Lithographic printing plate precursors

Abstract

A lithographic printing plate precursor comprising a photopolymerizable coating and an overcoat which is passivated after imagewise exposure and preheating of the precursor.

Description

Technical Field

[0001] The present invention relates to novel overcoat compositions for improving the sunlight stability of photopolymer printing plates. Background Art

[0002] Lithographic printing presses use a so-called printing master, for example a printing plate mounted on a cylinder of the printing press. The master carries a lithographic image on its surface, and the print is obtained by applying ink to the image and then transferring the ink from the master to a receiving material (usually paper). In conventional so-called "wet" lithography, ink and an aqueous fountain solution (also called dampening 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 driographic printing, the lithographic image consists of areas that accept ink and areas that resist ink adhesion (ink repelling), and in the waterless driographic printing process, only ink is supplied to the master.

[0003] So-called "analog" printing plates are usually obtained by first applying the so-called computer-to-film (CtF) method, in which various prepress steps, such as font selection, scanning, color separation, screening, color registration, typesetting and imposition, are completed digitally, and each color selection is transferred to a printing film using an imagesetter. After processing, the film can be used as a mask for the exposure of an imaging material called a printing plate precursor, and a printing plate that can be used as a master is obtained after processing the printing plate. Since about 1995, the so-called "computer-to-plate" (CtP) method has received a lot of attention. This method (also called "direct platemaking") bypasses the production of film because the digital file is directly transferred to the printing plate precursor with the help of a plate recorder. The printing plate precursor used for CtP is usually called a digital printing plate.

[0004] The support of the lithographic printing plate is usually an aluminum support which has a hydrophilic surface or is provided with a hydrophilic layer thereon. Such a hydrophilic surface and / or hydrophilic layer should improve the water receptivity of the non-printing areas of the lithographic printing plate and the repulsion of the printing ink in these areas. During development, the soluble part of the coating should be easily removed, whereby the surface of the support remains free of residues, so that a clean background area is obtained during printing.

[0005] Digital plates can be roughly divided into three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates containing a photopolymerizable composition that hardens when exposed to light; and (iii) thermal plates, whose imaging mechanism is triggered by heat or the conversion of light to heat.

[0006] Photopolymer printing plates rely on a working mechanism whereby a coating (which generally comprises a free-radically polymerizable compound) hardens upon exposure to light, optionally followed by a heating step to enhance or accelerate the process. "Hardening" means that the coating becomes insoluble or non-dispersible in the developer solution and can be achieved by polymerization and / or crosslinking of the photosensitive coating upon exposure to light. Conventional photopolymer printing plates are exposed with ultraviolet light from a lamp through a separate photomask film having a predetermined imaging pattern, which is placed between the light source and the printing plate. Nowadays, laser sources are generally used to imagewise expose a printing plate precursor that is sensitive to the corresponding laser wavelength, which allows the photomask film to be eliminated. However, such laser-imageable printing plates often have the disadvantage of limited indoor light stability and / or should have a high photosensitivity (sensitivity) due to the limited power of current laser imagers. Photopolymer printing plate precursors may be sensitive to blue, green or red light (ie, wavelengths in the range 450-750 nm), to violet light (ie, wavelengths in the range 350-450 nm), or to infrared light (ie, wavelengths in the range 750-1500 nm).

[0007] Violet laser-sensitive photopolymer printing plates commonly used in the newspaper market usually have high sensitivity because of the fast imaging speed or photosensitivity required. Since such high-sensitivity printing plates usually have limited white room light stability, violet laser-sensitive printing plates usually require yellow light to operate. In fact, printing plates with faster photosensitivity usually have a shorter operating window under white room light, while printing plates with longer operating windows under white room light usually have slower photosensitivity.

[0008] Photopolymer printing plates typically contain polymerizable monomers, a binder, a photoinitiator, and a sensitizing dye. Typically, a top layer or protective overcoat is required over the imageable layer to act as an oxygen barrier layer to maintain the photosensitivity of the printing plate. The top layer typically includes a water-soluble polymer or a water-swellable polymer, such as, for example, polyvinyl alcohol. In addition to acting as a barrier to oxygen, the top layer should preferably be easily removable during washout and sufficiently transparent to actinic radiation (e.g., 300-450 nm, or 450-750 nm, or 750-1500 nm).

[0009] The classical workflow of a photopolymer printing plate involves first an exposure step of the photopolymer printing plate precursor in a plate-setter, followed by an optional preheating step, a washing step of the protective outer coating, an alkaline development step, and a rinsing and gumming step. In the past few years, there has been a clear move towards a simplified workflow, in which the washing and gumming steps are performed in a single step. Alternatively, on-press processing has become very popular, in which the printing plate is mounted on a printing press and the coating is developed by interaction with the fountain solution and ink supplied to the cylinder during the operation of the printing press. During the first operation of the printing press, the non-image area is removed from the support and the non-printing area of ​​the printing plate is thereby defined. Therefore, on-press developable printing plates are designed to be developed on a lithographic printing press in a printing room that is usually under white room light. However, as discussed above, the photopolymer printing plates sensitive to violet lasers disclosed in the prior art generally have limited white room light stability and must be operated in "safe" light. "Safe" light means light having a specific wavelength range that does not cause hardening of the coating for at least the time required for continuous operation and washing of the printing plate. For most light-sensitive coatings, safe light is yellow or red.

[0010] There is great interest in the art for photopolymer printing plates sensitive to violet lasers having both fast photospeeds and a wide operating window under white room light conditions.

