Patent ID: 8323547

Claim:
A method of manufacturing a microporous polyethylene film, comprising the steps of: melt-mixing a resin mixture, containing 20-55 weight % of polyethylene (component I) and 80-45 weight % of a diluent (component II) having a liquid-liquid phase separation property from the polyethylene (the component I) at a liquid-liquid separation temperature of 160-280° C., at a temperature higher than the liquid-liquid phase separation temperature to make a molten material of thermodynamic single phase, carrying out liquid-liquid phase separation by passing the molten material of single phase through a phase separating zone which is maintained at a temperature which is lower than the temperature of liquid-liquid phase separation −10° C. for a residence time of longer than 30 seconds; molding the liquid-liquid separated molten material in the form of a sheet; stretching the sheet in the form of a film; extracting the diluent (component II) from the film to make a microporous polyethylene film and drying the microporous polyethylene film; and heat-setting the dried microporous polyethylene film by removing residual stress to have a shrinkage percentage of the film of 5% or less.