Patent GB-625027-A (GB)
Title: Improvements in or relating to fluorescent screens
IPC: G21K4/00
Filed: 1946-05-10
Abstract: A fluorescent screen, for example an X-ray intensifying screen, has an outer surface coated with a layer of an acid polysilicic acid ester/hydroxylated organic polymer composition.  The hydroxylated organic polymer preferably contains a large number of vinyl alcohol units per molecule.  These units may be directly joined to each other as in the case of polyvinyl alcohol or may have various other groups in the chain as in the case of interpolymers with unsaturated compounds such as styrene and olefins, e.g. ethylene.  Some of the hydroxyl groups may be esterified or etherified with lower acyl or alkyl groups, e.g. acetyl, propionyl, methoxy and ethoxy groups.  The composition contains silica in the proportion of 1 to 6 parts by weight per part of polyvinyl alcohol, preferably 1 to 3.5 parts by weight. Other organic polymers which can be used are partially hydrolysed polyvinyl acetate, polyvinyl acetals, cellulose ethers and esters, nitrocellulose, condensation polyesters and other types of polymers prepared from ingredients containing free alcoholic hydroxyl groups not involved in formation of the polymer. Additional materials can be as described in Specifications 591,592, [Group IV], and 596,005.  The acid polysilicic acid ester may be a partially hydrolysed tetraethyl silicate.  Solutions of acid polysilicic acid esters of monohydric alcohols can be prepared by acidifying an aqueous sodium silicate solution, mixing the resulting relatively low molecular weight polysilicic acid with the esterifying monohydric alcohol and removing water from the solution.  Alcohols of the kinds disclosed in U.S.A. Specification 2,395,880 can be used.  Polysilicic acid esters as described in Specification 290,717, [Class 95], and German Specification 659,814 such as silicic acid ethyl, propyl and butyl esters prepared by introducing 1 part of silicon tetrachloride into 5 parts of the appropriate alcohol can be used.  The layer on the screen is preferably formed by applying to the outer surface of the screen a solution of the ester and polymer in a volatile organic solvent.  The fluorescent screen may be coated with a layer of a resin, cellulose derivative or superpolymer before the coating layer is applied.  After the solvent has evaporated, the screen may be heated or baked at an elevated temperature, say 60-100 DEG  C., which should not exceed the softening point of the binding agent for the fluorescent material.  Rapid formation of a highly polymerized polysilicate coating may be promoted by carrying out the drying, baking or ageing operations in a moist atmosphere containing an amine such as ammonia or by flushing the dry, coated surface with an aqueous solution of an amine.  In one example, a coating solution comprising polyvinyl alcohol solution, silicate solution, ethanol and n-butanol was caused to flow down the face of an inclined fluorescent screen composed of calcium tungstate in a cellulose nitrate binder having a cellulose nitrate over-coating which was adhesively secured to a paper board backing.  The screen was then cured at a temperature of about 80 DEG  C.  Fluorescent screens composed of lead and/or barium sulphate, zinc sulphide, zinc cadmium sulphide or barium chlorofluoride can also be used.ALSO:A fluorescent screen, for example an X-ray intensifying screen, has an outer surface coated with a layer of an acid polysilicic acid ester/hydroxylated organic polymer composition.  The hydroxylated organic polymer preferably contains a large number of vinyl alcohol units per molecule.  These units may be directly joined to each other as in the case of polyvinyl alcohol or may have various other groups in the chain as in the case of inter-polymers with unsaturated compounds such as styrene and olefins, e.g. ethylene.  Some of the hydroxyl groups may be esterified or etherified with lower acyl or alkyl groups, e.g. acetyl, propionyl, methoxy and ethoxy groups.  The composition contains silica in the proportion of 1 to 6 parts by weight per part of polyvinyl alcohol, preferably 1 to 3.5 parts by weight. Other organic polymers which can be used are partially hydrolysed polyvinyl acetate, polyvinyl acetals, cellulose ethers and esters, nitrocellulose, condensation polyesters and other types of polymers prepared from ingredients containing free alcoholic hydroxyl groups not involved in formation of the polymer. Additional materials can be as described in Specifications 591,592 and 596,005.  The acid polysilicic acid ester may be a partially hydrolysed tetraethyl silicate.  Solutions of acid polysilicic acid esters of monohydric alcohols can be prepared by acidifying an aqueous sodium silicate solution, mixing the resulting relatively low molecular weight polysilicic acid with the esterifying monohydric alcohol and removing water from the solution.  Alcohols of the kinds described in U.S.A. Specification 2,395,880 can be used.  Polysilicic acid esters as described in Specification 290,717, [Class 95], and German Specification 659,814 such as silicic acid ethyl propyl and butyl esters prepared by introducing 1 part of silica tetrachloride into 5 parts of the appropriate alcohol can be used.  The layer on the screen is preferably formed by applying to the outer surface of the screen a solution of the ester and polymer in a volatile organic solvent.  The fluorescent screen may be coated with a layer of a resin, cellulose derivative or super-polymer before the coating layer is applied.  After the solvent has evaporated the screen may be heated or baked at an elevated temperature, say 60 DEG -100 DEG  C., which should not exceed the softening point of the binding agent for the fluorescent material.  Rapid formation of a highly polymerized polysilicate coating may be promoted by carrying out the drying, baking or ageing operations in a moist atmosphere containing an amine such as ammonia, or by flushing the dry, coated surface with an aqueous solution of an amine.  In one example a coating solution comprising polyvinyl alcohol solution, silicate solution, ethanol and n-butanol was caused to flow down the face of an inclined fluorescent screen composed of calcium tungstate in a cellulose nitrate binder having a cellulose nitrate over-coating which was adhesively secured to a paper board backing.  The screen was then cured at a temperature of about 80 DEG  C.  Fluorescent screens composed of lead and/or barium sulphate, zinc sulphide, zinc cadmium sulphide or barium chlorofluoride can also be used.