A thermal sensitive recording sheet

Abstract: A thermally sensitive recording sheet which comprises on a substrate, a thermally sensitive colour developing layer comprising: (a) a colourless or pale coloured basic leuco dye; (b) an organic colour developer of formula (I): wherein A is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, each of R 1 , R 2 and R 3 , which may be the same or different, is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, or R 1 and R 2 form, together with the carbon atoms to which they are attached, an aromatic ring, and Y is sulfur or oxygen; or an organic colour developer of formula (II): wherein each of R 1 , R 2 , R 3 and Y represent the same groups as above; and B represents the same groups as A; and (c) 0.01-2 parts, based on 1 part of colour developer of formula (I) or (II), of a stabiliser which is a diphenylsulfone derivative of formula (III):

Description: This invention relates to a thermal sensitive recording sheet which has an excellent image stability. In general, a thermal recording sheet can be obtained by the following procedure: A colourless or pale coloured basic leuco dye and a colour developer such as a phenolic compound are separately ground to fine particles, then mixed together and dispersed. By adding a binder, a filler, a sensitizer, a slip agent, and other additives to the said mixture, a coating colour can be obtained. The obtained coating colour is coated on a substrate such as paper, synthetic paper, none-woven cloth, film or plastic, thus a thermal sensitive recording sheet is prepared, which develops colour by a momentary chemical reaction caused by heating with a thermal pen, a thermal head, a hot stamp, laser light or the like. These thermal recording sheets can be applied in a wide variety of fields such as measuring recorders, terminal printers for computers, facsimiles, automatic ticket vendors, ATM (Automatic Teller Machines of bank). HT (Handy Terminal), bar code labels or the like. However, in view of recent advances in the range of applications for such recording apparatus, and the corresponding demand for high performance, there is a requirement for thermally sensitive recording sheets of higher quality. For instance, besides high speed recording a requirement of the thermally sensitive recording sheet is to achieve high density and clear colour images with lower thermal energy. Another requirement is to possess good preservation such as light resistance, weather resistance and oil resistance. Conventional examples of thermally sensitive recording sheets include, for instance, thermally sensitive recording materials disclosed in Japanese Patent publication 4160/68 or Japanese Patent publication 14039/70. However, since the response speed of these conventional thermally sensitive recording materials is slow, a sufficient colour developing density cannot be expected when the recording speed is high. In order to overcome this problem the development of highly sensitive dyes such as leuco dyes, using 3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluorane (Japanese Patent Laid-open publication 109120/74) or 3-dibutylamino-6-methyl-7-anilinofluorane (Japanese Patent Laid-open publication 190891/84) has been proposed. Further attempts to obtain high speed recording and a high sensitivity have involved using a substance which has good colour developing ability such as 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane (Japanese Patent Laid-open publication 106456/84), 1.5-bis(4-hydroxyphenylthio)-3-oxaheptane (Japanese Patent Laid-open publication 116262/84) and 4-hydroxy-4'-isopropoxy diphenylsulfone (Japanese Patent Laid-open publication 106456/84). Meanwhile, the present inventor has already proposed a thermally sensitive recording sheet which uses a newly developed derivative of urea or thiourea as a colour developer. Said thermally sensitive recording sheet can be used for high energy printing, but in the case of lower energy printing or high speed printing, insufficient colour developing density is obtained. Furthermore, the