Heat-sensitive recording media

Abstract

The present invention provides at least The supporters and an undercoat layer on the support layer; a heat-sensitive color layer on the undercoat layer; a protective layer on the heat-sensitive color layer; and the undercoat layer contains at least one kind of plastic hollow particles as an organic filler; The plastic hollow particles contain hollow particles in an amount of at least 20% by mass relative to the total mass of the hollow particles, and have a hollow ratio of 60% or more, the hollow ratio being the percentage of the inner diameter of the hollow particles relative to the outer diameter of the hollow particles; The heat-sensitive color layer comprises at least two types of color developers selected from the group consisting of (1) N-phenylureido-phenyl-benzenesulfonamide structure, (2) N-phenylureido-phenyl-oxysulfonyl-aryl structure, and / or (3) N,Nt-di-([arylsulfonyloxyl]-phenyl)urea structure. The present invention relates to the use of such heat-sensitive recording media in food packaging.

Description

[Technical Field]

[0001] In one aspect, the present invention relates to a heat-sensitive recording medium. In another aspect, the present invention relates to a label for attachment to a product comprising the heat-sensitive recording medium of the present invention, and to a consumer product package having attached thereto the heat-sensitive recording medium or label of the present invention. [Background technology]

[0002] Thermosensitive recording media are known to use colorant systems. A pigment, such as a leuco dye, in one layer of the media 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 in thermal paper. However, in this context, it is preferable to avoid the use of phenols, particularly bisphenol-A, bisphenol-S, and their derivatives, particularly 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 colour developers are presented in, for example, EP-A-3395583 and EP-A-3677569. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] European Patent Application Publication No. 2923851 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-150764 [Patent Document 3] European Patent Application Publication No. 3670205 [Patent Document 4] European Patent Application Publication No. 3395583 [Patent Document 5] European Patent Application Publication No. 3677569 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when N-phenylureido-phenyl-benzenesulfonamide is used as a color developer in the heat-sensitive color layer of a heat-sensitive recording medium, the dynamic sensitivity still does not reach the same level as that achieved with bisphenol-based color developers. Although several solutions have been proposed to address this issue, such as combining it with sensitizers or other color developers, 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 a digital printing process. In this field, extremely high heat resistance, exceeding 110°C, is desired due to the printing process using heated rolls. Processing a heat-sensitive recording medium that does not have sufficient heat resistance in a digital printing machine can result in undesirable color development. It is difficult to simultaneously achieve both dynamic sensitivity and extremely high heat resistance.

[0005] An 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 a preferred embodiment of the present invention, high background heat resistance up to 110°C is also achieved. [Means for solving the problem]

[0006] In view of solving the above-mentioned problems, in one aspect, the present invention provides a thermosensitive recording medium, comprising at least: The supporters and an undercoat layer on the support layer; a heat-sensitive color layer on the undercoat layer; a protective layer on the heat-sensitive color layer; and the undercoat layer contains at least one type of plastic hollow particles as an organic filler, the plastic hollow particles comprising at least 20% by mass of hollow particles relative to the mass of all hollow particles, and having a hollowness of 60% or more, the hollowness being a percentage ratio of the inner diameter of the hollow particles to the outer diameter of the hollow particles; The heat-sensitive color layer contains at least two types of color developers selected from the following general formulas (1), (2), and (3):

[0007] [ka]

[0008] [ka]

[0009] [ka] Including, Here, in formula (1), R 1 ~R 3 each independently represents a hydrogen atom, a halogen atom, a C1 to C6 alkyl group, a C1 to C6 alkoxyl group, or a C1 to C6 fluoroalkyl group, In equation (2), R 3 represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group, In formula (3), R represents an alkyl group, and n represents an integer of 0 to 3, relating to the thermosensitive recording medium.

[0010] In the thermosensitive recording medium of the present invention, it is preferred that at least 20% of the hollow particles in the undercoat layer have a void ratio of 80% or more. In a preferred embodiment, the void 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 color layer preferably further comprises 1,3-diphenylurea, which has been recognized as a useful stabilizer (storability improver) in the context of the present invention.

[0012] In another aspect, the present invention relates to consumer product packaging having attached or incorporated therein the heat-sensitive recording medium of the present invention.

[0013] In particular, food packaging incorporating the heat-sensitive recording medium of the present invention is contemplated. The heat-sensitive recording medium of the present invention may be converted into a label, for example, by laminating, die-cutting, and pre-printing, and then installed in the food packaging. The consumer product packaging may be partially or completely transparent, flexible or rigid, and may contain one or more perishable food items, such as prepared meals or bento boxes. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram of an illustrative, non-limiting example of a heat-sensitive recording medium according to an embodiment of the present invention. In this particular, non-limiting embodiment, in the heat-sensitive recording medium (1), a heat-sensitive color layer (12) is disposed on a support layer (13), and an undercoat layer (14) is incorporated between the support layer (13) and the heat-sensitive color layer (12). The heat-sensitive color layer (12) is also in contact with a protective layer (11) on the opposite side of the heat-sensitive color layer (12) from the undercoat layer (14) and the support layer (13). [Figure 2]1 is a schematic diagram showing an illustrative, non-limiting example of a heat-sensitive recording medium according to another embodiment of the present invention, where the arrangement is similar to the embodiment shown in FIG. 1, but here two successive protective layers, designated (11a) and (11b), are applied to the heat-sensitive color layer (12). (11a) is the lower protective layer and (11b) is the upper protective layer. DETAILED DESCRIPTION OF THE INVENTION

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

[0016] Possible supports include those made of fine 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 heat-sensitive recording material and cannot be generally specified, but is preferably 30 μm to 250 μm, more preferably 50 μm to 200 μm.

