Thermal recording medium

Abstract

An object of the present invention is to provide a thermosensitive recording medium which is suppressed from curling and has good transparency. A thermosensitive recording medium 1 has a substrate 2, a thermosensitive recording layer 3, and a protective layer 4 laminated in this order on one side of the substrate 2, and a backcoat layer 5 formed on the other side of the substrate 2, the substrate 2 being made of a transparent resin film, and the backcoat layer 5 containing a core-shell type resin and a polyamide epichlorohydrin resin. The protective layer 4 preferably includes a topcoat layer 6 and an intermediate layer 7 formed between the topcoat layer 6 and the thermosensitive recording layer 3.

Description

[Technical field]

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

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

[0003] When such a thermal recording medium is used, for example, as a label or packaging film for containers containing various foods, the contents of the container are hidden by the label or packaging film, making it difficult for consumers to see the contents.

[0004] Therefore, the applicant of the present invention has constructed a thermal recording medium so that the labels and packaging films are transparent. In other words, the applicant has proposed a thermal recording medium with excellent transparency so that the labels and packaging films can be made transparent so that the contents of the container can be confirmed (for example, see Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6202599 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, the problem of plastic waste has become more serious, and moving away from plastic has become important as an environmental measure, with a demand to reduce the amount of plastic used.

[0007] For this reason, it is desirable to reduce the amount of plastic used by making the base film thinner, even in the thermal recording medium proposed in the above Patent Document 1.

[0008] However, when the base film is made thin, the influence of stress due to the thermosensitive recording layer formed on the base film becomes stronger, which causes a problem that the thermosensitive recording medium curls toward the printing surface on which the thermosensitive recording layer is formed.

[0009] The present invention has been made in view of the above circumstances, and has an object to provide a thermosensitive recording medium which has good transparency and is suppressed from curling. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention is configured as follows.

[0011] (1) The thermal recording medium of the present invention is a thermal recording medium in which a substrate, a thermal recording layer, and a protective layer are laminated in this order on one side of the substrate, and a backcoat layer is formed on the other side of the substrate, the substrate being made of a transparent resin film, and the backcoat layer contains a core-shell type resin and a polyamide epichlorohydrin resin.

[0012] According to the thermal recording medium of the present invention, the backcoat layer contains a core-shell type resin, and therefore has better film-forming properties than general-purpose emulsions. In general, in the case of general-purpose emulsions, the emulsion is a particle, and after drying, the particles fuse together to form a film. On the other hand, in the case of core-shell type resins, the shell part having a water-soluble portion is partially dissolved before drying and is not a particle, so that it is considered that the film-forming properties are improved. Therefore, even if the thermal recording layer and the protective layer are laminated, they can withstand these stresses, and as a result, curling can be suppressed. In addition, the backcoat layer contains a polyamide epichlorohydrin resin as a crosslinking agent for crosslinking the core-shell type resin, and therefore, as described later, the transparency is improved compared to oxazoline-based crosslinking agents.

[0013] (2) In a preferred embodiment of the present invention, the protective layer includes a topcoat layer and an intermediate layer formed between the topcoat layer and the thermosensitive recording layer.

[0014] According to this embodiment, by providing the top coat layer, the matching of the thermosensitive recording layer with respect to the thermal head is improved, and color development in the thermosensitive recording layer can be performed appropriately.

[0015] Furthermore, by providing an intermediate layer, it is possible to provide a barrier property against water and oil.

[0016] (3) In another embodiment of the present invention, the intermediate layer contains a core-shell resin and a polyamide-epichlorohydrin resin.

[0017] According to this embodiment, the intermediate layer contains a core-shell type resin, which is a resin having a water-soluble portion. Therefore, when the coating liquid for forming the intermediate layer is applied onto the thermosensitive recording layer and dried, the core-shell type resin having a water-soluble portion permeates into the thermosensitive recording layer to form a smooth intermediate layer. This suppresses diffuse reflection of light in the thermosensitive recording layer, improving the transparency of the thermosensitive recording medium.

[0018] In addition, a core-shell resin is used for the intermediate layer, which is a protective layer on one side of the substrate, and a backcoat layer on the other side of the substrate, and a polyamide epichlorohydrin resin, which is a crosslinking agent, is used. This offsets the stress on one side of the substrate and the stress on the other side of the substrate, thereby effectively suppressing curling.

[0019] (4) In still another embodiment of the present invention, the dynamic surface tension of the coating liquid for forming the backcoat layer at 50 msec is 51 mN / m or less.

[0020] According to this embodiment, the dynamic surface tension of the coating liquid for forming the backcoat layer at 50 msec is 51 mN / m or less, so that the coating suitability to the substrate is good. This results in a uniform surface quality of the coated surface without unevenness. As a result, there is no unevenness in transparency and stress is also obtained.

[0021] (5) In another embodiment of the present invention, the substrate has a thickness of 10 μm or more and 50 μm or less.

[0022] According to this embodiment, the transparent resin film as the base material has a thin thickness of 50 μm or less, so that the amount of resin material used can be reduced, leading to a plastic-free environment. Effect of the Invention

[0023] Thus, according to the present invention, the backcoat layer contains a core-shell type resin, and therefore has better film-forming properties than general-purpose emulsions. Therefore, even if a thermal recording layer and a protective layer are laminated, the backcoat layer can withstand the stresses of these layers, and as a result, curling can be suppressed. In addition, the backcoat layer contains a polyamide epichlorohydrin resin as a crosslinking agent for crosslinking the core-shell type resin, and therefore has improved transparency compared to oxazoline-based crosslinking agents, as described below. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic cross-sectional view of a thermosensitive recording medium according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0026] 1 is a schematic cross-sectional view of a thermosensitive recording medium according to one embodiment of the present invention. In the thermosensitive recording medium 1 of this embodiment, a thermosensitive recording layer 3 that develops color when heated is laminated on one surface, that is, the upper surface, of a substrate 2, and a protective layer 4 is laminated on the surface of the thermosensitive recording layer 3, while a backcoat layer 5 is formed on the other surface, that is, the lower surface, of the substrate 2. The protective layer 4 includes a topcoat layer 6 that constitutes a surface layer, and an intermediate layer 7 that is formed between the topcoat layer 6 and the thermosensitive recording layer 3.

[0027] The structure of each layer will be described below.

[0028] [Base material] As the substrate 2, a transparent synthetic resin film, such as a polypropylene film, a polyethylene terephthalate film, a polystyrene film, a polycarbonate film, or the like, can be used. The substrate 2 may be a single layer or a multi-layer. The thickness of such a film is not particularly limited, but for example, a thickness of about 10 μm to 100 μm is preferable since it provides excellent coatability to the substrate 2 and transparency of the substrate 2. In order to reduce the amount of resin material used and to eliminate the use of plastics, the thickness of the substrate 2 is preferably 50 μm or less, and more preferably 40 μm or less.

[0029] [Thermal recording layer] Materials for forming the thermosensitive recording layer 3 include a color former that changes color when heated, a color developer, a filler, a binder, a lubricant, and the like.

[0030] It is preferable to use materials with a fine particle size for each material in order to improve the transparency of the thermal recording medium 1. Such materials with a fine particle size can suppress diffuse reflection of light and improve the transparency of the thermal recording medium.

