Thermal recording material

Abstract

To provide a thermal recording material having superior heat resistance and coatability, and to provide an underlayer coating for producing such thermal recording material.SOLUTION: A thermal recording medium 1 of the present invention comprises, in this order, a substrate 2, an underlayer 3, and a thermal layer 4, wherein the underlayer 3 contains hollow particles, an alkylene oxide adduct of acetylene glycol, and a binding agent. Preferably, the HLB value of the alkylene oxide adduct of acetylene glycol is 5 or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a heat-sensitive recording medium.

Background Art

[0002] A heat-sensitive recording medium develops color by a chemical reaction upon heating with a thermal head or the like, and a recorded image is obtained. It is used not only as a recording medium for facsimiles, automatic ticket vending machines, and scientific measuring instruments, but also widely as a heat-sensitive recording label for POS systems in retail stores and the like, and receipt paper.

[0003] In recent years, with the increase in ready-to-eat meals, the demand for heat-resistant heat-sensitive recording media that can withstand heating in a microwave oven has been increasing. Conventionally, in order to obtain such a heat-sensitive recording medium, it is known that blending a highly heat-resistant color former into the thermal layer is effective. However, since such a color former generally has low sensitivity, heat resistance and sensitivity have been in a trade-off relationship. Therefore, in order to achieve both heat resistance and sensitivity, a heat-sensitive recording medium has been proposed in which, in addition to a thermal layer containing a highly heat-resistant color former, an underlayer containing hollow particles is provided between the base material and the thermal layer. However, when hollow particles are blended into the underlayer, the low-specific gravity hollow particles tend to aggregate in the underlayer coating, resulting in a problem of deterioration in coatability.

[0004] In order to solve such problems, a heat-sensitive recording medium has been proposed in which a specific copolymer is blended into the underlayer to improve the dispersibility of the hollow particles in the underlayer coating, thereby improving the coatability. For example, Patent Document 1 discloses that the liquid stability over time of an underlayer liquid is improved by using a block copolymer of ethylene oxide and propylene oxide, and Patent Document 2 discloses that a saponified product of a diacetoneacrylamide-fatty acid vinyl ester copolymer having a degree of polymerization of 400 to 1,100 has excellent coatability of the underlayer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the study on the thermal recording medium that achieves both heat resistance and coatability is not yet sufficient, and further improvement is required.

[0007] Therefore, an object of the present invention is to provide a thermal recording medium excellent in heat resistance and coatability.

Means for Solving the Problems

[0008] As a result of intensive studies to solve the above problems, the present inventors have found that a thermal recording medium excellent in heat resistance and coatability can be obtained by blending hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder in an underlayer. The present invention has been completed based on these findings.

[0009] That is, the present invention provides a thermal recording medium including a base material, an underlayer, and a thermal layer in this order, wherein the underlayer contains hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder. Furthermore, the present invention provides an underlayer paint for forming the underlayer.

[0010] The HLB value of the alkylene oxide adduct of acetylene glycol is preferably 5 or more. By having such a configuration, the coatability can be further improved.

[0011] The thermal layer preferably contains an N,N'- diarylurea derivative. By having such a configuration, the heat resistance can be further improved.

[0012] The above N,N'- diarylurea derivative is preferably 3-[(3- phenylureido)phenyl]-4- methylbenzenesulfonate. By having such a configuration, it can be more excellent in heat resistance.

[0013] The above base material may be synthetic paper. Different from paper composed of fibers such as high- quality paper, synthetic paper is composed of resin and thus has excellent water resistance. On the other hand, since the surface of synthetic paper is smooth and has lower wettability of the surface compared with paper, it is more difficult to uniformly apply (coat) the coating for the underlayer containing hollow particles than for high- quality paper or the like. However, in the case of the thermal recording medium in the present embodiment, even when such synthetic paper is used as the base material, it can provide excellent coatability and can also be compatible with water resistance.

[0014] From the viewpoint of further improving water resistance, the content ratio of the polyvinyl alcohol (PVA)- based resin in the above underlayer is preferably 50% by mass or less with respect to 100% by mass of the dry mass of the above underlayer. Although the PVA- based resin is a general- purpose material for forming the underlayer, because of its high hydrophilicity, moisture is likely to penetrate and the water resistance of the thermal recording medium can be reduced. However, in the case of the thermal recording medium in the present embodiment, by having the above configuration, it is possible to provide a thermal recording medium excellent not only in heat resistance and coatability but also in water resistance.

[0015] The above underlayer may not substantially contain a block copolymer of ethylene oxide and propylene oxide. The block copolymer of ethylene oxide and propylene oxide is a copolymer known from Patent Document 1, but even without actively using such a copolymer, it is possible to provide a thermal recording medium excellent in heat resistance and coatability.

[0016] The present invention also provides an undercoat paint for forming the undercoat in a thermal recording medium including a base material, an undercoat, and a thermal layer in this order. The undercoat paint includes a solvent, hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder. The surface tension of a mixture obtained by mixing the solvent and the alkylene oxide adduct of acetylene glycol at the following mass ratio is 45 mN / m or less. By forming the undercoat using such an undercoat paint, a thermal recording medium excellent in heat resistance and coatability can be provided. · Mass ratio: When the total mass of the undercoat paint is (100 + X) parts by mass and the mass of the alkylene oxide adduct of acetylene glycol in the undercoat paint is X parts by mass, the solvent is 100 parts by mass and the alkylene oxide adduct of acetylene glycol is X parts by mass.

[0017] The hollow particles may be blended as a granular composition. Generally, hollow particles are preferably used as an emulsion composition containing additives such as a dispersant. However, in the undercoat paint of the present embodiment, even when blended as a granular composition, they can be stably dispersed in the undercoat paint, so that the coatability can be further improved.

Advantages of the Invention

[0018] According to the present invention, a thermal recording medium excellent in heat resistance and coatability can be provided.

Brief Description of the Drawings

[0019]

Figure 1

Embodiments for Carrying Out the Invention

[0020] [Thermal Recording Medium] The thermal recording medium of the present invention includes a base material, an underlayer, and a thermal layer in this order. Further, the underlayer contains hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder.

[0021] The thermal recording medium of the present invention may include other layers other than those described above. Examples of the other layers include an intermediate layer, a top coat layer, a back coat layer, and the like. When the thermal recording medium includes a top coat layer, it includes a base material, an underlayer, a thermal layer, and a top coat layer in this order. When the thermal recording medium includes an intermediate layer and a top coat layer, it includes a base material, an underlayer, a thermal layer, an intermediate layer, and a top coat layer in this order. When the thermal recording medium includes a back coat layer, it includes a back coat layer, a base material, an underlayer, and a thermal layer in this order. Note that for any layer, another layer may be further provided between the layers.

[0022] The thermal recording medium of the present invention preferably includes a top coat layer as the other layer, and more preferably includes an intermediate layer and a top coat layer.

[0023] 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.

[0024] FIG. 1 is a schematic cross-sectional view showing an embodiment of the thermal recording medium of the present invention.

[0025] As shown in FIG. 1, the thermal recording medium 1 of the present embodiment includes a sheet-like base material 2, an underlayer 3, a thermal layer 4, an intermediate layer 5, and a top coat layer 6 in this order. In the present embodiment, the base material 2 and the underlayer 3 are laminated so as to be in direct contact. Also, the underlayer 3 and the thermal layer 4 are laminated so as to be in direct contact. Also, the thermal layer 4 and the intermediate layer 5 are laminated so as to be in direct contact. Also, the intermediate layer 5 and the top coat layer 6 are laminated so as to be in direct contact.

[0026] (Base material) In this embodiment, the base material 2 functions as a support of the thermal recording medium 1. As the base material 2, for example, fine paper, art paper, coated paper, kraft paper, papers such as laminated paper obtained by laminating a thermoplastic resin such as polyethylene on these paper bases, synthetic paper, and porous materials such as non-woven fabrics can be used. Further, for example, synthetic resin films such as polypropylene film, polyethylene terephthalate film, polystyrene film, and polycarbonate film can be used. Further, the base material 2 may be a single layer or a multi-layer. The thickness of the base material 2 is not particularly limited, but for example, 5 μm to 150 μm is preferable, and more preferably 10 μm to 100 μm. If the thickness is within the above range, it tends to be excellent in coatability and supportability and is preferable.

[0027] Here, the synthetic paper means a support produced mainly from a synthetic resin. The synthetic paper is composed of a resin, unlike paper composed of fibers such as fine paper, and thus is excellent in water resistance and the like. On the other hand, since the synthetic paper is composed of a resin, its surface is smooth and the wettability of the surface is lower than that of paper. For this reason, it is generally considered more difficult to uniformly coat a coating material for an underlayer containing a granular raw material such as hollow particles on synthetic paper than on fine paper or the like. However, in the case of the thermal recording medium in this embodiment, even when such synthetic paper is used as the base material, excellent coatability can be provided, so that excellent water resistance and coatability can be achieved simultaneously.

