Planographic printing plate precursor, method for producing the same, and method for producing a planographic printing plate

Abstract

A lithographic printing plate precursor is disclosed, which comprises a support and a coating containing a photopolymerizable layer, the photopolymerizable layer comprising a polymerizable compound, a photoinitiator, and an infrared absorbing compound comprising a long-chain straight-chain or branched alkyl group.

Description

TECHNICAL FIELD

[0001] The present invention relates to a novel lithographic printing plate precursor. BACKGROUND ART

[0002] Lithographic printing generally involves the use of a so-called printing forme, such as a printing plate mounted on a cylinder of a rotary printing press. The forme bears a lithographic printing image on its surface, and a printed product is obtained by applying ink to the image and then transferring the ink from the forme to a receiving material, which is usually paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) is supplied to a lithographic printing image composed of oleophilic (or hydrophobic, i.e., accepting ink and repelling water) regions and hydrophilic (or oleophobic, i.e., accepting water and repelling ink) regions. In so-called waterless offset printing, the lithographic printing image is composed of regions accepting ink and anti-set-off ink (repelling ink), and during waterless offset printing, only ink is supplied to the forme.

[0003] Lithographic printing formes are generally obtained by imagewise exposure and processing of a radiation-sensitive layer on a lithographic printing support. Imaging and processing turn a so-called lithographic printing plate precursor into a printing plate or forme. The radiation-sensitive coating is generally imagewise exposed to heat or light by a digitally modulated exposure device, such as a laser, to trigger physical and / or chemical processes, such as ablation, polymerization, insolubilization by crosslinking of polymers or by coalescence of particles of a thermoplastic polymer latex, solubilization by disruption of intermolecular interactions or by increasing the permeability of a development barrier layer. Although some plate precursors are capable of producing a lithographic printing image immediately after exposure, the most popular lithographic printing plate precursors require wet processing because exposure creates a difference in solubility or a difference in dissolution rate in a developer between the exposed and unexposed regions of the coating. In a positive-working lithographic printing plate precursor, the exposed regions of the coating dissolve in the developer, while the unexposed regions remain resistant to the developer. In a negative-working lithographic printing plate precursor, the unexposed regions of the coating dissolve in the developer, while the exposed regions remain resistant to the developer. Most lithographic printing plate precursors contain a hydrophobic coating on a hydrophilic support, such that the regions remaining resistant to the developer define the ink-accepting regions of the plate and thus the printing regions of the plate, while the hydrophilic support is exposed by dissolution of the coating in the developer in the non-printing regions.

[0004] Photopolymer plates rely on a working mechanism whereby a coating, which typically comprises a free-radically polymerizable compound, hardens upon exposure. "Hardening" means that the coating becomes insoluble or non-dispersible in the developing solution and can be achieved by polymerization and / or crosslinking of the photosensitive coating upon exposure to light and / or heat. The photopolymer plate precursor can be sensitized to blue light, green light or red light (i.e., in the wavelength range between 450 - 750 nm), to violet light (i.e., in the wavelength range between 300 - 450 nm) or to infrared light (i.e., in the wavelength range between 750 - 1500 nm). Optionally, the exposure step is followed by a heating step to enhance or accelerate the polymerization and / or crosslinking reaction.

[0005] Typically, it is required that the top layer or protective top coating on the imageable layer act as an oxygen barrier to provide the desired sensitivity to the plate. The top layer typically comprises a water-soluble or water-swellable polymer, such as polyvinyl alcohol. In addition to acting as a barrier to oxygen, the top layer should preferably be easily removable during processing and be sufficiently transparent to actinic radiation (e.g., 300 - 450 nm or 450 - 750 nm or 750 - 1500 nm).

[0006] The classical workflow for photopolymer plates includes: first, an exposure step of the photopolymer plate precursor in a violet or infrared plate-making machine, followed by an optional preheating step, a washing step of the protective top coating, an alkaline development step, and a rinsing and coating step. However, there have been significant developments in the direction of simplified workflows, where the preheating step and / or the washing step are eliminated, and where the processing and coating steps are carried out in a single step, or where processing is carried out with a neutral gum and then coated in a second step. Alternatively, in-press processing has become very popular, where the plate is installed on the printing press and the coating is developed by interaction with the dampening solution and / or ink supplied to the plate during the operation of the printing press. During the first run of the printing press, the non-image areas are removed from the carrier and thus define the non-printing areas of the plate.

[0007] In order to be able to evaluate the image quality of a lithographic printing plate, such as image resolution and detail rendering (usually measured with a densitometer), before installing the lithographic printing plate on a printing press, the lithographic printing plate precursor typically contains a colorant, such as a dye or a pigment, in the coating. However, for a machined photopolymer lithographic printing plate and thus without developing the plate before installing the plate on a printing press, it is not possible to pre-check and identify the plate including the colorant. A solution has been provided in the art by including a component capable of forming a so-called "printed image" (i.e., an image visible before processing) upon exposure into the coating. A thermosensitive photopolymer lithographic printing plate may include a dye that absorbs in the visible light wavelength range and changes color upon heating, thereby forming a printed image. For example, the thermochromic dye technology involves designing IR dyes containing thermally cleavable groups, thereby obtaining a color shift upon exposure to heat and / or light, as disclosed in WO2019 / 219560.

[0008] EP 1 484 177 discloses a lithographic printing method that includes the step of imagewise exposing a lithographic printing plate precursor to infrared light, the lithographic printing plate precursor including an imaging layer containing an infrared absorber, a salt of a sulfonium ion as a polymerization initiator with a specific anion, and a polymeric binder.

[0009] EP 1 203 660 discloses a lithographic printing plate precursor that contains an imaging layer containing a thermal radical generator, a polymethine dye, and fine particles containing a compound having a free-radically polymerizable group and / or microcapsules encapsulating a compound having a free-radically polymerizable group.

[0010] JP2004 / 117713 discloses a planographic printing plate that includes an imaging layer including a liquid crystal compound having a polymerizable group.

[0011] Thermophotopolymer printing plates based on thermally induced physical and / or chemical reactions typically contain a thermosensitive coating that includes an IR dye as a photo-thermal conversion compound. Upon exposure to heat and / or exposure to infrared light, the heat generated triggers the imaging mechanism of the thermosensitive coating. These IR dyes exhibit strong absorption in the IR wavelength and are ideally uniformly distributed throughout the thermosensitive coating. However, due to limited solubility, for example, in common coating solvents, it has been observed that the dyes may be non-uniformly distributed and / or even form aggregates and / or crystals in the coating of the plate. As a result, defects or so-called "artefacts" may occur throughout the coating and may deteriorate the lithographic properties of the plate, such as adhesion problems, reduced run cycles, and / or lower plate sensitivity.

[0012] In summary, there is still a need for a photopolymer-based lithographic printing plate that provides improved contrast between the image area and the background area during imaging and is preferably designed for in-machine development of coating formulations. Summary of the Invention

[0013] Accordingly, an object of the present invention is to provide a thermal negative-working lithographic printing plate precursor that provides a printing plate having excellent lithographic printing properties in terms of both sensitivity and printing life, thereby significantly reducing the formation of aggregates and / or crystals.

[0014] This object is achieved by the printing plate precursor of the present invention. The printing plate precursor of the present invention is specifically characterized in that it contains a photopolymerizable layer, and the photopolymerizable layer includes a first infrared absorbing dye according to formula (I)

[0015]

[0016] wherein

[0017] Z and Z' represent -S-, -CR a R b -, or -CH=CH-; R a and R b represent alkyl, aralkyl or aryl;

[0018] T and T' independently represent hydrogen, halogen, alkyl, alkoxy, cyano, -CO2R k -, -CONR l R m -, -SO2R n -, -SO2NR o R p or an optionally substituted ring-forming benzene ring, wherein R l , R m represent hydrogen, an optionally substituted alkyl or aryl, R n represents an optionally substituted alkyl or aryl, and R o and R p represent hydrogen, an optionally substituted alkyl or aryl,

[0019] Rz represents a straight-chain or branched-chain alkyl containing 6 or more carbon atoms, and

[0020] W - represents a counterion to obtain an electrically neutral compound.

[0021] According to the present invention, it has surprisingly been found that by incorporating an infrared absorbing compound according to formula I into the photopolymerizable layer, a printing plate having improved sensitivity and printing life is obtained, thereby significantly reducing the formation of aggregates and / or crystals.

[0022] Development is preferably carried out by treating the precursor with a gum solution, yet more preferably by mounting the precursor on the plate cylinder of a lithographic press and rotating the plate cylinder while feeding dampening solution and / or ink to the precursor.

[0023] Another object of the present invention is to provide a method for preparing a lithographic printing plate, which comprises the following steps:

[0024] - exposing the printing plate precursor comprising the coating as defined above to heat and / or IR radiation in an imagewise manner, thereby forming a lithographic printing image consisting of image areas and non-image areas;

[0025] - developing the exposed precursor.

