Process for preparing microporous polyethylene film by uniaxial cold and hot stretching

A process for producing a microporous polyethylene film, comprising:

• (a) melt extruding at a drawdown ratio of about 20:1 to about 200:1 a polyethylene resin having a density of at least approximately 0.960 gm./cm..sup.3 and having at least 99% by weight ethylene to form an extruded precursor film;
• (b) annealing said extruded film at a temperature in the range of from about 10.degree. C. to about 25.degree. C. less than the crystalline melting point of the resin so as to improve the crystallinity of the extruded precursor film and so as to form an unstretched annealed precursor film;
• (c) uniaxially cold stretching said annealed precursor film at a temperature in the range of from about -20.degree. C. to about 70.degree. C. at a cold stretching rate of between about 380 percent per minute and about 440 percent per minute, based on the length of said annealed precursor film, to achieve a cold stretched length of between about 190 percent and 200 percent, said cold stretched length being based on the length of said annealed precursor film, to form a cold stretched precursor film;
• (d) hot stretching said cold stretched precursor film, in the same uniaxial direction as said cold stretching, at a temperature in the range of from above the temperature in (c) to a temperature in the range of from about 10.degree. C. to about 25.degree. C. less than the crystalline melting point of the resin so as to maintain the crystallinity of the cold stretched precursor film at a hot stretching rate of about 105 percent per minute, based on the length of the annealed precursor film, to achieve a hot stretched length of about 240 percent, said hot stretched length being based on the length of said annealed precursor film, to form a hot stretched precursor film;
• and
• (e) heat relaxing said hot stretched precursor film under tension at a temperature in the range of from above the temperature in (c) to less than or equal to the temperature in (b) so as to maintain the crystallinity of the hot stretched precursor film to achieve a decrease in length of said hot stretched precursor film, in the same uniaxial direction as said cold stretching, of between 0 percent and about 30 percent, said decrease in length being based on the length of said annealed precursor film, so as to form a microporous polyethylene film.