JP7470516B2 - Thermal recording medium - Google Patents

Description

This invention relates to a thermal recording medium that utilizes a color-developing reaction between a colorless or light-colored electron-donating leuco dye (hereinafter also referred to as "leuco dye") and an electron-accepting developer (hereinafter also referred to as "developer"), and has excellent resistance to disinfection gel.

Generally, a thermal recording medium is prepared by coating a coating liquid containing a colorless or light-colored leuco dye and a color developer on a support such as paper, synthetic paper, film, plastic, etc., and a recorded image is obtained by instantaneous chemical reaction caused by heating with a thermal head, hot stamp, heat pen, laser light, etc. Thermal recording media are widely used as recording media for facsimiles, computer terminal printers, automatic ticket vending machines, measurement recorders, receipts at supermarkets and convenience stores, etc.
In recent years, the use of thermal recording media has expanded to a variety of applications, such as for various tickets, receipts, labels, bank ATMs, gas and electricity meter reading, car and horse racing betting tickets and other coupons, and medical labels. As a result, a variety of performance properties are required, such as water resistance, resistance to plasticizers in the image area, heat resistance, oil resistance, and solvent resistance in the blank areas, and storage stability of the image and blank areas under harsh conditions.
In response to such demands, a technology has been developed in which a protective layer is provided on the thermal recording layer of a thermal recording medium and a specific binder/crosslinking agent is contained in the protective layer, thereby improving the water resistance of the thermal recording medium in particular (Patent Document 1). Also, a urea compound has been disclosed as a color developer for improving the required performance of the thermal recording medium, such as color density, whiteness, and storage stability of the printed portion (Patent Document 2).

International Publication WO2006/075467 International Publication WO2019/044462

Disinfection is carried out in the medical and nursing care fields to prevent infectious diseases, but due to the recent antibacterial boom, disinfectant gels are not only used in medical and nursing care sites, but also in various places such as restaurant kitchens and homes. In order to improve the applicability and usability of such disinfectant gels to the skin, they contain various ingredients such as water retaining agents in addition to alcohol, which makes the alcohol less likely to volatilize and remains on the surface for a long time after application, thereby maintaining its function. On the other hand, when disinfectant gel is attached to a thermal recording medium, the alcohol does not volatilize immediately, but penetrates while remaining on the surface, reacting with the thermal recording layer and causing the blank area to become colored.
Therefore, an object of the present invention is to provide a thermal recording medium that, among the various properties required for a thermal recording medium, has excellent resistance to disinfection gel.

The present inventors have previously investigated technologies that are thought to be effective for such purposes (e.g., Patent Documents 1 and 2), but have not been able to obtain a thermal recording medium with sufficient resistance to disinfection gel (see Comparative Examples 1 and 2 described below).
As a result of extensive research, the inventors have discovered that the above-mentioned problems can be solved by incorporating a specific urea compound in the thermal recording layer of a thermal recording material, and further incorporating a specific combination of a binder and a crosslinking agent in the protective layer provided on the thermal recording layer, thereby completing the present invention.

（式中、Ｒ^１は、置換若しくは無置換のアルキル基、アラルキル基又はアリール基を表し、Ｒ^２は、水素原子又はアルキル基を表す。） That is, the present invention is a thermosensitive recording medium comprising a support on which a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer is provided, and a protective layer is provided on the thermosensitive recording layer, the thermosensitive recording layer containing a urea compound represented by the following general formula (Chemical Formula 1) as the electron-accepting developer, and the protective layer containing a carboxyl group-containing resin as a binder, and further containing an epichlorohydrin-based resin and a polyamine/polyamide-based resin (excluding those contained in the epichlorohydrin-based resin) as a crosslinking agent.

(In the formula, R ¹ represents a substituted or unsubstituted alkyl group, aralkyl group, or aryl group, and R ² represents a hydrogen atom or an alkyl group.)

The present invention provides a thermal recording medium that has good color development performance and good resistance to disinfectant gel.

The thermosensitive recording medium of the present invention has a thermosensitive recording layer on a support, and further has a protective layer on the thermosensitive recording layer.
Various materials that can be used in the thermosensitive recording layer of the present invention are exemplified below, but binders, crosslinking agents, pigments, etc. can also be used in other coating layers that are provided as necessary, as long as they do not impair the desired effects for the above-mentioned problems.

The heat-sensitive recording layer of the present invention contains a urea compound represented by the following formula (Chemical Formula 1) as a developer.

In the general formula (Chemical Formula 1), R ¹ represents an alkyl group, an aralkyl group, or an aryl group, which may be substituted or unsubstituted. The alkyl group is, for example, a linear, branched, or alicyclic alkyl group, and preferably has 1 to 12 carbon atoms. The aralkyl group preferably has 7 to 12 carbon atoms, and the aryl group preferably has 6 to 12 carbon atoms. When these are substituted, the substituent is preferably an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or a halogen atom. The multiple R ^1s may be the same or different.
The positions of R ¹ -SO ₂ -O- in the benzene ring of the general formula (Chemical Formula 1) may be the same or different, and are preferably the 3-position, 4-position or 5-position.

