WO2024143345A1

WO2024143345A1 - Heat-sensitive recording body

Info

Publication number: WO2024143345A1
Application number: PCT/JP2023/046609
Authority: WO
Inventors: 尚竹村; 直信杉山
Original assignee: 王子ホールディングス株式会社
Priority date: 2022-12-27
Filing date: 2023-12-26
Publication date: 2024-07-04

Abstract

Disclosed is a heat-sensitive recording body having, on a support body, a heat-sensitive recording layer that contains a leuco dye, a developer, and an adhesive, wherein the heat-sensitive recording body is characterized by containing an olefin-acrylic acid-based copolymer salt and a styrene-butadiene-based latex as the adhesive, and by containing at least one non-phenolic developer selected from the group consisting of an N,N'-diarylurea-based compound represented by general formula (1) (In formula (1): R each represents a C1-12 alkyl group, a C7-12 aralkyl group, or a C6-12 aryl group, the aralkyl group and the aryl group may be substituted with a C1-12 alkyl group, a C1-12 alkoxy group, a C6-12 aryl group, or a halogen atom, and the plurality of R may be the same or different; and A¹ each represents a hydrogen atom or a C1-4 alkyl group, and the plurality of A¹ may be the same or different.), a compound represented by general formula (2) (In formula (2), R¹ to R⁵ may be the same or different and each represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkyl carbonyloxy group, an aryl carbonyloxy group, an alkyl carbonylamino group, an aryl carbonylamino group, an alkyl sulfonylamino group, an aryl sulfonylamino group, a monoalkylamino group, a dialkylamino group, or an aryl amino group.), and a compound represented by formula (3) as the developer.

Description

Thermal recording medium

The present invention relates to a thermal recording medium.

Thermal recording media, which can produce a recorded image by causing a leuco dye and a developer to react with heat energy to produce a color on at least one side of a support generally made of paper, synthetic paper, or synthetic resin film, have the advantages of being compact, inexpensive, and easy to maintain, and are widely used as recording media for facsimiles, automatic ticket machines, and scientific measuring instruments, and as output media for various printers and plotters, such as POS labels, CAD, and CRT medical images. Of these, those that use synthetic paper or synthetic resin film as a support are excellent in terms of water resistance and image quality, and are therefore being used significantly as output media for various printers for food labels, medical labels, medical images, etc.

However, compared to paper bases, film bases such as synthetic paper are not impregnated with the paint that constitutes the layer formed on the support, and the adhesion between the base material and the thermosensitive recording layer, and between the thermosensitive recording layer and the protective layer, are more likely to decrease than when paper is used as the support. Peeling can occur due to this lack of adhesion, particularly peeling from the base material due to the elution of water-soluble components when wet, and even more so when mechanical external forces are applied. As a result, the printed recording area peels off, causing problems such as poor reading by barcode readers.

In order to improve the adhesion between the synthetic paper base material and the thermal recording layer, Patent Documents 1 and 2 propose a thermal recording medium in which a styrene-butadiene copolymer is added to the thermal recording layer. Although this has excellent adhesion, the coating peels off when immersed in water, so further improvements are required.

Japanese Patent Application Laid-Open No. 8-175007 Japanese Patent Application Laid-Open No. 9-226246

The main objective of the present invention is to provide a thermal recording medium that is free from the risk of printing transferring to the printing surface and has excellent heat resistance to background fogging.

As a result of extensive research by the inventors to achieve the above object, they discovered that the above problems could be solved by using a thermal recording medium having a thermal recording layer containing a leuco dye, a color developer, and an adhesive on a support, which contains an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as the adhesive, and a specific non-phenolic color developer as the color developer, and thus completed the present invention. That is, the present invention relates to the following thermal recording medium.

Item 1: A thermal recording medium having a thermal recording layer on a support, the thermal recording layer containing a leuco dye, a developer and an adhesive, the adhesive containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex, and the developer containing a compound represented by the following general formula (1):

(wherein R represents an alkyl group having 1 to 12 carbon atoms, an aralkyl group having 7 to 12 carbon atoms, or an aryl group having 6 to 12 carbon atoms, and the aralkyl group and the aryl group may be substituted with an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or a halogen atom, and a plurality of R may be the same or different; A ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and a plurality of A ¹ may be the same or different), an N,N'-diaryl urea compound represented by the following general formula (2):

(wherein R ¹ to R ⁵ are the same or different and each represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group), and a compound represented by the following formula (3):

1. A thermal recording medium comprising at least one non-phenolic color developer selected from the group consisting of compounds represented by the formula:
Item 2: The N,N'-diaryl urea compound represented by the general formula (1) is N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea 2. The thermal recording medium according to item 1, wherein the urea is at least one selected from the group consisting of N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea.
Item 3: The thermal recording medium according to item 1 or 2, which contains N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea as the N,N'-diaryl urea compound represented by the general formula (1).
Item 4: The thermal recording medium according to any one of Items 1 to 3, which contains 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate as the compound represented by the general formula (2).
Item 5: The thermosensitive recording medium according to any one of Items 1 to 4, wherein the content of the olefin-acrylic acid copolymer salt is 4 to 25% by mass, and the content of the styrene-butadiene latex is 15 to 40% by mass, based on the total solid content of the thermosensitive recording layer.
Item 6: The thermal recording medium according to any one of Items 1 to 5, wherein the content of the olefin-acrylic acid copolymer salt is 10 to 150 parts by mass with respect to 100 parts by mass of the styrene-butadiene latex.
Item 7: The thermosensitive recording medium according to any one of Items 1 to 6, wherein the total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is 25 to 50% by mass of the total solid content of the thermosensitive recording layer.
Item 8: The thermal recording medium according to any one of items 1 to 7, wherein the support is synthetic paper or a synthetic resin film.
Item 9: The thermosensitive recording medium according to any one of Items 1 to 8, wherein the support contains a polyolefin resin.
Item 10: The thermosensitive recording medium according to any one of items 1 to 9, further comprising a protective layer on the thermosensitive recording layer, the protective layer containing acetoacetyl-modified polyvinyl alcohol as an adhesive.

