Chapter I

Introduction *

2 Chapter 1

I .I. Definition of composites

Composite is a combined material created by synthetic assembly of two or

more components, a selected filler or reinforcing agent and a compatible

matrix binder (i.e., a resin) in order to obtain specific characteristics and

properties. The components of composites do not dissolve or otherwise

merge completely into each other. Nevertheless, they do act in concert.

The components as well as the interface between them can usually be

physically identified and it is the behavior and properties of the interface

that generally control the properties of the composite. The properties of a

composite cannot be achieved by any of the components acting alone.

The goal in creating a composite is to combine similar or dissimilar

materials in order to develop specific properties that are related to desired

characteristics. Composites are used to produce a variety of economical,

efficient and sophisticated items ranging from toys and tennis rackets to

insulation shields and miniature printed circuits for spacecrafts [I -81.

1. 2. Constituent Materials

I. 2.1. Matrices

The matrix of a composite material has two functions: it is a binder that

holds the reinforcement in place, it transfers external load to the

reinforcement and it protects the reinforcement from adverse

environmental effects. Moreover, the matrix redistributes the load to

surrounding fibres when an individual fibre fractures and laterally supports

the fibres to prevent buckling in compression.

Commodity plastics are used in nonstructural applications, such as food

packing and toys, while other polymers classified as engineering plastics

meaning that they posses properties that enable them to replace traditional

construction materials, eg. metals and wood, in load bearing applications

Introduction 3

even without reinforcement. Thermoplastics are almost exclusively used

when no reinforcement is included and dominates also when short fibres

are incorporated. However thermosets clearly dominate in structural

composite applications, although thermoplastics have lately received

increased attention in continuous fibre reinforced composite applications

due to a number of attractive potential applications.

1.2.1 .I. Thermoplastic polymer matrices

One of the main features of amorphous thermoplastics is that they are

soluble in common industrial solvents. The reinforcement can be

impregnated with a low viscosity solution avoiding the problem of high melt

viscosity. Amorphous thermoplastics have good surface finish since they

do not shrink much when they solidify and there is no differential shrinkage

by the presence of crystalline regions. Semicrystalline polymers usually

have good solvent resistance due to the crystalline regions, which prevent

dissolution of entire molecular structure. The crystallinity also improves

hig h-tem perature performance and long-term properties like creep.

Semicrystalline polymers shrink more than amorphous ones upon

solidification; the higher the crystallinity, the higher the density change

between melt and solid. Due to difference in shrinkage between

amorphous and crystalline components, the surface of semicrystalline

thermoplastic is not as good as amorphous ones. Since solvent normally

cannot be used to dissolve semicrystalline polymers, reinforcement due to

impregnation is extremely difficult. A brief overview of some of the

common thermoplastic polymers used as matrices is given below.

Polyethylene (PE)

PE can be both commodity and engineering plastic depending on grade,

but is rarely used as composite matrix due to low temperature tolerance

and modest mechanical properties. Polyethylene has the highest degree of

4 Chapter 1 --

crystallinity due to its simple, regular and flexible molecular structure, thus

enabling it to be used well above its glass transition temperature.

Polypropylene (PP)

Like PE, PP can be used as both commodity and engineering plastic

depending on grade. PP is the chemically least complex and' cheapest polymer

commonly used as polymer matrix. Both homopolymer and copolymer (PP

copolymerised with PE units to improve toughness) version of PP are available.

PP has become the most common thermoplastic matrix in mass produced

structural composite applications including automobile components.

Polyamides (PA)

One of the best-known thermoplastic polymer families is the polyamides

(PA), often called nylons. In contrast to PE and PP, PA may be used at

moderately increased temperatures, thus greatly improving its usefulness

as matrix. The biggest drawback of some common PA grades is that they

are hygroscopic, i.e. absorb water. In composite applications, PAS are

normally reinforced with glass fibres and used in applications similar to

glass fibre reinforced PP, but where higher temperature tolerance and

improved mechanical properties are required.

Thermoplastic polyesters

Thermoplastic polyesters like poly (ethylene terephthalate) (PET) and poly

(butylene terephthalate) (PBT) are also used as composite matrices. The

properties of PBT and PET are similar to PA, but lacking hygroscopic

disadvantage.

Polyketones

Among the poiyketones like poly (ether ketone) (PEK), poly (ether ketone

ketone) PEKK, the most common is poly (ether ether ketone) (PEEK). The

polyketones possess high mechanical properties, high temperature

Introduction 5

tolerance, good solvent resistance and high price, and in composite

applications PEK is restricted in critical high performance applications.

Polysulp hones

Polysulphones like polysulphone (PSU), pol yether sulphone (PES) and

poly(aryl suplhone) ( PAS) are high performance amorphous polymers with

good tolerance to high temperature and fire. In composite applications

polysulphones are reinforced with glass or carbon fibres and used in the

same type of applications as polyketones.

Thermoplastic polyim ides

-Thermoplastic polyimides like poly (ether imide) (PEI), polyirnide (PI) and

poly (amide imide) (PAI), despite being amorphous, are very tolerant to

solvents and environmental exposure and offer good mechanical

properties with the disadvantages of very high melt viscosities and high

price. In composite applications the members of polyimide family are

reinforced with glass or carbon fibres and are used in same type of

applications as polyketones and polysulfones.

1.2.1.2. Thermoset polymer matrices

Most common thermosets used as composite matrices are unsaturated

polyesters, epoxies, vinyl esters, and phenolics.

Unsaturated polyesters

The workhorse of thermoset matrices is unsaturated polyester (UP), which

offers an attractive combination of low price, reasonably good properties

and uncomplicated processing. Whereas unsaturated polyester

formulations have drawbacks in terms of poor temperature and UV light

tolerance, they can be modified to suit various applications through various

additives and variations in resin compositions. The generic traits of

unsaturated polyesters include good mechanical properties, low viscosity,

6 Chapter 1

uncomplicated crosslinking requirements and low price. The main

drawbacks are low temperature tolerance, significant shrinkage from

crosslinking, and notable health problems. Glass fibre and polyester matrix

combination are a good match in terms of both performance and 3rice.

Epoxies

Where mechanical properties and temperature tolerance of unsaturated

polyesters are no longer suffice, epoxies (EP) are often used due to their

significant superiority in these respects. Epoxies are most often seen in

fields where the cost tolerance is the highest, eg. aerospace, defense and

sports applications. As in the case of polyester resins, the spectrum of

properties which are achievable with resin formulations and additives is

higher in epoxies. Significance of epoxies are low viscosity, low shrinkage,

and the fact that they adhere well to the reinforcement fibres make them a

unique matrix. The major drawbacks of epoxies are high price, toxicity, and

complex processing requirements, which often include elevated

temperature and consolidation pressure, thus translating into costly

manufacturing operations. Epoxies are used with all kinds of

reinforcements, but most commonly together with carbon fibres, since this

combination offers an attractive blend of properties and cost.

Vinyl esters

Vinyl esters are developed in an attempt to combine fast and simple

crosslinking of unsaturated polyesters with mechanical and thermal

properties of epoxies. Being an unsaturated polyester epoxy compromise,

vinyl esters are more likely to be used in applications where unsaturated

polyester does not quite fulfill. Applications and reinforcement areas are

the same as for unsaturated polyesters, but where somewhat improved

properties are required. An application area in which vinylesters have been

successful is a corrosive industrial environment.

Introduction 7

Phenolics

Phenolics were invented in the beginning of 2oth century and have since

been used extensively in unreinforced and short fibre reinforced

applications. The prime advantage of phenolics over other composite

matrices is their excellent hig h-temperature and fire tolerance corn bined

with low smoke emission and reasonable price. The major disadvantages

include mediocre mechanical properties, especially brittleness and the fact

that they have been impossible to pigment. Phenolic composites, which

nearly exclusively are glass fibre reinforced, are most likely found in

applications where the structural requirements are modest, but where high

temperature and fire tolerance is valued.

Other thermosets

Whilst the aforementioned thermosets are the most commonly used ones

in composite applications, there are several other thermoset candidate

matrices. Among these are polyurethanes (PUR), natural and synthetic

rubbers, imide-based thermosets, and cyanate esters, where the latter are

fairly introduced for high performance applications.

I .2.2. Reinforcements

Reinforcement is the constituent that primarily carries the structural loads

to which the composite is subjected. The reinforcement therefore, to a

significant degree, determines the strength and stiffness of the composite.

Composite reinforcement may be in the form of fibres, particles or

whiskers. 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-lpm. Whiskers are pure

single crystals manufactured through chemical vapor deposition and thus

8 Chapter 7

have preferred directions. Whiskers are more or less randomly arranged in

the matrix, whisker reinforced composites are likely to be considered as

macroscopically isotropic. The most common types of fibrous

rqinforcements used in composite applications are glass, carbon and

aramid.

Glass fibres

Glass fibres dominate as reinforcement in high performance composite

applications due to appealing combination of good properties and low cost.

The major ingredient of glass fibre is silica, which is mixed with varying

degrees of other oxides. The diameter of individual fibre varies between 3

and 25 pm and in composite applications it is commonly in the range of 10-

20 pm.

Several different glass compositions are available, the most common being

E and S glass, where E denotes electrical and S high strength. E glass

offers excellent electrical properties and durability and is general-purpose

grade. S glass and R glass are similar and offer improved stiffness and

strength as well as high temperature tolerance.

Properties characteristics of glass fibres are advantageous such as high

strength, very good tolerance to high temperatures and corrosive

environments, radar transparency and low prize. Disadvantages include

relatively low stiffness, moisture sensitivity and abrasiveness. Glass fibres

are amorphous and therefore isotropic.

Carbon fibres

Carbon fibres are corn mercially manufactured from three different

precursors, rayon, polyacrylonitrile, and petroleum pitch. The fibres are

initially drawn and oxidized at temperatures below 4 0 0 ' ~ to crosslink them

and then carbonized above 8 0 0 ' ~ in a process called pyrolysis.

Graphitization is then carried out to further eliminate impurities and

improve crystallinity.

The carbon atoms are covalently bonded together in graphene layers,

which are held together by secondary dispersion bonds. The properties of

carbon fibres are due to the strong covalent carbon-carbon bonds within

the graphene layers. While carbon fibres have the highest strength and

stiffness of many composite reinforcements, only high strength or high

modulus can be normally attained in the same fibre and is accompanied by

fairly low strain to failure. Other advantages include tolerance to high

temperature and corrosive environments, as well as lack of moisture

sensitivity. The major disadvantage of carbon fibre is their high price, while

others include brittleness and conductivity. Carbon fibre reinforcement

dominates in high performance applications due to its outstanding

mechanical properties combined with low weight. The carbon fibre

reinforcement mostly combined with epoxies finds applications mainly in

aerospace field.

Aramid fibres

Aramids, aromatic polyamides are members of polyamide family. Due to

high degree of crystallinity and rigid molecular structure, the temperature

tolerance of aromatic polyamide is very good. Among several different aramid

grades available for composite applications are Kevlar 29,49 and 1 49.

Advantages of aramid fibres are very good mechanical properties,

especially toughness, moderately high damage tolerance, corrosion

resistance and good electrical properties. The disadvantages of aramid

fibres include high price, moisture sensitivity and difficulty to cut the fibres

due to its high toughness. ,.

