Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2002-08-19
2004-06-29
Le, Hoa Van (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S309000, C430S331000
Reexamination Certificate
active
06756183
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for preparing a lithographic printing plate, and specifically to the plate-making method wherein a presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as “PS plate”) is developed by a specific developer. More specifically, the present invention relates to the plate-making method wherein a photopolymerizable presensitized plate for use in making a lithographic printing plate (hereinafter possibly referred to as “photopolymerizable PS plate”) is developed by a specific developer, which method never shows any reduction of developing characteristics due to the elapse of a long period of time and repeated use of the developer, can ensure a good ability to develop the non-image area (non-exposed area) of the lithographic printing plate, prevents the non-image area from scumming, does not damage the image area (exposed area) of the plate due to the development, ensures the formation of firm images and allows the achievement of high printing durability.
BACKGROUND OF THE INVENTION
The developers widely used conventionally for developing a photopolymerizable PS plate may roughly be divided into the following three groups: a) non-aqueous developers mainly comprising organic solvents; b) aqueous developers mainly comprising inorganic alkalis; and c) aqueous developers mainly comprising organic bases.
Among these, those presently used include, for instance, aqueous developers b) and c) at the request of the environmental protection.
Characteristic properties of these two developers will be detailed below. The inorganic alkaline developer b) is characterized in that it usually comprises a silicate and has a pH value in the proximity to 12 in order to hydrophilize the surface of a substrate of the printing plate after the development.
This silicate component is essential for the subsequent hydrophilization treatment or for preventing the non-image area from being contaminated by ink during printing, i.e., for the prevention of scumming.
For instance, there have been known a developer having a pH value of not less than 12 such as those disclosed in Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. Hei 8-248643 and a developer having a pH value of not more than 12 such as those disclosed in J.P. KOKAI No. Hei 11-65129. However, the former or the developer having a pH value of not less than 12 is liable to dissolve aluminum currently used as a substrate for the printing plate and, in particular, in case of a dot (small dot) occupying a small image area, it may dissolve the aluminum substrate immediately below the image area due to the so-called side etching phenomenon and this results in a phenomenon in which small dots are removed from the substrate (skipping of small dots) during printing. In other words, this developer suffers from such a problem that it may significantly deteriorate the printing durability of the resulting printing plate.
Moreover, the latter or the developer having a pH value of not more than 12 is excellent in the foregoing printing durability and an ability of preventing scumming, but it is liable to cause reduction of the pH value when continuing the developing treatment over a long period of time due to, for instance, the effect of carbon dioxide present in the air and the developer causes separation of a silicate during the development and this in turn leads to the occurrence of such a novel problem that it never allows any stable development.
In addition to the foregoing, J.P. KOKAI Nos. Sho 61-109052, 2000-81711 and Hei 11-65126 and West German Patent No. 1,984,605 disclose developers free of any silicate, but all of them were not only inferior in the prevention of scumming to those comprising silicates, but also could simultaneously satisfy the requirements for the printing durability and the prevention of scumming, only with great difficulty.
On the other hand, there have been known, as the organic base-containing developer c), for instance, those each comprising an organic amine such as ethanolamine and an alcoholic organic solvent such as benzyl alcohol as an auxiliary developer. In this case, however, such a developer undoubtedly has a low pH value on the order of 10 and therefore, it is not susceptible to the influence of carbon dioxide and is excellent in the processing stability. However, it also suffers from problems in that it is not only disadvantageous in the hydrophilization of the substrate, but also has an extremely high ability of penetration into the image area, that it adversely affects, in particular, small dots and that it is accompanied by the deterioration of the image-forming ability or such a phenomenon that small dots are removed from the substrate during development.
In other words, under the existing circumstances, there is not proposed any developer suitably used for developing the photopolymerizable PS plate, while taking into consideration, for instance, the image-forming ability, prevention of any scumming and printing durability as well as the processing stability.
Regarding the composition of the developer, it has been known that factors such as the presence of a silicate, the level of pH and the presence of an inorganic or organic alkali may exert a great influence on the developing phenomenon, but any combination thereof conventionally proposed has never permitted the solution of the foregoing problems at all. In particular, when a developer has a relatively lower pH value of approximately 12, such developer suffers greater change in a pH value, which change is associated with carbon dioxide gas and alkali consumption by PS plates, and the developer is susceptible to processing amount of PS plates, lag in adding a replenisher and a concentration of carbon dioxide in a room.
With respect to the processing stability, it is quite critical to consider not only the above-mentioned problem on the silicate-separation associated with the reduction of pH, but also stabilization of pH value of the developer in a relatively lower pH range. In other words, there has been desired for the development of a developer for a photopolymerizable PS plate, which can satisfy the requirements for the foregoing image-forming ability, for both the prevention of scumming and the printing durability and for the processing stability. In addition, in view of improving the prevention of scumming, a desensitizing treatment after a development is contributory thereto and it is necessary to ensure a sufficient volume of a coating by the desensitizing treatment.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for preparing a lithographic printing plate, which can satisfy both the requirements for the prevention of scumming and the printing durability on the resultant printing plate without impairing the image-forming ability, and which can improve the processing stability which is associated with a pH change of the developer.
SUMMARY OF THE INVENTION
The inventors of this invention have conducted various studies, recognized that the lower pH value of a developer is advantageous so as to avoid the impairment of the image-forming ability of the developer, while searched for suppressing a pH change of the developer, which change is associated with alkali consumption, and then found that an inclusion of a compound having a dissociation constant pka of from 10 to 13 into a developer is effective in suppressing the pH change of the developer, and the development performance is sufficiently exhibited and assured, and the scumming is favorably eliminated, even if the developer has a relatively lower pH value.
Accordingly, the present invention relates to a method for preparing a lithographic printing plate comprising the steps of imagewise exposing, to light, a presensitized plate for use in making a lithographic printing plate, which comprises a grained and anodized substrate provided thereon with a photopolymerizable light-sensitive layer containing a compound having at le
Burns Doane Swecker & Mathis L.L.P.
Le Hoa Van
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