Lithographic printing plate precursors

Abstract

A lithographic printing plate precursor is disclosed, comprising a support and a coating, the coating comprising (i) a photopolymerizable layer comprising a polymerizable compound and a photoinitiator, and a top layer provided on top of the photopolymerizable layer; characterized in that the top layer comprises a non-polymeric multifunctional compound in an amount between 2 mg / m² and 80 mg / m², which comprises at least two carboxyl groups and / or carboxylate groups.

Description

Technical Field

[0001] The present invention relates to novel lithographic printing plate precursors. Background Art

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

[0003] Lithographic printing plates are usually obtained by image exposure and processing of a radiation-sensitive layer on a lithographic support. Imaging and processing make the so-called lithographic printing plate precursor a printing plate or a base. The radiation-sensitive coating is usually 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, insolubility by crosslinking of polymers or by coagulation of particles of thermoplastic polymer latex, solubilization by destroying intermolecular interactions or by increasing the permeability of the development barrier layer. Although some plate precursors are able to produce lithographic images immediately after exposure, the most popular lithographic printing plate precursors require wet processing because exposure produces a difference in solubility or a difference in dissolution rate in the developer between the exposed and unexposed areas of the coating. In positive-working lithographic printing plate precursors, the exposed areas of the coating dissolve in the developer, while the 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 contain a hydrophobic coating on a hydrophilic support, so that the areas remaining resistant to the developer define the ink-receiving, and therefore printing, areas of the plate, while the hydrophilic support is exposed by dissolution of the coating in the developer in the non-printing areas.

[0004] Photopolymer printing plates rely on a working mechanism whereby a coating, typically comprising a free radical polymerizable compound, hardens upon exposure. "Hardening" refers to the coating becoming insoluble or non-dispersible in a developer 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 may be sensitive to blue, green 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, a heating step is performed after the exposure step to enhance or accelerate the polymerization and / or crosslinking reaction.

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

[0006] 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 an optical densitometer) before mounting the lithographic printing plate on a printing press, lithographic printing plate precursors usually contain a colorant, such as a dye or pigment, in a coating. After processing, such a colorant provides a contrast between the image area containing the colorant and the hydrophilic support from which the coating has been removed, which enables the end user to evaluate the image quality and / or determine whether the precursor has been exposed to light. Furthermore, in addition to allowing the evaluation of image quality, a high contrast between the image and the hydrophilic support is required to obtain good image registration (alignment) of different printing plates in multicolor printing to ensure the sharpness (resolution) of the image and the correct rendering of the colors in the rendered image.

[0007] However, for photopolymer lithographic printing plates that are machined, and therefore without developing the plate before mounting it on a printing press, it is not possible to inspect and authenticate the plate including the colorant in advance. A solution has been provided in the art by including components into the coating that are able to form a so-called "print-out image" upon exposure, i.e. an image that is visible prior to processing. However, in these materials, the photoinitiating system is generally a reactive component that induces the formation of a print-out image upon exposure and can therefore reduce lithographic printing differences.

[0008] For violet-sensitive photopolymer systems, the formation of printed images is disclosed, for example, in US 3,359,109, US 3,042,515, US 4,258,123, US 4,139,390, US 5,141,839, US 5,141,842, US 4,232,106, US 4,425,424, US 5,030,548, US 4,598,036, EP 434 968, WO 96 / 35143 and US 2003 / 68575.

[0009] For thermosensitive photopolymer lithographic printing plates, the formation of printed images is also known. Such plates are usually exposed by IR lasers according to the image, and in addition to IR dyes as light-heat conversion compounds, they often contain dyes that absorb in the visible wavelength range and change color when heated. This color change can be obtained, for example, with a thermally decomposable dye that bleaches when heated, such as disclosed in EP 897 134, EP 925 916, WO 96 / 35143, EP 1 300 241. Alternatively, this heat-induced color change can be the result of the maximum absorption shift of the dye absorbed in the visible wavelength range, as disclosed in EP 1 502 736 and EP 419 095. A problem associated with these prior art materials is that when the printed image is formed by heat-induced reduction in visible light absorption or by conversion from a highly pigmented coating to a weakly pigmented coating, the printed image obtained is characterized by only a low contrast between exposed and unexposed areas, requiring high levels of dye, and / or increasing the risk of contamination of processing equipment (e.g., developer rinse sections).

[0010] Colorants providing contrast obtained from so-called leuco dyes, which change color when pH, temperature, UV light, etc., change, have been widely used in the art. Leuco dye technology involves a conversion between two chemical forms, one of which is colorless. If the color change is caused by, for example, pH or temperature, the conversion is reversible. Irreversible changes are usually based on redox reactions.

[0011] Thermochromic dye technology, for example disclosed in WO2019 / 219560, involves designing IR dyes containing thermally dissociable groups, thereby obtaining a color shift upon exposure to heat and / or light.

[0012] The classical workflow of photopolymer plates comprises: first an exposure step of a photopolymer printing 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 gumming step. However, there is a clear move towards a simplified workflow, in which the preheating step and / or the washing step are eliminated, and in which the processing and gumming steps are carried out in a single step, or in which processing is carried out with a neutral gum and then gumming in a second step. Alternatively, on-machine processing has become very popular, in which the plate is mounted on a printing press and the coating is developed by interaction with a fountain solution and / or ink supplied to the plate during the operation of the printing press. During the first rotation of the printing press, the non-image areas are removed from the support and thus define the non-printing areas of the plate.

[0013] In particular, printing plates designed for on-machine processing require lithographic coatings that are sufficiently dissolved or dispersed on the printing press so that good removal (complete removal of the coating at the non-printing areas of the image) is obtained. Exposure to high humidity environments usually leads to toning (acceptance of ink in the non-image areas). In addition, in addition to excellent removal behavior, the press life of such printing plates should also be optimized. In fact, both press life and removal performance are determined by the interaction between the coating and the substrate: optimal press life requires sufficient adhesion between the substrate and the coating in the image area, while good removal requires minimal interaction of the coating with the substrate in the non-image area during processing. Therefore, maximizing the removal performance may lead to reduced image adhesion and, therefore, reduced press life.

[0014] Therefore, there remains a need for printing plate precursors that meet both press life and clean-up requirements. Summary of the invention

[0015] It was an object of the present invention to provide negative-working printing plate precursors which are characterized by both adequate cleaning behavior and a high press life.

[0016] This object is achieved by the printing plate precursor defined in claim 1 and the preferred embodiments defined in the dependent claims. The printing plate material of the invention has the particular feature that it contains a coating comprising at least two layers, wherein the top layer comprises at least one non-polymeric polyfunctional acid and / or its salt.

[0017] According to the present invention, it has surprisingly been found that by including a non-polymeric polyfunctional acid and / or a salt thereof in the top layer, the removal behavior, i.e. the removal of non-image areas, is significantly improved, whereby the occurrence of toning on paper prints is greatly reduced compared to printing plates without the non-polymeric polyfunctional acid and / or a salt thereof in the top layer. This reduction is particularly observed for printing plate precursors comprising a hydrophobic binder in the top layer.

[0018] 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. Specific embodiments of the present invention are also defined in the dependent claims. DETAILED DESCRIPTION

[0019] Lithographic printing plate precursors

[0020] The lithographic printing plate precursor according to the invention is negative-working, i.e. after exposure and development, the unexposed 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 a support 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 with hydrophilic properties are those with a higher affinity for aqueous solutions than for (oleophilic) inks; areas with hydrophobic properties are those with a higher affinity for (oleophilic) inks than for aqueous solutions.

[0021] "Hardened" 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, hereinafter also referred to as "preheating", the plate precursor is preferably heated at a temperature of about 80° C. to 150° C. and preferably during a residence time of about 5 seconds to 1 minute.

