A spontaneously colour changing type thermal sensitive recording medium

Abstract: A spontaneously colour changing type thermal sensitive recording medium, the colour of recorded pattern on said medium gradually changing and the pattern just after having been developed can easily be distinguished from that of one day passed comprising a thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive recording layer comprising (a) a colorless or pale color dye precursor which is a triphenylmethane-based leuco dye; (b) a reddish colored developing leuco dye having a maximum absorption wavelength from 450 to 560nm; and (c) an organic color developer which is a 4-hydroxybenzoic ester of the formula (I) wherein R is an unsubstituted or substituted C 1 -C 7 alkyl group or a benzyl group is described.

Description: The present invention relates to a spontaneously colour changing type thermally sensitive recording medium which utilizes a colour developing reaction between an electron donating colour developing compound and an electron accepting compound. A 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 with said dye precursor was disclosed in JP-A-45-14035 and has been widely used. 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 advantages in comparison with other conventional recording methods, namely it is noiseless during recording, no developing and printing procedure is needed, it is maintenance free, the apparatus is of relatively low price and compact and the recorded pattern is very vivid. Therefore, with the growth of the information industry the use of this method has expanded, for instance in facsimiles and computers, in many kinds of measuring equipment and labels. For these uses a highly concentrated recording density and non-fading of features by time lapse, light, water or solvent are desired. The recent growth in fields which use thermally sensitive recording media has required the development of new thermally sensitive recording media with a recorded pattern which fades out or changes spontaneously and which, just after developing, can be easily distinguished from a pattern recorded one day or two days previously. Such media are required for use in entrance tickets to amusement parks or ski lift passes, for instance. JP-A-2-258286 and JP-A-3-65383, a thermally sensitive recording medium whose colour pattern fades out after recording and which uses a pigment which changes colour by an oxidation-reduction state change is disclosed. Further, in JP-A-52-140483 a medium which uses hydroxy benzoic esters is disclosed. However, since the recorded pattern of the media disclosed in above mentioned documents has a tendency to fade out within several hours or one day after developing, such media cannot be used in practice as recording media for information. Thus, the object of this invention is to provide a spontaneously colour changing type thermally sensitive recording medium. The colour of the recorded pattern on said medium gradually changes and the pattern just after developing can easily be distinguished from that of one day ago. Accordingly the present invention provides a spontaneously colour changing type thermally sensitive recording medium which comprises, on a substrate, a thermally sensitive recording layer comprising (a) a colourless or a pale colour dye precursor which is a triphenylmethane-based leuco dye; (b) a reddish colour developing leuco dye having a maximum absorption wavelength from 450 to 560 nm; and (c) an organic colour developer which is a 4-hydroxybenzoic acid ester of formula (I): wherein R is an unsubstituted or substituted C 1 -C 7 alkyl group or a benzyl group. A C 1 -C 7 group may be, for instance, a C 1 -C 4 alkyl group such as methyl, ethyl, ipropyl, n-propyl, s-butyl, n-butyl or t-butyl. The alkyl group may in turn be substituted by any conventional substituent such as hydroxy, halogen, C 1 -C 6 alkoxy, amino, nitro or CF 3. Generally, a thermally sensitive recording medium is composed of a dye precursor which is an electron donor and a colour precursor which is an electron acceptor. Between said dye precursor and colour developer electrons are transferred by thermal fusion, and by this electron transfer a complex is formed and a pattern is developed. When a 4-hydroxybenzoic ester represented by general formula (I) is used as the colour developer and a triphenylmethane-based leuco dye and a red colour developing leuco dye whose maximum absorption wavelength is from 450 to 560 nm are used as the dye precursor, as disclosed in this invention, the interaction between the colour developer and the triphenylmethane-based leuco