PHOTOSENSITIVE RESIN PRINTING MATERIAL FOR LETTERSET PRINTING

Abstract: A photosenstive resin printing material for letterset printing using, as a member for supporting a photosenitive resin layer, a steel plate which has a tensile strength of 57 kg/mm² or more, an elongation factor of 8% or less, and a thickness machine or easily removed therefrom, and which exhibits excellent smoothness on the printing surface, ex­ cellent picture reproducibility and excellent printing durabil­ ity.

Description: This invention relates to a photosensitive resin printing material for letterset printing. In particular, this invention relates to a photosensitive resin printing material for letterset printing which can provide a printing plate which can be easily handled e.g. in the plate making step, especially can be easily mounted on the cylinder of a rotary press machine and easily removed therefrom, and which is resistant to deformation at the plate surface. Accordingly, the material assists in keeping smoothness of the plate surface, and can provide prints with high image-reproducibility. Lithography has become a main technology for printing in place of a relief printing, but there remain to be solved some problems in lithography which are caused by dampening water used in the printing step. For instance, an emulsification of oily ink cannot be avoided. The resulting state of emulsion changes after start of printing and it takes a relatively long time till the state of emulsion can be stabilized, which results in formation of a lot of surplus printed papers. In particular, in a multi-colour process time is required for stabilizing the state of emulsion necessary for each printing of each colour and, accordingly, surplus printed papers are formed in an amount proportional to the numbers of colour. Another problem of lithography is that it cannot be used for obtaining prints which have a high level of dimension accuracy because the printing papers are deformed by dampening water used in the printing step. Recently an ink with greater oiliness has been developed and put into practice. Such an ink can reduce the above-mentioned defects caused by dampening water used in the printing step to some degree, but it has such a disadvantage that it needs a special type of printing paper. On the other hand, there is known the letterset printing which may be called relief offset printing, which comprises transferring an image onto a blanket and printing by using the imaged blanket. This letterset printing technology has the advantages of both being superior in printing durability and image reproducibility compared with lithography, and causing less surplus printing papers than the lithography does, and, therefore, has been put into practice. The conventional photosensitive resin printing plate used for letterset printing, however, has the following defect owing to the plate structure, and therefore has not been widely used compared with lithography. Namely, in the conventional photosensitive resin relief printing plate, the usual type of rolled steel plate or aluminum plate are used as a support for a photosensitive resin layer and this support is necessary to have a thickness more than 0.15 mm in order to support the photocured photosensitive resin which constitutes the image-transferred portions, that is the printing area, strongly and with a high stability and to maintain smoothness of the plate surface by preventing the plate from bending or breaking, and this photosensitive resin layer must have a thickness more than 0.2 mm in order to prevent the portions on which image is not transferred, that is the non-printing area, from being stained with ink. Accordingly, the total thickness of the plate becomes more than 0.35 mm (0.15 mm + 0.20 mm) and such a plate can be said to be too thick compared with the lithography plate which generally has a thickness less than 0.35 mm. That is why the photosensitive resin relief printing plate is difficult to mount on the conventional offset printing machine and needs a special type of cylinder if such a mounting is to be done, which constitutes a great obstacle when attempting to apply a photosensitive resin relief printing plate for letterset printing. Accordingly, a photosensitive resin relief printing plate is mainly used for direct printing. GB-A-1088535 describes printing plates for, inter alia , offset printing having photosensitive resin layers on aluminium, iron or mangnesium foil of thickness 0.05 to 5 mm, particularly aluminium foil of 0.4 mm thickness. An object of the present invention is to provide a photosensitive resin printing plate for letterset printing which can be applied to a conventional lithography offset printing machine without a substantial remodeling thereto except taking off water-supplying means. Another object of the present invention is to provide a photosensitive resin printing material for letterset printing which can provide a printing plate which is resistant to deformation on the surface thereof and can produce prints having a high image quality. The present invention lies in a photosensitive resin printing material for letterset printing which uses as a support for a photosensitive resin layer a steel plate which has a tensile strength of 57 kg/mm² or more, preferably more than 60 kg/mm² and has an elongation of 8 % or less, preferably less than 5 %, and has a thickness falling within the range of 0.03 mm to 0.12 mm, preferably 0.05 mm to 0.10 mm. The thickness of the photosensitive resin layer is in the range of 0.10 mm to 0.50 mm, preferably 0.15 mm to 0.45 mm. The thickness of the photosensitive resin printing material according to the present invention is preferably 0.35 mm or less. Thus by the present invention, a photosensitive resin printing material for letterset printing having the desired properties aimed at by the present invention has been provided by using the steel plate having both a specified property and a specified thickness as a support for photosensitive resin layer. In the conventional type of printing plate for letterset printing, there has been used a steel plate support for photosensitive resin layer having a