Lithographic printing plate precursors

Abstract

Disclosed is a lithographic printing plate precursor comprising a support and a coating comprising (i) a photopolymerizable layer comprising a polymerizable compound, a photoinitiator, and an asymmetrically substituted infrared absorbing compound.

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 master, such as a printing plate mounted on a rotary printing press cylinder. The master carries a lithographic printing plate image on its surface, and the printed product is obtained by applying ink to the image and then transferring the ink from the master to a receiving material, generally paper. In conventional lithographic printing, ink and an aqueous fountain solution (also called a dampening liquid) are supplied to the lithographic image, which consists of oleophilic (or hydrophobic, i.e., ink-receptive and water-repellent) areas and hydrophilic (or oleophobic, i.e., water-receptive and ink-repellent) areas. In so-called waterless offset printing, the lithographic image consists of ink-receptive and ink-resistant (ink-repelling) areas, and during waterless offset printing, only ink is supplied to the master.

[0003] Lithographic printing masters are typically produced by image-wise exposure and processing of a radiation-sensitive layer on a lithographic support. This imaging and processing transforms the so-called lithographic printing plate precursor into a printing plate or master. The radiation-sensitive coating is image-wise exposed to heat or light, typically with the aid of a digitally modulated exposure device such as a laser, triggering physical and / or chemical processes such as ablation, polymerization, insolubilization by polymer crosslinking or particle agglomeration of thermoplastic polymer latexes, solubilization by disrupting intermolecular interactions, or by increasing the permeability of the developer barrier. While some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing because exposure creates a solubility difference or dissolution rate difference in the developer between 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 unexposed areas remain resistant to the developer. In negative-working lithographic printing plate precursors, the unexposed areas of the coating dissolve in the developer, while the exposed areas remain resistant to the developer. Most lithographic printing plate precursors comprise a hydrophobic coating on a hydrophilic support, such that areas remaining resistant to the developer define the ink receptive, and hence printing, areas of the printing plate, while the hydrophilic support is revealed in the non-printing areas by dissolution of the coating in the developer.

[0004] Photopolymer printing plates rely on a working mechanism whereby a coating, typically comprising a free radical polymerizable compound, hardens upon exposure. "Hardening" means that the coating becomes insoluble or non-dispersible in a developer solution and can be achieved by polymerizing and / or crosslinking the photosensitive coating upon exposure to light and / or heat. Photopolymer plate precursors can be sensitive to blue, green, or red light (i.e., wavelengths in the range of 450 to 750 nm), violet light (i.e., wavelengths in the range of 300 to 450 nm), or infrared light (i.e., wavelengths in the range of 750 to 1500 nm). Optionally, the exposure step is followed by a heating step to enhance or accelerate the polymerization and / or crosslinking reaction.

[0005] Typically, a topcoat or protective overcoat is required over the imageable layer to act as an oxygen barrier, thereby providing the desired sensitivity to the printing plate. The topcoat typically comprises a water-soluble or water-swellable polymer, such as polyvinyl alcohol. In addition to acting as an oxygen barrier, the topcoat should preferably be easily removable during processing and sufficiently transparent to actinic radiation, for example, 300 to 450 nm, 450 to 750 nm, or 750 to 1500 nm.

[0006] The traditional workflow for photopolymer printing plates involves first exposing the photopolymer printing plate precursor in a UV or infrared platesetter, followed by an optional preheating step, a washing step for the protective overcoat, an alkaline development step, and a rinsing and gumming step. However, there is now a significant move towards simplifying the workflow, where the preheating and / or washing steps are eliminated, and where the processing and gumming steps are performed in a single step, or processing is performed with a neutral gum followed by gumming in a second step. Alternatively, on-press processing is becoming popular, where the plate is mounted on a printing press and the coating is developed by interaction with fountain solution and / or ink supplied to the plate during a press run. During the first press run, the non-image areas are removed from the support, thereby defining the non-printing area of ​​the plate.

[0007] In order to be able to evaluate the image quality, such as image resolution and detail rendering (usually measured with a densitometer) before the lithographic printing plate is mounted on the printing press, the lithographic printing plate precursor usually includes a colorant, such as a dye or pigment, in the coating. However, for photopolymer lithographic printing plates that are not subjected to plate development before machining and therefore mounting the printing plate on the printing press, preliminary inspection and identification of the printing plate including the colorant is impossible. A solution has been provided in the art by including in the coating a component that is capable of forming a so-called "printed image" upon exposure, that is, the image is visible before processing. Thermosensitive photopolymer lithographic printing plates may include dyes that absorb in the visible light wavelength range and change color when heated to form a printed image. For example, thermochromic dye technology involves the design of IR dyes containing thermally cleavable groups, whereby a color shift is obtained upon exposure to heat and / or light, as disclosed in WO2019 / 219560.

[0008] EP 1 507 170 discloses a polymerizable composition comprising an ester group-containing dye soluble in an organic solvent and an alkaline aqueous solution and having absorption in the range of 700 to 1200 nm; a radical polymerization initiator; a compound having an ethylenically unsaturated bond; and a binder polymer.

[0009] EP 1 502 735 discloses a lithographic printing plate precursor comprising a recording layer comprising a polyurethane compound soluble or swellable in water or an aqueous alkaline solution and a free-radical initiator.

[0010] EP 1556227 discloses an IR-sensitive composition comprising an initiator system comprising a material capable of absorbing IR radiation, a compound capable of generating free radicals and a specific hetero-substituted arylacetic acid co-initiator compound.

[0011] Thermophotopolymer printing plates based on heat-induced physical and / or chemical reactions typically include a thermosensitive coating containing an infrared dye as a light-to-heat conversion compound. Upon exposure to heat and / or infrared light, the generated heat triggers the thermosensitive coating's imaging mechanism. These infrared dyes exhibit strong absorption in IR wavelengths and are ideally evenly distributed throughout the thermosensitive coating. However, due to, for example, possible limited solubility in common coating solvents, it has been observed that the dyes may become unevenly distributed in the coating of the printing plate and / or even form aggregates and / or crystals in the coating of the printing plate. As a result, defects or so-called "artifacts" may appear throughout the coating and may worsen the lithographic properties of the printing plate, such as adhesion problems, shortened run lengths, and / or lower sensitivity of the printing plate.

[0012] In summary, despite the solutions provided in the art, there is still an urgent need for thermophotopolymer printing plates characterized by improved sensitivity and press life, preferably obtained by machining. Summary of the Invention

[0013] It is therefore an object of the present invention to provide a thermal negative working lithographic printing plate precursor which provides a printing plate having excellent lithographic properties both in terms of sensitivity and run length.

[0014] This object is achieved by the printing plate precursor defined in the present invention. The printing plate precursor of the present invention is particularly characterized in that it comprises a photopolymerizable layer containing an asymmetrically substituted infrared-absorbing compound. According to the present invention, it has surprisingly been found that by incorporating an asymmetrically substituted infrared-absorbing compound into the photopolymerizable layer, a printing plate having improved sensitivity and run length is obtained, whereby the formation of aggregates and / or crystals is significantly reduced.

[0015] Another object of the present invention is to provide a method for manufacturing a lithographic printing plate, the method comprising the steps of:

[0016] - image-wise exposing the printing plate precursor as defined above comprising the coating to heat and / or IR radiation, thereby forming a lithographic printing image consisting of image areas and non-image areas;

[0017] - Visualize the exposed precursor.

[0018] Development is preferably performed by treating the precursor with a gum solution, but more preferably by mounting the precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding fountain solution and / or ink to the precursor.

[0019] Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention. DETAILED DESCRIPTION

[0020] Lithographic printing plate precursors

[0021] The lithographic printing plate precursor according to the invention is negative-working, i.e., after exposure and development, the non-exposed areas of the coating are removed from the support and define hydrophilic (non-printing) areas, while the exposed coating is not removed from the support and defines oleophilic (printing) areas. The hydrophilic areas are defined by the support having a hydrophilic surface or provided with a hydrophilic layer. The hydrophobic areas are defined by the coating that hardens upon exposure (optionally followed by a heating step). Areas having hydrophilic properties are those that have a higher affinity for aqueous solutions than for oleophilic inks; areas having hydrophobic properties are those that have a higher affinity for oleophilic inks than for aqueous solutions.

[0022] "Hardening" means that the coating becomes insoluble or non-dispersible in the developer solution and can be achieved by polymerizing and / or crosslinking the photosensitive coating, optionally followed by a heating step to enhance or accelerate the polymerization and / or crosslinking reaction. In this optional heating step (hereinafter also referred to as "preheating"), the printing plate precursor is preferably heated at a temperature of about 80° C. to 150° C., and preferably during a dwell time of about 5 seconds to 1 minute.

