JP2024055636A - Thermal recording medium - Google Patents

Abstract

The object of the present invention is to provide a thermal recording medium having excellent color development and print storage stability.
[Solution] A thermosensitive recording medium 1 has a structure in which a thermosensitive recording layer 3 is laminated on a substrate 2. The thermosensitive recording layer 3 contains a color former, a color developer, and a storage stability improver. The thermosensitive recording layer 3 contains a diphenyl sulfone compound represented by the following formula (1) as the color developer, and a compound having three or more phenolic hydroxyl groups in the molecule as the storage stability improver.
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(The definitions of each symbol in the above formula (1) are as described in the specification.)
[Selected Figure] Figure 1

Description

The present invention relates to a thermal recording medium, and more specifically, to a thermal recording medium with excellent color development and print preservation properties.

Thermal recording media develop color through a chemical reaction when heated by a thermal head or the like, producing a recorded image. They are used for a wide range of purposes, including as recording media for facsimiles, automatic ticket vending machines, and scientific measuring instruments, as well as thermal recording labels for POS systems in retail stores, receipt paper, etc.

As mentioned above, thermal recording media are widely used. For this reason, various performance features are required of thermal recording media. For example, when reading a barcode with a barcode reader, color development is required that ensures good reading accuracy. In addition, excellent print preservation properties are also required so that the color density of the printed area does not decrease easily even when stored for long periods in harsh environments such as high temperature and high humidity.

As a thermal recording medium with excellent color development, water resistance, and background color development resistance, for example, a thermal recording medium has been proposed in which a thermal recording layer containing a colorless or light-colored electron-donating leuco dye and an electron-accepting developer is provided on a support, and the thermal recording layer contains a diphenyl sulfone compound such as 4-hydroxy-4'-propoxydiphenyl sulfone as the developer (see, for example, Patent Document 1).

JP 2010-111037 A

Thermal recording media using diphenyl sulfone compounds, which are the color developers used in the above-mentioned Patent Document 1, have the problem that the color density of the printed area may decrease when stored for a long period of time under harsh environments such as high temperature and high humidity, resulting in poor print preservation.

The present invention has been made in view of the above circumstances, and has an object to provide a thermal recording medium which has excellent color development properties, is resistant to deterioration in color density, and has excellent print storage stability.
In the specification, "color development" means the ability of the plain areas of the thermal recording medium to turn black when heated, and "print preservation" means the ability of the density of the colored printed areas (color development density) to remain stable under various storage conditions.

As a result of intensive research to achieve the above object, the inventors of the present application have found that by blending a specific diphenyl sulfone compound as a color developer in the thermal recording layer, and a compound having three or more phenolic hydroxyl groups in the molecule as a storage stability improver, it is possible to provide a thermal recording medium that has excellent color development properties, is less likely to decrease in color density, and also has excellent print storage stability. The present invention was completed based on this knowledge.

That is, one aspect of the present invention provides a thermosensitive recording medium in which a thermosensitive recording layer is laminated on a substrate. In the thermosensitive recording medium of the present invention, the thermosensitive recording layer contains a color former, a color developer, and a storage stability improver.

（式（１）中、Ｒ¹、Ｒ²、Ｒ³、Ｒ⁴、Ｒ⁵、Ｒ⁶、Ｒ⁷、及びＲ⁸は、それぞれ独立して、水素原子、又は置換基を示す。Ｒ⁹は、水素原子、アルキル基、又はアルケニル基を示す。） The thermal recording medium of the present invention contains a diphenylsulfone compound represented by the following formula (1) as the developer.

(In formula (1), R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ each independently represent a hydrogen atom or a substituent. R ⁹ represents a hydrogen atom, an alkyl group, or an alkenyl group.)

When the thermal recording medium of the present invention contains the diphenyl sulfone compound represented by the above formula (1) as the color developer, a thermal recording medium with excellent color development properties can be provided.

The thermal recording medium of the present invention contains a compound having three or more phenolic hydroxyl groups in the molecule as the storage stability improver. This configuration provides a thermal recording medium with excellent print storage stability.

In one embodiment of the thermal recording medium of the present invention, it is preferable that the developer further contains a urea-urethane compound represented by the following formula (2).

When the thermal recording medium of the present invention further contains a urea-urethane compound represented by the above formula (2) as the color developer, it is possible to provide a thermal recording medium that has excellent color development properties, and also has excellent print storage stability with less loss in color density.

In another embodiment of the thermal recording medium of the present invention, the content of the diphenyl sulfone compound in the entire thermal recording layer is preferably 10% by mass or more and 40% by mass or less. This configuration provides a thermal recording medium with excellent color development properties.

In another embodiment of the thermal recording medium of the present invention, the content of the compound having three or more phenolic hydroxyl groups in the molecule relative to the entire thermal recording layer is preferably 1% by mass or more and 10% by mass or less. This configuration makes it possible to provide a thermal recording medium with excellent print storage stability.

The present invention provides a thermal recording medium that has excellent color development and print storage stability, with less loss of color density.

FIG. 1 is a schematic cross-sectional view showing one embodiment of a thermal recording medium of the present invention.

The thermal recording medium of the present invention has a laminated structure in which a thermal recording layer is laminated on a substrate. In the thermal recording medium of the present invention, the thermal recording layer contains a color former, a color developer, and a storage stability improver.

The thermal recording medium of the present invention contains the diphenylsulfone compound represented by the above formula (1) as the developer.
The thermal recording medium of the present invention further contains a compound having three or more phenolic hydroxyl groups in the molecule as the storage stability improving agent.

One embodiment of the thermal recording medium of the present invention will be described in detail below with reference to the drawings, but the present invention is not limited to the following embodiments.

Figure 1 is a schematic cross-sectional view showing one embodiment of the thermal recording medium of the present invention.

As shown in FIG. 1, the thermal recording medium 1 of this embodiment has a layered structure in which an undercoat layer 6, a thermal recording layer 3, an intermediate layer 4, and a topcoat layer 5 are layered in this order on a sheet-like substrate 2.

In this embodiment, the substrate 2 functions as a support for the thermal recording medium 1. As the substrate 2, for example, high-quality paper, art paper, coated paper, craft paper, laminated paper obtained by laminating a thermoplastic resin such as polyethylene to these paper substrates, synthetic paper, and porous materials such as nonwoven fabric can be used. Transparent synthetic resin films, for example, polypropylene films, polyethylene terephthalate films, polystyrene films, polycarbonate films, and the like can also be used. The thickness of the substrate 2 is not particularly limited, but when the thickness of the substrate 2 is adjusted to about 10 μm to 100 μm, a substrate 2 with excellent coatability can be obtained. Also, a substrate 2 with excellent transparency can be obtained.

