FIELD OF THE INVENTION
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The present invention relates to a desensitizing solution to be used for direct imaging lithographic printing plates, more particularly, to a desensitizing solution which can prevent the scum attributable to fog toner, and which hardly undergoes deterioration in desensitizing power even after long-term storage.
BACKGROUND OF THE INVENTION
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A direct imaging lithographic printing plate (direct imaging master) has an image-receiving layer which contains zinc oxide as a main component. The zinc oxide in the image-receiving layer performs the function of receiving toner to form an image and the function of combining with a desensitizing agent used in a desensitizing step to change its property from ink-receptivity (lipophilic) to water wettability (hydrophilic), thereby enabling the printing.
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In case of the direct imaging lithographic printing plate, an ink-receptive image is formed on the image-receiving layer containing zinc oxide by handwriting, thermal transfer, electrostatic transfer or the like.
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After the image formation on the foregoing lithographic plate, and before the printing operation, the image-formed plate is subjected to a process with a desensitizing solution (etching process) in order to make the zinc oxide in the non-image area water wettable. Also, a solution prepared by diluting the desensitizing solution with water by a factor of from 3 to 10 is generally used as a dampening solution for lithographic printing plate when offset printing is performed.
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As the processing solutions to be used in the desensitizing step, various desensitization processing solutions are known. Of those conventional ones, a desensitizing solution containing a ferrocyanide ion and a phosphoric acid salt as main components has been used widely. In such a processing solution, Zn ion is eluted due to acidity of phosphoric acid, and combines with a ferrocyanide ion to form a salt hardly soluble in water. This salt-formation reaction is utilized for converting the non-image area into an area having water wettability (hydrophilicity).
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The processing solution as described above can fully exhibit its desensitizing power when it is applied to a general direct master using wet toner in the image formation and the direct imaging master on which an image is formed by handwriting or thermal transfer. However, when it is applied to a master of the type wherein plate-making is carried out in electrostatic transfer system using a dry toner such as of a laser printer or the like which has become popular in recent years, its desensitizing power is insufficient, whereby the master of such a type suffers considerable extent of scum generation in the non-image area.
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In the plate-making method as described above, it is unavoidable that toner particles scattered upon toner development adhere to the non-image area. In the case of development with a dry toner, in particular, it is difficult by conventional processing solutions to give water wettability (hydrophilicity) to the spots to which the scattered toner particles have adhered as fog, because the particle size of toner is large as much as 1 µm or above, or 5 µm or above in some cases. As a result, a considerable extent of scum arising from the fog is formed in the non-image area.
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In order to prevent the scum formed in the manner described above, water wettability-processing solutions were proposed, such as one containing a ferrocyanide and a phosphate in a specific range of the ratio (JP-A-4-368888), and one further containing a sulfite and a chelating agent (JP-A-5-338371) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the processing solutions are still insufficient in prevention of fog scum.
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Further, when conventional desensitizing solutions are stored for a long time, a ferrocyanide decomposes and as a result, precipitates are formed and their desensitizing power is lowered, thereby resulting in generation of fog scum to a more considerable extent.
SUMMARY OF THE INVENTION
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Therefore, an object of the present invention is to provide a desensitizing solution, which is to be used for a direct imaging lithographic printing plate, and has a sufficiently high desensitizing power even when the lithographic printing plate is an electrostatic transfer-type direct imaging zinc oxide master, and a high storage stability as well.
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For the purposes of effecting improvements in the capabilities of the desensitizing solution as mentioned above in giving water wettability to fog toner, and enhancing the stability of the desensitizing solution as well, the present inventors have made intensive studies on the composition of desensitizing solutions and properties of the printing plates obtained using them. As a result, they have succeeded in finding a desensitizing solution which has a marked desensitizing power, hardly generates precipitates even after long-term storage, and hardly causes deterioration in desensitizing power, thus achieving the present invention.
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Namely, the object of the present invention is attained by a desensitizing solution used for a direct imaging lithographic printing plate, which comprises a ferrocyanide, a phosphate and from 2.3 to 15 % by weight of a hydroxy acid compound and has a pH value in the range of from 2.4 to 4.3.
