A thermally sensitive recording medium

Abstract: A thermally sensitive recording medium comprising, on a substrate, one or more layers including a thermally sensitive color developing layer which comprises a colorless or pale colored basic leuco dye and an organic color developer wherein at least one of said one or more layers comprises, as an ultraviolet ray absorbent, an aqueous emulsion type polymer which is an emulsion copolymer comprising (a) an ultraviolet ray absorbing monomer, (b) a vinyl compound monomer that can copolymerize with said monomer (a), (c) a hydrophilic monomer, and (d) a reactive emulsifier.

Description: The present invention relates to a thermally sensitive recording medium that has an excellent light resistance, and excels in the appearance of the unrecorded portion and color developing ability and further, prevents adherence of depositions and occurrence of sticking. In general, a thermally sensitive recording medium is obtained by the following procedure. Namely, a colorless or pale colored basic leuco dye and an organic color developer such as phenolic compound are respectively ground to fine particles and mixed together, a binder, a filler, a sensitizer, a lubricant and other additives are added, and a coating is obtained. The obtained coating is coated on a substrate such as paper, synthetic paper, film or plastic, thus a thermally sensitive recording medium is obtained. The thermally sensitive recording medium develops color by an instantaneous chemical reaction by heating with a thermal pen, a thermal head, a hot stamp or a laser ray, and a recorded image can be obtained. These thermally sensitive recording media are widely used in a field of a recorder of measuring equipment, a terminal printer of computer, a facsimile, a bar cord labeling machine and an automatic ticket vending machine. Recently, as the applications of a thermally sensitive recording medium, uses for various kinds of caption label, indication medium such as poster and ticket are becoming more popular. However, since the stabilities to light and to heat of these kinds of thermally sensitive recording media are low, occurrences of the following problems are pointed out. That is, when said thermally sensitive recording medium is exposed to room light or to sun light, or left in high humid conditions for the long term, the background color of said medium changes to yellowish color and the appearance is spoiled, or the stability of recorded image is deteriorated, therefore, the commodity image of the thermally sensitive recording medium is remarkably damaged. For the purpose of improving the light resistance of a thermally sensitive recording medium, a method to contain an inorganic filler that has an ultraviolet ray screening effect in a thermally sensitive recording layer or a protecting layer which covers said thermally sensitive recording layer is described in Japanese Patent Laid Open Publication 62-18626 or in Japanese Patent Laid Open Publication 6-64324. However, in the case of an inorganic filler that has an ultraviolet ray screening effect such as zinc oxide, titanium dioxide or cerium oxide, although the ultraviolet ray screening effect of these compounds is high, the transmittance of visible radiation is low. Therefore, the color developing part is concealed and the printed image density becomes low, and if the containing amount of these compounds is reduced to prevent the occurrence of the above mentioned problems, the screening effect for ultraviolet rays is deteriorated and becomes not practical. Further, in the case of cerium oxide, since cerium oxide itself has light yellowish color, it is not suited as a material for a thermally sensitive recording medium. Meanwhile, for the purpose of improving the light resistance, the method to contain fine ground particles of an ultraviolet ray absorbent in a thermally sensitive recording layer or a protecting layer which covers said thermally sensitive recording layer is disclosed (Japanese Patent Laid Open Publication 50-104650, 55-55891, 55-93492 and 58-87093). However, for the purpose of obtaining a sufficient light resistance by containing fine ground particles of an ultraviolet absorbent in a thermally sensitive recording layer, it is necessary to add a large amount of ultraviolet ray absorbent, because the absorbing effect and screening effect for ultraviolet rays are poor. Therefore, when these kinds of thermally sensitive recording media are kept in high temperature conditions, new problems such as background fogging or deterioration of the recorded image are caused, and the properties necessary for the thermally sensitive recording medium are damaged. Further, when fine ground particles of ultraviolet ray absorbent are contained in a protecting layer, and if the ultraviolet ray absorbents are the conventional type, these agents have low melting points and can be easily molten by the heat of a thermal head, and cause the problem of fine adherence of depositions or occurrence of sticking that stains the thermal head by long time printing. Furthermore, the ultraviolet ray absorbent is extracted from the protecting layer by the effect of plasticizer or oil, and consequently, the preserving ability of the recorded Image is deteriorated. And, in a case when the aqueous ultraviolet ray absorbent is used, as described in Japanese Patent Laid Open Publication 7-17131, since ions such as sodium salt are formed at the dissolving process of aqueous ultraviolet ray absorbent, the defect that the thermal head is damaged electrichemically is pointed out. Further, the method to obtain a thermally sensitive recording medium that has good preserving stability and light resistance by containing microcapsules of ultraviolet ray absorbent in a protective layer is disclosed in Japanese Patent Laid Open Publication 5-155134. However, in a case of this method, the preparation of microcapsule in which fine particles of ultraviolet ray absorbent are immobilized is necessary, and to improve the absorbing efficiency of ultraviolet rays, it is necessary to make the size of the microcapsule smaller. However, the size of the microcapsule is restricted by the preparation method and the size of the microcapsule has the smallest limit. Therefore, to obtain a sufficient light resistance, the necessary amount of the microcapsules that contain ultraviolet ray absorbent is increased, and the cost for the manufacturing of the thermally sensitive recording medium rises. Further, since the microcapsule in which ultraviolet ray absorbent is immobilized must be stabilized to heat and to pressure, the wall thickness of the microcapsule must be thicker, and it is one of the grounds of cost rising. Recently, a thermally sensitive recording medium, which uses an ultraviolet ray absorbent prepared by polymerization of benzotriazole molecule or benzophenone molecule which has an ultraviolet 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 and Japanese Patent Laid Open Publication 10-36371. Still more, the thermally sensitive recording medium containing an ultraviolet ray absorbent of an emulsified copolymer composed of an ultraviolet ray absorbing monomer, a vinyl compound monomer which can copolymerise with said monomer and a bridgeable monomer in a thermally sensitive recording layer is described in Japanese Patent Laid Open Publication 6-73368. However, when the thermally sensitive recording media in which the compounds disclosed in these publications for patent are used, various problems mentioned below are caused, and on ultraviolet ray absorbent which has a well balanced quality has not yet been obtained. Namely, problems of conventional ultraviolet ray absorbents for example; the appearance of unrecorded portion of thermally sensitive recording media are deteriorated because the color of materials itself is not so good; when used in a thermally sensitive layer, the recorded image density is deteriorated because it is necessary to add large amount of them to obtain a sufficient effect; and when used in a protective layer, the barrier ability reduces because these compounds are easily dissolved in plasticizer or oil; further the adherence of depositions or occurrence