Solvent stretch process for preparing a microporous film

In a process for preparing an open-celled microporous film by (1) contacting a non-porous precursor film having both an amorphous component and a crystalline component with a swelling agent comprising a non-aqueous solvent having a Hildebrand solubility parameter at or near that of the precursor film, for a time sufficient to permit adsorption of the swelling agent into the film, (2) stretching said precursor film in at least one direction while in contact with the swelling agent, and (3) removing said swelling agent while maintaining said film in its stretched state the improvement which comprises:

• (a) providing said precursor film from a homopolymer of polyethylene or mixtures thereof characterized by a melt index of from about 3 to about 20, a density of not less than about 0.960 gm/cc, a molecular weight distribution ratio (M.sub.w /M.sub.n) of from about 3.8 to about 13, an average gel count of not greater than about 2.0 per 90 square inches of precursor film area and a crystallization time at 120.degree. C. of less than about 70 seconds;
• (b) uniaxially stretching the precursor film of (a) having a thickness of from about 0.75 to about 15 mils, while in contact with the swelling agent, at a degree of stretch of from about 70 to about 300%, a strain rate of from about 5 to about 100%/minute, and a temperature of from about 80.degree. to about 95.degree. C. to yield a uniaxially stretched microporous film having a Gurley value of not less than about 2 seconds per mil of film thickness;
• and
• (c) transverse stretching the microporous film of (b) in a direction perpendicular to the uniaxial direction of stretch to increase the permeability of the microporous film of (b) without causing splitting of the resulting biaxially stretched microporous film.