Thermal recording medium

Abstract

Thermal recording medium. [Solution] The heat-sensitive recording medium comprises a support layer, an undercoat layer on the support layer, a heat-sensitive colored layer on the undercoat layer, and a protective layer on the heat-sensitive colored layer, wherein the undercoat layer contains at least one type of plastic hollow particles as an organic filler, the plastic hollow particles comprise at least 20% by mass of hollow particles relative to the total mass of the hollow particles, and the hollow ratio is 60% or more, the hollow ratio being the percentage of the inner diameter of the hollow particles to the outer diameter of the hollow particles, and the heat-sensitive colored layer contains at least two types of colorants from the group having (1) an N-phenylureido-phenyl-benzenesulfonamide structure, (2) an N-phenylureido-phenyl-oxysulfonyl-aryl structure, and / or (3) an N,Nt-di-([arylsulfonyloxyl]-phenyl)urea structure.

Description

Technical Field

[0001] In one of its aspects, the present invention relates to a heat-sensitive recording medium. In another aspect, the present invention relates to a label for attaching to a product including the heat-sensitive recording medium of the present invention, and a consumer product package to which the heat-sensitive recording medium or label of the present invention is attached.

Background Art

[0002] It is known to use a coloring agent system in heat-sensitive recording media. A dye, such as a leuco dye, within one layer of the medium reacts with another component, a so-called "developer", upon application of heat to produce a colored product. Regarding the leuco dye-developer pair, phenols can be used as developers for thermal paper. However, in this context, it is particularly preferred to avoid the use of phenols, especially bisphenol-A, bisphenol-S and their derivatives, for environmental and human health reasons. For example, in European Patent Application Publication No. 2923851, Japanese Patent Application Publication No. 2015-150764 and European Patent Application Publication No. 3670205, N-phenylureido-phenyl-benzenesulfonamide has been proposed as a non-phenolic developer in this context. Also, some structures of non-phenolic developers are presented, for example, in European Patent Application Publication No. 3395583 and European Patent Application Publication No. 3677569.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0004] However, when N-phenylureidophenylbenzenesulfonamide is used as a developer in the heat-sensitive colored layer of a heat-sensitive recording medium, dynamic sensitivity is still not achieved at the same level as with bisphenol-based developers. Several measures have been proposed to improve this problem, such as combinations with sensitizers or other developers, but it is difficult to maintain higher heat resistance while increasing dynamic sensitivity. There is a market for converting heat-sensitive recording media into labels using digital printing processes. In this field, extremely high heat resistance, exceeding 110°C, is desired due to the printing process via heated rolls. Processing heat-sensitive recording media that do not have sufficient heat resistance with a digital printing press can result in undesirable color development. It is difficult to solve this problem of simultaneously providing dynamic sensitivity and extremely high heat resistance.

[0005] The object of the present invention is to provide a heat-sensitive recording medium that achieves good dynamic sensitivity and good plasticizer resistance, oil resistance, and alcohol resistance of printed images. In preferred embodiments of the present invention, high heat resistance of background up to 110°C is also achieved. [Means for solving the problem]

[0006] In view of solving the aforementioned problems, in one embodiment, the present invention provides a thermal recording medium, which comprises at least, Supporting layer, The undercoat layer on the aforementioned support layer, The heat-sensitive colored layer on the undercoat layer, A protective layer on the aforementioned heat-sensitive colored layer, It has, The undercoat layer comprises at least one type of plastic hollow particle as an organic filler, wherein the plastic hollow particle comprises at least 20% by mass of hollow particles relative to the total mass of hollow particles, and has a hollow ratio of 60% or more, and the hollow ratio is the percentage ratio of the inner diameter of the hollow particle to the outer diameter of the hollow particle. The heat-sensitive colored layer comprises at least two color developers selected from the following general formulas (1), (2), and (3):

[0007] [ka]

[0008] [ka]

[0009] [ka] Includes, Here, in equation (1), R 1 ~R 3 Each of these independently represents a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxyl group, and a C1-C6 fluoroalkyl group. In equation (2), R 3 This represents a hydrogen atom, halogen atom, nitro group, amino group, alkyl group, alkoxy group, aryloxy group, alkylcarbonyloxy group, arylcarbonyloxy group, alkylcarbonylamino group, arylcarbonylamino group, alkylsulfonylamino group, arylsulfonylamino group, monoalkylamino group, dialkylamino group, or arylamino group. The present invention relates to a heat-sensitive recording medium in formula (3), where R represents an alkyl group and n represents an integer from 0 to 3.

[0010] In the heat-sensitive recording medium of the present invention, it is preferable that at least 20% of the hollow particles in the undercoat layer have a hollowness ratio of 80% or more. In a preferred embodiment, the hollowness ratio of the hollow particles in the undercoat layer may be at least 85%, or at least 90%.

[0011] In the heat-sensitive recording medium of the present invention, the heat-sensitive colored layer preferably further contains 1,3-diphenylurea. This has been recognized as a beneficial stabilizer (storage-improving agent) in the context of the present invention.

[0012] In another aspect, the present invention relates to a consumer product package to which the thermal recording medium of the present invention is attached or incorporated.

[0013] In particular, food packaging incorporating the heat-sensitive recording medium of the present invention is envisioned. The heat-sensitive recording medium of the present invention may be converted into a label, for example, by lamination, die-cutting, and pre-printing, and then placed on food packaging. Consumer product packaging may be partially or completely transparent, flexible or rigid, and may contain one or more fresh food items, such as prepared foods or bento boxes. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram of an exemplary, non-limiting example of a thermal recording medium according to an embodiment of the present invention. In this particular non-limiting embodiment, in the thermal recording medium (1), a thermal coloring layer (12) is disposed on a support layer (13), and an undercoat layer (14) is incorporated between the support layer (13) and the thermal coloring layer (12). The thermal coloring layer (12) is in contact with a protective layer (11) on the opposite side of the thermal coloring layer (12) from the undercoat layer (14) and the support layer (13). [Figure 2]This is a schematic diagram showing an exemplary, non-limiting example of a heat-sensitive recording medium according to another embodiment of the present invention. Here, the arrangement is similar to the embodiment shown in Figure 1, but here two consecutive protective layers, designated (11a) and (11b), are applied to the heat-sensitive colored layer (12). (11a) is the lower protective layer and (11b) is the upper protective layer. [Modes for carrying out the invention]

[0015] (Support layer) The support layer (which may also be referred to as the "substrate") in the heat-sensitive recording medium of the present invention is not particularly limited and can be appropriately selected depending on the purpose. This support layer may be transparent or opaque.

[0016] Possible supports include those made of high-quality paper, recycled pulp (containing 50% or more recycled pulp), synthetic paper, polyethylene film, and laminated paper. The thickness of the paper layer depends on the composition of the layer and the application of the thermal recording material and cannot be specified in general terms, but is preferably 30 μm to 250 μm, and more preferably 50 μm to 200 μm.

[0017] The transparent support may also be used in the form of a polymer material existing as a thin film. The total light transmittance of the transparent film is preferably at least 60%, more preferably at least 70%, and particularly preferably at least 90%. A suitable film exhibits a haze value of less than 3. The transparent film may also be colored. The thickness of the transparent film is preferably 20 μm to 100 μm, more preferably 40 μm to 70 μm.

[0018] Film materials used for transparent supports may be selected from the group consisting of ionomer film (IO), polyethylene film (PE), poly(vinyl chloride) film (PVC), poly(vinylidene chloride) film (PVDC), poly(vinyl alcohol) film (PVA), polypropylene film (PP) including biaxially oriented polypropylene (BOPP), polyester film, poly(ethylene terephthalate) film (PET), poly(ethylene naphthalate) film (PEN), polycarbonate film (PC), polystyrene film (PS), polyacrylonitrile film (PAN), ethylene-vinyl acetate copolymer film (EVA), ethylene-vinyl alcohol copolymer film (EMAA), nylon film (NY), polyamide film (PA), triacetylcellulose film (TAC), norbornene film (NB), and arton film. Other possibilities include polyethylene (PE) and polymethyl methacrylate (PMMA).

[0019] (Undercoat layer) In general, in the field of thermal recording media, it is understood by those skilled in the art that the term "undercoat" refers to the layer between the support and the thermal coloring layer. The term "underlayer" may be used synonymously with "undercoat layer" by those skilled in the art.

[0020] When present in a heat-sensitive recording medium of the present invention, the undercoat layer typically contains a binder resin.

[0021] As the binder resin used in the undercoat layer, a water-dispersible resin or a water-soluble resin may be used. Specific examples include conventionally known water-soluble polymers and aqueous polymer emulsions.

