Heat-sensitive positive working lithographic printing plate precursor with a high resistance to chemicals

摘要

Heat-sensitive element comprising (a) an optionally pre-treated substrate (b) a positive working heat-sensitive coating comprising (i) at least one novolak resin, (ii) at least one component which reduces the aqueous alkaline developer solubility of novolak, wherein said reduction in solubility is reversed upon the application of heat, and (iii) at least one acidic polyvinyl acetal comprising the structural units (A), (B), (C) (A) (B) (C) and (D) wherein (D) is at least one unit selected from (D-1), (D-2), and (D-3): (D-1) (D-2) (D-3) wherein R¹ is a hydrogen atom or a C₁-C₄ alkyl group, R² is a hydrogen atom or a C₁-C₁₈ alkyl group, R³ is a hydrogen atom or a C₁-C₄ alkyl group, R⁴ is a hydrogen atom or a C₁-C₄ alkyl group, R⁵ is -COOH, -(CH₂)_a-COOH, -O-(CH₂)_a-COOH, -SO₃H, -PO₃H₂ or -PO₄H₂, a is an integer from 1 to 8, and X is selected from -(CR⁶R⁷)_n- and -CR⁸-CR⁹- wherein n is an integer of 1 to 6, each R⁶ and R⁷ is independently selected from a hydrogen atom and C₁-C₆ alkyl group, and R⁸ and R⁹ are independently selected from a hydrogen atom and a C₁-C₆ alkyl group or R⁸ and R⁹ together with the two carbon atoms to which they are bonded, form an optionally substituted aryl or heteroaryl group, wherein components (i) and (ii) do not have to be present as separate substances but may be used in the form of an appropriately functionalized novolak.