Heat-sensitive recording medium, method of producing heat-sensitive recording medium, and article including heat-sensitive recording medium

Abstract

A heat-sensitive recording medium includes a support, a heat-sensitive recording layer including a leuco dye and a developer disposed directly or indirectly on the support, and a protective layer disposed directly or indirectly on the heat-sensitive recording layer. The color developing agent includes a first color developing agent having a specific structure and a second color developing agent having a specific structure. The protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth) acrylic acid copolymer.

Description

TECHNICAL FIELD

[0001] The disclosure herein relates generally to a heat-sensitive recording medium, a method of producing a heat-sensitive recording medium, and an article. BACKGROUND

[0002] Heat-sensitive recording media can be easily recorded at low cost in a short time using a relatively simple apparatus. Therefore, heat-sensitive recording media have been widely applied to various fields, such as a point-of-sale (POS) system field for fresh food, packaged food, or ready-made food, a copying field (e.g., books and documents), a communication field (e.g., facsimile), a ticket publishing field (e.g., ticket machines and receipt publishing), an aviation field (baggage tags used at airports), or a medical field (e.g., medicine boxes and medicine bottles).

[0003] A heat-sensitive recording medium includes an electron-donating dye (leuco dye) and an electron-accepting color developer (color developer). As the color developer, a phenolic color developer including a phenolic skeleton (e.g., 4,4’-isopropylidene diphenol) and a color developer not including a phenolic skeleton (hereinafter can be referred to as “non-phenolic color developer”) can be used. Among the color developers listed above, a heat-sensitive recording medium using a non-phenolic color developer has been studied to minimize possible environmental and health hazards.

[0004] As a heat-sensitive recording medium using a non-phenolic color developer, for example, a heat-sensitive recording material is disclosed, which includes two urea compounds having a specific structure, such as N-phenylureido-phenyl-benzenesulfonamide, as an electron-accepting color developer (see, for example, PTL 1).

[0005] The present disclosure aims to provide a heat-sensitive recording medium that achieves excellent adhesion between a surface of a production layer (an outermost surface layer) of the heat-sensitive recording medium and a UV-curable ink printed on the outermost surface layer by flexographic printing, and achieves excellent plasticizer resistance in a printed area.

[0006] LIST OF CITATIONS

[0007] PATENT LITERATURE

[0008] PTL 1: Japanese Patent No. 7146147 SUMMARY

[0009] In one embodiment, a heat-sensitive recording medium includes a support, a heat-sensitive recording layer disposed directly or indirectly on the support, and a protective layer disposed directly or indirectly on the heat-sensitive recording layer. The heat-sensitive recording layer includes a leuco dye and a color developer. The color developer includes a first color developer and a second color developer. The first color developer includes a urea compound represented by general formula (I). The second color developer includes at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV).

[0010] [Chemical Formula 1]

[0011]

[0012] In General Formula (I), R 1 to R 3 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group.

[0013] [Chemical Formula 2]

[0014]

[0015] [Chemical Formula 3]

[0016]

[0017] In General Formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group.

[0018] [Chemical Formula 4]

[0019]

[0020] In General Formula (IV), R 1 to R 5 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group. The protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.

[0021] The present disclosure can provide a thermal recording medium that achieves excellent adhesion between a surface (an outermost surface layer) of a production layer of the thermal recording medium and a UV curable ink printed on the outermost surface layer by flexographic printing, and achieves excellent plasticizer resistance of a printed area. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a cross-sectional view illustrating an example of a thermal recording medium of the present disclosure.

[0023] Figure 2 is a schematic view illustrating an example of an article including the thermal recording medium of the present disclosure. DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present application will be described with reference to the accompanying drawings. Furthermore, the present application is not limited to these embodiments, but various changes and modifications can be made thereto without departing from the scope of the present application. In the present specification, unless otherwise specified, a numerical value range designated includes the upper limit and lower limit of the range.

[0025] <Thermal recording medium>

[0026] The thermal recording medium of the present disclosure includes, in order, a support, a thermal recording layer directly or indirectly on the support, and a protective layer directly or indirectly on the thermal recording layer. The thermal recording layer includes one or more specific urea compounds as a color developer, and the protective layer includes one or more specific acrylic resins. As needed, the thermal recording medium can further include other layers.

[0027] In the present specification, the phrase "a layer A is directly or indirectly provided on a layer B" or "a layer A is directly or indirectly on B" refers to an embodiment in which there is no layer between the layer A and the layer B, and an embodiment in which there is one or more layers between the layer A and the layer B.

[0028] (Support)

[0029] The shape, structure, size, and material of the support of the thermal recording medium of the present disclosure are not particularly limited, and can be appropriately selected according to the intended purpose. Examples of the shape of the support include flat plate and sheet. The structure of the support can be a single layer structure or a laminated structure. The size of the support can be appropriately selected according to the size of the thermal recording medium.

[0030] For example, as the support, in addition to typical paper, synthetic paper and plastic films such as polyethylene, transparent polyethylene terephthalate, polypropylene, and vinyl chloride can be used. In the case where a plastic film is used as the support, surface treatment such as matte treatment and corona treatment can be performed on the surface of the support to improve the fixability of the coating liquid. Among the above-listed examples, oriented polyethylene terephthalate sheet is preferred because the oriented polyethylene terephthalate sheet has excellent strength, heat resistance, and dimensional stability. Furthermore, the support can be a white opaque film obtained by adding a white constituent material or a white filler to a plastic film material to form a film, or the support can be a foam sheet obtained by foaming. The support can be a laminate including any combination of the above-described materials. Examples of the laminate include a laminate of cellulose fiber and synthetic paper, a laminate of cellulose fiber and a plastic film, and a laminate of a plastic film and synthetic paper.

[0031] When the heat-sensitive recording medium is used as a packaging material for food, the support is preferably a clear film because the contents inside the package can be easily observed. In the present specification, the definition of the term "clear" or "transparent" is not particularly limited, as long as the index related to the transparency of the film, haze (turbidity), is about 10% or less. For example, the haze of the clear film is preferably 5% or less.

[0032] The average thickness of the support is appropriately selected according to the intended purpose. The average thickness of the support is preferably 3 μm (micrometers) to 300 μm (micrometers) in view of the transparency and ease of processing. When the average thickness of the support is 3 μm (micrometers) to 300 μm (micrometers), the support has sufficient rigidity, making the processing of the support easy, while maintaining the required strength and transparency.

[0033] (Heat-sensitive recording layer)

[0034] The heat-sensitive recording layer of the heat-sensitive recording medium of the present disclosure includes a leuco dye and a color developer, wherein the color developer includes a first color developer and a second color developer. The first color developer includes a urea compound represented by general formula (I). The second color developer includes at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV).

[0035] ((Leuco dye))

[0036] The leuco dye is not particularly limited, and can be appropriately selected from leuco dyes commonly used for heat-sensitive recording media according to the intended purpose. Preferred examples of the leuco dye include leuco compounds of triphenylmethane-based dyes, fluoran-based dyes, phenothiazine-based dyes, gallein-based dyes, spiropyran-based dyes, and indophenolphthalein-based dyes.

