METHOD OF PREPARING A PRINTING PLATE

Abstract: A method for preparing a lithographic printing plate which involves providing a plate precursor comprising a grained and anodised aluminium substrate coated with a metallic layer, preferably a silver layer, imagewise exposing this precursor by means of a high intensity laser beam, and treating the exposed surface of the plate with a chemical composition in order to enhance the oleophilic character of image areas and the hydrophilic character of non-image areas. On exposure of the plate precursor, removal of the metallic layer occurs in the exposed areas. The method provide press ready plates showing high image quality, good press properties and high durability on press, whilst eliminating the requirement of the use of intermediate film and developer chemistry.

Description: Method of preparing a printing plate This invention relates to the formation of images directly from electronically composed digital sources and is particularly concerned with the formation of images on lithographic printing plate precursors. More particularly, the invention relates to lithographic printing plate precursors which incorporate an imaging layer comprising metallic silver, and a method of preparing lithographic printing plates which does not require the use of chemical treatments. Lithographic printing is a process of printing from surfaces which have been prepared in such a way that certain areas are capable of accepting ink (oleophilic areas), whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas. Plates for use in lithographic printing processes may be prepared using a photographic material that is made imagewise receptive or repellent to ink upon photo-exposure of the photographic material and subsequent chemical treatment. However, this method of preparation, which is based on photographic processing techniques, involves several steps, and therefore requires a considerable amount of time, effort and expense. Consequently it has. for many years, been a long term aim in the printing industry to form images directly from an electronically composed digital database, ie by a so- called "computer-to-plate" system. The advantages of such a system over the traditional methods of making printing plates are: (i) the elimination of costly intermediate silver film and processing chemicals; (ii) a saving of time; and (iii) the ability to automate the system with consequent reduction in labour costs. The introduction of laser technology provided the first opportunity to form an image directly on a printing plate precursor by scanning a laser beam across the surface of the precursor and modulating the beam so as to effectively turn it on and off. In this way, radiation sensitive plates comprising a high sensitivity polymer coating have been exposed to laser beams produced by water cooled UV argon-ion lasers and electrophotographic plates having sensitivities stretching into the visible spectral region have been successfully exposed using low powered air-cooled argon-ion, helium-neon and semiconductor laser devices. Imaging systems are also available which involve a sandwich structure which, on exposure to a heat generating infra-red laser beam, undergoes selective (imagewise) delamination and subsequent transfer of materials. Such so-called peel-apart systems are generally used as replacements for silver halide films. A digital imaging technique has been described in US Patent No 4911075 whereby a so-called driographic plate which does not require dampening with an aqueous fountain solution to wet the non-image areas during printing is produced by means of a spark discharge. In this case, a plate precursor comprising an ink-repellent coating containing electrically conductive particles coated on a conductive substrate is used and the coating is ablatively removed from the substrate. Unfortunately, however, the ablative spark discharge provides images having relatively poor resolution. It is known to improve this feature by the use of lasers to obtain high resolution ablation as described, for example, by P E Dyer in "Laser Ablation of Polymers" (Chapter 14 of "Photochemical Processing of Electronic Materials", Academic Press, 1992, p359-385). Until recently, imaging via this method generally involved the use of high power carbon dioxide or excimer lasers. Unfortunately, such lasers are not well-suited to printing applications because of their high power consumption and excessive cost, and the requirement for high pressure gas handling systems. Recent developments have, however, led to the availability of more suitable infra-red diode lasers, which are compact, highly efficient and very economical solid state devices. High power versions of such lasers, which are capable of delivering up to 3000 mJ/cm 2 , are now commercially available. Coatings