No single fiber is known that functions effectively by itself as an asbestos replacement in friction materials. Short fibers have become recognized as a new and interesting class of materials for composites. Data presented indicates that EMCOR® 66 ultra-short fibers when combined with longer fibers such as steel wool, glass fiber, and aramid, produced friction materials that process well and are stable at higher temperatures.
THE LAST TWO DECADES have produced some rather dramatic changes in the formulation of friction materials for the braking systems of automobiles and trucks. There has been a shift in the direction of better heat resistance, higher coefficients of friction, and extended durability in brake pads and linings. Asbestos, a key ingredient in organic brake linings, is being replaced to allow brake engineers to meet these performance goals. Furthermore, chrysotile asbestos in friction materials is thought to represent a serious health risk. Accordingly, many U.S. friction material manufacturers have been searching for asbestos replacements which do not pose serious health problems. Replacements have included glass fiber, steel wool, aramid fiber, and a number of mineral fiber types, In general, friction material development with these fibers has come close to meeting most of the requirements. Difficulties in performance, noise, and high costs have been the primary deficiencies of many of the non-asbestos formulations tried to date.
Semi-metallic friction materials, an early asbestos-free type, were described by Aldrich (1) as a compromise between organic and sintered metal friction types. They were developed in Europe during World War II because of difficulties in importing asbestos and appeared in the early 1950's in the MGB disc pad made by Girling in England. Having minimal organic content, semi-metallics offered improved friction stability and high temperature wear resistance. While semi-metallics are still being used in automotive disc pads, they have made no major inroads into other phases of the business.
Aramid fiber has been suggested by Loken (2) but the high cost of this asbestos replacement and difficulties in getting uniform incorporation of the chopped fiber or pulp have limited its widespread market penetration.
Other fibers such as wollastonite, graphitic fiber, and processed mineral wool have seen limited usefulness in friction materials.
It is the thesis of this paper that no single synthetic or naturally occurring acicular or platy material can, by itself, adequately replace asbestos in friction materials. The approach advocated herein is to use a combination of one or more long fibers with an ultra-short fiber. The short fiber is needed to reinforce the areas between and around the long fibers, and to contribute other important mechanical and frictional properties to the composite.
This paper will, as a rule, not differentiate between original equipment and aftermarket friction materials or between disc pad and drum lining composites. This is because the basic principles of asbestos replacement by long and short fiber mixtures remain the same regardless of application.