[0011] US 8,148,048 discloses a method for mechanically stripping the outer coating of a rapid lithographic printing plate comprising a substrate, a photosensitive layer and an outer coating, thereby improving the daylight stability of the printing plate.

[0012] US 6,420,089 discloses a printing plate precursor which is developable on-press with the aid of fountain solution and ink in daylight or similar conditions and which comprises an overcoat comprising a sulphonated azo dye which does not reduce the photosensitivity of the precursor nor other properties such as developability, ink receptivity and storage stability.

[0013] US 2009 / 0274979 discloses a device for removing the outer coating of a photosensitive printing plate, which includes a structure for contacting the printing plate with water, preferably under the condition of brushing or rubbing. The device for removing the outer coating is preferably connected to a laser imager so that the printing plate can be imaged on the laser imager and then directly transferred to the device for removing the outer coating to remove the outer coating.

[0014] US 2015 / 0177618 discloses the use of a light-blocking material that prevents a portion of indoor light in the wavelength range of 200 to 780 nm from reaching a photosensitive layer, thereby achieving improved indoor light stability.

[0015] US 2009 / 170040 discloses a method of treating a laser-exposed photosensitive lithographic printing plate with a passivation solution and then subjecting it to overall irradiation, wherein the passivation solution can passivate the light hardening ability of the photosensitive layer in the non-hardened area, so that overall irradiation after laser exposure does not cause hardening of the photosensitive layer in the non-hardened area.

[0016] US 2008 / 280232 discloses laser exposed lithographic printing plates which can be freely handled under white room light during press development.The printing plate is passivated by applying a passivating agent, heat or general radiation to the plate as a whole which does not cause hardening of the photosensitive layer.

[0017] Designing laser-sensitive, preferably on-press processable, photopolymer printing plates having both fast photospeed and a wide operating window (in terms of time) under normal pressroom lighting conditions (ie, white room light) remains a challenge. SUMMARY OF THE INVENTION

[0019] It is an object of the present invention to provide negative-working, preferably press-processable lithographic printing plate precursors comprising a photopolymerizable layer which is characterized by a high photospeed and which can be freely handled after exposure and before / during development under white room light.

[0020] This object is achieved by a printing plate precursor as defined in claim 1 and by the preferred embodiments defined in the dependent claims. The invention has the particular feature that the printing plate material comprises an outer coating which is at least partially permeable to oxygen after imagewise exposure and heating.

[0021] Another object of the present invention is to provide a method for preparing a lithographic printing plate, comprising the steps of:

[0022] - imagewise exposing the printing plate precursor comprising the coating as defined above to light radiation;

[0023] - heating the exposed precursor;

[0024] - Developing the exposed precursor.

[0025] 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.

[0026] Implementation Description

[0027] External coating

[0028] The lithographic precursor of the present invention comprises a photopolymerizable coating on a support and a top layer provided thereon. The top layer acts as an oxygen barrier layer and comprises a water-soluble or water-swellable polymer or a binder.

[0029] The top layer should be an effective oxygen barrier layer that is well adhered to the photopolymerizable layer or optional other layers of the coating. The top layer is also referred to hereinafter as a "protective layer", "overcoat layer or overcoat". In addition to acting as a barrier to oxygen, the overcoat is preferably easily removable during washout and sufficiently transparent to actinic radiation (e.g., 300-450 nm or 700-900 nm or 750-1200 nm).

[0030] The outer coating can be any film-forming material, which is substantially transparent to imaging light and at least comprises a compound capable of producing volatile decomposition products (gases) when heated. The temperature at which the compound can produce volatile decomposition products (gases) is preferably between 80°C and 300°C, more preferably between 100°C and 250°C, and most preferably between 120°C and 200°C. The application period (i.e., residence time) of the heat is preferably between 2 seconds and 30 minutes, more preferably between 10 seconds and 15 minutes, and most preferably between 15 seconds and 10 minutes. The heating step (also referred to as "preheating step") can be implemented in a preheating unit comprising a heating element, the heating element comprising a heating source, such as, for example, a radiation source (IR lamp), heated air or a heated roller.

[0031] In this heat treatment step, the gases formed (at least partially) reduce the oxygen barrier properties of the top layer, as a result of which the white room light stability of the printing plate precursor is significantly enhanced. In fact, the oxygen at least partially passivates the coating, thereby substantially preventing (unwanted) polymerization of the non-image areas before development due to exposure of these areas to white room light. Unwanted polymerization at the non-image areas may lead to graying, i.e., inadequate cleaning behavior, which leads to on-press toning, i.e., ink receptivity in the non-image areas.

[0032] The term "passivation" means that the photosensitive layer has a reduced hardening rate or even becomes unable to harden under white room light. Thus, the photosensitive layer can be hardened under room light before the heating or passivation step and becomes unable to harden or has a reduced hardening rate under room light after the heating or passivation step.

[0033] "Hardening" means that the coating becomes insoluble or non-dispersible in a developing solution and can be achieved by polymerization and / or cross-linking of the photosensitive coating.

[0034] In addition to passivating the unexposed areas, this heating step may also enhance or accelerate the polymerization and / or cross-linking reactions at the exposed (hardened) areas.