The thermal recording medium of the present invention has no risk of printing transferring to the printing surface and has excellent heat resistance to background fogging.

In this specification, the expression "comprise" includes the concepts of "comprise," "consist essentially of," and "consist only of."

In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

Latex in the present invention includes the state of a gel or a dried film formed by drying a dispersion medium.

[Support]
The support in the present invention is not particularly limited in type, shape, size, etc., and can be appropriately selected from, for example, high-quality paper (acidic paper, neutral paper), medium-quality paper, coated paper, art paper, cast-coated paper, glassine paper, resin-laminated paper, polyolefin-based synthetic paper, synthetic fiber paper, nonwoven fabric, synthetic resin film, etc., as well as various transparent supports, etc. The thickness of the support is not particularly limited, and is usually about 20 to 200 μm.

The support in the present invention is preferably a synthetic paper or a synthetic resin film. There are no particular limitations on the synthetic paper or synthetic resin film used as the support in the present invention, and examples of the substrate include polyolefin films such as polyethylene and polypropylene, polyester films such as polystyrene film, polyethylene terephthalate, and polybutylene terephthalate, and cellulose derivative films such as cellulose triacetate. The support may also contain a pigment. For example, a synthetic paper can be used that is manufactured by heating and kneading a polyolefin resin and a white inorganic pigment, extruding the mixture from a die, stretching the mixture in the vertical direction, laminating one or two layers of a film made of a polyolefin resin and a white inorganic pigment on both sides of the mixture, and stretching the mixture in the horizontal direction to make the mixture semi-transparent or opaque. Since such synthetic paper contains a polyolefin resin, it still has poor adhesion to the thermal recording layer, but since voids are formed by the pigment, it has both cushioning and heat insulation properties and is excellent in color development and image quality. Although it provides an anchoring effect for the thermal recording layer, it also contains water that penetrates into the voids, reducing water resistance, limiting its use as a support. However, according to the present invention, this effect can be fully exerted, allowing it to be used favorably.

[Thermal recording layer]
The thermosensitive recording layer in the present invention contains an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as adhesives, and is formed on a support. This makes it possible to obtain excellent water resistance, water-resistant blocking properties, and adhesion between the support and the thermosensitive recording layer. In addition to obtaining water-resistant blocking properties, there is no risk of the print being transferred to the printing surface due to the synergistic effect with a specific non-phenolic color developer described later.

The olefin-acrylic acid copolymer salt used in the present invention has high water resistance due to the high hydrophobicity of the olefin portion. In addition, the carboxyl group portion of the olefin-acrylic acid copolymer salt has excellent adhesion to the film and can exhibit high binding properties.

Examples of the olefin component in the olefin-acrylic acid copolymer salt include ethylene, propylene, butylene, and isobutylene. Among these, ethylene-acrylic acid copolymer resins are preferably used from the viewpoints of water resistance and adhesion. The ratio of the olefin component in the olefin-acrylic acid copolymer salt is preferably about 50 to 90 mol%.

The weight-average molecular weight of the olefin-acrylic acid copolymer salt is preferably 20,000 to 200,000, and more preferably 30,000 to 150,000. By making the molecular weight 20,000 or more, it is possible to improve water resistance and adhesion. On the other hand, by making the molecular weight 200,000 or less, it is possible to achieve excellent film-forming properties and improved adhesion. The weight-average molecular weight is measured by gel permeation chromatography using polystyrene as a standard sample.

Examples of the salt of the olefin-acrylic acid copolymer salt include alkali metal salts such as lithium, potassium, and sodium, and ammonium salts. Among these, ammonium salts are preferred from the viewpoint of improving water dispersibility and water resistance.

The content of the olefin-acrylic acid copolymer salt is preferably 4 to 25% by mass, more preferably 5 to 20% by mass, and even more preferably 5 to 15% by mass, of the total solid content of the thermal recording layer. By making it 4% by mass or more, water resistance can be improved. On the other hand, by making it 25% by mass or less, color development can be improved.

Olefin-acrylic acid copolymer salts are commercially available as, for example, Chemipearl S100, S650, S75N, etc. manufactured by Mitsui Chemicals, Inc., and Hi-Tec S3121, S8512, etc. manufactured by Toho Chemical Industry Co., Ltd., and can be easily obtained and used in the form of emulsion.

The styrene-butadiene latex used in the present invention is preferably a latex in which the content of butadiene monomer is 20 to 45% by mass of the total monomer solid content and the gel content is 70 to 85%. By making the copolymerization rate of butadiene 20% by mass or more, the film-forming properties can be improved. On the other hand, by making it 45% by mass or less, the cohesive force can be increased and the dry pick strength can be improved. Furthermore, by making the gel content 70% or more, the adhesive strength can be improved. Furthermore, the content of styrene monomer constituting the styrene-butadiene latex is preferably about 40 to 70% by mass from the viewpoint of increasing the stability of the latex.

The styrene-butadiene latex used in the present invention has styrene and butadiene as its main constituent monomers, but the monomer composition can further contain at least one of various unsaturated carboxylic acid monomers for the purpose of modification. Specific examples of such unsaturated carboxylic acid monomers include itaconic acid, maleic acid, acrylic acid, methacrylic acid, fumaric acid, dicarboxylic acid anhydrides, dicarboxylic acid monoalkyl esters, etc. It is desirable to include these monomers in the monomer composition in a proportion of 0.5 to 10% by mass. Furthermore, the styrene-butadiene latex monomer composition can contain other monomers. Specific examples of such monomers include aromatic vinyl monomers such as α-methylstyrene, vinyltoluene, and dimethylstyrene, acrylate monomers such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate, methacrylate monomers such as methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, and 2-hydroxypropyl methacrylate, and monomers containing a glycidyl group such as glycidyl acrylate and glycidyl methacrylate. These monomers can be used in an amount of about 0 to 35% by mass, as long as they do not impair the effects of the present invention.

