Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C1/00—Forme preparation
  • B41C1/10—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme
  • B41C1/1008—Forme preparation for lithographic printing; Master sheets for transferring a lithographic image to the forme by removal or destruction of lithographic material on the lithographic support, e.g. by laser or spark ablation; by the use of materials rendered soluble or insoluble by heat exposure, e.g. by heat produced from a light to heat transforming system; by on-the-press exposure or on-the-press development, e.g. by the fountain of photolithographic materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/04—Negative working, i.e. the non-exposed (non-imaged) areas are removed
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/06—Developable by an alkaline solution
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/22—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by organic non-macromolecular additives, e.g. dyes, UV-absorbers, plasticisers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/24—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by a macromolecular compound or binder obtained by reactions involving carbon-to-carbon unsaturated bonds, e.g. acrylics, vinyl polymers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41C—PROCESSES FOR THE MANUFACTURE OR REPRODUCTION OF PRINTING SURFACES
  • B41C2210/00—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation
  • B41C2210/26—Preparation or type or constituents of the imaging layers, in relation to lithographic printing forme preparation characterised by a macromolecular compound or binder obtained by reactions not involving carbon-to-carbon unsaturated bonds

Definitions

• the present invention relates to a method for making a lithographic printing plate by exposing a heat-sensitive, negative working lithographic printing plate precursor to infrared light, thereby forming a visible image immediately after the exposure.
• Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the 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.
• 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.
• 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.
• Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor.
• plate precursor an imaging material
• heat-sensitive printing plate precursors have become very popular in the late 1990s.
• thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask.
• the material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
• a (physico-)chemical process such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
• the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating.
• the coating typically comprises a hydrophobic layer of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure.
• solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support.
• Plates comprising an infrared-sensitized photopolymerizable or infrared-sensitized photocurable coating are typical examples of negative-working thermal plates. Such plates have been described in e.g. US4997745, US5514521, US5275917, EP-A 611997, US5705309 and EP-A 889363. Infrared-sensitized photopolymer plates which are especially designed for on-press processing have been described in e.g. EP-A 1315998, US2002/177074, US6576401, EP-A 1495866, EP-A 1506855 EP-A 1516724, EP-A 1518672 and EP-A 1520694. Such plates are developed by mounting the exposed precursor on the plate cylinder of a lithographic printing press and starting a press run : the non-exposed areas of the coating are removed by the ink and/or fountain that is supplied to the plate.
• the exposed plate precursor shows a visible image even before being processed, i.e. a print-out image.
• a print-out image This enables the end-user to establish immediately whether or not the precursor has already been exposed, to inspect the image quality on the printing plate and to distinguish the different color selections.
• Photopolymer plates that produce a print-out image are known in the art, e.g. as disclosed in US 3,359,109, US 3,042,515, US 4,258,123, US 4,139,390, US 5,141,839, US 5,141,842, US 4,232,106, US 4,425,424, US 5,030,548, US 4,598,036, EP 0 434 968, WO 96/35143 and US 2003/68575.
• the print-out image formation is triggered by the photoinitiating system which also induces the photopolymerization of the image-recording layer.
• the print-out image formation leads to a reduced sensitivity of the lithographic imaging process.
• a color change can be obtained by the addition of a heat-decomposable dye which is bleached upon heating.
• a heat-decomposable dye which is bleached upon heating.
• Such materials are disclosed in e.g. DD 213 530, EP 897 134, EP 0 925 916, WO 96/35143, EP 1 300 241 and EP 1 508 440.
• Another approach is followed in EP 0 925 916 and US 2004/134365 wherein IR dyes are disclosed of which the light absorption changes upon IRradiation.
• the IR dyes exhibit, beside strong absorption in IR wavelength range, also side-absorption in the visible wavelength range. Due to IR-exposure the IR dye decomposes and a print-out image is obtained by the reduction of this side-absorption in the visible wavelength range.
• a problem of these prior art materials is that the print-out image is formed by a heat-induced reduction of the visible light absorption of the coating or by a switch from a highly colored to a weakly colored coating.
• Such bleaching processes produce print-out images which are characterized by a low contrast between the exposed and the non-exposed areas. A better contrast can be obtained by forming -rather than bleaching- a dye upon exposure or by a color switch from one hue to another upon exposure.
• EP-A 1502736 discloses printing plates comprising a dye of which the absorption maximum shifts by at least 50 nm upon heating.
• EP-A 1508440 discloses printing plates comprising an infrared dye as light-to-heat converter and a dye precursor which forms a visible color upon heating.
