Chapter 1

INTRODUCTION

Abstract

This chapter describes the classification and characterisation of composites.

A brief literature survey has been given to have an insight into the current

research works in the subject. Description about green composites and

nanocomposites is made. The scope and major objectives of the present work

have also been highlighted.

Result of this chapter has been communicated for publication in Progress in Polymer Science

2 Chapter 1

1.1 Composites

Developments of more powerful and effective energy sources and maximum

available motive power from them have been a source of constant enquiry.

All technical developments have in fact been centred around these two

vistas. The properties of engineering materials have lent a dependable help in

obtaining motive power from energy sources. For instance, more mechanical

energy is obtained from a gas turbine when the gases stand at a higher

temperature. But the performance of materials in gas turbines is decided by

the maximum workable temperature. There are situations where a suitable

reactor vessel is available in order to with stand the corrosive environment in

the vessel. Many chemical reactions are feasible only then. Many serious

demands of operation or use are successfully met with by seeking

engineering materials. Light weight and stiffness are essential characteristics

for air crafts, aerospace and industries. The structure must be capable to

retain the special properties at high temperature. High strength and corrosion

resistance are unavoidable factors in pressure vessel technology. Weight is

an additional factor to be considered in the case of large pressure vessels. It

is literally impossible for a single material to cater to these requirements at

the same time. When a designer confronts such a problem the best solution is

offered by composite materials.

Composites have gained tremendous industrial importance during the past

few decades and are now considered as engineering materials win an ever-

increasing bonding applications ranging from household articles of everyday

use to supersonic aircrafts and satellites.

During 1896, aeroplane seats and fuel tanks were made of natural fibres with

a small content of polymeric binders. (1). As early as 1908, the first

Introduction 3

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).

Composite materials are engineered materials made from two or more

constituent materials with significantly different physical or chemical

properties which remain separate and distinct on a macroscopic level within

the finished structure. Composites are made up of individual materials

referred to as constituent materials. All the constituents in the composite

retain their identities and do not dissolve or completely merge into each

other. The composite material should be created to obtain properties which

would not be achieved by any of the components acting alone. There are two

categories of constituent materials: The two phases are called matrix and

reinforcement. Matrix is the continuous phase into which the other phase,

often, called the reinforcement or dispersed phase is embedded. The region

between the continuous and dispersed phase is known as the interface. At least

one portion of each type is required. The matrix material surrounds and

supports the reinforcement materials by maintaining their relative positions.

The matrix is the less strong phase being strengthened by the stronger

reinforcing phase. The matrix acts as the bulk material and transfers load

between reinforcement. The reinforcements impart their special mechanical

and physical properties to enhance the matrix properties. A synergism

produces material properties unavailable from the individual constituent

materials, while the wide variety of matrix and strengthening materials

allows the designer of the product or structure to choose an optimum

combination. The matrix also has an additional role, which is to protect the

4 Chapter 1

reinforcement from the environment, abrasion and impact. The matrix has

three main functions.

Functions.

1. To hold the reinforcement in the correct orientation.

2. To protect and reinforcement from damage.

3. To transfer applied loads into the reinforcement.

Reinforcement can have various geometry like particles, fillers, fibres, flakes

or whiskers. The reinforcement provides the strength and stiffness properties

to composite. The form and arrangement of the fillers as they are introduced

to the mould can vary significantly. They can be arranged as short strands of

randomly oriented whiskers, a bundle of fibres, a unidirectional fabric, a

woven fabric, a braid (tubular) fabric or a multi- axial fabric. Reinforcement

may be used in several different forms or arrangements, depending on the

application and manufacturing route. Particles have no preferred directions

and are mainly a means to improve properties or lower cost of isotropic

material. Particles have length to diameter ratios of order unity and dimension

that range from that of a fibre diameter to several millimeters. Whiskers have

length to diameter ratios of order 1000 and diameter of order 0.1-1µm.

Whiskers are pure single crystals manufactured through chemical vapor

deposition and thus have preferred directions. Whiskers are more or less

randomly arranged in the matrix and whisker reinforced composites are likely

to be considered as macroscopically isotropic.

The composites have been classified generally into metal matrix, ceramic

matrix and polymer matrix composites based on the matrix material that

constitutes the composite. Because of the low processing temperature, the

Introduction 5

polymer matrix composites are much easier to fabricate than metal matrix and

ceramic matrix composites. Among the various polymer matrix composites,

fibre reinforced polymer composites have gained much importance in various

fields due to high strength to weight ratio. The most common types of

synthetic fibrous inforcements used in composite applications are glass, carbon

and aramid polyester, nylon and rayon (2-6).

The composites have been classified generally into metal matrix, ceramic

matrix and polymer matrix composites based on the matrix material that

constitutes the composite. Because of the low processing temperature, the

polymer matrix composites are much easier to fabricate than metal matrix and

ceramic matrix composites. Among the various polymer matrix composites,

fibre reinforced polymer composites have gained much importance in various

fields due to high strength to weight ratio. The plant kingdom offers a wealth

of potential resources for composite production. Recently, a large number of

studies have been published in this field (7-11).

1.2 Constituents

The most common types of fibrous reinforcements used in composite

applications are glass, carbon and aramid. Natural fibres have received much

attention from materials scientists and engineers in the past decades because

they are less expensive, lightweight, non-toxicity, ease of recyclability and

biodegradable (12) Light weight materials that involve bio fibre composite

materials are revolutionalising the materials field (13).

1.2.1 Fibres

Plant kingdom contributes profoundly to composite production. In the early

times textiles, ropes, canvas and paper were made of local natural fibres, like

flax and hemp. India continued to use natural fibres, mainly jute fibres as

6 Chapter 1

reinforcements for composites. The growing interest in lignocellulosic fibres

is mainly due to their economical production and also due to the reduction of

the total mass of the composite as a result of the low density of natural fibre.

They also present safer handling and working conditions compared to

synthetic reinforcements. The most interesting aspect about natural fibres is

their positive environmental impact. Biofibres are a renewable resource with

production requiring little energy.

Natural fibres are subdivided based on their origin, coming from plants, animals

or minerals. All vegetable fibres are composed of cellulose whereas fibres of

animal origin consists of proteins(14). low-cost polymeric composites with

reasonable mechanical and tribological properties were successfully developed

using untreated sugarcane fibre (15). Asbestos is an example for mineral fibre.

Plant fibres are composite material designed by the nature. Plant fibres include

bast fibres, leaf fibres, seed/ fruit fibres. Bast consists of a wood core surrounded

by a stem. Within the stem, there are a number of fibre bundles, each containing

individual fibre cells or filaments. Fibres extracted from stems after a process

called retting (16). Examples include flax, hemp, jute, kenaf, and ramie. Leaf

fibres such as sisal, abaca, banana and henequen are coarser than bast fibres.

Cotton is the most common seed fibre. Other examples include coir and oil

palm. Other source of lignocellulosics can be from agricultural residues such as

rice hulls from a rice processing plant, sun flower seed hulls from an oil

processing unit and bagasse from a sugar mill. The properties of natural fibres

vary considerably depending on the fibre diameter, structure, degree of

polymerization, crystal structure and finally whether the fibres are taken from

the plant stem, leaf or seed, and on the growing conditions.

Introduction 7

Jute, sisal, banana and coir (coconut fibre), the major source of natural fibres,

are grown in many parts of India. Some of them have aspect ratios (ratio of

length to diameter) > 1000 and can be woven easily. Sisal and banana fibres

are cellulose-rich (> 65%) and show tensile strength, modulus and failure

strain comparable with other cellulose-rich fibres like jute and flax whereas the

lignin-rich (> 40%) coir fibre is relatively weak and possess high failure strain.

These fibres are extensively used for cordage, sacks, fishnets, matting and

rope, and as filling for mattresses and cushions (e.g. rubberized coir).

Cellulosic fibres are obtained from different parts of plants, e.g. jute and

ramie are obtained from the stem; sisal, banana and pineapple from the leaf;

cotton from the seed; coir from the fruit, and so on.

1.2.1.a Types of natural fibres

Flax:

Flax, Linum usitatissimum, is one of the bast fibres grown chiefly in the

USSR, Poland, France, Belgium and Ireland. The plant cultivated mostly for

its oil-bearing seed (linseed), is also an important source of a vegetable fibre.

Flax plants range in height from 12 to 40 inches, and have shallow taproots.

The same species is used for both fibre and seed, with breeding of

specialized cultivars for the two different products. The seed-producing

varieties have shorter stems and are heavily branched. The fibre varieties

pursue stem development resulting in a taller plant more sparsely branched.

Kenaf:

Kenaf, Hibiscus cannabinus, originating from Africa, has traditionally been a

source of bast fibre in India, China, The Commonwealth of Independent States,

8 Chapter 1

Iran, Nigeria, and Thailand. Kenaf is a newer crop to the United States that shows

good potential as a raw material for use in composite products. Presently, around

4,300 acres of kenaf are cultivated in the United States. 2,000 acres are grown in

Mississippi, 1,200 acres in Texas, 560 acres in California, with lesser amounts in

Louisiana, New Mexico, and Georgia. The plant is a herbaceous annual growing

in single stem to heights of 1 – 4 m. Traditionally, kenaf has been known as a

cordage crop or jute substitute. Research on kenaf first began in the United States

in 1957 and has continued sporadically since that time. Newer advances in

decortications equipment which separates the core from the bast fibre combined

with fibre shortages has renewed recent interest in kenaf as a fibre source.

Hemp:

Hemp fibre is extracted from the plant Cannabis sativa that originated in

Central Asia. From there it spread to China where it is thought to have been

grown for 4,500 years. The plant originally grown for its fibre, came to be

noticed for its narcotic qualities by around 900 BC. Hemp is a strong,

durable, though harsh bast or phloem fibre, having a core, which is

characteristic of hardwood fibre. The bast portion is typically 14%. Hemp is

an annual plant, which at maturity develops a rigid, woody stem ranging in

height from 1.2 - 5 m, and having a diameter from 4 to 20 mm. Generally

hemp is regarded as a tough plant that grows quickly and produces abundant

seed and readily adapts to different niches or areas. Cordage is an area in

which some bast fibres, especially jute, sisal, and hemp, have historically

been strong. Consequently, pultrusion with its use of thread-like material is

especially applicable to these fibres. Cordage machinery having a spectrum

of sophistication and associated processing costs is available to convert these

bast fibre bundles into the appropriate form for such pultrusion products.

Introduction 9

Sisal

This fibre is extracted from the leaves of the plant Agave sisalana which is

widely cultivated in the Western Hemisphere, Africa and Asia. It accounts

for almost half of the total production of all textile fibres. Of the total world

production of 0.6 million tons, India’s share is only 3000 tons. In Kerala,

about 50,000 kg of sisal fibres are extracted every year from the leaves. The

agaves have rosettes of long and narrow fleshy leaves, which grow from a

central bud. As the leaves mature, they gradually spread out horizontally and

are 1-2 m long, 10-15 cm wide and about 6 mm thick at the centre. The

fibres embedded longitudinally in the leaves are most abundant near the leaf

surfaces. Though the leaves contain about 90 % moisture the fleshy pulp is

very firm and the leaves are rigid. Generally by a mechanical decortication

process, i.e, by scrapping away the pulpy material from the leaves the fibre is

removed. In the decortication process, the leaves are fed through sets of

crushing rollers. The crushed leaves are held firmly at their centres and both

ends are passed between pairs of metal drums on which blades are mounted

to scrape away the pulp, and the centers are scraped in the same way. The

fibre strands are then washed and dried.

Coir

Coir is the fibre extracted from the outer husks of coconuts obtained from

coconut tree (Cocos nucifera) and are referred to as 'coco mats' or 'coconut

matting.' The fibres are often obtained by the retting of the outer part of the

fruit followed by drying. More recently with great popularity of natural

flooring it has made its mark as 'Coir carpeting' Coir weaving is a centuries old

craft with the vast majority of the world's production based in the Alleppey

region in Kerala in the south western part of India. Coir products are 100%

10 Chapter 1

natural. Coir carpeting is an 'In-fashion' furnishing accessory, hardwearing,

attractive, warm underfoot, sound insulating, mothproof and rot & fungi

resistant, flame retardant, anti-static, and with regular treatment far more easily

kept clean than its detractors would suggest. Coir pith constitutes about 70 %

of coconut husk with very low density. About 500,000 tons of coconut pith are

produced in India, per annum. Coir fibre is naturally hygroscopic, breathing in

and giving out atmosphere moisture, thus helping to maintain an equable

humidity in the building. This is also the reason why it is always advisable to

fully adhere the material to the sub-floor to avoid stretching or shrinkage. The

coarse structural fibres in coconut fruits, called coir, are composed of vascular

bundles and are similar to the rather stiff, hard fibres obtained from monocot

leaves (manila hemp, sisal, henequen). Because they are tough and naturally

resistant to seawater, coconut fibres are used to make floor mats, heavy cord,

and the coarse nets used in shellfish aquaculture.

Jute

The chief sources of commercial jute are two Indian species (C. capsularis

and C. olitorius), grown primarily in the Ganges and Brahmaputra valleys.

Although jute adapts well to loamy soil in any hot and humid region,

cultivation and harvesting require abundant cheap labour, and India remains

the unrivalled world producer as well as the chief fibre processor. Calcutta is

the main centre. Europe and the United States import large quantities of jute

fibre and clothe; Dundee, Scotland, is also a major jute-textile manufacturer.

The fibre strands in the bark are 6 to 10 ft long (2-3 m) and are separated

from the woody stalk centres by retting. The fibre deteriorates quickly. Due

to its uneven diameter and comparatively low cellulose content, jute fibre is

relatively weak, about 90% is spun into yarn for fabrics; the better qualities

Introduction 11

supply burlap and the poorer grades are used for baling and sacking (e.g.,

gunny sacks). It is also used for twine, rope, carpet, linoleum backing and

insulation. The discarded lower ends, called jute butts, are used for paper

manufacture. This plant, cultivated in India from ancient times, made its

appearance in Western commerce only by about 1830.

Oil palm

Oil palm is one of the most economical and very high potential oil producing

crops. It belongs to the species Elaeis guineensis under the family

Palmaceae, and is commonly found in the tropical forests of West Africa. Its

major industrial cultivation is in the south-east Asian countries such as

Malaysia and Indonesia. Large-scale cultivation has come up in Latin

America. Oil palm cultivation in India has been accelerated with a view to

attain self-sufficiency in oil production. Oil palm empty fruit bunch

(OPEFB) fibre and oil palm mesocarp fibre are two important types of

fibrous materials left in the palm-oil mill. OPEFB is obtained after the

removal of oil seeds from fruit bunch for oil extraction. The average yield of

OPEFB fibre is about 400 g per bunch. The fibres are extracted by retting

followed by cleaning and drying.

