Textile Fibers: A Comparative Overview Fibers from natural sources, twisted by hand into yarns, and then woven into textile fabrics, constitute a materials technology which dates back over 10 000 years. Apart from hand-tools, the technology changed little until the industrial revolution, with the invention of power machines concentrated in 1775–1825. The available fibers, namely cotton, some fibers extracted from the stems or leaves of plants, wool, other hairs, and silk, remained unchanged for another 100 years. All except silk were short fibers, with staple lengths of about 1–10 cm. These fibers had to be twisted into yarns. Even silk filaments were of finite length and had to be ‘‘thrown’’ together into longer yarns, which were smooth and lustrous in contrast to the hairy staple-fiber yarns. Advances in chemistry led to solutions of cellulose derivatives, which could be extruded through multiple holes, coagulated, and regenerated as continuous filament yarns of effectively infinite length. For a time, these were known as artificial silk. The most successful, iscose rayon, was first commercially produced in 1905. The recognition of the idea of macromolecules in the 1920s led to manufactured, synthetic yarns of several vinyl polymers, but the major invention was nylon, which became commercial in 1938. Polyester followed 10 years later. The two other major synthetic fibers of this first generation were acrylics and polypropylene. A second generation of high-performance fibers started with the aramid fiber, Kelar, followed by high-modulus polyethylene. Elastomeric fibers, such as Lycra, were another development. The uses of textile fibers fall into three categories. Both clothing and furnishing fabrics are somewhat unusual in materials technology, since color and the other esthetic features of pattern and feel, which are determined by the fiber and textile structures, are as important as the functional requirements for cover, protection, warmth, and durability. The third category of technical textiles includes some old and simple uses, ranging from ropes to wiping cloths, but is becoming of increasing importance in demanding engineering and medical applications. All the fibers mentioned above are polymeric materials, and they, with some other polymer fibers of more limited or specialist interest, comprise the main scope of this article. Inorganic fibers, namely glass, ceramic, carbon, and metallic, although they can be formed into textile structures, will only be mentioned briefly. [The sizes of fibers are most conveniently expressed in terms of linear density, namely mass per unit length, rather than area of cross-section or diameter. The SI unit is the tex, which is grams per kilometer (g kmV"). The submultiple decitex (dtex) is widely used; an older unit, denier, is grams per 9000 m. This leads to the mechanical quantities specific stress, modulus, and
strength (tenacity) being expressed as (force\linear density). The preferred unit is newtons per tex (N texV"), which is dimensionally equivalent and equal to gigapascals per gram per cubic centimeter (GPa gV" cm$), kilojoules per gram (kJ gV"), or, in a link to wave propagation velocity, the square of kilometers per second (km sV")#.]
1. Fibers and Fiber Types 1.1 Fibers as a Material Form Fibers, which are materials in a one-dimensional form characterized by flexibility, fineness, and high ratio of length to thickness (McIntyre and Daniels 1995), perform many functions in living organisms and are used in a wide variety of manufactured structures. A list of values of the fiber form, some of which are more relevant to other fiber uses than to textiles, includes: (i) Combination of flexibility with strength, in contrast to the usual association of stiffness and strength. (ii) Crack-stopping at the discontinuities, which gives strength to composites. (iii) High surface area, which is important in absorption. (iv) Large included volume between fibers in textile structures. (v) Continuity over long lengths, which is vital in optical fibers. (vi) Ability to form networks. (vii) Structural control: in natural fibers, by genetics; in manufactured fibers, due to the rapid heating, cooling, evaporation, and stress changes. (viii) Small defect size (less than fiber diameters), typified by the high strength of glass fibers. (ix) Ability to modify chemistry or introduce additives to give specific properties. (x) Control of end-use performance at several levels: chemical constitution, fiber fine structure, macroscopic fiber form, yarn structure, fabric structure, etc.
1.2 Characteristic Features of Textile Fibers For clothing and household textiles, the common fiber requirements are: Appropriate dimensions: commonly, textile fibers are in the range 1–20 dtex, which gives diameters of 5–50 µm. Coarser forms, typically 0n1–1 mm, are known as monofilaments. Microfibers, down to 0n1 dtex ("3 µm diameter) or less, have been introduced in recent years. Nanofibers, with linear densities from 10V$–10V( dtex (ca. 3–300 nm), have been produced by electro-spinning, which involves the extrusion of polymer solutions or melts in a high electric field (Reneker and Chun 1996). 1
Textile Fibers: A Comparative Overview Table 1 Textile fiber statistics.
Fiber type Natural Cotton Other plant fibers Flax Sunn hemp True hemp Jute and jute-like Sisal and henequen Others Wool Silk Manufactured Regenerated cellulosic Viscose Industrial yarn Regular yarn Staple Acetate Yarn Cigarette tow Synthetic polymer World capacity PET Yarn Staple Nylon Yarn Staple Acrylic Yarn Staple Others (except olefin) Yarn Staple World production Olefin Yarn Staple Slit film US production Polyester Industrial yarn ‘‘Textile’’ yarn Staple Nylon Industrial yarn Carpet yarn ‘‘Textile’’ yarn Staple Glass
Annual world demand or production 1997 (i1000 t) 19 453a 4648b 633 81 27 3266 342 299 1429a (scoured 840) 86a 2309a (capacity 3172) 80 318 1220 107 584 21 683a 10 810 9383 4843 706 6 3286 201 190 1696 1012 1891 1844 202 546 1097 1288 130 609 188 362 2570
a Demand from Fiber Organon June 1997. b Production from FAO, FAO Yearbook, Production, 19??, Food and Agriculture Organization of the United Nations, Rome, Vol. 51. Other figures estimated from other sources.
Mechanical, thermal, and chemical stability in everyday use. 2
Accessibility to dyestuffs for coloration. An intermediate range of elastic extensibility; break extensions from 5 to 50%. The only materials that have been found to satisfy these requirements for natural or manufactured fibers are partially oriented, partially crystalline, linear polymers. For special purposes, fibers outside this range of requirements may be polymeric or inorganic. Highstretch, elastomeric fibers have break extensions up to 500%, and high-modulus, high-tenacity (HM-HT) fibers down to 2%. Coloration is not needed for engineering fibers. Higher temperature or chemical resistance or other special properties are needed for some applications. 1.3 Chemical and Manufacturing Classification and Importance The general textile fiber market is satisfied by six chemical types: (i) cellulose, in cotton, other plant fibers, and rayon and its derivative cellulose acetate; (ii) proteins in wool, hairs, and silk; (iii) polyamides, mainly nylon 6 and 66; (iv) polyesters, mainly polyethylene terephthalate (PET or 2GT); (v) polyacrylonitrile (PAN) acrylic fibers, and other vinyl polymers; (vi) polyolefins, mainly polypropylene. In addition, the elastomeric fibers are polyurethanes. Aramids (aromatic polyamides) and other polymers are used as HM-HT fibers, for high-temperature resistance, and for other special purposes. Table 1 gives statistics, which show the relative importance of the different materials, with some indication of the division between different forms and applications of the same material. Cotton, other natural cellulose fibers, wool, and hairs are laid down over weeks in living cells. Silk is extruded as an aqueous solution stream, which dries into the filaments. In spider silk, the rate of extrusion may be as high as 10 cm minV". Manufactured polymer fibers are produced at rates of 100– 10 000 m minV". There are several methods of production. From polymer solutions, either wet-spinning into a coagulating bath, sometimes involving a chemical reaction, or dry-spinning with evaporation may be used. Thermoplastic polymers enable the faster melt-spinning process to be used. High-modulus polyethylene (HMPE) fibers are made by gelspinning. Inorganic fibers are dealt with in detail in other articles, particularly in relation to composites. Glass fibers have been known since ancient times, and became of major importance, manufactured in various ways, in the twentieth century. They are occasionally used in clothing or furnishing fabrics, but their major uses are in technical textiles and composite preforms (see Glass Fibers). Optical fibers may be of glass or
Textile Fibers: A Comparative Overview Table 2 Typical properties of textile fibers.
