IR-sensitive positive working lithographic printing plate precursor

Abstract

 IR-sensitive element comprising
(a) a substrate with a hydrophilic surface, and
(b) a positive working coating comprising
(i) at least 40 wt.-%, based on the dry weight of the coating, of at least one polymer which comprises phenolic OH groups and/or sulfonamide groups and is soluble in aqueous alkaline developer,
(ii) at least one arylpolysiloxane resin having a hydroxy content of at least 1 wt.-% based on the weight of the resin and a glass transition temperature in excess of 60°C,
(iii) at least one substance capable of absorbing radiation of a wavelength from the range of 650 to 1,300 nm and converting it into heat, and
(iv) optionally at least one further component selected from print-out dyes, plasticizers, surfactants, inorganic fillers, antioxidants, contrast dyes and pigments, and polymer particles.

Description

[0001] The present invention relates to IR-sensitive positive working elements, in particular IR-sensitive printing plate precursors whose coating comprises an arylpolysiloxan containing silanol groups; the invention furthermore relates to a process for their production and a process for imaging such elements.

[0002] Lithographic printing is based on the immiscibility of oil and water, wherein the oily material or the printing ink is preferably accepted by the image area, and the water or fountain solution is preferably accepted by the non-image area. When an appropriately produced surface is moistened with water and a printing ink is applied, the background or non-image area accepts the water and repels the printing ink, while the image area accepts the printing ink and repels the water. The printing ink in the image area is then transferred to the surface of a material such as paper, fabric and the like, on which the image is to be formed. Generally, however, the printing ink is first transferred to an intermediate material, referred to as blanket, which then in turn transfers the printing ink onto the surface of the material on which the image is to be formed; this technique is referred to as offset lithography.

[0003] A frequently used type of lithographic printing plate precursor (in this connection, the term printing plate precursor refers to a coated printing plate prior to exposure and developing) comprises a photosensitive coating applied onto a substrate on aluminum basis. The coating can react to radiation such that the exposed portion becomes so soluble that it is removed during the developing process. Such a plate is referred to as positive working. On the other hand, a plate is referred to as negative working if the exposed portion of the coating is hardened by the radiation. In both cases, the remaining image area accepts printing ink, i.e. is oleophilic, and the non-image area (background) accepts water, i.e. is hydrophilic. The differentiation between image and non-image areas takes place during exposure.

[0004] In conventional plates, a film containing the information to be transferred is attached to the printing plate precursor under vacuum in order to guarantee good contact. The plate is then exposed by means of a radiation source, part of which is comprised of UV radiation. When a positive plate is used, the area on the film corresponding to the image on the plate is so opaque that the light does not affect the plate, while the area on the film corresponding to the non-image area is clear and allows light to permeate the coating, whose solubility increases. In the case of a negative plate, the opposite takes place: The area on the film corresponding to the image on the plate is clear, while the non-image area is opaque. The coating beneath the clear film area is hardened due to the incident light, while the area not affected by the light is removed during developing. The light-hardened surface of a negative working plate is therefore oleophilic and accepts printing ink, while the non-image area that used to be coated with the coating removed by the developer is desensitized and therefore hydrophilic.

[0005] For several decades, positive working commercial printing plate precursors were characterized by the use of alkali-soluble phenolic resins and naphthoquinone diazide derivatives; imaging was carried out by means of UV radiation.

[0006] Recent developments in the field of lithographic printing plate precursors have led to radiation-sensitive compositions suitable for the production of printing plate precursors which can be addressed directly by lasers. The digital image-forming information can be used to convey an image onto a printing plate precursor without the use of a film, as is common in conventional plates.

[0007] One example of a positive working, direct laser addressable printing plate precursor is described in US-A-4,708,925. The patent describes a lithographic printing plate precursor whose imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. As described in the patent, the interaction between the phenolic resin and the onium salt results in an alkali solvent resistance of the composition, which restores the alkali solubility by photolytic decomposition of the onium salt. The printing plate precursor can be used as a precursor of a positive working printing plate or as a precursor of a negative printing plate, if additional process steps are added between exposure and developing, as described in detail in British patent no. 2,082,339. The printing plate precursors described in US-A-4,708,925 are UV-sensitive and can additionally be sensitized to visible and IR radiation.

[0008] Another example of a direct laser addressable printing plate precursor that can be used as a positive working system is described in US-A-5,372,907 and US-A-5,491,046. These two patents describe the decomposition of a latent Bronsted acid by radiation in order to increase solubility of the resin matrix upon image-wise exposure. As in the case of the printing plate precursor described in US-A-4,708,925, these systems can also be used as negative working systems in combination with additional process steps between imaging and developing. In the case of the negative working printing plate precursors, the decomposition products are subsequently used to catalyze a crosslinking reaction between the resins in order to render the layer of the irradiated areas insoluble, which requires a heating step prior to developing. As is the case in US-A-4,708,925, these printing plate precursors per se are sensitive to UV radiation due to the acid-forming materials used therein.