[0028] (Underlayer) In this embodiment, the underlayer 3 functions as a layer having functions such as heat insulation and cushioning properties for preventing the dissipation of heat given from the thermal head and / or a layer for enhancing the adhesion between the base material 2 and the thermal layer 4. The underlayer 3 contains at least hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder. The materials other than the above for forming the underlayer 3 are not particularly limited, and for example, fillers, surfactants, and the like can be used.

[0029] The average particle diameter of the above hollow particles is not particularly limited. For example, it is preferably 0.5 μm or more, more preferably 1 μm or more, still more preferably 3 μm or more, and particularly preferably 5 μm or more. Also, the average particle diameter of the above hollow particles is preferably 100 μm or less, more preferably 50 μm or less, and still more preferably 30 μm or less. If it is within such a range, the effect of improving the heat insulation property of the underlayer 3 can be sufficiently obtained, which is preferable from the viewpoint of heat resistance. In this specification, the "average particle diameter" refers to the particle diameter (D50, median diameter) at the integrated value of 50% in the particle size distribution measured by the laser diffraction / scattering method. The measurement of the average particle diameter by the laser diffraction / scattering method can be performed, for example, using the product name "MT3300EX-II" manufactured by Microtrac Bell Corporation. Hereinafter, the "average particle diameter" refers to the median diameter measured by the above method.

[0030] The hollow ratio of the above hollow particles is not particularly limited. For example, it is preferably 50% by volume or more, more preferably 70% by volume or more, still more preferably 80% by volume or more, still more preferably 90% by volume or more, still more preferably 95% by volume or more, and particularly preferably 99% by volume or more. If it is within such a range, the heat insulation property of the underlayer 3 is further improved, and even a color former with high heat resistance can be efficiently colored, so that the heat resistance and sensitivity of the thermal recording medium 1 can be compatible. Furthermore, regarding the sensitizer generally blended in the thermal layer 4 for improving sensitivity, the amount of its use can be reduced.

[0031] Here, the hollow ratio of the hollow particles is calculated by the following formula. Hollow ratio = {(volume of voids) / (volume of hollow particles)} × 100

[0032] In this embodiment, the content ratio of the hollow particles in the underlayer 3 is preferably 5% by mass or more, more preferably 10% by mass or more, still more preferably 15% by mass or more, and particularly preferably 20% by mass or more with respect to 100% by mass of the dry mass of the underlayer 3. Further, the content ratio of the hollow particles in the underlayer 3 is preferably 90% by mass or less, more preferably 80% by mass or less, still more preferably 70% by mass or less, still more preferably 60% by mass or less, still more preferably 50% by mass or less, still more preferably 40% by mass or less, and particularly preferably 30% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3. If the content ratio of the hollow particles in the underlayer 3 is 5% by mass or more, the heat insulation property of the underlayer 3 is further improved, and even a color former with high heat resistance can develop color efficiently, so that the heat resistance and sensitivity of the thermal recording medium 1 can be compatible. Further, if the content ratio of the hollow particles in the underlayer 3 is 90% by mass or less, the adhesion between the base material 2 and the thermal layer 4 can be further improved.

[0033] In this specification, the "dry mass" means the mass of the non-volatile components (solid content) excluding solvents (volatile components) such as water contained in the coating liquid and its raw materials.

[0034] The material constituting the above hollow particles is not particularly limited, and examples thereof include thermoplastic resins. Examples of the thermoplastic resin include acrylic resins such as acrylic resin, acrylonitrile resin, acrylic-polyvinylidene chloride resin, acrylic-polyvinylidene chloride-acrylonitrile resin, acrylic-acrylonitrile resin, styrene-acrylic resin, acrylic-urethane resin, acrylic-amide resin, vinyl acetate-acrylic resin; polyvinylidene chloride resins such as polyvinylidene chloride resin, polyvinylidene chloride-acrylonitrile resin; polystyrene resins; urea-formalin resins; polyolefin resins; polyvinyl chloride resins; polyvinyl acetate resins; polybutadiene resins and the like. Further, these resins may be modified resins modified by known methods. These hollow particles can be used alone or in combination of two or more.

[0035] In this specification, the "acrylic resin" means a resin obtained by polymerizing an acrylic monomer (acrylic resin), or a resin obtained by copolymerizing an acrylic monomer and another monomer (a monomer other than an acrylic monomer copolymerizable with the acrylic monomer). Here, two or more of the above other monomers may be used. Further, when simply described as "acrylic", unless otherwise specified, it means (meth)acrylic acid (salt) and / or (meth)acrylate. Here, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid, and "(meth)acrylic acid (salt)" means (meth)acrylic acid and / or (meth)acrylate salt.

[0036] The salt in the above (meth)acrylate 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 diethylenetriamine salt and diethylenetriamine salt; alkali metal salts such as lithium, sodium, and potassium; alkaline earth metal salts such as magnesium and calcium; polyvalent metal salts such as zinc and iron. These salts can be used alone or in combination of two or more.

[0037] The composition for forming the above hollow particles is not particularly limited, and examples thereof include granular compositions. Generally, from the viewpoint of excellent handleability and dispersibility, hollow particles are often preferably used as an emulsion composition containing a dispersant or the like, but in this embodiment, a granular composition not containing a dispersant or the like can also be preferably used. Therefore, in this embodiment, the hollow particles may be blended as a granular composition with respect to the underlayer 3.

[0038] The content of the above hollow particles is not particularly limited, and examples thereof include water, air, liquid gas, gas, etc. The above liquid gas is typically a low-boiling volatile compound. Note that low-boiling means, for example, having a boiling point of 90°C or lower at normal pressure. Examples of the above low-boiling volatile compounds include isobutane, isopentane, petroleum ether, n-butane, n-pentane, n-hexane, cyclopentane, cyclohexane, trichlorofluoromethane, dichlorofluoromethane, butylene, methylene chloride, etc. Further, the above gas refers to the vaporized form of the above liquid gas. These contents can be used alone or in combination of two or more.

[0039] When the above hollow particles contain a liquid gas and / or a gas as the content, they may be referred to as thermally expandable hollow particles. Thermally expandable hollow particles, when heated, cause the resin forming the outer shell to soften, and further the gas in the content expands, so that they can be obtained as hollow particles with a thin outer shell and a high hollowness ratio. Therefore, from the viewpoint of achieving both heat resistance and sensitivity of the thermal recording medium 1 by further improving the heat insulation property of the underlayer 3, the above hollow particles are preferably thermally expandable hollow particles. Also, from the viewpoint of forming an underlayer 3 with high heat insulation effect and cushioning property, the above thermally expandable hollow particles are more preferably expanded thermally expandable hollow particles.

[0040] The underlayer 3 in the present embodiment may contain fillers other than hollow particles. For example, inorganic fillers such as kaolin, calcined kaolin, aluminum oxide, aluminum silicate, heavy calcium carbonate, light calcium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolinite, diatomaceous earth, white carbon, magnesium carbonate, magnesium oxide, magnesium hydroxide, aluminum hydroxide, zinc oxide; organic fillers such as polystyrene resin particles, urea-formalin resin particles, polyolefin resin particles, etc. These fillers can be used alone or in combination of two or more.

[0041] The structure of the alkylene oxide adduct of the above-mentioned acetylene glycol is not particularly limited as long as it has an acetylene bond in the molecule, and any compound in which an alkylene oxide is added to the glycol (two hydroxyl groups in the molecule) site can be appropriately used. As the alkylene oxide adduct of the above-mentioned acetylene glycol, an ethylene oxide and / or propylene oxide adduct of acetylene glycol is preferable, and an ethylene oxide adduct of acetylene glycol is more preferable.

[0042] As the above-mentioned acetylene glycol, for example, at least one selected from the group consisting of 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol, and 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol is preferable, at least one selected from the group consisting of 2,4,7,9-tetramethyl-5-decyne-4,7-diol and 3,6-dimethyl-4-octyne-3,6-diol is more preferable, and 2,4,7,9-tetramethyl-5-decyne-4,7-diol is particularly preferable. These alkylene oxide adducts of acetylene glycol can be used alone or in combination of two or more.

[0043] The HLB value of the alkylene oxide adduct of the above-mentioned acetylene glycol is not particularly limited. For example, 5 or more is preferable, more preferably 6 or more, and even more preferably 7 or more. Also, the HLB value of the alkylene oxide adduct of acetylene glycol is preferably 14 or less, more preferably 12 or less, even more preferably 10 or less, and particularly preferably 9 or less. If the HLB value is 4 or more, the wettability is more excellent, so the dispersibility of the hollow particles is improved, and as a result, the coating property can be further improved. If the HLB value is 14 or less, the foaming power tends to be further reduced, the foaming during paint preparation and coating is suppressed, and as a result, the coating property can be further improved.

