EP4676749A1

EP4676749A1 - Thermosensitive recording medium, method for producing thermosensitive recording medium, and article comprising the thermosensitive recording medium

Info

Publication number: EP4676749A1
Application number: EP24712606.3A
Authority: EP
Inventors: Daiki IWATA
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2023-03-07
Filing date: 2024-02-28
Publication date: 2026-01-14
Also published as: JP2024126554A; CN120835838A; KR20250151555A; WO2024185628A1

Abstract

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

Description

THERMOSENSITIVE RECORDING MEDIUM, METHOD FOR PRODUCING THERMOSENSITIVE RECORDING MEDIUM, AND ARTICLE COMPRISING THE THERMOSENSITIVE RECORDING MEDIUM

　　　　The disclosures herein generally relate to a thermosensitive recording medium, a method for producing a thermosensitive recording medium, and an article.

　　　　Thermosensitive recording media easily enable recording within a short period at low cost using a relatively simple device. Therefore, thermosensitive recording media have been widely used in various fields, such as the field of point of sale (POS) systems for fresh food products, packaged food products, or ready-made food products, the field of copying (e.g., books and documents), the communication field (e.g., facsimiles), the field of ticket publication (e.g., ticket machines and publication of receipts), the aviation field (luggage tags used in airports), or the medical field (e.g., pill cases and pill bottles).

A thermosensitive recording medium includes an electron-donating dye (leuco dye) and an electron-accepting color developer (color developer). As a color developer, a phenolic color developer including a phenol skeleton (e.g., 4,4'-isopropylidenediphenol) and a color developer that does not include a phenol skeleton (may be referred to as a "non-phenolic color developer" hereinafter) are available. Among the above-listed color developers, thermosensitive recording media using a non-phenolic color developer have been studied to minimize possible environmental and health hazards.

　　　　As a thermosensitive recording medium using a non-phenolic color developer, for example, a thermosensitive recording material including two urea compounds having certain structures, such as N-phenylureido-phenyl-benzenesulfonamide, as an electron-accepting color developer, is disclosed (see, for example, PTL 1).

　　　　The present disclosure aims to provide a thermosensitive recording medium that achieves excellent adhesion between a surface of a productive layer (outermost surface layer) of the thermosensitive recording medium and a UV-curable ink printed on the outermost surface layer by flexographic printing, and achieves excellent plasticizer resistance of the printed area.

Japanese Patent No. 7146147

　　　　In one embodiment, a thermosensitive recording medium includes a support, a thermosensitive recording layer disposed directly or indirectly on the support, and a protective layer disposed directly or indirectly on the thermosensitive recording layer. The thermosensitive 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 the group consisting 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).
In General Formula (I), R¹ to R³ 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.
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.
In General Formula (IV), R¹ to R⁵ 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. The protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.

　　　　The present disclosure can provide a thermosensitive recording medium that achieves excellent adhesion between a surface of a productive layer (outermost surface layer) of the thermosensitive recording medium and a UV-curable ink printed on the outermost surface layer by flexographic printing, and achieves excellent plasticizer resistance of the printed area.

FIG. 1 is a cross-sectional view illustrating an example of the thermosensitive recording medium of the present disclosure. FIG. 2 is a schematic view illustrating an example of an article including the thermosensitive recording medium of the present disclosure.

