On the basis of origin, the textile raw materials are classified into two main categories as shown in Fig. 1.
Natural fibers are those provided by Nature in ready-made form and need only to be extracted On the other hand, man-made fibers are generated by humans from the things which were not in fiber form previously .
2 Natural Fibers
Natural fibers are divided into three main classes according to the nature of source (origin), i. e. vegetable fibers, animal fibers, and mineral fibers as shown in Fig. 2. Natural fibers such as hemp, kenaf, jute, sisal, banana, flax, oil palm, etc. have been in considerable demand in recent years due to their eco-friendly and renewable nature. In addition, the natural fibers have low density, better mechanical and thermal properties and are biodegradable.
Vegetable fibers include the most important of the entire textile fibers “cotton” together with flax, hemp, jute, sisal and other fibers which are produced by plants. They are cellulose based, the material used by nature as structural material in the plant world. They can be collected from different parts of plants and are hence classified on the basis of their source of collection from the plant. Animal fibers include wool and other hair-like fibers and also fibers such as silk, produced by silk worms. These animal fibers are protein based, the complex material which most of animal body is made of. Mineral fibers are of less importance in the textile trade. Asbestos is the most useful fiber of this class. The outstanding property of asbestos fiber is its resistance to heat and burning. They are also highly resistant to acids, alkalis, and other chemicals. These fibers are used to make special fire-proof and industrial fabrics.
2.1 Cotton Fibers
Cotton is a soft, staple fiber that grows in a protective capsule known as boll around the seeds of cotton plant. The fiber is spun into yarn and used to make a soft, breathable textile, which is the most widely used form of textile for clothing. The earliest evidence of using cotton is from India and the date assigned to this fabric is 3000 B. C. There were also excavations of cotton fabrics of comparable age in Southern America. Cotton cultivation first spread from India to Egypt, China, and the South Pacific. Even though cotton fiber had been known already in Southern America, the large-scale cotton cultivation in Northern America began in the 16th century with the arrival of colonists to southern parts of today’s United States. The largest rise in cotton production is connected with the invention of saw-tooth cotton gin by Eli Whitney in 1793. With this new technology, it was possible to produce more cotton fiber, resulting in big changes in the spinning and weaving industry.
Cotton picking by hand is still practical in nearly all countries. The common practice in hand picking is to pick the seed cotton and the boll and put it into a sack. An experienced adult can pick 300 lbs. of seed cotton per day under normal conditions. In case of automatic picking, two type of pickers are used, namely stripper and spindle picker. Hand picking is advantageous to machine picking, as fibers are picked only from the completely mature capsules. After picking, the cotton is taken to the ginning factory where fibers are separated from the seeds. The beaters used for ginning are either saw gin or roller gin. Saw gin is more economical due to advanced automation and mechanization. The seeds separated are used to extract oil for edibles or soap / candle manufacture. A pound of seed cotton can be obtained from 50–100 bolls depending on the nature of plant and condition under which it is grown.
The classification of cotton is done on the bases of fineness, staple length, maturity, degree of contamination, and strength. The fineness of fiber is denoted in dtex, i.e. number of grams per 1000 meter. The fineness of cotton fiber is most commonly expressed in terms of micronaire value, i. e. number of microgram per inch. The staple length of fiber determines the fineness of yarn. More the length of cotton fiber, finer is the yarn produced. Immature and weak cotton has a cloudy appearance, while mature cotton appears bright and has a thick cell wall. These parameters describe that fibers should be sufficiently flexible to accommodate the continuous rearrangement in spinning; have a high length to diameter ratio, permit effective consolidation and inter-fiber coherence.
