Heat-sensitive recording medium

Abstract

The purpose of the present invention is to provide a heat-sensitive recording medium having excellent transparency while suppressing curling. A heat-sensitive recording material (1) in which a base material (2), a heat-sensitive recording layer (3), and a protective layer (4) are formed in this order on one surface of the base material (2), and a back coat layer (5) containing a core-shell resin and a polyamide epichlorohydrin resin is formed on the other surface of the base material (2). The protective layer (4) preferably includes a top coat layer (6) and an intermediate layer (7) formed between the top coat layer (6) and the heat-sensitive recording layer (3).

Description

Technical Field

[0001] The present invention relates to a thermosensitive recording medium. Background Art

[0002] Thermal recording media develop color when heated by a thermal head, producing a recorded image. Thermal recording media are used in a wide range of applications, including printers for output in fax machines, ticket vending machines, scientific measuring instruments, and CRT medical measuring instruments.

[0003] When this thermosensitive recording medium is used as a label or packaging film for various containers containing food or the like, the contents of the container are hidden by the label or packaging film, making it difficult for consumers to confirm the contents.

[0004] Therefore, the applicant of this case designed a thermosensitive recording medium to make the film used for labels and packaging transparent. Specifically, to enable identification of the contents of a container, the film used for labels and packaging should be transparent, and a thermosensitive recording medium with excellent transparency was proposed (see, for example, Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 6202599 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] In recent years, the problem of plastic waste has become increasingly serious. As an environmental measure, de-plasticization has received attention and calls for reducing the use of plastics.

[0010] Therefore, in the thermosensitive recording material proposed in Patent Document 1, it is also desired to reduce the thickness of the base film and thus reduce the amount of plastic used.

[0011] However, when the base film is thinned, the influence of stress from the thermosensitive recording layer formed on the base film increases, which causes the thermosensitive recording medium to curl toward the printed surface having the thermosensitive recording layer formed thereon.

[0012] The present invention has been made in view of such actual circumstances, and an object of the present invention is to provide a thermosensitive recording medium having excellent transparency and suppressed curling.

[0013] Means for solving problems

[0014] In order to achieve the above-mentioned object, the present invention is constructed as follows.

[0015] (1) The thermosensitive recording medium of the present invention comprises a substrate, a thermosensitive recording layer, and a protective layer laminated in this order on one surface of a substrate, and a back coating layer formed on the other surface of the substrate, wherein the substrate comprises a transparent resin film, and the back coating layer comprises a core-shell resin and a polyamide epichlorohydrin resin.

[0016] According to thermosensitive recording medium of the present invention, because back coating layer contains core-shell type resin, therefore compared with general emulsion, film forming property is good.Generally speaking, under the situation of general emulsion, emulsion is particle, and after drying, fusion between particle and becomes film.Think, under the situation of core-shell type resin, because the shell portion with water-soluble part partially dissolves before drying, not particle, so film forming property further improves.Therefore, even laminated thermosensitive recording layer and protective layer, also can tolerate their stress, as a result, can suppress curling.In addition, because above-mentioned back coating layer contains polyamide epichlorohydrin resin as the cross-linking agent that is used to make core-shell type resin crosslinked, therefore compared with the cross-linking agent of oxazoline system as described later, transparency improves.

[0017] (2) In a preferred embodiment of the present invention, the protective layer includes a top coat layer and an intermediate layer formed between the top coat layer and the thermosensitive recording layer.

[0018] According to this embodiment, since the top coat layer is provided, the compatibility of the thermal recording layer with the thermal head can be improved, and color development in the thermal recording layer can be appropriately performed.

[0019] In addition, the intermediate layer provides a barrier property against water and oil.

[0020] (3) In another embodiment of the present invention, the intermediate layer contains a core-shell resin and a polyamide epichlorohydrin resin.

[0021] According to this embodiment, the intermediate layer contains a core-shell resin having a water-soluble portion. Therefore, when the coating liquid for forming the intermediate layer is applied to the thermosensitive recording layer and then dried, the core-shell resin having a water-soluble portion permeates the thermosensitive recording layer, forming a smooth intermediate layer. This suppresses diffuse reflection of light from the thermosensitive recording layer, improving the transparency of the thermosensitive recording medium.

[0022] Furthermore, a core-shell resin is used in the intermediate layer serving as a protective layer on one side of the substrate and the back coating layer on the other side of the substrate, along with a polyamide-epichlorohydrin resin as a crosslinking agent. This offsets the stress on one side of the substrate and the other side, effectively suppressing curling.

[0023] (4) In another embodiment of the present invention, the dynamic surface tension of the coating liquid for forming the back coat layer at 50 msec is 51 mN / m or less.

[0024] According to this embodiment, since the dynamic surface tension of the coating liquid used for back coating formation at 50msec is below 51mN / m, the coating adaptability to the substrate becomes good. Thus, the coating surface becomes a uniform surface quality without unevenness. As a result, there is no unevenness in transparency and stress can also be obtained.

[0025] (5) In another embodiment of the present invention, the thickness of the substrate is 10 μm or more and 50 μm or less.

[0026] According to this embodiment, since the thickness of the transparent resin film serving as the base material is as thin as 50 μm or less, the amount of resin material used can be reduced, thereby achieving plastic elimination.

[0027] Effects of the Invention

[0028] Like this according to the present invention, because back coating layer contains core-shell type resin, therefore compare film-forming property good with general emulsion.Therefore, even laminated thermosensitive recording layer and protective layer, also can tolerate their stress, as a result, can suppress curling.In addition, because above-mentioned back coating layer contains polyamide epichlorohydrin resin as the cross-linking agent that is used to make the cross-linking of core-shell type resin, therefore compare transparency and improve with the cross-linking agent of oxazoline system as described later. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic cross-sectional view of a thermosensitive recording medium according to one embodiment of the present invention. DETAILED DESCRIPTION

[0030] Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

[0031] Figure 1 This is a schematic cross-sectional view of a thermosensitive recording medium according to one embodiment of the present invention. This embodiment comprises a thermosensitive recording medium 1 having a thermosensitive recording layer 3 that develops color upon heating laminated on the upper surface of one side of a substrate 2. Furthermore, a protective layer 4 is laminated on the surface of the thermosensitive recording layer 3, and a backcoat layer 5 is formed on the lower surface of the other side of the substrate 2. The protective layer 4 includes a topcoat layer 6 constituting a surface layer and an intermediate layer 7 formed between the topcoat layer 6 and the thermosensitive recording layer 3.

[0032] Hereinafter, the configuration of each layer will be described.

[0033] [Base material]

[0034] As the substrate 2, a transparent synthetic resin film can be used, such as a polypropylene film, polyethylene terephthalate film, polystyrene film, or polycarbonate film. Furthermore, the substrate 2 may be a single layer or a multilayer. The thickness of the film is not particularly limited, but a thickness of approximately 10 μm to 100 μm is preferred, as it provides excellent coating properties and transparency on the substrate 2. Furthermore, to reduce the amount of resin material used and achieve deplasticization, the thickness of the substrate 2 is preferably 50 μm or less, and more preferably 40 μm or less.

[0035] [Thermosensitive recording layer]

[0036] Materials forming the thermosensitive recording layer 3 include a coloring agent that develops color upon heating, a developer, a filler, a binder, a lubricant, and the like.

[0037] In order to improve the transparency of the thermosensitive recording material 1, it is preferable to use materials with small particle sizes. If the materials have such small particle sizes, diffuse reflection of light is suppressed, and the transparency of the thermosensitive recording material can be improved.