[0044] The number average molecular weight of the alkylene oxide adduct of the above acetylene glycol is not particularly limited. For example, it is preferably 200 or more, more preferably 250 or more, and even more preferably 300 or more. Also, the number average molecular weight of the alkylene oxide adduct of acetylene glycol is preferably 1500 or less, more preferably 1000 or less, and even more preferably 800 or less. If the number average molecular weight is 200 or more, the wettability can be more excellent, so the dispersibility of the hollow particles is improved, and as a result, the coating property can be further improved. If the number average molecular weight is 1500 or less, it is excellent in diffusibility in the paint and can quickly reduce the surface tension, so it tends to be excellent in wettability and / or defoaming property, and as a result, the coating property can be further improved.

[0045] When the structure of the above number average molecular weight is clearly understood, it can be calculated by calculation, but it can also be measured by gel permeation chromatography (GPC). GPC may be measured according to a conventional method. As the measurement conditions, for example, a GPC device (for example, a device "HLC-8020GPC" manufactured by Tosoh Corporation, etc.) is used, and a GPC column (for example, columns "TSKgel SuperHZM-H", "TSKgel SuperHZ4000", "TSKgel SuperHZ200" (all are trade names, 4.6mmID·15cm) manufactured by Tosoh Corporation, etc.) is used, tetrahydrofuran (THF) is used as the eluent, and the set temperature of the column oven is 40°C for measurement. Also, for calculating the number average molecular weight, a known standard substance may be used. For example, it can be calculated using standard polystyrene.

[0046] In this embodiment, the content ratio of the alkylene oxide adduct of acetylene glycol in the underlayer 3 is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and still more preferably 0.2% by mass or more, based on 100% by mass of the dry mass of the underlayer 3. Further, the content ratio of the alkylene oxide adduct of acetylene glycol in the underlayer 3 is preferably 3% by mass or less, more preferably 2% by mass or less, and still more preferably 1.8% by mass or less, based on 100% by mass of the dry mass of the underlayer 3. If the content ratio of the alkylene oxide adduct of acetylene glycol in the underlayer 3 is 0.01% by mass or more, the dispersibility of the hollow particles is further improved, and as a result, the coatability can be further improved. Also, if the content ratio of the alkylene oxide adduct of acetylene glycol in the underlayer 3 is 3% by mass or less, repelling during coating is more suppressed, and as a result, the coatability can be further improved.

[0047] The underlayer 3 in this embodiment may contain a surfactant other than the alkylene oxide adduct of acetylene glycol, and for example, may contain a nonionic surfactant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, a fluorine-based surfactant, etc.

[0048] As the above nonionic surfactant, for example, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ester, polyoxyethylene sorbitan fatty acid ester, etc. can be used.

[0049] As the above anionic surfactant, for example, polyoxyethylene alkyl ether sulfate or its salt, polyoxyethylene alkyl ether acetate or its salt, dodecylbenzenesulfonic acid or its salt, alkyl sulfate or its salt, alkanesulfonic acid or its salt, etc. can be used.

[0050] In this embodiment, the mass ratio of the alkylene oxide adduct of acetylene glycol to the hollow particles in the underlayer 3 (dry mass of the alkylene oxide adduct of acetylene glycol in the underlayer 3 / dry mass of the hollow particles in the underlayer 3) is preferably, for example, 0.005 or more, more preferably 0.007 or more, and even more preferably 0.010 or more. Also, the mass ratio of the alkylene oxide adduct of acetylene glycol to the hollow particles in the underlayer 3 (dry mass of the alkylene oxide adduct of acetylene glycol in the underlayer 3 / dry mass of the hollow particles in the underlayer 3) is preferably, for example, 0.5 or less, more preferably 0.1 or less, and even more preferably 0.075 or less. If the mass ratio is 0.005 or more, the dispersibility of the hollow particles can be further improved, and as a result, the coatability can be further improved. Also, if the mass ratio is 0.5 or less, the addition amount of the alkylene oxide adduct of acetylene glycol becomes a more appropriate range, and repelling during coating can be more suppressed, and as a result, the coatability can be further improved.

[0051] Generally, a block copolymer of ethylene oxide and propylene oxide is a compound that can have excellent dispersibility. However, since it is a compound with a relatively large molecular weight, it tends to take time for adsorption and orientation at the interface, and may inhibit the improvement of wettability and defoaming properties. Therefore, from the viewpoint of improving coatability by improving the wettability and defoaming properties of the paint, the underlayer 3 may not substantially contain a block copolymer of ethylene oxide and propylene oxide. In this specification, "not substantially containing" means not actively blending, and for example, excludes cases where it inevitably exists. Specifically, the content ratio of the block copolymer of ethylene oxide and propylene oxide in the underlayer 3 of this embodiment is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.01% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3.

[0052] The binder is not particularly limited. For example, acrylic resins such as acrylic resin, styrene-acrylic resin, acrylic-urethane resin, acrylic-amide resin, vinyl acetate-acrylic resin; styrene-butadiene (SBR) resin; polyvinyl alcohol resins such as fully saponified polyvinyl alcohol resin and partially saponified polyvinyl alcohol resin can be mentioned. Further, these resins may be modified resins modified by known methods. Among them, from the viewpoint of further improving the water resistance of the thermal recording body 1 and the adhesion between the base material 2 and the thermal layer 4, at least one selected from the group consisting of acrylic resins and SBR is preferable, and acrylic resins are more preferable. Further, as the acrylic resin, at least one selected from the group consisting of acrylic resin, styrene-acrylic resin, acrylic-urethane resin, acrylic-amide resin, and vinyl acetate-acrylic resin is preferable, and at least one selected from the group consisting of acrylic resin, styrene-acrylic resin, and acrylic-urethane resin is more preferable, at least one selected from the group consisting of acrylic resin and styrene-acrylic resin is still more preferable, and styrene-acrylic resin is particularly preferable. The composition forming the binder may be an emulsion or a solution. From the viewpoint of excellent coatability, it is more preferably an emulsion. These binders can be used alone or in combination of two or more.

[0053] Generally, polyvinyl alcohol-based resins tend to be excellent in coatability and dispersibility of other materials and are general-purpose binders. However, due to their high hydrophilicity, they may reduce the water resistance of the thermal recording medium 1. The alkylene oxide adduct of acetylene glycol contained in the underlayer 3 of the present embodiment is excellent in the dispersibility of hollow particles, and thus can provide excellent coatability even without containing the above polyvinyl alcohol-based resin. Therefore, from the viewpoint of obtaining a thermal recording medium 1 excellent in water resistance, the content ratio of the polyvinyl alcohol-based resin in the underlayer 3 is preferably 50% by mass or less, more preferably 40% by mass or less, still more preferably 30% by mass or less, still more preferably 20% by mass or less, and particularly preferably 10% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3. Further, the underlayer 3 may not substantially contain a polyvinyl alcohol-based resin. Specifically, the content ratio of the polyvinyl alcohol-based resin in the underlayer 3 of the present embodiment is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, still more preferably 0.01% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3.

[0054] The glass transition temperature (Tg) of the above binder is not particularly limited. For example, it is preferably -20°C or higher, more preferably -10°C or higher, still more preferably -5°C or higher, and particularly preferably 0°C or higher. Further, the glass transition temperature (Tg) of the above binder is preferably 50°C or lower, more preferably 40°C or lower, and still more preferably 30°C or lower.

[0055] The acid value of the above binder is not particularly limited. However, from the viewpoint of further improving the barrier properties such as the water resistance of the thermal recording medium 1, for example, it is preferably 200 mgKOH / g or less, more preferably 150 mgKOH / g or less, still more preferably 100 mgKOH / g or less, still more preferably 50 mgKOH / g or less, and particularly preferably 30 mgKOH / g or less. Similarly, when an acrylic resin is included as the above binder, from the viewpoint of further improving the barrier properties such as the water resistance of the thermal recording medium 1, the acid value of the above acrylic resin is preferably within the above range. The acid value can be determined by a test method based on, for example, JIS K0070:1992 for the dried dispersant.

[0056] In addition, examples of binders other than the above include starch and its derivatives; cellulose derivatives such as methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose; polyvinylpyrrolidone; styrene-maleic anhydride copolymer; isobutylene-maleic anhydride copolymer; and water-soluble polymers such as alginates, gelatin, and casein.

