PURPOSE: To produce a microporous polyolefin film having a suitable cell size at a high efficeincy and a low cost by forming a gel sheet of a polyolefin soln. contg. an ultrahigh-molecular weight component, thermally stretching the sheet, and removing the remaining solvent.
CONSTITUTION: A homogeneous polyolefin soln. is obtd. by feeding a crystalline polyolefin homopolymer, two-stage polymer, or copolymer contg. at least 1wt% component having a mol.wt. of 7×105 or higher and a mol.wt. distribution of 5-300 or their blend, if necessary, together with additives such as an antioxidant, to an extruder, melting the polymer, feeding a good solvent through a later feed port in a wt. ratio of the polymer to the solvent of (10:90) to (80:20), and kneading them. The soln. is extruded at 140-250°C into a sheet and cooled to give a gel sheet, which is heated, biaxially stretched, washed with an easily volatile solvent to remove the good solvent to the amt. of the residual solvent of 1wt.% or lower, dried, and thermally set at a temp. in the range from the crystal dispersion temp. to the m.p., thus giving a microporous film having voids of 35-95%, a mean open cell size of 0.001-0.5μm, and a tensile strength of 500g/cm2 or higher.