WO2004022337A1

WO2004022337A1 - Stabilized infrared-sensitive elements

Info

Publication number: WO2004022337A1
Application number: PCT/EP2003/009836
Authority: WO
Inventors: Tobias Wittig; Hans-Joachim Timpe; Anja Klie
Original assignee: Kodak Polychrome Graphics Gmbh
Priority date: 2002-09-05
Filing date: 2003-09-04
Publication date: 2004-03-18
Also published as: AU2003267044A1; DE10241165A1; DE10241165B4

Abstract

Heat-sensitive element comprising (a) an optionally pre-treated substrate (b) a heat-sensitive coating comprising (i) at least one polymeric binder, (ii) at least one C-C unsaturated free-radical polymerizable monomer and/or oligomer and/or polymer, (iii) an initiator system for free-radical polymerization, comprising at least one IR absorber capable of absorbing radiation in the range of 800 to 1,200 nm and at least one polyhaloalkyl-substituted compound and (iv) at least one stabilizer selected from 1,10-phenanthroline, 8-quinoline thiol, 2,2'-bipyridyl, 8-hydroxyquinoline, α-benzoin oxime, 1,1,4,7,10,10hexamethyltriethylene tetraamine, 1,5-diphenylthiocarbazane, benzo-15crown-5, oxalic acid bis-(cyclohexylidenehydrazine), acid addition salts and hydrates of the above-mentioned compounds, as well as mixtures thereof.

Description

STABILIZED INFRARED-SENSITIVE ELEMENTS

The present invention relates to infrared-sensitive (in the following referred to as IR- sensitive), negative working elements, in particular elements whose IR-sensitive coating comprises a stabilizer; the invention furthermore relates to a process for their production and a process for imaging such elements.

The technical field of 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.

A frequently used type of lithographic printing plate precursor (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.

In conventional plates, a film containing the information to be transferred is attached to the printing plate precursor in order to guarantee good contact under vacuum. 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.

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.

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.

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.

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.

Printing plate precursors which can be image-wise exposed with IR lasers are furthermore known from EP-A-0 672 544, EP-A-0 672 954 as well as US-A-5,491 ,046 and EP-A-0 819 985. These precursors are negative-working and after image-wise exposure they also require a preheating step within a very narrow temperature range which causes a partial crosslinking of the image layer. In order to meet the highest requirements regarding the number of copies and to show sufficient resistance to printing chamber chemicals, the plates are subjected to baking, i.e. an additional heating step.

DE-C-199 06823 describes IR-sensitive negative working printing plate precursors with a long shelf-life, providing a consistently high number of copies and exhibiting a high degree of resistance to developing chemicals, which are also characterized by a high degree of IR-sensitivity and processability in daylight. The radiation-sensitive coating of these printing plates comprises an initiator system for free-radical polymerization, comprising an IR dye, a polyhaloalkyl-substituted compound and a polycarboxylic acid with an aromatic unit substituted with a heteroatom (N, O or S). These printing plate precursors as well require a heating step prior to developing in order to effect complete curing of the layer.

It has been found that printing plate precursors whose coating comprises a polyhaloalkyl-substituted compound, a polycarboxylic acid as defined above and a C-C unsaturated bond may show point-shaped curing upon heating. If such curing took place e.g. in the non-image areas (background areas), those fine points would undesirably be transferred to the printed sheet/the paper during printing.

It is the object of the present invention to provide IR-sensitive elements with polyhaloalkyl-substituted compounds, which do not show any undesired curing during heating, but are still characterized by a long shelf-life, providing a continuously high number of copies, resistance to developing chemicals, high degree of IR sensitivity and processability in daylight.

This object is achieved by an IR-sensitive element comprising

(a) an optionally pre-treated substrate

(b) an IR-sensitive coating comprising (i) at least one polymeric binder,

(ii) at least one C-C unsaturated free-radical polymerizable monomer and/or oligomer and/or polymer,

(iii) an IR-sensitive initiator system for free-radical polymerization, comprising at least one IR absorber capable of absorbing radiation in the range of 800 to 1 ,200 nm and at least one polyhaloalkyl- substituted compound and

(iv) at least one stabilizer selected from 1 ,10-phenanthroline, 8-quinoline thiol, 2,2'-bipyridyI, 8-hydroxyquinoline, α-benzoin oxime, 1 ,1 ,4,7,10,10-hexamethyltriethylene tetraamine, 1 ,5-diphenylthio- carbazone, benzo-15-crown-5, oxalic acid bis-(cyclohexylidene- hydrazine), acid addition salts and hydrates of the above-mentioned compounds, as well as mixtures thereof.

The inventors do not wish to restrict themselves to one theory; however, they assume that the undesired curing of the printing plate precursor described above is due to an initiation of the of polymerization of the C-C unsaturated compounds by a contamination of the substrate and/or the coating with heavy metals/heavy metal ions, such as e.g. copper, lead, nickel, manganese, chromium and cobalt and that these heavy metals/heavy metal ions are rendered harmless by the stabilizer functioning as an initiator in the present invention.

In the production of printing plate precursor with aluminum substrates there are e.g. several heavy metal/heavy metal ion contamination sources, such as abrasion contacts during discharging of the substrate, the electrolyte solution and "burnt" electrode material during anodizing. Contamination may also occur due to the corrosion of ferrous parts of the processing plant. Moreover, e.g. iron is what is referred to as an "environmental element" (i.e. it is practically omnipresent) so that ultra-high purity conditions would be necessary to completely rule out any contamination; however, the production of printing plate precursors under ultra-high purity conditions would lead to an unacceptable drastic increase in the production costs.

However, the present invention is not restricted to IR-sensitive elements with aluminum substrates; the use of the stabilizer also benefits other substrate materials that may be contaminated with heavy metals/heavy metal ions during the pre-treatment of the substrate, coating and/or drying.

The IR-sensitive elements of the present invention will be described in more detail in the following.