[0037] The colorless dye is not particularly limited, and can be appropriately selected depending on the intended purpose. Examples of the colorless dye can include 3,3-bis(p- dimethylaminophenyl)-phthalimidine, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalimidine (synonym: crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)-6- diethylaminophthalimidine, 3,3-bis(p-dimethylaminophenyl)-6-chlorophthalimidine, 3,3- bis(p-dibutylaminophenyl)phthalimidine, 3-cyclohexylamino-6-chlorofluoran, 3- dimethylamino-5,7-dimethylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7- methylfluoran, 3-diethylamino-7,8-benzofluoran, 3-diethylamino-6-methyl-7- chlorofluoran, 3-(N-p-tolyl-N-ethylamino)-6-methyl-7-anilinofluoran, 2-[N-(3'- trifluoromethylphenyl)amino]-6-diethylaminofluoran, 2-[3,6-bis(diethylamino)-9-(o- chloroanilino)xanthenylbenzoic acid lactam], 3-diethylamino-6-methyl-7-(m- trichloromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3- pyrrolidinyl-6-methyl-7-anilinofluoran, 3-di-n-butylamino-7-o-chloroanilinofluoran, 3-N- methyl-N-n-pentylamino-6-methyl-7-anilinofluoran, 3-N-methyl-N-cyclohexylamino-6- methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N,N- diethylamino)-5-methyl-7-(N,N-diethylamino)fluoran, 3-(N,N-diethylamino)-5-methyl- 7-(N,N-di-n-propylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N- di-n-butylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N- diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5- methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N- dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N- diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5- methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N- dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N- diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5- methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N- dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino)fluoran, 3-(N,N-N-diphenylmethylamino)fluoran, benzoyl leuco dye methylene blue, 6'-chloro-8'-methoxy- benzindolinospiropyran, 6'-bromo-3'-methoxy-benzindolinospiropyran, 3-(2'-hydroxy-4'- dimethylaminophenyl)-3-(2'-methoxy-5'-chlorophenyl)phthalimidine, 3-(2'-hydroxy-4'- dimethylaminophenyl)-3-(2'-methoxy-5'-nitrophenyl)phthalimidine, 3-(2'-hydroxy-4'- diethylaminophenyl)-3-(2'-methoxy-5'-methylphenyl)phthalimidine, 3-(2'-methoxy-4'- dimethylaminophenyl)-3-(2'-hydroxy-4'-chloro-5'-methylphenyl)phthalimidine, 3-(N- ethyl-N-tetrahydrofurfuryl)amino-6-methyl-7-anilinofluoran, 3-N-ethyl-N-(2- ethoxypropyl)amino-6-methyl-7-anilinofluoran, 3-N-methyl-N-isobutyl-6-methyl-7- anilinofluoran, 3-morpholino-7-(N-propyl-trifluoromethylanilino)fluoran, 3-pyrrolidino-7- trifluoromethylanilino)fluoran, 3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino) fluoran, 3-pyrrolidino-7-(di-p-chlorophenyl)methylamino)fluoran, 3-diethylamino-5-chloro-7-(α- phenylethylamino)fluoran, 3-(N-ethyl-p-toluidino)-7-(α-phenylethylamino)fluoran, 3- diethylamino-7-(o-methoxycarbonylphenylamino)fluoran, 3-diethylamino-5-methyl-7-(α- phenylethylamino)fluoran, 3-diethylamino-7-piperidino fluoran, 2-chloro-3-(N- methyltoluidino)-7-(p-n-butylanilino)fluoran, 3-di-n-butylamino-6-methyl-7-anilinofluoran, 3,6-bis(dimethylamino)fluorenespiro(9,3')-6'-dimethylaminophthalimidine, 3-(N-benzyl-N- cyclohexylamino)-5,6-benzo-7-α-naphthylamino-4'-bromofluoran, 3-diethylamino-6-chloro- 7-anilinofluoran, 3-diethylamino-6-methyl-7-m-xylylaminofluoran, 4',5'-benzo, 3-N- methyl-N-isopropyl-6-methyl-7-anilinofluoran, 3-N-ethyl-N-isopentyl-6-methyl-7- anilinofluoran, 3-diethylamino-6-methyl-7-(2',4'-dimethylanilino)fluorane, 3-morpholino-7-(N-propyl-trifluoromethylanilino)fluorane, 3-pyrrolidino-7-trifluoromethylanilino fluorane, 3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino)fluorane, 3-pyrrolidino-7-(di-p-chlorophenyl)methylamino fluorane, 3-diethylamino-5-chloro-(a-phenylethylamino)fluorane, 3-(N-ethyl-p-toluidino)-7-(a-phenylethylamino)fluorane, 3-diethylamino-7-(o-methoxycarbonylphenylamino)fluorane, 3-diethylamino-5-methyl-7-(a-phenylethylamino)fluorane, 3-diethylamino-7-piperidino fluorane, 2-chloro-3-(N-methyltoluidino)-7-(p-n-butylanilino)fluorane, 3,6-bis(diethylamino)fluorene spiro(9,3')-6'-dimethylamino phthalide, 3-(N-benzyl-N-cyclohexylamino)-5,6-benzo-7-a-naphthylamino-4'-bromo fluorane, 3-diethylamino-6-chloro-7-anilino fluorane, 3-N-ethyl-N-(2-ethoxypropyl)amino-6-methyl-7-anilino fluorane, 3-N-ethyl-N-tetrahydrofurfurylamino-6-methyl-7-anilino fluorane, 3-(p-dimethylaminophenyl)-3-[1,1-bis(p-dimethylaminophenyl)ethen-2-yl]phthalide, 3-(p-dimethylaminophenyl)-3-[1,1-bis(p-dimethylaminophenyl)ethen-2-yl]-6- dimethylaminophthalide, 3-(p-dimethylaminophenyl)-3-(1-p-dimethylaminophenyl-1- phenylethen-2-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(1-p-dimethylaminophenyl-1-p-chlorophenylethen-2-yl)-6-dimethylaminophthalide, 3-(4'-dimethylamino-2'-methoxy)-3-(1"-p-dimethylaminophenyl-1"-p-chlorophenyl-1",3"-butadien-4"-yl)benzophthalide, 3-(4'-dimethylamino-2'-benzyloxy)-3-(1"-p-dimethylaminophenyl-1"-phenyl-1",3"-butadien-4"-yl)benzophthalide, 3-dimethylamino-6-dimethylamino-fluorene-9-spiro-3'-(6'-dimethylamino)phthalide, 3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide, 3-bis[1,1-bis(4-pyrrolidinophenyl)ethen-2-yl]-5,6-dichloro-4,7-dibromophthalide, bis(p-dimethylaminostyryl)-1-naphthalenesulfonylmethane, and bis(p-dimethylaminostyryl)-1-p-tolylsulfonylmethane. The above-listed examples can be used alone or in combination.

[0038] The mass of the leuco dye can be 3 to 30 mass% with respect to 100 mass% of the total mass of the heat-sensitive recording layer.

[0039] (color developer)

[0040] The color developer included in the heat-sensitive layer of the heat-sensitive recording medium of the present disclosure is a non-phenolic color developer. The non-phenolic color developer refers to a color developer that does not include a phenolic skeleton. Since the heat-sensitive recording medium of the present disclosure includes a non-phenolic color developer, it is not necessary to use a phenolic color developer that can act as an endocrine disruptor chemical. Therefore, the heat-sensitive recording medium has excellent environmental friendliness.

[0041] The color developer includes a first color developer and a second color developer, both of which are non-phenolic color developers.

[0042] The first color developer includes at least one urea compound represented by general formula (I). The urea compound represented by general formula (I) preferably constitutes the first color developer.

[0043] [Chemical Formula 5]

[0044]

[0045] In general formula (I), R 1 to R 3 are each independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group.

[0046] In one embodiment, the urea compound represented by general formula (I) preferably includes a compound represented by the following structural formula (1). The compound represented by structural formula (1) is a compound represented by general formula (I) in which R 1 , R 2 , and R 3 are each a hydrogen atom.

[0047] [Chemical Formula 6]

[0048]

[0049] The second color developer includes at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV). The second color developer preferably consists of a urea compound represented by structural formula (II), a urea compound represented by general formula (III), or a urea compound represented by general formula (IV). Furthermore, the second color developer can consist of two or three selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV).

[0050] [Chemical Formula 7]

[0051]

[0052] [Chemical Formula 8]

[0053]

[0054] In General Formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group.

[0055] [Chemical Formula 9]

[0056]

[0057] In General Formula (IV), R 1 to R 5 are each independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group.

[0058] The second color developer preferably includes a urea compound represented by Structural Formula (2) as the urea compound represented by General Formula (III).

[0059] [Chemical Formula 10]

[0060]

[0061] The second color developer preferably includes a urea compound represented by Structural Formula (3) as the urea compound represented by General Formula (IV).

[0062] [Chemical Formula 11]

[0063]

[0064] When the urea compound represented by General Formula (I) is determined to be the first urea compound and the urea compound represented by Structural Formula (II) and any one of General Formulas (III) and (IV) is determined to be the second urea compound, the amount of the second urea compound is preferably 0.02 parts by mass to 2.0 parts by mass, more preferably 0.1 parts by mass to 1.0 parts by mass, and still more preferably 0.25 parts by mass to 0.5 parts by mass, relative to 1.0 parts by mass of the first urea compound in the heat-sensitive recording layer.

[0065] The amount of the color developer is preferably 0.5 parts by mass to 10 parts by mass, and more preferably 1 part by mass to 5 parts by mass, relative to 1.0 parts by mass of the leuco dye.

[0066] Even when any one to three of the urea compound represented by Structural Formula (II), the urea compound represented by General Formula (III), and the urea compound represented by General Formula (IV) are mixed and combined with the urea compound represented by General Formula (I) and used as a color developer, the amount of the color developer with respect to the leuco dye is also the same as described above. The amount of the urea compound represented by General Formula (I) is preferably within the range described above with respect to the sum of the amount of any one of the urea compound represented by Structural Formula (II), the urea compound represented by General Formula (III), and the urea compound represented by General Formula (IV). For example, the urea compound represented by General Formula (I) is determined to be a first urea compound; the urea compound represented by Structural Formula (II) is determined to be a second (2-1) urea compound; the urea compound represented by General Formula (III) is determined to be a second (2-2) urea compound; and the urea compound represented by General Formula (IV) is determined to be a second (2-3) urea compound. The total amount (mass parts) of the first urea compound, the second (2-1) urea compound, the second (2-2) urea compound, and the second (2-3) urea compound with respect to 1 mass part of the leuco dye is preferably 0.5 mass parts to 10 mass parts, more preferably 1 mass part to 5 mass parts.

[0067] The color developer can also include other color developers that are generally used, as long as the effects obtainable by the thermal recording medium of the present disclosure are not adversely affected. As the other color developers, various electron-accepting compounds and oxidizing agents that are capable of coloring the leuco dye can be appropriately used.

[0068] Specific examples of the other color developer are not particularly limited, and can be appropriately selected from color developers available in the related art according to the intended purpose. Examples of the other color developer described above include 4,4'-isopropylidene bisphenol, 4,4'-isopropylidene bis(o-methylphenol), 4,4'-sec-butylidene bisphenol, 4,4'-isopropylidene bis(2-tert-butylphenol), zinc p-nitrobenzoate complex, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanurate, 2,2-(3,4'-dihydroxydiphenyl)propane, bis(4-hydroxy-3-methylphenyl)sulfide, 4'{6-(p-methoxyphenoxy)ethoxy}salicylate, 1,7-bis(4-hydroxyphenylthio)-3,5-dioxapane, 1,5-bis(4-hydroxyphenylthio)-5-oxapentane, monobenzyl monocalcium phthalate, 4,4'-cyclohexylidene bisphenol, 4,4'-isopropylidene bis(2-chlorophenol), 4,4'-diphenol sulfone, 4-isopropoxy-4'-hydroxydiphenyl sulfone, 4-benzyloxy-4'-hydroxydiphenyl sulfone, 4,4'-diphenol sulfoxide, isopropyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, benzyl protocatechuate, stearyl gallate, lauryl gallate, octyl gallate, 1,3-bis(4-hydroxyphenylthio)-propane, N,N'-diphenylthiourea, N,N'-di(m-chlorophenyl)thiourea, salicylic anilide, methyl bis-(4'-hydroxyphenyl)acetate, benzyl bis-(4-hydroxyphenyl)acetate, 1,3-bis(4-hydroxy cumyl)benzene, 1,4-bis(4-hydroxy cumyl)benzene, 2,4'-diphenol sulfone, 2,2'-diallyl-4,4'-diphenol sulfone, 3,4-dihydroxyphenyl-4'-methyl diphenyl sulfone, zinc 1-acetoxy-2-naphthoate, zinc 2-acetoxy-1-naphthoate, zinc 2-acetoxy-3-naphthoate, α,α bis(4-hydroxyphenyl)-α-methyltoluene, and an antipyrine complex of zinc and thiocyanate. The above-listed examples can be used alone or in combination.

[0069] The amount of the additional color developer can be appropriately selected according to the intended purpose, provided that the effects obtainable by the heat-sensitive recording medium of the present disclosure are not adversely affected.

