Thermal recording medium

Abstract

To provide a thermosensitive recording body which is excellent in image definition, especially excellent in barcode readability under a low temperature environment and suppresses a problem of tailing during printing under a severe environment.SOLUTION: There is provided a thermosensitive recording body which has a thermosensitive recording layer comprising a colorless or light-colored electron donating leuco dye and an electron acceptive developer on a support and a protective layer composed of a binder and a pigment on the thermosensitive recording layer, wherein the thermosensitive recording layer contains a sulfone compound, the sulfone compound is represented by the following formula (wherein, R1 represents a hydrogen atom or a hydroxyl group, R2 and R3 represent a hydrogen atom, an alkyl group or the like, m represents an integer of 1 to 3) and the protective layer contains an organic pigment having a volume 50% average particle diameter (D50) of 3 μm or more, as measured by a laser diffraction / scattering method, as a pigment.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a thermosensitive recording medium that has good image definition in a low-temperature environment, particularly good suitability for reading bar codes, and that suppresses trailing when printing in a harsh environment (hereinafter referred to as "print trailing"). [Background technology]

[0002] Generally, a thermal recording medium is formed by applying a coating liquid containing 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") to a support such as paper, synthetic paper, film, plastic, etc., to form a thermal recording layer, and the color develops through an instantaneous chemical reaction caused by heating with a thermal head, hot stamp, heat pen, laser light, etc., to obtain a recorded image. Thermal recording media are widely used as recording media for facsimiles, computer terminal printers, automatic ticket vending machines, measurement recorders, receipts from supermarkets and convenience stores, etc. In recent years, printers have become smaller and more portable, and printing is often done outdoors. It is therefore important that printing is performed normally and that the print is preserved even when the printer is used in colder environments than ever before or in harsh conditions (such as the high temperature inside a car in midsummer). For example, when an image is printed in a cold environment such as winter, the image may become distorted and lack definition. In particular, when a barcode is printed in a cold environment, it may be visible to the naked eye, but cannot be read by a reader, resulting in a problem of insufficient barcode readability. To improve this, a number of different pigments have been used in the thermal recording layer (Patent Document 1, etc.). On the other hand, a thermosensitive recording medium (Patent Document 2) has been disclosed in which a specific sensitizer and stabilizer are used in combination to improve the storage stability of the image area. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication 2007-190908 [Patent Document 2] International Publication WO2004 / 002748 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been a demand for thermal recording media to have high image definition in a low-temperature environment, particularly suitable for reading bar codes. In the present invention, a low-temperature environment refers to a temperature range of 0°C to -20°C. In order to solve the above-mentioned problems under such circumstances, the present inventors have discovered that this problem can be solved by providing a protective layer on the thermosensitive recording layer and having this protective layer contain an inorganic pigment having a particle size within a specific range, in particular amorphous silica or calcium carbonate, as a pigment. In practice, the barcode on the thermal recording medium is mechanically read by a barcode reader (barcode verifier) ​​and not by human visual inspection. Therefore, in the present invention, whether or not the barcode is properly printed on the thermal recording medium (barcode readability) is evaluated using the barcode reader (barcode verifier) ​​and not by human visual inspection. However, it was found that this thermal recording medium often has a problem of print tailing under harsh environments (see Comparative Example 1 (FIG. 2(D)) described later). This print tailing refers to the problem of tailing occurring at the end of printing when printing on the thermal recording medium. In the present invention, a harsh environment refers to, for example, high temperature and / or high humidity conditions, where high temperature refers, for example, to 70° C. or higher and high humidity refers, for example, to 80% RH or higher. [Means for solving the problem]

[0005] As a result of extensive investigations, the present inventors have found that the above problems can be solved by incorporating a specific sulfone compound as a color developer in the heat-sensitive recording layer in addition to the above-mentioned configuration, and have thus completed the present invention. That is, the present invention provides a thermosensitive recording medium having a support, a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer, and a protective layer made of a binder and a pigment on the thermosensitive recording layer, the thermosensitive recording layer containing a sulfone compound as an electron-accepting developer, 4-hydroxy-4'-benzyloxydiphenyl sulfone The protective layer is a thermal recording medium containing, as a pigment, an inorganic pigment having a volume 50% average particle diameter (D50) of 3 μm or more as measured by a laser diffraction / scattering method. Effect of the Invention