dye is small and the complex (blue colour developing substance) formed by thermal fusion between said two compounds gradually decomposes at room temperature. Therefore, the colour tone changes from blue to red. In one embodiment the invention relates to the spontaneously colour changing type thermally sensitive recording medium as described above wherein the colour difference b* value prescribed by Japanese Industrial Standard JIS-Z-8729 of the developed pattern just after development is less than 0 and the colour difference b* 24 hours after development is greater than b* just after development by more than 10. The colour difference b* value is a parameter which indicates the intensity of blue colour. Bigger minus numerical value indicate a deeper blue colour whereas minus numerical numbers close to zero indicate a lighter blue. When the value turns positive the colour turns a yellowish or red colour. In the present invention the colour difference b* value is used as an index, and the difference between the b* value of a pattern just after developing and that after a time lapse is regulated, which is an effective means to measure the degree (state) of colour change. If the b* value increases by more than 10 compared to that just after developing the difference can be easily distinguished and is sufficient to accomplish the object of this invention. Desirably, if the increase of b* is by more than 20, a thermally sensitive recording medium which has good discrimination can be obtained. In another embodiment the thermally sensitive recording medium is one whose content of triphenylmethane-based leuco dye is from 0.07 to 1 parts to 1 part of colour developer and the content of reddish colour developing leuco dye having maximum adsorption wave length at 450 to 560 nm is 0.05 to 1 part of triphenylmethane-based leuco dye. In general, in inventions in the thermally sensitive recording medium field, the main object is to improve colour sensitivity or to improve preserving stability. This is to prevent a fading out or vanishing of the recorded pattern. The improvement of preserving stability of the recorded pattern is contrary to the spontaneously colour changing phenomenon which is the object of this invention. However, also in this invention the recorded pattern must have an adequate colour developing density which is sufficient for the actual use as a thermally sensitive recording medium. To obtain a spontaneously colour changing type thermally sensitive recording medium which satisfies both colour developing density and colour changing tendency in good balance, it is effective in this invention to use 0.07 to 1 parts of triphenylmethane-based leuco dye to 1 part of 4-hydroxybenzoic ester which is a colour developer and to use 0.05 to 1 parts of a red colour developing leuco dye whose maximum absorption wavelength is from 450 to 560 nm to 1 part of triphenylmethane-based leuco dye. Further, in this invention, since it is necessary to distinguish the recorded pattern of several hours ago from that just after development by the naked eye, the colour tone change from blue to red is an important factor which affects the degree of colour change. In a preferred embodiment the content of triphenylmethane based leuco dye (a) is 0.07 to 1 parts per 1 part of colour developer (c). More preferably in this embodiment the content of the reddish colour developing leuco dye (b) is 0.05 - 1 parts to 1 part of the triphenylmethane-based leuco dye (a). Using these proportions a thermally sensitive recording medium which vividly and distinguishably changes colour of the recorded pattern is obtained. When the content of triphenylmethane-based leuco dye is greater than 0.07 parts to 1 part of colour developer, sufficient colour developing sensitivity and adequate colour changing speed of the recorded pattern can be obtained and the colour tone change is distinguishable. And when the content of triphenylmethane-based leuco is smaller than 1 part to 1 part of colour developer, the colour tone change of a recorded pattern 24 or 48 hours after developing is very clear when compared to the recorded pattern just after developing. The colour difference b* value becomes about 20 and the discrimination between a recorded pattern several hours after development and a recorded pattern just after development is good. When the content of triphenylmethane-based leuco dye is small, as shown in the results of Example 17, the colour difference b* value 20 minutes after development increases about 20 points and the colour tone change is vivid. However, from the view point of an information recording medium, the