[0022] There are no particular limitations on the water-soluble polymer used as the binder resin in the undercoat layer, and it can be appropriately selected depending on the intended purpose. Examples include polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose, polyvinylpyrrolidone, alkali salts of styrene-maleic anhydride copolymers, alkali salts of isobutylene-maleic anhydride copolymers, sodium alginate, gelatin, and casein. These may be used alone or in combination. A particularly preferred binder material for the undercoat layer of the present invention is polyvinyl alcohol.

[0023] There are no particular limitations on the aqueous polymer emulsion that can be used as the binder resin for the undercoat layer, and it can be appropriately selected depending on the intended purpose. Examples include, for example, styrene-butadiene copolymer latex, and emulsions such as vinyl acetate resin, acrylic resin, and polyurethane resin. These may be used alone or in combination. In the present invention, a particularly preferred embodiment consists of a combination of polyvinyl alcohol as the binder material and a styrene-butadiene copolymer added as an aqueous polymer emulsion.

[0024] When an undercoat layer is used in the heat-sensitive recording medium of the present invention, an inorganic filler may or may not be used. When an inorganic filler is used, examples include aluminum hydroxide, calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silica, barium sulfate, talc, kaolin, alumina, and clay. These may be used individually or in combination. Among these, aluminum hydroxide, calcium carbonate, kaolin, and clay are preferred in terms of the liquid properties of the coating solution, the stability of the dispersed particles, and water solubility.

[0025] In a preferred heat-sensitive recording medium according to the present invention, the undercoat layer contains an inorganic filler, the inorganic filler having an oil absorption capacity of 60 g / 100 g or less. Such low oil absorption is preferable in terms of the liquid properties described above. In this regard, a particularly preferred inorganic filler for the undercoat layer is uncalcined kaolin, and in this context, calcined kaolin is preferred.

[0026] Hollow particles are used in the undercoat layer of the heat-sensitive recording medium of the present invention.

[0027] For all hollow particles, the hollowness ratio, expressed as a percentage (%), is (inner diameter of the hollow particle / outer diameter of the hollow particle) × 100.

[0028] Each of these hollow particles has a shell made of a thermoplastic resin, which may contain air or other gases inside, and typically has a volume-average particle diameter of 1 μm to 10 μm. Most commonly, the shell has a thermoplastic resin made of polystyrene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyacrylic acid esters and polymethacrylate esters (polymethacrylates), polyacrylonitrile, polybutadiene, and copolymers thereof.

[0029] In the heat-sensitive recording medium of the present invention, it is preferable that at least 20% of the hollow particles in the undercoat layer have a hollowness ratio of 80% or more. In a preferred embodiment, the hollowness ratio of the hollow particles in the undercoat layer may be at least 85%, or at least 90%.

[0030] In a preferred embodiment of the heat-sensitive recording medium of the present invention, the proportion of all pigments (inorganic fillers and organic fillers) in the undercoat layer is at least 20% by weight and 80% by weight or less, relative to the total dry weight of the undercoat layer.

[0031] In the thermal recording medium of the present invention, when an undercoat layer is used, the amount of undercoat is 0.4 g / m². 2 From 10g / m 2 A suitable amount is 0.6 g / m2 From 4g / m 2 This is preferable.

[0032] In the present invention, the thickness of the undercoat layer varies depending on the composition of the undercoat layer and the intended use of the heat-sensitive recording material, and cannot be specified in general terms, but it is preferably 0.5 μm to 15 μm, and more preferably 0.8 μm to 6 μm.

[0033] (Thermosensitive colored layer) In the heat-sensitive recording medium of the present invention, the heat-sensitive colored layer is disposed on a transparent support layer, and the heat-sensitive colored layer contains a leuco dye and a color developer. In the present invention, an undercoat layer (or a plurality of undercoat layers) is located between the transparent support layer and the heat-sensitive colored layer.

[0034] The heat-sensitive colored layer contains a coloring agent system, in which a dye such as a leuco dye in one layer of the medium reacts with another component, a so-called "color developer," when heat is applied to produce a colored product.

[0035] Leuco dyes are electron-donating compounds that can be used alone or in combination of two or more. However, leuco dyes themselves are colorless or pale pigment precursors, and commonly known leuco compounds can be used. Examples of leuco compounds include, for example, triphenylmethanephthalide compounds, triarylmethane compounds, fluorane compounds, phenothiazine compounds, thiofluorane compounds, xanthene compounds, indophthalyl compounds, spiropyran compounds, azaphthalide compounds, chlormenopirazole compounds, methine compounds, rhodamine anilinolactam compounds, rhodamine lactam compounds, quinazoline compounds, diazaxanthene compounds, and bislactone compounds. When considering color development, background cast, and image fading due to moisture, heat, and light irradiation, specifically, 2-anilino-3-methyl-6-diethylaminofluorane, 2-anilino-3-methyl-6-(di-n-butylamino)fluorane, 2-anilino-3-methyl-6-(di-n-pentylamino)fluorane, 2-anilino-3-methyl-6-(Nn-propyl-N-methylamino)fluorane, 2-anilino-3-methyl-6-(N-isopropyl-N-methylamino)fluorane, 2-anilino-3-methyl-6-(N-isobutyl-N-methylamino)fluorane, 2-anilino-3-methyl-6-(Nn-amyl-N-methylamino)fluorane, 2-anilino-3-methyl-6-(Nn-amyl-N-ethylamino)fluorane, 2-anilino-3-methyl-6-(N-sec-butyl-N-ethylamino)fluorane, 2-anilino-3-methyl-6-(Nn-amyl-N-ethylamino)fluorane, 2-anilino-3- Methyl-6-(N-iso-amyl-N-ethylamino)fluorane, 2-anilino-3-methyl-6-(N-cyclohexyl-N-methylamino)fluorane, 2-anilino-3-methyl-6-(N-ethyl-p-toluidino)fluorane, 2-anilino-3-methyl-6-(N-methyl-p-toluidino)fluorane, 2-(m-trichloromethylanilino)-3-methyl-6-diethylaminofluorane, 2-(m-trifluoromethylanilino)-3-methyl-6-diethylaminofluorane, 2-(m-trifluoromethylanilino) -3-methyl-6-(N-cyclohexyl-N-methylamino)fluorane, 2-(2,4-dimethylanilino)-3-methyl-6-diethylaminofluorane, 2-(N-ethyl-p-toluidino)-3-methyl-6-(N-ethylamino)fluorane, 2-(N-methyl-p-toluidino)-3-methyl-6-(N-propyl-p-toluidino)fluorane, 2-anilino-6-(Nn-hexyl-N-ethylamino)fluorane, 2-(o-chloranilino)-6-diethylaminofluorane, 2-(o-bromoanilino)-6-di Ethylaminofluorane, 2-(o-chloranilino)-6-dibutylaminofluorane, 2-(o-fluoroanilino)-6-dibutylaminofluorane, 2-(m-trifluoromethylanilino)-6-diethylaminofluorane, 2-(p-acetylanilino)-6-(Nn-amyl-Nn-butylamino)fluorane, 2-benzylamino-6-(N-ethyl-p-toluidino)fluorane, 2-benzylamino-6-(N-methyl-2,4-dimethylanilino)fluorane, 2-benzylamino-6-(N-ethyl--2,4-dimethylanilino)fluorane, 2-benzylamino-6-(N-methyl-p-toluidino)fluorane, 2-dibenzylamino-6-(N-ethyl-p-toluidino)fluorane, 2-(di-p-methylbenzylamino)-6-(N-ethyl-p-toluidino)fluorane, 2-(p-phenylethylamino)-6-(N-ethyl-p-toluidino)fluorane, 2-methylamino-6-(N-methylanilino)fluorane, 2-methylamino-6-(N-ethylanilino)fluorane, 2-methylamino-6-(N-propylanilino)fluorane, 2-ethyl Amino-6-(N-methyl-p-toluidino)fluorane, 2-methylamino-6-(N-methyl-2,4-dimethylanilino)fluorane, 2-ethylamino-6-(N-methyl-2,4-dimethylanilino)fluorane, 2-dimethylamino-6-(N-methylanilino)fluorane, 2-dimethylamino-6-(N-ethylanilino)fluorane, 2-ethylamino-6-(N-methyl-p-toluidino)fluorane, benzoleucomethylene blue, 2-[3,6-bis(diethylamino)]-6-(o-chloranilino)xantylbenzoate lactam, 2-[3, 6-Bis(diethylamino)]-9-(o-chloranilino)xantylbenzoate lactam, 3,3-bis(p-dimethylaminophenyl)phthalide, 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)-6-diethylaminophthalide, 3,3-bis(p-dimethylaminophenyl)-6-chlorphthalide, 3,3-bis(p-dibutylaminophenyl)phthalide, 3-(2-methoxy-4-dimethylaminophenyl )-3-(2-hydroxy4,5-chlorophenyl)phthalide, 3-(2-hydroxy-4-dimethylaminophenyl)-3-(2-methoxy-5-chlorophenyl)phthalide, 3-(2-hydroxy-4-dimethoxyaminophenyl)-3-(2-methoxy-5-chlorophenyl)phthalide, 3-(2-hydroxy-4-dimethoxyaminophenyl)-3-(2-methoxy-5-nitrophenyl)phthalide, 3-(2-hydroxy-4-diethylaminophenyl)-3-(2-methoxy-5-methylphenyl)phthalide, 3,Compounds such as 6-bis(dimethylamino)fluorenespir(9,3’)-6’-dimethylaminophthalide, 6’-chloro-8’-methoxy-benzoindolinospiropyran, and 6’-bromo-2’-methoxybenzoindolinospiropyran are included. These may be used alone or in combination.,

