Post on 07-Jun-2020
Chapter 1
Introduction
Abstract
This chapter deals with the different types of composite systems and also
offers a review on the literature on the various aspects of bio fibers and
their composites. Natural fiber reiforced composites are finding
applications in many fields like construction industry, automotive industry,
electronic and biomedical applications. The classification of composites
into micro composites, green composites and green nanocomposites has
been discussed. New developments dealing with cellulose-based nano
composites have been presented. The importance of the interface in
determining the properties of the composite has been reviewed. A review
of the earlier works in the field of natural fiber reinforced rubber composite
systems and current status have been included. The applications of fiber
reinforced rubber composites have been discussed. Finally the scope and
major objectives of the investigation have been highlighted.
Part of this chapter has been communicated to Progressive Polymer Science.
2 Chapter 1
1.1. Composites
A composite is a heterogeneous material created by the synthetic
assembly of two or more components, constituting selected fillers or
reinforcing fibers and a compatible matrix, in order to obtain specific
characteristics and properties. In the broadest form, composites are the
result of embedding high strength, high stiffness fibers of one material in a
surrounding matrix of another material (1). A composite material has at
least one continuous phase (binding matrix) and one or more dispersed
phases (fillers / fibers / reinforcements). In this the fibers are principal load
carrying members where as matrix keeps them in the desired location and
orientation and act as a load transfer medium between them. Therefore,
they are rightfully claiming a prominent role in the weight sensitive
structural engineering applications in recent times. The composites exhibit
superior properties as compared to individual constituents. They are
superior to all other known structural materials in specific strength and
stiffness, fatigue strength, corrosion and abrasion resistance, high
temperature resistance properties, energy absorption (shock absorption)
capacity etc. Mud bricks reinforced with straw and laminated woods were
known to have been made hundreds of years B.C. The more recent
composites emerged after Second World War .Contemporary composites
range from glass fiber reinforced automobile bodies to SiC particulate
reinforced aluminum for lightweight space and military application.
Composites can be classified based on the type of binding matrix as,
ceramic matrix, metal matrix and polymer matrix composites. Because of
the low processing temperature, the polymer matrix composites are much
easier to fabricate than metal matrix and ceramic matrix composites (2).
Composite materials with polymeric matrices are emerging as strong
candidates for load bearing structural applications in the defense,
Introduction 3
aerospace and automobile sectors. Depending on the type of dispersed
phase composites are classified as fibrous, laminated and particulate.
Fibrous composites consist of fibers (short or discontinuous and randomly
arranged) in a matrix. Bonding together layers of planar reinforcement with
resin forms laminated composites. In particulate composites, particles
have no preferred directions and are mainly used to improve properties or
lower the cost of isotropic materials. Talc, mica, or CaCO3 filled polymers,
and rocket propellants etc are some examples for particulate fillers.
The importance of fiber filled composites arises due to their high
strength and stiffness per weight. Lightweight structures, which involve
fiber composite materials, are revolutionizing the material world. Fiber
reinforcement represents a physical rather than a chemical means of
changing a material to suit various engineering applications. Fiber
composites are anisotropic in properties having outstanding properties in
one direction. Fiber reinforced composite materials offer a combination of
strength and modulus that are either comparable to or better than many
traditional metallic materials (3). Because of their low specific gravities,
high strength-weight ratios, high modulus weight ratios, polymer
composites are superior to ceramic and metal composites. Among the
various polymer composites, fiber filled composites gained much
importance in various fields like aircraft, military, space, automotive,
marine, construction, and sporting good applications (4). Figure 1.1
represents honeycomb an example for composite material.
4 Chapter 1
Figure 1.1 Honeycomb composite materials used for building purposes
(Ref: Lindblad M.S., Liu Y., Albertsson A. C., Ranucci E., Karlsson S., Adv.Polym. Sci.,157,139, 2002)
Fibrous composites can be further subdivided on the basis of natural
or synthetic fiber. Natural fiber composites are now emerging as a realistic
alternative to wood filled and glass reinforced plastics. Eco friendly bio
composites have the potential to be the new material to the 21st century
and be a partial solution to many global environmental problems (5).
1.2 Natural (lignocellulosic) fibers
The plant kingdom offers a wealth of potential candidates for
composite production. Lignocellulosic materials are among the world’s
renewable materials and contribute significantly to the world’s economy.
Natural fibers as a substitute for glass fibers in composite components,
have gained interest in the last decade, in many areas including the
housing, automobile, furniture and aerospace sector (6). With the
exception of synthetic polymers, most economically important products,
such as paper, cordage (cords and rope) and textiles are derived from
Introduction 5
plant fibers. Natural fibers can be divided in to vegetable, animal and
mineral fibers as given in Fig. 1.2. The bio fiber world is full of examples
where cells or groups of cells are ‘designed’ for strength and stiffness. All
vegetable fibers are composed of cellulose; where as fibers of animal
origin consist of proteins (7). Plant fibers are composite materials designed
by the nature.
Figure1.2 .Classification of natural fibers based on origin
(Ref: Bledzki A.K., Gassan J., Prog. Polym.Sci., 24, 221, 1999)
Plant fibers can be generally classified as bast (or stem or soft or
sclerenchyma) fibers, leaf or hard fibers, seed, fruit, wood, cereal and
other grass fibers. Normally bast fibers are found in the inner bast tissue of
6 Chapter 1
certain plant stems, eg. jute, ramie, kenaf and flax. The leaf fibers are
coarser than the bast fibers, eg. sisal, abaca, banana and henequen. The
stiffness is relatively high and it is often applied as binder twines and in
composite applications. But non-structural fibers found in fruit and seeds
like that of cotton, coir, oil palm, kapok etc. are not assembled in bundles
(8). These fibers originate as hairs borne on the seeds or inner walls of the
fruit, where each fiber consists of a single, long, narrow cell. Also the
cellulose fibrils are wound around the fiber rather parallel to the
longitudinal axis. Figure1.3 (a-k) shows some of the main fibers used in
different forms in biodegradable polymer composites (9).
Figure1.3 Lignocellulosic reinforcements (a) Banana; (b) sugarcane bagasse; (c) curauá; (d) flax; (e) hemp; (f) jute; (g) sisal; (h) kenaf. Typical pattern of reinforcements used in the hybrid natural fiber based biodegradable composite synthesis. (i) Jute fabric; (j) ramie–cotton fabric. (k) jute–cotton fabric
(Ref: Satyanarayana K. G., Gregorio G.C., Wypych A.F., Progress in Polym. Sci.,
34, 982, 2009)
Introduction 7
Fibers are elongate cells with tapering ends with very thick, heavily
lignified cell walls (Fig. 1.4). Fiber cells are dead at maturity and function
as support tissue in plant stems and roots. The lumen or cavity inside
mature, dead fiber cells is very small when viewed in cross section. Many
natural fibers have hollow space (lumen) resulting in low densities and
have nodes at irregular distances that divide the fibers in to individual cells
(10). The natural fibers are basically a rigid, crystalline cellulose micro fibril
reinforced amorphous lignin and hemicelluloses matrix.
The chemical composition of numerous natural fibers, most of which
are used in composite technology are given in Table1.1. The climatic
conditions, age and digestion process influence the chemical composition
and the structure of the fibers. Most plant fibers except cotton are
composed of cellulose, hemi cellulose, lignin, waxes and some water-
soluble compounds, where cellulose, hemi cellulose, and lignin are the
major constituents. The hydrogen bonds and other linkages provide the
necessary strength to the fibers. The microfibrillar angle , cellulose content
and moisture content determine the mechanical properties of the cellulose
based natural fibers (11). The higher cellulose content leads to greater
tensile strength. The percentage composition of cellulose varies for
different fibers. Generally the fibers contain 60-80% cellulose, 5-12% lignin
and up to 20% moisture. For technical applications, natural fibers have to
be modified.
Figure 1.4 Longitudinal section and cross section of a fiber cell.
8 Chapter 1
Table 1.1 Chemical composition, moisture content and microfibrillar angle of vegetable fibers
(Ref: Mohanty A.K., Misra M., Hinrichsen G., Macromol. Mater.Engin., 276, 1, 2000)
Fiber Cellulose
(wt%) Hemicelluloses
(wt %) Lignin (wt%)
Pectin (wt%)
Moisture Content (wt %)
Waxes (wt%)
Microfibrillar Angle (deg)
Flax 71 18.6-20.6 2.2 2.3 8-12 1.7 5-10
Hemp 70-74 17.9-22.4 3.7 5.7 0.9 6.2-12 0.8 2-6.2
Jute 61-71.5 13.6-20.4 12 -13 0.2 12.5-13.7
0.5 8
Kenaf 45-57 21.5 8 - 13 3-5
Ramie 68.6-76 13.1-16.7 0.6– .7 1.9 7.5-17 0.3 7.5
Nettle 86 11-17
Sisal 66-78 10-14 10 -14 10 10-22 2 10-22
Henequen 77.6 4 -8 13.1
PALF 70-82 5– 12.7 11.8 14
Banana 63-64 10 5 10-12
Abaca 56-63 12-13 1 5-10
Oil palm
EFB 65 19 42
Oil palm
mesocarp 60 11 46
Cotton 85-90 5.7 0-1 7.8-8.5 0.6
Coir 32-43 0.15-0.25 40-45 3-4 8 30-49
Cereal straw
38-45 15 -31 12-20 8
Introduction 9
Natural fibers are suitable for the reinforcement of thermoplastics and
thermosets due to their relative high strength and stiffness and low density
(12). Table1.2 gives the mechanical properties of different natural fibers as
compared to various synthetic fibers. As can be seen from the table,
tensile strength of glass fibers is substantially higher than that of natural
fibers even though the modulus is of the same order. However, when the
specific modulus of natural fibers (modulus/specific gravity) is considered,
the natural fibers show values that are comparable to or better than those
of glass fibers (13). These higher specific properties are one of the major
advantages of using natural fiber composites for applications wherein the
desired properties also include weight reduction. The range of property of
natural fibers can be attributed to the difference in fiber structure due to the
overall environmental conditions such as area of growth, its climate and
the age of plant. The technical digestion of the fiber is another important
factor that determines the structure as well as the characteristic values of
the fibers. Natural fibers can be processed in different ways to yield
reinforcing elements having different properties. The elastic modulus of
bulk natural fibers such as wood is about 10GPa. Cellulose fiber with
moduli up to 40GPa can be separated from wood by methods like
chemical pulping process. These fibers can be further subdivided by
hydrolysis followed by mechanical disintegration in to microfibrils with an
elastic modulus of 70GPa (14). Theoretical calculation of the elastic
modulus of the cellulose nano crystals gives a moduli value of 250GPa.
The elastic modulus values of cellulose nano crystals are comparable to
those of high performance synthetic fibers and even higher than that of
aluminium, glass fibers. High elastic modulus and tensile strength show
that cellulose possesses a potential ability to replace glass fiber, and it can
be a good candidate for the reinforcement fiber of the composite without
taking the density in to consideration (15).
10 Chapter 1
Density values of some natural fibers are given in Table 1.2. It is
seen that most of them lower by about 40-50% than that of glass fibers
commonly used in composites. Natural fibers are lighter and hence
attractive materials to substitute glass fibers, since their specific properties
are comparable to those of glass fibers. The density of natural fibers
decreases as a result of some chemical treatments.
Table 1.2 Mechanical properties of natural fibers as compared to various synthetic fibers
(Ref: Bledzki A.K., Reihmane S., Gassan J., J. Appl. Polym.Sci., 59, 1329, 1996)
Fiber Density (g/cm 3)
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 - 11.4 593 11.0
Soft wood kraft 1.5 - 1000 40.0
Oil Palm OPEFB
Mesocarp fiber 1.4 14
17
248
80
2
0.5
E glass 2.5 2.5 2000-3500 70.0
S-glass 2.5 2.8 4570 86.0
Aramid
(Normal) 1.4 3.3-3.7 3000-3150 63.0-67.0
Carbon
(Standard) 1.4 1.4-1.8 4000 230.0-240.0
Introduction 11
1.2.1 Micro structure of plant fibers
The major component of most plant fibers is cellulose (α- cellulose).
Cellulose is a linear macromolecule consisting of D-anhydroglucopyranose
units, joined together by β-1-4-glycosidic bonds with a degree of
polymerization (DP) of around 10000 (Cotton-7000, Flax-8000, Rame-
6500). The Haworth projection formula for cellulose is given by in Fig. 1.5.
O
CH2OHH
HO
OH
H OH
H H
O
H
H
O
CH2OH
HOH H
OH
H
OH
HO
HH
OHH
OH
H
CH2OH
O
H
H
OH
HOHH
CH2OH
O
H
n
1
23
4
5
6
1
23
4
5
6
1 1
2
2
5
5
44
3
3
6
6
OH
Macromolecules of Cellulose
Figure 1.5 Haworth projection formulas for glucose
(Ref: Bledzki A.K., Gassan J., Prog.Polym.Sci., 24, 221, 1999)
In other words, cellulose can be considered as a syndiotactic polyacetal
of glucose. Terminal hydroxyl groups are present at both ends of the cellulose
chain molecule (16). However, these groups are quite different in nature. The
C (1) hydroxyl at one end of the molecule is an aldehyde hydrate group with
reducing activity and originates from the formation of the pyranose ring
through an intermolecular hemiacetal reaction. In contrast to this, the C (4)
hydroxyl on the other end of the chain is an alcoholic hydroxyl and as such
non-reducing.
