GB1083665A - Biaxially oriented polyethylene terephthalate films - Google Patents

Abstract

1,083,665. Biaxially orienting polyethylene terephthalate films. IMPERIAL CHEMICAL INDUSTRIES Ltd. Feb. 16, 1966 [March 5, 1965], No. 9501/65. Heading B5B. A process in which an amorphous polyethylene terephthalate film is drawn at a temperature above its second order transition point, first in its machine and then uniformly in the transverse direction is characterized in that the film is drawn in its machine direction to a draw ratio of from 3À75 : 1 to 4À75 : 1 the film being held for such drawing at a temperature so little above its second order transistion point and for so short a time that the density of the film after drawing in its machine direction does not exceed 1À365. The second order transition point of polyethylene terephthalates varies slightly, but is about 78‹ C. The transverse draw may be conducted at a temperature not greater than 130‹ C. and preferably not greater than 120‹ C. In the two examples the film is drawn in the machine direction at a draw ratio of 4À2 : 1 at 90‹ C. and the transverse drawing is conducted at 110‹ C. and 117‹ C. respectively to corresponding draw ratios 3À8 : 1 and 3À65 : 1. The film may be drawn in the machine direction by passing it between or round a pair or series of slow rolls heated to the desired drawing temperature and then between or round a pair or series of fast rolls at least the first of which is cooled, the two pairs or series of rolls being closely spaced so that the film is supported upon the rolls over most of the region in which it is subjected to tension. The film may be heated by means of radiant heat applied to as small a length of film, in the desired drawing region, as is consistent with the need to heat the film to the desired drawing temperature. The biaxially stretched film may be heat set at a temperature in the range 150-230‹ C.