The method for producing styrene-butadiene latex is not particularly limited, and any known emulsion polymerization method, such as continuous emulsion polymerization or batch emulsion polymerization, can be used. In addition, various known additives used in general emulsion polymerization, such as emulsifiers, chain transfer agents, polymerization initiators, electrolytes, and chelating agents, can be used as appropriate, and the polymerization temperature can be either high or low.

The content of styrene-butadiene latex is preferably 15 to 40% by mass, more preferably 15 to 30% by mass, of the total solid content of the thermal recording layer. By making it 15% by mass or more, adhesion can be improved. On the other hand, by making it 40% by mass or less, color development can be improved.

The total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is preferably 25 to 50% by mass, more preferably 25 to 40% by mass, and even more preferably 25 to 30% by mass, of the total solid content of the thermal recording layer. By making it 25% by mass or more, it is possible to improve water resistance, water blocking resistance, and adhesion. On the other hand, by making it 50% by mass or less, it is possible to improve color development.

The content of the olefin-acrylic acid copolymer salt relative to the styrene-butadiene latex in the thermal recording layer is preferably 10 to 150 parts by mass, more preferably 20 to 100 parts by mass, and even more preferably 30 to 90 parts by mass, relative to 100 parts by mass of the styrene-butadiene latex. By making the olefin-acrylic acid copolymer salt 10 parts by mass or more, it is possible to improve water resistance and water blocking resistance. On the other hand, by making it 150 parts by mass or less, it is possible to improve color development and obtain a synergistic effect on adhesion.

The adhesives used in the thermal recording layer in the present invention are olefin-acrylic acid copolymer salt and styrene-butadiene latex, but various other known adhesives may be used as necessary within the scope of not impairing the effects of the present invention. Examples of other adhesives include starches such as oxidized starch, acid-modified starch, phosphate-modified starch, enzyme-modified starch, cation-modified starch, esterified starch, etherified starch, and vinyl acetate-modified grafted starch; cellulose derivatives such as methylcellulose, ethylcellulose, carboxymethylcellulose, methoxycellulose, hydroxyethylcellulose, and hydroxypropylmethylcellulose; polyvinyl alcohols such as fully (or partially) saponified polyvinyl alcohol, silicon-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and acetoacetyl-modified polyvinyl alcohol; Examples of water-soluble adhesives include sodium polyacrylate, polyacrylamide, polyvinylpyrrolidone, acrylic acid amide-acrylic acid ester copolymer, acrylic acid amide-acrylic acid ester-methacrylic acid copolymer, styrene-maleic anhydride copolymer alkali salt, isobutylene-maleic anhydride copolymer alkali salt, sodium alginate, gelatin, and casein; and water-dispersible adhesives such as latexes of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid ester, polybutyl methacrylate, vinyl chloride-vinyl acetate copolymer, ethylene-vinyl acetate copolymer, and styrene-butadiene-acrylic copolymer.

In the present invention, various known leuco dyes can be used as the leuco dye contained in the thermal recording layer. For example, leuco dyes that give a black color include 3-diethylamino-6-methyl-7-anilinofluoran, 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 3-di(n-pentyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-(N-isoamyl-N-ethylamino)-7-(o-chloroanilino)fluoran, At least one of the following can be used: 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-ethylamino]-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-methylamino]-6-methyl-7-anilinofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-di(n-butylamino)-7-(2-chloroanilino)fluoran, etc. If necessary, the leuco dye may be a leuco dye that develops a color tone different from black, such as red, reddish purple, orange, blue, or green.

The leuco dyes used in the present invention are not limited to these, and two or more types can be used in combination. The content of such leuco dyes is preferably about 3 to 30 mass % of the total solid content of the thermal recording layer.

In the present invention, when the leuco dye is used in a solid fine particle state, the leuco dye is ground using water as a dispersion medium by various wet grinding machines such as a sand grinder, attritor, ball mill, or cobo mill, and the ground product is dispersed in a dispersion medium together with water-soluble synthetic polymer compounds such as polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, modified polyvinyl alcohol such as sulfone-modified polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, hydroxypropyl methyl cellulose, styrene-maleic anhydride copolymer salts and derivatives thereof, as well as surfactants, defoamers, etc., as necessary, to form a dispersion, which can be used to prepare a coating liquid for a thermal recording layer.

Alternatively, the leuco dye can be dissolved in a solvent, the solution can be emulsified and dispersed in water using the above-mentioned water-soluble polymer as a stabilizer, and the solvent can be evaporated from the emulsion to turn the leuco dye into solid fine particles for use. In either case, the average particle size of the dispersed particles of the leuco dye used in the solid fine particle state is preferably 0.2 to 3.0 μm to obtain appropriate color development sensitivity, and more preferably 0.3 to 1.0 μm.

In the present invention, in addition to using the leuco dye in the solid fine particle state described above, it can also be used as composite particles consisting of an organic polymer and a leuco dye. Composite particles can be prepared by known methods. For example, a method for preparing composite particles in which the organic polymer is at least one of polyurea and polyurea-polyurethane will be described. The composite particles are prepared by dissolving and mixing the leuco dye and a polymer-forming raw material that forms at least one of polyurea and polyurea-polyurethane by polymerization in a water-insoluble organic solvent having a boiling point of 100°C or less, emulsifying and dispersing this organic solvent solution in a hydrophilic protective colloid solution such as polyvinyl alcohol so that the average particle size is about 0.5 to 3 μm, and then mixing a reactive substance such as polyamine if necessary, heating this emulsion dispersion to volatilize and remove the organic solvent, and then polymerizing the polymer-forming raw material, or by dissolving the leuco dye in the polymer-forming raw material, emulsifying and dispersing this solution to an average particle size of about 0.5 to 3 μm using the method described above, and then polymerizing the polymer-forming raw material.

