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ManuscriFundamentals of Fibres and different Fibre Structures  as reinforcement in compositespt
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ManuscriFundamentals of Fibres and different Fibre Structures as reinforcement in compositespt

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Fibre reinforced composites are gaining importance as engineering materials due to their 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 and reinforcement, but also on structural form of reinforcement and its impact on interface created between reinforcement and matrix. For optimizing the property requirements of composites, therefore, proper thought must also be given to the structural form to be chosen for reinforcement. The textile fibres are used in composites as fibres themselves, or in the form of yarns, straight, twisted, inter-twined, laced, woven, knitted, non-woven or three – dimensionally structured by weaving or knitting techniques.

In this paper latest developments in natural as well as man-made fibres are discussed. Similarly the importance of fibres, yarns and fabric structure is explained to make the tailor-made composite.

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  • 1. Fundamentals of Fibres and different Fibre Structures as reinforcement in composites Dr.A.B.Talele HIMSON ENGINEERING PVT LTD., SURAT Abstract The most common man-made composites can be divided into three main groups, viz, Polymer Matrix Composites, Metal Matrix Composites and Ceramic Matrix Composites. Among these composites polymer matrix composites are the most common type which are also known as FRP – Fibre Reinforced Polymers/Plastics. In this type of composites a polymer based resin is used as matrix and a variety of polymers such as glass, carbon, aramid etc are used as the reinforcement. Fibre reinforced composites are gaining importance as engineering materials due to their 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 and reinforcement, but also on structural form of reinforcement and its impact on interface created between reinforcement and matrix. For optimizing the property requirements of composites, therefore, proper thought must also be given to the structural form to be chosen for reinforcement. The textile fibres are used in composites as fibres themselves, or in the form of yarns, straight, twisted, inter-twined, laced, woven, knitted, non-woven or three – dimensionally structured by weaving or knitting techniques. In this paper latest developments in natural as well as man-made fibres are discussed. Similarly the importance of fibres, yarns and fabric structure is explained to make the tailor-made composite. 1. Introduction In general composites material consists of more than two components were the identity of both components is not lost. The composites differ from blends in as much that the property the property contribution of each component is distinctly felt and most of the times one component over-dominates the properties of other components. Composites today are being used in all walks of life and are equally applied both in daily use applications as well as high tech applications. The basic advantage of 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 namely, flexible and rigid although numerous intermittent stages are also available. From the point of view of applications, this differ in as much that in the first instance the matrix plays a major role in application properties whereas the reinforcement plays secondary role of supportive nature. In case of latter the major contribution towards its property is derived from reinforcement and matrix plays mainly the job of holding reinforcement together. In any composite material the final properties of the composites are dependent on the properties of not only reinforcement and matrix material but also on the interface between strata. While in most composites 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 nature 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 also changing its physical attributes in various ways. 1
  • 2. The reinforcing material, depending up on the textile from given prior to composting, will not only modify the basic properties and their contribution to the final composites but will also very largely 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 the composites is discussed. 2. Type of fibres Every since the dawn of civilization man’s quest for comfortable clothing has been endlessly going on. A natural fibre like cotton, wool and silk have served the mankind for centuries and are in use even today, however, there is a growing need for synthetic substitutes of natural fibres. This is so 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 fibrous polymer sources such as a wood pulp for cellulosic fibres or casein for protein fibres. In later years attempts were made to produce fibrous polymers from basic chemicals and use them for producing 100% synthetic fibres or man-made fibres. Among different man-made fibres polyester and nylon fibres have proved to be the most popular due to their unique characteristics like durability, ease of care and excellent dimensional stability. However today the situation is dramatically changing due to the increasing affluence of the society, which