Lithographic photopolymer printing plate precursors with improved daylight stability

Abstract

A lithographic printing plate precursor is disclosed, comprising a support, a photopolymerizable image recording layer and an outer coating layer comprising a low molecular free radical inhibitor. After image-wise exposure, the printing plate is heated, whereby the free radical inhibitor diffuses from the outer coating layer into the image recording layer, resulting in an increase in the sunlight stability of the exposed and heated precursor. The printing plate is particularly suitable for machining.

Description

Technical Field

[0001] The present invention relates to a lithographic printing plate precursor which functions by photopolymerization or photocrosslinking and has improved daylight stability. Preferred embodiments are suitable for machining on a machine. Background Art

[0002] Lithographic printing generally involves the use of a so-called printing master, such as a printing plate mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface, and the print is obtained by applying ink to the image and then transferring the ink from the master to a receiving material, which is generally paper. In conventional lithographic printing, ink and an aqueous dampening solution (also called fountain solution) are supplied to the lithographic image, which consists of oleophilic (or hydrophobic, i.e., accepting ink and repelling water) areas and hydrophilic (or oleophobic, i.e., accepting water and repelling ink) areas.

[0003] The lithographic printing master is usually obtained by image exposure and development of a so-called lithographic printing plate precursor, which comprises a heat-sensitive or photosensitive coating on a lithographic support. The coating is usually exposed to heat or light by means of a digitally modulated exposure device such as a laser, triggering (physical) chemical processes in the coating, such as ablation, reduced solubility by polymerization, by polymer crosslinking or by coagulation of particles of a thermoplastic polymer latex, solubilization by destroying intermolecular interactions or by increasing the permeability of the development barrier layer. Although some plate precursors are able to produce a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing because exposure produces a difference in solubility or a difference in dissolution rate in the developer between the exposed and unexposed areas of the coating. In positive-working lithographic printing plate precursors, the exposed areas of the coating dissolve in the developer, while the non-exposed areas remain resistant to the developer. In negative-working lithographic printing plate precursors, the non-exposed areas of the coating dissolve in the developer, while the exposed areas are resistant to the developer. Most lithographic printing plate precursors contain a hydrophobic coating on a hydrophilic support so that the portions of the image that are resistant to the developer define the areas of the plate that accept ink and are therefore the printing areas (also called image areas), while the hydrophilic support is revealed by dissolution of the coating in the developer at the non-printing (non-image) areas.

[0004] The coating of a photopolymer printing plate typically comprises a negative-working image-recording layer and a protective overcoat. Upon image-wise exposure to light or heat, the image-recording layer undergoes a chemical reaction whereby the layer hardens, i.e. becomes insoluble or non-dispersible in the developer solution by photopolymerization and / or photocrosslinking. The overcoat protects the image-recording layer from scratches or contamination. Photopolymer printing plates that function by free radical polymerization or crosslinking typically contain an overcoat which also acts as an oxygen barrier, which increases the sensitivity of the printing plate by reducing the quenching of free radicals by oxygen, which are generated in the image-recording layer by image-wise exposure.

[0005] Photopolymer printing plate precursors are typically sensitive to ultraviolet (UV) light, in particular near-UV light with a wavelength in the range of 300 to 400 nm; violet light, i.e. a wavelength in the range of 400-450 nm; blue, green or red light, i.e. a wavelength in the range of 450 nm to 750 nm; and / or infrared (IR) light, in particular near-IR light, i.e. a wavelength in the range of 750 to 1500 nm.

[0006] The conventional method of manufacturing a photopolymer printing plate comprises a step of first imagewise exposing the printing plate precursor with a laser, followed by a so-called "preheating" step to enhance the polymerization and / or crosslinking reaction of free radicals (generated by exposure in the image-recording layer), a washing step to remove the protective outer coating, an alkaline development step to remove the unexposed areas of the image-recording layer, and a rinsing and gumming step. In the past few years, the market has partly moved towards a more simplified and more sustainable workflow, in which the preheating step and / or the washing step are omitted or in which the development and gumming are performed in a single step. Alternatively, on-machine processing has attracted market interest, in which the printing plate is mounted on a printing press and the image is developed by interaction with dampening fluid and ink supplied to the printing plate during the operation of the printing press.

[0007] In contrast to conventional printing plates processed off-press, the on-press processed printing plates should be sufficiently resistant to sunlight, otherwise the unexposed areas of the image may harden when the printing plate is stored and handled until it is developed on the printing press. Sunlight hardening leads to fogging, i.e. undesired absorption of ink by the printing plate in non-image areas. Several measures have been described in the prior art to increase the sunlight stability, i.e. reduce the sensitivity of the image recording layer to sunlight:

[0008] - Mechanical stripping of the outer coating after image-wise exposure, as described, for example, in US2009208874;

[0009] - Adding sunlight-absorbing dyes to the outer coating, as described in, for example, US6420089, US20150177618 and US20070160935;

[0010] - deactivation of the image-recording layer by liquid treatment between image-wise exposure and on-press development, as described, for example, in US2009170040;

[0011] - deactivation of the image-recording layer by heating or total exposure to radiation between image-wise exposure and on-press development, as described in US20080280233;

[0012] - Adding a blowing agent to the outer coating, which decomposes upon heating to form a gas, thereby increasing the oxygen permeability of the outer coating, as described in WO 2019 / 076584.

[0013] WO96 / 34314 and WO96 / 34317 disclose machine-processable printing plates in which the outer coating comprises a polymeric free radical inhibitor, known as a free radical quencher; since the polymer cannot diffuse into the image recording layer, only the interface where the components of the outer coating and the components of the image recording layer are mixed with each other is deactivated by the polymeric free radical inhibitor.

[0014] EP2165829A discloses a machine-processable printing plate, wherein the outer coating comprises a free radical inhibitor, which preferably comprises a functional group capable of interacting with an inorganic layered compound also contained in the outer coating. Since the polymerization inhibitor is adsorbed on the inorganic layered compound, photopolymerization around the compound is inhibited, thereby facilitating its removal during on-press development. However, the interaction with the layered compound prevents the free radical inhibitor from diffusing into the image recording layer, so it has no effect on the daylight stability of the printing plate.

[0015] In summary, the known measures for reducing the daylight sensitivity of photopolymer printing plates are either not sufficiently effective or too cumbersome, for example because they generate waste or require additional equipment that is not available in a conventional platemaking workflow; or they involve liquid treatments that are contrary to the concept of a "process-free" (i.e., machine-processable) printing plate; or they also reduce the sensitivity of the image-recording layer to the radiation used for image-wise exposure. Summary of the invention

[0016] It is therefore an object of the present invention to provide a lithographic printing plate precursor having improved daylight stability between imagewise exposure and processing, preferably machining. It is a further object of the present invention to provide said improved daylight stability in a simple and efficient manner, without liquid handling, without generating additional waste, and without substantially affecting the sensitivity of the image recording layer before imagewise exposure. This object is achieved by a printing plate and a method as defined in the claims.

[0017] According to the present invention, there is provided a printing plate precursor comprising an outer coating, the outer coating comprising a free radical inhibitor capable of diffusing into an image recording layer by applying heat. Since the polymer cannot diffuse in the dry matrix of the coating of the printing plate precursor, the free radical inhibitor is a low molecular weight compound, i.e. an organic compound having a molecular weight of less than 1000 Daltons. The low molecular weight free radical inhibitor may be present in the outer coating as is, or may be released from the polymer into the outer coating when heat is applied. By heating the printing plate precursor after image exposure, the low molecular weight free radical inhibitor diffuses into the image recording layer and thereby reduces its sensitivity to sunlight. As a result, a preferred embodiment of the present invention comprises a coating in which the non-image area remains soluble and / or dispersible in the developer after being exposed to sunlight at an intensity of 500 lux for at least one hour.

[0018] According to a highly preferred embodiment of the present invention, the printing plate precursor comprises means for reducing the degree of mixing and / or premature diffusion of free radical inhibitors between the image-recording layer and the overcoat layer. Such means include (i) attachment of the inhibitor to a high molecular weight compound which retains the inhibitor in the overcoat layer but releases low molecular weight inhibitors upon heating; (ii) a barrier layer between the image-recording layer and the overcoat layer; and (iii) a barrier layer introduced into the image-recording layer by incorporating into the image-recording layer a compound of the formula:

[0019]

[0020] Where n represents an integer equal to 0 or 1, L 1 represents a divalent linking group, and * represents the position of connection to the carbon atom of the remaining structure. DETAILED DESCRIPTION

[0021] definition

[0022] The term "daylight" as used herein refers to simulated ambient white light as described in the Examples.