stability of the recorded image towards the plasticizer included in the polyvinylchloride film is not sufficient. It is therefore unlikely that such a thermally sensitive recording sheet will be of practical use. Other disclosures of recording media include JP 6336082 which teaches, disposed on a substrate, a recording layer comprising a colourless or light-coloured basic dye, 4-benzyloxy-4'-(2-methyl-2,3-epoxypropyloxy)diphenyl sulphone and, as developer, a dihydroxydiphenylsulphone. EP-A-0 699662, which was published after the present priority date, discloses a thermal recording sheet comprising a layer which comprises a basic dye, a sulphonyl(thio)urea derivative as developer and, optionally a stabiliser based on a metal salt of phthalic acid monobenzyl ester or p-nitrobenzoic acid. The present invention seeks to provide a thermally sensitive recording sheet which has excellent image stability. Accordingly the invention provides a thermally sensitive recording sheet which comprises, on a substrate, a thermally sensitive colour developing layer comprising: (a) a colourless or pale coloured basic leuco dye; (b) an organic colour developer of formula (I): wherein A is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, each of R 1 , R 2 and R 3 , which may be the same or different, is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, or R 1 and R 2 form, together with the carbon atoms to which they are attached, an aromatic ring, and Y is sulfur or oxygen; or an organic colour developer of formula (II): wherein B as defined above for A, each of R 1 , R 2 and R 3 , which may be the same or different, is as defined above. and Y is as defined above (c) 0.01-2 parts, based on 1 part of colour developer of formula (I) or (II), of a stabiliser of formula (III): and (d) a binder. A C 1 -C 4 alkyl group is, for example, methyl, ethyl, i-propyl, n-propyl, t-butyl, s-butyl or n-butyl. A C 1 -C 4 alkoxy group is, for example, methoxy, ethoxy, i-propoxy, n-propoxy, t-butoxy, s-butoxy or n-butoxy. When R 1 and R 2 form, together with the carbon atoms to which they are attached, an aromatic ring that aromatic ring is, for example, a benzene ring which forms a naphthalene ring system with the benzene ring to which it is attached. In the present invention, the derivative of urea (thiourea) indicated by general formula (I) or (II) possesses stronger acidity than a phenolic compound, which is a well-known colour developer, and exhibits very strong colour developing ability with respect to a basic leuco dye. The reason for this is the presence of two electron withdrawing groups, e.g. a sulfonyl group, a carbonyl group or a thiocarbonyl group, and a benzene ring between, which cause the electron density surrounding the nitrogen atom to be remarkably reduced and consequently results a strong colour developing ability to a basic leuco dye. Further in this invention, since the molecule of stabilizer of formula (III) possesses a sulfone group, when it is fused by heat together with the organic colour developer of formula (I) or (II), it reduces the electron density surrounding the nitrogen still further and consequently helps improve the preservable stability of the recorded image. As examples of a derivative of urea (thiourea) used in this invention, following compounds are mentioned, however, it is not intended to be limited to them. This derivatives of urea (thiourea) can be used alone or by mixing with others. N-(benzenesulfonyl)- p -(phenylureylene)benzamide N-(4-toluenesulfonyl)- p -(phenylureylene)benzamide N-(4-ethylphenylsulfonyl)- p -(phenylureylene)benzamide N-(4- n -propylphenylsulfonyl)- p -(phenylureylene)benzamide N-(4-iso-propylphenylsulfonyl)- p -(phenylureylene)benzamide N-(t-butylphenylsulfonyl)- p -(phenylureylene)benzamide N-(4-methoxyphenylsulfonyl)- p -(phenylureylene)benzamide N-(4-ethoxyphenylsulfonyl)- p -(phenylureylene)benzamide N-(4-hydroxyphenylsulfonyl)- p -(phenylureylene)benzamide N-(2-hydroxyphenylsulfonyl)- p -(phenylureylene)benzamide N-(4-nitrophenylsulfonyl)- p -(phenylureylene)benzamide