Examples of the alkyl group include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, cyclopentyl, hexyl, cyclohexyl, 2-ethylhexyl, and lauryl.

Examples of the aralkyl group include benzyl, 1-phenylethyl, 2-phenylethyl, 3-phenylpropyl, p-methylbenzyl, m-methylbenzyl, m-ethylbenzyl, p-ethylbenzyl, p-i-propylbenzyl, p-t-butylbenzyl, p-methoxybenzyl, m-methoxybenzyl, o-methoxybenzyl, m,p-di-methoxybenzyl, p-ethoxy-m-methoxybenzyl, p-phenylmethylbenzyl, p-cumylbenzyl, p-phenylbenzyl, o-phenylbenzyl, m-phenylbenzyl, p-tolylbenzyl, m-tolylbenzyl, o-tolylbenzyl, and p-chlorobenzyl groups, and the like.

Examples of the aryl group include phenyl, p-tolyl, m-tolyl, o-tolyl, 2,5-dimethylphenyl, 2,4-dimethylphenyl, 3,5-dimethylphenyl, 2,3-dimethylphenyl, 3,4-dimethylphenyl, mesitylene, p-ethylphenyl, p-i-propylphenyl, p-t-butylphenyl, p-methoxyphenyl, 3,4-dimethoxyphenyl, p-ethoxyphenyl, p-chlorophenyl, 1-naphthyl, 2-naphthyl, and t-butylated naphthyl, and other unsubstituted or aryl groups substituted with alkyl groups, alkoxy groups, aralkyl groups, aryl groups, or halogen atoms.

^R2 represents a hydrogen atom or an alkyl group, preferably a hydrogen atom. The alkyl group is preferably an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, or a t-butyl group.
The positions of ^R2 in the benzene ring of the general formula (Chemical Formula 1) may be the same or different, and are preferably the 3-position, 4-position or 5-position.

一般式（化２）中、Ｒ^３はアルキル基又はアルコキシ基、好ましくはアルキル基であり、ｎは０～３、好ましくは０～２、より好ましくは０～１の整数を表す。このアルキル基の炭素数は、例えば、１～１２、好ましくは１～８、より好ましくは１～４である。
一般式（化２）のベンゼン環中のＲ^３の位置は、同じであっても異なってもよく、好適には３位、４位又は５位、好ましくは４位である。 The urea compound of the present invention is preferably a urea compound represented by the following general formula (Chemical Formula 2).

In the general formula (Chemical Formula 2), ^R3 is an alkyl group or an alkoxy group, preferably an alkyl group, and n is an integer of 0 to 3, preferably 0 to 2, more preferably 0 to 1. The number of carbon atoms in this alkyl group is, for example, 1 to 12, preferably 1 to 8, more preferably 1 to 4.
The position of ^R3 in the benzene ring of the general formula (Chemical Formula 2) may be the same or different, and is suitably the 3-position, 4-position or 5-position, preferably the 4-position.

In addition, examples of the urea compound of the present invention include N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-ethyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-5-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-propyl-phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3 -(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(p-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(1-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-isopropylbenzenesulfonyloxy)phenyl]urea, N,N' -di-[3-(p-t-butylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m,p-dimethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-butoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-cumyl benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(p-cumylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-chlorobenzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, and the like, but are not limited thereto.

The content (solid content) of the urea compound represented by the general formula (Chemical Formula 1) in the thermal recording layer of the present invention is 1.0 to 50.0% by weight, preferably 1.0 to 40.0% by weight.

The thermal recording layer of the present invention may use a color developer other than the urea compound represented by the general formula (Chemical Formula 1). Examples of such color developers include inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate, 4,4'-isopropylidenediphenol, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenyl sulfide, hydroquinone monobenzyl ether, 4-hydroxybenzoate benzyl, 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, and 4-hydroxy-4'-isopropoxydiphenyl sulfone. 4-hydroxyphenyl-4'-n-propoxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, the phenol condensation composition described in JP-A-2003-154760, the aminobenzenesulfonamide derivative described in JP-A-8-59603, bis(4-hydroxyphenylthioethoxy)phenyl bis(p-hydroxyphenyl)butyl acetate, bis(p-hydroxyphenyl)methyl acetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, di(4-hydroxy-3-methylphenyl)sulfide, 2,2'-thiobis(3-tert-octylphenol), 2,2'-thiobis(4-tert-octylphenol), compounds described in WO02/081229 or JP2002-301873A, and N,N' Examples of the color developer include thiourea compounds such as 1-di-m-chlorophenylthiourea, p-chlorobenzoic acid, stearyl gallate, bis[zinc 4-(n-octyloxycarbonylamino)salicylate] dihydrate, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, and 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, and salts of these aromatic carboxylic acids with polyvalent metal salts such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, and nickel, as well as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids. These color developers can be used alone or in combination of two or more. 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane is available, for example, under the trade name JKY-214 manufactured by API Corporation, and the phenol condensation composition described in JP 2003-154760 A is available, for example, under the trade name JKY-224 manufactured by API Corporation. Compounds described in WO 02/081229 and the like are available under the trade names NKK-395 and D-100 manufactured by Nippon Soda Co., Ltd. In addition, metal chelate-type color-developing components such as higher fatty acid metal double salts and polyvalent hydroxy aromatic compounds described in JP 10-258577 A can also be included.

