PHOTOSENSITIVE COMPOSITION

Abstract: Described is a photosensitive composition comprising: (a) a vinyl alcohol polymer containing the following structural unit A and having a saponification degree of a vinyl ester unit of not less than 70 mol%: wherein R¹, R² and R³ each represents H or a hydrocarbon group having 1 to 4 carbon atoms; (b) a radically polymerizable ethylenic unsaturated compound; and, (c) a photopolymerization initiator, having flexibility and impact resistance which can maintain sufficient softness but are not broken particularly even at low temperature and low humidity so that resin plates showing high printing quality can be obtained from the photosensitive composition.

Description: The present invention relates to photosensitive compositions. Compositions comprising vinyl alcohol polymers, radically polymerizable ethylenic unsaturated compounds and photopolymerization initiators are termed photosensitive compositions and have been widely used as, for example, plate-making materials for letterpress plates. The aforesaid photosensitive compositions are formed into a sheet form (termed photosensitive resin plate) on a support such as a metal plate or film plate, then exposed to light and developed to make resin plate-making materials. Photosensitive compositions using vinyl alcohol polymers have been widely used in the printing art because unhardened regions can be readily dissolved out with water after exposure to make printing operation easy (Published Examined Japanese Patent Application Nos. 39401/71 and 3041/75, cf. US-A-3 801 328). As described above, currently used photosensitive compositions comprising vinyl alcohol polymers are advan­ tageous in many aspects and are used in large quantities as materials for pattern plates particularly because the surface hardness of plate-making resins is high. However, resin plates of vinyl alcohol polymer type are generally inferior in flexibility and impact resistance so that often the resin plates are broken upon printing particularly in the winter season when temperatures and humidity are low. Further due to great changes in circumstances of printing technology in recent years, high quality of printing characters is required also for vinyl alcohol polymer resin plates. An object of the present invention is to provide photosensitive compositions having flexibility and impact resistance which maintain sufficient softness but do not break particularly even under low temperature and low humidity and can thus give resin plates having high printing quality. This object can be achieved by providing photosensitive compositions comprising (a) a radically polymerizable ethylenic unsaturated compound; and, (b) a photopolymerisation initiators. The photosensitive compositions are characterized by further comprising (c) a vinyl alcohol polymer containing the following structural unit A and having an ester saponification degree of not less than 70 mol%: wherein R¹, R² and R³ each represents H or a hydrocarbon group having 1 to 4 carbon atoms. The structural unit A is more specifically shown by the formulae: The content is 1 to 50 mol%, preferably 2 to 30 mol%. As will be later described, the structural unit A dominates the softness of the resin plate obtained; with less than 1 mol%, the effect is extremely small and when it is introduced in an amount exceeding 50 mol%, the polymerization degree is greatly reduced and therefore, the aforesaid range is preferred. Those having a polymerization degree in a range of 50 to 2000 can be used without any problem but the polymerization degree of 100 to 1000 is more preferred. Furthermore, the ester saponification degree is not less than 70 mol% in view of dimensional stability of resin plates prior to photohardening. The PVA polymer used in present invention can be obtained by copolymerizing vinyl esters such as vinyl acetate, with monomers represented by the following general formulae and by saponifying the resulting copolymer : or, wherein R¹, R² and R³ each represents H or a hydrocarbon group having 1 to 4 carbon atoms, and R 12 represents a CH₃ group or a C₂H₅ group. Examples of such monomers include : of these, CH₂=C-CH₂OCOCH₃ and are advantageously used due to their good copolymerizability with vinyl esters such as vinyl acetate. In case that the monomers represented by the general formulae described above are copolymerized with vinyl esters such as vinyl acetate, monomers copolymerizable therewith can also be copolymerized in a small quantity unless the object of the present invention is injured. Examples of these monomers include α-olefins such as ethylene, propylene and isobutene; acrylic