Thermally sensitive recording medium

Abstract: A thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive color developing layer comprising a colorless or a pale colored dye precursor and a color developer and which gives a developed image having one of the following three combinations of color difference L* value, a* value and b* value, as regulated by JIS-Z-8729: L* is 30 to 50, a* is -15 to 0 and b* is -30 to 10; or L* is 30 to 50, a* is 0 to 10 and b* is -30 to 0; or L* is 30 to 50, a* is 10 to 20 and b* is -30 to -15. Preferably the dye precursor comprises a compound selected from 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide and 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, and a leuco dye which develops a black colour with a maximum absorption wave length of 420-480nm and 550-640nm.

Description: The present invention relates to a thermally sensitive recording medium which develops navy blue or grayish blue color. Generally, the thermally sensitive recording medium possessing a thermally sensitive recording layer mainly comprising a colorless or a pale colored electron donating dye precursor (hereinafter shortened to dye precursor) and a color developer which develops color when heated together with said dye precursor was disclosed in Japanese Patent publication 45-14035 and had been widely utilized practically. As a recording apparatus for this thermally sensitive recording medium, a thermally printer to which a thermal head is installed can be used. The recording method mentioned above has strong points in comparison with other conventional recording methods, namely, noiseless during recording, a developing and a printing procedure are not needed, maintenance free, apparatus is relatively low price and compact and a recorded pattern is very vivid. Therefore, along with the growth of information industry, the application of this method is widely expanded, for instance, applications for a facsimile or a computer, for many kinds of measuring equipment and for a label. The developed color image of these thermally sensitive recording medium is mainly black color, however, a red color developing type, a blue color developing type, a green color developing type, a full color developing type and a dual color developing type are also well known. The developed color of thermally sensitive recording medium is comparatively sharp and closed to photograph, and recently used as an output means of image which is taken by a camera attached to a game machine. Accompanied with the extension of uses, a thermally sensitive recording medium which develops neutral color such as navy blue color or grayish blue color is becoming to be desired. In the conventional field of thermally sensitive recording medium, it is commonly observed that the developed color changes to navy blue color or grayish blue color by the effect of environment such as sun light and temperature, or by the effect of chemicals, however, this color change is a phenomenon which is recognized as a problem caused by poor colour preserving. And the thermally sensitive recording medium whose original developed colour is navy blue or grayish blue color is not developed yet. JP 05278328 discloses a heat-sensitive recording material which develops a black color using a combination of a black leuco dye and a blue or red leuco dye. JP 07108764 discloses a thermal recording material which develops a bluish black color which contains a black color forming leuco dye and a blue color forming leuco dye. JP 05169828 and JP 05169827 disclose a thermally sensitive recording medium which develops a bluish black or blackish blue image and contains a specific kind of fluorene derivative and a specific kink of azaphthalide derivative as the dyes. EP 0 561 738 A discloses particular fluorene dyes and their use in thermally sensitive recording materials to give a red or orange color. EP 0 356 386 A discloses particular phthalide dyes and their use in thermally sensitive recording materials to give an intensive blue or violet blue color. The object of this invention is to provide a thermally sensitive recording medium which has a sufficient color density and develops navy blue or grayish blue color. The invention provides a thermally sensitive recording medium having a thermally sensitive color developing layer containing a colorless or a pale colored dye precursor selected from 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide and 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, together with at least one leuco dye which develops a black color with a maximum absorption wavelength, measured in 99% acetic acid solution of 420-480 and 550-640 nm, wherein the content of said leuco dye is 0.05-1 parts per 1 part of the 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, and a color developer on a substrate, wherein the color difference L* value, a* value and b* value regulated by JIS-Z-8729 of developed image of said thermally sensitive recording medium are within the regions of, A. L* value is 30 - 50, a* value is -15 - 0 and b* value is -30 - 10, B. L* value is 30 - 50, a* value is 0 - 10 and b* value is -30 - 0, or C. L* value is 30 - 50, a* value is 10 - 20 and b* value is -30 - -15, The color difference L* value is a parameter which indicates brightness, and when the plus of L* is big, the brightness becomes strong. The color difference a* value is a parameter which indicates green color, and when the minus value of a* is big, the greenish tone is strong. And when the value is close to 0, the greenish tone becomes weak. Further, plus a* value indicates the reddish tone. Meanwhile, the color difference b* value is a parameter which indicates blue tone and when the minus value of b* is big, the bluish tone is strong. When the value is close to 0, the bluish tone becomes weak, and plus b* value indicates yellowish tone. When the color difference L* value, a* value and b* value of developed image of said thermally sensitive recording medium of this invention are within the afore mentioned regions, the required navy blue or grayish blue color can be obtained. Further, when the color difference L* value is 30 to 45, a* value color is -15-0 and b* value is -25 - 10 or a* value color is 0 - 10 and b* value is -25 - 0, more vivid color tone can be obtained and desirable. On the contrary, when these color difference values are out of the regions regulated in this invention, the color of developed image becomes blue or black and the aimed color tone can not be obtained. In this invention, the term of navy blue or grayish blue means dim and dark neutral color tone developing, which is disclosed e.g. from page 108 to 111, in items "46 SOFT & DULL BLUE" and "47 GRAYISH BLUE" of "Color one point 10, color naming and it's episode" (Japan Standard Society, issued on November 19, 1993). These colors are expressed as dim blue or dark navy blue by JIS common name, or are expressed as indigo or gray blue by idiomatic color naming. Further, the expression of color becomes different by a subjectivity of inspector or by an illumination, and in this invention, the expression of navy blue or grayish blue color are typically used containing commonly expressed navy blue, cyan color or ultramarine color, however, not limited to them. In this invention, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide and 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide are the leuco dye which develops bluish color, and the maximum absorption wave length of the dyes used in this invention is measured in 99% acetic acid solution. Thus, by the combination use of leuco dyes whose maximum absorption wave length are different, the thermally sensitive recording medium which develops neutral color such as navy blue color or grayish blue color can be easily obtained maintaining sufficient color developing density. In the invention the thermally sensitive recording medium contains 0.05∼1 parts of black color developing leuco dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm to 1 part of 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxy phenyl) -3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide When the content of black color developing leuco dye is smaller than 0.05 parts to 1 part of bluish color developing leuco dye, the aimed color tone can be obtained, however, the bluish color becomes slightly strong. On the contrary, when the content is bigger than 1 part, the black color becomes slightly strong. Therefore, it is desirable to contain the black color developing leuco dye by above mentioned ratio to the blue color developing leuco dye to obtain the thermally sensitive recording medium whose developed image is vivid navy blue or grayish blue color and the contrast of image is good. In a further aspect of the invention the thermally sensitive recording recording medium which develops