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With the use of the desensitizing solution of the present invention in etching process of a direct imaging master precursor using zinc oxide, and with the use of a solution obtained by diluting the desensitizing solution of the present invention by a factor of 3 to 10 as a dampening solution for printing, it became possible to perform printing without suffering from the generation of scum, which is attributable to fog toner on the master caused by the use of a dry toner having a large particle size in plate-making processing of the master, with good inking characteristics on the image area.
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Although there may be a case where a hydroxy acid compound is added to a conventional desensitizing solution mainly composed of a ferrocyanide and a phosphate, the purpose of addition of the compound is to make it serve as a pH buffer. On the other hand, according to the present invention, an hydroxy acid compound is used for the purpose of preventing the generation of scum, which is bought about by the use of a dry toner having a large particle size, and it is added in a large amount beyond the conventional conception.
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The mechanisms of the reactions taking place in the present invention, though they are not completely elucidated, are supposed as follows. That is, a large quantity of Zn ions are eluted because of a low pH, i.e., a high hydrogen ion concentration of the desensitizing solution of the present invention, so that the water-wettable matter thus formed is large in quantity. Further, a temporary chelating reaction takes place between the eluded Zn ions and hydroxy acid molecules which have a relatively strong coordination power over Zn ions. Thus, the water-wettable precipitate forming reaction of Zn ions with ferrocyanide ions is slightly retarded, whereby it becomes possible to deposit the water-wettable matter so as to cover the fog toner as well as the non-image area. These and other reactions produce a synergistic effect to enable the prevention of fog scum.
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It is further supposed that Fe3+ or Fe2+ ions produced by the decomposition of ferrocyanide ions, which cause the generation of precipitates upon long-term storage and the lowering of the desensitizing power, are masked by the chelating effect of a hydroxy acid present in a large quantity, and as a result, the stability of the desensitizing solution upon long-term storage is enhanced.
DETAILED DESCRIPTION OF THE INVENTION
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The hydroxy acid compound to be used in the present invention is a compound having both a carboxyl group and a hydroxy group per molecule, and typically represented by the following formula (I):
(HO)m―R―(COOH)n (I)
wherein R represents an organic residue, such as an alkyl, alkenyl, aryl, aralkyl or the like group; and n and m are each an integer of 10 or less.
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Specific examples of the compound of formula (I) include aliphatic hydroxy acids such as glycolic acid (HOCH2COOH), lactic acid (CH3CH(OH)COOH), hydroacrylic acid (HOCH2CH2COOH), α-hydroxylbutyric acid (CH3CH2CH(OH)COOH), glyceric acid (HOCH2CH(OH)COOH), tartronic acid (HOCH(COOH)2), malic acid (HOOCCH2CH(OH)COOH), tartaric acid (HOOC(OH)CH―CH(OH)COOH), and citric acid (HOOCCH2―C(OH)(COOH)―CH2COOH); and aromatic hydroxy acids such as salicylic acids (o-, m- and p-C6H4(OH)(COOH)), gallic acid (3,4,5-trihydroxybenzoic acid), mandelic acid (C6H5CH(OH)(COOH)), and tropic acid (C6H5CH(CH2OH)(COOH)). The specific examples of the compound of formula (I) further include salts of the above-cited acids, such as sodium salts, potassium salts and ammonium salts.
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Preferred hydroxy acid compounds in the present invention are citric acid, tartaric acid, malic acid and lactic acid, with citric acid and tartaric acid being more preferred.
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In the present invention, the hydroxy acid compound is added in an amount of from 2.3 to 15 % by weight, preferably from 2.5 to 8 % by weight, based on the total weight of the solution. As far as the amount of the hydroxy acid compound added is 2.3 % by weight or more, the desensitizing solution can show a satisfactory power in the prevention of fog scum.
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It is highly important to the present invention that the pH value of the desensitizing solution be controlled properly, i.e., within in the range of from 2.4 to 4.3, preferably in the range of from 3.0 to 4.0, and particularly in the range of from 3.5 to 3.8. When the desensitizing solution has a pH value higher than 4.3, its fog scum preventing effect thereof lowers, whereas when the desensitizing solution has a pH value lower than 2.4, it tends to make troubles on a printing machine (e.g., roller stripping, over-emulsification of ink, etc.), and the storage stability thereof is lowered.