of sticking caused by the weak heat resistance, are pointed out. As mentioned above, the method to provide a light resistance to a thermally sensitive recording medium by an ultraviolet absorbent has been carried into effect for long time, however, a sufficient method has not been developed yet. The object of the present invention is to provide a thermally sensitive recording medium that has good appearance of unrecorded portion and color developing ability and further that the defects such as adherence of deposition or occurrence of sticking is prevented. The inventors of the present invention have conduced an intensive study and developed the thermally sensitive recording medium of this invention. That is, the invention of claim 1 is a thermally sensitive recording medium that possesses a thermally sensitive color developing layer containing colorless or pale colored basic leuco dye and an organic color developer as main components on a substrate, wherein said thermally sensitive recording medium contains at least one kind of aqueous emulsion type polymer ultraviolet ray absorbent composed of an emulsion copolymerized compound of (a) ultraviolet ray absorbing monomer, (b) vinyl compound monomer that can copolymerize with said monomer, (c) hydrophilic monomer and (d) reactive emulsifier, in the form of an inner bridged type polymer particle. And, the invention of claim 2 is the t erma y sensitive recording medium of claim 1, wherein the glass transition temperature Tg of said emulsion copolymerized compound is not less than 50°C. The invention of claim 3 is the thermally sensitive recording medium of claim 1 or claim 2, wherein said ultraviolet ray absorbing monomer which composes the aqueous emulsion type polymer ultraviolet ray absorbent is a compound represented by general formula I and/or general formulaII, wherein, R 1 represents hydrogen atom, alkyl group or alkoxy group of carbon number 1-6, R 2 represents alkylene group or oxyalkylene group of carbon number 1-10, or a direct bond (in this case X combines with benzene ring directly), X represents ester bond, amide bond, ether bond or urethane bond and R 3 represents hydrogen atom or lower alkyl group, wherein, R 4 represents hydrogen, halogen or methyl group, R 5 represents hydrogen or hydrocarbon group of carbon number 1-6, R 6 represents alkylene group of carbon number 1-10 or C 1-10 oxyalkylene group or a direct bond (in this case Y combines with benzene ring directly), Y represents ester bond, amide bond, ether bond or urethane bond, R 7 represents alkylene group of carbon number 1-8, aminoalkylene group, alkyl group possessing hydroxide group at side chain or not exists (in this case Y combines with C directly), and R 8 represents hydrogen or lower alkyl group. The invention of claim 4 is characterized in that the reactive emulsifier (d) is a polymerizable vinyl compound. As the (a) ultraviolet ray absorbing monomer, any kind of monomer having ultraviolet ray absorbing ability can be used. However, desirably (a) is a polymerizable vinyl compound composed of a chemically bonded product of a 2-hydroxybenzophenone group in which an alkyl group or an alkoxy group of carbon number 1-6 can be substituted or 2-hydroxybenzotriazole group in which a hydrocarbon group of carbon number 1-6 can be substituted. As a specific example of said polymerizable vinyl compound, compounds indicated by (1) or (2) can be mentioned. However it is not intended to be limited to these compounds, further these kinds of ultraviolet absorbing monomers can be used in combination. (1) 2-hydroxybenzophenone derivatives represented by general formula (I). wherein, R 1 represents hydrogen atom, alkyl group or alkoxy group of carbon number 1-6, R 2 represents alkylene group or oxyalkylene group of carbon number 1-10, or a direct bond in this case, X combines with benzene ring directly), X represents ester bond, amide bond, ether bond or urethane bond and R 3 represents hydrogen atom or lower alkyl group. The monomer represented by general formula (I) can be obtained, for example, by reacting an ultraviolet ray absorbing compound having a functional group such as BP-R 2 -OH (BP: 2-hydroxybenzophenone structure) with a polymerizative vinyl compound having a functional group such as CH 2 =CR 3 -COOH, then introducing an ultraviolet ray absorbing compound residue into polymerizative vinyl compound by means of ester bond X (-COO-).As specific examples of a monomer compound represented by said general formula (I), 2-hydroxy-4-acryloyloxybenzophenone, 2-hydroxy-4-methacryloyloxybenzophenone, 2-hydroxy-4-(2-acroyloxy)ethoxybenzophenone, 2-hydroxy-4-(2-methacryloyloxy)ethoxybenzophenone, 2-hydroxy-4-(2-methyl-2-acroyloxy)ethoxybenzophenone or others can be mentioned (2) 2-hydroxybenzotriazole derivatives represented by general formula (II), wherein R 4 represents hydrogen, halogen or methyl group, R 5 represents hydrogen or hydrocarbon group of carbon number 1-6, R 6 represents alkylene group of carbon number 1-10 or oxyalkylene group or a direct bond (in this case, Y combines with benzene ring directly), Y represents ester bond, amide bond, ether bond or urethane bond, R 7 represents alkylene group of carbon number 1-8, C 1-8 aminoalkylene group, alkyl group possessing hydroxide group at side chain or a direct bond (in this case, Y combines with C directly) and R 8 represents hydrogen or lower alkyl group. A lower alkyl group is a C 1 -C 6 , preferably C 1 -C 4 , alkyl group. It is, for example, methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, iso-butyl or tert-butyl.As specific examples of a monomer compound represented by general formula (II), 2-[2'-hydroxy-5'-(methacryloyloxy)phenyl]benzotriazole, 2-[2'-hydroxy-5'(acryloyloxy)phenyl]benzotriazole, 2-[2'-hydroxy-3'-t-butyl-5'-(methacryloyloxy)phenyl]benzotriazole, 2-[2'-hydroxy-3'-methyl-5'-(acryloyloxy)phenyl]benzotriazole, 2-[2'-hydroxy-5'-(methacryloyloxypropyl)phenyl]-5-chlorobenzotriazole, 2-[2'-hydroxy-5'-(methacryloyloxyethyl)phenyl]benzotriazole, 2-[2'-hydroxy-5'-(acryloyloxyethyl)phenyl]benzotriazole, 2-[2'-hydroxy-3'-t-butyl-5'-(methacryloyloxyethyl)phenyl]benzotriazole, 2-[2'-hydroxy-3'-methyl-5'-(acryloyloxyethyl)phenyl]benzotriazole, 2-[2'-hydroxy-5'-(methacryloyloxypropyl)phenyl]-5-chlorobenzotriazole, 2-[2'-hydroxy-5'-(acrytoyloxybutyl)phenyl]-5-methylbenzotriazole, [2-hydroxy-3-t-butyl-5-(acryloyloxyethoxycarbonylethyl)phenyl]benzotriazole or others can be mentioned. As the other vinyl compound monomer that can copolymerize with said ultraviolet ray absorbing monomer (b) (hereinafter shortened to copolymerization monomer), acrylonitrile, acrylic acid alkyl ester, methacrylic acid alkyl ester, alkyl vinyl ether, alkyl vinyl ester or styrene can be mentioned, and although the carbon number of these compounds is not restricted, desirable carbon number is 1-18. As specific examples of these compounds following compounds can be mentioned: (1) acrylic acid alkyl ester, methacrylic acid alkyl ester. methylacrylate, ethylacrylate, butylacrylate, 2-ethylhexylacrylate, octylacrylate, laurylacrylate, stearylacrylate, methylmethacrylate, butylmethacrylate, t-butylmethacrylate, 2-ethylhexylmethacrylate or others. (2) alkyl vinylether. methylvinylether, ethylvinylether, propylvinylether, stearylvinylether or others. (3) alkylvinylester vinylacetate, ethylvinyl, butylvinyl, 2-ethylhexylvinyl or others. Copolymerization ratio of copolymerization monomer is from 5 to 69 % by weight to total weight of monomer. In the present invention, by the use of these with a hydrophilic monomer (c), resistance to solvent and solubility with hydrophilic resin is further improved and a durable ultraviolet ray absorbing polymer film can be formed. As a hydrophilic monomer, although any kind of monomer which has a hydrophilic reactive functional group can be used, desirably a hydroxy group containing monomer or a carboxylic acid containing monomer can be mentioned. As specific examples, the following compounds can be mentioned. (1) Hydroxy group containing monomer: 2-hydroxyethylmethacrylate, 2-hydroxyethylacrylate, hydroxypropyl methacrylate or others. (2) Carboxylic acid containing monomer: acrylic acid, methacrylic acid