navy blue or grayish blue color, and as mentioned above, the aimed color tone of this invention is not limited to the expression of navy blue or grayish blue color. As a leuco dye which develops blue color used in this invention, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxy phenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide are used. These mentioned leuco dyes can be used alone or used together with. As a leuco dye which develops black color which is used in this invention, the leuco dye whose maximum absorption wave length in 99% acetic acid solution is from 420∼480 nm and 550∼640 nm can be used. As the concrete example, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(o.p-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-m-methylanilinofluoran, 3-diethylamino-6-methyl-7-n-octhylaminofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-(o-fluoroanilino)fluoran, 3-diethylamino-6-methyl-7-(p-n-butylanilino)fluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-(o.p-dimethylanilino)fluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dibutylamino-7-(o-fluoroanilino)fluoran, 3-di-n-penthylamino-6-methyl-7-anilinofluoran, 3-di-n-penthylamino-7-(m-trifluoromethylanilino)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-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofrufurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran and 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran can be mentioned, however, not limited to them. And these leuco dyes can be used alone or as a mixture of two or more. As an organic color developer which can be used in this invention, bis-phenol A type, 4-hydroxyphthalic acid ester type, 4-hydroxyphthalic acid diester type, phthalic acid monoester type, bis-(hydroxyphenyl)sulfide type, 4-hydroxyphenylarylsulfone type, 4-hydroxyphenylarylsulfonate type, 1 ,3-di[2-(hydroxyphenyl) -2-propyl]benzene type, 4-hydroxybenzoyloxybenzoic acid ester type and bisphenolsulfones which are disclosed in Japanese Patent Laid-Open Publication 3-207688 or Japanese Patent Laid-Open Publication 5-24366 can be mentioned. The typical concrete well known examples are shown below, however, not intended to be limited to them. These developers can be used alone or as a mixture of two or more. <bisphenol A type> 4,4'-isopropylidenediphenol (another name is bisphenol A), 4,4'-cyclohexylidenediphenol, p,p'-(1-methyl-n-hexylidene)diphenol, 1,7-di(hydroxyphenylthio)-3,5-dioxaheptane. <4-hydroxybenzoic ester type> 4-hydroxybenzyl benzoate, 4-hydroxyethyl benzoate, 4-hydroxypropyl benzoate, 4-hydroxyisopropyl benzoate, 4-hydroxybutyl benzoate, 4-hydroxyisobutyl benzoate, 4-hydroxymethylbenzyl benzoate. <4-hydroxyphthalic acid diester type> 4-hydroxydimethyl phthalate, 4-hydroxydiisopropyl phthalate, 4-hydroxydibenzyl phthalate, 4-hydroxydihexyl phthalate. <phthalic acid monoester type> monobenzyl phthalate, monocyclohexyl phthalate, monophenyl phthalate, monomethylphenyl phthalate, monoethylphenyl phthalate, monopropylbenzyl phthalate, monohalogenbenzyl phthalate, monoethoxybenzyl phthalate. <bis-(hydroxyphenyl)sulfide type> bis-(4-hydroxy-3-tert-butyl-6-methylphenyl)sulfide, bis-(4-hydroxy-2,5-dimethylphenyl)sulfide, bis-(4-hydroxy-2-methyl-5-ethylphenyl)sulfide, bis-(4-hydroxy-2-methyl-5-isopropylphenyl)sulfide, bis-(4-hydroxy-2,3-dimethylphenyl)sulfide, bis-(4-hydroxy-2,5-dimethylphenyl)sulfide, bis-(4-hydroxy-2, 5-diisopropylphenyl)sulfide, bis-(4-hydroxy-2,3,6-trimethylphenyl)sulfide, bis-(2,4,5-trihydroxyphenyl)sulfide, bis-(4-hydroxy-2-cyclohexyl-5-methylphenyl)sulfide, bis-(2,3,4-trihydroxyphenyl)sulfide, bis-(4,5-dihydroxy-2-tert-butylphenyl)sulfide, bis-(4-hydroxy-2,5-diphenylphenyl)sulfide, bis-(4-hydroxy-2-tert-octyl-5-methylphenyl)sulfide. <4-hydroxyphenylarylsulfone type> 4-hydroxy-4'-isopropoxydiphenylsulfone, 4-hydroxy-4'-n-butyloxydiphenylsulfone, 4-hydroxy-4'-n-propoxydiphenylsulfone. <4-hydroxyphenylarylsulfonate type> 4-hydroxyphenylbenzenesulfonate, 4-hydroxyphenyl-p-tolylsulfonate, 4-hydroxyphenylmethylenesulfonate, 4-hydroxyphenyl-p-chlorobenzenesulfonate, 4-hydroxyphenyl-p-tert-butylbenzenesulfonate, 4-hydroxyphenyl-p-isopropoxybenzenesulfonate, 4-hydroxyphenyl-1'-naphthalenesulfonate, 4-hydroxyphenyl-2'-naphthalenesulfonate. <1,3-di[2-(hydroxyphenyl)-2-propyl]benzene type> 1,3-di[2-(4-hydroxyphenyl)-2-propyl]benzene, 1,3-di[2-(4-hydroxy-3-alkylphenyl)-2-propyl]benzene, 1,3-di[2-(2,4-dihydroxyphenyl)-2-propyl]benzene, 