Chapter 1

Introduction

Abstract

Rubber has many advantages. But since its strength is

low, fibres are used to reinforce rubbers. This chapter

describes the classification of composites, factors

influencing the performance of composites, mechanism

of reinforcement, structure and properties of coir, etc.

2

Rubber is a versatile and adaptable material that has been successfully used for

many years. A wide variety of flexible products are being made from natural or

synthetic rubbers1. The natural resilience of rubber is related to molecular

flexibility, amorphous structure and very low intermolecular attraction. Hence

rubber exhibits swift and easy response to force and release of force2.

Rubber is defined as a material that is capable of recovering from large

deformations quickly and forcibly and can be modified to a state in which it is

essentially insoluble but can swell in solvents such as benzene, methyl ethyl

ketone and ethanol-toluene azeotrope3. A rubber in this modified state retracts

within one minute to less than 1.5 times its original length after being stretched

(at a temperature of 18-20°C) to twice its length and held for one minute before

release. Rubber is unique due to the following properties.

• Its elastic strain is much higher than that of metal. Hence it can function

at high strains.

• It is stretched rapidly even under small load to about 1000% elongation.

• On releasing the applied forces, rubber retracts rapidly almost fully.

1.1 Natural Rubber

There are different types of rubbers including natural rubber (NR) and a variety

of synthetic rubbers. NR is obtained from rubber tree (Hevea brasiliensis) in

the form of field latex. Approximate composition of the field latex is given in

Table 1.1.

Table 1.1: Composition of field latex

Constituents Quantity

Rubber 33%

Water 60%

Protein 2-3%

Fatty acids 1-3%

Sugars 1%

Ash content 1%

Trace elements (Cu, Mn) 2-3 ppm Impurities (dirt, sand) 8-10 ppm

3

Among various rubbers, natural rubber (cis-1,4-polyisoprene) is very important

since it possesses the general features of other rubbers in addition to the

following highly peculiar characteristics. NR is produced by plants; so it is

renewable, inexpensive and creates no health hazard problems. It possesses

high tensile strength and modulus due to strain-induced crystallization. It

shows superior building tack, which is essential in many products like tyres,

hoses, belts etc. It possesses good crack propagation resistance also.

The field latex is concentrated by centrifugation, creaming or electro-

decantation. Generally the latex (either field or concentrated) is coagulated by

formic or acetic acid. A few classes of dry rubber are given in Table 1.2.

Table 1.2 Grading of dry rubber

Type Grading

method

Grades

1. Ribbed smoked sheet

2. White and pale crepe

3. Estate brown crepe

4. Compo crepe

5. Thin brown crepe

6. Thick blanket crepe

7. Technically specified

rubber

8. Other types and grades

Visual

”

”

”

”

”

Technical

specifications

As per

specifications

of the buyer

RSS 1X, No. 1, No. 2,

No. 3, No. 4, No. 5

No. 1X, No. 1, No. 2, No. 3

No. 1X, No. 2X, No. 3X

No. 1, No. 2, No. 3

No. 1, No. 2, No. 3

No. 1, No. 2, No. 3

SMR-L, SMR-CV, SMR GP

TSR-5, TSR-10, TSR-50

ISNR-3CV, ISNR-10, ISNR-50

Air-dried sheet, skim rubber,

Superior processing rubber

Visual aspects are made use of in conventional grading. The darker the rubber,

the lower the grade due to more dirt content. For example, RSS No. 4 is

considerably darker than RSS No.1. Other grading criteria such as the presence

or absence of rust, bubbles, mold, wet spots etc., are subjective in nature.

4

Technical grading is done according to the composition of the rubber, source

material, initial plasiticity etc. The main criterion is the dirt content, which is

the residue left after the rubber sample was dissolved in an appropriate solvent,

washed through a 45 µm sieve and dried. Table 1.3 gives the ISNR

specifications for natural rubber. Letter abbreviations identify the rubber

source, viz., SMR- Standard Malaysian Rubber, ISNR- Indian Standard

Natural Rubber.

Table 1.3 ISNR specifications for technically graded natural rubber

Property Grade 5 Grade 10 Grade 20 Grade 50

Dirt, % max. 0.05 0.10 0.20 0.50

Ash, % max. 0.60 0.75 1.00 1.50

Copper, % max. 0.0008 0.0008 0.0008 0.0008

Manganese, % max. 0.0010 0.0012 0.0015 0.0025 Volatile matter, % max. 0.080 0.080 0.080 0.080

Nitrogen, % max. 0.60 0.60 0.60 0.60

, % min. 0.25 0.25 0.25 0.25

Initial plasticity, min 40 40 35 30

Plasticity retention index, min 60 50 40 30

In addition to the different grades, certain modified forms of natural rubber are

also available.

