SILVER HALIDE PHOTOGRAPHIC MATERIAL

1. Field of the invention.

The present invention relates to a silver halide photographic material sensitive to light sources emitting light between 600 and 850nm. The present invention further relates to a method for obtaining images with said photographic material and to a method for obtaining a lithographic printing plate with said photographic material.

2. Background of the invention.

Silver halide photographic materials sensitive to light between 600 and 850nm are commonly employed as phototypesetting materials for use in phototypesetters equiped with a laser, laser diode or LED emitting in the red and/or infrared of the spectrum. Todate on the market phototypesetters usually work with a He/Ne laser (632nm), laser diode (680nm) or LED (670nm or 780nm) . Since these light sources emit in a very narrow spectral range photographic materials were developed for each of the above mentioned light sources.

Photographic phototypesetting materials include photographic films and papers used in a process for preparing a lithographic printing plate and silver salt diffusion transfer based lithographic printing plates disclosed in e.g. ϋS-P-4.501.811 and US-P-4.784.933. With the latter materials a lithographic printing plate is immediately obtained without the need of a contact exposure or camera exposure.

A universal photographic material sensitive to each of the common light sources in the spectrum from 600 to 850 nm would be much more convenient because such a material would be useful in any phototypesetter operating with a light source emitting light between 600nm and 850nm. Such a universal photographic material would thus require a silver halide emulsion sensitized both in the red and infrared region of the spectrum. In principal this would be

possible by sensitizing the silver halide emulsion with a number of sensitizing dyes so that the spectrum between 600nm and 850nm is coverred.

However several problems may arise when one tries to prepare such a photographic material. For example due to the difference in output energy between the different light sources there are different requirements for the silver halide emulsion. Thus a silver halide emulsion for use with LED ^Λ s that show a low output energy requires the intrinsic sensitivity of the silver halide emulsion to be high. The intrinsic sensitivity of these emulsions can be increased by increasing the bromide content of the silver halide. Such a practice reduces however the speed of development of the silver halide emulsion and in case the photographic material is to be used in a silver salt diffusion transfer process it also reduces the transfer speed of the silver halide. On the other hand silver halide emulsions for use in phototypesetters with He/Ne lasers that show a high intensity do not require such a high intrinsic sensitivity so that silver halide emulsions of high chloride content can be used.

Furthermore due to their large structure I.R.-sensitizing dyes cover laxge portions of the silver halide grain thus reducing the dissolution rate of the grain and therefore the speed of development. Adding a red sensitizing dye to the emulsion would thus decrease the speed of development even more. At the same time the stability of the silver halide emulsion is decreased because sensitizing dyes and especially infrared sensitizing dyes tend to fog the silver halide as disclosed in "The theory of the photographic process" by T.H. James, 4th edition, page 269.

It is further disclosed in e.g. US-P-4.784.933 that infrared sensitizing dyes may desorb from the silver halide grain during preparation of the silver halide emulsion.

For these reasons it was believed that a universal photographic material sensitized with a mixture of sensitizing dyes would not yield the desired sensitivity level for the light sources employed in the spectrum from 600nm to 850nm i.e. to the above mentioned laser and LED-devices.

3. Summary of the invention.

It is an object of the present invention to provide a universal photographic material sensitive to the spectral range of 600 to 850nm so that said material is suitable for use with different laser and LED-devices emitting in said spectral range.

It is another object of the present invention to provide an image with said universal photographic material.

It is a third object of the present invention to provide a method for obtaining a lithographic printing plate with said universal photographic material.

Other objects will become clear from the description hereinafter.

According to the present invention a universal photographic material is provided comprising on a support a silver halide emulsion sensitized to light of wavelengths between 600nm and 850nm characterized in that said silver halide emulsion comprises one or more red sensitizing dyes having a maximum absorption between 620nm and 680nm, an infrared sensitizing dye having a maximum absorption between 700nm and 850nm and a supersensitizing compound showing no substantial absorption in said wavelength range from 600nm to 850nm.

According to the present invention there is also provided a method for obtaining an image using the above defined photographic material.

