Thermally sensitive recording medium

Abstract: A thermally sensitive recording medium comprising, on a substrate, one or more layers including a thermally sensitive recording layer which comprises an electron donating compound and an electron accepting compound, wherein at least one of said one or more layers comprises, as an ultra violet ray absorbing agent, at least one benzoxadinone compound of formula (1) wherein, R represents hydrogen, hydroxyl, C 1 -C 8 alkyl, C 1 -C 6 alkoxy, C 1 -C 6 acyl, C 1 -C 6 acyloxy, C 1 -C 6 alkoxy carbonyl, carboxyl, halogen or nitro and X represents a divalent group or a direct bond.

Description: The present invention relates to a thermally sensitive recording medium which uses benzoxazinone compound as an ultraviolet ray absorbing agent. Generally, in a thermally sensitive recording method, a recording image is obtained by the following procedure. A thermally sensitive layer mainly composed by colourless or pale coloured dye precursor which is ordinarily an electron donating compound and a colour developer which is an electron accepting compound is prepared on a substrate, and a recording image is developed by an instantaneous chemical reaction caused by heating said layer using a thermal head, a thermal pen or a laser beam. And these techniques are disclosed in Japanese Patent Publication 43-4160 and in Japanese Patent Publication 45-14039. These thermally sensitive recording media do not need procedures for development and fixing, and very clear images can be obtained by a simple apparatus. And, the apparatus is not only relatively cheap and compact, but also has merits of easy maintenance and does not generate noise, which is widely seen in a measuring recorder, a thermal printer of computer, a facsimile, an automatic ticket vender or a bar cord label. As an use of this thermally sensitive recording medium, recently the use for an indication medium such as an indicating label or a poster is becoming more popular. However, this kind of thermally sensitive recording medium does not have good stability against light and heat. Therefore, when the thermally sensitive recording medium is exposed to indoor light or sun light, or kept at a higher temperature for a long term, the ground colour changes to a yellowish colour and the appearance is deteriorated, further the stability of the recorded medium is spoiled. Consequently the qualitative image of the thermally sensitive recording medium is affected. To prevent the ground colour from changing to a yellowish colour, various methods which add fine ground particles of an ultra violet ray absorbing agent to a colour developing layer or to a protecting layer are published (for example, Japanese Patent Laid Open Publication 50-104650, Japanese Patent Laid Open Publication 55-55891, Japanese Patent Laid Open Publication 55-93492 and Japanese Patent Laid Open Publication 58-87093). However, in the case of this method which obtains sufficient light resistance by adding fine ground particles of an ultra violet ray absorbing agent to the thermally sensitive recording layer, it is necessary to add a large amount of fine ground particles of an ultra violet ray absorbing agent, because the ultra violet ray absorbing efficiency of and the shading efficiency of it are poor. When this kind of thermally sensitive recording medium is kept under high temperature conditions, new problems such as a background fogging or deterioration of recorded density is caused, and the properties of the thermally sensitive recording medium are spoiled. Further, when fine ground particles of ultra violet ray absorbing agent are contained in a protecting layer, and if the ultra violet ray absorbing agents are the conventional type, these agents have low melting points and can be easily molten by the heat of the thermal head, and result in the problem of fine refuse or sticking which spoil the thermal head for long term printing. Furthermore, the ultra violet ray absorbing agent is extracted from the protecting layer by the effect of plasticizer or oil, and consequently, the preserving ability of the recorded image is deteriorated. In Japanese Patent 61-57198, a method to add an ultra violet ray absorbing agent which is made water soluble by introducing a sodium sulfonate group to the thermally sensitive recording layer and/or protecting layer is proposed. However, this method not only has a problem affecting pH or fluidity of solution at the production stage, but also causes problems of ground colour developing and the thermal head spoiling. This method can not be said as a practical method. Further, the method to obtain a thermally sensitive recording medium which has good preserving stability and light resistance by containing micro capsules of ultra violet ray absorbing agent is disclosed in Japanese Patent Laid Open Publication 5-155134. However, in this method, the preparation of microcapsules in which fine particles of ultra violet ray absorbing agent are immobilized is necessary, and to improve the absorbing efficiency of ultra violet rays it is necessary to make the size of the microcapsules smaller. However, the size of the microcapsules is restricted by the preparation method. Therefore, for the generation of sufficient light resistance, the necessary amount of the microcapsules that contain ultra violet ray absorber is increased and the cost of the thermally sensitive recording medium is increased. Further, since the microcapsule in which ultra violet absorbing agent is immobilized must be stabilized to heat and pressure, the wall thickness of the microcapsule must be thicker, and this increases costs. Recently, a thermally sensitive recording medium which uses an ultra violet