[0006] According to the present invention, it is possible to provide a thermal recording medium that has color development performance, has good image definition in low temperature environments, particularly good suitability for barcode reading, and does not produce print tailing even in harsh environments. [Brief description of the drawings]

[0007] [Figure 1] 1 shows prints of a thermal recording medium in a low temperature environment, where A shows the print of Example 1 and B shows the print of Comparative Example 5. [Diagram 2] The figures show some of the prints of the thermal recording medium in a harsh environment. C shows the print of Example 1, and D shows the print of Comparative Example 1. Print trailing is observed in the print of D (Comparative Example 1). BEST MODE FOR CARRYING OUT THEINVENTION

[0008] The thermosensitive recording medium of the present invention has a thermosensitive recording layer containing a leuco dye and a color developer on a support, and a protective layer on the thermosensitive recording layer. The thermosensitive recording layer contains a specific sulfone compound as a color developer. The thermosensitive recording layer may further contain a sensitizer, a binder, a pigment, a crosslinking agent, an image stabilizer, and other components. The support can be appropriately selected from conventionally known supports such as paper, recycled paper, synthetic paper, film, plastic film, foamed plastic film, nonwoven fabric, etc., depending on the desired quality of the thermal recording medium, and is not particularly limited. A composite sheet combining these may also be used as the support.

[0009] The sulfone compound used in the present invention is represented by the following general formula (Chemical Formula 1). [ka] In this formula, R 1 is a hydrogen atom or a hydroxyl group, preferably a hydroxyl group. Also R 2 and R 3 are each independently a hydrogen atom, an alkyl group or an alkoxy group having 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms. More preferably, R 2 and R 3 At least one of R is a hydrogen atom, and more preferably R 2 and R 3 are both hydrogen atoms. m is an integer of 1 to 3, preferably 1 or 2, and more preferably 1.

[0010] Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, an isopentyl group, a neopentyl group, a tert-pentyl group, a 1-methylbutyl group, a 2-methylbutyl group, a 1,2-dimethylpropyl group, a hexyl group, an isohexyl group, a 1-methylpentyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 1,1-dimethylbutyl group, a 1,2-dimethylbutyl group, a 2,2- Examples of such alkyl groups include linear or branched alkyl groups having 1 to 6 carbon atoms, such as a dimethylbutyl group, a 1,3-dimethylbutyl group, a 2,3-dimethylbutyl group, a 3,3-dimethylbutyl group, a 1-ethylbutyl group, a 2-ethylbutyl group, a 1,1,2-trimethylpropyl group, a 1,2,2-trimethylpropyl group, a 1-ethyl-1-methylpropyl group, and a 1-ethyl-2-methylpropyl group. Preferred are alkyl groups having 1 to 3 carbon atoms, that is, a methyl group, an ethyl group, a propyl group, and an isopropyl group. Examples of the alkoxy group include linear or branched alkoxy groups having 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy (amyloxy), isopentyloxy, tert-pentyloxy, neopentyloxy, 2-methylbutoxy, 1,2-dimethylpropoxy, 1-ethylpropoxy, and hexyloxy, and are preferably methoxy, ethoxy, propoxy, or isopropoxy.

[0011] A preferred sulfone compound for use in the present invention is represented by the following general formula (Chemical Formula 2). [ka] In this formula, R 2 , R 3 and m is defined as above. More preferred sulfone compounds for use in the present invention include 4-hydroxy-4'-benzyloxydiphenyl sulfone, 4-hydroxy-4'-phenethyloxydiphenyl sulfone, and 4-hydroxy-4'-(3-phenylpropoxy)diphenyl sulfone, and most preferred is 4-hydroxy-4'-benzyloxydiphenyl sulfone.

[0012] The thermosensitive recording layer of the present invention may contain a developer other than the above-mentioned sulfone compound as a developer, provided that the content of the sulfone compound in the thermosensitive recording layer is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 90% by weight or more, and particularly preferably 100% by weight of the total developer (including the sulfone compound), i.e., the total developer contained in the thermosensitive recording layer is the above-mentioned sulfone compound.

