Method for preparing microporous polyolefin film by thermally-induced phase separation method

A method for preparing a microporous polyolefin film comprising:

• (a) a step of injecting a composition comprising polyolefin 30 to 60 wt % and a diluent mixture 40 to 70 wt % together into an extruding machine, and melting and kneading thereof to prepare a single phase melt, wherein the diluent mixture comprises a diluent, which can make liquid-liquid phase separation with the polyolefin thermodynamically and a supplementary diluent forming a thermodynamic single phase with the polyolefin, and weight ratio of the diluent and the supplementary diluent is 3:7 to 7:3;
• and
• (b) a step of extruding the melt while conducting liquid-liquid phase separation by passing through a section having the temperature below the liquid-liquid phase separation temperature and forming thereof in the form of a sheet, wherein Hansen solubility parameter distance (Ra), expressed by the following formula of the diluent mixture to the polyolefin is 4.0 to 6.5 (J/cm3)1/2:
• Ra (J/cm3)1/2=[4*(δDP−δDD)2+(δPP−δPD)2+(δHP−δHD)2]1/2
• wherein δDP, δPP and δHP represent polyolefin solubility parameter generated by non-polar dispersion energy, polyolefin solubility parameter generated by polar energy by permanent dipole moment and polyolefin solubility parameter generated by energy by hydrogen bonds, respectively;
• and
• δDD, δPD and δHD represent diluent solubility parameter generated by non-polar dispersion energy, diluent solubility parameter generated by polar energy by permanent dipole moment and diluent solubility parameter generated by energy by hydrogen bonds, respectively;
• stretching the formed sheet to prepare a film;
• extracting the diluent and the supplementary diluent from the stretched film by using an organic solvent;
• drying the film;
• and
• heat-setting the dried film to prepare a microporous polyolefin film,
• wherein the diluent comprises ester of trimethylolpropane or ester of triethyleneglycol,
• wherein the supplementary diluent comprises at least one diluent selected from the group consisting of liquid paraffin, paraffin oil, mineral oil and paraffin wax.