Correction fluid for electrophoto plates for offset printing

Abstract: The invention relates to a hydrophilic stannous chelate compound characterised by having an acidic phosphate ester of an aliphatic alcohol as a ligand, being soluble in an aqueous acidic solution and becoming insoluble when the pH value of the said aqueous acidic solution containing the compound is raised. The invention also includes a correction fluid containing same and to a method of erasing the unnecessary line image of an electrophoto plate for offset printing.

Description: The present invention relates to a correction fluid which may partly erase the line image part on an electrophoto plate for offset printing. Erasure of line image part, as referred to herein, means that the line image part is lipophobicated to eliminate the ink-sensitivity therefrom. A correction fluid which has the function of lipophobicating the unnecessary line image part on an electrophoto plate for offset printing thereby to eliminate the ink-affinity from the part has been developed along with the development of electrophoto plates. The history of the development of such correction fluids is as follows: (a) Initially, a lipophilic powdery toner which is electrically charged by itself was sprayed over the charged part (line image part) of a master paper plate as coated with a photoelectroconductive zinc oxide (hereinafter referred to as "electrostatic master paper plate"), the toner was then fused onto the electrostatic master paper plate under heat, and thereafter the resulting electrostatic master paper plate lipophobicated to finally prepare an electrophoto plate for offset printing. Correction of the unnecessary line image part was effected by rubbing and removing the toner on the unnecessary line image part with cotton fibres or the like before the toner was hot-fused onto the electrostatic master paper plate. (b) In the next stage of development, almost the same process as that of the above-mentioned initial stage has been conducted for preparing the electrophotoplate for offset printing. However, because of progress in the development of the platemaking devices used for the process, the step of spraying the toner over the line image part of the electrostatic master paper plate and the step of hot-fusing the toner onto the electrostatic master paper plate have come to be effected continuously. Accordingly, removal of the unnecessary toner prior to thermal fusion of the toner onto the electrostatic master paper plate could no longer be effected. Under the above conditions, therefore, various correction fluids have been developed. (1) The correction fluid as first developed was an organic solvent which may dissolve the toner and which has the property of not dissolving the binder in the zinc oxide layer of the electrostatic master paper plate. In other words, the toner as hot-fused onto the electrostatic master paper plate is dissolved by the correction fluid and the correction fluid is wiped off whereby the toner is removed from the line image part. However, a correction fluid of this kind has a drawback that it requires much time and troublesome labour for correction. (2) The correction fluid as next developed was one consisting of ammonium phosphate as a hydrophilicating liquid and a heavy metal ion such as stannous ion as added to the liquid (refer to Japanese Patent Publication No.48-5681). The correction fluid forms a hydrophilic and water-insoluble precipitate. Where the unnecessary line image part is coated with the precipitate, the coated part becomes hydrophilic and is therefore no more sensitive to ink. Both the above-mentioned correction fluids (1) and (2) are used after the toner has been fixed onto the electrostatic master paper plate but before the electrostatic master paper plate is lipophobicated. However, these days a large-sized electrostatic master paper plate is often used. Such large-sized electrostatic master paper plates are inconvenient in that they are handled only with difficulty and "oil stains" as well as "fingerprints marks" often adhere to the paper plate during handling thereof. If such "oil stains" and the like adhere to the hydrophilic non-line image part, the part would be sensitive to ink to thereby cause formation of some undesirable stains in the prints. In order to prevent this as much as possible, a platemaker which has a lipophobicating device as combined therewith and which may therefore automatically lipophobicate the master plate has been developed. The electrostatic master paper plate as processed by the platemaker of this type is always already lipophobicated. If the above-mentioned correction fluids (1) and (2) are applied to such an already lipophobicated electrostatic master paper plate, the correction fluid must be coated on the unnecessary line image part and the electrostatic master paper plate has to be lipophobicated a second time. However, the advantage of the automatic lipophobication of the electrostatic master paper plate