• Deproteinised natural rubber (DPNR)- with 0.070% nitrogen content,

low water absorption, high reproducibility

• Oil extended natural rubber (OENR)- freeze resistant rubber,

possibility of high loading of filler and oil

• Cyclic natural rubber- more adhesive property, rubber to metal

bonding, surface coatings

1.2 Vulcanization

Even though NR is advantageous in many respects, it bears certain

disadvantages; its strength is lower than that of other classical materials. Hence

the products based on NR often suffer from tearing, breakage, puncture and

5

creep. Also it possesses poor crack initiation resistance, ozone resistance and

oil resistance.

These drawbacks are overcome by a number of methods. In conventional

rubber technology, cross-linking or vulcanization is one of the methods to

improve the above properties. The method was invented by Goodyear in 1839.

The word is originated from vulcan, the Greek God of fire and also means the

volcanic origin of sulphur.

Vulcanization is a chemical process in which the rubber is heated with sulphur,

accelerator, activator and promoter at 140-160 ºC. The process involves the

formation of crosslinks between long rubber molecules leading to a 3-D

network so as to achieve the properties such as elasticity, resilience, tensile

strength, viscosity, hardness and weather resistance. Use of accelerator alone

gives only a small increase in crosslink efficiency. Maximum efficiency is

achieved by using accelerators together with a metal oxide and fatty acid; ZnO

and strearic acid are the most commonly used. The selection of the suitable

vulcanization system depends up on the nature of rubber and service conditions

of the article.

Vulcanization changes the material from a formable viscoelastic to a highly

elastic substance capable of returning essentially to its original shape after very

large deformation. However even though the elastomers can produce elastic

products, their strength is low. Hence it is necessary to increase its strength

further by incorporating fillers having different physical forms. This leads to

the production of a special class of material called composites.

1.3 Composites

Only a few polymers are used in their pure form, instead most of them are

combined with small quantity of some other materials known as compounding

ingredients4. Besides these ingredients, a variety of particulate or fibrous

materials are mixed with polymers resulting in heterogeneous, multiphase

system. These materials are known as particulate filled and fibre reinforced

composites respectively. The schematic representation of composites is given

in Figure 1.1.

6

Hence composite can be defined as a material, which consists of two or more

chemically different phases separated by a distinct interface. The different

phases are combined judiciously to achieve a system with more useful

structural or functional properties non-attainable by any of the constituents

alone. The basic difference between blends and composite is that the two main

constituents in the composite remain recognizable while these may not be

recognizable in blends.

In addition, three other criteria have to be satisfied by a composite material. At

first, both the constituents are to be present in reasonable proportions, viz.,

greater than 5%. Secondly, the constituent phases should possess different

properties so that properties of the composite are noticeably different from the

properties of both the constituents. Hence polymeric products, which contain

compounding ingredients, do not satisfy either of these criteria and so cannot

be considered as composites.

Lastly, a man-made composite is produced by intimately mixing the

constituents by various means. Hence an alloy with a two-phase

microstructure, which is produced during solidification from a homogeneous

melt or by a subsequent heat treatment in the solid state, is not normally

classified as a composite.

7

Owing to the high strength-to-weight and stiffness–to-weight ratios possessed

by composites, these have been used in aviation and later used in other areas

such as the leisure and sports industry5. Conventional polymer composite

science and engineering has concentrated upon making products with the

following properties6.

• Improved mechanical properties such as modulus, strength, hardness,

abrasion resistance and toughness

• Improved processability

• Improved degradation resistance during fabrication and service

• Modified thermal properties such as the expansion coefficient and

conductivity

• Reduced shrinkage

1.4 Natural Composites

There are many examples of composite materials used in our everyday life, like

wood, concrete, tyres etc. Surprisingly, the most important polymeric

composites are found in Nature and these are known as natural composites.

Living species like plants and animals are constituted by composites that are

known as bio-composites7. The constituents of a few natural composites are

given in Table 1.4.

Table 1.4. Constituents of natural and man-made composites

Composite Main constituents

Wood Lignin, cellulose

Bone Hydroxy apatite, collagen

Cermet Ceramics, metal

Concrete Cement, stone chips, gravel, water

Reinforced concrete Concrete, steel rod

The connective tissues in mammals belong to the most advanced polymer

composites known to mankind where the fibrous protein, collagen is the

reinforcement. It functions both as soft and hard connective tissue. In bones it

acts as a binder for the inorganic hydroxyapatite, which are small rod-like

8

crystals with dimension of the order of 5x5x50 nm. In tendon it acts as a

reinforced elastomeric cable between muscle and bone where the collagen

fibres are glued together by a gel like proteoglycan matrix. The outstanding

toughness of this system is due to its composite nature. Such composite

structures have been established in intestine, articular cartilage, vertebral disc

and aorta. Similarly, the ductility and other remarkable properties of wood are

due to its hierarchical composite structure.

1.5 Man-made Composites

The first man-made polymeric composite appeared around 500 B. C. in the

Middle East where pitch was used as a binder for reeds in boat building. Bricks

made from mud, reinforced with straw were used in ancient civilizations.

Concrete is also a composite of stones (aggregate) held together by cement,

which is the most common building material. The constituents of certain man-

made composites are given in Table 1.4.

