and
antidegradants (they reduce ageing process) those are mainly fillers (they are making
rubbers not only cheaper but they positively influence also some of their commercial
properties) and also additives allowing compounding of all necessary powder or liquid
ingredients to the rubbers, very often marked as auxiliary processing additives
(Matador, 2007).
Presently, a lot of rubber types are on the market that can be divided into more groups in
accordance with different criterion (e.g. saturated and unsaturated, natural and synthetic,
polar and non-polar, crystallising and non-crystallising, etc.). From view of their usage
and basic properties these can be also divided into (Momoh et al., 2016a, b):
(i) Rubbers for general use-they have properties complying with
requirements of more products, often also with different properties, they
are relatively cheap, produced and consumed in big volume.
(ii) Special rubbers- except of basic elastic properties, they have at least one
special property, e.g. Ageing resistance, resistance against chemicals,
resistance against swelling in non-polar oils, resistance against high or
low temperatures etc. Normally, they are produced and consumed in
lower volume than general rubbers and they are significantly more
expensive.
18
In professional literature and also in practice the rubbers are named besides commercial
names also with abbreviations. The abbreviation consists of a number of capital letters.
The last letter of appropriate abbreviation characterises typical atom or group that is
present in the rubber macromolecule (Gilman et al., 2005):
M – Rubbers with saturated hydrocarbon chain of methylene type
N – Rubbers containing nitrogen in polymer chain
O – Rubbers containing oxygen in polymer chain
Q – Rubbers containing oxygen and silica in polymer chain
R – Rubbers with unsaturated hydrocarbon polymer chain (diene)
T – Rubbers containing sulphur in polymer chain
U – Rubbers containing carbon, oxygen and nitrogen in polymer chain
Z – Rubbers containing phosphor and nitrogen in polymer chain.
Other letters of the abbreviation characterise monomers, the rubber was produced from.
For example, in accordance with that, the SBR abbreviation means butadiene-styrene
rubber, CR is chloroprene rubber, EPM is ethylene-propylene rubber, BR is butadiene
rubber etc. Also some other letters can create a part of abbreviation, and these closer
characterise appropriate rubber, e.g. OE-SBR is oil extended styrene-butadiene rubber,
L-SBR means styrene-butadiene rubber produced by polymerisation in solution, H-NBR
is hydrogenated acrylonitrile-butadiene rubber, CIIR is chlorinated isobutene-isoprene
rubber, etc. (Gilman et al., 2005; Matador, 2007).
Natural rubber has vegetable origin. It is created by enzymatic processes in many plants,
belonging mainly to families of Euphorbiacea, Compositea, Moracea and Apocynacea.
19
It is industrially achieved mainly from the tree called Hevea brasiliensis belonging to
Euphorbiacea family. It is grown in plantation way in warm (average monthly
temperature of 25-28oC) and humid (humidity around 80%) climate of South-Eastern
Asia (Malaysia, India, China, Sri Lanka, Vietnam), in western Africa (Nigeria,
Cambodia) and in north part of South America (Brazil, Guatemela). Annual production
of rubber is presently varying around 3000-3500kg per 1ha and it depends on weather,
soil quality, used stimulation means, age of trees and other external factors. The first
source of rubber is sucrose that is created from carbon oxide and water during
photosynthesis process. In the first biosynthesis stage the acetyl-coenzyme A is created
from it and this is changed into isopentyl-pyrophosphate through mevalone acid and the
rubber in form of latex is generated by polymerisation. The rubber is achieved from it
by means of coagulation (Matador, 2007; Daniel et al., 2009).
Natural rubber obtained from Hevea brasiliensis is practically pure poly-cis-1, 4
isoprene (contains more than 99.9% of cis-1, 4 structural units) from chemical view. At
the end of its macromolecules there may be bonded also non-isoprene structural units,
mainly proteins, amino acids and phospholipids, in macromolecules backbone those
may be also epoxide, ester, aldehydes, eventually and lactone groups. Also part of nonrubber
additives that are present in latex is remaining in rubber. Their content may be
different but generally it is varying in range 5-10%. In spite of their small amount in
rubber they have significant influence on its properties and they represent one reason of
different properties of natural rubber and its synthetic equivalent (IR) (Daniel et al.,
2009).