[0026] Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention. Specific embodiments of the present invention are also defined in the dependent claims. Detailed Description

[0027] Planographic printing plate precursor

[0028] The lithographic printing plate precursor according to the present invention is negative-working, i.e., after exposure and development, the unexposed areas of the coating are removed from the carrier and define hydrophilic (non-printing) areas, while the exposed coating is not removed from the carrier and defines oleophilic (printing) areas. The hydrophilic areas are defined by a carrier having a hydrophilic surface or provided with a hydrophilic layer. The hydrophobic areas are defined by the coating, which hardens upon exposure, optionally followed by a heating step. Areas having hydrophilic properties refer to areas having a higher affinity for aqueous solutions than for oleophilic inks; areas having hydrophobic properties refer to areas having a higher affinity for oleophilic inks than for aqueous solutions.

[0029] "Hardening" means that the coating becomes insoluble or non-dispersible in the developing solution and can be achieved by polymerization and / or crosslinking of the photosensitive coating, optionally followed by a heating step to enhance or accelerate the polymerization and / or crosslinking reaction. In this optional heating step, also referred to hereinafter as "preheating", the plate precursor is preferably heated at a temperature of about 80 °C to 150 °C and preferably for a residence time of about 5 seconds to 1 minute.

[0030] The coating contains at least one layer comprising a photopolymerizable composition, which layer is also referred to as a "photopolymerizable layer". The coating may further contain a top layer provided on top of the photopolymerizable layer. Optionally, the coating may include other layers, such as intermediate layers, adhesion improvement layers and / or other layers located between the carrier and the photopolymerizable layer and / or between the optional top layer and the photopolymerizable layer.

[0031] The coating of the printing plate precursor is most preferably capable of on-press development with dampening solution and / or ink.

[0032] The printing plate of the present invention is preferably exposed at a low energy density, i.e., below 190 mJ / m²; preferably between 70 - 190 mJ / m²; more preferably between 75 - 150 mJ / m², and most preferably between 80 - 120 mJ / m².

[0033] Photopolymer coating

[0034] Photopolymerizable compound

[0035] The coating has at least one layer comprising a photopolymerizable composition, which layer is also referred to as the "photopolymerizable layer". The coating may include an intermediate layer located between the carrier and the photopolymerizable layer. The photopolymerizable layer comprises at least one polymerizable compound, a photoinitiator, a first infrared absorbing compound, and optionally a binder. The coating thickness of the photopolymerizable layer is preferably between 0.2 - 5.0 g / m 2 and more preferably between 0.4 - 3.0 g / m 2 and most preferably in the range between 0.6 - 1.5 g / m 2 range.

[0036] According to a preferred embodiment of the present invention, the polymerizable compound is a polymerizable monomer or oligomer comprising at least one terminal ethylenically unsaturated group, which is also referred to hereinafter as a "radically polymerizable monomer". Polymerization involves linking the radically polymerizable monomers together. Suitable radically polymerizable monomers include, for example, polyfunctional (meth)acrylate monomers (such as the (meth)acrylates of ethylene glycol, trimethylolpropane, pentaerythritol, ethylene glycol, ethoxylated trimethylolpropane, urethane (meth)acrylates) and oligomeric amine di(meth)acrylates. In addition to the (meth)acrylate group, (meth)acrylic monomers may also have other ethylenically unsaturated groups or epoxy groups. (Meth)acrylate monomers may also contain acidic (such as carboxylic acid or phosphoric acid) or basic (such as amine) functional groups.

[0037] Suitable radically polymerizable monomers are disclosed in

[0042] and

[0050] of EP 2 916 171.

[0038] Initiator

[0039] According to the present invention, any free radical initiator capable of generating free radicals either directly or in the presence of a sensitizer is a suitable initiator, also referred to herein as a photoinitiator. Suitable examples of photoinitiators include onium salts, compounds containing a carbon-halogen bond (such as [1,3,5]triazines having a trihalomethyl group), organic peroxides, aromatic ketones, thio compounds, azo polymerization initiators, azide compounds, ketoxime esters, hexaaryldiimidazoles, metallocenes, active ester compounds, borate esters, and quinone diazides. Among them, from the viewpoint of storage stability, onium salts are preferred, particularly iodonium salts and / or sulfonium salts.

[0040] More particularly suitable free radical initiators include, for example, derivatives of acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one); benzophenone; benzoin; ketocoumarins (such as 3-benzoyl-7-methoxycoumarin and 7-methoxycoumarin); xanthone; thioxanthone; benzoin or alkyl-substituted anthraquinones; onium salts (such as diaryliodonium hexafluoroantimonate salts, diaryliodonium trifluoromethanesulfonate salts, (4-(2-hydroxytetradecyloxy)phenyl)phenyl iodonium hexafluoroantimonate, triarylsulfonium hexafluorophosphate, triarylsulfonium p-toluenesulfonate, (3-phenylpropan-2-oneyl)triarylphosphonium hexafluoroantimonate salts and N-ethoxy(2-methyl)pyridinium hexafluorophosphate salts, and onium salts as described in U.S. Patent Nos. 5,955,238, 6,037,098 and 5,629,354); borates (such as tetrabutylammonium triphenyl(n-butyl)borate, tetraethylammonium triphenyl(n-butyl)borate, diphenyliodonium tetraphenylborate, diphenyliodonium tetraphenylborate (wherein the phenyl groups of the iodonium salt are substituted with groups containing at least six carbon atoms) and triphenyl(n-butyl)borate triphenylsulfonium, and borates as described in U.S. Patent Nos. 6,232,038 and 6,218,076); haloalkyl-substituted s-triazines (such as 2,4-bis(trichloromethyl)-6-(p-methoxy-styryl)-s-triazine, 2,4-bis(trichloromethyl)-6-(4-methoxy-naphthalen-1-yl)-s-triazine, 2,4-bis(trichloromethyl)-6-piperonyl-s-triazine and 2,4-bis(trichloromethyl)-6-[(4-ethoxy-ethylideneoxy)-benz-1-yl]-s-triazine and s-triazines as described in U.S. Patent Nos. 5,955,238, 6,037,098, 6,010,824 and 5,629,354); and titanocene (bis(etha.9-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl)titanium). Preferred free radical initiators are onium salts, borates and s-triazines. Preferred onium salts are diaryliodonium salts and triarylsulfonium salts. Preferred borates are triarylalkyl borates. Preferred s-triazines are trichloromethyl-substituted s-triazines. These initiators may have optional substituents and may be used alone or in combination.

[0041] Optionally substituted trihaloalkyl sulfones are particularly preferred initiators, wherein the halogen independently represents bromine, chlorine or iodine, and the sulfone is a compound containing a sulfonyl functional group attached to two carbon atoms. The most preferred initiator is tribromomethyl phenyl sulfone. More details about this initiator can be found in paragraphs

[0029] to

[0040] of patent application WO2019 / 179995.

[0042] Relative to the total dry weight of the components in the photopolymerizable composition, the amount of the initiator is generally in the range of 0.05-30% by weight, preferably 0.1-15% by weight, and most preferably 0.2-10% by weight.

[0043] The photopolymerizable layer may also contain a co-initiator. Generally, the co-initiator is used in combination with a free radical initiator. Suitable co-initiators for the photopolymer coating are disclosed in US 6,410,205, US 5,049,479, EP 1 079 276, EP1369 232, EP 1 369 231, EP 1 341 040, US 2003 / 0124460, EP 1 241 002, EP 1 288 720 and in reference books including the cited documents: Chemistry&Technology UV&EB formulation forcoatings, inks&paints, Volume 3, Photoinitiators for Free Radical and CationicPolymerisation, K.K. Dietliker, edited by P.K.T. Oldring, 1991, ISBN 0 947798161. As described in EP 107 792, specific co-initiators may be present in the photopolymerizable layer to further increase the sensitivity. Preferred co-initiators are disclosed in EP 2 916 171

[0051] .

[0044] Very high sensitivity can be obtained by including a fluorescent brightening agent in the coating. Suitable examples of fluorescent brightening agents as sensitizers are described on page 24, line 20 to page 39 of WO 2005 / 109103. Useful sensitizers may be selected from the sensitizing dyes disclosed in US 6,410,205, US 5,049,479, EP 1 079 276, EP 1369 232, EP 1 369 231, EP 1 341040, US 2003 / 0124460, EP 1 241 002 and EP 1 288 720.

[0045] As described in EP 107 792, specific coinitiators may be present in the photopolymerizable layer to further improve the sensitivity. Preferred coinitiators are sulfur compounds, especially thiols, such as 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 4-methyl-3-propyl-1,2,4-triazoline-5-thione, 4-methyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3,5-dimercapto-1,2,4-triazole, 4-n-decyl-3,5-dimercapto-1,2,4-triazole, 5-phenyl-2-mercapto-1,3,4-oxadiazole, 5-methylthio-1,3,4-thiadiazoline-2-thione, 5-hexylthio-1,3,4-thiadiazoline-2-thione, mercaptophenyltetrazole, pentaerythritol mercaptopropionate, 3-mercapto-neopentanetetrayl butyrate, pentaerythritol tetrakis(thioglycolate). Other preferred coinitiators are the polythiols disclosed in WO 2006 / 048443 and WO 2006 / 048445. These polythiols can be used in combination with the above thiols (such as 2-mercaptobenzothiazole).

[0046] Infrared absorbing compound

[0047] The photopolymerizable layer comprises an infrared light absorbing compound according to formula I. The infrared light absorbing compound is preferably a dye that absorbs light between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm.

