AU4144500A - Thermal recording material - Google Patents

Description

SPECIFICATION Title of the Invention Thermal Recording Material Field of Technology This invention relates to a thermal recording material comprising a support and a thermosensitive color-developing layer provided thereon including a leuco dye which is colorless or light-colored at normal temperature, an organic acid substance which reacts upon heating with the leuco dye to develop color and a sensitizer. Background Technoloagy Thermal recording materials comprising a support and a thermosensitive color-developing layer provided thereon including a leuco dye, an organic acid substance and a sensitizer are used in many fields such as computer outputs, printers of small-sized electronic calculators, recorders of various measuring instruments, facsimile, ticket vending machines, thermographic copying machines and labels. An increasingly wider use of thermal recording materials has created a stronger demand for improved performance and reduced price. A large number of proposals have been made on new leuco dyes, organic acid substances or sensitizers for improvement of performace, but it is difficult 1 for them to satisfy fully a variety of requirements. Acid compounds based on diphenyl sulfone have been proposed in JP (Japanese Patent) 63-46067 B (1988), JP 07-12749 B (1995) and JP 08 333329 A (1996). However, the use of these compounds, even in combination with general-purpose sensitizers, was unable to satisfy requirements for properties such as thermal response and stability of recorded images, particularly moisture resistance. Among organic acid substances, 4,4'-dihydroxydiphenyl sulfone is known to be relatively resistant to plasticizers; for example, JP 57-11088 A (1982), JP 58-119893 A (1983) and JP 61-160292 A (1986) disclose that the use of 4,4'-dihydroxydiphenyl sulfone as an organic acid substance improves the stability of colored images or that of the texture against plasticizers. On account of its high melting point and poor compatibility with general purpose dyes and sensitizers, however, 4,4'-dihydroxydiphenyl sulfone had a defect of developing color in low concentration. JP 08-72406 A (1996) proposes the use of 4,4'-dihydroxydiphenyl sulfone as an organic acid substance together with an acylacetanilide as a sensitizer. The use of the two together was able to provide a thermal recording material with good color-developing sensitivity and plasticizer resistance on one hand, but with reduced heat resistance of the texture on the other, and the material was not quite satisfactory in practical use. Moreover, JP 61-246088 A (1986) proposes the use of a 4-substituted biphenyl as a sensitizer of good color-developing sensitivty and low fogging 2 of the texture; however, bisphenol A and bis(3-allyl-4-hydroxyphenyl) sulfone are solely used here as organic acid substances and there is nothing to teach how to provide thermal recording materials exhibiting good stability of images, in particular, good plasticizer resistance. This invention has been devised in view of the aforementioned problems and its object is to provide a thermal recording material which exhibits good color-developing sensitivity and whiteness of the texture and develops little color in the texture in weathering tests such as wet heat test, plasticizer resistance test, and heat resistance test. Another object of this invention is to provide a thermal recording material exhibiting outstanding stability of recorded images such as storage stability under wet heat conditions and in contact with plasticizers. Still another object of this invention is to provide a thermal recording material with a good balance of the aforementioned properties. Disclosure of the Invention Accordingly, in a thermal recording material comprising a support and a thermosensitive color-developing layer provided thereon including a leuco dye which is colorless or light-colored at normal temperature, an organic acid substance which reacts upon heating with the leuco dye to develop color and a sensitizer, this invention relates to a thermal recording material wherein said thermosensitive color-developing layer includes one kind or two kinds or more of diphenyl sulfone derivatives selected from a group of 4 3 hydroxydiphenyl sulfone derivatives represented by the following