A NEGATIVE-WORKING LITHOGRAPHIC PRINTING PLATE PRECURSOR

Abstract

 A lithographic printing plate precursor is disclosed which includes a support and a coating including a photopolymerisable layer including a polymerisable compound and an infrared absorbing dye, and a protective overcoat layer provided above the photopolymerisable layer which includes a borate compound.

Description

Technical Field

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

Background Art

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

[0003] Lithographic printing masters are generally obtained by the image-wise exposure and processing of a radiation sensitive layer on a lithographic support. Imaging and processing renders the so-called lithographic printing plate precursor into a printing plate or master. Image-wise exposure of the radiation sensitive coating to heat or light, typically by means of a digitally modulated exposure device such as a laser, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing since the exposure produces a difference in solubility or difference in rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working lithographic plate precursors, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working lithographic plate precursors, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most lithographic plate precursors contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, hence printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas.

[0004] Photopolymer printing plates rely on a working-mechanism whereby the coating - which typically includes free radically polymerisable compounds - hardens upon exposure. "Hardens" means that the coating becomes insoluble or non-dispersible in the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating upon exposure to light. Photopolymer plate precursors can be sensitized to blue, green or red light i.e. wavelengths ranging between 450 and 750 nm, to violet light i.e. wavelengths ranging between 350 and 450 nm or to infrared light i.e. wavelengths ranging between 750 and 1500 nm. Optionally, the exposure step is followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction.

[0005] In general, a toplayer or protective overcoat layer over the imageable layer is required to act as an oxygen barrier to provide the desired sensitivity to the plate. A protective overcoat layer typically includes water-soluble or water-swellable polymers such as for example polyvinyl alcohol. Besides acting as barrier for oxygen, the protective overcoat layer should best be easily removable during processing and be sufficiently transparent for actinic radiation, e.g. from 300 to 450 nm or from 450 to 750 nm or from 750 to 1500 nm.

[0006] The classical workflow of photopolymer plates involves first an exposure step of the photopolymer printing plate precursor in a violet or infrared platesetter, followed by an optional pre-heat step, a wash step of the protective overcoat layer, an alkaline developing step, and a rinse and gum step. Over the past years, there is a clear evolution in the direction of a simplified workflow where the pre-heat step and/or wash step are eliminated and where the processing and gumming step are carried out in one single step or where processing is carried out with a neutral gum and then gummed in a second step. Alternatively, on-press processing wherein the plate is mounted on the press and the coating layer is developed by interaction with the fountain and ink that are supplied to the plate during the press run, has become very popular. During the first runs of the press, the non-image areas are removed from the support and thereby define the non-printing areas of the plate.

[0007] Thermal photopolymer printing plates which are based on a heat-induced physical and/or chemical reaction typically contain a heat-sensitive coating comprising an infrared dye as light-to-heat conversion compound. Upon exposure to heat and/or to infrared light, the generated heat triggers the imaging mechanism of the heat-sensitive coating. These infrared dyes exhibit strong absorption in the IR wavelength and are ideally evenly distributed throughout the heat-sensitive coating. However, many of the cationic infrared dyes that are typically used exhibit a strong tendency to crystallize in the presence of borate anions. Negative-working printing plate precursors often contain borate compounds in their coatings for example to improve the sensitivity and press life of the printing or as a counter ion of photoinitiators e.g. iodonium compounds. Due to the crystallization tendency, defects or so-called "artefacts" occur throughout the coating and may deteriorate the lithographic properties of the plate such as for example a lower sensitivity of the printing plate. Disturbed sensitivity results in modified screen rendering - i.e. reduced dot gain - on both the printing plate and on the printed sheet. Moreover, crystallization also leads to severe problems during manufacturing of the precursors; e.g. accumulation of crystallized compounds on the "rollers" for guiding the precursors through the coating facility. Therefore, there is a need to maintain and/or improve the sensitivity of negative-working printing plate precursors including borate compounds without the occurrence of undesirable crystallization.

[0008] WO2006/127313 discloses a negative-working printing plate precursor which includes a radically polymerisable element, an initiator system and a tetraarylborate salt.

[0009] EP 1 467 250 discloses a photosensitive composition containing an infrared absorber, a borate initiator compound including at least one alkyl group, a polymerizable compound, a binder polymer, and a compound having a weight average molecular weight of not more than 3,000 and containing at least one carboxylic acid group.

[0010] EP 3 768 513 and EP 3 768 514 disclose a printing plate material including a coating comprising a photopolymerisable composition including a polymerization initiator which is a trihaloalkyl sulfone initiator and a borate compound.

[0011] WO2013/043421 solves the crystallization problem of salts formed between infrared radiation absorbing cations and tetraphenyl borate anions by selecting specific infrared radiation absorbing dyes which form salts which have a solubility in 2-methoxy propanol at 20°C greater than or equal to 3.5 g/l.

Summary of invention

[0012] The object to maintain and/or improve the sensitivity of negative-working printing plate precursors including borate compounds without the occurrence of undesirable crystallization is realised by the printing plate precursor defined in claim 1 with preferred embodiments defined in the dependent claims. The printing plate precursor has the specific feature that it contains a coating comprising at least two layers; i.e. a photopolymerisable layer including a cationic IR absorbing dye and a toplayer including at least one borate compound.

[0013] According to the current invention, it was surprisingly found that although the borate compound is mainly present in a layer other than the photopolymerisable layer, the sensitivity of the printing plate precursor and press life are maintained, while the formation of aggregates and/or crystals are highly reduced. Indeed, it was observed that the formation of a salt - between the cationic infrared dye and the borate compound - which often exhibits a reduced solubility in the coating and tends to crystallize, is highly reduced. In other words, the separation of the cationic IR dye and the borate compound in two different layers, provides a reduced tendency of the dye to crystallize in the coating during and/or after production.

[0014] 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 invention are also defined in the dependent claims.

Description of embodiments

The lithographic printing plate precursor

[0015] The lithographic printing plate precursor according to the present invention is negative-working, i.e. after exposure and development the non-exposed areas of the coating are removed from the support and define hydrophilic (non-printing or non image) areas, whereas the exposed coating is not removed from the support and defines oleophilic (printing or image) areas. The hydrophilic areas are defined by the support which has a hydrophilic surface or is provided with a hydrophilic layer. The hydrophobic areas are defined by the coating, hardened upon exposing, optionally followed by a heating step. Areas having hydrophilic properties means areas having a higher affinity for an aqueous solution than for an oleophilic ink; areas having hydrophobic properties means areas having a higher affinity for an oleophilic ink than for an aqueous solution.

[0016] "Hardened" means that the coating becomes insoluble or non-dispersible for the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating upon the exposure step, optionally followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction. In this optional heating step, hereinafter also referred to as "pre-heat", the plate precursor is heated, preferably at a temperature of about 80°C to 150°C and preferably during a dwell time of about 5 seconds to 1 minute.