Mode for Carrying Out the Invention

In the following, embodiments of the present invention will be described with reference to the accompanying drawings. Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. In the present specification, a numerical range specified includes the upper limit and the lower limit of the described range, unless otherwise stated.
　　　　　　
<Thermosensitive recording medium>
　　The thermosensitive recording medium of the present disclosure includes a support, a thermosensitive recording layer directly or indirectly on the support, and a protective layer directly or indirectly on the thermosensitive recording layer in this order. The thermosensitive recording layer includes one or more certain urea compounds as a color developer, and the protective layer includes one or more certain acrylic resins. The thermosensitive recording medium may further include other layers, as necessary.
　　In the present specification, the phrase "a layer A being directly or indirectly disposed on a layer B" or "a layer A being directly or indirectly on a layer B" refers to both an embodiment where no layer is present between the layer A and the layer B, and an embodiment where one or more layers are present between the layer A and the layer B.
(Support)
　　A shape, structure, size, and material of the support of the thermosensitive recording medium of the present disclosure are not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the shape of the support include flat plate shapes and sheet shapes. The structure of the support may be a single layer structure or a laminate structure. The size of the support may be appropriately selected according to a size of the thermosensitive recording medium.
　　　　As the support, for example, as well as typical paper, synthetic paper and plastic films, such as polyethylene, transparent polyethylene terephthalate, polypropylene, and vinyl chloride, may be used. In the case where a plastic film is used as the support, a surface treatment, such as matte processing and corona processing, may be performed on the surface of the support to improve fixability of a coating liquid. Among the above-listed examples, an oriented polyethylene terephthalate sheet is preferred because the oriented polyethylene terephthalate sheet has excellent strength, heat resistance, and dimensional stability. Moreover, the support may be a white opaque film obtained by adding a white constituent material or white fillers to a plastic film material to form a film, or the support may be a foam sheet obtained by foaming. The support may be a laminate including any combination of the above-listed materials. Examples of the laminate include a laminate of cellulose fibers and synthetic paper, a laminate of cellulose fibers and a plastic film, and a laminate of a plastic film and synthetic paper.
　　　　When the thermosensitive recording medium is used on a packaging material of a food product, the support is preferably a clear film because 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 haze (cloudiness), which is an index associated with transparency of films, is approximately 10% or less. For example, the haze of the clear film is preferably 5% or less.
　　　　An average thickness of the support is appropriately selected according to the intended purpose. In view of transparency and easiness of processing, the average thickness of the support is preferably from 3 μm (micrometers) to 300 μm (micrometers). When the average thickness of the support is from 3 μm (micrometers) to 300 μm (micrometers), the support has adequate rigidness that makes processing of the support easy, while retaining desired strength and transparency.
　　　　　　
(Thermosensitive recording layer)
　　The thermosensitive recording layer of the thermosensitive recording medium of the present disclosure includes a leuco dye and a color developer, where 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 the group consisting 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).
((Leuco dye))
The leuco dye is not particularly limited, and may be appropriately selected from leuco dyes typically used in thermosensitive 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, auramine-based dyes, spiropyran-based dyes, and indolinophthalide-based dyes.
　　　　The leuco dye is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the leuco dye include 3,3-bis(p-dimethylaminophenyl)-phthalide, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (synonym: crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)-6-diethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)-6-chlorophthalide, 3,3-bis(p-dibutylaminophenyl)phthalide, 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)xanthyl benzoic acid lactam}, 3-diethylamino-6-methyl-7-(m-trichloromethylalinino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-di-n-butylamino-7-o-chloroanilino)fluoran, 3-N-methyl-N,n-amylamino-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-dibenzylamino)fluoran, benzoyl leuco methylene blue, 6'-chloro-8'-methoxy-benzoindolino-spiropyran, 6'-bromo-3'-methoxy-benzoindolino-spiropyran, 3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-chlorophenyl)phthalide, 3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-nitrophenyl)phthalide, 3-(2'-hydroxy-4'-diethylaminophenyl)-3-(2'-methoxy-5'-methylphenyl)phthalide, 3-(2'-methoxy-4'-dimethylaminophenyl)-3-(2'-hydroxy-4'-chloro-5'-methylphenyl)phthalide, 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-mopholino-7-(N-propyl-trifluoromethylanilino)fluoran, 3-pyrrolidino-7-trifluoromethylanilinofluoran, 3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino)fluoran, 3-pyrrolidino-7-(di-p-chlorophenyl)methylaminofluoran, 3-diethylamino-5-chloro-7-(alpha-phenylethylamino)fluoran, 3-(N-ethyl-p-toluidino)-7-(alpha-phenylethylamino)fluoran, 3-diethylamino-7-(o-methoxycarbonylphenylamino)fluoran, 3-diethylamino-5-methyl-7-(alpha-phenylethylamino)fluoran, 3-diethylamino-7-piperidinofluoran, 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'-dimethylaminophthalide, 3-(N-benzyl-N-cyclohexylamino)-5,6-benz-7-alpha-naphthylamino-4'-bromofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-methyl-7-mesidino-4',5'-benzofluoran, 3-N-methyl-N-isopropyl-6-methyl-7-anilinofluoran, 3-N-ethyl-N-isoamyl-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(2',4'-dimethylanilino)fluoran, 3-mopholino-7-(N-propyl-trifluoromethylanilino)fluoran, 3-pyrrolidino-7-trifluoromethylanilinofluoran, 3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino)fluoran, 3-pyrrolidino-7-(di-p-chlorophenyl)methylaminofluoran, 3-diethylamino-5-chloro-(alpha-phenylethylamino)fluoran, 3-(N-ethyl-p-toluidino)-7-(alpha-phenylethylamino)fluoran, 3-diethylamino-7-(o-methoxycarbonylphenylamino)fluoran, 3-diethylamino-5-methyl-7-(alpha-phenylethylamino)fluoran, 3-diethylamino-7-piperidinofluoran, 2-chloro-3-(N-methyltoluidino)-7-(p-N-butylanilino)fluoran, 3,6-bis(dimethylamino)fluorenespiro(9,3')-6'-dimethylaminophthalide, 3-(N-benzyl-N-cyclohexylamino)-5,6-benzo-7-alpha-naphthylamino-4'-bromofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-N-ethyl-N-(-2-ethoxypropyl)amino-6-methyl-7-anilinofluoran, 3-N-ethyl-N-tetrahydrofurfurylamino-6-methyl-7-anilinofluoran, 3-p-dimethylaminophenyl)-3-{1,1-bis(p-dimethylaminophenyl)ethylen-2-yl}phthalide, 3-(p-dimethylaminophenyl)-3-{1,1-bis(p-dimethylaminophenyl)ethylen-2-yl}-6-dimethylaminophthalide, 3-(p-dimethylaminophenyl)-3-(1-p-dimethylaminophenyl-1-phenylethylen-2-yl)phthalide, 3-(p-dimethylaminophenyl)-3-(1-p-dimethylaminophenyl-1-p-chlorophenylethylen-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-prydinophenyl)ethylen-2-yl}-5,6-dichloro-4,7-dibromophthalide, bis(p-dimethylaminostyryl)-1-napthalenesulfonylmethane, and bis(p-dimethylaminostyryl)-1-p-tolylsulfonylmethane. The above-listed examples may be used alone or in combination.
　　　　A mass of the leuco dye relative to 100% by mass of a total mass of the thermosensitive recording layer may be from 3% by mass to 30% by mass.
　　　　
((Color developer))
The color developer included in the thermosensitive layer of the thermosensitive recording medium of the present disclosure is a non-phenolic color developer. The non-phenolic color developer means a color developer that does not include a phenol skeleton. Since the thermosensitive recording medium of the present disclosure includes a non-phenolic color developer, it is not necessary to use a phenolic color developer, which may function as an endocrine-disrupting chemical. Therefore, the thermosensitive recording medium has excellent environmental friendliness.
　　　　The color developer includes a first color developer and a second color developer, both of which are non-phenolic color developers.
　　　　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.
　　In General Formula (I), R¹ to R³ 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.
　　　　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 the structural formula (1) is a compound represented by General Formula (I), in which R¹, R², and R³ are all hydrogen atoms.
　　　　
　　　　The second color developer includes at least one selected from the group consisting 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 second color developer is preferably composed 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). Moreover, the second color developer may be composed of two or three selected from 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).
　　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.
　　In General Formula (IV), R¹ to R⁵ 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.
　　　　The second color developer preferably includes, as the urea compound represented by General Formula (III), a urea compound represented by Structural Formula (2).
　　　　
　　　　The second color developer preferably includes, as the urea compound represented by General Formula (IV), a urea compound represented by Structural Formula (3).
　　　　
　　　　When the urea compound represented by General Formula (I) is determined as a first urea compound and the urea compound represented by any of Structural Formula (II) and the general formulae (III) and (IV) is determined as a second urea compound, an amount of the second urea compound relative to 1.0 parts by mass of the first urea compound in the thermosensitive recording layer is preferably from 0.02 parts by mass to 2.0 parts by mass, more preferably from 0.1 parts by mass to 1.0 parts by mass, and yet more preferably from 0.25 parts by mass to 0.5 parts by mass.
　　　　An amount of the color developer relative to 1.0 parts by mass of the leuco dye is preferably from 0.5 parts by mass to 10 parts by mass, and more preferably from 1 part by mass to 5 parts by mass.