[0023] The term "aryl" herein is preferably phenyl, benzyl, tolyl, o-, m- or p-xylyl, naphthyl, anthracenyl, phenanthrenyl and / or a combination thereof. Heteroaryl is preferably a monocyclic or polycyclic aromatic ring, which contains carbon atoms and one or more heteroatoms in the ring structure, preferably 1 to 4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulfur. Preferred examples thereof include optionally substituted furyl, pyridyl, pyrimidyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thienyl, tetrazolyl, thiazolyl, (1,2,3) triazolyl, (1,2,4) triazolyl, thiadiazolyl, thiophenyl and / or a combination thereof. Optionally substituted heteroaryl is preferably a five-membered or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulfur atoms, selenium atoms or a combination thereof. Examples thereof include furan, thiophene, pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole, oxazole, isoxazole, thiazole, isothiazole, thiadiazole, oxadiazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, 1,2,4-triazine or 1,2,3-triazine, benzofuran, benzothiophene, indole, indazole, benzoxazole, quinoline, quinazoline, benzimidazole or benzotriazole.

[0024] The term "alkyl" herein refers to all possible variations of the number of carbon atoms in the alkyl group, i.e., methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tert-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, etc. Preferably, the alkyl group is C1-C 20Alkyl; more preferably, the alkyl group is C1-C6 alkyl. Most preferably, the alkyl group is methyl. Cycloalkyl groups include, for example, substituted or unsubstituted cyclopropyl, cyclopentyl, cyclohexyl, 4-methylcyclohexyl and cyclooctyl.

[0025] The term "substituted", e.g., substituted alkyl, means that the alkyl group may be substituted with atoms other than the atoms normally present in such groups (i.e., carbon and hydrogen). For example, the substituted alkyl group may include a halogen atom or a thiol group. Unsubstituted alkyl groups contain only carbon and hydrogen atoms.

[0026] The optional substituents represent alkyl, cycloalkyl, alkenyl or cycloalkenyl, alkynyl, aryl or heteroaryl, alkaryl or aralkyl, alkoxy (e.g., methoxy, ethoxy, isopropoxy, tert-butoxy, (2-hydroxytetradecyl)oxy, and various other straight and branched alkyleneoxyalkoxy groups); aryloxy, thioalkyl, thioaryl or thioheteroaryl, hydroxy, -SH, carboxylic acid or its alkyl ester, sulfonic acid or its alkyl ester, phosphonic acid or its alkyl ester, phosphoric acid or its alkyl ester, amino, sulfonamido, amide, nitro, nitrile, halogen (e.g., fluorine, chlorine or bromine), or combinations thereof.

[0027] Suitable alkenyl groups herein are preferably C2-C6 alkenyl groups, such as vinyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, isopropenyl, isobutenyl, isopentenyl, neopentenyl, 1-methylbutenyl, isohexenyl, cyclopentenylcyclohexenyl and methylcyclohexenyl.

[0028] Suitable alkynyl groups herein are preferably C2-C6 alkynyl groups; suitable aralkyl groups are preferably phenyl or naphthyl groups including one, two, three or more C1-C6 alkyl groups; suitable aralkyl groups are preferably C1-C6 alkyl groups including aryl groups (preferably phenyl or naphthyl groups).

[0029] The cyclic group or cyclic structure herein includes at least one ring structure, and may be a monocyclic or polycyclic group, meaning one or more rings fused together.

[0030] Lithographic printing plate precursors

[0031] The lithographic printing plate precursor of the present invention comprises a support and a coating provided thereon, the coating comprising an image recording layer and an outer coating. Although the outer coating is provided on top of the image recording layer as a separate layer, there may be a partial interface in which the outer coating and image recording layer components are mixed. The coating on the support may also include one or more additional layers, such as an undercoat between the support and the image recording layer, which may be designed to increase the adhesion of the printing area of ​​the coating to the support and / or facilitate the removal of the non-printing area of ​​the coating during processing; or a barrier layer between the image recording layer and the outer coating to avoid mixing of components of different layers during coating or to avoid diffusion of components between layers during storage, which will affect the shelf life of the printing plate precursor.

[0032] Carrier

[0033] The support preferably has a hydrophilic surface or is provided with a hydrophilic layer. Most preferred is a granulated and anodized aluminum support known in the art. Suitable supports are disclosed, for example, in EP1843203 (paragraphs

[0066] to

[0075] ). The surface roughness obtained after the granulation step, expressed as the arithmetic mean center line roughness Ra (ISO 4287 / 1 or DIN 4762), can vary between 0.05 and 1.5 μm. The Ra value of the aluminum support is preferably less than 0.50 μm, more preferably less than 0.40 μm and most preferably less than 0.30 μm. The lower limit of the Ra value is preferably about 0.1 μm. By anodizing the aluminum support, an Al2O3 layer is formed, the weight (g / m 2 Al2O3) can be in the range of 1 to 8 g / m 2 More preferably, it is between 2 and 3 g / m 2 between.

[0034] The granulated and anodized aluminum support may be subjected to a so-called post-anodic treatment and / or pore widening treatment. Suitable examples of post-anodic treatments are treatments with poly(vinylphosphonic acid) or derivatives thereof, with poly(acrylic acid), with potassium fluorozirconate or phosphates, with alkali metal silicates or combinations thereof. Alternatively, the support may be treated with adhesion promoting compounds, such as those described in

[0010] of EP1788434 and WO 2013 / 182328.

[0035] Besides aluminium supports, it is also possible to use plastic supports, for example polyester supports, which are provided with one or more hydrophilic layers as disclosed, for example, in EP 1 025 992 .

[0036] Image recording layer

[0037] The image-recording layer is cathodogenic, i.e. the unexposed image-recording layer is soluble or dispersible in a suitable developer, whereas the exposed image-recording layer becomes resistant to the developer due to hardening caused by exposure. Preferred embodiments are suitable for on-press development and comprise an image-recording layer which can be completely removed from the support by interaction with ink and / or fountain fluid at the start of a printing press run and which becomes resistant to ink and fountain fluid by exposure.

[0038] The hardening of the image-recording layer upon exposure is produced by a photopolymerizable and / or photocrosslinkable composition that is sensitive to ultraviolet (external) or infrared light. The peak sensitivity of the composition may be above 420 nm, but better daylight stability may be obtained using compositions with a peak sensitivity at shorter wavelengths, preferably below 420 nm, and more preferably below 410 nm. The availability of laser diodes emitting in the near-ultraviolet wavelength range (e.g. 365 or 375 nm) makes compositions with a peak sensitivity outside the visible wavelength range (i.e. below 400 nm) particularly advantageous. According to another embodiment, the peak sensitivity of the composition is in the IR wavelength range, preferably in near-IR light with a wavelength of 750 to 1100 nm, and more preferably 780 to 850 nm.

[0039] The coating thickness of the image-recording layer is preferably from 0.2 to 5.0 g / m², more preferably from 0.4 to 3.0 g / m², and most preferably from 0.6 to 1.5 g / m².

[0040] Polymerizable or cross-linkable composition

[0041] Preferred photopolymerizable or photocrosslinkable compositions include a polymerizable or crosslinkable compound, an initiator, an ultraviolet (external) sensitizer and / or an infrared sensitizer, and a polymer binder.

[0042] The polymerizable or crosslinkable compound is preferably a monomer or oligomer comprising at least one terminal olefinic group, hereinafter also referred to as a "free radical polymerizable monomer", while the polymerization initiator is a compound capable of generating free radicals upon exposure, optionally in the presence of a sensitizer (hereinafter referred to as a "free radical initiator").

[0043] Suitable free radical polymerizable monomers include, for example, multifunctional (meth) acrylate monomers, such as (meth) acrylates of ethylene glycol, trimethylolpropane, pentaerythritol, ethoxylated ethylene glycol and ethoxytrimethylolpropane, multifunctional urethane (meth) acrylates and epoxy (meth) acrylates, and oligoamine diacrylates. In addition to the (meth) acrylate groups, the (meth) acrylic acid monomers may also have additional double bonds or epoxide groups. The (meth) acrylate monomers may also contain acidic (e.g., carboxylic acid) or basic (e.g., amine) groups. Suitable free radical polymerizable monomers are disclosed in

[0042] and

[0050] of EP2916171 and are incorporated herein by reference.

[0044] Suitable free radical initiators are described in WO 2005 / 111727, page 15, line 17 to page 16, line 11 and in EP 1 091 247. Preferred free radical initiators are, for example, hexaarylbisimidazole compounds (HABI; dimers of triarylimidazoles), aromatic ketones, organic peroxides, thio compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds and compounds having a carbon-halogen bond.

[0045] Preferred free radical initiators are optionally substituted trihaloalkylsulfone compounds (hereinafter referred to as "THS" compounds), wherein halogen independently represents bromine, chlorine or iodine, and sulfone is a compound containing a sulfonyl group attached to two carbon atoms ( ). More preferably, the THS compound is an optionally substituted trihaloalkyl-(hetero)aryl sulfone, i.e. a compound in which the sulfonyl group is connected to an optionally substituted trihaloalkyl and an optionally substituted aryl or an optionally substituted heteroaryl. The aryl group is preferably an optionally substituted phenyl, benzyl, tolyl or o-, m- or p-xylyl, naphthyl, anthracenyl, phenanthrenyl and / or a combination thereof. The heteroaryl group is preferably an optionally substituted monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably 1-4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulfur. Preferred examples thereof include furan, thiophene, pyrrole, pyrazole, imidazole, 1,2,3- or 1,2,4-triazole, tetrazole, oxazole, isoxazole, thiazole, isothiazole, thiadiazole, oxadiazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-, 1,2,4- or 1,2,3-triazine, benzofuran, benzothiophene, indole, indazole, benzoxazole, quinoline, quinazoline, benzimidazole or benzotriazole. The most preferred THS compound is optionally substituted tribromomethyl aryl sulfone, and most preferably optionally substituted tribromomethyl phenyl sulfone.