Polyethylene fibres

Polyethylene fibres are having tensile properties like aramid fibres dictated

by the properties of covalent bonds of molecular backbones. But due to

lower density of PE fibres, their specific strength and modulus are higher

and comparable to carbon fibres. The main drawback of polyethylene

fibres is poor temperature tolerance and poor matrix compatibility. The

potential applications of PE fibres include batlistic protection and sporting

goods.

Other fibres

Several specialty fibres are used in different applications for example

offering extra high temperature tolerance, radar transparency etc. Fibrous

reinforcements used with polymer matrices include boron and ceramic

fibres, metal wires, as well as natural fibres like jute sisal, coir etc.

I .3. Natural fibres

The preservation of our environment requires that we stop developing

materials that wilt like many plastics last indefinitely. Industry, especially

the automotive sector, which is an enormous user of bulk materials would like

long- lived materials that nevertheless degrade back into the environment

when they are no longer needed. Reinforced plastics based on natural mainly

plant derived substances show the promise of providing this.

The natural fibres have the potential to reduce vehicle weight (up to 40%)

compared with glass fibre, which accounts for the majority of automotive

corn ponents. While satisfying increasingly stringent environmental criteria

much less energy is used in growing, harvesting and preparing natural

fibres than in producing glass fiber.

Generations of seamen prized the tensile strength of coir, sisal, flak, jete,

kapok and other natural fibres in ropes and sails. The introduction of the

In froduction 1 I

natural fibres also improves physical advantages including improved

stiffness, strength, ductility, toughness etc. The introduction of natural fibre

mat can also reduce the number and size of voids formed in composites

during cure of the thermosetting resin because naturally hydrophilic fibres

absorb moisture produced by cure reaction.

In addition to their high strength and stiffness per weight and environmental

virtues the advantages of natural fibre composites are acoustic insulation,

easier health and safety management, rapid production by compression or

injection molding and potentially lower cost [9]. Natural fibre composites are

likely to be environmentally superior to glass fibre composites in most

cases for the following reasons: (1) natural fibre production has lower

environmental impacts compared to glass fibre production; (2) natural fibre

composites have higher fiber content for equivalent performance, reducing

more polluting base polymer content; (3) the light-weight natural fibre

composites improve fuel efficiency and reduce emissions in the use phase of

the component, especially in auto applications; and (4) end of life incineration of

natural fibres results in recovered energy and carbon credits [lo]. Burgueno

et al. reported that cellular biocomposite beams and plates not only have

the potential to serve as primary load bearing components, but that with

additional research they can compete with conventional structural

materials [I I]. Material characterization verified that the short-term

stiffness of biocomposites competes with E-glass com posites, while

strength, temperature and moisture properties are much lower.

Composites containing recycled plastics and wood fibre offer an interesting

combination of properties, as well as lower cost than competitive materials,

especially those based on synthetic fibres. By permitting use of moderately

contaminated recycled plastics rather than requiring the use of virgin resin, #

these materials provide an additional market for recycled plastics, thereby

helping to reduce waste disposal burdens. Composites can also be

12 Chapter 1

fabricated using recycled wood fibre, such as recovered paper fibre,

providing an additional market outlet for recovered paper and thus further

waste diversion benefits [I 21. A comparison of properties of various natural

and synthetic fibres is given in Table I.

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

Density Elongation Tensile Young's Fibre

(glcm3) (%) strength modulus (MPa) (GPa)

Cotton I .5-1.6

Jute I .3

Flax 1.5

Hemp - Ramie - Sisal 1.5

Coir 1.2

Viscose

Soft wood kraft I .5

E glass 2.5

S-glass 2.5

Aramid 1.4

(Normal)

Carbon I .4 (Standard)

[Ref. : AK Bledzki, S Reihmane, J Gassan, J Appl Polym Sci, 59 (1996) 13291

Introduction 13

I .4. Types of natural fibres

Natural fibres are grouped into three types, bast fibres, leaf fibers and seed

hair fibres depending upon their source.

1.4.1. Bast 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. Stronger composites are obtained if the bundles are pre-treated

in a way that removes the lignin between the cells. Boiling in alkali is one

of the ways to separate cells. Bast fibres are usually grown in warm

climates. Eg. Bagasse, Flax, Hemp, Jute, Kenaf, Ramie.

a. Bagasse

Bagasse is a by-product from the sugar-cane industry and it grows

extensively in tropical countries like Brazil, India, Pakistan, Indonesia,

Philippines and Jamaica. Bagasse consists of the sheath and pith of the

cane stalks. Bagasse fibre reinforced composites have been used as

materials for structural components.

b. Flax

This fibre is extracted from the plant Linum usitatissimum L. that is grown

chiefly in the USSR, Poland, France, Belgium and Ireland. The plant is

cultivated mostly for its oil-bearing seed (linseed), although it is also an

important source of a vegetable fibre. Flax is an annual plant with a

slender, grayish green stem growing to a height of 90-120 cm. The plants

are pulled by hand or machine for highest yield and quality although

mowing is practiced for some grades. After deseeding, the straw is retted

i-e. the fibres are liberated through enzymatic action on pectinous binding

material in the stem, dew or water retting is also employed. The fibre is

then hackled for alignment and final cleaning.

14 -- -. - .- --- Chapter I

c. Hemp

The hemp fibre is extracted from the plant Cannabis sativa that originated

in Central Asia. The plant grows readily in temperate and tropical climates

but its commercial production for fibre is in China and Eastern Europe. The

hemp plant is grown for fibre from the stem, for oil from the seeds or for

drugs from the flowers or leaves. The mature hemp stalk is harvested at

the proper time to ensure highest quality and yield by hand cutting and

spreading or by a harvester-spreader for dew-retting. The retted and dried

stem is further treated by either hand breaking and hackling to remove the

woody stem portion or by mechanical breaking. The grading is by color,

lustre, density, spinning quality and strength.

d. Jute

The jute fibre is obtained from two species of the annual herbaceous plant

Corchorus capsularis of Indo-Burma origin and C. olitorius from Africa. The

major jute production areas are India and Bangladesh. The two sources of

jute are differentiated by their seeds and seed pods. The C. capsularis is

round-podded and is called white jute; the C. olitorius is long podded and

is known as tossa or daisee. The plants are harvested by hand at an early

seed stage using knives and the stems are left on the ground for several

days to promote defoliation. The defoliated stems are bundled and taken

for wet retting in canals, ditches, or ponds for periods of 10-20 days. The

jute is then baled for shipment to domestic users.

e. Kenaf

The kenaf fibre is extracted from the plant Hibiscus cannabinus which

belongs to the Malvaceae family. The plant is native to Egypt and USSR.

The plant is a herbaceous annual growing in single stem to heights of 1-4 m.

Kenaf is harvested as flowering begins and it can be hand cut, mowed or

pulled. It is often stem-retted followed by hand or mechanical stripping and

Introduction --- 15

washing. Kenaf is graded into three grades based on color, uniformity,

strength and cleanliness.

f. Ramie

This fibre is produced from the stems of Bochmeria n k a , a nettle native to

central and western China but growing in regions varying from temperate

to tropical including China, Brazil, Taiwan and Japan. Another name for the

raw fibre is China grass. The plant grows 1-2 m high or higher with stems

8-16 mm in diameter. The ramie fibre is contained in the bark and is

usually extracted by hand stripping and scraping. The raw ramie fibres

contain 25-35 % plant gums and small quantities of parenchyma cells that

must be removed before the fibres can be spun. The plant is cut green and

defoliated manually and bast ribbons are stripped from the woody stem.

Ramie fibres are graded according the length, color and cleanliness.

1.4.2. Leaf fibres

In general, the leaf fibres are coarser than the bast fibres. eg. sisal, abaca,

banana and henequen. Within the total production of leaf fibres, sisal is the

most important which is obtained from the agave plant. The stiffness is

relatively high and it is often applied as binder twines. The abaca fibre that

is from the banana plant is durable and resistant to seawater.

a. Banana

Banana fibre is extracted from the plant Musa sapienturn. Each of the

stalks is 2.7-6.7 m tall with a trunk 10-20 cm wide at the base. The sheaths

before expanding are 2-4 m long, 13-20 cm wide and about 10 mm thick at

the center. The fibres run lengthwise in the sheaths. The sheaths vary in

length and width and in color. The mature stalks are cut off at the roots and

at a point just below where they begin to expand. In certain countries4he

stripping method is used while modern plantations have replaced this hand

16 - - - -

Chapter 1

pulling operation with power driven pulling machines. The annual

production of banana is 200 x l o 3 tons and the main countries of origin are

India, Indonesia and Philippines.

b. Sisal

Sisal is extracted from the leaves of the plant Agave sisalana that is widely

cultivated in the Western Hemisphere, Africa and Asia. The agaves have

rosettes of fleshy leaves, usually long and narrow, which grow out from a

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

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

fibres are embedded longitudinally in the leaves and are most abundant

near the leaf surfaces. The leaves contain about 90% moisture but the

fleshy pulp is very firm and the leaves are rigid. The fibre is removed when

the leaves are cut because dry fibres adhere to the pulp. The fibre is

removed by scraping away the pulpy material, generally by a mechanical

decorticat ion process.

In the decortication process, the leaves are fed through sets of crushing

rollers. The crushed leaves are held firmly at their centers 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.

c. Pineapple

Pineapple leaf fibres at present are a waste product of pineapple

cultivation. The fibers are extracted from the leaves of the plant Ananus

cosomus belonging to the Bromeliaceae family. The fibre is extracted by

hand scraping after beating the leaves to break up the pulpy tissue or after

a retting process that partially ferments and softens the leaves.

Introduction - - -

17

I .4.3. Seed fibres

Cotton is the most common seed fibre and is used for textile all over the

world. Other seed fibres are applied in less demanding applications such

as stuffing of upholstery. Coir is an exception to this and is used to make

mats and ropes. Other examples are kapok and oil palm.

a. Coir

The fibre is contained in the husk of the coconut tree (Cocos nucifera) that

is positioned beneath the outer covering of the fruit and envelops the

kernel or coconut. Sri Lanka and India are important producers of coir. The

fruits are gathered just short of ripeness and the husks are broken by hand

or by use of a bursting machine. The extraction of the fibre involves retting

at the edges of rivers and also in pits or in modern operations in concrete

tanks. The retted husks are beaten with sticks to remove extraneous

matter and the dried fibre is suitable for spinning. The fibres can also be

removed from the husk by a decorticating machine in connection with

copra production.

In the extraction of coir fibre from coconut husk, a large quantity of coir

waste is obtained. The coir waste consists of coconut pith resulting from

the retting process, broken fibres from coir spinning and shear waste from

mats and matting industries. Coir pith constitutes about 70% of coconut

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

produced in India.

b. Oil palm

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

producing crops. It belongs to the species Eiaeis guineensis under the

family Palmaceae, and is commonly found in the tropical forests of West

Africa. Its major industrial cultivation is in the Southeast Asian countries

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

Latin America. Oil palm cultivation in India has come up 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 fiber

is about 4009 per bunch. The fibers are extracted by retting followed by

cleaning and drying.

I .5. Microstructure of natural cellulose fibres

The chemical composition and cell structure of natural fibres are quite complex.