　　　　The amount of the color developer relative to the leuco dye is the same as described above, 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 with the urea compound represented by General Formula (I) and used in combination as the color developer. A sum of the amount of the urea compound represented by General Formula (I) and the amount of any 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) is preferably within the above-mentioned range. For example, the urea compound represented by General Formula (I) is determined as a first urea compound; the urea compound represented by Structural Formula (II) is determined as a second (2-1) urea compound; the urea compound represented by General Formula (III) is determined as a second (2-2) urea compound; and the urea compound represented by General Formula (IV) is determined as a second (2-3) urea compound. A total amount (parts by mass) of the first urea compound, the second (2-1) urea compound, the second (2-2) urea compound, and the second (2-3) urea compound relative to 1 part by mass of the leuco dye is preferably from 0.5 parts by mass to 10 parts by mass, and more preferably from 1 part by mass to 5 parts by mass.
　　　　The color developer may further include typically used other color developers, as long as the effects obtainable by the thermosensitive recording medium of the present disclosure are not adversely affected. As other color developers, various electron-accepting compounds and oxidants capable of coloring a leuco dye can be suitably used.
　　　　Specific examples of other color developers are not particularly limited, and may be appropriately selected from color developers available from the related art according to the intended purpose. Examples of the above-mentioned other color developers include 4,4'-isopropylidenebisphenol, 4,4'-isopropylidenebis(o-methylphenol), 4,4'-sec-butylidenebisphenol, 4,4'-isopropylidenebis(2-tert-butylphenol), zinc p-nitrobenzoic acid 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-dioxaheptane, 1,5-bis(4-hydroxyphenylthio)-5-oxapentan, monocalcium monobenzyl phthalate, 4,4'-cyclohexylidenediphenol, 4,4'-isopropylidenebis(2-chlorophenol), 4,4'-diphenolsulfone, 4-isopropoxy-4'-hydroxydiphenylsulfone, 4-benzyloxy-4'-hydroxydiphenylsulfone, 4,4'-diphenolsulfoxide, 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, salicylanilide, methyl bis-(4'-hydroxyphenyl)acetate, benzyl bis-(4-hydroxyphenyl)acetate, 1,3-bis(4-hydroxycumyl)benzene, 1,4-bis(4-hydroxycumyl)benzene, 2,4'-diphenolsulfone, 2,2'-diallyl-4,4'-diphenolsulfone, 3,4-dihydroxyphenyl-4'-methyldiphenylsulfone, zinc 1-acetyloxy-2-naphthoate, zinc 2-acetyloxy-1-naphthoate, zinc 2-acetyloxy-3-naphthoate, alpha,alpha-bis(4-hydroxyphenyl)-alpha-methyltoluene, and an antipyrine complex of zinc and thiocyanate. The above-listed examples may be used alone or in combination.
　　　　An amount of additional color developers may be appropriately selected according to the intended purpose, as long as the effects obtainable by the thermosensitive recording medium of the present disclosure are not adversely affected.
　　A total amount of other color developers than the urea compound represented by General Formula (I), 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) is preferably less than 2 parts by mass, and more preferably less than 0.5 parts by mass, relative to 1 part by mass of the leuco dye in the thermosensitive recording layer.
　　　　The thermosensitive recording layer may not substantially include other color developers than the urea compound represented by General Formula (I), 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).
　　　　　　
　　　　((Thermofusible material))
　　The thermosensitive recording layer may include various thermofusible materials as a sensitizer. Examples of the thermofusible 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, terphenyl, triphenylmethane, benzyl p-benzyloxybenzoate, beta-benzyloxynapthalene, phenyl beta-naphthoate, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate, diphenyl carbonate, dibenzyl terephthalate, dimethyl terephthalate, 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 1,4-dibenzyloxynaphthalene, 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, dibenzoylmethane, 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-aryloxybiphenyl, p-propargyloxybiphenyl, dibenzoyloxymethane, 1,3-dibenzoyloxypropane, dibenzyldisulfide, 1,1-diphenylethanol, 1,1-diphenylpropanol, p-(benzyloxy)benzyl alcohol, 1,3-diphenoxy-2-propanol, N-octadecylcarbamoyl-p-methoxycarbonylbenzene, N-octadecylcarbamoylbenzene, dibenzyl oxalate, and 1,5-bis(p-methoxyphenyloxy)-3-oxapentane. The above-listed examples may be used alone or in combination.
((Binder resin))
　　The thermosensitive recording layer may include a binder resin. The binder resin is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the binder resin include: water soluble polymers, such as polyvinyl alcohol resins, starch or derivatives of starch, cellulose derivatives (e.g., hydroxy methyl cellulose, hydroxy ethyl cellulose, carboxy methyl cellulose, methyl cellulose, and ethyl cellulose), sodium polyacrylate, polyvinyl pyrrolidone, acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid terpolymers, alkali salts of styrene-maleic acid anhydride copolymers, alkali salts of isobutylene-maleic acid anhydride copolymers, polyacrylamide, sodium alginate, gelatin, and casein; emulsions, such as polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylate, vinyl chloride-vinyl acetate copolymers, polybutyl methacrylate, and ethylene-vinyl acetate copolymers; and latex, such as styrene-butadiene copolymers and styrene-butadiene-acryl copolymers. The above-listed examples may be used alone or in combination. Among the above-listed examples, the binder resin is particularly preferably a styrene-butadiene copolymer (SBR) for improving water resistance.
((Other components))
　　The thermosensitive recording layer may further include other components, as necessary. Examples of the above-mentioned other components include auxiliary additives, surfactants, lubricants, and fillers.
-Auxiliary additives-
　　As auxiliary additives, for example, various hindered phenol compounds or hindered amine compounds, which are electron-accepting compounds but exhibits a relatively low coloring power, may be added. Specific examples of the auxiliary additives include 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidene bis(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.
-Lubricants-
　　Examples of the lubricants include higher fatty acids and metal salts of higher fatty acids, higher fatty acid amides, higher fatty acid esters, animal wax, vegetable wax, mineral wax, and petroleum wax.
-Fillers-
　　Examples of the fillers include: inorganic powder, 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 powder, such as urea -formalin resins, styrene-methacrylic acid copolymers, polystyrene resins, and vinylidene chloride resins.
(Method for producing thermosensitive recording layer)
　　A method for producing the thermosensitive recording layer is not particularly limited. The thermosensitive recording layer may be formed by any method typically used in the related art.
　　The method for producing the thermosensitive recording layer includes, for example, grinding and dispersing a leuco dye, a color developer, a binder resin, and other components by a disperser (e.g., a ball mill, an attritor, and a sand mill) until diameters of dispersed particles reach the range of from 0.1 μm (micrometers) to 3 μm (micrometers), followed by mixing the resulting mixture with a sensitizer and optional fillers, to prepare a thermosensitive recording layer coating liquid. The color developer includes a first color developer and a second color developer, where the first color developer is a urea compound represented by General Formula (I), and the second color developer is at least one selected from the group consisting 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 obtained thermosensitive recording layer coating liquid is applied onto a support, followed by drying, to form a thermosensitive recording layer.
　　The coating method is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the coating method include blade coating, gravure coating, gravure offset coating, bar coating, roll coating, knife coating, air knife coating, comma coating, U-comma coating, AKKU coating, smoothing coating, microgravure coating, reverse roll coating, four or five roll coating, dip coating, single layer curtain coating, simultaneous multilayer curtain coating, slide coating, and die coating.
　　A deposition amount of the thermosensitive recording layer after drying is not particularly limited, and may be appropriately selected according to the intended purpose. The deposition amount of the thermosensitive recording layer after drying is preferably from 0.5 g/m² to 20.0 g/m², and more preferably from 1.0 g/m² to 10.0 g/m².
(Protective layer)
　　The protective layer is disposed directly or indirectly on the thermosensitive recording layer. The protective layer includes a styrene-(meth)acrylic acid copolymer or a styrene-maleic acid copolymer, may further include a crosslinking agent and other components, as necessary.
((Styrene-(meth)acrylic acid copolymer))
　　The styrene-(meth)acrylic acid copolymer is not particularly limited, and may be appropriately selected from styrene-(meth)acrylic acid copolymers available in the related art. The styrene-(meth)acrylic acid copolymer may be appropriately synthesized, or may be selected from commercial products.
　　In the case where the styrene-(meth)acrylic acid copolymer is synthesized, a copolymer obtained by copolymerizing a (meth)acrylic acid, a (meth)acrylic acid ester, and a styrene monomer may be used.
　　The (meth)acrylic acid ester is not particularly limited, and may be appropriately selected according to the intended purpose. Monomers or oligomers of the (meth)acrylic acid ester typically used for UV curable resins or electron beam-curable resins may be suitably used. Among the above-listed examples, the (meth)acrylic acid ester having a flexible structure is preferred, and an aliphatic compound is more preferred. Among aromatic compounds, an aromatic compound having a chain structure is preferred. Moreover, a monofunctional monomer or bifunctional monomer is more preferred compared to trifunctional or higher polyfunctional monomers.　　
　　Examples of the (meth)acrylic acid ester include alkyl group-containing (meth)acrylic acid alkyl esters, alkyl group-containing amino(meth)acrylic acid esters, glycol di(meth)acrylic acid esters, allyl (meth)acrylic acid esters, trimethylolpropane tri(meth)acrylic acid ester, glycidyl(meth)acrylic acid ester, acrylamide, diacetone acrylamide, (meth)acrylonitrile, benzyl(meth)acrylic acid ester, dimethylaminoethyl(meth)acrylic acid ester methyl chloride salts, allyl(meth)acrylate, trimethylolpropane tri (meth)acrylate, and glycidyl(meth)acrylate. The above-listed examples may be used alone or in combination.