[0013] Examples of color developers other than the above-mentioned sulfone compounds used in the present invention 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, 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 1-[4-(4-hydroxyphenylsulfonyl)furan] [4-(4-isopropoxyphenylsulfonyl)phenoxy]butane, the phenol condensation composition described in JP-A-2003-154760, the aminobenzenesulfonamide derivative described in JP-A-8-59603, bis(4-hydroxyphenylthioethoxy)methane, 1,5-di(4-hydroxyphenylthio)-3-oxapentane, bis(p-hydroxyphenyl)butyl acetate, bis(p-hydroxyphenyl)methyl acetate, 1,1-bis(4-hydroxyphenyl)ethyl acetate, phenyl)-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,Examples of the color developers include thiourea compounds such as N'-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 TOMILAC214 manufactured by Mitsubishi Chemical Corporation, and the phenol condensation composition described in JP-A-2003-154760 is available, for example, under the trade name TOMILAC224 manufactured by Mitsubishi Chemical Corporation. Compounds described in WO02 / 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-A-10-258577 can also be contained.

[0014] In addition, the thermosensitive recording layer of the present invention may contain, as a developer, for example, a crosslinked phenolic compound represented by the following general formula (Chemical Formula 3) or a urea-urethane compound represented by the following general formula (Chemical Formula 4). [ka] The phenolic compound represented by the general formula (Chemical Formula 3) is available, for example, as D-90 manufactured by Nippon Soda Co., Ltd. [ka] Specifically, the urea-urethane compounds are three types represented by the following formulas (Chemical Formula 5) to (Chemical Formula 7), which may be used alone or in combination of two or more types.

[0015] [ka]

[0016] [ka]

[0017] [ka]

[0018] Next, various materials that can be used in the thermal recording layer of the thermal recording body of the present invention will be exemplified. However, binders, crosslinking agents, pigments, etc. can also be used in coating layers other than the thermal recording layer that are provided as needed, i.e., protective layers, undercoat layers, etc., as long as they do not impede the desired effects for the above-mentioned problems.

[0019] The leuco dye used in the present invention can be any known leuco dye in the field of conventional pressure-sensitive or heat-sensitive recording paper, and is not particularly limited, but triphenylmethane compounds, fluoran compounds, fluorene compounds, divinyl compounds, etc. are preferred. Specific examples of representative colorless or light-colored leuco dyes (dye precursors) are shown below. These dye precursors may be used alone or in combination of two or more.

[0020] <Triphenylmethane leuco dye> 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)

[0021] <Fluoran-based leuco dye> 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-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;3-Dibutylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran; 3-Dibutylamino-6-methyl-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-p-toluidino)-6-methyl-7-anilinofluoran; 3-(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-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;

[0022] <Fluorene-based leuco dye> 3,6,6'-Tris(dimethylamino)spiro〔fluorene-9,3´-phthalide〕; 3,6,6'-Tris(diethylamino)spiro〔fluorene-9,3´-phthalide〕

[0023] <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

[0024] <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-(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

[0025] Sensitizers that can be used in the thermal recording medium of the present invention include conventionally known sensitizers, such as fatty acid amides such as stearic acid amide and palmitic acid amide, ethylene bisamide, montanic acid wax, polyethylene wax, 1,2-di-(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 Examples of the sensitizer include, but are not limited to, 1-α-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, p-nitrobenzoic acid methyl, p-toluenesulfonic acid phenyl, etc. These sensitizers may be used alone or in combination of two or more kinds.

[0026] Examples of binders that can be used in the present invention include polyvinyl alcohols such as fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, acetoacetyl-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, silanol-modified polyvinyl alcohol, cation-modified polyvinyl alcohol, and terminal alkyl-modified polyvinyl alcohol; cellulose ethers such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, and acetyl cellulose, and derivatives thereof; starch, enzyme-modified starch, thermochemically modified starch, oxidized starch, esterified starch, etherified starch (e.g., hydroxyethylated starch), and cationized starch. Examples of the starches include polyacrylamides, cationic polyacrylamides, anionic polyacrylamides, amphoteric polyacrylamides, and other polyacrylamides, urethane resins such as polyester polyurethane resins, polyether polyurethane resins, and polyurethane ionomer resins, acrylic resins consisting of (meth)acrylic acid and monomer components (excluding olefins) copolymerizable with (meth)acrylic acid, styrene-butadiene copolymers, styrene-butadiene-acrylonitrile copolymers, and styrene-butadiene-acrylic copolymers, and other styrene-butadiene resins, polyvinyl acetate, vinyl chloride-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, and other polyolefin resins, polyvinyl chloride, polyvinylidene chloride, polyacrylic esters, gum arabic, polyvinyl butyral, polystyrene and copolymers thereof, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These may be used alone or in combination of two or more.