According to the present invention there is also provided a method for obtaining a lithographic printing plate with the above defined photographic material.

4. Detailed description.

It has been found that a photographic material comprising a silver halide emulsion sensitized with a red sensitizing dye, an infrared sensitizing dye and a supersensitizer shows a sufficient sensitivity for laser and LED-devices in the spectral range from 600nm to 850nm. Thus the same material can be used for exposure with e.g. a He/Ne laser, a IR-emitting LED, a laser diode emitting at 680nm etc..

By the maximum absorption of the red and infrared dye is meant the maximum absorption of the separate dye determined in a silver halide emulsion.

Red sensitizing dyes suitable for use in accordance with the present invention are the red sensitizing dyes commonly employed e.g. the dyes described by F.M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine dyes, complex merocyanine dyes. Preferred red sensitizing dyes correspond to one of the following formulas (I) (II) or (III) :

wherein each of X and Z independently represents S or Se, Y represents S, Se or NR with R representing an alkyl, cycloalkyl, aryl and that may be substituted, each of R and R independently represents C^-C^ alkyl, C^-^ alkyl substituted with a COOM or SOg M group with M representing hydrogen, a metal cation or NH _* -_ , R2 and

R each independently represents hydrogen, an alkyl, aralkyl or aryl each of which may be substituted, R and R each independently represents an alkyl, aryl, aralkyl each of which may be substituted and R and R may also be linked to each other to form a ring that may contain one or more heteroatoms and that may be aromatic, T represents one or more substituents selected from the group of alkyl, aryl, aralkyl, halogen, -CN, ester, hydroxyl, -TSU^ _r -NHR, ___2 with R as defined above, fused-on rings and alkoxy and T may also be absent, and A- represents an anion, e.g. F~, Cl~, Br~, I~, toluene sulphonate, ClO^ ^" etc..

1 A m. wherein R , R , R , A and T have the same meaning as defined above, each of G and L independently represents S, Se or C(CHg) and Q independently from T may have one of the significances given for T.

wherein Z, Y, R , R , R , R , R and R have the same meaning as defined above.

Examples of formula (I) (II) and (III) are listed in table 1.

Table 1

(2)

CO

^•• CO

The red sensitizing dye is preferably added to the silver halide emulsion in an amount of 1*10 mol/mol of silver halide to 1*10 mol/mol of silver halide. When amounts less than 1*10 mol/mol of silver halide are used the desired sensitivity in the red and/or far red part of the spectrum i.e. from 600 to 700 may not be obtained while amounts larger than 1*10 mol/mol of silver halide may yield silver halide emulsion of low stability due to fogging. The optimal amount of red sensitizing dye to be used in accordance with the present invention depends on the silver halide composition, average grain size and crystal habit of the silver halide grain, type of sensitizing dyes etc..

Suitable infra-red sensitizing dyes for use in accordance with the present invention are disclosed in i.a. DS-P Nos 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, 3,623,881 and 3,695,888. Preferred infra-red sensitizing dyes

correspond to general formula (IV)

wherein T, Q, R , R , and A~ have the same meaning as defined above,

Z 1 and X1 each independently represents O, S or Se, RQ, R10 and R1 ^■ . each independently represents an alkyl, aryl, aralkyl or a cycloalkyl each of which may contain one or more hetero-atoms and

Q 1 (] that may be substituted, a halogen or hydrogen and or R may be linked to R 13 and 11 and R12 each independently represents hydrogen or an alkyl and that may be linked to each other.

Examples of dyes corresponding to formula (IV) are listed in table 2.

Table 2

The infrared sensitizing dye is preferably added to the silver halide emulsion in an amount of 1*10 mol/mol of silver halide to 3*10 mol/mol of silver halide. When amounts less than 1*10 mol/mol of silver halide are used the desired sensitivity in the infrared part of the spectrum i.e. above 700nm may not be obtained while amounts larger than 3*10 mol/mol of silver halide may yield silver halide emulsion of low stability due to fogging. The optimal amount of infrared sensitizing dye to be used in accordance with the present invention depends on the silver halide composition, average grain size and crystal habit of the silver halide grain, type of sensitizing dyes etc..