Each fibre is essentially a composite, in which rigid cellulose microfibrils are

immersed in a soft lignin and hemicellulose matrix (Figure 1 . I .).

Physically each fibre cell is made up of four main parts namely the primary

wall, the thick secondary wall, the tertiary wall and lumen. The cell consists

of several layers of fibrillar structure consisting of fibrillate. In the primary

wall, the fibrillate has a reticulated structure. In the outer secondary wall

(s l ) which is located inside the primary wall, the fibrillate are arranged in

spirals with a spiral angle in relation to the longitudinal axis of the cell. The

fibrillate in the inner secondary wall (s2) of spiral fibre have a sharper slope

18' to 25'. The thin innermost territory wall has parallel fabrillar structure

and enclosed lumen. Then fibrillate area in turn built up of microfibrillate

with a thickness of about 20 nm. The microfibrillate are composed of

cellulose molecular chain with a thickness of 0.7 nm and a length of few

micrometers [6]. The micro fibrillar angle in plant fibres plays an important

role in determining the important behavior of the composites as shown in

Figure 1.2.

Introduction 19

Secondary wa!l S3 Lumen

Secondary wall S2

econdary wall S l

Disorderly arranged crystalline cellulose

and hemicelluIose microfibrils networks

Figure 7 . 7 . Structural constitution of natural vegetable fibre cell, in which the secondary wall S2 makes up -80% of the total thickness and then act ass the main load bearing component

[Ref.: M Rong, Comp Sci Technol, 61 (2001) 14371

1 Sisal

0 10 2G 30 40 51

Microfibrillar angle (a")

Figure 7.2. Work of fracture of natural fibre composites plotted against microfibrillar angle of fibres.

[Ref.: C Pavithran, PS Mukharjee, M Brahrnakumar, AD Damodaran, J Mat Sci Lett, 6 (1 987) 8821

20 Chapter 1

Besides, the microfibrils are helically wound along fibre axis to form

ultimate hollow cells. Uncoiling of these spirally oriented fibres consumes

large energy and is the predominant failure mode. As a result a

pretreatment of the fibres would result in chemical and structural changes

not only on fibre surface but also on distinct cells, which in turn influences

the fibres and composites as well.

In fact, a piece of natural fibre is not a single filament like a carbon or glass

fibre but a bundle of cellular aggregate consisting of more than 100

irregular hexagonal hollow ultimate cells [Figure 1-31.

Cuticular layer Epidermis \

Palisade tissue

Chloroph

Bunched fibers

Vascular bundle

Spongy tissue

Figure 1.3. (a) Schematic drawing of the cross section of a piece of sisal leaf

[Ref.: M Rong, Comp Sci Technol, 61 (2001) 14371

The binder that separates the cells makes the governing contribution to

stress development in fibre bundles. Therefore two types of interface

Introduction -, .. --

2 I

should be considered when discussing natural fibre composites: one

between the fibre bundle and the matrix and the other between the cells. A

higher binding strength at former interface than the later one is highly

desired so that for ultimate cells to decohere from each other, probably

leading to interfibrillar failure and uncoiling of helical fibrils. The composites

would be remarkably strengthened and toughened in this way.

Ultimate cells

\

Figure 1. 3. (b) Schematic drawing of the inter construction of a bunched sisal fibre

[Ref.: M Rong, Comp Sci Technol, 61 (2001) 14371

I .6. Chemical composition of natural fibres

The chemical composition of natural fibers varies depending on the type of

fibre. Primarily fibres contain cellulose, hemicellulose and lignin. The properties

of each constituent contribute to the overall properties of the fibre.

Hemicel lulose is responsible for the biodegradation, moisture absorption and

thermal degradation of the fibre and it shows least resistance where as ligni'n is

thermally stable, but is responsible for the UV degradation.

22 Chapter I

Cellulose

Cellulose is a polymer derived from glucose: P-D glucopyranose. Glucose

is just one of a number of monosachrides having the same chemical

composition C6HI2o6 i.e, a hexose sugar having five hydroxyl groups

which are responsible for their solubility in water. They can be represented

schematically by the general formula,

CH20H- CHOH - CHOH - CHOH - CHOH - CHO

The sugars exist predominantly as a six membered ring structure where

the C, carbon is linked to C, carbon through an oxygen atom. Cellulose is

a natural polymer containing thousands of p-D-glucose units linked by

glucosidic linkages (C-0-C) at the C, and Cq position (Figurel. 4)

I I

;a Cellobiose -I

Figure 1.4. Structure of ce/lu/use

Each unit is rotated through 180' with respect to its neighbors so that structure

repeats itself every two units. The pair of units is called cellobiose and since

cellulose is made up of repeating cellobiose units, cellulose is technically a

polymer of cellobiose rather than a-D glucose. With cellulose, the polymer

chains part together alongside one another in a highly regular manner. The

degree of polymerization of cellulose is high and in consequence there will be

few chain ends to disrupt the crystalline arrangement. According to Gram [I41

Introduction 23

the degree of polymerization in sisal fibre is about 25000. In this crystalline area

penetration by other molecules such as water or other chemicals is difficult. At

other points in the fibres however, the arrangement is much more random and

disorganized and there are no crystallized structures. This region is known as

amorphous region.

Hemicellulose

The hemicelluloses are also polymers often of very similar sugars eg.

galactose and mannose and rarely if ever crystalline, the principal

constituent sugars, which are found in hemicellulose, are pentose sugars

(five carbon sugars). The hemicellulose presents a relatively low degree of

polymerization and is soluble in alkali. According to Gram [I41

hemicelluloses occur mainly in the primary wall and their degree of

polymerization in sisal fibre lies between 50 and 200.

Lignin

Lignin has an aromatic structure, which is almost insoluble in most solvents.

No regular structure for lignin has been demonstrated. It is totally amorphous.

According to Gram [I41 lignin can be broken down or leached out in an

alkaline environment. Almost 25% of the total lignin is found in the middle

lamella. Since the middle lamella is very thin the concentration of lignin is also

correspondingly high.

1.7. Major issues of natural fibres

I .7.1. Moisture absorption of fibres

The lignocellulosic natural fibers are hydrophilic and absorb moisture. The

swelling behavior of natural fibres is generally affected by its morphology

as well as physical and chemical structures. Biofibres change their

dimensions with varying moisture content because the cell wall polym,ers

contain hydroxyl and other oxygen containing groups, which attract

24 Chapter 1

moisture thorough hydrogen bonding [I 61. The hemicelluloses are mainly

responsible for moisture sorption, but the accessible cellulose, noncrystalline

cellulose, lignin, and surface of crystalline cellulose also play major roles.

Lignocellulosics shrink as they lose moisture. Water penetration through natural

fibre can be explained by capillarity action [17]. The cross section of the fibres

becomes the main access to the penetrating water. The waxy materials present

on the fibre surface helps to retain the water molecules on the fibre. The porous

nature of natural fibre accounts for the large initial uptake at the capillary region.

The hydroxyl groups (-OH) in the cellulose, hemicellulose and lignin build a

large amount of hydrogen bonds between the macromolecules in the plant fibre

cell wall. Subjecting the plant fibres to humidity causes the bonds to break. The

hydroxyl groups then form new hydrogen bonds with water molecules, which

induce swelling [I 81.

The swelling of cell wall exerts very large forces. The theoretical value of

these forces is about 165 MPa. However the actual swelling pressure is

reported to be half of the calculated value. Schematic representation of

swelling process in cellulose is given in Figure 1.5.

Ctll wall - I J --- -AT--- OH-, OH.\

2) ' P H

" OH,-'^ ' OH. .+'

h H F

?-H- H 'H --.--.-- 2-H - - .- 'OH

H Cell wall -.-I----- -.. - OH ,L--

OH 7-- OH 0 1 1

Figure 7.5. Schematic representation of swelling process in ce/lulose fibre [Ref. : LY Mwaikam bo, MP Ansell, J Appl Polym Sci, 84 (2002) 22221

Introduction 25

Generally the moisture content of natural fibres vary between 5-1 0%. This

can lead to dimensional variations in composites and also affect the

mechanical properties of composites. Therefore the removal of moisture

from fibres is very essential before the preparation of composites. The

moisture absorption of natural fibres can be reduced by proper surface

modifications.

I .7.2.Thermal stability of natural fibres

Natural fibres are complex mixtures of organic materials and as a result,

thermal treatment leads to a variety of physical and chemical changes. The

thermal stability of natural fibres can be studied by thermogravimetric

analysis (TGA).

As mentioned before, natural fibre is composed mainly of cellulose,

hemicellulose and lignin. Each of the three major components has its own

characteristic properties with respect to thermal degradation, which are

based in 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 behaves differently if they are isolated or if they are

intimately combined within each single cell of the fibre structure [20].

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

at slower rate than the other constituents. This is shown in figure 1.6a. The

TGA curve indicates that the beginning of the natural fibre degradation

occurs at approximately 1 8 0 ° ~ , but the rate of degradation is always lower

than that of the cellulose (Figure. 1.6b).

26 -

Chapter 7

Figure 7.6. TG (percentage of weight) and D TG (derivative) Vs. Temperature curves. (a) lignin, (b) cellulose.

[Ref. : N E Marcovich, MM Reboredo, MI Aranguren, Thermochemia Acta, 372 (2001) 451

This process has been described by Shukry and Girgis [21] who also

presented an analysis of the products of the degradation. Figure 1 -6. b shows

the TG and DTG vs. temperature curves of pure cellulose. It is noticed that

the weight loss in the cellulose sample is negligible below 3 0 0 ' ~ (water

Introduction - - - - - - -. - - - 27 ,. - .-. .

desorption being of no concern here). However above that temperature the

cellulose begins to degrade fast and at above 4 0 0 ' ~ only the residual char

is found. Beall [22] has described this process as the loss of hydroxyl

groups and depolymerisation of cellulose to anhydroglucose units.

The natural fibres start degrading at about 240 '~ . The thermal degradation

of lignocellulosic materials has been reviewed by Tinh et al. in detail for

modified and unmodified materials [23,24]. The thermal degradation of

natural fibres is a two-stage process, one in the temperature range 20-

2 2 0 ' ~ and another in the range 280-300'~. The low temperature

degradation process is associated with degradation of hemicellulose where

as the high temperature process is due to lignin. The apparent activation

energies for the two processes are about 28 and 35kCal/rnol, which

correspond to the degradation of hemicellulose and lignin, respectively.

The degradation of natural fibres is a crucial aspect in the development of

natural fibre composites and thus has a bearing on the curing temperature

in the case of thermosets and extrusion temperature in thermoplastic

composites [25,26,27].

1.7.3. Biodegradation and photodegradation of natural fibres

The lignocellulosic natural fibres are degraded biologically because

organisms recognize the carbohydrate polymers, mainly hemicelluloses in

the cell wall and have very specific enzyme systems capable of

hydrolyzing these polymers into digestible units [28]. Lignocellulosics

exposed outdoors undergo photochemical degradation caused by

ultraviolet light. Resistance to biodegradation and UV radiation can be

improved by bonding chemicals to the cell wall polymers or by adding

polymer to the cell matrix.

Figure 1.7 demonstrates how the components of lignocellulosics interact in

various ways. Biodegradation of the high molecular weight cellulose

28 Chapter 1

weakens the lignocellulosic cell wall because crystalline cellulose is

primarily responsible for the strength of lignocellulosics [29].