general formula (1) HO \S OR 0 (i) (wherein R1 is an alkyl, aralkyl or aryl group), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone and 4-hydroxydipheny sulfone derivatives represented by the general formula (2) 00 HO - - Y-o O Y- oH oo n (2) (wherein Y is a divalent organic group and n is an integer in the range 0-6) as an organic acid substance and a 4-substituted biphenyl derivative represented by the following general formula (3) 0 \ / - / C-R2 (wherein R2 is an alkyl or aryl group) as a sensitizer. This invention will be described in detail below. A leuco dye to be used as a color former in this invention is a substance which is colorless or light-colored at normal temperature and reacts upon heating with an organic acid substance to develop color. The following are 4 examples of such a leuco dye: triarylmethane dyes such as 3,3-bis(p dimethylaminophenyl)-6-dimethylaminophthalide, T-anilino-3' dibutylamino-6'-methylfluoran, 3-(p-dimethylaminophenyl)-3-(1,2-dimethyl 3-indolyl)phthalide, 3,3 -bis(9 -ethyl- 3 -carbazolyl) -5- dimethylaminophthalide and 3,3-bis(2-phenyl-3-indolyl)-5-dimethylaminophthalide; diphenylmethane dyes such as 4,4'-bis(dimethylaminobenzhydryl) benzyl ether; thiazine dyes such as benzoyl leucomethylene blue; spiro dyes such as 3-methylspirodinaphthopyran; and fluoran dyes, leucoauramine dyes, indoline dyes and indigo dyes. These leuco dyes can be used singly or as a mixture of two kinds or more. In this invention, one kind or two kinds or more of diphenyl sulfone derivatives selected from a group of 4-hydroxydiphenyl sulfone derivatives represented by the aforementioned general formula (1), 4,4' dihydroxydiphenyl sulfone, 2,4'-dihydxoxydiphenyl sulfone and 4 hydroxydipheny sulfone derivatives represented by the general formula (2) are incoporated as an organic acid substance in the thermosensitive color developing layer. In the aforementioned general formula (1), R1 is an alkyl, aralkyl or aryl group, preferably a lower alkyl group with 1-5 carbon atoms, benzyl group, a lower alkyl-substituted benzyl group, phenyl group or a lower alkyl substituted phenyl group. Preferable concrete examples of 4-hydroxydiphenyl sulfone derivatives represented by the aforementioned general formula (1) are Compound 1 and 5 Compound 2 shown below. [Compound 1: melting point 1299C] 0 HO -0/ S C /OCH(CH3)2 0 [Compound 2: melting point 154 0 C] HO S OCH2CH2CH3 0 Regarding the method of preparation and preferable examples of 4 hydroxydiphenyl sulfone derivatives represented by the aforementioned general formula (2), reference may be made to the method and compounds described in the aforementioned JP 08-333329 A (1996). The reaction of 4,4' dihydroxydiphenyl sulfone with a compound represented by X-Y-X (wherein X is a halogen such as chlorine and bromine and Y is as defined earlier) can be utilized for the preparation. It is even possible to prepare preferentially a compound of the general formula (1) wherein n = 0 if the amount of X-Y-X is limited to below 0.5 times that of 4,4'-dihydroxydiphenyl sulfone on a mole basis. In case the amount of X-Y-X is in the range 0.5-0.9 times in mole, the reaction yields a mixture of compounds of the general formula (1) wherein n ranges from 1 to 6 depending on the mole ratio. The unreacted 4,4' dihydroxydiphenyl sulfone differs in solubility in an aqueous alkaline solution from the compounds of the general formula (1) wherein n ranges 6 from 0 to 6 and the two can be separated by utilizing this property. The compounds of the general formula (1) differing in n can be separated from one another by the aforementioned procedure or recrystallization, but they can also be used as a mixture without separation. It is naturally to be understood that the compounds to be used in this invention are not limited to those prepared by the aforementioned procedure. In the general formula (2), Y is an arbitray divalent organic group, for example, an aliphatic hydrocarbon group, a hydrocarbon group containing one or more heteroatoms in the main molecular chain, and a hydrocarbon group containing one or more aromatic rings in the main molecular chain. However, preferable examples are the following divalent organic groups cited in the aforementioned JP 08-333329 A (1996): a saturated or unsaturated divalent hydrocarbon group with 1-12 carbon