In this embodiment, the undercoat layer 6 has functions such as heat insulation to prevent the heat applied from the thermal head from dissipating, and cushioning properties. However, if synthetic paper is used as the substrate 2, the undercoat layer 6 does not need to be provided because the synthetic paper itself has heat insulation and cushioning properties.

The undercoat layer 6 is formed, for example, by adding hollow particles as a filler to a binder.

By providing such an insulating undercoat layer 6 on the thermal recording medium 1, the printing sensitivity is improved. This makes it possible to suppress an increase in the voltage applied to the thermal head, and as a result, to suppress burn-in of the thermal head.

The average particle diameter of the hollow particles added as a filler to the undercoat layer 6 is preferably 1 μm to 10 μm. This range improves the heat insulating properties of the undercoat layer 6. Here, the average particle diameter is the weight average particle diameter measured by a laser diffraction method. Measurement of the average particle diameter by the laser diffraction method can be performed, for example, using a product name "MT3300EX-II" manufactured by Microtrack Bell.

The hollow ratio of the hollow particles is preferably 30% to 99%. Within this range, the heat insulating properties of the undercoat layer 6 are improved. The higher the hollow ratio of the hollow particles, the higher the heat insulating effect. This allows the color former to effectively develop color with a small amount of heat. In other words, increasing the hollow ratio improves the print quality of the thermal recording medium 1.

Here, the hollow ratio of the hollow particles is calculated by the following formula.
Hollow ratio={(void volume)/(hollow particle volume)}×100

The content of hollow particles in the undercoat layer 6 is preferably 40 to 90 parts by weight per 100 parts by weight of the undercoat layer.

The material that constitutes the hollow particles is, for example, a thermoplastic resin. Examples of such thermoplastic resins include polystyrene-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, polyvinyl acetate-based resins, polyacrylic ester-based resins, polyacrylonitrile-based resins, and polybutadiene-based resins.

The filler for the undercoat layer 6 may be other than hollow particles. Examples include calcined kaolin, aluminum oxide, aluminum silicate, heavy calcium carbonate, light calcium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolinite, diatomaceous earth, white carbon, magnesium carbonate, magnesium oxide, magnesium hydroxide, zinc oxide, polystyrene resin particles, urea-formaldehyde resin particles, polyolefin resin particles, etc. These fillers may be used alone or in combination of two or more.

Examples of binders contained in the undercoat layer 6 include acrylic-styrene copolymers, styrene-butadiene copolymers, acrylic-butadiene-styrene copolymers, vinyl acetate resins, vinyl acetate-acrylic acid copolymers, styrene-acrylic ester copolymers, acrylic ester resins, polyurethane resins, etc.

As the binder, water-soluble polymers such as polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose, sodium polyacrylate, polyvinylpyrrolidone, acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid terpolymers, alkali salts of styrene-maleic anhydride copolymers, alkali salts of isobutylene-maleic anhydride copolymers, polyacrylamide, sodium alginate, gelatin, and casein may also be used.

The mixing ratio of hollow particles to binder (hollow particles/binder; dry weight ratio) can be appropriately selected according to the particle size of the hollow particles. For example, it can be 8/2 when the average particle size of the hollow particles is 1 μm, and 2/8 when the average particle size is 10 μm. When the average particle size is 1 to 10 μm, it can be appropriately adjusted between 8/2 and 2/8.

The coating amount (dry weight) of the undercoat layer 6 is preferably 1 g/m ² to 10 g/m ² .

The thickness of the undercoat layer 6 is preferably 1 μm to 20 μm.

When the coating amount and thickness of the undercoat layer 6 are adjusted to the above range, the undercoat layer 6 will adequately perform its insulating function.

In this embodiment, the thermal recording layer 3 is a layer that develops color through a chemical reaction when heated by a thermal head or the like, and forms a recorded image on the thermal recording medium 1. In this embodiment, the thermal recording layer 3 contains a color former, a color developer, and a storage stability improver.

As for the color former, the color former that changes color upon heating is a component that changes color through a chemical reaction when heated by a thermal head or the like, and forms a recorded image on the thermal recording medium 1 of this embodiment. As the color former that changes color upon heating, a commonly used known leuco dye can be used. Examples of this leuco dye include 3-(N-isobutyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-isopentyl-N-ethyl)amino-6-methyl-7-o-chloroanilinofluoran, 3-(N-methyl-N-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-p-toluidino)-6-methyl-7-anilinofluoran, 3- (N-ethyl-N-isopentyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethoxypropyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-cyclohexyl-N-methyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-n-propyl)amino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-a Nilinofluoran, 3-diethylamino-6-methyl-7-p-toluidinofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-8-methylfluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-chlorofluoran, 3-dibutylamino-6-methyl-7-bromofluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dipentylamino-6-methyl-7-anilinofluoran, 3-dimethylamino-5-methyl-7-methylfluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, crystal violet lactone, etc. can be used alone or in combination of two or more.

The particle size of the color former is preferably 0.1 to 1.0 μm. Since the color former melts and reacts, the larger the particle size, the slower the reaction and the lower the sensitivity characteristics. On the other hand, as the particle size becomes smaller, the risk of coloring at an unexpected temperature due to the heat generated when drying the paint increases. In this embodiment, by setting the particle size of the color former within the above range, the sensitivity characteristics and coloring temperature of the color former can be appropriately adjusted. Here, the particle size refers to the 50% average particle size measured using a microtrack laser analysis/scattering particle size analyzer.

In this embodiment, in order to obtain excellent color development, it is preferable that the color former is contained in an amount of about 5 to 20 mass % relative to the entire thermal recording layer 3. It is preferable that the color developer described below is contained in a ratio of 1 to 3 parts by dry weight to 1 part by color former.

（式（１）中、Ｒ¹、Ｒ²、Ｒ³、Ｒ⁴、Ｒ⁵、Ｒ⁶、Ｒ⁷、及びＲ⁸は、それぞれ独立して、水素原子、又は置換基を示す。Ｒ⁹は、水素原子、アルキル基、又はアルケニル基を示す。） In this embodiment, the thermosensitive recording layer 3 contains, as the color developer, a diphenylsulfone compound represented by the following formula (1).

(In formula (1), R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ each independently represent a hydrogen atom or a substituent. R ⁹ represents a hydrogen atom, an alkyl group, or an alkenyl group.)

When the diphenyl sulfone compound represented by the above formula (1) is heated, the phenolic hydroxyl group supplies a proton to the leuco dye as described above to cause it to develop color, and it is believed that the phenolic hydroxyl group also forms an intermolecular hydrogen bond with the leuco dye to maintain the colored state.