20
CH3 H
C ═ C
CH2 CH2 CH2 CH2
n
Figure. 1.1 Poly-cis-1, 4-isoprene
Natural rubber achieved from fresh latex and immediately dried-out after coagulation
contain small portion of gel, too. Gel-rubber has higher content of nitrogen and minerals
in comparison with sol-rubber, which leads to vision that rubber chains are more
branched in gel-rubber and they are mutually connected with proteins through hydrogen
bridges. This assumption is approved also by discovery that content of gel-rubber in
deproteinised rubber is much lower. Average amount of side branches per one rubber
macromolecule is varying approximately from 1 to 6 and it is higher in macromolecules
with higher molecular weight. Accompaniment of the natural rubber storing is gradual
increasing of its viscosity that is externally shown by its hardening. Reason of this
phenomenon is not well known, but it is accredited to cross-linking reactions of nonrubber
groups present in its macromolecules (Ansarifar et al., 2005).
NR latex in Hevea brasiliensis is located in latex vessels to be founded in various parts
of the tree. The lowest occurring is in the wood and the highest in the secondary
phloem. There are the vessels aligned to spirals in concentric circles close to cambium.
It is obtained from them by tapping based on cutting of the tree bark by special knife
under approximate angle of 30o. Latex spontaneously flows out of this slot, because it
occurs in spurges under hydrostatic pressure of 1-1.9 MPa. It is collected into special
bowls (Daniel et al., 2009; De Rosa et al., 2010).
21
Natural rubber latex is a colloid system having the rubber particles dispersed in water.
Latex particle size is varying approximately from 0.05 to 3μm. In fresh latex they have
mainly spherical shape that is under their aggregation gradually changed to pear shape
(section through rubber particles). Besides these also small amounts of proteins,
resinous matters (including lipids), hydrocarbons and mineral substances are present in
NR latex. Part of these non-rubbery matters, mainly proteins and lipids, is surrounded
by a surface of rubbery particles and gives them negative charge, which assures the
latex stability (Dhakal et al., 2007; Matador, 2007).
Specific weight of fresh NR latex is 0.96 -0.98g/cm3 and its pH is varying within 6.5-
7.0. It coagulates by standing on the air, and for this reason it must be stabilised. The
most often used item for this stabilisation is ammonia (HA latex- maximum 0.7% NH3)
or its combinations (LA latex – 0.2% NH3) with secondary stabilizers, such as
dithiocarbamates, combination of Tetramethylthiuram disulphide and ZnO, Lauric or
boric acid. These are added to latex only in very small amounts, normally 0.01 -0.05%.
Some rubber products (e.g. foam rubber, gloves, condoms, glues) are produced directly
from latex. The latex is modified for these reasons to have higher dry rubber content
(DRC) values (minimum 60- 65% of rubber). It is performed by means of its
concentrations, the most often by centrifugation and sedimentation, but also water
evaporation, thickening and electro-decantation is used. During these operations also
eventual dirtiness and non-rubbery additives are removed from rubber besides increase
of the dry rubber content in latex (Matador, 2007; Daniel et al, 2009).
Mastication is a process during which the elastic rubber achieves plastic properties.
During mastication breaking of chemical bonds in its macromolecules take place by
means of high shear forces. This process results in the decreasing of molecular weight
and viscosity of rubber and consequently it becomes treatable. Mastication of NR is
22
performed either at low temperature on mills or at higher temperature in closed mixers,
often in the presence of peptisers (they act as donors of electrons or hydrogen), that
increase its efficiency. Besides mechanical degradation of rubbery macromolecules also
their oxidation degradation occurs in this process and its rate is upgraded with
mastication temperature increasing. Mastication of the NR is the most efficient at
temperatures below 60-70oC and above 120-130oC, its efficiency is low in interval
between these temperatures (Gilman et al., 2005).
