Chapter

1

INTRODUCTION

Review of recent developments in the field of composite research, theory of

reinforcement and characterisation techniques of composites is presented in this

chapter. The scope, objective and plan of the thesis is also presented in this

chapter.

1.1 General introduction Composites consist of two or more physically and chemically distinct components

and offer a combination of properties and a diversity of applications unobtainable

with metals, ceramics or polymer alone. In composites, one of the components,

usually the reinforcement has superior mechanical properties and has a definite

interface between the matrix and reinforcement usually with zero thickness1.

Composites do occur in nature--e.g. in tree trunks, spider webs, and mollusk shells.

Fig.1.1 Composition of a

tree wood (Ref. CCM

home page, University of

Delaware, Newark)

A tree (Fig.1.1) is a good example of a natural

composite, consisting of cellulose (the fibrous

material) and lignin (a natural polymer) forming the

woody cell walls and the cementing (reinforcing)

material between them. The three important historical

steps through which the modern composites

developed were the commercial availability of

fibre glass filaments in 1935, the development of

strong aramid fibres in the late 1960’s and early

1970’s and the development of analytical methods for structures developed from

these fibres 2-4.

Based on the matrix material used composites can be classified into metal, ceramic

and polymer composites1. Among these, polymer composites possess the

advantages of easier processing and fabrication than metal and ceramic

composites. The structure, properties and applications of various composites were

reported by a number of researchers5-20 all over the world. Among the various

Short Sisal Fibre Reinforced Polystyrene Composites 4

polymer composites, fibre reinforced composites gained much importance in

various fields due to high strength to weight ratio.

1.2 Fibre reinforced composites

Fibre reinforced composites consist of fibres of high strength and modulus

embedded in a matrix with distinct interfaces between them. Fibre reinforcement

improves the stiffness and the strength of the matrix. In the case of polymers that

are not tough in the non- reinforced form, the toughness may also increase21. The

fibre reinforced composites exhibit anisotropy in properties. The maximum

improvement in mechanical properties is obtained with continuous fibre

reinforcement. However, short fibre reinforced composites offer many advantages

like ease of fabrication, low production cost and possibility of making complex

shaped articles, over continuous fibre reinforcement21. The performance of the

composite is controlled by the fibres and depends on factors like aspect ratio,

orientation of fibres and fibre–matrix adhesion. Discontinuous fibre reinforced

composite form an important category of materials used in engineering

applications. The use of fibre reinforced plastic (FRP) composites for the

production of rebars and pre stressing tendons in civil engineering and

transportation applications are becoming increasingly important in recent years22.

Major constituents in a fibre reinforced composite material are the reinforcing

fibres and a matrix, which act as a binder for the fibres. Coupling agents and

coatings used to improve the wettability of the fibre with the matrix and fillers

Introduction 5

used to reduce the cost and improve the dimensional stability are the other

commonly found constituents in a composite.

1.3 Factors influencing the properties of fibre reinforced composites

1.3.1 Strength, modulus and chemical stability of fibre and matrix

In fibre reinforced composites fibres are the main load carrying members and the

matrix keeps them in the desired orientation and location. The final properties of

fibre- reinforced composite depend on the strength and modulus of the reinforcing

fibre23-25. Choice of the matrix depends on the final requirements of the product

and other factors like cost, fabrication process, environmental conditions and

chemical resistance of the matrix. The function of the matrix in a composite will

vary depending on how the composite is stressed 26. For compressive loading, the

matrix prevents the fibres from buckling and provides a stress transfer medium, so

that when an individual fibre breaks, it does not loss its load carrying capacity. The

physical properties of the matrix that influence the behaviour of the composites are

shrinkage during cure, modulus of elasticity, ultimate elongation, tensile and

flexural strength and compression and fracture toughness.

1.3.2 Fibre length, loading and orientation

Fibre length, loading and orientation play important roles in determining the

ultimate properties of the fibre reinforced composites. There are several studies on

the effect of fibre length and fibre orientation on the tensile strength of the short

fibre composites27. In the case of short fibre reinforced composites, there exist a

critical aspect ratio at which the properties are maximised. The critical aspect ratio

depends on the volume fraction of the fibre and also on the ratio of the modulus of

Short Sisal Fibre Reinforced Polystyrene Composites 6

the fibre and matrix28. At low volume fraction, the fibres play no major role and

the strength of the composite is matrix dominated. Above a critical volume fraction

of the fibre, the strength of the composite increases. The critical volume fraction

depends on the fibre aspect ratio and found to decrease with increase in aspect

ratio. At low fibre content, the critical aspect ratio remain almost constant and

show sharp decrease at higher volume fraction. The critical aspect ratio is given by

the equation

)1.1.(....................2 i

f

cDL

τσ

=⎟⎠⎞

⎜⎝⎛

where,

L - length of fibre

D - diameter of fibre

(L/D)c - critical aspect ratio

σf - tensile strength of fibre

τi - fibre-matrix interfacial shear strength

A critical fibre length may be defined as the minimum fibre length in which the

maximum allowable fibre stress can be achieved. Fig 1.2 shows the variation of

fibre stress along the fibre length in a fibre –matrix composite.

The increase in fibre length above critical fibre length does not contribute to the

increase in composite strength. However, a decrease in fibre length below the

critical fibre length results in a decrease in composite strength. When all the fibres

are below critical fibre length, the fibres act only as filler and the strength of the

composite decreases. As the critical aspect ratio depends on the efficiency of

Introduction 7

stress transfer from the matrix to fibre the critical aspect ratio decreases with

improvement in fibre –matrix adhesion.

In the case of fibre reinforced composites there is an optimum spacing between the

fibres at which the fibre tensile strength is fully exploited29 and below which the

structure starts to disintegrate under loading before the tensile failure. The spacing

between the fibres is controlled by the volume fraction of the fibre and fibre

dispersion in the composite.

stressFibre

l>(l/d)cl=(l/d)cl<(l/d)c

Fig.1.2. Variations in fibre stress at a fibre/matrix interface along the fibre length

[Ref. L.J.Broutman and R. H. Krock, Modern Composite Materials, Addison-

Wesley Publishing]

Orientation of the fibre also affects the composite strength and other properties of

the composites. The reinforcement provided by each individual fibre depends on

the orientation with respect to the loading axis. Longitudinally oriented fibre

composites in which the fibres are oriented in the direction of applied forces, the

composites are inherently anisotropic, and the maximum stress and reinforcement

are achieved in the direction of fibre orientation. In the case of transversely

Short Sisal Fibre Reinforced Polystyrene Composites 8

oriented fibre composite, fibre reinforcement is virtually absent and fracture occurs

at a very low tensile stress, which is usually lower than the strength of the matrix.

In the case of randomly oriented composites the strength lies between these two

extremes.

Although the tensile strength of longitudinally oriented fibre composites are very

high the compressive strength shows lower values due to the bukling of the

fibre 30-31.

Fig.1.3. Fibre micro buckling modes in a unidirectional continuous fibre composite under

longitudinal compressive loading: (a) extensional mode and (b) shear mode

[Ref. P. K. Mallick, Fibre Reinforced Composites: Materials, Manufacturing and Design,

Marcel Dekker, Inc., 270 Madison Avenue, New York, 1988, p. 95]

Fig 1.3 shows the two possible microbukling modes viz. extensional mode and

shear mode observed in fibre composites. The extensional mode of microbukling

occurs at low fibre volume fractions (Vf<0.2) and creates an extensional strain in

the matrix. The shear mode of microbuckling occurs at high fibre volume fractions

and creates a shear strain in the matrix. In the case of transversely oriented fibre

Introduction 9

composites the compressive strength is limited to the matrix strength and is always

lower than that of longitudinally oriented fibre composites. Randomly oriented

fibre composites prepared by randomly orienting the fibre or by making multi

layered laminates with layers having different fibre orientation are isotropic

composites with the same properties in all directions. Fibre orientation can be

assessed by micro radiography, optical diffraction methods and scanning electron

microscope examination of the fracture surfaces.

1.3.3 Presence of voids

During the incorporation of fibres into the matrix or manufacture of laminates, air

or other volatiles may be trapped in the material. The trapped air or volatiles exist

in the cured laminates as microvoids and may significantly affect the mechanical

properties of the composites. There are two types of voids in composite materials

(a) voids formed along individual fibres and (b) voids formed between lamina and

in resin rich regions. Shrinkages during cure of the resin and the cooling rate play

important role in void formation. The void content in a composite is calculated

using the equation

V = 100 (Td - Md )/ Td …………………….(1.2)

Where, Td is the theoretical composite density, Md the measured composite density

and V is the void content in volume per cent. A high void content (over 20% by

volume) usually leads to lower fatigue resistance, greater susceptibility to water

diffusion, and increased scattering in mechanical properties. The volatiles

Short Sisal Fibre Reinforced Polystyrene Composites 10

produced during the curing cycle in thermosetting resins and during melt

processing operation in thermoplastic polymers may also result in the production

of voids in composites.

1.3.4 Fibre –matrix interface

The bond strength between the reinforcing fibres and the surrounding matrix is

of crucial importance for many mechanical and physical properties of the

composites. Interface cracks initiates composite damage and hence change the

composite properties. While most of the properties deteriorate, the transverse

impact properties are improved due to the dissipation of some impact energy by the

cracks 32. The characteristics of the interface are dependent on the bonding at the

interface and the physical and chemical characteristics of the constituents. The

interface effects are seen as a type of adhesion phenomenon and are often

interpreted in terms of surface structure of the bonded materials. The important

surface factors are wettability, surface free energy, the polar groups on the surface

and roughness of the material to be bonded33.

An interface is considered as a region in which the fibre and matrix phase are

chemically and/or mechanically combined or otherwise indistinct. It may be a

diffusion zone, a nucleation zone, a chemical reaction zone or any combination of

these. The interphase not only includes the two dimensional regions of contact

between fibre and matrix (interface) but also incorporates the region of some finite

thickness extending to both sides of the interface in both the fibre and matrix34. An

ideal model of an interphase is given in Fig.1.4. Effective reinforcement of

polymer matrices by any fibre requires good stress transfer at the interface 35. Load

Introduction 11

applied directly to the matrix at the surface of the composite is transferred to the

fibre 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.