[0057] In the present embodiment, the content ratio of the binder in the underlayer 3 is preferably 20% by mass or more, more preferably 30% by mass or more, still more preferably 40% by mass or more, still more preferably 50% by mass or more, still more preferably 60% by mass or more, and particularly preferably 70% by mass or more with respect to 100% by mass of the dry mass of the underlayer 3. Also, the content ratio of the binder in the underlayer 3 is preferably 95% by mass or less, more preferably 90% by mass or less, and still more preferably 80% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3. If the content ratio of the binder in the underlayer 3 is 20% by mass or more, the adhesion between the base material 2 and the thermal layer 4 can be further improved. Also, if the content ratio of the binder in the underlayer 3 is 95% by mass or less, the heat insulation property of the underlayer 3 is further improved, and even a color former with high heat resistance can develop color efficiently, so that the heat resistance and sensitivity of the thermal recording medium 1 can be compatible.

[0058] The underlayer 3 of this embodiment may contain other components other than those described above. Examples of the other components include a dispersant, a preservative, and the like. The content ratio of the other components in the underlayer 3 is preferably 30% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, still more preferably 5% by mass or less, and particularly preferably 1% by mass or less with respect to 100% by mass of the dry mass of the underlayer 3.

[0059] Examples of the dispersant include styrene-acrylic copolymers such as styrene-(meth)acrylic acid (salt) copolymer, styrene-(meth)acrylate copolymer, styrene-(meth)acrylic acid (salt)-(meth)acrylate copolymer; styrene-maleic acid (salt) copolymer, styrene-maleic acid ester, styrene-maleic acid (salt)-maleic acid ester copolymer, and the like can be used. In addition, among the resins described as the binder above, resins with a relatively high acid value can be used as a dispersant. This is presumably because a high acid value means having many anionic functional groups such as carboxy groups, and such anionic functional groups generally tend to exhibit dispersibility with respect to other materials. These dispersants can be used alone or in combination of two or more.

[0060] From the viewpoint of further improving the dispersibility of, for example, dyes, developers, etc., the acid value of the above dispersant is preferably more than 100 mgKOH / g, more preferably more than 150 mgKOH / g, still more preferably more than 200 mgKOH / g. In this specification, "more than" means "greater than".

[0061] Since the above dispersant is generally a compound with high hydrophilicity, it may reduce the water resistance of the thermal recording medium 1. Therefore, from the viewpoint of obtaining a thermal recording medium 1 with excellent water resistance, the content ratio of the dispersant in the underlayer 3 is preferably 30% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, still more preferably 5% by mass or less, and particularly preferably 1% by mass or less, based on 100% by mass of the dry mass of the underlayer 3. Further, the underlayer 3 may not substantially contain a dispersant. Specifically, the content ratio of the dispersant in the underlayer 3 of the present embodiment is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and still more preferably 0.01% by mass or less, based on 100% by mass of the dry mass of the underlayer 3.

[0062] Generally, when the underlayer contains hollow particles, a dispersant is often used to disperse the hollow particles in the coating material. In the present embodiment, since it contains an alkylene oxide adduct of acetyleneglycol, the hollow particles can be well dispersed in the coating material without using a dispersant. Therefore, the content ratio of the dispersant in the underlayer 3 may be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on 100% by mass of the dry mass of the underlayer 3. Further, the underlayer 3 may not substantially contain a dispersant. Specifically, the content ratio of the dispersant in the underlayer 3 of the present embodiment may be 0.1% by mass or less, 0.05% by mass or less, or 0.01% by mass or less, based on 100% by mass of the dry mass of the underlayer 3.

[0063] The coating amount (dry mass) of the above underlayer 3 is, for example, preferably 0.3 g / m 2 ~10 g / m 2 and more preferably 0.5 g / m 2 ~5.0 g / m 2 is.

[0064] The thickness of the above underlayer 3 is, for example, preferably 1 μm to 50 μm, and more preferably 5 μm to 40 μm.

[0065] When the coating amount and / or thickness of the underlayer 3 in the present embodiment are within the above ranges, the adhesion between the base material 2 and the thermal layer 4 can be further improved.

[0066] (Thermal layer) In the present embodiment, the thermal layer 4 functions as a layer that develops color when heated by a thermal head or the like. Examples of the material for forming the thermal layer 4 include dyes, developers, preservative improvers, fillers, dispersants, crosslinking agents, lubricants, sensitizers, binders, surfactants, and preservatives.

[0067] Examples of such dyes include 3-dibutylamino-6-methyl-7-anilinofluoran, 2-aniline-3-methyl-6-(N-methyl-p-toluidino)fluoran, 3-(N-isobutyl-N-ethyl)amino-6-methyl-7-anilinofluoran, 3-(N-isopentyl-N-ethyl)amino-6-methyl-7-o-chloroanilinofluoran, 3-(N-methyl-N-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-p-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-N-n-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. These dyes can be used alone or in combination of two or more.

[0068] The average particle diameter of the above dye is preferably 0.1 to 1.0 μm. Since the color former melts and reacts, as the particle diameter increases, the reaction becomes sluggish and the sensitivity characteristics decrease. On the other hand, as the particle diameter decreases, the risk of unexpected color development at a temperature due to the heat when drying the paint increases. In this embodiment, by setting the average particle diameter of the dye within the above range, the sensitivity characteristics and the color development temperature of the dye can be adjusted more appropriately.

[0069] The above color developer is not particularly limited, and known ones can be used. From the perspective of environmental protection, when the use of a color developer having a phenol skeleton (phenolic color developer) is restricted, a color developer having no phenol skeleton (non-phenolic color developer) can be used. If there is no such restriction, a non-phenolic color developer and / or a phenolic color developer can be used. These color developers can be used alone or in combination of two or more.

[0070] Examples of the above non-phenolic color developer include compounds having an N,N'-diarylurea skeleton, that is, N,N'-diarylurea derivatives and the like. As the above N,N'-diarylurea derivative, it is preferably an N,N'-diarylurea derivative having one or more sulfonic acid ester structures (-S(=O) 2 -O-) in the molecule. Further, as the above N,N'-diarylurea derivative, it may be an N,N'-diarylurea derivative having one or more sulfonic acid ester structures in the molecule (excluding urethane urea compounds). Specific examples of the N,N'-diarylurea derivative include compounds represented by the following formula (1) and / or compounds represented by the following formula (2).

Chemical formula

Chemical formula

[0071] As the above-mentioned "substituent", an organic group other than a hydrogen atom can be used without particular limitation. For example, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryl group, an aryloxy group, an alkylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, an arylamino group, etc. can be mentioned.

[0072] Examples of the above-mentioned "halogen atom" include a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom.

[0073] Examples of the above-mentioned "alkyl (group)" include linear or branched alkyl groups having 1 to 12 carbon atoms, such as methyl group, ethyl group, normal propyl group, isopropyl group, normal butyl group, isobutyl group, secondary butyl group, tertiary butyl group, normal pentyl group, isopentyl group, tertiary pentyl group, neopentyl group, 2,3-dimethylpropyl group, 1-ethylpropyl group, 1-methylbutyl group, 2-methylbutyl group, normal hexyl group, isohexyl group, 2-hexyl group, 3-hexyl group, 2-methylpentyl group, 3-methylpentyl group, normal heptyl group, normal octyl group, normal nonyl group, normal decyl, normal undecyl group, normal dodecyl group, etc.

[0074] Examples of the above-mentioned "alkoxy group" include linear or branched alkoxy groups having 1 to 8 carbon atoms, such as methoxy group, ethoxy group, normal propoxy group, isopropoxy group, normal butoxy group, secondary butoxy group, tertiary butoxy group, normal pentyloxy group, isopentyloxy group, tertiary pentyloxy group, neopentyloxy group, 2,3-dimethylpropyloxy group, 1-ethylpropyloxy group, 1-methylbutyloxy group, normal hexyloxy group, isohexyloxy group, normal heptyloxy group, normal octyloxy group, etc.

[0075] Examples of the above-mentioned "aryl (group)" include aromatic hydrocarbon groups having 6 to 10 carbon atoms, such as phenyl group, 1-naphthyl group, 2-naphthyl group, etc.

[0076] In the above-mentioned "dialkylamino group", the two alkyl groups may be the same or different.

[0077] Specific examples of the compound represented by the formula (1) include N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, and the like.

[0078] Specific examples of the compound represented by the formula (2) include 3-[(3-phenylureido)phenyl]-4-methylbenzenesulfonate, and the like.

[0079] Examples of the non-phenolic color former other than those mentioned above include urethane-urea compounds; urea compounds such as 2,2-bis[(4-methyl-3-phenoxycarbonylaminophenyl)urea]diphenylsulfone, 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, 2'-(3-phenylureido)benzenesulfonanilide, N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea. In this specification, the "urethane-urea compound" means a compound having one or more urea bonds (-NH-C(=O)-NH-) and one or more urethane bonds (-NH-C(=O)-O-) in the molecule, and the "urea compound" means a compound having one or more urea bonds in the molecule.