[0023] The coating comprises at least one layer comprising a photopolymerizable composition, which layer is also referred to as a "photopolymerizable layer". The coating may also comprise a top layer provided on top of the photopolymerizable layer. The coating may optionally comprise further layers, such as intermediate layers, adhesion-improving layers and / or other layers, located between the support and the photopolymerizable layer and / or between the optional top layer and the photopolymerizable layer.

[0024] The coating of the printing plate precursor is most preferably developable on-press with fountain solution and / or ink.

[0025] The printing plates of the present invention are preferably exposed at a low energy density, i.e. below 190 mJ / m 2 ; preferably 70 to 190 mJ / m 2 More preferably, between 75 and 150 mJ / m 2 between 80 and 120 mJ / m 2 between.

[0026] Photopolymer coating

[0027] Photopolymerizable compounds

[0028] The coating has at least one layer comprising a photopolymerizable composition, said layer also being referred to as a “photopolymerizable layer.” The coating may comprise an intermediate layer between the support and the photopolymerizable layer.

[0029] 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 in the range of 0.2 to 5.0 g / m 2 between 0.4 and 3.0 g / m 2 between 0.6 and 1.5 g / m 2 The range between.

[0030] According to a preferred embodiment of the present invention, the polymerizable compound is a polymerizable monomer or oligomer containing at least one terminal ethylenically unsaturated group, hereinafter also referred to as a "free radical polymerizable monomer". Polymerization involves linking the free radical polymerizable monomers together. Suitable free radical polymerizable monomers include, for example, multifunctional (meth)acrylate monomers (such as (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, the (meth)acrylate monomer may also have other ethylenically unsaturated groups or epoxide groups. The (meth)acrylate monomer may also contain acidic (such as carboxylic acid or phosphoric acid) or basic (such as amine) functional groups.

[0031] Suitable free-radically polymerizable monomers are disclosed in EP 2 916 171 at

[0042] and

[0050] .

[0032] initiator

[0033] According to the present invention, any free radical initiator capable of generating free radicals upon exposure, 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 carbon-halogen bonds (such as [1,3,5] triazines with trihalomethyl groups), organic peroxides, aromatic ketones, thio compounds, azo polymerization initiators, azides, ketoxime esters, hexaarylbisimidazoles, metallocenes, active ester compounds, borate esters (salts), and quinonediazide. Among these, onium salts, especially iodonium and / or sulfonium salts, are preferred in view of storage stability.

[0034] More specific 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; benzil; ketocoumarins (such as 3-benzoyl-7-methoxycoumarin and 7-methoxycoumarin); xanthone; thioxanthone; benzoin or alkyl-substituted anthraquinone; onium salts (such as diaryliodonium hexafluoroantimonate, diaryliodonium trifluoromethanesulfonate, hexafluoroantimonate (4-(2- methylthio)phenyl) benzoyl ... triarylsulfonium hexafluorophosphate, triarylsulfonium p-toluenesulfonate, (3-phenylpropan-2-onyl)triarylphosphonium hexafluoroantimonate, and N-ethoxy(2-methyl)pyridinium hexafluorophosphate, and onium salts as described in U.S. Patent Nos. 5,955,238, 6,037,098, and 5,629,354); borate salts (e.g., tetrabutylammonium triphenyl(n-butyl)borate, tetraethylammonium triphenyl(n-butyl)borate, diphenylphosphonium tetraphenylborate, tetraethylammon ... and tetraethylammonium tetraphenylborate. diphenyl iodonium phenylborates (wherein the phenyl group of the iodonium salt is substituted with a group comprising at least six carbon atoms) and triphenylsulfonium triphenyl(n-butyl)borates and borates as described in U.S. Pat. Nos. 6,232,038 and 6,218,076); haloalkyl-substituted s-triazines (such as 2,4-bis(trichloromethyl)-6-(p-methoxy-phenylvinyl)-s-triazine, 2,4-bis(trichloromethyl)-6-(4-methoxy-naphthalen-1-yl)-s-triazine, 2,4-bis(

[0015] In some embodiments, the present invention provides an initiator comprising: a bis(trichloromethyl)-6-piperidinyl-s-triazine and 2,4-bis(trichloromethyl)-6-[(4-ethoxy-vinyloxy)-phenyl-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). Onium salts, borates, and s-triazines are preferred free radical initiators. Diaryliodonium salts and triarylsulfonium salts are preferred onium salts. Triarylalkylborates are preferred borates. Trichloromethyl-substituted s-triazines are preferred s-triazines. These initiators may have optional substituents and may be used alone or in combination.

[0035] Optionally substituted trihaloalkylsulfones are particularly preferred initiators, wherein the halogen independently represents bromine, chlorine or iodine, and the sulfone is a compound comprising a sulfonyl functional group attached to two carbon atoms. Tribromomethylphenylsulfone is the most preferred initiator. More details about this initiator can be found in patent application WO2019 / 179995, paragraphs

[0029] to

[0040] .

[0036] The amount of initiator is generally in the range of 0.05 to 30 wt%, preferably 0.1 to 15 wt%, most preferably 0.2 to 10 wt%, relative to the total dry weight of the components in the photopolymerizable composition.

[0037] The photopolymerizable layer may also contain a coinitiator. Typically, the coinitiator is used in combination with a free radical initiator. Suitable coinitiators for photopolymer coatings are disclosed in US 6,410,205, US 5,049,479, EP 1 079 276, EP 1 369 232, EP 1 369 231, EP 1 341 040, US 2003 / 0124460, EP 1 241 002, EP 1 288 720, and in the reference book Chemistry & Technology UV & EB formulation for coatings, inks & paints – Volume 3 – Photoinitiators for Free Radical and Cationic Polymerisation, edited by KK Dietliker – PKT Oldring – 1991 – ISBN 0947798161. Specific coinitiators as described in EP 107 792 may be present in the photopolymerizable layer to further increase the sensitivity. Preferred coinitiators are disclosed in EP 2 916 171

[0051] .

[0038] Very high sensitivity can be achieved by including an optical brightener in the coating. Suitable examples of optical brighteners as sensitizers are described in WO 2005 / 109103, page 24, lines 20 to 39. Useful sensitizers can be selected from the sensitizing dyes disclosed in US Pat. No. 6,410,205, US Pat. No. 5,049,479, EP 1 079 276, EP 1 369 232, EP 1 369 231, EP 1 341 040, US Pat. No. 2003 / 0124460, EP 1 241 002 and EP 1 288 720.

[0039] Specific coinitiators as described in EP 107 792 may be present in the photopolymerizable layer to further increase the sensitivity. Preferred coinitiators are sulfur compounds, especially thiols, such as 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 4-methyl-3-propyl-1,2,4-triazolin-5-thione, 4-methyl-3-n-heptyl-1,2,4-triazolin-5-thione, 4-phenyl-3-n-heptyl-1,2,4-triazolin-5-thione, 4-phenyl-3,5-dimercapto ... 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, mercaptophenyltetrazolyl, pentaerythritol mercaptopropionate, butyric acid-3-mercapto-neopentane tetraester, pentaerythritol tetrakis(thioglycolate). Other preferred coinitiators are polythiols as disclosed in WO 2006 / 048443 and WO 2006 / 048445. These polythiols can be used in combination with the above-mentioned thiols, for example 2-mercaptobenzothiazole.

[0040] Infrared absorbing compounds

[0041] The photopolymerizable layer includes an asymmetrically substituted infrared light absorbing compound. 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, and more preferably between 800 nm and 1100 nm. The asymmetrically substituted infrared absorbing dye is represented by Formula I:

[0042]

[0043] Formula I

[0044] in

[0045] Z and Z' independently represent -S-, -CR a R b - or -CH=CH-; R a and R b represents an alkyl group, an aralkyl group or an aryl group; preferably Z and Z' represent -CR a R b -, where R a and R b represents an alkyl group, most preferably a methyl or ethyl group;

[0046] A represents halogen, optionally substituted aliphatic hydrocarbon group, optionally substituted (hetero)aryl group or -NR 1 R 2 , where R 1 and R 2independently represents hydrogen, optionally substituted aliphatic hydrocarbon group or optionally substituted (hetero)aryl group and / or combinations thereof;

[0047] Q represents the atoms necessary for ring formation;

[0048] T and T' independently represent hydrogen, alkyl, halogen, alkoxy, cyano, -CO2R n 、-CONR k R m 、-SO2R n 、-SO2NR o R p or an optionally substituted cyclized benzene ring, wherein R k 、R m represents hydrogen, an optionally substituted alkyl or aryl group, R n represents an optionally substituted alkyl or aryl group, and R o and R p represents hydrogen, optionally substituted alkyl or aryl;

[0049] W - represents a counterion to obtain a neutral compound; and

[0050] R z and R z’ independently represents an optionally substituted alkyl group;

[0051] It is characterized by R z and R z’ are different.

