Heat-sensitive recording medium

Abstract

The invention relates to a heat-sensitive recording medium comprising at least:-a support layer,-an undercoat layer on the support layer,-a heat-sensitive pigmented layer on the undercoat layer,-a protective layer on the heat-sensitive pigmented layer, the undercoat layer comprising at least one type of plastic hollow particles as organic filler, and the protective layer comprising at least one type of plastic hollow particles as organic filler. The plastic hollow particles comprise at least 20 mass% of the hollow particles with respect to the mass of all the hollow particles, having a hollowness ratio of 60% or greater, the hollowness ratio being a percentage of the inner diameter of the hollow particles to the outer diameter of the hollow particles; and-the heat-sensitive colored layer comprises at least two types of color developing agents from the group consisting of (1) an N-phenylureido-phenyl-benzenesulfonamide structure, (2) an N-phenylureido-phenyl-oxysulfonyl-aryl structure, and / or (3) an N, N '-di-([arylsulfonyloxy]-phenyl) urea structure. The invention relates to the use of such a heat-sensitive recording medium in food packaging.

Description

Technical Field

[0001] The present invention relates in one of its aspects to a thermosensitive recording medium. In a further aspect, the present invention relates to a label for attachment to a product containing the thermosensitive recording medium of the present invention, and to consumer product packaging to which the thermosensitive recording medium or label of the present invention has been attached. Background Art

[0002] Thermal recording media are known, and they use colorant systems, wherein when heat is applied, a dye in one layer of the medium, such as a leuco dye, reacts with another component, so-called " developer ", to produce a colored product. With regard to leuco dye-developer pairs, phenols can be successfully used as developers in thermal paper (thermal paper). However, particularly for environmental and human health reasons, it is preferred to avoid using phenols, especially bisphenol-A, bisphenol-S, and derivatives thereof in this context. For example, N-phenylurea-phenyl-benzenesulfonamide has been proposed as the non-phenol developer in this context in EP 2 923 851, JP-2015-150764 and EP3670205. In addition, a variety of structures of non-phenol developers have been proposed, such as in EP3395583 and EP3677569.

[0003] Citation List

[0004] Patent Literature

[0005] [PTL 1] EP 2 923 851

[0006] [PTL 2] JP-2015-150764

[0007] [PTL 3] EP 3670205

[0008] [PTL 4] EP 3395583

[0009] [PTL 5] EP 3677569 Summary of the Invention

[0010] Technical issues

[0011] However, when using N-phenylureido-phenyl-benzenesulfonamide as a developer in the heat-sensitive coloring layer of heat-sensitive recording media, it still fails to achieve the same level of dynamic sensitivity as bisphenol-based developers. Various countermeasures have been proposed to improve this issue, such as combining it with sensitizers or other developers. However, it is difficult to simultaneously improve dynamic sensitivity and maintain high heat resistance. There is a market for converting heat-sensitive recording media into labels through digital printing processes. In this field, due to the printing process using heated rollers, extremely high heat resistance, such as heat resistance above 110°C, is desired. When heat-sensitive recording media without sufficient heat resistance are processed in digital printing presses, undesirable color development may occur. It is difficult to simultaneously address this compatibility issue of dynamic sensitivity and provide very high heat resistance.

[0012] The object of the present invention is to provide a thermosensitive recording medium which achieves good dynamic sensitivity of the printed image and good plasticizer resistance, oil resistance and alcohol resistance. In a preferred embodiment of the invention, a high heat resistance up to a background of 110° C. is also achieved.

[0013] Solution to the problem

[0014] To solve the above problems, the present invention relates in one aspect to a thermosensitive recording medium comprising at least:

[0015] - support layer,

[0016] - a primer layer on said support layer,

[0017] - a thermosensitive coloring layer on the base coat,

[0018] - a protective layer on the thermosensitive coloring layer,

[0019] in

[0020] - the undercoat layer contains at least one type of plastic hollow particles as an organic filler, the plastic hollow particles accounting for at least 20% by mass of the hollow particles relative to the mass of all hollow particles, having a hollow ratio of 60% or greater, the hollow ratio being the percentage of the inner diameter of the hollow particles to the outer diameter of the hollow particles; and

[0021] - The heat-sensitive coloring layer contains at least two types of color developers selected from the group consisting of general formulas (1), (2) and (3):

[0022] [Chemistry 1]

[0023]

[0024] [Chemistry 2]

[0025]

[0026] [Chemistry 3]

[0027]

[0028] in,

[0029] - In formula (1), R1 to R3 each independently represent a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group;

[0030] -In formula (2), R 3 is 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;

[0031] - In formula (3), R represents an alkyl group, and n represents an integer ranging from 0 to 3.

[0032] In the thermosensitive recording medium of the present invention, the at least 20% of the hollow particles of the undercoat layer preferably have a hollow ratio of 80% or greater. In a preferred embodiment, the hollow ratio of the hollow particles of the undercoat layer may be at least 85%, or at least 90%.

[0033] In the thermosensitive recording medium of the present invention, the thermosensitive coloring layer preferably further comprises 1,3-diphenylurea, which has been found to be a useful stabilizer (preservability-improving agent) in the context of the present invention.

[0034] In another aspect, the present invention relates to consumer product packaging to which the thermosensitive recording medium of the present invention has been attached or incorporated. In particular, food packaging incorporating the thermosensitive recording medium of the present invention is contemplated. The thermosensitive recording medium of the present invention can be converted into a label, for example, by lamination, die-cutting, and pre-printing, and then placed on the food packaging. The consumer product packaging can be partially or completely transparent, flexible, or rigid, and can contain one or more perishable food products, such as prepared foods or boxed lunches. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] [ Figure 1 ] Figure 1Schematic diagram of an illustrative, non-limiting example of a thermosensitive recording medium according to an embodiment of the present invention. In this specific, non-limiting embodiment, in a thermosensitive recording medium (1), a thermosensitive coloring layer (12) is placed on a support layer (13), with an undercoat layer (14) incorporated between the support layer (13) and the thermosensitive coloring layer (12). The thermosensitive coloring layer (12) is also in contact with a protective layer (11) on the opposite side of the thermosensitive coloring layer (12) relative to the undercoat layer (14) and the support layer (13).

[0036] [ Figure 2 ] Figure 2 is a schematic diagram of another illustrative, non-limiting example of a thermosensitive recording medium according to a further embodiment of the present invention. Here, the arrangement is similar to Figure 1 The embodiment shown in , except that two consecutive protective layers, here numbered ( 11 a ) and ( 11 b ), ( 11 a ) being the lower protective layer and ( 11 b ) being the upper protective layer, are applied to the thermosensitive coloring layer ( 12 ). DETAILED DESCRIPTION

[0037] Support layer

[0038] The support layer (which may also be referred to as "substrate") in the thermosensitive recording medium of the present invention is appropriately selected depending on the intended purpose without any particular limitation. The support layer may be transparent or opaque.

[0039] Possible supports include those made of wood-free paper, recycled pulp (containing 50% or more recycled pulp), synthetic paper, polyethylene film, and laminated paper. The thickness of the paper layer varies depending on the composition of the layer and the intended use of the heat-sensitive recording material and cannot be simply specified, but is preferably 30 μm to 250 μm, more preferably 50 μm to 200 μm.

[0040] The transparent support can also be used in the form of a polymeric material in the form of a film. The total light transmittance of the transparent film is preferably at least 60%, more preferably at least 70%, and most preferably at least 90%. Preferred films exhibit a haze value of less than 3. The transparent film can also be colored. The thickness of the transparent film is preferably 20 μm to 100 μm, more preferably 40 μm to 70 μm.

