JP2015071268A - Thermosensitive recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermosensitive recording medium that is excellent in recorded color development properties and recorded image quality and is excellent in recording performance after long-term storage after it is processed into a thermosensitive recording label.SOLUTION: In a thermosensitive recording medium having an undercoat layer containing a pigment and a binder on a paper substrate and a thermosensitive recording layer containing a leuco dye and a coloring agent on the undercoat layer, at least one kind selected from an ammonium salt of a styrene/maleic anhydride copolymer and an ammonium salt of a styrene/acrylic acid copolymer as a binder contained in the undercoat layer is contained in the range of not less than 0.5 pt.mass to less than 5 pts.mass based on 100 pts.mass of the pigment contained in the undercoat layer.

Description

  The present invention relates to a heat-sensitive recording material utilizing a color development reaction between a leuco dye and a colorant.

  In general, a heat-sensitive recording medium is usually a colorless or light-colored electron-donating leuco dye and an electron-accepting colorant such as a phenolic compound are ground and dispersed into fine particles, and then mixed together to form a binder, a filling A coating liquid obtained by adding an agent, a sensitizer, a lubricant and other auxiliary agents is coated on a sheet-like support such as paper, synthetic paper, or a plastic film, a thermal head, Color is generated by an instantaneous chemical reaction by heating with a hot stamp, a thermal pen, laser light or the like, and a recorded image is obtained. Thermosensitive recording media are used in a wide range such as facsimiles, computer terminal printers, automatic ticket vending machines, and measurement recorders. In recent years, the use of labels with adhesive processing on the back surface has been increasing, and POS labels and process control labels have a long period of time from adhesive processing to use, and are applied to the back surface during storage. The plasticizer or emulsifier contained in the ink passes through the support and reaches the heat-sensitive recording surface, causing problems such as remarkably lowering recording performance such as recording color development and recording image quality, or causing white spots in printing. .

  In order to solve the above problem, a barrier layer containing polyvinyl alcohol and a pigment having a high saponification degree and a pigment is provided between the support and the pressure-sensitive adhesive layer (see Patent Document 1), and a styrene-butadiene copolymer is provided on the back of the support. A back coat layer containing a polymer and a pigment is provided (see Patent Document 2), and a back layer containing a water-soluble acrylic resin as a main component is provided on the side of the support opposite to the thermosensitive recording layer (patent) Reference 3), and a back coat layer containing a styrene-acrylic resin having a glass transition point of 10 ° C. or less and a 0.01% by weight aqueous solution having a transmittance at 280 nm of 2.0% or less (see Patent Document 4) ) Etc. have been proposed, but at present, satisfactory results have not been obtained.

Japanese Patent Laid-Open No. 5-4450 JP-A-9-150576 JP 2004-284089 A JP 2009-255309 A

  The main object of the present invention is to provide a heat-sensitive recording material excellent in recording color development and recording image quality and excellent in recording performance even after long-term storage after processing into a heat-sensitive recording label.

  As a result of intensive studies, the inventors have made the undercoat layer contain a specific amount of at least one selected from an ammonium salt of a styrene-maleic anhydride copolymer and an ammonium salt of a styrene-acrylic acid copolymer. The inventors have found that the above problems can be solved, and have completed the present invention. That is, the present invention relates to the following thermal recording material.

  Item 1: A heat-sensitive recording material comprising a paper support on which an undercoat layer containing a pigment and a binder and a heat-sensitive recording layer containing a leuco dye and a colorant on the undercoat layer are contained in the undercoat layer. As a binder, at least one selected from an ammonium salt of a styrene-maleic anhydride copolymer and an ammonium salt of a styrene-acrylic acid copolymer is used in an amount of 0.5 mass relative to 100 parts by mass of the pigment contained in the undercoat layer. A heat-sensitive recording material characterized by containing in an amount of not less than 5 parts and less than 5 parts by mass.

  Item 2: The content of at least one selected from an ammonium salt of the styrene-maleic anhydride copolymer and an ammonium salt of the styrene-acrylic acid copolymer with respect to 100 parts by mass of the pigment contained in the undercoat layer Item 2. The heat-sensitive recording material according to Item 1, which is 1 to 4 parts by mass.

