JPH0517879B2 - - Google Patents

Description

[Detailed description of the invention]

(A) Industrial Application Field The present invention relates to a processing liquid used for desensitizing an electrophotographic offset master. (B) Prior art An electrophotographic offset master has a photosensitive layer on a conductive support in which a photoconductive substance such as zinc oxide is dispersed in a resin binder, and the layer is charged, exposed, and
After the development and fixing steps, a lipophilic image is formed on the photosensitive layer and treated with a desensitizing solution. Conventionally, processing liquids used in electrophotographic offset masters have been mainly composed of hexacyanoferric salts such as ferrocyanate and ferricyanide salts. In particular, processing liquids containing iron hexacyanosalts as a main component have a wide latitude depending on the processing conditions; for example, even if an automated processing device performs one or two passes, the processing liquid can also be manually applied to the plate material. Even with hand etching, it was possible to produce prints with good adhesion. However, hexacyanoferric salts may be decomposed by heat or light to generate free cyanide, and desensitizing treatment liquids containing hexacyanoferric salts as a main component have had problems in terms of pollution. For this reason, a cyanide-free type desensitizing solution containing inositohexalic acid ester or its salt as a main ingredient has been proposed, but a solution of sufficiently satisfactory quality has not yet been completed. A disadvantage of using inositohexalic acid ester or its salt as a desensitizing main agent is that the rate of desensitizing film formation is slow. A specific example is hand etching, which involves soaking absorbent cotton or sponge in a desensitizing solution and hand-painting it onto the surface of the plate. The difference in desensitization processing ability with the cyan type is not so noticeable, but when processed using automated desensitization processing equipment (e.g. Mitsubishi Paper Etching Converter OH-11, etc.), there is a difference in ability with the cyan type. There were cases where printed matter was smudged. On the other hand, in consideration of these points, a processing solution with a high film formation rate is disclosed in Japanese Patent Application Laid-Open No. 59-207290.
This is a combination of inositohexalic acid ester or its salt and acidic colloidal silicic acid having a SiOH group and a particle size of 40 mμ or less, and has good instant film-forming properties. However, if this processing liquid is passed twice through an automated desensitizing device and the processing liquid is applied to the surface of the plate material by hand, the desensitizing process will not be applied to the image area (toner area). This has the disadvantage that the ink does not adhere to the image area. (C) Purpose of the Invention The present invention aims to improve the slow desensitizing film formation rate of cyanide-free type processing solutions such as inositohexalic acid esters or their salts, and is easy to manufacture. and the quality is stable,
A further object of the present invention is to provide a desensitizing liquid for lithographic printing that does not cause background smudges on printed matter and has good ink coverage in one pass through an automated desensitizing device. (D) Structure of the Invention As a result of intensive study to solve the above problems, the inventors discovered the following method and completed the present invention. That is, the present invention comprises a desensitizing solution containing dihydrogen phosphate and acidic colloidal silicic acid having a SiOH group and a particle size of 40 mμ or less, and adjusting the pH of the solution to 3.0 to 6.0. The dihydrogen phosphate referred to in the present invention is, for example,
NaH ₂ PO ₄ , KH ₂ PO ₄ , NH ₄ H ₂ PO ₄ , Ca
(H ₂ PO ₄ ) ₂ , Mg (H ₂ PO ₄ ) ₂ , Al (H ₂ PO ₄ ) ₃ , Fe
(H ₂ PO ₄ ) _3, etc., and the concentration in the treatment liquid is 1.0
g/~300 g/ is preferred, and a particularly preferred range is 5.0 g/~50 g/. If the amount is less than this range, the desensitizing ability will not be sufficient and scumming will occur on printed matter, and if it is more than this range, the desensitizing ability will be too strong and ink coverage will be poor. On the other hand, acidic colloidal silicic acid having a SiOH group and having a particle size of 40 mμ or less includes so-called colloidal silica, water glass, etc., and surface-modified products thereof. The concentration to be added to these processing solutions is preferably 0.1g/~30g/,
A particularly preferred range is 1.0 g/-20 g/.
