GB721605A - Water-repellent aerogels - Google Patents

Abstract

Water-repellent cellulose aerogels are formed by adding to a gel thereof containing a non-aqueous liquid phase, organic material which on subsequent heating of the gel in an autoclave above the critical temperature of the liquid phase, forms a p water-repellent film on the surfaces of the gel structure, heating the gel and releasing the autoclave pressure slowly to remove the vaporized liquid phase. The organic film forming material may comprise equivalent amounts of polyhydric alcohols or partially esterified alcohols such as glycerol, glycol, glycerolmonostearate, and polybasic acids such as phthalic, succinic, malic, or citric. The equivalent anhydrides or condensation products of these bases and acids may also be used. Other alcohols as constituent of the film-forming material may be ethylene glycol, butanediol, ethylidene glycol, mesitylene glycol, propylene-propanediol, pinacone, tetraphenylbenzopinacol, and sugars such as pentoses, hexoses and glucoses and also erythritol and sorbitol. Further film-forming materials are aminosilanes such as methyl or butyl amino silanes, polystyrene, siloxanes and also drying oils, natural resins, synthetic resins and cellulose derivatives such as the acetate, acetate-butyrate or ethyl or benzyl cellulose. The amount of such substances may be from 10-100 per cent by weight of the dry weight of the gel. The original gel containing a non-aqueous liquid phase is formed by displacing water from gels by lower alkyl ketones, alcohols, fatty acids or their anhydrides especially acetone and ethyl alcohol, or ammonia, pyridine or monodi- or tertiary alkyl substituted amines. If desired the displacement of water from the aerogel may be carried out in stages using different organic solvents, each of which displaces the solvent already present in the gel, the last displacing solvent being capable of dissolving the film-forming materials subsequently added. Using such a method the last displacing solvent may be gasolines, petroleum solvents, benzene or toluene. According to an example, cellulose was regenerated by acidifying viscose solution with dilute sulphuric acid and after washing with water the gel was solvent exchanged with acetone. To the resulting paste was added citric acid and glycerol monoricinoleate and the whole heated at 260 DEG C. and 1600 p.s.i. pressure. The acetone was released at 250 DEG C. and the bulk density of the product was 0.249 gms. per cc.ALSO:Water repellent aerogels are formed by adding to a gel containing a non-aqueous liquid phase, organic material which on subsequent heating of the gel in an autoclave above the critical temperature of the liquid phase, forms a water repellent film on the surfaces of the gel structure, heating the gel and releasing the autoclave pressure slowly to remove the vaporized liquid phase. The organic film forming material may comprise equivalent amounts of polyhydric alcohols or partially esterified alcohols such as glycerol, glycol, glycerol-monostearate, and polybasic acids such as phthalic, succinic, malic, or citric. The equivalent anhydrides or condensation products of these bases and acids may also be used. Other alcohols as constituent of the film forming material may be ethylene glycol, butanediol, ethylidene glycol, mesitylene glycol, propylene-propanediol, pinacone, tetraphenyl-benzopinacol, and sugars such as pentoses, hexoses and glucoses and also erythritol and sorbitol. Further film-forming materials are aminosilanes such as methyl, or butyl amino silanes, polystyrene and siloxanes and also drying oils, natural resins, synthetic resins and cellulose derivatives such as the acetate, acetate-butyrate, or ethyl or benzyl cellulose. The amount of such substances may be from 10-100 per cent by weight of the dry weight of the gel. The original gel containing a non-aqueous liquid phase is formed by displacing water from gels by lower alkyl ketones, alcohols, fatty acids or their anhydrides, especially acetone and ethyl alcohol, or ammonia, pyridine or monodi- or tertiary alkyl substituted amines. If desired the displacement of water from the aerogel may be carried out in stages using different organic solvents each of which displaces the solvent already present in the gel, the last displacing solvent being capable of dissolving the film forming materials subsequently added. Using such a method the last displacing solvent may be gasolines, petroleum solvents, benzene or toluene. According to an example, silica acetone gel was made by reacting sodium silicate and sulphuric acid to produce a dispersion of silicic acid with a pH of 6.5 (phosphoric acetic or hydrochloric acid may also be used, the final pH being between 5 and 9). After aging for 24 hours the gel formed was agitated with water to form a smooth paste which was filtered and silica was suspended in acetone or alcohol. The silica now wet with a mixture of water and acetone or alcohol was placed in a continuous liquid extractor where a continuous distillation of the acetone or alcohol was performed. Water was removed from the distillate before recycling and the extraction was continued until all the water was removed from the silica. A large number of examples are provided showing such a gel treated with the film forming agents referred to above, the bulk density of the product lying between 0.06 and 0.18 grams/cc. Similarly, alumina, prepared by precipitating a solution of aluminium sulphate with ammonia, washing and solvent extracting with acetone, may be treated to form a water repellent aerogel of a bulk density of 0.15 grams./c.c.