As the "substituent", any organic group other than a hydrogen atom can be used without any particular limitation, and examples thereof include a halogen atom, a nitro group, an amino group, an alkyl group, an alkenyl group, an alkoxy group, an aryl group, an aryloxy group, an alkylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, and an arylamino group.

The above "halogen atom" includes a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom.

Examples of the above "alkyl group" and "alkyl" include linear or branched alkyl groups having 1 to 12 carbon atoms, such as methyl, ethyl, normal propyl, isopropyl, normal butyl, isobutyl, secondary butyl, tertiary butyl, normal pentyl, isopentyl, tertiary pentyl, neopentyl, 2,3-dimethylpropyl, 1-ethylpropyl, 1-methylbutyl, 2-methylbutyl, normal hexyl, isohexyl, 2-hexyl, 3-hexyl, 2-methylpentyl, 3-methylpentyl, normal heptyl, normal octyl, normal nonyl, normal decyl, normal undecyl, and normal dodecyl.

Examples of the "alkenyl group" include straight-chain or branched alkenyl groups having 2 to 6 carbon atoms, such as vinyl, allyl, isopropenyl, 1-butenyl, 2-butenyl, 2-methyl-2-propenyl, 1-methyl-2-propenyl, 2-methyl-1-propenyl, pentenyl, 1-hexenyl, and 3,3-dimethyl-1-butenyl.

Examples of the "alkoxy group" include linear or branched alkoxy groups having 1 to 8 carbon atoms, such as methoxy, ethoxy, normal propoxy, isopropoxy, normal butoxy, secondary butoxy, tertiary butoxy, normal pentyloxy, isopentyloxy, tertiary pentyloxy, neopentyloxy, 2,3-dimethylpropyloxy, 1-ethylpropyloxy, 1-methylbutyloxy, normal hexyloxy, isohexyloxy, normal heptyloxy, and normal octyloxy.

The above "aryl group" and "aryl" include, for example, aromatic hydrocarbon groups having 6 to 10 carbon atoms, such as a phenyl group, a 1-naphthyl group, and a 2-naphthyl group.

In the above "dialkylamino group," the two alkyl groups may be the same or different.

As the diphenylsulfone compound represented by formula (1), a compound represented by the following formula (1a) is preferred from the viewpoint of imparting excellent color development to the thermal recording medium 1.

In formula (1a), R ¹⁰ , R ¹¹ , and R ¹² each independently represent a hydrogen atom, an alkyl group, or an alkenyl group, where, when R ¹² is a hydrogen atom, R ¹⁰ and R ¹¹ each independently represent an alkyl group or an alkenyl group, and when R ¹² is an alkyl group or an alkenyl group, R ¹⁰ and R ¹¹ each independently represent a hydrogen atom.

Specific examples of the compound represented by the above formula (1a) include 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, 4-hydroxy-4'-ethoxydiphenyl sulfone, and 4-hydroxy-4'-n-butoxydiphenyl sulfone. In particular, 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, and bis(3-allyl-4-hydroxyphenyl)sulfone are preferred.

In this embodiment, the thermal recording layer 3 may contain a single diphenyl sulfone compound represented by formula (1), or may contain two or more types of diphenyl sulfone compounds.

In this embodiment, the content of the diphenyl sulfone compound represented by formula (1) in the entire thermal recording layer 3 is preferably 10% by mass or more and 40% by mass or less. A configuration in which the content of the diphenyl sulfone compound is 10% by mass or more is preferable in that it can prevent poor color development (low optical density) caused by a shortage of color developer. Also, a configuration in which the content of the diphenyl sulfone compound is 40% by mass or less is preferable in that it can prevent poor color development (low optical density) caused by an excess of color developer (i.e., a shortage of dye).

In this embodiment, the thermosensitive recording layer 3 preferably further contains a urea-urethane compound represented by the following formula (2) as a developer. That is, the thermosensitive recording layer 3 preferably contains a urea-urethane compound represented by the following formula (2) in addition to the diphenyl sulfone compound represented by formula (1).

By containing a urea urethane compound represented by formula (2) in addition to the diphenyl sulfone compound represented by formula (1) in the thermal recording layer, the reaction efficiency with the leuco dye is increased, making it easier to generate an electron transfer complex and making it difficult for a reverse reaction to occur, resulting in excellent color development of the thermal recording medium, less reduction in color development density, and excellent print preservation properties.

The urea-urethane compound represented by formula (2) used as a color developer in the present invention is specifically the three types represented by the following formulas (2a) to (2c), which may be used alone or in combination of two or more types.

In this embodiment, the content of the urea-urethane compound represented by formula (2) in the entire thermal recording layer 3 is preferably 1% by mass or more and 20% by mass or less. A configuration in which the content of the urea-urethane compound is 1% by mass or more is preferable in that it can prevent poor color development (low optical density) due to a shortage of color developer, can suppress a decrease in color development density, and provides excellent print storage stability. A configuration in which the content of the urea-urethane compound is 20% by mass or less is preferable in that it can prevent poor color development (low optical density) due to an excess of color developer (i.e., a shortage of dye).

In this embodiment, when the thermal recording layer 3 contains a urea urethane compound represented by formula (2), the content ratio of the urea urethane compound represented by formula (2) to the diphenyl sulfone compound represented by formula (1) (urea urethane compound/diphenyl sulfone compound) is preferably 1/20 to 1/1. A configuration in which the content ratio is 1/1 or less is preferable in that it can prevent poor color development (low optical density). Also, a configuration in which the content ratio is 1/20 or more is preferable in that it can suppress a decrease in color development density and provide excellent print storage stability.

In this embodiment, the thermosensitive recording layer 3 may contain, as a developer, a developer (other developer) other than the diphenyl sulfone compound represented by formula (1) and the urea urethane compound represented by formula (2) within a range that does not impair the effects of the present invention.
Other color developers include, for example, 1,1-bis(p-hydroxyphenyl)cyclohexane, 1,1-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)butane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 2,2'-methylenebis(4-chlorophenol), 2,2-bis(4-hydroxyphenyl)-4-methylpentane, poly(4-hydroxybenzoic acid), 4-hydroxybenzoic acid benzyl, 2,4-bis(phenylsulfonyl)phenol, α-{4-[(4-hydroxyphenyl)sulfonyl]phenyl}-ω-hydroxypoly(polymerization degree n=1 to 7)(oxyethyleneoxyethyleneoxy-p-phenylenesulfonyl-p-phenylene), 3,5-bis(α-methylbenzyl)salicylic acid, bis[4-( n-octyloxycarbonylamino)salicylate zinc], 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, 4-hydroxybenzenesulfonanilide, 2'-(3-phenylureido)benzenesulfonanilide, N-(2-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio]acetamide, N-(4-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio]acetamide, 4-[[4-[4-[[4-(1-methylethoxy)phenyl]sulfonylphenoxy]butoxy]phenyl]sulfonyl]phenol, 4-tert-butylphenol-formaldehyde polycondensate, N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea, 1-phenyl-3-(4-methylphenylsulfonyl)urea, and the like.