absorbing agent prepared by polymerization of a benzotriazole molecule or a benzophenone molecule which has an ultra violet ray absorbing ability, is proposed in Japanese Patent Laid Open Publication 7-314894, Japanese Patent Laid Open Publication 9-221487, Japanese Patent Laid Open Publication 9-268183, Japanese Patent Laid Open Publication 9-314496, Japanese Patent Laid Open Publication 10-71770 (equivalent to EP-A-0826511) and Japanese Patent Laid Open Publication 10-36371. But; the improvement effect for refuse and sticking by use of the compounds disclosed in these documents is not sufficient. As mentioned above, the method to provide a light resistance to a thermally sensitive recording medium by an ultraviolet absorbing agent has been carried out for long time, however, a sufficient method is not yet developed. EP-A-0068327 provides a photo-stabilised polymer composition comprising a polymer and an ultraviolet absorber which may be chosen from cyclic imino esters, The polymer composition is used in the preparation of a moulded article or an extruded film. The object of the invention is to provide a thermally sensitive recording medium which has a good light resistance at the recording part and the ground part, a good runnability, and which is good at image preservation ability, has resistance to plasticizer, oil or solvent, without spoiling the basic properties of a thermally sensitive recording medium. The inventors of the present invention have conducted an intensive study and have found that the thermally sensitive recording medium which has a good resistance to light in the recording part and a good resistance to yellowish colour change of the ground part can be obtained by using a benzoxazinone compound represented by the following general formula (1) as an ultraviolet ray absorbing agent in a thermally sensitive recording medium which applies colour developing reaction between an electron donating compound and an electron accepting compound. In the formula, R represents hydrogen atom, hydroxyl group, alkyl group of carbon number 1-8, alkoxy group of carbon number 1-6, acyl group of carbon number 1-6, acyloxy group of carbon number 1-6, alkoxy carbonyl group of carbon number 1-6, carboxyl group, halogen atom or nitro group. Especially, hydrogen atom, hydroxyl group, alkyl group of carbon number 1-4, alkoxy group of carbon number 1-4, acyl group of carbon number 1-4, alkoxy carbonyl group of carbon number 1-4 or halogen atom are desirable. X represents a divalent group or a direct bond, and desirably is an aromatic hydrocarbon of carbon number 6-12. Additionally, the inventors of this invention have found that the following problems which occurs when a conventional ultra violet ray absorbing agent is used can be prevented by use of this benzoxadinone compound. That is, the ground colour interference when blended in the recording layer, the deterioration of running recording ability by the generation of refuse and sticking when blended in the protecting layer and the deterioration of resistance to plasticizer or oil can be prevented. The benzoxadinone compound of Formula (I) is not easily molten by heating with a thermal sensitive head, because the compound has a high melting point. Therefore, the thermal sensitive recording medium which contains the compound of Formula (I) can prevent perfectly the generation of refuse and sticking. Further, the solubility of the benzoxadinone compound is very poor. Therefore, the thermally sensitive recording medium containing the compound of Formula (I) has an advantage that when it is contacted with plasticizer or oil, the compound does not ooze out from the protecting layer. The benzoxadinone compound of Formula (I) and the thermally sensitive recording medium containing said compound are illustrated in detail as follows. The benzoxadinone compound represented by above mentioned general formula (1) can be prepared by well known synthetic methods, for example by the method disclosed in Japanese Patent Laid Open Publication 58-194854. Namely, anthranitic acid derivatives represented by following general formula (2) and halogenated dicarboxylic acid represented by following general formula (3) are stirred or heated and stirred together with refluxing in an aqueous solution of an inorganic alkaline compound such as sodium carbonate for several minutes or several hours at room temperature, and bisamidecarboxylic acid is obtained. The obtained compound is reacted with acetic anhydride, then the benzoxadinone compound represented by the above mentioned general formula (1) can be prepared. In the formula, R represents hydrogen atom, hydroxyl group, alkyl group of carbon number 1-8, alkoxy group of carbon number 1-6, acyl group of carbon number 1-6, acyloxy group of carbon number 1-6, alkoxy carbonyl group of carbon number 1-6, carboxyl group, halogen atom or nitro group. Especially, hydrogen atom, hydroxyl group, alkyl group of carbon number 1-4, alkoxy group of carbon number 1-4, acyl group of carbon number 1-4, alkoxy carbonyl group of carbon number 1-4 or halogen atom are desirable. In the above mentioned general formula, X represents a divalent group or a direct bond, and is desirably an aromatic hydrocarbon of carbon number 6-12. As specific examples of the benzoxazinone compound represented by above mentioned general formula (1), the following compounds can be mentioned. However it is not intended to be limited to them, and these compounds can be used alone or can be used in combination. 