METHOD FOR PRODUCING MICROPOROUS POLYOLEFIN MEMBRANE

1 . A method for producing a microporous polyolefin membrane comprising the steps of extruding a melt blend of a polyethylene resin, polypropylene having a mass-average molecular weight of 1×10 5 or more, and an ethylene content of 5% by mass or less, and a membrane-forming solvent through a die, slowly cooling the resultant extrudate to form a gel-like sheet, stretching the gel-like sheet, and removing said membrane-forming solvent, the slow-cooling speed of said extrudate being 30° C./second or less.

2 . The method for producing a microporous polyolefin membrane according to claim 1 , wherein said melt blend is extruded from a double-screw extruder, wherein the number of screw revolution Ns in said double-screw extruder is 300 rpm or more, and wherein a ratio Q/Ns of the amount of the extrudate Q (kg/h) to the number of screw revolution Ns (rpm) is 0.3 kg/h/rpm or less.

3 . The method for producing a microporous polyolefin membrane according to claim 1 , wherein said polyethylene resin comprises ultra-high-molecular-weight polyethylene having a mass-average molecular weight of 5×10 5 or more, and high-density polyethylene having a mass-average molecular weight of 7×10 4 or more and less than 533 10 5 , wherein the content of said ultra-high-molecular-weight polyethylene is 1% by mass or more per the total amount (100% by mass) of said polyethylene resin and said polypropylene, and wherein the content of said polypropylene is 1 to 30% by mass.

4 . The method for producing a microporous polyolefin membrane according to claim 1 , wherein after said membrane-forming solvent is removed, it is stretched again to 1.1 to 2.5 fold in at least one direction at a temperature ranging from the crystal dispersion temperature of said polyethylene resin to the melting point of said polyethylene resin +10° C.