　　The alkyl group-containing (meth)acrylic acid alkyl ester is not particularly limited, and may be appropriately selected according to the intended purpose. For example, the alkyl group-containing (meth)acrylic acid alkyl ester including 1 to 18 carbon atoms is preferred, and the alkyl group-containing (meth)acrylic acid alkyl ester including 3 to 15 carbon atoms is more preferred. Specific examples of the alkyl group-containing (meth)acrylic acid alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl(meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, lauryl (meth)acrylate, and stearyl (meth)acrylate.
　　When the alkyl group is short (the number of carbon atoms in the alkyl group is small), a resulting (meth)acrylic resin may not have adequate flexibility. When the alkyl group is very long, methylene chains in side chain are regularly aligned to reduce flexibility of a resulting (meth)acrylic resin.
　　The alkyl group-containing amino(meth)acrylic acid ester is not particularly limited, and may be appropriately selected according to the intended purpose. For example, the alkyl group-containing amino(meth)acrylic acid ester is preferably an alkyl group-containing amino(meth)acrylic acid ester including 1 to 5 carbon atoms. Specific examples of the alkyl group-containing amino(meth)acrylic acid ester include dimethylaminoethyl(meth)acrylic acid ester.
　　The glycol di(meth)acrylic acid ester is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the glycol di(meth)acrylic acid ester include ethylene glycol di(meth)acrylic acid ester, and butylene glycol di(meth)acrylic acid ester.
　　Examples of the styrene monomer include vinyl allenes, such as styrene, alpha-methylstyrene, monochlorostyrene, and dichlorostyrene.
((Styrene-maleic acid copolymer))
　　The styrene-maleic acid copolymer is not particularly limited, and may be appropriately selected from styrene-maleic acid copolymers available in the related art. The styrene-maleic acid copolymer may be appropriately synthesized, or may be selected from commercial products.
　　In the case where the styrene-maleic acid copolymer is a synthesized product, the styrene-maleic acid copolymer is preferably any of copolymers obtained by copolymerizing maleic acid anhydride, maleic acid ester, and a styrene monomer.
　　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. may be used.
　　The above-listed styrene-(meth)acrylic acid copolymers and styrene-maleic acid copolymers may be used alone or in combination. Among the above-listed examples, a styrene-acrylic acid copolymer or a styrene-maleic acid copolymer is particularly preferred in view of binding ability with resin included in a UV flexographic ink.
　　The styrene-(meth)acrylic acid copolymer and the styrene-maleic acid copolymer may be a water-soluble type or an emulsion type. The both types can achieve the same level of adhesion between an ink and an outermost surface of a resulting thermosensitive recording medium. However, use of the emulsion type also have an adverse effect of reducing barrier properties (e.g., plasticizer resistance and oil resistance). Therefore, the water-soluble type is more preferably used.
　　An amount of the styrene-(meth)acrylic acid copolymer or styrene-maleic acid copolymer in the protective layer is preferably from 5% by mass to 50% by mass, and more preferably from 40% by mass to 60% by mass, relative to 100% by mass of a total amount of solids of the protective layer. When the amount of the styrene-(meth)acrylic acid copolymer or styrene-maleic acid copolymer is from 5% by mass to 50% by mass, adequate adhesion between a UV flexographic ink and an outermost surface layer of a resulting thermosensitive recording medium is achieved, and adhesiveness (sticking) of the protective layer is obtained in a low temperature and low humidity environment.
　　An acid value of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer is preferably from 190 mgKOH/g to 300 mgKOH/g. When the acid value is from 190 mgKOH/g to 300 mgKOH/g, a sufficient effect is exhibited to achieve desired adhesion between a UV flexographic ink and an outermost surface layer of a resulting thermosensitive recording medium.
　　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.
　　A mass average molecular weight of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer is preferably 20,000 or greater, and more preferably from 30,000 to 300,000. When the mass average molecular weight is 20,000 or greater, a printed image area of a resulting thermosensitive recording medium achieves sufficient plasticizer resistance.
((Crosslinking agent))
　　The crosslinking agent is not particularly limited, and may be appropriately selected according to the intended purpose. For example, crosslinking agents available in the related art, such as glyoxal, melamine, aziridine compounds, polyamide epichlorohydrin compounds, zirconium carbonate compounds, ethylene diamine, and oxazoline compounds, may be used. In view of water resistance at the time when a thermosensitive recording medium is rubbed with a finger after being immersed in water, an oxazoline compound or a polyamide epichlorohydrin is particularly suitable.
　　An amount of the crosslinking agent may vary depending on a degree of modification of the crosslinking agent with a functional group, and the crosslinking agent used. The amount of the crosslinking agent is preferably from 0.1 parts by mass to 1.0 parts by mass, and more preferably from 0.2 parts by mass to 0.8 parts by mass, relative to 1.0 parts by mass of the styrene-(meth)acrylic acid copolymer or the styrene-maleic acid copolymer.
((Other components))
　　The protective layer may include fillers as other components. Examples of the fillers include: inorganic powder, such as aluminum hydroxide, calcium carbonate, silica, zinc oxide, titanium oxide, zinc hydroxide, barium sulfate, clay, talc, surface-treated calcium, and surface-treated silica; and organic powder, such as urea-formalin resin, styrene-methacrylic acid copolymers, and polystyrene resins. Among the above-listed examples, aluminum hydroxide and calcium carbonate are particularly preferred because excellent abrasion resistance against a thermal head is achieved when printing is performed over a long period.
((Method for forming protective layer))
　　A method for forming the protective layer is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the method for forming the 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, smoothing coating, microgravure coating, reverse roll coating, four or five roll coating, dip coating, single layer curtain coating, simultaneous multilayer curtain coating, slide coating, and die coating. Among the above-listed examples, simultaneous multilayer curtain coating is particularly preferred in view of coating efficiency.
　　A coating amount of the protective layer on a dry basis is not particularly limited, and may be appropriately selected according to the intended purpose. The coating amount of the protective layer is preferably from 0.5 g/m² to 5.0 g/m², and more preferably from 1.5 g/m² to 3.5 g/m². When the coating amount of the protective layer on a dry basis is from 0.5 g/m² to 5.0 g/m², the protective layer has a sufficient function to reduce color fading of a printed image due to external chemicals, such as oil, plasticizers, and water, and the protective layer can improve a function of the protective layer, such as coloring sensitivity.
(Other layers)
　　Other layers included in the thermosensitive recording medium of the present disclosure are not particularly limited, and may be appropriately selected according to the intended purpose. As the above-mentioned other layers, for example, a backing layer, an undercoating layer, an intermediate layer, and/or an adhesive layer may be used.
((Backing layer))
　　The backing layer may be disposed on a side of the support where the thermosensitive recording layer is not disposed, as necessary. The backing layer includes a filler and a binder resin, and may further include other components, such as a lubricant and a color pigment, as necessary.
　　As the filler, for example, inorganic fillers or organic fillers may be used.
　　Examples of the inorganic fillers include carbonates, silicates, metal oxides, and sulfuric acid compounds.
　　Examples of the organic fillers include silicone resins, cellulose, 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 polymethylmethacrylate resins.
　　The binder resin is not particularly limited, and may be appropriately selected according to the intended purpose. For example, the binder resin identical to the binder resin of the thermosensitive recording layer may be used.
　　　　An average thickness of the backing layer is not particularly limited, and may be appropriately selected according to the intended purpose. The average thickness of the backing layer is preferably from 0.1 μm (micrometers) to 20 μm (micrometers), and more preferably from 0.3 μm (micrometers) to 10 μm (micrometers).
　　　　
((Undercoating layer))
　　The undercoating layer may be disposed between the support and the thermosensitive recording layer. The undercoating layer is not particularly limited, and may be appropriately selected according to the intended purpose. The undercoating layer preferably includes an adhesive resin and hollow thermoplastic resin particles, and may further include other components, as necessary.
　　Each of the hollow thermoplastic resin particles has a shell formed of thermoplastic resin, and includes air or another gas inside the shell. The hollow thermoplastic resin particles may be microporous particles including cells filled with air or a gas.
　　An average particle diameter (mean particle size) of the hollow thermoplastic resin particles is not particularly limited, and may be appropriately selected according to the intended purpose. The average particle diameter of the hollow thermoplastic resin particles is preferably from 0.2 μm (micrometers) to 20 μm (micrometers), and more preferably from 2 μm (micrometers) to 5 μm (micrometers). When the average particle diameter of the hollow thermoplastic resin particles is 0.2 μm (micrometers) or greater, formation of hollow thermoplastic resin particles each having a hollow structure can be technically possible so that a function as the undercoating layer can be adequately exhibited. When the average particle diameter of the hollow thermoplastic resin particles is 20 μm (micrometers) or less, desired smoothness of a surface of a resulting undercoating layer can be achieved after coating and drying, so that a thermosensitive recording layer can be uniformly coated without increasing an amount of a thermosensitive recording layer coating liquid used to achieve a uniform coating. Therefore, the average particle diameter of the hollow thermoplastic resin particles preferably has a monodisperse distribution peak, as well as falling within the above-mentioned range.