or others. The desirable copolymerization ratio of a hydrophilic monomer is from 1 to 20% by weight to total monomer weight. When the ratio exceeds 20 wt%, polymerization becomes difficult because of the problem of copolymerization with an ultraviolet ray absorbing monomer. The aqueous emulsion type polymer ultraviolet ray absorbent used in this invention can be obtained by mixing each monomer with emulsifier in aqueous solvent and by emulsion plolymerization using a aqueous polymerization initiator. The emulsion copolymer desirably has a relatively high glass transition temperature (Tg), and when the Tg is not less than 50°C, good heat resistance of film (durability to sticking) can be obtained. Desirably the Tg is not less than 70°C and more desirably the Tg is not less than 80°C. In the present invention, by adjusting the monomer ratio for emulsion copolymerization, the aqueous emulsion type polymer ultraviolet ray absorbent whose Tg is not less than 50°C can be easily obtained. As the reactive emulsifier, any kind of conventional reactive emulsifier used in conventional emulsion polymerization involving radical polymerization can be used, However, a polymerizable vinyl compound is desirably used. These reactive emulsifiers can be used alone or in combination. Since these emulsifiers are introduced into the copolymer obtained at the emulsion polymerization, bad influences on the water resistance and on the transparency of film, which usually occur in non-reactive emulsifier after the formation of film, are effectively prevented. As specific examples of the reactive emulsifier, Latemul S-180, S-180A, S-120A (products of Kao Co., Ltd.), Aqualon HS-10, HS-20, RN-20 series (Daiichi Kogyo Seiyaku Co., Ltd.), Eleminor JS-2 (Sanyo Kasei Industries Co., Ltd.) and Adekaria soap NE-30 (Asahi Denka Industries Co., Ltd.) can be mentioned. From the viewpoint of color developing ability of the thermally sensitive recording medium, anion type reactive emulsifier is desirably used. The desirable amount of the reactive emulsifier to be used is from 0.5 to 5 weight parts to 100 weight parts of total monomer amount. If the amount of use is over than 5 weight parts, the effect on a basic leuco dye is too strong and can interfere with the color developing, and the water resistance of the film is weakened. Generally, conventional emulsifiers, which are used for the emulsification, surround and contact the whole surface of the emulsified material. Meanwhile, the reactive emulsifier used in this invention reacts with the emulsified material and is introduced into it, and does not remain in the original form of emulsifier. Therefore, the aqueous emulsion type polymer ultraviolet absorbent of this invention becomes soap free, and does not have a bad influence on the color developing material. As the polymerization initiator, peroxide such as persulfate, percarbonate or perborate, or azo type aqueous polymerization initiator such as 2,2'-azobis[2-(2-imidazoline-2-yl)propane]hydrochloride, 2,2'-azobis(2-methylpropionamidine)hydrochloride, 2,2'-azobis[N-(2-hydroxyethyl)-2-methylpropionamidine]hydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidine-2-yl)propane]hydrochloride and others can be used. Desirable amount of aqueous polymerization initiator is from 0.05 to 1 wt % to total weight of monomer. The shape of the emulsion copolymer that composes the aqueous emulsion type polymer ultraviolet ray absorbent used in this invention is an inner bridged type polymer particle, and the desirable average particle size is from 500nm to 80nm, preferably over 100nm and more preferably not less than 180nm. The upper limit of particle size is desirably smaller than 400nm and more desirably does not exceed 300nm. The aqueous emulsion type polymer ultraviolet ray absorbent of this invention can be obtained by emulsion polymerization using the above mentioned components and examples of the actual preparation method are mentioned below. (Preparation Example 1) 301g of monomer mixture composed of 1g of Aqualon HS-20 (product of Daiichi Kogyo Seiyaku Co., ltd.), 50g of 2-hydroxy-4-(2-methacryloxy)ethoxy benzophenone, 10g of acrylic acid 2-hydroxyethyl, 40g of methytmethacrylate and 200g of deionized water is prepared, and 100g of said mixture is poured into a glass reactor to which a thermometer, a stirrer, a reflux condenser, a nitrogen gas induction tube and a dropping funnel are attached and emulsified for 30 minutes at a temperature of 70°C. Then, as the aqueous polymerization initiator, an aqueous solution that is prepared by adding 0.5g of 2,2'-azobis[2-(2-imidazoline-2-yl)propane]hydrochloride to 33g of deionized. water is added to said glass reactor, then immediately after, the remaining amount of the monomer mixture is dropped into the glass reactor constantly over 90 minutes, and polymerization is carried out at a temperature of 70°C. After all of the monomer mixture is dropped, the content in the glass reactor is ripened for 90 minutes at a temperature of 70°C, thus the aqueous emulsion type polymer ultraviolet ray absorbent of Preparation Example 1 is obtained. Further, the aqueous emulsion type polymer ultraviolet ray absorbents of Preparation Examples 2-8 are obtained by changing an ultraviolet ray absorbing monomer, a hydrophilic monomer, a copolymerization monomer, a reactive emulsifier and aqueous polymerization initiator as mentioned in Table 1. The number in parenthesis appearring in the table indicates the amount to be used of each of the components. A constant amount of 50g of the ultraviolet ray absorbing monomer is used in each Preparation Example. In the present invention, by providing at least one kind of aqueous emulsion type polymer ultraviolet ray absorbent in at least one layer of a thermally sensitive recording medium, which is specified below, properties of the thermally sensitive recording medium such as light resistance, appearance of the unrecorded portion and color developing ability are remarkably improved, and defects such as adhering of depositions and occurrence of sticking are prevented effectively, in comparison with the thermally sensitive recording medium containing ultraviolet ray absorbents or the ultraviolet ray screening agents which are proposed in the prior art. The thermally sensitive recording medium of this invention preferably possesses a thermally sensitive recording layer containing a basic leuco dye and organic color developer on a substrate, further, possesses an under coating layer between said thermally sensitive recording layer and substrate, a protecting layer on the thermally sensitive recording layer and a middle layer between the thermally sensitive recording layer and a protecting layer when the need arises. When the thermally sensitive recording medium has a protecting layer and the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent is contained in said protecting layer, the barrier ability is remarkably improved besides above mentioned properties, in comparison with the thermally sensitive recording media that contain conventional ultraviolet ray absorbents. That is, the ultraviolet ray absorbing ability of the aqueous emulsion type polymer ultraviolet ray absorbent used in this invention is excellent because of the large amount of ultraviolet ray absorbing monomer contained in the polymer and the aqueous emulsion type polymer ultraviolet ray absorbent used in this invention provides good light resistance even with a small amount. Therefore, the defects such as deterioration of color developing density or background fogging caused by the use of a large amount of an ultraviolet ray absorbent can be prevented. Still more, by the use of reactive emulsifier, the effect on the color developing materials becomes very small, and good color developing ability and excellent appearance of unreacted portion can be obtained. Furthermore, by the use of hydrophilic monomer, since the solubility with aqueous binder is good and solubility with plasticizer, oil or organic solvent is poor, when it is contained in a protecting layer, the deterioration of the barrier function does not occur and resistance to chemicals is excellent. Still further, if the glass transition