Patent ID: 8058348

Claim:
A polyolefin microporous film comprising a polyethylene and a polypropylene having a viscosity average molecular weight of 100,000 or more, wherein the polyolefin microporous film has a content of the polypropylene of 4 to 30 wt % and a terminal vinyl group concentration of 2 or more and 20 or less per 10,000 carbon atoms in the polyolefin constituting the microporous film as determined by infrared spectroscopy, wherein the MD film thickness deviation (%) of the polyolefin microporous film is 3% or less, the MD film thickness deviation being determined according to the following procedure: 10 strips measuring 1000 mm (MD) by 30 mm (TD) were cut from a polyolefin microporous film, superimposed on one another and measured for film thickness at 20 locations in a central region of the TD at 50 mm intervals in the MD and Maximum, Minimum and Mean values of the film thickness were determined from the film thicknesses measured and used in the following equation MD Film Thickness Deviation (%)=(Maximum Valueâˆ’Minimum Value)/10/Mean Film ThicknessÃ—100.