DE69712989T2 - Radiation sensitive imaging element and method of making lithographic printing plates using the same - Google Patents

Description

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a radiation-sensitive material for producing a lithographic printing plate. The present invention further relates to a method for producing a printing plate using this radiation-sensitive material.

2. GENERAL STATE OF THE ART

Lithographic printing is the process of printing from specially prepared surfaces, certain areas of which will attract lithographic ink and other areas will repel the ink when wetted with water. The ink-attracting areas form the printing image areas, and the ink-repelling areas form the background areas.

In the field of photolithography, a photographic material is made to attract oily or greasy colors in the photo-exposed areas (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.

In the production of conventional lithographic printing plates, also referred to as surface lithoplates or planographic printing plates, a support which has an affinity for water or has acquired such affinity through chemical processing is coated with a thin layer of a radiation-sensitive composition. Suitable layers with a radiation-sensitive composition are light-sensitive polymer layers which contain diazo compounds, dichromate-sensitized hydrophilic colloids and a variety of synthetic photopolymers. In particular, diazo-sensitized layer combinations are widely used.

During the image-wise exposure of such a light-sensitive layer, the exposed image areas become insoluble and the non-exposed areas remain soluble. The printing plate is then treated with a suitable liquid designed to remove the diazonium salt or diazo resin contained in the non-exposed areas.

On the other hand, there are also processes in which image-generating elements that are more sensitive to heat than to radiation are used to produce printing plates. The radiation-sensitive image-generating elements used to produce a printing plate as described above have the particular disadvantage that they must be protected from light. Furthermore, they are less suitable for direct printing plate imaging (computer-to-plate technology). There is a clear trend in the market towards heat-sensitive printing plate precursors.

EP-A 95202871.0, 95202872.8, 95202873.6 and 95202874.4 disclose a process for producing a lithographic printing plate characterized by the following steps.

(1) imagewise exposure to light from a heat-sensitive imaging element containing (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) and developing an image-wise exposed imaging element thus obtained by rinsing with tap water.

EP-A 625 728 discloses a lithographic printing plate with an image-forming layer which is sensitive to UV and IR radiation and can be both positive-working and negative-working. This image-forming layer contains (1) a resole resin, (2) a novolak resin, (3) a latent Brönsted acid and (4) a substance which absorbs infrared radiation. By exposure to UV or IR light (830 nm) and subsequent conventional PS plate development, a positive-working lithographic printing plate is obtained. If the plate is baked before the development stage (60 s at 100°C), a negative-working printing plate is obtained.

EP-A 483 693 discloses a radiation-sensitive colored resin composition for the formation of color images which (a) an acid-curable resin-based material, (b) a photoreactive acid-releasing agent, and (c) a pigment.

US-P 4,708,925 discloses a positive-working printing plate with a photosensitive composition containing (1) an alkali-soluble novolak resin and (2) an onium salt and optionally a spectral IR sensitizer. By imagewise irradiation of this printing plate with UV light, visible light or IR light and a subsequent development step with an alkaline solution, a positive-working printing plate is obtained.

3. BRIEF DESCRIPTION OF THE PRESENT INVENTION

The object of the present invention is to provide an imaging element which is infrared and heat sensitive in order to simplify digital imaging by exposure to infrared radiation or by means of a thermal head.

Another object of the present invention is to provide an imaging element that is sensitive to UV light to facilitate optical imaging by exposure to ultraviolet radiation through an imaging original.

Yet another object of the present invention is to provide an imaging element that is substantially insensitive to visible radiation to simplify room light handling.

Yet another object of the present invention is to provide a method for conveniently producing a high quality negative working lithographic printing plate using the above imaging element.

Further objects of the present invention will become apparent from the following description.

The objects of the invention are achieved by a radiation-sensitive imaging element which contains, on a hydrophilic surface of a lithographic support, an image-forming layer comprising (1) an alkali-soluble or alkali-swellable resin having a phenolic hydroxyl group, (2) a latent Brönsted acid and (3) a substance which absorbs infrared radiation, characterized in that the imaging layer contains an amino crosslinking agent, and with the proviso that the infrared radiation absorbing substance is not a carbon black pigment.

