INTRODUCTION

Chapter I

Abstract

This chapter deals with a review of the literature on the various aspects of biofibres

and biocomposites. Biocomposites are finding applications in many fields ranging from

construction industry to automotive indusby. The pros and cons of using biofibres are

enumerated. The classification of biocomposites into green composites, hybrid

biocomposites and texhle biocomposites has been discussed. New d evdopments

dealing with cellulose based nanocomposites and electrospinning of nanofibres have

been presented. The importance of the interface in determining the properks of the

composite and various methods of characterization have been reviewed. The

mechanics and properties of short fibre and biofibre reinforced natural rubber

composites have been discussed. Finally, the applications of fibre reinforced rubber

composites have been highlighted.

INTRODUCTION

1 .I .I COMPOSITES

A composite can be defiried as a material having two or more

chemically distinct phases, which at the microscopic scale are separated by a

distinct interface. We can also say thal a composite is a combination of

properties of two or more components held together by the same type of matrix.

In essence, a composite is a commodity having superior properties than the

individual constituents. Composites, the wonder materials with light weight, high

strength to weight ratio and stiffness properties have come a long way in

replacing the conventional materials such as metals and wood!

The classification of composites is represented in Figure 1.1.1

Composites can be mainly categorized on the basis of matrix and type of

reinforcement. The classifications according to matrix type are ceramic matrix

composites (CMC), polymer matrix composites (PMC) and metal matrix

composites (MMC). The classifications according to type of reinforcement are

particulate composites (composed of particles), fibrous composites (composed of

fibres) and laminate composites (composed of laminates). Fibrous composites

can be further sub-divided on the basis of qatural I biofibre or synthetic fibre.

EioEbre encompassing composites are referred to as biocomposites.

Biocomposites can be again divided Qn the basis of matrix i.e. non-biodegradable

matrix and biodegradable matrix. Biocomposites made from natural I biofibre and

biodegradable polymers are referred to as green composites. These can be

further sub-divided as hybrid composites and textile composites. Hybrid

composites comprise of a combination of two or more types of fibres?

In fibrous composites, fibres are the load carrying members, while the

surrounding matrix keeps them in desired location and orientation. The matrix

acls as a load transfer medium, provides shape to the composite structure and

protects the fibres from environmental damage. The matrix also gives toughness

and compressional strength to the composite. The selection of suitable fibres is

determined by the required value of stiffness and tensile strength of the

composite. Other deciding factors are thermal stability, adhesion of fibres and

matrix, dynamic and long term behaviour, price and processing costs.

Natural or8iofibre I Composite BIOCOMPOSITE

Synthetic fibre 1 composite

Bio fiber-petroleum Biofiber-bioplastic (PLA) based plastic (PE, PP) GREEN COMPOSITE i s

HYBRID/TEXTILE BIOCOMPOSITE [Fibre blendinglmatrix blending]

Figure 1 .I .I Classification of composites

1 .I .2 BlOFlBRESl LIGNOCELLULOSIC FIBRESINATURAL FIBRES

Natural fibres are subdivided based on their origins, coming from plants,

animals or minerals. All piant fibres are composed of cellulose while animal

fibres consist of proteins (hair, silk and wool). Plant fibres include bast (or stem

or soft sclerenchyma) fibres, leaf or hard fibres, seed, fruit, wood, cereal straw

and other grass fibres. Over the last few years, a number of researchers have

been involved in investigating the exploitation of natural fibres as load bearing

constituents in composite materials. The use of such materials in composites

has increased due to their relative cheapness, their ability to recycle and for the

fact that they can compete well in terms of strength per weight of material.

Natural fibres can be considered as naturally occurring composites consisting

mainly of cellulose fibrils embedded in lignin matrix. The cellulose fibrils are

aligned along the length of the fibre, which render maximum tensile and flexural

strengths, in addition to providing rigidity. The reinforcing efficiency of natural

fibre is related to the nature of cellulose and its crystallinity. The main

components of natural fibres are cellulose (a-cellulose), hemicellulose, lignin,

pectins and waxes.

Cellulose is a natural polymer .consisting of D-anhydroglucose

(C6tt1105) repeating units joined by e-1,4-glycosidic linkages at CI & C4

position3. The degree of polymerization (DP) is around 10,000. Each repeating

unit contains three hydroxyl groups. These hydroxyl groups and their ability to

hydrogen bond play a major role in directing the crystalline packing and also

govern the physical properties of cellulose. Solid cellulose forms a

microcrystalline structure with regions of high order i.e. crystalline regions and

regions of low order i.e. amorphous regions. Cellulose is also formed of slender

rod like crystalline microfibrils. The crystal nature (monoclinic sphenodic) of

naturally occurring cellulose is known as cellulose I. Cellulose is resistant to

strong alkali (1 7.5 wt%) but is easily hydrolyzed by acid to water-soluble sugars.

Cellulose is relatively resistant to oxidizing agents.

Hemicellulose is not a form of cellulose and the name is a misnomer.

They comprise a group of polysaccharides composed of a combination of 5- and

6- carbon ring sugars. Hemicellulose differs from cellulose in three aspects.

Firstly, they contain several different sugar units whereas cellulose contains

only 1,4--P-D-glucopyranose units. Secondly, they exhibit a considerable

degree of chain branching containing pendant side groups giving rise to its non

crystalline nature, whereas cellulose is a linear polymer. Thirdly, the degree of

polymerization of native cellulose is 10-100 times higher than that of

hemicellulose. DP of hemicellulose is around 50 to 300. Hemicelluloses form

the supportive matrix for cellulose microfibrils. Hemicellulose is very hydrophitic,

soluble in alkali and easily hydrolyzed in acids.

Lignin is a complex hydrocarbon polymer with both aliphatic and

aromatic constituents. They are totally insoluble in most solvents and cannot be

broken down to monomeric units. Lignin is totany amorphous and hydrophobic

in nature. It is the compound that gives rigidity to the plants. It is thought to be a

complex, three-dimensional copolymer of aliphatic and aromatic constituents

with very high molecular weight. Hydroxyl, methoxyl and carbonyl groups have

been identified. Lignin has been found to contain five hydroxyl and five methoxyl

groups per building unit. It is believed that the structural units of lignin molecule

are derivatives of 4-hydroxy-3-methoxy phenylpropane3. The main difficulty in

lignin chemistry is that no method has been established by which it is possible

to isolate lignin in its native state from the fibre. Lignin is considered to be a

thermoplastic polymer exhibiting a glass transition temperature of around 90°C

and melting temperature of around 170°C4. It is not hydrolyzed by acids, but

soluble in hot alkali, readily oxidized and easily condensable with phenols.

Pectins are a collective name for heteropolysaccarides. They give

plants flexibility. Waxes make up the last part of fibres and they consist of

different types of alcohols.

Biofibres can be considered to be composites of hollow cellulose fibrils

held together by a lignin and hemicellulose matrix? The cell wall in a fibre is not

a homogenous membrane (Figure 1 . I .2). Each fibril has a complex, layered

structure consisting of a thin primary wall that is the first layer deposited during

cell growth encircling a secondary wall. The secondary wall is made up of three

layers and the thick middle layer determines the mechanical properties of the

fibre. The middle layer consists of a series of helically wound cellular microfibrils

formed from long chain cellulose molecules: the angle belween the fibre axis

and the microfibrils is called the microfibrillar angle. The characteristic value for

this parameter varies from one fibre to another. +

Secondary wall S3 Lumen

'. Helically

crystatline microfibrils \\-

i ,. Secor~da r y wall 52

Spiral anqle

Secondary ,#all S1 of cellulose

Amorphous rogion mainly consrsting of lignin a n d hemicellulose

O~sarderly arrr nge --, crysta i l~ne celltllos

~ ~ l i c r o f ; b r ~ l s netwot

Figure 1.1.2 Structure of biofibre

[Reference: Rong M.Z. et al., Comp. Sci. Tech., 61, 1437, 20011

Such microfibrils have typically a diameter of about 10-30 nrn and are

made up of 30 to 100 cellulose molecules in extended chain conformation and

provide mechanical strength lo the fibre. The amorphous matrix phase in a

cell wall is very complex and consists of hemicellulose, lignin and in some

cases pectin. The hemicellulose molecules are hydrogen bonded to cellulose

and act as cementing matrix between the cellulose microfibrils, forming the

cellulose-hemicellulose network, which is thought to be the main structural

component of the fibre cell. The hydrophobic lignin network affects the

properties of other network in a way that it acts as a coupling agent and

increases the stiffness of the celluloselhemicellulose composite.

The structure, microfibrillar angle, cell dimensions,defects and the

chemical composition of fibres are the most important variables that determine

the overall properties of the fibres7. Generally, tensile strength and Young's

modulus of fibres increases with increasing cellulose con tent. The microfibrillar

angle determines the stiffness of the fibres. Plant fibres are more ductile if the

microfibrils have a spiral orientation to the fibre axis. If the microfibrils are

oriented parallel to the fibre axis, the fibres will be rigid, inflexible and have high

tensile strength. Some of the importanl biofibres are listed in Table 1 . I . I

Table 1.1.18 List of important biofibres

[Reference: Eichorn et al. J. Mat. Sci., 36, 2107, 20011

Fibre source Abaca Bagasse Bamboo Banana

-.

Broom root

Cantala Caroa China jute Coir

I Dale palm 1 Phoenix Dactylifera Leaf 1

Species

Musa texfis

(> 1 250 species) ~ u s a indica

- -

Muhlenbergia macroura

Cotton Curaua

1 Flax 1 Linum usitatissimurn I Stem

Origin

Leaf Grass Grass Leaf Root

Agave canfala Neoglaziovia variegata Abutilontheophrasti Cocos nucifera

I Hemp I Cannabis safiva I Stem

Leaf Leaf Stem Fruit

Gossypium sp. Ananas erectifoiius

r enequen 1 Agave fourcroydes 1 Leaf _) lsora Helicteres isora Stern

Seed Leaf

- -

I lstle / Samue~a carnerosana r ~ e a f 1 r -- I I

1 Corchorus caosularis 1 Stem I Kapok I Ceiba penfranda I Fruit 1

pz - 1 H i b i s ~ n a ~ u s 1 F5 I

Pueraria thunbergiana Mauritius hemp Furcraea gigantea Nettle Urtica dioica Stem

1 0 j 1 7 e ; s gujn;e*Tt -1 Piassava A ttalea funifera Leaf Pineapple Ananus comosus Leaf Phormium Phormium tenas Leaf

1 Rosetle 1 Hibiscus sabdariffa I Stem I 1 Ramie -1 - - Boehmeria nivea Istern I 1 Sansevieria Sansevieria 1 Leaf I

(Bowstring hemp) I sisal I Agave sis~lana I ~ e a f /

Sponge gourd Straw (Cereal)

I Wood 1 (>10,000 species) 1 Stem

1.1.2.1 Biofibres: Advantages 8 Disadvantages

Sun hemp Cadillol Urena

The growing interest in lignocellulosic fibres is mainly due b their economical

Luffa cylinderica

production with few requirements for equipment and low specific weight, which results

Stalk Crorolaria juncea Urena lobata

in a higher specific strength and stiffness when compared to glass reinforced

Stem

composites. They also present safer handling and working conditions compared to

synthetic reinforcements. Biofibres are nonabrasive to mixing and molding

equipment, which can contribute to significant cost reductions. The most interesting

aspect about natural fibres is their positive environmental impact. Biofibres are a

renewable resource with production requiring little energy. They are carbon dioxide

neutral i.e. they do not return excess carbon dioxide into the atmosphere when they

are cornposted or combusted. The processing atmosphere is friendly with better

working conditions and therefore there will be reduced dermal and respiratory

irritation. Biofibres possess high electrical resistance. Thermal recycling is also

possible. The hollow cellular structure provides good acoustic insulating properties.

The worldwide availability is an additional factor.

The inherent polar and hydrophilic nature of lignocellulosic fibres and the

non-polar characleristics of most thermoplastics results in compounding difficulties

leading to non-uniform dispersion of fibres within the matrix which impairs the

efticiency of the composite. This is a major disadvantage of natural fibre reinforced

composites. Another problem is that the processing temperature of composites is

restricted to 200°C as vegetable fibres undergo degradation at higher temperatures;

this restricts the choice of matrix material. Another setback is the high moisture

absorption of natural fibres leading to swelling and presence of voids at the interface,

which results in poor mechanical properties and reduces dimensional stability of

composites. Another restriction to ihe successful exploitation of bioftbres for durable

composite application is low microbial resistance and susceptibility to rotting. These

properties pose serious problems during shipping, storage and composite processing.

The non-uniformity and variation of dimensions and of their mechanical properties

(even between individual plants in the same cultivation) poses another serious

problem.

It is quite clear that the advantages outweigh the disadvantages and most of

the shortcomings have remedial measures in the form of chemical treatments. The

lignocellulosic fibres have an advantage over synthetic ones since they buckle rather

than break during processing and fabrication. In addition, cellulose possesses a

flattened oval cross section that enhances stress transfer by presenting an effectively

higher aspect ratio.

Researchers9 at the Institute of Natural Fibres, Poland have developed fire

resistant upholstery using fire-relarded flax nonwoven (LinFR) FT. The non-woven

used in the composites plays the role of fire barrier that reduces the vulnerability of

filling material to the development and spread of fire. The softness of the upholstery

system is also enhanced. In another interesting study, researchers10 investigated ihe

influence of natural and synthetic fibres which covered the forearm muscles on the

activity of motor units in these muscles. The electrophysiological parameters of motor

units were measured with electromyographical methods. The results indicated that

temporaly covering of examined muscles in the forearm with the synthetic clothing

changed the pattern of motor units activity of muscles. Clothing made of natural fibre

did not evoke this kind of effect.

Recently, natural cellulose fibres having properties between cotton and linen

and suitable for various industrial applications were extracted from comhusksil. High

quality cellulosic fibres from comhusks mean food, clothing and other major industrial

products from the same source without the need for any additional natural resources.

Using cornhusks for fibrous applications would save the land and other natural

resources required to grow fibre crops and will conserve the non-renewable

petroleum sources required to produce synthetic fibres. More than 9 million tons of

natural cellulose fibres with a potential sale value of $19 billion and with a value

addition of at least $12 billion could be produced from the cornhusks available every

year. Utilizing comhusks as a source for natural cellulose fibres will significantly

benefit the agriculture, fibre, food and energy needs of the future and will also benefit

the environment. The structure and composition of the natural cellulose fibres

obtained from cornstalks were found to be different from that of the common bast

fibres such as flax and kenaf. Cornstalk fibres were found to have relatively lower

percentage of crystallinity but similar microfibrillar angle as that of the common bast

fi bresl?