[0017] The coating contains a toplayer - also referred to herein as overcoat or protective overcoat layer - and at least one layer including a photopolymerisable composition, said layer is also referred to as the "photopolymerisable layer". The photopolymerisable layer has a coating thickness preferably ranging between 0.2 and 5.0 g/m2, more preferably between 0.4 and 3.0 g/m2, most preferably between 0.6 and 2.2 g/m2.

[0018] The protective overcoat layer is provided on top of the photopolymerisable layer. The coating may further include other layers such as for example an intermediate layer, located between the support and the photopolymerisable layer and/or between the top layer and the photopolymerisable layer, an adhesion improving layer and/or other layers.

[0019] The printing plate of the present invention is characterized that it can be exposed at a low energy density, i.e. below 190 mJ/m²; preferably between 70 mJ/m² and 150 mJ/m²; more preferably between 75 mJ/m² and 120 mJ/m² and most preferably of maximum 80 mJ/m².

Toplayer or protective overcoat layer

[0020] The coating includes a toplayer or protective overcoat layer which preferably acts as an oxygen barrier layer. Low molecular weight substances present in the air may deteriorate or even inhibit image formation and therefore a protective overcoat layer is applied to the coating.The protective overcoat layer should preferably be easily removable during development, adhere sufficiently to the photopolymerisable layer or optional other layers of the coating and should preferably not inhibit the transmission of light during exposure. The protective overcoat layer is provided on top of the photopolymerisable layer.

[0021] The protective overcoat layer of the current invention includes a borate compound i.e. a chemical compound including a borate anion and a cation as counterion. Preferably the borate anion is a tetrahedral boron anion. The borate compound is preferably represented by the following Formula I:

wherein Rb¹,Rb²,Rb³ and Rb⁴ are independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted aryl or heteroaryl group; and M⁺ is an alkali metal cation such as e.g. Li⁺, Na⁺, K⁺, an optional substituted diaryliodonium, an optional substituted triarylsulfonium, an optional substituted alkoxy- or aryloxypyridinium, or an optional substituted ammonium, preferably according to Formula II:

wherein

R_n¹,R_n² and R_n³ are independently an optionally substituted aliphatic hydrocarbon group, an optionally substituted aryl or heteroaryl group or a halogen atom.

[0022] In a preferred embodiment Rb¹,Rb²,Rb³ and Rb⁴ are independently an optionally substituted aryl or heteroaryl group. More preferably, Rb¹,Rb²,Rb³ and Rb⁴ are independently an optionally substituted aryl group. The aryl is preferably selected from an optionally substituted phenyl, benzyl, tolyl or an ortho- meta- or para-xylyl group, naphtyl, anthracenyl or phenanthrenyl.

[0023] In a highly preferred embodiment the borate compound includes at least one optionally substituted phenyl group, more preferably at least two optionally substituted phenyl groups, even more preferably at least three optionally substituted phenyl groups and most preferably four optionally substituted phenyl groups. Suitable borate compounds are described in US 6,232,038 column 23 to 28.

[0024] The optional substituted diaryliodonium, triarylsulfonium and alkoxy- or aryloxypyridinium borate compounds also function as radical polymerization initiators. Specific examples include optional substituted diphenyliodonium and triphenylsulfonium borate salts more preferred tetraphenylborate salts.

[0025] Suitable ammonium borate compounds include tetrabutylammonium triphenyl(n-butyl)borate and tetraethylammonium triphenyl(n-butyl)borate. Preferred iodonium borate compounds include butt are not limited to, 4-octyloxyphenyl phenyliodonium tetraphenylborate, [ 4-[(2-hydroxytetradecyl)-oxy]phenyl]phenyliodonium tetraphenylborate, bis( 4-t-butylphenyl)iodonium tetraphenylborate, 4-methylphenyl-4 '-hexylphenyliodonium tetraphenylborate, 4-methylphenyl-4' - cyclohexylpheny liodonium tetraphenylborate, bis(t-butylphenyl)iodonium tetrakis(pentafluorophenyl)borate, 4-hexylphenyl-phenyliodonium tetraphenylborate, 4-cyclohexylphenyl-phenyliodonium tetraphenylborate, 2- methyl-4-t-butylphenyl-4 '-methylphenyliodonium tetraphenylborate, 4-methylphenyl-4 '-dodecylphenyliodonium tetrakis (4-fluorophenyl)borate, bis (dodecylphenyl)iodonium tetrakis(pentafluorophenyl)borate, and bis( 4-tbutylphenyl) iodonium tetrakis(I-imidazolyl)borate. Useful compounds include 5 bis( 4-t-butylphenyl)iodonium tetraphenylborate, 4-methylphenyl-4 ' - hexylphenyliodonium tetraphenylborate, 2-methyl-4-t-butylphenyl-4 ' - methylphenyliodonium tetraphenylborate, and 4-methylphenyl-4 ' - cyclohexylphenyliodonium tetraphenylborate. Mixtures of two or more of these compounds can also be used in the initiator composition.

[0026] The borate compound may be used singly or in admixture of two or more thereof. With respect to the content, the borate compound is preferably present in an amount comprised between 0.05 and 30 % by weight, more preferably between 0.1 and 25% by weight, and most preferably from 0.3 and 15% by weight relative to the whole of solids of the overcoat layer.

[0027] The protective overcoat layer may include a heat sensitive color-forming IR dye - i.e an infrared absorbing compound which is capable of forming a colored compound whereby a print-out image is formed upon exposure to infrared light and/or heat. The heat sensitive color-forming IR dye is preferably a thermochromic infrared absorbing dye, also referred to herein as thermochromic IR dye. The thermochromic IR dye has a main absorption in the infrared wavelength range of the electromagnetic spectrum - i.e. a wavelength range between about 750nm and 1500nm - and does preferably not have a substantial light absorption in the visible wavelength range of the electromagnetic spectrum - i.e. a wavelength range between 390nm and 700nm. The heat sensitive color-forming IR dye includes at least one thermocleavable group which is transformed by a chemical reaction, induced by exposure to IR radiation or heat, into a group which is a stronger electron-donor. As a result, the exposed thermochromic IR dye absorbs substantially more light in the visible wavelength range of the electromagnetic spectrum, or in other words, the thermochromic IR dye undergoes a hypsochromic shift whereby a visible image is formed, also referred to as print-out image. The formation of this print-out image is clearly different from a process where a compound changes from an essentially colorless compound into a pale-colored to colored compound. These compounds typically change absorption from the UV wavelength range of the electromagnetic spectrum to the visible wavelength range of the electromagnetic spectrum, i.e. these compounds typically have a batochromic shift. The contrast of the print-out image obtained by such a process is much weaker compared to the color-forming process described above of the heat sensitive color-forming IR dyes.