[0009] EP-A-0 823 327 describes IR-sensitive printing plate precursors whose radiation-sensitive layer comprises, in addition to an IR absorber and a polymer such as for example novolak, a substance that decreases the solubility of the composition in an alkaline developer. Amongst others, sulfonic acid esters, phosphoric acid esters, aromatic carboxylic acid esters, carboxylic acid anhydrides, aromatic ketones and aldehydes, aromatic amines and aromatic ethers are mentioned as such "insolubilizers". These printing plate precursors show a high degree of IR sensitivity and do not require additional steps between exposure and developing; furthermore, they can be handled under normal lighting conditions (daylight with a certain portion of UV radiation), i.e. they do not require yellow light.

[0010] EP-A-1 241 003 describes imageable elements with a positive working thermally imageable layer comprising a binder and an insolubilizer, and an overcoat layer comprising material that reduces the alkali-solubility of phenolic resins. Cationic and non-ionic surface-active materials, such as polyethoxylated, polypropoxylated and poly(ethoxylated/propoxylated) compounds, are mentioned as material for the overcoat layer.

[0011] WO 99/21725 discloses IR-sensitive positive working printing plate precursors whose heat-sensitive layer comprises a substance that improves the resistance of the unheated areas to an attack by the alkaline developer; this substance is selected from compounds with polyalkylene oxide units, siloxanes, as well as esters, ethers and amides of polyvalent alcohols, preferably siloxanes. These printing plate precursors as well are characterized by a high degree of IR sensitivity and can be handled under normal daylight conditions. The use of conventional siloxanes described in this document, however, results in the formation of precipitates in the processor.

[0012] In WO 2005/016645 heat-sensitive elements are disclosed where a (C₄-C₂₀ alkyl)phenol novolak is used instead of the siloxanes used in WO 99/21725. However, said novolaks are insoluble in aqueous alkaline developer and therefore their use also results in the formation of precipitates in the processor.

[0013] It is therefore the object of the present invention to provide IR-sensitive elements such as lithographic printing plate precursors which can be developed with conventional aqueous alkaline developers without the formation of precipitates in the processor. Besides this the coating of the imaged elements should be bakeable in order to have the possibility to further improve the abrasion resistance - and therefore the print run length ― if required.

[0014] It is furthermore an object of the present invention to provide a process for the production of such elements as well as a process for imaging such elements.

[0015] The first object is surprisingly achieved by an IR-sensitive element comprising:

(a) a substrate with a hydrophilic surface, and

(b) a positive working coating comprising

(i) at least 40 wt.-%, based on the dry weight of the coating, of at least one polymer which comprises phenolic OH groups and/or sulfonamide groups and is soluble in aqueous alkaline developer,

(ii) at least one arylpolysiloxane resin having a hydroxy content of at least 1 wt.-% based on the weight of the resin and a glass transition temperature in excess of 60°C,

(iii) at least one substance capable of absorbing radiation of a wavelength from the range of 650 to 1,300 nm and converting it into heat, and

(iv) optionally at least one further component selected from print-out dyes, plasticizers, surfactants, inorganic fillers, antioxidants, contrast dyes and pigments, polymer particles and carboxylic acid derivatives of a cellulose polymer.

[0016] According to one embodiment component (i) of the coating is selected from novolak resins, functionalized novolak resins, polyvinylphenol resins, polyvinyl cresols, poly(meth)acrylates with phenolic and/or sulfonamide side groups and phenolic polyvinylacetals.

[0017] The process according to the invention for imaging these elements comprises the following steps:

(a) providing an element as defined above

(b) image-wise exposure of the element to IR radiation and

(c) removing the exposed areas of the coating with an aqueous alkaline developer.

[0018] The IR-sensitive elements of the present invention can for example be printing plate precursors (in particular precursors of lithographic printing plates), printed circuit boards for integrated circuits or photomasks. The IR-sensitive compositions can also be used for producing reliefs to be used as printing forms, screens and the like.

[0019] A dimensionally stable plate or foil-shaped material is preferably used as a substrate in the production of printing plate precursors. Preferably, a material is used as dimensionally stable plate or foil-shaped material that has already been used as a substrate for printing matters. Examples of such substrates include paper, paper coated with plastic materials (such as polyethylene, polypropylene, polystyrene), a metal plate or foil, such as e.g. aluminum (including aluminum alloys), zinc and copper plates, plastic films made e.g. from cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose acetate, cellulose acetatebutyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate and polyvinyl acetate, and a laminated material made from paper or a plastic film and one of the above-mentioned metals, or a paper/plastic film that has been metallized by vapor deposition. Among these substrates, an aluminum plate or foil is especially preferred since it shows a remarkable degree of dimensional stability; is inexpensive and furthermore exhibits excellent adhesion to the coating. Furthermore, a composite film can be used wherein an aluminum foil has been laminated onto a polyethylene terephthalate film.

[0020] A metal substrate, in particular an aluminum substrate, is preferably subjected to a surface treatment, for example graining by brushing in a dry state or brushing with abrasive suspensions, or electrochemical graining, e.g. by means of a hydrochloric acid electrolyte, and optionally anodizing.

[0021] Furthermore, in order to improve the hydrophilic properties of the surface of the metal substrate that has been grained and optionally anodized in sulfuric acid or phosphoric acid, the metal substrate can be subjected to an aftertreatment with an aqueous solution of e.g. sodium silicate, calcium zirconium fluoride, polyvinyl phosphonic acid or phosphoric acid. Within the framework of the present invention, the term "substrate" also encompasses an optionally pretreated substrate exhibiting, for example, a hydrophilizing layer (also known as "interlayer") on its surface.