Bulk fibre

Fibre /InterphaseInterface

Fibre/Matrix Interface

Matrix/interphase Interface

Matrix

Fig. 1.4. Ideal model of an interphase in a fibre reinforced composite

[Ref. P. J. Herrera and L. T. Drzal, Composites, 23(1), 2,1992]

In the case of perfect adhesion between the fibre and matrix, the failure occurs only

in tension with no fibre debonding and leads to a catastrophic failure of the

composite28. The various modes of fracture in the case of perfect adhesion between

the fibre and matrix can be schematically represented as in Fig.1.5(a-d) and this

figure show two successive deformation schemes obtained in a longitudinal x-y

plain passing through the centre of the lattice. Fig 1.5a shows the tensile failure of

the matrix near the fibre ends. Further straining of the composite leads to

transverse propagation of those matrix cracks with eventual fibre breaking near

Short Sisal Fibre Reinforced Polystyrene Composites 12

Fig-1.5a Fig-1.5b

Fig-1.5c Fig-1.5d

Fig 1.5(a),(b) Typical deformation schemes for l/d =20 at two different strain

values (a) 0.035 (b) 0.075 (assuming perfect adhesion at the fibre –matrix

interface),13(c),(d), Typical deformation schemes for l/d =1 at two different

strain values (a) 0.035 (b) 0.075 (assuming perfect adhesion at the fibre –

matrix interface)

(Ref.- Y.Termonia, J.Mater. Sci., 25, 4644, 1990.)

Introduction 13

Fig 1.6a Fig 1.6b

Fig 1.6c Fig-1.6d

Fig 1.6(a),(b) Typical deformation schemes for l/d =20 at two different strain

values (a) 0.025 (b) 0.075 (assuming poor adhesion at the fibre –matrix

interface),14(c),(d), Typical deformation schemes for l/d =1 at two different

strain values (a) 0.08 (b) 0.12 (assuming perfect adhesion at the fibre –matrix

interface)

(Ref.- Y.Termonia, J.Mater. Sci., 25, 4644, 1990.)

Short Sisal Fibre Reinforced Polystyrene Composites 14

catastrophic failure (Fig.1.5b). Figs.1.5c and 1.5d show the failure of a particulate

reinforced composite, and shows that the local matrix cracks appearing at “fibre

ends”(Fig.1.5c) easily merge transversely and catastrophic failure of the composite

occurs with no fibre fracture (Fig 1.5d).

The deformation behaviour of the composite with poor interfacial adhesion

between the fibre and matrix can be represented as in Fig. 1.6a and 1.6b. In this

case tensile failure of the matrix, near the fibre ends is the only mode of fracture

observed (Fig1.6a) and there is a high concentration of the stress building up at the

fibre- matrix interface. Near catastrophic failure (Fig 1.6b), the matrix starts to fail

in shear and progressive debonding of the fibres is seen to occur.

The case of particulates with poor adhesion is described in Fig1.6c and 1.6d.

Again, composite fracture is initiated through matrix failure at “fibre ends”

(Fig.1.6c). Due to the high concentration of ‘fibre ends’, these matrix cracks easily

merge transversely, leading to catastrophic failure (Fig.1.14d). However, no

debonding is seen to occur along the direction of applied load.

Fibre –matrix interfaces can be characterized using different techniques like

electron spectroscopy for chemical analysis (ESCA)36, X-ray photoelectron

spectroscopy (XPS)37,38, photoacoustic spectroscopy34, X-ray diffraction studies39,

electron induced vibration spectroscopy40, solid state 13C NMR spectroscopy41, and

Fourier transform infrared spectroscopy42.

Introduction 15

Solid state 13C NMR spectroscopy using cross polarisation and magic angle

spinning is useful in characterising wood –polymer composites since detailed

information can be obtained from solid samples. These include composition, glass

transition temperature, melting transition, percent crystallinity, and number and

type of crystalline phases41. Thomson and Dwight38 have investigated the use of X-

ray photoelectron spectroscopy as a tool for the rapid characterisation of the sizing

and interphase in glass fibre reinforcement. They also used XPS analysis to give

new insights into the in situ nature of the coating on glass. Zadorecki and

Rohnhult36 used ESCA technique to evaluate the cellulose surface modified with

triazine coupling agents and found that the coupling agent is concentrated more on

the surface rather than uniformly distributed throughout the fibre. Kszayawoko et

al.42 used FTIR data to identify the nature of bonds between wood fibre and MAPP

during the study of modification of wood fibre by MAPP. Tillman et al.43 used

atomic force microscopy to study the interphase characteristics of polymer

composites.

Fibre pull-out tests using one or a few single filaments can also be used for the

characterization of interface. These methods44-50 include embedded single fibre

tension test, the embedded single fibre compression test, the micro debond test,

single fibre pull-out test, the bead pull-off test, the short beam shear test, the

Iosipesu shear test, the transverse tension test and the transverse flexural test.

Although these methods, do not represent the real state of stresses in the real

composites they provide a fruitful evaluation of interface performances.

Short Sisal Fibre Reinforced Polystyrene Composites 16

Table1.1 Matrix materials used in composites

(Ref. P. K. Mallick, Fibre Reinforced Composites- Materials, Manufacturing

and Design, Marcel Dekker, Inc, New York, 1988.)

Polymeric

Thermoset polymers (resins)

Epoxies: Principally used in aerospace and aircraft applications

Polyesters, vinyl esters: Commonly used in automotive, marine, chemical, and

electrical applications.

Phenolics: used in bulk moulding compounds

Polyimides, polybenzimidazole, polyphenylquinoxalin: for high-temperature

applications in aerospace applications (temperature range: 250-4000C).

Thermoplastic polymers

Nylons (such as nylon 6, nylon 6,6, thermoplastic polyesters (such as

PET,PBT), polycarbonate, polyacetals: used with discontinuous fibres in

imjection moulded articles.

Polyamide-imide, polyether-ether ketone, polysulfone, polyphenylene

sulfide,polyether imide: suitable for moderately high temperature applications

with continuous fibres.

Metallic

Aluminium and its alloys, titanium alloys, magnesium alloys, nickel -based

super alloys, stainless steel: suitable for high temperature applications

(temperature range 300-5000C )

Ceramic

Aluminium oxide, carbon, silicone carbide, silicone nitride : suitable for

high temperature applications.

Introduction 17

1.4 Matrix materials used in composites

The role of a matrix in a composite material is to (a) transfer stresses between the

fibres, (b) provide a barrier against any adverse environment and (c) protect the

surface of the fibres from mechanical abrasion.

Although, the matrix plays only a minor role in the tensile load-carrying capacity

of the composite structure, matrix considerably influences the inter laminar shear51

and in-plane shear properties of the composite material. While the inter laminar

shear strength is an important design consideration for structure under bending

loads, in-plane shear strength is important under torsional loads. The matrix

provides lateral support against the possible fibre buckling under compressive

loading and hence influences, to some extent, the compressive strength of the

composite material. The interaction between the fibre and matrix is also important

in designing damage- tolerant structures. Finally, the processability and defects in

a composite material are influenced by the physical characteristics such as

viscosity, melting point and curing temperature of the matrix. Table 1.1 lists

various matrix materials in use. Among these, thermoset polymers, such as epoxies

and polyesters, find commercial interest due to the easy processability of these

materials and metallic matrices are primarily used for high temperature application.

1.5 Fibres used in composites

Reinforcement used in a composite may be in the form of long fibres, particles,

flakes, whiskers, discontinuous fibres, continuous fibres or sheets. Fibres are the

major constituents in a fibre-reinforced composite material and occupy the largest

Short Sisal Fibre Reinforced Polystyrene Composites 18

Table1.2 - Commercial fibres and their Properties

(Ref. P.K.Mallik, Fibre-Reinforced Composites: materials, manufacturing, and design,

Marcel Decker, Inc. New York, 1988.)

Fibre Typical

diameter

(μm)

Specific

gravity

Tensile

modulus

(GPa)

Tensile

strength

(GPa)

Strain

to

failure

(%)

Poison’s

ratio

Glass

E-glass

S-glass

10

( round)

10

( round)

2.54

2.49

72.4

86.9

3.45

4.30

4.8

5.0

0.2

0.22

PAN-

carbon

T-300

Pitch

carbon

P-55

P-100

7(round)

10

10

1.79

2.0

2.15

228

380

690

3.2

1.90

2.2

1.4

0.5

0.31

0.2

-

-

Kevlar-

49

11.9

(round)

1.45

131

3.62

2.8

0.35

Boron 140

(round)

2.7

393 3.1 0.79 0.2

SiC 133

(round)

3.08 400 3.44 0.84 -

Al2O3 20

(round)

3.95 379.3 1.90 0.4 -

Introduction 19

volume fraction in a composite laminate. The major portions of the load acting on

the composite material is shared by the fibres and hence a proper selection of the

type, amount, and orientation of the fibres are critical factors deciding the

properties of the composite by controlling the following characteristic of the

composite laminate.

a. Specific gravity

b. Tensile strength and modulus

c. Compressive strength and modulus

d. Fatigue strength as well as fatigue failure mechanism

e. Electrical and thermal conductivity

f. cost

Fibres can be classified in to man made fibres and natural fibres based on their

origin. Table 1.2 lists the commercially available fibres and their properties 51.

1.6 Fillers and other additives used in composites

Fillers are added to composite material to reduce cost, increase stiffness (modulus),

reduce mould shrinkage, control viscosity and /or to produce smoother surface.

Fillers commonly used in are calcium carbonate, clay, mica and glass

microspheres. Although fillers increase the modulus of an unreinforced matrix,

they tend to reduce its strength and impact resistance. In addition to fillers,

toughners, colorants, flame retardants, and ultraviolet absorbers may also be added

to the polymer matrix.

1.7 Natural fibres as reinforcement in composite

Natural fibres like jute, silk, sisal etc. appear to gain importance as reinforcement

in composites in recent years. Natural fibres can be classified based on their origin

Short Sisal Fibre Reinforced Polystyrene Composites 20

as coming from plants, animals or minerals and plant or vegetable fibres. Among

plant fibres hairs (cotton, kapok), fibre–sheaf of dicotylic plants or vessel sheaf of

monocotylic plants (flax, hemp, jute, ramie) and hard fibres (sisal, henequen, coir)

are the generally available fibres in nature. One of the important drawbacks of

these materials is the lack of availability of large quantities of these fibres with

well-defined mechanical properties. Moreover, for technical oriented applications,

these fibres have to be modified or prepared regarding

a. Homogenisation of the fibres properties

b. Degree of elementarisation and degumming

c. Degree of polymerisation and crystallization

d. Good adhesion between the fibre and matrix

e. Moisture repellence

f. Flame retardants

However, there are several advantages for these natural fibres over glass fibres.

These include:

a. Plant fibres are a renewable raw material and their availability

is more or less unlimited

b. When natural fibre reinforced plastics were subjected to, at the

end of their life cycle, to a combustion process or land fill, the

released amount of CO2 of the fibres are neutral with respect to

the assimilated amount during their growth.

Introduction 21

c. The abrasive nature of natural fibres is much lower than that of

glass fibres, which leads to advantages with regard to technical,

material recycling or process of composite materials.

d. Natural fibre reinforced plastics using biodegradable polymers

as matrix is the most environmental friendly materials, which

can be composted at the end of their life cycle.

However, the over all physical properties of these composites are far away from

glass fibre reinforced thermoplastics. More over, a balance between the life

performance and biodegradation has to be developed.

1.7.1 Mechanical properties of natural fibres

Natural fibres are suitable for the reinforcement of thermoplastics and thermosets

due to their relative high strength and stiffness and low density52.