The objects of the invention are also achieved by a method for producing a negative-working lithographic printing plate characterized by the following steps:

(a) imagewise or informationally exposing an imaging element as described above to ultraviolet light, infrared light or heat,

(b) heating the exposed imaging element to reduce the solubility in the exposed areas, and

(c) developing the exposed and heated imaging element with an aqueous alkaline developer solution to remove the non-exposed areas and thereby obtain a lithographic printing plate.

4. DETAILED DESCRIPTION OF THE PRESENT INVENTION

We have found that imaging elements which are imageable with ultraviolet and infrared radiation and heat and which give high quality lithographic printing plates can be obtained using an imaging element as described above according to the process of the invention.

In order to be used as a negative-working printing plate, the printing plate must be exposed imagewise to activating radiation and heated to reduce the solubility in the exposed areas and then brought into contact with an aqueous alkaline developer solution to remove the non-exposed areas. In contrast to the printing plates known from EP-A 625 728, the requirement to use both a resole resin and a novolak resin is eliminated and the use of an alkali-soluble or alkali-swellable resin with a phenolic hydroxyl group and an amino cross-linking agent is a top priority.

While it is not the applicants' wish to limit the effect of their invention by any theory, the starting point is that the present invention is based on a chemical amplification mechanism with acid catalysis, which Heating of the exposed plate occurs. This mechanism causes a reduction in the solubility of the exposed areas by hardening the resin mixture.

The effect of the printing plate as a negative-working printing plate depends to a critical extent on the use of a mixture of an amino cross-linking agent and an alkali-soluble or alkali-quenchable resin with a phenolic hydroxyl group, because by using one of these compounds alone no usable developed image is obtained.

As a starting point, exposure of the printing plate to infrared radiation causes decomposition of both the infrared radiation absorbing substance and the latent Bronsted acid in the exposed areas. It is believed that the decomposition products catalyze a reaction between the amino crosslinking agent and the alkali-soluble or alkali-swellable resin having a phenolic hydroxyl group to form a matrix which, upon heating, is insoluble in an aqueous alkaline developer solution.

Exposure to ultraviolet radiation also causes a decomposition of the latent Bronsted acid, yielding a Bronsted acid that catalyzes the matrix-forming reaction between the amino crosslinking agent and the alkali-soluble or alkali-swellable resin containing a phenolic hydroxyl group. The printing plate has a high spectral sensitivity to both ultraviolet radiation and infrared radiation. In other words, it can be exposed at two different wavelengths.

An amino crosslinking agent according to the invention is preferably a compound obtained by condensing a substance containing an amino group with formaldehyde. The amino crosslinking agent has functional groups bonded in pairs to the amino nitrogens. The three most common paired groups correspond to the following formulas:

-N(CH2 OR)2 , -N(CH2 OH)CH2 OR, -N(H)CH2 OR

where R is usually a low molecular weight alkyl group such as a methyl group, an ethyl group, a butyl group or an isobutyl group.

A preferred amino crosslinking agent is a compound from the group consisting of melamine-formaldehyde resins, (thio)urea-formaldehyde resins, guanamine-formaldehyde resins, benzoguanamine-formaldehyde resins and glycoluril-formaldehyde resins. Certain of these compounds are sold under the registered trademarks CYMEL or DYNOMIN by Dyno Cyanamid.

Suitable resins containing phenolic hydroxyl groups for use in an image-forming layer according to the invention include, for example, synthetic novolak resins such as ALNOVOL, a registered trademark of Reichold Hoechst, and DUREZ, a registered trademark of OxyChem, and synthetic polyvinylphenols such as MARUKA LYNCUR M, a registered trademark of Dyno Cyanamid.

The third essential ingredient of the radiation-sensitive composition of the invention is a latent Bronsted acid. The term "latent Bronsted acid" indicates a precursor which forms a Bronsted acid upon decomposition. It is believed that the Bronsted acid catalyzes the matrix-forming reaction between the amino crosslinking agent and the resin with phenolic hydroxyl groups. Typical examples of Bronsted acids suitable for this purpose are sulfonic acids, e.g. trifluoromethanesulfonic acid and hexafluorophosphoric acid.

