GB887346A - Linear polyester films and processes for their production - Google Patents

Abstract

Voided or lacunose films are produced by drawing a film of a crystallizable linear polyester, particularly polyethylene terephthalate, which has not been completely oriented in the drawing direction, so that it necks down at a speed between 1 foot per minute and the speed which causes the film to break, the drawing being carried out in the drawing direction in a heated zone wherein the film is heated to a temperature below its second-order transition temperature, and the speed of drawing being sufficiently high and the temperature being so chosen as to lead to the production of voids, taking into account the crystallinity of the film and the applied draw ratio. Voided or lacunose films are defined as films containing internal cavities running in the direction of drawing. The draw ratio used must be equal to the natural draw ratio of the film at the drawing temperature and speed of haul-off of the film from the heated zone necessary to maintain the neck substantially stationary within the drawing zone. The speed of haul-off or speed of drawing should preferably be at least 10 feet per minute. The initial film preferably has a degree of crystallinity of less than 40% and especially less than 10%. The natural draw ratio is defined as the draw ratio when the film is drawn through a spontaneously produced neck. The degree of crystallinity is determined by the X-ray diffraction method. In the case of polyethylene terephthalate films, the temperature of the heated drawing zone when neck drawing according to the invention may usefully be in the range 20 DEG C. above the ambient temperature up to 70 DEG C., and the applied draw ratio (for initially undrawn film) will then vary between about 4:1 and about 5,75:1 depending on the temperature and the speed of draw used. Under these conditions films having initially a low degree of orientation transverse to the direction of necking draw are rendered pearlescent or opalescent. In order to produce translucent voided polyethylene terephthalate films, the quenched film may be biaxially drawn, either simultaneously or consecutively in two directions at right angles, preferably at a temperature around or above the second-order transition point such that extensive crystallization (e.g. to above 40% crystallinity) does not occur, e.g. in the range 70 DEG -100 DEG C., preferably 75 DEG -90 DEG C. The film may be drawn to an extent of at least 2:1, but less than that leading to complete orientation, conveniently to about 3:1 in each direction. The film is then neck drawn according to the invention. Alternatively, the quenched film may be drawn in one direction to about 3:1 to 4:1 at a temperature within the above ranges and then neck drawn according to the invention in the transverse direction. The films produced may be "heat shrunk" at 80 DEG -150 DEG C. while allowing them to shrink freely.