The thermal recording layer in the present invention contains at least one non-phenolic developer selected from the group consisting of N,N'-diaryl urea compounds represented by the general formula (1), compounds represented by the general formula (2), and compounds represented by the formula (3). This eliminates the risk of the print being transferred to the printed surface. Here, the phenomenon of the print being transferred to the printed surface refers to the phenomenon in which, when two printed surfaces bearing pattern prints, such as a solid black checkerboard print or a barcode, are placed together in a water-wet state and subjected to a blocking test, then dried and peeled off, the prints are reflected on the other printed surface, or more simply, on the blank, unprinted areas of the printed surface, as an inverted mirror image, although at a low density. Observation of the printed surface after peeling reveals no changes such as swelling or dissolution of the surface, and no evidence of cohesive failure or delamination of the thermal recording layer or the protective layer provided as necessary, suggesting that the developer is being affected in some way by moisture. If the support is synthetic paper or synthetic resin film, moisture does not penetrate into the support, but tends to remain on the surface opposite the support, and this tends to produce a noticeable effect that can be discerned visually. Thermal recording media that cause this phenomenon are problematic because they impair the quality of the printed matter and lead to poor reading of the printed information.

In the general formula (1), the alkyl group having 1 to 12 carbon atoms represented by R may be any of linear, branched, and alicyclic, and is preferably an alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms. Examples of the alkyl group having 1 to 12 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a t-butyl group, a cyclopentyl group, a hexyl group, a cyclohexyl group, a 2-ethylhexyl group, and a lauryl group. The alkyl group here also includes the alkyl portion of an alkoxy group having 1 to 12 carbon atoms.
The aralkyl group means an arylalkyl group. Examples of the aralkyl group having 7 to 12 carbon atoms include a benzyl group, a 1-phenylethyl group, a 2-phenylethyl group, and a 3-phenylpropyl group.
The aryl group refers to a monocyclic or polycyclic group consisting of a 5- or 6-membered aromatic hydrocarbon ring. Examples of the aryl group having 6 to 12 carbon atoms include a phenyl group, a 1-naphthyl group, and a 2-naphthyl group. The aryl group in this case also includes the aryl portion of an aralkyl group.
Halogen atoms include fluorine, chlorine, bromine and iodine.
In general formula (1), the substitution positions of multiple R-SO ₃ - may be the same or different. The substitution position is preferably position 3, 4 or 5, and more preferably position 3. In addition, when the aralkyl group having 7 to 12 carbon atoms and the aryl group having 6 to 12 carbon atoms of R have a substituent, the number of the substituent is not particularly limited and is, for example, 1 to 4.
The alkyl group having 1 to 4 carbon atoms for ^A1 may be either linear or branched, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, and a t-butyl group.
The substitution positions of the multiple A ¹ 's may be the same or different. The substitution position is preferably the 3-position, 4-position or 5-position.

N,N'-diaryl urea compounds represented by general formula (1) are not particularly limited, but include N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, At least one selected from the group consisting of N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea is preferred. Among these, N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea is preferred.

The content of the N,N'-diaryl urea compound is not particularly limited and may be adjusted according to the leuco dye used. In general, it is preferably 0.5 parts by mass or more relative to 1 part by mass of leuco dye, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, even more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more. On the other hand, the content of the N,N'-diaryl urea compound is preferably 10 parts by mass or less relative to 1 part by mass of leuco dye, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less. By making it 0.5 parts by mass or more, it is possible to improve the recording performance. On the other hand, by making it 10 parts by mass or less, it is possible to effectively suppress heat-resistant background fogging in a high-temperature environment.