Nowadays a tremendous interest is given to polymer composites because of the

growing demand for lightweight, high performance materials coupled with an

attempt to reduce the increasing cost of energy and petrochemical feed stocks.

In principle, there is no limitation on the number of possible permutations of

matrix and filler that can be combined to produce a composite. However

depending upon the end use application, only a limited number of composites

can be selected for a particular purpose.

1.6 Reinforcement in Composites

Composites can be classified on the basis of different criteria like the

reinforcing nature of filler, shape of filler, distribution of filler, nature of matrix

etc. Fillers can be classified depends on whether it is reinforcing or non-

reinforcing. The reinforcement is harder, stronger and stiffer. The mechanical

properties of the composites are a function of shape and dimensions of the

reinforcement.

The reinforcing filler can be divided into fibrous or particulate structures. At

least one of the dimensions of the reinforcement is small; say less than 500

9

microns and sometimes only have the order of a micron. The shape of the

reinforcing particles can be spherical, cubic, platelet or any regular or irregular

geometry. Particulate reinforcements have dimensions that are approximately

equal in all directions. Certain particulate fillers like clay, silica etc. are non-

reinforcing whereas carbon black and many of the fibres are reinforcing in

nature.

The hardness increases with the incorporation of any filler at a reasonable

loading. Hence increase in hardness is not a measure of reinforcement. Also in

the case of rubbers tensile strength alone or modulus alone cannot be taken as a

criterion for reinforcement. Hence a reinforcing filler in rubber can be defined

as the material that is capable of increasing the low strain modulus, failure

properties like tensile strength, tear strength, abrasion resistance etc and to

improve the heat dissipation properties indicated by damping factor of the final

vulcanizate.

But particulate fillers are disadvantageous in certain respects mainly due to its

lower strength compared to that of fibres and secondly due to the lack of

directional property of the composite. Thirdly these possess enhanced

processing difficulty at high loading.

1.7 Reinforcing Mechanism of Particulate Fillers

Many theories have been put forward regarding the mechanism of

reinforcement in rubbers. Contributions of researchers have yet to produce a

consensus on the fundamental nature of reinforcement. The most important

obstacle is that it is not practical to dissect rubber vulcanizate to define the

polymer-filler interactions, which exist during service conditions. Many have

postulated that chemical interactions occur between filler and elastomer during

mixing and/or vulcanization. Studies on the mechanism of carbon black

reinforcement in rubber vulcanizate have been conducted8.

Figure 1.2 shows the schematic representation of reinforcing mechanism by

particulate fillers. The molecules in the cylinder A, cannot take all the

conformations that are theoretically possible because of the presence of filler

particles. Hence greater force has to be applied to cylinder A, in order to

10

observe the same extension for both A and B. Thus it is clear that greater

breaking force is needed for rubber containing filler particles. Also it is

obvious that the reinforcement capability increases with volume fraction of

filler.

But usually simple physical inclusion of filler in a matrix of non-polar polymer

weakens the polymer and the former just acts as a diluent. Polymer-filler

interactions are to be optimized in order to achieve certain properties of rubber

compounds9,10,11

. For example, the modulus and abrasion resistance of rubber

compounds containing graphitised carbon black, where reaction sites are

removed, is much lower than that containing ungraphitised carbon black.

Hence strong filler-rubber interaction is important in two ways.

• As a means of pulling apart filler agglomerates during mixing. This leads

to good dispersion and opposing any tendency toward subsequent particle

re- agglomeration.

• Contributes to the variation of mechanical properties such as modulus,

resilience and extensibility of the vulcanizate.

But it should be noted that excessive bonding is undesirable and if this

happens prior to vulcanization it may result in the formation of a continuous

11

network of rubber and filler. This makes the processing difficult or even

impossible. If it were developed during vulcanization, the effect in the extreme

would be to adsorb the polymer completely on the filler surface and cause a

drastic reduction in extensibility.

1.8 Fibre Reinforced Composites

In order to compensate for the drawbacks of particulate filled composites, fibre

reinforced composites (FRC) were developed. These are the most important

class among composites. These are used when high specific strength and

specific modulus are needed for the article. Hence the article based on FRC

possesses high stiffness associated with lightweight. FRC mainly comprises of

3 phases; matrix, fibre and the interface between these two phases.

The constituent that is continuous and is often (but not always) present in

greater quantity is termed as Matrix. It transmits the applied force to the fibre

reinforcement. It binds fibres together and keeps fibres apart to retard crack

propagation. It also protects the fibres from damage due to mechanical

abrasion or chemical reactions under the influence of environmental

conditions. Fibres exhibit high strength when the composite is pulled axially. It

is the function of the matrix to prevent fibres from bending and buckling,

which may occur during processing and service life. In rubber composites, the

rubbery matrix must satisfy the following primary criteria during its selection:

• It must be fluid enough to flow into the fabric easily, wet all the fibre

surfaces, and separate them from each other so that they will not suffer

from fibre-fibre friction

• It must be a very flexible rubbery material to give sufficient flexibility and

softness for the finished product.

