Asbestos friction materials

Asbestos friction materials introduced in 1908 contained up to 65% of short chrysotile asbestos fibers that served as a cost-effective, heat-resistant, insulating reinforcement and friction enhancer. Asbestos formulations were widely employed in drum and disk formulations for several decades until health and safety concerns triggered efforts to find suitable substitutes. These materials are being universally phased out in favor of nonasbestos formulations. 

A sintered friction material is composed of a metal matrix, generally mainly copper, to which a number of other metals such as tin, zinc, lead, and iron are added. Important constituents include graphite and friction-producing components such as siHca, emery, or asbestos. 

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... 

Asbestos fibers have also been widely used for the fabrication of papers and felts for flooring and roofing products, pipeline wrapping, electrical insulation, etc. Asbestos textiles, comprising yam, thread, cloth, tape, or tope, also found wide apphcation in thermal and electrical insulation, friction products in brake or clutch pads, etc. In recent years, some of these appHcations have decreased to various extents, although others remain fairly active, typically in friction materials. 

Prior to the mid-1970s, the most common type of friction materials in use in brakes and clutches for normal duty for original equipment installations and for the aftermarket were termed organics. These materials usually contained about 30—40 wt % of organic components and were asbestos-based (3). 

After the mid-1970s, the downsizing of North American vehicles and the introduction of front wheel drive vehicles brought about the widespread usage of a new class of friction materials (4) called semimetallics, also called semimets and carbon—metaUics. Because of the allegedly adverse health effects associated with asbestos [1332-21 -4] (qv) fibers, a second new class of friction materials called nonasbestos organics (NAOs) came about (5). Such materials are called either asbestos-free or nonasbestos friction materials (2). 

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). 

In order to define the extent of emissions from automotive brakes and clutches, a study was carried out in which specially designed wear debris collectors were built for the dmm brake, the disk brake, and the clutch of a popular U.S. vehicle (1). The vehicle was driven through various test cycles to determine the extent and type of brake emissions generated under all driving conditions. Typical original equipment and aftermarket friction materials were evaluated. Brake relines were made to simulate consumer practices. The wear debris was analyzed by a combination of optical and electron microscopy to ascertain the asbestos content and its particle size distribution. It was found that more than 99.7% of the asbestos was converted to a nonfibrous form and... 

Future brakes must satisfy health standards and most vehicle manufacturers have moved toward removing all asbestos from brakes. Lighter weight rotors and caUpers based on aluminum-based metal-matrix materials are also on the horizon for lighter vehicles requiring a whole new family of compatible friction materials. 

A. E. Anderson, "Brake Systems Performance—Effects of Eiber Types and Concentrations", Proceedings from Fibers in Friction Materials Symposium, Asbestos Institute, Atiantic City, N.J., Oct. 1987. 

It is explained that the remaining uses of white asbestos (asbestos cement, friction materials, seals and gaskets, textiles and composites) are likely to become prohibited in the UK in the near future. This article examines in detail the choices available for its replacement. 

Substitutes for asbestos are constantly being developed (EPA 19891). Nonasbestos friction materials are currently being used in disc brake pads, and substitutes have been developed for drum brake linings. Substitutes include fibers made of carbon, steel, cellulose, ceramics, glass, and wollastonite and organic fibers made from aramid, polyethylene, polypropylene, and polytetrafluoroethylene (USGS 2000). No single substitute was as versatile and as cost effective as asbestos. 

Berry G, Newhouse ML. 1983. Mortality of workers manufacturing friction materials using asbestos. 

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