[0036] When the amount of the leuco dye contained in the heat-sensitive coloring layer is based on 100% of the total mass of the heat-sensitive coloring layer, it is preferably from 3% by mass to 30% by mass.,

[0037] As color developers, various electron-accepting materials that can react with the above-mentioned leuco dye upon heating to develop color are known. For example, they include phenolic compounds, organic or inorganic acidic compounds, and their esters or salts.,

[0038] In the present invention, the heat-sensitive coloring layer has at least two types of color developers. In the heat-sensitive coloring layer of the heat-sensitive recording medium of the present invention, at least two types, or actually all three types, are present. The three types of color developers most preferably correspond to compounds having an N-phenylureido-phenyl-benzenesulfonamide structure, an N-phenylureido-phenyl-oxy-sulfonyl-aryl structure, or an N,N’-di-([arylsulfonyloxy]-phenyl)urea structure.,

[0039] Regarding the type (1) color developer, in the case of the N-phenylureido-phenyl-benzenesulfonamide structure, a generally preferred compound has the following structure:

[0040]

Chemical formula

[0041] Such color developers can be prepared, for example, according to the synthesis method described in European Patent Application Publication No. EP2923851. Most preferably, the aromatic ring bonded to the SO2 group in formula (1) above is (R 1 ) has no substituents, or has one Me group or one Cl group, or has -NHAc or -OMe. Most preferably, the central ring is assumed to be (R 3 ) has a Me group as a substituent. An aromatic ring linked via a urea group (the left side of the figure above may represent -OMe, -F, -CF3, Cl, and CH3(Me) substituents).

[0042] In a particularly preferred embodiment, the heat-sensitive colored layer of the heat-sensitive recording medium of the present invention has a color developer represented by the following formula (4):

[0043] [ka] (4) Therefore, the above compound, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, is a suitable color developer of general formula (1). This is because, in the above general formula (1), R 1 , R 2 and R 3 This corresponds to the case where all atoms are hydrogen atoms.

[0044] The color developer of type (2) has an N-phenylureido-phenyl-oxysulfonyl-aryl structure:

[0045] [ka] Here R 3This is a hydrogen atom, halogen atom, nitro group, amino group, alkyl group, alkoxy group, aryloxy group, alkylcarbonyloxy group, arylcarbonyloxy group, alkylcarbonylamino group, arylcarbonylamino group, alkylsulfonylamino group, arylsulfonylamino group, monoalkylamino group, dialkylamino group, or arylamino group.

[0046] Such color developers can be prepared, for example, by the synthesis method described in European Patent Application Publication No. 3395583. Most preferably, R 3 is a methyl group. Most preferably in the present invention, the developer (2) is -O-SO2-(phenyl)-para-R with respect to the -NH-CO-NH-phenyl substituent. 3 It has a structure in which the group is in the meta position and R3 = methyl.

[0047] The color developer of type (3) has an N,N'-di-([arylsulfonyloxy]-phenyl)urea structure:

[0048] [ka] Here, in formula (3), R represents an alkyl group and n represents an integer from 0 to 3. Such a color developer can be prepared, for example, by the synthesis method described in European Patent Application Publication No. 3677569. In the present invention, most preferably, the color developer of formula (3) has a structure in which each (R)n system is composed of a single methyl group in the para position relative to the -SO2-O group.

[0049] In the heat-sensitive colored layer, the mixing ratio of the developer to the leuco dye is preferably 0.5 to 10 parts by mass of the developer per 1 part by mass of the leuco dye, and more preferably 1 to 5 parts by mass of the developer.

[0050] In the present invention, various other known color developers can be used as desired, as long as they do not impair the effects of the present invention. These other color developers are color developers that contain various electron-accepting compounds and oxidizing agents capable of developing leuco dyes.Examples include 4,4'-isopropylidenebisphenol, 4,4'-isopropylidenebis(o-methylphenol), 4,4'-secondary butylidenebisphenol, 4,4'-isopropylidenebis(2-tert-butylphenol), zinc p-nitrobenzoate, 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-oxapentane, monocalcium monobenzylphthalate, 4,4'-cyclohexyllidenediphenol, 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, benzylprotocatechuic acid, stearyl gallate, lauryl gallate, octyl gallate, 1,3-bis(4- This includes antipyrine complexes of 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, p-bis(4-hydroxyphenyl)-p-methyltoluene, and zinc thiocyanate. These may be used alone or in combination.

[0051] Among other color developers that can be added to the heat-sensitive colored layer of a heat-sensitive recording medium, the following urea urethane and D90 commercial products should also be considered:

[0052] [ka] Urea urethane compounds

[0053] [ka] D90 The amount of additional colorants is suitably selected according to the intended purpose, as long as it does not impair the effects of the present invention. In preferred embodiments, the total mass of other colorants other than those of formula (1), (2), or (3) is less than 2 parts by mass, and more preferably less than 0.5 parts by mass, per 1 part by mass of the leuco dye in the heat-sensitive colored layer. In certain embodiments of the present invention, other colorants other than those of formula (1), (2), or (3) may not be substantially present in the heat-sensitive colored layer.

[0054] If necessary, various known stabilizers (preservatives) can be used, as long as they do not impair the effects of the present invention. Most commonly, these stabilizers are hindered phenol compounds or hindered amine compounds. The latter type of electron-accepting compound has relatively low coloring power and can be optionally added to the heat-sensitive recording layer as a pre-additive. Specific examples include 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidenebis(6-tert-butyl-2-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and 4,4'-thiobis(6-tert-butyl-2-methylpheno This includes 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.

[0055] In addition to the leuco dyes, color developers, and stabilizers mentioned above, other materials commonly used in thermal recording materials, such as binders, fillers, sensitizers, crosslinking agents, pigments, surfactants, fluorescent whitening agents, and lubricants, may be added to the heat-sensitive colored layer as appropriate.

[0056] A binder may be used as needed to improve the adhesion and applicability of the layer. There are no particular restrictions on the binder, and it can be appropriately selected depending on the purpose. Specific examples of binder resins include starches, hydroxyethylcellulose, methylcellulose, carboxymethylcellulose, gelatin, casein, gum arabic, polyvinyl alcohol, salts of diisobutylene-maleic anhydride copolymers, salts of styrene-maleic anhydride copolymers, salts of ethylene-acrylic acid copolymers, salts of styrene-acrylic copolymers, and salt emulsions of styrene-butadiene copolymers.

[0057] The filler is not limited in any way and is suitably selected according to the intended purpose. Examples include inorganic pigments such as calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silica, aluminum hydroxide, barium sulfate, talc, kaolin, alumina, and clay, and well-known organic pigments. Among these, acidic pigments (those that exhibit acidity in aqueous solution) such as silica, alumina, and kaolin are preferred, and silica is particularly preferred from the viewpoint of color density. In the framework of the present invention, calcined kaolin is preferred. In the heat-sensitive colored layer of the heat-sensitive recording medium according to the present invention, it is preferable to use an inorganic filler having a high oil absorption capacity, such as 80 g / 100 g or more. Examples of suitable fillers in this context for the heat-sensitive colored layer are calcined kaolin or amorphous silica.

