GB1129366A - Lithographic plates and method of making such plates - Google Patents

Abstract

1,129,366. Photographic processes for producing lithographic plates. MINNESOTA MINING & MFG. CO. Sept.21, 1965 [Sept. 21, 1964], No.38810/65. Headings G2C and G2M. An image-receiving material capable of conversion to a lithographic plate comprises a backing having an oleophilic surface coated a liquid permeable hydrophilic layer containing a reduction promoter and this layer becoming friable and removable on reduction of metal ions to the metal; premature reduction of metal ions being prevented by either (i) limiting the reduction promoter to the oelophilic/hydrophilic interface and leaving the upper part of hydrophilic layer free of reduction promoter or (ii) adding a humectant such as glycerin to the layer. Lithographic plates are produced by selectively permeating a metal salt through a hydrophilic layer containing reduction nuclei, the hydrophilic layer being coated on an oleophilic surface of a backing, chemically reducing the salt to its metal at least at the hydrophilic/oleophilic interface, bleaching to remove the metal image and then removing the hydrophilic layer in the areas of formation of the metal image. The oleophilic backing is generally a paper support coated with an oleophilic layer. Selective permeation is achieved by using an imagewise exposed silver halide emulsion which is either coated on, or contacted with the hydrophilic layer reduction being achieved by the silver salt diffusion transfer process. In the examples, a paper support coated with a polyethylene or polypropylene layer or a polyester support is coated with an oleophilic layer of a vinyl chloride/vinyl acetate copolymer contaning titanium dioxide and overcoated with a hydrophilic silica layer containing colloidal silver. The silica layer either contains a humectant such as glycerine or is overcoated with a further silica layer not containing colloida silver so that the silver salt is precipitated at the oleophilic/hydrophilic interface. A silver halide emulsion may be coated on top of the silica layer and the material imagewise exposed and developed to produce a positive silver image at the interface by the diffusion transfer process or an exposed silver halide emulsion may be placed on top of the silica layer to produce a positive silver image at the interface development. The emulsion layer is removed where necessary and the positive silver image bleached out. Silica is now removed by rubbing and washing from the areas which contained the positive image to leave a lithographic plate; the bond between silica and the oleophilic layer being weakened by the formation of the silver image.