[0041] The film material used in the transparent support can be selected from: 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 (bi-oriented) polypropylene (BOPP)), polyester film, poly (ethylene terephthalate) film (PET), polyethylene 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), norbornane film (NB) and Arton film. Other possibilities include polyethylene (PE) and polymethyl methacrylate (PMMA).

[0042] <Base Coating>

[0043] Generally, in the technical field of thermosensitive recording media, the expression "priming" is understood by those skilled in the art to mean a layer between the support and the thermosensitive coloring layer. The expression "under layer" can also be used by those skilled in the art as a synonym for "priming layer".

[0044] If present in the thermosensitive recording media of the present invention, the undercoat layer will typically contain a binder resin.

[0045] As the binder resin to be used in the undercoat layer, a water-dispersible resin or a water-soluble resin can be used, and specific examples thereof include conventionally known water-soluble polymers and aqueous polymer emulsions.

[0046] The water-soluble polymer that can be used in the binder resin in the undercoat layer can be suitably selected without any restriction according to the intended purpose. Its example includes polyvinyl alcohol, starch and derivatives thereof, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose and ethylcellulose, polyvinyl pyrrolidone, alkali metal salts of styrene-maleic anhydride copolymer, alkali metal salts of isobutylene-maleic anhydride copolymer, sodium alginate, gelatin, and casein. These can be used alone or in combination. The particularly preferred binder material for undercoat layer of the present invention is polyvinyl alcohol.

[0047] The aqueous polymer emulsion that can be used as the binder resin in the primer layer can be appropriately selected according to the intended purpose without any limitation. Examples include latexes such as styrene-butadiene copolymers, and emulsions such as vinyl acetate resins, acryl-based resins, and polyurethane resins. These can be used alone or in combination. In the present invention, a particularly preferred embodiment is a combination of polyvinyl alcohol as the binder material and a styrene-butadiene copolymer added as the aqueous polymer emulsion.

[0048] If an undercoat layer is used in the thermosensitive recording medium of the present invention, an inorganic filler may be used or may be omitted from the undercoat layer. If an inorganic filler is used, examples thereof include aluminum hydroxide, calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silicon dioxide, barium sulfate, talc, kaolin, alumina, and clay. These may be used alone or in combination. Of these, aluminum hydroxide, calcium carbonate, kaolin, and clay are preferred in terms of liquid properties in the coating liquid, stability of dispersed particles, and water solubility.

[0049] 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 preferred in terms of liquid properties as described above. In this regard, a particularly preferred inorganic filler for the undercoat layer is non-calcined kaolin, preferably calcined kaolin in this context.

[0050] In the undercoat layer of the thermosensitive recording medium according to the present invention, hollow particles are used.

[0051] For all hollow particles, the hollowness ratio given as a percentage (%) is (inner diameter of the hollow particle / outer diameter of the hollow particle)×100.

[0052] Such hollow particles each may have a shell made of a thermoplastic resin and contain air or other gas therein, typically having a volume average particle diameter of 1 μm to 10 μm, most commonly having a thermoplastic resin as a shell, made of polystyrene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyacrylate and polymethacrylate (polymethacrylate), polyacrylonitrile and polybutadiene, and copolymer resins thereof.

[0053] In the thermosensitive recording medium of the present invention, at least 20% of the hollow particles of the undercoat layer preferably have a hollow ratio of 80% or greater. In a preferred embodiment, the hollow ratio of the hollow particles of the undercoat layer may be at least 85%, or at least 90%.

[0054] In a preferred embodiment of the thermosensitive recording medium according to the present invention, the total pigment ratio (inorganic filler and organic filler) in the undercoat layer is at least 20 wt % and at most 80 wt % relative to the total dry weight of the undercoat layer.

[0055] If an undercoat layer is used in the thermosensitive recording medium of the present invention, the amount of its deposition is suitably 0.4 g / m 2 Up to 10g / m 2 , more preferably 0.6 g / m 2 Up to 4 g / m 2 .

[0056] The thickness of the undercoat layer in the present invention, if used, varies depending on the composition of the layer and the intended use of the thermosensitive recording material and cannot be simply specified, but is preferably 0.5 to 15 μm, more preferably 0.8 to 6 μm.

[0057] <Thermosensitive Coloring Layer>

[0058] In the thermosensitive recording medium of the present invention, the thermosensitive coloring layer is located on the transparent support layer, and the thermosensitive coloring layer contains a leuco dye and a developer. In the present invention, an undercoat layer (or multiple undercoats) exists between the transparent support layer and the thermosensitive coloring layer.

[0059] Thermosensitive color layers contain a colorant system in which a dye, for example a leuco dye, in one layer of the medium reacts with another component (the so-called "developer") upon application of heat to produce a colored product.

[0060] Leuco dye is a compound showing electron donating properties, and can be used individually or in combination of two or more species. However, leuco dye itself is a colorless or light-colored dye precursor, and conventionally known leuco compounds can be used. The example of leuco compound includes triphenylmethanephthalide compound, triarylmethane compound, fluoran compound, phenothiazine compound, thiofluoran (thiofluoran) compound, xanthene (xanthene) compound, indolephthalic acid (indophthalyl) compound, spiropyran compound, azaphthalide (azaphthalide) compound, chloromethylpyrazole compound, methine (methyne) compound, rhodamine anilinolactum compound, rhodamine lactam (rhodamine lactuam) compound, quinazoline compound, diazaxanthene compound, dilactone compound. Considering coloring properties, background fogging and the image fading due to moisture, heat or light radiation, the specific examples of such compound are as follows:

[0061] 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-(N-n-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-(N-n-pentyl-N-methylamino)fluoran, 2-anilino- 3-methyl-6-(N-sec-butyl-N-ethylamino) fluoran, 2-anilino-3-methyl-6-(N-n-pentyl-N-ethylamino) fluoran, 2-anilino-3-methyl-6-(N-isopentyl-N-ethylamino) fluoran, 2-anilino-3-methyl-6-(N-cyclohexyl-N-methylamino) fluoran, 2-anilino-3-methyl-6-(N-ethyl-p-toluidinyl) fluoran, 2-anilino-3-methyl-6-(N-methyl-p-toluidinyl) fluoran, 2-(m-trichloromethylanilino)-3-methyl-6-diethylamino fluoran, 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-toluidinyl)-3-methyl-6-(N-ethylanilino)fluoran, 2-(N-methyl-p-toluidinyl)-3-methyl-6-(N-propyl-p-toluidinyl)fluoran, 2-anilino-6-(N-n-hexyl-N-ethylamino)fluoran, 2-(o-chloroanilino) 2-Benzylamino-6-(N-ethyl-p-toluidinyl)fluoran, 2-benzylamino-6-(N-methyl-2,4-dimethylanilino)fluoran, 2-benz ...fluoran, 2-dibenzylamino-6-(N-methyl-p-toluidinyl)fluoran, 2-dibenzylamino-6-(N-ethyl-p-toluidinyl)fluoran, 2-(di-p-methylbenzylamino)-6-(N-ethyl-p-toluidinyl)fluoran, 2-(α-phenylethylamino)-6-(N-ethyl-p-toluidinyl)fluoran, 2-methylamino-6-(N-methylanilino)fluoran, 2-methylamino-6-(N-ethylanilino)fluoran, 2-methylamino-6-(N-propylanilino)fluoran, 2-ethylamino-6-(N-methyl 2-(N-methyl-2,4-dimethylanilino)fluoran, 2-(2-ethylamino-6-(N-methyl-2,4-dimethylanilino)fluoran, 2-dimethylamino-6-(N-methylanilino)fluoran, 2-dimethylamino-6-(N-ethylanilino)fluoran, 2-diethylamino-6-(N-methyl-p-toluidinyl)fluoran, benzoleuco-methylene blue, 2-[3,6-bis(diethylamino)]-6-(o-chloroanilino)xanthenylbenzoic acid lactam, 2-[3,6-bis(diethylamino)]-6-(o-chloroanilino)xanthenylbenzoic acid lactam, )]-9-(o-chloroanilino)xanthenylbenzoic 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-chlorophthalide, 3,3-bis(p-dibutylaminophenyl)phthalide, 3-(2-methoxy-4-dimethylaminophenyl)-3-(2-hydroxy-4,5-dichlorophenyl)phthalide, 3-(2-hydroxy-4-dimethylaminophenyl)-3-(2-methylaminophenyl)phthalide 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, 3,6-bis(dimethylamino)fluorenespiro(9,3')-6'-dimethylaminophthalide, 6'-chloro-8'-methoxy-benzoindolinol spiropyran, and 6'-bromo-2'-methoxybenzoindolinol spiropyran. These can be used alone or in combination.