  Item 3: The heat-sensitive recording material according to Item 1 or 2, further comprising a protective layer containing a pigment and a binder on the heat-sensitive recording layer.

  The heat-sensitive recording material of the present invention is excellent in recording color development and recording image quality, and excellent in recording performance even after long-term storage after processing into a heat-sensitive recording label.

  In the present invention, a specific amount of at least one selected from an ammonium salt of a styrene-maleic anhydride copolymer and an ammonium salt of a styrene-acrylic acid copolymer is contained as a specific binder in the undercoat layer. By providing such an undercoat layer, excellent recording color developability is obtained, and even after processing into a thermosensitive recording label, it prevents the penetration of the plasticizer, emulsifier, etc. contained in the adhesive layer into the thermosensitive recording layer, This is considered to suppress a decrease in recording performance after long-term storage. Moreover, since it is an ammonium salt, there is no fear that the thermal head will cause electrical corrosion due to sodium salt or the like. Furthermore, if the binder is an emulsion type, the binder bleeds upward during the drying of the undercoat layer, and it is difficult for the pigment to sink to the paper support side to form a uniform layer, which may impair the resistance to the adhesive layer. However, since the specific binder in the present invention is a solution type, it can form a relatively uniform layer and exhibit barrier properties.

  The content of at least one selected from an ammonium salt of a styrene-maleic anhydride copolymer and an ammonium salt of a styrene-acrylic acid copolymer is 0.5 with respect to 100 parts by mass of the pigment contained in the undercoat layer. It is in the range of not less than 5 parts by mass. More preferably, it is 1-4 mass parts. If the amount is less than 0.5 parts by mass, the recording colorability and the recording image quality after the thermal recording label processing is deteriorated. When the amount is 5 parts by mass or more, the recording color developability is lowered, and the sensitivity of the heat-sensitive recording layer may be lowered.

  In the undercoat layer of the present invention, other binders can be used in combination as long as the effects of the present invention are not impaired. Examples of other binders include polyvinyl alcohol, modified polyvinyl alcohol, starch, modified starch, starch-vinyl acetate graft copolymer, casein, gelatin, polyacrylamide, polyamide, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, and styrene-maleic anhydride. Examples thereof include a sodium salt of a copolymer, a sodium salt of a styrene-acrylic acid copolymer, a styrene-butadiene latex, a polyurethane latex, and an acrylic latex. Although the total content rate of a binder is not specifically limited, About 5-30 mass% is preferable in the total solid of a subbing layer, and about 10-20 mass% is more preferable.

  The pigment contained in the undercoat layer is not particularly limited, but oil-absorbing pigments and / or organic hollow particles and / or thermally expandable particles having an oil absorption of 70 ml / 100 g or more, particularly about 80 to 150 ml / 100 g are preferable. Here, the oil absorption is a value determined according to the method of JIS K 5101. Various oil-absorbing pigments can be used, and specific examples include inorganic pigments such as calcined kaolin, amorphous silica, light calcium carbonate, and talc. The average particle diameter of the primary particles of these oil-absorbing pigments is preferably about 0.01 to 5 μm, particularly about 0.02 to 3 μm. The content of the oil-absorbing pigment is not particularly limited, but is preferably about 2 to 95% by mass and more preferably about 5 to 90% by mass in the total solid content of the undercoat layer.

  Examples of the organic hollow particles include conventionally known particles, for example, particles having a hollow ratio of about 50 to 99% in which the film material is made of an acrylic resin, a styrene resin, a vinylidene chloride resin, or the like. Here, the hollowness is a value obtained by (d / D) × 100. In the formula, d represents the inner diameter of the organic hollow particles, and D represents the outer shape of the organic hollow particles. The average particle diameter of the organic hollow particles is preferably about 0.5 to 10 μm, more preferably about 1 to 3 μm. Although the content rate of the said organic hollow particle is not specifically limited, About 2-90 mass% is preferable in the total solid of a subbing layer, and about 5-70 mass% is more preferable.