BIOLOGICAL DEGRADATION

Hemicellulose>~~Accessible Crystalline Cellulose > Non-crystalline Cellulose>>>>Crystalline Cellulose>>>>> Lig nin

MOISTURE SORPTION

Hemicellulose~~Accessi ble Crystalline Cellulose >>> Non-crystalline Cellulose > Lignin >>>Crystalline Cellulose

ULTRAVIOLET DEGRADATION

Lignin>>>>>Hemicellulose>Accessible Cellulose>Non Crystalline Cellulose>>> Crystalline Cellulose

THERMAL DEGRADATION

Hemicellulose > Cellulose>>>>>Lignin

STRENGTH

crystalline Cellulose~~noncrystaline Cellullose+Hemicellulose- lignin>Lignin

Figure7.7. Cell Wall polymers responsible for lignocellulosic properties 1281

[Ref.: RM Rowell, Emerging Technologies for Materials and Chemicals, R.M. Rowell, TP Schultz, RNarayan, Eds.12 (1 992) 4761

Due to degradation of cellulose the strength gets lost. Photochemical

degradation by ultraviolet light occurs when lignocellulosics are exposed to

outdoor. This degradation primarily takes place in lignin component, which

is responsible for the characteristic color changes [30]. The surface

becomes richer in cellulose content as the lignin degrades. In comparison

to lignin, cellulose is much less susceptible to UV degradation.

After the lignin is degraded, the poorly bonded carbohydrate-rich fibres erode

easily from the surface, which exposes new lignin to further degradative

Introduction 29

reactions. It is important to note that hemicellulose and cellulose of

~ignocellulosic fibres are degraded by heat much before the lignin 1301. The

lignin component contributes to char formation, and the charred layer helps to

insulate the lignocellulosic from further thermal degradation.

1.8. Fibre-matrix interface and interfacial modifications

The term interface has been defined as the boundary region between two

phases in contact. The composition, structure or properties of the interface

may be variable across the region and may also differ from the composition,

structure or properties of either of the two contacting phases [31]. This

interfacial region exhibits a complex interplay of physical and chemical

factors that exerts a considerable influence on and controls the properties

of reinforced or filled composites. The interfacial interaction depends on

the fibre aspect ratio, strength of interaction, anisotropy, orientation,

aggregation etc. [32-381. Extensive research has been done on the

interfacial shear strength (ISS) of man made fibres (carbon and boron

fibres, polymeric fibres and different types of glass fibres) 139-421 and for

natural fibres [43-461 by using methods such as fibre pull out tests, critical

fibre length and microbond tests.

Natural fibres are incompatible with the hydrophobic polymer matrix and have a

tendency to form aggregates. They are hydrophilic fibres and so not resistant to

moisture. To eliminate the problems related to high water absorption, treatment

of fibers with hydrophobic aliphatic and cyclic structures has been attempted.

These structures contain reactive functional groups that are capable of bonding

to the reactive groups in the matrix polymer, e.g., the carboxyl group of the

polymer resin. Thus modification of natural fibers is attempted to make the

fibres hydrophobic and to improve interfacial adhesion between the fibre and

the matrix polymer [47-611.

30 Chapter I

In addition to the surface treatment of fibres, use of a cornpatibilizer or a

coupling agent for effective stress transfer across the interface can also be

explored [62-671. The compatibilizer can be a polymer with functional

groups grafted into the chain of the polymer. The coupling agents are

tetrafunctional organornetallic compounds based on silicon, titanium, and

zirconium and are commonly known as silane, ziroconate, or titanate

coupling agents. Table 1.2. presents the structures, functional groups, and

applications of a few commercial coupling agents.

SI. No. Functional Gmup Chemical Structure A p p l i e P~lymer

E l a s f m : p9lydhylens, silicone elasto- mers. UP. PE. PP. E P M . EPR

EP Elastorner~, especially butyk ebbmers. ep on y, phenolic and rnetm~re. PC. PVC. UR

Unsahted polyestem, PE, PP. EPDk E r n

Wnsatuded @yestea, PA. PC, FUR, MF, PF. PI. MPF

All polymers PS, add~im to mine s i l w EP, PUR. SBR, E M

Polyokfins. ABS, ptumlicss. polyestem. PVC. poly~tern=. styrenics

Potybbefins, AES, phmlics. polyesters. W C . polyurethane. stpnics

Table 1.2. Structures, functional groups and applications of a few commercial coupling agents.

[Ref.: DN Saheb, JP Jog, Advances in Polym Technol, 18 (1999) 3511

Most of the silaneltitanatelzirconate coupling agents can be represented

as R-(CH2)-X (OR-)n, where X = Si, Ti, or Zr, n= 0-3, OR is the hydrolysable

alkoxy group, and R and R' are the functional organic groups. For example,

Introduction 3 1

for triazine coupling agents, triazine derivatives form a covalent bond with

cellulose fibres, schematically represented as given in Figure 1.8.

Figure 1.8. Triazine derivative of cellulose fib re

[Ref.: AK Bledzki, S Reihmane, J Gassan, J Appl Polym Sci, 34 (1 987) 1 3251

The reduction of the water absorption of cellulose fibres and their

composites treated with triazine derivatives is explained by [68-691.

(1) Reducing the number of cellulose hydroxyl groups which are

available for water uptake,

(2) Reducing the hydrophilicity of the fibre surface, and

(3) Restraint of the swelling of the fibre, by creating a crosslinked

network due to covalent bonding, between the matrix and fibre.

The effect of two silanebased coupling agents, Gamma-aminopropyltriethoxysilane

(GS) and dichlorodiethylsilane (DCS), on radiata pine wood fibre wkh and without

pretreatment using sodium hydroxide was investigated using XPS and solid

state Si NMR, by Pickering et al. [70]. Results based on the elemental and

functional composition of the untreated and treated wood fibres indic,ate

that modification of the fibre surface has occurred for most of the samples.

Concentrations of silicon were measured to be 2.3, 3.2 and 2.2 wt% for GS

treated without sodium hydroxide pre-treatment, DCS treated without

sodium hydroxide pre-treatment and DCS treated with sodium hydroxide

pre-treatment, respectively. NMR analysis has given firm evidence of a

reaction producing ether linkages between the hydroxyl groups on the

wood fibre and silane for treatment with DCS as given in Figure 1.9 [71].

Pedro et al. [72] found that preimpregnation of cellulose fibres in a LDPElxylene

solution and use of a silane coupling agent result in small increment in

mechanical properties of LDPE, reinforced with green cellulosic fibre

composites, which is attributed to an improvement in the interface between the

fibres and the matrix. The fibre treatment also improved the shear properties of

the composite and fibre dispersion in the matrix.

Figure 7.9 a. NMR spectrum of wood fibres treated with DCS without pre-treatment

[Ref.: NE Zafeiropoulos, DR. Williams, CA Baillie, FL. Matthews, Compo Part A, 33 (2002) 10831

Introduction 33

Figure 1.9. 6. Silanol reacted with hydroxyl groups on wood fibre

(Ref.: NE Zafeiropoulos, DR. Williams, CA Baillie, FL. Matthews, Compo Part A, 33 (2002) 10831

lshak et al. [73] did some chemical treatments to improve the mechanical

properties of oil palm empty fruit bunch (EFB) filled with high-density

polyethylene (HDPE) composites. They used two types of coupling agents i.e.,

Saminopropyl trimethoxy silane (3APM) and 3-aminopropyltriethoxysilane

(3APE) and two types of corn pati bilbers, polypropylene-acrylic acid (P PAA)

and poly propylene ethylene acrylic acid (PPEAA). The study of

mechanical properties showed that in the case of coupl~ng agent, the

incorporation of 3-APM has produced composites with superior properties

as compared to 3 APE, due to better interaction between the chemical

reactive groups in the 3-APMm with both EFB and HDPE. For

compatibilised composite systems, the enhancement in the mechanical

properties of PPEAA treated composites gives a clear indication that the

presence of ethyl end group has promoted a better interaction and

corn patibilization between PPEAA and H DPE matrix. However the

efficiency of either 3-APM or PPEAA in enhancing the fillerlmatrix

interaction was hindered by the presence of lignin surrounding the

hollocellulose fibres.

The use of silane coupling agent enhanced the tensile properties and tear

strength of bamboo fibres filled rubber composites [74]. The silane-

coupling agent is believed to improve the surface functionality of bamboo

fibers and subsequently enabled bamboo fibres to bond chemically to the

34 Chapter 7

rubber matrix. The wetting of bamboo fibres in rubber matrix is also

improved by the use of coupling agent. According to Krysztafkiewicz and

Domka [75], the use of silanes permits within a shorter vulcanization time

an increase in rubber bound sulphur and gives vulcanizates of increased

strength.

The surface modification of cellulosic fibres was carried out by

Abdelrnouleh et at. using organofunctional silane coupling agents in an

ethanollwater medium [76]. Silane coupling agents adsorbed from diluted

solution on cellulose fibre surfaces, followed by heat treatment, were

shown to condense both among themselves and with the OH groups of the

substrate to give Si-0-Si and Si-0-C couplings, respectively. These

reactions ensured efficient and irreversible chemical bonding of the silane

onto the cellulose surface. This double modification represents an effective

approach to the optimization of quality of the fibrelmatrix interface for

composite materials containing natural fibres.

Jana and Prieto [77] used two silane coupling agents silquest0 and A

2120 and hydroxy methylated resorcinol coupling agents. The coupling

agents showed the epoxy coupling curing reactions and promoted

complete coverage of wood flour particles by crosslinked epoxy and a

hydroxymethylated resourcinol-coupling agent provided better morphology

around wood flour particles.

Natural fibres are chemically treated to remove lignin, pectin, waxy

substances and natural oil covering the external surface of the fibre cell

wall (Figure 1 . l o a.). Alkalization gives a rough surface topography to the

fibre (figure 1.10 b). It also changes the fine structure of native cellulose I

to cellulose 11 [78]. The increase in the percentage crystallinity index of

alkali treated fibres occurs because of the removal of cementing materials,

which leads to better packing of the cellulose chain. In addition, the

Introduction 35

treatment with NaOH leads to a decrease in the spiral angle and an

increase in molecular orientation. A fair amount of randomness is

introduced in the orientation of crystallites by the removal of non-cellulosic

matter. The elastic modulus of the fibre is expected to increase with

increasing degree of molecular orientation. Well-oriented cellulosic fibres

such as flax have much higher Young's modulus than fibres with medium

orientation such as cotton [79].

To explain the mechanism of alkalization a model of ramie fibre structure

consisting of crystalline oriented amorphous, highly distorted crystalline,

and amorphous regions respectively are given in Figure I . l l a . The

amorphous and highly distorted crystalline regions lie on the fringe of the

crystallites that may be coupled to the adjacent crystallites through the

oriented amorphous zones containing cellulose chains of both polarities.

