GB805102A - Modified silica aerogels and silicone rubbers containing the same - Google Patents

Abstract

Partially or completely hydrophobic silica aerogel particles are prepared by coating particles of silica aerogel having an acid number, as defined, below 0.8 with from 1-20 wt. per cent of a silicone oil consisting of an organo-polysiloxane containing an average of at least two alkyl groups directly attached to each silicon atom, and simultaneously or subsequently comminuting the coated particles. The aerogel may have an acid number 0.1-0.79, surface area 80-600 sq. m./grm., bulk density less than 10 lb./cu. ft., and particle size 0.01-15 microns, and may be prepared by adding sodium silicate solution to sulphuric acid solution with agitation. The resulting hydrosol is stirred with three separate additions of ethyl alcohol to form an alcosol, which is centrifuged to remove sodium sulphate, neutralized with aqueous sodium bicarbonate, and rapidly heated in an autoclave above the critical temperature of the water-alcohol vehicle. The silicone oils may be methyl silicone oils prepared by hydrolysis of dimethyl dichlorosilane, by cohydrolysis of dimethyldichlorosilane and trimethyl chlorosilane, or by catalytic equilibration of a mixture of cyclic dimethyl siloxane and hexamethylsiloxane with a minor amount of sulphuric acid. The aerogel may be immersed in an anhydrous solution of the silicone oil in an organic solvent and comminuted in an air grinding mill, or freshly prepared aerogel in the autoclave, and prior to removel therefrom, may be subjected to silicone vapour, or to a spray of oil or a solution of oil, and then comminuted. Alternatively, the aerogel during comminution in an air mill at 80-300 DEG F. may be treated with a fine spray of oil or solution of oil. The coated aerogel may be improved, as regards its hydrophobic properties, by heating at 80-600 DEG C. at atmospheric or sub-atmospheric pressure for 5 mins.-24 hrs. The coated aerogel is used as a reinforcing filler for silicone gums or rubbers. Specifications 649,896 and 654,850 are referred to.ALSO:Partially or completely hydrophobic silica aerogel particles, prepared by coating particles of silica aerogel having an acid number, as defined, below 0.8 with from 1-20 wt. per cent of a silicone oil consisting of an organo-polysiloxane containing an average of at least two alkyl groups directly attached to each silicon atom, and simultaneously or subsequently comminuting the coated particles (see Group III), are used as a reinforcing filler for silicone gums or rubbers. The aerogel is incorporated in a silicone gum, in amount 10-55 parts per 100 parts gum, in the presence of 1-6 parts benzoyl peroxide, or tertiary butyl perbenzoate. The composition is aged 5-100 days at 60-100 DEG F., remilled, and vulcanized at 110-225 DEG C. The silicone gum may be a dimethylpolysiloxane prepared by heating the hydrolysate of dimethyl dichlorosilane in the presence of a small amount of iron chloride, potassium or sodium hydroxide, or sulphuric acid, or the copolymer of at least 90 per cent dimethyl siloxane and not more than 10 per cent diphenyl siloxane. Specifications 649,896 and 654,850, [both in Group III], are referred to.