The supersensitizing compound used in accordance with the present invention is a compound that shows no substantial absorption in the sensitivity range of the dye mixture used in accordance with the present invention and whereby said supersensitizing compound increases the sensitivity of at least one of the dyes constituting said dye mixture. Suitable supersensitizing compounds for use in accordance with the present invention are disclosed in e.g. Research Disclosure Vol 289, May 1988, item 28952, US-P-5.009.991, US-P-4.910.129, US-P-2.785.058, US-P-3.695.888; US-P-3.457.078 etc.. Said supersensitizing compound or compounds are preferably used in a total amount of 5*10 mol/mol silver halide to 7*10 mol/mol of silver halide most preferably between 1*10 and 6*10 mol/mol of silver halide. Too small amounts of supersensitizing compound may yield silver halide emulsion of low sensitivity and low stability while too large amounts of supersensitizing compound(s) may also yield low sensitivity due to a reduced development rate of the silver halide.

Preferably used supersensitizing compounds are of the l-phenyl-5- mercapto-tetrazole type or of the 2-mercaptobenzthiazole type. Specific examples of supersensitizing compounds are:

Spectral sensitization of the silver halide emulsion is preferably carried out as a last step in the preparation of the silver halide emulsion. The red sensitizing dye, infrared sensitizing dye and supersensitizing compound may be sequentially added in any order or simultaneously to the silver halide emulsion during sensitization. However the supersensitizer is preferably added first followed by the red sensitizing dye.

The photographic silver halide emulsions can be prepared from soluble silver salts and soluble halides according to different methods as described e.g. by P. Glafkides in "Chimie et Physique

Pho ographique", Paul Montel, Paris (1967), by G.F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966) , and by V.L. Zelikman et al in "Making and Coating Photographic Emulsion", The Focal Press, London (1966) .

The photographic silver halide emulsions used according to the present invention can be prepared by mixing the halide and silver solutions in partially or fully controlled conditions of temperature, concentrations, sequence of addition, and rates of addition. The silver halide can be precipitated according to the single-jet method or the double-jet method.

The silver halide particles of the photographic emulsions used according to the present invention may have a regular crystalline form such as a cubic or octahedral form or they may have a transition form. They may also have an irregular crystalline form such as a spherical form or a tabular form, or may otherwise have a composite crystal form comprising a mixture of said regular and irregular crystalline forms.

To take most advantage of the present invention the emulsion or emulsions preferably consist principally of silver chloride preferably at least 70 mole% while a fraction of silver bromide may be present ranging from 1 mole% to 30 mole%. It has been found that when too large amounts of bromide are present in the silver halide composition sensitivity of the photographic material drops probably due to a reduced rate of development of the silver halide as a consequence of the large coverage of the grains with sensitizing dyes and supersensitizing compound. When silver chlorobromide emulsions are used they preferably belong to the core/shell type well known to those skilled in the art in the sense that substantially all the bromide is concentrated in the core. This core contains preferably 10 to 40 % of the total silver halide precipitated, while the shell consists preferably of 60 to 90 % of the total silver halide precipitated. The silver halide may also contain small amounts of iodide i.e. upto 3%.

The average size of the silver halide grains may range from 0.10 to 0.70 μm , preferably from 0.25 to 0.45 μm.

The size distribution of the silver halide particles of the photographic emulsions to be used according to the present invention

can be homodisperse or heterodisperse. A homodisperse size distribution is obtained when 95% of the grains have a size that does not deviate more than 30% from the average grain size.

Preferably during the precipitation stage Iridium and/or

Rhodium containing compounds or a mixture of both are added. The concentration of these added compounds ranges from 10 —8 to 10—3 mole per mole of AgNOg, preferably between 10 —7 and 10—6 mole per mole of

AgNOo. This results in the building in in the silver halide crystal lattice of minor amounts of Iridium and/or Rhodium, so-called

Iridium and/or Rhodium dopants. As known to those skilled in the art numerous scientific and patent publications disclose the addition of

Iridium or Rhodium containing compounds or compounds containing other elements of Group VIII of the Periodic System during emulsion preparation.

