| 摘要 |
1,088,834. Electro-photographic processes for producing printing plates. GEVAERT-AGFA N.V. Oct.22, 1964, No.43223/64. Headings G2H and G2M. A hydrophobic photo-conductive surface is given a temporary or permanent hydrophilic character, in the configuration of an electrical latent image, by the imagewise or overall application of a liquid to it, after or during charging. Preferably, the liquid, applied overall, wets the charged areas, which it converts to a hydrophilic character, and is repelled from the uncharged areas. The liquid may, however, be deposited only in the charged areas, through the influence of the charge on a capillary applicator. Alternatively, the potential of charging electrodes can be varied to cause wetting in either the more conductive or less conductive parts of a latent conductivity image. The mere presence of an aqueous wetting liquid may be sufficient to repel lithographic ink from the non-image areas. Preferably, however, the liquid has a permanent hydrophilizing effect and is acidic to react with photo-conductive zinc oxide to form a hydrophilic precipitate. In the examples, photoconductive zinc oxide in a binder of polyvinylacetate or a copolymer of vinylacetate/laurate or vinylacetate/stearate is coated on a sheet aluminium or paper support. After treatment with liquid applied by means of a helically grooved roller Figs. 1 and 2 (not shown) the surface is contacted immediately with lithographic ink. The hydrophilizing liquids used include aqueous solutions of acid cupric chloride, hydrochloric and tannic acids, aluminium sulphate, ferric chloride, oxalic, tannic and phosphoric acids and potassium ferrocyanide. The following aqueous solutions are also referred to in this connection: ammonium and sodium phosphates, potassium ferricyanide and tartaric and fluorobutyric acids. Thickening agents used include: gum arabic, carboxymethyl cellulose, casein and polyvinylpyrrolidone. Other additives include dyes, surface active agents, water miscible solvents, soluble salts, hygroscopic salts and glycerine. Alternative binders for photo-conductive zinc oxide include polyvinyl butyral and silicone resins. The hydrophobic nature of the photoconductive surface may be increased by the application of a lithographic lacquer. A photo-conductive layer may be overcoated with a hydrophobic layer. |
