LIPOPHOBICATING SOLUTION FOR ELECTROPHOTOGRAPHIC PLATES FOR OFFSET PRINTING

Abstract: There is provided a lipophobicating solution for electrophotographic plates for offset printing which comprises (a) phytic acid, (b) two or more dicarboxylic acids selected from those represented by the formula HOOC-(CH₂) n -COOH (where n is 1 to 6) and phthalic acid.

Description: The present invention relates to a lipophobicating solution for electrophotographic plates for offset printing. More particularly, it relates to an improvement of a lipophobicating solution containing phytic acid as the chelating agent. This lipophobicating solution is applied to a plate material (referred to as master paper hereinafter) produced by coating the surface of a support (e.g., paper) with zinc oxide to form the lipophilic image areas by electrophotography on a plate for offset printing. Among the lipophobicating solutions for electrophotographic plates for offset printing, the one based on phytic acid (see FR-A- 1 558 045) is being proposed in place of the one based on ferrocyan ion because the former causes no harm, permits the use of cyanin blue-based ink and facilitates pH adjustment. However, it is known that a lipophobicating solution prepared simply by diluting phytic acid is not of practical value because it soon causes scumming to master paper (as shown in Comparative Example 1 given later). In an attempt to eliminate this disadvantage, it has been proposed to use for example a lipophobicating solution composed of phytic acid, a complex or salt of an aminocarboxylic acid, and a polyhydroxyl-carboxylic acid (as shown in Conventional Example 1 given later). Another example is composed of a metal salt of phytic acid, a water-soluble fluoride, an alkali metal salt of dicarboxylic acid, and an alkali metal salt of phosphoric acid (as shown in Conventional Example 2 given later). It is alleged that these lipophobicating solutions provide master paper capable of thousands or even tens of thousands of impressions without scumming. The present inventors have prepared lipophobicating solutions according to the formulations given in the Conventional Examples and examined them for their effect. It was found that the master paper treated with them causes slight scumming even at the beginning of printing (as shown in Table 1 given later). These results indicate that contrary to the above allegation that the conventional lipophobicating solutions have not overcome the disadvantage inherent in the practical use of phytic acid. It is the first object of the present invention to provide a lipophobicating solution of practical use which does not cause scumming at all although it contains phytic acid as in the conventional lipophobicating solution containing phytic acid as a chelating agent. Another object of the present invention is to provide a lipophobicating solution which affords durable master paper capable of printing without scumming (for consecutive 30 days, 3000 impressions every day). It is a further object of the present invention to provide a lipophobicating solution containing dicarboxylic acids as a subsidiary material which is inexpensive and readily available. It is also an object of the present invention to provide a lipophobicating solution which can be easily prepared from phytic acid and two or more dicarboxylic acids in a broad mixing ratio. These objects are achieved by the present invention by providing a lipophobicating solution for electrophotographic plates for offset printing which contains phytic acid as a chelating agent and two or more dicarboxylic acids selected from those of the formula HOOC-(CH₂) n -COOH (where n is 1 to 6) and phthalic acid. Referring to the accompanying drawings. Fig. 1 is a graph showing the relationship between the mixing ratio (mol%) of phthatlic acid to adipic acid and the number of impressions; Fig. 2 is a graph showing the relationship between the mixing ratio of phytic acid to malonic acid and adipic acid (0.8:1) by mol) and the number of impressions. The following are detailed descriptions of the constituents of the lipophobicating solution of this invention. (a) Phytic acid (also called inositol hexaphosphate) makes the nonimage areas lipophobic, preventing the sticking of oily ink, on account of its hydroxyl groups at the 2- and 6-positions or at the 3- and 5-positions that form a chelate compound with zinc on the nonimage areas formed on master paper by electrophotography. Phytic acid occurs in nature in the seeds of many cereal grains, and it is harmless to human bodies. (b) The dicarboxylic acids are auxiliaries for the phytic acid. They include, for example, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, cork acid, and phthalic acid. Two or more of such dicarboxylic acids may be used. Those dicarboxylic acids of the above formula in which n = 7 or more do not have satisfactory solubility in water. Oxalic acid (in which n = O) is not effective in achieving the objects of the invention. Phthalic acid may be added in the form of its anhydride. Isophthalic acid and terephthalic acid are not desirable because they react with phytic acid to form precipitates. The dicarboxylic acids may be added in an amount of about 60 to 100 mol % or more based on the phytic acid (calculated as anhydrous), see Fig. 2. When the mol % of dicarboxylic acids is less than 60, the lipophobicating solution is generally poor in durability; although more than 100 mol % of the dicarboxylic acids may be used, increasing the mol % to more than 100 does not produce any more improvement in durability. Whilst not wishing to be bound by any specific theory the Applicants believe that the combination of phytic acid used with two or more of the abovementioned dicarboxylic acids produces the desired effect through the following mechanism: (i) The dicarboxylic acids (excluding terephthalic acid, isophthalic acid, and oxalic acid) also form chelate compounds with zinc. (ii) The two or more dicarboxylic acids produce a buffer action and continue forming chelating compounds even when the lipophobicating solution changes in its properties such as concentration and pH. (c) The lipophobicating solution of this invention may also contain one or more of the following additives as desired. Pastes such as starch (including soluble starch and dextrin) and derivatives thereof, cellulose derivatives, sodium polyacrylate, gum arabic, and pullulan which are in common use; Wetting agents such as ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, polypropylene glycol, glycerin, sorbitol, glucose, and sugar; Inorganic acids and salts thereof such as phosphoric acid, metaphosphoric acid, nitric acid, silicate acid, and metasilicic acid, and organic acids and salts thereof such as citric acid and tannic acid which are commonly used for printing chemicals; Preservatives such as salicyclic acid, benzoic acid, and dehydroacetic acid; Auxiliaries such as benzene, toluene, and aromatic sulfonic acid and salts thereof; Interfacial tension adjusting agents such as lower alcohols, ethers, ketones, and cellosolves; Coloring dyes. Examples The present invention will be further illustrated by reference to the following Examples. Different kinds of lipophobicating solutions were prepared from 1000 parts by weight of water, 150 parts by weight of 50% phytic acid, and the indicated amount of dicarboxylic acids as shown in Table 1. The resulting solution was adjusted to pH 4. (The phytic acid used is a product of Mitsui Toatsu Chemicals, Inc.) Each of the lipophobicating solutions was applied to master paper which had undergone platemaking. On the other hand, the lipophobicating solution was diluted 10 times with water, and the diluted solution was used as a dampening solution. The performance of the lipophobicating solutions was evaluated by actual printing with an offset printing machine (Besty 1800 CD, made by Toko Co., Ltd. and 2800 CD may be Ryobi Co., Ltd.), deep blue ink ("Master blue" made by Nikken Kagaku Kenkyusho), and neutral paper (with 20% ash, made by Hokeutsu Paper Mills, Ltd.). Lipophobicating solutions in Comparative Examples and Conventional Examples were prepared according to the following formulations. The same phytic acid was used in the Comparative Examples as in the Examples. In both Examples and Comparative Examples, each master paper was used for printing of 3000 impressions, and the resulting printed paper was visually examined for scumming in the nonimage areas. The printing of 3000 impressions was performed once a day for 30 consecutive days, lipophobicating solution being left in the working place under normal ambient conditions until next day, so that the experiments were carried out under actual printing conditions. The results are shown in Table 1. It is noted from Table 1 that the lipophobicating solutions in the Examples did not cause scumming even after printing of 3000 x 30 impressions. Fig. 1 is a graph showing the relationship between the mixing ratio (mol%) of phthalic acid to adipic acid and the number of impressions. The total amount of phthalic acid and adipic acid was kept constant (about 0.156 mol and 26 parts by weight). This result suggests that the mixing ratio of the dicarboxylic acids is not critical. Fig. 2 is a graph showing the relationship between the mixing ratio of the total amount of malonic acid and adipic acid (0.8:1 in mole = 1:2 in parts by weight) and the number of impressions. It is noted from Fig. 1 and Fig. 2 that the amount of dicarboxylic acids should be more than 0.067 mol (or more than 60 mol%) for phytic acid (150 g of 50% phytic acid = 0.113 mol). The results in Fig. 1 and 2 were obtained by experiments in which the lipophobicating solution was prepared from 1000 parts by weight of water, 150 parts by weight of phytic acid, and the indicated amount of dicarboxylic acid. In the experiments that gave the results in Figs. 1 and 2, no scumming occurred after printing of 3000 x 30 impressions. In examples shown in Table 2, the lipophobicating solutions contain some conventional additives in addition to phytic acid and the dicarboxylic acids. As in the above-mentioned examples, they did not cause scumming even after printing of 3000 x 30 impressions.