• the CIE 1976 color system is described in e.g. "Colorimetry, CIE 116-1995: Industrial Colour Difference Evaluation", or in "Measuring Colour” by R.W.G. Hunt, second edition, edited in 1992 by Ellis Horwood Limited, England.
• the coating on the support may comprise one or more layer(s). Coating a plurality of layers may be carried out by the simultaneous coating of a plurality of coating solutions with a multi-layer coating head, using the known coating techniques, or by the subsequent coating of a plurality of coating solutions using single-layer coating heads, either with or without intermediate drying (respectively wet-on-dry or wet-on-wet).
• the coating preferably contains a compound, sometimes referred to as "sensitizer", which absorbs infrared light and converts the absorbed infrared light into heat.
• infrared absorbers can be used in combination with the color-forming infrared dyes discussed below.
• the color-forming infrared dye is the only infrared absorber in the coating. Since the coating is essentially colorless or only slightly colored, it is preferred that the infrared dye does not show a substantial light absorption in the visible wavelength range. For the same reason, the amount of dyes or pigments that show visible light absorption in the coating is kept low, so that the material has a CIE 1976 lightness value L* nonexp which is not less than 50, preferably not less than 60.
• Preferred plate precursors (support + coating) for use in the method of the present invention have a visual density in the nonexposed state of at most 0.6, more preferably at most 0.5.
• the amount of infrared absorbing agent in the coating is preferably between 0.25 and 25.0 % by weight, more preferably between 0.5 and 20.0 % by weight.
• the infrared absorbing compound can be present in the image-recording layer and/or an optional other layer.
• the infrared absorbing agent is present in the image-recording layer of the coating, its concentration is preferably at least 6 % by weight, more preferably at least 8 % by weight, relative to the weight of all the components in the image-recording layer.
• Preferred IR absorbing compounds are organic dyes having an absorption between 750 nm and 1300 nm, preferably between 780 nm and 1200 nm, more preferably between 800 nm and 1100 nm, such as cyanine, merocyanine, indoaniline, oxonol, pyrilium and squarilium dyes. Examples of suitable IR absorbers may be found in EP 1 359 008, including the references cited therein.
• the coating may also contain one or more additional layer(s) besides the image-recording layer.
• additional layer already discussed above - i.e. an optional light-absorbing layer comprising one or more compounds that are capable of converting infrared light into heat -
• the coating may further comprise for example an adhesion-improving layer between the image-recording layer and the support.
• compounds may be present which are capable of interacting with the surface of the support, e.g. the compounds described in EP-A 1495866, EP-A 1500498 and EP-A 1520694.
• the coating may further contain additional ingredients.
• additional ingredients may be present in the image-recording layer or in on optional other layer.
• additional binders polymer particles such as matting agents and spacers, surfactants such as perfluoro surfactants, fillers such as silicon or titanium dioxide particles, development inhibitors or development accelerators are well-known components of lithographic coatings.
• Typical infrared-sensitized photopolymerizable or photocurable compositions have been described in e.g. EP-A 1315998, US2002/177074, US6576401, EP-A 1495866, EP-A 1506855 EP-A 1516724, EP-A 1518672 and EP-A 1520694.
• the coating thickness of the image-recording layer is preferably between 0.1 and 4.0 g/m 2 , more preferably between 0.4 and 2.0 g/m 2 .
• the photopolymerizable composition typically comprises a polymerizable monomer or oligomer and an initiator capable of hardening said monomer or oligomer and, preferably, a sensitizer capable of absorbing light used in the image-wise exposure step.
• the composition comprises a polymerizable monomer or oligomer comprising at least one epoxy or vinyl ether functional group and an initiator which is a Bronsted acid generator capable of generating a free acid upon exposure, hereinafter also referred to as "cationic photoinitiator" or "cationic initiator”.
• Suitable polyfunctional epoxy monomers include, for example, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohex-ane carboxylate, bis-(3,4 - epoxycyclohexymethyl) adipate, difunctional bisphenol A epichlorohydrin epoxy resin and multifunctional epichlorohydrinitetraphenylol ethane epoxy resin.
• Suitable cationic photoinitiators include, for example, triarylsulfonium hexafluoroantimonate, triarylsulfonium hexafluorophosphate, diaryliodonium hexafluoroantimonate, and haloalkyl substituted s-triazine. It is noted that most cationic initiators are also free radical initiators because, in addition to generating a free acid, they also generate free radicals during photo or thermal decomposition.
• the polymerizable monomer or oligomer is an ethylenically unsaturated compound, having at least one terminal ethylenic group, hereinafter also referred to as “free-radical polymerizable monomer”, and said initiator is a compound which is capable of generating free radical upon exposure, hereinafter also referred to as “free radical initiator”.
