Thermal recording material

Abstract

To provide a thermal recording material that exhibits superior stability in color development in a printed portion.SOLUTION: A thermal recording material has a thermal recording layer comprising at least a leuco dye, a developer, and a sensitizer on a support, wherein the developer comprises at least two non-phenolic developers, and the sensitizer comprises a first sensitizer, which is a saturated fatty acid amide represented by the general formula (1), or comprises both the first sensitizer, which is a saturated fatty acid amide represented by the general formula (1), and a second sensitizer (where R represents an alkyl group having 15 to 21 carbon atoms).SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Thermosensitive recording media, which record color images by utilizing the thermal color-developing reaction between colorless or light-colored leuco dyes and phenols or organic acids, are widely used. Because color images are formed simply by heating, these thermal recording media offer advantages such as compact recording devices, easy maintenance, and low noise. For these reasons, thermal recording media are widely used as information recording materials in a variety of applications, including label printers and other issuing machines, automatic ticket vending machines, CD / ATMs, order slip output devices in restaurants, and data output devices in scientific research equipment.

[0003] Generally, color developers with phenolic hydroxyl groups have drawbacks such as poor thermal response and poor water resistance of printed areas, and phenolic compounds such as bisphenol A have endocrine problems. Therefore, there is a growing demand, particularly in Europe, for thermal recording paper that uses non-phenolic color developers, and various new non-phenolic color developers are being developed.

[0004] For example, Patent Document 1 proposes a thermal recording medium having a support and a thermal recording layer containing at least a leuco dye and a color developer, the color developer being N-[2-(3-phenylureido)phenyl]benzenesulfonamide, a non-phenolic color developer, and further containing at least one sensitizer selected from 2-naphthylbenzyl ether, di-p-chlorobenzyl oxalate, di-p-methylbenzyl oxalate, 1,2-di(3-methylphenoxy)ethane, 1,2-diphenoxyethane, and diphenyl sulfone, and a saturated fatty acid amide. It also reports that the thermal recording medium has high recording density and excellent resistance to heat-induced background fogging in high-temperature environments.

[0005] Patent Document 2 proposes a thermosensitive recording material comprising a support and a thermosensitive recording layer containing a color-forming compound and a color-developing compound, wherein the color-developing compound contained in the thermosensitive recording layer contains 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, a non-phenolic color developer, and at least one color-developing compound selected from diphenylsulfone compounds and non-phenolic compounds. It also reports that the thermosensitive recording material has excellent storage stability of the printed area and the background.

[0006] However, in thermal recording media using non-phenolic color developers, there is a problem that the density change immediately after printing of high gradation color density is large, impairing the gradation reproducibility of the recorded image, and the current situation is that satisfactory results are not necessarily obtained in terms of color development stability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-226848 [Patent Document 2] Japanese Patent Publication No. 2021-100798 Summary of the Invention [Problem to be solved by the invention]

[0008] A main object of the present invention is to provide a thermosensitive recording medium having excellent color development stability in the printed area. [Means for solving the problem]

[0009] The present inventors have conducted extensive research in light of the above-mentioned prior art and have come to solve the above-mentioned problems. That is, the present invention relates to the following thermosensitive recording medium. Item 1: A thermosensitive recording medium having a thermosensitive recording layer containing at least a leuco dye, a color developer, and a sensitizer on a support, The developer contains at least two non-phenolic developers, The sensitizer contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer, or contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer and a second sensitizer: A thermal recording medium characterized by: [ka] (In the formula, R represents an alkyl group having 15 to 21 carbon atoms.) Item 2: The thermal recording medium according to Item 1, wherein the color developers comprise at least two non-phenolic color developers selected from the group consisting of Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Item 3: The thermal recording medium according to Item 1, containing, as the developer, Np-tolylsulfonyl-N'-3-(p-trisulfonyloxy)phenylurea and at least one selected from N-[2-(3-phenylureido)phenyl]benzenesulfonamide and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Item 4: The thermosensitive recording medium according to Item 1, wherein the color developers are Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Item 5: The thermosensitive recording medium according to any one of Items 1 to 4, wherein the saturated fatty acid amide is stearic acid amide. Item 6: The thermosensitive recording material according to any one of Items 1 to 5, wherein the second sensitizer is at least one selected from the group consisting of diphenoxyethane compounds and diphenyl sulfones. Item 7: The thermosensitive recording material according to any one of Items 1 to 6, wherein the second sensitizer is 1,2-diphenoxyethane or diphenyl sulfone. Item 8: The thermosensitive recording medium according to any one of Items 1 to 7, wherein the saturated fatty acid amide accounts for 60 to 80% by mass of the total amount of the sensitizer. Item 9: The thermosensitive recording medium according to any one of Items 1 to 8, wherein the second sensitizer is contained in a ratio of 1 part by mass or less to 1 part by mass of the first sensitizer. Item 10: The thermosensitive recording medium according to any one of Items 1 to 9, wherein the support is synthetic paper. Item 11: The thermosensitive recording medium according to any one of Items 1 to 10, which has an undercoat layer between the support and the thermosensitive recording layer. [Effects of the Invention]

