JPH10147066A - Dichroic thermal recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dichroic thermal recording medium having hard occurrence of fault color developing part such as blur, haze or color turbidity and excellent preservability of a recorded image. SOLUTION: In the dichroic thermal recording medium comprising a high temperature color developing layer and a low temperature color developing layer both containing leuco dye, developer and binder as main components and sequentially laminated on a support in such a manner that the medium is selectively color developed in red or black under the control of thermal energy, the developers of the high and low temperature layers contain the same, the dye of the low temperature layer contains red leuco dye for developing in red at low temperature by an operation of the developer with compatibility with the developer, and the dye of the high temperature layer contains black leuco dye for developing in black at high temperature by an operation of the developer with lower compatibility with the developer than that of the red leuco dye.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-color heat-sensitive recording medium which selectively develops a red or black color system by controlling thermal energy, and more particularly to the generation of defective color portions such as bleeding, blurring and color turbidity. The present invention relates to an improvement in a two-color heat-sensitive recording medium which is less likely to occur and has excellent preservability of recorded images.

[0002]

2. Description of the Related Art Hitherto, heat-sensitive recording media having a color-forming layer mainly composed of a leuco dye, a developer and a binder have been widely known. Since the thermosensitive recording medium is relatively inexpensive, and the printer is compact and relatively easy to maintain, it is used as a recording medium for facsimile and various terminal printers, and in other wide fields.

[0003] The required performance and quality have been diversified with the expansion of applications, and in recent years, higher sensitivity, multicolor recording, and the like have been demanded. In particular, since multicolor recording has a wide range of applications, many recording media are currently proposed.

[0004] Conventional two-color heat-sensitive recording media are roughly classified into two types: a mixed color type and a decolored type. That is,
A two-color thermosensitive recording medium of a mixed color type is formed by sequentially laminating a low-temperature coloring layer and a high-temperature coloring layer in which two or more leuco dyes having different color tones and a developer having a different melting point are combined on a support. It is. In this method, the low-temperature coloring layer is colored at the time of low-temperature heating, and both the low-temperature coloring layer and the high-temperature coloring layer are colored at the time of high-temperature heating to obtain a mixed color.

On the other hand, the decoloring type two-color thermal recording medium has a low-temperature coloring layer and a high-temperature coloring layer similarly to the above-described mixed-color two-color thermal recording medium. Is added thereto. In addition, only the coloring of the high-temperature coloring layer was obtained at the time of high-temperature heating by using the erasing agent in combination.

[0006]

By the way, the two-color type thermosensitive recording medium of the mixed color type has a structure in which a low-temperature coloring layer and a high-temperature coloring layer having different coloring temperatures are sequentially laminated on a support. In addition, defective coloring portions such as bleeding and blurring are likely to occur in both coloring layers, and the recorded image is unclear. Furthermore, when a black leuco dye, which is usually used as a leuco dye in the high-temperature coloring layer, is applied, a part of the high-temperature coloring layer also develops color at the time of low-temperature heating, and there is a problem that the low-temperature coloring image becomes turbid. Was.

A compound having a melting point of 200 ° C. or higher as a color developer in the high-temperature color-forming layer, for example, as disclosed in JP-A-4-3
The problem of color turbidity can be solved by applying 2,4'-dihydroxydiphenylsulfone or the like described in JP-A-29186, but when such a high-melting-point developer is applied. Another disadvantage is that the thermal sensitivity of the high-temperature coloring layer is reduced.

On the other hand, the decoloring type two-color heat-sensitive recording medium has an advantage that a clear color-developed image can be obtained at high and low temperatures, respectively, by adding an erasing agent to the low-temperature color-forming layer. There was a problem that the preservability of the recorded image was not always sufficient due to the incorporation of the erasing agent.

Incidentally, the above-mentioned color turbidity phenomenon at the time of low-temperature heating in the mixed-color type is not limited to the mixed-color type, but has occurred slightly in the decoloring type two-color thermal recording medium.

The present invention has been made in view of such problems, and the problems to be solved are blurring, blurring,
An object of the present invention is to provide a two-color heat-sensitive recording medium in which defective color-developing portions such as color turbidity are unlikely to occur and which have excellent storage stability of recorded images.