[0046] The amount of THS initiator generally ranges from 0.1 to 30 wt. %, preferably from 0.5 to 10 wt. %, most preferably from 2 to 7 wt. %, relative to the total dry weight of the non-volatile components of the photopolymerizable or photocrosslinkable composition.

[0047] Another group of preferred free radical initiators are onium salts, in particular iodonium salts and sulfonium salts or mixtures thereof. Suitable iodonium salt classes are optionally substituted diaryl iodonium salts or diheteroaryl iodonium salts. Specific examples of diaryl iodonium salts include diphenyl iodonium, 4-methoxyphenyl-4-(2-methylpropyl)phenyl iodonium, 4-chlorophenyl-4-phenyl iodonium, 4-(2-methylpropyl)phenyl-tolyl iodonium, 4-hexyloxyphenyl-2,4,6-trimethoxyphenyl iodonium, 4-hexyloxyphenyl-2,4-diethoxyphenyl iodonium, 4-octyloxyphenyl-6-trimethoxyphenyl iodonium, bis(4-tert-butylphenyl)iodonium and bis(4- isopropylphenyl)iodonium, 4-octyloxyphenylphenyliodonium, [4-[(2-hydroxytetradecyl)oxy]phenyl]phenyliodonium, 4-methylphenyl-4'-hexylphenyliodonium, tetraphenylborate, 4-methylphenyl-4'-cyclohexylphenyliodonium, 4-hexylphenyl-phenyliodonium, 4-methylphenyl-4'-cyclohexylphenyliodonium, 4-cyclohexylphenyl-phenyliodonium, 2-methyl-4-tert-butylphenyl-4'-methylphenyliodonium.

[0048] Preferred examples of the triarylsulfonium salt include triphenylsulfonium, dialkylbenzoylsulfonium, dialkyl-4-hydroxyphenylsulfonium, bis(4-chlorophenyl)phenylsulfonium, triphenylsulfonium benzoylformate, bis(4-chlorophenyl)phenylsulfonium benzoylformate, bis(4-chlorophenyl)-4-methylphenylsulfonium, bis(4-chlorophenyl)-4-methylphenylsulfonium, tris(4-chlorophenyl)sulfonium, tris(2,4-dichlorophenyl)sulfonium, bis(2,4-dichlorophenyl)phenylsulfonium and bis(2,4-dichlorophenyl)-4-methoxyphenylsulfonium.

[0049] A suitable counterion for the onium salt is, for example, PF6 - 、SbF6 - 、AsF6 - 、Ph4B - The onium salt is preferably present in the coating in an amount of 1 to 25 wt%, more preferably in an amount of 5 to 20 wt%, and most preferably in an amount of 10 to 16 wt%, all based on the total dry weight of the photopolymerizable and / or photocrosslinkable layer.

[0050] The image recording layer may also include a coinitiator used in combination with a free radical initiator. Suitable coinitiators for photopolymer coatings are disclosed in US6,410,205; US5,049,479; EP1079276, EP107792, EP1369232, EP1369231, EP1341040, US2003 / 0124460, EP1241002, EP1288720 and references, including the cited reference: KK Dietliker's Chemistry & Technology UV & EB formulation for coatings, inks & paints - Volume 3 - Photoinitiators for Free Radical and CationicPolymerization, edited by PKT Oldring (1991; ISBN 0947798161). Preferred coinitiators are disclosed in EP2916171 (paragraph

[0051] ) and are incorporated herein by reference.

[0051] Suitable ultraviolet (external) sensitizers are dyes having a light absorption peak in the wavelength range of 320nm to 500nm, preferably 350 to 450nm, and more preferably 360 to 420nm. Suitable (near) infrared sensitizers are dyes having a light absorption peak in the wavelength range of 750 to 1100nm, preferably 780 to 850nm, and more preferably 810 to 830nm. The best sunlight stability can be obtained by using a sensitizer with an absorption peak below 400nm and / or above 750nm. The absorption peak wavelength is a value measured in the dry matrix of the printing plate precursor coating.

[0052] Suitable ultraviolet (outside) sensitizer is for example fluorene, thioxanthone, (keto-) coumarin, pyrilium or thiopyrilium dye.Preferred dye has general structure Sty-Ar-Sty, wherein each " Sty " group is optionally substituted styryl (C6H5-CH=CH-), and Ar is optionally substituted aryl or optionally substituted heteroaryl, and it forms conjugated system with Sty group.Two Sty groups can be identical or different.The example of Ar is preferably derived from two or more combinations in benzene, naphthalene, anthracene, fluorene, biphenyl, carbazole, furan, dibenzofuran, thiophene, dibenzothiophene, dithienylthiophene, oxadiazole, thiadiazole, pyridine, pyrimidine and these groups, and it can be identical or different.Wherein Ar is the dyestuff of biphenyl or phenyl is most preferred, and these dyestuffs will be respectively referred to as distyryl biphenyl compound and distyryl benzene compound in this article.

[0053] Highly preferred UV (extra) sensitizers are distyrylbiphenyl compounds and distyrylbenzene compounds having structures according to the following formulae UV-I and UV-II, respectively:

[0054]

[0055] Where R 1' To R 5' and R 1'' To R 5'' independently represent hydrogen, alkyl, alkoxy, cyano or halogen;

[0056]

[0057] Where R 1 To R 14 independently represent hydrogen, alkyl, alkoxy, cyano or halogen.

[0058] R in formula UV-I 1' To R 5' or R 1'' To R 5'' One of the preferably represents an alkoxy group having more than 1 carbon atom.

[0059] More preferably, R in formula UV-I 1' , R 5' , R 1'' , R 5'' independently represent hydrogen, fluorine or chlorine, and R in formula UV-I 2' To R 4' and R 2'' To R 4'' independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have 3 to 15 carbon atoms.

[0060] Even more preferably, R in formula UV-I 1' , R 5' , R 1'' , R 5 represents hydrogen, and R in formula UV-I 2' To R 4' and R 2'' To R 4'' independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have 3 to 15 carbon atoms.

[0061] Most preferably, R in formula UV-I 2' , R 4' , R 2'' , R 4'' represents a methoxy group, and R in formula UV-I 3 ' and R 3'' independently represents a branched alkoxy group having 3 to 15 carbon atoms.

[0062] R in formula UV-II 1 To R 10 One of the preferably represents an alkoxy group having more than 1 carbon atom.

[0063] More preferably, R in formula UV-II 1 , R 5 , R 6 , R 10 , R 11 , R 12 , R 13 and R 14 independently represent hydrogen, fluorine or chlorine, and R in formula UV-II 2 To R 4 and R 7 To R 9 independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have 3 to 15 carbon atoms.

[0064] Even more preferably, R in formula UV-II 1 , R 5 , R 6 and R 10 represents hydrogen, and R in formula UV-II 2 To R 4 and R 7 To R 9 independently represent an alkoxy group; and at least two of the alkoxy groups are branched and have 3 to 15 carbon atoms.

[0065] Most preferably, R in formula UV-II 2 , R 4 , R 7 and R 9 represents a methoxy group, and R in formula UV-II 3 and R 8 and independently represent a branched alkoxy group having 3 to 15 carbon atoms.

[0066] The following compounds are examples of highly preferred sensitizers according to formula UV-I or UV-II:

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] The total amount of the ultraviolet sensitizer contained is preferably 0.1 to 25 wt %, more preferably 0.5 to 20 wt %, and most preferably 1.0 to 15 wt %, relative to the total dry weight of the image recording layer.

[0079] Suitable near infrared sensitizers include IR light absorbing dyes and pigments. Preferred pigments are carbon black. Preferred IR dyes have a light absorption peak of 750nm to 1300nm, more preferably 780nm to 1100nm, and most preferably 800nm ​​to 850nm. Suitable IR dyes are merocyanines, indoanilines, oxonoles, pyrilium dyes, squarilium dyes and cyanine dyes, particularly heptamethine cyanine dyes. Examples of suitable IR dyes are described in, for example, EP823327, EP978376, EP1029667, EP1053868, EP1093934; EP1359008; WO97 / 39894 and WO00 / 29214. Highly preferred IR dyes produce visible images immediately after image exposure, such as those disclosed in EP1736312, EP1910082 and WO2019 / 219560. Such thermochromic IR dyes can also be used in outer coatings.

[0080] Mixtures of sensitizers may also be used, for example mixtures of two or more of the above dyes, or mixtures of the above dyes with other sensitizers. The total concentration of the sensitizer(s) is preferably 0.25 to 25.0 wt %, more preferably 0.5 to 20.0 wt % and most preferably 1.0 to 10.0 wt % relative to the total dry weight of the image-recording layer.

[0081] The adhesive can be selected from a wide range of organic polymers. Mixtures of different adhesives can also be used. Useful adhesives are described in WO2005 / 111727, page 17, line 21 to page 19, line 30, EP1043627, paragraph

[0013] , and WO2005 / 029187, page 16, line 26 to page 18, line 11.

