Patent ID: 8003204

Claim:
A microporous polyolefin multilayer film comprising a polyethylene layer comprising a polyethylene having a melting temperature of 125° C. or higher, and a polypropylene and heat-resistant layer comprises 20-60 wt % of propylene having a melting temperature of 160° C. or higher and 80-40 wt % of a heat-resistant resin having a melting temperature of 170° C. or higher, wherein the film has a thickness of 9-30 μm, a piercing strength of 0.15 N/μm or more, a permeability of 1.5×10 -5 Darcy or more and a melt fracture temperature of 170° C. or higher, in which the pores of the polyethylene layer are micropores that are fabricated by stretching and extracting after separating the phase of polyethylene and the diluent and have an average size of 0.1 μm or smaller; and the pores of the polypropylene and heat-resistant layer are macropores that are fabricated by cracking of the interface between the polypropylene and heat-resistant resin and have an average size of 1-50 μm.