When the thermosensitive recording layer of the present invention contains a color developer other than the urea compound represented by the general formula (Chemical Formula 1), the total content (solid content) of the urea compound represented by the general formula (Chemical Formula 1) relative to all the color developers contained in the thermosensitive recording layer (including the urea compound represented by the general formula (Chemical Formula 1)) is preferably 50% by weight or more, more preferably 90% by weight or more.

The leuco dye used in the present invention can be any dye known in the field of conventional pressure-sensitive or heat-sensitive recording paper, and is not particularly limited. However, triphenylmethane compounds, fluoran compounds, fluorene compounds, divinyl compounds, etc. are preferred. Specific examples of representative colorless or light-colored dyes (dye precursors) are shown below. These dye precursors may be used alone or in combination of two or more.

<Triphenylmethane leuco dyes>
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (also known as crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)phthalide (also known as malachite green lactone)

<Fluoran-based leuco dyes>
3-diethylamino-6-methylfluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(o-fluoroanilino)fluoran, 3-diethylamino-6-methyl-7-(m-methylanilino)fluoran, 3-diethylamino-6-methyl-7 -n-octylanilinofluoran, 3-diethylamino-6-methyl-7-n-octylaminofluoran, 3-diethylamino-6-methyl-7-benzylaminofluoran, 3-diethylamino-6-methyl-7-dibenzylaminofluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-chloro-7-p-methylanilinofluoran, 3-diethylamino-6-ethoxyethyl-7-anilinofluoran, 3-diethylamino-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-(p-chloroanilino)fluoran, 3-diethylamino-7-(o-fluoroanilino)fluoran, 3-diethylamino-benzo[a]fluoran, 3-diethylamino-benzo[c]fluoran, 3-dibutylamino-6-methyl-fluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-fluoroanilino)fluoran fluoran, 3-dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran, 3-dibutylamino-6-methyl-7-chlorofluoran, 3-dibutylamino-6-ethoxyethyl-7-anilinofluoran, 3-dibutylamino-6-chloro-7-anilinofluoran, 3-dibutylamino-6-methyl-7-p-methylanilinofluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dibutylamino-7-(o-fluoroanilino)fluoran, 3-di-n-pentylamino-6-methyl-7-anilinofluoran, 3-di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-di-n-pentylamino- 7-(m-trifluoromethylanilino)fluoran, 3-di-n-pentylamino-6-chloro-7-anilinofluoran, 3-di-n-pentylamino-7-(p-chloroanilino)fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-xylamino)-6-methyl-7-(p-chloroanilino)fluoran, 3-(N-ethyl 1-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran, 3-cyclohexylamino-6-chlorofluoran, 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)- 3-diethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran, 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 No-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, 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran

<Fluorene-based leuco dyes>
3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide], 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide]

<Divinyl 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

＜Other＞
3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide, 3,6-bis(diethylamino)fluoran-γ-(3′-nitro)anilinolactam, 3,6-bis(diethylamino)fluoran-γ-(3′-nitro)anilinolactam, Fluoran-γ-(4'-nitro)anilinolactam, 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]-methylmalonic acid dimethyl ester

The sensitizer used in the present invention may be a conventionally known sensitizer. Examples of such sensitizers include fatty acid amides such as stearic acid amide and palmitic acid amide, ethylene bisamide, montanic acid wax, polyethylene wax, 1,2-bis-(3-methylphenoxy)ethane, p-benzylbiphenyl, β-benzyloxynaphthalene, 4-biphenyl-p-tolyl ether, m-terphenyl, 1,2-diphenoxyethane, dibenzyl oxalate, di(p-chlorobenzyl) oxalate, di(p-methylbenzyl) oxalate, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-p-tolyl carbonate, phenyl-α- Examples of the sensitizers include naphthyl carbonate, 1,4-diethoxynaphthalene, 1-hydroxy-2-naphthoic acid phenyl ester, o-xylene-bis-(phenyl ether), 4-(m-methylphenoxymethyl)biphenyl, 4,4'-ethylenedioxy-bis-benzoic acid dibenzyl ester, dibenzoyloxymethane, 1,2-di(3-methylphenoxy)ethylene, bis[2-(4-methoxy-phenoxy)ethyl]ether, methyl p-nitrobenzoate, phenyl p-toluenesulfonate, o-toluenesulfonamide, and p-toluenesulfonamide. These sensitizers may be used alone or in combination of two or more.

