Stretched microporous material

A method for producing stretched microporous material comprising:

• (a) forming a sheet from a mixture comprising
• (1) essentially linear ultrahigh molecular weight polyolefin which is essentially linear ultrahigh molecular weight polyethylene having an intrinsic viscosity of at least about 18 deciliters/gram, essentially linear ultrahigh molecular weight polypropylene having an intrinsic viscosity of at least about 6 deciliters/gram, or a mixture thereof,
• (2) finely divided particulate substantially water-insoluble siliceous filler, the weight ratio of said filler to polymer in the mixture being in the range of from about 1:1 to about 9:1, and
• (3) processing plasticizer which is a liquid at room temperature;
• (b) substantially removing said processing plasticizer from said sheet to form precursor microporous material having at least
• (1) a matrix consisting essentially of said essentially linear ultrahigh molecular weight polyolefin,
• (2) finely divided particulate substantially water-insoluble siliceous filler distributed throughout said matrix, said filler constituting from about 50 percent to about 90 percent by weight of said precursor microporous material, and
• (3) a network of interconnecting pores communicating throughout said precursor microporous material, said pores constituting from about 35 percent to about 80 percent by volume of said precursor microporous material;
• and
• (c) stretching said precursor microporous material in at least one stretching direction to a stretch ratio of at least about 1.5 to produce stretched microporous material which is dimensionally stable at room temperature, which has a stretch ratio in said stretching direction of at least about 1.5, and which has at least
• (1) a matrix consisting essentially of said essentially linear ultrahigh molecular weight polyolefin, said matrix of said stretched microporous material comprising regions of stretch-induced molecularly oriented ultrahigh molecular weight polyolefin distributed throughout said matrix of said stretched microporous material,
• (2) said filler distributed throughout said matrix of said stretched microporous material, and
• (3) a network of interconnecting pores communicating throughout said stretched microporous material, said pores of said stretched microporous material constituting more than 80 percent by volume of said stretched microporous material.