Patent GB-510724-A (GB)
Title: Improvements in or relating to windows and the like
IPC: E06B7/12
Filed: 1938-02-08
Abstract: 510,724. Films ; cellulose derivatives ; compound sheets. MOSS, W. H. Feb. 8, 1938, No. 3946. [Class 2 (ii)] [Also in Group VIII] A transparent window closure or the like, for an article in the use of which the closure will be exposed to conditions of temperature and humidity tending to cause fogging by condensation of moisture thereon, is composed of a layer of a transparent, water-resistant, organic film-forming substance, and adherent thereto at least one outer layer of a transparent hydrotropic film-forming ester or ether of cellulose or of a polymerized unsaturated organic compound. The film-forming substances of the basic and outer layers may be esters or ethers derived from the same ester-forming or ether-forming components. The ester or ether of the outer layer may be soluble in aqueous solutions of an organic liquid, e.g., acetone, containing 50 per cent. or more of water, while the ester of the basic layer is insoluble in such aqueous solutions containing more than 15 per cent. of water. The hydrotropic layer should be such that it may be held close to the mouth and breathed upon several times without dimming. The closures may be in the form of eye-pieces for respirators, or goggles, wind-screens, windows or observation panels for closed vehicles, aircraft, or gun turrets. The basic sheet and the hydrotropic sheet may be obtained by casting, spraying, or extrusion methods or by blocking and slicing. Adhesion of the layers may be effected by means of heat and pressure, preferably in the presence of solvents, or liquids which become solvents at elevated temperatures. The hydrotropic layer may be cast or sprayed on to the basic sheet. The transparent closure may be cut from the composite sheet and, if required, may be given any desired curvature in a heated press. The closure may be provided with a hydrotropic surface on both sides. Hydrotropic cellulose acetate may be obtained by ripening a primary cellulose acetate solution for a prolonged period with water until it attains solubility in 50 per cent. aqueous acetone solution. Cellulose acetate for the base layer may be obtained by carrying out the acetylation under conditions such that degradation of the cellulose molecule is avoided and then ripening to acetone-solubility, e.g., to an acetyl value of 51-54 per cent. calculated as acetic acid. Other methods of making hydrotropic cellulose acetate include the use of higher temperatures, lower proportion of diluent, higher proportion of catalyst, and other conditions which lead to partial degradation of the cellulose molecule. The cellulose may be pretreated with mineral acids to effect partial degradation. Plasticisers may be present in the outer as well as the basic sheets. For the outer sheets plasticisers or softeners with affinity for water are preferably used, e.g., dimethyl-, dibutyl-, or diethyl tartrate, dioxy-ethyl phthalate, ethyl acetanilide, sulphonamides, sulphonanilides, glycol, glycerol, or diacetone alcohol. Synthetic resins with affinity for water may be present, e.g., those obtained by condensing di- or polyhydroxy alcohols with hydroxy polycarboxylic acids or formals or esters of such acids. Condensation products of glycerol with citric acid or methylene citric acid may be used. The basic and outer layers may comprise cellulose esters such as acetate, butyrate, acetate-propionate, or nitrate ; or cellulose ethers, or polymerised unsaturated compounds, such as polyvinyl acetate, chloracetate, or chloride, or polyacrylic esters. In an example, hydrotropic cellulose acetate is made by pretreating cellulose with warm acetic acid for several hours, acetylating with acetic acid, acetic anhydride and sulphuric acid, and ripening after adding water until the product is soluble 'in 40 per cent. acetic acid. The cellulose acetate is dissolved in glycerol, diethylene glycol, dioxane, and water, and the solution is flowed on to a sheet comprising acetone-soluble cellulose acetate and dimethyl phthalate. The sheet is dried and polished in a hot press, and then eye-pieces or other transparent closures are cut from it. In another example, a sheet of hydrotropic cellulose acetate moistened with an alcoholic solution of polyvinyl acetate is applied to a sheet of acetonesoluble cellulose acetate similarly moistened. The assembly is pressed in a platen press. The sheet is used for making observation panels for aircraft or windows of closed motor vehicles. Specifications 253,833, [Class 52 (v)], 453,833, and 503,834 are referred to.