Solid cellulose has a semi crystalline structure, i.e., consists of highly
crystalline and amorphous regions. The chemical character of the cellulose
molecule is determined by the sensitivity of the β-glucosidic linkages between
the glucose repeating units to hydrolytic attack and by the presence of three
reactive hydroxyl groups, one primary and two secondary, in each of the base
12 Chapter 1
units. These reactive hydroxyl groups are able to undergo etherification and
esterification reactions (17).
Cellulose is a natural polymer with high strength and stiffness per unit of
weight and it is the building material of long fibrous cells. The main cause for
the relative stiffness and rigidity of the cellulose molecule is the intermolecular
hydrogen bonding which is reflected in its high viscosity in solution, its high
tendency to crystallize and the ability to form fibrillar strands. The β-glucosidic
linkage further favors the chain stiffness (18). Thus cellulose is a natural
polymer with high strength and stiffness per unit weight, and is the building
material of long fibrous cells.
Hemi celluloses are polysaccharides composed of a combination of 5
and 6-ring carbon sugars. Natural fibers contain hemicelluloses, which
consists of a group of polysaccharides that remains associated with the
cellulose after lignin has been removed. The hemicelluloses differ from
cellulose in that they contain several sugar units where as cellulose contains
only glucopyranose units. Hemi cellulose also exhibits considerable chain
branching where as cellulose is strictly linear (19). The degree of
polymerization of native cellulose is also ten to one hundred times higher than
that of hemicelluloses. Unlike cellulose, the constituents of hemicelluloses
differ from plant to plant. Hemi celluloses form the supportive matrix for
cellulose micro fibrils. Hemi cellulose is very hydrophilic and soluble in alkali
and easily hydrolyzed in acids.
Lignin is the compound that gives rigidity to the plants. It is thought to be
a complex, three-dimensional copolymer of aliphatic and aromatic
constituents with very high molecular weight. It is built up by oxidative
coupling of three major C6-C3 (phenylpropanoid) units, namely syringyl
alcohol, guaiacyl alcohol and p-coumaryl alcohol which forms a randomized
structure in a tri-dimensional network inside the cell walls. The major inter unit
Introduction 13
linkage is an aryl-aryl ether type. Lignin has been found to contain five
hydroxyl and five methoxyl groups per building unit (20). It is believed that the
structural units of a lignin molecule are derivatives of 4-hydroxy-3-
methoxyphenyl propane. Lignin is amorphous and hydrophobic in nature.
Lignin forms the matrix sheet around the fibers that holds the natural structure
together. The mechanical properties of lignin, however, are lower than that of
cellulose. In addition to these, pectin and waxes make up parts of the fiber.
Plant fibers are bundles of elongated thick walled dead plant cells. The
cell walls are formed from oriented reinforcing semi crystalline cellulose micro
fibrils embedded in a hemicelluloses/lignin matrix of varying composition. Such
micro fibrils have typically a diameter of about 10-30µm in diameter, each
composed of 30 to 200 cellulose molecules in extended chain conformation and
provide mechanical strength to the fiber (21). Figure1.6 shows the arrangement
of fibrils and microfibrils in the cell walls of a plant fiber.
Figure1.6 (a) Representation of cellulose micro fibril (b) elementary fibrillar unit
(Ref: Goda K., Sreekala M.S., Gomes A., Kaji T., Ohgi J., Compos. Part A, Appl. Sci and
Manuf., 37, 2213, 2006)
Cellulose
Microfibrils
Fiber
fiber microfibril
14 Chapter 1
Cellulose is synthesized by cellulose synthase, an enzyme complex
located in the cell membrane, which simultaneously synthesize a number
of parallel cellulose chains forming an elementary fibrillar unit, called a
micellar strand (Fig. 1.7). Several of these strands are most often
combined into a larger micro fibril, which conventionally is considered to
be the smallest unit of cellulose chains (22). The number of cellulose
chains in a microfibril varies between 30 and 200 depending on the type
of plant fiber. In some regions of the microfibrils the molecular structure is
highly ordered by intermolecular hydrogen bonds linking the cellulose
chains together in a crystalline arrangement, and accordingly, the
ordered regions are denoted crystalline regions and the less ordered
regions are denoted amorphous regions (23). In one theory, the so-called
fringe-micellar theory, the amorphous regions are thought to be located
inside the microfibrils where the ends of single cellulose chains are
disrupting the crystalline arrangement (Fig.1.8). In another theory the
amorphous regions are thought to merely reflect the higher free energy of
cellulose molecules at the surface of the microfibrils (24). The degree of
crystallinity varies with the type of plant fiber; e.g. for wood fibers it is
between 60 and 70%, whereas it is between 40 and 45 % for cotton
fibers. Moreover, physical and chemical treatments of plant fibers are
known to change the degree of crystallinity (25).
Cellulose is not uniformly crystalline but with amorphous regions of
low degree of order. Cellulose micro fibrils consist of predominantly
crystalline cellulose core. This crystalline cellulose core is covered with a
sheath of para crystalline polyglucosan material surrounded by
hemicelluloses. The amorphous matrix phase in a cell wall is very complex
and consists of hemi cellulose, lignin and in some cases pectin. The hemi
cellulose molecules are hydrogen bonded to cellulose and act as a
cementing matrix between the cellulose micro fibrils, forming the cellulose
Introduction 15
hemi cellulose network, which is thought to be the main structural
component of the fiber cell (26). The hydrophobic lignin network affects the
properties of other network in a way that it acts as a coupling agent and
increases the stiffness of the cellulose/hemi cellulose composite.
Micellar strand
Globule + rosette
Figure1.7 Section of a plant fiber cell membrane showing a cellulose synthase enzyme complex, synthesizing a micellar strand
(Ref: Favier V., Chanzy H., Cavaille J.Y., Macromolecules, 28,6365, 1995)
Figure 1.8 Depiction of the fringe-micellar theory showing how crystalline and amorphous regions are repeatedly located next to each other along the cellulose microfibril
(Ref:http://www.mhhe.com/biosci/pae/botany_map/images/cd.gif)
Assuming a cylindrical cell model for the structure of natural fibers, it
is possible to explain the observed strength and fracture mode of fibers
16 Chapter 1
(Fig. 1.9). Natural fibers can be considered to be composites of hollow
cellulose fibrils held together by a lignin and hemi cellulose matrix. Each
fibril has a complex, layered structure consisting of a primary thin wall that
is the first layer deposited during cell growth encircling a secondary wall.
The secondary wall is made up of three layers and the thick middle layer
determines the mechanical properties of the fiber. The middle layer
consists of a series of helically wound cellulose micro fibrils formed from
long chain cellulose molecules, the angle between the fiber axis and the
microfibrils is called the microfibrillar angle. The characteristic value for this
parameter differs from one fiber to another (27). All walls are fibrous in
nature and vary in thickness, composition and orientation. The primary wall
is normally thin and basically cellulosic, but gets lignified on growth. On the
other hand, the secondary walls, being mainly cellulosic crystalline regions
consisting of small crystallites called micro fibrils in the fiber, are separated
by a noncrystalline region and constitute a longer part of the cell wall (28).
Table1.1 gives the microfibril angle of few natural fibers. The microfibrillar
angle and cellulose content determine the mechanical properties of the
cellulose based natural fibers. In general, fiber strength increases, with
increasing cellulose content and decreasing spiral angle with respect to
fiber axis (29). Thus, a natural fiber can be considered as a natural
composite with cellulosic crystallites embedded with some orientation in a
lignaceous matrix.
Mechanical properties of fibers depend on the amount of cellulose,
degree of polymerization of cellulose and on the microfibrill angle (30).
Fibers with higher cellulose content and lower degree of microfibril angle
exhibit higher tensile strength and modulus.
Introduction 17
Figure 1.9 Structure of bio fiber
(Ref: Wambua P.,Ivens J.,Verpoest I.,Compos.Sci.Technol.,63,1259,2003)
1.2.2 Cellulose micro fibrils
Cellulose is found not to be uniformly crystalline. However, the
ordered regions are extensively distributed throughout the material, and
these regions are called crystallites. The thread like entity, which arises
from the linear association of these components, is called the micro fibril; it
forms the basic structural unit of the plant cell wall. These micro fibrils are
found to be 10-30nm wide, less than this in width, indefinitely long
containing 2-30,000 cellulose molecules in cross section. Their structure
consists of predominantly crystalline cellulose core.
Figure 1.10 depicts SEM photomicrographs showing the fibril
dimensions. From these photomicrographs, it is clear that the fibrils have
a very broad size (diameter) distribution, ranging from a few hundred
nano-meters to the micron level. Micro/nano sized bamboo fibrils used
can be roughly categorized as a mixture of nano-fibrils and micro-fibrils
based on their diameters. The fibril lengths also show a broad
distribution. In addition, the fibrils, especially the nano-fibrils, show
entanglements and branchings that form a network by splitting at
18 Chapter 1
different locations along the length as can be seen in the figure. Fibrils
were randomly orientated. The interconnected fibril network can
contribute to improve the mechanical properties of the composites due to
the high interlocking density facilitating load transfers among fibrils and
the excellent mechanical properties of the fibrils.
Figure 1.10 SEM photomicrographs of the micro/nano-sized bamboo fibrils
(Ref: Huang X., Netravali A., Compos. Sci. Technol., 69, 1009, 2009)
These are covered with a sheath of para crystalline polyglucosan
material surrounded by hemicelluloses. In most natural fibers, these micro
fibrils orient themselves at an angle to the fiber axis called the micro fibril
angle. The ultimate mechanical properties of the natural fibers are found to
be dependent on the microfibrillar angle. Gassan et al. (31) have done
calculation of the elastic properties of natural fibers. Two models were
developed to calculate the elastic properties. It was found that modulus in
fiber axis decreases with increasing spiral angles as well as the degree of
anisotropy, while the shear modulus reached a maximum value for a spiral
angle of 45°.
Cellulose exists in the plant cell wall in the form of thin threads with
an indefinite length. Such threads are cellulose micro fibrils, playing an
Introduction 19
important role in the chemical, physical and mechanical properties of
plant fibers and wood. Microscopists and crystallographers studied the
green algae valonia as an excellent material for the ultra structural study
of the cellulose micro fibril (32). A discrepancy in the size of the
crystalline regions of cellulose, obtained by X-ray diffractometry and
electron microscopy, led to differing concepts as to the molecular
organization of micro fibrils. Frey-wyssling (33) regarded the micro fibril
itself as being made up of a number of crystallites, each of which was
separated by a para crystalline region and later termed “elementary
fibril”. The term elementary fibril is therefore applied to the smallest
cellulosic strand. Scanning Electron micrographic studies of the
disintegrated micro fibrils showing crystalline nature of cellulose micro
fibrils taken by diffraction contrast in the bright field mode are given in
Figure 1.11. These were obtained by diffraction contrast in the bright field
mode for an epoxy resin embedded resin. The crystalline regions are
shown as dark zones due to electron diffraction. Thus cellulose micro
fibrils have a highly crystalline nature.
Cellulose microfibers were isolated from bagasse Atomic Force
Microscopy (AFM) studies illustrated the fiber bundle morphology in
cellulose MFs isolated from bagasse. In Fig. 1.12 the whole microfibrillar
bundles, as well as individual nanofibers are shown. Increased
magnification of microfibrillar bundles reveals nanometer-scale (30nm)
structures.
20 Chapter 1
Figure 1.11 Electron micrograph of ultra thin transverse section of cellulose microfibrills.
(Reprinted from Wood and Cellulose Chemistry by N.David et al. Marcel Dekker
NY, 1991)
Figure1.12 AFM of microfibrillar bundles of bagasse observed to be composed of nanometer-sized (30nm) nanofibers
(Ref: Bhattacharya D., Germinario L.T., Winter W.T., Carbohydrate Polymers, 73,
371, 2008)
Introduction 21
Reports on the characterization and the make up of the elementary
fibrils and their association to establish the fiber structure-usually called
fibrillar on fringed fibril structure are there in the literature (34). According
to this concept, the elementary fibril is formed by the association of many
cellulose molecules, which are linked together in repeating lengths along
their chains. In this way, a strand of elementary crystallites is held together
by parts of the long molecules reaching from one crystallite to the next
through less ordered interlinking regions. Molecular transition from one
crystallite strand to an adjacent one is possible, in principle. Apparently in
natural fibers this occurs only to a minor extent whereas in man made
cellulosic fibers such molecular transitions occur more frequently.
a b
Figure 1.13 Transmission electron micrograph of a dilute suspension of (a) sugar beet cellulose and( b) microfibril tunicin
(Reprinted from Recent Research Developments in Macromolecular Research, Dufresne A, 3, 455, 1988)
The internal cohesion inside the elementary fibrils is being
established by the transition of the long cellulose chain molecules from
crystallite to crystallite. The coherence of the fibrils in their secondary
aggregations is given either by hydrogen bonds at close contact points or
22 Chapter 1
by diverging molecules. Access into this structure is given by large voids
formed by imperfect axial orientation of the fibrillar aggregates, interspaces
of nanometer dimensions between the fibrils in the fibrillar aggregations,
and by the less ordered interlinking regions between the crystallites inside
the elementary fibrils. Dufresne (35) has reported on whiskers obtained
from a variety of natural and living sources. Cellulose microfibrils and
cellulose whisker suspension were obtained from sugar beet root or potato
pulp and from tunicin. Typical electron micrographs obtained from dilute
suspensions of tunicin whiskers and sugar beet cellulose micro fibrils are
shown in Fig. 1.13 (a) and (b). Individual micro fibrils are almost 5 nm in
width and length is much higher, leading to a practically infinite aspect ratio
of this filler. These can be used as reinforcing phase in a polymer matrix.
