| abstract |
A polyolefin microporous film comprising polyethylene and polypropylene having a viscosity average molecular weight of 100,000 or higher, the polypropylene being contained in an amount of 4 wt % or more and the terminal vinyl group concentration per 10,000 carbon atoms in the polyolefin constituting the microporous film being 2 or more as measured by infrared spectroscopy. The microporous film has both of a good film breaking resistance and a low thermal shrinkage, is excellent in fuse property, and has a uniform film thickness. Particularly, the microporous film can retain safety in the oven safety test at 150° C. for a battery which is required for use as a lithium ion battery separator. |