Ionic latent Brönsted acids can be used according to the invention. Examples of such acids include onium salts, in particular iodonium, sulfonium, phosphonium, selenonium, diazonium and arsonium salts.

The ionic latent Brönsted acids that can be used are those corresponding to the following formula:

X+R1 R2 R3 R4 W&supmin;

When X is iodine, R₃ and R₄ are lone pairs of electrons and R₁ and R₂ are each independently an aryl group or a substituted aryl group. When X is XS or Se, R₄ is a lone pair of electrons and R₁, R₂ and R₃ are independently each represent an aryl group, a substituted aryl group, an aliphatic group or a substituted aliphatic group. When XP or As, R₁, R₂, R₃ and R₄ may each independently represent an aryl group, a substituted aryl group, an aliphatic group or a substituted aliphatic group. W may represent BF₄, CF₃SO₃, SbF₆, CCl₃CO₂, ClO₄, AsF₆, PF₆ or any corresponding acid having a pH of less than 3.

Any of the onium salts described in US-P 4,708,925 can be used as a latent Bronsted acid in the present invention. These include, but are not limited to, iodonium, sulfonium, phosphonium, bromonium, chloronium, oxysulfoxonium, oxysulfonium, sulfoxonium, selenonium, telluronium and arsonium salts.

The use of diazonium salts as latent Brönsted acids is particularly preferred according to the invention because, compared to other latent Brönsted acids, they provide the same sensitivity in the infrared range and a higher sensitivity in the ultraviolet range.

Typical examples of particularly useful onium salts include: diphenyliodonium hexafluorophosphate, triphenylsulfonium hexafluoroantimonate, phenylmethyl orthocyanobenzylsulfonium trifluoromethanesulfonate and 2-methoxy-4-aminophenyldiazonium hexafluorophosphate.

Non-ionic latent Brönsted acids can also be used according to the invention. Examples of these acids include compounds of the formula:

RCH2X, RCHX2, RCX3, R(CH2X)2 and R(CH&sub2;X)&sub3;

in which X is Cl, Br, F or CF₃SO₃ and R is an aromatic group or an aliphatic group.

Other suitable non-ionic latent Brönsted acids are haloalkyl-substituted s-triazines as described in EP-A 672 954, o-quinonediazides, photoacid-generating substances having a protecting group of the o-nitrobenzyl type as described by S. Hayase et al. in Polymer Sci., 25, 573 (1987), the compounds which undergo photodecomposition to produce a sulfonic acid such as iminosulfonates as described by M. Tunooka et al. in Polymer Preprints, Japan, 35 (8), disulfone compounds as described in JP-Pi 61-166544, α-sulfonyloxyketones, α-hydroxymethylbenzoinsulfonates, nitrobenzylsulfonates, α-sulfonylacetophenones and sulfonylimides, the preparation of these last compounds being known from numerous well-known references in the literature, the compounds which undergo photodecomposition to produce a phosphonic acid, a partially esterified phosphoric acid or a phosphoric acid, such as nitrobenzyl phosphates or nitrobenzylphosphonates, as described by M. Rubinstein et al. in Tetrahedron Letters, 17, 1445 (1975), benzoin phosphates or benzoin phosphonates as described by M. Pirrung and S. Shuey in J. Org. Chem., 59, 3890 (1994), pyrene methyl phosphates or pyrene methyl phosphonates, imino phosphates or imino phosphonates and imido phosphates or imido phosphonates, the preparation of these last compounds being based on numerous well-known references in the literature.

Also suitable are compounds in which the above-mentioned photosensitive acid precursors are embedded in a main chain or a side chain of a polymer. Examples of such compounds include the compounds mentioned by M. E. Woodhouse et al. in J. Am. Chem. Soc., 104, 5586 (1982), the compounds mentioned by S. P. Pappas et al. in J. Imaging Sci., 30 (5), 218 (1986), etc.

The fourth essential ingredient of the radiation-sensitive composition according to the invention is an infrared absorber, which sensitizes the composition to infrared radiation and turns the plate into a directly laser-addressable printing plate which can be imaged by exposure to a laser emitting in the infrared range.