Density (g cmV$)
Melting point (mC )
Moisture at 65% relative humidity, 20 mC (%)
" 200 (chars)
" 200 (chars)
Acetate (secondary) Wool Silk Nylon 6\66
1n30 1n34 1n14
130 (decomposes) 175 (chars) 215\260
14–18 10 4
Acrylic Polypropylene Para-aramid HMPE
1n19 0n91 1n44 0n97
200 (sticks) 165 500 (decomposes) 147
0n4 1–2 0 7–1n2 0
Tenacity (N texV")
Break extension (%)
Work of rupture (J gV")
0n19 0n45 0n18 0n41 0n13
5n6 6n8 27 12 24
5n1 15 31 28 22
0n14 0n38 0n29 0n84 0n47 0n82 0n27 0n65 2n1–1n6 2n6–3n5
43 23 46 20 37 13 25 17 4n4–2n5 3n8–2n7
38 60 77 100 119 60 47 71 45–30
Initial modulus (N texV") 3n9 7n3 4n8 8n8 3n6 2n1 7n3 0n6 6–9 8n8 10–17 6n2 7n1 51–98 90–170
From Morton and Hearle (1993), and manufacturers’ data.
organic polymers (see Glass Optical Fibers). Lowgrade ceramic fibers, used for insulation, are made by melt processes. High-performance ceramic fibers, such as those based on silicon carbide or alumina, may be formed as polymer fibers, with the inorganic elements either incorporated in the polymer or mixed as compounds in the polymer solution, and then heat treated for conversion to ceramic; their main use is in composites (see Ceramic Fibers from Polymer Precursors; Spun (Slurry and Sol–Gel) Ceramic Fibers). Other ceramic fibers, such as boron or silicon carbide, may be made by chemical vapor deposition (see CVD Monofilaments). The highest strengths are achieved with single-crystal whiskers (see Whiskers). Most carbon fibers (see Carbon Fibers) are made by heat treatments of acrylic fibers, but rayon can also be used, and pitch-based fibers have somewhat different structure and properties. Moderate heat treatment gives thermally resistant fibers used in protective clothing, and higher temperature treatments give high-strength, high-modulus fibers, which are mainly used in composites (see Fibrous Reinforcements for Composites : Oeriew) but can also be used, for example, in medical textiles. Fine metal wires can be regarded as fibers, and may be used for decorative or physical reasons (see Metallic Filaments).
1.4 Comparatie Properties Table 2 lists typical properties of the more important textile fibers. Where two sets of mechanical properties are given, these are typical of different types, but the available range may be greater. Figure 1 shows stress–strain curves. These values are for ‘‘dry’’ fibers,
namely fibers in equilibrium at a standard atmosphere of 65% relative humidity and 20 mC. Roughly, the fibers can be divided into five groups: low strength, low extension, e.g. rock wools; high strength, low extension, e.g. aramid and HMPE; tough, due to combined strength and extension, e.g. nylon, polyester, and polypropylene; less tough, lower combined strength and extension, e.g. cotton, wool, and rayon; and ery high extension, e.g. Lycra. However, for the natural fibers, properties vary with variety and growth conditions, and, for manufactured fibers of any given type, the degree of orientation imposed allows for a range from lower strength\higher extension to higher strength\lower extension. The densities of textile fibers are typically in the range 1–1n5 g cmV$, which gives them a considerable weight advantage compared to many structural materials. Cellulose, protein, and, to a lesser extent, nylon absorb appreciable quantities of water, which causes swelling and changes properties. When dry, the polymeric textile fibers are insulators. As water is absorbed the dielectric constant increases, reaching high values at high humidities; this allows ions to dissociate, and lowers the specific resistance to about 10$ Ω cm in moisture-absorbing fibers at high humidity. The addition of additives to fibers gives special properties, for example, flame resistance, antimicrobial action, and ultraviolet protection. Most textile fibers will stand temperatures up to 200 mC, but at some higher temperature they melt, char, or chemically decompose in other ways. The thermoplastic fibers can be ‘‘permanently’’ heat set at temperatures appreciably below their melting points, and temporarily set at lower-temperature transitions. Similar temporary setting in cellulose and protein fibers is strongly influenced by wetting and drying. 3
Textile Fibers: A Comparative Overview
Specific stress ( N tex–1)
Specific stress ( N tex–1)
Figure 1 Typical stress–strain properties of textile fibers at 65% relative humidity and 20 mC. (a) Natural fibers and first-generation manufactured fibers. (b) HM-HT fibers. (Sources: Morton and Hearle 1993 and manufacturers’ data.)
Figure 2 Principal routes for assembly of fibers into fabrics.
1.5 Textile Processes and Products Figure 2 outlines the major ways in which textile fibers are assembled into fabrics, which then have to be fabricated into the wide range of final products. In order to give yarns any strength, the natural staple fibers have to be twisted, entangled or bonded together. This is not necessary for continuous filaments, though limited interlacing or twisting is used to give some cohesion to the yarns. In order to get a rougher ‘‘natural’’ fabric character, coarse tows are made and cut into staple fibers for spinning, often in blends with natural fibers. Texturing processes can be used to set or trap filaments in helical or looped paths, in order to give more stretch and bulk to continuous filament yarns. Most textiles are planar sheet materials. In woven fabrics, warp (lengthwise) yarns pass under and over weft (fill, crosswise) yarns. In braids, interlacing occurs between diagonal sets of yarns. In knits, yarns are 4
looped together in neighboring rows, either across the length in weft-knitting or along the length in warpknitting. Nonwoven fabrics are more-or-less random sheets of fibers, which are held together by adhesive bonding, entanglement, or stitching. One-dimensional assemblies are used in cords and ropes. Three-dimensional fabrics are used as composite preforms, either in the form of shaped sheets or thick structures, but also have other applications, such as, respectively, knitted garments and conveyor belts. Dyeing of fibers, yarns, or fabrics in the wide range of needed colors involves selection of dyestuffs to match the chemical composition and structural accessibility of fibers. Acid, basic, and other reactive dyes are readily taken up by cellulose, protein, and nylon fibers, which are swollen in water. Mordanting fibers with metal salts allow dyes that form organometallic complexes to be used. Vat dyes are insoluble in water, but become soluble on reduction; fabric is reoxidized after dyeing. Disperse dyes, which are sparingly soluble in water, are used for thermoplastic fibers. Chemical and mechanical finishing of fabrics is used to stabilize dimensions, modify surfaces, and impart special functions.