[0010] The thermosensitive recording medium of the present invention has excellent color stability in the printed area, and its sensitivity at low gradation levels is at a level that does not pose any practical problems. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this specification, the expression "comprise" includes the concepts of "comprise," "consist essentially of," and "consist only of."

[0012] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] The latex in the present invention includes a gel or a dried film formed by drying a dispersion medium.

[0014] The present invention provides a thermosensitive recording medium having a thermosensitive recording layer containing at least a leuco dye, a color developer, and a sensitizer on a support, The developer contains at least two non-phenolic developers, The sensitizer contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer, or contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer and a second sensitizer: It is characterized by: [ka] (In the formula, R represents an alkyl group having 15 to 21 carbon atoms.)

[0015] [Support] The support in the present invention is not particularly limited in type, shape, size, etc., and can be appropriately selected from various transparent supports, such as high-quality paper (acid paper, neutral paper), medium-quality paper, coated paper, art paper, cast-coated paper, glassine paper, resin-laminated paper, synthetic paper (e.g., polyolefin-based synthetic paper), synthetic fiber paper, nonwoven fabric, synthetic resin film, etc., and is preferably synthetic paper. The thickness of the support is not particularly limited, and is usually about 20 to 200 μm. The density of the support is also not particularly limited, and is 0.60 to 0.85 g / cm. 3 The degree is preferable.

[0016] [Thermal recording layer] (leuco dye) The thermosensitive recording layer of the thermosensitive recording medium of the present invention may contain various known colorless or light-colored leuco dyes. Specific examples of such leuco dyes are listed below.

[0017] Specific examples of leuco dyes include blue-coloring dyes such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, and fluoran; green dyes such as 3-(N-ethyl-Np-tolyl)amino-7-N-methylanilinofluoran, 3-diethylamino-7-anilinofluoran, 3-diethylamino-7-dibenzylaminofluoran, and rhodamine B-anilinolactam; Red color-forming dyes such as 3,6-bis(diethylamino)fluoran-γ-anilinolactam, 3-cyclohexylamino-6-chlorofluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-7-chlorofluoran, 3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexyl)amino-6-methyl-7-anilinofluoran, 3-diethyl ... -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-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-(N-isoamyl-N-ethylamino)-7-(o-chloroanilino)fluoran, 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3- (Nn-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, 4,4'-bis-dimethylaminobenzhydrin benzyl ether, N-2,4,5-Trichlorophenylleucoauramine, 3-diethylamino-7-butylaminofluoran, 3-ethyl-tolylamino-6-methyl-7-anilinofluoran, 3-cyclohexyl-methylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-(β-ethoxyethyl)aminofluoran, 3-diethylamino-6-chloro-7-(γ-chloropropyl)aminofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-(N-isopropyl)aminofluoran Soamyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-dibutylamino-7-chloroanilinofluoran, 3-diethylamino-7-(o-chlorophenylamino)fluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-(p-toluidino)fluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino -6-chloro-7-anilinofluoran, 3-dimethylamino-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 2,2-bis{4-[6'-(N-cyclohexyl-N-methylamino)-3'-methylspiro[phthalido-3,9'-xanthene]-2'-ylamino]phenyl}propane, 3-diethylamino-7-(3'-trifluoromethylphenyl)aminofluoran, etc. Color dyes, 3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3-p-(p-dimethylaminoanilino)anilino-6-methyl-7-chlorofluoran, 3-p-(p-chloroanilino)anilino-6-methyl-7-chlorofluoran, 3,Examples of such dyes include dyes with absorption wavelengths in the near-infrared region, such as 6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)phthalide. However, the present invention is not limited to these examples, and two or more compounds can be used in combination as needed.

