Lithographic printing plate precursor

Details Lithographic printing plate precursor Lithographic printing plate precursor

2000-04-19

2002-09-24

Ashton, Rosemary (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C101S453000, C430S302000

Reexamination Certificate

active

06455224

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a negative lithographic printing plate precursor comprising a support having a hydrophilic surface, and a hydrophilic image-forming layer. More specifically, the present invention relates to a lithographic printing plate precursor capable of manufacturing a plate by scan exposure based on digital signals, ensuring high sensitivity and long press life, and providing a printed matter free of residual color or staining. According to a preferred embodiment, the lithographic printing plate precursor can be developed with water or an aqueous solution or can be mounted and subjected to printing in a printing machine without passing through development.
BACKGROUND OF THE INVENTION
In general, the printing plate comprises an ink-receptive (i.e., a lithographic) image area for receiving ink during the printing process and a hydrophilic non-image area for receiving a fountain solution. For the original plate (i.e., the precursor) of such a lithographic printing plate, a PS plate comprising a hydrophilic support having provided thereon an ink-receptive (i.e., a lipophilic) photosensitive resin layer (ink-receptive layer) has heretofore been widely used. According to the plate-making process therefor, a plate is usually subjected to mask exposure through a lithographic film and then the non-image area is dissolved and removed with a developer to obtain a desired printing plate.
In recent years, digitization technology of electrically processing, storing and outputting image information using a computer has been widely popularized. To cope with this digitization technology, various new methods for outputting an image have been proposed and are actually used. To keep up with this tendency, demands are increasing for a computer-to-plate technique where a printing plate can be directly produced by scanning a ray having high directivity, such as laser ray, according to digitized image information and a lithographic film can be dispensed with. Thus, it is an important technical problem to obtain a printing plate precursor (i.e., sometimes, called “a printing original plate”) suitable therefor.
In the plate-making process of conventional PS plates, the step of dissolving and removing the non-image area after the exposure is indispensable. This additional wet processing as an indispensable step is another problem demanded to overcome in conventional techniques. Particularly in recent years, to take scrupulous care of the global environment is a great concern in the whole industry. From both of the environmental aspect and the process rationalization aspect to keep up with the digitization, demands for a simple processing, a dry processing or no processing are more keenly increasing.
From this standpoint, the following method has been proposed for eliminating the above-described processing step in conventional techniques. That is, a plate-making system for lithographic printing plate, called on-press developing system, has been proposed, where a photosensitive layer capable of allowing the non-image area of the printing original plate to be removed during the usual printing process is used and the plate after the exposure is developed on a printing press to obtain a final printing plate without passing through a development step. To speak more specifically, for example, a method of using a photosensitive layer soluble in a fountain solution or an ink solvent and dynamically removing the non-image area by the contact with the impression cylinder or blanket cylinder in the printing machine is known. However, if a conventional PS plate is applied to this printing plate in the on-press developing system, the original plate must be stored under completely light-shielding and/or constant temperature conditions until it is mounted on a printing press because the photosensitive layer is not fixed after the exposure.
In recent years, some solid lasers having high output have become available at a low cost, such as semiconductor laser and YAG laser. With this progress, a method of using such a laser is taking charge of highly promising means for solving the above-described technical problem. In the high power density exposure system using these high output solid lasers, various phenomena different from the photoreaction occurring in the photosensitive material system for the low to medium power density exposure can be used. More specifically, various structural changes such as chemical change, phase change and morphology change can be used. The recording system by this high power density exposure is called “heat-mode recording”. This is because in the high power density exposure system, the light energy absorbed by the photosensitive material is converted into heat in many cases and the heat generated is believed to bring about a desired phenomenon.
This heat-mode recording system is greatly advantageous in that fixing of an image after the exposure is not an essential matter.
More specifically, the phenomena used for the recording of an image on a heat-mode photosensitive material substantially do not occur in the exposure to light having an ordinary intensity or at an ordinary ambient temperature, therefore, fixing of an image after the exposure is not necessary. By virtue of this, a system may be established, for example, where a photosensitive layer insolubilized or solubilized by the heat-mode exposure is used, as a result, even when the layer after the imagewise exposure is exposed to the environmental light for an arbitrary time and then developed (removal of non-image area), the image obtained does not undergo any change.
By using this heat-mode recording, a lithographic printing original plate suitable for the above-described on-press developing system may be obtained.
As one preferred example of the method for manufacturing a lithographic printing plate according to the heat-mode recording, a method of providing a hydrophilic image-forming layer on a hydrophilic support, imagewise exposing it by heat-mode exposure to cause changes in the solubility and dispersibility of the hydrophilic layer and if desired, removing the unexposed area by wet development has been proposed.
For the original plate of this type, an image-forming material comprising a support having thereon a photosensitive layer containing a compound capable of generating an acid by the irradiation of an active ray, a compound having at least one bond capable of cross-linking in the presence of an acid and a cyanine dye having a specific chemical structure is disclosed, for example, in JP-A-10-239834 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, the photosensitive layer disclosed is not sufficiently high in the heat sensitivity and exhibits conspicuously low sensitivity in the heat-mode scan exposure. Furthermore, a problem is present in that the discrimination in the hydrophilicity/hydrophobicity before and after the exposure, namely, the change in the solubility is small. If the discrimination is small as such, the plate-making by the on-press developing system cannot be substantially performed.
Conventional original plates of heat-mode system have another serious problem, that is, the non-image area is poor in the resistance against staining or the image area is low in the strength. In other words, improvements are necessary in the point that the change in solubility of the image-forming layer upon exposure is small near the support as compared with the change near the surface of the image-forming layer. In the original plate of heat-mode system, heat generation at the heat-mode exposure is attributable to light absorption of a light absorbent in the recording layer. Therefore, the quantity of heat generated is large on the surface of recording layer and small near the support. As a result, the degree of change in solubility of the recording layer is relatively low near the support and the ink-receptive layer in the exposed area which must provide a hydrophobic ink-receptive layer is sometimes removed during the developm

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