1.2.3 Advantages and disadvantages of vegetable fib ers
In the last decade, there is a growing interest in natural reinforced
composites because of their high performance in terms of mechanical
properties, significant processing advantages, chemical resistance, and
low cost/low density ratio. On the other hand, for environmental reasons,
there is an increased interest in replacing reinforcement materials
(inorganic fillers and fibers) with renewable organic materials (36). Plant
fibers have many significant advantages over synthetic fibers (Table 1.3).
Natural fibers are emerging as low cost, lightweight and apparently
environmentally superior alternatives to glass fibers in composites. Some
plant fibers are low cost, low density, have high specific strength, and
young’s modulus and possess ease of formability. Considering there lower
densities, renewable nature and high specific strength, it has been proved
that natural fibers can compete with E-glass, a commonly used
reinforcement. Bio fibers are non-abrasive to processing equipment and
compounding parts, which can contribute to significant cost reduction (37).
Introduction 23
A further advantage is that plant fibers cause no skin irritation during
handling and use. Also the amount of energy necessary for the production
of plant fiber textiles and fabrics was estimated to be 80% lower than for
the production of glass fibers. Because of their hollow and cellular nature,
natural fibers perform as acoustic and thermal insulators and exhibit
reduced bulk density
Table 1.3 Main advantages and disadvantages of lignocellulosic fibers.
(Ref: Bledzki A.K., Gassan J., Progress in Polym.Sci., 24, 221, 1999)
Advantages Disadvantages
Low cost High moisture adsorption
Renewable Poor microbial resistance
Low-density Low thermal resistance
Nonabrasive Local and seasonal quality variations
Low energy consumption Demand and supply cycles
High specific properties
High strength and elasticity modulus
No skin irritations
No residues when incinerated
Fast absorption/ desorption of water
Good thermal conductivity
Biodegradability
24 Chapter 1
Plant fiber products are environmentally friendly. Compared to glass
fibers, which are potentially toxic, natural fibers offer other advantages.
Production of glass fibers releases CO2 into the atmosphere, along with
NOX and SOX gases and dust, which can be a health hazard (38). Bio
fibers can minimize harmful pollutants and their eventual breakdown is
environmentally benign. Thus the ecological advantages of biofibers are
the reduction of CO2 emission, biodegradability and recycling property.
Developing countries face the problem of disposing products made using
synthetic fibers. Green labeled products benefit present and future
humankind by preserving limited natural resources and reducing green
house gas emissions (39).
1.2.4 Limitations of natural fibers
In spite of the advantages of plant fibers in making ‘green
composites’, there are some limitations to using plant fibers that must be
considered in making composite materials. For instance, plant fibers can
vary in quality and properties depending on factors such as source, age,
processing techniques, rainfall and other growing conditions. Another
problem is that the processing temperature of composite is limited to
2000C as natural fibers undergo degradation at higher temperatures (40).
Consequently, thermoplastic and thermosetting materials with processing
temperatures in excess of 2000C are used with thermally stable materials
such as glass fibers and are generally unsuitable as matrix materials for
plant fiber based composites. The main disadvantage of plant fibers is their
high moisture absorption, which leads to swelling of the fibers and
subsequently causes dimensional variations of the final material. Because
of irregular fiber geometry, modeling of discrete volumes of ‘green
‘composites is lacking (41).
Introduction 25
Bonding of the matrix material to the fiber surface is a critical factor in
making composites with improved properties. Stronger bonding between
the fiber surface and the matrix material ensures a greater transfer of
stress from the matrix to the fiber component during deformation (42).
Bonding may be inherently weak between hydrophilic plant fibers and
hydrophobic matrix materials. This results in non-uniform dispersion of
fibers within the matrix, which impairs the efficiency of the composite.
Cycles of fiber shrinkage and swelling can further affect bond strength in
cases where the plant fibers contact water or are exposed to changes in
humidity. Another restriction to the successful exploitation of natural fibers
for durable composite application is low microbial resistance and
susceptibility to rotting. Natural fibers tend to yellow upon exposure to
sunlight and moisture and extended exposure results in loss of strength.
Bond strength may also be affected by fiber dispersion (43). Natural fibers
that form into clumps or otherwise are in direct contact with other fibers
have less surface area in contact with the matrix material and are less able
to absorb stress from the matrix (44). The result is a composite with poor
mechanical properties.
1.3 Natural fiber reinforced polymer composites
In recent years, natural fiber reinforced polymer composites are
superior to synthetic fiber reinforced composites in properties such as
enhanced biodegradability, combustibility, lightweight, non-toxicity,
decreased environmental pollution, low cost, ease of recyclability etc (45).
These advantages place the natural fiber composites among the high
performance composites having economical and environmental
advantages. Synthetic fibers such as nylon, rayon, aramid, glass, polyester
and carbon are extensively used for the reinforcement of plastics (46-51).
Nevertheless, these materials are expensive and are nonrenewable
26 Chapter 1
resources. Because of the uncertainties prevailing in the supply and price
of petroleum-based products, there is every need to use the naturally
occurring alternatives. Lightweight materials that involve bio fiber
composite materials are revolutionalising the materials field (52).
Concerning their intrinsic properties, natural fibers have a specific weight
half that of glass fibers and a tensile modulus for the ultimate fibril almost
as high as for aramid fibers. More over they cause no damage by abrasion
to the processing machines as glass fibers do, which have a high amount
of ashes on combustion (53).
There are numerous fields in which natural fibers can be used along
with polymeric matrices. Natural fiber composites are attracting more
attention as alternative building material, especially as wood substitute in
the developing countries. Various attempts have been made recently to
rationally utilize abundantly available natural fibers such as banana, sisal,
coir, oil palm, hemp and wood fibers in polymer matrices like polyester,
epoxy and phenolics to be used as building materials for an assortment of
applications (54-59). Notable contribution in this field is the construction of
school building using jute fiber reinforced polyester in Bangladesh. In the
1980s building panels and roofing sheets from bagasse / phenolics were
installed in houses in Jamaica, Ghana and Philippines (60). Attempts to
prepare wall panels and roofing sheets using jute/ polyester/ epoxy/
polyurethane resin for temporary shelters, bunker houses, storage silos,
post office boxes, helmets, and roofing sheets made from coir / polyester
have drawn considerable attention (61-64). Use of natural fibers as
reinforcement in a cementing matrix has also been practiced for making
low cost building materials such as panels, claddings, roofing sheets and
tiles, slabs, and beams (65). Hybridization with glass fibers provides a
method to improve the mechanical properties of natural fiber composites
and its effect in different modes of stress depends on the design and
Introduction 27
construction of the composite. Composite laminates/panels can be
prepared using nonwoven/woven/sisal/jute/coir mats and unsaturated
polyester/phenolic/polyurethane resin by a compression molding technique
(66). Modifications of rice husks surfaces by steam and sodium hydroxide
(NaOH) were carried out in order to study the effects of these on the
surface functional groups properties and performances of the composite
panels bonded with phenol formaldehyde (PF) resin by Ndazi (67) et al.
Surface treatment of coir (Cocos nucifera) fibers and its influence on the
fibers’ physico-mechanical properties was done by Rahman et al. (68)
Surface treatment improved the fiber matrix adhesion in composites. Agro-
residue such as wheat straw, corn stalk and corn cob reinforced high-
density polyethylene composites as an alternative to wood fibers was
studied by Panthapulakkal and Sain (69). Fiber characterization and
analysis of composite properties showed that wheat strawfilled HDPE
composites exhibited superior mechanical properties compared to
cornstalk, corncob and even wood flour filled HDPE, where as cornstalk
showed comparable mechanical properties to that of wood flour-HDPE
composite.
The curing characteristics and mechanical properties of alkali treated
grass fiber filled natural rubber composites and effect of coupling agent
was studied by Debasish et al.(70). Increasing the amount of fibers
resulted in the composites having reduced tensile strength but increased
modulus. The better mechanical properties of the 400-mesh grass-fiber-
filled natural rubber composite showed that the rubber/fiber interface was
improved by the addition of resorcinol formaldehyde latex (RFL) as
bonding agent for this particular formulation.
Short randomly oriented intimately mixed banana and sisal hybrid
fiber-reinforced polyester composites having varying volume fraction of
28 Chapter 1
fiber were fabricated by compression molding (CM) and resin transfer
molding (RTM) techniques by keeping the volume ratio of banana and
sisal, 1:1. The static mechanical properties such as tensile, flexural, and
impact behavior were studied by Idicula et al. (71). The dynamic
mechanical properties were also evaluated. Resin transfer molded
composites showed enhanced static and dynamic mechanical properties,
compared with the compression molded samples. The dynamic
mechanical properties of microfibers of oil palm-reinforced acrylonitrile
butadiene rubber (NBR) composites were investigated as a function of
fiber content, temperature, treatment, and frequency by Joseph et al. (72).
The storage modulus (E ) was found to increase with weight fraction of
microfibrils due to the increased stiffness imparted by the strong adhesion
between the polar matrix and the hydrophilic microfibrils. The damping
properties were found to decrease with increase in fiber loading.The
influence of oil palm empty fruit bunch (OPEFB) fiber and oil palm empty
fruit bunches grafted with poly(methyl methacrylate) (OPEFB-g-PMMA) on
the tensile properties of poly(vinyl chloride) (PVC) was investigated by
Bakar et al. (73). A comparison with the composite filled with the ungrafted
OPEFB fiber showed that the tensile strength and elongation at break
increased, whereas Young’s modulus decreased, with the incorporation of
20 phr OPEFB-g-PMMA fiber into the PVC matrix. The trend of the tensile
properties obtained in this study was supported by functional group
analysis, glass transition temperature measurements and surface
morphological analysis.
Mechanical properties of natural fiber/polyamide composites was
studied by Alvarez et al. (74).They used high performance thermoplastic
matrices such as polyamides instead of the commonly used polyolefins to
develop natural fiber composites for substituting glass fibers without
renouncing to their mechanical properties. For this purpose, different
Introduction 29
natural fibers such as flax, jute, pure cellulose and wood pulps have been
melting compounded with different polyamides to analyze the effect of fiber
content on mechanical properties. Flexural and tensile modulus and
strength of composites were analyzed finding an increase in the
mechanical properties compared with the unreinforced matrix that turns
natural fibers into a considerable reinforcement offering a wealth of
possibilities for industrial applications.
A new route for the preparation of cellulose triacetate (CTA) optical
films from the biomass of ramie fiber has been found with environmental
benefits by Fan et al. (75). CTA with a degree of substitution (DS) of 2.81-
2.92 was prepared by the reaction of acetic anhydride with ramie fiber
catalyzed by sulfuric acid in acetic acid solution at 55 °C. The CTA film
was prepared by casting the solution of CTA dissolving in dichloromethane
on the culture disk via spreading the solution through a syringe. The
structure and properties of CTA and its film were investigated by Fourier
transform infrared (FT-IR), ultraviolet (UV), X-ray diffraction (XRD), nuclear
magnetic resonance (NMR), thermogravimetric analysis (TGA), differential
scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and
titration. It was found that the CTA films prepared from ramie fiber shows a
high transparency of 89% and excellent mechanical properties with stress
measurements of 31.04-47.80 MPa and strain of 3.99-5.22%. The CTA
films prepared from ramie fiber are suitable as protective films for the liquid
crystal displays (LCD).
As a novel piezoelectric material, the mechanical and piezoelectric
properties of cellulose electro-active paper (EAPap) were studied by Wool
et al. (76). Young’s modulus of piezoelectric EAPap was dependant on the
material orientation as compared with other EAP materials. The highest
Young’s modulus was obtained at 0° direction, while the highest direct
30 Chapter 1
piezoelectric charge constant was achieved at 45°. By measuring the
induced output voltage from the thin piezoelectric cellulose film under the
applied impact force, they demonstrated that piezoelectric EAPap film has
a potential for sensor applications (Fig. 1.14)
Figure 1.14 Piezoelectric cellulose EAPap sample. Au electrode was deposited on both sides of the EAPap.
(Ref:Sang-Woo L., Joo-Hyung K., Jaehwan K., Heung Soo K., Chinese Sci. Bull., 54, 2703, 2009)
Effect of fiber surface modification on the mechanical and water
absorption characteristics of sisal/polyester composites fabricated by resin
transfer molding was investigated by Sreekumar et al. (77). Sisal fibers were
subjected to various chemical and physical modifications such as
mercerization, permanganate treatment, benzoylation and silanization to
improve the interfacial bonding with matrix. Composites were prepared by
these fibers as reinforcement, using resin transfer molding (RTM). The
mechanical properties such as tensile, flexural and impact strength were
Introduction 31
examined. Mercerized fiber-reinforced composites showed 36% of increase
in tensile strength and 53% in Young’s modulus while the permanganate
treated fiber-reinforced composites performed 25% increase in flexural
strength The water absorption study of these composites at different
temperature revealed that it is less for the treated fiber-reinforced
composites at all temperatures than to the untreated one.