[0082] The image-recording layer may also include additional ingredients such as leuco dyes that form a visible image upon imagewise exposure, particles to protect the layer from mechanical damage, adhesion promoting compounds, and even small amounts of free radical inhibitors, which may be the same as or different from those in the overcoat layer. Various surfactants may also be added to the image-recording layer to allow or enhance its developability.

[0083] The particles mentioned to protect the layer from mechanical damage (e.g. scratches caused by manual handling or plate handling equipment) may be inorganic particles, organic particles or fillers, such as described in US 7,108,956. More details of suitable spacer particles described in

[0053] to

[0056] of EP2916171A are incorporated herein by reference.

[0084] Adhesion-promoting compounds are compounds which are capable of interacting with the carrier, preferably compounds having addition-polymerizable ethylenically unsaturated bonds and functional groups which are capable of interacting with the carrier. "Interaction" is understood to be any type of physical and / or chemical reaction or process in which a bond is formed between the functional group and the carrier, which bond may be a covalent bond, an ionic bond, a complex bond, a coordinate bond or a hydrogen bond and may be formed by adsorption processes, chemical reactions, acid-base reactions, complex formation reactions or reactions of chelating groups or ligands.

[0085] The adhesion promoting compound may be selected from at least one low molecular weight compound or polymeric compound as described in EP851299A, page 3, line 22 to page 4, line 1, EP1500498A, page 7, paragraph

[0023] to page 20, paragraph

[0052] , EP1495866A, page 5, paragraph

[0030] to page 11, paragraph

[0049] , EP1091251A, page 3, paragraph

[0014] to page 20, paragraph

[0018] and EP1520694A, page 6, paragraph

[0023] to page 19, paragraph

[0060] . Preferred compounds are those which contain phosphate or phosphonate groups as functional groups capable of adsorbing on an aluminum support and which comprise groups reactive with addition polymerizable olefinic double bonds, in particular those described in EP851299A, page 3, line 22 to page 4, line 1 and EP1500498A, page 7, paragraph

[0023] to page 20, paragraph

[0052] . Also preferred are those compounds containing tri-alkyl-oxy-silane groups, also referred to below as "trialkoxysilane" groups, in which the alkyl group is preferably methyl or ethyl, or in which the trialkoxysilane groups are at least partially hydrolyzed to silanol groups, as functional groups capable of adsorption on the support, in particular silane coupling agents having addition polymerizable olefinic double bond reactive groups, as described in EP1557262A, page 49, paragraph

[0279] and EP1495866A, page 5, paragraph

[0030] to page 11, paragraph

[0049] . Adhesion-promoting compounds are also described in EP2916171A

[0058] , which is incorporated herein by reference.

[0086] The adhesion promoting compound may be present in the image-recording layer in an amount in the range from 1 to 50 wt%, preferably from 3 to 30 wt%, more preferably from 5 to 20 wt% based on the dry weight of the layer.

[0087] The adhesion promoting compound may be present in the optional intermediate layer (primer) in an amount of at least 25 wt %, preferably at least 50 wt %, more preferably at least 75 wt % of the layer dry weight. Alternatively, the intermediate layer may consist entirely of the adhesion promoting compound.

[0088] In a highly preferred embodiment, the image recording layer further comprises means for reducing the degree of mixing between the image recording layer and the overcoat layer or for reducing the degree of premature diffusion of low molecular free radical inhibitors from the overcoat layer (i.e. before the heating step) into the image recording layer. These means are described in separate sections below.

[0089] External coating

[0090] The topcoat is preferably soluble or dispersible in water so that it can be easily removed by an aqueous developer or dampening fluid during on-press development. Therefore, the topcoat preferably includes a hydrophilic binder. Preferred binders that can be used in the top layer are disclosed in WO2005 / 029190 (page 36, line 3 to page 39, line 25), US 2007 / 0020563 (paragraph

[0158] ) and EP1288720A (paragraphs

[0148] and

[0149] ), including the references cited in these patent applications.

[0091] The most preferred binder for the outer coating is poly(vinyl alcohol) and / or a poly(vinyl alcohol) derivative. The degree of hydrolysis of the poly(vinyl alcohol) is preferably in the range of 74 mol % to 99 mol %, more preferably in the range of 88 mol % to 98 mol %. The viscosity value of the poly(vinyl alcohol) is related to the molecular weight and is measured according to DIN 53015 at 20° C. as a 4 wt % aqueous solution, preferably in the range of 1 to 26, more preferably in the range of 2 to 15, most preferably in the range of 2 to 10.

[0092] Mixtures of hydrophilic binders may also be used, for example mixtures of two or more water-soluble polymers, for example mixtures of poly(vinyl alcohol) and poly(vinyl pyrrolidone), or mixtures of poly(vinyl alcohol) and / or poly(vinyl ene alcohol) derivatives having different hydrolysis and viscosity values. Modified poly(vinyl alcohol)s, for example poly(vinyl alcohol)s having carboxyl and / or sulfonic acid groups, may also be used, preferably together with unmodified poly(vinyl alcohol).

[0093] The coating thickness of the outer coating is preferably 0.15 to 1.75 g / m 2 , more preferably 0.20 to 1.3 g / m 2 , most preferably 0.25 to 1.0 g / m 2 After application of the overcoat, the wet layer is dried at moderate temperatures, preferably below 100° C., more preferably below 80° C., and most preferably below 60° C. High drying temperatures may result in diffusion of free radical inhibitors into the image-recording layer and are therefore preferably avoided.

[0094] In addition to the free radical inhibitors described below, the overcoat may include other ingredients, such as anionic surfactants, for example sodium alkyl sulfates or alkyl sulfonates, sodium dioctyl sulfosuccinate, sodium dodecylbenzene sulfonate and ammonium lauryl sulfate; amphoteric surfactants, for example alkylamino carboxylates and alkylamino dicarboxylates; nonionic surfactants, for example polyethylene glycol, polypropylene glycol, and copolymers of ethylene glycol and propylene glycol, polysiloxane surfactants, perfluorocarbon surfactants, alkylphenyl ethylene oxide condensates, alkoxylated alkylenediamines, as disclosed in EP1085380A (paragraphs

[0021] and

[0022] ); and various additives, for example glycerol, pigments, matting agents or wetting agents as disclosed in EP2916171A, and / or (inorganic) organic acids, such as the acids disclosed in EP2149071A, page 27, lines 1 to 21. Microparticles may also be added to the overcoat, for example, to reduce the tack or moisture sensitivity of the printing plate.

[0095] In a highly preferred embodiment of the present invention, the outer coating further comprises a thermochromic dye which produces a visible image when imagewise exposed to IR light, as disclosed in WO2019 / 219560.

[0096] Low molecular weight free radical inhibitors

[0097] According to the present invention, the outer coating further comprises a free radical inhibitor, which is preferably water-soluble, because the outer coating is usually applied from an aqueous coating solution. A mixture of water and an organic solvent (e.g., alcohol) can also be used to improve the solubility of the free radical inhibitor. A poorly soluble inhibitor can also be added to the coating solution of the outer coating as a dispersion.

[0098] The free radical inhibitor is a low molecular weight compound, i.e. an organic compound having a molecular weight of less than 1000 Daltons. The low molecular weight free radical inhibitor may be added to the outer coating as is, or may be a component of a polymer compound which releases the low molecular weight free radical inhibitor upon heating. In both embodiments, the low molecular weight free radical inhibitor is able to diffuse from the outer coating to the image recording layer, whereby the daylight sensitivity of the image recording layer is reduced, preferably to the extent that it can withstand daylight at an intensity of 500 lux for at least one hour without fogging. A more preferred embodiment shows a reduction in daylight sensitivity such that the image recording layer can withstand daylight at an intensity of 500 lux for at least 2 hours without fogging, even more preferably for at least 4 hours, and most preferably for at least 12 hours.

[0099] Many free radical inhibitors suitable for use in the present invention have been described in the prior art as components of the image-recording layer, where they are added to increase the shelf life (i.e. storage before use) of the printing plate precursor. Typical inhibitors include free radical scavenging groups, which are capable of deactivating free radicals generated in the image-recording layer by exposure to light. Polyfunctional inhibitors comprising two or more free radical scavenging groups may also be used. Suitable scavenging groups are, for example, oximes, phenols, nitro, nitroxyl, nitroso, nitrones, hydroxamic acids and amide oximes.

[0100] Specific examples of free radical inhibitors for use in the present invention include quinones or quinone methides, such as substituted or unsubstituted benzoquinone; phenolic compounds, such as substituted or unsubstituted phenols or hydroquinone; amino compounds, such as N,N'-tetraethyl-p-phenylenediamine; sulfur compounds, such as tetraalkylthiourea disulfide; N-oxides, such as substituted or unsubstituted pyridine-N-oxides; thiocyanates, such as ammonium thiocyanate; nitrites, such as sodium nitrite; phenothiazines; nitro or nitroso compounds; 2-mercaptobenzothiazole; 2-mercaptobenzoxazole; and 2-mercaptobenzimidazole.