[0017] The transparent support may also be used in the form of a polymeric material present in the form of 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%. Suitable films exhibit 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] The film material used for the transparent support can 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), triacetyl cellulose film (TAC), norbornene film (NB), and Arton film. Other possibilities include polyethylene (PE) and polymethyl methacrylate (PMMA).

[0019] (undercoat layer) Generally, in the technical field of thermosensitive recording media, the term "undercoat" is understood by those skilled in the art to refer to a layer between the support and the thermosensitive color layer. The term "underlayer" can be used synonymously with "undercoat layer" by those skilled in the art.

[0020] If present in the thermosensitive recording medium of the present invention, the undercoat layer typically comprises a binder resin.

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

[0022] The water-soluble polymer used as the binder resin in the undercoat layer is not particularly limited and can be appropriately selected depending on the intended purpose. Examples include polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose, 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] The aqueous polymer emulsion that can be used as the binder resin for the undercoat layer is not particularly limited and can be appropriately selected depending on the intended purpose. Examples include latex of styrene-butadiene copolymer, and emulsions of vinyl acetate resin, acrylic resin, and polyurethane resin. These may be used alone or in combination. In a particularly preferred embodiment of the present invention, polyvinyl alcohol is used as the binder material in combination with styrene-butadiene copolymer added as an aqueous polymer emulsion.

[0024] When an undercoat layer is used in the thermosensitive recording medium of the present invention, an inorganic filler may be used or may be omitted. When an inorganic filler is used, examples thereof include aluminum hydroxide, calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silica, barium sulfate, talc, kaolin, alumina, clay, etc. These may be used alone or in combination. Among these, aluminum hydroxide, calcium carbonate, kaolin, and clay are preferred in terms of the liquid properties of the coating liquid, the stability of dispersed particles, and water solubility.

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

[0026] The undercoat layer of the thermosensitive recording medium of the present invention contains hollow particles.

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

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

[0029] In the thermosensitive recording medium of the present invention, it is preferred that at least 20% of the hollow particles in the undercoat layer have a void ratio of 80% or more. In a preferred embodiment, the void 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 based on the total dry weight of the undercoat layer.

[0031] When an undercoat layer is used in the heat-sensitive recording medium of the present invention, the coating amount is 0.4 g / m 2 to 10g / m 2 is preferred, and 0.6 g / m2 to 4g / m 2 is more preferred.

[0032] When used 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 generally defined, but is preferably 0.5 μm to 15 μm, and more preferably 0.8 μm to 6 μm.

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

[0034] A heat-sensitive color layer comprises a colorant system in which a pigment, such as a leuco dye, in one layer of the medium reacts with another component, the so-called "developer," upon the application of heat to produce a colored product.

[0035] Leuco dyes are compounds that exhibit electron-donating properties and can be used alone or in combination of two or more. However, leuco dyes themselves are colorless or light-colored dye precursors, and commonly known leuco compounds can be used. Examples of leuco compounds include triphenylmethanephthalide compounds, triarylmethane compounds, fluoran compounds, phenothiazine compounds, thiofluoran 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. Considering color development, background fogging, and image fading due to moisture, heat, and light exposure, the following compounds are specifically selected: 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-(di-n-butylamino)fluoran, 2-anilino-3-methyl-6-(di-n-pentylamino)fluoran, 2-anilino-3-methyl-6-(Nn-propyl-N-methylamino)fluoran, 2-anilino-3-methyl-6-(N-isopropyl-N-methylamino)fluoran, 2-anilino-3-methyl-6-(N-isobutyl-N-methylamino)fluoran, 2-anilino-3-methyl-6-(Nn-amyl-N-methylamino)fluoran, 2-anilino-3-methyl-6-(Nn-amyl-N-ethylamino)fluoran, 2-anilino-3-methyl-6-(N-sec-butyl-N-ethylamino)fluoran, 2-anilino-3-methyl-6-(Nn-amyl-N-ethylamino)fluoran, 2-anilino-3- Methyl-6-(N-isoamyl-N-ethylamino)fluoran, 2-anilino-3-methyl-6-(N-cyclohexyl-N-methylamino)fluoran, 2-anilino-3-methyl-6-(N-ethyl-p-toluidino)fluoran, 2-anilino-3-methyl-6-(N-methyl-p-toluidino)fluoran, 2-(m-trichloromethylanilino)-3-methyl-6-diethylaminofluoran, 2-(m-trifluoromethylanilino)-3-methyl-6-diethylaminofluoran, 2-(m-trifluoromethylanilino) -3-methyl-6-(N-cyclohexyl-N-methylamino)fluoran, 2-(2,4-dimethylanilino)-3-methyl-6-diethylaminofluoran, 2-(N-ethyl-p-toluidino)-3-methyl-6-(N-ethylamino)fluoran, 2-(N-methyl-p-toluidino)-3-methyl-6-(N-propyl-p-toluidino)fluoran, 2-anilino-6-(Nn-hexyl-N-ethylamino)fluoran, 2-(o-chloroanilino)-6-diethylaminofluoran, 2-(o-bromoanilino)-6-di Ethylaminofluoran, 2-(o-chloroanilino)-6-dibutylaminofluoran, 2-(o-fluoroanilino)-6-dibutylaminofluoran, 2-(m-trifluoromethylanilino)-6-diethylaminofluoran, 2-(p-acetylanilino)-6-(Nn-amyl-Nn-butylamino)fluoran, 2-benzylamino-6-(N-ethyl-p-toluidino)fluoran, 2-benzylamino-6-(N-methyl-2,4-dimethylanilino)fluoran, 2-benzylamino-6-(N-ethyl-2,4-dimethylanilino)fluoran, 2-benzylamino-6-(N-methyl-p-toluidino)fluoran, 2-dibenzylamino-6-(N-ethyl-p-toluidino)fluoran, 2-(di-p-methylbenzylamino)-6-(N-ethyl-p-toluidino)fluoran, 2-(p-phenylethylamino)-6-(N-ethyl-p-toluidino)fluoran, 2-methylamino-6-(N-methylanilino)fluoran, 2-methylamino-6-(N-ethylanilino)fluoran, 2-methylamino-6-(N-propylanilino)fluoran, 2-ethyl Amino-6-(N-methyl-p-toluidino)fluoran, 2-methylamino-6-(N-methyl-2,4-dimethylanilino)fluoran, 2-ethylamino-6-(N-methyl-2,4-dimethylanilino)fluoran, 2-dimethylamino-6-(N-methylanilino)fluoran, 2-dimethylamino-6-(N-ethylanilino)fluoran, 2-ethylamino-6-(N-methyl-p-toluidino)fluoran, benzoleuco methylene blue, 2-[3,6-bis(diethylamino)]-6-(o-chloroanilino)xanthylbenzoic acid lactam, 2-[3, 6-bis(diethylamino)]-9-(o-chloranilino)xanthylbenzoic acid 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-hydroxy-4,5-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,These include compounds such as 6-bis(dimethylamino)fluorene spiro(9,3')-6'-dimethylaminophthalide, 6'-chloro-8'-methoxy-benzoindolinospiropyran, and 6'-bromo-2'-methoxybenzoindolinospiropyran. These may be used alone or in combination.