During 1896, aeroplane seats and fuel-tanks were made of natural
fibers with a small content of polymeric binders (78). As early as 1908, the
first composite materials were applied for the fabrication of large quantities
of sheets, tubes and pipes for electronic purposes (paper or cotton to
reinforce sheets, made of phenol- or melamine-formaldehyde resins).
Earlier, textiles, ropes, canvas and also paper were made of local natural
fibers, such as flax and hemp. India continued to use natural fibers, mainly
jute-fibers, as reinforcements for composites. Pipes, pultruded profiles, and
panels with polyester matrices, were produced with these fibers (79).
Natural fibers, as construction materials for buildings were known long
before.
The automotive industry is in the driving seat of green composites
because it is here that the need is greatest. The European Union’s end-of-
life of vehicles directive requires that by 2015, all new vehicles should be
95% recyclable. Faced with pressures to produce fuel-efficient, low-
polluting vehicles, the industry has used fiber reinforced plastic composites
to make its products lighter (Fig. 1.15). With their low density, compared to
other synthetic fibers, natural fibers help in reducing the weight of
automobile parts by 40%. Lower body weight helps in reducing fuel
consumption. The use of natural fibers in automobile industries has grown
rapidly over the last 5 years (80).
32 Chapter 1
Figure1.15 The interior parts of the Mercedes A-20 made by Natural Mat Thermoplastic
(Reprinted from Paper ID1414: Proc ICCM-13, China July, W. D. Brouwer, 2001)
Virtually all of the major car manufactures in Germany (Daimler
Chrysler/ Ford/ Mercedes/ BMW/Audi group and Opel) now use natural
fiber composites in applications such as head rests, boot liners, parcel
shelves, door liners, seat backs, sun roof interior shields etc (81). There is
a good potential of using natural fiber– thermo set composites as an
engineering material in automobiles in Australia. In this new era of
ecological importance scientists are aiming to make every component of
vehicles either recyclable or biodegradable.
1.4 Green composites
The environmental problem of solid waste disposal has become an
important issue due to the huge volumes of non-biodegradable waste
currently stored in landfills. Lately, there has been an increased interest in
the use of biopolymers due to more environmentally aware consumers,
increased price of crude oil and the concern about global warming.
Biopolymers are naturally occurring polymers that are found in all living
Introduction 33
organisms. The use of biopolymers will have a less harmful effect on our
environment compared to the use of fossil fuel based commodity plastics
(82). Biopolymers are based on renewable resources and will degrade to
form carbon dioxide, water and biomass. The amount of carbon dioxide
released during degradation is the same amount as the renewable
resource harnessed during its cultivation. As a result carbon dioxide will
not accumulate in the atmosphere due to the use of biopolymers (83).
Biopolymers can today be retrieved from for example agricultural
feedstock, marine fauna and microbial activities. Waste products from
industries can also be utilized to produce biopolymers, for example waste
from agriculture and marine food industries. Biopolymers, polymers
synthesized by nature such as starch and polysaccharides are an obvious
alternative to oil-derived plastics (84). Biodegradable composites from
sugar beet pulp and poly (lactic acid) (PLA) were prepared by compression
heating (85). The resultant thermoplastics had a lower density, but they
had a tensile strength similar to that of pure PLA. The composite
thermoplastics showed suitable properties for potential use as light weight
construction materials.
The innovation in the development of materials from biopolymers are
the preservation of fossil-based raw materials, complete biological
degradability, the reduction in volume garbage and compostability in the
natural environment as well as the application possibilities of agricultural
resources for the production of biomaterials are the major causes why
polymeric composites from renewable resources have attracted great interest
not only from academic point of view but also for industrial applications (86).
Products made using biopolymers are shown in Fig. 1.16 (a – e).
These include a drinking cup (Fig. 1.16a) made of starch, bags (Fig. 1.16b)
made of corn starch, “green pens” (Fig. 1.16c) made of starch based
34 Chapter 1
material, razors (Fig. 1.16d) and foamed trays (Fig. 1.16e) made of
thermoplastic starch.
Figure 1.16 Biopolymer products: (a) disposable cup; (b) compostable bag; (c) writing “green” pens; (d) shaving razor heads; (e) foamed trays
(Ref: Satyanarayana K. G., Gregorio G.C., Wypych A.F., Progress in Polym. Sci.,
34, 982, 2009)
The real challenge of biodegradable polymers lies in finding more
applications in order to achieve economies of scale. Research efforts are
currently being harnessed in developing a new class of fully biodegradable
“green” composites by combining (natural / bio) fibers with biodegradable
resins. High-performance biomass-based plastics that consist of poly (lactic
acid) (PLA) and kenaf fiber, which fixates CO2 efficiently were prepared by
Introduction 35
Serizava et al.(87). Adding this fiber to PLA greatly increases its heat
resistance (distortion temperature under load) and modulus and also
enhances itscrystallization, so the ease of molding this material is improved.
These composites (PLA/kenaf fiber and PLA/kenaf fiber/flexibilizer) show
good practical characteristics for housing materials of electronic products
in comparison with petroleum-based plastics used in housing such as
glass fiber-reinforced acrylonitrile-butadien-styrene (ABS) resin.
The major attractions of green composites are that they are
environmentally friendly, fully degradable and sustainable, that is, they are
truly ‘green.’ After use they can be easily disposed of or composted without
harming the environment. Baille (88) has exclusively dealt with the design
and life cycle assessment of green composites. Composites may be used
effectively in many applications such as in mass-produced consumer
products with short life cycles or products intended for one-time or short-
term use before disposal. Green composites may also be used for indoor
applications with a useful life of several years (89). A number of natural and
biodegradable matrices that are available for use in such green composites
are listed in Table1.4. Starch and modified resins have also been used as
matrix to form green composites.
The reinforcement of biofibers in green composites has been
highlighted in four families (90). Except the fourth family, which is of fossil
origin, most polymers (family 1–3) are obtained from renewable resources
(biomass). The first family is agro-polymers (e.g., polysaccharides)
obtained from biomass by fractionation. The second and third families are
polyesters, obtained respectively by fermentation from biomass or from
genetically modified plants (e.g., polyhydroxyalkanoate: PHA) and by
synthesis from monomers obtained from biomass (e.g., polylactic acid:
PLA). The fourth family comprises polyesters, totally synthesized by the
36 Chapter 1
petrochemical process (e.g., polycaprolactone: PCL, polyesteramide: PEA,
aliphatic or aromatic copolyesters). A large number of these biodegradable
polymers (biopolymers) are commercially available. They show a range of
properties and can compete with non-biodegradable polymers in different
industrial fields (e.g., packaging).
Table 1.4 Natural and synthetic biodegradable polymer resins
(Ref: Stevens E.S, Green plastics, Princeton University press, Princeton 2002)
Natural Synthetic
1. Polysaccharides 1. Poly(amides)
Starch 2. Poly(amide-enamines)
Cellulose 3. Poly(anhydrides)
Chitin 4. Poly(vinyl alcohol)
Alginates 5. Poly(ethylene-co-vinyl alcohol)
Carrageenan 6. Poly(vinyl acetate)
7. Polyesters
2. Proteins Poly(caprolactone)
Protein from grains Poly(glycolic acid)
Collagen / Gelatin Poly(lactic acid)
Casein, albumin, fibrogen, silks, elastin
3. Polyesters 8. Poly(ethylene oxide)
Poly hydroxyalkanoates, copolymers 9. Poly(urethanes)
4. Other Polymers 10. Poly(phosphazines)
Lignin 11. Poly(imino carbomates)
Natural Rubber 12. Poly(acrylates)
Introduction 37
Another important biocomposites category is based on agro-
polymers matrixes, mainly focused on starchy materials. Plasticized starch,
the so-called ‘thermoplastic starch’ (TPS) is obtained after disruption and
plasticization of native starch, with water and plasticizer (e.g. polyol) by
applying thermo mechanical energy in a continuous extrusion process
(91). Unfortunately, TPS shows some drawbacks such as a strong
hydrophilic character (water sensitive), rather poor mechanical properties
compared to conventional polymers and an important post-processing
variation of the properties. TPS properties reach equilibrium only after
several weeks. To improve these material weaknesses, TPS is usually
associated with others compounds.
Biodegradable composite from polyhydroxybutyrate co-hydroxy
valerate (PHBV) and wood fiber were fabricated using extrusion followed
by injection molding (92). Different compositions of the composites were
studied in respect to the mechanical, thermo mechanical and
morphological aspects. Wood fiber embodied in PHBV matrix gave an
appreciable rise in tensile and flexural moduli. The storage modulus was
also improved with wood fiber addition in to PHBV. Coefficient of linear
thermal expansion (CLTE) and notch impact strength of PHBV was
uniformly reduced with the fiber reinforcement. Thermal stability of the
PHBV reduced insignificantly with the wood fiber incorporation where as
the heat deflection temperature (HDT) improved to an extent. Figure
1.17(a) and (b) shows the SEM photomicrographs exhibited the existence
of interfacial interaction between wood fiber and PHBV thus providing a
good compatibility between the two.
38 Chapter 1
Figure1.17 SEM photomicrographs of impact-fractured samples of PHBV–wood (60:40) composite: (a) 60- 200µm (b) 200-100µm
(Ref: Singh S., Mohanty A. K., Compos. Sci. Technol., 67, 1753, 2007)
Researchers had investigated the effect of stearic acid on tensile
and thermal properties of ramie fiber-reinforced soy protein isolate (SPI)
resin green composites (93). It was observed that part of the stearic
acid crystallized in SPI resin and that the crystallizability was affected
by the addition of glycerol as a plasticizer. The fabricated green
composite was found to have enormous potential for certain indoors
applications.
Cellulose fiber/chitosan biodegradable rod (CF/CS rod) with layer by
layer structure having good mechanical properties and excellent X-ray
developing capability was successfully constructed via in-situ precipitated
method by Wang et al. (94). As the ratio of CF to CS was 0.2/20 (wt/wt),
the bending strength and bending modulus arrived at 124.1 MPa and 4.3
GPa, respectively, were significantly improved compared with pure CS rod.
TGA indicated that the thermal stability of CS rod could be enhanced by
mixing with CF, but fiber and matrix are partially compatible. Thus, CF/CS
rod has great potential to be used as internal fixation of bone fracture.
Introduction 39
Micro/nano-sized bamboo fibrils (MBF) and a modified soy protein
resin were used to fabricate environmentally friendly composites by Huang
and Netravali (95). With the incorporation of MBF the fracture stress and
Young’s modulus of the soy protein concentrate (SPC) increased
significantly. With the addition of 30 parts of MBF (SPC is 100 parts, based
on weight), the fracture stress and Young’s modulus were increased from
20.2 MPa to 59.3 MPa and from 596 MPa to 1816 MPa, respectively. As a
result, the toughness of the MBF reinforced SPC increased. The
environment-friendly, fully biodegradable green composites, based on
MBF and modified SPC resins, have excellent properties and great
potential to replace the traditional petroleum-based materials in many
applications.
The production and the mechanical characteristics of composites
made completely of renewable raw materials was conducted by Graupner
et al. (96). Composites of different kinds of natural fibers like cotton, hemp,
kenaf and man-made cellulose fibers (Lyocell) with various characteristics
were processed with a fiber mass proportion of 40% and poly (lactic acid)
(PLA) by compression moulding. Additionally, composites were made of
fiber mixtures (hemp/kenaf, hemp/Lyocell). The composites were tested for
tensile strength, elongation at break, young’s modulus and charpy impact
strength. Their characteristics varied markedly depending on the
characteristics of the raw fibers and fiber bundles and fiber mixtures used.
While kenaf and hemp/PLA composites showed very high tensile strength
and young’s modulus values, cotton/PLA showed good impact
characteristics. Lyocell/PLA composites combined both, high tensile
strength and young’s modulus with high impact strength. Thus, the
composites could be applied in various fields, each meeting different
requirements.
40 Chapter 1
1.5 Cellulose nano composites
Recent advances in nanotechnology have enabled materials and
devices to be fabricated at the nanoscale. One of the motivations for the
miniaturization process of materials is the superior mechanical properties
that nano-sized materials possess as compared to bulk materials.
“Nanotechnology relates to the understanding and control of matter at
dimensions of roughly 1 to 100 nanometers (97). Nano composites are a
new class of materials filled with molecular (nano meters) size mineral
particles instead of conventional scale fillers. In the case of nano
composites significant improvement can be observed in properties like
modulus, thermal stability, heat distortion temperature, fire retardancy,
barrier properties, dimensional stability, surface hardness etc (98). A good
example for a nano composite in nature is natural bone consists of
approximately 30% matrix material and 70% nano sized mineral (99).
Nanocomposites can demonstrate significant improvements compared
to virgin polymers with the content of the modified nano filler in the 2-10 wt.