1.1.3. BIOCOMPOSITES

The history of mankind has witnessed several surges in the field of research

an3 development. The rampant use of pelroleurn products has created a twin

dilemma; depletion of petroleum resources and entrapment of plastics in the food

chain and environment. The increasing pollution caused by the use of plastics and

emissions during incineration is affecting the food we eat, water we drink, air we

breathe and threatening the greatest right of human beings, the right to live. The

exhaustive use of petroleum based resources has initiated the efforts to develop

biodegradable plastics. This is based on renewable biobased plant and agricultural

products that can compete in the markets currently dominated by petroleum based

products. The production of 100 % biobased materials as substitute for petroleum

based products is not an economical solution. A more viable solution would be to

combine petroleum and biobased resources to develop a cost-effective product

having immense applications. Biopolymers or synthetic polymers reinforced with

natural or bio fibres (termed as biocomposites) are a viable alternative to glass fibre

composites. Scientists are looking at the various possibilities of combining biofibres

such as sisal, flax, hemp, jute, banana, wood and various grasses with polymer

matrices from non-renewable and renewable resources to form composite materials

to make the biocomposite revolution a reality'?

Broadly defined, biocomposites are composite materials made from

naturallbio fibre and petroleum derived non-biodegradable polymers (PP,PE) or

biodegradable polymers (PLA, PHA) . The latter categoly i.e. biocomposites derived

from plant derived fibre (naturallbiofibre) and croplbioderived plastic

(biopolymerlbioplastic) are likely to be more eco-friendly and such composites are

termed as green composites.

The best known renewable resources capable of making biodegradable

plastics are starch and cellulose14. Starch is one of the least expensive biodegradable

materials available in the world market today. It is a versatile polymer with immense

potential for use in non-food industries. Cellulose from trees and cotton plants is a

substitute for petroleum feedstocks to make cellulose plastics. Another aspect that

has gained global attention is the development of biodegradable plastics from

vegetable oils like soybean oil, peanut oil, walnut oil, sesame oil and sunflower oil.

Green composites from soy protein based bioplaslics and natural fibres show

potential for rigid packing and housing and transportation applicationsl5. Fish oil

based polymers have also attracted the attention of researchers due to their high

degree of unsaturation. Fish oil based polymers also possess unique good damping

and shape memory properties16.

The history of fibre reinforced plastics began in 1908 with cellulose fibre in

phenolics, later extending to urea and melamine and reaching commodity status with

glass fibre reinforced plastics. The fibre-reinforced composites market is now a

multibilliondollar business" (Figure 1 . I .3). Though hailed as a miraculous discovery

long back, plastic products now enjoy an ambiguous reputation. Scientists at the

BioComposites Centre at the University of Wales, Bangor are developing high-quality

packaging of goods, using starch from corn and potatoes to tackle the problem of

high cost waste disposal'*. Researchers19 are also exploring the aspects of producing

plastics from limonene which is extracted from cilrus. After decades of obscurity,

biofibre reinforced composites are being touted as the material of the millennium.

Electronic components

_I-

products 31 %

Automotives

Figure 1.1.3 Fibre reinforced plastic composites used in 2002-2.28 X l o 9 lb. [Reference: Adapted from Plast. News. August 26,20021

1 .I .3.1 Classification of Biocomposites 1 .I .3.1 .I Green Composites

Research efforts are currently being harnessed in developing a new class of

fully biodegradable green' composites by combining (natural I bio) fibres with

biodegradable resins2Q. The major attractions about green composites are that they

are environmentally-friendly , fully degradable and sustainable, i.e. they are truly

'green' in every way. At the end of their life they can be easily disposed of or

cornposted without harming the environment. The design and life cycle assessment

of green composites have been exclusively dealt with by Baillie.2t Green composites

may be used effectively in many applications such as mass produced consumer

products with short life cycles or products intended for one time or short time use

before disposal, Green composites may also be used for indoor applications with a

useful life of several years. A number of natural and biodegradable matrices hat are

available for use in such green cornposi tes22 are listed in Table 1.1.2

Table 1.1.2 20 Natural and biodegradable matrices

[Reference: Stevens E.S., Green Plastics, Princeton University Press, Princeton 20021

Biodegradable polymer matrices -

Natural

1 Chitin

Synthetic

1. Polysaccharides

Starch

Cellulose

1 4. Poly(viny1 alcohol)

1. Poly (amides)

3. Poly (amide-enamines)

1 2. Proteins 1 5. PoIy (vinyl acetate) I 1 Collagen/ Gelatin 1 6. Polyesters

I Casein, Albumin,Fibrogen, Silks / 6.1 Poly (glycolic acid)

1 4. Other Polymers

3. Polyesters

Polyh yd roxyalkanoates

1 6.4 Poly (orthoesters)

6.2 Poly (lactic acid)

6.3 Poly (caprolactone)

I Lipids 1 8. Poly (phosphazines) I Lig n in

Shellac

Natural rubber

7..Poly (ethylene oxides)

The reinforcement of biofibres in green composites has been

highlighted by Bismarck et al.5 Researchers recently investigated the effect of

stearic acid on tensile and thermal properties of ramie fibre reinforced soy

protein isolate (SPI) resin green composites23. It was observed that part of the

stearic acid cryslallized in SPI resin and that the crystallizability was affected by

the addition of glycerol as a plasticizer. The fabricated green composite was

found to have enormous potential for certain indoor applications.

The interfacial adhesion in the composites was apparent from SEM

photomicrographs of the fracture tensile surfaces of the ramielSPl and ramie1

modified soy protein isolate (MSPI) composiles tested in the axial direction is

shown in Figure 1.1.4 a & b respectively. Some of the fibres at the fracture

surface do not show any resin, where as a few other fibres clearly show resin

sticking on the surface. The presence of resin on some fibre surfaces for the

SPI and MSPI resins clearly indicates good interfacial interaction between the

fibre and the resins.

Figure 1.1.4 (a & b) SEM photomicrographs of the fracture tensile surfaces of untreated and treated composites

(Reference: Lodha and Netravali, Cornp. Sci. Tech. 65, 121 1, 20051

In an interesting study, researchers24 modified soy flour (SF) by cross-

linking it with glutaraldehyde (GA). The cross-link'ed soy flour (CSF) polymer

was characterized for its tensile and thermal properties. The effect of glycerol

on the mechanical properties of the soy flour was also characterized and

optimized. CSF polymer showed improved tensile properties and thermal

stability, compared to unmodified SF resin, for use as a resin to fabricate

composites. Unidirectional green composites using flax yarn and CSF resin

were fabricated and characterized for their tensile and flexural properties. The

composite specimens exhibited fracture stress and Young's modulus of 259.5

MPa and 3.71 GPa, respectively, and flexural strength of 174 MPa, in the

longitudinal direction.

Recently green composites were fabricated using pineapple leaf fibre and

soy based plastic25. The addilion of compatabilizer (polyester amide grafted

g lycid yl methacry late (PEA-g-G MA) was seen to increase the mechanical

properties of composites. In another interesting study involving biocomposites,

the effect of alkali treatment on the thermal properties of Indian grass fibre

reinforced soy protein green composites was studied by the same

The thermal stability of biodegradable composite films prepared from

blends of poly(viny1 alcohol), cornstarch, and lignocellulosic fibre was investigated

by Imam e l al.27. Thermogravimetric analysis (TGA) indicated the suitability of

formulations for melt processing, and for application as mulch films in fields at

much higher temperatures. The study also revealed that both starch and

lignocellulosic fibre degraded much more rapidly than PVA. The addition of fibre

to the formulations was found to enhance the PVA degradation. In another

interesting study, the thermal behavior of green composites fabricated from

bagasse fibre and polyvinyl alcohol was investigated by Fernandes et a128. They

observed an increase of thermal stability upon incorporation of bagasse fibre.

In another study biocomposites*9 were fabricated using a non-woven

fibre mat (90% hemp fibre with 10% thermoplastic polyester binder) as

reinforcement, and unsaturated polyester (UPE) resin as well as blends of UPE

and functionalized vegetable oils as the polymer matrix. All composites were

made with 30% volume fraction of fibre, which was optimized earlier. The

structure-property relationships of this system as well as the thermo-

mechanical properties of these composites were measured. The notched lzod

impact strength of biocomposites from biobased resin blends of UPE and

functionalized vegetable oil and industrial hemp fibre mat were enhanced by

90% as compared to that of the pure UP€-industrial hemp fibre mat composites.

The tests also showed an improvemenl in the tensile properties of the

composite as a result of the incorporation of the derivatized vegetable oil.

Researchers have also developed green composites from jute fabric

and Biopol composites30. Chemical modification of the fabric was carried out lo

improve interfacial properties. A significant amount of research has been done

at the German Aerospace Centre (DLR) in Braunschweg on biodegradable

plastics and composites31. Starch and modified resins have also been used as

matrix to form green composites32.

A major hurdle to the commercialization of green composites is the high

cost of biodegradable matrices. Most of the biodegradable resins cost

significantly more than the commonly used resins. Inexpensive production of

oils for resins through biotechnology would certainly be of help in expediting

their commercialization. The manufacturing costs can also be reduced by faster.

better and more efficient processing.

Efforts are on the anvil for the development of "advanced green

composites" made out of high strength protein fibres (spider silk) and

biodegradable matrices Biotechnology is being used to increase the yield of

specific triglycerides and oils in beans for producing resins. These resins will be

inexpensive compared to those available today and if suitably modified, could

be biodegradable. Research is also being conducted to develop new pathways

to synthesize inexpensive biodegradable resins33 with better mechanical

properties and thermal stability using nanote~hnology3~.

1 .I .3.1.2 Hybrid Biocomposites

The incorporation of several different types of fibres into a single matrix

has led to the development of hybrid biocomposites. The behavior of hybrid

composites is a weighed sum of the individual components in which there is a

more favorable balance between the inherent advantages and disadvantages.

Also, using a hybrid composite that contains two or more types of fibre, the

advantages of one type of fibre could complement with what are lacking in the

other. As a consequence, a balance in cost and performance could be achieved

through proper material design? The properties of a hybrid composite mainly

depend upon the fibre content, length of ind iv id~a l fibres, orientation, extent of

intermingling of fibres, fibre to matrix bonding and arrangement of both the

fibres. The strength of the hybrid composite is also dependent on the failure

strain of individual fibres. Maximum hybrid results are obtained when the fibres

are highly strain compatible36.

The properties of the hybrid system consisting of two components can

be predicted by the rule of mixtures.

where PI{ is the property to be investigated, PI the corresponding property of the

first system and P2 the corresponding property of the second system. VI and V?

are the relative hybrid volume fractions of the first and second system and

A positive or negative hybrid effect is defined as a positive or negative

deviation of a certain mechanical property from the rule of hybrid mixture.

The term hybrid effect has been used to describe the phenomenon of an

apparent synergistic improvement in the properties of a composite containing

two or more types of fibre? The selection of the components that make up the

hybrid composite is determined by the purpose of hybridization, requirements

imposed on the material or the construction being designed. The problem of

selecting the type of compatible fibres and the level of their properties is of

prime importance when designing and producing hybrid composites. The

successful use of hybrid composites is determined by the chemical, mechanical

and physical stability of the fibre I matrix system.

Hybrid biocomposites can be designed by the combination of a

synthetic fibre and natural fibre (biofibre) in a matrix and a combination of two

natural fibre I biofibre in a matrix. Hybridization with glass fibre provides a

method to improve the mechanical properties of natural fibre composites and its

effect in different modes of stress depends on the design and construction of

the composites38. The effect of hybridization of glass fibre in thermoset

biocom posi tes has been discussed in detai139.

The tensile and impact behavior of oil palm fibre-glass fibre-reinforced

epoxy resin was investigated by Bakar et al.40. The hybridization of oil palm

fibres with glass fibres increased the tensile strength, Young's modulus, and

elongation at break of the hybrid composites. A negative hybrid effect was

observed for the tensile strength and Young's modulus while a positive hybrid

effect was observed for the elongation at break of the hybrid composites. The

impact strength of the hybrid composites increased with the addition of glass

fibres.

Cellular biocomposile cores fabricated from industrial hemp or flax

fibres with unsaturated polyester were hybridized with woven jute, chopped

glass, and unidirectional carbon fabrics4'. Material characterizalion showed

improved stifiness, strength, and moisture-absorption stability, while flexural

tests on laboratory-scale plates demonstrated enhanced structural behavior.

These hybrid cellular biofibre-based composites were found to provide an

economic and environmentally friendlier alternative to entry-level synthetic

composites.

Hybridization also has a profound effect on the water absorption

property of composites.

An attempt to study the moisture uptake characteristics of hybrid

systems was performed by Mishra et aI.42. The composite systems chosen were

sisal I glass and pineapple lglass fibre reinforced polyester composites.

Composites were prepared by varying the concentration of glass fibre and by

subjecting the bio-fibres to different chemical treatments. The authors observed

that water uptake of hybrid composites were less than that of unhybridized

composites.

A comparative study of the water absorption of the glass fibre (7 wt. %)

1 natural biofibre (13 wt. %) with that of non-hybrid composites is given in Table

1 .I .3 where a lowering in water absorption is evident4?

Table 1.1.3 Comparative study of the water absorption of the glass fibre I natural biofibre with that of non-hybrid composites

[Reference: Rout J. et at., Composites Science & Technology 61, 1303,20011

T- - Water Absorption %

Sample Non hybrid [Coir-Polyester composite]

(20 wt. %)

Hybrid [CoirlGlass -

Polyester composite]

/ Untreated

I PMMA grafted (5 %) I 3.98 1 2.663 1 Alkali- treated (5 %)

8,53

4.994 I 3.147 1 5.186

PAN grafted (10 Oh)

/ Bleached 1 5 -8 I 3.718 1 1 Cyanoethylated 1 3.6

The novel development of a photofabrication process of biofibre

composites, based on oil palm empty fruit bunch fibres was recently reported44.

The process consisled of the following steps: (1) Lhe preparation of nonwoven

mat of biofibre, either alone or in combination with glass and nylon; (2) drying

the mat; (3) preparation of photocurable resin matrix, consisting of vinyl ester

and photoinitiator; (4) impregnation of the mat by photocurable resin; and (5)

4.119

3.138

2.997

irradiation of the impregnated mat by UV radiation to effect the cure of the

composite. Oil palm fibre, glass, and nylon fibres were mixed in different

proportions. A "mixture experimental design" was used to generate

experimental compositions of the reinforcing fibres and to model dependency of

the response variables on the components through mathematical relationships.

Scientists45 at the Affordable Composites from Renewable Resources

(ACRES) program at the University of Delaware investigated the mechanical

properties of glasslflax hybrid composites based on a novel modified soybean

oil matrix material. Composites with different glasslflax ratios and different fibre

arrangements were made using a modified soybean oil matrix material. The

fibre arrangement was varied to make symmetric and unsymmetric composites.

The latter were tested in different modes in flexural tests and drop weight

impact tests. The mechanical properties of the composites were found to

depend upon the glasslflax ratio and the arrangement of fibres in the composite.

On proper selection of the arrangement of fibres in the composite, the glass

fibres and flax fibres were found to act synergistically resulting in an improved

flexural and impact performance.