[0036] The amount of the leuco dye contained in the heat-sensitive color layer is preferably 3% by mass to 30% by mass, assuming the total mass of the heat-sensitive color layer to be 100%.

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

[0038] In the present invention, the heat-sensitive color layer contains at least two types of color developers. At least two types, or in fact all three types, are present in the heat-sensitive color layer of the heat-sensitive recording medium of the present invention. 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) developer, in the case of an N-phenylureido-phenyl-benzenesulfonamide structure, a typical suitable compound has the following structure:

[0040] [ka] (1) where R 1 ~R 3 each independently represents a hydrogen atom, a halogen atom, a C1 to C6 alkyl group, a C1 to C6 alkoxyl group, or a C1 to C6 fluoroalkyl group.

[0041] Such a developer can be prepared, for example, according to the synthesis method described in European Patent Application Publication No. EP 2923851. Most preferably, the aromatic ring bonded to the SO group in the above formula (1) is (R 1 ) substituent, or one Me group or one Cl group, or -NHAc or -OMe. Most preferably, the central ring is 3 ) substituent. The aromatic ring (left side in the diagram above) connected via the urea group may represent -OMe, -F, -CF3, Cl, and CH3(Me) substituents.

[0042] In a particularly preferred embodiment, the heat-sensitive color layer of the heat-sensitive recording medium of the present invention comprises 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 the general formula (1). 1 , R 2 and R 3 corresponds to the case where all are hydrogen atoms.

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

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

[0046] Such developers can be prepared, for example, by the synthetic methods described in EP 3395583. Most preferably, R 3 is a methyl group. Most preferably in the present invention, the developer (2) is -O-SO2-(phenyl)-para-R relative to the -NH-CO-NH-phenyl substituent. 3 The group is in the meta position and has a structure where R3=methyl.

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

[0048] [ka] In formula (3), R represents an alkyl group and n represents an integer from 0 to 3. Such a developer can be prepared, for example, by the synthesis method described in EP 3677569. In the present invention, the developer of formula (3) most preferably has a structure in which each (R)n system is constituted by a single methyl group in the para position relative to the -SO2-O group.

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

[0050] In the present invention, various other known color developers can be used as desired as long as the effects of the present invention are not impaired. These other color developers are color developers containing various electron-accepting compounds and oxidizing agents that can cause leuco dyes to develop color.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 monobenzyl phthalate, 4,4'-cyclohexylidenediphenol, 4,4'-isopropylidenebis(2 - Chlorophenol), 4,4'-diphenol sulfone, 4-isopropoxy-4'-hydroxydiphenyl sulfone, 4-benzyloxy-4'-hydroxydiphenyl sulfone, 4,4'-diphenol sulfoxide, isopropyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, benzyl protocatechuate, stearyl gallate, lauryl gallate, octyl gallate, 1,3-bis(4 - hydroxyphenylthiopropane, 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'-diphenol sulfone, 2,2'-diallyl-4,4'-diphenol sulfone, 3,4-dihydroxyphenyl-4'-methyldiphenyl sulfone, zinc 1-acetyloxy-2-naphthoate, zinc 2-acetyloxy-1-naphthoate, zinc 2-acetyloxy-3-naphthoate, p-bis(4-hydroxyphenyl)-p-methyltoluene, and antipyrine complex of zinc thiocyanate, which may be used alone or in combination.

[0051] Among other alternative color developers that can be added to the heat-sensitive color layer of the heat-sensitive recording medium, the following urea-urethane and D90 commercial products also come into consideration:

[0052] [ka] Urea urethane compounds

[0053] [ka] D90 The amount of additional color developer can be suitably selected depending on the intended purpose, as long as it does not impair the effects of the present invention. In a preferred embodiment, the total mass of other color developers other than those of formula (1), (2), or (3) is less than 2 parts by mass per part by mass of the leuco dye in the heat-sensitive color layer, and more preferably less than 0.5 parts by mass per part by mass of the leuco dye in the heat-sensitive color layer. In a specific embodiment of the present invention, other color developers than those of formula (1), (2), or (3) may be substantially absent in the heat-sensitive color layer.

[0054] If necessary, various known stabilizers (storability improvers) 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 a relatively low coloring power and can be optionally added to the heat-sensitive recording layer as a preliminary additive. Specific examples thereof include 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 4,4'-butylidenebis(6-tert-butyl-2-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 4,4'-thiobis(6-tert-butyl-2-methylphenol), and 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.