[0022] The coating contains a top layer and at least one layer comprising a photopolymerizable composition, which layer is also referred to as a "photopolymerizable layer". The top layer is provided on top of the photopolymerizable layer. The coating may further comprise other layers, such as intermediate layers, adhesion-improving layers, hydrophilic layers and / or other layers, located between the support and the photopolymerizable layer and / or between the top layer and the photopolymerizable layer.

[0023] The printing plate of the invention is characterized in that it can be exposed at a low energy density, ie 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.

[0024] Top floor

[0025] The coating includes a topcoat or protective topcoat which may act as an oxygen barrier. Low molecular weight species present in air may degrade or even inhibit image formation, and therefore a topcoat is applied to the coating. The topcoat should preferably be easily removed during development, adhere well to the photopolymerizable layer or optional other layers of the coating, and preferably should not inhibit light transmission during exposure. The topcoat is provided on top of the photopolymerizable layer.

[0026] The top layer comprises at least one non-polymeric polyfunctional acid and / or salt thereof, further also referred to as "polyfunctional compound". The polyfunctional compound is a non-polymeric compound and comprises at least two, more preferably at least three, and most preferably three carboxylic acid and / or carboxylate groups. The polyfunctional compound is preferably an aliphatic or aromatic organic polyfunctional compound. In the context of the present invention, a non-polymeric compound means that the compound does not comprise more than two repeating units.

[0027] Suitable aliphatic organic polyfunctional compounds include tartaric acid, succinic acid, malonic acid, oxalic acid, glutaric acid, aconitic acid, trisaccharic acid, citric acid and isocitric acid, and / or salts thereof.

[0028] Suitable aromatic organic polyfunctional compounds include phthalic acid, isophthalic acid, terephthalic acid and trimellitic acid and / or their salts.

[0029] The polyfunctional compound is preferably an aliphatic organic polyfunctional compound.Most preferably, the polyfunctional compound is selected from citric acid or isocitric acid, and / or their salts, ie citrate and isocitrate.

[0030] The polyfunctional compound is present in the top layer in an amount between 2 mg / m² and 80 mg / m². Preferably the polyfunctional compound is present in the top layer in an amount between 4 mg / m² and 50 mg / m². More preferably the polyfunctional compound is present in the top layer in an amount between 6 mg / m² and 30 mg / m². Most preferably the polyfunctional compound is present in the top layer in an amount between 8 mg / m² and 20 mg / m². The concentration of the polyfunctional compound should be sufficient to prevent toning, which occurs especially in humid conditions, and preferably not too high to avoid a negative impact on print life.

[0031] The top layer preferably further comprises a hydrophobic polymer also referred to as a "hydrophobic binder". The hydrophobic polymer is preferably a halogenated polymer comprising at least one halogen. The hydrophobic polymer is a polymer that is preferably insoluble or non-swellable in water (i.e. at about neutral pH). The hydrophobic binder is preferably not crosslinked or only slightly crosslinked. The hydrophobic polymer is preferably in the form of particles. The hydrophobic polymer is preferably used in the top layer in the form of a dispersion; i.e. an emulsion or a suspension. A dispersion of particles in an aqueous medium is preferred.

[0032] A hydrophobic binder in the top layer shows superior resistance to damage in the imaged areas caused by plate handling compared to printing plates without such a binder in the top layer. More details on hydrophobic binders can be found in EP 3 587 113.

[0033] The average particle size is preferably between 10 nm and 1000 nm, more preferably between 25 nm and 250 nm, even more preferably between 30 nm and 200 nm, and most preferably between 50 nm and 175 nm. In this article, particle size is defined as the particle diameter measured by photon correlation spectroscopy, also referred to as quasi-elasticity or dynamic light scattering. This technology is a convenient method for measuring particle size, and the value of the measured particle size is well matched with the particle size measured by transmission electron microscopy (TEM), as disclosed by Stanley D.Duke et al. in Calibration of Spherical Particles by Light Scattering, inTechnical Note-002B, May 15, 2000 (revised on 1 / 3 / 2000, from Particulate Science and Technology 7, a paper disclosed in 223-228 pages (1989)).

[0034] The amount of hydrophobic binder in the top layer is preferably between 30 and 96 wt.-%, more preferably between 40 and 90 wt.-%, and most preferably between 50 and 85 wt.-%, relative to the total weight of the top layer.

[0035] The hydrophobic binder preferably includes at least one monomer unit derived from vinyl and / or vinylidene monomers (preferably vinylidene monomers). The hydrophobic binder can be a homopolymer or a copolymer. Highly preferred copolymers. Copolymers are preferably random copolymers, gradient copolymers or block copolymers. Block copolymers are preferably block copolymers, graft copolymers or star polymers in which the polymer chain is attached to the core. Suitable examples of vinyl monomers include vinyl halides, such as vinyl chloride, vinyl bromide or vinyl iodide. Suitable examples of vinylidene monomers include halogens, such as fluorine, chlorine, bromine or iodine, i.e. vinyl halides, such as vinylidene fluoride, vinylidene chloride, vinylidene bromide or vinylidene iodide.

[0036] In highly preferred embodiments, the hydrophobic binder includes at least one monomer unit derived from a vinylidene monomer. Suitable vinylidene monomers include vinylidene halides, such as vinylidene fluoride, vinylidene chloride, vinylidene bromide and / or vinylidene iodide. Most preferably, the hydrophobic binder includes at least one monomer unit derived from vinylidene fluoride and / or vinylidene chloride, most preferably derived from vinylidene chloride, also referred to herein as PVDC binder. The hydrophobic binder is preferably included between 60 wt % and 95 wt %, more preferably between 65 wt % and 90 wt %, and most preferably between 70 wt % and 85 wt % of the monomer unit derived from a vinylidene monomer.

[0037] It has been observed that the presence of a hydrophobic binder, which is preferably a particulate, non-hygroscopic binder, in the top layer increases the susceptibility to toning behavior when exposed to high humidity. It has surprisingly been found that by including a polyfunctional compound according to the invention in the top layer, the occurrence of such toning is greatly reduced.

[0038] Hydrophobic binder can be synthesized by conventional known methods based on addition polymerization.The number average molecular weight (Mn) for polymer of the present invention is preferably 5.000 g / mol to 1.000.000 g / mol, more preferably 10.000 g / mol to 500.000 g / mol, and most preferably in the scope of 20.000 g / mol to 150.000 g / mol.The weight average molecular weight (Mw) for polymer of the present invention is preferably 10.000 g / mol to 400.000 g / mol, more preferably 70.000 g / mol to 350.000 g / mol, and most preferably in the scope of 100.000 g / mol to 250.000 g / mol.Number average molecular weight (Mn) and weight average molecular weight (Mw) are determined separately by size exclusion chromatography using a mixture of THF and 5 % by weight acetic acid as eluent and polystyrene as calibration standard.

[0039] Hydrophobic binder for the present invention is preferably copolymer, such as gradient copolymer, it shows the gradual change of monomer composition from main one monomer to main another monomer; Or there is no random copolymer of continuous variation in composition.Hydrophobic binder can comprise other monomer units except vinyl and / or vinylidene monomer units as defined above.Hydrophobic binder is preferably included between 5 wt % and 40 wt %, more preferably between 10 wt % and 30 wt %, and most preferably these other monomer units between 15 wt % and 25 wt %.All amounts of monomer as a whole (expressed as wt % in this article) refer to the sum of all monomer units of copolymer.Other monomer units are defined as follows.

