Fiber composites

7/29/2019 Fiber composites

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Role of Fibres, Yarns and Fabric Structures

On Properties of composites

Dr.A.B.Talele

HIMSON TEXTILE ENGG.PVT.LTD.SURAT

Abstract

Fibre reinforced composites are gaining importance as engineering materials due to the

excellent ability to be tailor made to suit specific end use application.

Properties of composites are not only decided by the properties of materials used for matrix an

reinforcement, but also on structural form of reinforcement and its impact on interface created betwe

reinforcement and matrix.For optimizing the property requirements of composites, therefore, prop

thought must also be given to the structural form to be chosen for reinforcement.The textile fibres aused in composites as fibres themselves, or in the form of yarns, straight, twisted, inte

twined,laced,woven,knitted,non-woven,or three dimensionally structured by weaving or knittitechniques.

In this paper the importance of fibres,yarns and fabric structure is explained to make the tailo

made composite.

1. Introduction

In general composites material consists of more than two components were the identity of bo

components is not lost.The composites differ from blends in as much that the property the proper

contribution of each component is distinctly felt and most of the times one component over-dominat

the properties of other components.Composites today are being used in all walks of life and are equalapplied both in daily use applications as well as high tech applications.The basic advantage

composites is its ability to be tailor made for specific applications.

From the point of view of properties composites could be classified into two categories namel

flexible and rigidAlthough numerous intermittent stages are also available.From the point of view

applications, this differ in as much that in the first instance the matrix plays a major role in applicatioproperties whereas the reinforcement plays secondary role of supportive nature.In case of latter t

major contribution towards its property is derived from reinforcement and matrix plays mainly the jo

of holding reinforcement together.

In any composite material the final properties of the composites are dependent on the propertiof not only reinforcement and matrix material but also on the interface between strata.While in mostcomposites reinforcement consists of textile materials such as yarn, fibre and fabrics, matrix is

invariably a pure polymeric.Thus, while the characteristics of matrix are decided by its chemical natur

and the modification done to it during the fabrication process,the reinforcement material which is pre-

formed has got wide scope of not only changing the polymeric structure of the base material,but alsochanging its physical attributes in various ways.


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The reinforcing material, depending up on the textile from given prior to composting, will nonly modify the basic properties and their contribution to the final composites but will also very large

influence the interface between the reinforcement and the matrix.

In the present paper the role of fibres, yarns and fabric structures , on the properties of t

composites is discussed .

2. Type of fibres

Every since the dawn of civilization mans quest for comfortable clothing has been endless

going on. A natural fibre like cotton, wool and silk have served the mankind for centuries and are use even today, however, there is a growing need for synthetic substitutes of natural fibres. This is

because of pressing demands on scare agricultural land to feed the rising population.

Earlier attempts at substituting natural fibres resorted to use of naturally available fibropolymer sources such as a wood pulp for cellulosic fibres or casein for protein fibres. In later yea

attempts were made to produce fibrous polymers from basic chemicals and use them for producin

100% synthetic fibres or man-made fibres. Among different man-made fibres polyester and nylofibres have proved to be the most popular due to their unique characteristics like durability, ease of ca

and excellent dimensional stability. However today the situation is dramatically changing due to th

increasing affluence of the society, which has resulted in greater demand for better quality aconvenience in all aspects of life including apparel textiles. Hence various new fibres have bee

developed. These developments in man-made fibres can be classified in three generations namely; (

diversification (2) invention and (3) sophistication as shown in Chart-1.

Although the first generation fibres,e.g.,polyester,nylons,acrylics,etc. became increasing

popular due to their unique characteristics,like durability,,ease of care and excellent dimension

stability,day by day the situation is dramatically changing because of the increasing awareness of t

consumer in selecting the fabrics for particular end users.Therefore ,new 2 nd generation fibres habeen introduced by physical and chemical modification of the normal fibres such as stap

fibres,profiled fibres,crimped fibres,microfine fibres,etc.These various modifications have played a roin making the man made fibres more pleasing to the eye and hand.Also recently ,new 3 rd generati

fibres are being introduced in the market,which are tailor-madefor specialised end users.

For the composites of the rigid variety the type of fibres have the main basic criteria of highe

tensile strength and modulus.From this point of view the fibres generally used in this kind

composites are Glass, asbestos, Boron, Silicon Carbide (SiC), Carbon Sulphire,etc.All the above fibr

indicated are of the inorganic nature.Apart from these,organic polymeric fibres such aramide,polyfinoyal sulphide,ployphynaylene sulphide,poly benzene salt,polymides,and ladd

polymers like polyquinoxoline,etc are also being used in these applications.

For most of the flexible and normal application composites, the reinforcement consists

natural fibres like jute, cotton or synthetic fibres like nylon, polyester, Poly -propylene, HDPE, etc.


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2.1 Physico-chemical properties of fibres

Properties of the fibres are dependent on the basic Chemical nature and structure of polymer

molecules of which these fibres consist.In addition to this, the properties also depends upon the natu

of arrangement of these molecules within the fibre structure.

