WO2009030279A1

WO2009030279A1 - A heat-sensitive lithographic printing plate precursor

Info

Publication number: WO2009030279A1
Application number: PCT/EP2007/059387
Authority: WO
Inventors: Hieronymus Andriessen; Tim Aerts
Original assignee: Agfa Graphics Nv
Priority date: 2007-09-07
Filing date: 2007-09-07
Publication date: 2009-03-12

Abstract

According to the present invention a printing plate precursor which is sensitive to near infrared light ranging from 700 nm to 1500 nm is disclosed. The printing plate precursor comprises an aluminum- silicon alloy support and a heat-sensitive coating, characterized in that the amount of silicon in said alloy ranges between 1% by weight and 20% by weight and that said support is capable of absorbing infrared light and subsequently converting it into heat at an extent sufficient to induce a lithographic image after exposing and optionally developing said precursor.

Description

A HEAT-SENSITIVE LITHOGRAPHIC PRINTING PLATE PRECURSOR

[DESCRIPTION] FIELD OF THE INVENTION [0001] The present invention relates to a heat-sensitive printing plate precursor.

BACKGROUND OF THE INVENTION

[0002] 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. In conventional, so-called "wet" 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] Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such 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 cross linking of a polymer, heat- induced solubilization, or by particle coagulation of a thermoplastic polymer latex. [0004] 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 an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the 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 . Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP- A 625,728.

[0005] Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer particle (latex), as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink.

[0006] Some of these thermal processes enable plate making without wet processing and are for example based on ablation of one or more layers of the coating. At the exposed areas the surface of an underlying layer is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating.

[0007] Other thermal processes which enable plate making without wet processing are for example processes based on a heat- induced hydrophilic/ oleophilic conversion of one or more layers of the coating so that at exposed areas a different affinity towards ink or fountain is created than at the surface of the unexposed coating.

[0008] US 6,250,225, US 6,357,353 and US 6,244,181 describe a negative working, non-ablative lithographic printing plate precursor which includes a metal support on to which a layer comprising a near infrared light absorbing compound for at least 50% by weight is provided.

[0009] US 6,715,420 discloses a printing plate precursor comprising a coating composition, such as an acrylic polymer, on an anodized aluminium substrate having a radiation- absorbing composition deposited in its pores and optionally a sealant layer provided thereon. Upon exposure to laser irradiation, the affinity of the coating for water and ink switches, or alternatively the coating is ablated, so that the underlying layer comprising the radiation-absorbing composition is revealed.

[0010] US 6,107,001 describes an imaging method wherein upon exposure to heat at least one layer which is provided on an anodized and colored aluminum support becomes irreversibly detached from said colored support at the exposed areas. During a cleaning step the exposed areas are removed. [0011] [0012] JP 2003-220773 describes a lithographic printing plate having an anodized aluminum alloy containing Si and Mn as a support on to which a coating including a radical polymerisable photopolymer composition is applied. Upon exposure to powerful laser light having a wavelength of 488 nm or 532 nm, ink accepting areas are obtained. The invention prevents irregular reflection of the laser light resulting in sharp images with good reproducibility.

[0013] Thermal 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. The IR dyes exhibit strong absorption in the IR wavelength and are typically distributed evenly throughout the heat- sensitive coating. As a result, the light intensity distribution is not constant over the whole thickness of the coating. In addition, the heat generated at the part of the coating closest to the support is dissipated faster due to the high thermal conductivity of aluminum. This effect is known in the art as the so-called heat-sink behaviour of aluminum. As a result of both effects, the heat generated within the coating is not homogeneously distributed which may lead to adhesion problems, a reduced run- length and/or a lower sensitivity of the printing plate.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a lithographic printing plate precursor comprising a heat- sensitive coating wherein the heat generated upon exposure to infrared light is homogeneously distributed throughout the coating or is even higher at the lower part of the coating, i.e. the part closest to the support.

[0015] It is a further object to provide a lithographic printing plate precursor comprising a heat-sensitive coating of which the imaging mechanism is triggered upon exposure to near infrared light whithout the requirement for the presence of an infrared absorbing agent.

[0016] These objects are realized by claim 1; i.e. a lithographic printing plate precursor which is sensitive to near infrared light ranging from 700 nm to 1500 nm comprising - -

an aluminum- silicon support and a heat- sensitive coating, characterized in that the amount of silicon in said alloy ranges between 1% by weight and 20% by weight and that said support is capable of absorbing infrared light and subsequently converting it into heat at an extent sufficient to induce a lithographic image after exposing and optionally developing said precursor.

[0017] It was surprisingly found that a printing plate precursor which comprises a heat-sensitive coating provided on an aluminum- silicon alloy support having an amount of silicon ranging between 1% by weight and 20% by weight is sensitive to near infrared light ranging from 700 ran to 1500 ran without the presence of an infrared light absorber in the coating. It was found that the aluminum- silicon alloy support enables a more efficient heat generation at its surface - i.e. the interface of the substrate with the heat-sensitive coating - resulting in an improved sensitivity, because the support is capable of absorbing infrared light and subsequently converting it into heat . This infrared absorption is even more pronounced when

₂ the anodic weight of the support is at least 2.5 g/m . The silicon present in the aluminum-silicon alloy support is preferably concentrated at the top of the support, i.e. the side of the support on to which the coating is applied. Furthermore, as the heat is more efficiently generated at the interface between the support and the coating, in the embodiment where the coating comprises thermoplastic polymer particles, switchable polymers or a photopolymerizable composition, the adhesion of the coating to the support will be improved rendering the image areas more resistant to development and to wear on the press.

