Polyester films and their preparation

Claim 1

IN A PROCESS OF MAKING A DIMENSIONALLY-STABLE, COATED POLYMER FILM WHICH COMPRISES FORMING A SHEET OF FILM FROM A MOLTEN HIGHLY POLYMERIC ESTER SUBSTANTIALLY COMPOSED OF THE POLYESTEIFICATION PRODUCT OF DICARBOXYLIC ACID AND A DIHYDRIC ALCOHOL, SAID ESTER HAVING A MELTING POINT FROM 150*C. TO 300*C. AND BEING CAPABLE OF BEING FORMED INTO FILAMENTS WHICH WHEN COLD-DRAWN SHOW BY CHARACTERISTIC X-RAY PATTERNS MOLECULAR ORIENTATION ALONG THE FIBER AXIS, COATING AT LEAST ONE SURFACE OF THE FILM WITH AN AQUEOUS DISPERSION OF A VINYLIDENE CHLORIDE COPOLYMER CONTAINING AT LEAST 35% BY WEIGHT OF VINYLIDENE CHLORIDE, DRYING THE COATED POLYMERIC ESTER FILM, BIAXIALLY ORIENTING THE COATED FILM BY STRETCHING IT AT AN ELEVATED TEMPERATURE, HEATSETTING THE COPOLYMER-COATED FILM AT A TEMPERATURE BETWEEN 150*C. AND 210*C. UNDER CONDITIONS SUCH THAT NO SHRINKAGE OCCURS, COATING THE LAYER OF COPOLYMER WITH AN AQUEOUS SOLUTION CONTAINING A WATER-PERMEABLE COLLOID AND DRYING THE RESULTING LAYER; THE STEP WHICH COMPRISES MODIFYING THE HEAT-SET FILM AFTER THE STRETCHING OPERATION BY HEATING IT TO A TEMPERATURE OF 110*C. TO 150*C. FOR A PERIOD OF 60 TO 300 SECONDS WHILE MAINTAINING SAID FILM UNDER A TENSION OF ABOUT 10 TO 295 POUNDS PER SQUARE INCH.