[0052] T and T' preferably independently represent hydrogen, an alkyl group (such as methyl or ethyl) or an optionally substituted cyclized benzene ring.

[0053] W - represents a counter ion that provides for the electrically neutral compound and can be selected from, for example, halogen, sulfonate, perfluorosulfonate, toluenesulfonate, tetrafluoro or tetraphenylborate, hexafluorophosphate, arylborate, arylsulfonate.

[0054] Q preferably represents -CHR'-CHR"-, -CR'=CR"- or -CHR'-CHR"-CHR'"'-, and R', R" and R'"' independently represent hydrogen, optionally substituted alkyl, cycloalkyl, aralkyl, alkaryl, aryl or heteroaryl, or R' and R" or R" and R'"' together form a cyclic structure.

[0055] Most preferably, Q is represented by formula II, III or IV:

[0056]

[0057] Formula II Formula III Formula IV

[0058] in

[0059] Ry and Ry' independently represent hydrogen, optionally substituted alkyl, aralkyl, alkaryl or aryl, or represent the atoms necessary to form a cyclic structure. In Formula II, Ry and Ry' preferably independently represent hydrogen or optionally substituted alkyl; in Formula III, Ry and Ry' preferably represent a cyclized ring, preferably as shown in Formula V:

[0060]

[0061] Formula V

[0062] In formulae II to V, * indicates the position of attachment to the rest of the dye.

[0063] As defined above, R z and R z’ Different from, and independently represent a straight chain or branched alkyl group. The straight chain or branched alkyl group preferably includes C1 to C 15 Alkyl groups of carbon atoms, more preferably straight chain or branched chain alkyl groups including C2 to C 12 Alkyl groups of carbon atoms, most preferably straight chain or branched chain alkyl groups are C4 to C 10 The linear or branched alkyl group is preferably selected from methyl, ethyl, propyl (n-propyl, isopropyl), butyl (n-butyl, isobutyl, tert-butyl), pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl or dodecyl, with the condition that R z and R z’ Different. Highly preferred R z represents a methyl group or an ethyl group, and R z’ represents octyl, nonyl or decyl.

[0064] Adhesives

[0065] The photopolymerizable layer preferably comprises a binder. The binder can be selected from a variety of organic polymers. Combinations of different binders can also be used. Useful binders are described, for example, in EP 1 043 627, paragraph

[0013] , WO 2005 / 111727, page 17, lines 21 to page 19, lines 30, and WO 2005 / 029187, page 16, lines 26 to page 18, lines 11. Also included are 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, acrylates or methacrylates, or mixtures thereof. Preferably, the discrete particles are particles suspended in the polymerizable composition. The presence of discrete particles tends to enhance the developability of the unexposed areas.

[0066] The thermoreactive polymer fine particles include thermoreactive groups such as an ethylenically unsaturated group, a cationically polymerizable group, an isocyanate group, an epoxy group, a vinyloxy group, and a functional group having an active hydrogen atom, a carboxyl group, a hydroxyl group, an amino group, or an acid anhydride.

[0067] The average particle size of the polymer fine particles is preferably 0.01 mm to 3.0 mm. As disclosed in EP 1 132 200, EP 1 724112, US 2004 / 106060, particulate polymers in the form of microcapsules, microgels or reactive microgels are suitable.

[0068] Other ingredients

[0069] The photopolymerizable layer may also contain 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 2916 171

[0053] to

[0056] .

[0070] The photopolymerizable layer may also comprise an inhibitor. Specific inhibitors for photopolymer coatings are disclosed in US 6,410,205, EP 1 288 720 and EP 1 749 240.

[0071] The photopolymerizable layer may further comprise an adhesion promoting compound. The adhesion promoting compound is a compound that can act on a carrier, preferably has an addition polymerizable ethylenically unsaturated bond and a compound that can interact with the carrier's functional group. "Interaction" should be understood as various types of physical and / or chemical reactions or processes, wherein a key is formed between the functional group and the carrier, and this key can be a covalent bond, an ionic bond, a complex bond, a coordinate bond or a hydrogen bond, and can be formed by an adsorption process, a chemical reaction, an acid-base reaction, a complex formation reaction or a chelating group or a ligand reaction. Adhesion promoting compounds are described in EP 2 916171

[0058] .

[0072] The photopolymerizable layer may comprise 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 the unpublished application EP19153178

[0069] to

[0085] .

[0073] The photopolymerizable layer may further comprise at least one borate compound. A 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, such as a diphenyl iodonium photoinitiator, and / or the counterion of the aforementioned infrared absorbing compounds or any other salt, such as sodium tetraphenylborate.

[0074] Preferably, the borate anion is a tetrahedral boron anion and can be represented by the following formula A:

[0075]

[0076] Formula A

[0077] where R b 1 、R b 2 、R b 3 and R b 4 are independently optionally substituted aliphatic hydrocarbon, optionally substituted aryl or heteroaryl; or, R b 1 、R b 2 、R b 3 and R b 4 Two or more of may be linked together to form a heterocyclic ring with the boron atom, such a ring may include up to seven carbon, nitrogen, oxygen and / or nitrogen atoms. b 1 、R b 2 、R b 3 and R b 4 R is independently an optionally substituted aryl or heteroaryl. b 1 、R b 2 、R b 3 and R b 4 Most preferably, the borate compound comprises 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.

[0078] 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.

[0079] 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-3-pyridinium groups, Methyl-4-pyridinium, N-alkyl-3,5-dimethyl-4-pyridinium, N-alkyl-3-pyridinium or N-alkyl-4-pyridinium, particularly preferably N-methyl-3-pyridinium, N-octyl-3-pyridinium, N-methyl-4-pyridinium or N-octyl-4-pyridinium, most preferably N-octyl-3-pyridinium or N-octyl-4-pyridinium.

[0080] The optionally substituted onium ion is preferably an ammonium ion represented by formula B:

[0081]

[0082] Formula B

[0083] in

[0084] R n 1 、R n 2 and R n 3 is independently an optionally substituted aliphatic hydrocarbon group, an optionally substituted aryl or heteroaryl group, or a halogen atom.

[0085] 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 electron-donating groups (e.g., alkyl or alkoxy groups), and asymmetric diphenyliodonium salts. The phenyl group of the iodonium ion is preferably substituted with a group containing at least six carbon atoms.

[0086] Specific examples of the borate compound including an iodine ion include 4-hexyloxyphenyl-2,4-diethoxyphenyliodine tetrafluoroborate, 4-octyloxyphenylphenyliodine tetraphenylborate, [4-[(2-hydroxytetradecyl)-oxy]phenyl]phenyliodine tetraphenylborate, bis(4-tert-butylphenyl)iodine tetraphenylborate, 4-methylphenyl-4'-hexylphenyliodine tetraphenylborate, 4-methylphenyl-4'-cyclohexylphenyliodine tetraphenylborate, bis(tert-butylphenyl)iodine tetraphenylborate, 4-hexylphenyl-phenyliodine tetraphenylborate, n-butyltriphenyl 4-Methylphenyl-4'-cyclohexylphenyl iodine borate, 4-cyclohexylphenyl-phenyl iodine tetraphenyl borate, 2-5-methyl-4-tert-butylphenyl-4'-methylphenyl iodine tetraphenyl borate, 4-methylphenyl-4'-pentylphenyl iodine tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, 4-methoxyphenyl-4'-cyclohexylphenyl iodine tetrakis(pentafluorophenyl)borate, 4-methylphenyl-4'-dodecylphenyl iodine tetrakis(4-fluorophenyl)borate, bis(dodecylphenyl) iodine tetrakis(pentafluorophenyl)borate, and bis(4-tert-butylphenyl) iodine tetrakis(imidazolyl)borate. Preferred compounds include bis(4-tert-butylphenyl)iodinium tetraphenylborate, 4-methylphenyl-4'-hexylphenyliodinium tetraphenylborate, 2-methyl-4-tert-butylphenyl-4'-methylphenyliodinium tetraphenylborate, and 4-methylphenyl-4'-cyclohexylphenyliodinium tetraphenylborate.

