Thermal recording medium

Abstract

An object of the present invention is to provide a thermosensitive recording medium having excellent transparency. The thermal recording medium of the present invention is characterized in that it is a thermal recording medium having a thermal layer containing a fatty acid wax. The fatty acid wax is preferably a solubilized fatty acid wax and / or an emulsion of fatty acid wax. The fatty acid wax is preferably a solubilized fatty acid wax. The solubilized fatty acid wax preferably contains a solubilizer.

Description

[Technical field]

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

[0002] Thermal recording media develop color through a chemical reaction when heated by a thermal head or the like, producing a recorded image. They are used for a wide range of purposes, not only as recording media for facsimiles, automatic ticket vending machines, and scientific measuring instruments, but also as thermal recording labels and receipt paper for POS systems in retail stores, etc.

[0003] In particular, when using thermal recording media as labels for food containers such as bento boxes or as food packaging films, there is a problem that the contents cannot be seen when an opaque thermal recording media is used, so there is a demand for thermal recording media with excellent transparency so that the contents can be clearly seen.

[0004] As a thermosensitive recording medium with excellent transparency, for example, a thermosensitive recording medium has been proposed in which the thermosensitive recording layer contains a dispersant having a carboxyl group and a crosslinking agent mainly composed of an epichlorohydrin-modified polyamide polyamine resin or an organic compound containing an oxazoline group, and it has been disclosed that transparency can be further improved by using a material with a fine particle size (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2021 / 106076 Summary of the Invention [Problem to be solved by the invention]

[0006] However, research into thermosensitive recording media with excellent transparency has not yet been sufficient, and there is room for further research.

[0007] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a thermosensitive recording medium having excellent transparency. [Means for solving the problem]

[0008] As a result of intensive research into solving the above problems, the present inventors have found that by incorporating a fatty acid-based wax in the thermal layer, it is possible to provide a thermal recording medium with excellent transparency. The present invention was completed based on these findings.

[0009] That is, the present invention provides a thermosensitive recording medium having a thermal layer containing a fatty acid-based wax. The thermosensitive recording medium has excellent transparency because the thermal layer contains a fatty acid-based wax.

[0010] The fatty acid wax is preferably a solubilized fatty acid wax and / or an emulsion of fatty acid wax, which can further improve transparency.

[0011] The fatty acid wax is preferably a solubilized fatty acid wax, which can further improve transparency.

[0012] The solubilized fatty acid wax preferably contains a solubilizer, which can further improve transparency.

[0013] The thermal layer preferably contains a surfactant and / or a solvent, which can further improve transparency.

[0014] It is preferable that the thermal layer further contains a wax other than the fatty acid wax and having an average particle size of 1 μm or less. By having such a constitution, the transparency can be further improved.

[0015] The content of the fatty acid wax is preferably 0.1% by mass or more and 5% by mass or less with respect to 100% by mass of the dry mass of the thermal layer. By having such a configuration, the transparency can be further improved.

[0016] The present invention also provides a method for producing a coating material for a thermal layer, comprising the steps of: preparing a coating material for a thermal layer using a solubilized fatty acid wax and / or an emulsion of a fatty acid wax; and forming a thermal layer by applying and drying the thermal layer coating material. The thermal recording medium produced by this method has excellent transparency. Effect of the Invention

[0017] According to the present invention, a thermosensitive recording medium having excellent transparency can be provided. [Brief description of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view showing one embodiment of a thermosensitive recording medium of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] [Thermal recording medium] The thermal recording medium of the present invention is a thermal recording medium having a thermal layer containing a fatty acid-based wax.

[0020] The thermal recording medium of the present invention may include a substrate and an anchor layer. When the thermal recording medium of the present invention includes a substrate and an anchor layer, the thermal recording medium of the present invention preferably includes the substrate, the anchor layer, and the thermal layer in this order. The thermal recording medium of the present invention may also include layers other than the above layers. Examples of layers other than the above layers include a backcoat layer, an intermediate layer, and a topcoat layer.

[0021] Hereinafter, an embodiment of the thermal recording medium of the present invention will be described in detail with reference to the drawings, but the present invention is not limited to the following embodiments.

[0022] FIG. 1 is a schematic cross-sectional view showing one embodiment of a thermosensitive recording medium of the present invention.

[0023] As shown in FIG. 1, the thermal recording medium 1 of this embodiment includes a substrate 2, an anchor layer 3, and a thermal layer 4 in this order.

[0024] (base material) In this embodiment, the substrate 2 functions as a support for the thermal recording medium 1. As the substrate 2, for example, a transparent synthetic resin film can be used. Examples of the transparent synthetic resin film include a polypropylene film, a polyethylene terephthalate film, a polystyrene film, and a polycarbonate film. The transparent synthetic resin film may be a biaxially oriented film or a film having heat sealability. The substrate 2 may be a single layer or a multilayer. The thickness of the substrate 2 is, for example, preferably 5 μm to 150 μm, and more preferably 10 μm to 100 μm. If the thickness is within the above range, it is preferable because of excellent coatability and transparency. Among the substrates described above, a biaxially oriented polypropylene (OPP) film and a polyethylene terephthalate (HS-PET) film having heat sealability can be preferably used from the viewpoint of improving transparency and imparting functionality.

[0025] (Anchor layer) In this embodiment, the anchor layer 3 functions as a layer that enhances the adhesion between the substrate 2 and the thermal layer 4. The material for forming the anchor layer 3 is not particularly limited, and only a binder can be used. The anchor layer 3 may also contain other components, such as a pigment. Furthermore, if the anchor layer is not required, it may not be provided.

