Herstellung von Flachdruckformen

1,237,192. Light- and heat-sensitive materials and processes. GEVAERT-AGFA N. V. June 19, 1968 [June 21, 1967], No. 28693/67. Addition to 1,139, 896. Heading G2C. A process for preparing printing masters according to claim 24 of parent Specification using a material in which the water-permeable layer is not covered by another layer and in which the silver halide emulsion is an internal silver halide emulsion which after imagewise exposure the unexposed silver halide is developed with a surface developer to form a reversal image of a density at least 0À1 above the background areas. The invention also relates to a light- and heat-sensitive material comprising a heat sensitive surface layer of thermoplastic particles in a hydrophilic binder and in the same or another layer, an internal silver halide emulsion so that on production of a silver image as above, the material can be uniformly exposed to render the heat-sensitive layer less waterpermeable. The heat-sensitive material and silver halide emulsion preferably constitute a single layer. The master obtained may be inked up with a normal lithographic ink or with an ink described in Specification 1, 142, 472 to produce a "reversed" planographic plate. The silver halide emulsion is preferably unsensitized and may contain a watersoluble bromide, a water-soluble iodide and a fog-inhibitor such as a tri- or tetra azoline-thione (of Specification 1, 150, 553). The developer preferably contains a fogging agent such as hydrazine, carlazic acid or bi-carbamic acid or salts thereof or alternatively development is carried out using a low intensity light-source. In the examples, an internal gelatino silver bromide emulsion containing cadmium bromide, potassium iodide, lead nitrate, 1 (0-methoxy-phenyl)-2-tetrazoline-5-thione, 2-mercaptrethyl-carbanitate, poly-N-vinyl pyrrolidene and polyethylene particles (constituting the heat-sensitive material) is coated on a support, imagewise exposed in an energetic "surface" developer which either contains carbazic acid or the development is carried out under diffuse light to produce a direct positive silver image. The material is now exposed to a flash-lamp to produce the printing master.