1. Field of the Invention
The present invention relates to a waterless planograhic printing plate capable of printing without using dampening water (hereinafter referred to as waterless printing plate), more particularly, to a waterless planographic printing plate which is usable for direct plate making and which is capable of directly making a printing plate using digital signals outputted from a computer or the like, and to a simplified plate making method by using the waterless printing plate.
2. Description of the Related Art
There have been known typographic printing, gravure printing, planographic offset printing methods and the like as conventional printing methods for performing printing. In recent years, printings using planographic printing plates have been increasing except for special printing fields. In the planographic printing, it has been known a printing plate using dampening water as an ink repellent component, in which a hydrophilic image pattern and a hydrophobic image pattern are formed on the surface of a printing plate, and a waterless printing plate without using dampening water, on the surface of which an ink repellent image pattern and an ink receiving image pattern are formed. The waterless printing plate has advantageous features such that (a) printing can be performed without skillfulness because dampening water is not required for printing, and (b) printing can economically performed in case that a small number of prints is required since the ink density is stable from the beginning of printing and an amount of spoilage is small, as compared with those of printing plates using dampening water.
As the computer technology develops, the plate making process which is a pre-printing process manually operated, has become digitally operated, and printing images has been processed as digital data. For this reason, in recent years, technologies capable of making a printing plate directly from digital image data without using a lithographic film (computer-to-plate technology) have been developed. Among these technologies, there have been known printing plate making technologies using dampening water. However, there have been known few waterless printing plate making technologies. An example in which plate making by using a waterless printing plate is performed by writing with laser beam has been known. In this method, the printing plate is exposed by a laser beam such as an infrared laser beam as a laser light source, and only the exposed area is heated by heat converted from the absorbed beam energy, so that the printing plate is subjected to local combustion and explosion reaction, thereby removing a silicone layer serving as an ink repellent layer, or facilitating removal of the silicone layer. Japanese Patent Publication 42-21879 describes an earliest disclosure of this type of method in which an ink repellent silicone layer of a printing plate is locally removed by irradiating a laser beam thereto, thereby forming a local ink adhesive portion to make a planographic printing plate for use in printing without using dampening water. Further, Japanese Patent Application Laid-Open (JP-A) No. 50-158405 and European Patent No. 0573091 disclose a method for making waterless planographic printing plate in which a printing plate having a silicone rubber surface layer is irradiated with laser light from a YAG laser as an infrared laser and a portion irradiated by the laser is removed to make a planographic printing plate for use in printing.
International Patent Application WO-9401280 discloses a thermal transfer plate making method for a waterless planographic printing plate, in which a printing plate having a silicone layer, a cover sheet being superposed thereon, is irradiated by a laser beam and the silicone layer irradiated by the laser beam is transferred to the cover sheet, thereafter, the cover sheet is peeled off and removed from the plate together with the transferred silicone layer to make a printing plate for use in printing. The specification of this patent appliction describes a method in which a binder which can be decomposed by an acid serving as a catalyst generated from a compound which can generate the acid by applying heat, the binder being added to the silicone layer.
As mentioned above, there have been proposed several plate making methods through converting laser light to thermal energy. However, there are problems to be solved. The above-described waterless planographic printing plates are low sensitive to laser light so that a number of laser light sources for writing are required to compensate the low sensitivity of the plates to achieve a practical recording speed. Consequently, the use of a number of laser light sources results in increase of cost of a writing apparatus. Accordingly, if the sensitivity of the printing plate to laser light, i.e., the sensitivity for writing is increased, the number of the laser light sources can be reduced and the cost of the writing apparatus can be reduces.
Further, the known waterless planographic printing plates above have drawbacks such that the edge sharpness of an image is deteriorated, depending on writing conditions. For example, in case that the output power of a laser source is increased in order to reduce irradiating time with laser light, the edge sharpness of an image is greatly deteriorated. Therefore, there has been an obstacle to reduce writing time.
Furthermore, in a system in which a known compound which generates an acid by the decomposition of the compound is used, the acid generating reaction and the heat decomposition reaction catalyzed by the acid cannot sufficiently proceed within a short period of time during radiation with laser light and the sensitivity of the plate is too low. Moreover, in order to improve separating characteristic between layers through the process of converting laser light to thermal energy, it has been necessary to employ an expensive infrared absorbing agent which is an acid generating agent, resulting in a high cost of printing plate. For example, Japanese Patent Application Laid-Open (JP-A) No. 63-88556 discloses a waterless planographic printing plate comprising: a photosensitive layer containing a compound which generates an acid by irradiating with light (light-acid generating agent), a compound which has a C--O--C bond decomposable by an acid and which changes its solubility by being decomposed, and a water soluble binder; and a silicone rubber layer through an intermediate layer containing amorphous silicic acid. After the printing plate has been exposed, the photosensitive layer is dissolved and removed by use of a developer, and at the same time, the silicone rubber layer thereon is removed to form an ink receiving portion. However, in this system, reactive compounds coexist in the photosensitive layer so that the storability of the printing plate is poor. Further, there have been various drawbacks such that processing of the plate is restricted to dissolving development which requires a solvent to remove the silicone rubber layer or the like, a simple separation development cannot be applicable to the system, and the printing plate is not sensitive to laser light in a longer wavelength region.