Lislin Wax and ceU wall

Cellulose (8) @)

Figure 7.7 0. Structure of (a) Untreated and (b) alkalized cellulose fibre

[Ref.: LY Mwaaikambo, MP Ansell, J Appli Polym Sci, 84 (2002) 22221

The cellulose I crystallite with parallel chain structure decreases in lateral

size on conversion to cellulose I lattice with antiparrallel chain structure as

shown in Figure l . l l b [80j. Superior mechanical properties of alkali

treated jute based biodegradable polyester amide based composites as

compared to untreated as well as bleached fabrics composites were

attributed to the fact that alkali treatment improves the fibre surface

characteristics by removing natural and artificial impurities, there by

producing a rough surface morphology [81].

washing NQOH - - '% X)rying

Cell~lose, I Cellulose I + - Cellulose I

Figure 7 . 7 7 . (a.) Schematic model of ramie fibre structure. (bl Mechanism of transformation of cellulose I to cellulose I7 where arrow indicates chain direction.

[Ref.: KP Sao, BK. Samantaray, S Bhattachrrjee, J Appl Polym Sci, 60

introduction 37

Boynard et al. [82] analyzed the effect of mercerization treatment to

improve the strength of fibrelpolyester interface. They found that although

the surface analysis shows that the treatments promote a clear removal of

outer surface layer of fibres with exposition of the inner fibrillar structure

and the consequent increase of the fibre surface area; only a secondary

increase on the mechanical properties was obtained. The slight increase

observed was attributed only to mechanical interlock.

The alkali treatment leads to fibre fibrillation, which increases the effective

surface area available for contact with matrix. The alkali treatment results

in improvement in the interfacial bonding by giving rise to additional sites of

mechanical interlocking there by promoting more resin-fibre interaction at

the intersurface [83].

The effect of alkalization and fibre alignment on the mechanical and

thermal properties of kenaf and hemp bast fibre reinforced polyester

composites were studied by Aziz and Ansell [84]. A small positive change

in fibre density was observed for both hemp and kenaf fibres after

6%NaOH treatment indicating cell wall densification. Samal et al. [85]

studied the effect of alkali treatment, denitrophenylation, nitration, and

combined diazocoupling cyanoethylation on the coir fibers and found that

the modified coir showed significant hydrophobicity, improved tensile

strength and moderate resistance to chemical reagents.

Hill et al. [86] studied benefit of fibre treatment by chemical modification of

fibers (acetylation) or the use of stirane or titanate coupling agents on the

mechanical properties of coir or oil palm reinforced polyester composites.

They found that acetylation of coir or oil palm fibres results in increase in

the ISS between the fibre and the matrix (polyester and styrene) and

increase in the mechanical properties of composites. The interfacial shear

strength between acetylated fibres and various polymer matrices:

38 Chapter 1

polyesters, epoxy and polystyrene showed that improvements in the I SS

were observed upon acetylation of the fibres. Whilst those with thermoset

systems show the highest magnitude in ISS, the greater improvement

upon acetylation was shown by polystyrene [46]. l ncreased hydrophobicity

of fibres has also increased the ISS values. The introduction of acetyl

group an the fibre surface has increased the ISS of the composites with

polystyrene (as compared to the unmodified) significantly than those of

epoxies and polyesters. This results in improved wetting of polystyrene on

to the unmodified fibre surface there by increasing the work of adhesion.

The work of adhesion is increased by an increase in the surface tensions

which can be explained in detail using Young Dupre equation as follows.

where y,-, is the surface energy between solid and the vapour y,-~, the

surface energy between the solid and drop and yl-,, that between the liquid

and the vapor.

The studies by Baiardo et al. show that acetylation of the fibre surface

remarkably increased the strength of a Bionolle/flax fibre composite, by

improving the interfacial strength of natural fibres [87].

George et al. [88] analyzed the improved interfacial interactions in the

chemically reinforced pineapple leaf fibre reinforced polyethylene

composites. They used various chemical treatments using reagents such

as NaOH, PMPPIC, silane, and peroxide to improve the interfacial bonding

and found that the treatment improved the mechanical properties. Among

the treatments PMPPIC treatment of fibre exhibits maximum interfacial

interactions due to the reduced hydrophilicity of the treated fibre as shown

Figure 1.12.

Introduction 39

C O c,= 0 1, I - -

Sd -

I N-M

Figure 7.12. Hypothetical structure of PMPPlC treated composites at the interfacial area of PALF and LPDE.

[Ref.: J George, SS Bhargawan, S Thomas, Comp Interfaces, 5,3 (1998) 2011

In order to improve the interaction between HDPE matrix and aspen fibres

maleated polypropylene (epolene-18) and gamma methacryloxy propyl

trimethoxy silane (silane A-174) coupling agents were used as

lignocellulose substrate modifiers [89]. The FTlR studies showed variation

of absorption in spectral bands related to water absorption, glycocydic bonds in

the cellulose backbone, hydroxyl and vinyl contents and formation of ether

bonds. The bridge effect brought about by the coupling agent was clear from

the scanning electron micrographs of the composites.

Wood fibre reinforced polypropylene composites of different fibre content

(40,50 and 60%by weight) were prepared and wood fibres (hard and long

fibre) were treated with compatibiliser (MAH-PP) to increase the interfacial

adhesion with the matrix, to improve the dispersion of the particles and to

40 Chapter 1

decrease the water sorption properties of the final composite by Bledzki

and Faruk [go].

Cellulose fibre/thermoplastic composites with ionic interphase were

prepared from modified cellulose fibres and poly(ethylene-co-methacrylic

acid) (PE-co-MA) by Cai et al. [91]. The cellulose fibre was treated by

using coupling agent or sodium hydroxide followed by introduction of ionic

quaternary ammonium groups on the fibre surface, which was then

compounded with the polymer having anionic groups (Figure 1.13). The

effect of the ionic interface on the physical and thermal dynamic properties

of the composite was investigated and an obvious improvement in

mechanical strength of the ionic-interface composites was observed due to

acid-base interactions. The improved adhesion was ascribed to the

interaction between cationic grafted groups at the cellulose fiber surface

and the anionic groups in the PE-co-MA.

The role of persulp hate induced graft copolymerisat ion of matrices of

acrylamide and methyl methacrylate at 5 0 ' ~ in modifying the mechanical

properties of jute fibers was studied by Ghosh et at. [93]. Results obtained

indicated that such a process admits a good scope for modification of

mechanical properties of jute fibre depending on degree of grafting

achieved and compositional variations of (1) final monomer mixture

(2) melts constituent jute itself consequent to selective removal of lignin

and hemicellulose to different extent from the fibre. Low to moderate

removal of cellulose is more effective than a similar degree of removal of

lignin from jute in rendering the fibre more amenable to vinyl grafting using

the mixed monomer system without being adversely affected with respect

to tensile properties.

Introduction 41 - - . , . . - --

Alkali cdlulose

Ilbl

Flgure 7 .7 3. Surface treatment of A PS following the formation of the cattontc quaternary ammonium termtnal group (a) at the ce//ulose fibre surface and, (b) alkali cellulose following the formation of catlonlc quaternary ammonium terminal group at the cellulose fibre surface.

[Ref.: X Cai, B Riedl, A Ait-Kadi, Compo Part A, 34 (2003) 10751

Keener et al. [93] showed that the physical properties of natural

fiberlpolyolefin corn posites can be greatly enhanced by Ma PO coupling

agents. Typical manufacturing processes necessitate that the molecular

weight of a MaPO decrease as the acid number increases. Optimizing .the

balance of these two MaPO properties results in a coupler for natural

42 -- -- Chapter 1

fiberlpotyolefin composites, which can yield 60% increases in flexural and

tensile strengths. Epolene G-3015 was shown to be an optimized coupler.

Newly developed MaPE couplers for the wood-pol yet hylene market were

shown to be superior to other potential polyolefin-coupling agents. Results

indicate that the new polyethylene couplers can double the tensile strength

and triple the impact properties compared to non-coupled blend of wood

and polyethylene. In all cases the optimum loading of the maleated

polyolefin coupling agents was 3% based on the total composite weight.

Li and Matuana [94] investigated the chemical reactions between cellulosic

materials and functionalized polyethylene and maleic anhydride

fictionalized polyethylene (maleated polyethylene) for surface modifications

and FTlR and XPS studies indicated that chemical bonds between the

hydroxyl groups of cellulosic materials and functional groups of coupling

agents occurred through esterification reactions as Figure 1.14.

ibS Figure 1.14. Proposed reaction schemes for the esterificatlon reaction

bet ween cellulosic materials and coupling agents: (a) cellulosic materials treated with maleated PE and (b) cellulosic rnatenal treated with acrylic acid functionalized PE

[Ref.: Q Li, LM Matuana, J Appl Polym Sci, 88 (2003) 2781.

Introduction 43

For improving the adhesion between the hydrophilic flax fibre and

hydrophobic propylene Cantero et al. 1951 treated the fibre with rnaliec

anhydride, polypropylene co-polymer and venyltrimethoxy silane. It was

found that the three treatments reduce the polar component of the surface

energy of the fibre. The treatments increase the contact angle of the fibres

with water therefore decreasing its polarity. MAPP seems to be the

optimum treatment for both flax fibre and pulp because it reaches a polar

component similar to that for PP, which is strongly non polar. The

mechanical properties of the composites were also in accordance with the

surface energy value.

Pedro et al. [96] found that preimpregnation of cellulose fibres in a LDPE

xylene solution and use of a silane coupling agent result in small increment

in mechanical properties of LDPE, reinforced with green cellulosic fiber

composites, which is attributed to an improvement in the interface between

the fibres and the matrix. The fibre treatment also improved the shear

properties of the composite and fiber dispersion in the matrix.

On comparing the mechanical strength values of AN-grafted and MMA

grafted jute polyester amide composites it was observed that AN-grafting

exhibits comparatively better results than MMA grafting. The enhanced

strength of the vinyl grafted fiber composites was attributed to an improved

compatibility between polymer matrix and jute fabrics through vinyl

[polymer] moieties on the fabric surface during the fibre modification

treatment resulting in better compatibility and adhesion between the fabrics

and matrix polymer 1841.

Properties of lignocellulosic fibre can be modified by graft copolymerisation

with vinyl monomers under selective and controlled conditions [97].

Polyacrylonitrile grafting onto chemically modified sisal fibre was done with a

view to improve the surface as well as bulk mechanical properties for its

44 Chapter 1

potential as reinforcing fibre for polymer composites [98]. [ 10i ] =0.008 mot-'L-'

and [cu2'' =0.002 mol-'L-' produced optimum grafting for use of 0.1 g

chemically modified sisal fibre and 1 ml AN at 60%. The best tensile

strength and modulus were obtained for low percentage of grafting. Saha

and Misra I993 changed the reaction variables upon the reaction of jute

fibre with acrylonitrile like concentration of sodium hydroxide, reaction time

and reaction temperature. It was found that fibre swollen with ethylene

diamine prior to cyanoethylation gave a higher yield of cyanoethylated jute.

Within the experimental range of concentration of NaOH (0.5 -lo%), the

extent of cyanoethylation increases with increase in concentration of

NaOH, but the cyanoethylation in the presence of water abruptly reduces

the rate of reaction. Hydroxide groups of both hollow cellulose and lignin

are simultaneously substituted by cyanoethyl group, but the reactivity of

hollow cellulose towards cyanoethylation is higher than lig nin.