Table 1.3 gives the mechanical properties of different natural fibres. This table

shows a range for the property values of natural fibres, which are much higher than

those of glass fibres and can be attributed to the difference in fibre structure due to

the overall environmental conditions such as area of growth, its climate, and the

age of plant 53,54. The technical digestion of the fibre is another important factor

that determines the structure as well as the characteristic values of the fibres.

Natural fibres can be processed in different ways to yield reinforcing elements

having different mechanical properties.

Fig 1.7 shows the elements of a natural fibre and their elastic modului. The elastic

modulus of bulk natural fibres such as wood is about 10GPa. Cellulose fibre with

moduli up to 40 GPa can be separated from wood by methods like chemical

Short Sisal Fibre Reinforced Polystyrene Composites 22

Table 1.3 Mechanical properties of different natural fibres

( Ref. A.K.Bledzki and J.Gassan, Progress in Polymer Science

,24,221,1999.

Fibre Density

(g/cc)

Elongation

(%)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Cotton 1.5-1.6 7.0-8.0 287-597 5.5-12.6

Jute 1.3 1.5-1.8 393-773 26.5

Flax 1.5 2.7-3.2 345-

1035

27.6

Hemp - 1.6 690 -

Ramie - 3.6-3.8 400-938 61.4-128

Sisal 1.5 2.0-2.5 511-635 9.4-22.0

Coir 1.2 30.0 175 4.0-6.0

Viscose

(cord)

- 11.4 593 11.0

Softwood

kraft

1.5 - 1000 40

Oil Palm

OPEFBa

Mesocarpb

fibre

1.4

14

17

248

80

2

0.5

a- obtained from fruit bunch after removal of oil seeds

b- obtained from oil seeds after oil extraction

.

Introduction 23

Structure Process Components Young’s

modulus

250GPa

70GPa

40GPa

10GPa

Crystallites

No existing technology

Hydrolysis followed by mechanical disintegration

Microfibrils

Single pulp fibre

Pulping

Wood

Fig.1.7 - Correlation between structure, process, resulting components and modulus. (Ref.- A.J. Michell, Wood cellulose-organic polymer composite, Asia Pacific, Adelaide, Vol.89, 19-21, 1989).

Short Sisal Fibre Reinforced Polystyrene Composites 24

chemical pulping process. These fibres can be further subdivided by

hydrolysis followed by mechanical disintegration into microfibrils with an

elastic modulus of 70 GPa. Theoretical calculation of the elastic modulus of

the cellulose chains gives a moduli value of 250GPa. However, there is no

technology available now to separate these microfibrils 55,56.

1.7.2 Chemical composition of natural fibres

Table1.4 shows the chemical composition of different cellulose fibres. The

climatic conditions, age and the digestion process influence the chemical

composition and the structure of the fibres.

Table 1.4- Composition of different cellulose based fibres (Ref.- J.Gassan and A.K.Bledzki, Die Angew. Makromol.Chem.

236,129,1996. )

Cotton Jute Flax Ramie Sisal

Cellulose 82.7 64.4 64.1 68.6 65.8

Hemi-

cellulose

5.7

12.0

16.7

13.1

12.0

Pectin 5.7 0.2 1.8 1.9 0.8

Lignin - 11.8 2.0 0.6 9.9

Water

soluble

1.0 1.1 3.9 5.5 1.2

Wax 0.6 0.5 1.5 0.3 0.3

Water 10 10 10.0 10.0 10

With the exception of cotton, the components of natural fibres are cellulose, hemi-

cellulose, lignin, pectin, waxes and water-soluble substances. Cellulose,

Introduction 25

hemicellulose and lignin are the basic components responsible for the physical

properties of the fibres.

(a) Cellulose

Cellulose is a linear condensation polymer consisting of D- anhydroglucopyranose

units joined together by β-1,4- glycosidic bonds. The pyranose rings are in the 4C1

conformation which means that the –CH2OH and –OH groups, as well as the

glycosidic bonds are equatorial with respect to the mean planes of the ring 57.

Fig.1.8 shows the Haworh projection formulae of the cellulose57.

The molecular structure of cellulose is responsible for its supramolecular structure

and this, in turn, determines many of its chemical and physical properties. In the

fully extended molecule, adjacent chain units are oriented by their mean planes at

an angle of 1800 to each other. Thus the repeating unit in cellulose is the

anhydrocellulobiose unit and the number of repeat units per molecule is half the

DP. This may be as high as 14000 in native cellulose, but purification procedures

usually reduces it to something in the order of 2500 57. Depending on the type of

natural fibres the length of the polymer chain varies and is clear from the DP

(Cotton-7000, Flax-8000, and Rame-6500).

Fig.1.8- Structure of a cellulose molecule.

(Ref.- T.P.Nevell, S.H. Zeronian, Cellulose Chemistry and its Applications, Wiley,

New York, 1985.)

Short Sisal Fibre Reinforced Polystyrene Composites 26

The type of cellulose present in the fibre also affects the mechanical properties of

the fibre as each cellulose has its own cell geometry. Solid cellulose forms a

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

regions of low order i.e amorphous regions. Naturally occurring cellulose

(cellulose I) crystallizes in monoclinic sphenodic structures. The molecular chains

are oriented in the fibre direction. The geometry of the elementary cell (Fig1.9)

depends on the type of cellulose.

Fig.1.9 – Lattice structure of elementary cells in cellulose (Ref..- J.Warwicker, J.Appl.Polym. Sci., 41,1,1969.).

(b) Hemicellulose

Hemicellulose comprises a group of polysaccharides (excluding pectin) that

remains associated with the cellulose after lignin has been removed. It contains

several different sugar units and exhibits a considerable degree of branching.

Unlike cellulose, the constituents of hemicellulose differ from plant to plant 57.

(c) Lignin

Lignins are complex hydrocarbon polymer with both aliphatic and aromatic

constituents57. The monomer units present in lignin are various ring substituted

Introduction 27

phenyl propanes linked together in ways which are still not fully understood and

the detailed structure differ from source to source. The mechanical properties are

lower than those of cellulose.

(d) Pectin

Pectin is a collective name for hetro polysaccharides which consists essentially of

polygalacturon acid and is soluble in water only after partial neutralization with

alkali or ammonium hydroxide 58.

(e) Waxes

Waxes make up the part of the fibres, which can be extracted with organic solvents

and consist of different types of alcohols which are insoluble in water as well as in

acids58.

1.7.3 Physical structure of natural and man made cellulose fibres

(a) Natural fibres

A single fibre of all natural fibres is made up of several cells which are formed out

of crystalline microfibrils based on cellulose and are connected to a layer by

amorphous lignin and hemicellulose. Multiple of such cells in one primary and

three secondary cell walls stick together to a multiple-layer- composites, the cell,

as given in Fig 1.10. These cell walls differ in their composition and in the

orientation (spiral angle) (Table1.4,1.5) of the cellulose microfibrils. The

characteristic values for these structural parameters vary from one natural fibre to

another as well as by physical and chemical fibre treatments such as mercerization

or acetylation.

Short Sisal Fibre Reinforced Polystyrene Composites 28

Lumen

Secondary walls (fibrils of cellulose in a lignin/ hemicellulose matrix

Primary wall (fibrils of cellulose in a ignin/hemicellulose matrix

Fig.1.10 -Constitution of a natural fibre cell schematic representation (Ref. M.K.Sridhar and G.Basavarajappa, Indian J.Text. Res. 7(9), 87,1982.)

Table 1.5 Structural parameters of different cellulose based fibres

Ref..- P.S.Mukhergee and K.G.Satyanarayana, J.Mater.Sci., 21,51,1986.).

Fibre Cellulose

content

(%)

Spiral

angle (0)

Cross

sectional

area x 10-2

mm2

Cell

length

(mm)

Aspect

ratio

(l/d)

Jute 61 8.0 0.12 2.3 110

Flax 71 10,0 0.12 20.0 1687

Hemp 78 6.2 0.06 23.0 960

Ramie 83 7.5 0.03 154.0 3500

Sisal 67 20.0 1.10 2.2 100

Coir 43 45.0 1.20 3.3 35

Introduction 29

The spiral angle of the fibrils and the content of the cellulose fibres are the factors

controlling the mechanical properties of the cellulose based natural fibres.

However, the strength of the natural fibre show only little dependency on structural

arrangements like cellulose content and spiral angle. Fibre strength is rather

affected by their defects. The model for the description of the stiffness of the

cellulose fibre developed by Hearle et al.59 is shown in Fig 1.11.

Crystallineregion

Non-crystallineregion

Fig.1.11-Model for the description of the stiffness of the fibre (a) layers in a 3D view, (b) layers in a 2D view (Ref.- J.W.S Hearle and J.T.Sparrow, J.Appl.Polym. Sci., 24,1857,1979.).

(b) Man made cellulose fibres

The mechanical properties of man-made cellulose depends on their structure on

different levels like (a) degree of polymerisation (DP) (b) crystal structure like type

of cellulose and defects (c) super molecular structure like degree of crystallinity (d)

Short Sisal Fibre Reinforced Polystyrene Composites 30

orientation of chains (non crystalline and crystalline regions) (e) void – structure

(void content, specific interface, void size) and (f) fibre diameter.

Generally, the tensile strength of these fibres is strongly influenced by the length of

the molecule as shown for viscose and acetate type fibres. A linear correlation with

a negative slope between the strength and 1/DP may be modified by orientation

effect, by variations of crystalline dimensions and crystallinity, by impurities and

probably by pores and non-uniform cross section of the fibres 55.

1.7.4 Surface characteristics

The properties of natural fibres and wood based composites are strongly influenced

by the surface properties of these fibres. The natural fibre or wood surface is a

complex heterogeneous polymer composed of cellulose, hemicelluloses and lignin.

The surface is influenced by polymer morphology, extractive chemicals and

processing conditions. Toussaint and Luner60 reported a rapid decrease of the

contact angle of water with the time for cellulose films reacted with alkyl ketone

dimer, while other test liquids such as glycerol, ethylene glycol and diidomethne a

constant contact angle was obtained after 2-5 minutes. The observed behaviour can

be attributed to the specific interaction between the cellulose surface and water

allowing water to penetrate into the cellulose causing the cellulose to swell thus

lowering the interfacial free energy and decreasing the contact angle. Lee and

Luner 61 also shows a decrease in contact angles of water for different kind of

(wood) lignin. A similar behaviour was observed for glycerol and formamide in

contact with lignin as well as cellulose, however, with only a slight decrease in

Introduction 31

contact angle. The use of different kinds of physical (corona discharge) and

chemical surface treatments (coupling agents such as silanes) leads to changes in

the surface structure of the fibres as well as changes in the surface energy.

1.7.5 Surface modification of natural fibres

As discussed earlier, the quality of the fibre-matrix interface is significant for the

application of natural fibres as reinforcement in plastics. Interface can be modified

by different physical and chemical methods and the efficiency of these treatments

can vary depending on the treatment 62.