stability of the recorded pattern is not good and is better suited to distinguishing a short term change in a recorded pattern rather than a long term change, of, for example, several days. When the content of triphenylmethane-based leuco dye is large, as shown in the results of Example 16, the difference between colour difference b* value 24 or 48 hours after the development and that just after development is bigger than 10, which is sufficient to be discriminated. However, in this case, since the colour change is in the same blue colour tone, the degree of colour change is not so obvious. Meanwhile, when the content of the reddish colour developing leuco dye whose maximum absorption wave length is 450 to 560 nm is bigger than 0.05 parts to 1 part of triphenylmethane-based leuco dye, the colour tone change by time lapse is vivid, and when the content of reddish colour developing leuco dye is smaller than 1 part to 1 part of triphenylmethane-based leuco dye, the initial colour of the developed pattern does not turn to red and the colour change degree becomes intense. For instance, as shown in the results of Example 19, when the contents of reddish colour developing leuco dye is small, the difference between the colour difference b* value 24 hours after development and that just after development is big and does not cause any problem in discrimination. However, in this case, since the colour change is in the same blue colour tone the degree of colour change is not so intensive. On the contrary, as shown in the results of Example 18, when the content of the reddish colour developing leuco dye is large and the red colour tone of the initial developed pattern is too deep the difference between the colour difference b* value of 24 hours after development and that just after development is bigger than 20, which is sufficient to be discriminated. However, in this case, since the colour tone change is from royal purple to purplish red, the colour tone change is slightly difficult to discriminate. Thus, in the present invention, by controlling the mixing ratio of the content of triphenylmethane-based leuco dye which is used as a dye precursor and the reddish colour developing leuco dye whose maximum absorption wave length is from 450 to 560 nm, the thermally sensitive recording medium can change colour changing speed or colour tone at need. In one aspect of the invention the recording medium comprises as components (c) and (a) respectively: 4-hydroxybenzoic acid-benzyl ester as a colour developer and 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophtalide as a dye precursor. A spontaneously colour changing type thermally sensitive recording medium of this invention can be obtained by the following procedure. That is, a thermally sensitive colour developing coating is prepared by dispersing an organic colour developer and a dye precursor together with a binder, adding a filler and other additives at need coating and drying it on a substrate. With respect to 4-hydroxybenzoic ester represented by general formula (I) of this invention, R in the general formula represents a hydric carbon group and as the concrete examples an unsubstituted or a substituted alkyl group or a benzyl group can be mentioned. Further, it is possible to insert a substituent group which does not obstruct the colour developing sensitivity or colour fading effect of R, and as concrete examples of such substituents, lower alkyl groups such as a methyl group or an ethyl group can be mentioned. Examples of a compound represented by general formula (I) include: 4-hydroxybenzoic acid methyl ester, 4-hydroxybenzoic acid ethyl ester, 4-hydroxybenzoic acid propyl ester, 4-hydroxybenzoic iso-propyl ester, 4-hydroxybenzoic acid butyl ester, 4-hydroxybenzoic acid iso-butyl ester, 4-hydroxybenzoic acid iso-amyl, 4-hydroxybenzoic acid hexyl ester, 4-hydroxybenzoic acid heptyl ester, 4-hydroxybenzoic acid benzyl ester, and 4-hydroxybenzoic acid methylbenzyl ester. However, is not intended to limit to these compounds. Among the compounds mentioned above, from the view point of easily procurement and good recording sensitivity, 4-hydroxybenzoic acid-benzyl ester can be preferably used. These 4-hydroxybenzoic esters can be used alone or by mixing. In this invention, a colour developer which prevents the colour changing property, for example the well known bis-phenol A, cannot be used. Examples of the triphenylmethane-based leuco dye used in this invention include: 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)-6-pyrrolydinophthalide, 3,3-bis(p-dimethylaminophenyl)phthalide, and 