Patent ID: 7635513

Claim:
A heat resistant microporous membrane characterized by: (1) comprising polyvinylidene fluoride resin spherulites; (2) having a maximum pore diameter of 10-150 nm as determined by a bubble point method; and (3) having a layer structure formed in the direction of membrane thickness, said layer structure containing 5-100% by thickness of a layer defined by the following (A) and 95-0% by thickness of a layer defined by the following (B): (A) a layer in which the spherulites are joined to each other without having boundaries and the micropores are intra-spherulitic voids, said intra-spherulitic voids do not form a layer of porous fused or compressed structure”, and (B) a layer in which void spaces are present with forming boundaries among the adjacent spherulites and the micropores are intra-spherulitic voids and interspherulitic voids.