[0062] The amount of the leuco dye contained in the thermosensitive coloring layer is preferably 3% by mass to 30% by mass with respect to the total mass of the thermosensitive coloring layer taken as 100%.

[0063] As the color developer, various electron-accepting materials are known that can react with the aforementioned leuco dye upon heating to develop color, such as phenolic compounds, organic or inorganic acidic compounds, and esters or salts thereof.

[0064] In the present invention, the thermosensitive coloring layer contains at least two types of color developers. Three types of color developers, at least two of which, or practically all three, are present in the thermosensitive coloring layer of the thermosensitive recording medium of the present invention, most preferably correspond to a compound having an N-phenylurea-phenyl-benzenesulfonamide structure, an N-phenylurea-phenyl-oxy-sulfonyl-aryl structure, or an N,N′-bis-([arylsulfonyloxy]-phenyl)urea structure.

[0065] Regarding the color developer of type (1) having an N-phenylureido-phenyl-benzenesulfonamide structure, generally preferred compounds have the following structure:

[0066] [Chemistry 4]

[0067] (1)

[0069] wherein R1 to R3 each independently represent a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group or a C1-C6 fluoroalkyl group.

[0070] Such developers can be prepared according to the synthesis methods disclosed in, for example, EP 2 923 851. Most preferably, the aromatic ring attached to the SO2 group in formula (1) above does not have (R 1 ) substituent, or has a Me group or a Cl group, or

[0071] -NHAc or -OMe. Most preferably, the above intermediate ring has a Me group as a possible (R 3 ) substituents. The aromatic ring connected via the urea group (on the left in the figure above) can show -OMe, -F, -CF3, Cl, and CH3(Me) substituents.

[0072] In a particularly preferred embodiment, the thermosensitive coloring layer of the thermosensitive recording medium of the present invention contains a color developer having the following formula (4):

[0073] [Chemistry 5]

[0074] (4)

[0076] Therefore, the above compound N-[2-(3-phenylureido)phenyl]benzenesulfonamide is a preferred color developer of the general formula (1). This corresponds to the above general formula (1), wherein R 1 、R 2 and R 3 They are all hydrogen atoms.

[0077] The developer of type (2) has the structure of N-phenylureido-phenyl-oxysulfonyl-aryl:

[0078] [Chemistry 6]

[0079]

[0080] In formula (2), R 3 It is 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.

[0081] Such developers can be prepared according to the synthesis methods disclosed in, for example, EP 3 395 583. Most preferably, R 3 Most preferably, in the present invention, the developer (2) has the following structure: wherein -O-SO2-(phenyl)-p-R 3 The group is in the meta position relative to the -NH-CO-NH-phenyl substituent, and R 3 = methyl.

[0082] The developer of type (3) has the structure of N,N'-bis-([arylsulfonyloxy]-phenyl)urea:

[0083] [Chemistry 7]

[0084]

[0085] wherein, in formula (3), R represents an alkyl group, and n represents an integer ranging from 0 to 3. Such a developer can be prepared according to the synthesis method disclosed, for example, in EP 3 677 569. Most preferably, in the present invention, the developer of formula (3) has the following structure: wherein each (R)n system consists of a single methyl group in the para position relative to the -SO2-O- group.

[0086] In the thermosensitive coloring layer, the mixing ratio of the developer to the leuco dye is such that the developer is preferably 0.5 to 10 parts by mass, more preferably 1 to 5 parts by mass, relative to 1 part by mass of the leuco dye.

[0087] In the present invention, various other known developers can be optionally used as long as the effect of the present invention is not impaired. These other developers are color developers, which include oxidants and various electron-accepting compounds that can color leuco dyes. Examples include 4,4'-isopropylidene bisphenol, 4,4'-isopropylidene bis(o-methylphenol), 4,4'-secondary butylidene bisphenol, 4,4'-isopropylidene bis(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-methylphenyl)propane ... 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane, 1,5-bis(4-hydroxyphenylthio)-5-oxapentane, calcium 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-hydroxyphenylthio)-propane, N,N'-diphenylthiourea, N,N'-bis(m-chlorophenyl)thiourea, salicylanilide, methyl bis-(4'-hydroxyphenyl)acetate, benzyl bis-(4-hydroxyphenyl)acetate, 1,3-bis(4-hydroxycumyl)propane )benzene, 1,4-bis(4-hydroxycumyl)benzene, 2,4'-diphenol sulfone, 2,2'-diallyl-4,4'-diphenol sulfone, 3,4-dihydroxyphenyl-4'-methyldiphenyl sulfone, 1-acetoxy-2-naphthoic acid zinc, 2-acetoxy-1-naphthoic acid zinc, 2-acetoxy-3-naphthoic acid zinc, α,α-bis(4-hydroxyphenyl)-α-methyltoluene, and an antipyrine complex of zinc thiocyanate. These can be used alone or in combination.

[0088] Among other further developers that can be added to the heat-sensitive coloring layer of the heat-sensitive recording medium, the following urea urethane and D90 commercial products may also come into consideration:

[0089] [Chemistry 8]

[0090]

[0091] Ureaurethane compounds

[0092] [Chemistry 9]

[0093]

[0094] D90

[0095] The amount of the additional developer is appropriately selected depending on the intended purpose, as long as the effects of the present invention are not impaired. In a preferred embodiment, the combined mass of the other developers other than those of formula (1), (2), or (3) is less than 2 parts by mass, more preferably less than 0.5 parts by mass, relative to 1 part by mass of the leuco dye in the thermosensitive coloring layer. In certain embodiments of the present invention, substantially no other developer other than those of formula (1), (2), or (3) may be present in the thermosensitive coloring layer.

[0096] Various known stabilizers (anticorrosive improvers) can be optionally used, as long as the effects of the present invention are not impaired. 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 an auxiliary additive. Specific examples thereof include 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 4,4′-butylenebis(6-tert-butyl-2-methylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 4,4′-thiobis(6-tert-butyl-2-methylphenol), tetrabromobisphenol A, tetrabromobisphenol S, 4,4-thiobis(2-methylphenol), 4,4′-thiobis(2-chlorophenol), tetrakis(1,2,2,6,6-pentamethyl-4-piperidinyl)-1,2,3,4-butanetetracarboxylate, and tetrakis(1,2,2,6,6-tetramethyl-4-piperidinyl)-1,2,3,4-butanetetracarboxylate.