% range. There are improvements in (a) mechanical properties, such as
tension, compression, bending and fracture (b) barrier properties, such as
permeability and solvent resistance(c) optical properties, such as ionic
conductivity (100). Other interesting properties exhibited by polymer-
nanocomposites include their increased thermal stability and ability to
promote flame retardancy at very low filling levels. The formation of a
thermal insulating and low permeability char from the polymer degradation
caused by a fire is responsible for these improved properties.
Organic and inorganic hybrid composite resins have attracted much
attention due to their excellent properties. Nano-composites formed by
adding nano-sized inorganic particles in organic polymers show significant
improvements in mechanical, physical and thermal properties (101). In
Introduction 41
general, nano-composites are defined as materials that consist of a nano-
meter scale (10-9 meter) phase mixed with another continuous phase.
Many nano-particles, when evenly dispersed, have shown their potential
ability to increase the overall mechanical properties of polymers with the
addition of only a small volume fraction of nano-particle. Figure 1.18 shows
the TEM image of nano crystalline cellulose. However, naturally occurring
clay nano-particles have been particularly found useful in improving
polymer properties.
Figure: 1.18 Transmission electron microscope (TEM) image of Nano Crystalline Cellulose (NCC)
(Ref: Bai W., Holbery J., Li K.,Cellulose, 16,455,2009)
Water bamboo husk reinforced poly (lactic acid) green composites
were prepared by Wang et al. (102). In this study, the powder obtained
from the water bamboo husk was added to poly(lacticacid) (PLA) to form
novel reinforced biodegradable composites. Morphologies, mechanical
properties, and heat resistance of these water bamboo powder reinforced
composites were investigated. The results indicate that the char yields
were increased as plant powder was incorporated to PLA. In addition, the
42 Chapter 1
mechanical properties were also enhanced due to the addition of powders.
The increments of storage moduli of PLA were about 50–200%.
Electrically conductive nanocomposites of polyaniline with poly (vinyl
alcohol) and methylcellulose was undertaken by Chattopadhyay et al.
(103). Electrically conductive nanocomposites of HCl-doped polyaniline
(PANI–HCl) nanocolloid particles with water-soluble and film-forming
polymers such as poly(vinyl alcohol) (PVA) and methylcellulose (MC) were
prepared by the redispersion of preformed MC-coated submicrometric
PANI–HCl particles in PVA and MC solutions under sonication for 1 h and
the casting of the films from the dispersions followed by drying The
composites showed low fp values at a volume fraction of PANI of 2.5x 10-2
in the PVA matrix and at a volume fraction of 3.7 x 10-2 in the MC matrix.
Finally, although the volume fraction of PANI needed to reach fp was
nearly the same for both types of matrices, the volume fraction required to
reach a high conductivity (10-2 S/cm) for the PVA matrix was significantly
lower than that required for the MC matrix.
Cellulose is the most abundant biomass resource and possesses
high ability from the viewpoint of mechanical and thermal properties.
Cellulose is a classical example of these reinforcing elements, which occur
as whisker like micro fibrils those are biosynthesized and deposited in a
continuous fashion. In many cases this mode of biogenesis leads to
crystalline micro fibrils that are almost defect free, with the consequence of
axial physical properties approaching hose of perfect crystals. The
application of this cellulose micro fibril has provided great interest and
expectations among material scientists.
Influence of cellulose as filler in vulcanized rubber composites was
analysed by Nunes and Mano (104). Co polymers of SBR, NBR and NR
were compounded with regenerated cellulose. The best results were given
Introduction 43
by the NR regenerated cellulose composites. Figure 1.19 exhibits cellulose
gels created nano whiskers isolated from cotton and microcrystalline
cellulose.
Figure 1.19 Pictures of cellulose gels created from nanowhiskers isolated from cotton (left) or microcrystalline cellulose (MCC, right).
(Ref: Capadona J. R., Shanmuganathan K.,Trittschuh S., Seidel S., Rowan S.J., Weder C., Biomacromolecules , 10, 712, 2009)
Researchers had studied the mechanical properties of
nanocomposites from sorbitol plasticized starch and tunicin whiskers (105).
Nanocomposite materials were obtained using sorbitol plasticized waxy
maize starch as matrix and tunicin whiskers as the reinforcement. The
effect of filler loading (0–25 wt% whiskers) and the relative humidity levels
(0–98%) on the mechanical behavior of the films are discussed for linear
and nonlinear deformation. The nanocomposites exhibit good mechanical
strength due to the strong interaction between tunicin whiskers, matrix,
plasticizer (sorbitol), and water, and due to the ability of the cellulose filler
to form a rigid three-dimensional network.
Chemically modified starch paste (MST) with poly butyl acrylate
(PBA) graft chains is investigated (106) as a reinforcing filler of rubber
through mixing and co- coagulating with natural rubber(NR) latex. Through
the comparison of mechanical properties and phase morphology, MST is
proved to be much superior to unmodified starch paste. MST showed
44 Chapter 1
obvious reinforcement effect on NR matrix by increasing tensile strength,
elongation at break and tear strength besides modulus and hardness.
Green composites reinforced with hemp nanocrystals in plasticized
starch were studied by Cao et al. (107). New nanocomposite films were
prepared from a mixed suspension of hemp cellulose nanocrystals (HCNs)
and thermoplastic starch, or plasticized starch (PS), by the casting and
evaporating method. Cellulose nanocrystals dispersed in the PS matrix
homogeneously and resulted in an increase in the glass-transition
temperature ascribed to the fact that the flexibility of the starch molecular
chains in the starch rich phase was reduced because of the strong
intermolecular interactions between the starch and stiff HCNs.
The cellulose microfibrils, which make the cellulose chains, can be
employed in the preparation of nanocomposites, which can be used in
various optical as well as biomedical applications. Depending on their
origin, these elements differ in lateral size, with diameter ranging from 2 to
20nm. These microfibrils on reaction with strong acids break down into short
crystalline rods or cellulose micro crystals (108). Natural fibers, which are rich
in cellulose, can be used as the starting material for the preparation of
cellulose micro fibrils. Cellulose microfibrils can be separated by methods like
cryo-crushing where the frozen pulp is crushed with liquid nitrogen. In
addition, methods to mechanically homogenize and stabilize food
ingredients could also be adopted for the preparation of cellulose
microfibrils. Steam explosion is another excellent process, which can be,
used to defibrillate the fiber bundles. This process being fast and well
controlled is well adapted for semi-retted fibers. Enzymatic hydrolysis of
cellulose is also accelerated by steam explosion. New innovations in nano
technology is because of the intellectual appeal of building blocks on the
nano meter scale and because the technical innovations permit to design
Introduction 45
and create new materials and structures with unprecedented flexibility,
improvements in their physical properties and significant industrial impact
(109). There are two reasons for change in material properties as the size
of the reinforcing phase are reduced down to the nanometer range.
Cellulose nano composites are a new class of materials of
considerable interest. The basic idea is to utilize cellulose micro fibrils in
new materials. Improved mechanical properties can be expected due to
the nanoscale distribution of the micro fibrils, high aspect ratio and the high
inherent stiffness of crystalline cellulose (≈130Gpa) (110). The major
difficulties are cellulose disintegration from the plant cell wall and
dispersion in a polymer matrix. The axial young’s modulus of cellulose has
been measured to be 137GPa. This is similar to aramid fibers. A typical
diameter of a wood based micro fibril is 30nm. The concept of cellulose
nano composites for load bearing applications is fairly new. Compared to
lignocellulose micro composites, we expect property enhancements due to
higher young’s modulus of pure cellulose reinforcement and more finely
distributed cellulose nano fibrils (111). A major application of cellulose
nano composites is as a binder in pharmaceutical tablets. A nano scale
reinforcement phase will have a very large surface area. Any morphology
or molecular mobility effects, which the cellulose surface may have on the
matrix, may there fore be very strong (112).
Biodegradable nanocomposites were successfully fabricated from
the thermoplastic corn starch (TPCS) and activated-montmorillonite
(MMT) by melt-intercalation by Huang et al. (113). It was revealed that
plasticized thermoplastic corn starch wase intercalated into the layers of
MMT successfully and layers of MMT were fully exfoliated and so formed
the exfoliated nanocomposites with MMT.
46 Chapter 1
New innovations in nano technology is because of the intellectual
appeal of building blocks on the nano meter scale and because the
technical innovations permit to design and create new materials and
structures with unprecedented flexibility, improvements in their physical
properties and significant industrial impact (114). There are two reasons
for change in material properties as the size of the reinforcing phase are
reduced down to the nanometer range: the large surface area associated
with nano particles results in many interfaces between the constituent
intermixed phases that play an important role on the macroscopic
properties. In addition, the mean distance between particles decreases as
the size is reduced, enhancing particle-particle interactions. The
occurrence of possible quantum effect, viz. change in magnetic effect,
optical or electrical properties (115).
Nanocomposites have several advantages over conventional
composites like efficient reinforcement with minimal loss of ductility and
impact strength, heat stability, flame retardancy and reduced smoke
emission, improved abrasion resistance, reduced shrinkage and residual
stress, and altered optical and electronic properties (116). It also
possesses remarkable improved properties like increased modulus,
increased gas barrier, increased heat distortion temperature, decreased
permeability to gases, water and hydrocarbons, increased thermal stability
and electrical conductivity in comparison to conventionally filled systems.
1.6 Fiber/matrix intreface
The interface plays an important role in natural fiber reinforced polymer
composites, since a strong interface is essential to have a good stress
transfer and therefore an effective reinforcement. Interface is generally
confined to the thickness of one molecular layer. The fiber/ matrix interface is
called the heart of the composite. The property of the interface controls the
Introduction 47
shear stress transfer between the matrix and the fibers. The mechanical
properties of the composites are controlled by interface/ interphase between
the fiber and matrix. The extent of interfacial adhesion is often interpreted in
terms of the surface structure of the bonded material i.e. surface factors such
as surface free energy, wetability, the polar group on the surface and surface
roughness of the material to be bonded. The properties of the interface are
intermediate between those of fiber and matrix. Superior mechanical
properties of the composites are clearly a function of how well the load
transfer occurs through the interface. The interfacial interaction depends on
the fiber aspect ratio, strength of interaction, anisotropy, orientation,
aggregation etc. A strong interface creates a material that displays good
strength and stiffness but which is very brittle in nature with easy crack
propagation through the matrix and fiber (117).
The regions separating fiber and matrix phases are chemically and
mechanically combined or otherwise indistinct is known as interphase. This
region was originally referred as interface but is now known as interphase
because of its three dimensional heterogeneous nature. It may be a
chemical reaction zone, nucleation zone, diffusion zone or any
combination of these. Matrix molecules can be anchored to the fiber
surface by chemical reaction or adsorption, which determines the strength
of interfacial adhesion. The interphase is also known as the mesophase
(Fig. 1.20). If the interface is weak, effective load distribution is not
achieved and the mechanical properties of the composite are impaired. On
the other hand, a strong interface can assure that the composite is able to
bear load even after several fibers are broken because the load can be
transferred to the intact portions of broken as well as unbroken fibers. A
poor interface is also a drawback in situations other than external
mechanical loading e.g. because of differential thermal expansions of fiber
and matrix, premature failure can occur at a weak interface when the
48 Chapter 1
composite is subjected to thermal stress. Thus adhesion between fiber and
matrix is a major factor in determining the response of the interface and its
integrity under stress. Researches have used many different techniques to
characterize the fiber / matrix interface. These include direct tests such as
single fiber composite (SFC) tests, single fiber pull out tests, micro
indentation, micro bead test etc. and indirect tests such as transverse and
longitudinal tensile test, flexural tests, inter laminar shear tests (ILSS),
impact tests, losipescu tests etc.
Figure1.20 Schematic model of interface
[Ref: Pothen L. A., PhD Thesis, Mahatma Gandhi University, kottayam, 2002)
The interface plays an important role in natural fiber reinforced
polymer composites, since a strong interface is essential to have good
stress transfer and therefore an effective reinforcement. Natural fibers are
incompatible with the hydrophobic polymer matrix and have a tendency to form
aggregates. They are hydrophilic fibers and so not resistant to moisture. This
Introduction 49
requires the development of strategies of surface modification to attain an
effective interaction between the fiber and polymer at the interface. Alkalization
of plant fibers has been found to change the surface topography of the
fibers and their crystallographic structure. Reports are there in the
literature on alkalization of natural fibers developing changes in the surface
morphology and increase in the availability of the hydroxyl groups
1.6.1 Characterization of the interfaces
The characterization of the interface gives the chemical
composition as well as information on interactions between fiber and
matrix. Various methods are available for characterization of the
interface.
1.6.2 Micro mechanical techniques
The extent of fiber-matrix interface bonding in terms of interfacial
shear strength(IFSS) can be determined by different micro-mechanical
tests such as single fiber pull out, micro debond test, micro-indentation
/micro-compression / fiber push out test and single fiber fragmentation
test (Fig. 1.21).
1.6.3 Single fiber pull out test
In the single fiber pull out test, the end of a fiber is embedded in a
large amount of matrix and pulled out from the matrix. The pull out test is
considered to be the best method of evaluating the interfacial shear
load as it can directly measure the interfacial shear strength between
the fiber and matrix independent of their properties. The interfacial
shear strength is a critical factor that controls the toughness,
mechanical properties and interlaminar shear strength of composite
materials. The fiber pull out problem has been investigated extensively
for purposes of studying the interfacial adhesion quality and elastic
50 Chapter 1
stress transfer between fibers and matrix (118). From the
load/displacement curves, the average interfacial shear strength
(IFSS) τ is given by
τ = F / π dl [1.1]
where F is the load needed to debond the fiber from the microbead and
d and l are the diameter and embedded length of the fiber respectively.