[0040] The hydrophobic binder may further comprise one or more other monomer units, which are preferably derived from an acrylate or methacrylate, such as an alkyl (meth)acrylate or an aryl (meth)acrylate, for example methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, benzyl (meth)acrylate, 2-phenylethyl (meth)acrylate, hydroxyethyl (meth)acrylate, phenyl (meth)acrylate or N-(4-methylpyridyl) (meth)acrylate; (meth)acrylic acid; (meth)acrylamide, such as (meth)acrylamide or N-alkyl or N-aryl (meth)acrylamide, such as N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-phenyl (meth)acrylamide, N-benzyl (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, N-(4-hydroxyphenyl) (Meth)acrylamide; (meth)acrylonitrile; styrene; substituted styrenes, such as 2-hydroxy-styrene, 3-hydroxy-styrene or 4-hydroxy-styrene, 4-carboxy-styrene esters; vinylpyridines, such as 2-vinylpyridine, 3-vinylpyridine, 4-vinylpyridine; substituted vinylpyridines, such as 4-methyl-2-vinylpyridine; vinyl acetate, optionally, the copolymerized vinyl acetate monomer units are at least partially hydrolyzed to form alcohol groups, and / or at least partially reacted with aldehyde compounds (such as formaldehyde or butyraldehyde) to form acetal or butyral groups; vinyl alcohol; vinyl nitrile; vinyl acetal; vinyl butyral; vinyl ethers, such as methyl vinyl ether; vinyl amides; N-alkylvinylamides, such as N-methylvinylamide, caprolactam, vinylpyrrolidone; maleic anhydride, maleimides, such as maleimide or N-alkyl or N-arylmaleimide, such as N-benzylmaleimide.

[0041] In a preferred embodiment, the binder further comprises monomer units selected from the group consisting of: (meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate or phenyl (meth)acrylate, hydroxyethyl (meth)acrylate or benzyl (meth)acrylate; vinyl nitrile or vinyl pyrrolidone.

[0042] The hydrophobic binder most preferably comprises methyl acrylate units and / or butyl acrylate units.

[0043] Particularly preferred PVDC polymers are Ixan™ and Diofan™ commercially available from Solvay, PVDC latex commercially available from Asahi-Kasei, Daran™ commercially available from Owensboro, Permax™ commercially available from Lubrizol. Some of these copolymer grades are not water-based, but can be dispersed in water via different dispersion techniques well known in the art to obtain water-based dispersions.

[0044] Other preferred binders that can be used for the top layer are disclosed in WO 2005 / 029190 (page 36, line 3 to page 39, line 25), US 2007 / 0020563 (paragraph

[0158] ) and EP 1 288 720 (paragraphs

[0148] and

[0149] ). The most preferred binders that can be used for the top layer are polyvinyl alcohol or polyvinyl alcohol / polyvinyl acetate copolymers. The copolymers preferably have 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 defined by measuring the viscosity of a 4 wt % aqueous solution at 20° C., as defined in DIN 53 015, and the viscosity value (mPas) is preferably in the range of 2-28, more preferably in the range of 2-15, most preferably in the range of 2-10. It is also possible to use modified polyvinyl alcohols or polyvinyl alcohol / polyvinyl acetate copolymers, for example polyvinyl alcohols or copolymers comprising carboxyl and / or sulfonic acid groups, preferably together with unmodified polyvinyl alcohols or polyvinyl alcohol / polyvinyl acetate copolymers.

[0045] The top layer may optionally include further ingredients, for example inorganic or organic acids, matting agents, surfactants, for example anionic surfactants, for example sodium alkyl sulfates or sodium alkyl sulfonates; amphoteric surfactants, for example alkylaminocarboxylates and alkylaminodicarboxylates; nonionic surfactants, for example polyoxyethylene alkylphenyl ethers, (co)polymers containing siloxane and / or perfluoroalkyl units and / or oligo(alkylene oxide) units; fillers, (organic) waxes, alkoxylated alkylenediamines as disclosed, for example, in EP 1 085 380 (paragraphs

[0021] and

[0022] ), glycerol, inorganic particles, pigments or wetting agents as disclosed in EP 2 916 171 and incorporated herein by reference.

[0046] 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, most preferably between 0.25-1.0 g / m². In a more preferred embodiment of the 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 ranging between 74 mol% and 99 mol% and a viscosity value in the range of 2-26 mPas as defined above.

[0047] Color Precursors

[0048] The coating preferably includes a color precursor that forms a colored compound when exposed to UV light, infrared light and / or heat, thereby forming a printed image. The contrast of the printed image can be defined as the difference between the optical density of the exposed area and the optical density of the unexposed area, and is preferably as high as possible. This enables the end user to immediately determine whether the precursor has been exposed, distinguish between different color options, and check the image quality on the plate precursor. The contrast of the printed image preferably increases with the increase of the optical density in the exposed area, and can be measured in reflectance using an optical densitometer equipped with several filters (e.g., cyan, magenta, yellow). It has been observed that when the color precursor is present in the top layer, defects and / or oxidation spots on the surface of the aluminum support are much more obvious.

[0049] At least one color precursor can be in the photopolymerizable layer and / or in the top layer and / or in the optional other layer. The photopolymerizable layer, the top layer and the optional other layer can include the same or different color precursors. Preferably, the color precursor is present in the top layer.

[0050] Many types of color precursors can be used as color-forming compounds in the present invention, for example: spiropyran leuco dyes, such as spirobenzopyrans (e.g., spiroindolinylbenzopyrans, spirobenzopyranobenzopyrans, 2,2-dialkylchromenes), spironaphthooxazines and spirothianynes; leucoquinone dyes; azines, such as oxazines, diazines, thiazines and phenazines; phthalide and benzopyrrolone type leuco dyes, such as triarylmethanephthalides (e.g., crystal violet lactone), diarylmethanephthalides, monoarylmethanephthalides, heterocyclic substituted phthalides, alkenyl substituted phthalides, bridged phthalides (e.g., spirofluorenephthalide and spirobenzoanthraphthalide) and bisphthalides; fluoran leuco dyes, such as fluorescein, rhodamine and p-aminophenol; triarylmethanes, such as leuco crystal violet; ketoazines; barbituric acid leuco dyes and thiobarbituric acid leuco dyes. Specific color precursors suitable for forming colored dyes upon exposure to heat and / or light are disclosed in WO2019 / 243037

[0035] to

[0060] .

[0051] Color precursors can be added at 0.01-0.1 g / m 2The amount is preferably 0.02-0.08 g / m 2 The amount is preferably 0.025-0.05 g / m 2 The amount exists in the top layer.

[0052] Highly preferred color precursors are IR leuco dyes, also referred to herein as IR thermochromic dyes or infrared thermochromic dyes, which have a predominant absorption in the infrared wavelength range of the electromagnetic spectrum (i.e., in the wavelength range between about 750-1500 nm), and preferably have no significant light absorption in the visible wavelength range of the electromagnetic spectrum (i.e., in the wavelength range between 390-700 nm). Preferred IR thermochromic dyes are disclosed in EP 1 736 312 and EP 1 910 082, and have a partial structure according to the following formula:

[0053]

[0054] wherein * indicates the connection of a part of the structure to the rest of the structure, and wherein at least one R d A group is a group which, by exposure to IR radiation or heat-induced chemical reaction, is converted to a group d A stronger electron donor group; or, at least one of the R d A group is a group which, by exposure to IR radiation or heat-induced chemical reaction, is converted to a group a The electron accepting group is preferably defined as having a Hammett σ parameter value (para-value) greater than or equal to 0.3, and the electron donor group is defined as having a Hammett σ parameter value less than or equal to 0.3. Details about the σ parameter value can be found in Chapman and Shorter, Correlation Analysis in Chemistry, Recent Advances, Plenum, New York, 1978, 439-540 pages.

[0055] The IR thermochromic dye preferably includes at least one thermally dissociable group which is converted to a stronger electron donor group by exposure to IR radiation or a heat-induced chemical reaction. As a result, the exposed IR thermochromic dye absorbs substantially more light in the visible wavelength range of the electromagnetic spectrum, or in other words, the IR thermochromic dye undergoes a blue shift, thereby forming a visible image, also referred to as a printout image.

[0056] The concentration of the IR thermochromic 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.

