GB551786A - Improvements relating to the production of sheets of organic film-forming substance - Google Patents

Abstract

551,786. Films, sheets &c. ; moulding plastic substances. DREYFUS, H. June 6, 1941, No. 7193. [Class 2 (ii)] [Also in Group V] Thick sheets of cellulose acetate or like organic film-forming substance are obtained by applying a solution of the film-forming substance in a volatile solvent to the smooth surface of a second film-forming substance which is insoluble in the solvent of the applied solution and which forms a coating on a rigid support, evaporating the bulk of the solvent, stripping the sheet so formed and then evaporating substantially all of the remaining volatile solvent. Several layers of film-forming solution may be applied over the first layer, the bulk of the volatile solvent being removed from each layer before deposition of the next layer. Instead of cellulose acetate the thick sheets may be produced from solutions of cellulose propionate or other cellulose organic derivative or of a polyvinyl compound such as polystyrene. The coating for the rigid support may be a water-soluble polyvinyl acetate obtained by partial hydrolysis of a polyvinyl acetate of higher acetyl content, or it may be methyl cellulose, water-soluble cellulose acetate, or polyacrylic acid. The rigid support may be a flat polished glass or metal plate. The dope may contain dyes, pigments, plasticizers, or fireproofing agents. Transparent, colourless, sheet materials suitable for observation panels of aircraft, wind shields, and windows for vehicles and the like may be obtained. In an example, a layer of saponified polyvinyl acetate is formed by pouring an aqueous solution thereof on to a polished glass plate fitted with removable side-walls and evaporating the water. One or more layers of the coating solution may be cast on to the plate. On the dried coating there is deposited a layer of cellulose acetate dope and this is set by removal of most of the solvent. Further layers of dope are deposited on the first layer, most or a considerable part of the solvent being removed from each layer before casting the next layer. The composite sheet is then stripped from the casting surface and substantially the whole of the remaining volatile solvent is evaporated. The sheets may be produced by a mass production method. Thus the coating solution may be flowed continuously on to a series of glass plates moving past the flowing-point and through a drying-tunnel. The dope is flowed continuously on to a series of coated plates moving past the flowing-point. The sheets may also be formed by a dipping method. A glass plate or other support is coated on both sides with saponified polyvinyl acetate and then dipped into the dope from which the sheets are to be formed. The dope layers are dried and further layers are formed by alternate dipping and drying steps. The composite sheets are then stripped from the two polyvinyl acetate surfaces, and the remaining solvent is then evaporated. Specifications 533,788, and 542,080 are referred to.