Patent ID: 8333946

Claim:
Partly hydrophobic fumed silica particles said partly hydrophobic fumed silica particles having a contact angle θ in air for water of less than 180°, a degree of coverage i of the surface of the silica with silylating agent residues, based on the total silica particle surface area, of 1%<τ<50%, a density of surface silanol groups SiOH ranging between a minimum of 0.9 and a maximum of 1.7 SiOH/nm 2 particle surface area, and having a carbon content of more than 0% and up to 2.0% by weight, and a methanol number of less than 30, said partly hydrophobic silica prepared by a process comprising silylating fumed silica particles prepared under anhydrous conditions, with at least one of I) and II) I) an organosilane of the formula R 1 n SiX 4-n where n is 1, 2 or 3 R 1 being a monovalent, optionally halogenated hydrocarbon radical having 1 to 24 carbon atoms, being identical or different at each occurrence, and being saturated, aromatic, monounsaturated, or polyunsaturated, X each independently being halogen, a nitrogen-containing radical, OR 2 , OCOR 2 , or O(CH 2 ) x OR 2 , R 2 being hydrogen or a monovalent hydrocarbon radical having 1 to 12 carbon atoms, and x being 1, 2 or 3; II) an organosiloxane composed of units of the formula (R 1 3 SiO 1/2 ), and/or (R 1 2 SiO 2/2 ), and/or (R 1 SiO 3/2 ) where R 1 is as defined above, or mixtures thereof, the number of these units in one organosiloxane being at least 2; and I and II being used in a total amount of from 0.015 mmol/g to 0.15 mmol/g per 100 m 2 /g of silica BET surface area measured by the BET method in accordance with DIN 66131 and 66132.