Process for production of biaxially oriented polypropylene film

Claim 1

A PROCESS FOR THE PRODUCTION OF BIAXIALLY ORIENTED FILM WHICH COMPRISES THE FOLLOWING CONTINUOUS, SEQUENTIAL STEPS: (A) EXTRUDING POLYPROPYLENE RESIN, HAVING AN ISOTATIC POLYPROPYLENE CONTENT OF GREATER THAN ABOUT 70% BY WEIGHT, AT A TEMPERATURE OF ABOUT 350* TO 550*F. FROM AN EXTRUSION ZONE IN A WEB THICNESS OF ABOUT 8 TO 12 TIMES FINISHED WEB THICKNESS AND AT A DRAW DOWN RATIO ABOVE ABOUT 7:1 TO OBTAIN MAXIMUM MELT ORIENTATION; (B) RAPIDLY COOLING SID EXTRUDED WEB IN A ZONE CONTAINING SUBSTANTIALLY A RIPPLE-FREE LIQUID QUENCH WHOSE UPPER SURFACE IS MAINTAINED AT A DISTANCE OF ABOUT 3/8 TO 2 INCHES FROM THE EXIT POINT OF THE WEB FROM SAID EXTRUSION ZONE; (C) WITHDRAWING SAID EXTRUDED WEB FROM THE LIQUID QUENCH AND STRETCHING THE WEB LIN A LONGITUDINAL DIRECTION AT A TEMPERATURE OF ABOUT 200* TO 300*F. TO A STRETCH RATIO OF ABOUT 1.2:1 TO 1.6:1; (D) HEATING THE LONGITUDINALLY STRETCHED WEB, WHILE SAID WEB IS UNDER LONGITUDINAL TENSION, TO A TEMPERATURE OF ABOUT 250* TO 300*F.; (E) STRETCHING THE HEATED LONGITUDINALLY STRETCHED WEB IN A TRANSVERSE DIRECTION AT A TEMPERATURE OF ABOUT 200* TO 300*F., WHILE SAID WEB IS UNDER LONGITUDINAL TENSION, TO OBTAIN A STRETCH RATIO OF ABOUT 6:1 TO 12:1; (F) COOLING THE RESULTING BIAXIALLY ORIENTED POLYPROPYLENE FILM TO ABOUT AMBIENT TEMPERATURE; AND (G) RECOVERING THE BIAXIALLY ORIENTED POLYPROPYLENE FILM.