tensile strength of 30 to 55 kg/mm² and an elongation more than 10 % and a thickness more than 0.15 mm, which are entirely different from those of the steel plate support of the present invention. In the present invention, the steel plate, has a tensile strength of 57 kg/mm² or more. If this tensile strength is less than 57 kg/mm², the plate cannot avoid unfavourable change in dimensions when the printing plate is mounted on a printing machine and while the printing is carried out. In the present invention, this support has an elongation of 8 % or less. If, the elongation is more than 8 %, the printing material tends to undergo deformation and to become uneven on the surface. Even if the plate has a tensile strength of 57 kg/mm² or more and an elongation of 8 % or less, the iron plate becomes too rigid to mount the printing plate on the cylinder if it has a thickness more than 0.12 mm. and the plate does not function as a sufficient support, namely becomes brittle if it has a thickness less than 0.03 mm. The object of the present invention cannot be attained by these both cases. By the present invention, there can be provided a letterset printing material having both the same thickness that an usual lithography printing material has, that is 0.24 mm or 0.30 mm, and superior properties. The printing material of the present invention can be applied to the printing according to the conventional sheet-fed offset press and offset rotary press. The mounting of the plate on the printing machine is carried out using a gripper bar system or an inserting and fitting system in the same manner as when using the lithography plate. A magnet fitting system using magnet cylinder is particularly favourable for the mounting of the plate of the present invention on the printing machine. As a photosensitive resin layer in the present invention, a conventional one can be used, and the composition obtained by adding photopolymerizable component, optical sensitizer, heat stabilizer, plasticizer and so on to the base polymer is usually used. As a base polymer there are used, for example, saponified polyvinylacetate, partially saponified polyvinylacetate, ethyleneoxide-modified resin of partially saponified polyvinylacetate, partially saponified vinylacetate/ethylene copolymer, solvent soluble polyamide, water soluble polyamide, polyetheresteramide, cellulose derivatives such as methylcellulose, ethylcellulose, hydroxyethylcellulose, and solvent soluble polyester. The photopolymerizable component used in the present invention is not restricted to any particular one. There can for example be used in the present invention any photopolymerizable component which has an ethylenic unsaturated bond and has an affinity to the base polymer or can be uniformly mixed with it. As an optical sensitizer these can be used any conventional one such as benzophenone type, benzyl type, benzylalkylketal type, acetophenone type, benzoinalkylether type, and anthraquinone type. There may be included in the photosensitive resin layer of the present invention the conventional additives such as heat stabilizer, plasticizer, surfactant, ultraviolet rays absorber, dyestaff, and pigment. Thickness of the photosensitive resin layer is preferably 0.10 mm or more. If the thickness is less than 0.10 mm, so-called scumming is caused by the adherence of ink to the hollows of the plate because the height difference between peaks and hollows of the plate becomes too small, and therefore good printing cannot be expected. And if the thickness of the plate is more than 0.50 mm, exposure shrinkage of the photosensitive resin layer becomes too large to be disregarded, which causes the plate to become curled to the direction of the plate surface and renders the plate difficult to mount on the plate-fitting device or cylinder. Accordingly, the thickness of the photosensitive resin layer of the present invention is preferably within the range of 0.10 mm to 0.50 mm, more preferably 0.15 mm to 0.45 mm, most preferably 0.20 mm to 0.40 mm. As an steel plate support for the photosensitive resin layer in the present invention there is used, for example, the non-annealing ultra-thin rolled steel plate which is generally called by zinc steel plate or tin steel plate which can be obtained by rolling zinc-plated or tin-plated steel sheet to the desired thickness. The preferred steel plate used in the present invention has a tensile strength of 60 Kg /mm² or more, an elongation of 5% or less, and a thickness within the range of 0.05 mm to 0.10mm. It is preferable that the surface of the support is treated for preventing it from getting rusty, for example, by means of plating treatment or the other chemical treatment, but the invention is not limited to this. In the present invention, the fixing between the photosensitive polymer layer and the support is usually carried out using an adhesive. Strong fixing between them can be attained using an adhesive selected from epoxy compound, epoxy resin, phenol resin, isocyanate resin, polyurethane, polyester, polyvinylacetate, or chloroprene rubber singly or a mixture thereof in a suitable ratio. A suitable film layer can be put on the surface of the photosensitive resin layer in order to prevent it from being damaged. As such a film there can be used, for example, polyester film, polypropyrene film, polyethylene film, and polyvinylidenechloride. Polyester film, especially polyethylene terephthalate film, is preferably used thanks to high dimension stability and superior oxygen permeability. If the matted film is used, it becomes easy to adhere the printing plate to a negative film in vacuum because the shape of the matted film is transferred onto the surface of the printing plate. Of course, the printing plate can be subjected to practical use without being covered with such a film. The photosensitive resin printing material of the present invention can be easily used as a letterset printing plate by being subjected to conventional exposure step and processing step. The most characteristic feature of the photosensitive resin printing material for letterset printing is that said photosensitive resin printing material not only can be easily mounted on and removed from the printing machine in the same manner as the lithography plate in spite of being a relief printing plate, but also is superior in image reproducibility and durability compared with the lithography plate printing, and, further, renders it possible to lessen surplus papers to a great degree compared with the lithography plate. Examples 1, 2 and Comparative Examples 1 to 3 The following steel plates (A) to (E) were coated with polyester/isocyanate type of adhesives in the amount sufficient to form a layer having a thickness of 0.02mm in a dry state : Chromium plate steel plate (A) (comparative example 1) tensile strength: 43 Kg/mm² elongation : 28.0% thickness : 0.20 mm Rolled steel plate (B) ( comparative example 2) tensile strength: 50Kg/mm² elongation : 15.3% thickness : 0. 