On-press processing of a UV or violet-sensitized lithographic printing plate
A method of lithographic plate making is disclosed wherein a plate precursor is exposed to laser radiation in the wavelength range from 350 to 450 nm, said plate precursor comprising a hydrophilic support and a photopolymerizable or photocrosslinkable image recording layer, and then processed on-press by supplying fountain and ink. The exposure produces a visible image having a 1976 CIELAB color distance ΔE between exposed and non-exposed areas of at least 2.5.
1. A method of making a lithographic printing plate comprising:
a) image-wise exposing a lithographic printing plate precursor to laser radiation in the wavelength range of from 350 to 450 nm at an energy density in the range from 1 to 150 mJ/cm 2 , said printing plate precursor comprising:
a hydrophilic support;
a negative-working image recording layer which comprises a polymerizable or crosslinkable compound, an initiator, a sensitizer, and a polymeric binder; and
optionally, an overcoat;
thereby producing a visible image consisting of exposed and non-exposed areas which have a CIELAB 1976 color distance ΔE of at least 2.5, wherein the color distance is measured prior to development; and
b) developing a lithographic printing plate by mounting the plate precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding fountain and/or ink to the plate precursor.
2. The method according to claim 1 wherein ΔE is at least 2, measured 24 hours after the exposure, during which 24 hour period the plate precursor is stored in the dark under ambient temperature and humidity conditions.
3. The method according to claim 1 wherein the laser radiation has a wavelength in the range of from 400 to 410 nm.
4. The method according to claim 1 wherein the non-exposed areas remain soluble or dispersible in fountain and/or ink upon exposure to daylight for at least 2 hours.
5. The method according to claim 1 wherein the precursor comprises substantially no leuco dyes.
6. The method according to claim 1 wherein the binder is a polymer having a glass transition temperature of at least 35° C.
7. The method according claim 6 wherein the binder is a polymer or a copolymer comprising monomeric units derived from an optionally substituted (meth)acrylate, an optionally substituted (meth)acrylamide, an optionally substituted styrene, or an optionally substituted vinyl naphthalene.
8. The method according to claim 1 wherein the initiator is an onium salt.
9. The method according to claim 8 wherein the initiator is a diaryliodonium salt.
10. The method according to claim 1 wherein the sensitizer is an optionally substituted fluorene or a dye having the general formula Sty-Ar-Sty wherein Sty is an optionally substituted styryl group and Ar is an optionally substituted (hetero-)aromatic group which forms a conjugated system with the Sty groups.
11. The method according to claim 10 wherein the initiator is a diaryliodonium salt.
12. The method according to claim 1 wherein the sensitizer is a distyryl-biphenyl or distyryl-benzene compound according to Formula I or Formula II, respectively:
wherein R 1″ to R 5′ and R 1″ to R 5″ independently represent hydrogen, an alkyl group, an alkoxy group, a cyano group, or a halogen;
wherein R 1 to R 14 independently represent hydrogen, an alkyl group, an alkoxy group, a cyano group, or a halogen.
13. The method according to claim 1 wherein the plate precursor comprises no overcoat or comprises an overcoat having a coating weight of less than 1.0 g/m 2 .
14. The method according to claim 1 wherein step b) is carried out by supplying only fountain to the plate precursor while starting the press and then switching on the ink supply.