In the general formula (2), examples of the halogen atom for R ¹ to R ⁵ include a fluorine atom, a chlorine atom and a bromine atom, with a fluorine atom and a chlorine atom being preferred.
The alkyl group may be linear, branched or cyclic, preferably linear or branched, more preferably linear. Usually, it is an alkyl group having 1 to 12 carbon atoms, preferably an alkyl group having 1 to 8 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 4 carbon atoms.
The alkoxy group may be linear, branched or cyclic, and is preferably a linear or branched alkoxy group, more preferably a linear alkoxy group. Usually, the alkoxy group has 1 to 12 carbon atoms, preferably an alkoxy group having 2 to 8 carbon atoms, more preferably an alkoxy group having 2 to 6 carbon atoms, and even more preferably an alkoxy group having 2 to 4 carbon atoms.
The alkylcarbonyloxy group may be linear, branched or cyclic, is preferably a linear or branched alkylcarbonyloxy group, and is more preferably a linear alkylcarbonyloxy group. Also, an alkylcarbonyloxy group having 1 to 10 carbon atoms is preferred.
The alkylcarbonylamino group may be linear, branched or cyclic, is preferably a linear or branched alkylcarbonylamino group, and is more preferably a linear alkylcarbonylamino group. Also, an alkylcarbonylamino group having 1 to 10 carbon atoms is preferred.
The alkylsulfonylamino group may be linear, branched or cyclic, is preferably a linear or branched alkylsulfonylamino group, and is more preferably a linear alkylsulfonylamino group. Also, an alkylsulfonylamino group having 1 to 10 carbon atoms is preferred.
The aryl group refers to a monocyclic or polycyclic group consisting of a 5- or 6-membered aromatic hydrocarbon ring. Examples of the aryl group include a phenyl group, a naphthyl group, and a biphenyl group.
The aryloxy group is preferably an aryloxy group having 6 to 12 carbon atoms. The arylcarbonyloxy group is preferably an arylcarbonyloxy group having 6 to 12 carbon atoms. The arylcarbonylamino group is preferably an arylcarbonylamino group having 6 to 12 carbon atoms. The arylsulfonylamino group is preferably an arylsulfonylamino group having 6 to 12 carbon atoms.
The monoalkylamino group may be linear, branched or cyclic, is preferably a linear or branched monoalkylamino group, and is more preferably a linear monoalkylamino group. Also, a monoalkylamino group having an alkyl group with 1 to 10 carbon atoms is preferred.
The dialkylamino group may be linear, branched or cyclic, preferably linear or branched, more preferably linear. Also, a dialkylamino group having an alkyl group with 1 to 10 carbon atoms is preferred.
The arylamino group may be a monoarylamino group or a diarylamino group, and is preferably a monoarylamino group having 6 to 12 carbon atoms.
Specific examples of the compound represented by general formula (2) include those in which R ¹ to R ⁵ are an alkyl group or a hydrogen atom, preferably those in which R ¹ to R ⁵ are a linear alkyl group having 1 to 8 carbon atoms or a hydrogen atom, more preferably those in which R ¹ to R ⁵ are a linear alkyl group having 1 to 4 carbon atoms or a hydrogen atom, and even more preferably those in which R ¹ to R ⁵ are a methyl group or a hydrogen atom.
Other specific examples of the compound represented by general formula (2) include those in which R ¹ , R ² , R ⁴ and R ⁵ are hydrogen atoms and R ³ is a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy 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, or an arylamino group (preferably a hydrogen atom or an alkyl group, more preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, even more preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and particularly preferably a methyl group).
The substitution position of the substituent bonded to one of the benzene rings of the diphenylurea structure in the general formula (2) can be the ortho-position, meta-position or para-position relative to the aminocarbonyl group on the benzene ring, preferably the ortho-position or meta-position, more preferably the meta-position.

The compound represented by general formula (2) is not particularly limited, but is preferably at least one selected from the group consisting of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 2-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, and 4-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Among these, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate is preferred.

The content of the compound represented by general formula (2) is not particularly limited and may be adjusted according to the leuco dye used. In general, it is preferably 0.5 parts by mass or more relative to 1 part by mass of leuco dye, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, even more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more. On the other hand, the content of the compound represented by general formula (2) is preferably 10 parts by mass or less relative to 1 part by mass of leuco dye, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less. By making it 0.5 parts by mass or more, it is possible to improve the recording performance. On the other hand, by making it 10 parts by mass or less, it is possible to effectively suppress heat-resistant background fogging in a high-temperature environment.

The content of the compound represented by the formula (3) is not particularly limited, and may be adjusted according to the leuco dye used. In general, it is preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, even more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more, relative to 1 part by mass of the leuco dye. On the other hand, the content of the compound represented by the formula (3) is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less, relative to 1 part by mass of the leuco dye. By making it 0.5 parts by mass or more, the recording performance can be improved. On the other hand, by making it 10 parts by mass or less, the heat-resistant background fogging in a high-temperature environment can be effectively suppressed.
As long as the effects of the present invention are not impaired, a developer other than the N,N'-diaryl urea compound represented by the general formula (1), the compound represented by the general formula (2), and the compound represented by the general formula (3) may be contained.

A sensitizer can also be incorporated into the thermal recording layer of the present invention. This can increase the recording sensitivity. Examples of sensitizers include stearic acid amide, methoxycarbonyl-N-stearic acid benzamild, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, behenic acid amide, methylene bisstearic acid amide, N-methylolstearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, diphenyl sulfone, benzyl p-benzyloxybenzoate, 1-hydroxy-2-phenyl naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, di-p-chlorobenzyl ester of oxalic acid, di-p-methylbenzyl ester of oxalic acid, dibenzyl ester of oxalic acid, p-tolyl biphenyl ether, di(p-methoxyphenoxyethyl)ether, 1,2-di(3-methylphenoxy)ethyl ... p-methylthiophenylbenzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, 1,2-diphenoxymethylbenzene, di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane, di-o-chlorobenzyl adipate, 1,2-bis(3,4-dimethylphenyl)ethane, 1,3-bis(2-naphthoxy)propane, diphenyl, benzophenone, and the like. Among these, 1,2-di(3-methylphenoxy)ethane is preferred. These can be used in combination to the extent that no problems occur. The content of the sensitizer should be an amount effective for sensitization, and is usually preferably 5 to 30% by mass, more preferably 10 to 30% by mass, and even more preferably 15 to 25% by mass of the total solid content of the thermal recording layer.

In the present invention, a storage stability improver can be further incorporated into the thermal recording layer, mainly to further improve the storage stability of the color image. Examples of such storage stability improvers include 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol, and 4,4'-[1,3-phenylenebis(1-methylethylidene)]bisphenol. At least one selected from phenol compounds such as 4-benzyloxyphenyl-4'-(2-methyl-2,3-epoxypropyloxy)phenylsulfone, epoxy compounds such as 4-(2-methyl-1,2-epoxyethyl)diphenylsulfone, and 4-(2-ethyl-1,2-epoxyethyl)diphenylsulfone, and isocyanuric acid compounds such as 1,3,5-tris(2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl)isocyanuric acid can be used. Of course, the present invention is not limited to these compounds, and two or more compounds can be used in combination as necessary.

When a storage stability improver is used, the amount used should be an amount effective for improving storage stability, and is usually preferably about 1 to 25% by mass, and more preferably about 5 to 20% by mass, of the total solid content of the thermal recording layer.