[0028] The concentration of the thermochromic IR dyes with respect to the total dry weight of the coating, may be from 0.1 %wt to 20.0 %wt, more preferably from 0.5 %wt to 15.0 %wt, most preferred from 1.0 %wt to 10.0 %wt.

[0029] Suitable examples of heat sensitive color-forming IR dyes are described in WO2019/21560 [0032] to [0042] and EP 1 910 082 pages 4 to 8, IRD-001 to IRD-101.

[0030] The protective overcoat layer may further include a binder. Preferred binders which can be used in the protective overcoat layer are polyvinyl alcohol.The polyvinylalcohol has preferably a hydrolysis degree ranging between 74 mol % and 99 mol %, more preferably between 80-98%. The weight average molecular weight of the polyvinylalcohol can be measured by the viscosity of an aqueous solution, 4 % by weight at 20°C as defined in DIN 53 015, and this viscosity number ranges preferably between 2 and 26, more preferably between 2 and 15, most preferably between 2 and 10.

[0031] The protective overcoat layer may include a halogenated polymer which is preferably a hydrophobic polymer, i.e. not soluble or swellable in water at about neutral pH. This binder may be used in the protective overcoat layer in the form of a dispersion; i.e. an emulsion or suspension. The amount of the halogenated binder in the protective overcoat layer may be between 30%wt and 96%wt, more preferably between 40%wt and 90%wt and most preferably between 50%wt and 85%wt. The halogenated binder preferably includes between 60 %wt and 95 %wt monomeric units derived from vinylidene monomers such as vinylidene fluoride, vinylidene chloride, vinylidene bromide and/or vinylidene iodide.

[0032] The protective overcoat layer may optionally include other ingredients such as inorganic or organic acids, matting agents, surfactants such as anionic surfactants, e.g. sodium alkyl sulphate or sodium alkyl sulphonate; amphoteric surfactants, e.g. alkylaminocarboxylate and alkylaminodicarboxylate; non-ionic surfactants, e.g. polyoxyethylene alkyl phenyl ether, (co)polymers comprising siloxane and/or perfluoroalkyl units and/or oligo(alkylene oxide) units; fillers; (organic) waxes; alkoxylated alkylene diamines as for example disclosed in EP 1 085 380 (paragraph [0021] and [0022]); glycerine; inorganic particles; pigments or wetting agents as disclosed in EP 2 916 171.

[0033] The coating thickness of the protective overcoat layer is between 0.10 and 1.75 g/m², preferably between 0.20 and 1.30 g/m², more preferably between 0.25 and 1.0 g/m² and most preferably between 0.30 and 0.80 g/m². In a more preferred embodiment of the present invention, the protective overcoat layer has a coating thickness between 0.25 and 1.75 g/m² and comprises a polyvinylalcohol having a hydrolysis degree ranging between 74 mol % and 99 mol % and a viscosity number as defined above ranging between 2 and 26 mPas.

Photopolymerisable layer

Polymerisable compound

[0034] The photopolymerisable layer of the lithographic printing plate precursor includes a polymerisable compound such as a polymerisable monomer or oligomer including at least one terminal ethylenic group, hereinafter also referred to as "free-radical polymerisable monomer". The polymerisation involves the linking together of the free-radical polymerisable monomers.

[0035] Suitable free-radical polymerisable monomers are disclosed in [0042] and [0050] of EP 2 916 171.

The initiator

[0036] The photopolymerizable layer includes at least one infrared light absorbing dyes which preferably absorbs light between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm. The infrared absorbing dye preferably contains a cyanine cation and a suitable counter ion and is preferably presented by Formula III:

wherein

Z and Z' independently represent -S-, -CRaRb- or -CH=CH-; Ra and Rb represent an alkyl, aralkyl or aryl group; preferably Z and Z' represent -CRaRb- wherein Ra and Rb represent an alkyl group, most preferably a methyl or ethyl group;

A represents a halogen, an optionally substituted aliphatic hydrocarbon group, an optionally substituted (hetero)aryl group or -NR1R2, wherein R1 and R2 independently represent hydrogen, an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, and/or combinations thereof;

Q represents the necessary atoms to form a ring;

T and T' independently represent hydrogen, alkyl, halogen, alkoxy, cyano,-CO₂R_n, -CONR_kR_m, -SO₂R_n, -SO₂NR_oR_p or an optionally substituted annulated benzene ring wherein R_k and R_m represent hydrogen, an optionally substituted alkyl or aryl group, R_n represents an optionally substituted alkyl or aryl group and R_o and R_p represent hydrogen, an optionally substituted alkyl or aryl group;

W- represents a counterion in order to obtain an electrically neutral compound; and

Rz and Rz' independently represent an optionally substituted alkyl group.

[0037] T and T' preferably independently represent hydrogen, an alkyl group such as methyl or ethyl; or an optionally substituted annulated benzene ring. Most preferred, T and T' independently represent an alkyl group such as methyl or ethyl.

[0038] W^- represents a counterion which provides an electrically neutral compound and may be selected from for example a halogen, a sulphonate, a perfluorosulphonate, a tosylate, a hexafluorophosphate, an arylborate, an arylsulphonate.

[0039] Q preferably represents -CHR'-CHR"-, -CR'=CR"- or -CHR'-CHR"-CHR‴- and R', R" and R‴ independently represent hydrogen, an optionally substituted alkyl, cycloalkyl, aralkyl, alkaryl, aryl or heteroaryl group, or R' and R" or R" and R‴ form together a cyclic structure.

[0040] Most preferably Q is represented by Formulae IV, V or VI:

wherein

Ry and Ry' independently represent hydrogen, an optionally substituted alkyl, aralkyl, alkaryl or aryl group or represent the necessary atoms to form a cyclic structure. Preferably, in Formula IV, Ry and Ry' independently represent hydrogen or an optionally substituted alkyl group; and in Formula V Ry and Ry' preferably represent an annulated ring, preferably as presented by Formula VII:

In Formulae IV to VII, * represents the linking positions to the rest of the dye.

[0041] Rz and Rz' independently represent a linear or branched alkyl group. The linear or branched alkyl group is preferably an alkyl group including C1 to C15 carbon atoms, more preferably the linear or branched alkyl group is an alkyl group including C2 to C12 carbon atoms and most preferably the linear or branched alkyl group is an alkyl group including C4 to C10 carbon atoms. The linear or branched alkyl group is preferably selected from a methyl, ethyl, propyl (n-propyl, i-propyl), butyl (n-butyl, i-butyl, t-butyl), pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl or dodecyl group.