Under a light microscope, cotton fibers are recognized by the presence of the lumen and convolutions, i. e. twists along the length of the fiber. Another unique feature of cotton fibers is the reversal in the direction of the spiral (fibril) structure or helix along the length of the fiber. The important characteristics of cotton fiber are given in Table 1.
|1–4 dtex/2.3–6.9 micronaire
Cotton fiber turns yellow at temperatures above 110 °C. It is not easily damaged by sunlight; gradual loss of strength occurs on longer exposure to sunlight. Being cellulosic in nature, it dissolves in the concentrated solution of acids, but has excellent resistant to alkalis. A strong caustic solution causes the fibers to swell. Fungus and bacteria attack and degrade cotton. It serves as food for fungus. It contains mineral nutrients (salts of Na, K, Mg, and Ca) and starch which promote growth of fungus and mildew. Mildew grows in high relative humidity. Bacteria and fungus discharge enzymes which attack the cellulose and convert it to sugar. For protection, cotton is treated with materials which either inhibit the growth or kill these microorganisms.
Blending is the process of mixing different fibers into a single yarn, generally to improve the strength of yarn and give it the desired properties. Cotton blends easily with other fibers; mostly with the polyester and viscose. Its strength and absorbency make it an ideal fabric for medical and personal hygiene products such as bandages and swabs. It has low thermal conductivity, and is, therefore, ideal material for both summer and winter clothing. In summer, it prevents the skin from heat, and in winter it preserves the warmth of body. Cotton is often used in the manufacture of curtains, tents, and tarpaulins. It is also preferred widely for apparel including blouses, shirts, dresses, children’s wear, active wear, separates, swimwear, suits, jackets, skirts, pants, sweaters, hosiery, and neckwear. Home fashion articles of cotton are curtains, draperies, bedspreads, sheets, towels, table cloths, table mats, and napkins. Some of the industrial applications of this fiber include ropes, bags, shoes, conveyor belts, filter cloth, medical supplies, etc.
Flax is probably the oldest textile fiber known to mankind. The fiber is obtained from the stem of the plant Linum usitatissimum which is 80–120 cm high. The flax woven fabric is also called linen. The first well-documented application of linen fabric was by the Egyptians to wrap their mummies. It was also found in graves in Egypt dating from before 5000 B. C. But even long before that time flax was used for various applications. At excavation sites of Stone Age dwellings in Switzerland, dated at approximately 7000 B. C., flax seeds, twines, and fishing nets were found. The flax plant is thought to have arrived in Europe with the first farmers, and in the Stone Age people were usually dressed in linen clothes . In the Netherlands the cultivation of flax has presumably existed continuously ever since 2500 B. C.
Presently, two types of flax plant are grown, for fibers and for seeds (oil). Fiber flax is optimized for the production of thin strong fibers, while seed flax gives far more linseed and coarser fibers. The cross breeding these two extreme types had resulted in the cultivation of oil–fiber linen named as combination linen. Flax grows in moderate climates and is presently cultivated in large parts of Western and Eastern Europe, Canada, USA, and Russia. The fiber flax grows in humid, moderate areas, while oil flax grows in dry, warm areas. The characteristics of flax differ depending on the sowing and growing conditions; affecting stem length, thickness, and the number of branching.
The harvesting of flax plant is done by pulling the stalk either by hand or using a mechanical puller. Sometimes the plants are cut close to the ground, but pulling is preferred in order to retain the longest fiber length. The flax stalk bundles are then allowed to dry. Rippling is the next process, resulting in the removal of flower heads and leaves from the stem. Next, the plants are spread over the ground for retting. It is the process by which the pectin layer, holding the fiber bundles together in stem, is broken down by the combined action of bacteria and moisture.
The types of retting commonly employed for flax are water retting, enzyme retting, and dew retting. In water retting, the bundles of flax stems are immersed in the running water (rivers) or standing water (ponds or specially prepared pits). The anaerobic bacteria cause fermentation, thus degrading pectin and other binder substances. The enzyme retting employs the use of warm water and enzymes to degrade pectin. This method was developed for the production of very fine fibers. It is a controlled but rather laborious process. In dew retting, the flax stems are spread over the field. The humidity in environment causes the growth of indigenous aerobic fungi which partly degrade the stem. It is an inexpensive process, taking about three to seven weeks depending on the weather conditions.