[0058] Typically, various thermoplastic materials are added to the heat-sensitive colored layer as sensitivity enhancers (sensitizers). In preferred embodiments of the present invention, no sensitizer is added to the heat-sensitive colored layer. In the present invention, it has been observed that the absence of a sensitizer improves heat resistance, particularly when measured at, for example, 110°C. In some cases, sensitizers can improve the coloring effect by melting under heat influence and providing a temporary solvent that promotes the reaction between the leuco dye and the color developer. It should be noted that when heat resistance is required, such as in labeling applications for cooked foods or digital printing processes, it is preferable that no thermoplastic material is added, or that a compound with a melting point of 90°C or higher is selected.

[0059] Examples of sensitizers include: (1) fatty acids such as stearic acid and behenic acid; (2) fatty acid amides such as stearamide and palmitamide; (3) metal salts of fatty acids such as zinc stearate, aluminum stearate, calcium stearate, zinc palmitate, and zinc behenate; (4) sensitizers having an N-octadecyl chain such as N-octadecylcarbamoyl-p-methoxycarbonylbenzene and N-octadecylcarbamoylbenzene; (5) polyphenyl hydrocarbons such as p-benzylbiphenyl, terphenyl, and triphenylmethane; (6) Benzyloxy derivatives, benzoates and naphthates, benzyl p-benzyloxybenzoate, β-benzyloxynaphthalene, phenyl β-naphthoate, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate, dibenzoylmethane, dibenzoyloxymethane, dibenzoyloxypropane; (7) carbonates such as diphenyl carbonate and glycol carbonate; (8) terephthalic acids such as dibenzyl terephthalate and dimethyl terephthalate; (9) dibenzyl oxalate, bis(4-methylbenzyl) oxalate, bis(4-chlorobenzyl) Oxalate ester sensitizers such as oxalate; (10) 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 1,4-dibenzyloxynaphthalene, 1,2-diphenoxyethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, 1,4-diphenoxy-2-butene, 1,2-bis(4-methoxyphenylthio)ethane, 1,3-bis(2-vinyloxyethoxy)benzene, 1,4-bis(2-vinyloxyethoxy)be Alkoxy and aryloxy sensitizers such as (11) 1,1-diphenylethanol, 1,1-diphenylpropanol, p-benzyloxybenzyl alcohol, 1,3-phenoxy-2-propanol; (11) Alcoholic sensitizers such as (11) 1,1-diphenylethanol, 1,1-diphenylpropanol, p-benzyloxybenzyl alcohol, 1,3-phenoxy-2-propanol;(12) Includes sulfur-based sensitizers such as 1,4-diphenylthiobutane, 1,4-diphenylthio-2-butene, and dibenzyl disulfide.

[0060] In preferred embodiments of the present invention, the heat-sensitive colored layer does not necessarily have to contain compounds known in the field of heat-sensitive recording media as sensitizers, such as compounds containing aryloxy groups (e.g., phenoxy, tolyloxy groups), alkoxy groups, oxaate groups, and benzyloxy groups. Therefore, it is particularly important that the heat-sensitive colored layer does not contain the type sensitizers shown in groups 6, 9, 10, and 11 above. The absence of such sensitizers can improve heat resistance, for example, as measured at 110°C.

[0061] The colored layer can be formed by generally known methods. In preferred embodiments, to avoid reactions between the components of the heat-sensitive colored layer, the dispersions are carried out separately, and then the liquids are mixed. When grinding other components together with the binder, this is usually done using a disperser such as a ball mill, attritor, or sand mill to a particle size of 0.2 μm to 3 μm, preferably 0.2 μm to 1 μm. The resulting dispersion is, if necessary, mixed with a filler and a heat-fused material (sensitizer) dispersion according to a predetermined composition, thereby preparing a coating solution for the heat-sensitive colored layer, which is then coated onto a support.

[0062] The thickness of the heat-sensitive colored layer varies depending on the composition and intended purpose of the heat-sensitive colored layer, and cannot be generalized, but it is preferably between 1 μm and 50 μm, and more preferably between 2 μm and 20 μm.

[0063] (protective layer) In this invention, at least one protective layer is provided on top of the heat-sensitive layer. Several different protective layers may be layered on top of each other, with attention paid to matching properties or barrier properties, respectively.

[0064] In the heat-sensitive recording medium of the present invention, at least one protective layer may suitably contain wax particles, and the average particle size is preferably at least 0.05 μm and 2.0 μm or less. When there are multiple protective layers, only the uppermost protective layer, which is furthest from the heat-sensitive coloring layer and on the surface exposed to the outside, can suitably contain wax particles. In a preferred embodiment in which the uppermost protective layer contains wax particles, the lower protective layers may or may not contain wax particles.

[0065] Regarding the average particle size of wax particles, this value is the median size (D) measured by laser diffraction using a laser diffraction particle size distribution analyzer. 50 This is obtained in the form of a method for measuring the average particle size. This measurement can be performed, for example, by the LA-950 instrument manufactured by HORIBA.

[0066] Preferably, when wax particles are used in the protective layer / top protective layer, the melting point of the wax is at least 80°C and 200°C or less. More preferably, the melting point of the wax is at least 90°C and 130°C or less, and most preferably at least 100°C and 120°C or less.

[0067] More preferably, the wax particle size is at least 0.1 μm and 0.5 μm or less.

[0068] In significant embodiments, the wax particles constitute at least 2.0% by weight and 20% by weight or less, relative to 100% by weight of the total components of the protective layer as a whole, and more preferably at least 5.0% by weight and 10% by weight or less, relative to 100% by weight of the total components of the protective layer as a whole.

[0069] The wax material of the wax particles in the present invention may be polyethylene wax, salts of higher fatty acids such as zinc stearate and calcium stearate, montanic acid ester wax, carnauba wax, paraffin wax, ester wax and its metal salts, higher fatty acid amides, higher fatty acid esters, animal-based waxes, plant-based waxes, mineral waxes, and petroleum-based waxes.

[0070] A particularly suitable wax material for the wax particles in the protective layer of the heat-sensitive recording medium of the present invention is polyethylene wax. Low-density or high-density polyethylene wax particles may also be used.

[0071] The protective layer typically includes at least a binder, and each of the protective layers may also include an inorganic filler and a surfactant.

[0072] There are no particular restrictions on the binder(s) of the protective layer, and they may be appropriately selected depending on the purpose. The same binder may be used in each protective layer, or different binders may be used in different protective layers. Examples of binders that can be used in the protective layer include polyvinyl alcohol, modified polyvinyl alcohol, starch and its derivatives, cellulose derivatives, polyvinylpyrrolidone, polyethyleneimine, sodium alginate, gelatin, and casein. Acrylic binders may also be used. Hydrophobic resins that can be used as binders in the protective layer include urethane resins, epoxy resins, vinyl acetate (co)polymers, vinylidene chloride (co)polymers, vinyl chloride (co)polymers, and styrene-butadiene copolymers, which are usually provided as aqueous emulsions during the preparation of the protective layer. A particularly suitable binder material for the protective layer of the present invention is polyvinyl alcohol.

[0073] The thickness of the protective layer preferably varies from 0.2 μm to 10 μm, and more preferably from 0.5 μm to 5 μm. In a non-limiting exemplary embodiment of the present invention, a protective layer with a thickness of 2.5 μm can be used during drying. When multiple protective layers are applied, each requires a smaller individual thickness. The preferred maximum cumulative thickness of all protective layers combined is 10 μm in the dried final product.

[0074] When an inorganic filler is included in the protective layer, there are no particular restrictions on the inorganic filler, and it can be appropriately selected according to the purpose. Examples of inorganic fillers include aluminum hydroxide, calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silica, barium sulfate, talc, kaolin, alumina, and clay. These may be used individually or in combination. Among these, aluminum hydroxide and calcium carbonate are particularly preferred. This is because protective layers containing such inorganic fillers provide excellent abrasion resistance to thermal heads during long-term printing. There are no particular restrictions on the amount of inorganic filler in the protective layer, and it is selected appropriately according to the intended purpose. The amount of inorganic filler depends on the type of filler, but it is preferably 50 parts by mass or more and 500 parts by mass or less per 100 parts by mass of binder resin.

[0075] In a significant embodiment of the present invention, the first protective layer is disposed on a heat-sensitive colored layer and contains a binder such as polyvinyl alcohol (PVA), provided that it does not contain wax particles. However, a second protective layer may be disposed on the first protective layer, so that the second protective layer does not directly contact the heat-sensitive colored layer, and the second protective layer contains wax particles and, if assumed, a filler such as an inorganic filler.