which may be imaged by means of ablation with infra-red radiation have previously been proposed. Thus, for example, a proofing film in which an image is formed by imagewise ablation of a coloured layer on to a receiver sheet is described in PCT Application No 90/12342. This system is, however, disadvantageous in requiring a physical transfer of material in the imaging step, and such methods tend to give rise to inferior image resolution. Much superior resolution is obtained by means of the ablation technique described in European Patent No 649374, wherein a driographic printing plate precursor is imaged digitally by means of an infra-red diode laser or a YAG laser, and the image is formed directly through the elimination of unwanted material. The technique involves exposing a plate precursor, incorporating an infra-red radiation ablatable coating covered with a transparent cover sheet, by directing the beam from an infrared laser at sequential areas of the coating so that the coating ablates and loses its ink repellancy in those areas to form an image, removing the cover sheet and ablation products, and inking the image. A heat mode recording material is disclosed in US Patent No 4034183 which comprises an anodised aluminium support coated with a hydrophilic layer. On imagewise exposure using a laser, the exposed areas are rendered hydrophobic, and thereby accept ink. Japanese patent application laid open to public inspection No 49-117102 (1974) discloses a method for producing printing plates wherein a metal is incorporated in the imaging layer of a printing plate precursor which is imaged by irradiation with a laser beam modulated by electric signals. Typically, the plate precursor comprises a metal base, such as aluminium, coated with a resin film, which is typically nitrocellulose, and on top of which has been provided a thin layer of copper. The resin and metal layers are removed in the laser-struck areas, thereby producing a printing plate. The disadvantage of this system, however, is that two types of laser beam irradiation are required in order to remove firstly the copper (eg by means of an argon-ion laser) and then the resin (eg with a carbon dioxide laser); hence, the necessary equipment is expensive. Subsequently a method of printing plate production which obviated the requirement for a second laser exposure was disclosed in Japanese patent application laid open to public inspection No 52-37104 (1977). Thus, a printing plate precursor comprising a support, typically aluminium, an anodic aluminium oxide layer, and a layer of brass, silver, graphite or, preferably, copper is exposed to a laser beam of high energy density in order to render the exposed areas hydrophilic to yield a printing plate. The printing plate precursor is, however, of rather low sensitivity and requires the use of a high energy laser for exposure. An alternative heat mode recording material for making a lithographic printing plate is disclosed in European Patent No 609941, which comprises a support having a hydrophilic surface, or provided with a hydrophilic layer, on which is coated a metallic layer, on top of which is a hydrophobic layer having a thickness of less than 50nm. A lithographic printing plate may be produced from the said material by imagewise exposing to actinic radiation, thereby rendering the exposed areas hydrophilic and repellent to greasy ink. Conversely, European Patent No 628409 discloses a heat mode recording material for making a lithographic printing plate which comprises a support and a metallic layer, on top of which is provided a hydrophilic layer having a thickness of less than 50nm. A lithographic printing plate is produced by imagewise exposing the material to actinic radiation in order to render the exposed areas hydrophobic and receptive to greasy ink. In each of the two foregoing heat mode recording materials, however, difficulties in printing will be encountered. On exposure of the materials to actinic radiation, the energy is converted to heat in the image areas by interaction with the metallic layer, thereby destroying the hydrophilicity or hydrophobicity - depending on the material employed - of the topmost layer in those areas. Consequently, the surface of the metallic layer becomes exposed, and the success of the printing operation is dependent upon differences in hydrophilicity and oleophilicity between the metallic surface and the hydrophilic or hydrophobic layer, as the case may be. Since the metallic layer functions as the hydrophobic surface in one case, and as the hydrophilic surface in the alternative case, it would be expected that such differences in hydrophilicity and oleophilicity would not be sufficiently clearly defined so as to provide a satisfactory printing surface. Furthermore, when a hydrophilic layer is present, and the metallic