N-(benzenesulfonyl)- p -(4-tolylureylene)benzamide N-(4-toluenesulfonyl)- p -(4-tolylureylene)benzamide N-(4-ethlphenylsulfonyl)- p -(4-tolylureylene)benzamide N-(4-iso-propylphenylsulfonyl)- p -(4-tolylureylene)benzamide N-( n -butylphenylsulfonyl)- p -(4-tolylureylene)benzamide N-(4-methoxyphenylsulfonyl)- p -(4-tolylureylene)benzamide N-(4-hydroxyphenylsulfonyl)- p -(4-tolyoureylene)benzamide N-(2-hydroxyphenylsulfonyl)- p -(4-tolyoureylene)benzamide N-(4-nitrophenylsulfonyl)- p -(4-tolylureylene)benzamide N-(benzenesulfonyl)- p -(phenylthioureylene)benzamide N-(4-toluenesulfonyl)- p -(phenylthioureylene)benzamide N-(4-ethylphenylsulfonyl)- p -(phenylthioureylene)benzamide N-(4-n-propylphenylsulfonyl)- p -(phenylthioureylene) benzamide N-(4-iso-propylphenylsulfonyl)- p -(phenylthioureylene) benzamide N-(4-methoxyphenylsulfonyl)- p -(phenylthioureylene)benzamide N-(4-ethoxyphenylsulfonyl)- p -(phenylthioureylene)benzamide N-(4-hydroxyphenylsulfonyl)- p -(phenylthioureylene)benzamide N-(2-hydroxyphenylsulfonyl)- p -(phenylthioureylene)benzamide N-(4-nitrophenylsulfonyl)- p -(phenylthioureylent)benzamide N-(benzenesulfonyl)- p -(4-tolylthioureylene)benzamide N-(4-toluenesulfonyl)- p -(4-tolylthioureylene)benzamide N-(4- n -propylphenylsulfonyl)- p -(4-tolylthioureylene) benzamide N-(4-methoxyphenylsulfonyl)- p -(4-methylphenylthioureylene) benzamide N-(4-hydroxyphenylsulfonyl)- p -(4-tolylthioureylene) benzamide N-(2-hydroxyphenylsulfonyl)- p -(4-methylphenylthioureylene) benzamide N-(4-nitrophenylsulfonyl)- p -(4-methylphenylthioureylene) benzamide N-(phenyl)-N'-( p -benzoylaminosulfonyl)phenylurea N-(phenyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(2-hydroxybenzoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(4-methoxybenzoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(4-ethoxybenzoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(4-n-propoxybenzoylaminosulfonyl)] phenylurea N-(phenyl)-N'-{ p -(4-iso-propoxybenzoylaminosulfonyl)} phenylurea N-(phenyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)}phenylurea N-(4-toluyl)-N'-(p-benzoylaminosulfonyl)phenylurea N-(4-toluyl)-N'-(p-(4-toluoylaminosulfonyl))phenylurea N-(4-toluyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylurea N-(3-toluyl)-N'-( p -benzoylaminosulfonyl)phenylurea N-(3-toluyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylurea N-(3-toluyl)-N'-{ p -(3-toluoylaninosulfonyl)}phenylurea N-(3-toluyl)-N'-{ p -(2-toluoylaminosulfonyl)}phenylurea N-(3-toluyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylurea N-(4-toluyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)} phenylurea N-(4-toluyl)-N'-{ p -(2-hydroxybenzoylaminosulfonyl)} phenylurea N-(4-toluyl)-N'-{ p -(4-methoxybenzoylaminosulfonyl)} phenylurea N-(4-toluyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)}phenylurea N-(phenyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylthiourea N-(phenyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylthiourea N-(phenyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)} phenylthiourea N-(phenyl)-N'-{ p -(4-methoxybenzoylaminosulfonyl)} phenylthiourea N-(phenyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)} phenylthiourea N-(4-toluyl)-N'-( p -benzoylaminosulfonyl)phenylthiourea N-(4-toluyl)-N'-{ p -(4-toluoylaminosulfonyl))phenylthiourea N-(4-toluyl)-N'-{ p -(3-toluoylaminosulfonyl)}phenylthiourea N-(4-toluyl)-N'-{ p -(2-naphthoylaminosulfonyl)} phenylthiourea N-(3-toluyl)-N'-( p -benzoylaminosulfonyl)phenylthiourea N-(3-toluyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylthiourea N-(3-toluyl)-N'-{ p -(3-toluoylaminosulfonyl)}phenylthiourea N-(3-toluyl)-N'-{ p -(2-naphthoylaminosulfonyl)} phenylthiourea N-(4-toluyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)} phenylthiourea N-(4-toluyl)-N'-{ p -(4-methoxybenzoylaminosulfonyl)} phenylthiourea N-(4-toluyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)} phenylthiourea In this invention, as a stabilizer, a derivative of diphenylsulfone indicated by general formula (III) is used. In this invention, if the proportion