Microporous films

A method of making a polymeric film which comprises:

• (a) mixing together a first component which is a halopolymer in which the repeating units are --(C.sub.n H.sub.2n)-- and --(C.sub.m X.sub.2m)-- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, more than 150 parts by weight per 100 parts by weight of the halopolymer of a second component which is an extractable salt comprising lithium carbonate, and not more than 80 parts by weight per 100 parts by weight of the halopolymer of a third component which is an extractable polymer selected from the group consisting of polyalkylene oxides and glycols, the extractable polymer being less viscous than the halopolymer and incompatible therewith when both are molten;
• (b) melt processing the mixture to form a film in which the extractable polymer has at least partially migrated to the surfaces;
• and
• (c) extracting at least some of the extractable salt to convert the film into a three-dimensional network structure including communicating pores and extracting at least some of said polymer to increase the number of pores opening through the major surfaces of the film, said film having a porosity of more than about 50% by volume.

A method of making a polymeric film which comprises:

• (a) mixing together a first component which is a halopolymer in which the repeating units are --(C.sub.n H.sub.2n)-- and --(C.sub.m X.sub.2m)-- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, more than 150 parts by weight per 100 parts by weight of said halopolymer of an extractable salt comprising lithium carbonate having a nominal upper limit of particle size of 15 microns, not more than 80 parts by weight per 100 parts by weight of the halopolymer of an extractable polymer selected from the group consisting of polyalkylene oxides and glycols, said extractable polymer being less viscous than said halopolymer and incompatible therewith when both are molten;
• (b) melt processing said mixture to form a film in which said extractable polymer has at least partially migrated to the surface, and further deforming said film to reduce its thickness to less than about 75 microns;
• and
• (c) extracting at least some of said extractable salt to convert said film into a three-dimensional network structure including communicating pores, and extracting at least some of said extractable polymer to increase the number of pores opening through the major surfaces of said film, said film having a mean pore size of greater than about 0.4 microns.

A method of making a polymeric film which comprises:

• (a) mixing together a first component comprising a halopolymer in which the repeating units are --(C.sub.n H.sub.2n)-- and --(C.sub.m X.sub.2m)-- in which each X independently represents fluorine or chlorine and the values of n and m are greater than one and less than six, more than 150 parts by weight per 100 parts by weight of said halopolymer of an extractable salt comprising lithium carbonate, not more than 80 parts by weight per 100 parts by weight of said halopolymer of an extractable polymer selected from the group consisting of polyalkylene oxides and glycols, said extractable polymer being less viscous than said halopolymer and incompatible therewith when both are molten, the molecular weight of said extractable polymer being less than about one million;
• (b) melt processing the mixture to form a film in which said extractable polymer has at least partially migrated to the surface and further deforming said film to reduce its thickness to less than about 75 microns;
• and
• (c) extracting at least some of said extractable salt to convert said film into a three-dimensional network structure including communicating pores, and extracting at least some of said extractable polymer to increase the number of pore openings through the major surfaces of said film, said film having a porosity of not less than about 55% by volume and an airflow through said film of at least about 200 cm.sup.3 cm.sup.-2 min.sup.-1 at 20 psi.