surface functions as the oleophilic areas of the plate, image areas will necessarily be printed from the metallic surface; such an arrangement is known to be unsatisfactory, and to result in difficulties in achieving acceptable printing quality. It is an object of the present invention to provide a lithographic printing plate having excellent printing properties, and a method of making said plate which obviates the requirement for the use of processing developers after exposure. It is a further object of the present invention to provide a method of preparing a lithographic printing plate which does not require the use of costly intermediate film and relies on direct-to-plate exposure techniques. It is a still further object of the present invention to provide a method of producing a lithographic printing plate in which a high quality image results from the ablation of a metallic layer from a hydrophilic support, thus providing a high degree of differentiation between hydrophilic and oleophilic areas. However, it is often found that further treatments to the plate surface are required in order to enhance the ink receptive properties of the metal surface in image areas and the hydrophilic properties of the exposed substrate, typically aluminium oxide, in the non-image areas. It is possible and, in many cases, desirable for such treatments to be applied prior to exposure of the plate, and even during the course of the manufacturing process. However, in terms of environmental considerations, it is desirable that the metallic surface of the lithographic plate precursor is substantially free from other chemicals prior to exposure by a high intensity laser beam. In this way it is possible to ensure that the ablated debris produced on exposure, during which imagewise removal of the metal in non-image areas occurs, comprises substantially the metal, and is free from other contaminants. As a consequence, collection of the debris is more easily achieved, and the level of environmental hazard is reduced. Subsequently, the exposed plate may be treated with various materials prior to use on the press in order to achieve improved press performance; in particular, it is often desirable to treat the plate such that the oleophilicity of the metallic image areas and the hydrophilicity of the non-image areas, comprising revealed substrate, is enhanced in order that good start-up and clean-up properties can be ensured. In practice, it has been found that even in cases where plate treatments are applied prior to exposure, subsequent retreatment after exposure is necessary in order to ensure good operation on press. According to the present invention, there is provided a method of preparing a lithographic printing plate, said method comprising: (a) providing a lithographic printing plate precursor comprising: (i) a grained and anodised aluminium substrate having coated thereon (ii) a metallic layer; (b) imagewise exposing said precursor by means of a high intensity laser beam: and (c) treating the exposed surface of the plate with a chemical composition in order that the oleophilic character of image areas and the hydrophilic character of non-image areas may be enhanced. The substrate employed in the present invention is an aluminium substrate which has been electrochemically grained and anodised on at least one surface in order to enhance its lithographic properties. Optionally, the aluminium may be laminated to other materials, such as paper or various plastics materials, in order to enhance its flexibility, whilst retaining the good dimensional stability associated with aluminium. The metallic layer, which is applied to the grained and anodised surface of the aluminium, may comprise any of several metals, specific examples of which include copper, bismuth and brass. Most preferably, however, the metallic layer comprises a silver layer. The thickness of the metallic layer is preferably from 20 nm to 200 nm most preferably from 40 nm to 100 nm. Various techniques are available for the application of the metallic layer to the grained and anodised aluminium substrate, including vapour or vacuum deposition or sputtering. In the case where the metal layer comprises a silver layer, however, the most preferred method for applying the layer involves the treatment of a silver halide photographic material according to the silver salt diffusion transfer process. In the diffusion transfer process, a silver halide emulsion layer is transformed, by treatment with a so-called silver halide solvent, into soluble silver complex compounds which are then allowed to diffuse into an image receiving layer and are reduced therein by means of a developing agent, generally in the presence of physical development nuclei, to form a metallic silver layer. Two such systems are available: a two sheet system in which