PROCESS FOR PRODUCING MICROPOROUS POLYETHYLENE FILM AND SEPARATOR FOR CELL

摘要

A polyethylene resin and a solvent for film formation are melt-kneaded to prepare a solution (A) having a resin concentration of 25-50 mass% and a solution (B) having a resin concentration of 10-30 mass%, provided that the resin concentration in the solution (A) is higher than the resin concentration in the solution (B). Subsequently, (a) the resin solutions (A) and (B) are simultaneously extruded through a die to superpose the resin solutions (A) and (B) and the extrudate is cooled to form a gel-state sheet, from which the solvent for film formation is removed. Alternatively, (b) the following steps are conducted: separately extruding the resin solutions (A) and (B) through respective dies, removing the solvent for film formation from the resultant gel-state sheets (A) and (B) to form microporous polyethylene films (A) and (B), and alternately bonding the microporous polyethylene films (A) and (B). Thus, a microporous polyethylene film in which the average pore diameter changes in the thickness direction and which has an excellent balance among permeability, mechanical properties, unsusceptibility to heat shrinkage, compressive resistance, electrolytic-liquid-absorbing properties, shutdown properties, and meltdown properties can be produced while easily regulating the distribution of the average pore diameter in the film-thickness direction.