[0087] The borate compound may be present in an amount comprised between 0.05 and 30 wt%, more preferably between 0.1 and 25 wt%, most preferably between 0.5 and 15 wt%, relative to the components of the photopolymerizable layer.

[0088] Various surfactants may be added to the photopolymerizable layer to allow or enhance the developability of the precursor, particularly with a gum solution. Both polymeric and small molecule surfactants are preferred, such as nonionic surfactants. Further details are described in EP 2 916 171

[0059] .

[0089] Top floor

[0090] The coating may include a topcoat or protective outer coating, which can act as an oxygen barrier. Low molecular weight substances present in the air can deteriorate or even inhibit image formation, so a topcoat is applied to the coating. The topcoat should preferably be easily removable during development, adhere well to the photopolymerizable layer or optional other layers of the coating, and should preferably not inhibit light transmission during exposure. The topcoat is preferably provided on top of the photopolymerizable layer.

[0091] The top layer preferably includes an infrared-absorbing compound capable of forming a colored compound upon exposure to infrared light and / or heat—thus forming a printed image. This infrared-absorbing compound is preferably an infrared-absorbing dye, or IR dye. The color-forming IR dye is also referred to herein as a secondary infrared-absorbing dye, a thermochromic infrared-absorbing dye, or a thermochromic IR dye. Thermochromic IR dyes primarily absorb in the infrared wavelength range of the electromagnetic spectrum—that is, between approximately 750 and 1500 nm—and preferably do not absorb significantly in the visible wavelength range of the electromagnetic spectrum—that is, between 390 and 700 nm. Thermochromic IR dyes preferably include at least one thermally cleavable group that, upon exposure to IR radiation or a heat-induced chemical reaction, is converted into a group that is a more potent electron donor. As a result, the exposed thermochromic IR dye absorbs significantly more light in the visible wavelength range of the electromagnetic spectrum, or in other words, undergoes a blue shift, thereby forming a visible image, also referred to as a printed image. The formation of this printed image is significantly different from the prior art process in which a compound changes from a substantially colorless compound to a pale to colored compound. These compounds typically shift absorption from the UV wavelength range of the electromagnetic spectrum to the visible wavelength range of the electromagnetic spectrum, i.e., these compounds typically have a bathochromic shift. Compared to the color forming processes described above for thermochromic IR dyes, the contrast of the printed image obtained by such a process is much weaker.

[0092] The contrast of the printed image can be defined as the difference between the optical density of the exposed areas and the optical density of the non-exposed areas and is preferably as high as possible. This allows the end user to immediately determine whether the precursor has been exposed and processed, to distinguish between different color selections, and to check the image quality on the printing plate precursor. The contrast of the printed image preferably increases with increasing optical density in the exposed areas, and reflectance can be measured using a densitometer equipped with several color filters (e.g., cyan, magenta, yellow).

[0093] The concentration of the thermochromic IR dye may be from 0.1 wt% to 20.0 wt%, more preferably from 0.5 wt% to 15.0 wt%, most preferably from 1.0 wt% to 10.0 wt%, relative to the total dry weight of the coating.

[0094] The second infrared absorbing dye is preferably represented by Formula VI

[0095]

[0096] Formula VI

[0097] in

[0098] Ar 1 and Ar 2 independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted cyclized benzene ring,

[0099] W 1 and W 2 independently represents a sulfur atom, an oxygen atom, NR*, wherein R* represents an optionally substituted alkyl group, NH or -CM 10 M 11 Group, where M 10 and M 11 independently represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; or wherein M 10 and M 11 Together contain the atoms necessary to form a cyclic structure (preferably a 5- or 6-membered ring);

[0100] M 1 and M 2 M represents independently hydrogen, an optionally substituted aliphatic hydrocarbon group or the atoms necessary to form an optionally substituted cyclic structure which may include an optionally substituted cyclized benzene ring. 1 and M 2 together contain the atoms necessary to form an optionally substituted cyclic structure, which may include an optionally substituted cyclized benzene 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;

[0101] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group;

[0102] M 5 、M 6 、M 7 and M 8 represents hydrogen, halogen or an optionally substituted aliphatic hydrocarbon group,

[0103] M 9 The chemical reaction induced by exposure to IR radiation or heat is converted into a 9 A group that is a stronger electron donor group; and said transition provides an increase in the cumulative light absorption of said dye between 350 and 700 nm;

[0104] and optionally one or more counterions to obtain a neutral compound.

[0105] Thermochromic IR dyes can be neutral, anionic or cationic, depending on the type of substituents and the number of each substituent.

[0106] In a preferred embodiment, the thermochromic IR dye is represented by formula VI above and includes M represented by one of the following groups: 9 :

[0107] -(N=CR 17)a –NR 5 -CO-R 4 ,

[0108] -(N=CR 17 )b -NR 5 -SO2-R 6 ,

[0109] -(N=CR 17 )c -NR 11 -SO-R 12 ,

[0110] -SO2-NR 15 R 16 and

[0111] -S-CH2-CR 7 (H) 1-d (R 8 ) d -NR 9 -COOR 18 ,

[0112] in

[0113] a, b, c and d are independently 0 or 1;

[0114] 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 atoms necessary to form a ring structure;

[0115] R 4 Indicates -OR 10 、-NR 13 R 14 or -CF3;

[0116] where R 10 represents an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group;

[0117] R 13 and R 14 independently represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R 13 and R 14 together contain the atoms necessary to form a ring structure;

[0118] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, -OR 10 、-NR 13 R14 or -CF3;

[0119] 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 With R 10 、R 13 and R 14 at least one of them together contains the atoms necessary to form a ring structure;

[0120] R 11 、R 15 and R 16 independently represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein R 15 and R 16 together contain the atoms necessary to form a ring structure;

[0121] R 12 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group;

[0122] R 7 and R 9 independently represent hydrogen or an optionally substituted aliphatic hydrocarbon group;

[0123] R 8 Indicates –COO- or –COOR 8’ , where R 8’ represents hydrogen, an alkali metal cation, an ammonium ion or a mono-, di-, tri- or tetra-alkylammonium ion;

[0124] R 18 represents an optionally substituted (hetero)aryl group or an α-branched aliphatic hydrocarbon group; and

[0125] Optionally, one or more counterions to obtain a neutral compound.

[0126] Most preferably, the thermochromic IR dye is represented by Formula VI, wherein

[0127] Ar 1 and Ar 2 independently represents an optionally substituted aryl group; optionally cyclized with an optionally substituted benzene ring,

[0128] W 1 and W 2 represents –C(CH3)2;

[0129] M 1 and M 2 together contain the atoms necessary to form an optionally substituted 5-membered ring which may include an optionally substituted cyclized benzene ring;

[0130] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group,

[0131] M 5 、M 6 、M 7 and M 8 represents hydrogen;

[0132] M 9 express

[0133] -NR 5 -CO-R 4

[0134] -NR 5 -SO2-R 6

[0135] -NR 11 -SO-R 12

[0136] -SO2-NR 15 R 16

[0137] where R 4 、R 5 、R 6 、R 11 、R 12 、R 15 and R 16 As defined above;

[0138] and optionally one or more counterions to obtain a neutral compound.

[0139] In a highly preferred embodiment, the thermochromic IR dye is represented by Formula VI, wherein

[0140] Ar 1 and Ar 2 independently represents an optionally substituted aryl group;

[0141] W 1 and W 2 represents –C(CH3)2;

[0142] M 1 and M 2 together contain the atoms necessary to form an optionally substituted 5-membered ring which may include an optionally substituted cyclized benzene ring;

[0143] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group,

[0144] M 5 、M 6 、M 7 and M 8 represents hydrogen;

[0145] M 9 express

[0146] -NR 5 -CO-R 4

[0147] -NR 5 -SO2-R 6

[0148] in

[0149] R 4 For-OR 10 , where R 10 is an optionally branched aliphatic hydrocarbon group;

[0150] R 5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group,

[0151] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; and

[0152] Optionally, one or more counterions to obtain a neutral compound.

[0153] The above-mentioned 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 VI contains at least one anion or acid group (e.g., -CO2H, -CONHSO2R h 、-SO2NHCOR i 、-SO2NHSO2R j , –PO3H2, –OPO3H2, –OSO3H, -S-SO3H or –SO3H) 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 a mono- or di- or tri- or tetra-alkylammonium salt. These anions or acid groups may be present in Ar 1 or Ar 2 The aromatic hydrocarbon group or cyclized benzene ring, or is present in M 3 or M 4 Other substituents may be selected from halogen atoms, cyano groups, sulfone groups, carbonyl groups or carboxylate groups. 3 or M 4At least one of them is terminated with at least one of these groups, more preferably with a -CO2H, -CONHSO2-Me, -SO2NHCO-Me, -SO2NHSO2-Me, -PO3H2 or -SO3H group or a corresponding salt thereof, wherein Me represents a methyl group.

