Thermally sensitive recording medium

Abstract: A thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive colour 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 - 40 and a colour difference b* value regulated by JIS-Z-8729 of 0 - 55. Preferably the developed image is coloured sepia or light brown.

Description: The present invention relates to a thermally sensitive recording medium which develops sepia or light brown colour. EP-A-0181698 provides a heat sensitive recording material comprising a heat sensitive colour-forming layer comprising 3,3'-dialkyl-4-4'-dihydroxydiphenylsulphone as an effective ingredient. 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 media is mainly black in 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. The developed colour of the 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 sepia colour or light brown colour is becoming desirable. In the conventional field of thermally sensitive recording media, it is commonly observed that he developed colour changes to sepia or light brown colour by the effect of environment such as sun light and temperature, or by the effect of chemicals, however, this colour change is a phenomenon which is recognised as a problem caused by poor colour preserving. And the thermally sensitive recording medium whose original developed colour is sepia or light brown is not developed yet. The object of this invention is to provide a thermally sensitive recording medium which has a sufficient colour density and develops sepia or light brown colour. The invention provides a thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive colour developing layer which comprises a colourless or pale coloured dye precursor comprising a leuco dye which develops an orange colour with a maximum absorption wavelength measured in 99% acetic acid solution of from 460 to 550 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 and a colour developer, and which gives a developed image having a colour difference a* value regulated by JIS-Z-8729 of from 0 to 40 and a colour difference b* value regulated by JIS-Z- 8729 of from 0 to 55. 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, plus a* values indicate 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 0∼40 and b* value is 0∼55, and the required sepia or light brown colour tone can be obtained. Further, more vivid tone can be obtained when colour difference a* value is 0∼40 and b* value is 3∼55, more desirable a* value is 0∼30 and b* value is 10∼40. When these colour difference values are out of the region regulated in this invention, the colour of developed image becomes orange or black and the aimed colour tone can not be obtained. In addition to the a* and b* values regulated according to this invention, the colour tone can also be indicated by the 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 30∼60, and more desirably to be 30∼50. In one aspect of the invention the dye precursor is composed by at least one kind of an orange colour developing leuco dye whose maximum absorption wavelength is 460 to 550 nm and at least one kind of a black colour developing leuco dye whose maximum absorption wave length is 420~480 nm and 550~640 nm. The maximum absorption wave length 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 sepia colour or light brown colour can be easily obtained In another aspect of the invention the thermally sensitive recording medium contains 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 orange colour developing leuco dye whose maximum absorption wave length is 460∼550 nm. When the content of black colour developing leuco dye is smaller than 0.05 parts to 1 part of orange colour developing leuco dye, the colour of developed image becomes reddish brown which is the aimed colour tone, however, the colour density becomes slightly low. This is not a problem in a practical use, however, 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 colour tone becomes slightly dark and looks like dark brown. Therefore, it is desirable to contain the black colour developing leuco dye by above mentioned ratio to the orange colour developing leuco dye to obtain the thermally sensitive recording medium whose developed image is vivid sepia or light brown colour and the contrast of image is good. In a further aspect of the invention the thermally sensitive recording medium contains at least one leuco dye selected from the group composed by 3-cyclohexylamino-6-chlorofluoran and 3-diethylamino-6,8-dimethylfluoran as an orange colour developing leuco dye whose maximum absorption wave length is 460~550 nm. The use of these leuco dye is effective to obtain the sepia or light brown colour which is the object of this invention. In another aspect or the invention the thermally sensitive recording medium displays the developed colour image of sepia or light brown colour. In this invention, the term of sepia or light brown colour means dim and dark neutral tone developing of yellowish red or light brown, which is disclosed e.g. from page 42 to 51 of "Colour