[0026] The binder contained in the anchor layer 3 is not particularly limited, and examples thereof include modified styrene-acrylic resin, acrylic emulsion, styrene-acrylic copolymer, modified styrene-butadiene latex, styrene-butadiene copolymer (SBR), acrylic-butadiene-styrene copolymer, vinyl acetate resin, vinyl acetate-acrylic acid copolymer, polyurethane resin, etc. Here, acrylic means acrylic acid (salt) and acrylic ester, and acrylic acid (salt) means acrylic acid and acrylic acid salt.

[0027] The binder is preferably one or more selected from the group consisting of modified styrene-acrylic resin, acrylic emulsion, styrene-acrylic copolymer, acrylic-butadiene-styrene copolymer, and vinyl acetate-acrylic acid copolymer, and more preferably one or more selected from the group consisting of modified styrene-acrylic resin and acrylic emulsion. Among them, when the modified styrene-acrylic resin is used, modified styrene-acrylic acid (salt) resin is preferred. These binders can be used alone or in combination of two or more.

[0028] The salt in the above acrylic acid salt is 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 salt, ethylamine salt, diethylamine salt, and triethylamine salt; polyamine salts such as diethyleneamine salt and diethylenetriamine salt; 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. Among them, ammonium salts are particularly preferred. These salts can be used alone or in combination of two or more.

[0029] Examples of binders that can be used other than those mentioned above include polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose, water-soluble polymers such as sodium polyacrylate, polyvinylpyrrolidone, acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid terpolymers, alkali salts of styrene-maleic anhydride copolymers, alkali salts of isobutylene-maleic anhydride copolymers, polyacrylamide, sodium alginate, gelatin, and casein. These binders can be used alone or in combination of two or more.

[0030] The content of the binder is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 50% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more, based on 100% by mass of the dry mass of the anchor layer 3. The content of the binder may be substantially 100% by mass, based on 100% by mass of the dry mass of the anchor layer 3. When the content of the binder is within the above range, it is preferable from the viewpoint of further increasing the adhesion between the substrate 2 and the thermal layer 4.

[0031] In this specification, the term "dry mass" refers to the mass of non-volatile components (solids) obtained by removing solvents (volatile components) such as water contained in the paint or its raw materials through drying.

[0032] The coating amount (dry mass) of the anchor layer 3 is preferably 0.3 g / m 2 ~5.0g / m 2 and more preferably 0.5 g / m 2 ~2.0g / m 2 It is.

[0033] The thickness of the anchor layer 3 is, for example, preferably 0.3 μm to 5.0 μm, and more preferably 0.5 μm to 2.0 μm.

[0034] In the present embodiment, when the coating amount and thickness of the anchor layer 3 are within the above ranges, the adhesion between the substrate 2 and the thermal layer 4 tends to be further improved, which is preferable.

[0035] In this embodiment, when the anchor layer 3 is provided, the material for forming the anchor layer 3 can be selected according to the type of the base material 2. For example, when a polypropylene film is used as the base material 2, the anchor layer 3 preferably contains a modified styrene-acrylic resin as a binder, more preferably a modified styrene-acrylic acid (salt) resin, and more preferably a modified styrene-acrylic acid (ammonium salt) resin. Moreover, from the viewpoints of improving transparency and imparting functionality, the polypropylene film is particularly preferably a biaxially oriented polypropylene (OPP) film.

[0036] In addition, when a polyethylene terephthalate film is used as the substrate 2, the anchor layer 3 preferably contains an acrylic emulsion as a binder. As the acrylic emulsion, for example, a material commercially available under the name "AQUENCE EPIX BC AD81B" (manufactured by Henkel Japan Co., Ltd.) can be used. In addition, the polyethylene terephthalate film is particularly preferably a polyethylene terephthalate (HS-PET) film having heat sealability from the viewpoint of improving transparency and imparting functionality. In the case of the combination of the substrate 2 and the anchor layer 3, the adhesion between the substrate 2 and the thermal layer 4 is further improved, and the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0037] (Thermal layer) In this embodiment, the thermal layer 4 functions as a layer that develops color when heated. The thermal layer 4 contains at least a fatty acid wax.

[0038] It is preferable to use materials with a small average particle size in order to improve the transparency of the thermal recording medium 1. By using materials with a small average particle size, diffuse reflection of light caused by the particles is suppressed, and the transparency of the thermal recording medium 1 tends to be improved.

[0039] In this specification, the average particle size refers to the particle size at 50% cumulative value in the particle size distribution measured by the laser diffraction / scattering method (D50, median size). The average particle size can be measured by the laser diffraction / scattering method using, for example, a Microtrack Bell product called "MT3300EX-II." Hereinafter, the "average particle size" refers to the median size measured by the above method.

[0040] The fatty acid wax has the function of improving the transparency of the thermosensitive recording medium. Specifically, when forming the thermal layer 4, the fatty acid wax penetrates into the gaps between the particles of the other components and seals the gaps, suppressing the diffuse reflection of light in the gaps, which is presumed to result in further improving the transparency of the thermosensitive recording medium. Examples of the fatty acid wax include fatty acids, fatty acid salts, fatty acid amides, and fatty acid esters. These fatty acid waxes can be used alone or in combination of two or more.