The flax bundles are then dried, and by now the fibers must have loosened from the stem. The stem is broken by passing the bundles between the fluted rollers. The broken stem parts are then removed from the fiber bundles in the scutching process. The scutching machine consists of two interpenetrating rollers equipped with three or more knives. The knives scrape along the fiber to remove the wooden stem. The scutched fiber bundles are still relatively coarse, thick and ribbon-shaped. After scutching, the fibers are combed (hackling process), producing a thinner fiber with circular fiber structure. The elementary fiber contains about 65–75 % cellulose, 15 % hemicellulose and 10-15 % pectin, along with 2-5 % of waxes. Other properties of flax fiber are given in Table 2.
The flax fiber does not provoke allergies, absorbs humidity, and allows the skin to breathe; therefore, it is preferred in the manufacture of summer articles. It can be washed many times without alteration; rather it becomes softer, something very important for articles of clothing and for daily use which requires frequent washing such as shirts. Linen has very low elasticity and the cloths do not deform. It has vast uses such as tableware, suiting, clothing apparel, surgical thread, sewing thread, decorative fabrics, bed linen, kitchen towels, high quality papers, handkerchief linen, shirting, upholstery, draperies, wall coverings, artist’s canvases, luggage fabrics, paneling, insulation, filtration, fabrics for light aviation use, automotive end uses, and reinforced plastics and composite materials, etc. The ability of flax fiber to absorb water rapidly is particularly useful in the towel trade.
Jute is known as the “golden fiber” due to its golden brown color and its importance. Jute belongs to bast fiber category and is normally spun in the form of coarse threads. Contrary to most vegetable fibers which consist mainly of cellulose, jute fibers are part cellulose and part lignin. Jute fiber offers strength, low cost, high durability, and versatility. It is a cheap natural fiber having variety of end uses, for example to make hessian sacks, garden twine, ropes, and carpets. It is most popular in the agriculture sector to control soil erosion, seed protection, and weed control. It is used for technical applications in the area of geotextiles. Jute is being replaced by synthetic materials for many of these uses, but the biodegradation and sustainability are the main advantages of jute over synthetic fibers.
The jute plant becomes ready for harvesting in 120 days, and is either pulled by hand or cut by sharp edge. These stems are then tied into bundles for retting. The process of retting involves immersion of the stem in water until the bacterial action makes it possible to release the fibers within the stalk. It takes about 12–25 days for completion of retting. Stripping means removal of jute fibers from stem. The most common method is manual stripping, performed by beating the bark gently with a wooden mallet, starting from stem base. The fibers are then separated and dried. The properties of jute fiber are given in Table 3.
|30–34 cN / tex
2.4 Other Vegetable Fibers
Other commonly used vegetable fibers include coir, oil palm, pineapple, banana, hemp, ramie, sisal, etc. Coir is a coarse, short fiber extracted from the outer shell of coconuts. Its low decomposition rate is a key advantage for making durable geotextiles. Coir fibers measure up to 35 cm in length with a diameter of 12–25 microns. Among vegetable fibers, coir has one of the highest concentrations of lignin, making it a stronger fiber . Brown coir is used in sacking, brushes, doormats, rugs, mattresses, insulation panels, and packaging.
Oil palm is the highest yielding edible oil crop in the world. The trunk, frond and empty fruit bunch (EFB) of the oil palm tree can be used for the extraction of lignocellulosic fibers. The EFB has a potential to yield up to 73 % fibers and hence it is preferable in terms of availability and cost.
Sisal fiber is derived from the leaves of the sisal plant. It is usually obtained by machine decortications in which the leaf is crushed between rollers and then mechanically scraped. The fiber is then washed and dried by mechanical or natural means. The dried fiber represents only 4 % of the total weight of the leaf. Once it is dried the fiber is mechanically double brushed. The lustrous strands, usually creamy white, average from 80 to 120 cm in length and 0.2 to 0.4 mm in diameter. Sisal fiber is fairly coarse and inflexible. It is valued for cordage (ropes, baler, binder twines, etc.) use because of its strength and durability. The higher-grade fiber after treatment is converted into yarns and used by the carpet industry.
