Surface treating compositions

An organopolysiloxane composition suitable for conferring release properties on substrates comprising 100 parts by weight of a linear diorganopolysiloxane in the form of a curable release composition and from 0.5 to 50 parts by weight based on the linear diorganopolysiloxane of a copolymer consisting essentially of the average general formula:

• ##STR17## where A is selected from the group consisting of R, R.sup.1 Q, alkenyl and hydroxy groups and hydrogen, Q is oxygen or sulphur, R is selected from the group consisting of (a) alkyl, aryl, alkaryl, aralkyl and cycloalkyl groups, (b) groups --R.sup.2 QR.sup.3 where R.sup.2 is a divalent organic group having at least 2 carbon atoms and is joined to silicon by a C-Si linkage and R.sup.3 is a monovalent hydrocarbon group and (c) hydrocarbonoxy groups joined to silicon by a C-Si linkage, R.sup.1 is a hydrocarbon or hydrocarbonoxy group;
• a and d, which will not normally be the same, are 0 or numbers not greater than 950, b is 0 or a number not greater than 999 and c is a number from 1 to 1000, a + b + c + d is not less than 20 and not greater than 1000, (b + c)/(a + b + c + d) is not less than 0.05, at least one of the groups R and R.sup.1 being an alkyl group of 8 to 30 carbon atoms, the proportion of said copolymer in the composition being sufficient to provide at least one of said alkyl groups for each 2000 silicon atoms in the composition.