Mastication of synthetic rubber is much less efficient, and thus they are either produced
with the molecular weight and viscosity suitable for their processing or they are
modified in final production stages by means of suitable oils (e.g. SBR extended by oil).
Advantage of such modification is in possibility to keep the high molecular weights,
that normally afford better physical-mechanical and dynamic properties to vulcanisates
and also processing of rubbers is good (Matador, 2007).
1.8 Processing Techniques of Natural Rubber from Latex
Latex is a white or slightly yellowish opaque liquid with a specific gravity, which varies
between 0.974 and 0.986. It is a weak lyophilic colloidal system of spherical or peer
shaped rubber globules suspended in an aqueous serum. The rubber globule is
surrounded by a protective layer of proteins and phospholipids, which impart the
lyophilic nature to latex. The stability of latex is due to the negative charge present on
the protective layer. Also it contains a variety of non-rubber constituents both organic
and inorganic, in addition to rubber (Flory and Rehner, 1943; Flory, 1953; Hosler,
1999). The proportion of these constituents may vary with clone, soil nutrition, climate
etc. Latex composition could be represented as shown in the Table 1.2
23
Table 1.2: Percentage Composition of Natural Rubber Latex
Composition Percent (%)
Rubber
Proteins
Ash
Resins
Sugars
Water
30– 40
2– 2.5
0.7– 0.9
1– 2.0
1– 1.5
55– 65
Fresh latex, as it comes out from the tree is slightly alkaline or neutral. It becomes
acidic rapidly due to bacterial action. The formation of organic acids neutralizes the
negative charge on rubber particles and the latex gradually gets coagulated on keeping.
Therefore, fresh latex cannot be kept for long without preservative treatment. Latex can
be processed into any of the following forms: Preserved field latex and latex
concentrate, sheet rubber, block rubber, crepe rubber. Field coagulum can be processed
only into crepe rubber or block rubber (Helge, 2000; Roger, 2002; Rowel, 2005)
1.8.1 Preserved field latex
Field latex is preserved using suitable preservative for long term storage. The
processing of preserved field latex consists essentially of adding the preservative
(usually ammonia, minimum 1%) to the sieved latex, bulking, settling, blending and
packing. Field latex can also be preserved with LATZ (low ammonia – TMTD – zinc
oxide) system (Zhang, 2004a).
1.8.2 Ribbed smoked sheet (RSS)
Latex is coagulated in suitable containers into thin slabs of coagulum and rolled through
a set of smooth rollers followed by a grooved set and dried to obtain sheet rubber.
Depending upon the drying method, sheet rubbers are classified into two: Ribbed
24
Smoked Sheets (RSS) and Air Dried Sheets (Pale Amber Unsmoked Sheets) (Chotirat et
al., 2007; Daniel et al., 2009).
For processing latex into sheet rubber, it is important that the latex collected is brought
to the processing centre before pre-coagulation sets in. In cases where the latex is found
to be prone to pre-coagulation, an anticoagulant is used. Latex brought to the centre is
strained through 40 and 60 mesh stainless steel sieves. The volume of latex is measured
with a standard vessel and a calibrated rod. The DRC is estimated with a metrolac,
which is a special type of hydrometer calibrated to directly read the DRC. However,
laboratory methods are employed for accurate determinations (Eiras and Pessan, 2009).
Latex is diluted in bulking tanks to a standard consistency of 1/2kg of dry rubber for
every 4 litres of the diluted latex (12.5% DRC). The diluted latex is allowed to stand in
the bulking tank for a fixed time (usually 15 to 20 minutes) for the heavy dirt particles
to sediment. The diluted latex is drawn out from the bulking tank without disturbing the
sediment layer of impurities into the coagulation pans or tanks. Four litres of latex is
usually transferred to each pan (Daniel et al., 2009; Eiras and Pessan, 2009).
