A HEAT SENSITIVE PRINTING PLATE PRECURSOR

Abstract: A lithographic printing plate precursor comprises a grained and anodised aluminium substrate coated with a metallic silver layer which is deposited by means of a silver salt diffusion transfer process in which the developer has a pH of at least 10. Imagewise exposure of the precursor by means of a high intensity laser beam allows for the direct provision of press-ready plates showing high image quality and clean background, good press properties and high durability on press without the requirement for the use of intermediate film and developer chemistry. The plate precursors show increased sensitivity on exposure, whereupon removal of the metallic layer occurs in the exposed areas.

Description: A HEAT SENSITIVE PRINTING PLATE PRECURSOR 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. Specifically, in the case of the present invention, the metallic layer is silver and it is an additional object of the invention to provide a lithographic printing plate precursor which shows improved sensitivity when exposed to a high intensity laser beam and which, following exposure, produces a lithographic printing plate showing enhanced durability during printing operations. It has been found that the most appropriate technique for the application of a silver layer to a hydrophilic support 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. The present inventors have observed that the optical reflective densities and absorptivities of the silver depositions produce by this method play a critical role in determining the amount of energy required to cause the deposited silver to become activated and subsequently ablated from a hydrophilic substrate surface. Furthermore, the inventors have observed that these optical properties are highly dependent on the composition of the developing agent which, therefore, has a significant effect upon the sensitivity of the silver layer. Specifically, it has been found that optimum sensitivity is achieved when the developer system has an alkaline pH value in excess of 10. In addition, the inventors have also found that lithographic printing plate precursors including metallic silver layers produced in this way also provide printing plates showing good clean-up properties in background areas and enhanced durability on press. According to a first aspect of the present invention, there is provided a lithographic printing plate precursor comprising: (i) a grained and anodised aluminium substrate, having coated thereon (ii) a metallic silver layer, deposited by means of a silver salt diffusion transfer process in which the developer has a pH of at least 10. 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 silver layer, deposited by means of a silver salt diffusion transfer process, has a thickness in the range of from 20 nm to 200 run, most preferably from 40 nm to lOO n-m. Two suitable diffusion transfer processes 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 being 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 exposure of 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 each case, the developer system has a pH in excess of 10, preferably between 10.5 and 13, most preferably between 11.0 and 12.5. Adjustment of the pH into the appropriate range is most conveniently achieved by the addition of an alkaline solution, such as aqueous sodium hydroxide, as appropriate. Corrective adjustments to reduce the pH, where necessary, are typically made by the addition of a relatively weak acid, such as glacial acetic acid. A typical developer composition could also include an antioxidant (preferably 5 - 15% by weight), for example sodium sulphite; a silver halide developing agent (preferably 1 - 3% by weight), such as hydroquinone or ascorbic acid; an auxiliary developing agent (preferably 0.5 - 1% by weight, such as l-phenyl-3-pyrazolidone; a silver halide solvent (preferably 0.5 - 2% by weight), for example, sodium thiosulphate or sodium thiocyanate; a development accelerator (preferably 1 - 4% by weight), such as 2-methylaminoethanol; a corrosion inhibitor (preferably 2 - 5% by weight), for example, glycerol; a sequestrant (preferably 0.5 - 2% by weight), such as ethylenediaminetetraacetic acid; and a surfactant (preferably 0.01 - 0.5% by weight), typically a non-ionic or anionic surfactant. According to a second aspect of the present invention, there is provided a method of preparing a lithographic printing plate, said method comprising a) providing a lithographic printing plate precursor as hereinbefore described; and b) imagewise exposing said precursor by means of a high intensity laser beam. 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 10 W 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. The plate precursors according to the first aspect of the invention show increased sensitivity on exposure. The plates prepared on exposure give good clean-up on press, allowing for efficient start-up of printing operations and giving copies free from background scumming. Additionally, long print runs may be achieved. Advantageously, the plate may be prepared for printing operations, prior to or following exposure, by treatment with a composition comprising a proteolytic enzyme, a silver oleophilising agent and a desensitising compound. In this way, it is possible to ensure good ink acceptance in image areas and a high degree of hydrophilicity in background areas, thus further improving start-up on press. Suitable enzymes for use in the above composition may include, for example, trypsin, pepsin, ficin, papain or the bacterial proteases or proteinases. Oleophilising compounds may be chosen from those disclosed on pages 105 to 106 of "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde, but mercapto compounds and cationic surfactants, such as quaternary ammonium compounds, are of particular value. Carbohydrates such as gum arabic, dextrin and inorganic polyphosphates such as sodium hexametaphosphate provide useful desensitising compounds in these compositions. Typically, the compositions comprise aqueous solutions containing from 0.1 % to 10.0% by weight of enzyme, from 0.05% to 5.0%> by weight of oleophilising compound and from 1.0% to 10.0% by weight of desensitising compound. The printing plate precursor and the method of the present invention provide press ready plates showing high image quality and clean background, good press properties and high durability on press without the requirement for the use of costly intermediate film and developer chemistry and the attendant materials. The following examples are illustrative of the invention, without placing any limitation on the scope thereof: EXAMPLE A sheet of aluminium metal was degreased in a 5% w/w aqueous solution of sodium hydroxide before being electrochemically grained with an alternating electric current in a mixture of acetic and hydrochloric acids according to the method disclosed in British Patent No 1598701, then cleaned with a 10% aqueous solution of phosphoric acid and finally anodised with a direct electric current in sulphuric acid. The sheet was then rinsed with water to remove residual acid, and a Carey Lea colloidal dispersion of silver was applied to the grained and anodised surface to give a coating weight of 1 mg/m 2 of silver, and this was then further coated with a gelatino-silver chlorobromide dispersion to give a coating weight of 4 g/m 2 and a silver coating weight of 1.6 g/m 2 . A series of eight diffusion transfer developers (A-H), having a range of pH values in the region of 10 to 13.5, was prepared. Each developer comprised an aqueous solution containing 11% w/w sodium sulphite, 2% w/w hydroquinone, 0.6% w/w Phenidone (l-phenyl-3-pyrazolidone), 1% w/w sodium thiosulphate and 3% w/w 2- methylaminoethanol. In each case pH adjustments were effected by the addition of aqueous sodium hydroxide and/or acetic acid. Samples of the above plate precursor were dipped into each of the developers at 20 °C for 20 seconds, and then rinsed with warm water to give a physically developed silver layer. The resulting assemblies of physically developed silver on a grained and anodised aluminium substrate were loaded onto a Gerber Crescent 42T internal drum Laser Platesetter fitted with an extraction system comprising a curved nozzle about 1cm from the plate surface, an air suction pump and a 0.3 μm HEPA filter for removal of ablation debris and imagewise exposed to a 10W YAG laser outputting at a wavelength of 1064 nm to ablatively remove the silver in background areas and thereby create an image. After exposure, the surface of each printing plate was treated with an aqueous solution comprising a proteolytic enzyme, an oleophilising agent and a desensitising gum prior to mounting on a printing press. This treatment was designed to ensure a good start up to printing operations with image areas showing high oleophilicity with good ink acceptance, and background non-image areas being clean and free from ink adhesion. In each case, the press was run to produce 20,000 good copies before being stopped with the plates fully inked. After a stopdown lasting 18 hours, the press was restarted and further copies were printed. For each sample of plate, the number of copies produced before a good quality copy was achieved was noted, both at the commencement of the print run and during the subsequent start-up following the stoppage after the production of 20,000 copies. The resulting data illustrated the clean-up properties of each plate. Subsequently, the total number of copies produced by each plate was also recorded, as a measure of plate durability. Additionally, the yield of physically developed silver deposited after treatment of the plate precursor with the diffusion transfer developer was measured in every case, together with the reflective optical density and absorptivity at a wavelength of 1064 nm, and the power density requirement from the laser. The results obtained are shown in Table 1, and illustrate the advantage of the invention. TABLE 1