2,2'-bis(4H-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-propyl-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-acethyl-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-propionyl-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-bis(4H-7-chloro-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-nitro-3,1-benzoxazin-4-one), 2,2'-bis(4H-6-carboxy-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-propyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-ethoxy-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-acethyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-propionyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-7-chloro-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-p-phenylenebis(4H-6-nitro-3,1-benzoxazin-4-one), 2,2'-p-phenylenbis(4H-7-methoxycarbonyl-3,1-benzoxazin-4-one), 2,2'-p-phenylenbis(4H-7-carboxy-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-ethoxy-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-acetyl-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-propionyl-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-7-chloro-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-nitro-3,1-benzoxazin-4-one), 2,2'-(2,6-naphthylene)bis(4H-6-carboxy-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-ethyl-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-methoxy-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-ethoxy-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-acetyl-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-propionyl-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-acetoxy-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-chloro-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-7-chloro-3,1-benzoozazine-4-on), 2,2'-(4,4'-biphenylene)bis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-nitro-3,1-benzoxazin-4-one), 2,2'-(4,4'-biphenylene)bis(4H-6-carboxy-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-propyl-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-acetyl-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-propionyl-3, 1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-7-chloro-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-nitro-3,1-benzoxazin-4-one), 2,2'-ethylenebis(4H-6-carboxy-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-propyl-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-ethoxy-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-acetyl-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-propionyl-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-7-chloro-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-propylenebis(4H-6-nitro-7-cloro-3,1-benzoxazin-4-one) 2,2'-propylenebis(4H-6-carboxy-7-chloro-3,1-benzoxazin-4-one) 2,2'-cyclohexylenebis(4H-6-hydroxy-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-methyl-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-ethyl-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-methoxy-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-ethoxy-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-acetyl-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-propionyl-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-acetoxy-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-chloro-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-bromo-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-iodo-3,1-benzoxazin-4-one), 2,2'-cyclohexylenebis(4H-6-carboxy-3,1-benzoxazin-4-one), and 2,2'-cyclohexylenebis(4H-6-nitro-3,1-benzoxazin-4-one) can be mentioned. Especially, from the view point of ultra violet absorbing effect, and easily synthetic, 2,2'-p-phenylenebis(4H-3,1-benzoxazin-4-one) is desirably used. The application of the benzoxadinone compound obtained as above to the thermally sensitive recording medium is illustrated. In general, a thermally sensitive recording medium can be obtained by preparing a thermally recording layer containing an electron donating compound and an electron accepting compound on a substrate. In the present invention, it is possible to accumulate a laminated protecting layer composed mainly of water soluble or water dispersible binder on a thermally recording layer, an intermediate layer between said thermally sensitive recording layer and said protecting layer, an undercoating layer mainly composed of filler and binder between substrate and said thermally sensitive recording layer, and a back coating layer at the reverse side of the substrate. The combination of structure which composes said accumulated laminate layer can be voluntarily changed according to the required quality level, and conventional well-known techniques in the thermally sensitive recording medium field can be applied. The important point of the thermally sensitive recording medium of this invention is to contain at least one benzoxadinone compound represented by said general formula (1) in one layer of the above mentioned layers. By use of said benzoxadinone compound, the resistance to light of the recording part and the ground part of the thermally sensitive recording medium are remarkably improved. Especially, from the view point of an absorbing and a shading effect for ultra violet rays, it is desirable to prepare a protecting layer on the thermally sensitive recording layer and to add the compound of Formula (I) to this layer. Further, the benzoxadinone compound of Formula (I) has a good stability at high temperature, does not generate fine refuse and does not cause a sticking problem by melting, and therefore, good runnability can be obtained. When the benzoxadinone compound of Formula (I) is blended into a thermally sensitive recording layer, the background fogging of the recording part at high temperature conditions is very small. And when the compound of Formula (I) is blended into a protecting layer, since the compound is not dissolved in plasticizer, oil or organic solvent, even