• Suitable free-radical polymerizable monomers include, for example, multifunctional (meth)acrylate monomers (such as (meth)acrylate esters of ethylene glycol, trimethylolpropane, pentaerythritol, ethoxylated ethylene glycol and ethoxylated trimethylolpropane, multifunctional urethanated (meth)acrylate, and epoxylated (meth)acrylate), and oligomeric amine diacrylates.
• the (meth)acrylic monomers may also have other double bond or epoxide group, in addition to (meth)acrylate group.
• the (meth)acrylate monomers may also contain an acidic (such as carboxylic acid) or basic (such as amine) functionality.
• Suitable free-radical initiators include, for example, the derivatives of acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone, and 2-methyl-1-[4-(methylthio) phenyl-2-morpholino propan-1-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,
• borate salts such as tetrabutylammonium triphenyl(n-butyl)borate, tetraethylammonium triphenyl(n-butyl)borate, diphenyliodonium tetraphenylborate, and triphenylsulfonium triphenyl(n-butyl)borate, and borate salts as described in U.S. Pat. Nos.
• haloalkyl substituted s-triazines such as 2,4-bis(trichloromethyl)-6-(p-methoxy-styryl)-s-triazine, 2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-1-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.
• titanocene bis(etha.9-2,4-cyclopentadien-1-yl) bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl) titanium.
• Onium salts, borate salts, and s-triazines are preferred free radical initiators.
• Diaryliodonium salts and triarylsulfonium salts are preferred onium salts.
• Triarylalkylborate salts are preferred borate salts.
• Trichloromethyl substituted s-triazines are preferred s-triazines.
• the polymerizable monomer or oligomer is a combination of a monomer or oligomer comprising at least one epoxy or vinyl ether functional group and a polymerizable ethylenically unsaturated compound, having at least one terminal ethylenic group
• said initiator is a combination of a cationic initiator and a free-radical initiator.
• a monomer or oligomer comprising at least one epoxy or vinyl ether functional group and a polymerizable ethylenically unsaturated compound, having at least one terminal ethylenic group can be the same compound wherein the compound contains both ethylenic group and epoxy or vinyl ether group.
• Examples of such compounds include epoxy functional acrylic monomers, such as glycidyl acrylate.
• the free radical initiator and the cationic initiator can be the same compound if the compound is capable of generating both free radical and free acid.
• Examples of such compounds include various onium salts such as diaryliodonium hexafluoroantimonate and s-triazines such as 2,4-bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)-phen-1-yl]-s-triazine which are capable of generating both free radical and free acid in the presence of a sensitizer.
• the photopolymerizable composition may also comprise a multifunctional monomer.
• This monomer contains at least two functional groups selected from an ethylenically unsaturated group and/or an epoxy or vinyl ether group.
• Particular multifunctional monomers for use in the photopolymer composition are disclosed in US 6,410,205 , US 5,049,479 , EP 1079276 , EP 1369232 , EP 1369231 EP 1341040 , US 2003/0124460 , EP 1241002 , EP 1288720 and in the reference book including the cited refences : Chemistry & Technology UV & EB formulation for coatings, inks & paints - Volume 2 - Prepolymers and Reactive Diluents for UV and EB Curable Formulations by N.S. Allen, M.A. Johnson, P.K.T. Oldring, M.S. Salim - Edited by P.K.T. Oldring - 1991 - ISBN 0 947798102.
• the photopolymerizable composition may also comprise a co-initiator.
• a co-initiator is used in combination with a free radical initiator and/or cationic initator.
• Particular coinitiators for use in the photopolymer composition are disclosed in US 6,410,205 , US 5,049,479 , EP 1079276 , 1369232 , EP 1369231 EP 1341040 , US 2003/0124460 , EP 1241002 , EP 1288720 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.
• the photopolymerizable composition may also comprise an inhibitor.
• Particular inhibitors for use in the photopolymer composition are disclosed in US 6,410,205 and EP 1288720.
• the photopolymerizable composition may also comprise a binder.
• the binder can be selected from a wide series of organic polymers. Compositions of different binders can also be used.
• Useful binders include for example chlorinated polyalkylene (in particular chlorinated polyethylene and chlorinated polypropylene), polymethacrylic acid alkyl esters or alkenyl esters (in particular polymethyl (meth)acrylate, polyethyl (meth)acrylate, polybutyl (meth)acrylate, polyisobutyl (meth)acrylate, polyhexyl (meth)acrylate, poly(2-ethylhexyl) (meth)acrylate and polyalkyl (meth)acrylate copolymers of (meth) acrylic acid alkyl esters or alkenyl esters with other copolymerizable monomers (in particular with (met)acrylonitrile, vinyl chloride, vinylidene chloride, styrene and/or but
• preferred binders are hydrophilic such as binders containing carboxylic groups, in particular copolymers containing monomeric units of ⁇ , ⁇ -unsaturated carboxylic acids or monomeric units of ⁇ , ⁇ -unsaturated dicarboxylic acids (preferably acrylic acid, methacrylic acid, crotonic acid, vinylacetic acid, maleic acid or itaconic acid).