one point 10, colour naming and it's episode" (Japan Standard Society, issued on November 19, 1993) in items of "13 DULL ORANGE", "14 REDDISH BROWN". "15 BROWN", "16 YELLOWISH BROWN" or "17 GRAYISH BROWN". These colours are expressed as dim yellowish red, dim red, dark red, dark yellowish red and dark gray by JIS common name, or are expressed as cinnamon colour, wheat colour, fox colour, brick colour, reddish brown, light brown, nut-brown colour, dark-brown colour, yellowish brown and earth-brown 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 sepia colour or light brown colour are typically used containing commonly expressed sepia or light brown colour, however, not limited to them. As a leuco dye which develops orange colour used in this invention, the leuco dye whose maximum absorption wave length in 99% acetic acid solution is from 460∼550 nm can be used. As the concrete examples, 3-cyclohexylamino-6-chlorofluoran and 3-diethylamino-6,8-dimethylfluoran 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. As a leuco dye which develops black colour 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-penthylamino-6-methyl-7-anilinofluoran, 3-di-n-penthylamino-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 as a mixture of two or more. For the purpose to adjust the colour tone, to the thermally sensitive recording medium of this invention, small amount of the well known reddish colour developing leuco dye can be added. As the concrete example of such kind of leuco dye, 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-buromofluoran, 3,6-bis(diethylamino)fluoran-γ-(4'-nitro)-anilinolactam, 3,3-bis(1-n-butyl-2-methylindole-3-yl)phthalide, 3,3-bis(1-n-octhyl-2-methylindole-3-yl)phthalide, 3,3-bis(1-ethyl-2-methylindole-3-yl)phthalide and 3,6-bis(diethylamino)fluoran-γ-anilinolactam can be mentioned, however, not limited to them. And the desirable adding amount is about 0.01∼0.05 parts to 1 part of the orange colour developing leuco dye. As an organic colour developer which can be used in this invention, bis-phenol A tyoe, 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-hydroxylsopropyl benzoate, 4-hydroxybutyl benzoate, 4-hydroxyisobutyl benzoate, 4-hydroxymethylbenzyl benzoate. <4-hydroxyphthalic acid diester type> dimethyl 4-hydroxyphthalate, diisopropyl 4-hydroxy phthalate, dibenzyl 4-hydroxyphthalate, dihexyl 4-hydroxyphthalate, <phthalic acid monoesters> 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-hydroxyphenylarylsulfonates> 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-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 bonzoate, 4-hydroxybenzoyloxy β-phenethyl benzoate, 4-hydroxybenzoyloxyphenyl benzoate, 4-hydroxybenzoyloxy α-naphthyl benzoate, 4-hydroxybenzoyloxy β-naphthyl benzoate, 4-hydroxybenzoyloxysec-butyl benzoate. <bisphenolsulfone type (1)> 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-hyydroxyphenyl-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-benzyloxybenzyl benzoate, 1-hydroxy-2-phenyl naphthoate, dibenzyl oxalate di-p-methylbenzyl oxalate, di-p-chlorobenzyl oxalate, 2-naphthylbenzylether, m-tarphenyl, p-benzylbiphenyl, 4-biphenyl-p-tolylether di(p-methoxyphenoxyethyl)ether 1,2-di(3-methylphenoxy)ethane 1,2-di(4-methylphonoxy)ethane 1,2-di(4-methoxyphenoxy)ethane 1,2-di(4-chlorophenoxy)ethane 1,2-diphenoxyethane 1-(4-methoxyphenoxy)-2-(2-methylphenoxy)ethane p-methyltiophenylbenzylether 1,4-di(phenyltio)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 those compounds. These sensitizers can be used alone or by mixing more than two kinds of them. As the binder used in the present invention, full saponificated polyvinyl alcohol of 200∼1900 polymerization degree, partial saponificatod polyvinyl alcohol, denatured polyvinyl alcohol by carboxyl, denatured polyvinyl alcohol by amide denatured polyvinyl alcohol by sulfonic acid denatured polyvinyl alcohol by butylal modified polyvinyl alcohol, derivatives of cellulose such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose and acetyl cellulose, copolymer of styrene-maleic unhydride, copolymer of styrene-butadiene, polyvinyl chloride, polyvinyl acetal, 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, benzophenon- or triazole- based ultra violet 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 fabrics, 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 0.5∼4 parts of filler to 1 part of colour developer and 5∼25 % of binder to the total amount of solid. When the orange colour developing leuco dye whose maximum absorption wave length is 460∼550 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 orange 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. Further, for the purpose to adjust the colour tone, reddish colour developing leuco dye can be added, and the desirable amount is 0.01∼0.05 parts to 1 part of orange colour developing leuco dye. And, the colour tone is slightly changeable by additives such as stabilizer, sensitizer or others, but the effect 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 