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As for the pH regulating agent, organic and inorganic salts and acids known as a pH buffer and a pH regulator, such as potassium hydroxide, sodium hydroxide, ammonia, etc., can be used.
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The ferrocyanide to be used in the present invention may be any of the potassium, sodium and ammonium salts, or may be a mixture of these salts. Although the present invention has no particular restriction as to the amount of a ferrocyanide to be added, it is preferred that the amount thereof in the desensitizing solution is from 0.5 to 10 % by weight, preferably from 1 to 7 % by weight, based on the total weight of the solution.
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When the amount of the ferrocyanide added is less than 0.5 % by weight, the resulting solution may not be sufficient in the desensitizing power, while the addition of a ferrocyanide in an amount larger than 10 % by weight causes no particular problem, but it is economically disadvantageous.
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The phosphate used in the present invention may be any of potassium phosphates, sodium phosphates and ammonium phosphates. Further, these salts may be any of the monobasic salts, dibasic salts and tribasic salts. Also, these salts may be used as a mixture of two or more thereof.
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The amount of the phosphate in the present desensitizing solution, though it has no particular limitation in the present invention, is preferably from 1 to 25 % by weight, particularly from 3 to 15 % by weight, based on the weight of the solution.
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When the amount of the phosphate added is less than 1 % by weight, the resulting solution may not be sufficient in the desensitizing power, while the addition of the phosphate in an amount larger than 25 % by weight causes no particular problem, but it is economically disadvantageous.
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The desensitizing solution of the present invention may further comprise a chelating agent. The chelating agent to be used is one having the function of masking iron ions which promote deterioration of the desensitizing solution. As for the chelating agent to be used in the present invention, aminocarboxylic acid compounds are especially effective, and representatives examples thereof include ethylenediaminetetraacetic acid, and its sodium, potassium and ammonium salts. It is preferred that the concentration of the chelating agent in the desensitizing solution is from 0.5 to 10 g/ℓ. When the chelating agent is added in an amount less than 0.5 g/ℓ, it may not be sufficient to exhibit the stabilization effect of the desensitizing processing solution, whereas when the amount added is greater than 10 g/ℓ, it produces adverse effect upon the scum preventing properties.
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In addition to the aforementioned ingredients, the desensitizing solution of the present invention can contain a wetting agent, a moisture retaining agent, a coloring agent, a preservative and so on in their respectively appropriate proportions.
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The present invention will now be illustrated in greater detail by reference to the following examples. However, the invention should not be construed as being limited to these examples.
EXAMPLE 1
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A desensitizing solution was prepared using the following ingredients in their respective amounts described below.
| Ingredients | Amount |
| Potassium ferrocyanide | 25 g |
| Monoammonium phosphate | 90 g |
| Citric acid | 30 g |
| Disodium ethylenediaminetetraacetate (EDTA-2Na) | 1.5 g |
| NH3 | to adjust the pH to 3.5 |
| Water | to make the total weight 1 kg |
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An plate precursor was obtained by forming a toner image directly on the surface of a commercial offset master by means of a laser printer, TN-7270 (trade name, a product of Toshiba Corp.). Next, an etching processor made by Ricoh Company Ltd. was filled with the foregoing desensitizing solution, and the foregoing plate precursor was etched therein twice to prepare a printing plate. The printing plate was set to a printing machine, Model 2850N, made by AM Co. and printing operation was performed using, as a dampening solution, a solution obtained by diluting the foregoing desensitizing solution by a factor of 6. The printed matters thus obtained were good in quality and free from scum attributable to fog toner.
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Further, the desensitizing solution prepared above was put in a glass flask, and allowed to stand for 3 months in an illuminated room. As a result, no precipitate was observed, and it was confirmed that the desensitizing solution had no change in its desensitizing power.
EXAMPLES 2 TO 5 AND COMPARATIVE EXAMPLES 1 TO 4
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Desensitizing solutions were prepared in the same manner as in Example 1, except for changing the formulation of the compositions as follows, and examined for their capabilities in the same procedures as in Example 1.