In this embodiment, the total content of the diphenyl sulfone compound represented by formula (1) and the urea urethane compound represented by formula (2) relative to the total amount of the color developer contained in the thermal recording layer 3 is not particularly limited, but from the viewpoint of improving color development and print storage stability, it is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

In this embodiment, the content of the diphenyl sulfone compound represented by formula (1) relative to the total amount of the color developer contained in the thermosensitive recording layer 3 is not particularly limited, but from the viewpoint of improving color development and print storage stability, it is preferably 50% by mass or more, more preferably 55% by mass or more, and even more preferably 60% by mass or more.

In this embodiment, when the thermosensitive recording layer 3 contains a urea-urethane compound represented by formula (2), the content of the urea-urethane compound represented by formula (2) relative to the total amount of the color developer contained in the thermosensitive recording layer 3 is not particularly limited, but from the viewpoint of improving color development and print storage stability, it is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more.

In this embodiment, the thermal recording layer 3 contains a compound having three or more phenolic hydroxyl groups in the molecule as a storage stability improver. The compound having three or more phenolic hydroxyl groups in the molecule is a compound that improves print storage stability by preventing oxidative decomposition of leuco dyes and color developers (including diphenyl sulfone compounds represented by formula (1) and urea urethane compounds represented by formula (2)) by capturing hydroperoxy radicals generated by air oxidation with the phenolic hydroxyl groups.

The number of phenolic hydroxyl groups in a compound having three or more phenolic hydroxyl groups in the molecule is not limited as long as it is three or more, but is, for example, three to five, preferably three to four, and more preferably three.

Specific examples of compounds having three or more phenolic hydroxyl groups in the molecule include 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and tris(2,6-dimethyl-4-t-butyl-3-hydroxybenzyl)isocyanurate. In this embodiment, the thermal recording layer 3 may contain a single compound having three or more phenolic hydroxyl groups in the molecule, or may contain two or more types of compounds.

In this embodiment, the content of the compound having three or more phenolic hydroxyl groups in the molecule relative to the entire thermosensitive recording layer 3 is not particularly limited, but is preferably 1% by mass or more and 10% by mass or less. A configuration in which the content of the compound having three or more phenolic hydroxyl groups in the molecule is 1% by mass or more is preferable in that it can impart excellent print preservability to the thermosensitive recording medium 1 of this embodiment. A configuration in which the content of the compound having three or more phenolic hydroxyl groups in the molecule is 10% by mass or less is preferable in that it can impart excellent color development to the thermosensitive recording medium 1 of this embodiment.

In this embodiment, the thermosensitive recording layer 3 may contain, as a storage stability improver, a compound having three or more phenolic hydroxyl groups in the molecule (other storage stability improver) within a range that does not impair the effects of the present invention.
Other storage property improvers include, for example, sodium-2,2'-methylenebis(4,6-di-t-butylphenyl)phosphite, 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 4-(2-methylglycyloxy)-4'-benzyloxydiphenylsulfone, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), diethylthiourea, zinc dibutyldithiocarbamate, and 4,4'-thiobis(6-t-butyl-m-cresol).

In this embodiment, the content of the compound having three or more phenolic hydroxyl groups in the molecule relative to the total amount of the storage stability improver contained in the thermal recording layer 3 is not particularly limited, but from the viewpoint of improving color development and print storage stability, it is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

Thermal recording layer 3 may also contain additives such as binders, sensitizers, lubricants, fillers, and pigments as necessary.

Examples of binders contained in the thermal recording layer 3 include polyvinyl alcohol, modified polyvinyl alcohol, starch, casein, gelatin, polyamide, polyacrylamide, modified polyacrylamide, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl acetate, polyacrylic acid esters, styrene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, diisobutylene-maleic anhydride copolymers, vinyl acetate-maleic anhydride copolymers, methyl vinyl-maleic anhydride copolymers, isopropylene-maleic anhydride copolymers, styrene-butadiene copolymers, polyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinyl acetate copolymers, polyurethane, polystyrene, polyvinylpyrrolidone, acrylic acid esters, acrylonitrile, methyl vinyl ether, and the like. These binders can be used alone or in combination of two or more.

Examples of sensitizers include stearic acid, stearic acid amide, stearic acid anilide, methylol stearic acid amide, methylene bis stearic acid amide, ethylene bis stearic acid amide, 1-benzyloxynaphthalene, 2-benzyloxynaphthalene, 2,6-diisopropylnaphthalene, 1,2-diphenoxyethane, 1,2-diphenoxymethylbenzene, 1,2-bis(3,4-dimethylphenol)ethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, di(p-chlorobenzyl) oxalate, di(p-methylbenzyl) oxalate, dibenzyl oxalate, p-benzylbiphenyl, m-terphenyl, diphenyl sulfone, benzyl p-benzyloxybenzoate, dibenzyl terephthalate, and p-toluenesulfonamide, which are solid at room temperature and preferably have a melting point of about 70°C or higher. These sensitizers can be used alone or in combination of two or more.

Examples of lubricants include paraffin wax, fatty acids such as oleic acid, polyolefin waxes such as polyethylene wax, metal soaps such as zinc stearate, ester waxes such as carnauba wax, and oils such as silicone oil and whale oil. These lubricants can be used alone or in combination of two or more kinds.

Examples of fillers include aluminum hydroxide, magnesium hydroxide, aluminum oxide, magnesium oxide, aluminum silicate, calcium carbonate, magnesium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolin, calcined kaolin, diatomaceous earth, white carbon, zinc oxide, silicon oxide, colloidal silica, polystyrene resin particles, urea-formalin resin particles, polyolefin resin particles, etc. These fillers can be used alone or in combination of two or more.

In this embodiment, by providing an intermediate layer 4 on the thermal recording layer 3, a thermal recording medium 1 having excellent water resistance, chemical resistance, plasticizer resistance, etc. can be obtained.

Examples of materials constituting the intermediate layer 4 include water-based resins such as polyvinyl alcohol, modified polyvinyl alcohol, starch, modified starch, casein, gelatin, glue, gum arabic, polyamide, polyacrylamide, modified polyacrylamide, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl acetate, polyacrylic acid ester, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, diisobutylene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, methyl vinyl-maleic anhydride copolymer, isopropylene-maleic anhydride copolymer, styrene-butadiene copolymer, maleic acid copolymer, polyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinyl acetate copolymer, polyurethane, polystyrene, polyvinylpyrrolidone, acrylic acid ester, acrylonitrile, and methyl vinyl ether polyvinyl alcohol. The term "water-based resin" refers to a resin component in which these compounds are dispersed in water or dissolved in water. These materials can be used alone or in combination of two or more.