[0048]

[0049] wherein

[0050] Z and Z’ represent -S-, -CR a R b -, or -CH=CH-; R a and R b represent alkyl, aralkyl or aryl; preferably Z and Z’ represent -CR a R b -, where R a and R b represent alkyl selected from methyl or ethyl;

[0051] T and T’ independently represent hydrogen, halogen, alkyl, alkoxy, cyano, -CO2R k -, -CONR l R m -, -SO2R n -, -SO2NR o R p or an optionally substituted ring-forming benzene ring, where Rl , R m represents hydrogen, an optionally substituted alkyl or aryl group, and R n represents an optionally substituted alkyl or aryl group, and R o and R p represent hydrogen, an optionally substituted alkyl or aryl group, and most preferably T and T' represent an optionally substituted benzene ring forming a ring;

[0052] Rz represents a straight-chain or branched-chain alkyl group containing 6 or more carbon atoms; and

[0053] W - represents a counterion to obtain an electrically neutral compound.

[0054] Preferably, Rz represents a straight-chain or branched-chain alkyl group containing 7 or more carbon atoms, more preferably the straight-chain or branched-chain alkyl group contains 8 or more carbon atoms, and most preferably the straight-chain or branched-chain alkyl group contains 10 or more carbon atoms.

[0055] More preferably, Rz represents a straight-chain or branched-chain alkyl group containing 6 - 10 carbon atoms, 7 - 10 carbon atoms, 8 - 10 carbon atoms, or 9 or 10 carbon atoms. Highly preferred Rz represents a straight-chain alkyl group containing 10 carbon atoms.

[0056] W - represents a counterion that provides an electrically neutral compound and can be selected from, for example, halogens, sulfonate, perfluorosulfonate, tosylate, tetrafluoroborate or tetraphenylborate, hexafluorophosphate, arylborate, arylsulfonate.

[0057] Binder

[0058] The photopolymerizable layer preferably includes a binder. The binder can be selected from a wide range of organic polymers. Combinations of different binders can also be used. Useful binders are described, for example, in paragraph

[0013] of EP 1043 627, lines 21 on page 17 to line 30 on page 19 of WO2005 / 111727, and lines 26 on page 16 to line 11 on page 18 of WO2005 / 029187. There are also particulate polymers, including homopolymers or copolymers prepared from monomers (such as ethylene, styrene, vinyl chloride, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, vinyl carbazole, acrylate or methacrylate or mixtures thereof). Preferably, the discrete particles are particles suspended in the polymerizable composition. The presence of discrete particles tends to promote the developability of the unexposed areas.

[0059] The thermally reactive polymer fine particles include thermally reactive groups such as ethylenically unsaturated groups, cationically polymerizable groups, isocyanate groups, epoxy groups, vinyloxy groups, and functional groups having active hydrogen atoms, carboxyl groups, hydroxyl groups, amino groups, or acid anhydrides.

[0060] The average particle size of the polymer fine particles is preferably from 0.01 mm to 3.0 mm. Particle polymers in the form of microcapsules, microgels or reactive microgels are suitable, as disclosed in EP 1 132 200, EP 1 724 112, US2004 / 106060.

[0061] Other components

[0062] The photopolymerizable layer may further comprise particles that increase the resistance of the coating to manual or mechanical damage. The particles may be inorganic particles, organic particles or fillers, as described, for example, in US 7,108,956. More details of suitable spacer particles are described in EP 2 916 171

[0053] to

[0056] .

[0063] The photopolymerizable layer may further comprise inhibitors. Specific inhibitors for use in photopolymer coatings are disclosed in US 6,410,205, EP 1 288 720 and EP 1 749 240.

[0064] The photopolymerizable layer may further comprise an adhesion promoting compound. An adhesion promoting compound is a compound capable of interacting with the carrier, preferably a compound having an ethylenically unsaturated bond capable of addition polymerization and a functional group capable of interacting with the carrier. "Interaction" is understood as each type of physical and / or chemical reaction or process by 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 coordination bond or a hydrogen bond, and which may be formed by an adsorption process, a chemical reaction, an acid-base reaction, a complex formation reaction or a reaction of a chelating group or ligand. Adhesion promoting compounds are described in EP 2 916 171

[0058] .

[0065] The photopolymerizable layer may include a leuco dye which forms a colored compound upon exposure to light and / or heat (preferably infrared light), thereby forming a printed image. More information on suitable leuco dyes can be found in

[0069] to

[0085] of the unpublished application EP19153178.

[0066] The photopolymerizable layer may further comprise at least one borate compound. The borate compound preferably refers to a compound comprising a borate anion and preferably a cation as a counterion. The borate anion may be derived from the counterion of a photoinitiator; for example, the counterion of a diphenyliodonium photoinitiator and / or the above-mentioned infrared absorbing compound or any other salt (such as sodium tetraphenylborate).

[0067] Preferably, the borate anion is a tetrahedral borate anion and can be represented by formula A below:

[0068]

[0069] wherein R b 1 , R b 2 , R b 3 and R b 4 are independently optionally substituted aliphatic hydrocarbon groups, optionally substituted aryl groups or heteroaryl groups; or, two or more of R b 1 , R b 2 , R b 3 and R b 4 can be combined with the boron atom to form a heterocyclic ring, such a ring may include up to seven carbon, nitrogen, oxygen and / or nitrogen atoms. Preferably, R b 1 , R b 2 , R b 3 and R b 4 are independently optionally substituted aryl groups or heteroaryl groups. More preferably, R b 1 , R b 2 , R b 3 and R b 4 are independently optionally substituted aryl groups. Most preferably, the borate compound includes at least one optionally substituted phenyl group, more preferably at least two optionally substituted phenyl groups, even more preferably at least three optionally substituted phenyl groups, and most preferably four optionally substituted phenyl groups.

[0070] M + is an alkali metal cation, such as Li + , Na + , K + or an optionally substituted onium ion. Examples of optionally substituted onium ions include pyridinium, ammonium, iodonium or sulfonium.

[0071] Examples of the pyridinium ion include N-alkyl-3-pyridinium groups, N-benzyl-3-pyridinium groups, N-(alkoxypolyalkyleneoxyalkyl)-3-pyridinium groups, N-alkoxycarbonylmethyl-3-pyridinium groups, N-alkyl-4-pyridinium groups, N-benzyl-4-pyridinium groups, N-(alkoxypolyalkyleneoxyalkyl)-4-pyridinium groups, N-alkoxycarbonylmethyl-4-pyridinium groups, N-alkyl-3,5-dimethyl-4-pyridinium, N-alkyl-3-pyridinium groups or N-alkyl-4-pyridinium, N-methyl-3-pyridinium, N-octyl-3-pyridinium, N-methyl-4-pyridinium, or particularly preferably N-octyl-4-pyridinium, and most preferably N-octyl-3-pyridinium groups or N-octyl-4-pyridinium groups.

[0072] The optionally substituted onium ion is preferably an ammonium ion represented by formula B:

[0073]

[0074] wherein

[0075] R n 1 、R n 2 and R n 3 are independently an optionally substituted aliphatic hydrocarbon group, an optionally substituted aryl or heteroaryl group or a halogen atom.

[0076] The optionally substituted onium ion is most preferably an iodonium ion; more preferably an optionally substituted diphenyliodonium salt. Particularly preferred are diphenyliodonium salts substituted with an electron-donating group (e.g., an alkyl or alkoxy group) and asymmetric diphenyliodonium salts. The phenyl group of the iodonium ion is preferably substituted with a group containing at least six carbon atoms.

[0077] Specific examples of borate compounds including iodonium ions include 4-hexyloxyphenyl-2,4-diethoxyphenyl iodonium tetrafluoroborate, 4-octyloxyphenylphenyl iodonium tetraphenylborate, [4-[(2-hydroxytetradecyl)-oxy]phenyl]phenyl iodonium tetraphenylborate, bis(4-tert-butylphenyl) iodonium tetraphenylborate, 4-methylphenyl-4'-hexylphenyl iodonium tetraphenylborate, 4-methylphenyl-4'-cyclohexylphenyl iodonium tetraphenylborate, bis(tert-butylphenyl) iodonium tetrakis(pentafluorophenyl)borate, 4-hexylphenyl-phenyl iodonium tetraphenylborate, 4-methylphenyl-4'-cyclohexylphenyl iodonium n-butyltriphenylborate, 4-cyclohexylphenyl-phenyl iodonium tetraphenylborate, 2-5-methyl-4-tert-butylphenyl-4'-methylphenyl iodonium tetraphenylborate, 4-methylphenyl-4'-pentylphenyl iodonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, 4-methoxyphenyl-4'-cyclohexylphenyl iodonium tetrakis(pentafluorophenyl)borate, 4-methylphenyl-4'-dodecylphenyl iodonium tetrakis(4-fluorophenyl)borate, bis(dodecylphenyl) iodonium tetrakis(pentafluorophenyl)borate, and bis(4-tert-butylphenyl) iodonium tetrakis(imidazolyl)borate. Preferred compounds include bis(4-tert-butylphenyl) iodonium tetraphenylborate, 4-methylphenyl-4'-hexylphenyl iodonium tetraphenylborate, 2-methyl-4-tert-butylphenyl-4'-methylphenyl iodonium tetraphenylborate, and 4-methylphenyl-4'-cyclohexylphenyl iodonium tetraphenylborate.