[0057] The IR thermochromic dye is preferably represented by formula I, II or III:

[0058]

[0059] in

[0060] Ar 1 ,Ar 2 and Ar 3 independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted cyclic benzene ring,

[0061] 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 is independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein M 10 and M 11 Together they contain the atoms necessary to form a ring structure (preferably a 5- or 6-membered ring);

[0062] W 3 represents a sulfur atom or -C(A 3 )=C(A 4 )-group,

[0063] W 4 represents a sulfur atom or -C(A 7 )=C(A 8 )-group,

[0064] M 1 and M 2 M independently represents hydrogen, an optionally substituted aliphatic hydrocarbon group or atoms together comprising the atoms required to form an optionally substituted cyclic structure, preferably M 1 and M 2 Together they comprise the atoms necessary to form an optionally substituted cyclic structure, which may comprise an optionally substituted cyclic 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;

[0065] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group;

[0066] M 5 、M 6 、M 7 and M 8 、M 16 and M 17 independently represent hydrogen, halogen or an optionally substituted aliphatic hydrocarbon group,

[0067] A 1 -A 8 independently represent hydrogen, a halogen atom, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein A 1 and A 2 , A 3 and A 4 , A 5 and A 6 , or A 7 and A 8 Each of which together comprises the atoms required to form a ring structure (preferably a 5- or 6-membered ring);

[0068] M 12 and M 13 and M 14 and M 15 independently represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein said M 14 、M 15 , A 5 or A 7 Two of the M together contain the atoms required to form at least one cyclic structure (preferably a 5- or 6-membered ring); the M 12 、M 13 , A 2 or A 4 Two of the two together contain the atoms required to form at least one cyclic structure (preferably a 5- or 6-membered ring);

[0069] M 9 is converted into a 9 A group that is a stronger electron donor group; and said conversion provides an increase in the integrated light absorption of said dye between 350-700 nm;

[0070] and optionally one or more counterions to obtain a neutral compound.

[0071] IR thermochromic dyes can be neutral dyes, anionic dyes or cationic dyes, depending on the type of substituents and the number of each substituent. In a preferred embodiment, the IR thermochromic dyes of formula I, II or III contain at least one anionic or acidic group, such as -CO 2 H, -CONHSO 2R h 、-SO 2 NHCOR i 、-SO 2 NHSO 2 R j ,-PO 3 H 2 ,-OPO 3 H 2 、-OSO 3 H, -S-SO 3 H or -SO 3 H groups or their corresponding salts, wherein R h , R i and R j is 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 ,Ar 2 or Ar 3 The aromatic hydrocarbon group or cyclic benzene ring, or exists on M 3 、M 4 or M 12 To M 15 On the aliphatic hydrocarbon group of M 12 To M 15 The other substituents may be selected from halogen atoms, cyano groups, sulfone groups, carbonyl groups or carboxylate groups.

[0072] In another preferred embodiment, M 3 、M 4 or M 12 To M 15 At least one of the groups is substituted at the terminal end with at least one of these groups, more preferably with -CO 2 H, -CONHSO 2 -Me, -SO 2 NHCO-Me, -SO 2 NHSO 2 -Me, -PO 3 H 2 or -SO 3 H group or its corresponding salt, wherein Me represents a methyl group.

[0073] In a preferred embodiment, the IR thermochromic dye represented by the above formula I, II or III comprises M represented by one of the following groups: 9 :

[0074] -(N=CR 17 )a-NR 5 -CO-R 4 ,

[0075] -(N=CR 17 )b-NR 5 -SO 2 -R 6 ,

[0076] -(N=CR 17 )c-NR 11 -SO-R 12 ,

[0077] -SO 2 -NR 15 R 16 and

[0078] -S-CH 2 -CR 7 (H) 1-d (R 8 ) d -NR 9 -COOR 18 ,

[0079] in

[0080] a, b, c and d are independently 0 or 1;

[0081] 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 they contain the atoms necessary to form a ring structure;

[0082] R 4 Indicates -OR 10 、-NR 13 R 14 or -CF 3 ;

[0083] Where R 10 represents an optionally substituted (hetero)aryl group or an optionally branched aliphatic hydrocarbon group;

[0084] 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 they contain the atoms necessary to form a ring structure;

[0085] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, -OR 10 、-NR 13R 14 or -CF 3 ;

[0086] R 5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group, SO 3 -group, -COOR 18 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 required to form a ring structure;

[0087] R 11 , R 15 and R 16 independently represent hydrogen, optionally substituted aliphatic hydrocarbon group or optionally substituted (hetero)aryl group, or wherein R 15 and R 16 Together they contain the atoms necessary to form a ring structure;

[0088] R 12 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group;

[0089] R 7 and R 9 independently represent hydrogen or an optionally substituted aliphatic hydrocarbon group;

[0090] 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;

[0091] R 18 represents an optionally substituted (hetero)aryl group or an α-branched aliphatic hydrocarbon group.

[0092] Suitable examples of IR thermochromic dyes for use in the present invention are described in EP 1 910 082, pages 4-8, IRD-001 to IRD-101, and are incorporated herein by reference.

[0093] In a highly preferred embodiment, the IR thermochromic dye is represented by Formula I

[0094]

[0095] Among them, Ar 1 ,Ar 2 , W 1 , W 2 and M 1 To M 9 As defined above.

[0096] Most preferably, the IR thermochromic dye is represented by Formula I, wherein

[0097] Ar 1 and Ar 2 independently represent an optionally substituted aryl group; optionally substituted benzene ring forms a ring,

[0098] W 1 and W 2 Indicates -C(CH 3 ) 2 ;

[0099] M 1 and M 2 together comprise the atoms required to form an optionally substituted 5-membered ring, which may comprise an optionally substituted cyclic benzene ring;

[0100] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group,

[0101] M 5 、M 6 、M 7 and M 8 represents hydrogen;

[0102] M 9 express

[0103] -NR 5 -CO-R 4

[0104] -NR 5 -SO 2 -R 6

[0105] -NR 11 -SO-R 12

[0106] -SO 2 -NR 15 R 16

[0107] Where R 4 , R 5 , R 6 , R 11 , R 12 , R 15 and R 16 As defined above;

[0108] and optionally one or more counterions to obtain an electrically neutral compound. Preferably, the IR dye contains at least one anionic group or an acid group, such as -CO 2 H, -CONHSO2 R h 、-SO 2 NHCOR i 、-SO 2 NHSO 2 R j ,-PO 3 H 2 ,-OPO 3 H 2 、-OSO 3 H, -SO 3 H or -S-SO 3 H groups or their corresponding salts, wherein R h , R i and R j is independently aryl or alkyl. More preferably, M 3 or M 4 At least one of the aliphatic hydrocarbon groups is substituted at a terminal with at least one of the anionic group or the acid group.

[0109] In a highly preferred embodiment, the IR thermochromic dye is represented by Formula I, wherein

[0110] Ar 1 and Ar 2 independently represent optionally substituted aryl;

[0111] W 1 and W 2 Indicates -C(CH 3 ) 2 ;

[0112] M 1 and M 2 together comprise the atoms required to form an optionally substituted 5-membered ring, which may comprise an optionally substituted cyclic benzene ring;

[0113] M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group,

[0114] M 5 、M 6 、M 7 and M 8 represents hydrogen;

[0115] M 9 express

[0116] -NR 5 -CO-R 4

[0117] -NR 5 -SO 2 -R 6

[0118] in

[0119] R 4 For-OR 10 , where R 10 is an optionally branched aliphatic hydrocarbon group;

[0120] R 5 represents hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group,

[0121] R 6 represents an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group; and

[0122] Optionally one or more counterions to obtain a neutral compound.

