Patent ID: 8710110

Claim:
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<1.0×10 6 , a second polyethylene having an Mw≧1.0×10 6 , and a polypropylene having an Mw≧5.0×10 5 and a ΔHm≧80.0 J/g; wherein the sum of the amounts of the polypropylene having an Mw≧5.0×10 5 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.