[0022] The details of the above-mentioned substrate pretreatment are known to the person skilled in the art.

[0023] The coating comprises at least 40 wt.-% of at least one polymer which comprises phenolic OH groups and/or sulfonamide groups and is soluble in aqueous alkaline developer. According to one embodiment, the polymer soluble in aqueous alkaline developer is selected from novolak resins, functionalized novolak resins, polyvinylphenol resins, polyvinyl cresols, poly(meth)acrylates with phenolic and/or sulfonamide side groups, phenolic polyvinylacetals and mixtures thereof. Polymers (including copolymers) with phenolic OH groups are preferred.

[0024] As used in the present invention, the term "(meth)acrylate" refers to both "acrylate" and "methacrylate"; the same applies analogously to "(meth)acrylic acid".

[0025] Novolak resins suitable for the present invention and soluble in aqueous alkaline developer (component (i)) are condensation products of one or more suitable phenols, e.g. phenol itself, m-cresol, o-cresol, p-cresol, 2,5-xylenol, 3,5-xylenol, resorcinol, pyrogallol, phenylphenol, diphenols (e.g. bisphenol-A), trisphenol, 1-naphthol and 2-naphthol with one or more suitable aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde and furfuraldehyde and/or ketones such as e.g. acetone, methyl ethyl ketone and methyl isobutyl ketone. The type of catalyst and the molar ratio of the reactants determine the molecular structure and thus the physical properties of the resin. Phenylphenol, xylenols, resorcinol and pyrogallol are preferably not used as the single phenol for condensation but rather in admixture with other phenols. An aldehyde/phenol ratio of about 0.5:1 to 1:1, preferably 0.5:1 to 0.8:1, and an acid catalyst are used in order to produce those phenolic resins known as "novolaks" and having a thermoplastic character. As used in the present application, however, the term "aqueous alkaline developer soluble novolak" should also encompass the phenolic resins known as "resols" which are obtained at higher aldehyde/phenol ratios and in the presence of alkaline catalysts as long as they are soluble in aqueous alkaline developers; however, resols are not preferred.

[0026] Novolaks suitable as component (i) can be prepared according to known processes or are commercially available. Preferably, the molecular weight (weight average determined by means of gel permeation chromatography using polystyrene as standard) is between 1,000 and 15,000, especially preferred between 1,500 and 10,000.

[0027] Functionalized novolaks can also be used as component (i) as long as they are soluble in aqueous alkaline developer. As used in the present invention, the term "functionalized novolaks" refers to novolaks wherein the OH group is esterified or etherified or has become part of a urethane bond due to reaction with an isocyanate. Examples of functionalized novolak resins include those of formula (I)

wherein
the groups R¹ and R² are independently selected from a hydrogen atom and a cyclic or straight-chain or branched saturated or unsaturated hydrocarbon group with preferably 1 to 22 carbon atoms (preferably hydrogen and C₁-C₄ alkyl),

R³

is a phenolic group derived from a novolak R³(OH)_k,

D

is a divalent cyclic or straight-chain or branched saturated or unsaturated hydrocarbon group with preferably 1 to 22 carbon atoms, which is derived from a diisocyanate of the formula D(NCO)₂ (e.g. isophorone diisocyanate, toluene-1,2-diisocyanate, 3-isocyanatomethyl-1-methyl-cyclohexylisocyanate),

m

is at least 1 and

k

is 1 or 2.

[0028] These functionalized novolaks of formula (I) are capable of forming multicenter hydrogen bonds, in particular a four-center hydrogen bond (also referred to as quadrupol H bonding, or QHB). Suitable QHB compounds are also described in US 6,320,018 B1 and US 6,506,536 B1.

[0029] Polyvinyl phenol resins suitable for the present invention are polymers of one or more hydroxystyrenes such as o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, 2-(o-hydroxyphenyl)propylene, 2-(m-hydroxyphenyl)propylene and 2-(p-hydroxyphenyl)-propylene. Such a hydroxystyrene can optionally comprise one or more additional substituents at the phenyl ring, such as e.g. a halogen atom (F, Cl, Br, I). It is important that the polyvinyl phenol resin is soluble in aqueous alkaline developers.

[0030] Polyvinyl phenol resins can be produced according to known processes. Usually, one or more hydroxystyrenes are polymerized in the presence of an initiator for free-radical or cationic polymerization.

[0031] The weight-average molecular weight of suitable polyvinyl phenol resins is preferably in the range of 1,000 to 100,000, more preferably 1,500 to 50,000.

[0032] Polyacrylates with sulfonamide side groups suitable for the present invention are for example those comprising structural units of the formulas (IIa) and/or (IIb) below:

        ― [CH₂―CH(CO―X¹―R⁴―SO₂NH―R⁵)]―     (IIa)

        ― [CH₂―CH(CO―X²―R^4a―NHSO₂―R^5a)]―     (IIb)

wherein

X¹ and X²

each represent O or NR⁶;

R⁴ and R^4a

each represent a substituted or unsubstituted alkylene group (preferably C₁-C₁₂), cycloalkylene group (preferably C₆-C₁₂), arylene group (preferably C₆-C₁₂) or aralkylene group (preferably C₇-C₁₄);

R⁵ and R⁶

each independently represent a hydrogen atom or a substituted or unsubstituted alkyl group (preferably C₁-C₁₂); cycloalkyl group (preferably C₆-C₁₂), aryl group (preferably C₆-C₁₂) or aralkyl group (preferably C₇-C₁₄);

R^5a

represents a substituted or unsubstituted alkyl group (preferably C₁-C₁₂), cycloalkyl group (preferably C₆-C₁₂), aryl group (preferably C₆-C₁₂) or aralkyl group (preferably C₇-C₁₄).