of a stabilizer indicated by general formula (III) is smaller than 0.01 parts based on 1 part of an organic colour developer indicated by general formula (I) and (II), the stabilizing effect is not sufficient, and if it is bigger than 2 parts based on 1 part of an organic colour developer the enough stabilizing effect can not be expected. Therefore, the proportion of the stabilizer is 0.01-2 parts based on 1 part of the organic colour developer. As the basic leuco dye used in this invention, triphenylmethane type, fluorane type, fluorene type and divinyl type dyes are desirably used, although the invention is not limited to these. Examples of them are indicated below. These dyes can be used alone or as mixtures. <triphenylmethane-based leuco dye> 3,3-bis( p -dimethylaminophenyl)-6-dimethylaminophthalide [another name is crystal violet lactone] <fluoran-based leuco dye(I)> 3-diethylamino-6-methyl-7-anilinofluoran 3-(N-ethyl- p -toluidino)-6-methyl-7-anilinofluoran 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran 3-diethylamino-6-methyl-7-( o , p -dimethylanilino)fluoran 3-pyrrolidino-6-methyl-7-anilinofluoran 3-piperidino-6-methyl-7-anilinofluoran 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran 3-diethylamino-7-( m -trifluoromethylanilino)fluoran 3-N- n -dibutylamino-6-methyl-7-anilinofluoran 3-N- n -dibutylamino-7-(o-chloroanilino)fluoran 3-( n -ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran 3-dibutylamino-6-methyl-7-( o,p -dimethylanilino)fluoran 3-( n -methyl-N-propylamino)-6-methyl-7-anilinofluoran 3-diethylamino-6-chloro-7-anilinofluoran 3-dibutylamino-7-( o -chloroanilino)fluoran 3-diethylamino-7-( o -chloroanilino)fluoran 3-diethylamino-6-methyl-chlorofluoran 3-diethylamino-6-methyl-fluoran 3-cyclohexylamino-6-chlorofluoran 3-diethylamino-benzo[a]-fluoran 3- n -dipentylamino-6-methyl-7-anilinofluoran 2-(4-oxo-hexyl)-3-dimethylamino-6-methyl-7-anilinofluoran 2-(4-oxo-hexyl)-3-diethylamino-6-methyl-7-anilinofluoran 2-(4-oxo-hexyl)-3-dipropylamino-6-methyl-7-anilinofluoran <fluorene-based leuco dye> 3,6',6'-tris(dimethylamino)spiro[fluorene-9, 3'-phthalide] 3,6',6'-tris(diethylamino)spiro[fluorene-9,3,-phthalide] <fluoran-based leuco dye(II)> 2-methyl-6- p -( p -dimethylaminophenyl)aminoanilinofluoran 2-methoxy-6- p -( p -dimethylaminophenyl)aminoanilinofluoran 2-chloro-3-methyl-6- p -( p -phenylaminophenyl) aminoanilinofluoran 2-chloro-6- p -( p -dimethylaminophenyl)aminoanilinofluoran 2-nitro-6- p -( p -diethylaminophenyl)aminoanilinofluoran 2-amino-6- p -( p -diethylaminophenyl)aminoanilinofluoran 2-diethylamino-6- p -( p -diethylaminophenyl) aminoanilinofluoran 2-phenyl-6-methyl-6- p -( p -phenylaminophenyl) aminoanilinofluoran 2-benzyl-6- p -( p -phenylaminophenyl)aminoanilinofluoran 2-hydroxy-6- p -( p -phenylaminophenyl)aminoanilinofluoran 3-methyl-6- p -( p -dimethylaminophenyl)aminoanilinofluoran 3-diethylamino-6- p -( p -diethylaminophenyl) aminoanilinofluoran 3-diethylamino-6- p -( p -dibutylaminophenyl) aminoanilinofluoran <divinyl-based leuco dye> 3,3-bis- [2-( p -dimethylaminophenyl)-2-( p -methoxyphenyl) ethenyl] -4,5,6,7-tetrabromophthalide 3,3-bis-[2-( p -dimethylaminophenyl)-2-( p -methoxyphenyl) ethenyl]-4,5, 6, 7-tetrachlorophthalide 3, 3-bis-[1,1-bis (4-pyrrolidinophenyl) ethylene-2-yl] -4, 5, 6, 7-tetrabromophthalide 3,3-bis-[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl) ethylene-2-yl] -4,5,6,7-tetrachlorophthalide <others> 1,1-bis-[2',2',2",2"-tetrakis-( p -dimethylaminophenyl)-ethenyl] -2, 2-dinitrileethane 1,1-bis-[2',2',2",2"-tetrakis-( p -dimethylaminophenyl)-ethenyl] -2- β -naphthoylethane 1,1-bis-[2',2',2",2"-tetrakis-( p -dimethylaminophenyl)-ethenyl]-2,2-diacetylethane bis-[2,2,2',2'-tetrakis-( p -dimethylaminophenyl)-ethenyl]-methymalonatedimethyl ester In the present invention, a known sensitizer can be used to the extent that it does not interfere with the effects of the invention. Examples of the sensitizer include the following compounds : fatty acid amide such as amide stearate or amide palmitate ethylene bisamide montane-based wax