3,3-bis(p-dimethylaminophenyl)-6-di-n-propylaminophthalide. However, is not intended to limit to these compounds. Among the compounds mentioned above, from the view point of easily procurement, and a good recording sensitivity and a good colour changing property when used together with aforementioned 4-hydroxybenzoic acid benzyl ester, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide is preferably used. These triphenylmethane-based leuco dyes can be used alone or by mixing more than two kinds of them. As a reddish colour developing leuco dye used in this invention, the leuco dye whose maximum absorption wave length measured in 99% acetic acid solution is 450∼560 nm can be used. Examples include: 3-diethylamino-6-methyl-7-chlorofuluorane (λmax : 499, 531 nm), 3-cyclohexylamino-6-chlorofuluorane (λmax : 475 nm), 3-diethylamino-benzo[a]fuluorane (λmax : 520 nm), 3-diethylamino-7-chlorofuluorane (λmax : 500, 532 nm), 3-diethylamino-7-methylfuluorane (λmax : 493, 525 nm), 3-N-ethyl-N-isoamylamino-benzo[a]fuluorane (λmax : 523 nm) 3-N-ethyl-N-p-metylphenylamino-7-methylfuluorane (λmax : 518 nm), 3-diethylamino-6,8-dimethylfuluorane (λmax : 495, 525 nm), 3-dibuthylamino-6-methyl-7-buromofuluorane (λmax : 501, 537 nm), 3,6-bis(diethylamino)fuluorane-λ-(4'-nitro)-anilinolactam (λmax : 560 nm), 3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide (λmax : 536 nm), 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide (λmax : 535 nm) and 3,6-bis(diethylamino)fuluorane-λ-anilinolactam (λmax : 557 nm). However, it is not intended to limit to them. The numerical value of λmax indicated in parentheses is the maximum absorption wave length in 99% acetic acid solution. These red colour developing leuco dyes can be used alone or by mixing more than two kinds of them to obtain a desired reddish colour such as red, magenta, vermilion, orange or purplish red. In this invention, a conventional well known sensitizer can be used to the extent that the desired effect of this invention is not prevented. Examples of the sensitizer include: 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-methoxyphenoxyetliyl)ether, 1,2-di(3-methylphenoxy)ethane, 1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxylphenoxy)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-cetophenetidide, 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 However, it is not intended to limit to these compounds. These sensitizers can be used alone or by mixing more than two kinds of them. As a binder to be used in this invention, full saponificated polyvinyl alcohol of 200∼1900 degree of polymerization, partially saponificated polyvinyl alcohol denatured polyvinyl alcohol such as ; denatured polyvinyl alcohol by carboxyl, denatured polyvinyl alcohol by amide, denatured polyvinyl alcohol by sulfonic acid and denatured polyvinyl alcohol by butylal, cellulose derivatives 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 acetal, polyacrylicamide, polyacrylic acid ester, polyvinylbutylal, polystyrene and copolymer of these polymer, polyamide resin, silicon resin, petroleum resin, terpene resin, ketone resin and cumarone resin can be mentioned as examples. These high molecular compounds can be used not only by dissolving in a solvent e.g. water, alcohol, ketone, ester or hydrocarbon but also in paste form by dispersing or emulsifying in water or other medium. These states can be used in combination according to the desired quality. 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-formalin resin, copolymer of styrene-methacrylilate acid, copolymer of styrene-butadiene and hollow plastic pigment can be mentioned. Further, a parting agent such as the metal salt of a fatty acid, a slipping agent such as wax, an ultra violet ray absorbent such as benzophenone-based or triazole-based, a water proof agent such as glyoxal, a dispersing agent, a deformer, an anti-oxidation agent and a fluorescent dye can be used as an additive. As a substrate, paper, synthetic paper, plastic film, non-woven cloth, metallic foil and a hybrid sheet composed by said substances can be used. Further, to improve friction resistance, an overcoat layer composed of a macromolecular substance can be provided on the surface of the thermal sensitive colour developing layer. Furthermore, to improve the colour sensitivity, an undercoat layer containing organic or inorganic filler can be provided between the colour developing layer and the substrate. The amount of colour developer and dye precursor, and the kind and amount of other additives to be used in the spontaneously colour changing type thermal sensitive recording medium of this invention are decided according to the required quality and recording features, and are not restricted. 