(a) Physical methods

Reinforcing fibres can be modified by physical and chemical methods. Physical

methods such as stretching63, calendaring64,65, thermotreatment66 and the

production of hybrid yarns67-68 do not change the chemical composition of the

fibres. Physical treatments change the structural and surface properties of the fibre

and thereby influence the mechanical bonding to polymers. Electric discharge

(corona, cold plasma) is another way of physical treatment. Corona treatment

activates surface oxidation and changes the surface energy of cellulose fibres69 and

increases the amount of aldehyde groups in wood surfaces 70. The mechanism of

improved fibre –matrix adhesion by plasma treatment also follows the same

mechanism and a verity of surface modifications can be achieved by suitably

selecting gases used for treatments and surface crosslinkings could be introduced,

surface energy could be increased or decreased, reactive free radicals 69 and groups

71 could be produced by plasma treatment. Electric discharge methods72 are known

Short Sisal Fibre Reinforced Polystyrene Composites 32

to be very effective for ‘non-active’ polymer substrates as polystyrerne,

polyethylene, polypropylene etc.

One of the earliest methods of cellulose fibre modification is mercirization57,63,72-74.

In this method the cellulose fibre is treated with alkali and the effect of alkali

treatment depends on the type and concentration of alkaline solution, its

temperature, time of treatment, tension of the material and on the additives 57,74.

Ray and Sarkar75 recently studied the effect of alkali treatment in jute fibres from

the weight loss, linear density, tenacity, modulus, FTIR and X-ray measurements.

They observed an improvement in tenacity and modulus of the fibre and a

reduction in the breaking strain after 8hr treatment. X-ray diffractograms showed

increase in crystallinity of the fibre only after 6 h treatment.

(b) Chemical methods

Cellulosic fibres being polar are inherently incompatable with hydrophobic

polymers76-78. Incorporating a third material that has properties intermediate

between those of the other two can reduce the incompatibility between two

materials. There are several mechanisms for the coupling in materials79. (a)

elimination of weak boundary layers (b) producing a tough flexible layer at the

interface, (c) by developing a highly cross linked interphase region with a modulus

intermediate between those of substrate and the polymer (d) by improving the

wetting between polymer and substrate (critical surface tension factor) (e) by

forming covalent bonds with both materials and (f) by changing the acidity of

substrate surface.

Introduction 33

(c) Impregnation of fibres

Fibre –matrix interaction can be improved by impregnation of the reinforcing fibre

with polymer matrixes that are compatible to the polymer. For this purpose

polymer solutions77,80 or dispersions 81 of low viscosity are used. Lack of good

solvents for a number of polymers is one of the major limitations of this method.

(d) Change of surface tension

The surface energy of fibres is related to the hydrophilicity of fibres82 and several

methods are reported in literature to decrease the hydrophilicity of the fibres. The

modification of wood-cellulose fibres with stearic acid83 decreases the

hydrophilicity and improves their dispersion in polypropylene. Treatment with

polyvinyl acetate65 and silanes are some other methods used to improve the fibre–

matrix adhesion by changing the surface tension of fibres.

(e) Chemical coupling

Chemical coupling is one of the important techniques used to improve the fibre-

matrix adhesion and in this method the fibre surface is treated with a compound

that forms a bridge of chemical bonds between the fibre and matrix.

(f) Graft copolymerization

Graft copolymerization initiated by the free radicals of the cellulose

molecules72,73,84 is an effective method of chemical modification of natural fibres.

The fibre is treated with an aqueous solution with selected ions and is exposed to

high energy radiation when the cellulose molecules crack and radical cites are

formed along the cellulose backbone. These sites when allowed to react with a

solution of the monomer compatible with the matrix, generates graft copolymers of

Short Sisal Fibre Reinforced Polystyrene Composites 34

the cellulose and the monomer. The resulting copolymer possesses properties

characteristic of both the fibrous cellulose and grafted polymer and the surface

energy of the fibre is increased to a level much closer to the surface energy of the

matrix which produces a better wettability and a higher interfacial adhesion in the

composite.

(g) Treatment with compounds containing methoxy groups

Chemical compounds which contain reactive methoxy group form stable covalent

as well as hydrogen bonds with cellulose fibres. The treatment of cellulose with

methanolamine compounds decreases the moisture up take and increases the wet

strength of reinforced plastics 85,86.

(h) Treatment with isocyanates

Improvement in the properties of natural fibre reinforced composites on treatment

with isocyanates was reported by various researchers87-90. Poly methylene-

polyphenyl isocyanate (PMPPIC) in the pure state or in solution in the plasticizer

can be used. PMPPIC is chemically bonded to the cellulose matrix through

covalent linkages.

R-N=C=O + H-O-Cell R-HN-C-O-Cell

O

In the case PMPPIC treated fibre –polystyrene composites both material contain

benzene ring and their delocalised π electrons provide strong interactions so that

there is an adhesion between PS and fibre. A hypothetical model of the interface

between the PMPPIC treated cellulose fibre and PS matrix is shown in Fig.1.12.

Introduction 35

OH

PS Matrix

PMPPIC treated fibre

C=O

NH

O

C=O

NH

O

C=O

NH

O

CH2

C=O

NH

O

CH2CH2

Fig.1.12 - A hypothetical model of the interface between PS matrix and PMPPIC treated cellulose fibre (Ref. D.Maldas, B.V.Kokta, C.J.Daneault, J. Appl. Polym. Sci, 37,751, 1989 . 87,1982. )

(i) Treatment with triazine coupling agents

N

N

N

Cl

Cl Cl

RNH2N

N

N

HN

Cl Cl

R

+ FIBER

FIBER

O

N

N

N

Cl

NH

R

OH OH

Fig.1.13 - A possible reaction between trazine coupling agent and cellulose fibre (Ref. P. Zadorecki and T.Ronnhult, J.Polym Sci., Part A, Polym. Chem. 24, 737,1986.)

Short Sisal Fibre Reinforced Polystyrene Composites 36

Triazine derivatives form covalent bonds with cellulose fibres as shown in

Fig.1.13. The observed moisture absorption of triazine treated cellulose fibre and

the composites are explained by (a) reduction in the number of cellulose hyroxyl

groups (b) reduction in the hydrophilicity of fibre surface and (c) reduction in the

swelling of the fibre due to the formation of cross linked network by covalent

bonding between the fibre and matrix91-92.

(j) Organo silanes as coupling agents

Organo silanes are the main group of coupling agents for glass- fibre reinforced

polymers and can be used as coupling agents with any polymer to the minerals

used in reinforced composites79,93.

Silane coupling agents can be represented by the general formula,

R1-(CH2)n –Si (OR2)3

The organo functional group R1 in the coupling agent causes the reaction with the

polymer and may be co-polymerization and /or the formation of interpenetrating

net work. The curing reaction of a silane treated substrate enhances the wetting by

the resin. The mechanism of reaction between the fibre and the silanes and the

formation of bond between silane treated fibre and polymer matrix is shown in

Fig.1.14.

Alkoxy silanes undergo hydrolysis, condensation and the bond formation takes

place by a base or acid catalysed mechanism. In addition to this reaction formation

of polysiloxane structure can also take place.

Introduction 37

O

OH

O

O

Si

R1

HO

OH

HO

OO

Si

R1

HO

HO

H

H

Silane treated fiber

H

R1Si (OR2)3 + 3H2O R1Si (OH)3

Fibre

+

Fig.1.14 - A possible reaction between silane coupling agent and cellulose fibre (Ref. P. K. Mallick, Fibre Reinforced Composites: Materials, Manufacturing and Design Marcel Dekker, Inc, New York, 1988.)

Contradictory to the glass fibre reinforced polymer composites, in the case of

unsaturated polyester composites reinforced with dichloro methyl vinyl silane

treated coir fibre94 a decrease in mechanical properties was observed. The

treatment of alkali treated sisal fibre with aminosilanes95, however, markedly

improves the moisture repellency of the composites.

1.8 Theory of reinforcement of short fibre reinforced composites

In the case of an elastic matrix, reinforced with uniaxially-oriented continuous

elastic fibre, the mechanical properties in the direction of the fibres are given by

the rule of mixtures96 as given in equations 1.3 and 1.4 below.

Ec = EfVf + EmVm (1.3)

Short Sisal Fibre Reinforced Polystyrene Composites 38

σcu = σfu Vf+ σmVm (1.4)

σ m = Em εuc (1.5)

Equations 1.3 and 1.4 may be written as

Ec = EfVf + Em (1-Vf ) ……… (1.6)

σcu = σfu Vf + Em εuc(1-Vf ) … (1.7)

In the case of misaligned fibres, the fibre contribution in equations 1.6 and 1.7 will

be reduced and the equations can be modified by incorporating an orientation

factor K, whose values lies in between 1 and 0.167.

Ec = KEfVf + Em (1-Vf ) …… (1.8)

σcu = Kσfu Vf+ Em εuc(1-Vf )… (1.9)

When the fibres are discontinuous, the fibre may carry the stress only by a shear

transfer process at the interface. Kelly and Tyson97 have proposed a model where,

there is a linear transfer of stress from the tip of the fibre to a maximum value

when the strain in the fibre is equal to that in the matrix. Equation1.10 relates the

maximum stress in the fibre to the fibre radius and the shear strength of the fibre-

matrix interface.

Introduction 39

)10.1.....(....................if

rLτσ

=

This equations suggest the existence of a critical fibre length Lc, which is the

length required for the maximum stress in the fibre to reach a fibre fracture stress

σuf . This may be written in the form of equation 1.11.

)11.1...(..........i

ucf

i

fuc

rErL

ττ

σ ∈==

When the fibre length is less than critical fibre length, the average strength in the

fibre at composite failure is given by the equation 1.12 and this is half the

maximum stress in the fibre.

)12.1......(..............................2rL

fτσ =

−

When the fibre length is greater than the critical fibre length, the average stress in

the fibre is given by equation 1.13.

)13.1.().........2

1(i

cfcff

LrE

Eτ

σ∈

−∈=−

Bowyer and Bader98 developed their model based on that at any value of composite

strain there is a critical fibre length Lc and fibre shorter than this will carry an

average stress as given in equation 1.12 and will be always lower than cfE ∈21 .

Fibres longer than this will carry an average stress as indicated by equation 1.13

and will be always greater than cfE ∈21 and Lc will be given by the equation 1.14.

Short Sisal Fibre Reinforced Polystyrene Composites 40

)14.1......(....................i

cfc

rEL

τ∈

=

In the case of misoriented fibres a correction factor as given in equation 1.8 and 1.9

must be used.

Short fibre reinforced composites usually contain a spectrum of fibres of different

lengths and at low strain all fibres will make a contribution to the reinforcement as

given in equation 1.13, since Lε is small. As the strain is increased progressively,

smaller proportion of the fibres will follow the equation 1.13 and an increasing

proportion will follow the equation1.12. The slopes of the load extension curve for

such a material are expected to decrease as the extension is increased. A

mathematical model of this behaviour may be constructed by a combination of the

concepts of equations 1.9,1.12,1.13 and 1.14.

