Conventionally, a rubber plate for use in flexographic printing has been produced by etching a metallic plate to produce an original plate, preparing a matrix plate from, e.g., a plastic by using the original plate, and pouring a rubber into the matrix plate, followed by pressing. However, this method has drawbacks in that numerous steps are needed, thus requiring too great a cost and too much time. In addition, this method also has drawbacks in that the produced rubber plate itself is low in thickness precision and, hence, it is necessary to raise the thickness precision by shaving the back side of the plate before use.
In order to solve this problem, there has recently been proposed a method of producing a flexographic printing plate directly from a photosensitive resin composition. According to this method, not only can the step for the above-mentioned shaving of the back side of the plate be eliminated, but also it is possible to obtain a printing plate capable of printing a pattern which is fine, as compared to a pattern which can be printed by the conventional rubber plate.
The production of a printing plate from such a photosensitive resin composition is performed by irradiating a photosensitive resin composition with actinic rays through a negative mask having a predetermined transparent pattern to thereby photocure only the resin portion exposed to rays in accordance with the transparent pattern, and then washing out the non-exposed portion with a developer, thereby forming a "relief portion" corresponding to the exposed portion and a "non-image portion" corresponding to the non-exposed portion. The thus formed relief portion is utilized as a relief of a printing plate.
A printing plate produced from a photosensitive resin composition, which plate is used in multicolor printing in many cases, has a disadvantage in that since the relief is transparent, the relief surface is difficult to distinguish, so that it is difficult to properly position a printing plate each time a printing plate is mounted onto the cylinder of a printing machine for printing an individual color.
As a method for alleviating this drawback, it is conceivable to simply color the photosensitive resin composition. By this method, there can be produced a difference in color strength between the relief portion and the non-image portion due to the difference in thickness therebetween. However, this method has drawbacks in that the color of the relief portion is essentially the same as the color of the non-image portion, so that only little effect can be attained with respect to the improvement of working efficiency and accuracy for positioning when the printing plate is mounted onto the cylinder of a printing machine. Thus, for greatly improving working efficiency and accuracy for positioning in fitting the printing plate to the cylinder of a printing machine, it is desired to provide a difference in color between the relief surface and the non-image portion. As a method for obtaining such a printing plate having different colors, reference is made to, for example, Japanese Patent Application Laid-Open Specification No. 58-125042, in which it is described that a mass of a photosensitive resin composition is inserted between a substrate and a multilayer cover element comprised of a cover film, a solvent-soluble polymer film and a photosensitive elastomeric composition containing a non-migratory dye having a contrasting color, and then subjected to calendering to thereby prepare a photosensitive structure, from which there can be obtained a printing plate having a relief surface portion (comprising a photocured elastomeric composition layer containing the non-migratory dye) having a color which is different from the color of. the lower portion. However, this method has a drawback in that the photocured elastomeric composition layer containing the non-migratory dye is likely to come off at its lower portion at the time of development, resulting in the formation of undesired bumps and dents on the relief surface. Thus, it is difficult to practically use the printing plate produced by this method.
On the other hand, Japanese Patent Application Laid-Open Specification No. 62-229127 describes a structure prepared by disposing a dyed layer comprising a photo-discolorable dye and a binder polymer on a photosensitive layer. In the above-mentioned structure, the provision of the dyed layer is intended to cause a portion of the dyed layer, which corresponds to an image portion, to discolor upon exposure, thereby facilitating the cutting of a non-image portion which is to be conducted before development. The selection of an appropriate type of dye makes it possible for the discolored dyed layer to resume its original color after development and drying. This prior art reference contains a description that a portion of the dyed layer which corresponds to the non-image portion does not undergo discoloration and, hence, remains low in light transmission, thereby being less affected by scattered light or the like, while a portion of the dyed layer which corresponds to the image portion is caused to be high in light transmission due to discoloration, thereby enabling formation of an image of high fidelity to the original image in the photosensitive layer. However, in this prior art, during the storage of an uncured plate after the preparation thereof before the actual use thereof for being exposed, the dye is likely to migrate from the dyed layer to the photosensitive resin composition, so that in a photocured plate produced therefrom both of the relief surface and the non-image portion are colored with the dye. Thus, a satisfactory contrast cannot be obtained between the relief portion and the non-image portion.
Further, Japanese Patent Application Laid-Open Specification No. 62-287234 describes a photosensitive resin layer obtained by uniformly incorporating a dye, which can discolor upon exposure to actinic radiation of ultraviolet or visible region and which can resume its original color upon development, into a composition comprising a polymer having rubber-like elasticity, an addition-polymerizable unsaturated monomer and a photopolymerization initiator. However, also in this case, the ultimate plate has its entire portion colored with the same color, so that a satisfactory contrast cannot be obtained between the relief portion and the non-image portion.
In this situation, the present inventor has made extensive and intensive studies with a view toward developing a method for obtaining a photocured resin product in which not only does a relief portion have a different color from the color of a non-relief portion, but also the contrast due to the color difference is stably maintained. As a result, it has been found that when an anthraquinone dye capable of exhibiting a color change to produce a fresh color upon being irradiated with an ultraviolet ray of a wavelength in the range of from 300 to 400 nm and of a quantity of larger than a threshold value in the presence of a radical producer capable of producing a free radical upon being irradiated with an ultraviolet ray having a wavelength in the range of from 300 to 400 nm, is incorporated into a photosensitive resin composition containing a radical producer and the photosensitive resin composition is irradiated with an ultraviolet ray of a quantity of larger than a threshold value through a negative mask having a transparent pattern, a photocured resin product is obtained having, formed therein, an image portion and a non-image portion which are different in color from each other and wherein the contrast between these portions due to the color difference is stably maintained. The present invention has been completed based on this finding.
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