Patent ID: 6878484

Claim:
A secondary battery separator made of a resin composition mainly containing a polyolefin having at least one hydrophilic functional group, characterized in that the resin composition has a content (ratio to a total resin amount) of a low-density polyethylene of 20 mass % or less, a piercing strength of 5 to 100 N, a Metsuke of 20 to 75 g/m 2 , a thickness of 15 to 150 μm, and an unneutralized hydrophilic functional group amount of 1×10 −3 to 5×10 −2 mol/m 2 , which amount is defined as follows: unneutralized hydrophilic functional group amount (mol/m 2 )=(total sulfur amount (ppm)×Metsuke (g/m 2 )/32.66/1000−ion exchange amount (meq/m 2 ))/1000.