• copolymers is to be understood in the context of the present invention as polymers containing units of at least 2 different monomers, thus also terpolymers and higher mixed polymers.
• Particular examples of useful copolymers are those containing units of (meth)acrylic acid and units of alkyl (meth)acrylates, allyl (meth)acrylates and/or (meth)acrylonitrile as well as copolymers containing units of crotonic acid and units of alkyl (meth)acrylates and/or (meth)acrylonitrile and vinylacetic acid/alkyl (meth)acrylate copolymers. Also suitable are copolymers containing units of maleic anhydride or maleic acid monoalkyl esters.
• copolymers containing units of maleic anhydride and styrene, unsaturated ethers or esters or unsaturated aliphatic hydrocarbons and the esterification products obtained from such copolymers are, for example, copolymers containing units of maleic anhydride and styrene, unsaturated ethers or esters or unsaturated aliphatic hydrocarbons and the esterification products obtained from such copolymers.
• Further suitable binders are products obtainable from the conversion of hydroxyl-containing polymers with intramolecular dicarboxylic anhydrides.
• Further useful binders are polymers in which groups with acid hydrogen atoms are present, some or all of which are converted with activated isocyanates. Examples of these polymers are products obtained by conversion of hydroxyl-containing polymers with aliphatic or aromatic sulfonyl isocyanates or phosphinic acid isocyanates.
• polymers with aliphatic or aromatic hydroxyl groups for example copolymers containing units of hydroxyalkyl (meth)acrylates, allyl alcohol, hydroxystyrene or vinyl alcohol, as well as epoxy resins, provided they carry a sufficient number of free OH groups.
• Particular useful binder and particular useful reactive binders are disclosed in EP 1 369 232, EP 1 369 231, EP 1 341 040, US 2003/0124460, EP 1 241 002, EP 1 288 720, US 6,027,857, US 6,171,735 and US 6,420,089.
• the organic polymers used as binders have a typical mean molecular weight M w between 600 and 200 000, preferably between 1 000 and 100 000. Preference is further given to polymers having an acid number between 10 to 250, preferably 20 to 200, or a hydroxyl number between 50 and 750, preferably between 100 and 500.
• the amount of binder(s) generally ranges from 10 to 90 % by weight, preferably 20 to 80 % by weight, relative to the total weight of the non-volatile components of the composition.
• Nonionic surfactants are preferred.
• Preferred nonionic surfactants are polymers and oligomers containing one or more polyether (such as polyethylene glycol, polypropylene glycol, and copolymer of ethylene glycol and propylene glycol) segments.
• nonionic surfactants are block copolymers of propylene glycol and ethylene glycol (also called block copolymer of propylene oxide and ethylene oxide); ethoxylated or propoxylated acrylate oligomers; and polyethoxylated alkylphenols and polyethoxylated fatty alcohols.
• the nonionic surfactant is preferably added in an amount ranging between 0.1 and 30% by weight of the photopolymerizable composition, more preferably between 0.5 and 20%, and most preferably between 1 and 15%.
• the coating is capable of producing a visible print-out image upon exposure to infrared light.
• the print-out image is characterized by a contrast between exposed and non-exposed areas which is quantified by the CIE 1976 lightness difference ⁇ L*, measured immediately after exposure, i.e. before the exposed plate precursor is developed, of at least 5.
• ⁇ L* is preferably at least 8, more preferably at least 10, and most preferably at least 15.
• the color difference ⁇ E between the non-exposed and exposed areas is at least 5, preferably at least 10, more preferably at least 15 and most preferably at least 18.
• the compounds, which produce the print-out image upon infrared exposure, can be present in the image-recording layer and/or optionally in (an)other layer(s) of the coating.
• the following embodiments of color forming compounds can be used :
• the interaction between D and Q may be a reaction whereby the compound Q and the dye D are covalently and/or ionically bound to each other, or whereby D and Q form a complex and/or one ore more hydrogen bonds.
• D and Q may be added to the coating solution of the image-recording layer or to another layer of the coating, e.g. a layer on top of the coating or an intermediate layer between the support and the image-recording layer or another intermediate layer between the top layer and the image-recording layer.