The reason why this range is preferable is the same as mentioned above. In an embodiment of the desensitizing treatment liquid for lithographic printing plates of the present invention, in addition to the above-mentioned components, it may contain at least one of the above-mentioned hydrophilic polymer, oxycarboxylic acid or its salt, and chelating agent. include. Hydrophilic polymers are water-soluble, have protective colloidal properties, and provide a protective and reinforcing effect to the hydrophilic layer. Examples include gum arabic, alginates, polyacrylates, polyvinyl alcohol,
Polyvinylpyrrolidone, carboxymethyl cellulose or its salt, dextrin, modified starch, etc., and the concentration in the treatment liquid is preferably 0.1 g/~50 g/, and a particularly preferable range is 1.0 g/~30 g/
It is. Oxycarboxylic acids include salicylic acid, malderic acid, lactic acid, gluconic acid, ascorbic acid, citric acid, etc., and the concentration in the treatment liquid is 0.5g/~100
g/, but a particularly preferable range is 1 g/
~40g/. A chelating agent refers to a substance that has the ability to further improve the desensitization ability of a processing liquid and also to prevent scumming during printing when the processing liquid of the present invention is diluted and used as a dampening liquid. Examples include sodium, potassium, and ammonium salts of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, iminodiacetic acid, nitrilotriacetic acid, pyrophosphoric acid, triphosphoric acid, etc., and the concentration in the treatment liquid is 0.1 g/~100
g/ is preferable, and a particularly preferable range is 5 g/
~50g/. Furthermore, suitable amounts of inorganic and organic acids and basic substances, as well as known preservatives and rust preventives, can be added as acidity regulators for the treatment liquid. When using the treatment liquid of the present invention, the pH of the liquid is
A range of 3.0 to 6.0 is desirable. Moreover, this treatment liquid can be diluted 4 to 10 times with water and used as a humidifying liquid. Next, the present invention will be explained with reference to Examples and Comparative Examples. Example 1 Potassium dihydrogen phosphate 3.0 g Acidic colloidal silicic acid (particle size 20 mμ) 0.2 g Diammonium citrate 1.0 g Glycerin 4.0 g Water was added to sufficiently dissolve 100 g, and the pH of the solution was adjusted to 4.5. Example 2 Ammonium dihydrogen phosphate 3.0 g Acidic colloidal silicic acid (particle size 20 mμ) 0.2 g Sodium gluconate 1.5 g Glycerin 4.0 g Water was added to sufficiently dissolve 100 g, and the pH of the solution was adjusted to 4.5. Example 3 Calcium dihydrogen phosphate 3.0 g Acidic colloidal silicic acid (particle size 20 mμ) 0.2 g Potassium phthalate 1.0 g Glycerin 4.0 g Water was added to sufficiently dissolve 100 g, and the PHH of the solution was adjusted to 4.5. Comparative Example 1 Inositohexalic acid ester 3.0g Acidic colloidal silicic acid (particle size 20mμ) 0.2g Diammonium citrate 1.0g Glycerin 4.0g Water was added to sufficiently dissolve 100g, and the pH of the solution was adjusted to 4.5. Comparative Example 2 Disodium hydrogen phosphate 3.0 g Acidic colloidal silicic acid (particle size 20 mμ) 0.2 g Sodium gluconate 1.0 g Glycerin 4.0 g Water was added to sufficiently dissolve 100 g, and the pH of the solution was adjusted to 4.5. Comparative Example 3 3.0 g of potassium dihydrogen phosphate, 0.2 g of acidic colloidal silicic acid (particle size 50 mμ), 1.0 g of diammonium citrate, 4.0 g of glycerin, and 91.8 g of distilled water were sufficiently dissolved to adjust the PHH of the solution to 6.1. On the other hand, after forming an image using a commercially available electrophotographic offset master according to a conventional method, etching converter OH-11 (manufactured by Mitsubishi Paper Industries) was used with each processing solution.
A one-time desensitization treatment was carried out at . Further, as the humidifying water, a solution diluted with a volume ratio of 1 part of the same treatment solution to 9 parts tap water was used. The table shows the results of printing with a small offset printing machine (Toko model 8:10, manufactured by Tokyo Aircraft Instruments).

[Table] As shown above, in Examples 1 to 3, there was no background smudge even after printing 5000 sheets, and the ink coverage at the beginning of printing was good, but in Comparative Example 1, no background smear occurred. However, the ink transfer was very poor, and in Comparative Examples 2 and 3, the printing became smeared after about 10 sheets, and printing could not be continued.

Claims (1)

[Claims] 1. Particle size 40 containing dihydrogen phosphate and SiOH group
A desensitizing treatment liquid for lithographic printing plates, characterized in that it contains acidic colloidal silicic acid of mμ or less. 2 Dihydrogen phosphate concentration is 1.0g/~300g/
, the acidic colloidal silicic acid concentration is 0.1g/~30
The desensitizing treatment liquid for lithographic printing plates according to claim 1, which is