It is assumed that the shear stress along the interface is uniform.
Though the single fiber pull-out has an apparently unrealistic character
when compared to the complexity of the composite material, much
information can be derived that is related to the most fundamental
aspects of the fiber/matrix mechanical interaction. The drawback of
single fiber pull out test is that it involves only a single fiber.
Figure 1.21 Four methods currently used for measuring the shear strength of interfaces.
(Ref: Jacob M., Joseph S., Pothen L. A., Thomas S., Compos. Interface, 12, 95, 2005)
Introduction 51
1.6.4 Micro-debond test
The microbond technique, a modification of the single-fiber-pullout,
involves depositing a micro-bead of the matrix onto a fiber.The fiber with
the micro-bead is then mounted in a micro-vise blades and the fiber is
pulled out. The interfacial shear strength is calculated from equation 1.1
The specimen preparation for the micro droplet test whereby a
single fiber is pulled out of a small droplet of resin suffers from several
difficulties. For instance, the reliability of the data is affected by the
shape of the droplet. Symmetric, round droplets are easier to test and
analyze than droplets with flat surfaces, produced when the specimens
solidify on a flat substrate. Also the size of the droplet is critical. If the
length of the droplet exceeds a critical value, the fiber will fracture prior
to debonding and pull out (119). An additional complication with some
thermoset materials is that the anticipated curing characteristics may
not manifest themselves in a droplet of small size, and hence
comparison on a microstructural level between micro and macro
specimens may not be possible. Another defect is that this test is not
applicable to matrices that are soft.
1.6.5 Microindentation test
The micro indentation test was initially developed for fiber
reinforced ceramics but has been extended to the other fiber-matrix
systems. It is also known as the fiber-push out test. This is the only
single fiber test, which is able to analyze actual composite specimens.
The use of a real composite allows a more realistic simulation of
thermal stresses, polymer morphology and the influence of neighboring
fibers. The presence of other fibers is an advantage, but it complicates
the calculation of stress state around the fiber and the choice of an
appropriate failure criterion. Nevertheless, the more realistic testing
52 Chapter 1
conditions of the micro-indentation method make it an attractive new
technique for many researchers. In this method, a compression force is
applied on a single fiber in a well-prepared specimen of a real
composite.
1.6.6 Single fiber fragmentation test
The SFF technique involves embedding a single fiber along the
centerline of a dog-bone shaped specimen of matrix material. The
specimen is then strained along the fiber axis. The shear in the resin
exerts a tensile stress on the fiber. At some stress the fiber fractures at
its weakest point. With increasing specimen strain the fiber fractures
repeatedly at different locations (120). No additional breaks can occur
when the fragments become so short that the shear transfer along the
length of the broken fiber can no longer make the tensile strength
higher to cause additional fractures. This fragment length at the end of
the test is known as critical length lc. The average IFSS is then
calculated using the force balance equation,
τ = d σf / 2 lc [1.2]
The ratio lc/d is called the critical aspect ratio. This is the most
realistic test from the point of view of the interfacial pressure. The fiber
is neither pushed nor pulled directly, and so fiber Poisson effects are
similar to that occurring in a fiber composite.
A disadvantage of this technique is that the failure strain of the
matrix must be much larger than the failure strain of the fiber to promote
multi-fragmentation of the fiber. This requires the use of matrices, which
can undergo large deformations. Consequently commercial resins
utilized in actual composite systems, which typically have low strains to
failure, cannot be used for this test. Therefore the interfacial shear
Introduction 53
strength determined is not directly applicable to the actual composite
system.
Another aspect is that friction plays an important role in the
debonding process and this is governed by two additional unknowns,
i.e. the coefficient of friction µ and the pressure across the interface P.
Although some progress has been made with this, values for debonding
rely rather heavily on the correctness of the assumptions about µ and P.
1.6.7 Microscopic techniques
Microscopic studies such as optical microscopy, scanning
electron microscopy (SEM), transmission electron microscopy (TEM)
and atomic force microscopy (AFM) can be used to study the
morphological changes on the surface and can predict the extent of
mechanical bonding at the interface. The adhesive strength of fiber
to various matrices can be determined by AFM studies. AFM is a
useful technique to determine the surface roughness of fibers. AFM
has become a technique of major interest in composite science. It’s
advantageous such as high resolution and non-destructivity offer a
unique possibility for the repetitive examinations (121). The
technique allows to determine surface profiles of heterogeneous
materials with a resolution down to the sub nanometer level and
without any specific sample preparation, and to directly measure
forces that dominate adhesion phenomena. The force modulation
mode gives a qualitative statement about the local sample surface
elasticity using an oscillating cantilever tip, which indents into the
sample surface. The amplitude of this deflection is measured as a
function of the tip position when the cantilever tip indents cyclically in
to the surface.
54 Chapter 1
In an interesting study Sgriccia et al. (122) characterized untreated
and treated surfaces of natural fibers using SEM. SEM indicated the
presence of silane on treated hemp and kenaf. The SEM images show that
during alkali treatment interfibrillar material, hemicellulose and lignin, is
etched away by the alkali treatment. Conventional SEM requires high
vacuum, dry specimens and usually electrically conductive surfaces. The
environmental scanning electron microscopy (ESEM) allows wet, oily and
electrically non-conductive specimens to be observed without special
preparation and at relatively high pressures.
1.6.8 Spectroscopic techniques .
Electron spectroscopy for chemical analysis (ESCA) also referred
to as X-ray photoelectron spectroscopy (XPS), fourier transform infrared
spectroscopy (FTIR), laser raman spectroscopy (LRS), nuclear
magnetic resonance (NMR) and photoacoustic spectroscopy have been
shown to be successful in polymer surface and interfacial
characterization.
ESCA technique has an information depth of 1-5 nm and therefore it
is capable of examining only the outer layers of surfaces of fibers. ESCA
has been used to determine the surface composition of cellulose and wood
fibers. Pothen et al. (123) investigated the change in the surface
composition of the raw and chemically modified banana fibers using XPS.
FTIR technique has also been used to characterize the surface of
fibers from which information can be obtained about the interfacial
adhesion. George et al. (124) characterized the interface and the modified
fiber surface of pineapple leaf surface reinforced polyethylene composites
using IR. The advantages of FTIR include a less experimental complexity
and a more interpretable spectrum.
Introduction 55
The development of laser raman spectroscopic (LRS) method has
led to the assessment of the stress field at the interface level. The
technique relies on the fact that Raman bands corresponding to the
vibrational modes of bonds in the fiber shift towards a lower wave number
upon the action of strain and stress and this are thought to be due to direct
molecular straining / stressing. This has been used to map stresses along
fibers embedded in matrix resin to determine interfacial shear stress.
Raman spectroscopy has also been used to investigate the deformation
micromechanics of natural and regenerated cellulose fibers (125).
High resolution nuclear magnetic resonance spectroscopy (NMR) is
a very powerful technique to measure and characterize polymer tactcity,
helicity, molecular weight, and composition and diffusion coefficient of
polymers. Solid state 13C NMR spectroscopy using cross polarization and
magic angle spinning is useful for characterizing polymer composites.
Detailed information that can be obtained includes composition, glass
transition temperature, melting transition, percent crystallinity and the
number and type of crystalline phases. In general solid state NMR involves
proton-carbon cross polarization to enhance the 13C signal high power
decoupling to eliminate dipolar line broadening due to protons and
spinning of the sample about the magic angle with respect to static field to
reduce chemical shift anisotropy effects. NMR resonance imaging has
been used as a method of void detection in carbon fiber reinforced
composites. Physicochemical and 13 C NMR characterization of different
treatment sequences on Kenaf bast fibers has been investigated by
Keshk et al. (126).The spectra of the extracted pulp confirmed the
presence of oxidized lignin in the pulp.
56 Chapter 1
1.6.9 Thermodynamic methods
The frequently used thermodynamic methods for characterization in
reinforced polymers are wettability study, inverse gas chromatography
measurement, zeta potential measurements etc. Contact angle
measurements have been used to characterize the thermodynamic
work of adhesion between solids and liquids as well as surface of
solids. Contact angle measurement is probably the most common
method of solid surface tension measurement. The three most
commonly used methods of contact angle measurement are the
sessile drop, the captive bubble and the Wilhelmy plate technique.
Rong et al. (127) reported the surface properties of sisal fibers by the
capillary rise technique using the Washburn analysis.
X-ray diffraction method is a powerful tool for detecting the stress
on the incorporated fiber in the composite under load in-situ and
nondestructively. Nishino et al. (128) evaluated the influence of silane
coupling agent on on Kenaf fiber reinforced PLLA. The stress on the
composite was effectively transferred to the incorporated kenaf fiber
through the matrix, because of the strong interaction between the
kenaf fiber and PLLA. The most frequently used technique for the
determination of thermodynamic and acid/base characteristics is
inverse gas chromatography (IGC). In IGC, compounds usually
solvents, of known properties, characterize the unknown fiber surface.
The surface energies of natural fibers can be investigated using IGC.
The surface energy of highly crystalline cellulose has been reported to
be between 60-66mJ/m2. William and Douglas (129) used dynamic
contact angle (DCA) analysis and IGC to probe the surface-chemical
changes in wood pulp fibers during recycling. The DCA measurements
revealed that the overall effect of recycling was an increase in the
Introduction 57
nonpolar component and a corresponding decrease in the surface free
energy of the polar component, hence resulting in a total surface free
energy that remains essentially unaltered.
Zeta potential technique has been found to be useful in
characterizing lignocellulosic fibers. Electrophoresis, electro-osmosis
and streaming potential are basically three electrokinetic phenomena
those are currently exploited to measure the zeta potential of the
surface of polymers. The presence of acidic or basic dissociable
surface functional groups can be estimated by measuring the pH
dependence of the zeta potential. The measurement of the pH
dependence of the zeta potential results in the qualitative
measurement of the acidity or basicity of solid surfaces. The influence
of chemical treatments on the electro kinetic properties of cellulose
fibers was reported by Shearly et al. (130).
1.6.10 Other techniques
The interface can also be characterized qualitatively by other
methods like dynamic mechanical analysis and stress relaxation
technique. Swelling techniques have been used to assess the level of
interfacial adhesion in the case of fiber reinforced elastomer
composites. The term stress relaxation denotes the process of
establishment of static equilibrium in a physical or physicochemical
system and its rate depends upon the probability of transition of
system from one stage of equilibrium to another. Meaningful data on
the behavior of materials can be obtained by accelerated testing
methods. Stress relaxation is one of the widely employed testing
methods for this since it represents the basic time dependent
response of material from which other time dependent responses.
58 Chapter 1
Dynamic mechanical analysis has been used to analyze the
interfacial adhesion in chemically treated oil palm/sisal fiber reinforced
NR composites by Jacob et al.(131).The dynamic modulus value and
the damping factor used to quantify interfacial adhesion were
investigated in hybrid composites with reference to the effect of
temperature and frequency. The extent of fiber alignment and strength
of fiber polymer interfacial adhesion can be evaluated from the
swelling measurements. The diffusion mechanism in polymers is
essentially connected with the ability of the polymer to provide
pathways for the solvent to progress in the form of randomly
generated voids. Information about the anisotropy caused by the
orientation of fibers and fiber/matrix adhesion can be obtained from
anisotropic swelling studies.
1.7 Natural rubber (NR)
Natural rubber (NR) is the most fascinating material known to
mankind, which accounts for its use in a variety of applications. It is an
elastic hydrocarbon polymer that naturally occurs as a milkycolloidal
suspension, or latex, in the sap of some plants. NR belongs to a class of
compounds known as elastomers. NR is a linear, long chain polymer known
chemically as cis-1, 4 polyisoprene. Like other high polymers, NR is made
up of molecules of different sizes with molecular weight ranging from 30000
to about 10 million. NR is obtained from rubber tree (Hevea brasiliensis) in
the form of field latex. Polyisoprene exists naturally in the form of two
stereoisomers, namely cis-1, 4- polyisoprene and trans-1, 4- polyisoprene
(Fig. 1.22). Due to the high structural regularity of cis-1, 4 polyisoprene, NR
tends to crystallize when stored at low temperature or when stretched. The
strain induced crystallization behavior gives natural rubber the unique high
tensile strength in pure gum or in non-reinforcing fillers.
Introduction 59
The physical properties of NR vulcanisates are dependent like other
elastomers on several variables such as compound viscosity, type and
amount of fillers, degree of filler dispersion, degree and type of cross
linking. NR has an intrinsic density of about 0.92 gm/cc. Raw natural
rubber contains about 93% rubber hydrocarbons.
CH2 C CH CH2
CH3
CHCH2CH2
CCH3
CH3C
CH2
CH2CH
Trans-1,4-polyisoprene
Isoprene
Cis-1,4-polyisoprene
n
n
Figure 1.22 Structure of natural rubber
The rest consists of inorganic salts and organic materials some of
which are natural antioxidants and accelerators. NR has excellent abrasion
resistance, especially under mild abrasive conditions. It has high
resilience, with values exceeding 90% in well-cured gum vulcanisates.
