Specialty fibers are engineered for specific uses that require exceptional strength, heat resistance and/or chemical resistance. They are generally niche products, but some are produced in large quantities.
Glass is the oldest, and most familiar, performance fiber. Fibers have been manufactured from glass since the 1930s. Although early versions were strong, they were relatively inflexible and not suitable for many textile applications. Today’s glass fibers offer a much wider range of properties and can be found in a wide range of end uses, such as insulation batting, fire resistant fabrics, and reinforcing materials for plastic composites. Items such as bathtub enclosures and boats, often referred to as “fiberglass” are, in reality, plastics (often crosslinked polyesters) with glass fiber reinforcement. And, of course, continuous filaments of optical quality glass have revolutionized the communications industry in recent years.
Carbon fiber may also be engineered for strength. Carbon fiber variants differ in flexibility, electrical conductivity, thermal and chemical resistance. Altering the production method allows carbon fiber to be made with the stiffness and high strength needed for reinforcement of plastic composites, or the softness and flexibility necessary for conversion into textile materials. The primary factors governing the physical properties are degree of carbonization (carbon content, usually greater than 92% by weight) and orientation of the layered carbon planes. Fibers are produced commercially with a wide range of crystalline and amorphous contents.
Because carbon cannot readily be shaped into fiber form, commercial carbon fibers are made by extrusion of some precursor material into filaments, followed by a carbonization process to convert the filaments into carbon. Different precursors and carbonization processes are used, depending on the desired product properties. Precursor fibers can be specially purified rayon (used in fabrication of the space shuttle), pitch (for reinforcement and other applications) or acrylics (for varied end uses). Since carbon fiber may be difficult to process, the precursor fiber may be converted into fabric form, which is then carbonized to produce the end product. The following materials are common precursors for carbon fiber:
Rayon, in either fiber or fabric form, is one precursor for carbon fiber. Specially purified rayon containing a dehydration catalyst (frequently a phosphorus compound) is subjected to heat treatment to dehydrate the cellulose structure. Higher temperature treatment and controlled oxidation produces carbonization. A third, higher temperature, treatment may also be used to further increase the carbon content. Many aerospace applications use rayon fabric to produce material with high thermal resistance but relatively low strength.
Acrylic fiber (based on polyacrylonitrile, or PAN) is the most common carbon precursor. The carbonization process is similar to that used for rayon, except that continuous tension is applied to produce a more highly oriented ladder structure and, thus, a fiber with greater tensile strength. Carbon fiber produced from PAN is most frequently used as reinforcement for a wide variety of plastic composites.
Pitch, a polyaromatic hydrocarbon material derived from petroleum or coal, is another common carbon fiber precursor. The pitch is converted into a liquid-crystal state prior to extrusion into fiber form. The shear forces during extrusion and subsequent drawing produce a filament with high molecular orientation in the direction of the fiber axis. This orientation is maintained during oxidation and high temperature carbonization. Carbon fiber can be produced in this way with a variety of strength and flexibility characteristics.
Aramids are among the best known of the high-performance, synthetic, organic fibers. Closely related to the nylons, aramids are polyamides derived from aromatic acids and amines. Because of the stability of the aromatic rings and the added strength of the amide linkages, due to conjugation with the aromatic structures, aramids exhibit higher tensile strength and thermal resistance than the aliphatic polyamides (nylons). The para- aramids, based on terephthalic acid and p-phenylene diamine, or p-aminobenzoic acid, exhibit higher strength and thermal resistance than those with the linkages in meta positions on the benzene rings. The greater degree of conjugation and more linear geometry of the para linkages, combined with the greater chain orientation derived from this linearity, are primarily responsible for the increased strength. The high impact resistance of the para-aramids makes them popular for “bullet-proof” body armor. For many less demanding applications, aramids may be blended with other fibers.
PBI (polybenzimidazole) is another fiber that takes advantage of the high stability of conjugated aromatic structures to produce high thermal resistance. The ladder-like structure of the polymer further increases the thermal stability. PBI is noted for its high cost, due both to high raw material costs and a demanding manufacturing process. The high degree of conjugation in the polymer structure imparts an orange color that cannot be removed by bleaching. When converted into fabric, it yields a soft hand with good moisture regain. PBI may be blended with aramid or other fibers to reduce cost and increase fabric strength.
PBO (polyphenylenebenzobisozazole) and PI (polyimide) are two other high-temperature fibers based on repeating aromatic structures. Both are recent additions to the market. PBO exhibits very good tensile strength and high modulus, which are useful in reinforcing applications. Polyimide’s temperature resistance and irregular cross-section make it a good candidate for hot gas filtration applications.
Sulfar (PPS, polyphenylene sulfide) exhibits moderate thermal stability but excellent chemical and fire resistance. It is used in a variety of filtration and other industrial applications.
Melamine fiber is primarily known for its inherent thermal resistance and outstanding heat blocking capability in direct flame applications. This high stability is due to the crosslinked nature of the polymer and the low thermal conductivity of melamine resin. In comparison to other performance fibers, melamine fiber offers an excellent value for products designed for direct flame contact and elevated temperature exposures. Moreover, the dielectric properties and cross section shape and distribution make it ideal for high temperature filtration applications. It is sometimes blended with aramid or other performance fibers to increase final fabric strength.
Fluoropolymer (PTFE, polytetrafluoroethylene) offers extremely high chemical resistance, coupled with good thermal stability. It also has an extremely low coefficient of friction, which can be either an advantage or disadvantage, depending on the use.
HDPE (high-density polyethylene) can be extruded using special technology to produce very high molecular orientation. The resulting fiber combines high strength, chemical resistance and good wear properties with light weight, making it highly desirable for applications ranging from cut-proof protective gear to marine ropes. Since it is lighter than water, ropes made of HDPE float. Its primary drawback is its low