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.

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

Furthermore, in order to improve the hydrophilic properties of the surface of the metal substrate that has been roughened and optionally anodically oxidized in sulfuric acid or phosphoric acid, the metal substrate can be subjected to an after-treatment with an aqueous solution of e.g. sodium silicate, calcium zirconium fluoride, polyvinylphosphonic acid or phosphoric acid. Within the framework of the present invention, the term "substrate" also encompasses an optionally pre-treated substrate exhibiting, for example, a hydrophilizing coating on its surface.

The details of the above-mentioned substrate pre-treatment are well known to the person skilled in the art.

Basically all polymers or polymer mixtures known in the art can be used as polymeric binders for the IR-sensitive coating. Linear organic polymers soluble or swellable in water or aqueous alkaline solutions are especially suitable. Suitable binders are described for example in EP-A-1 170 123. Acrylic acid copolymers, methacrylic acid copolymers, itaconic acid copolymers, crotonic acid copolymers, maleic acid copolymers, partially esterified copolymers of maleic acid and acidic cellulose derivatives are particularly suitable. Preferably, the polymers have a weight-average molecular weight in the range of 10,000 to 1 ,000,000 (determined by means of GPC). In view of possible problems occurring in connection with ink acceptance during the printing process, it is preferred that the used polymer have an acid number of > 70 mg KOH/g, or, when polymer mixtures are used, that the arithmetic average of the individual acid numbers be > 70 mg KOH/g. A polymer or polymer mixture with an acid number of > 110 mg KOH/g is preferred; especially preferred is an acid number between 140 and 160 mg KOH/g. The content of the polymeric binder in the heat-sensitive coating preferably accounts for 30 to 60 wt.-%, more preferably 35 to 45 wt.-%, based on the dry layer weight.

All monomers, oligomers and polymers with C-C double and triple bonds which are free- radical polymerizable and comprise at least one C-C double or triple bond can be used as C-C unsaturated monomers, oligomers and polymers. Such compounds are well known to the person skilled in the art and can be used in the ^"present invention without any particular limitations. Esters of acrylic and methacrylic acids, itaconic acid, crotonic acid, isocrotonic acid, maleic acid and fumaric acid with one or more unsaturated groups in the form of monomers, oligomers or prepolymers are preferred. They may be present in solid or liquid form, with solid and highly viscous forms being preferred. Compounds suitable as monomers include for instance trimethylol propane triacrylate and methacrylate, pentaerythrite triacrylate and methacrylate, dipentaerythritemono hydroxy pentaacrylate and methacrylate, dipentaerythrite hexaacrylate and methacrylate, pentaerythrite tetraacrylate and methacrylate, ditrimethylol propane tetraacrylate and methacrylate, diethyleneglycol diacrylate and methacrylate, triethyleneglycol diacrylate and methacrylate or tetraethyleneglycol diacrylate and methacrylate. Suitable oligomers and/or prepolymers are urethane acrylates and methacrylates, epoxide acrylates and methacrylates, polyester acrylates . and methacrylates, polyether acrylates and methacrylates or unsaturated polyester resins.

In addition to monomers and/or oligomers, use can also be made of polymers comprising free-radical polymerizable C-C double bonds in the main or side chains. Examples thereof include reaction products of maleic acid anhydride olefin copolymers and hydroxyalkyl(meth)acrylates; polyesters comprising an allylalcoholester group; reaction products of polymeric polyalcohols and isocyanato(meth)acrylates; unsaturated polyesters; (meth)acrylate-terminated polystyrenes, poly(meth)acrylic acid ester, poly(meth)acry!ic acids, poly(meth)acrylamides and polyethers. In this connection, the prefix "(meth)" indicates that both derivatives of acrylic acid and of methacrylic acid can be used.

Additional suitable C-C unsaturated free-radical polymerizable compounds are described e.g. in EP-A-1 176007.

The free-radical polymerizable monomers, oligomers or polymers are preferably present in an amount of 35 to 90 wt.-%; if monomers/oligomers are used, 45 to 60 wt.-%, based on the dry layer weight of the IR-sensitive coating, are especially preferred.

The initiator system of the IR-sensitive element of the present invention comprises one or more polyhaloalkyl-substituted compounds. These are compounds which comprise either one polyhalogenated or several monohalogenated alkyl substituents.

The halogenated alkyl group preferably has 1 to 3 carbon atoms; especially preferred is a halogenated methyl group.

Preferably, fluorine, chlorine or bromine atoms are used as halogen atoms; especially preferred are chlorine and bromine atoms. The absorption properties of the polyhaloalkyl-substituted compound fundamentally determine the daylight stability of the IR-sensitive elements. Compounds having a UV/VIS absorption maximum in the range of about 300 to 750 nm result in elements which can no longer be completely developed after the element has been kept in daylight for 6 to 8 minutes and then been reheated. As a principle, such elements can be image-wise exposed not only with IR but also with UV radiation. Therefore, if an element with a high degree of daylight stability is desired, polyhaloalkyl-substituted compounds are preferred which do not show any essential UV/VIS absorption at > 330 nm.

Examples of especially suitable polyhaloalkyl-substituted compounds for the elements of the present invention include:

Tribromomethylphenylsulfone,

1 ,2,3,4-tetrabromo-n-butane,

2-(4-methoxyphenyl)-4,6-bis(trichloromethyl)-s-triazine,

2-(4-chlorophenyl)-4,6-bis-(trichloromethyl)-s-triazine,

2-phenyl-4,6-bis(trichloromethyl)-s-triazine,

2,4,6-tri-(trichloromethyl)-s-triazine, and

2,4,6-tri-(tribromomethyl)-s-triazine.

The polyhaloalkyl-substituted compound is preferably present in the IR-sensitive coating in an amount of from 2 to 15 wt.-%, based on the dry layer weight; especially preferred in an amount of from 4 to 7 wt.-%.