Claims: CLAIMS 1. A lithographic printing plate precursor comprising : (i) a grained and anodised aluminium substrate having coated thereon (ii) a metallic silver layer deposited by means of a silver salt diffusion transfer process in which the developer has a pH of at least 10. 2. A lithographic printing plate precursor as defined in claim 1 wherein said metallic silver layer has a thickness of from 20 nm to 200 nm. 3. A lithographic printing plate precursor as defined in claim 1 or 2 wherein said developer has a pH in the range between 11.0 and 12.5. 4. A lithographic printing plate precursor as defined in claims 1-3 wherein said developer comprises an alkaline solution containing an antioxidant, a silver halide developing agent, an auxiliary developing agent, a silver halide solvent and a development accelerator. 5. A lithographic printing plate precursor as defined in claim 4 wherein said developer comprises an aqueous solution of sodium hydroxide which contains sodium sulphite, hydroquinone, l-phenyl-3-pyrazolidone, sodium thiosulphate and 2-methylaminoethanol. 6. A method of preparing a lithographic printing plate, said method comprising : (a) providing a lithographic printing plate precursor as defined in any of claims 1-5 ; and (b) imagewise exposing said precursor by means of a high intensity laser beam. 7. A method as defined in claim 6 wherein, prior to or following said imagewise exposure, said plate precursor or plate is treated with a solution comprising a proteolytic enzyme, a silver oleophilising agent and a desensitising compound.

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