a silver halide emulsion layer is provided on one element, and a physical development nuclei layer is provided on a second element, the two elements are placed in contact in the presence of developing agent(s) and silver halide solvent(s) in the presence of an alkaline processing liquid, and subsequently peeled apart to provide a metallic silver layer on the second element; and a single sheet system wherein the element is provided with a physical development nuclei layer, a silver halide emulsion layer is provided on top thereof, the element is treated with developing agent(s) and silver halide solvent(s) in the presence of an alkaline processing liquid, and the element is washed to remove spent emulsion layer and leave a metallic silver layer which is formed in the layer containing physical development nuclei. Alternatively, the diffusion transfer process may be used to apply a metallic silver layer by overall exposing a positive working silver halide emulsion layer to form a latent negative image which is then developed in contact with a physical development nuclei layer to form a metallic silver layer. Again, the process may be carried out using either a single sheet or a double sheet system. The principles of the silver complex diffusion transfer process are fully described in the publication "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde, The Focal Press, London and New York, 1972, and further detail may be gleaned by reference thereto. In order to prepare a lithographic printing plate, in accordance with the method of the present invention, the precursor is imaged by a beam of radiation, preferably from a laser operating in the infra-red region of the spectrum. Examples of suitable infra-red lasers include semiconductor lasers and YAG lasers, for example the Gerber Crescent 42T Platesetter with a 10W YAG laser outputting at 1064 nm. Exposure to the beam of radiation causes ablation of the metallic layer to occur in the radiation-struck areas. Following exposure, the plate is treated with a composition comprising materials which enhance the oleophilic character of the metal forming the image areas and also improve the hydrophilic nature of the exposed anodic aluminium oxide layer comprising the non-image areas. Typical components would include enzymes, oleophilic agents and desensitising compounds. Additional components, having a beneficial effect on other aspects of plate performance, may also be included, for example, storage gums and surfactants; other typical ingredients could include pH buffering agents and biocides. Generally, the compositions comprise aqueous solutions of the various components. Suitable enzymes which may be incorporated in the finishing compositions utilised in the present invention include proteolytic enzymes such as trypsin, pepsin, ficin, papain or the bacterial proteases or proteinases. The enzyme is generally present in the composition in an amount of from 0.1% to 10.0% by weight. Various oleophilising agents are suitable for use in the present invention and, in particular, the materials disclosed on pages 105 to 106 of "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde find such application. However, mercapto compounds and cationic surfactants such as quaternary ammonium compounds are of especial value. Such materials are generally present at a level of from 0.05% to 5.0% by weight of the total composition. Several desensitising agents are of value in the finishing compositions finding application in the method of the present invention, with carbohydrates such as gum arabic, dextrin and inorganic polyphosphates such as sodium hexametaphosphate being particularly effective in ensuring that background areas remain free from contamination. The desensitisers are typically incorporated in the finishing compositions at a level of from 1.0% to 10.0% by weight. It will be apparent that the presence of materials such as gums, surfactants, quaternary ammonium compounds and mercapto compounds on the plate surface prior to exposure would result in the ablation of many pyrolysis products of unknown composition and hazard during exposure in a high temperature environment. However, by eliminating such products, plate preparation can be undertaken in a far safer manner. Following exposure, however, it is possible to apply a suitable finishing composition to ensure that the press performance of plates is excellent in terms of start-up, cleanup, image oleophilicity, durability and quality of the printed image. The method of the present invention also provides press ready plates showing high image quality, good press properties and high durability on press without the requirement for the use of costly intermediate film and developer chemistry and the attendant inconvenience resulting from the use of these materials. The following example