temperature (Tg) is high, heat resistance is improved and even if it is contained in a protecting layer, defects, which are observed at the recording process, such as adhering of depositions and occurrence of sticking caused on a thermal head, do not occur. That is, also the recording runnability is improved. Yet further, in general, in the case where an ultraviolet ray absorbent is used in a thermally sensitive recording medium, when an ultraviolet ray absorbent of larger average particle size is used, a problem of recorded image fogging (the state that the recorded image becomes white, cloudy or hazy) can readily occur, and therefore, it is said that the smaller particle size is better. However, in a case of an ultraviolet ray absorbent of aqueous emulsion type polymer used in this invention, even if an ultraviolet ray absorbent of relatively large particle size is used, a clear recorded image in which the problem of recorded image fogging is dissolved, can be obtained. Although the reason why is not made clear yet, since the aqueous emulsion type polymer ultraviolet absorbent of this invention contains hydrophilic monomer as an important ingredient, it is guessed that said aqueous emulsion type polymer ultraviolet absorbent has good compatibility with the aqueous binder and indicates good transparency when film is formed. In the present invention, by containing the above mentioned special aqueous emulsion type polymer ultraviolet ray absorbent in a thermally sensitive recording layer, or, in an under coating layer, a middle layer or a protecting layer, which are prepared as needed, the desired effect can be obtained. Said special aqueous emulsion type polymer ultraviolet ray absorbent can be contained in one layer or two or more layers, and the combination of layers is not restricted. Further, the number of layers of thermally sensitive layer, undercoating layer, middle layer and protecting layer is not respectively restricted to only one layer. Each layer can be multiply prepared. The adding amount and the position (layer) of ultraviolet ray absorbent to the thermally sensitive medium depend on the required qualities of the thermally sensitive medium, However, from the viewpoint of absorbing efficiency of ultraviolet rays, it is preferable to blend the ultraviolet ray absorbent in an outer layer, namely, the thermally sensitive layer is better than the under coating layer and the protecting layer is better than the thermally sensitive layer. The containing amount of the ultraviolet ray absorbent of this invention to the under coating layer is desirably from 0.1 to 80 wt% to the dry weight of the under coating layer, and more desirably is from 1 to 50 wt%. If the containing amount is under 0.1 wt%, a sufficient light resistance cannot be obtained. If the containing amount is over 80 wt%, the adhering strength between substrate and under coating layer is deteriorated, and consequently the surface strength of the thermally sensitive recording medium is weakened. Therefore, in this case, it is not suited to a use which requires cellopick strength and printability. With regard to the thermally sensitive recording layer, the containing amount of the ultraviolet ray absorbent of this invention is desirably from 0.1 to 25 wt% to the dry weight of the thermally sensitive recording layer, and more desirably from 1 to 20 wt%. If the containing amount is under 0.1 wt%, a sufficient light resistance can not be obtained. If the containing amount is over 25 wt%, the influence on the deterioration of color developing density and background fogging becomes serious. The containing amount of the ultraviolet ray absorbent in the protective layer is desirably from 0.1 to 30 wt% to the dry weight of the protective layer, and more desirably is from 1 to 25 wt%. If the containing amount is under 0.1 wt%, a sufficient light resistance cannot be obtained. If the containing amount is over 30 wt%, the barrier ability becomes insufficient. Further, in the present invention, for further improvment in the light resistance, it is possible to use other conventional and well-known ultraviolet ray absorbent or ultraviolet ray screening agents together with the aqueous emulsion type polymer ultraviolet ray absorbent used herein in an amount sufficient to retain the effect of this invention. As conventional ultraviolet ray absorbents, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbuthyl)-6-(2H-benzotriazole-2-yl)phenol, 2(2'-hydroxy-3'-t-buthyl-5'-methyl-phenyl)5-chloro-benzotriazole and 2,2'-p-phenylenebis(4H-3,1-benzoxazine-4-on) and others can be mentioned. As ultraviolet screening agents, a calcined particle having triple layers composed of flaky pigment whose refractive index is from 1.5 to 1.6 coated by insoluble cerium compound and amorphous silica, inorganic powder obtained by drying, calcining the double layer structure composed of insoluble cerium compound coated with amorphous silica and others can be mentioned. In the present invention, the kind of basic leuco dye to be used in the thermally sensitive recording layer is not restricted. However, a leuco dye such as triphenyl methane type, fluoran type, azaphtalide type or fluorene type can be desirably used. Specific examples of these leuco dyes are mentioned below. These basic leuco dyes can be used alone or in combination. <Triphenylmethane type leuco dyes> 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide [another name is Crystal Violet Lactone] 3,3-bis(p-dimethylaminophenyl)phthalide [another name is Malachite Green Lactone] <Fluoran type leuco dyes> 3-diethylamino-6-methylfluoran 3-diethylamino-6-methyl-7-anilinofluoran 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran 3-diethylamino-6-methyl-7-chlorofluoran 3-diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran 3-diethylamino-6-methyl-7-(o-chloroanilino)fluoran 3-diethylamino-6-methyl-7-(p-chloroanilino)fluoran 3-diethylamino-6-methyl-7-(o-fluoroanilino)fluoran 3-diethylamino-6-methyl-7-(m-methylanilino)fluoran 3-diethylamino-6-methyl-7-n-octylanilinofluoran 3-diethylamino-6-methyl-7-n-octylaminofluoran 3-diethylamino-6-methyl-7-benzylanilinofluoran 3-diethylamino-6-methyl-7-dibenzylanilinofluoran 3-diethylamino-6-chloro-7-methylfluoran 3-diethylamino-6-chloro-7-anilinofluoran 3-diethylamino-6-chloro-7-p-methylanilinofluoran 3-diethylamino-6-ethoxyethyl-7-anilinofluoran 3-diethylamino-7-methylfluoran 3-diethylamino-7-chlorofluoran 3-diethylamino-7-(m-trifluoromethylanilino)fluoran 3-diethylamino-7-(o-chloroanilino)fluoran 3-diethylamino-7-(p-chloroanilino)fluoran 3-diethylamino-7-(o-fluoroanilino)fluoran 3-diethylamino-benzo[a]fluoran 3-diethylamino-benzo[c]fluoran 3-dibutylamino-6-methyl-fluoran 3-dibutylamino-6-methyl-7-anilinofluoran 3-dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran 3-dibutylamino-6-methyl-7-(o-chloroanilino)fluoran 3-dibutylamino-6-methyl-7-(p-chloroanilino)fluoran 3-dibutylamino-6-methyl-7-(o-fluoroanilino)fluoran 3-dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran 3-dibutylamino-6-methyl-chlorofluoran 3-dibutylamino-6-ethoxyethyl-7-ani linofluoran 3-dibutylamino-6-chloro-7-anilinofluoran 3-dibutylamino-6-methyl-7-p-methylanilinofluoran 3-dibutylamino-7-(o-chloroanilino)fluoran 3-dibutylamino-7-(o-fluoroanilino)fluoran 3-di-n-pentylamino-6-methyl-7-anilinofluoran 3-di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluoran 3-di-n-pentylamino-7-(m-trifluoromethylaniliono)fluoran 3-di-n-pentylamino-6-chloro-7-anilinofluoran 3-di-n-pentylamino-7-(p-chloroanilino)fluoran 3-pyrrolidino-6-methyl-7-anilinofluoran 3-piperidino-6-methyl-7-anilinofluoran 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran 3-(N-ethyt-N-xylamino)-6-methyl-7-(p-chloroanilino)fluoran 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran 3-(N-ethy)-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran 3-cyclohexylamino-6-chlorofluoran 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran 2-(4-oxahexyl)-3-diethylamino-6-methyl-7-anilinofluoran 2-(4-oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran 2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran 2-methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran 2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran 2-chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran 2-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran 2-amino-6-p-(p-diethylaminophenyl)aminoanilinofluoran 2-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran 2-phenyl-6-metyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran 2-benzyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran 2-hydroxy-6-p-(p-phenylaminophenyl)aminoanilinofluoran 3-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran 3-diethylamino-6-p-(p-dibutylaminophenyl)aminoanilinofluoran 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran <Fluorene type leuco dyes> 3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide] 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide] <Divinyl type leuco dyes> 3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetra bromophthalide 3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetra chlorophthalide 3,3-bis-[1,1-bis(4-pyrrolidinophenyl)ethylen-2-yl]-4,5,6,7-tetrabromophthalide 3,3-bis-[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrac hlorophthalide <Others> 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-aza phthalide 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-aza phthalide 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide 3,6-bis(diethylamino)fluoran-γ-(3'-nitro)anilinolactam 3,6-bis(diethylamino)fluoran-γ-(4'-nitro)anilinolactam 1,1-bis-[2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-dinitrilethane 1,1-bis-[2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2-β-naphthoyl ethane 1,1-bis-[2',2',2",2"-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-diacetylethane bis-[2,2,2',2'-tetrakis-(p-dimethylaminophenyl)-ethenyl]-methylmalonicacid dimethylester. As an organic color developer used in a thermally sensitive recording layer of the recording medium of this invention, well-known color developers can be mentioned as follows. However, it is not intended to be limited to them. Inorganic acidic substances such as activated clay, colloidal silica or aluminium silicate, 4,4'-isopropylidenediphenol 1,1-bis(hydroxyphenyl)cyclohexane 2,2-bis(hydroxyphenyl)-4-methylpentane 4,4'-dihydroxydiphenylsulfido hydroquinonemonobenzylether 4-hydroxybenzoicbenzyl 4,4'-dihydroxydiphenylsulfone 2,4'-dihydroxydiphenylsulfone 4-hydroxy-4'-isopropoxydiphenylsulfone 4-hydroxy-4-n-propoxydiphenylsulfone bis(3-allyl-4-hydroxyphenyl)sulfone 4-hydroxy-4'-methyldiphenylsulfone 4-hydroxyphenyl-4'-benzyloxyphenylsulfone 3,4-dihydroxyphenyl-4'-methylphenylsulfone Derivative of aminobenzenesulfone amide described in Japanese Patent Laid Open Publication 8-59603, bis(4-hydroxyphenylthioethoxy)methane 1,5-di(4-hydroxyphenylthio)-3-oxapentane bis(p-hydroxyphenyl)butyl acetate bis(p-hydroxyphenyl)methyl acetate 1,1-bis(4-hydroxyphenyl)-1-phenylethane 1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethy)]benzene 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene di(4-hydroxy-3-methylphenyl)sulfido 2,2'-thio-bis(3-tert-octylphenol) 2,2'-thio-bis(4-tert-octylphenol) Phenolic compound such as diphenylsulfone bridged type compound described in WO97/16420, thio urea compound such as N,N'-di-m-chlorophenylthiourea, p-chloro benzoic acid, gallic acid stearyl aromatic carboxylic acid such as bis[4-(n-octyloxycarbonylamino)zinc salicylate]2 hydrate, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid or 5-[p-(2-p-methoxyphenoxy)cumyl]salicylic acid and metallic salts of these aromatic carboxylic acids with zinc, magnesium, aluminium, calcium, titanium, manganese, tin or nickel, further, antipyrine complexes of zinc thiocyanate and complex zinc salts of terephtalic aldehyde acid and other aromatic carboxylic acids can be mentioned. These organic color developers can be used alone or in combination. Further, a metal chelate type color developing component such as high fatty acid metal complex salt described in Japanese Patent Laid Open Publication 10-258577 or multivalent hydroxy aromatic compound can be used. In the present invention, for the purpose of improving the thermally recording color developing sensitivity, a sensitizer can be contained in the thermally sensitive recording layer. As sensitizers, any kinds of compounds that are conventionally used in a thermally sensitive recording medium may be used. For example, stearic acid amide, fatty acid amide such as palmitic acid amide, ethylenebisamide, montanewax, polyethylenewax, dibenzyl terephthalate, p-benzyloxy benzyl benzoate, di-p-tolylcarbonate, p-benzylbiphenyl, phenyl α - naphthylcarbonate, 1,4-diethoxy naphthalene, 1-hydroxy-2-phenylnaphthoate, 1,2-di-(3-methylphenoxy)ethane, oxalic acid di(p-methylbenzyl), β-benzyloxy naphthalene, 4-biphenyl-p-tolylether, o-xylene-bis-(phenylether), 4-(m-methyl phenoxymethyl)biphenyl, p-toluenesulfoneamide and o- toluenesulfoneamide can be added. These sensitizers can be used alone or in combination. In the present invention, mainly for the purpose of improving the preservative ability of the color developed recorded image, an image stabilizer can be contained in the thermally sensitive recording layer. As the image stabilizer, for example, at least one compound selected from the group composed of phenol type compound such as 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol, 4,4'-[1,3-phenylenebis(1-methylidene)]bisphenol, 4,4'-butylidene(6-t-butyl-3-methylphenol) or 2,2'-di-t-butyl-5,5'-dimethyl-4,4'-sulfonyldiphenol, epoxy compound such as 4-benzyloxyphenyl-4'-(2-metyl-2,3-epoxypropyloxy)phenylsulfone, 4-(2-methyl-1,2-epoxyethyl)diphenylsulfone or 4-(2-ethyl-1,2-epoxyethyl)diphenylsulfone and isocyanuric acid compound such as 1,3,5-tris(2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl)isocyanuric acid can be used. These image stabilizers can be used alone or in combination. As the binder which may be used in the present invention, full saponificated polyvinyl alcohol of 200-1900 polymerization degree, partial saponificated polyvinyl alcohol, denatured polyvinyl alcohol by carboxyl, denatured polyvinyl alcohol by amide, denatured polyvinyl alcohol by sulfonic acid denatured polyvinyl alcohol by butyral or other modified polyvinyl alcohol, derivatives of cellulose such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose and acetyl cellulose, copolymer of styrene-maleic acid anhydride, copolymer of styrene-butadiene, polyvinyl chloride, polyvinyl acetate, polyacrylamide, polyacrylic acid ester, polyvinylbutyral, polystyrene or copolymers thereof, polyamide resin, silicone resin, petroleum resin, terpene resin, ketone resin and coumarone resin can be illustrated. These macromolecule compounds can be applied by being dissolved into solvents such as water, alcohol, ketone, ester or hydrocarbon or by being dispersed in water or other medium under an emulsion state or a paste state, and these forms of application can be used in combination according to the quality requirement. As a filler which can be used in this invention, an inorganic filler such as silica, calcium carbonate, clay, kaoline, calcined kaoline, diatomaceous earth, synthetic aluminum silicate, talc, zinc oxide, titanium dioxide, aluminum hydroxide, zinc hydroxide, barium sulfate, magnesium carbonate, surface treated calcium carbonate or silica, and an organic filler such as urea-formalin resin, styrene-methacrylic acid copolymer resin, polystyrene resin and vinylidene chloride can be mentioned. Further, a parting agent such as metallic salts of fatty acid, a lubricant such as waxes, a water resistance agent such as glyoxal, a dispersing agent, a defoamer, and fluorescent dye, which are well-known materials used in a thermally sensitive recording medium can be preferably used. The preparing method of the coating for a thermally sensitive recording layer of this invention is not restricted. However, the amount of basic leuco dye, organic color developer and kind and amount of other component is decided according to the required properties and recording aptitude. The desirable amount of organic color developer is from 1 to 8 parts, the desirable amount of filler is from 1 to 20 parts to 1 part of basic leuco dye and a desirable amount of binder is from 5 to 25% to the total