[0035] The compound capable of producing volatile decomposition products is preferably a thermally decomposable compound that produces at least one gas when heated. The gas is preferably non-condensable at room temperature, such as, for example, carbon dioxide, carbon monoxide, hydrogen and / or ammonia; or an inert gas, such as nitrogen, is produced. The most preferred compounds are so-called blowing agents; more information about blowing agents can be found in George Wypych's "Handbook of foaming and blowing agents", 1st edition, February 2017, Chapter 10. These compounds capable of producing volatile decomposition products are preferably included in the coating at a level of between 10 wt% and 60 wt%, more preferably between 15 wt% and 50 wt%, and most preferably between 20 wt% and 40 wt%. Specific compounds include, but are not limited to, for example, carbonates, such as ammonium carbonate and ammonium bicarbonate; and carbamates, such as ammonium carbamate, which undergo decomposition at about 60°C to produce carbon dioxide and / or ammonia; and hydrazides, such as benzenesulfonyl hydrazide or p-toluenesulfonyl hydrazide, which decompose to form nitrogen. The compounds mentioned are preferably initially dissolved or dispersed, so that they are added in the form of a solution.

[0036] The outer coating preferably further comprises a hydrophilic binder. Preferred binders that can be used in the top layer are disclosed in WO2005 / 029190 (page 36, line 3 to page 39, line 25), US 2007 / 0020563 (paragraph

[0158] ) and EP 1288 720 (paragraphs

[0148] and

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

[0037] The most preferred binder for the top layer is polyvinyl alcohol and / or a derivative of polyvinyl alcohol. The polyvinyl alcohol preferably has a degree of hydrolysis in the range of 74 mol% to 99 mol%, more preferably in the range of 88 mol% to 98 mol%. The weight average molecular weight of the polyvinyl alcohol can be measured by the viscosity of an aqueous solution (4 wt%) at 20°C as defined in DIN 53 015, and the viscosity value is preferably in the range of 3 to 26, more preferably in the range of 3 to 15, and most preferably in the range of 3 to 10.

[0038] Mixtures of polyvinyl alcohols and / or polyvinyl alcohol derivatives having different hydrolysis and viscosity values ​​may also be used, for example a combination of two or more water-soluble polymers, for example a combination of polyvinyl alcohol and polyvinyl pyrrolidone. Modified polyvinyl alcohols (e.g. polyvinyl alcohols having carboxyl and / or sulfonic acid groups) may also be used, preferably together with unmodified polyvinyl alcohol.

[0039] The coating thickness of the top layer is preferably between 0.25 g / m 2 and 1.75 g / m 2More preferably, between 0.25 g / m 2 and 1.3 g / m 2 between 0.25 g / m 2 and 1.0 g / m 2 In a more preferred embodiment of the present invention, the optional top layer has a thickness of between 0.25 g / m 2 and 1.75 g / m 2 and comprising polyvinyl alcohol having a degree of hydrolysis ranging from 74 mol% to 99 mol% and a viscosity value as defined above ranging from 3 to 26.

[0040] The outer coating may contain other ingredients, such as anionic surfactants, for example sodium alkyl sulfates or alkyl sulfonates, sodium dioctyl sulfosuccinate, sodium dodecylbenzenesulfonate and ammonium lauryl sulfate; amphoteric surfactants, such as alkylaminocarboxylates and alkylamino-dicarboxylates; nonionic surfactants, such as polyethylene glycol, polypropylene glycol and copolymers of ethylene glycol and propylene glycol; polysiloxane surfactants, perfluorocarbon surfactants, alkylphenyl ethylene oxide condensates, alkoxylated alkylenediamines disclosed in EP 1 085 380 (paragraphs

[0021] and

[0022] ); glycerol, inorganic particles, acids, pigments, etc. Various organic or inorganic microparticles can be added to the outer coating, for example to reduce the viscosity or moisture sensitivity of the printing plate. The acid is preferably water-soluble and has a low molecular weight. The acid can be an inorganic acid or an organic acid as disclosed in EP 2 149 071, page 27, lines 1-21. The outer coating may optionally include other ingredients, for example, matting agents or wetting agents as disclosed in EP 2 916 171 and incorporated herein by reference.

[0041] The outer coating may be formed on the printing plate by coating a solution or dispersion, or by laminating a polymer film. Coating may be performed by any known method, such as roller coating, slot coating, curtain coating, Mayer rod coating, dip coating or spraying; preferably roller coating or slot coating. The applied outer coating is preferably further dried to remove water and / or any solvent by forced hot air drying, radiation drying or a combination of forced air drying and radiation drying. The solution or dispersion containing a film-forming polymer suitable for forming the outer coating may be a water-soluble polymer solution (containing a water-soluble polymer) or a polymer dispersion (including polymer emulsions and latexes, containing water-insoluble polymers, with or without the addition of water-soluble polymers), preferably a water-soluble polymer solution. Various additives such as other surfactants, wetting agents, defoamers, leveling agents and dispersants may be added to the outer coating to facilitate, for example, the coating and / or development process.

[0042] The polymer film suitable for lamination onto the photosensitive layer to form the outer coating layer can be any polymer film, for example, a polymer film based on polyester (including polyethylene terephthalate), polyethylene, polypropylene, ethylene / propylene copolymer, polystyrene, polyvinyl butyrol, polynitrile, polyamide, polyimide, polyvinyl chloride, polycarbonate, polymethyl methacrylate, acrylate / styrene copolymer, methacrylate / styrene copolymer, polyurethane, epoxy-amine polymer, nylon, Teflon, cellulose and its film-forming derivatives, and polyvinyl acetate. Polyester, polyethylene and polypropylene are preferred polymer films; and polyester is more preferred.

[0043] The overcoat is preferably removed on-press using a fountain solution during on-press development.

[0044] Lithographic printing plate precursors

[0045] 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 represent areas with a higher affinity for aqueous solutions than for oleophilic inks; areas with hydrophobic properties represent areas with a higher affinity for oleophilic inks than for aqueous solutions. "Hardening" means that the coating becomes insoluble or non-dispersible in the developer solution, and can be achieved by polymerization and / or crosslinking of the photosensitive coating, which can be enhanced by a heating step as described above.