[0038] Specifically, as the leuco dye used as a coloring agent, 3-(N-isobutyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-isopentyl-N-ethyl)amino-6-methyl-7-o-chloroanilinofluoran, 3-(N-ethyl-N-p-toluidinyl)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isopentyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethoxypropyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-cyclohexyl-N-methyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-n-propyl)amino-6-methyl-7-anilinofluoran, 3-dibutylamino 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-p-toluidinylfluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-8-methylfluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-chlorofluoran, 3-dibutylamino-6-methyl-7-bromofluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dipentylamino-6-methyl-7-anilinofluoran, 3-dimethylamino-5-methyl-7-methylfluoran, 3-pyrrolidinyl-6-methyl-7-anilinofluoran, crystal violet lactone, etc. These leuco dyes can be used alone or in combination of two or more.

[0039] The particle size of the coloring agent is preferably 0.1 μm to 1.0 μm. The particle size herein refers to the 50% average particle size measured using a Microtrac laser light scattering particle size analyzer. Hereinafter, the term "particle size" refers to the 50% average particle size measured using the Microtrac laser light scattering particle size analyzer.

[0040] As the color developer, for example, 1,1-bis(p-hydroxyphenyl)cyclohexane, 1,1-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)butane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 2,2'-methylenebis(4-chlorophenol), 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 2,4'-dihydroxydiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-phenyl sulfone can be used. n-Propoxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl) sulfone, poly(4-hydroxybenzoic acid), benzyl 4-hydroxybenzoate, 2,4-bis(phenylsulfonyl)phenol, α-{4-[(4-hydroxyphenyl)sulfonyl]phenyl}-ω-hydroxypoly((polymerization degree n = 1 to 7))(oxyethylene 2,2-bis[(4-methyl-3-phenoxycarbonylaminophenyl)urea]diphenylsulfone, 3,5-bis(α-methylbenzyl)salicylic acid, bis[zinc 4-(n-octyloxycarbonylamino)salicylate], 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, 4-hydroxybenzenesulfonylanilide, 2'-(3-phenylureido)benzenesulfonylanilide, N-(2-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio] Acetamide, N-(4-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio]acetamide, 4-[[4-[4-[4-[[4-(1-methylethoxy)phenyl]sulfonylphenoxy]butoxy]phenyl]sulfonyl]phenol, 4-tert-butylphenol-formaldehyde polycondensate, N-(p-toluenesulfonyl)N'-(3-p-toluenesulfonyloxyphenyl)urea, 1-phenyl-3-(4-methylphenylsulfonyl)urea, etc., preferably have a particle size of 0.1 μm or more and 1.0 μm or less. These color developers can be used alone or in combination of two or more.

[0041] Examples of the fillers include aluminum hydroxide, magnesium hydroxide, aluminum oxide, magnesium oxide, aluminum silicate, calcium carbonate, magnesium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolin, calcined kaolin, diatomaceous earth, white carbon black, zinc oxide, silicon oxide, colloidal silica, polystyrene resin particles, urea-formaldehyde resin particles, and polyolefin resin particles. These fillers preferably have a particle size of 1.0 μm or less. These fillers may be used alone or in combination of two or more.

[0042] Examples of the binder include polyvinyl alcohol, modified polyvinyl alcohol, starch, casein, gelatin, polyamide, polyacrylamide, modified polyacrylamide, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl acetate, polyacrylate, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, diisobutylene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, methylvinyl-maleic anhydride copolymer, isopropylene-maleic anhydride copolymer, styrene-butadiene copolymer, polyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinyl acetate copolymer, polyurethane, polystyrene, polyvinyl pyrrolidone, acrylic ester, acrylonitrile, and methyl vinyl ether. These binders may be used alone or in combination of two or more.

[0043] Examples of the sensitizer include stearic acid, stearic acid amide, stearic acid aniline, hydroxymethyl stearic acid amide, methylene bis stearic acid amide, ethylene bis stearic acid amide, 1-benzyloxynaphthalene, 2-benzyloxynaphthalene, 2,6-diisopropylnaphthalene, 1,2-diphenoxyethane, 1,2-diphenoxymethylbenzene, 1,2-bis(3,4-dimethylphenyl)ethane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bis(4-methylphenoxy)ethane, di(p-chlorobenzyl) oxalate, di(p-methylbenzyl) oxalate, dibenzyl oxalate, p-benzylbiphenyl, m-terphenyl, diphenyl sulfone, benzyl p-benzyloxybenzoate, dibenzyl terephthalate, and p-toluenesulfonamide. These sensitizers may be used alone or in combination of two or more.

[0044] Examples of the lubricant include paraffin wax, fatty acids such as oleic acid, polyolefin waxes such as polyethylene wax, metal soaps such as zinc stearate, ester waxes such as carnauba wax, silicone oil, and oils such as whale oil. These lubricants preferably have a particle size of 0.5 μm or less. These lubricants may be used alone or in combination of two or more.

[0045] In order to improve the transparency of the thermosensitive recording material 1, it is known that it is particularly effective to include paraffin wax in the thermosensitive recording layer 3. The paraffin wax has a low melting point lower than the color development temperature of the thermosensitive recording layer 3, preferably lower than 80°C, more preferably lower than 50°C.

[0046] The particle size of the low melting point paraffin wax is preferably 0.5 μm or less as described above. The content of the paraffin wax is preferably 0.1 g / m2 on a dry weight basis, for example. 2 ~1.0g / m 2 .

[0047] By including the low-melting-point paraffin wax, when the coating liquid for forming the thermosensitive recording layer 3 is applied to the substrate 2 and dried, the low-melting-point paraffin wax melts and enters and fills the gaps between the uneven surfaces of the particles constituting the thermosensitive recording layer 3. This suppresses diffuse reflection of light in the thermosensitive recording layer 3, and improves the transparency of the thermosensitive recording medium 1.

[0048] [Protective layer]

[0049] The protective layer 4 of this embodiment includes a top coat layer 6 and an intermediate layer 7 .

[0050] The intermediate layer 7 has a barrier property against water and oil, and improves the water resistance and chemical resistance of the thermal recording layer 3. The top coat layer 6 improves the compatibility of the thermal recording layer 3 with the thermal head, and enables the color development in the thermal recording layer 3 to proceed appropriately.

[0051] Intermediate layer 7 is primarily formed of a resin. The resin may be a water-soluble resin or a water-insoluble resin. Specifically, examples include acrylic resins such as acrylic resins, styrene-acrylic resins, acrylic-urethane resins, acrylic-amide resins, and vinyl acetate-acrylic resins; epichlorohydrin resins such as polyamide-epichlorohydrin resins, polyamine-epichlorohydrin resins, and polyamide-polyamine-epichlorohydrin resins; maleic resins such as maleic resins, styrene-maleic resins, and olefin-maleic resins; styrene-butadiene (SBR) resins; acrylonitrile-butadiene-styrene resins; vinyl acetate resins; and polyvinyl alcohol resins such as fully saponified polyvinyl alcohol resins, partially saponified polyvinyl alcohol resins, diacetone-modified polyvinyl alcohol resins, acetoacetyl-modified polyvinyl alcohol resins, sulfonic acid-modified polyvinyl alcohol resins, olefin-modified polyvinyl alcohol resins, acrylonitrile-modified polyvinyl alcohol resins, pyrrolidone-modified polyvinyl alcohol resins, silanol-modified polyvinyl alcohol resins, and cation-modified polyvinyl alcohol resins. These resins may also be modified resins modified by known methods. These resins may be used alone or in combination of two or more. The compositions forming these resins may be solid, emulsions, or solutions. From the perspective of excellent coating properties, emulsions or solutions are preferred.