Saha et al. [I 001 cyanoethylated jute fibres using acrylonitrile monomer,

which react with hydroxyl group of fibre constituents. The degree of

cyanoethylation to different extents were varied by the reaction time. The

moisture regain, water absorption and thickness swelling of jute fibre

based composites were significantly reduced by cyanoethylation of fibre.

The dimensional stability of composite was also improved by the

cyanoethylation of jute fibre. A significant improvement on the tensile and

the flexural properties of jute PP composite was also observed which is

associated with improved bonding at the interface between jute fibre and

polyester. The improved mechanical properties of cyanoethylated jute

polyester amide composites may be due to bonding of 13 cyanoethyl group

of fibre with the polymer matrix thereby improving the fibre matrix

interaction [83].

Introduction 45

Reddy and Bhaduri modified cellulose fibre by cyanoethylation [ A 011. The

moisture regain, water absorption and thickness swelling of jute fibre based

polyester composites were significantly reduced when cyanoethylated fibre

was used [102].

A study of interface in treated and untreated flax fibreIPP composite

showed that both acetytation and stearic acid treatment improved the

stress transfer efficiency at the fibre matrix interface, where the treatment

time for stearic acid treatment was very low [103]. It was found that

stearation for 90 hours deteriorated the interface because the longer time

of treatment deteriorated the fibre strength and extent of stearic acid

present on the fibre surface may have acted more as a lubricant than a

compatibiliser. It was suggested that the removal of the outer layer from

the surface of the green flax fibres enhanced the interface because the

outer layer may act as a weak boundary layer leading to a premature

debonding. For dew retted flax fibers the stearic acid treatment has

improved the interfacial stress transfer efficiency through inter-

entanglement of the stearic acid chains with the iPP chains.

Rout et al. [I041 chemically treated surface of coir fibres by dewaxing, using an

alkal~ treatment, vinyl grafting, with methyl methacrylate and cyanoethylation.

The morphology studies showed the removal of tyloses from the surface of coir

as a result of alkali treatment, resulting in a rough fibre surface with regularly

spaced parts. At a lower percentage of grafting (PMMA) the surface became

more or less uniform, while the surface of the coir fibres with a higher

percentage of grafting were increasingly covered with grafted material, resulting

in canal like cavities between the overgrowths of grafted material on the unit

cells. Cyanoethylated coir fibre surfaces showed an insufficient deposit of

cyanoethyl groups. The morphological features correlated with the mechanical

properties of modified fibres.

46 Chapter

Sisal fibres were chemically treated with a two-step treatment, first sodium

sulphate aqueous solution and then acetic anhydride (acetic acid mixture) to

promote adhesion to a polyester resin matrix [105]. It was found that the

chemical treatment improved fibre matrix interaction as revealed by brittle

behavior of composites reinforced with treated fibres. Though the treatment

improved the composite behavior in relation to moisture, the water absorption

capacity of the composites was increased by the treatment. This should be due

to the failure to remove all the unreacted hydrophilic species left by treatment or

to the formation of acetyl cellulose micro tubes in the treated fibre.

The effects of various chemical modifications (dewaxing, alkali treatment,

cyano ethylation and vinyl monomer grafting) of jute fabrics as means of

improving its suitability as a reinforcement in biopol based composites

were done by Mohanthy et at. [I061 the fibrelmatrix interaction for

corn pati bilised systems were superior to those for uncompati bilised

system. The alkali treatment and low grafting of acrylonitrile results in

better mechanical properties of composites.

Etherification and esterification reactions were done on steam exploded

flax fibres to decrease the natural hydrophilic character of cellulose,

without the occurrence of any structural changes that may impart the good

mechanical properties of native fibres [I 071. Independent of the su bstituent

nature, chemically modified fibres exhibited a thermal stability comparable to

that of native cellulose. Rozman et al. [I081 employed lignin as a

compatibilizer in the coconut fibre- polypropylene composite. Since lignin

contains both polar hydroxyl groups and non polar hydrocarbon and benzene

rings it can play a role in enhancing the compatibility between both

components. The composite with lignin as compatibilizer processed higher

flexural properties as compared to the control composites. Lignin also

reduced water absorption and thickness swelling of the composites. But

Introduction 47

composites with maleic anhydride modified polypropylene as compatibilizer

displayed greater mechanical properties than those with lignin.

Development, optimisation and characterisation of two treatments; acetylation

and stearation of flax fibre was done by Zafeiropoulos et al. [103]. The two

treatments were applied on two grades of flax fibres (green and dew retted

flax). Three characterization techniques were applied on the treated and

untreated fibres; X-ray diffraction, scanning electron microscopy, and

inverse gas chromatography. It was found that both treatments result in a

removal of non-crystalline constituents of the fibres, and alter the

characteristics of the surface topography. It was also found that both

treatments change the fibre surface free energy, with acetylation

increasing it and stearation decreasing it. The surface free energies of

treated and untreated flax fibres are given in Figure I. 15.

Butyrated kraft lignin was added to an unsaturated thermosetting resin,

consisting of a mixture of acrylated epoxidized soybean oil (AESO) and styrene.

Composites were made by the vacuum assisted transfer molding process with

varying amounts of butyrated kraft lignin dissolved in the unsaturated resin

system. These results suggest that addition of lignin-BA to the AESOlstyrene

resin results in a significant improvement in the natural fibre-AESOlstyrene

interface, by compatibilizing the resin and the natural fibre [I 091.

48 Chapter 7

DR GR ACDR AGGR STOR STGR

Figure 1. 75. Surface free energy (dispersive component) of treated and untreated flax fibres

[Ref.: W Thielemans, RP Wool, Comp Pad A, 35 (2004) 3271

The feasibility of using recycled plastic and wood particles from chromated

copper arsenate (CCA)-treated wood removed from service was

investigated by Kamdem et al. [I 101. The higher modulus of elasticity and

modulus of rupture from CCA-treated material were attributed to the

increased thermal coefficient of the solid deposits rich in copper chromium

and arsenic present in the cell wall of the recycled CCA-treated wood. The

biological durability and the photo-protection properties were improved for

samples containing recycled CCA-treated wood.

Improvement of the interfacial properties of composites consisting of

poly(3-hydroxybutyrate)and flax fibres was provided by addition of

4,4'-thiodiphenol (TDP)at various concentrations up to 1 O%v/v.The additive

TDP is known to form hydrogen bonds with many functional groups. The

use of TDP in PHB-flax fibre composites as a hydrogen-bonding additive

was successful in achieving beneficial properties. Peak shifts in the

hydroxy and carboxyl group regions in FTlR spectroscopy were observed

for the composites with TDP, which indicate that, some degree of

lntrodu ction 49

H-bonding between the carboxyl group of PHB and the OH group of the

fibres. The thermal stability of the fibres and the composites with TDP

showed an improvement with higher degradation temperatures [ I 1 I].

Conventional compatibilization techniques involve wet chemical methods

for the modification of lignocellulosic components and involve mainly the

primary and secondary hydroxyl groups of the polysaccharide chain [I 12-1 141.

Cold plasma chemistry opens up a new way for the surface modification of

materials for composites and other applicat~ons [I 15-1 171. The energies of

neutral and charged plasma species, including electrons, ions of either

polarity, free radicals, excited species, atoms, molecules, and photons are

comparable to bond energies involved in all organic compounds and as a

consequence, both the gas phase and surface molecular fragmentation

and recombination processes can be controlled by selecting the proper

plasma parameters [I 12, I 181.

Cold plasma enhanced functionalisation reactions will only be adequate for

the modification of components for melt processing compounding if the

surface area of the substrate is very large relative to its volume, which is

crucial for the development of interaction between the fibre and matrix

[I 19,1201. Interfacial properties and microfailure degradation mechanisms

of the oxygen plasma treated biodegradable poly(p -dioxanone) (PPDO)

fiber/poly(l -lactide)(PLLA)composites were investigated for the orthopedic

applications as implant materials using micromechanical technique and

nondestructive acoustic emission (AE). PLLA oriented in melt state was

brittle and their mechanical strength was not high, whereas PPDO fibre

appeared high mechanical strength and flexibility [I 211.

1.9. Fracture mechanism of composite failure

The fracture behavior of natural fibre is much more complex than that of

the plan of failure obtained for synthetic fibres such as glass, carbon, and

50 Chapter I

so on. Mukhopadhyay [122] studied the fracture of jute fibres at various

test lengths and various rates of extension. They observed that at shorter

test length [2 cm], the diameter variation along the fibre axis being less, the

stress distribution was better, the probability of developing cracks at any

point was less and hence, the incidence of slippage washpredominant. At a

higher test length (4 cm) and higher rate of extension catastrophic fracture

was observed. At a lower rate of extension (2 mmlmin) slippage type

failures mixed with occasional sharp brakes at weak points were evident.

Mukherjee and Satyanarayana [I231 classified the fracture behavior of

various cellulose fibres into two groups. The fracture behavior of natural

fibres largely affects the fracture behavior of natural fiber-reinforced

composites. Sanadi et al. [I 241 examined the impact-fractured surface of

sun hem plpolyester composites and observed fi brillar pullout with the

plastic deformation of the matrix. Pavithran et al. [I51 examined the impact

fractured surface of sisal-, PALF-, banana- and coir- reinforced polyester

composites. They explained the variation in the impact properties of

various natural fibre composites in terms of the microfibrillar angle of the

fibre. The fracture surface of strawlpolyester composites was studied by

White and Ansell [125]. They observed that tensile failures were relatively

flat and transverse, where as the bend failure surfaces were considerably

greater in area and indicated that extensive shear delamination had

occurred at the fibrelresin interface. Roe and Ansell [I 261 showed that the

tensile fracture of 1 5% jutelpolyester composites were mostly brittle with

very little fibre pullout. They observed that with the increase in jute content,

the pullout lengths were increased and the crack paths were

macroscopically longer and more complex, involving delamination between

the strands of jute fibre.

Fibre plays an important role in the toughening process of composites.

Such large toughness enhancement has come from a number of

Introduction 51

deformation and failure mechanism acting in the notch tip process zone

(type 1 toughening mechanisms) and in the crack wake zone (type 2

toughening mechanisms).

Crack tip zone process include:

Plastic deformation of the thermoptastic matrix; an example is by

the nucleation, growth and coalescence of micro-voids indicated by

'stress whitening';

Delamination cracking at or in front of the crack tip between piles

(layers) of fibre and matrix and at polymer-polymer interfaces.

Crack wake mechanisms include;

Crack bridging by the flax fibre;

Crack bridging by highly ductile m icroscopic-sized ligaments of

polymer;

Fibre slippage, fibre deformation, cracking, splitting and fracture

and fibre pull-out

The deformation and fracture of a natural fibre (flax fibre) /recycled polymer

(HDPE) composite was done by Singletone et al. [I 271. Figure I . l6a. shows a

schematic representation of the mode of failure of a typical short fibre-

reinforced thermoplastic composite. During the fracture process, as crack

propagation takes place, several types of energy-absorbing mechanisms,

which are either matrix related (such as crazing and shear yielding) and/ or

fibre related (such as pullout, debonding, fibre fracture) will contribute to

the toughness of the composites. In addition, the dispersion of the fibres

will allow better interaction between the fibres and the matrix.

Introduction 53

This will obviously increase the efficiency of stress transfer from the matrix

to the fibres. Thus, enhancement in stiffness and strength of the short fibre

composites could also be realized. In the case of composites, the

existence of final bundles in the final molded composite product is believed

to give a significant contribution to the mode of failure of the composites.