[0042] The photopolymerisable layer of the lithographic printing plate precursor may further comprise one or more photoinitiators. In the event that the overcoat layer does not include a borate compound which functions as a photoinitiator, it is preferred that a photoinitiator is present in the photopolymerisable layer. Any free radical initiator capable of generating free radicals upon exposure directly or in the presence of a sensitizer, is a suitable initiator, also referred to herein as photoinitiator. Suitable examples of photoinitiators include onium salts such as described above in [0022] and [0023], carbon-halogen bond-containing compounds such as [1,3,5] triazines having trihalomethyl groups, optionally substituted trihaloalkyl sulfones, organic peroxides, aromatic ketones, thio compounds, azo based polymerization initiators, azide compounds, ketooxime esters, hexaarylbisimidazoles, metallocenes, active ester compounds and quinonediazides. Of these, onium salts, especially iodonium and/or sulfonium salts are preferable in view of storage stability.

[0043] More specific suitable free-radical initiators include, for example, the derivatives of acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone, and 2-methyl-I-[4-(methylthio) phenyll-2-morpholino propan-I-one); benzophenone; benzil; ketocoumarin (such as 3-benzoyl-7-methoxy coumarin and 7-methoxy coumarin); xanthone; thioxanthone; benzoin or an alkyl-substituted anthraquinone; onium salts (such as diaryliodonium hexafluoroantimonate, diaryliodonium triflate, (4-(2-hydroxytetradecyl-oxy)-phenyl) phenyliodonium hexafluoroantimonate, triarylsulfonium hexafluorophosphate, triarylsulfonium p-toluenesulfonate, (3-phenylpropan-2-onyl) triaryl phosphonium 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); haloalkyl substituted s-triazines (such as 2,4-bis(trichloromethyl)-6-(p-methoxystyryl)-s-triazine, 2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-I-yl)-s-triazine, 2,4-bis(trichloromethyl)-6-piperonyl-s- triazine, and 2,4-bis(trichloromethyl)-6-[(4 -ethoxy-ethylenoxy)-phen-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(etha.9-2,4-cyclopentadien-1-yl)-bis[2,6-difluoro-3-(IH-pyrrol-1-yl)phenyl) titanium). These initiators may have optional substituents and may be used alone or in combination.

[0044] Optionally substituted trihaloalkyl sulfones wherein halo independently represents fluoro, bromo, chloro or iodo and sulfone is a chemical compound containing a sulfonyl functional group attached to two carbon atoms, are particularly preferred initiators. Tribromomethyl phenyl sulfones are most preferred photoinitiators. More details concerning this initiator can be found in WO2019/179995 paragraphs [0029] to [0040].

[0045] The amount of the photoinitiator typically ranges from 0.05 to 30 % by weight, preferably from 0.1 to 15 % by weight, most preferably from 0.2 to 10 % by weight relative to the total dry weight of the components in the photopolymerisable composition.

[0046] A very high sensitivity can be obtained by the combination of an optical brightener as sensitizer and a polymerisation initiator.

[0047] The photopolymerisable layer may also comprise a co-initiator. Typically, a co-initiator is used in combination with a free radical initiator. Suitable co-initiators for use in the photopolymer coating 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 refences: Chemistry & Technology UV & EB formulation for coatings, inks & paints - Volume 3 -Photoinitiators for Free Radical and Cationic Polymerisation by K.K. Dietliker - Edited by P.K.T. Oldring -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].

[0048] A very high sensitivity can be obtained by including a sensitizer such as for example an optical brightener in the coating. Suitable examples of optical brighteners 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 1 369 231, EP 1 341 040, US 2003/0124460, EP 1 241 002 and EP 1 288 720.

[0049] 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 sulfur-compounds, especially thiols like e.g. 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 4-methyl-3-propyl-1,2,4-triazoline-5-thione, 4-methyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3-n-heptyl-1,2,4-triazoline-5-thione, 4-phenyl-3,5-dimercapto-1,2,4-triazole, 4-n-decyl-3,5-dimercapto-1,2,4-triazole, 5-phenyl-2-mercapto-1,3,4-oxadiazole, 5-methylthio-1,3,4-thiadiazoline-2-thione, 5-hexylthio-1,3,4-thiadiazoline-2-thione, mercaptophenyltetrazole, pentaerythritol mercaptopropionate, butyric acid-3-mercapto-neopentanetetrayl ester, pentaerythritol tetra(thioglycolate). Other preferred co-initiators are polythioles as disclosed in WO 2006/048443 and WO 2006/048445. These polythiols may be used in combination with the above described thiols, e.g. 2-mercaptobenzothiazole.

Other ingredients

[0050] The photopolymerizable layer preferably includes a binder. The binder can be selected from a wide series of organic polymers. Compositions of different binders can also be used. Useful binders are described in WO2005/111727 page 17 line 21 to page 19 line 30, EP 1 043 627 in paragraph [0013] and in WO2005/029187 page 16 line 26 to page 18 line 11. Also suitable are particulate shaped polymers including homopolymers or copolymers prepared from monomers such as ethylene, styrene, vinyl chloride, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, vinyl carbazole, acrylate or methacrylate, or mixtures thereof.

[0051] Thermally reactive polymer fine particles including a thermally reactive group such as an ethylenically unsaturated group, a cationic polymerizable group, an isocyanate group, an epoxy group, a vinyloxy group, and a functional group having an active hydrogen atom, a carboxy group, a hydroxy group, an amino group or an acid anhydride, may be included in the coating.

[0052] The average particle diameter of the polymer fine particle is preferably 0.01 micron to 3.0 micron. Particulate polymers in the form of microcapsules, microgels or reactive microgels are suitable as disclosed in EP 1 132 200; EP 1 724 112; US 2004/106060.

[0053] The photopolymerisable layer may also comprise particles which increase the resistance of the coating against 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 are described in EP 2 916 171 [0053] to [0056].

[0054] The photopolymerizable layer may also comprise an inhibitor. Particular inhibitors for use in the photopolymer coating are disclosed in US 6,410,205, EP 1 288 720 and EP 1 749 240.

[0055] The photopolymerizable layer may further comprise an adhesion promoting compound. 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. Under "interacting" is understood each type of physical and/or chemical reaction or process whereby, between the functional group and the support, a bond is formed which can be a covalent bond, an ionic bond, a complex bond, a coordinate bond or a hydrogen-bond, and which can be formed by an adsorption process, a chemical reaction, an acid-base reaction, a complex-forming reaction or a reaction of a chelating group or a ligand. The adhesion promoting compounds described in EP 2 916 171 [0058] are suitable examples.

[0056] Various surfactants may be added into the photopolymerisable layer to allow or enhance the developability of the precursor; especially developing with a gum solution. Both polymeric and small molecule surfactants for example nonionic surfactants are preferred. More details are described in EP 2 916 171 [0059].

Definitions

[0057] An aliphatic hydrocarbon group preferably represents an alkyl, cycloalkyl, alkenyl, cyclo alkenyl or alkynyl group; suitable groups thereof are described below. An aromatic hydrocarbon group preferably represents a hetero(aryl) group; suitable hetero(aryl) groups - i.e. suitable aryl or heteroaryl groups - are described below.