Good resistance to flexing and fatigue together with high resilience makes
NR useful in applications such as heavy vehicle tires where cyclic
stressing is involved. Compression set and related process such as creep
are poorer in NR than in synthetic polyisoprene. In addition to this, NR has
excellent tensile and tear properties, good green strength and building tack
(Table1.5). However, NR is not very resistant to oxidation, ozone-
weathering variety of solvents and chemicals mainly due to its unsaturated
chain structure and non-polarity (132).
60 Chapter 1
Rubber is a unique engineering material because unlike other
engineering solids, it has high elastic deformability and an almost
theoretical value for poison’s ratio (0.5). It is currently used in bridge
bearings, medical devices, springs, anti vibration mountings to prevent
earthquakes and other suspension systems.
1.7.1 Nitrile rubber (acrylonitrile butadiene rubbe r) NBR.
Nitrile rubbers are co polymers of butadiene and acrylo nitrile. Some
trade names are: Mill-Right N, Nipol, Krynac and Europrene It is
manufactured synthetically by emulsion polymerization technique.
Acrylonitrile butadiene rubber (NBR) is a family of unsaturated copolymers
of 2-propenenitrile and various butadiene monomers (1,2-butadiene and
1,3-butadiene). The production of this rubber was first stated in Germany
in 1934 by I.G. Faber industries, under the trade name of Buna N. The
structure of NBR is depicted in Fig. 1.23.
R
H2CCH
CHCH2
CH2CH
C N
CH2trans1,4BDAcrylo nitrile
Figure 1.23 Structure of NBR
(Ref; Blow C.M., Hepburn C., Rubb. Technol. and manufacture, 2nd ed.,1982)
The acrylonitrile content (ACN) is one of two primary criteria defining
each specific NBR grade. The ACN level, by reason of polarity, determines
several basic properties, such as oil and solvent resistance, low
temperature flexibility/glass transition temperature, and abrasion
resistance (133). High ACN content provides improved solvent, oil and
abrasion resistance, along with higher glass transition temperature. Table
1.5 summarizes most of the common properties for conventional NBR
Introduction 61
polymers. The direction of the arrows signifies an increase/improvement in
the values.
Table 1.5 NBR properties – relationship to acrylonitrile Content
NBR with Lower NBR with Higher
Acrlonitrile content Acrylonitrile content
Process ability
Cure Rate w/Sulfur Cure System
Oil/Fuel Resistance
Compatibility w/Polar Polymers
Air/Gas Impermeability
Tensile Strength
Abrasion Resistance
Heat-Aging
Cure Rate w/Peroxide Cure System
Compression Set
Resilience
Hysteresis
Low Temperature Flexibility
The greatest characteristics of NBR are its excellent oil resistance;
fuel resistance and solvent resistance, NBR fall in to the special purpose
oil resistant rubbers. Various properties of nitrile polymers are directly
62 Chapter 1
related to the proportion of acrylonitrile in the rubber. As the ACN content
increases, the resistance to fuel and oil increases. These characteristics
combined with their good abrasion and water-resistant qualities make
them suitable for use in a wide variety of applications with heat resistant
requirements to 1490C. Nitrile rubber is a versatile raw material with
unique oil, fuel and chemical resistant characteristics that make it
suitable for many dynamic applications such as hose industry,
automotive, marine and aircraft fuel lines, water handling applications,
rubber rollers for printing, gasket for transformers, oil transport pipes,
boots, adhesives modifier for plastics etc. A larger use of nirile rubber is
the oil drilling industry in such items as blowout preventor, down-hole
packers, drill pipe protectors, pump piston elements and rotary drilling
hose. Powder and particulate form of nitrile rubber are especially useful
in cement, adhesions, binders for cork gaskets and brake limiting.
Another interesting application for nitrile rubber is in plastic modification
to improve impact strength and flexibility. Its resilience makes NBR the
perfect material for disposable lab, cleaning, and examination gloves. In
the automotive industry, it is used to make fuel and oil handling hoses,
seals and grommets. NBR’s ability to withstand a range of temperatures
from −40°C to +120°C makes it an ideal material for extreme automotive
applications. Acrylonitrile butadiene is also used to create moulded
goods, footwear, adhesives, sealants, sponge,expanded foams and floor
mats (134).
Compared to natural rubber, nitrile rubber is more resistant to oils
and acids, but has inferior strength and flexibility. Nitrile rubber is
generally resistant to aliphatic hydrocarbons. However (like natural
rubber), it can be attacked by ozone, aromatichydrocarbons, ketones,
esters and aldehydes.
Introduction 63
1.8 Oil palm fiber
The oil palm is a tropical palm tree. The oil palms (Elaeis) comprise
two species of the Arecaceae, or palm family. The African Oil Palm
Elaeis guineensis is native to west Africa, occurring between Angola and
Gambia, while the American Oil Palm Elaeis Oleifera is native to tropical
Central America and South America. The history of palm oil can be
traced back to the days of the Egyptian paraohs 5000 years B.C. They
are used in commercial agriculture in the production of palm oil,now it is
a major cash crop cultivated through out the tropics especially in
Malaysia, India, Indonesia. Oil palm cultivation in India has risen to the
level of attaining possible self-sufficiency in oil production. Oil palm
empty fruit bunch (OPEFB) fiber and oil palm mesocarp fiber are two
important types of fibrous materials left in the palm oil mill. OPEFB is
obtained as a waste material in the palm oil mill after the removal of oil
seeds for oil extraction. Figure1. 24 show the photograph of an empty
fruit bunch. Variety of other fillers such as shells, mesocarp fibers, oil
palm stems etc. is available from the tree. Oil palm stem has been
utilized as cellulosic raw material in the production of panel products
such as particleboard, fiberboard, and mineral bonded particleboard,
plywood and furniture.
The explosive expansion of oil palm plantation in India and Malaysia
has generated enormous amounts of vegetable waste, creating problem in
replanting operations and tremendous environmental concerns. It is
reported that Malasia alone produced during the recent past years about
30 million tones annually of oil palm biomass, including trunks, fronds, and
empty fruit bunches. But the only utilization of this lignocellulosic material
is as boiler fuel and in the preparation of potassium fertilizers. At present
protection of environment has become a global issue and scientists are
64 Chapter 1
working to find out ways to demarcate the use of synthetic materials. As a
result there is great interest on the utilization of natural materials for the
development of green composites (135).
Figure1.24 Photograph of oil palm empty fruit bunch (OPEFB)
(Ref: www.armadilloproducts .com)
In India palm oil mills are situated in Kerala and Andhra pradesh. In
Kerala we have vast source of these cellulosic fiber but the full potential of
these fibers as reinforcing agent has not yet been realized. Besides much
lower expense, renewable nature and much lower energy requirement for
the production and processing of oil palm empty fruit bunch fiber, make it
an attractive fiber for use as reinforcement in resin based composites.
Introduction 65
Oil palm fiber is separated from the empty fruit bunch by a
bacteriological process known as retting, which involves soaking the bunch
in water for about one month. During retting the phenolic material is
removed which gives strength to the fiber bundles. This loosens the fiber
interior and results in easy extraction. Then fiber is separated from the
pithy material of the fibrous mesocarp and empty fruit bunch. Retting
yields fiber with the desired color. The efficiency of retting depends upon
many factors like the nature of water, temperature, rate of removal of foul
water and the stress that the bunches are subjected to during retting. In
Kerala, oil palm fiber is mainly obtained from oil palm India Ltd.
Kulathupuzha, Kollam,Kerala.
Oil palm, a monocot, possess a hard peripheral layer enclosing a
central region consisting of fiber bundles embedded in a parenchymatous
tissue. This fiber has 10mm of length with the average filament diameter of
0.1mm. Chemical composition of various vegetable fibers varies greatly
between plants and within specific fibers depending on genetic
characteristics, parts of the plant, growth, harvesting and fiber preparation
conditions. The chemical composition and micro fibrillate structure of fibers
determine its properties. Comparison of properties of oil palm fibers with
other natural fibers is given in Table1.1.
The fiber strands from the oil palm stem, fronds and fruit bunches are
a good source of raw material for the production of various grades of paper
product. Oil palm fiber filler has the potential to be used as filler / extender
for the formulation of urea formaldehyde adhesives used in plywood
making. Oil palm empty fruit bunch fiber was used to prepare cost effective
and environmental friendly composite materials in house hold articles,
automobile parts, building materials etc.
66 Chapter 1
1.9 Natural fiber reinforced rubber composites
Short fibers are used to reinforce polymers in order to improve or
modify certain mechanical properties of the matrix for specific applications
or to reduce the cost of the fabricated products. The reinforcement of a
rubber with fibers combines the elastic behavior of the rubber with the
strength and stiffness of the reinforcing fiber. It is well known that
incorporation of short fibers into rubber compounds imparts good strength
and stiffness to the rubber matrix (136). Many researchers have reported
the processing advantages and improvements in the mechanical
properties of the short fiber reinforced rubber composites (137).
The properties and performance of short fiber-reinforced rubber
composites depend on several factors such as nature and concentration of
the fiber, its aspect ratio, orientation, and the degree of adhesion of the
fiber to the rubber matrix. Various synthetic fibers such as glass, rayon,
nylon, asbestos, aramid and cellulose have been studied as reinforcement
in both natural and synthetic rubber matrices (138).
Guth et al. (139) derived an equation for calculating the modulus
of a fiber-reinforced matrix, applicable to fiber-reinforced rubbers, which
has been much quoted. This equation is commonly referred to as the
‘Guth-Gold’ equation and is expressed as:
G = Go (1+ 0.67 fc + 1.62 f 2c2) [1.3]
Where G = modulus of composite material
Go = modulus of matrix material
f = length to diameter ratio (aspect ratio) of fiber
c = volume concentration of fiber
Introduction 67
When the fiber aspect ratio is in the range 10-50, moduli ratio of
102- 103 can be achieved if there is good adhesion between the fiber
and matrix. A later modification taking into account the fiber orientation
is the Boustany -Coran equation:
Ecomp = Er {1+ Kf Vf [ 26 + 0.85 ( l/d ) ]} [1.4]
Where Ecomp = modulus of the composite
Er = modulus of rubber
K = constant
f = a function of fiber orientation
l/d = fiber aspect ratio
Vf = fiber volume fraction
Lopattananon et al. (140) reported on the use of pineapple leaf
fiber as rubber reinforcement. They claimed that this fiber being
naturally available in short lengths it would circumvent the complicated
route in making short fiber from synthetic polymer and thus would be
more cost effective. The authors reported that the oriented cellulose
fibers gave an increase in tensile strength but the most important
changes were an increased modulus and reduced elongation at break.
The modulus of the fiber-reinforced materials as defined by young's
modulus, showed a steady increase with increasing fiber concentration.
Tensile strength however first fell with increasing fiber level and then
increased beyond that of the matrix material at higher fiber loadings.
This was explained that at low concentrations the matrix is not
restrained by enough fibers and high strains occur in the matrix at low
stresses, causing debonding. The debonded fibers thus reduce the
matrix strength.
68 Chapter 1
Natural fiber reinforced composites may become a viable alternative
to those which use glass fibers as reinforcement .Short fiber reinforced
rubber composites are of tremendous importance both in structural
applications and in the area of research and development .Composites in
which short fibers are oriented uniaxially in an elastomer have a
combination of good strength and stiffness from the fibrous and elasticity
from the matrix. The natural fibers gained importance because of the fibers
have renewable nature, low cost, easily availability, low density, bio
degradability and low energy consumption. The stiffness and modulus of
short fiber composite increase with fiber concentration though it may not
be necessarily linear. Tear strength is highly dependent on fiber loading
and is seen to increase with concentration.
The mechanical properties of lignocellulosic fiber reinforced
natural rubber composites have been extensively studied. It has been
reported by Dzyura (141) that the minimum amount of fibers to restrain
the matrix is smaller if the matrix strength is higher. Natural rubber is a
very strong matrix because of its strain-induced crystallization.
Generally it has been seen that the tensile strength initially drops down
to a certain amount of fiber and then increases. The minimum volume of
fiber is known as the critical volume above which the fiber reinforces the
matrix. The critical volume varies with the nature of fiber and matrix,
fiber aspect ratio and fiber / matrix interfacial adhesion.
At low fiber concentrations, the fiber acts as a flaw in the rubber
matrix and the matrix is not restrained by enough fibers causing highly
localized strains to occur in the matrix at low stress. This makes the
bond between fiber and rubber to break leaving the matrix diluted by
non-reinforcing debonded fibers. As the fiber concentration increases,
the stress is more evenly distributed and the strength of composite
Introduction 69
increases. The incorporation of fiber into rubber matrix increases the
hardness of the composite, which is related to strength and toughness.
The close packing of fibers in the compounds increases the density
while resilience decreases.