2. Cellulose and Related Fibers 2.1 Basic Chemistry, Formation, and Properties Cellulose, which is the main structural material in plants, is a condensation polymer formed by bio-
Textile Fibers: A Comparative Overview synthesis from carbon dioxide and water with glucose as the intermediate monomer. The chemical formula is H O
CH2OH HO C
In terms of physical fiber structure and properties, the important molecular features are shown in Fig. 3. The long chain, which has a natural degree of polymerization (DP) of over 10 000, has direction. In the crystal form, cellulose I, in natural fibers, the chains are parallel, but mercerization (see Sect. 2.2) or solution and regeneration converts this form to cellulose II with antiparallel chains. The molecule is ribbonlike, which allows for easy twisting and relatively easy bending in one plane. The hydroxyl groups, which stick out of the chain, form hydrogen bonds within or between chains and with absorbed water. Biosynthesis occurs at enzyme complexes within plant cells. At each complex, glucose units add on to about 30 growing molecules, which automatically crystallize as fibrils with a width of 3n5 nm. In this sense, natural cellulose fibers can be regarded as 100% crystalline. The lower fiber density, absorption of water, x-ray diffraction, and chemical evidence, which suggest the presence of around one-third disordered material, can be attributed to imperfect packing of fine fibrils with high surface area. Under genetic control, the fibrils are laid down in helical arrays, specific to particular plants, to form cell walls. The processing of cellulose to make regenerated fibers reduces the DP to between 250 in cheaper forms and 600 in improved forms. The amount of disordered material increases to about two-thirds, which doubles the water absorption. Studies of the dynamic mechanical properties of viscose rayon, which are also generally applicable to other cellulose fibers, show glass-to-rubber transitions above 200 mC when dry and below 0 mC when wet. Consequently, wetting and drying leads to formation of creases. These can be reduced by cross-linking treatments with formaldehyde resins, which have the negative effect of making the fibers more brittle and less resistant to wear. Swelling in water makes for easy dyeing and chemical finishing of cellulose fibers (see also Cellulose : Chemistry and Technology). 2.2 Cotton Cotton accounts for almost half the world’s fiber usage. Until challenged in price by polyester, its combination of low cost and useful properties made it the commodity fiber, which could be used in every
application, including demanding technical textiles. Now, it is used more selectively, particularly for its comfort and its aesthetic and environmental appeal in clothing and household textiles. The cotton fiber is a single plant cell, which is the seed hair of the genus Gossypium. Plant breeding has led to many varieties from different natural species. Genetic engineering is now introducing new traits. Many fibers grow from each of many seeds within the cotton boll, which forms after flowering. When the fibers are mature, the boll opens, and the fibers dry, ready for picking. Early opening, due to frost or disease, leads to thin-walled immature fibers. Cell formation starts with a primary wall, which grows to the full fiber dimensions in a few days. In the next 24–30 days, the secondary wall, which constitutes the main part of the fiber, is laid down in layers inside the primary wall with a helical angle of about 21m. At intervals, the helix reverses sense from left-handed to right-handed. At maturity, an open lumen remains at the center of the circular fiber. Drying causes the fiber to collapse into a flatter, bean-shaped cross-section. The helix reversals lead to the fiber taking the form of a convoluted ribbon. Natural waxes are present on the surface of the fiber. Raw fiber is slightly yellowish, but becomes white after bleaching. Some special varieties contain green or brown pigments. Cotton fiber dimensions range from short, coarse fibers (1n5 cm, 3 dtex) to long, fine fibers (5 cm, 1 dtex), with the bulk of the crop around 2n5 cm, 2 dtex. The stress–strain curve of cotton, as shown in Fig. 1, is slightly concave upward, and can be explained by the elastic modulus of the crystal lattice, modified by the effects of the helix angle, twisting at helix reversals, and removal of convolutions (Hearle and Sparrow 1979). The elastic recovery progressively decreases as extension increases, reaching a value of about 0n4 near break in dry cotton. In the dry state, hydrogen bonding between fibrils resists shear, which occurs when a helical assembly is extended. The absorption of water introduces mobility between fibrils, and leads to a reduction in modulus and increase in break extension as humidity increases. Cotton, and other plant fibers, are unusual in having a higher strength wet than dry, due to the relief of internal stresses. The form of break also changes (Hearle et al. 1998). The tenacity and break extension of the weaker Indian cottons, which have been largely replaced by American varieties, have values of 0n19 N texV" and 5n6%, compared to 0n45 N texV" and 6n8% for strong Sea Island cottons. Typical values for the bulk of the crop are 0n35 N texV" and 7%. Mercerization is a treatment of cotton with a sodium hydroxide solution. This disrupts the crystals, swelling and plasticizing the fibers but leaving sufficient cohesion to retain fiber identity and allow processing. The effects are reversed, though with changes in structure, when the caustic soda is washed out. If shrinkage is allowed, mercerized cotton will be weaker 5
Textile Fibers: A Comparative Overview
5 × 10–4 µm
Figure 3 Essential features of the cellulose molecule.
and more extensible, but stretching under tension in the process increases fiber strength and reduces extensibility. A main reason for mercerization is that it converts the fibers to unconvoluted round crosssections with a smaller central lumen, thus increasing the fiber luster. 2.3 Other Natural Plant Fibers Wood provides the commonest plant fibers, and is the raw material for regenerated cellulose fibers, but wood fibers are too short for textile processing (unless paper is regarded as a wet-laid nonwoven textile (see Paper : History of Deelopment; Pulp and Paper : Wood Sources). Kapok, which is a hollow fiber used for buoyancy, is a seed fiber, and the coarse fiber coir, from the outer coat of the coconut, is a fruit fiber, but both have limited textile usage. Apart from cotton, the main textile plant fibers are extracted from stems (bast fibers) (e.g. flax, sunn hemp, true hemp, jute, kenaf, ramie) or stiff leaves (leaf fibers) (e.g. abaca (Manila hemp), sisal, henequen, phormium tenax (New Zealand flax)). At different times and locations, fibers from many other plants have been used. Currently, there is interest in promoting the use of waste products, such as pineapple fiber. The bast fibers were once of major importance, particularly before the supply of cotton from America, for clothing and other textiles; and the leaf fibers were dominant in ropes and cordage. However, with the additional competition from manufactured fibers, their use is now limited. The lower-cost fibers have inferior properties, and the superior fibers are expensive to produce. Bast and leaf fibers are multicellular. The fiber ultimates are small and short, but in bundles they provide the reinforcement to stems and leaves. Extraction of the fibers starts with retting, which is a biological or chemical attack on the non-fibrous tissues, and is followed by mechanical beating and further textile processing. The size of the fibers, as they are used in yarns and fabrics, depends on the severity of the treatments. The helix angle in these fibers is lower than in cotton, typically around 10m, so that when, as in flax and ramie, they are almost pure cellulose, they have greater strength and lower break extensions than cotton. Other fibers, such as jute, contain substantial fractions of noncellulosic material, lignins and hemicelluloses, which lower strength and break extension. Flax, which is used to make linen, and ramie are 6
high-quality fibers with good color and an attractive appearance and handle in fabrics. True hemp (Cannabis satia) was a major European and American fiber, which was largely displaced, though attempts are being made to revive its production. Jute was widely used for low-grade uses, such as sacking and carpet backing, but, in developed markets, has lost out to polypropylene. Trials for other uses are being made in jute-producing countries. In cordage, abaca and sisal were dominant, but have been replaced by synthetic fibers. 2.4 Regenerated Celluloses The first factory to manufacture textile fibers, by extruding and regenerating cellulose nitrate (nitrocellulose) dissolved in ether and alcohol, opened in 1890 but this method is no longer used. The second process continues in limited production in Japan. Cellulose forms a soluble complex with cuprammonium hydroxide. The solution is extruded into a dilute acid bath, which precipitates the cellulose fibers. The third process, which in a variety of forms is now dominant, began commercial production in 1905 as iscose rayon. However, there are environmental problems with the viscose process, and it is doubtful if building a new, clean factory would be economically viable. This has led to research into organic solvents for cellulose, and semicommercial production of lyocell fibers began in 1988. These are likely to become more important. Other methods, such as making acetate yarns, stretching and then regenerating cellulose fibers, have been used at various times. Marketing of cuprammonium and viscose rayon started with continuous filament yarns as a cheap ‘‘artificial silk.’’ Waste yarn was chopped into short lengths for processing into spun yarns on cotton and wool machinery. In the 1930s, high-tenacity yarns began to replace cotton in tire reinforcement, and the production of thick tows for conversion into staple fibers began. By 1950, staple had exceeded filament yarn production, and new variants with improved properties were introduced. Other developments and changing economics mean that rayon is no longer a low-cost commodity fiber, but is used to give added value either alone or in blends with polyester and cotton. Viscose is an alkaline solution of sodium cellulose xanthate, which is made by reacting cellulose with sodium hydroxide and carbon disulfide. After filtering and aging, it is extruded into an acid bath. The filaments coagulate as regenerated cellulose fibers, which are stretched to increase strength and reduce extensibility. An early discovery was that the addition of zinc salts to the coagulating bath improved properties, by causing the regeneration to pass through an intermediate stage as zinc cellulose xanthate. Because hydrogen ions move faster than zinc ions, the zinc route occurred near the surface but the direct route