[0041] Examples of the fatty acid include higher fatty acids having 10 to 22 carbon atoms. Examples of the higher fatty acids include higher saturated fatty acids having 10 to 22 carbon atoms, such as capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and behenic acid; and higher unsaturated fatty acids having 10 to 22 carbon atoms, such as palmitoleic acid, oleic acid, elaidic acid, linoleic acid, and linolenic acid. These fatty acids may have a substituent. Examples of the substituent include a hydroxyl group.

[0042] Examples of the fatty acid salt include salts obtained by neutralizing the fatty acids. Examples of the salt include 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. Specific examples of the fatty acid salt include sodium stearate, potassium stearate, magnesium stearate, calcium stearate, zinc stearate, iron stearate, sodium oleate, potassium oleate, magnesium oleate, calcium oleate, zinc oleate, and iron oleate.

[0043] Examples of the fatty acid amide include the amidation reaction product of the fatty acid with ammonium or amine. Examples of the amine include diamines such as ethylenediamine and hexamethylenediamine. Specific examples of the fatty acid amide include capric acid amide, lauric acid amide, myristic acid amide, palmitic acid amide, stearic acid amide, arachidic acid amide, behenic acid amide, palmitoleic acid amide, oleic acid amide, elaidic acid amide, linoleic acid amide, linolenic acid amide, N,N'-ethylenebis(stearic acid amide), N,N'-hexamethylenebis(stearic acid amide), etc.

[0044] Examples of the fatty acid ester include esterification products of the fatty acids with monohydric and / or polyhydric alcohols. Examples of the monohydric and / or polyhydric alcohols include monohydric alcohols such as ethanol and propanol, and polyhydric alcohols such as propylene glycol, glycerin, and sorbitol.

[0045] In this embodiment, the fatty acid wax is preferably a solubilized fatty acid wax and / or an emulsion-type fatty acid wax, and more preferably a solubilized fatty acid wax. In this specification, the solubilized fatty acid wax means that it is soluble in the thermal layer or the paint for the thermal layer. By using the solubilized fatty acid wax and / or the emulsion-type fatty acid wax, the transparency of the thermal recording medium tends to be improved. In particular, the solubilized fatty acid wax is soluble in the thermal layer or the paint for the thermal layer, and thus the transparency of the thermal recording medium tends to be improved.

[0046] The solubilized fatty acid wax may be soluble in the thermal layer or thermal layer paint alone, or may be soluble in the thermal layer or thermal layer paint as a mixture of the fatty acid wax and the solubilizer. Examples of the solubilizer include surfactants, solvents, and solvents used in thermal layer paints. Examples of the solvents used in the thermal layer paint include organic solvents and water, and water is preferred. When the solubilized fatty acid wax contains a solvent used in the thermal layer paint, it may contain a part of the solvent or the entire amount of the solvent. From the viewpoint of ease of handling and ease of solubilization, the solubilized fatty acid wax preferably contains a fatty acid wax and a solubilizer. The solubilizer is preferably one or more selected from the group consisting of surfactants, solvents, and solvents used in thermal layer paints. In addition, the solubilizer preferably contains a surfactant and / or a solvent and a portion of the solvent of the paint for the thermal layer, and it is also preferable that the solubilizer contains a surfactant and / or a solvent but does not contain the solvent of the paint for the thermal layer.

[0047] The above-mentioned emulsion-like fatty acid wax is a compound that emulsifies the above-mentioned fatty acid wax by itself, and may be one that disperses in the solvent of the thermal layer paint by itself to form an emulsion, or one that disperses in the solvent of the thermal layer paint by the aid of a surfactant or the like to form an emulsion. In this specification, the emulsion-like fatty acid wax means a fatty acid wax that stably forms an emulsion. Furthermore, stably forming an emulsion means that, for example, when left to stand for 24 hours at room temperature and normal pressure, no changes in state such as separation or precipitation of components are observed, and the emulsion state can be maintained.

[0048] In this embodiment, the thermal layer 4 may contain other components than the fatty acid wax, such as dyes, color developers, fillers, binders, dispersants, crosslinking agents, waxes other than the fatty acid wax, surfactants, solvents, etc. Among them, it is preferable that the thermal layer 4 contains a surfactant and / or a solvent. The surfactant is not particularly limited, and may be any component that has a function of improving the solubility of the fatty acid wax. Examples of the surfactant include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. These surfactants may be used alone or in combination of two or more.

[0049] Examples of the anionic surfactant include alkylbenzenesulfonic acid and / or its salt, alkylsulfonic acid and / or its salt, alkyl sulfate and / or its salt, alkyl ether sulfate and / or its salt, etc. Examples of the alkyl group of the anionic surfactant include an alkyl group having 10 to 22 carbon atoms. Examples of the alkyl group include a saturated hydrocarbon group and an unsaturated hydrocarbon group. The alkyl group may have a substituent. Examples of the substituent include a hydroxyl group. Examples of the salt include ammonium salts such as ammonia; alkanolamine salts such as triethanolamine; alkali metal salts such as lithium, sodium, potassium; alkaline earth metal salts such as magnesium and calcium; and polyvalent metal salts such as zinc and iron.

[0050] Examples of the nonionic surfactant include polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, polyoxyalkylene alkyl ether, polyoxyethylene alkyl ester, polyoxyethylene polyoxypropylene glycol, sorbitan alkyl ester, polyoxyethylene sorbitan alkyl ester, glycerin alkyl ester, polyoxyethylene glycerin alkyl ester, polyoxyethylene hydrogenated castor oil, etc. The alkyl group of the nonionic surfactant is not particularly limited, and examples thereof include alkyl groups having 10 to 22 carbon atoms. Examples of the alkyl group include saturated hydrocarbon groups and unsaturated hydrocarbon groups. The alkyl group may have a substituent. Examples of the substituent include a hydroxyl group, etc.