[0101] Preferred free radical inhibitors for use in the present invention are nitroxyl compounds, i.e. compounds containing a free radical >N-O•, such as compounds disclosed in EP0828195A and WO2006 / 024621. More preferred free radical inhibitors contain sterically hindered nitroxyl groups, such as:

[0102]

[0103] wherein R is a substituent.

[0104] The specific examples are as follows:

[0105] Di-tert-butylnitroxyl,

[0106] 1-oxy-2,2,6,6-tetramethylpiperidine,

[0107] 1-Oxy-2,2,6,6-tetramethylpiperidin-4-01,

[0108] 1-oxy-2,2,6,6-tetramethylpiperidin-4-one,

[0109] 1-oxy-2,2,6,6-tetramethylpiperidin-4-yl acetate,

[0110] 1-oxy-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate,

[0111] 1-oxy-2,2,6,6-tetramethylpiperidin-4-yl stearate,

[0112] 1-oxy-2,2,6,6-tetramethylpiperidin-4-ylbenzoate,

[0113] 1-Oxy-2,2,6,6-tetramethylpiperidin-4-yl 4-tert-butylbenzoate,

[0114] Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate,

[0115] Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate,

[0116] Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)sebacate,

[0117] Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butylmalonate,

[0118] Bis(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl)phthalate,

[0119] Bis(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate,

[0120] Bis(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate,

[0121] Bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexahydroterephthalate,

[0122] N,N'-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide,

[0123] N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-caprolactam,

[0124] N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-dodecylsuccinimide,

[0125] 2,4,6-Tris(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl isocyanurate,

[0126] 2,4,6-tri-[N-butyl-N-(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl]-s-triazine, or

[0127] 4,4'-Ethylidenebis(1-oxy-2,2,6,6-tetramethylpiperazin-3-one).

[0128] Highly preferred inhibitors are bis(1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) sebacate (hereinafter "Compound A"), 4-benzylidene-2,6-di-tert-butyl-cyclohexa-2,5-dienone and 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine.

[0129] Since the free radical inhibitor diffuses into the image recording layer when heated after image exposure, it can be added to the overcoat layer at a much higher concentration than when added directly to the image recording layer. The prior art discloses that when the free radical inhibitor is added directly to the image recording layer, its concentration is very low: generally less than 1% by weight, more preferably less than 0.2% by weight relative to the image recording layer as a whole. Since a typical image recording layer has a surface area of ​​about 1 g / m 2 The amount of free radical inhibitor used in the prior art is usually less than 10 mg / m 2 However, in the present invention, the low molecular free radical inhibitor can be used at a concentration higher than 10 mg / m 2 , preferably at least 20 mg / m 2 , more preferably at least 40 mg / m 2 The concentration of the free radical inhibitor may be added (or released by heating, as described in more detail below) to the outer coating. Some embodiments may even be used at a concentration of free radical inhibitor greater than 60 mg / m 2 When such high amounts are used in the image-recording layer, its sensitivity to light for image-wise exposure will be too low and the material will be of no practical use.

[0130] Means for reducing the degree of mixing and / or premature diffusion between the image-recording layer and the overcoat layer

[0131] In a preferred embodiment of the invention, the printing plate precursor comprises means for reducing the extent of mixing of the components of the image-recording layer and the overcoat layer during coating and / or means for reducing the extent of premature diffusion (i.e. migration before the heating step) of the free radical inhibitor from the overcoat layer into the image-recording layer during storage. Both effects result in a lower sensitivity of the image-recording layer to light for image-wise exposure due to the presence of the free radical inhibitor in the photopolymerizable and / or photocrosslinkable composition of the image-recording layer.

[0132] The extent of mixing and / or premature diffusion depends on various factors. When the image-recording layer and the overcoat are applied as solutions in an immiscible solvent system (e.g., the recording layer is coated from an organic solvent or a mixture of organic solvents, while the overcoat is coated from an aqueous solution), the extent of mixing is expected to be lower than when the layers are coated from similar solvent systems. Premature diffusion after coating may also depend on the type of binder in the layer or the presence of fillers, such as inorganic particles, which may hinder diffusion of the components.

[0133] A method of achieving less mixing and / or less premature diffusion comprises applying the outer coating alone on a temporary carrier, for example a plastic carrier provided with a peeling layer, and then laminating the dry outer coating on the dry image recording layer and peeling off the temporary carrier. Before applying the outer coating, a barrier layer can also be applied to the image recording layer. This barrier layer can in principle have any composition as long as it dissolves well in the developer because it is mainly used as a spacer between the image recording layer and the outer coating. These two embodiments can also be combined, by coating the outer coating and the barrier layer on a temporary carrier, laminating the material to the image recording layer so that the barrier layer is between the image recording layer and the outer coating, and then peeling off the temporary carrier.

[0134] In a preferred embodiment, the extent of mixing or premature diffusion may be reduced by adding a compound according to the following formula I to the image-recording layer:

[0135]

[0136] Where n is an integer equal to 0 or 1, L 1 represents a divalent linking group, and * represents the position of attachment to a carbon atom of the remaining structure.

[0137] The compounds of formula I are referred to herein as "blocking compounds" because they are integrated into the image recording layer as a barrier to mixing and diffusion. The synthesis of these compounds is described in WO2014 / 198820 and WO2014 / 18823. The amount of blocking compounds in the image recording layer is preferably higher than 1% by weight, more preferably higher than 2% by weight, and most preferably higher than 5% by weight, relative to the total weight of all ingredients in the photopolymerizable layer. The amount is preferably less than 50% by weight, more preferably 8 to 40% by weight, and most preferably 10 to 20% by weight, relative to the total weight of all ingredients in the photopolymerizable layer.

[0138] The blocking compound can be a monomer, an oligomer (i.e. a structure comprising a limited number of monomers, such as two, three or four repeating units) or a polymer (i.e. a structure comprising more than four repeating units). The blocking compound contains at least one moiety according to formula I, preferably 2 to 150 moieties according to formula I, more preferably 2 to 100 moieties according to formula I and most preferably 2, 3 or 4 moieties according to formula I.

[0139] The divalent linking group L in formula I 1It is preferably selected from optionally substituted alkylene, cycloalkylene, arylene or heteroarylene, -O, -CO-, -CO-O-, -O-CO-, -CO-NH-, -NH-CO-, -NH-CO-O-, -O-CO-NH-, -NH-CO-NH-, -NH-CS-NH-, -CO-NR'-, -NR''-CO-, -NH-CS-NH-, -SO-, -SO2-, -SO2-NH-, -NH-SO2-, -CH=N-, -NH-NH-, -N+(CH3)2-, -S-, -SS- and / or combinations thereof, wherein R' and R'' each independently represent optionally substituted alkyl, aryl or heteroaryl. Substituents optionally present on the alkylene, arylene or heteroarylene groups may be represented by an alkyl group, such as methyl, ethyl, propyl or isopropyl, and substituents include, for example, oxygen or sulfur; a halogen, such as a fluorine, chlorine, bromine or iodine atom; a hydroxyl group; an amino group; an alkoxy group such as a methoxy or ethoxy group or a (di)alkylamino group.

[0140] More preferably, the divalent linking group L 1 is a divalent aliphatic group including a straight or branched carbon chain or an alicyclic, non-aromatic ring. Optionally, the aliphatic linking group may contain substituents including, for example, oxygen or sulfur; alkyl groups such as, for example, methyl, ethyl, propyl or isopropyl, and halogens such as fluorine, chlorine, bromine or iodine atoms. Most preferably, the linking group L 1 represents an optionally substituted alkylene or cycloalkylene. The substituent optionally present on the alkylene or cycloalkylene may be represented by an alkyl group such as methyl, ethyl, propyl or isopropyl, or a halogen such as fluorine, chlorine, bromine or iodine atom.

[0141] Preferred barrier compounds are represented by Formula II:

[0142] (Formula II)

[0143] Where R 1 and R 2 independently represents a group including a free radical polymerizable group; n represents an integer equal to 0 or 1; L 2 and L 3 independently represent a divalent linking group; and L 1 has the same meaning as in formula I.

[0144] R 1 and R 2 The free radical polymerizable groups in can be the same or different. Suitable examples are acrylate, methacrylate, acrylamide, methacrylamide, styryl or vinyl, which can be substituted separately. Acrylate and methacrylate groups are particularly preferred. Optional substituents are, for example, halogens such as fluorine, chlorine, bromine or iodine atoms, or alkyls such as methyl, ethyl, propyl or isopropyl.

[0145] Linking group L 2 and L 3 Preferably independently represents as above for L 1 Preferably, the linking group L of formula I and II 1 Contains no tertiary amine groups, as these groups are often responsible for staining in non-image areas of the plate after processing. Contains linking groups L containing aromatic ring structures 1 Compounds of may be of less interest due to their limited solubility in the photopolymer coating solution.

[0146] Highly preferred blocking compounds are oxalamide derivatives, ie compounds according to formula I or II, wherein n = 0. In the most preferred embodiment, the oxalamide derivative is represented by formula III:

[0147] (Formula III)

[0148] Where R 3 and R 4 independently represent terminal groups, and L 4 and L 5 independently represents an optionally substituted divalent linking group, which is preferably the above for group L 2 and L 3 Defined groups.