This equation can be written as

σc = CX + CY + Z (1.15)

The first term is the contribution of the sub-critical fibres, the second term that of

the super-critical fibres, and the third term that of the matrix. Equations 1.16,1.17

and 1.18 gives the values of X, Y and Z respectively.

)16.1.........(........................................2r

vLX iii

LLi τ∈⟨

Σ=

)17.1.......(..........).........2

1(i

cfcf

LL

j

i

LrE

EYτ∈

−∈Σ=∈⟩

)18.1....(..............................).........1( fcm VEZ −∈=

Introduction 41

The spectrum of fibre lengths in the composites are considered to be divided into

subfractions with subcritical fractions denoted by Li with volume fractions Vi and

supercritical fractions denoted by Lj with volume fractions Vj. The strength of the

composite is then given by combining the above three terms as follows

( ) )19.1..(..........12

12 fcmj

ij

cfcf

iiic VEV

LrE

ErVL

C −∈+⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ ∈−∈Σ+Σ=

ττ

σ

The values of Ef , Em and r can be measured and the relation between σc and εc can

be obtained from a tensile test. The values of C and τi are generally not known, but

the fibre length distribution can be determined. The values of C and τ can be

determined from the tensile test data based on the assumption that the orientation

factor C is independent of the strain and is the same for all fibre lengths at least at

small strains. Bowyer and Bader determined the value of C and τ using a computer

programme.

1.9 Micromechanics of deformation for discontinuous fibre composites

1.9.1 The Cox model

Many models have been proposed to describe the micro mechanics of fibre

reinforcement. Hollister and Thomas99, Asloun et al.100 and Chow101 have made

comprehensive reviews of these models. The simplest model of the

micromechanics of short fibre reinforced composite is that developed by Cox 102

and based on shear- lag mechanism observed in these composites. According to

Cox, in shear lag analysis, the main aspects of controlling the properties of the

composite are critical fibre length of the fibre and fibre- matrix interfacial shear

Short Sisal Fibre Reinforced Polystyrene Composites 42

strength. The shear lag theory provides solution to the tensile and shear stress

distribution along the length of a fibre embedded in a matrix, provided some

simplifications are made. These assumptions are;

a) the fibre and the matrix remain elastic in their mechanical response

b) the interface between the fibre and matrix is perfect

c) no axial load is transmitted through the fibre ends.

Based on these assumptions, the applied stress placed on the matrix parallel to the

fibre long axis results in a tensile stress distribution along the fibre length given by

)20.1.....(....................

21cosh

21cosh

1

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛ −

−∈=l

xlE mff

β

βσ

This can be rearranged in terms of fibre strain to give

)21.1.....(....................

21cosh

21cosh

1

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛ −

−=∈∈l

xl

mf

β

β

The shear stress distribution along the fibre length is given by

( ))22.1......(..........

21cosh

21sinh

ln12

2/1

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛ −

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

⎟⎠⎞

⎜⎝⎛+

∈=

l

xl

rRvE

EE

mf

mmff

β

βτ

The value of β is given by

( ))23.1.......(....................

)ln(12

2/1

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

+=

rRvE

Ed

mf

mβ

Introduction 43

In the case of a square fibre packed system, the inter-fibre spacing can be related to

the volume fraction of the fibre by

)24.1...(....................4

2/1

⎟⎟⎠

⎞⎜⎜⎝

⎛=

fVrR π

and hence β can be rewritten as

( ))25.1.......(....................

4ln1

2

2/1

2/1

⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛+

=

fmf

m

VvE

Ed π

β

The process of load transfer between the matrix and fibres occurs by shear about

the fibre ends. About each fibre end, the interfacial shear stress is at a maximum,

which rapidly decays to zero about the fibre centre. The tensile stress is zero about

the fibre ends and rapidly reaches a maximum value about the fibre centre, where

the fibre strain matches to that applied to the matrix.

In short fibre reinforced composites, there are regions about both fibre ends that do

not reach the same strain as that applied to the matrix so that the reinforcing

efficiency is less than that achieved for a continuous fibre composite. The term

‘critical fibre length’ is frequently used to describe the region about both fibre

ends, where the process of shear stress transfer occurs and the tensile stress in the

fibre builds up to reach a maximum value. The exact value of the critical fibre

length, Lc will depend on the definitions used. For the purpose of measurement the

critical fibre length, Lc, has been defined as the length of the fibre required to reach

0.9 of the maximum fibre strain, about each fibre end 103 in a long fibre. Galiotis et

Short Sisal Fibre Reinforced Polystyrene Composites 44

al.106 and other researchers99,105 shows that equation 1.21 can be reduced provided

the fibre modulus is greater than the matrix modulus and the fibre length is

reasonably long .

( ) ( )[ ]{ } )26.1.....(....................1expexp1 xxmf −−−−−=∈∈ ββ

The ratio of / reaches the value of 0.9 at a certain positions of x along the

fibre length. Consequently equation 1.26 can be solved for x to obtain the critical

fibre length. Thus

f∈ m∈

f∈ / = 0.9 at the positions for x when x = 2.303/ β. Direct

substitution of these values of x reduces 1- exp(-βx)to the value of 0.9.

m∈

Consequently, the critical fibre length, Lc can be directly determined from the

definition of β such that Lc = 2x.

( ))27.1........(....................

4ln

1303.2

2/12/12/1

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡ +=

fm

mfc VE

vEdL π

Equation 1.27 can be simplified to

)28.1........(....................1ln

2/12/12/1

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡=

fm

fdc VE

EKL

The critical fibre length can be seen to depend upon the fibre diameter, the square

root of the ratio of the tensile moduli and on a complicated term on the inverse of

the fibre volume fraction. The term Kd represents a constant, involving the

collection of all the constant terms in equation 1.25 multiplied together.

There are several other analytical models that give similar expressions for critical

fibre length. For the model proposed by Rosen99,106 by taking the same definitions

Introduction 45

of critical fibre length ( lc =2x where x gives the result f∈ / m∈ = 0.9 ), the critical

fibre length can be given by

( ))29.1........(....................

112/1

2/1

2/12/1

⎥⎥⎦

⎤

⎢⎢⎣

⎡ −⎥⎦

⎤⎢⎣

⎡ +=

f

f

m

mfc V

VE

vEdL

This equation also shows that the critical fibre length depends upon the fibre

diameter, the square root of the ratio of the tensile moduli and on a complicated

term for the fibre volume fraction.

A finite difference analysis solution to this problem by Termonia107, however,

gives an expression that showed a different dependency on the ratio of the tensile

moduli for the fibre and matrix.

)30.1...(..............................m

fdc E

EKL =

1.9.2 Stiffness of discontinuous fibre composites

Fibres with length less than the critical fibre length in SFRC do not carry the

maximum tensile stress possible due to the lack of strains match in both the phases

of the composite (about the centre and of the fibre and the matrix). Fibres having

lengths much higher than the critical fibre length contribute to the stiffness of the

composite essentially in the same way as continuous fibres. The relative efficiency

in stiffening the composite depends on the lengths of the fibre present in the

composites and a continuous fibre composite where all the fibre length is available

to carry the maximum tensile stress is considered as 100% reinforcing efficient.

For fibre lengths between very small (fibre lengths in the order of fibre diameter)

and continuous the reinforcing efficiency lies between zero and 100%.

Short Sisal Fibre Reinforced Polystyrene Composites 46

The critical fibre length will depend upon the fibre /matrix parameters and the rule

of mixtures to the stiffness (equation 1.27) can be modified by incorporating a

correction factor to account for the lengths of the fibres not fully contributing to

the stiffness of the composites due to the shear stress transfer between each fibre

and the matrix. This term ηl is defined as

)31.1..(..............................

21

21tanh

1⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

−=l

ll

β

βη

The term β is given by equation 1.23 and the stiffness of the composite is given

by the equation

Ec = lη EfVf + Em (1-Vf ) (1.32)

For fibre lengths that are very long, the term lη tends towards unity and the

equation reduces to the simple rule of mixtures for continuous fibre composites. In

the case of very small fibre length the model breaks down and the situations

becomes that of particulate reinforcement. In this case, this equation cannot be

applied and within the two limits of fibre length described the Cox equation shows

agreement with real values. Cox’s shear lag analysis, however, has two major

disadvantages. The first one is that stress amplification effects at the fibre ends are

not taken into account, and the second is that the matrix tensile stress possesses no

radial dependence.

Robinson and Robinson108 discuss the application of equation 1.34 for a number

of unidirectional discontinuous fibre composites and came to the following

conclusions. For a given fibre diameter and fibre length the stiffest composite

Introduction 47

occurs with the highest matrix modulus value. As the fibre length increases, the

predicted modulus rises until reaching a plateau value, which is effectively the

continuous fibre composite value with 100% reinforcing efficiency. Comparison of

the predicted composite modulus values at different fibre diameters shows that at

smaller fibre lengths (L<0.5mm) the smaller fibre diameter composites produces

the stiffest predicted modulus at identical fibre lengths. At higher fibre lengths (L

>1mm) also the smaller fibre diameter produces the stiffest composite reaching a

plateau region earlier than for larger fibre diameter and this effect becomes

particularly important for composites with lower matrix modulus values.

Considering the dependence of reinforcement efficiency of a discontinuous fibre

composite upon the fibre length, fibre diameter, and the matrix modulus it is

possible to calculate the mean fibre lengths and diameters required for a high

performing composite material for a given choice of matrix.

1.9.3 Effect of fibre length and fibre orientation distribution on the

tensile strength of SFRP

Fu and Lauke109 developed an analytical method to study the effect of fibre length

and fibre orientation distribution on the tensile strength of SFRP. The mechanical

properties of the injection moulded SFRP depends on the fibre length distribution

(FLD) and the fibre orientation distribution (FOD)110-115. The fibre length

distribution can be described with a probability density function, f(L), so that

f(L)dL and F(L) are the probability density that the length of the fibre is between

L and L+dL and the length of the fibre is less than or equal to L, respectively. Then

the relationship of f(L) and F(L) is

Short Sisal Fibre Reinforced Polystyrene Composites 48

∫=L

dxxfLF0

)()( and ∫∝

=0

)33.1.....(..........1)( dxxf

Tung distribution116 given by the equation 1.34 can be used to describe the fibre

length distribution in SFRP.