has resulted in greater demand for better quality and convenience in all aspects of life including apparel textiles. Hence various new fibres have been developed. These developments in man-made fibres can be classified in three generations namely; (1) diversification (2) invention and (3) sophistication as shown in Chart-1. Although the first generation fibres,e.g.,polyester,nylons,acrylics,etc. became increasingly popular due to their unique characteristics, like durability, ease of care and excellent dimensional stability, day by day the situation is dramatically changing because of the increasing awareness of the consumer in selecting the fabrics for particular end users. Therefore ,new 2nd generation fibres have been introduced by physical and chemical modification of the normal fibres – such as staple fibres,profiled fibres,crimped fibres,microfine fibres,etc.These various modifications have played a role in making the man made fibres more pleasing to the eye and hand. Also recently, new 3rd generation fibres are being introduced in the market, which are tailor-madefor specialized end users. For the composites of the rigid variety the type of fibres have the main basic criteria of highest tensile strength and modulus.From this point of view the fibres generally used in this kind of composites are Glass, asbestos, Boron, Silicon Carbide (SiC), Carbon Sulphire,etc.All the above fibres indicated are of the inorganic nature.Apart from these,organic polymeric fibres such as aramide,polyfinoyal sulphide,ployphynaylene sulphide,poly benzene salt,polymides,and ladder polymers like polyquinoxoline,etc are also being used in these applications. For most of the flexible and normal application composites, the reinforcement consists of natural fibres like jute, cotton or synthetic fibres like nylon, polyester, Poly -propylene, HDPE, etc. 2
  • 3. 2.1 Physico-chemical properties of fibres Properties of the fibres are dependent on the basic Chemical nature and structure of polymeric molecules of which these fibres consist.In addition to this, the properties also depends upon the nature 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 is naturally dependent and be modified only on to a limited extent. Whereas in case of synthetic polymeric fibres like polyester and polyamides etc.By changing the basic monomers, various polymeric compositions are possible to be arrived at.In addition to this, during the formation of the process of these fibres,fibre structure can be modified according to desired applications by appropriate thermo-mechanical treatment. Depending upon the polymer molecular structure such as its basic rigidity, side groups, isotacticity and presence or absence of the chemical, moieties which may have internal attractions,thermal and physical properties of the polymer are decided.Thus,for example ,for a polymer like polypropylene which has very high flexibility of the basic chain,very very stereo regular arrangement of CH2 groups along the chain length and relatively low attraction between intermolecular chain,the fibre made from such substance has relatively low elastic modulus,high elongation and very low thermal stability(strength loss at elevated temperature).On the other hand,aromatic polyamide,/fibres like Kevlar and Normex,which have higher rigidity benzene groups within this structure and very high attraction powers between molecular chains are twice as strong as conventional Organic polyester and nylon fibres and five times as strong as steel wires and also exhibits high thermal 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 by optimization of molecular weight and arrangement of molecules within the fibre.If the molecules are drawn out to their maximum possible extent, then they crystallize with each other due to their stereo structure help to maximize the contribution of molecular strength to the fibre structure. Molecular orientation and crystallinity is dependent on drawing.When proper drawing temperature and strain rates, the strength and modulus optimizes the process of drawing can be considerably increased. Thus, for example, normal textile nylon and polyester yarns have tenacities in the range of 4 to 5 gm/d. It is possible to suitably control the process to obtain tenacities as high as 8 to 10 gm/d. In addition to drawing, controlled annealing of the fibres or fabrics also is done to optimize on crystallinity and therefore, the thermal stability. Properties of fibres related to inter fibre friction and interface between fibres and matrix can also be modified by physical modification of cross-sectional shapes of fibres during its manufacture. Thus, one can produce circular, triangular, pentagonal and dumbbell shapes. Alternatively one can also make hollow fibres if specific properties of low density is desired. Interfacing properties of fibres can also cane be modified. By subsequent post-treatment to the fibres,yarns or fabrics which would help to increase interface between reinforcement and matrix by chemical or physical modification of the surface of the fibres. While selecting the material for reinforcement, the important criteria are not only breaking strength and modulus but also its proportion to material density and cost effectiveness. Table II gives relative value s for some of the major reinforcing materials.It can be seen from this table that Nylon 6 and glass are more cost effective from the point of view of stress while aramide has slightly higher cost 3