[0078] Relative to the components of the photopolymerizable layer, the borate compound may be present in an amount between 0.05 and 30% by weight, more preferably between 0.1 and 25% by weight, and most preferably between 0.5 and 15% by weight.

[0079] Various surfactants can be added to the photopolymerizable layer to allow or enhance the developability of the precursor; in particular, development with a gum solution. Both polymeric and small molecule surfactants are preferred, such as nonionic surfactants. More details are described in EP 2 916 171

[0059] .

[0080] Top layer

[0081] The coating may include a top layer or a protective topcoat, which may act as an oxygen barrier. Low molecular weight substances present in the air may deteriorate or even inhibit image formation, and thus the top layer is applied to the coating. The top layer should preferably be easily removable during development, adhere well to the photopolymerizable layer or an optional other layer of the coating, and preferably should not inhibit light transmission during exposure. The top layer is preferably provided on top of the photopolymerizable layer.

[0082] The top layer preferably includes an infrared absorbing compound capable of forming a colored compound, thereby forming a printed image upon exposure to infrared light and / or heat. The infrared absorbing compound is preferably an infrared absorbing dye or an IR dye. The color-forming IR dye is also referred to herein as a second infrared absorbing dye, a thermochromic infrared absorbing dye, or a thermochromic IR dye. The thermochromic IR dye has a main absorption in the infrared wavelength range of the electromagnetic spectrum (i.e., the wavelength range between about 750 - 1500 nm) and preferably has no significant light absorption in the visible wavelength range of the electromagnetic spectrum (i.e., the wavelength range between 390 - 700 nm). The thermochromic IR dye preferably includes at least one thermally cleavable group, which is converted into a group that is a stronger electron donor through a chemical reaction induced by exposure to IR radiation or heat. As a result, the exposed thermochromic IR dye absorbs substantially more light in the visible wavelength range of the electromagnetic spectrum, or in other words, the thermochromic dye undergoes a blue shift, thereby forming a visible image, also referred to as a printed image. The formation of such a printed image is significantly different from the methods of the prior art, in which the compound changes from a substantially colorless compound to a light-colored to colored compound. These compounds generally change their absorption from the UV wavelength range to the visible wavelength range of the electromagnetic spectrum, i.e., these compounds generally have a red shift. Compared with the color-forming method of the thermochromic IR dye described above, the contrast of the printed image obtained by such a method is much weaker.

[0083] The contrast of the printed image can be defined as the difference between the optical density in the exposed area and the optical density in the unexposed area, and is preferably as high as possible. This enables the end user to immediately determine whether the precursor has been exposed and processed, to distinguish different color selections, and to check the image quality on the plate precursor. The contrast of the printed image preferably increases with the increase in the optical density in the exposed area and can be measured by reflectance using a densitometer equipped with several filters (e.g., cyan, magenta, yellow).

[0084] The concentration of the thermochromic IR dye, relative to the total dry weight of the coating, can be from 0.1 wt% to 20.0 wt%, more preferably from 0.5 wt% to 15.0 wt%, and most preferably from 1.0 wt% to 10.0 wt%.

[0085] The second infrared absorbing dye is represented by formula II

[0086]

[0087] wherein

[0088] Ar 1 and Ar 2 independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted ring-forming benzene ring,

[0089] W 1 and W 2 independently represent a sulfur atom, an oxygen atom, N*, where R* represents an optionally substituted alkyl group, NH, or -CM 10 M 11 group, where M 10 and M 11 independently are an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; or where M 10 and M 11 together contain the necessary atoms to form a cyclic structure (preferably a 5 - or 6 - membered ring);

[0090] M 1 and M 2 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or together contain the necessary atoms to form an optionally substituted cyclic structure, which optionally substituted cyclic structure may contain an optionally substituted benzene ring forming the ring, preferably M 1 and M 2 together contain the necessary atoms to form an optionally substituted cyclic structure, which optionally substituted cyclic structure may contain an optionally substituted benzene ring forming the ring, preferably a 5 - or 6 - membered ring, more preferably a 5 - membered ring, most preferably a 5 - membered ring having a cyclic structure of 5 carbon atoms;

[0091] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group;

[0092] M 5 、M 6 、M 7 and M 8 independently represent hydrogen, a halogen, or an optionally substituted aliphatic hydrocarbon group,

[0093] M 9 is a group that is converted into a stronger electron donor than the said M 9 by a chemical reaction induced by exposure to IR radiation or heat; and said conversion provides an increase in the integrated light absorption of the dye between 350 - 700 nm.

[0094] and optionally one or more counterions to obtain a neutral compound.

[0095] The thermochromic IR dye can be a neutral, anionic, or cationic dye, depending on the type of substituents and the number of each substituent.

[0096] In a preferred embodiment, the thermochromic IR dye is represented by formula II above and includes M represented by one of the following groups 9 :

[0097] -(N=CR 17 )a-NR5 -CO-R 4 ,

[0098] -(N=CR 17 )b-NR 5 -SO2-R 6 ,

[0099] -(N=CR 17 )c-NR 11 -SO-R 12 ,

[0100] -SO2-NR 15 R 16 ,and

[0101] -S-CH2-CR 7 (H) 1-d (R 8 ) d -NR 9 -COOR 18 ,

[0102] in

[0103] a, b, c and d are independently 0 or 1;

[0104] R 17 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R 17 and R 5 or R 17 and R 11 together contain the necessary atoms to form a ring structure;

[0105] R 4 Indicates -OR 10 、-NR 13 R 14 or -CF3;

[0106] Where R 10 represents an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group;

[0107] R 13 and R 14 independently represent hydrogen, optionally substituted aliphatic hydrocarbon group or optionally substituted (hetero)aryl group, or wherein R 13 and R 14 together contain the necessary atoms to form a ring structure;

[0108] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, -OR 10 、-NR 13 R 14or -CF3;

[0109] R 5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group, a SO3- group, a -COOR 18 group or an optionally substituted (hetero)aryl group, or wherein R 5 together with R 10 、R 13 and R 14 contains together with at least one of the necessary atoms to form a cyclic structure;

[0110] R 11 、R 15 and R 16 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R 15 and R 16 contains together the necessary atoms to form a cyclic structure;

[0111] R 12 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group;

[0112] R 7 and R 9 independently represent hydrogen or an optionally substituted aliphatic hydrocarbon group;

[0113] R 8 represents -COO- or -COOR 8’ wherein R 8’ represents hydrogen, an alkali metal cation, an ammonium ion or a mono- or di- or tri- or tetra-alkylammonium ion;

[0114] R 18 represents an optionally substituted (hetero)aryl group or an α-branched aliphatic hydrocarbon group; and

[0115] optionally one or more counterions to obtain an electrically neutral compound.

[0116] Most preferably, the thermochromic IR dye is represented by formula II, wherein

[0117] Ar 1 and Ar 2 independently represent an optionally substituted aryl group; optionally forming a ring with an optionally substituted benzene ring,

[0118] W 1 and W 2 represent -C(CH3)2;

[0119] M 1 and M 2 together contain the necessary atoms to form an optionally substituted 5-membered ring which may contain an optionally substituted benzene ring forming the ring;

[0120] M 3 and M 4 each independently represents an optionally substituted aliphatic hydrocarbon group,

[0121] M 5 、M 6 、M 7 and M 8 represents hydrogen;

[0122] M 9 represents

[0123] -NR 5 -CO-R 4

[0124] -NR 5 -SO2-R 6

[0125] -NR 11 -SO-R 12

[0126] -SO2-NR 15 R 16

[0127] wherein R 4 、R 5 、R 6 、R 11 、R 12 、R 15 and R 16 are as defined above;

[0128] and optionally one or more counterions to obtain an electrically neutral compound.

[0129] In a highly preferred embodiment, the thermochromic IR dye is represented by formula II, wherein

[0130] Ar 1 and Ar 2 each independently represents an optionally substituted aryl group;

[0131] W 1 and W 2 represents -C(CH3)2;

[0132] M 1 and M 2 together contain the necessary atoms to form an optionally substituted 5-membered ring that may contain an optionally substituted ring-forming benzene ring;

[0133] M 3 and M 4 each independently represents an optionally substituted aliphatic hydrocarbon group,

[0134] M 5 、M6 , M 7 and M 8 represent hydrogen;

[0135] M 9 represents

[0136] -NR 5 -CO-R 4

[0137] -NR 5 -SO2-R 6

[0138] wherein

[0139] R 4 is -OR 10 , where R 10 is an optionally branched aliphatic hydrocarbon group;

[0140] R 5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group,

[0141] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; and optionally one or more counterions to obtain an electrically neutral compound.

[0142] The above thermochromic IR dyes can be neutral, anionic or cationic dyes, depending on the type of substituents and the number of each substituent. In a preferred embodiment, the dye of formula II comprises at least one anionic group or acidic group (e.g., -CO2H, -CONHSO2R h , -SO2NHCOR i , -SO2NHSO2R j , -PO3H2, -OPO3H2, -OSO3H, -S-SO3H or -SO3H groups) or their corresponding salts, wherein R h , R i and R j are independently aryl or alkyl, preferably methyl, and wherein the salt is preferably an alkali metal salt or an ammonium salt, including mono- or di- or tri- or tetra-alkylammonium salts. These anionic groups or acidic groups can be present on the aromatic hydrocarbon group or the ring-forming benzene ring of Ar 1 or Ar 2 or on the aliphatic hydrocarbon group of M 3 or M 4 . Other substituents can be selected from halogen atoms, cyano groups, sulfone groups, carbonyl groups or carboxylate groups. Preferably, M 3 or M 4At least one of them is substituted at the end by at least one of these groups, more preferably by a -CO2H, -CONHSO2-Me, -SO2NHCO-Me, -SO2NHSO2-Me, -PO3H2 or -SO3H group or its corresponding salt, where Me represents methyl.