[0011]

The inventors of the present invention have conducted intensive studies with the aim of realizing this two-color heat-sensitive recording medium. As a result, a conventional two-color heat-sensitive recording medium having a low-temperature coloring layer and a high-temperature coloring layer has been obtained. A red leuco dye comprising the same color developer as the color developer in each color developing layer of the recording medium, and which is compatible with this color developer and develops a red color at a low temperature by the action of the color developer. Is applied as a leuco dye in the low-temperature color-forming layer, and the compatibility with the above-described color developer is lower than that of the red-type leuco dye. The present inventors have found that the above object can be achieved by applying the present invention as a leuco dye, thereby completing the present invention.

That is, according to the first aspect of the present invention, a high-temperature coloring layer and a low-temperature coloring layer, which are mainly composed of a leuco dye, a color developer and a binder and are sequentially laminated, are provided on a support to control thermal energy. Assuming a two-color heat-sensitive recording medium that selectively develops a red or black system, the developer in the high-temperature coloring layer and the low-temperature coloring layer is composed of the same developer, and the low-temperature coloring is performed. The leuco dye in the layer is composed of a red leuco dye which is compatible with the above-mentioned developer and develops a red color at a low temperature by the action of the developer, and the leuco dye in the high-temperature color developing layer is the above-mentioned developer. It is characterized by being composed of a black leuco dye which has a lower compatibility with the colorant than the red leuco dye and develops a black color at a high temperature by the action of the developer.

According to the two-color heat-sensitive recording medium of the first aspect of the invention, the red leuco dye in the low-temperature coloring layer having high compatibility with the color developer develops color when heated at a low temperature. Since the black leuco dye in the high-temperature coloring layer having low compatibility with the color developer does not form a color, the heated portion of the recording medium exhibits a red-colored color in the low-temperature coloring layer. On the other hand, at the time of high-temperature heating, the black-system leuco dye in the high-temperature color-forming layer also develops color because the compatibility of the black-system leuco dye with the color developer increases with an increase in temperature, and the heated portion of the recording medium is heated at the low-temperature-color-forming layer. Of the red system and the black system of the high-temperature coloring layer. Therefore, the heating portion of the recording medium can be colored red or black by controlling the heat energy, and the recording medium can be used as a two-color thermal recording medium.

The two-color heat-sensitive recording medium has a structure in which a high-temperature coloring layer and a low-temperature coloring layer having different coloring temperatures are sequentially laminated on a support. Are composed of the same color developing agent, and the color developing sensitivity of the leuco dye in each color forming layer is different due to the difference in compatibility with the color developing agent, so that bleeding, blurring,
It is unlikely that a defective coloring portion such as color turbidity occurs, and the recording medium has sufficient preservability of the recorded image because an erasing agent that acts upon heating at a high temperature is not applied.

Here, the level of the compatibility of the leuco dye with the developer is related to the chemical structure of the leuco dye, and the color development sensitivity of the leuco dye is higher as the compatibility with the developer is higher (ie, the higher the compatibility with the developer). The more compatible leuco dyes develop color at lower temperatures). The color development sensitivity of the leuco dye can be qualitatively predicted from the melting point of the leuco dye and its solubility in toluene. That is, the leuco dye having high solubility in toluene has high compatibility with the color developer, and the leuco dye having low solubility in toluene has low compatibility with the color developer. In the two-color thermosensitive recording medium according to the first aspect of the present invention, the black leuco dye which has a lower compatibility with the color developer than the red leuco dye and develops a black color at a high temperature by the action of the color developer may be used. , 200
It is desirable to use a leuco dye having a melting point of not less than ° C. and a solubility in toluene of 1% or less. Claim 2
Has been made for such a technical reason.

That is, the invention according to claim 2 is based on the two-color heat-sensitive recording medium according to the invention described in claim 1, and
The black leuco dye is characterized by being composed of a leuco dye having a melting point of 00 ° C. or more and a solubility in toluene of 1% or less.

[0017]

Embodiments of the present invention will be described below in detail.