)exp()( 1 bb aLabLLf −= − for L>0 (1.34)

Here, a and b are scale and shape parameters respectively. Combination of

equation 1.33 and 1.34 gives the cumulative distribution function, F(L), as

) for L >0 (1.35) exp(1)( baLLF −−=

From eqn.1.34 the mean fibre length can be deduced as

∫∝

− +Γ==0

/1 )36.1.........().........1/1()( badLLLfL bmean

where (x) is the gamma function and the most probable length (mode length)

L

Γ

mode is obtained as

[ ] )37.1(............................../1/1 /1mod

be abaL −=

The fibre orientation distribution function (g (θ)) is given by

[ ] [ ]

[ ] [ ])38.1....(....................

cossin

cossin)(1212

1212

max

min

θθθ

θθθ θ

θ

dg

qp

qp

−−

−−

∫=

where, θ is the fibre orientation angle and p and q are the shape parameters which

can be used to determine the shape of the distribution curve. Also, p ≥1/2, q ≥1/2

and 0≤ θmin ≤ θ ≤ θmax ≤ π/2 . The cumulative distribution function of fibre

orientation is given by

Introduction 49

[ ] [ ]

[ ] [ ])39.1....(....................

cossin

cossin)()(

min

max

min

min

1212

1212

∫∫

∫

−−

−−

==θ

ϑθ

θ

θ

θ

θθθ

θθθ

θθθd

ddgG

qp

qp

When a fibre crosses the crack plane and the fibre orients parallel to the normal of

the crack plane or the applied load (Fig.1.15a) the bridging stress, σf of the fibre

across the crack is given by

fif rLs /2 τσ = for Ls < Lc/2 ……………..(1.40)

Fig-1.15-Schematic drawing of a fibre across a crack: (a) the fibre which orients

parallel to the crack plane normal; (b) the fibre which crosses obliquely with the

crack plane (Ref.- S.Y.Fu, B. Lauke, Comp. Sci. Technol., 56, 1179, 1996.)

(b) F

F(a)

F

F

and fuf σσ = for Ls ≥ Lc/2 ……………………(1.41)

where, Ls is the length of the shorter fibre segment and iτ denotes the interfacial

shear strength which is assumed to be constant.

Short Sisal Fibre Reinforced Polystyrene Composites 50

When the fibre crosses obliquely with a crack plane (Fig1.15b), the bridging stress,

σfθ is given by 117-121

)exp()/(2 μθτσ θ fisf rL= for Ls < Lcθ/2 ………(1.42)

where, θ is the angle between the fibre and the crack plane normal and μ is the

snubbing friction coefficient between the fibre and the matrix at the crossing point

and Lcθ denotes the critical fibre length for an obliquely crossed fibre.

When Ls ≥ Lcθ/2 the bridging stress of the oblique fibre is given by 122-123

θθ σσ fuf = for Ls ≥ Lcθ/2 ………………(1.43)

where, fuθ denotes the fracture stress of the oblique fibres

In the case of brittle fibres θσ fu is given by 122-123

[ ] )44.1..(........................................tan1 θσσ θ Afufu −=

where, σfu denotes the ultimate strength of the fibre and A is a constant for a

specific fibre /matrix system. Considering the snubbing friction effect and the fibre

flexural effect the critical length of oblique fibres derived from eqns. 1.42-1.44 as

[ ] )45.1.....(....................).........exp(/tan1 μθθθ ALL cc −=

If the effect of snubbing friction is neglected , i.e μ = 0, the equn. 1.45 becomes

[ ] )46.1....(....................tan1 θθ ALL cc −=

When the effect of fibre flexural is neglected, i.e A = 0, then equn.1.45 becomes

)47.1......(....................).........exp(/ μθθ cc LL =

and if both the snubbing friction effect and the fibre flexural effect are neglected

i.e. μ = 0 and A = 0 the equn. 1.45 becomes

Introduction 51

)48.1......(..............................cc LL =θ

In the case of perfectly aligned fibre composites (Fig. 1.16a) the average bridging

stress of fibres across the crack is given by the equns. 1.49-1.50.

θ

F

F(a)

(b)

F

F

Fig-1.16-Schematic drawing of a crack which has developed across fibres : (a)

the fibres aligned in the direction perpendicular to the cross section; (b) the

fibres crossed obliquely with the cross section. (Ref.- S.Y.Fu, B. Lauke, Comp.

Sci. Technol., 56, 1179, 1996.)

When the fibre length L is less than Lc

)49.1.........().........2/( cfuf LLσσ =

where,

ficfu rL /τσ =

When the fibre length, L is greater than Lc the average bridging stress is given by

)2/1( LLcfuf −= σσ ……………..(1.50)

Short Sisal Fibre Reinforced Polystyrene Composites 52

In the case of composites with fibres oriented obliquely at an angle θ with the

crack plane (Fig.1.16b), the average bridging stress of fibres crossing the crack is

given by

cfuf LL 2/)exp(μθσσ θ = for L< Lcθ ……………….(1.51) and

)2/1( LLcfuf θθθ σσ −= for L ≥ Lcθ. ……………… ..(1.52)

The strength of the composite is contributed by all the fibres of length from Lmin to

Lmax and the orientation angle from θmin to θmax. and the matrix and is given by

)53.1....(..............................max

min

max

∑ ∑= =

+=θ

θθθ σσσ

L

LLmmficu

miin

VV

where, Vi is the volume of subfraction of fibres of the fibres of length from L to

L+dL and an orientation angle from θ to θ + dθ given by the equn.

( ) ( )[ ] )54.1(............................../ meanifi NLLNVV =

where, Vf = volume fraction of fibre in composite, N = total number of fibres in

the composite, and Ni = number of fibres with length from L to L+dL and

orientation angle from θ to θ + dθ .

and )(Lf )(θg , the functions of the fibre length distribution and the fibre

orientation distribution respectively are related to the number of fibres by the equn.

)55.1......(`..............................)()(/ θθ dLdgLfNNi =

Combining these equns we get

)56.1........(..........)/)(()(max

min

max

min

mmfmean

L

Lfcu VdLdLLgLfV σθσθσ θ

θ

θ

+= ∫ ∫

Substituting equns. 1.44,1.45,1.51, and 1.52 in equn. 1.56 we get the strength of

SFRP:

Introduction 53

)57.1.....(..........21 mmfufcu VV σσχχσ +=

where,

)58.1........(

))exp(2/()tan1(1)(tan1)(/)(()(

)exp())2/()(/)(()(

max

min

max

min

max

min

max

min

21

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

−−−

+

=

∫ ∫

∫ ∫

θμθθθθ

θμθθ

χχθ

θ

θ

θ

dLdLALALLgLf

dLdxLLLLgLf

cmean

L

L

cmean

L

L

The larger the value of 21χχ , the higher is the strength of the composite. When θ =

0, 1χ is equal to 1 and is the case of unidirectionally aligned short fibre

composites. The fibre length factor in this case is given by

[ ] [ ] )59.1().........()()2/(1)/()()()2/(min

22 LdLfLLLLLdLfLLL

xma

c

c L

Lcmean

L

Lmeanc ∫∫ −+=χ

Fu et al.109 discussed in detail the use of these equations to predict the properties

of SFRP and compare the theoretical values with experimental results of a number

of SFRP and came to the following conclusions. The strength of the SFRP

increases rapidly with the increase of mean fibre length at small mean fibre lengths

(in the vicinity of critical fibre length, Lc) and approaches a plateau level as the

mean fibre length increases to mean fibre length >5Lc. The composite strength

increases with the decrease of critical fibre length and hence with the increase of

interfacial adhesion strength and slightly with the decrease of the mode fibre

length. When the ratios of Lmean/Lc and Lmod/Lc are the same for a given fibre–

matrix system, the strength of SFRP will be the same no matter how large Lmean,

Lmod and Lc are. In general the strength of the composites increases with increase

of fibre orientation coefficient, )(θf , and the decrease of mean fibre orientation

Short Sisal Fibre Reinforced Polystyrene Composites 54

angle. However, when the fibre orientation coefficients are the same, the strength

of the composite increases with the increase of mean fibre orientation angle for

0)( >θf and increases with the decrease of mean fibre length for 0)( <θf . No

direct relationship was found between the strength of SFRP and the most probable

fibre orientation angle. The effect of the snubbing friction between the fibre and

the strength of the SFRP is small. The inclined tensile strength of fibres has a

significant effect on the tensile strength of composites.

1.10 Natural fibre reinforced composites

The use of natural fibres as reinforcement is considerably growing in recent years

due to their biodegradation124 and relatively high strength and stiffness along with

low density.

1.10.1 Mechanical behaviour of natural fibre reinforced plastics

(a) Creep and stress relaxation behaviour

There are only few reports available in literature on the creep behaviour of

natural fibre reinforced plastics. Tobias125 studied the influence of fibre content on

the stress rupture life of short abaca fibre reinforced thermoset composites

containing 30 and 40% fibre and showed that both composites fail by rupture. An

increase in sustained constant stress reduces the life of the abaca –fibre reinforced

composite materials. However, in composite materials, under sustained stress there

is an increase in rupture strength and a decrease in rate of degradation with

increase of fibre content. Varghese et al.126 reported the stress relaxation behaviour

of short sisal fibre reinforced natural rubber composites with and without coupling

agents. In the absence of coupling agents, the rate of relaxation of these composites

Introduction 55

increased with strain level. In the presence of coupling agents the rate of relaxation

remains almost unchanged with strain level and can be attributed to the strong

fibre–rubber interface.

(b) Cyclic dynamic behaviour

Gassan et al.52, 127,128 reported the cyclic dynamic material behaviour of natural

fibre reinforced thermosets and thermoplastics. Improved fibre-matrix adhesion

due to a coupling agent such as MAH grafted PP leads to a higher dynamic

strength. In the case of unmodified jute- polypropylene composites, the progress of

damage is nearly independent of fibre content. In the case of MAH grafted

polypropylene modified jute fibre reinforced PP composites a considerable

increase in dynamic mechanical strength is observed. The results of the

investigation suggested that the damage of the jute –polypropylene composites of

both modified and un modified fibre does not occur spontaneously but occurs

continuously with the increasing stress.

(c) Impact behaviour

There are only a few studies on the impact behaviour of natural fibre reinforced

plastics compared with glass fibre reinforced plastics. Pavithran et.al129 determined

the fracture energies for sisal, pineapple, banana and coconut fibre polyester

composites by Charpy method. They found that, except for coconut fibre-polyester

composites, the fracture energy increases with increase in fibre toughness. Natural

fibre reinforced plastics with fibres which has a high spiral angle of the fibrils

indicated a higher composite- fracture –toughness rather than those with small

fibril angles and accordingly sisal fibre with spiral angle 250 show the optimum

Short Sisal Fibre Reinforced Polystyrene Composites 56

impact properties. The specific toughness of sisal-UHMPE composites shows

approximately 25% lower values than glass fibre reinforced composites containing

the same fibre content. Tobias130 examined the influence of fibre content and fibre

length for banana-epoxy composites and an increase in impact strength was

reported with increasing fibre content (fibre length –20mm). Smaller fibre lengths

leads to a higher impact strength at constant fibre content in contradiction with the

increase of impact strength of glass fibre reinforced polypropylene composites

with increase of fibre length. The studies of impact strength of jute-polypropylene

composites with and without coupling agent (MAH grafted PP) shows that the

damaging initiation is shifted to higher forces with strong fibre-matrix adhesion at

a load perpendicular to the fibre. The dissipation factor of the modified jute-PP

composite is smaller than that of unmodified jute fibre-polypropylene composites.