  When the oil absorbing pigment and the organic hollow particles are used in combination, the oil absorbing pigment and the organic hollow particles are used within the above content ratio, and the total amount of the oil absorbing pigment and the organic hollow particles is the total solid content of the undercoat layer. The amount is preferably about 5 to 90% by weight, more preferably about 10 to 80% by weight.

The undercoat layer can be formed, for example, by applying and drying an undercoat layer coating solution containing water, a pigment, a specific binder, and various additives as necessary, on a paper support. The coating amount of the undercoat layer is not particularly limited, but is preferably adjusted in the range of ^{2 to} 30 g / m ² , more preferably 4 to 15 g / m ² in terms of dry weight.

  The leuco dye used in the heat-sensitive recording layer of the present invention is not particularly limited as long as it is used for general heat-sensitive recording paper. Specific examples of the leuco dye include, for example, (1) triarylmethane compounds such as 3,3-bis (P-dimethylaminophenyl) -6-dimethylaminophthalide. (2) Examples of diphenylmethane compounds include 4,4'-bis-dimethylaminobenzhydrin benzyl ether and N-2,4,5-trichlorophenylleucooramine. (3) Examples of xanthene compounds include rhodamine B-anilinolactam, 3-diethylamino-7-dibenzylaminofluorane, 3-diethylamino-7-butylaminofluorane, 3-diethylamino-7- (2-chloroanilino). Fluorane, 3-piperidino-6-methyl-7-anilinofluorane, 3-ethyl-tolylamino-6-methyl-7-anilinofluorane, 3-cyclohexyl-methylamino-6-methyl-7-anilinofluor Oran, 3-diethylamino-6-chloro-7- (β-ethoxyethyl) aminofluorane, 3-diethylamino-6-chloro-7- (γ-chloropropyl) aminofluorane, 3-di (n-butyl) Amino-6-methyl-7-anilinofluorane, 3-diethylamino-6-methyl-7-ani Linofluorane, 3- (N-isoamyl-N-ethylamino) -6-methyl-7-anilinofluorane, 3-ethyl-isoamylamino-6-methyl-7-anilinofluorane, 3-dibutylamino-7 -Chloroanilinofluorane and the like. These leuco dyes can be used alone or in combination of two or more, and are appropriately selected and used depending on the use of the thermal recording material and desired properties. The content ratio of the leuco dye may be appropriately selected depending on the colorant to be used, but is preferably about 3 to 50% by mass, particularly about 5 to 40% by mass, based on the total solid content of the heat-sensitive recording layer. preferable.

  Examples of the colorant include 4,4′-isopropylidenediphenol, 4,4′-cyclohexylidenediphenyl, 1,1-bis (4-hydroxyphenyl) -ethane, 1,1-bis (4-hydroxy). Phenyl) -1-phenylethane, 4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone, 4-hydroxy-4′-isopropoxydiphenylsulfone, 4-hydroxy-4′-n-propoxydiphenylsulfone 4-hydroxy-4′-allyloxydiphenylsulfone, 3,3′-diallyl-4,4′-dihydroxydiphenylsulfone, 2,2′-bis [4- (4-hydroxyphenyl) phenoxy] diethyl ether, N -P-Toluenesulfonyl-N'-3- (p-toluenesulfonyloxy Phenylurea, 4-hydroxybenzoic acid benzyl ester, N, N′-di-m-chlorophenylthiourea, Np-tolylsulfonyl-N′-phenylurea, 4,4′-bis (p-tolylsulfonylaminocarbonyl) Amino) diphenylmethane, 4- [2- (p-methoxyphenoxy) ethyloxy] salicylic acid zinc, 4- {3- (p-tolylsulfonyl) propyloxy] salicylic acid zinc, 5- [p- (2-p-methoxyphenoxyethoxy) ) Cumyl] zinc salicylate and the like.

  The content of the colorant may be appropriately selected depending on the leuco dye used, but generally 1 to 10 parts by weight, more preferably 1.5 to 5 parts by weight with respect to 1 part by weight of the leuco dye. It is.