is illustrative of the invention, without placing any limit on the scope thereof: EXAMPLE Samples of a commercially available Howson SILVERLITH* SDB printing plate, supplied by DuPont Printing and Publishing, were processed without exposure through an automatic processor by means of the diffusion transfer reversal method, in accordance with the recommendation of the manufacturer, but the final stage of applying a specified finishing gum was varied in order to illustrate the invention. Rather than applying the finishing gum specified by the manufacturer, the following finishing compositions were employed: Plate Finisher 1 None 2 Water 3 l% w/w Alcalase ® 2.5L 4 10% w/w Dextrin + 5% w/w polyethylene glycol 200 + 0.5% w/w boric acid + 1% w/w Triethanolamine + 2% w/w Alcalase ® 2.5L + 0.1% w/w 2-mercapto-5-heptyl-l,3,4- oxadiazole. Alcalase ® 2.5L is a proprietary bacterial protease. The printing plate precursors were separately loaded on to a Gerber Crescent 42T Platesetter and fully (blanketwise) exposed to a 10 W YAG laser outputting at a wavelength of 1064 nm and peak power density of 6.5% mW/cm 2 . The debris and pyrolysis products generated on exposure were collected and analysed. In each case, silver was recovered, with the following materials also being detected: Plate Principal By-products 1 None 2 None 3 1 , 4-dioxane, methanol, acetone 4 85 separate materials, including acetaldehyde, acetone, methanol, toluene, 3-methylpyridine, dimethylpyridine, methyl benzoate, benzonitrile, phenyl isopropyl ketone, biphenyl, phenol. Further plate samples were produced in an identical fashion except that, after loading on to a Gerber Crescent 42T internal drum Laser Platesetter fitted with an extraction system comprising a curved nozzle about 1cm from the surface of the plate, an air suction pump, and a 0.3 μm HEPA filter for removal of ablation debris, the plates were imagewise, rather than blanket, exposed according to the above parameters. Following exposure, the plates were treated with a finishing composition comprising 5% w/w sodium hexametaphosphate, 2% w/w Triton XI 00 ® (a commercial surfactant), 2% w/w Alcalase ® 2.5L, 5% w/w sorbitol, 5% w/w polyethylene glycol 200 and 0.1% w/w l-octyl-5-mercapto-l,2,3,4-tetrazole. Each of the plates produced 80,000 good copies when loaded on to a Drent Web Offset press.

Claims: CLAIMS 1. A method of preparing a lithographic printing plate, said method comprising : (a) providing a lithographic printing plate precursor comprising : (i) a grained and anodised aluminium substrate having coated thereon (ii) a metallic layer ; (b) imagewise exposing said precursor by means of a high intensity laser beam ; and (c) treating the exposed surface of the plate with a chemical composition in order that the oleophilic character of image areas and the hydrophilic character of non-image areas may be enhanced. 2. A method as defined in claim 1 wherein said metallic layer comprised in said lithographic printing plate precursor comprises a silver layer. 3. A method as defined in claim 2 wherein said silver layer is applied by means of the silver salt diffusion transfer process. 4. A method as defined in claim 1, 2 or 3 wherein said metallic layer comprised in said lithographic printing plate precursor has a thickness of from 20 nm to 200 nm. 5. A method as defined in claims 1-4 wherein the exposed surface of said lithographic printing plate is treated with a chemical composition which comprises an enzyme, an oleophilic agent and a desensitising compound in aqueous solution. 6. A method as defined in claim 5 wherein said chemical composition comprises an aqueous solution in which said enzyme is present at a level of from 0.1% to 10.0% by weight, said oleophilic agent is present at a level of from 0.05% to 5.0% by weight and said desensitising compound is present at a level of from 1.0% to 10.0% by weight. 7. A method as defined in claim 5 or 6 wherein said enzyme comprises a proteolytic enzyme. 8. A method as defined in claim 7 wherein said proteolytic enzyme comprises trypsin, pepsin, ficin, papain or a bacterial protease or proteinase. 9. A method as defined in claims 5-8 wherein said oleophilic agent comprises a mercapto compound or a cationic surfactant. 10. A method as defined in claim 9 wherein said cationic surfactant comprises a quaternary ammonium compound. 11. A method as defined in claims 5-10 wherein said desensitising compound comprises a carbohydrate or an inorganic polyphosphate. 12. A method as defined in claim 11 wherein said carbohydrate comprises gum arabic or dextrin.

Inventors: WATKISS PHILIP JOHN, WATKISS, PHILIP, JOHN
Applicants: AGFA GEVAERT NV, WATKISS PHILIP JOHN, AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP, WATKISS, PHILIP, JOHN
Application year: 1998
Main classification: B41N1
IPC codes: B41C1, B41N1, G03F7