First, the constituent materials of the two-color thermosensitive recording medium according to the present invention will be sequentially described. As a black leuco dye, for example, a base having a melting point of 200 ° C. or more and a solubility in toluene of 1% or less is used. Sexual leuco dyes can be used, but are not limited to specific compounds. Specific examples include the following compounds. For example, 3-
Diethylamino-7- (o-chloroanilino) fluoran (trade name: TH-106, manufactured by Hodogaya Chemical Co., Ltd., melting point: 21)
4.5-216.5 ° C.), 3-diethylamino-7-
(O-fluoroanilino) fluoran (Nippon Kayaku Co., Ltd.)
Trade name TG-11, melting point; 215 ° C or higher), 3- (N-
Ethyl-p-toluidino) -6-methyl-7-anilinofluoran (trade name: ETAC, manufactured by Yamada Chemical Co., melting point: 2)
06-208 ° C.), dimer of 3- (N-methyl-N-cyclohexylamino) -6-methyl-7-anilinofluoran (Papyrol SD-12, trade name, manufactured by Showa Denko KK)
0, melting point: 215 ° C. to 219 ° C.) and fluoran black leuco dye (trade name: CP101, manufactured by Yamamoto Chemical Co., melting point: 206 ° C.). Naturally, these leuco dyes are limited to these. Instead, any leuco dye having the above characteristics can be applied. Further, it is also possible to use two or more kinds as needed.

On the other hand, red leuco dyes include orange to
Any red-based basic leuco dye can be used. Specific examples include the following compounds. For example, 3-diethylamino-6-methyl-7-
Chlorofluorane, 3-cyclohexylamino-6-chlorofluorane, 3-diethylamino-7-chlorofluorane, 3-diethylamino-7,8-benzofluoran, 3- (N-ethyl-p-toluidino) -7 -Methylfluoran, 3-diethylamino-7-methylfluoran, 3- (N-ethyl-N-isoamylamino) -7-
Fluoran dyes such as methylfluoran, rhodamine-
Lactam-based dyes such as B-anilinolactam, rhodamine (p-nitroanilino) lactam, rhodamine (o-chloroanilino) lactam, 3- (p-dimethylaminophenyl) -3- (2-methylindol-3-yl) phthalide , 3,3-bis (1,2-dimethylindole-
3-yl) -5-dimethylaminophthalide, 3,3-bis (1,2-dimethylindol-3-yl) -6-dimethylaminophthalide, 3,3-bis (2-phenylindole-3- Yl) -6-dimethylaminophthalide,
3,3-bis (1-ethyl-2-methylindole-3
-Yl) phthalide, 3,3-bis (1-n-butyl-2)
-Methylindol-3-yl) phthalide and other triallylmethane dyes and the like, but of course, the present invention is not limited to these leuco dyes, and two or more dyes may be used in combination as necessary. It is also possible.

Next, various known compounds can be used as a developer to be incorporated in each color-forming layer. Specific examples include the following phenolic compounds, organic or inorganic acidic compounds, and esters and salts thereof. That is, 4-t-butylphenol, 4-hydroxydiphenoxide, α-naphthol, β-naphthol, 4-hydroxyacetophenol, 4-t-octylcatechol, 2,2′-methylenebis (4-methyl-6-t- Isobutylphenol), 4,4'-isopropylidenebis (2-t-butylphenol), 4,4'-sec-
Butylidene diphenol, 4-phenylphenol, bisphenol A, bisphenol S, bisphenol F, 2,2'-methylenebis (4-chlorophenol), 4,4'-cyclohexylidenediphenol, bisphenol methyl acetate, bisphenol butyl acetate, Phenolic compounds such as benzyl-hydroxybenzoate and dimethyl 4-hydroxyphthalate, benzoic acid, p-hydroxybenzoic acid, 3,5-di-t-butylsalicylic acid, salicylic acid, 3,5-di-α-methylbenzyl Organic acid compounds such as salicylic acid, 3-benzylsalicylic acid, and 3-isopropylsalicylic acid, and polyvalent metal salts thereof, and the like, are of course not limited to these color developers, Use two or more types according to Both are possible.

Further, as the binder applied to the present invention, a generally used polymer resin which is soluble in water or an organic solvent can be used. Specific examples of such a polymer resin include polyvinyl alcohol, methyl cellulose, cellulose acetate, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl pyrrolidone, gelatin, casein, starches, sodium polyacrylate, styrene-maleic acid copolymer and the like. Water-soluble polymer, polystyrene, polyvinyl chloride, linear saturated polyester, homo- or copolymer of methacrylic resin such as polymethyl methacrylate or polyethyl methacrylate, polyurethane, polybutyral, nitrocellulose, ethylcellulose, epoxy resin, etc. And a solvent-soluble polymer.