  These leuco dyes and colorants generally use water as a dispersion medium, and by various wet pulverizers such as a ball mill, a coball mill, an attritor, a vertical or horizontal sand mill, polyacrylamide, polyvinyl pyrrolidone, polyvinyl alcohol, methyl cellulose, And a water-soluble synthetic polymer compound such as a styrene-maleic anhydride copolymer salt or the like and other surfactants to form a dispersion, which is then used to prepare a thermal recording layer coating solution.

  Other materials that make up the heat-sensitive recording layer include binders, sensitizers, inorganic or organic pigments, waxes, metal soaps, and UV absorbers, preservatives, fluorescent dyes, and coloring dyes as necessary. Can be mentioned. Water-soluble binders are generally used, and polyvinyl alcohol, modified polyvinyl alcohol, polyamide, styrene-acrylic acid copolymer salt, styrene-maleic anhydride copolymer salt, ethylene-maleic anhydride copolymer salt Water-soluble synthetic polymer compounds such as diisobutylene-maleic anhydride copolymer salts and the like, and water-soluble natural polymer compounds such as methylcellulose, other cellulose derivatives, starch and derivatives thereof, gelatin, and casein, or the like Derivatives. In addition, a water resistance-imparting agent may be added for the purpose of imparting water resistance to these binders, or a synthetic polymer emulsion, specifically SBR latex, acrylic resin emulsion, or the like may be added.

  The heat-sensitive recording layer may contain a storability improving agent. Thereby, the storage stability of a recording part can be improved. Specific examples of such preservability improvers include, for example, 2,2′-ethylidenebis (4,6-di-tert-butylphenol), 4,4′-thiobis (2-methyl-6-tert-butylphenol), 1 1,1,3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 1,1,3-tris (2-methyl-4-hydroxy-5-cyclohexylphenyl) butane, 2,2 Hindered phenol compounds such as bis (4-hydroxy-3,5-dimethylphenyl) propane, 4,4′-diglycidyloxydiphenylsulfone, 4-benzyloxy-4 ′-(2-methylglycidyloxy) diphenylsulfone , Diglycidyl terephthalate, cresol novolac epoxy resin, phenol novolac epoxy resin, vinyl Epoxy compounds such as bisphenol A type epoxy resin, N, N'-di-2-naphthyl -p- phenylenediamine, bis (4-ethylene iminocarbonyl aminophenyl) include methane and the like.

  The content of the preservability improver may be an effective amount for improving the preservability, but is usually preferably about 1 to 30% by mass, and preferably about 5 to 20% by mass in the total solid content of the thermosensitive recording layer. Is more preferable.

  The heat-sensitive recording layer may contain a sensitizer. Thereby, the recording sensitivity can be improved. Specific examples of the sensitizer include, for example, stearamide, methylenebis stearamide, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, diphenyl sulfone, 1,2-diphenoxymethylbenzene, 2-naphthylbenzyl ether, m-terphenyl, p-benzylbiphenyl, p-tolylbiphenyl 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-methylthiophenylbenzyl ether, 1,4-di (phen Ruthio) butane, p-acetotoluizide, p-acetophenidide, N-acetoacetyl-p-toluidine, di (β-biphenylethoxy) benzene, oxalic acid di-p-chlorobenzyl ester, oxalic acid di-p-methyl Examples thereof include benzyl ester and oxalic acid dibenzyl ester.

  The content of the sensitizer may be an amount effective for sensitization, but is usually preferably about 2 to 40% by mass, more preferably about 5 to 25% by mass in the total solid content of the heat-sensitive recording layer. preferable.

  Inorganic pigments include calcium carbonate, kaolin, calcined kaolin, talc, wax, diatomaceous earth, aluminum oxide, aluminum hydroxide, magnesium hydroxide, magnesia, titanium dioxide, barium carbonate, barium sulfate, finely divided silicic acid, calcium silicate, aluminum silicate, etc. However, examples of organic pigments include urea, phenol, epoxy, styrene, nylon, polyethylene, melamine, and benzoguanamine resin.