[0058] The term "alkyl" herein means all variants possible 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 tertiary-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 tertiary-butyl, n-pentyl, n-hexyl, chloromethyl, trichloromethyl, iso-propyl, iso-butyl, iso-pentyl, neo-pentyl, 1-methylbutyl and iso-hexyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and methylcyclohexyl groups. Preferably, the alkyl group is a C1 to C6-alkyl group.

[0059] A suitable alkenyl group is preferably a C2 to C6-alkenyl group such as an ethenyl, n-propenyl, n-butenyl, n-pentenyl, n-hexenyl, iso-propenyl, isobutenyl, iso-pentenyl, neo-pentenyl, 1-methylbutenyl, iso-hexenyl, cyclopentenyl, cyclohexenyl and methylcyclohexenyl group.

[0060] A suitable alkynyl group is preferably a C2 to C6-alkynyl group; a suitable aralkyl group is preferably a phenyl group or naphthyl group including one, two, three or more C1 to C6-alkyl groups; a suitable alkaryl group is preferably a C1 to C6-alkyl group including an aryl group, preferably a phenyl group or naphthyl group.

[0061] A cyclic group or cyclic structure includes at least one ring structure and may be a monocyclic- or polycyclic group, meaning one or more rings annulated or fused together.

[0062] Examples of suitable aryl groups may be represented by for example an optionally substituted phenyl, benzyl, tolyl or an ortho- meta- or para-xylyl group, an optionally substituted naphtyl, anthracenyl, phenanthrenyl, and/or combinations thereof. The heteroaryl group is preferably a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably, 1 to 4 heteroatoms, independently selected from nitrogen, oxygen, selenium and sulphur. Preferred examples thereof include an optionally substituted furyl, pyridinyl, pyrimidyl, pyrazoyl, imidazoyl, oxazoyl, isoxazoyl, thienyl, tetrazoyl, thiazoyl, (1,2,3)triazoyl, (1,2,4)triazoyl, thiadiazoyl, thiofenyl group and/or combinations thereof.

[0063] Halogens are selected from fluorine, chlorine, bromine or iodine.

[0064] The term "substituted", in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms.

[0065] The optional substituents on the alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aralkyl, alkaryl, aryl and heteroaryl group are preferably selected from -F, -Cl, -Br, -I, -OH, -SH, -CN, -NO2, an alkyl group such as a methyl or ethyl group, an alkoxy group such as a methoxy or an ethoxy group, an aryloxy group, a carboxylic acid group or an alkyl ester thereof, a sulphonic acid group or an alkyl ester thereof, a phosphonic acid group or an alkyl ester thereof, a phosphoric acid group or an an ester such as an alkyl ester such as methyl ester or ethyl ester, a thioalkyl group, a thioaryl group, thioheteroaryl, -SH, a thioether such as a thioalkyl or thioaryl, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester, sulphonamide, an amino, ethenyl, alkenyl, alkynyl, cycloalkyl, alkaryl, aralkyl, aryl, heteroaryl or heteroalicyclic group and/or combinations thereof.

Support

[0066] The lithographic printing plate used in the present invention comprises a support which has a hydrophilic surface or which is provided with a hydrophilic layer. The support is preferably a grained and anodized aluminium support, well known in the art. Suitable supports are for example disclosed in EP 1 843 203 (paragraphs [0066] to [0075]). The surface roughness, obtained after the graining step, is often expressed as arithmetical mean center-line roughness Ra (ISO 4287/1 or DIN 4762) and may vary between 0.05 and 1.5 µm. The aluminum support of the current invention has preferably an Ra value below 0.45 µm, more preferably below 0.40 µm and most preferably below 0.30 µm. The lower limit of the Ra value is preferably about 0.1 µm. More details concerning the preferred Ra values of the surface of the grained and anodized aluminum support are described in EP 1 356 926. By anodising the aluminum support, an Al₂O₃ layer is formed and the anodic weight (g/m² Al₂O₃ formed on the aluminum surface) varies between 1 and 8 g/m². The anodic weight is preferably ≥ 3 g/m², more preferably ≥ 3.5 g/m² and most preferably ≥ 4.0 g/m². The anodisation of the support may be carried out in one, two or more steps.

[0067] The grained and anodized aluminium support may be subjected to so-called post-anodic treatments, for example a treatment with polyvinylphosphonic acid or derivatives thereof, a treatment with polyacrylic acid, a treatment with potassium fluorozirconate or a phosphate, a treatment with an alkali metal silicate, or combinations thereof. Alternatively, the support may be treated with an adhesion promoting compound such as those described in EP 1 788 434 in [0010] and in WO 2013/182328. However, for a precursor optimized to be used without a pre-heat step it is preferred to use a grained and anodized aluminium support without any post-anodic treatment.

[0068] Besides an aluminium support, a plastic support, for example a polyester support, provided with one or more hydrophilic layers as disclosed in for example EP 1 025 992 may also be used.

[0069] The lithographic printing plate precursor can be prepared by (i) applying on a support the coating as described above and (ii) drying the precursor.

Exposure step

[0070] The printing plate precursor is preferably image-wise exposed by a laser emitting IR-light and/or UV light. Preferably, the image-wise exposing step is carried out off-press in a platesetter, i.e. an exposure apparatus suitable for image-wise exposing the precursor with a laser such as a laser diode, emitting around 830 nm or a Nd YAG laser emitting around 1060 nm, or by a conventional exposure in contact with a mask. In a preferred embodiment of the present invention, the precursor is image-wise exposed by a laser emitting IR-light.

Preheat step

[0071] After the exposing step, the precursor may be pre-heated in a preheating unit, preferably at a temperature of about 80°C to 150°C and preferably during a dwell time of about 5 seconds to 1 minute. This preheating unit may comprise a heating element, preferably an IR-lamp, an UV-lamp, heated air or a heated roll. Such a preheat step can be used for printing plate precursors comprising a photopolymerisable composition to enhance or to speed-up the polymerization and/or crosslinking reaction.

Development step

[0072] Subsequently to the exposing step or the preheat step, when a preheat step is present, the plate precursor may be processed (developed). Before developing the imaged precursor, a pre-rinse step might be carried out especially for the negative-working lithographic printing precursors having a protective oxygen barrier or topcoat. This pre-rinse step can be carried out in a stand-alone apparatus or by manually rinsing the imaged precursor with water or the pre-rinse step can be carried out in a washing unit that is integrated in a processor used for developing the imaged precursor. The washing liquid is preferably water, more preferably tap water. More details concerning the wash step are described in EP 1 788 434 in [0026].

[0073] During the development step, the non-exposed areas of the image-recording layer are at least partially removed without essentially removing the exposed areas. The processing liquid, also referred to as developer, can be applied to the plate e.g. by rubbing with an impregnated pad, by dipping, immersing, coating, spincoating, spraying, pouring-on, either by hand or in an automatic processing apparatus. The treatment with a processing liquid may be combined with mechanical rubbing, e.g. 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 temperatures between 20 and 40 °C in automated processing units.

