MICROPOROUS POLYOLEFIN MEMBRANE, ITS PRODUCTION METHOD AND BATTERY SEPARATOR

1 . A microporous polyolefin membrane comprising a polyethylene resin, and polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 5×10 4 or less being 5% or less by mass of said polypropylene.

2 . A method for producing a microporous polyolefin membrane comprising the steps of (1) extruding a molten blend of a polyethylene resin, polypropylene having a weight-average molecular weight of 6×10 5 or more, and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 5×10 4 or less being 5% or less by mass of said polypropylene, and a membrane-forming solvent from a die, (2) cooling the resultant extrudate to a gel-like sheet, and (3) conducting the stretching of said gel-like sheet and the removal of said membrane-forming solvent one or more times in an arbitrary order.

3 . The method for producing a microporous polyolefin membrane according to claim 2 , wherein said step (3) is removing said membrane-forming solvent after the stretching of said gel-like sheet, stretching said gel-like sheet after the removal of said membrane-forming solvent, or conducting the stretching of said gel-like sheet and the removal of said membrane-forming solvent in this order and then stretching said gel-like sheet.

4 . A battery separator formed by a microporous polyolefin membrane comprising a polyethylene resin, polypropylene having a weight-average molecular weight of 6×10 5 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 5×10 4 or less being 5% or less by mass of said polypropylene.