Another innovative approach to hybrid Eornposites is the incorporation

of two natural fibres I biofibre in a matrix system. Recently, the mechanical

performance of short randomly oriented banana and sisal hybrid fibre reinforced

polyester composites46 was investigated with reference to the relative volume

fraction of the two fibres at a constant total fibre loading of 0.40 volume fraction

(Vr), keeping banana as the skin material and sisal as the core material. A

positive hybrid effect was observed in the flexural strength (Figure 1.1.5) and

flexural modulus of the hybrid composites. The tensile strength of the

composites showed a positive hybrid effect when the relative volume fraction of

the two fibres was varied, and maximum tensile strenglh was found to be in the

hybrid composite having a ratio of banana and sisal 4 : 1.

Volume fraction of the fibre (banana+sisal)

Figure 1.1.5 Variation of flexural strength with volume fraction of fibre

[Reference: ldicula M., et al. J. Appl. Polym. Sci. 96, 5,1699,20051

As a continuation to ihe above study the dynamic and static mechanical

properties of randomly oriented intimately mixed banana and sisal hybrid fibre

reinforced polyester composites were reported47. Maximum stress transfer

between the fibre and matrix was obtained in composites having volume ratio of

banana and sisal as 33. The storage modulus was found to increase with fibre

volume fraction above the glass transition temperature of the composites.

Researchers have also explored the incorporation of a fruit and leaf

fibre in natural rubber48. The addition of sisal and coir fibres offered good

rcinforccmen t and resulted in improvement of properties.

1 .I .3.1.3 Textile Biocomposites

The development of textile technologies such as weaving, knitting and

braiding has resulted in the formation of composites that have superior

mechanical properties, as continuous orientation of fibres is not restricted at

any point.

In applications where more than one fibre orientation is required, a

fabric combining 0" and 90" fibre orientations is useful. Woven fabrics are

produced by the interlacing of warp (0") fibres and weft (90") fibres in a regular

pattern or weave style. The fabric's integrity is maintained by the mechanical

interlocking of the fibres. Drape (the ability of a fabric to conform lo a complex

surface), surface smoothness and stability of a fabric are controlled primarily by

the weave style.

1 .I ,3,1.3.1 Common Weave Architectures

The following is a description of some of the more commonly found

weave styles:

Plain

Each warp fibre passes alternately under and over each weft fibre. The

fabric is symmetrical, with good stability and reasonable porosity. However, it is

the most difficult of the weaves to drape, and the high level of fibre crimp

imparts relatively low mechanical properties compared with the other weave

styles. With large fibres (high (ex) this weave style gives excessive crimp and

therefore it is not generally used for very heavy fabrics.

One or more warp fibres alternately weave over and under two or more

weft fibres in a regular repeated manner. 'This produces the visual effect of a

straight or broken diagonal 'rib' to the fabric. Superior wet out and drape is seen

in the twill weave over the plain weave with only a small reduction in stability.

With reduced crimp, the fabric also has a smoother surface and slightly higher

mechanical properties.

Satin

Satin weaves are fundamentally twill weaves modified to produce fewer

intersections of warp and weft. The 'harness' number used in the designation

(typically 4 , 5 and 8) is the total number of fibres crossed and passed under,

before the fibre repeats the pattern. A 'crowsfoot' weave is a form of satin

weave with a different stagger in the repeat pattern. Satin weaves are very flat,

have good wet out and a high degree of drape. The low crimp gives good

mechanical properties. Satin weaves allow fibres to be woven in the closest

proximity and can produce fabrics with a close 'tight' weave. However, the

style's low stability and asymmetry needs to be considered. The asymmetry

causes one face of the fabric to have fibre running predominantly in the warp

direction while the other face has fibres running predominantly in the weft

direction.

Basket

Basket weave is fundamentally the same as plain weave except that two

or more warp fibres alternately interlace with. two or more weft fibres. An

arrangement of two warps crossing two wefts is designated 2x2 basket, but the

arrangement of fibre need not be symmetrical, Therefore it is possible to have

8x2, 5x4, etc. Basket weave is flatter, and, though less crimp, stronger than a

plain weave, but less stable. It must be used on heavy weight fabrics made with

thick (high tex) fibres to avoid excessive crimping.

28 Cltrrp frr 1

Leno

Leno weave improves the stability in 'open' fabrics which have a low

fibre count. A form of plain weave in which adjacent warp fibres are twisted

around consecutive weft fibres to form a spiral pair, effectively 'locking' each

weft in place. Fabrics in leno weave are normally used in con-junction with other

weave styles because if used alone their openness could not produce an

effective composite component.

Mock Leno

A version of plain weave in which occasional warp fibres, at regular

intervals but usually several fibres apart, deviate from the alternate under-over

interlacing and instead interlace every two or more fibres. This happens with

similar frequency in the weft direction, and the overall effect is a fabric with

increased thickness, rougher surface, and additional porosity.

Researchers have looked into tensile strength of ramie-cotton hybrid

fibre reinforced polyester cornposites49. They observed that tensile behaviour

was dominated by volume fraction of ramie fibres aligned in the tesl direclion.

The fabric and diameter of the thread did not play any role in tensile

characteristics. Cotton fabric was found to have minor reinforcement effect due

to weak cottonlpolyester interface. Similar studies were performed by

Mwaikambo and Bisandaso on kapok-cotton fibre reinforced polyester

composites.

Pothen et al.51 conducted tensile and impact studies of woven sisal

fabric reinforced polyester composites prepared by RTM technique. It was found

that the weave architecture was a crucial factor in determining the response of

the composites. Researchers have studied the micromechanics of moisture

diffusion in woven cornposites52. The weave pattern of the fabric was found to

have a profound effect on the water uptake of the composites. They observed

that woven composites exhibited quicker diffusion than that of a unidirectional

laminate with the same overall fibre volume fraction. The plain weave with a

lenticular tow and large waviness was seen to exhibit the quickest diffusion

process

Novolac type phenolic composites reinforced with jutelcotton hybrid

woven fabrics were fabricated and its properties were investigated as a function

of fibre orientation and rovinglfabric characteristics53. Results showed that the

composite properties were strongly influenced by test direction and

rovingslfa bric characteristics. The an isotropy degree was shown to increase

with test angle and to strongly depend on the type of architecture of fabric used,

i.e., jute rovings diameter, relative fibre content, etc. The best overall

mechanical properties were obtained for the composites tested along the jute

rovings direction. Composites tested at 45" and 90' with respect to the jute

roving direction exhibited a controlled brittle failure combined with a successive

fibre pullout, while those tested in the longitudinal direction (0") exhibited a

catastrophic failure mode. The researchers are of the opinion that jute fibre

promotes a higher reinforcing effect and cotton fibre avoids catastrophic failure.

Therefore, this combination of natural fibres is suitable to produce composites

for lightweight structural applications.

The thermal diffusivity, thermal conductivity and specific heat of

jutelcotton, sisallcotton and ramielcotton hybrid fabric-reinforced unsaturated

polyester composites were inveslgated by Alsina et a15? These properties were 4

measured both parallel and perpendicular to the plane of the fabrics. The

results obtained show that higher values were obtained parallel to the plane of

the fibres. Sisallcotton composites showed a particular behavior, with thermal

properties very close to those of the resin matrix. The thermal properties of the

fabrics, i.e. without any resin, were also evaluated and were used to predict the

properties of the composites from the theoretical series and parallel model

equations. The effect of fabric pre-drying on the thermal properties of the

composites was also evaluated. The results showed that the drying procedure

used did not bring any relevant change in the properties evaluated.

1.1.3.2 Applications of Biocomposites

The use of biofibre reinforced composites has extended to almost all

fields. Recently three-layer particleboards were produced from a mixture of

sunflower stalks and poplar wood at certain ratios utilizing urea-formaldehyde

(UF) adhesives. Panels with a density of 0.7glcm3 were manufactured with the

ratios of 25, 50, and 75 percent particles from sunflower stalks or poplar. Panels

were subjected to various tests for physical properties. Results show that all the

panels provide properties required by the normal standards for general purpose-

use particleboardsss.

Bio-based composite roof slructures were successfully fabricated from

soy oil-based resin and cellulose fibres in the form of paper sheets made from

recycled card board boxes. This recycled paper was previously tested in

composite sheets and structural unit beams and was found to give the required

stiffness and strength required for roof construction56.

In a study encompassing many applications, the flame retardancy of

biodegradable polymers and biocomposites was inves tigateda. For the

comparison, flame retarded lignocellulosic filler reinforced biocomposites were

prepared using polypropylene (PP), polyurethane (PUR) and fully biodegradable

starch matrices. The compatibility of wood flake with PP was improved by

application of an alkoxy silane based reactive surfactant. The silylation

improved not only the compatibility but also the thermal stability of the wood

flake according to TG measurements. Raman spectroscopic analysis of the

silylated product showed that the improved thermal stability is the result of

reduced ratio of the amorphous phase of cellulose. The phosphorus additives in

flame retarded PUK biocomposites, comprising waste bio fillers and recycled

polyol, proved to be very effective because both the matrix and the filler

components participate in mechanism of flame retardancy.

Researchers58 developed a new low dielectric constant material suited to

electronic materials applications using hollow keratin fibres and chemically

modified soybean oil. The unusual low value of dielectric constant was due to the

air present in the hollow microcrystalline keratin fibres and the triglyceride

molecules. The authors are of the opinion that the low cost composite made from

avian sources and plant oil has the potential to replace the dielectrics in

microchips and circuit boards in the ever growing electronics materials field. In an

extension of the above study the authors have also observed that the coefficient

of thermal expansion (CTE) of the composite was low enough for electronic

applications and similar to the value of silicon materials or polyimides used in

printed circuit boards?

Plasticlwood fibre composites are being used in a large number of

applications in decks, docks, window frames and molded panel components60. It

has been reported that 460 million pounds of plasticlwood fibre composites

were produced in 199961. Statistics show that the production of these

composites in 2001 has increased to 700 million pounds62. Over the last three

decades considerable research has been committed to finding an alternative

fibre to replace asbestos in fibre cement products. Australian research was

centred on natural fibres and ultimately it was a natural fibre, wood pulp fibre,

which was responsible for the greatest replacement of asbestos in the

beleaguered global fibre cement industry63. As these fibres are cheap and

readily available the energy required for the processing of these composites is

low; also the incorporation of random vegetable fibres in cement matrices

requires only a small number of trained personnel in the construction industry.

Recently, researchers64 have explored the use of bamboo fibre as reinforcement

in structural concrete elements. Pulp from eucalyptus waste and residual sisal

and coir fibres have also been studied as a replacement for asbestos in roofing

components65. The use of cactus pulp as a stabilising agent to improve the

behaviour of the sisal fibre reinforced soil has been studied66. Chopped barley

straw has also been used as a suitable reinforcement for composite soil67.

Biocomposites also offer immense opportunities for an increasing role

as alternate material, especially as wood substitutes in the furniture market68.

Yet biofibre as construction material for buildings were known long before. For

centuries, mixtures of straw and loam, dried in the sun, were employed as

construction composite in Egypt69. Pipes, pultruded profiles and panels with

polyester matrices are also quite popular. Large projects have been promoted

where jute-reinforced polyester resins are used for buildings, e.g. the Madras

House and grain elevators. Today, a renaissance in the use of natural fibres as

reinforcement in technical applications is taking place, mainly in the automobile

and packaging industries (e.g., egg boxes).

In the automotive industry70, cotton fibres embedded in polyester matrix

was used in the body of the East German "Trabant" car. The use of flax fibres in

car disk brakes to replace asbestos fibres is also another example. Daimler-

Benz has been exploring the idea of replacing glass fibres with natural fibres in

automotive components since 1991. A subsidiary of the company, Mercedes

8enz pioneered this concept with the "Beleem project" based in Sao Paolo,

Brazil. In this case, coconut fibres were used in the commercial vehicles over a

9 year period. Mercedes also used jute-based door panels in its E-class

vehicles in 1996. In September 2000, Daimler Chrysler began using natural

fibres for their vehicle production. The bast fibres are primarily used in

automotive applications because they exhibit greatest strength. The other

advantages of using bast fibres in the automotive industry include weight

savings of between 10 and 30 X and corresponding cost savings. Recent

studies have also indicated that hemp-based natural fibre mat thermoplastics

are promising candidates in automotive applications where high specific

stiffness is required?

Virtually all the major car manufacturers in Germany (Daimler Chrysler,

Mercedes, Volkswagen, Audi Group, BMW, Ford and Opel) now use

biocomposites in various applications. Interior trim components such as

dashboards and door panels using polypropylene and natural fibres are

produced by Johnson Controls, lnc for Daimler Chrysler72. In 2000 Audi

launched the A2 midrange car in which door trim panels were made of

polyurethane reinforced with mixed flaxlsisal mat. DaimlerChrysler has now

increased its research and development in flax reinforced pol yes ter composites

for exterior applications73.

The end of life vehicle (ELV) directive in Europe states that by 2015,

vehicles must be constructed of 95 % recyclable materials, with 85 % recoverable

through reuse or mechanical recycling and 10 % through energy recovery or

thermal recycling74. This will definitely lead to an increased use of biofibres. The

diverse range of products utilizing biofibres and biobased resins derived from soy

beans is giving life to a new generation of composites for a number of

applications like hurricane-resistant housing and structures75.

1 .I .3.3 Designing Biocomposites

Although biofibre reinforced polymer composites are gaining interest,

the challenge is to replace conventional glass reinforced plastics with

biocomposites that exhibit structural and functional stability during storage and

use and yet are susceptible to environmental degradation upon disposal. An

interesting approach in fabricating biocomposites of superior and desired

properties include efficient and cost effective chemical modification of fibre,

matrix mod ifica lion by functional izing and blending and efficient processing

techniques. (Figure 1 . I .6)

Another interesting concept is that of "engineered natural fibres" to

obtain superior strength biocomposites76.This concept explores the suitable

blending of bast (stem) and leaf fibres. The judicious selection of blends of

biofibres is based on the fact that the correct blend achieves optimum balance

in mechanical properlies for e.g., the combination of bast and leaf fibre is

expected to provide a stiffness-toughness balance in the resulting

biocom posites. •

/ EFFICIENT BIOCOMPOSITE PROCESSING

Figure 1.1.6 'Tricorner approach in designing of high performance biocornposites.

[Reference: Adapted from Natural fibres, Biopolyrners and Biocomposites, 37, Edited by Mohanty A.K., Misra M., Drzal L.T., CRC Press, 20051

1.1.4. ELECTROSPJNNING (ELSP)

Electrospinning has been recognised as one of the most efficient

techniques for the fabrication of nanopolymer fibres. When the diameter of

polymer fibre is shrunk from micrometer to nanometre range, amazing new

properties are exhibited in the nanopolymer fibres. The large surface area of the

fibres contributes to various applications ranging from medical prosthesis to

composite preparation. In addition, the electrospun fibres find applications in

various other fields such as tissue template, protective clothing, cosmetics, drug

delivery, conducting nanofibres, sensors, and optical shutters etc.