[0074] In a preferred embodiment, the processing step as described above is replaced by an on-press processing whereby the imaged precursor is mounted on a press and processed on-press by rotating said plate cylinder while feeding dampening liquid and/or ink to the coating of the precursor to remove the unexposed areas from the support. In a preferred embodiment, only dampening liquid is supplied to the plate during start-up of the press. After a number of revolutions of the plate cylinder, preferably less than 50 and most preferably less than 5 revolutions, also the ink supply is switched on. In an alternative embodiment, supply of dampening liquid and ink can be started simultaneously or only ink can be supplied during a number of revolutions before switching on the supply of dampening liquid.

[0075] The processing step may also be performed by combining embodiments described above, e.g. combining development with a processing liquid with development on-press by applying ink and/or fountain.

Processing liquid

[0076] Most preferably, the processing liquid is a gum solution whereby during the development step the non-exposed areas of the photopolymerisable layer are removed from the support. At the same time, the plate may be gummed; i.e. in a single step. The development with a gum solution has the additional benefit that, due to the remaining gum on the plate in the non-exposed areas, an additional gumming step to protect the surface of the support in the non-printing areas may be omitted. As a result, the precursor may be processed and gummed in one single step which involves a less complex 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. the gum solution, used in the second gumming unit and present in the second tank, overflows from the second tank to the first tank when gum replenishing solution is added in the second gumming unit or when the gum solution in the second gumming unit is used once-only, i.e. only starting gum solution is used to develop the precursor in this second gumming unit by preferably a spraying or jetting technique. More details concerning such gum development is described in EP1 788 444. A gum solution is typically an aqueous liquid which comprises one or more surface protective compounds that are capable of protecting the lithographic image of a printing plate against contamination, e.g. by oxidation, fingerprints, fats, oils or dust, or damaging, e.g. by scratches during handling of the plate. Suitable examples of such surface protective compounds are film-forming hydrophilic polymers or surfactants. The layer that remains on the plate after treatment with the gum solution preferably comprises between 0.005 and 20 g/m² of the surface protective compound, more preferably between 0.010 and 10 g/m², most preferably between 0.020 and 5 g/m². More details concerning the surface protective compounds in the gum solution can be found in WO 2007/057348 page 9 line 3 to page 11 line 6. As the developed plate precursor is developed and gummed in one step, there is no need to post-treat the processed plate. However, after the gumming step a finishing gum may be applied.

[0077] The gum solution preferably has a pH between 1.5 and 10, more preferably between 2 and 9 and most preferably between 2.5 and 7. A suitable gum solution is described in for example EP 1 342 568 in [0008] to [0022] and WO2005/111727. In the current invention, the pH of the gum solution is preferably about 4 or below 4, more preferably 3 or below 3 and most preferably 2 or below 2. The gum solution may further comprise an inorganic salt, an anionic surfactant, a wetting agent, a chelate compound, an antiseptic compound, an anti-foaming compound and/or an ink receptivity agent and/or combinations thereof. More details about these additional ingredients are described in WO 2007/057348 page 11 line 22 to page 14 line 19.

[0078] The processing liquid may be an alkaline developer or solvent-based developer. Suitable alkaline developers have been described in US2005/0162505. An alkaline developer is an aqueous solution which has a pH of at least 11, more typically at least 12, preferably from 12 to 14. Alkaline developers typically contain alkaline agents to obtain high pH values can be inorganic or organic alkaline agents. The developers can comprise anionic, non-ionic and amphoteric surfactants (up to 3% on the total composition weight); biocides (antimicrobial and/or antifungal agents), antifoaming agents or chelating agents (such as alkali gluconates), and thickening agents (water soluble or water dispersible polyhydroxy compounds such as glycerine or polyethylene glycol).

[0079] In the event that the processing liquid is an alkaline solution and/or has a pH of more than 4, it is required to perform an additional treatment of the processed plate with a liquid having a pH of about 4 or below 4. This liquid may comprise for example at least one non-polymeric acid or polyfunctional acid and/or their salt, such as for example compounds including carboxylic acid and/or carboxylate groups. The polyfunctional compound is preferably an aliphatic or aromatic organic polyfunctional compound. In the context of this invention non-polymeric means that the compound does not include more than two repeating units.

[0080] After processing, a finishing solution may be applied to provide a protective layer on the processed printing plate. A finishing solution typically includes an acid e.g. citric acid, a buffer, a biocide and a suitable binder such as patato dextrine.

[0081] The processing liquid and finishing solution may be in the form of a powder, granule or tablet which is able to dissolve in a liquid and preferably contains an effervescence generating compound such as a bicarbonate salt. Dissolution of the powder, granule or tablet in a liquid results in a solution suitable for developing a lithographic printing plate precursor. Suitable examples are described in WO2020/074258 especially in [0040] to [0047].

Drying and baking step

[0082] 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 the drying unit which may contain at least one heating element selected from an IR-lamp, an UV-lamp, a heated metal roller or heated air.

[0083] After drying the plate can optionally be heated in a baking unit. More details concerning the heating in a baking unit can be found in WO 2007/057348 page 44 line 26 to page 45 line 20.

[0084] The printing plate thus obtained can be used for conventional, so-called wet offset printing, in which ink and an aqueous dampening liquid is supplied to the plate.

EXAMPLES

Example 1

1. Preparation of the printing plates PP-01 to PP-07.

Preparation of the aluminium support S-01

[0085] A 0.3 mm thick aluminium foil was degreased by spraying with an aqueous solution containing 26 g/l NaOH at 65°C for 2 seconds and rinsed with demineralised water for 1.5 seconds. The foil was then electrochemically grained during 10 seconds using an alternating current in an aqueous solution containing 15 g/l HCl, 15 g/l SO₄^2- ions and 5 g/l Al³⁺ ions at a temperature of 37°C and a current density of about 100 A/dm². Afterwards, the aluminium foil was then desmutted by etching with an aqueous solution containing 5.5 g/l of NaOH at 36°C for 2 seconds and rinsed with demineralised water for 2 seconds. The foil was subsequently subjected to anodic oxidation during 15 seconds in an aqueous solution containing 145 g/l of sulfuric acid at a temperature of 50°C and a current density of 17 A/dm², then washed with demineralised water for 11 seconds and dried at 120°C for 5 seconds.

[0086] The support thus obtained was characterized by a surface roughness Ra of 0.96-1.08 µm (measured with interferometer NT1100) and had an oxide weight of about 3.0 g/m².