Pineapple leaf

The fibres are extracted from the leaves of the plant Ananus comous

belonging to the Bromeliaceae family. They are short-lived perennials or

biennials with a very short main axis and a rosette of leaves. The long,

fleshy, fibrous leaves are sword shaped, dark green and taper to a fine point;

they often have spiny margins. They reach a length of 1 meter or long and

are 5-8 cm wide, grooved on the upper surface and clasps the main axis

12 Chapter 1

closely at the base. These pineapple leaves are a source of very strong

durable fibre. The fibres are shining white when extracted, flexible and is of

good quality.

Banana

Banana fibre, obtained from the pseudo stem of the banana plant, Musa

Sapientum, is another bast fibre. The fibres are extracted either by retting or by

scraping the pithy material with a wooden scraper. The fibres are then washed in

water and hung in the shade to dry. Fibres can be extracted before or after

harvesting the fruit. But, the fibres obtained after harvesting the fruit have better

properties compared to the ones obtained before harvesting. The fibre property

also varies depending on the season and the region in which the plants grow. In

India, approaximately 1.5 million acres of land is for under banana plantation, and

this yields about 3x105 tons of fibre. In Kerala, about 5 lakh acres of land is under

banana cultivation. Normally 600 trees are grown in an acre of land. A stem

weighing about 37 kg yields about 1 kg of good quality fibre as a by product.

Recent reports indicate that plant-based natural fibres can very well be used

as reinforcement in polymer composites, replacing to some extent more

expensive and non-renewable synthetic fibres such as glass (17, 18). Gastón

et al. (19) attempted to understand how the main processing variables are

affected when glass fibres are replaced by natural fibres in reinforced

plastics. In this publication, a jute fabric was characterised in terms of its

saturated and unsaturated permeability. Oaksman (20) found that fluid

absorption and swelling are mechanisms present in natural fibres that reduce

both permeabilities. Fluid absorption removes fluid from the main stream as

it travels through the reinforcement, acting as a sink component and thus

Introduction 13

decreasing flow velocity during the unsaturated flow. Also, the saturation of

the natural fibres cause swelling, reducing the porosity and increasing flow

resistance during saturated flow.

There are many examples of the use of cellulosic fibres in their native

condition like sisal, coir, jute, banana, palm, flax, cotton, and paper for

reinforcement of different thermoplastic and thermosetting materials like

phenol formaldehyde, unsaturated polyester, epoxy, polyethylene, cement,

natural rubber etc.

Different geometries of these fibres, both singly and in combination with

glass, have been employed for fabrication of uni-axial, bi-axial and randomly

oriented composites. Amongst these ligno-cellulosic fibres, jute contains a

fairly high proportion of stiff natural cellulose.

Retted fibres of jute have three principal chemical constituents, namely, α-

cellulose, hemicellulose and lignin. In addition, they contain minor

constituents such as fats and waxes, inorganic (mineral) matter, nitrogenous

matter and traces of pigments like β-carotene and xanthophyll.

Several studies of fibre composition and morphology have found that

cellulose content and microfibril angle tend to control the mechanical

properties of cellulosic fibres. Higher cellulose content and lower microfibril

angle result in higher work of fracture in impact testing.

Sisal and banana fibres show better reinforcing efficiency than coir and the

specific strength properties of the composites are comparable to those of

glass fibre reinforced plastics (GRP). On the other hand, coir fibre, despite

having low strength and modulus, improves the impact resistance of

polyester due to its large strain energy absorption.

14 Chapter 1

Although the tensile strength and Young’s modulus of jute are lower than

those of glass fibres, the specific modulus of jute fibre is superior to that of

glass and when compared on modulus per cost basis, jute is far superior. The

specific strength per unit cost of jute, too, approaches that of glass.

Therefore, where high strength is not a priority, jute may be used to fully or

partially replace glass fibre. The need for using jute fibres in place of the

traditional glass fibre partly or fully as reinforcing agents in composites

stems from its lower specific gravity (1.29) and higher specific modulus (40

GPa) of jute compared with those of glass (2.5 & 30 GPa respectively).

Apart from much lower cost and renewable nature of jute, much lower

energy requirement for the production of jute (only 2% of that for glass)

makes it attractive as a reinforcing fibre in composites.

Lignocellulosic fibres offer many advantages that make them more attractive as

fillers and reinforcements for plastics. These advantages include low density,

low cost, less abrasiveness, higher specific properties, toxinless, easiness to

handle, no health problems like glass fibre which can cause skin irritations

during processing and respiratory diseases when the fibrous dust is inhailed (21).

There are also some environmental and socio-economic advantages that cannot

be ignored. These include biodegradability, the wide variety of fibres available

around the world, the generation of rural jobs, stimulation of non-food

agricultural/farm based economy, low energy consumption, and low energy of

utilization. Despite the advantages mentioned above, the fibres exhibit some

undesirable characteristics, which result in inferior composite properties. These

include the thermal instability above 200oC and high moisture absorption which

can affect both dimensional stability and mechanical properties of the

composite. The hydrophilic nature of lignocellulosic fibres is also responsible

Introduction 15

for insufficient fibre dispersion, poor fibre/matrix compatibility and interfacial

adhesion of the fibres with some hydrophobic thermoplastic matrices. Their low

microbial resistance and susceptibility to rotting limits the service life of

lignocellulosic/thermoplastic composites particularly for outdoor applications

(22). In addition to the above, lignocellulosic fibres also exhibit non uniformity

and variability of their dimensions as well as mechanical properties. However,

natural fibres are undergoing high tech revolution in replacing synthetic

materials from housing to clothing, touching every aspect of our lives. Pipes,

pultruded profiles and panels with polyester matrices were produced with

these fibres (23).

The development of commercially viable “green products” based on natural

resources for both matrices and reinforcements for a wide range of

applications is on the rise. This effort includes new pathways to produce

natural polymers with better mechanical properties and thermal stability

using nanotechnology and use of natural polymers to make biodegradable

plastics and their composites with lignocellulosic fibres (24-30).

1.2.2 Chemical composition of lignocellulosic fibre

The chemical composition and structure of plant fibres depend to a large

extent on the climatic conditions, age and the digestion process of the plant,

which they are derived from. The wt % value of each components in some

plant fibres are presented in table 1.1 (31-32)

16 Chapter 1

Table 1.1 Composition of some plant fibres

Component

Plant fibres

Cotton Jute Flax Hemp Coir Ramie Pine- apple

leaf Banana

Cellulose (wt%) 82.7 61-71.5 64.1-71 70.2-74.4 36-43 68.6- 76.2 70 - 82 63 - 64

Hemicellulose

(wt%) 5.7 13.6- 20.4 16.7- 20.6 17.9-22.4 0.15-0.25 13.1- 16.7 - 19

Pectin (wt%) 5.7 0.2 1.8 - 2.3 0.9 3-4 1.9 - -

Lignin (wt%) - 12 - 13 1.7 - 2.0 3.7- 5.7 41 - 45 0.6-0.7 5-12 5

Waxes (wt%) 0.6 0.5 1.5 - 1.7 0.8 - 0.3 - -

Moisture content(%)

10 10-12.6 10 10.8 8 8 11.8 10 -12

Microfibril angle(%) 8 10 6.2 20.0 41 - 45 7.5 14.0 11

[Reference: J. G.Cook, Handbook of Textile Fibre and Natural Fibres, 4th Ed., Morrow Publishing, England, 1968]

The physical properties of fibres mainly depend on cellulose (content and

orientation of molecules) hemi cellulose and lignin. Hemicelluloses and

pectin are responsible for the bio-degradation, moisture absorption and

thermal degradation of fibre. Individual fibre properties and the structure of

fibre can vary widely depending upon the plant, part of the stem, age,

extraction technique, moisture content etc.

The major component of most plant fibres is cellulose (α-cellulose). Cellulose

is a linear macromolecule consisting of D-anhydroglucose repeating units

joined by β-1, 4-glycosidic linkages with a degree of polymerization (DP) of

around 10,000 (8). Each repeating unit contains three hydroxyl groups. These

Introduction 17

hydroxyl groups and their ability to hydrogen bond play a major role in

directing the crystalline packing and also govern the physical properties of

cellulose materials (8). A single or elementary plant fibre is a single cell

typically of a length from 1to 50 mm and a diameter of around 10-50 µm.

Plant fibres are like microscopic tubes i.e. cell walls surrounding the central

lumen. The lumen contributes to the water uptake behavior of plant fibres (33).

The fibre consists of several cell walls. These cell walls are formed from

oriented reinforcing semicrystalline cellulose microfibrils embedded in a

hemicellulose-lignin matrix of varying composition. Such microfibrils have

typically a diameter of about 10-30 nm, and are made up of 30-100 cellulose

molecules in extended chain conformation and provide mechanical strength to

the fibre. Fig. 1.1 shows the arrangement of fibrils, microfibrils and cellulose

in the cell walls of a plant fibre.

Figure 1.1 Arrangement of fibrils, microfibrils and cellulose in the cell walls

(Ref: Ph.D Thesis of Shirly Annie Paul, Mahatma Gandhi University, India, 2008)

18 Chapter 1

Cellulose

The molecular structure of cellulose, which is responsible for its

supramolecular structure determines many of its chemical and physical

properties. In the fully extended molecule, the adjacent chain units are

oriented by their mean planes at the angle of 1800 to each other. Thus, the

repeating unit in cellulose is the anhydrocellulobiose unit and the number of

repeating units per molecule is half the DP. This may be as high as 14000 in

native cellulose. The structure of cellulose is given figure 1.2.

Figure 1.2. Structure of cellulose

(Reference: A.K.Bledzki, J.Gassan, Prog. Polym. Sci., 24, 221, 1999)

As the mechanical properties of natural fibres depend on the cellulose type,

each type of cellulose has its own cell geometry and the geometrical

conditions determine the mechanical properties. Solid cellulose forms a

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

regions and regions of low order i.e. amorphous regions. Cellulose is also

formed of slender rod like crystalline microfibrils. The crystal nature

(monoclinic sphenodic) of naturally occurring cellulose is known as

cellulose I. Cellulose is resistant to strong alkali (17.5 wt%) but is easily

hydrolyzed by acid to water-soluble sugars. Cellulose is relatively

resistant to oxidizing agents.

Introduction 19

Hemicellulose

Hemicellulose is not a form of cellulose at all. It comprises a group of

polysaccharides (excluding pectin) that remains associated with the cellulose

after lignin has been removed. The hemicellulose differs from cellulose in

three important aspects. In the first place they contain several different sugar

units whereas cellulose contains only 1,4-β-D-glucopyranose units. Secondly

they exhibit a considerable degree of chain branching, where as cellulose is

strictly a linear polymer.

Thirdly, the degree of polymerization of native cellulose is ten to one

hundred times higher than that of hemicellulose. Unlike cellulose, the

constituents of hemicellulose differ from plant to plant (34). Figure 1.3

depicts the structure of hemicellulose.

Figure 1.3 Structure of hemicellulose

(Reference: A.K Bledzki, J.Gassan, Prog. Polym. Sci., 24, 221, 1999)

Lignins

Lignins are complex hydrocarbon polymers with both aliphatic and aromatic

constituents. (35).

Their chief monomer units are various ring-substituted phenyl propanes

linked together in ways, which are still not fully understood. The mechanical

properties are lower than those of cellulose (36).

20 Chapter 1

Figure 1.4 Structure of lignin

(Reference: A.K Bledzki, J.Gassan, Prog. Polym. Sci., 24, 221, 1999)

Lignin is totally amorphous and hydrophobic in nature. It is the

compound that gives rigidity to the plants. Lignin is considered to be a

thermoplastic polymer exhibiting a glass transition temperature of around

90°C and melting temperature of around 170°C (37).

It is not hydrolyzed by acids, but soluble in hot alkali, readily oxidized

and easily condensable with phenol (35). The structure is presented in

figure 1.4.

Pectins and waxes

Pectin is a collective name for heteropolysaccharides, which consist

essentially of polygalacturon acid. Pectin is soluble in water only after a

partial neutralisation with alkali or ammonium hydroxide. It provides

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

of different types of alcohols, which are insoluble in water as well as in

several acids.

Introduction 21

1.2.3 Micro structure of plant fibres

The cell wall in a fibre is not a homogenous membrane. Each fibril has a

complex, layered structure consisting of a thin primary wall that is the

first layer deposited during cell growth encircling a secondary wall. The

secondary wall is made up of three layers and the thick middle layer

determines the mechanical properties of the fibre. The middle layer

consists of a series of helically wound cellular microfibrils formed from

long chain cellulose molecules: the angle between the fibre axis and the

microfibrils is called the microfibrillar angle. The characteristic value for

this parameter varies from one fibre to another. Figure 1.5 shows the

structure of a biofibre.

Figure 1.5. Microstructure of plant fibre

(Ref: M.Z. Rong et al., Comp. Sci. Tech., 61, 1437, 2001)

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

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

This provides mechanical strength to the fibre. The amorphous matrix

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

and in some cases pectin. The hemicellulose molecules are hydrogen

22 Chapter 1

bonded to cellulose and act as cementing matrix between the cellulose

microfibrils, forming the cellulose-hemicellulose network, which is thought

to be the main structural component of the fibre cell. The hydrophobic lignin

network affects the properties of other network in a way that it acts as a

coupling agent and increases the stiffness of the cellulose/ hemicellulose

composite.

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

chemical composition of fibres are the most important variables that

determine the overall properties of the fibres (38).

The microfibrillar angle, cellulose content and moisture content determine

the mechanical properties of the cellulose based natural fibres (39).

Generally, tensile strength and Young’s modulus of fibres increase with

increasing cellulose content. The microfibrillar angle determines the stiffness

of the fibres. Plant fibres are more ductile if the microfibrils have a spiral

orientation to the fibre axis. If the microfibrils are oriented parallel to the

fibre axis, the fibres will be rigid, inflexible and have high tensile

strength.When the specific modulus of natural fibres is considered, they

show comparable or better value than those of glass fibres (40).

Unlike man-made synthetic fibres, natural fibres, extracted from plants have

various geometry. The fibre length and diameter are main parameters, which

have certain influence on the reinforcing capabilities in composites. This

geometrical pattern of the extracted fibres varies not only from one plant to

another but vary from one part of the palnt to other. Table 1.2 gives an idea

on the structural parameters of different cellulosic fibres (41).