[0080] In the present embodiment, from the viewpoint of further improving color developability and print storage stability, it is preferable that the non-phenolic color developer contains a urethane-urea compound. Further, it is preferable that the non-phenolic color developer contains a urethane-urea compound in addition to an N,N'- diarylurea derivative (excluding urethane-urea compounds) having one or more sulfonic acid ester structures in the molecule. By using two or more non-phenolic color developers in combination as described above, it is possible to impart more excellent color developability and print storage stability to the thermal recording medium 1 than when used alone.

[0081] Examples of the urethane-urea compound include urethane-urea compounds having two or more urea bonds and two or more urethane bonds in the molecule. Specifically, a urethane-urea compound represented by the following formula (3) can be mentioned.

Chemical formula

[0082] As the urea urethane compound represented by the formula (3), from the viewpoint of further improving the color developability and print storage property, the compound represented by the following formula (3a) can be mentioned. Specifically, the compound represented by the following formula (3b) can be mentioned. [Chemical formula] (Each symbol in the formula (3a) is the same as that in the formula (3).) [Chemical formula]

[0083] Since the above thermal layer 4 contains a urea urethane compound, the reaction efficiency between the dye and the developer is increased, an electron transfer complex is easily generated, and a reverse reaction is less likely to occur. Therefore, the color developability of the thermal recording material becomes more excellent, and further the color density is less likely to decrease. Thus, it is considered to be excellent in print storage property, particularly in plasticizer resistance.

[0084] Examples of the phenolic color former include 2,6-bis[(2-hydroxy-5-methylphenyl)methyl]-4-methylphenol, 2,6-bis[(4-hydroxy-2,3,5-trimethylphenyl)methyl]-4-methylphenol, 2,6-bis[(hydroxy-phenyl)methyl]-butylphenol, 1,1-bis(p-hydroxyphenyl)cyclohexane, 1,1-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)propane, 2,2-bis(p-hydroxyphenyl)butane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 2,2'-methylenebis(4-chlorophenol), 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 2,4'-dihydroxydiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, bis(3-allyl-4-hydroxyphenyl) sulfone, poly(4-hydroxybenzoic acid), benzyl 4-hydroxybenzoate, 2,4-bis(phenylsulfonyl)phenol, α-{4-[(4-hydroxyphenyl)sulfonyl]phenyl}-ω-hydroxy poly(degree of polymerization n = 1 to 7)(oxyethylene oxyethylene oxy-p-phenylene sulfonyl-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-toluenesulfonylaminocarbonylamino)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-[[4-(1-methylethoxy)phenyl]sulfonylphenoxy]butoxy]phenyl]sulfonyl]phenol, 4-tert-butylphenol·formaldehyde polycondensate, etc. are mentioned.,

[0085] The average particle diameter of the above color former is preferably 0.1 to 1.0 μm, more preferably 0.3 to 1.0 μm, and still more preferably 0.4 to 1.0 μm. If the average particle diameter is within the above range, the heat resistance of the thermal recording medium 1 can be further improved.,

[0086] Examples of the above storage stability improver include sodium-2,2'-methylenebis(4,6-di-t-butylphenyl)phosphite, 4,4, butylidenebis(3-methyl-6-t-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, tris(2,6-dimethyl-4-t-butyl-3-hydroxybenzyl)isocyanurate, 4-(2-methylglycyloxy)-4'-benzyloxydiphenylsulfone, 2,2’-methylenebis(4-methyl-6-t-butylphenol), 2,2’-methylenebis(4-ethyl-6-t-butylphenol), diethylthiourea, zinc dibutyldithiocarbamate, 4,4’-thiobis(6-t-butyl-m-cresol), etc. These storage stability improvers can be used alone or in combination of two or more. Also, known ultraviolet absorbers, etc. may be added.,

[0087] Examples of the above-mentioned filler include inorganic fillers such as kaolin, calcined kaolin, aluminum oxide, aluminum silicate, heavy calcium carbonate, light calcium carbonate, titanium oxide, barium sulfate, silica gel, activated clay, talc, clay, kaolinite, diatomaceous earth, white carbon, magnesium carbonate, magnesium oxide, magnesium hydroxide, aluminum hydroxide, zinc oxide; and organic fillers such as hollow particles, polystyrene resin particles, urea-formalin resin particles, polyolefin resin particles. These fillers can be used alone or in combination of two or more.

[0088] Examples of the above-mentioned crosslinking agent include organic crosslinking agents such as cationic crosslinking agents and non-cationic crosslinking agents; and inorganic crosslinking agents such as zirconium carbonate. Examples of the above-mentioned cationic crosslinking agent include epichlorohydrin-based resins such as polyamide epichlorohydrin resin, polyamine epichlorohydrin resin, polyamide polyamine epichlorohydrin resin. Examples of the above-mentioned non-cationic crosslinking agent include oxazoline-based compounds such as oxazoline group-containing polymers. These crosslinking agents can be used alone or in combination of two or more.

[0089] Examples of the above-mentioned lubricant include higher fatty acid metal salts such as zinc stearate and calcium stearate, paraffin, oxidized paraffin, polyethylene, oxidized polyethylene. These lubricants can be used alone or in combination of two or more.

[0090] Examples of the above sensitizer include stearic acid amide, methoxycarbonyl-N-benzamyl stearate, N-benzoyl stearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, behenic acid amide, methylene bisstearic acid amide, N-methylol stearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, benzyl p-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, di-p-chlorobenzyl oxalate, di-p-methylbenzyl oxalate, dibenzyl oxalate, p-tolyl biphenyl ether, di(p-methoxyphenoxyethyl) ether, 1,2-di(3-methylphenoxy)ethane, 1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane, 1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane, 1-(4-methoxyphenoxy)-2-(3-methylphenoxy)ethane, p-methylthiophenyl benzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetylacetyl-p-toluidide, di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane, diphenyl sulfone, paraffin, oxidized paraffin, polyethylene, oxidized polyethylene, and the like. These sensitizers can be used alone or in combination of two or more kinds.

[0091] In this embodiment, since the underlayer 3 contains hollow particles and the thermal recording medium 1 has excellent sensitivity, the content ratio of the above sensitizer can be reduced. Therefore, the content ratio of the sensitizer in the thermal layer 4 may be 10% by mass or less, 5% by mass or less, or 1% by mass or less with respect to 100% by mass of the dry mass of the thermal layer 4. Further, the thermal layer 4 may not substantially contain a sensitizer. Specifically, the content ratio of the sensitizer in the thermal layer 4 of this embodiment may be 0.1% by mass or less, 0.05% by mass or less, or 0.01% by mass or less with respect to 100% by mass of the dry mass of the thermal layer 4.

[0092] Examples of the above binder include the same ones as those of the underlayer. These binders can be used alone or in combination of two or more.

[0093] Examples of the above dispersant include the same ones as those of the underlayer. These dispersants can be used alone or in combination of two or more.

[0094] Examples of the above surfactant include the same ones as those of the underlayer. These surfactants can be used alone or in combination of two or more.

[0095] The coating amount (dry mass) of the above thermal layer 4 is, for example, preferably 0.3 g / m 2 ~10 g / m 2 and more preferably 0.5 g / m 2 ~5.0 g / m 2 is.

[0096] The thickness of the above thermal layer 4 is, for example, preferably 0.3 μm to 20 μm, and more preferably 0.5 μm to 5.0 μm.

[0097] When the coating amount and thickness of the thermal layer 4 in this embodiment are within the above ranges, it is preferable from the viewpoint that the color developability and the like of the obtained thermal recording medium 1 are more excellent.

[0098] (Intermediate layer) In this embodiment, by providing the intermediate layer 5 on the thermal layer 4, the barrier properties (such as water resistance, oil resistance, chemical resistance, plasticizer resistance, etc.) of the thermal recording medium 1 are further improved. When the intermediate layer is not required, it may not be provided. The material for forming the intermediate layer 5 is not particularly limited, and for example, it can be formed of a resin.

[0099] The resin is not particularly limited, and for example, the same resins as those mentioned as the binder for the underlayer can be mentioned. These resins can be used alone or in combination of two or more. Further, the intermediate layer 5 may contain other components, for example, it may contain a crosslinking agent, a surfactant, a preservative, etc.

[0100] The resin may be a core-shell type resin in which hydrophobic core particles are coated with a water-soluble shell polymer. As the core-shell type resin, for example, those commercially available under the name of Barrier Star (manufactured by Mitsui Chemicals, Inc.) can be used.

[0101] As the crosslinking agent, for example, the same ones as those for the thermal layer can be mentioned. These crosslinking agents can be used alone or in combination of two or more.