[0033] Such polyacrylates and starting monomers and comonomers for their production are described in detail in EP-A-0 544 264 (pages 3 to 5).

[0034] Polymethacrylates analogous to the polyacrylates of the formulas (IIa) and (IIb) can also be used according to the present invention.

[0035] Polyacrylates with sulfonamide side groups which additionally comprise a urea group in the side chain can be used as well. Such polyacrylates are for example described in EP-A-0 737 896 and exhibit the following structural unit (IIc):

wherein

X³

is a substituted or unsubstituted alkylene group (preferably C₁-C₁₂), cycloalkylene group (preferably C₆-C₁₂), arylene group (preferably C₆-C₁₂) or aralkylene group (preferably C₇-C₁₄), and

X⁴

is a substituted or unsubstituted arylene group (preferably C₆-C₁₂).

[0036] Polymethacrylates analogous to the polyacrylates of formula (IIc) can also be used in the present invention.

[0037] The polyacrylates of formula (IId) with urea groups and phenolic OH mentioned in EP-A-0 737 896 can also be used:

wherein

X³ and X⁴

are as described above.

[0038] Polymethacrylates analogous to the polyacrylates of formula (IId) can also be used in the present invention.

[0039] The weight-average molecular weight of suitable poly(meth)acrylates with sulfonamide side groups and/or phenolic side groups is preferably 2,000 to 300,000.

[0040] Phenolic polyvinylacetals suitable for the present invention comprise the following structural units (A), (B) and (C)

wherein R¹⁶ is H or C₁-C₄ alkyl
and a is an integer from 1 to 5.

[0041] The phenolic polyvinylacetal can optionally further comprise at least one of structural units (D), (E) and (F)

wherein R¹⁷ is C₁-C₁₈ alkyl,
c is an integer from 1 to 5 and
d is an integer from 1 to 3.

[0042] Preferably R¹⁶ is CH₃.
Preferably a is an integer from 1 to 3, more preferably a is 1. If a = 1, the hydroxy group is preferably in p-position.
c is preferably an integer from 1 to 3, more preferably 1. If c = 1, the carboxyl group is preferably in p-position.
If d = 1, the ―O―tosyl group is preferably in p-position.
R¹⁷ is preferably C₁-C₁₁ alkyl, for instance methyl, n-butyl and n-undecyl.

[0043] According to one embodiment the phenolic polyvinylacetal consists of units (A), (B), (C) and (E); according to another embodiment it additionally contains unit (D) and/or (F).

[0044] Preferably the phenolic polyvinylacetals have an acid number of 70 mg KOH/g polymer or less, more preferably 50 mg KOH/g polymer or less and particularly 30 mg KOH/g polymer or less; the acid number indicates the number of mg KOH necessary for neutralizing 1 g polymer by titration.

[0045] In the present invention also phenolic polyvinylacetals with several different units (B) and/or (C) and/or (D) and/or (E) and/or (F) can be used.

[0046] The molar ratio of units (A) to (F) is not limited, however, the following amounts are preferred:

unit (A): 10 to 60 mol% (more preferably 15 to 40 mol%)

unit (B): 0.1 to 30 mol% (more preferably 1 to 15 mol%)

unit (C): 10 to 80 mol% (more preferably 40 to 60 mol%)

unit (D): 0 to 50 mol% (more preferably 10 to 30 mol%)

unit (E): 0 to 50 mol% (more preferably 10 to 30 mol%)

unit (F): 0 to 20 mol% (more preferably 0 to 5 mol%)

[0047] If several different units (B) are present, the above amounts refer to the total of units (B); the same applies to units (C), (D), (E) and (F).

[0048] The phenolic polyvinylacetals can be prepared according to known processes; they are for instance described in detail in US 5,169,897, DE-B-34 04 366 and DE-A-100 11 096.

[0049] The degree of hydrolysation of the vinylalcohol/vinylacetat-copolymers used for the preparation of the polyvinylacetals is preferably 70 to 98 mol% (more preferably 80 to 98 %) and their molecular weight (weight average) M_w is preferably 20,000 to 130,000 g/mol (more preferably 35,000 to 130,000 g/mol).

[0050] Based on the dry weight of the coating, the amount of polymer soluble in aqueous alkaline developer is at least 40 wt.-%, preferably at least 50 wt.-%, more preferred at least 70 wt.-% and particularly preferred at least 80 wt.-%. Usually the amount does not exceed 95 wt.-%, more preferred 85 wt.-%.

[0051] In the framework of the present invention, the dry weight of the coating is equated with the solids content of the coating composition(s) used for the production of the coating.

