Abstract: A process for the manufacture of thin layers based on polyurethane elastomers. A liquid reaction mixture essentially consisting of (A) polyesters, polyethers, polyester urethanes or polyether urethanes, all of which contain at least two hydroxyl groups in the molecule, and diisocyanates or (B) prepolymers obtained from the hydroxyl-containing compounds mentioned under (A) above with an excess of diisocyanates, said prepolymers containing at least two isocyanate groups in the molecule, and at least one of the compounds mentioned under (A) above which are capable of reacting with isocyanates, to which reaction mixture a minor quantity of substantially compatible monomers having at least two photocrosslinkable C--C multiple bonds and optionally a photopolymerization initiator have been added, is applied to a substrate to form a coating thereon, which coating is then gelled by irradiation and finally thermocured.

Abstract: The invention relates to special acetals of benzoin compounds which act as photoinitiators, and to a technically simple manufacturing process involving the reaction of an appropriate .alpha.- or .beta.-hydroxyketone with an .alpha.,.beta.-olefinically unsaturated ether in the presence of an acid catalyst. The acetals are particularly suitable for use as photoinitiators in the preparation of photopolymer printing plates.

Abstract: Compositions polymerizable by ultraviolet irradiation which contain photopolymerizable monomers possessing at least one polymerizable carbon-carbon multiple bond and monoketals of the formula ##STR1## as photoinitiators. The compositions have a particularly long shelf life and show very little yellowing after curing by irradiation.

Abstract: Laminates for the production of relief plates for flexographic printing, by imagewise exposure and washing out with a developer solution have, on a base optionally provided with an elastomeric layer, a photo-crosslinkable layer containing a photoinitiator and consisting of a polyurethane elastomer which has been prepared in a special manner from an aliphatic saturated polyesterglycol, an aliphatic diisocyanate, but-1-ene-3,4-diol and, if desired, another diol and which has a Shore A hardness of 30 to 90.degree. in the photo-crosslinked state, the elastomer being optionally mixed with a photo-crosslinked monomer.