Introduction 23

Table 1.2. Structural parameters of different fibres

Fibre Spiral Angle(θo)

Cross sectional area Ax10-2

(mm2)

Cell-length L(mm)

L/D ratio (D is the cell diameter)

Jute 8 0.12 2.3 110

Flax 10 0.12 20 1687

Hemp 6.2 0.06 23 960

Ramie 7.5 0.03 124 3500

Sisal 20 1.10 2.2 100

Coir 45 1.20 3.3 35

(E. T. N. Bisanda, M. P. Ansell, J. Mater. Sci., 27, 1690-1700, (1992)

Mechanical properties of natural fibres, like geometrical properties, vary to a

large extent. Natural fibres exhibit considerable variation in diameter along

with the individual bundles. Quality as well as most of their properties

depend on the factors like size, maturity as well as the processing methods

adopted for the extraction of fibres. Properties such as tensile strength,

modulus etc. strongly depend on the internal structure and chemical

composition of fibres. Table 1.3 gives us a comparison regarding the

mechanical properties of natural fibres and synthetic fibres (30).

24 Chapter 1

Table 1.3. Mechanical properties of natural fibres as compared to various synthetic fibres

Fibre Density (g/cm3)

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

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

(Ref: AK Bledzki, S Reihmane, J Gassan, J Appl Polym Sci, 29, 1329, (1996)

1.2.4 Thermal stability of natural fibres

Thermogravimetry is one of the most widely used techniques to monitor the

composition and structural dependence on the thermal degradation of natural

cellulose fibre. This is because the different compositions and supramolecular

structures of cellulose behave differently when undergoing thermal

degradation. Natural fibre is composed mainly of cellulose, hemicelluloses and

Introduction 25

lignin. Each of the three major components has its own characteristic

properties with respect to thermal degradation, which are based on polymer

composition. However, the microstructure and three-dimensional nature of

natural fibre are variables that also play important roles in terms of their

effects on combustion behavior. Thus, the individual chemical components of

fibre behave differently if they are isolated or if they are intimately combined

within each single cell of the fibre structure (42).

Lignin, specifically the low molecular weight protolignin, degrades first and

at a slower rate than the other constituents.

1.2.5 Advantages and disadvantages of plant fibres

Plant fibres are advantageous to synthetic fibres due to the following reasons

(45). These fibres are mainly based on cellulose, which is the most abundant

material on our planet and so are inexpensive. They are produced to the

extent of about 4.4 x 1014 kg annually by plants and are renewable. The

growing interest in lignocellulosic fibres is mainly due to their economical

production with few requirements for equipment and low specific weight,

which results in a higher specific strength and stiffness when compared to

glass reinforced composites. They are carbon dioxide neutral i.e. they do not

return excess carbon dioxide into the atmosphere when they are composted

or subjected to combustion. They are amenable to physical, chemical and

mechanical modifications. They possess high flexibility and their breaking

resistance is high. Hence they undergo bending rather than breakage unlike

synthetic fibres (44).

The chemical structure includes rigid cellulose fibres immersed in soft lignin

matrix. Rigid cellulose possesses very high modulus of rigidity among

26 Chapter 1

polymers. Their rough surfaces lead to mechanical anchorage and this

increases fibre-matrix adhesion. Plant fibres are nonabrasive to mixing and

moulding equipment, which can contribute to significant cost reductions. The

most interesting aspect about natural fibres is their positive environmental

impact and they cause no health hazards. They are susceptible to microbial

decomposition, including mildew, aerobic bacteria, fungi, moths, carpet

beetles, termites and rot. Hence these are biodegradable. Biofibres possess

high electrical resistance. Thermal recycling is also possible. The hollow

cellular structure provides good acoustic insulating properties. With the

exception of mineral fibres, all natural fibres have an affinity for water. This

strong affinity results in the swelling of the fibres, which facilitates dyeing in

watery solutions.

Although plant fibres have several advantages over synthetic fibres they have

some disadvantages resulting from their chemical composition and structure.

There is a major drawback associated with the application of natural fibres

for reinforcement of resin matrices. Due to presence of hydroxy and other

polar groups in various constituents of natural fibre, the moisture uptake is

high (approx. 12.5% at 65% relative humidity & 20oC) by dry fibre. All this

leads to:

• poor wettability with resin and

• weak interfacial bonding between the fibre and relatively more

hydrophobic matrices.

Environmental performance of such composites is generally poor due to

delamination under humid conditions. With increase in relative humidity up

to 70%, the tenacity and Young’s modulus of jute increases but beyond 70%,

Introduction 27

a decrease is observed. Thus, it is essential to pre-treat the fibre so that its

moisture absorption is reduced and the wettability by the resin is improved.

1.2.6 Modification of natural fibre

In order to develop composites with better mechanical properties and

environmental performance, it is necessary to impart hydrophobicity to the

natural fibres by chemical reaction with suitable coupling agents or by

coating with appropriate resins.

Such surface modification of fibre does not only decrease moisture

adsorption, but also concomitantly increases wettability of fibres with resin

and improve the interfacial bond strength, which are critical factors for

obtaining better mechanical properties of composites.

Modification of jute and other natural cellulosic fibres can be done by

following means:

• Chemical means

• Coating with polymeric solutions and

• Graft co-polymerization.

Natural fibre is chemically treated with isopropyl triisostearoyl titanate

(abbreviated as titanate), γ -aminopropyl trimethoxy silane (abbreviated as

silane), sebacoyl chloride (SC), and toluene di-isocynate (TDI). All these

reagents are expected to block the hydroxy groups of jute thus making the fibres

more hydrophobic. These surface modifiers penetrate and deposit into lumens of

cell wall of fibre, minimizing the possible extent of moisture ingress.

Polymeric coating of natural fibre with phenol-formaldehyde or resorcinol

formaldehyde resins by different approaches are highly effective in enhancing

28 Chapter 1

the reinforcing character, giving as high as 20-40% improvement in flexural

strength and 40-60% improvement in flexural modulus. These modifications

improve the fibre-resin wettability and lead to enhanced bonding.

Natural fibre such as jute can be graft co-polymerized with vinyl monomers

such as methyl methacrylate, ethyl acrylate, styrene, vinyl acetate,

acrylonitrile & acrylamide in presence of different redox initiator systems

such as vanadium-cyclohexanol, vanadium-cyclohexanone etc.

Grafting of poly-acrylonitrile (10-25%) imparts 10-30% improvement in

flexural strength and flexural modulus of the composites. Grafting of

polymethylmethacrylate is also effective in this respect, though to a lower

degree.

1.3 Matrix

The constituent that is continuous and is often (but not always) present in

greater quantity is termed as matrix. Matrix provides toughness and ductility

to the composite. They have a strong influence on several mechanical

properties of the composite such as transverse modulus and strength, shear

properties and properties of compression. They also bind the dispersion

phase together and cause them to act as a team in resisting failure or

deformation under an applied load. The main role of the matrix is to transmit

and distribute stresses along the reinforcement phase. Physical and chemical

characteristics of the matrix such as melting or curing temperature, viscosity,

and reactivity with fibres influence the choice of fabrication process.

Commonly used matrix materials include polymers, metals, and ceramics.

Introduction 29

1.3.1. Polymers

Polymers are the most widely used matrix materials for fibre composites.

Their chief advantages are low cost, easy processability, good chemical

resistance and low specific gravity. On the other hand, low strength, low

modulus and low operating temperatures limit their use. They are also

degraded by prolonged exposure to ultraviolet light as well as some solvents.

1.3.1.1. Thermosetting, thermoplastic and rubbery polymers

Based on their structure and behaviour, polymers can be classified as

thermoplastics or thermosets. The thermoplastics are incorporated into the

composite system by melting and then they are solidified by cooling; the

physical reaction being reversible in nature. It consists of linear or branched-

chain molecules having strong intramolecular bonds but weak intermolecular

bonds. They can be reshaped by application of heat and pressure and are

either semicrystalline or amorphous in structure. Examples include

polyethylene, polystyrene, nylons, polycarbonate, polyacetals, polyamide-

imide, polyether-ether ketone, and polysulfone polyphenylene sulfide and

polyether imide. Thermosetting plastics have crosslinked or network

structures with covalent bonds between all molecules. They do not soften but

decompose on heating. Once solidified by cross-linking process, they cannot

be reshaped. Common examples of thermosetting polymers include epoxies,

polyesters, phenolics, ureas, melamine, silicone and polyimides.

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

30 Chapter 1

suspension systems. Polymerisation of rubber yields highly branched

polymers. Branching supports good tack and a strong bond in adhesive

applications. The physically entangled structure of this kind of polymer also

provides a significant improvement in tear strength. The natural resistance to

flow makes them excellent candidates for compression molding and

sponge.Other applications are thin walled or complex extrusions where shape

retention is important.

1.3.1.2 Elastomers

Elastomers are amorphous polymers existing above their glass transition

temperature, so that considerable segmental motion is possible. At ambient

temperatures, rubbers are thus relatively soft and deformable. Their primary

uses are for seals, adhesives and molded flexible parts. Natural rubber (NR)

is an elastomer (an elastic hydrocarbon polymer) that was originally derived

from a milky colloidal suspension, or latex, found in the sap of some plants.

Natural rubber is a unique biomass. It is the only one polymeric hydrocarbon

among many biopolymers whose source is almost Hevea brasiliensis. NR is

an indispensable material for many industrial and household applications,1

which is constituted of cis-1,4-polyisoprene (ca. 94%) and non-rubber

components such as proteins (ca. 2%) and lipid (ca. 3%). The versatility of

the plant-derived NR is mainly due to its outstanding tensile properties and

the good crack growth resistance.

Crude rubber is a tough and an elastic solid. It becomes soft and sticky as the

temperature rises. The purified form of natural rubber is the chemical

polyisoprene which can also be produced synthetically. The structure of

natural rubber is given below.

Introduction 31

There may be as many as 11,000 to 20,000 isoprene units in a polymer chain

of natural rubber. Its specific gravity is ~0.915. Natural rubber has a broad

bimodal molecular weight distribution. The polydispersity or the ratio of

weight-average molecular weight to number-average molecular weight,

Mw/Mn, can be as high 9.0 for some variety of natural rubber. Natural rubber

is used extensively in many applications and products as is synthetic rubber.

The use of natural rubber (NR) is widespread in everyday life through many

household or industrial applications, thanks to its outstanding physical

properties such as high stress at break and good crack growth resistance.

1.3.1.3. Polyester resin

The workhorse of thermoset matrices is unsaturated polyester, which offers an

attractive combination of low price, reasonably good properties and

uncomplicated processing. A polyester resin is unsaturated (reactive) polyester

solid dissolved in a polymerizable monomer. Unsaturated polyesters are long

chain linear polymers containing a number of carbon double bonds. They are

made by a condensation reaction between a glycol and an unsaturated dibasic

acid (maleic or fumaric). The polymerizable monomer such as styrene which

also contains carbon double bonds, act as cross linking agent by bridging

adjacent polyester molecules at their unsaturation points. The monomer also acts

as a diluent, reduces viscosity, and makes it easier to process. The curing or

cross linking process is initiated by adding a small quantity of a catalyst like

organic peroxide or an aliphatic azo compound. Since there is no by-product of

the reaction, the curing is done at room temperature or elevated temperature

32 Chapter 1

with or without application of pressure. The structure of typical polyester made

from maleic acid and diethylene glycol is shown below.

HOC

OCH CH C

O

OH+ HOCH2CH2OCH2CH2OH

HO CH2CH2OCH2CH2OC CH CHC O n

O O

H2O+H

(Ref: B.D.Agarwal., L.J Broutman. Analysis and Performance of Fibre Composites, John Wiley and Sons. Inc. 1990 )

The length of the molecule or degree of polymerization may vary. The resin

will generally be a solid but is dissolved in a monomer such as styrene. The

solution viscosity can be controlled by the percentage of styrene. The

conversion from liquid to solid occurs through the use of a free radical

initiator or curing agent. The styrene monomer cross-links react with the

double bond in the polyester backbone above to form a network polymer as

indicted below.

CCOO OOCC CCOO OOCC CCOO OOCCOOCCR CCOO

CCOO OOCC CCOO OOCC CCOO OOCCOOCCR CCOO

(St) (St) (St) (St)

(St)(St)(St)(St)

(St) (St) (St) (St)

CH2 CH

St = Styrene link

Introduction 33

1.3.1.4 Epoxy resin

Epoxies are generally seen in fields where the cost tolerance is the highest,

eg. aerospace, defense and sports applications. Epoxy resins are low

molecular weight organic liquids containing a number of epoxide groups,

which are three-membered rings with one oxygen and two carbon atoms. The

most common process for producing epoxies is the reaction of

epichlorohydrin with bisphenol-A amino or acid compounds, and cross-

linking is obtained by introducing chemicals that react with the epoxy and

hydroxy groups between the adjacent chains. The chemical reaction to form

epoxy resin pre-polymer is as shown below.

CH2 CHCH2Cl + OHO

2 HO C

CH3

CH3

CH2 CHCH2O O

OCH2CH2CH CH2+ 2HCl

OC

CH3

CH3

Epoxy systems, like polyesters, can be cured at room temperature, but quite

often heat is added to accelerate and improve curing. The choice of curing

agent dictates whether a room temperature or elevated temperature cure is

required.

1.3.1.5 Phenolic resin

Phenolic resins have been in commercial use longer than any other synthetic

polymer except cellulose nitrate. In contrast to the latter, however, the sales

of phenolic resins continued to rise at about 15% per year, reaching a peak of

about 1.3 billion-lb in 1979. Phenols react with aldehydes to give

34 Chapter 1

condensation products if there are free positions on the benzene ring ortho

and para to the hydroxyl group. Formaldehyde is by far the most reactive

aldehyde and is used almost exclusively in commercial production. By far

the largest use of the phenolics is in heat-setting adhesives for plywood.

Generally the interaction of cellulose fibre with PF resin is splendid due to

the hydrophilic nature of cellulose and PF resin. This is shown schematically

in Fig. 1.6.

Jackson et al. (45) investigated the toughening of phenolic thermoset and its

composites reinforced with sisal fibres, using hydroxyl-terminated

polybutadiene rubber (HTPB) as both impact modifier and coupling agent.

Substantial increase in the impact strength of the thermoset was achieved by

the addition 10% of HTPB.