atoms, a divalent hydrocarbon group with 1-8 carbon atoms containing an ether linkage or a group represented by -R- + -R- (wherein R is methylene or ethylene group and 1 is a benzene ring); more preferably, an alkylene group with 1-4 carbon atoms, a group represented by -R'-O-R'- (wherein R' is an alkylene group with 1-4 carbon atoms) or a group represented by -R- 6 -R- (wherein R is methylene group and 0 is a benzene ring); n is an integer of 0-6, preferably n is 0 or 1-3. Preferable concrete examples of 4-hydroxydiphenyl sulfone derivatives represented by the general formula (2) are compounds of the following formula such as Compound 3 (n = 0), Compound 4 (n = 1) and Compound 5 7 (n = 2). o 0 HO O CH 2

CH

2 0CH 2

CH

2 -0 0 O 0n 0

-CH

2

CH

2 0CH 2

CH

2 -0-- OH 0 A diphenyl sulfone derivative to be used as an organic acid substance is one kind or two kinds or more of compounds selected from a group of 4 hydroxydiphenyl sulfone derivatives represented by the aforementioned general formula (1), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone and 4-hydroxydiphenyl sulfone derivatives represented by the general formula (2). The amount of the organic acid substance is 1-6 parts by weight, preferably 1.5-2.5 parts by weight, per 1 part by weight of the leuco dye, although it varies with the kind of leuco dye and sensitizer in use. At least one kind of the aforementioned diphenyl sulfone derivatives may be used singly or as a mixture with another organic acid substance. Concrete examples of such another organic acid substance is a phenolic compound such as bisphenol A and a carboxylic acid such as benzoic acid and phthalic acid and it is used in an amount less than 50 wt%, preferably less than 10 wt%, of the total organic acid substances. In this invention, one kind or two kinds or more of 4-substituted biphenyl derivatives represented by the aforementioned general formula (3) are incorporated as a sensitizer in the thermosensitive color-developing layer. In the general formula (3), R2 is an alkyl or aryl group and, 8 preferably, a group to give the drivative in question a melting point in the range 50-200 9C such as a lower alkyl group with 1-5 carbon atoms, phenyl group and a lower alkyl-substituted phenyl group. Preferable concrete examples of a sensitizer represented by the general formula (3) for use in this invention are the following: [Compound 6: melting point 12 19C] 0 C-CH3 [Compound 7: melting point 96 *Cl 0 \ / \ / C-CH2CH3 [Compound 8: melting point 100C] O These compounds can be used singly or as a mixture of two kinds or more. The amount used is normally 1-6 parts by weight, preferably 1.5-2.5 parts by weight, per 1 part by weight of the leuco dye, although the amount varies with the kind of leuco dye and organic acid substance to be used. A compound represented by the aforementioned general formula (3) may be used singly or as a mixture with another sensitizer. Examples of such 9 another sensitizer include the following compounds: nitrogen-containing compounds such as stearamide, palmitamide, linolic acid amide and stearic acid anilide; esters such as benzyl 4-hydroxybenzoate, benzyl 4 benzyloxybenzoate, phenyl 2-naphthoate, benzyl 2-naphthoate, phenyl 1 hydroxy-2-naphthoate, dibenzyl oxalate, di(4-methylbenzyl) oxalate, dibenzyl terephthalate, n-butyl isophthalate and phenyl p-toluenesulfonate; aromatic compounds such as 4-benzylbiphenyl, m-terphenyl, fluorene, fluoranthene, 2,6-diisopropylnaphthalene, 3-benzylacenaphthene, 1,2 bis(3,4-dimethylphenyl)ethane, 1,2-bis(2,4-dimethylphenyl)ethane and 1,2 bis(2,4,5-trimethylphenyl)ethane; and ethers and sulfur-containing compounds such as 2-benzyloxynaphthalene, 1,4-diethoxynaphthalene, 1,2 diphenoxyethane, 1,2-diphenoxybenzene, 1,4-diphenoxybenzene, 4-(4' methylphenoxy)biphenyl, diphenyl sulfone, 4-methyldiphenyl sulfone, 4,4' diisopropoxydiphenyl sulfone, 4,4'-diphenoxydiphenyl thioether, benzyl 4 methylthiophenyl ether and 1,2-bis(phenoxymethyl)benzene. It is advantageous to add such another sensitizer in an amount less than 50 wt%, preferably less than 10 wt%, of the total sensitzers and maintain the melting point after mixing in the range 50-2009C. Moreover, for the purpose of improving the storage stability of the colored region still further, it is allowable to use epoxy resin and the metal salt of an organic acid substance such as zinc stearate and zinc salicylate, although they are slightly less responsive to heat, together with the aforementioned compounds. 