[0097] In addition to the above-mentioned leuco dye, developer and stabilizer, other materials conventionally used in heat-sensitive recording materials such as binders, fillers, sensitizers, crosslinking agents, pigments, surfactants, fluorescent brighteners and lubricants may be appropriately added to the heat-sensitive coloring layer.

[0098] If necessary, an adhesive can be used to improve the adhesiveness and coatability of the layer. The adhesive is appropriately selected according to the intended purpose without any restriction. Specific examples of the adhesive resin include starch, 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-acryloyl copolymers, and salt emulsions of styrene-butadiene copolymers.

[0099] Filler is suitably selected without any restriction according to the intended purpose. Its example comprises inorganic pigment such as calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silicon dioxide, aluminum hydroxide, barium sulfate, talc, kaolin, aluminum oxide and clay, and conventionally known organic pigment. Among these, acidic pigment (showing those of acidity in aqueous solution) such as silicon dioxide, aluminum oxide and kaolin are preferred, wherein from the perspective of the color density of color development, silicon dioxide is particularly preferred. Calcined kaolin is preferred in the framework of the present invention. In the heat-sensitive coloring layer according to the heat-sensitive recording medium of the present invention, it is preferred to use an inorganic filler with high oil absorption such as 80 g / 100 g or larger. The example of the suitable filler for the heat-sensitive coloring layer in this context is calcined kaolin or amorphous silicon dioxide.

[0100] Various thermoplastic materials are conventionally added to heat-sensitive coloring layers as sensitivity-improving agents (sensitizers). In the present invention, in a preferred embodiment, no sensitizer is added to the heat-sensitive coloring layer. It has been observed in the present invention that the absence of any sensitizer can particularly allow for an increase in heat resistance, for example as measured at 110°C. In some cases, a sensitizer can improve the coloring effect by melting under the influence of heat, thereby providing a temporary solvent that promotes the reaction between the leuco dye and the developer. It should be noted that in cases where heat resistance is required, such as for labeling ready-to-eat foods or digital printing processes, it is preferable to not add a thermoplastic material or to select a compound with a melting point of 90°C or higher.

[0101] 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, N-octadecylcarbamoylbenzene; (5) polyphenyl hydrocarbons such as p-benzylbiphenyl, terphenyl, triphenylmethane; (6) benzyloxy derivatives such as benzophenone, ... Bio-, benzoic acid esters and naphthoic acid esters such as benzyl p-benzyloxybenzoate, β-benzyloxynaphthalene, β-phenyl naphthoate, 1-hydroxy-2-phenyl naphthoate, 1-hydroxy-2-methyl naphthoate, dibenzoylmethane, dibenzoyloxymethane, dibenzoyloxypropane; (7) carbonates such as diphenyl carbonate, ethylene carbonate; (8) terephthalic acid esters such as dibenzyl terephthalate, dimethyl terephthalate; (9) oxalate sensitizers such as dibenzyl oxalate, bis(4-methylbenzyl) oxalate and oxalate. (10) alkoxy and aryloxy sensitizers such as 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)benzene, -(2-vinyloxyethoxy)biphenyl, p-aryloxybiphenyl, p-propargyloxybiphenyl, 1,2-bis(4-methoxyphenoxy)propane, 1,5-bis(4-methoxyphenoxy)-3-oxapentane; (11) alcohol sensitizers such as 1,1-diphenylethanol, 1,1-diphenylpropanol, p-benzyloxybenzyl alcohol, 1,3-phenoxy-2-propanol; (12) sulfur-based sensitizers such as 1,4-diphenylthiobutane, 1,4-diphenylthio-2-butene, dibenzyl disulfide;

[0102] In a preferred embodiment of the present invention, the thermosensitive color layer may not contain compounds known in the field of thermosensitive recording media as sensitizers, such as compounds containing aryloxy groups (e.g., phenoxy, tolyloxy), alkoxy groups, oxalate groups, and benzyloxy groups. Therefore, it is particularly interesting that the thermosensitive color layer does not contain sensitizers of the types shown in Groups 6, 9, 10, and 11 above. The absence of such sensitizers can particularly allow for an increase in heat resistance, for example, as measured at 110°C.

[0103] The coloring layer can be formed by conventional methods. To avoid reactions between the components of the heat-sensitive coloring layer, in a preferred embodiment, they are dispersed separately and then mixed. Grinding with the binder and other components is typically performed using a disperser such as a ball mill, atritter, or sand mill to achieve a particle size of 0.2 to 3 μm, preferably 0.2 to 1 μm. If necessary, the resulting dispersion is mixed with a filler and a heat-melting material (sensitizer) dispersion according to a predetermined formula to prepare a coating solution for the heat-sensitive coloring layer. This coating solution is then applied to a support.

[0104] The thickness of the thermosensitive coloring layer varies depending on the composition of the thermosensitive coloring layer and the intended use of the thermosensitive recording material and cannot be simply specified, but it is preferably 1 μm to 50 μm, more preferably 2 μm to 20 μm.

[0105] <Protective layer>

[0106] In the present invention, at least one protective layer is provided on the heat-sensitive layer. A plurality of different protective layers can be superimposed on each other to focus more on matching or blocking properties, respectively.

[0107] At least one protective layer in the thermosensitive recording medium of the present invention may suitably contain wax particles, preferably having an average particle size of at least 0.05 and at most 2.0 μm. When more than one protective layer is present, only the uppermost protective layer, i.e., the protective layer furthest from the thermosensitive coloring layer and on the surface exposed to the outside, may suitably contain wax particles. In a preferred embodiment, when the uppermost protective layer contains wax particles, the underlying protective layers may or may not contain wax particles.

[0108] Regarding the average particle size of the wax particles, the values ​​are as obtained in the method for measuring the average particle size, expressed as the median size (D 50 ) in the form of, as measured by laser diffraction using a laser diffraction particle size distribution analyzer. The measurement can be performed, for example, by an LA-950 machine produced by HORIBA LA-950.

[0109] Preferably, if wax particles are used in the protective layer / uppermost protective layer, the wax has a melting point of at least 80°C and at most 200°C. More preferably, the wax has a melting point of at least 90°C and at most 130°C, most preferably at least 100°C and at most 120°C.

[0110] More preferably, the wax particle size is at least 0.1 μm and at most 0.5 μm.

[0111] In an advantageous embodiment, the wax particles constitute at least 2.0% by weight and at most 20% by weight, relative to 100% by weight of all components of the protective layer as a whole, more preferably at least 5.0% by weight and at most 10% by weight, relative to 100% by weight of all components of the protective layer as a whole.

[0112] The wax material of the wax particles in the present invention can be polyethylene wax, salts of higher fatty acids such as zinc stearate and calcium stearate, montanate 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.

[0113] A particularly preferred wax material for the wax particles used in the protective layer of the thermosensitive recording medium of the present invention is polyethylene wax. Either low-density or high-density polyethylene wax particles can be used.

[0114] The protective layers typically contain at least one binder, and the protective layers each may contain an inorganic filler and a surfactant.

