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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