[0052] It should be noted that, in this specification, the term "acrylic resin" refers to a resin obtained by polymerizing an acrylic monomer (acrylic resin), or a resin obtained by copolymerizing an acrylic monomer with another monomer (a monomer other than an acrylic monomer copolymerizable with the acrylic monomer). The other monomers may be two or more. Furthermore, when simply referred to as "acrylic," unless otherwise specified, it refers to (meth)acrylic acid (salt) and / or (meth)acrylic acid ester. Here, "(meth)acrylic acid" refers to acrylic acid and / or methacrylic acid, and "(meth)acrylic acid (salt)" refers to (meth)acrylic acid and / or (meth)acrylic acid salt.

[0053] The salts in the (meth)acrylate salts are not particularly limited, and examples thereof include ammonium salts such as ammonia; alkanolamine salts such as triethanolamine, diethanolamine, and monoethanolamine; alkylamine salts such as methylamine, ethylamine, diethylamine, and triethylamine; polyamine salts such as diethyleneamine and diethylenetriamine; alkali metal salts such as lithium, sodium, and potassium; alkaline earth metal salts such as magnesium and calcium; and polyvalent metal salts such as zinc and iron. These salts may be used alone or in combination of two or more.

[0054] To improve the transparency of the thermosensitive recording material 1, the resin of the intermediate layer 7 can be a resin having a water-soluble portion. For example, polyvinyl alcohol (PVA), which is a resin having hydroxyl groups as hydrophilic structural units, or a resin having carboxyl groups as hydrophilic structural units (carboxyl-containing resins) can be used. In this specification, the term "carboxyl-containing resin" refers to a resin containing carboxyl groups in the structure of the polymer or copolymer forming the resin. The carboxyl groups in the carboxyl-containing resin can be free carboxyl groups or acid anhydride groups (specifically, dicarboxylic anhydride groups). Furthermore, the acid anhydride groups can be partially ring-opened to convert to carboxyl groups. In the carboxyl-containing resin, some or all of the carboxyl groups can be neutralized with a base. Furthermore, to efficiently form a crosslinked structure with the crosslinking agent described below and further improve film-forming properties, a resin having a reactive structural unit is preferred. In the present invention, a "reactive structural unit" refers to a structural unit that is reactive enough to form a crosslinked structure with other materials. Representative examples include carboxyl groups, azetidinium rings, and oxazoline groups.

[0055] The intermediate layer 7 of this embodiment preferably contains at least one selected from a carboxyl group-containing resin and an epichlorohydrin-based resin. From the perspectives of forming a crosslinked structure (described below), further improving film-forming properties, improving transparency, and suppressing curling, the carboxyl group-containing resin and the epichlorohydrin-based resin are more preferably contained.

[0056] The carboxyl group-containing resin is preferably at least one selected from acrylic resins and maleic resins, more preferably acrylic resins. The acrylic resin is preferably at least one selected from acrylic resins, styrene-acrylic resins, acrylic-polyurethane resins, acrylic-amide resins, and vinyl acetate-acrylic resins, more preferably at least one selected from acrylic resins, styrene-acrylic resins, and acrylic-polyurethane resins, and even more preferably at least one selected from acrylic resins and styrene-acrylic resins.

[0057] The epichlorohydrin resin is preferably at least one selected from polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, and polyamide polyamine epichlorohydrin resins, more preferably at least one selected from polyamide epichlorohydrin resins and polyamide polyamine epichlorohydrin resins, and still more preferably a polyamide epichlorohydrin resin.

[0058] Alternatively, to further enhance the transparency and curl suppression of the thermosensitive recording medium 1, the resin of the intermediate layer 7 preferably comprises a core-shell resin, i.e., a core-shell resin, in which hydrophobic core particles are coated with a water-soluble shell polymer. The core-shell resin is not particularly limited; for example, a core-shell resin containing carboxyl groups (core-shell carboxyl resin) is preferred. The core-shell carboxyl resin is considered to contain carboxyl groups within the structure of at least the water-soluble shell polymer. Generally speaking, a core-shell resin is formed by a multi-stage polymerization reaction of hydrophobic core particles and a water-soluble shell polymer. The resins forming the core particles and the shell polymer can be the same as those described above. Therefore, all descriptions of the resins described above can be used with respect to the core-shell resin.

[0059] Specifically, as the above-mentioned core-shell type carboxyl-containing resin, it is preferably selected from at least one of core-shell type acrylic resins and core-shell type maleic acid resins, and more preferably core-shell type acrylic resins. As the above-mentioned core-shell type acrylic resin, it is preferably selected from at least one of core-shell type acrylic resins, core-shell type styrene-acrylic resins, core-shell type acrylic-polyurethane resins, core-shell type acrylic-amide resins, and core-shell type vinyl acetate-acrylic resins, more preferably selected from at least one of core-shell type acrylic resins, core-shell type styrene-acrylic resins, and core-shell type acrylic-polyurethane resins, further preferably selected from at least one of core-shell type acrylic resins and core-shell type styrene-acrylic resins, and particularly preferably core-shell type acrylic resins. It should be noted that as the core-shell type acrylic resin, for example, a resin sold on the market under the name of Barrierstar (Mitsui Chemicals) can be cited. It should be noted that the above-mentioned epichlorohydrin-based resin is a resin that is not a core-shell structure, that is, a non-core-shell type resin.

[0060] Water-soluble polyvinyl alcohol and core-shell acrylic resins have excellent film-forming properties. Furthermore, when the coating liquid for forming the intermediate layer is applied to the thermosensitive recording layer 3 and dried, the water-soluble portion of the resin enters the thermosensitive recording layer 3, forming a smooth intermediate layer 7. This suppresses diffuse reflection of light from the thermosensitive recording layer 3, improving the transparency of the thermosensitive recording layer 3.

[0061] In addition, even if it is a resin with a water-soluble portion, if the film-forming property is poor, it is also impossible to fully suppress curling. In this regard, the core-shell type resin is a structure in which hydrophobic core particles are coated with a water-soluble shell polymer, and has good film-forming property compared to general emulsions. Generally speaking, in the case of general emulsions, the emulsion is a particle, and after drying, the particles fuse to form a film. On the other hand, it can be considered that in the case of a core-shell type resin, the shell portion with a water-soluble portion partially dissolves before drying and is not a particle, so the film-forming property is further improved. Therefore, even if a back coating layer is stacked, it can withstand its stress, and as a result, it is effective in suppressing curling.

[0062] The intermediate layer 7 of this embodiment preferably contains a crosslinking agent. As the above-mentioned crosslinking agent, for example, organic crosslinking agents such as cationic crosslinking agents and non-cationic crosslinking agents; inorganic crosslinking agents such as zirconium carbonate, etc. can be cited. As the above-mentioned cationic crosslinking agent, for example, epichlorohydrin resins such as polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, and polyamide polyamine epichlorohydrin resins can be cited. As the above-mentioned non-cationic crosslinking agent, oxazoline compounds such as polymers containing oxazoline groups can be cited. As the above-mentioned crosslinking agent, a cationic crosslinking agent is preferred, and an epichlorohydrin resin is more preferred. Here, the preferred types of epichlorohydrin resins are the same as the examples cited above. These crosslinking agents can be used alone or in combination of two or more.

[0063] Generally, crosslinking agents have reactive structural units as described above. The reactive structural units in the crosslinking agent react with reactive structural units in other materials to form a crosslinked structure, further improving film-forming properties and enhancing transparency and curling suppression. For example, when the crosslinking agent is an epichlorohydrin-based resin, the azetidinium ring (AZR) in the epichlorohydrin-based resin acts as a reactive structural unit and reacts with the other material (e.g., the carboxyl groups in the aforementioned carboxyl-containing resin) to form a crosslinked structure. Thus, to further improve film-forming properties, the interlayer 7 of this embodiment preferably comprises a carboxyl-containing resin and an epichlorohydrin-based resin. Furthermore, it is also preferred to comprise a core-shell-type carboxyl-containing resin and an epichlorohydrin-based resin. Furthermore, it is more preferred to comprise a core-shell-type carboxyl-containing resin and an epichlorohydrin-based resin. Preferred types of the carboxyl-containing resin, the core-shell-type carboxyl-containing resin, and the epichlorohydrin-based resin are the same as those described above for the carboxyl-containing resin, the core-shell-type carboxyl-containing resin, and the epichlorohydrin-based resin.