[0070] The total amount of the other color developer other than the urea compounds represented by General Formula (I), the urea compounds represented by Structural Formula (II), the urea compounds represented by General Formula (III), and the urea compounds represented by General Formula (IV) is preferably less than 2 parts by mass, more preferably less than 0.5 parts by mass, with respect to 1 part by mass of the leuco dye in the heat-sensitive recording layer.

[0071] The heat-sensitive recording layer can essentially not include other color developers other than the urea compounds represented by general formula (I), the urea compounds represented by structural formula (II), the urea compounds represented by general formula (III), and the urea compounds represented by general formula (IV).

[0072] ((hot-melt material))

[0073] The heat-sensitive recording layer can include various hot-melt materials as a sensitizer. Examples of the hot-melt material include fatty acids such as stearic acid and behenic acid; fatty acid amides such as stearic acid amide and palmitic acid amide; fatty acid metal salts such as zinc stearate, aluminum stearate, calcium stearate, zinc palmitate, and zinc behenate; p-benzylbiphenyl, triphenylbenzene, triphenylmethane, benzyl p-benzyloxybenzoate, β-benzyloxy naphthalene, phenyl β-naphthoate, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate, diphenyl carbonate, dibenzyl p-terephthalate, dimethyl p-terephthalate, 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 1,4-dibenyloxynaphthalene, 1,2-bis(phenoxy)ethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, 1,4-bis(phenoxy)butane, 1,4-bis(phenoxy)-2-butene, 1,2-bis(4-methoxyphenylthio)ethane, dibenzoyl methane, 1,4-bis(phenylthio)butane, 1,4-bis(phenylthio)-2-butene, 1,2-bis(4-methoxyphenylthio)ethane, 1,3-bis(2-vinyloxyethoxy)benzene, 1,4-bis(2-vinyloxyethoxy)benzene, p-(2-vinyloxyethoxy) biphenyl, p-aryloxy biphenyl, p-propynoxy biphenyl, dibenzoyloxymethane, 1,3-dibenzoyloxypropane, dibenzyl disulfide, 1,1-diphenyl ethanol, 1,1-diphenyl propanol, p-(benzyloxy)benzyl alcohol, 1,3-diphenoxy-2-propanol, N-octadecylcarbamoyl-p-methoxycarbonylbenzene, N-octadecylcarbamoylbenzene, dibenzyl oxalate, and 1,5-bis(p-methoxyphenoxy)-3-oxapentane. The above-listed examples can be used alone or in combination.

[0074] ((adhesive resin))

[0075] The heat-sensitive recording layer can include a binding resin. The binding resin is not particularly limited and can be appropriately selected depending on the intended purpose. Examples of the binding resin include: water-soluble polymers such as polyvinyl alcohol resin, starch or starch derivatives, cellulose derivatives (such as hydroxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose), sodium polyacrylate, polyvinylpyrrolidone, acrylamide-acrylate copolymer, acrylamide-acrylate-methacrylate terpolymer, alkali metal salt of styrene-maleic anhydride copolymer, alkali metal salt of isobutylene-maleic anhydride polymer, polyacrylamide, sodium alginate, gelatin, and casein; emulsions such as polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylate, vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, and ethylene-vinyl acetate copolymer; and latex such as styrene-butadiene copolymer and styrene-butadiene-acrylate copolymer. The above-listed examples can be used alone or in combination. Among the above-listed examples, the binding resin is particularly preferably styrene-butadiene copolymer (SBR) in order to improve water resistance.

[0076] ((Other Components)

[0077] The heat-sensitive recording layer can further include other components as needed. Examples of the above-mentioned other components include auxiliary additives, surfactants, lubricants, and fillers.

[0078] -Auxiliary Additives-

[0079] As the auxiliary additive, for example, various hindered phenol compounds or hindered amine compounds, which are electron-accepting compounds, but exhibit relatively low coloring ability, can be added. Specific examples of the auxiliary additive include 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidenebis(6-tert-butyl-2-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 4,4'-thiobis(6-tert-butyl-2-methylphenol), tetrabromobisphenol A, tetrabromobisphenol S, 4,4-thiobis(2-methylphenol), 4,4'-thiobis(2-chlorophenol), tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate, and tetrakis(1,2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate.

[0080] -Lubricants-

[0081] Examples of the lubricant include higher fatty acids and metal salts of higher fatty acids, higher fatty acid amides, higher fatty acid esters, animal waxes, vegetable waxes, mineral waxes, and petroleum waxes.

[0082] -Fillers-

[0083] Examples of the filler include inorganic powders such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clay, kaolin, talc, surface-treated calcium and surface-treated silica; and organic powders such as urea-formaldehyde resin, styrene-methacrylic acid copolymer, polystyrene resin and vinylidene chloride resin.

[0084] (Method for producing thermosensitive recording layer)

[0085] The method for producing the thermosensitive recording layer is not particularly limited. The thermosensitive recording layer can be formed by any method generally used in the related art.

[0086] The method for producing a heat-sensitive recording layer includes, for example, grinding and dispersing a leuco dye, a developer, a binder resin, and other components using a disperser (e.g., a ball mill, a grinder, or a sand mill) until the diameter of the dispersed particles reaches a range of 0.1 μm (micrometer) to 3 μm (micrometer), and then mixing the resulting mixture with a sensitizer and an optional filler to prepare a heat-sensitive recording layer coating liquid. The developer includes a first developer and a second developer, wherein the first developer is a urea compound represented by general formula (I), and the second developer is at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV). The resulting heat-sensitive recording layer coating liquid is applied to a support and then dried to form the heat-sensitive recording layer.

[0087] The coating method is not particularly limited and can be appropriately selected according to the intended purpose. Examples of coating methods include blade coating, gravure coating, gravure offset coating, rod coating, roll coating, knife coating, air knife coating, comma coating, U-comma coating, AKKU coating, smooth coating, micro gravure coating, reverse roll coating, four-roll or five-roll coating, dip coating, single-layer curtain coating, simultaneous multi-layer curtain coating, slide coating, and die coating.

[0088] The deposition amount of the thermosensitive recording layer after drying is not particularly limited and can be appropriately selected depending on the intended purpose. The deposition amount of the thermosensitive recording layer after drying is preferably 0.5 g / m 2 Up to 20.0g / m 2 , more preferably 1.0 g / m 2 Up to 10.0g / m 2 .

[0089] (Protective layer)

[0090] The protective layer is directly or indirectly provided on the thermosensitive recording layer. The protective layer comprises a styrene-(meth)acrylic acid copolymer or a styrene-maleic acid copolymer, and may further comprise a crosslinking agent and other components as necessary.

[0091] ((styrene-(meth)acrylic acid copolymer))

[0092] The styrene-(meth)acrylic acid copolymer is not particularly limited, and can be appropriately selected from styrene-(meth)acrylic acid copolymers available in the related art. The styrene-(meth)acrylic acid copolymer can be appropriately synthesized, or can be selected from commercial products.

[0093] In the case of synthesizing the styrene-(meth)acrylic acid copolymer, a copolymer obtained by copolymerizing (meth)acrylic acid, (meth)acrylic ester, and styrene monomers can be used.

[0094] The (meth)acrylic ester is not particularly limited, and can be appropriately selected according to the intended purpose. Monomers or oligomers of (meth)acrylic esters commonly used for UV-curable resins or electron beam-curable resins can be appropriately used. Among the above-listed examples, (meth)acrylic esters having a flexible structure are preferred, and aliphatic compounds are more preferred. Among aromatic compounds, aromatic compounds having a chain structure are preferred. Furthermore, monofunctional monomers or difunctional monomers are more preferred than trifunctional or higher polyfunctional monomers.

[0095] Examples of the (meth)acrylic ester include alkyl group-containing alkyl (meth)acrylate, alkyl group-containing amino (meth)acrylate, glycol di(meth)acrylate, allyl (meth)acrylate, trimethylolpropane tri(meth)acrylate, glycidyl (meth)acrylate, acrylamide, diacetone acrylamide, (meth)acrylonitrile, benzyl (meth)acrylate, dimethylaminoethyl (meth)acrylate chloride methanesulfonate, allyl (meth)acrylate, trimethylolpropane tri(meth)acrylate, and glycidyl (meth)acrylate. The above-listed examples can be used alone or in combination.

[0096] The alkyl group-containing alkyl (meth)acrylate is not particularly limited, and can be appropriately selected according to the intended purpose. For example, alkyl group-containing alkyl (meth)acrylates having 1 to 18 carbon atoms are preferred, and alkyl group-containing alkyl (meth)acrylates having 3 to 15 carbon atoms are more preferred. Specific examples of the alkyl group-containing alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate.

[0097] When the alkyl group is short (the number of carbon atoms in the alkyl group is small), the resulting (meth)acrylic resin can not have sufficient flexibility. When the alkyl group is long, the methylene chains in the side chains can regularly align to reduce the flexibility of the resulting (meth)acrylic resin.

[0098] The alkyl group-containing amino (meth) acrylate is not particularly limited and can be appropriately selected depending on the intended purpose. For example, the alkyl group-containing amino (meth) acrylate is preferably an alkyl group-containing amino (meth) acrylate containing 1 to 5 carbon atoms. Specific examples of the alkyl group-containing amino (meth) acrylate include dimethylaminoethyl (meth) acrylate.

[0099] The diol di(meth) acrylate is not particularly limited and can be appropriately selected depending on the intended purpose. Examples of the diol di(meth) acrylate include ethylene glycol di(meth) acrylate and butanediol di(meth) acrylate.

[0100] Examples of the styrene monomer include vinyl propylenes such as styrene, a-methylstyrene, monochlorostyrene, and dichlorostyrene.

[0101] (styrene-maleic acid copolymer)

[0102] The styrene-maleic acid copolymer is not particularly limited and can be appropriately selected from styrene-maleic acid copolymers available in the related art. The styrene-maleic acid copolymer can be appropriately synthesized or selected from commercial products.

[0103] In the case where the styrene-maleic acid copolymer is a synthetic product, the styrene-maleic acid copolymer is preferably any copolymer obtained by copolymerizing maleic anhydride, maleic acid ester, and a styrene monomer.