[0018] The content of the leuco dye is not particularly limited, and is preferably about 3 to 30% by mass, more preferably about 5 to 25% by mass, and even more preferably about 7 to 25% by mass, of the total solid content of the thermosensitive recording layer. By making it 3% by mass or more, the color-developing ability can be improved and the print density can be improved. By making it 30% by mass or less, the heat resistance can be improved.

[0019] (developer) The thermosensitive recording layer of the thermosensitive recording medium of the present invention contains at least two non-phenolic color developers as color developers. By combining the non-phenolic color developers with a saturated fatty acid amide sensitizer, excellent color stability of the printed area can be achieved. Furthermore, sensitivity at low gradations can be improved to a practically acceptable level, and heat resistance and humidity resistance can also be improved. Specific examples of such non-phenolic color developers include urea urethane derivatives such as Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 4,4'-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, N,N'-di[3-(p-toluenesulfonyl)oxy]phenylurea, 5-(N-3-methylphenyl-sulfonamido)-(N',N''-bis-(3-methylphenyl)-isophthalic acid amide, and 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone represented by the following general formula (2), and diphenylurea.

[0020] [ka]

[0021] The developer preferably contains at least two non-phenolic developers selected from the group consisting of Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Because this also improves heat resistance and humidity resistance, the developer more preferably contains at least one selected from Np-tolylsulfonyl-N'-3-(p-trisulfonyloxy)phenylurea and N-[2-(3-phenylureido)phenyl]benzenesulfonamide and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. In order to further improve moisture resistance, it is even more preferable to contain three non-phenolic color developers, namely, Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, as color developers.

[0022] The content (total amount) of the non-phenolic color developer is not particularly limited and can be adjusted depending on the leuco dye used. Generally, it is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, even more preferably 1.5 parts by weight or more, even more preferably 2.0 parts by weight or more, and particularly preferably 2.5 parts by weight or more, per 1 part by weight of leuco dye. Furthermore, the content (total amount) of the non-phenolic color developer is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, even more preferably 4.5 parts by weight or less, and particularly preferably 4.0 parts by weight or less, per 1 part by weight of leuco dye. By setting the content to 0.5 parts by weight or more, recording performance can be improved. On the other hand, by setting the content to 10 parts by weight or less, background fogging in high-temperature environments can be effectively suppressed.

[0023] Other color developers may be contained as long as they do not impair the effects of the present invention. Specific examples of other color developers include 4-tert-butylphenol, 4-acetylphenol, 4-tert-octylphenol, 4,4'-sec-butylidenediphenol, 4-phenylphenol, 4,4'-dihydroxydiphenylmethane, 4,4'-isopropylidenediphenol, 4,4'-cyclohexylidene diphenyl, 4,4'-cyclohexylidene diphenol, 1,1-bis(4-hydroxyphenyl)-ethane, and 1,1-bis(4-hydroxyphenyl)-1-phenylethane. 4,4'-Dihydroxydiphenyl sulfide, 4,4'-thiobis(3-methyl-6-tert-butylphenol), 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropanol 4-hydroxy-4'-n-propoxydiphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, 4-hydroxy-4'-benzyloxydiphenyl sulfone, 3,3'-diallyl-4,4'-dihydroxydiphenyl sulfone, butyl bis(p-hydroxyphenyl)acetate, methyl bis(p-hydroxyphenyl)acetate, hydroquinone monobenzyl ether, bis(3-allyl-4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl Sulfon, 4-allyloxy-4'-hydroxydiphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 4-hydroxybenzophenone, dimethyl 4-hydroxyphthalate, methyl 4-hydroxybenzoate, propyl 4-hydroxybenzoate, sec-butyl 4-hydroxybenzoate, phenyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate ester, tolyl 4-hydroxybenzoate, chlorophenyl 4-hydroxybenzoate, 4,phenolic compounds such as 4'-dihydroxydiphenyl ether, and aromatic carboxylic acids such as benzoic acid, p-chlorobenzoic acid, p-tert-butylbenzoic acid, trichlorobenzoic acid, terephthalic acid, salicylic acid, 3-tert-butylsalicylic acid, 3-isopropylsalicylic acid, 3-benzylsalicylic acid, 3-(α-methylbenzyl)salicylic acid, 3,5-di-tert-butylsalicylic acid, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, and zinc 4-[3-(p-tolylsulfonyl)propyloxy]salicylate, and mixtures of these phenolic compounds, aromatic carboxylic acids with, for example, zinc, magnesium, aluminum, calcium Examples of suitable organic acidic substances include salts with polyvalent metals such as titanium, manganese, tin, and nickel, as well as organic acidic substances such as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids, urea compounds such as Np-toluenesulfonyl-N'-p-butoxycarbonylphenylurea and Np-tolylsulfonyl-N'-phenylurea, thiourea compounds such as N,N'-di-m-chlorophenylthiourea, organic compounds having a -SONH- bond in the molecule such as N-(p-toluenesulfonyl)carbamoyl acid p-cumylphenyl ester, N-(p-toluenesulfonyl)carbamoyl acid p-benzyloxyphenyl ester, and N-(o-toluoyl)-p-toluenesulfamide, and inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate.