[0064] The content of the above resin is preferably 50% by mass or greater, more preferably 60% by mass or greater, even more preferably 70% by mass or greater, even more preferably 80% by mass or greater, and particularly preferably 90% by mass or greater, relative to 100% by mass of the dry mass of the intermediate layer 7. Within this range, film-forming properties are further improved, and transparency and curl suppression can be further enhanced.

[0065] The content of the core-shell resin relative to 100% by mass of the dry mass of the resin in the intermediate layer 7 is preferably 50% by mass or greater, more preferably 60% by mass or greater, and even more preferably 70% by mass or greater. Furthermore, the content of the core-shell resin relative to 100% by mass of the dry mass of the resin in the intermediate layer 7 is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. Within this range, film-forming properties are further improved, and transparency and curl suppression can be further enhanced. Furthermore, the content of the core-shell resin relative to 100% by mass of the dry mass of the intermediate layer 7 is preferably within the above range.

[0066] The content of the crosslinking agent is preferably 3% by mass or greater, more preferably 6% by mass or greater, and even more preferably 10% by mass or greater, relative to 100% by mass of the dry mass of the resin in the intermediate layer 7. Furthermore, the content of the crosslinking agent is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, relative to 100% by mass of the dry mass of the resin in the intermediate layer 7. Within this range, film-forming properties are further improved, and transparency and curl suppression can be further enhanced. Furthermore, the content of the crosslinking agent relative to 100% by mass of the dry mass of the intermediate layer 7 is preferably within the above range.

[0067] In addition, the intermediate layer 7 may further contain other materials than those mentioned above. Examples of other materials include wetting agents and the like.

[0068] The coating amount (dry mass) of the intermediate layer 7 is preferably 0.3 g / m 2 ~10g / m 2 , more preferably 0.5g / m 2 ~5.0g / m 2 , more preferably 1.0 g / m 2 ~4.0g / m 2 .

[0069] In this embodiment, the intermediate layer 7 is made of a core-shell acrylic resin similar to the back coat layer 5, and a polyamide epichlorohydrin resin is used as a crosslinking agent. This will be described in detail later with reference to examples.

[0070] The top coat layer 6 is made of a binder to which a filler, a lubricant, a cross-linking agent, and the like are added.

[0071] Examples of the binder resin include acrylic resins. Other resins include those listed in the section regarding the intermediate layer 7. These resins may be used alone or in combination of two or more. Examples of the lubricant include polyethylene and zinc stearate. Other lubricants include those listed in the section regarding the thermosensitive recording layer 3. These lubricants may be used alone or in combination of two or more.

[0072] Examples of the crosslinking agent include zirconium carbonate, etc. Other crosslinking agents include those mentioned in the section regarding the intermediate layer 7. These crosslinking agents may be used alone or in combination of two or more.

[0073] Examples of the filler include colloidal silica, calcium carbonate, polymethyl methacrylate (PMMA), and polystyrene (PS). Other fillers include those listed above for the thermosensitive recording layer 3. These fillers may be used alone or in combination of two or more.

[0074] The particle size of these fillers is preferably 1.0 μm or less. In order to improve transparency, colloidal silica with a small particle size is preferably used as the filler.

[0075] [Back coating]

[0076] The back coat layer 5 formed on the non-printing side of the substrate 2 opposite to the thermosensitive recording layer 3 is mainly composed of resin, including at least a core-shell resin and a polyamide epichlorohydrin resin.

[0077] As the resin contained in the back coating layer 5 of this embodiment, the content of recording in the project of the intermediate layer 7 can all be quoted.In addition, as the resin contained in the back coating layer 5, it can be the resin with water-soluble part, also can be the resin without water-soluble part.Wherein, preferably have the resin with water-soluble part.Herein, as the resin with water-soluble part, can enumerate polyvinyl alcohol as mentioned above, though in this case, transparency can improve, yet the water resistance of polyvinyl alcohol is poor, therefore under many wet environments, back coating layer might peel off.

[0078] By further improving the water resistance of the thermosensitive recording medium 1, curl suppression can be further improved. Therefore, in this embodiment, the back coating layer 5 contains a core-shell resin as the resin having a water-soluble portion. The core-shell resin is not particularly limited, and for example, a core-shell carboxyl-containing resin is preferred. The preferred types of the core-shell carboxyl-containing resin are the same as the examples given in the item of the intermediate layer 7. That is, the core-shell carboxyl-containing resin is preferably selected from at least one of a core-shell acrylic resin and a core-shell maleic acid resin, and more preferably a core-shell acrylic resin. The core-shell acrylic resin is preferably at least one selected from the group consisting of core-shell acrylic resins, core-shell styrene-acrylic resins, core-shell acrylic-polyurethane resins, core-shell acrylic-amide resins, and core-shell vinyl acetate-acrylic resins. It is more preferably at least one selected from the group consisting of core-shell acrylic resins, core-shell styrene-acrylic resins, and core-shell acrylic-polyurethane resins. It is further preferably at least one selected from the group consisting of core-shell acrylic resins and core-shell styrene-acrylic resins. Core-shell acrylic resins are particularly preferred.

[0079] In addition, a polyamide epichlorohydrin resin is used as a crosslinking agent for crosslinking the core-shell acrylic resin.

[0080] Even if it is the resin with water-soluble part, if film forming property is poor, then also can't suppress curling fully.In this regard, the acrylic resin of core-shell type is coated with the water-soluble shell polymer of hydrophobic core particle, and compared with general emulsion, film forming property is good.Therefore, even laminated heat-sensitive recording layer and protective layer, also can tolerate their stress, as a result, is effective for the inhibition of curling.In addition, owing to having hydrophobic core, therefore also can not have the situation of water resistance deterioration.

[0081] Furthermore, by using a polyamide epichlorohydrin resin as a crosslinking agent for crosslinking a core-shell acrylic resin, as shown in a comparative transparency test described later, the transparency is improved compared to an oxazoline-based crosslinking agent.

[0082] The content of the above resin is preferably 50% by mass or more, more preferably 60% by mass or more, further preferably 70% by mass or more, further preferably 80% by mass or more, and particularly preferably 90% by mass or more relative to 100% by mass of the dry mass of the back coating layer 5. When within the above range, film-forming properties are further improved, and transparency and curling suppression can be further enhanced.

[0083] The content ratio of the above-mentioned core-shell type resin relative to 100% by mass of the dry mass of the resin in the back coating layer 5 is preferably 50% by mass or more, more preferably 60% by mass or more, and further preferably 70% by mass or more. In addition, the content ratio of the above-mentioned core-shell type resin relative to 100% by mass of the dry mass of the resin in the back coating layer 5 is preferably 99% by mass or less, more preferably 95% by mass or less, and further preferably 90% by mass or less. When it is within the above range, the film-forming property becomes better, and the transparency and curling suppression can be further improved. In addition, the content ratio of the above-mentioned core-shell type resin relative to 100% by mass of the dry mass of the back coating layer 5 is preferably within the above range.

[0084] The content of the cross-linking agent is preferably 3% by mass or more relative to 100% by mass of the dry mass of the resin in the back coating layer 5, more preferably 6% by mass or more, and further preferably 10% by mass or more. In addition, the content of the cross-linking agent is preferably 40% by mass or less relative to 100% by mass of the dry mass of the resin in the back coating layer 5, more preferably 30% by mass or less, and further preferably 20% by mass or less. When it is within the above range, film-forming property becomes better, and transparency and curling suppression can be further improved. In addition, the content of the cross-linking agent relative to 100% by mass of the dry mass of the back coating layer 5 is preferably within the above range.

