Patent ID: 7332144

Claim:
A hydrophobic silica fine powder prepared by I. hydrolysis or oxidative firing in a flame of an (organo)halosilane or (organo)alkoxysilane to produce a hydrophilic silica fine powder; II. premixing the hydrophilic silica fine powder with a hydrophobizing agent in the form of a dimer diol siloxane which is a 1,3-dihydroxy-1,1,3,3-tetraorganodisiloxane of the following formula: wherein R is at least one group independently selected from the group consisting of methyl, ethyl, vinyl, trifluoropropyl, and trimethylsiloxy groups, and the number of silicon atoms per molecule is from 2 to 6; III. mixing the hydrophobizing agent with the powder in a ball-mediated mill for the intended purpose of achieving dispersion and for effecting cleavage or disintegration and consolidation; and thereafter IV. heating at 100° C. to 300° C. in an ammonia or amine-containing atmosphere, to form a silica fine powder having —O—Si—O—Si—O— structure on the surface thereof, which is formed with the dimer diol siloxane through hydrolysis condensation of silanol on the dimer diol siloxane with silanol on the surface of the hydrophilic silica fine powder; and wherein the hydrophobic silica fine powder has the following properties: an aerated bulk density of 190 to 250 g/l; a specific surface area of 60 to 300 m 2/ g; a primary particle diameter of 10 to 120 nm; a degree of hydrophobization of 71 to 80 as measured by methanol titration; and remaining stable in compositions without undergoing an increase of viscosity with time when the hydrophobic silica fine powder is used as a material for RTV silicone compositions; and wherein the hydrophobic silica fine powder has the following surface structure on a part of the surface of the silica fine powder: wherein R is defined as above.