[0051] The solvent is not particularly limited, and may be any component that has the function of improving the solubility of the fatty acid wax. Examples of the solvent include glycol ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, and propylene glycol; ethers such as diisopropyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and 1,4-dioxane; alcohols such as methanol, ethanol, n-propanol, isopropanol, 2-ethylhexanol, and benzyl alcohol; and amines such as monoethanolamine, diethanolamine, and triethanolamine. Among these, from the viewpoint of handling and solubilizing the fatty acid wax, the solvent is preferably one or more selected from the group consisting of glycol ether, glycol, ether, and alcohol, more preferably one or more selected from the group consisting of glycol ether and glycol, and even more preferably glycol ether. When the solvent is a glycol ether, diethylene glycol monobutyl ether is particularly preferred. These solvents can be used alone or in combination of two or more.

[0052] In this specification, a compound that corresponds to a fatty acid wax and may also correspond to other components such as a surfactant or a solvent is classified only as a fatty acid wax and not as other components, and a compound that does not correspond to a fatty acid wax and may correspond to a surfactant or a solvent is classified only as a surfactant and not as a solvent.

[0053] The average particle size of the fatty acid wax is preferably 1.0 μm or less. When the average particle size of the fatty acid wax is within the above range, the transparency of the thermosensitive recording medium 1 tends to be further improved, which is preferable.

[0054] The content of the fatty acid wax is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, based on 100% by mass of the dry mass of the thermal layer 4. The content of the fatty acid wax is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less, based on 100% by mass of the dry mass of the thermal layer 4. When the content of the fatty acid wax is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0055] In this specification, the "content of fatty acid wax" means the total content of the dry mass of the fatty acid wax and the dry mass of the soluble agent when the fatty acid wax contains a solubilizer. Also, when the fatty acid wax is in an emulsion state and contains a surfactant, it means the content of the dry mass of the emulsion containing the surfactant.

[0056] In this embodiment, from the viewpoint of further improving the long-term water resistance of the thermal recording medium 1, the thermal layer 4 preferably contains a wax (other wax) other than the fatty acid wax. Examples of the wax other than the fatty acid wax that can be used include polystyrene, polyethylene, and paraffin. Among them, from the viewpoint of further improving the long-term water resistance and transparency of the thermal recording medium 1, the wax other than the fatty acid wax is preferably one or more selected from the group consisting of polystyrene and polyethylene, and more preferably polystyrene. These waxes can be used alone or in combination of two or more.

[0057] The average particle diameter of the wax other than the fatty acid wax is preferably 0.1 μm or more. The average particle diameter of the wax other than the fatty acid wax is preferably 1.0 μm or less, more preferably 0.6 μm or less, even more preferably 0.55 μm or less, even more preferably 0.4 μm or less, and particularly preferably 0.3 μm or less. When the average particle diameter of the wax other than the fatty acid wax is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0058] The content of the wax other than the fatty acid wax is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, based on 100% by mass of the dry mass of the thermal layer 4. The content of the wax other than the fatty acid wax is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less, based on 100% by mass of the dry mass of the thermal layer 4. When the content of the wax other than the fatty acid wax is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0059] In this embodiment, the types of wax contained in the thermal layer 4 are, for example, preferably 1 to 5 types, more preferably 1 to 2 types, and further preferably 2 types. When the types of wax are two, a particularly preferred combination is one each of fatty acid wax and polystyrene.

[0060] The ratio of the dry mass of the wax other than the fatty acid wax when the dry mass of the fatty acid wax is taken as 1 (dry mass of the wax other than the fatty acid wax / dry mass of the fatty acid wax) is preferably 0.1 or more, more preferably 0.5 or more. In addition, the ratio of the dry mass of the wax other than the fatty acid wax when the dry mass of the fatty acid wax is taken as 1 (dry mass of the wax other than the fatty acid wax / dry mass of the fatty acid wax) is preferably 10 or less, more preferably 5 or less, and even more preferably 2 or less. In addition, the ratio of the dry mass of the wax other than the fatty acid wax when the dry mass of the fatty acid wax is taken as 1 (dry mass of the wax other than the fatty acid wax / dry mass of the fatty acid wax) is particularly preferably 1. When the ratio of the dry mass of the wax other than the fatty acid wax to the dry mass of the fatty acid wax is within the above range, the transparency and long-term water resistance of the thermal recording medium 1 tend to be further improved, which is preferable.

[0061] The total wax content is preferably 0.2% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more, based on 100% by mass of the dry mass of the thermal layer 4. The total wax content is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of the dry mass of the thermal layer 4. When the total wax content is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0062] Examples of the dye include 3-dibutylamino-6-methyl-7-anilinofluoran, 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-diethyl Amino-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 dyes can be used alone or in combination of two or more.