Textile Fibers: A Comparative Overview operated in the center, giving a skin–core structure. Removal of excess solvent led to a collapse into a serrated cross-section. The fine structure of ‘‘standard’’ viscose rayon is probably a classical fringed micelle structure, with little if any crystallographic chain folding. Chain molecules link crystalline micelles through tie-segments in amorphous regions. The crystals hold the structure together, deformation and rupture occurring in the disordered material. Chain ends between micelles reduce strength, but their effect can be minimized by increasing molecular weight, which is done for superior fibers, and by reducing the size of micelles and the intervening lengths. The last explains the influence of zinc: the skin structure has a finer texture than the core. For general textile purposes, moderate or low strength and a break extension around 20% was suitable, but for tire cords the stretch in manufacture was increased to give higher strength and reduced extension. The early tire cord yarns were skin-plus-core fibers, but the process was then modified to increase the skin thickness and eventually make stronger, ‘‘all-skin’’ fibers with a round cross-section. Similar techniques were later applied to more extensible staple fibers. By forcing the formation of an asymmetric skin, which contracts differentially, it is possible to make fibers that crimp spontaneously on drying. In the 1950s, modifications, such as extrusion into a bath of a weak acid, made it possible to coagulate fibers of the cellulose derivative, stretch these, and then regenerate the fibers. This gives a fibrillar, instead of a micellar, fine structure. The resulting high-wetmodulus (HWM) fibers, which are also known as polynosic fibers, are closer to cotton in properties. Other developments included the production of hollow filaments, which give added bulk or can be collapsed in to ribbon forms, bulky multilobal fibers from shaped spinneret holes, deep-dyeing fibers, flame-retardant fibers, and superfine fibers (" 0n5 dtex). In the dry state, the tensile stress–strain curve of standard viscose rayon, as included in Fig. 1, shows an initial stiff region, with a yield at 1% or 2% extension to a lower-modulus section, which stiffens somewhat toward break. This is a micellar composite, so deformation occurs in the amorphous regions with only limited stiffening by the crystallites. Extension is initially restricted by the hydrogen bonds between cellulose molecules. Yield occurs when these bonds break. In the wet state, mobile water molecules replace the direct hydrogen bonds, and the low modulus yield region effectively starts at the origin. The fibrillar structure of the HWM fibers gives a parallel composite, in which the crystals stiffen the structure. This has some effect when dry, giving more cotton-like properties, and a major increase in wet modulus. Lyocell fibers are made by dissolving cellulose in a concentrated amine oxide, N-methylmorpholine-N-
oxide [7529–22–8] (NMMO), solution, and then extruding into a dilute NMMO solution, which precipitates the cellulose. With effective recycling, the process is environmentally and economically attractive. Due to their high DP and fibrillar structure, lyocell fibers, such as Tencel, have good properties. Fibrillation, namely the splitting off of fine fibrils from the fiber surface, occurs easily. This is advantageous in enabling fabrics to be finished to ‘‘peach skin’’ textures and in bonding hydro-entangled nonwovens, but, if not wanted, its extent can be controlled by process adjustments.
2.5 Cellulose Acetate Treatment of cellulose with acetic acid and acetic anhydride converts it into cellulose acetate. Partial acetylation, with one in six hydroxyl groups remaining, gives secondary acetate, which is soluble in acetone and can be dry-spun into fibers. Due to their poor crystallinity, secondary acetate fibers are comparatively weak and extensible. Their low modulus means that fabrics have low bending stiffness, which is useful when soft, drapable materials are in fashion. Acetate fibers have low moisture absorption and are thermoplastic. The largest usage of acetate fibers is now in cigarette filters. Complete acetylation gives cellulose triacetate, for which more expensive solvents are needed. Triacetate fibers, although weaker than nylon or polyester, have the same heat-setting advantages, and found a market when the synthetic fibers were expensive.
2.6 Alginate Fibers and Other Polysaccharides Salts of alginic acid, derived from seaweed and withCOOH replacing the -CH OH of cellulose, can be made into fibers. Their main# use is as calcium alginate fibers, which are soluble in hot, soapy water, and can be used to give temporary strength to a fabric. Calcium and sodium alginate fibers, and the other polysaccharides, chitin and chitosan, are used in wound dressings and other medical textiles.
3. Protein Fibers 3.1 Basic Protein Chemistry The basic repeat unit in proteins is H
The diversity of proteins is due to the choice and order 7
Textile Fibers: A Comparative Overview of side groups -R, which derive from 20 amino acids. The side-groups are inert (-H and hydrocarbons), acidic, basic, hydroxyl-containing, and more complex. Of particular importance are two anomalous forms. Proline is a ring structure, which joins on to the mainchain carbon in place of -H and distorts chain geometry. Cysteine is formed as -CH SH, but can be # forms crossoxidized to cystine, -CH SSCH -, which # # links within or between chains. In addition to the cystine cross-links, covalent isopeptide bonds between lysine and glutamine, acid–base links, and hydrogen bonds influence fiber properties. The residue sequence, which is controlled genetically, causes protein molecules to take up specific geometric forms. Some protein molecules, or parts of molecules, crystallize in one or other of two lattice forms. In variants of the α helix of simple polypeptides, the chains follow helical arrays, with internal hydrogen bonds and different chemical and geometrical repeats. Intermediate filaments (IFs) contain 32 chains, consisting of coiled-coil dimers, packed in tetramers and then octamers to form the microfibrils of wool. Alternatively, proteins crystallize as β sheets of extended molecules linked by hydrogen bonds between neighboring chains. Under tension, intermediate filaments undergo a reversible α β crystal structure transition, with an extension of 120% for an ideal α helix, but down to 80% in wool. Another class of proteins are globular, formed by individual chains folding into specific but irregular forms with internal cross-links.
3.2 Wool Wool was formerly a more expensive commodity fiber, used for a wide variety of purposes. It is more resilient and durable than cotton, and can be made into warm, attractive clothing, upholstery, and carpets. Some of the attributes of wool have been replicated in synthetic fibers, and it must now compete on quality and tradition. Production costs are high, and farmers can change to other products. It has been demonstrated that genetic engineering can change the structure and properties of wool, though not yet in beneficial ways. This may be a route to a future for different wools with characters aimed at particular high-quality markets. Wool fibers are slightly elliptical, with mean diameters between 15 µm and 50 µm. Finer fibers command a higher price. A typical length of Australian Merino wools (" 22 µm) is 100 mm and of coarse New Zealand wool (" 35 µm), used in carpets from sheep grown mainly for meat, is 150 mm. Wool, which is multicellular, grows under genetic control in a sequence of stages out of skin follicles at a rate of around 100 mm per year, and has the most complex structure of any textile fiber. Characteristic features are present at many levels. Chemically, there are families of keratin proteins of types I and II and of 8
a variety of keratin-associated proteins (KAPs). The major part of the keratin I and II molecules pack into crystalline IFs, but there are also terminal groups (tails), which are rich in cysteine and project from the IFs. The KAPs are globular proteins, some of which are rich in cysteine while others are rich in glycine and tyrosine. The fine structure consists of a parallel assembly of IFs (microfibrils) with a diameter of about 7 nm, spaced about 10 nm apart in a matrix of KAPs. The IF structure, which is laid down first, is now well documented, but there is considerable uncertainty about the matrix, which is laid down later. The final actions are keratinization, which converts cysteine into cystine, and coating the fiber surface with a lipid layer. Some of the cystine cross-links will be within globular KAPs, but it is likely that some will be between globules and also to the tails of the IF proteins, which act as spacers between IFs. In this scenario, the matrix would act as a fairly strongly cross-linked rubber with a maximum extension of about 50%. Most of the fiber consists of spindle-shaped cells held together by the cell membrane complex (CMC), which is rich in lipids. Cells in this central cortex are of two types, separated by a boundary, which rotates along the fiber. In the para-cortex the IFs are oriented parallel to the fiber axis; in the ortho-cortex the IFs are associated in helical arrays as macrofibrils. On the surface there are several layers of a cuticle, which contains scale cells, overlapping like tiles on a roof. The structural features determine the specific properties of wool. The surface scales cause a directional frictional effect, which leads to felting (fiber entanglement by wet agitation) as movement is easier with scales than against scales. Felting is a defect, which causes shrinkage and can be avoided by chemical removal of scales, but can also be used to make fabric. Differential shrinkage of ortho- and para-cortex on drying causes fibers to bend, and the rotation of the boundary converts this into helical crimp, which gives bulk to wool yarns. The CMC generally holds the cellular composite together, but if it is weak it may allow slip and reduce fiber strength. The many amino acid side-groups present allow for a range of chemical treatments, including the introduction of covalent cross-links to give crease-resistance. The tensile stress–strain curve of wet wool is stiff up to 2% extension, but then yields with an almost constant stress up to 30% extension, followed by a sharp change to a stiffer postyield region up to break at about 50% extension. A feature, which is unusual for yielding materials, is that recovery is complete from 30% extension or less, though on a curve at a lower stress level. Above 30% there is a small loss of recovery. In the dry state the yield stress increases, and recovery is incomplete. However, subsequent wetting will restore the fiber to its initial state. This is an important factor in the durability and appearance retention of wool fabrics.