Pigments that can be used in the present invention include kaolin, calcined kaolin, calcium carbonate, aluminum oxide, titanium oxide, magnesium carbonate, aluminum silicate, magnesium silicate, calcium silicate, aluminum hydroxide, silica, etc., and can be used in combination depending on the required quality.

Examples of binders used in the thermal recording layer of the present invention include fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, acetoacetylated polyvinyl alcohol, carboxy-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, olefin-modified polyvinyl alcohol, nitrile-modified polyvinyl alcohol, pyrrolidone-modified polyvinyl alcohol, silicone-modified polyvinyl alcohol, other modified polyvinyl alcohols, hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, styrene-maleic anhydride copolymer, styrene-butadiene copolymer, as well as cellulose derivatives such as ethyl cellulose and acetyl cellulose, casein, gum arabic, oxidized starch, etherified starch, dialdehyde starch, esterified starch, polyvinyl chloride, polyvinyl acetate, polyacrylamide, polyacrylic acid esters, polyvinyl butyral, polystyrene and copolymers thereof, polyamide resins, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These polymeric substances can be used by dissolving them in solvents such as water, alcohol, ketones, esters, and hydrocarbons, or by dispersing them in water or other media in an emulsion or paste form, and can also be used in combination depending on the required quality.

A crosslinking agent may be used in combination in the heat-sensitive recording layer of the present invention. Examples of the crosslinking agent include epichlorohydrin resins such as polyamine epichlorohydrin resins and polyamide epichlorohydrin resins, polyamide urea resins, polyalkylene polyamine resins, polyalkylene polyamide resins, polyamine polyurea resins, modified polyamine resins, modified polyamide resins, polyalkylene polyamine urea formalin resins, and polyalkylene polyamine polyamide polyurea resins, polyamine/polyamide resins such as glyoxal, methylol melamine, melamine formaldehyde resins, melamine urea resins, potassium persulfate, ammonium persulfate, sodium persulfate, ferric chloride, magnesium chloride, borax, boric acid, alum, and ammonium chloride.

It is preferable that the heat-sensitive recording layer of the present invention contains a carboxyl group-containing resin as a binder and an epichlorohydrin-based resin as a crosslinking agent, since this provides particularly good water resistance.

The carboxyl group-containing resin used in the heat-sensitive recording layer of the present invention may be any resin as long as it mainly has a carboxyl group, and examples thereof include acrylic resins containing monofunctional acrylic monomers having a carboxyl group, such as methacrylic acid, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, dimethylaminoethyl methacrylate, glycidyl methacrylate, and tetrahydrofurfuryl methacrylate, oxidized starch, carboxymethyl cellulose, and carboxy-modified polyvinyl alcohol in which a carboxyl group has been introduced into polyvinyl alcohol.
In particular, when the carboxyl group-containing resin is carboxy-modified polyvinyl alcohol, the plasticizer resistance of the image area and the heat resistance of the blank area are further improved, which is presumably because, in addition to the above-mentioned crosslinking reaction, the cationic site of the polyamine/polyamide resin crosslinks with the carboxyl group of the carboxy-modified polyvinyl alcohol.

The carboxy-modified polyvinyl alcohol used in the thermal recording layer of the present invention is obtained as a reaction product of polyvinyl alcohol with a polycarboxylic acid such as fumaric acid, phthalic anhydride, mellitic anhydride, or itaconic anhydride, or an ester of these reaction products, or a saponification product of a copolymer of vinyl acetate with an ethylenically unsaturated dicarboxylic acid such as maleic acid, fumaric acid, itaconic acid, crotonic acid, acrylic acid, or methacrylic acid. Specific examples include the manufacturing methods exemplified in Examples 1 or 4 of JP-A-53-91995. The saponification degree of the carboxy-modified polyvinyl alcohol is preferably 72 to 100 mol%, and the polymerization degree is preferably 500 to 2400, and more preferably 1000 to 2000.

The epichlorohydrin resin used in the heat-sensitive recording layer of the present invention is a resin characterized by containing an epoxy group in the molecule, and examples thereof include the above-mentioned polyamide epichlorohydrin resin and polyamine epichlorohydrin resin.
As the amine present in the main chain of the epichlorohydrin resin, primary to quaternary amines can be used, and there is no particular limitation. Furthermore, since the water resistance is good, the degree of cationization is preferably 5 meq/g·Solid or less (measured at pH 7) and the molecular weight is preferably 500,000 or more. These epichlorohydrin resins may be used alone or in combination of two or more kinds.
Specific examples of epichlorohydrin resins used in the heat-sensitive recording layer of the present invention include Sumirez Resin 650(30), Sumirez Resin 675A, Sumirez Resin 6615 (all manufactured by Sumitomo Chemical Co., Ltd.), WS4002, WS4020, WS4024, WS4030, WS4046, WS4010, CP8970 (all manufactured by Seiko PMC Corporation), and the like.

