The present invention relates to a negative-working, light-sensitive mixture which contains a diazonium salt condensation product, a polymerizable compound, a photoinitiator and a binder, and to a recording material prepared from the mixture, which recording material is particularly suitable for preparing planographic printing plates.
Combinations of photopolymerizable mixtures with negative-working diazo compounds, in particular diazonium salt polycondensation products, are known from German Offenlegungsschriften No. 2,361,931, No. 2,903,270 and No. 3,007,212 (corresponding to British patent application No. 2,044,788). In such mixtures the diazo compound itself acts as a photoinitiator for the polymerization of the monomer, or an additional photoinitiator is added. These mixtures are said to provide particular advantages of better copy resolution, compared with pure photopolymerizable layers, and an increased print run compared with layers that contain only diazo compounds as light-sensitive substances. Moreover, in certain cases it is possible to carry out the development with aqueous solutions or even with pure water.
A similar mixture which contains a binder having crosslinkable allyl side groups is described in European patent application No. 104,863. The advantage of this mixture is said to be, in particular, that the light sensitivity of recording materials prepared using the mixture has a lower temperature dependence. A further advantage is said to be the lower dependence of the light sensitivity on the time gap between exposure and development. But offset printing plates prepared from light-sensitive mixtures as disclosed by the European patent publication have the disadvantages of a relatively low run stability and an undesirably high dot gain during printing.
The examples of the European patent publication indicate that a light-sensitive layer normally has an additional oxygen barrier layer applied to it. This entails the disadvantages of an additional preparative operation and lower image resolution in the copy. In addition, a stronger dot gain is frequently observed as a consequence of halation. Without the oxygen barrier layer, the reciprocity of the layer is impaired and the light sensitivity is usually reduced; also, the light-sensitive layer itself has a stronger tackiness than the barrier layer.
U.S. Pat. No. 4,659,645 (priority date: July 10, 1984) describes a light-sensitive mixture comprising the above-mentioned constituents and, as the diazonium salt condensation product, a polycondensation product containing repeat units AN.sub.2 X and B which are bonded to one another through intermediate moieties, preferably methylene groups. A denotes the radical of an aromatic diazonium compound which is condensable with formaldehyde, and B denotes the radical of a compound which is free of diazonium groups and condensable with formaldehyde. In particular, B is the radical of an aromatic amine, a phenol, a phenol ether, an aromatic thioether, an aromatic hydrocarbon, an aromatic heterocyclic compound or an organic acid amide.
Another U.S. Pat. No. 4,631,245 (priority date: February 8, 1984) describes a light-sensitive mixture, said to be particularly useful for preparing planographic printing plates, that contains a diazonium salt polycondensation product of the composition mentioned in the preceding paragraph and, as a binder, a reaction product of an intramolecular anhydride of an organic polycarboxylic acid with a hydroxyl-containing synthetic polymer. According to the application, the polymer should have no other functional groups that are capable of reacting with acid anhydrides.