• It should contribute to the mechanical strength of the final product.

There are several commercial polymers like natural rubber, vinyl plastisols,

silicone rubber oligomers, polyurethane oligomers etc., which more or less

meet the above criteria12.

12

Fibre is defined as any single unit of matter characterized by flexibility,

fineness and high aspect ratio13. It is a slender filament that is longer than 100

µm or the aspect ratio greater than 10. Fibres are dispersed throughout the

polymeric matrix to increase its rigidity and strength further and especially to

add impact strength, which often lack in rigid matrices14

. Fibre is the major

load-bearing component in the composite15

. Wide ranges of crystalline (rayon,

kevlar, PET) and amorphous (glass, boron, silica, asbestos) materials are used

as fibres16.

While glass and steel fibres are obvious candidates, experts have found that

many organic fibres offer a superior balance of modulus, strength, impact

resistance, vibration resistance, thermal properties, electrical properties and

light weight, making them materials of choice in many high-performance

composites.

Brittle materials are able to sustain a high stress, but these fail in a catastrophic

manner without warning. Moreover, the stress at which nominally identical

specimens fail can vary markedly. The brittleness and variation in strength is

due to the failure initiated at flaws, which normally occur at the surface. The

larger the failure-initiating flaw, the lower the failure stress. The failure

initiates at the largest flaw. The smaller the volume of the material, the smaller

the average size of flaws ad greater the strength. Hence the fracture stress of a

material in its fibrous form is greater than that of a larger volume of the same

material. The properties of certain fibres and their monolithic counterparts are

given in Table 1.517

.

Hence a fibre possesses high strength because of stretch-orientation of

molecules or crystals, special skin effects and lower probability of finding a

flaw in its small cross-sectional area. Thus any polymer in fibre form has much

higher modulus and strength than the same polymer in solid molded form. For

example, bulk graphite is a brittle material with tensile strength of about 70

MPa where as graphite fibres have a tensile strength of about 5500 MPa18.

Thus all such fibrous reinforcements are relatively of high tensile strength, as

compared to the polymeric matrices in which they are dispersed, typically 1-2

orders of greater magnitude.

13

Table 1.5: Properties of certain fibres and their monolithic counterparts

Young’s

modulus

GPa

Strength a

GPa

Alumina : fibre (Saffil RF) 300 2000

: monolithic 382 332

Carbon: fibre (IM) 230 3200

: monolithic 10 20

Glass : fibre (E) 76 1500 : monolithic 19 100

a Tensile and flexural strength for fibre and monolithic respectively

Apart from strength, the flexibility of fibres is important during the selection of

reinforcement as it determines whether the fibres may be easily woven or not

and influences the choice of method for composite manufacture. The flexibility

of a fibre is given by equation 1.1.

4

fD

fE

cyFlexibilit =

where c is a constant, Ef and D f are the modulus and diameter of the fibre

respectively. Now-a-days synthetic fibres have been replaced by their

environmentally friendly alternative natural fibres such as jute, cotton, sisal,

flax, hemp, pineapple fibre, oil palm fibre and coir19.

Interface is generally confined to the thickness of one molecular layer. The

high performance of FRCs results from the unique interaction between

polymeric matrix and reinforcing fibre. If the interface is weak and fails, the

composite is worthless. Thus great effort has been expended to insure that the

matrix-fibre interface is as strong as possible. Theory and practice with the

interface has been studied20

.

The interface in certain systems has enough natural affinity due to polar,

hydrogen-bonding, electronic attraction or primary covalent bonding. However

the following methods are generally adopted to increase the interfacial

adhesion.

(1.1)

14

• Utilization of coupling agents

• Surface modification of fibre by chemical or physical methods

• Coating of fibre with appropriate resins

• Graft co-polymerization

• Utilization of bonding system

In most systems it is necessary to use a coupling agent to create good stress

transfer across the interface without failure. Coupling agents like titanates (e.g.

isopropyl triisostearoyl titanate), silanes (e.g. γ-aminopropyl trimethoxy

silane)21

, isocyantes (e.g.toluene diisocyante), sebacoyl chloride etc. are used in

natural fibre composites. These react and thereby block the hydroxyl groups of

the natural fibre leading to higher hydrophobicity. The effect of various

chemical or physical methods is also similar to this. Also the resin coating and

graft co-polymer on the surface penetrate and deposit into the holes of the fibre

decreasing the moisture absorption.

Some other bonding agents are used which can react directly with the matrix,

thus creating primary covalent bonding between fibre and matrix, and giving

the maximum interfacial stress transfer and strength. Incorporation of HRH

(hydrated silica, resorcinol and hexa) bonding system is a commonly adopted

method to enhance interfacial adhesion in rubber composites.

1.9 Factors Influencing the Fibre Reinforcement

When fibres are used as reinforcement they extend the useful range of polymer

properties. At the same time, they make the analysis of the FRC more complex.

