Preparing method of microporous polyolefin film through effective extrusion

A process for preparing a microporous polyolefin film, which comprises the steps of

• (a) mixing 15-55% by weight of polyolefin (Component I), and 85-45% by weight of a diluent (Component II) which forms thermodynamic single phase with the polyolefin and a diluent (Component III) which can undergo thermodynamic liquid-liquid phase separation with the polyolefin by separately injecting them into an extruder, wherein Components I and II are injected/mixed at a location before the ⅓ of the extruder, and Component (III) at a location between ⅓ and the ⅔ of the extruder, and the temperature after the location of injecting Component (III) is raised above the temperature of liquid-liquid phase separation of the mixture of Components (I), (II) and (III) to make the mixture into thermodynamic single phase within the extruder;
• (b) passing the melt through the section wherein the extrusion temperature is not higher than the liquid-liquid phase separation temperature and provide a sheet;
• and
• (c) extracting Components (II) and (III) from the sheet, and drying.