In the thermal recording layer of the present invention, 4,4'-butylidene (6-t-butyl-3-methylphenol), 2,2'-di-t-butyl-5,5'-dimethyl-4,4'-sulfonyldiphenol, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, etc. can be added as stabilizers to improve the oil resistance of the image area within a range that does not impair the desired effect for the above-mentioned problems. In addition, benzophenone-based or triazole-based ultraviolet absorbers, dispersants, antifoaming agents, antioxidants, fluorescent dyes, and lubricants such as fatty acid metal salts such as zinc stearate and calcium stearate, waxes, and silicone resins can be used.

The types and amounts of the leuco dye, developer, sensitizer, and other various components used in the thermal recording layer of the present invention are determined according to the required performance and recording suitability, and are not particularly limited, but typically, 0.5 to 10 parts by weight of developer, 0.1 to 10 parts by weight of sensitizer, 0.5 to 20 parts by weight of pigment, 0.01 to 10 parts by weight of stabilizer, and 0.01 to 10 parts by weight of other components are used per 1 part by weight of leuco dye. The binder is preferably about 5 to 25% by weight of the solid content of the thermal recording layer.

In the present invention, the leuco dye, developer, and materials added as necessary are pulverized to particle sizes of several microns or less using a grinding machine such as a ball mill, attritor, or sand grinder, or an appropriate emulsifying device, and a binder and various additives depending on the purpose are added to make a coating liquid. The solvent used for this coating liquid can be water or alcohol, and the solid content is about 20 to 40% by weight.

The protective layer of the present invention contains the above-mentioned carboxyl group-containing resin, particularly the above-mentioned carboxy-modified polyvinyl alcohol, as a binder, and further contains the above-mentioned epichlorohydrin-based resin and polyamine/polyamide-based resin (excluding those contained in epichlorohydrin-based resin) as a crosslinking agent, thereby further improving the disinfection gel resistance of the thermal recording medium.

This polyamine/polyamide resin does not have an epoxy group in the molecule, and examples thereof include polyamide urea resin, polyalkylene polyamine resin, polyalkylene polyamide resin, polyamine polyurea resin, modified polyamine resin, modified polyamide resin, polyalkylene polyamine urea formalin resin, polyalkylene polyamine polyamide polyurea resin, etc., which can be used alone or in combination. Specific examples of polyamine/polyamide resins include Sumilaze Resin 302 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumilaze Resin 712 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumilaze Resin 703 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumilaze Resin 636 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumilaze Resin SPI-100 (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin), Sumilaze Resin SPI-102A (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin), Sumilaze Resin SPI-106N (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin), and Sumilaze Resin SPI-106N (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin). (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamide resin), Sumirez Resin SPI-203 (50) (manufactured by Sumitomo Chemical Co., Ltd.), Sumirez Resin SPI-198 (manufactured by Sumitomo Chemical Co., Ltd.), Printive A-700, Printive A-600 (all manufactured by Asahi Kasei Corporation), PA6500, PA6504, PA6634, PA6638, PA6640, PA6644, PA6646, PA6654, PA6702, PA6704 (manufactured by Seiko PMC Co., Ltd.: polyalkylene polyamine polyamide polyurea resin), CP8994 (manufactured by Seiko PMC Co., Ltd.: polyethyleneimine resin), etc. Although there are no particular limitations, it is preferable to use polyamine resins (polyalkylene polyamine resins, polyamine polyurea resins, modified polyamine resins, polyalkylene polyamine urea formalin resins, polyalkylene polyamine polyamide polyurea resins) because of their good print density.

When epichlorohydrin resin and polyamine/polyamide resin are used together with a carboxyl group-containing resin in the protective layer, the content is preferably 1 to 100 parts by weight, more preferably 5 to 50 parts by weight, and even more preferably 10 to 40 parts by weight, respectively, per 100 parts by weight of the carboxyl group-containing resin.

As the pigment used in the protective layer, any of the pigments that can be used in the above-mentioned heat-sensitive recording layer can be used as appropriate, but kaolin, calcined kaolin, aluminum hydroxide, and silica are preferred, and it is more preferred that the protective layer contains kaolin and/or aluminum hydroxide. One or more of these pigments may be used. The aspect ratio of this kaolin is preferably 20 or more, more preferably 30 or more. By using a pigment with an aspect ratio of 20 or more, the coverage of the heat-sensitive recording layer is improved.

The content (solid content) of the binder in the protective layer is preferably 20% by weight or more, and more preferably about 20 to 80% by weight. When the protective layer contains a pigment, the content of the pigment and binder is preferably about 30 to 300 parts by weight in terms of solid content of the binder per 100 parts by weight of the pigment.
The coating solution for the protective layer may contain various auxiliary agents such as crosslinking agents, lubricants, stabilizers, ultraviolet absorbers, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc. that can be used in the heat-sensitive recording layer described above, as required.

