a microporous , polymeric film is prepared by dissolving methylene - chloride - soluble , film - forming polymers completely in methylene chloride . then an aqueous phase is prepared containing non - methylene - chloride - soluble , water - soluble thickeners and , if desired , additives , and brought to a viscosity of about 0 . 07 to 0 . 5 pa s . the aqueous phase is then dispersed in the polymer solution and the resultant paste is spread onto a support material or intermediate support , the methylene chloride is evaporated , the resultant membrane is transferred to the actual support , and finally the water is removed by drying .