1.8.3 Coagulation
Formic acid or acetic is generally used for coagulation. The quantity of acid required for
satisfactory coagulation depends on various factors like the amount and type of
anticoagulant used the duration of coagulation, the season and the nature of the latex.
The acid requirement may slightly change under varying conditions and can be fixed up
by experience. Only diluted acid should be used for coagulation and should be
thoroughly mixed with latex (Daniel et al., 2009). Catalyst AC and Sulphuric acid are
also used by growers. Catalyst AC is a dry powder and comparatively a safe coagulant.
25
Normally, 100cm3 of a 5 percent solution of this chemical is enough for making a ½kg
sheet (Nasir and Choo, 1989).
Since sulphuric acid is highly corrosive, care should be taken in its handling and
dilution. 300 ml of a 0.5% solution of the acid is required for same day sheeting and 250
ml for next day sheeting. Coagulum from latex often shows a tendency for surface
darkening. To prevent this, a small quantity of sodium bisulphate (1.2g per kg DRC),
dissolved in water may be added to the diluted latex before coagulation. After
coagulation, the coagulum is removed from the pan or tank and thoroughly washed in
running water (Daniel et al., 2009). They are rolled either in a sheeting battery or
smooth rollers to a thickness of 3 mm and finally passed through the grooved roller.
White sheeting, the coagulum is continuously washed. The sheets are again washed in
running water in a tank. Mould growth on sheet rubber can be prevented by treating
freshly machined sheet in a dilute solution of paranitrophenol (PNP). The concentration
of PNP is 0.05 to 0.1% in water. 100 litres of the solution will be sufficient for treating
100 sheets. The wet sheets are allowed to drip on reapers arranged in a well-ventilated
dripping shed (Nasir and Choo, 1989; Daniel et al., 2009).
1.9 Types of Natural Rubber
Apart from ribbed smoked sheet (RSS), which forms a major group type of natural
rubber as described above; other six (6) types are described thus: Ribbed smoked sheet
(RSS), white and pale crepe, crepe rubber, thin brown rubber, technically specified
rubber, superior processing rubber, Hevea crumb rubber (Daniel et al., 2009).
1.9.1 White and pale crepe
Selected field latex is treated with sodium bisulphate and then strained several times.
The latex is diluted with water to 25% weight of DRC. A dilute solution of acetic acid is
26
added. Reflection rich in pigments floats-off the surface, which is removed and
bleached. Formic acid is added to the bleach portion to reduce the pH to 4.5. It is then
poured into aluminum partitioned batteries to coagulate into slabs. The slabs are
processed in mills under water and are converted into thick blankets which are
transferred to macerator mills. It undergoes shearing and mastication. The slabs are then
reduced to 1–3 mm thickness. On smoother finishing mills, the sheets are dried at (31–
34)0C for several days. This is graded in the basis of colour, smell, dust, specs, sand and
foreign materials (El-Tayeb and Nasir, 2007; George et al., 2001).
1.9.2 Crepe rubber
The latex is coagulated and washed in washing mills; (16–20) % latex yields a soft,
cohesive coagulate which on standing gives a clear serum. The residue is washed with
water on closely spaced rollers. The thin sheets are dried at 17oC for two hours in
vacuum driers. It is then cooled to a brownish colour. Initial concentrated rubber is
obtained from undiluted latex. This is graded according to colour, purity and strength
(Golub et al., 1962; George, 2000; Ghosh, 2007).
1.9.3 Thin brown rubber
The raw materials used are fresh cup lump, wet slab, unsmoked sheets or high-grade
scraps. These are milled with water on grooved wash mills. Then they are dried for
three weeks in large open drying sheds. The resultant sheets may be thin brown or thick
brown. These are graded according to colour and contamination (Ghosh, 2007).
