Thermally sensitive recording medium

Abstract: A thermally sensitive recording medium which comprises, on a substrate, a developing layer comprising a colourless or a pale coloured dye precursor and a colour developer, and which gives a developed image having a colour difference a* value regulated by JIS-Z-8729 of 0- 50, and a colour difference b* value regulated by JIS-Z-8729 of -15 - 10. Preferably the developed image is coloured russet or wine red.

Description: The present invention relates to a thermally sensitive recording medium which develops russet or wine red colour. Generally, the thermally sensitive recording medium possessing a thermally sensitive recording layer mainly comprising a colourless or a pale coloured electron donating dye precursor (hereinafter shortened to dye precursor) and a colour developer which develops colour 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 thermal 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 colour image of these thermally sensitive recording medium is mainly black colour, however, a red colour developing type, a blue colour developing type, a green colour developing type, a full colour developing type and a dual colour developing type are also well known. EP-A-0058345 discloses a heat sensitive recording sheet having a heat sensitive layer comprising a basic colourless dye and dimethylterephthalate. The developed colour of thermally sensitive recording medium is comparatively sharp and close to a 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 colour such as russet colour or wine red colour is becoming desirable. However, in the conventional field of thermally sensitive recording media, there was not a thermally sensitive recording medium which develops these neutral colours. The object of the invention is to provide a thermally sensitive recording medium which has a sufficient colour density and develops russet or wine red. The invention provides a thermally sensitive recording medium which comprises, on a substrate, a developing layer comprising a colourless or pale coloured dye precursor which comprises a leuco dye which develops a red colour with a maximum absorption wavelength, measured in 99% acetic acid solution, of from 480 to 570 nm and a leuco dye which develops a black colour with a maximum absorption wavelength measured in 99% acetic acid solution, of from 420 to 480 nm and from 550 to 640 nm together with a colour developer, and which gives a developed image having a colour difference a* value regulated by JIS-Z-8729 of from 0 to 50, and a colour difference b* value regulated by JIS-Z-8729 of from -15 to 10. The colour difference a* value is a parameter which indicates green colour, and when the minus value of a* is big, the greenish tone is strong. And when the value becomes close to 0, the greenish tone becomes weak. Further, a plus a* value indicates the reddish tone. Meanwhile, the colour 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 a plus b* value indicates yellowish tone. In the thermally sensitive recording medium of this invention, the colour difference a* value of developed image is from 0 to 50 and b* value is from -15 to 0, and the required russet or wine red colour tone can be obtained. Further, a more vivid tone can be obtained when colour difference a* value is 10∼50 and b* value is -15∼10, more desirable a* value is 15∼45 and b* value is -10∼0. When these colour difference values are out of the region regulated in this invention, the colour of developed image becomes red or black and the aimed colour tone can not be obtained. In addition to a* and b* value regulating in this invention, the colour tone can be also indicated by L* value which displays brightness. L* value is not limited in this invention, however, when L* value is too low, colour tone becomes dark. Therefore, desirably the practical L* value is to be 20∼60, and more desirably to be 20∼50. In one aspect of the invention the dye precursor is composed by at least one kind of a red colour developing leuco dye whose maximum absorption wavelength is 480-570 nm and at least one kind of a black colour developing leuco dye whose maximum absorption wavelength is 420-480 nm 550-640nm. The maximum absorption wavelength of 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 colour such as russet colour or wine red colour can be easily obtained. In another aspect of the invention the thermally sensitive recording medium comprises 0.05-1 parts of black colour developing leuco dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm to 1 part of red colour developing leuco dye whose maximum absorption wave length is 480∼570 nm. When the content of black colour developing leuco dye is smaller than 0.05 parts to 1 part of red colour developing leuco dye, the aimed colour tone can be obtained, however, the colour density becomes slightly low. This is not a problem in a practical use, but the contrast of developed image is slightly bad. In a meanwhile, when the content is bigger