with the newly developed platemaker would then be lost. It is therefore desirable to provide a correction fluid which can eliminate the ink-sensitivity from the unnecessary line image part at any stage before or after the lipophobication treatment of the electrostatic master paper plate. To this end the present inventor tried to use various substances which are extremely highly hydrophilic and which are insoluble in water, such as acrylic acid polymers, metal oxides, silicate oxides and various proteins, as a correction fluid. However, it has been found that these materials did not have a sufficient adhesion power to the line image part and therefore have the drawback that the layer of the substance coating the line image part peels off or the layer breaks down after the plate has been used for printing for a longer period of time. Therefore, correction liquids comprising such substances are not practical. JP-A-55-103382 discloses specific stannous alkylphosphate salts and their use as resist or discharging agents in textile printing processes. It is an object of the present invention to provide correction fluid practically usable at any stage before or after lipophobication of the electrostatic master paper plate. In order to attain this object, a stannous chelate compound having an acidic phosphate ester of an aliphatic alcohol as a ligand in which the proportion of ester/stannous ion is from 0.5/1.0 to 1.0/1.0 in view of chelating equivalency therebetween is utilized in accordance with the present invention. The invention also provides a correction fluid in which a said stannous chelate compound is dissolved in an aqueous acidic solution, in which the stannous ion is present in an amount of 3 to 8% by weight based on the total weight of the aqueous solution. In use, the aqueous solution is coated on the part to be corrected of the electrostatic master paper plate. After pH value of the aqueous solution is elevated, the said stannous chelate compound precipitates out to form a hydrophilic and water-insoluble layer on the part to be corrected of the electrostatic master paper plate. Afterwards, the moisture content in the wetting water applied to the plate is adsorbed to the layer of the said chelate compound to form an aqueous film thereon, so that the part is no longer sensitive to ink. In order to provide a correction fluid the pH value of which may be easily elevated and which comprises a stannous chelate compound dissolved in an aqueous acidic solution, the acidic group of the aqueous acidic solution of the invention is preferably volatile. In order to provide a correction fluid which is difficult to hydrolyze and which will adhere well to an electrostatic master paper plate, in accordance with the present invention, a polar solvent is preferably incorporated into the correction fluid which contains the stannous chelate compound dissolved in an aqueous acid solution. In order to provide a correction fluid which can smoothly be complexed or chelated with a lipophobicating solution when it is used prior to lipophobication of an electrostatic master paper plate, in accordance with the present invention, a dicarboxylic acid is preferably incorporated into the correction fluid containing a stannous chelate compound dissolved in an aqueous acidic solution. Acidic phosphate esters of aliphatic alcohols for use in the present invention include, for example, acidic phosphate esters of aliphatic lower alcohols, e.g. those having 1 to 3 carbon atoms such as acidic methyl phosphate (H₂PO₄CH₃), acidic ethyl phosphate (H₂PO₄CH₂CH₃), acidic n-propyl phosphate (H₂PO₄(CH₂)₂CH₃) or acidic isopropyl phosphate (H₂PO₄CH(CH₃)₂), as well as acidic phosphate esters of alicyclic alcohols such as acidic inositol phosphate or phytic acid (C₆H₆(H₂PO₄)₆), etc. One or more of such esters can be used as the chelating agent for forming the stannous chelate compound of the invention. In forming the chelate compound, the proportion of ester/stannous ion is from 0.5/1.0 to 1.0/1.0, from the viewpoint of the chelating equivalency therebetween. Most preferably, the chelating equivalency between them is 1/1. If the chelating equivalency of the ester is more than 1.0 or less than 0.5, the corrected part may unfavourably be stained. The phosphate esters of aliphatic alcohols are ligands for the stannous ion. The stannous ion having the acidic ester of aliphatic alcohol as a ligand is a water-insoluble white chelate precipitate having an extremely high hydrophilicity in an aqueous solution. However, if the hydrogen ion concentration in the aqueous solution is increased, (the pH value of the solution decreases), the precipitate is converted into a water-soluble chelate so that the precipitate redissolves. If the hydrogen ion concentration of the solution is then lowered (the pH value thereof is elevated), the chelate becomes water-insoluble and again forms a precipitate. The content of the stannous ion in the acidic solution is from 3 to 8% by weight, preferably 