The transparency of the above resins can be improved by using resins with water-soluble portions, such as polyvinyl alcohol (PVA) resins, which are resins that have hydroxyl groups as hydrophilic structural units, or resins with a core-shell structure in which hydrophobic core particles are coated with a water-soluble shell polymer, such as core-shell acrylic resins.

As a core-shell type resin, for example, a core-shell type acrylic resin commercially available under the name "Barrierstar (manufactured by Mitsui Chemicals)" can be used.

The coating amount (dry weight) of the intermediate layer 4 is preferably 0.3 g/m ² to 10 g/m ² .

The topcoat layer 5 improves the thermal head suitability of the thermal recording medium 1 for use with a thermal head, ensuring smooth color development of the thermal recording layer 3. Specifically, this means that color development of the thermal recording layer 3 is carried out in such a way that defects such as the accumulation of deposits on the thermal head and distortion of the surface of the thermal recording medium 1 due to heat are minimized.

In this embodiment, the top coat layer 5 of the thermal recording medium 1 plays a role in reducing wear on the thermal head and not shortening the life of the thermal head without adding elastic particles or the like. This means improving the thermal head suitability. It is also necessary to improve the sticking resistance of the top coat layer 5 against the thermal head. Here, sticking resistance means that the components of the top layer of the thermal recording medium are less likely to melt due to the heat of the thermal head and stick to the thermal head. More specifically, it means that the thermal recording medium is less likely to have problems such as partial non-printing or distortion of the printed surface.

The top coat layer 5 of this embodiment has depressions on its surface, evaporation holes due to evaporation of water, and cracks. This reduces the contact area between the surface of the top coat layer 5 and the thermal head.

In this way, in order to create recesses, particularly cracks, on the surface of the topcoat layer 5, a coating liquid containing hydrophobic resin particles is used as the coating liquid for forming the topcoat layer 5.

That is, the topcoat layer 5 uses an emulsion of hydrophobic resin particles as a binder; in this embodiment, an emulsion of hydrophobic acrylic resin particles dispersed in water.

In this way, an emulsion of hydrophobic resin particles is used as the binder for the topcoat layer 5, and no water-soluble polymer is used.

When a coating liquid containing a water-soluble polymer is applied and dried, it does not easily aggregate, forming a flexible coating film, so cracks due to shrinkage do not occur in the topcoat layer 5.

In contrast, when an emulsion of hydrophobic resin particles is applied and dried, the hydrophobic resin particles evaporate, causing them to aggregate and shrink, resulting in cracks that become depressions on the surface of the topcoat layer 5.

These cracks are formed by the shrinkage caused by the aggregation of hydrophobic resin particles, so they remain in the top coat layer 5 and do not reach the intermediate layer 4.

In addition, in this embodiment, in order to form evaporation holes on the surface of the topcoat layer 5 due to the evaporation of water, which become depressions, the three layers of the thermal recording layer 3, the intermediate layer 4, and the topcoat layer 5 are simultaneously coated using a curtain coater.

In the curtain coater, the coating liquids for forming the thermal recording layer 3, intermediate layer 4, and topcoat layer 5 are ejected from multiple slits and layered, and the layered coating liquids are continuously run. At this time, they are allowed to fall freely onto the undercoat layer 6 that has been formed in advance on the substrate 2 and coated.

In this simultaneous coating of three layers using a curtain coater, when the top coat layer 5 dries, the hydrophobic resin particles begin to aggregate as described above, causing cracks, through which water vapor escapes, and the semi-dry intermediate layer 4 and thermosensitive recording layer 3 dry and solidify. Most of the water vapor in the intermediate layer 4 and thermosensitive recording layer 3 is released from the cracks, but some water vapor forms evaporation holes in the top coat layer 5 and is released. For this reason, cracks and evaporation holes are formed near the top coat layer 5.

In this embodiment, the evaporation holes formed in the top coat layer 5 are limited to the intermediate layer 4. Therefore, even if oil or the like adheres to the surface of the top coat layer 5, which is the uppermost layer, it does not reach the thermal recording layer 3, and the thermal recording layer 3 does not discolor or otherwise deteriorate.

The topcoat layer 5 is made by adding additives such as lubricants, crosslinking agents, dispersants, defoamers, and water-resistant agents to the binder as necessary.

Examples of resin binders include acrylic resins. Examples of lubricants include polyethylene and zinc stearate. Examples of cross-linking agents include zirconium carbonate.

Examples of fillers include aluminum hydroxide, aluminum oxide, aluminum silicate, heavy calcium carbonate, light calcium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolinite, diatomaceous earth, white carbon, magnesium carbonate, magnesium oxide, magnesium hydroxide, zinc oxide, polystyrene resin particles, urea-formaldehyde resin particles, polyolefin resin particles, etc. These fillers can be used alone or in combination of two or more. The particle size of the filler contained in the top coat layer 5 is preferably 1.0 μm or less.

In this embodiment, the thermal recording medium 1 is manufactured using a water-dispersed suspension as the coating liquid for forming the topcoat layer 5, which is a mixture of an emulsion of hydrophobic acrylic resin dispersed in water, polyethylene wax as a lubricant, and calcium carbonate as a pigment in a dry mass ratio of 4:3:3.

The coating amount (dry weight) of the top coat layer 5 is 1 g/m ² .

According to this embodiment, as described above, the surface of the topcoat layer 5, which is the uppermost layer of the thermal recording medium 1, has cracks that form depressions and moisture evaporation holes, so that the surface of the topcoat layer 5 becomes uneven. This reduces the contact area between the topcoat layer 5 and the thermal head, reduces wear on the thermal head, improves thermal head suitability, and improves sticking resistance.

The thickness of the top coat layer 5 is adjusted to, for example, less than 1 μm. In this embodiment, it is adjusted to about 0.8 μm. As a result, the distance from the surface of the top coat layer 5 to the thermal recording layer 3 is short, so that heat from the thermal head is efficiently conducted to the thermal recording layer 3. In addition, the thin thickness contributes to cost reduction.

Furthermore, because the cracks on the surface of the topcoat layer 5 propagate in the thickness direction inside the topcoat layer 5, the cracks cause the topcoat layer 5 to be divided in a direction perpendicular to the thickness direction, i.e., horizontally. This suppresses the dissipation of heat from the thermal head in the horizontal direction. As a result, the heat from the thermal head is efficiently conducted to the underlying thermal recording layer 3 located in the thickness direction.

To reduce the contact area between the top coat layer 5 and the thermal head, it is preferable that the roughly circular moisture evaporation holes have an average diameter of 2 μm or more.