The emulsions can be chemically sensitized e.g. by adding sulphur-containing compounds during the chemical ripening stage e.g. allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Also reducing agents e.g. the tin compounds described in BE-P 493,464 and 568,687, and polyamines such as diethylene triamine or derivatives of aminomethane-sulphonic acid can be used as chemical sensitizers. Other suitable chemical sensitizers are noble metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium. This method of chemical sensitization has been described in the article of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951) .

The silver halide emulsions may contain the usual stabilizers e.g. homopolar or salt-like compounds of mercury with aromatic or heteroσyclic rings such as mercaptotriazoles, simple mercury salts, sulphonium mercury double salts and other mercury compounds. Other suitable stabilizers are azaindenes, preferably tetra- or penta-azaindenes, especially those substituted with hydroxy or amino groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr. Photophys. Photochem. 47, 2-27 (1952) . Other suitable stabilizers are i.a. heterocyclic mercapto compounds e.g. phenylmercaptotetrazole, quaternary benzothiazole derivatives, and benzotriazole. Preferred compounds are mercapto substituted pyrimidine derivatives as disclosed in US-P 3,692,527.

The silver halide emulsions may contain pH controlling ingredients. Preferably the emulsion layer is coated at a pH value below the isoelectric point of the gelatin to improve the stability characteristics of the coated layer. Other ingredients such as antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin may be present. The silver halide emulsion layer may comprise light-screening dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. US-P 4,092,168, US-P 4,311,787 and DE-P 2,453,217.

In an especially preferred embodiment the emulsion layer contciined in the imaging element contains a compound which comprises in its molecular structure a group capable of adsorbing to silver halide and a group capable of reducing silver halide. Compounds of this kind have been disclosed in EP-A-449340. In this way a combination of a stabilizing and a development activating function in one compound is achieved. A preferred compound belonging to this class is represented by the following formula :

(22)

More details about the composition, preparation and coating of silver halide emulsions can be found in e.g. Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109. In addition to the above described emulsion layer other hydrophilic colloid layers in water permeable relationship with these layers may be present. For example it is especially advantageous to include a base-layer between the support and the photosensitive silver halide emulsion layer. In a preferred

embodiment of the present invention said base-layer serves as an an ihalation layer. This layer can therefore contain the same light-absorbing dyes as described above for the emulsion layer ; as alternative finely divided carbon black can be used for the same antihalation purposes as described in US-P 2,327,828. On the other hand, in order to gain sensitivity, light reflecting pigments, e.g. titaniumdioxide can be present. Further this layer can contain hardening agents, matting agents, e.g. silica particles, and wetting agents. At least part of these matting agents and/or light reflection pigments may also be present in the silver halide emulsion layer the most part however preferably being present in said base-layer. As a further alternative the light reflecting pigments may be present in a separate layer provided between the antihalation layer and the photosensitive silver halide emulsion layer.

In a preferred embodiment in connection with the present invention a backing layer is provided at the non-light sensitive side of the support. This layer which can serve as anti-curl layer can contain i.a. matting agents e.g. silica particles, lubricants, antistatic agents, light absorbing dyes, opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and wetting agents. The backing layer can consist of one single layer or a double layer pack.

The hydrophilic layers usually contain gelatin as hydrophilic colloid binder. Mixtures of different gelatins with different viscosities can be used to adjust the rheological properties of the layer. Like the emulsion layer the other hydrophilic layers are coated preferably at a pH value below the isoelectric point of the gelatin. But instead of or together with gelatin, use can be made of one or more other natural and/or synthetic hydrophilic colloids, e.g. albumin, casein, zein, polyvinyl alcohol, alginic acids or salts thereof, cellulose derivatives such as carboxymethyl cellulose, modified gelatin, e.g. phthaloyl gelatin etc.

The hydrophilic layers of the photographic element, especially when the binder used is gelatin, can be hardened with appropriate hardening agents such as those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type e.g.

l,3-vinylsulphonyl-2-propanol, chromium salts e.g. chromium acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and ethyloldimethylhydantoin, dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and muσophenoxychloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts of the type, described in US 4,063,952.