In case of natural fibres like cotton and jute, basic polymer is cellulose and its arrangement

naturally dependent and be modified only on to a limited extent.Whereas in case of synthetic polymerfibres like polyester and polyamides etc.By changing the basic monomers, various polymercompositions are possible to be arrived at.In addition to this, during the formation of the process

these fibres,fibre structure can be modified according to desired applications by appropriate therm

mechanical treatment.

Depending upon the polymer molecular structure such as its basic rigidity, side group

isotacticity and presence or absence of the chemical, moieties which may have internattractions,thermal and physical properties of the polymer are decided.Thus,for example ,for a polym

like polypropylene which has very high flexibility of the basic chain,very very stereo regul

arrangement of CH2 groups along the chain length and relatively low attraction between intermolecul

chain,the fibre made from such substance has relatively low elastic modulus,high elongation and velow thermal stability(strength loss at elevated temperature).On the other hand,aromat

polyamide,/fibres like Kevlar and Normex,which have higher rigidity benzene groups within th

structure and very high attraction powers between molecular chains are twice as strong as conventionOrganic polyester and nylon fibres and five times as strong as steel wires and also exhibits high therm

stability.Physical properties of some of the industrial fibres are listed in table 1

The basic high strength and high modulus in polymeric materials can also be obtained b

optimization of molecular weight and arrangement of molecules within the fibre.If the molecules a

drawn out to their maximum possible extent, then they crystallize with each other due to their sterestructure help to maximize the contribution of molecular strength to the fibre structure.

Molecular orientation and crystallinity is dependent on drawing.When proper drawi

temperature and strain rates, the strength and modulus optimizes the process of drawing can bconsiderably increased. Thus, for example, normal textile nylon and polyester yarns have tenacities

the range of 4 to 5 gm/d. It is possible to suitably control the process to obtain tenacities as high as 8 t

10 gm/d. In addition to drawing, controlled annealing of the fibres or fabrics also is done to optimion crystallinity and therefore, the thermal stability.

Properties of fibres related to inter fibre friction and interface between fibres and matrix calso be modified by physical modification of cross-sectional shapes of fibres during its manufactur

Thus, one can produce circular, triangular, pentagonal and dumbbell shapes. Alternatively one can al

make hollow fibres if specific properties of low density is desired. Interfacing properties of fibres caalso cane be modified.

By subsequent post-treatment to the fibres,yarns or fabrics which would help to increa

interface between reinforcement and matrix by chemical or physical modification of the surface of tfibres.

While selecting the material for reinforcement, the important criteria are not only breakinstrength and modulus but also its proportion to material density and cost effectiveness. Table II giv

relative value s for some of the major reinforcing materials.It can be seen from this table that Nylon

and glass are more cost effective from the point of view of stress while aramide has slightly higher co


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but, very high stress per unit density ratio.For air transport applications where every kilo of weigreduced adds to profitability by virtue of additional kilo of cargo carried, high stress to density ratio

even relatively higher cost becomes an attractive proposition from the point of overall profitability.

3. Yarn structure and its Effect on Properties

Textile yarns are broadly classified into two classes namely, spun yarns and filameyarns.These differ not only in their manufacturing processes but also in their properties. Natural fibr

like cotton, jute, etc. are having a finite length and to make these into yarn, arrangement of fibres

required to be made coherent by application of twist.This process is itself called spinning and the yarthus produced is called spun yarns.Spun yarns can also be made from synthetic polymeric materials bfirst making continuous filaments which are subsequently chopped into fibres and then are spun b

methodology similar to those used for natural fibres.In case of filament yarns just taking predetermin

number of filaments together makes the yarn.As shown in Fig.1 the physical structure of spun yadiffers vastly from filament yarns.This not only affects the correlation of fibre strength to yarn streng

but also the fibre density in the yarn as shown in Table III.These factors of the yarn structure al

contribute very realty to the interface between reinforcement and matrix.Because of the close packinin case of multi-filament Yarns the area of the interface available is mostly Restricted to the surface

the yarn itself whereas in Case of spun yarns it includes inter fibre spaces Within the yarn also.

addition to this,the protruding Fibres from the spun yarn surface also infiltrate Matrix and giv

additional interfacing area.Thus,when the intrinsic strength of the fibres much higher than trequirement of the composite and where the failure of the composite may be due to po

interfacing,additional interface could be obtained by using spun yarns of the same polymer instead

continuous filament yarns.

The Basic fibre properties are further modified when the Fibres are spun into yarns.This is

because of the fact that the correlation of the fibre properties to the Yarn properties is dependent on thyarn structure.In case of spun yarns where the twist is an Integral part of the yarn manufacturing

process itself, the correlation of the fibre strength to the yarn strength increases with the increasing

twist initially upto a point due to increased frictional cohesion between the fibres but beyond a pointincrease in the cohesive forces is reduced and simultaneously obliquity of the fibre to the yarn axis

increases reducing the correlation of the fibre strength to the yarn strength.