The IR-sensitive element of the present invention can be image-wise exposed with IR radiation. For this purpose, it is necessary that the IR-sensitive coating of the elements of the present invention comprise at least one substance capable of absorbing IR radiation in the range of about 800 to 1 ,200 nm. In the following, these substances are referred to simply as "IR absorber". It should be noted at this point that imaging can also be carried out by means of UV light or visible light if the polyhalo-substituted compound has an absorption maximum in the range of about 300 to 600 nm.

In the present invention, the IR absorbers are preferably selected from the class of triarylamine dyes, thiazolium dyes, indolium dyes, oxazolium dyes, cyanine dyes, polyaniline dyes, polypyrrol dyes, polythiophene dyes and phthalocyanine dyes and pigments.

Suitable IR absorbers include e.g. the following compounds:

©NH₂

In a preferred embodiment of the present invention, an IR absorber of formula (D) is used

wherein: each X independently represents S, O, NR or C(alkyl)₂; each R¹ independently represents an alkyl group;

R² represent a halogen atom, SR, OR or NR₂; each R³ represents a hydrogen atom, an alkyl group, OR, SR or NR₂ or a halogen atom; R 3 can also be benzofused;

A^" represents an anion; represents an optional carbocyclic five- or six-membered ring;

R represents an alkyl or aryl group; in the case of NR R can also be H; each n can independently be 0, 1 , 2 or 3. These IR absorbers absorb in the range of 800 to 1 ,200 nm; IR absorbers of the formula (D) which absorb in the range of 810 to 860 nm are preferred.

X is preferably a C(alkyl)₂ group, wherein the alkyl group preferably has 1 to 3 carbon atoms.

R¹ is preferably an alkyl group with 1 to 4 carbon atoms.

R² is preferably SR.

R³ is preferably a hydrogen atom.

R is preferably a phenyl group.

The broken line preferably represents the rest of a ring with 5 or 6 carbon atoms.

The counterion A^" is preferably a chloride ion or a tosylate anion.

IR absorbers with a symmetrical structure (D) are especially preferred.

Examples of especially preferred IR absorbers 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 - cycIohexen-1-yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumchloride,

2-[2-[2-thiophenyl-3-[2-(1 ,3-dihydrq-1 ,3,3-trimethyl-2H-indole-2-ylidene)-ethylidene]-1 - cyclopenten-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 -cyclohexen-1 -yl]-ethenyl]-1 ,3,3-trimethyl-1 H-benzo[e]-indolium-tosylate, and

2-[2-[2-chloro-3-[2-ethyl-3H-benzthiazole-2-ylidene)-ethylidene]-1-cyclohexen-1-yl]- ethenyl]-3-ethyl-benzthiazolium-tosylate.

The IR absorber is preferably present in the IR-sensitive coating in an amount of from 0.5 to 8 wt.-%, based dry layer weight; especially preferred is an amount of from 1.0 to 2.5 wt.-%. It is essential to the present invention that in addition to the free-radical polymerizable compounds, IR absorbers and polyhaloalkyl-substited substances, the IR-sensitive layer comprise a stabilizer which reduces or preferably completely prevents the undesired curing of the unexposed areas of the layer upon heating. It is desirable that the stabilizer does not considerably affect the sensitivity and developability of the IR- sensitive element; preferably, these properties are not affected at all by the stabilizer.

The inventors have surprisingly found that the addition of a stabilizer selected from 1 ,10- phenanthroline, 8-quinoline thiol, 2,2'-bipyridyl, 8-hydroxyquinoline, α-benzoin oxime, 1 ,1 ,4,7,10,10-hexamethylthethylene tetraamine, 1 ,5-diphenylthiocarbazone, benzo-15- crown-5, oxalic acid bis-(cyclohexylidene-hydrazine), acid addition salts and hydrates of the above-mentioned compounds, as well as mixtures of two or more of the above compounds reduces undesired curing during heating, and in some cases prevents such curing altogether. The use of 1 ,10-phenanthroline, 8-quinoline thiol, 1,5- diphenylthiocarbazone, 2,2'-bipyridyl and 1 ,1 ,4,7,10,10-hexamethyltriethylene tetraamine, their acid addition salts and hydrates, as well as of stabilizer mixtures comprising at least one of these compounds, is preferred. Especially preferred are 1 ,10- phenanthroline and 8-quinoline thiol and their acid addition salts and hydrates. Most preferred are 1 ,10-phenanthroline, acid addition salts and hydrates thereof since it was found that in addition to the stabilizing effect these compounds improve the adhesion of the IR sensitive coating to the substrate.

All the above-mentioned stabilizers that can be used in the present invention are complexing agents but not free-radical scavengers; the latter characteristic is of particular importance so that the desired curing by free-radical polymerization in the image areas will not be affected.

The person skilled in the art can easily determine the suitable amount of stabilizer by means of simple tests (see also Examples 17 to 20 in the experimental part). Preferably, the amount is 0.1 to 5 wt.-% based on the dry layer weight of the IR-sensitive coating, more preferably 0.2 to 2 wt.-% and particularly preferred 0.4 to 1.4 wt.-%.

In a preferred embodiment, the initiator system comprises, in addition to the IR absorber and the polyhaloalkyl-substituted compound, at least one carboxylic acid of the general formula (A) R⁴-(CR⁵R⁶)_a-Y-CH₂COOH (A)

wherein

Y is selected from O, S or NR⁷, wherein R⁷ represents a hydrogen atom, a C₁-C-₆ alkyl group, a group CH₂CH₂COOH or a C₁-C₅ alkyl group substituted with -COOH;

R⁴, R⁵ and R⁶ are each independently selected from a hydrogen atom,

alkyl group, substituted or unsubstituted aryl group, -COOH, or NR⁸CH₂COOH, wherein R⁸ is selected from -CH₂COOH, -CH₂OH and -(CH₂)N(CH₂)COOH; and a is 0, 1 , 2 or 3.