[0018] Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The support of the printing plate of the present invention is an aluminum- silicon alloy. Aluminum- silicon alloys are commercially available and may contain up to 70 % by weight silicon depending on their specific use. The amount of silicon in the alloy used in the present invention is at least 0.1% by weight, preferably at least 0.5 % by weight, more preferably at least 5 %wt and most preferably at least 8 %wt . The amount of silicon in the alloy preferably ranges between 1% by weight and 20% by weight, more preferably the amount ranges between 5% by weight and 15% by weight and most preferably between 8% by weight and 15% by weight. Silicon has a very low solubility in aluminium and therefore precipitates in the aluminum as substantially pure silicon. Aluminium- silicon alloys having about 11 % by weight of Si are the best known alloys as they are formed at the lowest melting temperature. Indeed, the binary phase diagram of aluminum and silicon which plots relative concentrations of aluminum and silicon along the X-axis and temperature along the Y-axis, shows an eutectic point around 11 % by weight of silicon at the eutectic temperature of about 580 ⁰C. The presence of silicon in aluminum increases the fluidity of the melt, decreases the contraction associated with solidification and reduces the overall weight of the alloy as it has a low density. Also other elements including Fe, Mn, Mg, Cr, Zn, Cu and Ti and/or mixtures thereof may be present in the aluminum- silicon alloy at low concentrations; typically the sum of these elements is below 5 %wt .

[0020] The support of the present invention has preferably a thickness ranging from 50 μm to 500 μm, more preferably a thickness ranging from 100 μm to 400 μm, most preferably a thickness ranging from 150 μm to 300 μm. [0021] The aluminum- silicon alloy support is preferably - -

electrocheraically grained and anodized. The graining and anodizing steps of the aluminum- silicon alloy give similar results compared to the graining and anodizing steps of aluminum alloys with a high purity. The aluminum- silicon alloy support is preferably grained by electrochemical graining, and anodized by means of anodizing techniques employing phosphoric acid or a sulphuric acid/phosphoric acid mixture. Methods of both graining and anodization of aluminum and aluminum alloys which are very well known in the art, are also suitable for anodizing the aluminum- silicon alloy support of the present invention.

[0022] By anodizing the aluminum- silicon alloy support, its abrasion resistance and hydrophilic nature are improved. The microstructure as well as the thickness of the oxide layer formed are determined by the anodizing step, the anodic weight

2 (g/m oxides formed on the aluminium surface) typically varies

2 between 1 and 20 g/m . Preferably the anodic weight is at

2 least 1.5 g/m , more preferably the anodic weight is at least

₂

2.5 g/m , even more preferably the anodic weight is at least 5

2 g/m and most preferably the anodic weight is at least 10 g/m .

[0023] The grained and anodized aluminium-silicon alloy support may be post- treated to improve the hydrophilic properties of its surface. For example, the anodized surface may be silicated by treating its surface with a sodium silicate solution at elevated temperature, e.g. 95°C. Alternatively, a phosphate treatment may be applied which involves treating the anodized surface with a phosphate solution that may further contain an inorganic fluoride. Further, the anodized surface may be rinsed with an organic acid and/or salt thereof, e.g. carboxylic acids, hydrocarboxylic acids, sulphonic acids or phosphonic acids, or their salts, e.g. succinates, phosphates, phosphonates, - —

sulphates, and sulphonates. This treatment may be carried out at room temperature or may be carried out at a slightly elevated temperature of about 30⁰C to 50⁰C. A further interesting treatment involves rinsing the anodized surface s with a bicarbonate solution. Still further, the anodized surface may be treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid esters ofo polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulfonated aliphatic aldehyde. It is further evident that one or more of these post treatments may be carried out alone or in combination. More detailed descriptions of these treatments are given in GB 1084070, DE5 4423140, DE 4417907, EP 659909, EP 537633, DE 4001466, EP A 292801, EP A 291760 and US 4458005.

[0024] The printing plate precursor of the present invention is sensitive to near infrared light and comprises a heat-sensitive coating. Near infrared light means infrared0 light having a wavelength in the range from about 700 nm to about 1500 nm. The imaging mechanism of the thermal printing plate precursor is triggered by infrared light absorption of the coating and/or the support which is subsequently converted into heat. The heat-sensitive coating comprises an image-5 recording layer and optionally other layers.

[0025] A first suitable example of such a printing plate precursor is a precursor comprising thermoplastic polymer particles which are preferably dispersed in a hydrophilic binder. Due to heat generated during the exposure step, theo thermoplastic polymer particles may fuse or coagulate so as to form a developer-resistant phase which corresponds to the printing areas of the printing plate. Coagulation may result from heat- induced coalescence, softening or melting of the thermoplastic polymer particles . The thermoplastic polymer5 particles are present in the coating of the printing plate precursor and preferably have an average particle size below 200 ran, more preferably between 10 nm and 100 nm. The thermoplastic polymer particles have preferably an average particle size comprised between 15 nm to 75 nm, more preferably between 25 nm and 65 nm and most preferably between 35 nm and 55 nm. The particle size is defined as the particle diameter, measured by Photon Correlation Spectrometry, also known as Quasi-Elastic or Dynamic Light-Scattering. The amount of thermoplastic polymer particles in the layer which comprises said particles is preferably at least 70% by weight, more preferably between 70% by weight and 85% by weight and more preferably between 75% by weight and 85% by weight. The weight percentage of the thermoplastic polymer particles is determined relative to the weight of all the components in the layer which comprises said particles.