[0143] Optional counterions for obtaining a neutral compound can be selected, for example, from halogens, sulfonates, perfluorosulfonates, tosylates, tetrafluoroborates, hexafluorophosphates, arylborates (such as tetraphenylborate), arylsulfonates; or cations, such as alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkylammonium salts.

[0144] The above thermochromic IR dyes can also be coupled to each other or to other IR dyes to form IR dye dimers or oligomers. In addition to covalent coupling between two or more thermochromic IR dyes, supramolecular complexes containing two or more thermochromic IR dyes can also be formed through ionic interactions. For example, dimers composed of two different IR dyes can be formed through the interaction between cationic and anionic IR dyes, as described, for example, in WO / 2004069938 and EP 1 466 728. IR dyes can also be ionically bonded to polymers, as described, for example, in EP 1582346, where an IR dye containing 2-4 sulfonate groups is ionically bonded to a polymer containing covalently attached ammonium, phosphonium, and sulfonium groups.

[0145] Supramolecular complexes containing two or more thermochromic IR dyes can also be formed through hydrogen bonding or dipole-dipole interactions.

[0146] Suitable examples of the thermochromic IR dyes for use in the present invention are described on pages 4-8 of EP 1 910 082, IRD-001 to IRD-101.

[0147] Particularly preferred thermochromic IR dyes are represented by one of the following formulas:

[0148]

[0149] where

[0150] X - represents halogen, sulfonate, perfluorosulfonate, tosylate, tetrafluoroborate, hexafluorophosphate, arylborate or arylsulfonate; and

[0151] R 3 、R 3’ independently represent optionally substituted alkyl, preferably methyl or ethyl; or ether group, preferably -CH2-CH2-O-CH3;

[0152]

[0153]

[0154]

[0155] Among them

[0156] M + =Li + 、Na + 、K + 、NH4 + 、R ’ R ’’ R ’’’ NH + wherein R ’ 、R ’’ 、R ’’’ independently represent hydrogen, optionally substituted alkyl or aryl;

[0157]

[0158]

[0159]

[0160] The color difference between the exposed area and the unexposed area of the coating, calculated from the L*a*b* values of the image area (exposed area) of the coating and the L*a*b* values of the non-image area (unexposed area) of the coating, is denoted as ΔE. When the coating of the present invention is exposed even at a low energy density (for example, between 70 - 190 mJ / m², more preferably between 75 - 150 mJ / m², and most preferably between 80 - 120 mJ / m²), a printed image characterized by a CIE 1976 color difference ΔE of at least 2, more preferably at least 2.5, and most preferably at least 3 is formed. According to the present invention, at a very low exposure energy (for example, below 150 mJ / m²), a CIE 1976 color difference ΔE of at least 2 is obtained. ΔE is the CIE 1976 color distance ΔE, which is defined by the pairwise Euclidean distances of the CIE L*a*b* color coordinates. The CIEL*a*b* color coordinates are obtained by reflectance measurement under 45 / 0 geometry (non-polarized), using a CIE 2° observer and D50 as the light source. More details are described in CIE S014-4 / E: 2007 Colourimetry - Part 4: CIE 1976 L*a*b* Colour Spaces and CIE publications and CIE S 014-1 / E:2006, CIE Standard Colourimetric Observers.

[0161] The CIE 1976 color coordinates L*, a*, and b* discussed in this document are part of the well-known tristimulus color coordinate CIE (Commission Internationale de l’Eclairage) system, which also includes an additional chromaticity value C* defined as C* = [(a)2+(b)2]1 / 2. The CIE 1976 color system is described, for example, in "Colorimetry, CIE 116-1995: Industrial Colour Difference Evaluation" or "Measuring Colour", R.W.G. Hunt, 2nd Edition, 1992, edited by Ellis Horwood Limited, England.

[0162] The CIE L*a*b* values discussed and reported in this document were measured using the ASTM E308-85 method.

[0163] The top layer may further include a binder. A preferred binder that can be used for the top layer is polyvinyl alcohol. The polyvinyl alcohol preferably has a degree of hydrolysis in the range of 74 mol% to 99 mol%, more preferably in the range of 80 - 98%. The weight-average molecular weight of the polyvinyl alcohol can be measured by the viscosity of a 4 wt% aqueous solution at 20 °C, as defined in DIN 53 015, and this viscosity value is preferably in the range of 2 - 26, more preferably in the range of 2 - 15, and most preferably in the range of 2 - 10.

[0164] The top layer may include a halogenated polymer, which is preferably a hydrophobic polymer, i.e., insoluble or non-swellable in water at about neutral pH. The binder can be used in the top layer in the form of a dispersion; i.e., an emulsion or a suspension. The amount of the halogenated binder in the top layer can be between 30 wt% and 96 wt%, more preferably between 40 wt% and 90 wt%, and most preferably between 50 wt% and 85 wt%. The halogenated binder preferably includes monomer units derived from vinylidene monomers (such as vinylidene fluoride, vinylidene chloride, vinylidene bromide, and / or vinylidene iodide) in the range of 60 wt% to 95 wt%.

[0165] The top layer may optionally include other components, such as inorganic or organic acids, matting agents, surfactants such as anionic surfactants, such as sodium alkyl sulfate or sodium alkyl sulfonate; amphoteric surfactants, such as alkyl aminocarboxylates and alkyl aminodicarboxylates; nonionic surfactants, such as polyoxyethylene alkyl phenyl ethers, (co)polymers comprising siloxane and / or perfluoroalkyl units and / or oligo(alkylene oxide) units; fillers; (organic) waxes; alkoxylated alkylenediamines, such as those disclosed in EP 1 085 380 (paragraphs

[0021] and

[0022] ); glycerol; inorganic particles; pigments or wetting agents, as disclosed in EP 2 916 171.

[0166] The coating thickness of the top layer is preferably between 0.10 - 1.75 g / m², more preferably between 0.20 - 1.3 g / m², and most preferably between 0.25 - 1.0 g / m². In a more preferred embodiment of the present invention, the coating thickness of the top layer is between 0.25 - 1.75 g / m² and comprises polyvinyl alcohol having a degree of hydrolysis in the range of 74 mol% to 99 mol% and a viscosity value as defined above in the range of 2 - 26 mPas.

[0167] Hydrophilic polymers in the protective top coating may cause a problematic increase in the viscosity of printing chemicals (such as dampening solution and / or developer solution). Therefore, the coating weight of the hydrophilic polymer and / or the thickness of the protective top coating are preferably not too high; for example, higher than the ranges given above.

[0168] Definition

[0169] The aliphatic hydrocarbon group preferably represents an alkyl group, cycloalkyl group, alkenyl group, cycloalkenyl group or alkynyl group; suitable groups are described below. The aromatic hydrocarbon group preferably represents a hetero(aryl); suitable hetero(aryl) (i.e., suitable aryl or heteroaryl) are described below.

[0170] As used herein, the term "alkyl" refers to all possible variants 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. Examples of suitable alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, 1-isobutyl, 2-isobutyl and tert-butyl, n-pentyl, n-hexyl, chloromethyl, trichloromethyl, isopropyl, isobutyl, isopentyl, neopentyl, 1-methylbutyl and isohexyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and methylcyclohexyl. Preferably, the alkyl group is a C1-C6-alkyl.

[0171] Suitable alkenyl groups are preferably C2-C6-alkenyl groups, such as vinyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, isopropenyl, isobutenyl, isopentenyl, neopentenyl, 1-methylbutenyl, isohexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl.

[0172] Suitable alkynyl groups are preferably C2-C6-alkynyl groups; suitable aralkyl groups are preferably phenyl or naphthyl groups comprising one, two, three or more C1-C6-alkyl groups; suitable alkaryl groups are preferably C1-C6-alkyl groups comprising an aryl group (preferably phenyl or naphthyl).

[0173] A cyclic group or cyclic structure comprises at least one ring structure and may be a monocyclic or polycyclic group, where the polycyclic group refers to one or more rings fused together.

[0174] Examples of suitable aryl groups may be represented, for example, by optionally substituted phenyl, benzyl, tolyl or o-, m- or p-xylene, optionally substituted naphthyl, anthracenyl, phenanthryl and / or combinations thereof. A heteroaryl group is preferably a monocyclic or polycyclic aromatic ring which contains carbon atoms and one or more heteroatoms, preferably 1-4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulfur, in the ring structure. Preferred examples thereof include optionally substituted furyl, pyridyl, pyrimidinyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thienyl, tetrazolyl, thiazolyl, (1,2,3)triazolyl, (1,2,4)triazolyl, thiadiazolyl, thienyl and / or combinations thereof.

[0175] A cyclic group or cyclic structure comprises at least one ring structure and may be a monocyclic or polycyclic group, where the polycyclic group refers to one or more rings fused together.

[0176] Halogen is selected from fluorine, chlorine, bromine or iodine.