[0027] Examples of pigments that can be used in the present invention include inorganic or organic fillers such as silica, calcium carbonate, kaolin, calcined kaolin, diatomaceous earth, talc, titanium oxide, aluminum hydroxide, etc. These may be used alone or in combination of two or more.

[0028] Examples of crosslinking agents that can be used in the present invention include glyoxal, methylol melamine, melamine formaldehyde resin, melamine urea resin, polyamine epichlorohydrin resin, polyamide epichlorohydrin resin, potassium persulfate, ammonium persulfate, sodium persulfate, ferric chloride, magnesium chloride, borax, boric acid, alum, and ammonium chloride agents.

[0029] In addition, in 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, 4-benzyloxy-4'-(2,3-epoxy-2-methylpropoxy)diphenylsulfone, etc. can also be used as image stabilizers that exhibit oil resistance effects and the like of recorded images within a range that does not impair the desired effects for the above-mentioned problems. In addition, benzophenone-based or triazole-based ultraviolet absorbents, lubricants, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc. may be used.

[0030] The types and amounts of the electron-donating leuco dye, electron-accepting developer, and other various components used in the thermal recording medium of the present invention are determined according to the required performance and recording suitability, and are not particularly limited; however, typically, 0.5 to 10 parts by weight of the electron-accepting developer and 0.5 to 10 parts by weight of the sensitizer are used per 1 part by weight of the electron-donating leuco dye. The types and amounts of other optional components such as binder, pigment, crosslinking agent, image stabilizer, and other components are determined according to the required performance and recording suitability and are not particularly limited, but usually, the binder is suitably used in an amount of about 5 to 50 parts by weight in solid content per 100 parts by weight of the thermosensitive recording layer (solid content), and the pigment is suitably used in an amount of about 0 to 50 parts by weight in solid content per 100 parts by weight of the thermosensitive recording layer (solid content).When a lubricant is used, the content of the lubricant is suitably used in an amount of about 5 to 10 parts by weight in solid content per 100 parts by weight of the thermosensitive recording layer (solid content).

[0031] The thermosensitive recording medium of the present invention has a protective layer on the thermosensitive recording layer, and this protective layer contains a binder and an inorganic pigment, and the inorganic pigment has a volume 50% average particle size (D50) of 3 μm or more, preferably 3 to 10 μm, more preferably 4 to 10 μm, and even more preferably 5 to 10 μm. This volume 50% average particle size (D50) is measured by a laser diffraction / scattering method. As a measuring device for the laser diffraction / scattering method, for example, a particle size distribution measuring device "Partica" by Horiba Ltd. or a particle size distribution measuring device "MASTER SIZER S" by Malvern Instruments can be used.

[0032] As the inorganic pigment, there can be used known inorganic pigments that have been generally used so far, such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, kaolin, calcined kaolin, clay, talc, etc., and these pigments may be used alone or in combination of two or more. Preferably, at least one selected from the group consisting of calcium carbonate and amorphous silica is used, and more preferably, calcium carbonate and amorphous silica are used.

[0033] The amorphous silica used in the present invention has a volume 50% average particle size (D50) of preferably 4 to 10 μm, more preferably 5 to 10 μm. Furthermore, by making the volume 90% average particle size (D90) of this amorphous silica, measured by a laser diffraction / scattering method, 15 μm or less, the definition of the printed image can be further improved.

[0034] In the present invention, the oil absorption of the amorphous silica is preferably 150 ml / 100 g or more, more preferably 150 to 400 ml / 100 g. The surface area of ​​the amorphous silica is 250 m 2 It is preferable that the molecular weight is not more than 1 / g. The oil absorption of the amorphous silica is measured in accordance with JIS K5101, and the specific surface area is measured in accordance with the BET method. Examples of such amorphous silica include Carplex 101 (manufactured by DSL Japan) and Mizukasil P526 (manufactured by Mizusawa Chemicals).

[0035] The calcium carbonate (hereinafter also referred to as "calcium carbonate") used in the present invention has a volume 50% average particle size (D50) of 3 μm or more, and preferably 3 to 10 μm. Examples of such calcium carbonate include Hakuenka PZ (manufactured by Shiraishi Calcium Co., Ltd., cubic calcium carbonate aggregate), PC / PCX (same company, spindle-shaped calcium carbonate), Callite SA (same company, aragonite-type calcium carbonate), and Tsunex E (same company, spindle-shaped calcium carbonate aggregate). In the present invention, calcium carbonate that is a spindle-shaped aggregate is preferred because it has good image definition in a low-temperature environment, particularly good suitability for reading barcodes. This spindle-shaped calcium carbonate aggregate has a shape formed by agglomerating spindle-shaped primary particles of calcium carbonate, so that the voids inside the particles are large and it is possible to maintain appropriate adhesion with the print head of a thermal printer during printing.