Preferably used hardening agents are of the aldehyde type. The hardening agents can be used in wide concentration range but are preferably used in an amount of 4% to 7% of the hydrophilic colloid. Different amounts of hardener can be used in the different layers of the imaging element or the hardening of one layer may be adjusted by the diffusion of a hardener from another layer.

The imaging element used according to the present invention may further comprise various kinds of sur ace-active agents in the photographic emulsion layer or in at least one other hydrophilic colloid layer. Suitable surface-active agents include non-ionic agents such as saponins, alkylene oxides e.g. polyethylene glycol, polyethylene glycol/polypropylene glycol condensation products, polyethylene glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkylamines or alkylamides, siliσone-polyethylene oxide adducts, glycidol derivatives, fatty acid esters of polyhydric alcohols and alkyl esters of saσcharides; anionic agents comprising an acid group such as a carboxy, sulpho, phospho, sulphuric or phosphoric ester group; ampholytic agents such as aminoacids, aminoalkyl sulphonic acids, aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides; and cationic agents such as alkylamine salts, aliphatic, aromatic, or heterocyclic quaternary ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or sulphonium salts. Preferably compounds containing perfluorinated alkyl groups are used» Such surface-active agents can be used for various purposes e.g. as coating aids, as compounds

preventing electric charges, as compounds improving slidability, as compounds facilitating dispersive emulsification and as compounds preventing or reducing adhesion.

The photographic material of the present invention may further comprise various other additives such as e.g. compounds improving the dimensional stability of the photographic element, UV-absorbers, spacing agents and plasticizers.

Suitable additives for improving the dimensional stability of the photographic element are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl (meth) acrylates, alkoxy(meth)aσrylates, glyσidyl (meth) crylates, (meth) acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth) acrylates, and styrene sulphonic acids.

Supports suitable for use in accordance with the present invention may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer, e.g. a polyethylene layer which optionally contains an anti-halation dye or pigment. It is also possible to use an organic resin support e.g. cellulose nitrate film, cellulose acetate film, pol (vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film, polyvinylchloride film or poly-Alpha-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a hydrophilic adhesion layer which can contain water insoluble particles such as silica or titanium dioxide. Metal supports such as e.g. aluminium, zinc, steel etc., may also be used in accordance with the photographic element of the present invention.

According to the method of the present invention the above described photographic material element is information-wise exposed using a scanning exposure beam between 600 and 850nm and subsequently developed in alkaline processing liquid in the presence of developing agents. The photographic element in connection with

the present invention may be exposed with phototypesetters operating with a laser, LED or laser diode emitting light between 600nm and 850nm. Examples of HeNe laser containing exposure units are the image-setters LINOTRONIC 300, marketed by LINOTYPE Co, and CG 9600, marketed by AGFA COMPUGRAPHIC, a division of AGFA CORPORATION. Exposure units provided with a laserdiode are LINOTRONIC 200, marketed by LINOTYPE Co, and CG 9400, marketed by AGFA COMPUGRAPHIC, a division of AGFA CORPORATION.

Silver halide developing agents for use in accordance with the present invention are preferably of the p-dihydroxybenzene type, e.g. hydroquinone, methylhydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a l-phenyl-3-pyrazolidinone-type developing agent and/or p-monomethylaminophenol. Particularly useful auxiliary developing agents are of the phenidone type e.g. l-phenyl-3-pyrazolidinone, l-phenyl-4-monomethyl-3-pyrazolidinone, and l-phenyl-4,4-dimethyl-3-pyrazolidinone. However other developing agents can be used. Said developing agents may be contained in an alkaline processing liquid but are preferably contained in one or more layers of the imaging element. In the latter case the alkaline processing liquid merely serves as an alkaline activating liquid.

The pH of said activating liquid is preferably between 12 and 14 and may be established by an organic and/or inorganic alkali agent. Examples of suitable alkali agents are e.g. sodium hydroxide, carbonates, secundary and/or tertiary alkanolamines, amines etc. or mixtures thereof.

