Casting films of polymers of meta-phenylene isophthalamide

Claim 1

A process for preparing a film of a wholly aromatic polyamide, at least 85% of the repeating units of which are meta-phenylene isophthalamide units, said polyamide having an inherent viscosity of at least 1.2 deciliters/g in concentrated sulfuric acid at 30° C. at a concentration of 0.5 gram of polymer per 100 cc of solution, said process comprising (1) casting onto a support a film of a solution which comprises (a) a liquid from the class consisting of N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone and dimethylsulfoxide, said polyamide having a solubility at 20° C. of at least 0.5% by weight in said liquid, (b) from about 0.2% to about 15% by weight based on said liquid of a salt from the class consisting of calcium bromide, sodium bromide, ammonium bromide, lithium bromide, lithium chloride, calcium chloride, and magnesium chloride, said salt being characterized in that it forms a solution in said liquid which has a specific resistance of less than about 5000 ohm-centimeters as measured on a solution of 1/100 of an equivalent of said salt in one liter of said liquid at 20° C., (c) from about 3 to about 30% by weight of said polyamide based on said liquid, and (d) up to 7% by weight of water based on the total weight of said liquid salt, polyamide and water; (2) removing said liquid from said film by vaporizing it with heat at a temperature no greater than 5° C. below the boiling point of said solution until the amount of said liquid in said film is in the range of 50 to 160% by weight based on said polyamide; (3) washing additional said liquid from said film with water until the amount of said liquid in said film is less than 3% by weight and of said salt in said film is less than 0.3% by weight, each based on said polyamide; (4) removing water remaining in said film from said film by vaporizing it with heat below a temperature of about 150° C. until the amount of water in said film is less than 15% by weight based on said polyamide while maintaining said film at constant length and width or permitting the film to shrink at most by about 12% in each of the length and width; (5) removing said film from said support at a time during any of steps (2), (3) or (4); and (6) heat treating said film at a temperature in the range of 400° to 300° C. for a period of time ranging from about 2 seconds to 2 minutes while maintaining the film at constant length and width or permitting the film to shrink by at most 20% in each of the length and width.