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 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-cyclohexylamino-6-chrolofluoran (shortened to Or-1 in Table) is used as an orange colour developing leuco dye whose maximum absorption wave length is 460∼550 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 an orange 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 orange colour developing leuco dye) 3-cyclohexylamino-6-chrolofluoran (Or-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-dibutyl-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, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye [Or-1]) 11.04 parts C solution (dispersion of black colour developing leuco dye [B-1]) 2.76 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 benzoyl ester (shortened to e ; Example 5) 4,4'-dihydroxydiphenylsulfone (shortened to f ; Example 6) are used instead of 4,4'-isopropyridendiphenyl (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'-isoprpoxydiphenylsulfone (b) are use. The mixing proportion of dispersion is mentioned below, mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 18.0 parts A solution (dispersion of colour developer [b]) 18.0 parts B solution (dispersion of orange colour developing leuco dye [Or-1]) 11.04 parts C solution (dispersion of black colour developing leuco dye [B-1]) 2.76 parts Kaoline clay (50% dispersion) 12.0 parts [Example 8] The thermally sensitive recording medium is prepared by the same procedure to Example 2. At the preparation of B solution. 3-diethylamino-6,8-dimethylfluoran (Or-2) is used instead of 3-cyclohexylamino-6-chrolofluoran (Or-1). [Example 9] The thermally sensitive recording medium is prepared by the same procedure to Example 2. As the orange colour developing leuco dye, 3-cyclohexylamino-6-chrolofluoran (Or-1) and 3-diethylamino-6,8-dimethylfluoran (Or-2) is used. The mixing proportion of dispersion is mentioned below, mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye[Or-1]) 5.52 parts B solution (dispersion of orange colour developing leuco dye [Or-2]) 5.52 parts C solution (dispersion of black colour developing leuco dye [B-1]) 2.76 parts Kaoline clay (50% dispersion) 12.0 parts [Example 10∼15] 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 10), 3-diethylamino-6-methyl-7-anilinofluoran (shortened to B-3 ; Example 11) 3-diethylamino-7-(m-trifluoromethylanilino)fluoran (Shortened to B-4 ; Example 12), 3-diethylamino-6-methyl-7-m-methylanilinofluoran (shortened to B-5 ; Example 13), 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran (shortened to B-6 ; Example 14) and 3-di-n-penthylamino-6-methyl-7-anilinofluoran (shortened to B-7, Example 15) are used instead of 3-dibutyl-6-methyl-7-anilinofluoran (B-1). [Example 16] 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, mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing Ieuco dye[Or-1]) 11.04 parts C solution (dispersion of black colour developing leuco dye [B-1]) 1.38 parts C solution (dispersion of black colour developing leuco dye [B-2]) 1.38 parts Kaoline clay (50% dispersion) 12.0 parts [Example 17] The thermally sensitive recording medium is prepared by the same procedure to Example 10. The mixing proportion of dispersion is mentioned below, mixed, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye[OR-1]) 12.88 parts C solution (dispersion of black colour developing leuco dye [B-2]) 0.92 parts Kaoline clay (50% dispersion) 12.0 parts [Example 18] The thermally sensitive recording medium is prepared by the same procedure to Example 10. The each dispersion are mixed by following mixing proportion. stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye[Or-1]) 8.28 parts C solution (dispersion of black colour developing leuco dye [B-2]) 5.52 parts Kaoline clay (50% dispersion) 12.0 parts [Example 19] The thermally sensitive recording medium is prepared by the same procedure to Example 10. The each dispersion are mixed by following mixing proportion. stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye[Or-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 20] The thermally sensitive recording medium is prepared by the same procedure to Example 10. The each dispersion are mixed by following mixing proportion, stirred and the coating is prepared. A solution (dispersion of colour developer [a]) 36.0 parts B solution (dispersion of orange colour developing leuco dye[Or- 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 [Example 21] The thermally sensitive recording medium is prepared by the same procedure to Example 10. At the preparation of coating, the dispersion of 3,3-bis(1-n-butyl-2-methylindol-3-il)phthalide which is reddish colour developing leuco dye is added. While, the proportion of reddish developing leuco dye to 1 part of orange colour developing leuco dye is 0.029 parts. The reddish colour developing leuco dye D solution of following blending proportion are ground in a wet condition to average diameter of 1 µm by means of a sand grinder. D solution (dispersion of reddish colour developing leuco dye) 