| 摘要 |
991,917. Rendering clear films translucent. MINNESOTA MINING & MANUFACTURING CO. May 19, 1961 [May 19, 1960], No. 18408/61. Drawings to Specification. Heading B5B. [Also in Division G2] Clear transparent smooth surfaced films which when creased and re-flattened are rendered opaque along the crease line, are rendered translucent by subjecting the film to a progressively advancing pronounced localized difference in the elongation of one surface of the film with respect to the opposite surface, for example, by passing the film under tension over and in intimate contact with a smooth edge which is at an angle to the direction of travel of the film in such a manner that the path of travel of the film undergoes an abrupt change of direction while the film is travelling over the edge. The film may be drawn over the edge two or more times and the opposite faces of the film may alternately be brought into contact with the edge in two or more passes in alternate directions. The film may additionally be uniformly stretched, either during its passage over the edge or subsequently. The treatment may be such that the light transparency of the film is reduced to not more than 10% of its original value. The density of the treated film may be at least 1% less than that of the untreated film. A particularly satisfactory material for the film is unplasticized polyvinyl chloride mixed with small proportions of stabilizers and waxy lubricants and may be essentially non - oriented. Alternatively the film may be formed by melt-extruding polypropylene or polyethylene terephthalate, or solventcasting polystyrene. Transparent polyamide and polyethylene films cannot be rendered translucent by the method of the invention. Only films which become opaque along the fold lines when creased can be rendered translucent by the method of the invention. |