[0104] In the case where a commercial product is used as the styrene-maleic acid copolymer, for example, "WC-M-1203" available from ARAKAWA CHEMICAL INDUSTRIES, LTD. can be used.

[0105] The styrene-(meth) acryl copolymers and the styrene-maleic acid copolymers listed above can be used alone or in combination. Among the examples listed above, the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer is particularly preferable in view of the binding ability to the resin contained in the UV flexographic ink.

[0106] The styrene-(meth) acryl copolymers and the styrene-maleic acid copolymers can be water-soluble or emulsion type. Both of these types can achieve the same level of adhesion between the ink and the outermost surface of the resulting heat-sensitive recording medium. However, the use of the emulsion type also has the adverse effect of reducing barrier properties (e.g., resistance to plasticizers and oil resistance). Therefore, the use of the water-soluble type is more preferable.

[0107] The amount of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer in the protective layer is preferably 5 to 50 mass% and more preferably 40 to 60 mass% relative to 100 mass% of the total amount of the solids of the protective layer. When the amount of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer is 5 to 50 mass%, sufficient adhesion between the UV flexographic ink and the outermost surface layer of the resulting thermosensitive recording medium is achieved, and the tackiness of the protective layer is obtained in a low-temperature and low-humidity environment.

[0108] The acid value of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer is preferably 190 to 300 mgKOH / g. When the acid value is 190 to 300 mgKOH / g, sufficient effects are exhibited to achieve the required adhesion between the UV flexographic ink and the outermost surface layer of the resulting thermosensitive recording medium.

[0109] For example, the acid value of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer can be measured according to the test method specified in JIS K0070.

[0110] The mass average molecular weight of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer is preferably 20,000 or more and more preferably 30,000 to 300,000. When the mass average molecular weight is 20,000 or more, the printed image area of the resulting thermosensitive recording medium achieves sufficient resistance to plasticizers.

[0111] ((crosslinking agent)

[0112] The crosslinking agent is not particularly limited and can be appropriately selected depending on the intended purpose. For example, crosslinking agents available in the related art, such as glyoxal, melamine, aziridine compounds, polyamide epichlorohydrin compounds, zirconium carbonate compounds, ethylenediamine, and oxazoline compounds, can be used. In view of water resistance when the thermosensitive recording medium is immersed in water and rubbed with a finger, oxazoline compounds or polyamide epichlorohydrin are particularly suitable.

[0113] The amount of the crosslinking agent can vary depending on the degree of modification of the crosslinking agent having a functional group and the crosslinking agent used. The amount of the crosslinking agent is preferably 0.1 to 1.0 parts by mass and more preferably 0.2 to 0.8 parts by mass relative to 1.0 part by mass of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer.

[0114] ((other components)

[0115] The protective layer can include a filler as the other component. Examples of the filler include: inorganic powders such as aluminum hydroxide, calcium carbonate, silicon dioxide, zinc oxide, titanium oxide, zinc hydroxide, barium sulfate, clay, talc, surface-treated calcium, and surface-treated silicon dioxide; and organic powders such as urea-formaldehyde resin, styrene-methacrylic acid copolymer, and polystyrene resin. Among the above-listed examples, aluminum hydroxide and calcium carbonate are particularly preferred because excellent wear resistance to a thermal head can be achieved when printing is performed for a long time.

[0116] ((Method of forming a protective layer)

[0117] The method of forming a protective layer is not particularly limited and can be appropriately selected depending on the intended purpose. Examples of the method of forming a protective layer include blade coating, gravure coating, gravure offset coating, bar coating, roll coating, knife coating, air knife coating, comma coating, U-comma coating, AKKU coating, smooth coating, microgravure coating, reverse roll coating, four- or five-roll coating, dip coating, single-layer curtain coating, simultaneous multi-layer curtain coating, slide coating, and die coating. Among the above-listed examples, simultaneous multi-layer curtain coating is particularly preferred from the viewpoint of coating efficiency.

[0118] The dry basis coating amount of the protective layer is not particularly limited and can be appropriately selected depending on the intended purpose. The coating amount of the protective layer is preferably 0.5 g / m 2 to 5.0 g / m 2 , more preferably 1.5 g / m 2 to 3.5 g / m 2 . When the dry basis coating amount of the protective layer is 0.5 g / m 2 to 5.0 g / m 2 , the protective layer has a sufficient function to reduce the discoloration of a printed image due to external chemical substances such as oil, plasticizer, and water, and the protective layer can improve the function of the protective layer such as color development sensitivity.

[0119] ((Other layer)

[0120] The other layer included in the thermal recording medium of the present disclosure is not particularly limited and can be appropriately selected depending on the intended purpose. As the above other layer, for example, a backing layer, a primer layer, an intermediate layer, and / or an adhesive layer can be used.

[0121] ((Backing layer)

[0122] A backing layer can be provided on the side of the support on which the thermal recording layer is not provided, as needed. The backing layer includes a filler and a binding resin, and can further include other components such as a lubricant and a color pigment, as needed.

[0123] As the filler, for example, an inorganic filler or an organic filler can be used.

[0124] Examples of the inorganic filler include carbonates, silicates, metal oxides, and sulfuric acid compounds.

[0125] Examples of the organic filler include silicone resins, celluloses, epoxy resins, nylon resins, phenol resins, polyurethane resins, urea resins, melamine resins, polyester resins, polycarbonate resins, styrene resins, acrylic resins, polyethylene resins, formaldehyde resins, and polymethyl methacrylate resins.

[0126] The adhesive resin is not particularly limited, and can be appropriately selected depending on the intended purpose. For example, the same adhesive resin as that of the heat-sensitive recording layer can be used.

[0127] The average thickness of the backing layer is not particularly limited, and can be appropriately selected depending on the intended purpose. The average thickness of the backing layer is preferably 0.1 μm (micrometer) to 20 μm (micrometer), more preferably 0.3 μm (micrometer) to 10 μm (micrometer).

[0128] ((undercoat layer))

[0129] The undercoat layer can be provided between the support and the heat-sensitive recording layer. The undercoat layer is not particularly limited, and can be appropriately selected depending on the intended purpose. The undercoat layer preferably includes an adhesive resin and hollow thermoplastic resin particles, and can further include other components as necessary.

[0130] Each of the hollow thermoplastic resin particles has a shell formed of a thermoplastic resin, and includes air or another gas inside the shell. The hollow thermoplastic resin particles can be microporous particles including cells filled with air or a gas.

[0131] The average particle diameter (average particle size) of the hollow thermoplastic resin particles is not particularly limited, and can be appropriately selected depending on the intended purpose. The average particle diameter of the hollow thermoplastic resin particles is preferably 0.2 μm (micrometer) to 20 μm (micrometer), more preferably 2 μm (micrometer) to 5 μm (micrometer). When the average particle diameter of the hollow thermoplastic resin particles is 0.2 μm (micrometer) or more, it is technically possible to form hollow thermoplastic resin particles each having a hollow structure, and thus it is possible to sufficiently exhibit the function as an undercoat layer. When the average particle diameter of the hollow thermoplastic resin particles is 20 μm (micrometer) or less, it is possible to achieve the desired surface smoothness of the resulting undercoat layer after coating and drying, and thus it is possible to uniformly coat the heat-sensitive recording layer without increasing the amount of the heat-sensitive recording layer coating liquid used to achieve uniform coating. Therefore, the average particle diameter of the hollow thermoplastic resin particles preferably has a monodisperse distribution peak, and falls within the above range.

[0132] The average particle diameter of the hollow thermoplastic resin particles is an arithmetic average calculated from the shape of the hollow thermoplastic resin particles. The average particle diameter is a volume average of the effective diameter. The average particle diameter is, for example, a median diameter determined in the following manner. The particle size distribution of the hollow thermoplastic resin particles is determined by laser diffraction / scattering or dynamic light scattering to obtain a particle size distribution curve, and the particle diameter at 50% (median diameter) based on the cumulative volume distribution from the smallest particle size is determined as the average particle diameter.

[0133] The hollow ratio of the hollow thermoplastic resin particles is not particularly limited and can be appropriately selected depending on the intended purpose. The hollow ratio is preferably 50% to 95%, more preferably 80% to 95%. When the hollow ratio is 30% or more, sufficient heat insulation performance is achieved so that the heat energy applied from the thermal head is desirably maintained without being released from the thermal recording medium via the support, thereby improving the sensitivity of the thermal recording medium. The hollow ratio is the ratio between the outer diameter of the hollow particle and the inner diameter (diameter of the hollow portion), represented by the following equation.

[0134] Hollow ratio [%] = (inner diameter of hollow particle / outer diameter of hollow particle) x 100

[0135] As described above, the hollow thermoplastic resin particles have a shell formed of a thermoplastic plastic resin. The thermoplastic resin is not particularly limited and can be appropriately selected depending on the intended purpose. Examples of the thermoplastic resin include styrene-acrylic acid resins, polystyrene resins, acrylic resins, polyethylene resins, polypropylene resins, polyacetal resins, chlorinated polyether resins, polyvinyl chloride resins, and copolymer resins including vinylidene chloride and acrylonitrile as main components. Among the above-listed examples, styrene-acrylic acid resins and copolymer resins including vinylidene chloride and acrylonitrile as main components are preferred in view of high hollow ratio, monodisperse particle size, and suitability for blade coating.

[0136] Furthermore, the thermoplastic resin is not particularly limited and can be appropriately selected depending on the intended purpose. Examples of the thermoplastic resin include phenol formaldehyde resins, urea-formaldehyde resins, melamine-formaldehyde resins, furan resins, unsaturated polyester resins synthesized by addition polymerization, and crosslinked MMA resins.

[0137] The coating amount of the hollow thermoplastic resin particles is not particularly limited and can be appropriately selected depending on the intended purpose. The coating amount of the hollow thermoplastic resin particles is preferably 0.1 to 3 g per 1 m2of the support in view of sensitivity and uniform coating. 2 The support is 1 g to 3 g. When the coating amount of the hollow thermoplastic resin particles is 0.1 to 3 g per 1 m2of the support, 2 The support is 1 g or more. When the coating amount of the hollow thermoplastic resin particles is 0.1 to 3 g per 1 m2of the support, 2 The support is 3 g or less. When the coating amount of the hollow thermoplastic resin particles is 0.1 to 3 g per 1 m2of the support, the desired binding ability of the resulting primer layer can be achieved.