Preferred are N-arylpolycarboxylic acids, in particular those of the following formula (B)

^ CH₂-COOH (B)

Ar-N

^ C_nH₂n-COOH

wherein Ar represents a mono-, poly- or unsubstituted aryl group and n is an integer from 1 to 5, and of formula (C)

wherein R⁴ represents a hydrogen atom or a C-i-Cβ alkyl group and k and m each represent an integer from 1 to 5.

Possible substituents of an aryl group are C₁-C₃ alkyl groups, C₁-C₃ alkoxy groups, C C₃ thioalkyl groups and halogen atoms. The aryl group can have 1 to 3 identical or different substituents. n is preferably 1 and Ar preferably represents a phenyl group.

In formula (C), m is preferably 1 and R⁴ preferably represents a hydrogen atom. Examples of such polycarboxylic acids include:

(p-Acetamidophenylimino)diacetic acid

3-(bis(carboxymethyl)amino)benzoic acid

4-(bis(carboxymethyl)amino)benzoic acid

2-[(carboxymethyl)phenylamino]benzoic acid

2-[(carboxymethyl)phenylamino]-5-methoxybenzoic acid

3-[bis(carboxymethyl)amino]-2-naphthalenecarboxylic acid

N-(4-aminophenyl)-N-(carboxymethyl)glycine

N,N'-1 ,3-phenylenebisglycine

N,N'-1 ,3-phenylenebis[N-(carboxymethyl)]glycine

N,N'-1 ,2-phenylenebis[N-(carboxymethyl)]glycine

N-(carboxymethyl -N-(4-methoxyphenyl)glycine N-(carboxymethyl -N-(3-methoxyphenyl)glycine N-(carboxymethyl -N-(3-hydroxyphenyl)gIycine N-(carboxymethyl -N-(3-chlorophenyl)glycine N-(carboxymethyl -N-(4-bromophenyl)glycine N-(carboxymethyl -N-(4-chlorophenyl)glycine N-(carboxymethyl -N-(2-chlorophenyl)glycine N-(carboxymethyl -N-(4-ethylphenyl)glycine N-(carboxymethyl -N-(2,3-dimethylphenyl)glycine N-(carboxymethyl -N-(3,4-dimethylphenyl)glycine N-(carboxymethyl -N-(3,5-dimethylphenyl)glycine N-(carboxymethyl -N-(2,4-dimethylphenyl)glycine N-(carboxymethyl N-(2,6-dimethylphenyl)glycine N-(carboxymethyl N-(4-formylphenyl)glycine N-(carboxymethyl N-ethylanthranilic acid N-(carboxymethyl N-propylanthranilic acid 5-bromo-N-(carboxymethyl)anthranilic acid N-(2-carboxyphenyl)glycine o-dianisidine-N,N,N',N'-tetraacetic acid

N,N'-[1 ,2-ethanediyl-bis-(oxy-2,1-phenylene)]bis[N-(carboxymethyl)glycine] 4-carboxyphenoxyacetic acid catechol-O,O'-diacetic acid 4-methylcatechol-O,O'-diacetic acid resorcinol-O,O'-diacetic acid hydroquinone-O,O'-diacetic acid α-carboxy-o-anisic acid 4,4'-isopropylydenediphenoxyacetic acid 2,2'-(dibenzofuran-2,8-diyldioxy)diacetic acid 2-(carboxymethylthio)benzoic acid 5-amino-2-(carboxymethylthio)benzoic acid 3-[(carboxymethyl)thio]-2-naphtalenecarboxylic acid.

The most preferred polycarboxylic acid is anilino diacetic acid.

The polycarboxylic acid is preferably present in the IR-sensitive coating in an amount of from 0 to 10 wt.-%, more preferably 1 to 10 wt.-% and especially preferred 1.5 to 3 wt- %, based on the dry layer weight of the IR-sensitive coating.

The IR-sensitive coating may furthermore comprise one or more dyes for staining the coating; suitable dyes are those that dissolve well in the solvent or solvent mixture used for coating or are easily introduced in the disperse form of a pigment. Suitable contrast dyes include inter alia rhodamine dyes, triarylmethane dyes, anthraquinone pigments and phthalocyanine dyes and/or pigments. The dyes are preferably present in the IR- sensitive composition in an amount of from 0.5 to 10 wt.-%, especially preferred in an amount of from 1 to 4 wt.-%, based on the dry layer weight.

The IR-sensitive coating may furthermore comprise one or more plasticizers. Suitable plasticizers include inter alia dibutyl phthalate, triaryl phosphate and dioctyl phthalate. If one or more plasticizers are used, their total amount preferably accounts for 0.25 to 2 wt.-% based on the dry layer weight.

In addition the IR-sensitive coating may comprise one or more mercapto compounds, for instance selected from aliphatic thiols and heterocyclic mercapto compounds. A suitable aliphatic thiol is for instance trimethylolpropane tris(3-mercaptopropionate). Suitable heterocyclic mercapto compounds include for instance compounds comprising an aromatic 5-membered heterocyclic ring bearing a thiol substituent, where the ring comprises a nitrogen atom and either at least one other nitrogen atom, or an oxygen atom or a sulfur atom, in which the sulfur, oxygen or second nitrogen is separated from the first nitrogen by one carbon atom, which bears the thiol group; examples are 3- mercapto-1 ,2,4-triazole; 3-mercapto-4-methyl-4H-1 ,2,4-triazole; 3-mercapto-5-(4- pyridyl)-1 H-1 ,2,4-triazole; 2-mercaptobenzimidazole; 2-mercaptobenzoxazole; 2- mercaptobenzothiazole; 6-ethoxy-2-mercaptobenzothiazole; 2-mercapto-5-methyl-1 ,3,4- thiadiazole; 2-mercapto-5-phenyl-1 ,3,4-oxadiazole; 2-mercapto-5-(4-pyridyl)-1 ,3,4- oxadiazole; 5-mercapto-3-methylthio-1 ,2,4-thiadiazole; 2-mercapto-5-methylthio-1 ,3,4- thiadiazole; 2-mercaptoimidazole; 2-mercapto-1-methylimidazole; 5-mercapto-1-methyl- 1 H-tetrazole; and 5-mercapto-1-phenyl-1H-tetrazole. Preferred heterocyclic mercapto compounds include 3-mercapto-1 ,2,4-triazole; 2-mercaptobenzimidazole; 2- mercaptobenzoxazole; 5-mercapto-3-methylthio-1,2,4-thiadiazole; and 2-mercapto-1- methylimidazole.

