초록 ▼

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

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.

대표청구항 ▼

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 fil

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 an olefin homopolymer or mixtures thereof characterized by a balance of properties of density, melt index, molecular weight distribution ratio (Mw/Mn), crystallization time, and a average gel count, sufficient to impart to the non-porous precursor film the potential to develop a Gurley value of less than about 2 seconds per mil of film thickness when stretched in a uniaxial direction in accordance with step (b); (b) unaxially stretching the precursor film of (a) while in contact with the swelling agent under conditions of degree of stretch, strain rate, temperature, and precursor film thickness which are controlled in a manner sufficient to yield a uniaxially stretched microporous film having a Gurley value of less than 2.0 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.