polyethylene wax dibenzyl terephthalate p -benzyloxybenzylbenzoate di- p -tolylcarbonate p -benzylbiphenyl phenyl-α-naphthylcarbonate 1,4-diethoxynaphthalene 1-hydroxy-2-naphthoeic acid phenyl ester 1,2-di-(3-methylphenoxy)ethane di( p -methylbenzyl)oxalate β-benzyloxynaphthalene 4-biphenyl- p -tolyleter o -xylylene-bis-(phenylether) 4-( m -methylphenoxymethyl)biphenyl These sensitizers can be used alone or as mixtures with others. Examples of the binder to be used in this invention include full saponificated polyvinylalcohol having 200-1900 polymerization degree, partially saponificated polyvinylalcohol, denatured polyvinylalcohol such as carboxy denatured polyvinylalchol, amide denatured polyvinylalcohol, sulfonic acid denatured polyvinylalcohol and butylal denatured polyvinylalcohol, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, copolymer of styrene-maleic anhydride, copolymer of styrene-butadiene, derivatives of cellulose such as ethylcellulose or acetyl-cellulose, polyvinylchloride, polyvinylacetate, polyacrylamide, polyacrylate, polyvinylbutylal, polystyrene and copolymers of these polymers, polyamide, resin, silicon resin, petroleum resin, terpene resin, ketone resin and cumarone resin. These kinds of macromolecule compound can be used by dissolving in water or in solvents such as alcohols, ketones or esters of hydrocarbons and also can be used by dispersing as an emulsion or paste in water or other solvent. These methods can be used in combination, if necessary. Examples of fillers to be used in this invention include inorganic or organic fillers such as silica, calcium carbonate, kaoline, calcined kaoline, diatomaceous earth, talc, titanium oxide, aluminum hydroxide, urea-formalin resin and hollow plastic pigment. Further, parting agents such as metallic salts of fatty acids, lubricants such as wax, ultra violet ray absorbers such as benzophenone or triazoles, waterproofing agents such as glyoxal, dispersing agents and deformers can additionally be used in this invention. Examples The present invention will be described in more detail in the following Examples. The thermally sensitive recording sheet of this invention can be obtained by the following procedure, that is; preparing the coating colour of the thermally sensitive colour developing layer by grinding and dispersing a colour less or pale-coloured basic leuco dye, a derivative of urea (thiourea) of the general formula (I) or (II) as an organic colour developer and a derivative of diphenylsulfone of the general formula (III) as a stabilizer together with a binder, and adding a filler and other additives as required, then coating the obtained coating colour on a substrate and drying it. The amount of the derivative of diphenylsulfone of general formula (III) to be added to one part of the organic developer of general formula (I) or (II) is 0.01-2 parts. The type and amount of organic developer, basic leuco dye and other additives are decided according to the required features and to the recording property. In general the preferred amounts of these compounds are as follows, although these are not intended to be limiting. That is, 0.1-2 parts of basic leuco dye and 0.5-4 parts of filler based on 1 part of organic developer of general formula (I) or (II). The preferred amount of binder is 5-25 % to the total amount of solid. By coating the coating colour having the above-mentioned composition on a substrate such as paper, non woven cloth, a film or a plastic, a thermally sensitive recording sheet of the invention can be obtained. Further, for the purpose of improving the preservability, an overcoating layer comprising macromolecules and including a filler can be prepared on the thermal sensitive colour developing layer. And, furthermore, to improve both preservability and sensitivity, an undercoat layer including an organic or inorganic filler can be provided under the thermally sensitive colour developing layer. The organic developer, basic leuco dye, sensitizer and other additives to be added are ground to fine particles smaller than one micrometre