[0085] The back coat layer 5 may further contain other materials besides those mentioned above. Examples of other materials include a wetting agent, a filler, a lubricant, and an antiseptic.

[0086] Examples of the wetting agent include nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants, and fluorine-based surfactants. These surfactants may be used alone or in combination of two or more.

[0087] As the nonionic surfactant, for example, acetylene glycol surfactants, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, polyoxyethylene sorbitan fatty acid esters, etc. can be used.

[0088] Examples of the acetylene glycol surfactant include acetylene glycol and alkylene oxide adducts of acetylene glycol. Examples of the acetylene glycol include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, 3,5-dimethyl-1-hexyne-3-diol, and 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol.

[0089] Examples of the anionic surfactant include polyoxyethylene alkyl ether sulfuric acid or its salts, polyoxyethylene alkyl ether acetic acid or its salts, dodecylbenzenesulfonic acid or its salts, alkylsulfuric acid or its salts, and alkanesulfonic acid or its salts.

[0090] Among these wetting agents, nonionic surfactants are preferred, acetylene glycol-based surfactants are more preferred, and alkylene oxide adducts of acetylene glycol are still more preferred.

[0091] The wetting agent can generally be adjusted for wettability by using its HLB value. From the viewpoint of further improving the coating properties of the back coat layer 5 , the HLB value of the wetting agent is preferably 4 or greater, more preferably 6 or greater, and even more preferably 6 or greater and 16 or less.

[0092] The coating amount (dry mass) of the back coating layer 5 is preferably 0.3 g / m 2 ~10g / m 2 , more preferably 0.5g / m 2 ~5.0g / m 2 , more preferably 1.0 g / m 2 ~4.0g / m 2 .

[0093] In this embodiment, from the viewpoint of further suppressing the curl of thermosensitive recording medium, when observing from the base material as support, preferably the stress that may produce in a face and the stress that may produce in the face of the side opposite to the above-mentioned face are more equal.Therefore, back coating layer 5 and intermediate layer 7 preferably all contain core-shell type resin, more preferably contain the core-shell type resin of the same kind, further preferably contain identical core-shell type resin.In addition, back coating layer 5 and intermediate layer 7 preferably all contain polyamide epichlorohydrin resin.In addition, back coating layer 5 and intermediate layer 7 preferably contain the core-shell type resin and polyamide epichlorohydrin resin of the same kind, more preferably contain identical core-shell type resin and polyamide epichlorohydrin resin.In addition, back coating layer 5 and intermediate layer 7 preferably contain core-shell type acrylic resin and polyamide epichlorohydrin resin, more preferably contain core-shell type acrylic resin and polyamide epichlorohydrin resin.

[0094] Here, the term "same type" means structural units having the same reactivity. For example, the core-shell resins of the same type refer to structural units having the same reactivity such as a carboxyl group derived from an acrylic monomer.

[0095] The mass ratio of the core-shell resin in the intermediate layer 7 to the core-shell resin in the back coat layer 5 (core-shell resin content per unit area (dry mass) in the intermediate layer 7 / core-shell resin content per unit area (dry mass) in the back coat layer 5) is preferably 0.2 to 4, more preferably 0.3 to 3, even more preferably 0.5 to 2, and particularly preferably 0.7 to 1.5. Within this range, when viewed from the substrate, the stress that may be generated on one surface and the stress that may be generated on the surface opposite to that surface are more equal, and curl suppression can be further improved.

[0096] The mass ratio of the polyamide-epichlorohydrin resin in the intermediate layer 7 to the polyamide-epichlorohydrin resin in the back coat layer 5 (content per unit area (dry mass) of the polyamide-epichlorohydrin resin in the intermediate layer 7 / content per unit area (dry mass) of the polyamide-epichlorohydrin resin in the back coat layer 5) is preferably 0.2 to 4, more preferably 0.3 to 3, even more preferably 0.5 to 2, and particularly preferably 0.7 to 1.5. Within this range, when viewed from the substrate, the stress that may be generated on one surface and the stress that may be generated on the surface opposite to the surface are more equal, and curling can be further suppressed.

[0097] The ratio of the coating weight (dry mass) of the intermediate layer 7 to the coating weight (dry mass) of the back coating layer 5 (coating weight (dry mass) per unit area of the intermediate layer 7 / coating weight (dry mass) per unit area of the back coating layer 5) is preferably 0.3 to 3, more preferably 0.5 to 2, and even more preferably 0.7 to 1.5. Within this range, when viewed from the substrate, the stress that may be generated on one surface and the stress that may be generated on the surface opposite to the above surface are more equal, and curling can be further suppressed.

[0098] When the thermosensitive recording material 1 (a laminate having a back coat layer 5 and a top coat layer 6 as its two end surfaces) of this embodiment is subjected to a curl test using the method described in the Examples, the curl height is preferably 6 mm or less under at least one of the three conditions specified in (B), and 6 mm or less under at least one of the conditions in the subsequent (C). It is more preferred that the curl be 6 mm or less under at least two of the three conditions specified in (B), and 6 mm or less under at least two of the conditions in the subsequent (C). It is even more preferred that the curl be 6 mm or less under all of the three conditions specified in (B), and 6 mm or less under all of the conditions in the subsequent (C).

[0099] Regarding the back coat layer 5 and substrate 2 included in the thermosensitive recording material 1 of this embodiment, the haze value of the laminate of the back coat layer 5 and substrate 2 is preferably 7 or less, more preferably 6 or less, and even more preferably 5 or less when measured by the method described in the Examples.

[0100] Regarding the back coating layer 5 and substrate 2 possessed by the thermosensitive recording material 1 of this embodiment, when a water-resistant ink adhesion test is performed on the laminate of the back coating layer 5 and the substrate 2 using the method described in the examples, preferably, no ink falls off after immersion in water for 1 minute, more preferably, no ink falls off after immersion in water for 2 minutes, and even more preferably, less than 50% of the ink falls off after immersion in water for 5 minutes.

[0101] Example

[0102] Hereinafter, the present invention will be described in further detail based on specific examples. However, the following examples do not limit the present invention in any way.

[0103] The inventors prepared coating solutions for forming a back coat layer by mixing the base agent, crosslinking agent, and wetting agent shown in Table 1 below in formulations No. 1 to No. 5. All formulations were prepared so that the solid content concentration was 18.5%.

[0104]

[0105] In formulation No. 1, 18 parts of a polyamide epichlorohydrin resin as a crosslinking agent and 1.3 parts of an acetylene glycol surfactant having an HLB value of 13 to 14 as a wetting agent are blended with 100 parts of a core-shell acrylic resin as a main component in a dry weight ratio.

[0106] Here, HLB (Hydrophile-Lipophile-Balance) is a value that indicates the degree of affinity of a surfactant for water and oil. An HLB value greater than 7 indicates strong hydrophilicity, while an HLB value less than 7 indicates strong hydrophobicity (lipophilicity).

[0107] In formulation No. 2, an oxazoline-based crosslinking agent, specifically oxazoline WS300 (manufactured by Nippon Shokubai Co., Ltd.), and 1.3 parts of an acetylene glycol-based surfactant having an HLB value of 13 to 14 as a wetting agent are added to 100 parts of a core-shell acrylic resin as a main component, in terms of a dry weight ratio.

[0108] In formulation No. 3, only the core-shell acrylic resin 100 is used as the main component, and no crosslinking agent or wetting agent is mixed.

[0109] In formulation No. 4, 18 of a polyamide epichlorohydrin resin as a crosslinking agent and 0.16 of an acetylene glycol surfactant having an HLB value of 8 as a wetting agent are blended with 100 of a core-shell acrylic resin as a main component in a dry weight ratio.