[0055] In addition to the above-mentioned leuco dye, developer, and stabilizer, other materials generally used in heat-sensitive recording materials, such as binders, fillers, sensitizers, crosslinking agents, pigments, surfactants, fluorescent brighteners, 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 coating properties of the layer. There are no particular limitations on the binder, and it may be appropriately selected depending on the purpose. Specific examples of binder resins include starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, 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 can be appropriately selected depending on 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, as well as 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 standpoint of color density. Within the framework of the present invention, calcined kaolin is preferred. In the heat-sensitive color layer of the heat-sensitive recording medium according to the present invention, it is preferred to use an inorganic filler with a high oil absorption, such as 80 g / 100 g or more. Examples of suitable fillers in this context for the heat-sensitive color layer include calcined kaolin and amorphous silica.

[0058] Typically, various thermoplastic materials are added to the heat-sensitive color layer as sensitivity enhancers (sensitizers). In a preferred embodiment of the present invention, no sensitizers are added to the heat-sensitive color layer. In the present invention, it has been observed that the absence of sensitizers improves heat resistance, especially when measured at, for example, 110°C. In some cases, sensitizers can improve the coloring effect by providing a temporary solvent that melts under the influence of heat and promotes the reaction between the leuco dye and the developer. It is noted that when heat resistance is required, such as in label applications for ready-to-eat foods or digital printing processes, it is preferable to not add thermoplastic materials or to select compounds with a melting point of 90°C or higher.

[0059] Examples of sensitizers include: (1) fatty acids such as stearic acid and behenic acid; (2) fatty acid amides such as stearic acid amide and palmitic acid amide; (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; and (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, glycol carbonate; (8) terephthalic acids such as dibenzyl terephthalate, dimethyl terephthalate; (9) dibenzyl oxalate, bis(4-methylbenzyl) oxalate, bis(4-chlorobenzyl) Oxalate ester sensitizers such as oxalates; (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)benzyloxy (11) Alkoxy- and aryloxy-based sensitizers such as benzene, p-(2-vinyloxyethoxy)biphenyl, p-aryloxybiphenyl, p-propargyloxybiphenyl, 1,2-bis(4-methoxyphenoxy)propane, and 1,5-bis(4-methoxyphenoxy)-3-oxapentane; (12) Alcohol-based sensitizers such as 1,1-diphenylethanol, 1,1-diphenylpropanol, p-benzyloxybenzyl alcohol, and 1,3-phenoxy-2-propanol;(12) Sulfur-based sensitizers such as 1,4-diphenylthiobutane, 1,4-diphenylthio-2-butene, and dibenzyl disulfide;

[0060] In a preferred embodiment of the present invention, the heat-sensitive color layer may be free of compounds known in the field of heat-sensitive recording media as sensitizers, such as compounds containing aryloxy groups (e.g., phenoxy and tolyloxy groups), alkoxy groups, oxaate groups, and benzyloxy groups. Therefore, it is particularly important that the heat-sensitive color layer does not contain sensitizers of the types shown in Groups 6, 9, 10, and 11 above. The absence of such sensitizers can improve heat resistance, particularly when measured at 110°C.

[0061] The color layer can be formed by a commonly known method. In a preferred embodiment, to avoid reactions between the components of the heat-sensitive color layer, the components are dispersed separately and then mixed. When grinding the binder and other components, a disperser such as a ball mill, attritor, or sand mill is typically used to obtain a particle size of 0.2 μm to 3 μm, preferably 0.2 μm to 1 μm. The resulting dispersion is optionally mixed with a filler and a heat-meltable material (sensitizer) dispersion according to a predetermined composition to prepare a coating liquid for the heat-sensitive color layer, which is then coated onto a support.

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

[0063] (protective layer) In the present invention, at least one protective layer is provided on the heat-sensitive layer. Several different protective layers may be superimposed on one another, each focusing on matching or barrier properties.

[0064] At least one protective layer in the thermosensitive recording medium of the present invention may suitably contain wax particles, and the average particle size is preferably at least 0.05 μm or more and 2.0 μm or less. When there are multiple protective layers, only the topmost protective layer, which is the surface furthest from the thermosensitive coloring layer and exposed to the outside, may suitably contain wax particles. In a preferred embodiment in which the topmost 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 50 ) is obtained in a method for measuring the average particle size. This measurement can be carried out, for example, by an LA-950 apparatus manufactured by HORIBA LA-950.

[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 not more than 200° C. More preferably, the melting point of the wax is at least 90° C. and not more than 130° C., and most preferably at least 100° C. and not more than 120° C.

[0067] More preferably, the wax particle size is at least 0.1 μm and not more than 0.5 μm.

[0068] In a significant embodiment, the wax particles comprise at least 2.0% by weight and no more than 20% by weight, based on 100% by weight of all components of the protective layer as a whole, and more preferably at least 5.0% by weight and no more than 10% by weight, based on 100% by weight of all 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, Montan acid ester wax, carnauba wax, paraffin wax, ester wax and metal salts thereof, higher fatty acid amides, higher fatty acid esters, animal waxes, vegetable waxes, mineral waxes, and petroleum waxes.

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

[0071] The protective layers usually contain at least a binder, and each of the protective layers may contain an inorganic filler and a surfactant.

[0072] The binder (or binders) for the protective layers are not particularly limited and can be selected appropriately 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 those that are typically provided as aqueous emulsions during the preparation of the protective layer, such as urethane resins, epoxy resins, vinyl acetate (co)polymers, vinylidene chloride (co)polymers, vinyl chloride (co)polymers, and styrene-butadiene copolymers. A particularly preferred 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, more preferably from 0.5 μm to 5 μm. In a non-limiting exemplary embodiment of the present invention, a protective layer having a thickness of 2.5 μm when dried can be used. If multiple protective layers are applied, a smaller individual thickness is required for each. The preferred maximum cumulative thickness of all protective layers combined is 10 μm in the dried final product.