[0149] Terminal group R 3 and R 4 Preferably, it is represented by hydrogen, optionally substituted alkyl or cycloalkyl, optionally substituted aryl, optionally substituted aralkyl or optionally substituted heteroaryl. Suitable alkyl groups include 1 or more carbon atoms, for example C1-C 22 Alkyl, more preferably C1-C 12Alkyl, and most preferably C1-C6 alkyl.Alkyl can be straight or branched, such as methyl, ethyl, propyl (n-propyl, isopropyl), butyl (n-butyl, isobutyl, tert-butyl), pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl or hexyl.Suitable cycloalkyl is a non-aromatic, carbon-containing homocyclic group, and can be monocyclic or polycyclic.Examples include cyclopentyl, cyclohexyl or adamantyl.Suitable aryl includes, for example, phenyl, naphthyl, benzyl, tolyl, o-, m- or p-xylene dimethyl, anthracenyl or phenanthrenyl.Suitable aralkyl includes, for example, phenyl or naphthyl, including one, two, three or more C1-C6 alkyl.Suitable heteroaryl is preferably a monocyclic or polycyclic aromatic ring containing carbon atoms and one or more heteroatoms in the ring structure.Preferably 1-4 heteroatoms are independently selected from nitrogen, oxygen, selenium and sulfur and / or combinations thereof. Examples include pyridyl, pyrimidinyl, pyrazolyl, triazinyl, imidazolyl, (1,2,3)- and (1,2,4)-triazolyl, tetrazolyl, furanyl, thienyl, isoxazolyl, thiazolyl and carbazolyl.

[0150] More preferably, R 3 and R 4 are independently represented by hydrogen or optionally substituted alkyl, aryl or aralkyl and / or combinations thereof. Most preferably, R 3 and R 4 Independently represents hydrogen or methyl. Alkyl, cycloalkyl, aralkyl, aryl or heteroaryl can include one or more substituents. The optional substituents on alkyl, cycloalkyl, aralkyl, aryl or heteroaryl are preferably selected from alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, 1-isobutyl, 2-isobutyl and tert-butyl; ester, amide, ether, thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate or sulfonamide, halogen such as fluorine, chlorine, bromine or iodine, -OH, -SH, -CN and -NO2 and / or its combination.

[0151] Specific examples of suitable barrier compounds for use in the present invention are given in Table A below.

[0152] Table A

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159] According to an alternative embodiment of the blocking means, the extent of premature diffusion and / or mixing can be reduced by attaching a low molecular free radical inhibitor to a polymer compound. Upon heating, the inhibitor is released from the polymer compound and allowed to diffuse into the image recording layer. Preferred embodiments are obtained by covalently binding the inhibitor to a polymer, encapsulating the inhibitor in a polymer capsule or by filling polymer particles with the inhibitor.

[0160] According to a first embodiment in which the low molecular free radical inhibitor is attached to the polymer, the inhibitor may be part of the polymer backbone, which releases the low molecular free radical inhibitor by thermally induced backbone degradation, or it may be part of a side group attached to the polymer backbone, which releases the low molecular free radical inhibitor by thermal side group degradation (e.g., by cleavage of a thermally fragile linker). Several thermally degradable groups are disclosed in the literature, including thermal sulfoxide elimination, thermal decomposition beta elimination of functional groups beta to electron withdrawing groups (e.g., esters, amides, and sulfones), and thermal decomposition cleavage of secondary esters and more preferably tertiary esters, tertiary carbamates, tertiary ethers, and tertiary carbonates of carboxylic acids, sulfonic acids, phosphonic acids, and phosphoric acids. Thermal degradation may be accelerated by acid catalysis, wherein the acid may be generated by thermal acid or photoacid. Typical thermally degradable resins optionally catalyzed by acid are disclosed in Okamura et al., Journal of Photopolymer Science and Technology, 24 (5), 561-564 (2011); Okamura et al., Reactive and Functional Polymers, 71 (4), 480-488 (2011); Fouassier JP, Allonas X., Basics and Applications of Photopolymerization Reactions, 2, 235-244 (2010)) and as resins in additive manufacturing (US20170120515, WO20170487100). Palmieri et al. disclose the use of acid-degradable resists in nanoimprint lithography (ACS Nano, 1 (4), 307-312 (2007)), generating low molecular weight radical inhibitors from polymer compounds.

[0161] In another embodiment, mixing and premature diffusion can be avoided or reduced by encapsulating the low molecular free radical inhibitor into microcapsules or nanocapsules, which are added to the outer coating and can release the inhibitor when heated. Microcapsules or nanocapsules are defined as core-shell structures, which include a polymer shell that is degradable or permeable when heated, surrounding a core containing the low molecular free radical inhibitor. The capsules are preferably prepared by interfacial polymerization (e.g., interfacial polycondensation). Interfacial polymerization is well known, for example, Zhang Y. and Rochefort D. (Journal of Microencapsulation, 29 (7), 636-649 (2012)) and Salitin (in Encapsulation Nanotechnologies, Vikas Mittal (ed.), Chapter 5, 137-173 (Scrivener Publishing LLC (2013)). Generally, interfacial polymerization requires that a lipophilic phase be dispersed in a continuous aqueous phase, or vice versa, to form an emulsion. Each phase contains at least one dissolved monomer (e.g., a first shell component in the lipophilic phase) that is capable of reacting with another monomer (a second shell component) dissolved in another phase (e.g., an aqueous phase). The two monomers meet and react rapidly at the interface of the emulsion droplets. The polymer thus formed is insoluble in both the aqueous phase and the lipophilic phase. As a result, the formed polymer tends to precipitate at the interface between the two phases, thereby forming a shell around the dispersed core. A more specific description of capsules suitable for the present invention can be found in WO2015 / 158654, WO2015 / 158592, and WO2016 / 184504, provided that the released component is replaced by the low molecular weight free radical inhibitor of the present invention.

[0162] According to yet another embodiment, mixing and premature diffusion can also be avoided or reduced by filling polymer particles with low molecular free radical inhibitors used in the present invention. The filled polymer particles are added to the outer coating and can release the inhibitor when heated. The polymer particles are preferably latex particles, i.e., polymer particles that form a stable aqueous dispersion. The polymer particles are preferably self-dispersible particles, which can be obtained by incorporating monomers containing carboxylic acid groups, sulfonic acid groups or phosphoric acid groups. For polyurethane particles, the self-dispersing group can be incorporated via diols and / or diamines with any one of phosphoric acid groups, sulfonic acid groups, N, N-disubstituted amino groups, carboxyl groups, neutralized phosphoric acid groups, neutralized sulfonic acid groups, neutralized N, N-disubstituted amino groups and neutralized carboxyl groups. The polymer particles can be filled with low molecular free radical inhibitors by the following method. First, the inhibitor is dissolved in a water-immiscible organic solvent having a boiling point below 100°C. The solution is then finely dispersed in water or an aqueous medium, for example, by a homogenizer, a microfluidizer or very high-speed stirring to form an oil-in-water emulsion. The emulsion thus obtained is then added to an aqueous medium containing dispersed polymer particles while stirring. The organic solvent is then distilled off, whereby the inhibitor is transferred from the organic solvent phase to the polymer particles as the polymer particles become insoluble in the aqueous phase. Specific examples of suitable polymer particles and solvents are described, for example, in WO2016 / 184504.

[0163] Exposure Steps

[0164] The method of the present invention comprises the step of exposing the printing plate precursor to UV, violet or IR light, preferably by a laser. UV and violet light are preferably radiation with a wavelength in the range of 350 to 450 nm, more preferably 360 to 420 nm, and most preferably 400 to 410 nm. Preferred UV and violet lasers are laser diodes, in particular gallium nitride diodes, emitting at 375 nm or 405 nm, respectively. Frequency-doubled gallium arsenide diodes emitting at 410 nm may also be used. IR light is preferably near-infrared radiation with a wavelength in the range of 750 to 1100 nm, more preferably 780 to 850 nm. Preferred IR lasers are laser diodes emitting at about 830 nm or Nd:YAG lasers emitting at 1064 nm.

[0165] The sensitivity of the printing plate precursor, defined as the energy density of the laser beam measured at the surface of the plate coating, which is necessary to make the image-recording layer resistant to the developer, is generally between 0.01 and 250 mJ / cm², more preferably between 0.1 and 10 mJ / cm² for plates sensitive to ultraviolet light and between 50 and 200 mJ / cm² for plates sensitive to infrared light. 2The optimum value depends not only on the wavelength but also on the nature and thickness of the overcoat: a plate with a thin overcoat is less protected against oxygen quenching and therefore requires more energy than a plate with a thick overcoat.

[0166] The degree of mixing and / or premature diffusion due to migration of low molecular free radical inhibitors from the overcoat layer into the image recording layer also affects the sensitivity of the printing plate precursor. In an ideal material, no mixing or premature diffusion occurs, so that the image recording layer is highly sensitive to light for image-wise exposure and becomes much less sensitive during the heating step that induces diffusion of the free radical inhibitors into the image recording layer. However, when mixing and / or premature diffusion occurs, it is preferred to include blocking means in the material, as described in the previous section.