4% by weight. If it is less than 3% by weight, the correcting power of the correction fluid may be insufficient. However, if it is more than 8% by weight, too much of the hydrophilic and water-insoluble chelate may be formed, resulting in the aqueous film over the corrected part being holed and the corrected part would possibly become sensitive to ink. Such stannous ion is preferably supplied to the aqueous acidic solution in the form of stannous chloride. This is because the compound stannous chloride may be free in the aqueous solution to release hydrochloric acid thereby to lower the pH value of the solution. Additionally, the thus released hydrochloric acid may easily be removed from the solution by drying and therefore the post-treatment is easy. The stannous ion may also be supplied in any other form of salt such as with an organic acid radical such as an acetic acid radical or propionic acid radical or an inorganic acid radical. The pH value of the aqueous solution containing the stannous chelate compound as dissolved therein may be determined in accordance with the ligand of the chelate compound. As a means of elevating the pH value of the aqueous solution containing the stannous chelate compound as dissolved therein, (1) the aqueous solution is neutralized with a base or (2) where the acid radical in the aqueous solution is volatile (for example, it is hydrochloric acid radical, acetic acid radical, propionic acid radical or the like), the aqueous solution is dried (for example, by leaving it as it is in the ambient atmosphere for a while or by applying a room temperature air or a hot air thereto) so as to remove the acid radical from the solution. The method of preparing the water-soluble stannous chelate compound for use in the present invention is not critical. For instance, a water-insoluble chelate compound may first be prepared, suspended in water and an acid added thereto to convert it into a water-soluble compound. Alternatively, the aqueous solution may be made acidic from the first to directly prepare a water-soluble chelate compound. It is preferred to incorporate an effective amount of a polar solvent such as an alcohol to the aqueous solution containing the stannous chelate compound dissolved therein, for the purpose of inhibiting hydrolysis of the stannous ion and of increasing the adhesiveness of the solution to the surface of zinc oxide. By addition of a polar solvent to the solution, the anchor effect of the solution is increased. Additionally, it is also preferred to incorporate an effective amount of a dicarboxylic acid, such as malonic acid, adipic acid, succinic acid, gluconic acid, p-toluolsulfonic acid or the like, to the solution for the purpose of smoothly effecting the complexing reaction or chelating reaction with ferrocyan or phytic acid. As explained in the above, the stannous chelate compound of an acidic phosphate ester of an aliphatic alcohol of the present invention is soluble in an aqueous acidic solution and, after the pH value of the compound-containing aqueous soltuion has been raised, the compound gives a precipitate in the solution. Since the thus formed precipitate is hydrophilic, an aqueous film is formed thereon when treated with a wetting water and the aqueous film so formed repells ink. Accordingly, an aqueous acidic solution of the chelate compound of the invention may be coated on the part to be corrected of an electrostatic master paper plate and the pH value of the chelate compound-containing aqueous acidic solution as applied to the part to be corrected of the electrostatic master paper plate raised, whereby the solution forms a hydrophilic and water-insoluble layer to effectively coat the electrostatic master paper plate part to be corrected. That is to say, the electrostatic master paper plate part to be corrected is lipophobicated by the layer coated thereover. Such treatment may be effected at any stage before or after the lipophobication of the electrostatic master paper plate to be treated. The precipitate layer derived from the chelate compound is strongly resistant to peeling off or breaking down from the electrostatic master paper plate during use of the master paper plate for offset printing. Therefore, the chelate compound-containing correction fluid of the invention may be applied not only to conventional electrostatic master paper plates but also to lipophobicated electrostatic master paper plates as processed by a platemaker combined with a lipophobicating device, with ease and without necessity of re-lipophobication treatment of the plates, whereby the unnecessary line image parts may be removed from the thus treated plates. The invention will be further illustrated by reference to the following examples. In the following examples, the "stannous chelate compound having an acidic phosphate ester of an aliphatic alcohol as the ligand" was prepared in the form as dissolved in an aqueous acidic solution. That is, the compound-containing solution is the correction fluid