Textile Fibers: A Comparative Overview The tensile properties have been explained (Chapman 1969) as the mechanics of a composite system. Up to 2% there is deformation of the IF crystal lattice with only a small contribution from the elastomeric matrix. At 2%, the α β transition is initiated in IFs and, locally, the IF stress drops from the critical value for nucleation of the β form to the equilibrium value for the α β transition, with stress transfer to the neighboring matrix, which extends to 30%. Only a limited zone can open, but the sequence proceeds with more zones opening until all the zones have joined up. Beyond 30% extension the matrix is stretched more, and the elastomer stress–strain curve is followed. Some cross-links break, which accounts for the loss of recovery and leads to fiber rupture in a granular form typical of a composite structure. Nucleation is not needed in recovery, which, below 30%, follows the elastomer curve with a shrinkage of opened zones until they disappear and the extension curve is rejoined below 1% extension. In the dry state, hydrogen bonding causes the matrix to have an initial stiff region, before yielding. The evidence is strong for the above explanation, but alternative views are still current (Feughelman 1997).
filament production. The silk from larvae that hatch out to continue the cycle is chopped into staple fiber for spun silk fabrics. There is also some use of wild tussah silk. Silk is a natural block copolymer. Segments of a simple repeating structure of glycine, alanine, serine, and tyrosine form β-crystalline regions linked by segments with more diverse chemistry. Cysteine is absent. Silk is the toughest (highest work of rupture and good recovery) of natural textile fibers, due to moderately high tenacity and break extension. This led to uses such as parachutes until it was superseded by nylon. Spider silks have not found textile uses, but have remarkable and variable properties. Different proteins are formed from the same glands for different purposes. Drag-line silk is extremely strong, and derives from an oriented liquid crystal solution, similar to production of HM-HT fibers such as Kelar. Other spider silks in the web are more extensible and elastic. Currently, there is interest in the genetic engineering of spider silk proteins by bacteria as a route to polymers for high-performance fibers from aqueous solutions, but other factors such as the form of the ‘‘spinneret’’ in the spider and the relatively slow extrusion rate may be critical (see Silk Produced by Engineered Bacteria).
3.3 Other Hair Fibers The hairs of other animals (and humans) have structures that are generally similar to wool but have specific differences. Coarse hairs, such as cow or horse, were used for undemanding uses, such as furniture stuffing, but also for special purposes, such as violin bows, where long fibers are needed. Other hairs, which find textile uses, are finer than wool or have other characteristics, which give particular qualities to fabrics. The main sources are varieties of goat, such as mohair and cashmere, camel, alpaca, vicuna, and angora rabbit.
3.5 Regenerated Protein Fibers Fibers can be regenerated from solutions of proteins, but the price\property ratio was not sufficiently favorable for continued commercial production. The fibers are weaker than wool and have poor elastic recovery. The most successful was made of casein from milk, but proteins from corn and ground-nuts were also used.
4. Melt-spun Synthetic Fibers 4.1 General Features: Polyamides and Polyesters 3.4 Silk and Spider Silks Silks differ from other natural fibers in not being formed as living cells, being extrusions of protein solutions which coagulate on drying. Silk used in the textile industry comes from the covering of the cocoon of the silk moth Bombyx mori. Pairs of triangular filaments (2–3 dtex) are stuck together by gum, and can be unreeled after soaking the cocoon in hot water. Each cocoon yields about 700 m of useful length of silk. As the only continuous filament yarn available to the textile industry until the twentieth century, silk gave expensive, lustrous fabrics. Even today, the shape of the fibers and their bulk and surface properties give special qualities, which command a high price in luxury fabrics. Production involves feeding mulberry leaves to silk worms, which then change to larvae within cocoons. Most cocoons are harvested for
The invention by Carothers of the polyamide nylon 66, which was commercialized by DuPont in 1938, marks the real start of a major synthetic fiber manufacturing industry. It was followed by nylon 6, invented in Germany, and by the polyester polyethylene terephthalate (PET), which has now overtaken nylon in world production. These are all polymers with melting points between 215 mC and 260 mC. Molten polymer is extruded, cooled, and solidified, and stretched (drawn) to give orientation. Continuous filament yarns contain 1 to about 1000 filaments. Staple fibers are cut from massive tows. In the first production method, polymer chips were melted, extruded and wound up at under 1000 m minV" as unoriented yarns, which were partially crystalline for nylon but amorphous for polyester. In a second stage, the undrawn yarn was stretched by draw 9
Textile Fibers: A Comparative Overview ratios of around W4. Later, combined spin–draw processes were introduced, with first-stage rollers at around 1000 m minV" and wind-up at 3000 m minV". A major change, particularly important in polyester, was the production of partially oriented yarn (POY) by direct wind-up at high speed. The orientation results from the rapid elongation and attenuation of the molten threadline before solidification. The necessary residual draw ratio of around W1n5 is imparted simultaneously with the following textile operations: yarn texturing or warp-drawing. Even higher-speed winding machinery, 5000–10 000 m minV", allows production of comparatively low-strength, high-extension yarns suitable for direct use in fabrics for some purposes. Continuous polymerization, which feeds molten polymer directly to extrusion, is now common. Other heat treatments, with tension or relaxation, can be applied to change the tensile, thermal shrinkage, and other properties. For general textile purposes, fibers of moderate strength and break extensions around 20% are produced, but for tire cords and other technical uses higher strengths and lower break extensions (" 10%) are used. Fiber cross-sections are usually circular, but other shapes, such as bulky trilobal fibers, can be formed from shaped spinneret holes. Finishes are applied to reduce electrostatic charging and to give frictional properties needed for different markets. Internal additives, such as delustrants, pigments, or conducting carbon or polymer, can be incorporated. For some uses, bicomponent fibers, with skin–core or side-by-side forms, or multicomponent fibers are produced. An important example of the latter is the islands-in-a-sea process which gives very fine microfibers embedded in a matrix, which can be removed. Nylon 6 and 66 and PET are characterized by polymers with long repeat units that contain flexible inert sequences alternating with more interactive sequences, which in PET are also stiffer. This gives rise to thermal transitions which have led these polymers to dominate the market for synthetic fibers. At some temperature around k100 mC, the amorphous regions of the polymer change from a fully glassy state to give some extensibility as freedom of rotation around C–C or similar bonds becomes possible. At around 100 mC, another second-order transition occurs, as hydrogen bond mobility develops in nylon and benzene ring interactions become free in PET. In addition to lowering the modulus, this opens up the structure for dyeing. At about 80 mC below the melting point the structure loosens up still more, probably by some form of crystal mobility, but possibly in PET by transesterification in amorphous tie molecules. Heat setting, namely the fixing of fibers within yarns and fabrics in new shapes, is an important attribute of nylon and polyester fibers. The intermediate transition at around 100 mC causes a temporary set, which can be released by reheating. The higher transition allows ‘‘permanent’’ heat-setting treatments to be applied to 10
