Patent ID: 7140496

Claim:
A multilayer microporous membrane containing a thermoplastic resin, said membrane comprising: a coarse structure layer with a higher open pore ratio and a fine structure layer with a lower open pore ratio, wherein said coarse structure layer is present at least in one membrane surface has a thickness of not less than 5.0 μm, and has an open pore ratio gradient that continuously decreases from the membrane surface toward said fine structure layer; said fine structure layer has a thickness of not less than 50% of the whole membrane thickness and has a microporous and homogeneous structure in which the open pore ratio is within the range of (an average open pore ratio of the fine structure layer) ±2.0% inclusive of both limits; said coarse structure layer and said fine structure layer are formed in one-piece; and the maximum pore diameter of said membrane determined by the bubble point method is 10–100 nm.