[0110] In formulation No. 5, 18 of a polyamide epichlorohydrin resin as a crosslinking agent and 0.32 of an acetylene glycol surfactant having an HLB value of 8 as a wetting agent are blended with 100 of a core-shell acrylic resin as a main component in a dry weight ratio.

[0111] The inventors of the present invention appropriately selected the coating solutions for forming a back coat layer having the above-mentioned formulations No. 1 to No. 5 to prepare samples, and conducted tests on curling, transparency, ink adhesion, and dynamic surface tension.

[0112] Each test will be described below.

[0113] Curl test

[0114] The inventors of this case respectively produced the above Figure 1 A comparative test was conducted on the curling of the thermosensitive recording medium of the example shown and the thermosensitive recording medium of the comparative example not having the back coat layer 5 .

[0115] In the examples, the back coat layer 5 was formed using the coating film for forming a back coat layer having the formulation No. 1 in Table 1 above.

[0116] The Examples and Comparative Examples have the same configuration except for the presence or absence of the back coat layer 5. That is, the configurations of the substrate 2, the thermosensitive recording layer 3, the intermediate layer 7, and the top coat layer 6 are common in the Examples and Comparative Examples.

[0117] The following specifically describes the configuration of the substrate 2 , thermosensitive recording layer 3 , intermediate layer 7 , and topcoat layer 6 . However, this configuration is similar to that of Patent Document 1 (Japanese Patent No. 6202599) previously proposed by the applicant of this application.

[0118] Therefore, the thermosensitive recording material having a configuration other than the back coat layer 5 has excellent transparency similar to that of the thermosensitive recording material described in Patent Document 1.

[0119] First, a PET (polyethylene terephthalate) film having a thickness of 25 μm was used as the substrate 2 .

[0120] Thermosensitive recording layer 3 uses 3,3'-diallyl-4,4'-dihydroxydiphenylsulfone with a particle size of 0.4 μm as a developer, and kaolin with a particle size of 0.4 μm as a filler. Furthermore, SBR with a glass transition temperature (Tg) of -3°C is used as a binder. Paraffin wax with a melting point of 46°C and a particle size of 0.2 μm is used as a lubricant. 2-anilino-3methyl-6-(N-methyl-p-toluidinyl)fluoran with a particle size of 0.5 μm is used as a dye.

[0121] The weight ratio of the developer, kaolin, SBR, paraffin, and dye in the dry state is 25, 10, 20, 4, and 12, and the total weight ratio is 74.

[0122] The prepared coating liquid for forming a heat-sensitive recording layer was prepared and applied to the PET film in an amount of 4.5 g / m2 in dry weight. 2 After coating in a manner of , drying is performed to obtain a thermosensitive recording layer 3 .

[0123] The intermediate layer 7 uses a core-shell acrylic resin as a binder, a polyamide epichlorohydrin resin as a crosslinking agent, and an acetylene glycol surfactant having an HLB value of 13 to 14 as a wetting agent.

[0124] The core-shell acrylic resin, the polyamide epichlorohydrin resin, and the acetylene glycol surfactant having an HLB value of 13 to 14 were used as weight ratios in a dry state, 100, 18, and 1.3, respectively, for a total of 119.3.

[0125] The intermediate layer-forming coating liquid was prepared and applied to the thermosensitive recording layer 3 in an amount of 2.0 g / m2 in dry weight. 2 After coating in a manner of , drying is performed to form the intermediate layer 7 .

[0126] The topcoat layer 6 uses polyethylene (PE) with a particle size of 0.12 μm and zinc stearate (St-Zn) with a particle size of 5.5 μm as lubricants. An acrylic resin is used as a binder (adhesive), and zirconium carbonate is used as a crosslinking agent. Colloidal silica with particle sizes of several nanometers and tens of nanometers is used as a filler.

[0127] The weight ratios of polyethylene (PE), zinc stearate (St-Zn), acrylic resin, zirconium carbonate, colloidal silica with a particle size of several nanometers, and colloidal silica with a particle size of tens of nanometers when dried are set to 10, 5, 50, 5, 15, and 30, with the total being 130.

[0128] The top coating liquid was prepared and applied on the intermediate layer 7 in a dry weight of 1.5 g / m 2 The top coating layer 6 is formed by coating and drying.

[0129] Thus, a thermosensitive recording material of a comparative example was obtained, that is, a thermosensitive recording material having excellent transparency described in Patent Document 1. This thermosensitive recording material was cut into pieces measuring 70 mm in the flow direction and 240 to 320 mm in the width direction to prepare a sample of the comparative example.

[0130] The thermosensitive recording layer 3, the intermediate layer 7 and the top coat layer 6 were formed on the substrate 2 in the same manner as above. Then, the lower surface of the substrate 2 was coated with a film having a dry weight of 2.0 g / m 2 The coating liquid for forming a back coat layer of the above-mentioned formulation No. 1 was applied and dried to form a back coat layer 5.

[0131] The thermosensitive recording material of the example was obtained in this manner. The thermosensitive recording material was cut into pieces of 70 mm in the flow direction and 240 to 320 mm in the width direction to prepare samples of the example.

[0132] The curling test was performed using the samples of the examples and comparative examples produced as described above. The curling test conditions and results are shown in Table 2 below.

[0133]

[0134] The curl test was performed as follows.

[0135] (A) Before the start of the test, each sample of the Examples and Comparative Examples used in the test was left to stand at a temperature of 23° C. and a relative humidity of 50% for 24 hours.

[0136] (B) Subsequently, each sample of the Examples and Comparative Examples was left for 24 hours under the conditions shown in Table 2, i.e., at a temperature of 30°C and at low, medium, and high humidity levels, specifically at relative humidity levels of 35%, 67%, and 98%, and the curling state of each sample was evaluated.

[0137] (C) Each sample of the Examples and Comparative Examples evaluated for curling in (B) above was further left at 23° C. and 50% relative humidity for 2 hours as shown in Table 2, and the curling state was evaluated again.

[0138] In the curling state evaluation, samples with a maximum lift height at the end of the sample, i.e., a curl height of 3 mm or less were marked as 0, samples with a curl height greater than 3 mm and less than 6 mm were marked as △, and samples with a curl height greater than 6 mm were marked as ×.

[0139] Regarding each sample of the Example having the back coating layer 5 formed thereon, the curl height after each sample was left for 24 hours under the conditions of (B) above was 3 mm or less.

[0140] In addition, regarding each sample of the Example in which the back coating layer 5 was formed, the curl height after the sample was left for 2 hours under the above-mentioned condition (C) was 0 or less of 3 mm.

[0141] In contrast, in the comparative example sample without a back coat layer, the curl height after being left for 24 hours under the conditions of temperature 30°C and relative humidity 35% in the above (B) was 7 mm or more.

[0142] Furthermore, under the above-mentioned condition (C), the curl height of the sample of the comparative example after being left for 2 hours was 7 mm or more, and was marked as ×.

[0143] In the samples of the comparative example in which the back coating layer 5 is not formed, there are cases where the curl height is greater than 7 mm. However, in the samples of the embodiment in which the back coating layer 5 is formed, even if the thickness of the base material is thin, such as a PET (polyethylene terephthalate) film with a thickness of 25 μm, the curl height is less than 3 mm, and there is no case where the curl height is greater than 3 mm.

[0144] In the Examples, the back coat layer contains as a main agent a core-shell acrylic resin having a water-soluble portion obtained by coating hydrophobic core particles with a water-soluble shell polymer as described above. Therefore, the back coat layer has good film-forming properties and can suppress curling.

[0145] Furthermore, since a polyamide epichlorohydrin resin is used as a cross-linking agent, the film exhibits excellent barrier properties, water resistance, and film-forming properties, and can suppress curling caused by changes in humidity under high temperature and high humidity conditions.