The schematic representation shown in Figure 1 . I 6. b illustrates the failure

process proposed based on the qualitative evidences from the SEM studies.

It is quite obvious that with the particle size in the range of 70-500 pm, pullout

is most likely to be the dominating mechanisms operating in the

composites. However, due to the nature of the lignocellulosic filler, which

tends to exist in the form of bundles, the strong inclination for fibre bundle

pullout to take place is inevitable. Thus, the energy absorbing capabilities

are not going to be as efficient as that of individual fibre pull out; and,

consequently the toughness enhancement is limited. In addition, the

presence of fibre bundles will also reduce the efficiency of stress transfer

from matrix to the hollow cellulose fibres. These will obviously lead to poor

stiffness and strength of natural fibre composites [73].

I .lo. Green composites

Significant research efforts are currently being spent in developing a new

class of fully biodegradable green composites by combining natural fibres

with biodegradable resins [I 28-1 331. These corn posites are environment

friendly, fully degradable and sustainable, i.e., they are truly green in every

way. At the end of their tife they can be easily disposed of or corn posted

without harming the environment. Typical life cycle of green composites is

given in Figure 1.17. The green composites may be used effectively in

many applications such as mass produced consumer products with short

life cycles of one or two years (nondurable) or products indented for one

term or short term (few times) use before disposal.

54 - - -. - - -, - Chapter I

A variety of natural and synthetic biodegradable resins are available for

use in green composites. A list of some of the biodegradable resins is

given in Table 1.3. Most of these resins degrade through enzymatic

reactions when exposed to a compost environment. Many will also

degrade in moistlwet outdoor environments through microbiallbacterial

attack. The mechanical properties of poly(3-hydroxy butyrate-co-3-

hydroxyvarelate) (PHBV) composites, reinforced with short abaca fibres

prepared by melt mixing and subsequent injection molding, were

investigated and compared with PHBV composites reinforced with glass

fibre (GF) by Shibata et al. [ I 341. The flexural and tensile properties of

PHBVltreated abaca composite were corn parable to those of PH BVIGF

composite, except for tensile modulus, compared with the same weight

fraction of fibre.

A review of biocomposites highlighting recent studies and developments in

natural fibres, bio-polymers, and various surface modifications of natural fibres

to improve fibrelmatrix adhesion is presented by Mohanthy et al. [135].

Luo and Netravali [I 36.1 371 used biopol" (Poly(hydroxy butyrate-co-

hydroxyvalerate) or PHBV resin from Monsanto company ) with pineapple

and henequen fibres to make green composites. The tensile and flexural

strengths of green composites were reported to be significantly higher in

grain direction varieties, even at a low fibre content of 28 %. There are also

reports on jute fanricl Biopol composites [139-1401. Significant work has

been done on soyprotein polymers, which may be available in the form of

soy flour, soy protein isolate (SPI) or soy protein concentrate (SPC) [141].

Starch and modified starch blends have also been used as resin to form

green composites. Goda et al [I421 used ramie fibres in the form of low

twist yarns and starch based resin to obtain green composites. These

composites also had strengths in the range of 250 MPa and would be

useful for structural applications.

Introduction 55

Mohanty et al. [I431 reported that between non-polar polypropylene matrix

and 30% plasticized polar cellulose acetate plastic (CAP) the latter proved

to be a much better matrix for hemp fiber (polar in nature) and thus

exhibited improved physico-mechanical properties. Fabricated by the same

processing method (extrusion and injection molding) with the same content

of hemp fibre (30 wt%) the CAP-hemp based biocomposite exhibited a

flexural strength of 78.3 MPa and modulus of elasticity of 5.6 GPa as

contrasted to 55.3 MPa and 3.7 GPa for the corresponding PP-hemp

based composite.

9 olymers

Green composrtes n After useful

life 1 Fjgure 7.17. Typical life cycle of green composites

[Ref. : AN Netravali, S Cha bba, Materials Today, April (2003) 221

56 Chapter f -

Table 7.3. A list of some of the biodegradable resins

[Ref.: AN Netravali, S Chabba, Materials Today, April (2003) 221

Natural Polymers '--bnthetic Polymers

1. Polysaccharides

Starch

Cellulose

Chitin

Pullulan

Levan

Konjac

Elsinana

2. Proteins

Collagen/Gelatin

Casein, albumin, fibrogen, silks

Elatins

Protein from grains

3. Polyesters

Polyhydroxyal kanoates

4. Other polymers

Lignin

Lipids

Shellac

Natural rubber

I. Poly(amides) ,

2. Poly(anhydrides)

3. Poly(am ide-enamines)

4. Poly(viny1 alcohol)

5. Poly( ethylene-co-vinyl alcohol)

6. Poly(viny1 acetate)

7. Polyesters

Poly(g1ycolic acid)

Poly(lactic acid)

Pol y(capro1actone)

Poly(ortho esters)

8. Poly(ethylene oxide)

9. Some poly( urethanes)

10. Poly(phosphazines)

1 1. Poly(imino carbamates)

I 2. Some poly(acrylates)

Introduction -- -- .- 57

The incorporation of a high content of natural fibre say up to 50 wt% or

even more in the biocomposite system along with suitable surface

treatments of natural fibers along with the use of coupling agents during

composite fabrications, can generate superior performance biocomposites.

Mwaikambo and Ansell [I441 studied the mechanical properties of hemp

fibre reinforced cashew nut shell liquid composites. They found that

combination of naturally occurring lignin-containing fibres with natural

monomers containing similar phenolic corn pounds provides a corn pati ble

interaction on polymerization and hence improved mechanical properties.

The mechanical properties of poly (8 hydroxy butyrate- co-8 hydroxy

varelate) (PHBV) composites, reinforced with short abaca fibres were

investigated and compared with PHBV composites reinforced with glass

fibre (GF) by Shibata et al. (1451. They found that the flexural and tensile

properties of PHBVItreated abaca composites were corn parable to those of

PHBVIGF composite except for tensile modulus, corn pared with same

weight fraction of fibre.

Oksman et al. [I461 used flax fibres as reinforcement in polymers based on

renewable raw materials. The polymer was polylactic acid (PLA), a

thermoplastic polymer made from lactic acid and has mainly been used for

biodegradable products, such as plastic bags and planting cups and found

that PLA can also be used as a matrix material in composites. The

mechanical properties of PLAIflax composites were compared to PPIflax.

The pure PLA has a tensile strength of 50 MPa and a modulus of 3.4 GPa

compared to 28 MPa and 1.3 GPa of pure PP. The tensile strength and

modulus of the composites are given in Figure 1.18. The addition of flax

fibres will not improve the tensile strength, which is an indication of poor

adhesion between the flax fibres and the matrix. The stress is not

transferred from the matrix to the stronger fibres.

Figure 7.78. Tensile strength and modulus of flax fibre reinforced PP and PLA composites

[Ref.: K Oksman, M Skrifvars, J.F.Selin, Comp Sci Technol, 63

1 .I I. Hybrid composites

Figure 1.19 shows schematically the fields occupied by two families of

materials, plotted on a chart with properties PI and P2 as axes. Within

each field a single member of that family is identified (materials M I and M2).

The figure shows what might be achieved by making a hybrid of the two.

There are four scenarios, each typical of a different class of hybrid. We

consider the case when large values of P I and P2 are desirable, low

Introduction 59

values not. Then, depending on the shapes of the materials and the way

they are combined, we may find any one of the following. "The best of

both" scenario (Point A). The ideal, often, is the creation of a hybrid with

the best properties of both components. There are examples, most

commonly when a bulk property of one material is combined with the

surface properties of another. Zinc coated steel has the strength and

toughness of steel with the corrosion resistance of zinc. Glassed pottery

exploits the formability and low cost of clay with the impermeability and

durability of glass. "The rule of mixturesJ' scenario (Point B). When bulk

properties are combined in a hybrid, as in structural composites, the best that

can be obtained is often the arithmetic average of the properties of the

components, weighted by their volume fractions. Thus unidirectional fibre

composites have an axial modulus (the one parallel to the fibres) that lies

close to the rule of mixtures. "The weaker link dominates" scenario (Point C).

Sometimes we have to live with a lesser compromise, typified by the stiffness

of particulate composites, in which the hybrid properties fall below those of a

rule of mixtures, lying closer to the harmonic than the arithmetic mean of the

properties. Although the gains are less spectacular, they can still be useful.

The "worst of both" scenario (point D) not something we want exhaustive.

Other com binations are possible, some relying on the physics of percolation,

others on atomistic effects. These will emerge below [147].

60 Chapter 1

,- -,-!am ily I I

-I-

Family 2 --,

Figure 7 .7 9 Schematic representation of properties of different hybrid corn binations

[Ref.: MF Ashby, YJM. Brechet, Acta Materialia, 51 (2003) 58011

The development of composite materials based on the reinforcement of

two or more fibres in a single matrix, which leads to the development of

hybrid composites with a great diversity of material properties. Research

revealed that the behavior of hybrid composites appears to be the weighed

sum of the individual components in which there is a more favorable

balance between the advantages and disadvantages inherent in any

composite material [I 28,148-1 501. It is generally accepted that the

properties of hybrid composite are controlled by factors such as nature of

matrix, nature, length and relative composition of reinforcements, fibre

matrix interface and hybrid design. Several advantages of natural fibre

incorporated hybrid composites are reported earlier [I 511. Sisal and glass

fibres are good examples of hybrid composites possessing very good

combined properties. [I 52-1 541. For sisal/glass/LDPE (low density

polyethylene) composites, the effects of fibre orientation, composition and

surface treatment on the mechanical properties have been studied. Due to

superior properties of glass fibres, the mechanical properties of hybrid

composites increase with increase in volume fraction of glass fibres.

Yang et at. [I551 studied the mechanical and interface properties of

sisallglass fibre reinforced poly(viny1 chloride), PVC hybrid composites

before and after immersion in water. It has been found that there exists a

positive hybrid effect for the flexural modulus and unnotched impact

strength. They also suggest that water might have a detrimental effect on

the fibrelmatrix interface leading to reduced properties. Thomas et al. [I 561

have studied properties of sisallsawdustlhybrid fibre composites with

phenol formaldehyde resin as a function of sisal fibre loading. It has been

seen that mechanical properties (tensile and flexural) increase with sisal

fibre content. This is due to the fact that the sisal fibre possesses

moderately higher strength and modulus than saw dust.

Mishra et al. [ I 571 studied the mechanical properties of biofibrelglass

reinforced polyester composites. They found that the addition of a small

amount of glass fibres to the pineapple leaf fibre and sisal fibre reinforced

polyester matrix enhanced the mechanical properties of resulting hybrid

composites [Figure 1.201. Optimum glass fibre loading for PALFIglass

hybrid polyester and sisallglass hybrid polyester are 8.6 and 5.7 weight %

respectively. It was also observed that the water absorption tendency of

composite decreases by process of hybridization and surface treatments of

biofibres. Rozman et al. [158] studied the tensile and flexural properties of

polypropyleneloil palmlglassfi brelhybrid composites and found that

incorporation of both fibres in to PP matrix has resulted in the reduction of

tensile and flexural strength. Both tensile and flexural strength have been

~mproved by the increasing level of overall fibre loading.