[0063] The average particle size of the dye is, for example, preferably 0.1 μm to 1.0 μm, more preferably 0.1 μm to 0.6 μm, even more preferably 0.1 μm to 0.55 μm, still more preferably 0.1 μm to 0.4 μm, and particularly preferably 0.1 μm to 0.3 μm. When the average particle size of the dye is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0064] The content of the dye is preferably 5% by mass or more and 50% by mass or less, more preferably 8% by mass or more and 35% by mass or less, and even more preferably 10% by mass or more and 25% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0065] Examples of the developer include 4-hydroxyphenyl(4'-n-propoxyphenyl)sulfone, 3-[(3-phenylureido)phenyl]-4-methylbenzenesulfonate, N,N-di-[3-(p-toluenesulfonyloxy)phenyl]urea, 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, and 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), 4-hydroxybenzoic acid benzyl, 2,4-bis(phenylsulfonyl)phenol, α-{4-[(4-hydroxyphenyl)sulfonyl]phenyl}-ω-hydroxypoly(polymerization degree 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. These color developers can be used alone or in combination of two or more.

[0066] The average particle size of the developer is, for example, preferably 0.1 μm to 1.0 μm, more preferably 0.1 μm to 0.6 μm, even more preferably 0.1 μm to 0.55 μm, still more preferably 0.1 μm to 0.4 μm, and particularly preferably 0.1 μm to 0.3 μm. When the average particle size of the developer is within the above range, the transparency of the thermal recording medium 1 tends to be further improved, which is preferable.

[0067] The content of the developer is preferably 20% by mass or more and 70% by mass or less, more preferably 25% by mass or more and 60% by mass or less, and even more preferably 30% by mass or more and 50% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0068] Examples of the filler that can be used include kaolin, calcined kaolin, aluminum hydroxide, magnesium hydroxide, aluminum oxide, magnesium oxide, aluminum silicate, calcium carbonate, magnesium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, diatomaceous earth, white carbon, zinc oxide, silicon oxide, colloidal silica, polystyrene resin particles, urea-formalin resin particles, polyolefin resin particles, etc. These fillers can be used alone or in combination of two or more.

[0069] The average particle size of the filler is, for example, preferably 0.1 μm to 1.0 μm, more preferably 0.1 μm to 0.6 μm, even more preferably 0.1 μm to 0.55 μm, still more preferably 0.1 μm to 0.4 μm, and particularly preferably 0.1 μm to 0.3 μm. When the average particle size of the filler is within the above range, the transparency of the thermosensitive recording medium 1 tends to be further improved, which is preferable.

[0070] The content of the filler is preferably 0.1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 20% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0071] Examples of the binder include styrene-butadiene copolymer (SBR), 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.

[0072] The content of the binder is preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 20% by mass or more and 30% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0073] The dispersant is preferably a dispersant having a carboxyl group. As the dispersant having a carboxyl group, styrene-acrylic polymers such as styrene-acrylic acid (salt) polymers and styrene-acrylic acid ester polymers; styrene-maleic acid (salt) resins, styrene-maleic acid ester resins, etc. can be used. Among them, styrene-acrylic polymers are preferred, styrene-acrylic acid (salt) polymers are more preferred, and styrene-acrylic acid (ammonium salt) polymers are even more preferred. These dispersants can be used alone or in combination of two or more kinds.

[0074] The salt of the acrylic acid salt contained in the dispersant having a carboxyl group 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. Among these, ammonium salts are particularly preferred.

[0075] The content of the dispersant is preferably 5% by mass or more and 30% by mass or less, more preferably 7% by mass or more and 18% by mass or less, and even more preferably 10% by mass or more and 15% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0076] As the crosslinking agent, for example, an epichlorohydrin-modified polyamide polyamine resin, an organic compound containing an oxazoline group, etc. These crosslinking agents can be used alone or in combination of two or more kinds.

[0077] The content of the crosslinking agent is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.3% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 2% by mass or less, relative to 100% by mass of the dry mass of the thermal layer 4.

[0078] When a dispersant having a carboxyl group is used as the dispersant and an epichlorohydrin-modified polyamide polyamine resin is used as the crosslinking agent, an azetidinium ring (AZR) in the epichlorohydrin-modified polyamide polyamine resin, which is the crosslinking agent, reacts with a carboxyl group of the dispersant to form a crosslinked structure in the thermal layer 4, which is thought to improve the film-forming property of the thermal layer 4 and fill voids in the thermal layer 4. As a result, the transparency of the thermal layer 4 is further improved, and a thermal recording medium 1 with excellent transparency can be obtained.

[0079] Furthermore, when a dispersant having a carboxyl group is used as the dispersant and an organic compound containing an oxazoline group is used as the crosslinking agent, the oxazoline group of the organic compound containing an oxazoline group, which is the crosslinking agent, reacts with the carboxyl group of the dispersant to form a crosslinked structure in the thermal layer 4, which is thought to improve the film-forming properties of the thermal layer 4 and fill voids in the thermal layer 4. As a result, the transparency of the thermal layer 4 is further improved, and a thermal recording medium 1 with excellent transparency can be obtained.

[0080] The coating amount (dry mass) of the thermal layer 4 is preferably, for example, 0.5 g / m 2 ~10.0g / m 2 and more preferably 1.0 g / m 2 ~8.0g / m 2 and more preferably 2.0 g / m 2 ~6.0g / m 2 and more preferably 3.0 g / m 2 ~6.0g / m 2 and more preferably 3.5 g / m 2 ~5.5g / m 2 and more preferably 4.0 g / m 2 ~5.0g / m 2 and particularly preferably 4.5 g / m 2 When the coating amount is within the above range, the transparency of the thermosensitive recording medium 1 tends to be further improved, which is preferable.

[0081] The thickness of the thermal layer 4 is, for example, preferably 2 μm to 6 μm, and more preferably 3 μm to 5 μm.