Banana plant not only gives the delicious fruit but also provides fiber for textile applications. The fiber is obtained after the fruit is harvested. The small pieces of banana plant trunk are put through a softening process for mechanical extraction of the fibers with subsequent bleaching and drying. The fiber obtained has appearance similar to silk which has become popular as banana silk fiber yarn. In the recent past, banana fiber had a very limited application for making items like ropes, mats, and some composite materials. With the increasing environmental awareness and importance of eco-friendly fabrics, it is finding applications in other fields such as apparels and home furnishings.
The stalk of the hemp plant produces two types of fibers: long (bast) fibers and short (core) fibers. Bast fibers can be cleaned, spun and then woven or knitted into many fabrics suitable for durable and comfortable clothing and housewares. Fabrics with at least 50% hemp content block the sun’s UV rays more effectively than the other fabrics. In comparison with cotton, hemp fibers are longer, stronger, more lustrous, absorbent, and more mildew resistant. Hemp textiles are extremely versatile – they are used in the production of clothing, shoes, apparel, canvas, rugs, and upholstery.
Ramie is one of the oldest vegetable fibers; used for mummy cloths in Egypt during 5000–3000 B. C. It belongs to the category of bast fibers and need chemical treatment to remove the gums and pectin found in the bark. The fiber is very fine like silk, and being naturally white in color does not need bleaching. Ramie is commonly used in clothing, tablecloths, napkins, and handkerchiefs. Outside the clothing industry, ramie is used in fish nets, canvas, upholstery fabrics, straw hats and fire hoses. Ramie is resistant to bacteria, mildew, alkalis, rotting, light, and insect attack. It is extremely absorbent and naturally stain resistant. On the other hand, it is low in elasticity, lacks resiliency, low abrasion resistance and wrinkles easily . A comparison of the properties of common natural fibers is provided in Table 4.
|1.5 g / cm3
|1.16 g / cm3
|1.46 g / cm3
|35–70 cN / tex
|40–70 cN / tex
|30–45 cN / tex
|12–18 cN / tex
Wool is an animal fiber obtained by shearing the fibrous covering of sheep and is produced in almost all parts of the world. Sheep are commonly shorn for their fleece once or twice a year and the raw wool obtained is known as fleece. An efficient shearer would remove the fleece from a sheep in 2 minutes. Wool is also removed from the pelts of slaughtered sheep by chemical treatment or bacterial action without damaging the hide. Raw wool is often dirty and contaminated with natural fats, grease and perspiration residues. All these impurities are removed during wool scouring and wool carbonising to get cleaned wool.
The breed of the sheep as well as the environmental conditions strongly affect the quality of wool. The wool also differs in fineness, length and purity depending on the body part of sheep from which it is taken. Wool may be broadly classified into fine wool, medium wool, long wool and carpet wool. Wool is usually spun into two types of yarn i. e. woolen and worsted. Woolen yarns are usually made from short staple fibers which are held loosely and given only a limited twist during spinning. Worsted yarns are much finer, regular, tightly twisted, and smoother than woolen. These are usually spun from longer staple fibers.
Wool fiber has a natural crimp due to its unique chemical and physical structure. This causes the fiber to bend and turn, giving wool an inherent three dimensional crimp . Because it is naturally elastic and resilient, wool imparts to all products that are made from it, many unique properties: rapid wrinkle recovery, durability, bulk, loft, warmth, and resistance to abrasion. Wool can easily absorb up to 30 % of its weight in moisture without feeling damp or clammy. Wool contains moisture in every fiber, it resists flame without chemical treatment. Instead of burning freely when touched by flame, wool chars and stops burning when it is removed from the source of the flame. Wool is self-extinguishing; it will not support combustion.
