A flexographic plate is a type of relief printing plates and generally includes an elastic relief plate made of rubber or photosensitive resin, to which a liquid ink is applied for printing. Flexographic plates can print on rough or curved surfaces and are thus widely used for printing images on wrapping, magazines, cardboards, labels,  and bottles. Flexographic plates were previously manufactured by pouring molten rubber in a mold and then curing the rubber or by manually carving a rubber plate. Neither of these techniques was suitable for producing accurate flexographic plates, however. Lately, the development of a new technique that uses curable resins to make flexographic plate materials has made the production of flexographic plates considerably simple.
A typical flexographic plate material of the newly developed type includes, from top to bottom, a surface protective layer; a layer of a curable resin composition that is curable by irradiating with an active energy ray and is composed of an elastomer, such as urethane rubber, butyl rubber, silicon rubber and ethylene propylene rubber, an ethylenic unsaturated compound and, if necessary, a photopolymerization initiator; an adhesive layer; and a substrate (See, for example, “Kankosei jushi no kiso to jitsuyo (Basics and applications of photosensitive resins)” Supervised by Kiyoshi Akamatsu, CMC Co. Ltd. (2001) 152-160).
In one process for producing a flexographic plate from such a flexographic plate material, a film carrying a negative image of a letter, diagram, picture, pattern, or any other image to be printed is first applied to the surface of the protective film opposite to the substrate. The negative film is then irradiated with an active energy  ray from above, so that the predetermined areas of the curable resin composition layer are selectively cured by the action of the active energy ray transmitted through the imaged area of the film and become insoluble to solvent. Subsequently, the negative film and the protective film are removed and a solvent is applied to remove the non-irradiated or uncured areas of the curable resin composition layer (development step) and thereby form an image area (i.e., image plate surface). This completes a flexographic plate (See, for example, “Kankosei jushi no kiso to jitsuyo (Basics and applications of photosensitive resins)” Supervised by Kiyoshi Akamatsu, CMC Co. Ltd. (2001) 152-160; Japanese Patent Publication No. S55-34415; U.S. Pat. No. 4,323,636; Japanese Patent Publication No. S51-43374; and Japanese Patent Application Laid-Open No. H2-108632).
In an effort to ensure formation of fine dots and lines on the flexographic plates and prevent chipping of the image plate surface during development, improvements have been made as to the type and proportion of the resin to be added to the curable resin composition. One example involves the use of a styrene-based block copolymer in which the part of the copolymer formed of a conjugated diene has a significant bound vinyl content (See, Japanese Patent Application Laid-Open No. H5-134410). In another example, a certain thermoplastic elastomer composed of a  monovinyl-substituted aromatic hydrocarbon and a conjugated diene is used in conjunction with a diene-based liquid rubber that has a high average proportion of bound vinyl units (See, Japanese Patent Application Laid-Open No. 2000-155418).
The technique described in Japanese Patent Publication No. S55-34415 employs crystalline 1,2-polybutadiene in conjunction with a polymer compound, such as polyisoprene rubber, that comprises as its constituents at least one of ethylene, butadiene and isoprene. A drawback of this technique is that the uncrosslinked rubber used in the resin makes the flexographic plate susceptible to deformation (cold flow) during storage or transportation of uncured plates. U.S. Pat. No. 4,323,636 and Japanese Patent Publication No. S51-43374 describes the use of a certain block copolymer (preferably a styrene-isoprene-styrene triblock copolymer or a styrene-butadiene-styrene triblock copolymer having a particular composition) that is a thermoplastic elastomer and in which the hard segments have a grass transition temperature of 25° C. or above. In these techniques, the part of the copolymer formed of polystyrene causes a cohesive force, which reduces deformation of uncured plates. However, the polystyrene blocks of the elastomer do not undergo crosslinking even when irradiated with an active energy ray, so that the uncrosslinked polystyrene  blocks causes a poor solvent resistance of the cured area. Consequently, the image area tends to swell when a solvent is applied to remove the uncured area, resulting in insufficient reproducibility and poor ink resistance of the flexographic plates. Another flaw of this technique is that the strength and extension of the cured area are insufficient especially for forming a fine image area and the resulting flexographic plate becomes less durable. This causes chipping in the edge of the image plate surface during the removal of the uncured area by washing with a solvent and, when necessary, a brush. As a result, the desired sharp image plate surface may not be obtained.
In the technique described in Japanese Patent Laid-Open Publication No. H2-108632, the flexibility of flexographic plates is increased by the use of a binder (preferably, a styrene-butadiene-styrene triblock copolymer) containing thermoplastic and elastomeric domains, in combination with a particular addition polymerizable ethylenic unsaturated monomer. Despite its improved flexibility, the resin according to this technique includes some part formed of a polystyrene block similar to the one described above, which makes the flexographic plate less resistant to solvent. The cured area of the flexographic plate obtained by this technique is not strong enough to form a fine image area.
Although both of the techniques described in  Japanese Patent Application Laid-Open No. H5-134410 and No. 2000-155418 have managed to improve the curability of the part of the styrene-based thermoplastic elastomer formed of conjugated diene units and have managed to increase the toughness of the resulting flexographic plate, the similar polystyrene block part present in the thermoplastic elastomers suppresses the solvent resistance and the flexographic plates are not operative enough to form a fine image area.
Accordingly, it is an object of the present invention to provide a curable resin composition suitable for the production of a flexographic plate material that allows printing on an article with rough surfaces, such as cardboard and recycled paper. The curable composition of the present invention can be cured to form strong and extendable areas and can thus be used to make flexographic plates that can form a sharp image plate surface even for a fine image. It is also an objective of the present invention to provide a flexographic plate material that uses the curable resin composition as its constituent.