1.9.4 Technically specified rubber (TSR)
The coagulum is milled in series of wash mills to obtain clear and homogenised rubber.
The wet crumbs are dried in air-circulating ovens at 1000C (Gent and Pulford, 1983).
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1.9.5 Superior processing rubber
This is a mixture of cross-linked (SPR) rubber and unmodified rubber. It is processed as
RSS, Pale crepe, brown crepe and air dried sheets. These rubbers are used as processing
aids in mixture with regular grade NR to give low die swell, smoother and faster
extrusion and improved calendaring (Gent, 1989).
1.9.6 Hevea crumb rubber
In Standard Malaysian Rubber (SMR) castor oil is added to field latex prior to
coagulation. It is then milled with castor oil, which acts as a crumbling agent. SMR is
then converted to fine crumbs. These are washed with water and dried at (80 – 100)0C
for 3hours. This is known as Hevea crumbs and consists of SMRCV, SMRLV,
SMRWF, SMR 5, SMR 10, SMR 20, SMR 50 and SMRGD grades (James and Burak,
2005).
1.10 Modification of natural rubber
There are various forms of NR rubber modification which may include the following
techniques: Filler Incorporation, hydrogenation, halogenations, cyclisation, resinous
addition, epoxidisation, grafting process, degraded inclusion, blending.
1.10.1 Filler incorporation
Fillers are most widely used additives in polymer compositions. They are used in all
plastics, natural and synthetic rubber and in coatings. Filler is an inert material added to
a polymer composition to improve its properties and/or to reduce its cost. On being
mixed with the resin, it forms a heterogeneous mixture which can be moulded under the
influence of heat or pressure or both. There are reinforcing fillers, active fillers and
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inactive fillers which may be black or non-black in colour; organic or inorganic in
nature (Iyasele and Okieimen, 2004).
1.10.2 Halogenation
NR can be chlorinated in a solution in latex and in solid state. Glass transition
temperature of chlorinated NR increases with increasing chlorine content. These are
light, cream coloured and thermoplastic products. It is used as adhesive and for textile
wafting, and sometimes in black coloured paint. NR undergoes reaction with hydrogen
halides. With hydrogen chloride (HCl) it produces a very hard white product (C5H4Cl)n;
which gives a transparent film (Kabir et al., 2006).
1.10.3 Cyclisation
NR on treatment with Sulphuric acid gives a hard brittle product. NR can be reacted
with sulphuric acid, sulphonyl chloride and other sulphur compounds. It can also be
cyclised with ZnCl2 and Chlorostanic acid. These are of two types, brown black similar
to balata and light and low molecular weight resins soluble in many solvents and
compatible with resins, oils and plasticizers (Kandem et al., 2004; Jurkowska et al.,
2006).
1.10.4 Resinous addition
Phenol formaldehyde resins are added during vulcanisation. These have low
compression set, excellent dynamic stability and good aging properties. Mooney
viscosity does not increase (Joseph et al., 2002; Joseph et al., 2003a, b).
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1.10.5 Epoxidisation
Trialkyl ethylene double bond in NR reacts with per acids to produce the epoxidised
product in high yields. A mixture of H2O2 and formic acid has been found to give
satisfactory results. Glass transition temperature increases by 1oC per mole of
epoxidation per mole of NR. Epoxidised natural rubber (ENR) has Tg near room
temperature. These are used as anti-skidding agents (Kim et al., 2006; Kim et al., 2007).
1.10.6 Grafting process
Styrene, vinyl acetate, acrylonitrate and methyl methacrylate can graft NR. Peroxides
and hydroperoxides initiate the reaction. These grafted products are used for reinforcing
vulcanisates and adhesives (Karmakar et al., 2007; Liang, 2011).
1.10.7 Degraded inclusion
Liquid NR on oxidation gives a dark viscous material, to be used in rubber processing
as a binder and processing aid (Guo, 2009).