[0177] The term "substituted" in, for example, substituted alkyl means that the alkyl may be substituted by other atoms in addition to the atoms normally present in such a group (i.e., carbon and hydrogen). For example, substituted alkyl may include halogen atoms or mercapto groups. Unsubstituted alkyl contains only carbon and hydrogen atoms.

[0178] Optional substituents on alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl are preferably selected from -Cl, -Br, -I, -OH, -SH, -CN, -NO2, alkyl (such as methyl or ethyl), alkoxy (such as methoxy or ethoxy), aryloxy, carboxyl or its alkyl ester, sulfonic acid group or its alkyl ester, phosphonic acid group or its alkyl ester, phosphoric acid group or ester (such as alkyl ester, such as methyl ester or ethyl ester), thioalkyl, thioaryl, thioheteroaryl, -SH, thioether (such as thioalkyl or thioaryl), ketone, aldehyde, sulfoxide, sulfone, sulfonate, sulfonamide, amino, vinyl, alkenyl, alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl or heterocycloalkyl and / or their combinations.

[0179] Carrier

[0180] The lithographic printing plate for use in the present invention comprises a support having a hydrophilic surface or provided with a hydrophilic layer. The support is preferably a granulated and anodized aluminum support well known in the art. Suitable supports are, for example, disclosed in EP 1 843 203 (paragraphs

[0066] to

[0075] ). The surface roughness obtained after the granulation step is usually expressed as the arithmetic mean center line roughness Ra (ISO 4287 / 1 or DIN 4762) and can vary between 0.05 - 1.5 μm. The aluminum substrate of the present invention preferably has an Ra value between 0.1 - 1.4 μm, more preferably between 0.3 - 1.0 μm, and most preferably between 0.4 - 0.9 μm. The lower limit of the Ra value is preferably about 0.1 μm. More details about the preferred Ra value of the surface of the granulated and anodized aluminum support are described in EP 1 356 926. By anodizing the aluminum support, an Al2O3 layer is formed, and the anodic weight (g / m 2 of Al2O3 formed on the aluminum surface) varies between 1 - 8 g / m 2 The anodic weight is preferably ≥ 2.0 g / m 2 , more preferably ≥ 2.5 g / m 2 , and most preferably ≥ 3.0 g / m 2 .

[0181] The granulated and anodized aluminum carriers can be subjected to a so-called post-anodization treatment, such as treatment with polyvinylphosphonic acid or its derivatives, treatment with polyacrylic acid or its derivatives, treatment with potassium hexafluorozirconate or potassium phosphate, treatment with alkali metal silicates, or combinations thereof. Treatment of the carrier edges as described, for example, in US 2017 / 320351 may be of concern to prevent printing edge appearance. Enlargement or sealing of the micropores of the anodized aluminum as disclosed in JP2001-253181A or JP2001-322365A can be carried out. Alternatively, the carrier can be treated with an adhesion-promoting compound, such as those described in

[0010] of EP 1788 434 and WO2013 / 182328. However, for precursors optimized for use without a preheating step, granulated and anodized aluminum carriers without any post-anodization treatment are preferably used.

[0182] In addition to aluminum carriers, plastic carriers, such as polyester carriers, provided with one or more hydrophilic layers as disclosed, for example, in EP 1 025 992 can also be used.

[0183] According to the present invention, there is also provided a method for preparing a negative-working lithographic printing plate, the method comprising the steps of: imagewise exposing a printing plate precursor, followed by developing the imagewise exposed precursor such that the unexposed areas are dissolved in a developer solution. Optionally, after the imaging step, a heating step is carried out to enhance or accelerate the polymerization and / or crosslinking reaction. The lithographic printing plate precursor can be prepared by (i) applying a coating as described above on a carrier and (ii) drying the precursor.

[0184] Exposure step

[0185] The printing plate precursor is preferably imagewise exposed by a laser emitting IR light. Preferably, the imagewise exposure step is carried out off-press in an exposing apparatus, i.e., an exposure instrument adapted to imagewise expose the precursor either by a laser (such as a laser diode emitting at about 830 nm or a Nd YAG laser emitting at about 1060 nm) or by conventional exposure in contact with a mask. In a preferred embodiment of the present invention, the precursor is imagewise exposed by a laser emitting IR light.

[0186] Preheating step

[0187] After the exposure step, the precursor can be preheated in a preheating unit, preferably at a temperature of about 80 °C to 150 °C and preferably during a residence time of about 5 seconds to 1 minute. The preheating unit can comprise heating elements, preferably IR lamps, UV lamps, heated air, or heated rollers. Such a preheating step can be used for printing plate precursors containing a photopolymerizable composition to enhance or accelerate the polymerization and / or crosslinking reaction.

[0188] Development step

[0189] After the exposure step or the preheating step (when a preheating step is present), the plate precursor can be processed (developed). Before developing the imaged precursor, a pre-rinsing step can be carried out, particularly for negative-working lithographic plate precursors having a protective oxygen barrier or a top coating. This pre-rinsing step can be carried out in a separate instrument, or by manually rinsing the imaged precursor with water, or the pre-rinsing step can be carried out in a washing unit integrated in a processor for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details regarding the washing step are described in

[0026] of EP 1788 434.

[0190] During the developing step, at least a portion of the unexposed areas of the image recording layer are removed, while the exposed areas are substantially not removed. The processing liquid (also referred to as developer) can be applied to the plate by hand or in an automatic processing instrument, for example, by rubbing with an impregnation pad, by dipping, immersion, coating, spin coating, spraying, pouring. Treatment with the processing liquid can be combined with mechanical friction (e.g., by a rotating brush). During the developing step, any water-soluble protective layer present is preferably also removed. Development is preferably carried out in an automated processing unit at a temperature between 20 - 40 °C.

[0191] In a highly preferred embodiment, the above-described processing step is replaced by in-machine processing, whereby the imaged precursor is mounted on a printing press and in-machine processed by rotating the plate cylinder while feeding dampening solution and / or ink to the coating of the precursor to remove the unexposed areas from the carrier. In a preferred embodiment, only the dampening solution is supplied to the plate during the start-up of the printing press, and after multiple rotations of the plate cylinder, the ink supply is also switched on. In an alternative embodiment, the supply of the dampening solution and the ink starts simultaneously, or only the ink can be supplied during multiple rotations before switching on the supply of the dampening solution. The supply of paper can be before, between, or after any ink and / or dampening feed step.

[0192] The processing step can also be carried out by combining the above-described embodiments, for example, by combining development with processing liquid with in-machine development by applying ink and / or dampening solution.

[0193] Processing solution

[0194] The processing liquid can be an alkaline developer or a solvent-based developer. Suitable alkaline developers have been described in US2005 / 0162505. The alkaline developer is an aqueous solution with a pH of at least 11, more typically at least 12, preferably 12 - 14. The alkaline developer typically contains an alkaline reagent to obtain a high pH value, which can be an inorganic or organic alkaline reagent. The developer can contain anionic, non-ionic, and amphoteric surfactants (up to 3% by total weight of the composition); biocides (antimicrobial agents and / or antifungal agents), defoamers, or chelating agents (such as alkaline gluconates) and thickeners (water-soluble or water-dispersible polyhydroxy compounds, such as glycerol or polyethylene glycol).

[0195] Preferably, the processing liquid is a gum solution, whereby during the developing step, the unexposed areas of the photopolymerizable layer are removed from the carrier, and the plate is gummed in a single step. Developing with a gum solution has the additional benefit that, due to the remaining gum in the unexposed areas on the plate, no additional gumming step is required to protect the carrier surface in the non-printing areas. As a result, the precursor is processed and gummed in a single step, which step involves a developing apparatus simpler than a developing apparatus comprising a developer tank, a rinsing section, and a gumming section. The gumming section can comprise at least one gumming unit or can comprise two or more gumming units. These gumming units can have the configuration of a cascade system, i.e., when a gum replenishment solution is added to the second gumming unit or when the gum solution in the second gumming unit is used only once, i.e., when developing the precursor in the second gumming unit using only the starting gum solution by preferably spraying or jetting techniques, the gum solution present in the second tank and used for the second gumming unit overflows from the second tank to the first tank. More details regarding such gum development are described in EP1 788 444.

[0196] The gum solution is generally an aqueous liquid containing one or more surface-protective compounds capable of protecting the lithographic image of the printing plate from contamination, such as contamination caused by oxidation, fingerprints, fats, oils, or dust, or from damage, such as damage caused by scratching during plate handling. Suitable examples of such surface-protective compounds are film-forming hydrophilic polymers or surfactants. The layer remaining on the plate after treatment with the gum solution preferably contains between 0.005 - 20 g / m 2 more preferably between 0.010 - 10 g / m 2 and most preferably between 0.020 - 5 g / m 2 of the surface-protective compound. More details regarding the surface-protective compound in the gum solution can be found on page 9, line 3 to page 11, line 6 of WO 2007 / 057348. Since the developed plate precursor is developed and gummed in one step, no post-treatment of the processed plate is required.

[0197] The preferred pH value of the glue solution is between 3 and 11, more preferably between 4 and 10, even more preferably between 5 and 9, and most preferably between 6 and 8. Suitable glue solutions are described, for example, in

[0008] to

[0022] of EP 1 342 568 and WO2005 / 111727. The glue solution may further contain inorganic salts, anionic surfactants, wetting agents, chelating compounds, preservative compounds, defoaming compounds and / or ink absorbers and / or combinations thereof. More details regarding these additional components are described on page 11, line 22 to page 14, line 19 of WO2007 / 057348.

