DE69806986T2 - Process for the production of positive working lithographic printing plates - Google Patents

Description

1. Technical field of the invention.

The present invention relates to a process for producing lithographic printing plates using a heat-sensitive imaging element. The present invention relates in particular to a process in which the heat-sensitive imaging element is exposed to an infrared laser with a pixel dwell time of less than 0.075 us in order to obtain highly sensitive positive-working printing plates.

2. General state of the art.

Lithographic printing is the process of printing from a plate with specially manufactured surfaces, certain areas of which will attract ink and other areas will repel the ink.

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 color-repellent 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 an 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. Diazo-sensitized layer combinations in particular 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-forming elements that are more sensitive to heat than to radiation are used to produce printing plates. The radiation-sensitive image-forming elements used to produce a printing plate as described above have the particular disadvantage that they must be protected from daylight. Furthermore, the stability of the sensitivity with regard to storage time and storage conditions is also problematic. There is a clear trend in the market towards heat-sensitive printing plate precursors.

For example, Research Disclosure No. 33303, January 1992, describes a heat-sensitive imaging element comprising a cross-linked hydrophilic layer of thermoplastic polymer particles and an infrared-absorbing pigment such as carbon black on a support. When exposed imagewise to an infrared laser, the thermoplastic polymer particles coagulate imagewise, rendering the surface of the imaging element ink-accepting at those areas without further development. A disadvantage of this process is the high susceptibility to damage of the resulting printing plate, since the non-printing areas can become ink-accepting when light pressure is applied to those areas. In addition, the lithographic performance of such a printing plate can be poor under critical conditions and such a printing plate will therefore have a limited lithographic printing latitude.

US-P 4,708,925 discloses an imaging element with a radiation-sensitive composition containing an alkali-soluble novolak resin and an onium salt and optionally an IR sensitizer. After image-wise irradiation of this imaging element with UV light - visible light - or IR light and a subsequent development step with an aqueous alkaline liquid, a positive-working or negative-working printing plate is obtained.

EP-A 625 728 discloses an imaging element comprising a layer sensitive to UV and IR radiation and can be both positive-working and negative-working. This layer contains a resol resin, a novolak resin, a latent Brönsted acid and a substance that absorbs infrared radiation.

EP-A 738 930 discloses an imaging element comprising a substrate and an imaging medium, the medium comprising (a) a compound absorbing at a first wavelength in the ultraviolet/blue region and (b) a dye absorbing at a second wavelength longer than the first wavelength, the absorption of the compound (a) at the first wavelength being bleached by irradiation at the second wavelength. Suitable exposure dwell times vary between about 1 µs and about 0.1 µs.

GB-1 492 070 discloses a planographic printing plate comprising a layer of material sensitive to ultraviolet light and coated thereon a second layer which is opaque to ultraviolet light and can be removed or made transparent to ultraviolet light by laser irradiation, but not ultraviolet laser radiation.

EP-A 464 270 provides a method in which a tape or a plate provided with radiation-sensitive areas is written with letters, the letters being written thereon by laser irradiation and, after developing the radiation-sensitive areas of the tape, a protective layer is applied to at least the areas of the tape to be written on, the letters to be written are written onto the protective layer by the laser beam and the material of the radiation-sensitive areas under the written areas of the protective layer is removed.

FR-1561957 discloses an imaging element comprising a radiation-sensitive layer comprising a binder and a liquid and/or solid material dispersed in the binder, the liquid and/or solid material being more hydrophobic than the binder.

EP-A 580 394 discloses various types of image forming devices in which lasers are used as exposure units.

EP-A 803 771, which represents the current state of the art according to Article 54 (3) EPC for only the contracting states DE, FR and GB, discloses an imaging element comprising an Ozasol N61 printing plate and a coating layer coated thereon which contains, inter alia, carbon black and nitrocellulose. After imagewise exposure with an infrared laser, the printing plate is wiped with a cotton pad to remove ablated material, then exposed to UV light and finally developed with an alkaline developer.

The heat-sensitive image generation systems discussed above can be imaged using rotary drum exposure units that work with one or more laser beams (internal or external drum scanners). This has the disadvantage that the sensitivity is not always satisfactory, especially when using internal drum scanners.

3. Brief description of the present invention.

The object of the present invention is to provide a process for the production of positive-working lithographic printing plates with excellent printing properties that can be developed in a practical and ecological manner.

A further object of the present invention is to provide a process for producing positive-working lithographic printing plates which have improved sensitivity and can be imaged on an internal drum scanner.

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

The objects of the invention are achieved by a process for producing highly sensitive lithographic printing plates characterized by the following steps: imagewise exposure of a positive heat-sensitive imaging element which contains a hydrophobic radiation-sensitive layer sensitive to infrared radiation on a lithographic base with a hydrophilic surface, with a wavelength range between 700 and 1500 nm emitting laser and, without any further processing except possible heat processing, developing the imaging element in an aqueous alkaline solution, characterized in that the infrared laser has a pixel residence time of less than 0.075 us.

After exposure, the heat-sensitive imaging element is developed in a simple wet cleaning process by rinsing with an aqueous alkaline solution.