Other component materials constituting the thermal recording layer may include auxiliary agents such as inorganic or organic pigments, crosslinking agents, waxes, metal soaps, water-resistant agents, dispersants, colored dyes, and fluorescent dyes, if necessary.

Various inorganic pigments can be used, but specific examples include inorganic pigments such as calcium carbonate, such as light calcium carbonate, aluminum hydroxide, clay, such as kaolin, and talc. Among these, at least one type selected from the group consisting of calcium carbonate, aluminum hydroxide, and clay is preferable. The content of the inorganic pigment can be selected from a wide range, but is preferably 2 to 40 mass %, more preferably 3 to 30 mass %, and even more preferably 4 to 20 mass % of the total solid content of the thermal recording layer.

The water resistance of the thermal recording layer can be improved by including a crosslinking agent in the thermal recording layer. Examples of crosslinking agents include aldehyde compounds such as glyoxal, polyamine compounds such as polyethyleneimine, epoxy compounds, polyamide resins, melamine resins, glyoxylates, dimethylol urea compounds, aziridine compounds, and blocked isocyanate compounds; inorganic compounds such as ammonium persulfate, ferric chloride, magnesium chloride, sodium tetraborate, and potassium tetraborate; boric acid, boric acid triesters, boron-based polymers, hydrazide compounds such as adipic acid dihydrazide, and glyoxylates. These may be used alone or in combination of two or more. The amount of crosslinking agent used is preferably about 0.2 to 5% by mass, and more preferably about 0.3 to 3% by mass, of the total solid content of the thermal recording layer.

The thermal recording layer is formed on a support by, for example, dispersing a leuco dye and a developer, optionally together with a sensitizer or storage improver, or separately, in water using various stirring/wet grinding machines such as ball mills, coball mills, attritors, and vertical and horizontal sand mills, together with water-soluble synthetic polymer compounds such as polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, styrene-maleic anhydride copolymer salt, and other surfactants to prepare respective dispersions, and then finely grinding the dispersions to an average particle size of 2 μm or less to prepare a thermal recording layer coating material prepared by mixing an adhesive and, if necessary, an auxiliary, etc., with the dispersions, and then drying the coating material. The coating amount of the thermal recording layer is not particularly limited, and is preferably about 1 to 12 g/m ² in terms of dry mass, more preferably 2 to 10 g/m ² , even more preferably 2.5 to 8 g/m ² , and particularly preferably 3 to 5.5 g/m ² . If necessary, the heat-sensitive recording layer can be formed in two or more separate layers, and the composition and coating amount of each layer may be the same or different.

[Protective Layer]
The thermal recording medium may also have a protective layer on the thermal recording layer as necessary. The protective layer preferably contains a pigment and an adhesive. Furthermore, the protective layer preferably contains a lubricant such as polyolefin wax or zinc stearate for the purpose of preventing sticking to the thermal head, and may also contain an ultraviolet absorbing agent. Furthermore, by providing a glossy protective layer, the added value of the product can be increased.

The pigments contained in the protective layer are not particularly limited, and examples include inorganic pigments such as amorphous silica, kaolin, clay, light calcium carbonate, heavy calcium carbonate, calcined kaolin, titanium oxide, magnesium carbonate, aluminum hydroxide, colloidal silica, and synthetic layered mica, and plastic pigments such as urea-formaldehyde resin filler.

The adhesive contained in the protective layer is not particularly limited, and a water-soluble or water-dispersible aqueous adhesive can be used. The adhesive can be appropriately selected from those that can be used in the thermal recording layer. Of these adhesives, various modified polyvinyl alcohols such as acetoacetyl-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and diacetone-modified polyvinyl alcohol are more preferably used. From the viewpoint of improving water blocking resistance, acetoacetyl-modified polyvinyl alcohol is even more preferable.

The protective layer is formed on the thermosensitive recording layer by, for example, applying a coating material for the protective layer prepared by mixing a pigment, an adhesive, and, if necessary, an auxiliary, etc., in water as a dispersion medium, and then drying the coating material. The amount of the coating material for the protective layer is not particularly limited, and is preferably about 0.3 to 15 g/ ^m2 in terms of dry mass, more preferably about 0.3 to 10 g/ ^m2 , even more preferably about 0.5 to 8 g/ ^m2 ^, particularly preferably about 1 to 8 g/m2, and even more preferably about 1 to 5 g/ ^m2 . The protective layer can be formed in two or more layers as necessary, and the composition and coating amount of each layer may be the same or different.

[Other layers]
In the present invention, it is preferable that the support has an adhesive layer on at least one side. This can increase the added value of the thermal recording medium. For example, the adhesive layer can be made into adhesive paper, remoistened adhesive paper, delayed tack paper, etc. by applying coating processing with an adhesive, remoistened adhesive, delayed tack type adhesive, etc. on one side. In addition, the surface of the support opposite to the thermal recording layer can be given the function of thermal transfer paper, inkjet recording paper, carbonless paper, electrostatic recording paper, xeography paper, etc., to make a recording paper capable of double-sided recording. Of course, it can also be made into a double-sided thermal recording medium. In addition, a back layer can be provided to suppress the penetration of oil and plasticizer from the back side of the thermal recording medium, to control curling, and to prevent static electricity. It is also possible to make a linerless label that does not require a release paper by coating processing a release layer containing silicone on the protective layer and coating processing a pressure sensitive adhesive on one side.

[Thermal recording medium]
The thermal recording medium can be manufactured by forming each of the above layers on a support. The method for forming each of the above layers on a support may be any of known coating methods such as the air knife method, blade method, gravure method, roll coater method, spray method, dip method, bar method, curtain method, slot die method, slide die method, and extrusion method. Each coating material may be applied and dried one by one to form each layer, or the same coating material may be applied in two or more layers. Furthermore, simultaneous multi-layer coating may be performed by applying two or more layers at the same time. After each layer is formed or after all layers are formed, the surface may be smoothed using known methods such as super calendaring and soft calendaring.