The infrared absorber can be a dye or a pigment, provided that the infrared absorber is not a carbon black pigment. A very wide range of such compounds is generally known to those skilled in the art. These include dyes or pigments of the squarylium, croconate, cyanine, merocyanine, indolizine, pyrylium and metal dithiolene classes. The infrared absorber is preferably fragmented upon exposure to activating radiation because the decomposition products reduce the contrast between image and non-image areas and thus improve the development process.

Additional infrared absorbers useful in accordance with the invention include those mentioned in US-P 5,166,024, issued November 24, 1992. As described in this '024 patent, particularly useful infrared absorbers are phthalocyanine pigments.

As explained at the outset, the four essential ingredients of the radiation-sensitive composition of the invention are an amino crosslinking agent, a resin with phenolic hydroxyl groups, a latent Bronsted acid and an infrared absorber. Other ingredients that can optionally be incorporated into the composition are colorants, stabilizers, additional sensitizers, exposure indicators and surfactants.

The thickness of the image-forming layer in the printing plates according to the invention can vary within wide limits. An appropriate dry thickness is generally between 0.5 and 10 µm, particularly preferably between 1 and 5 µm.

The radiation-sensitive imaging element is prepared by dissolving or dispersing the amino crosslinking agent, the resin containing phenolic hydroxyl groups, the latent Bronsted acid and the infrared absorber in appropriate proportions in a suitable solvent and applying them to the support by well-known coating techniques such as spin coating or cascade coating. Preferred solvents include acetone, methyl ethyl ketone and 1-methoxy-2-propanol.

The amino crosslinking agent is preferably present in an amount between 0.5 and 10 wt.%, more preferably between 1 and 6 wt.%, and most preferably between 2.0 and 4.0 wt.% in the casting composition.

The resin having phenolic hydroxyl groups is preferably present in the casting composition in an amount between 1.0 and 20 wt.%, more preferably between 3.5 and 9 wt.%, and most preferably between 5.0 and 7.5 wt.%.

The latent Bronsted acid is preferably present in the casting composition in an amount between 0.1 and 2 wt.%, more preferably between 0.25 and 0.9 wt.%, and most preferably between 0.35 and 0.70 wt.%.

The infrared absorber is preferably present in the casting composition in an amount between 0.1 and 1.5 wt.%, more preferably between 0.15 and 1.0 wt.%, and most preferably between 0.20 and 0.40 wt.%.

All of the above weight percentages of amino crosslinker, phenolic hydroxyl resin, latent Bronsted acid and infrared absorber are weight percentages of the dry materials in the casting dispersion prior to its application to the substrate.

Suitable drying conditions for drying the layer include heating for 30 seconds to 10 minutes at a temperature between 20ºC and 150ºC.

According to one embodiment of the invention, the lithographic support may be an anodized aluminum support. A particularly preferred lithographic support is an electrochemically grained and anodized aluminum support. An anodized aluminum support in the present invention may be subjected to processing to improve the hydrophilic properties of the support surface. For example, the aluminum support may be siliconized by processing the support surface with a sodium silicate solution at an elevated temperature, e.g. 95°C. Alternatively, phosphate processing may be carried out, wherein the alumina surface is processed with a phosphate solution optionally further containing an inorganic fluoride. Furthermore, the alumina surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or at a slightly elevated temperature between about 30°C and 50°C. An interesting method consists in treating the aluminum oxide surface with polyvinylphosphonic acid, as described in DE-OS 2 607 207. Another interesting method consists in rinsing the aluminum oxide surface with a bicarbonate solution. It is also obvious that one or more of these post-treatments can be carried out separately or in combination.

According to a further embodiment of the invention, the lithographic support comprises a flexible support, such as a paper support or a plastic film, which is coated with a cross-linked hydrophilic layer. A particularly A suitable crosslinked hydrophilic layer can be obtained from a hydrophilic binder crosslinked with a crosslinking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed tetraalkyl orthosilicate. The latter binder is particularly preferred.

Hydrophilic (co)polymers such as homopolymers and copolymers of vinyl alcohol, acrylamide, methylolacrylamide, methylolmethacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride-vinyl methyl ether copolymers are suitable as hydrophilic binders. The hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably higher than or equal to the hydrophilicity of at least 60% by weight, preferably at least 80% by weight, hydrolyzed polyvinyl acetate.