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. Also, it is preferable to use 0.07∼1 parts of triphenylmethane-based leuco dye to 1 part of colour developer although this amount can be adjusted according to the desired recording sensitivity, colour changing speed and colour tone. Further, the desirable amount of a reddish colour developing leuco dye whose maximum absorption wave length is 450∼560 nm is 0.05∼1 parts to 1 part of triphenylmethane-based leuco dye, and also this amount can be adjusted according to the desired recording sensitivity, colour changing speed and colour tone. These colour developer, dye and other additives which are added as required are ground to fine particles smaller than several micrometres in diameter by means of a pulverizer such as a ball mill, an attritor or a sand grinder, or by means of an adequate emulsifying device, 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 reverseλroll 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 spontaneously colour changing type thermal sensitive recording medium- The spontaneously colour changing type thermal 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 spontaneously colour changing type thermal sensitive recording medium of this invention in which 4-hydroxybenzoic acid benzyl ester (hereinafter shortened to BZ) is used as a colour developer, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (shortened to CVL) is used as a triphenylmethane-based leuco dye and 3,3-bis( 1-n-buthyl-2-methylindol-3-yl)phthalide (shortened to Red 40) is used as a reddish colour developing dye whose maximum absorption wave length is 450∼560 nm. Dispersion of colour developer (A solution), dispersion of triphenylmethane-based leuco dye (B solution) and reddish leuco dye (C solution) prepared by 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 colour developer) 4-hydroxybenzoic acid benzyl ester (BZ) 6.0 parts 10% aqueous solution of polyvinylalcohol 18.8 parts water 11.2 parts B solution (dispersion of triphenylmethane-based leuco dye) 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide(CVL) 1.0 parts 10% aqueous solution of polyvinylalcohol 2.3 parts water 1.3 parts C solution (dispersion of reddish leuco dye) 3,3-bis(1-n-buthyl-2-methylindol-3-yl)phthalide (Red 40) 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 [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 4.6 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 paper is processed by a super calendar to surface smoothness of 500∼600 second and the spontaneously colour changing type thermal sensitive recording medium of 6.0 g/m 2 coating amount can be obtained. Example 2 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 2.3 parts Kaoline clay (50% dispersion) 12.0 parts Example 3 The thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 1.38 parts Kaoline clay (50% dispersion) 12.0 parts Example 4 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 18.4 parts C solution (dispersion of reddish leuco dye [Red 40]) 1.38 parts Kaoline clay (50% dispersion) 12.0 parts Example 5 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 23.0 parts C solution (dispersion of reddish leuco dye [Red 40]) 1.38 parts Kaoline clay (50% dispersion) 12.0 parts Example 6 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 2.3 parts C solution (dispersion of reddish leuco dye [Red 40]) 0.23 parts Kaoline clay (50% dispersion) 12.0 parts Example 7∼8 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. At the preparation of A solution, 4-hydroxybenzoic acid ethyl ester (shortened to Et) and 4-hydroxybenzoic acid methyl ester (shortened to Me) are used instead of 4-hydroxybenzoic acid benzyl ester (BZ) Example 9 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. In this Example, 4-hydroxybenzoic acid benzyl ester (BZ) and 4-hydroxybenzoic acid methyl ester (Me) are used together with as the colour developer. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 18.0 parts (dispersion of colour developer [Me]) 18.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts Example 10∼11 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. At the preparation of B solution, 3,3-bis(p-dimethylaminophenyl)-6-pyrrolydinophthalide and 3,3-bis(p-dimethylaminophenyl)phthalide are used instead of CVL. Example 12 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. In this Example, CVL and 3,3-bis(p-dimethylaminophenyl)phthalide are used together with as triphenylmethane-based leuco dye. