Hydrophobic silica fine powder and making method

Abstract

A hydrophobic silica fine powder is prepared by premixing a hydrophilic silica fine powder with a dimer diol siloxane or cyclic siloxane as a hydrophobizing agent, mixing them in a ball mill for achieving dispersion and for achieving cleavage or disintegration and consolidation, and thereafter heating at 100-300° C. in an ammonia or amine-containing atmosphere. The powder has an aerated bulk density of 100-300 g/l, a specific surface area of 40-300 m 2 /g, a primary particle diameter of 10-120 nm, and a degree of hydrophobization of 40-80 as measured by methanol titration and is less bulky, easy to handle and disperse and stable in a kneaded mixture.

1 . A method for preparing a hydrophobic silica fine powder, comprising the steps of: 
 premixing a hydrophilic silica fine powder obtained by hydrolysis or oxidative firing in flame of an (organo)halosilane or (organo)alkoxysilane with a hydrophobizing agent in the form of a dimer diol siloxane and/or cyclic siloxane,    mixing the hydrophobizing agent with the powder in a ball-mediated mill for achieving dispersion and for effecting cleavage or disintegration and consolidation, and    thereafter heating at 100° C. to 300° C. in an ammonia or amine-containing atmosphere,    the resulting hydrophobic silica fine powder having an aerated bulk density of 100 to 300 g/l, a specific surface area of 40 to 300 m 2 /g, a primary particle diameter of 10 to 120 nm, and a degree of hydrophobization of 40 to 80 as measured by methanol titration.    
     
     
         2 . The method of  claim 1  wherein the dimer diol siloxane is 1,3-dimethyl-1,3-bis(trimethylsiloxy)-1,3-dihydroxydisiloxane, and the cyclic siloxane is hexamethylcyclotrisiloxane or octamethylcyclotetrasiloxane.  
     
     
         3 . The method of  claim 1  wherein in the premixing step, 1.7×10 −6  to 10×10 −6  mol of the hydrophobizing agent is added per square meter of silica surface area.  
     
     
         4 . The method of  claim 1  wherein the ammonia or amine-containing atmosphere is established by flowing an ammonia-containing gas, adding ammonia water or adding a water-soluble amine.