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

1. 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 40 to 70 wt %, which can make liquid-liquid phase separation with the polyolefin thermodynamically, into an extruding machine, and melting and kneading thereof to prepare a single phase melt;

(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;

preparing a film by stretching the formed sheet to prepare a film;

extracting the 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 Total Hansen solubility parameter of the diluent is higher than Total Hansen solubility parameter of the polyolefin,

wherein Total Hansen solubility parameter of the diluent is higher than Total Hansen solubility parameter of the polyolefin by 0.1 to 3.0 (J/cm 3 ) 1/2 , and

wherein the diluent comprises an ester of trimethylolpropane or an ester of triethyleneglycol.

2. The method for preparing a microporous polyolefin film according to claim 1 , wherein the temperature difference (delta T) between a T-die of the extruding machine and a casting roll when extruding is 120 to 160° C.

3. The method for preparing a microporous polyolefin film according to claim 1 , wherein the composition further comprises a supplementary diluent forming a thermodynamic single phase with the polyolefin.

4. The method for preparing a microporous polyolefin film according to claim 3 , wherein the supplementary diluent comprises at least one selected from the group consisting of liquid paraffin, paraffin oil, mineral oil and paraffin wax.

5. The method for preparing a microporous polyolefin film according to claim 3 , wherein weight ratio of the diluent to the supplementary diluent is 3:7 to 7:3.

6. The method for preparing a microporous polyolefin film according to claim 1 , wherein the liquid-liquid phase separation forms uniform pore size by slowly lowering the temperature of the melt (T-die temperature) beyond a filter of the extruding machine to 200° C., and by controlling retention time.

7. The method for preparing a microporous polyolefin film according to claim 2 , which increases crystallinity and forms uniform pore size by lowering the temperature difference (delta T) between the T-die and the casting roll of the extruding machine when extruding.

8. The method for preparing a microporous polyolefin film according to claim 1 , which further comprises a step of aging the prepared microporous polyolefin film.