Introduction
From aircraft structures and protective apparel to filters, insulation, and decorative fabrics, high-performance and specialty fibers support products that are often taken for granted. These fibers are selected when strength, stiffness, heat resistance, chemical resistance, or a special visual effect justifies the added cost. Their main applications are found in industry, building and construction, law enforcement, the military, transportation, recreational equipment, and geotextiles. As these segments of the textile industry grow, more graduates of textile programs are likely to find work in the management, manufacture, and distribution of these materials.
High-Strength, High-Modulus Fibers
Much of the development and interest in high-performance fibers has focused on those with both high tenacity and high modulus. They are very strong and usually very stiff, with strong resistance to bending and stretching. Some of these fibers are also very thermally stable, so they are used in heat-resistant products as well.
Carbon Fibers
Carbon fibers are technically produced from precursors such as polyacrylonitrile or rayon, or other organic base fibers. They are characterized by extremely high modulus and tensile strength, depending on the temperature used in the graphitization stage. These fibers are extensively used in industrial applications such as aircraft structures and space applications. Because of their high-performance structure, carbon fibers are used as reinforcements embedded in suitable resins such as epoxide, polyester, phenolic, polyphenylene, and polyimide.
There are two main methods for the industrial production of carbon fibers. One is based on polyacrylonitrile, with polymerization, wet spinning, drawing, oxidation, carbonization, and graphite annealing. The other is based on mesophase pitch, with thermal treatment, melt spinning, oxidation, carbonization, and graphitization annealing. The carbonization and graphitization annealing serve as surface treatments, and the graphitization step is done at temperatures of up to 3000°C. The main objective of both methods is to arrange graphite layers in the fiber direction. For carbon fibers, the carbon content is supposed to be at least 90 percent.
Carbon fibers can also be produced from viscose fibers. This process has little market relevance at present and is mainly used to manufacture textiles for medical applications.
Among the exceptional properties of carbon fibers are high tenacity, high modulus of elasticity, high brittleness, low creeping tendency, chemically inert behavior, low thermal expansion, and good electrical conductivity. The carbon layers are responsible for the high tenacity and E-modulus, and the free electrons between the layers are responsible for the excellent electric conductivity.
For various applications, carbon fiber staple yarns are processed into woven and noncrimp fabrics, which are used for building components and as prepregs. Textile structures of carbon filaments are used in composite plastics and with fiber-reinforced concrete. Short fibers down to 6 mm in length are also used.
Glass Fiber
Glass is an inorganic nonmetallic material. Generally, the glass state is defined as the frozen state of a supercooled and solidified liquid. It results from suppression of the crystallization of a melt. It is also known as fiberglass, a material made from extremely fine fibers of glass. Glass fiber is formed when thin strands of silica-based or other formulation glass are extruded into many fine fibers by textile processing.
The glass is basically an alkali calcium silicate. Common glass is lime-soda glass, produced by fusion of silicon dioxide, calcium oxide, and sodium carbonate. Soda ash can be replaced by potash, and the glass produced this way is called potash glass. Silicate glass is composed of a network of SiO2 and silicate. Depending on composition, fiber diameter, and additional components, A-glass, C-glass, D-glass, E-glass, ECR-glass, R-glass, S-glass, and some other special types are distinguished. A network modifier is a technical term for an atom that modifies the glass network and changes the glass properties. The basis of textile-grade glass fibers is silica, SiO2. In its pure form, it exists as a polymer, (SiO2)n. To induce crystallization, it must be heated to higher temperatures.
The fiber is manufactured by drawing molten glass. At present, drag spinning is most often used, with a market share of about 90 percent for all glass fibers. The glass melt is kept at a temperature of about 1250 to 1350°C. Due to gravitational forces, the glass exits through the spinneret, which is perforated with holes of about 1 to 2 mm in diameter. The number of capillaries varies between 400 and 2400. After exiting the spinneret, the glass filaments are drafted mechanically and continuously at high speed. The extremely high pull-off velocity makes an additional cooling device necessary, so cooling fins are arranged between the filaments on the lower side of the jet. After the cooling phase, an aqueous liquid or oil is applied to the glass filaments. This step is called sizing. Then the filaments are wound onto a cone.