[0031] Specifically, the leuco dyes that are color formers include 3-(N-isobutyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-isopentyl-N-ethyl)amino-6-methyl-7-o-chloroanilinofluoran, 3-(N-ethyl-Np-toluidino)-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-Nn-propyl)amino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3 -diethylamino-6-methyl-7-p-toluidinofluoran, 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-pyrrolidino-6-methyl-7-anilinofluoran, crystal violet lactone, etc. can be used. These leuco dyes can be used alone or in combination of two or more.

[0032] The particle size of the color former is preferably 0.1 μm to 1.0 μm. Here, the particle size refers to the 50% average particle size measured by a microtrack laser analysis / scattering type particle size analyzer. Hereinafter, "particle size" refers to the 50% average particle size measured by this microtrack laser analysis / scattering type particle size analyzer.

[0033] Examples of the developer include 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'-dihydroxydiphenylsulfone, 4,4'-dihydroxydiphenylsulfone, 4-hydroxy-4'-n-propoxydiphenylsulfone, 4-hydroxy-4'-isopropoxydiphenylsulfone, 4-hydroxy-4'-methyldiphenylsulfone, 4-hydroxyfur ... phenyl-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(degree of polymerization n=1-7)(oxyethyleneoxyethyleneoxy-p-phenylenesulfonyl-p-phenylene)2,2-bis[(4-methyl-3-phenoxycarbonylaminophenyl)urea]diphenyl sulfone, 3,5-bis(α-methylbenzyl)salicylic acid, bis[4-(n-octyloxycarbonylamino)salicylic acid zinc], 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, 4-hydroxybenzenesulfonanilide, 2'-(3-phenylureido)benzenesulfonanilide, N-(2-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio]acetamide, N-(4-hydroxyphenyl)-2-[(4-hydroxyphenyl)thio]acetamide, 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. can be used, and their particle size is preferably 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.

[0034] Examples of the filler 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, zinc oxide, silicon oxide, colloidal silica, polystyrene resin particles, urea-formalin resin particles, polyolefin resin particles, etc., and the particle size thereof is preferably 1.0 μm or less. These fillers can be used alone or in combination of two or more.

[0035] 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, polyacrylic acid ester, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride copolymer, diisobutylene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, methyl vinyl-maleic anhydride copolymer, isopropylene-maleic anhydride copolymer, styrene-butadiene copolymer, polyvinyl chloride, polyvinylidene chloride, vinyl chloride-vinyl acetate copolymer, polyurethane, polystyrene, polyvinylpyrrolidone, acrylic acid ester, acrylonitrile, methyl vinyl ether, etc. These binders can be used alone or in combination of two or more.

[0036] Examples of the sensitizer that can be used include stearic acid, stearic acid amide, stearic acid anilide, methylol 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-dimethylphenol)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, diphenylsulfone, benzyl p-benzyloxybenzoate, dibenzyl terephthalate, and p-toluenesulfonamide. These sensitizers can be used alone or in combination of two or more.

[0037] As the lubricant, for example, 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, oils such as whale oil, etc., and the particle size thereof is preferably 0.5 μm or less. These lubricants can be used alone or in combination of two or more kinds.

[0038] It is known that it is particularly effective to incorporate paraffin wax into the thermosensitive recording layer 3 in order to improve the transparency of the thermosensitive recording medium 1. This paraffin has a low melting point below the color-developing temperature of the thermosensitive recording layer 3, preferably below 80°C, and more preferably below 50°C.

[0039] The particle size of the low melting point paraffin is preferably 0.5 μm or less as described above. The content of the paraffin is, for example, 0.1 g / m 2 ~1.0g / m 2 It is preferable that:

[0040] By incorporating low-melting point paraffin in this manner, when the coating liquid for forming the thermosensitive recording layer 3 is applied onto the substrate 2 and the coating liquid dries, the low-melting point paraffin melts and penetrates into and fills gaps in the unevenness of the surfaces of the particles that make up the thermosensitive recording layer 3. This suppresses diffuse reflection of light in the thermosensitive recording layer 3, and the transparency of the thermosensitive recording medium 1 can be improved.

[0041] [Protective layer] The protective layer 4 in this embodiment comprises a topcoat layer 6 and an intermediate layer 7 .

[0042] The intermediate layer 7 has a barrier property against water and oil, and improves the water resistance and chemical resistance of the thermosensitive recording layer 3. The topcoat layer 6 improves the matching of the thermosensitive recording layer 3 with a thermal head, allowing the thermosensitive recording layer 3 to appropriately develop color.

[0043] The intermediate layer 7 is mainly formed of a resin. The above-mentioned resin may be, for example, a resin having a water-soluble portion, or a resin not having a water-soluble portion. Specifically, for example, acrylic resins such as acrylic resin, styrene-acrylic resin, acrylic-urethane resin, acrylic-amide resin, vinyl acetate-acrylic resin, etc.; epichlorohydrin resins such as polyamide epichlorohydrin resin, polyamine epichlorohydrin resin, polyamide polyamine epichlorohydrin resin, etc.; maleic acid resins such as maleic acid resin, styrene-maleic acid resin, olefin-maleic acid resin, etc.; styrene-butadiene (SBR) resin; acrylonitrile-butadiene-styrene resin; vinyl acetate resin; polyvinyl alcohol resins such as fully saponified polyvinyl alcohol resin, partially saponified polyvinyl alcohol resin, diacetone-modified polyvinyl alcohol resin, acetoacetyl-modified polyvinyl alcohol resin, sulfonic acid-modified polyvinyl alcohol resin, olefin-modified polyvinyl alcohol resin, nitrile-modified polyvinyl alcohol resin, pyrrolidone-modified polyvinyl alcohol resin, silanol-modified polyvinyl alcohol resin, cation-modified polyvinyl alcohol resin, etc. may be mentioned. In addition, these resins may be modified resins modified by known methods. These resins can be used alone or in combination of two or more. The composition forming these resins may be a solid, an emulsion, or a solution, although from the viewpoint of excellent coatability, an emulsion or a solution is preferred.

[0044] In this specification, the term "acrylic resin" refers to a resin (acrylic resin) obtained by polymerizing an acrylic monomer, or a resin obtained by copolymerizing an acrylic monomer with another monomer (a monomer other than the acrylic monomer that is copolymerizable with the acrylic monomer). Here, the other monomer may be two or more kinds. In addition, when simply described as "acrylic", it means (meth)acrylic acid (salt) and / or (meth)acrylic acid ester unless otherwise specified. Here, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid, and "(meth)acrylic acid (salt)" means (meth)acrylic acid and / or (meth)acrylic acid salt.

[0045] The salt in the (meth)acrylic acid salt is not particularly limited, and examples thereof include ammonium salts such as ammonia, alkanolamine salts such as triethanolamine, diethanolamine, monoethanolamine, alkylamine salts such as methylamine salt, ethylamine salt, diethylamine salt, triethylamine salt, polyamine salts such as diethyleneamine salt, diethylenetriamine salt, alkali metal salts such as lithium, sodium, potassium, alkaline earth metal salts such as magnesium, calcium, polyvalent metal salts such as zinc, iron, etc. These salts can be used alone or in combination of two or more.

