Patent GB-1190698-A (GB)
Title: Process for the Production of Stretched Films Consisting of Synthetic Thermoplastic High-Polymer Material.
IPC: B29C55/02
Filed: 1968-07-19
Abstract: 1,190,698. Stretching films of synthetic polymeric material. KALLE A.G. 19 July, 1968 [21 July, 1967], No. 34545/68. Heading B5B. Stretched films consisting of synthetic thermoplastic high-polymer material are produced by a process wherein an " easily separable film laminate " of at least two such materials, which are stretchable under the same conditions, is stretched in such a manner that the laminate retains its capacity for separation and each component of the laminate retains its laminar form, after which the laminate is separated into individual stretched films. An " easily separable film laminate " is defined as a laminate of which the force required to separate the components of the laminate by overcoming the adhesion forces between the individual layers is lower than the force that would produce a permanent extension of either or any components of the laminate. At least one component of the laminate may have a tensionelongation curve of the type shown in Fig. 1 of the drawing and at least one other component of the laminate may have a tension-elongation curve of the type shown in Fig. 2 of the drawing, the laminate itself having a tensionelongation curve of the type shown in Fig. 3. In the figures the elongation (#), measured at 90‹ C., is plotted against the tension (#) to which the film samples are subjected. The films constituting the laminate may consist of the same polymer, e.g. polyethylene terephthalate, or different polymers, e.g. polyethylene terephthalate and polyethylene having a density of 0À915 to 0À930. The laminate may be stretched in the longitudinal and transverse directions. Stretching of the temporary laminate is generally performed in a temperature range between the glass transition temperature and the melting temperature of the polymeric material of the individual films. The individual unstretched films of the laminate may have a thickness of 20 to 500 microns, preferably 50 to 350 microns. If the films of the laminate are chemically similar, they generally have the same thickness but if they are dissimilar, it is advantageous that the film or films having the better stretching properties should be thicker. The selection of the films for processing according to the invention and the process itself should be such that the single films are stretchable in the same temperature range, the elongation at break of any single film is not exceeded during the stretching step, and the adhesion to one another of the films or components of the laminate must be retained to the extent that each film can itself retain its laminar form and not become distorted during the stretching process. The laminates may be formed by pressing together two or more preformed films at an elevated temperature or by extruding the molten film forming materials simultaneously through two or more adjacent slot-shaped extrusion orifices or two or more concentric annular extrusion orifices, the two or more individual films thereby formed being brought into contact with one another while still molten or at a temperature high enough to be plastic and adhesive, the resulting laminate being then cooled and stretched. The films used may include films of polystyrene, or of plasticized polyvinyl chloride.