The infrared-sensitive coating preferably comprise about 0 to about 10 wt%, preferably about 0.5 to about 5 wt%, of the mercapto compound or mixture of mercapto compounds, based on the dry layer weight.

Further additives that may be present in the IR-sensitive coating are surface-active agents such as non-ionic, anionic and cationic surfactants and mixtures thereof. Examples of suitable surfactants include sorbitol-tri-stearate, glycerol monostearate, polyoxyethylene nonylether, alkyl-di-(aminoethyl)-glycine, 2-alky-N-carboxyethyl- imidazoliumbetaine and perfluoro compounds. The total amount of surface-active agents is preferably in the range of 0.05 to 0.5 wt.-% based on the dry layer weight.

The IR-sensitive elements of the present invention can be produced as follows:

The optionally pre-treated substrate is coated with the IR-sensitive composition from an organic solvent or solvent mixture such that dry layer weights in the range of 0.5 to 4 g/m², preferably 0.8 to 3 g/m², are obtained. This can be done by means of common coating methods such as coating with doctor blades and centrifugal coating.

In the case of lithographic printing plate precursors, an oxygen-impermeable layer, as it is known in the art, is applied on top of the IR-sensitive layer, e.g. a layer of polyvinyl alcohol, polyvinyl alcohol/polyvinyl acetate copolymers, polyvinyl pyrrolidon, polyvinyl pyrrolidon/polyvinyl acetate copolymers, polyvinyl methylether, polyacrylic acid and gelatine. The dry layer weight of the oxygen-impermeable layer is preferably 0.1 to 4 g/m², more preferably 0.3 to 2 g/m². The thus obtained IR-sensitive elements can be exposed e.g. with semiconductor lasers or laser diodes which emit in the range of about 800 to 1 ,200 nm. Such a laser beam 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 digitalized information in the computer. In the case of lithographic printing plate precursors, such elements are therefore referred to as computer-to-plate (ctp) printing plates.

The image-wise exposed elements such as printing plate precursors are then developed with a commercially available aqueous alkaline developer, wherein the exposed areas remain on the substrate and the non-exposed areas are removed.

After the element has been image-wise exposed, it is briefly heated to a temperature of 85 to 135°C in order to effect an even more complete curing of the exposed areas. Depending on the temperature applied, this only takes 20 to 100 seconds. Then the element is developed as known to the person skilled in the art.

If the developed elements of the present invention are printing plates, they are usually treated with a preservative ("gumming") after developing. The preservatives are aqueous solutions of hydrophilic polymers, such as e.g. polyvinyl alcohol, wetting agents and other additives.

The invention is described in more detail in the following examples.

Examples

Since the heavy metal/heavy metal ion contamination of printing plate precursors lies in the ppm range and furthermore varies, it is difficult, if not impossible, to compare different precursors with respect to undesired curing upon heating.

The inventors have therefore chosen the following method to demonstrate the surprising effect of their invention:

A known amount of metal salt was added to the coating solution in the production of the printing plate precursors in order to simulate contamination and to be able to draw a meaningful comparison of precursors with and without stabilizer. Suppression of coating residue caused by copper(ll)-ions by means of added stabilizers

(Examples 1 to 10)

A coating solution was prepared from the following components:

100 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting 1 -methyl-2,4-bis-isocyanate benzene (Desmodur N100™ available from the company Bayer) with hydroxy ethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted

36.6 g Jagotex MA 2814 (methacrylic acid copolymer available from the company Ernst Jager GmbH & Co., having an acid number of 120 mg KOH/g)

36.6 g loncryl 683 (acrylic acid copolymer available from the company SC Johnson & Sδhne Inc., having an acid number of 175 mg KOH/g)

16.8 g Sartomer 355 (multi-functional acrylic monomer available from the company Sartomer Co., Inc.)

9.0 g 2-(4-methoxyphenyl)-4,6-(trichloromethyl)-s-triazine

2.5 g crystal violet

4.8 g anilino diacetic acid

1.8 g 2-[2-[2-phenylhtio-3-[1 ,3-dihydro-1 ,3,3-trimethyl-2H-indole-2-ylidene)- ethylidene]-1 -cyclohexen-1 -yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumchloride.

These components were dissolved under stirring in 1000 g of a mixture comprising 900 g 1 -methoxy-2-propanol and 100 g acetone. After filtration, the solution was divided into 10 samples of 38 g each and the components listed in Table 1 were added. After another filtration, each of the solutions was applied to an electrochemically grained and anodized aluminum foil that was subjected to a treatment using an aqueous solution of polyvinyl phosphonic acid by means of common methods, and the coating was dried for 4 minutes at 90°C. The dry layer weight of the IR-sensitive layer was approximately 2 g/m².

Then, a second layer of 2 g/m² dry layer weight was applied, using a coating solution comprising the following components:

42.5 g polyvinyl alcohol (Airvol 203™ available from Air Products; 12 wt.-% residual acetyl groups) 7.5 g polyvinyl imidazole (PVI available from Panchim) 170 g water

Again, drying took place for 5 minutes at 90 °C.