[0074] When an inorganic filler is contained in the protective layer, the inorganic filler is not particularly limited and may be appropriately selected depending on 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 alone or in combination. Among these, aluminum hydroxide and calcium carbonate are particularly preferred. This is because a protective layer containing such an inorganic filler provides excellent abrasion resistance to a thermal head during long-term printing. The amount of inorganic filler in the protective layer is not particularly limited and may be appropriately selected depending on the intended purpose. The amount of inorganic filler varies depending on the type of filler, but is preferably 50 parts by mass or more and 500 parts by mass or less per 100 parts by mass of the binder resin.

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

[0076] The method for forming the first protective layer, the second protective layer, and subsequent protective layers is not particularly limited and 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 the protective layer in the present invention and can also be used when applying the heat-sensitive color layer.

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

[0078] The backing layer may further include other ingredients such as fillers, lubricants, and antistatic agents.

[0079] The binder resin may be either a water-dispersible resin or a water-soluble resin. Specific examples include conventionally known water-soluble polymers and aqueous polymer emulsions.

[0080] The water-soluble polymer is not limited in any way and may be suitably selected depending on the intended purpose. Examples thereof 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] The aqueous polymer emulsion is not particularly limited and may be appropriately selected depending on the purpose. Examples include latexes such as styrene-butadiene copolymers, and emulsions of vinyl acetate resins, acrylic resins (e.g., acrylic acid-acrylic acid ester copolymer latexes), (meth)acrylamide resins, and polyurethane resins. These may be used alone or in combination.

[0082] The crosslinking agent is not particularly limited and can be selected appropriately depending on the purpose. Examples include polyamine compounds such as ethylenediamine; polyaldehyde 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; polyvalent 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 in the crosslinking agent, but is preferably 0.1 to 100 parts by weight, more preferably 1 to 100 parts by weight, per 100 parts by weight of binder resin.

[0083] The filler may be either an inorganic filler or an organic filler. 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] Examples of the antistatic agent include commonly used ionically conductive antistatic agents and electronically conductive antistatic agents. Specific examples of ionically 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 metal compounds such as conductive tin and antimony oxide, and conductive polymers such as polyaniline. Among these antistatic agents, polystyrene sulfonate in particular reacts with aziridine to improve water resistance through crosslinking. Furthermore, salts copolymerized with maleic acid are advantageous in that they have antistatic properties and also improve water resistance.

[0085] The method for forming the back layer is not particularly limited and may be appropriately selected depending on the purpose. The back layer is preferably formed by applying a back layer coating liquid to the support.

[0086] The coating method is not particularly limited and may be appropriately selected depending on the purpose, examples of which include blade coating, roll coating, wire bar coating, die coating, and curtain coating.

[0087] The thickness of the back layer is not particularly limited and may be appropriately selected depending on the purpose, but is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 5 μm.

[0088] (viscous layer) The thermosensitive recording medium of the present invention may be provided with a tacky layer, also called an adhesive layer, although such a tacky layer is not essential to the present invention and may be optionally selected.

[0089] The support layer or backing layer (back layer) may be provided with an adhesive layer on the surface opposite to the surface on which the protective layer is formed. For example, in a typical application of the present invention, the adhesive layer may assist in attaching the thermosensitive recording medium to a food package. Therefore, the thermosensitive recording medium of the present invention may be provided with an adhesive surface attached to the support or backing layer, which is useful when providing a label with an adhesive layer. A peelable liner may then be attached to the adhesive layer and removed before the label is finally attached to the product to be labeled. The adhesive layer may also provide antistatic properties. The method for forming the adhesive layer is not particularly limited. Examples of methods include a general coating method and a lamination method. The average thickness of the adhesive layer is not particularly limited and may be selected appropriately depending on the purpose, but is preferably 0.1 μm or more and 20 μm or less.

[0090] The adhesive layer material is not particularly limited and may be selected appropriately depending on the purpose. Examples of adhesive layer materials include urea resins, melamine resins, phenolic resins, epoxy resins, vinyl acetate resins, vinyl acetate-acrylic 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 ester copolymers, methacrylic ester copolymers, natural rubber, cyanoacrylate resins, and silicone resins. These materials may be used alone or in combination. These materials may be crosslinked with a crosslinking agent. The adhesive layer material may be a hot-melt type. In one embodiment of the present invention, a 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 of the heat-sensitive recording media of the embodiments of the present invention using an image recording unit that is either a thermal head or a laser.

[0092] There are no particular limitations on the shape, structure, and size of the thermal head, and they may be selected appropriately depending on the purpose.

[0093] The laser is not particularly limited and can be selected depending on the intended purpose. In one preferred embodiment, a CO2 laser emitting light having a wavelength of 9.3 μm to 10.6 μm can be used. By using a CO2 laser emitting light having a wavelength of 9.3 μm to 10.6 μm, a good laser-printed image can be obtained without using a photothermal conversion agent such as a phthalocyanine pigment. Other laser types, such as a FLDA (fiber laser diode array), may also be used. [Example]

[0094] The present invention will be specifically described 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 "% by mass".

[0095] Example 1 A heat-sensitive recording medium was formed according to the following procedure.

