In general, the lithographic printing plate is constituted of a lipophilic imaging area to receive ink in the printing step and a hydrophilic non-imaging area to receive dampening water applied thereto prior to the inking step. For making such a lithographic printing plate, a presensitized plate (abbreviated as "PS plate" hereinafter) comprising a hydrophilic support and a ink-receptive photopolymer layer provided thereon has been widely used as a lithographic printing plate precursor. In a conventional method adopted therein for making the intended printing plate, mask exposure is generally carried out via a lith film, and then the non-imaging area is dissolved and removed with a developer.
In recent years, the technology to digitize image information has been widely spread, wherein the image information is electronically processed, stored and outputted by the use of a computer. And a variety of new image-output systems which can keep up with such digitization technology have become practical. Under these circumstances, it has been anxiously awaited to develop the computer-to-plate technology which enables the direct platemaking to be performed by scanning highly directional actinic rays, such as laser beams, corresponding to the digitized image information, but not using a lith film. And the production of printing plate precursors suitable for such technology has been one of important technical problems.
On the other hand, the conventional process of making a printing plate by the use of a PS plate necessitates a step of removing the non-imaging area by dissolution after exposure and, in general, further requires an after-processing step of washing the development-processed printing plate with wash water, a rinsing solution containing a surfactant or a desensitizing solution containing gum arabic and a starch derivative. Such an additional wet processing requirement has been recognized as leaving room for improvement. Lately in particular, consideration of global environment has been a matter of great concern of the whole industrial world. From the viewpoints of friendliness to the environment and streamlining the platemaking process accompanied by the digitization of image information, it has been desired more strongly than ever to render the processing steps for platemaking simple, dry or unnecessary.
With respect to the method of making a printing plate by means of scanning exposure, the utilization of actinic rays having high energy density, such as electron beams and high-output laser, has been proposed in addition to the utilization of a high speed photosensitive material. In recent years, it has become possible to get high-output solid laser devices, such as a semiconductor laser device and a YAG laser device, at low prices. As a result, the bright future of computer-to-plate systems utilizing such solid laser devices has come to be understood. The characteristic of high energy density exposure systems consists in that various phenomena, other than the photo reactions taking place in the low to medium energy density exposure-utilized photosensitive materials, can be applied to development. Specifically, not only a chemical change but also a structural change, such as change in phase or form, can be utilized for development. In general, such a high energy density exposure-utilized recording system is referred to as heat mode recording. This is because, in many high energy density exposure systems, it is believed that the energy of light absorbed by a photosensitive material is converted into heat, and the heat thus produced causes the intended development. The heat mode recording system has a great advantage in having potentialities for making the processing steps simple, dry or unnecessary. These potentialities are based on that the phenomena utilized for the image recording in a heat mode photosensitive material don't occur in a substantial sense under exposure to ordinary intensity of light or under temperatures of ordinary environment, so that no step for fixing images is required after exposure.
As a desirable method of making a lithographic printing plate on a basis of heat mode recording, a method proposal was advanced, wherein a precursor constituted of a water-receptive layer and an ink-receptive layer is subjected to heat mode exposure and only one layer of them is removed imagewise, thereby developing an imagewise difference between water-receptive and ink-receptive areas. This method can provide the precursor for printing plate showing relatively good printing properties in addition to the possibility of having scanning exposure suitability and rendering processing steps unnecessary or dry.
With respect to examples of such a lithographic printing plate precursor, JP-A-5-77574, JP-A-4-125189, U.S. Pat. No. 5,187,047 and JP-A-62-195646 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose using sulfonated polyolefin films as plate material requiring no development-processing and making printing plates through changes in hydrophilic properties of the film surface by thermal writing. More specifically, those systems form images through the desulfonation of sulfonic acid groups caused in the sensitive material surface by thermal writing.
In addition, U.S. Pat. No. 4,081,572 discloses the method of forming images through the dehydration ring closure caused in the polymers having carboxylic acid groups by exposure to heat or laser beams.
All those plate materials are hydrophilic films before exposure, but can be converted into hydrophobic ones by exposure. In other words, they are examples of the so-called polarity conversion negative press plate. The characteristic thereof is no need for development-processing.
However, the plate materials used in those conventional arts are lacking in thermal reactivity, so that it takes a long time to form images therein due to low sensitivity. Further, those materials have small discrimination between hydrophilic and hydrophobic areas, so that the printing plates made therefrom have nothing but insufficient water-receptivity or low image strength. In other words, sensitive materials which can afford satisfying sensitivity, scum resistance and press life cannot be obtained by those conventional arts.