[0154] In order to obtain electrically neutral compounds, optional counterions may be selected from, for example, halogens, sulfonates, perfluorosulfonates, toluenesulfonates, tetrafluoroborate, hexafluorophosphate, arylborates (e.g., tetraphenylborate), arylsulfonates; or cations such as alkali metal salts or ammonium salts, including mono-, di-, tri-, or tetra-alkylammonium salts.

[0155] The thermochromic IR dyes mentioned above 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 comprising two or more thermochromic IR dyes can also be formed by ionic interactions. For example, dimers consisting of two different IR dyes can be formed by interactions 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 1 582 346, in which an IR dye containing 2 to 4 sulfonate groups is ionically bonded to a polymer containing covalently attached ammonium, phosphonium, and sulfonium groups.

[0156] Supramolecular complexes containing two or more thermochromic IR dyes can also be formed through hydrogen bonding or dipole-dipole interactions.

[0157] Suitable examples of thermochromic IR dyes for use in the present invention are described in EP 1 910 082, pages 4 to 8, IRD-001 to IRD-101.

[0158] Particularly preferred thermochromic IR dyes are represented by one of the following formulae:

[0159]

[0160] in

[0161] X - represents a halogen, a sulfonate, a perfluorosulfonate, a toluenesulfonate, a tetrafluoroborate, a hexafluorophosphate, an arylborate or an arylsulfonate; and

[0162] R 3 , R 3’ independently represents an optionally substituted alkyl group, preferably a methyl or ethyl group; or an ether group, preferably -CH2-CH2-O-CH3;

[0163]

[0164]

[0165]

[0166] in

[0167] M + = Li + , Na + , K + , NH4 + , R ’ R ’’ R ’’’ NH + , where R ’ , R ’’ , R ’’’ independently represents hydrogen, optionally substituted alkyl or aryl;

[0168]

[0169]

[0170]

[0171] Formula VIII

[0172] The most preferred thermochromic IR dye is represented by Formula VIII.

[0173] The color difference between the exposed and non-exposed areas of the coating is expressed as ΔE, 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 (non-exposed area) of the coating. When the coating of the present invention is exposed, even at a low energy density, for example, between 70 and 190 mJ / m 2 between 75 and 150 mJ / m 2 between 80 and 120 mJ / m 2 The printed image formed is characterized by a CIE 1976 color difference ΔE of at least 2, more preferably at least 2.5, most preferably at least 3. According to the present invention, at very low exposure energies, for example below 150 mJ / m 2, obtaining a CIE 1976 color difference, ΔE, of at least 2. ΔE is the CIE 1976 color distance, ΔE, defined as the pairwise Euclidean distance of the CIE L*a*b* color coordinates. The CIEL*a*b* color coordinates are derived from reflectance measurements in 45 / 0 geometry (unpolarized) with a CIE 2° observer and D50 as the illuminant. Further 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.

[0174] The CIE 1976 color coordinates L*, a*, and b* discussed herein are part of the well-known CIE (Commission Internationale de l'Eclairage) tristimulus coordinate 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 Color Difference Evaluation," or "Measuring Color," R.W.G. Hunt, 2nd edition, edited by Ellis Horwood Limited, England, 1992.

[0175] The CIE L*a*b* values ​​discussed and reported herein have been measured according to ASTM E308-85.

[0176] The top layer may also contain a binder. A preferred binder for the top layer is polyvinyl alcohol. The degree of hydrolysis of the polyvinyl alcohol is preferably between 74 mol% and 99 mol%, more preferably between 80 mol% and 98 mol%. 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. This viscosity value is preferably in the range of 2 to 26, more preferably in the range of 2 to 15, and most preferably in the range of 2 to 10.

[0177] The top layer may comprise a halogenated polymer, preferably a hydrophobic polymer, i.e., insoluble or non-swellable in water at approximately neutral pH. Such an adhesive may be used in the top layer in the form of a dispersion, i.e., an emulsion or suspension. The amount of the halogenated adhesive in the top layer may be between 30% and 96% by weight, more preferably between 40% and 90% by weight, and most preferably between 50% and 85% by weight. The halogenated adhesive preferably comprises between 60% and 95% by weight of monomeric units derived from vinylidene monomers, such as vinylidene fluoride, vinylidene chloride, vinylidene bromide, and / or vinylidene iodide.

[0178] The top layer may optionally comprise further ingredients, such as inorganic or organic acids; matting agents; surfactants, such as anionic surfactants (e.g. sodium alkyl sulfates or sodium alkyl sulfonates), amphoteric surfactants (e.g. alkylaminocarboxylates and alkylaminodicarboxylates), nonionic surfactants (e.g. polyoxyethylene alkylphenyl 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, such as those disclosed in EP 2 916 171.

[0179] The coating thickness of the top layer is preferably between 0.10 and 1.75 g / m 2 between 0.20 and 1.3 g / m 2 between 0.25 and 1.0 g / m 2 In a more preferred embodiment of the present invention, the top layer has a thickness of 0.25 to 1.75 g / m 2 and comprising polyvinyl alcohol having a degree of hydrolysis ranging between 74 mol % and 99 mol % and a viscosity value as defined above ranging between 2 and 26 mPas.

[0180] The hydrophilic polymer in the protective overcoat layer may lead to a problematic increase in the viscosity of the printing chemicals (e.g., fountain solution and / or developer solution). Therefore, the coating weight of the hydrophilic polymer and / or the thickness of the protective overcoat layer should preferably not be too high, e.g., above the ranges specified above.

[0181] definition

[0182] Aliphatic hydrocarbon groups preferably represent alkyl, cycloalkyl, alkenyl, cycloalkenyl or alkynyl groups; suitable groups are described below. Aromatic hydrocarbon groups preferably represent hetero(aryl) groups; suitable hetero(aryl) groups - i.e. suitable aryl or heteroaryl groups - are described below.

[0183] In this document, the term "alkyl" refers to all possible variations for each number of carbon atoms in the alkyl group, i.e., methyl, ethyl; for 3 carbon atoms: n-propyl and isopropyl; for 4 carbon atoms: n-butyl, isobutyl and tert-butyl; for 5 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-dimethylpropyl, 2,2-dimethylpropyl and 2-methylbutyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and methylcyclohexyl. Preferably, the alkyl group is a C1 to C6 alkyl group.

[0184] Suitable alkenyl groups are preferably C2 to C6-alkenyl groups, for example ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, isopropenyl, isobutenyl, isopentenyl, neopentenyl, 1-methylbutenyl, isohexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl.

[0185] Suitable alkynyl is preferably C2 to C6-alkynyl, suitable aralkyl is preferably phenyl or naphthyl including one, two, three or more C1 to C6-alkyl groups, suitable alkaryl is preferably C1 to C6-alkyl including aryl, and aryl is preferably phenyl or naphthyl.

[0186] The cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic or polycyclic group, which means that one or more rings are cyclized or fused together.

[0187] Suitable examples of aryl groups can be represented by, for example, optionally substituted phenyl, benzyl, tolyl or o-, m- or p-xylyl, optionally substituted naphthyl, anthracenyl, phenanthrenyl and / or combinations thereof. Heteroaryl is preferably a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably comprising 1 to 4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulphur. Preferred examples include optionally substituted furyl, pyridyl, pyrimidyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thienyl (thienyl), tetrazolyl, triazolyl, (1,2,3) triazolyl, (1,2,4) triazolyl, thiadiazolyl, thienyl (thiofenyl) and / or combinations thereof.

[0188] "Halogen" is selected from fluorine, chlorine, bromine or iodine.

[0189] The term "substituted," for example, in a substituted alkyl group, means that the alkyl group may be substituted with atoms other than the atoms normally present in such a group (i.e., carbon and hydrogen). For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

[0190] Optional substituents on alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl groups are preferably selected from -F, -Cl, -Br, -I, -OH, -SH, -CN, -NO2, alkyl (such as methyl or ethyl), alkoxy (such as methoxy or ethoxy), aryloxy, carboxylic acid groups or alkyl esters thereof, sulfonic acid groups or alkyl esters thereof, phosphonic acid groups or alkyl esters thereof, phosphoric acid groups or esters (such as alkyl esters, for example methyl or ethyl), alkylthio, arylthio, heteroarylthio, -SH, thioether (such as alkylthio or arylthio), ketone, aldehyde, sulfoxide, sulfone, sulfonate, sulfonamide, amino, vinyl, alkenyl, alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl or heteroalicyclic groups and / or combinations thereof.

