In the production of composite materials, for example friction materials for use as brake linings, clutch faces, and the like, fibrous materials are used to bind the composition together. The reinforcing fibers not only impart desirable characteristics to the final product, they also provide "green strength" during preforming of the composite wherein the composition mixture is preliminarily compacted or densified prior to final pressing and curing. (Pre-forming of compositions for friction materials is well known in the art, see for example Searfoss et al U.S. Pat. No. 4,150,011 and Gallagher et al U.S. Pat. No. 4,374,211, to which reference may be had for further background.)
For many years asbestos appeared to be the ideal reinforcing material for composite friction materials. It is inexpensive, extremely durable, and its fiber bundles can easily be "opened" to provide a fiber mass which displays a large surface area per unit weight. This in turn provides strong engagement with and binding of frictional compositions.
However, the controversy concerning the possible carcinogenic effect of asbestos prompted attempts to develop alternative materials. This has proven very difficult in practice. Many substitute materials have been suggested and tried, but very few of them have proven satisfactory in commercial practice. Two principal reasons for the lack of success have been the fact that other fibers have not, with few exceptions, provided anywhere near the preformability and reinforcing properties of asbestos fibers; and those which do are undesirably expensive. For example, the aramid synthetic fibers, such as those sold by DuPont under the trademark "Kevlar", are available in a so-called "pulp" form which has a high degree of fibrillation, but its high cost has hindered widespread use. On the other hand, fibers such as acrylic, nylon, fiberglass, wollastonite, steel, mineral fibers, ceramic fibers, cotton and polyester, are less expensive than Kevlar, but it has not been possible to provide them in forms with sufficient degrees of fibrillation to reinforce as effectively as Kevlar.
There has thus been a strong demand for a lower cost fiber which can be fibrillated to a degree equivalent to that of Kevlar pulp fiber. Extensive research programs have been undertaken to develop such an alternative, but so far without commercial utility and practicality.
USPTO Backgrounds
USPTO Backgrounds