weight of solid. Said basic leuco dye, organic color developer and other additives which are added as needed are ground to fine particles smaller than several microns diameter by means of a grinder such as a ball mill, an attriter or a sand grinder, or by means of an adequate emulsifying apparatus, then the binder and additives such as aqueous emulsion type polymer ultraviolet ray absorbent of this invention are added and the coating is prepared. The method of forming the thermally sensitive recording layer is not restricted, and the coating of the above mentioned recipe is coated over the substrate by means of a blade coater, an air knife coater, a bar coater or a reverse roll coater and dried, thus the thermally sensitive recording medium can be obtained. In the thermally sensitive recording medium, an undercoating layer can be prepared between the thermally sensitive recording layer and the substrate. In this case, usually the undercoating layer contains a binder and a filler. As the binder, filler and other additives, materials that are disclosed as the components of thermally sensitive recording layer can be preferably used according to the required qualities. Calcined kaolin, well-known hollow particles used for undercoating layers of conventional thermally sensitive recording medium such as fine hollow particles with hull described in Japanese Patent Publication 3-54074 or cup shape hollow polymer particles described in Japanese Patent Laid Open Publication 10-258577 have an excellent adiabatic effect and are desirable for the filler used for the undercoating layer. In the present invention, the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent can be preferably blended to the undercoating layer when the need arises. The preparing method of a coating for the undercoating layer is not restricted. In general, a binder and a filler are poured into water, which is a dispersing medium, then the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent and a defoamer are added as needed and mixed. Thus the coating for the undercoating layer is prepared. The method of forming an undercoating layer is not restricted, and for example, a coating for the undercoating layer is coated on the substrate by various coating methods in the same way as the forming method of the above mentioned thermally sensitive recording layer, dried and the undercoating layer is formed. In the thermally sensitive recording medium, in the case of providing a protecting layer on the thermally sensitive recording layer, usually, such kind of protecting layer contains a binder and a filler. As the binder, filler and other additives, materials that are disclosed as components of the thermally sensitive recording layer can be preferably used according to the required qualities. In this case, it is desirable to add a bridging agent so as to provide water resistance to the protecting layer. In the present invention, the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent can be preferably blended to the protecting layer as needed. The preparing method of a coating for the protecting layer is not restricted. In general, a binder and a filler are poured into water, which is a dispersing medium, then the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent, a filler and a lubricant are added as needed and mixed. Thus the coating for the protecting layer is prepared. The method of forming a protecting layer is not restricted, and for example, a coating for the protecting layer is coated on the thermally sensitive recording layer by various coating methods in the same way as the forming method of the above mentioned thermally sensitive recording layer, dried and the protecting layer is formed. In the thermally sensitive recording medium, a middle layer can be prepared between the thermally sensitive recording layer and a protecting layer. In this case, usually the middle layer contains a binder and a filler. As the binder, filler and other additives, materials that are disclosed as the components of thermally sensitive recording layer can be preferably used according to the required qualities. In this case, it is desirable to add a bridging agent so as to provide water resistance to the middle layer. In the present invention, the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent can be preferably blended to the middle layer as needed. The preparing method of a coating for the middle layer is not restricted. In general, a binder and a filler are poured into water, which is a dispersing medium, then the above mentioned aqueous emulsion type polymer ultraviolet ray absorbent, a filler and a lubricant are added as needed and mixed. Thus the coating for the middle layer is prepared. The method of forming a middle layer is not restricted, and for example, a coating for the middle layer is coated on the thermally sensitive recording layer by various coating methods in the same way as the forming method of the above mentioned thermally sensitive recording layer, dried and the middle layer is formed. As a substrate to be used in a thermally sensitive recording medium of this invention, any kinds of materials that can maintain the thermally sensitive recording layer and other layers provided in case of need, such as paper (acidic paper, neutralized paper), recycled paper, plastic film, synthetic paper, nonwoven fabrics or metal vaporized sheet can be used. EXAMPLE The thermally sensitive recording medium of this invention will be illustrated more specifically by Examples. However, it is not intended to be limited to them. In the Examples and Comparative Examples, the term "parts" indicates weight part. Example 1-8 Examples 1-8 are the examples that the aqueous emulsion type polymer ultraviolet ray absorbent of this invention is used in a protecting layer. As an ultraviolet ray absorbent, compounds shown in Preparation Examples 1-8 are used independently. (1) Forming of thermally sensitive recording layer Each solution of following recipe is separately ground in wet condition to average particle diameter of 1 µm by a sand grinder. A solution (dispersion of color developer) 2,4'-dihydroxydiphenylsulfone 6.0 parts 10%polyvinyl alcohol aqueous solution 18.8 parts water 11.2 parts B solution (dispersion of basic leuco dye) 3-di-butylamino-6-methyl-7-anilinofluoran(ODB-2) 2.0 parts 10%polyvinyl alcohol aqueous solution 4.6 parts water 2.6 parts C solution (dispersion of sensitizer) Para-benzylbiphenyl 4.0 parts 10% polyvinyl alcohol aqueous solution 5.0 parts water 3.0 parts Then obtained dispersions are mixed by following ratio and stirred, and the coating for thermally sensitive recording layer is obtained. A solution 36.0 parts B solution 9.2 parts C solution 12.0 parts Kaolin clay(50% dispersion) 12.0 parts The obtained coating is coated on a surface of 50g/m 2 woodfree paper composed of 80% LBKP and 20% NBKP so as the coating amount to be 6.0g/m 2 and dried, and a thermally sensitive recording layer is obtained. (2) Forming of a protecting layer The materials mentioned below are mixed and the coatings for protecting layer are prepared. 