GB1373862A - Method of fabricating a temporary apertured mask electrode for a colour television tube - Google Patents

Abstract

1373862 Luminescent screens RCA CORPORATION 2 Aug 1972 [2 Aug 1971] 36109/72 Addition to 1311313 Heading HID In the manufacture of a colour television screen by exposure of a photo-resist through a shadow mask which is used in the completed tube the apertures in the shadow mask are made smaller for the photo-resist process by coating at least one surface of the mask with a film material having a thermally reversible viscosity that is lowered by heating and then heating the coated surface until the lower viscosity of the film causes the film to break and to collect in annular deposits around the inner periphery of each mask aperture. The film material may comprise polyvinyl alcohol, an aromatic compound, water, and a plasticizer selected from the groups of glycerines, ethers, ethanolamine salts, thiocyanates, glycols, and formamides. One suitable composition comprises 3% polyvinyl alcohol, 1% red dye, 0À4% wetting agent and the balance water. This has a viscosity/temperature curve as shown at 77 in Fig. 6, as compared with that shown at 80 for a known thermally irreversible composition. When a mask coated with the material is heated, the viscosity drops rapidly down to the point 82 where the film breaks due to surface tension; with continued heating the viscosity increases as shown at 83 and the peripheral regions of the coating on the wall of the apertures harden. It is preferable to heat the mask in a closed chamber so that the vapour pressure increases and the viscosity follows the curve 84. This has a relatively flat minimum region 85 so that differences in temperature over the surface of the mask make little difference to the reduction in size of the apertures by the coating. When 5% ethylene glycol is added to the above composition and the mask is heated in a closed chamber the viscosity follows the curve 79. Two other suitable compositions comprise 5% polyvinyl alcohol, 1% dye, 44% water, and 50% ethylene glycol or glycerine. A convex mask may be rotated slowly about its centre whilst its convex surface extends down into a tank holding the liquid film-forming material, and then tilted to spread the liquid over the surface without forcing it through the apertures. Alternatively the mask may be rotated with its convex surface upwards whilst the material is flowed over the mask, and then rotated more rapidly to remove excess liquid. After the screen has been made the temporary coating may be removed by baking or chemical dissolution to restore the mask to its original condition. This process may be carried out at the same time as a similar process is used to remove resist material from the screen, by baking both at once or by applying dissolution techniques to both at once.