FIBERSBy definition (ASTM), a fiber has a length at least 100 times its diameter or width, and its length must be at least 0.2 in (0.5 cm). Length also determines whether a fiber is classified as staple or filament. Filaments are long and/or continuous fibers. Staple fibers are relatively short and, in practical applications, range from under 1 to 6 in (2.5 to 15.2 cm) long (except for rope, where the fibers can run to several feet). Of the natural fibers, only silk exists in filament form, while synthetics are produced as both staple and filaments.
The internal, microscopic structure of fibers is basically no different from that of other polymeric materials. Each fiber is composed of an aggregate of thousands of polymer molecules. However, in contrast to bulk plastic forms, the polymers in fibers are generally longer and aligned linearly, more or less parallel to the fiber axis. Thus fibers are generally more crystalline than bulk forms.
Also in contrast to bulk forms, fibers are used not alone, but either in assemblies or aggregates such as yarn or textiles or as a constituent with other materials, such as in composites. Also, compared with other materials, the properties and behavior of both fibers and textile forms are more critically dependent on their geometry. Hence fibers are sometimes characterized as tiny microscopic beams, and as such, their structural properties are dependent on such factors as cross-sectional area and shape, and length. The cross-sectional shape and diameter of fibers vary widely. Glass, nylon, Dynel, and Dacron, for example, are essentially circular. Some other synthetics are oval, while others are irregular and serrated round. Cotton fibers are round tubes, and silk is triangular.
Fiber diameters range from about 394 to 1,575 (iin (10 to 40 (xm) in diameter. Because of the irregular cross section of many fibers, it is common practice to specify diameter or cross-sectional area in terms of fineness, which is defined as a weight-to-length or linear density relationship. One exception is wool, which is graded in micrometers. The common measure of linear density is the denier, which is the weight in grams of a 29,530-ft (9,000-m) length of fiber. Another measure is the tex, which is defined in grams per kilometer. A millitex is in grams per 1,000 km.
Of course, the linear density, or denier, is also directly related to fiber density. This is expressed as the denier/density value, commonly referred to as denier per unit density, which represents the equivalent denier for a fiber with the same cross-sectional area and a density of 1.The cross-sectional diameter or area generally has a major influence on fiber and textile properties. It affects, for example, yarn packing, weave tightness, fabric stiffness, fabric thickness and weight, and cost relationships. Similarly, the cross-sectional shape affects yarn packing, stiffness, and twisting characteristics. It also affects the surface area, which in turn determines the fiber contact area, air permeability, and other properties.
Most fibers are nonmetallic: mostly glass (E and S types), acrylic, carbon or graphite (polyacrylonitrile-, or PAN-, and pitch-based), meta- and para-aramids, melamine, polyethylene, nylon, quartz, ceramic, glass-ceramic, and hybrid combinations, such as copolymers. Many of these are used to reinforce plastics, producing polymer-matrix composites (PMCs). For PMCs in general, glass fibers are the most widely used. They can be short, or chopped, that is about 0.5 in long and 1 to 2 in or continuous in length. For high-performance products notably aerospace applications but also sports equipment, carbon or graphite fiber, such as Thornel 300, are the most common. Plastic optical fibers, with polymethylmethacrylate as the core and, generally, a fluoropolymer coating, are used in illuminating signs and displays.
Poly-p-phenylene benzobisoxazole (PBO) fiber, made by Toyobo (Japan) under license from Dow Chemical, was originally patented by Stanford Research Institute and produced in a U.S. Air Force program as polybenzazole (PBZ). The tensile strength and modulus are said to be twice those of the aramid fiber Kevlar. It also has excellent thermal stability, not decomposing until 1200°F (649°C), or 180°F (100°C) higher than the aramid, and high resistance to creep, chemicals, abrasion, and cutting. Miraflex fiber, from Owens Corning, consists of two forms of glass fiber, fused together into a filament that is randomly twisted along the length for softness, flexibility, and resiliency, unlike conventional straight and rigid fibers. Its tensile strength is 100,000 to 150,000 lb/in2 (690 to 1034 MPa), half of which is retained at 600°F (316°C). Twintex Direct Composite, from Vetrotex Certain Teed Corp., is a glass-fiber roving with intermixed polypropylene filaments for use as dry prepreg for thermoplastic composites. Compression-moldable fabrics of the roving are also available and fabrics of 60% glass and 40% polypropylene are more than twice as impact resistant as glass-mat thermoplastics.