[0026] The thermoplastic polymer particles are preferably hydrophobic polymers selected from polyethylene, poly (vinyl) chloride , poiymethyl (meth) acryiate , polyethyi

(meth) acrylate, poyvinylidene chloride, poly (meth) acrylonitrile, polyvinylcarbazole, polystyrene or copolymers thereof. According to a preferred embodiment, the thermoplastic polymer particles comprise polystyrene or derivatives thereof, mixtures comprising polystyrene and poly (meth) acrylonitrile or derivatives thereof, or copolymers comprising polystyrene and poly (meth) acrylonitrile or derivatives thereof . The latter copolymers may comprise at least 50% by weight of polystyrene, and more preferably at least 65% by weight of polystyrene. In order to obtain sufficient resistivity towards organic chemicals such as hydrocarbons used in plate cleaners, the thermoplastic polymer particles preferably comprise at least 5% by weight of nitrogen containing units as described in EP 1 219 416, more preferably at least 30% by weight of nitrogen containing units, such as (meth) acrylonitrile . According to the most preferred embodiment, the thermoplastic polymer particles - ~

consist essentially of styrene and acrylonitrile units in a weight ratio between 1:1 and 5:1 (styrene : acrylonitrile) , e.g. in a 2:1 ratio .

[0027] The weight average molecular weight of the thermoplastic polymer particles may range from 5,000 to 1,000,000 g/mol.

[0028] The thermoplastic polymer particles present in the coating can be applied onto the lithographic base in the form of a dispersion in an aqueous coating liquid and may be prepared by the methods disclosed in US 3,476,937 or EP 1 217 010. Another method especially suitable for preparing an aqueous dispersion of the thermoplastic polymer particles comprises : dissolving the thermoplastic polymer in an organic water immiscible solvent, dispersing the thus obtained solution in water or in an aqueous medium and removing the organic solvent by evaporation.

[0029] The coating further comprises a hydrophilic binder0 which is preferably soluble in an aqueous developer. Examples of suitable hydrophilxc binders are homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate and maleics anhydride/vinylmethylether copolymers.

[0030] Also of interest are the coatings as disclosed in unpublished European Patent Application no's. 06122415.0 and 06122423.4 comprising thermoplastic polymer particles having an average particle diameter from 15 nm to 75 nm, more0 preferably from 25 to 55 nm, most preferably from 35 nm to 45 nm (measured by Photon Correlation Spectrometry) and either a compound comprising an aromatic moiety and at least one acidic group or salt thereof and having a most bathochromic light absorption peak at a wavelenght between 300 nm and 450 nm, or a compound with a specified structure and a most bathochromic light absorption peak at a wavelength between 451 and 750 nm. [0031] A printing plate precursor comprising a coating including hydrophobic thermoplastic polymer particles, a hydrophilic binder and a tetra-alkyl orthosilicate crosslinking agent, as disclosed in Research Disclosure no. 33303, January (1992), is also a suitable example.

[0032] In a second suitable embodiment, the printing plate is positive working and relies on heat-induced solubilization of an oleophilic resin. The oleophilic resin is preferably a polymer that is soluble in an aqueous developer, more preferably an aqueous alkaline developing solution with a pH between 7.5 and 14. Preferred polymers are phenolic resins e.g. novolac, resoles, polyvinyl phenols and carboxy substituted polymers . Typical examples of these polymers are described in DE-A-4007428 , DE-A-4027301 and DE-A-4445820. Further, the oleophilic resin may be a phenolic resin wherein the phenyl group or the hydroxy group is chemically modified with an organic substituent. The phenolic resins which are chemically modified with an organic substituent may exhibit an increased chemical resistance against printing chemicals such as fountain solutions or press chemicals such as plate cleaners. Examples of such chemically modified phenolic resins are described in EP-A 0 934 822, EP-A 1 072 432, US 5 641 608, EP-A 0 982 123, WO 99/01795, EP-A 02 102 446, EP-A 02 102 444, EP-A 02 102 445, EP-A 02 102 443, EP-A 03 102 522. The modified resins described in EP-A 02 102 446, are preferred, especially those resins wherein the phenyl -group of said phenolic resin is substituted with a group having the structure -N=N-Q, wherein the -N=N- group is covalently bound to a carbon atom of the phenyl group and wherein Q is an aromatic group.

[0033] In the latter embodiment the coating may comprise a second layer that comprises a polymer or copolymer (i.e. - -

(co) polymer) comprising at least one monomeric unit that comprises at least one sulfonamide group. This layer is located between the layer described above comprising the oleophilic resin and the hydrophilic support. Hereinafter, ^xa (co) polymer comprising at least one monomeric unit that comprises at least one sulfonamide group' is also referred to as "a sulphonamide (co) polymer" . The sulphonamide (co)polymer is preferably alkali soluble. The sulphonamide group is preferably represented by -NR-SO₂-, -SO₂-NR- or -SO₂-NRR' wherein R and R' each independently represent hydrogen or an organic substituent.

[0034] Sulfonamide (co) polymers are preferably high molecular weight compounds prepared by homopolymerization of monomeric units containing at least one sulfonamide group or by copolymerization of such monomeric units and other polymerizable monomeric units. Examples of monomeric units containing at least one sulfonamide group include monomeric units further containing at least one poiymerizabie unsaturated bond such as an acryloyl, allyl or vinyloxy group. Suitable examples are disclosed in U.S. 5,141,838, EP 1545878, EP 909,657, EP 0 894 622 and EP 1,120,246. Examples of monomeric units copolymerized with the monomeric units containing at least one sulfonamide group include monomeric units as disclosed in EP 1,262,318, EP 1,275,498, EP 909,657, EP 1,120,246,EP 0 894 622 and EP 1,400,351.

