Patent ID: 7186440

Claim:
A process for producing a hydrophobic silica powder comprising the steps for hydrophobic treatment of: adding to a silica sol in mixed solvent having a silica concentration of 5 to 50 mass % obtained by mixing an aqueous silica sol containing hydrophilic colloidal silica having a specific surface area of 5.5 to 550 m 2 /g with a hydrophilic organic solvent in a mass ratio of 0.12 to 2.5 based on water in the aqueous silica sol, a disilazane compound of formula (1) (R 1 3 Si) 2 NH (1) wherein each R 1 is C 1-6 alkyl group or phenyl group that is selected independently of one another, in an amount of 0.1 to 20 mmol per surface area 100 m 2 of the hydrophilic colloidal silica, to obtain a mixture of the silica sol in mixed solvent and the disilazane compound; and heating the mixture at a temperature of 50 to 100° C. for aging to obtain a slurry dispersion of hydrophobic treated colloidal silica.