[0102] As the surfactant, for example, the same ones as those for the underlayer can be mentioned. These surfactants can be used alone or in combination of two or more.

[0103] The coating amount (dry mass) of the intermediate layer 5 is, for example, preferably 0.3 g / m 2 ~10 g / m 2 and more preferably 0.5 g / m 2 ~5.0 g / m 2 .

[0104] The thickness of the intermediate layer 5 is, for example, preferably 0.3 μm to 20 μm, and more preferably 0.5 μm to 5.0 μm.

[0105] When the coating amount and thickness of the intermediate layer 5 in the present embodiment are within the above ranges, it is preferable from the viewpoint that the water resistance, chemical resistance, plasticizer resistance, etc. of the obtained thermal recording medium 1 are more excellent.

[0106] (Topcoat layer) In the present embodiment, the topcoat layer 6 functions as a layer that further improves the color development property of the thermal layer 4 by improving the matching property of the thermal recording medium 1 with respect to the thermal head. If the topcoat layer is not necessary, it may not be provided. The material for forming the topcoat layer 6 is not particularly limited, and for example, it can be formed using a binder, a crosslinking agent, a filler, a lubricant, a surfactant, a preservative, etc. Further, the topcoat layer 6 may contain other components.

[0107] Examples of the binder include the same ones as those of the underlayer. These binders can be used alone or in combination of two or more.

[0108] Examples of the crosslinking agent include the same ones as those of the thermal layer. These crosslinking agents can be used alone or in combination of two or more.

[0109] Examples of the filler include colloidal silica, calcium carbonate, zirconium carbonate, polymethyl methacrylate (PMMA), polystyrene (PS), etc. The particle diameter of these fillers is preferably 1.0 μm or less.

[0110] Examples of the surfactant include the same ones as those of the underlayer. These surfactants can be used alone or in combination of two or more.

[0111] The coating amount (dry mass) of the topcoat layer 6 is, for example, preferably 0.3 g / m 2 ~10 g / m 2 and more preferably 0.5 g / m 2 ~5.0 g / m 2 That is.

[0112] The thickness of the top coat layer 6 is, for example, preferably 0.3 μm to 20 μm, more preferably 0.5 μm to 5.0 μm.

[0113] When the coating amount and thickness of the top coat layer 6 in the present embodiment are within the above ranges, it is preferable from the viewpoint that the color developability etc. of the obtained heat-sensitive recording medium 1 are more excellent.

[0114] [Method for manufacturing heat-sensitive recording medium] The method for manufacturing the heat-sensitive recording medium of the present invention is not particularly limited. For example, by dispersing the components contained in the underlayer in a solvent such as water, an underlayer coating material can be prepared by a known or conventional method, and a thermal layer coating material can be prepared by a similar method. Next, the underlayer coating material is applied to the surface of the substrate by a known or conventional method, and then the underlayer can be formed by drying by a known or conventional method. Next, the thermal layer coating material is applied to the surface of the underlayer by a known or conventional method, and then the thermal layer can be formed by drying by a known or conventional method.

[0115] [Step for preparing underlayer coating material] The underlayer coating material of the present embodiment contains a solvent, hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder. In the step for preparing the underlayer coating material, each material may be prepared by previously dispersing all of them in the same solvent. Also, when containing materials that can react with each other and / or materials with low dispersibility, they may be prepared as separate dispersions and then the respective dispersions may be mixed to obtain the underlayer coating material. At that time, other materials may be added to any of the dispersions, some of the dispersions, or all of the dispersions. The method for preparing the underlayer coating material is not particularly limited, and examples include crushing treatments using stirring, ultrasonic treatment, ball mill, bead mill, sand mill, high-pressure homogenizer, etc. These methods can be used alone or in combination of two or more.

[0116] Examples of the solvent include water and organic solvents. Examples of the organic solvent include alcohols such as ethanol, methanol, 1-propanol, 2-propanol, n-butanol, isobutyl alcohol, and t-butanol; ethers such as diethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, acetylacetone, and cyclohexanone; amides such as dimethylformamide, N,N-dimethylacetoacetamide, and N-methylpyrrolidone; and glycols such as ethylene glycol, diethylene glycol, and propylene glycol. The above solvents can be used alone or in combination of two or more. Among them, water is preferred from the viewpoints of safety and ease of handling.

[0117] The composition forming the above material is not particularly limited, and may be, for example, a lump, a granular, an emulsion, a suspension, or a solution. In general, from the viewpoint of excellent handling and dispersibility, any one of emulsion, suspension, and solution compositions can be preferably used. In particular, when hollow particles are generally blended as a granular composition in the underlayer paint, they aggregate in the paint and become lumps, which often leads to deterioration of coating properties, such as causing unevenness and streaks during coating. However, in this embodiment, even if the hollow particles are a granular composition, they can be preferably blended in the underlayer paint. This is thought to be because the alkylene oxide adduct of acetylene glycol can disperse the hollow particles well in the underlayer paint. Specifically, even if the hollow particles, which are a granular composition, are blended as they are without being mixed with a solvent and a dispersant in advance to form a composition such as an emulsion or suspension, an underlayer paint in which the hollow particles are well dispersed can be obtained.

[0118] In this embodiment, the surface tension of the mixture obtained by mixing the above solvent and the alkylene oxide adduct of the above acetylene glycol at the following mass ratio is 45 mN / m or less, preferably 40 mN / m or less, more preferably 35 mN / m or less, and particularly preferably 30 mN / m or less. Further, the surface tension of the mixture obtained by mixing the above solvent and the alkylene oxide adduct of the above acetylene glycol at the following mass ratio is preferably 20 mN / m or more, more preferably 25 mN / m or more, and still more preferably 27 mN / m or more. ·Mass ratio: When the total mass of the above underlayer paint is (100 + X) parts by mass and the mass of the alkylene oxide adduct of the above acetylene glycol in the above underlayer paint is X parts by mass, the solvent is 100 parts by mass and the alkylene oxide adduct of acetylene glycol is X parts by mass.

[0119] If the above surface tension is 45 mN / m or less, the wettability is more excellent, so that the coating property can be further improved. Further, if the above surface tension is 25 mN / m or more, the repellency of the underlayer paint can be more suppressed. In addition, in this specification, "surface tension" means the surface tension at a surface age of 50 ms measured by the bubble pressure method. Such surface tension can be measured by a surface tensiometer (manufactured by KRUSS, equipment name: BP2).

[0120] (Coating process of underlayer paint) In the coating process of the underlayer paint, as methods for coating the underlayer paint obtained above, there are methods such as directly coating on the substrate, or coating on a release liner or the like and then transferring it to the substrate. The coating method is not particularly limited, and examples include air knife coating, bar blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, die coating, gravure coating, etc. Also, manual coating using a wire bar may be used. These methods can be used alone or in combination of two or more. The line speed during coating is not particularly limited, and for example, 75 m / min or less is preferable, and 60 m / min or less is more preferable.

[0121] (Drying process of the underlayer paint) The method for drying the underlayer paint coated above is not particularly limited, and examples include heat drying, room temperature drying, vacuum drying, etc. By drying the paint coated by these methods, an underlayer can be formed. These methods can be used alone or in combination of two or more.

[0122] (Preparation process of the thermal layer paint) The preparation process of the thermal layer paint can employ the above-mentioned preparation process of the underlayer paint. Among them, it is preferable to prepare a color former and a developer that can react with each other as separate dispersions and then mix the two to obtain the thermal layer paint. At that time, other components may be added to either the dispersion containing the dye or the dispersion containing the developer, or to both.

[0123] (Coating process of the thermal layer paint) The coating process of the thermal layer paint can employ the above-mentioned coating process of the underlayer paint.

[0124] (Drying process of the thermal layer paint) The drying process of the thermal layer paint can employ the above-mentioned drying process of the underlayer paint.