  • 4. but, very high stress per unit density ratio.For air transport applications where every kilo of weight reduced adds to profitability by virtue of additional kilo of cargo carried, high stress to density ratio at 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 filament yarns.These differ not only in their manufacturing processes but also in their properties. Natural fibres like cotton, jute, etc. are having a finite length and to make these into yarn, arrangement of fibres is required to be made coherent by application of twist.This process is itself called spinning and the yarns thus produced is called spun yarns.Spun yarns can also be made from synthetic polymeric materials by first making continuous filaments which are subsequently chopped into fibres and then are spun by methodology similar to those used for natural fibres.In case of filament yarns just taking predetermined number of filaments together makes the yarn.As shown in Fig.1 the physical structure of spun yarn differs vastly from filament yarns.This not only affects the correlation of fibre strength to yarn strength but also the fibre density in the yarn as shown in Table III.These factors of the yarn structure also contribute very realty to the interface between reinforcement and matrix.Because of the close packing in case of multi-filament Yarns the area of the interface available is mostly Restricted to the surface of the yarn itself whereas in Case of spun yarns it includes inter fibre spaces Within the yarn also.In addition to this,the protruding Fibres from the spun yarn surface also infiltrate Matrix and gives additional interfacing area.Thus,when the intrinsic strength of the fibres much higher than the requirement of the composite and where the failure of the composite may be due to poor interfacing,additional interface could be obtained by using spun yarns of the same polymer instead of 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 the yarn 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 point increase 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, twisting, texturising, air texturising, interlacing, etc.Each of these processes create different kinds of geometrical configuration of fibres within yarn structure as shown in Fig.1 out of these processes, doubling and twisting are mainly applicable for spun yarns whereas processes like texturising, air texturising, 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 in either ‘Z’ (clock wise) or ‘S’ (anti clockwise) direction.Every yarns which have been already twisted can be taken together and given subsequent treatment of twisting to create twine or rope structures.Both the direction and amount of twist change not only the tensile properties such as tenacity and elongation but also the surface geometry by aligning individual fibres either along or obliquity to the yarns. In case of multi filament yarns filament bundle is simply an arrangement of parallel filaments with perhaps differential length and differential individual filament characteristics. Because the correlation of the filament strength to the yarn bundle strength depends on non-uniformity, such yarns 4
  • 5. when loaded gives multiple stepwise filament breaks.When such yarns are twisted together, naturally there is certain increment in their strength due to better load sharing. However ,further increase in 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 filaments tend to come very close to each other leaving no space between filaments.This kind of structure creates very low interface in the composite structure due to inability of matrix to penetrate with in the yarn, especially when the yarns are twisted.It is possible to avoid this by contouring the filaments so that the possibility of the filaments so that the possibility of their coming near to each other is reduced.This process is called texturising of yarn.Broadly texturising can be divided into two categories namely where the filaments are individually grouped but remains separate from each other and another in which filaments are entangled in such a manner that their contour geometry is permanent in nature.Air texturising and interlacing achieve the latter objective while false twist texturised yarn has a very high stretchability in it. Structure of the air texturised yarn is similar to spun yarn by virtue of the loops protruding out of the yarn structure. 3.1.3 Combinations While generally in composite structures today only twisted yarns are being used,it is felt that substantial modification of the linear construction carried out by texturising and intermingling techniques could be gainfully employed for creating better interface due to hollow geometry obtained by these techniques,especially if the yarns textured by these are superimposed by twist so as to make the geometry more stable.Such constructions are readily being employed in fabric manufacturing for developing various kinds of fabric textures and feels. 