GB1125581A - Ribbed films - Google Patents

Abstract

1,125,581. Ribbed films. COURTAULDS Ltd. 31 Dec., 1965 [1 Jan., 1965], No. 98/65. Heading B5B. A film comprising a linear crystalline polymer and having on each major surface a plurality of parallel ribs which are substantially straight when the film is flat, a major proportion of the polymer constituting each rib being oriented longitudinally of that rib, and the ribs on one surface being angularly disposed relative to those on the other surface, is made by forming a linear crystalline polymer into a film having a plurality of parallel ribs on each major surface, the ribs on one surface being angularly disposed relative to those on the other surface, stretching the film longitudinally of the ribs on one surface and, subsequently, stretching the film in the direction of the other ribs, whereby a major proportion of the polymer constituting each rib is oriented longitudinally of that rib. Preferably, the ribs on one surface are at right-angles to those on the other surface when the film is flat, and such a film may be made by forming the polymer into a smooth sheet which is embossed with the ribs on each major surface before being stretched, suitably by passing between two heated embossing rollers, one of which has coaxial grooves and the other longitudinal grooves in the peripheral surface. Such a film may have perforations in the form of parallel splits, located at interstices defined by adjacent pairs of crossing ribs. The stretching process may be simple cold-drawing or it may comprise simultaneously heating and drawing the film. When the second stretch is itself incapable of causing the thinner sections to split, this may be effected mechanically, e.g. by rupturing the sections with points or violently moving one rib relatively to its neighbour, without disrupting the individual ribs appreciably. The extruded film may be composed of a thermoplastic polymer or a mixture of two more polymers, either homogeneous or heterogeneous. Specified polymers include linear polyethylene, stereoregular polypropylene, stereoregular polystyrene, nylon 6, nylon 11, nylon 66, nylon 88, polyethylene terephthalate, and cellulose acetate. Regenerated cellulose may be used if a plasticizer is added to render the polymer thermoplastic. Such a plasticizer may be removed wholly or partially before stretching. Polyacrylonitrile and its copolymers, especially with vinylidene chloride, may also be used in the presence of a plasticizer. Film so made and perforated has an open weave or scrim appearance and in physical properties resembles woven fabrics. Using an intimate mixture of incompatible polymers results in a multi-fibrillar structure in the ribs and thinner sections, the fibrils of a rib running longitudinally thereof and the fibrils of a thinner section running in the direction of the stretch first imposed in making the film. The multi-fibrillar character of the thinner sections encourages the fibrillation of these sections during the second stretching stage or the mechanical treatment. In an example, polypropylene film is so treated.