[0146] Comparative test of transparency

[0147] The coating solutions of formulation No. 1, 2, and 3 in the coating solution for forming the back coating layer in Table 1 were prepared at a dry weight of 2.5 g / m 2 The coating solution was applied to a 25 μm thick PET (polyethylene terephthalate) film as a substrate and dried, and the transparency was evaluated. The transparency of the substrate alone without the coating solution was also evaluated.

[0148] The transparency was evaluated by measuring the haze in accordance with JIS K 7136.

[0149] The measurement results are shown in Table 3 below.

[0150]

[0151] Table 3 also shows the total light transmittance (TT), parallel line transmittance (PT), and diffuse (scattered) light component (DIF) of the following formula for calculating the haze.

[0152] HAZE (%) = DIF / TT × 100

[0153] = (TT-PT) / TT×100

[0154] As shown in Table 3, the sample obtained by applying the coating solution for forming a back coating layer of Compound No. 1, i.e., the coating solution for forming a back coating layer comprising a core-shell acrylic resin as a main component, a polyamide epichlorohydrin resin as a crosslinking agent, and an acetylene glycol-based surfactant as shown in Table 1 above, had a haze value of 4.35 (%), indicating the highest transparency.

[0155] The sample obtained by applying the coating solution for forming a back coating layer with formulation No. 3, i.e., the coating solution for forming a back coating layer containing only the core-shell acrylic resin as the main agent and no crosslinking agent or surfactant as shown in Table 1 above, had a haze value of 4.92 (%), indicating the second highest transparency.

[0156] In contrast, the sample obtained by applying the coating solution for forming a back coating layer with formulation No. 2, i.e., the coating solution for forming a back coating layer in which an oxazoline-based crosslinking agent and an acetylene glycol-based surfactant are blended with a core-shell acrylic resin as a main agent as shown in Table 1, had the highest haze value of 8.28 (%), indicating low transparency.

[0157] Note that the haze value of only the substrate to which the coating liquid was not applied, that is, only the PET (polyethylene terephthalate) film, was 3.48 (%).

[0158] It is found that the haze value of 4.35 of sample No. 1 using a polyamide epichlorohydrin resin as a crosslinking agent is lower than the haze value of 8.28 (%) of sample No. 2 using an oxazoline-based crosslinking agent, indicating good transparency.

[0159] Compared to the haze value of 4.92 of Formulation No. 3 containing only a core-shell acrylic resin without a crosslinking agent, the haze value of 4.35 of Formulation No. 1 containing a polyamide epichlorohydrin resin as a crosslinking agent is lower, indicating good transparency.

[0160] The sample obtained by applying the coating solution for forming a back coat layer of Compound No. 1 containing a core-shell acrylic resin and a polyamide epichlorohydrin resin as a crosslinking agent had the lowest haze value and the best transparency.

[0161] <Ink Adhesion Test>

[0162] The coating solutions of formulation No. 1, 2, and 3 in the coating solution for forming the back coating layer in Table 1 were prepared at a dry weight of 2.0 g / m 2 The sample was then coated onto a PET (polyethylene terephthalate) film substrate. The coated surface was then printed with gravure ink for surface printing. The gravure ink used was the Raijin series from DIC Graphics Co., Ltd.

[0163] Regarding ink adhesion, an ink adhesion test and a water-resistant ink adhesion test when immersed in water were conducted.

[0164] In the ink adhesion test, a transparent tape was attached to the printed surface of the sample, and the presence or absence of ink falling off when peeled in a 180° direction was determined. Samples with no ink falling off were rated as 0.

[0165] In the water-resistant ink adhesion test, samples were immersed in water for 1 minute, removed, and gently wiped to remove the water. Transparent tape was affixed to the printed surface, and the presence of ink detachment when peeled off at 180° was evaluated. Samples with no detachment were rated 0, samples with less than 50% detachment were rated △, and samples with 50% or more detachment were rated ×.

[0166] Separately, immerse the sample in water for 2 minutes, remove it, and gently wipe off the water. Attach transparent tape to the printed surface and examine whether the ink falls off when peeled off in a 180° direction.

[0167] Similarly, the presence or absence of ink shedding was determined for each of the sample immersed in water and taken out after 5 minutes and the sample immersed in water and taken out after 10 minutes.

[0168] Specifically, in the water-resistant ink adhesion test, the sample was immersed in water for four different times: 1 minute, 2 minutes, 5 minutes, and 10 minutes.

[0169] The test results are shown in Table 4 below.

[0170]

[0171] In the ink adhesion test, the samples using the coating solutions for forming the back coat layer of formulations No. 1, 2, and 3 all showed no ink detachment, and all were rated as 0.

[0172] In a test of water-resistant ink adhesion, samples coated with back coating solution No. 3 (the back coating solution shown in Table 1, containing only a core-shell acrylic resin as the main component and no crosslinking agent or surfactant) showed ink detachment of less than 50% after immersion in water for 1 minute (a positive rating). Furthermore, samples immersed in water for 2 minutes, 5 minutes, and 10 minutes all showed ink detachment of 50% or more (a negative rating).

[0173] In contrast, samples coated with back coating layer coating solution No. 1, a back coating layer coating solution comprising a core-shell acrylic resin as a main component, a polyamide epichlorohydrin resin as a crosslinking agent, and an acetylene glycol-based surfactant, showed no ink detachment after immersion in water for 1 minute or 2 minutes, both rated as ○. Furthermore, ink detachment in the sample immersed in water for 5 minutes was less than 50%, rated as △. Ink detachment in the sample immersed in water for 10 minutes was greater than 50%, rated as ×.

[0174] In the sample obtained by mixing back coating film No. 2, a coating solution for forming a back coating layer comprising a core-shell acrylic resin as a main component, an oxazoline-based crosslinker, and an acetylene glycol-based surfactant, similar to the back coating solution No. 1, no ink detachment was observed in the sample immersed in water for 1 minute or for 2 minutes, both receiving a rating of ○. Furthermore, the sample immersed in water for 5 minutes exhibited ink detachment of less than 50%, receiving a rating of △. The sample immersed in water for 10 minutes exhibited ink detachment of more than 50%, receiving a rating of ×.

[0175] Compared to the sample coated with the coating liquid for forming a back coat layer of the composition No. 3 containing only a core-shell acrylic resin, the sample coated with the coating liquid for forming a back coat layer of the composition No. 1 containing a crosslinking agent and the sample coated with the coating liquid for forming a back coat layer of the composition No. 2 had better water-resistant ink adhesion.

[0176] As described above, the sample obtained by applying the coating liquid for forming a back coat layer of formulation No. 1 using a polyamide epichlorohydrin resin as a crosslinking agent and the sample obtained by applying the coating liquid for forming a back coat layer of formulation No. 2 using an oxazoline-based crosslinking agent were equivalent in terms of ink adhesion.

[0177] <Dynamic surface tension measurement test>

[0178] The dynamic surface tension of the coating solutions Nos. 1, 3, 4, and 5, among those used to form the back coating layer in Table 1, was measured. This dynamic surface tension was measured using the maximum bubble pressure method (bubble pressure method). Specifically, a KRUSS Bubble Pressure BP-2 dynamic surface tension meter was used.

[0179] In addition, the coating liquids of No. 1, 3, 4, and 5 were mixed at a dry weight of 2.5 g / m 2 The coating was applied to a 25 μm thick PET (polyethylene terephthalate) film substrate, dried at 50°C for 1 minute, and the surface quality of the coated surface was visually evaluated. Specifically, samples with a uniform, good coating surface without unevenness were rated as 0, while samples with uneven thickness and thin areas were rated as △.