The thermosensitive recording medium of the present invention may have an undercoat layer between the support and the thermosensitive recording layer, which is mainly composed of a binder and a pigment.
The binder used in the undercoat layer may be an emulsion of a water-soluble polymer or a hydrophobic polymer that is generally used.Specific examples include cellulose derivatives such as polyvinyl alcohol, polyvinyl acetal, hydroxyethyl cellulose, methyl cellulose, and carboxymethyl cellulose, starch and its derivatives, sodium polyacrylate, polyvinylpyrrolidone, acrylic acid amide/acrylic acid ester copolymer, acrylic acid amide/acrylic acid ester/methacrylic acid copolymer, styrene/maleic anhydride copolymer alkali salt, isobutylene/maleic anhydride copolymer alkali salt, polyacrylamide, sodium alginate, gelatin, and casein, and emulsions of hydrophobic polymers such as polyvinyl acetate, polyurethane, styrene/butadiene copolymer, polyacrylic acid, polyacrylic acid ester, vinyl chloride/vinyl acetate copolymer, polybutyl methacrylate, ethylene/vinyl acetate copolymer, and styrene/butadiene/acrylic copolymer.These binders may be used alone or in combination of two or more.

The pigment used in the undercoat layer may be any known pigment that has been generally used, such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, calcined kaolin, clay, talc, or other inorganic pigments. These pigments may be used alone or in combination.
The amount of the pigment in the undercoat layer is usually 50 to 95 parts by weight, preferably 70 to 90 parts by weight, based on 100 parts by weight of the total solid content.
The coating solution for the undercoat layer may contain various auxiliary agents, such as dispersants, plasticizers, pH adjusters, defoamers, water retention agents, preservatives, coloring dyes, and ultraviolet protection agents, as required.

In the present invention, the means for coating the thermosensitive recording layer and the coating layers other than the thermosensitive recording layer, i.e., the protective layer, undercoat layer, etc., is not particularly limited, and can be applied according to well-known conventional techniques. For example, an off-machine coater or an on-machine coater equipped with various coaters such as an air knife coater, a rod blade coater, a bent blade coater, a bevel blade coater, a roll coater, or a curtain coater can be appropriately selected and used.
The coating weights of the thermosensitive recording layer and the coating layers other than the thermosensitive recording layer are determined according to the required performance and recording suitability, and are not particularly limited. However, the typical coating weight of the thermosensitive recording layer is about 2 to 12 g/ ^m2 in solid content, and the coating weight of the protective layer is preferably 0.5 to 5.0 g/ ^m2 in solid content.
Furthermore, various known techniques in the field of thermal recording media can be added as necessary, such as smoothing treatment such as supercalendering after coating of each coating layer.

The following examples illustrate the present invention but are not intended to limit it. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified.

Each dispersion and coating solution was prepared as follows.
The following mixture was stirred and dispersed to prepare a coating liquid for the undercoat layer.
Coating fluid for undercoat layer
Calcined kaolin (manufactured by BASF, product name: Ansilex 90)
100.0 parts styrene-butadiene copolymer latex (manufactured by Zeon Corporation,
Product name: ST5526 (solid content 48%) 10.0 parts Water 50.0 parts

Next, the developer dispersions (Liquid A1, Liquid A2), leuco dye dispersion (Liquid B), and sensitizer dispersion (Liquid C) with the following compositions were separately wet-ground in a sand grinder until the average particle size was 0.5 μm.

Developer dispersion liquid (A1 liquid)
N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea
6.0 parts fully saponified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., trade name:
PVA117, solid content 10%) 5.0 parts Water 1.5 parts

完全ケン化型ポリビニルアルコール水溶液（ＰＶＡ１１７） ５．０部
水 １．５部 Developer dispersion liquid (A2 liquid)
A phenolic compound represented by the following formula (Chemical Formula 3) (manufactured by Nippon Soda Co., Ltd., product name: D90)
6.0 parts

Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Leuco dye dispersion (liquid B)
3-Dibutylamino-6-methyl-7-anilinofluoran (manufactured by Yamamoto Chemical Industry Co., Ltd., trade name: ODB-2) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts Water 1.5 parts

Sensitizer dispersion (liquid C)
1,2-di(3-methylphenoxy)ethane (manufactured by Sankosha, product name: KS232)
6.0 parts fully saponified polyvinyl alcohol aqueous solution (PVA117 5.0 parts water 1.5 parts