• D and Q may first be allowed to form the interaction product DQ which is then added to a coating solution or D and Q may be coated in separate layers and are then allowed to interact by diffusion between said layers.
• the interaction between D and Q reduces the white light absorption of the dye D. Said reduction of the white light absorption of D due to the interaction with Q can be measured by comparing the white light optical density of a coating comprising D and Q with the white light optical density of the same coating without Q. Upon image-wise exposure to infrared light, said interaction product DQ at least partially releases a dye, possibly D, and thereby forms the visible print-out image.
• the dye D is a di- or tri-arylmethane dye wherein an aryl group is substituted with an amino group, hereinafter also referred to as a "amino substituted di- or tri-aryl methane dye".
• amino substituted di- or tri-aryl methane dyes are given in the following list. In this list the dyes are mentioned in their reduced form (also called “leuco form” or “leuco dye") having one or two hydrogen atoms, the removal of which together with one or two electrons produces the dyes D which are suitable in the present invention.
• the dye D is an amino substituted di- or tri-arylmethane dye, having at least one hydrophilic group.
• hydrophilic groups are selected from sulphonic acid group, carboxylic acid group, phosphoric acid group or phosphonic acid group or salts thereof, such as alkali metal salts or ammonium salt; most preferred hydrophilic group is sulphonic acid group or salt thereof.
• the dye D is a cationic dye.
• Cationic dyes are dyes which carry a positive charge in their molecule.
• Preferred cationic dyes are dyes having a positive charge in the chromophore moiety of the molecule.
• More preferred cationic dyes are dyes having a positive charge in the chromophore moiety and having a hydrophilic group in a side chain of the chromophoric moiety. Examples of cationic dyes are those mentioned by R. Raue in the Ullmann's Encyclopedia of Industrial Chemistry, edited by Wiley-VCH, volume A5, p. 369-373 (1986).
• the dye D may also be incorporated into a polymer, comprising at least one monomeric unit having a dye D which is covalently or ionically bound to the monomeric unit by a linking group.
• the dye D in such a polymer is preferably an amino substituted di- or tri-arylmethane dye.
• the nucleophilic compound Q is preferably a compound having one or more electron-rich sites such as an unshared pair of electrons or ions, the negative end of a polar bond, or ⁇ -electrons, and this compound is able to donate electrons to, or to share electrons with dye D.
• Nucleophilic compounds are usually organic compounds comprising a hetero-atom such as O, S, N or P.
• the nucleophilicity of the compound Q is preferably high enough to form the interaction product DQ, especially when DQ is formed in situ in the coating at low pH, e.g. pH ⁇ 7. Upon heating, the interaction product DQ releases at least partially a dye and this release is less likely when the nucleophilicity is high.
• the nucleophilicity of Q is preferably sufficiently low to maintain its leaving capability upon heating (formation of dye); on the other hand, the nucleophilicity of Q is preferably sufficiently high to be able to from a leuco dye adduct DQ, even at a low pH value (pH ⁇ 7); as a result, the compound Q of the present invention exhibits preferably an acceptable compromise as to its nucleophilicity.
• Preferred nucleophilic compounds Q are compounds comprising a thiol group. More preferred nucleophilic compounds are compounds comprising a thiol group and an aminoacid group. Specific examples of suitable compounds comprising a thiol group are the following :
• nucleophilic compounds are :
• the nucleophilic compound Q may also be incorporated into a polymer, comprising at least one monomeric unit having a nucleophilic group which is covalently or ionically bound to the monomeric unit by a linking group.
• the nucleophilic group in such a polymer is preferably a thiol group. Examples of monomeric units having a nucleophilic group are the following :
• polymers comprising a monomeric unit having a nucleophilic group are :
• the indices m and n represent the number of monomeric units in the polymer and usually the monomeric unit with a nucleophilic group is present in an amount of at least 1 unit, preferably between 2 to 100, more preferably between 5 and 20.
• the nucleophilic compound Q and the dye D may both be present in the same compound, e.g. a compound wherein Q and D are covalently coupled by a linking group such as an alkylene group.
• a linking group such as an alkylene group.
• Another example of such a compound is a polymer comprising at least one monomeric unit having a nucleophilic group and at least one monomeric unit having a dye D.
• the nucleophilic group in this polymer is preferably a thiol group and the dye D is preferably an amino substituted di-or tri-arylmethane dye.
