Vitreous refractory fibers

Fibrous materials may be naturally occurring or synthetically manufactured by thermal or chemical processes (Fig. 1) (see Fibers, survey). Refractory fibers are generally used in industrial appHcations at temperatures between 1000°C and 2800°C. These fibers may be oxides or nonoxides, vitreous or polycrystalline, and may be produced as whiskers, continuous filaments, or loose wool products. 

Fiber chemistry determines whether the material is an oxide or nonoxide and can also influence its vitreous or polycrystalline physical form. Refractory fibers generally have diameters ranging from submicrometer to 10 )J.m, and lengths, as manufactured, may range from millimeters to continuous filaments. 

Aluminosilicate Fibers. Vitreous alurninosihcate fibers, more commonly known as refractory ceramic fibers (RCF), belong to a class of materials known as synthetic vitreous fibers. Fiber glass and mineral wool are also classified as synthetic vitreous fibers, and together represent 98% of this product group. RCFs were discovered in 1942 (18) but were not used commercially until 1953. Typical chemical and physical properties of these materials are shown in Table 3. 

Sihca and aluminosihcate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobaUte (1). Cristobahte is a form of crystalline siUca that can cause siUcosis, a form of pneumoconiosis. lARC has deterrnined that cristobaUte should be classified as 2A, a probable carcinogen. The amount of cristobahte formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobahte. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Brown DM, Fisher C, Donaldson K. 1998. Free radical activity of s mthetic vitreous fibers Iron chelation inhibits hydroxy radical generation by refractory ceramic fiber. J Toxicol Environ Health 53 545-561. 

Vitreous silica optical fibers for modern telecommunication systems must be made from ultrapure materials. Glasses produced from melts are incapable of reaching the quality levels required by these fibers. The glasses used in current fibers are produced in situ as the preform is formed by vapor deposition processes. Since the raw materials are liquids, purification by distillation can radically reduce impurity contents. Furthermore, since the glass never contacts crucible or refractory materials, the purity of the glass is maintained throughout the process. 

FIGURE 6.36 Fiber-blowing process used to make refractory ceramic fibers. [Man-Made Vitreous Fibers Nomenclature, Chemical and Physical Properties, Nomenclature Committee of TIMA Inc., p. 16 (NAIMA—North American Insulation Manufacturers Assoc.).]... 

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