[0036] The content (solid content) of the inorganic pigment in the protective layer is preferably 40 to 65% by weight, and more preferably 45 to 60% by weight. When the protective layer contains a pigment (including inorganic and organic pigments) other than amorphous silica and calcium carbonate, the content (solid content) of at least one of amorphous silica and calcium carbonate in the protective layer is preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 90% by weight or more, and particularly preferably 100% by weight, based on the total amount of pigment. When the protective layer contains amorphous silica and calcium carbonate, the weight ratio of amorphous silica / calcium carbonate is preferably 20 / 80 to 80 / 20, and more preferably 40 / 60 to 60 / 40.

[0037] As the binder used in the protective layer, polyvinyl alcohols and acrylic resins are preferred among the binders usable in the heat-sensitive recording layer described above. As the polyvinyl alcohols, fully saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, and acetoacetyl-modified polyvinyl alcohol are preferred. The acrylic resin contains (meth)acrylic acid and a monomer component copolymerizable with (meth)acrylic acid, and the amount of (meth)acrylic acid is preferably 1 to 10 parts by weight per 100 parts by weight of the acrylic resin. (Meth)acrylic acid is alkali-soluble, and has the property of turning the acrylic resin into a water-soluble resin by adding a neutralizing agent. By changing the acrylic resin into a water-soluble resin, the binding to the pigment is significantly improved, particularly when the protective layer contains a pigment, and a protective layer having excellent strength even when a large amount of pigment is contained can be formed. Examples of components copolymerizable with (meth)acrylic acid include alkyl acrylate resins such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and octyl (meth)acrylate, as well as epoxy resins, silicone resins, modified alkyl acrylate resins such as the alkyl acrylate resins modified with styrene or a derivative thereof, (meth)acrylonitrile, acrylic esters, and hydroxyalkyl acrylic esters, but it is particularly preferable to blend (meth)acrylonitrile and / or methyl methacrylate. It is preferable to blend 15 to 70 parts of (meth)acrylonitrile in 100 parts of the non-core-shell type acrylic resin. It is also preferable to include 20 to 80 parts of methyl methacrylate in 100 parts of the non-core-shell type acrylic resin. When (meth)acrylonitrile and methyl methacrylate are contained, it is preferable to mix 15 to 18 parts of (meth)acrylonitrile and 20 to 80 parts of methyl methacrylate in 100 parts of the non-core-shell type acrylic resin. The acrylic resin is preferably a non-core-shell type acrylic resin. The glass transition point (Tg) of this acrylic resin is preferably not higher than 95° C., and more preferably higher than 50° C. The Tg is measured by differential scanning calorimetry (DSC).

[0038] In addition, in order to improve water resistance, etc., the protective layer preferably contains a carboxyl group-containing resin as a binder, and may further contain an epichlorohydrin-based resin and / or a polyamine / polyamide-based resin (excluding those contained in epichlorohydrin-based resins). Examples of the carboxyl group-containing resin include the above-mentioned carboxy-modified polyvinyl alcohol, acrylic resins, oxidized starch, and carboxymethyl cellulose.

[0039] This carboxy-modified polyvinyl alcohol can be obtained, for example, 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 as 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 production methods include those exemplified in 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, more preferably 1000 to 2000.

[0040] The epichlorohydrin resin is a resin characterized by containing an epoxy group in the molecule, and examples thereof include polyamide epichlorohydrin resin and polyamine epichlorohydrin resin, which can be used alone or in combination. In addition, primary to quaternary amines can be used as the amines present in the main chain of the epichlorohydrin resin, and there is no particular limitation. Furthermore, since the water resistance is good, the cationization degree and molecular weight are preferably 5 meq / g·Solid or less (measured at pH 7) and 500,000 or more. Specific examples of epichlorohydrin resins 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 Co., Ltd.), etc.