if the surface of the protecting layer is contacted with organic solvents, the deterioration of barrier function caused by oozing out of the ultra violet ray absorbing agent is not observed. That is, the remarkable improvement effect can be obtained on the preserving ability of image. When the benzoxadinone compound of Formula (I) is used in the thermally sensitive recording medium, it is desirable that the average particle size of the benzoxadinone compound is smaller than 2µm to obtain the desired effect, and more desirably to be smaller than 0.5µm. When the average particle size is smaller than 0.5µm, not only is the ultra violet ray absorbing effect remarkably improved, but also a more clear recorded image can be obtained, because the recorded image is not shaded if the compound is contained in the protecting layer. As the method to grind the average particle size of the benzoxadinone compound, the following method can be mentioned. That is, in general, water is used as the dispersing medium and dispersing agent and defoaming agent are added as needed, then the mixture is ground by stirring and pulverized by a ball mill, an attriter or a sand grinder. Especially, a sand grinder is preferably used, and the desirable average particle size of dispersed medium is smaller than 1mm. The benzoxadinone compound of Formula (I) can be used together with other kinds of well known ultra violet ray absorbing agent provided does not hurt the desired effect of the present invention. Further, it is desirable to use an ultra violet absorbing agent such as, nickel bis(octylphenyl)sulfide, [2,2'-thiobis(4-tert-octylphenolate)]-n-butylamine nickel, nickel-complex-3,5'-di-tert-butyl-4-hydroxybenzyl-phosphoric acid monoethylate, nickel dibutyldithiocarbamate and benzoate type quencher, or hindered amine type light stabilizer such as, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate can be used together with the benzoxadinone compound. As the thermally sensitive recording method which uses an electron'donating compound and an electron accepting compound, for example, a combination of dye precursor and colour developer, diazonium salt and coupler, transition element such as iron and chelate compound, aromatic isocyanate compound and imino compound can be mentioned. Since the combination of dye precursor and colour developer gives excellent results in colour density and recording sensitivity, the thermally sensitive recording medium which uses a colour developing reaction between dye precursor and colour developing agent is illustrated in detail below. As the dye precursor which is used in this invention, various kind of well known compound can be used. These compounds can be used alone or together, and preferably used concerning the use and the required properties. Specific examples are mentioned below. However, it is not intended to be limited to them. (Mark in parenthesis indicates commercialized name) (1) Triarylmethane compound 3,3'-bis(4-dimethylaminophenyl)-6-dimethylaminophthalide, [crystalvioletlactone, CVL] 3-(4-dimethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)phthalide, 3,3'-bis(2-(4-dimethylaminophenyl)-2-(4-methoxyphenyl)ethenyl)-4,5,6,7-tetrachlorophthalide [NIR-Black], 3,3'-bis(4-dimethylaminophenyl)phthalide [MGL], 3-(4-dimethylaminophenyl)-3-(1,2-dimethylindol-3-yl)phthalide, 3-(4-dimethylaminophenyl)-3-(2-phenylindol-3-yl)phthalide, 3,3'-bis(4-ethylcarbazol-3-yl)-3-dimethylaminophthalide, 3,3'-bis(1-ethyl-2-methylindol-3-yl)phthalide [Indolylred], 3,3'-bis(2-phenylindol-3-yl)-5-dimethylaminophthalide, tris(4-dimethylaminophenyl)methane [LCV], can be mentioned. (2) Diphenylmethane type compound 4,4-bis(dimethylamino)benzhydrylbenzylether, N-halophenyl-leucoauramine and N-2,4,5-trichlorophenylleucoauramine can be mentioned. (3) Xanthene type compound rhodamine B-anilinolactam, 3-diethylamino-7-dibenzylaminofluorane, 3-diethylamino-7-butylaminofluorane, 3-diethylamino-7-anilinofluorane [Green-2], 3-diethylamino-7-(2-chloroanilino)fluorane, 3-dibutylamino-7-(2-chloroanilino)fluorane [TH-107], 3-diethylamino-7-(3-trifluoromethylanilino)fluorane [Black-100], 3-diethylamino-6-methyl-7-anilinofluorane [ODB], 3-dibutylamino-6-methyl-7-anilinofluorane [ODB-2], 3-piperidino-6-methyl-7-anilinofluorane, 3-(N-isoamyl-N-ethylamino)-6-methyl-7-anilinofluorane [S-205], 3-(N-ethyl-N-tolylamino)-6-methyl-7-anilinofluorane, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane [PSD-150], 3-diethylamino-6-chloro-7-(β-ethoxyethylamino)fluorane, 3-diethylamino-6-chloro-7-(γ-chloropropylamino)fluorane, 3-cyclohexylamino-6-chlorofluorane [OR-55], 3-diethylamino-6-chloro-7-anilinofluorane, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-7-phenylfluorane can be mentioned. (4) Thiazine type compound benzoylleucomethyleneblue and p-nitrobenzoylleucomethyleneblue can be mentioned. (5) Spiro type compound 3-methylspirobinaphthopyrane, 3-ethylspirobinaphthopyrane, 3-benzylspirobinaphthopyrane and 3-methylnaphtho-(6'-methoxybenzo)spiropyrane can be mentioned. (6) Pentadiene type compound 1,1,5,5-tetrakis(4-dimethylaminophenyl)-3-methoxy-1,4-pentadiene and 1,1,5,5-tetrakis(4-dimethylaminophenyl)-1,4-pentadiene can be mentioned. As the developer to be used in this invention an inorganic acidic compound such as activated clay, attapulgite, colloidal silica and aluminum silicate, and a 4-hydroxybenzoic ester type such as benzyl 4-hydroxybenzoate, ethyl 4-hydroxybenzoate, n-propyl 4-hydroxybenzoate, isopropyl 4-hydroxybenzoate, butyl 4-hydroxybenzoate, a 4-hydroxy phthalic acid diester type such as, dimethyl 4-hydroxy phthalate, diisopropyl 4-hydroxy phthalate, dibenzyl 4-hydroxy phthalate, dihexyl 4-hydroxy phthalate, a phthalic monoester type such