Photopolymerisable layer

[0087] The printing plate precursors PPP-01 to PPP-07 were prepared by first coating onto the above described support S-01 the photosensitive compositions PL-01 to PL-04 as defined in Table 1. The components were dissolved in a mixture of 35% by volume of MEK and 60% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied to obtain a coating weight of 1.24 g/m² after drying at 120°C for 1 minute in a circulation oven.

Table 1: Composition of the photopolymersisable layers (mg/m²)

Ingredients mg/m2	PL-01	PL-02	PL-03	PL-04
Sartomer CN104 (1)	235.9	235.9	235.9	235.9
Mono Z 1620 (2)	235.9	235.9	235.9	235.9
Albritect CP30 (3)	30.3	30.3	30.3	30.3
Initiator (4)	31.5	31.5	31.5	31.5
IR-01 (5)	22.8	22.8	0	0
IR-02 (6)	0	0	22.8	22.8
Pigment dispersion (7)	124.3	124.3	124.3	124.3
Tegoglide 410 (8)	1.5	1.5	1.5	1.5
SR348L (9)	90.6	90.6	90.6	90.6
Sipomer PAM 100 (10)	172.0	172.0.	172.0	172.0
Ruco Coat EC4811 (11)	291.9	291.9	291.9	291.9
Borate compound (12)	3.1	0.0	0.0	5.0

(1) Tris (2-hydroxy ethyl) isocyanurate triacrylate commercially available from Sartomer/ Arkema; (2) Monomer commercially available from Clariant; (3) Poly(acrylic acid) poly(vinylphosphonic acid) 70/30 copolymer from RHODIA; (4) 4-hydroxyphenyl tribromomethyl sulfone; (5) S2025, infrared absorbing dye commercially available from Few Chemicals; (6) S2539, infrared absorbing dye commercially available from Few Chemicals; (7) Hostaperm Blue P-BFS 60 mg/m2 (commercially available from Clariant) stabilised with Disperbyk 182 60mg/m2 (Commercially available from Byk Chemicals); (8) Polyether siloxane copolymer commercially available from Evonik Resource Efficiency GmbH; (9) Surfactant commercially available from Arkema BV; (10) Phosphate ester of polyethylene glycol monomethacrylate available from Rhodia; (11) Non-ionic aliphatic polyether polyurethane commercially available from Rudolf GmbH; (12) Sodium tetraphenylborate commercially available from Charkit Chemical Corporation.

Protective overcoat layer

[0088] On top of the photopolymerisable layer, an aqueous solution with the composition as defined in Table 2 was coated on the printing plate precursors and subsequently dried at 110°C for 2 minutes. Printing plate precursors PPP-01 to PPP-07 were obtained (Table 3).

Table 2: Composition of protective overcoat layers (mg/m²)

Ingredients mg/m2	OC-01	OC-02	OC-03	OC-04	OC-05
Mowiol 4-88 (1)	668.9	271.3	271.3	271.3	271.3
Mowiol 4-98 (2)	205.6	205.6	205.6	205.6	205.6
Luvitec K30 (3)	209.7	209.7	209.7	209.7	209.7
Surfactant (4)	10.4	10.4.	10.4	10.4	10.4
Borate compound (5)	0	3.5	4.0	5.0	5.8

(1) Partially hydrolyzed polyvinylalcohol commercially available from Kuraray; (2) Fully hydrolyzed polyvinylalcohols commercially available from Kuraray; (3) Polyvinylpyrrolidone commercially available from BASF; (4) Akypoprox RLM 80V, commercially available from Chemische Fabrik Chem Y GMBH; (5) Sodium tetra phenyl borate commercially available from Charkit Chemical Corporation.

Tabel 3: Printing plate precursors PPP-01 to PPP-07

Printing Plate Precursor	Photopolymerisable layer	Overcoat
PPP-01 Comparative	PL-01	OC-01
PPP-02 Inventive	PL-02	OC-02
PPP-03 Inventive	PL-02	OC-03
PPP-04 Inventive	PL-02	OC-04
PPP-05 Inventive	PL-02	OC-05
PPP-06 Inventive	PL-03	OC-04
PPP-07 Comparative	PL-04	OC-01

2. Imaging

[0089] The obtained printing plate precursors PPP-01 to PPP-07 were imaged at 2400 dpi with an Avalon N8 20 image setter (900rpm and 60% laser power) commercially available from ECO3 BV with a 2x2 checkerboard pattern and equipped with a 830 nm IR laser diode, at energy densities of 90 mJ/cm².

3. Processing

[0090] After imaging the plates were washed out in an Adamas cleanout unit with Adamas Pro gum, commercially available from ECO3 BV. Printing plates PP-01 to PP-07 were obtained.

4. Crystallization behaviour

[0091] After processing the printing plates are visually checked for screen density artefacts. The occurrence of crystallization was tested by visual assessment of the screen density of the obtained printing plates. If crystallization occurred, the screen density at these areas is lower or even gone compared to the non crystal containing areas. If no differences in screen density are observed - i.e. the screen rendering after processing is uniform without image screen damages - the plate is free from crystallization. The results are summarized in Table 4.

Table 4 Crystallisation results

Printing Plate	Photo layer	Overcoat	Crystallization behaviour
	Borate compound
PP-01 Comparative	Yes	No	Artefacts clearly visual
PP-02 Inventive	No	Yes	Uniform
PP-03 Inventive	No	Yes	Uniform
PP-04 Inventive	No	Yes	Uniform
PP-05 Inventive	No	Yes	Uniform
PP-06 Inventive	No	Yes	Uniform
PP-07 Comparative	Yes	No	Artefacts clearly visual

[0092] The results in Table 4 show that the printing plates including the borate compound in the overcoat do not show any crystallisation behaviour while the printing plates including the borate compound in the photopolymerisable layer show artefacts indicating crystallisation defects.

Example 2

1. Preparation of the printing plates PP-08 and PP-09.

Photopolymerisable layer

[0093] The printing plate precursors PPP-08 and PP-09 were prepared by first coating onto the above described support S-01 respectively the photosensitive compositions PL-05 and PL-06 as defined in Table 5. The components were dissolved in a mixture of 35% by volume of MEK and 60% by volume of Dowanol PM (1-methoxy-2-propanol, commercially available from DOW CHEMICAL Company). The coating solution was applied to obtain a coating weight of 1.24 g/m² after drying at 120°C for 1 minute in a circulation oven.

Table 5: Composition of the photopolymerisable layers PL-05 and PL-06

Ingredients mg/m2	PL-05	PL-06
FST 510 (1)	192	192
CN 104 (2)	130	130
Borate compound (3)	30	0
Aerosil 150 (4)	85	85
IR-02 (5)	19	19
Ruco coat EC4811 (6)	182	182
Tegoglide 410 (7)	1.5	1.5
Sipomer PAM 100 (8)	130	130

(1) Reaction product from 1 mole of 2,2,4-trimethylhexamethylenediisocyanate and 2 moles of hydroxyethyl-methacrylate commercially available from AZ Electronics as a 82 wt.% solution in MEK; (2) Reaction product of acrylic acid and bisphenol A diglycidyl ether; (3) Bis(4-t-butylphenyl)iodonium tetraphenylborate commercially available from Merck; (4) Aerosil 150 is a hydrophilic fumed silica commercially available from Evonik Resource Efficiency GmbH (5) to (8) see Table 1.