[0041] 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 Sumirez Resin 302 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumirez Resin 712 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumirez Resin 703 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumirez Resin 636 (manufactured by Sumitomo Chemical Co., Ltd.: polyamine polyurea resin), Sumirez Resin SPI-100 (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin), Sumirez Resin SPI-102A (manufactured by Sumitomo Chemical Co., Ltd.: modified polyamine resin), Sumirez Resin SPI-106N (manufactured by Sumitomo Chemical Co., Ltd.: Co., Ltd.: modified polyamide resin), Sumilaze Resin SPI-203(50) (Sumitomo Chemical Co., Ltd.), Sumilaze Resin SPI-198 (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 (all manufactured by Seiko PMC Corporation: polyalkylene polyamine polyamide polyurea resin), CP8994 (manufactured by Seiko PMC Corporation: polyethyleneimine resin), and the like. 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 they provide good print density.

[0042] In addition to the above, the protective layer of the present invention may contain other components such as a crosslinking agent that can be used in the above-mentioned heat-sensitive recording layer, and, if necessary, a surfactant, a viscosity modifier, etc., within the range that does not impair the desired effects.

[0043] The total amount of binder and pigment in the protective layer is usually 80 to 100% by weight, preferably 90 to 100% by weight, in terms of solid content, and the amount of binder is preferably about 30 to 300 parts by weight per 100 parts by weight of pigment. The content of each of the components other than the binder and the pigment in the protective layer does not exceed 15% by weight, and preferably does not exceed 10% by weight.

[0044] The thermosensitive recording medium of the present invention may optionally have an undercoat layer between the support and the thermosensitive recording layer, or a backcoat layer may be provided on the opposite side of the support to the thermosensitive recording layer. In addition, a coating layer appropriately configured according to the purpose of use may be provided between these layers.

[0045] Each coating layer that is provided arbitrarily other than the heat-sensitive recording layer and the protective layer may contain the above-mentioned binders, pigments, crosslinking agents and other components within the range that does not impair the desired effects.

[0046] The coating method for the thermosensitive recording layer, protective layer, and other coating layers is not particularly limited, and any conventionally known method can be used, such as curtain coating, air knife coating, bar blade coating, rod blade coating, bent blade coating, bevel blade coating, roll coating, or spray coating.

[0047] The coating weight of the thermal recording layer, protective layer, and other coating layers is determined according to the required performance and recording suitability, and is not particularly limited. For example, the general coating weight of the thermal recording layer is 2 to 12 g / m2 in terms of solid content. 2 The typical coating weight of the protective layer is 1 to 5 g / m2 in solids. 2 That's about it. Furthermore, after each layer is applied, various known techniques in the field of thermal recording media, such as smoothing treatment by supercalendering, may be applied as necessary. EXAMPLES

[0048] The present invention will be illustrated by the following examples, but is not intended to limit the present invention. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified.

[0049] 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 (BASF product name: Ansilex 90) 100.0 copies Styrene-butadiene copolymer latex (manufactured by Zeon Corporation, Product name: ST5526, solid content 48%) 10.0 parts Water 50.0 parts

[0050] The developer dispersions (Liquids A1 to A3), leuco dye dispersion (Liquid B), and sensitizer dispersion (Liquid C) having the following compositions were each prepared by wet grinding in a sand grinder until the average particle size reached 0.5 μm.

[0051] Developer dispersion liquid (A1 liquid) 4-Hydroxy-4'-benzyloxydiphenyl sulfone (manufactured by NICCA CHEMICAL CO., LTD., product name: BPS-MA3) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., product name: PVA 117, solids content 10%) 5.0 parts 1.5 parts water Developer dispersion liquid (A2 liquid) 4-Hydroxy-4'-isopropoxydiphenyl sulfone (Mitsubishi Chemical Corporation, product name: NYDS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water Developer dispersion liquid (A3 liquid) 4,4'-Dihydroxydiphenyl sulfone (Konishi Chemical Co., Ltd., Product name: BS-FD) 6.0 copies Completely saponified polyvinyl alcohol aqueous solution (PVA117 5.0 parts 1.5 parts water

[0052] Leuco dye dispersion (liquid B) 3-Dibutylamino-6-methyl-7-anilinofluoran (Yamamoto Chemical Industry Co., Ltd., product name: ODB-2) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water

[0053] Sensitizer dispersion (liquid C) Diphenyl sulfone (Volant, trade name: DPS) 6.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 5.0 parts 1.5 parts water

[0054] 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 liquid) 36.0 parts Leuco dye dispersion (liquid B) 18.0 parts Sensitizer dispersion (liquid C) 9.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 25.0 parts

