GB811066A - Biaxially oriented films - Google Patents

Abstract

<PICT:0811066/IV (a)/1> <PICT:0811066/IV (a)/2> A film of a crystallizable organic thermoplastic material that has been bascially oriented is further drawn along one of the axes of the previous orientation. Partial shrinkage may be allowed to occur during the further drawing, in a direction transverse to the drawing direction. The initial biaxial orientation may be produced by drawing a substantially amorphous film simultaneously or consecutively in two directions mutually at right angles. In the latter method the amorphous film is preferably drawn first in the direction in which it is later to be further drawn. The biaxially oriented film may be heat set, preferably at 150-230 DEG C., before or after the further drawing. Polyethylene terephthalate may be the organic thermoplastic material, which may be drawn in two directions at right angles to a draw ratio of 2:1 to 5:1 in each direction, preferably a ratio of 2.5:1 to 4:1, the draw ratios in the two directions being the same or different, and further drawn in the direction in which greater strength is required to a ratio of 1.05:1 to 4:1, preferably 1.5:1 to 3:1. When biaxial orientation is produced by simultaneous drawing, temperatures of 70-120 DEG C. may be used, and further drawing may be carried out at 80-200 DEG C. Using consecutive drawing, corresponding temperatures may be 70-90 DEG C. for initial drawing in the direction in which the film is later to be further drawn, 80-90 DEG C. for the drawing at right angles thereto, and 100-220 DEG C. for the final drawing. Polyethylene terephthalate films treated in this way have relatively high tensile strength in one direction, coupled with adequate strength in the transverse direction, and are particularly useful as photographic film base. As shown in Fig. 1, a flat film 1 is drawn longitudinally between rolls 2, 3, then transversely in stenter 5. Further longitudinal drawing takes place between rolls 7, 8, and the film then passes to a heat setting oven 9 and an edge-trimming device 10. In Fig. 2, a tubular film 11 is stretched longitudinally between nip rolls 12, 16, and simultaneously transversely at 15 by the pressure of air introduced through probe 13, the film being heated to drawing temperature by heater 14. The film is further heated by heater 17 and further stretched longitudinally between nip rolls 16, 18, partial transverse shrinkage being obtained at 20 by regulation of the pressure of air introduced through probe 19. Heater 17 also serves to heat-set the film, which is slit after passing nip rolls 18, and then wound.