The fiber is also spun from manufactured glass pellets. Pellets are melted in a spinning bath and drawn through spinnerets at a speed of 2000 to 3000 m/min or more. The drawn filaments are immediately cooled by a suitable method, and the fiber attains a solid state. Further drawing is not possible. The finer the fiber, the faster the cooling and formation of the solid fiber. A frozen-in induced orientation birefringence exists in glass fibers, which is difficult to determine. Structural changes in the fiber are caused by ageing or reheating. Structural changes take place, and at the same time density is increased along with contraction. The exceptional structure of the fiber is one reason for the higher strength of the fiber when compared to solid glass.
The first type of glass used for fiber was soda-lime glass. By trapping air within them, blocks of glass fiber make a good thermal insulation material, with a thermal conductivity of 0.05 W/m. Because glass has an amorphous structure, its properties are the same along the length. Humidity is an important factor in tensile strength. If adsorbed, humidity can worsen microscopic cracks and defects and lessen tenacity. It has no effect on exposure to sunlight even after extended periods. In commercial use, fiber diameters are important criteria along with cost. For general reinforcing purposes, fiber diameters of 9 to 11 μm are required, but due to cost, diameters of 13 to 15 μm are also sometimes used. Glass fibers find application in chemistry, chemical technology, insulation of machines, pipes and containers, sound insulation, electronics, boat making, and similar uses. Fabrics made with glass fibers are used in interior furnishings wherever high safety requirements apply, such as in ships, hotels, cinemas, and wall coverings.
Polyphenylene Benzobisoxazole Fibers
PBO fibers are recently developed fibers composed of the polymer polyphenylene benzobisoxazole. Like the aramids, PBO is a rigid rod polymer that is extremely stiff and forms highly ordered crystalline structures. PBO is synthesized from benzene dihydrochloride and terephthalic acid, the acid also used in the manufacture of polyester. It is spun from a strong acid solution through an air gap into a water bath. Zylon® PBO fibers are manufactured by the Toyobo Company in Japan.
Because of its rigid structure, PBO exhibits very high strength and modulus, as well as excellent thermal stability. Its tenacity and modulus are the highest of the present-day commercial high-performance fibers. PBO does not melt and will not decompose below temperatures of 1,200°F. PBO fabrics are light and flexible but are not easily dyed, and they are a gold color as produced. The fibers are appropriate for many industrial end uses such as high-performance tires, high-temperature devices, sporting goods, ropes, and cables. At the same time, the heat resistance and light weight of PBO make it a good choice for protective clothing for firefighters, industrial workers, and race car drivers.
Ceramic Fibers
Composed of aluminum or other oxides, ceramic fibers have lower tensile strength than many inorganic fibers but higher compressive strength. This makes them useful as reinforcing materials in aircraft, space shuttles, and submarines, where high pressures are encountered.
Boron Fibers
Made by depositing boron vapor onto fine tungsten wire, these fibers have uses in reinforcement, especially of aluminum. Cloth made in part from boron fibers was used in the drill stems for collecting lunar rock samples during NASA’s moon missions.
Heat-Resistant Fibers
Some fibers, natural and manufactured, organic and inorganic, have extremely high or no melting points. They thus provide high thermal stability or resistance to heat. Several of the fibers discussed as high tenacity and high modulus are also very heat resistant. The primary end uses of the fibers described below are determined by their thermal stability.
Asbestos
Asbestos is no longer being made into textile products, but it is of historical interest both because it is the only natural mineral fiber and because it still remains as an insulating material in many buildings. Because it is naturally fireproof, it was used alone or in blends to make textile products in which fire resistance was important, for example, in insulating materials, protective clothing, fire curtains for theaters, and household products such as ironing board covers or pot holders. Continued inhalation of asbestos fiber can cause serious lung disease, and asbestos fiber is carcinogenic. For this reason, it is no longer used in textile applications in the United States, although it may still be used in other countries. Asbestos fiber insulation, which was placed in many commercial buildings in the 1950s and 1960s, must be removed if renovation or structural modification of these buildings occurs. This is usually expensive.