[0198] Drying and baking step

[0199] After the processing step, the plate can be dried in a drying unit. In a preferred embodiment, the plate is dried by heating the plate in a drying unit which may contain at least one heating element selected from IR lamps, UV lamps, heated metal rollers or heated air.

[0200] After drying, the plate can optionally be heated in a baking unit. More details regarding heating in the baking unit can be found in WO 2007 / 057348 on page 44, line 26 to page 45, line 20.

[0201] The printing plate thus obtained can be used for conventional so-called wet offset printing, in which ink and aqueous dampening solution are supplied to the plate. Another suitable printing method uses so-called single-fluid ink without dampening solution. Suitable single-fluid inks have been described in US 4,045,232, US 4,981,517 and US 6,140,392. In a most preferred embodiment, the single-fluid ink contains an ink phase (also called a hydrophobic or lipophilic phase) and a polyol phase, as described in WO 00 / 32705. Examples

[0202] Example 1

[0203] 1. Synthesis of infrared-absorbing dyes

[0204] Infrared-absorbing dye IR-03

[0205] Step 1: Synthesis of 3-butyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate

[0206] 115.1 g (0.55 mol) of 1,1,2-trimethyl-1H-benzo[e]indole was dissolved in 90 ml of N-butyl-pyrrolidone, and the mixture was heated to 130 °C. From 70 °C, 1,1,2-trimethyl-1H-benzo[e]indole was completely dissolved in the mixture. At 130 °C, 201 g (0.88 mol) of butyl tosylate was added over 90 minutes. The reaction was continued at 130 °C for 4 hours. The reaction mixture was cooled to 100 °C, and 660 ml of ethyl acetate was added over 20 minutes. The reaction mixture was cooled to room temperature, and 3-butyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate crystallized out from the medium. 3-Butyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate was separated by filtration, washed with 660 ml of ethyl acetate and dried. 211 g (yield: 88%) of 3-butyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate was isolated.

[0207] Step 2: Synthesis of IR-03

[0208] 13.8 g (32 mmol) of 3-butyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate and 5.4 g (15 mmol) of N-[(3-anilinomethylene)-2-chloro-1-cyclohexen-1-yl)methylene]aniline monohydrochloride were dissolved in 10.7 g of ethanol. 1.8 g (30 mmol) of acetic acid was added, followed by 3.9 g (39 mmol) of triethylamine. The reaction was continued at 25 °C for 1 hour. A further 2.7 g (45 mmol) of acetic acid was added, followed by 90 ml of methyl tert-butyl ether. The reaction mixture was cooled to 10 °C, and NIR-C4 was crystallized at 10 °C for 2 hours. NIR-C4 was separated by filtration, washed with a 1 / 1 mixture of 50 ml of ethyl acetate and methyl tert-butyl ether, washed with 50 ml of methyl tert-butyl ether and dried. 8.15 g (yield: 65%) of IR-03 was isolated.

[0209] Infrared absorption dye IR-07

[0210] Step 1: Synthesis of decyl tosylate

[0211] 1.292 kg (8 mol) of decanol and 1.54 kg of tosyl chloride were dissolved in 4 l of isopropyl acetate. 899.5 g (8.8 mol) of triethylamine was added at a rate of 10 ml / minute. The reaction was continued at room temperature for 92 hours. The precipitated triethylamine chloro-hydrate was removed by filtration and washed with 2 l of isopropyl acetate. The combined organic fractions were extracted twice with 2 l of 10 wt% aqueous sodium chloride solution and once with 1 l of water. The organic fractions were dried over MgSO4 and evaporated under reduced pressure. 2272 g (yield: 91%) of decyl tosylate was isolated and used for alkylation without further purification.

[0212] Step 2: Synthesis of 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate

[0213] 1068 g (5 mol) of 1,1,2-trimethyl-1H-benzo[e]indole and 2029 g (6.5 mol) of decyl tosylate were dissolved in 1500 ml of sulfolane. The reaction mixture was heated to 125 °C. From 50 °C onwards, all components were completely dissolved in the reaction mixture. The reaction was continued at 125 °C for 6 hours. The reaction mixture was cooled to 75 °C and 10 l of ethyl acetate was added. The reaction mixture was cooled to room temperature and stirred at room temperature for 16 hours. The precipitated crude 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate was separated by filtration, washed several times with ethyl acetate and dried. 1818 g (yield: 70%) of 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate was isolated.

[0214] Step 3: Synthesis of IR-07

[0215] 50.3 g (0.14 mol) of N-[(3-anilinomethylene)-2-chloro-1-cyclohexen-1-yl)methylene]aniline monohydrochloride and 146.1 g (0.28 mol) of 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium tosylate were dissolved in 500 ml of methanol. 114.3 g (0.84 mol) of sodium acetate trihydrate and 42 g (0.7 mol) of acetic acid were added and the reaction was continued at room temperature for 5 hours. NIR-C10 was separated by filtration and washed with 500 ml of methyl tert-butyl ether. NIR-C10 was treated twice with 1 l of water and once with 500 ml of methyl tert-butyl ether, and separated by filtration each time. NIR-C10 was dried. 111.6 g (yield: 83%) of IR-07 was isolated.

[0216] The remaining IR dyes can be synthesized by those skilled in the art based on the synthesis given above.

[0217] 2. Preparation of the printing plate precursor

[0218] Preparation of the aluminum support S-01

[0219] A 0.3 mm thick aluminum foil was degreased by spraying with an aqueous solution containing 26 g / l NaOH at 65 °C for 2 seconds and rinsed with softened water for 1.5 seconds. Then, at a temperature of 37 °C and a current density of about 100 A / dm 2 in an aqueous solution containing 15 g / l HCl, 15 g / l SO4 2- ions and 5 g / l Al 3+ ions, the foil was electrochemically grained for 10 seconds using alternating current. Then, the aluminum foil was decontaminated by etching with an aqueous solution containing 5.5 g / l NaOH at 36 °C for 2 seconds and rinsed with softened water for 2 seconds. Subsequently, at a temperature of 50 °C and a current density of 17 A / dm 2 the foil was subjected to anodization in an aqueous solution containing 145 g / l sulfuric acid for 15 seconds, then washed with softened water for 11 seconds and dried at 120 °C for 5 seconds.

[0220] Preparation of the comparative printing plates PP-01 to PP-04 and the printing plates PP-05 to PP-07 of the present invention

[0221] Photopolymerizable layer

[0222] Photopolymerizable layers PL-01 to PL-08 were produced by coating the components defined in Table 1 onto the above support S-01, the components being dissolved in a mixture of 35% by volume of MEK and 65% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied at a wet coating thickness of 30 μm and then dried in a circulating oven at 120 °C for 1 minute.

[0223] Table 1: Composition of the photosensitive layer

[0224]

[0225] (1) FST 510 is the reaction product of 1 mole of 2,2,4-trimethylhexamethylene diisocyanate and 2 moles of 2-hydroxyethyl methacrylate and is commercially available from AZ Electronics as an 82 wt% solution in MEK;

[0226] (2) CN 104 is an epoxy acrylate oligomer and is commercially available from Arkema;

[0227] (3) Ini-01 is 4-hydroxyphenyl-tribromomethyl-sulfone

[0228] (4) The IR dye is an infrared-absorbing compound represented by the following structure, where the R-group substitutions are summarized in Table 2:

[0229]

[0230] Table 1: R-groups of different IR dyes

[0231] IR dye R group Photolayer IR-01 <![CDATA[-CH3]]> PL-01 IR-02 <![CDATA[-C2H5]]> PL-02 IR-03 <![CDATA[-C4H9]]> PL-03 IR-04 <![CDATA[-C6H 13 > PL-04 IR-05 <![CDATA[-C7H 15 > PL-05 IR-06 <![CDATA[-C8H 17 > PL-06 IR-07 <![CDATA[-C 10 H 21 > PL-07

[0232] The IR dye is synthesized based on the synthesis given above;

[0233] (5) Ruco Coat EC4811 is a nonionic aliphatic polyether polyurethane and is commercially available from Rudolf GmbH;

[0234] (6) Tegoglide 410 is a polyether siloxane copolymer and is commercially available from Evonik Resource Efficiency GmbH;

[0235] (7) JPA 528 is polyethylene glycol monomethacrylate acid phosphate and is commercially available from Johoku Chemical Co., Ltd.;

[0236] (8) Albritect CP 30 is a copolymer of vinyl phosphonic acid and acrylic acid and is commercially available as a 20 wt% aqueous dispersion from Rhodia;

[0237] (9) Aerosil R972 is a hydrophobic fumed silica and is commercially available from Evonik Resource Efficiency GmbH.

[0238] Protective topcoat

[0239] On top of the photosensitive layer, an aqueous solution (40 µm wet film) having the composition defined in Table 3 is coated and dried at 110 °C for 2 minutes. The plate precursors PPP-01 to PPP-07 are obtained (see Table 4 below).