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of dye precursor [CVL]) 6.9 parts B solution (dispersion of dye precursor [3,3-bis(p-dimethylamino-phenyl)phthalide]) 6.9 parts C solution (dispersion of reddish leuco dye [Red 40] 4.6 parts Example 13∼14 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. At the preparation of C solution, 3-cyclohexylamino-6-chlorofuruoran (shortened to Orange 100) and 3,3-bis(1-ethylmethylindol-3-yl)phtalide (shortened to Indolyl Red) are used instead of Red 40. Example 15 The spontaneously colour changing type thermal sensitive recording medium is prepared by the same procedure to Example 1. In this Example, Red 40 and Orange 100 are used together with as the reddish leuco dye. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of dye precursor [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 2.3 parts C solution (dispersion of reddish leuco dye [Indolyl Red ]) 2.3 parts Kaoline clay (50% dispersion) 12.0 parts Example 16 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 32.2 parts C solution (dispersion of reddish leuco dye [Red 40]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts Example 17 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 1.38 parts C solution (dispersion of reddish leuco dye [Red 40]) 0.69 parts Kaoline clay (50% dispersion) 12.0 parts Example 18 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 16.56 parts Kaoline clay (50% dispersion) 12.0 parts Example 19 The spontaneously colour changing thermal sensitive recording medium is prepared by the same procedure to Example 1. The mixing proportion of dispersion is mentioned below. A solution (dispersion of colour developer [BZ]) 36.0 parts B solution (dispersion oftriphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 0.138 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 1 Comparative Example 1 is an example to use bis-phenol A (BPA) as a colour developer. The thermal sensitive recording medium for Comparative Example is prepared by the same procedure to Example 1. Dispersion of BPA (D solution) is ground in a wet condition to average diameter of 1 µm by means of a sand grinder. D solution (dispersion of colour developer [BPA]) bisphenol A (BPA) 6.0 parts 10% polyvinylalchol aqueous solution 18.8 parts water 11.2 parts Then the resulting dispersion are mixed together by the proportion below and the coating is prepared (0.17 parts of dye to 1 part of colour developer). D solution (dispersion of colour developer [BPA]) 36.0 parts B solution (dispersion of triphenylmethane-based leuco dye [CVL]) 13.8 parts C solution (dispersion of reddish leuco dye [Red 40]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts Comparative Example 2 Comparative Example 2 is an example to use 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide (shortened to Blue 63) instead of triphenylmethane-based leuco dye. The thermal sensitive recording medium for Comparative Example is prepared by the same procedure to Example 1. Dispersion of Blue63 (E solution) is ground in a wet condition to average diameter of 1µm by means of a sand grinder. E solution (dispersion of dye precursor) Blue 63 1.0 parts 10% polyvinylalchol aqueous solution 2.3 parts water 1.3 parts Then the resulting dispersion are mixed together by the proportion below and the coating is prepared (0.17 parts of dye to 1 part of colour developer). D solution (dispersion of colour developer [BZ]) 36.0 parts E solution (dispersion of dye precursor [Blue 63]) 4.6 parts C solution (dispersion of reddish leuco dye [Red 40]) 4.6 parts Kaoline clay (50% dispersion) 12.0 parts -Method for evaluation- Using a thermal sensitive printer (product of MARKPOINT Co., Ltd.) in which a thermal head KM2004-A3 (product of ROMH Co., Ltd.) is installed, recording tests are carried out on prepared spontaneously colour changing type thermal sensitive recording medium by 0.137 mJ/dot impressive energy. The colour difference b* value prescript in JIS-Z-8729 is measured by colour difference meter (CR-121 ; product of Minolta Co., Ltd.), and this value is established as an initial colour difference b* value. The specimen on which a pattern is recorded are left for 20 minutes, 24 hours and 48 hours in the room kept at 20° C and 60% RH, then the colour difference b* value of recorded position is measured, thus the degree of colour changing is evaluated. Further the appearance evaluation test by naked eyes of the operator is carried out. In the appearance evaluation test, mark A indicates that the specimen which has passed 24 hours from the development can easily be distinguished from that of just after development, mark B indicates that the specimen which has passed 48 hours