Porous vinylidene fluoride polymer membrane and process for its preparation

Claim 1

A process for preparing an ultrafiltration membrane of a resinous, film-forming polymer selected from the group consisting of (a) vinylidene fluoride homopolymer, (b) copolymers of at least 70 weight percent of vinylidene fluoride and up to 30 weight percent tri- or tetrafluoroethylene, and (c) a blend of no less than about 75 weight percent of either (a) or (b) with no greater than about 25 percent of a thermoplastic methyl methacrylate polymer said membrane having a surface layer containing a multiplicity of pores having a diameter within the range of from about 1 to about 100 millimicrons, and a support layer thicker than and having less resistence to fluid flow than said surface layer, said process comprising casting on a smooth solid surface a solution of said resinous, film forming polymer having a polymer concentration ranging from about 12 to about 35 weight percent, about 65 to about 80 weight percent of a solvent selected from the group consisting of triethyl phosphate, hexamethyl phosphoramide, 1-methyl-2-pyrrolidinone, dimethyl acetamide, dimethyl formamide, propylene carbonate and mixtures of said solvents, and from about 1 to about 10 weight percent of a non-solvent for said polymer selected from the group consisting of glycerol, ethylene glycol, phosphoric acid and mixtures thereof, to form a sheet, evaporating a portion of said liquid mixture from said sheet, and contacting said sheet with a liquid gelation medium.