in size by means of a pulverizer such as a ball mill, an attritor or a sand grinder, or by means of emulsifying apparatus. The binder and other additives are added as required and thus the coating colour is prepared. In the following Examples and Comparative Examples the term "parts" means "parts by weight". EXAMPLE 1 (experiment No. 1-32) Solution A (dispersion of dye) 3-N-n-dibuthylamino-6-methyl-7-anilinofluoran 2.0 parts 10% polyvinylalcohol aqueous solution 4.6 parts water 2.5 parts Solution B (dispersion of colour developer) colour developer (refer to table 1 and 2) 6.0 parts 10% polyvinylalcohol aqueous solution 18.8 parts water 11.2 parts Solution C (dispersion of stabilizer) stabilizer 4-benziloxy-4'-(2, 3-epoxy-2-methylpropoxy) diphenyl sulfone 4.0 parts 10% polyvinylalcohol aqueous solution 12.5 parts water 7.5 parts These three kinds of dispersion are separately ground to average diameter of 1 µ m by means of a sand grinder. Then the resulting dispersion are mixed together in the proportion below so as to prepare the coating colour. Solution A (dispersion dye) 9.1 parts Solution B (dispersion of colour developer) 36.0 parts Solution C (dispersion of stabilizer) 24.0 parts Kaoline clay (50% dispersion) 12.0 parts The prepared coating colours are applied to one side of 50g/m 2 sheet paper so as obtain a coating weight of 5.0g /m 2 amount, dried up and the sheet is processed by a super calender to surface smoothness of 400-500 second. Thus the thermal sensitive recording paper can be obtained. COMPARATIVE EXAMPLE 1 (experiment No.33-37) Solution A (dispersion of dye) 3-N-n-dibuthylamino-6-methyl-7-anilinofluoran 2.0 parts 10% polyvinylalcohol aqueous solution 4.6 parts water 2.5 parts Solution D (dispersion of colour developer) colour developer (refer to table 5) 6.0 parts 10% polyvinylalcohol aqueous solution 18.8 parts water 11.2 parts These two kinds of dispersion are separately ground to average diameter of 1 µ m by means of a sand grinder. Then the resulting dispersion are mixed together in the proportion below so as to prepare the coating colour. Solution A (dispersion dye) 9.1 parts Solution D (dispersion of colour developer) 36.0 parts Kaoline clay (50% dispersion) 12.0 parts The prepared coating colours are applied to one side of 50g/m 2 sheet paper so as obtain a coating weight of 5.0g /m 2 amount, dried up and the sheet is processed by a super calender to surface smoothness of 400-500 second. Thus the thermal sensitive recording paper can be obtained. COMPARATIVE EXAMPLE 2 (experiment No.38-42) Solution A (dispersion of dye) 3-N-n-dibuthylamino-6-methyl-7-anilinofluoran 2.0 parts 10% polyvinylalcohol aqueous solution 4.6 parts water 2.5 parts Solution E (dispersion of colour developer) colour developer (refer to table 5) 6.0 parts 10% polyvinylalcohol aqueous solution 18.8 parts water 11.2 parts Solution C (dispersion of stabilizer) stabilizer 4-benziloxy-4'-(2,3-epoxy-2-methylpropoxy) diphenyl sulfone 4.0 parts 10% polyvinylalcohol aqueous solution 12.5 parts water 7.5 parts These three kinds of dispersion are separately ground to average diameter of 1 µ m by means of a sand grinder. Then the resulting dispersion are mixed together in the proportion below so as to prepare the coating colour. Solution A (dispersion dye) 9.1 parts Solution E (dispersion of colour developer) 36.0 parts Solution C (dispersion of stabilizer) 24.0 parts Kaoline clay (50% dispersion) 12.0 parts The prepared coating colours are applied to one side of 50g/m 2 sheet paper so as obtain a coating weight of 5.0g /m 2 amount, dried up and the sheet is processed by a super calender to surface smoothness of 400-500 second. Thus the thermal sensitive recording paper can be obtained. Quality evaluation tests mentioned below are carried out on thermal sensitive recording papers prepared in above mentioned Examples and Comparative Examples. The obtained results are summarized in table 3, 4 and 6. In the tables, stabilizer (III) indicates compound of 4-benziloxy-4'-(2,3-epoxy-2-methylpropoxy)diphenylsulfone. Remark (1) resistance against plasticizer ; are carried out using TH-PMD (thermal sensitive recording paper testing apparatus, to which thermal head [Kyosera Ltd.] is installed) made by Ohkura Denki Ltd., by 0. 