  Examples of the wax include paraffin wax, carnauba wax, microcrystalline wax, polyethylene wax, and higher fatty acid ester wax. Examples of the metal soap include higher fatty acid polyvalent metal salts, that is, zinc stearate, aluminum stearate, calcium stearate, zinc oleate and the like.

The heat-sensitive recording layer is generally a heat-sensitive recording layer coating solution prepared by mixing a leuco dye and a colorant, if necessary a dispersion of a storability improver, a sensitizer, and various materials, using water as a medium. It can be formed by applying and drying on the undercoat layer. The coating amount of the heat-sensitive recording layer is not particularly limited, but is preferably ² to 10 g / m ^{2 and} more preferably ³ to 8 g / m ² .

  In order to further improve the storage characteristics of the recording part, a protective layer containing a pigment and a binder as main components may be provided on the thermosensitive recording layer. Examples of the binder used in the protective layer include binders contained in the above-mentioned heat-sensitive recording layer. Among these, in particular, modified polyvinyl alcohols such as acetoacetyl-modified polyvinyl alcohol, silicon-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, and carboxy-modified polyvinyl alcohol, which are modified polyvinyl alcohols, and acrylic resins have improved chemical resistance in the recording area. In addition, it is preferable because it is easy to impart water resistance. In particular, acetoacetyl-modified polyvinyl alcohol and diacetone-modified polyvinyl alcohol are preferable.

  Further, the acrylic resin used in the protective layer is not particularly limited. For example, acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, cyclohexyl acrylate, acrylic acid- 2-ethylhexyl, hydroxyethyl acrylate, aminoethyl acrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, cyclohexyl methacrylate, methacrylate-2-cyclohexyl, methacrylate-tert-butyl In addition, it is sufficient that at least one of acrylic monomers such as aminoethyl methacrylate, acrylamide, and acrylonitrile is contained as a component constituting the resin.

  The amount of the acrylic monomer component is preferably 10% by mass or more based on the total solid amount of the acrylic resin. Further, in the acrylic resin, other monomers such as ethylene, styrene, butadiene, isobutylene, maleic anhydride and the like may be copolymerized in addition to the acrylic monomer.

  Although it does not specifically limit as a content rate of the binder in a protective layer, For example, about 10 mass% or more is preferable in the total solid of a protective layer, More preferably, it is 20 mass% or more. Barrier property can be improved by setting it as 10 mass% or more. Furthermore, pigments, auxiliaries and the like added to the heat-sensitive recording layer can be used in the protective layer.

The protective layer can be generally formed by applying and drying a protective layer coating solution containing a pigment and a binder on a heat-sensitive recording layer using water as a medium. The coating amount of the protective layer is not particularly limited, preferably about 0.5 to 10 g / ^{m 2,} about 1 to 5 g / ^{m 2} is more preferable.

  Various additives such as a cross-linking agent, an oil-repellent agent, an antifoaming agent, a surfactant, a viscosity modifier, a fluorescent dye, and a coloring agent can be added to the undercoat layer, the heat-sensitive recording layer, and if necessary, the protective layer. In the present invention, a surface smoothing process such as supercalendering can be performed after the formation of each layer and in an arbitrary process after the formation of all the layers. It is also possible to provide a coating layer on the back surface of the support for the purpose of curl control and the like.

  The method for forming each layer in the present invention is not particularly limited. For example, by appropriate coating methods such as bar coating, air knife coating, varibar blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, die coating, and the like. For example, it is formed by applying and drying an undercoat layer coating solution on a support and then applying and drying a thermal recording layer coating solution and, if necessary, a protective layer coating solution on the undercoat layer. .

  The undercoat layer is preferably a layer formed by a blade coating method. As a result, the heat-sensitive recording layer having a uniform thickness can be formed without the unevenness of the support, and the recording sensitivity can be increased, and the barrier property of the protective layer provided can be improved if necessary. The blade coating method is not limited to a coating method using a blade represented by a bevel type or a vent type, and includes a rod blade method, a bill blade method, and the like.

  The paper support in the present invention is not particularly limited, and for example, papers such as high-quality paper, medium-quality paper, glossy paper, art paper, coated paper, cast-coated paper, and glassine paper are appropriately selected and used. be able to.