as, monobenzyl phthalate, monocyclohexyl phthalate, monophenyl phthalate, monomethylphenyl phthalate, a bishydroxyphenylsulfide type such as, bis(4-hydroxy-3-tert-butyl-6-methylphenyl)sulfide, bis(4-hydroxy-2,5-dimethylphenyl)sulfide, bis(4-hydroxy-5-ethyl-2-methylphenyl)sulfide, a bisphenol type such as, 3,4-bisphenol A, 1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)propane [bisphenol A], bis(4-hydroxyphenyl)methane [bisphenol F], 2,2-bis(4-hydroxyphenyl)hexane, tetramethyl bisphenol A, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,4-bis(2-(4-hydroxyphenyl)propyl)benzene, 1,3-bis(2-(4-hydroxyphenyl)propyl)benzene, 1,4-bis(4-hydroxylphenyl)cyclohexane, 2,2'-bis-(4-hydroxy-3-isopropylphenyl)propane, 1,4-bis(1-(4-(2-(4-hydxoxyphenyl)-2-propyl)phenyl)ethyl)benzene, a 4-hydroxyphenylarylsulfone type such as, 4-hydroxy-4'-isopropoxydiphenylsulfone [D-8], 4-hydroxy-4'-methoxydiphenylsulfone, 4-hydroxy-4'-n-propoxydiphenylsulfone, bishydroxyphenylsulfone type such as, bis(4-hydroxyphenyl)sulfone [bisphenol S], tetramethylbisphenol S, bis(3-ethyl-4-hydroxyphenyl)sulfone, bis(3-propyl-4-hydroxyphenyl)sulfone, bis(3-isopropyl-4-hydroxyphenyl)sulfone, bis(3-tert-butyl-4-hydroxy-6-methylphenyl)sulfone, bis(3-chloro-4-hydroxyphenyl)sulfone, bis(3-bromo-4-hydroxyphenyl)sulfone, 2-hydroxyphenyl-4'-hydroxyphenylsulfone a 4-hydroxyphenylarylsulfonate type such as, 4-hydroxybenzenesulfonate, 4-hydroxyphenyl-p-tolylsulfonate, 4-hydroxyphenyl-p-chlorobenzenesulfonate, a 4-hydroxybenzoyloxybenzoic ester type such as, benzyl 4-hydroxybenzoyloxybenzoate, ethyl 4-hydroxybenzoyloxybenzoate, normal-propyl 4-hydroxybenzoyloxybenzoate, isopropyl 4-hydroxybenzoyloxybenzoate, butyl 4-hydroxybenzoyloxybenzoate, a benzophenone type such as, 2,4-dihydroxybenzophenone, α,α'-bis-(3-methyl-4-hydroxyphenyl)-m-diisopropylbenzoplenone, 2,3,4,4'-tetrahydroxybenzophenone, a phenolic compound such as, N-stearyl-p-aminophenol, 4-hydroxysalicylanilide, 4,4'-dihydroxydiphenylether, n-butyl-bis(hydroxyphenyl)acetate, α,α',α"-tris-(4-hydroxyphenyl)-1,3,5-triisopropylbenzene, stearyl gallate, 4,4'-thiobis(6-t-butyl-m-cresol), 2,2-bis(3-allyl-4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfide, bis(4-hydroxy-3-methylphenyl)sulfide, p-tert-butylphenol, p-phenylphenol, p-benzylphenol, 1-naphthol, 2-naphthol, a thio urea compound such as, N,N'-di-m-chlorophenylthiourea, an aromatic carboxylic acid such as, benzoic acid, p-tert-butyl benzoic acid, trichloro benzoic acid, 3-sec-butyl-4-hydroxy benzoic acid, 3-cyclohexyl-4-hydroxy benzoic acid, 3,5-dimethyl-4-hydroxy benzoic acid, terephthalic acid, salicylic acid, 3-isopropylsalicylic acid, 3-tert-butylsalicylic acid, 4-(2-(p-methoxyphenoxy)ethyloxy)salicylic acid, 4-(3-(p-tolylsulfonyl)propyloxy)salicylic acid and 5-(p-(2-(p-methoxyphenoxy)ethoxy)cumyl)salicylic acid, and a metallic salt of these aromatic carboxylic acids with a polyvalent metal such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin or nickel. Further, an organic acidic compound such as, antipyrene complex of zinc thiocyanate and complex zinc salt of terephthalaldehydic acid with an other aromatic carboxylic acid can be mentioned. These compounds can be used alone or can be used together. In general, in a thermally sensitive recording medium, for the purpose of improving the sensitivity, a sensitizer is used. In the thermally sensitive recording medium of this invention, a sensitizer can be added to the thermally sensitive recording layer as needed. Specific examples are mentioned below. However, it is not intended to be limited to them. These sensitizers can be used alone, or in combination: stearamide, methoxycarbonyl-N-stearamide, N-benzoyl stearamide, N-eicosamide, Ethylenebis stearamide, behenamide, methylenebis stearamide, methylolamide, N-methylol stearamide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, benzyl p-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthote, dibenzyl oxalate, di-p-methylbenzyl oxalate, di-p-chlorobenzyl oxalate, 2-naphthylbenzylether, m-tarphenyl, p-benzylbiphenyl, tolylbiphenylether, di(p-methoxyphenoxyethyl)ether, 1,2-di(3-methylphenoxy)ethane, 1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane, 1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane, 1-(4-methoxyphenoxy)-2-(2-methylphenoxy)ethane, p-methylthiophenylbenzylether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, di(biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene and 1-isopropylphenyl-2-phenylethan can be mentioned. In general, from 0.1 to 10 weight parts of these sensitizer can be used to 1 weight part of dye precursor. To the thermally sensitive recording medium of this invention, a preserving stabilizer can be added for the stabilization at the preservation. As specific examples of the preserving stabilizer, a hindered phenol compound such as 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 4,4'-butylidenebis(2-tert-butyl-5-methylphenol), 4,4'-thiobis(2-tert-butyl-5-methylphenol), 2,2'-thiobis(6-tert-butyl-4-methylphenol) and 2,2'-methylenebis(6-tert-butyl-4-methylphenol), 4-benzyloxy-4'-(2-methylglycidyloxy)diphenylsulfone and sodium-2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate can be mentioned. In general, from 0.1 to 10 weight parts of these preserving stabilizer can be used to 1 weight part of dye precursor. As specific examples of a binder which may be added to each layer, a water soluble binder such as starch, hydroxyethyl cellulose, methylcellulose, carboxymethyl cellulose, gelatine, casein, acacia, polyvinyl alcohol, carboxy modified polyvinyl alcohol, acetoacetyl group modified polyvinyl alcohol, silicon modified polyvinyl alcohol, isobutylene-maleic anhydride copolymer alkaline