62 Chapter

Figure 1.20. Variation of (a) tensile strength and (b) flexural strength by the addition of small amount of glass fibres to the pineapple leaf fibre

[Ref.: S Mishra et al. Comp. Sci Technol, 63 (2003) 13771

Junior et al. [I591 used plain weave hybrid ramie-cotton fabrics as

reinforcement in polyester matrix composites. Ramie fibres have a great

potential as fibre reinforcement in resin matrix composite materials.

Composites with 45% of ramie fibers showed an increase on the tensile

Introduction 63 - -- .. , -- -

strength over the bare polyester resin of up to 338%. Neither the fabric nor

the diameter of the thread used played an important role in the tensile

properties of the composites analyzed. The tensile behavior was

dominated by the volume fraction of the ramie fibres aligned with the test

direction. Cotton had a minor reinforcement effect. This behavior was

attributed to the weak cotton-polyester interface as well as poor cotton

alignment. As expected, when disposed transversally to the test direction,

ramie fibres had a deleterious effect on the tensile behavior of the

composites. This behavior was also attributed to poor fibrelmatrix

interface.

Hybrid composites comprising glass fibre mat (7 wt.%), coir fibre mat

(1 3 wt %) and polyester resin matrix were prepared by Rout et al. [I 601.

Hybrid composites containing surface modified coir fibres showed

significant improvement in flexural strength. Water absorption studies of

coirlpolyester and hybrid composites showed significant reduction in water

absorption due to surface modifications of coir fibres. They predicated that

with coir fibre reinforcement the best properties of glass fibre reinforced

plastics couldn't be achieved, because of a large difference between the

modulus of elasticity of the glass and coir fibres. Nevertheless, surface

modified coir fibres and polyester matrix could be molded into a cost

effective but value added composite materials. Figure 21 (a) shows the

glass bundles with polyester matrix in untreated coir hybrid composites.

This clearly shows the incompatibility between the untreated coir with glass

fibres. Figure 1. 21 (b) and (c) show the impact fractured surfaces of 10%

AN-grafted fibre and bleached (65%) coi r hybrid corn posites. Both the

SEM micrographs show good adhesion of fibres with matrix. Cracks are

developed in the matrix due to stronger interaction of fibre with matrix

materials.

64 Chapter 1

Figure 1.21. SEM of impact fractured surfaces ofr(a)untreated coir hybrid composite;(b)A N-grafted (1 O%)coir hybrid composite; (c) bleached (65 %) coir hybrid composite.

[Ref.: J Rout, M Misra, SS Tripathy, SK Nayak, AK Mohanty, Comp Sci Technol, 61 (2001) 13031

1.12. Cellulose microfibrils reinforced composites

Due to their biological origin, cellulose fibres display a unique structural

hierarchy: they are composed of an assembly of microfibrils, which in turn

consist of a number of cellulose molecules [161-1651. These molecules,

which constitute the basic common element of all celluloses, consist of

long linear chains of poly-~-(l-4)-~-gluco& residues organized in perfect

stereoregular configuration. During biosynthesis, these chains themselves

get packed into slender microfibrils of extreme length, whose diameters

Introduction 65

range from 2 to 20 nm depending on the sample origin [I 66,1671. Within each

microfi bril, the cellulose molecules are organized in a crystalline order, which

results from a regular network of intra-molecular hydrogen bonds.

In native cellulose, one distinguishes two types of crystal structures,

namely cellulose la and If3 where the cellulose chains are nearly packed in

the same way, but in different overall symmetry [168]. Within a given

microfibril, the cellulose molecules are organized in a perfect parallel mode

without any chain folding. Thus, each microfibril can be considered as a

polymer whisker having mechanical properties approaching those of the

theoretical properties of the cellulose crystal.

The use of cellulose microfibrils as a new type of raw material that could

be used in a number of applications ranging from particles for plastic

reinforcement to gel forming and thickening agent has been reported in a

number of papers and patents [I 69-1 871. Methods have been developed

to extract microfibrils not only from wood pulp fibres [169, 1701 but also

from parenchymal cell walls that constitute major leftovers from the food

industry [ I 71,172,1761.

The cellulose microfibrils, which make the cellulose chains, can be

employed in the preparation of nanocomposites, which can be used in

various optical as well as biomedical applications. Depending on their

origin, these elements differ in lateral size, with diameter ranging from 2 to

20nm. These microfibrils on reaction with strong acids break down into short

crystalline rods or cellulose micro crystals [177,178]. Natural fibres, which

are rich in cellulose, can be used as the starting material for the preparation of

cellulose micro fibrils. Cellulose microfibrils can be separated by methods like

cryo-crushing where the frozen pulp is crushed with liquid nitrogen [I 791. In

addition, methods to mechanically homogenize and stabilize food

ingredients could also be adopted for the preparation of cellulose

66 Chapter 1

microfibrils [ I 801. Steam explosion is another excellent process, which can

be, used to defibrillate the fibre bundles [ I 81 -1 831. This process being fast

and well controlled is well adapted for semi-retted fibres. Enzymatic

hydrolysis of cellulose is also accelerated by steam explosion. Grunert and

Winter [I 861 developed cellulose nanocrystals from bacterial cellulose. The

surfaces of the nanocrystals were modified by trimethyl silynation. The

unreacted and the surface trimethyl silylated crystals were exploited as the

particulate phase in nanocomposites with crosslinked polydirnethyl

siloxane as the matrix material. They found that cellulose microfibril could

be successfully incorporated into various polymer matrices for the

preparation of nanocomposites.

A mild silylation protocol was devised to surface silylate dispersed

microfibrils from parenchymal cell cellulose by Gousse et al. [187]. These

silylated rnicrofibrils, which had the same morphological features as those

of the underivatized samples, were dispersible into non-polar solvent to

yield stable suspensions that did not flocculate. By silylation, the

microfibrils have acquired an inherent flexibility, with the result that their

suspensions present the rheological behavior of polymer solutions.

An examination of the morphology of the cellulose microfibrils before and

after silylation is interesting. Typical microfi brils before and after

derivatization are shown in Figure1.22. Figure 1.22 (a) corresponds to the

initial un-reacted sample. It consists of a random dispersion of long and

rather stiff microfibrils that occur either individually or packed into bundles.

Whereas, the isolated rnicrofibrils have diameters of only 2 to 3 nm, the

bundles are substantially wider as they contain variable numbers of

microfibrils, ranging from a few to several tens. Figure 1.22 (b)

corresponds to a reaction where the molar ratio of initial chlorosilane~ to

surface AGU was of 2 and the resulting Ds was of 0.36. This sample,

Introduction 67

which consists also of isolated and bundled microfibrils, has the same

features as the sample shown in Figure 1.22 (a). Figure 1.22 (c) the sample

where the molar ratio of initial chlorosilane to surface AGU was of 4.

Figure 1.22. Morphology of the cellulose microfibrils before and after silylation

[Ref.: Gousse', ii Chanzy, ML Cerrada, E Fleury, Polymer, 45 (2004) 15691

This sample is partially solubilized in THF but the remaining microfibrils are

flocculated in this solvent. These microfibrils shown in Figure I .22(c) are

drastically different from. those in the other two figures. Indeed these

microfibrils are no longer straight, but have become clumped and crumpled P

as if they had recoiled under the action of silylation. In this sample, the

absence of individual microfibril is also significant.

68 -- . - -. Chapter 1

1.13. Scope and objectives of the present Work

In the present work banana fibre has been used as reinforcement in

phenol formaldehyde resin matrix. Banana fibres abundantly available in

the state of Kerala, India are currently being under utilized. These fibres

have high cellulose content and low microfibrillar angle (1 1 O). These fibres,

if put to better use, will contribute for the development of the economy of

the state. For the successful design of a composite material from banana

fibre several parameters such as fibre length, fibre loading, fibre surface

modifications, hybridization with other fibres and fibrelmatrix adhesion

have to be optimized. To our knowledge no serious attempt has been

made so far to make use of banana fibre as reinforcement in PF resin.

Previous reports have shown that the natural fibres are susceptible to

moisture absorption, thermal degradation, and biodegradation [I 88-1 921.

Therefore, strategies need to be worked out to reduce these limitations.

This includes analysis of different processes such as surface modification

of fibres with different chemicals, hybridization with synthetic fibres and

using cellulose microfibril separated from banana fibre as reinforcement in

the resin. The present study focuses on the following aspects.

1.13.1. A comparison of banana fibrelPF and glass fibre1PF corn posites

The properties of banana fibreIPF composites have been compared with

those of glass1PF composites. The effects of fibre length and fibre loading

on the performance of both bananalPF and glass1PF composites have

been analyzed. The interfacial properties and specific properties of both

the composites were also calculated and compared.

1 . I 3.2. Surface modifications

Banana fibre has been given different surface treatments and the effect. of

these treatments on the surface morphology has been analyzed by SEM

Introduction - - . . . -.

69

studies. The effect of fibre surface modifications on the properties of the

composites has been evaluated.

I .13.3. Hybrid composites

Effects of hybridization using glass fibres on the properties of banana fibre

reinforced PF composites have been investigated. Hybrid composites were

prepared by varying the hybrid ratio and hybrid layering patterns.

1 A3.4. Dynamic mechanical properties of the composites

The dynamic mechanical properties of the banana/PF composites, glassIPF

composites bananalglass/hybrid corn posites and chemically treated fibre

reinforced PF composites were analyzed. The effect of frequency and

temperature on storage modulus, loss modulus and damping behavior of

the composites have been evaluated. The glass transition temperature

was also measured from the DMA studies.

I .I 3.5. Thermal degradation behavior of composites

The examination of thermal degradation behavior of treated and untreated

fibres was done by thermogravimetric analysis. The effect of chemical

treatment on the thermal stability of the fibre was determined from TG and

DTG curves. The effect of fibre loading and fibre treatments on the thermal

stability of the composites have also been analyzed.

I .I 3.6. Water absorption

The water absorption studies of banana fibreIPF composites, glass fibreIPF

composites and hybrid fibre reinforced composites were carried out at

different temperatures. The effect of chemical treatments on the water

absorption behavior of the composites was analyzed. The mole percent water

uptake of the composites at different temperatures was calculated. The

diffusion coefficient, sorption coefficient and permeation coefficient of the

composites have been calculated from diffusion experiments.

1 .I 3.7. Electrical properties

The electrical properties of banana fibrelPF composites were studied with

special reference to the effects of fibre loading, fibre treatment and

hybridization using glass fibre. The dielectric constant, volume resistivity

and loss factor of the different composite samples have been evaluated.

I .I 3.8. Aging studies

The effects of chemical treatments and hybridization using glass fibres on

the aging and weathering resistance of the composites have been

evaluated. The percentage weight change and change in tensile properties

before and after aging have been analysed.

1 .I 3.9. M icrofi bril reinforced composites

Cellulose microfibrils were extracted from banana fibre and have been

used as reinforcement in PF resin. The extent of fibrillation has been

evaluated from SEM studies. The effect of fibril loading on the mechanical

and dynamic mechanical properties and thermal stability of PF composites

has been analyzed. The fibrelmatrix interactions and fracture mechanism

in the composites have been investigated by SEM studies

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