10 In the cases where a high degree of storage stability is required, it is desirable to incorporate a publicly known preservative in a thermal recording material of this invention as occasion demands. Examples of such a preservative include phenolic compounds such as tris(2,6-dimethyl-4-tert-butyl-3-hydroxybenzy) isocyanurate, tris(3,5-di tert-butyl-4-hydroxybenzyl) isocyanurate, tris[2-(3-(3,5-di-tert-butyl-4 hydroxyphenyl)propionyloxyl ethyll isocyanurate, 1,3,5-tris(3,5-dimethyl-4 hydroxybenzyl)-2,4,6-trimethylbenzene, 1,1,3-tris(2-methyl-4-hydroxy-5 tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5 cyclohexylphenyl)butane, 4,4'-butylidenebis(2-tert-butyl-5-methyl)phenol, 4,4'-thiobis(2-tert-butyl-5-methyl)phenol and 2,2'-methylenebis(6-tert-butyl 4-methyl)phenol, epoxy compounds such as 4-benzyloxy-4'-(2 methylglycidyloxy)diphenyl sulfone and 4,4'-diglycidyloxydiphenyl sulfone, and sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate. Preferable preservatives are the aforementioned phenolic and epoxy compounds. These preservatives are normally used in an amount of 0.1-10 parts by weight per 1 part by weight of the leuco dye. In the cases where a still higher degree of storage stability, particularly plasticizer resistance, is required, it is advantageous to provide an overcoat layer on the thermosensitive color-developing layer of this invention. The overcoat layer is formed by a suitably selected procedure in order to manifest the desired performance. For example, a coating fluid composed of a publicly known resin component such as poly(vinyl alcohol) is applied to a 11 suitable thickness by a known coating method. However, provision of an overcoat layer, particularly a somewhat thicker one, tends to lower the sensitivity and proper selection of the coating conditions is important. Furthermore, in the cases where long-term weather resistance is required, it is desirable to add an ultraviolet absorber to the thermosensitive color-developing layer and/or the overcoat layer in a thermal recording material of this invention. Examples of ultraviolet absorbers are the following compounds: 2-hydroxybenzophenone derivatives such as 2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone, 2 hydroxy-4-octoxybenzophenone and 5,5'-methylenebis(2-hydroxy-4 methoxy)benzophenone; 2-(2-hydroxyphenyl)benzotriazole derivatives such as 2-(2-hydroxy-5-methylphenyl)benzotriazole, 2-(2-hydroxy-5-tert octylphenyl)benzotriazole, 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5 chlorobenzotriazole, 2-(2-hydroxy-3,5-dicumylphenyl)benzotriazole, 2,2' methylenebis(4-tert-octyl-6-benzotriazolyl)phenol and polyethylene glycol 2 (2-hydroxy-3-tert-butyl-5-carboxyphenyl)benzotriazole ester; benzoates such as phenyl salicylate, resorcinol monobenzoate, 2,4-di-tert-butylphenyl 3,5-di tert-butyl-4-hydroxybenzoate and hexadecyl 3,5-di-tert-butyl-4 hydroxybenzoate; substituted oxanilides such as 2 -ethyl-2'- ethoxyoxanilide and 2-ethoxy-4'-dodecyloxanilide; cyanoacrylates such as ethyl a -cyano 3 , 3 -diphenylacrylate and methyl 2-cyano-3-methyl-3-(p methoxyphenyl)acrylate; and derivatives of triazole and triazine such as 2 (2-hydroxy-4-octoxyphenyl)-s-triazine, 2-(2-hydroxy-4-methoxyphenyl)-4,6 12 diphenyl-s-triazine and 2-(2-hydroxy-4-propoxy-5-methyl)-4,6-bis(2,4-di-tert butylphenyl)-s-triazine. It is allowable to add a variety of additives such as binders and white pigments to thermal recording materials of this invention depending upon the end uses. Binders fix the leuco dye and the organic acid substance in the segregated state from each other while dispersed on fine particles and their examples are poly(vinyl alcohol) (PVA), latex, methylcellulose, carboxymethylcellulose, poly(acrylic acid), casein, gelatin, starch and their derivatives. White pigments are added for whiteness of the thermosensitive color-developing layer, slipperiness of writing devices and sticking and their examples are calcium carbonate, kaolin, clay, talc and titanium oxide. They are applied, either mixed or separately, to the support such as paper and film in the formation of the thermosensitive color-developing layer. Preferred Embodiments of the