4. Detailed description of the present invention.

It has been found that in the present invention, using a heat-sensitive imaging element as described above, high quality positive working lithographic printing plates with improved sensitivity can be obtained by exposing the imaging element to an infrared laser with a pixel dwell time of less than 0.075 µs, preferably of at most 0.05 µs. The printing plates are produced in an ecologically acceptable manner.

The imagewise exposure according to the invention is an imagewise scanning exposure using a laser emitting in the infrared or near infrared range, i.e. in the wavelength range between 700 and 1500 nm. Laser diodes emitting in the near infrared range are particularly preferred.

An internal drum scanner or flatbed scanner is used as the imaging device for image-wise laser exposure of the heat-sensitive imaging element, whereby the imaging element is exposed to a laser beam or a limited number of laser beams with a short pixel dwell time (0.01 µs to 0.07 µs).

The layer sensitive to infrared radiation contains an infrared-absorbing compound and a binder resin. Particularly useful infrared-absorbing compounds are, for example, infrared dyes, metal carbides, metal borides, metal nitrides, metal carbonitrides, oxides with a bronze structure and oxides with a structure related to the bronze family but without the A component, e.g. WO2,9. Preferably, however, as Carbon black is an infrared-absorbing compound. Suitable binder resins are cellulose and cellulose derivatives, cellulose esters, e.g. cellulose acetate, cellulose nitrate, etc., a copolymer of vinylidene chloride and acrylonitrile, poly(meth)acrylates, polyvinyl chloride, phenolic resins, e.g. phenol formaldehyde, cresol formaldehyde, novolak, etc., (aromatic or aliphatic) polyesters, polystyrenes, polycarbonates, e.g. polycarbonates containing bisphenol A, and copolymers or terpolymers of the above elements.

In the present invention, a thermally curable layer can be inserted between the infrared radiation-sensitive layer and the lithographic base, which layer contains a self-curing hydrophobic polymer or a hardener and a hydrophobic polymer and is soluble in an aqueous developer solution, particularly preferably in an aqueous alkaline developer solution with a pH value preferably between 7.5 and 14. The alkali-soluble binders used in this layer are preferably hydrophobic binders such as those used in conventional positive or negative working PS plates, e.g. novolak, polyvinylphenols, carboxyl-substituted polymers, etc. Typical examples of these polymers are described in DE-A 40 07 428, DE-A 40 27 301 and DE-A 4 445 820. The hydrophobic binder used in the present invention is further characterized by insolubility in water and partial solubility/swellability in an alkaline solution and/or partial solubility in water when combined with a cosolvent. Furthermore, this layer which is soluble in an aqueous alkaline solution is preferably a layer desensitized to visible light or UV light, which is thermally curable and ink-attracting. This layer desensitized to visible light or UV light does not contain any radiation-sensitive ingredients such as diazo compounds, photoacids, photoinitiators, quinonediazides, sensitizers, etc., which absorb in the wavelength range between 250 nm and 650 nm.

According to an 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. The anodized In the present invention, aluminum support can be subjected to processing to improve the hydrophilic properties of the support surface. For example, the aluminum support can be silicated by treating the support surface with a sodium silicate solution at an elevated temperature, e.g. 95°C. Alternatively, phosphate processing can be carried out, whereby the aluminum oxide surface is treated with a phosphate solution optionally further containing an inorganic fluoride. Furthermore, the aluminum oxide surface can be rinsed with a citric acid or citrate solution. This treatment can be carried out at room temperature or at a slightly elevated temperature between about 30°C and 50°C. Another interesting method is to rinse the aluminum oxide surface with a bicarbonate solution. Furthermore, it is obvious that one or more of these post-treatments can be carried out separately or in combination.

According to a further embodiment of the present invention, the lithographic support comprises a flexible support, such as a paper support or a plastic film, coated with a cross-linked hydrophilic layer. A particularly suitable cross-linked hydrophilic layer can be obtained from a hydrophilic binder cross-linked with a cross-linking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed tetraalkyl orthosilicate. The latter binder is 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, 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 cross-linked 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 that serve as storage sites 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 at least 500 m²/g.

After image-wise exposure, the heat-sensitive imaging element is developed by rinsing with an aqueous alkaline developer solution without any additional processing other than heat processing. This development is a simple cleaning process using developer solutions with a pH between 7.5 and 14, such as the developer solutions used for developing conventional positive or negative working presensitized printing plates. No additional preprocessing steps such as prebaking are carried out.

After wet development of the image-wise exposed imaging element with an aqueous alkaline solution and drying, the resulting printing plate can be used as a printing plate without further processing. However, to improve the print run stability, the printing plate can be baked at a temperature between 200ºC and 250ºC for a period of 5 minutes to 1 minute.

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

EXAMPLES Example 1: positive-working thermal plate that can be imaged with a pixel dwell time < 0.075 us Production of the lithographic printing plate

An IR-sensitive composition based on a gas black dispersion containing the following ingredients in parts by weight is coated onto an Ozasol N61 printing plate (negative-working printing plate from AGFA).