Claims: A lipophobicating aqueous solution comprising phytic acid as a chelating agent for electrophotographic plates for offset printing characterized in that the solution further comprises two or more dicarboxylic acids selected from those represented by the formula HOOC-(CH₂) n -COOH, where n is 1 to 6, and phthalic acid. A lipophobicating solution as claimed in claim 1 characterized in that the amount of dicarboxylic acids is at least 60 mol% based on the phytic acid calculated as anhydrous. A lipophobicating solution as claimed in claim 1 or 2 characterized in that the amount of dicarboxylic acids is not more than 100 mol% based on the phytic acid calculated as anhydrous. A lipophobicating solution as claimed in any of the preceding claims characterized in that when phthalic acid is present it is added in the form of phthalic anhydride. A lipophobicating solution as claimed in any of the preceding claims characterized by the presence of a pH adjusting agent. A lipophobicating solution as claimed in any of the preceding claims characterized by the inclusion of one or more additives selected from pastes, wetting agents, inorganic acids or salts thereof, preservatives, auxiliaries, interfacial tension adjusting agents and colouring dyes.

Inventors: YOSHIDA, HAJIME
Applicants: NIKKEN CHEMICAL LABORATORY CO, LTD.
Application year: 1986
Main classification: G03G13
IPC codes: G03G13, B41N3