Microporous polymeric films and process for their manufacture

A soft, tear resistant, microporous film substatially free of cracks detectable with an electron microscope and consisting essentially of a polymer matrix and a particulate filler material dispersed therein, said film having a water vapor permeability of at least ##EQU4## according to ASTM 96-66BW (inverted cup method) and possessing hydrostatic resistance to the penetration of liquid water of at least 0.25 kPa,

• the film composition consisting essentially of the following components:
• (a) a matrix of a thermoplastic orientable polymer characterized by
• (1) a hydrophobic surface, as indicated by a contact angle between said surface and a droplet of water of at least about 40.degree.;
• (2) a glass transition temperature, Tg, of less than about 25.degree. C.;
• (3) a Vicat softening point, Tv, of more than about 25.degree. C.;
• (4) ultimate elongation at some temperature between Tg and Tv of at least 100%;
• and
• (5) a flexural modulus at 25.degree. C. of less than about 200 MPa;
• said thermoplastic orientable matrix polymer being a polymer selected from the group consisting of copolymers of ethylene and copolyesters;
• (b) about 20-40%, based on the volume of the final composition, of a particulate filler chemically nonreactive with the polymer matrix and dispersed therein, said filler being characterized by
• (1) a softening point sufficiently high to avoid particle distortion at the lowest temperature at which the matrix polyer has sufficient fluidity to be satisfactorily melt-blended;
• (2) an aspect ratio of less than about 3;
• (3) an average equivalent spherical diameter of about 0.05-50 .mu.m;
• (4) an at most negligible electrostatic or dipole attraction to the matrix polymer;
• and
• (5) a surface tension which either has at the lowest temperature at which the matrix polymer is sufficiently fluid to be satisfactorily melt-blended a value at most about equal to that or the matrix polymer or can be corrected to have at that temperature at most such a value by the adsorption on its surface of a third component (c);
• said component (c) being an organic compound, which is liquid at the lowest temperature at which the matrix polymer is sufficiently fluid to be melt-blended and is capable at that temperature of being adsorbed on the surface of filler (b) to the extent of at least about 0.5.times.10.sup.-5 mole of component (c) per square meter of surface area of filler (b), as calculated from the average equivalent spherical diameter of filler (b), and being present in the blend in an amount at least sufficient to satisfy this requirement;
• said component (c) being characterized by
• (1) at most neglible solubility in the matrix polymer at the melting temperature of the matrix polymer;
• (2) chemical inertness to the matrix polymer;
• (3) a number average molecular weight of about 100-10,000;
• and
• (4) a surface tension which at the temperature at which component (c) is liquid has a value at most about equal to that of the matrix polymer.

A process for making a soft, tear resistant, microporous film consisting essentially of a polymer matrix and a particulate filler material dispersed therein, said film being substantially free of cracks detectable with an electron microscope and having a water vapor permeability of at least ##EQU5## according to ASTM 96-66BW (inverted cup method) and possessing hydrostatic resistance to the penetration of liquid water of at least 0.25 kPa,

• said process comprising stretching at a temperature no higher than about 10.degree. C. above the Vicat softening point but at least 10.degree. C. above the glass transition temperature of the matrix polymer a precursor film at most very low porosity and consisting essentially of the following components:
• (a) a matrix of a thermoplastic orientable polymer characterized by
• (1) a hydrophobic surface, as indicated by a contact angle between said surface and a droplet of water of at least about 40.degree.;
• (2) a glass transition temperature, Tg, of less than about 25.degree. C.;
• (3) a Vicat softening point, Tv, of more than about 25.degree. C.;
• (4) ultimate elongation at some temperature between Tg and Tv of at least 100%;
• and
• (5) a flexural modulus at 25.degree. C. of less than about 200 MPa;
• said thermoplastic orientable matrix polymer being a polymer selected from the group consisting of ethylene copolymers and copolyesters;
• (b) about 20-40%, based on the volume of the final composition, of a particulate filler chemically nonreactive with the polymer matrix and dispersed therein by melt-blending, said filler being characterized by
• (1) a sufficiently high softening temperature to avoid particle distortion at the lowest temperature at which the matrix polymer is sufficiently fluid to be satisfactorily melt-blended;
• (2) an aspect ratio of less than about 3;
• (3) an average equivalent spherical diameter of about 0.05-50 .mu.m;
• (4) an at most negligible electrostatic or dipole attraction to the matrix polymer;
• and
• (5) a surface tension which either has at the lowest temperature at which the matrix polymer is sufficiently fluid to be satisfactorily melt-blended a value at most about equal to that of the matrix polymer or can be corrected to have under the blending conditions at most such a value by the adsorption on its surface of a third component (c);
• said component (c) being an organic compound, which is liquid at the lowest temperature at which the matrix polymer is sufficiently fluid to be satisfactorily melt-blended and is capable at that temperature of being adsorbed on the surface of filler (b) to the extent of at least about 0.5.times.10.sup.-5 mole of component (c) per square meter of surface area of filler (b), as calculated from the average equivalent spherical diameter of filler (b), and being present in the blend in an amount at least sufficient to satisfy this requirement;
• said component (c) being characterized by
• (1) at most negligible solubility in the matrix polymer at the melt-blending temperature;
• (2) chemical inertness to the matrix polymer;
• (3) a number average molecular weight of about 100-10,000;
• and
• (4) a surface tension which at the temperature at which component (c) is liquid has a value at most about equal to that of the matrix polymer;
• and recovering the resulting microporous film.