1,3-di[2-(2-hydroxy-5-methylpheny)-2-propyl]benzene. <resorcinol type> 1,3-dihydroxy-6(α,α-dimethylbenzyl)-benzene. <4-hydroxybenzoyloxybenzoic acid ester type> 4-hydroxybenzoyloxybenzyl benzoate, 4-hydroxybenzoyloxymethyl benzoate, 4-hydroxybenzoyloxyethyl benzoate, 4-hydroxybenzoyloxypropyl benzoate, 4-hydroxybenzoyloxybutyl benzoate, 4-hydroxybenzoyloxyisopropyl benzoate, 4-hydroxybenzoyloxytert-butyl benzoate, 4-hydroxybenzoyloxyhexyl benzoate, 4-hydroxybenzoyloxyoctyl benzoate, 4-hydroxybenzoyloxynonyl benzoate, 4-hydroxybenzoyloxycyclohexyl benzoate, 4-hydroxybenzoyloxy β-phenethyl benzoate, 4-hydroxybenzoyloxyphenyl benzoate, 4-hydroxybenzoyloxy α-naphthyl benzoate, 4-hydroxybenzoyloxy β-naphthyl benzoate, 4-hydroxybenzoyloxysec-butyl benzoate <bisphenolsulfone type (I)> bis-(3-1-butyl-4-hydroxy-6-methylphenyl)sulfone, bis-(3-ethyl-4-hydroxyphenyl)sulfone, bis-(3-propyl-4-hydroxyphenyl)sulfone, bis-(3-methyl-4-hydroxyphenyl)sulfone, bis-(2-isopropyl-4-hydroxyphenyl)sulfone, bis-(2-ethyl-4-hydroxyphenyl)sulfone, bis-(3-chloro-4-hydroxyphenyl)sulfone, bis-(2,3-dimethyl-4-hydroxyphenyl)sulfone. bis-(2,5-dimethyl-4-hydroxyphenyl)sulfone, bis-(3-methoxy-4-hydroxyphenyl)sulfone, 4-hydroxyphenyl-2'-ethyl-4'-hydroxyphenylsulfone, 4-hydroxyphenyl-2'-isopropyl-4'-hydroxyphenylsulfone, 4-hydroxyphenyl-3'-isopropyl-4'-hydroxyphenylsulfone, 4-hydroxyphenyl-3'-sec-butyl-4'-hydroxyphenylsulfone, 3-chloro-4-hydroxyphenyl-3'-isopropyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-butylphenyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-aminophenyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-isopropylphenyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-octylphenyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-butylphenyl-3'-chloro-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-butylphenyl-3'-methyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-butylphenyl-3'-isopropyl-4'-hydroxyphenylsulfone, 2-hydroxy-5-t-butylphenyl-2'-methyl-4'-hydroxyphenylsulfone. <bisphenolsulfone type (II)> 4,4'-sulfonyldiphenol, 2,4'-sulfonyldiphenol, 3,3'-dichloro-4,4'-sulfonyldiphenol, 3,3'-dibromo-4,4'-sulfonyldiphenol, 3,3',5,5'-tetrabromo-4,4'-sulfonyldiphenol, 3,3'-diamino-4,4'-sulfonyldiphenol. <others> p-tert-butylphenol, 2, 4-dihydroxybenzophenone, novolac type phenolic resin, 4-hydroxyacetophenone, p-phenylphenol, benzyl-4-hydroxyphenylacetate, p-benzylphenol. In the present invention, since the use of a color developer which has plural phenolic hydroxyl groups causes a problem of ground color contamination (ground color developing) by aqueous coating or by humidity in atmosphere, mono-phenol type color developer is preferably used when more high ground color stability is required. Especially, mono-phenol sulfone type color developer represented by above mentioned 4-hydroxyphenylarylsulfone contains sulfonyl group in molecular. A strong electron accepted portion is formed by an electron attractive of this sulfonyl group, indicates strong reactivity with dye precursor and performs an excellent color developing ability, further the obtained thermally recording medium is also superior to the stability of ground color. In this invention, a conventional well known sensitizer can be used in the limitation in which the desired effect of this invention is not prevented. As an example of the sensitizer, stearic acid amide, palmitic acid amide, methoxycarbonyl-N-benzamidestearate, N-benzoylstearic acid amide, N-eicosenoic acid amide, ethylene-bis-stearic acid amide, behenic acid amide, methylene-bis-stearic acid amide, methylolamide, N-methylolstearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, p-benzyloxybenzylbenzoate, 1-hydroxy-2-phenylnaphthoate, dibenzyloxalate di-p-methylbenzyloxalate, di-p-chlorobenzyloxalate, 2-naphthylbenzylether, m-terphenyl, p-benzylbiphenyl, 4-biphenyl-p-tolylether 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)buthane p-acetotoluidide p-acetophenetidide, N-acetoacetyl-p-toluidine, di-(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane 1,2-bis(phenoxymethyl)benzene p-toluenesulfonamide, o-toluenesulfonamide, di-p-tolylcarbonate and phenyl-α-naphtylcarbonate can be mentioned, however is not intended to be limited to these compounds. These sensitizers can be used alone or as a mixture of two or more. As the binder 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 and denatured polyvinyl