High modulus silicone rubber

A method of making a silicone rubber composition consisting essentially of

• I. mixing
• A. 100 parts by weight of polydiorganosiloxane gum where the organic radicals are selected from methyl, phenyl, vinyl, ethyl and 3,3,3-trifluoropropyl and mixtures thereof, the polydiorganosiloxane gum having at least 50 percent of the organic radicals being methyl and no more than 2 percent being vinyl, the polydiorganosiloxane gum being endblocked with hydroxyl radicals or triorganosiloxy units where the organic radicals are defined above,
• B. from 25 to 75 parts by weight of reinforcing silica filler,
• C. from 5 to 20 parts by weight of hydroxylated diorganosilicon compound which has an average of about 2 hydroxyl radicals per molecule, at least 3 weight percent silicon-bonded hydroxyl radicals based on the weight of the weight of the hydroxylated diorganosilicon compound, and the organic radicals are selected from a combination of methyl, phenyl and vinyl, there being from 10 to 50 percent phenyl radicals, from 2 to 20 percent vinyl radicals and the remainder being methyl radicals,
• D. from 20 to 80 parts by weight fiberized blast furnace slag fibers having an average length of from 0.0001 to 0.0005 meter and a diameter of 0.000001 to 0.00001 meter, and
• E. from 0 to 10 parts by weight of alkoxy silicon compound having at least 25 weight percent silicon-bonded alkoxy radical selected from methoxy, ethoxy and propoxy and any remaining monovalent organic groups attached to silicon atoms being bonded through silicon-carbon bonds and being selected from methyl, ethyl and vinyl, to obtain a mixture and,
• Ii heating the mixture from (I) under reduced pressure for at least 30 minutes at a temperature from 100.degree. to 200.degree. C.