10%polyvinyl alcohol aqueous solution 60.0 parts aluminum hydroxide (50% dispersion) 30.0 parts zinc stearate 10.0 parts water 50.0 parts compounds of Preparation Examples 1∼8 (30%) 20.0 parts The obtained coatings are coated on a thermally sensitive recording layer so as the coating amount to be 4.0g/m 2 and dried, and the protecting layers are obtained. The sheets are processed by a super calendar so as the Bekk smoothness to be 200-600 seconds, and the thermally sensitive recording media are obtained. Example 9 In Example 9, the ultraviolet ray absorbent of this invention is used in a thermally sensitive recording layer. At the formation of thermally sensitive recording layer of Example 1, 10.0 parts of ultraviolet ray absorbent of Preparation Example 8 (30%) is added to the coating for thermally sensitive recording layer, and by same process as Example 1 except not preparing a protecting layer, a thermally sensitive recording medium is obtained. Example 10 In Example 10, the ultraviolet ray absorbent of this invention is used in a thermally sensitive recording layer and a protecting layer. At the formation of thermally sensitive layer of Example 8, by same process as Example 8 except adding 10.0 parts of ultraviolet ray absorbent of Preparation Example 2 (30%) to the coating for thermally sensitive recording layer, a thermally sensitive recording medium is obtained. Example 11 In Example 11, the ultraviolet ray absorbent of this invention is used in an undercoating layer and a protecting layer. By same process as Example 8 except forming an undercoating layer as mentioned below, a thermally sensitive recording medium is obtained. Said undercoating layer is formed between substrate and thermally sensitive recording layer and contains the ultraviolet ray absorbent of Preparing Example 4. Forming of undercoating layer Forming of undercoating layer 10%polyvinyl alcohol aqueous solution 150 parts calcined kaolin (40% dispersion) 250 parts compound of Preparation Example 4 (30%) 50 parts Above mentioned materials are mixed by above mentioned ratio and stirred, and the coating for undercoating layer is obtained. The obtained coating is coated on a surface of 50g/m 2 woodfree paper composed of 80% LBKP and 20% NBKP so as the coating amount to be 5.0g/m 2 and dried, and an undercoating layer is formed. Example 12 In Example 12, the ultraviolet ray absorbent of this invention is used in an undercoating layer, a thermally sensitive recording layer and a protecting layer. By same process as Example 11, except adding the ultraviolet ray absorbent of the Preparation Example 2 (30%) to the coating for thermally sensitive recording layer, a thermally sensitive recording medium is obtained. Example 13 At the formation of an undercoating layer of Example 11, D solution is added to a coating for undercoating layer instead of 50 parts of ultraviolet ray absorbent of Preparation Example 4 (30%), and at the formation of a thermally sensitive recording layer, by same process as Example 11, except adding E solution to the coating for thermally sensitive recording layer, a thermally sensitive recording medium is obtained. D solution (dispersion of ultraviolet absorbent 1) 2(2'-hydroxy-3'-t-butyl-5'-methyl-phenyl)5-chloro-benzotriazole 15.0 parts 10% polyvinylalcohol aqueous solution 30.0 parts water 30.0 parts The solution of above mentioned recipe is ground in wet condition to average particle diameter of 1 µm by a sand grinder. E solution (dispersion of ultraviolet absorbent 2) 2,2'-methylenebis[4-(1,1,3,3,-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol] 3.0 parts 10% polyvinylalcohol aqueous solution 6.0 parts water 6.0 parts The solution of above mentioned recipe is ground in wet condition to average particle diameter of 1 µm by a sand grinder. Example 14 At the formation of a thermally sensitive recording layer of Example 8, by same process as Example 8, except adding F solution to the coating for thermally sensitive recording layer, a thermally sensitive recording medium is obtained. F solution (dispersion of ultraviolet ray absorbent 3) 2,2'-p-phenylenebis(4H-3,1-benzoxazine-4-on) 3.0 parts 10% polyvinylalcohol aqueous solution 6.0 parts water 6.0 parts The solution of above mentioned recipe is ground in wet condition to average particle diameter of 1 µm by a sand grinder. Example 15 At the formation of an undercoating layer of Example 11, by same process as Example 8, except adding G solution instead of 50 parts of compound of Preparation Example 4, a thermally sensitive recording medium is prepared. G solution(dispersion of ultraviolet ray screening agent) Ultraviolet ray screening agent of Producing Example mentioned below 15.0 parts 10% polyvinylalcohol aqueous solution 30.0 parts water 30.0 parts The solution of above mentioned recipe is ground in wet condition to average particle diameter of 1 µm by a sand grinder. (Producing Example) 500g of flaky silica, which is a material for nucleus of an ultraviolet ray screening agent is poured into 10 liter of water, then dispersed for one hour by means of DISPAMILL (product of Hosokawa Micron). the obtained dispersion is heated to 80°C, and 264g of aqueous solution of cerium nitrate (CeO 2 content is 19 wt%, insoluble cerium compound sediment amount converted in CeO 2 is 10 wt%) is dropped in with constant stirring. Then, the dispersion is neutralized by sodium hydroxide solution to pH7-9, cerium hydroxide is accumulated on the surface of silica so as to form a coating layer. The obtained product is filtrated, rinsed by water and dried up, then ground, and flake type cerium coated silica pigment is obtained. The obtained flake type cerium coated silica pigment is put into 10 liter of water and dispersed for one hour by means of DISPAMILL. The obtained dispersion is heated to 80°C, and 348g of No.3 sodium silicate (SiO 2 content is 28.5 wt%, amorphous silica sediment amount converted into SiO 2 is 18 wt%) is dropped in with constant stirring. Diluted sulfuric acid is added to maintain pH of solution in 9-11, continue the constant stirring for another one hour, then pH of solution is adjusted to 6-8 using sulfuric acid. The obtained product is filtrated, rinsed by water and dried up, then ground and flake type cerium-silica coated silica pigment is obtained. Further, this pigment is calcined at the temperature of 500°C for two hours and the ultraviolet ray screening agent of triple layer structure (CeO 2 10 wt%-SiO 2 18 wt%) is obtained. Example 16 Example 16 is the example which uses a recycled paper as a substrate. By same process as Example 8 except using a mechanical paper (50 g/m 2 ) composed of 85% of waste paper pulp and 15% of NBKP instead of a woodfree paper (50 g/m 2 ) composed of 80% of LBKP and 20% of NBKP, a thermally sensitive recording medium is prepared. Comparative Example 1 Comparative Example 1 is the example in which no ultraviolet ray absorbent is used. At the formation of a protecting layer of Example 1, by same process as Example 1, except not adding an ultraviolet ray absorbent, a thermally sensitive recording medium is prepared. Comparative Example 2 At the formation of a thermally sensitive recording layer of Example 9, by same process as Example 9, except using compound 1 (30%) shown in Table 2 instead of using compound of Preparation Example 8, a thermally sensitive recording medium is prepared. Still more, all compounds described in Table 2 are prepared in accordance with Preparation Example 1. The compound 1 is an ultraviolet ray absorbent that non-reactive emulsifier is used at the preparation. Comparative Examples 3 and 4 At the formation of a protecting layer of Example 1, by same process as Example 1 except using compounds 3 and 4 (30% each) shown in Table 2 instead of using the compound of Preparation Example 1, thermally sensitive recording media are prepared. Further, the compound 3 is an ultraviolet ray absorbent such that a hydrophilic monomer is not used in the preparation, and the compound 4 is an ultraviolet ray absorbent such that a hydrophobic bridgeable monomer is used instead of a hydrophilic monomer in the preparation. [Evaluation of color developing] Thermal recording is carried out on the prepared thermally sensitive recording media using TH-PMD (printing tester for thermally sensitive recording paper, thermal head of Kyocera Co., Ltd. Is installed), which is a product of Ohkura Denki Co., by 0.42mJ/dot impressed energy. Image density of recorded part is measured by means of a Macbeth densitometer (RD-914, amber filter used). (density: Da) [Evaluation of light resistance] (1) The printed thermally sensitive recording medium is treated by Carbon ark Fade Meter (Toyo Seiki BH) for 12 hours, and the density of recorded part is measured by above mentioned method (density: Db). And the density of background is measured by changing the filter to a blue filter (density: Dc). (2) Evaluation was carried out by following numerical formula. Light resistance of recorded part (remaining ratio) : Db/Da × 100=Rd(%) Light resistance of background part : Dc Bigger Rd value indicates better light resistance of recorded part. Actually it is desirable to be not less than 80%. And when Dc value is small, it indicates that the light resistance of background is good. Actually, it is desirable not to exceed 