　　The average particles diameter of the hollow thermoplastic resin particles is the arithmetic mean calculated from the outer shapes of the hollow thermoplastic resin particles. The average particle diameter is a volume average particle diameter of effective diameters. The average particle diameter is a median diameter determined, for example, in the following manner. A 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 a particle diameter at 50% (median diameter) of the cumulative volume-based distribution from the smallest particle size is determined as the average particle diameter.
　　A hollow ratio of the hollow thermoplastic resin particles is not particularly limited, and may be appropriately selected according to the intended purpose. The hollow ratio is preferably from 50% to 95%, and more preferably from 80% to 95%. When the hollow ratio is 30% or greater, sufficient thermal insulation properties are achieved so that thermal energy applied from a thermal head is desirably retained without releasing from the thermosensitive recording medium via the support so that sensitivity of the thermosensitive recording medium is improved. The hollow ratio is a ratio between an outer diameter and an inner diameter (a diameter of a hollow part) of the hollow particle, and is represented by the following equation.
Hollow ratio [%]=(inner diameter of hollow particle/outer diameter of hollow particle) × 100
　　As described above, the hollow thermoplastic resin particles have shells formed of a thermoplastic resin. The thermoplastic resin is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the thermoplastic resin include styrene-acrylic 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, a styrene-acrylic resin and a copolymer resin including vinylidene chloride and acrylonitrile as main components are preferred in view of a high hollow ratio, monodisperse particle size, and suitability for blade coating.
　　Moreover, the thermoplastic resin is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the thermoplastic resin include phenol-formaldehyde resins, urea -formaldehyde resins, melamine-formaldehyde resins, furan resins, unsaturated polyester resin synthesized by addition polymerization, and crosslinked MMA resins.
　　A coating amount of the hollow thermoplastic resin particles is not particularly limited, and may be appropriately selected according to the intended purpose. In view of sensitivity and uniform coating, the coating amount of the hollow thermoplastic resin particles is preferably from 1 g to 3 g per 1 m² of the support. When the coating amount of the hollow thermoplastic resin particles is 1 g or greater per 1 m² of the support, sufficient sensitivity is achieved. When the coating amount of the hollow thermoplastic resin particles is 3 g or less per 1 m² of the support, desired binding ability of a resulting undercoating layer can be achieved.
((Intermediate layer))
　　For example, the intermediate layer may be disposed between the protective layer and the thermosensitive recording layer. The intermediate layer typically includes at least a binder, and may further include an inorganic filler and a surfactant.
　　The binder of the intermediate layer (one layer or two or more layers) is not particularly limited, and may be appropriately selected according to the intended purpose. In the case where two or more intermediate layers are disposed, the same binder may be used in all the intermediate layers, or different binders may be used in different intermediate layers. Examples of the binder used in the intermediate layer include polyvinyl alcohol, modified polyvinyl alcohol, starch and derivatives of starch, cellulose derivatives, polyvinyl pyrrolidone, polyethylene imide, sodium alginate, gelatin, casein, and acrylic binders.
　　Moreover, a hydrophobic resin may be used as the binder resin of the intermediate layer. The hydrophobic resin used as the binder of the intermediate layer may be provided as 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 binder resin of the intermediate layer is particularly preferably polyvinyl alcohol or modified polyvinyl alcohol.
　　A thickness of the intermediate layer is preferably from 0.2 μm (micrometers) to 10 μm (micrometers), and more preferably from 0.5 μm (micrometers) to 5 μm (micrometers). When two or more intermediate layers are disposed, a total thickness of all the intermediate layers on a dry basis is preferably 5 μm (micrometers) or less.
((Adhesive layer))
　　For example, the adhesive layer may be disposed at a side of the thermosensitive recording layer opposite to the side where the protective layer is formed.
　　A material of the adhesive layer is not particularly limited, and may be appropriately selected according to 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-acryl 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, acrylic acid ester copolymers, methacrylic acid ester copolymers, natural rubber, cyanoacrylate resins, and silicone resins. The above-listed examples may be used alone or in combination.
　　In typical use of the thermosensitive recording medium of the present disclosure, for example, the adhesive layer aids to adhere the thermosensitive recording medium to a package of a food product. Since the thermosensitive recording medium of the present disclosure may include an adhesive surface provided on the support or the backing layer, the thermosensitive recording medium is used as an effective recording medium for providing a label having an adhesive layer. The adhesive layer may be provided with a releasable liner that is removed before adhering the thermosensitive recording medium to an article with a label. Moreover, the adhesive layer may have anti-charging properties.
　　A method for forming the adhesive layer is not particularly limited. As the method for forming the adhesive layer, a typical coating method or laminating method may be used.
　　An average thickness of the adhesive layer is not particularly limited, and may be appropriately selected according to the intended purpose. The average thickness of the adhesive layer may be from 0.1 μm (micrometers) to 20 μm (micrometers).
　　A method for producing the thermosensitive recording medium of the present disclosure is not particularly limited. The thermosensitive recording medium may be produced by any method available in the related art. The method includes formation of a thermosensitive recording layer, formation of a protective layer, and lamination of the thermosensitive recording layer and the protective layer. The formation of the thermosensitive recording layer and the formation of the protective layer may be performed by the above-described methods. The lamination of the thermosensitive recording layer and the protective layer is not particularly limited, as long as the protective layer is directly or indirectly laminated on the thermosensitive recording layer. A method for laminating the thermosensitive recording layer and the protective layer is not particularly limited.
　　The method for producing the thermosensitive recording medium of the present disclosure is preferably a method where a thermosensitive recording layer and other layers, such as a protective layer, are coated by simultaneous multilayer curtain coating, or a method where at least one intermediate layer is disposed between a thermosensitive recording layer and other layers, and the above-mentioned layers are coated by simultaneous multilayer curtain coating, in view of cost efficiency and improvement in uniformity of coating layers.
　　As described above, the thermosensitive recording medium of the present disclosure includes a support, a thermosensitive recording layer, and a protective layer. The thermosensitive recording layer includes a color developer includes a first color developer and a second color developer, where the first color developer is a urea compound represented by General Formula (I) and the second color developer is at least one selected from the group consisting 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 including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer. In the thermosensitive recording medium of the embodiment, the first color developer and the second color developer are used in combination as the color developer in the thermosensitive recording layer, and the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer is used in the protective layer. As the thermosensitive recording medium of the above-described embodiment is printed by flexographic printing using a UV-curable ink, therefore, adhesion between the printed ink and the protective layer, which is the outermost surface layer of the thermosensitive recording medium, is increased to minimize peeling of the ink when external force is applied. Accordingly, the thermosensitive recording medium can achieve excellent adhesion between a surface of the protective layer (an outermost surface) of the thermosensitive recording medium and a UV curable ink printed on the outermost surface layer by UV flexographic printing, and can achieve excellent plasticizer resistance of the printed area.
　　A molecular weight of the copolymer in the protective layer of the thermosensitive recording medium of the present disclosure is 30,000 or greater. Use of the copolymer having molecular weight of 30,000 or greater in the protective layer can enhance plasticizer resistance of the thermosensitive recording medium so that peeling of a printed area can be minimized.
　　An acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer of the thermosensitive recording medium of the present disclosure is 190 mgKOH/g or greater. Since the acid value of the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer of the thermosensitive recording medium is 190 mgKOH/g or greater, adhesion of an ink can be improved so that peeling of a printed area can be minimized.
　　An amount of the copolymer of the protective layer of the thermosensitive recording medium of the present disclosure is from 20% by mass to 100% by mass, relative to 100% by mass of a total amount of the resin component of the protective layer. Since the amount of the copolymer is in the above-mentioned range, adhesion of an ink can be improved and plasticizer resistance can be improved, so that peeling of a printed area can be minimized.