[0115] The adhesive of protective layer (separately) is suitably selected without any restriction according to the intended purpose, and the same adhesive can be used in each protective layer or different adhesives can be used in an independent protective layer. The example of the adhesive that can be used in protective layer comprises polyvinyl alcohol, modified polyvinyl alcohol, starch and derivatives thereof, cellulose derivatives, polyvinyl pyrrolidone, polyethyleneimine, sodium alginate, gelatin and casein. Also acrylic adhesives can be used. The hydrophobic resin that can be used as the adhesive in protective layer is included in the hydrophobic resin that is typically provided as aqueous emulsion during the preparation period of protective layer, for example polyurethane resin, epoxy resin, vinyl acetate (to) polymer, vinylidene chloride (to) polymer, vinyl chloride (to) polymer and styrene-butadiene copolymer. The particularly preferred adhesive material for protective layer of the present invention is polyvinyl alcohol.

[0116] The thickness of the protective layer preferably ranges 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 dry can be used. In the case of applying multiple protective layers, a lower individual thickness will be required for each protective layer. For the dried final product, the preferred maximum cumulative thickness of the sum of all protective layers is 10 μm.

[0117] The inorganic filler in the protective layer (if used) is appropriately selected according to the intended purpose without any restrictions. Examples of inorganic fillers include aluminum hydroxide, calcium carbonate, aluminum oxide, zinc oxide, titanium dioxide, silicon dioxide, barium sulfate, talc, kaolin, alumina and clay. These can be used alone or in combination. Among these, aluminum hydroxide and calcium carbonate are particularly preferred because, when printing for a long time, relative to a thermal head, a protective layer containing such an inorganic filler has excellent wear resistance. The amount of the inorganic filler in the protective layer is appropriately selected according to the intended purpose without any restrictions. The amount of the inorganic filler depends on the type of filler, but is preferably 50 to 500 parts by mass relative to 100 parts by mass of the binder resin.

[0118] In a preferred embodiment of the present invention, a first protective layer is applied over the heat-sensitive color layer and, while containing a binder such as polyvinyl alcohol (PVA), does not contain wax particles. However, a second protective layer can be applied over the first protective layer so that it is not in direct contact with the heat-sensitive color layer, the second protective layer containing wax particles and possibly a filler such as an inorganic filler.

[0119] The method for forming the first, second or subsequent protective layers is appropriately selected according to the intended purpose without any restriction. Examples include blade coating, roller coating, wire rod coating, die coating and curtain coating. Such methods can be used to apply other layers of the heat-sensitive recording medium of the present invention, such as an undercoat layer. Curtain coating is a preferred method for applying the protective layer in the present invention and can also be used to apply a heat-sensitive coloring layer.

[0120] <Back layer>

[0121] A backing layer (which may also be referred to as a "backing layer") may be provided below the support layer in the thermosensitive coloring layer of the present invention. However, such a backing layer is not essential in the present invention and is merely optional. In one embodiment, the thermosensitive recording medium may contain a backing layer containing a pigment, a binder resin, and preferably a crosslinking agent. The backing layer (if present) will be arranged on the surface of the support layer opposite to the surface of the support layer where the thermosensitive layer is arranged, or, if such an undercoat layer is present, opposite to the surface of the support layer where the undercoat layer is arranged, the undercoat layer being located between the support layer and the thermosensitive layer.

[0122] The backing layer may further contain other components such as fillers, lubricants and antistatic agents.

[0123] As the binder resin, any of a water-dispersible resin and a water-soluble resin can be used, and specific examples thereof include conventionally known water-soluble polymers and aqueous polymer emulsions.

[0124] The water-soluble polymer is appropriately selected according to the intended purpose without any restriction. Examples thereof include polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose and ethylcellulose, polyvinylpyrrolidone, alkali metal salts of styrene-maleic anhydride copolymers, alkali metal salts of isobutylene-maleic anhydride copolymers, sodium alginate, gelatin, and casein. These can be used alone or in combination.

[0125] The aqueous polymer emulsion is appropriately selected depending on the intended purpose without any limitation. Examples thereof include latexes such as styrene-butadiene copolymers; and emulsions such as vinyl acetate resins, acryl-based resins (e.g., acrylic acid-acrylate copolymer latexes), (meth)acrylamide-based resins, and polyurethane resins. These may be used alone or in combination.

[0126] The cross-linking agent is appropriately selected according to the intended purpose without any restriction. Examples thereof include: polyvalent amine compounds such as ethylenediamine; polyvalent 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 and phenol); polyfunctional epoxy compounds; polyvalent metal salts (such as Al, Ti, Zr and Mg); titanium lactate; and boric acid. The amount of the cross-linking agent varies according to the amount and type of the functional group of the cross-linking agent, but is preferably 0.1 to 100 parts by mass, more preferably 1 to 100 parts by mass, relative to 100 parts by mass of the binder resin.

[0127] As the filler, either an inorganic filler or an organic filler can be used. 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, phenol resins, polyurethane resins, urea resins, melamine resins, polyester resins, polycarbonate resins, styrene resins, polyethylene resins, and formaldehyde resins.

[0128] The antistatic agent can be selected, for example, from conventionally used ion-conductive antistatic agents and electron-conductive antistatic agents. Specific examples of ion-conductive antistatic agents include inorganic salts such as sodium chloride; anionic polymers such as sodium polystyrene sulfonate; and resins containing quaternary ammonium salts as electrolyte cations. Specific examples of electron-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 sulfonates react particularly well with aziridine, thereby improving the water resistance achieved by crosslinking. In addition, salts copolymerized with maleic acid are effective because they have antistatic properties and also improve water resistance.

[0129] A method for forming the back layer is appropriately selected depending on the intended purpose without any restriction.The back layer is preferably formed by applying a coating liquid for the back layer to a support.

[0130] The coating method is appropriately selected depending on the intended purpose without any restriction, and examples thereof include knife coating, roll coating, wire bar coating, die coating, and curtain coating.

[0131] The thickness of the back layer is appropriately selected depending on the intended purpose without any limitation. It is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 5 μm.

[0132] <Viscous layer>

[0133] A viscous layer, also referred to as an adhesive layer, may be provided in the thermosensitive recording medium of the present invention. However, such a viscous layer is not essential in the present invention but is merely optional.

[0134] The adhesive layer can be provided on the surface of the support layer or backing layer (back layer), which is opposite to the surface of the support layer or backing layer (back layer) on which the protective layer is formed. The adhesive layer can, for example, help attach the thermosensitive recording medium to food packaging in typical applications of the present invention. Therefore, the thermosensitive recording medium of the present invention can be provided with an adhesive surface attached to the support or backing layer, which is useful in order to provide a label with an adhesive layer. A removable liner can then be attached to the adhesive layer to be removed before finally attaching to the product to be labeled. The adhesive layer can also provide antistatic properties. There is no particular limitation on the method for forming the adhesive layer. Examples of the method include conventional coating methods and lamination methods. The average thickness of the adhesive layer is not particularly limited and can be appropriately selected according to the intended purpose, and is preferably 0.1 μm or larger but 20 μm or smaller.

[0135] The material of viscous layer is not particularly limited, and can be suitably selected according to intended purpose.The example of the material of viscous layer comprises urea resin, melamine resin, phenol resin, epoxy resin, based on the resin of vinyl acetate, based on the copolymer of vinyl acetate-acrylic acid, ethylene-vinyl acetate copolymer, based on the resin of acrylic acid, based on the resin of polyvinyl ether, based on the copolymer of vinyl chloride-vinyl acetate, based on the resin of polystyrene, based on the resin of polyester, based on the resin of polyurethane, based on the resin of polyamide, based on the resin of chlorinated polyolefin, based on the resin of polyvinyl butyral, based on the copolymer of acrylate, based on the copolymer of methacrylate, natural rubber, based on the resin of cyanoacrylate and based on the resin of organosilicon.Can use one of these materials alone or can use two or more of these materials in combination.These materials can be cross-linked by means of a cross-linking agent.The material of viscous layer can be hot melt type.In one aspect of the present invention, the label comprising the heat-sensitive recording medium of the present invention is in the form of organosilicon linerless (SLL) label.