[0191] Asymmetrically substituted infrared light absorbing compounds herein refer to compounds having two different sides or halves.

[0192] carrier

[0193] The lithographic printing plate used in the present invention comprises a support having a hydrophilic surface or provided with a hydrophilic layer. The support is preferably a roughened and anodized aluminum support well known in the art. Suitable supports are disclosed, for example, in EP 1 843 2033 (paragraphs

[0066] to

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

[0194] The roughened and anodized aluminum support may be subjected to a so-called post-anodic treatment, for example, treatment with polyvinylphosphonic acid or its derivatives, treatment with polyacrylic acid or its derivatives, treatment with potassium fluorozirconate or potassium phosphate, treatment with an alkali metal silicate, or a combination thereof. Treatment of the support edge, as described, for example, in US 2017 / 320351, can advantageously prevent the appearance of printed edges. Enlargement or sealing of the micropores of the anodized aluminum, as disclosed in JP 2001-253181 A or JP 2001-322365 A, can be performed. Alternatively, the support may be treated with an adhesion-promoting compound, such as those described in

[0010] of EP 1 788 434 and in WO 2013 / 182328. However, for precursors optimized for use without a preheating step, it is preferred to use the roughened and anodized aluminum support without any post-anodic treatment.

[0195] Besides aluminum 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.

[0196] 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 image-wise exposing a printing plate precursor, and subsequently developing the image-wise exposed precursor so that the non-exposed areas dissolve in a developer. Optionally, a heating step is performed after the imaging step to promote or accelerate polymerization and / or crosslinking reactions. The lithographic printing plate precursor can be prepared by (i) applying the above-described coating to a support, and (ii) drying the precursor.

[0197] Exposure Steps

[0198] The printing plate precursor is preferably image-wise exposed by a laser emitting IR light. The image-wise exposure step is preferably carried out off-press in a plate-setter, i.e., an exposure apparatus suitable for image-wise exposure of the precursor with 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 via contact with a mask. In a preferred embodiment of the invention, the precursor is image-wise exposed by a laser emitting IR light.

[0199] Preheating steps

[0200] After the exposure step, the precursor can be preheated in a preheating unit, preferably at a temperature of about 80° C. to 150° C., preferably during a residence time of about 5 seconds to 1 minute. This preheating unit can comprise a heating element, preferably an IR lamp, a UV lamp, heated air or a heated roller. Such a preheating step can be used for printing plate precursors comprising a photopolymerizable composition in order to promote or accelerate the polymerization and / or crosslinking reaction.

[0201] Development step

[0202] After the exposure step or the preheating step, when present, the printing plate precursor can be processed (developed). Before developing the imaged precursor, a pre-rinsing step can be performed, particularly for negative-working lithographic printing precursors having a protective oxygen barrier or overcoat. This pre-rinsing step can be performed in a separate unit, or by manually rinsing the imaged precursor with water, or the pre-rinsing step can be performed in a washing unit integrated into the processing machine for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. Further details regarding the washing step are described in

[0026] of EP 1 788 434.

[0203] During the development step, the non-exposed areas of the image-recording layer are at least partially removed, while the exposed areas are substantially not removed. A processing liquid, also known as a developer, is applied to the printing plate, for example by rubbing with an impregnated pad, by dipping, immersing, coating, spin-coating, spraying, or pouring onto it. This can be done manually or in an automated processing unit. Treatment with the processing liquid can be combined with mechanical rubbing, for example by a rotating brush. During the development step, any water-soluble protective layer present is also preferably removed. Development is preferably carried out in an automated processing unit at a temperature between 20 and 40°C.

[0204] In a highly preferred embodiment, the processing steps described above are replaced by on-machine processing, whereby the imaged precursor is mounted on a printing press and on-machine processed by rotating the plate cylinder while simultaneously feeding fountain solution and / or ink to the precursor coating to remove non-exposed areas from the support. In a preferred embodiment, only fountain solution is fed to the printing plate during startup of the printing press, and the ink supply is also turned on after a few revolutions of the plate cylinder. In an alternative embodiment, the fountain solution and ink supply are turned on simultaneously, or only ink may be fed during a few revolutions before the fountain solution supply is turned on. The paper feed may be before, during, or after any ink and / or fountain solution supply steps.

[0205] The processing steps can also be performed by combining the above-described embodiments, for example combining development with a processing fluid with on-press development by applying ink and / or fountain solution.

[0206] Processing fluid

[0207] The processing fluid may be an alkaline developer or a solvent-based developer. Suitable alkaline developers are described in US 2005 / 0162505. Alkaline developers are aqueous solutions having a pH of at least 11, more typically at least 12, and preferably between 12 and 14. Alkaline developers typically contain an alkaline agent to achieve a high pH, ​​which may be inorganic or organic. The developer may contain anionic, nonionic, and amphoteric surfactants (up to 3% by weight of the total composition); biocides (antimicrobial and / or antifungal agents), defoamers or chelating agents (e.g., alkali metal gluconates), and thickeners (water-soluble or water-dispersible polyols, such as glycerol or polyethylene glycol).

[0208] The processing liquid is preferably a gum solution, whereby during the development step, the non-exposed areas of the photopolymerizable layer are removed from the support and the printing plate is gummed in a single step. Development with a gum solution has the additional benefit that, due to residual gum on the printing plate in the non-exposed areas, no additional gumming step is required to protect the support surface in the non-printing areas. As a result, the precursor is processed and gummed in a single step, which involves a simpler development apparatus than a development apparatus comprising a developer tank, a rinse section, and a gumming section. The gumming section may comprise at least one gumming unit, or may comprise two or more gumming units. These gumming units may have a cascade system configuration, i.e., when a gum replenishing solution is added to a second gumming unit, or when the gum solution in the second gumming unit is used only once, i.e., when the precursor is developed in this second gumming unit using only the starting gum solution, preferably by spraying or jetting technology, the gum solution used for the second gumming unit and present in the second tank overflows from the second tank to the first tank. More details about such gum development are described in EP 1 788 444.

[0209] The gum solution is generally an aqueous liquid containing one or more surface protective compounds that are capable of protecting the lithographic image of the printing plate from contamination (e.g. oxidation, fingerprints, fat, oil or dust) or damage (e.g. scratches during handling of the printing plate). Suitable examples of such surface protective compounds are film-forming hydrophilic polymers or surfactants. The layer remaining on the printing plate after treatment with the gum solution preferably contains from 0.005 to 20 g / m 2 Surface protection compound between 0.010 and 10 g / m 2 between 0.020 and 5 g / m 2 More details on surface-protective compounds in the gum solution can be found in WO 2007 / 057348, page 9, line 3 to page 11, line 6. Since the developed printing plate precursor is developed and gummed in one step, there is no need for after-treatment of the processed printing plate.

[0210] The glue solution preferably has a pH value 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 WO 2005 / 111727. The glue solution may also contain an inorganic salt, an anionic surfactant, a wetting agent, a chelating compound, an antiseptic compound, a defoaming compound and / or an ink absorbent and / or a combination thereof. More details about these additional ingredients are described in WO 2007 / 057348, page 11, line 22 to page 14, line 19.

[0211] Drying and baking steps

[0212] After the processing step, the printing plate can be dried in a drying unit. In a preferred embodiment, the plate is dried by heating the printing plate in a drying unit, which may comprise at least one heating element selected from IR lamps, UV lamps, heated metal rollers or heated air.

[0213] After drying, the printing plate can optionally be heated in a baking unit. More details about heating in a baking unit can be found in WO 2007 / 057348, page 44, line 26 to page 45, line 20.

[0214] The printing plate thus obtained can be used for conventional so-called wet offset printing, in which ink and an aqueous fountain solution are supplied to the printing plate. Another suitable printing method uses so-called single-fluid inks, without fountain solution. Suitable single-fluid inks are described in US 4,045,232, US 4,981,517 and US 6,140,392. In a most preferred embodiment, the single-fluid ink comprises an ink phase (also called a hydrophobic or oleophilic phase) and a polyol phase, as described in WO 00 / 32705. Example

[0215] Example 1

[0216] 1. Synthesis of infrared absorbing dye IR-02

[0217] Step 1: Synthesis of 3-ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate

[0218] 3-Ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium iodide was prepared according to Xing et al. (Macromolecular Chemistry and Physics, 214(5), 578-588 (2013)).