When the composites have no coupling agents a part of the impact energy is

degraded in the fibre- matrix interface by debonding and friction effects. At these

conditions a multiple–impact load leads to a decrease in the loss of energy until a

more or less levelling off is observed. In the case of unmodified fibre composites

this situation of composite damage is less pronounced.

1.10.2 Environmental effects on the properties of natural fibre reinforced

composites

The application of natural fibre and natural fibre reinforced composites depends on

the environmental conditions influencing the ageing and degrading effects. Natural

fibres undergo degradation in acid and alkaline solution and also by UV radiations

and can be controlled by using suitable modifications. Unmodified cellulose fibre

Introduction 57

is normally degraded by enzymes after about 6-12 months and can be extended

for more than two years through suitable treatment.

Table 1.6 Influence of chemicals on the bending strength of

PP-sisal composites.

(Ref.-R.Seizer, SAMPE-The Materials and Process Society, Kaiserslautern,

28 March 1995.)

Chemicals/time Bending

strength (MPa)

Bending

modulus (GPa)

Reference

samples

30 1.5

NaOH/50h 24 1.1

NaOH/500h 18 1.05

HCl/ 50h 20 1.35

HCl/500h 15 1.4

Dry-stored fibres show little changes in the mechanical properties especially

strength and elongation at break for a period of 2.5 years58. Lower temperatures,

such as –700C results in lower strength and can be minimized by previous drying.

Higher temperature, such as 100-1300C leads to notable degradation in the strength

of cotton fibres after 80 days. In composites, moisture content results in lower

mechanical properties and the effect is more pronounced in the case of ocean rather

than fresh water. The low resistance of natural fibres, against environmental factors

decisively affects the mechanical properties of the composites131 (Table1.6).

Short Sisal Fibre Reinforced Polystyrene Composites 58

1.10.3 Processing methods for wood and natural fibre reinforced plastics

(a) Natural fibre reinforced thermosets

The choice of manufacturing technique for the fabrication of fibre reinforced

composites material is related to the performance, economics and application of

the materials. Other factors that decide the selection of processing techniques are

number of component required, complexity of the product, number of moulded

surfaces and type of reinforcement. The important methods currently used for the

production of polymer matrix composites can be classified as matched die mould,

contact mould, filament winding and pultrusion51. All these methods involve the

transformation of uncured or partially cured fibre reinforced thermosetting

polymers into composite parts or structures by curing the material at elevated

temperatures and pressures for a predetermined length of time. The magnitude of

these two process parameters and their duration significantly affects the

performance of the product. The cure cycle, the length of time required to properly

cure the article, depends on a number of factors including resin chemistry, catalyst

reactivity, cure temperature and the presence of inhibitors or accelerators. In

addition to this, for natural fibre reinforced composites, drying of fibre before

processing is an important factor, because water on the fibre surface acts like a

separating agent in the fibre-matrix interface. Moreover, evaporation of water

during processing leads to voids in the matrix that deteriorate the properties. In the

case of jute-epoxy composites a 10% increase of tensile strength on pre-drying of

Introduction 59

fibres (moisture content 1%) compared to minimally dried fibres (moisture content

10%) were reported 52.

The important characteristics of an ideal processing technique is

a. High productivity

b. Minimum material cost

c. Maximum geometrical flexibility

d. Maximum property flexibility

e. Minimum finishing requirements and

f. Reliable and quality manufacture

The materials usually moulded are reinforced plastics, either in the form of

separate resin and reinforcements or in the form of composites like sheet moulding

or bulk moulding prepergs. Sheet moulding compounds are thin sheets of fibres

pre- compounded with a thermoset resin and are generally used for compression

moulding process. SMCs are classified into SMC –R, SMC-CR and XMC

depending on the orientation of fibre in the compound. SMC-R contains randomly

oriented discontinuous fibres and SMC-CR contains a layer of unidirectional fibres

on top of a layer of randomly oriented discontinuous fibres. XMC contains

continuous fibres arranged in X pattern with interlaced fibre angle between 5 and

700. Additionally, it may also contain randomly oriented discontinuous fibres

interspersed with the continuous fibres. In contrast to SMC, BMC contain

randomly oriented chopped fibres with rather lower fibre content, shorter fibre

length and lower viscosity. A bulk moulding compound is prepared by mixing

chopped strands or a particulate reinforcement with a premixed resin containing

Short Sisal Fibre Reinforced Polystyrene Composites 60

fillers, thickners, catalyst and other additives. Prepergs are prepared by pre-

impregnating fibre fabrics with resin followed by partial curing. Prepergs yield

superior products having all kinds of shapes with uniform resin content and

consistent quality. Pultrusion is a continuous moulding process for producing long

straight members of constant cross sectional area and in this method continuous

strand rovings and mats are pulled from one end of the line into a resin bath that

contains the liquid resin, curing agent and other ingredients such as colorants, UV

stabilizer and fire retardant. Several layers of mats or woven rovings are added at

or near the outer surface to improve the transverse strength. The fibre –resin stream

is pulled first through a series of performers and then through a long preheated die.

The performers distribute the fibre uniformly, squeeze out the excess resin and

bring the material to its final shape. Final shaping, compaction and curing takes

place in the heated die. In filament winding process, a band of continuous resin

impregnated rovings or monofilaments is wrapped around a rotating mandrel and

cured to produce axisymmetric hollow parts. Reinforced reaction injection

moulding (RRIM) techniques also find application in the processing of composites.

In this method a rapid curing resin system, involving two components mixed

immediately before injection, is used. Fibres are either placed in the closed mould

before resin is injected or added as short chopped fibres to one of the resin

components to form a slurry before injection. The liquid mix fills the closed mould

under low pressure and polymerisation of the monomers takes place in situ. Large

and fairly complex shapes can be moulded by this technique.

Introduction 61

b) Natural fibre reinforced thermoplastics

In the case of natural fibre reinforced thermoplastic composites only those

thermoplastics whose processing temperature do not exceed 2300C, the

degradation temperature of most of the natural fibre, could be used as matrix.

Technical thermoplastics such as polyamides polyesters and polycarbonates

require processing temperature higher than 2500C and are therefore not desirable

for composites. The important methods used for the production fibre reinforced

thermoplastics are the fibre mat reinforcement132 technique and the ”Express “

processing method133. In the fibre –mat process natural fibres are converted to mats

by stitching together layers of fibre, which has been crushed before. The fibre-mats

are then melt coated in a double coil coating press with a heat and cool press zone

where the fibre mats are brought together with the polypropylene melt between the

two circulating steel bands. In the hot press-zone wetting of the mats with the

thermoplastics takes place and in the cooling press zone the laminate is cooled to

get the final product. In the extrusion press moulding (express-processing) process

natural fibre non-wovens and thermoplastic melt films are alternatively deposited

and moulded in a tempered mould. The thermoplastic melt film is laid on by a

mobile extruder. The structural order consists of three layers, two layers of non-

wovens on the bottom and one on top, with the polymer melt in between.

1.11 Sisal fibre reinforced composites

Sisal is one of the most important leaf fibres and of the total world

production of 0.6 million tonnes India’s share is only 3000 tonnes134. Sisal fibre is

strong and is commercially exploited as a cordage fibre. Compared to synthetic

Short Sisal Fibre Reinforced Polystyrene Composites 62

fibres, the price of sisal fibre is very low135. It is about one-ninth of that of glass

and one-five hundredth of carbon fibre. Among the natural fibres sisal fibre came

next to jute in specific price (modulus per unit cost). A good sisal plant yields

about 205 leaves134. Four verities have been reported from India viz. sisalana,

vergrose, istle and kanthala.

Table1.7 Locations of sisal fibre production in and around Kerala.

(Ref. G.Kalaprasad, Ph.D. Thesis, Mahatma Gandhi

University, Kottayam, Kerala, India, 1999)

Kerala Tamilnadu

(Kanyakumari)

Anchal Ancvhugramam

Ayoor Kanjirode

Kaliyakkamla Kattathurni

Kilimanoor Kiliyoor

Paranodode Marthandam

Parassala Mulgumoodu

Poovachal Nagercoil

Punaloor Arumanai

The fibre content of the leaf varies with the type of sisal, its source and age.

Among the verity available in India vergrose and kanthala have higher fibre

content than the other two types. The typical composition of the leaf is fibre 4%,

cuticle 0.75%, other dry matter 8%and moisture 87.25%. Thus the normal fibre

yield from a leaf is about 3% by weight. In and around Kerala, about 50 tonnes of

Introduction 63

sisal fibres are extracted every year from the leaves and the important location of

sisal fibre extraction in Kerala are listed in Table1.7.

The extraction of fibres is done in the cottage industries supported by Khadi and

Village Industries Commission. Normally the leaves are collected once in every

three months134 and each tree gives five to six leaves at a time.

Table 1.8 Extraction methods, amount and length of various plant fibres

(Ref. J. G. Cook, Hand book of Textile Fibre and Natural Fibres, 4th edn., Morrow Publishing, England, 1968)

Fibre Method Amount Length

(mm) Banana Manual/raspodar 1.5% wt of

stem 300-900

Coir Retting/Mechanical 8wt% of nut (this weighs about 0.1 kg)

75-150

Jute Retting and beating/chemical

3-4 wt % of stem

1500

Linseed Retting / Dry scratching

20-25 wt % of dry straw

-

Mesta Retting and beating/chemical

Same as jute -

Palmyarah By hand (by beating)

0.5 mg per stalk

300-600

Pineapple By hand/ decorticator

2.5-3.5 wt% of green leaves

900-1500

Ramie decorticator 2.5-3.5 wt% of bark

900-1200

Sisal Manual (beating)/ microbial retting/ decorticator

3-4 wt % of green leaves

900-1200

Sunhemp Manual/ retting 2-4% of green stalk.

The extraction is carried out either by retting or by hand scrapping or using a

raspador machine. The fibres after extraction are washed in water and dried in the

Short Sisal Fibre Reinforced Polystyrene Composites 64

sun before use. Sisal fibres are 0.6 –1.2 meter long, 100-300 μm in diameter and

cramy white to yellow in colour depending on the processing techniques

used134,136. These fibres are relatively stiff and flexible and are multi cellular with

cells having different shapes from rectangular to cylindrical. The ends of the fibre

are broad and blunt. Thick cells are polygonal in cross section with thicker cell

walls and well-defined lumen, which varies in size. In some cases the lumens are

wider than the cell walls, often packed with tiny globules and is polygonal in cross

section with rounded ends. The length of the cell is normally 5-6 x 10-3 m and

width 5-40 x 10-3 m giving an L/D ratio of 150. Interlaced with individual cells are

occasional spiral vessels and parenchymatous cells containing single calcium

oxalate crystals which, often long as 4.6 x 10-1 m. The major chemical constituent

of this fibre is cellulose (66-72%), lignin (10-14%), hemicellulose (12%), and

moisture (10%)134.The extraction methods, amount and length of different natural

fibres are given in Table 1.8.