1.10.8 Blending
This could be binary or ternary blending; where NR can be blended with polyolefin or
polystyrenes. This gives a thermoplastic rubber (Chand et al., 1987).
1.11 Basic Properties of Natural Rubber
(i) Crude rubber is a tough and elastic solid. It becomes soft and sticky as
the temperature rises.
(ii) Its specific gravity is 0.92.
(iii) The most important property of natural rubber is its elasticity. When
stretched, it expands and attains its original state, when released. This is
30
due to its coil-like structure. The molecules straighten out when stretched
and when released, they coil up again. Therefore applying a stress can
easily deform rubber. Note that when this stress is removed, it retains its
original shape (Ekebafe et al., 2009).
(iv) Raw natural rubber has elasticity over a narrow range of temperature
from 10 to 60 degrees centigrade. Because of this, articles made of raw
natural rubber do not work well in hot weather.
(v) Raw natural rubber has low tensile strength and abrasion resistant.
(vi) It absorbs large quantities of water.
(vii) It is insoluble in water, alcohol, acetone, dilute acids and alkalis.
(viii) It is insoluble in ether, carbon disulphide, carbon tetrachloride, petrol and
turpentine when vulcanized.
(ix) Pure rubber is a transparent, amorphous solid, which on stretching or
prolonged cooling becomes crystalline (Choi et al., 2003; Egwaikhide et
al., 2007a,b; Ekebafe et al., 2009)
1.11.1 Comparison between raw natural rubber and vulcanised natural rubber
Most of the shortcomings/weakness of raw natural rubber has its corrections in the
vulcanised rubber as indicated in Table 1.3 (Hull and Clyne, 1996):
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Table 1.3: Comparison of Raw Natural Rubber (RNR) with Vulcanised Natural Rubber
(VNR)
Raw Natural Rubber (RNR) Vulcanised Natural Rubber (VNR)
Comparatively hard and non-sticky
High tensile strength and very
Strong
High elasticity
Can be used over a wide range of
Temperature from (40 – 100)0C
High abrasion resistance
Absorbs a small amount of water
Insoluble in all the usual solvents.
Soft and sticky
Low tensile strength and not very
Strong
Low elasticity
Can be used over a narrow range of
Temperature from (10 – 60)0C
Low abrasion resistance
Absorbs a large amount of water
Soluble in solvents like ether,
carbon disulphide, carbon
tetrachloride, petrol and turpentine.
1.11.2 Hardening of natural rubber
Rubber can be hardened by adding carbon black as filler (solid substance added for
strength and to reduce cost) to it during the vulcanisation process. This increases the
strength and abrasion resistance of natural rubber. Carbon black when mixed with
rubber in rubber tyres makes them more durable and cuts the cost. The vulcanised and
hardened rubber is used to make tyres and tubes of automobiles and conveyor belts for
industrial use. Vulcanised natural rubber could also be useful for rubber bands, football
bladders and gloves (Ishak and Bakar, 1995; Igwe and Ejim, 2011; Husseinsyah and
Mostapha, 2011).
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1.12 Application of Natural Rubber
Rubber is versatile and finds a wide range of application in the production of the
following:
(i) Pneumatic tyres and tubes
(ii) Belting-conveyors, transmissions and V-belts
(iii)Hose-hand mode and braided
(iv) Footwear sole-micro-cellular, unit, resin rubber, DVP/DIP
(v) Cable-insulation and sheath
(vi) Coated fabric and calendared sheeting
(vii) Moulded items like seats, gaskets, auto components
(viii) Rubber to metal bonded components
(ix) Rubber rollers
(x) Extruded items like tubing, weather strips
(xi) Adhesives
(xii) Latex products – dipped goods, threads and foams (Honday, 1966; Hon
and Shiraishi, 2001).
