Correction fluid for a diazo-based lithographic printing plate

Abstract: The present invention provides a correction fluid comprising a mixture of water and a water miscible organic solvent and an inorganic fluoride in a concentration from 0.5% by weight to 4% by weight. The present invention also provides a method for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate using said correction fluid.

Description: 1. Field of the invention. The present invention relates to a correction fluid for correcting a diazo-based lithographic printing plate and a method for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkylorthosilicate. 2. Background of the invention. Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas. In the art of photolithography, a photographic material is made image-wise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophillic background. In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophillic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. These systems have been extensively reviewed by Kosar J. in "Light-Sensitive Systems", Wiley, New York, 1965, Chapter 7. A generally used negative-working diazo-sensitized system is based on the capability of diazo compounds to harden a polymer when exposed to ultraviolet and blue radiation. Diazo compounds which have been used for the preparation of lithographic printing plates based on their hardening properties are e.g. diazonium salts whose photolysis products can harden polymers (natural colloids or synthetic resins) directly and diazonium polymers. Although polymers containing diazonium groups have a large structure they may remain water soluble owing to the presence of the ionic diazonium groups. When these groups are destroyed by exposure to light an insoluble resin is formed. During a development step subsequent to the information-wise exposure the diazo resin or diazonium salt will be dissolved and removed by a suitable solvent, e.g. water, in the non-exposed or insufficient exposed parts of the lithographic printing plate precursor. The hydrophillic or oleophobic surface of the plate will thus be in the non-exposed or insufficient exposed parts while the oleophillic diazo resin will remain in the exposed parts. Several types of supports can be used for the manufacturing of a diazo-sensitized lithographic printing plate. Common supports are metal supports like Al or Zn. Other supports that are employed are polyester film supports and paper supports coated with a hydrophillic layer to form the hydrophillic background of the printing plate. A typical hydrophillic layer in these systems is a layer containing polyvinyl alcohol and hydrolysed tetraethyl orthosilicate and preferably also silicium dioxide and/or titanium dioxide as described in e.g. GB-P-1419512, FR-P-2300354, US-P-3971660 and 4284705. It is common practice that the imaged lithographic printing plate is corrected. i.e. some of the imaged areas are selectively deleted before mounting it on an offset press for example for correcting misspellings, punctuation errors etc.. Deletion of parts of the imaged areas can be accomplished by applying a hydrophillic coating over the imaged areas thus rendering them hydrophillic. However this method has the disadvantage that the applied hydrophillic coating vanishes during printing. Another method for deleting parts of an image on a lithographic printing plate is the etching or removal of image areas. The latter method is commonly employed for correcting diazo-based lithographic printing plates having a metal support. The correction fluid used in accordance with this method mostly comprises fluorine containing compounds. See for example US-P-3.248.332, FR-A-2027332 and the Japanese unexamined patent applications laid open to public numbers 172147/61, 211041/61 198157/61 and 27190/62. The correction fluid for correcting diazo-based lithographic printing plates having a metal support are not suited for correcting diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer as described above because the hydrophillic layer is strongly attacked by the correction fluid so that the hydrophobic polyester film base is exposed. 3. Summary of the invention. It is an object of the present invention to provide a correction fluid suitable for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate. It is another object of the present invention to provide a method for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate. Still other objects of the present invention will become clear from the description hereinafter. According to the present invention there is provided a correction fluid comprising a mixture of water and a water miscible organic solvent and an inorganic fluoride in a concentration from 0.5% by weight to 4% by weight, the inorganic fluoride being potassium hydrogendifluoride or ammonium fluoride. According to the present invention there is provided a method for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate using a correction fluid comprising a mixture of water and a water miscible organic solvent and an inorganic fluoride in a concentration from 0.5% by weight to 4% by weight. 4. Detailed description of the invention. Diazo-based lithographic printing plates having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate are disclosed in FR-P-2300354, US-P-3971660, US-P-4284705 and EP-A-0450199 (prior art under Art. 54(3)(4)). It has been found that these lithographic printing plates can be corrected without severely damaging the hydrophillic layer by using a correction fluid comprising an inorganic fluoride dissolved in a mixture of water and a water miscible organic solvent. When the amount of inorganic fluoride is larger than 4% by weight the hydrophillic layer is dramatically damaged while a concentration smaller than 0.5% by weight does not yield a sufficient rapid correction of the image. Most preferably the content of inorganic fluoride is between 1% by weight and 3% by weight. Inorganic fluorides suitable for use in accordance with the present invention are e.g. alkali metal fluorides, alkaline-earth metal fluorides, ammonium fluoride etc. Preferably used are potassium hydrogendifluoride or ammonium fluoride. Suitable water miscible organic solvents for use in accordance with the present invention are e.g. ethylene glycol diacetate, diethylene glycol dimethyl ether, diethylene glycol dimethyl ether, lactone, N-vinyl pyrrolidone, dimethylformamide, methylglycol, 2-methoxy-propan-1-ol, 1-methoxy-propan-2-ol, 2-ethoxy-ethanol or mixtures thereof. Methylglycol is preferred for ecological reasons. The ratio by volume of water miscible organic solvent to water is preferably from 9:1 to 1:1. Too large an amount of water affects the hydrophillic layer while too small an amount does not allow sufficient fluoride to be dissolved and does not yield a sufficiently rapid correction of the image areas. The most preferred range of ratios of water miscible organic solvent to water is from 5:1 to 2:1. The pH of the correction fluid is preferably between 4 and 8 and most preferably around 7. When the pH of the correction is too low the correction fluid appears to be too aggressive so that the hydrophillic layer of the lithographic printing plate is damaged. The correction fluid is preferably bufferred around the desired pH. Suitable buffers for use in accordance with the present invention are e.g. phosphate buffers, citrate buffers, acetate buffers etc. Citrate buffers are preferably used. The correction fluid in accordance with the present invention may also comprise thickening agents e.g. carboxy methylcellulose, hydroxypropyl cellulose, high molecular weight compounds e.g. a vinyl acetate maleic acid anhydride copolymer, polyacrylamides etc.. According to the method of the present invention a diazo-based lithographic printing plate as disclosed in EP-A-0450199 or US-P-3971660 is information-wise exposed and subsequently developed as disclosed in these applications. The obtained imaged lithographic printing plate is subsequently corrected in the image areas using a correction fluid as described above. The correction fluid may be applied e.g. by wiping the image areas with a cloth wetted with the correction fluid. Alternatively the correction fluid may be present in a correction stylus to be used for correcting the image on the lithographic printing plate. After a short reaction time, i.e. from 1 second to 1 minute depending upon the strength of the particular composition of the correction fluid and the condition of the plate surface, the surface is wiped clean and the diazo compound thereby removed. The following example illustrate the present invention without limiting it thereto. The percentages given are by weight unless otherwise stated. EXAMPLE A lithographic printing plate was prepared as described in EP-A-0450199. Part of the image was removed by applying with a brush a correction fluid containing 2% ammoniumfluoride dissolved in a mixture of water and methylglycol in a ratio by volume of 1 part of water to 3 parts of methylglycol. The pH of the correction fluid was 7. The correction fluid was left on the image part to be removed for 10 to 30s and was then removed together with the image part by a wet cotton pad. No severe damage of the hydrophillic layer of the plate and no ink acceptance was noticed on the image parts removed during printing. Similar results were obtained with a correction fluid containing 2% of potassium hydrogendifluoride in a mixture of water and methylglycol in a ratio by volume of 1 part of water to 3 parts of methylglycol. The pH of the latter correction fluid was 4.