[0046] Support

[0047] The lithographic printing plate used in the present invention comprises a support having a hydrophilic surface or being provided with a hydrophilic layer. The support is preferably a roughened and anodized aluminum support, which is well 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 roughening step is generally expressed as the arithmetic mean centerline roughness Ra (ISO 4287 / 1 or DIN 4762), and may vary between 0.05 μm and 1.5 μm. The aluminum substrate of the present invention has preferably below 0.45 μm, more preferably below 0.40 μm, and most preferably below 0.30 μm Ra value. The lower limit of the Ra value is preferably about 0.1 μm. More details about the preferred Ra value of the roughened and anodized aluminum support surface are described in EP 1 356 926. The Al2O3 layer is formed by anodizing the aluminum support and the anode weight (Al2O3 formed on the aluminum surface, g / m 2 ) between 1 g / m 2 and 8 g / m 2 The anode weight is preferably ≥3 g / m 2 , more preferably ≥3.5 g / m 2 , and most preferably ≥4.0 g / m 2 .

[0048] The roughened and anodized aluminum support can be subjected to a so-called post-anodization treatment to improve the hydrophilic properties of its surface. For example, the surface of the aluminum support can be silicated by treating it with a sodium silicate solution at an elevated temperature (e.g., 95°C). Alternatively, a phosphate treatment can be applied, which involves treating the aluminum oxide surface with a phosphate solution that may further contain an inorganic fluoride. In addition, the aluminum oxide surface can be rinsed with citric acid or a citrate solution. The treatment can be carried out at room temperature or can be carried out at a slightly elevated temperature of about 30 to 50°C. Other interesting treatments include rinsing the aluminum oxide surface with a bicarbonate solution. Still in addition, the aluminum oxide surface can be treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphate esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfates of polyvinyl alcohol, and acetals of polyvinyl alcohol formed by reacting with sulfonated aliphatic aldehydes.

[0049] In the present invention, post-treatment of the aluminum oxide surface with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfates of polyvinyl alcohol and / or acetals of polyvinyl alcohol formed by reaction with sulfonated aliphatic aldehydes is particularly preferred. Post-treatment of the aluminum oxide surface with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol and / or polyvinylsulfonic acid is more preferred. Most preferred is treatment of the aluminum oxide surface with polyvinylphosphonic acid.

[0050] Another useful post-anodic treatment may be with a solution of polyacrylic acid or a polymer comprising at least 30 mol % of acrylic acid monomer units, such as GLASCOL E15 (polyacrylic acid, commercially available from Ciba Speciality Chemicals).

[0051] Alternatively, the support may be treated with adhesion-promoting compounds, such as those described in EP 1 788 434 (paragraph

[0010] ) and WO 2013 / 182328.

[0052] Besides aluminium supports it is also possible to use plastic supports, for example polyester supports, which are provided with one or more hydrophilic layers as disclosed, for example, in EP 1 025 992.

[0053] Photopolymer coating

[0054] The coating has at least one layer comprising a photopolymerizable composition, which layer is also referred to as a "photopolymerizable layer". The photopolymerizable layer preferably has a density of preferably between 0.2 g / m 2 ~5.0 g / m 2 range, more preferably between 0.4 g / m 2 ~3.0 g / m 2 range, most preferably between 0.6 g / m 2 ~2.2 g / m 2 The coating may include further layers between the support and the photopolymerizable layer, such as, for example, an adhesion improving layer.

[0055] The photopolymerizable composition preferably comprises a polymerizable compound. The polymerizable compound may be a monomer or oligomer comprising at least one epoxy or vinyl ether functional group, and the polymerization initiator is a Bronsted acid generator capable of generating a free acid, optionally in the presence of a sensitizer. Suitable multifunctional epoxy monomers and suitable Bronsted acid generators are disclosed in WO2014 / 198820

[048] and

[049] .

[0056] The polymerizable compound is most preferably a polymerizable monomer or oligomer comprising at least one olefinic terminal group, hereinafter also referred to as a "free radical polymerizable monomer." Polymerization involves linking the free radical polymerizable monomers together.

[0057] Suitable free radical polymerizable monomers are disclosed in

[0050] and

[0051] of WO 2014 / 198820 and are incorporated herein by reference.

[0058] Initiator

[0059] In a highly preferred embodiment, the coating preferably contains a free radical initiator, which is capable of generating free radicals directly upon exposure or in the presence of a sensitizer. Suitable free radical initiators are described in WO 2005 / 111727 (from page 15, line 17 to page 16, line 11) and EP 1 091 247, and may include, for example, hexaaryl-bisimidazole compounds (HABI; dimers of triaryl-imidazoles), aromatic ketones, aromatic onium salts, organic peroxides, thio compounds, keto-oxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, and other compounds having carbon-halogen bonds.

[0060] The coating (preferably a photopolymerizable layer) may also contain a coinitiator. Typically, a coinitiator is used in combination with a free radical initiator. Suitable coinitiators for use in 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 following references (including cited literature): KK Dietliker's Chemistry & Technology UV & EB formulation for coatings, inks & paints – Volume 3 – Photoinitiators for Free Radical and Cationic Polymerisation – PKT Oldring, ed. – 1991 – ISBN 0 947798161. Specific co-initiators as described in EP 107 792 may be present in the photopolymerizable layer to further increase the sensitivity. Preferred co-initiators are disclosed in EP 2 916 171

[0051] and are incorporated herein by reference.