[0046] In order to improve the transparency of the thermal recording medium 1, the resin of the intermediate layer 7 may be a resin having a water-soluble portion, for example, polyvinyl alcohol (PVA) which is a resin having a hydroxyl group as a hydrophilic structural unit, or a resin having a carboxyl group as a hydrophilic structural unit (carboxyl group-containing resin). In this specification, the term "carboxyl group-containing resin" refers to a resin containing a carboxyl group in the structure of a polymer or copolymer forming the resin. The carboxyl group in the carboxyl group-containing resin may be a free carboxyl group or an acid anhydride group (specifically, a dicarboxylic acid anhydride group). Furthermore, the acid anhydride group may be partially ring-opened to become a carboxyl group. In the carboxyl group-containing resin, a part or all of the carboxyl group may be neutralized with an alkali. In order to efficiently form a crosslinked structure with a crosslinking agent described later and further improve the film-forming property, a resin having a reactive structural unit is preferable. In the present invention, the "reactive structural unit" refers to a structural unit having a reactivity capable of forming a crosslinked structure with other materials, and representative examples include a carboxyl group, an azetidinium ring, and an oxazoline group.

[0047] The intermediate layer 7 of this embodiment preferably contains at least one resin selected from the group consisting of a carboxyl group-containing resin and an epichlorohydrin-based resin, and more preferably contains a carboxyl group-containing resin and an epichlorohydrin-based resin from the viewpoints of forming a crosslinked structure described below, further improving film-forming properties, and improving transparency and curl suppression.

[0048] The carboxyl group-containing resin is preferably at least one selected from the group consisting of acrylic resins and maleic acid resins, more preferably acrylic resins. The acrylic resin is preferably at least one selected from the group consisting of acrylic resins, styrene-acrylic resins, acrylic-urethane resins, acrylic-amide resins, and vinyl acetate-acrylic resins, more preferably at least one selected from the group consisting of acrylic resins, styrene-acrylic resins, and acrylic-urethane resins, and even more preferably at least one selected from the group consisting of acrylic resins and styrene-acrylic resins.

[0049] The epichlorohydrin resin is preferably at least one selected from the group consisting of polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, and polyamide polyamine epichlorohydrin resins, more preferably at least one selected from the group consisting of polyamide epichlorohydrin resins and polyamide polyamine epichlorohydrin resins, and even more preferably a polyamide epichlorohydrin resin.

[0050] Alternatively, from the viewpoint of further improving the transparency and curl suppression of the thermal recording medium 1, the resin of the intermediate layer 7 preferably contains a resin having a core-shell structure in which a hydrophobic core particle is coated with a water-soluble shell polymer, that is, a core-shell type resin. The core-shell type resin is not particularly limited, and for example, a carboxyl group-containing resin having a core-shell structure (core-shell type carboxyl group-containing resin) is preferable. Here, the core-shell type carboxyl group-containing resin is considered to contain a carboxyl group at least in the structure of the water-soluble shell polymer. In general, the core-shell type resin is a resin obtained by forming a hydrophobic core particle and a water-soluble shell polymer by a multi-stage polymerization reaction, and the resins forming the core particle and the shell polymer are the same as those described above. For this reason, the resin in the core-shell type resin may be all of the above descriptions regarding the resin.

[0051] Specifically, the above-mentioned core-shell type carboxyl group-containing resin is preferably at least one selected from the group consisting of a core-shell type acrylic resin and a core-shell type maleic acid resin, and more preferably a core-shell type acrylic resin. The above-mentioned core-shell type acrylic resin is preferably at least one selected from the group consisting of a core-shell type acrylic resin, a core-shell type styrene-acrylic resin, a core-shell type acrylic-urethane resin, a core-shell type acrylic-amide resin, and a core-shell type vinyl acetate-acrylic resin, and more preferably at least one selected from the group consisting of a core-shell type acrylic resin, a core-shell type styrene-acrylic resin, and a core-shell type acrylic-urethane resin, and more preferably at least one selected from the group consisting of a core-shell type acrylic resin and a core-shell type styrene-acrylic resin, and particularly preferably a core-shell type acrylic resin. In addition, examples of the core-shell type acrylic resin include resins commercially available under the name Barrier Star (manufactured by Mitsui Chemicals). In addition, the above-mentioned epichlorohydrin resin is a resin that does not have a core-shell structure, that is, a non-core-shell type resin.

[0052] Water-soluble polyvinyl alcohol and core-shell type acrylic resin have good film-forming properties. When the coating liquid for forming the intermediate layer is applied onto the thermosensitive recording layer 3 and dried, the resin having a water-soluble portion permeates into the thermosensitive recording layer 3 to form a smooth intermediate layer 7. This suppresses diffuse reflection of light in the thermosensitive recording layer 3, improving the transparency of the thermosensitive recording layer 3.

[0053] In addition, even if the resin has a water-soluble portion, if the film-forming property is poor, curling cannot be sufficiently suppressed. In this respect, the above-mentioned core-shell type resin has a structure in which a hydrophobic core particle is coated with a water-soluble shell polymer, and has better film-forming property than a general-purpose emulsion. In general, in the case of a general-purpose emulsion, the emulsion is a particle, and after drying, the particles are fused together to form a film. On the other hand, in the case of a core-shell type resin, the shell part having a water-soluble portion is partially dissolved before drying and is not a particle, so it is considered that the film-forming property is further improved. Therefore, even if a backcoat layer is laminated, it can withstand the stress, and as a result, it is effective in suppressing curling.

[0054] The intermediate layer 7 of this embodiment preferably contains a crosslinking agent. Examples of the crosslinking agent include organic crosslinking agents such as cationic crosslinking agents and non-cationic crosslinking agents; inorganic crosslinking agents such as zirconium carbonate, and the like. Examples of the cationic crosslinking agent include epichlorohydrin resins such as polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, and polyamide polyamine epichlorohydrin resins. Examples of the non-cationic crosslinking agent include oxazoline compounds such as oxazoline group-containing polymers. Examples of the crosslinking agent include cationic crosslinking agents, and more preferably epichlorohydrin resins. Here, the preferred types of epichlorohydrin resins are the same as those listed above. These crosslinking agents can be used alone or in combination of two or more.

[0055] Generally, the crosslinking agent has a reactive structural unit as described above. The reactive structural unit in the crosslinking agent reacts with the reactive structural unit in other materials to form a crosslinked structure, which can improve the film-forming property, transparency, and curl suppression. For example, when the crosslinking agent is an epichlorohydrin resin, the azetidinium ring (AZR) in the epichlorohydrin resin reacts as a reactive structural unit with other materials (for example, the carboxy group in the carboxy group-containing resin) to form a crosslinked structure. Thus, from the viewpoint of further improving the film-forming property, the intermediate layer 7 of this embodiment preferably contains a carboxy group-containing resin and an epichlorohydrin resin. It is also preferable to contain a core-shell type resin and an epichlorohydrin type resin. Furthermore, it is more preferable to contain a core-shell type carboxy group-containing resin and an epichlorohydrin type resin. Here, preferred types of the carboxy group-containing resin, the core-shell type carboxy group-containing resin, and the epichlorohydrin-based resin are the same as those described for the carboxy group-containing resin, the core-shell type carboxy group-containing resin, and the epichlorohydrin-based resin.

[0056] The content of the above resin is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, still more 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 intermediate layer 7. When it is within the above range, the film-formability is improved, and the transparency and curl suppression can be further improved.

