Novel open-celled microporous film

Claim 2

The process of claim 2 wherein the cold stretching and the hot stretching are conducted at an extension ratio between about 0.10:1 and 0.99:1, and wherein the polymer is selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides.

Claim 3

A process for the preparation of an open-celled microporous polypropylene film characterized by having reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 30 percent, a pore size of less than 5000 Angstroms a nitrogen flux of greater than 35.4 and a breaking elongation of 50 to 150 percent, which comprises: cold stretching at a temperature between 10*C. and 70*C. a non-porous, crystalline, elastic, polypropylene film to develop porous surface regions perpendicular to the stretch direction, the elastic polypropylene film having an initial crystallinity of at least 30 percent and an initial elastic recovery from a 50 percent strain of at least 50 percent at 25*C., hot stretching the resulting unrelaxed cold stretched film at a temperature between 130*C. and 150*C. to a total stretch of 10 to 300 percent of the original length of the elastic film and at an extension ratio between 0.50:1 and 0.97:1 to develop pore spaces elongated parallel to the stretch direction, the cold stretching and the hot stretching being conducted in the same stretch direction, and thereafter heating the resulting microporous film at substantially constant length at a temperature between about 130*C. and 160*C.

Claim 4

The process of claim 4 wherein the polypropylene has a molecular weight of from 200,000 to 500,000 and a melt index of from about 0.5 to 30, and wherein the film is subjected to a total stretch of 50 to 150 percent of its original length and at an extension ratio between 0.60:1 and 0.95:1.

Claim 5

A process according to claim 5 wherein the resulting polypropylene film has a bulk density of about 58 to 70 percent of the corresponding polypropylene film have no open-celled structure, a nitrogen flux of about a 50 to 200 and a surface area of about 30 to 110 sq.m/cc.