[0096] 1) A coating liquid for an undercoat layer was applied onto the substrate, thereby forming an undercoat layer (2 g / m as a dry mass). 2 In this example, a thickness of approximately 60 g / m 2 The coating liquid formulation for the undercoat layer was as follows: Coating liquid formulation for undercoat layer Preparation of the coating solution for the underlayer: [Liquid A1] Undercoat liquid 1 -------------------------------------------------------------------------------- Microscopic hollow spherical 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, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Dow AF-1570, styrene / acrylic copolymer resin, solid content 17.5%, average particle diameter 1.6 μm, hollow ratio 65% 3) Solid content: 50.0% 4) Fully hydrolyzed PVA.

[0097] 2) A coating liquid for a heat-sensitive recording layer was applied onto the undercoat layer to form a heat-sensitive recording layer.

[0098] For the preparation of the coating liquid for the heat-sensitive color layer, the following composition was prepared: [Liquid B] Pigment dispersion liquid -------------------------------------------------------------------------------- 2-anilino-3-methyl-6-(di-n-butylamino)fluoran (dye) 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] Developer dispersion liquid 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] Developer dispersion liquid 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 above general formula (2) has the following specific structure:

[0100] [ka] [Liquid D1] Filler dispersion liquid 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: 105 to 120g / 100g 2) Itacon-modified polyvinyl alcohol, manufactured by Kuraray Co., Ltd. Using a sand mill, the above-mentioned [Liquid B], [Liquid C1], [Liquid C2], and [Liquid D1] were dispersed so that the particles contained in each liquid had an average particle diameter of 1 μm or less, thereby preparing a pigment dispersion [Liquid B], a color developer dispersion [Liquid C1], [Liquid C2], and a 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 color layer coating liquid (Liquid E1).

[0102] Liquid E1 was uniformly applied to the undercoat layer to form a heat-sensitive color layer. The coating amount of the heat-sensitive layer was 0.4 g / m on a dry basis. 2 and then dried, thereby forming a heat-sensitive color layer.

[0103] 3) On the heat-sensitive color layer, coating liquids for a two-layered protective layer (first coating liquid and second coating liquid) were applied so that the lower protective layer made of the first coating liquid was underneath the upper protective layer made of the second coating liquid, thereby forming a two-layered protective layer on the heat-sensitive recording layer. The upper and lower protective layers were each coated in a dry weight of 1 g / m 2 , 1g / m 2 The coating solution for the two-layer protective layer had the following formulation, which was 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 dispersion formulation 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 for 48 hours at 50° C. In addition to this process, the samples were calendered at 20 kgF and then subjected to quality evaluation.

[0104] (For Examples 2 to 13) In Examples 2 to 12, the thermosensitive recording media were each prepared according to Example 1 except as indicated 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] (Developer dispersion liquid 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 liquid 2 -------------------------------------------------------------------------------- amorphous silica 1) 32 copies 10% itacon-modified polyvinyl alcohol aqueous solution 2) 32 copies Water 36 parts -------------------------------------------------------------------------------- 1) Mizusawa Industrial Chemicals Mizusil P-527, oil absorption: 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] Developer dispersion liquid 1 10 parts [Liquid C2] Developer dispersion liquid 2 10 parts [Liquid C3] Developer dispersion 3 10 parts [Liquid D1] Filler dispersion liquid 1 31 parts Water Pigment dispersion 29 parts -------------------------------------------------------------------------------- Thus, in Examples 1 to 3, different combinations of color developers are compared. In Example 4, a combination of three different types of color developers is 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 color layer is tested.

[0106] Example 14 In Example 14, a heat-sensitive recording medium was produced in the same manner as in Example 1, except that the coating liquid for the heat-sensitive colored layer, [Liquid E3], was prepared by adding the following [Liquid I]. [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 coloring layer liquid 3 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C1] Developer dispersion liquid 1 15 parts [Liquid C2] Developer dispersion liquid 2 15 parts [Liquid I] Preservability improved dispersion liquid 3 parts [Liquid D1] Filler dispersion liquid 1 29 parts Water 28 parts -------------------------------------------------------------------------------- Examples 15 to 22 In Examples 15 to 22, thermosensitive recording media were prepared in accordance with Example 1, except for the changes shown in Table 1 below. [Liquid A2], [Liquid A3], [Liquid A4], [Liquid A5], [Liquid A6], [Liquid A7], [Liquid A8], and [Liquid A9] were prepared as follows.

[0107] (Example 15) [Liquid A2] Undercoat Liquid 2 -------------------------------------------------------------------------------- Microscopic hollow spherical 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, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 3) Solid content: 50.0% 4) Fully hydrolyzed PVA.

[0108] (In the case of Examples 16 and 17) [Liquid A3] Undercoat Liquid 3 -------------------------------------------------------------------------------- Microscopic hollow spherical 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, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 3) Solid content: 50.0% 4) Fully hydrolyzed PVA.

[0109] (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, solid content 17.5%, average particle diameter 1.6 μm, hollow ratio 65% 2) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 3) Solid content: 50.0% 4) Fully hydrolyzed PVA.

[0110] (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) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 2) Solid content: 50.0% 3) Fully hydrolyzed PVA.

[0111] (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: Dow styrene / acrylic copolymer resin, solid content 17.5%, average particle diameter 1.6 μm, hollowness 65% 2) Solid content: 50.0% 3) Fully hydrolyzed PVA.

[0112] (Example 21) [Liquid A7] Undercoat Liquid 7 -------------------------------------------------------------------------------- Microscopic hollow spherical 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, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 3) UW-90 4) Solid content: 50.0% 5) Fully hydrolyzed PVA.

[0113] (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, solid content 17.5%, average particle diameter 1.6 μm, hollow ratio 65% 2) Matsumoto Yushi Pharmaceutical Co., Ltd. R-500, solid content 33.0%, hollow ratio 90% 3) UW-90 4) Solid content: 50.0% 5) Fully hydrolyzed PVA.