2. Protective overcoat layer

[0094] On top of the photopolymerisable layer, an aqueous solution with the composition as defined in Table 6 was coated on the printing plate precursors, and dried at 110°C for 2 minutes. Printing plate precursors PPP-08 and PPP-09 were obtained (Table 7).

Table 6: Composition of protective overcoat layers

Ingredients mg/m2	OC-06	OC-07
Mowiol 4-88 (1)	271.73	271.3
Mowiol 4-98 (2)	163.82	163.82
Ebotec MB SF (3)	1.0	1.0
Advantage S (4)	8.99	8.99
Lutensol A8 (5)	4.76	4.76
Borate compound (6)	0	30

(1) Mowiol 4-88 is a partially hydrolyzed polyvinylalcohol commercially available from Kuraray; (2) Mowiol 4-98 and Mowiol 28-99 are fully hydrolyzed polyvinylalcohols commercially available from Kuraray; (3) Ebotec MB SF is 2.5-10% 2-bromo-2-nitro-1,3-propanediol commercially available from Bode Chemie Hamburg; (4) Poly[vinylpyrrolidone-c-vinylcaprolactam- dimethylaminoethyl methacrylate (5) Lutensol A8 is a surface active agent commercially available from BASF; (6) Bis(4-t-butylphenyl)iodonium BPh4 commercially available from Merck.

Tabel 7: Printing plate precursors PPP-08 and PPP-09

Printing Plate Precursor	Photopolymerisable layer	Overcoat
PPP-08 Comparative	PL-07	OC-06
PPP-09 Inventive	PL-08	OC-07

3. Imaging

[0095] The printing plate precursors PPP-08 and PPP-09 were imaged at 2400 dpi with a Creo Trendsetter 3244 thermal platesetter (200 lpi Agfa Balanced Screening (ABS)), commercially available from Kodak and equipped with a 830 nm IR laser diode, at energy densities between 50 and 150 mJ/cm2.

4. Processing

[0096] The printing plate precursors PPP-08 and PPP-09 were mounted on a Heidelberg GTO52 dalhgren press using K+E Skinnex 800 SPEED IK black ink (trademark of BASF Druckfarben GmbH) and 4 wt% Prima FS303 SF (trademark of Agfa Graphics) and 8% isopropanol in water as fountain solution. A compressible blanket was used and printing was performed on non-coated offset paper. Prior to paper feeding, 10 press revolution with only the dampening system followed by 5 revolutions with only the inking rollers was performed.

5. Press life Results

[0097] The press life of the obtained printing plates PP-08 and PP-09 was evaluated on a Heidelberg GTO 46 printing press (commercially available from Heidelberg) by monitoring every 5.000 impressions up to 90.000 impressions the rendition (density) on the printed sheet of a test pattern with a nominal tone value of 40% (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 is defined as the point where the density of the 40% test pattern starts to drop.

[0098] The press life test revealed that a similar, excellent press life result was obtained for both the printing plates PP-08 and PP-09. Thus, the presence of the borate initiator in the protective overcoat layer does not significantly affect the crosslinking and/or polymerization behaviour of the photopolymerisable layer, and the robustness of the printing plate PP-09 on the press - i.e. the press life - remains unchanged.

Claims

1. A negative-working lithographic printing plate precursor including a support and a coating comprising a photopolymerisable layer including a polymerisable compound and an infrared absorbing dye, and a protective overcoat layer provided above the photopolymerisable layer;
characterized in that the overcoat layer includes a borate compound.

2. The printing plate precursor according to claim 1 wherein the borate compound is represented by Formula I:

wherein Rb¹,Rb²,Rb³ and Rb⁴ are independently an optionally substituted aliphatic hydrocarbon group, an optionally substituted aryl or heteroaryl group; and M⁺ is an alkali metal cation selected from Li⁺, Na⁺, K⁺, an optional substituted diaryliodonium, an optional substituted triarylsulfonium or an optional substituted ammonium, preferably according to Formula II:

wherein

R_n¹,R_n² and R_n³ are independently an optionally substituted aliphatic hydrocarbon group, an optionally substituted aryl or heteroaryl group or a halogen atom.

3. The printing plate precursor according to claims 1 or 2 wherein Rb¹,Rb²,Rb³ and Rb⁴ are independently an optionally substituted aryl or heteroaryl group.

4. The printing plate precursor according to any of the preceding claims wherein Rb¹, Rb², Rb³ and Rb⁴ are independently an optionally substituted phenyl, benzyl, tolyl or an ortho- meta- or para-xylyl group, naphtyl, anthracenyl or phenanthrenyl.

5. The printing plate precursor according to any of the preceding claims wherein M⁺ is an alkali metal cation selected from Li⁺, Na⁺ and K⁺.

6. The printing plate precursor according to any of the preceding claims wherein the borate compound is a free radical initiator capable of generating free radicals upon exposure to infrared radiation.

7. The printing plate precursor according to any of the preceding claims wherein the borate compound is an onium compound selected from diaryliodonium borates, triarylsulfoniurn borates, and alkoxy- or aryloxypyridinium borates.

8. The printing plate precursor according to any of the preceding claims wherein the borate compound is an onium compound selected from diphenyliodonium borates or triphenylsulfoniurn borates.

9. The printing plate precursor according to any of the preceding claims wherein the infrared absorbing dye includes a cyanine cation and a counter ion which provides an electrically neutral compound.

10. The printing plate precursor according to any of the preceding claims wherein the photopolymerisable layer further includes a photoinitiator.

11. The printing plate precursor according to claim 10 wherein the photoinitiator is an onium compound selected from diaryliodonium borates, triarylsulfoniurn borates, and alkoxy- or aryloxypyridinium borates.

12. A method for making a negative-working lithographic printing plate precursor comprising the steps of:

- applying the coating as defined in claims 1 to 11 on a support,

- drying the precursor.

13. A method for making a negative-working lithographic printing plate comprising the steps of:

- image-wise exposing the lithographic printing plate precursor as defined in claims 1 to 11,

- optionally subjecting the lithographic plate precursor to heat;

- developing the exposed printing plate precursor.

14. The method according to claim 13 wherein the printing plate is developed off-press by treating the precursor with a developing solution thereby removing the non-exposed areas of the coating from the support.

15. The method according to claim 13 wherein the printing plate is developed by mounting the precursor on a plate cylinder of a lithographic printing press and rotating the plate cylinder while feeding dampening liquid and/or ink to the precursor thereby removing the non-exposed areas of the coating from the support.