1.13 Statement of Research Problem
The increasing health challenges in the use of carbon black with its high nitrosamines
contents in rubber compounding have become worrisome in recent times. An attempt at
finding and developing alternative filler materials to combat all major disadvantages of
carbon black in rubber reinforcement is a major focus of this work. A polymeric matrix
33
material with better filler/matrix interaction together especially from natural and
renewable agro-byproducts is a promising venture in compound design because of
larger surface area, and greater aspect ratio, with fascinating properties (Cao et al.,
2009; Momoh et al., 2016b and Momoh et al., 2017 a, b).
Being environmentally friendly, applications of coconut palm wastes offer new
technology and entrepreneurial opportunities for several sectors, such as aerospace,
automotive, electronics, and agro-technology industries. Hybrid agro-based composites
that exploit the synergy between natural materials in a reinforced rubber can lead to
improved properties along with maintaining environmental appeal.
1.14 Aims of the Research
(i) This research work is intended to present workable and applicative
information about coconut palm wastes which include the shells and fibres
composites with specific concern to harnessing them, removing them away
from the environment as wastes, moving them from waste to wealth,
characterising them, evaluating their potentials and appropriately modifying
them to extend the frontiers and modern technological advancements
through the scientific exploitation of renewably abundant and available
materials.
(ii) The far reaching implication is geared towards products development
especially in automobile, engineering and specialty applications. The target
will be to eradicate modern challenges in the utilisation of composites by
appropriate formulation designs and product optimisation.
34
1.15 Research Objectives of the Study
(i) This research work seeks to broaden the horizon for rubber product
development through reinforcement of composites from renewable
agricultural wastes.
(ii) This work also sort the use of carbonisation as an appropriate
modification method for morphological re-orientation of coconut palm
waste to alleviate certain inherent weaknesses as stated in the
Justification.
(iii) The health implication of the use of non-renewable mineral fillers like
carbon black for rubber reinforcements was a major driving force for this
research study.
(iv) This work seek to utilised modern and high-tech analytical laboratory
equipment to analyse and evaluate the possible extent of modification
attained through carbonisation using standard measurement test methods.
1.16 Scope/Limitations of the Study
In order to fulfill the stated aim and objectives, the scope of the research will cover the
following studies:
(i) Sourcing and gathering of coconut palm shells and fibres. Thorough washing in
water to remove sands and debris. Oven drying to remove moisture retained during
the washing operations.
(ii) Physical modification of the coconut shell and fibre by carbonisation process.
(iii)Grounding of both raw and treated shell/fibre. Particle filtering and sizing
operations to achieve 100 μm.
35
(iv) Characterisation of the raw and treated fillers in terms of pH value, iodine
adsorption number to determine surface area reactivity, loss on ignition, bulk
density, ash content, fibre dimensions, moisture content, oil absorption and
particle size determination.
(v) Formulation design, mixing and compounding in a two roll mill to achieve a
compounded mix in the form of sheets.
(vi) Rheological determination using oscillating disk rheometer, (ODR Model 2000.
This will enable the determination of processing/cure characteristics parameters
such as temperature of cure, press pressure, time of cure, optimum cure
determination; scorch time, modulus control and reversion cure as well as mix
viscosity of melt.
(vii) Press curing to stabilise crosslink formation.
(viii) Mechanical evaluation of hardness, abrasion resistance index measurements,
compressive strength determination, tensile strength, modulus, elongation at break
and flexural strength analysis.
(ix) Chemical sorption evaluation to determine extent of crosslink interaction between
filler and rubber matrix.
(x) Qualitative evaluative analysis on composites using Fourier transform infra-red
spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction study
(XRD), X-ray fluorescence analysis (XRF), thermal degradation and stability
measurement using thermogravimetric analysis (TGA).
(xi) Development of an engineering product from achieved best formulation using the
modified coconut shell and fibre.
36
(xii) Property comparison between formulated and manufactured engineering product
with such standard product in the commercial market and usage by evaluative
field/product analysis.
(xiii) Mathematical modelling using analysis of variance (ANOVA) for between
subject factors of mechanical and chemical sorption properties with sample
modification through carbonisation temperature.