nylon 66 and PET yarns and fabrics between 180 mC and 240 mC, when fibers start to stick together. The temperatures are lower for nylon 6. Setting processes are used in the production of textured yarns and to fix creases or otherwise stabilize fabrics. In nylon, it is necessary to go to more severe conditions to overcome a prior treatment, but PET can be reset at any temperature in the range. Superficially, the extrusion, attenuation, and solidification of a molten threadline looks like a steady process, and the macroscopic relations between forces, elongation, and rheology and between heat transfer and temperature have been extensively studied. However, at the molecular level the motions may be more complicated. The semicrystalline structure is established at high speeds (1 ms corresponds to 5 cm at 3000 m minV"). In addition to polymer dynamics in situations far from equilibrium, the phenomena will involve aspects of the emerging science of complexity, chaos, fractals, kinetics of nonhomogeneous processes, nonlinear irreversible thermodynamics, and, possibly, quantum superposition, but it is not easy to see how to apply these ideas. Despite studies by many analytical techniques, there is still considerable uncertainty about the fine structure of these fibers. Degrees of orientation, which vary with stretch in processing, can be measured. Crystallinity can be estimated on the basis of a scale from crystal to amorphous of a parameter such as density, and is always close to 50% in fibers as used. Some indications of the size, spacing, and linkage of crystals can be obtained, but the details remain obscure. Partly this is due to the many ways in which molecules can be arranged in a partially oriented, semicrystalline structure, and partly to the fact that the structures formed and modified by annealing at temperatures close to the melting point will vary with processing history. There have not been comprehensive investigations and interpretation of well-characterized samples by the several available techniques. Almost all pictures drawn to illustrate views of structure have been simplistic and not quantitative. The common view is that the fine structure is a modified fringed micelle form. Crystallites, with a mix of fringing and folding at their ends, are stacked in a pseudofibrillar array and linked by tie molecules, as suggested in Fig. 4(a). Figure 4(b) is an attempt to be more specific on the structure of a nylon 6 fiber and, while retaining the main features of more idealized representations of the common view, introduces some variability and three-dimensionality. Other views have been suggested. The long repeat length means that defects, such as those that occur in polyethylene due to additional units, are not possible without disruption of crystals, but, in contrast to Figs. 4(a) and (b) and similar pictures, there could be defect forms in which molecules follow paths across crystallites and emerge from the sides. More rapid quenching may give continuous structures of partial order, such as illus-
Textile Fibers: A Comparative Overview trated in Fig. 4(c), in which the boundaries between crystalline and amorphous regions are less well defined. The partial order and molecular characteristics give strong, extensible nylon and polyester fibers. Tensile rupture is a ductile break in which a crack opens to a V notch, before final transverse rupture. Apart from the range of possibilities for each fiber type, there will be generic differences between nylon 6, nylon 66, and PET. Cracks induced by shear stresses move across nylon fibers at an angle of about 10m but are parallel to the axis in polyester fibers, which suggests that crystallites may have higher aspect ratios. The structural differences, together with the stiffening effect and different intermolecular forces of the benzene ring in amorphous regions, lead to the significant differences between nylon and polyester in properties and uses. 4.2 Polyamides: Nylon 6 and 66 The chemical formulae of nylon 66 and two repeats of nylon 6 are O
–CCH2CH2CH2CH2CNHCH2CH2CH2CH2CH2CH2NH– Nylon 66
is reduced. Near the transition temperature the dissipation factor (tan δ) due to internal friction, increases to over 0n1, and above the transition the modulus is lower. The first uses of nylon in parachute fabrics and ladies’ stockings resulted from its toughness and durability. After 1945, markets developed for nylon as the first ‘‘ease-of-care ,’’ ‘‘wash-and-wear’’ fabric, due to the ability to give a permanent set. Setting also led to high-stretch yarns, which were made by twisting, heat setting, and untwisting, at first in separate steps and then by continuous false twisting. Nylon production was overtaken by polyester in the 1970s, and nylon is now used selectively for applications for which its properties are well suited. These features are low modulus, high work of rupture, good durability when dry, and resilient recovery from large deformations. In addition to underwear and some other items of clothing, there is a large market for carpet yarns, which can be made either from staple nylon spun in the same way as wool or from bulked continuous filament yarns (BCF) made by a jet-screen buckling and setting process. Industrial uses, such as ropes, come when a limited number of applications of high forces, for example in the shock loading of climbing ropes, are expected. When wet, nylon has poor wear resistance in applications such as marine ropes, due to shear cracks from surface abrasion under repeated cycling at moderate loads.
–CCH2CH2CH2CH2CH2NHCCH2CH2CH2CH2CH2NH– Two repeats of nylon 6
The same units are present with a slight rearrangement in two repeats of nylon 6 as in one repeat of nylon 66. The main difference is that the nylon 6 molecule has direction, which is absent due to the symmetry of nylon 66. This affects the possibilities for crystal packing, and is probably the reason for the lower melting point of nylon 6. Although some sigmoidal curvature can be put into the tensile stress–strain curve of nylon fibers by temporary setting, the curves are usually almost linear up to a yield region close to the break stress. The modulus is comparatively low. Depending on the last treatment by the manufacturer, nylon fibers will often shrink by 10% in boiling water, which lowers the modulus. Alternatively, setting under tension may increase length by 10% and raise the modulus. Moderately high strength and break extension combine to make nylon a tough, durable fiber. Elastic recovery is good, being around 70% from near break stress, but time-dependent. Nylon shows an appreciable amount of primary (recoverable) creep and stress relaxation. Due to the presence of -CONH- groups, nylon absorbs water, which leads to changes in properties. The glass transition drops from near 100 mC when dry to near 0 mC when wet, and strength
4.3 Other Nylons Polyamides with more -CH - groups, such as nylon 11 # melting points, density or nylon 610, will have lower and moisture absorption, and will tend to be more extensible. They have mainly found application in thick monofilaments as bristles. Reducing the number of -CH - groups, for example in nylon 4, will give a # stiffer fiber, which is advantageous for some applications. Attempts have been made to follow this route, but there is a large barrier of lower price in the larger volume of nylon 6 and 66 to overcome. In the 1970s, DuPont made a more expensive, luxury fiber, QIANA, from the polymer of an amine with two cyclic rings, trans,trans-di(4-aminocyclohexyl)methane, with dibasic acids having 6–10 -CH - groups; but sales did # not justify continuing production. 4.4 Polyethylene Terephthalate PET (or 2GT) is the ester of ethylene glycol and terephthalic acid, which are polymerized either directly or through other intermediates, and has the chemical formula O
Textile Fibers: A Comparative Overview
Figure 4 (a) Schematic view of nylon fine structure, intended to explain the small-angle X-ray diffraction pattern by the angled lattice repeat. (b) Fine structure of nylon 6, as interpreted from SAXS and WAXD data (Murthy et al. 1990). (c) Alternative continuous partially ordered structure.
The -COCCOC- sequence acts in a similar way to the -CH - sequence in nylon. The benzene rings make the # molecule stiffer, and form intermolecular cross-links analogous to hydrogen bonds. PET has a good combination of properties for a wide range of textile products. Depending on the molecular orientation, combinations of moderate strength and high break extension or high strength and moderate break extension give a high work of rupture. The initial modulus is about three times that of nylon, which is beneficial in many applications. Elastic recovery from high loads is less than for nylon, but from strains under tensions up to about half the break load is better. The extent of creep is small. The glass transition is above 100 mC and, since PET absorbs little water, is almost the same wet and dry. This means that creasing is not imposed by normal washing and drying. ‘‘Permanent’’ heat setting can be achieved repetitively above 180 mC. Twist-textured PET yarns are processed continuously over two heaters. The first stage of twist–set–untwist gives a yarn with a potential for high contraction, like the nylon stretch yarns, but second-stage setting after contraction by around 15% gives a yarn with high bulk but limited stretch. The majority of polyester filament yarns are produced as POY yarns at around 3000 m minV". This speed induces an incipient crystallinity, which prevents the rapid structural aging of yarns spun at lower 12
speeds. Lower-strength yarns, for undemanding end uses in flat-filament fabrics, can be produced directly at speeds from 5000 m minV" to 10 000 m minV". Staple fiber is cut from large tows, which have been hot-stretched in wet conditions and crimped by passing through stuffer boxes. Continuous filament webs are extruded and thermally bonded as nonwoven fabrics. However, there is considerable diversity beyond the simplest round cross-section fibers of PET homopolymer, which are used in the commodity apparel textile market. Microfibers and shaped fibers give special effects in handle, drape and other aesthetic characteristics, particularly in the Shingosen fabrics from Japan. Examples are the peach-skin feel from microfibers and the characteristic luster and rustle of silk from three-petal shaped fibers. Copolymers are used to enhance dyeability, or to give differential dyeability by incorporating groups attractive to acidic or basic dyes. Pilling, which can be a nuisance, is reduced by using weaker fibers, which allow pills to break off. Industrial grades are processed to give strengths that are the highest of any except the newer highperformance fibers, and special finishes are applied to meet needs such as promotion of adhesion to rubber in tire cords or low-friction to reduce internal abrasion in marine rope grades.