Invention This invention will be described concretely below with reference to examples and comparative examples. Example 1 (1) Preparation of Liquid A A mixture of 11.5 parts by weight of Compound 1 as an organic acid substance, 11.5 parts by weight of Compound 6 as a sensitizer and 46 parts by weight of a 5 wt% aqueous solution of PVA was ground in a sand mill to 13 prepare Liquid A or a dispersion of a powder with an average particle diameter of 0.8 y m. (2) Preparation of Liquid B A mixture of 5.5 parts by weight of 7'-anilino-3'-dibutylamino-6' methylfluoran and 49.5 parts by weight of a 5 wt% aqeuous solution of PVA was ground in a paint shaker to prepare Liquid B or a suspension of a powder with an average particle diameter of 0.8 u m. (3) Preparation of thermal recording paper A coating fluid was prepared by mixing 20 parts by weight of Liquid A, 10 parts by weight of Liquid B, 1 part by weight of a dispersion of zinc stearate (Hydrin Z-7 available from Chukyo Yushi Co., Ltd.), 4 parts by weight of an emulsion of paraffin wax (Hydrin P-7 available from Chukyo Yushi Co., Ltd.) and 11.5 parts by weight of a 10 wt% aqueous solution of PVA and the coating fluid was applied to a paper substrate and dried to give thermal recording paper with a coating weight of 6 g/m 2 after drying. (4) Testing method for color development The thermal recording paper thus prepared was printed and tested for dynamic color development and at the same time measured for the concentration of developed color of the texture while printing the specimens for testing sensitivity and moisture resistance at 24 V and 1.0 ms and those for testing plasticizer resistance at 27 V and 1.9 ms. The dynamic color development test was carried out with the use of a printing tester (available from Ohkura Electric Co., Ltd.) and the concentration of developed color was 14 measured by a Macbeth reflection densitometer, Model RD-914. (5) Testing method for moisture resistance The thermal recording paper which had been tested for dynamic color development was stored in a thermo-hygrostat (50 0 C, relative humidity 90%) for 24 hours and the concentration of developed color was measured by a Macbeth reflection densitometer, Model RD-914. The surviving rate was calculated as follows: Surviving rate = (A-B)/C wherein A is the concentration of dynamically developed color after the moisture resistance test, B is the value obtained by subtracting the concentration of developed color of the texture before the moisture resistance test from that after the moisture resistance test, and C is the concentration of dynamically developed color before the moisture resistance test. (6) Testing method for plasticizer resistance The entire printed surface of the thermal recording paper after the dynamic color development test was covered in close contact with polyvinyl chloride wrapping film. The paper and the wrapping film were stored in a dryer (40 0 C) for 24 hours and the concentration of developed color in the printed region was measured by a Macbeth reflection densitometer, Model RD-914. The surviving rate was calculated as follows: Surviving rate = (D - E)/F wherein D is the concentration of dynamically developed color after the 15 plasticizer resistance test, E is the value obtained by subtracting the concentration of developed color of the texture before the plasticizer resistance test from that after the plasticizer resistance test, and F is the concentration of dynamically developed color before the plasticizer resistance test. Example 2 Thermal recording paper was prepared as in Example 1 except using Compound 2 in place of Compound 1 in the preparation of Liquid A and it was tested as in Example 1. Example 3 Thermal recording paper was prepared as in Example 1 except using Compound 7 in place of Compound 6 in the preparation of Liquid A and it was tested as in Example 1. Example 4 Thermal recording paper was prepared as in Example 1 except using Compound 8 in place of Compound 6 in the preparation of Liquid A and it was tested as in Example 1. Comparative Example 1 Thermal recording paper was prepared as in Example 1 except using para-benzylbiphenyl in place of Compound 6 in the preparation of Liquid A and it was tested as in Example 1. Comparative Example 2 16 Thermal recording paper was prepared as in Example 1 except using bisphenol A in place of Compound 1 in the preparation of Liquid A and it was tested as in Example 1. The results of the tests for moisture resistance and plasticizer resistance are respectively shown in Tables 1 and 2. [Table 1 1 Dynamic color After moisture resistance test development test 24V, 1. Oms Texture Printed Texture Printed Surviving region region rate (%) Ex.1 0. 