Next, the dispersions were mixed in the following ratio to prepare coating solutions 1 and 2 for the thermal recording layer.
Coating solution for thermal recording layer 1
Developer dispersion (liquid A1) 10.0 parts Leuco dye dispersion (liquid B) 5.0 parts Sensitizer dispersion (liquid C) 3.0 parts Carboxy-modified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., product name: KL318,
Solid content 10% < Degree of polymerization: about 1700, Degree of saponification: 95 to 99 mol%, Sodium acetate:
3% or less) 25.0 parts Polyamide epichlorohydrin resin (Seiko PMC Corporation, product name: WS4020,
Solid content 25% (Cationization degree: 2.7, molecular weight: 2.2 million, quaternary amine)
0.5 parts
Coating solution for thermal recording layer 2
Developer dispersion (liquid A1) 10.0 parts Leuco dye dispersion (liquid B) 5.0 parts Sensitizer dispersion (liquid C) 3.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 25.0 parts

Next, the following components were mixed in the following proportions to prepare a coating solution for the protective layer.
Protective layer coating solution 1
Carboxy-modified polyvinyl alcohol aqueous solution (KL318) 40.0 parts Polyamide epichlorohydrin resin (WS4020) 4.0 parts Modified polyamine resin (manufactured by Sumitomo Chemical Co., Ltd., trade name: Sumirez Resin SPI-
102A, solid content 45%) 2.2 parts Aluminum hydroxide dispersion (manufactured by Martinsberg, trade name:
Martyfin OL, solid content 50%) 9.0 parts Zinc stearate (manufactured by Chukyo Yushi Co., Ltd., trade name: Hydrin Z-7-30,
Solid content 30%) 2.0 parts
Protective layer coating solution 2
Carboxy-modified polyvinyl alcohol aqueous solution (Kuraray Co., Ltd., product name: KL118,
Solid content 10% < Degree of polymerization: about 1700, Degree of saponification: 95 to 99 mol%, Sodium acetate:
3% or less>) 40.0 parts Polyamide epichlorohydrin resin (WS4020) 4.0 parts Modified polyamine resin (Sumirez resin SPI-102A) 2.2 parts Aluminum hydroxide dispersion (Martifin OL) 9.0 parts Zinc stearate (Hydrin Z-7-30) 2.0 parts
Protective layer coating solution 3
Completely saponified polyvinyl alcohol aqueous solution (PVA117) 40.0 parts Glyoxal aqueous solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., solid content 40%) 3.0 parts Aluminum hydroxide dispersion (Martifin OL) 9.0 parts Zinc stearate (Hydrin Z-7-30) 2.0 parts

[Example 1]
The undercoat layer coating liquid was applied to one side of a support (fine paper with a basis weight of 47 g/ ^m2 ) by the bent blade method so that the coating amount was 10.0 g/ ^m2 in solid content, and then dried to obtain an undercoat layer coated paper.
Onto the undercoat layer of this undercoat layer coated paper, the thermosensitive recording layer coating solution 1 was applied by a rod blade method so as to give a coating amount of 6.0 g/ ^m2 in solid content, and then dried to obtain a thermosensitive recording layer coated paper.
The protective layer coating solution 1 was applied onto the thermal recording layer of this thermal recording layer coated paper by the rod blade method so that the coating amount was 3.0 g/ ^m2 in solid content, and then the coating was dried and processed with a super calendar to achieve a smoothness of 500 to 1000 seconds to produce a thermal recording medium.
[Example 2]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea.
[Example 3]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea.

[Example 4]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea.
[Example 5]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea.
[Example 6]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea.
[Example 7]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea.

[Example 8]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea.
[Example 9]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea.
[Example 10]
A thermal recording medium was prepared in the same manner as in Example 1, except that the N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea in the A1 solution used in the thermal recording layer coating solution 1 was changed to N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea.
[Example 11]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the coating solution 1 for the thermosensitive recording layer was changed to the coating solution 2 for the thermosensitive recording layer.
[Example 12]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the amount of A1 solution used in the thermosensitive recording layer coating solution 1 was changed from 10.0 parts to 12.5 parts.
[Example 13]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the amount of A1 solution used in the thermosensitive recording layer coating solution 1 was changed from 10.0 parts to 5.5 parts.
[Example 14]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the protective layer coating solution 1 was changed to the protective layer coating solution 2.

[Comparative Example 1]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the A1 liquid used in the coating liquid 1 for the thermosensitive recording layer was changed to the A2 liquid.
[Comparative Example 2]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the protective layer coating solution 1 was changed to the protective layer coating solution 3.