[0114] (Reference Examples 1 to 5) In Reference Examples 1 to 5, heat-sensitive recording media were prepared according to Example 1, except for the changes shown in the following Table 1. Therefore, three types of sensitizers were added to the heat-sensitive color 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] Developer dispersion liquid 15 parts [Liquid C] Developer dispersion liquid 15 parts [Liquid J] Sensitizer dispersion 3 parts [Liquid D1] Filler dispersion liquid 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, 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 color layer liquid.

[0116] (Comparative Examples 6, 7 and 14) [Liquid C4] Developer dispersion liquid 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 manufactured 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, thermosensitive recording media were prepared according to Example 1, except for the changes shown in Table 1 below. Therefore, the following three types of undercoat layer liquids were tested: [Liquid A12], [Liquid A13], and [Liquid A14].

[0118] (Comparative Example 8) [Liquid Al2] Undercoat Liquid 12 -------------------------------------------------------------------------------- Micro-spherical hollow plastic particles1) 36 parts Styrene / butadiene copolymer latex 2) 10 parts 6 parts of 10% aqueous polyvinyl alcohol solution3) Water 48 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Solid content: 50.0% 3) Fully hydrolyzed PVA.

[0119] (Comparative Example 9) [Liquid A13] Undercoat Liquid 13 -------------------------------------------------------------------------------- Microscopic hollow spherical 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, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) UW-90 3) Solid content: 50.0% 4) Fully hydrolyzed PVA.

[0120] (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) Solid content: 50.0% 3) Fully hydrolyzed PVA.

[0121] (Comparative Examples 11 to 14) In Comparative Examples 11 to 14, thermosensitive recording media were prepared according to Example 1, except for the changes shown in the following Table 1. Therefore, each developer dispersion was used without being combined with any other. [Liquid E5] Heat-sensitive coloring layer liquid 5 -------------------------------------------------------------------------------- [Liquid B] Pigment dispersion 10 parts [Liquid C] Developer dispersion liquid 31 parts [Liquid D1] Filler dispersion liquid 1 31 parts Water 28 parts -------------------------------------------------------------------------------- (Comparative Example 15) In Comparative Example 15, a thermosensitive recording medium was produced according to Example 1, except for the changes shown in Table 1 below. Therefore, the undercoat layer liquid [Liquid A15] was used instead of [Liquid A1]. [Liquid A15] Undercoat Liquid 15 -------------------------------------------------------------------------------- Microscopic hollow spherical plastic particles 1) 33 copies Spherical hollow plastic particles 2) 22 copies Styrene / butadiene copolymer latex 3) 2nd part 10% aqueous polyvinyl alcohol solution 4) 9 parts Water 34 parts -------------------------------------------------------------------------------- 1) Dow AF-1055, styrene / acrylic copolymer resin, solid content 26.5%, average particle diameter 1 μm, hollow ratio 55% 2) Dow AF-1570, styrene / acrylic copolymer resin, solid content 17.5%, average particle diameter 1.6 μm, hollow ratio 65% 3) Solid content: 50.0% 4) Fully 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, so that no protective layer was coated on the heat-sensitive color layer.

[0123] (Evaluation method) Still image density was evaluated as follows: Using a thermal gradient tester (HG-100, manufactured by Toyo Seiki Seisakusho), thermal energy was applied from 140°C to 180°C for 1 second at a pressure of 0.36 kgf. The black optical density was measured using a spectrophotometer (device name: Exact, manufactured by X-Rite). The maximum value was recorded as the still image density. This value is preferably 1.30 or higher. Rank I: Optical density greater than 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 Dynamic image density was assessed as follows: Images were printed using a ZEBRA 110Xi4 printer at a printing speed of 304 mm / sec (12 in / sec) with applied energies ranging from +24 to +30. A spectrophotometer (device name: Exact, manufactured by X-Rite) was used to measure the black optical density at each energy level. The maximum value was recorded as the video density. This value is preferably 1.20 or greater: Rank I: Optical density greater than 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 The decodability of the barcode was evaluated as follows: A picket-type barcode was printed using a ZEBRA 110Xi4 printer at a print speed of 304 mm / s (12 in / s) with an applied energy of +15 (bars printed parallel to the printing direction). The barcode was read using a barcode verifier (OMRON, MICROSCAN LVS-9580) to evaluate its decodability. The results were recorded according to the ANSI symbol grade. A high value (maximum 4.0) is preferred. Dynamic sensitivity was evaluated as follows: Images were printed using a MarkPoint MK2 printer at a printing speed of 102 mm / sec (4 in / sec) with an applied energy of 8.88 mJ. The black optical density was measured using a spectrophotometer (device name: Exact, manufactured by X-Rite). This value is preferably 1.20 or greater: Rank I: Optical density greater than 1.20 Rank II: Optical density between 1.00 and 1.19 Rank III: Optical density less than 0.99 Background 110°C heat resistance was evaluated as follows: Using a thermal gradient tester (HG-100, manufactured by Toyo Seiki Seisakusho), thermal energy of 110°C was applied for 1 second at a pressure of 0.36 kgf. 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 greater than 0.36 Background heat resistance at 100°C and 90°C was assessed as follows: The prepared samples were treated at 100°C or 90°C for 1 hour. The black optical density was measured using a spectrophotometer (device 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 greater than 0.36 The plasticizer resistance of the printed images was evaluated as follows: Using a thermal printing simulator (OHKURA ELECTRIC TH-PMD), images were generated on the prepared samples with 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 polyvinyl chloride (PVC, Shin-Etsu Polymer) sheet at 40°C for 15 hours. The black optical density of the printed image was measured using a spectrophotometer (Exact, X-Rite). The image retention rate was then calculated as follows: Preservation rate (%) = [degree of optical black density after test] / [degree of optical black density before test] x 100 This value should preferably be 90% or higher: Rank I: Save rate over 90% Rank II: Save rate between 70% and 89% Rank III: Save rate less than 70% The oil resistance of the printed images was evaluated as follows: Using a thermal recording simulator (OHKURA ELECTRIC TH-PMD), images were formed on the prepared samples with a head power of 0.45 w / dot, an interval of 0.1 msec, and a pulse width (applied energy) of 1.0 msec. A drop of cottonseed oil was placed on each sample and spread with a cotton swab. The samples were then kept at 40°C for 15 hours. The black optical density of the printed images was measured using a spectrophotometer (Exact, X-Rite). The image retention rate was then calculated as follows: Preservation rate (%) = [degree of optical black density after test] / [degree of optical black density before test] x 100 This value is preferably 90% or more. Rank I: Save rate over 90% Rank II: Save rate between 70% and 89% Rank III: Save rate less than 70% The ethanol (EtOH) resistance of the printed images was evaluated as follows: Using a thermal recording simulator (TH-PMD manufactured by Okura Electric Co., Ltd.), images were formed on the prepared samples with 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 99% by weight EtOH was applied to each sample. The samples were dried at 22°C for 3 hours. The black optical density of the printed images was measured using a spectrophotometer (device name: Exact, manufactured by X-Rite). The image retention rate was then calculated as follows: Preservation rate (%) = [degree of optical black density after test] / [degree of optical black density before test] x 100 This value should preferably be 70% or higher: Rank I: Save rate over 70% Rank II: Save rate between 50% and 99% Rank III: Save rate less than 50% The level of anchorage of the layer was assessed as follows: A piece of Scotch tape (NICHIBAN CT405AP-18) was placed on the surface of each sample. The tape was removed in three steps: Step 1) Slowly peel back at a 180° angle Step 2) Slowly peel back at a 90° angle Step 3) Quickly peel back at a 90° angle The adhesion level was evaluated according to the following ranking: Rank III: Layer peeled off in the first step Rank II: Layer peeled off in the second step Rank I: The layer peeled off in the third step, or there was no peeling (substrate damage) Finally, the scores for each item were used to calculate the overall score according to the following rules: Rank I (or A for barcode) = 3 points Rank II (or B, C for barcodes) = 1 point Rank III (or F for barcode) = -3 points.