0 8 1. 0 3 0. 0 9 0. 6 1 5 8 Ex.2 0. 0 7 1. 0 5 0. 0 8 0. 5 8 5 4 Ex.3 0. 0 7 1. 0 2 0. 0 8 0. 6 3 6 1 Ex.4 0. 0 7 0. 9 9 0. 0 9 0. 6 8 6 7 Comp.Ex.1 0. 0 9 1. 0 1 0. 1 2 0. 3 1 2 8 Comp.Ex.2 0. 1 0 0. 9 9 0. 1 5 0. 2 2 1 7 17 [Table 2 ] Dynamic color After plasticizer resistance test development test 27V, 1. 9ms Texture Printed Texture Printed Surviving region region rate (%) Ex.1 0. 08 1. 35 0. 10 0. 58 41 Ex.2 0. 07 1. 38 0. 0 9 0. 6 3 44 Ex.3 0. 07 1. 31 0. 10 0. 53 38 Ex.4 0. 07 1. 28 0. 0 9 0. 48 36 Comp.Ex.1 0. 0 9 1. 3 3 0. 1 3 0. 3 9 2 6 Comp.Ex.2 0. 1 0 1. 3 0 0. 1 4 0. 2 0 1 2 Example 5 Liquid A was prepared as in Example 1 except using 11.5 parts by weight of 2,4'-dihydroxydiphenyl sulfone in place of Compound 1. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. Example 6 Liquid A was prepared as in Example 5 except using Compound 7 in place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. Example 7 Liquid A was prepared as in Example 5 except using Compound 8 in place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. 18 Comparative example 3 Liquid A was prepared as in Example 5 except using para benzylbiphenyl in place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. The results of the tests for moisture resistance and plasticizer resistance are respectively shown in Tables 3 and 4. [Table 3 ] Dynamic color After moisture resistance test development test 2 4V, 1. Oms Texture Printed Texture Printed Surviving region region rate (%) Ex.5 0. 0 7 0. 9 3 0. 10 0. 58 5 9 Ex.6 0. 0 8 0. 9 0 0. 0 9 0. 5 5 6 0 Ex.7 0. 08 0. 8 3 0. 10 0. 53 61 Comp.Ex.3 0. 07 0. 6 2 0. 0 9 0. 42 6 5 19 [Table 4 1 Dynamic color After plasticizer resistance test development test 2 7V, 1. 9ms Texture Printed Texture Printed Surviving region region rate (%) Ex.5 0. 0 7 1. 31 0. 10 0. 4 3 31 Ex.6 0. 0 8 1. 2 9 0. 10 0. 4 5 3 3 Ex.7 0. 0 8 1. 3 1 0. 1 1 0. 4 4 3 1 Comp.Ex.3 0. 0 7 1. 2 1 0. 0 9 0. 2 8 2 1 Example 8 A mixture of 11.5 parts by weight of Compound 3 as an organic acid substance, 11.5 parts by weight of Compound 6 as a sensitizer and 46 parts by weight of a 5 wt% aqueous solution of PVA was ground in a sand mill to prepare Liquid A or a dispersion of a powder with an average particle diameter of 0.8 g m. Using Liquid A this prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. Example 9 Liquid A was prepared as in Example 8 except using a 75:20:5 mixture by weight of Compound 3, Compound 4 and Compound 5 in place of Compound 3. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 1 and it was tested as in Example 1. Example 10 Liquid A was prepared as in Example 9 except using Compound 7 in 20 place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 8 and it was tested as in Example 1. Example 11 Liquid A was prepared as in Example 9 except using Compound 8 in place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 8 and it was tested as in Example 1. Comparative Example 4 Liquid A was prepared as in Example 9 except using para benzylbiphenyl in place of Compound 6. Using Liquid A thus prepared, thermal recording paper was prepared as in Example 8 and it was tested as in Example 1. The results of the tests for moisture resistance and plasticizer resistance are respectively shown in Tables 5 and 6. [Table 5 1 Dynamic color After moisture resistance test development test 2 4V, 1. Oms Texture Printed Texture Printed Surviving region region rate (%) Ex.8 0. 0 9 1. 0 3 0. 1 2 0. 25 17 Ex.9 0. 08 0. 96 0. 11 0. 33 25 Ex.10 0. 08 0. 9 3 0. 1 1 0. 32 24 Ex.11 0. 08 0. 7 2 0. 1 2 0. 27 31 Comp.Ex.4 0. 0 8 0. 4 4 0. 0 9 0. 2 5 3 4 21 [Table 6 1 Dynamic color After plasticizer resistance test development test 2 7V, 1. 9ms Texture Printed Texture Printed Surviving rate (%) region region Ex.8 0. 10 1. 27 0. 10 : 0. 96 76 Ex.9 0. 10 1. 20 0. 10 1. 18 9 7 Ex.10 0. 11 :1. 1 7 0. 1 1 1. 1 7 9 9 Ex.11 0. 1 0 1. 1 2 0. 1 0 1. 1 1 9 7 Comp.Ex.4 0. 1 0 0. 9 8 0. 1 0 0. 