Polybenzimidazole
Polybenzimidazole, or PBI, is defined by the FTC as a manufactured fiber in which the fiber-forming substance is a long-chain aromatic polymer having recurring imidazole groups as an integral part of the polymer chain. PBI entered commercial production in May 1983. The fiber is manufactured by first making the PBI polymer and then dissolving it in dimethyl acetamide. The solution is dry spun into an oxygen-free atmosphere to evaporate the solvent.
PBI is nonflammable in air, emits little or no toxic gases or smoke up to temperatures of 1,040°F, and has excellent resistance to acids, organic solvents, and fuels. It has a low modulus and high moisture regain, making it more comfortable for apparel than some other thermally resistant fibers. The largest present end use of PBI is in civilian and military protective apparel. It is being used as a successful alternative to asbestos in protective gloves, gaskets, and packing materials in industrial applications and to construct fireblocking material in places such as aircraft. PBI is currently produced by Celanese Corporation in staple form.
Sulfar
Another heat-resistant fiber that was previously made was sulfar, defined by the FTC as a manufactured fiber in which the fiber-forming substance is a long synthetic polysulfide in which at least 85 percent of the sulfide linkages are attached directly to two aromatic rings. The fiber had tenacity, elongation, modulus, elastic recovery, and moisture regain that were satisfactory for textile applications. Chemical resistance was excellent.
Sulfar fibers were manufactured under the trademark name Ryton® by Amoco Fabrics and Fibers Company and intended for use in filtration and protective clothing. While it is currently not seen much in textile products, sulfar is still used as a heat-resistant plastic.
Melamine
A newer generic fiber is melamine, recognized by the FTC as a manufactured fiber in which the fiber-forming substance is a synthetic polymer composed of at least 50 percent by weight of a cross-linked melamine polymer. Unlike many synthetic manufactured fibers, melamine is not thermoplastic. Rather, it is thermosetting. This means that, once formed, the fibers do not deform when heat is imposed. The polymers are in a three-dimensional crosslinked network, similar to the material in melamine dinnerware and Formica countertops.
Because of the rigid structure, melamine fibers have low thermal conductivity and high heat resistance. They are weak, however, compared to fibers made from rigid-rod polymers and have a modulus similar to cotton. Melamine fibers are very resistant to chemical degradation. They have applications in protective clothing for firefighters and industrial workers, in airplane seating, upholstered furniture, and high-temperature filters, and they are cheaper than some other fibers for these products. Melamine is produced by BASF Corporation under the brand name Basofil®.
Novoloid
Novoloid is a manufactured fiber containing at least 85 percent by weight of a crosslinked novolac, a three-dimensional amorphous network of phenol-aldehyde. Because the polymer is composed of only carbon, oxygen, and hydrogen, the products of decomposition at high temperature are not hazardous. This is an attractive feature of novoloid fabrics. In addition, because the fiber has very low thermal conductivity, it protects from both heat and cold.
Currently manufactured by Gun-ei Chemical Industry in Japan under the brand name Kynol®, novoloid fibers are used in heat-resistant clothing and industrial felts and fabrics. They are also good precursors for carbon fibers, and novoloid fabrics can be converted to activated carbon for filtration.
Other Specialty Fibers
Metallic Fibers
Metallic fibers were the first manufactured fibers. Gold and silver threads have long been used to decorate costly garments, tapestries, carpets, and the like. These threads were made either by cutting thin sheets of metal into narrow strips and weaving these strips into decorative patterns or by winding a thin filament of metal around a central core of another material.
Examination of historic costumes and fabrics reveals the disadvantage of using metals in this way. Except for gold, the metals tended to tarnish and become discolored. Yarns were relatively weak and broke readily. Durability was limited, and, of course, the cost of precious metals was high. Fashion fabrics today make use of a variety of metallic fibers that are both inexpensive and decorative. Metallic fibers are also used in products for purely practical purposes. Many decorative metallic fibers are made in the form of large monofilaments.
