SILVER MASTER PLATE RECOVERY SOLUTION

Abstract: The invention relates to a silver master plate recovery solution for recovering the ink receptivity of an image area on a silver salt photographic plate for offset printing by reducing the silver oxide layer of said image area, said silver master plate recovery solution characterised in that it comprises: (a) a hydrazine component for reducing said silver oxide layer of said image area; (b) a quaternary ammonium salt cationic activator which enters the minute pores on said image area to lower the polarity of said image area which had been made porous by said hydrazine component; and (c) water.

Description: The present invention relates to a silver master plate recovery solution for recovering the ink receptivity of an image area on silver salt photographic plates for offset printing by reducing the silver oxide layer of the image area. The silver salt photographic plate for offset printing (hereinunder referred to as "silver master plate") is a plate material produced by coating the surface of a support such as paper and plastic film with silver halide and by an electro diffusion transfer process this plate forms a smooth silver surface which constitutes a lipophilic image area for offset printing. The surface of the silver master plate is divided into image areas and non-image areas as in a general metal photographic plate. The image area, which is a smooth metal silver surface (the polarity is theoretically zero), is lipophilic without any affinity for water. In other words, the printing area is receptive to ink. The non-image area is made of gelatin having a polarity and is hydrophilic without any affinity for oil. In other words, the non-image area is repellent to ink. In order to perform offset printing, it is necessary to maintain a low polarity on the image areas and a high polarity on the non-image areas. The silver surface, which constitutes an image area on a silver master plate, begins to be oxidized immediately after plate making. Since silver oxide has a high polarity, the polarity of the oxidized image area becomes too high to maintain the lipophilic nature of the image area which results inter alia as non-uniformity of printing. Accordingly, the guaranteed stock life of a conventional silver master plate is only substantially eight hours, which brings about the following problems: (1) the operation of offset printing is troublesomely restricted by the short stock life, and (2) a silver master plate which has passed the stock life becomes unusable and has to be discarded. Since such problems are caused by the silver oxide layer in the image area, they will be eliminated by reducing the silver oxide layer by a reducing agent (silver master plate recovery solution). From JP-A-6363643 a stable quaternary alkylhydroxyalkylammonium hydroxide solution is known, said solution containing a quaternary alkylhydroxyalkylammonium hydroxide, such as cholire, and hydrazine to improve its long-term storage ability as a developer for resists. However, no silver master plate recovery solution which recovers the lipophilic nature of the image area by reducing the silver oxide layer has been known. This is because no conventional reducing agent can satisfy the following conditions (a), (b) and (c): (a) To have a reducing ability in the pH range of 3 to 6. In performing offset printing, it is inevitable that ink and water (fountain solution) are kneaded at a high speed by various rollers on the silver master plate. At this time, if the pH value of the fountain solution on the silver master plate is too high, the ink on the plate would be saponificated into a state of being compatible with water, namely, being emulsified. On the other hand, if the pH value of the fountain solution on the silver master plate is too low, there is a possibility of causing a phenomenon of roller stripping (ink fails to adhere to and distribute uniformly on the metal rollers) or chalking (ink pigment is not properly bound to the paper and can be easily ruffed off). It is therefore generally necessary to adjust the pH value of the dampening water on the silver master plate within the range of 3 to 6. (b) To have a equivalent reducing ability in the pH range of 3 to 6 It is difficult to fix the pH value of the fountain solution in a practical offset printing operation. Therefore, if the reducing ability of a reducing agent is controlled by the variation of the pH value of the print-immersing water, the reducing agent is not suitable for the practical offset printing. (c) Not to make the image area (silver surface) porous after reduction According to the investigation undertaken by the present inventors, an aqueous hydrazine solution has been selected as a silver master plate recovery solution that satisfies the conditions (a) and (b). However, when the silver oxide layer on an image area is reduced by hydrazine, the surface of the image area becomes porous. The surface then physically produces a polarity and, as a result, water is physically adsorbed on the surface of the image area. In other words, the lipophilic nature (receptivity to ink) of the image area is lowered. An object of this invention is to provide a silver master plate recovery solution which will recover the lipophilic nature of an image area on a silver master plate which has passed the stock life by reducing the image area. Another object of the present invention is to provide a silver master plate recovery solution which does not exert any deleterious influence on the gelatin layer on a non-image area and the unoxidized silver surface of an image area at the time of reducing the image area on a silver master plate which has passed the stock life. These objects are achieved by the present invention, which provides a silver master plate recovery solution, characterised in that it comprises: (a) a hydrazine component for reducing said silver oxide layer of said image area; (b) a quaternary ammonium salt cationic activator which enters the minute pores on said image area to lower the polarity of said image area which had been made porous by said hydrazine component; and (c) water. The quaternary ammonium salt has a polar group which is attracted into the minute pores in the image area and an elongated non-polar group which aligns itself on the surface of said plate. The present invention will be illustrated in more detail hereinafter. (1) A hydrazine component has the ability to reduce silver oxide in the pH range of 3 to 6 even when the pH value varies within this range, as described above. Such a hydrazine component is dissolved in a solution in the form of hydrazine or an acid salt or a hydrate of hydrazine. As examples of an acid salt of hydrazine, there are inorganic acid salts such as hydrazine sulfate and hydrazine hydrochloride and organic acid salts such as hydrazine acetate and hydrazine oxalate. The amount of hydrazine present in the recovery solution is preferably 1.0 to 3.5 wt.