The present invention will be explained in more detail with reference to examples, but the present invention is not limited thereto. Unless otherwise specified, "parts" and "%" refer to "parts by mass" and "% by mass", respectively. The average particle size was measured using a laser diffraction particle size distribution analyzer SALD2200 (manufactured by Shimadzu Corporation). Here, the average particle size refers to the median diameter (D50).

Example 1
(1) Preparation of leuco dye dispersion (liquid A) 40 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization 500, degree of saponification 88%), and 20 parts of water were mixed and pulverized using a sand mill (sand grinder, manufactured by Imex Corporation) to an average particle size of 0.5 μm, to obtain a leuco dye dispersion (liquid A).

(2) Preparation of color developer dispersion liquid (liquid B) 40 parts of N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization 500, degree of saponification 88%), and 20 parts of water were mixed and ground using a sand mill (sand grinder manufactured by Imex Co., Ltd.) to an average particle diameter of 1.0 μm to obtain a color developer dispersion liquid (liquid B).

(3) Preparation of sensitizer dispersion liquid (liquid C) 40 parts of 1,2-di(3-methylphenoxy)ethane (product name: KS-232, manufactured by Sankosha Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization 500, degree of saponification 88%), and 20 parts of water were mixed and pulverized using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) to an average particle size of 1.0 μm, thereby obtaining a sensitizer dispersion liquid (liquid C).

(4) Preparation of a coating material for a thermosensitive recording layer 34.1 parts of solution A, 68.2 parts of solution B, 45.5 parts of solution C, 26 parts of an olefin-acrylic acid copolymer salt (product name: Hitec S-3121, an aqueous dispersion of ethylene-acrylic acid copolymer ammonia salt, manufactured by Toho Chemical Industry Co., Ltd., solid content 26.5%), 41.7 parts of a styrene-butadiene latex (product name: Smartex PA-9281, manufactured by Nippon A&L Co., Ltd., solid content concentration 48%), 5 parts of calcium carbonate (product name: Brilliant 15, manufactured by Shiraishi Kogyo Co., Ltd.), 0.5 parts of adipic acid dihydrazide (manufactured by Otsuka Chemical Co., Ltd.), and 200 parts of water were mixed and stirred to obtain a coating material for a thermosensitive recording layer.

(5) Preparation of protective layer coating material A composition consisting of 208 parts of a 12% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenex Z-200, manufactured by Nippon Gosei Co., Ltd.), 55 parts of kaolin (trade name: HYDRAGLOSS 90, manufactured by KaMin LLC), 2.5 parts of polyethylene wax (trade name: Chemipearl W-400, manufactured by Mitsui Chemicals, Inc., solid content concentration: 40%), 20.8 parts of zinc stearate (trade name: Hydrin Z-8-36, manufactured by Chukyo Yushi Co., Ltd., solid content concentration: 36%), and 150 parts of water was mixed and stirred to obtain a protective layer coating material.

(6) Preparation of a thermosensitive recording medium A synthetic paper (product name: YUPO FPG95, paper thickness 95 μm, manufactured by Yupo Corporation) was produced by heating and kneading a polyolefin resin and calcium carbonate as a white inorganic pigment, extruding the mixture from a die, stretching the mixture in the longitudinal direction, laminating films made of a polyolefin resin and calcium carbonate as a white inorganic pigment on both sides of the mixture, and stretching the mixture in the transverse direction to make it opaque. A thermosensitive recording layer paint was applied to one side of the synthetic paper so that the dry mass was 4.0 g/ ^m2 and dried to form a thermosensitive recording layer, and a protective layer paint was applied thereon so that the dry mass was 2.5 g/ ^m2 and dried to form a protective layer, and the surface was smoothed with a super calendar to obtain a thermosensitive recording medium.

Example 2
(7) Preparation of color developer dispersion liquid (liquid D) 40 parts of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization 500, degree of saponification 88%), and 20 parts of water were mixed and pulverized using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) to an average particle size of 1.0 μm to obtain a color developer dispersion liquid (liquid D).

A thermal recording medium was obtained in the same manner as in Example 1, except that in preparing the coating material for the thermal recording layer in Example 1, Liquid D was used instead of Liquid B.

Example 3
(8) Preparation of color developer dispersion liquid (liquid E) 40 parts of the compound represented by the formula (3), 40 parts of a 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and 20 parts of water were mixed, and the mixture was ground using a sand mill (sand grinder manufactured by Imex Co., Ltd.) to an average particle diameter of 1.0 μm to obtain a color developer dispersion liquid (liquid E).

A thermal recording medium was obtained in the same manner as in Example 1, except that in preparing the coating material for the thermal recording layer in Example 1, Liquid E was used instead of Liquid B.

(Comparative Example 1)
(9) Preparation of color developer dispersion liquid (Liquid F) 40 parts of N-p-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea (product name: PF-201, manufactured by Solenis), 40 parts of a 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and 20 parts of water were mixed and pulverized using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) to an average particle size of 1.0 μm to obtain a color developer dispersion liquid (Liquid F).

(10) Preparation of color developer dispersion liquid (Liquid G) 40 parts of 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone (product name: UU, manufactured by Chemipro Chemical Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and 20 parts of water were mixed and ground using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) to an average particle size of 1.0 μm, to obtain a color developer dispersion liquid (Liquid G).

A thermal recording medium was obtained in the same manner as in Example 1, except that in the preparation of the coating material for the thermal recording layer in Example 1, 56.8 parts of F liquid and 11.4 parts of G liquid were used instead of 68.2 parts of B liquid.