[0125] When the heat-sensitive recording medium of the present invention includes a substrate, an underlayer, and other layers other than the thermal layer, the method for forming the other layers may incorporate the above content. For example, as shown in FIG. 1, as a method for manufacturing the heat-sensitive recording medium 1 including a substrate 2, an underlayer 3, a thermal layer 4, an intermediate layer 5, and a top coat layer 6 in this order, for the laminate in which the substrate 2, the underlayer 3, and the thermal layer 4 obtained in the above steps are laminated in this order, an intermediate layer 5 is further formed on the thermal layer 4, and then a top coat layer 6 is formed on the intermediate layer 5, whereby it can be manufactured. As a method for preparing the paint for the intermediate layer and the paint for the top coat layer, the paint may be prepared for each layer. For example, all the materials contained in the layer may be dispersed in the same solvent in advance for preparation, or when it contains components that react with each other, each material may be prepared as a separate dispersion and then the two are mixed and prepared. The above preparation method is not particularly limited, and examples include crushing treatment using stirring, ultrasonic treatment, ball mill, bead mill, sand mill, high-pressure homogenizer, etc. These methods can be used alone or in combination of two or more. As a method for coating the paint for the intermediate layer obtained above on the thermal layer 4, for example, a method of directly coating on the thermal layer 4 can be mentioned. The method for coating the paint for the intermediate layer is not particularly limited, and examples include air knife coating, bar blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, die coating, gravure coating, etc. Also, manual coating using a wire bar may be used. These methods can be used alone or in combination of two or more. Next, the intermediate layer 5 can be formed on the thermal layer 4 by drying the paint for the intermediate layer coated above. Examples of the above drying method include heat drying, normal temperature drying, vacuum drying, etc. These methods can be used alone or in combination of two or more. Next, the top coat layer is formed on the intermediate layer 5 in the same procedure as above, whereby the heat-sensitive recording medium 1 shown in FIG. 1 can be obtained.

[0126] In the method for manufacturing the thermosensitive recording material of the present invention, each layer may be simultaneously coated in multiple layers using, for example, a curtain coater or the like, or may be sequentially formed individually. Further, some layers may be simultaneously coated and some layers may be sequentially formed individually.

Example

[0127] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by these examples and is limited only by the description in the claims.

[0128] A thermosensitive recording material 1 in which an underlayer 3, a thermal layer 4, an intermediate layer 5, and a top coat layer 6 are laminated in this order on a substrate 2 was produced by the following steps. (Example 1) (Production of thermosensitive recording material) <Underlayer> According to the following composition, a coating material for the underlayer was prepared by a conventional method. Next, using a wire bar set at a line speed of 60 m / min, the above coating material for the underlayer was coated on a synthetic paper (material: polypropylene, thickness: 90 μm) as the substrate 2 by a conventional method and dried, whereby the coating amount was 3.0 g / m as the dry mass 2 and an underlayer 3 having a thickness of 10.0 μm was formed. ·Composition of the coating material for the underlayer Water as a solvent: 80.05 parts by mass Hollow particles (porosity: 99% by volume, material: acrylonitrile resin, average particle diameter: 10 μm, thermally expandable hollow particles, expanded ones): 4.75 parts by mass Styrene-acrylic resin as a binder (acid value: 100 mgKOH / g or less): 15.2 parts by mass Ethylene oxide adduct of acetylene glycol (HLB value: 8): 0.05 parts by mass

[0129] [Evaluation of coatability] In the evaluation of the coating property, the surface state when the above-mentioned underlayer paint was applied onto the above-mentioned synthetic paper was visually observed and evaluated based on the following criteria. In this specification, "repellency of the paint" means a state in which the applied paint bounces off the coating surface, resulting in coating unevenness. The above evaluation results are shown in Table 1 described later. <Evaluation Criteria for Coating Property of Underlayer> ○: No unevenness due to aggregation of hollow particles, coating streaks, or repellency of the paint occurs, and the coating property is very good. △: Repellency of the paint occurs, but no unevenness due to aggregation of hollow particles or coating streaks occurs, and the coating property is good. ×: Unevenness due to aggregation of hollow particles and coating streaks occur, and the coating property is poor.

[0130] [Evaluation of Dispersibility (Floating Height of Hollow Particles)] In the evaluation of dispersibility, among the materials formulated in the above-mentioned underlayer paint, the above solvent, the above hollow particles, and the ethylene oxide adduct of the above acetylene glycol were mixed at the following mass ratios to prepare a mixture. Next, 25 g of the above mixture was added to a cylindrical glass container (inner diameter: 15 mm), the upper part of the container was sealed with a rubber stopper, and then the contents were uniformly mixed by shaking and allowed to stand for 3 minutes. Thereafter, the floating height of the hollow particles was visually measured. · Composition of the mixture for dispersibility evaluation (preparation of a diluted solution with a solid content of 0.25%) Water as the solvent: 99.75 parts by mass Hollow particles: 0.25 parts by mass Ethylene oxide adduct of acetylene glycol: 0.05 parts by mass

[0131] In this specification, the "floating height of hollow particles" means the distance between the hollow particle at the highest position in the liquid and the hollow particle at the lowest position among the hollow particles floating in the liquid. For example, when the dispersibility of the hollow particles is low, the hollow particles aggregate densely into lumps and float, so the distance between the hollow particle at the highest position in the liquid and the hollow particle at the lowest position becomes small, and the value of the floating height of the hollow particles tends to be small. On the other hand, when the dispersibility of the hollow particles is high, the hollow particles are dispersed in the liquid, so the value of the floating height of the hollow particles tends to be large. The above evaluation results are shown in Table 1 described later. In addition, in the evaluation of the above dispersibility and the evaluation of the surface tension described later, the same ethylene oxide adduct of acetylene glycol contained in the underlayer was used. That is, in the examples described later, when different alkylene oxide adducts of acetylene glycol were used, the dispersibility and the surface tension were evaluated using the alkylene oxide adduct of acetylene glycol used in that example.

[0132] [Evaluation of Surface Tension] In the evaluation of surface tension, among the materials formulated in the underlayer paint, the above solvent and the ethylene oxide adduct of acetylene glycol were mixed at the following mass ratio to prepare a mixture. Next, the surface tension when the surface age of the above mixture was 50 ms was measured with a surface tensiometer (manufactured by KRUSS, instrument name: BP2). · Composition of the mixture for surface tension measurement Water as a solvent: 100 parts by mass Ethylene oxide adduct of acetylene glycol: 0.05 parts by mass

[0133] <Thermal layer> According to the following composition, a developer dispersion liquid, Agent A, was prepared by a conventional method. Then, Agent B containing a part of the above Agent A and other materials was prepared. Then, Agent B and all the remaining materials were mixed to prepare a thermal layer paint. Next, it was coated on the above underlayer 3 by a conventional method and dried so that the coating amount was 4.0 g / m as the dry mass. 2A thermal layer 4 with a thickness of 4 μm was formed. Note that the numerical values of each of the following compounding materials indicate the mass ratio during drying, unless otherwise specified. ·Composition of Agent A 3-[(3-Phenylureido)phenyl]-4-methylbenzenesulfonate (average particle diameter: 0.6 μm) as a color former: 90.2% by mass Acrylic resin (acid value: 240 mgKOH / g) as a dispersant: 9.0% by mass Ethylene oxide adduct of acetylene glycol: 0.1% by mass Mineral oil-based surfactant: 0.3% by mass Carboxylic acid-based copolymer as a thickener: 0.4% by mass Water as a solvent: appropriate amount (the amount that makes the solid content concentration of Agent A approximately 39% by mass) ·Composition of Agent B Agent A: 50.8% by mass Hollow particles: 13.2% by mass Urea urethane compound as a color former: 13.7% by mass SBR emulsion as a binder: 20.5% by mass Polyethylene wax emulsion (average particle diameter: 1.0 μm): 1.8% by mass Water as a solvent: appropriate amount (the amount that makes the solid content concentration of Agent B approximately 36% by mass) ·Composition of the thermal layer paint Agent B: 84.6% by mass 3-Dibutylamino-6-methyl-7-anilinofluoran as a dye: 14.0% by mass Polyamide epichlorohydrin-based resin as a crosslinking agent: 1.0% by mass Ethylene oxide adduct of acetylene glycol: 0.4% by mass Water as a solvent: appropriate amount (the amount that makes the solid content concentration of the thermal layer paint approximately 27% by mass)

[0134] <Intermediate layer> According to the following composition, water was used as a solvent, and an intermediate layer paint was prepared by a conventional method. Then, it was coated on the above thermal layer 4 by a conventional method and dried, so that the coating amount was 1.6 g / m as the dry mass 2and an intermediate layer 5 with a thickness of 1.6 μm was formed. The numerical values of each of the following compounding materials indicate the mass ratio during drying. · Composition of the paint for the intermediate layer Core-shell type acrylic resin: 93.9% by mass Polyamide epichlorohydrin resin: 5.6% by mass Ethylene oxide adduct of acetylene glycol: 0.5% by mass

[0135] <Top coat layer> According to the following composition, water was used as a solvent, and a paint for the top coat layer was prepared by a conventional method. Then, it was applied onto the above intermediate layer 5 by a conventional method and dried, so that the coating amount was 1.6 g / m as the dry mass 2 and a top coat layer 6 with a thickness of 1.6 μm was formed, and the heat-sensitive recording medium of Example 1 was produced. The numerical values of each of the following compounding materials indicate the mass ratio during drying. · Composition of the paint for the top coat layer Non-core-shell type acrylic resin: 40.2% by mass Calcium carbonate: 30.6% by mass Polymethyl methacrylate (PMMA): 6.8% by mass Zinc stearate wax: 11.1% by mass Polyethylene wax emulsion (average particle diameter: 1.0 μm): 4.9% by mass Ammonium zirconium carbonate: 4.8% by mass Surfactant with an alkylene oxide adduct of acetylene glycol formulation: 1.6% by mass

[0136] (Comparative Example 1) In the formation process of the underlayer, the evaluation of dispersibility, and the evaluation of surface tension, except that the alkylene oxide adduct of acetylene glycol was changed to not be used, a heat-sensitive recording medium 1 of Comparative Example 1 was produced in the same manner as in Example 1 above and subjected to each evaluation. The evaluation results at this time are shown in Tables 1 and 2.