0.15. [Evaluation of sticking] On the prepared thermally sensitive recording medium, image is printed by Panafax UF-22, which is a product of Panasonic, by transmitting mode, and the printing sound and the sticking of printed image are sensually evaluated. Evaluation : o ○ quiet, ○ sounds slightly, actually no problem, × noisy, stick pattern appears on image It is desirable that the sound is low and the stick pattern is rare. [Evaluation of printing depositions] After 40 sheets of B4 size manuscript with vertical stripes pattern are copied using Panafax UF-22, which is a product of Panasonic, by copy mode, and the adherence of printing depositions to thermal head is evaluated by naked eyes of the inspector. It is desirable that the amount of adhered deposition is little. Evaluation : ○ very little, Δ found out, × very much [Evaluation of barrier ability] This evaluation test is carried out only on the thermally sensitive recording medium that has a protecting layer. On the surface of protecting layer of obtained thermally sensitive recording medium, a drop of ethylacetate is dropped using a syringe and enlarged to a circle of 3cm diameter by cotton cloth. After one minute, degree of color developing of thermally sensitive recording layer is evaluated by naked eyes of the inspector. It is desirable that the degree of color developing is low. Evaluation : o ○ not developed, ○ almost not developed, × area of color developed part is bigger than that of color undeveloped part Results obtained by above mentioned evaluation tests are summarized in table 3. "-" mark that appears in table 3 indicates that the evaluation test is not carried out. Test results of qualities Example Comp. Example color developing ability light resistance sticking deposition at printing barrier ability Da Rd(%) Dc 1 1.18 85.1 0.10 o ○ ○ o ○ 2 1.20 83.9 0.11 o ○ ○ o ○ 3 1.19 86.7 0.12 o ○ ○ o ○ 4 1.19 85.0 0.12 o ○ ○ o ○ 5 1.19 83.8 0.10 ○ ○ o ○ 6 1.21 84.9 0.11 o ○ ○ o ○ 7 1.20 86.3 0.10 ○ ○ o ○ 8 1.21 87.0 0.10 o ○ ○ o ○ 9 1.24 80.6 0.14 o ○ ○ - 10 1.17 88.5 0.09 o ○ ○ o ○ 11 1.26 88.2 0.09 o ○ ○ o ○ 12 1.23 91.6 0.08 o ○ ○ o ○ 13 1.25 95.3 0.08 o ○ ○ o ○ 14 1.18 91.4 0.11 o ○ ○ o ○ 15 1.24 92.0 0.10 o ○ ○ o ○ 16 1.19 80.5 0.14 o ○ ○ o ○ Co.Ex.1 1.19 30.0 0.32 o ○ ○ o ○ Co.Ex.2 0.89 85.1 0.14 o ○ ○ - Co.Ex.3 1.19 82.7 0.10 ○ ○ × Co.Ex.4 1.17 81.5 0.11 ○ ○ × As clearly understood from above mentioned Table 3, when an aqueous emulsion type polymer ultraviolet ray absorbent of this invention is used to a thermally sensitive recording medium, good light resistance can be obtained, and also color developing ability, occurrence of sticking and generation of printing depositions adherence are remarkably improved. Especially, in regard to sticking, when Tg of the ultraviolet ray absorbent used in the outermost layer of printing side of thermally sensitive recording medium is high, evaluation of sticking becomes better. Examples 1, 2, 3 and 8-16 in which an ultraviolet ray absorbent having Tg of not less than 80°C is used are superior to Examples 5 and 7 in which an ultraviolet ray absorbent having Tg of 50-70°C from the view point of sticking sound. Sticking condition of Examples 4 and 6 using an ultraviolet ray absorbent having Tg of 70-80°C is middle level between said two groups. When a thermally sensitive recording medium has a protecting layer and an ultraviolet ray absorbent of this invention is used, the thermally sensitive recording medium displays excellent barrier ability. Meanwhile, in a case that an ultraviolet ray absorbent is not blended, as obvious from Comparative Example 1, the light resistance becomes very poor. Further, concerning the case of Comparative Examples in which other ultraviolet ray absorbents are used, that is, in a case of Comparative Example 2 in which non-reactive emulsifier is used, the color developing ability is inferior and in the case of Comparative Example 3 and 4 in which hydrophilic monomer is not contained, the barrier ability is inferior. As mentioned above, by using the ultraviolet ray absorbent of aqueous emulsion type polymer described herein a thermally sensitive recording medium having good light resistance, good color developing ability and less generations of sticking and printing depositions can be obtained. By this invention, a thermally sensitive recording medium having good light resistance, appearance of unrecorded portion and color developing ability, and less generations of sticking and printing depositions can be provided.

Claims: A thermally sensitive recording medium comprising, on a substrate, one or more layers including a thermally sensitive color developing layer which comprises a colorless or pale colored basic leuco dye and an organic color developer wherein at least one of said one or more layers comprises, as an ultraviolet ray absorbent, an aqueous emulsion type polymer which is an inner bridged type polymer particle emulsion comprising (a) an ultraviolet ray absorbing monomer, (b) a vinyl compound monomer that can copolymerize with said monomer (a), (c) a hydrophilic monomer, and (d) a reactive emulsifier. A medium according to claim 1, wherein the emulsion copolymer has a glass transition temperature Tg of not less than 50°C. A medium according to claim 1 or 2, wherein the ultraviolet ray absorbing monomer (a) is a compound of formula I, wherein, R 1 is hydrogen, C 1 -C 6 alkyl or C 1 -C 6 alkoxy, R 2 is C 1 -C 10 alkylene, C 1 -C 10 oxyalkylene or is a direct bond, X is an ester, amide, ether or urethane linkage and R 3 is hydrogen or C 1-6 alkyl, and/or a compound of formula II wherein, R 4 is hydrogen, halogen or methyl, R 5 is hydrogen or a C 1 -C 6 hydrocarbon, R 6 is C 1 -C 10 alkylene, C 1 -C 10 oxyalkylene or a direct bond, Y is an ester, amide, ether or urethane linkage, R 7 is C 1 -C 8 alkylene, C 1 -C 8 aminoalkylene, C 1 -C 6 alkyl substituted by hydroxide or a direct bond and R 8 is hydrogen or C 1-6 alkyl. A medium according to claim 1, 2 or 3, wherein the reactive emulsifier (d) is a polymerizable vinyl compound. A medium according to any one of the preceding claims wherein the emulsion copolymer comprises a compound of formula (I) and a compound of formula (II) as defined in claim 3. A medium according to any one of the preceding claims wherein said one or more layers comprise one or more of a protecting layer disposed on the thermally sensitive color developing layer; and an undercoating layer between the substrate and the thermally sensitive color developing layer. A medium according to any one of the preceding claims which comprises, in order on the substrate: (a) an undercoating layer; (b) the thermally sensitive color developing layer; (c) a middle layer; and (d) a protecting layer. A medium according to any one of the preceding claims wherein the ultraviolet ray absorbent is present in the thermally sensitive color developing layer, an undercoating layer between said thermally sensitive color developing layer and the substrate, a protecting layer on the thermally sensitive color developing layer or a middle layer between the thermally sensitive color developing layer and the protecting layer. A medium according to any one of the preceding claims wherein the ultraviolet ray absorbent is present in a protecting layer on the thermally sensitive color developing layer. A medium according to any one of the preceding claims which further comprises an additional ultraviolet absorbing agent other than an ultraviolet absorbing agent as defined in claim 1.

Inventors: HAMADA KAORU, KAWABATA HIROYUKI, KIMURA YOSHIHIDE, SATO REIKO, SUGIMORI SEIJI, YAMAMOTO TAKAICHI, HAMADA, KAORU, KAWABATA, HIROYUKI, KIMURA, YOSHIHIDE, SATO, REIKO, SUGIMORI, SEIJI, YAMAMOTO, TAKAICHI, Hamada, Kaoru, c/o Res. Lab. Product Dev. of, Kawabata, Hiroyuki, c/o Ipposha Oil Ind. Co., Ltd., Kimura, Yoshihide, c/o Res. Lab. Product dev. of, Sato, Reiko, c/o Res. Lab. of Product Dev. of, Sugimori, Seiji, c/o Ipposha Oil Ind. Co., Ltd., Yamamoto, Takaichi, c/o Ipposha Oil Ind. Co. Ltd.
Applicants: IPPOSHA OIL IND CO LTD, JUJO PAPER CO LTD, IPPOSHA OIL INDUSTRIES CO. LTD., NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 2000
Main classification: B41M5
IPC codes: B41M5