Electrospinning has recently drawn strong attention in biomedical

engineering77, providing a basis for the fabrication of unique matrices and

scaffolds for tissue engineering. ELSP is a spinning method that can produce

polymer fibres with diameters ranging from several micros to 100 nm or less

under a high-voltage electrostatic field operated between a metallic nozzle of a

syringe and a metallic collector in air. The fibres are typically deposited in the

form of a non-woven fabric onto a target metallic collector through a random

projected jet of polymer solution, the so-called cone or instable jet. The

accumulated charges on the polymer solution ejected from the nozzle induce

the radial charge repulsion in the electric field, which induces the instable jet. If

the random deposition process of fibres in the instability jet is appropriately

con trolled on the mesoscopic scale, the higher-ordered spatial placement of the

nano- and microfibres in the ELSP fabrics will be realized. In addition, the high-

speed movement of the collector or nozzle may produce oriented meshes and

more hig her-ordered macroscopic devices.

Researchers recently developed two novel ELSP techniques:

multilayering ELSP and (multicomponent) mixing ELSP, both of which are

composed of different nano- and rnicrofibres78. In the rnultilayering ELSP, after

electrospinning the first polymer, the second polymer is sequentially electrospun

on the same target collector (See Figure 1.1.7). Such a sequential spinning

process can produce a multilayered fibre mesh, in which a hierarchically

ordered structure composed of different kinds of polymer mesh could be

obtained. For the mixing ELSP, two different polymers are simultaneously

electrospun from different syringes under special conditions. The produced

polymer fibres are mixed on the same target collector, resulting in the formation

of a mixed fibre mesh.

Figure 1.1.7

[Reference: Kidoaki et at. Biornaterials 26, 37, 20051

Tremendous amount of studies are being undertaken in the field of

efectrospinning. The effect of molecular weight on fibrous PVA prepared by

electrospinning has been reported by Koski et a179. Zarkoob e l a1.80 has reporled

on the structure and property of electrospun silk fibres. The preparation of

PVAlZirconiumoxychloride composite fibres by using sol-gel process and

electrospinning was reported by Shao et ata1. In the above work, fibres with

diameter 50-200 nm could be obtained.

Recently scientists82 have looked into the governing parameters in the

electrospinning of polymer fibres. Researchers have studied the mechanical

characterisation of electrospun gelatin nanofibres83. Another innovative

approach has been the preparation of blend biodegradable nanofibrous non-

woven mats via multi jet electro spinninga4. Recently a leading group85 at the

Chonbuk National University, Republic of Korea has reported on the preparation

of nano to submicron fibres of ruthenium doped titanium dioxide lpoly (vinyl

acetate) hybrid fibres by electrospinning.

Another interesting study was the electrospinning of chitosan nanofibres

from aqueous chitosan solution using concentrated acetic acid solution as a

solvent. A uniform nanofibrous mat of average fibre diameter of 130 nm was

obtained from 7% chitosan solution in aqueous 90% acetic acid solution in an

electric field of 4 kVlcm. An aqueous acetic acid concentration higher than 30%

was prerequisite for chitosan nanofibre formation, because more concentrated

acetic acid in water progressively decreased surface tension of the chitosan

solution and concomitantly increased charge density of jet without significant

effect on solution viscositya6.

In an innovative study, ultrafine gelatin (Gt) fibres were successfully

produced with the use of the electrospinning technique by using a fluorinated

alcohol of 2,2,2-trifluoroethanol (TFE) as the dissolving solvent87.

1.1.5. CELLULOSE BASED NANOCOMPOSITES

The concept of nanostructured materials design is gaining widespread

importance among the scientific cornmunityaa. The strong reinforcement effects

at low volume fraction resulted in a lremendous interest from the industry and

research circles. With this as an inspiration, the potential of nanoscale cellulose

structures as reinforcement in novel composite materials was extremely

interesting. The concept of cellulose nanocomposites for load bearing

applications is fairly new. Property enhancements are expected due to higher

Young's modulus of pure cellulose reinforcement and finely distributed

reinforcing microfibrils. A major problem in the commercial use of cellulose

microfibrils in structural materials is the disintegration of cellulose from plant cell

wall at reasonable cost and without severe degradation. Another major problem

is dispersion of cellulose microfibrils in a polymer matrix.

A simple model of the cellulose microfibril structure is presented in

Figure 1 . I .8. Cellulose nanocomposites are usually fabricated by utilizing these

microfibrils of 10-50 nm on width as reinforcement in a polymer matrix. Although

many studies provide detailed knowledge regarding the morphology and

crystallography of different types of cellulose, the Young's modulus of

microfibrils from different sources and subjected to different types of hydrolysis

is seldom discussed.

L=! 3nm

Ceilulose elementary fibril

Figure 1.1.8 Model of the cellulose microfibril structure [Reference: Berglund L., Natural fibres, Biopolymers and Biocornposites, 808, Edited by

Mohanty A.K., Misra M., Drzal L.T., CRC Press, 20051

The research group at CERMAV-CN~S has presented numerous

studies based on cellulose whiskers reinforced nanocomposites89. The whiskers

were disintegrated from an edible tunicate, a sea animal. Another area of

interest is that of microfibrillaled cellulose (MFC) nanocomposites. MFC based

on parenchyma cells (i.e. sugar beets*, potato tuber) tend to show small

microfibril diameter and the fibrillar structures resemble swirled mats of

connected, microfibrils rather than discrete rods. A review of the recent

research into cellulosic whiskers, their properties and their applicalion in

nanocomposite field has been presented by the same groupql.

Recently scientists92 achieved uniform polypyrrole nanocoating on

natural cellulose fibres without disrupting the hierarchical network structures of

individual cellulose fibres by means of polymerization-induced adsorption

(Figure 1.1.9).

Cellul~$e fiber coated with Cellulose C b e r coated wilh polypyrrde layer titania and polypyrrole layers

Figure 1.1.9

[Reference: Huang et al,, Chemical Communications, 13,1717, 20051

High performance composites from low-cost plant primary cell walls

were developed by Bruce et al93. Purified cell wall fragments and cellulose

microfibrils were developed from Swede root to form novel composite materials.

The composites fabricated from purified cell wall fragments and acrylic matrix

gave the best tensile properties. Researchers94 have also reinforced natural

rubber with nanofibres of sepiolite. The composites also contained silica

particles generated in situ by the sol-gel process. The level of reinforcement

was assessed from mechanical and orientation behaviour.

Algae are also a potential source of cellulose for use in composite

materials. Researchers have concentrated on the morphology of cellulose

powder from Caldophora spatza algae from Ihe Baltic sea95. The effective

stiffness of the algae MFC (which is the combined effect of the microfibril

modulus and its aspect ratio) was found to be higher than for the wheat straw

cellulose. Bacterial or microbial cellulose (BC) have also found their way as

reinforcement in composites. Cellulose can be produced from some bacteria

such as A.xylinum, vinegar or acetic acid. Bacterial cellulose is an extracellular

product which is excreted into the culture medium. Acetic acid bacteria are not

photosynthetic but can convert glucose, sugar, glycerol to pure cellulose.

Bacterial cellulose has been frequently studied to clarify the mechanism for

biosynthesis96. Researchers have looked into the development of cellutose

nanocomposites based on bacterial cellulose and cellulose acetate butyrateg7.

Though BC composites have evoked widespread interest, the industrial use of

BC composites requires the development of efficient large-scale fermentation

technology.

Nanoscale-modifed plant structures such as paper or bast fibres also

present a different approach lo preparation of nanocomposite structures, An

interesting study concentrated on the preserving the microfibril organization of

wood veneer in composites98. Pine veneer was used as the starting material.

The basic idea was to increase the volume fraction of cellulose microfibrils by

partially removing the lignin and hemicellulose wood polymers and by

compressing the veneer. Phenol formaldehyde resin (PF) was used to preserve

the compressed microstructure of the material and to bond microfibrils.

1.1.6. INTERFACE

A strong interface is critical for superior mechanical properties of the

composites. Interface has been dubbed as the "heart" of the composite. The

term interface is defined as a two dimensional region between the fibre and

matrix having properties intermediate between those of fibre and matrix. Matrix

molecules can be anchored to the fibre surface by chemical reaction or

adsorption, which determine the extent of interfacial adhesion. In certain cases,

the interface may be composed of additional constituent as a bonding agent or

as an interlayer between the two components of the composite.

The region separating the bulk polymer from the fibrous reinforcement

is of utmost importance to load transfer. This region was originally dubbed as

interface but is now viewed as interphase because of its three dimensional

heterogeneous nature. It is not a distinct phase, as the interphase does not

have a clear boundary. It is more accurately viewed as a transition region that

possesses neither the properties of fibre nor those of the matrix. An

interphase that is softer than the surrounding polymer would result in lower

overall stiffness and strength, but greater resistance to fracture. On the other

hand an interphase that is stiffer than the surrpunding polymer would give the

composite less fracture resistance but make it very strong and stiff. The

nature of the interphase varies with the specific composite system. The

interphase is generally thought to Lhin (less than 5 pm) with the differences in

property between bulk polymer and the interphase very subtle. The

developments of atomic force microscope and nano-indentation devices have

facilitated the investigation of the interphasegg.

The word interphase is used as a general term to categorize the

polymeric region surrounding a fibre. It consists of polymeric material made

from the chemical interaction of sizing or coating on the fibre and the bulk

matrix during the curing process. The interphase is also known as the

mesophase (Figure 1 .1.10).

- BULK U4tRD:

p O L v u U l d ,-. o l r r c w x

P R r n R T I M

r m cauarpv ~ E R U ~ L . '---FI~wT-

Q i m J ' a L . "-#aC.Q - M Y

U E C W l [ j U . BULK F l E R

FIBER-MATRIX INTERPHASE ..,-,,

Figure 1.1.10 Schematic model of interface

[Reference: Laly A Pothen, Ph.D Thesis]

The strength and toughness of fibre-reinforced materials are

determined by the interface between the reinforcing fibres and matrix.

Extensive research has been carried out in order to understand the nature of

the interfacial bond and ils characterization. A strong interface creates a

material that displays exemplary strength and stiffness but which is very

brittle in nature with easy crack propagation through the matrix and fibre. A

weaker interface reduces the efficiency of stress transfer from the matrix to

the fibre and as a result the strength and stiffness are not high.

The fibre1 matrix interface in a continuous filament composite transfers

an externally applied load to fibres themselves. Load applied directly to the

matrix at the surface of the composite is transferred to the fibres nearest the

surface and continues from fibre to fibre via matrix and interface. If the

interface is weak, effective load distribution is not achieved and the

mechanical properties of the composite are impaired. On the other hand, a

strong interface can assure that the composite is able to bear load even after

several fibres are broken because the load can be transferred to the intact

portions of broken as well as unbroken fibres. A poor interface is also a

drawback in situations other than external mechanical loading e.g. because of

differential thermal expansions of fibre and matrix, premature failure can occur

at a weak interface when the composite is subjected to thermal stress. Thus,

adhesion between fibre and matrix is a major factor in determining the

response of the interface and its integrity under stress. Additionally the

interface maybe more vulnerable to moisture and solvents than are fibre and

matrix. The fracture of an interface between two materials depends on the

geometry, the constitutive properties of the adherends and the details of

bonding across the interface.

1.1.6.1 Characterization Techniques

The characterization of the interface gives the chemical composition

as well as information on interactions between fibre and matrix. Various

methods are available for characterization of the interface.

1. Micro-mechanical, Techniques

The extent of fibre-matrix interface bonding in terms of interfacial

shear strength can be determined by different micro-mechanical testssuch as

single fibre pull out, micro debond test, micro-indentation /micro-compression 1

fibre push out test and single fibre fragmentation test.

2. Spectroscopic Techniques.

Electron spectroscopy for chemical analysis 1 X-ray photoelectron

spectroscopy (ESCAI XPS), fourier transform spectroscopy (FTIR), Laser

Raman Spectroscopy (LRS), nuclear magnetic resonance (NMR) and

photoacoustic spectroscopy have been shown to be successful in polymer

surface and interfacial characterization.

3. Microscopic Techniques

Microscopic studies such as optical microscopy, scanning electron

microscopy (SEM), transmission electron microscopy (TEM) and atomic

force microscopy (AFM) can be used to study the morphological changes on

the surface and can predict the extent of mechanical bonding at the

interface. The adhesive strength of fibre to various matrices can be

determined by AFM studies.

4. Thermodynamic Methods

The frequently used thermodynamic methods for characterization in

reinforced polymers are wettability study, idverse gas chromatography

measurement, zeta potential measurements etc. Contact angle measurements

have been used to characterize the thermodynamic work of adhesion be tween

solids and liquids as well as surface of solids.

5. Other Techniques

The interface can also be characterized qualitatively by other methods

like dynamic mechanical analysis and stress relaxation technique. Swelling

techniques have been used to assess the level of interfacial adhesion in the

case of fibre reinforced elastomer composites.

1 .I .6.1 .I Micromechanical Techniques

(a) Single fibre pull out test

One of the most common methods is to measure the force required to

pull out a fibre embedded in a matrix. This method, in addition to its relative

simplicity of sample preparation and measurement is expected to give realistic

information when one considers the pull out of fibres from the fracture

surfaces of composites. The two main aspects are the debonding which

destroys the bond between fibre and matrix and the other is the pull out in

which a fibre is extracted from a broken matrix against friction. The pull out

test is considered to be the best method of evaluating the interfacial shear

load as it can directly measure the interfacial shear strength between the fibre

and matrix independent of their properties. The interfacial shear strength is a

critical factor that controls the toughness, mechanical properties and

interlaminar shear strength of composite materials. The fibre pull out problem

has been investigated extensively for purposes of studying the interfacial

adhesion quality and elastic slress transfer between fibres and matrix. From

the loadldisplacernent curves, the average interfacial shear strength (IFSS) r

is given by

where F is the load needed to debond the fibre from the microbead and d and

I are the diameter and embedded length of the fibre respectively. It is assumed

that the shear stress along the interface is uniform. Though the single fibre

pull-out has an apparently unrealistic character when compared to the

complexity of the composite material, much information can be derived that is

related to the most fundamental aspects of the fibretmatrix mechanical

interaction. The drawback of single fibre pull out test is that it involves only a

single fibre. As the role of neighboring fibres is not taken into account, the

thermal stresses and the polymer morphology around the fibre are not the

same as in a real composite. Real composites contain multiple fibres and the

pull out fibre is surrounded by a composite medium. In single fibre model

composites, the effect of the composite medium surrounding the pull out fibre

has been ignored. Therefore, owing to the influence of the composite medium,

surrounding the pull out fibre, the interfacial debonding process in multi-fibre

composites and the interfacial properties obtained would very likely deviate

from those of the single-fibre composite pull out test.

