Treatment of glass for high temperature resistance
TECHNICAL FIELD 
     This invention pertains to a method of treating glass fibers to improve their resistance to high temperatures. 
     In one of its more specific aspects, this invention pertains to a method of improving the stability of glass fibers when subjected to high temperatures. 
     BACKGROUND OF THE INVENTION 
     Various methods of improving the high temperature resistance of vitreous materials are well known. Particularly important in this field are those methods in which a refractory metal oxide is deposited on the surface of glass fibers. 
     U.S. Pat. No. 2,901,379 is directed to the use of metal oxides having a melting point of at least 1000° C. and defines as suitable oxides Fe 2  O 3 , MgO, Cr 2  O 3  and the like. 
     U.S. Pat. No. 3,232,782 suggests coatings consisting essentially of at least one compound selected from the group consisting of the oxides and silicates of chromium, aluminum, zirconium, titanium, and metals of Group II of the Periodic Table having an atomic number from 4 to 12. 
     U.S. Pat. No. 3,449,137 suggests the fusing into the glass of specific amounts of chromic oxide with a very limited range of relative proportions of alumina and silica. 
     U.S. Pat. No. 4,169,182 states that effective substances for absorbing radiant heat energy are metals such as Cu, Co, Fe, Cr, Mn and Ni and these materials can be either mixed in the glass raw materials and thereafter forming the mixture into fibers, or by forming a coating film of such materials or their oxides on the glass fibers. 
     The deposition of such metals on glass fibers, while improving the temperature resistance of the fibers, produced fibers having limited flexural properties. 
     BRIEF SUMMARY OF THE INVENTION 
     There has now been discovered a method of improving the flexural strength of glass fibers which involves, prior to the deposition on the surface thereof, a refractory material, the leaching of magnesium and aluminum therefrom by contacting the fiber with trans-1,2-diaminocyclohexane tetraacetic acid. The leached fiber can then be contacted, successively, with colloidal silica, an aqueous solution of the salt of the refractory metal, ammonium hydroxide and, again if desired, with colloidal silica to improve the flexural properties of the fiber at high temperatures. 
     In one embodiment of the invention, the contact with the colloidal silica is made before the hydrolysis step employing the ammonium hydroxide, i.e., the colloidal silica is in the CrCl 3 .6H 2  O solution. 
     In another embodiment of this invention, tetraethyl-ortho-silicate is substituted for colloidal silica. 
     DETAILED DESCRIPTION 
     The method of this invention is applicable to any vitreous material employed as glass fiber insulation. It is particularly applicable to glass fibers possessing magnesium ions on their surface. 
     By refractory materials are meant oxides and silicates of chromium, aluminum, zirconium, titanium, and metals of Group II of the Periodic Table having an atomic number from 4 to 12. 
     The method of this invention is carried out by contacting the size-free glass with the trans-1,2-diaminocyclohexane tetraacetic acid (DAA) in the form of about a 1 to about 3 weight percent aqueous solution for a period of about 24 to about 72 hours. 
     Preferably, the DAA will be in the form of a buffered solution comprising from about 1 to about 3 weight percent DAA, and about 7 weight percent of an aqueous buffer solution comprising about 6.5 weight percent ammonium chloride, and about 57 weight percent concentrated ammonium hydroxide, the pH of the buffer and DAA solution being 10 and 9, respectively. 
     The solution containing the glass fibers should be sealed to prevent loss of ammonia. 
     After so treating the glass fibers, they are drained dry and contacted with the colloidal silica. 
     Any suitable colloidal silica can be employed. One such silica is commercially available as Ludox AS-40. This material is an aqueous colloidal silica sol available from duPont of Wilmington, Del. and comprises about 40 percent by weight SiO 2  in water and is stabilized with about 2 percent ammonia. 
     The glass fibers are contacted with the colloidal silica in the &#34;as received&#34; state for a period of time sufficient to wet out the glass fibers. 
     The fibers are drained dry and then immersed in a 7 to 9 weight percent aqueous solution of the refractory salt (having an oxide equivalent of from 1 to 3 weight percent on the fiber) for a period of time sufficient to wet the glass fibers. The fibers are removed from the solution and drained of excess solution. 
     If desired, the contact of the glass fibers with the colloidal silica and the refractory salt can be made simultaneously by immersing the glass fibers in a combination of the colloidal silica and the aqueous solution of the refractory salt. 
     The glass fibers are then immersed in concentrated ammonium hydroxide for a period sufficient to wet them, and then in colloidal silica, in that order, the latter contact being in the same manner and under the same conditions as originally contacted as set forth above. 
    
    
     The following example demonstrates the method of this invention. 
     EXAMPLE I 
     To a slurry of 3.6 grams of trans-1,2-diaminocyclohexane tetraacetic acid monohydrate in 100 ml. water were added about 5 ml. of a buffer for the purpose of dissolving the acid. 
     The buffer was comprised of 32 g. ammonium chloride, 285 ml. concentrated ammonium hydroxide and 500 ml. of distilled water. 
     After addition of the buffer, 100 ml. of distilled water were added to the solution. 
     A sample of size-free S-glass was soaked in the solution for three days in a closed container. The glass was then removed and washed to remove ammonia. 
     The washed glass was soaked in a solution containing 2 g. Ludox AS-40, 2.1 g. CrCl 3 .6H 2  O and 25 ml. water. After complete contact of the glass with the solution, the glass was removed and subsequently immersed in concentrated ammonium hydroxide. The glass was then oven dried. 
     The glass was heat soaked at 1600° F. (871° C.) for three days after which time the glass was still quite flexible. 
     The sample was compared to a sample similarly treated except that the pre-soaking with the trans-1,2-diaminocyclohexane tetraacetic acid was omitted. 
     The pre-soaked sample was decidedly superior after comparable heat soak treatment. 
     It will be evident from the foregoing that various modifications can be made to this invention. However, such are considered within the scope of the invention.