The amount of crosslinking agent, in particular tetraalkyl orthosilicate, is preferably at least 0.2 parts by weight per part by weight of hydrophilic binder, is preferably between 0.5 and 5 parts by weight, particularly preferably between 1.0 part by weight and 3 parts by weight per part by weight of hydrophilic binder.

A crosslinked hydrophilic layer in a lithographic support used according to this embodiment preferably also contains substances which improve the mechanical strength and porosity of the layer. For this purpose, colloidal silica can be used. The colloidal silica can be used in the form of any commercially available water dispersion of colloidal silica with, for example, an average particle size of up to 40 nm, e.g. 20 nm. In addition, inert particles with a larger grain size than the colloidal silica can be added, e.g. silica prepared as described in J. Colloid and Interface Sci., Volume 26, 1968, pages 62 to 69, by Stöber, or alumina particles or particles with an average diameter of at least 100 nm, which are particles of titanium dioxide or other heavy metal oxides. By embedding these particles, the surface of the cross-linked hydrophilic layer is given a uniform rough texture with microscopic peaks and valleys which serve as storage points for water in background areas.

The thickness of a cross-linked hydrophilic layer in a lithographic support used according to this embodiment can vary between 0.2 µm and 25 µm and is preferably between 1 µm and 10 µm.

Particular examples of suitable crosslinked hydrophilic layers usable according to the invention are described in EP-A 601 240, GB-P 1 419 512, FR-P 2 300 354, US-P 3 971 660, US-P 4 284 705 and EP-A 514 490.

As a flexible support of a lithographic support according to this embodiment, a plastic film is particularly preferred, e.g. a subbed polyethylene terephthalate film, cellulose acetate film, polystyrene film, polycarbonate film, etc. The plastic film support can be opaque or translucent.

A polyester film carrier coated with an adhesion-improving layer is particularly preferred. Adhesion-improving layers particularly suitable for use according to the invention contain a hydrophilic binder and colloidal silica, as described in EP-A 619 524, EP-A 620 502 and EP-A 619 525. The amount of silica in the adhesion-improving layer is preferably between 200 mg/m² and 750 mg/m². Furthermore, the ratio of silica to hydrophilic binder is preferably above 1 and the specific surface area of the colloidal silica is preferably at least 300 m²/g, particularly preferably 500 m²/g.

The lithographic printing plates according to the invention can be exposed with a laser diode emitting radiation in the near infrared spectral range. Such laser diodes combine the advantages of low cost and low energy consumption. The lithographic printing plates can also be exposed with conventional ultraviolet radiation sources. These include pure carbon arc lamps, mercury vapor lamps, fluorescent lamps, tungsten filament lamps and flash lamps.

A particularly suitable exposure device is a laser diode with a maximum output of about 830 nm. Such a device is usually able to irradiate both the latent Brönsted acid and the infrared absorber in the exposed The products obtained during thermal decomposition are predominantly strong acids, which cause effective cross-linking of the resins and thereby change their solubility in aqueous alkaline developer solutions.

As described above, the image-wise exposed printing plate is heated during a step called the post-exposure baking step or NBE step. The heating step is carried out for between 15 and 300 seconds at a temperature between 70ºC and 150ºC. Particularly preferred is heating for between 30 and 90 seconds at a temperature between 80ºC and 135ºC. After NBE is complete, the plate is processed either manually or mechanically in an aqueous alkaline developer solution until the non-image areas are removed. This typically takes 30 to 120 seconds. A preferred aqueous alkaline developer solution is a silicate solution such as a 6% by weight aqueous solution of sodium metasilicate. A commercially available silicate solution that can be used for this purpose is KODAK AQUA-IMAGE POSITIVE DEVELOPER MX-1406-1, sold by Eastman Kodak Company, or Fuji PS Plate Developer DP-5, sold by Fuji Company. Following contact with the aqueous alkaline developer solution, the plate is usually processed with a finishing agent such as gum arabic.

The number of prints available depends mainly on the use of a post-processing bake. If no bake is used, the plate will typically give 60,000 to 70,000 prints, while a plate subjected to a 5-minute post-processing bake at 250ºC will typically give 300,000 to 350,000 prints. The number of prints available before wear can also be improved by increasing the coating weight.