[0061] Very high sensitivity can be obtained by including a sensitizer (such as, for example, 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. Other preferred sensitizers are sensitizers that absorb blue light, green light or red light, which have an absorption spectrum between 450 nm and 750 nm. Useful sensitizers can be selected from sensitizing dyes 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 and EP 1 288 720.

[0062] Infrared absorbing compounds

[0063] In another preferred embodiment, the coating may include an infrared absorbing compound. The IR absorbing compound may be an infrared light absorbing dye or pigment. Preferably, an infrared light absorbing dye, also referred to herein as an IR dye. The infrared light absorbing dye preferably has an absorption spectrum between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm. The IR absorbing compound absorbs infrared light and converts the absorbed energy into heat.

[0064] The concentration of the IR dye is preferably 0.25 wt% to 25.0 wt%, more preferably 0.5 wt% to 20.0 wt%, most preferably 1.0 wt% to 10.0 wt%, relative to the total dry weight of the coating.

[0065] Infrared absorbing compounds may be present in the photopolymerizable layer and / or the outer coating and / or in optional other layers. Preferred IR absorbing compounds are dyes such as cyanines, merocyanines, indoanilines, oxonols, pyrilium and squarilium dyes, or pigments such as carbon black. Examples of suitable IR absorbers are described in, for example, EP 823 327, EP 978 376, EP 1 029 667, EP 1 053 868, EP 1 093 934, WO 97 / 39894 and WO 00 / 29214. Particularly preferred dyes are heptamethinecyane dyes, especially dyes disclosed in EP 1 359 008, paragraphs

[0030] to

[0032] .

[0066] Adhesives

[0067] The coating preferably includes an adhesive, preferably, the adhesive is present in the photopolymerizable layer. The adhesive may be selected from a wide range of organic polymers. Combinations of different adhesives may also be used. Useful adhesives are described in WO2005 / 111727, page 17, line 21 to page 19, line 30, EP 1 043 627, paragraph

[0013] , and WO2005 / 029187, page 16, line 26 to page 18, line 11. Suitable adhesives include hydrophilic adhesives, such as homopolymers and copolymers of vinyl alcohol, acrylamide, hydroxymethyl acrylamide, hydroxymethyl methacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate; and maleic anhydride / vinyl methyl ether copolymers.

[0068] Other ingredients

[0069] Optionally, the coating may further contain additional ingredients. These ingredients may be present in the photopolymerizable layer or in the optional other layers.

[0070] For example, 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 EP 2 916 171

[0053] -

[0056] are incorporated herein by reference.

[0071] The photopolymerizable layer may also contain inhibitors. Particular inhibitors for use in photopolymer coatings are disclosed in US Pat. No. 6,410,205, EP 1 288 720 and EP 1 749 240.

[0072] The photopolymerizable layer may further comprise an adhesion promoting compound, which can improve the adhesion between the coating and the support and the durability of the printing plate during printing. The adhesion promoting compound is a compound capable of interacting with the support, preferably a compound having an addition polymerizable ethylenically unsaturated bond and a functional group capable of interacting with the support, such as a phosphate group, a phosphonate group and a trialkoxysilane group. The compound may be present in the photopolymerizable layer or in an intermediate layer between the support and the photopolymerizable layer. Suitable examples thereof are disclosed in

[0010] of EP 1 788 434, WO 2013 / 182328, EP 851 299, EP 1 091 251, US 2004 / 214105, EP 1 491 356, US 2005 / 39620, EP 1 495 866, EP 1 500 498, EP 1 520 694 and EP 1 557 262, EP 2 212 746 and EP 2007 / 059379.

[0073] Various surfactants may be added to the photopolymerizable layer to allow or enhance the developability of the precursor; particularly development using a gum solution. Both high molecular weight surfactants and low molecular weight surfactants, such as nonionic surfactants, are preferred. Further details are described in EP 2 916 171

[0059] and are incorporated herein by reference.

[0074] Preferably, the coating includes a printout agent, i.e., a compound that can change the color of the coating when exposed. After the precursor is imagewise exposed, a visible image can be produced, also referred to as a "printout image". The printout agent can be a compound described in paragraphs

[0116] to

[0119] on pages 19 and 20 of EP-A-1 491 356 and paragraphs

[0168] to

[0172] on page 17 of US 2005 / 008971. A preferred printout agent is a compound described in EP 1 765 592 (from line 1 on page 9 to line 27 on page 20). More preferably, an IR dye as described in EP 1 736 312 (from line 32 on page 5 to line 9 on page 32). The contrast of the image formed after imagewise exposure and washing enables the end user to immediately determine whether the precursor has been exposed and washed, to distinguish between different color selections and to check the image quality on the printing plate precursor. The color type of the colorant may also be important in order to obtain a good visual contrast for a human observer.Preferred colors of the colorant are cyan or blue, ie under blue we understand colors that appear blue to the human eye.

[0075] Method for preparing a lithographic printing plate precursor

[0076] According to the present invention, there is also provided a method for preparing a negative-working lithographic printing plate, comprising the steps of: imagewise exposing a printing plate precursor of the present invention, and then developing the imagewise exposed precursor so that the unexposed areas are dissolved in a developer solution. The lithographic printing plate precursor may be prepared by: (i) applying a coating as described above on a support as described above; and (ii) drying the precursor.