[0138] ((intermediate layer))

[0139] For example, an intermediate layer can be provided between the protective layer and the heat-sensitive recording layer. The intermediate layer generally includes at least a binder, and can further include an inorganic filler and a surfactant.

[0140] The binder of the intermediate layer (one layer or two or more layers) is not particularly limited, and can be appropriately selected depending on the intended purpose. In the case where two or more intermediate layers are provided, the same binder can be used in all of the intermediate layers, or different binders can be used in different intermediate layers. Examples of the binder for the intermediate layer include polyvinyl alcohol, modified polyvinyl alcohol, starch and starch derivatives, cellulose derivatives, polyvinylpyrrolidone, polyvinyl imidazole, sodium alginate, gelatin, casein, and acrylic acid binders.

[0141] Further, a hydrophobic resin can be used as the adhesive resin of the intermediate layer. The hydrophobic resin used as the binder of the intermediate layer can be provided in the form of an emulsion or a water-soluble form. Examples of the hydrophobic resin used as the binder of the intermediate layer include urethane resins, epoxy resins, vinyl acetate (co)polymers, vinylidene chloride (co)polymers, vinyl chloride (co)polymers, and styrene-butadiene copolymers. The adhesive resin of the intermediate layer is particularly preferably polyvinyl alcohol or modified polyvinyl alcohol.

[0142] The thickness of the intermediate layer is preferably 0.2 pm (micrometers) to 10 pm (micrometers), and more preferably 0.5 pm (micrometers) to 5 pm (micrometers). When two or more intermediate layers are provided, the total thickness of all of the intermediate layers on a dry basis is preferably 5 pm (micrometers) or less.

[0143] ((adhesive layer))

[0144] For example, the adhesive layer can be provided on the side of the heat-sensitive recording layer opposite to the side on which the protective layer is formed.

[0145] The material of the adhesive layer is not particularly limited, and can be appropriately selected depending on the intended purpose. Examples of the material of the adhesive layer include urea resins, melamine resins, phenol resins, epoxy resins, vinyl acetate resins, vinyl acetate-acrylic acid copolymers, ethylene-vinyl acetate copolymers, acrylic resins, polyvinyl ether resins, vinyl chloride-vinyl acetate copolymers, polystyrene resins, polyester resins, polyurethane resins, polyamide resins, chlorinated polyolefin resins, polyvinyl butyral resins, acrylate copolymers, methacrylate copolymers, natural rubber, cyanoacrylate resins, and silicone resins. The above-listed examples can be used alone or in combination.

[0146] For example, in typical use of the heat-sensitive recording medium of the present disclosure, the adhesive layer helps to adhere the heat-sensitive recording medium to a food packaging. Since the heat-sensitive recording medium of the present disclosure can include an adhesive surface provided on the carrier or backing layer, the heat-sensitive recording medium serves as an effective recording medium for providing a label having an adhesive layer. The adhesive layer can be provided with a releasable liner, which is removed before adhering the heat-sensitive recording medium to an article having a label. In addition, the adhesive layer can have an anti-charging property.

[0147] The method of forming the adhesive layer is not particularly limited. As the method of forming the adhesive layer, a typical coating method or a lamination method can be used.

[0148] The average thickness of the adhesive layer is not particularly limited and can be appropriately selected according to the intended purpose. The average thickness of the adhesive layer can be from 0.1 μm (micrometer) to 20 μm (micrometer).

[0149] The method for producing the heat-sensitive recording medium of the present disclosure is not particularly limited. The heat-sensitive recording medium can be produced by any method available in the related art. The method includes forming a heat-sensitive recording layer, forming a protective layer, and laminating the heat-sensitive recording layer and the protective layer. The formation of the heat-sensitive recording layer and the formation of the protective layer can be performed by the above-described methods. The lamination of the heat-sensitive recording layer and the protective layer is not particularly limited as long as the protective layer is laminated directly or indirectly on the heat-sensitive recording layer. The method of laminating the heat-sensitive recording layer and the protective layer is not particularly limited.

[0150] In view of cost efficiency and improvement in coating uniformity, the method for producing the heat-sensitive recording medium of the present disclosure is preferably a method of coating the heat-sensitive recording layer and other layers (such as the protective layer) by simultaneous multi-layer curtain coating, or a method of providing at least one intermediate layer between the heat-sensitive recording layer and other layers, and coating the above-described layers by simultaneous multi-layer curtain coating.

[0151] As described above, the thermal recording medium of the present disclosure includes a support, a thermal recording layer, and a protective layer. The thermal recording layer includes a color developer, the color developer including a first color developer and a second color developer, wherein the first color developer is a urea compound represented by general formula (I), and the second color developer is at least one of a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV). The protective layer includes a resin, the resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer. In the thermal recording medium of the present embodiment, the first color developer and the second color developer are used in combination as the color developer in the thermal recording layer, and the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer is used in the protective layer. Since the thermal recording medium of the above-described embodiment is printed by using UV curable ink for flexographic printing, the adhesion between the printing ink and the protective layer, which is the outermost surface layer of the thermal recording medium, is increased to minimize the peeling of the ink when an external force is applied. Therefore, the thermal recording medium can achieve excellent adhesion between the surface of the protective layer (the outermost surface) of the thermal recording medium and the UV curable ink printed on the outermost surface layer by UV flexographic printing, and can achieve excellent plasticizer resistance of the printed area.

[0152] The molecular weight of the copolymer in the protective layer of the thermal recording medium of the present disclosure is 30,000 or more. The use of the copolymer having a molecular weight of 30,000 or more in the protective layer can improve the plasticizer resistance of the thermal recording medium, and thus can minimize the peeling of the printed area.

[0153] The acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer of the thermal recording medium of the present disclosure is 190 mgKOH / g or more. Since the acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer of the thermal recording medium is 190 mgKOH / g or more, the adhesion of the ink can be improved, and thus the peeling of the printed area can be minimized.

[0154] The amount of the copolymer in the protective layer of the thermal recording medium of the present disclosure is 20 to 100 mass% with respect to 100 mass% of the total amount of the resin component of the protective layer. Since the amount of the copolymer is within the above-described range, the adhesion of the ink can be improved, and the plasticizer resistance can be improved, and thus the peeling of the printed area can be minimized.

[0155] In the heat-sensitive recording layer of the heat-sensitive recording medium of the present disclosure, the mass ratio of the urea compound represented by Structural Formula (II), General Formula (III), or General Formula (IV) to the urea compound represented by General Formula (I) is 0.02 to 2.0. Due to the mass ratio being within the above range, the adhesion of the ink can be improved, the resistance to plasticizers can be improved, and thus the peeling of the printed area can be minimized.

[0156] The heat-sensitive recording medium of the present disclosure further includes a backing layer located on the side of the heat-sensitive recording layer opposite the side on which the support is disposed. The use of a backing layer in the heat-sensitive recording medium can improve the strength and dimensional stability of the heat-sensitive recording medium.

[0157] The heat-sensitive recording medium of the present disclosure further includes a primer layer located between the support and the heat-sensitive recording layer. The use of a primer layer in the heat-sensitive recording medium contributes to the heat-insulating properties of the heat-sensitive recording medium, thereby preventing the thermal energy transferred to the support from being released from the heat-sensitive recording medium through the support to improve the sensitivity of the heat-sensitive recording medium.

[0158] The primer layer of the heat-sensitive recording medium of the present disclosure includes hollow particles. Due to the use of hollow particles in the primer layer, the heat-insulating properties of the heat-sensitive recording medium are reliably exhibited, thereby preventing the thermal energy transferred to the support from being released from the heat-sensitive recording medium through the support to further improve the sensitivity of the heat-sensitive recording medium.

[0159] The heat-sensitive recording medium of the present disclosure further includes a protective layer directly or indirectly on the heat-sensitive recording layer. Due to the provision of the protective layer, the heat-sensitive recording layer of the heat-sensitive recording medium can be protected from external damage to improve the durability of the heat-sensitive recording layer.

[0160] The embodiments of the heat-sensitive recording medium are not particularly limited and can be appropriately selected according to the intended purpose. For example, the heat-sensitive recording medium can be used as a label as is. Alternatively, a print layer on which information such as letters, marks, images, bar codes, and QR codes (registered trademark) are printed is provided on the protective layer or the support. Furthermore, the heat-sensitive recording medium can be in the form in which an adhesive layer is provided on the side of the support opposite the side on which the heat-sensitive recording layer is disposed.

[0161] The shape of the heat-sensitive recording medium is not particularly limited and can be appropriately selected according to the intended purpose. Examples of the shape of the heat-sensitive recording medium include labels, sheets, and rolls.

[0162] Specific examples of the embodiments of the heat-sensitive recording medium of the present disclosure will be described below.

[0163] (Heat-sensitive recording label)

[0164] One embodiment of the heat-sensitive recording medium is a heat-sensitive recording label including a support, a heat-sensitive recording layer disposed directly or indirectly on the support, an adhesive layer disposed on the side of the support opposite to the side on which the heat-sensitive recording layer is disposed, and a release liner disposed on the adhesive layer. The heat-sensitive recording label can further include other layers as needed. The adhesive layer can be applied over the entire area of the label or over a partial area of the label.

[0165] (Heat-sensitive recording medium without liner)

[0166] The heat-sensitive recording medium without liner can be used as an embodiment including a release layer (with a release layer) or an embodiment not including a release layer (without a release layer).

[0167] ((Heat-sensitive recording medium with release layer)

[0168] The heat-sensitive recording medium of the present disclosure is a heat-sensitive recording medium without liner in which a release layer is disposed on the outermost layer of the side (surface) on which the heat-sensitive recording layer is disposed with respect to the support, and an adhesive layer is disposed on the opposite side (back) of the side on which the heat-sensitive recording layer is disposed with respect to the support. The heat-sensitive recording medium can further include other layers as needed.

[0169] The release layer is preferably a layer formed using a material having appropriate peelability with respect to the adhesive layer, and is particularly preferably a layer including a siloxane.