The polymer itself has many variables that can affect its properties. In addition

to these, other extra variables also exist among which a few are mentioned

below.

• Chemical nature of matrix

• Chemical nature of fibre

• Length of fibre

• Aspect ratio of fibre

• Distribution of fibre

• Orientation of fibre

15

• Surface texture of fibre

• Volume fraction of fibre

• Interfacial adhesion

• Type of moulding

• Moulding conditions

• Void content etc.

1.10 Rule of Mixtures

The properties of a composite are strongly influenced by its composition. The

composition can be expressed either using weight fraction (wf), which is

relevant to fabrication or using volume fraction (vf), which is relevant in

property calculations. The volume fraction and weight fraction of the filler can

be calculated using the following equations.

composite ofweight

filler ofweight =

fw

composite of volume

filler of volume

fv =

1.11 Classification of FRC

FRCs can be classified on the basis of different aspects among which the fibre

length is a very important parameter. Thus FRCs are grouped into two: long

(continuous) fibre reinforced composites and short (discontinuous) fibre

reinforced composites. In continuous fibre composites, fibres are oriented to

produce enhanced strength properties in one direction.

The length of a short fibre is neither too high to allow individual fibres to

entangle with each other nor too low for the fibres to lose their fibrous nature.

The reinforcement is uniform in the case of composites containing well-

dispersed short fibres. There is a clear distinction between the behaviour of

continuous and short fibre composites. End effects dominate in the behaviour

of short fibre composites and so they do not act as good reinforcing agents. The

(1.3)

(1.2)

16

schematic representation of the classification of composites is given in Table

1.6.

1.12 Short Fibre Reinforced Rubber Composites

Short fibre reinforced rubber composites were developed to fill the property

gap in between the long fibre reinforced rubber composites and particulate

filler reinforced rubber compounds, i.e., mainly to achieve the high

performance of the fibre coupled with the easy processability and elasticity of

the rubber.

The use of short fibres in rubbers is not new; tyre retreaders have been using

chopped cotton and rayon fibres in camel back for quite some time.

Composites in which short fibres are oriented uniaxially in an elastomer have a

combination of good strength and stiffness from the fibres and elasticity from

the matrix. These composites are being used for the fabrication of a wide

variety of products such as V-belts, hoses and complex shaped articles22,23

.

17

Advantages of short fibre reinforced rubber composites24,25,26,27

1) Processing advantages

a. Short fibres can be incorporated easily into the rubber compound

along with other additives

b. These are amenable to standard rubber processing operations such

as extrusion, calendering, compression moulding, injection

moulding and transfer moulding

c. These provide high green strength and high dimensional stability

during fabrication

2) Design flexibility

a. Fibres can be placed in any direction depends on requirement

b. Complex shaped articles other than hose, tyres, belts etc. can be

fabricated using these composites, which is quite impractical to

accomplish with long fibre reinforced composites.

3) Mechanical properties

a. Exceptional specific strength and specific stiffness

b. Reduced shrinkage in molded products

c. Controlled damping properties

d. Improved solvent swell resistance

e. Increased abrasion resistance

f. Improved cut and tear resistance

g. Improved creep resistance

h. Anisotropy in properties

4) Economy

a. Continuous fibres are expensive than short fibres

b. Economically more viable since dipping, wrapping, laying and

placing of the fibres associated with long fibre reinforced

composites are avoided

Disadvantages of short fibre reinforced rubber composites

1) Difficulties in achieving uniform dispersion

2) Fibre breakage during processing

3) Difficulties in handling and bonding

18

4) Filament winding, autoclave and vacuum bag processing techniques are

generally used for continuous fibre reinforced composites. But these are

difficult to achieve using short fibre reinforced composites.

1.13 Reinforcing Mechanism of Short Fibres

The reinforcing mechanism of fibre in a unidirectional composite can be

explained as follows. The composite satisfies equation 1.4, when a tensile or

compressive load is applied parallel to the fibre direction. This equation is

applicable under perfect conditions of adhesion between fibre and matrix.

where εc, εf and εm are the strain in composite, fibre and matrix respectively. If

it is assumed that both fibres and matrix behave elastically, then the following

equations can be applied.

σf = Ef εf

σm = Em εm

Hence

where σc, σf and σm are the stress developed in composite, fibre and matrix

respectively. Similarly, Ec, Ef and Em are the modulus of composite, fibre and

matrix respectively. Generally Ef is greater than Em and so the stress in the

fibre is greater than that in the matrix. Thus the fibre can bear a major part of

the applied load.

In the analysis of long fibre-reinforced composites, any effect associated with

fibre ends are neglected. But in the case of short fibre reinforced composites,

the end effects become progressively significant due to the decrease in aspect

ratio of the fibre. This results in the reduction of fibre efficiency in reinforcing

the matrix and also causes an early fracture of the composite.

m

m

f

f

c

c

EEE

σσσ==

mfc εεε == (1.4)

(1.5)

(1.7)

(1.6)

19

Consider an oriented fibre composite in which fibres are aligned parallel to the

direction of application of the force. A single fibre is embedded in a matrix of

lower modulus. Imagine perpendicular lines running through the fibre-matrix

interface in a continuous manner in the unstressed state as shown in Figure 1.3.

