Lithographic printing plate precursor

Details Lithographic printing plate precursor Lithographic printing plate precursor

2001-04-19

2003-07-22

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Microcapsule, process, composition, or product

C430S270100, C430S302000, C101S453000

Reexamination Certificate

active

06596455

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a negative-working lithographic printing plate precursor comprising a substrate having a water-receptive surface and a water-receptive image-forming layer. More specifically, the present invention is concerned with a lithographic printing plate precursor which can be subjected to platemaking by scanning exposure based on digital signals, has high sensitivity, and ensures high printing durability and scumming resistance in the printed matters.
BACKGROUND OF THE INVENTION
In general, a lithographic printing plate comprises an oleophilic image area capable of accepting ink in the process of printing and a hydrophilic non-image area capable of accepting a fountain solution. As a precursor for such a lithographic printing plate, the so-called PS plate having an oleophilic photosensitive resin layer (an ink-receptive layer) on a hydrophilic substrate has so far been used prevailingly. From such a PS plate, the intended printing plate has been generally made by subjecting the PS plate to mask exposure via a lith film and then processing the exposed PS plate with a developer to dissolve and remove the non-image area.
On the other hand, recent years have seen a proliferation of digitization technology for electronically processing, storing and outputting image information by the use of a computer And a variety of new image output systems ready for such digitization technology have come to be in practical use. Under these circumstances, it has been longed to develop computer-to-plate (CTP) technology for making a printing plate directly without the mediation of a lith film by scanning highly directional active radiation, such as laser beams, in response to digitized image information, and so it is an important technological problem to design printing plate precursors appropriate for the CTP technology.
On the other hand, removal of non-image areas by dissolution after exposure is indispensable for the conventional plate making from a PS plate, so obviation of the necessity for such an additional wet processing is another target for improvement of the prior art. Lately in particular, consideration of world's ecology has become a big concern in all of industries. Therefore, it has been more strongly desired than ever to simplify the processing, effect the processing in a dry process, or make the processing unnecessary from both the viewpoints of ecology and process streamlining associated with the foregoing digitization.
As a method of eliminating a conventional processing process, the following proposal has been made. To be concrete, the proposed mode comprises using a photosensitive layer that enables non-image areas of the printing plate precursor to be removed during a usual printing process. By the use of such a photosensitive layer, the printing plate precursor can be mounted on a printing machine without development process after exposure, and developed on the machine to provide a final printing plate. Such a system of making a lithographic printing plate is referred to as “on-press development system”. More specifically, the on-press development can be effected, e.g., by the use of a photosensitive layer capable of dissolving in a fountain solution or an ink solvent and mechanical removal by contact with an impression cylinder or a blanket cylinder installed in a printing machine. However, application of the on-press development system to conventional PS plates has raised a big problem that the printing plate precursors require to be preserved under perfectly light-tight and/or isothermal condition even after exposure, e.g., until the they are mounted in a printing machine, because the photosensitive layer thereof is still in an unfixed condition after exposure.
In the context of the above described technological problems, the method of using a high-output laser, such as semiconductor laser or solid laser (e.g., YAG laser), has become a promising method for plate making by scanning exposure, because such lasers have come to be available at low prices in recent years. In the high-power density exposure system using such a high-output laser, it is possible to utilize various phenomena other than the photo reactions utilized in heretofore known photosensitive materials having suitability for low to medium-power density exposure. For instance, not only chemical changes but also structural changes, such as changes in phase and form, can be utilized as such phenomena. In general, the recording system utilizing such a high-power density exposure is called “heat-mode recording system”. This is because, in many of high-power density exposure systems, it is believed that the light energy absorbed in photosensitive materials is converted to heat and the heat thus generated causes the intended phenomena.
A great advantage of such a heat-mode recording system is in that the fixation of image after exposure is not essential.
More specifically, the phenomena utilized for image recording in heat-mode photosensitive materials don't take place in a substantial sense under exposure to light of ordinary intensity or at ordinary environmental temperature, so that the fixation of images after exposure is not essential. Accordingly, the use of photosensitive layers rendered insoluble or soluble by heat-mode exposure makes possible systems capable of producing images by imagewise exposure but undergoing no changes in the images by development (removal of non-image areas) after exposure to, e.g. environmental light for an arbitrary period of time.
According to heat-mode recording, therefore, it becomes possible to obtain lithographic printing plate precursors appropriate for the on-press development described above.
As a suitable method for production of a lithographic printing plate on a basis of heat-mode recording, one method has been put forth that a water-receptive image-forming layer provided on a water-receptive substrate is subjected to imagewise heat-mode exposure, and thereby the exposed area thereof undergoes a change in solubility or dispersibility and the non-exposed area thereof is removed by wet-process development, if needed.
However, the hitherto known printing plate precursors for heat-mode recording have another big problem that the non-image areas thereof are liable to generate scumming or the image areas thereof are low in mechanical strength. In other words, it is necessary to overcome a drawback that the image-forming layer has a smaller solubility change in the part near to the substrate than in the part near to its surface when subjected to heat-mode exposure. More specifically, heat generated by heat-mode exposure in a printing plate precursor of heat-mode recording system is based on light absorption by a light absorbent in the recording layer of the printing plate precursor, so that the quantity of heat generated is great in the surface part of the recording layer and small in the vicinity of the substrate and, therefore, the extent of a change in solubility of the recording layer becomes relatively small in the vicinity of the substrate. As a result, in the case of negative-working printing plate precursors of heat-mode recording type, removal of exposed areas to fundamentally provide a hydrophobic ink-receptive layer has frequently occurred during development and/or in the process of printing. Such a removal of ink-receptive image areas of negative-working printing plate precursors produces a deterioration in printing durability. This problem becomes worse in particular when a metallic sheet having high printing suitability and high thermal conductivity, such as an aluminum sheet, is used as substrate, because thermal diffusion is promoted due to high thermal conductivity of such a substrate and thereby the temperature rise in the vicinity of the substrate is further hindered. In order to achieve a sufficient solubility change in the vicinity of a substrate, it was necessary to apply extremely high exposure energy or carry out after-treatment, such as heating after exposure.
For instance, Japanese Patent 2,938

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