[0052] The arylpolysiloxanes used in the present invention have a glass transition temperature Tg of more than 60°C (preferably at least 65°C, more preferably at least 70°C; preferably up to 200°C) and a hydroxyl content of at least 1 % by weight (preferably at least 3 % by weight); preferably the hydroxyl content is up to 8 % by weight, more preferably up to 6 %. Their molecular weight is preferably from 500 to 10.000 g/mol with particular preference from 500 to 6000 g/mol. In the framework of the present invention the hydroxyl content relates to OH groups present as silanol groups.

[0053] The arylpolysiloxanes can be prepared by reacting identical or different alkoxysilanes of the general formula (III)

         R_xSi(OR')_y     (III)

with each R being independently selected from a hydrogen atom, and optionally substituted aryl, with the proviso that at least one R is phenyl, each R' being independently selected from hydrogen and optionally substituted alkyl, x being from 1 to 3 and y being from 3 to 1, with x + y = 4
in the presence of acid, water and organic solvent having a boiling point above that of water, the acid used being removed by distillation at the end.

[0054] Each R is preferably an optionally substituted aryl, in particular phenyl.

[0055] Each R' is preferably independently H, methyl, ethyl, butyl or dimethylol butyl (or other residues of polyhydric alcohols).

[0056] The arylpolysiloxanes used in the present invention are for instance described in US 6,552,151 B1, DE 198 57 348 and WO 00/35994 which are incorporated herein by reference. Processes for their production are also described in detail in said documents. A suitable product is commercially available under the tradename Silres® IC 836 from Wacker Chemie GmbH, Germany.

[0057] The above described arylpolysiloxanes are characterized by a relatively high content of OH groups compared to other siloxanes. This is believed to allow crosslinking of the siloxanes at a temperature of 230°C or above which would mean that crosslinking occurs at the imaged precursor when it is baked resulting in an improved print run length. On the other hand the high content of OH groups seems to result in better solubility of the unbaked coating in aqueous alkaline developers so that sludge formation in the processor can be avoided.

[0058] The arylpolysiloxane is preferably present in the positive working coating in an amount of at least 0,05 and less than 60 wt.-% based on the dry coating weight, more preferably 0,1 to 20 wt.-%.

[0059] The chemical structure of the IR absorber is not particularly restricted as long as it is capable of converting the absorbed radiation into heat. It is preferred that the IR absorber shows an essential absorption in the range of 650 nm to 1,300 nm, preferably 750 to 1,120 nm, and preferably exhibits an absorption maximum in that range. IR absorbers showing an absorption maximum in the range of 800 to 1,100 nm are especially preferred. It is furthermore preferred that the IR absorber essentially does not absorb radiation in the UV range. The absorbers are selected e.g. from carbon black, phthalocyanine dyes and pigments, and dyes and pigments from the polythiophene, squarylium, thiazolium, croconate, merocyanine, cyanine, indolizine, pyrylium or the metaldithioline class, preferably from the cyanine class. The compounds mentioned in Table 1 of US-A-6,326,122 are e.g. suitable IR absorbers. Further examples can be found in US-A-4,327,169, US-A-4,756,993, US-A-5,156,938, WO 00/29214, US-B-6,410,207 and EP-A-1 176 007.

[0060] According to one embodiment, a cyanine dye of the formula (IV)

is used, wherein

each Z

independently represents S, O, NR^a or C(alkyl)₂;

each R'

independently represents an alkyl group, an alkylsulfonate group or an alkylammonium group;

R"

represents a halogen atom, SR^a, OR^a, SO₂R^a or NR^a₂;

each R"'

independently represents a hydrogen atom, an alkyl group, -COOR^a, -OR^a, -SR^a, -NR^a₂ or a halogen atom; R"' can also be a benzofused ring;

A^-

represents an anion;

R^a

represents a hydrogen atom, an alkyl or aryl group;

each b

is independently 0, 1, 2 or 3; and

R^b und R^c

are both hydrogen or together with the carbon atoms to which they are bonded form a carbocyclic 5- or 6-membered ring.

[0061] If R' represents an alkylsulfonate group, an inner salt can form so that no anion A^- is necessary. If R' represents an alkylammonium group, a second counterion is needed which is the same as or different from A^-.

Z

is preferably a C(alkyl)₂ group.

R'

is preferably an alkyl group with 1 to 4 carbon atoms.

R"

is preferably a halogen atom or SR^a.

R"'

is preferably a hydrogen atom.

R^a

is preferably an optionally substituted phenyl group or an optionally substituted heteroaromatic group.

[0062] R^b and R^c preferably form a carbocyclic ring together with the carbon atoms to which they are bonded.

[0063] The counterion A^- is preferably a chloride ion, trifluoromethylsulfonate or a tosylate anion.

[0064] Of the IR dyes of formula (II), dyes with a symmetrical structure are especially preferred. Examples of especially preferred dyes include:

2-[2-[2-Phenylsulfonyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indole-2-ylidene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indoliumchloride,

2-[2-[2-thiophenyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indole-2-ylidene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-3H-indoliumchloride,

2-[2-[2-thiophenyl-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-indole-2-ylidene)-ethylidene]-1-cyclopentene-1-yl]-ethenyl]-1,3,3 -trimethyl-3H-indoliumtosylate,

2-[2-[2-chloro-3-[2-(1,3-dihydro-1,3,3-trimethyl-2H-benzo[e]-indole-2-ylidene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-1,3,3-trimethyl-1H-benzo[e]-indolium-tosylate and

2-[2-[2-chloro-3-[2-ethyl-(3H-benzthiazol-2-ylidene)-ethylidene]-1-cyclohexene-1-yl]-ethenyl]-3-ethyl-benzthiazolium-tosylate.