The matrix and the fibre will experience different tensile strains because of

their different moduli. When the composite is loaded axially, the longitudinal

strain in the matrix will be higher than that in the adjacent fibre due to lower

modulus of the former. When force is applied, the imaginary vertical lines in

the continuous fibre composite will not be distorted (Figure 1.3a).

But these lines in short fibre composite will be distorted as in Figure 1.3c

because at the region of fibre ends, the matrix will be deformed more than that

in the region along the fibre. This difference in longitudinal strains creates a

shear stress distribution around the fibres in the direction of the fibre axis and

so the fibre is stressed in tension. The applied load is transferred from matrix to

fibre across the interface because of this shear stress distribution.

When mechanical force is applied to the polymeric matrix, it spreads smoothly

through the matrix until it reaches the matrix-fibre interface. If the interface is

well bonded, the stress is transferred across it into the fibre and then spreads

20

throughout the fibre. This process occurs at all the fibre-matrix interfaces in the

composite. Thus it is obvious that load will be transferred to the fibre only if

the interface is strong and a perfect bond exists between the two constituents.

Hence strong interface is a must for high reinforcing efficiency.

1.14 Natural Fibres

These are one of the major renewable resource materials throughout the world,

especially in the tropics. There are about 2000 species of useful fibre plants in

various parts of the world and these are used for many applications. Certain

developing countries utilize locally grown fibres as a substitute for expensive

synthetic fibres. Natural fibres can be classified according to their origin (Table

1.7).

There have been attempts to use natural fibre reinforced polymer composites

from 1868 for making seats in aircraft and later in 1940-46 for bearings and

fuselage in aircraft and ships. Recently, natural fibres are used for making

hoses, V-belts and oil seals. The natural fibre reinforced composites possess

characteristic properties such as high electrical resistance, impact resistance,

21

fracture energy, good thermal, acoustic and insulating properties in addition to

specific strength comparable to glass fibre reinforced composites.

Vegetable or cellulose-based class includes important fibres such as cotton,

flax and jute. Chemically, all vegetable fibres consist mainly of cellulose,

although they also contain varying amounts of substances like hemicellulose,

lignin, pectins and waxes. Animal fibres such as wool, mohair and silk; consist

exclusively of proteins and constitute the fur or hair that serves as the

protective epidermal covering of animals. Silk is an exception to this, which is

exuded by the larvae of moths and is used to spin their cocoons. An important

fibre in the mineral class is asbestos.

Vegetable fibres can be divided into smaller groups, based on their origin

within the plant i.e., bast, leaf and fruit/seed varieties. Normally, bast fibres are

found in the inner bast tissue of certain plant stems. e.g., hemp, jute, flax, ramie

and kenaf. These are made up of overlapping cells of bundles in which fibres

are bonded together by pectin. Older bundles are larger, more lignified and so

stiffer. Abaca, henequen, banana and sisal are fibres occurring as a part of the

fibrovascular system of the leaves. Bast and leaf fibres are commonly used in

composite applications.

But non-structural fibres found in fruit and seeds like that of cotton, kapok, oil

palm, coir etc. are not assembled as bundles. These fibres originate as hairs

borne on the seeds or inner walls of the fruit, where each fibre consists of a

single, long, narrow cell. Also the cellulose fibrils are wound around the fibre

rather than parallel to the longitudinal axis.

1.15 Structure of Vegetable Fibre

Cell is the primary structural unit of plants. It is said that fibre is the suitable

name for the cell wall of a single cell of high aspect ratio, whether or not that

cell which produced the cell wall is still living28. Hence fibre is the smallest

intact unit of the fibrous material in the plant. The single fibre has a diameter

of around 10-20 µm.

22

From the living cell, cellulose is produced as micro-fibrils of 5 nm in diameter,

each composed of 30 to 100 cellulose molecules in extended chain

conformation and provides mechanical strength to the fibre. These micro-

fibrils fuse into larger micro-fibrils of 20 nm diameter, which are seen in the

electron microscope image of the cell wall. A good orientation of the micro-

fibrils along with high cellulose content is essential for obtaining a fibre with

good mechanical properties.

The microstructure of natural fibres comprises of different hierarchical

structures and so it is extremely complicated 29

. Each fibre cell is constituted by

four concentric layers viz. primary wall, outer secondary wall, middle

secondary wall and inner secondary wall. Primary wall is porous and these

pores act as diffusion paths of water through the wall. It is initially cellulosic

but become lignified on growth. It consists also of pectin and other non-

structural carbohydrates.