Silk is a protein fiber of insect origin, being produced as a fine filament of long length from the body fluid of silkworm (Bombyx Mori). The silkworms eat only the leaves of mulberry tree. Silk is a polypeptide, formed from four different amino acids. Silk fibers are relatively stiff and show good to excellent recovery from deformation depending on the temperature and humidity conditions. These fibers exhibit favorable heat-insulating properties but owing to their moderate electrical resistivity, they tend to build up static charge.
The four stages in the life cycle of a silkworm are egg, caterpillar, larva (cocoon), and butterfly. Caterpillars come out of the eggs after hatching for 12 days. During the growth period of the caterpillar, fresh mulberry leaves are its food. After 35 days, the caterpillars are ready for spinning silk. They stop eating and produce their cocoon in a few days. Silkworm makes its cocoon from a twin filament that extrudes from two silk glands in its head. These filaments are coated and glued together by gummy substance called sericin. The worm gradually gets covered and captivated in a strongly structured cocoon made from continuous silk strand (may be up to a mile in length). This filament silk is unwind from the cocoons. The properties of silk fiber are given in Table 5.
|1.37 g / cm3
|25–50 cN / tex
The fibroin of silk is decomposed by concentrated acids into the constituent amino acids. Silk is more resistant to alkalis and organic solvents, except hydrogen bond-breaking solvents. Continuous exposure of silk fiber to sunlight results in strength loss. It begins to yellow at high temperatures and disintegrates above 165 °C. The moisture absorption results in a temporary 10–25 % strength loss of silk fiber. It is easily attacked by moth and mildew.
3 Man-made Fibers
Man-made fibers are classified into synthetic and regenerated fibers as shown in Fig. 3. The polymers used for the spinning of synthetic fibers are chemical based, while regenerated fibers are derived from a natural polymer, most commonly cellulose.
3.1 Spinning of Man-made Fibers
There are three most common techniques employed in the production of man-made fibers namely wet spinning, melt spinning, and dry spinning as shown in Fig. 4. These techniques vary in the method of liquefying the raw material (powder or pellet). The term spinning here defines the extrusion process of liquefied polymer through spinnerets to solidify in a continuous flow. The melt spinning is a simple transformation of the physical state; however, it can be applied only to polymer having a melting temperature, e. g. PA 6, PA 6.6, PES, PP, etc. In melt spinning, the extruded polymer is transformed directly into a filament owing to its fast cooling, and cross-sectional form remains unchanged .
In case of solution spinning, the polymer is dissolved in variable concentrations according to the kind of polymer and of solvent to produce a viscous liquid (dope). It is used for the polymers that degrade thermally at a temperature lower than melting point (cellulosic fibers). The extruded filaments are subject to structural changes due to solvent extraction from the polymer mass. The solution spinning is further divided into type, dry spinning, and wet spinning. In dry spinning, the solvent is removed by flow of warm gas directed to the extruded filaments. The wet spinning method involves introduction of extruded polymeric viscose into a coagulation bath, where water behaves as a solvent for the polymer solvent and as a non-solvent for the polymer mass.
4 Regenerated Fibers
The basic element of a cellulose macromolecule is glucose. The empirical formula of cellulose is (C6H10O5)n, where n represents the number of glucose molecules constituting the cellulose macromolecule and is called the degree of polymerization (DP). The a-cellulose (insoluble in cold dilute NaOH) has a DP greater than 200, while ß-cellulose (hemicelluloses soluble in cold dilute NaOH) has DP less than 200. Wood pulp is used as the raw material for cellulose, and is refined to increase the percentage of a-cellulose. The percentage of up to 99 % can be obtained depending on the cleaning method. The regenerated fibers are produced according to the viscose spinning method.
4.1 Viscose Fiber
The production of viscose fiber involves the process of solution spinning. The viscose solution for spinning is prepared by treating cellulose with NaOH producing alkali cellulose. This alkali cellulose reacts with carbon disulphide to give cellulose xanthate, which on dissolution in NaOH gives viscose solution. This solution is extruded from the spinneret into the spinning bath. The composition of spinning bath, its temperature, and spinning speed are adjusted according to the type of fiber to be spun. The solidification into yarn takes place in the spinning bath. These spun fibers are drawn to achieve a regular orientation of the chain molecules.