alcohol by butylal, derivatives of cellulose such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose and acetyl cellulose, copolymer of styrene-maleic anhydride, copolymer of styrene-butadiene, polyvinyl chloride, polyvinyl acetate, polyacrylicamide, polyacrylic acid ester, polyvinylbutylal, polystyrene or copolymer of them,polyamide resin, silicon resin, petroleum resin, terpene resin, ketone resin and cumarone resin can be illustrated. These macro molecule compounds can be applied by being dissolved into solvents such as water, alcohol, ketone, ester or hydrocarbon or by being dispersed in water or othere medium under an emulsion state or a paste state and these forms of application can be used in combination according to the quality requirement. In the present invention, it is also possible to add known stabilizers based on metal salts (Ca, Zn) of p-nitrobenzoic acid or metal salts (Ca, Zn) of monobenzylphthalate, which have an effect to endow the recorded image with oil resistance, as much as the desire effect on the object of the present invention is not hindered. As a filler which can be used in this invention, an inorganic or an organic filler such as silica, calcium carbonate, kaoline, calcined kaoline, diatomaceous earth, talc, titanium oxide, zinc oxide, aluminum hydroxide, polystyrene resin, urea-formaldehyde resin, copolymer of styrene-methacrylic acid, copolymer of styrene-butadiene and hollow plastic pigment can be mentioned. Further, a parting agent such as metallic salt of fatty acid, a slipping agent such as wax, bezophenon- or triazole-based ultraviolet absorbers, water proof agent such as glyoxal, dispersing agent, defoamers anti-oxidation agent and fluorescent dye can be used as an additive. As a substrate, paper, synthetic paper, plastic film, plastic foam film, nonwoven fabric, recycled paper, metallic foil and a complex of these material can be used. Further, for the purpose to improve a friction resistance and an image preserving ability, an overcoat layer composed by high polymer composition can be prepared on the surface of thermally sensitive color developing layer. Furthermore, for the purpose to improve the color sensitivity, an undercoat layer containing organic or inorganic filler can be prepared between color developing layer and substrate. The amount of color developer and dye precursor, the kind and amount of other additives to be used to the thermally sensitive recording medium of this invention are decided according to the required quality and recording feature, and not restricted. However, in general, it is preferable to use 0.5∼4 parts of filler to 1 part of color developer and 5∼25 % of binder to the total amount of solid. The mixing ratio of 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-il)-4-azaphthalide and the black color developing leuco dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm is decided by the required color tone, however, the medium contains 0.05∼1 parts of black color developing leuco dye to 1 part of blue color developing leuco dye and it is desirable that the total parts of the leuco dye is 0,1∼2 parts to 1 part of organic color developer. The color tone of developed image slightly changes by a stabilizer, a sensitizer and other additives, however the influence of it is not so remarkable. These color developer, dye and other additives which are added at need are ground to the fine particles smaller than several microns diameter by means of a pulverizer such as a ball mill, an attriter or a sand grinder, or by means of an adequate emulsifying apparatus, then binder and other additives are added at need, thus the coating is prepared. As a method to coat the coating, a hand coating, a size press coating method, a roll coating method, an air knife coating method, a blend coating method, a flow coating method, a comma direct method, a gravure direct method, a gravure reverse method and a reverse·roll coating method can be mentioned. Further, the method to dry up after sputtering, spraying or dipping can also be used. EXAMPLES AND COMPARATIVE EXAMPLES 〈Preparation of thermally sensitive recording medium〉 The spontaneously color changing type thermally sensitive recording medium of this invention is illustrated by following Examples. In Examples, terms of parts and % indicate parts by weight and weight %. Example 1 Example 1 is an example of the thermally sensitive recording medium of this invention in