[0065] The following compounds are also IR absorbers suitable for use in the present invention:

[0066] The IR absorber is present in the IR-sensitive coating in an amount of at least 0.1 wt.%, based on the dry weight of the coating, more preferred at least 1 wt.-%, still more preferred at least 1.5 wt.-%. Usually, the amount of IR absorber does not exceed 25 wt.-%, more preferred 20 wt.-% and most preferred 15 wt.-%. A single IR absorber or a mixture of two or more can be present; in the latter case, the amounts given refer to the total amount of all IR absorbers.

[0067] The amount of IR absorber to be used also has to be considered in connection with the dry layer thickness of the coating. Preferably, it should be selected such that the optical density of the coating - measured for example on a transparent polyester film - preferably shows values between 0.4 and 20 at the wavelength of the incident IR radiation.

[0068] Furthermore, the coating can also comprise carboxylic acid derivatives of a cellulose polymer. Suitable derivatives include reaction products of a cellulose polymer, such as e.g. a cellulose alkanoate, and a carboxylic acid or in particular an acid anhydride, wherein the carboxylic acid and the anhydride are preferably of the formulas (V) and (Va), respectively

wherein

Y

is selected from

        ― (CR⁶R⁷)_k― and ―CR⁸=CR⁹―

wherein

k

is an integer from 1 to 6,

each R⁶ and R⁷

is independently selected from a hydrogen atom and a C₁-C₆ (preferably C₁-C₄) alkyl group (if k>1 not all groups R⁶ have to be the same nor do all groups R⁷ have to be the same), and

R⁸ and R⁹

are independently selected from a hydrogen atom and a C₁-C₆ (preferably C₁-C₄) alkyl group or R⁸ and R⁹, together with the two carbon atoms to which they are bonded, form an optionally substituted aryl or heteroaryl group.

[0069] It is especially preferred that Y be selected from:

        ― CR¹⁰R¹¹―CR¹²R¹³― ; ―CR¹⁴= CR¹⁵―







wherein R¹⁰ to R¹⁵ are each independently selected from a hydrogen atom and a C₁-C₆ alkyl group.

[0070] Such carboxylic acid derivatives of a cellulose polymer are for example described in EP-A-1 101 607 in paragraphs [0024] to [0037].

[0071] The commercially available derivatives such as cellulose acetate phthalate (CAP), cellulose acetate hydrogen phthalate (CAHPh), cellulose acetate trimellitate (CAT), cellulose acetate propionate and cellulose acetate butyrate should be mentioned in particular in this connection.

[0072] The amount of cellulose carboxylic acid derivatives in the coating ― if they are present ― can account for up to 15 wt.-%, based on the dry weight of the coating, preferably up to 10 wt.-% and especially preferred up to 5 wt.-%.

[0073] The acid number of the cellulose carboxylic acid derivative is preferably at least 50 mg KOH/g polymer, more preferably at least 80 mg KOH/g polymer and most preferred at least 100 mg KOH/g polymer. Preferably, the acid number does not exceed 210 mg KOH/g polymer. The acid number indicates the number of mg KOH necessary for neutralizing 1 g polymer by titration.

[0074] The coating can also comprise polymer particles with an average particle diameter of preferably 0.5 to 5 µm.

[0075] The coating can furthermore comprise dyes or pigments having a high absorption in the visible spectral range in order to increase contrast. Suitable dyes and pigments are those that dissolve well in the solvent or solvent mixture used for coating or can easily be introduced in the disperse form of a pigment. Suitable contrast dyes include inter alia rhodamine dyes, triarylmethane dyes such as Victoria blue R and Victoria blue BO, crystal violet and methyl violet, anthraquinone pigments, azo pigments and phthalocyanine dyes and/or pigments. The dyes are preferably present in the coating in an amount of from 0.5 to 15 wt.-%, especially preferred in an amount of from 1.5 to 7 wt.-%, based on the dry weight of the coating.

[0076] Furthermore, the layer can comprise surfactants (e.g. anionic, cationic, amphoteric or non-ionic tensides or mixtures thereof). Suitable examples include fluorine-containing polymers, polymers with ethylene oxide and/or propylene oxide groups, sorbitol-tri-stearate and alkyl-di-(aminoethyl)-glycines. They are preferably present in an amount of 0 to 10 wt.-%, based on the dry weight of the coating, especially preferred 0.2 to 5 wt.-%.

[0077] Further optional components of the radiation-sensitive composition are e.g. inorganic fillers such as e.g. Al₂O₃ and SiO₂ (they are preferably present in an amount of 0 to 20 wt.-%, based on the dry weight of the coating, especially preferred 0.1 to 5 wt.-%).

[0078] The coating can also comprise print-out dyes such as crystal violet lactone or photochromic dyes (e.g. spiropyrans etc.). They are preferably present in an amount of 0 to 15 wt.-% based on the dry weight of the coating, especially preferred 0.5 to 5 wt.-%.