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide (RED) 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 orange colour developing leuco dye [Or-1]) 11.04 parts C solution (dispersion of black colour developing leuco dye [B-2]) 2.76 parts D solution (dispersion of reddish colour developing leuco dye [RED]) 0.322 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 B solution, 3,6-bis(diethylamino)fluoran-γ-(4'-nitro)-anilinolactam (shortened to Red 2) is used instead of 3-cyclohexylamino-6-chrolofluoran (Or-1). The dispersion of Red 2 (D solution) is ground in a wet condition to average diameter of 1 µm by means of a sand grinder. D solution (dispersion of reddish leuco dye) 3,6-bis(diethylamino)fluoran-γ-(4'-nitro)-anilinolactum (Red 2) 1.0 part 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 I) solution (dispersion of reddish colour developing leuco dye [Red2]) 11.04 parts C solution (dispersion of black colour developing leuco dye [B-2]) 2.76 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 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 density of developed colour (O.D. in Tables) is measured by Macbeth densito meter (RD-914, blue filter is used). The obtained results are summarized in Table 1 and Table 2. Kinds of colour developer and dye experiment No. colour developer colour developing dye orange black Example 1 a Or-1 B-1(0.25) Example 2 b Or-1 B-1(0.25) Example 3 c Or-1 B-1(0.25) Example 4 d Or-1 B-1(0.25) Example 5 e Or-1 B-1(0.25) Example 6 f Or-1 B-1(0.25) Example 7 a/b Or-1 B-1(0.25) Example 8 b Or-2 B-1(0.25) Example 9 b Or-1/Or-2 B-1(0.25) Example 10 b Or-1 B-2(0.25) Example 11 b Or-1 B-3(0.25) Example 12 b Or-1 B-4(0.25) Example 13 b Or- 1 B-5(0.25) Example 14 b Or-1 B-6(0.25) Example 15 b Or-1 B-7(0.25) Example 16 b Or-1 B-1/B-2(0.25) Example 17 b Or- 1 B-2(0.07) Example 18 b Or-1 B-2(0.67) Example 19 b Or-1 B-2(0.03) Example 20 b Or-1 B-2(1.7) Example 21 b Or-1 B-2(0.25) Comp. Example 1 b Red2 B-2(0.25) a*, b*, tone and density after printed experiment No. a* b* colour tone O.D Example 1 17.3 19.0 light brown 1.28 Example 2 11.2 14.7 light brown 1.30 Example 3 8.7 12.9 light brown 1.25 Example 4 12.2 15.6 light brown 1.24 Example 5 9.8 17.4 light brown 1.26 Example 6 11.1 14.9 light brown 1.29 Example 7 13.7 16.6 light brown 1.30 Example 8 12.8 15.8 light brown 1.28 Example 9 12.2 16.4 light brown 1.29 Example 10 16.0 15.6 light brown 1.32 Example 11 15.0 14.2 light brown 1.26 Example 12 12.5 15.2 light brown 1.27 Example 13 6.0 19.5 light brown 1.29 Example 14 11.9 17.4 light brown 1.28 Example 15 12.5 17.6 light brown 1.26 Example 16 14.3 15.1 light brown 1.32 Example 17 24.6 35.4 reddish brown 1.16 Example 18 7.5 6.6 dark brown 1.35 Example 19 34.7 50.6 reddish brown 0.95 Example 20 4.2 1.4 dark brown 1.36 Example 21 14.3 15.0 light brown 1.29 Comp. Example 1 28.7 -12.4 purple 1.26 <Evaluation results> Examples 1∼20 of this invention, are the examples which use orange colour developing leuco dye whose maximum absorption wave length is 460∼550 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∼40, and that of colour difference b* value are within the region of 0∼55, and the colour tone of these Examples are sepia colour or light brown. 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. [Effect of the invention] The thermally sensitive recording medium of this invention, has a sufficient colour developing density and develops sepia colour or light brown 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 thermally sensitive colour developing layer which comprises a colourless or pale coloured dye precursor comprising a leuco dye which develops an orange colour with a maximum absorption wavelength measured in 99% acetic acid solution of from 460 to 550 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 and a colour developer, and which gives a developed image having a colour difference a* value regulated by JIS-Z-8729 of from 0 to 40 and a colour difference b* value regulated by JIS-Z- 8729 of from 0 to 55. 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 an orange colour. A recording medium according to claim 1 or 2 wherein the leuco dye which develops an orange colour is selected from 3-cyclohexylamino-6-chlorofluoran and 3-diethylamino-6,8-dimethylfluoran. A recording medium according to any one of the preceding claims, wherein the developed image is coloured sepia, light brown, or dim yellowish red, dim red, dark red, dark yellowish red or dark grey according to JIS common name. A recording medium according to any of the preceding claims, wherein the colour developing layer includes a leuco dye which develops a reddish colour. A recording medium according to claim 5, wherein the leuco dye which develops a reddish colour is contained in an amount of from 0.01 to 0.05 parts to 1 part of the leuco dye that develops an orange colour. A recording medium according to any 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 inorganic filler or an organic filler.

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