[0035] Suitable examples of sulfonamide (co) polymers and/or their method of preparation are disclosed in EP-A 933 682, EP- A 982 123, EP-A 1 072 432, WO 99/63407 and EP 1,400,351. [0036] A highly preferred example of a sulfonamide (co) polymer is a homopolymer or copolymer comprising a structural unit represented by the following general formula (D :

(D wherein:

R¹ represents hydrogen or a hydrocarbon group having up to 12 carbon atoms; preferably R¹ represents hydrogen or a methyl group ;

X¹ represents a single bond or a divalent linking group. The divalent linking group may have up to 20 carbon atoms and may contain at least one atom selected from C, H, N, O and S.

Preferred divalent linking groups are a linear alkylene group having 1 to 18 carbon atoms, a linear, branched, or cyclic group having 3 to 18 carbon atoms, an alkynylene group having 2 to 18 carbon atoms and an arylene group having 6 to 20 atoms, -0-, -S-, -CO-, -CO-O-, -0-C0-, -CS-, -NR¹¹R¹-, -CO- NR^h-, -NR^h-CO-, -NR^h-C0-O-, -O-CO-NR*¹- , -NR¹^-CO-NR¹-, -NR^h-CS- NR¹-, a phenylene group, a naphtalene group, an anthracene group, a heterocyclic group, or combinations thereof, wherein R^h and R¹ each independently represent hydrogen or an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclic, aryl, heteroaryl, aralkyl or heteroaralkyl group. Preferred substituents on the latter groups are an alkoxy group having up to 12 carbon atoms, a halogen or a hydroxyl group. Preferably X¹ is a methylene group, an ethylene group, a propylene group, a butylene group, an isopropylene group, cyclohexylene group, a phenylene group, a tolylene group or a biphenylene group;

Y¹ is a bivalent sulphonamide group represented by -NR^j- - -

SO₂- or

-SO₂-NR^k- wherein R^j and R^k each independently represent hydrogen, an optionally substituted alkyl, alkanoyl, alkenyl, alkynyl, cycloalkyl, heterocyclic, aryl, heteroaryl, aralkyl or heteroaralkyl group or a group of the formula -C (=N) -NH-R², wherein R² represents hydrogen or an optionally substituted alkyl or aryl group;

Z¹ represents a bi-, tri- or quadrivalent linking group or a terminal group. When Z¹ is a bi-, tri- or quadrivalent linking group, the remaining 1 to 3 bonds of Z¹ are linked to Y¹ forming crosslinked structural units.

When Z¹ is a terminal group, it is preferably represented by hydrogen or an optionally substituted linear, branched, or cyclic alkylene or alkyl group having 1 to 18 carbon atoms such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t-butyl group, a s -butyl group, a pentyl group, a hexyl group, a cyclopentyl group, a cyclohexyl group, an octyl group, an optionally substituted arylene or aryl group having 6 to 20 carbon atoms,- an optionally substituted hetero-arylene or heteroaryl group,- a linear, branched, or cyclic alkenylene or alkenyl group having 2 to 18 carbon atoms, a linear, branched, or cyclic alkynylene or alkynyl group having 2 to 18 carbon atom or an alkoxy group.

When Z is a bi, tri- or quadrivalent linking group, it is preferably represented by an above mentioned terminal group of which hydrogen atoms in numbers corresponding to the valence are eliminated there from. [0037] Examples of preferred substituents optionally present on the groups representing Z¹ are an alkyl group having up to 12 carbon atoms, an alkoxy group having up to 12 carbon atoms, a halogen atom or a hydroxyl group. [0038] The structural unit represented by the general formula (I) has preferably the following groups: X¹ represents an alkylene, cyclohexylene, phenylene or tolylene group, -0-, -S-, -CO-, -CO-O-, -0-C0-, -CS-, -NR¹¹R¹-, -CO-NR^h-, -NR^h-C0-, -NR^h-CO-O-, -O-CO-NR^h-, -NR^h-CO-NRⁱ- , -NR^h- CS-NR¹-, or combinations thereof, and wherein R^h and R¹ each independently represent hydrogen or an optionally substituted alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclic, aryl, heteroaryl, aralkyl or heteroaralkyl group. Preferred substituents on the latter groups are an alkoxy group having up to 12 carbon atoms, a halogen or a hydroxy1 group;

Y¹ is a bivalent sulphonamide group represented by -NR^j-SO₂-, -SO₂-NR^k- wherein R^j and R^k each independently represent hydrogen, an optionally substituted alkyl, alkanoyl, alkenyl, alkynyl, cycloalkyl, heterocyclic, aryl, heteroaryl, aralkyl or heteroaralkyl group;

Z¹ is a terminal group represented by hydrogen, an alkyl group such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t- butyl group, a s -butyl group, a pentyl group, a hexyl group, a cyclopentyl group, a cyclohexyl group or an octyl group, a benzyl group, an optionally substituted aryl or heteroaryl group, a naphtyl group, an anthracenyl group, a pyridyl group, an allyl group or a vinyl group.

[0039] Specific preferred examples of sulphonamide (co) polymers are polymers comprising N- (p-aminosulfonylphenyl) (meth) acrylamide, N- (m-aminosulfonylphenyl) (meth) acrylamide and/or N- (o-aminosulfonylphenyl) (meth) acrylamide . A particularly preferred sulphonamide (co) polymer is a polymer comprising N- (p-aminosulphonylphenyl) methacrylamide wherein the sulphonamide group comprises an optionally substituted straight, branched, cyclic or heterocyclic alkyl group, an optionally substituted aryl group or an optionally substituted heteroaryl group. [0040] The layer comprising the sulphonamide (co) polymer may further comprise additional hydrophobic binders such as a phenolic resin (e.g. novolac, resoles or polyvinyl phenols), a chemically modified phenolic resin or a polymer containing a s carboxyl group, a nitrile group or a maleimide group.