The reinforcement of Isora fiber in natural rubber has been
extensively studied (142). Isora fibers are present in the bark of the
Helicteres isora plant and are separated by retting process. Isora fiber
resembles jute in appearance but surpasses it in strength, durability and
lustre. A series of short-isora-fiber-reinforced natural rubber composites
were prepared by the incorporation of fibers of different lengths (6, 10, and
14 mm) at 15 phr loading and at different concentrations (10, 20, 30, and
40 phr) with a 10 mm fiber length mixes containing fiber loadings of 30 phr
with bonding agent (resorcinol-formaldehyde [RF] resin) showed
mechanical properties superior to all other composites. Isora fiber was
seen to have immense potential as reinforcement in natural rubber.
Pineapple (143) and jute fiber (144) have also found their way as a
potential reinforcement in natural rubber.
Chemically modified starch paste (MST) with polybutylacrylate (PBA)
graft chains is investigated as a reinforcing filler of rubber through mixing
and co-coagulating with natural rubber (NR) latex by Liu et al. (145).
Through the comparison of mechanical properties and phase morphology,
MST is proved to be much superior to unmodified starch paste. In contrast,
optimum MST shows obvious reinforcement effect on NR matrix by
increasing tensile strength, elongation at break and tears strength besides
modulus and hardness. Moreover, fine starch dispersion and strong
interfacial interaction are achieved in NR/MST composites.
The physico-mechanical properties of oil palm wood flour (OPWF)
filled natural rubber composites were investigated by Ismail and Jaffri
70 Chapter 1
(146). Increasing OPWP loading in natural rubber compounds resulted in
reduction of tensile strength, tear strength and elongation at break but
increased tensile modulus and hardness. The incorporation of OPWF has
also resulted in the reduction of fatigue life.
Cure characteristics, mechanical properties and morphological
studies of linoleum flour-filled NBR compounds were investigated by
Biagiotti et al. (147). The linoleum flour was obtained from wastes of
linoleum Production. Four filler percentages (10, 20, 30 and 40 phr) were
added to the rubber. Linoleum flour appears to be the most effective,
showing the lower curing time values, eventually leading to a premature
vulcanization of linoleum-filled NBR systems. The mechanical
characteristics of vulcanized compounds show the stiffening effect of all
fillers, underlined by the increment of the elastic modulus. The positive
preliminary results reported in this paper demonstrate the feasibility of
using the wastes of linoleum production as accelerating agents for the
kinetics of vulcanization of rubbery systems, and also reflect the important
value of the final products due to their positive environmental impact.
Thomas and co-workers have also investigated the effect of chemical
modification of sisal fiber (148) and coir fiber (149) in natural rubber. It
was found that modification of fiber resulted in superior mechanical
properties.
Microcrystalline cellulose (MCC) was investigated to partially replace
silica in rubber composites (150). The partial replacement of silica with
MCC significantly reduced the energy required for dispersion of fillers in
rubber matrix and lowered the internal temperature during the
compounding. Moreover, the partial replacement of silica with MCC
reduced mooney viscosity, apparent shear stress, and apparent shear
Introduction 71
viscosity of the rubber composites, which facilitated the manufacturing
process of the rubber composites.
The advantages of using short fibers are that they can be easily
incorporated into the rubber compound. Short fibers also provide high
green strength and dimensional stability during fabrication. Almost all
standard rubber processing operations such as extrusion, calendering,
compression moulding, injection moulding and transfer moulding can be
used for fabrication of composites. Short fiber composites also possess
design flexibility resulting in the fabrication of complex shaped articles.
These composites possess high specific strength, reduced shrinkage,
controlled damping properties, improved swell resistance, increased
abrasion resistance and creep resistance. They are also more
economically viable since dipping, wrapping, laying and placing of fibers
associated with long fiber composites are avoided.
The disadvantages of short fiber reinforced rubber composites are
that it is quite difficult to achieve uniform dispersion, the problem of fiber
breakage during processing (though biofibers undergo less breakage
than synthetic ones). Also certain processing techniques like filament
winding, autoclave and vacuum bag processing techniques cannot be
used for short fiber composites.
1.10 Processing
Some of the common and important processing technique for rubber
composites is given below.
1.10.1 Milling
The first step in milling is to oven-dry the whole fiber to reduce
moisture to below 0.1%. The fibers can also be modified by chemical
treatments to make it more compatible with the rubber matrix. The second
72 Chapter 1
step is the mixing of the treated fiber into the rubber formulation during the
rubber compounding operation in an intensive (banbury) or two roll mills.
The product from this step is a homogenous rubber compound reinforced
with fiber. The compound is heated on a mill roll into manageable sheets
for handling. The final process step is the compression molding at elevated
temperature and pressure to cure the rubber. During mixing in a two roll
mill, high shear forces get developed leading to fiber breakage and the
breakage pattern be studied by means of fiber length distribution curve. It
has been reported that the breakage is more common for synthetic fibers
than natural fibers.
1.10.2. Calendering
Calendering is the process of forming a continuous sheet by
squeezing the material between two or more parallel rolls having the same
or different speeds, the sheets or films produced is suitable for sealant
applications. The sheet surface may be smooth or textured depending on
the roller surfacing. The addition of fibers renders a rubber compound
less elastomeric and extensible and therefore the technique of
calendering is quite difficult.
1.10.3. Extrusion
Extrusion is used for the production of blown films, pipes, wide-
width films, filaments, sheets, rods, hoses and straps. Rubber extrusion
is assisted by the lower rigidity of the organic fiber reinforcement
commonly used. In this area, a number of processability criteria take on
added importance with short fiber reinforced rubber. These are the
direction of fiber orientation, surface appearance, and flow balancing in
the die to minimize tearing and the control of downstream post-die
sizing operations. Elastomeric matrices possess the advantage of green
strength that is essential to allow a free-surface forming operation such
Introduction 73
as extrusion. A unique application of this technology for extruding short
fiber reinforced hose allows the hose to be curved into a shape as it
emerges from the expanding die by mechanically adjusting the die
geometry to effect local variations in the flow uniformity within the die
itself. Another attractive feature is that the fiber reinforced green hose
retains its shape during handling and the bend areas are nearly as
strong as the straight sections. At present, hose reinforced with oriented
natural short length cellulose fiber is available commercially in Europe.
1.10.4. Injection moulding
Injection moulding is a major processing technique for converting
thermoplastics and thermosetting materials into all types of products.
The moulding of short fiber-reinforced rubber composites follows closely
the technology of plastic composites. The natural fiber reinforced
rubbers are less abrasive to machine and tool surfaces causing less
wear than is common with synthetic fiber-plastic resins. The highly
automated injection moulding requires fibers that are shorter and less
concentrated than in compression moulding. An advantage of injection
moulding is its less labor-intensive operation under well-controlled
sequencing for good reproducibility at low cost. The typical parts that
are made from short fiber reinforced rubber composites include
diaphragms, gaskets and certain flexible automotive parts.
1.11 Applications of fiber reinforced rubber compos ites.
Bio based composites with their constituents developed from
renewable resources are being developed and its applications has
extended to almost all fields. The automotive industry over the past
years developed various new components based on natural fiber
composites (151). In Europe, plant fiber composites are mainly used by
the automotive industry. In 1996 the total reported use of natural fibers
74 Chapter 1
did not exceed 4.0 kiloton; by 1999 this has increased to more than 21
kiloton; as reported by the suppliers to the European automotive
industry. Projections for 2010 suggest that the total applications of bio
fibers in the European sector could rise more than 100,000 ton by 2010.
Short fibers have the potential for reinforcing low-performance tires.
In automotive and truck tires they find application in better abrasion
resistance for the chafer strip and in improved cut resistance to treads
especially for trucks and OTR vehicles. Bio based roof structures were
successfully fabricated from soy oil based resin and cellulose fibers in the
form of paper sheets made from recycled card board boxes (152). This
recycled paper was previously tested in composite sheets and structural
unit beams and was found to give the required stiffness and strength
required for roof construction. Recently, researchers have explored the use
of bamboo fiber as reinforcement in structural concrete elements. Pulp
from eucalyptus waste and residual sisal and coir fibers have also been
used as a replacement for asbestos in roofing sheets. Other uses are as
belts diaphragms and gaskets. Some of the other applications are hose,
dock and ship fenders and general uses such as belts, tires and other
industrial articles. Short fibers can reinforce and stiffen rubber in fenders
and other impact applications in accordance with simple design equations.
1.12 Importance of the work
Short fiber reinforced rubber composites are of tremendous
importance both in structural applications and in the area of research and
development. Composites in which short fibers are oriented uniaxially in an
elastomer have a combination of good strength and stiffness from the
fibrous and elasticity from the matrix. Oil palm fiber is the waste by-product
that is amassed after palm oil production. The utilization of oil palm fiber is
therefore an ecological and economical answer to the problem of waste
Introduction 75
disposal. These fibers are relatively stiff and are multicellular, with cells
having different shapes. Oil palm fiber is highly resistant to bacterial
damage. The reinforcement of natural rubber with oil palm fiber was based
on parameters such as fiber length, fiber ratio, fiber loading and fiber
surface modification. Also without reinforcement, natural rubber and nitrile
rubber possess low modulus and stiffness and cannot be used for
structural applications.
Cellulose is the most abundant biomass resource and possesses
high ability from the view point of mechanical and thermal properties.
Cellulose is a classical example of these reinforcing elements, which
occur as whisker like micro fibrils those are biosynthesized and
deposited in a continuous fashion. In many cases this mode of
biogenesis leads to crystalline micro fibrils that are almost defect free,
with the consequence of axial physical properties approaching those of
perfect crystals. The application of this cellulose micro fibril has provided
great interest and expectations among material scientists. Processing
and characterization of starch-cellulose micro fibril composites were
investigated by Angellier et al. (153). The mechanical properties and
water absorption behavior of the resulting composites were found to be
superior to starch particles. Literature survey shows that very few works
are reported about micro fiber reinforced rubber composites. The
preservation of our environment requires that we stop developing
materials that will, like many plastics, last indefinitely. Reinforced plastics
based on natural, mainly plant-derived substances show promise of
providing this and may turn out to be one of the material revolutions of
this century (154). Cellulose based fibers meet such requirements in
almost every aspect. The applications of cellulose fibers as
reinforcements however have been greatly hindered by the lack of the
requisite forms of the fibers (155). The use of agricultural by-products
76 Chapter 1
solves the problem of disposal of agricultural waste. Micro fibers are
separated from oil palm empty fruit bunch fibers by a newer technique
known as steam explosion method. The use of cellulose microfibrils as
reinforcement in polymer matrix is a new and emerging field. Microfibril
composites are a relatively new class of composites that exhibit some
unique and out standing properties with respect to their conventional
composites (156). Research work is going on with other natural fibers
like jute, banana, hemp, flax, sisal etc. Till now, no work has been done
by reinforcing oil palm micro fibers in NR and NBR matrix. These micro
fibers can be used as a reinforcing material in natural rubber and nitrile
rubber by compression molding process. Major attraction is natural
rubber and reinforcing fiber is naturally available. Hence products based
on this work will be cheaper and sustainable. Nirtile rubber reinforced
fiber composites have excellent oil resistance properties and hence
products based on this work have many industrial applications.
Therefore, a detailed and in-depth investigation has been carried out to
study the properties of randomly oriented oil palm micro fiber reinforced
natural rubber and nitrile rubber bio composites
1.13 Scope of the present work
The present work aims at investigating the prospect of using oil palm
micro and macro fiber as reinforcement in NR and NBR matrix. Oil palm
fiber, a waste product of oil palm cultivation, is abundantly available in the
state of Kerala. In India large acres of land is cultivated with oil palm
plantations, which yield tons of the fiber. Oil palm fiber obtained from the
empty fruit bunch has good mechanical properties. In general, the strength
of a fiber increases with increasing cellulose content and decreasing spiral
angle with respect to the spiral axis. The major constituents of oil palm
fiber are cellulose (64%), hemi cellulose (4%) and lignin (19%).These
Introduction 77
properties can be advantageously utilized in reinforcing with NR and NBR
which can increase the stiffness of the matrix and can favor good
mechanical properties. Research work is going on with other natural
fibers like sisal, jute, flax, hemp, kenaf etc. Till now, no systematic work
has been done on micro fibril reinforced rubber composites. Presently oil
palm fibers are underutilized. If they are put to better use they can improve
oil palm cultivation and hence the economy of our nation. This can lead to
their appropriate choice as reinforcement in polymeric and cement
matrices.
1.14 Major Objectives
The objectives of the present work include:
1. Preparation of oil palm micro fibers and its chemical modification.
2. Characterization of oil palm micro fibers by FTIR, solvatochromism
and scanning electron microscopy (SEM).
3. Cure characteristics and green strength measurements of oil palm
macro/micro fiber reinforced NR and NBR composites.
4. Evaluation of the mechanical properties of macro/micro fiber
reinforced NR and NBR composites.
5. Investigation on the dynamic mechanical properties of micro fiber
reinforced NR and NBR composites as a function of fiber loading,
treatment and frequency.
6. Thermogravimetric analysis of micro fiber reinforced NR and NBR
composites with special reference to the fiber loading and
treatment.
7. Solvent sorption behavior of micro fiber reinforced NR and NBR
composites.
78 Chapter 1
8. Ultra violet (UV) ageing and bio-degradation studies of micro fiber
reinforced NR and NBR composites.
9. Evaluation of the dielectric properties of the micro fiber reinforced
NR and NBR composites.
10. Thermo physical characterization of the micro fiber reinforced NR
and NBR composites.
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