GB783867A - Improvements in or relating to organopolysiloxane elastomers - Google Patents

Abstract

A finely divided solid organosilicon compound having a bulk density of less than 0.3 g. per cc., for use as a filler in making optically clear siloxane elastomers, as a flattening agent in paints and as a filler in life preservers, is a copolymer composed of (a) from 1 to 50 mol. per cent of RSiO 3/2 units in which R is an aliphatic hydrocarbon radical containing less than 5 carbon atoms; (b) from 4 to 30 mol. per cent of MenSiO 4-n/2 units in which n is 2 or 3 and Me is a methyl radical; and (c) from 46 to 95 mol. per cent of SiO2 units. The compounds are best prepared by the method described in Specification 783,868. A modification of this process involves replacing the water in the co-hydrogel with an organic solvent, e.g. toluene or methylene chloride, and reacting the resulting organogel with dimethyldichloro- or trimethylmonochloro-silane or the corresponding alkoxy silanes or combinations thereof, with subsequent evaporation of the solvent. Transparent elastomers are made from compositions consisting essentially of (1) an organopolysiloxane convertible to the solid elastic state of the general formula R1mSiO 4-m/2 in which each R1 is a monovalent hydrocarbon radical containing less than 10 carbon atoms and m has an average value from 1.9 to 2.1 and at least 50 mol. per cent of the polymer units are dimethylsiloxane units; and (2) as filler the above specified organosilicon compound, preferably in amount of 5 to 50 parts by weight per 100 parts of polymer. The elastomers are made by mixing the polymer, filler and vulcanizing agent, and thereafter vulcanizing the mixture. Thus the compositions may be vulcanized by heating them with organic peroxides, e.g. benzoyl peroxide, tertiary butylperbenzoate and halogenated benzoyl peroxide, or by mixing them with polyalkoxysilicates or hydrogen-containing siloxanes, e.g. methylhydrogensiloxane, together with catalysts such as lead octoate or dibutyl-tin-di-acetate, and allowing the mixture to stand at room temperature. Stabilizers, compression set additives and oxidation inhibitors may also be added. In examples: (1) a cohydrogel composed of SiO2 and PrSiO 3/2 (Pr = propyl) units was cut into small pieces and stirred for 1 hour with conc. hydrochloric acid and isopropyl alcohol and for a further 1 1/2 hours after the addition of hexamethyldisiloxane; after removal of the aqueous phase, which had separated out, the resulting organogel was dried and heated at 110 DEG C.; (2), (3), (4) a cohydrogel containing silicon-bonded methyl; or (6) vinyl radicals was reacted in a similar manner; (5), (9) the products of examples (1) to (4) were used as fillers in making transparent siloxane elastomers; (7) a cohydrogel composed of SiO2 and CH3SiO 3/2 units was extracted with isopropyl alcohol and then with toluene to give an organogel which was mixed with toluene and dimethyldichlorosilane and allowed to stand for 3 days; excess silane, HCl and toluene were then removed to yield a powder which was then used as a filler according to Example (5); (8) a co-organogel containing CH3 radicals was reacted in the manner or Example (7) with trimethylchlorosilane, and the product was used in accordance with Example (5). Specification 607,427 and U.S.A. Specification 2,610,167 also are referred to.