Microporous membranes having increased pore densities and process for

Abstract

Disclosed are a process for forming an open-celled microporous membrane (e.g., in film or fiber form) and a novel microporous membrane formed thereby. The process generally includes sequential cold and hot stretching operations, whereby the cold stretching operation is preferably accomplished in a plurality of discrete cold stretching steps. In this regard, the total cold stretch extension is of greater than about 30 percent, and advantageously greater than about 40 percent based upon the initial unstretched length of the nonporous precursor, with this total cold stretch ratio being distributed among a plurality of discrete cold stretching steps. The novel membranes of this invention are characterized by decreased pore size and increased pore densities. For example, the membranes of this invention will have an average pore radius as determined by mercury porosimetry of less than about 0.040 micron. The membranes of this invention, moreover, exhibit a distinctive 'bluish' color hue (i.e., a Macbeth Coloreye.TM. b* value of less than about -10 as determined by placing a single ply sample of the membrane against a black background).