　　A 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) in the thermosensitive recording layer of the thermosensitive recording medium of the present disclosure is from 0.02 to 2.0. Since the mass ratio is within the above-mentioned range, adhesion of an ink can be increased and plasticizer resistance can be improved, so that peeling of a printed area can be minimized.
　　The thermosensitive recording medium of the present disclosure further includes a backing layer at a side of the thermosensitive recording layer opposite to the side where the support is disposed. Use of the backing layer in the thermosensitive recording medium can improve the strength and dimensional stability of the thermosensitive recording medium.
　　The thermosensitive recording medium of the present disclosure further includes an undercoating layer between the support and the thermosensitive recording layer. Use of the undercoating layer in the thermosensitive recording medium contributes to thermal insulation properties of the thermosensitive recording medium so that thermal energy transmitted to the support is prevented from being released from the thermosensitive recording medium via the support to improve sensitivity of the thermosensitive recording medium.
　　The undercoating layer of the thermosensitive recording medium of the present disclosure includes hollow particles. Since the hollow particles are used in the undercoating layer, thermal insulation properties of the thermosensitive recording medium are securely exhibited so that thermal energy transmitted to the support is prevented from being released from the thermosensitive recording medium via the support to further improve sensitivity of the thermosensitive recording medium.
　　The thermosensitive recording medium of the present disclosure further includes a protective layer directly or indirectly on the thermosensitive recording layer. Since the protective layer is disposed, the thermosensitive recording layer of the thermosensitive recording medium can be protected from external damages to improve durability of the thermosensitive recording layer.
　　An embodiment of the thermosensitive recording medium is not particularly limited, and may be appropriately selected according to the intended purpose. For example, the thermosensitive recording medium may be used as it is as a label. Alternatively, a print layer where information of two-dimensional codes, such as letters, marks, images, barcodes, and QR code (registered trademark), is printed is disposed on the protective layer or the support. Moreover, the thermosensitive recording medium may be in a form where an adhesive layer is provided at the side of the support opposite to the side where the thermosensitive recording layer is disposed.
　　A shape of the thermosensitive recording medium is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the shape of the thermosensitive recording medium include labels, sheets, and rolls.
　　Concrete examples of embodiments of the thermosensitive recording medium of the present disclosure will be described hereinafter.
(Thermosensitive recording label)
　　One embodiment of the thermosensitive recording medium is a thermosensitive recording label, which includes a support, a thermosensitive recording layer disposed directly or indirectly on the support, an adhesive layer disposed at a side of the support opposite to the side where the thermosensitive recording layer is disposed, and a release liner disposed on the adhesive layer. The thermosensitive recording label may further include other layers, as necessary. The adhesive layer may be applied over an entire area of the label, or may be applied on a partial area of the label.
(Linerless thermosensitive recording medium)
　　The linerless thermosensitive recording medium may be used as an embodiment including a release layer (with a release layer), or an embodiment that does not include a release layer (without a release layer).
((Thermosensitive recording medium with release layer))
　　The thermosensitive recording medium of the present disclosure is a linerless thermosensitive recording medium, in which a release layer is disposed on an outermost layer at the side (surface) where the thermosensitive recording layer is disposed relative to the support, and an adhesive layer is disposed at an opposite side (back surface) to the side where the thermosensitive recording layer is disposed relative to the support. The thermosensitive recording medium may further include other layers, as necessary.
　　The release layer is preferably a layer formed using a material having suitable releasability against the adhesive layer, and particularly preferably a layer including silicone.
　　The linerless thermosensitive recording medium may be handled as a roll that is prepared by winding up the thermosensitive recording medium in a manner such that the adhesive layer is stacked on the release layer.
((Thermosensitive recording medium without release layer))
　　The thermosensitive recording medium of the present disclosure is a linerless thermosensitive recording medium that includes an adhesive layer at the side of the support opposite to the side where the thermosensitive recording layer is disposed. The adhesive layer is a thermosensitive adhesive layer that exhibits adhesion as heated. The linerless thermosensitive recording medium may further include other layers, as necessary.
　　　　The thermosensitive adhesive layer includes a thermoplastic resin and a thermofusible material, and may further include a tackifier, as necessary.
　　
　　　　The thermoplastic resin imparts tackiness and adhesion. Since the thermofusible material is a solid at room temperature, the thermofusible material does not impart plasticity to a resin. As the thermofusible material is heated and melted, the melted thermofusible material makes the resin swell or soften to impart tackiness. The tackifier has a function of improving tackiness. As the thermoplastic resin, the thermofusible material, and the tackifier, any thermoplastic resin, thermofusible material, and tackifier typically used may be used.
(Thermosensitive magnetic recording paper)
The thermosensitive recording medium of the present disclosure is a thermosensitive magnetic recording sheet that includes a magnetic recording layer at a side of the support opposite to the side where the thermosensitive recording layer is disposed. The thermosensitive magnetic recording sheet may further include other layers, as necessary.
　　　　The magnetic recording layer may be formed by coating the support with the magnetic recording layer using a magnetic material (e.g., iron oxide and barium ferrite) and a resin (e.g., a vinyl chloride resin, a urethane resin, and a nylon resin). Alternatively, the magnetic recording layer may 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 where the thermosensitive recording layer is disposed, but the magnetic recording layer may be disposed between the support and the thermosensitive recording layer. The magnetic recording layer may be formed on at least a partial area of the thermosensitive recording layer.
(Recording method)
　　A recording method using the thermosensitive recording medium of the present disclosure is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the recording method include recording methods using a thermal head or laser.
　　　　A shape, structure, and size of the thermal head are not particularly limited, and may be appropriately selected according to the intended purpose.
　　　　The laser used is not particularly limited, and may be appropriately selected according to the intended purpose. Examples of the laser include CO₂ lasers emitting light having a wavelength of 9.3 μm (micrometers) to 10.6 μm (micrometers), and semiconductor lasers.
((Use))
　　The thermosensitive recording medium of the present disclosure has high coloring sensitivity and achieves high image density, is free from a phenolic color developer, and has excellent resistance to skin protective agents (e.g., hand cream), oil, and heat. For example, the thermosensitive recording medium may be used in various fields, such as the field of point of sale (POS) systems for fresh food products, packaged food products, or ready-made food products, the field of copying (e.g., books and documents), the communication field (e.g., facsimiles), the field of ticket publication (e.g., ticket machines and publication of receipts), the aviation field (luggage tags used in airports), or the medical field (e.g., medical containers, such as pill cases and pill bottles). Since the thermosensitive recording medium of the present disclosure has excellent resistance to a skin protective agent, the thermosensitive recording medium is particularly preferably used for the medical field.
((Article))
　　The article of the present disclosure includes an article main body and the thermosensitive recording medium of the present disclosure disposed on the article main body. As the thermosensitive recording medium, the above-described thermosensitive recording medium of the present disclosure is suitably used. The article including the thermosensitive recording medium of the present disclosure disposed on the article main body means that the thermosensitive recording medium of the present disclosure is adhered to, attached to, or mounted on the article main body.
　　The article of the present disclosure is not particularly limited, except that the article includes the thermosensitive recording medium of the present disclosure. The article may be appropriately selected according to the intended purpose. Examples of the article include packing materials, packaging materials, wrapping paper, and containers. Specific examples of the article include packaging materials for fresh food products, packaged food products, ready-made food products, books, and documents, and medical containers (e.g., pill cases and pill bottles).
　　Fig. 1 is a cross-sectional view illustrating an example of the thermosensitive recording medium of the present disclosure. As illustrated in Fig. 1, the thermosensitive recording medium 1 of the present disclosure includes an adhesive layer 11, a backing layer 12, a support 13, an undercoating layer 14, a thermosensitive recording layer 15, an intermediate layer 16, and a protective layer 17 in this order.
　　Fig. 2 is a schematic view illustrating an example of an article including the thermosensitive recording medium of the present disclosure.
　　As described above, embodiments of the present invention have been described. The embodiments are described merely as examples, and the present invention is not limited to these embodiments. Various variations and modifications may be made to the embodiments without departing from the scope of the present invention. The embodiments and modified embodiments are within the scope of the present invention, as well as being within the scope equivalent to the invention defined in the scope of claims.

