GB765904A - Process for treating thermoplastic polymeric films - Google Patents

Abstract

A process for the production of molecularly oriented thermoplastic polymeric films, especially applicable to films of a thickness "as cast" greater than 75 mm., comprises stretching the film and thereafter rolling it in a direction substantially at right angles to the stretching direction. The polymer may be stretched, at temperatures up to 50 DEG C. higher than the second order transition temperature, to up to four times its original length and, within the same temperature range, may then be rolled in the direction at right angles to the stretch until the film is reduced to as little as one quarter of the thickness before rolling. The film may then be heat-set at an elevated temperature, below the crystalline melting temperature, under conditions which do not allow of shrinkage. Two classes of polymers, both having second order transition temperatures above 0 DEG C. but substantially below the crystalline melting temperature, may be treated, viz., (a) those which are readily quenchable to an amorphous form, such as polymers of ethylene terepthalate, hexamethylene adipamide or caproamide and copolymers of ethylene terephthalate and ethylene isophthalate; and (b) those which are not more than transiently obtainable in an amorphous form, such as polyvinyl fluoride, polyethylene and polymerized formaldehyde. Details are given of polyethylene terephthalate films treated by the process, together with methods for measuring the second order transition temperature of polymers. Other polymers of class (a) which may be treated are those of ethylene - 2,6 - naphthalate, tetramethylene-1,2 - dioxybenzoate, and ethylene - 1,5-naphthalate. Specifications 578,079, [Group IV], 695,567, 743,503 and 753,299 are referred to.