[0055] <Coating solution for thermal recording layer 2> Developer dispersion liquid (A1 liquid) 36.0 parts Leuco dye dispersion (liquid B) 18.0 parts Sensitizer dispersion (liquid C) 9.0 parts Spindle-shaped aggregated calcium carbonate dispersion (Shiraishi Calcium Co., Ltd., product name: TSUNEX E, D50: 5.8 μm, solid content 50%) 9.0 parts Amorphous silica dispersion (DSL Japan, product name: Carplex 101, D50: 6.5 μm, solid content 25%) 22.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 25.0 parts Zinc stearate (manufactured by Chukyo Yushi Co., Ltd., product name: Hydrin Z-7-30, Solid content 30%) 2.0 parts

[0056] Next, protective layer coating solutions 1 to 3 were prepared by mixing the ingredients in the following proportions. <Protective layer coating solution 1> Spindle-aggregated calcium carbonate dispersion (Tsunex E) 9.0 parts Amorphous silica dispersion (Carplex 101) 22.0 parts Acrylic resin (Mitsui Chemicals, product name: ASN1004K, Solid content 18%) 30.0 parts Zinc stearate (Hydrin Z-7-30) 2.0 parts

[0057] <Protective layer coating solution 2> Spindle-aggregated calcium carbonate dispersion (Tsunex E) 6.6 parts Amorphous silica dispersion (Carplex 101) 10.8 parts Carboxy-modified polyvinyl alcohol aqueous solution (manufactured by Kuraray Co., Ltd., product name: KL318, solid content 10%) 50.0 parts Polyamide epichlorohydrin resin (manufactured by Seiko PMC Corporation, product name: WS4030, solids content 25%) 4.0 parts Zinc stearate (Hydrin Z-7-30) 2.0 parts

[0058] <Protective layer coating solution 3> Spindle-aggregated calcium carbonate dispersion (Tsunex E) 9.0 parts Amorphous silica dispersion (Carplex 101) 22.0 parts Completely saponified polyvinyl alcohol aqueous solution (PVA117) 50.0 parts Zinc stearate (Hydrin Z-7-30) 2.0 parts

[0059] [Example 1] Support (basis weight 47g / m2 The coating solution for the undercoat layer is applied to one side of the paper (high-quality paper) at a coating weight of 10.0 g / m2 (solid content). 2 After coating by the bent blade method so that the above-mentioned coating was performed, the coated paper was dried to obtain a coated paper with an undercoat layer. On the undercoat layer of this undercoat layer coated paper, the coating solution 1 for the thermal recording layer was applied in a coating amount of 6.0 g / m2 in terms of solid content. 2 After coating by the rod blade method so as to obtain a thermal recording layer coated paper, the coating was dried. The protective layer coating solution 1 was applied to the thermal recording layer of the thermal recording layer coated paper in a coating amount of 3.0 g / m2 in terms of solid content. 2 After coating by the rod blade method so as to obtain the above-mentioned coating composition, the coating composition was dried and then processed with a super calendar so as to obtain a smoothness of 100 to 500 seconds to prepare a thermal recording medium. Even when this thermal recording medium was used for printing under harsh environments, no print trailing was observed (FIG. 2(C)), as in Comparative Example 1 (FIG. 2(D)) described later.

[0060] [Example 2] A thermal recording medium was prepared in the same manner as in Example 1, except that in the protective layer coating solution 1, the amount of spindle-aggregated calcium carbonate dispersion was changed to 11.2 parts and the amount of amorphous silica dispersion was changed to 28.8 parts. [Example 3] A thermal recording medium was prepared in the same manner as in Example 3, except that in the protective layer coating solution 2, the amount of spindle-aggregated calcium carbonate dispersion was changed to 4.2 parts and the amount of amorphous silica dispersion was changed to 7.2 parts. [Example 4] 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. [Example 5] A thermal recording medium was prepared in the same manner as in Example 1, except that in the protective layer coating solution 1, no spindle-aggregated calcium carbonate dispersion was added, instead 12.0 parts of aluminum hydroxide dispersion (manufactured by Toshin Chemical Industry Co., Ltd., product name: Hijilite H32, D50: 8.0 μm, solids content 50%) was added, and the amount of amorphous silica dispersion was changed to 40.0 parts.