[0167] Imagewise exposure can be carried out under a safelight which does not emit in the wavelength range to which the image recording layer has substantial sensitivity, such as a yellow safelight. Platemaking equipment for automatic plate handling and exposure in complete darkness is available from numerous suppliers. Most platemaking machines are mechanically connected to a processing device so that the plates are automatically developed immediately after exposure. The heating step according to the invention can be carried out in the platemaking machine, in the processing device or in an intermediate heating unit. Preferably, the method of the invention is carried out on an automatic platemaking line comprising a platemaking machine comprising one or more plate supply boxes and mechanically connected to a processing machine, which is equipped with a preheating unit ("pre" means before development), in which the heating step according to the invention is carried out.

[0168] Heating steps

[0169] According to the invention, the printing plate precursor is heated after imagewise exposure in order to induce diffusion of the free radical inhibitor from the overcoat layer into the image-recording layer, thereby reducing its daylight sensitivity. The heating may be produced by any means, such as an IR lamp, heated air or one or more heated contact rollers. In a preferred embodiment, the heating is carried out in an oven, such as an oven similar to those commonly used in the prior art for "preheating" printing plates before development. The printing plate precursor may be heated at a temperature of 80°C to 300°C, preferably 100°C to 250°C, and more preferably 120°C to 200°C. The heating time is preferably 2 seconds to 30 minutes, more preferably 10 seconds to 15 minutes, and most preferably 1 to 10 minutes.

[0170] In addition to global heating of the plate, heat can also be applied selectively, i.e., only the portions of the plate that are not image-wise exposed to ultraviolet (UV) or infrared light. The exposed portions actually benefit from high daylight sensitivity, since exposure to daylight after image-wise exposure induces further hardening of the image, resulting in longer run lengths on the printing press. It is therefore advantageous not to apply global heating to induce diffusion of free radical inhibitors, but to apply selective heating of only the non-image portions, for example by a scanning infrared laser emitting at a wavelength to which the image recording layer is substantially insensitive. Such an IR laser can be integrated in the platemaking equipment used for image-wise exposure. By adding dyes or pigments, the infrared light can be absorbed by the outer coating, converting the absorbed light into heat. The scanning infrared laser can simultaneously produce a visible image in the outer coating, for example by bleaching a visible dye or converting a colorless or thermochromic dye into a more colored form. Global heating and selective heating can also be combined in a single step.

[0171] Processing

[0172] After the image-wise exposing step and the heating step, the printing plate precursor is processed (developed), which involves treatment with a processing liquid (developer) to remove the non-image areas of the coating from the support without substantially removing the image areas. The image areas may be removed by dissolving and / or dispersing the coating components into the developer and / or by mechanically breaking the coating from the support. As a result, a lithographic image of the printing areas and the non-printing areas is obtained.

[0173] The overcoat is preferably removed together with the non-image areas of the recording layer in a single process. Alternatively, a pre-cleaning step may be performed to remove the overcoat before the image recording layer is developed. The pre-cleaning step may be performed in a separate device or by manually rinsing the printing plate precursor with water or the pre-cleaning step may be performed in a cleaning unit integrated in the processing machine for developing the image recording layer. The washing liquid is preferably water, more preferably tap water. More details about the washing step are described, for example, in EP1788434, paragraph

[0026] .

[0174] Any developer known in the prior art for processing free radical polymerizable or crosslinkable image recording layers may be used, such as alkaline developers or solvent-based developers described in US2005 / 0162505. More preferably, the developer is a colloidal aqueous solution capable of simultaneously developing and gumming the image in a single step, as described in WO2005 / 111727. The colloidal developer preferably has a pH close to neutral, for example 5 to 9.

[0175] The developer can be applied to the printing plate in various ways, for example, by wiping the developer onto the printing plate with an impregnated pad, by dipping or immersing the printing plate in the developer, by applying, spraying or pouring the developer onto the printing plate manually or in an automatic processing device (processor). The treatment with the developer can also be combined with mechanical abrasion of the coating, for example using a sponge or a rotating brush. As described in WO2007 / 057349, off-press development is preferably carried out in a processor comprising a dual developer unit in a cascade configuration.

[0176] The development of the lithographic image can also be carried out on-press, by mounting the printing plate precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding dampening solution and / or ink to the precursor. Before mounting the printing plate precursor on the printing press, on-press processing is preferably carried out without any other wet processing. The dampening solution and / or ink then acts as a developer to remove the non-image areas of the coating from the support. Alternatively, the printing plate precursor may first be subjected to off-press wet processing, for example to remove the topcoat, before being mounted on a plate cylinder for further on-press development.

[0177] In a preferred on-press development method, the removed coating is preferably transferred to the paper together with the ink so that the dampening fluid supply of the printing press is not contaminated. In order to reduce paper waste, the image is preferably fully developed (i.e., the non-printing areas of the printing plate are completely removed and the printing areas are completely absorbed by the ink) within the first 50 revolutions of the plate cylinder, more preferably within the first 20 revolutions. Some printing plate precursors are better developed on-press when dampening fluid is only supplied to the printing plate within the first 60 seconds, more preferably the first 30 seconds, and most preferably the first 15 seconds after the start of the printing press, and then the ink supply is also turned on. In alternative embodiments, the supply of dampening fluid and ink can be started simultaneously, or only ink is supplied during a certain number of revolutions before the dampening fluid supply is turned on. Example

[0178] Ingredients

[0179] Alberdingk U180 A 50% by weight aqueous aliphatic polyester polyurethane dispersion from Alberdingk Boley.

[0180] Albritect CP 30 A 20% by weight aqueous dispersion of a copolymer of vinylphosphonic acid and acrylic acid from Rhodia.

[0181] S-Lec BX35Z A copolymer of vinyl alcohol, vinyl acetate, vinyl butyral and vinyl acetal from Sekisui Chemical Co. Ltd.

[0182] Mowiol 4-88 M from Kuraray 27000 parts (about 88%) of the hydrolyzed poly(vinyl alcohol) were obtained.

[0183] CN 104 Epoxy acrylate oligomer from Arkema.

[0184] FST 510 Diurethane dimethacrylate from AZ Electronic Materials GmbH.

[0185] SR368 Tris(2-hydroxyethyl)isocyanurate triacrylate from Sartomer (Arkema Group).

[0186] HABI 1-2 A dimer of a triarylimidazole from Hodogaya Chemical having the formula:

[0187]

[0188] Fluomix A mixture of the following sensitizers (synthesis described in WO2008 / 145529):

[0189]

[0190] Tegoglide 410 Surfactant from Evonik Tego Chemie GmbH.

[0191] Lutensol A8 Surfactant from BASF.

[0192] Ebotec MB-SF Biocide from Bode Chemie Hamburg GmbH.

[0193] JPA-528 A mixture of phosphate esters of polyethylene glycol monomethacrylate from Johoku Chemical Company Ltd.

[0194] Sipomer PAM 100 A mixture of phosphate esters of polyethylene glycol monomethacrylate from Rhodia.

[0195] Leuco Crystal Violet Compound with the formula:

[0196]

[0197] Compound A Irgastab UV-10, a low molecular weight free radical inhibitor from Ciba Geigy:

[0198]

[0199] Compound B is an oxalamide derivative having the following structure:

[0200]

[0201] Examples 1 to 8

[0202] Examples 1 to 4 comprise image-recording layers PL-1 to PL-4, respectively. These were produced by coating a solution of the components in Table 1 onto a conventional aluminium support which had been electrochemically grained with hydrochloric acid and anodised with sulphuric acid without further anodic post-treatment. The support had a surface roughness Ra of about 0.40 µm (measured by interferometry) and an aluminium oxide weight of 3.0 g / m 2 The components were dissolved in a mixture of 36 wt % methyl ethyl ketone, 63 wt % 1-methoxy-2-propanol and 1 wt % water. Each coating solution was applied at a wet coating thickness of 30 μm and then dried in a circulation oven at 120° C. for 1 minute.

[0203] Half of each of the materials PL-1 to PL-4 was provided with an overcoat layer OC-1 on top of the image-recording layer, thereby obtaining Examples 1 to 4. The other half was provided with an overcoat layer OC-2, to produce Examples 5 to 8. In contrast to OC-1, OC-2 contained the low molecular free radical inhibitor compound A. Each overcoat layer was applied as an aqueous solution of the composition defined in Table 2 (wet coating thickness 20 µm) and then dried at 50°C for 5 minutes.

[0204] Table 1

[0205]

[0206] Table 2

[0207]

[0208] (*) Added as an aqueous dispersion in water containing 5 wt% Mowiol 4-88 and 5 wt% Compound A in water

[0209] The materials were exposed to simulate daylight by flood exposure with fluorescent tubes TL-D58W / 840 from Philips (color code = 840; correlated color temperature = 4000K; color rendering index = 82). The light intensity measured at the coated surface of the printing plate precursor was about 1000 lux. During the exposure, the printing plate precursor was covered with a step wedge with a light transmittance ranging from 100% (step 1 at 1000 lux) to 0.3% (step 18 at 3 lux). After 60 minutes of exposure, the printing plates were processed with Violet CF GUM-NP developer in an Azura C95 processor at a speed of 60 cm / min and a temperature of 21ºC (all processing materials from Agfa NV, Belgium). Materials that develop well in this processing system are generally also suitable for on-machine processing.