[0136] Image recording method

[0137] The image recording method can be used to record an image on the thermosensitive recording medium of any embodiment of the present invention using an image recording unit that is any one of a thermal head and a laser.

[0138] The thermal head is appropriately selected depending on the intended purpose without any limitation as to its shape, structure, and size.

[0139] The laser can be selected without limitation depending on the intended purpose. In a preferred embodiment, a CO2 laser emitting light with a wavelength of 9.3 μm to 10.6 μm can be used. By using a CO2 laser emitting light with a wavelength of 9.3 μm to 10.6 μm, satisfactory laser-printed images can be obtained without using a photothermal conversion agent such as a phthalocyanine pigment. Other laser types, such as FLDA (fiber laser diode array), can also be used.

[0140] Example

[0141] Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples. However, it should be noted that the present invention is not limited to these Examples in any way. It should be noted that in the following Examples, unless otherwise specified, the unit "part" means "part by mass" and the unit "%" means "% by mass".

[0142] Example 1

[0143] The thermosensitive recording medium was produced according to the following steps.

[0144] 1) A coating solution for an undercoat layer was applied to a substrate to produce an undercoat layer (having a 2 g / m 2 In this example, a dry mass of about 60 g / m was used. 2 The coating liquid for the undercoat layer has the following formula (prescription).

[0145] Formulation of coating solution for base coat

[0146] Preparation of coating solution for lower layer:

[0147]

[0148] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow rate: 55%

[0149] 2) AF-1570 from Dow, a styrene / acryl copolymer resin, solid content: 17.5%, average particle size: 1.6 μm, hollow ratio: 65%

[0150] 3) Solid content :50.0%

[0151] 4) Fully hydrolyzed PVA

[0152] 2) A coating liquid for a thermal recording layer is applied on the undercoat layer, thereby producing a thermal recording layer.

[0153] Regarding the preparation of the coating solution for the heat-sensitive coloring layer, the following composition was prepared:

[0154]

[0155] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0156]

[0157] 1) NKK-1304 from Nippon Soda Co., Ltd.

[0158] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0159]

[0160] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0161] The compound of the general formula (2) mentioned above has a specific structure:

[0162] [Chemistry 10]

[0163]

[0164]

[0165] 1) Ansilex 93 from BASF, oil absorption: 105-120 g / 100 g

[0166] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0167] [Liquid B], [Liquid C1], [Liquid C2], and [Liquid D1], each having the aforementioned composition, were each dispersed using a sand mill so that the particles contained in each liquid had an average particle size diameter of 1 μm or less, thereby preparing a dye dispersion [Liquid B], a developer dispersion [Liquid C1], [Liquid C2], and a filler dispersion [Liquid D1]. [Liquid B], [Liquid C1], [Liquid C2], and [Liquid D1] were then mixed in the following ratio:

[0168] This mixture was placed under stirring, thereby preparing a coating liquid for a thermosensitive coloring layer [Liquid E1].

[0169] [Liquid E1] is uniformly applied to the undercoat layer to form a thermosensitive coloring layer.

[0170] The heat-sensitive layer is applied in an amount such as to produce 0.4 g / m² on a dry basis. 2 The coating weight of the dye is then dried to form a heat-sensitive coloring layer.

[0171] 3) A coating liquid for a double protective layer (a first coating liquid and a second coating liquid) is applied on the thermosensitive coloring layer so that the lower protective layer formed by the first coating liquid is present below the upper protective layer formed by the second coating liquid, thereby producing a double protective layer on the thermal recording layer. The upper protective layer and the lower protective layer each have a dry basis weight of 1 g / m 2 and 1g / m 2 The coating solution for the double-layer protective layer was formulated as follows; they were then dried.

[0172]

[0173] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0174] 2) KYMENE-920 from SOLENIS

[0175]

[0176] 1) See the dispersion formulation below

[0177] 2) Ultralube E-842N from Keim Additec GmbH

[0178] 3) KYMENE-920 from SOLENIS

[0179] 4) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0180]

[0181] 1) Apy-100 from Nabaltec GmbH

[0182] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0183] After coating, the samples were aged for 48 hours at 50° C. After this process, the samples were calendered at 20 kgF before quality evaluation.

[0184] Examples 2 to 13

[0185] In Examples 2 to 12, a thermosensitive recording medium was prepared in each case according to Example 1 except that changes were made as shown in Table 1 below.

[0186] [Liquid C3], [Liquid D2], and [Liquid E2] were respectively prepared as follows:

[0187] For Examples 2 to 4 and Examples 9 to 12

[0188]

[0189] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0190] The compound of the general formula (3) mentioned above has a specific structure:

[0191] [Chemistry 11]

[0192]

[0193] For Example 13

[0194]

[0195] 1) Mizukasil P-527 from Mizusawa Industrial Chemicals, Ltd., oil absorption: 140-180 g / 100 g

[0196] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0197] For Example 4

[0198]

[0199] Thus, different combinations of developers were compared in Examples 1 to 3. A combination of three different types of developers was tested in Example 4. Examples 5 to 12 tested the effect of the ratio between two different types of developers. Example 13 tested the effect of another type of inorganic filler in the heat-sensitive coloring layer.

[0200] Example 14

[0201] In Example 14, a thermosensitive recording medium was prepared according to Example 1, except that the following [Liquid I] was added thereto to prepare a coating liquid [Liquid E3] for a thermosensitive coloring layer.

[0202]

[0203] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0204]

[0205] Examples 15 to 22

[0206] In Examples 15 to 22, a thermosensitive recording medium was prepared in each case according to Example 1 except that changes were made as shown in Table 1 below.

[0207] [Liquid A2], [Liquid A3], [Liquid A4], [Liquid A5], [Liquid A6], [Liquid A7], [Liquid A8], and [Liquid A9] were respectively prepared as follows:

[0208] For Example 15

[0209]

[0210] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0211] 2) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0212] 3) Solid content: 50.0%

[0213] 4) Fully hydrolyzed PVA

[0214] For Examples 16 and 17

[0215]

[0216] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0217] 2) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0218] 3) Solid content: 50.0%

[0219] 4) Fully hydrolyzed PVA

[0220] For Example 18

[0221]

[0222] 1) AF-1570 from Dow, a styrene / acryl copolymer resin, solid content: 17.5%, average particle size: 1.6 μm, hollow ratio: 65%

[0223] 2) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0224] 3) Solid content: 50.0%

[0225] 4) Fully hydrolyzed PVA

[0226] For Example 19

[0227]

[0228] 1) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0229] 2) Solid content: 50.0%

[0230] 3) Fully hydrolyzed PVA

[0231] For Example 20

[0232]

[0233] 1) AF-1570 from Dow, a styrene / acryl copolymer resin, solid content: 17.5%, average particle size: 1.6 μm, hollow ratio: 65%

[0234] 2) Solid content: 50.0%

[0235] 3) Fully hydrolyzed PVA

[0236] For Example 21

[0237]

[0238] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0239] 2) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0240] 3) UW-90

[0241] 4) Solid content: 50.0%

[0242] 5) Fully hydrolyzed PVA

[0243] For Example 22

[0244]

[0245] 1) AF-1570 from Dow, a styrene / acryl copolymer resin, solid content: 17.5%, average particle size: 1.6 μm, hollow ratio: 65%

[0246] 2) R-500 from Matsumoto Yushi-Seiyaku Co., Ltd., solid content: 33.0%, hollow ratio: 90%

[0247] 3) UW-90

[0248] 4) Solid content: 50.0%

[0249] 5) Fully hydrolyzed PVA

[0250] Reference Examples 1 to 5

[0251] In Reference Examples 1 to 5, in each case, a thermosensitive recording medium was prepared according to Example 1 except that changes were made as shown in the following Table 1. Thus, three types of sensitizers were respectively added to the thermosensitive coloring layer liquid.