%, more preferably 1.2 to 3.0 wt.% based on the total amount of solution. With less than 1.0 wt.% of hydrazine the reduction, namely the silver master plate recovery operation, takes too much time, while if more than 3.5 wt.% hydrazine is used there is a possibility of causing scumming on the non-image areas of the silver master plate. (2) Suitable quaternary ammonium salt cationic activators are represented by the following general formula: wherein R₁, R₂, R₃ and R₄ each represent the same or a different alkyl group having 1 to 20 carbon atoms; B represents an inorganic acid group such as chlorine or a sulfate group, or an organic acid group such as an acetate group, oxalic group, propionate group, butyrate group, benzoate group or phthalate group; and n represents an atomic value or a radical number of 1 to 2. Thus B n- represents an inorganic anion such as a chloride ion or sulfate ion, or an organic anion such as an acetate ion, oxalic ion, propionate ion, butyrate ion, benzoate ion or phthalate ion. The cationic activator enters minute pores produced on the surface of the image areas and the long-chain portions of the above formula structure align themselves with each other on the surface of the image areas thereby lowering the polarity of the surface of the image areas and recovering the lipophilic nature of these surfaces. The cation activator may be present in an amount of from 3.0 to 20.0 wt.% based on the hydrazine component, but 5.0 to 10.0 wt.% is preferable. If the amount of the cation activator is less than 3.0 wt.% of the hydrazine component all the minute pores on the image area cannot be filled. On the other hand, if it exceeds 20.0 wt.% of the hydrazine component, a part of the cationic activator remains in the image areas to cause scumming. The amount of cationic activator may be from 0.1 to 0.3 wt.% based on the total amount of solution. (3) It is possible to add, if necessary, (a) a gelatin layer reinforcing agent such as a strong electrolyte which does not deteriorate silver on the image areas, e.g., ammonium chloride, sodium chloride, potassium chloride, ammonium sulfate, sodium sulfate and potassium sulfate, or a lower alcohol, in order to reinforce the gelatin layer of the non-image areas. (b) a moisture absorption improver such as a saccharide and a polyvalent alcohol, in order to improve the moisture absorption of the gelatin layer, and (c) a starching agent, wetting agent, antiseptic, interfacial tension regulator, dye, etc. The invention will be further illustrated by reference to the following Examples and Comparative Examples which are purely illustrative. Solutions having a composition shown in Table 1 or 2 were partially applied to a silver master plate which had been exposed to air for 350 hours after plate making process. Each solution was applied to the silver master plate by impregnating cotton with the solution and wiping the surface of the silver master plate. In Tables 1 and 2, Arcurd is a trade name of a cationic activator produced by Lion Fat and Oil Co., Ltd., and an experimental product is a cationic activator prepared by the present inventors. The structural formula of each of the cationic activators is shown in the following. The values in the brackets represent the concentrations (wt%) of the respective cationic activators (aqueous solutions). The amount in wt.% of the cationic activator shown in Table 1 and 2 is the dry weight thereof (i.e. the water content is eliminated from the commercial or experimental product). For example, Arcurd 2C-75 is an aqueous 75% solution. The amount of Arcurd 2C-75 solution used in Example 1 is 0.15 ÷ 0.75 = 0.2 (wt.%). The amount of hydrazine in wt.% is also shown by the dry weight. Each solution shown in Tables 1 and 2 was applied to a silver master plate, and immediately thereafter, the silver master plate was set in an offset printing machine Besty 1800 CD (trade name: produced by Toko Inc.) to print sheets of alkali paper having an ash content of 20% (produced by Hokuetsu Paper Mills, Ltd.) with a black ink, Master Black N (trade name: produced by Nikken Chemical Laboratory Co., Ltd.) A printed image was visually observed from the start of printing operation at the portion of a printed matter which corresponded to the silver master plate with the solution in Example applied thereto. On the 25th printed sheet, a clear image free from scumming was obtained, and thereafter the printed images were continuously clear. A printed image was also visually observed from the start of printing operation at the portion of a printed matter which corresponded to the silver master plate with the solution is Comparative Example applied thereto. In Comparative Exaimples 1 and 2, however, since the receptivity to ink was poor, no clear image was obtained even after 25 sheets had been printed. It was not until more than 100 sheets were printed that a clear printed image was obtaind. On the other hand, in Comparative Example 3, scumming was caused and the scumming did not disappear even after 50 sheets were printed. No printed image was visually observed at the portion of a printed matter which corresponded to the silver master plate with no solution applied thereto even after 50 sheets, much less 25 sheets, had been printed. Although a printed image was observed on and after about the 150th sheet, the printed image did not become clear even after 200 sheets had been printed. As described above, when an ordinaly offset printing was made after a silver master recovery solution of the present invention which contains 3.0 to 20.0 wt% of a quaternary ammonium salt cationic activator on the basis of hydrazine, which functions as a reducing agent, was applied to a silver master plate which had passed the stock life, a clear printed image was obtained on the 25th paper from the start of printing. When a silver master plate within the stock time was used, a clear printed image was also obtained, ordinarily, on the 25th paper from the start of printing. No scumming was caused on a printed matter. This is because the solution of the present invention does not exert any deleterious influence on the gelatin layer and the unoixdized silver surface of a non-image area.