[0124] In the following, "SBR" stands for styrene-butadiene resin, "Dev" stands for developer, and "Pig" stands for pigment. The "ratio" in Tables 1a to 1d stands for dry mass ratio. "Pigment ratio in undercoat layer" stands for the weight ratio of all pigments (inorganic filler and organic filler) in the undercoat layer to the total dry weight of the undercoat layer. "Hollow fraction 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 hollow fraction greater than 60% and 80%, respectively, relative 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 from European Patent Application No. 23305230.7, filed February 22, 2023, the entire contents of which are incorporated herein by reference. [Explanation of symbols]

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

Claims

1. A thermosensitive recording medium comprising at least: The supporters and an undercoat layer on the support layer; a heat-sensitive color layer on the undercoat layer; a protective layer on the heat-sensitive color layer; and the undercoat layer contains at least one type of plastic hollow particles as an organic filler, the plastic hollow particles comprising at least 20% by mass of hollow particles relative to the mass of all hollow particles, and having a hollowness of 60% or more, the hollowness being a percentage ratio of the inner diameter of the hollow particles to the outer diameter of the hollow particles; The heat-sensitive color layer contains at least two types of color developers selected from the following general formulas (1), (2), and (3): 【Chemistry 1】 【Chemistry 2】 【Transformation 3】 Including, Here, in equation (1), R 1 ~R 3 each independently represents a hydrogen atom, a halogen atom, a C1 to C6 alkyl group, a C1 to C6 alkoxyl group, or a C1 to C6 fluoroalkyl group, In equation (2), R 3 represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group, In formula (3), R represents an alkyl group, and n represents an integer of 0 to 3.

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

3. 3. The heat-sensitive recording medium according to claim 1, wherein the heat-sensitive color layer contains N-[2-(3-phenylureido)phenyl]benzenesulfonamide as a color developer.

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

5. 5. The heat-sensitive recording medium according to claim 1, wherein the heat-sensitive color layer further contains 1,3-diphenylurea.

6. 6. The heat-sensitive recording medium according to claim 1, wherein the undercoat layer contains an inorganic filler having an oil absorption of 60 g / 100 g or less.

7. 7. A heat-sensitive recording medium according to claim 1, 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, based on the total dry weight of the undercoat layer.

8. The developer (1) is represented by the following general formula (4): 【Chemistry 4】 The heat-sensitive recording medium according to any one of claims 1 to 7, comprising:

9. The developer (2) is -O-SO 2 -(phenyl)-para-R 3 group is meta to the -NH-CO-NH-phenyl substituent, and R 3 9. The heat-sensitive recording medium according to claim 1, wherein the structure is: methyl.

10. In the developer of general formula (3), each (R)n group is -SO 2 10. The heat-sensitive recording medium according to claim 1, which is constituted by a single methyl group in the para position relative to the -O group.

11. 11. The heat-sensitive recording medium according to claim 1, wherein the heat-sensitive colored layer contains at least one inorganic filler, and the inorganic filler has an oil absorption of 80 g / 100 g or more.

12. 12. The heat-sensitive recording medium according to claim 1, wherein the heat-sensitive color layer does not contain a sensitizer.

13. 13. The heat-sensitive recording medium according to claim 1, wherein the heat-sensitive color layer does not contain sensitizers in the form of benzyloxy derivatives, oxalate sensitizers, alkoxy or aryloxy sensitizers, and / or alcohol sensitizers.

14. A food package comprising the 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 a food package.

Images