Examples

　　The present disclosure will be described hereinafter by way of Examples and Comparative Examples. Examples should not be construed as to limit the scope of the present disclosure.
<Preparation of each coating liquid>
(Preparation of undercoating layer coating liquid)
　　The following materials were mixed and stirred to prepare an undercoating layer coating liquid.
Hollow particles (copolymer of acrylonitrile, methacrylonitrile, and isobornyl methacrylate, hollow ratio of 90%, volume average particle diameter of 4.4 μm (micrometers), solid content of 33% by mass): 20 parts by mass
Styrene/butadiene copolymer latex (solid content of 47.5% by mass): 20 parts by mass
10% by mass polyvinyl alcohol aqueous solution (PVA117, available from): 20 parts by mass
Ion-exchanged water: 40 parts by mass
(Preparation of thermosensitive recording layer coating liquid)
　　A leuco dye dispersion liquid [Liquid A], a main color developer dispersion liquid [Liquid B], auxiliary color developer dispersion liquids [Liquid C1] to [Liquid C4], and a sensitizer dispersion liquid [Liquid D], each including the following materials, were dispersed and prepared, respectively, by a sand grinder so that [Liquid A] had an average particle diameter of 0.5 μm (micrometers), [Liquid B] had an average particle diameter of 1.0 μm (micrometer), [Liquid C1] to [Liquid C3] each had an average particle diameter of 1.0 μm (micrometer), and [Liquid D] had an average particle diameter of 1.0 μm (micrometer).
　　Next, [Liquid A], [Liquid B], any of [Liquid C1] to [Liquid C3], [Liquid D], and a 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution each in the predetermined amount were mixed and stirred to prepare a thermosensitive recording layer coating liquid. The amounts of [Liquid A], [Liquid B], any of [Liquid C1] to [Liquid C3], [Liquid D], and the 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution in each of Examples and Comparative Examples are presented in Tables 1 to 3.
((Materials of [Liquid A] (leuco dye dispersion liquid)))
Leuco dye (3-dibutylamino-6-methyl-7-anilinofluoran): 20 parts by mass
10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from KURARAY CO., LTD.): 40 parts by mass
Surfactant (Newcol 290, available from Nippon Nyukazai Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 40 parts by mass
((Materials of [Liquid B] (main color developer dispersion liquid)))
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
10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from KURARAY CO., LTD.): 20 parts by mass
Amorphous silica (MIZUKASIL P527, available from Mizusawa Industrial Chemicals, Ltd.): 15 parts by mass
Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 65 parts by mass
((Materials of [Liquid C1] (auxiliary color developer dispersion liquid)))
Second (2-1) urea compound represented by Structural Formula (II) (UU, available from Chemipro Kasei Kaisha, Ltd.): 32 parts by mass
10% by mass sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass
Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 36 parts by mass
((Materials of [Liquid C2] (auxiliary color developer dispersion liquid)))
Second (2-2) urea compound represented by General Formula (III) (TG-MD, available from Nippon Kayaku Co., Ltd.): 32 parts by mass
10% by mass sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass
Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 36 parts by mass
((Materials of [Liquid C3] (auxiliary color developer dispersion liquid)))
Second (2-3) urea compound represented by General Formula (IV) (S-176, available from SANKO CO., LTD.): 32 parts by mass
10% by mass sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 32 parts by mass
Surfactant (PD-001, Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 36 parts by mass
((Materials of [Liquid C4] (auxiliary color developer dispersion liquid)))
Isopropyl (D-8, available from Nippon Soda Co., Ltd.): 32 parts by mass
10% by mass sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, Nihon Gosei Kako Co., Ltd.): 32 parts by mass
Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.2 parts by mass
Ion-exchanged water: 36 parts by mass
((Materials of [Liquid D] (sensitizer dispersion liquid)))
1,2-di(3-methylphenoxy)ethane (KS-232, available from Sanko Co., Ltd.): 10 parts by mass
10% by mass sulfone-modified polyvinyl alcohol aqueous solution (Gohseran L-3266, available from Nihon Gosei Kako Co., Ltd.): 10 parts by mass
Surfactant (PD-001, available from Nissan Chemical Industry Co., Ltd., solid content of 100% by mass): 0.1 parts by mass
Ion-exchanged water: 50 parts by mass
(Preparation of [Liquid F] (protective layer coating liquid))
　　By a sand grinder, 30 parts by mass of aluminum hydroxide, 30 parts by mass of a 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from KURARAY CO., LTD.), and 40 parts by mass of ion-exchanged water were stirred and dispersed in a manner that a volume average particle diameter of the dispersed particles was 0.5 μm (micrometers), to thereby obtain [Liquid E].
　　Next, the following materials were mixed and stirred in the following manner to prepare a protective layer coating liquid [Liquid F].
(Materials of [Liquid F](protective layer coating liquid))
[Liquid E]: 30 parts by mass
10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 (JONCRYL 61J, available from BASF SE, acid value of 195, molecular weight of 12,000, diluted with water to adjust nonvolatile content to 10% by mass): 50 parts by mass
Oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content of 10% by mass): 20 parts by mass
Montanic acid ester wax dispersion liquid (solid content of 30% by mass): 5 parts by mass
Ion-exchanged water: 15 parts by mass
<Production of thermosensitive recording medium>
(Example 1)
(Amount of each component included in thermosensitive recording layer coating liquid)
　　An amount of [Liquid A] included in the thermosensitive recording layer coating liquid was 15 parts by mass; an amount of [Liquid B] included in the thermosensitive recording layer coating liquid was 54 parts by mass; an amount of [Liquid C1] included in the thermosensitive recording layer coating liquid was 8 parts by mass; an amount of [Liquid D] included in the thermosensitive recording layer coating liquid was 8 parts by mass; and an amount of the 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution included in the thermosensitive recording layer coating liquid was 15 parts by mass.
((Production of thermosensitive recording medium))
　　The undercoating layer coating liquid was applied onto a surface of paper having a basis weight of 60 g/m² serving as a support so that a deposition amount of the undercoating layer coating liquid on a dry basis was 3.0 g/m². The applied undercoating layer coating liquid was dried to form an undercoating layer on the support. The thermosensitive recording layer coating liquid was applied onto the undercoating layer so that a deposition amount of the thermosensitive recording layer coating liquid on a dry basis was 3.0 g/m². The applied thermosensitive recording layer coating liquid was dried to form a thermosensitive recording layer on the undercoating layer. The protective layer coating liquid was applied onto the thermosensitive recording layer so that a deposition amount of the protective layer coating liquid on a dry basis was 2.5 g/m². The applied protective layer coating liquid was dried to form a protective layer on the thermosensitive recording layer to thereby obtain a thermosensitive recording medium.
　　Subsequently, a surface treatment was performed on the resulting thermosensitive recording medium by supercalendaring to achieve the surface smoothness of from 1,500 seconds to 2,500 seconds. The resulting thermosensitive recording medium was sealed in a highly dense polyethylene bag, and was cured in the atmosphere of 40 degrees Celsius for the predetermined time period.
(Example 2)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass styrene-acrylic acid copolymer resin aqueous solution 2 (PL-2249, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 180, molecular weight of 38,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
(Example 3)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 190, molecular weight of 50,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
(Example 4)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that [Liquid C1] used in the thermosensitive recording layer coating liquid was replaced with [Liquid C2].
(Example 5)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that [Liquid C1] used in the thermosensitive recording layer coating liquid was replaced with [Liquid C3].
(Example 6)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass styrene-maleic acid copolymer resin aqueous solution 1 (3500H, available from Polyscope, acid value of 300, molecular weight of 80,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
(Example 7)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 used in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass styrene-maleic acid copolymer resin aqueous solution 2 (WC-M-1203, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 240, molecular weight of 30,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
(Example 8)
　　A thermosensitive 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 G].
((Materials of protective layer coating liquid [Liquid G]))
[Liquid E]: 30 parts by mass
10% by mass styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 190, molecular weight of 50,000, diluted with water to a nonvolatile component content to 10% by mass): 10 parts by mass
10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution (25-88KL, available from KURARAY CO., LTD.): 40 parts by mass
Oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content of 10% by mass): 20 parts by mass
Montanic acid ester wax dispersion liquid (solid content of 30% by mass): 5 parts by mass
Ion-exchanged water: 15 parts by mass
(Example 9)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid C1] in the thermosensitive recording layer coating liquid was changed to 1 part by mass.
(Example 10)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid C1] in the thermosensitive recording layer coating liquid was changed to 66 parts by mass.
(Example 11)
　　A thermosensitive 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 thermosensitive recording layer coating liquid.
(Example 12)
　　A thermosensitive 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].
((Materials of protective layer coating liquid [Liquid H]))
[Liquid E]: 30 parts by mass
10% by mass styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 190, molecular weight of 50,000, diluted with water to adjust a nonvolatile component content to 10% by mass): 135 parts by mass
Oxazoline-based crosslinking agent (EPOCROS WS-500, available from NIPPON SHOKUBAI CO., LTD., solid content of 10% by mass): 20 parts by mass
Montanic acid ester wax dispersion liquid (solid content of 30% by mass): 5 parts by mass
Ion-exchanged water: 15 parts by mass
(Example 13)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that the amount of [Liquid C1] in the thermosensitive recording layer coating liquid was changed to 33 parts by mass.
(Comparative Example 1)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that [Liquid C1] was not added to the thermosensitive recording layer coating liquid, and the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution 1 (25-88KL, available from KURARAY CO., LTD.).
(Comparative Example 2)
　　A thermosensitive recording medium was produced in the same manner as in Comparative Example 1, except that the 10% by mass itaconic acid-modified polyvinyl alcohol aqueous solution in the protective layer coating liquid was replaced with a 10% by mass styrene-acrylic acid copolymer resin aqueous solution 3 (WC-M-1205, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 190, molecular weight of 50,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
(Comparative Example 3)
　　A thermosensitive recording medium was produced in the same manner as in Example 3, except that [Liquid C1] in the thermosensitive recording layer coating liquid was replaced with [Liquid C4].
(Comparative Example 4)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the protective layer coating liquid [Liquid F] was replaced with the protective layer coating liquid [Liquid F] used in Comparative Example 1.
(Comparative Example 5)
　　A thermosensitive recording medium was produced in the same manner as in Example 1, except that the 10% by mass styrene-acrylic acid copolymer resin aqueous solution 1 in the protective layer coating liquid [Liquid F] was replaced with a 10% by mass styrene-acrylic acid copolymer resin aqueous solution 4 (WC-M-1212, available from ARAKAWA CHEMICAL INDUSTRIES, LTD., acid value of 130, molecular weight of 30,000, diluted with water to adjust a nonvolatile component content to 10% by mass).
　　The materials used as the first color developer and the second color developer included as non-phenolic color developers in the thermosensitive recording layer coating liquid, and the ratio (second color developer/first color developer) between the amount of the first color developer and the amount of the second color developer are presented in Tables 1 to 3. The material for forming the protective layer, the form, acid value, and molecular weight of the material, a ratio of the amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer relative to a total amount of solids of the protective layer are presented in Tables 1 to 3. The ratio of the amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer relative to the total amount of solids of the protective layer is calculated by dividing the solid amount of the styrene-acrylic acid copolymer or the styrene-maleic acid copolymer with the total amount of the solids of the protective layer.
　　In Tables 1 to 3, as materials used as the first color developer and the second color developer, N-[2-(3-phenylureido)phenyl]benzenesulfonamide (the first urea compound represented by General Formula (I)) is presented as "NKK," the second (2-1) urea compound represented by Structural Formula (II) is presented as "UU," the second (2-2) urea compound represented by General Formula (III) is presented as "TG-MD," the second (2-3) urea compound represented by General Formula (IV) is presented as "S-176," and the isopropyl is presented as "D-8."
<Evaluations of properties>
　　Next, the properties of each of the produced thermosensitive recording media were evaluated. As the properties of the thermosensitive recording medium, ink adhesion of UV flexographic printing and plasticizer resistance were evaluated. Evaluation results of the both properties are presented in Tables 1 to 3.
(Ink adhesion of UV flexographic printing)
((UV flexographic printing))
　　A solid image was printed by a flexographic printer (product name: RK FlexiProof 100-UV, available from RK PRINT COAT INSTRUMENTS) using an ink (SICURA Nutriflex 10 Yellow, available from Siegwerk Druckfarbe AG & Co. KGaA) so that an deposition amount of the ink was 5 cm³/m², followed by irradiating the formed image with UV under the following UV irradiation conditions to cure the ink to perform UV flexographic printing.
((UV irradiation conditions))
UV irradiation device: TOSURE 2000 (Model number: KUV-20261-1X), available from TOSHIBA DENZAI CO., LTD.
UV irradiation conditions: maximum emission state (from 10 A to 12 A as measured by an ammeter), irradiation was performed 5 times at the irradiation speed of 5 m/min
((Ink adhesion test))
　　After performing the above-described printing on the thermosensitive recording medium, a piece of a cellophane tape (CT18, available from NICHIBAN CO., LTD.) having a width of 18 mm was adhered on the thermosensitive recording medium along the direction of the printing. During the adhesion of the tape, the tape was sufficiently pressed down with a finger so that air was not trapped below the tape. The tape was peeled off from the thermosensitive recording medium at the following peeling steps, and the state of peeling of the printed ink was evaluated according to the following ink adhesion ranks.
-Peeling steps-
1^st step: Slowly peeled off from an angle of 180 degrees
2^nd step: Slowly peeled off from an angle of 90 degrees
3^rd step: Quickly peeled off from an angle of 90 degrees
-Ink adhesion ranks-
A: There was no peeling observed in the printed area.
B: The peeling of the ink occurred at the 3^rd step.
C: The peeling of the ink occurred at the 2^nd step.
D: The peeling of the ink occurred at the 1^st step.
(Plasticizer resistance)
　　The thermosensitive recording medium was printed by a heat gradient tester (available from Toyo Seiki Seisaku-sho, Ltd.) at 2 kg/cm² for 1 second with a thermal block heated at a temperature that achieved the saturated density of a pre-test image sample of the thermosensitive 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 sheets of a polyvinyl chloride film (available from Shin-Etsu Polymer Co., Ltd.) were placed over the pre-test image sample, followed by applying a load of 5 kg and leaving to stand for 15 hours at 50 degrees Celsius. After the standing, the image density was measured by the Macbeth densitometer. The plasticizer resistance was evaluated based on the image density residual rate of the post-test sample relative to the density value of the pre-test image sample according to the following evaluation criteria.
Image density residual rate [%] = (image density after test)/(image density before test) × 100
A: The image density residual rate was 85% or greater
B: The image density residual rate was from 60% to 84%
C: The image density residual rate was from 30% to 59%
D: The image density residual rate was 29% or less
　　It was found from the results of Tables 1 and 2 that all the thermosensitive recording media of Examples 1 to 13 had the excellent results on the ink adhesion of UV flexographic printing and plasticizer resistance and achieved the properties desired for practical applications. Conversely, it was found from Table 3 that the thermosensitive recording media of Comparative Examples 1 to 5 did not achieve desired properties at least for the ink adhesion of UV flexographic printing and the results were not desirable.
　　It was found from Tables 1 to 3 that excellent ink adhesion of UV flexographic printing and plasticizer resistance could be achieved when the thermosensitive recording media of Examples each included, as non-phenolic color developers, certain benzene sulfonamide, which was the first urea compound, and the second urea compound having a certain structure other than the benzene sulfonamide in the thermosensitive recording layer, and the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer. According to the thermosensitive recording media of Examples, therefore, a thermosensitive recording medium having high image storage stability of a printed area after flexographic printing can be provided.
　　For example, embodiments of the present disclosure are as follows.
　　<1>　　A thermosensitive recording medium, including:
　　a support;
　　a thermosensitive recording layer including a leuco dye and a color developer, disposed directly or indirectly on the support; and
　　a protective layer disposed directly or indirectly on the thermosensitive recording layer,
　　wherein the color developer includes a first color developer including a urea compound represented by General Formula (I) and a second color including at least one selected from the group consisting 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),
where, in General Formula (I), R¹ to R³ 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,
where, 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,
where, in General Formula (IV), R¹ to R⁵ 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, and
wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.
　　<2>　　The thermosensitive recording medium according to <1>,
wherein the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer has a molecular weight of 30,000 or greater.
　　<3> 　　The thermosensitive recording medium according to <1> or <2>,
wherein the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer has an acid value of 190 mgKOH/g or greater.
　　<4>　　The thermosensitive recording medium according to any one of <1> to <3>,
wherein an amount of the styrene-(meth)acrylic acid copolymer is from 5% by mass to 50% by mass relative to a total amount of solids of the protective layer.
　　<5>　　The thermosensitive recording medium according to any one of <1> to <4>,
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) relative to 1 part by mass of the urea compound represented by General Formula (I) in the thermosensitive recording layer is from 0.02 to 2.0.
　　<6>　　The thermosensitive recording medium according to any one of <1> to <5>, further including:
a backing layer at a side of the support opposite to a side of the support where the thermosensitive recording layer is disposed.
　　<7> 　　The thermosensitive recording medium according to any one of <1> to <6>, further including:
an undercoating layer between the support and the thermosensitive recording layer.
　　<8>　　The thermosensitive recording medium according to <7>,
wherein the undercoating layer includes hollow particles.
　　<9> 　　A method for producing a thermosensitive recording medium, the method including:
　　forming a thermosensitive recording layer directly or indirectly on a support, where the thermosensitive recording layer includes a first color developer including a urea compound represented by General Formula (I) and a second color developer including at least one selected from the group consisting 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),
where, in General Formula (I), R¹ to R³ 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,
where, 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,
where, in General Formula (IV), R¹ to R⁵ 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; and
　　forming a protective layer on directly or indirectly on the thermosensitive recording layer, where the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.
　　<10>　　The method according to <9>, further including:
　　forming a backing layer at a side of the support opposite to a side where the thermosensitive recording layer is disposed.
　　<11>　　An article, including:
　　an article main body; and
　　the thermosensitive recording medium of any one of <1> to <8> disposed on the article main body.
　　The present application is based on and claims priority to Japanese patent application No. 2023-034972 filed on March 7, 2023, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.