• the R d group which is transformed by a chemical reaction, is selected from the list consisting of
• Preferred classes of such color forming IR-dyes have a structure according to one of the following formulae II, III or IV: wherein Ar 1 , Ar 2 and Ar 3 are independently an optionally substituted aromatic hydrocarbon group or an aromatic hydrocarbon group with an annulated benzene ring which is optionally substituted, W 1 and W 2 are independently a sulfur atom or a -CM 10 M 11 group wherein M 10 and M 11 are independently an optionally substituted aliphatic hydrocarbon group or an optionally substituted (hetero)aryl group, or wherein M 10 and M 11 together comprise the necessary atoms to form a cyclic structure, preferably a 5- or 6-membered ring, M 1 and M 2 are independently an optionally substituted aliphatic hydrocarbon group or wherein M 1 and M 2 together comprise the necessary atoms to form an optionally substituted cyclic structure, preferably a 5- or 6-membered ring, more preferably a 5-membered ring, most preferably a 5-membered ring having
• the color-forming infrared dye can be a neutral, an anionic or a cationic dye depending on the type of the substituting groups and the number of each of the substituting groups.
• the dye of formula II, III or IV has at least one anionic or acid group, selected from the list consisting of -CO 2 H, - CONHSO 2 R h , -SO 2 NHCOR i , -SO 2 NHSO 2 R j , -PO 3 H 2 , -OPO 3 H 2 , -OSO 3 H or -SO 3 H groups or their corresponding salts, wherein R h , R i and R j are independently an aryl or an alkyl group, preferably a methyl group, and wherein the salts are preferably alkali metal salts or ammonium salts, including mono- or di- or tri- or tetra-alkyl ammonium salts.
• anionic or acid groups may be present on the aromatic hydrocarbon group or the annulated benzene ring of Ar 1 , Ar 2 or Ar 3 , or on the aliphatic hydrocarbon group of M 3 , M 4 or M 12 to M 15 , or on the (hetero)aryl group of M 12 to M 15 .
• Other substituting groups can be selected from a halogen atom, a cyano group, a sulphone group, a carbonyl group or a carboxylic ester group.
• each of the aliphatic hydrocarbon groups of M 3 , M 4 or M 12 to M 15 is terminally substituted with at least one of these groups, more preferably with -CO 2 H, - CONHSO 2 -Me, -SO 2 NHCO-Me, -SO 2 NHSO 2 -Me, -PO 3 H 2 or -SO 3 H groups or their corresponding salt, wherein Me represents a methyl group.
• the color-forming IR-dye has a structure according to one of the following formulae : wherein Q is O, S, -CR s R t or -COOR u wherein R s , R t and R u are independently a hydrogen atom or an alkyl group; and the other groups have the same meaning as defined in formula II, III and IV.
• color-forming IR-dyes that are suitable for use in the present invention are the following :
• the support of the lithographic printing plate precursor used in the method of the present invention is a grained and anodized aluminum support.
• the aluminum support is characterized by a lightness value L* sup not less than 70, more preferably even not less than 75.
• the a* sup and b* sup values of the surface of the aluminum are as low as possible : a* sup and b* sup are each in the range between -4 and +4, preferably between -1.5 and +1.5 and more preferably between -1 and +1.
• a low surface roughness also contributes to a higher contrast of the print-out image :
• a preferred aluminum surface is characterized by an arithmetical mean roughness Ra, measured according to DIN4768, between 0.1 and 0.7, more preferably between 0.1 and 0.5 and most preferably between 0.15 and 0.40.
• the aluminum substrates used in the present invention include those composed of substantially pure aluminum and aluminum alloys.
• Aluminum alloys include alloys of aluminum and materials such as silicon, copper, manganese, magnesium, chromium, zinc, lead, bismuth or nickel.
• the substrate is preferably degreased to remove milling oils.
• Degreasing is preferably conducted by passing the substrate through an aqueous solution of an alkali hydroxide, such as sodium hydroxide which is present in the solution at a concentration of from about 5 to about 50 g/l.
• the solution is preferably maintained at about 40 to about 100°C.
• Degreasing may be conducted at from about 1 to about 180 seconds.
• the substrate may be rinsed with water and/or chemically etched. This is preferably done by passing the substrate through a second aqueous solution of an alkali hydroxide, such as sodium hydroxide which is present in the solution at a concentration of from about 5 to about 25 g/l.
• the solution is preferably maintained at about 40 to about 100°C.
• Chemical etching is typically conducted between about 10 to about 180 seconds.
• the substrate is then preferably electrochemically grained.
• Electrochemical graining is preferably done by electrolyzing the substrate in an aqueous solution of nitric or hydrochloric acid at a concentration of from about 8 g/l to about 20 g/l, preferably from about 10 g/l to about 16 g/l and most preferably from about 12 to about 15 g/l.