1.17 Justification/Significance of the Study
(a) The possible advantages, such as reduced tool wear, low cost, and low density
per unit volume and acceptable specific strength, along with their sustainable
renewable and degradable features are some of the important properties of the
coconut palm wastes, which make them suitable to use as filler in rubber
composites (Egwaikhide et al., 2007).
(b) Synthetic filler materials, such as carbon black, carbon fibres, glass fibres create
severe ecological, and health hazard problems in their use in rubber
compounding. A suitable replacement from natural alternatives like coconut
palm wastes is necessitated (Okieimen et al., 2003a,b).
(c) Easy approaches and schemes are readily established to supplement and
appropriately modify certain inherent deficiencies such as; poor fibre/matrix
interactions, water resistance, and relatively lower durability. Although weaker
interfacial bonds between highly hydrophilic natural fillers and hydrophobic,
non-polar organophilic rubber matrix, often leads to considerable decrease in the
properties of the composites and, thus, significantly obstructs their industrial
utilisation and production. However, the surface of the natural coconut filler can
be easily modified and this can be achieved by physical, mechanical and/or
chemical means via carbonisation (Nemour, 1999; Newman, 2008).
37
(d) A perfect substitutes to traditional or conventional automobile and bridge
bearing materials are the fibre reinforced rubber composites due to a number of
factors which include: higher strength and stiffness with reference to specific
gravity; better resistance to corrosion, natural hazardous environments, no
conductivity, non-toxicity and lower life-cycle costs; and higher fatigue strength
and impact energy absorption capacity (Myhre and Mackillop, 2002; Osman et
al., 2010).
(e) When coconut palm wastes, especially the fibres are used to prepare
construction or building modules, then, under developed or developing
countries, with rural regions, will tend to enforce the cultivation of required
manufacturing crops and this would be empowering to address their own
housing, poverty and financial issues without any outsider support. They become
a way of contributing to the economic growth, entrepreneurial development and
job creation.
(f) An attempt is being made for the utilisation of locally manufactured starting
materials, goods and Nigerian service companies in production operations,
projects and well engineering.
1.18 Potential Contributions to Knowledge
The purpose of scientific research is to add to the body of knowledge. Results obtained
from this thesis will precisely address:
(a) Further information on polymer network structure via filler modification will be
contributed to existing and inadequate literature.
(b) The carbonisation process for the modification of the coconut palm wastes
which is a novel approach is intended to show that the removal/depletion of
38
lignocellulose components and moisture which are prominent features that
hinders the use of most agricultural wastes and by-products in polymer matrix
reinforcements could be readily dealt with giving a better interactions between
agro-based fillers and polymer matrix of composites.
(c) The elemental compositional analysis has shown that at least eighteen (18)
metal oxides are present both at trace and macro levels in the coconut palm
wastes; and carbonisation increased the potassium oxide (K2O) component in
preference to all other minor and major participating metal oxides.
(d) Modification of the coconut palm waste through carbonisation clearly showed a
direct impact on the molecular re-orientation of the shell and the fibre as seen in
the particle aggregations, glassy-like strands depicted by SEM and the increase
in crystalline index shown by X-ray diffractions evaluated.
(e) The optimised formulation design has been proven to be effective and efficient
in imparting appropriate reinforcements to certain engineering wares such as the
produced motor cycle vibration dampener and industrial oil seals for bambury
mixers, popularly called two-roll mill used in rubber mastication and mixing.
Needed mechanical properties were greatly improved upon by good filler-matrix
interactions in the composites.
(f) The viability of coconut palm wastes, otherwise seen as environmental nuisance
would be validated and profitably converted from waste to wealth through the
appropriate techniques to be exploited this thesis.
(g) Statistical predictions and modelling of mechanical and chemical sorption
properties in response to product modification through filler carbonisation shall
be made available. Levels of significance and non-significance by means of
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ANOVA evaluative studies could encourage further mathematical modelling
using other means.
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