[0082] The thermosensitive recording medium 1 of this embodiment has the above-mentioned structure and is therefore excellent in transparency.

[0083] In the thermal recording medium 1 of this embodiment, the haze value of the laminate having the substrate 2 and the thermal layer 4 on both end faces is preferably 40% or less, more preferably 38% or less, and even more preferably 37% or less, when measured by the method described in the Examples. If the haze value of the laminate having the substrate 2 and the thermal layer 4 on both end faces is within the above range, it is preferable because the transparency is more excellent.

[0084] [Method of manufacturing the thermal recording medium] The method for producing the thermal recording medium of the present invention is not particularly limited. For example, the thermal layer can be produced by dispersing the components contained in the thermal layer in a solvent such as water to prepare a coating material for the thermal layer by a known or conventional method, applying the coating material by a known or conventional method, and then drying the coating material by a known or conventional method.

[0085] (Preparation process) In the preparation step, the thermal layer coating material may be prepared by dispersing all of the components in the same solvent in advance. Alternatively, a color former and a color developer that react with each other may be prepared as separate dispersions and then mixed to prepare the thermal layer coating material. In this case, the other components may be added to either the dispersion containing the dye or the dispersion containing the color developer, or to both. The method for preparing the coating material is not particularly limited, and examples of the method include stirring, ultrasonic treatment, and crushing treatment using a ball mill, a bead mill, a sand mill, a high-pressure homogenizer, etc. These methods may be used alone or in combination of two or more.

[0086] In the method for producing a thermal recording medium of the present invention, from the viewpoint of further improving the transparency of the thermal recording medium, it is preferable to include a step of preparing a coating material for the thermal layer using a solubilized fatty acid wax and / or an emulsion-type fatty acid wax. Of these, it is more preferable to include a step of preparing a coating material for the thermal layer using a solubilized fatty acid wax. By microparticulating or solubilizing the fatty acid wax into an emulsion-type, the permeability of the fatty acid wax into the gaps between the particles of other components is improved, and the gaps are more sealed, so that the diffuse reflection of light in the gaps is more suppressed, and as a result, it is presumed that the transparency of the thermal recording medium is further improved.

[0087] The method for obtaining the solubilized fatty acid wax is not particularly limited, and for example, the solubilizer can be added to the fatty acid wax and mixed. For the solubilizer, all of the contents described in the section on [thermosensitive recording medium] can be used. The method for mixing is not particularly limited, and examples include stirring, ultrasonic treatment, and crushing treatment using a ball mill, a bead mill, a sand mill, a high-pressure homogenizer, etc. These methods can be used alone or in combination of two or more.

[0088] The solubilized fatty acid wax is preferably a transparent liquid. For example, the fatty acid wax and the solubilizer can be mixed to prepare a transparent liquid solubilized fatty acid wax. Such a solubilized fatty acid wax can improve the permeability into the thermal layer or the thermal layer coating material, and therefore can improve the transparency of the thermal recording medium. Moreover, unlike an emulsion in which the wax exists as particles in the medium, such a solubilized fatty acid wax does not exist as particles, and therefore can improve the transparency of the thermal recording medium.

[0089] (Coating process) In the coating step, the coating material for the thermal layer obtained above can be applied by directly coating the substrate, or by coating a release liner or the like and then transferring the coating material to the substrate. The coating method is not particularly limited, and examples thereof include air knife coating, barrier blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, die coating, gravure coating, etc. These methods can be used alone or in combination of two or more.

[0090] (drying process) The method for drying the applied coating material is not particularly limited, and examples thereof include heat drying, room temperature drying, vacuum drying, etc. By drying the applied coating material by these methods, a thermal layer can be formed. These methods can be used alone or in combination of two or more.

[0091] When the thermal recording medium of the present invention has layers other than the thermal layer, the above content can be used as a method for forming the other layers. For example, as shown in FIG. 1, a thermal recording medium 1 having a substrate 2, an anchor layer 3, and a thermal layer 4 in this order can be manufactured by forming the anchor layer 3 on the substrate 2, and then forming the thermal layer 4 on the anchor layer 3. The above-mentioned coating material for the anchor layer may be prepared by dispersing all of the components contained in the anchor layer 3 in the same solvent in advance, or, when components that react with each other are included, each component may be prepared as a separate dispersion and then mixed to prepare the two. The above-mentioned preparation method is not particularly limited, and examples thereof include stirring, ultrasonic treatment, and crushing treatment using a ball mill, a bead mill, a sand mill, a high-pressure homogenizer, etc. These methods can be used alone or in combination of two or more. The above-mentioned coating material for the anchor layer can be applied to the substrate 2 by, for example, directly applying the coating material to the substrate 2. The method of applying the coating material for the anchor layer is not particularly limited, and examples thereof include air knife coating, barber blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, die coating, and gravure coating. These methods can be used alone or in combination of two or more. Next, the coating material for the anchor layer applied as above is dried to form an anchor layer 3 on the substrate 2. Examples of the drying method include heat drying, room temperature drying, and vacuum drying. These methods can be used alone or in combination of two or more. Next, a thermal layer 4 is formed on the anchor layer 3 in the same manner as above, to obtain the thermal recording material 1 shown in FIG. 1.