of the invention. The proportion of the respective components in the aqueous solution is shown in Table 1 below. In proportions where the amount of the hydrochloric acid is lower than that indicated in Table 1, the chelate compound may precipitate out. Needless to say, the amount of the hydrochloric acid may be increased. The composition of Comparative Example 1 is the same as the basic composition of the correction fluid which is most popularly sold in the commercial market at the present time, and the commercial correction fluid is used prior to lipophobication. Comparative Examples 2 and 3 show the effect of the content of the stannous ion to be incorporated into the correction fluid. Comparative Examples 4 and 5 illustrate the effect of the proportion of the stannous ion to the ligand. The correction fluids as indicated in Table 1 were used as mentioned below. First, an electrostatic master paper plate was processed by an ordinary electrophotomechanical process to prepare a processed plate. The processed plate was introduced into an etching processer, using a commercial ferrocyan or phytic acid-containing lipophobicating solution, whereby the plate was lipophobicated. Each of the correction fluids of Examples 1 to 9 was coated on the line image part of the thus lipophobicated processed plate and dried thereon by allowing it to stand as it was for several minutes. Afterwards, the processed plate was set in a printing machine and printing was started immediately,whereupon the printing ink first adhered to the complete surface of the processed plate as the plate was in a dry state, but after water was applied thereto, the surface of the plate gradually became clear to give normal prints. In the procedure, the printing ink did not adhere to the part as coated with the dry layer of the correction fluid. Even after 3000 copies were printed, no copies were stained. Accordingly, the ink-insensitiveness of the part as coated with the dry layer of the correction fluid of each Example was well maintained during the printing procedure. On the other hand, where the correction fluid of Comparative Example 1 was used (1), the printing ink could not be removed from the surface as coated with the correction fluid even after water was applied thereto during running of the printing machine and, as a result, the complete surface of the plate as coated with the correction fluid was stained with the printing ink. After continuous printing with the plate, the ink could still not be removed from the coated surface. Next, after the correction fluid of each Example was coated and dried on the processed plate, (2) the plate was previously wetted with a wetting water prior to initiation of printing, whereby normal prints were obtained immediately after the initiation of printing. Precisely, since no ink adhered to the correction fluid-coated part, there was no trouble even after printing of 3000 copies. On the other hand, however, where the correction fluid of Comparative Example 1 was used in the same procedure, a noticeable amount of the printing ink still remained on the correction fluid-coated surface, although the degree of the stains to be caused by the remaining ink was not so much as the above-mentioned case (1). Therefore, the correction fluid was not practically usable. Additionally, after 100 copies were printed, the correction fluid of Comparative Example 1 as coated on the printing plate was lost, and the unnecessary line images appeared on the printing copies. Where the correction fluids of Comparative Examples 2 to 5 were tested in the same manner as above, a noticeable amount of the printing ink also remained on the corrected part and the printed copies were thereby stained. Where the correction fluid of each Example was coated and dried on the line image part of the processed plate prior to the lipophobication treatment of the plate and thereafter the thus coated plate was lipophobicated, no stain occurred in the corrected parts of the printed copies. This was same even after 3000 copies were printed. The property of the correction fluid of each Example is comparable to that of the correction fluid of Comapative Example 1 in this respect. Table 2 below shows correction fluids of Examples 10, 11 and 12, which were prepared by adding an adhesion accelerator and/or a complexing reaction accelerator to the correction fluid of Example 2. Where the correction fluid of each of these Examples was partly coated and dried on an electrostatic master paper plate, no ink adhered to the coated part. The thus coated plate was used in printing, and normal 3000 copies were obtained from the initial stage of the printing procedure. In the above-mentioned printing operation, the number of the copies to be printed with the processed plate was limited to 3000 copies in view of the life of the electrostatic master paper plate used. However, it may be expected that the alcohol-containing correction fluids of Examples 10 and 12 would have a higher durability