Basofil melamine fiber, from BASF Corp., is a thermoset product made by mixing a proprietary monomer with melamine and formaldehyde precursors to improve ductility and then polymerizing the material as the fiber is formed. Heat resistant and flame retardant, the fiber does not melt but begins to decompose at 698°F (370°C).Securus fiber, from Allied Signal Performance Fibers, is a copolymer of polyethylene terephthalate (PET) and polycaprolactone
made into fabric and used for auto seat belts. Basalt fiber, produced by Kompozit Ltd. (Ukraine) and Sudogda Fiber Glass Co. (Russia), is said to be comparable in properties to glass fiber and potentially comparable in cost. Because of its alkali resistance, reinforcing concrete is a possible application, currently an emerging market for carbon and graphite fiber in infrastructure.
Single-crystal sapphire fiber, from Saphikon, Inc., combines low-temperature toughness with high-temperature creep resistance and is a candidate for reinforcing nickel aluminides. At the National Aeronautics and Space Administration Glenn Research Center, this fiber has been made by a melt-modulation technique, resulting in a tensile strength of 725,000 to 870,000 lb/in2 (5000 to 6000 MPa) at 80°F (27°C) compared with 290,000 to 435,000 lb/in2 (2000 to 3000 MPa) for the commercial fiber made by the edge-defined growth technique. Zentron fiber, from Owens Corning, is an aluminosilicate glass material compatible for use with epoxy and vinyl ester resins. Its tensile strength is 50% greater than that of E-glass fiber and 15% more than that of S-glass fiber. As a reinforcement for PMCs, it imparts 20 to 25% greater impact resistance than the E-glass and 5 to 10% more than the S-glass. Nextel 440 fiber is a glass-ceramic fiber made by the sol-gel technique. Its modulus is about 2.6 times that of E-glass fiber but less than that of Thornel 300.
Metal fibers are more limited, including boron, stainless steel, iron and iron alloys, iron-chromium alloys, nickel alloys, and titanium alloys. They are made in many ways: wire drawing, bundle drawing, wire cutting, melt extrusion or extraction, chemical vapor deposition (boron), and electrolytic deposition. Common bundle-drawn fibers measure 0.00016 to 0.00088 in (4 to 22 (im) in diameter, but finer and larger ones are available. Applications include aerospace, thermal insulation, protective clothing, conductive plastics and textiles, sound absorption, filter media, and microwave detection, notes Bekaert Fiber Technologies (Belgium). For structural aerospace applications, boron fiber is used in both epoxy and aluminum matrixes, forming PMCs and MMCs (metal-matrix composites). Feltmetal, or Fibermetal, sheet is available in stainless steel, titanium alloy, and iron-chromium-yttrium alloy from Technetics Corp. The fiber, 0.0004 to 0.0010 in (0.01 to 0.25 mm) thick, are bonded by sintering at all contact points.
FILTER FABRICS. Any fabric used for filtering liquids, gases, or vapors, but, because of the heat and chemical resistance usually required, generally synthetic or metal fibers. Weave is an important
consideration. Plain weave permits maximum interlacings, and a tight weave gives high impermeability to particles. Twill weave has lower interlacings in sharp diagonal lines and gives a more selective porosity for some materials. Satin weave has fewer interlacings, is spaced widely but regularly, and is used for dust collection and gaseous filtration.
Fibers are chosen for their particular chemical resistance, heat resistance, and strength. Dacron has good acid resistance except for concentrated sulfuric or nitric acid. It can be used to 325°F (162°C). High-density polyethylene has good strength and abrasion resistance, and its smooth surface minimizes clogging of the filter, but it has an operating temperature only to 230°F (110°C). Polypropylene can be used to 275°F (134°C). Nylon gives high strength and abrasion resistance. It has high solvent resistance, but low acid resistance. Its operating limit is about 250°F (121°C). Teflon is exceptionally resistant to a wide variety of chemicals. It can be operated above 400°F (204°C), and its waxy, nonsticking surface prevents clogging and makes it easy to clean, but the fiber is available only in single-filament form.
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