1.11.1 Sisal fibre reinforced thermoset composites

Incorporation of sisal fibre in to thermosetting plastics have been reported by

various researchers137-140. Paramasivam and Abdulkalam137 have investigated the

feasibility of developing polymer based composites using sisal fibres due to the

low cost of production of composites and amenability of these fibres to winding,

laminating and other fabrication process. It was found that the fabrication of these

composites was fairly easy and production cost is relatively low. Winding of

cylinders with longitudinal or helical hoop reinforcement was successfully carried

out. Reinforcement of epoxy resin with sisal fibre yield composites with tensile

Introduction 65

strength 250-300 MPa and is nearly half the strength of glass-fibre epoxy

composites of the same composition. Because of the low density of sisal fibre, the

specific strength of sisal composite was comparable with that of glass composites.

The unidirectional modulus of sisal fibre –epoxy composite was found to be about

8.5 Gpa. This means that incorporation of abundantly available natural fibres in

polymer matrix could be used as a means for the development of consumer goods,

low cost housing and civil engineering structures. Satyanarayana et al.141 have

studied the mechanical properties of chopped sisal fibre –polyester composites and

indicate a value of 1.90 for specific modulus compared with 2.71 for glass fibre

reinforced composites and the specific strength was of the same order as that of

poly ester resin (31-41 MPa). The impact strength of the composite (30J/m2) is

about three times higher than that of neat polyester and about 30% lower than that

of glass fibre reinforced polyester composites. Pavithran et al.138,139 studied the

impact properties of oriented sisal fibre-polyester composites and showed that sisal

fibre composites shows the maximum work of fracture followed by pineapple fibre

composite and banana and coir fibre composite showed comparatively low work of

fracture (Table 1.9).

Bisanda and Ansell95 studied the effect of silane and alkali treatments on the

mechanical and physical properties of sisal -epoxy composites. It is reported that

the incorporation of sisal fibres in epoxy resin produces stiff composites and silane

treatment of alkali treated fibre provides improved wettability, mechanical

properties and water resistance.

Short Sisal Fibre Reinforced Polystyrene Composites 66

Table 1.9. Mechanical properties of natural fibres and work of fracture of their polyester composites. [ Ref. C.Pavithran, P.S.Mukherjee, M.Brahmakumar

and A.D.Damodaran, J. Mater. Sci. Lett., 6,8821987]

Fibre properties Composite properties

Fibre type Tensile Strength (MPa)

Elongation at break (%)

Tough- ness

(MNm-2)

Fibre pull-out layer

(mm)

Work of fracture (KJ/m-2)

Sisal 580 4.3 1250 3.5 98.7

Pineapple 640 2.4 970 2.2 79.5

Banana 540 3.0 816 1.9 51.6

Coir 140 25.0 3200 1.1 43.5

The influence of interfacial adhesion on the mechanical and fracture behaviour of

various thermosets resin matrices (polyester, epoxy, phenol- formaldehyde) and

thermoplastic matrix (low density polyethylene) as a function of fibre length and

fibre loading were reported by Joseph et al.140. They observed that all the

composites showed a general trend of increasing properties with fibre loading.

However, the optimum length of the fibre required toobtain increase in properties

varied with the type of matrix.

Veluraja et al.142 reported a novel composite material based on tamarind seed gum

and sisal fibre and developed techniques for the improvement of the strength of the

composite by a process of humidification and this composite material find

application in false roofing and room partitioning. Dhalke et al.143 developed

composites based on plant polyols and showed that the properties of sisal fibre -

polyurethane system is comparable to that of standard polyether system. Recently,

Introduction 67

Bai et al144 have studied the failure mechanisms of continuous sisal fibre reinforced

epoxy matrix composite and showed that sisal fibre bundle/ epoxy interface had a

moderate strength with a comparatively small adhesive strength between the micro

tubular fibre and bonding material.

1.11.2 Sisal fibre –reinforced thermoplastic composites

Various workers have reported the use of sisal fibre for the reinforcement of

thermoplastics during recent years. Joseph et al.145-147 studied the mechanical,

rheological, and viscoelastic properties of short sisal fibre reinforced low density

poly ethylene composites as a function of processing methods, fibre content, fibre

length and fibre –matrix interface modification. They have reported that the fibre

breakage occurs during the melt mixing and can be avoided by adopting a solution

mixing method. It is also reported that fibre modification by suitable method

considerably affects the properties of the composites. The use of short sisal and

glass hybrid fibre for the reinforcement of low density polyethylene has been

reported by Kalaprasad et al.148. They observed that the addition of a small volume

fraction (0.03) of glass fibre into the above system enhanced the tensile strength of

the longitudinally oriented fibre composites by more than 80%. The electrical

properties of coir and sisal fibre reinforced low density polyethylene have reported

by Paul et al.149-150. They observed an increase in dielectric constant and

conductivity of the composite with increase in fibre loading. Selzer151 studied the

effect of environmental influences on the mechanical properties sisal fibre

reinforced polymer composites. The effect of moisture, acid and alkali attacks on

Short Sisal Fibre Reinforced Polystyrene Composites 68

the mechanical properties of sisal fibre-polypropylene composites have been

examined.

LeThi et al.152 have studied the mechanical properties of sisal fibre reinforced

polypropylene composites produced by reactive extrusion and reported that the

grafting of the fibres by PP graft –maleic anhydride enhance the impact strength

and breaking strength of the composites. Effects of fibre surface modification on

the fibre –matrix bond strength of sisal fibre reinforced polyethylene composites

was studied by Gonzaleez et al.153 and reported an increase in interfacial shear

strength between the fibre and matrix by morphological and silane modification of

the fibre surface. Yang et al.154 studied the effect of fibre and matrix modification

(thermal treatment, acetylation and coupling agents) on the mechanical properties

of sisal/PVC composites and shows that there is no improvement in mechanical

properties on treatment.

1.11.3 Sisal fibre reinforced gypsum and cement matrices

There are several studies155-161 on the use of sisal fibre for the reinforcement of

gypsum and cement composites. Bessell and Mutuli160 studied the interfacial bond

strength of sisal/cement composite and shows that the interfacial bond strength of

sisal fibre –cement composite is lower than that of other composites due to the

moisture absorption of sisal fibre from cement leading to very poor interface.

Swift161 studied the mechanical properties of sisal fibre –cement composite and

concluded that a composite material formed by sisal fibre and cement is suitable

for applications in several structures like earthquake-resistant adobe structures for

houses, roofing sheets and tiles, grain storage bins and water ducts.

Introduction 69

Hernandez et al.156 studied the fire behaviour of short sisal fibre reinforced gypsum

composite and found that addition of sisal fibre increase the fire insulating

performance and delays the occurrence of surface fissuration.

1.11.4 Sisal fibre reinforced rubber matrices

The use of sisal fibre for the reinforcement of elastomers gain importance in recent

years due to the advantages in processing and low cost coupled with high strength.

Varghese et al.162 studied the mechanical properties of acetylated and untreated

short sisal fibre reinforced natural rubber composites and found that acetylation

improves the adhesion between the fibre and rubber. They also reported the effects

of different bonding agent on the physical and mechanical properties of the natural

rubber –sisal fibre composites. Kumar and Thomas163-164 have investigated the

processing behaviour and mechanical properties of short sisal fibre reinforced

styrene butadiene rubber (SBR) composites.

The effect of adhesion on the equilibrium swelling of short sisal fibre reinforced

natural rubber composites in a series of normal alkanes were reported by Varghese

et al.165 and showed that increased fibre content and adhesion reduce the swelling

considerably.

1.12 Scope and objective of the present work

Composite materials based on thermoplastics are now becoming more important

due to their processing advantages. Though commodity resins such as PVC, PS, PE

and PP have properties that matches the requirements of high volume end uses,

price escalations combined with the sporadic and possible future shortage of resins

and petroleum feed stocks established the urgent need for utilization of fillers and

Short Sisal Fibre Reinforced Polystyrene Composites 70

reinforcements in these resins. Development of composite materials is one of the

ways for extending the available volume of these materials with improvement in

properties. These improvements are often associated with economic advantages

such as lower raw material cost and faster moulding cycles as a result of increased

thermal conductivity and fewer rejects due to warpage 26.

In the present work, short sisal fibre is used as a reinforcement to achieve

improved mechanical properties along with cost reduction and improved

biodegradation. Sisal fibre is one of the most widely used natural fibres and a large

quantity of this economic and renewable resource is still under utilized. In the

present study the use of this abundantly available material as reinforcement in

polystyrene have been investigated. The use of sisal fibre as a source of raw

material in plastic industry not only provides a renewable source but also generate

a non –food source of economic development for farming and rural areas.

Moreover the use of short fibre provides easy processability and can be processed

using machineries currently used in thermoplastic industry.

1.13 Plan of the thesis

The major objectives of the thesis are listed below.

Characterisation of the mechanical properties of sisal fibre-polystyrene

composites. The mechanical properties of short sisal fibre reinforced polystyrene composites

were studied as a function of fibre loading, fibre length and fibre orientation and a

comparison was made between theoretical and experimental tensile properties.

Introduction 71

The role of fibre/ matrix interaction on the mechanical properties of sisal

fibre -polystyrene composites

The fibre/matrix interface was modified by various fibre modifications and the role

of improved fibre/matrix interactions on the mechanical properties of composites

were studied. IR spectroscopy, 13CNMR spectroscopy and scanning electron

microscopy were used to characterise the modified fibre surface. The improved

fibre matrix adhesion was characterised by scanning electron microscopy.

Rheological behaviour of sisal fibre -polystyrene composites

The rheological behaviour of PS-sisal fibre composites were investigated with

reference to fibre loading, fibre length, fibre modification and shear rate. The

extrudate morphology was studied using optical and scanning electron microscopy.

Thermal and dynamic mechanical properties of the sisal fibre-polystyrene

composites.

Thermal and dynamic mechanical properties of the sisal fibre-PS composites were

investigated as a function of fibre length, fibre lading, fibre orientation and fibre/

matrix interface modification. The dependence of storage modulus, loss modulus

and damping factor of composites on the frequencies and temperature were also

studied.

Dielectric properties of the sisal fibre- Polystyrene composites

Dielectric properties of the PS-sisal composites were studied as a function of fibre

loading, fibre length and fibre modification. Dielectric constant, dielectric

conductivity and loss factor of the composites have been studied.

Short Sisal Fibre Reinforced Polystyrene Composites 72

Water sorption and environmental degradation behaviour of the sisal fibre-

polystyrene composite.

The water sorption behaviour and the effect of different extreme conditions on the

tensile properties of the composites were studied as a function of fibre loading and

fibre modification.

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