• nitric acid aluminum nitrate is also added to the solution and if hydrochloric acid is used, then aluminum chloride and/or sulfate may be added to the solution.
• the aluminum salt is preferably added in an amount of from about 5 to about 100 g/l, more preferably from about 20 to about 80 g/l and most preferably from about 30 to about 60 g/l.
• the graining is preferably conducted in either direct or alternating current, however alternating current is most preferred. Graining is performed at a charge density of from about 5 to about 100 coulombs/dm 2 , preferably from about 40 to about 90 coulombs/dm 2 . Graining is done for from about 5 seconds to about 5 minutes.
• the substrate is then preferably rinsed and desmutted, e.g. with an aqueous solution of sulfuric or phosphoric acid in an amount of 100 to 300 g/l at a temperature for 40 to 100 °C for 2 to 10 seconds and again rinsed with water.
• Desmutting can also be carried out with an aqueous alkaline solution, e.g. a solution of an alkali hydroxide, such as sodium hydroxide which is present in the solution at a concentration of from about 5 to about 50 g/l.
• the solution is preferably maintained at about 40 to about 100°C.
• the substrate is then anodized, preferably by electrolytically treating the substrate in an aqueous solution of sulfuric or phosphoric acid having a concentration of from about 100 to about 300 g/l at a temperature of from about 40 to about 100°C. Sulfuric acid is most preferred.
• Anodizing preferably takes place for about from 5 seconds to about 5 minutes at a charge density from about 20 to about 100 coulombs/dm 2 .
• the anodization preferably produces an anodic oxide weight of from about 0.1 to about 5 g/m 2 , preferably from 3.0 to 4.5 g/m 2 .
• the substrate is then preferably washed with water and post-treated with an aqueous solutions of a hydrophilizing compound such as alkali silicate, silicic acid, Group IV-B metal fluorides, the alkali metal salts, polyvinylphosphonic acid, polyvinylmethyl phosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulfonated aliphatic aldehyde.
• a hydrophilizing compound such as alkali silicate, silicic acid, Group IV-B metal fluorides, the alkali metal salts, polyvinylphosphonic acid, polyvinylmethyl phosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid
• hydrophilizing compounds are poly(acrylic acid), the alkali zirconium fluorides, such as potassium zirconium hexafluoride, or hydrofluozirconic acid. Such hydrophilizing compounds are used in concentrations of from about 0.01 to about 10% by volume. A preferred concentration range is from about 0.05 to about 5% and the most preferred range is from about 0.1 to about 1%.
• the printing plate precursor of the present invention is image-wise exposed with infrared light, preferably near infrared light.
• the infrared light is preferably converted into heat by an IR light absorbing compound as discussed above.
• the image-recording layer preferably has a low sensitivity towards visible light.
• the material can be kept in ambient daylight at an intensity corresponding to normal working conditions without the need for a safe light environment during at least 4 hours, more preferably at least 12 hours.
• the printing plate precursors of the present invention can be exposed to infrared light by means of e.g. LEDs or an infrared laser.
• the light used for the exposure is a laser emitting near infrared light having a wavelength in the range from about 700 to about 1500 nm, e.g. a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser.
• the energy density of the infrared light used in the exposure step is 250 mJ/cm 2 or less, more preferably 200 mJ/cm 2 or less and most preferably 150 mJ/cm 2 or less.
• an energy density in the range between 70 and 250 mJ/cm 2 is sufficient to induce the color change according to the present invention.
• said energy density range is between 100 and 200 mJ/cm 2 and most preferably between 100 and 150 mJ/cm 2 .
• the photopolymerizable or photocurable composition is hardened, so as to form a hydrophobic phase which corresponds to the printing areas of the printing plate.
• “hardened” means that the coating becomes insoluble or non-dispersible for the developer and may be achieved through polymerization and/or crosslinking of the photosensitive coating, optionally followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction.
• this optionally heating step hereinafter also referred to as "pre-heat"
• the exposed plate precursor is heated before being developed, preferably at a temperature of about 80°C to 150°C and preferably during a dwell time of about 5 seconds to 1 minute.
• the non-exposed areas of the image-recording layer are removed, preferably without essentially removing the exposed areas, i.e. without affecting the exposed areas to an extent that renders the ink-acceptance of the exposed areas unacceptable.
• the developer may be plain water, an aqueous solution such as a gum solution, an alkaline developer, a solvent-based developer, etc.
• the development step may also be carried out on-press, i.e. while the exposed precursor is mounted on the plate cylinder of a lithographic printing press, by rotating said plate cylinder while feeding dampening liquid and/or ink to the coating of the precursor.
• 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 10, also the ink supply is switched on.
• 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.
• 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 during printing.