  The present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. Unless otherwise specified, “part” and “%” indicate “part by mass” and “% by mass”, respectively. The average particle diameter is a median diameter measured by a laser diffraction particle size distribution analyzer SALD2200 (manufactured by Shimadzu Corporation).

Example 1
-Preparation of coating solution for undercoat layer Styrene-butadiene copolymer was obtained by dispersing 85 parts of calcined kaolin (trade name: Ansilex, manufactured by BASF, oil absorption 90 ml / 100 g) in 100 parts of water. 40 parts of latex (solid content concentration 50%), 50 parts of 10% aqueous solution of oxidized starch, 1 part of carboxymethylcellulose (trade name: Cellogen AG gum, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), styrene-maleic anhydride copolymer A composition comprising 15 parts of an ammonium salt (Polymaron WR300DS, solid concentration 20%, manufactured by Arakawa Chemical Co., Ltd.) was mixed and stirred to obtain a coating solution for an undercoat layer.

Preparation of leuco dye dispersion (liquid A) 3- (N-ethyl-p-toluidino) -6-methyl-7-anilinofluorane 10 parts, 5 parts of 5% aqueous solution of methylcellulose, and 15 parts of water The composition was pulverized with a sand mill until the average particle size became 0.5 μm to obtain Liquid A.

-Preparation of colorant dispersion (Liquid B) A composition comprising 10 parts of 2,4'-dihydroxydiphenylsulfone, 5 parts of a 5% aqueous solution of methylcellulose, and 15 parts of water was sand milled with an average particle size of 0.8 µm. The liquid B was obtained by pulverizing.

-Preparation of sensitizer dispersion (liquid C) A composition comprising 20 parts of di-p-methylbenzyl oxalate, 5 parts of a 5% aqueous solution of methylcellulose, and 55 parts of water was mixed with a sand mill and the average particle size was 0.8 μm. C liquid was obtained by pulverizing.

-Preparation of coating solution for heat-sensitive recording layer A part 25 parts, B part 50 parts, C part 50 parts, fine particle amorphous silica dispersion (trade name: Silojet 703A, average particle diameter 0.3μ, solid content concentration 20% 20 parts of a 20% aqueous solution of oxidized starch, and 50 parts of a 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: GOHSEFIMAR Z-200, manufactured by Nippon Synthetic Chemical Industry), A composition comprising 2 parts of a polyethylene dispersion (trade name: Chemipearl W-400, solid content concentration 20%, manufactured by Mitsui Chemicals) was mixed and stirred to obtain a thermal recording layer coating solution.

-Preparation of coating solution for protective layer Dispersion obtained by dispersing 50 parts of kaolin (trade name: UW-90, manufactured by BASF) in 100 parts of water, acetoacetyl-modified polyvinyl alcohol (trade name: Gohsefaimer Z) -200, supra) 600 parts of 10% aqueous solution and zinc stearate (trade name: Hydrin Z-8-36, solid content concentration 36%, manufactured by Chukyo Yushi Co., Ltd.) are mixed and stirred for coating for protective layer. A liquid was obtained.

-Preparation of thermosensitive recording material An undercoat layer coating solution was applied on one side of a high-quality paper having a basis weight of 50 g / m ² so that the coating amount after drying was 5.0 g / m ² and dried to form an undercoat layer. Formed. A thermal recording layer was formed by applying and drying the thermal recording layer coating solution on the undercoat layer so that the coating amount after drying was 4.0 g / m ² . The protective layer coating solution was applied and dried on the heat-sensitive recording layer so that the coating amount after drying the protective layer coating solution was 1.5 g / m ² to form a protective layer. Thereafter, a smoothing process was performed with a super calendar under a pressurizing condition of a linear pressure of 78 N / m to obtain a heat-sensitive recording material.

Example 2
A thermosensitive recording material was obtained in the same manner as in Example 1 except that the amount of ammonium salt of the styrene-maleic acid copolymer was changed to 5 parts instead of 15 parts in the preparation of the undercoat layer coating solution of Example 1. It was.