The spun fibers are further treated to remove impurities, increase brightness, and improve adhesive and frictional properties. These treatments may include washing, bleaching, application of some finish, etc. The spinning process parameters allow the manipulation of properties to a wider level, producing certain fibers like high tenacity viscose, highly crimped viscose, hollow fibers, modal, etc. By the addition of suitable chemicals in viscose solution or spinning bath, spin dyed, flame retardant fibers, etc. can be produced.
4.2 Acetate Fiber
The raw material for the production of acetate fibers is also cellulose, but they are composed of cellulose ester. The cellulose is mixed with acetic anhydride and glacial acetic acid under addition of wet splitting chemicals. The spinning is carried out according to the dry spinning technique. The spinning solution is transported to the spinning pump and extruded through spinneret. The filaments exiting the spinneret are passed through stream of warm air in the quench duct which leads to evaporation of solvent acetone and alcohol and freezing of filaments. During the passage, the filaments are drawn, combined, oiled, and wound onto bobbins. The properties of common regenerated fibers are given in Table 6.
|15–30 cN / tex
|16–25 cN / tex
|20–40 cN / tex
|1.52 g / cm3
|1.52 g / cm3
|1.29–1.33 g / cm3
|8–12 cN / tex
|40–60 cN / tex
|8 cN / tex
5 Synthetic Fibers
The synthetic fibers are result of the extensive research to improve the properties of naturally occurring animal and vegetable fibers. These synthetic fibers are produced by the extrusion of a polymeric material having synthetic origin through spinneret into air or water. This fiber forming polymers are obtained generally from petro chemicals. Therefore, these fibers are called synthetic fibers.
Nylon 6.6 was the first synthetic fiber produced in 1935 in USA. A parallel development in Germany led to the production of continuous filament (Nylon 6) in 1939 . Nylon fibers are made up of linear macromolecules whose structural units are linked by the amide (–NH–CO–) group. Therefore, these fibers are termed as the polyamides. The most common way for the production of nylon polymers is by the condensation of diamines with diacids. In nylon 6 the molecules are directional with all of the amide links in a particular direction, e. g. –NH–CO–, while in nylon 6.6 there is a reversal in the order of alternate amide linkages.
–NH–CH2–CH2–CH2–CH2–CH2–CH2–NH –OC– CH2–CH2–CH2–CH2–CH2–CO–
The nylon fibers are produced by the extrusion of molten polymer and no solvents are involved. The polymer chips are melted by a heated grid, or by an extruder where a screw forces the chips along a heated tube. The molten polymer is fed to a controlled metering pump which helps to control the linear density of the fiber. The molten polymer is fed to the spinneret at a temperature of 280–300oC and against a pressure as high as 50–70 MPa. The spinneret is a stainless steel plate, 5 mm or more in thickness, having a number of small holes (100–400 μm in diameter). The number of holes corresponds to the number of filaments required in the final yarn. The polymer emerging from the spinneret holes is drawn down by a take up reel and undergoes a considerable acceleration. The accelerated filaments solidify in the cool air. The properties of nylon fiber are given in Table 7.
|20–35 cN / tex
|15–35 cN / tex
|40–60 cN / tex
|40–60 cN / tex
At a temperature of 21 °C with an R. H. of 65 %, nylon 6.6 or 6 contains 3.5–4.5 % of water by weight as a proportion of the mass of dry fiber. Exposure of nylon 6.6 and 6 to air at temperatures above 100 °C results in a loss of both tenacity and breaking elongation. Exposure to sunlight and other sources of UV radiation also leads to deterioration in the properties of nylons. Nylons are only slowly affected by water at the boiling point. Nylon 6.6 is inert to alkali solution while sensitive to acids. Nylon 6 and 6.6 are inert to most common organic solvents, but they do dissolve in concentrated formic acid and phenols.