[0041] The dissolution behavior of the coating of the latter embodiment in the developer can be fine-tuned by optional solubility regulating components. More particularly, development accelerators and development inhibitors can beo used.

[0042] Development accelerators are compounds which act as dissolution promoters because they are capable of increasing the dissolution rate of the coating. For example, cyclic acid anhydrides as described in U.S. 4,115,128, phenols or organics acids as described in JP 60-88,942 and 2-96,755, can be used in order to improve the aqueous developability .

[0043] Developer resistance means, also called development inhibitors are capable of delaying the dissolution of the unexposed areas during processing. The dissolution inhibitingo effect is preferably reversed by heating, so that the dissolution of the exposed areas is not substantially delayed and a large dissolution differential between exposed and unexposed areas can thereby be obtained. The compounds described in e.g. EP-A 823 327 and WO97/39894 are believed to5 act as dissolution inhibitors due to interaction, e.g. by hydrogen bridge formation, with the alkali-soluble resin (s) in the coating. Inhibitors of this type typically comprise at least one hydrogen bridge forming group such as nitrogen atoms, onium groups, carbonyl (-C0-), sulfinyl (-S0-) oro sulfonyl (-SO₂-) groups and a large hydrophobic moiety such as one or more aromatic rings. Some of the compounds mentioned below, e.g. infrared dyes such as cyanines and contrast dyes such as quaternized triarylmethane dyes can also act as a dissolution inhibitor. Other suitable inhibitors improve the5 developer resistance because they delay the penetration of the aqueous alkaline developer into the coating. Preferred examples include (i) a polymeric material which is insoluble in or impenetrable by the developer, e.g. a hydrophobic or water-repellent polymer or copolymer; or polymers comprising siloxane (silicones) and/or perfluoroalkyl units; (ii) bifunctional compounds such as surfactants comprising a polar group and a hydrophobic group such as a long chain hydrocarbon group, a poly- or oligosiloxane and/or a perfluorinated hydrocarbon group and (iii) bifunctional block- copolymers comprising a polar block such as a poly- or oligo (alkylene oxide) and a hydrophobic block such as a long chain hydrocarbon group, a poly- or oligosiloxane and/or a perfluorinated hydrocarbon group.

[0044] More details concerning development accelerators and development inhibitors can be found in patent applications WO

2004/182,268; WO 2005/058,605; EP 1 543 958; EP 1 588 847.

[0045] The coating may optionally contain a compound which absorbs infrared light and converts the absorbed energy into heat. 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. Preferred IR absorbing compounds are dyes such as cyanine, merocyanine, indoaniline, oxonol, pyrilium and squarilium dyes or pigments such as carbon black. Examples of suitable infrared absorbers are described in e.g. EP 823 327, EP 978 376, EP 1 029 667, EP 1 053 868, EP 1 093 934; WO 97/39894 and WO 00/29214. Infrared absorbing dyes which become intensively colored after exposure by infrared irradiation or heating and thereby form a visible image, are also of interest and are extensively described in EP 1 614 541 and PCT 2006/063327. Another preferred IR compound is the following cyanine dye IR-I: [0046] 1 -

IR- I

[0047] The heat-sensitive printing plate precursor can be image-wise exposed with near infrared light ranging between about 700 nm and about 1500 nm. The infrared light may be converted into heat by an infrared light absorbing compound as discussed above and/or by the aluminum- silicon alloy support.

[0048] The printing plate precursor 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 nm to about 1500 nm, e.g. a semiconductor laser diode, a Nd: YAG or a Nd: YLF laser.

[0049] Two types of laser- exposure apparatuses are commonly used: internal (ITD) and external drum (XTD) platesetters . ITD plate- setters for thermal plates are typically characterized by a very high scan speed up to 500 m/s and may require a laser power of several Watts. XTD plate-setters for thermal plates having a typical laser power from about 200 mW to about 1 W operate at a lower scan speed, e.g. from 0.1 to 10 m/s. An XTD platesetter equipped with one or more laserdiodes emitting in the wavelength range between 750 and 850 nm is an especially preferred embodiment for the method of the present invention.

[0050] The known plate-setters can be used as an off-press exposure apparatus, which offers the benefit of reduced press down-time. XTD plate-setter configurations can also be used for on-press exposure, offering the benefit of immediate registration in a multi-color press. More technical details of on-press exposure apparatuses are described in e.g. US 5,174,205 and US 5,163,368.

[0051] After exposure, the precursor can be developed by means of a suitable processing liquid, such as an aqueous alkaline solution, whereby the non- image areas of the coating are removed; the development step may be combined with mechanical rubbing, e.g. by using a rotating brush. During development, any water-soluble protective layer present is also removed. The heat-sensitive printing plate precursors based on polymer particle coalescence, can also be developed using plain water or aqueous solutions, e.g. a gumming solution. The 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 or damaging. Suitable examples of such compounds are film- forming hydrophilic polymers or surfactants. The gum solution has preferably a pH from 4 to 10, more preferably from 5 to 8. Preferred gum solutions are described in EP 1 342 568. Alternatively, such printing plate precursors can after exposure directly be mounted on a printing press and be developed on-press by supplying ink and/or fountain to the precursor.