[0079] The coating can furthermore comprise antioxidants such as e.g. mercapto compounds (2-mercaptobenzimidazole, 2-mercaptobenzthiazole, 2-mercaptobenzoxazole and 3-mercapto-1,2,4-triazole), and triphenylphosphate. They are preferably used in an amount of 0 to 15 wt.-%, based on the dry weight, especially preferred 0.5 to 5 wt.-%.

[0080] According to one embodiment, the coating is applied onto the optionally pretreated substrate from a solution of all components in a polar organic solvent or solvent mixture (e.g. alcohols such as methanol, n- and iso-propanol, n-and iso-butanol; ketones such as methyl ethyl ketone, methyl propyl ketone, cyclohexanone; multifunctional alcohols and derivatives thereof, such as ethylene glycol monomethyl ether and monoethyl ether, propylene glycol monomethyl ether and monoethyl ether; esters such as methyl lactate and ethyl lactate) and dried. This can be carried out by means of common coating methods such as coating with doctor blades, spin coating, and the like.

[0081] The dry weight of the coating in lithographic printing plate precursors is preferably 0.5 to 4.0 g/m², especially preferred 1 to 3 g/m².

[0082] Imaging can be carried out by means of IR irradiation, e.g. in the form of semiconductor lasers or laser diodes which emit in the range of 650 to 1,300 nm, preferably 750 to 1,120 nm. Such laser radiation can be digitally controlled via a computer, i.e. it can be turned on or off so that an image-wise exposure of the plates can be effected via stored digitized information in the computer which results in so-called computer-to-plate (ctp) printing plates. All image-setting units with IR lasers known to the person skilled in the art can be used for this purpose.

[0083] The image-wise exposed elements such as e.g. printing plate precursors are developed with an aqueous alkaline developer, which typically has a pH value in the range of 10 to 14. For this purpose, commercially available developers can be used.

[0084] The developed printing plates can additionally be subjected to a baking step in order to further increase the abrasion resistance of the printing areas; however, the printing plates according to the present invention do not necessarily have to be subjected to such a treatment since they can be used for printing a large number of copies without any deterioration in quality.

[0085] Under typical processing conditions for printing plates, the IR-sensitive elements of the present invention are preferably not sensitive to visible light and the UV portion of daylight so that they can be processed under white light and do not require yellow light conditions.

[0086] The present invention is described in more detailed in the following examples; however, they are not intended to restrict the invention in any way.

Examples

Glossary

[0087] 

6564 LB

- cresol-phenol novolak from Bakelite AG, Germany

PD 494A

- m/p cresol novolak from Borden Chemicals

Trump dye

- 830 nm IR-dye from Eastman Kodak

Projet 825

- 808 nm IR-dye from Avecia

crystal violet

- triphenylmethane contrast dye from Aldrich

CAHPh

- cellulose acetate hydrogen phthalate, polymer from Eastman Kodak

SP-1077

- p-octylphenol novolak from Schenectady Europe, France

Dowanol PM

- propylene glycol monomethylether (solvent from Dow Chemical)

MEK

- methyl ethyl ketone

PVPA

- polyvinylphosphonic acid

Example 1 and Comparative Examples 1 to 3

[0088] A 10 wt/wt.-% coating solution was made by dissolving 6564LB, PD494, Projet 825, Trump dye, crystal violet, CAHPh and hydrophobic polymer (see Table 1) (wt/wt.-% of total solids: 65, 23.5, 0.5, 1, 2, 2, 6) in a mix of Dowanol PM and MEK (80:20 wt/wt.-%). The solution was coated on a substrate (aluminum foil, electrolytically grained to an Ra of 0.45 µm, anodized to obtain 3 g/m² aluminum oxide; PVPA interlayer) by means of a wire bar and dried first by hot air, followed by 105°C for 90 sec to obtain a dry coating weight of 1.5 g/m².

[0089] The dried plate was then conditioned, i.e. covered with a 50 g/m² interleaving paper and kept for 3 days at 55°C.

[0090] The properties of the resulting plates were determined by using the following tests:

Test 1:

water repellency (hydrophobicity): by putting a drop of water on the surface and evaluating the contact angle (results see Table 1);

Test 2:

resistence to Goldstar® DC developer: by putting a drop of the developer at 25°C onto the plate surface and noting the time for the first attack and complete coating removal (results see Table 1);

Test 3:

resistence to loaded Goldstar® DC (20 g PD 140 were dissolved in 11 Goldstar® DC developer): test analoguous to test 2 (results see Table 1);

Test 4:

Test on sludge formation in the developer bath.
A 10 wt/wt.-% solution of 30 % hydrophobic polymer and 70 % 6564LB in Dowanol PM was coated by means of a wire bar and first dried by hot air and then in an oven at 105°C for 90 sec to give a dry coating weight of 1.5 g/m². The

substrate was an aluminum foil grained to 0.5 µm, anodized and interlayered with PVPA.
0,12 m² of each plate was immersed for 40 sec in 30 ml of developer. The resulting loaded developer was left overnight and then checked for precipitates.