(b) Micro-debond test

The microbond technique, which is a modification of the single fibre pull

out, involves depositing a micro-bead of the matrix onto a fibre. The fibre with

the micro-bead is then mounted in a micro-vise by placing the microbead under

the micro-vise blades and the fibre is pulled out. The interfacial shear strength

is calculated from equation 1 .I .3.

The specimen preparation for the micro droplet test whereby a single

fibre is pulled out of a small droplet of resin suffers from several difficulties. For

instance, the reliability of the data is affected by the shape of the droplet*

Symmetric, round droplets are easier to test and analyze than droplets with flat

surfaces, produced when the specimens solidify on a flat substrate. Also the

size of the droplet is critical. If the length of the droplet exceeds a critical value,

the fibre will fracture prior to debonding and pull out. An additional complication

with some thermoset materials is that the anticipated curing characteristics may

not manifest themselves in a droplet of small size, and hence comparison on a

microstructural level between micro and macro specimens may not be possible.

Another defect is that this test is not applicable to matrices that are soft.

(c) Microindentation test

The micro indentation test was initially developed for fibre -reinforced

ceramics but has been extended to the other fibre-matrix systems. It is also

known as the fibre-push out test. This is the only single fibre test, which is able

to analyze actual composite specimens. The use of a real composite allows a

more realistic simulation of thermal stresses, polymer morphology and the

influence of neighboring fibres. The presence of other fibres is an advantage,

but it complicates the calculation of stress state around the fibre and the choice

of an appropriate failure criterion. Nevertheless, the more realis tic tes ling

conditions of the micro-indentation method make it an attractive new technique

for many researchers. In this method, a compression force is applied on a single

fibre in a well-prepared specimen of a real composite.

(d) Single fibre fragmentation test

The SFF technique involves embedding a single fibre along the

centerline of a dog-bone shaped specimen of matrix material. The specimen is

then strained along the fibre axis. The shear in the resin exerts a tensile stress

on the fibre. At some stress the fibre fractures at its weakest point. With

increasing specimen strain the fibre fractures repeatedly at different locations.

No additional breaks can occur when the fragments become so short that the

shear transfer along the length of the broken fibre can no longer make the

tensile strength higher to cause additional fractures. This fragment length at the

end of the test is known as critical length l c . The average IFSS is then

calculated using the force balance equation:

The ratio I, Id is called the critical aspect ratio. This is the most realistic

test from the point of view of the interfacial pressure. The fibre is neither pushed

nor pulled directly, and so fibre Poisson effects are similar to that occurring in a

fibre composite.

A disadvantage of this technique is that the failure strain of the matrix

must be much larger than the failure strain of the fibre to promote multi-

fragmentation of the fibre. This requires the use of matrices, which can undergo

large deformations. Consequently commercial resins utilized in actual

composite systems, which typically have low strains to failure, cannot be used

for this test. Therefore the interfacial shear strength determined is not directly

applicable to the actual composite system.

Another aspect is that friction plays an important role in the debonding

process and this is governed by two additional u~knowns, i.e, the coefficient of

friction 11 and the pressure across the interface P. Although some progress has

been made with this, value for debonding rely rather heavily on the correctness

of the assumptions about 11 and P.

1.1.6.2 Characterization of Interfaces in Biocomposites

Tremendous amount of work has been carried out for the past twenty

years on the characterization of the fibre I matrix interface. The key issue has

been the development of mathematical models that by taking into account the

stress transfer at the interface would provide the calculalion of a parameter that

could assess the interfacial bond. Biofibres unlike synthetic ones are very

heterogeneous ~naterials and are still not completely understood. A major

problem for the natural fibres is the fibre diameter, which is not constant even for

the same single fibre. Therefore the characterization of interfaces in natural fibre

reinforced composites is a difficult task.

The role and the importance of interfaces and interphases in

rnulticomponenl materials have been enumerated by Pukanszky1004 Recently

researchers have looked into the applicability of different micromechanical tests

to interface strength characterization and the advantages and disadvantages of

stress based and energy based rnodels'o! In an interesting studyjO2, the

correlation between post-mortem fracture surface morphology of short fibre

reinforced thermoplastics and interfacial adhesion was presented. The authors

are of the opinion that fracture surface morphology is dependent on both fibre-

matrix interfacial adhesion strength and matrix shear yield strength. Jacob et

al.Io3 have reviewed the recent advances in characterization of interfaces in

biofibre reinforced composites.

The interfacial morphology in wood-fibre and maleated HDPE

composites was studied by Lu et allo4. FTlR and ESCA analyses presented the

evidence of a chemical bridge between the wood fibre and polymeric matrix

through esterification. The interface effects on the mechanical properties and

fracture toughness of sisal fibre reinforced vinyl-es ter composites were

invesligated by Li et allo5. The interfacial shear strength was calculated by

single fibre pull out technique. It can be clearly seen from Figure 1 . I .ll that after

fibre surface treatments by using different chemical agents (except silane-I ), the

IFSS was greatly improved. KMn04 treatment showed the best effect and the

IFSS between permanganate-treated sisal fibre and vinyl-ester resin was almost

three limes that of the untreated fibre, then followed by siiane-2 and DCP

treatments. For silane-I treated sisal fibres, IFSS was similar to that of

untreated sisal fibre reinforced vinyl-ester.

Untreated Silane 1 DCP Silane 2 KMnO,

Figure 1.1.11 Variation of interfacial shear strength with chemical modification

[Reference: Li et al., Comp. Interf., 12,l-2,141, 20051

Gassan'" has attempted an interesting study on fibre and interface

parameters affecting the fatigue behaviour of natural fibre reinforced composites.

Composites comprising of flax and jute yams and wovens as reinforcements for epoxy

resin, polyester and polypropylene were analyzed. The studies revealed that when the

interface bonding is weak, debonding and frictional sliding occur readily upon crack

extension. Upon cyclic loading of composites, the frictional sliding at the interface

changes with interface properties such as roughness. Furthermore, unidirectional

composites were found to be less sensitive lo fatigue induced damage than woven

reinforced ones.

A study utilizing single fibre pull-out was atiempted by VandeVelde et al.lo7.

The authors used dew retted hackled long flax treated with propyltrimethoxy silane,

phenyl isocyanate and maleic anhydride polypropylene. The studies revealed that

composite prepared with flax fibre treated with MAA-PP exhibited the highest interfacial

shear strength. Joseph et al.108 have conducted an interesting study on the comparison

of interfacial properties of banana fibre and glass fibre reinforced phenol-formaldehyde

composites. They observed that the interfacial shear strength is higher for banana I PF

system than for glass IPF system. This was attributed to the hydrophilic nature of

cellulose and PF resin. Hydrophilicity of fibre arises from the hydroxyl groups of lignin

and cellulose which can easily form bonds with methyl01 and phenolic hydroxyl groups

of the resole resulting in a strong interlocking between the two. In the case of glass IPF

composites this kind of bonding is not possible and hence the interfacial shear strength

is low.

The interfacial adhesion of flax fibre reinforced polypropylene and

polypropyleneelhylene propylene diene terpolymer blends were investigated by

Manchado e l al.l@ In this study both the matrices were modified with maleic anhydride.

Single fibre pullout tests were conducted to investigate the extent of interfacial

adhesion. They observed that the addition of small proportions of maleic anhydride to

the matrices significantly increased the shear strength. The authors were of the opinion

that introduction of functional groups in the matrix reduced the interfacial stress

mncen tra tions preventing fi bre-fibre interactions which are responsible for premature

composite failure. Also, in presence of rnaleic anhydride functional groups the

esterification of flax fibres takes place increasing the surface energy of the fibres to a

level closer to that of the matrix. Hence, a better wettability and interfacial adhesion was

obtained.

Zafeiropoulos et al.1'0 characterized the interface in flax fibre reinforced

polypropylene composites by means of single fibre fragmentation test. Flax fibre was

modified by means of two surface treatments: acetylation and stearation. The authors

observed that acetylation improved the stress transfer efficiency at the interface. Stearic

acid treatment was also found to improve the stress transfer efficiency but only for lower

times. They also found that stearation for longer durations deteriorated the interface.

This was attributed to decrease of fibre strength upon treatment of longer duration and

the fact that excess of stearic acid acted more as a lubricant than as a compatabiliser.

In an interesting study, dynamic mechanical analysis has been used to analyse

the interfacial adhesion in banana fibre reinforced polyester composites by Pothan and

Thomas"'. The authors observed an additional peak in their tan delta curve and have

reported it to be due to an additional polymer layer which probably is the interphase.

The authors also observed that there was increase of storage modulus with treatment.

Slorage modulus is found to be directly proportional to the interfacial strength of

composite. Also the T, was found to shift to higher temperatures for treated composiles

indicative of good interfacial. adhesion.

The effect of different chemical modifications on the interfacial characteristics

of aspen fibre reinforced HDPE composite was conducted by Colom et al.112 The

interaction between aspen fibres and HDPE was improved by the addition of two

coupling agents; maleated polyethylene (epolene C-18) and y-methacryloxy-propyl

trimethoxy silane (silane A-174). Interfacial morphology was studied by means of FTIR.

1.1.7. NATURAL RUBBER [NR]

Natural rubber (NU) belongs to a class of compounds known as elastomers.

The Mayans in the Western Hemisphere used NR for centuries before it was introduced

into Europe by Columbus. The term rubber was however coined by Joseph Priestly.

Natural rubber is indispensable in our daily lives. The main uses of NR are

concentrated in four key areas, namely: medical devices, industrial products, domestic

and recreational goods and foremost automobile products. The current elastomer

consumption in the world is 18 million tonnes per year. NR supplies about one-third of

the world demand for elastomers. It is also used as an industrial raw material. NR is a

naturally occurring elastomeric polymer of isoprene ( 2-methyl-1,Sbutadiene). It can be

extracted from latex of only one kind of tree, the Hevea braziliensis. Hevea rubber is

produced in many tropical regions of Southeast Asia, Africa and Central and South

America. There is practically only one other potential source of NR, that is the guayule

shrub (Parthenium argentaturn). Polyisoprene exists naturally in the form of two stereo-

isomers, namely cis-l,4-plyisoprene and trans-1.4-poly isoprene ( Figure 1 .I . I 2)

Trans-l,4-polyi soprene

Figure 1.1.12 Structure of natural rubber

Chemically NR is cis 1,4-polyisoprene. A linear, long chain polymer with

repeating isoprenic units (CsHa), it has a density of 0.93 at 20" C . Natural trans-

1, 4-polyisoprene is a crystalline thermopiastic polymer, which is mainly used in

golf ball covers, root canal fillings in dentistry, special adhesives and to a lesser

extent in wire and cable coverings. It is obtained as gutta percha from

Palaquium oblongofolium in South East Asia. I I is much harder and less soluble

than Hevea rubber. Certain trees like Chicle (Achras sapota) produce latex,

which is a physical mixture of cis and trans polyisoprene. Cis-I , 4-polyisoprene

can be produced synthetically by stereospecific polymerization reaction using

Zieglar-Natta-hetergenous catalyst. The important components of NR are given

in Table 1.1.4113

Table 1.1.4 Typical Analysis of Natural Rubber [Reference: Barfow F.W., Rubber Compounding, Principles, Materials and Techniques,

M. Dekker, 19981

1 components 1 YO

1 Rubber hydrocarbon

1 Acetone extract 1 2.9

1 Protein

1 Moisture --

Ash 0.4

The most important factor governing the properties of polyisoprene is

the stereoregularity of the polymer chain. The very unique characteristic of NU

is the ability to crystaltize under strain, the phenomenon known as strain-

induced crystallization~~4. Stretching of vulcanizates of polyisoprene having at

least 90 % cis content leads to crystallization, which in turn leads to

strengthening of rubber.

Another interesting aspect of isoprene rubbers is their low hystereses

giving low heat build up during flexing. The combination of high tensile

properties and low hysteresis explains the requirement of NR as the primary

rubber in heavy vehicle tyres. In addition to this, NR has excellent tensile and

tear properties, good green strength and building tack. However, NR is not very

resistant to oxidation, ozone weathering and a wide range of chemicals and

solvents, mainly due to its unsaturated chain structure and non-polarity.

1.1.7.1 Grades and Grading115

All types of NR that are not modified (such as oil extended NR) or

technically specified rubbers (TSR1s) are considered to be international grades.

Grade designations usually use color or how the rubber was made; typical

grade description are pale crepe, rubber smoked sheet and thin brown crepe.

The main disadvantage of this system is that grading is done on visual aspects.

Almost exclusively, the darker the rubber, the lower the grade. Other grading

criteria such as the presence or absence of rust, bubbles, mould and cut spots

are subjective in nature. Perhaps the most valid assumption is that the darker

the rubber the more dirt it contains. In the 19601s, Malaysia developed a grading

scheme that was more sophisticated and useful to customers. A major criticism

of NR is the large and varying dirt content. The cornerstone of the new system

was grading according to dirt content measured in hundredths of 1%. For eg:

Standard Malaysian rubber (SMR) is a rubber whose dirt conlent does not

exceed 0.5%. Dirt is considered to be the residue on a 45pm sieve after a

rubber sample has been dissolved in an appropriate solvent, washed through

the sieve and dried. The specification has other parameters including source

material for the grade, ash and nitrogen content, volatile matter, plasticity

retention index (PRI) and initial plasticity. Acceptance of these standards is

vigorous and other rubber producing countries followed suit. Letter

abbreviations identify the rubber source, SMR denotes rubber from Malaysia,

SIR indicates Indonesian product, SSR indicates Singapore product and ISNR

denotes Indian Standard Natural Rubber.

1.1.7.2 Modifications of Natural Rubber

The main modifications of NR are:

Deprotenized rubber (DPNR) - This is very useful when low water

absorption is wanted, vulcanizates with low creep are needed or more than

ordinary reproducibility is required. Normally NR has between 0.25 and 0.5 %

nitrogen as protein; deprotenized rubber has only about 0.07%. A drawback is

that since protein matter in the rubber accelerates cure, deprotenized rubber

requires more acceleration. DPNR is made by treating NU latex with bioenzyme

which hydrolyzes the proteins to water-soluble forms. A protease like bacillus

subtilis is used at about 0.3 phr. When the enzymolysis is completed the latex is

diluted to 3 % total solids and coagulated by adding a mixture of phosphoric and

sulphuric acid. The coagulated rubber is then pressed free of most of the water,

crumbed, dried and baled.

Another modification is oil extended NR (OENR). There are three ways

lo make this kind of rubber a) co-coagulation of latex with oil emulsion

b) Banbury mixing of oil and rubber c) allowing the rubber to absorb the oil in

pans until almost all is absorbed, then milling to incorporate the remaining oil.