[0052] More details concerning the development step can be found in for example EP 1 614 538, EP 1 614 539, EP 1 614 540 and WO/2004071767.

[0053] In addition to the above discussed thermal materials, also heat-sensitive coatings which contain a photopolymerizable composition that hardens upon exposure to infrared light can be used in the printing plate precursor of the present invention. With "harden" is meant that the coating becomes insoluble or non-dispersible for the developer and may be achieved through polymerization and/or crosslinking of the photosensitive coating. The printing plates of interest are - -

those which can be sensitized with an infrared laser diode (830 nm) or a Nd-YAG laser (1060 nm) . In this type of materials, due to the heat generated during the exposure step, a photo-polymerisation reaction occurs and a hydrophobic phase which corresponds to the printing areas of the printing plate is formed. Such printing plate precursors typically comprise a coating containing a photocurable composition comprising a free radical initiator (as disclosed in for example US 5,955,238; US 6,037,098; US 5,629,354; US 6,232,038; US 6,218,076; US 5,955,238; US 6,037,098; US 6,010,824; US 5,629,354; DE 1,470,154; EP 024 629; EP 107 792; US 4,410,621; EP 215 453; DE 3,211,312 and EP A 1 091 247) a polymerizable compound (as disclosed in EP 1 614 541, EP 1 349 006, WO2005/109103, WO2007/057409 , WO2007/057413 , WO2007/057348 and WO2007/057333 and a polymeric binder (as disclosed in for example US2004/0260050 , US2005/0003285 ; US2005/0123853; 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; US 6,420,089; EP 152 819; EP 1 043 627; US 6,899,994; US2004/0260050 ; US 2005/0003285; US2005/0170286 ; US2005/0123853; US2004/0260050 ; US2005/0003285 ; US 2004/0260050; US 2005/0003285; US 2005/0123853 and US2005/0123853) . Other ingredients such as sensitizers, co- initiators, adhesion promoting compounds, colorants, surfactants and/or printing out agents may optionally be added. Heat-sensitive photopolymerizable compositions, i.e. compositions comprising initiators and/or initiator systems that generate free radicals upon thermal decomposition, are preferred in the present invention. Suitable examples of such initiators and/or initiator systems as disclosed in EP 1 176 007 include onium salts, trihalomethyl substituated triazine compounds, peroxides, azo-type polymerization initiators, azide compounds, quinonediazide compounds, and metallocene compounds . [0054] Typically, a photopolymer plate is processed in alkaline developer (see above) and subsequently gummed. Alternatively, the exposed photopolymer plate can also be developed by applying a gum solution to the coating whereby the non-exposed areas are removed. Suitable gumming solutions are described in WO/2005/111727. After the exposure step, the imaged precursor can also be directly mounted on a press and processed on-press by applying ink and/or fountain solution. Methods for preparing such plates are disclosed in WO 93/05446, US 6,027,857, US 6,171,735, US 6,420,089, US 6,071,675, US 6,245,481, US 6,387,595, US 6,482,571, US 6,576,401, US 6,548,222, WO 03/087939, US 2003/16577 and US 2004/13968.

[0055] In a further preferred embodiment, the heat- sensitive coating may also comprise a switchable polymer which is capable of changing the hydrophilic/hydrophobic-balance of the surface of the heat- sensitive coating due to heat generated during the exposure step with infared radiation. Usually such printing plates can be used directly on the printing press, but an additional wet developing step such as an on-press developing step or an off -press developing step (as discussed above) , may be used.

[0056] Typical examples of such systems are the thermally induced cleavage of labile groups pendant from a polymer backbone as described in WO 92/9934 and EP 652 483 and EP 1 787 810, polymeric systems which ablate from the support or which depolymerise upon heating, the thermal cyclodehydration of polyamic acids with hydrazide groups as described in US 4,081,572, the thermally induced destruction or generation of a charge on polymers as described in EP 200 488, the thermally induced rupture of microcapsels and the subsequent reaction the encapsulated material with other compounds of the coating as described in US 5,569,573, EP 646 476, WO 94/2395, WO 98/29258, the image-wise crosslinking of a water-soluble bottom layer with a phenolic top layer as described in JP 10069089, the heat- sensitive hyperbranched polymers containing heat- sensitive active end groups as described in US 6,162,578, and the polarity switchable image- forming materials as described in EP 1 129 861.

[0057] To protect the surface of the coating of the heat sensitive printing plate precursors described above, in particular from mechanical damage, a protective layer may also optionally be applied. The protective layer generally comprises at least one water-soluble binder, such as polyvinyl alcohol, polyvinylpyrrolidone, partially hydrolyzed polyvinyl acetates, gelatin, carbohydrates or hydroxyethylcellulose, and can be produced in any known manner such as from an aqueous solution or dispersion which may, if required, contain small amounts - i.e. less than 5% by weight based on the total weight of the coating solvents for the protective layer - of organic solvents . The thickness of the protective layer can suitably be any amount, advantageously up to 5.0 μm, preferably from 0.1 to 3.0 μm, particularly preferably from 0.15 to 1.0 μm. [0058] Optionally, the coating may further contain additional ingredients such as surfactants, especially perfluoro surfactants, silicon or titanium dioxide particles or polymers particles such as matting agents and spacers.