than 1 part, the contrast of developed image is good, but the black colour tone becomes slightly strong. Therefore, it is desirable to contain the black colour developing leuco dye by above mentioned ratio to the red colour developing leuco dye to obtain the thermally sensitive recording medium whose developed image is vivid russet or wine red colour and the contrast of image is good. In a further aspect of the invention the thermally sensitive recording medium comprises at least one kind of chemical compound indicated by general formula (I) as an red colour developing leuco dye whose maximum absorption wave length is 480∼570 nm. where R is an unsubstituted or substituted C 1-8 alkyl group. The use of these leuco dye is effective to obtain the russet or wine red colour which is the object of this invention. Further, the preserving ability of the developed image is improved, and especially the resistance to the plasticizer is remarkably improved. The reason for above mentioned improvement can not be clearly explained, however, in this invention, it is assumable that the polar of the compound represented by general formula (1) is high, and the solubility of developed image formed by the reaction with a colour developer to the plasticizer becomes low. In another aspect of the invention the thermally sensitive recording medium displays the developed colour image of russet or wine red colour. In this invention, the term of russet or wine red colour means dim and dark neutral tone developing of red, which is disclosed e.g. from page 32 to 33, in item "8 Dull & Dark red" of "Colour one point 10, colour naming and it's episode" (Japan Standard Society, issued on November 19, 1993). These colours are expressed as dim red or dark red by JIS common name, or are expressed as russet, reddish brown or garnet colour by idiomatic colour naming. Further, the expression of colour becomes different by a subjectivity of inspector or by an illumination, and in this invention, the expression of russet colour or wine red colour are typically used containing commonly expressed wine red colour, rose colour or reddish purple colour, however, not limited to them. As a leuco dye which develops red colour used in this invention, the leuco dye whose maximum absorption wave length in 99% acetic acid solution is from 480∼570 nm can be used. As the concrete example, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-benzo[a]fluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-methylfluoran, 3-N-ethyl-N-isoamylamino- benzo[a]fluoran, 3-N-ethyl-N-p-methylphenylamino-7-methylfluoran, 3-dibutylamino-6-methyl-7-bromofluoran, 3,6-bis(diethylamino)fluoran- γ-(4'-nitro)-anilinolactam, 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalido, 3,3-bis(1-n-octyl-2-methylindol-3-yl)phthalido, 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalido and 3,6-bis(diethylamino)fluoran-γ-anilinolactam, can be mentioned, however, not limited to them. And these leuco dyes can be used alone or can be used together with. Among above mentioned chemicals, 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalido, 3,3-bis(1-n-octyl-2-methylindol-3-yl)phthalido and 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalido are the compound indicated by above mentioned formula (1). As a leuco dye which develops black colour which is used in this invention, the leuco dye whose maximum absorption wave length in 99% acetic acid solution is 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-fuluoroanilino)fluoran, 3-di-n-pentylamino-6-methyl-7-anilinofluoran, 3-di-n-pentylamino-7-(m-trifuluoromethylanilino)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 can be used as a mixture of two or more. In this invention, it is possible to add a small amount of leuco dye which develops orange colour as to obtain a desired colour tone. The desirable amount to be added is less than 0.05 parts of orange colour developing leuco dye to 1 part of red colour developing leuco dye. As the concrete examples of a leuco dye which develops orange colour, 3-cyclohexylamino-6-chlorofluoran and 3-diethylamino-6,8-dimethylfluoran can be mentioned, however, not limited to them. As an organic colour 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 bisphenolsulfone type 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-buthoxydiphenylsulfone, 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-propyllbenzene, 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-methylphenyl)-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-hydroxybenzoyloxy sec-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'-hydroxyphenyl sulfone, 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 colour developer which has plural phenolic hydroxyl groups causes a problem of ground colour contamination (ground colour developing) by aqueous coating or by humidity in atmosphere, mono-phenol type colour developer is preferably used when more high ground colour stability is required. Especially, mono-phenol sulfone type colour 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 colour developing ability, further the obtained thermally recording medium is also superior to the stability of ground colour. 