The secondary wall is developed on to the inner surface of the primary wall,

which comprises of a number of cylindrical, long, sheath-like and anisotropic

cellulosic micro-fibrils. These are surrounded and joined by a loose and

complicated macromolecular network of lignin-hemicellulose matrix. All the

secondary walls are fibrillar in nature and vary in composition and orientation

of the micro-fibrils. The micro-fibrils present in the middle secondary wall are

spirally arranged about the fibre axis at an angle, which varies from fibre to

fibre and cell to cell in a fibre30

. The lumen in the centre of the fibre contributes

to the water uptake properties of the fibre31

.

The primary walls of adjacent cells are held together by the intercellular

cementing material called middle lamella. This amorphous matrix phase is very

complex and consists of various compounds, including hemicellulose, lignin

and pectin.

1.16 Advantages of Vegetable Fibres

Vegetable fibres are advantageous to synthetic fibres because of the following

reasons 32

.

23

1. These fibres are mainly based on cellulose which is the most abundant

material on our planet and so these are inexpensive

2. These are produced to the extent of about 4.4 x 1014 kg annually by plants

and so it is renewable

3. These derive their carbon from atmosphere instead of petroleum or natural

gas and so less fuel consumption

4. These are amenable to physical, chemical and mechanical modification

5. These possess low density and so light weight articles with increased

stiffness can be made

6. These possess high flexibility and the breaking resistance is high. Hence

these undergo bending rather than breakage. A comparison can be obtained

from Table 1.8.

Table 1.8: Comparison of the properties of natural and synthetic fibres33

Fibre Density

(gcm-3)

Elastic

modulus

(GPa)

Tensile

strength

(MPa)

Elongation-

at-break

(%)

Coir 1.15 4-6 131-175 15-40

Banana 1.35 8-20 529 -759 1- 3.5 Sisal 1.45 9-12 568-640 3-7

Flax 1.52 100 1100 1.8

Pineapple 1.44 43-81 413-1627 0.8-1.6

Jute 1.45 2.5 - 13 460-533 1-1.2 E-glass 2.54 76 1500 4.8

Carbon

(high strength)

1.79 230 3200 1.0

Kevlar 49 1.45 124 2800 2.0

7. The chemical structure includes rigid cellulose fibres immersed in soft

lignin matrix. Rigid cellulose possesses very high modulus of rigidity

among polymers

8. These are non-abrasive and so render low machine wear than with mineral

reinforcements

9. These cause no health hazards

24

10. These are susceptible to microbial decomposition, including mildew,

aerobic bacteria (those that live only in oxygen), fungi, moths, carpet

beetles, termites and rot. Hence these are biodegradable

11. The hollow nature of fibre imparts acoustic insulation properties

12. These can be incinerated for energy recovery since they possess a good

calorific value

13. The rough surface leads to mechanical anchorage and this increases fibre-

matrix adhesion

14. Cellulose fibres tend to agglomerate during mixing and so these need

dispersive mixing rather than distributive mixing which is required by

brittle fibres such as glass or carbon

15. With the exception of mineral fibres, all natural fibres have an affinity for

water. This strong affinity results in swelling of the fibres, which facilitates

dyeing in watery solutions.

1.17 Disadvantages of Vegetable Fibres

1. Except for wood, the diameter of natural fibres is about 10 times larger

than that of synthetic fibres. This results in increased critical length in short

fibre reinforced composites and decrease in load bearing capability

2. These exhibit broad distribution of properties; for certain fibres standard

deviation of the order of 50% is observed

3. High degree of porosity due to the empty lumens of the ultimate cells

4. Low thermal stability

5. High moisture absorption and poor dimensional stability. Due to

hydrophilicity, ligno-cellulosic fibres absorb considerable amount of water

by forming hydrogen bonding between water and hydroxyl groups of

cellulose, lignin and hemi-cellulose in the cell wall34,35

6. Weak interfacial bonding with the matrix and so delamination occurs

7. Low strength.

8. Susceptibility to rotting

9. Natural fibres tend to yellow upon exposure to sunlight and moisture and

extended exposure results in loss of strength

25

1.18 Separation of Coir Fibre

Coir is a lignocellulosic fibre obtained from the fibrous mesocarp of coconuts,

the fruit of coconut tree (Cocos nucifera) cultivated extensively in the tropical

countries. Coir fibre is obtained from the fruit by dehusking coconut. The

husks are then subjected to a bacteriological process known as retting which

involves soaking the husks in saline backwaters for about 4 to 12 months.

During retting the phenolic material is removed, which gives strength to the

outer covering of the husk. This loosens the fibre interior and results in easy

extraction. Then coir is separated from the pithy material of the fibrous

mesocarp of the retted husk. Retting yields fibre with the desired colour. The

efficiency of retting depends upon many factors like the nature of water,

temperature, rate of removal of foul water and the stresses that the husks are

subjected to during retting. The Central Coir Research Institute of India at

Alappuzha, Kerala, developed a method by which the period of retting could

be reduced from 10 to 3 months36.

1.19 Structure of Coir Fibre

Coir is a hard fibre with a length in the range of 15 to 30 cm depending on the

size of the coconut37. The physical properties, chemical composition and cost

of coir fibre are compared with that of other natural fibres in Table 1.9.