The samples of Examples 1 to 10 were placed in an commercially available MercuryNews processor (available from Kodak Polychrome Graphics LLC) equipped with a heating section, a pre-wash section, an immersion type developing bath, a section for rinsing with water, a gumming and a drying section. The processor was filled with developer 980 (Kodak Polychrome Graphics LLC). The following parameters were applied for developing the plate samples: speed 120 cm/rnin, heating 650 digits, pre- wash 0.5 l/m² plate, temperature of the developing bath 23 ± 1 °C.

After developing, the plates were evaluated visually. The results are listed in Table 4.

Table 1 : Compounds added in Examples 1 to 10 (in parts by weight)

¹⁾ solution of 0.075 g copper(ll)-sulfate in 100 g water (deionized) Suppression of coating residue caused by various heavy metal ions by means of 1,10-phenanthroline

(Examples 11 to 16)

A coating solution was prepared from the following components:

50.0 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting 1 -methyI-2,4-bis-isocyanate benzene (Desmodur N100™ available from the company Bayer) with hydroxy ethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted

18.3 g Jagotex MA 2814 (methacrylic acid copolymer available from the company Ernst Jager GmbH & Co., having an acid number of 120 mg KOH/g)

18.3 g loncryl 683 (acrylic acid copolymer available from the company SC Johnson & Sδhne Inc., having an acid number of 175 mg KOH/g)

8.4 g Sartomer 355 (multi-functional acrylic monomer available from the company

Sartomer Co., Inc.)

4.5 g 2-(4-methoxyphenyl)-4,6-(trichloromethyl)-s-triazine 1.25 g crystal violet

2.4 g anilino diacetic acid

0.9 g 2-[2-[2-phenylhtio-3-[1 ,3-dihydro-1 ,3,3-trimethyl-2H-indole-2-ylidene)- ethylidene]-1 -cyclohexen-1 -yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumchloride.

These components were dissolved under stirring in 500 g of a mixture comprising 450 g 1-methoxy-2-propanol and 50 g acetone.

After filtration, 3 samples of 38 g each of this mixture were prepared and 0.15 g of an aqueous solution of a heavy-metal salt as per Table 2 was added; see Examples 11 to 13 (comparative). The subsequent process steps were carried out as described for Examples 1 to 10.

In Examples 14 to 16, 0.4 g 1 ,10-phenanthroline-monohydrate were added to 400 g of the above coating mixture. Then, 3 samples of 38 g each were prepared from this solution and 0.15 g of an aqueous solution of 7.5 g of the heavy-metal salt as listed in Table 2 in 100 g water (deionized) was added. Further processing was carried out as described for Examples 1 to 10. After developing, the samples were examined visually with respect to coating residues. The results are listed in Table 4. As the data show, the coating residue caused by the iron, cobalt and chromium ions was completely suppressed by the addition of 1 ,10- phenanthroline-monohydrate.

Table 2: Heavy-metal salts used in Examples 11 to 16

comparative

Suppression of coating residue caused by copper(ll) ions by means of different concentrations of

8-quinoline thiol hydrochloride

(Examples 17 to 20)

A coating solution was prepared from the following components:

25.0 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting 1 -methyl-2,4-bis-isocyanate benzene (Desmodur N100™ available from the company Bayer) with hydroxy ethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted

9.2 g Jagotex MA 2814 (methacrylic acid copolymer available from the company Ernst Jager GmbH & Co., having an acid number of 120 mg KOH/g)

9.2 g loncryl 683 (acrylic acid copolymer available from the company SC Johnson & Sδhne Inc., having an acid number of 175 mg KOH/g)

4.2 g Sartomer 355 (multi-functional acrylic monomer available from the company Sartomer Co., Inc.)2.3 g 2-(4-methoxyphenyl)-4,6-(trichloromethyI)-s- triazine

0.6 g crystal violet 1.2 g anilino diacetic acid

0.5 g 2-[2-[2-phenylhtio-3-[1 ,3-dihydro-1 ,3,3-trimethyl-2H-indole-2-ylidene)- ethylidene]-1 -cyclohexen-1 -yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumchloride 0.63 g of a solution of 0.075 g copper(ll)-sulfate in 100 g water (deionized)

These components were dissolved under stirring in 250 g of a mixture comprising 225 g 1 -methoxy-2-propanol and 25 g acetone.

After filtration, 4 samples of 57 g each of this mixture were prepared and 8-quinoline thiol hydrochloride was added in the concentrations given in Table 3. The subsequent process steps were carried out as described for Examples 1 to 10.

After developing, the samples were examined visually with respect to coating residues. The results are listed in Table 4. The data show that while at a concentration of below 0.25 wt.-%, 8-quinoline thiol hydrochloride shows suppressing activity, it was quite low, and at concentrations higher than about 1.0 wt.-%, IR sensitivity was somewhat affected.

Table 3: Concentration of 8-quinoline thiol hydrochloride in Examples 3 and 17 to 20 (in parts by weight)

Example 21 (comparative)

A coating solution was prepared from the following components:

2.5 g Scripset 540™ (butylsemiester of maleic acid anhydride/styrene copolymer available from the company Monsanto Co.)

0.55 g dipentaerythritol-pentaacrylate

3.4 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting 1 -methyl-2,4-bis-isocyanate benzene (Desmodur N100™ available from the company Bayer) with hydroxy ethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted 0.8 g anilino diacetic acid 0.32 g 2-[2-[2-chloro-3-[2-ethyl-(3H-benzothiazole-2-ylidene)-1 -cyclohexene-1 -yl]- ethenyl]-3-ethyl-benzthiazolium-tosylate 0.32 g tribromomethylphenylsulfone 0.1 g crystal violet

0.25 g of a solution of 0.075 g copper(ll)-sulfate in 100 g water (deionized)

These components were dissolved under stirring in 40 ml of a mixture of 90 vol.-% 1- methoxy-2-propanol and 10 vol.-% acetone.

Further processing was carried out as described for Examples 1 to 10.

After developing, the samples were examined visually with respect to coating residues. The results are listed in Table 4. The data show that even in compositions with a different IR dye, monomer and polymeric binder, copper(II)-sulfate leads to considerable coating residue.