4. Fabrics in composites As can be envisaged, filaments,fibres and yarns are linear structures and therefore,composite forms wound tend to have the structural bias for their properties unless the yarns or fibres are laminated in multi-directional formats while fabricating the composites could be classified into four categories namely,discrete ,linear ,laminar and integrated .For the first type the fibres in a chopped form are dispersed randomly or in pre organized manner within the composites whereas in case of linear reinforcement the yarns are laid unidirectional. When one desires more concrete multi-axial stress bearing capacities within the composites one has to modify yarns by various interlacing technique such as plan weaving, trial-axial weaving, braiding, knitting and non-wovens.Essentially all the above techniques tend to make arrangement of reinforcement within the composites so as to give bias in more than one direction. Fig.2. Illustrates various kinds of textile structures. Generally, yarns are fabricated into various textile structures with two distinct objectives viz.Ease of handling during composite manufacturer or to add specific properties to the composites.When the objective is only to obtain ease of handling during composite manufacture, the fabric structures used are such that predominantly stress bearing elements are kept straight and linear. Thus, their contribution to the stress bearing capacity of the composite is unaffected By the structure of the fabric.When the Objective is to give multi-axial stress bearing Capacity to the composite, the fabrics may have Specific construction details, which add not only multi-directionality to the stress 5
  • 6. bearing capacity but also add additional capacity to withstand repeated loading With fully recoverable strength.The extent of this Elastic property is decided by the fabric geometry Thus ,for example ,knitted structure will give very high deferability whereas woven structures will give limited reformability.Solid woven Multi-layer structures not only give elasticity to the structure but also certain amount of compressibility In The direction perpendicular to the fabric axis. Three Dimensional and multi-dimensional fabrics are Specifically used for making formed 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 of interface. When the construction for the fabric is extremely dense,penetration of a matrix within the fabric structure is difficult and hence,peeling probabilities exist.This has to be taken care of while selecting the fabric geometry for specific end users. REFERENCES 1. Wake W C and Wooton D B ,”Textile Reinforcement of Elastomers”,Applied Science Publishers,London ,1982 2. Chou Tsu Wei and Ko Frank K,Composite Material Series,Vol 3,” Textile Structural Composites”,Elsevier Publishers,1989. 3. Moder E and Freitag K H,Composites,21(5),1990,p397 4. Manocha L M,Bahl O P and Singh Y K Tanso,140,1989,p255 5. Muller H,Chemifestern /Textilindustrie,40/92,12,1990,T-175 6. Parrinello L M,Tappi Journal,74(1),1991,p85 7. Tech Textile Ltd,Advanced Composite Bulletine,January ,1991,p5 8. Yoon Ho Takahashi K,Kojima K,and Kon Y,Sen-I ,Gokkaishi,47(2),1991,p76 9. Planck H, Chemifestern /Textilindustrie,Industrial Textiles,41/93/5,1991,t141 10. Lambillotte B D,J of Coated Fabrics,18,1,1989,p162 11. Institute Fur Textile & Verfahrennstechnik Denkendrof & Plank H,Textile Prexis,International ,44,7,1988,p86 12 Kalantar J and Drzal L T,J of Mat Sci,25,(10) ,1990.pp4186,p86 13 Norgolish J M,Plast Ind News (Jap),34,no 6,1988,p86 14 Lenug C K Y and Li V C ,J of Mat Sci,Letters,9,(10),1990,p1140 6
  • 7. bearing capacity but also add additional capacity to withstand repeated loading With fully recoverable strength.The extent of this Elastic property is decided by the fabric geometry Thus ,for example ,knitted structure will give very high deferability whereas woven structures will give limited reformability.Solid woven Multi-layer structures not only give elasticity to the structure but also certain amount of compressibility In The direction perpendicular to the fabric axis. Three Dimensional and multi-dimensional fabrics are Specifically used for making formed 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 of interface. When the construction for the fabric is extremely dense,penetration of a matrix within the fabric structure is difficult and hence,peeling probabilities exist.This has to be taken care of while selecting the fabric geometry for specific end users. REFERENCES 1. Wake W C and Wooton D B ,”Textile Reinforcement of Elastomers”,Applied Science Publishers,London ,1982 2. Chou Tsu Wei and Ko Frank K,Composite Material Series,Vol 3,” Textile Structural Composites”,Elsevier Publishers,1989. 3. Moder E and Freitag K H,Composites,21(5),1990,p397 4. Manocha L M,Bahl O P and Singh Y K Tanso,140,1989,p255 5. Muller H,Chemifestern /Textilindustrie,40/92,12,1990,T-175 6. Parrinello L M,Tappi Journal,74(1),1991,p85 7. Tech Textile Ltd,Advanced Composite Bulletine,January ,1991,p5 8. Yoon Ho Takahashi K,Kojima K,and Kon Y,Sen-I ,Gokkaishi,47(2),1991,p76 9. Planck H, Chemifestern /Textilindustrie,Industrial Textiles,41/93/5,1991,t141 10. Lambillotte B D,J of Coated Fabrics,18,1,1989,p162 11. Institute Fur Textile & Verfahrennstechnik Denkendrof & Plank H,Textile Prexis,International ,44,7,1988,p86 12 Kalantar J and Drzal L T,J of Mat Sci,25,(10) ,1990.pp4186,p86 13 Norgolish J M,Plast Ind News (Jap),34,no 6,1988,p86 14 Lenug C K Y and Li V C ,J of Mat Sci,Letters,9,(10),1990,p1140 6