[0076] There are no particular limitations on the method for forming the first protective layer, the second protective layer, and subsequent protective layers, and these can be appropriately selected depending on the intended purpose. Examples include blade coating, roll coating, wire bar coating, die coating, and curtain coating. Other layers of the heat-sensitive recording medium of the present invention, such as an undercoat layer, can be applied using such methods. Curtain coating is a preferred method for applying protective layers in the present invention and can also be used when applying a heat-sensitive colored layer.

[0077] (Back layer) In the heat-sensitive colored layer of the present invention, a back layer (also referred to as a "backing layer") may be provided below the support layer. However, such a back layer is not essential in the present invention and is optional only when necessary. The heat-sensitive recording medium of the present invention may have a back layer comprising a pigment, a binder resin, and preferably a crosslinking agent. If present, the back layer is located on the surface of the support layer opposite to the surface of the support layer on which the heat-sensitive layer is placed, or, if an undercoat layer is present, it is located on the surface between the support and the heat-sensitive layer, opposite to the surface of the support layer on which the undercoat layer is placed.

[0078] The back layer may also contain other components such as fillers, lubricants, and antistatic agents.

[0079] Regarding the binder resin, either a water-dispersible resin or a water-soluble resin can be used. Specific examples include conventionally known water-soluble polymers and aqueous polymer emulsions.

[0080] There are no limitations on the water-soluble polymers, and they are suitably selected depending on the intended purpose. Examples include polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose, polyvinylpyrrolidone, alkali salts of styrene-maleic anhydride copolymers, alkali salts of isobutylene-maleic anhydride copolymers, sodium alginate, gelatin, and casein. These may be used alone or in combination.

[0081] There are no particular restrictions on aqueous polymer emulsions, and they are selected appropriately depending on the purpose. Examples include latex such as styrene-butadiene copolymers; and emulsions such as vinyl acetate resins, acrylic resins (e.g., acrylic acid-acrylic acid ester copolymer latex), (meth)acrylamide resins, and polyurethane resins. These may be used alone or in combination.

[0082] There are no particular restrictions on the crosslinking agent, and it is selected appropriately depending on the purpose. Examples include polyhydric amine compounds such as ethylenediamine; polyhydric aldehyde compounds such as glyoxal, glutaraldehyde, and dialdehyde; dihydrazide compounds such as adipic acid dihydrazide and phthalic acid dihydrazide; polyamide-epichlorohydrin compounds; water-soluble methylol compounds (urea, melamine, phenol); polyfunctional epoxy compounds; polyhydric metal salts (e.g., Al, Ti, Zr, Mg); titanium lactate; and boric acid. The amount of crosslinking agent depends on the amount and type of functional groups of the crosslinking agent, but is preferably 0.1 to 100 parts by mass, and more preferably 1 to 100 parts by mass, per 100 parts by mass of binder resin.

[0083] Either inorganic or organic fillers may be used as fillers. Examples of inorganic fillers include carbonates, silicates, metal oxides, and sulfate compounds. Examples of organic fillers include silicone resins, cellulose resins, epoxy resins, nylon resins, phenolic resins, polyurethane resins, urea resins, melamine resins, polyester resins, polycarbonate resins, styrene resins, polyethylene resins, and formaldehyde resins.

[0084] Antistatic agents can include, for example, commonly used ionic conductive antistatic agents and electronically conductive antistatic agents. Specific examples of ionic conductive antistatic agents include inorganic salts such as sodium chloride; anionic polymers such as sodium polystyrene sulfonate; and resins containing quaternary ammonium salts, which are electrolyte cations. Specific examples of electronically conductive antistatic agents include conductive tin, conductive metal compounds such as antimony oxide, and conductive polymers such as polyaniline. Among these antistatic agents, polystyrene sulfonates are particularly noteworthy because they react with aziridine and improve water resistance through crosslinking. Salts copolymerized with maleic acid are also significant in that they possess antistatic properties and improved water resistance.

[0085] There are no particular restrictions on the method for forming the back layer, and it can be appropriately selected depending on the purpose. The back layer is preferably formed by applying a back layer coating liquid to the support.

[0086] There are no particular restrictions on the coating method, and it is selected appropriately depending on the purpose. Examples include blade coating, roll coating, wire bar coating, die coating, and curtain coating.

[0087] There are no particular restrictions on the thickness of the back layer, and it can be appropriately selected depending on the purpose. Preferably, it is 0.1 μm to 10 μm, and more preferably 0.5 μm to 5 μm.

[0088] (viscous layer) The heat-sensitive recording medium of the present invention may include a viscous layer, also called an adhesive layer. However, such a viscous layer is not essential in the present invention and is optional.

[0089] A viscous layer may be provided on the surface opposite to the side on which the protective layer of the support layer or backing layer is formed. The viscous layer can, for example, assist in attaching the heat-sensitive recording medium to food packaging in a typical application of the present invention. Thus, the heat-sensitive recording medium of the present invention may be provided with an adhesive surface attached to the support or backing layer, which is significant when providing a label having an adhesive layer. A peelable liner can then be attached to the adhesive layer and removed before it is finally attached to the product to which the label is to be applied. The viscous layer may also provide antistatic properties. The method for forming the viscous layer is not particularly limited. Examples of methods include general coating methods and lamination methods. There is no particular limit to the average thickness of the viscous layer, and it is appropriately selected depending on the purpose, but is preferably 0.1 μm to 20 μm.

[0090] There are no particular restrictions on the material of the viscous layer, and it can be appropriately used depending on the purpose. Examples of materials for the viscous layer include urea resin, melamine resin, phenolic resin, epoxy resin, vinyl acetate resin, vinyl acetate-acrylic copolymer, ethylene-vinyl acetate copolymer, acrylic resin, polyvinyl ether resin, vinyl chloride-vinyl acetate copolymer, polystyrene resin, polyester resin, polyurethane resin, polyamide resin, chlorinated polyolefin resin, polyvinyl butyral resin, acrylic acid ester copolymer, methacrylic acid ester copolymer, natural rubber, cyanoacrylate resin, and silicone resin. One of these materials may be used alone, or two or more may be used in combination. These materials may be crosslinked with a crosslinking agent. The material of the viscous layer may be of the hot melt type. In one aspect of the present invention, the label containing the heat-sensitive recording medium of the present invention is in the form of a silicone linerless (SLL) label.

[0091] (Image recording method) The image recording method is used to record an image on any thermal recording medium according to the embodiments of the present invention, using an image recording unit which is either a thermal head or a laser.

[0092] There are no particular restrictions on the shape, structure, and size of the thermal head; it can be selected appropriately depending on the purpose.

[0093] There are no particular restrictions on the laser, and it can be selected according to the intended purpose. In one preferred embodiment, a CO2 laser emitting light with a wavelength of 9.3 μm to 10.6 μm may be used. By using a CO2 laser emitting light with a wavelength of 9.3 μm to 10.6 μm, good laser-printed images can be obtained without using photothermal converters such as phthalocyanine pigments. Other types of lasers, such as FLDA (Fiber Laser Diode Array), may also be used. [Examples]

[0094] The present invention will be described in detail below based on examples and comparative examples. However, it should be noted that the present invention is not limited to these examples. In the following examples, unless otherwise specified, "parts" means "parts by mass" and "%" means "percent mass".

[0095] (Example 1) A thermal recording medium was formed by following the procedure below.

[0096] 1) Apply the coating liquid for the undercoat layer onto the substrate, thereby creating an undercoat layer (2 g / m² as dry mass). 2 ) formed. In this example, approximately 60 g / m² 2 High-quality paper with the following basis weight was used. The formulation of the coating liquid for the undercoat layer is as follows: Formulation of coating liquid for the undercoat layer Preparation of the coating solution for the base layer: [Liquid A1] Undercoat Liquid 1 -------------------------------------------------------------------------------- Microscopic spherical hollow plastic particles 1) 24 copies Spherical hollow plastic particles 2) 16 copies Styrene / butadiene copolymer latex 3) 10 copies 10% aqueous polyvinyl alcohol solution 4) 6 parts Water 44 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) Dow AF-1570, styrene / acrylic copolymer resin, solids content 17.5%, average particle diameter 1.6 μm, hollowness 65% 3) Solids content: 50.0% 4) Completely hydrolyzed PVA.

[0097] 2) A coating liquid for the thermal recording layer was applied to the undercoat layer, thereby creating a thermal recording layer.