[0123] Preferably, the IR dye contains at least one anionic group or acid group, such as -CO 2 H, -CONHSO 2 R h 、-SO 2 NHCOR i 、-SO 2 NHSO 2 R j ,-PO 3 H 2 ,-OPO 3 H 2 、-OSO 3 H, -SO 3 H or -S-SO 3 H groups or their corresponding salts, wherein R h , R i and R j is independently aryl or alkyl. More preferably, M 3 or M 4 At least one of the aliphatic hydrocarbon groups is substituted at the terminal with at least one of the anionic groups or acid groups. The salt is preferably an alkali metal salt or an ammonium salt, including a mono- or di- or tri- or tetra-alkylammonium salt.

[0124] Optional counterions for obtaining electrically neutral compounds may be selected, for example, from halogens, sulfonates, perfluorosulfonates, toluenesulfonates, tetrafluoroborate, hexafluorophosphate, arylborates, arylsulfonates; or cations, such as alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkylammonium salts.

[0125] Particularly preferred IR thermochromic dyes are represented by one of the following formulae IV to XI:

[0126]

[0127] in

[0128] X - represents a halogen, a sulfonate, a perfluorosulfonate, a toluenesulfonate, a tetrafluoroborate, a hexafluorophosphate, an arylborate or an arylsulfonate; and

[0129] R 3 , R 3’ independently represents an optionally substituted alkyl group, preferably a methyl or ethyl group; or an ether group, preferably -CH 2 -CH 2 -O-CH 3 ;

[0130]

[0131]

[0132]

[0133] in

[0134] M + = Li + 、Na + , K + NH 4 + , R ’ R ’’ R ’’’ NH + , where R ’ , R ’’ , R ’’’ independently represents hydrogen, optionally substituted alkyl or aryl.

[0135]

[0136]

[0137]

[0138] definition

[0139] 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 (ie, suitable aryl or heteroaryl groups) are described below.

[0140] The term "alkyl" refers in this context to all possible variations for each 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 C 1 -C 6 -alkyl.

[0141] Suitable alkenyl groups are preferably C 2 -C 6 -alkenyl, for example ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, isopropenyl, isobutenyl, isopentenyl, neopentenyl, 1-methylbutenyl, isohexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl.

[0142] Suitable alkynyl groups are preferably C 2 -C 6 -alkynyl; suitable aralkyl groups preferably include one, two, three or more C 1 -C 6 Suitable alkaryl groups are preferably C including aryl groups (preferably phenyl or naphthyl). 1 -C 6 -alkyl.

[0143] The cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic or polycyclic group, meaning one or more rings fused together.

[0144] The example of suitable aryl can be represented by for example optionally substituted phenyl, benzyl, tolyl or o-, m- or p-xylyl, optionally substituted naphthyl, anthracenyl, phenanthryl and / or its combination.Heteroaryl is preferably a monocyclic or polycyclic aromatic ring, which contains carbon atoms and one or more heteroatoms in the ring structure, preferably 1-4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulfur.Its preferred examples include optionally substituted furyl, pyridyl, pyrimidyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thienyl, tetrazolyl, thiazolyl, (1,2,3) triazolyl, (1,2,4) triazolyl, thiadiazolyl, thienyl and / or its combination.

[0145] The cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic or polycyclic group, meaning one or more rings fused together.

[0146] Halogen is selected from fluorine, chlorine, bromine or iodine.

[0147] The term "substituted", in, for example, a substituted alkyl, means that the alkyl group may be substituted with atoms other than the atoms normally present in such groups (i.e., carbon and hydrogen). For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

[0148] The optional substituents on the alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl groups are preferably selected from hydroxy, -Cl, -Br, -I, -OH, -SH, -CN, -NO 2 , alkyl (e.g., methyl or ethyl), alkoxy (e.g., methoxy or ethoxy), aryloxy, carboxylic acid or its alkyl ester, sulfonic acid or its alkyl ester, phosphonic acid or its alkyl ester, phosphate or ester (e.g., alkyl ester, such as methyl or ethyl ester), thioalkyl, thioaryl, thioheteroaryl, -SH, thioether (e.g., thioalkyl or thioaryl), ketone, aldehyde, sulfoxide, sulfone, sulfonate, sulfonamide, amino, vinyl, alkenyl, alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl or heteroalicyclic group and / or combinations thereof.

[0149] Carrier

[0150] 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 granular and anodized aluminum support known in the art. Suitable supports are disclosed, for example, 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 / 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 on the preferred Ra values ​​of the granular and anodized aluminum support surface are described in EP 1 356 926. By anodizing the aluminum support, the Al 2 O 3 layer, and the anode weight (g / m 2 , Al formed on the aluminum surface 2 O 3 ) in the range of 1-8 g / m 2The anode weight is preferably ≥2.0 g / m 2 , more preferably ≥2.5 g / m 2 , and most preferably ≥ 3.0 g / m 2 .

[0151] The granular and anodized aluminum support can be subjected to a so-called post-anodization treatment, for example, treatment with polyvinylphosphonic acid or a derivative thereof, treatment with polyacrylic acid or a derivative thereof, treatment with potassium fluorozirconate or potassium phosphate, treatment with an alkali metal silicate, or a combination thereof. The micropores of the anodized aluminum can be enlarged or sealed, as disclosed in JP2001-253181A or JP2001-322365A. Treatment of the edge of the support as described in, for example, US 2017 / 320351 may be advantageous to prevent the occurrence of printed edges. Alternatively, the support can be treated with an adhesion-promoting compound (such as those described in

[0010] of EP 1 788434 and WO 2013 / 182328). However, for a precursor optimized for use without a preheating step, it is preferred to use a granular and anodized aluminum support without any post-anodization treatment.

[0152] 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 .

[0153] Photopolymer coating

[0154] Photopolymerizable compounds

[0155] 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.

[0156] The photopolymerizable layer comprises at least one polymerizable 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 More preferably, between 0.4 and 3.0 g / m 2 The most preferred range is 0.6-2.2 g / m 2 in the range between.

[0157] 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, also referred to hereinafter as a "free radical polymerizable monomer". Polymerization comprises linking together free radical polymerizable monomers. Suitable free radical polymerizable monomers include, for example, mono- or multifunctional (meth)acrylate monomers (e.g. (meth)acrylates of ethylene glycol, trimethylolpropane, pentaerythritol, ethylene glycol, ethoxylated trimethylolpropane, urethane (meth)acrylates) and oligomeric amine diacrylates. In addition to the (meth)acrylate groups, the (meth)acrylate monomers may also have other ethylenically unsaturated groups or epoxy groups. The (meth)acrylate monomers may also contain acidic (e.g. carboxylic acid or phosphoric acid) or basic (e.g. amine) functionality.

[0158] Suitable free-radically polymerizable monomers are disclosed in

[0042] and

[0050] of EP 2 916 171 and are incorporated herein by reference.

[0159] Initiator

[0160] According to the present invention, any free radical initiator capable of generating free radicals in direct exposure or in the presence of a sensitizer is a suitable initiator. Suitable examples of initiators include onium salts, compounds containing carbon-halogen bonds (e.g., [1,3,5] triazines with trihalomethyl groups), organic peroxides, aromatic ketones, thio compounds, azo polymerization initiators, azides, ketoxime esters, hexaaryl diimidazoles, metallocenes, active ester compounds, borates, and quinone diazides. Among them, from the viewpoint of storage stability, onium salts are preferred, particularly iodonium salts and / or sulfonium salts.