The mixing ratio of each component is not particularly limited. For example, the amount of the leuco dye is 0.1 to 2 parts by weight, and the amount of the binder is 2 to 1 part by weight of the developer. It is preferably at most part by weight.

Next, the method for producing a two-color heat-sensitive recording medium according to the present invention will be described below, but it goes without saying that the method for producing a two-color heat-sensitive recording medium according to the present invention is limited to the method described below. Not something. First, a binder is dissolved in a coating liquid composed of a leuco dye and a developer uniformly dispersed or dissolved in water or an organic solvent to prepare a high-temperature coloring layer coating liquid and a low-temperature coloring layer coating liquid, respectively. The color forming layer coating liquid is coated on a support such as paper, synthetic paper, plastic film or sheet by bar coating, blade coating,
An example is a method in which a high-temperature coloring layer and a low-temperature coloring layer are sequentially formed by coating by an application method such as air knife application, gravure application, roll coating application and the like, followed by drying. On this occasion,
A preferable coating amount after drying is, for example, 3 to 10 g /
m ² .

Further, the leuco dye and the developer may be present in the color-forming layer in a state of being encapsulated in microcapsules. Various known methods can be applied as the microencapsulation method at this time, and examples thereof include an interfacial polymerization method, an in-situ polymerization method, a coacervation method, a spray drying method, and a submerged drying method.

A protective layer may be provided on the low-temperature coloring layer of the support for the purpose of improving thermal head matching (adhesion to the thermal head, sticking) or imparting durability to the low-temperature coloring layer. As the protective layer component, a polymer resin applied to the binder of the low-temperature coloring layer, or a thermosetting resin, an electron beam-curable resin, a silicon-based resin, and talc, carbonic acid dispersed in these resins. Examples include combinations of inorganic fillers such as calcium, titanium oxide, and silicon oxide, organic fillers such as fluorine-based resin, silicon resin, and urea-formalin resin, and lubricants such as waxes.

Further, in each color-forming layer, in addition to a red-based leuco dye or a black-based leuco dye, a color developer, a binder, and other additives used for ordinary thermosensitive recording paper, may be added. Specific examples include dispersants such as surfactants, color image stabilizers, antioxidants, light stabilizers, and inorganic or organic pigments. Further, a melting point depressant for adjusting the thermal sensitivity of the color forming layer may be contained. Specific examples include higher fatty acid amides such as stearic acid amide and palmitic acid amide, benzenesulfonic acid amides, benzyloxynaphthalene, benzylbiphenyl and the like.

The support for the two-color heat-sensitive recording medium according to the present invention is not particularly limited, and papers such as high-quality paper and coated paper, synthetic paper, and plastic films such as polyester, PS, and PP. And sheets, papers laminated with a synthetic resin, and the like. Further, after applying and drying each color-forming layer or protective layer on the support, if necessary, a smoothing treatment such as super-calendering may be applied.

[0028]

Embodiments of the present invention will be described below.

In the examples, "parts" indicates parts by weight.

Example 1 (A) Preparation of Coloring Layer Coating Solution Composition of Solution A (Composition of Leuco Dye Solution for Red System Low Temperature Coloring) 3-Diethylamino-7-chlorofluorane 10 parts 10% aqueous solution of polyvinyl alcohol 10 parts Water 30 parts Composition of solution B (composition of leuco dye solution for black system high-temperature coloring) 3- (N-ethyl-p-toluidino) -6-methyl 10 parts -7-anilinofluoran 10% aqueous solution of polyvinyl alcohol 10 parts Water 30 parts Composition of liquid C (composition of developer solution) Bisphenol A 10 parts Calcium carbonate 10 parts 10% aqueous polyvinyl alcohol solution 20 parts Water 60 parts Each mixture composed of the above compositions is ground for 3 hours using a sand grinder. -Dispersion was performed to prepare solution A, solution B, and solution C, respectively.