[0024] Further examples include diphenyl sulfone derivatives represented by the following general formula (3): Of course, the compounds are not limited to these, and two or more compounds can be used in combination as needed.

[0025] [ka] (In the formula, n represents an integer of 1 to 6.)

[0026] (sensitizer) In the present invention, the thermosensitive recording layer contains a saturated fatty acid amide represented by general formula (1) as a first sensitizer, or a saturated fatty acid amide represented by general formula (1) as a first sensitizer and a second sensitizer. This improves the color development stability of the printed area. Furthermore, sensitivity at low gradations can be maintained at a practically acceptable level. Furthermore, the gradation reproducibility of highlight areas of the image is excellent.

[0027] The saturated fatty acid amide represented by the general formula (1) is preferably at least one selected from palmitic acid amide, stearic acid amide, arachidic acid amide, and behenic acid amide, and more preferably stearic acid amide.

[0028] The content of the sensitizer is not particularly limited, and is usually preferably about 2 to 40% by mass, more preferably about 10 to 35% by mass, of the total solid content of the thermosensitive recording layer. By making it 2% by mass or more, it is possible to increase the recording sensitivity. On the other hand, by making it 40% by mass or less, it is possible to improve the heat resistance.

[0029] The second sensitizer is not particularly limited, and examples thereof include 2-naphthylbenzyl ether, di-p-chlorobenzyl oxalate, di-p-methylbenzyl oxalate, diphenoxyethane compounds (1,2-di(3-methylphenoxy)ethane, 1,2-diphenoxyethane, etc.), diphenyl sulfone, dimethyl terephthalate, and 3,3'-diaminodiphenyl sulfone. Among these, diphenoxyethane compounds and diphenyl sulfone are preferred, and 1,2-diphenoxyethane and diphenyl sulfone are more preferred from the viewpoint of excellent color development stability of the printed portion. These may be used alone or in combination of two or more.

[0030] When the sensitizer contains a saturated fatty acid amide represented by general formula (1) as a first sensitizer and a second sensitizer, it is preferable to contain the second sensitizer in a ratio of 1 part by mass or less per 1 part by mass of the first sensitizer. By containing the second sensitizer in a ratio of 1 part by mass or less, the color development stability of the printed area can be improved. It is preferable to contain the second sensitizer in a ratio of less than 1 part by mass, more preferably 0.8 parts by mass or less, and even more preferably 0.6 parts by mass or less. On the other hand, it is preferable to contain the second sensitizer in a ratio of 0.05 parts by mass or more. To improve sensitivity at low gradations, it is more preferable to contain the second sensitizer in a ratio of 0.1 parts by mass or more, and more preferably 0.4 parts by mass or more.

[0031] The proportion of saturated fatty acid represented by general formula (1) in the total amount of sensitizer is not particularly limited, but is preferably 60 to 80% by mass. By making it 60% by mass or more, the color development stability of the printed part can be improved. By making it 80% by mass or less, the sensitivity at low gradation can be improved.