9 4 9 6 Example 12 (1) Preparation of Liquid A A mixture of 20 g of 7'-anilino-3'-dibutylamino-6'-methylfluoran and 100 g of a 10% aqueous solution of poly(vinyl alcohol) was ground thoroughly in a ball mill to prepare Liquid A or a suspension of a powder with an average particle diameter of 0.8 y m. (2) Preparation of Liquid B A mixture of 20 g of 4,4'-dihydroxydiphenyl sulfone as an organic acid substance and 100 g of a 10% aqueous solution of poly(vinyl alcohol) was ground thoroughly in a ball mill to prepare Liquid B or a suspension of a powder with an average particle diameter of 0.8 g m. (3) Preparation of Liquid C 22 A mixture of 20 g of Compound 6 as a sensitizer and 100 g of a 10% aqueous solution of poly(vinyl alcohol) was ground thoroughly in a ball mill to prepare Liquid C or a suspension of a powder with an average particle diameter of 0.8 u m. (4) Preparation of Liquid D A mixture of 10 g of 1,1,3-tris(2-methyl-4-hydroxy-5 cyclohexylphenyl)butane as a preservative and 100 g of a 10% aqueous solution of poly(vinyl alcohol) was ground thoroughly in a ball mill to prepare Liquid D or a suspension of a powder with an average particle diameter of 0.8 g m. (5) Preparation of Liquid E A mixture of 10 g of zinc stearate, 1 g of dimethylolurea, 1 g of calcium carbonate and 100 g of a 10% aqueous solution of poly(vinyl alcohol) was ground thoroughly in a ball mill to prepare Liquid E or a suspension of a powder with an average particle diameter of 0.8 [t m. (6) Preparation of Liquid F Liquid F was prepared in the same manner as Liquid E except further adding 10 g of 2,4-dihydroxybenzophenone as an ultraviolet absorber. (7) Preparation of thermal recording paper To 200 g of a 1:2:2:1 mixture by weight of the aforementioned Liquids A, B, C and E was added 50 g of calcium carbonate and the mixture was thoroughly dispersed. The resulting coating fluid was applied to a paper substrate and dried to give thermal recording paper with a coating weight of 23 6 g/m 2 after drying. The results of various tests are shown in Table 7. (8) Color development test The dynamic color development test (24 V, 1.0 ms) was performed on the thermal recording papers thus prepared and the concentration of developed color was measured for the printed region and the texture. The dynamic color development test was performed with the use of a printing tester (available from Ohkura Electric Co., Ltd.) and the concentration of developed color was measured by a Macbeth reflection densitometer, Model RD-914. (9) Test for moisture resistance The thermal recording paper which had been tested for dynamic color development was stored in a thermo-hygrostat (50 0 C, relative humidity 90%) for 24 hours and the concentration of developed color in the printed region and texture was measured by a Macbeth reflection densitometer, Model RD-914. (10) Test for plasticizer resistance The whole surface of the printed region of the thermal recording paper which had been tested for dynamic color development was covered in close contact with poly(vinyl chloride) wrapping film. The test specimen was stored in a dryer (40 0 C) for 24 hours and the concentration of developed color of the printed region and texture was measured by a Macbeth reflection densitometer, Model RD-914. (11) Test for heat resistance 24 The thermal recording paper which had been tested for dynamic color development was stored in a constant-temperature dryer (80 0 C) for 24 hours and the concentration of developed color of the printed region and texture was measured by a Macbeth reflection densitometer, Model RD-914. (12) Test for light resistance The thermal recording paper which had been tested for dynamic color development was irradiated with light in a fadeometer for 12 hours and the concentration of developed color of the printed region and texture was measured by a Macbeth reflection densitometer, Model RD-914. Example 13 Thermal recording paper was prepared as in Example 12 except using Compound 7 in place of Compound 6 in the preparation of Liquid C and it was tested as in Example 12. Example 14 Thermal recording paper was prepared as in Example 12 except using Compound 8 in place of Compound 6 in the preparation of Liquid C and it was tested as in Example 12. Example 15 Thermal recording paper was prepared as in Example 12 except using a 1:1 mixture of 4,4'-dihydroxydiphenyl sulfone and 4-isopropoxy-4' hydroxydiphenyl sulfone as an organic acid substance in the preparation of Liquid B and it was tested as in Example 12. 