Since the printing plate of the invention is sensitive to infrared light, it can conveniently be used to produce halftone or halftone images using a suitable infrared radiation source such as a laser diode emitting in the infrared range. Since the printing plate of the invention is also sensitive to ultraviolet light, it can also conveniently be imaged to produce halftone or halftone images using ultraviolet exposure through a suitable imaging original such as a silver halide film. to produce halftone or halftone images. Due to these properties, the same printing plate can be used in a device for entering electronic data by writing with a laser or in a type of device commonly used for ultraviolet exposure of lithographic printing plates. Digital or electronic imaging techniques can thus be easily combined with conventional optical imaging techniques, ie both types of imaging can be used on the same printing plate. This makes it possible, if necessary, to add information not available in an electronic format by optical imaging techniques in order to complete the imaging of the lithographic printing plate.

The present invention will now be illustrated by the following examples, but is not limited thereto. All parts, percentages and ratios are by weight, unless otherwise indicated.

EXAMPLE 1 Preparation of the lithographic support

A 0.20 mm thick aluminium foil is degreased by immersing the foil in an aqueous solution containing 5 g/l sodium hydroxide at 50ºC and rinsed with demineralised water. The foil is then electrochemically grained with alternating current at a temperature of 35ºC and a current density of 1,200 A/m² in an aqueous solution containing 4 g/l hydrochloric acid, 4 g/l hydroboric acid and 5 g/l aluminium ions to obtain a surface topography with an arithmetic mean roughness Ra of 0.5 µm.

After rinsing with demineralized water, the aluminium foil is etched with an aqueous solution containing 300 g/l sulfuric acid for 180 s at 60ºC and then rinsed with demineralized water for 30 s at 25ºC.

The film is then placed in an aqueous solution containing 200 g/l sulphuric acid at a temperature of 45ºC, a voltage of about 10 V and a current density of 150 A/m² for about 300 s. anodized to obtain an anodic oxidation film containing 3.00 g/m² Al₂O₃, washed with demineralized water, then post-treated with a solution containing 20 g/l sodium bicarbonate for 30 s at 40ºC, then rinsed with demineralized water for 120 s at 20ºC and dried.

Manufacturing of the imaging element

The lithographic aluminum support is coated with the IR-sensitive layer from a 10 wt. % solution of 90 wt. % methyl ethyl ketone and 10 wt. % methanol in a wet layer thickness of 30 µm. The IR-sensitive layer has the following composition after drying.

63 wt% MARUKA LYNCUR M H-2 (homopolymer of polyvinylphenol from Maruzen Co), 29 wt% CYMEL 303 (hexamethoxymethylmelamine from Dyno Cyanamid), 5 wt% TRIAZINE S (2,4,6-(trichloromethyl)-s-triazine from PCAS) and 3 wt% IR Absorber I. IR Absorber I

Production of the printing plate

The infrared light-sensitive imaging element is scanned by a laser diode emitting at 830 nm (scanning speed 2 m/s, beam width 9.6 µm and laser power on the plate surface between 60 and 120 mW).

After imaging, the plate is heated in an oven at 115ºC for 150 seconds and then allowed to cool to room temperature. The imaging element is then processed with Fuji PS Plate Developer DP-5 to remove the unexposed areas, thereby obtaining a negative-working lithographic plate.

The image obtained on the lithographic support can be used for printing on a conventional offset press using a conventional printing ink and a conventional fountain solution. Excellent prints are obtained.

EXAMPLE 2

A lithographic printing plate is prepared as described for the lithographic printing plate of Example 1, but with the difference that IR absorber II is used instead of IR absorber I and the imaged plate is heated to 135ºC instead of 115ºC before development. The results obtained are similar to the results of Example 1. IR absorber II

EXAMPLE 3

A lithographic printing plate is prepared as described for the lithographic printing plate of Example 1, but with the difference that 9-fluorenilidene iminotosylate is used instead of TRIAZINE S. The results obtained are similar to the results of Example 1.