The average diameter of the evaporation holes is calculated by observing the surface of the topcoat layer 5 with an electron microscope (SEM) and measuring the diameter of the evaporation holes per unit area, for example, ¹ mm2. The number of evaporation holes is preferably, for example, 30 or more, and more preferably 40 or more, per ^mm2 , with the average diameter being 5 μm or more.

In the thermal recording medium 1 of this embodiment, by adjusting the composition of the topcoat layer 5, for example, the surface of the topcoat layer 5 can be made to have a large number of evaporation holes and a small number of cracks. Alternatively, the surface of the topcoat layer 5 can be made to have only a large number of evaporation holes without any cracks.

In this embodiment, the three layers of the thermal recording layer 3, intermediate layer 4, and top coat layer 5 are simultaneously coated in multiple layers using a curtain coater, but this is not limited to simultaneous multi-layer coating, and each of the thermal recording layer 3, intermediate layer 4, and top coat layer 5 may be formed individually and sequentially.

In this embodiment, the undercoat layer 6 and the intermediate layer 4 are formed on the substrate 2, but in other embodiments of the present invention, at least one of the undercoat layer 6 and the intermediate layer 4 may be omitted.

The thermal recording medium of the above embodiment has a thermal recording layer with the above-mentioned configuration, and therefore has excellent color development, is less likely to lose color density, and has excellent print storage stability.

The dynamic sensitivity (OD value) of the printed portion of the thermal recording medium of this embodiment at 0.104 mj/dot is preferably 0.8 or more, and more preferably 0.9 or more, from the viewpoint of excellent color development.

The dynamic sensitivity (OD value) of the printed portion of the thermal recording medium of this embodiment at 0.144 mj/dot is preferably 1.3 or more, and more preferably 1.4 or more, from the viewpoint of excellent color development.

After printing with 0.171 mj/dot on the thermal recording medium of this embodiment, the color density (OD value) of the printed portion that is immersed in water at 23°C for 24 hours and then dried is preferably 1.2 or more, and more preferably 1.3 or more, from the viewpoint of excellent print preservation properties.

The thermal recording medium of this embodiment is printed at 0.171 mj/dot, then immersed in a 1% aqueous solution of hypochlorous acid for 15 minutes, and dried. From the viewpoint of excellent print preservation, the color density (OD value) of the printed portion is preferably 1.2 or more, and more preferably 1.25 or more.

After printing with the thermal recording medium of this embodiment at 0.171 mj/dot, the color density (OD value) of the printed area when left to stand for 24 hours under conditions of 70°C and 100% RH is preferably 1.0 or more, and more preferably 1.1 or more, from the viewpoint of excellent print preservation properties.

After printing with the thermal recording medium of this embodiment at 0.104 mj/dot, the color density (OD value) of the printed portion left to stand for 3 days under 40°C drying conditions is preferably 0.5 or more, and more preferably 0.55 or more, from the viewpoint of excellent print preservation.

After printing with the thermal recording medium of this embodiment at 0.104 mj/dot, the color density (OD value) of the printed portion left to stand for 7 days under 40°C drying conditions is preferably 0.4 or more, and more preferably 0.45 or more, from the viewpoint of excellent print preservation.

The dynamic sensitivity (OD value) and color density (OD value) are measured in the examples below, and higher values indicate better color development and print preservation properties.

In the following examples and comparative examples, a thermal recording medium was prepared containing a diphenyl sulfone compound and a compound having three or more phenolic hydroxyl groups in the molecule in the thermal recording layer, and the color development, water resistance, chemical resistance (hypochlorite resistance), moist heat resistance, and print preservation over time were evaluated. Note that the present invention is not limited to these examples.

(Examples 1 to 4, Comparative Examples 1 to 3)
(Preparation of Thermal Recording Medium)
<Undercoat layer>
An ^undercoat layer coating liquid prepared by mixing and stirring a composition containing 70 parts by mass of hollow particles (solid content concentration 26.5%, Ropeake HP-1055: Rohm and Haas Japan Co., Ltd.), 10 parts by mass of modified styrene butadiene latex (solid content concentration 49%), and 20 parts by mass of water was applied onto a substrate of fine paper (thickness: 80 μm) with a basis weight of 70 g/m, and then dried to form an undercoat layer with a coating weight of 3.0 g/m when ^dried and a thickness of 5 μm.

<Thermal recording layer>
The coating liquid for forming the thermosensitive recording layer shown in Table 1 was prepared, and the prepared coating liquid for forming the thermosensitive recording layer was applied onto the above-mentioned undercoat layer so that the coating amount was 4.0 g/ ^m2 in terms of dry weight, and then dried to form a thermosensitive recording layer with a thickness of 3.5 μm on the undercoat layer. In Table 1, the numerical values of each compounding material indicate the weight ratio when dried.

In addition, the compounding materials used were 3-dibutylamino-6-methyl-7-anilinofluorolane with a particle size of 0.6 to 0.7 μm as the leuco dye, 4-hydroxy-4'-n-propoxydiphenylsulfone (product name "Tomilac KN", manufactured by API Corporation) as the developer 1, and a urea urethane compound represented by the above formula (2) (product name "UU", manufactured by Chemipro Chemicals) as the developer 2. In addition, the storage improver 1 used was 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane (trade name "DH37", manufactured by ADEKA Corporation), the storage improver 2 used was 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane (trade name "DH43", manufactured by ADEKA Corporation), and the storage improver 3 used was 4,4'-butylidenebis(3-methyl-6-t-butylphenol) (trade name "Yoshinox BB", manufactured by Mitsubishi Chemical Corporation).

The binder used was a styrene-acrylic copolymer emulsion, the pigment used was calcium carbonate (dispersed in a 5% aqueous solution of sodium hexametaphosphate to give a dispersion with a solids concentration of 30%), and the lubricant used was zinc stearate emulsion.

<Middle class>
An acrylic emulsion (solid concentration 30%) was applied onto the above-mentioned heat-sensitive recording layer and then dried to form an intermediate layer having a dry coating weight of 1.6 g/m ² and a thickness of 1.5 μm.

<Topcoat layer>
A liquid obtained by mixing and stirring 40 parts by mass of acrylic emulsion (solid concentration 20%), 5 parts by mass of calcium carbonate, 15 parts by mass of polyethylene wax (solid concentration 40%), and 40 parts by mass of water was applied onto the intermediate layer and dried to form a top coat layer with a coating weight of 1.0 g/ ^m2 and a thickness of 0.9 μm when dried.

The thermal recording media of Examples 1 to 4 and Comparative Examples 1 to 3 were produced using the above method.