[0077] Exposure steps

[0078] The printing plate precursor is preferably exposed to laser light. Preferably, the imagewise exposure step is performed off-press in a plate-setter, i.e., an exposure apparatus suitable for imagewise exposure of the precursor with a laser (e.g., a laser diode emitting at about 830 nm; a Nd YAG laser emitting at about 1060 nm; a violet laser emitting at about 400 nm; or a gas laser, such as 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. Preferably, the precursor is imagewise exposed by a laser emitting IR light (i.e., with a wavelength in the range of 750 nm to 1500 nm) or violet light (i.e., with a wavelength in the range of 350 nm to 450 nm), most preferably by a laser emitting violet light.

[0079] Preheating Steps

[0080] After the exposure step, the precursor is preheated as described above, whereby the compounds capable of generating volatile decomposition products generate gas, and the polymerization and / or crosslinking reaction of the photopolymerizable layer may also be enhanced. The heating is preferably at a temperature between 80° C. and 300° C., more preferably between 100° C. and 250° C., and most preferably between 120° C. and 200° C. The application period (i.e., the residence time) of the heat is preferably between 2 seconds and 30 minutes, more preferably between 10 seconds and 15 minutes, and most preferably between 15 seconds and 10 minutes. The heating step may be implemented in a preheating unit comprising a heating element, such as, for example, an IR lamp, a UV lamp, heated air or a heated roller.

[0081] Development Steps

[0082] After the exposure step and the preheating step, the printing plate precursor may be rinsed. Before developing the imaged precursor, a pre-rinsing step may be performed to remove protective oxygen barriers or top coatings. The pre-rinsing step may be performed in a separate device, or by manually rinsing the imaged precursor with water, or the pre-rinsing step may be performed in a washing unit integrated into a developer for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details about the washing step are described in EP 1 788 434

[0026] .

[0083] During the development step, the unexposed areas of the image-recording layer are at least partially removed without substantially removing the exposed areas. The washing liquid (also referred to as developer) can be applied to the printing plate by hand or in an automatic washing device, for example by rubbing with an impregnated pad, by dipping, immersing, coating, spinning, spraying, pouring. The treatment with the washing 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 at a temperature of between 20° C. and 40° C. in an automated washing unit.

[0084] In a highly preferred embodiment, the washing step as described above is replaced by an on-press washing, whereby the imaged precursor is mounted on the printing press and washed on-press by rotating the plate cylinder while supplying dampening solution and / or ink to the coating of the precursor to remove unexposed areas from the support. In a preferred embodiment, during the start-up of the printing press, only dampening solution is supplied to the printing plate. After several revolutions of the plate cylinder (preferably less than 50 revolutions, and most preferably less than 5 revolutions), the ink supply is also switched on. In alternative embodiments, the supply of dampening solution and ink may be started simultaneously, or only ink may be supplied during several revolutions before the supply of dampening solution is switched on.

[0085] The washing step may also be implemented by combining the embodiments described above, for example, combining development with a washing solution with on-press development by applying ink and / or fountain solution.

[0086] Flushing fluid

[0087] The rinse solution 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 to 14. Alkaline developers typically contain an alkaline agent to obtain a high pH value, which may be an inorganic alkaline agent or an organic alkaline agent. The developer may comprise 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. alkali metal gluconates) and thickeners (water-soluble or water-dispersible polyhydroxy compounds, such as glycerol or polyethylene glycol).

[0088] Preferably, the rinse solution is a glue solution, whereby during the development step, the unexposed areas of the photopolymerizable layer are removed from the support, and the printing plate is glued in a single step. The development using the glue solution has an additional benefit, that is, due to the residual glue in the unexposed area on the printing plate, no additional glue step is required to protect the support surface in the non-printing area. As a result, the precursor is rinsed and glued in a single step, which involves a simpler developing device compared to a developing device comprising a developer tank, a rinsing section and a glue section. The glue section may include at least one glue unit or may include two or more glue units. These glue units may have the structure of a cascade system, that is, when the glue replenishing solution is added to the second glue unit or when the glue solution in the second glue unit is used only once (that is, preferably by spraying or jetting technology using only the starting glue solution in the second glue unit to develop the precursor), the glue solution used in the second glue unit and present in the second tank overflows from the second tank to the first tank. Further details on such gum development are described in EP 1 788 444 .

[0089] The gum solution is generally an aqueous liquid comprising one or more surface protective compounds which are capable of protecting the lithographic image of the printing plate from contamination (e.g. by oxidation, fingerprints, fats, oils or dust) or damage (e.g. by scratches during operation 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 comprises between 0.005 g / m 2 and 20 g / m 2 More preferably, between 0.010 g / m 2 and 10 g / m 2 between 0.020 g / m 2 and 5 g / m 2 Further details on 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 printing plate precursor is developed and gummed in one step, the washed printing plate does not require post-treatment.

[0090] 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 in, for example, EP 1 342 568

[0008] to

[0022] and WO 2005 / 111727. The glue solution may further comprise an inorganic salt, an anionic surfactant, a wetting agent, a chelating compound, an antiseptic compound, a defoaming compound and / or an ink receptor 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.

[0091] Drying and baking steps

[0092] After the rinsing step, the printing plate may be dried in a drying unit. In a preferred embodiment, the printing plate is dried by heating the printing 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.

[0093] After drying, the printing plate can optionally be heated in a baking unit. More details on heating in a baking unit can be found in WO 2007 / 057348, page 44, line 26 to page 45, line 20.

