Method of making microporous material

1. A process for improving the wet out time of an uncoated microporous membrane comprising the steps of:

providing an ultrahigh molecular weight polyethylene (UHMWPE);

providing a particulate silica filler;

providing a processing plasticizer;

mixing UHMWPE, filler and processing plasticizer together to form a mixture, having a weight ratio of filler to UHMWPE of from 1:9 to 15:1 by weight;

extruding said mixture to form a sheet;

calendering said sheet;

extracting all or part of said processing plasticizer from said sheet to produce a matrix comprising UHMWPE and said particulate filler, the filler being distributed throughout said matrix, to produce a microporous matrix sheet; and

calendering said microporous matrix sheet to produce a microporous membrane;

where said microporous membrane exhibits a reduction in wet out time of 50% or more over said microporous matrix sheet.

2. The process for producing an uncoated microporous membrane according to claim 1 further comprising the steps of:

stretching said microporous matrix sheet in at least one stretching direction to a stretch ratio of at least about 1.5 to produce a stretched microporous matrix sheet prior to the calendering step;

calendering said stretched microporous matrix sheet to produce a microporous membrane;

where said microporous membrane exhibits a reduction in wet out time of 50% or more over said stretched microporous matrix sheet.

3. The method for producing a microporous material according to claim 1 where said step of extruding said mixture to form a sheet includes casting or blowing.

4. A process for improving pore uniformity in a microporous membrane comprising the steps of:

providing an ultrahigh molecular weight polyethylene (UHMWPE);

providing a particulate silica filler;

providing a processing plasticizer;

mixing UHMWPE, filler and processing plasticizer together to form a mixture, having a weight ratio of filler to UHMWPE of from 1:9 to 15:1 by weight;

extruding said mixture to form a sheet;

calendering said sheet;

extracting all or part of said processing plasticizer from said sheet to produce a matrix comprising UHMWPE and said particulate filler, the filler being distributed throughout said matrix, to produce a microporous matrix sheet; and

calendering said microporous matrix sheet to produce a microporous membrane;

where said microporous membrane has no pores greater in size than 0.5 micrometers; and

where median pore size is between, or equal to, 0.01 to 0.3 micrometers and the pores vary in size by plus or minus 0.2 micrometers or less.

5. A process for improving pore uniformity in a microporous membrane comprising the steps of:

providing an ultrahigh molecular weight polyethylene (UHMWPE);

providing a particulate silica filler;

providing a processing plasticizer;

mixing UHMWPE, filler and processing plasticizer together to form a mixture, having a weight ratio of filler to UHMWPE of from 1:9 to 15:1 by weight;

extruding said mixture to form a sheet;

calendering said sheet;

extracting all or part of said processing plasticizer from said sheet to produce a matrix comprising UHMWPE and said particulate filler, the filler being distributed throughout said matrix, to produce a microporous matrix sheet; and

calendering said microporous matrix sheet to produce a microporous membrane;

where said microporous membrane has no pores greater in size than 1.0 micrometers; and

where change in volume divided by log d for the pores of this microporous membrane is less than 2 cc/g for the entire pore distribution.