POROUS MEMBRANE, BATTERY SEPARATOR MADE THEREOF AND PRODUCTION OF THE MEMBRANE

摘要

PROBLEM TO BE SOLVED: To provide a porous membrane having high air permeability and high mechanical strengths. SOLUTION: A nonporous film formed from a crystalline polymer is pre- stretched to a degree of reduction of thickness of 5-35% at a temperature from (Tm-50 deg.C) to Tm deg.C, while it is kept in the nonporous state. The film is then heat-treated and stretched to form a porous membrane. Preferably, the above stretching is carried out in two stages one of which consists of stretching at a temperature as low as -20 to 80 deg.C and the other of which consists of stretching at a temperature as high as (Tm-40) deg.C to Tm deg.C. It is desirable that the crystalline polymer is at least either of polyethylene and polypropylene. The porous membrane should have a porosity of 30-95% and have a tensile strength of 2,050kg/cm<2> or above in at least one direction.