Process for biaxially producing oriented tubular polyethylene terephthalate films by simultaneous stretching

Claim 1

A PROCESS FOR PRODUCING SIMULTANEOUSLY BIAXIALLY ORIENTED TUBULAR POLYETHYLENE TEREPHTHALATE FILMS BY SIMULTANEOUS STRETCHING WHICH COMPRISES EXTRUDING A MOLTEN POLYETHYLENE TEREPHTHALATE RESIN FROM AN ANNULAR DIE, INTRODUCING THE COOLED TUBULAR NON-STRETCHED FILM BETWEEN TWO PAIRS OF SPEED CONTROLLING DEVICES WHICH HAVE A DIFFERENT CIRCUMFERENTIAL SPEED, AND EXPANDING IN A STRETCHING ZONE BY APPLYING GAS PRESSURE INSIDE THE TUBULAR FILM WHILE HEATI*G, BY WHICH THE TUBULAR FILM IS SIMULTANEOUSLY BIAXIALLY STRETCHED 2.5 TO 4.0 TIMES IN BOTH DIRECTIONS, WHEREIN SAID PROCESS IS CHARACTERIZED BY: THE CRYSTALLINITY OF THE NON-STRETCHED FILM IS 3% TO 18%; THE LENGTH OF THE STRETCHING ZONE FROM THE STARTING POINT FOR STRETCHING THE FILM TO THE END POINT OF STRETCHING THEREOF IS KEPT IN THE RANGE OF 1.0-1.5 TIMES OF THE MAXIMUM EXPANSION DIAMETER OF THE STRETCHED FILM; THE FILM TEMPERATURE AT THE INITIAL ONE-THIRD OF THE STRETCHING ZONE IS KEPT AT 83*-108*C; THE FILM TEMPERATURE IS CONTINOUSLY DECREASED IN THE REMAIN: ING TWO-THIRDS OF SAID STRETCHING ZONE; AND THE FILM TEMPERATURE AT THE STRETCHING END POINT IS KEPT AT 70*-78*C.