[0170] The heat-sensitive recording medium without liner can be handled as a roll prepared by winding the heat-sensitive recording medium in a manner in which the adhesive layer is stacked on the release layer.

[0171] ((Heat-sensitive recording medium without release layer)

[0172] The heat-sensitive recording medium of the present disclosure is a heat-sensitive recording medium without liner including an adhesive layer on the side of the support opposite to the side on which the heat-sensitive recording layer is disposed. The adhesive layer is a heat-sensitive adhesive layer that exhibits tackiness upon heating. The heat-sensitive recording medium without liner can further include other layers as needed.

[0173] The heat-sensitive adhesive layer includes a thermoplastic resin and a hot-melt material, and can further include a tackifier as needed.

[0174] The thermoplastic resin imparts tackiness and adhesion. Since the hot-melt material is a solid at room temperature, the hot-melt material does not impart plasticity to the resin. When the hot-melt material is heated and melted, the melted hot-melt material swells or softens the resin to impart tackiness. The tackifier has a function of increasing tackiness. As the thermoplastic resin, the hot-melt material, and the tackifier, any thermoplastic material, hot-melt material, and tackifier that are generally used can be used.

[0175] (Heat-sensitive magnetic recording paper)

[0176] The heat-sensitive recording medium of the present disclosure is a heat-sensitive magnetic recording sheet including a magnetic recording layer on a side of a support opposite to a side on which a heat-sensitive recording layer is provided. The heat-sensitive magnetic recording sheet can further include other layers as needed.

[0177] The magnetic recording layer can be formed by coating the magnetic recording layer on the support using a magnetic material (e.g., iron oxide and barium ferrite) and a resin (e.g., vinyl chloride resin, urethane resin, and nylon resin). Alternatively, the magnetic recording layer can be formed by vapor deposition or sputtering without using a resin. The magnetic recording layer is preferably formed on a side of the support opposite to the side on which the heat-sensitive recording layer is provided, but the magnetic recording layer can be provided between the support and the heat-sensitive recording layer. The magnetic recording layer can be formed on at least a part of the area of the heat-sensitive recording layer.

[0178] (Recording method)

[0179] The recording method using the heat-sensitive recording medium of the present disclosure is not particularly limited, and can be appropriately selected according to the intended purpose. Examples of the recording method include a recording method using a heat-sensitive head or a laser.

[0180] The shape, structure, and size of the heat-sensitive head are not particularly limited, and can be appropriately selected according to the intended purpose.

[0181] The laser used is not particularly limited, and can be appropriately selected according to the intended purpose. Examples of the laser include a CO2 laser that emits light having a wavelength of 9.3 μm (micrometer) to 10.6 μm (micrometer) and a semiconductor laser.

[0182] ((Use))

[0183] The heat-sensitive recording medium of the present disclosure has high color development sensitivity and achieves high image density, does not contain a phenolic color developer, and has excellent resistance to skin protectants (e.g., hand cream), oil, and heat. For example, the heat-sensitive recording medium can be used in various fields, such as a point-of-sale (POS) system field for fresh foods, packaged foods, or ready-made foods, a copying field (e.g., books and documents), a communication field (e.g., facsimile), a ticket publication field (e.g., ticket vending machines and receipt publication), an aviation field (baggage tags used at airports), or a medical field (e.g., medical containers such as medicine boxes and medicine bottles). Since the heat-sensitive recording medium of the present disclosure has excellent resistance to skin protectants, the heat-sensitive recording medium is particularly preferably used in the medical field.

[0184] ((Article))

[0185] The article of the present disclosure includes an article main body and the heat-sensitive recording medium of the present disclosure provided on the article main body. As the heat-sensitive recording medium, the above-described heat-sensitive recording medium of the present disclosure is appropriately used. The article including the heat-sensitive recording medium of the present disclosure provided on the article main body means that the heat-sensitive recording medium of the present disclosure is adhered, attached, or mounted on the article main body.

[0186] The article of the present disclosure is not particularly limited except that the article includes the heat-sensitive recording medium of the present disclosure. The article can be appropriately selected in accordance with the intended purpose. Examples of the article include packaging materials, wrapping materials, wrapping papers, and containers. Specific examples of the article include packaging materials for fresh foods, packaged foods, ready-made foods, books and documents, and medical containers such as medicine boxes and medicine bottles.

[0187] Figure 1 is a cross-sectional view showing an example of the heat-sensitive recording medium of the present disclosure. As shown in Figure 1 The heat-sensitive recording medium 1 of the present disclosure includes, in order, an adhesive layer 11, a backing layer 12, a carrier 13, a primer layer 14, a heat-sensitive recording layer 15, an intermediate layer 16, and a protective layer 17.

[0188] Figure 2 is a schematic view showing an example of an article including the heat-sensitive recording medium of the present disclosure.

[0189] As described above, the embodiments of the present disclosure have been described. The embodiments are described only as examples, and the present disclosure is not limited to these embodiments. Various changes and modifications can be made to the embodiments without departing from the scope of the present disclosure. The embodiments and the modified embodiments are within the scope of the present disclosure, and within the scope defined by the claims of the present disclosure, and the range equivalent to the present disclosure.

[0190] Examples

[0191] The present disclosure will be described below by way of examples and comparative examples. The examples should not be construed as limiting the scope of the present disclosure.

[0192] <Preparation of each coating liquid>

[0193] (Preparation of primer layer coating liquid)

[0194] The following materials were mixed and stirred to prepare a primer layer coating liquid.

[0195] Hollow particles (copolymer of acrylonitrile, methacrylonitrile, and isobornyl methacrylate, hollow ratio 90%, volume average particle diameter 4.4 μm (micrometers), solid content 33 mass %): 20 parts by mass

[0196] Styrene / butadiene copolymer latex (solid content 47.5 mass %): 20 parts by mass

[0197] 10 mass% of a polyvinyl alcohol aqueous solution (PVA 117, available from Kuraray Co., Ltd.): 20 parts by mass

[0198] Ion-exchanged water: 40 parts by mass (preparation of a thermosensitive recording layer coating liquid)

[0199] Each of the colorless dye dispersion liquid [Liquid A], the primary color developer dispersion liquid [Liquid B], the auxiliary color developer dispersion liquids [Liquid C1] to [Liquid C4], and the sensitizer dispersion liquid [Liquid D] was dispersed and prepared by a sand mill, respectively, so that the average particle diameter of [Liquid A] was 0.5 μm (micrometer), the average particle diameter of [Liquid B] was 1.0 μm (micrometer), the average particle diameters of [Liquid C1] to [Liquid C3] were each 1.0 μm (micrometer), and the average particle diameter of [Liquid D] was 1.0 μm (micrometer).

[0200] Next, a predetermined amount of each of [Liquid A], [Liquid B], any one of [Liquid C1] to [Liquid C3], [Liquid D], and a 10 mass% itaconic acid-modified polyvinyl alcohol aqueous solution was mixed and stirred to prepare a thermosensitive recording layer coating liquid. The amounts of [Liquid A], [Liquid B], any one of [Liquid C1] to [Liquid C3], [Liquid D], and the 10 mass% itaconic acid-modified polyvinyl alcohol aqueous solution in the examples and comparative examples are shown in Tables 1 to 3.

[0201] (Materials of [Liquid A] (colorless dye dispersion liquid))

[0202] Colorless dye (3-dibutylamino-6-methyl-7-anilinofluorane): 20 parts by mass

[0203] 10 mass% of an itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from Kuraray Co., Ltd.): 40 parts by mass

[0204] Surfactant (Newcol 290, available from Nippon Nyukazai Co., Ltd., solid content: 100 mass%): 0.2 parts by mass

[0205] Ion-exchanged water: 40 parts by mass (materials of [Liquid B] (primary color developer dispersion liquid))

[0206] N-[2-(3-phenylureido)phenyl]benzenesulfonamide (first urea compound represented by General Formula (I), NKK-1304, available from Nippon Soda Co., Ltd.): 20 parts by mass

[0207] 10 mass% of an itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from Kuraray Co., Ltd.): 20 parts by mass

[0208] Amorphous silica (MIZUKASIL P527, available from Mizusawa Industrial Chemicals, Ltd.): 15 parts by mass

[0209] Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content: 100% by mass): 0.2 parts by mass

[0210] Ion-exchanged water: 65 parts by mass (material of [Liquid C1] (auxiliary color developer dispersion liquid))

[0211] Second (2-1) urea compound (UU, available from Chemipro Kasei Kaisha, Ltd.) represented by Structural Formula (II): 32 parts by mass

[0212] 10% by mass aqueous solution of a sulfone-modified polyvinyl alcohol (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass

[0213] Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content: 100% by mass): 0.2 parts by mass

[0214] Ion-exchanged water: 36 parts by mass (material of [Liquid C2] (auxiliary color developer dispersion liquid))

[0215] Second (2-2) urea compound (TG-MD, available from Nippon Kayaku Co., Ltd.) represented by General Formula (III): 32 parts by mass

[0216] 10% by mass aqueous solution of a sulfone-modified polyvinyl alcohol (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass

[0217] Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content: 100% by mass): 0.2 parts by mass

[0218] Ion-exchanged water: 36 parts by mass (material of [Liquid C3] (auxiliary color developer dispersion liquid))

[0219] Second (2-3) urea compound (S-176, available from SANKO CO., LTD.) represented by General Formula (IV): 32 parts by mass

[0220] 10 mass% of a sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass

[0221] Surfactant (PD-001, Nissan Chemical Industry Co., Ltd., solid content: 100 mass%) : 0.2 parts by mass

[0222] Ion exchange water: 36 parts by mass (material of [Liquid C4] (auxiliary color developer dispersion liquid))

[0223] Isopropyl (D-8, available from Nippon Soda Co., Ltd.): 32 parts by mass

[0224] 10 mass% of a sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, Nihon Gosei Kako Co., Ltd.): 32 parts by mass

[0225] Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content: 100 mass%) : 0.2 parts by mass

[0226] Ion exchange water: 36 parts by mass (material of [Liquid D] (sensitizer dispersion liquid))

[0227] 1,2-bis(3-methylphenoxy)ethane (KS-232, available from Sanko Co., Ltd.): 10 parts by mass

[0228] 10 mass% of a sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 10 parts by mass

[0229] Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content: 100 mass%) : 0.1 parts by mass

[0230] Ion exchange water: 50 parts by mass (preparation of [Liquid F] (protective layer coating liquid))

[0231] [LIQUID E] was obtained by stirring and dispersing 30 parts by mass of aluminum hydroxide, 30 parts by mass of a 10 mass% aqueous solution of itaconic acid-modified polyvinyl alcohol (25-88KL, available from KURARAY CO., LTD.), and 40 parts by mass of ion exchange water by a sand mill so that the volume average particle diameter of the dispersed particles was 0.5 μm (micrometer).