[0061] [Comparative Example 1] A thermosensitive recording medium was prepared in the same manner as in Example 1, except that in the coating solution 1 for thermosensitive recording layer, the developer dispersion liquid (A1 liquid) was changed to a developer dispersion liquid (A2 liquid). When this thermal recording medium was used to print in a harsh environment, a trail was observed. In other words, in Figure 2(D), a thin print trail (print trail) was observed parallel to the movement of the printer after the printing was completed. [Comparative Example 2] A thermal recording medium was prepared in the same manner as in Example 1, except that in the protective layer coating solution 1, no spindle-aggregated calcium carbonate dispersion was added, instead 12.0 parts of aluminum hydroxide dispersion (Martinsberg, product name: Martifin OL, D50: 1.5 μm, solids content 50%) was added, and the amount of amorphous silica dispersion was changed to 40.0 parts.

[0062] [Comparative Example 3] A thermal recording medium was prepared in the same manner as in Example 1, except that in the protective layer coating solution 1, the spindle-aggregated calcium carbonate dispersion and the amorphous silica dispersion were not mixed, and 20.0 parts of amorphous silica dispersion (manufactured by Mizusawa Chemical Industries, Ltd., product name: P604, D50: 2.0 μm, solid content 25%) was mixed. [Comparative Example 4] A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the thermosensitive recording layer coating solution 1 was changed to the thermosensitive recording layer coating solution 2 and no protective layer was provided. [Comparative Example 5] A thermal recording medium was prepared in the same manner as in Comparative Example 4, except that in the thermal recording layer coating solution 2, the amount of spindle-aggregated calcium carbonate dispersion was changed to 27.0 parts and the amount of amorphous silica dispersion was changed to 66.0 parts.

[0063] 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 an applied energy of 0.35 mJ / dot and a printing speed of 50 mm / sec using a TH-PMD (thermal recording paper print tester, equipped with a Kyocera thermal head) 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).

[0064] <Barcode reading ability> Using the prepared thermal recording medium, a barcode (CODE39) was printed vertically (the printer head movement direction and the barcode were perpendicular) using a Zebra 140XiIII label printer under the following two environmental conditions at a print level of +10 and a print speed of 15.2 cm / sec (6 in / sec). The printed barcode was then read using a barcode verifier (Honeywell, QCPC600, light source 640 nm) to evaluate the barcode readability. The evaluation results were recorded using the ANSI standard symbol grade. (1) 23℃ (2) -10℃ The symbol grade is calculated by dividing the barcode into 10 parts vertically to the bars, conducting a reading test once for each part, and expressing the average value on a 5-point scale from A (excellent), B, C, D, to F (poor). Fig. 1 shows the printout of the thermal recording medium (2) in an environment of -10°C (A: Example 1, B: Comparative Example 5). As mentioned above, the barcode readability is not suitable for evaluation by human visual inspection, but is evaluated using a barcode reader (barcode verifier).

[0065] <Print tail> The thermal recording media thus prepared were printed using a Zebra 140XiIII label printer at a print level of +10 and a print speed of 15.2 cm / sec (6 in / sec) under environmental conditions of 50°C and 90% Rh. The resulting print was visually evaluated for tailing at the end of the print, using the following criteria: Good: No trailing Fair: slight tailing Unacceptable: Strong tailing

[0066] The results are shown in the table below. [Table 1]

Claims

1. A thermal recording medium having a support, a thermal recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer, and a protective layer on the thermal recording layer, the protective layer being made of a binder and a pigment, the thermal recording layer containing a sulfone compound as an electron-accepting developer, the sulfone compound being 4-hydroxy-4'-benzyloxydiphenylsulfone, and the protective layer containing an inorganic pigment as a pigment, the volume 50% average particle size (D50) of which is 3 μm or more as measured by a laser diffraction / scattering method.

2. A thermal recording medium as described in claim 1, wherein the inorganic pigment has a volume 50% average particle diameter (D50) of 3 to 10 μm.

3. A thermal recording medium as described in claim 1 or 2, wherein the inorganic pigment is at least one selected from the group consisting of calcium carbonate and amorphous silica.

4. A thermal recording medium described in any one of claims 1 to 3, wherein the thermal recording layer contains an electron accepting developer other than the sulfone compound, and the content of the sulfone compound in the thermal recording layer is 50 weight % or more of the total developer (including the sulfone compound).

5. A thermal recording medium described in any one of claims 1 to 4, wherein the inorganic pigment is spindle-aggregated calcium carbonate.

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