Process for producing composite membrane

Claim 1

A process for producing a composite membrane which comprises: bringing an organic solution of a cross-linkable silicone resin represented by the formula: ##STR11## wherein X 1 , X 2 , X 3 , X 4 , X 5 and X 6 each is a monovalent organic group, with the proviso that at least three groups thereof present at both ends of the molecule each is a reactive group selected from the group consisting of an acyloxy group, an alkoxy group, a ketoxime group, an alkyl-substituted amino group and an amide group, R 2 and R 3 each is an organic group selected from the group consisting of an alkyl group, a halogenated alkyl group, a vinyl group and an aryl group, and n is an integer of from 400 to 1,000, into contact with the surface of a dense layer of an anisotropic membrane made of a polyimide resin comprising a dense surface layer and a porous layer continuously connected to the dense surface layer, said polyimide resin consisting essentially of a repeating unit represented by the formula: ##STR12## wherein A is a tetravalent aliphatic or aromatic group, and R 1 is a divalent aromatic, alicyclic or aliphatic hydrocarbon group, or a divalent organic group comprising the hydrocarbon groups linked together through a divalent organic linking group, and cross-linking the silicon resin to form a cross-linked silicone resin thin film on the dense surface layer of the anisotropic membrane, wherein the average molecular weight of the cross-linkable silicone resin is equivalent to or greater than the molecular cut-off of the polyimide resin anisotropic membrane and wherein the cross-linkable silicone resin is cross-linkable at room temperature.