GB890005A - Improvements in or relating to the stretching of organic polymer film - Google Patents

Abstract

Continuous film of organic thermoplastic polymeric material, particularly polyethylene terephthalate, is longitudinally stretched (as by being passed between two spaced pairs of nip rolls, of which the second pair rotate at a higher peripheral speed than the first) while the film is at a temperature Ts at which molecular orientation occurs, and the film moving between the pairs of feed and draw rolls is uniformly cooled to a temperature Tq beginning abruptly at a predetermined point between the pairs of rolls, Tq being a temperature at which the applied stress is not sufficient to deform permanently the already stretched film past its elastic limit. In the case of polyethylene terephthalate films Ts may be within the range 80 DEG -155 DEG C., preferably 80 DEG -100 DEG C. In the latter case Tq may be below the second order transition temperature of the polymer. The film is preferably cooled to the temperature Tq at a point nearer to the feed rolls than to the draw rolls, e.g. by means of a narrow blast of cold air impinging on the film across its width. Alternatively the film may be cooled to the temperature Tq by contact with one or more rollers which are cooled by means of a cooling fluid. The film may be heated to temperature Ts by means of feed rolls heated to this temperature. The draw rolls may be maintained either at room temperature or at an elevated temperature (e.g. 100 DEG -225 DEG C. in the case of polyethylene terephthalate film), if simultaneous stretching and heat setting of the film is desired. The process prevents too great a reduction in film width and may be applied to the stretching of narrow films. Specification 890,004 is referred to.