[0210] The printing plate is then visually inspected to determine at which step fogging of the image-recording layer begins, i.e., the image-recording layer is resistant to the developer and is therefore not completely removed from the support by the developer, due to sufficient polymerization induced by sunlight. In Table 3, the sunlight stability is expressed as the maximum amount of sunlight that the material can withstand without fogging (light intensity in lux multiplied by time in minutes): for example, Example 1 shows no more fogging starting from step 14, which corresponds to a light intensity of 11 lux; the sunlight stability of Example 1 is therefore 11 lux·60 minutes=660 lux·minutes.

[0211] The light intensity of a typical office or print shop environment is about 500 lux, so a value of 660 lux·min indicates that the material is clearly not sunlight stable, as fogging begins after just over one minute of sunlight exposure (660 / 500=1.3). Table 3 shows that none of Examples 1 to 8 are sunlight stable if no heating step is performed prior to sunlight exposure: the best result (Example 5: 4500 lux·min) represents a material that begins to fog after only 9 minutes in ambient light of 500 lux.

[0212] The sunlight exposure test was then repeated with further samples identical to Examples 1 to 8, which were first subjected to heating in an oven with forced air circulation at a temperature of 130° C. for 5 minutes and then subjected to sunlight exposure and processing as described above. The results in Table 3 show that Examples 1 to 4, which do not contain a low molecular free radical inhibitor, also remain highly sensitive to sunlight after heating, whereas the heating step reduces the light sensitivity of Examples 5 to 8 to such an extent that the printing plates become sunlight stable, since no fogging is observed at wedge step 1 (1000 lux during 60 minutes). These results show that the heating step enables compound A to diffuse into the image-recording layer and thereby reduce its light sensitivity to a level that allows the exposed and heated printing plates to be processed in a typical environment (500 lux) for at least 2 hours (60000 lux·min=500 lux·120 minutes).

[0213] Table 3

[0214]

[0215] (*) The numbers in brackets are the amount of compound B, in mg / m 2 .

[0216] (**) The numbers in brackets are the amount of compound A, in mg / m 2 .

[0217] However, it should be noted that Examples 5 to 8 are fairly insensitive to the laser (before heating), due to a combination of the large amount of compound A in the overcoat layer and a certain degree of mixing and / or premature diffusion thereof in the image recording layer. As will be shown in the following Examples 9 to 13, by using a large amount of barrier compound B in the image recording layer, the degree of mixing and / or premature diffusion can be reduced.

[0218] Examples 9 to 13

[0219] The compositions of Examples 9 to 13 are shown in Table 4. The coating and drying conditions were the same as for Examples 1 to 8. The sunlight stability was evaluated in the same manner as for Examples 1 to 8 above, with the proviso that the exposure time was only 30 minutes instead of 60 minutes. In addition, the violet laser (405 nm) sensitivity of these Examples was also tested by using a Polaris platesetter (trademark of Agfa NV, Belgium) at 0.081 mJ / cm 2 The energy density of the laser was evaluated by exposing a solid image on a fresh (unheated) sample through a step wedge and then processing the sample in the same manner as the sunlight stability test described above. The last column of Table 4 shows the number of steps required to obtain 50% color density (produced by conversion of Crystal Violet Leuco Dye) relative to the color density of the fully exposed area. Higher step numbers indicate higher laser sensitivity.

[0220] Table 4

[0221]

[0222] (*) The numbers in brackets are the amount of compound B, in mg / m 2 .

[0223] (**) The numbers in brackets are the amount of compound A, in mg / m 2 .

[0224] The results of Examples 9 and 10 in Table 4 show that when a large amount of Compound A is added to the overcoat layer, no image is formed upon exposure to a violet laser (at the low energy density used in this experiment) due to mixing and / or diffusion into the image recording layer. In contrast to Examples 9 and 10, Examples 11 to 13 include a blocking compound B in the image recording layer, which reduces the extent of mixing and / or diffusion, thereby maintaining acceptable laser sensitivity, while significantly improving sunlight stability by the heating step (Example 13 compared to 10).

[0225] Examples 14 to 17

[0226] A solution of the components of Table 5 in a mixture of 36 wt. % methyl ethyl ketone, 61 wt. % 1-methoxy-2-propanol and 3 wt. % water was coated on the same support as used in Examples 1-8. Each coating solution PL 6 and PL 7 was applied at a wet coating thickness of 30 μm and then dried in a circulation oven at 120°C for 1 minute. Examples 14, 15 and 16 were obtained by coating OC-1, OC-3 and OC4 (defined in Tables 2 and 6) on top of the image-recording layer PL-6, respectively. Example 17 was obtained by coating OC-1 on PL-7. All of the topcoats were applied at a wet coating thickness of 20 μm and dried at 50°C for 5 minutes.

[0227] Table 5

[0228]

[0229] (*) The pigment dispersion in Table 4 contains 20 wt% of Pigment Blue 60, which has the following formula (CAS Registry Number 81-77-6),

[0230]

[0231] It is dispersed with 10 wt % Bykjet-9152 and 10 wt % Disperbyk-182 (both dispersants from Byk Chemie GmbH) in a mixture of 6.2 wt % 2-methoxy-1-methylethyl-acetate, 3.5 wt % propoxypropanol, 3.5 wt % n-butyl acetate and 46.8 wt % 1-methoxy-2-propanol.

[0232] Table 6

[0233]

[0234] The sunlight stability of these examples was tested in the same manner as for Examples 1 to 8 above. The results are summarized in Table 7, where the sunlight stability is expressed as defined above, i.e., the maximum amount of simulated sunlight (lux·min) that the printing plate precursor can withstand without fogging.

[0235] Table 7

[0236]

[0237] (*) The numbers in brackets are the amount of compound A, in mg / m 2 .

[0238] The amount of Compound A in the outer coating of Example 15 was too low to have a significant effect on sunlight stability (the results of Examples 14 and 15 were approximately the same within experimental error). Example 16 contained 5 times more Compound A, and this amount produced good sunlight stability of at least 60,000 lux·min (i.e., at least 2 hours at 500 lux) when heated.

[0239] Example 17 is a comparative example in which the same amount of Compound A as in Example 16 is added to the image-recording layer instead of the overcoat layer. The good daylight stability of Example 17 gives the wrong impression that it performs well, but the material is very insensitive to light of any wavelength and therefore cannot even record images when exposed in the practical setting of a typical platemaking machine. This shows that, according to the present invention, the large amounts of free radical inhibitors required to produce good daylight stability cannot be added to the image-recording layer but should be added to the overcoat layer.

Claims

1. A lithographic printing plate precursor comprising a support and provided thereon in order: - a negative-working image-recording layer which undergoes free radical polymerization or cross-linking reaction when exposed to ultraviolet, violet or infrared light; - an outer coating comprising a low molecular free radical inhibitor, which is an organic compound with a molecular weight of less than 1000 Daltons, or a polymer that releases said low molecular free radical inhibitor upon heating, It is characterized in that Upon heating, the low-molecular free-radical inhibitor can diffuse into the image-recording layer, thereby rendering the image-recording layer less sensitive to sunlight than before the heating.

2. The lithographic printing plate precursor according to claim 1, wherein the image-recording layer has a sunlight stability of at least 30,000 lux-min after heating.

3. The lithographic printing plate precursor according to claim 1, wherein the image-recording layer has a sunlight stability of at least 60,000 lux-min after heating.

4. A lithographic printing plate precursor according to claim 1 or 2, wherein the low molecular free radical inhibitor is a nitroxyl compound.

5. A lithographic printing plate precursor according to claim 1 or 2, wherein the low molecular free radical inhibitor is present in an amount of at least 10 mg / m 2 An amount of the compound is present or released in the outer coating.

6. A lithographic printing plate precursor according to claim 1 or 2, wherein the image recording layer comprises one or more compounds having a chemical structure according to the following formula: Where n is an integer equal to 0 or 1, L 1 represents a divalent linking group, and * represents a chemical bond to the remaining structure.

7. A method for manufacturing a lithographic printing plate, comprising the steps of: (a) imagewise exposing a lithographic printing plate precursor as defined in any one of the preceding claims to ultraviolet, violet or infrared light; (b) heating the lithographic printing plate precursor, thereby inducing diffusion of the low-molecular free radical inhibitor into the image-recording layer and making the image-recording layer less sensitive to sunlight than before said heating; (c) Processing of lithographic printing plate precursors.

8. The method according to claim 7, wherein step (c) is performed by mounting the lithographic printing plate precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while supplying dampening solution and / or ink to the lithographic printing plate precursor.

9. The method according to claim 8, wherein during step (b), the lithographic printing plate precursor is heated at a temperature of 120 to 200°C for 1 to 10 minutes.

10. The method of claim 9, wherein the image-recording layer has a sunlight stability of at least 30,000 lux-min after the heating step (b).

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