GB719600A - Fluorescent screens - Google Patents

Abstract

A fluorescent screen comprises a sheet support, a layer thereon of particles of a fluorescent material dispersed in a binding medium of mixed cellulose nitrate and a water-insoluble cellulose ether and superimposed on the fluorescent layer, a thin layer of a hydrophobic cellulose carboxylic acid ester of which the ester groups are predominantly acetate. The fluorescent material may be calcium tungstate, barium lead sulphate mixed crystals, zinc cadmium sulphide mixed crystals, magnesium tungstate and zinc silicate. The ratio of mixture of binding agents to fluorescent material can be 1:4-50 by weight, the ratio of cellulose ether to cellulose nitrate can be 0.1-3:1 and the thickness of the cellulose ester coating can be 0.002-0.05 mm. The cellulose ether may be a lower alkyl ether of cellulose wherein the alkyl group contains 1 to 7 carbon atoms or may be benzyl cellulose. Examples of cellulose ethers used are methyl cellulose, ethyl cellulose, benzyl cellulose or mixtures of one or more of methyl cellulose and benzyl cellulose with ethyl cellulose or with each other. The sheet support may be cardboard to which a solution of cellulose nitrate in ester or ketone solvents, e.g. methyl acetate, ethyl acetate or acetone, can be applied. The cardboard support can be replaced by other flexible or stiff supports such as plastics, e.g. cellulose derivatives, super polymers, e.g. polyvinyl chloride, polyamides, e.g. nylon and polyesters, e.g. polyethylene terephthalate; aluminium and aluminium alloys. The sheet supports when composed of plastic materials may contain pigment such as TiO2, ZnO, ZnS and ZrO2, or dyes (list given) in order to make them opaque to light. A backing coat may be applied to the fluorescent layer and serve as an adhesive layer for the card or plastic sheet support, and may contain any of the foregoing pigments to reduce transmission or increase reflection of the fluorescent light. Suitable adhesives layers are polyvinyl acetate and magnesium oxide-pigmented cellulose nitrate or ethyl cellulose. The various layers are superimposed on a smooth surface such as a glass plate, the fluorescent material layer including a high boiling point solvent for cellulose acetate, and the adherent layers are then stripped from the surface. Several high boiling point solvents for cellulose acetate are mentioned, the one preferred being the methyl ether of ethylene glycol monoacetate. Specifications 1718/39 (as open to inspection under Sect. 91), 538,178, [Group XL], and 623,386, [Group XL (a)], are referred to.