• Another suitable printing method uses so-called single-fluid ink without a dampening liquid.
• Suitable single-fluid inks have been described in US 4,045,232; US 4,981,517 and US 6,140,392.
• the single-fluid ink comprises an ink phase, also called the hydrophobic or oleophilic phase, and a polyol phase as described in WO 00/32705.
• the single-fluid ink can also be used for the on-press development of the exposed precursor. In that embodiment, no dampening liquid is required in the development step.
• L*, a*, b* values were measured following the ASTM E308-85 method, based on illuminant D65.
• a 0.3 mm thick aluminum foil was degreased by spraying with an aqueous solution containing 26 g/l of NaOH at 65°C for 2 seconds and rinsed with demineralized 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 of HCl, 15 g/l of SO 4 2 - ions and 5 g/l of Al 3+ ions at a temperature of 37°C and a current density of about 100 A/dm 2 .
• the aluminum 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 demineralized 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/dm 2 , then washed with demineralized water for 11 seconds and post-treated for 3 seconds by spraying a solution containing 2.2 g/l of polyvinylphosphonic acid at 70°C, rinsed with demineralized water for 1 seconds and dried at 120°C for 5 seconds.
• the support thus obtained was characterized by a surface roughness Ra of 0.37 ⁇ m and had an anodic weight of 3.0 g/m 2 .
• the coating solution defined in Table 1 was prepared and coated on support AS-1. After drying, the thickness of the layer was 1.5 g/m 2 .
• the aqueous solution defined in Table 2 was coated and then dried at 110°C for 2 minutes to obtain a top coat having a dry thickness of 2.0 g/m 2 .
• the lightness value L* nonexp of the complete material PPP-33 (support + image-recording layer + top coat) was 67. Its ISO visual density was 0.43.
• Plate precursor PPP-33 which comprises color-forming IR dye IRD-004 in the top coat, was exposed with a Creo Trendsetter IR laser (830 nm) at 275 mJ/cm 2 .
• Creo Trendsetter IR laser 830 nm
• a high-contrast print-out image was observed : the exposed areas were dark blue as opposed to the pale green background color of the non-exposed areas.
• ⁇ L* of the print-out image was 10.6 and ⁇ E was 12.3.
• the aqueous solution defined in Table 4 was coated and then dried at 110°C for 2 minutes to obtain a top coat having a dry thickness of 2.0 g/m 2 .
• Table 4 Composition (g) of the coating solution of the protective top coat
• Ingredient PPP-34 PPP-35 partially hydrolyzed polyvinylalcohol (degree of hydrolysis 88 %, viscosity 4 mPa ⁇ s in a solution of 4 wt.% at 20 °C). 17.03 partially hydrolyzed polyvinylalcohol (degree of hydrolysis 88 %, viscosity 8 mPa ⁇ s in a solution of 4 wt.% at 20 °C).
• PPP-34 and 35 were exposed with a Creo Trendsetter IR laser (830 nm) at 275 mJ/cm 2 .
• PPP-34 and PPP-35 showed a high-contrast print-out image : the exposed areas were dark blue as opposed to the green background color of the non-exposed areas.
• ⁇ L* of the print-out image obtained with PPP-34 and -35 was 9.0 and 14.9 respectively.
• ⁇ E was 11.0 and 15.6 respectively.
• a coating solution was prepared by mixing the ingredients as specified in Table 5. This composition was coated on support AS-1 and was dried at 105 °C . The resulting thickness of the layer was 1.5 g/m 2 .
• Table 5 Composition of the coating solution of the image-recording layer. Ingredient Parts per weight (g) (A) 5.77 (B) 3.14 (C) 0.13 (D) 0.34 (E) 0.57 (F) 16.72 (G) 33.32
• the aqueous solution defined in table 6 was coated and then dried at 120°C for 2 minutes to obtain a top coat having a dry thickness of 0.80 g/m 2 .
• Table 6 Composition (g) of the coating solution of the top coat layers OC-01 and OC-02.
• Ingredient OC-01 (Example 4) OC-02 (Example 5) (L) 3.02 3.02 (N) 7.56 - (O) - 7.56 (M) 22.67 22.67 (P) 39.73 39.73 (H) 27.02 27.02
• the ISO visual density was 0.49 and 0.51 respectively.
• the plate precursors of Examples 4 and 5 were exposed with a Creo Trendsetter 3244T (plate setter available from Creo, Burnaby, Canada), operating at 300 mJ/cm 2 and 150 rpm.
• the ⁇ L* values of the print-out image were 10.4 and 6.3 respectively.
• the ⁇ E values were 17.4 and 8.1 respectively.

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