Method for producing flexographic printing forms by thermal development

A process for the production of flexographic printing plates by thermal development, in which the starting material used is a photopolymerizable flexographic printing element which comprises, arranged one on top of the other, at least

• a dimensionally stable substrate,
• at least one photopolymerizable relief-forming layer, at least comprising an elastomeric binder, ethylenically unsaturated monomers, plasticizer and photoinitiator, and
• the thermal development process comprises at least the following steps:
• (a) imagewise exposure of the photopolymerizable relief-forming layer to actinic radiation to produce polymerized and unpolymerized portions of the relief-forming layer,
• (b) heating of the relief-forming layer to a temperature of from 40 to 200° C. after imagewise exposure to soften the unpolymerized portions of the relief-forming layer,
• (c) removal of the softened, unpolymerized portions of the relief-forming layer to form a printing relief,
• wherein steps (b) and (c) are performed simultaneously or sequentially (b) then (c);
• and
• wherein the elastomeric binder is at least one styrene/butadiene block copolymer having a molecular weight Mw of from 80 000 to 250 000 g/mol and a styrene content of from 15 to 35% by weight, based on the binder, the proportion of butadiene present in 1,2-linked form being at least 15% by weight, based on the binder, and the amount of the styrene/butadiene block copolymer is from 35 to 50% by weight and that of the plasticizer is from 25 to 50% by weight, based in each case on the sum of all components of the relief-forming layer.