[0098] Regarding the preparation of a coating solution for a heat-sensitive colored layer, the following composition was prepared: [Liquid B] Pigment dispersion liquid -------------------------------------------------------------------------------- 2-Anilino-3-methyl-6-(di-n-butylamino)fluorane (pigment) 32 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 1) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid C1] Chromate Dispersion 1 -------------------------------------------------------------------------------- N-[2-(3-phenylureido)phenyl]benzenesulfonamide 1) 32 copies (General formula (1) = "Formula (1)") 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) NKK-1304 manufactured by Nippon Soda Co., Ltd. 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid C2] Chromate Dispersion 2 -------------------------------------------------------------------------------- General formula (2) (“Formula (2)”) 32 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 1) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd.

[0099] The compound represented by the general formula (2) above has the following specific structure:

[0100] [ka] [Liquid D1 Solution] Filler Dispersion 1 -------------------------------------------------------------------------------- Calcined kaolin 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) BASF Ansilex 93, oil absorption capacity: 105 to 120g / 100g 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. Using a sand mill, [Liquid B], [Liquid C1], [Liquid C2], and [Liquid D1] of the above composition were dispersed in each liquid such that the particles contained in each liquid had an average particle diameter of 1 μm or less, thereby preparing the dye dispersion [Liquid B], the color developer dispersion [Liquid C1], [Liquid C2], and the filler dispersion [Liquid D1]. Next, [Liquid B], [Liquid C1], [Liquid C2], and [Liquid D1] were mixed in the following proportions.

[0101] This mixture was stirred to prepare a heat-sensitive colored layer coating liquid [Liquid E1].

[0102] A heat-sensitive colored layer was formed by uniformly applying [Liquid E1] to the undercoat layer. The coating amount of the heat-sensitive layer was 0.4 g / m² on a dry basis. 2 The amount of pigment coating was adjusted to the desired level, and then dried to form a heat-sensitive colored layer.

[0103] 3) A two-layer protective coating solution (first coating solution and second coating solution) was applied to the heat-sensitive colored layer so that the lower protective layer, composed of the first coating solution, was below the upper protective layer, composed of the second coating solution, thereby forming a two-layer protective layer on the heat-sensitive recording layer. The upper and lower protective layers each had a dry weight of 1 g / m². 2 , 1g / m 2 The thickness was as follows. The formulation of the coating liquid for the two-layer protective layer was as follows. These were then dried: [Liquid F] First protective layer liquid -------------------------------------------------------------------------------- 10% Itacon-modified polyvinyl alcohol aqueous solution 1) 70 copies 20% polyamide epichlorohydrin 2) 15 copies Water 15 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. 2) SOLENIS KYMENE-920 [Liquid G] Second protective layer liquid -------------------------------------------------------------------------------- [Liquid H] 1) 18 copies High-density polyethylene wax 2) 2nd part 20% polyamide epichlorohydrin 3) 4 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 4 ) 20 copies Water 56 parts -------------------------------------------------------------------------------- 1) See below for the formulation of the dispersion. 2) Ultralube E-842N manufactured by Keim Additec GmbH 3) KYMENE-920 manufactured by SOLENIS 4) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid H] Filler dispersion for protective layer -------------------------------------------------------------------------------- Apy-100 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Apy-100 manufactured by Nabaltec GmbH 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. After coating, the samples were aged at 50°C for 48 hours. In addition to this process, the samples were calendered with 20 kgF and then subjected to quality evaluation.

[0104] (Regarding Examples 2 through 13) In Examples 2 to 12, the thermal recording media were prepared according to Example 1, except for the points shown in Table 1 below. Liquid C3, Liquid D2, and Liquid E2 were prepared as follows: Examples 2 to 4 and Examples 9 to 12 [Liquid C3] (Color developer dispersion 3) -------------------------------------------------------------------------------- General formula (3) (“Formula (3)”) 32 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 1 ) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. The compound represented by the above general formula (3) has the following specific structure:

[0105] [ka] (Regarding Example 13) [Liquid D2] Filler dispersion 2 -------------------------------------------------------------------------------- Amorphous silica 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Mizukasil P-527, manufactured by Mizusawa Industrial Chemicals, oil absorption capacity: 140 to 180 g / 100 g 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. (Regarding Example 4) [Liquid E2] Heat-sensitive colored layer liquid 2 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C1] Chromogenizer dispersion 1 10 parts [Liquid C2] Chromogenizer dispersion 2 10 parts [Liquid C3] Developer dispersion 3 10 parts [Liquid D1] Filler dispersion 1 31 parts Water Pigment dispersion 29 parts -------------------------------------------------------------------------------- Therefore, in Examples 1 to 3, different combinations of color developers were compared. In Example 4, three different combinations of color developers are tested. In Examples 5 to 12, the effect of the ratio between two different types of color developers is tested. In Example 13, the effect of another type of inorganic filler in the heat-sensitive colored layer is tested.

[0106] (Example 14) In Example 14, a heat-sensitive recording medium was prepared according to Example 1, except that the following [Liquid I] was added to prepare the coating liquid [Liquid E3] for the heat-sensitive colored layer. [Liquid I] Preservability improved dispersion liquid -------------------------------------------------------------------------------- 1,3-Diphenylurea 32 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 1) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid E3] Heat-sensitive colored layer liquid 3 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C1] Chromogenic agent dispersion 1 15 parts [Liquid C2] Chromogenizer dispersion 2 15 parts [Liquid I] Preservability improved dispersion liquid 3 parts [Liquid D1] Filler dispersion 1 29 parts Water 28 parts -------------------------------------------------------------------------------- (Examples 15 to 22) In Examples 15 to 22, thermal recording media were fabricated according to Example 1, except for the changes described in Table 1 below. Liquids A2, A3, A4, A5, A6, A7, A8, and A9 were prepared as follows:

[0107] (In the case of Example 15) [Liquid A2] Undercoat Liquid 2 -------------------------------------------------------------------------------- Microscopic spherical hollow plastic particles 1) 26 copies Coarse spherical hollow plastic particles 2) 7 parts Styrene / butadiene copolymer latex 3) 10 copies 10% aqueous polyvinyl alcohol solution 4) 7 parts 50 parts water -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 3) Solids content: 50.0% 4) Completely hydrolyzed PVA.

[0108] (In the case of Examples 16 and 17) [Liquid A3] Undercoat Liquid 3 -------------------------------------------------------------------------------- Microscopic spherical hollow plastic particles 1) 24 copies Coarse spherical hollow plastic particles 2) 10 copies Styrene / butadiene copolymer latex 3) 10 copies 10% aqueous polyvinyl alcohol solution 4) 6 parts 50 parts water -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 3) Solids content: 50.0% 4) Completely hydrolyzed PVA.

[0109] (In the case of Example 18) [Liquid A4] Undercoat Liquid 4 -------------------------------------------------------------------------------- Spherical hollow plastic particles 1) 25 copies Coarse spherical hollow plastic particles 2) 11 parts Styrene / butadiene copolymer latex 3) 11 parts 10% aqueous polyvinyl alcohol solution 4) 7 parts Water 46 parts -------------------------------------------------------------------------------- 1) Dow AF-1570, styrene / acrylic copolymer resin, solids content 17.5%, average particle diameter 1.6 μm, hollowness 65% 2) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 3) Solids content: 50.0% 4) Completely hydrolyzed PVA.

[0110] (In the case of Example 19) [Liquid A5] Undercoat Liquid 5 -------------------------------------------------------------------------------- Coarse spherical hollow plastic particles 1) 17 copies Styrene / butadiene copolymer latex 2) 17 copies 10% aqueous polyvinyl alcohol solution 3) 11 parts Water 55 parts -------------------------------------------------------------------------------- 1) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 2) Solids content: 50.0% 3) Completely hydrolyzed PVA.

[0111] (In the case of Example 20) [Liquid A6] Undercoat Liquid 6 -------------------------------------------------------------------------------- Spherical hollow plastic particles 1) 48 copies Styrene / butadiene copolymer latex 2) 10 copies 10% aqueous polyvinyl alcohol solution 3) 6 parts Water 36 parts -------------------------------------------------------------------------------- 1) AF-1570: Manufactured by Dow, styrene / acrylic copolymer resin, solids content 17.5%, average particle diameter 1.6 μm, hollowness 65% 2) Solids content: 50.0% 3) Completely hydrolyzed PVA.

[0112] (In the case of Example 21) [Liquid A7] Undercoat Liquid 7 -------------------------------------------------------------------------------- Microscopic spherical hollow plastic particles 1) 12 parts Coarse spherical hollow plastic particles 2) 10 copies Uncalcined kaolin 3) 3rd part Styrene / butadiene copolymer latex4) 10 copies 10% aqueous polyvinyl alcohol solution 5) 6 parts Water 59 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 3) UW-90 4) Solids content: 50.0% 5) Completely hydrolyzed PVA.