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-tarphenyl, 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, polyvinylbutyral, 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, benzophenon- or triazole- based ultra violet absorbers, water proof agent such as glyoxal, dispersing agent, deformers, 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 colour developing layer. Furthermore, for the purpose to improve the colour sensitivity, an undercoat layer containing organic or inorganic filler can be prepared between colour developing layer and substrate. The amount of colour 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 limited. However, in general, it is preferable to use 0.4∼4 parts of filler to 1 part of colour developer and 5∼25 % of binder to the total amount of solid. When the red colour developing leuco dye whose maximum absorption wave length is 480∼570 nm and the black colour developing leuco dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm as a dye precursor are used, the mixing ratio is decided by the required colour tone, however, it is desirable to contain 0.05∼1 parts of black colour developing leuco dye to 1 part of red colour developing leuco dye and the desirable total parts of these leuco dye is 0.1∼2 parts to 1 part of organic colour developer. The colour tone of developed image slightly changes by a stabilizer, a sensitizer and other additives, however the influence of it is not so remarkable. These colour developer, dye and other additives which are added at need are ground to the fine particles smaller than several micrometres in 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. The spontaneously colour 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 colour developer, 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide, (shortened to R-1 in Table) is used as an red colour developing leuco dye whose maximum absorption wave length is 480∼570 nm and 3-dibutyl-6-methyl-7-anilinofluoran (shortened to B-1 in Table) is used as a black colour developing dye whose maximum absorption wave length is 420∼480 nm and 550∼640 nm. Dispersion of colour developer (A solution), dispersion of a red colour developing leuco dye (B solution) and a black colour developing leuco dye (C solution) prepared by following blending proportion are separately ground in a wet condition to average diameter of 1µm by means of a sand grinder. A solution (dispersion of colour developer) 4,4'-isopropyridenediphenol (a) 6.0 parts 10% aqueous solution of polyvinylalcohol 18.8 parts water 11.2 parts B solution (dispersion of red colour developing leuco dye) 3,3-bis(1-n-butyl-2-methylindol-3-yl)phtalide (R-1) 1.0 parts 10% aqueous solution of polyvinylalcohol 2.3 parts water 1.3 parts C solution (dispersion of black colour 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 below and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye [R-1]) 10.58 parts C solution (dispersion of black colour 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∼6 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-hydroxy-4'-buthoxydiphenylsulfone (shortened to d ; Example 4) and 4-hydroxybenzoic acid benzyl ester (shortened to e ; Example 5) 4,4'-dihydroxydiphenylsulfone (shortened to f; Example 6) are used instead of 4,4'-isopropyridendiphenol (a) Example 7 The thermally sensitive recording medium is prepared by the same procedure to Example 1. As the colour developer, 4,4'-isopropyridenediphenol (a) and 4-hydroxy-4'-isopropoxydiphenylsulfone (b) are use. The mixing proportion of dispersion is mentioned below, and the coating is prepared. A solution (dispersion of colour developer [a]) 18.0 parts A solution (disperdion of colour developer [b]) 18.0 parts B solution (dispersion of red colour developing leuco dye [R-1]) 10.58 parts C solution (dispersion of black colour developing leuco dye [B-1]) 3.22 parts Kaoline clay (50% dispersion) 12.0 parts Example 8∼10 The thermally sensitive recording media are prepared by the same procedure to Example 2. At the preparation of B solution, 3,3-bis(1-ethyl-2-methylindole-3-yl)phthalide (shortened to R-2 ; Example 8), 3-diethylamino-6-methyl-7-chlorofluoran (shortened to R-3 ; Example 9) and 3-diethylamino-benzo[a]fluoran (shortened to R-4 ; Example 10) are used instead of 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide (R-1). Example 11 The thermally sensitive recording medium is prepared by the same procedure to Example 2. As the red colour developing leuco dye, 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide (R-1) and 3,3-bis(1-ethyl-2-methylindole-3-yl)phthalide (R-2) are used. The mixing proportion of dispersion is mentioned below, and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye [R-1]) 5.29 parts B solution (dispersion of red colour developing leuco