Polyethylene terephthalate (PET), also called polyester fiber dominates the world synthetic fibers industry. They are inexpensive, easily produced from petrochemical sources, and have a desirable range of physical properties. They are strong, lightweight, and wrinkle-resistant, having good wash–wear properties. Polyester, produced by the condensation polymerization of a dicarboxylic acid with a diol, contains in-chain ester units as their essential polymer-forming chain linkage .
Alcohol (R–OH) + Carboxylic acid (R–COOH) → Ester
where R is an alkyl group.
The filament polyester fiber is produced by melt spinning under different conditions. The polymer is melted at a temperature some 15–25 °C above its melting temperature in a screw-extruder. The polymer melt is precisely metered by a positive displacement gear pump which delivers a fixed amount of polymer melt per revolution. The molten polymer is forced through tiny holes (0.180–0.400 mm) in the spinneret plate. The polymer solidifies as it emerges from the spinneret. The cooling process is accelerated by controlled flow of air. The polymer is also drawn down, i. e. stretched in semi-molten state to induce molecular order and orientation in the fiber. The properties of polyester are given in Table 8.
|800–1000 cN / tex
|40–60 cN / tex
|1.22–1.38 g / cm3
5.3 Other Synthetic Fibers
The most common among other synthetic fibers are polyolefins, acrylics, and elastane. Olefin fiber is a manufactured fiber. In these fibers, the fiber-forming substance is any long-chain synthetic polymer composed of at least 85 % by weight of ethylene, propylene, or other olefin monomers. Olefin fiber is a generic description that covers thermoplastic fibers derived from olefins. Polypropylene (PP) and polyethylene (PE) are the two most common members of the family. The polypropylene yields greatest volume of fiber for a given weight, because of its low specific gravity i.e. 0.90–0.91 g/cm3. Polypropylene does not exhibit a static behavior in normal circumstance. Like other synthetic fibers i. e. nylon, acrylic, and polyester – polypropylene fibers are not attacked by bacteria or micro-organisms; they are also moth-proof and rot-proof and are inherently resistant to the growth of mildew and mold. Polypropylene is hydrophobic and will not absorb water in the fiber. A comparison of the properties is given in the Table 9.
|32–65 cN / tex
|15–60 cN / tex
|4–12 cN / tex
|0.95–0.96 g / cm3
|0.9 g / cm3
|1.1–1.3 g / cm3
|15–30 cN / tex
|13–15 cN / tex
|0.05–0.1 cN / tex
5.4 Glass Fiber
Glass is a non-metallic fiber, widely used as industrial material these days. Generally the glass state is defined as the frozen state of a super cooled and thus solidified liquid. The basic raw materials for glass fiber include a variety of natural minerals and manufactured chemicals. The major ingredients are silica sand, limestone, and soda ash. Silica sand is used as the glass former, while soda ash and limestone help to lower the melting temperature. A low coefficient of thermal expansion combined with low thermal conductivity properties make glass fiber a dimensionally stable material that rapidly dissipates heat as compared to asbestos and organic fibers.
They are produced by direct melting, involving processes of batching, melting, spinning, coating, drying, and packaging. Batching is the initial state of glass manufacture, material quantities are thoroughly mixed at this stage. The mixture is then taken to furnace at a high temperature of 1400 °C for melting. This temperature is high enough to convert the sand and other ingredients into molten state. The molten glass then flows into the refiner, where its temperature is reduced to 1370 °C. Spinning of glass fiber involves a combination of extrusion and attenuation. During this process the molten glass passes out through a bushing with very fine orifices. Bushing plates are heated electronically, and their temperature is controlled to maintain a constant viscosity. Water jets are used to cool the filaments as they exit the bushing at roughly 1204 °C.
Attenuation is the process of mechanically drawing the extruded streams of molten glass into filaments, with a diameter ranging from 4 μm to 34 μm. A highspeed winder is used to provide tension, and draw the molten stream into thin filaments. In the final stage, a chemical coating of lubricants, binders and/or coupling agents is applied to the filaments. The lubrication will help to protect the filaments from abrasion when collected and wound into packages. The packages, still wet from water cooling and sizing, are then dried in an oven. Afterwards, the filaments are ready for further processing into chopped fiber, roving, or yarn.