Table 1

Example	hydrophobic polymer	test 1	test 2	test 3	test 4
1	Silres IC 8361)	rounded	good resistance	good resistance	clear solution
comparative 1	Silikophen2)	rounded	good resistance	good resistance	turbid solution and flaky precipitate
comparative 2	p-octylphenol novolak	rounded	good resistance	good resistance	turbid solution and flaky precipitate
comparative 3	none	flat	worse than Example 1	worse than Example 1	no image obtained but complete coating was removed

1) phenylethoxysiloxane, silanol groups 3 to 4.5 wt.-%; softening point 65 to 85°C; available from Wacker 2) phenylmethylsiloxane, containing methoxy groups and butoxy groups as well as 2 % methylol groups, but no silanol groups; available from Tego Chemie.

[0091] The conditioned plates were imaged in a Creo Trendsetter (830 nm) at 9,5 W and 100 rpm using the UGRA/FOGRA digital plate wedge V2.4 EPS. The exposed plates were then developed in a Mercury processor with Goldstar® (750 mm/min; 22.5°C). The wedge was evaluated with a Gretag densitometer D19C using a Yule-Nielsen factor of 1.15. The 50 % screen was measured to have 50 % for the plate of Example 1 and Comparative Examples 1 and 2. This shows that a plate according to the invention was as good as that of the comparative examples with respect to speed and resolution but - as is apparent from Table 1 - does not result in sludge formation in the processor.

Claims

1. IR-sensitive element comprising

(a) a substrate with a hydrophilic surface, and

(b) a positive working coating comprising

(i) at least 40 wt.-%, based on the dry weight of the coating, of at least one polymer which comprises phenolic OH groups and/or sulfonamide groups and is soluble in aqueous alkaline developer,

(ii) at least one arylpolysiloxane resin having a hydroxy content of at least 1 wt.-% based on the weight of the resin and a glass transition temperature in excess of 60°C,

(iii) at least one substance capable of absorbing radiation of a wavelength from the range of 650 to 1,300 nm and converting it into heat, and

(iv) optionally at least one further component selected from print-out dyes, plasticizers, surfactants, inorganic fillers, antioxidants, contrast dyes and pigments, and polymer particles.

2. IR-sensitive element according to claim 1, wherein component (i) is selected from novolak resins, functionalized novolak resins, polyvinylphenol resins, polyvinyl cresols, poly(meth)acrylates with phenolic and/or sulfonamide side groups, phenolic polyvinylacetals and mixtures thereof.

3. IR-sensitive element according to claim 1 or 2, wherein component (i) is a novolak resin or a mixture of novolak resins.

4. IR-sensitive element according to any of claims 1 to 3, wherein component (i) is a cresol novolak, a cresol-phenol novolak or a mixture thereof.

5. IR-sensitive element according to any of claims 1 to 4, wherein component (ii) is a phenylpolysiloxane.

6. 5. IR-sensitive element according to any of claims 1 to 5, wherein the hydroxy content is at least 3 wt.-%.

7. IR-sensitive element according to any of claims 1 to 6, wherein the IR-sensitive coating further comprises a carboxylic acid derivative of a cellulose polymer.

8. IR-sensitive alement according to any of claims 1 to 7, wherein component (iii) comprises a cyanin dye of formula (IV)

wherein
each Z independently represents S, O, NR^a or C(alkyl)₂;
each R' independently represents an alkyl group, an alkylsulfonate group or an alkylammonium group;
R" represents a halogen atom, SR^a, OR^a, SO₂R^a or NR^a₂;
each R"' independently represents a hydrogen atom, an alkyl group, -COOR^a, - OR^a, -SR^a,-NR^a₂ or a halogen atom or a benzofused ring;
A^- represents an anion;
R^a represents a hydrogen atom, an alkyl or aryl group;
each b is independently 0, 1, 2 or 3; and
R^b und R^c are both hydrogen or together with the carbon atoms to which they are bonded form a carbocyclic 5- or 6-membered ring.

9. IR-sensitive element according to any of claims 1 to 8, wherein the at least one polymer soluble in aqueous alkaline developer is present in an amount of 50 to 95 wt.-%, based on the dry weight of the coating.

10. IR-sensitive element according to any of claims 1 to 9, wherein the at least one acrylpolysiloxane is present in an amount of at least 0.05 wt.-%, based on the dry weight of the coating.

11. IR-sensitive element according to any of claims 1 to 10, wherein the element is a lithographic printing plate precursor.

12. IR-sensitive element according to claim 11, wherein the substrate is an aluminum substrate which prior to being coated with the IR-sensitive coating was subjected to at least one treatment selected from (a) mechanical and/or chemical graining, (b) anodizing and (c) hydrophilizing.

13. IR-sensitive element according to claim 11 or 12, wherein the dry weight of the coating is 0.5 to 4.0 g/m².

14. Process for the production of a IR-sensitive element as defmed in any of claims 1 to 13 comprising:

(a) applying a solution comprising components (i), (ii), (iii) and optionally (iv) as defined in any of claims 1 to 8 to a substrate with a hydrophilic surface, and

(b) drying.

15. Process for imaging a IR-sensitive element comprising:

(a) image-wise exposure of an IR-sensitive element as defined in any one of claims 1 to 13 to IR radiation and

(b) removing the exposed areas of the coating with an aqueous alkaline developer.