Asbestos, properties

Fig. 2.8A. After Hodgson, A. A. (1979). Chemistry and physics of asbestos. Fig. 3.2, p. 76. In L. Michaels and S. S. Chissick, eds. Asbestos Properties, Applications and Hazards. John Wiley and Sons, New York. With permission of the publisher.

Hodgson, AA. in L. Michaels and S.S. Oiissick, eds. Asbestos Properties, Applications and Hazards. Vol. 1, John Wiley Sons, Inc., New York, NY, 1979 AA. Hodgson, Scientific Advances in Asbestos, 1967 to 1985, Anjalena Publication, Crowthome, UK. 1986. 

Zussman, J., The Mineralogy of Asbestos, Asbestos Properties, Applications and Hazards, pp. 45-67 Wiley, 1979. 

With the exception of glass fiber, asbestos (qv), and the specialty metallic and ceramic fibers, textile fibers are a class of soHd organic polymers distinguishable from other polymers by their physical properties and characteristic geometric dimensions (see Glass Refractory fibers). The physical properties of textile fibers, and indeed of all materials, are a reflection of molecular stmcture and intermolecular organization. The abiUty of certain polymers to form fibers can be traced to several stmctural features at different levels of organization rather than to any one particular molecular property. 

Table 4. Properties of Dolanit Asbestos Replacement Fibers...

PBI is being marketed as a replacement for asbestos and as a high temperature filtration fabric with exceUent textile apparel properties. The synthesis of whoUy aromatic polybenzimidazoles with improved thermal stabUities was reported in 1961 (12). The Non-MetaUic Materials and Manufacturing Technology Division of the U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, awarded a contract to the Narmco Research and Development Division of the Whittaker Corp. for development of these materials into high temperature adhesives and laminates. 

Other fibrous and porous materials used for sound-absorbing treatments include wood, cellulose, and metal fibers foamed gypsum or Pordand cement combined with other materials and sintered metals. Wood fibers can be combined with binders and dame-retardent chemicals. Metal fibers and sintered metals can be manufactured with finely controlled physical properties. They usually are made for appHcations involving severe chemical or physical environments, although some sintered metal materials have found their way into architectural appHcations. Prior to concerns regarding its carcinogenic properties, asbestos fiber had been used extensively in spray-on acoustical treatments. 

The main use of lead metaborate is in glazes on pottery, porcelain, and chinaware, as weU as in enamels for cast iron. Other appHcations include as radiation-shielding plastics, as a gelatinous thermal insulator containing asbestos fibers for neutron shielding, and as an additive to improve the properties of semiconducting materials used in thermistors (137). 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

Since asbestos fibers are all siUcates, they exhibit several other common properties, such as incombustibiUty, thermal stabiUty, resistance to biodegradation, chemical inertia toward most chemicals, and low electrical conductivity. 

Tlie microscopic and macroscopic properties of asbestos fibers stem from their intrinsic, and sometimes unique, crystalline features. As with all siUcate minerals, the basic building blocks of asbestos fibers are the siUcate tetraliedra wliicli may occur as double chains, as in the ampliiboles, or in... 

Table 4. Physical and Chemical Properties of Asbestos Fibers...

Other Bulk Physical Properties. The hardness of asbestos fibers is comparable to that of other crystalline or glassy siHcates. Compared to glass fibers, amphiboles have similar hardness values, while chrysotile shows lower hardness values. 

The choice of a particular mining method depends on a number of parameters, typically the physical properties of the host matrix, the fiber content of the ore, the amount of sterile materials, the presence of contaminants, and the extent of potential fiber degradation during the various mining operations (33). However, most of the asbestos mining operations are of the open pit type, using bench drilling techniques. 

The fiber extraction (milling) process must be chosen so as to optimize recovery of the fibers in the ore, while minimizing reduction of fiber length. Since the asbestos fibers have a chemical composition similar to that of the host rock, the separation processes must rely on differences in the physical properties between the fibers and the host rock rather than on differences in their chemical properties (33). 

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

Finally, other properties of asbestos fibers may be evaluated depending on the envisaged appHcation. Typically, the grits and spicule content, the magnetic susceptibiUty (magnetic rating), the content in soluble chlorides, and the humidity level may be of a particular interest in specific appHcations. 

The main characteristic properties of asbestos fibers that can be exploited in industrial appHcations (8) are their thermal, electrical, and sound insulation nonflammabiUty matrix reinforcement (cement, plastic, and resins) adsorption capacity (filtration, Hquid sterilization) wear and friction properties (friction materials) and chemical inertia (except in acids). These properties have led to several main classes of industrial products or appHcations... 

The reinforcing properties of asbestos fibers have been widely exploited in asbestos—cement products mosdy for the constmction industry and sanitation (sheets, pipes). Into the 1990s, asbestos—cement products represent by fat (f 70%) the largest industrial consumption of asbestos fibers. 

Table 7 Hsts some of the materials and fibers that have been suggested or used ia the development of asbestos-free products. These are Hsted by categories, and no attempt is made to provide a detailed coverage of the respective properties of these materials. Table 7 also provides an estimate of the cost ranges of asbestos fibers and several types of substitution materials.

The primary constituent of practically ah. asbestos—organic friction materials was asbestos fiber, with smah quantities of other fibrous reinforcement material. Asbestos was chosen because of its thermal stabhity, its relatively high friction, and its reinforcing properties. Because asbestos alone did not offer ah of the desked properties, other materials cahed property modifiers were added to provide desked levels of friction, wear, fade, recovery, noise, and rotor compatibihty. A reski bkider held the other materials together. This bkider is not completely neutral and makes contributions to the friction and wear characteristics of the composite. The more commonly used kigredients can be found ki various patents (6—9). 

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