salt, styrene-maleic anhydride copolymer alkaline salt, ethylene-maleic anhydride copolymer alkaline salt and styrene-acrylic acid copolymer alkaline salt, a latex such as styrene-butadiene copolymer, acrylonitrile-butadiene copolymer and methylacrylate-butadiene copolymer, a water dispersion binder such as urea resin, melamine resin, amide resin and polyurethane resin can be mentioned. At least one kind of these binders can be used in the region of from 15 to 80 weight % to the total amount of solid of thermally sensitive recording layer, protecting layer, intermediate layer and under layer. As a filler an inorganic filler such as activated clay, clay, calcinated clay, diatomaceous earth, talc, kaolin, calcinated kaolin, calcium carbonate; magnesium carbonate, barium carbonate, titanium oxide, zinc oxide, silicon oxide or aluminum hydroxide and organic filler such as urea-formadehyde resin, polystyrene resin and phenol resin can be used. To prevent the wearing out of the head and sticking, a thermal soluble substance can be added. As specific examples, animal wax such as beeswax or shellac wax, vegetable wax such as carnauba wax, mineral wax such as montan wax, petroleum wax such as microlite waxes, higher fatty acid amides such as higher fatty acid polyhydric alcohol ester and stearamide, higher fatty acid metal salt such as zinc stearate or calcium stearate, synthetic wax such as higher amine, condensed product of fatty acid and amine, condensed product of aromatic and amine, synthetic paraffin such as chlorinated paraffin, paraffin oxide, higher chain glycol, 3,4-epoxyhexahydrodialkyl phthalate, polyethylene or polyethylene oxide can be mentioned. Further, a dispersing agent such as sodium dioctylsulfosuccinate, a surface active detergent, a defoaming agent, a fluorescent brightening agent, a waterproof agent and a slipping agent can be added as needed. As a substrate of the thermally sensitive recording medium, paper such as high quality paper, middle quality paper and coated paper can be mainly used, and various kinds of nonwoven cloth, plastic film, synthetic paper and metal foil can be used, further the complex sheet of these materials can be used voluntarily. The thermally sensitive recording medium of this invention can be prepared by use of above mentioned materials by a conventional well known method. The preparation method of a coating for each layer of thermally sensitive recording medium is not restricted, and in general, water is used as a dispersing medium, a binder and filler or slipping agent which is added as needed are added to benzoxadinone compound of Formula (I) and mixed and stirred, thus the coating can be prepared. A method to obtain an aqueous coating by mixing dispersions containing a dye precursor or a colour developing agent which are prepared separately using a sand grinder, an attriter or a ball mill, or a method to obtain an aqueous coating after dye precursor or colour developing agent is immobilized in a microcapsule are well known. The mixing ratio of dye precursor and colour developer is decided according to the kind of dye precursor and colour developer and not restricted, however, from 1 to 50 weight parts, desirably from 2 to 10 weight parts of colour developer to 1 part of dye precursor is used. The ratio of the benzoxadinone compound of Formula (I) to be contained is desirably from 1 to 50wt%, more desirably from 10 to 40 wt% to the total amount of solid of thermally sensitive recording layer, protecting layer, intermediate layer and under coating layer. When the amount of the compound is smaller than 1 wt% to the total amount of solid, the light resistance of the recording part and the ground part is remarkably deteriorated, and when is bigger than 50 wt%, the viscosity of the coating increases on aging and is not suited as a coating. In the case where the compound is contained in a protecting layer, when the ratio is bigger than 50 wt%, the film forming ability of the protecting layer is affected and the image preservility is remarkably deteriorated. The forming method of each layer of the thermally sensitive recording medium is not restricted, and an air knife coating, a Vali bar blade coating, a pure blade coating, a rod blade coating, a Shore-dwell coating, a curtain coating and a dye coating can be used. For example, the coating for a thermally sensitive recording layer is coated over the surface of the substrate and dried up, then the coating for a protecting layer is coated over the thermally sensitive recording layer and dried up. The coating amount of the coating for a thermally sensitive recording layer is from 2 to 12g/m 2 , desirably from 3 to 10g/m 2 by dry weight, and the coating amount for an intermediate layer or a protecting layer is from 0.1 to 15g/m 2 , desirably from 0.5 to 7g/m 2 by dry weight. Further, it is possible to prepare a back coating layer at the back surface of the substrate of the thermally sensitive recording medium of this invention to improve the preserving ability. And after each layer is formed, a treatment to obtain flatness such as a super calendar can be carried out, or adhesion can be mounted over the back surface of the thermally sensitive recording medium to prepare an adhesive label. Furthermore, various well known techniques such as magnetic thermally sensitive layer, printing coating layer or thermally transferring recording layer can be applied to this invention. EXAMPLES AND COMPARATIVE EXAMPLES The present invention will be illustrated more clearly by Examples. However, it is not intended to be limited to them. In the Examples and Comparative Examples, "parts" and "%" indicates weight parts and wt%. <preparation of thermally sensitive recording medium> Example 1 (1) preparation of a thermally sensitive recording layer A solution (dispersion of colour developer) 4-hydroxy-4'-isopropoxydiphenylsulfone 6.0 parts 10% aqueous solution of polyvinyl alcohol 20.0 parts water 17.