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.
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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 –
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.
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