GB658979A - Organosiloxane films and film-forming compositions containing polysiloxanes - Google Patents

Abstract

A liquid cellulosic composition suitable for preparing films and laminated sheet material comprises (1) an organosiloxane polymer comprising organo-substituted silicon oxide units, at least 50 per cent of the total number of silicon atoms being present as mono-organosilsesquioxane units RSiO1.5 (R being a mono-valent hydrocarbon radical), any remaining silicon atoms being present as silicon oxide units having 3, 2 or no carbon-silicon bonds and which contains an average of 0.8-1.7 hydro-carbon radicals per silicon atom, 10-60 per cent of all the hydrocarbon radicals being phenyl radicals and the remainder being alkyl radicals; (2) an ethyl cellulose containing 45-52 per cent of ethoxy radicals, and (3) less than 5 per cent by weight (calculated on total solids) of a setting catalyst for siloxane resins, in a common organic solvent. The latter may consist of or contain an aromatic hydrocarbon such as benzene and a film may be prepared by applying the composition on an impervious surface, removing the solvent, curing the film by heating, and thereafter stripping it from the surface. Laminated sheet material may be similarly prepared (see Group VIII). The composition may comprise 60-80 parts by weight of the organosiloxane polymer and 40-20 parts of the ethyl cellulose dissolved in an aromatic hydrocarbon solvent. Examples of organo-substituted silicon oxide units having 3, 2 or no carbon-silicon bonds are R3SiO0.5, R2SiO and Si(O0.5)4, R being a monovalent hydrocarbon radical, such as a methyl, ethyl, propyl, butyl or phenyl radical, or combinations thereof. The organosiloxanes are preferably produced by the hydrolysis of mixed monosilanes, but they may also be produced by the separate hydrolysis of individual silanes and the limited interpolymerization of the hydrolysis products in the desired proportions. The hydrolysates may be bodied by heating a solution of the hydrolysate to produce a resin of increased viscosity. The hydrolysates, the bodied resins or a mixture of the two may be used in the preparation of films or laminated products. Preferred setting catalysts are basic organic compounds such as the ethanolamines, conventional paint driers such as the heavy metal salts of organic acids, or mixtures of both types of compounds. An anti-oxidant may be present in the filmforming composition in a proportion of less than 5 per cent by weight based on total solids, specified anti-oxidants being diphenyl amine, p,p1-dihydroxydiphenylamine, p,p1-di-ethoxy-diphenyl amine and N,N1-diphenyl-1,4-phenylene di-amine or the anti-oxidants disclosed in Specification 594,154. The composition may contain up to 3 per cent by weight (based on total solids) of a parting agent such as one of the higher alkyl carboxylic acids, e.g. stearic acid, particularly if the film is to be formed on a metal casting surface. The film-forming composition may be cast as a thin coating on to an endless band or drum, the casting surface of which may be metal, rubber, or glass and may be coated with a composition impervious to the film solvent. The film solution on the casting surface may be passed through a heating zone at 75-210 DEG C. to evaporate the solvent and cure the film to a non-tacky state. The cured film is stripped from the coating surface in the form of sheets. In an example 67 per cent by weight of a siloxane resin in the form of a toluene solution, 26.5 of ethyl cellulose (containing 48.5 per cent by weight of ethoxyl groups) in solution in a benzene-methanol mixture, 2 of diphenylamine, 3 of triethanolamine and 1.5 of stearic acid are mixed, the resultant solution cast on polished glass plates and the films dried in an oven at 118 DEG C. for 30 minutes, and after cooling stripped from the glass surface. The siloxane resin used is a copolymer having the composition CH3SiO1.5 32 mol per cent, C6H5SiO1.5 34 mol per cent, and (CH3)2SiO 34 mol per cent, prepared by the cohydrolysis of the corresponding hydrolysable silanes in a toluene-water hydrolysis medium. Data are given for the electrical properties of films prepared according to the above example and a similar example. Films, tapes and laminates prepared from the compositions of the invention are stated to be more heat-resistant than films &c. prepared from conventional organic materials and to retain their dielectric properties better than glass fibre cloth. Specifications 563,995, 596,768, 597,834, 598,099, 598,100, 608,956, 643,018 and 650,208, and U.S.A. Specifications 2,371,068, 2,449,572, 2,460,799, 2,518,160 and 2,521,672 are referred to. The Specification as open to inspection under Sect. 91 comprises also film-forming compositions containing organosiloxane polymers in which the nature and range of proportions of the aryl radicals is not specified, the hydrocarbon radical/silicon atom range of ratios of the organosiloxane polymers other than the mono-organosilsesquioxane units is not stated, the range of proportions of etherifying radicals of the cellulose ether component is not given, and the etherifying radicals in the cellulose ether may be propyl, benzyl and benzyl ethyl. Examples are given of the preparation of films from compositions containing only a siloxane resin and an ethyl cellulose with and without a cobalt naphthenate drier dissolved in a mixture of solvents. This subject-matter does not appear in the Specification as accepted.