Figure 1.6 Schematic representation of interaction of cellulose fibre with PF resin (Ref: B. M. Cherian, Unpublished Work)

H2C

CH2

O

CH2

CH2

O

CH2

CH2

O

CH2

o

O

O

CH2OH o

O

O

CH2OH

O

o

O

CH2OH

o

O

O

HOH2Co

O

O

HOH2C

Oo

O

HOH2C

H2C

CH2

HO

CH2

CH2

CH2

H2C

CH2

Cured Composite

o

HO OH

O

HOH2C

CH2 O

H OH

CH2

O

H

CH2 O H

o

OHHO

O

CH2OH o

OHHO

O

CH2OH

HO

CH2

O

H

CH2

O

H

O H

o

HO

O

HOH2C

Resole PF

Curing

Introduction 35

1.3.1.6 Other Thermosetting polymers

The important classes of amino resins are the condensation products of urea

and of melamine with formaldehyde. In general, the melamine resins have

somewhat better properties but are higher in price. A distinct advantage of

the amino resins over the phenolics is the fact that they are clear and

colourless, so that objects of light or pastel colour can be produced. Urethane

polymers contain the group –NHCOO- and are formed through the reaction

of a diisocyanate and a glycol. In the production of urethane foams, excess

isocyanate groups in the polymer react with water or carboxylic acids to

produce carbondioxide, blowing the foam, at the same time that crosslinking

is effected. Thermosetting polyurethane moulding compounds fabricated by

reaction injection moulding provide a new use for this polymer type,

fabricated by what may become a major new processing technique. Silicone

polymers are particularly noted for their stability at temperatures as high as

1500oC. The variety of products available ranges from liquids through

greases and waxes to resins and rubbers. Silicone polymers are produced by

intermolecular condensation of silanols, which are formed from the halide or

alkoxy intermediates by hydrolysis.

1.4 Natural fibre reinforced polymer composites

Natural fibre are now emerging as a realistic alternative to wood filled and

glass reinforced plastics. Eco friendly and bio composites are partial solution

to many global environmental problems. 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 (46).

Natural fibres have a specific weight half that of glass fibres and a tensile

36 Chapter 1

modulus almost as high as that for aramid fibres. They cause no damage by

abrasion to the processing machines as glass fibres do. (1, 47-49)

They can very well be used as reinforcement in polymer composites

replacing the more expensive and non-renewable synthetic fibres (47).

Abundant availability of natural fibres such as jute, coir, sisal, pineapple,

ramie, bamboo, banana etc., has focussed on the development of natural fibre

composites primarily to explore value-added application avenues. Such

natural fibre composites are well suited as wood substitutes in the housing &

construction sector. The developments in composite material after meeting

the challenges of aerospace sector have cascaded down for catering to

domestic and industrial applications. Composites, the wonder material with

light-weight, high strength-to-weight ratio and stiffness properties have come

a long way in replacing the conventional materials like metals, woods

etc.The material scientists all over the world focused their attention on

natural composites reinforced with jute, sisal, coir (coconut fibre), pineapple

etc. primarily to cut down the cost of raw materials.The natural fibre

composites can be very cost-effective material especially for building &

construction industry (panels, false ceilings, partition boards etc.), packaging,

automobile & railway coach interiors and storage devices.

The different combinations of composites are thermoplastic matrix

reinforced with natural fibres, thermosetting matrix reinforced with natural

fibres and elastomers reinforced with natural fibres. In the composites, based

on synthetic plastic matrix reinforced with natural fibres, synthetic plastics,

both thermosets and thermoplastics, are being used as matrix. Mechanical

performances of synthetic plastics are considered as sufficient for many

structural applications. Environmental impact of fully synthetic composites

Introduction 37

can be substantially reduced by the replacement of synthetic reinforcement

by natural fibres.Various attempts have been made recently to utilize

abundantly available natural fibres like banana, sisal, coir, oil palm, hemp

and wood fibres in polymer matrices like polyester, epoxy and phenolics to

be used as building materials for an assortment of applications (50-53).

Building panels and roofing sheets from bagasse /phenolics were installed in

houses in Jamaica, Ghana and Philippines (54).

Attempts were made to prepare wall panels and roofing sheets using

jute/polyester/epoxy/polyurathaneresin for temporary shelters, bunker

houses, post office boxes and helmets (55).

Composite laminates/panels can be prepared using non-woven/woven/sisal/

jute/coir mats and unsaturated polyester/phenolic/ polyurethane resin by a

compression moulding technique (55).

Composites of good performance formed from non-woven mats of flax and

hemp fibres and natural resin matrices have been prepared by Pizzi et al.

(56). Both higher density thin composites as well as lower density thicker

composites have been prepared.

Short randomly oriented intimately mixed banana and sisal hybrid fibre

reinforced polyester composites having varying volume fractions of fibre

were fabricated by compression moulding and resin transfer moulding

techniques by keeping the volume ratio of banana and sisal,1:1 and the

mechnical properties were evaluated. Resin transfer moulded composites

showed improved ststic snd dynamic mechanical properties compared with

that of compression moulded composites (57).

38 Chapter 1

The influence of oil palm empty fruit bunch fibre grafted with poly methyl

methacrylate on the tensile properties of poly vinyl chloride was investigated

by Baker et al. (58).

Alvarez et al. (59) studied the mechanical properties of natural fibres and

high performance thermoplastic matrices like polyamides composites to

develop natural fibre composites for substituting glass fibres without

renouncing their mechanical properties. They used different natural fibres

like flax, jute, pure cellulose and wood pulps and melt compounded with

different polyamides to anlyse the effect of fibre content on mechanical

properties. The properties were found to be improved in comparison with

unreinforced matrix. These natural fibre reinforced composites offer a wealth

of possibilities for industrial applications. The cellulose triacetate optical

films prepared from ramie fibre are suitable as protective films for the liquid

crystal displays. They showed a high transparency of 89% with excellent

mechanical properties (60).

Ford Motor Company has been researching for natural fibres applications in

vehicles since the 1930s. Gerard Mougin et al. studied natural-fibres

composites for the automotive industry (61).

Natural fibres are low-cost, recyclable, and eco-friendly materials. Due to

eco-friendly and bio-degradability characteristics of these natural fibres, they

are considered as strong candidates to replace the conventional glass and

carbon fibres.

Due to an occurrence of a wide variety of natural fibres in the country, Indian

researchers have directed efforts for quite some time in developing

innovative natural fibre composites for various applications. Development of

Introduction 39

diversified composite materials as wood substitutes is being considered an

attractive solution with a view to conserve forest resources.

1.4.1 Different types of lignocellulosic fibre composites

Most plastics themselves are not suitable for load bearing applications due to

their lack of sufficient strength, stiffness and dimensional stability. However

vegetable fibres possess sufficient strength and stiffness but are difficult to

use in load bearing applications by themselves because of their fibrous

structure. In lignocellulosic fibre reinforced composites, the fibres serve as

reinforcement by giving strength and stiffness to the structure while the

plastic matrix serve as the adhesive to hold the fibres in place so that suitable

structural components can be made. The matrix for the lignocellulosic fibres

includes thermosets, thermoplastics and rubber. Different plant fibres and

wood fibres are found to be interesting reinforcements for rubber,

thermoplastics and thermosets (62-65).

1.5 Thermoplastic composites

Thermoplastic polymers constitute an important class of materials with a

wide variety of applications. Thermoplastic natural fibre composites are

gaining acceptance due to renewed interest in the environment. The trend

towards recycling, protection of natural resources and biodegradability is the

driving force behind the increased use of natural fibre thermoplastic

composites. The surface treatments of bagasse fibre (BF) with benzoic acid

as a surface/interface modifier and the mechanical properties of BF/Poly

Vinyl Chloride (PVC) composites were studied by Zheng et al. (66). The

experimental results indicated that the ratio of PVC/BF, the content of

benzoic acid and the processing temperature had a significant effect on the

40 Chapter 1

mechanical properties of the composite. The effect of silane coupling agents

on the mechanical and morphological properties of luffa fibre/PP composites

was studied by Demir et al. (67). They reported that the use of silane

coupling agents increased the interfacial adhesion between luffa fibre and PP.

They used AFM to study the morphology of the fibres and revealed that the

use of silane coupling agents decreased the surface roughness of the fibres.

Effect of a novel compatibilizer with isocyanate functional group on

mechanical properties of wood-fibre reinforced PP composites was studied by

Karmarker et al. (68). The addition of compatibilizer resulted in a greater

increase of the tensile properties. Arbelaiz et al. (69) prepared composites with

biodegradable thermoplastic polymer poly (ε-caprolactone) and flax fibre.

They studied the thermal and mechanical properties of the composites and

reported that the addition of flax fibres and the compatibilizer increased the

mechanical properties of the composite while the thermal properties of the

composite decreased with the addition of flax fibres and matrix modifiers.

Dynamic mechanical and thermal properties of MAPE treated jute/HDPE

composite were investigated by Mohanty et al. (70). They observed observed

an increase in storage modulus and thermal stability of HDPE with the

addition of fibre reinforcement and MAPE.

1.6 Thermoset composites

Natural fibre thermoset composites are now finding extensive uses in various

fields from household articles to automobiles. Thermosets are brittle at room

temperature and have low fracture toughness values and also due to

crosslinking, reheating cannot reshape the thermosets. But there are many

advantages resulting from the presence of crosslinks. Thermosets can be used

at high temperatures and have better creep properties than thermoplatics and

Introduction 41

also thermosets are more resistant to a chemical attack than thermoplastics.

Polyester, epoxy, phenolics and vinyl ester resins are commonly used for

preparing such natural fibre composites. A wide variety of natural fibres like

coir, sisal, oil palm, banana, jute, pineapple and sun hemp with thermoset

matrices have been studied by several authors (70-75). Fabrication techniques

suitable for manufacturing natural fibre reinforced thermoset composites

include the hand lay up technique for unidirectional fibres / mats / fabric, sheet

moulding (SMC)/bulk moulding (BMC) for short and chopped fibres, filament

winding and pultrusion for continuous fibres. Thermo physical properties of

natural fibre reinforced polyester composites was reported by Idicula et al.

(76). The results showed that chemical treatment of the fibres reduces the

composite thermal contact resistance. Hybridisation of natural fibre with glass

fibre allows a significantly better heat transport ability of the composite. In

another interesting study, the static and dynamic mechanical properties of

vinyl ester resin matrix composite reinforced with shellac treated jute yarns

was investigated by Ray and Gupta (77). They observed an increase in static

and dynamic mechanical properties of the composites treated with 1% shellac

solution. The role of fibre/matrix interaction on the dynamic mechanical

properties of chemically treated BF/polyester composites was investigated by

Pothan et al. (78). Increased dynamic modulus values and low damping

values of the composite showed the improved interactions between the fibre

and the matrix. The use of short palm tree lignocellulosic fibres as a

reinforcing phase in polyester and epoxy matrices has been reported by

Kaddami et al. (79). The morphology and the mechanical properties of the

resulting composites were characterised using scanning electron microscopy

analysis, differential scanning calorimetry, dynamical mechanical analysis

and three-point bending tests. It was shown that the interfacial adhesion was

42 Chapter 1

better in the case of epoxy-based composites. In order to improve interfacial

adhesion, the esterification of the lignocellulosic filler in alkaline medium

was performed using acetic and maleic anhydrides. They concluded that such

type of chemical modification, which led to a change in the chemical

composition of the filler, only succeeded to improve mechanical properties

of the epoxy-based composites.

1.7 Rubber composites

Fibre reinforced rubber composites are important both in the end use

applications and in the area of research and development. These composites

exhibit the combined behavior of the soft, elastic rubber matrix and the stiff

strong fibrous reinforcement. Although both short and long fibres are potentially

useful as reinforcement for rubbers, the use of short fibres are preferred in

product such as V-belts and hoses because of their easy processability and high

green strength in addition to the possibility of producing complex shaped

articles. The principal class of rubbers that have been used for short fibre

composites are natural rubber (NR), styrene- butadiene rubber (SBR), butyl

rubber (IIR), butadiene rubber(BR), nitrile rubber (NBR), chloroprene rubber

(CR), ethylene polypropylene diene rubber (EPDM), polyurethane rubber,

silicon rubber and thermoplastic elastomers. The most widely used rubber

matrix is natural rubber. The primary effect of short fibre reinforcement on the

mechanical properties of natural rubber composite include increased modulus,

increased strength with good bonding at high fibre concentration, decreased

elongation at break, greatly improved creep resistance, increased hardness and a

substantial improvement in cut, tear and puncture resistance of the composite.

Various synthetic fibres such as glass, rayon, nylon, asbestos, aramid and

cellulose have been studied as reinforcement in both synthetic and natural

Introduction 43

rubber composites (52). Incorporation of short fibres into rubber compounds

imparts good strength and stiffness to the rubber matrix (80). Byars (81)

reported the processing advantages and improvements in the mechanical

properties of the short fibre. The reinforcement of coir fibre in natural rubber

has been extensively studied by Geethamma et al. (82). It was seen that tensile

strength decreases sharply with increase in fibre loading up to 30 phr, and then

showed a slight increase for composites containing 40 and 60phr fibre loading.

In an interesting study, researchers have used a novel fibre isora in natural

rubber (83). Swelling behaviour of isora/natural rubber composite in aromatic

and aliphatic solvents were studied. Researchers have also designed novel

rubber biocomposites by using a combination of leaf and fruit fibre in natural

rubber (84). The incorporation of sisal and coir fibre in NR was shown to

increase the dielectric constant of the composites.

1.8 Textile composites

Textile composites form another important class of composites. Woven fabric

reinforced composites are the most widely used form of textile structural

reinforcement (85,86). The increased interest in textile reinforcements is due to

several factors like their strength, lower production cost and improved

mechanical properties when compared to their non-woven counterparts.

Moreover, textile structural composites are associated with near net shape and

cost effective manufacturing process. The process of weaving in which the

fabric is formed by interlacing warp and weft (fill) strands/yarns forms woven

fabrics. Lateral cohesion is a serious problem encountered in the preparation of

the reinforcing elements, but this can be overcome through woven

reinforcements. Twisted yarns have been reported to increase the lateral

cohesion of the filaments as well as facilitate their easier handling (87). By

44 Chapter 1

twisting the yarns the possible micro damages within the yarn can be

localized, leading to a possible decrease in the failure strength of the yarn.

Their use in the fabrication of structures with high mechanical performance

is increasing in the field of aeronautics, naval construction and automobile

engineering (88). Since they provide excellent integrity and conformability

for advanced structural composite applications, woven fabrics are viable and

attractive as reinforcements. The major driving force for the increased use of

woven fabrics, compared to their non-woven counterparts, are excellent

drapeability (allowing complex shapes to be formed), reduced manufacturing

costs (e.g. a single two-dimensional biaxial fabric replaces two non-woven

plies) (89) and increased resistance to impact damage (improved compressive

strengths after impact follow from a reduction in the area of impact damage).