Description of Reference Numeral

1 thermosensitive recording medium
2 packaging material for fresh food products
11 adhesive layer
12 backing layer
13 support
14 undercoating layer
15 thermosensitive recording layer
16 intermediate layer
17 protective layer

Claims

　　A thermosensitive recording medium, comprising:
　　a support;
　　a thermosensitive recording layer including a leuco dye and a color developer, disposed directly or indirectly on the support; and
　　a protective layer disposed directly or indirectly on the thermosensitive recording layer,
　　wherein the color developer includes a first color developer including a urea compound represented by General Formula (I) and a second color including at least one selected from the group consisting 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),
where, in General Formula (I), R¹ to R³ 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,
where, 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,
where, in General Formula (IV), R¹ to R⁵ 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, and
wherein the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.
　　The thermosensitive recording medium according to claim 1,
wherein the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer has a molecular weight of 30,000 or greater.
　　The thermosensitive recording medium according to claim 1,
wherein the styrene-maleic acid copolymer or the styrene-(meth)acrylic acid copolymer in the protective layer has an acid value of 190 mgKOH/g or greater.
　　The thermosensitive recording medium according to claim 1,
wherein an amount of the styrene-(meth)acrylic acid copolymer is from 5% by mass to 50% by mass relative to a total amount of solids of the protective layer.
　　The thermosensitive 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) relative to 1 part by mass of the urea compound represented by General Formula (I) in the thermosensitive recording layer is from 0.02 to 2.0.
　　The thermosensitive recording medium according to claim 1, further comprising:
a backing layer at a side of the support opposite to a side of the support where the thermosensitive recording layer is disposed.
　　The thermosensitive recording medium according to claim 1, further comprising:
an undercoating layer between the support and the thermosensitive recording layer.
　　The thermosensitive recording medium according to claim 7,
wherein the undercoating layer includes hollow particles.
　　A method for producing a thermosensitive recording medium, the method comprising:
　　forming a thermosensitive recording layer directly or indirectly on a support, where the thermosensitive recording layer includes a first color developer including a urea compound represented by General Formula (I) and a second color developer including at least one selected from the group consisting 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),
where, in General Formula (I), R¹ to R³ 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,
where, 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,
where, in General Formula (IV), R¹ to R⁵ 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; and
　　forming a protective layer directly or indirectly on the thermosensitive recording layer, where the protective layer includes a resin including a styrene-maleic acid copolymer or a styrene-(meth)acrylic acid copolymer.
　　The method according to claim 9, further comprising:
　　forming a backing layer at a side of the support opposite to a side where the thermosensitive recording layer is disposed.
　　An article, comprising:
　　an article main body; and
　　the thermosensitive recording medium of claim 1 disposed on the article main body.