Patent ID: 7988895

Claim:
A method for producing a microporous polyethylene membrane having an average pore diameter changing in a thickness direction, comprising the steps of melt-blending at least a polyethylene resin comprising ultra-high-molecular-weight polyethylene and high-density polyethylene, and a membrane-forming solvent to prepare a polyethylene resin solution A having a resin concentration of 25 to 50% by mass, melt-blending at least a polyethylene resin comprising the same ultra-high-molecular-weight polyethylene and high-density polyethylene as those in said polyethylene resin solution A, and a membrane-forming solvent to prepare a polyethylene resin solution B having a resin concentration of 10 to 30% by mass, the resin concentration in said polyethylene resin solution A being higher than that in said polyethylene resin solution B; simultaneously extruding said polyethylene resin solutions A and B through a die; cooling the resultant laminate extrudate to provide a gel-like sheet; and removing said membrane-forming solvent from said gel-like sheet.