EXAMPLE 2
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| Potassium ferrocyanide |
20 g |
| Monopotassium phosphate |
100 g |
| Citric acid |
25 g |
| EDTA-2Na |
1 g |
| NH3 |
to adjust the pH to 3.8 |
| Water |
to make the total weight 1 kg |
EXAMPLE 3
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| Potassium ferrocyanide |
30 g |
| Monoammonium phosphate |
110 g |
| Tartaric acid |
35 g |
| EDTA-2Na |
1 g |
| Glycerine |
100 g |
| NH3 |
to adjust the pH to 3.7 |
| Water |
to make the total weight 1 kg |
EXAMPLE 4
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| Sodium ferrocyanide |
40 g |
| Monopotassium phosphate |
140 g |
| Malic acid |
55 g |
| EDTA-2Na |
1 g |
| NH3 |
to adjust the pH to 3.6 |
| Water |
to make the total weight 1 kg |
EXAMPLE 5
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| Potassium ferrocyanide |
50 g |
| Monosodium phosphate |
70 g |
| lactic acid |
45 g |
| Glycerine |
50 g |
| NH3 |
to adjust the pH to 3.5 |
| Water |
to make the total weight 1 kg |
COMPARATIVE EXAMPLE 1
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| Potassium ferrocyanide |
25 g |
| Monoammonium phosphate |
90 g |
| Citric acid |
15 g |
| NH3 |
to adjust the pH to 3.5 |
| Water |
to make the total weight 1 kg |
COMPARATIVE EXAMPLE 2
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| Potassium ferrocyanide |
16 g |
| Monoammonium phosphate |
120 g |
| Sodium sulfite |
10 g |
| EDTA-2Na |
1.5 g |
| Citric acid |
10 g |
| NH3 |
to adjust the pH to 3.8 |
| Water |
to make the total weight 1 kg |
Comparative Example 3
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| Potassium ferrocyanide |
20 g |
| Monoammonium phosphate |
100 g |
| EDTA-2Na |
1.0 g |
| Citric acid |
25 g |
| NH3 |
to adjust the pH to 4.4 |
| Water |
to make the total weight 1 kg |
Comparative Example 4
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| Potassium ferrocyanide |
30 g |
| Monoammonium phosphate |
170 g |
| EDTA-2Na |
1.5 g |
| NH3 |
to adjust the pH to 3.7 |
| Water |
to make the total weight 1 kg |
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The examination results in Examples 1 to 5 and Comparative Examples 1 to 4 are shown together in Table 1.
TABLE 1 | | Fog Scum | Fog Scum After Storage for 3 months | Generation of Precipitates upon Storage |
| Example 1 | A | A | B |
| Example 2 | A | A | A |
| Example 3 | A | A | A |
| Example 4 | A | A | A |
| Example 5 | A | A | A |
| Comparative Example 1 | E | E | C |
| Comparative Example 2 | D | E | F |
| Comparative Example 3 | F | F | A |
| Comparative Example 4 | F | F | E |
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The evaluation criteria as used in Table 1 for evaluating the examination results are as follows.
Fog Scum and Fog Scum After Storage for 3 months:
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- A:
- No fog scum was observed at all.
- B:
- Fog scum was faintly found by the observation with great caution, but it was a negligible level.
- C:
- Fog scum was found by the observation with caution.
- D:
- Fog scum was found at a glance.
- E:
- Fog scum was found in a considerable degree.
- F:
- Fog scum was found in a extremely serious degree over the entire surface of the printed matters.
Generation of Precipitates upon Storage:
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- A:
- No precipitates was formed.
- B:
- Precipitates were slightly formed.
- C:
- A small amount of precipitates were formed.
- D:
- A large amount of precipitates were formed.
- F:
- A considerable large amount of precipitates were formed.
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As can be seen from the examination result shown in Table 1, the desensitizing solutions having their pH values within the range specified in the present invention and containing hydroxy acids in large quantities as specified in the present invention (Examples 1 to 5) were free from fog scum and had excellent printing performance, and underwent no deterioration in their capabilities upon long-term storage. On the other hand, the desensitizing solutions wherein a hydroxy acid was absent or present in a small quantity and the desensitizing solution whose pH was beyond the range specified in the present invention caused fog scum, and underwent precipitation and deterioration in scum preventing capability upon long-term storage.
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That is, the present invention provides the desensitizing solution which is well suited for direct imaging lithographic printing plates, and which has a sufficient desensitizing power to a zinc oxide direct imaging master of a electrostatic transfer type using a toner of a large particle size and also has a high storage stability.
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While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.