(Comparative Example 2)
(11) Preparation of color developer dispersion liquid (Liquid H) 40 parts of 4-allyloxy-4'-hydroxydiphenyl sulfone (product name: BPS-MAE, manufactured by NICCA Chemical Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and 20 parts of water were mixed and ground using a sand mill (sand grinder, manufactured by IMEX Co., Ltd.) to an average particle size of 1.0 μm to obtain a color developer dispersion liquid (Liquid H).

(12) Preparation of color developer dispersion liquid (liquid I) 40 parts of a diphenylsulfone crosslinked compound (product name: D-90, manufactured by Nippon Soda Co., Ltd.) represented by the following general formula (4), 40 parts of a 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and 20 parts of water were mixed and pulverized using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) until the average particle size became 1.0 μm, to obtain a color developer dispersion liquid (liquid I).

(In the formula, n represents an integer of 1 to 6.)

A thermal recording medium was obtained in the same manner as in Example 1, except that in the preparation of the coating material for the thermal recording layer in Example 1, 56.8 parts of Liquid H and 11.4 parts of Liquid I were used instead of 68.2 parts of Liquid B.

The thermal recording medium thus obtained was evaluated as follows. The results are shown in Table 1.

[Recording Density]
Using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermal recording medium was recorded at an applied energy of 0.25 mJ/dot at which the maximum color density was obtained, and the reflection density of the recorded portion was measured with a spectrodensitometer (X-Rite 504, manufactured by X-Rite Co., Ltd.). The evaluation criteria for the recording density were as follows.
Density 1.30 or more: The print is clear and excellent.
Density 1.10 or more and less than 1.30: Barcode reading is good and there is no problem in practical use.
Density less than 1.10: Barcode readability is slightly poor, and there is a problem in practical use.

[Heat resistance at 100°C]
Using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermal recording medium was recorded with an applied energy of 0.25 mJ/dot (maximum color development density), and the obtained pieces of paper were cut into pieces 9 cm long and 4 cm wide. The obtained pieces of paper were subjected to a process of standing in a dryer at 100°C for 1 hour. The reflection density of the background part (unprinted white part) before and after the process and the recorded part after the process were measured with a spectrodensitometer (X-Rite504, manufactured by X-Rite Co., Ltd.). The evaluation criteria for the background part were as follows. The evaluation criteria for the recorded part were based on the above-mentioned recording density.
Background density is 0.10 or less: excellent heat resistance.
Background density is 0.20 or less and more than 0.10: no problem in practical use.
Background density exceeds 0.20: Heat resistance of the background is large and the heat resistance is poor.

[Transfer test]
Using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermal recording medium was recorded with an applied energy of 0.25 mJ/dot (maximum color development density), and the obtained paper piece was cut into a length of 9 cm and a width of 4 cm, and two identical pieces of paper were prepared. Water was applied to the printed surface side of each of the obtained paper pieces with a spray bottle, and the two printed surfaces were joined together. A 1 kg weight was placed on the joined test pieces, and the pieces were left to stand for three days in an environment of 23 ° C. and 65% RH. After standing for three days, the test pieces and the weight were left to stand for one day in a dryer at 40 ° C. to evaporate the moisture. After evaporating the moisture, the two test pieces were peeled off, and it was visually observed whether the print of the other was transferred to the printed surface of the other. The evaluation criteria for print transfer were as follows.
A: There is absolutely no transfer of the print when visually observed.
B: Transfer of the print is visually observed.

The thermal recording medium of the present invention is free from the risk of printing transferring to the printing surface and has excellent resistance to heat and background fogging, so it fully meets the demand for improved performance in applications such as handheld terminals and delivery slips that require water-blocking resistance and are used in harsh environments.

Claims

The thermosensitive recording medium has a thermosensitive recording layer on a support, the thermosensitive recording layer containing a leuco dye, a developer and an adhesive, the adhesive containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex, and the developer containing a compound represented by the following general formula (1):

(wherein R represents an alkyl group having 1 to 12 carbon atoms, an aralkyl group having 7 to 12 carbon atoms, or an aryl group having 6 to 12 carbon atoms, and the aralkyl group and the aryl group may be substituted with an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or a halogen atom, and a plurality of R may be the same or different; A 1 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, and a plurality of A 1 may be the same or different), an N,N'-diaryl urea compound represented by the following general formula (2):

(wherein R 1 to R 5 are the same or different and each represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group), and a compound represented by the following formula (3):

1. A thermal recording medium comprising at least one non-phenolic color developer selected from the group consisting of compounds represented by the formula:
The N,N'-diaryl urea compounds represented by the general formula (1) are N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, and N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea. , N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea.
The thermal recording medium according to claim 1, which contains N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea as the N,N'-diaryl urea compound represented by the general formula (1).
The thermal recording medium according to claim 1, which contains 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate as the compound represented by the general formula (2).
The thermal recording medium according to claim 1, wherein the content of the olefin-acrylic acid copolymer salt is 4 to 25% by mass, and the content of the styrene-butadiene latex is 15 to 40% by mass, based on the total solid content of the thermal recording layer.
The thermal recording medium according to claim 1, wherein the content of the olefin-acrylic acid copolymer salt is 10 to 150 parts by mass per 100 parts by mass of the styrene-butadiene latex.
The thermal recording medium according to claim 1, wherein the total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is 25 to 50% by mass of the total solid content of the thermal recording layer.
The thermal recording medium according to claim 1, wherein the support is synthetic paper or a synthetic resin film.
The thermal recording medium according to claim 1, wherein the support comprises a polyolefin resin.
The thermal recording medium according to claim 1, which has a protective layer on the thermal recording layer, the protective layer containing acetoacetyl-modified polyvinyl alcohol as an adhesive.