(a) Field of the Invention
This invention concerns high temperature fibers and shrink resistant articles manufactured therefrom.
(b) History of the Prior Art
In the prior art, amorphous or polycrystalline fibers were manufactured either from molten ceramic materials or from materials which would convert to ceramic materials upon the application of sufficient heat. Examples of such fibers are mineral wool manufactured by blowing fibers from molten slag obtained as a by-product from metal refining, glass wool manufactured by blowing fibers from molten clay, silica and alumina; refined mineral fibers blown from molten iron silicates; ceramic fibers made from molten aluminum silicates; drawn glass fibers manufactured from alkali metal silicates, alkaline earth metal silicates and borosilicates; and spun fibers from inorganic or organic compositions which convert to ceramic fibers upon the application of heat such as solutions of aluminum chlorhydrate.
Of the foregoing fibers, aluminum silicate, silica and alumina-silica fibers are generally considered to have the highest temperature resistance.
Such fibers, however, have disadvantages which have never been completely overcome. In particular, the heat resistance is still not as high as is desired and shrink resistance of fiber articles manufactured from the fibers was not as good as desired.
Numerous approaches have been taken to improve heat resistance and shrink resistance in inorganic fibers at high temperatures. The most common of such approaches has been to incorporate additives into the composition from which the fibers are made. Among such additives, chromium oxide has been used to increase temperature resistance. For example, U.S. Pat. Nos. 3,007,806 to Hartwig and 3,449,137 to Ekdahl; U.S. Pat. No. 4,125,406 to Sowman and British patent specification No. 495,654 all disclose that chromium oxide can be incorporated into a fusion containing silica and alumina followed by formation of fibers from the fused material.
While the incorporation of the chromium oxide into the fibers results in fibers having higher temperature resistance and improvement in shrink resistance of the fibers when formed into mats or other fiber articles, the process has serious disadvantages. In particular, oxygen is released by the chromium oxide in the melt which creates corrosion problems and results in the presence of chromium metal which creates an unstable fiber product.
U.S. Pat. No. 3,019,117 to Labino discloses that a mass of glass fibers may be dipped into a saturated solution of a metal nitrate such as chromium, iron, nickel or cobalt nitrates followed by drying the fiber mass and subjecting the fiber mass to sufficient pressure and temperature to fuse the fibers. While the resulting fiber block has reasonably good temperature resistance, the flexibility and shrink resistance at elevated temperatures is not as good as desired. This is believed, in accordance with the present invention, to be due to migration of the solution prior to complete drying which results in non-uniform coating of the fibers. U.S. Pat. No. 2,839,424 to Labino discloses that fibers of an alkali silicate could be treated with an acidic salt such as an aqueous solution of calcium, zinc or barium chloride so that the alkaline metal of the chloride replaces at least a portion of the alkali metal in the fiber followed by heating to drive off water and treating the resulting fibers with a solution such as chromic anhydride to fill the resulting pores in the fiber with chromic oxide upon heating to a sufficient temperature. The resulting fiber is characterized by pockets of chromic oxide and according to the patent, is resistant to temperatures in excess of 2200.degree. F. and, under certain circumstances, when the fibers are completely covered, i.e., encased, with chromic oxide, will resist temperatures of about 3000.degree. C. without deformation. It is to be noted that the process for treating the fibers is complex utilizing at least two liquid treatment steps and numerous drying and heating steps. It is to be further noted that the only specific teachings with respect to a method for obtaining complete coverage of the fiber with chromium oxide is by incorporating the fiber into a refractory brick structure containing between about 6 to about 16 percent of temperature-resistant oxides such as chromium oxide by weight of product. Such a brick structure is not flexible. Additionally, the fibers discussed in U.S. Pat. No. 2,839,424 are fibers of an alkali silicate which is a substance suitable for utilization in accordance with the process of the Labino patent but which is highly undesirable if resistance to humidity and chemical action is to be obtained.