The alkaline processing liquid preferably also contains a preserving agent having antioxidation activity, e.g. sulphite ions provided e.g. by sodium or potassium sulphite. For example, the aqueous alkaline solution comprises sodium sulphite in an amount ranging from 0.15 to 1.0 mol/1. Further may be present a thickening agent, e.g. hydroxyethylσellulose and carboxymethylcellulose, fog inhibiting agents, e.g. potassium bromide, potassium iodide and a benzotriazole which is known to improve the printing endurance, calcium-sequestering compounds, anti-sludge agents, and hardeners including latent hardeners.

Development acceleration can be accomplished with the aid of

various compounds to the alkaline processing liquid and/or one or more layers of the photographic element, preferably polyalkylene derivatives having a molecular weight of at least 400 such as those described in e.g. US-P 3,038,805 - 4,038,075 - 4,292,400 - 4,975,354.

The above described development step is preferably followed by a washing step, a fixing step and another washing or stabilizing step. The first washing step may be omitted.

The photographic material of the present invention may also be used in the silver salt diffusion transfer process. The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in US-P- 2352014 and the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde - The Focal Press -London and New York, (1972) .

According to the DTR process, a silver complex salt is image- wise transferred by diffusion from the image-wise exposed silver halide emulsion layer into an image receiving layer, where it is converted to a silver image usually in the presence of physical development nuclei. For this purpose, the image-wise exposed silver halide emulsion layer is developed in the presence of a developing agent and non-developed silver halide is converted by means of a silver halide complexing agent into a soluble silver complex salt while in contact with an image receiving layer.

At the exposed areas of the silver halide emulsion layer the silver halide is developed (chemical development) and thus cannot be dissolved anymore to diffuse to the receiving layer.

At the unexposed areas of the silver halide emulsion layer the silver halide is converted to a soluble silver complex salt and is transferred to the receiving layer, where it forms a silver image usually in the presence of physical development nuclei.

Suitable silver complexing agents also called silver halide solvents for use in accordance with the present invention are e.g. thiosulphate or thioσyanate. Further interesting silver halide complexing agents, are cyclic imides, preferably combined with alkanolamines, as described in US-P 4,297,430 and US-P 4,355,090 and 2-mercaρtobenzoic acid derivatives as described in US-P 4,297,429,

preferably combined with alkanolamines or with cyclic imides and alkanolamines. Said silver halide solvent(s) can be present in one or more layers comprised in the imaging element but are preferably comprised in the alkaline processing liquid.

Preferred development nuclei for use in accordance with the present invention are sulphides of heavy metals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, and zinc. Especially suitable development nuclei in connection with the present invention are palladium sulphide nuclei. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans, and tin (II) halides. Heavy metals, preferably silver, gold, platinum, palladium, and mercury can be used in colloidal form.

A DTR—image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. in US-P 4,297,429, US-P 4,297,430, US-P 4,501,811 or US-P 5,059,508.

The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two—sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.

According to a preferred embodiment of the present invention an imaging element is provided that can be imaged according to the DTR- method so that a lithographic printing plate can be obtained. Said imaging element comprises on a support in the order given a silver halide emulsion layer sensitized to light from 600nm to 850nm and a physical development nuclei layer. Preferably the imaging element also comprises a base layer between the support and the silver halide emulsion layer as described above. A further intermediate

layer between the silver halide emulsion layer and the layer containing physical development nuclei may also be provided.

A matting agent is preferably included in said base layer and optionally in small amounts i.e. from 1 to 20% by weight is the silver halide emulsion layer. When the matting agent is included in the silver halide emulsion layer it is preferable added to the emulsion after spectral sensitization of the silver halide emulsion to avoid adsorption the sensitizer to the matting agent. Suitable matting agents for use in accordance with the present embodiment are water insoluble inorganic or organic particles having an average diameter between lμm and 10μm most preferably between 4μm and 8μm. A preferred matting agent is silica.

The layer containing physical development nuclei is preferably free of hydrophilic binder but may comprise small amounts upto 30% by weight of the total weight of said layer of a hydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicity of the surface.