[0219] 40 g (0.109 mol) of 3-ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium iodide was suspended in 300 ml of ethyl acetate. 16.7 ml (0.12 mol) of triethylamine was added and the mixture was stirred for 1 hour. The reaction mixture was extracted with 300 ml of water. The ethyl acetate portion was separated and dried over MgSO4. A solution of 22.8 g (0.12 mol) of p-toluenesulfonic acid monohydrate in 200 ml of ethyl acetate was added dropwise. The crude precipitated 3-ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate was separated by filtration, treated with 300 ml of ethyl acetate, separated by filtration, and dried. 35.5 g (y: 79.5%) of 3-ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate was isolated.

[0220] Step 2: Synthesis of 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate

[0221] Decyl tosylate was prepared as follows.

[0222] 1.292 kg (8 mol) of decanol and 1.54 kg of tosyl chloride were dissolved in 4 liters of isopropyl acetate. 899.5 g (8.8 mol) of triethylamine were added at a rate of 10 ml per minute. The reaction was continued at room temperature for 92 hours. The precipitated triethylamine chloride was removed by filtration and washed with 2 liters of isopropyl acetate. The combined organic fractions were extracted twice with 2 liters of 10% by weight sodium chloride aqueous solution and once with 1 liter of water. The organic fractions were dried over MgSO and evaporated under reduced pressure. 2272 g (y: 91%) of decyl tosylate were isolated and used for alkylation without further purification.

[0223] Step 2: Dissolve 1068 g (5 mol) of 1,1,2-trimethyl-1H-benz[e]indole and 2029 g (6.5 mol) of decyl toluenesulfonate in 1500 ml of sulfolane. Heat the reaction mixture to 125°C. From 50°C onward, all components completely dissolve in the reaction mixture. Continue the reaction at 125°C for 6 hours. Cool the reaction mixture to 75°C, and add 10 liters of ethyl acetate. Cool the reaction mixture to room temperature and stir at room temperature for 16 hours. Isolate the precipitated crude 3-decyl-1,1,2-trimethyl-1H-benz[e]-indol-3-ium toluenesulfonate by filtration, wash several times with ethyl acetate, and dry. Isolate 1818 g (y: 70%) of 3-decyl-1,1,2-trimethyl-1H-benz[e]-indol-3-ium toluenesulfonate.

[0224] Step 3: Synthesis of 5-bromo-2-[(E)-2-[(3Z)-3-[(2Z)-2-(5-bromo-1-butyl-3,3-dimethyl-indolin-2-ylidene)ethylidene]-2-chloro-cyclohexen-1-yl]vinyl]-1-butyl-3,3-dimethyl-indol-1-ium iodide

[0225] 3.8 g (9.3 mmol) of 3-ethyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate, 4.9 g (9.3 mmol) of 3-decyl-1,1,2-trimethyl-1H-benzo[e]-indol-3-ium toluenesulfonate and 2.4 g (13.95 mmol) of 2-chloro-1-formyl-3-(hydroxymethylene)cyclohexene were dissolved in 30 ml of ethanol. A mixture of 2.8 g (27.9 mmol) of triethylamine and 9.5 g (93 mmol) of acetic anhydride was added. The temperature of the reaction mixture was raised to 60° C. The reaction was continued at 60° C. for 90 minutes. The reaction mixture was cooled to 20° C. and 200 ml of dichloromethane was added. The mixture was extracted twice with 500 ml of water. The dichloromethane was exchanged for methyl tert-butyl ether using distillation. After the solvent exchange, the dye mixture precipitated. The methyl tert-butyl ether was decanted and the residue was dissolved in 50 ml of dichloromethane. The dichloromethane was evaporated to dryness, and 6.5 g of a dry residue was isolated, which contained 5-bromo-2-[(E)-2-[(3Z)-3-[(2Z)-2-(5-bromo-1-butyl-3,3-dimethyl-indolin-2-ylidene)ethylidene]-2-chloro-cyclohexen-1-yl]vinyl]-1-butyl-3,3-dimethyl-indol-1-ium iodide. The mixture was used for the lithographic printing plate precursor according to the present invention without further purification.

[0226] 2. Preparation of Printing Plate Precursors

[0227] Preparation of aluminum support S-01

[0228] The aluminum foil was degreased by spraying it with an aqueous solution containing 26 g / l NaOH at 65°C for 2 seconds and rinsing it with demineralized water for 1.5 seconds. 2 The current density is in the presence of 15g / lHCl, 15g / l SO4 2- ions and 5g / l Al 3+ The foil was electrochemically roughened in an aqueous solution containing 5.5 g / l NaOH for 10 seconds. The aluminum foil was then decontaminated by etching it at 36°C for 2 seconds with an aqueous solution containing 5.5 g / l NaOH and rinsing with demineralized water for 2 seconds. Subsequently, the aluminum foil was decontaminated at a temperature of 50°C and a temperature of 17 A / dm 2The foil was anodized during 15 seconds at a current density of 145 g / l of sulfuric acid in an aqueous solution, then washed with demineralized water for 11 seconds and dried at 120° C. for 5 seconds.

[0229] The carrier thus obtained has a surface roughness Ra characteristic of 0.35-0.4 μm (measured with an interferometer NT1100) and a surface roughness of 3.0 g / m 2 The weight of oxides.

[0230] Preparation of comparative printing plate PP-01 and inventive printing plate PP-02

[0231] Photopolymerizable layer

[0232] Photopolymerizable layers PL-01 and PL-02 were prepared by coating the components defined in Table 1, dissolved in a mixture of 35% by volume MEK and 65% by volume Dowanol PM (1-methoxy-2-propanol, available from DOWCHEMICAL Company), onto the above-mentioned support S-01. The coating solution was applied to a wet coating thickness of 30 μm and then dried in a circulation oven at 120° C. for 1 minute.

[0233] Table 1: Composition of the photosensitive layer

[0234]

[0235] (1) FST 510 is the reaction product of 1 mole of 2,2,4-trimethylhexamethylene diisocyanate and 2 moles of hydroxyethyl methacrylate and is available from AZ Electronics as an 82 wt% solution in MEK;

[0236] (2) CN 104 is an epoxy acrylate oligomer commercially available from Arkema;

[0237] (3) Ini-01 is 4-hydroxyphenyl-tribromomethyl sulfone

[0238] (4) IR-01 (available from FEW CHEMICALS) and IR-02 are infrared absorbing compounds represented by the following structures, where the R group substitutions are summarized in Table 2:

[0239]

[0240] Table 2: R groups of different IR dyes

[0241] IR dyes R Group R' group IR-01 <![CDATA[-C2H5]]> <![CDATA[-C2H5]]> IR-02 <![CDATA[-C2H5]]> <![CDATA[-C 10 H 21 ]]>

[0242] (5) Ruco Coat EC4811 is a nonionic aliphatic polyether polyurethane commercially available from Rudolf GmbH;

[0243] (6) Tegoglide 410 is a polyether siloxane copolymer commercially available from Evonik Resource Efficiency GmbH;

[0244] (7) JPA 528 is polyethylene glycol monomethacrylate acid phosphate, commercially available from Johoku Chemical Co., Ltd.;

[0245] (8) Albritect CP 30, a copolymer of vinylphosphonic acid and acrylic acid, commercially available from Rhodia as a 20 wt% aqueous dispersion;

[0246] (9) Aerosil R972 is a hydrophobic fumed silica commercially available from Evonik Resource Efficiency GmbH.

[0247] Protective outer coating

[0248] On top of the photosensitive layer, an aqueous solution having the composition defined in Table 3 was applied (40 μm wet film) and dried for 2 minutes at 110° C. Printing plate precursors PP-01 and PP-02 were obtained (see Table 4 below).

[0249] Table 3: Composition of protective outer coating OC

[0250]

[0251] (1) Mowiol 4-88™ is a partially hydrolyzed polyvinyl alcohol commercially available from Kuraray;

[0252] (2) Diofan A050 is a polyvinylidene chloride commercially available from Solvay;

[0253] (3) Acticide LA1206™ is a biocide commercially available from Thor;

[0254] (4) Lutensol A8™ is a surfactant commercially available from BASF;

[0255] (5) IR-03 is an infrared absorbing dye represented by the following structure:

[0256]

[0257] Table 4: Printing plate precursors PPP-01 and PPP-02

[0258]

[0259] 3. Crystal Formation

[0260] Crystallization test

[0261] The resulting printing plate precursor was then treated with contact pressure using a rubber wheel (2 cm diameter) to simulate roller pressure and initiate crystal formation of the IR dye in the imageable layer. Following the contact pressure, the printing plate precursor was subjected to a steam chamber test in Dowanol PM (a saturated atmosphere of 2-methoxypropanol). After 3 days of treatment, the lithographic printing plate precursor was visually inspected for IR dye crystal formation at 8x optical magnification.