Claims: A correction fluid for correcting a lithographic printing plate comprising a mixture of water and a water miscible organic solvent and an inorganic fluoride in a concentration from 0.5% by weight to 4% by weight, said inorganic fluoride being potassium hydrogendifluoride or ammonium fluoride. A correction fluid according to claim 1 wherein said water miscible organic solvent is dimethylformamide, methylglycol, 2-methoxy-propan-1-ol, 1-methoxy-propan-2-ol or 2-ethoxy-ethanol or a mixture thereof. A correction fluid according to claims 1 or 2 wherein the ratio by volume of said water miscible solvent to water is from 9:1 to 1:1. A correction fluid according to any of the above claims wherein the concentration of said inorganic fluoride is between 1% and 3% by weight. A correction fluid according to any of the above claims wherein the pH of said correction fluid is between 4 and 8. A method for correcting a diazo-based lithographic printing plate having a polyester film support coated with a hydrophillic layer comprising polyvinyl alcohol hardened with a tetraalkyl orthosilicate using a correction fluid comprising a mixture of water and a water miscible organic solvent and an inorganic fluoride in a concentration from 0.5% by weight to 4% by weight. A method according to claim 6 wherein said inorganic fluoride is potassium hydrogendifluoride or ammonium fluoride.

Inventors: VERMEERSCH JOAN TRIPHON, VERMEERSCH JOAN, TRIPHON
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP
Application year: 1991
Main classification: B41N3
IPC codes: G03F7, B41N3