[0240] Table 2: Composition of the protective topcoat OC

[0241]

[0242] (1) Mowiol 4-88TM is a partially hydrolyzed polyvinyl alcohol and is commercially available from Kuraray;

[0243] (2) Diofan A050 is polyvinylidene chloride and is commercially available from Solvay;

[0244] (3) Acticide LA1206TM is a biocide and is commercially available from Thor;

[0245] (4) Lutensol A8TM is a surfactant and is commercially available from BASF;

[0246] (5) IR-08 is an infrared absorption dye represented by the following structure:

[0247]

[0248] Table 4: Plate precursors PPP-01 to PPP-07

[0249]

[0250] 3. Crystal formation

[0251] Crystallization test

[0252] Subsequently, the obtained plate precursors were treated with a rubber wheel (2 cm in diameter) under contact pressure to simulate roll pressure and initiate crystal formation of the IR dye in the imageable layer. After contact pressure, the plate precursors were subjected to a steam chamber test in Dowanol PM (2-methoxypropanol). After 3 days of treatment, the crystal formation of the IR dye in the lithographic plate precursor was visually inspected under 8x optical magnification.

[0253] In addition, samples of each plate precursor PPP-01 to PPP-07 were subjected to an accelerated aging test for 14 days in a climate chamber at 40 °C and 80% relative humidity.

[0254] After the aging test, the crystal formation of the IR dye in the aged precursor samples and the unaged precursor samples was visually inspected under 8x optical magnification.

[0255] Results of the crystallization test

[0256] The results of crystal formation (i.e., frosting) in the coatings of plate precursors PPP-01 to PPP-07 are summarized in Table 5.

[0257] Table 5: Results of the crystal formation test

[0258]

[0259] * Rating of the amount of crystals observed after 3 days in the steam chamber test;

[0260] ** Rating of the amount of crystals observed after 4 days in the steam chamber test;

[0261] ***Rating of the amount of crystals observed after 14 days in a climatic chamber at 40 °C and 80% relative humidity (RH);

[0262] wherein

[0263] A = no crystals observed;

[0264] B = some crystals observed;

[0265] C = many crystals observed.

[0266] The results in Table 5 show that:

[0267] · Comparative plates PPP-01 to PPP-03 had many crystal formations after both three and four days of exposure in the steam chamber and after exposure in the climatic chamber;

[0268] · Plates PPP-04 to PPP-07 of the present invention had no crystal formation after three days of exposure in the steam chamber and slightly more crystal formation (PPP-04 to PPP-06) after the four-day steam chamber test, except for plate PPP-07 of the present invention (which includes dye IR-07 containing a C10 substituent) which showed no crystal formation in all three tests.

[0269] 4. Imaging

[0270] Subsequently, both aged and unaged plate precursors PPP-01 to PPP-07 were imaged at 2400 dpi with a High Power Creo 40W TE38 thermal plate setter TM (200 lpi Agfa Balanced Screening (ABS)), which is commercially available from Kodak and equipped with an 830 nm IR laser diode with an energy density of 130 mJ / cm².

[0271] 5. ΔE Measurement

[0272] Laboratory measurements were made with a GretagMacBeth Spectro Eye reflectance spectrophotometer, which was set to: D50 (illuminant), 2° (observer), no filter; commercially available from GretagMacBeth. The total color difference ΔE is a single value that takes into account the differences between the L, a*, and b* values of the image area and the non-image area:

[0273]

[0274] The higher the total color difference ΔE, the better the contrast obtained. The contrast between the image area and the non-image area results in the appearance of a printed image.

[0275] 6. Printing

[0276] After imaging, the printing plate was installed on a Heidelberg GTO 46 printing press. Each printing job was started using K+E Skinnex 800SPEED IK black ink (a trademark of BASF Druckfarben GmbH) and 4 wt% Prima FS303 SF (a trademark of AgfaGraphics) and 8% isopropyl alcohol in water as the dampening solution. A compressible blanket was used, and printing was performed on uncoated offset paper.

[0277] Before sheet feeding, 10 rotations of the printing press were made using only the dampening system, followed by 5 rotations using only the ink rollers. The printing results are summarized in Table 6.

[0278] 7. Results of printing properties

[0279] The results of the printing properties ΔE, cleanout, and printing life are summarized in Table 6.

[0280] Table 6: Results of ΔE, cleanout, and printing life

[0281]

[0282] *The printing life is a rating of the printing robustness of the printing plate, where

[0283] A = no wear after 40,000 impressions,

[0284] B = some wear after 40,000 impressions; and

[0285] C = severe wear after 40,000 impressions.

[0286] The results of the printing properties in Table 6 show that:

[0287] - The printing plates of the present invention and the comparative printing plates PP-01 to PP-07 show excellent in-machine developability (cleanout);

[0288] - The unaged printing plates of the present invention and the unaged comparative printing plates show good printing life performance, while compared with the printing life performance of the aged printing plates of the present invention, the printing life performance of the aged comparative printing plates deteriorates more;

[0289] - Compared with the image contrast of the comparative printing plates PP-01 to PP-03, the printed image contrast produced by the printing plates PP-04 to PP-07 of the present invention is higher.

Claims

1. A lithographic printing plate precursor comprising a coating on a support, the coating comprising a photopolymerizable layer, the photopolymerizable layer comprising a polymerizable compound, a photoinitiator, and a first infrared absorbing dye according to formula I wherein Z and Z’ represent -S-, -CR a R b -, or -CH=CH-; R a and R b represent alkyl, aralkyl or aryl; T and T' independently represent hydrogen, halogen, alkyl, alkoxy, cyano, -CO2R k , -CONR l R m , -SO2R n , -SO2NR o R p or an optionally substituted ring-forming benzene ring, where R l , R m represents hydrogen, optionally substituted alkyl or aryl, R n represents optionally substituted alkyl or aryl, and R o and R p represent hydrogen, optionally substituted alkyl or aryl, W - represents a counter ion to obtain a neutral compound; and Rz represents a straight-chain or branched-chain alkyl group; It is characterized in that the straight-chain or branched-chain alkyl group contains 6 to 10 carbon atoms.

2. The lithographic printing plate precursor according to claim 1, wherein Rz represents a straight-chain alkyl group containing 6 to 10 carbon atoms.

3. The lithographic printing plate precursor according to claim 1 or 2, wherein the photoinitiator is an optionally substituted trihaloalkyl sulfone compound.

4. The lithographic printing plate precursor according to claim 1 or 2, wherein the coating further comprises a top layer provided above the photopolymerizable layer.

5. The lithographic printing plate precursor according to claim 4, wherein the top layer comprises a second infrared absorbing dye, the second infrared absorbing dye comprising a thermally cleavable group that transforms into a group that is a stronger electron donor upon exposure to heat and / or IR radiation and is capable of forming a printed image upon exposure to heat and / or IR radiation.

6. The lithographic printing plate precursor according to claim 5, wherein the second infrared absorbing dye is represented by formula II wherein Ar 1 and Ar 2 each independently represents an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted ring-forming benzene ring, W 1 and W 2 each independently represents a sulfur atom, an oxygen atom, NR*, where R* represents an optionally substituted alkyl group, NH or -CM 10 M 11 group, where M 10 and M 11 each independently represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted aryl or heteroaryl group; or where M 10 and M 11 together contain the necessary atoms to form a cyclic structure; M 1 and M 2 each independently represents hydrogen, an optionally substituted aliphatic hydrocarbon group or together constitute essential atoms forming an optionally substituted cyclic structure, and the optionally substituted cyclic structure optionally contains an optionally substituted benzene ring forming the ring; M 3 and M 4 each independently represents an optionally substituted aliphatic hydrocarbon group; M 5 、M 6 、M 7 and M 8 represent hydrogen, halogen or an optionally substituted aliphatic hydrocarbon group, M 9 is a group that is converted into a stronger electron donor than said M by a chemical reaction induced by exposure to IR radiation or heat; and said conversion increases the integrated optical absorption of said dye between 350 - 700 nm; 9 ​ and optionally one or more counterions to obtain a neutral compound.

7. The lithographic printing plate precursor according to claim 6, wherein M 1 and M 2 together constitute essential atoms for forming an optionally substituted cyclic structure, and the optionally substituted cyclic structure optionally contains an optionally substituted benzene ring forming the ring.

8. The lithographic printing plate precursor according to claim 7, wherein the optionally substituted cyclic structure is a 5-membered or 6-membered ring.

9. The lithographic printing plate precursor according to claim 8, wherein the optionally substituted cyclic structure is a 5-membered ring having a cyclic structure with 5 carbon atoms.

10. The lithographic printing plate precursor according to claim 6, wherein the thickness of the top layer is between 0.1 g / m 2 and 1.75 g / m 2 .

11. A method for preparing a lithographic printing plate precursor, comprising the steps of - coating a photopolymerizable layer on a support, the photopolymerizable layer comprising a polymerizable compound, a photoinitiator, and a first infrared absorbing dye according to formula I as defined in any one of claims 1 and 2, and - drying the precursor.

12. A method for preparing a lithographic printing plate, comprising the steps of - imagewise exposing the lithographic printing plate precursor as defined in any one of the preceding claims 1 - 10 to heat and / or IR radiation, thereby forming a lithographic printing image consisting of an image area and a non-image area, - developing the exposed precursor.

13. The method according to claim 12, wherein the precursor is developed by mounting the lithographic 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.

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