Example 22

Example 21 was repeated, however, 0.1 g 1 ,10-phenanthroline-monohydrate was added to the composition. The corresponding data are shown in Table 4, and it can be inferred that an addition of this compound completely suppresses any coating residue.

Example 23 (comparative)

A coating solution was prepared from the following components:

1.6 g CAP™ (cellulose acetate phthalate available from the company Eastman

Kodak; acid number 130 mg KOH/g) 1.6 g terpolymer (methacrylic acid copolymer available from the company Panchim; acid number 130 mg KOH/g) 0.72 g dipentaerythritol-pentaacrylate 3.6 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting 1-methyl-2,4-bis-isocyanate benzene (Desmodur N100™ available from the company Bayer) with hydroxy ethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted

0.2 g 3,4-dimethoxyphenylthio acetic acid

0.32 g 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-1 H-benzo[e]- indoliumtosylate

0.35 g 2-(phenyl)-4,6-bis-(trichloromethyl)-s-triazine

0.1 g crystal violet

0.25 g of a solution of 0.075 g copper(ll)-sulfate in 100 g water (deionized)

These components were dissolved under stirring in 50 ml of a mixture of 90 vol.-% 1- methoxy-2-propanol and 10 vol.-% acetone.

Further processing was carried out as described for Examples 1 to 10.

After developing, the samples were examined visually with respect to coating residues. The results are listed in Table 4. The data show that even in compositions with a different IR dye, monomer, polymeric binder and components of IR-sensitive initiator systems, copper(ll)-sulfate causes considerable coating residue.

Example 24

Example 23 was repeated, however, 0.1 g 1 ,10-phenanthroline-monohydrate was added to the composition. The corresponding data are shown in Table 4; it can be inferred that an addition of this compound completely suppresses any coating residue.

Exposure and printing tests

Pieces of the plates coated in Examples 2, 3, 14 to 16, 19, 20, 22 and 24 were exposed on a Creo Trendsetter 3244 image-setter (available from the company Creo) with an 830 nm laser diode. An UGRA/FOGRA Postscript control strip Version 2.0 EPS (available from UGRA), which contains different elements for evaluating the quality of the copies, was used for imaging. Developing of the exposed samples was carried out as described above. The following criteria were examined for evaluating the copies obtained after heating and developing: The reproduction quality of 1 and 2 pixel elements, the optical density of solid regions and the optical density of checkerboard patterns of the pixel elements. A D19C/D 196 densitometer available from Gretag/Macbeth was used for evaluating the density of solid regions and grid points. The exposure results of the above-mentioned examples are shown in Table 4.

It has been found that the use of the stabilizer according to the present invention did not fundamentally affect IR sensitivity.

In order to evaluate their printing behavior, samples of the printing plates obtained in Examples 2 and 3 were mounted in a sheet-fed offset press and used for printing. The image areas accepted the printing ink without any problems and the paper copies did not show any toning in the non-image areas. After 250,000 high-quality copies, printing was discontinued; however, the plates could have been used for more prints.

Table 4: Degree of coating residue and IR sensitivity

¹' laser energy (in mJ/cm²) needed for maximum optical density ²^ no imaging after exposure with 200 mJ/cm² ³ elements were attacked very little ' elements were attacked little

Claims

C lai ms

1. IR-sensitive element comprising

(a) an optionally pre-treated substrate

(b) an IR-sensitive coating comprising (i) at least one polymeric binder,

(iv) at least one stabilizer selected from 1 ,10-phenanthroline, 8-quinoline thiol, 2,2'-bipyridyl, 8-hydroxyquinoline, α-benzoin oxime, 1 ,1 ,4,7,10,10-hexamethyltriethylene tetraamine, 1 ,5-diphenylthio- carbazone, benzo-15-crown-5, oxalic acid bis-(cydohexylidene- hydrazine), acid addition salts and hydrates of the above-mentioned compounds, as well as mixtures thereof.

2. IR-sensitive element according to claim 1 , wherein the IR absorber is selected from the class of triarylamine dyes, thiazolium dyes, indolium dyes, oxazolium dyes, cyanine dyes, polyaniline dyes, polypyrrol dyes, polythiophene dyes and phthalocyanine dyes and pigments.

3. IR-sensitive element according to claim 2, wherein the IR absorber is a cyanine dye of formula (D)

wherein every X independently represents S, O, NR or C(alkyl)₂; every R¹ independently represents an alkyl group; R² represents a halogen atom, SR, OR or NR₂; every R³ represents a hydrogen atom, an alkyl group, OR, SR or NR₂ or a halogen atom; R³ can also be benzofused A^' represents an anion; represents an optional carbocyclic five- or six-membered ring; R represents an alkyl or aryl group; in the case of NR₂ R can also be H; each n can independently be 0, 1 , 2 or 3.

4. IR-sensitive element according to claim 3, wherein the IR dye is 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 -cyclopentene-1 -yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumtosylate, 2-[2-[2-thiophenyl-3-[2-(1 ,3-dihydro-1 ,3,3-trimethyl-2H-indoIe-2-ylidene)- ethylidene]-1 -cyclohexene-1 -yl]-ethenyl]-1 ,3,3-trimethyl-3H-indoliumchloride, 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-1 H-benzo[e]-indolium- tosylate or 2-[2-[2-chloro-3-[2-ethyl-(3H-benzothiazole-2-ylidene)-ethylidene]-1 - cyclohexene-1 -yl]-ethenyl]-3-ethyl-benzthiazolium-tosylate.

5. IR-sensitive element according to any of claims 1 to 4, wherein the polyhaloalkyl- substituted compound is selected from:

2-phenyl-4,6-bis(trichloromethyl)-s-triazine, 1 ,2,3,4-tetrabromo-n-butane, 2-(4- methoxyphenyl)-4,6-bis(trichloromethyl)-s-triazine, 2-(4-chlorophenyl)-4,6-bis-(tri- chloromethyl)-s-triazine, tribromomethylphenylsulfone, 2,4,6-tri-(trichloromethyl)- s-triazine, and 2,4,6-tri-(tribromomethyl)-s-thazine.