[0057] The content of the core-shell resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on 100% by mass of the dry mass of the resin in the intermediate layer 7. The content of the core-shell resin is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less, based on 100% by mass of the dry mass of the resin in the intermediate layer 7. If it is within the above range, the film formability can be improved, and the transparency and curl suppression can be further improved. The content of the core-shell resin is preferably within the above range, based on 100% by mass of the dry mass of the intermediate layer 7.

[0058] The content of the crosslinking agent is preferably 3% by mass or more, more preferably 6% by mass or more, and even more preferably 10% by mass or more, based on 100% by mass of the dry mass of the resin in the intermediate layer 7. 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, based on 100% by mass of the dry mass of the resin in the intermediate layer 7. If it is within the above range, the film formability can be improved, and the transparency and curl suppression can be further improved. The content of the crosslinking agent is preferably within the above range, based on 100% by mass of the dry mass of the intermediate layer 7.

[0059] The intermediate layer 7 may further contain materials other than those mentioned above. Examples of the other materials include a wetting agent.

[0060] The coating amount (dry mass) of the intermediate layer 7 is, for example, 0.3 g / m 2 ~10g / m 2 is preferable, and more preferably 0.5 g / m 2 ~5.0g / m 2 , and more preferably 1.0 g / m 2 ~4.0g / m 2 It is.

[0061] In this embodiment, the intermediate layer 7 uses a core-shell type acrylic resin as in the backcoat layer 5, and also uses a polyamide epichlorohydrin resin as a crosslinking agent. This will be described in detail later with reference to examples.

[0062] The topcoat layer 6 is made of a binder to which a filler, a lubricant, a crosslinking agent, etc. are added.

[0063] Examples of the resin that is the binder include acrylic resin. Other resins include the resins listed in the section on the intermediate layer 7. These resins can be used alone or in combination of two or more. Examples of the lubricant include polyethylene and zinc stearate. Other lubricants include the lubricants listed in the section on the thermal recording layer 3. These lubricants can be used alone or in combination of two or more.

[0064] The crosslinking agent may be, for example, zirconium carbonate. Other crosslinking agents may also be used as those listed in the section on the intermediate layer 7. These crosslinking agents may be used alone or in combination of two or more.

[0065] Examples of the filler include colloidal silica, calcium carbonate, polymethyl methacrylate (PMMA), polystyrene (PS), etc. As other fillers, the fillers listed in the section on the thermal recording layer 3 may also be used. These fillers may be used alone or in combination of two or more.

[0066] The particle size of these fillers is preferably 1.0 μm or less. In order to improve transparency, colloidal silica having a small particle size is preferred as the filler.

[0067] [Backcoat layer] The backcoat layer 5 formed on the non-printing surface side of the substrate 2 opposite to the thermosensitive recording layer 3 is mainly formed of a resin and contains at least a core-shell type resin and a polyamide-epichlorohydrin resin. Note that the polyamide-epichlorohydrin resin is a non-core-shell type resin.

[0068] As the resin contained in the backcoat layer 5 of this embodiment, all of the contents described in the section on the intermediate layer 7 can be used. In addition, the resin contained in the backcoat layer 5 may be a resin having a water-soluble portion or a resin not having a water-soluble portion. Among them, a resin having a water-soluble portion is preferable. Here, as mentioned above, an example of a resin having a water-soluble portion is polyvinyl alcohol. In this case, although the transparency can be improved, polyvinyl alcohol has poor water resistance, and therefore there is a risk that the backcoat layer will peel off in a humid environment.

[0069] In this embodiment, the backcoat layer 5 contains a core-shell type resin as a resin having a water-soluble portion, since curling can be suppressed by further improving the water resistance of the thermal recording medium 1. The core-shell type resin is not particularly limited, and for example, a core-shell type carboxyl group-containing resin is preferable. The preferred types of the core-shell type carboxyl group-containing resin are the same as those listed in the section on the intermediate layer 7. That is, the core-shell type carboxyl group-containing resin is preferably at least one selected from the group consisting of a core-shell type acrylic resin and a core-shell type maleic acid resin, and more preferably a core-shell type acrylic resin. The core-shell type acrylic resin is preferably at least one selected from the group consisting of a core-shell type acrylic resin, a core-shell type styrene-acrylic resin, a core-shell type acrylic-urethane resin, a core-shell type acrylic-amide resin, and a core-shell type vinyl acetate-acrylic resin, more preferably at least one selected from the group consisting of a core-shell type acrylic resin, a core-shell type styrene-acrylic resin, and a core-shell type acrylic-urethane resin, still more preferably at least one selected from the group consisting of a core-shell type acrylic resin and a core-shell type styrene-acrylic resin, and particularly preferably a core-shell type acrylic resin.

[0070] In addition, a polyamide epichlorohydrin resin is used as a crosslinking agent for crosslinking the core-shell type acrylic resin.

[0071] Even if the resin has a water-soluble portion, it cannot sufficiently suppress curling if the film-forming property is poor. In this respect, the core-shell type acrylic resin has hydrophobic core particles coated with a water-soluble shell polymer, and has better film-forming property than general-purpose emulsions. Therefore, even if a thermal recording layer and a protective layer are laminated, it can withstand the stress of these layers, and as a result, it is effective in suppressing curling. In addition, since it has a hydrophobic core, water resistance is not deteriorated.

[0072] Furthermore, by using a polyamide-epichlorohydrin resin as a crosslinking agent for crosslinking the core-shell type acrylic resin, transparency is improved compared to that of an oxazoline-based crosslinking agent, as shown in the transparency comparison test described later.

[0073] The content of the above resin is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, still more 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 backcoat layer 5. When it is within the above range, the film-forming property is improved, and the transparency and curl suppression can be further improved.

[0074] The content of the core-shell type resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on 100% by mass of the dry mass of the resin in the backcoat layer 5. The content of the core-shell type resin is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less, based on 100% by mass of the dry mass of the resin in the backcoat layer 5. When it is within the above range, the film-forming property is improved, and the transparency and curl suppression can be further improved. The content of the core-shell type resin is preferably within the above range, based on 100% by mass of the dry mass of the backcoat layer 5.

[0075] The content of the crosslinking agent is preferably 3% by mass or more, more preferably 6% by mass or more, and even more preferably 10% by mass or more, based on 100% by mass of the dry mass of the resin in the backcoat layer 5. 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, based on 100% by mass of the dry mass of the resin in the backcoat layer 5. When it is within the above range, the film-forming property is improved, and the transparency and curl suppression can be further improved. The content of the crosslinking agent is preferably within the above range, based on 100% by mass of the dry mass of the backcoat layer 5.

[0076] The backcoat layer 5 may further contain other materials in addition to those mentioned above. Examples of the other materials include a wetting agent, a filler, a lubricant, and a preservative.

[0077] Examples of usable wetting agents include nonionic surfactants, anionic surfactants, cationic surfactants, amphoteric surfactants, fluorine-based surfactants, etc. These surfactants can be used alone or in combination of two or more.

[0078] Examples of the nonionic surfactant that can be used include acetylene glycol surfactants, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan fatty acid esters.

[0079] The acetylene glycol surfactant may, for example, be acetylene glycol or an alkylene oxide adduct of acetylene glycol. The acetylene glycol may, for example, be 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, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, etc.