than the correction fluid of Example 2 bacause of the hydrolysis-inhibiting effect and the anchor effect of the alcohol added. Additionally, it may also be expected that the dicarboxylic acid-containing correction fluids of Examples 11 and 12 would have a higher durability than the correction fluid of Example 2 when they are used prior to the lipophobication treatment of the electrostatic master paper plate, since the complexing reaction with the ferrocyan or phytic acid is accelerated. Example 10 Example 11 Example 12 Stannous Chloride (stannous ion) 6.0 (3.2) 6.0 (3.2) 6.0 (3.2) Hydrochloric Acid 18.0 18.0 18.0 Water 22.0 22.0 22.0 Phytic Acid (Chelate equivalency) 13.5 [1] 13.5 [1] 13.5 [1] Ethanol 34.0 - 34.0 Malonic Acid - 1.0 1.0 Results ○ ○ ○

Claims: A hydrophilic stannous chelate compound characterised by having an acidic phosphate ester of an aliphatic alcohol as a ligand and in that the proportion of the said acidic phosphate ester of an aliphatic alcohol to the said stannous ion is from 0.5/1.0 to 1.0/1.0 in view of the chelating equivalency therebetween. The stannous chelate compound as claimed in claim 1, characterised in that the proportion of the said acidic phosphate ester of an aliphatic alcohol to the said stannous ion is 1.0/1.0 in view of the chelating equivalency therebetween. The stannous chelate compound as claimed in claim 1 or 2, characterised in that the said ligand is one or more selected from acidic phosphate esters of aliphatic alcohols having 1 to 3 carbon atoms and acidic phosphate esters of alicyclic alcohols. The stannous chelate compound as claimed in claim 3, characterised in that the said acidic phsophate ester of an aliphatic lower alcohol is at least one member selected from the group consisting of acidic methyl phosphate, acidic ethyl phosphate, acidic propyl phosphate and acidic isopropyl phosphate. The stannous chelate compound as claimed in claim 3, characterised in that the said acidic phosphate ester of an alicyclic alcohol is phytic acid. A correction fluid for correcting an unnecessary line image part of an electrostatic master paper plate, characterised by comprising: (a) an aqueous acidic solution, and (b) a hydrophilic stannous chelate compound as claimed in any of claims 1 to 5, and (c) in that the content of the stannous ion of the said stannous chelate compound is from 3 to 8% by weight based on the total weight of the aqueous acidic solution. The correction fluid as claimed in claim 6, wherein the content of the stannous ion of the said stannous chelate compound is 4% by weight based on the total weight of aqueous acidic solution. The correction fluid as claimed in claim 6 or 7, characterised in that the acid radical of the said aqueous acidic solution is volatile. The correction fluid as claimed in claim 8, wherein the said volatile acid radical is selected from hydrochloric acid, acetic acid and propionic acid. The correction fluid as claimed in any of claims 6 to 9, characterised by further containing a polar solvent. The correction fluid as claimed in claim 10, characterised in that the said polar solvent is an alcohol. The correction fluid as claimed in any of claims 6 to 11, characterised by further containing a dicarboxylic acid selected from malonic acid, adipic acid, succinic acid, gluconic acid and p-toluolsulfonic acid. A method of erasing the unnecessary line image part of an electrophoto plate for offset printing, comprising: (a) a correction fluid-preparing step, where a stannous chelate compound as claimed in any of claims 1 to 5 having an acidic phosphate ester of an aliphatic alcohol as a ligand is dissolved in an aqueous acidic solution to prepare a correction fluid in which the content of the stannous ion contained in the said stannous chelate compound is from 3 to 8% by weight based on the total weight of the aqueous solution; (b) a coating step, where the correction fluid is coated on the unnecessary line image part of an electrophoto plate for offset printing; and (c) a precipitation step, where the pH value of the said correction fluid is elevated so that the said stannous chelate compound is precipitated on the said line image part. The method of erasing the unnecessary line image part of an electrophoto plate for offset printing as claimed in claim 13, characterised in that the pH of the correction fluid is raised by adding an alkali thereto whereby to effect said precipitation step. The method as claimed in claim 13, characterised in that in the said correction fluid-preparing step, the said stannous chelate compound is dissolved in an aqueous acidic solution containing a volatile acid radical and the said volatile acid radical is evaporated from the correction fluid so as to elevate the pH value of the fluid in the said precipitation step.

Inventors: YOSHIDA HAJIME NO YAMAMOTO-, YOSHIDA, HAJIME NO. 5 YAMAMOTO-201
Applicants: NIKKEN CHEMICAL LAB, NIKKEN CHEMICAL LABORATORY CO, LTD.
Application year: 1990
Main classification: C07F9
IPC codes: C07F9, B41N3