Preferably used supports in connection with the present embodiment are paper supports or resin supports e.g. polyester film supports.

To obtain a lithographic printing plate the above described DTR- imaging element is information-wise exposed using a scanning exposure in the spectrum of 600nm to 850nm and is subsequently developed with an alkaline processing liquid in the presence of developing agent(s) and silver halide solvent (s). Said development step is preferably followed by a neutralization of the surface of the imaged element by guiding the element through a neutralization liquid having a pH between 5 and 7. The neutralization liquid preferably contains a buffer e.g. a phosphate buffer, a citrate buffer or mixture thereof. The neutralization solution can further contain baσtericides, e.g. phenol, thymol or

5-bromo-5-nitro-l,3-dioxan as described in EP 0,150,517. The liquid can also contain substances which influence the hydrophobic / hydrophilic balance of the printing plate obtained after processing of the DTR element, e.g. silica. Finally the neutralization solution can contain wetting agents, preferably compounds containing perfluorinated alkyl groups.

To improve the differentiation between the hydrophobic silver

image and the hydrophilic background the alkaline processing liquid and/or neutralization liquid preferably contain one or more hydrophobizing agents, e.g. those described in US-P 3,776,728, and US-P 4,563,410. Preferred compounds are 5-n-heptyl-2-mercapto-l,3,4,-oxadiazol and 3-mercapto-4-acetamido-5-n.heptyl-1,2,4-triazole.

According to an alternative embodiment of the present invention a lithographic printing plate can be obtained by means of the DTR- process using an imaging element comprising in the order given a grained and anodized aluminium support, an optional layer of physical development nuclei and a silver halide emulsion layer sensitized to light of 600nm to 850nm. The imaging element of the present embodiment may be imaged using an information-wise exposure as described above followed by a development step in the presence of development agent(s) and silver halide solvent(s) so that a silver image is formed in the physical development nuclei layer or directly on the aluminium support. Subsequently the silver halide emulsion layer and any other optional hydrophilic layers are removed by rinsing the imaged element with water so that the silver image is exposed. Finally the hydrophobic character of the silver image is preferably improved using a finishing liquid comprising hydrophobizing agents as described above.

To facilate the removal of the silver halide emulsion layer it is advantageous to provide a hydrophilic layer between the aluminium support and the silver halide emulsion layer. Preferably used hydrophilic layers for this purpose are layers comprising a hydrophilic non-proteinic film-forming polymers e.g. polyvinyl alcohol, polymer beads e.g. poly(meth)acrylate beads or mixtures thereof. Such type of layers are disclosed in EP-A-483415 and EP-A-410500.

The present invention is illustrated by the following examples without limiting it thereto. All parts are by weight unless otherwise specified. The spectral sensitizers and superserisitizers used in the following examples are shown in the description above and are referred to by the number mentioned besides .their structure.

EXAMPLE 1

A gelatin silver halide emulsion was prepared using the double jet precipitation by slowly mixing whilst stirring an aqueous solution of AgNOg having a concentration of 2 mole/1, and an aqueous solution having a concentration of 1.7 mole/1 of NaCl, 0.48 mole/1 of KBr and 0.001 mole/1 of KI. Before the precipitation 5*10 mole/1 of sodium hexachlororhodaat was added to the silver nitrate solution. In a second part of the precipitation an aqueous solution of AgNOg having a concentration of 1 mole/1 was slowly mixed with a aqueous solution of NaCl at a concentration of 1.3 mole/1.

The temperature during the silver halide formation was 55°C. The obtained core-shell emulsion was cooled, flocculated and washed. Gelatin was added in an amount sufficient to reach a ratio of 2/3 by weight of gelatin to silver halide, expressed as equivalent amount of silver nitrate.

Subsequently a chemical ripening was carried out in a conventional way, known to those skilled in the art, using thiosulphate and gold salts.

The emulsion was sensitised for the red and infrared spectral regions by using compounds (15) and (1) following the procedure : first compound (19) the supersensitizing compound is added, then the I.R. sensitizing dye (15) followed by the red sensitizing dye (1) . Finally compound (22) mentioned above was added. The amounts were varied as indicated in Table 3.