Textile Fibers: A Comparative Overview Owing to the ease of manufacture, large volume, and good properties, PET polyester has become the low-cost fiber for a universal range of purposes. This can be shown by the list below of chapters in the applications section of a book celebrating 50 years of polyester (Brunnschweiler and Hearle 1993). Each chapter covers many products. The last item in the list is typical in going from cheap disposable nonwoven surgeons’ gowns and sanitary products to expensive knitted tubes for arterial grafts and braided replacements for ligaments. Woven and knitted apparel fabrics. Man-made suedes and leathers. Household products. Fashion and function in Italy. Textiles for the home. The revolution in sewing threads. Competing with feathers and hair: the Fiberfill story. Tyres and industrial textiles. Polyester inside cars. Forming paper: a success for monofilaments. Ropes and nets. Building with polyester fabrics. Geotextiles. Composites: a lost opportunity or an exciting future. Medical textiles: perfect fit for a critical use. Once five times the price of cotton, the PET commodity fibers and yarns are now similar or lower in price and account for about 25% of world fiber production. The use of the cheapest PET fibers in lower-quality fabrics has given polyester a poor image. This is not justified when premium fibers are used in quality fabrics. It is possible to combine excellent aesthetics with durability and ease-of-care. 4.5 Other Polyesters Analogues of PET (2GT), derived from other glycols with more -CH - groups in the chain, give fibers with broadly similar# properties. Their viability depends largely on economic factors. There has been commercial production of 4GT. Genetic engineering of bacteria to convert sugar from corn into trimethylene glycol opens a route to 3GT. Polyethylene naphthalate (PEN) fibers, produced in experimental quantities in the 1990s, are made from the double-ringed naphthalene analogue of terephthalic acid. This development results from the commercialization of an economic route to intermediate chemical production. The fibers have a higher modulus and a higher melting point than PET, which is useful for some industrial applications. 4.6 Polyolefins The hydrocarbon polyolefins, polyethylene (PE) and polypropylene (PP), have the chemical formulae –CH2–
These are short repeat units without interactive
groups. The polymers can be melt spun into fibers, but, except for toughness, strength, and break extension, do not share the similarities of nylon and polyester. Their density and melting points are lower; recovery is poorer; fiber feel is waxy; moisture absorption is zero; chemically they are inert and resistant to solvents; dyeing is impossible without modification; and coloration has to be achieved by adding pigments to the melt. The single large glass transition in PE makes it undesirably soft for most textile purposes, and the approach to melting cause its properties to fall off below 100 mC. The single transition in PP spans room temperature, which makes the fiber stiffer, but gives large energy losses in deformation. PE has not penetrated the general textile fiber market. Thick monofilaments are used as an alternative to natural fibers in canvas fabrics and in ropes. Tyek, which can be regarded as a stiff nonwoven textile or a tough paper, is flash spun from solution to give a fibrillated web, which is bonded by heat. Gel spun HMPE fibers are described in the highperformance fiber section below. PP is more widely used, but 40% of its textile use is in the form of slit film, which is used for coarse fabrics such as carpet backing; there are similar uses for Typar, which is a directly produced, thermally bonded, continuous-filament web. The hydrophobic character of PP has been promoted as advantageous in thermal underwear. For nonwovens, its ability to be thermally bonded without undue compression of fiber webs is superior to polyester. However, PP continuous-filament yarns and staple fibers have mostly found textile markets where lower cost offsets poorer properties. Major uses are disposable nonwovens, and cheaper carpets, upholstery, and ropes. PP yarns are easy to produce, so that ropemakers, for example, have their own production facilities. 5. Solution-spun Synthetic Fibers 5.1 Polyacrylonitrile (Acrylic) Polyacrylonitrile (PAN) was the third major synthetic fiber of the 1950s, but has not achieved the wide usage of nylon and polyester. Apart from providing precursor yarns for carbon fibers, PAN is almost entirely produced as staple fiber. The major uses are in warm, bulky fabrics, such as knitwear, blankets, and pile fabrics, as an alternative to wool. Its synthetic-fiber carpet market has been lost to nylon. The basic chemical formula of PAN is –CH2CH(CN)–
In order to make dyeing possible, acrylic fibers (specified as 85% PAN) are copolymers with added reactive groups. Acrylic fibers can be melt spun, but are usually produced from solution by wet or dry 13
Textile Fibers: A Comparative Overview spinning. The diversity of materials and processes leads to differences in properties in fibers from different manufacturers and in different options. Acrylic fibers are weaker than nylon or polyester, yield at 2% extension, and have poor recovery from higher extensions. Breaks are granular. They are regarded as quasicrystalline, and have a major secondorder transition near 100 mC. Fibers which have been highly stretched, or broken, show a high shrinkage when heated. Blending of no-shrink or preshrunk and high-shrink fibers is a way of making bulky yarns.
5.2 Other Solution-spun Fibers Modacrylic fibers contain less PAN and more copolymer groups. Properties are generally similar to those of acrylic fibers, but the copolymer is used to give special properties such as flame resistance. Vinyl and vinylidene polymers and copolymers were the first polymers to be used as fibers, but have not achieved widespread use, as can be seen from the small figure in Table 2 for ‘‘others ,’’ which also includes some high-performance fibers. Polyinyl chloride (PVC) fibers are produced in the same way as acrylic fibers, and have somewhat similar properties. Polyvinyl alcohol (PVA) fibers have good properties, and have been produced in substantial quantities in Japan and China, but the cost\performance ratio has not worked out as favorably as some have expected.
6. High-performance and Specialty Synthetic Fibers 6.1 General Features Following the success of nylon, polyester, and acrylic fibers, research moved to a second generation of highperformance, synthetic-polymer fibers. The major drive was for HM-HT fibers for industrial uses such as tire cords and for advanced composites and other defense-related applications. The structural requirement is to have long polymer chains (high molecular weight), fully extended and oriented parallel to the fiber axis. They will usually, but not necessarily, be highly crystalline. Two routes were found. The first uses stiff, interactive polymers, which form liquid crystals in solution or melt. These become oriented during extrusion. From solution, the technique is dryjet, wet-bath spinning, with a gap between the end of the spinneret and the coagulating bath. The second route uses flexible, inert polymers, which form an orientable gel in concentrated solution. Unfortunately, no method has been found to produce the right structure in nylon or PET fibers, which would have technical and economic advantage. Another need was to produce fibers with high temperature resistance. Glass, carbon, and ceramic fibers are high-modulus and high-temperature fibers, 14
but, as mentioned in the introduction, are not the concern of this article. Another group of advanced polymer fibers has special physical or chemical properties.