The prepared thermosensitive recording medium was evaluated as follows.
<Color development performance (print density)>
A checkered pattern was printed on the prepared thermal recording medium at a printing speed of 50 mm/sec and an applied energy of 0.41 mJ/dot using a TH-PMD (thermal recording paper print tester, equipped with a thermal head manufactured by Kyocera Corporation) manufactured by Okura Electric Co., Ltd. The print density of the printed area was measured with a Macbeth densitometer (RD-914, using an amber filter) to evaluate the color development performance (print density).
<Disinfection gel resistance>
Reckitt's Muse disinfectant hand gel (ethanol 76.9-81.4%) was applied with a cotton swab to the blank areas of the thermal recording sheets prepared, and the sheets were left to stand for 24 hours under environmental conditions of 23°C x 50% RH, after which they were visually evaluated according to the following criteria.
Excellent: No color at all Fair: Slight coloration Poor: Strong coloration

<Heat resistance>
The thermal recording surface of the prepared thermal recording medium was brought into contact with a 110° C. iron plate for 5 seconds. The density of the non-printed area (blank area) was measured with a Macbeth densitometer (RD-914, using an amber filter), and the background color development value was calculated from the difference in the value before and after the treatment, and the discoloration of the non-printed area (blank area) was evaluated according to the following criteria.
Background color value = (density of non-printed area after processing) - (density of non-printed area before processing)
Excellent: Background color value is less than 0.1. Passable: Background color value is 0.1 or more and less than 0.2. Poor: Background color value is 0.2 or more. <Solvent barrier properties>
Ethanol (99.5%) was applied with a cotton swab to the blank portion of the prepared thermal recording medium, which was then left to stand for 24 hours under environmental conditions of 23° C.×50% RH, and then visually evaluated according to the following criteria.
Excellent: No color at all Fair: Slight coloration Poor: Strong coloration

<Water resistance>
The thermal recording medium thus prepared was subjected to a thermal recording test using a TH-PMD (thermal recording paper print tester, manufactured by Okura Electric Co., Ltd.).
A checkered pattern was printed using a thermal head manufactured by Kyocera Corporation (equipped with an applied energy of 0.41 mJ/dot and a printing speed of 50 mm/sec). The printed thermal recording medium was immersed in tap water for 24 hours under environmental conditions of 23°C and 50% RH. The print density of the printed area was measured with a Macbeth densitometer (RD-914, using an amber filter), and the remaining rate was calculated from the values before and after the treatment to evaluate the water resistance.
Residual rate (%)=(print density of printed part after processing/print density of printed part before processing)×100
Excellent: Residual rate is 90% or more. Passable: Residual rate is 70% or more, but less than 90%. Unacceptable: Residual rate is less than 70%.

<Water-resistant blocking>
For the thermosensitive recording medium thus prepared, 10 ml of tap water was dropped onto the surface of the protective layer, the medium was folded in half so that the surface of the protective layer was on the inside, a load of 10 gf/ ^cm2 was applied, and the medium was left to stand for 12 hours, after which it was peeled off. The peeling of the protective layer and thermosensitive recording layer in the area where the water was dropped was visually evaluated according to the following criteria.
Excellent: No blocking occurs at all, and the protective layer and the thermosensitive recording layer do not peel off at all. Fair: Blocking occurs, and the protective layer peels off slightly, but the thermosensitive recording layer does not peel off. Poor: Severe blocking occurs, and the protective layer and the thermosensitive recording layer peel off, or the thermosensitive recording medium is destroyed when peeling off. <Wet friction>
The surface of the protective layer of the prepared thermosensitive recording medium was rubbed back and forth 50 times with tap water on a finger, and peeling of the protective layer and thermosensitive recording layer was visually evaluated according to the following criteria.
Excellent: The protective layer and the thermosensitive recording layer are not peeled off at all. Fair: The protective layer is slightly peeled off, but the thermosensitive recording layer is not peeled off. Poor: The protective layer and the thermosensitive recording layer are peeled off.

The results are shown in the table below. The developer values in the table indicate the parts blended.

Claims (6)

A thermosensitive recording medium comprising a support on which a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer is provided, and a protective layer on the thermosensitive recording layer, the thermosensitive recording layer containing a urea compound represented by the following general formula (Chemical Formula 1) as the electron-accepting developer, and the protective layer containing a carboxyl group-containing resin as a binder, and further containing an epichlorohydrin-based resin and a polyamine/polyamide-based resin (excluding those contained in the epichlorohydrin-based resin) as a crosslinking agent.

(In the formula, R ¹ represents a substituted or unsubstituted alkyl group, aralkyl group, or aryl group, and R ² represents a hydrogen atom or an alkyl group.) 2. The thermal recording medium according to claim 1, wherein the urea compound is represented by the following general formula (Chemical Formula 2).

(In the formula, ^R3 represents an alkyl group or an alkoxy group, and n represents an integer of 0 to 3.) 3. The thermal recording material according to claim 2, wherein in the urea compound, R ³ represents an alkyl group having 1 to 4 carbon atoms, n represents an integer of 0 to 1, and R ³ is located at the 4-position in the benzene ring. The thermal recording medium according to any one of claims 1 to 3, wherein the content (solid content) of the urea compound in the thermal recording layer is 1.0 to 50.0% by weight. The thermal recording medium according to any one of claims 1 to 4, wherein the protective layer contains kaolin and/or aluminum hydroxide as a pigment. The thermal recording medium according to any one of claims 1 to 5, wherein the thermal recording layer further contains a carboxyl group-containing resin and an epichlorohydrin-based resin.