38mj/dot impressive energy. Image density of the recorded portion of each specimen is measured by a Macbeth densitometer (RD-914, an amber filter is used), and the obtained results are regarded as the image density of untreated specimen. Wrapping film of polyvinylchloride (High Wrap KMA ; product of Mitsui Toatsu Ltd.) is wound around a paper tube to form a single layer, recorded sheet of the thermal sensitive recording paper is stuck on it, then the wrapping film of polyvinylchloride is wound over the sheet to form a quarto layer. This specimen is left for 4 hours in the chamber of 40°C, then the Macbeth density of image portion is measured and the obtained results are regarded as the image density after treated. The Results of Quality Evaluation Tests Test No. colour developer stabilizer Ex.1 1 N-(benzenesulfonyl)- p -(phenylureylene)benzamide (III) 2 N-(4-toluenesulfonyl)- p -(phenylureylene)benzamide (III) 3 N-(benzenesulfonyl)- p -(phenylthioureylene)benzamide (III) 4 N-(4-toluenesulfonyl)- p -(phenylthioureylene)benzamide (III) 5 N-(4-methoxyphenylsulfonyl)- p -(phenylureylene)benzamide (III) 6 N-(2-hydroxyphenylsulfonyl)- p -(phenylureylene)benzamide (III) 7 N-(4-toluenesulfonyl)- p -(4-tolylureylene)benzamide (III) 8 N-( n -butylphenylsulfonyl)- p -(4-tolylureylene)benzamide (III) 9 N-(2-hydroxyphenylsulfonyl)- p -(4-tolylureylene)benzamide (III) 12 N-(4-toluenesulfonyl)- p -(phenylureylene)benzamide (III) 13 N-(4-iso-propylphenylsulfonyl)- p -(phenylthioureylene)benzamide (III) 14 N-(4-hydroxyphenylsulfonyl)- p -(phenylthioureylene)benzamide (III) 15 N-(benzenesulfonyl)- p -(4-tolylthiourne)benzamide (III) 16 N-(4-methoxyphenylsulfonyl)- p -(4-methylphenylthioureylene)benzamide (III) The Results of Quality Evaluation Tests Test No. colour developer stabilizer Ex.1 17 N-(phenyl-N'-( p -benzoylaminosulfonyl)phenylurea (III) 18 N-(phenyl)-N'-{ p -(benzoylaminosulfonyl)} (III) 19 N-(phenyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)}phenylurea (III) 20 N-(phenyl)-N'-{ p -(4-ethoxybenzoylaminosulfonyl)}phenylurea (III) 21 N-(phenyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)}phenylurea (III) 22 N-(4-toluyl)-N'-{ p -(2-naphthoilaminosulfonyl)}phenylurea (III) 23 N-(3-toluyl)-N'-{ p -(3-toluoylaminosulfonyl)}phenylurea (III) 24 N-(4-toluyl)-N'{ p -(4-hydroxybenzoylaminosulfonyl)}phenylurea (III) 25 N-(4-toluyl)-N'-{ p -(4-nitrobenzoylaminosulfonyl)}phenylurea (III) 26 N-(phenyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylthiourea (III) 27 N-(phenyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylthiourea (III) 28 N-(phenyl)-N'-{ p -(4-hydroxybenzoylaminosulfonyl)}phenylthiourea (III) 29 N-(phenyl)-N'-{ p -(4-methoxybenzoylaminosulfonyl)}phenylthiourea (III) 30 N-(4-toluyl)-N'-{ p -(2-naphthoylaminosulfonyl)}phenylthiourea (III) 31 N-(3-toluyl)-N'-( p -benzoylaminosulfonyl)phenylthiourea (III) 32 N-(3-toluyl)-N'-{ p -(4-toluoylaminosulfonyl)}phenylthiourea (III) The Results of Quality Evaluation Tests Test No. resistance against plasticizer (1) untreated treated Example-1 1 1.19 0.81 2 1.19 0.76 3 1.18 0.72 4 1.25 0.73 5 1.19 0.76 6 1.26 0.73 7 1.18 0.76 8 1.22 0.74 9 1.17 0.79 12 1.22 0.71 13 1.19 0.75 14 1.25 0.81 15 1.23 0.79 16 1.21 0.78 The Results of Quality Evaluation Tests Test No. resistance against plasticizer (1) untreated treated Example-2 17 1.21 0.81 18 1.22 0.75 19 1.23 0.78 20 1.19 0.84 21 1.18 0.75 22 1.20 0.76 23 1.21 0.71 24 1.19 0.72 25 1.23 0.77 26 1.24 0.74 27 1.20 0.73 28 1.18 0.76 29 1.24 0.74 30 1.23 0.79 31 1.20 0.77 32 1.18 0.75 The Results of Quality Evaluation Tests Test No. colour developer stabilizer Com. Ex.1 33 N-(benzenesulfonyl)- p -(phenylthioureylene)benzamide none 34 N-(4-toluenesulfonyl)- p -phenylthioureylene)benzamide none 35 N-(benzenesulfonyl)- p -(phenylureylene)benzamide none 36 N-(4-toluenesulfonyl)- p -phenylthioureylene)benzamide none 37 