These woven fabric composite materials have better out-of-plane stiffness,

strength and toughness than laminate composites. They also have easier

handling in production quality. Properties of woven flax fibre reinforced

recycled HDPE composites were studied by Foulk and co-workers (90). Fabrics

were treated with maleic anhydride, silane and enzyme to promote interaction

between polymer and fibres. Compared to recycled HDPE, mechanical

properties of composites materials demonstrated significant increase in tensile

strength and modulus of elasticity. The mechanical properties and fracture

surface morphology of woven date palm fibre (DPF) reinforced polyester resin

composites were investigated by Wazzan (91). Laminates with different

orientation and volume fraction of reinforcement were prepared using resin

transfer moulding (RTM) processing technique. The woven DPF reinforced

composites recorded a tensile strength of 76.9 MPa. The effect of fibre surface

treatments (silane and permanganate treatments) on tensile strength and

modulus of sisal textile reinforced vinyl-ester resin composites was investigated

Introduction 45

by Li et al. (92). Chemical modification of fabric has not been able to make a

significant improvement in tensile properties. Thomas and co-workers (93, 94)

recently reported the mechanical properties as well as the moisture sorption

characteristics of textile sisal reinforced natural rubber composites. Sisal fabric

was subjected to various chemical treatments. Tensile strength was found to

decrease with all chemical modifications except for composites prepared with

heat-treated sisal fabric. Water uptake has been found to be the maximum for

textile composites containing sisal fabric treated with 4% NaOH.

Compared to unwoven, unidirectional composites, the woven fabric

composites provide more balanced properties, higher impact resistance,

easier handling and lower fabrication cost, particularly for parts with

complex shape. A typical plain weave is represented as shown in Fig 1.7.

Figure 1.7 Typical plain weave fabric

(Reference: Laly A Pothen, Ph.D Thesis, Mahatma Gandhi University,2002)

Along with dimensional stability, WF (woven fabric) composites offer

higher impact resistance and toughness compared to UD (unidirectional)

composites; however, these advantages are obtained at the cost of in-plane

stiffness and strength. Woven-fabric reinforcements are available in several

forms as shown in Fig. 1.8.

46 Chapter 1

Figure 1.8 Various perform architectures

(Ref: Laly A Pothen, Ph.D Thesis, Mahatma Gandhi University,2002)

1.9 Interface

The behaviour of a composite material is explained on the basis of the

combined behaviour of the reinforcing element, polymeric matrix and the

fibre/matrix interface. The fibre/matrix interface is very important in the case

of natural fibre composites. To attain, superior mechanical properties, the

interfacial adhesion should be strong. Matrix molecules can be anchored to the

fibre surface by chemical reaction or adsorption, which determine the extent of

interfacial adhesion. The region separating the bulk polymer from the fibrous

reinforcement is of utmost importance in load transferring. This region though

originally dubbed as interface is now viewed as interphase due to its three

dimensional heterogeneous nature. The developments in atomic force

microscope (AFM) and nano-indentation devices have facilitated the

investigation of the interphase (95)

The schematic representation of the composite interphase is shown in Fig. 1.9.

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

Introduction 47

surrounding a fibre. It consists of polymeric material made from the chemical

interaction of sizing or coating on the fibre and the bulk matrix during the curing

process. The interphase is also known as the mesophase.

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

the interfacial bonding and its characterisation. A strong interface creates a

material that displays exemplary strength and stiffness but is very brittle in

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

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

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

Figure 1.9 Schematic model of interphase

(Ref: Laly A Pothen, Ph.D Thesis, Mahatma Gandhi University,2002)

There are different methods for improving the efficiency of adhesion

between matrix and fibre. Different physical methods, such as stretching,

calendering, thermo- treatment and the production of hydrid yarns can be

used to increase the fibre/matrix interactions. They do not change the

chemical composition of fibres, but cause changes in the structural and

surface properties of fibres and there–by influence the mechanical bonding to

48 Chapter 1

polymer matrices. Electric discharge (corona, cold plasma) and UV radiation

are the other ways for physical treatment.

Chemical methods are also used to strengthen the interface.Strongly

polarized cellulose fibres are inherently incompatible with hydrophobic

polymers (96, 97). When two materials are incompatible, it is often possible

to bring about compatibility by introducing a third material, which has the

properties intermediate between those of the other two. An important

chemical modification method is the chemical coupling method, which

improves the interfacial adhesion. The fibre surface is treated with

compound, which forms a bridge of chemical bonds between fibres and the

matrix. Fonseca et.al. (98) reported a comparison on the impact and tensile

properties after the polyester matrix modification using silane coupling

agent, flame retardant system, and a blend of the silane and the flame

retardant system, showing that silane coupling agent could act as a good

plasticizer. In addition to improving the mechanical properties of natural

fibre reinforced composites, it was possible to minimize the moisture

sensitivity, by using silanes as coupling agents (99). The improved moisture

resistance caused by the application of the coupling agent could be explained

by an improved fibre/matrix adhesion.

1.10 Fabrication of composites

Engineered composite materials must be formed to shape. The matrix

material can be introduced to the reinforcement before or after the

reinforcement material is placed into the mold cavity or onto the mold

surface. The matrix material experiences a melding event, after which the

part shape is essentially set. Depending upon the nature of the matrix

material, this melding event can occur in various ways such as chemical

Introduction 49

polymerization or solidification from the melted state. A variety of molding

methods can be used according to the end-item design requirements. In

general, the reinforcing and matrix materials are combined, compacted and

processed to undergo a melding event. After the melding event, the part

shape is essentially set, although it can deform under certain process

conditions. For a thermoset polymeric matrix material, the melding event is a

curing reaction that is initiated by the application of additional heat or

chemical reactivity such as organic peroxide. For a thermoplastic polymeric

matrix material, the melding event is solidification from the melted state. For

a metal matrix material such as titanium foil, the melding event is a fusing at

high pressure and a temperature near the melt point.The fabrication and

shaping of composites into finished products often combines the formation

of the material itself during the fabrication process. The formation of the

composite involves the combination of the matrix and fibre such that the

matrix impregnates, surrounds, and wets the fibre. The important processing

methods for thermosetting polymers involve hand lay-up, Bag moulding

process, Filament winding, Pultrusion, Bulk moulding, Sheet moulding,

Compression moulding, Resin Transfer moulding etc.

1.10.1 Hand lay-up

The oldest, simplest and the most commonly used method for the

manufacture of both small and large reinforced products is the Hand lay-up

technique. A flat surface, a cavity or a positive shaped mould, made from

wood, metal, plastics, or a combination of these materials may be used for

the hand lay-up method. Fibre reinforcements and resin are placed manually

against the mould surface. The layers of materials placed against the mould

control thickness.

50 Chapter 1

1.10.2 Bag moulding process

It is one of the most versatile processes used in manufacturing composite parts.

In bag moulding process, the lamina is laid up in a mould and resin is spread or

coated, covered with a flexible diaphragm or bag and cured with heat and

pressure. After the required curing cycle, the materials become an integrated

moulded part shaped to the desired configuration. Three basic moulding

methods involved are pressure bag, vacuum bag, and autoclave. Vacuum bag

and autoclave methods are used to produce the most bag-moulded parts. Their

main advantages are that the tooling is relatively inexpensive and the basic

curing equipment can be used for the unlimited variety of shaped parts. The

disadvantage of the pressure bag system is that the tooling is relatively

expensive because it is combined with the curing pressure system and can be

used only for the specific part for which it is designed.

1.10.3 Spray lay-up

In this a stream of chopped fibres and liquid resin are sprayed in a mould

cavity. A specialized spray gun is used to apply the chopped fibre and resin

to the tool. The direction of fibre is random. Uniformity for the surface

occurs. Void content is less when compared to hand lay-up.

1.10.4 Pultrusion

Pultrusion has some similarities to aluminium extrusion or thermoplastic

extrusion. It is an automated process for manufacturing composite materials into

continuous, constant-cross-section profiles. In this technique, the product is

pulled from the die rather than forced out by pressure. A large number of

profiles such as rods, tubes and various structural shapes can be produced using

appropriate dies. Profiles may have high strength and stiffness in the length

direction, with fibre content as high as 60-65% by volume.

Introduction 51

1.10.5 Filament winding

Filament winding is a technique used for the manufacture of surfaces of

revolution such as pipes, tubes, cylinders, and spheres and is frequently used

for the construction of large tanks and pipe work for the chemical industry.

High-speed precise lay down of continuous reinforcement in predescribed

patterns is the basis of the filament winding method. Continuous

reinforcements in the form of roving are fed from a multiplicity of creels. A

creel is a metallic shelf holding roving packages at desired intervals and

designed for pulling roving from the inside of the package.

1.10.6 Preformed moulding compounds

A large number of reinforced thermosetting resin products are made by

matched die moulding processes such as hot-press compression moulding,

injection moulding and transfer moulding. Matched–die moulding can be a

wet process but it is most convenient to use a preformed moulding

compound or premix to which all necessary ingredients are added. This

enables the attainment of faster production rate. Moulding compounds can

be divided into three broad categories: dough moulding, sheet moulding and

Prepegs.

1.10.7 Dough moulding compounds

It consists of dough like or putty like mixture of a resin, fibre reinforcement

and filler to which pigments and other materials are added.

1.10.8 Sheet moulding compounds

These compounds are produced as flat sheets and are invariably based on

unsaturated polyester resin systems reinforced with chopped glass fibres

although carbon and/or aramid can also be used separately or as hybrids.

52 Chapter 1

1.10.9 Prepegs

This is the short form for preimpregnated fibre- reinforced plastics. Prepegs

consist of roving, woven fabric, and continuous uni-directional fibre

reinforcement sheets or random chopped-fibre sheets impregnated with a

partially cured resin system. These differ from sheet moulding compounds in

those thickening agents, additives, fillers are rarely used.

1.10.10 Vacuum infusion moulding

The composite industry, under new environmental constraints, is looking for

alternative technologies to replace the existing open mould techniques. The

vacuum infusion mounding process is considered as promising alternative

since it involves low tooling cost and allows complete elimination of volatile

organic compound. The process is similar to resin transfer mouldling (RTM),

however a polymeric film often referred to as vacuum bag replaces the stiff

mould cover. The film is sealed against the lower half of the mould at the

periphery. Air expelled from the mould cavity results in the compression of

reinforcements by the atmospheric pressure present on the outer side of the

polymeric film. Finally, the resign impregnates the mould cavity usually

through a resin distribution channel.

1.10.11 Compression moulding

Compression moulding (CM) is a technique used to manufacture a large

number of composite components. The reason for its dominance is that CM

is most cost effective techniques for long production series of products. In

CM, a set of matching male and female mould halves is used (Fig 1.10). To

mould a component the material is placed on the lower mould half, the

mould is then rapidly and forcefully closed using hydraulic press to force its

flow to fill the mould. The mould is heated to ensure that cross- linking is

Introduction 53

initiated and completed. When the part is dimensionally stable, it is

demoulded. CM is the most popular method in the production of automobile

and track parts. It also finds applications in the development of containers

and housings, electrical and machinery components and bathroom interiors.

Various thermoplastic polymers and rubbers are successfully used as

matrices for the fabrication of natural fibre reinforced composites by using

CM (100-102).

Figure 1. 10 Schematic representation of compression molding

(Ref: www.naniwagousei.com/english/gijutsu/press.html)

54 Chapter 1

Many thermosetting plastics are also being used for the fabrication of

composites, which include epoxies, polyesters, phenolics, urreas, melamine,

silicone and polyimides. Among them, polyester resin is the most widely

used matrix for the fabrication of natural fibre reinforced composites. This is

principally due to low cost, resultant good mechanical properties and the

ease of handling of polyester resins. Excellent reports on the development

and characteristics of the natural fibre reinforced thermosetting polymeric

systems exist in literature. (103-107).

The mechanical properties of composites made by CM have been reported by

various authors (108-111)

Analysis of the energy–absorption mechanisms during impact fracture

showed that the fibre pull-out and interface fracture were the major

contributors to the high toughness of these composites.

1.10.12 Resin transfer moulding

Resin Transfer Molding (RTM) is a novel processing technique which

bridges the gap between the traditional techniques like labour intensive hand

lay up process and capital intensive compression molding (CM). All the

processes mentioned above are either slow or therefore suitable only for low

volume production, or are restricted to the manufacture of parts with only

very simple geometries. In addition the processes are labour intensive and the

parts produced have only relatively low volume fraction (typically 20-40%)

and can vary widely in mechanical performance. Tool turn around times is

also slow. A process that has received considerable attention in recent years

is resin transfer moulding. Resin transfer moulding (RTM) has the potential

of becoming a dominant low cost process for the fabrication of large,

Introduction 55

integrated, high performance products for the consumer segment of the

economy and ultimately for segments now dominated by the higher precision

laminated fabrication techniques Figure1.11 illustrates the schematic

representation of RTM. RTM is a process using a two-sided mold set that

forms both surfaces of the panel. The lower side is a rigid mold. The upper

side can be a rigid or flexible mold. Flexible molds can be made from

composite materials, silicone or extruded polymer films such as nylon. The

two sides fit together to produce a mold cavity. The distinguishing feature of

resin transfer molding is that the reinforcement materials are placed into this

cavity and the mold set is closed prior to the introduction of matrix material.

The liquid resin is injected into the closed mould to impregnate the

reinforcement. The resin is often injected at the lower point and fills mould

upwards to reduce the entrapped air. When the resin starts to leak into the

resin trap the tube is clamped to minimize resin loss. Most of the works

reported in the area of RTM are the simulation works and modeling

regarding design of the mould, mould filling, flow rate of resin, curing of

resin, prediction of formation of voids etc. Jihua Gou et al. (112) prepared

Carbon fibre reinforced polymer matrix composites increasingly used for

aircraft structures using RTM technique. Exequiel Rodríguez et al.(113) have

done work on characterisation of composites based on natural and glass

fibres obtained by RTM. Yehai Yan et al. (114) developed a thermosetting

resin system for RTM based on novolak and BMI. The cured resin system

showed good mechanical properties, high heat resistance, and low water

absorption. Rouison et al. (115) prepared hemp/kenaf fibre reinforced

unsaturated polyester composites of various fibre content upto 20.6 vol%

using RTM method. A good wetting of fibres was observed. In this process,

a dry reinforcement material that has been cut and shaped into a preformed

56 Chapter 1

piece, generally called a preform, is placed in a prepared mould cavity. The

resin is often injected at the lowest point and fills the mould upward to

reduce the entrapping of air. When the resin starts to leak into the resin trap,

the tube is clamped to minimize resin loss. Since the process takes place in a

closed mould, the evolution of styrene will be less. RTM does not involve

large injection pressure. Injection time ranges from few minutes for small

and simple components to hours for large, complex components with high

fibre content. When excess resin begins to flow from the vent areas of the

mould, the resin flow is stopped and the moulded component begins to cure.

Once the composite develops sufficient green strength it can be removed

from the tool and post cured (116). One of the major advantages of RTM is

the modest requirements on the mould since relatively low pressures and

temperatures are encountered.