3.1 Physical modification of the yarns

Yarns as they are spun are further modifiable by various processes like doubling, twistin

texturising, air texturising, interlacing, etc.Each of these processes create different kinds of geometricconfiguration of fibres within yarn structure as shown in Fig.1 out of these processes, doubling an

twisting are mainly applicable for spun yarns whereas processes like texturising, air texturisin

interlacing, etc are used for continuous filament yarns.

3.1.1 Twisting

Yarns both spun as well as filament could be twisted in various ways.They can be twisted either Z (clock wise) or S (anti clockwise) direction.Every yarns which have been already twist

can be taken together and given subsequent treatment of twisting to create twine or ro

structures.Both the direction and amount of twist change not only the tensile properties such as tenaciand elongation but also the surface geometry by aligning individual fibres either along or obliquity

the yarns.

In case of multi filament yarns filament bundle is simply an arrangement of parallel filamen

with perhaps differential length and differential individual filament characteristics. Because t

correlation of the filament strength to the yarn bundle strength depends on non-uniformity, such yar


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when loaded gives multiple stepwise filament breaks.When such yarns are twisted together, naturalthere is certain increment in their strength due to better load sharing. However ,further increase

twisted only tends to reduce tenacity and increase elongation in the yarn.

3.1.2 Texturising

Essentially as spun filament yarns are straight rod like and therefore, in a bundle the filamen

tend to come very close to each other leaving no space between filaments.This kind of structure creat

very low interface in the composite structure due to inability of matrix to penetrate with in the yarespecially when the yarns are twisted.It is possible to avoid this by contouring the filaments so that thpossibility of the filaments so that the possibility of their coming near to each other is reduced.Th

process is called texturising of yarn.Broadly texturising can be divided into two categories name

where the filaments are individually grouped but remains separate from each other and another which filaments are entangled in such a manner that their contour geometry is permanent in nature.A

texturising and interlacing achieve the latter objective while false twist texturised yarn has a very hig

stretchability in it.

Structure of the air texturised yarn is similar to spun yarn by virtue of the loops protruding o

of the yarn structure.

3.1.3 Combinations

While generally in composite structures today only twisted yarns are being used,it is felt thsubstantial modification of the linear construction carried out by texturising and intermingli

techniques could be gainfully employed for creating better interface due to hollow geometry obtaine

by these techniques,especially if the yarns textured by these are superimposed by twist so as to ma

the geometry more stable.Such constructions are readily being employed in fabric manufacturing fdeveloping various kinds of fabric textures and feels.

4. Fabrics in composites

As can be envisaged, filaments,fibres and yarns are linear structures and therefore,composi

forms wound tend to have the structural bias for their properties unless the yarns or fibres are laminatin multi-directional formats while fabricating the composites could be classified into four categori

namely,discrete ,linear ,laminar and integrated .For the first type the fibres in a chopped form a

dispersed randomly or in pre organized manner within the composites whereas in case of linereinforcement the yarns are laid unidirectional.

When one desires more concrete multi-axial stress bearing capacities within the composites ohas to modify yarns by various interlacing technique such as plan weaving, trial-axial weavin

braiding, knitting and non-wovens.Essentially all the above techniques tend to make arrangement

reinforcement within the composites so as to give bias in more than one direction. Fig.2. Illustratvarious kinds of textile structures.

Generally, yarns are fabricated into various textile structures with two distinct objectiv

viz.Ease of handling during composite manufacturer or to add specific properties to tcomposites.When the objective is only to obtain ease of handling during composite manufacture, t

fabric structures used are such that predominantly stress bearing elements are kept straight and linea

Thus, their contribution to the stress bearing capacity of the composite is unaffected By the structure the fabric.When the Objective is to give multi-axial stress bearing Capacity to the composite, th

fabrics may have Specific construction details, which add not only multi-directionality to the stre


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bearing capacity but also add additional capacity to withstand repeated loading With fully recoverabstrength.The extent of this Elastic property is decided by the fabric geometry Thus ,for examp

,knitted structure will give very high deferability whereas woven structures will give limit

reformability.Solid woven Multi-layer structures not only give elasticity to the structure but also certa

amount of compressibility In The direction perpendicular to the fabric axis.

Three Dimensional and multi-dimensional fabrics are Specifically used for making forme

composites, which are expected to have stresses imposed on it in various Directions.

While designing fabrics for composite, fabric structure is important from the point of view interface. When the construction for the fabric is extremely dense,penetration of a matrix within thfabric structure is difficult and hence,peeling probabilities exist.This has to be taken care of whi

selecting the fabric geometry for specific end users.

Acknowledgement The authour is thankfull to Shri.Devendrabhai Bachkaniwala,Director,BorsaraMachines for giving co-operation and guidance in presentation of this paper.

REFERENCES

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7. Tech Textile Ltd,Advanced Composite Bulletine,January ,1991,p58. Yoon Ho Takahashi K,Kojima K,and Kon Y,Sen-I ,Gokkaishi,47(2),1991,p76

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Fiber composites