acid, metha­ crylic acid, crotonic acid, maleic acid, itaconic acid, maleic anhydride or, salts or alkyl esters thereof; acrylo­ nitrile, methacrylonitrile, acrylamide, methacrylamide N,N-­ dimethylacrylamide, alkyl vinyl ethers, N-vinylpyrrolidone, vinyl chloride, vinylidene chloride and vinyl versatate, but the monomers are not necessarily limited thereto. As the radically polymerizable ethylenic unsaturated compound used in the present invention invention, any monomer is usable as long as it is capable of radical polymerization in the presence of a photopolymeriza­ tion initiator. However, those having good compatibility with the PVA polymers containing the strutural unit A and showing an ester saponification degree of not less than 70 mol% and the photopolymerization initiators are preferably used. These radically polymerizable ethylenic unsaturated compounds are, for example represented by general formulae [I], [II], [III], and [IV] described below. Specific examples of compounds represented by general formula [I] described above are shown below. Of these, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 18, 20, 22, 23, 24 and 25 are more preferred (wherein n=1-20). Specific examples of compounds represented by general formula [II] described above are shown below. Of these, 28, 29, 30, 32, 33, 34, 35, 38 and 39 are more preferred (wherein n=1-20). Specific examples of compounds represented by general formula [III] described above are shown below. Of these, 40, 42, 44 and 46 are more preferred (wherein r=1-5). Specific examples of compounds represented by general formula [IV] described above are shown below. Of these, 50, 51, 52 and 53 are more preferred. In addition to those represented by general formulae [I], [II], [III] and [IV] described above, polyfunctional monomers such as trimethylol­ propane dimethacrylate, trimethylolpropane trimethacrylate, triacryl formal, triallyl cyanurate and triallyl isocyanurate can be used as the radically polymerizable ethylenic unsaturated compound, singly or in combination. These monomers are used singly or as admixture. Examples of the photopolymerization initiators which can be used in the present invention include benzophenone, benzoin, p,p′-dimethylbenzoin, benzoyl methyl ether, benzoin isopropyl ether, acyloin or a substituted aromatic acyloin or acyloin alkyl ethers, diacetyl, benzyl, keto­ aldonyl compounds, 1,1′-azocyclohexanecarbonitrile, azonitrile, 9,10-anthraquinone, chloroanthraquinone, 1,4-­ naphthoquinone on 9,10-phenanthrenequinone. In the photosensitive compositions of the present invention preferably used are (a) in amounts ranging from 20 to 250 parts by weight and 0.01 to 15 parts by weight of (b) based on 100 parts by weight of (c). Further components other than (a), (b) and (c), for example, unmodified vinyl alcohol polymers and polyamide polymers may also be used within such a range that does not injure the effect of the present invention. The photosensitive compositions of the present inven­ tion may also contain known thermal polymerization inhibitors or stabilizers during storage for purposes of improving stability during storage and, if necessary and desired, may also contain other additives such as colorants, pigments, plasticizers. The photosensitive compositions have flexibility and impact resistance and can sufficiently maintain their softness, but are not broken even at low temperature and low humidity and also give resin plates showing high printing quality. As has been described hereinbefore, the object can be achieved by the use of the vinyl alcohol polymer containing the aforesaid structural unit A and showing an ester saponification degree of not less than 70 mol%. It is assumed that the introduction of the structural unit A would increase the softness of the polymer itself and in addition thereto, cause improved change in compatibility with the polymerizable monomer to impart flexibility and impact resistance to the resin plate after photohardening and thus softness could be obtained. Hereafter the photosensitive compositions of the present invention will be described. with reference to the examples but is not deemed to be limited thereto, wherein "parts" and "%" are all "parts by weight" and "% by weight", unless otherwise indicated. Synthesis Example of Vinyl Alcohol Polymer (Polymer A in Table 1): In a reactor were charged 354 parts of vinyl acetate (hereafter merely referred to as VAc), 46 parts of allyl acetate (hereafter merely referred to as AAc) and 500 parts of methanol. After the content was sufficiently replaced with nitrogen, the external temperature was raised to 65°C. When the inner temperature