GB814740A - Vinyl-substituted silica aerogels - Google Patents

Abstract

A vinyl-modified silica aerogel is made by (1) cohydrolysing a vinylchlorosilane and a silicon tetrahalide to yield a gel; (2) dissolving the gel in aqueous alkali metal hydroxide; (3) treating the resulting solution with an acid, e.g. HCl or H2SO4, while maintaining the temperature below room temperature, until the pH is from 2 to 3 (4) precipitating the alkali metal salt formed from step (3) with methanol or ethanol and removing the precipitate by filtration; (5) ensuring the removal of all alkali metal ions by passing the solution through an ion-exchange resin; (6) autoclaving the alcoholic solution at a temperature and pressure at least equal to the critical constants of the mixture; and (7) thereafter releasing the vapours formed in the pressure reactor while continuing to maintain the temperature between 275 DEG and 375 DEG C. The vinyltrichloro-, divinyldichloro- or trivinylchlorosilane is used in amount, e.g. of 5 to 50 per cent, by weight of the total vinylsilane-silicon tetrahalide (preferably the tetrachloride) mixture. An example is given. Specifications 607,426 and 607,427, [both in Group IV (a)], are referred to.ALSO:A vinyl-modified silica aerogel is made by (1) cohydrolysing a vinyl chlorosilane and a silicon tetrahalide to yield a gel; (2) dissolving the gel in aqueous alkali metal hydroxide; (3) treating the resulting solution with an acid, e.g. HCl or H2SO4, while maintaining the temperature below room temperature, until the pH is from 2 to 3; (4) precipitating the alkali metal salt formed from step (3) with methanol or ethanol and removing the precipitate by filtration; (5) ensuring the removal of all alkali metal ions by passing the solution through an ion-exchange resin; (6) autoclaving the alcoholic solution at a temperature and pressure at least equal to the critical constants of the mixture; and (7) thereafter releasing the vapours formed in the pressure reactor while continuing to maintain the temperature between 275 DEG and 375 DEG C. Vinyl silanes specified are vinyltrichloro-, divinyldichloro- and trivinylchloro-silanes, and are used in amount e.g. of 5 to 50 per cent by weight of the total mixture of vinyl chlorosilane and silicon tetrahalide, which is preferably the chloride. An example is given. The products may be used for reinforcing silicone rubbers containing Si-bonded vinyl or allyl radicals, natural or synthetic rubber-containing ethylenic unsaturation (e.g. butyl, GR-S and perbunan), or addition polymers. Thus, the aerogels may be copolymerized with monomers, e.g. styrene, methyl- or ethyl-acrylate, methyl methacrylate or acrylonitrile, or with vinyl-containing polysiloxanes, in the presence of inorganic or organic peroxides, e.g. benzoyl peroxide and t-butyl-perbenzoate, or in the presence of S in the case of ethylenically unsaturated rubbers, bulk or emulsion polymerization techniques being used. Uses: the reinforced silicone rubbers find use as gasket material or electrical insulation. Specifications 607,426, 607,427, 785,384 and 797,606 are referred to.