Photopolymerizable mixture and photopolymerizable copying material produced therewith

#This invention relates to a photopolymerizable mixture which contains a polymeric binder, an ethylenically unsaturated polymerizable compound and a photoinitiator, and to a copying material for the production of relief images, in particular printing forms and photoresists, which copying material is composed of a layer support and a photopolymerizable layer of the mixture.

#Photopolymerizable mixtures and materials of the indicated generic type are known. When they are processed, it is necessary to eliminate the influence of atmospheric oxygen on the free-radical polymerization during exposure, or to suppress this as far as possible. In most cases, this is effected by covering the photopolymerizable layer with a cover sheet (U.S. Pat. No. 3,060,026) which can be peeled off and is impermeable to oxygen, or with a cover layer which is impermeable to oxygen and is soluble in the developer (U.S. Pat. No. 3,458,311). In this way, oxygen is prevented, during the light-induced polymerization, from diffusing into the layer and leading there to chain-stopping reactions which result in an incomplete polymerization and hence a lower light sensitivity. Due to the oxygen inhibition, in particular, the relationship between the incident amount of light and the exposure action, for example the degree of light curing--the reciprocity of the copying material is lost.

#The disadvantage of the first-mentioned known materials is the additional working step which is required for the application of a cover film or cover layer. Moreover, the image resolution on contact printing deteriorates when self-supporting cover films are used, which as a rule have a thickness of about 20 .mu.m. Cover layers which are soluble in the developer entail a more rapid consumption of the latter and an additional sewage contamination. Attempts have therefore been made to produce photopolymerizable materials which display an adequate light sensitivity in imagewise polymerization even without a cover layer.

#The invention is explained in more detail by reference to the Examples which follow and in which parts by weight (pbw) and parts by volume (pbv) have the same relationship as g and ml.sup.3. Unless otherwise stated, percentages are percent by weight.

#(a) As the layer carrier for printing plates, electrochemically roughened and anodized aluminum with an oxide layer of 3 g/m.sup.2 is used, which is coated with a solution of the following composition:

#11.7 parts by weight of a 33.4% concentration solution of a methyl methacrylate/methacrylic acid copolymer, having an acid number of 110 and a mean molecular weight of 35,000, in methyl ethyl ketone,