05 mm Annealed zinc steel plate (C) (comparative example 3) tensile strength: 58Kg/mm² elongation : 12.0% thickness : 0.08 mm Zinc steel plate (D) (example 1) tensile strength: 72Kg/mm² elongation : 1.3% thickness : 0.05 mm Zinc steel plate (E) (example 2) tensile strength: 65Kg/mm² elongation : 1.7% thickness : 0.10 mm The adhesive-coated steel plates were kept for five minutes in an oven at 150 °C to remove the solvent. Thus, five supports (A) to (E) were prepared. Next, the photosensitive polymer solution was prepared by solving partially saponified polyvinylacetate having a saponifization degree of 75 mol% and an approximate polymerization degree of 600 into a mixture of ethanol and water (50/50 by weight ), and then adding to the resulting solution propylene glycol diglycidylether diacrylate as a photopolymerizable monomer, benzyldimethyl ketal as an optical sensitizer, and diethylene glycol as a plasticizer. The above adhesive-treated steel plates (A) to (E) were coated with the resulting solution in the amount sufficient to form a layer of 0.20mm in dry state and then kept in the oven of 50 °C for two hours to remove the solvent. Thus photosensitive resin printing materials (A) to (E) were obtained. On these photosensitive resin printing materials the negative film of usual type of account slip was adhered in vacuum and then the resulting products were subjected to exposure for two hours using an ultra-high pressure mercury lamp and then were subjected to processing by using a brush-type processer filled with neutral water until the non-cured photosensitive resin layer was completely resolved out. After that, the resulting plates were completely dryed in the oven at 50 °C for ten minutes. Five kinds of printing plate for letterset printing thus prepared were fitted on the conventional offset rotary press and the printing was carried out. The result is shown in Table 1. ○ mark described in the item "Evaluation" means that the product was superior in the corresponding property, while X mark means that the product has a defect in the corresponding property. From the result shown in Table 1 it has been substantiated that the printing plates obtained by using the supports (D) and (E) of the present invention were superior in the smoothness of the surface and could be easily mounted on and removed from the cylinder, and, further, from said printing plate there were obtained excellent prints. Example 3 to 6 and Comparative examples 4 and 5 As a base polymer there was employed water swelling copolyamide, that is, nylon 6/6· 6/PGDA · 6 (copolymerization ratio : 20/20/60) which was derived from adipic acid, hexamethylene diamine, and diamine obtained by cyanoethylation and hydrogenation of polyethyleneglycol (this diamine is abbreviated to PGDA). This base polymer was solved into a mixed solvent of ethanol and water (70/30 by weight) and, then, into the resulting solution there were added and mixed propyleneglycol diglycidylether acrylate and pentaerithritol triacrylate as photopolymerizable monomer and benzyl dimethylketal as photosensitizer. The same supports (A), (B), (D) and (E) as employed in the examples 1,2 and comparative examples 1,2, which have been already treated with an adhesive, were coated with the resulting polymer solution in the amount sufficient to have a thickness of 0.23mm after being dried and were dried for three hours in the oven of 50 °C. Further, the same support (E) of example 2, which has been already treated with an adhesive, was coated with the resulting polymer solution in the amount sufficient to have a thickness of 0.31 mm after being dried and was dried for five hours in the oven at 50 °C. From thus prepared photosensitive resin printing materials the letterset printing plates were obtained in the same manner as the examples 1, 2 and comparative examples 1 to 3, and then, the printing test thereof was carried out. The result is shown in Table 2. ○, X marks described in the item "Evaluation" mean that the product was superior and inferior respectively in the corresponding property. From these results it is apparent that the printing plates obtained using the supports (D), (E) of the present invention are excellent for letterset printing. THe photosensitive resin printing material for letterset printing of the present invention can be easily handled and is superior in property and can be applied effectively to letterset printing.

Claims: A photosensitive resin printing material for letterset printing which comprises a photosensitive polymer layer and a support therefor in the form of a metal plate having a thickness in the range of 0.03 mm to 0.12 mm, characterized in that said metal plate is a steel plate which has a tensile strength of 57 kg/mm² or more and an elongation of 8% or less and in that said photosensitive polymer layer has a thickness in the range of 0.10 mm to 0.50 mm. A printing material according to claim 1, wherein said steel plate has a tensile strength of 60 kg/mm² or more and an elongation of 5% or less. A printing material according to claim 1 or claim 2, wherein said photosensitive polymer layer has a thickness in the range of 0.20 mm to 0.40 mm.

Inventors: AKAMA, TADASHI, TAKAHASHI, KUNIYUKI, TANAKA, SEIZABURO
Applicants: TORAY INDUSTRIES, INC.
Application year: 1985
Main classification: G03F7
IPC codes: G03F7, B41N1