[0161] More particularly suitable free radical initiators include, for example, derivatives of acetophenone (e.g., 2,2-dimethoxy-2-phenylacetophenone and 2-methyl-1-[4-(methylthio)phenyl-2-morpholinopropan-1-one); benzophenone; benzil; ketocoumarins (e.g., 3-benzoyl-7-methoxycoumarin and 7-methoxycoumarin); xanthone; thioxanthone; benzoin or alkyl-substituted anthraquinone; onium salts (e.g., diaryliodonium hexafluoroantimonate, difluoromethanesulfonate); aryliodonium, (4-(2-hydroxytetradecyloxy)-phenyl)phenyliodonium hexafluoroantimonate, 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. Pat. Nos. 5,955,238, 6,037,098, and 5,629,354); borates (e.g., tetrabutylammonium triphenyl(n-butyl)borate , tetraethylammonium triphenyl(n-butyl)borate, diphenyliodonium tetraphenylborate, and triphenylsulfonium triphenyl(n-butyl)borate, and borates as described in U.S. Pat. Nos. 6,232,038 and 6,218,076); haloalkyl-substituted s-triazines (e.g., 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(trichloromethyl)-6-piperonyl-s-triazine and 2,4-bis(trichloromethyl)-6-[(4-ethoxy-ethyleneoxy)-phenyl-1-yl]-s-triazine and s-triazines as described in U.S. Pat. Nos. 5,955,238, 6,037,098, 6,010,824 and 5,629,354); and titanocene (bis(ethyl 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 be used alone or in combination.

[0162] Optionally substituted trihaloalkyl sulfones are particularly preferred initiators, wherein halogen independently represents bromine, chlorine or iodine, and sulfone is a compound containing a sulfonyl functional group attached to two carbon atoms. Tribromomethylphenyl sulfone is the most preferred initiator. More details about this initiator can be found in paragraphs

[0029] to

[0040] of the unpublished co-pending application EP 18163285.2.

[0163] The amount of initiator is generally in the range of 0.1-30 wt%, preferably 0.5-15 wt%, most preferably 2-10 wt%, relative to the total weight of the non-volatile components of the photopolymerizable composition.

[0164] Very high sensitivity can be obtained by combining a fluorescent brightener as a sensitizer and a polymerization initiator.

[0165] 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 including the cited literature: Chemistry & Technology UV & EB formulation for coatings, inks & paints – Volume 3 – Photoinitiators for Free Radical and Cationic Polymerisation by KK Dietliker – PKT Oldring ed. – 1991 – ISBN 0947798161. Specific co-initiators may be present in the photopolymerizable layer to further increase the sensitivity, as described in EP 107 792. Preferred co-initiators are disclosed in EP 2 916 171

[0051] and are incorporated herein by reference.

[0166] Very high sensitivity can be obtained by including a sensitizer (e.g., a fluorescent whitening agent) in the coating. Suitable examples of fluorescent whitening agents as sensitizers are described in WO 2005 / 109103, page 24, line 20 to page 39. Useful sensitizers can be selected from the sensitizing dyes disclosed in US 6,410,205; US 5,049,479; EP 1 079 276, EP 1 369 232, EP 1 369231, EP 1 341 040, US 2003 / 0124460, EP 1 241 002, and EP 1 288 720.

[0167] Specific coinitiators may be present in the photopolymerizable layer to further increase the sensitivity, as described in EP 107 792. Preferred coinitiators are sulfur compounds, especially thiols, such as 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 4-methyl-3-propyl-1,2,4-triazol-5-thione, 4-methyl-3-n-heptyl-1,2,4-triazol-5-thione, 4-phenyl-3-n-heptyl-1,2,4-triazol-5-thione, 4-phenyl-3,5-dithiophene. 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-thiadiazole-2-thione, 5-hexylthio-1,3,4-thiadiazole-2-thione, mercaptophenyltetrazolyl, pentaerythritol mercaptopropionate, 3-mercaptopentyl butyrate, pentaerythritol tetrakis(thioglycolic acid). 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 (e.g. 2-mercaptobenzothiazole).

[0168] The photopolymerizable layer may optionally include a violet or infrared light absorbing dye as a sensitizer. The infrared light absorbing dye absorbs light between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm. Particularly preferred sensitizers are the heptamethine cyanine dyes disclosed in paragraphs

[0030] to

[0032] of EP 1 359 008.

[0169] Binder

[0170] The photopolymerizable layer preferably includes a binder. The binder can be selected from a wide range of organic polymers. Compositions of different binders can also be used. Useful binders are described, for example, in paragraph

[0013] of EP 1 043 627, page 17, line 21 to page 19, line 30 of WO 2005 / 111727, and page 16, line 26 to page 18, line 11 of WO 2005 / 029187.

[0171] A PVDC binder as described above may optionally be present in the photopolymerizable layer.

[0172] The photopolymerizable layer may include discrete particles, i.e., 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 promote the developability of the unexposed areas.

[0173] The thermo-reactive polymer fine particles include a thermo-reactive group 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.

[0174] Specific examples of polymer binders according to this embodiment are described in US 6,899,994, US 2004 / 0260050, US 2005 / 0003285, US 2005 / 0170286, US 2005 / 0123853 and EP 2 916 171 at

[0029] ,

[0030] and

[0031] . Other suitable binders described in EP 2 471 655, EP 2 492 748 and EP 2 660 068 include multifunctional thiols having 6-10 functional groups as a core (central skeleton) and polymer chains connected to the core via sulfide bonds. In addition to the polymer binder of this embodiment, the imageable layer may optionally contain one or more co-binders. Typical co-binders are water-soluble or water-dispersible polymers, such as cellulose derivatives, polyvinyl alcohol, polyacrylic acid, poly(meth)acrylic acid, polyvinyl pyrrolidone, polylactide, polyvinylphosphonic acid, synthetic copolymers, such as copolymers of alkoxy polyethylene glycol (meth)acrylates. Specific examples of co-binders are described in US 2004 / 0260050, US 2005 / 0003285 and US 2005 / 0123853. The printing plate precursor according to this embodiment, its imageable layer comprises a binder and an optional co-binder, and is described in more detail in US 2004 / 0260050, US 2005 / 0003285 and US 2005 / 0123853.

[0175] The average particle size of the polymer fine particles is preferably 0.01 mm to 3.0 mm.Particulate polymers in the form of microcapsules, microgels or reactive microgels are suitable, as disclosed in EP 1 132 200, EP 1 724 112 and US 2004 / 106060.

[0176] Other ingredients

[0177] 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, such as described in, for example, US 7,108,956. More details of suitable spacer particles described in

[0053] to

[0056] of EP 2 916 171 are incorporated herein by reference.

[0178] The photopolymerizable layer may also contain inhibitors. Specific inhibitors for photopolymer coatings are disclosed in US 6,410,205, EP 1 288 720 and EP 1 749 240.

[0179] The photopolymerizable layer may further comprise an adhesion promoting compound. The adhesion promoting compound is a compound that can interact with the carrier, preferably has an ethylenically unsaturated bond that can be addition polymerized and a compound that can interact with the carrier's functional group." interaction " is understood to be a physical and / or chemical reaction or process of each type, thus forming a key between the functional group and the carrier, the key can be a covalent bond, an ionic bond, a complex bond, a coordination bond or a hydrogen bond, and it 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. The adhesion promoting compound described in EP 2 916 171

[0058] is incorporated herein by reference.

[0180] Various surfactants may 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 (e.g. nonionic surfactants) are preferred. Further details are described in

[0059] of EP 2 916 171 and are incorporated herein by reference.

[0181] Exposure Steps

[0182] The printing plate precursor is preferably imagewise exposed off-press in a plate-making machine, i.e. an exposure apparatus adapted to imagewise expose the precursor with a laser, e.g. a laser diode emitting about 830 nm or a Nd YAG laser emitting about 1060 nm, a violet laser emitting about 400 nm or a gas laser, e.g. an Ar laser, or with a digitally modulated UV exposure device (using e.g. a digital mirror device), or by conventional exposure in contact with a mask. In a preferred embodiment of the invention, the precursor is imagewise exposed by a laser emitting IR light or violet light, more preferably by a laser emitting IR light.

[0183] Preheating Steps

[0184] After the exposure step, the precursor may 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 may comprise a heating element, preferably an IR lamp, a UV lamp, heated air or a heated roller. Such a preheating step may be used for a printing plate precursor comprising a photopolymerizable composition to enhance or accelerate the polymerization and / or crosslinking reaction.