[0252]

[0253] 1) KS-232 from SANKO Co., Ltd.

[0254] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0255]

[0256] 1) HS-3520 from Dainippon Ink & Chemicals

[0257] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0258]

[0259] 1) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0260]

[0261] Reference Example 1 corresponds to Example 21 with the addition of KS-232, Reference Example 2 corresponds to Example 17 with the addition of KS-232, Reference Example 3 corresponds to Example 2 with the addition of KS-232, Reference Example 4 corresponds to Example 21 with the addition of HS-3520, and Reference Example 5 corresponds to Example 1 with the addition of stearamide.

[0262] Comparative Examples 6 and 7

[0263] In Comparative Examples 6 and 7, a thermosensitive recording medium was prepared in each case according to Example 1 except for changes as shown in the following Table 1. Therefore, another type of developer was combined with [Liquid C1] and [Liquid C2], respectively, in the thermosensitive coloring layer liquid.

[0264] For Comparative Examples 6, 7 and 14

[0265]

[0266] 1) Pergafast 201 from SOLENIS

[0267] 2) Itaconic acid-modified polyvinyl alcohol from Kuraray

[0268] Pergafast 201 has the following structure:

[0269] [Chemistry 12]

[0270]

[0271] Comparative Examples 8 to 10

[0272] In Comparative Examples 8 to 10, a thermosensitive recording medium was prepared according to Example 1 in each case except for changes as shown in the following Table 1. Therefore, the following three types of undercoat liquids [Liquid A12], [Liquid A13], and [Liquid A14] were tested.

[0273] For Comparative Example 8

[0274]

[0275] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0276] 2) Solid content: 50.0%

[0277] 3) Fully hydrolyzed PVA

[0278] For Comparative Example 9

[0279]

[0280] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0281] 2) UW-90

[0282] 3) Solid content: 50.0%

[0283] 4) Fully hydrolyzed PVA

[0284] For Comparative Example 10

[0285]

[0286] 1) UW-90

[0287] 2) Solid content: 50.0%

[0288] 3) Fully hydrolyzed PVA

[0289] Comparative Examples 11 to 14

[0290] In Comparative Examples 11 to 14, a thermosensitive recording medium was prepared in each case according to Example 1 except that changes were made as shown in the following Table 1. Therefore, each developer dispersion was used without being combined with the others.

[0291]

[0292] Comparative Example 15

[0293] In Comparative Example 15, a thermosensitive recording medium was prepared according to Example 1 except for the changes shown in Table 1 below.

[0294] Therefore, the undercoat liquid [Liquid A15] was used instead of [Liquid A1].

[0295]

[0296] 1) AF-1055 from Dow, a styrene / acryl copolymer resin, solid content: 26.5%, average particle size: 1 μm, hollow ratio: 55%

[0297] 2) AF-1570 from Dow, a styrene / acryl copolymer resin, solid content: 17.5%, average particle size: 1.6 μm, hollow ratio: 65%

[0298] 3) Solid content: 50.0%

[0299] 4) Fully hydrolyzed PVA

[0300] Comparative Example 16

[0301] In Comparative Example 16, a thermosensitive recording medium was prepared according to Example 1 except for the changes shown in Table 1 below.

[0302] Therefore, the protective layer is not coated on the thermosensitive coloring layer.

[0303] Evaluation Methods

[0304] Static image density was evaluated as follows:

[0305] Heat energy of 140°C to 180°C was applied for 1 second under a pressure of 0.36 kgf using a thermal gradient tester (HG-100, available from Toyo Seiki Seisaku-sho, Ltd.). The black optical density was measured using a spectrophotometer (instrument name: Exact, available from X-Rite, Inc.). The maximum value was recorded as the static image density. A value of 1.30 or greater was preferred.

[0306] Grade I: Optical density greater than 1.20

[0307] Grade II: Optical density 1.00 to 1.19

[0308] Level III: Optical density less than 0.99

[0309] Dynamic image density was evaluated as follows:

[0310] Images were printed using a ZEBRA 110Xi4 printer at a print speed of 304 mm / s (12 in / s) using an applied energy of +24 to +30. Black optical density at each energy was measured using a spectrophotometer (instrument name: Exact, available from X-Rite, Inc.). The maximum value was recorded as the dynamic image density. A value of 1.20 or greater was preferred.

[0311] Grade I: Optical density greater than 1.20

[0312] Grade II: Optical density 1.00 to 1.19

[0313] Level III: Optical density less than 0.99

[0314] Barcode decodability is evaluated as follows:

[0315] A picket-type barcode was printed using a ZEBRA 110Xi4 printer with an applied energy of +15 at a print speed of 304 mm / sec (12 inches / sec) (the bar was printed parallel to the print direction). The barcode was read using a barcode verifier (MICROSCAN LVS-9580, available from OMRON Corporation) to evaluate its decodability. The results are reported according to the ANSI symbology scale. The higher the value (maximum 4.0), the better.

[0316] Dynamic sensitivity evaluation is as follows:

[0317] 8.88 mJ / mm using a MarkPoint MK2 printer 2 The image was printed at a print speed of 102 mm / sec (4 in / sec) with an applied energy of 100%. The black optical density was measured using a spectrophotometer (instrument name = Exact, available from X-Rite, Inc.). The value is preferably 1.20 or greater.

[0318] Grade I: Optical density greater than 1.20

[0319] Grade II: Optical density 1.00 to 1.19

[0320] Level III: Optical density less than 0.99

[0321] The 110°C heat resistance of the background is evaluated as follows:

[0322] Heat energy was applied at 110°C for 1 second using a thermal gradient tester (HG-100, available from Toyo Seiki Seisaku-sho, Ltd.) at a pressure of 0.36 kgf. Black optical density was measured using a spectrophotometer (instrument name: Exact, available from X-Rite, Inc.). The value was preferably 0.25 or less.

[0323] Class I: Optical density of 0.25 or less

[0324] Grade II: Optical density 0.26 to 0.35

[0325] Grade III: Optical density higher than 0.36

[0326] Background The heat resistance at 100°C and 90°C was evaluated as follows:

[0327] The prepared samples were treated at 100° C. or 90° C. for 1 hour, respectively. The black optical density was measured using a spectrophotometer (instrument name = Exact, available from X-Rite, Inc.) and the value was preferably 0.25 or less.