[0180] The measurement results of dynamic surface tension and the evaluation results of surface quality are shown in Table 5 below.

[0181]

[0182] The back coating layer-forming coating liquid of Formulation No. 4 in Table 1, comprising 18 grams of a polyamide-epichlorohydrin resin as a crosslinking agent and 0.16 grams of an acetylene glycol-based surfactant having an HLB value of 8 as a wetting agent, relative to 100 grams of a core-shell acrylic resin as a main component, at a dry weight ratio, had a dynamic surface tension of 51 mN / m at 50 msec. This back coating layer-forming coating liquid of Formulation No. 4 exhibited excellent coating suitability, with the coated surface being uniform and free of unevenness, resulting in a rating of "Good (0)."

[0183] The coating liquid for forming a back coating layer of formulation No. 5 in Table 1, i.e., a coating liquid for forming a back coating layer obtained by blending 18 polyamide epichlorohydrin resin as a crosslinking agent and 0.32 acetylene glycol surfactant having an HLB value of 8 as a wetting agent with respect to 100 core-shell acrylic resin as a main component at a dry weight ratio, had a dynamic surface tension of 46 mN / m at 50 msec.

[0184] The coating liquid for forming a back coating layer of formulation No. 1 in Table 1, i.e., a coating liquid for forming a back coating layer obtained by blending 18 polyamide epichlorohydrin resin as a crosslinking agent and 1.3 acetylene glycol surfactant having an HLB value of 13 to 14 as a wetting agent with respect to 100 core-shell acrylic resin as a main component at a weight ratio when dried, had a dynamic surface tension of 35 mN / m at 50 msec.

[0185] The dynamic surface tension at 50msec is lower than the dynamic surface tension of the coating liquid for forming the back coating layer of No. 4. The coating adaptability of the coating liquid for forming the back coating layer of No. 5 and the coating liquid for forming the back coating layer of No. 1 are both good, and their coating surfaces are not uneven but uniform, and are all good 0.

[0186] In contrast, the back coating layer-forming coating liquid of Formulation No. 3 in Table 1, which contains only the core-shell acrylic resin 100 as a main component and no crosslinking agent or wetting agent, has a dynamic surface tension of 58 mN / m at 50 msec, which is higher than the dynamic surface tension of 51 mN / m of the back coating layer-forming coating liquid of Formulation No. 4. This back coating layer-forming coating liquid of Formulation No. 3 has poor coating suitability, and the thickness of the coated surface is uneven, with thin portions (Δ).

[0187] In order to obtain good coating suitability and a uniform surface quality of the coated surface as described above, the dynamic surface tension of the coating liquid for forming the back coat layer at 50 msec is preferably 51 mN / m or less.

[0188] As described above, according to this embodiment, the back coat layer contains a core-shell acrylic resin and a polyamide-epichlorohydrin resin, resulting in excellent film-forming properties and reduced curling even on thin substrates. Since the transparent resin film serving as the substrate can be thinned in this manner, the amount of resin material used can be reduced, achieving de-plasticization.

[0189] Furthermore, since the polyamide epichlorohydrin resin is contained as a crosslinking agent for crosslinking the core-shell acrylic resin, the transparency is improved compared to an oxazoline-based crosslinking agent.

[0190] Due to the inclusion Figure 1 The thermosensitive recording material including the substrate 2, the thermosensitive recording layer 3, the top coat layer 6 and the intermediate layer 7 other than the back coat layer 5 has excellent transparency similarly to the thermosensitive recording material described in the above-mentioned Patent Document 1. Therefore, the thermosensitive recording material 1 of this embodiment having the back coat layer 5 also has excellent transparency.

[0191] Therefore, when the thermosensitive recording material 1 of the present embodiment is attached to a container as, for example, a packaging film, the contents of the container can be visually confirmed through the film.

[0192] As another embodiment of the present invention, in order to improve Figure 1 In order to improve the adhesion between the substrate 2 and the back coating layer 5, an anchor layer may be provided.

[0193] As a summary of the above, the configuration of the present invention and its modifications are described below.

[0194] [Supplementary Note 1] A thermosensitive recording medium comprising a substrate, a thermosensitive recording layer, and a protective layer laminated in this order on one surface of a substrate, and a back coating layer formed on the other surface of the substrate, wherein the substrate comprises a transparent resin film, and the back coating layer comprises a core-shell resin and a polyamide epichlorohydrin resin.

[0195] [Supplementary Note 2] The thermosensitive recording material according to Supplementary Note 1, wherein the protective layer includes a top coat layer and an intermediate layer formed between the top coat layer and the thermosensitive recording layer.

[0196] [Supplementary Note 3] The thermosensitive recording material according to Supplementary Note 2, wherein the intermediate layer contains a core-shell resin and a crosslinking agent.

[0197] [Supplementary Note 4] The thermosensitive recording material according to Supplementary Note 3, wherein the crosslinking agent is a polyamide epichlorohydrin resin.

[0198] [Supplementary Note 5] The thermosensitive recording material according to any one of Supplementary Notes 1 to 4, wherein the content of the core-shell resin in the back coat layer is 50% by mass or more relative to 100% by mass of the dry mass of the back coat layer.

[0199] [Supplementary Note 6] The thermosensitive recording material according to any one of Supplementary Notes 1 to 5, wherein the content of the polyamide epichlorohydrin resin in the back coat layer is 3% by mass or more relative to 100% by mass of the dry mass of the back coat layer.

[0200] [Supplementary Note 7] The thermosensitive recording material according to any one of Supplementary Notes 3 to 6, wherein the content of the core-shell resin in the intermediate layer is 50% by mass or more relative to 100% by mass of the dry mass of the intermediate layer.

[0201] [Supplementary Note 8] The thermosensitive recording material according to any one of Supplementary Notes 3 to 7, wherein the content of the crosslinking agent in the intermediate layer is 3% by mass or more relative to 100% by mass of the dry mass of the intermediate layer.

[0202] [Supplementary Note 9] The thermosensitive recording material according to any one of Supplementary Notes 3 to 8, wherein the back coat layer and the intermediate layer contain the same core-shell resin.

[0203] [Supplementary Note 10] The thermosensitive recording material according to Supplementary Note 9, wherein the core-shell type resin of the same type is a core-shell type acrylic resin.

[0204] [Supplementary Note 11] The thermosensitive recording material according to any one of Supplementary Notes 1 to 10, wherein the coating liquid for forming the back coat layer has a dynamic surface tension of 51 mN / m or less at 50 msec.

[0205] [Supplementary Note 12] The thermosensitive recording material according to any one of Supplementary Notes 1 to 11, wherein the thickness of the substrate is 10 μm to 50 μm.

[0206] Description of Reference Numerals

[0207] 1 Thermosensitive recording medium, 2 substrate, 3 thermosensitive recording layer, 4 protective layer, 5 back coating layer, 6 top coating layer, 7 intermediate layer.

Claims

1. A thermosensitive recording medium comprising a substrate, a thermosensitive recording layer, and a protective layer laminated in this order on one side of a substrate, and a back coating layer formed on the other side of the substrate, The substrate comprises a transparent resin film, The back coating layer contains a core-shell resin and a polyamide epichlorohydrin resin.

2. The thermosensitive recording medium according to claim 1, wherein The protective layer includes a top coat layer and an intermediate layer formed between the top coat layer and the thermosensitive recording layer.

3. The thermosensitive recording medium according to claim 2, wherein The intermediate layer contains core-shell resin and polyamide epichlorohydrin resin.

4. The thermosensitive recording medium according to claim 3, wherein The dynamic surface tension of the coating liquid for forming the back coat layer at 50 msec is 51 mN / m or less.

5. The thermosensitive recording material according to any one of claims 1 to 4, wherein The thickness of the substrate is 10 μm or more and 50 μm or less.

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