Figure 1.11 Schematic representation of RTM technique

(Ref : Schmachtenberg et al. Polym. Test., 24 , 330 2005)

Introduction 57

1.10.12.1 Design of the mould

In the case of mould designing, it is very important to look into the position

number of gates and vents. An understanding of the location of gates and vents

gives an idea for a mould designer about the fibre impregnation to ensure the

complete wetting of the preform. The mould design can affect the other

important parameters such as mould-filling time, flow-front shape and

pressure distribution in resin. In order to effectively control the resin flow, the

selections of the locations of auxiliary gates is as important as flow rate.

Hence, it is essential to optimise the locations of auxiliary or additional gates

for flow control in RTM.

Neilson reported that the flow rate control using multiple gates were effective

for steering flow during the mould filling stage (117-118).

1.10.12.2 Resin flow

During mould filling, due to the chemical reaction in resin, resultant viscosity

increases with filling time. High performance composite parts require high

fibre fractions and long flow paths, which result in large flow resistance and

extremely long mould filling times. Consequently a premature gelation of

resin may occur that leads to an incomplete mould filling. Therefore, accurate

prediction and precise control of resin viscosity in mould filling process is

critical for achieving consistency in product quality and process efficiency. A

complete characterisation of the reaction kinetics and viscosity change of resin

during mould filling stage can provide useful information on the relation

between the processing variables and material properties.

In RTM, the flow of resin is mainly governed by the well-known Darcy’s law

(119). The viscosity of resin and permeability of preform and two important

58 Chapter 1

parameters in Darcy’s Law that relate to proper fibre wet-out and the

impregnation rate of preform (120). However, many authors have questioned

the validity and applicability of Darcy’s law. Visconti et al. (121) identified

differences in measured permeability brought about by changes in injection

pressure. The use of RTM equipment itself is also a most efficient way to

measure the in-plane permeability of fibrous reinforcements. The simplicity of

the set-up and the ease with which more reliable and reproducible results can

be determined and said to be the major advantages of the rectilinear flow

experiments over the radial ones. Amico et al. (122) addresed the inter

relationship between the permeability and capillary pressure in in-plane

infilteration of assemblies of woven glass fabrics. When a corrected

permeability was calculated taking into account the capillary pressure effect,

the permeability proved to be independent of the injection pressure and the

permeating fluid.

Kiuna et al. (123) did a cure modelling for the viscosity of two part

epoxy/amine resin, focussing in the low extents of cure, the most important

region for the mould filling stage in RTM.

1.10.12.3 Void formation

The study of void formation is one of the interesting areas in RTM. The main

sources of void formation in RTM process are the following.

* Flow front coalescence and the race tracking where the resin would flow

through the the high permeability regions faster, and surround the low

perme ability regions to form dry spots.

* Partial evapouration of mould releasing agents, volatilization of

dissolved gases, moisture and volatile chemical species during curing.

Introduction 59

* Air entrapment in the fibre bundles which is governed by the process

parameters and material characteristics.

In the above, different methods can be employed to remove the air from

the mould. These can be classified into two major groups:

* Vented moulds in which the advancing flow fronts expel the air through

the vents. Continous peripheral venting and discrete venting fall into

this category.

* Sealed moulds in which the air is removed from the mould before the

begining of impregnation. Vacuum impregnation and vapour purging fall

into this category.

According to the nature of perform, two types of voids can be

observed: macroscopic/inter-bundle voids and microscopc/intra - bundle voids.

If the perform is in the form of mat, capillary flow will occur in the tows and

cannel flow will occur within the pore space. Depending on the injection

pressure, one flow is likely to lead the other. At low pressures (below those

commonly used in RTM) channel flow will lead. A schematic diagram of the

void formation in perform is given in figure 1.12.

Figure 1.12 Schematic diagram of the void formation in perform

60 Chapter 1

When the resin in the pore space leads that within the tows, complex

transverse micro flows occur, wicking resin from the pore space into the tows

ahead of the capillary flow. This action form voids within the tows. Further

capillary flow causes the volume of the flow to reduce, increasing their

internal pressure until equilibrium is reached. Once a void has formed its

volume can change due to the following effects.

* Changes in vapour mass (solvents, condensation produts) and the vapour

transfer across the void /material interface.

* Pressure change inside the void due to temperature and pressure change

in the material.

* Thermal expansion due to temperature gradient in resin.

An extensive review on the effect of voids on the mechanical performance was

carried out by Judd and Wright (124). They indicated that the voids reduced

the properties like interlaminar shear strength (ILSS), longitudinal and

transverse flexural modulus, longitudinal and transverse tensile strength and

modulus, compressive strength and modulus, fatigue resistance and high

temperature resistance. It was also shown that the dielectric strength of

composites was reduced with increasing void content (125). Bowles and

Frimpong (126) found a 20% dcrease at 5% void for a unidirectional carbon

fibre composites.Voids could also affect the long term performance of fibre

reinforced composites by increasing the moisture absorption resulting in a

degradation of the fibre/matrix interface.

From the review it is clear that an increase in void content destroys the

properties of composites. The prediction of dry spot locations is important and

is also essential for moulding designing. Models which consider the effects of

Introduction 61

vapour transport and changes in temperature/pressure have been developed by

Springer and Loos (127,128) and by Kardos et al. (129). Voids can also form

during the curing of thermosetting resins by the reaction of residual solvent,

catalyst,resin and the binder, causing homogeneous or heterogeneous

nucleation and growth through out the material . However, this influence on

void content is minor in comparison to that of gas/vapour entrapment during

flow. As pointed out in the earlier section regardless of the type of

reinforcement or resin and its viscosity, the void content of composites

produced in RTM can be significantly reduced bythe application of vacuum to

the mould during injections. If the vacuum is applied in the pore space ahead

of the flow, the voids formed will eventually collapse, as they will have no

internal pressure to support them. Vacuum can have a beneficial influence on

the void content only if the mould is vacuum tight. If this is not ensured, air

will be drawn into he resin in the flow region that is below atmospheric

pressure and the void content may be increased.

Problems associated with poor void distribution are believed to be more likely

in the case of convergent flow. It is feasible that when two or more flow fronts

meet, their driving pressure will cancel each other (either partly or wholly),

causing flow to diminish or cease. The voids previously created by mechanical

entrapment at the fronts will then remain in that region causing an area of high

void content. This problem is also expected to be exacerbated by further void

formation when the uneven, fingering flow fronts meet, creating isolated

unwanted pockets in RTM. Kang et al. (130) developed a mathematical model

to analyse the void formation during RTM process. The model recognizes

non-uniform velocity fields resulting from heterogeneous microstructure of

fibre preform .Although a few simplifying assumptions have been introduced,

62 Chapter 1

the model still accounts for the effect of fibre preform, resin velocity and

capillary pressure. With proper calibration, it can predict the size and content

of voids within fibre tows as well as between them. The model predicts the

same trends as exhibited by the one-dimensional RTM experiments.

1.10.12.4 Fibre reinforced polymer composites fabricated by RTM

Glass and natural fibres can be used as effective reinforcements for the

fabrication of polymer composites by RTM. The main studies in this area can

be summarized as below:

Rouison et al. (131) prepared hemp/kenaf fibre reinforced unsaturated polyster

composites of various fibre content upto 20.6 vol% using RTM method. A

good wetting of the fibres was observed. As the fibre content increased the

resin injection time increased dramatically due to the low permeability of mat.

Dansiri et al. (132) incorporated kenaf fibre in polybenzoxazine (PBZX) resin

matrix to form a unidirectional reinforced composites containing 20 wt% fibre

by RTM technique. Two types of benzoxazine monomers were used as resin

mixtures: benzoxazines based on bisphenol-A/aniline (BA-a) and

phenol/aniline (ph-a). The effects of varying BA-a:ph-a ratio in the resin

mixture and curing conditions on the mechanical properties of pure PBZX

resin and kenaf/PBZX composites were studied. The flexural strength of the

pure PBZX resin increased with increasing ratio of BA-a:ph-a, however, the

impact strength increased only slightly. PBZX resin had a lower water

absorption and higher flexural modulus, compared to unsaturated polyster

resin. PBZX composites with 20 wt % fibre content had lowered the flexural

and impact strengths, but gave higher flexural moduli compared to unsaturated

polyester resin composites with the same fibre content. Richardson et al. (133)

carried out flow visualisation experiments using a transparent RTM mould for

Introduction 63

hemp mat in phenolic composites. With an increase in the fibre concentration,

the fibre washing effect was gradually reduced due to the consequential

improvement of clamping condition. Edge flow was less sensitive to injection

pressure variations. However, fibre concentrations had a dramatic influence.

The deployment of performs larger than the mould eliminated the problem.

Changes in injection pressure lead to corresponding variations in injection

time. An increase in the number of layers of non-woven hemp leads to

prolonged injection time.

Rowell et al. (134) evaluated the mechanical properties of jute and glass fibre

reinforced polyester composites fabricated by RTM under identical procedure

and parameters. The properties of jute composites were then compared with

those of glass fibre reinforced composites. The results proved that jute fibre

reinforced composites might be used in a large number of applications where

the higher cost and higher mechanical properties of glass composites were not

appreciated. O’Dell (135) also investigated the jute and glass fibre reinforced

composites prepared by RTM technique. Sebes et al. (136) obtained very good

result for hemp fibre reinforced polyester composites. Oksman (137)

manufactured high quality flax fibre reinforced composites using RTM

process. Good flow properties were observed in the mold and composites

having higher fibre content were fabricated. The resultant composites were

found to have very impressive mechanical properties also.

Boi et al.(138) studied the effect of the elastomer structure on the toughening

of highly cross-linked epoxy systems in RTM process using two kinds of

elastomers: (1) a reactive liquid elastomer based on carboxyl-terminated

butadiene acrylonitrile copolymers (CTBN), (2) a preformed core-shell rubber

(CSR). The introduction of CTBN rubber caused a modification in glass

64 Chapter 1

transition temperature (Tg) due to the miscibility in the epoxy matrix, whereas

CSR particles did not. During cure, these elastomers affected the

morphological, rheological and dielectric behaviour of epoxy/amine systems.

Warrior et al. (139) investigated the effects of resin properties and resin

processing parameters on the crush behaviour of thermoset composite tubes

manufactured by RTM. Vinyl ester typically showed an increase in energy

absorption over polyster for similar degrees of cure and in a mix of compatible

polyester and vinyl ester resins. Post-curing was shown to result in a large

increase in all propertis, even for short curing period. Cure temperature did not

change the properties for the systems studied but had the capacity to reduce the

cycle times.

1.11 Green composites

Researchers are currently making their effort in developing a new class of

fully biodegradable ‘green’ composites by combining natural fibres with

biodegradable resins. They constitute a loosely defined family of polymers

that are designed to degrade through the action of living microorganisms. The

major attractions of green composites are that they are environmental 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.

Green composites may be used effectively in many applications such as in

mass-produced consumer products with short lifecycles 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 (140). The

design and life cycle assessment of green composites have been exclusively

dealt with by Baillie (141). Cellulose fibre/Chitosan biodegradable rod with

good mechanical properties and exellent X-ray developing capability was

Introduction 65

successfully constructed via in-situ precipitated method by Wang et al. (142).

Micro/ nano sized bamboo fibrils and a modified soy protein resin were used

to fabricate environmentally friendly composites by Huang et al. (143) The

fracture stress and Young’s modulus of soy protein concentrate were increased

significantly with the incorporation of the bamboo nano fibrils. Graupner et

al. (144) conducted the mechanical characteristics of composites made of

renewable raw materials like cotton, hemp, kenaf and man-made cellulose

fibres with PLA by compression moulding. These composites gave improved

mechanical properties.

A number of natural and biodegradable matrices that are available for use in

such green composites are listed in Table 1.4. 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). Another important category of

biocomposites is based on agro-polymer matrixes, mainly focussed 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 thermomechanical energy in a continuous

extrusion process. 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. Starch and modified resins have also

been used as matrix to form green composites (145).

66 Chapter 1

Table 1.4 Natural, synthetic and biodegradable matrices

Biodegradable polymer matrices

Natural Synthetic

1. Polysaccharides

Starch

Cellulose

Chitin

2. Proteins

Collagen/ gelatin

Casein, albumin, fibrogen,silks

3. Polyesters

Polyhydroxyalkanoates

4. Other polymers

Lignin

Lipids

Shellac

Natural rubber

1. Poly(amides)

2. Poly( anhydrides)

3. Poly ( amide-enamines)

4. Poly ( vinyl alcohol)

5. Poly ( vinyl acetate)

6. Polyesters

Poly ( glycolic acid)

Poly ( lactic acid)

Poly( caprolactone)

Poly ( orthoesters)

7. Poly ( ethylene oxides)

8. Poly ( phosphazines)

(Ref : M.J. Jacob et al. Compos. Interfaces, 14,733, 2007)

1.11.1 Advanced green composites

The most common term used for high strength composites made using

graphite, aramid and glass fibres are ‘advanced’ composites. To obtain high

strength composites three factors are considered to be critical: high strength

fibres, resin with excellent mechanical properties and high fibre/resin

interfacial shear strength. Most current green composites, however, use

plant-based fibres that are not as strong as these high strength fibres. As a

result, most composites have strengths in the range of 300 MPa as described

above. However, there are efforts in the direction of creating high strength

green fibres based on cellulose and protein. A significant amount of research

Introduction 67

has been done to understand the amino acid sequence in spider silk and to

create fibres with similar properties. However, these efforts have not yielded

fibres with high strength as of today. 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 (146). The spider silk protein is then expressed in the goat milk

which is purified and spun into fibres. As this technology matures stronger

fibres can be expected. To make advanced green composites Netravali and

Chabba (147) used cellulose fibres spun using liquid crystalline (LC) solutions

prepared by dissolving cellulose in phosphoric acid. The dry-jet wet spinning

technique similar to what is used to spin aramid fibres such as Kevlar was used

in spinning these fibres. The strengths of these LC cellulose fibres were in the

range of 1700 MPa, which is by far the highest strength achieved for cellulose-

based fibres. To fabricate advanced composites Huang and Netravali (148)

developed soy protein concentrate based resins that were modified using a cross-

linking polycarboxylic acid to form interpenetrating network-like resin systems

with excellent mechanical properties. Biopolymers are based on renewable

resources and will degrade to form carbon dioxide, water and biomass. The

amount of carbondioxide released during degradation is the same amount as the

renewable resource harnessed during its cultivation. As a result carbondioxide

will not accumulate in the atmosphere due to the use of biopolymers (149).

1.12 Cellulose Nanocomposites

Nanotechnology is now recognized as one of the most promising areas for

technological development in the 21st century. In materials research, the

68 Chapter 1

development of polymer nanocomposites is rapidly emerging as a

multidisciplinary research activity whose results could broaden the

applications of polymers to the great benefit of many different industries.

