Abstract: A multilayer membrane is composed of a carrier layer, a porous supporting layer and a separating layer. The carrier layer consists of fibers of polyamide, polyvinylidene difluoride, polyester or glass; the porous supporting layer consists of a polysulfone, polyvinylidene difluoride, polyethersulfone, polyimide, polyvinyl alcohol, polyurethane, polyacrylonitrile or polyether imide; the separating layer consists of polyvinyl alcohol. The separating layer is nonporous and has a structure having a high microcrystalline content. A process of manufacturing that membrane is also described.

Abstract: The process of making a multilayer membrane composed of a carrier layer, a porous supporting layer and a separating layer includes making a modified polyvinyl alcohol by decomposing a polyvinyl ester or ether until the proportion of the vinyl alcohol monomer units in the modified polyvinyl alcohol corresponds to that in a material consisting of 80 to 95% of the polyvinyl ester or ether and 20 to 5% of a polyvinyl alcohol obtained by completely decomposing the polyvinyl ester or ether; preparing a composite material consisting of the carrier layer and the porous supporting layer, preparing a dilute aqueous solution containing 1 to 10% by weight of the modified polyvinyl alcohol; preferably storing the dilute solution at temperatures from -10.degree. to +10.degree. C., preferably from 0.degree. to 5.degree. C.