which 4,4'-isopropyridenediphenol (bisphenol A, shortened to a in Table) is used as a color developer, 3,3-bis(p-dimethylaminophenyl)-6-dimethyl phthalide (shortened to C-1 in Table) is used as a blue color developing leuco dye and 3-dibutyl-6-methyl-7-anilinofluoran (shortened to B-1 in Table) is used as a black color developing dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm. Dispersion of color developer (A solution), dispersion of a blue color developing leuco dye (B solution) and a black color developing leuco dye (C solution) of following blending proportion are separately ground in a wet condition to average diameter of 1 µm by means of a sand grinder. A solution (dispersion of color developer) 4,4'-isopropyridenediphenol (a) 6.0 parts 10% aqueous solution of polyvinylalcohol 18.8 parts water 11.2 parts B solution (dispersion of blue color developing leuco dye) 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (C-1) 1.0 parts 10% aqueous solution of polyvinylalcohol 2.3 parts water 1.3 parts C solution (dispersion of black color developing leuco dye) 3-dibutylamino-6-methyl-7-anilinofluoran (B-1) 1.0 parts 10% aqueous solution of polyvinylalcohol 2.3 parts water 1.3 parts Then the resulting dispersion are mixed together by the proportion indicated below, mixed and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye [C-1]) 10.58 parts C solution (dispersion of black color developing leuco dye [B-1]) 3.22 parts Kaoline clay (50% dispersion) 12.0 parts The prepared coating is applied to one side of 50g/m 2 substrate paper and dried up, then the sheet is processed by a super calendar to surface smoothness of 500∼600 second and the thermally sensitive recording medium of 6.0 g/m 2 coating amount can be obtained. Example 2∼5 The thermally sensitive recording media are prepared by the same procedure to Example 1. At the preparation of A solution, 4-hydroxy-4'-isoprpoxydiphenylsulfone (shortened to b ; Example 2), 4-hydroxy-4'-propoxydiphenylsulfone (shortened to c ; Example 3), 4-hydroxybenzoic acid benzyl ester (shortened to d ; Example 4) and 4,4'-dihydroxydiphenylsulfone (shortened to e ; Example 5) are used instead of 4,4'-isopropyridendiphenol (a) Example 6 The thermally sensitive recording medium is prepared by the same procedure to Example 1. As the color developer, 4,4'-isopropyridenediphenol (a) and 4-hydroxy-4'-isopropoxydiphenylsulfone (b) are use. The mixing proportion of dispersion is mentioned below, stirred and the coating is prepared. A solution (dispersion of color developer [a]) 18.0 parts (dispersion of color developer [b]) 18.0 parts B solution (dispersion of blue color developing leuco dye [C-1]) 10.58 parts C solution (dispersion of black color developing leuco dye [B-1]) 3.22 parts Kaoline clay (50% dispersion) 12.0 parts Example 7 The thermally sensitive recording medium is prepared by the same procedure to Example 2. At the preparation of B solution, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide (C-2) is used as the blue leuco dye instead of 3,3-bis(p-dimethylaminophenyl)-6-dimethylamino phthalide(C-1). Example 8 The thermally sensitive recording medium is prepared by the same procedure to Example 2. As the blue leuco dye, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (C-1) and 3-(4-diethyl amino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide (C-2) are used. The mixing proportion of dispersion is mentioned below, and mixed, stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 5.29 parts B solution (dispersion of blue color developing leuco dye [C-2]) 5.29 parts C solution (dispersion of black color developing leuco dye [B-1]) 3.22 parts Kaoline clay (50% dispersion) 12.0 parts Example 9∼14 The thermally sensitive recording media are prepared by the same procedure to Example 2. At the preparation of C solution, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran (shortened to B-2 ; Example 9), 3-diethylamino-6-methyl-7-anilinofluoran (shortened to B-3 ; Example 10) 3-diethylamino-7-(m-trifluoromethylanilino)fluoran (shortened to B-4 ; Example 11), 3-diethylamino-6-methyl-7-m-methylanilinofluoran (shortened to B-5 ; Example 12), 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran (shortened to B-6 ; Example 13) and 3-di-n-pentylamino-6-methyl-7-anilinofluoran (shortened to B-7, Example 14) are used instead of 3-dibutylamino-6-methyl-7-anilinofluoran (B-1). Example 15 The thermally sensitive