Solvent stretch process for preparing a microporous film

Abstract

The present invention provides an improvement in a 'solvent stretch' process which renders it possible to transverse stretch a microporous film prepared from a precursor film which has been previously uniaxially stretched in a direction perpendicular to the intended transverse stretching direction to thereby improve the permeability of the microporous film while also improving the balance of mechanical properties thereof in both directions of stretch. The improvement lies in uniaxially stretching the precursor film to form a microporous film having a specifically defined permeability as measured by the Gurley test. The requisite permeability of the uniaxially stretched microporous film is obtained by selecting the polymer employed to prepare the precursor film based on certain specifically defined property requirements and by employing specifically defined process conditions during uniaxial stretching. The uniaxially stretched microporous film having the requisite permeability can then be transverse stretched under certain conditions without the likelihood of developing splits or breaks in the biaxially stretched microporous film.