from development can be distinguished from that of just after development and others are ranked as mark C. The obtained results are summarized in Table 1 and Table 2. A list of colour developer and dye precursor Test number colour developer Leuco dye colour density just after developed triphenyl methane Reddish Example 1 BZ CVL(0.5) Red 40(0.33) 1.35 Example 2 BZ CVL(0.5) Red 40(0.17) 1.43 Example 3 BZ CVL(0.5) Red 40(0.1) 1.41 Example 4 BZ CVL(0.67) Red 40(0.075) 1.42 Example 5 BZ CVL(0.83) Red 40(0.06) 1.39 Example 6 BZ CVL(0.083) Red 40(0.1) 1.25 Example 7 Et CVL(0.5) Red 40(0.33) 1.27 Example 8 Me CVL(0.5) Red 40(0.33) 1.26 Example 9 BZ/Me CVL(0.5) Red 40(0.33) 1.32 Example 10 BZ (0.5) Red 40(0.33) 1.20 Example 11 BZ (0.5) Red 40(0.33) 1.18 Example 12 BZ (0.5) Red 40(0.33) 1.29 Example 13 BZ CVL(0.5) Orange100(0.33) 1.39 Example 14 BZ CVL(0.5) Indolyl Red(0.33) 1.34 Example 15 BZ CVL(0.5) Red 40/Indolyl Red(0.33) 1.35 Example 16 BZ CVL(1.17) Red 40(0.14) 1.40 Example 17 BZ CVL(0.05) Red 40(0.5) 1.18 Example 18 BZ CVL(0.5) Red 40(1.2) 1.43 Example 19 BZ CVL(0.5) Red 40(0.01) 1.41 Com.Ex.1 BPA CVL(0.5) Red 40(1) 1.42 Com.Ex.2 BZ Blue63(0.17) Red 40(1) 1.47 Change of colour difference b* and colour tone after developed Test Number after printed 20 minutes after 24 hours after 48 hours after evaluation by naked eye Example 1 -54.9(B) -54.9(B) -5.2(PR) 2.7(R) A Example 2 -54.3(B) -54.3(B) -9.1(PR) 4.2(R) A Example 3 -60.7(B) -60.7(B) -20.7(RP) -1.3(PR) A Example 4 -60.8(B) -60.8(B) -18.7(RP) -1.6(PR) A Example 5 -62.7(B) -62.7(B) -24.5(PR) -3.9(RP) A Example 6 -38.9(B) -36.6(B) 2.3(R) 4.1(R) A Example 7 -40.5(B) -38.6(B) 2.8(R) 4.7(R) A Example 8 -43.7(B) -41.7(B) 2.5(R) 4.6(R) A Example 9 -45.9(B) -45.3(B) -1.6(PR) 5.0(R) A Example 10 -46.3(B) -45.3(B) 2.3(R) 4.4(R) A Example 11 -44.5(B) -43.6(B) 2.8(R) 4.9(R) A Example 12 -45.9(B) -45.6(B) -0.5(R) 5.0(R) A Example 13 -22.9(DB) -22.9(DB) 32.0(OR) 42.3(OR) A Example 14 -48.6(B) -48.6(B) -4.4(PR) 4.6(R) A Example 15 -50.7(B) -50.7(B) -6.1(PR) 3.7(R) A Example 16 -65.6(B) -65.2(B) -49.6(B) -38.7(B) B Example 17 -38.2(B) -18.7(RP) 2.8(R) 5.1(R) A Example 18 -20.8(RP) -20.8(RP) -1.4(PR) 6.9(R) B Example 19 -61.4(B) -61.4(B) -25.8(B) -3.9(RP) B Com.Ex. 1 -51.8(B) -51.8(B) -51.8(B) -51.8(B) C Com.Ex. 2 -52.8(B) -52.8(B) -52.8(B) -52.8(B) C (Remarks) : Mark in parenthesis indicates colour tone by appearance evaluation as follows, (B):blue, (R):red, (PR) : purplish red, (RP) : royal purple, (DB) : dark blue, (OR) : orange -Evaluation results- As obviously understood from the results of Table.1 and Table.2, the Examples 1∼19 of this invention are the substantial examples which use 4-hydroxybenzoic acid esters represented by general formula (λ), triphenylmethane-based leuco dye and reddish colour developing leuco dye whose maximum absorption wave length is 450∼560 nm, and the degree of colour change of 24 hours passed to that of just after development superiors to that of the Comparative Examples which do not use 4-hydroxybenzoic acid esters or triphenylmethane-based leuco dye. Further, in the Examples 1∼15 which contain 0.07∼1 parts of triphenylmethane-based leuco dye to 1 part of colour developer and 0.05∼1 parts of reddish colour developing leuco dye to 1 part of triphenylmethane-based leuco dye, there is not so big difference in the colour difference b* value between that of just after development and that of 20 minutes passed and the stability of recorded pattern is good. And, the difference between colour difference b* value of 24 after development and that of just after development is bigger than 20, further the colour tone change is distinguishable by appearance. Therefore, the thermal sensitive recording medium of this invention is suited for the application which is necessary to distinguish the recorded pattern which is just after development from that of one or two days passed. In the Example 17 which contains 0.05 parts of triphenylmethane-based leuco dye to 1 part of colour developer, the colour difference b* value of 20 minutes passed after development increases about 20 points and the colour tone change by appearance is visible, however, the stability of image is slightly bad and is suited to the application to distinguish short term change of recorded pattern rather than long term change such as after several days. In the Example 16 which contain 1.17 parts of triphenylmethane-based leuco dye to 1 part of colour developer, although the colour change is in same blue colour tone and the degree of colour change is not so obvious, the difference between colour difference b* value of 24 or 48 hours passed from