dye [R-2]) 5.29 parts C solution (dispersion of black colour developing leuco dye [B-1]) 3.22 parts Kaoline clay (50% dispersion) 12.0 parts Example 12∼17 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 12), 3-diethylamino-6-methyl-7-anilinofluoran (shortened toB-3 ; Example 13) 3-diethylamino-7-(m-trifluoromethylanilino)fluoran (shortened to B-4 ; Example 14), 3-diethylamino-6-methyl-7-m-methylanilinofluoran (shortened to B-5 ; Example 15), 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran (shortened to B-6 ; Example 16) and 3-di-n-pentylamino-6-methyl-7-anilinofluoran (shortened to B-7, Example 17) are used instead of 3-dibutylamino-6-methyl-7-anilinofluoran (B-1). Example 18 The thermally sensitive recording medium is prepared by the same procedure to Example 2. As the black colour 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 the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye[R-1]) 10.58 parts C solution (dispersion of black colour developing leuco dye [B-1]) 1.61 parts C solution (dispersion of black colour developing leuco dye [B-2]) 1.61 parts Kaoline clay (50% dispersion) 12.0 parts Example 19 The thermally sensitive recording medium is prepared by the same procedure to Example 12. The mixing proportion of dispersion is mentioned below, and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye[R-1]) 12.88 parts C solution (dispersions of black colour developing leuco dye [B-2]) 0.92 parts Kaoline clay (50% dispersion) 12.0 parts Example 20 The thermally sensitive recording medium is prepared by the same procedure to Example 12. The each dispersion are mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye[R-1]) 9.2 parts C solution (dispersion of black colour developing leuco dye [B-2]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts Example 21 The thermally sensitive recording medium is prepared by the same procedure to Example 12. The each dispersion are mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye[R-1]) 13.34 parts C solution (dispersion of black colour developing leuco dye [B-2]) 0.46 parts Kaoline clay (50% dispersion) 12.0 parts Example 22 The thermally sensitive recording medium is prepared by the same procedure to Example 12. The each dispersion are mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of red colour developing leuco dye[R-1]) 5.06 parts C solution (dispersion of black colour 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 1. At the preparation of coating, the dispersion of 3-cyclohexylamino-6-chlorofluoran (shortened to Or) which is orange colour developing leuco dye whose maximum absorption wave length is 475 nm is added instead of 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide (R-1). The dispersion of Or (D solution) is ground in a wet condition to average diameter of 1 µm by means of a sand grinder. D solution (dispersion of orange colour developing leuco dye) 3-cyclohexylamino-6-chlorofluoran (Or) 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 below and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts D solution (dispersion of orange colour developing leuco dye [Or]) 10.58 parts C solution (dispersion of black colour developing leuco dye [B-2]) 3.22 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 which installs Kyosera Thermal head) recording tests are carried out on prepared spontaneously colour changing type thermally sensitive recording medium by 0.41 mJ/dot impressive energy. The colour difference a* and b* value prescript in JIS-Z-8729 is measured by colour difference meter (NF999 ; product of Nihon Denshoku Kogyo Co., Ltd.). C standard light regulated by JIS-Z-8720-1983 is used and measured by angle 2 degree. Further, the colour tone of developed image is evaluated by the naked eyes of the inspector. The density of developed colour is measured by Macbeth densito meter (RD-914, cyan filter is used) and the measured date is used as the density of untreated specimen. The colour developed specimen sheet of thermally sensitive recording medium is stuck on a paper tube on which surface a single layer of wrapping sheet of vinyl chloride is wound, further triple layers of said wrapping sheet are wound on the specimen sheet. After left for 4 hours at room temperature, Macbeth density of the colour image part is measured, and the resistance to a plasticizer is evaluated. The obtained results are summarized in Tables 1 and 2. (Remarks) In Table 1, numerical value in parenthesis shows the blending proportion of a black colour developing dye to a red colour developing dye, and only in a case of Comparative Example 1, it shows the blending proportion of a black colour developing dye to an orange colour developing dye. Further, in Table 2, the colour tone is that of evaluated by the naked eyes of the inspector. Kinds of colour developer and dye experiment No. colour developer colour developing dye red black Example 1 a R-1 B-1(0.3) Example 2 b R-1 B-1(0.3) Example 3 c R-1 B-1(0.3) Example 4 d R-1 B-1(0.3) Example 5 e R-1 