[0094] The printing plate thus obtained can be used for conventional so-called wet offset printing, in which ink and an aqueous dampening solution are supplied to the printing plate. Another suitable printing method uses a so-called single-fluid ink without dampening solution. Suitable single-fluid inks have been described in US 4,045,232, US 4,981,517 and US 6,140,392. In a most preferred embodiment, as described in WO 00 / 32705, the single-fluid ink comprises an ink phase (also called a hydrophobic phase or an oleophilic phase) and a polyol phase. Example

[0095] Preparation of lithographic supports S-01 and S-02

[0096] Support S-01

[0097] 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 rinsing with demineralized water for 1.5 seconds. 2 At a current density of 15 g / lHCl, 15 g / l SO4 2- ions and 5 g / l Al 3+The foil was electrochemically roughened using alternating current in an aqueous solution of ions for 10 seconds. The foil was then stripped of the black film by etching with an aqueous solution containing 5.5 g / l NaOH at 36°C for 2 seconds and rinsed with demineralized water for 2 seconds. The foil was then subjected to a temperature of 50°C and 17 A / dm 2 The sample was subjected to anodization in an aqueous solution containing 145 g / l sulfuric acid at a current density of 1.5 Å for 15 seconds, then washed with demineralized water for 11 seconds and dried at 120° C. for 5 seconds.

[0098] The support thus obtained is characterized by a surface roughness Ra of 0.35-0.4 μm (measured using interferometer NT1100) and a surface roughness of 3.0 g / m 2 The anode weight.

[0099] Support S-02

[0100] 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 rinsing with demineralized water for 1.5 seconds. 2 At a current density of 15 g / lHCl, 15 g / l SO4 2- ions and 5 g / l Al 3+ The foil was electrochemically roughened using alternating current in an aqueous solution of ions for 10 seconds. The foil was then stripped of the black film by etching with an aqueous solution containing 5.5 g / l NaOH at 36°C for 2 seconds and rinsed with demineralized water for 2 seconds. The foil was then subjected to a temperature of 50°C and 17 A / dm 2 The samples were anodized in an aqueous solution containing 145 g / l sulfuric acid for 15 s at a current density of 200 Å, then washed with demineralized water for 11 s and post-treated with a solution containing 1.1 g / l PVPA at 70°C for 3 s (by spraying), rinsed with demineralized water for 1 s and dried at 120°C for 5 s.

[0101] The support thus obtained is characterized by a surface roughness Ra of 0.35-0.4 μm (measured using interferometer NT1100) and a surface roughness of 3.0 g / m 2 The anode weight.

[0102] Preparation of printing plate precursors

[0103] Photosensitive layer

[0104] Printing plate precursors were prepared by coating the photopolymerizable layers PL-01 and PL-02 defined in Table 1 dissolved in a mixture of 36% by volume of MEK and 64% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOWCHEMICAL Company) onto the supports S-01 and S-02 described above. The coating solutions PL-01 and PL-02 were applied at a wet coating thickness of 30 μm and then dried in a circulation oven at 120° C. for 1 minute.

[0105] Table 1: Dry coating weight of photosensitive layers PL-01 and PL-02

[0106]

[0107] (1) Tegoglide 410™ is a surfactant commercially available from Evonik Tego Chemie GmbH;

[0108]

[0109] (2) GOHSEFIMER L 5407 is commercially available from ALDRICH;

[0110]

[0111] (3) FST510 is a diurethane dimethacrylate commercially available from AZ Electronic;

[0112]

[0113] (4) SR368 is commercially available from Sartomer;

[0114]

[0115] (5) HABI is 2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole commercially available from SUMITOMO;

[0116]

[0117] (6) Fluomix is ​​a mixture of purple sensitizers consisting of the following compounds:

[0118]

[0119] 15 wt%

[0120]

[0121] 38 wt%

[0122]

[0123] 47 wt%

[0124] (7) MBI is 2-mercapto-benzimidazole;

[0125] (8) Aerosil 150 is commercially available from DEGUSSA and has a specific surface area (BET) of 135-165 m 2 / g of silica particles;

[0126] (9) DISI 43236 / 2 is a pigment dispersion of 20% MK9352 (DCC Blue A3RN, Indanthrone Blue, commercially available from DOMINION COLOUR CORPORATION) and 20% DISPERBYK 182 (commercially available from BYK Chemie GmbH) in Dowanol PM (1-methoxy-2-propanol commercially available from DOW CHEMICAL Company);

[0127] (10) Albritect CP30 is a poly(acrylic acid) poly(vinylphosphonic acid) 70 / 30 copolymer commercially available from RHODIA;

[0128] (11) Sipomer PAM 100 is a phosphate ester of polyethylene glycol monomethacrylate commercially available from RHODIA.

[0129] Top floor

[0130] An aqueous solution comprising the composition as defined in Table 2 was coated (40 μm wet thickness) on top of the photosensitive layer and dried at 120° C. for 2 minutes.

[0131] Table 2: Composition of topcoats OC-01 to OC-04

[0132]

[0133] (1) Mowiol 4-88™ is a partially hydrolyzed polyvinyl alcohol commercially available from Kuraray;

[0134] (2) IF-01 is a dispersion of 10% (synthetic compound) and 5% Poval 4 88 commercially available from Kuraray;

[0135] (3) IF-02 is a dispersion of 10% Celogen OT (p.p'-oxybis(benzenesulfonylhydrazide), commercially available from Chempoint) and 5% Poval 4 88 (commercially available from Kuraray);

[0136]

[0137]

[0138]

[0139] (4) Lutensol A8™ is a surfactant commercially available from BASF.

[0140] Printing plate precursors PPP-01 to PPP-06 were obtained. Table 3 gives an overview of the inventive and comparative printing plate precursors PPP-01 to PPP-06.