[0059] Any coating method can be used for applying two or more coating solutions to the hydrophilic surface of the support. The multi-layer coating can be applied by coating/drying each layer consecutively or by the simultaneous coating of several coating solutions at once. In the drying step, the volatile solvents are removed from the coating until the coating is self-supporting and dry to the touch. However it is not necessary (and may not even be possible) to remove all the solvent in the drying step. Indeed the residual solvent content may be regarded as an additional composition variable by means of which the composition may be optimized. Drying is typically carried out by blowing hot air onto the - -

coating, typically at a temperature of at least 70⁰C, suitably 80-15O⁰C and especially 90-140°C. Also infrared lamps can be used. The drying time may typically be 15-600 seconds. [0060] Between coating and drying, or after the drying step, a heat treatment and subsequent cooling may provide additional benefits, as described in WO99/21715, EP-A 1074386, EP-A 1074889, WO00/29214, and WO/04030923, WO/04030924, WO/04030925.

[0061] The printing plates thus obtained can be used for conventional, so-called wet offset printing, in which ink and an aqueous dampening liquid are supplied to the plate. 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. In a most preferred embodiment, 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.

EXAMPLES

1. Preparation of the substrates .

The substrates AA1050 and AlSiIO commercially available form Hydro Aluminium were used and their composition is given in Table 1.

Table 1: Composition of the substrates.

: specified m the U.S. Aluminum Association Standard, the balance is essentially aluminum.

a. Graining and anodisation of the AA1050-substrates . AA1050 foils having a thickness of 190 μm were used. The samples were pre-treated in a 5% NaOH solution at 50⁰C during 10 seconds and rinsed with water. Subsequently the samples were treated with 1.5% Nubuclean 60 (trademark of Chemos GmbH, Germany) at 60⁰C during 10 seconds and rinsed with water. The samples were then electrochemically grained in 6 g/1 HCl solution at room temperature, 20 A/dm² AC and 700 C/dm².

After the graining step, the samples were again rinsed with water. Subsequently the grained samples were anodized at room temperature in a 145 g/1 H₂SO₄ solution at 4 A/dm² DC during a varying period resulting in three different Al-oxide thicknesses estimated by microscopic analysis according to Table 2. After the anodizing, the samples were rinsed with water and dried at room temperature . The substrates were subsequently dipped in a 10 g/1 solution of polyvinyl phosphonic acid during 5 seconds at 80⁰C, rinsed with water and dried at room temperature. The substrates 01, 02 and 03 were obtained.

Table 2: Anodizing step.

b. Graining and anodisation of the AlSilO-substrates .

An AlSiIO alloy sheet commercially available from Hydro Aluminium was clad onto an AA1050 sheet. This resulted in a total sheet thickness of 240 μm (190 μm of the AA1050 sheet + 50 μm of the AlSiIO sheet) .

These clad foils were pre-treated in a 1.5% Nubuclean 60 (trademark of Chemos GmbH, Germany) at 60⁰C during 10 seconds and rinsed with water. The samples were then electrochemically grained in 12 g/1 HCl solution at room temperature, 50 A/dm² AC and 1000 C/dm².

After the graining step, the samples were again rinsed - -

with water. Subsequently the grained samples were anodized at room temperature in a 145 g/1 H₂SO₄ solution at 4 A/dm² DC during a varying period resulting in four different oxide thicknesses (Table 3) .

After the anodizing step, the samples were rinsed with water and dried at room temperature .

The substrates were subsequently dipped in a 10 g/1 solution of polyvinyl phosphonic acid during 5 seconds at 80⁰C, rinsed with water and dried at room temperature. The substrates 04, 05, 06 and 07 were obtained.

Visual inspection of these substrates shows a more pronounced blackening with thicker anodic layers indicating an increasing absorption in the IR spectral region of the oxide layer .

Table 3 : Anodizing time and resulting anodic layer thickness .

Substrate Anodizing time Anodic layer

(S) thickness (g/m²)

Substrate 04 75 1.5

Substrate 05 250 5

Substrate 06 500 10

Substrate 07 750 15

2. Preparation of the printing plate precursors.

a. Preparation of the thermoplastic polymer particles .

The polymer emulsion was prepared by means of a so-called 'seeded emulsion polymerization' technique wherein a part of the monomers, together with the surfactant, are brought into the reactor, before the initiator is added. All surfactant (2.15 wt. % relative to the total monomer amount) is present in the reactor before the reaction is started. In a 400 1 double- jacketed reactor, 17.2 kg of a 10% sodium dodecyl sulphate solution (Texapon K12 obtained from Cognis) and 243.4 kg of demineralised water was added. The reactor was put under inert atmosphere by 3 times vacuum/ nitrogen exchanging and heated - -

to 75 ⁰C. In another flask the monomer mixture was prepared by mixing 53.04 kg of styrene and 27.0 kg of acrylonitrile . 3.2 1 of the monomer mixture was added to the reactor and stirred during 15 min. at 75 ⁰C to homogeneously disperse the 'seed' monomer fraction. Then 6.67 kg of a 2% aqueous solution of sodium persulphate was added (33% of the total initiator amount) . After another 5 min. at 75 ⁰C, the reactor was heated up to 80 ⁰C in 30 min. At 80 ⁰C, the monomer and initiator dosage was started. The monomer mixture (85 1) of acrylonitrile (26.0 kg) and styrene (51.2 kg) was added during 3 hours . Simultaneously with the monomer addition an aqueous persulphate solution was added (13.33 kg of a 2% aqueous Na₂S₂O₈ solution) while keeping the reactor at 80 ⁰C. Since the reaction is slightly exothermic the reactor jacket was cooled until 74 ⁰C, in order to keep the reactor content at 80 ⁰C. After the monomer dosage, the reactor temperature was set to 82 ⁰C and stirred during 30 min. To reduce the amount of residual monomer a redox- initiation system was added: 340 g sodium formaldehyde sulphoxylate dihydrate (SFS) dissolved in 22.81 kg water and 570 g of a 70 wt . % t-butyl hydro peroxide (TBHP) diluted with 4.8 kg of water. The aqueous solutions of SFS and TBHP were added separately during 2 hours and 20 min. The reaction was then heated for another 10 min. at 82 ⁰C followed by cooling to 20 ⁰C. 760 g of a 5 wt . % aqueous solution of 5-bromo-5-nitro-l , 3-dioxane was added as biocide and the latex was filtered using a 5 micron filter.