[0185] Development Steps

[0186] After the exposure step or the preheating step, when there is a preheating step, the plate precursor can be processed (developed). Before developing the imaged precursor, a pre-rinsing step can be performed, in particular for negative-working lithographic precursors with a protective oxygen barrier or topcoat. This pre-rinsing step can be performed in a separate instrument, or by manually rinsing the imaged precursor with water, or the pre-rinsing step can be performed in a washing unit, which is integrated in the processor for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details about the washing step are described in

[0026] of EP1 788 434.

[0187] During the development step, the unexposed areas of the image-recording layer are at least partially removed, while the exposed areas are substantially not removed. The processing liquid (also called developer) can be applied to the plate by hand or in an automated processing apparatus, for example by rubbing with an impregnated pad, by dipping, immersion, coating, spin coating, spraying, pouring. The 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 preferably also removed. The development is preferably carried out in an automated processing unit at a temperature between 20 and 40°C.

[0188] 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 processed on-press by rotating the plate cylinder while feeding fountain solution and / or ink to the coating of the precursor to remove unexposed areas from the support. In a preferred embodiment, the supply of fountain solution and ink is started simultaneously, or only ink may be supplied during a number of rotations before the supply of fountain solution is switched on. In an alternative embodiment, only fountain solution is supplied to the plate during the start-up of the printing press, and the ink supply is also switched on after a number of rotations of the plate cylinder.

[0189] The processing step 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.

[0190] Processing fluid

[0191] The processing fluid may be an alkaline developer or a solvent-based developer. Suitable alkaline developers have been described in US2005 / 0162505. Alkaline developers are aqueous solutions having a pH of at least 11, more typically at least 12, preferably 12-14. Alkaline developers typically contain an alkaline agent to obtain a high pH value, which may be an inorganic or organic alkaline agent. The developer may contain anionic surfactants, nonionic surfactants and amphoteric surfactants (up to 3% of the total weight of the composition); biocides (antimicrobial agents and / or antifungal agents), defoamers or chelating agents (e.g. alkaline gluconates) and thickeners (water-soluble or water-dispersible polyols, such as glycerol or polyethylene glycol).

[0192] Preferably, the processing liquid is a gum solution, whereby during the development step, the unexposed areas of the photopolymerizable layer are removed from the support and the plate is gummed in a single step. Development with a gum solution has the additional benefit that, due to the residual gum in the unexposed areas on the plate, no additional gumming step is required to protect the support surface in the non-printing area. As a result, the precursor is processed and gummed in a single step, which involves a simpler developing apparatus than a developing apparatus comprising a developer tank, a rinsing 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 the configuration of a cascade system, i.e., when a gum replenishing 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 the precursor is developed in the second gumming unit using only the starting gum solution by preferably a spray or jet technique, the gum solution used in 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 EP1 788 444.

[0193] The gum solution is generally an aqueous liquid containing one or more surface protective compounds which are capable of protecting the lithographic image of the printing plate from contamination, for example by oxidation, fingerprints, fat, oil or dust, or from damage, for example by scratches during the handling of the plate. 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 0.005 to 20 g / m 2 More preferably, between 0.010-10 g / m 2 The most preferred range is 0.020-5 g / m 2 More details about the 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 plate precursor is developed and gummed in one step, no post-treatment of the processed plate is required.

[0194] The preferred pH value of the glue solution is between 3-11, more preferably between 4-10, even more preferably between 5-9, and most preferably between 6-8. Suitable glue solutions are described in for example

[0008] to

[0022] of EP 1 342 568 and WO2005 / 111727. The glue solution may further comprise an inorganic salt, an anionic surfactant, a wetting agent, a chelating compound, a preservative compound, a defoaming compound and / or an ink receptor and / or a combination thereof. More details about these other compositions are described in WO 2007 / 057348, page 11, row 22, to page 14, row 19.

[0195] Drying and baking steps

[0196] After the processing step, the plate may 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.

[0197] After drying, the plate can optionally be heated in a baking unit. Further details on heating in a baking unit can be found in WO 2007 / 057348, page 44, line 26 to page 45, line 20.

[0198] The lithographic printing plate precursor of the invention may be prepared by (i) applying a coating as described above on a support as described above and (ii) drying the precursor. In a preferred embodiment, the support is a granular and anodized support which has not been subjected to any post-anodization treatment as described above.

[0199] According to the present invention, there is also provided a method for preparing a negative-working lithographic printing plate, the method comprising exposing a printing plate precursor imagewise, followed by developing the imagewise exposed precursor so that the unexposed areas are dissolved in a developer solution. The development is preferably carried out 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 a fountain solution and / or ink to the precursor. Optionally, after the imaging step, a heating step is performed to enhance or accelerate the polymerization and / or crosslinking reaction. The lithographic printing plate precursor can be prepared as follows: (i) applying a coating as described above on a support and (ii) drying the precursor. Any coating method can be used to apply one or more coating solutions to the hydrophilic surface of the support. The multilayer coating can be applied by coating / drying each layer continuously or by applying several coating solutions at the same time. In the drying step, the volatile solvent is removed from the coating until the coating is self-supporting and dry to the touch.

[0200] 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 plate. Another suitable printing method uses a so-called single-fluid ink without a fountain 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 comprises an ink phase (also called a hydrophobic phase or an oleophilic phase) and a polyol phase (as described in WO 00 / 32705). Example

[0201] Example 1

[0202] 1. Preparation of Printing Plate Precursors

[0203] Preparation of aluminum support S-01

[0204] 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 demineralized water for 1.5 seconds. 2 At a current density of 15 g / l HCl, 15 g / l SO 4 2- ions and 5 g / l Al 3+ The foil was electrochemically grained using alternating current in an aqueous solution of ions for 10 seconds. The foil was then decontaminated by etching with an aqueous solution containing 5.5 g / l NaOH at 36°C for 2 seconds and rinsing with demineralized water for 2 seconds. The foil was then decontaminated by etching with an aqueous solution containing 5.5 g / l NaOH at 36°C for 2 seconds and rinsing with demineralized water for 2 seconds. 2 The foil was subjected to anodization in an aqueous solution containing 145 g / l of sulfuric acid at a current density of 100 Å for 15 seconds, then washed with demineralized water for 11 seconds and dried at 120° C. for 5 seconds.

[0205] The support thus obtained is characterized by a surface roughness Ra of 0.35-0.4 μm (measured with interferometer NT1100) and an oxide weight of 3.0 g / m 2 .

[0206] Preparation of the printing plates PP-02 to PP-05 according to the invention and the comparative printing plate PP-01

[0207] Photopolymerizable layer

[0208] The printing plate precursor was produced by coating the components defined in Table 1 dissolved in a mixture of 35 volume % MEK and 65 volume % Dowanol PM (1-methoxy-2-propanol, commercially available from DOWCHEMICAL Company) onto the above-mentioned support S-01. The coating solution was applied at a wet coating thickness of 30 μm and then dried in a circulation oven at 120° C. for 1 minute.

[0209] Table 1: Composition of the photosensitive layer

[0210]

[0211] 1) FST 510 is the reaction product of 1 mole of 2,2,4-trimethylhexamethylene diisocyanate and 2 moles of hydroxyethyl methacrylate, commercially available from AZ Electronics as an 82 wt% solution in MEK;

[0212] 2) CN 104 is an epoxy acrylate oligomer commercially available from Arkema;

[0213] 3) Initiator-01 is bis(4-tert-butylphenyl)-iodonium tetraphenylborate

[0214] 4) S2539 is an infrared absorbing dye commercially available from FEW Chemicals

[0215]

[0216] 5) Ruco coat EC4811 is a polyether polyurethane commercially available from Rudolf Chemistry

[0217] 6) Tegoglide 410 is a surfactant commercially available from Evonik Tego Chemie GmbH;

[0218] 7) Sipomer PAM 100 is a methacrylate phosphonate commercially available from Rhodia;

[0219] 8) Albritect CP 30 is a copolymer of vinylphosphonic acid and acrylic acid, commercially available as a 20 wt% aqueous dispersion from Rhodia.

