Process for the manufacture of polypropylene films

Abstract

Abstract 1,069,450. Making polypropylene films. KALLE A.G. Sept. 28, 1964 [Sept. 28, 1963], No. 39432/64. Heading B5B. In a continuous process for making shrinkable films of polypropylene by extrusion of predominantly isotactic polypropylene at a temperature of 190‹ to 325‹ C. to form a sheet of melt, quenching the extruded melt to below 90‹ C., to form a film having a thickness greater than 0À3 mm. and a minimum crystallinity of 60% stretching the film in the longitudinal and transverse directions and cooling the biaxially stretched film under tension to below 40‹ C., the film is (a) preheated to a temperature ranging from (T 1 -5‹ C.) to T 1 (T 1 = 130‹ to 140‹ C.); (b) further heated in contact with a metal roller maintained at a peripheral surface temperature T 2 (T 2 = 140‹ to 150‹ C.); (c) immediately longitudinally stretched to 5 to 7 times its original length without further heating or artificial cooling; (d) immediately cooled by contact with a cooled metal roller to about 20‹ C.; (e) reheated to a temperature from (T 3 -5‹ C.) to T 3 (T 3 = 155‹ to 165‹ C.); (f) transversely stretched to 8 to 13 times its width after longitudinal stretching at an ambient temperature of T 4 (T 4 = 150‹ to 160‹ C.), the relationship T 1 <T 2 <T 4 <T 3 being observed; and (g) cooled under tension to a temperature below 40‹ C. The heating of the film prior to the longitudinal stretching step may be effected by directing on to it a jet of heated air or by contact with heated rollers, or by passing it through a bath of a heated liquid. The specific gravity at 20‹ C. of the polypropylene used is preferably 0À90 to 0À91 (usually 0À906). The Reduced Specific Viscosity of the polypropylene (i.e. its specific viscosity at zero concentration) is preferably greater than 2, particularly 3 to 4. The R.S.V. value is determined in the usual manner at 135‹ C., using a 0À1% solution of the polymer in cisdecahydronaphthalene stabilized with 0À5% of phenyl-b-naphthylamine. The melt index of the polypropylene used is advantageously 3 to 15 g./10 minutes. The heating of the film prior to its being longitudinally stretched is preferably effected by passing the film through the air space of a heated chamber or of a chamber through which a current of heated air is passed, the relationship between S, the length in metres of the path of travel of the film through the chamber, V the speed of the film in metres per minute, and D, the thickness of the film in millimetres, being such that S/V.d is not less than 0À5.

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