6.2 Para-aramid Fibers The aramid fiber Kelar, polyphenylene terephthalamide (PPTA), was the first successful HM-HT polymer fiber. It is a para-aramid, with benzene rings from dibasic acid and diamine joined along the chain in 1,4 positions. The chemical formula is O –C
The polymer cannot be melted, but degrades chemically at around 500 mC. To be spun, it is dissolved in concentrated sulfuric acid and extruded into an alkali bath. The fiber, as first produced, is highly crystalline and highly, but not perfectly, oriented. The structure consists of pleated sheets, which are slightly off-axis. This structure causes the stress–strain curve to be concave upward, and results in some creep under low loads. Further heat treatments under tension increase orientation and give higher modulus forms. Other techniques can reduce modulus, as needed in some applications. Covalent bonds give the high axial properties, including a tensile strength around 3 GPa, but intermolecular forces are weak. Hydrogen bonding between -CONH- groups occurs only in one plane. Tensile rupture results from axial splits, which run across the fiber, giving long fibrillar forms. The shear modulus is low. Axial yield occurs at about 0n4 GPa. Although the low compressive strength is a disadvantage in many ways, it has the advantage that the fibers are not brittle in bending. Yielding on the inside of a bend allows fibers to bend back on themselves through 180m without breaking. Kink bands can be seen internally with polarized light microscopy and externally with electron microscopy. Although a few repetitions of sharp bending do not cause major damage, a few thousand cycles will cause breakage by flexural fatigue. This is seen as axial compression fatigue when component yarns in ropes are allowed to go into compression. The nature of the tensile break and the compressive yielding lead to high-energy absorption in structural impacts. Technora is a copolymer aramid fiber in which half the p-phenylene diamine is substituted by 4,4h-diaminodiphenyl ether. Its properties are similar to PPTA fibers, but with differences which may be advantageous in some applications. It was hoped that para-aramid fibers would find a large market in tire cords. However, apart from the
Textile Fibers: A Comparative Overview high price, they are stronger than is needed for the general market. Going from cotton to rayon to nylon and polyester, it was possible to reduce the amount of cord needed as strength was increased. However, there is a limit to the sparseness of cords within the tire, so that advantage could not be taken of the added strength of aramids. Their use depends on applications where high strength and high stiffness justify a high price. This includes: heavy-duty tires; light, strong ropes and industrial fabrics; ballistic impact resistance; and advanced composites. 6.3 Other Liquid Crystal Fibers The para-aramid polybenzamide (PBA), which is the analog of nylon 6, and other polymers with benzene rings in the chain are alternatives to PPTA, which is the nylon 66 analog, but have not been commercialized as of 2000. Superior properties come from polymers with more complicated rings, which were the subject of US Air Force research. One of these, polybenzoxazole (PBO), has been commercialized by Toyobo as Zylon. The chemical formula is O
Production methods, structure, and properties are similar to those of aramids, but the modulus and strength are higher. Experimentally, M5 fibers with higher shear modulus and strength and higher compressive yield stress have been produced from a polymer with a similar ring structure to PBO, but with -OH side-groups giving hydrogen bonding in both planes. HM-HT cellulose fibers have been made from solutions in phosphoric acid. Vectran is a melt-spun fiber, based on the fully aromatic polyester polyphenylene terephthalamide, which has the following chemical formula: O C
Limited copolymerization gives a thermotropic liquidcrystal material, which orients in flow through the spinneret. Melt spinning is much cheaper than solution spinning with strong acids, but this is offset by the need for an expensive subsequent heat treatment to build high molecular weight. The properties of Vectran are generally similar to aramids, but with specific differences, such as an absence of creep, which are beneficial in some applications.
6.4 High-modulus Polyethylene HMPE fibers are made by gel spinning of ultra-high molecular weight polyethylene. The extruded gels, which form in concentrated solutions, can be stretched to give highly oriented, chain-extended, highly crystalline fibers. The strength and modulus are higher than for aramids, but the fibers have the disadvantage of a low melting point. Properties start to fall off appreciably as temperatures rise above 40 mC. Substantial creep is due to molecules sliding over one another. The rate of creep increases with tension, and ultimately leads to creep rupture. Post-treatments can increase modulus and reduce creep. There are major differences in the incidence of creep among the different grades from different manufacturers. In one set of tests, one HMPE yarn broke after 4 days at 30% of break load, whereas another lasted 123 days. Improved grades have been introduced since these tests were carried out. The converse of creep failure is the excellent resistance to high-speed loading. HMPE fibers have low compressive strength, but are somewhat more resistant to axial compression fatigue than aramids. Gel spun HMPE fibers find applications in ballistic protection, in high-performance ropes and industrial fabrics, and in some composites. An alternative route to HMPE fibers is super drawing of melt-extruded polymer. However, the strength is about half that of gel-spun fibers, and commercial production has not been successful. 6.5 Thermally Resistant Fibers The meta-aramid fiber Nomex, which is poly-mphenylene isophthalamide, has the chemical formula O –C
The 1,3 links between benzene rings gives a less compact molecule than the 1,4 para-aramids. The polymer can be spun into fibers with mechanical properties comparable to general textile fibers. The advantages of Nomex are excellent thermal stability, flame resistance, and good dielectric properties. It is used in industrial apparel, aircraft upholstery, composites, and electrical insulation subject to high temperature. Other linear polymers, such as poly-m-phenylene benzimidazole and aromatic polyimides, can be used to make thermally resistant fibers. Flame-retardant fibers, with the FTC designation noaloid, are made from cross-linked phenol–formaldehyde polymers. 6.6 Elastomeric Fibers Elastic threads can be made from natural or synthetic rubbers, but it is not possible to make good fine fibers. 15
Textile Fibers: A Comparative Overview Spandex fibers, such as Lycra, are made from segmented polyurethanes from the reaction of a diisocyanate with polyethers or polyesters. Amorphous soft segments give the elastic extension, and crystalline hard segments cross-link the structure and hold it together. The fibers are relatively weak, though the true stress at break is high, and have elastic extensions over 400%. They are resistant to physical and chemical damage in use, and are incorporated in fabrics when high stretch is required.
expensive, account for about 0n5%. The diversity is mainly achieved by form, shape, size, process parameters, copolymerization, additives, and finishes. See also: Carbon Fibers; Cellulose : Chemistry and Technology; Ceramic Fibers from Polymer Precursors; Fibrous Reinforcements for Composites : Overview; CVD Monofilaments; Glass Fibers; Paper : History of Development; Metallic Filaments; Glass Optical Fibers; Silk Produced by Engineered Bacteria; Spun (Slurry and Sol–Gel) Ceramic Fibers; Whiskers; Pulp and Paper : Wood Sources
6.7 Fibers with Special Functions There are a variety of fibers used in specialized applications, because of particular chemical and physical properties. Teflon, polytetrafluorethylene (PTFE), which cannot be melted or dissolved, is spun into fibers by heat sintering a dispersion of particles. The fibers are used where low friction or high chemical resistance are needed. In addition to the use of Teflon and polyesters in arterial grafts, several new fibers are being used for medical purposes. For example, fibers of polyglycolic acid are used for sutures, because they are assimilated by the body after the limited period for which they are needed to function. A great variety of smart fibers are now being explored, and some are in commercial use. They are mostly made by incorporating additives. By way of example, the range seen on a visit to one Japanese company in 1993 comprised: exothermic, electrically conducting yarns, which can be incorporated as resistance elements to control temperature in fabrics; antistatic fibers for clean rooms; polypeptides that absorb and disperse water; fibers that melt and bind; filaments that vary in thickness to match natural nonuniformity; fibers with zirconium carbide that warm by absorbing near infrared and emitting farinfrared radiation; chitin fibers for wound dressing; activated carbon fibers to absorb and desorb chemicals; amorphous metal fibers with high strength and special magnetic properties; and glass fibers with special coatings for printed circuits.
7. Conclusion Textile fibers exist in great variety and are used for many applications in a diversity of structures. Much more could be written on their place in materials science and technology than can be included in this article. However, in another sense, the materials are limited. Table 2 shows that about 80% of the world’s textile fibers are now based on cellulose or polyester, 18% on nylon, polyacrylonitrile and polypropylene, and 2% on proteins. Some vinyl polymers, which have not achieved wide market acceptance, and the newer high-performance and specialty fibers, which are 16
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Textile Fibers: A Comparative Overview
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