recording medium is prepared by the same procedure to Example 2. As the black color developing leuco dye, 3-dibutyl-6-methyl-7-anilinofluoran (B-1) and 3-(N-ethyl-N-isoamylamino)-6-methyl -7-anilinofluoran (shortened to B-2) are used. The mixing proportion of dispersion is mentioned below, and mixed, stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 10.58 parts C solution (dispersion of black color developing leuco dye [B-1]) 1.61parts C solution (dispersion of black color developing leuco dye [B-2]) 1.61 parts Kaoline clay (50% dispersion) 12.0 parts Example 16 The thermally sensitive recording medium is prepared by the same procedure to Example 9. The mixing proportion of dispersion is mentioned below, mixed and stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 12.88 parts C solution (dispersion of black color developing leuco dye [B-2]) 0.92 parts Kaoline clay (50% dispersion) 12.0 parts Example 17 The thermally sensitive recording medium is prepared by the same procedure to Example 9. The mixing proportion of dispersion is mentioned below, mixed and stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 9.2 parts C solution (dispersion of black color developing leuco dye [B-2]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 18 The thermally sensitive recording medium is prepared by the same procedure to Example 9. The mixing proportion of dispersion is mentioned below, mixed and stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 13.34 parts C solution (dispersion of black color developing leuco dye [B-2]) 0.46 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 19 The thermally sensitive recording medium is prepared by the same procedure to Example 9. The mixing proportion of dispersion is mentioned below, mixed and stirred and the coating is prepared. A solution (dispersion of color developer [a]) 36.0 parts B solution (dispersion of blue color developing leuco dye[C-1]) 5.06 parts C solution (dispersion of black color developing leuco dye [B-2]) 8.74 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 1 The thermally sensitive recording medium is prepared by the same procedure to Example 9. At the preparation of coating layer, C solution is not added. The mixing proportion of each dispersion is mentioned below, and mixed, stirred and the coating is prepared. A solution (dispersion of color developer [b]) 36.0 parts B solution (dispersion of blue color developing leuco dye [C-1]) 13.8 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 2 The thermally sensitive recording medium is prepared by the same procedure to Example 9. At the preparation of coating layer, B solution is not added. The mixing proportion of each dispersion is mentioned below, and mixed, stirred and the coating is prepared. A solution (dispersion of color developer [b]) 36.0 parts C solution (dispersion of black color developing leuco dye [B-2]) 13.8 parts Kaoline clay (50% dispersion) 12.0 parts < Method for Estimation > Using a thermally sensitive printer TH-PMD (product of Ohkura Electric Co., Ltd. Thermally recording paper printing tester in which Kyocera Thermal head is installed) recording tests are carried out on prepared spontaneously color changing type thermally sensitive recording medium by 0.41 mj/dot impressive energy. The color difference L*, a* and b* value prescript in JIS-Z-8729 are measured by color tone difference meter (NF-999 ; product of Nihon Denshoku Kogyo Co., Ltd.). Further, the density of developed color (O.D. in Tables) is measured by Macbeth densito meter (RD-914, red filter is used). The obtained results are summarized in Table 1 and Table 2. In Table 1, numerical value in parenthesis shows the blending proportion of a black color developing dye to a blue color developing dye. Kinds of color developer and dye experiment No. color developer color developing dye blue black Example 1 a C-1 B-1(0.3) Example 2 b C-1 B-1(0.3) Example 3 c C-1 B-1(0.3) Example 4 d C-1 B-1(0.3) Example 5 e C-1 B-1(0.3) Example 6 a/b C-1 B-1(0.3) Example 7 b C-2 B-1(0.3) Example 8 b C-1/C-2 B-1(0.3) Example 9 b C-1 B-2(0.3) Example 10 b C-1 B-3(0.3) Example 11 b C-1 B-4(0.3) Example 12 b C-1 B-5(0.3) Example 13 b C-1 B-6(0.3) Example 14 b C-1 B-7(0.3) Example 15 b C-1 B-1/B-2(0.3) Example 16 b C-1 B-2(0.07) Example 17 b C-1 B-2(0.5) Comp. Example 18 b C-1 H-2(0.03) Comp. Example 19 b C-1 B-2(1.7) Comp. Example 1 b C-1 - Comp. Example 2 b - B-2 L*, a*, b*value, tone