[0328] Class I: Optical density of 0.25 or less

[0329] Grade II: Optical density 0.26 to 0.35

[0330] Grade III: Optical density higher than 0.36

[0331] The plasticizer resistance of the printed images was evaluated as follows:

[0332] An image was produced on the prepared sample using a thermal recording simulator (TH-PMD, available from Ohkura Denki) with a pulse width (applied energy) of 1.0 millisecond (msec), an interval of 0.1 millisecond, and a head power of 0.45 W / dot. The printed sample was kept in contact with a polyvinyl chloride sheet (PVC produced by Shin-Etsu Polymer Co., Ltd.) at 40°C for 15 hours. The black optical density of the printed image was measured using a spectrophotometer (instrument name = Exact, available from X-Rite, Inc.). The image preservation rate was then calculated as follows:

[0333] Preservation rate (%) = [the degree of optical black density after the test] / [the degree of optical black density before the test]

[0334] The value is preferably 90% or higher.

[0335] Level I: 90% or higher preservation rate

[0336] Level II: 70% to 89% preservation rate

[0337] Level III: Preservation rate less than 70%

[0338] The oil resistance of the printed images was evaluated as follows:

[0339] Images were generated on the prepared samples using a thermal recording simulator (TH-PMD, available from Ohkura Denki) with a pulse width (applied energy) of 1.0 millisecond and an interval of 0.1 millisecond at a head power of 0.45 w / point. A drop of cottonseed oil was applied to each sample and spread with a piece of cotton. The samples were then kept at 40°C for 15 hours. The black optical density of the printed image was measured using a spectrophotometer (instrument name = Exact, purchased from X-Rite, Inc.). The image preservation rate was then calculated as follows:

[0340] Preservation rate (%) = [the degree of optical black density after the test] / [the degree of optical black density before the test]

[0341] The value is preferably 90% or higher.

[0342] Level I: 90% or higher preservation rate

[0343] Level II: 70% to 89% preservation rate

[0344] Level III: Preservation rate less than 70%

[0345] The ethanol (EtOH) resistance of the printed images was evaluated as follows:

[0346] Images were generated on the prepared samples using a thermal recording simulator (TH-PMD, available from Ohkura Denki) with a pulse width (applied energy) of 1.0 milliseconds, an interval of 0.1 milliseconds, and a head power of 0.45 w / point. A drop of 99% by mass EtOH was applied to each sample. The samples were kept at 22°C for 3 hours to dry. The black optical density of the printed image was measured using a spectrophotometer (instrument name = Exact, purchased from X-Rite, Inc.). The image preservation rate was then calculated as follows:

[0347] Preservation rate (%) = [the degree of optical black density after the test] / [the degree of optical black density before the test]

[0348] The value is preferably 70% or higher.

[0349] Level I: Preservation rate 70% or higher

[0350] Level II: 50% to 99% preservation rate

[0351] Level III: Preservation rate less than 50%

[0352] The anchorage level of the layer is evaluated as follows:

[0353] A piece of scotch tape (NICHIBAN CT405AP-18) was applied to the surface of each sample. The tape was repeatedly peeled off (repeeled) in the following three steps.

[0354] Step 1) Repeat the peeling motion slowly in 180° steps

[0355] Step 2) Repeat the peeling slowly in 90° steps

[0356] Step 3) Repeat the peeling process in 90° steps

[0357] The anchoring level was evaluated according to the following scale.

[0358] Level III: Layers peeled off in step 1

[0359] Level II: Layers peeled off in step 2

[0360] Level I: Layer peeling in step 3 or no peeling (base fracture)

[0361] Finally, calculate the total score using the scores for each item following the rules below.

[0362] Level I (or A for barcodes) = 3 points

[0363] Level II (or B, C for barcodes) = 1 point

[0364] Level III (or F for barcodes) = -3 points

[0365] In the following, "SBR" stands for styrene-butadiene resin, "Dev" stands for developer, and "Pig" stands for pigment. The "ratios" in Tables 1a to 1d represent dry ratios by mass. "Pigment ratio in primer" gives the total pigment ratio (inorganic filler and organic filler) by weight in the primer, relative to the total dry weight of the primer. "Hollow in pigment" gives the dry ratio of hollow particles in the base layer by weight, relative to the total dry weight of the pigment in the primer. ">60% hollow" and ">80% hollow" give the ratios of hollow particles having a hollow ratio higher than 60% and 80%, respectively, relative to the total dry weight of the hollow particles in the primer.

[0366] [Table 1a]

[0367]

[0368] [Table 1b]

[0369]

[0370] [Table 1c]

[0371]

[0372] [Table 1d]

[0373]

[0374] [Table 1e]

[0375]

[0376] [Table 1f]

[0377]

[0378] [Table 1g]

[0379]

[0380] [Table 1h]

[0381]

[0382] Reference Signs List

[0383] 1: Thermal recording media

[0384] 11, 11a, 11b: Protective layer

[0385] 12: Thermal coloring layer

[0386] 13: Support layer

[0387] 14: Base coat

[0388] This application is based upon and claims the benefit of priority from European priority application No. 23305230.7, filed on February 22, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A thermosensitive recording medium comprising at least: - support layer, - a primer layer on said support layer, - a thermosensitive coloring layer on the base coat, - a protective layer on the thermosensitive coloring layer, in - the undercoat layer contains at least one type of plastic hollow particles as an organic filler, the plastic hollow particles accounting for at least 20% by mass of the hollow particles relative to the mass of all hollow particles, having a hollow ratio of 60% or greater, 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 coloring layer contains at least two types of color developers selected from the group consisting of general formulas (1), (2) and (3): [Chemistry 13] [Chemistry 14] [Chemistry 15] in, - In formula (1), R1 to R3 each independently represent a hydrogen atom, a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, or a C1-C6 fluoroalkyl group; -In formula (2), R 3 is 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 ranging from 0 to 3. 2 . The thermosensitive recording medium according to claim 1 , wherein the at least 20% of the hollow particles have a hollow ratio of 80% or more. 3 . The thermosensitive recording medium according to claim 1 , wherein the thermosensitive coloring layer contains N-[2-(3-phenylureido)phenyl]benzenesulfonamide as a developer.

4. The thermosensitive recording medium according to claim 3, wherein the thermosensitive coloring layer contains N-[2-(3-phenylureido)phenyl]benzenesulfonamide and a compound of the general formula (2) as a developer. 5 . The thermosensitive recording medium according to claim 1 , wherein the thermosensitive coloring layer further contains 1,3-diphenylurea. 6 . The thermosensitive 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. The thermosensitive recording medium according to claim 1, wherein the total pigment ratio (inorganic filler and organic filler) in the undercoat layer is at least 20 wt% and at most 80 wt% relative to the total dry weight of the undercoat layer.

8. The thermosensitive recording medium according to any one of claims 1 to 7, wherein the developer (1) has the following formula (4): [Chemistry 16] (4)。 9. The thermosensitive recording medium according to any one of claims 1 to 8, wherein the developer (2) has the following structure: wherein -O-SO2-(phenyl)-p-R 3 The group is in the meta position relative to the -NH-CO-NH-phenyl substituent, and R 3 = methyl.

10. The thermosensitive recording medium according to any one of claims 1 to 9, wherein in the developer of formula (3), each (R)n system consists of a single methyl group in the para position relative to the -SO2-O- group.

11. The thermosensitive recording medium according to any one of claims 1 to 10, wherein the thermosensitive coloring layer contains at least one type of inorganic filler having an oil absorption of 80 g / 100 g or more. 12 . The thermosensitive recording medium according to claim 1 , wherein the thermosensitive coloring layer does not contain a sensitizer.

13. The thermosensitive recording medium according to any one of claims 1 to 12, wherein the thermosensitive coloring 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.

14. A food package comprising the thermosensitive recording medium according to any one of claims 1 to 13.

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

Images