[0262] Furthermore, each sample of the printing plate precursors PPP-01 and PPP-02 was subjected to an accelerated aging test in a climate chamber at 40° C. and 80% relative humidity for 14 days. After this aging test, the precursor samples were visually inspected for crystal formation of the IR dye at 8x optical magnification.

[0263] Crystallization test results

[0264] The results of crystal formation (ie blooming) in the coatings of the printing plate precursors PPP-01 and PPP-02 are summarized in Table 5.

[0265] Table 5: Results of crystal formation tests

[0266]

[0267] *Rating based on the number of crystals observed after 3 days in a steam chamber test, **Rating based on the number of crystals observed after 14 days in a climate chamber at 40°C and 80% relative humidity (RH);

[0268] A = no crystals observed;

[0269] B = some crystals observed;

[0270] C = Many crystals observed.

[0271] The results in Table 5 show that the comparative printing plate precursor PPP-01 had a lot of crystal formation after 3 days of exposure in the steam chamber and 14 days in the climate chamber, whereas the inventive printing plate precursor PPP-02 had no crystal formation after these tests.

[0272] Imaging

[0273] Aged and non-aged printing plate precursors PPP-01 and PPP-02 were then processed at 2400 dpi using a high power Creo 40WTE38 Thermal Platesetter™ (200 lpi Agfa Balanced Screening (ABS)) at 130 mJ / cm 2 Energy density imaging, platesetters are commercially available from Kodak and are equipped with an 830 nm IR laser diode.

[0274] ΔE measurement

[0275] Laboratory measurements were performed using a GretagMacBeth SpectroEye reflectance spectrophotometer, set to D50 (illuminant), 2° (observer), no color filters; 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 and non-image areas:

[0276]

[0277] The higher the total color difference ΔE, the better the contrast achieved. The contrast between image and non-image areas causes the printed image to appear.

[0278] ΔE measurement results

[0279] The ΔE measurement results are given in Table 6.

[0280] Table 6: ΔE measurement results

[0281]

[0282] The results in Table 6 show that the inventive printing plate precursor PPP-02 produces a printed image with significantly higher contrast than the image contrast of the comparative printing plate precursor PPP-01.

[0283] print

[0284] After imaging, the printing plates were mounted on a Heidelberg GTO 46 printing press. Each print run was initiated using K+E Skinnex 800SPEED IK black ink (trademark of BASF Druckfarben GmbH) and a fountain solution of 4% by weight Prima FS303SF (trademark of Agfa Graphics) and 8% isopropyl alcohol in water. A compressible blanket was used and printing was performed on uncoated offset paper.

[0285] Before paper feeding, 10 revolutions of printing were performed using only the dampening system, followed by 5 revolutions using only the inking roller. The printing results are summarized in Table 7.

[0286] Clearance and run length print performance results

[0287] The results of the clearance and run length printing performance are summarized in Table 7.

[0288] Table 7: Clearance and Run Strength Results

[0289]

[0290] *Printing run is a rating of the printing stability of a printing plate

[0291] A = No wear after 40,000 impressions

[0292] B = some wear after 40.000 prints;

[0293] C = Severe wear after 40,000 prints

[0294] **Climate chamber exposure, see above

[0295] The printing performance results in Table 7 show:

[0296] - The inventive and comparative printing plates have good on-press developability (clearance);

[0297] - The non-aged inventive and non-aged comparative printing plates have good press run properties, whereas the press run properties of the comparative printing plates after aging deteriorate more than those of the inventive printing plates.

Claims

1. A lithographic printing plate precursor comprising a coating on a support, the coating comprising a photopolymerizable layer comprising a polymerizable compound, a photoinitiator and an infrared absorbing dye having a structure according to formula I in Z and Z' independently represent -S-, -CR a R b - or -CH=CH-; R a and R b represents an alkyl group, an aralkyl group or an aryl group; A represents halogen, optionally substituted aliphatic hydrocarbon group, optionally substituted (hetero)aryl group or -NR 1 R 2 , where R 1 and R 2 independently represents hydrogen, optionally substituted aliphatic hydrocarbon group or optionally substituted (hetero)aryl group and / or combinations thereof; Q represents the atoms necessary for ring formation; T and T' independently represent hydrogen, alkyl, halogen, alkoxy, cyano, -CO2R n 、-CONR k R m 、-SO2R n 、-SO2NR o R p or an optionally substituted cyclized benzene ring, wherein R k 、R m represents hydrogen, an optionally substituted alkyl or aryl group, R n represents an optionally substituted alkyl or aryl group, and R o and R p represents hydrogen, optionally substituted alkyl or aryl; W - represents a counterion to obtain a neutral compound; and R z and R z’ independently represents an optionally substituted alkyl group; It is characterized by R z and R z’ are different.

2. A printing plate precursor according to claim 1, wherein R a and R b represents an alkyl group.

3. A printing plate precursor according to claim 2, wherein R a and R b It represents a methyl or ethyl group.

4. A printing plate precursor according to claim 1 , wherein Q is represented by formula II, III or IV: in Ry and Ry' independently represent hydrogen, optionally substituted alkyl, aralkyl, alkaryl or aryl, or the atoms necessary to form a cyclic structure.

5. A printing plate precursor according to any one of claims 1 to 4, wherein R z and R z’ Independently represents C1 to C 15 A linear or branched alkyl group of carbon atoms, provided that R z and R z’ are different.

6. A printing plate precursor according to any one of claims 1 to 4, wherein Rz represents methyl or ethyl, and Rz' represents octyl, nonyl or decyl.

7. A printing plate precursor according to any one of claims 1 to 4, wherein the coating further comprises a top layer provided on the photopolymerizable layer.

8. A printing plate precursor according to claim 7, wherein the top layer comprises a second infrared absorbing dye, the second infrared absorbing dye comprising a thermally cleavable group, which is converted into a group that is a stronger electron donor when exposed to heat and / or IR radiation and is capable of forming a printed image when exposed to heat and / or IR radiation.

9. A printing plate precursor according to claim 8, wherein the second infrared absorbing dye is represented by formula VI in Ar 1 and Ar 2 W independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted cyclized benzene ring, 1 and W 2 independently represents a sulfur atom, an oxygen atom, NR*, wherein R* represents an optionally substituted alkyl group, NH or -CM 10 M 11 Group, where M 10 and M 11 independently represent an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; M 1 and M 2 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or the atoms necessary to form an optionally substituted cyclic structure comprising an optionally substituted cyclized benzene ring; M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group; M 5 、M 6 、M 7 and M 8 represents hydrogen, halogen or an optionally substituted aliphatic hydrocarbon group, M 9 is converted into a chemical reaction induced by exposure to IR radiation or heat 9 is a more powerful electron donor group; and the transition provides an increase in the cumulative light absorption of the dye between 350 and 700 nm; and optionally one or more counterions to obtain a neutral compound.

10. A printing plate precursor according to claim 9, wherein said M 1 and M 2 Together they comprise the atoms necessary to form an optionally substituted cyclic structure comprising an optionally substituted cyclized benzene ring.

11. A printing plate precursor according to claim 9, wherein the cyclic structure comprises a 5- or 6-membered ring.

12. A printing plate precursor according to claim 9, wherein the cyclic structure comprises a 5-membered ring.

13. The printing plate precursor according to claim 9, wherein the cyclic structure comprises a 5-membered ring having a cyclic structure of 5 carbon atoms.

14. A method for preparing a printing plate precursor, the method comprising the steps of: - coating on the support (i) a photopolymerizable layer comprising a polymerizable compound, a photoinitiator and an infrared absorbing dye as defined in any one of claims 1 to 13, and - Allow the precursor to dry.

15. A method for preparing a printing plate, the method comprising the steps of: - exposing a printing plate precursor as defined in any one of the preceding claims 1 to 13 to heat and / or IR radiation in an image-wise manner, thereby forming a lithographic printing image consisting of image areas and non-image areas, - developing the exposed precursor.

16. A method according to claim 15, wherein the precursor is developed by mounting the precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding fountain solution and / or ink to the precursor.

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