GB740573A - Improvements in or relating to organopolysiloxane adhesive compositions and adhesive tapes - Google Patents

Abstract

An adhesive composition suitable inter alia for making pressure-sensitive tapes (see Group VIII), comprises a mixture of (a) 5 to 70 per cent by weight of a benzene-soluble copolymer resin consisting of SiO2 and R3SiO 1/2 units and from 0 to 0.2 R2SiO units per SiO2 unit, R being alkyl up to three carbon atoms or phenyl and at least 90 per cent of the R radicals being alkyl, and the ratio of R3SiO 1/2 plus R2SiO units to SiO2 units being from 0.6 : 1 to 0.9 : 1, and (b) 95 to 30 per cent of a benzene soluble organosiloxane having a viscosity of at least one million cs. and having the formula Rl2SiO where R1 is methyl or phenyl and at least 90 per cent of the R1 radicals are methyl. The composition may also contain up to 10 per cent by weight of a titanium orthoester or a hydrocarbon-soluble partial hydrolysate thereof, the organic radicals in the ester being aliphatic or hydroxylated aliphatic having less than 20 carbon atoms. The copolymers (a) may be made by the co-hydrolysis of the appropriate chloro- or alkoxy-silanes in the presence of a water-miscible solvent, but are preferably prepared by the process of Specification 706,719. The compositions may contain curing catalysts such as alkyl zirconates and borates, metal alkoxides, metal salts of carboxylic acids and alkoxy silanes such as ethyl silicate, and fillers, e.g. ferric oxide, titanium dioxide and zinc oxide, up to 3 per cent of the composition. The compositions are prepared by mixing the ingredients alone or in a common solvent such as an aromatic or petroleum hydrocarbon solvent. After coating any suitable surface with the composition the composition is cured by air drying or heating at temperatures up to 300 DEG C. The adhesives will adhere to practically any solid surface including fluorocarbon polymers and polyethylene. A typical copolymer (a) was prepared as follows:-450 grams of isopropanol was added to a solution of 432 grams sodium silicate in 864 grams water acidified with 252 grams concentrated hydrochloric acid, followed by 222 grams trimethylchlorosilane and 288 grams hexamethylsiloxane, and the mixture refluxed for half-an-hour. The resulting resin was separated as a toluene solution. The resin had a (CH3)3Si:SiO2 ratio of 0.8. In examples 1, 2, 4 and 5 compositions containing various quantities of resin (a) prepared as above and a high molecular weight dimethylpolysiloxane or phenyl methylpolysiloxane dissolved in a petroleum hydrocarbon solvent were applied to glass tapes. The adhesive strengths were tested either after removing the solvent by air drying or by drying at 250 DEG C. for 5 minutes. In example 4, 2 per cent (of the total polymer weight) of various titanium esters were added. In example 5 various curing catalysts were employed with heat treatment at 150 DEG or 250 DEG C. for five minutes. In example 6 various other backings for adhesive tapes were employed. In example 7 a different resin (a) was employed.