[0092] In the method for producing a thermal recording medium of the present invention, each layer may be simultaneously coated in multiple layers by, for example, a curtain coater, or may be formed individually and successively. Also, some layers may be simultaneously coated and some layers may be formed individually and successively. EXAMPLES

[0093] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples, and is limited only by the claims. It should be noted that Examples 3 and 4 should be read as Reference Examples 1 and 2.

[0094] A thermosensitive recording medium having an anchor layer and a thermal layer laminated in this order on a substrate was prepared by the following process. Example 1 (Preparation of thermal recording medium) <Anchor layer> A coating material for the anchor layer was prepared by adding water to a modified styrene-acrylic acid (ammonium salt) resin (manufactured by BASF Japan Ltd., product name: Joncryl PDX7430) so that the dry mass was 10% by mass. The coating material for the anchor layer was applied to a biaxially oriented polypropylene (OPP) film (thickness: 40 μm) as a substrate, and then dried to obtain a coating amount of 0.5 to 2.0 g / m2 in terms of dry mass. 2 An anchor layer having a thickness of 0.5 to 2 μm was formed.

[0095] <Thermal layer> The paint for the thermal layer was prepared by using water as a solvent and adjusting the dry mass of each compounding material to the composition shown in Table 1 to 23% by mass. The paint was then applied onto the anchor layer and dried at 50°C to obtain a coating amount of 4.0 g / m2 in terms of dry mass. 2 A thermal layer of this was formed to produce a thermal recording medium of Example 1. In Table 1, the numerical value of each compounding material indicates the mass ratio after drying (dry mass). Furthermore, unless otherwise specified, when the compounding material (fatty acid-based wax, etc.) contains a surfactant and a solvent, the numerical value of each compounding material in Table 1 indicates the mass ratio when dried, including the surfactant and the solvent.

[0096] As a compounding material, wax 1 was used, which was a solubilized fatty acid wax (manufactured by Nisshin Chemical Laboratory Co., Ltd., product name: R-053D). The wax 1 contained a fatty acid wax as a main component, an anionic surfactant and a polyoxyethylene alkyl ether as surfactants, and water as a solvent. The wax 1 was also easily soluble in water. The dye used was 3-dibutylamino-6-methyl-7-anilinofluoran with an average particle size of 0.15 μm, the developer used was 4-hydroxyphenyl(4'-n-propoxyphenyl)sulfone (manufactured by Mitsubishi Chemical Corporation, product name: Tomilac KN) with an average particle size of 0.15 μm, the filler used was kaolin with an average particle size of 0.4 μm, the binder used was SBR with a glass transition temperature Tg of -3°C, the dispersant used was a styrene-acrylic acid (ammonium salt) polymer, and the crosslinking agent used was epichlorohydrin-modified polyamide polyamine resin.

[0097] Example 2 The thermal recording medium of Example 2 was prepared in the same manner as Example 1 above, except that in the thermal layer, Wax 5 (polystyrene wax with an average particle diameter of 0.2 μm) was used as a wax other than the fatty acid-based wax, and the amounts of each compounded material were adjusted so that the dry mass of each compounded material had the composition shown in Table 1.

[0098] Example 3 A thermosensitive recording medium of Example 3 was prepared in the same manner as in Example 2, except that in the thermal layer, wax 1 was replaced with an emulsion of wax 2, a fatty acid-based wax (manufactured by Chukyo Yushi Co., Ltd., product name: High Micron L-271). The wax 2 contained fatty acid amide as the main component at 20% to 30% by dry weight, triethanolamine as a solvent at less than 1.0% by dry weight, and water as a solvent. The wax 2 was dispersible in water. The average particle diameter of the fine particles contained in the emulsion of wax 2 was measured and found to be 0.87 μm.

[0099] Example 4 A thermal recording medium of Example 4 was prepared in the same manner as in Example 2, except that in the thermal layer, wax 1 was changed to an emulsion of wax 3, a fatty acid wax (manufactured by Nissin Chemical Laboratory Co., Ltd., product name: New Wax E-50). The wax 3 contained a fatty acid wax as a main component, 10% to 20% by dry mass of solid paraffin as other components, and water as a solvent. The wax 3 was dispersible in water. The average particle diameter of the fine particles contained in the emulsion of wax 3 was measured and found to be 0.18 μm.

[0100] Example 5 A thermal recording medium of Example 5 was prepared in the same manner as in Example 2, except that in the thermal layer, Wax 1 was replaced with Wax 4, a solubilized fatty acid wax. Wax 4 was prepared by adding 3 g of diethylene glycol monobutyl ether as a solvent to 1 g of solid matter obtained by heating an emulsion-like fatty acid wax of Wax 3 (manufactured by Nisshin Chemical Laboratory Co., Ltd., product name: New Wax E-50) at 50°C and distilling off water, and this was used as the solubilized fatty acid wax. In Example 5, the diethylene glycol butyl ether is treated as a solvent for solubilizing the fatty acid wax, so the value of Wax 4 in Example 5 in Table 1 indicates the ratio of the dry mass including the diethylene glycol butyl ether.

[0101] The above wax 1 and wax 4 were in a transparent liquid state.

[0102] Comparative Example 1 A thermosensitive recording medium of Comparative Example 1 was prepared in the same manner as in Example 2, except that in the thermal layer, Wax 1 was changed to Wax 6 and Wax 5 to Wax 7. Wax 6 was a polyethylene wax with an average particle size of 1.3 μm, and Wax 7 was paraffin with a melting point of 46° C. and an average particle size of 0.3 μm.

[0103] By the above-mentioned method, the thermosensitive recording media of Examples 1 to 5 and Comparative Example 1 were prepared and used for the measurement of the haze value described below.