(Dynamic Sensitivity Evaluation)
In the dynamic sensitivity test, printing was performed on each thermal recording medium in each Example and Comparative Example with different printing energies, and the optical density (OD value of the printed portion) at each printing energy was measured. From the measurement results, the dynamic sensitivity of each thermal recording medium in each Example and Comparative Example was evaluated. The procedure of the dynamic sensitivity test is described below.

The thermal recording medium was printed using a thermal paper printing test device (Okura Engineering, product name: Pulse Simulator TH-M2/PP) at a print speed of 50 mm/sec, applied voltage of 17.0 V, head resistance of 870 Ω, pulse width of 0.488 to 1.394 ms, and print energy of 0.104 mJ/dot and 0.144 mJ/dot. The optical density (OD value) under these print energy conditions was measured using a spectrophotometer (X-rite, product name: eXact).

The measurement results from the above test are shown in Table 2. In the measurement results in Table 2, a larger optical density (OD value) value indicates more color development, and a smaller value indicates insufficient color development. For example, if the optical density (OD value) value is large despite the low printing energy, it is evaluated as having "good color development." On the other hand, if the optical density (OD value) value is small despite the high printing energy, it is evaluated as having "poor color development." In other words, the dynamic sensitivity test is an evaluation of color development.

(Water resistance evaluation)
In the water resistance evaluation, the optical density (OD value of the printed portion) of the printed portion of each thermosensitive recording medium in each Example and Comparative Example was measured by immersing the printed portion in water. The water resistance of each thermosensitive recording medium in each Example and Comparative Example was evaluated based on the measurement results. The procedure for evaluating water resistance will be described below. The above optical density includes the meaning of color density. The same applies to the optical density in the water resistance test, hypochlorous acid resistance test, moist heat resistance test, and aged print storage test.

The thermal recording medium was printed using a thermal paper printing test device (manufactured by Okura Engineering, product name: Pulse Simulator TH-M2/PP) at a printing speed of 50 mm/sec, applied voltage of 17.0 V, head resistance of 870 Ω, pulse width of 0.488 to 1.394 ms, and printing energy of 0.171 mJ/dot.

The printed thermal recording medium sample was immersed in water at 23°C for 24 hours and then dried.

The optical density (OD value of the printed area) of the thermal recording medium sample before and after the test was measured using a spectrophotometer (product name: eXact, manufactured by Videojet X-Rite Inc.).

The measurement results of the above test are shown in Table 2. In the measurement results in Table 2, when the optical density (OD value) of the printed part is large (i.e., the light reflectance is low), the color density is maintained (the color is maintained black), and when the value is small (i.e., the light reflectance is high), the color density is reduced. In other words, it represents the degree of reduction in the color density of the printed part when the thermal recording medium is immersed in water. Therefore, the water resistance of the printed part can be confirmed by checking whether the colored part disappears when immersed in water. Specifically, when the optical density (OD value) of the printed part is large, it means that the printed part has excellent water resistance.

(Hypochlorous acid resistance evaluation)
In the hypochlorous acid resistance evaluation, the optical density (OD value of the printed part) of each thermosensitive recording medium in each embodiment and each comparative example is measured by immersing the printed part in hypochlorous acid aqueous solution.From this measurement result, the hypochlorous acid resistance of each thermosensitive recording medium in each embodiment and each comparative example is evaluated.The procedure of hypochlorous acid resistance evaluation is described below.

The thermal recording medium was printed using a thermal paper printing test device (manufactured by Okura Engineering, product name: Pulse Simulator TH-M2/PP) at a printing speed of 50 mm/sec, applied voltage of 17.0 V, head resistance of 870 Ω, pulse width of 0.488 to 1.394 ms, and printing energy of 0.171 mJ/dot.

The printed thermal recording medium sample was immersed in a 1% aqueous solution of hypochlorous acid at 23°C for 15 minutes and then dried.

The optical density (OD value of the printed area) of the thermal recording medium sample before and after the test was measured using a spectrophotometer (product name: eXact, manufactured by Videojet X-Rite Inc.).

The measurement results of the above test are shown in Table 2. In the measurement results in Table 2, as in the water resistance test above, when the optical density (OD value) of the printed part is large (i.e., the light reflectance is low), the color density is maintained (the color is maintained black), and when the value is small (i.e., the light reflectance is high), the color density is decreased. In other words, it represents the degree of decrease in the color density of the printed part when the thermal recording medium is immersed in a hypochlorous acid aqueous solution. Therefore, the hypochlorous acid resistance evaluation of the printed part can be confirmed by the fact that the colored part does not disappear when immersed in a hypochlorous acid aqueous solution. Specifically, when the optical density (OD value) value is large, it means that the printed part has excellent hypochlorous acid resistance.

(Evaluation of humidity and heat resistance)
In the evaluation of the moist heat resistance, the optical density (OD value of the printed part) was measured for the printed part of each thermal recording medium stored under moist heat conditions in each Example and Comparative Example. The moist heat resistance of each thermal recording medium in each Example and Comparative Example was evaluated based on the measurement results. The procedure for evaluating moist heat resistance is described below.

The thermal recording medium was printed using a thermal paper printing test device (manufactured by Okura Engineering, product name: Pulse Simulator TH-M2/PP) at a printing speed of 50 mm/sec, applied voltage of 17.0 V, head resistance of 870 Ω, pulse width of 0.488 to 1.394 ms, and printing energy of 0.171 mJ/dot.

The printed thermal recording medium sample was left to stand for 24 hours under conditions of 70°C and 100% RH.

The optical density (OD value of the printed area) of the thermal recording medium sample before the test and after it was left to stand was measured using a spectrophotometer (product name: eXact, manufactured by Videojet X-Rite Inc.).

The measurement results of the above test are shown in Table 2. In the measurement results in Table 2, as with the water resistance test above, when the optical density (OD value) of the printed part is large (i.e., the light reflectance is low), the color density is maintained (the color is maintained black), and when the value is small (i.e., the light reflectance is high), the color density is decreased. In other words, it represents the degree of decrease in the color density of the printed part when the thermal recording medium is left to stand under moist heat conditions of 70°C and 100% RH. Therefore, the moist heat resistance evaluation of the printed part can be confirmed by whether the color part does not disappear under the moist heat conditions. Specifically, when the optical density (OD value) value is large, it means that the printed part has excellent moist heat resistance.

(Evaluation of print preservation over time)
In the evaluation of print preservation stability over time, the optical density (OD value of the printed part) was measured for the printed part of each thermosensitive recording medium in each Example and Comparative Example, which was stored under heat drying conditions. From the measurement results, the print preservation stability over time of each thermosensitive recording medium in each Example and Comparative Example was evaluated. The procedure for evaluating the print preservation stability over time is described below.