25 Example 16 Thermal recording paper was prepared as in Example 12 except using Liquid D in place of Liquid E in the preparation of a coating fluid and it was tested as in Example 12. Example 17 Liquid E was applied to the thermal recording paper obtained in Example 12 until the amount of coating after drying became 3 g/m 2 and dried to give thermal recording paper with an overcoat layer. The paper with the overcoat was tested as in Example 12. Example 18 Thermal recording paper was prepared as in Example 17 except using liquid D in place of Liquid E in the preparation of a coating fluid, thereafter overcoated as in Example 17, and it was tested as in Example 17. Example 19 Thermal recording paper was prepared as in Example 18, overcoated as in Example 18 except using liquid F in place of Liquid E, and tested as in Example 18. Example 20 Thermal recording paper was prepared as in Example 12 except using 2,4'-dihydroxydiphenyl sulfone as an organic acid substance in the preparation of Liquid B and using Liquid D in place of Liquid E in the preparation of a coating fluid and it was tested as in Example 12. Example 21 26 Thermal recording paper was prepared as in Example 12 except using 2,4'-dihydroxydiphenyl sulfone as an organic acid substance in the preparation of Liquid B, then coated with Liquid E until the weight of coating became 3 g/m 2 after drying, and dried to give thermal recording paper with an overcoat layer. The paper with the overcoat layer was tested as in Example 12. Example 22 Thermal recording paper was prepared as in Example 21 except using Liquid D in place of Liquid E in the preparation of a coating fluid, then overcoated as in Example 21, and tested as in Example 21. Example 23 Thermal recording paper was prepared as in Example 22, overcoated as in Example 22 except using Liquid F in place of Liquid E, and tested as in Example 22. Comparative Example 5 Thermal recording paper was prepared as in Example 12 except using para-benzylbiphenyl in place of Compound 6 in the preparation of Liquid C and tested as in Example 12. Comparative Example 6 Thermal recording paper was prepared as in Example 12 except using acetoacetanilide in place of Compound 6 in the preparation of Liquid C and tested as in Example 12. 27 The test results are summarized and shown in Table 7. [Table 7 1 Dynamic color Moisture Plasticizer Heat Light development test resistance test resistance test resistance test resistance test Texture/Printed Texture/Printed Texture/Printed Texture/Printed Texture/Printed region region region region region Ex. 12 0. 08 1.01 0.09 0.62 0.10 0.77 0. 12 1.08 0. 13 0.98 Ex. 13 0.07 0.97 0.09 0. 55 0.09 0.68 0.08 1.04 0. 12 0.90 Ex. 14 0.07 0.91 0.08 0. 56 0.09 0.60 0.08 1.00 0. 15 0.88 Ex. 15 0.09 1.10 0. 10 0.82 0.09 0. 79 0. 15 1.13 0. 18 1.05 Ex. 16 0.08 1.02 0.08 0. 71 0.09 0.78 0.15 1.03 0. 14 0.98 Ex. 17 0.08 0.96 0.08 0. 92 0. 10 0.93 0.13 0.98 0. 13 0.94 Ex. 18 0.07 0.99 0.09 0.95 0.09 0.96 0. 18 1.02 0. 12 0.97 Ex. 19 0.08 0.98 0. 10 0. 97 0. 11 0.92 0. 12 1.00 0.09 0.93 Ex. 20 0.08 1.09 0.09 0. 83 0.09 0.85 0. 13 1.01 0. 12 1.04 Ex. 21 0.08 1.07 0.09 1. 05 0. 10 1.00 0. 14 1.02 0. 13 1.01 Ex.22 0.09 1.05 0.09 1.03 0.10 1.02 0.13 1.03 0.13 1.01 Ex.23 0.09 1.04 0.09 1.01 0.10 1.01 0.13 1.00 0.09 1.01 Ex.24 0.10 0.98 0.15 0.48 0.13 0.78 0.88 1.02 0.18 0.90 Comp. 0.07 0.42 0.08 0.22 0.09 0.26 0.12 0.48 0.10 0.38 Ex.5 Comp. 0. 10 0.98 0. 15 0. 48 0. 13 0.78 0.88 1.02 0. 18 0.90 Ex. 6 Industrial Applicability A combination of an organic acid substance and a sensitizer of this invention gives a thermal recording material which is well balanced in properties and exhibits outstanding color-developing sensitivity, whiteness 28 of the texture after printing and also after long-term storage and good storage stability of images in exposure to moist heat or in contact with plasticizers. In consequence, this thermal recording material performs excellently in storage of images such as printed pictures, letters and symbols and is useful for recording media such as receipts, tickets, highway cards, baggage tags and barcode labels. Because of its good resistance to plasticizers, it is also useful for labels on food packages. 29