and density after printed experiment No. L* a* b* color tone O.D. Example 1 30.5 -5.1 -0.6 grayish blue 1.27 Example 2 38.5 -0.7 0.1 grayish blue 1.25 Example 3 34.7 4.2 -1.7 grayish blue 1.23 Example 4 36.0 -2.1 0.4 grayish blue 1.28 Example 5 39.0 -0.9 -0.3 grayish blue 1.25 Example 6 32.6 1.8 -0.4 grayish blue 1.26 Example 7 30.4 6.8 -0.8 dark blue 1.30 Example 8 35.3 2.3 -0.5 navy blue 1.29 Example 9 39.4 -2.5 -0.1 grayish blue 1.30 Example 10 30.6 -4.5 -0.5 grayish blue 1.29 Example 11 30.0 -3.5 -1.9 grayish blue 1.28 Example 12 31.0 -6.0 -0.9 grayish blue 1.30 Example 13 33.0 -8.1 3.4 grayish blue 1.27 Example 14 33.1 -7.1 3.5 grayish blue 1.26 Example 15 39.1 -1.7 0.1 grayish blue 1.30 Example 16 40.7 2.1 -15.7 grayish blue 1.32 Example 17 35.4 -5.6 2.2 grayish blue 1.26 Comp. Example 18 40.8 3.1 -20.2 bright grayish blue 1.34 Comp. Example 19 31.4 -8.9 5.3 dark grayish blue 1.24 Comp. Example 1 46.3 36.0 -69.0 blue 1.40 Comp. Example 2 26.0 -1.5 1.9 black 1.32 〈Evaluation results〉 Examples 1∼19 of this invention, are the examples which use 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxy phenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide and black color developing leuco dye whose absorption maximum wave length is 420∼480 nm and 550∼640 nm. The color difference L* value, a* value and b* value regulated by JIS-Z-8729 of developed image of these Examples are within the regions of, A. L* value is 30∼50, a* value is -15∼0 and b* value is -30∼10, B. L* value is 30∼50, a* value is 0∼10 and b* value is -30∼0 or C. L* value is 30∼50, a* value color is 10∼20, b* value is -30∼-15, and the color tone of these Examples are navy blue or grayish blue. On the contrary, color difference a* value and b* value of Comparative Example 1 are out of the region regulated by this invention, and L* value of Comparative Example 2 is out of the region regulated by this invention. In both cases, the aimed color can not be obtained. Effect of the invention The thermally sensitive recording medium of this invention, has a sufficient color developing density and develops navy blue color or grayish blue color, therefore it is suited to be used in a field where these color tone are desired.

Claims: A thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive color developing layer comprising a colorless or a pale colored dye precursor selected from 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide and 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, together with at least one leuco dye which develops a black color with a maximum absorption wave length, measured in 99% acetic acid solution, of 420-480 nm and 550-640 nm, wherein the content of said black leuco dye is 0.05-1 parts to 1 part of the 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide or 3-(4-diethylamino-2-ethoxyphenyl) -3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, and a color developer and which gives a developed image having one of the following three combinations of color difference L* value, a* value and b* value, as regulated by JIS-Z-8729: L* is 30 to 50, a* is -15 to 0 and b* is -30 to 10; or L* is 30 to 50, a* is 0 to 10 and b* is -30 to 0; or L* is 30 to 50, a* is 10 to 20 and b* is -30 to -15. A recording medium according to claim 1 wherein the developed image is colored navy blue, grayish blue, or dim blue or dark navy blue according to the JIS common name. A recording medium according to claim 1 or 2 wherein the color developing layer further includes a sensitizer, binder, stabilizer based on metal salts of p-nitrobenzoic acid or monobenzylphthalate, filler or parting agent. A recording medium according to any one of the preceding claims which comprises, on the surface of the color developing layer, an overcoat layer comprising a polymeric composition. A recording medium according to any one of the preceding claims which comprises, between the color developing layer and the substrate, an undercoat layer comprising an inorganic filler.

Inventors: HAMADA KAORU, KIMURA YOSHIHIDE, MIDORIKAWA YOSHIMI, NAGAI TOMOAKI, HAMADA, KAORU, KIMURA, YOSHIHIDE, MIDORIKAWA, YOSHIMI, NAGAI, TOMOAKI, Hamada, Kaoru, Research Lab. of Product Dev. of, Kimura, Yoshihide, Res. Lab. of Product Dev. of, Midorikawa, Yoshimi, Res. Lab. of Product Dev. of, Nagai, Tomoaki, Nippon Paper Industries Co., Ltd.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1999
Main classification: B41M5
IPC codes: B41M5