Ethyl acetate 579.7

Butyl acetate 386.5

Special Black 250 (gas black from Degussa) 16.7

Nitrocellulose E950 (available from Wolff Walsrode) 12.3

Solsperse 5000 (wetting agent available from ICI) 0.3

Solsperse 28000 (wetting agent available from ICI) 1.7

Cymel 301 (melamine hardener available from Dyno Cyanamid) 2.3

p-Toluenesulfonic acid 0.5

The UV-sensitive layer of the Ozasol N61 printing plate is applied together with the IR-sensitive composition using a roller coater in a wet film thickness of 20 µm.

Exposure with a pixel dwell time < 0.075 us

The IR-sensitive printing plate is scanned with a Nd-YAG infrared laser emitting at 1064 nm in an internal drum configuration (scanning speed 218 m/s, pixel dwell time 0.05 us, beam width 14 um and laser bar on the surface of the imaging element varying between 2 and 6 W). After this exposure, part of the IR-sensitive mask has disappeared in the areas where the laser radiation has struck.

The imaging element is then developed with Ozasol EN143 (AGFA developer solution), removing the IR-imaged parts and obtaining a positive printing plate. The measured sensitivity is 60 mJ/cm².

After processing, the printing plate is clamped into a GTO46 offset press and used in a printing cycle using K + E 123W as printing ink and Rotamatic as fountain solution. Good printing quality is achieved without ink absorption in the IR-imaged areas.

Example 2: positive-working thermal plate based on an alkali-soluble binder Infrared laser exposure with short pixel dwell time (0.05 us) Production of the lithographic base

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

Manufacturing of the imaging element

A 5 wt.% solution of MARUKA LYNCUR M H-2 (homopolymer of polyvinylphenol from Maruzen Co.) in methyl ethyl ketone is first applied to a lithographic base in a wet film thickness of 20 μm. This film is dried for 10 minutes at 40ºC.

The IR-sensitive composition based on a gas black dispersion containing the following ingredients in parts by weight is then poured onto this layer in a wet layer thickness of 20 μm.

Ethyl acetate 579.7

Butyl acetate 386.5

Special Black 250 (gas black from Degussa) 16.7

Nitrocellulose E950 (available from Wolff Walsrode) 12.3

Solsperse 5000 (wetting agent available from ICI) 0.3

Solsperse 28000 (wetting agent available from ICI) 1.7

Cymel 301 (melamine hardener available from Dyno Cyanamid) 2.3

p-Toluenesulfonic acid 0.5

The IR-sensitive layer is dried at 120ºC for 2 minutes.

Image-wise exposure and processing of the imaging element

The IR-sensitive printing plate is scanned with a Nd-YAG infrared laser emitting at 1064 nm in an inner drum configuration (scanning speed 218 m/s, pixel dwell time 0.05 us, beam width 14 um and laser bar on the surface of the imaging element varying between 2 and 6 W). After this exposure, a part of the IR-sensitive mask is in the areas where the laser radiation hit.

The imaging element is then developed with Ozasol EP26 (AGFA developer solution), removing the IR-imaged parts and obtaining a positive printing plate. The measured sensitivity is 45 mJ/cm².

After processing, the printing plate is clamped into a GTO46 offset press and used in a printing cycle using K + E 123W as printing ink and Rotamatic as fountain solution. Good printing quality is achieved without ink absorption in the IR-imaged areas.

Claims (10)

1. A process for producing high-sensitivity lithographic printing plates characterized by the following steps: imagewise exposure of a positive heat-sensitive imaging element containing a hydrophobic radiation-sensitive layer sensitive to infrared radiation on a lithographic base with a hydrophilic surface with a laser emitting in the wavelength range between 700 and 1500 nm and, without any further processing except for any heat processing, developing the imaging element in an aqueous alkaline solution, characterized in that the infrared laser has a pixel residence time of less than 0.075 µs. 2. Method according to claim 1, characterized in that the heat-sensitive imaging element is exposed on an internal drum scanner or flatbed scanner. 3. Method according to one of claims 1 or 2, characterized in that the heat-sensitive imaging element is developed in a simple wet cleaning process by rinsing with an aqueous alkaline developer solution. 4. Method according to one of the preceding claims, characterized in that the hydrophobic layer sensitive to infrared radiation contains an infrared-absorbing compound and a binder resin. 5. Process according to one of the preceding claims, characterized in that the hydrophobic layer sensitive to infrared radiation contains nitrocellulose. 6. A method according to any one of the preceding claims, characterized in that the imaging element is disposed between the lithographic base and the hydrophobic Infrared radiation sensitive layer contains a thermally curable layer which is soluble in an aqueous developer solution and contains a self-curing hydrophobic polymer or a hardener and a hydrophobic polymer. 7. Process according to claim 6, characterized in that the self-curing hydrophobic polymer is a phenolic resin. 8. Process according to one of claims 6 or 7, characterized in that the thermally curable layer is a layer desensitized to visible light or UV light. 9. Process according to one of claims 6 to 8, characterized in that the thermally curable layer is ink-attracting. 10. A heat-sensitive imaging element according to any one of claims 7 to 9, characterized in that the thermally curable layer is ink-accepting.