Example 3
In the preparation of the coating liquid for the undercoat layer of Example 1, instead of the ammonium salt of styrene-maleic acid copolymer, ammonium salt of styrene-acrylic acid copolymer (Polymaron PM326, solid content concentration 20%, Arakawa Chemical Co., Ltd.) A heat-sensitive recording material was obtained in the same manner as in Example 1 except for using

Example 4
A thermosensitive recording material was obtained in the same manner as in Example 1 except that the amount of ammonium salt of the styrene-maleic acid copolymer was changed to 3 parts instead of 15 parts in the preparation of the undercoat layer coating solution of Example 1. It was.

Example 5
A thermosensitive recording material was obtained in the same manner as in Example 1 except that the amount of ammonium salt of the styrene-maleic acid copolymer was changed to 20 parts instead of 15 parts in the preparation of the undercoat layer coating solution of Example 1. It was.

Comparative Example 1
A thermosensitive recording material was obtained in the same manner as in Example 1 except that 15 parts of ammonium salt of styrene-maleic acid copolymer was not added in the preparation of the undercoat layer coating solution of Example 1.

Comparative Example 2
A thermosensitive recording material was obtained in the same manner as in Example 1 except that the amount of ammonium salt of the styrene-maleic acid copolymer was changed to 40 parts instead of 15 parts in the preparation of the undercoat layer coating solution of Example 1. It was.

Comparative Example 3
In the preparation of the coating solution for the undercoat layer of Example 1, instead of the ammonium salt of the styrene-maleic acid copolymer, the sodium salt of isobutylene-maleic anhydride copolymer (Isoban 600SF35, solid content concentration 20%, Kuraray Co., Ltd.) A heat-sensitive recording material was obtained in the same manner as in Example 1 except for using

  The thermosensitive recording material thus obtained was evaluated as follows. The results were as shown in Table 1.

(Recording color before thermal recording label processing)
Using a label printer (trade name: L-2000, manufactured by Ishida Co., Ltd.), a checkered pattern is recorded with each thermal recording medium, and the optical density (recording density) of the recording portion is measured by a spectrocolorimetric optical densitometer [trade name: X-rite 939 type, manufactured by X-rite, Inc.].

(Recording image quality before thermal recording label processing)
The image quality of the recorded image obtained by the above-described evaluation of recording color development was visually observed and evaluated according to the following criteria.
A: There are almost no white spots in the image.
○: There are some white spots in the image, but there is no practical problem.
X: The image has many white spots.

(Recording color after thermal recording label processing)
Apply an acrylic resin-based adhesive to the opposite side (back side) of the recording surface of each thermal recording medium, and then apply the silicone-coated release surface of a release sheet based on fine paper to the thermal recording. Label processing was performed, and after storage for 7 days under an environment of 40 ° C. and 90% RH as an acceleration condition, the optical density of the recording portion was measured in the same manner as the evaluation of the recording color before the thermal recording label processing.

(Recording image quality after thermal recording label processing)
The image quality of the recorded image obtained by the evaluation of the recording color development after the thermal recording label processing was evaluated in the same manner as the evaluation of the recording image quality before the thermal recording label processing.

Claims (3)

  In a heat-sensitive recording medium comprising an undercoat layer containing a pigment and a binder on a paper support, and a heat-sensitive recording layer containing a leuco dye and a colorant on the undercoat layer, styrene is used as a binder contained in the undercoat layer. -At least one selected from ammonium salt of maleic anhydride copolymer and ammonium salt of styrene-acrylic acid copolymer, 0.5 parts by mass or more with respect to 100 parts by mass of pigment contained in the undercoat layer, A heat-sensitive recording material characterized by containing in a range of less than 5 parts by mass.   The content of at least one selected from the ammonium salt of the styrene-maleic anhydride copolymer and the ammonium salt of the styrene-acrylic acid copolymer is 1 to 100 parts by mass with respect to 100 parts by mass of the pigment contained in the undercoat layer. The heat-sensitive recording material according to claim 1, which is 4 parts by mass.   The heat-sensitive recording material according to claim 1, further comprising a protective layer containing a pigment and a binder on the heat-sensitive recording layer.