[0232] Next, the following materials were mixed and stirred in the following manner to prepare a protective layer coating liquid [LIQUID F].

[0233] (Materials of [LIQUID F] (protective layer coating liquid))

[0234] [LIQUID E]: 30 parts by mass

[0235] A 10 mass% aqueous solution of styrene-acrylic acid copolymer resin 1 (JONCRYL 61J, available from BASF SE, acid value 195, molecular weight 12000, diluted with water so that the nonvolatile content was adjusted to 10 mass%): 50 parts by mass

[0236] An oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content 10 mass%): 20 parts by mass

[0237] A montanic acid ester wax dispersion liquid (solid content 30 mass%): 5 parts by mass

[0238] Ion exchange water: 15 parts by mass

[0239] <Production of the thermosensitive recording medium>

[0240] (Example 1)

[0241] (Content of each component contained in the thermosensitive recording layer coating liquid)

[0242] The amount of [LIQUID A] contained in the thermosensitive recording layer coating liquid was 15 parts by mass; the amount of [LIQUID B] contained in the thermosensitive recording layer coating liquid was 54 parts by mass; the amount of [LIQUID C1] contained in the thermosensitive recording layer coating liquid was 8 parts by mass; the amount of [LIQUID D] contained in the thermosensitive recording layer coating liquid was 8 parts by mass; and the amount of the 10 mass% aqueous solution of itaconic acid-modified polyvinyl alcohol contained in the thermosensitive recording layer coating liquid was 15 parts by mass.

[0243] ((Production of the thermosensitive recording medium))

[0244] The primer layer coating liquid was applied to the surface of a paper sheet serving as a support having a basis weight of 60 g / m 2 so that the dry basis deposition amount of the primer layer coating liquid was 3.0 g / m 2The applied primer coating liquid was dried to form a primer coating layer on the support. A heat-sensitive recording layer coating liquid was applied to the primer coating layer so that the dry basis deposition amount of the heat-sensitive recording layer coating liquid was 3.0 g / m2 2 The applied heat-sensitive recording layer coating liquid was dried to form a heat-sensitive recording layer on the primer coating layer. A protective layer coating liquid was applied to the heat-sensitive recording layer so that the dry basis deposition amount of the protective layer coating liquid was 2.5 g / m 2 The applied protective layer coating liquid was dried to form a protective layer on the heat-sensitive recording layer, thereby obtaining a heat-sensitive recording medium.

[0245] Subsequently, the obtained heat-sensitive recording medium was subjected to surface treatment by supercalendering to achieve a surface smoothness of 1500 seconds to 2500 seconds. The obtained heat-sensitive recording medium was sealed in a high-density polyethylene bag and cured in an atmosphere of 40 degrees Celsius for a predetermined period of time.

[0246] (Example 2)

[0247] A heat-sensitive recording medium was produced in the same manner as in Example 1, except that 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 2 (PL-2249, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 180, molecular weight 38000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of the styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F].

[0248] (Example 3)

[0249] A heat-sensitive recording medium was produced in the same manner as in Example 1, except that 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 190, molecular weight 50000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of the styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F].

[0250] (Example 4)

[0251] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that [Liquid C2] was used instead of [Liquid CI] used in the heat-sensitive recording layer coating liquid.

[0252] (Example 5)

[0253] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that [Liquid C3] was used instead of [Liquid CI] used in the heat-sensitive recording layer coating liquid.

[0254] (Example 6)

[0255] A heat-sensitive recording medium was produced in the same manner as in Example 1 except that 10 mass% of a styrene-maleic acid copolymer resin aqueous solution 1 (3500H, available from Polyscope, acid value 300, molecular weight 80000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [liquid F].

[0256] (Example 7)

[0257] A heat-sensitive recording medium was produced in the same manner as in Example 1 except that 10 mass% of a styrene-maleic acid copolymer resin aqueous solution 2 (WC-M-1203, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 240, molecular weight 30000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [liquid F].

[0258] (Example 8)

[0259] A heat-sensitive recording medium was produced in the same manner as in Example 3 except that the following protective layer coating liquid [liquid G] was used instead of the protective layer coating liquid [liquid F].

[0260] ((Materials of the protective layer coating liquid [liquid G])

[0261] [liquid E]: 30 parts by mass

[0262] 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 190, molecular weight 50000, diluted with water to a non-volatile component content of 10 mass%): 10 parts by mass

[0263] 10 mass% of a itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from KURARAY CO., LTD.): 40 parts by mass

[0264] Oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content 10 mass%): 20 parts by mass

[0265] Lignite acid ester wax dispersion liquid (solid content 30 mass%): 5 parts by mass

[0266] Ion exchange water: 15 parts by mass (Example 9)

[0267] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid Cl] in the heat-sensitive recording layer coating liquid was changed to 1 part by mass.

[0268] (Example 10)

[0269] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid Cl] in the heat-sensitive recording layer coating liquid was changed to 66 parts by mass.

[0270] (Example 11)

[0271] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that 8 parts by mass of [Liquid C2] was further added to the heat-sensitive recording layer coating liquid.

[0272] (Example 12)

[0273] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that the protective layer coating liquid [Liquid F] was replaced with the following protective layer coating liquid [Liquid H].

[0274] ((Materials of protective layer coating liquid [Liquid H])

[0275] [Liquid E]: 30 parts by mass

[0276] 10 mass% styrene-acrylic copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 190, molecular weight 50000, diluted with water to adjust the non-volatile component content to 10 mass%): 135 parts by mass

[0277] Oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content 10 mass%): 20 parts by mass

[0278] Lignite acid ester wax dispersion liquid (solid content 30 mass%): 5 parts by mass

[0279] Ion exchange water: 15 parts by mass (Example 13)

[0280] A heat-sensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid Cl] in the heat-sensitive recording layer coating liquid was changed to 33 parts by mass.

[0281] (Comparative Example 1)

[0282] A heat-sensitive recording medium was produced in the same manner as in Example 1 except that 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 190, molecular weight 50000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of the itaconic acid-modified polyvinyl alcohol aqueous solution in the protective layer coating liquid.

[0283] (Comparative Example 2)

[0284] A heat-sensitive recording medium was produced in the same manner as in Comparative Example 1 except that 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 190, molecular weight 50000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of the itaconic acid-modified polyvinyl alcohol aqueous solution in the protective layer coating liquid.

[0285] (Comparative Example 3)

[0286] A heat-sensitive recording medium was produced in the same manner as in Example 3 except that [liquid C4] was used instead of [liquid C1] in the heat-sensitive recording layer coating liquid.

[0287] (Comparative Example 4)

[0288] A heat-sensitive recording medium was produced in the same manner as in Example 1 except that the protective layer coating liquid [liquid F] used in Comparative Example 1 was used instead of the protective layer coating liquid [liquid F].

[0289] (Comparative Example 5)

[0290] A heat-sensitive recording medium was produced in the same manner as in Example 1 except that 10 mass% of a styrene-acrylic acid copolymer resin aqueous solution 4 (WC-M-1212, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value 130, molecular weight 30000, diluted with water to adjust the non-volatile component content to 10 mass%) was used instead of 10 mass% of the styrene-acrylic acid copolymer resin aqueous solution 1 in the protective layer coating liquid [liquid F].

[0291] The materials used as the first color developing agent and the second color developing agent, and the ratio between the amount of the first color developing agent and the amount of the second color developing agent (second color developing agent / first color developing agent) included in the heat-sensitive recording layer coating liquid as the non-phenolic color developing agent are listed in Tables 1 to 3. The material used to form the protective layer, the form of the material, the acid value, and the molecular weight, and the ratio of the amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer with respect to the total amount of the solids of the protective layer are listed in Tables 1 to 3. The ratio of the amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer with respect to the total amount of the solids of the protective layer is calculated by dividing the solid amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer by the total amount of the solids of the protective layer.

[0292] In Tables 1 to 3, as the materials used as the first color developing agent and the second color developing agent, N-[2-(3-phenylureido)phenyl]benzenesulfonamide (the first urea compound represented by general formula (I)) is represented as “NKK”, the second (2-1) urea compound represented by structural formula (II) is represented as “UU”, the second (2-2) urea compound represented by general formula (III) is represented as “TG-MD”, the second (2-3) urea compound represented by general formula (IV) is represented as “S-176”, and isopropyl is represented as “D-8”.

[0293] <Performance Evaluation>

[0294] Next, the performance of each of the heat-sensitive recording media produced was evaluated. As the performance of the heat-sensitive recording media, the ink adhesion of UV flexographic printing and the plasticizer resistance were evaluated. The evaluation results of the two performances are listed in Tables 1 to 3.

[0295] (Ink adhesion of UV flexographic printing)

[0296] ((UV flexographic printing))

[0297] A solid image was printed using an ink (SICURA Nutriflex 10 Yellow, available from Siegwerk Druckfarbe AG & Co. KGaA) by a flexographic printing machine (product name: RK FlexiProof 100-UV, available from RK PRINT COATING INSTRUMENTS) so that the deposition amount of the ink was 5 cm 3 / m 2 and then the formed image was subjected to UV irradiation under the following UV irradiation conditions to cure the ink to perform UV flexographic printing.

[0298] ((UV irradiation conditions))

[0299] UV irradiation device: TOSURE 2000 (Model: KUV-20261-1X), available from TOSHIBA DENZAI CO., LTD.