Forming fibers from metal is difficult, therefore metal fibers are largely limited to those made from steel, aluminum, iron, nickel, and cobalt-based superalloys. These particular metals lend themselves most readily to fiber formation. The manufacture of fibers from metal is usually done by several methods. A rod of metal is used as the starting material, and from it fine-dimension metal wire is drawn to form fibers. A more common method begins with the formation of metal foil that is sheared or slit into fibers. Copper scrubs, commonly used in cleaning, demonstrate this method on a macro scale. The metal foil can be coated with a colored adhesive and transparent film for added color and durability of the split fibers. A commonly used and inexpensive decorative fiber is aluminum coated with polyester. In a more costly process, gold can be plated on aluminum-covered yarns of polyester or other fibers.
Most applications of metal fibers are for aerospace and other industrial applications where products such as filters, seals, abrasives, and insulation are used. The automotive industry uses metal fibers in tires and brake linings. There are limited applications of metal fibers in consumer products, particularly in certain household textile products, decorative apparel, military uniform decorations, and others. Superfine filaments of stainless steel and aluminum are made and added to fabrics in a number of ways. Metal fibers are used as the core of yarns, wrapped around other core yarns, or used alone. Steel, copper, and aluminum fibers are blended into some industrial carpets where they cut down on static electricity buildup and have the side effect of decreasing flammability. As the heat of a fire increases, the metallic fibers conduct some of the heat away from the fire, thereby decreasing the heat of the flame. This tends to slow the rate of burning or lower temperatures below the kindling point. Other projected end uses for metal fibers include upholstery, blankets, work clothing, and blends with polyester for hospital gowns.
Chlorofibers
Chlorofibers, made from polymers containing chlorine atoms, were among the first synthetic fibers produced in Europe and the United States. With the development of nylon, acrylic, and polyester, which have better overall properties, chlorofibers became limited to a few end uses based on the distinctive characteristics of each.
Saran
Saran is defined by the FTC as a manufactured fiber in which the fiber-forming substance is any long-chain synthetic polymer composed of at least 80 percent by weight of vinylidene chloride units. Introduced by Dow Chemical Company in 1941, saran is no longer made in fiber form in the United States. It is much more commonly seen as the plastic film used for the storage of food.
Saran is a stiff, nondrapable fiber with a number of specialized uses. It has a higher specific gravity than most other synthetics and has good strength and excellent elastic recovery and resiliency. The chlorine in the polymer structure, like modacrylic, imparts a level of flame resistance to the fibers. Saran fibers were used for a time in outdoor furniture and upholstery for public conveyances because of their chemical and flame resistance. Lighter-weight fibers such as acrylic and polypropylene have supplanted saran for some of these applications. Other uses are as diverse as filters, scouring pads, fishing nets, and doll hair. For the latter, the shiny character of the fibers is appealing. Saran is manufactured under the trade name Saran® by Asahi Kasei of Japan.
Vinyon
The FTC defines vinyon as a manufactured fiber in which the fiber-forming substance is any long-chain synthetic polymer composed of at least 85 percent by weight of vinyl chloride units. They are called polyvinyl chloride, or PVC, fibers in other countries.
The low melting point of vinyon has prevented its use to any extent in apparel in the United States but has made it useful as a heat-sensitive binder in nonwoven applications. One very familiar such product is tea bags. Vinyon fibers are also used in industrial products because of their high resistance to chemicals, and indeed, this property has determined the use of PVC polymer in plastic pipes. Rhovyl® vinyon fibers are manufactured in France and provided to customers for use in a wide variety of products including underwear, socks, bedding, filters, and flame-resistant wall coverings and drapes.
Fluoropolymer
Polymers containing fluorine, based on the substance polytetrafluoroethylene, have been used for some time in textile and other applications. The FTC has now approved the generic name of fluoropolymer for these fibers. The polymeric material may be formed into fibers by the emulsion spinning process or may be formed into sheets, extruded in molded form, or applied as a coating to other substances.