[0080] Examples of the anionic surfactant that can be used include polyoxyethylene alkyl ether sulfate or a salt thereof, polyoxyethylene alkyl ether acetate or a salt thereof, dodecylbenzenesulfonic acid or a salt thereof, alkyl sulfate or a salt thereof, and alkanesulfonic acid or a salt thereof.

[0081] Of the wetting agents, nonionic surfactants are preferred, acetylene glycol surfactants are more preferred, and alkylene oxide adducts of acetylene glycol are even more preferred.

[0082] In general, the wettability of the wetting agent can be adjusted by its HLB value. From the viewpoint of further improving the coatability of the backcoat layer 5, the HLB value of the wetting agent is preferably 4 or more, more preferably 6 or more, and even more preferably 6 or more and 16 or less.

[0083] The coating amount (dry mass) of the backcoat layer 5 is preferably, for example, 0.3 g / m 2 ~10g / m 2 and more preferably 0.5 g / m 2 ~5.0g / m 2 and more preferably 1.0 g / m 2 ~4.0g / m 2 It is.

[0084] In this embodiment, from the viewpoint of suppressing curling of the thermal recording medium, it is preferable that the stress that may occur on one side and the stress that may occur on the opposite side from the above side are more equal when observed from the substrate, which is the support. For this reason, it is preferable that both the backcoat layer 5 and the intermediate layer 7 contain a core-shell type resin, more preferably the same type of core-shell type resin, and even more preferably the same core-shell type resin. It is also preferable that both the backcoat layer 5 and the intermediate layer 7 contain a polyamide epichlorohydrin resin. It is also preferable that both the backcoat layer 5 and the intermediate layer 7 contain the same type of core-shell type resin and a polyamide epichlorohydrin resin, and more preferably the same core-shell type resin and a polyamide epichlorohydrin resin. It is also preferable that the backcoat layer 5 and the intermediate layer 7 contain a core-shell type acrylic resin and a polyamide epichlorohydrin resin, and more preferably the core-shell type acrylic resin and a polyamide epichlorohydrin resin. Here, the term "same kind" means that they have a common reactive structural unit. The same kind of core-shell resin means that they have a common reactive structural unit such as a carboxyl group derived from an acrylic monomer.

[0085] The mass ratio of the core-shell resin in the intermediate layer 7 to the core-shell resin in the backcoat layer 5 (content per unit area of ​​the core-shell resin in the intermediate layer 7 (dry mass) / content per unit area of ​​the core-shell resin in the backcoat layer 5 (dry mass)) 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. When within the above range, the stress that can be generated on one side and the stress that can be generated on the opposite side when observed from the substrate become more equal, and curl suppression can be further improved.

[0086] The mass ratio of the polyamide epichlorohydrin resin in the intermediate layer 7 to the polyamide epichlorohydrin resin in the backcoat layer 5 (content per unit area of ​​polyamide epichlorohydrin resin in intermediate layer 7 (dry mass) / content per unit area of ​​polyamide epichlorohydrin resin in backcoat layer 5 (dry mass)) 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. When within the above range, the stress that can be generated on one side and the stress that can be generated on the side opposite to the above side when observed from the substrate become more equal, and curl suppression can be further improved.

[0087] The ratio of the coating weight (dry mass) of the intermediate layer 7 to the coating weight (dry mass) of the backcoat layer 5 (coating weight (dry mass) per unit area of ​​intermediate layer 7 / coating weight (dry mass) per unit area of ​​backcoat layer 5) is preferably 0.3 to 3, more preferably 0.5 to 2, and even more preferably 0.7 to 1.5. When within the above range, the stress that can be generated on one side and the stress that can be generated on the opposite side when observed from the substrate become more equal, and curl suppression can be further improved.

[0088] When a curl test is carried out on the thermosensitive recording medium 1 (a laminate having a backcoat layer 5 and a topcoat layer 6 on both end surfaces) in this embodiment by the method described in the Examples, it is preferable that the curl height is 6 mm or less under at least one of the three conditions specified in (B), and that the curl is 6 mm or less under at least one of the following conditions in (C). It is more preferable that the curl is 6 mm or less under at least two of the three conditions specified in (B), and that the curl is 6 mm or less under at least two of the following conditions in (C). It is even more preferable that the curl is 6 mm or less under all of the three conditions specified in (B), and that the curl is 6 mm or less under all of the following conditions in (C).

[0089] With regard to the backcoat layer 5 and substrate 2 provided in the thermal recording medium 1 in this embodiment, the haze value of the laminate of the backcoat layer 5 and the 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.

[0090] With respect to the backcoat layer 5 and substrate 2 provided in the thermal recording medium 1 in this embodiment, when a waterproof ink adhesion test is carried out on a laminate of the backcoat layer 5 and the substrate 2 by the method described in the Examples, it is preferable that 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. EXAMPLES

[0091] Next, the present invention will be described in more detail based on specific examples, but the following examples are not intended to limit the present invention in any way.

[0092] The present inventors prepared coating solutions for forming a backcoat layer by blending the base agent, crosslinking agent, and wetting agent shown in Table 1 below as shown in Blends No. 1 to No. 5. All blends were prepared so that the solid content concentration was 18.5%.

[0093] [Table 1]

[0094] In Blend No. 1, 100 parts of the main component, core-shell acrylic resin, were blended with 18 parts of the cross-linking agent, polyamide epichlorohydrin resin, and 1.3 parts of the wetting agent, an acetylene glycol-based surfactant with an HLB value of 13 to 14, by weight when dry.

[0095] Here, HLB (Hydrophile-Lipophile-Balance) is a value that indicates the degree of affinity of a surfactant for water and oil. If the HLB value is greater than 7, it is highly hydrophilic, and if it is less than 7, it is highly hydrophobic (lipophilic).

[0096] In Blend No. 2, the main ingredient, core-shell acrylic resin, was blended with 100 parts by weight when dry, of an oxazoline-based crosslinking agent, specifically, Oxazoline WS300 (manufactured by Nippon Shokubai Co., Ltd.), and 1.3 parts by weight of an acetylene glycol-based surfactant with an HLB value of 13-14 as a wetting agent.

[0097] In Blend No. 3, only the main component, the core-shell type acrylic resin 100, was used, and no crosslinking agent or wetting agent was added.

[0098] In Blend No. 4, 100 parts of the main component, core-shell acrylic resin, 18 parts of the cross-linking agent, polyamide epichlorohydrin resin, and 0.16 parts of the wetting agent, acetylene glycol-based surfactant with an HLB value of 8, were blended together by weight when dry.

[0099] In Blend No. 5, the base resin, core-shell acrylic resin, was mixed with 18 parts polyamide epichlorohydrin resin, which was the cross-linking agent, and 0.32 parts acetylene glycol surfactant with an HLB value of 8, which was the wetting agent, by weight when dry.

[0100] The present inventors prepared samples by appropriately selecting the coating liquids for forming the backcoat layer from the above Blends No. 1 to No. 5, and conducted tests on curl, transparency, ink adhesion, and dynamic surface tension.

[0101] Each test will be explained below. <Curl test> The present inventors prepared a thermosensitive recording medium of the embodiment shown in FIG. 1 and a thermosensitive recording medium of a comparative example not having a backcoat layer 5, and carried out a comparative test regarding curling of both thermosensitive recording mediums.

[0102] In the example, the backcoat layer 5 was formed by using a coating film for forming the backcoat layer having the blend No. 1 in Table 1 above.