development and that of just after development is bigger than 10 and is sufficient for the practical use. Example 19 which contains 0.01 parts of reddish colour developing leuco dye to 1 part of triphenylmethane-based leuco dye, the colour tone change by appearance is slightly difficult to distinguish, however, the difference between colour difference b* value of 24 hours passed after development and that of just after development is bigger than 20 and has a sufficient discrimination. And, Example 18 which contains 1.2 parts of reddish colour developing leuco dye to 1 part of triphenylmethane-based leuco dye, since the colour of just after development is reddish, the colour tone change is slightly difficult to distinguish by appearance evaluation, however, the difference between colour difference b* value of 24 hours passed after development and that of just after development is bigger than 20, it is sufficient for discrimination. The thermal sensitive recording medium of this invention, even if the colour tone change is not so vivid, since the colour change progresses by time lapse, it can be used for the application which is necessary to be distinguished after long term lapse. The thermal sensitive recording medium of Examples 16, 18 and 19 are the thermal sensitive recording medium which are suited for the purpose to distinguish the recorded pattern of long term passed from that of just after developed lather than one or two days passed. The spontaneously colour changing type thermal sensitive recording medium of this invention has a sufficient colour developing sensitivity and the colour fades out slowly, therefore the recorded pattern of one or two days passed can easily distinguished from that of just after printed. Therefore, the thermal sensitive recording medium of this invention fulfils it's function as the recording medium same as the conventional thermal sensitive recording medium for several hours after development, and after one or two days the colour obviously changes and has a remarkable distinguishing feature. The thermal sensitive recording medium of this invention can be applied to the use which prevent the reuse of an used ticket such as a lift riding ticket of a skiing ground or as entrance ticket of an amusement park, further the use which is necessary to distinguish from that issued on the previous day such as a statement of delivery.

Claims: A spontaneously colour changing type thermally sensitive recording medium which comprises , on a substrate, a thermally sensitive recording layer comprising (a) a colorless or pale colour dye precursor which is a triphenylmethane-based leuco dye; (b) a reddish colour developing leuco dye having a maximum absorption wavelength from 450 to 560 nm; and (c) an organic color developer which is a 4-hydroxybenzoic acid ester of the formula (I): wherein R is an unsubstituted or substituted C 1 -C 7 alkyl group or a benzyl group. A recording medium according to claim 1 which gives a developed pattern wherein the color difference b* value prescribed by Japanese Industrial Standard-Z-8729 just after development is less than 0 and the color difference b* 24 hours after development is greater than the value of b* just after development by at least 10. A recording medium according to claim 1 or 2 wherein the content of the triphenylmethane-based leuco dye is 0.07-1 parts to 1 part of the colour developer (c). A recording material according to claim 1, 2 or 3 wherein the content of reddish colour developing leuco dye (b) is 0.05 - 1 parts to 1 part of the triphenylmethane-based leuco dye (a). A recording medium according to any one of the preceding claims wherein the 4-hydroxybenzoic acid ester of formula (I) is 4-hydroxybenzoic acid benzyl ester and the triphenylmethane-based leuco dye (a) is 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide. A recording material according to any one of the preceding claims wherein the recording layer further includes a sensitizer, binder, filler, or parting agent. A recording material according to any one of the preceding claims which further includes, on the surface of the colour developing layer, an overcoat layer. A recording material according to any one of the preceding claims which further includes, between the color developing layer and the substrate, an undercoat layer containing an inorganic or organic filler.

Inventors: FUKUCHI TADAKAZU, HAMADA KAORU, NAGAI TOMOAKI, SATO REIKO, WAKITA YUTAKA, FUKUCHI, TADAKAZU, HAMADA, KAORU, NAGAI, TOMOAKI, SATO, REIKO, WAKITA, YUTAKA
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1998
Main classification: B41M5
IPC codes: B41M5