MICROPOROUS POLYOLEFIN COMPOSITION MEMBRANE, PRODUCTION METHOD THEREOF AND BATTERY SEPARATOR

摘要

A microporous polyolefin composite membrane, preferably for use as a battery separator, comprising a microporous polyolefin membrane and a polyolefin nonwoven fabric laminated on at least one surface of the microporous polyolefin membrane. The composite membrane has a thickness of 25 to 200 .mu.m, a porosity of 30 to 70%, an air permeability of 100 to 2000 sec/100cc and a surface opening area ratio of 50 to 90% on at least one outer surface thereof. The microporous polyolefin membrane comprises a matrix polyolefin component which is a polyolefin having a weight average molecular weight of 5 x 10 5 or more or a polyolefin mixture containing the polyolefin having a weight average molecular weight of 5 x 10 5 or more, and has a porosity of 30 to 95%, an air permeability of 100 to 2000 sec/100cc, an average open pore diameter of 0.001 to 1 .mu.m and a tensile strength at break of 500 kg/cm2 or more. The microporous polyolefin membrane may further comprise a shutdown polymer component to shut down the pores, thereby making the composite membrane impermeable. The polyolefin nonwoven fabric comprises fine fibers and has an air permeability of 0.1 to 100 sec/100cc and a basis weight of 5 to 50 g/m2. The polyolefin nonwoven fabric prevents the composite membrane from melting down at a low temperature thereby preventing the short-circuit between the electrodes.