During the past decade there has been a growing interest in incorporating

cellulose whiskers as nano reinforcement in polymer matrixes. The

reinforcing ability of the cellulose whiskers lies in their high surface area and

good mechanical properties. However, to obtain a significant increase in

material properties, the whiskers should be well separated and evenly

distributed in the matrix material (150). Properties of nanofibrillated

cellulose from different raw materials and its reinforcement potential were

studied by Zimmermann et al. (151). They showed homogeneous network

structures. Rodriguez et al. (152) prepared sisal cellulose whiskers reinforced

polyvinyl acetate nanocomposites. Statistical analysis of the sisal whisker length

and diameter resulted in average values of 250 nm and 4 nm, respectively.

Bondeson et al. (153) isolated nanocrystals from microcrystalline cellulose by

acid hydrolysis. With a sulfuric acid concentration of 63.5% (w/w), it was

possible to obtain cellulose nanocrystals/whiskers with a length between 200

and 400 nm and a width less than 10 nm in approximately 2 h with a yield of

30% (of initial weight). Bhatnagar et al. (154) prepared cellulose nanofibres

from various sources such as flax bast fibres, hemp fibres, kraft pulp, and

rutabaga, by chemical treatments followed by innovative mechanical

techniques. The nanofibres thus obtained have diameters between 5 and 60 nm.

The ultrastructure of cellulose nanofibres is investigated by atomic force

microscopy and transmission electron microscopy. Figure 1.13 shows the TEM

electron micrograph of nano fibres prepared from different natural fibres.

Introduction 69

Guoxin (155) proposed to measure the elastoplastic properties of micro- and

nano-fibres by a normal indentation technique in which the vertically aligned

fibres were embedded in an elastic matrix. The extracted material properties

agreed well with those measured from the parallel experiments on human

hair and glass fibres.

Figure 1.13 (A)Transmission electron micrograph of rutabaga nanofibres negatively stained. (B) atomic force micrograph of flax bast nanofibres. (C) atomic force micrograph of hemp nanofibres and (D) atomic force micrograph of bleached kraft pulp nanofibres.

(Ref.: A. Bhatnagar, M. Sain., J. Reinf. Plasti. and Compos.24, 1259, 2005)

Ok-Kyung Park et al. (156) Studied the effects of the surface treatment on

the properties of polyaniline coated carbon nanotubes/epoxy composites. The

effects of a surface treatment of carbon nanotubes (CNTs) on the electrical

70 Chapter 1

conductivity and the hydrophilicity of a polyaniline (PAni) coated CNTs

(PAni-CNTs)/epoxy (EP) composites were examined. The surface of the

CNTs was treated with various chemicals, such as acid mixtures

(HNO3:H2SO4), potassium persulfate (KPS) and sodium dodecyl sulfate

(SDS), to improve their dispersion and reactivity with PAni. The electrical

conductivity and hydrophilicity of PAni-CNTs and their EP composites were

strongly affected by the surface treatment of the CNTs.

Szu-Hui Lim et al.(157) Reported the impact fracture behaviour of nylon 6-based

ternary nanocomposites. Jia-Lin Tsai et al. (158) studied the Characterizing elastic

properties of carbon nanotubes/polyimide nanocomposites using multi-scale

simulation. Dufresne et al. (159) extracted both nanowhiskers and microfibrillated

cellulose (MFC) from sisal and reinforced with polycaprolactone (PCL). They

reported the influence of the nanoparticle’s nature on the mechanical and thermal

properties of the ensuing nanocomposites. The surface of both the nanoparticles

was chemically modified to improve their compatibilization with the polymeric

matrix. N-Octadecyl isocyanate (C18H37NCO) was used as the grafting agent.

PCL nanocomposite films reinforced with sisal whiskers or MFC (raw or

chemically modified) were prepared by film casting. The thermal behavior (Tg,

Tm, Tc, and degree of crystallinity) and the mechanical properties of the

nanocomposites in both the linear and the nonlinear range were determined using

differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA),

and tensile tests, respectively. Significant differences were reported according to

the nature of the nanoparticle and amount of nanofillers used as reinforcement.

It was also proved that the chemical treatment clearly improves the ultimate

properties of the nanocomposites. Figure 1.14 shows the transmission electron

micrographs of sisal whiskers.

Introduction 71

Figure 1.14 Transmission electron micrographs of sisal whiskers.

(Ref: G. Siqueira, J. Bras, A.Dufresne, Biomacromolecules, 10, 425–432, 2009)

Figure 1.15 shows the SEM image of sisal MFC (A) and optical microscopy

image of sisal microfibrillated cellulose (B). (159)

Figure 1.15 SEM image sisal MFC (A) and optical microscopy image of

sisal microfibrillated cellulose (B).

(Ref: G. Siqueira, J. Bras, A.Dufresne, Biomacromolecules, 10, 425–432 , 2009)

72 Chapter 1

Novel polyurethane nanocomposites based on toluene diisocyanate,

poly(propylene glycol), various hyperbranchedpolymers (HBPs), and layered

silicate were synthesized with the aim of determining the effect of the layered

silicate loading and the functionality of HBP on the structure and properties of

polyurethane nanocomposites (160). The microstructure of the nanocomposites

was investigated by X-ray diffraction analysis and high-resolution transmission

electron microscopy. It was found that exfoliated morphology and good

dispersion were obtained up to 8 phr clay loading for all of the nanocomposites.

Nearly 100% increment in tensilestrength, 2-fold increase in the lap shear

strength, 200% increment in the peel strength, and 120% increment in the

storage modulus along with a dramatic improvement in thermal stability were

observed with the addition of 8 phr clay, over the pristine polyurethane.

1.13 Composite characterisation

The characterisation of the interface gives much relevant information on the

interaction between the fibres and the matrix in acomposites system. Studies

of the interfacial contributions in wood fibre reinforced polyurethane

composites were conducted by Rials et al. (161). The chemical and physical

characterisation of the interface have been discussed by Kim and Hodzic

(162). The authors are of the opinion that the gap between physicochemical

investigation and bulk material testing is being bridged by the

implementation of novel techniques such as nano-indentation, nano-scratch

tests and atomic force microscopy. The methods those are available for the

characterisation of the fibre/matrix interface is as following.

Introduction 73

1.13.1 Micromechanical techniques

For the micromechanical characterisation of the interface properties, fibre

pullout, fragmentation and micro-identation tests are usuallly used. These

techniques do not simulate a situation that can be found as much in a real

composite material but the loading conditions are relatively simple so that

the interface properties can be rather easily derived out of these tests. All

these techniques evaluate the bonding strength between the fibres and the

matrix. The drawback of single fibre pull out tests is that it involves only a

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

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

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

pullout fibres are surrounded by a composite medium. In single fibre model

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

been ignored. Therefore,owing to the influene of the composite medium,

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

fibre cmposites and the interfacial properties obtained there from, would very

likely deviate from those of the single fibre composite pull out test.

1.13.2 Microbond test

The procedure involves the deposition of a small amount of resin onto the

surface of a fibre in the form of one or morediscrete micro-droplets. The

droplets form concentrically around the fibre in the shape of ellipsoids and retain

their shape after appropriate curing. Once cured, the microdroplet dimensions

and the fibre diameter are measured with the aid of an optical microscope. The

embedded length is fixed by the diameter of the micro-droplet along the fibre

axis, which is dependent on the amount of resin deposited on the fibre. Problem

that is associated with the microbond technique is that the maximum debonding

74 Chapter 1

force value is influenced by interfacial friction in already debonded regions,

and,therefore, these parameters are not purely ‘adhesional’ but depend ,in an

intricate way, on interfacial adhesion and friction. The specimen preparation for

the microdroplet test whereby a single fibre is pulled out of a resin suffers from

sveral difficulties. For instance, the reliability of the data is affected by the shape

of the droplet. Symmetric, round droplets are easier to test and analyse than

droplets with flat surfaces, produced when the specimen solidify on a flat

substrate. Also, the size of the doplet is critical .If the length of the droplet

exceeds a critical value, the fibre will fracture prior to debonding and pullout.

An additional complicaton with some thermoset material is that the anticipated

curing characteristic may not manifest themselves in a droplet of small size, and

the hence comparison on a microsructural level between micro and macro

specimens may not be possible. Another defect is that this test is not applicable

to matrices that are soft.

1.13.3 Fragmentation test/Single fibre composite (SFC) test

In this method, a single fibre is embedded in a polymer and broken into

small pieces. The fibre is neither pushed nor pulled directly, and so fibre

poisson effects are similar to that occurring in a fibre composite. Unlike the

other methods, it produces only one result for the interfacial shear stress,

which is the average for the many fragments produced. The use of Raman

spectral lines has made it possible to estimate fibre stains and hence the

interfacial shear stress can be estimated directly from the fibre Young’s

modulus. This makes the test much more powerful. In the SFC, the failure

strain of the matrix must be much larger than the failure strain of the fibre to

promote multi-fragmentation of the fibre. This requires the use of matrices,

which can undergo large deformations. Consequently, commercial resins

Introduction 75

utilised in actual composite systems which typically have low strains to

failure cannot be used for this test. Therefore the interfacial shear strength

determined is not directly applicable to the actual composite system. Another

problem is that the embedding matrix can inhibit fibre fracture, which

intiates from surface flaws. The magnitude of this effect is dependent on the

type of matrix or embedding resin. Therefore, it is important to utilise the

actual fibre and resin of a given composites system to determine the

interfacial shear strength.

1.13.4 Spectroscopic techniques

1.13.4.1 Electron spetroscopy for the chemical analysis (ESCA/XPS)

ESCA is an extremely powerful tool for studying solid surfaces. This

technique has an information depth of 1-5nm and therefore it is capable of

examining only the outer layers or surfaces of fibres. Carbon fibres and

coated glass fibres have been characterised by XPS. ESCA has been used to

determine the surface composition of cellulose and wood fibres. Pothan et al.

(163) investigated the change in the surface composition of the raw and

chemically modified banana fibres using XPS. Surface characterisation by

XPS showed the presnce of numerous elements on the surface of the fibre.

1.13.4.2 Fourier transform infrarred spectroscopy (FTIR)

FTIR technique has also been used to characterise the surface of fibres from

which information can be obtained about the interfacial adhesion. George et al.

(164) characterized the interface and the fibre surface of pineapple leaf fibre

reinforced polyethylene composites using IR. The effect of different

chemical modification on the interfacial characterisatics of spen finer

reinforced HDPE composite was examined by Colom et al. (165). The

76 Chapter 1

advantages of FTIR include a less experimental complexity and a more

easily interpretable spectrum. It is also much more precise than infrared

technique. Disadvantages are lower sensitivity and the necessity to use

compact samples.

1.13.4.3 Laser Raman spetroscopy (LRS)

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 bond in the

fibre shift towards a lower wave number upon the action of strain and stress

and this is thought to be due to direct molecular straining/stressing. This has

been used to map stresses along fibres embedded in matrix resin to determine

the interfacial shear stress. Raman spectroscopy has also been used to

investigate the deformation micromechanics of natural and regenerated

cellulose fibres (166). Studies have also been carried out in the case of

composites but it was difficult to evaluate the properties of the interface.

1.13.4.4 Nuclear magnetic resonance spectroscopy (NMR)

High resolution NMR spectroscopy is a very powerful technique to measure

and characterise polymer tacticity, helicity and molecular weight,

composition and diffusion coefficient of polymers. Solid state 13C NMR

spectroscopy using cross polymerization and magic angle spinning is

especially useful for characterizing wood and wood/polymer composites.

Detailed information that can be obtained includes composition, Tg, melting

transitions, percent crystallinity and 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

Introduction 77

broadening due to protons and spinning of the sample about the magic angle

with respect to static field to reduce chemical shift anisotropy effects.

Imaging NMR, with improvement of lateral resolution, will offer many

opportunities for non-destructive studies of interfaces. NMR resonance

imaging has also been used as a method of void detection in carbon fibre

reinforced polymer composites.

1.13.5 Microscopic techniques

1.13.5.1 Environmental scanning electron microscopy (ESEM)

Conventional SEM requires high vacuum, dry specimens, and electrically

conductive surfaces. Coating the surface of non-conductive specimens with a

conducting material such as gold or carbon has been necessary. This has

precluded the viewing of wet or oily materials. The environmental SEM

allows wet, oily and electrically non-conductive specimens to be observed

without special preparation and at relatively high pressure. The ESEM has

proven useful for investigating wood-polymer interaction at fracture surfaces

and polymer distribution in the wood-polymer composites (WPC’s).

1.13.5.2 Atomic force microscopy (AFM)

AFM is a useful technique to determine the surface roughness of fibres. The

force modulation mode gives a qualitative statement about the local sample

surface elasticity using an oscillating cantilever tip that indents into the

sample surface. The amplitude of this deflection is measured as a function of

the tip position when the cantiliver tip indents cyclically into the surface.

Mader and co-workers (167) used this technique to determine the physical

poperties of the interfaces. In an interesting study, Dupres et al. (168)

investigated the wetting behaviour and electrostatic properties of hair fibre

78 Chapter 1

by AFM. Surfae potential imaging was used to characterise local polarities.

The wetting properties were analysed by imaging hair samples on which

different liquids were spread. The authors observed that non-polar liquids

preferentially wetted the cuticile edges of the hair fibre while polar liquids

did not wet any region of the cuticle. Water was found to penetrate into the

bulk of hair fibre.

1.13.5.3 Transition electron microscopy (TEM)

The transmission electron microscope operates on the same basic principles

as the light microscope but uses electrons instead of light. The physical and

chemical properties of nanophase materials rely on their crystal and surface

structures. Transmission electron microscopy (TEM) is a powerful and

unique technique for structure characterization. The most important

application of TEM is the atomic-resolution real-space imaging of

nanoparticles. By forming a nanometer size electron probe, TEM is unique in

identifying and quantifying the chemical and electronic structure of

individual nanocrystals. Electron energy-loss spectroscopy analysis of the

solid-state effects and mapping the valence states are even more attractive. In

situ TEM is demonstrated for characterizing and measuring the

thermodynamic, electric, and mechanical properties of individual

nanostructures, from which the structure−property relationship can be

registered with a specific nanoparticle/structure.

1.13.6 Other techniques

1.13.6.1 Contact angle measurements

Contact angle measurement is probably the most common method of solid

surface tension measurement. The three most commonly used methods of

Introduction 79

contact angle measurement are the sessile drop, the captive bubble and the

Wilhelmy plate technique. In the sessile drop experiment, a droplet of a

properly purified liquid is put on the solid surface by means of a syringe or a

micropipette. The droplets generally observed by a low magnification

microscope, and the resulting contact angle is measured by a goniometer fitted

in the eyepiece. In the captive bubble technique, a liquid drop is put on the

sample surface immersed in a liquid medium by means of a U- shaped needle.