Claims: A silver master plate recovery solution for recovering the ink receptivity of an image area on a silver salt photographic plate for offset printing by reducing the silver oxide layer of said image area, said silver master plate recovery solution characterised in that it comprises: (a) a hydrazine component for reducing said silver oxide layer of said image area; (b) a quaternary ammonium salt cationic activator which enters the minute pores on said image area to lower the polarity of said image area which had been made porous by said hydrazine component; and (c) water. A silver master plate recovery solution as claimed in claim 1, characterised in that the hydrazine component is present in an amount of from 1.0 to 3.5 wt.% based on the total amount of said silver master plate recovery solution. A silver master plate recovery solution as claimed in claim 2, characterised in that the hydrazine component is present in an amount of from 1.2 to 3.0 wt.% based on the total amount of said silver master plate recovery solution. A silver master plate recovery solution as claimed in claim 1, 2 or 3, characterised in that the quaternary ammonium salt cationic activator is present in an amount of from 3.0 to 20.0 wt.% based on the weight of said hydrazine component. A silver master plate recovery solution as claimed in claim 4, characterised in that the quaternary ammonium salt cationic activator is present in an amount of from 5.0 to 10.0 wt.% based on the weight of said hydrazine component. A silver master plate recovery solution as claimed in any of claims 1 to 5, characterised in that the quaternary ammonium salt has a polar group which is attracted into said minute pores present in the surface of the image area of said master plate and an elongated non-polar group which aligns itself on the surface of the said plate. A silver master plate recovery solution as claimed in any of claims 1 to 6, characterised in that said quaternary ammonium salt cationic activator is represented by the following general formula: wherein R₁, R₂, R₃ and R₄ each represent the same or a different alkyl group having 1 to 20 carbon atoms, B represents an acid group, and n represents an atomic value or a radical number of 1 to 2. A silver master plate recovery solution as claimed in claim 7, wherein B n- represents an inorganic anion. A silver master plate recovery solution as claimed in claim 8, characterised in that the inorganic anion is selected from the group consisting of chloride ion and sulfate ion. A silver master plate recovery solution as claimed in claim 7, characterised in that B n- represents an organic anion. A silver master plate recovery solution as claimed in claim 10, characterised in that the organic anion is selected from the group consisting of an acetate ion, oxalic ion, propionate ion, butyrate ion, benzoate ion and phthalate ion. A silver master plate recovery solution as claimed in any of the preceding claims, which further comprises at least one additive selected from the group consisting of a gelatin layer reinforcing agent, an agent for improving the moisutre absorption of said gelatin layer, a starching agent, a wetting agent, an antiseptic, an interfacial tension regulator and a dye. A silver master plate recovery solution as claimed in claim 12, characterised in that said gelatin layer reinforcing agent is selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, ammonium sulfate, sodium sulfate, potassium sulfate and a lower alcohol. A silver master plate recovery solution as claimed in claim 12 or 13, wherein said agent for improving the moisture absorption of said gelatin layer is selected from the group consisting of a saccharide and a polyvalent alcohol.

Inventors: KAINUMA, OSAMU, KAMADA, TOKUICHI, YOSHIDA, HAJIME
Applicants: NIKKEN CHEMICAL LABORATORY CO, LTD.
Application year: 1989
Main classification: B41N3
IPC codes: B41N3, G03F7