[0137] (Examples 2 to 7) In the preparation process of the underlayer coating, the evaluation of dispersibility, and the evaluation of surface tension, except that the compounding amount of the ethylene oxide adduct of acetylene glycol was changed from 0.05 parts by mass to 0.10 parts by mass in Example 2, 0.15 parts by mass in Example 3, 0.20 parts by mass in Example 4, 0.25 parts by mass in Example 5, 0.30 parts by mass in Example 6, and 0.40 parts by mass in Example 7, respectively, the heat-sensitive recording bodies 1 of Examples 2 to 7 were produced in the same manner as in Example 1 above and subjected to each evaluation. The evaluation results at this time are shown in Table 1.

[0138]

Table 1

[0139] (Example 8) In the formation process of the underlayer, the evaluation of dispersibility, and the evaluation of surface tension, except that the ethylene oxide adduct of acetylene glycol (HLB value: 8) was changed to the ethylene oxide adduct of acetylene glycol (HLB value: 7 - 9), the heat-sensitive recording body 1 of Example 8 was produced in the same manner as in Example 1 above and subjected to each evaluation. The evaluation results at this time are shown in Table 2.

[0140] (Examples 9 - 14) In the preparation process of the underlayer coating, the evaluation of dispersibility, and the evaluation of surface tension, except that the compounding amount of the ethylene oxide adduct of acetylene glycol was changed from 0.05 parts by mass to 0.10 parts by mass in Example 2, 0.15 parts by mass in Example 3, 0.20 parts by mass in Example 4, 0.25 parts by mass in Example 5, 0.30 parts by mass in Example 6, and 0.40 parts by mass in Example 7, respectively, the heat-sensitive recording bodies 1 of Examples 9 to 14 were produced in the same manner as in Example 8 above and subjected to each evaluation. The evaluation results at this time are shown in Table 2.

[0141]

Table 2

[0142] As shown in Table 1, when the ethylene oxide adduct of acetylene glycol was not incorporated into the undercoat paint (Comparative Example 1), the floating height of the hollow particles in the evaluation of dispersibility was low, indicating that the dispersibility of the hollow particles was low. Therefore, when the undercoat paint was applied to the substrate 2, aggregated hollow particles were visually observed as irregularities, and thus the coatability was determined to be poor. On the other hand, when the ethylene oxide adduct of acetylene glycol (HLB value: 8) was used (Examples 1 to 7), a correlation was found between the content ratio of the ethylene oxide adduct of acetylene glycol and the dispersibility and surface tension. At this time, the coatability of the undercoat paint was good or very good. Among them, in Examples 1 to 6, in addition to suppressing the aggregation of the hollow particles, the repellency of the paint was also suppressed, and it was evaluated as having particularly excellent coatability.

[0143] As shown in Table 2, the same tendency as in Table 1 was also observed when the ethylene oxide adduct of acetylene glycol (HLB value: 7 to 9) was used. That is, in the present embodiment, it was confirmed that by containing the alkylene oxide adduct of acetylene glycol in the undercoat paint, the aggregation of the hollow particles was suppressed, and good coatability was also exhibited with respect to the synthetic paper. Among them, it was confirmed that when the surface tension of the undercoat paint was 27 mN / m or more, the repellency of the paint was more suppressed, and the coatability with respect to the synthetic paper was more excellent.

[0144] As a summary above, the configuration of the present invention and its variations are appended below. [Appendix 1] A thermal recording medium including a substrate, an undercoat layer, and a thermal layer in this order, wherein the undercoat layer contains hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder. [Appendix 2] The thermal recording medium according to Appendix 1, wherein the HLB value of the alkylene oxide adduct of acetylene glycol is 5 or more. [Appendix 3] The thermal recording medium according to Appendix 1 or 2, wherein the thermal layer contains an N,N'- diarylurea derivative. [Supplementary Note 4] The heat-sensitive recording material according to Supplementary Note 3, wherein the N, N'- diarylurea derivative is 3 - [(3 - phenylureido)phenyl] - 4 - methylbenzenesulfonate. [Supplementary Note 5] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 4, wherein the base material is synthetic paper. [Supplementary Note 6] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 5, wherein the content ratio of the polyvinyl alcohol-based resin in the underlayer is 50% by mass or less with respect to 100% by mass of the dry mass of the underlayer. [Supplementary Note 7] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 6, wherein the underlayer substantially does not contain a block copolymer of ethylene oxide and propylene oxide. [Supplementary Note 8] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 7, wherein the average particle diameter of the hollow particles is 5 μm or more and 50 μm or less. [Supplementary Note 9] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 8, wherein the mass ratio of the alkylene oxide adduct of acetylene glycol to the hollow particles in the underlayer (dry mass of the alkylene oxide adduct of acetylene glycol in the underlayer / dry mass of the hollow particles in the underlayer) is 0.005 or more and 0.5 or less. [Supplementary Note 10] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 9, wherein the binder is at least one selected from the group consisting of an acrylic resin and SBR. [Supplementary Note 11] The heat-sensitive recording material according to any one of Supplementary Notes 1 to 10, wherein the acid value of the binder is 100 mgKOH / g or less. [Supplementary Note 12] An underlayer coating material for forming the underlayer in a heat-sensitive recording material including a base material, an underlayer, and a thermal layer in this order, the underlayer coating material containing a solvent, hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder, and the surface tension of a mixture obtained by mixing the solvent and the alkylene oxide adduct of acetylene glycol at the following mass ratio is 45 mN / m or less. Underlayer coating material. ·Mass ratio: When the total mass of the paint for the underlayer is (100 + X) parts by mass, and the mass of the alkylene oxide adduct of acetylene glycol in the paint for the underlayer is X parts by mass, the solvent is 100 parts by mass and the alkylene oxide adduct of acetylene glycol is X parts by mass. [Appendix 13] The hollow particles are incorporated as a granular composition into the paint for the underlayer described in Appendix 12.

Explanation of reference numerals

[0145] 1 Thermal recording medium 2 Substrate 3 Underlayer 4 Thermal layer 5 Intermediate layer 6 Topcoat layer

Claims

1. A heat-sensitive recording medium comprising a base material, an underlayer, and a thermal layer in this order, wherein the underlayer contains hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder, the heat-sensitive recording medium.

2. The heat-sensitive recording medium according to claim 1, wherein the HLB value of the alkylene oxide adduct of acetylene glycol is 5 or more.

3. The heat-sensitive recording medium according to claim 1 or 2, wherein the thermal layer contains an N,N'- diarylurea derivative.

4. The heat-sensitive recording medium according to claim 3, wherein the N,N'- diarylurea derivative is 3-[(3-phenylureido)phenyl]-4-methylbenzenesulfonate.

5. The heat-sensitive recording medium according to claim 1 or 2, wherein the base material is synthetic paper.

6. The heat-sensitive recording medium according to claim 1 or 2, wherein the content ratio of the polyvinyl alcohol-based resin in the underlayer is 50% by mass or less based on 100% by mass of the dry mass of the underlayer.

7. The heat-sensitive recording medium according to claim 1 or 2, wherein the underlayer does not substantially contain a block copolymer of ethylene oxide and propylene oxide.

8. An underlayer coating material for forming the underlayer in a heat-sensitive recording medium comprising a base material, an underlayer, and a thermal layer in this order, wherein the underlayer coating material contains a solvent, hollow particles, an alkylene oxide adduct of acetylene glycol, and a binder, and the surface tension of a mixture obtained by mixing the solvent and the alkylene oxide adduct of acetylene glycol at the following mass ratio is 45 mN / m or less, the underlayer coating material. ・Mass ratio: When the total mass of the underlayer coating material is (100 + X) parts by mass and the mass of the alkylene oxide adduct of acetylene glycol in the underlayer coating material is X parts by mass, the solvent is 100 parts by mass and the alkylene oxide adduct of acetylene glycol is X parts by mass.

9. The underlayer coating material according to claim 8, wherein the hollow particles are blended as a granular composition.

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