[0103] The examples and comparative examples have the same configuration, except for the presence or absence of the backcoat layer 5. That is, the configurations of the substrate 2, the thermosensitive recording layer 3, the intermediate layer 7, and the topcoat layer 6 are common between the examples and comparative examples.

[0104] The structures of the substrate 2, the thermosensitive recording layer 3, the intermediate layer 7 and the topcoat layer 6 will be described in detail below, but this structure is the same as that of the above-mentioned Patent Document 1 (Patent Publication No. 6202599) previously proposed by the present applicant.

[0105] Therefore, the thermosensitive recording medium except for the backcoat layer 5 has excellent transparency similar to that of the thermosensitive recording medium described in Patent Document 1.

[0106] First, as the base material 2, a PET (polyethylene terephthalate) film having a thickness of 25 μm was used.

[0107] The thermosensitive recording layer 3 used 3,3'-diallyl-4,4'-dihydroxydiphenylsulfone with a particle size of 0.4 μm as the developer, and kaolin with a particle size of 0.4 μm as the filler. The binder used SBR with a glass transition temperature Tg of -3°C. The lubricant used paraffin with a melting point of 46°C and a particle size of 0.2 μm. The dye used was 2-aniline-3 methyl-6-(N-methyl-p-toluidino)fluoran with a particle size of 0.5 μm.

[0108] The weight ratios of the developer, kaolin, SBR, paraffin, and dye when dried were 25, 10, 20, 4, and 12, respectively, for a total of 74.

[0109] A coating solution for forming a thermal recording layer having this composition was prepared, and applied onto the PET film in a coating amount of 4.5 g / m2 in terms of dry weight. 2 After coating, the mixture was dried to obtain a thermosensitive recording layer 3.

[0110] The intermediate layer 7 used a core-shell type acrylic resin as a binder, a polyamide epichlorohydrin resin as a cross-linking agent, and an acetylene glycol surfactant with an HLB value of 13 to 14 as a wetting agent.

[0111] The weight ratios when dried were 100 for core-shell acrylic resin, 18 for polyamide epichlorohydrin resin, and 1.3 for acetylene glycol surfactant with an HLB value of 13 to 14, for a total of 119.3.

[0112] A coating solution for forming an intermediate layer having this composition was prepared, and applied onto the thermal recording layer 3 in a coating amount of 2.0 g / m2 in terms of dry weight. 2 After coating so as to obtain the above-mentioned composition, the composition was dried to form an intermediate layer 7.

[0113] The topcoat layer 6 used 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, acrylic resin as binder, and zirconium carbonate as cross-linking agent, and colloidal silica with a particle size of several nm and colloidal silica with a particle size of several tens of nm as fillers.

[0114] The weight ratios when dry were 10, 5, 50, 5, 15, and 30 of polyethylene (PE), zinc stearate (St-Zn), acrylic resin, zirconium carbonate, colloidal silica with a particle size of several nm, and colloidal silica with a particle size of several tens of nm, for a total of 130.

[0115] A coating solution for forming a topcoat layer having this composition was prepared, and applied to the intermediate layer 7 in an amount of 1.5 g / m2 in terms of dry weight. 2 The coating was then dried so as to form a top coat layer 6.

[0116] In this way, a comparative thermosensitive recording medium was obtained, that is, a thermosensitive recording medium having excellent transparency as described in Patent Document 1. This thermosensitive recording medium was cut into a size of 70 mm in the machine direction × 240 to 320 mm in the width direction to prepare a comparative sample.

[0117] In the same manner as above, a heat-sensitive recording layer 3, an intermediate layer 7, and a top coat layer 6 were formed on the substrate 2, and further, the coating liquid for forming the back coat layer of the above-mentioned Blend No. 1 was applied to the lower surface side of the substrate 2 in an amount of 2.0 g / m2 in terms of dry weight. 2 The coating was then dried so as to form a backcoat layer 5.

[0118] In this way, a thermosensitive recording medium was obtained as an example. This thermosensitive recording medium was cut into a size of 70 mm in the machine direction x 240 to 320 mm in the width direction to prepare a sample of the example.

[0119] A curl test was carried out using the samples of the embodiment and the comparative example prepared as described above. The conditions and results of this curl test are shown in Table 2 below.

[0120] [Table 2]

[0121] The curl test was carried out as follows.

[0122] (A) Before the start of the test, each sample of the Examples and Comparative Examples to be used in the test was left to stand for 24 hours under conditions of a temperature of 23° C. and a relative humidity of 50%.

[0123] (B) Then, each sample of the Examples and Comparative Examples was left for 24 hours under the conditions shown in Table 2, i.e., a temperature of 30 degrees and low, medium, and high humidity (specifically, relative humidity of 35%, 67%, and 98%), and the curl state of each sample was evaluated.

[0124] (C) Each sample of the Examples and Comparative Examples for which the curl state was evaluated in (B) above was left for a further 2 hours under conditions of a temperature of 23°C and a relative humidity of 50%, as shown in Table 2, and the curl state was evaluated again.

[0125] The curl condition was evaluated as follows: a curl height, which is the maximum lift height of the sample end, of 3 mm or less was rated as ◯; a curl height of more than 3 mm but less than 6 mm was △; and a curl height of more than 6 mm was x.

[0126] In each of the samples of the examples in which the backcoat layer 5 was formed, the curl height after each sample was left for 24 hours under each of the above conditions (B) was rated as ◯, ie, 3 mm or less.

[0127] Moreover, in each of the samples of the examples in which the backcoat layer 5 was formed, the curl height after the sample was left for 2 hours under the above condition (C) was 3 mm or less, which was evaluated as ◯.

[0128] In contrast, in the comparative example sample not having a backcoat layer, the curl height after being left for 24 hours under the conditions of 30°C temperature and 35% relative humidity among the above (B) conditions was rated as x, meaning that the curl height was 7 mm or more.

[0129] Furthermore, under the above condition (C), the comparative sample was left for 2 hours, after which the curl height was 7 mm or more, rated as x.

[0130] Thus, in the comparative example samples not having a backcoat layer 5, the curl height could be 7 mm or more, whereas in the example samples having a backcoat layer 5, even when the substrate was thin, such as a 25 μm thick PET (polyethylene terephthalate) film, the curl height was rated as ◯ (0) (3 mm or less) and never exceeded 3 mm.

[0131] In the examples, as described above, the backcoat layer contains, as the main component, a core-shell type acrylic resin having a water-soluble portion in which hydrophobic core particles are coated with a water-soluble shell polymer, and therefore has good film-forming properties and can suppress curling.

[0132] In addition, since polyamide epichlorohydrin resin is used as a crosslinking agent, it has excellent barrier properties, water resistance, and film-forming properties, and can suppress curling due to high temperature and humidity or changes in humidity.

[0133] <Transparency comparison test> Among the coating solutions for forming the backcoat layer in Table 1, the coating solutions of formulations 1, 2, and 3 were applied to a PET (polyethylene terephthalate) film having a thickness of 25 μm as a substrate, with a dry weight of 2.5 g / m 2 The transparency of the substrate without the coating liquid was also evaluated.

[0134] The transparency was evaluated by measuring the haze in accordance with JIS K 7136.

[0135] The measurement results are shown in Table 3 below.

[0136] [Table 3]

[0137] Table 3 also shows the total light transmittance (TT), parallel light transmittance (PT), and diffuse (scattered) light component (DIF) used in the following formula for calculating haze.