Recently rubber and oil have been mixed using an extruder. A comparatively

new modification of NR is epoxidized NR. The rubber molecule is partially

epoxidised with the epoxy groups randomly distributed along the molecular

chain. The main advantage of ENR over NR is improved oil resistance and low

gas permeability. Another new modification is thermoplastic natural rubbers

(TPNR). These are physical blends of natural rubber and polypropylene, mixed

in different proportions to give rubbers with different stiffness properties. They

are suitable for injection moulding into products for automotive applications

such as flexible sight shields and bumper components. Many other chemically

modified forms of natural rubber were available in the past. These included

cyclized rubber, chlorinated rubber, hydrochlorinated rubber, isomerized or an

anticrystallizing rubber and depolymerised rubber.

1.1.8 SHORT FIBRE REINFORCED RUBBER COMPOSI'TES

Short fibre rubber composite can be defined as a compounded rubber d

matrix containing discontinuous fibres that are distributed within the rubber to

form a reinforcement phase. A tremendous amount of research has been

carried out on natural and synthetic fibre reinforced rubber composites. The

reinforcement of an elastomer with fibres combines the elastic behavior of

rubber with the strength and sliffness of the reinforcing fibre. Short fibres are

also used to improve or modify certain thermodynamic properties of the rubber

for specific applications or to reduce the cost of the fabricated articles.

According lo O'Connorll6, the limitations of using short fibres in rubber

compounding applications have been due to difficulties in achieving uniform

dispersion of fibres, fibre breakage during processing and difficulties in

handling, incorporating and bonding the short fibres into the rubber matrix. In

order to use a fibre-reinforced elastomeric composite most effectively, a basic

understanding must be obtained of how the various properties depend on the

fibre properties, the matrix properties and the processing methods13. The early

use of fibres (bast fibres) in rubber was merely as a cheapening aid. The

presence of fibres in a rubber increases the modulus but other improvements

are also sought with fibre addition. The possibility of much improved properties

brought about by the alignment of fibres, to give a composite with some

measure of anisotropy is also of great interest.

Gulh et al.ll7 derived an equation for calculating the modulus of a fibre-

reinforced matrix, applicable to fibre-reinforced rubbers, which has been much

quoted. This equation is commonly referred to as the 'Guth-Gold' equation and

is expressed as:

where G = modulus of composite material

Go = modulus of matrix material

f = length to diameter ratio (aspect ratio) of fibre

c = volume concentration of fibre

When the fibre aspect ratio is in the range 10-50, moduli ratio of

102- 103 can be achieved if there is good adhesion between the fibre and

matrix. A later modification taking into account the fibre orientation is the

Boustany -Corm equation:

where €cornp = Modulus of the composite

El = Modulus of rubber

K = constant

f = a function of fibre orientation

Ild = fibre aspect ratio

Vr = fibre volume fraction

Anthoine et all18 dealt mainly with the use of natural short length

cellulose fibre as rubber reinforcement. They claimed thal this fibre being

naturally available in short lengths (from wood pulp) it would circumvent the

complicated route in making short fibre from synthetic polymer and thus would

be more cost effective. The authors reported that the oriented cellulose fibres

gave an increase in tensile strength but the most important changes were an

increased modulus and reduced elongation at break. The modulus of the fibre-

reinforced materials as defined by Young's modulus, showed a steady increase

with increasing fibre concentration. Tensile strength however first fell with

increasing fibre level and then increased beyond that of the matrix material at

higher fibre loadings. This was explained that at low concentrations the matrix is

not restrained by enough fibres and high strains occur in the matrix at low

stresses, causing debonding. The matrix strength is thus reduced by the

debonded fibres.

The advantages of using short fibres are that they can be easily

incorporated into the rubber compound. Short fibres also provide high green

strength and dimensional stability during fabrication. Almos l all standard rubber

processing operations such as extrusion, calendering, compression moulding,

injection moulding & transfer moulding can be used for fabrication of

composites. Short fibre composites also possess design flexibility resulting in

the fabricalion of complex shaped articles. These composites possess high

specific strength, reduced shrinkage, controlled damping properties, improved

swell resistance, increased abrasion resistance and creep resistance. They are

also more economically viable since dipping, wrapping, laying and placing of

fibres associated with long fibre composites are avoided.

'The disadvantages of short fibre reinforced rubber composites are that

it is quite difficult to achieve uniform dispersion, the problem of fibre breakage

during processing (though biofibres undergo less breakage than synthetic

ones). Also certain processing techniques like filament winding, autoclave and

vacuum bag processing techniques cannot be used for short fibre composites.

1.1.8.1 Theory of Reinforcement

The most common assumption in the case of short fibre reinforced

elastomers is that either the stress state or the strain is uniform everywhere in

the deformed composite material, Stiffness is usually considered in terms of the

linear elastic Young's modulus. Even though elastomers do not exhibit

substantial linear elastic regions of deformation in comparison to their useful

extensions, the analysis of elastomer-based composites can proceed on this

basis, because in the reinforced state, linear elasticity is more closely followed.

In the case of reinforcements with fibres of sufficiently high aspect ratio, it is

found that as long as a substantial orientation is obtained in the direction of the

applied stress, the assumption of constant strain is valid.

Thus, linear elasticity is generally applicable, but it must be realized that

in actuality the matrix exists in a rather complex state of non-uniform tension

and shear. The high elastic moduli of the fibres (> 14GPa) differ widely from

those of the elaslomeric matrix (generally about 3 MPa), which might yield a l o 3 greater Young's modulus ratio Ef I Em than for a hard plastic with the same fibre

and have an effect on reducing the efficiency of reinforcement.

The tensile stress generated in a fibre-reinforced elastomer will in

general be distributed belween the fibre (f) and matrix (m) phases,

where c refers to the fibre-rubber composite. If each phase is assumed to be

linearly elastic, this relationship can be further written as

Gc = Et Et $f + E m Ern $rn

where E is the Young's modulus, E the tensile elongation, and 4 the volume-

fraction of each phase.

Although the micromechanical stress state is highly complex in a non-

uniform composite, the role of discontinuous fibres in providing reinforcement

can be gained by considering them to be well aligned in the direction of applied

stress. It is through the shear stresses applied through the matrix that a tensile

stress is generated in the fibre. As these shearing stresses are larger at the

fibre ends and diminish to zero in the midsection of the fibre, the tensile stress

builds up from a small value given by the matrix-supported tensile stress at the

fibre ends toward a constant elevated value in its midsection. The fibre tensile

stress does not continue to increase in unbounded form with increasing

distance from the fibre end but will rather reach a peak at the point where the

fibre strength is reached or will plateau at some high level due to the eventual

decay of the shear stress as the matrix shear deformation goes to zero in

regions of fully developed stress state between fibres. In general, it has been

found that low aspect ratio, non-uniform dispersion and partial or improper fibre

orientation limit the stress development in short-fibre composites.

1 .I 3 . 2 Factors Affecting Reinforcement

'There are many parameters which affect the performance of a fibre-

reinforced rubber composite"? The degree and type of adhesion cannot be

estimated quantitatively even though its importance is well recognized. Aspect

ratio has a considerable effect on composite properties, hence it is important to

conserve fibre length as much as possible during rubber composite processing

operations. Fibre aspect ratio must be in the range of 100-200 for optimum

effectiveness. Fibre orientation however has the largest effect on composite

properties. During processing, the fibres tend to orient along the flow direction

causing mechanical properties to vary in different directions. 4

Poor fibre dispersion results in a loose bundle, embracing an effectively

lower aspect ratio with less reinforcing potential than a single fibre. In addition,

the bundle itself may be low in strength due to poor adhesion. Both the above

factors reduce the overall strength of the composite. The development of

strength in a composite depends on the existence of a strong interface, which

can be enhanced through the selection of an optimum bonding system. A

suitable reaction or a high degree of physical compatibility with both the fibre

66 - -. --

CIt (~pter 1

and matrix is required. Though the presence of a covalent bond would increase

the levels of adhesion tremendously, many of the bonding systems rely only

upon physical forces. The extent of physical forces is quite large in the case of

polar polymers and hydrophilic surfaces. The typical bonding systems tried out

in the case of most cellulosic fibres is resorcinol-based adhesives such as

resorcinol-formaldehyde-latex (RFL) cord-dip formulation or resorcinol-

hexamethylene tetramine-hydrated silica (HRH) bonding system. Another

version substitutes hexamethoxyrnethyl-melamine as the methylene bridge

donor. Isocyanate based resins and organofunctional silanes or tilanales act as

good bonding systems for the more polar classes of fibre-rubber composites.

1.1.8.3. Biofibre Reinforced Natural Rubber Composites

The primary effects of short fibre reinforcement on the mechanical

properties of natural rubber composites include increased modulus, increased

strength with good bonding at high fibre concentrations, decreased elongation

at failure, greatly improved creep resistance over particulate-filled rubber,

increased hardness and a substantial improvement in cut, tear and puncture

resistance. Hysteresis and fatigue strength are also improved. When fibres are

aligned parallel to the stress direction, tensile strength develops a characteristic +

drop with increasing fibre volume contenl until a critical fibre level is reached.

Higher reinforcement concentrations generally cause the strength to increase.

The critical concentration level at which the unreinforced matrix strength is

recovered varies directly with the critical fibre aspect ratio. The stiffness and

modulus of short fibre composite increase with fibre concentration though it may

not be necessarily linear. Tear strength is highly dependent on fibre loading and

is seen to increase with concentration.

The mechanical properties of lignocellulosic fibre reinforced natural

rubber composites have been extensively studied. It has been reported by

Dzyura119 that the minimum amount of fibres to restrain the matrix is smaller if

the matrix strength is higher. Natural rubber is a very strong matrix because of

its strain-induced crystallization. Generally it has been seen that the tensile

strength initially drops down to a cerlain amount of fibre and then increases.

The minimum volume of fibre is known as the critical volume above which the

fibre reinforces the matrix, The critical volume varies with the nature of fibre and

matrix, fibre aspect ratio and fibre I matrix interfacial adhesion.

At low fibre concentrations, the fibre acts as a flaw in the rubber matrix

and the matrix is not restrained by enough fibres causing highly localized strains

to occur in the matrix at low stress. This makes the bond between fibre and

rubber to break leaving the matrix diluted by non-reinforcing debonded fibres.

As the fibre concentration increases, the stress is more evenly distributed and

the strength of composite increases. 'The incorporation of fibre in to rubber

matrix increases the hardness of the composite, which is related to strength and

toughness. The close packing of fibres in the compounds increases the density

while resilience decreases.

The reinforcement of coir fibre in natural rubber has been extensively

studiedl20. Upon incorporation of coir fibre, it was seen that the tensile strength

decreased sharply with increase in fibre loading up to 30 phr and then showed a

slight increase for composites containing 40 and 60 phr fibre loading. This trend

was observed in both longitudinal and transverse directions. The extent of coir

fibre orientation from green strength measurements was also determined. It was

6 8 - .--."

Clt npter I

observed that orientation was lowest when fibre loading was small and

increased with loading. Maximum orientation was observed at 30 phr loading.

Researchers have also investigated the reinforcement effects of a leaf

fibre - sisal fibre -in natural rubber'*'. They observed that the tensile strength

decreased up to 17.5 % volume loading and then increased. The tear strength

and modulus values showed a consistent increase with loading. Thomas and

co-workers have also investigated the effect of chemical modificalion of banana

fibre in natural rubber. It was found that modification of banana fibre resulted in

superior mechanical properties122.

Attempts to incorporate oil palm fibre in rubber matrix have also been

successful. 'The effect of fibre concentration on the mechanical properties of oil

palm reinforced natural rubber composites was investigated by lsmail e t a1.123 They

observed the general trend of reduction in tensile and tear strength with increasing

fibre concentration. The modulus and hardness of composites registered an

increase with fibre loading. Another interesting study by the same group'24 reported

on the fatigue and hysteresis behavior of oil palm wood flour filled natural rubber

composites. It was observed that the composite with the highest loading was the

most sensitive towards changes in strain energy and exhibited the highest

hysteresis.

An interesting report on the reinforcement effect of grass fibre - bagasse

- in natural rubber was presented by Nassar et aI.'25 Aging experiments

revealed tensile strength retention of 97 %. Scientists have also developed

composites comprising of kenaf fibre and natural rubber126. An increase in

rheometric and mechanical properties was observed. The kenaf fibre loaded

composites also possessed good thermal stability and swelling resistance.

Pineapple127 and jute fibre128 have also found their way as a potential

reinforcement in natural rubber. In an interesting study, researchers have used

a novel fibre isora fibre-in natural rubber129. lsora fibres are present in the bark of the

Helicteres isora plant and are separated by retting process, lsora fibre resembles jute in

appearance but surpasses it in strength, durability and lustre. The effects of different

chemical treatments, including mercerisation, acetylation , benzo yla tion and treatment with

toluene diisocyanate and silane coupling agents, on isora fibre properties and mechanical

properties were analyzed, lsora fibre was seen to have immense potential as

reinforcement in natural rubber. The variation of tensile strength with chemical

modification is given in Figure 1.1 -13.

Figure 1.1.13 Variation of tensile strength with chemical modification

[Reference: Mathew et al., Prog. Rubb. Plast. Recycl. Tech., 20, 4, 337, 20041

Researchers have also designed novel rubber biocomposites by using a

combination of leaf and fruit fibre in natural rubber-130. The incorporation of sisal and

coif fibre in NR was seen to increase the dielectric constant of the composites.

These hybrid biocomposites were found to have enormous applications as antistatic

agents. In another interesting study, the preparation of composites comprising of

waste paper in natural rubber along with boron carbide and paraffin wax, for

radiation shielding applications, was investigated13'.

1.8.4 Biofibre ! Natural Rubber Adhesion

The compatibility of hydrophobic rubber matrix and h ydrophilic cellulose

fibre can be enhanced through the modification of polymer or fibre surface. The

extent of adhesion is usually increased by the use of bonding agents and chemical

modification of fibres. The common bonding systems employed are: silica-phenol

formaldehyde and resorcinol-hexamethylenetetramine-silica. Silica is believed to

act as controller for resin formation and helps in developing adhesion between

rubber and fibre. The importance of silica is still a matter of controversy.

Studies by Murty et al.132 have indicated that silica is an essential

component for enhancing the development of adhesion between fibre and rubber in

the case of jute-NR composiles. But studies by Varghese el al.1" indicated that

bonding between sisal and NR can be enhanced by use of resorcinol and

hexamethylenetetramine in the ratio 53.2. It was found that adhesion imparted by

this system was found to be better than that of normal HRH system.

Another interesting report134 claimed that silica was not essential in producing

good adhesion between coir fibre and NU. It was observed that the tensile strength of

mixes that did not contain silica was significantly higher than that of the mixes containing

silica. Thus, it is clear that nature of both rubber matrix and reinforcing fibre determines

whether silica is needed or not as one of the components.

The adhesion of short oil palm fibre with natural rubber was

investigated135 using various bonding agents like phenol-formaldehyde;

resorcinolformaldehyde: silica; hexarnethylenetetrarnine: resorcinolformaldehyde:

silica. It was observed that different bonding agents gave different cure

characteristics and mechanical properties. Better mechanical properties were

obtained for the bonding system RF: Sil : Hex ( 5:2:5).