[0141] Table 3: Overview of printing plate precursors PPP-01 to PPP-06

[0142]

[0143] 3. Heat Treatment

[0144] The printing plate precursors PPP-01 to PPP-06 were treated in a circulation oven at 160°C for 5 minutes. Each printing plate was placed in the same position of the heating furnace and removed from the heating furnace after the heat treatment. Heraeus UT 6120 furnace with forced air circulation TM Used for heat treatment.

[0145] When the printing plate precursor has cooled to room temperature, the obtained printing plate precursor is exposed to white light (see below).

[0146] Unheated printing plate precursor PPP-06 (reference example) was also exposed to white light (see below).

[0147] White light stability test

[0148] In a first step, the printing plate precursor was covered with a grey wedge (comprising steps of a constant of 0.15) and subsequently exposed to light (fluorescent tube TL-D58W / 840) for a period of 5 minutes. The illumination generated by the fluorescent tube (measured at the printing plate surface) was about 1000 Lux.

[0149] Table 4 gives the correlation between the steps on the gray wedge, the transmission of light and the amount of Lux. For example, step 1 has no filter and the light passes completely, which produces 1000 Lux, while in step 2 only 71% of the light passes, which produces 708 Lux.

[0150] Table 4: Correlation between steps on the grey wedge and the resulting transmission

[0151]

[0152] In a second step, after exposure (5 minutes), the plate precursor obtained is washed (see below) and the steps of the wedges where no polymerization of the coating has occurred are determined. The areas on the support where polymerization has occurred are visible as colored areas. The lower the step value, the better the white light stability of the printing plate precursor. In practice, the best results are obtained when no steps are observed or in other words no polymerization has occurred at step 1, which means that the sample is stable for at least 5 minutes at 1000 Lux. If, for example, the step of the wedge where no polymerization has occurred is step 4, the sample is stable for 5 minutes at 350 Lux (35% of the total illumination).

[0153] White light stability is defined as the amount of Lux multiplied by the exposure time, so the white light stability of the latter example is 350 Lux x 5 minutes = 1750 Lux*min.

[0154] development

[0155] The printing plate precursors PPP-01 to PPP-06 were processed in an Azura C95 processor™ available from Agfa Graphics NV (speed 60 cm / min at 21° C.) using Violet CF GUM-NP™ commercially available from Agfa Graphics NV to remove the coating in the non-image areas from the support. Printing plates PP-01 to PP-06 were obtained.

[0156] result

[0157] The results of the white light stability test are summarized in Table 5 below.

[0158] Table 5: White light stability of printed plates PP-01 to PP-05*

[0159]

[0160] * Heat treatment for 5 minutes

[0161] The results in Table 5 show that the white light stability of the inventive printing plates (ie, PP-03 and PP-05) that include the compounds of the present invention in the overcoat layer is significantly higher than that of the comparative printing plates (PP-01 and PP-04).

[0162] The effect of the support

[0163] The white light stability results of the printing plate PP-06 are given in Table 6. PP-06 was prepared using support S-02, which was post-treated with a hydrophilic polymer. This example includes a printing plate that has not been heat treated as a reference example.

[0164] Table 6: White light stability of printed plate PP-06

[0165]

[0166] The results in Table 6 show that PP-06 has a photostability of greater than 5000 Lux after heat treatment and illustrate the significant effect of post-treatment of the support.

Claims

1. A method for preparing a printing plate, comprising the following steps: - imagewise exposing the printing plate precursor to heat and / or light; - heating the precursor; - developing said precursor, The printing plate precursor comprises, on a support, a photopolymerizable coating and an overcoat provided on said coating, characterised in that said overcoat comprises a compound capable of generating volatile decomposition products during the step of heating said precursor.

2. The method according to claim 1, wherein the precursor is exposed with a laser having a wavelength between 350 nm and 450 nm.

3. The method according to claim 1 or 2, wherein the precursor is heated at a temperature between 80°C and 300°C.

4. The method of claim 1 or 2, wherein the precursor is heated for a residence time of between 2 seconds and 30 minutes.

5. The method according to claim 1 or 2, wherein the developing step is performed on-press by mounting the exposed precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while supplying dampening fluid and / or ink to the coating.

6. The method according to claim 1 or 2, wherein the compound capable of producing a volatile decomposition product is a thermally decomposable compound.

7. The method of claim 1 or 2, wherein the compound capable of generating a volatile decomposition product is a blowing agent.

8. The method of claim 6, wherein the thermally decomposable compound is selected from the group consisting of carbonates, carbamates and hydrazides.

9. The method according to claim 6, wherein the thermally decomposable compound is selected from ammonium carbonate, ammonium bicarbonate, ammonium carbamate, benzenesulfonyl hydrazide or p-toluenesulfonyl hydrazide.

10. The method of claim 1 or 2, wherein the volatile decomposition product is a gas that is non-condensable at room temperature.

11. The method of claim 1 or 2, wherein the volatile decomposition product is an inert gas.

12. The method according to claim 1 or 2, wherein the volatile decomposition products are selected from carbon dioxide, carbon monoxide, ammonia, oxygen, hydrogen and / or nitrogen.

13. The method of claim 1 or 2, wherein the compound capable of producing a volatile decomposition product is present in the coating at a level between 10 wt% and 60 wt%.

14. The method of claim 1 or 2, wherein the photopolymerizable coating comprises a polymerizable compound and a free radical initiator.

15. The method according to claim 1 or 2, wherein the support is roughened and anodized aluminum, which is post-treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphate esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfate esters of polyvinyl alcohol and / or acetals of polyvinyl alcohol formed by reaction with sulfonated aliphatic aldehydes.

Images