reached 60°C, 100 parts of methanol containing 8.6 parts of 2,2′-azobisisobutyronitrile were added thereto to initiate polymerization. After the polymerization was continued for 5 hours (conversion to polymer, 51.2%), the reactor was cooled and the remaining VAc was distilled out of the system together with methanol, while supplementing methanol under reduced pressure. Thus, the methanol solution of (VAc-AAc) copolymer was obtained. A part of the methanol solution was poured into ethyl ether to precipitate the copolymer and the resulting copolymer was purified by repeated reprecipitation with ethyl ether from an acetone solution. With respect to the resultant copolymer, a polymerization degree ( p ) was determined by measuring [η] in acetone at 30°C and using the equation: [η] = 7.94 × 10 -3 × ( p ) 0.62 and ( p ) showed 250. Next, the methanol solution was added with a methanol solution of NaOH in a copolymer concentration of 50% and a molar ratio of [NaOH]/([VAc]+[AAc])=0.007 , whereby saponification was performed at 40°C for 1 hour to give the saponification product of (VAc-AAc) copolymer (hereafter referred to as Polymer A). From H¹-NMR (500 MHz), it was noted that the saponification product was composed of unit (simply referred to as VA), (simply referred to as VAc), unit (simply referred to as AA) and (simply referred to as AAc), wherein VA, VAc, AA and AAc were 72.7 mol%, 17.6 mol%, 7.6 mol% and 2.1 mol%, respectively. Therefore, the ester saponification degree was 80.3 mol%. Polymer B to Polymer E shown in Table 1 were synthesized in a similar manner. Examples 1 to 5 These components described above were homogeneously dissolved while stirring them in a flask with heating at 90°C. After defoaming, the solution was applied to an aluminum plate and a polyester film, which were dried at 80°C for 30 minutes to give photosensitive resin plates in a thickness of 0.5 mm. A negative was closely contacted with the resin plate obtained on the aluminum plate and irradiated with a mercury lamp of 2 kW having a wavelength of 320 to 400 mm for 3 minutes from a distance of 75 cm. After the irradiation, the unexposed portions were washed with water at 25°C for 5 minutes to observe dot reproducibility. On the other hand, the resin plate obtained on the polyester film was irradiated with a mercury lamp of 2 kW having a wavelength of 320 to 400 mm from a distance of 75 cm for 10 minutes to harden entirely. Thereafter the polyester film was peeled apart. Ten sheets of the resin plate were laminated and with respect to the resulting laminate, Shore hardness (by a modification of JIS K6301-­ 1975: [5.2] spring hardness test model A) was measured at 20°C and 37.5% RH. The results are shown in Table 2. Comparative Example 1 A test was performed in a similar manner as described above except that PVA-205 (polymerization degree of 550, ester saponification degree of 88 mol% manufactured by KURARAY CO., LTD.) was used as a vinyl alcohol polymer. The results are shown in Table 2.

Claims: 1. Photosensitive compositions comprising: (a) a radically polymerizable ethylenic unsaturated compound; and, (b) a photopolymerization initiator; characterized by further comprising (c) a vinyl alcohol polymer containing the following structural unit A and having an ester saponification degree of not less than 70 mol%: wherein R¹, R² and R³ each represents H or a hydrocarbon group having 1 to 4 carbon atoms. 2. Photosensitive compositions as claimed in claim 1 wherein R¹, R² and R³ each represents H or CH₃. 3. Photosensitive compositions as claimed in claim 1 wherein R¹, R² and R³ each represents H. 4. Photosensitive compositions as claimed in claim 1 wherein R¹ represents H, and R² and R³ each represents CH₃. 5. Photosensitive compositions as claimed in claim 1 wherein said radically polymerizable ethylenic unsaturated compound (a) is at least one selected from the groups of the following general formulae [I], [II], [III] and [IV] : 6. Photosensitive compositions as claimed in claim 1 comprising: (a) 20 to 250 parts by weight of the radically poly­ merizable ethylenic unsaturated compound; (b) 0.01 to 15 parts by weight of the photopolymeri­ zation initiator; and (c) 100 parts by weight of the vinyl alcohol polymer containing the structural unit A. 7. Photosensitive compositions as claimed in claim 1 wherein the content of said structural unit A is 1 to 50 mol%. 8. Use of the photosensitive compositions as claimed in claim 1 as plate-making material for letterpress plates.

Inventors: OKAYA, TAKUJI, SATO, TOSHIAKI, YAMAUCHI, JUNNOSUKE
Applicants: KURARAY CO. LTD.
Application year: 1986
Main classification: G03F7
IPC codes: G03F7