[0138] HAZE(%)=DIF / TT×100 =(TT-PT) / TT×100 As shown in Table 3, the haze value of the sample coated with the backcoat layer forming coating liquid of formulation No. 1, i.e., the backcoat layer forming coating liquid containing the core-shell type acrylic resin as the main agent, the polyamide epichlorohydrin resin as the crosslinking agent, and the acetylene glycol-based surfactant as shown in Table 1 above, was 4.35 (%), which was the most transparent.

[0139] The haze value of the sample coated with the backcoat layer forming coating liquid of Formulation No. 3, i.e., the backcoat layer forming coating liquid containing only the core-shell type acrylic resin as the main component and no crosslinking agent or surfactant as shown in Table 1 above, was 4.92(%), which was the second most transparent.

[0140] In contrast, the haze value of the sample coated with the backcoat layer forming coating liquid of Formulation No. 2, i.e., the backcoat layer forming coating liquid containing a core-shell acrylic resin as the main component, an oxazoline-based crosslinking agent, and an acetylene glycol-based surfactant as shown in Table 1 above, was 8.28%, which was the highest and had the lowest transparency.

[0141] The haze value of the substrate alone, on which no coating liquid was applied, that is, the PET (polyethylene terephthalate) film alone, was 3.48(%).

[0142] As shown above, the haze value of the sample of Blend No. 1, which used a polyamide-epichlorohydrin resin as a crosslinking agent, was 4.35, compared to the haze value of 8.28 (%) of the sample of Blend No. 2, which used an oxazoline-based crosslinking agent, indicating that the sample has a lower haze value and better transparency.

[0143] Compared to the haze value of 4.92 for Blend No. 3, which contains only a core-shell acrylic resin without using a crosslinking agent, Blend No. 1, which uses a polyamide-epichlorohydrin resin as a crosslinking agent, has a haze value of 4.35, which is lower and has better transparency.

[0144] Thus, the sample coated with the coating liquid for forming the backcoat layer of Blend No. 1, which contains a core-shell type acrylic resin and a polyamide epichlorohydrin resin as a crosslinking agent, had the lowest haze value and the best transparency.

[0145] <Ink adhesion test> Among the coating solutions for forming the backcoat layer in Table 1, the coating solutions of Formulation Nos. 1, 2, and 3 were applied to a PET (polyethylene terephthalate) film as a substrate at a dry weight of 2.0 g / m 2 The coated surface was printed with gravure ink for surface printing to prepare a sample. The gravure ink used was the Raizin series manufactured by DIC Graphics Corporation.

[0146] Regarding the ink adhesion, an ink adhesion test and a waterproof ink adhesion test when immersed in water were carried out.

[0147] In the ink adhesion test, cellophane tape was applied to the printed surface of the sample, and the tape was peeled off at an angle of 180 degrees to determine whether or not the ink had come off. Those that did not come off were marked as "good."

[0148] In the waterproof ink adhesion test, the sample is immersed in water, taken out after 1 minute, and lightly wiped to remove the moisture. Cellophane tape is applied to the printed surface, and when it is peeled off at a 180° angle, the presence or absence of ink removal is judged. No ink removal is rated as ◯, less than 50% removal is △, and removal of 50% or more is ×.

[0149] The sample was also immersed in water for 2 minutes, removed, and lightly wiped dry. Cellophane tape was applied to the printed surface, and the tape was peeled off at an angle of 180 degrees to determine whether the ink had come off.

[0150] Similarly, a sample was immersed in water and taken out after 5 minutes, and a sample was immersed in water and taken out after 10 minutes, and the presence or absence of ink removal was evaluated.

[0151] That is, in the waterproof ink adhesion test, the samples were immersed in water for four different times: 1 minute, 2 minutes, 5 minutes, and 10 minutes.

[0152] The test results are shown in Table 4 below.

[0153] [Table 4]

[0154] In the ink adhesion test, all of the samples using the coating liquids for forming the backcoat layer of Formulation Nos. 1, 2, and 3 showed no ink peeling and were rated as good.

[0155] In the waterproof ink adhesion test, the sample coated with the backcoat layer forming coating liquid of Formulation No. 3, i.e., the backcoat layer forming coating liquid containing only the main component, the core-shell type acrylic resin, and neither a crosslinking agent nor a surfactant, shown in Table 1 above, was immersed in water for 1 minute, and the ink loss was rated as △, with less than 50% of the ink coming off. Also, the samples immersed in water for 2 minutes, 5 minutes, and 10 minutes were all immersed in water, and the ink loss was rated as ×, with more than 50% of the ink coming off.

[0156] In contrast, in the case of samples coated with the coating liquid for forming the backcoat layer of Blend No. 1, that is, the coating liquid for forming the backcoat layer containing the core-shell acrylic resin as the main component, the polyamide epichlorohydrin resin as the crosslinking agent, and the acetylene glycol surfactant, the samples immersed in water for 1 minute and the samples immersed in water for 2 minutes showed no ink loss and were rated as ○. The samples immersed in water for 5 minutes showed less than 50% ink loss and were rated as △. The samples immersed in water for 10 minutes showed more than 50% ink loss and were rated as ×.

[0157] In the coating film for forming the backcoat layer of Formulation No. 2, that is, the sample coated with the coating liquid for forming the backcoat layer, which is a core-shell type acrylic resin as the main component, an oxazoline-based crosslinking agent, and an acetylene glycol-based surfactant, similarly to the coating liquid for forming the backcoat layer of Formulation No. 1, the sample immersed in water for 1 minute and the sample immersed in water for 2 minutes did not lose any ink and were rated as ○. In addition, the sample immersed in water for 5 minutes was rated as △, with less than 50% of the ink falling off. The sample immersed in water for 10 minutes was rated as ×, with more than 50% of the ink falling off.

[0158] Thus, compared to the sample coated with the coating liquid for forming a backcoat layer of Formulation No. 3 containing only a core-shell type acrylic resin, the sample coated with the coating liquid for forming a backcoat layer of Formulation No. 1 using a crosslinking agent and the sample coated with the coating liquid for forming a backcoat layer of Formulation No. 2 had better waterproof ink adhesion.

[0159] As described above, the ink adhesion was equivalent between the sample coated with the coating liquid for forming the backcoat layer of formulation No. 1, which used a polyamide epichlorohydrin resin as a crosslinking agent, and the sample coated with the coating liquid for forming the backcoat layer of formulation No. 2, which used an oxazoline-based crosslinking agent.

[0160] <Dynamic surface tension measurement test> The dynamic surface tensions of the coating liquids for forming the backcoat layer in Table 1 above were measured for the coating liquids of formulations 1, 3, 4, and 5. The dynamic surface tensions were measured by the maximum bubble pressure method (bubble pressure method). Specifically, the measurements were performed using a bubble pressure type dynamic surface tensiometer BP-2 manufactured by KRUSS.

[0161] In addition, the coating solutions of the above blends Nos. 1, 3, 4, and 5 were applied to a PET (polyethylene terephthalate) film having a thickness of 25 μm as a substrate at a dry weight of 2.5 g / m 2 The coating was then dried at 50°C for 1 minute, and the quality of the coated surface was visually evaluated. Specifically, a coating surface without unevenness, that was homogeneous and good, was rated as ◯, and a coating surface with uneven thickness and thin areas was rated as △.