It is an inorganic material and does not burn or support combustion, retaining approximately 25 % of its initial strength at 540 °C. Most chemicals have little or no effect on glass fiber. The inorganic glass textile fibers will not mildew or deteriorate. Glass fibers are affected by hydrofluoric, hot phosphoric acids and strong alkaline substances. It is an excellent material for electrical insulation. The combination of properties such as low moisture absorption, high strength, heat resistance and low dielectric constant makes fiber glass fabrics ideal as reinforcement for printed circuit boards and insulating varnishes.
The high strength-to-weight ratio of glass fiber makes it a superior material in applications where high strength and minimum weight are required. In textile form, this strength can be unidirectional or bidirectional, allowing flexibility in design and cost. It is extensively used in automotive market, civil construction, sporting goods, aviation and aerospace, boats and marine, electronics, home and wind energy. They are also used in the manufacture of structural composites, printed circuit boards and a wide range of special-purpose products.
5.5 Carbon Fiber
A carbon fiber is a long, thin strand about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the axis of the fiber. This crystal alignment makes the fiber incredibly strong. Several thousand carbon fibers are joined together to form a yarn. Carbon fibers were developed in the 1950s, by heating strands of rayon until they carbonized. This process was inefficient, as the resulting fibers contained only about 20 % carbon and had low strength and stiffness properties. In the early 1960s, a process was developed using PAN as a raw material. This produced a carbon fiber that contained about 55 % carbon and had much better properties.
During the 1970s, experimental work to find alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch derived from oil processing. These fibers contained about 85 % carbon and had excellent flexural strength. But they had limited compression strength and were not widely accepted. About 90 % of the carbon fibers produced are made from PAN. The remaining 10 % are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. A typical process used to form carbon fibers from PAN includes spinning, stabilization, carbonizing, surface treating, and sizing.
In the spinning process, acrylonitrile powder is mixed with another material like methyl acrylate or methyl methacrylate, and is reacted with a catalyst in a suspension to form PAN plastic. It is then either spun into fibers via coagulation, or heated and pumped through tiny jets into a chamber where the solvents evaporate producing a solid fiber. The fibers are then washed and stretched to the desired fiber diameter. These fibers are stabilized by heating in the presence of air to about 200–300 °C for 30–120 minutes. The fibers pick up oxygen from the air and rearrange the atomic bonding pattern, resulting into a thermally stable ladder bonding.
The stabilized fibers are heated to a temperature of 1000–3000 °C for several minutes in a furnace filled with a gas mixture, other than oxygen. The lack of oxygen prevents the fibers from burning at very high temperature. Heated fibers lose their non-carbon atoms in the form of various gases like water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. The remaining carbon atoms form tightly bonded carbon crystals that are aligned parallel to the long axis of the fiber. The surface treatment of these fibers is performed to improve the fiber bonding properties. In sizing process the fibers are coated to protect them from damage during winding or weaving. Typical coating materials include epoxy, polyester, nylon, urethane, and others. The coated fibers are wound onto cylinders called bobbins.
The carbon fibers are an important part of many products, and new applications are being developed every year. Carbon fiber-reinforced composite materials are used in the automotive and aerospace industry, sports and many other components where light weight and high strength are needed. Carbon fibers have high electric conductivity (volumetric impedance) and at the same time have excellent EMI shielding property. This successfully brings CFRP (Carbon fiber reinforced plastics) to the field of EMI shielding. Carbon fibers have low heat expansion ratio and high dimensional stability, and sustains its mechanical performances even under high temperature region. CFRP is superior to steel or glass fiber reinforced plastics (GFRP) in its specific tensile strength and specific elastic modulus (specific rigidity). Fatigue resistance of Carbon fiber surpasses that of other structural material.
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