Methods of producing the membranes and the uses of membranes as battery separator films

A process of producing a microporous membrane comprising:

• (1) extruding a mixture of diluent and polymer to form an extrudate, the polymer comprising a first polyethylene having an Mw<1.0×106, a second polyethylene having an Mw≧1.0×106, and a polypropylene having an Mw≧5.0×105 and a ΔHm≧80.0 J/g;
• wherein the sum of the amounts of the polypropylene having an Mw≧5.0×105 and a ΔHm≧80.0J/g and the second polyethylene is ≧15.0 wt. %, all the weight percents being based on the total weight of the polymer in the mixture;
• and
• (2) processing the extrudate into the microporous membrane having a thickness ≧12.0 μm by stretching the extrudate in at least one planar direction at a temperature of about 108.0 to 116.0° C. after removing the solvent to a magnification factor of ≧1.1 and excludes any stretching of the extrudate after said step of removing the solvent at a magnification factor of ≧1.1 and removing at least a portion of the diluent from the extrudate and having a Normalized Pin Puncture strength from 326 to 350 mN/μm;
• wherein processing comprises:
• (a) biaxially stretching the extrudate in at least one planar direction at a magnification factor of from 9-fold to 49-fold in area, while exposing the extrudate to a temperature in the range of 90.0° C. to 125.0° C.,
• (b) removing at least a portion of the diluent from the extrudate,
• (c) cooling the extrudate,
• (d) subjecting the membrane to a thermal treatment, and
• (e) removing any remaining volatile species from the membrane.