[0113] (In the case of Example 22) [Liquid A8] Undercoat Liquid 8 -------------------------------------------------------------------------------- Spherical hollow plastic particles 1) 20 copies Coarse spherical hollow plastic particles 2 ) 9 copies Uncalcined kaolin 3) 3rd part Styrene / butadiene copolymer latex 4) 9 parts 10% aqueous polyvinyl alcohol solution 5) 6 parts Water 53 parts -------------------------------------------------------------------------------- 1) Dow AF-1570, styrene / acrylic copolymer resin, solids content 17.5%, average particle diameter 1.6 μm, hollowness 65% 2) R-500 manufactured by Matsumoto Oil & Fat Pharmaceutical Co., Ltd., solid content 33.0%, hollow ratio 90% 3) UW-90 4) Solids content: 50.0% 5) Completely hydrolyzed PVA.

[0114] (Reference examples 1 to 5) In Reference Examples 1 to 5, the heat-sensitive recording media were prepared according to Example 1, except for the changes shown in Table 1 below. Accordingly, three types of sensitizers were added to the heat-sensitive colored layer liquid, respectively. [Liquid J1] Sensitizer dispersion 1 -------------------------------------------------------------------------------- 1,2-bis(3-methylphenoxy)ethane 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Sanko KS-232 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid J2] Sensitizer dispersion 2 -------------------------------------------------------------------------------- Bis(4-methylbenzyl)oxalate 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) HS-3520 manufactured by Dainippon Ink and Chemicals, Inc. 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid J3] Sensitizer dispersion 3 -------------------------------------------------------------------------------- Stearic acid amide 32 parts 10% Itacon-modified polyvinyl alcohol aqueous solution 1) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. [Liquid E4] Heat-sensitive colored layer liquid 4 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C] Chromogenizer dispersion 15 parts [Liquid C] Chromogenizer dispersion 15 parts [Liquid J] Sensitizer dispersion, 3 parts [Liquid D1] Filler dispersion 1 29 parts Water 28 parts -------------------------------------------------------------------------------- Here, Reference Example 1 corresponds to Example 21 in which KS-232 was added, Reference Example 2 corresponds to Example 17 in which KS-232 was added, Reference Example 3 corresponds to Example 2 in which KS-232 was added, Reference Example 4 corresponds to Example 21 in which HS-3520 was added, and Reference Example 5 corresponds to Example 1 in which stearic acid amide was added.

[0115] (Comparative Examples 6 and 7) In Comparative Examples 6 and 7, the heat-sensitive recording media were prepared according to Example 1, except for the changes shown in Table 1 below. Therefore, different types of color developers were combined with [Liquid C1] and [Liquid C2], respectively, in the heat-sensitive colored layer liquid.

[0116] (In the case of Comparative Examples 6, 7, and 14) [Liquid C4] Chromate Dispersion 4 -------------------------------------------------------------------------------- N-(p-toluenesulfonyl)-N'-(3-(p-toluenesulfonyloxy)phenyl)urea 1) 32 copies 10% Itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Pergafast 201 by SOLENIS 2) Itacon-modified polyvinyl alcohol manufactured by Kuraray Co., Ltd. Pergafast 201 has the following structure:

[0117] [ka] (Comparative Examples 8 to 10) In Comparative Examples 8 to 10, thermal recording media were prepared according to Example 1, except for the changes shown in Table 1 below. Accordingly, the following three types of undercoat layer liquids [Liquid A12], [Liquid A13], and [Liquid A14] were tested.

[0118] (In the case of Comparative Example 8) [Liquid Al2] Undercoat Liquid 12 -------------------------------------------------------------------------------- Fine spherical hollow plastic particles 1) 36 parts Styrene / butadiene copolymer latex 2) 10 parts 10% aqueous polyvinyl alcohol solution 3) 6 parts Water 48 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) Solids content: 50.0% 3) Completely hydrolyzed PVA.

[0119] (Comparative Example 9) [Liquid A13] Undercoat Liquid 13 -------------------------------------------------------------------------------- Microscopic spherical hollow plastic particles 1) 24 copies Uncalcined kaolin 2) 3rd part Styrene / butadiene copolymer latex 3) 10 copies 10% aqueous polyvinyl alcohol solution 4) 6 parts Water 57 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solids content 26.5%, average particle diameter 1 μm, hollowness 55% 2) UW-90 3) Solids content: 50.0% 4) Completely hydrolyzed PVA.

[0120] (In the case of Comparative Example 10) [Liquid A14] Undercoat Liquid 14 -------------------------------------------------------------------------------- Uncalcined kaolin 1) 10 copies Styrene / butadiene copolymer latex 2) 10 copies 10% aqueous polyvinyl alcohol solution 3) 6 parts Water 74 parts -------------------------------------------------------------------------------- 1) UW-90 2) Solids content: 50.0% 3) Completely hydrolyzed PVA.

[0121] (Comparative Examples 11 to 14) In Comparative Examples 11 to 14, the heat-sensitive recording media were prepared according to Example 1, except for the changes shown in Table 1 below. Therefore, each developer dispersion was used independently of others. [Liquid E5] Heat-sensitive colored layer liquid 5 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C] Chromogenic agent dispersion 31 parts [Liquid D1] Filler dispersion 1 31 parts Water 28 parts -------------------------------------------------------------------------------- (Comparative Example 15) In Comparative Example 15, a heat-sensitive recording medium was produced according to Example 1, except for the changes shown in Table 1 below. Therefore, instead of [Liquid A1], an undercoat layer liquid [Liquid A15] was used. [Liquid A15] Undercoat liquid 15 -------------------------------------------------------------------------------- Fine spherical hollow plastic particles 1) 33 parts Spherical hollow plastic particles 2) 22 parts Latex of styrene / butadiene copolymer 3) 2 parts 10% aqueous polyvinyl alcohol solution 4) 9 parts Water 34 parts -------------------------------------------------------------------------------- 1) AF-1055 manufactured by Dow, styrene / acrylic copolymer resin, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) AF-1570 manufactured by Dow, styrene / acrylic copolymer resin, solid content 17.5%, average particle diameter 1.6 μm, hollow ratio 65% 3) Solid content: 50.0% 4) Completely hydrolyzed PVA.

[0122] (Comparative Example 16) In Comparative Example 16, a heat-sensitive recording medium was prepared according to Example 1, except for the changes shown in Table 1 below. Therefore, the protective layer was not coated on the heat-sensitive coloring layer.