Next, when the weight ratio of the liquid A and the liquid C is A: C = 1
By mixing and stirring so that the ratio becomes 0:40, a red system low-temperature coloring layer coating solution is prepared, and the weight ratio of the B solution and the C solution is B: C =
The mixture was stirred and mixed at a ratio of 10:80 to prepare a black system high-temperature coloring layer coating solution. (B) Preparation of two-color heat-sensitive recording medium The black-based high-temperature coloring layer coating solution and the red-based low-temperature coloring layer coating solution prepared in (A) were weighed in the order of 50 g / m ^2.
Each having a coating weight of 7 g / m ² (black high-temperature coloring layer coating solution) after drying on a high-quality paper using a Mayer bar, 3.5
g / m ² (red system low temperature coloring layer coating solution) at 60 ° C
, And dried to form a high-temperature coloring layer and a low-temperature coloring layer in that order.
142 (measured in accordance with Example 142) to obtain a two-color thermosensitive recording medium according to Example 1.

Example 2 The composition of the solution B of Example 1 (composition of the leuco dye solution for black system high-temperature color development) was determined to be 3- (N
-Ethyl-p-toluidino) -6-methyl-7-anilinofluoran instead of 3-diethylamino-7- (o
A dichroic thermosensitive recording medium was obtained in the same manner as in Example 1 except that (chloroanilino) fluorane was applied.

Example 3 The composition of the solution B of Example 1 (composition of the leuco dye solution for black system high-temperature color development) was determined to be 3- (N
-Ethyl-p-toluidino) -6-methyl-7-anilinofluoran instead of 3-diethylamino-7- (o
A two-color heat-sensitive recording medium was obtained in the same manner as in Example 1 except that (fluoroanilino) fluorane was applied.

Example 4 (A) Preparation of Coloring Layer Coating Solution Composition of Solution A (Composition of Leuco Dye Solution for Red System Low Temperature Coloring) 3,3-Bis (1-n-butyl-2-methylindole) -3- 10 parts Il) phthalide 5% methylcellulose aqueous solution 5 parts Water 25 parts Composition of solution B (composition of leuco dye solution for black system high-temperature coloring) 3- (N-methyl-N-cyclohexylammono) 10 parts -6 -Methyl-7-anilinofluoran dimer (Papyrol SD-120, trade name, manufactured by Showa Denko KK) 10% aqueous polyvinyl alcohol 5 parts Water 25 parts Composition of liquid C (composition of developer) 2,2 '-Diallyl-4,4'-diphenol sulfone 10 parts Silicon oxide (Mizukasil P-527 manufactured by Mizusawa Chemical Co., Ltd.) 10 parts Ionomer (Hydran AP-40 manufactured by DIC) 50 parts Ethanol 5 parts The mixture composed of the above-mentioned composition was pulverized and dispersed using a sand grinder so that the average particle diameter was about 2 μm, respectively, to prepare Liquid A, Liquid B, and Liquid C.

Next, when the weight ratio of the liquid A and the liquid C is A: C = 1
By mixing and stirring so that the ratio becomes 0:40, a red system low-temperature coloring layer coating solution is prepared, and the weight ratio of the B solution and the C solution is B: C =
The mixture was stirred and mixed at a ratio of 10:80 to prepare a black system high-temperature coloring layer coating solution. (B) Preparation of a two-color heat-sensitive recording medium A black PET high-temperature coloring layer coating liquid and a red low-temperature coloring layer coating liquid prepared in (A) were sequentially applied to a white PET sheet having a thickness of 188 μm. The conditions at 60 ° C. are such that the coating amount after drying with a Mayer bar is 8 g / m ² (black-based high-temperature coloring layer coating solution) and 4 g / m ² (red-based low-temperature coloring layer coating solution). To form a high-temperature coloring layer and a low-temperature coloring layer.

Next, on this low-temperature coloring layer, a UV offset ink (T & K TO
A protective layer was formed by UV offset printing of TOP-OP varnish 1) (trade name, manufactured by KA) so that the average film thickness was 2 μm, to obtain a two-color thermal recording medium according to Example 4.

Example 5 The composition of the solution B of Example 4 (the composition of the leuco dye solution for black system high-temperature coloring) was changed to 3- (N
-Methyl-N-cyclohexylamino) -6-methyl-
Example 1 except that a fluoran-based black leuco dye (trade name CP101, manufactured by Yamamoto Chemical Co., Ltd.) was used instead of the dimer of 7-anilinofluoran (trade name, papyrol SD-120, manufactured by Showa Denko KK). In the same manner as in Example 4, a two-color heat-sensitive recording medium was obtained.