[0220] Protective top coating

[0221] On top of the photosensitive layer, a dispersion in water (40 μm) with the composition defined in Table 2 was coated and dried for 2 minutes at 110° C. Printing plate precursors PP-01 to PP-05 were obtained (Table 3).

[0222] Table 2: Composition of protective top coatings OC-01 to OC-05

[0223]

[0224] 1) Mowiol 4-88 is a partially hydrolyzed polyvinyl alcohol commercially available from Kuraray;

[0225] 2) Lutensol A8™ is a surfactant commercially available from BASF;

[0226] 3) IR-01 is a thermochromic infrared absorbing dye having the following formula:

[0227]

[0228] 4) PVDC-1 is Diofan A050, and PVDC-2 is Diofan A602, both of which are polyvinylidene chloride latexes commercially available from Solvay.

[0229] 2. Humidity treatment

[0230] In order to evaluate the toning behavior of the printing plate precursors when exposed to high relative humidity, the printing plate precursors were subjected to high relative humidity in a climate chamber. The climate chamber parameters were set at 75% relative humidity and a temperature of 26°C, and the printing plate precursors were stored for a period of 5 days. Each printing plate precursor was placed so that the coating surface was maximally exposed to the environment. After a period of five days, the printing plate precursors were removed from the climate chamber and imaged as described below.

[0231] 3. Imaging

[0232] The laser was then processed using a High Power Creo 40W TE38 thermal platesetter commercially available from Kodak and equipped with an 830 nm IR laser diode with an energy density of 120 mJ / cm². TM (200 lpi Agfa Balanced Screening (ABS)) The obtained printing plate precursors with and without humidity treatment were imaged at 2400 dpi.

[0233] 4. Processing and printing

[0234] The imaged printing plates were mounted on a Heidelberg GTO 52 printing press. Each print run was started using K+E Skinnex 800 SPEEDIK black ink (trademark of BASF Druckfarben GmbH) and 4 wt. % Prima FS303 SF (trademark of Agfa Graphics) and 8% isopropyl alcohol in water as a dampening solution. A compressible blanket was used and printing was performed on uncoated offset paper.

[0235] Before feeding the paper, 10 press revolutions were carried out in which only the dampening system was used, followed by 5 revolutions in which the inking rollers were used.

[0236] Up to 250 sheets were printed and the 250th sheet was visually evaluated for the presence of toning (yes = "-" / no = "+") (ie, acceptance of ink in non-image areas).

[0237] 5. Results

[0238] Color Grading

[0239] The results of the visual assessment of the toning in the image area are given in Table 3.

[0240] Table 3: Color grading results

[0241]

[0242] *See above

[0243] **Visual evaluation of the image area of ​​the 250th sheet (see above):

[0244] -: There is toning in the image area

[0245] +: No color correction in the image area

[0246] The results in Table 3 show that after exposing the printing plates according to the invention (PP-02 to PP-05) to a high humidity environment, the printing plates showed no toning in the image area. The comparative printing plate PP-01 showed unacceptable toning in the image area after humidity treatment.

[0247] Printing life

[0248] The press life of each printing plate was evaluated by monitoring the reproduction (density) of a test pattern with a nominal tone value of 40% on the printed paper every 5.000 impressions (200 lpi ABS (Agfa Balanced Screening)) using a Gretag-MacBeth D19C (commercially available from GretagMacbeth AG, magenta filter setting). The press life of each printing plate was defined as the point at which the density of the 40% test pattern began to decrease. The result of the press life test is a measure of the press life of the plate. The results are given in Table 4.

[0249] Table 4: Print life results

[0250]

[0251] Excellent press life results were obtained for printing plates PP-01 to PP-03. Printing plates PP-04 and PP-05 comprising higher amounts of citrate showed reduced press life compared to printing plates comprising lower amounts of citrate.

Claims

1. A negative-working lithographic printing plate precursor comprising an aluminum support and a coating comprising a photopolymerizable layer and a top layer provided on top of the photopolymerizable layer; Features The top layer comprises a non-polymeric multifunctional compound comprising at least two carboxyl and / or carboxylate groups in an amount between 2 mg / m² and 80 mg / m².

2. A printing plate precursor according to claim 1, wherein the photopolymerizable layer further comprises a hydrophobic binder.

3. A printing plate precursor according to claim 2, wherein the non-polymeric multifunctional compound comprises at least three carboxyl and / or carboxylate groups.

4. A printing plate precursor according to any one of claims 2 to 3, wherein the non-polymeric multifunctional compound is an aliphatic organic acid and / or salt.

5. A printing plate precursor according to claim 4, wherein the aliphatic organic acid and / or salt is selected from citric acid, aconitic acid, isocitric acid and triglyceride, and / or salts thereof.

6. A printing plate precursor according to any one of claims 2 to 5, wherein the hydrophobic binder is present in the top layer in an amount between 30% and 96% by weight relative to the total weight of the top layer.

7. A printing plate precursor according to any one of claims 2 to 6, wherein the hydrophobic binder comprises monomer units derived from vinyl monomers and / or vinylidene monomers.

8. A printing plate precursor according to claim 1, wherein the non-polymeric multifunctional compound comprises at least three carboxyl and / or carboxylate groups.

9. A printing plate precursor according to claim 1 or 8, wherein the non-polymeric multifunctional compound is an aliphatic organic acid and / or salt.

10. A printing plate precursor according to claim 9, wherein the aliphatic organic acid and / or salt is selected from citric acid, aconitic acid, isocitric acid and triglyceride, and / or salts thereof.

11. A printing plate precursor according to any one of the preceding claims, wherein the photopolymerizable layer and / or the top layer comprises a colour precursor.

12. A printing plate precursor according to claim 11, wherein the color precursor is an infrared thermochromic dye, which includes a thermally dissociable group, which is converted into a stronger electron donor group when exposed to heat and / or IR radiation and is capable of forming a printed image when exposed to heat and / or IR radiation.

13. The printing plate precursor according to claim 12, wherein the infrared thermochromic dye is represented by formula I: in Ar 1 and Ar 2 independently represent an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group having an optionally substituted cyclic benzene ring, 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 is independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein M 10 and M 11 Together they contain the atoms necessary to form a cyclic structure, preferably a 5- or 6-membered ring; M 1 and M 2 M independently represents hydrogen, an optionally substituted aliphatic hydrocarbon group or together comprises the atoms required to form an optionally substituted cyclic structure, the cyclic structure may comprise an optionally substituted cyclic benzene ring, preferably M 1 and M 2 Together they comprise the atoms necessary to form an optionally substituted cyclic structure, which may comprise an optionally substituted cyclic 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; M 3 and M 4 independently represent an optionally substituted aliphatic hydrocarbon group; M 5 、M 6 、M 7 and M 8 independently represent hydrogen, halogen or an optionally substituted aliphatic hydrocarbon group, M 9 is converted into a 9 A group that is a stronger electron donor group; and said conversion provides an increase in the integrated light absorption of said dye between 350-700 nm; and optionally one or more counterions to obtain a neutral compound.

14. A method for producing a negative-working printing plate precursor comprising the following steps - coating on a support (i) a photopolymerizable layer comprising a polymerizable compound and a photoinitiator, and (ii) a top layer as defined in any one of the preceding claims provided on said photopolymerizable layer, - Drying the precursor.

15. A method for manufacturing a negative-working printing plate comprising the following steps - image-wise exposing a printing plate precursor as defined in any of the preceding claims to heat and / or light radiation, thereby forming a lithographic image consisting of image areas and non-image areas, - developing said exposed precursor.

16. The method according to claim 15, wherein the energy density of the optical radiation is between 75 mJ / cm² and 150 mJ / cm².

17. The method according to claim 15 or 16, 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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