| 摘要 |
PROBLEM TO BE SOLVED: To prepare a microporous film which maintains basic properties as a separator for secondary batteries and has extremely high piercing strength and tensile fracture strength and low TD shrinkage by biaxially stretching a molded sheet comprising a polyolefin and a plasticizer, extracting the plasticizer from the sheet, stretching the sheet, and applying shrinkage relaxation in the TD direction. SOLUTION: A mixture comprising a polyolefin with a viscosity average molecular weight of 150,000-1,000,000 and a plasticizer is melted and kneaded, molded into a sheet, cooled and solidified. This sheet is inserted 10-100 mm inside from the edge into holding chucks of an extruder to hold the sheet to biaxially stretch the sheet. Subsequently, the solvent is extracted and the sheet is stretched at least in one direction and shrinkage relaxation is applied in the TD direction. The micorporous film has a porosity of 20-70%, an air permeability of 1-2,000 sec, a piercing strength of 1,000-3,000 g/25μm, a tensile fracture strength of 1,700-7,000 kg/cm2, a TD maximum shrinkage of 0-15 kg/cm2, an average pore diameter of 0.01-0.08μm and a flexion ratio of pores of 2.5-7.0. |