Table 1.9: Comparison of coir and other natural fibres

Fibre Diameter

(µm)

Micro-

fibrillar

angle

(°)

Cellulose/

lignin

content

(%)

Cost

relative

to coir

Coir 100-460 30-49 43/45 1

Sisal 50-200 10-22 67/12 1.5

Jute 1.45 8.1 63/12 2

Pineapple fibre 20-80 14 -18 81/12 1.5

Banana fibre 80-250 11 65/5 3

26

Coir is a multicellular fibre that contains 30 to 300 or more cells in its cross

section, which is polygonal to round in shape. These cells are 12-14 µm in

diameter and are arranged around the central pore that is called lacuna. Coir is

coated with a waxy cuticle layer made up of fatty acids and their condensation

products.

Cellulose is made by plants in which the long chain-like molecules are linearly

aligned. This gives linear strength combined with lateral flexibility to the plant

skeleton. Hence it allows the plant to grow tall but to yield without snapping to

lateral forces such as bending in the wind. Cellulose forms micro-fibrils, which

are in turn helically wound and ultimately hollow fibres are formed. Cellulose

is a polymer of anhydro-β-D-glucopyranose units linked by 1,4-oxygen

linkages. Cellulose is polymorphic and exist in two crystalline forms

cellulose-I and cellulose-II as shown in Figure 1.4.

27

The cellulose-I is highly crystalline with about 80% crystallinity. Degree of

polymerization of cellulose-I is in the range of 4000-5000 anhydro glucose

units with a molecular weight up to 800000. The crystallinity of the fibre is due

to the presence of crystalline cellulose. In the less crystalline (60%) cellulose-

II, every other D-glucosyl residue is rotated by 1800. Hence cellulose-II more

nearly represents the actural 3-D structure of cellulose. But structure-I is more

commonly used to represent cellulose. It is stiff and straight due to internal

hydrogen bonding.

Pure α-cellulose is soluble in 17.5% NaOH solution and precipitates upon

acidification. But β-cellulose does not soluble in 17.5% NaOH solution. The β-

cellulose is having low degree of polymerization. The γ-cellulose is completely

soluble at any pH and is considered to be hemicelluloses.

Hemicellulose is branched cellulose and this forms hydrogen bond to cellulose

and acts as cross-linking molecule between the cellulose micro-fibrils.

Hemicellulose consists of hexosans such as glucosan, galactan and mannan,

pentosans like araban, xylan and polyuronides of gluconic acid. Chain length

of hemicellulose is much less than that of cellulose and so its molecular weight

is less. Hence these contribute very less to the tensile strength of the fibre. The

hydroxyl groups in lignin and the carbonyl groups in polyuronide

hemicellulose are linked together by ester linkages.

Pectins are polyuronides and consist exclusively of galacturonic units. Pectins

decrease as plant tissues mature. They may undergo transformation to

hemicellulose and lignin. The cellulose-hemicellulose (hollow cellulose)

network formed is thought to be the main structural component of the fibre

cell. The lignin network increases the strength of this network.

Lignin is the amorphous cementing material, which consists of phenyl propane

derived structural units as shown in Figure 1.5. The structure of lignin is

complicated by the various ways in which these units are joined. The lignin

content increases as the plant matures and the fibre becomes stiffer and

tougher.

28

1.20 Properties of Coir Fibre

Coir possesses the advantages of a lignocellulosic fibre as already mentioned in

section 1.16. In addition coir possesses the following advantages.

1. Possesses high weather resistance (durability) and hardwearing quality due

to higher amount of lignin. Hence it is used for making a wide variety of

floor-furnishing materials, yarn, rope etc.

2. The density of coir is 1.15 g/cm3, which is less than that of glass fibre, i.e.,

2.54 g/cm3. Hence it can be successfully used to produce lightweight

composites.

3. Coir fibres absorb water to a lesser extent compared to all the other natural

fibres. This is due to its less cellulose but more lignin content. Lignin

contains higher amount of non-polar hydrocarbon chains and aromatic

29

rings. This factor is reflected in the entire chemical, physical and

mechanical properties of coir fibre and its composites.

4. Rosin acts as water repellent and in certain cases bonding agent

5. Ability to stretch beyond its elastic limit without rupture due to helical

arrangement of micro-fibrils at 45º

6. The high wet strength value is advantageous when the product is used in

humid conditions

7. Natural resilience

8. The cost of coir fibre is comparatively less

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Books (1972) 3 ASTM D 1566- 856

4 Elias H.G. and Vonwinkel F., New Commercial Polymers, Gordon &

Breach Science Publishers (1986) 5 Manson J. A. and Sperling L. H., Polymer Blends and Composites,

Plenum Press, Ch. 12 (1976) 6 George L., Handbook of Composites, Van Nostrand Reinhold, NY (1982)

7 Baer E. and Moet A., (Eds.) High Performance Polymers, Structure,

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30

19

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