[0162] The measurement results of dynamic surface tension and the evaluation results of surface quality are shown in Table 5 below.

[0163] [Table 5]

[0164] The coating liquid for forming a backcoat layer of Blend No. 4 in Table 1 above, that is, the coating liquid for forming a backcoat layer, which is a blend of 100% of the core-shell acrylic resin as the main component, 18% of the polyamide epichlorohydrin resin as the crosslinking agent, and 0.16% of the acetylene glycol surfactant with an HLB value of 8 as the wetting agent, by weight ratio when dried, had a dynamic surface tension of 51 mN / m at 50 msec. The coating liquid for forming a backcoat layer of Blend No. 4 had good coating suitability, and the coated surface was uniform without unevenness, and was rated as good.

[0165] The coating liquid for forming a backcoat layer of Blend No. 5 in Table 1 above, i.e., a coating liquid for forming a backcoat layer containing 100 parts of the core-shell type acrylic resin as the main component, 18 parts of the polyamide epichlorohydrin resin as the crosslinking agent, and 0.32 parts of the acetylene glycol-based surfactant with an HLB value of 8 as the wetting agent, by weight ratio when dried, had a dynamic surface tension of 46 mN / m at 50 msec.

[0166] In addition, the coating liquid for forming a backcoat layer of Blend No. 1 in Table 1 above, i.e., a coating liquid for forming a backcoat layer blended with 100 parts of the core-shell type acrylic resin as the main component, 18 parts of the polyamide epichlorohydrin resin as the crosslinking agent, and 1.3 parts of the acetylene glycol-based surfactant with an HLB value of 13 to 14 as the wetting agent, by weight ratio when dried, had a dynamic surface tension of 35 mN / m at 50 msec.

[0167] The coating liquid for forming a backcoat layer of Formulation No. 5, which has a dynamic surface tension at 50 msec lower than the dynamic surface tension of 51 mN / m of the coating liquid for forming a backcoat layer of Formulation No. 4, and the coating liquid for forming a backcoat layer of Formulation No. 1 both had good coatability, and the coated surfaces were uniform without unevenness, and were rated as good (good).

[0168] In contrast, the coating liquid for forming a backcoat layer of Blend No. 3 in Table 1 above, that is, the coating liquid for forming a backcoat layer containing only the main component, core-shell type acrylic resin 100, without blending a crosslinking agent or a wetting agent, had a dynamic surface tension of 58 mN / m at 50 msec, which was higher than the dynamic surface tension of 51 mN / m of the coating liquid for forming a backcoat layer of Blend No. 4. This coating liquid for forming a backcoat layer of Blend No. 3 had poor coatability, and the coated surface was rated △, with uneven thickness and thin areas.

[0169] In order to obtain a coating surface with good coatability and uniform surface quality as described above, the dynamic surface tension of the coating liquid for forming the backcoat layer at 50 msec is preferably 51 mN / m or less.

[0170] As described above, according to the present embodiment, the backcoat layer contains a core-shell type acrylic resin and a polyamide epichlorohydrin resin, and therefore has good film-forming properties and can suppress curling even if the substrate is thin. Since the substrate, a transparent resin film, can be made thin, the amount of resin material used can be reduced, and plastic-free can be achieved.

[0171] In addition, since the film contains a polyamide epichlorohydrin resin as a crosslinking agent for crosslinking the core-shell type acrylic resin, the transparency is improved compared to when an oxazoline-based crosslinking agent is used.

[0172] The thermosensitive recording material consisting of the substrate 2, thermosensitive recording layer 3, top coat layer 6 and intermediate layer 7 other than the backcoat layer 5 in Figure 1 has excellent transparency, similar to the thermosensitive recording material described in Patent Document 1 above, and therefore the thermosensitive recording material 1 of this embodiment having the backcoat layer 5 formed thereon also has excellent transparency.

[0173] Therefore, when the thermal recording medium 1 of this embodiment is attached to a container as a packaging film or the like, the contents of the container can be visually confirmed through the film or the like.

[0174] In another embodiment of the present invention, an anchor layer may be provided to enhance adhesion between the substrate 2 and the backcoat layer 5 in FIG.

[0175] In summary, the configuration of the present invention and its variations are described below. [Appendix 1] A thermal recording medium in which a substrate, a thermal recording layer, and a protective layer are laminated in this order on one side of the substrate, and a backcoat layer is formed on the other side of the substrate, the substrate being made of a transparent resin film, and the backcoat layer contains a core-shell type resin and a polyamide epichlorohydrin resin. [Appendix 2] The thermosensitive recording material according to appendix 1, wherein the protective layer includes a topcoat layer and an intermediate layer formed between the topcoat layer and the thermosensitive recording layer. [Appendix 3] The thermal recording material according to Appendix 2, wherein the intermediate layer contains a core-shell type resin and a crosslinking agent. [Appendix 4] The thermosensitive recording material according to Appendix 3, wherein the crosslinking agent is a polyamide epichlorohydrin resin. [Appendix 5] The thermosensitive recording material according to any one of Appendices 1 to 4, wherein the content of the core-shell type resin in the backcoat layer is 50% by mass or more relative to 100% by mass of the dry mass of the backcoat layer. [Appendix 6] A thermosensitive recording material according to any one of Appendices 1 to 5, wherein the content of the polyamide epichlorohydrin resin in the backcoat layer is 3% by mass or more relative to 100% by mass of the dry mass of the backcoat layer. [Appendix 7] The thermosensitive recording medium according to any one of Appendices 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. [Appendix 8] The thermosensitive recording medium according to any one of Appendices 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. [Appendix 9] The thermosensitive recording material according to any one of Appendices 3 to 8, wherein the backcoat layer and the intermediate layer contain the same type of core-shell type resin. [Appendix 10] The thermosensitive recording material according to Appendix 9, wherein the same type of core-shell type resin is a core-shell type acrylic resin. [Appendix 11] The thermosensitive recording material according to any one of Appendices 1 to 10, wherein the coating liquid for forming the backcoat layer has a dynamic surface tension of 51 mN / m or less at 50 msec. [Appendix 12] The thermosensitive recording medium according to any one of Appendices 1 to 11, wherein the thickness of the substrate is 10 μm or more and 50 μm or less. [Explanation of symbols]

[0176] 1 Thermal recording medium 2 Base material 3. Thermal recording layer 4 protective layer 5 Backcoat layer 6 Topcoat Layer 7. Middle Tier

Claims

1. A thermal recording medium comprising a substrate, a thermal recording layer, and a protective layer laminated in this order on one surface of the substrate, and a backcoat layer formed on the other surface of the substrate, The substrate is made of a transparent resin film, the protective layer includes a topcoat layer and an intermediate layer formed between the topcoat layer and the thermal recording layer, The backcoat layer contains a core-shell resin and a polyamide-epichlorohydrin resin. Thermal recording medium.

2. The intermediate layer contains a core-shell resin and a polyamide epichlorohydrin resin.

2. The thermosensitive recording medium according to claim 1.

3. The dynamic surface tension of the coating liquid for forming the backcoat layer at 50 msec is 51 mN / m or less. The thermosensitive recording medium according to claim 2.

4. The thickness of the substrate is 10 μm or more and 50 μm or less.

4. The thermosensitive recording medium according to claim 1.