Lithographic printing plate precursor

Details Lithographic printing plate precursor Lithographic printing plate precursor

2001-01-10

2004-05-25

Ashton, Rosemary (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Microcapsule, process, composition, or product

C430S270100, C430S302000, C430S303000

Reexamination Certificate

active

06740464

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 plate-making by scan exposure based on digital signals, ensuring high sensitivity and long press life, and providing a printed matter free of residual color and staining.
BACKGROUND OF THE INVENTION
In general, the lithographic printing plate comprises an ink-receptive image area for receiving ink during the printing process and a hydrophilic non-image area for receiving a fountain solution. As the lithographic printing plate precursor for use in the manufacture of such a lithographic printing plate, a PS plate comprising a hydrophilic support having provided thereon an ink-receptive photosensitive resin layer (ink-receptive layer) has heretofore been widely used. For manufacturing a lithographic printing plate, the lithographic printing plate precursor is usually subjected to mask exposure through a lith 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 electronically processing, storing and outputting image information using a computer has been widely popularized. To cope with this digitization technology, various new systems 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 (CTP) technique where a printing plate is directly produced by scanning an active radiation having high directivity, such as laser light, according to digitized image information without intervention of a lith film. Thus, it is an important technical problem to obtain a printing plate precursor 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 be overcome in conventional techniques. Particularly in recent years, a great concern in industry as a whole is to give careful consideration to the global environment. 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 no processing are more keenly increasing.
From this standpoint, the following method has been proposed for dispensing with the above-described processing step in conventional techniques. That is, the method is a plate-making system where a photosensitive layer capable of allowing the non-image area of the printing plate precursor to be removed during the usual printing process is used and the plate after the exposure is developed on a printing machine 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 development system, the printing plate precursor must be stored under completely light-shielding and/or constant temperature conditions until it is mounted on a printing machine because the photosensitive layer is not fixed after the exposure.
In recent years, some solid lasers having high output become available at a low cost, such as semiconductor laser and YAG laser. With this progress, a method of using such a laser is expected as 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 conventional photosensitive material systems for low to medium power density exposure can be used. That is, 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 usually 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 do not substantially 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, for example, a system may be established, where a photosensitive layer capable of being insolubilized or solubilized by the heat-mode exposure is used and even when the layer after the imagewise exposure is exposed to the environmental light for an arbitrary time period and then developed (removal of non-image area), the image obtained can be free of any change.
By using this heat-mode recording, a lithographic printing plate precursor suitable for the above-described on-press development 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.
Conventional printing plate precursors using the heat-mode system have, however, a 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 printing plate precursor using the heat-mode system, generation of heat at the heat-mode exposure is attributable to light absorption by a light absorbent in the recording layer. Accordingly, the quantity of heat generated is large on the surface of recording layer and small near the support. Therefore, the degree of change in solubility of the recording layer is relatively low near the support, as a result, the ink-receptive layer in the exposed area which must provide a hydrophobic ink-receptive layer is sometimes removed during the development and/or printing process. If the ink-receptive layer in the image area of a negative printing plate precursor is removed, the printing performance encounters a problem of short press life. In particular, when a metal support having high thermal conductivity preferred in view of suitability for printing, such as A
1
, is used, the temperature near the support is much more prevented from increasing due to the thermal diffusion and the above-described problem comes out more seriously. For obtaining a sufficiently large change in solubility near the substrate, an extremely large exposure energy is necessary or an after-treatment such as heating must be performed after the exposure.
For example, Japanese Patent 2938397 describes a method of heat-fusing thermoplastic hydrophobic polymer fine particles by infrared laser exposure to form an image, fixing the plate on a cylinder of a printing machine and performing on-press development with fountain solution and/or ink. This method of forming an image by mere heat fusion ensures good on-press developability, however, when a heat-sensitive layer is provided directly on an aluminum substrate, the heat generated is deprived of by the aluminum substrate

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