EXAMPLE 4

A lithographic printing plate is prepared as described for the lithographic printing plate of Example 1, but with the difference that NOVOLAK N3 (phenol-formaldehyde resin from Soc. Belge de l'Azote) is used instead of MARUKA LYNCUR M H-2 and the imaged plate is heated to 90°C instead of 115°C before development. The results obtained are similar to those of Example 1.

EXAMPLE 5

A lithographic printing plate is prepared as described for the lithographic printing plate of Example 1, but with the difference that ALNOVOL PN430 (phenol-formaldehyde resin from Reichold Hoechst) is used instead of MARUKA LYNCUR M H-2 and the imaged plate is heated to 110ºC instead of 115ºC before development. The results obtained are similar to the results of Example 1.

EXAMPLE 6

A lithographic printing plate is prepared as described for the lithographic printing plate of Example 1, but with the difference that ALNOVOL PN844 (phenol-formaldehyde resin from Reichold Hoechst) is used instead of MARUKA LYNCUR M H-2 and the imaged plate is heated to 110ºC instead of 115ºC before development. The results obtained are similar to those of Example 1.

EXAMPLE 7

A test object with a 60 grid (60 lines/cm) and fine positive and negative lines is placed in layer-side contact on the imaging element of Example 1, after which the imaging element is exposed to ultraviolet radiation through the object.

The exposed plate is then heated in an oven at 115ºC for 150 seconds, after which the heated plate is allowed to cool to room temperature. The imaging element is then processed with Fuji PS Plate Developer DP-5 to remove the unexposed areas, thereby obtaining a negative-working lithographic printing plate.

The image obtained on the lithographic support can be used for printing on a conventional offset press using a conventional printing ink and a conventional fountain solution. Excellent prints are obtained.

Claims (10)

1. A radiation-sensitive imaging element comprising on a hydrophilic surface of a lithographic support an imaging layer comprising (1) an alkali-soluble or alkali-quenchable resin having a phenolic hydroxyl group, (2) a latent Bronsted acid and (3) an infrared radiation absorbing substance, characterized in that the imaging layer contains an amino crosslinking agent, and with the proviso that the infrared radiation absorbing substance is not a carbon black pigment. 2. A radiation-sensitive imaging element according to claim 1, characterized in that the amino crosslinking agent is a compound selected from the group consisting of melamine-formaldehyde resins, (thio)urea-formaldehyde resins, guanamine-formaldehyde resins, benzoguanamine-formaldehyde resins and glycoluril-formaldehyde resins. 3. A radiation-sensitive imaging element according to claim 1 or 2, characterized in that a novolac resin or a polyvinylphenol resin is used as the alkali-soluble or alkali-quenchable resin having phenolic hydroxyl groups. 4. A radiation-sensitive imaging element according to any one of claims 1 to 3, characterized in that the latent Bronsted acid is an ionic latent Bronsted acid. 5. A radiation-sensitive imaging element according to claim 4, characterized in that the ionic latent Bronsted acid is an iodonium, sulfonium, selenonium, diazonium or arsonium salt. 6. A radiation-sensitive imaging element according to any one of claims 1 to 3, characterized in that the latent Bronsted acid is a non-ionic latent Bronsted acid. 7. A radiation-sensitive imaging element according to claim 6, characterized in that the non-ionic latent Bronsted acid is a haloalkyl-substituted s-triazine. 8. A radiation-sensitive imaging element according to any one of claims 1 to 7, characterized in that the infrared absorber is a dye or pigment from the group consisting of squarylium, croconate, cyanine, merocyanine, indolizine, pyrylium and metal dithiolene compounds. 9. A radiation-sensitive imaging element according to any one of claims 1 to 8, characterized in that the lithographic support is an anodized aluminum support or comprises a flexible support with a crosslinked hydrophilic layer coated thereon. 10. A process for producing a negative-working lithographic printing plate, characterized by the following steps: (a) imagewise or informationally exposing an imaging element according to any one of claims 1 to 9 to ultraviolet light, infrared light or heat, (b) heating the exposed imaging element to reduce the solubility in the exposed areas, and (c) developing the exposed and heated imaging element with an aqueous alkaline developer solution to remove the non-exposed areas and thereby obtain a lithographic printing plate.