Advancing and receding angles are measured as in the above technique. In the

Wilhelmy plate technique the advancing andreceding angles are calculated

from the force exerted as the sample is immersed or withdrawn from a liquid.

The main advantages of the Wilhelmy plate technique over more conventional

methods are the control of the interfacial velocity and that measurement can

also be performed on small diameter fibres.

1.13.6.2 Zeta potential measurements

Electrophoresis, electro-osmosis and streaming potential are basically three

electro kinetic phenomena those are currently exploited to measure the zeta

potential of the surface of polymers. All these phenomena have in common a

distinguishable moving and a stationary phase. In electrophoresis and

electro-osmosis an electric field is applied and the movement of colloidal

particles (electrophoresis) or of a solution (electro-osmosis) is observed. In

the streaming potential or streaming current measurements the liquid phase is

moved by an applied pressure and the ensuring potential or current is

measured. For every measurements of the concentration or pH dependence

needs a well-defined starting point. For most fibre materials, the zeta

potential shows a parabolic curve like trend, which is caused by the

adsorption properties of the solid for potential determining ions as well as by

80 Chapter 1

specifically , adsorbed ions. Since the zeta potential as a property of the

electrochemical double layer cannot depend on the geometry of the

investigated capillary bundle, like a fibre bundle, it is necessary to

characterise the pore geometry as well. The presence of acidic or basic

dissociable surface functional groups can be estiminated by measuring the

pH dependence of the zeta potential. Zeta potential measurement provides

interesting results when applied to the effect of coupling agents in the filling

and reinforcement of polymers.

1.13.6.3 Solvatochromism

The sum of all possible interactions between a surface group and an adsorbed

molecule or ion is termed as the surface polarity. In the well-behaved regular

solvents the thermal motions of the solvent molecules surrounding the

solvatochromic probe dye causes an average polarity. Consequently, a

specific probe dye reflects the specific sum of interactions being operative

between the relevant surface groups and the relevant sites of the probe

molecule. The thermal motions at the surface of the solid material attached to

the probe dye will be lower than that of the probe dye contacting the liquid.

The probe dye should interact with the site on the surface corresponding to a

minimum of the energy.

1.13.6.4 Stress relaxation

The term relaxation denotes the process of establishment of statistic

equilibrium in a physical or phsico- chemical system and its rate depends

upon the probability of transition of system from one stage of equilibrium to

another. Meaningful data on the behaviour of the material can be obtained by

accelerated testing methods. Stress relaxation is one of the widely employed

Introduction 81

testing methods for this since it represents the basic time -dependent

response of material from which other time-dependent responses. The stress

relaxation modulus of polymers is increased by rigid fillers and decreased by

elastomers ones upto the point where dewetting or crazing becomes

pronounced. The rate of stress relaxation for the rigid and elastomeric fillers

increases after the onset of dewetting. The stress relaxation rate has been

chosen a way of ranking adhesion between fibres and matrix. The slope of

the stress relaxation curve is chosen as a measure of the level of adhesion

between the fibres and matrix.

1.13.6.5 Inverse gas chromatography

The most frequently used technique for the determination of thermodynamic

and acid/base characteristic is inverse gas chromatography. In IGC,

compounds usually solvents, of known properties, characterise the unknown

fibre surface. IGC measurements can be carried out in two different ways.

Usually linear or ideal IGC infinite concentration of n-alkane are injected

into the column containing the fibre to be characterised. The net retention

volume (VN) can be calculated by:

VN = (tr -to ) Fjo 1.1

where tr is the retention time and t0 the refrence time, F the flow rate of the

carrier gas and jo is a correction factor taking into account the pressure

difference between the two ends column. The dispersion component of the

surface tension of the filler can be calculated from the retention volumes of

n-alkanes:

-RTInVn = Na (γLV γds )

1/2 1.2

82 Chapter 1

Where Vn is the net retention of the alkane, a is the surface area of the

adsorbed molecule, γLV the surface tension of the solvent and N is the

Avogadro number. The product of RT and the logarithm of the retention

volume of normal alkanes is a linear function of (γLV)1/2. If the measurements

are carried out with polar solvents, the deviation from this line is

proportional to the acid/base interaction potential of the solid surface.

Adsorption isotherm of the probe compound on the fibre surface can be

determined with non-linear or finite dilution IGC.

When the surface of a solid is only partially wetted by a liquid, it forms a

droplet with a definite contact angle (θ). The interaction of the components is

expressed by the Young’s equation:

γsv = γsl + γlv cos θ 1.3

where γsv is the surface tension of the solid in contact with the vapour of the

liquid, γlv is the surface tension of the liquid and γsl the interfacial tension.

Due to wetting, γsv is smaller than the surface tension of the solid measured

in vacuum (γso ),the difference is the spreading pressure ( π e) i.e.:

γso - γsv = π e 1.4

The value of π e is very small for low energy surface, but it cannot be

neglected for fibres. On the contrary πe can be used for the calculation of the

thermodynamics characteristics of their surface.The spreading pressure can

be determined from the adsorption isotherm in the following way.

πe = RT τ dln P 1.5

where P is the vapour pressure and τ the moles of vapour adsorbed on a unit

volume of the fibre. If the measurement is carried out with polar solvents the

Introduction 83

dispersion component of the surface tension of the fibre can be determined

from the spreading pressure:

πe = 2 (γs d γLv) 1 /2 - 2γLv 1.6

This equation is derived from the Young’s, Dupre and the Fowkes equation

by assuming complete wetting cos θ = 0. Measurements with polar solvents

give the polar components of the surface tension, but acid/base constants and

the corresponding work of adhesion can also be calculated from them:

WAB Spec = 2 γLV,p + πe, p – 2 ( γds γ d

LV,p) ½ 1.7

where WAB Spec is the polar or acid/base component and the p subscript

indicates a polar solvent. Finite dilution IGC is more tedious to carry out

than the linear IGC.

1.13.6.6 Swelling studies

The extent of fibre alignment and strength of fibre-polymer interfacial

adhesion can be analysed 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. As the void formation decreases with fibre

addition, the solvent uptake also decreases. Information about anisotropy

caused by the orientation of fibres and matrix/fibre adhesion can be obtained

from anisotropic swelling studies. This provides an idea about the extent of

fibre orientation also.

1.14 Applications of natural fibre reinforced composites

The potential applications of the natural fibre reinforces plastic that can be

produced with this technology are countless. Some areas of interest are:

84 Chapter 1

Automotive industry: fibre reinforced plastics can be used for inner and outer

parts. The advantages are that these composites are cheaper, more sustainable

and environmentally friendly than glass fibre reinforced composites. The

resulting products from those composites can be both re-used and incinerated

and do not have to be land filled as glass fibre compounds, which can help in

developing cars according to the EU End-of-Life directive. In interior

automotive applications they have the advantage of not injuring passengers

when an accident occurs because the fibres are soft and not harsh as glass

fibres.The total reported use of natural fibres has increased to more than 21

kiloton as stated by the suppliers to the European automotive industry (169).

In the past decade, natural-fibre composites with thermoplastic and thermoset

matrices have been embraced by European car manufacturers and suppliers for

door panels, seat backs, headliners, package trays, dashboards, and interior parts.

Natural fibres such as kenaf, hemp, flax, jute, and sisal offer such benefits as

reductions in weight, cost, and CO2, less reliance on foreign oil sources, and

recyclability. However, several major technical considerations must be

addressed before the engineering, scientific, and commercial communities gain

the confidence to enable wide-scale acceptance, particularly in exterior parts

where a Class A surface finish is required. Challenges include the

homogenization of the fibre's properties and a full understanding of the degree

of polymerization and crystallization, adhesion between the fibre and matrix,

moisture repellence, and flame-retardant properties, to name but a few. (170).

Packaging industry: for instance for light weight pallets. The advantages of

composite material compared to wood are weight reduction, which safes fuel

during transport, and re-use: chip-implementation makes it easy to trace them

and they can be remelted into new products at the end of their life-cycle.

Introduction 85

Consumer products: any injection moulded product. The advantages are

reduction of plastic's use, re-use and flame retandancy. Examples include

housing of household appliances like computers, refrigerators and cell phones.

The fibre structure of the composite makes these products become less

vulnerable for fire and they can be rejected to new products at the end of their

life-cycle. In addition, high fibre loads results in significant material cost

reduction.

Building and construction industry: for instance profiles and roofings. The

advantages are cost reduction, flame retardancy and re-use. The fibre

structure of the composite makes these products become less vulnerable for

fire and they can be remelted into new products at the end of their life-cycle.

In addition, high fibre loads results in significant material cost reduction.

In the last two decades, the developments in the field of natural fibre-

polymeric composites have grown from laboratory scale fundamental

research to industrial implementation. Nowadays, the use of natural fibres

especially in the automotive industry has become rather common practice.

Successfully implemented examples include both natural fibre-thermoset and

thermoplastic composites for interior applications like door panels, trim

parts, seatings and parcel shelves. Driven by environmental and cost

considerations slowly the focus shifts from thermoset- to thermoplastic-

based natural fibre composites. Natural fibre-reinforced composites are also

used in bioengineering and environmental engineering applications (142,171,

172). When glass fibre composites are replaced by natural fibre

composites,there will be a reduction in the weight by 10-20Kg. This can

bring about a significant reduction in fuel consumption leading to a positive

effect on the environment (173). Cherian et al. isolated nano cellulose from

86 Chapter 1

pineapple leaf fibres by steam explosion and reported that they have wide

range of biomedical and biotechnological applications (174).

1.15 Major challenges/gap

Utilization of lightweight natural fibres offers the potential to replace a large

segment of the glass and mineral fillers in numerous transportation

structures, both internal and external. In the past decade, natural-fibre

composites with thermoplastic and thermoset matrices have been embraced

by auto manufacturers and suppliers for interior door panels, seat backs,

headliners, package trays, dashboards, etc. Natural fibres such as kenaf,

hemp, flax, jute, and sisal are providing automobile reinforcement due to

such drivers as weight reduction, cost, CO2 reduction, less reliance on

foreign oil sources, recyclability, with the added benefit that these fibre

sources are “green” or eco-friendly. In the present work banana fibre has

been used as reinforcement in phenol formaldehyde resin matrix. Generally

the interaction of cellulose fibre with PF resin is splendid due to the

hydrophilic nature of cellulose and PF resin.

For the successful design of a composite material from banana fibre several

parameters such as fibre length, fibre loading, fibre surface modifications,

and fibre/matrix adhesion have to be optimized. To our knowledge no

serious attempt has been made so far to compare the efficiency of the CM

and RTM techniques for the production of banana fibre/ phenol

formaldehyde resin composite systems. Previous reports have shown that the

natural fibres are susceptible to moisture absorption, thermal degradation,

and biodegradation (175-179).

Introduction 87

Therefore, strategies need to be worked out to reduce these limitations. The

major aim of the present research work is to conduct a detailed investigation on

the development and characterisation of banana fibre/ phenol formaldehyde

resin composite systems fabricated by CM and RTM techniques. This includes

analysis of different processes such as surface modification of fibres with

different chemicals and characterisation using cellulose microfibrils and

nanofibrils separated from banana fibre as reinforcement in the resin. The

present study focuses on the following aspects.

1. A comparison on the properties of banana raw fibre, microfibril and

nanofibril.

2. Surface modifications of fibre and the effect of these modifications on

the surface morphology has been analysed by Fourier Transform

Infrared spectroscopy, Contact angle measurements, Solvatochromism

and electrokinetic studies.

3. The mechanical properties and viscoelastic properties were investigated

in detail to evaluate the fibre/matrix interactions and nature of

interface.

4. Water absorption is an important characteristic of composites for

applications where contact with moisture becomes essential.

5. Thermal stability of the raw fibre and cellulose micro and nanofibrils

was determined from TG and DTG curves.

The use of cellulose microfibrils and nanofibrils as reinforcement in polymer

matrix is a new and emerging field. These fibrils can be used to process

microfibril and nanofibril composite materials using a polymer as the matrix.

These composites are a relatively new class of composites that exhibit some

88 Chapter 1

unique and outstanding properties with respect to their conventional

composite. The present study also deals with the development of banana

microfibrils and nanofibrils from raw banana fibre by steam explosion

process followed by acid hydrolysis and fabrication of composites with phenol

formaldehyde matrix. The experimental details, results, discussion and

conclusions are presented.

1.16 Scope and objectives of the work

Composites based on thermosetting resins are now becoming popular due to

their processing advantages. Phenol formaldehyde (PF) is a commodity

polymer that has properties which can be extended into the applications of

high performance polymers. PF has become a universal polymer and it is

applied in each of the areas where special properties are required. PF provides

most of the advantages with regard to economic (price), ecological (recycling

behavior) and higher thermal stability (technical requirements). PF has low

density, sterilizability, good surface hardness, good abrasion resistance,

excellent electrical insulation properties and is available in many grades

differing in molar mass.

In the present work, banana fibre has been used as reinforcement in PF matrix.

Banana fibre, a waste product of banana cultivation, is abundantly available in

the state of Kerala. In India about 1.5M acres of land is cutivated with banana

plantations which yield about 3 x 105 tons of the fibre. A large quantity of this

renewable fibre resource is being underutilized. Banana fibre is obtained from

the sheath of banana plants. The cellulosic fibre obtained from the pseudo-

stem of banana plant (Musa sepientum) is a bast fibre with relatively good

mechanical properties. Table 2.1 lists the mechanical properties of banana

fibre. X-ray studies on the fibre has shown that the structure is that of cellulose

Introduction 89

I. The fibre has a high degree of crystallinity, with good alignment of

crystallites parallel to the fibre axis. The crystalline cellulose in banana fibre is

arranged in the form of a helix at an angle of fibre 11–12o. When compared to

other natural fibres, banana fibre has good mechanical properties. In general,

the strength of a fibre increases with increasing cellulose content and

decreasing spiral angle with respect to the fibre axis. The major chemical

constituents of the fibre are cellulose (63–64%), hemicellulose (19%), and

lignin (5%). These properties can be advantageously utilized in reinforcing PF

which can increase the stiffness of PF and can favour good mechanical

properties.

The major objectives take account of the following

1. Utilisation of banana fibres as a reinforcement potential in

phenolformaldehyde composites

2. Isolation of cellulose fibrils

3. Surface modification and characterisation by different techniques.

4. Fabrication of composites using surface modified banana fibres by

RTM and CM

5. Comparative study of RTM and CM fabricated composites.

6. Fabrication of micro and nano composites.

7. Characterisation of the fabricated composites by different techniques.

90 Chapter 1

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