B-1(0.3) Example 6 f R-1 B-1(0.3) Example 7 a/b R-1 B-1(0.3) Example 8 b R-2 B-1(0.3) Example 9 b R-3 B-1(0.3) Example 10 b R-4 B-1(0.3) Example 11 b R-1/R-2 B-1(0.3) Example 12 b R-1 B-2(0.3) Example 13 b R-1 B-3(0.3) Example 14 b R-1 B-4(0.3) Example 15 b R-1 B-5(0.3) Example 16 b R-1 B-6(0.3) Example 17 b R-1 B-7(0.3) Example 18 b R-1 B-1/B-2(0.3) Example 19 b R-1 B-2(0.07) Example 20 b R-1 B-2(0.5) Example 21 b R-1 B-2(0.03) Example 22 b R-1 B-2(1.73) Comp. Example 1 b Or B-2(0.3) a*, b*, tone and density after printed experiment No. a* b* colour tone untreated resistance to plasticizer Example 1 26.4 -3.8 wine red 1.38 0.41 Example 2 30.8 -3.1 wine red 1.40 0.75 Example 3 35.7 -4.9 wine red 1.35 0.39 Example 4 32.2 -2.2 wine red 1.34 0.40 Example 5 29.4 -2.8 wine red 1.36 0.39 Example 6 31.3 -3.3 wine red 1.39 0.7 Example 7 33.3 -3.6 wine red 1.40 0.60 Example 8 30.7 -2.9 wine red 1.40 0.74 Example 9 23.7 0.9 wine red 1.36 0.10 Example 10 23.2 0.4 wine red 1.37 0.11 Example 11 31.0 -2.9 wine red 1.41 0.76 Example 12 29.0 -3.3 wine red 1.47 0.77 Example 13 30.0 -4.7 wine red 1.42 0.80 Example 14 27.5 -3.7 wine red 1.41 0.65 Example 15 21.0 0.6 wine red 1.43 0.78 Example 16 27.9 -1.4 wine red 1.40 0.64 Example 17 28.5 -1.3 wine red 1.39 0.62 Example 18 31.3 -1.5 wine red 1.44 0.78 Example 19 40.3 -6.3 bright wine red 1.41 0.80 Example 20 18.8 -2.0 russet 1.46 0.65 Example 21 42.8 -7.4 bright wine red 1.30 0.82 Example 22 9.7 -1.7 bright wine red 1.37 0.56 Comp. Example 1 15.3 16.2 light brown 1.31 0.11 Evaluation result Examples 1∼22 of this invention, are the examples which use red colour developing leuco dye whose maximum absorption wave length is 480∼570 nm and black colour developing leuco dye whose absorption maximum wave length is 420 ∼480 nm and 550∼640 nm. The colour difference a* value of thermally sensitive recording medium of these Examples are within the region of 0∼50, and that of colour difference b* value are within the region of -15∼10, and the colour tone of these Examples are russet colour or wine red. On the contrary, colour difference a* value and b* value of Comparative Example 1 are out of the region regulated by this invention and the aimed colour can not be obtained. And, the Examples 1∼8, 11∼22 in which the dye represented by general formula (1) is used have a better resistance to a plasticizer than the Examples 9 and 10 in which said dye is not used. Effect of the invention The thermally sensitive recording medium of this invention, has a sufficient colour developing density and develops russet colour or wine red colour, therefore it is suited to be used in a field where these colour tone are desired.

Claims: A thermally sensitive recording medium which comprises, on a substrate, a developing layer comprising a colourless or pale coloured dye precursor which comprises a leuco dye which develops a red colour with a maximum absorption wavelength, measured in 99% acetic acid solution, of from 480 to 570 nm and a leuco dye which develops a black colour with a maximum absorption wavelength measured in 99% acetic acid solution, of from 420 to 480 nm and from 550 to 640 nm together with a colour developer, and which gives a developed image having a colour difference a* value regulated by JIS-Z-8729 of from 0 to 50, and a colour difference b* value regulated by JIS-Z-8729 of from -15 to 10. A recording medium according to claim 1, wherein the leuco dye which develops a black colour is contained in an amount of from 0.05 to 1 parts to 1 part of the leuco dye which develops a red colour. A recording medium according to claim 1 or 2, wherein the leuco dye which develops a red colour comprises a compound of formula (I): wherein R is an unsubstituted or substituted C 1 -C 8 alkyl group. A recording medium according to any one of the preceding claims wherein the developed image is coloured russet, wine red, or dim red or dark red according to JIS common name. A recording medium according to any one of the preceding claims wherein the colour developing layer further includes a sensitizer, a binder, a stabilizer based on metal salts of p-nitrobenzoic acid or monobenzylphthalate, a filler or a parting agent. A recording medium according to any one of the preceding claims which comprises, on the surface of the colour developing layer, an overcoat layer comprising a polymeric composition. A recording medium according to any one of the preceding claims which comprises, between the colour developing layer and the substrate, an undercoat layer comprising an organic or inorganic filler.

Inventors: HAMADA KAORU, KIMURA YOSHIHIDE, MIDORIKAWA YOSHIMI, NAGAI TOMOAKI, HAMADA, KAORU, KIMURA, YOSHIHIDE, MIDORIKAWA, YOSHIMI, NAGAI, TOMOAKI, Hamada, Kaoru, c/o Nippon Paper Ind. Co. Ltd., Kimura, Yoshihide, c/o Nippon Paper Ind. Co. Ltd., Midorikawa, Yoshimi, c/o Nippon Paper Ind. Co. Ltd, Nagai, Tomoaki, c/o Nippon Paper Ind. Co. Ltd.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1999
Main classification: B41M5
IPC codes: B41M5