[Evaluation Test] Next, the dynamic color development sensitivity characteristics of the two-color heat-sensitive recording media according to the respective examples were tested by the following measuring methods.

(Recording conditions) Apparatus: Megatronics thermal simulator, thermal head; Kyocera thin film head (8 dots / m
m, 1672 Ω) Applied voltage: 24 V fixed, pulse modulation, printing speed: 4 msec / line The applied voltage was pulse-controlled under the above recording conditions and recorded, and the low-temperature coloring energy and high-temperature coloring energy were determined.

The color density at each energy was measured using a Macbeth reflection densitometer (trade name: RD- manufactured by Macbeth).
918). The results are shown in Table 1 below.

The "coloring density" was evaluated by measuring the density of a color tone corresponding to red and black, and the "black fog" during heating at low temperature was evaluated by measuring the blue density. If the black fog density exceeds 0.25, the sharpness of the low-temperature color image is significantly impaired.

Further, "red bleeding" during high-temperature color development
Was visually observed.

Table 1 shows the results.

[0044]

[Table 1]

[Confirmation] As is clear from the results in Table 1,
The two-color thermosensitive recording medium according to each embodiment, a red image without color mixture fog is formed in the low-temperature coloring image,
The recording sensitivity of the high-temperature coloring image is good, and a black image without red bleeding is formed.

Accordingly, it was confirmed that a very clear two-color image could be obtained.

[0047]

According to the two-color thermosensitive recording medium according to the first and second aspects of the present invention, the developers in the high-temperature coloring layer and the low-temperature coloring layer are composed of the same developer. The leuco dye in the low-temperature coloring layer is composed of a red leuco dye that is compatible with the above-described developer and develops a red color at a low temperature by the action of the developing agent, and the leuco dye in the high-temperature coloring layer is Compatibility with the developer is lower than that of the red leuco dye, and the color developer is composed of a black leuco dye that develops a black color at a high temperature due to the action of the developer. The red-based leuco dye in the low-temperature coloring layer, which is highly compatible with the developer, develops color, whereas the black leuco dye in the high-temperature coloring layer, which is less compatible with the developer, does not develop color. The layer has a reddish color.

On the other hand, when heated at a high temperature, the compatibility of the black leuco dye with the developer increases with the temperature rise, so that the black leuco dye in the high-temperature coloring layer also develops a color,
The heated portion of the recording medium has a mixed color of a red system of the low-temperature coloring layer and a black system of the high-temperature coloring layer.

Accordingly, the heating portion of the recording medium can be colored in a red system or a black system by controlling the thermal energy, which has the effect of being usable as a two-color thermosensitive recording medium.

The two-color heat-sensitive recording medium has a structure in which a high-temperature coloring layer and a low-temperature coloring layer having different coloring temperatures are sequentially laminated on a support. Are composed of the same color developing agent, and the color developing sensitivity of the leuco dye in each color forming layer is different due to the difference in compatibility with the color developing agent, so that bleeding, blurring,
The occurrence of defective coloring portions such as color turbidity is unlikely to occur, and the recording medium has an effect of providing sufficient preservability of the recorded image because an erasing agent that acts at the time of high-temperature heating is not applied.

Claims (2)

[Claims] 1. A support comprising a leuco dye, a color developer and a binder as main components and a high-temperature coloring layer and a low-temperature coloring layer sequentially laminated on a support, and selectively controlling red energy or black color by controlling thermal energy. In a two-color heat-sensitive recording medium that develops a system, the developer in the high-temperature coloring layer and the low-temperature coloring layer is composed of the same developer, and the leuco dye in the low-temperature coloring layer is the developer. It is composed of a red leuco dye that is compatible with and has a red color at a low temperature due to the action of the developer, and the leuco dye in the high-temperature coloring layer has a red leuco compatibility with the developer. A two-color thermosensitive recording medium comprising a black leuco dye which is lower in color than a dye and develops a black color at a high temperature by the action of a developer. 2. The black leuco dye according to claim 1, wherein the leuco dye has a melting point of 200 ° C. or higher and a solubility in toluene of 1% or less. Color thermal recording medium.