This resulted in a dispersion with a solid content of 20.68 %wt and a pH of 3.25.

b. Preparation of the coating solution.

The coating solution was prepared by adding the above dispersion to a polyacrylic acid (PAA) solution followed by stirring during 10 minutes. Subsequently after another 10 - -

minutes of stirring a surfactant solution was added and the coating dispersion was stirred for another 30 minutes.

Subsequently the pH was adjusted to a value of 3.6 with a diluted ammonia solution (ca 3%) .

c . Preparation of the printing plate precursors .

The coating solution was subsequently coated on the substrates 1 - 7 as described above with a coating knife at a wet thickness of 30 μm. The coatings were dried at 60⁰C. Table

4 lists the resulting dry coating weight of the different components of the printing plate precursors .

Table 4: dry coating weight.

(1) Styrene-acrylonitrile latex with a particle diameter 0_PCs of 59 nm; particle diameter measured by Photon Correlation Spectroscopy according to ISO 13321 (first edition, 1996-07-01) with a Brookhaven BI- 90 analyzer from Brookhaven Instrument Company, Holtsville, NY,

USA;

(2) Polyacrylic acid from Ciba Specialty Chemicals; added to the coating solution as a 5 wt% aqueous solution;

(3) Zonyl FSO 100 is a fluorsurfactant commercially available from Dupont .

3. Exposure and printing steps.

The printing plate precursors 01 to 07 were exposed with a

Creo Trend-Setter 3244 (40W) (plate-setter, trademark from Creo, Burnaby, Canada) , operating at a varying density (750 -

500 - 300 - 200 - 100 mJ/cm²) at 150 rotations per minute

(rpm) with a 200 line per inch (lpi) screen.

The exposed printing plate precursors were directly mounted on a GTO46 printing press (available from Heidelberger - -

Druckmaschinen AG) without any processing or pre-treatment . A compressible blanket was used and printing was done with the fountain Agfa Prima FSlOl (trademark of Agfa) and K+E 800 black ink (trademark of K&E) .

The following start-up procedure was used : first 5 revolutions with the dampening rollers engaged followed with 5 revolutions with both the dampening and ink rollers engaged and finally the printing was started. 1000 prints were made on 80 g offset paper.

Table 5: Printing results of PP-Ol to PP- 07.

* : plate sensitivity (mJ/cm ) : the minimum exposure energy density required to render the measured optical density on the one -thousandth print on paper of the 1 pixel x 1 pixel (lxl) checkerboards (CHKB) equal to the measured optical density of the 8 pixel x 8 pixel (8x8) checkerboards (CHKB) on the one-thousandth print on paper. At a resolution of 2400 dots per inch (dpi), one pixel measures theoretically 10.56 μm x 10.56 μm. - -

[0062] From table 5 it can be concluded that thicker anodic oxide layers result in higher sensitivies. No printed image is observed for the AA1050 substrates, whereas the AlSiIO substrates clearly show an image . The results obtained for the printing plates having a substrate having an anodic layer thicknes greater than 1.5 μm, are superior.

[0063] The results indicate that the printing plate precursors having an aluminum- silicon alloy substrate are able to generate sufficient heat to allow for latex coalescence upon IR- laser irradiation without the presence of an infrared absorbing agent .

Claims

[CLAIMS]

1. A lithographic printing plate precursor sensitive to near infrared light ranging from 700 nm to 1500 nm comprising an aluminum- silicon alloy support and a heat-sensitive coating, characterized in that the amount of silicon in said alloy ranges between 1% by weight and 20% by weight and that said support is capable of absorbing infrared light and subsequently converting it into heat at an extent sufficient to induce a lithographic image after exposing and optionally developing said precursor.

2. A printing plate precursor according to claim 1 wherein the amount of silicon in the alloy ranges between 5% by weight and 15% by weight.

3. A printing plate precursor according to claims 1 or 2 wherein the heat-sensitive coating does not contain an infrared absorbing agent .

4. A printing plate precursor according to any of the preceding claims wherein the aluminum- silicon alloy support is grained and anodized and has an anodic weight

2 of at least 2.5 g/m .

5. A printing plate precursor according to claim 4 wherein the grained and anodized support has an anodic weight of

2 at least 5 g/m .

6. A printing plate precursor according to any of the preceding claims wherein the coating comprises thermoplastic polymer particles dispersed in a hydrophilic binder .

7. A printing plate precursor according to any of the preceding claims 1 to 5 wherein the coating comprises an alkali- soluble polymeric binder. - -

8. A printing plate precursor according to claim 7 wherein the alkali-soluble polymeric binder is a phenolic resin.

9. A printing plate precursor according to any of the preceding claims 1 to 5 wherein the coating comprises a photopolymerizable composition.

10. A method for making a lithographic printing plate precursor comprising the steps of

(i) providing an aluminum- silicon alloy support as defined in any of the preceding claims 1 to 5 ;

(ϋ) applying a heat- sensitive coating onto said support ;

(iii) drying the coated support.