GB1340687A - Thermoplastic film with a degree of planarity and process for the production thereof - Google Patents

Abstract

1340687 Films KALLE AG 23 Dec 1970 [27 Dec 1969] 61102/70 Heading B5B A uni- or bi-axially oriented film having a deviation from planarity of at most 0À25 per mille is prepared by extruding a molten thermoplastic material to form a film, stretching the film in one or two directions to orient it and heat setting the oriented film at a range of temperatures below its m.pt., at least part of the heat setting process being performed at a steadily decreasing temperature given by the equation T/T F = 1-m.S/Smax where T F is the initial heat setting temperature, Smax is the overall period during which the temperature decreases, T is the temperature of the film at any given time S during the heat setting procedure and m is the gradient of the temperature curve and has a constant or increasing value between 0À2 and 0À6. Preferred thermoplastic materials are polyesters, particularly polyethylene terephthalate or a mixed polymer or block copolymer of polyethylene terephthalate with polyesters of the formula In examples films of PET are biaxially stretched at a temperature of 120‹ C. and conveyed through a heat setting channel in which for at least part of the length of the channel the temperature decreased uniformly according to the above equation from 210‹ C. to 160‹ C. (Example 2, m = 0À24); from 200‹ to 140‹ C. (Example 3, m = 0À6) and from 150‹ C. to 90‹ C. (Example 5, m = 0À4). Examples 2 and 5 are two stage heat setting processes in which the temperature decreases from the higher temperature only during the last quarter of the channel and Example 3 is a three stage process in which the heat setting temperature is constant at 200‹ C. for the first half of the channel, decreases uniformly to 140‹ C. in the next quarter and decreases to 55‹ C. in the final quarter of the channel.