[0123] (Evaluation method) The still image density was evaluated as follows: A thermal gradient test apparatus (HG-100, manufactured by Toyo Seiki Seisakusho) was used to apply thermal energy from 140°C to 180°C for 1 second at a pressure of 0.36 kgf. A spectrophotometer (Exact, manufactured by X-Rite) was used to measure the black optical density. The maximum value was recorded as the still image density. This value is preferably 1.30 or higher. Rank I: Optical density exceeds 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 Dynamic image density was evaluated as follows: Images were printed using a ZEBRA 110Xi4 printer at a print speed of 304 mm / second (12 inches / second) with applied energies ranging from +24 to +30. The black optical density at each energy level was measured using a spectrophotometer (Exact, manufactured by X-Rite). The maximum value was recorded as the motion density. A value of 1.20 or higher is preferable. Rank I: Optical density exceeds 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 The decodeability of the barcode was evaluated as follows: Using a ZEBRA 110Xi4 printer, picket-type barcodes were printed at a print speed of 304 mm / second (12 inches / second) with an applied energy of +15 (bars printed parallel to the printing direction). The barcodes were read using a barcode verifier (OMRON, MICROSCAN LVS-9580) and their decodeability was evaluated. The results were recorded according to the ANSI symbolic grade. A high value is preferable (maximum 4.0). Dynamic sensitivity was evaluated as follows: An image was printed using a MarkPoint MK2 printer at a print speed of 102 mm / second (4 inches / second) with an applied energy of 8.88 mJ. The black optical density was measured using a spectrophotometer (Exact, manufactured by X-Rite). A value of 1.20 or higher is preferable. Rank I: Optical density exceeds 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 The background heat resistance at 110°C was evaluated as follows: A thermal gradient tester (HG-100, manufactured by Toyo Seiki Seisakusho) was used to apply a thermal energy of 110°C at a pressure of 0.36 kgf for 1 second. The black optical density was measured using a spectrophotometer (instrument name = Exact, manufactured by X-Rite). This value is preferably 0.25 or less. Rank I: Optical density is 0.25 or less Rank II: Optical density between 0.26 and 0.35 Rank III: Optical density is higher than 0.36 The background heat resistance at 100°C and 90°C was evaluated as follows: The prepared samples were treated at 100°C or 90°C for 1 hour, respectively. The black optical density was measured using a spectrophotometer (Exact, manufactured by X-Rite). A value of 0.25 or less is preferred. Rank I: Optical density is 0.25 or less Rank II: Optical density between 0.26 and 0.35 Rank III: Optical density is higher than 0.36 The plasticizer resistance of printed images was evaluated as follows: Using a thermal recording simulator (TH-PMD manufactured by Okura Electric), an image was generated on the preparation sample at a head power of 0.45 w / dot, an interval of 0.1 msec, and a pulse width (applied energy) of 1.0 msec. The printed sample was maintained in contact with a sheet of polyvinyl chloride (PVC manufactured by Shin-Etsu Polymer) at 40°C for 15 hours. The black optical density of the printed image was measured using a spectrophotometer (device name = Exact, manufactured by X-Rite). Subsequently, the image retention rate was calculated as follows: Retention rate (%) = [Degree of optical black density after the test] / [Degree of optical black density before the test] × 100 This value is preferably 90% or more: Rank I: Retention rate is 90% or more Rank II: Retention rate is from 70% to 89% Rank III: Retention rate is less than 70% The oil resistance of the printed image was evaluated as follows: Using a thermal recording simulator (TH-PMD manufactured by Okura Electric), an image was formed on the adjustment sample at a head power of 0.45 w / dot, an interval of 0.1 msec, and a pulse width (applied energy) of 1.0 msec. One drop of cottonseed oil was dropped onto each sample and spread with a cotton piece. Subsequently, the sample was maintained at 40°C for 15 hours. The black optical density of the printed image was measured using a spectrophotometer (device name = Exact, manufactured by X-Rite). Subsequently, the image retention rate was calculated as follows: Retention rate (%) = [Degree of optical black density after the test] / [Degree of optical black density before the test] × 100 This value is preferably 90% or more. Rank I: Retention rate is 90% or more Rank II: Retention rate is from 70% to 89% Rank III: Retention rate is less than 70% The ethanol (EtOH) resistance of printed images was evaluated as follows: Using a thermal recording simulator (TH-PMD, Okura Electric Co., Ltd.), images were formed on the prepared samples under a head power of 0.45 w / dot, with pulses of 0.1 msec intervals and a pulse width (applied energy) of 1.0 msec. One drop of 99% by mass EtOH was added to each sample. The samples were dried by holding them at 22°C for 3 hours. The black optical density of the printed images was measured using a spectrophotometer (Exact, X-Rite). Subsequently, the image preservation rate was calculated as follows: Preservation rate (%) = [Degree of optical black density after testing] / [Degree of optical black density before testing] × 100 This value is preferably 70% or higher: Rank I: Preservation rate of 70% or higher Rank II: Preservation rate between 50% and 99% Rank III: Preservation rate less than 50% The layer anchorage level was evaluated as follows: A piece of Scotch tape (NICHIBAN CT405AP-18) was placed on the surface of each sample. The tape was peeled off in the following three stages: Step 1) Slowly peel it off again in a 180° direction. Step 2) Slowly peel it off again at a 90° angle. Step 3) Quickly re-peel towards a 90° angle. The level of adhesion was evaluated according to the following ranks: Rank III: Layers are peeled off in the first step. Rank II: Layers are peeled off in the second step. Rank I: Layer delamination occurs in the third step, or no delamination occurs (substrate damage). Finally, the total score was calculated using the points for each item, according to the following rules: Rank I (or A in the case of a barcode) = 3 points Rank II (or B, C in the case of a barcode) = 1 point Rank III (or F if it's a barcode) = -3 points.

[0124] In the following, "SBR" represents styrene-butadiene resin, "Dev" represents a color developer, and "Pig" represents a pigment. In Tables 1a to 1d, "Ratio" represents the dry mass ratio. "Pigment ratio in undercoat layer" represents the weight ratio of all pigments (inorganic and organic fillers) in the undercoat layer to the total dry weight of the undercoat layer. "Hollowness ratio in pigment" indicates the ratio of the dry weight of hollow particles in the undercoat layer to the total dry weight of pigments in the undercoat layer. ">60% hollow" and ">80% hollow" indicate the proportion of hollow particles with a hollowness ratio higher than 60% and 80%, respectively, to the total dry weight of hollow particles in the undercoat layer.

[0125] [Table 1a]

[0126] [Table 1b]

[0127] [Table 1c]

[0128] [Table 1d]

[0129] [Table 1e]

[0130] [Table 1f]

[0131] [Table 1g]

[0132] [Table 1h] This application claims priority to European Patent Application No. 23305230.7, filed on 22 February 2023, the entire contents of which are incorporated herein by reference. [Explanation of symbols]

[0133] 1: Thermal recording medium 11, 11a, 11b: Protective layer 12: Heat-sensitive colored layer 13:Support layer 14: Undercoat layer

Claims

1. A thermal recording medium, wherein at least, Supporting layer, The undercoat layer on the aforementioned support layer, The heat-sensitive colored layer on the undercoat layer, A protective layer on the aforementioned heat-sensitive colored layer, It has, The undercoat layer comprises at least one type of plastic hollow particle as an organic filler, wherein the plastic hollow particle comprises at least 20% by mass of hollow particles relative to the total mass of hollow particles, and has a hollow ratio of 60% or more, and the hollow ratio is the percentage ratio of the inner diameter of the hollow particle to the outer diameter of the hollow particle. The heat-sensitive colored layer comprises at least two color developers selected from the following general formulas (1), (2), and (3): 【Chemistry 1】 【Chemistry 2】 【Transformation 3】 Includes, Here, in equation (1), R 1 ~R 3 Each of these independently represents a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxyl group, and a C1-C6 fluoroalkyl group. In equation (2), R 3 This represents a hydrogen atom, halogen atom, nitro group, amino group, alkyl group, alkoxy group, aryloxy group, alkylcarbonyloxy group, arylcarbonyloxy group, alkylcarbonylamino group, arylcarbonylamino group, alkylsulfonylamino group, arylsulfonylamino group, monoalkylamino group, dialkylamino group, or arylamino group. In formula (3), R represents an alkyl group and n represents an integer from 0 to 3, wherein the recording medium is a heat-sensitive recording medium.

2. The heat-sensitive recording medium according to claim 1, wherein at least 20% of the hollow particles have a hollowness ratio of 80% or more.

3. The heat-sensitive colored layer comprises N-[2-(3-phenylureido)phenyl]benzenesulfonamide as a color developer, according to claim 1 or 2.

4. The heat-sensitive colored layer comprises N-[2-(3-phenylureido)phenyl]benzenesulfonamide and a compound of general formula (2) as a color developer, according to claim 3.

5. The heat-sensitive colored layer further comprises 1,3-diphenylurea, according to any one of claims 1 to 4.

6. The undercoat layer comprises an inorganic filler having an oil absorption capacity of 60 g / 100 g or less, as described in any one of claims 1 to 5.

7. The thermal recording medium according to any one of claims 1 to 6, wherein the total pigment ratio (inorganic filler and organic filler) in the undercoat layer is 20% by weight or more and 80% by weight or less with respect to the total dry weight of the undercoat layer.

8. The aforementioned color developer (1) is given by the following general formula (4) 【Chemistry 4】 A thermal recording medium according to any one of claims 1 to 7, having the following:

9. The aforementioned color developer (2) is -O-SO 2 -(phenyl)-para-R 3 The group is in the meta position relative to the -NH-CO-NH-phenyl substituent, R 3 A heat-sensitive recording medium according to any one of claims 1 to 8, having a structure that is methyl.

10. In the color developer of general formula (3), each (R)n system is -SO 2 A thermosensitive recording medium according to any one of claims 1 to 9, comprising a single methyl group located in the para position relative to the -O group.

11. The heat-sensitive colored layer comprises at least one inorganic filler, the inorganic filler having an oil absorption capacity of 80 g / 100 g or more, according to any one of claims 1 to 10.

12. The heat-sensitive colored layer is free of a sensitizer, as described in any one of claims 1 to 11.

13. The heat-sensitive colored layer does not contain a sensitizer in the form of a benzyloxy derivative, an oxalate sensitizer, an alkoxy or aryloxy sensitizer, and / or an alcohol sensitizer, according to any one of claims 1 to 12.

14. A food package comprising a heat-sensitive recording medium according to any one of claims 1 to 13.

15. Use of the heat-sensitive recording medium according to any one of claims 1 to 13 in food packaging.

Images