The present invention relates to a negative working photosensitive composition comprising a polymeric diazonium salt and a polymeric binder, and to a recording material prepared therewith which is particularly suitable for producing lithographic printing plates.
It is known to increase the service life (print run) of an offset printing form by baking, i.e., by heating the printing form to a temperature between about 150.degree. C. and 250.degree. C. This process is employed, in particular, for positive working printing plates based on naphthoquinone diazides and, as binders, novolaks or resols (phenol resins), as disclosed by British Patent Specification No. 1,154,749.
Polymeric diazonium salts, particularly diazonium salt polycondensation products, have been employed very successfully as photosensitive compounds in negative working printing plates. But it has been almost impossible heretofore to achieve a substantial increase in print run by baking these plates. In some cases, baking has even led to a decrease in print run, possibly due to a fragmentation of macromolecules in the printing layer.
It has also shown that the phenol resins, and in particular the novolaks, are required as binders in positive working layers in order to achieve an optimum increase in print run. These binders are, however, unsuited in practice for use in the above-mentioned negative working layers. It has even been found that photosensitive compositions comprising an excess of phenol resin and diazonium salts which are per se negative working turn into positive working compositions (see West German Auslegeschrift No. 20 65 732, Examples 54 and 55).
So that negative working printing plates may nevertheless be baked, it has been proposed to reinforce the developed printing plate with a coating of phenol resin-containing lacquer, which lacquer adheres to the image areas only, and then to bake the plate. This method is described in British Patent Specification No. 1,151,199. It has not gained general acceptance, however.
European Patent Application No. 0 111 273 describes a photosensitive composition that comprises a binder which is soluble in aqueous-alkaline solutions, a photosensitive substance and, as the crosslinking agent, a melamine derivative. The preferred binders are novolaks. No satisfactory crosslinking is achieved when customary copolymers of (meth)acrylic acid and (meth)acrylic acid esters are used.
French Pat. No. 2,485,759 recommends epoxy resins as crosslinking additives to diazo-diphenylamine condensation products. Freshly prepared layers of this kind can be successfully baked, but they have a poor shelf life.
West German Patent Application No. P 33 29 443 describes photopolymerizable compositions comprised of polymerizable, ethylenically unsaturated compounds, photoinitiators and polymeric binders. At least part of the binders employed are polymers having crosslinking lateral groups of the formula --CH.sub.2 OR, in which R denotes an alkyl group, acyl group or hydroxyalkyl group. The disclosed compositions are used for the production of thermally curable solder masks.