Fluoropolymer fibers have excellent chemical and flame resistance, are usable over a wide temperature range, resist abrasion, and are nonabsorbent. Strength is low to moderate, and melting points vary from low to high depending on the particular polymer. Industrial uses include pump and valve packing, gaskets, filtration materials, bearings, and office copy equipment. Fluoropolymer fibers are expensive and so are confined to products where the cost is justified. Consumers may know Teflon®, a fluorocarbon manufactured by DuPont, as a coating for cooking utensils. Teflon® is also the trade name for DuPont’s fluoropolymer fibers. Teflon® fluorocarbon membranes with very tiny openings, or pores, are used in making Gore-Tex® fabrics, which have wide use in outdoor apparel. Albany International produces several fluoropolymer fibers.
Vinal
The FTC defines vinal as a manufactured fiber in which the fiber-forming substance is any long-chain synthetic polymer composed of at least 50 percent by weight of vinyl alcohol units and in which the total of the vinyl alcohol units and any one or more of the various acetal units is at least 85 percent by weight of the fiber. In some countries vinal fibers are called polyvinyl alcohol, or PVA, fibers. Most of the development of vinal has taken place in Japan. The fiber has a melting point close to that of nylon 6, around 425°F, and extremely good chemical resistance, and is especially resistant to rot-producing microorganisms. The use of vinal fibers is largely in industrial applications, although some blends of vinal fiber with cotton, rayon, or silk have been used abroad.
In a preliminary step during manufacture, a soluble form of vinal fiber is created. Insoluble PVA fibers for regular use are formed by crosslinking with formaldehyde. The water-soluble form of vinal fibers has found specialty end uses. Soluble yarns of PVA may be used as support yarns in fabric constructions where open, sheer, or lacelike effects are desired but are not attainable through normal weaving processes. The fabric is woven then laundered, causing the soluble yarns to dissolve, leaving only the insoluble yarns in a lacy, open pattern. Narrow strips of lace can be separated from a wider web by including water-soluble PVA at intervals. Soluble yarns may also be used when socks are manufactured in a continuous string with a few rows of stitches between the toe of one sock and the top of the next. The socks are cut apart, and when subjected to finishing processes, the remaining soluble threads dissolve out, leaving a smoothly finished edge. Only hot water is required to dissolve the PVA fibers. Medical devices are another growing field for soluble PVA fibers. Examples are dissolving sutures and temporary tissue supports that allow the body to grow replacement ligaments and then disappear.
The Future of High-Performance and Specialty Fibers
Hearle alludes to three generations of manufactured fibers. The first generation comprised those commercialized during the first three-quarters of the twentieth century, those with which we are most familiar. Second-generation fibers, referred to as high-performance fibers and described in this chapter, appeared through the last quarter of the century. While there will continue to be improvements in the high-tenacity, high-modulus, and chemically resistant fibers, the new third generation will be the smart fibers. A good description of these is active systems that sense and react to environmental stimuli, such as those from mechanical, thermal, chemical, magnetic, or others.
Many of these new fibers and materials will be developed for military usage initially, and this is a significant source of funding for research. Development is driven also by the revolution in information technology. The first wave of these materials has sensors, or even electrical circuits, included in fabrics and garments, that behave as wearable computers.
As scientists further tackle the challenge of developing third-generation fibers, they are looking more closely at nature’s creatures that are sophisticated manufacturing machines. Examples are silkworms and spiders, which can synthesize and spin very complex fibers in one operation. They work at the nanoscale level, assembling polymer molecules with special features for protection and survival. Analytical instruments today allow scientists and engineers to work on this level too, incorporating the sensors and conductivity into the polymer. The electrical activity can not only transmit information, but can also induce activity in artificial muscles.
A mandate in developing third-generation fibers will most likely be environmental compatibility and sustainability. Mimicking natural fiber production will yield materials that are biodegradable. As protecting the environment becomes increasingly important, fibers made from recycled materials and environmentally friendly processes are likely to thrive.
Conclusion
High-performance and specialty fibers are chosen when demanding end uses require strength, thermal stability, chemical resistance, or a distinctive appearance. Carbon and glass fibers serve high-strength applications, while PBI, melamine, novoloid, and other heat-resistant fibers support protective and industrial products. Metallic fibers, chlorofibers, fluoropolymer fibers, and vinal each fill narrower but important roles in decoration, filtration, flame resistance, and soluble constructions. As the field moves ahead, smart fibers and more sustainable production methods are likely to shape the next stage of textile development.