A photographic DTR monosheet material was prepared as follows. One side of a film support is coated with two layers by double layer curtain coating. The layer nearer to the support is the antihalation layer and the other is the emulsion layer. The emulsion was coated

2 at an amount of silver halide corresponding to 1.5 g AgNOg/m . This

2 emulsion layer contained 0.1 g/m of l-phenyl-3-pyrazolidinon and

1.0 g/m of gelatin.

The antihalation layer contained carbon black, silica particles of 5 micron average size and gelatin at 3 g/m . The gelatin was lime-treated, substantially free of calcium ions (1000 ppm or less) and of the high viscosity type (not less than 85 .Pa.s at 40°C for

a 10% solution) .

After drying these layers were subjected to a temperature of

40°C for 5 days and then overcoated with a layer containing PdS o 2 nuclei hydrochinon at 0.4 g/ and formaldehyde at 100 mg/m .

The following processing solutions were prepared :

Activator solution sodium hydroxide sodium sulphite anh. potassium thiocyanate 3-mercapto-4-acetamido-

5-n.heptyl-l,2,4-triazole water to make

Neutralization solution citric acid 10 g sodium citrate 35 g cysteine 1 g sodium sulphite anh. 5 g phenol 50 mg water to make 11

Dampening- solution water 880 ml citric acid 6 g boric acid 8.4 g sodium sulphate anh. 25 g ethyleneglycol 100 g colloidal silica 28 g

The above described DTR material was image-wise exposed in image setters working with a HeNe laser (632 nm) or with laser diodes (at 670 nm or at 780-800 nm) . Subsequently the material was treated with the described activator solution for 10 seconds at 30°C, thereupon treated with the described neutralization solution at 25°C and finally dried.

Sensitivity of the photographic materials was measured and is

expressed as the relative log It value determined at density 1.0.

A lower value indicates a higher sensitivity and vice versa.

A decrease by 0.3 units indicates a doubling of the sensitivity.

The printing plate thus prepared was mounted on an offset printing machine (AB DICK 350 CD - trade name for offset printing machine manufactured by AB DICK Co. ) . During the printing run the described dampening solution was used in each case.

As table 3 shows the mixing of the different sensitizing dyes gives the desired sensitivity in the spectral region 630 to 850 nm.

Table 3 : Influence of the amounts of sensitizing dyes on the spectral sensitivity various wavelengths.

Experiment Compound Sensitivity at( )

(19) (15) (1) 630nm 670nm 780-800nm

- __.R

( ) expressed as 10 mol/mol AgNOg i*i*

( ) expressed as log It

EXAMPLE 2

An imaging element was prepared as in example 1 with the exception that the emulsion was sensitized using the following procedure. First compound (19), the supersensitizing compound is added, then the IR sensitizing dye (15) , followed by a mixture of red sensitizing dyes (1) + (13) . The amounts are varied as indicated in table 4. The extra addition of dye (13) increases the sensitivity at 670 nm.

Table 4 :Influence of the amounts of sensitizing dyes on the spectral sensitivity of various wavelengths.

( ) expressed as 10 mol/mol AgNOg

Table 5 :Influence of mixing different sensitizing dyes on printing

Experiment Printing endurance stain

7 >7000 no

9 >7000 no

As table 5 shows no adverse effects of the mixing of sensitizing dyes have been seen on printing. All plates have a good printing endurance with no occurence of staining.

EXAMPLE 3

An imaging element was prepared as described in example 2 with the exception however that the supersensitizer was replaced with the supersensitizing compound (20) or (21) .

Table 6 represents the concentrations of the red sensitizing dyes (compound (13)+(1)), infrared sensitizing dye (compound (15)) and the concentration of the supersensitizing contpound together with the sensitivity obtained.

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Table 6: Influence of the nature of the supersensitizing compound.

*

SS-compound Compound Sensitivity

_ __R expressed as 10 mol/mol AgNOg

It can be seen that the nature of the supersensitizing agent can be changed without substantial influence on the sensitivity in the spectral range from 600nm to 820nm.