GB803262A - Organopolysiloxanes having pressure-sensitive adhesive properties - Google Patents

Abstract

A pressure-sensitive adhesive is made by polymerizing a cyclic organopolysiloxane of the formula (R2SiO)n wherein each R is a monovalent hydrocarbon radical and n is at least 3, treating the product with steam to introduce silicon-bonded hydroxyl groups into the molecule, heating the product in the presence of a catalyst until a viscosity between 200,000 and 3 X 106 c.p. at 25 DEG C. is obtained, and intercondensing from 0.5 to 6 parts by weight of the resulting viscous polymer with one part by weight of the product of cohydrolyses of a hydrolysable trialkyl silane and an alkyl silicate. The silanes specified are of the formula R3SiX wherein each R is a methyl, ethyl, propyl, isopropyl, butyl or isobutyl radical and X is halogen, alkoxy or acyloxy. The alkyl silicate may be monomeric or polymeric having a viscosity preferably below 0.5 X 106 centipoises. The co-hydrolysis is effected by adding a solution of the silane and silicate, e.g. in toluene or xylene, to water, the silicate preferably being used in amounts of 1 to 2 mols. per mol of silane. Up to 5 per cent by weight of R3SiX of other silanes, e.g. (CH3)2SiCl2 and CH3SiCl3, may be added to the reactants in making the co-hydrolysate. The cyclic siloxanes disclosed contain specified alkyl, aryl, alkaryl, aralkyl, haloaryl or alkenyl radicals, and are converted to the desired viscous polymers by heating, e.g. at 125 DEG to 150 DEG C., with re-arrangement catalysts such as potassium-, caesium-, tetramethylammonium- or tetrabutyl-phosphonium-hydroxide; the resulting polymer is then terminally hydroxylated by blowing steam across the surface of, or through the polymer and, if desired, is further heated to obtain a higher viscosity material and, finally the re-arrangement catalyst is inactivated or neutralized, e.g. by adding triphenyl phosphate as claimed in Specification 772,088. The adhesive is then made by mixing the viscous hydroxylated polymer either with the cohydrolysate and heating at 100 DEG to 150 DEG C. to remove the solvent, or with the cohydrolysate free of solvent. Examples are given. Uses.-The adhesives when backed with suitable sheet or tape material may be used for insulating conductors, sealing closures, gaskets and in masking operations.