N-(4-methoxyphenylsulfonyl)- p -(phenylureylene)benzamide none Com. Ex.2 38 4,4'-dihydroxydiphenylsulfone (III) 39 4-hydroxy-4'-isopropoxydiphenylsulfone (III) 40 4-hydroxy-4'- n -butoxydiphenylsulfone (III) 41 4,4'-isopropylidenediphenol (III) 42 4-hydroxy-4'- n -propoxydiphenylsulfone (III) The Results of Quality Evaluation Tests Test No. resistance against plasticizer (1) untreated treated Com. Ex.1 33 1.18 0.26 34 1.19 0.28 35 1.17 0.27 36 1.18 0.29 37 1.16 0.28 Com. Ex.2 38 1.21 0.23 39 1.23 0.22 40 1.24 0.23 41 1.22 0.22 42 1.25 0.20 As clearly understood from the test results shown in table 3, 4 and 6, the specimens of thermal sensitive recording sheets prepared by Examples 1 of this invention which use the colour developer indicated by general formula (I) and (II) and the stabilizer indicated by general formula (III) have a remarkably superior image stability towards a plasticizer On the contrary, the effects of image stability towards a plasticizer of the specimens prepared by Comparative Example 1 which do not use the stabilizer of formula (III), and the specimens prepared by Comparative Example 2 which use a typical well-known phenolic colour developer (for instance, bisphenol A) in combination with a stabilizer of general formula (III) are inferior, and the image density after treatment is low. Therefore, it is clear that an excellent effect can be expected only when using the compound of general formula (I) or (II) together with the compound of general formula (III). In the present invention, the use of a derivative of urea (thiourea) of formula (I) or (II) as a colour developer, and a derivative of formula (III) as a stabilizer, results in a sharp recording image which has good preserving stability being formed even at a high speed and under high density recording conditions.

Claims: A thermally sensitive recording sheet which comprises, on a substrate, a thermally sensitive colour developing layer comprising: (a) a colourless or pale coloured basic leuco dye; (b) an organic colour developer of formula (I): wherein A is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, each of R 1 , R 2 and R 3 , which may be the same or different, is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy or nitro, or R 1 and R 2 form, together with the carbon atoms to which they are attached, an aromatic ring, and Y is sulfur or oxygen; or an organic colour developer of formula (II): wherein B is as defined above for A, each of R 1 , R 2 and R 3 , which may be the same or different, is as defined above and Y is as defined above; (c) 0.01-2 parts, based on 1 part of colour developer of formula (I) or (II), of a stabiliser of formula (III): and (d) a binder. A recording sheet according to claim 1 wherein, in the organic colour developer of formula (I) or (II), A or B, respectively, is hydrogen, methyl, ethyl, n-propyl, i-propyl, t-butyl, methoxy, ethoxy, n-propoxy, i-propoxy, hydroxy or nitro. A recording sheet according to claim 1 or 2 wherein, in the organic colour developer of formula (I) or (II), each of R 1 , R 2 and R 3 , which may be the same or different, is hydrogen, methyl, ethyl, n-propyl, i-propyl, t-butyl, methoxy, ethoxy, n-propoxy, i-propoxy, hydroxy or nitro. A recording sheet according to any one of the preceding claims wherein the colourless or pale coloured basic leuco dye is selected from fluoran-based leuco dyes, triphenylmethane-based leuco dyes, fluorene-based leuco dyes and divinyl-based leuco dyes. A recording sheet according to any one of the preceding claims which further includes, between the thermally sensitive colour developing layer and the substrate, an undercoating layer which comprises an organic or inorganic filler. A recording sheet according to any one of the preceding claims wherein the thermally sensitive colour developing layer is present on the substrate in a coating weight of 5.0 g/m 2 .

Inventors: MINAMI TOSHIAKI, MINAMI, TOSHIAKI, Minami, Toshiaki, Nippon Paper Ind. Co., Ltd.
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1997
Main classification: B41M5
IPC codes: B41M5