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. B solution (dispersion of dye) 3-dibutylamino-6-methyl-7-anilinofluorane 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. Then the dispersions are mixed in the following ratio and the coating is prepared. A solution 43.0 parts B solution 10.0 parts 30% aqueous dispersion of kaolin clay 10.0 parts The prepared coating is coated over one side surface of a 50g/m 2 substrate in a coating amount of 6.0g/m 2 , dried and the thermally sensitive recording layer is prepared. (2) preparation of a protecting layer (containing ultra violet absorbing agent) C solution (containing ultra violet absorbing agent). 2,2'-p-phenylenebis(4H-3,1-benzoxazin-4-one) 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. Then the dispersions are mixed in the following ratio and the coating is prepared. 10% aqueous solution of polyvinyl alcohol 26.2 parts aluminium hydroxide (50% aqueous dispersion) 6.0 parts C solution 10.0 parts water for dilution 48.1 parts 25% aqueous solution of fluorescent dye 0.1 parts The prepared coating is coated over the thermally sensitive recording layer in a coating amount of 3.0g/m 2 , dried and treated by super calendaring, then the thermally sensitive recording medium is obtained. Example 2 (1) preparation of a thermally sensitive recording layer (containing ultra violet absorbing agent) A solution (dispersion of colour developer) 4-hydroxy-4'-isopropoxydiphenylsulfone 6.0 parts 10% aqueous solution of polyvinyl alcohol 20.0 parts water 17.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. B solution (dispersion of dye) 3-dibutylamino-6-methyl-7-anilinofluorane 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. C solution (containing ultra violet absorbing agent) 2,2'-p-phenylenebis(4H-3,1-benzoxazin-4-one) 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. Then the dispersions are mixed in the following ratio and the coating is prepared. A solution 43.0 parts B solution 10.0 parts C solution 10.0 parts 30% aqueous dispersion of kaolin clay 10.0 parts The prepared coating is coated over one side surface of a 50g/m 2 substrate in a coating amount of 6.0g/m 2 , dried and the thermally sensitive recording layer is prepared. (2) preparation of a protecting layer The dispersions are mixed in the following ratio and the coating is prepared. 10% aqueous solution of polyvinyl alcohol 60.0 parts aluminium hydroxide (50% aqueous solution) 50.0 parts zinc stearate 10.0 parts water for dilution 50.0 parts 25% aqueous solution of fluorescent dye 0.5 parts The prepared coating is coated over the thermally sensitive recording layer in a coating amount of 3.0g/m 2 , dried and treated by super calendaring, then the thermally sensitive recording medium is obtained. Example 3 (1) preparation of a thermally sensitive recording layer A solution (dispersion of colour developer) 4-hydroxy-4'-isopropoxydiphenylsulfone 6.0 parts 10% aqueous solution of polyvinyl alcohol 20.0 parts water 17.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. B solution (dispersion of dye) 3-dibutylamino-6-methyl-7-anilinofluorane 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. Then the dispersions are mixed in the following ratio and the coating is prepared. A solution 43.0 parts B solution 10.0 parts 30% aqueous dispersion of kaolin clay 10.0 parts The prepared coating is coated over one side surface of a 50g/m 2 substrate in a coating amount of 6.0g/m 2 , dried and the thermally sensitive recording layer is prepared. (2) preparation of an intermediate layer (containing ultra violet absorbing agent) The dispersions are mixed in the following ratio and the coating is prepared. 2,2'-p-phenylenebis(4H-3,1-benzoxazin-4-one) 3.0 parts 10% aqueous solution of polyvinyl alcohol 5.0 parts water 2.0 parts The mixture of the above mentioned compounds is ground to an average particle diameter of 1µm by a sand grinder. The obtained coating is coated over the thermally sensitive recording layer in a coating amount of 3.0g/m 2 , dried then the intermediate layer is formed. (3) preparation of protecting layer 10% aqueous solution of polyvinyl alcohol 60.0 parts aluminium hydroxide (50% aqueous solution) 50.0 parts zinc stearate 10.0 parts water for dilution 50.0 parts 25% aqueous solution of fluorescent dye 0.5 parts The prepared coating is coated over the thermally sensitive recording layer in a coating amount of 3.0g/m 2 , dried and treated by super calendaring, then the thermally sensitive recording medium is obtained. Example 4 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-chloro-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 5 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-7-chloro-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 6 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-methyl-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 7 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-acetoxy-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 8 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-bromo-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 9 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-iodo-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 10 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-7-nitro-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 11 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-p-phenylenebis(4H-6-carboxy-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Example 