6. IR-sensitive element according to any of claims 1 to 5, wherein the initiator system additionally comprises at least one carboxylic acid of the general formula (A)

R⁴-(CR⁵R⁶)_a-Y-CH₂COOH (A)

wherein

Y is selected from O, S or NR⁷, wherein R⁷ represents a hydrogen atom, a Cι-C₆ alkyl group, a group CH₂CH₂COOH or a C1-C5 alkyl group substituted with

-COOH; R⁴, R⁵ and R⁶ are each independently selected from a hydrogen atom, C1.-C₆ alkyl group, substituted or unsubstituted aryl group, -COOH, or NR⁸CH₂COOH, wherein R⁸ is selected from -CH₂COOH, -CH₂OH and -(CH₂)N(CH₂)COOH; and a is 0, 1 , 2 or 3.

7. IR-sensitive element according to claim 6, wherein the polycarboxylic acid is a compound of the formula (B)

^ CH₂-COOH (B)

Ar-N

^ C_nH_2n-COOH

wherein Ar represents a mono-, poly- or unsubstituted aryl group and n is an integer from 1 to 5, or a compound of the formula (C)

wherein R⁴ represents a hydrogen atom or a C1_.-C-₆ alkyl group, and k and m each are an integer from 1 to 5.

8. IR-sensitive element according to claim 6 or 7, wherein the polycarboxylic acid is anilino diacetic acid.

9. IR-sensitive element according to any of claims 1 to 8, wherein the stabilizer is selected from 1 ,10-phenanthroline, 8-quinoline thiol, their acid addition salts and hydrates as well as mixtures of at least one of these compounds and at least one further stabilizer selected from the other stabilizers listed in claim 1.

10. IR-sensitive element according to any of claims 1 to 9, wherein the stabilizer is present in an amount of 0.1 to 5 wt.-%, based on the dry layer weight of the IR- sensitive coating.

11. IR-sensitive element according to any of claims 1 to 10, wherein the substrate is an optionally pre-treated aluminum plate or foil.

12. IR-sensitive element according to any of claims 1 to 11 , wherein the IR-sensitive coating additionally comprises at least one additive selected from surfactants, contrast dyes or pigments and plasticizers.

13. IR-sensitive element according to any of claims 1 to 12, wherein an oxygen- impermeable overcoat is provided on the IR-sensitive layer.

14. IR-sensitive element according to any of claims 1 to 13, wherein the polymeric binder has an acid number of >70 mg KOH/g polymer.

15. IR-sensitive composition for producing a heat-sensitive element as defined in any of claims 1 to 14 comprising

(i) at least one polymeric binder,

(iii) an initiator system for free-radical polymerization, comprising at least one IR absorber capable of absorbing radiation in the range of 800 to 1,200 nm and- at least one polyhaloalkyl-substituted compound and

(iv) at least one stabilizer selected from 1 ,10-phenanthroline, 8-quinoline thiol, 2,2'-bipyridyl, 8-hydroxyquinoline, α-benzoin oxime, 1, 1,4,7, 10,10-hexa- methyltriethylene tetraamine, 1 ,5-diphenylthiocarbazone, benzo-15-crown-5, oxalic acid bis-(cyclohexylidene-hydrazine), acid addition salts and hydrates of the above-mentioned compounds, as well as mixtures thereof and

(v) an organic solvent or solvent mixture.

16. IR-sensitive composition according to claim 14, wherein the initiator system additionally comprises at least one carboxylic acid of the general formula (A)

R⁴-(CR⁵R⁶)_a-Y-CH₂COOH (A)

wherein Y is selected from O, S or NR⁷, wherein R⁷ represents a hydrogen atom, a OpCβ alkyl group, a group CH CH₂COOH or a C₁-C₅ alkyl group substituted with

-COOH;

R⁴, R⁵ and R⁶ are each independently selected from a hydrogen atom, C C₆ alkyl group, substituted or unsubstituted aryl group, -COOH, or NR⁸CH₂COOH, wherein R⁸ is selected from -CH₂COOH, -CH₂OH and -(CH₂)N(CH₂)COOH; and a is 0, 1 , 2 or 3.

17. IR-sensitive composition according to claims 15 and 16, said composition additionally comprising at least one additive selected from surfactants, contrast dyes and pigments and plasticizers.

18. Process for the production of an IR-sensitive element as defined in any of claims 1 to 14 comprising:

(a) providing an optionally pre-treated substrate,

(b) applying an IR-sensitive composition as defined in any of claims 15 to 17,

(c) drying and

(d) optionally applying an oxygen-impermeable overcoat and drying.

19. Process for providing an imaged element comprising:

(a) providing an IR-sensitive element as defined in any of claims 1 to 14,

(b) image-wise exposure of the element and

(c) subsequent development of the element obtained in step (b) with an aqueous alkaline developer.

20. Use of a compound selected from 1 ,10-phenanthroline, 8-quinoline thiol, 2,2'- bipyridyl, 8-hydroxyquinoline, α-benzoin oxime, 1 ,1 , 4,7, 10,10-hexamethyltri- ethylene tetraamine, 1 ,5-diphenylthiocarbazone, benzo-15-crown-5, oxalic acid bis- (cyclohexylidene-hydrazine), acid addition salts and hydrates of the above- mentioned compounds, as well as mixtures thereof for reducing or preventing heavy metal or heavy metal ion-induced curing of the IR-sensitive layer in areas of IR-sensitive elements that have not been exposed image-wise, wherein the IR- sensitive coating of said elements comprises a polyhaloalkyl-substituted compound, an IR absorber and C-C unsaturated free-radical polymerizable monomers and/or oligomers and/or polymers.