[0104] (Haze value measurement) The haze value was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name: NDH7000) from the thermal layer side of the thermal recording medium (a laminate having a substrate and a thermal layer on both end faces) prepared above. The haze values ​​obtained for the thermal recording mediums prepared in each Example and Comparative Example are shown in Table 1.

[0105] The above measurements were carried out according to the method specified in JIS K7136:2000.

[0106] [Table 1]

[0107] The following can be confirmed from Table 1.

[0108] The thermal recording materials of Examples 1 to 5, which contain a fatty acid wax as the wax in the thermal layer, had smaller haze values ​​than the thermal recording material of Comparative Example 1, which does not contain a fatty acid wax. This confirmed the tendency for the transparency of the thermal recording material to be improved by using a fatty acid wax as the wax. Furthermore, the thermal recording material of Example 5, which was obtained using wax 4, which was obtained by solubilizing wax 3 with a solvent to obtain a solubilized fatty acid wax, had a smaller haze value than the thermal recording material of Example 4, which was obtained using wax 3, which was an emulsion-like fatty acid wax, and this confirmed the improvement in transparency. Although the detailed mechanism of the improvement in transparency by solubilizing the fatty acid wax before use is unknown, it is presumed that the solubilization of wax 3, which existed as particles of a size that can be confirmed by the naked eye, improved the penetration into the gaps between the particles of the blended material.

[0109] In summary, the configuration of the present invention and variations thereof are described below. [Appendix 1] A thermal recording medium having a thermal layer containing a fatty acid-based wax. [Appendix 2] The thermal recording medium according to appendix 1, wherein the fatty acid wax is a solubilized fatty acid wax and / or an emulsion of fatty acid wax. [Appendix 3] The thermosensitive recording material according to any one of Appendices 1 to 2, wherein the fatty acid wax is a solubilized fatty acid wax. [Appendix 4] The thermosensitive recording material according to any one of Appendices 2 to 3, wherein the solubilized fatty acid wax contains a solubilizer. [Appendix 5] The thermosensitive recording material according to any one of Appendices 1 to 4, wherein the solubilized fatty acid wax is a transparent liquid. [Appendix 6] The thermal recording medium according to any one of Appendices 1 to 5, wherein the thermal layer contains a surfactant and / or a solvent. [Appendix 7] The heat sensor according to any one of Appendices 1 to 6, wherein the thermal layer further contains a wax other than the fatty acid wax and having an average particle diameter of 1 μm or less. [Appendix 8] The heat sensor according to appendix 7, wherein the wax other than the fatty acid wax is at least one of polystyrene, polyethylene, and paraffin. [Appendix 9] The thermal recording medium according to any one of Appendices 1 to 8, wherein the content of the fatty acid wax is from 0.1% by mass to 5% by mass, relative to 100% by mass of the dry mass of the thermal layer. [Appendix 10] A method for producing a thermal recording medium according to any one of Appendices 1 to 9, comprising the steps of preparing a thermal layer coating material using a solubilized fatty acid wax and / or an emulsion of fatty acid wax, and applying and drying the thermal layer coating material to form a thermal layer. [Appendix 11] A method for producing a thermal recording medium according to any one of Appendices 1 to 9, comprising the steps of preparing a thermal layer coating material using a transparent liquid solubilized fatty acid wax, and applying and drying the thermal layer coating material to form a thermal layer. [Industrial Applicability]

[0110] The thermosensitive recording medium of the present invention has excellent transparency and can be particularly suitably used in applications where visibility of the contents is required, such as labels for food containers such as lunch boxes and food packaging films. [Explanation of symbols]

[0111] 1 Thermal recording medium 2 Base material 3 Anchor Layer 4 Thermal Layer

Claims

1. A thermal recording medium comprising a substrate which is a transparent synthetic resin film and a thermal layer which contains a fatty acid-based wax.

2. 2. The thermosensitive recording material according to claim 1, wherein the fatty acid wax is a solubilized fatty acid wax and / or an emulsion of fatty acid wax.

3. 2. The thermosensitive recording medium according to claim 1, wherein the fatty acid wax is a solubilized fatty acid wax.

4. 4. The thermosensitive recording material according to claim 3, wherein the solubilized fatty acid wax contains a solubilizing agent.

5. A thermal recording medium as described in claim 3, wherein the solubilized fatty acid-based wax is a transparent liquid.

6. A thermal recording medium as described in claim 1, wherein the average particle diameter of the fatty acid-based wax is 1.0 μm or less.

7. 7. The thermosensitive recording medium according to claim 1, wherein the thermal layer contains a surfactant and / or a solvent.

8. 7. The thermosensitive recording medium according to claim 1, wherein the thermal layer further contains a wax other than the fatty acid wax, the wax having an average particle size of 1 μm or less.

9. A thermal recording medium as described in Claim 8, wherein the average particle diameter of the wax other than the fatty acid-based wax is 0.6 μm or less.

10. 7. The thermosensitive recording medium according to claim 1, wherein the content of the fatty acid wax is 0.1% by mass or more and 5% by mass or less relative to 100% by mass of the dry mass of the thermal layer.

11. A step of preparing a coating material for a thermal layer using a solubilized fatty acid wax and / or an emulsion of a fatty acid wax; The method for producing a thermal recording medium according to any one of claims 1 to 6, further comprising the step of forming a thermal layer by applying and drying the thermal layer coating material.