12 The thermally sensitive recording medium is prepared by the same procedure to Example 1, except that 2,2'-(4,4'-biphenylene)bis(4H-3,1-benzoxazin-4-one) is used instead of the ultra violet ray absorbing agent. Comparative Example 1 The thermally sensitive recording medium is prepared by the same procedure as Example 1, without using the ultra violet ray absorbing agent. Comparative Example 2 The thermally sensitive recording medium is prepared by the same procedure as Example 1, except that 2,2'-methylenebis[6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutylphenol) which is a conventional ultra violet absorbing agent is used instead of the ultra violet ray absorbing agent. Quality and ability measurements are carried out on the specimen obtained in the above mentioned Examples and Comparative Examples. The results are summarized in Table.1. (1) Colour developing ability : The density of image recorded by UBI Easy Coder 201 IIE (product of UBI Co., Ltd.) 0.26mj/dot is measured by Machbeth density meter umber filter. (2) Light resistance : Thermally sensitive recording medium printed by said condition is treated with a Xenon lamp weather meter for 24 hours, then the densities of the recording part and the ground part are measured by Machbeth density meter umber filter. (radiate illumination intensity : 67W/m 2 , integrating radiate illumination intensity : 5348kj/m 2 ) (3) Fine refuse stuck to head : The stain of the thermal head after printing by UBI EasyCoder 201 HE (product of UBI Co., Ltd.) 0.41mj/dot is evaluated by the naked eye of the inspector. Evaluation standard o ○ : no refuse is observed ○ : small amount of refuse is observed, but negligible × : much refuse is observed Specimen before test after test refuse stuck to head colour developed part ground part colour developed part ground part Example 1 1.46 0.09 1.32 0.10 o ○ Example 2 1.50 0.09 1.37 0.11 o ○ Example 3 1.48 0.09 1.38 0.10 o ○ Example 4 1.29 0.09 1.26 0.11 o ○ Example 5 1.41 0.09 1.29 0.11 o ○ Example 6 1.37 0.09 1.30 0.09 o ○ Example 7 1.51 0.09 1.39 0.09 o ○ Example 8 1.39 0.09 1.30 0.15 o ○ Example 9 1.29 0.09 1.17 0.09 o ○ Example 10 1.23 0.09 1.21 0.09 o ○ Example 11 1.49 0.09 1.35 0.11 o ○ Example 12 1.49 0.09 1.32 0.09 o ○ Co. Expl. 1 1.50 0.10 1.20 0.24 o ○ Co. Expl. 2 1.49 0.10 1.35 0.13 × Effect of the invention The thermally sensitive recording medium of the invention which uses benzoxadinone compound represented by general formula (1) has a good light resistance of the recording part and the ground part, does not change colour even if exposed to sunlight or to a high temperature atmosphere for a long time and the appearance is not hurt. Further, since the sticking of head refuse at recording action is controlled, the runnability is good. Furthermore, since it has a good image preserving ability such as resistance to plasticizer, it can be said to be a very useful thermal sensitive recording medium.

Claims: A thermally sensitive recording medium comprising, on a substrate, one or more layers including a thermally sensitive recording layer which comprises an electron donating compound and an electron accepting compound, wherein at least one of said one or more layers comprises, as an ultra violet ray absorbing agent, at least one benzoxadinone compound of formula (1) wherein, R represents hydrogen, hydroxyl, C 1 -C 8 alkyl, C 1 -C 6 alkoxy, C 1 -C 6 acyl, C 1 -C 6 acyloxy, C 1 -C 6 alkoxy carbonyl, carboxyl, halogen or nitro and X represents a divalent group or a direct bond. A recording medium according to claim 1 wherein the said one or more layers comprise one or more of a protecting layer, disposed on the thermally sensitive recording layer; an intermediate layer, between the protecting layer and the thermally sensitive recording layer; and an undercoating layer, between the substrate and the thermally sensitive recording layer. A recording medium according to claim 1 or 2 which comprises, in order on the substrate, an undercoating layer, the thermally sensitive recording layer, an intermediate layer and a protecting layer. A recording medium according to any one of the preceding claims, wherein the benzoxadinone compound of formula (1) is 2,2'-p-phenylenebis (4H-3,1-benzoxazin-4-one). A recording medium according to any one of the preceding claims, wherein the at least one benzoxadinone compound of formula (1) is present in the thermally sensitive recording layer, or in a protecting layer formed on said thermally sensitive recording layer, or in an intermediate layer formed between said thermally sensitive recording layer and said protecting layer. A recording medium according to any one of the preceding claims wherein the benzoxadinone of formula (1) has an average particle size of less than 2 µm. A recording medium according to any one of the preceding claims which further comprises an additional ultra violet ray absorbing agent other than a compound of formula (1). A recording medium according to any one of the preceding claims wherein the compound of formula (1) forms from 1 to 50% by weight of the total amount of solids in the thermally sensitive recording layer, the protecting layer, the intermediate layer or the undercoating layer. A recording medium according to any one of the preceding claims which further comprises a back coating layer on the reverse side of the substrate.

Inventors: DATE TAKASHI, OHASHI REIJI, RYU YUKIKO, DATE, TAKASHI, OHASHI, REIJI, RYU, YUKIKO, Date, Takashi, Nippon Paper Industries Co.,Ltd., Ohashi, Reiji, Nippon Paper Industries Co.,Ltd., Ryu, Yukiko, Nippon Paper Industries Co.,Ltd.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1999
Main classification: B41M5
IPC codes: B41M5, C08K5, C09K3