Equilibrium swelling technique was used by Mathew et aIa136 to study the

interfacial adhesion in isora fibre reinforced natural rubber composites. The authors

observed that Ihe presence of bonding agent increased the interfacial adhesion

between the fibre and matrix. The composites containing bonding agents were less

prone lo solvent sorption (Figure 1.1.14)

Without bonding agent

With bonding agent

Figure 1.1.14 Variation of Qt with chemical modification

[Reference: Mathew L. et a!. In Proceedings from ICBC 2005, March 21-23, 61, 20051

72 Clt np frr 1

Another interesting study, using the swelling technique to estimate

interfacial adhesion was reported in the case of sisal I coir fibre reinforced

natural rubber compositesl3? The bonding agent added mixes showed

enhanced restriction to swelling and it was seen that the ratio of change in

volume fraction of rubber before and after swelling to the volume fraction of

rubber before swelling (V, - VrNo) was lower for bonding agent added

composites, when compared to an unbonded one.

1 .I .9. PROCESSING138

Some of the common and important processing techniques for rubber

composites are given below.

a. Milling

The first step in mil ling is to oven-dry the whole fibre lo reduce moisture

to below 0.1 %. The fibres can also be modified by chemical treatments to make

it more compatible with the rubber matrix. The second step is the mixing of the

treated fibre into the rubber formulation during the rubber compounding

operation in an intensive (banbury) or two roll mill. The product from this step is

a homogenous rubber compound reinforced with fibre. The compound is heated

on a mill roll into manageable sheets for handling. The final process step is the

compression molding at elevated temperature and pressure to cure the rubber.

During mixing in a two roll mill, high shear forces get developed leading to fibre

breakage and the breakage pattern be studied by means of fibre length

distribution curve. It has been reported that the breakage is more common for

synthetic fibres than natural fibres.

b. Calendering

Calendering usually converts the rubber mix into a uniform continuous

sheet. The product of this step is a roll of thin flexible rubber that can be

trimmed with scissors or knives to fit into any desired mould. The addition of

fibres renders a rubber compound less elastomeric and extensible and therefore

the technique of calendering is quite difficult.

c. Injection moulding

The moulding of short fibre-reinforced rubber composites follows closely

the technology of plastic composites. The natural fibre reinforced rubbers are

less abrasive to machine and loo1 surfaces causing less wear than is common

with synthetic fibre-plastic resins. The highly automated injection moulding

requires fibres that are shorter and less concentrated than in compression

moulding. An advantage of injection moulding is its less labor intensive

operation under well-controlled sequencing for good reproducibility at low cost.

The typical parts that are made from short fibre reinforced rubber composites

include diaphragms, gaskets and certain flexible automotive parts.

d. Extrusion

Rubber extrusion is assisted by the lower rigidity of the organic fibre

reinforcement commonly used. In this area, a number of processability criteria

take on added importance with short fibre reinforced rubber. These are (he

direction of fibre orientation, surface appearance, and flow balancing in the die

to minimize tearing and ihe control of downstream post-die sizing operations.

Elastomeric matrices possess the advantage of green strength that is essential

to allow a free-surface forming operation such as extrusion. A unique

application of this technology for extruding short fibre reinforced hose allows the

hose to be curved into a shape as i t emerges from the expanding die by

mechanically adjusting the die geometry to effect local variations in the flow

uniformity within the die itself. Another attractive feature is that the fibre

reinforced green hose retains its shape during handling and the bend areas are

nearly as strong as the straight sections. At present, hose reinforced with

oriented natural short length cellulose fibre is available commercially in Europe.

I .I .I 0. APPLICATIONS OF FIBRE REINFORCED RUBBER COMPOSITES

Short fibres have the potential for reinforcing low-performance tires. In

automotive and truck tires they find application in better abrasion resistance for the

chafer strip and in improved cut resistance to treads especially for trucks and OTR

vehicles. As short fibres have higher green strength and cut, tear and puncture

resistance they can be used for sheeting. Some applications for such reinforced

sheeting are in roofing membranes. Short fibres can be utilized as the sole

reinforcement for a moderate -performance hose or as an auxiliary reinforcement

with cord constructions. They can provide stiffening to soft inner tubes for the

application of metal braids and can extend hose life by bridging the stresses across

weakened filaments. Other uses are as belts diaphragms and gaskets. Some of the

other applications are roofing; hose; dock and ship fenders and general uses such

as belts, tires and other industrial articles, Short fibres can reinforce and stiffen

rubber in fenders and other impact applications in accordance with simple design

equations.

1.1.11 IMPORTANCE OF THE WORK

1.1.1 1 .I Sisal and Oil Palm Fibres

The use of agricultural by-products solves the problem of disposal of

agricultural waste. Sisal and oil palm fibres are extremely promising materials

because of the high tensile strength of sisal fibre139 and hardness of oil palm fibre.

Therefore, the composite comprising of these two fibres is anticipated to exhibit the

above desirable properties of the individual constituents. Oil palm fibre is the waste

by-product that is amassed after palm oil production140. The utilization of oil palm

fibre is therefore an ecological and economical answer to the problem of waste

disposal.

Another major attraction is that both the matrix and reinforcing fibres are

naturally available. Hence, products based on this work will be both cheaper

and sustainable.

1 .I .11.2. Scope of the present work

The present work deals with the fabrication of hybrid and textile

biocomposites. Designing biocomposites comprising of different blends of

b iofi bres will mutt in high performance biocompssites. Research work is going on with other

natural fibres like jute, flax, hemp, kenaf etc. A unique combination of s'ial and oil palm fibres

in natural rubber matrix has been expbred. Sisal fibre has relatively high cellulosic mtent and

low rnimfibrillar angle Mile oil palm fibre passess high hardness value. Presently they are

underutilized and can be considered lo be a g o d candidate as reinforcement in pdymeric

and cement mabices. Careful analysis of the literature Indicates that no systematic studies

have been reported on hybnd composites of sisal and oil palm.

1 .I -12 Major Objectives

The objectives of the present work include

1.1.12.1. Chemical modification of sisal and oil palm fibres and woven sisal fabric

The inherent polar and hydrophilic nature of lignocellulosic fibres and

the non-polar characteristics of most polymeric matrices results in weak bonding

between fibre and matrix which impairs the efficiency of the composite. Another

major setback of natural fibres is their high moisture absorption leading to

swelling and presence of voids at the interface, which results in poor

mechanical properties. This can be remedied by the addition of bonding agents

and chemical modification of natural fibres which decreases the hydrophilic

character of the fibres thereby reducing their affinity for moisture and

consequently increasing fibrel matrix interaction.

1.1.12.2. Optimisation of sisal/ oil palm length, ratio and loading in the preparation of hybrid biocomposites

Composites were prepared by varying the lengths of sisal and oil palm.

The fibre ratio and loading were also changed. 'The mechanical properties were

evaluated to choose the optimum fibre length, ratio and loading.

1.1.12.3. Processability characteristics and green strength measurements

The elastic properties of polymer melts are of prime importance in

processing and thereby in the fabrication of polymer products. Hence, studies

were conducted using Monsanto R-100 rheometer. Green strength

measurements also provide information about processa bility .

1.1.12.4. Macroscale examination of the* composites to evaluate fiber- matrix interactions and swelling measurements

The mechanical properties of composite are clear indications of the

strength of the interface and the fibrel matrix interaction. The fracture

mechanism is an indication of the interfacial adhesion in a composite. Therefore

a comprehensive analysis of the macroscopic properties of the composite is

very important. Equilibrium and anisotropic swelling measurements reveal a

great deal about the crosslink density and fibre orientation of composites.

1 . I . 12.5. Analysis of dynamic mechanical properties of hybrid biocomposites

The nature of fibrel matrix interaction can be evaluated on the basis of

dynamic mechanic analysis. The nature of modulus curves and damping peaks

are good indications of fibrel matrix interactions and nature of interface.

1 .I .I 2.6. Thermogravimetric analysis of hybrid biocomposites

Thermal analysis gives an indication about the thermal stability of the

composites. Generally it has been seen that the incorporation of plant fibres into

different ma trices increases the thermal stability of the system. The kinetics of

thermal degradation of the composites was also evaluated. The kinetics of

thermal degradation of the composites provides a clear idea regarding their

performance under different thermal environments.

1 .I .12.7. Water sorption behaviour of hybrid biocomposites

'The water sorption behaviour of cornposiles is an extremely important

characteristic for applications where contact with moisture becomes essential.

Generally it has been seen that chemical modification of fibres reduces the

water uptake of the composites.

1.12.8. Solvent sorption behaviour of hybrid biocomposites

Diffusion in organic solvents is an important study in elastorneric

compounds because it can be used as a measure of their long term behaviour in

a liquid environment. It can also be an indirect estimation of the interfacial

adhesion in rubber composites. The factors that affect swelling are type of solvent

used, temperature, structure of rubber compound and presence of fillers or fibres.

1 .I .12.9. Ageing and Bio-degradation studies

The information of the mechanical behaviour of natural fibre reinforced

composites during long term environmental exposure is crucial to be utilized in

outdoor applications. Also, the accumulation of plastic wastes causing

environmental pollution has led to an increasing interest and demand for

biodegradable polymers"? Biodegradable polymers can be divided into two

classes: completely biodegradable polymers and bio-disintegrable polymers.

Completely biodegradable polymers are known to be the best materials for

reduction of environmental pollution caused by plastic waste but they often have

inferior properties. A bio-disintegrable polymer, on the other hand, is a blend

material composed of a completely biodegradable polymer and a non-

biodegradable polymer

1 .I .12.10 Dielectric measurements of hybrid biocomposites

For a given polymer, the electrical properties are determined by the

amount and type of conductive additives. Parameters like dielectric constant,

volume resistivity and dielectric loss factor of the composites were evaluated as

a function of fibre loading, frequency and chemical modification of fibres. These

parameters are an indication of the conducting capacity of the composites.

1 .I .12.11. Stress relaxation behaviour of hybrid biocomposites

Long term behaviour of the composites can be evaluated by studying

stress relaxation behaviour of the composites. The stress relaxation properties

of rubber composites are important as they can influence the useful life of

components made from such a polymer system. Of particular interest is the

understanding of the changes in properties with time and whether the

mechanisms responsible for stress relaxation are able to induce other material

property changes.

1 .I $1 2.1 2. Fabricat ion of textile biocomposi tes f rom woven s isa l fabric and natural rubber

Woven fabric composites are widely used in laminated composites for

engineering and aerospace applications. Sisal fabric - natural rubber textile

composites were prepared by sandwiching a single layer of woven sisal fabric

between two layers of pre-weighed rubber sheets which was then compression

moulded. The mechanical and viscoelastic properties of the textile composites

were evaluated as a function of chemical modification of the fabric.

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Chapter 2

Experimental

Abstract

This chapter deals with the details of materials used, chemical modification

used on fibres, biocornposite preparation and the various experiments done for

evaluating the performance of hybrid sisal and oil palm fibre reinforced natural

1 rubber biocomposites and woven sisal fabric reinforced natural rubber textile

biocomposites.

EXPERIMENTAL 1.2.1. Materials

Natural rubber used for the study was ISNR 3L grade obtained from

Rubber Research Institute of India, KO ttayam, Kerala. The molecular weight,

molecular weight distribution and non-rubber constituents of natural rubber are

affected by clonal variation, season, use of yield stimulants, tapping system and

method of preparation! Hence rubber obtained from the same lot has been used in

this study. The properties are given in Table 1.2.1. All the other ingredients were of

commercial grade and were obtained from local sources. The various silanes such

as silane A174, silane A1 100, silane A151 were obtained from Sigma-Aldrich, India.

Silane F8261 was obtained from ABCR MBH and Co, Mumbai.

Table 1.2.1 Properties of rubber

I Dirt content, % by mass 0.03 1

1 Nitrogen, % by mass 1 0.30 1 Volatile matter, % by mass 0.50

/ Initial plasticity number, Po 1 38 1 Ash, % by mass

- - - - - - 1 Plasticity reten tion index 1 78

0.40

Sisal fibre was obtained from Sheeba Fibres, Poovancode, Tamil Nadu. Oil

palm fibre was obtained from Oil Palm lndia Limited, Punalur, Kerala. Sisal rabric

was hand-woven in collaboration with Khadi Village Industries Corporation,

Sreekariyam, Thiruvananthapuram, India.

1.2.2. Fibre Preparation

Sisal and oil palm fibres were first separated from undesirable foreign

matter and pith material. The fibres were then chopped lo different lengths viz.,

2,6,10 and 15 mm. The chemical constituents and physical properties of fibres2 are

given in Table 1.2.2. (a) & 1.2.2.(b)

A unidirectional type of sisal fabric weave having a count of 20 was used.

The properties of sisal fabric are given in Table 1.2.3.

Table 1.2.2. (a) Properties of Sisal Fibre

Chemical constituents (%)

I Lignin

Hemicellulose

I Wax

10

1 Ash

Physical properties d sisal fibre -

1 Diameter (pm)

I Tensile strength ( MPa)

1 Young's modulus (GPa)

I Microfibrillar angle (")

Table 1.2.2. (b) Properties of Oil Palm Fibre

I Chemical constituents (%)

1 Cellulose

I Hemicellulose

1 - Physical properties of oil palm fibre 1

I Tensile strength ( MPa) I 248 I ( Diameter (prn) 150-500

Young's modulus (GPa)

1 Microfibrillar angle (") 1 46 1

3.2

Elongation at break (%)

Table 1.2.3 Properties of sisal fabric

25

, properties of sisal fabric ,, , , b .

- . I .ir, , ,, ' ' ' , , , "' . : - ; ' , , . , " _ , , :.. <

1

Yarn distance (warp)

Yarn distance

-

Twist (turns per mm)

•

3 mm

Area density (slm2)

1.2.3. Chemical modification of sisal, oil palm fibres and sisal fabric

1.2.3.1 Alkali treatment

Sisal and oil palm fibres of lengths 10 mm and 6 mm were treated for 1

hour with NaOH solutions of concentrations 0.5,1,2,4 and 8 % respectively. The

fibres were further washed with water containing acetic acid. Finally the fibres

were washed again with fresh water and dried in an oven.

Sisal fabric was treated for 1 hour with NaOH solution of concentration

4 %. The fabric was further washed with water containing acetic acid. Finally,

the fabric was washed again with fresh water and dried in an oven at 70°C until

the fabric was completely dry.

1.2.3.2 Silane treatment

The silanes used were Silane F8261 [1,1,2,2-Perfluorooctyl triethoxy

silane], Silane A1 100 [y-Aminopropyltriethoxy silane ] and Silane A151 [vinyl

triethoxy silane]. 0.4 % of the respective silanes was prepared by mixing with an

ethanol I water mixture in the ratio 6 1 4 and was allowed to stand for 1 hour.

