GB780215A - Process of casting and stretching film - Google Patents

Abstract

Films of thermoplastic, film-forming, synthetic linear organic polymeric material are produced by casting the material in the form of a substantially symmetrical film having a tapered contour and longitudinally stretching the film over stretching rolls, the tapered contour of the film and the stretching conditions being such that the stretched film is of substantially uniform thickness from edge to edge. The film is cast in such a manner that a transverse section across its full width is substantially symmetrical about the longitudinal axis of the film and that the central portion of a transverse section of the film is thicker than the edge portions. The width of each of these edge portions preferably constitutes at least 15 per cent, e.g. 15-20 per cent, of the total width of the film. The minimum thickness at the edges may be 91-92 per cent of the maximum thickness of the film, which may be at least 10 mils. The film may be stretched longitudinally 2-4 times its original length and the temperature of the surfaces of the stretching rolls may be 80-90 DEG C. The film material is preferably polyethylene glycol terephthalate but may also be a polyamide, a polyvinylidene chloride or a polystyrene. In an example a melt of amorphous polyethylene glycol terephthalate prepared as described in Specification 578,079, [Group IV], is extruded through an extrusion hopper, the lips of which are adjusted so as to form a tapered film having a width of 19.25 inches and a central thickness of 0.075 inch, the thickness of the film at its edges being 8-9 per cent less. The extruded material is cast on to the surface of a positively driven quenching drum and is fed into a longitudinal stretching section composed of three groups of horizontal, parallel, chromium-plated polished rolls whose surfaces are maintained at 80-90 DEG C. The first group of five rolls is positively driven at a peripheral speed of 5.3 yards per minute and the third group of five rolls at 16 yards per minute, the intermediate group of nine rolls being idler rolls. The film is reduced in width by about 14 per cent. The film is finally transversely stretched in an enclosed tentering apparatus maintained at 95-100 DEG C. to three times its original width. A long list of applications of such polyethylene glycol terephthalate film is given. Specification 461,236, [Group IV], and U.S.A. Specification 2,071,250 also are referred to.