The thermal recording medium was printed using a thermal paper printing test device (manufactured by Okura Engineering, product name: Pulse Simulator TH-M2/PP) at a printing speed of 50 mm/sec, applied voltage of 17.0 V, head resistance of 870 Ω, pulse width of 0.488 to 1.394 ms, and printing energy of 0.104 mJ/dot.

The printed thermal recording medium samples were left to stand under dry conditions at 40°C for 3 or 7 days.

The optical density (OD value of the printed area) of the thermal recording medium sample before the test and after it was left to stand was measured using a spectrophotometer (product name: eXact, manufactured by Videojet X-Rite Inc.).

The measurement results of the above test are shown in Table 3. In the measurement results in Table 3, as with the water resistance test above, when the optical density (OD value) of the printed part is large (i.e., the light reflectance is low), the color density is maintained (the color is maintained black), and when the value is small (i.e., the light reflectance is high), the color density is decreased. In other words, it represents the degree of decrease in the color density of the printed part when the thermal recording medium is stored under the above heat drying conditions. Therefore, the evaluation of the print preservation over time of the printed part can be confirmed by whether the color part does not disappear by storage under the above heat drying conditions. Specifically, when the optical density (OD value) value of the printed part is large, it means that the printed part has excellent print preservation over time.

<Verification results>
The results shown in Tables 2 and 3 confirm the following:
Comparative Example 1, which contained only a diphenyl sulfone compound as a color developer and did not contain a urea urethane compound or a storage stability improver, and Comparative Example 2, which contained a diphenyl sulfone compound and a urea urethane compound as a color developer but did not contain a storage stability improver, showed a large decrease in the OD value of the printed area and were inferior in print storage stability in all of water resistance, hypochlorite resistance, moist heat resistance, and print storage stability over time, compared to the examples which contained a diphenyl sulfone compound and a urea urethane compound as a color developer and a compound having three phenolic hydroxyl groups in the molecule as a storage stability improver.
On the other hand, Comparative Example 3, which contained a diphenyl sulfone compound and a urea urethane compound as a color developer and a compound having two phenolic hydroxyl groups in the molecule as a storage stability improver, showed color development, water resistance, and print storage stability over time equivalent to those of the Examples, but in terms of hypochlorite resistance and moist heat resistance, the OD value of the printed area was lower than that of the Examples.
From the above results, it can be seen that a thermal recording medium having a thermal recording layer containing a diphenyl sulfone compound and a urea urethane compound as a developer and a compound having three phenolic hydroxyl groups in its molecule as a storage stability improver has excellent color development properties, is less likely to reduce color density, and also has excellent print storage stability.

As explained above, the present invention is particularly useful for thermal recording media on which barcodes and the like are printed.

（式（１）中、Ｒ¹、Ｒ²、Ｒ³、Ｒ⁴、Ｒ⁵、Ｒ⁶、Ｒ⁷、及びＲ⁸は、それぞれ独立して、水素原子、又は置換基を示す。Ｒ⁹は、水素原子、アルキル基、又はアルケニル基を示す。）
前記保存性向上剤として、分子内にフェノール性水酸基を３個以上有する化合物を含有することを特徴とする、感熱記録体。
〔付記２〕
前記顕色剤として、さらに、下記式（２）で表されるウレアウレタン化合物を含有する、付記１に記載の感熱記録体。

〔付記３〕
前記感熱記録層の全体に対する前記ジフェニルスルホン化合物の含有量が、１０質量％以上４０質量％以下である、付記１又は２に記載の感熱記録体。
〔付記４〕
前記感熱記録層の全体に対する前記分子内にフェノール性水酸基を３個以上有する化合物の含有量が、１質量％以上１０質量％以下である、付記１～３のいずれか１つに記載の感熱記録体。 Variations of the present invention are described below.
[Appendix 1]
A thermal recording medium having a thermal recording layer laminated on a substrate,
the thermal recording layer contains a color former, a color developer, and a storage stability improving agent;
The color developer contains a diphenyl sulfone compound represented by the following formula (1):

(In formula (1), R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ each independently represent a hydrogen atom or a substituent. R ⁹ represents a hydrogen atom, an alkyl group, or an alkenyl group.)
A heat-sensitive recording medium comprising, as the storage stability improving agent, a compound having three or more phenolic hydroxyl groups in the molecule.
[Appendix 2]
2. The thermal recording material according to claim 1, further comprising a urea-urethane compound represented by the following formula (2) as the developer:

[Appendix 3]
3. The thermosensitive recording medium according to claim 1, wherein the content of the diphenyl sulfone compound in the thermosensitive recording layer is 10% by mass or more and 40% by mass or less.
[Appendix 4]
4. The thermosensitive recording medium according to claim 1, wherein the content of the compound having three or more phenolic hydroxyl groups in the molecule is 1% by mass or more and 10% by mass or less based on the entire thermosensitive recording layer.

Reference Signs List 1: Thermosensitive recording medium 2: Substrate 3: Thermosensitive recording layer 4: Intermediate layer 5: Topcoat layer 6: Undercoat layer

The thermal recording medium of the present invention contains, as the storage stability improving agent, at least one compound selected from the group consisting of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and tris(2,6-dimethyl-4-t-butyl-3-hydroxybenzyl)isocyanurate . With this constitution, a thermal recording medium having excellent print storage stability can be provided.

In another embodiment of the thermosensitive recording medium of the present invention, the content of the at least one compound selected from the group consisting of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and tris(2,6-dimethyl-4-t-butyl-3-hydroxybenzyl)isocyanurate is preferably 1% by mass or more and 10% by mass or less based on the entire thermosensitive recording layer. According to this configuration, a thermosensitive recording medium having excellent print storage stability can be provided.

Claims (4)

A thermal recording medium having a thermal recording layer laminated on a substrate,
the thermal recording layer contains a color former, a color developer, and a storage stability improving agent;
The color developer contains a diphenyl sulfone compound represented by the following formula (1):

(In formula (1), R ¹ , R ² , R ³ , R ⁴ , R ⁵ , R ⁶ , R ⁷ , and R ⁸ each independently represent a hydrogen atom or a substituent. R ⁹ represents a hydrogen atom, an alkyl group, or an alkenyl group.)
A heat-sensitive recording medium comprising, as the storage stability improving agent, a compound having three or more phenolic hydroxyl groups in the molecule. 2. The thermal recording medium according to claim 1, further comprising, as the developer, a urea-urethane compound represented by the following formula (2):

The thermosensitive recording medium according to claim 1 or 2, wherein the content of the diphenyl sulfone compound in the thermosensitive recording layer is 10% by mass or more and 40% by mass or less. The thermosensitive recording medium according to claim 1 or 2, wherein the content of the compound having three or more phenolic hydroxyl groups in the molecule relative to the entire thermosensitive recording layer is 1% by mass or more and 10% by mass or less.

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