[0300] UV irradiation conditions: maximum emission state (10 A to 12 A, as measured by an ammeter), irradiation at a speed of 5 m / min for 5 times ((ink adhesion test))

[0301] After the above printing on the thermosensitive recording medium, a glassine tape (CT18, available from Nichiban Co., Ltd.) having a width of 18 mm was adhered to the thermosensitive recording medium along the printing direction. During the tape adhering, the tape was sufficiently pressed with a finger so that air would not be trapped under the tape. The tape was peeled from the thermosensitive recording medium in the following peeling step, and the peeling state of the printed ink was evaluated according to the following ink adhesion grade.

[0302] NICHIBAN CO., LTD. obtained) was adhered to the thermosensitive recording medium along the printing direction. During the tape adhering, the tape was sufficiently pressed with a finger so that air would not be trapped under the tape. The tape was peeled from the thermosensitive recording medium in the following peeling step, and the peeling state of the printed ink was evaluated according to the following ink adhesion grade.

[0303] - Peeling Step -

[0304] First Step: Slowly peeled from an angle of 180 degrees

[0305] Second Step: Slowly peeled from an angle of 90 degrees

[0306] Third Step: Quickly peeled from an angle of 90 degrees

[0307] - Ink Adhesion Grade -

[0308] A: No peeling was observed in the printed area.

[0309] B: Peeling of the ink occurred in the third step.

[0310] C: Peeling of the ink occurred in the second step.

[0311] D: Peeling of the ink occurred in the first step.

[0312] (Anti-Plasticizer Resistance)

[0313] The anti-plasticizer resistance was evaluated by a hot gradient tester (available from Toyo Seiki Seisaku-sho, Ltd.) at 2 kg / cm 2The printed thermal sensitive recording medium was left for 1 second, where the thermal module was heated at a temperature to reach the saturation density of the pre-test image sample of the thermal sensitive recording medium to produce the pre-test image sample. The printed density of the pre-test image sample was measured by Macbeth densitometer RD-914. Three pieces of polyvinyl chloride film (available from Shin-Etsu Polymer Co., Ltd.) were placed on top of the pre-test image sample, then a load of 5 kg was applied and left for 15 hours at 50 degrees Celsius. After the standing, the image density was measured by the Macbeth densitometer. The resistance to plasticizer was evaluated based on the residual rate of the image density of the post-test sample relative to the density value of the pre-test image sample according to the following evaluation criteria.

[0314] Image density residual rate [%] = (post-test image density) / (pre-test image density) x 100

[0315] A: Image density residual rate is 85% or more

[0316] B: Image density residual rate is 60% to 84%

[0317] C: Image density residual rate is 30% to 59%

[0318] D: Image density residual rate is 29% or less

[0319] [Table 1]

[0320]

[0321] [Table 2]

[0322]

[0323] [Table 3]

[0324]

[0325] From the results of Table 1 and Table 2, it was found that all of the thermal sensitive recording media of Examples 1 to 13 had excellent results in terms of ink adhesion of UV flexographic printing and resistance to plasticizer, and achieved the performance desired for practical applications. On the contrary, from Table 3, it was found that the thermal sensitive recording media of Comparative Examples 1 to 5 failed to achieve the desired performance at least in terms of ink adhesion of UV flexographic printing, and the results were not desirable.

[0326] It is found from Tables 1 to 3 that, when the heat-sensitive recording media of the Examples each contain, in the heat-sensitive recording layer, as a non-phenolic color developer, a certain benzene sulfonyl amide which is a first urea compound, and a second urea compound having a certain structure other than the benzene sulfonyl amide, and, in the protective layer, a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer. Thus, according to the heat-sensitive recording media of the Examples, it is possible to provide a heat-sensitive recording medium having high image storage stability in the printed area after flexographic printing.

[0327] For example, the embodiments of the present disclosure are as follows.

[0328] <1> A heat-sensitive recording medium comprising:

[0329] a support;

[0330] a heat-sensitive recording layer directly or indirectly provided on the support, the heat-sensitive recording layer including a leuco dye and a color developer; and

[0331] a protective layer directly or indirectly provided on the heat-sensitive recording layer,

[0332] wherein the color developer includes a first color developer and a second color developer, the first color developer including a urea compound represented by general formula (I), and the second color developer including at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV),

[0333] [Chemical Formula 12]

[0334]

[0335] wherein, in general formula (I), R 1 to R 3 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group,

[0336] [Chemical Formula 13]

[0337]

[0338] [Chemical Formula 14]

[0339]

[0340] wherein, in general formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group,

[0341] [Chemical Formula 15]

[0342]

[0343] wherein, in general formula (IV), R 1 to R 5 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group, and

[0344] wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.

[0345] <2> The heat-sensitive recording medium according to <1>,

[0346] wherein a molecular weight of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer is 30000 or more.

[0347] <3> The heat-sensitive recording medium according to <1> or <2>,

[0348] wherein an acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer is 190 mgKOH / g or more.

[0349] <4> The heat-sensitive recording medium according to any one of <1> to <3>,

[0350] wherein an amount of the styrene-(meth)acrylic acid copolymer is 5 to 50 mass% with respect to a total amount of solids of the protective layer.

[0351] <5> The heat-sensitive recording medium according to any one of <1> to <4>,

[0352] wherein, in the heat-sensitive recording layer, a mass ratio of the urea compound represented by structural formula (II), the urea compound represented by general formula (III), or the urea compound represented by general formula (IV) with respect to 1 mass part of the urea compound represented by general formula (I) is 0.02 to 2.0.

[0353] <6> The heat-sensitive recording medium according to any one of <1> to <5>, further comprising:

[0354] a backing layer on a side of the support opposite to a side of the support on which the heat-sensitive recording layer is provided.

[0355] <7> The heat-sensitive recording medium according to any one of <1> to <6>, further comprising:

[0356] a primer layer between the support and the heat-sensitive recording layer.

[0357] <8> The heat-sensitive recording medium according to <7>,

[0358] wherein the primer layer includes hollow particles.

[0359] <9> A method of producing a heat-sensitive recording medium, the method comprising:

[0360] forming a heat-sensitive recording layer directly or indirectly on a support, wherein the heat-sensitive recording layer includes a first color developer and a second color developer, the first color developer including a urea compound represented by general formula (I), and the second color developer including at least one of a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV),

[0361] [Chemical Formula 16]

[0362]

[0363] wherein, in general formula (I), R 1 to R 3 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group,

[0364] [Chemical Formula 17]

[0365]

[0366] [Chemical Formula 18]

[0367]

[0368] wherein, in general formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group,

[0369] [Chemical Formula 19]

[0370]

[0371] wherein, in general formula (IV), R 1 to R 5 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group; and

[0372] forming a protective layer directly or indirectly on the heat-sensitive recording layer, wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.

[0373] <10> The method according to <9>, further comprising:

[0374] A backing layer is formed on the side of the support opposite to the side on which the heat-sensitive recording layer is provided.

[0375] <11> An article comprising:

[0376] An article body; and

[0377] The heat-sensitive recording medium according to any one of <1> to <8> provided on the article body.

[0378] This application is based on and claims priority to Japanese Patent Application No. 2023-034972, filed on March 7, 2023, in the Japan Patent Office, the entire content of which is incorporated herein by reference.

[0379] [Description of Reference Signs]

[0380] 1 Heat-sensitive recording medium

[0381] 2 Packaging material for fresh food

[0382] 11 Adhesive layer

[0383] 12 Backing layer

[0384] 13 Support

[0385] 14 Primer layer

[0386] 15 Heat-sensitive recording layer

[0387] 16 Intermediate layer

[0388] 17 Protective layer

Claims

1. A thermal recording medium, comprising: a support; a thermal recording layer provided directly or indirectly on the support, which includes a leuco dye and a color developer; and a protective layer provided directly or indirectly on the thermal recording layer, wherein the color developer includes a first color developer and a second color developer, the first color developer includes a urea compound represented by general formula (I), and the second color developer includes at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV), [Chemical Formula 1] wherein, in the general formula (I), R 1 to R 3 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group or a C1-C6 fluoroalkyl group, [Chemical Formula 2] [Chemical Formula 3] wherein, in general formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group, [Chemical Formula 4] wherein, in the general formula (IV), R 1 to R 5 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group or a C1-C6 fluoroalkyl group, and wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer. 2.The thermal recording medium according to claim 1, wherein a molecular weight of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer is 30000 or more. 3.The thermal recording medium according to claim 1, wherein an acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer is 190 mgKOH / g or more. 4.The thermal recording medium according to claim 1, wherein an amount of the styrene-(meth)acrylic acid copolymer is 5 to 50 mass% with respect to a total amount of solids of the protective layer. 5.The thermal recording medium according to claim 1, wherein a mass ratio of the urea compound represented by structural formula (II), the urea compound represented by general formula (III), or the urea compound represented by general formula (IV) to 1 mass part of the urea compound represented by general formula (I) in the thermal recording layer is 0.02 to 2.

0. 6.The thermal recording medium according to claim 1, further comprising: a backing layer on a side of the support opposite to a side on which the thermal recording layer is provided. 7.The thermal recording medium according to claim 1, further comprising: a primer layer between the support and the thermal recording layer. 8.The thermal recording medium according to claim 7, wherein the primer layer includes hollow particles. 9.A method of producing a thermal recording medium, the method comprising: forming a thermal recording layer directly or indirectly on a support, wherein the thermal recording layer includes a first color developer and a second color developer, the first color developer includes a urea compound represented by general formula (I), and the second color developer includes at least one selected from a urea compound represented by structural formula (II), a urea compound represented by general formula (III), and a urea compound represented by general formula (IV), [Chemical Formula 5] wherein, in the general formula (I), R 1 to R 3 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group or a C1-C6 fluoroalkyl group, [Chemical Formula 6] [Chemical Formula 7] wherein, in general formula (III), each R is independently a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group, [Chemical Formula 8] wherein, in the general formula (IV), R 1 to R 5 each independently is a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group; and forming a protective layer directly or indirectly on the heat-sensitive recording layer, wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.

10. The method of claim 9, further comprising: forming a backing layer on a side of the support opposite the side on which the heat-sensitive recording layer is disposed.

11. An article comprising: an article body; and the heat-sensitive recording medium of claim 1 disposed on the article body.

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