The pH of the solution was maintained at 4 with the addition of acetic acid. Sisal

and oil palm fibres were immersed in 1% NaOH 'solution and then dipped in the

silane solution and were allowed to stand for 1.5 hours. The ethanol I water

mixture was drained out and the fibres were dried in air and then in an oven at

70°C until the fibres were completely dry.

The silanes used for chemically modifying sisal fabric were Silane A1 100 [y-

Arninopropyltriethoxy silane] and Silane A174 [y-Metharyloxypropyltrimethoxysilane].

Sisal fabric was immersed in 1 % NaOH solution and then soaked in 0.4 %

silane solution and were allowed to stand for 1.5 hours. The ethanol - water

mixture was drained out and the fabric was dried in air and then in an oven at

70°C until the fabric was completely dry.

The structures of the silane coupling agents are given in Figure 1.2.1

Figure 1.2.1 Structures of silane coupling agents

1.2.3.3. Thermal treatment

Thermal treatment was carried out by keeping the woven sisal fabric in

the oven for 8 hours at 150°C. The fabric, directly from the oven was used for

composite preparation.

1.2.4. Spectroscopic Techniques

The surfaces of the treated sisal and oil palm fibres were characterized

by Perkin Elmer spectrometer model 1720-X FTIR. Fibres were ground well and

embedded in KBr crystals before taking the spectra.

1.2.5. Preparation of hybrid biocomposites and textile biocomposites

Formulation of mixes [hybrid biocomposites and textile biocomposites]

used in the present investigation are shown in Tables 1.2.4 (a, b & c) and

Table 1.2.5. Natural rubber was masticated on a two-roll mixing mill (1 50 x 300

mm) for 2 minutes followed by the addition of the ingredients. The nip-gap, mill

roll, speed ratio, and the number of passes were kept the same in all the mixes.

The samples were milled for sufficient time to disperse the fibres in the matrix at

a mill opening of 1.25 mm.

The bonding system consisting of resorcinol and hexamethylene

tetramine was incorporated for mixes containing treated fibres. The mixed sisal

and oil palm fibres (untreated and chemically treated) were added at the end of

the mixing process, taking care to maintain the direction of compound flow, so

that the majority of fibres followed the direction of the flow.

The sisal fabric - natural rubber textile composites were prepared by

sandwiching a single layer of sisal fabric between two layers of pre-weighed

rubber sheets which was then compression moulded at 150" C for 8 minutes. A

schematic sketch is given in Part Ill, Chapter I.

Table 1.2.4. (a) Formulation of hybrid biocomposites [mixes A to €1

I Mixes (phr) by weight 1

a- 2,2,4 -trimethyl-1,2-dihydroxy quinoline

Ingredients

Natural Rubber

ZnO

Stearic acid

TDQa

CBSb

Sulphur

Sisal fibre Fibre length, ( 1 Omm)

Oil palm fibre Fibre length

(6 mm)

b- N-cyclohexyl-2-benzothiazyl sulphenamide

Gum

100.0

5.0

1.5

1 .O

0.6

2.5

B

100.0

5.0

1.5

1 .O

0.6

2.5

5.0

5.0

C

100.0

5.0

1.5

1 .O

0.6

2.5

10.0

10.0

-

A

100.0

5.0

1.5

1 .O

0.6

2.5

15.0

15.0

D

100,O

5.0

1.5

1 .O

0.6

2.5

20.0

20.0

E

100.0

5 .O

1.5

1 .O

0.6

2.5

25.0

25.0

-

Table 1.2.4. (b) Formulation of hybrid biocomposites [mixes ( I to R)]

a - 2, 2, 4-trimethyl-l , 2-di hydroxy quinoline

b- N-cyclohexyl-2-benzothiazyl sulphenamide

Ingredients I Q R J ---

Natural Rubber

ZnO

Stearic acid

Resorcinol

Hexamethylene tetramine

TDQa

CBSb

Sulphur

Sisal fibre Fibre length (10 mm)

Treatment

Oil palm fibre Fibre lenglh (6 mm)

Treatment

5.0

1.5

7.5

4.8

1 .O

0.6

2.5

15.0

0.4%

Silane

A151

15.0

0.4%

Silane

A151

100.0

5.0

1.5

7.5

4.8

1.0

0.6

2.5

15.0

1 % NaOH

1 h r

15.0

1 %

NaOH

1 hr

100.0

5.0

1.5

7.5

4.8

1.0

0.6

2.5

15.0

0.5 % NaOH

1h r

15.0

0.5 %

NaOH 1 hr

1.5

7.5

4.8

1 ,O

0.6

2.5

15,O

0.4%

Silane

A1100

15.0

0.4% Silane

A1100

Mixes

K

100.0

5.0

1.5

7.5

4.8

1 .O

0.6

2.5

15.0

2 %

NaOH

1hr

15.0

2 %

NaOH 1 hr

( phr) by

L

weight

P

5 .O

I .5

7.5

4.8

1 .O

0.6

2.5

15.0

4 % NaOH

1 h r

i5 .0 .

4 %

NaOH

I hr

100.0- 5.0

1.5

7.5

4.8

1 .O

0.6

2.5

15.0

0.4 %

Silane

F8261

15.0

0.4 %

Silane

F8261

Table 1.2.4 (c) Formulation of hybrid biocomposites [mixes (A1 to A5)]

Sulphur

lngredien ts

Natural Rubber

ZnO

Stearic acid

Sisal fibre Fibre length (10 mm)

Oil palm fibre Fibre length (6 mm)

I Mixes [phr] by weight

- - -

a - 2, 2,4-trimethyl-1, 2-dihydroxy quinoline

b- N-cyclohexyl-2-benzothiatyl sulphenamide

A1

100.0

5.0

1.5

A4

100.0

5.0

1.5

A5

100.0

5.0

1.5

A2

100.0

5.0

1.5

A3

100.0

5.0

1.5

Table I .2.5. Formulation of textile biocomposites

a - 2, 2, 4-trimethyl-1, Zdihydroxy quinoline

b - N-cyclohexyl-2-benzothiazyl sulphenamide

Ingredients

Natural Rubber

ZnO - , . - - . --

Stearic acid

Resorcinol

Hexamethylene tetramine

TDQa

CBSb

Sulphur

Sisal fabric

Treatment

Mixes [phr] by weight

TT

100.0

5.0

1.5

7.5 -. . .

4.8

1 ,O

0.6

2.5

J

Thermal

Gum

100.0

5.0

1.5

1.0

0.6

2.5

TBA

100.0 .

5,O

1.5 --

7.5

4.8

1 .O

0.6

2.5

J

4% NaOH .

1 hr

T

100.0

5.0

1,5

.-

1.0

0.6

2.5

J

TBAS

100.0 - . --

5 .O

1.5

7.5

4.8

1 .O

0.6

2.5

J

Silane

A1100

TB

100.0

5.0

1,5

7.5

4.8

1 ,O

0.6

2.5

J

TBMS

100.0

5.0

1.5

7.5

4,8

1 .O

0.6 -

2.5

J

Silane

A174

1.2.6. Measurement of Properties

1.2.6.1. Scanning electron microscopic studies (SEM)

The fracture morphology of the composites was observed by means of

SEM. Scanning electron microscopic studies were conducted using JEOL, JSM

5800 to analyse the fracture behaviour of the composites. The fracture ends of

the tensile and tear specimens of the composites were mounted on aluminium

stubs and gold coated to avoid electrical charging during examination.

1.2.6.2. Fibre Breakage Analysis

'The fibre breakage analysis of hybrid biocornposites was carried out by

dissolving l g of the uncured composite in toluene, followed by separation of

fibres from Re solution. The distribution in fibre length was determined using a

traveling microscope.

1.2.6.3. Processing Characteristics

The rheological properties of the hybrid biocomposites under very low

shear rate were evaluated using an Oscillating Disc Rheorneter (model

Monsanto R-100). This consists of a bi-conical shaped disc. The upper and

lower dies constituting the cavities were maintained, at 150°C. The cavity was

opened and closed pneumatically. The torque required to oscillate the rotor and

thus shearing stress experienced by rubber was measured by a stress

transducer consisting of strain gauges.

Two circular pieces of the vulcanizable rubber compound having

approximately 30 rnrn in diameter, 12.5 mm in thickness or equivalent to a volume

of 8 cm3, was cut and introduced into the test cavity. The disc is oscillated through

small rotational amplitude, 3", at a frequency of 100 oscillations per minute and

this action exerts a low shear strain on the test piece.

The force required to oscillate the disc was continuously recorded as a

function of time. This force is proportional to the shear modulus (stiffness) of the

test piece. This stiffness first decreased as it warms up and then increased due

to the increased cross-link density as a result of vulcanization. The test was

completed when the recorded torque increased to an equilibrium value. The

time required to obtain a cure curve is a function of the characteristics of the

rubber compound and the test temperature3.

1.2.6.4. Green Strength Measurements

Green strength measurements of hybrid biocomposites were carried

using dumb-bell shaped samples obtained from unvulcanized composites on an

lnstron Universal Testing Machine (UTM) at a stretching rate of 500 %min-1.

1.2.6.5. Mechanical Property Measurements

Stress-strain measurements were carried out at a crosshead speed of

500 mmlmin. Tensile strength and tear strength was measured according to

ASTM methods D412-98 and D624-00 respectively using lnstron UTM. The

tensile tests were done using dumbbell samples cut at different angles with

respect to the orientation of fibres. Tear testing was done using crescent

shaped specimens. Minimum of four samples were tested in each case and the *

average value is reported.

1.2.6.6. Swelling Studies

1.2.6.6.1. Anisotropic Swelling

Anisotropic swelling studies of sisal - oil palm hybrid fibre reinforced

natural rubber biocomposites were carried out using rectangular samples cut at

different angles with respect to orientation of the fibre from the specimen sheets

and swollen in toluene at room temperature for 3 days. Length, breadth and

thickness of the samples were measured before and after swelling.

1.2.6.6.2. Equilibrium Swelling

Equilibrium swelling measurements of the composites was performed in

toluene at room temperature. Circular shaped samples having a thickness of 2

mm, punched out of specimen sheets were used. The samples were allowed lo

swell for 72 h in toluene. The weight of the samples before and after equilibrium

swelling was measured.

1.2.6.7. Hardness and Abrasion Resistance Studies

Wear properties such as hardness and abrasion resistance of hybrid

and textile biocomposites were determined. Hardness measurements were 4

measured by Shore A type Durometer according to ASTM D2240-03. Abrasion

resistance was measured on an abrader according to DIN 53516.

1.2.6.8. Dynamic Mechanical Analysis

DMA measurements were carried out on Universal V2.6D TA

Instruments. The test specimen was clamped between the ends of two parallel

arms, which are mounted on low force flexure pivots allowing motion only in

horizontal plane. The samples were measured at an operating frequency of

0.1,l and 10 Hz and a heating rate of 2°C I rnin. The samples were evaluated in

the temperature range from -80°C to +40°C.

1.2.6.9. Thermogravimetric Studies

Thermal analysis of the composites was carried out by using a

Universal V2.3C TA Instrument with temperature programmed at 20°C I min

heating rate, from room temperature to 700 " C in presence of nitrogen.

1.2.6.1 0. Water Sorption Experiments

Water sorption is evaluated in terms of weight increase for composite

specimens immersed in distilled water at temperatures 30, 50 & 70 "C. Circular

specimens were dried in vacuum at room temperature for two days and the

weight of dried specimen was measured using an electronic balance. The

thickness of the samples was also measured. The weighed specimens were

then immersed in distilled water at different temperatures. The specimens were

periodically removed from water bath and the surface moisture was wiped off.

The weight gain of the specimen has been measured as a function of lime until

equilibrium or saturated state of water uptake' has been reached. Moisture

absorption was determined by weighing the specimen on an electronic balance.

The molar percentage uptake Qt for water by 1009 of the polymer was plotted

against square root of time. The Qt value is expressed as:

where Me (w) is the mass of water sorbed by the sample, M,(w) is the relative

molecular mass of water (18) and M~(s) is the initial mass of the sample. When

equilibrium was reached, QI was taken as the molar percentage uptake at

infinite time i.e. Q,.

1.2.6.1 1. Solvent Sorption Measurements

Solvent sorption is evaluated in terms of weigh1 increase for composite

specimens immersed in solvents benzene, toluene and xylene at room

temperature. Circular specimens were dried in vacuum at room temperature for

two days and the weight of dried specimen was measured using an electronic

balance. The thickness of the samples was also measured. The weighed

specimens were then immersed in the solvents. The specimens were

periodically removed from water bath and the surface solvent was wiped off.

The weight gain of ihe specimen has been measured as a function of time until

equilibrium or saturated state of solvent uptake has been reached. The molar

percentage uptake Qt for the composite samples was determined using the

following equation:

where W2 is the weight of the sample after swelling , W1 is the weight of the

sample before swelling and M, is the molecular mass of the solvent. The

sorption data were evaluated by plotting the mole percentage uptake of the

composite versus square root of time for different solvents.

E-vperir~r ert t r r l --- - 10s

1.2.6.1 2. Ageing and Bio-degradation Studies

Biodegradation experiments were carried out by soil burial

experiments. This test was performed for 6 and 12 months of soil exposure.

Dumbbell tensile specimens were completely buried in natural soil at 90%

water holding capacity (WHC) and a 50% soil moisture content. The samples

were in permanent contact with the soil. The soil test was set up using plastic

bins. All the specimens were vertically buried in the bins, using spatula and

forceps. Tensile strength of the exposed samples was measured according to

ASTM methods D412-98.

The resistance to ageing was determined by keeping tensile specimens

in an air circulated oven, maintained at a temperature of 90°C for seven days.

The tensile properties of the samples were determined after the ageing period

and compared with those of the original samples.

1.2.6.1 3. Dielectric Measurements

Rectangular specimens of 1.9 mm thickness were used. Samples

were prepared by cutting from the composite specimens using a die. The

test samples were coated with conductive graphite paint on either side and

copper wires are fixed on both sides of the samples as electrodes. The

capacitance, resistance and dielectric loss factor were measured directly at

room temperature using an Impedance Analyzer by varying the frequencies

(500Hz - 6 MHz).

1.2.6.14. Stress relaxation Experiments

This experiment was carried out on a Zwick universal testing machine

(model 1465) (Zwick EmbH & Company,Ulm-Ein, Germany) in uniaxial tension

mode at 25°C. The dumbbell specimens were extended to different strain

levels, namely, 5 and 50%. When the appropriate strain was reached, it was

maintained, and the stress was recorded for a time span of 10,800 seconds.

Graphs were plotted with stress at a specific time (or) divided by the maximum

stress when the required strain was attained (00) versus time. The slope was

obtained by regression analysis of the data for the best fit to a straight line.

References:

1 . Subramanyam A,. Rubber Chem. Technol. 45 346 1972

2, Bismarck A., Mishra S., Lampke T., Natural Fibres, Biopolymers, and

Biocomposites, Edited by Mohanty A.K., Misra M., Drzal L.T., 39, 2005

3. ASTM D 2084-87