[0032] Other sensitizers may be contained as long as they do not impair the effects of the present invention. Specific examples of other sensitizers include methoxycarbonyl-N-stearic acid benzamyl, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, methylene bisstearic acid amide, N-methylolstearic acid amide, dibenzyl terephthalate, dioctyl terephthalate, benzyl p-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthoate, m-terphenyl, p-benzylbiphenyl, dibenzyl oxalate, p-tolylbiphenyl ether, di(p-methoxyphenoxyethyl)ether, 1,2-di(4-methylphenoxy)ethane, and 1,2-di(4-methoxyphenoxy)ethane. , 1,2-di(4-chlorophenoxy)ethane, 1-(4-methoxyphenoxy)-2-(3-methylphenoxy)ethane, 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, etc. These can be used in combination as long as no problems occur.

[0033] (Storage improver) In the present invention, the heat-sensitive recording layer may further contain a storage stability improver, 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)phenyl sulfone, epoxy compounds such as 4-(2-methyl-1,2-epoxyethyl)diphenyl sulfone, and 4-(2-ethyl-1,2-epoxyethyl)diphenyl sulfone, 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 compounds are not limited to these, and two or more compounds can be used in combination as needed.

[0034] 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 30% by mass, and more preferably about 5 to 20% by mass, of the total solid content of the thermosensitive recording layer.

[0035] (waxes) The thermosensitive recording layer of the present invention may contain waxes, such as paraffin wax, carnauba wax, microcrystalline wax, polyolefin wax, and polyethylene wax; higher fatty acid amides such as ethylene bisstearic acid amide, higher fatty acid esters, and derivatives thereof; and higher fatty acid polyvalent metal salts such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate.

[0036] (pigment) The thermosensitive recording layer of the present invention may contain a pigment. Examples of pigments include inorganic pigments such as clay, calcined clay, diatomaceous earth, talc, kaolin, calcined kaolin, light or heavy calcium carbonate, precipitated calcium carbonate, magnesium carbonate, zinc oxide, aluminum oxide, aluminum hydroxide, magnesium hydroxide, titanium dioxide, barium sulfate, zinc sulfate, amorphous silica, amorphous calcium silicate, and colloidal silica, as well as organic pigments such as melamine resin, urea-formalin resin, polyethylene, nylon, styrene, acrylic, and hydrocarbons. The pigment content is preferably an amount that does not reduce the color density, i.e., 50% by mass or less of the total solid content of the thermosensitive recording layer.

[0037] Other components constituting the heat-sensitive recording layer include adhesives, and if necessary, auxiliary agents such as crosslinking agents, water-resistant agents, dispersants, colored dyes, and fluorescent dyes can be used.

[0038] (glue) The adhesive used in the coating liquid for the thermosensitive recording layer may be, for example, either a water-soluble adhesive or a water-dispersible adhesive. Examples of water-soluble adhesives include modified polyvinyl alcohols such as polyvinyl alcohol, carboxy-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, and silicon-modified polyvinyl alcohol, starch and its derivatives, cellulose derivatives such as methoxycellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, methylcellulose, and ethylcellulose, sodium polyacrylate, polyvinylpyrrolidone, polyamide, diisobutylene-maleic anhydride copolymer salt, styrene-acrylic acid copolymer salt, styrene-maleic anhydride copolymer salt, ethylene-maleic anhydride copolymer salt, acrylamide-acrylic acid ester copolymer, acrylamide-acrylic acid ester-methacrylic acid copolymer, polyacrylamide, sodium alginate, gelatin, casein, and gum arabic. Examples of water-dispersible adhesives include emulsions of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic ester, vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, ethylene-vinyl acetate copolymer, etc., or latexes of water-insoluble polymers such as styrene-butadiene copolymer, styrene-butadiene-acrylic copolymer, etc. One type of adhesive can be used alone, or two or more types can be used in combination. The content of the adhesive can be selected from a wide range, but is generally blended in a range of preferably about 5 to 30 mass %, more preferably about 10 to 20 mass %, of the total solid content of the thermosensitive recording layer.

[0039] (Crosslinking agent) A crosslinking agent that hardens the adhesive of the thermosensitive recording layer or other layers can be incorporated into the thermosensitive recording layer. This improves the water resistance of the thermosensitive 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, 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, and glyoxylates. These may be used alone or in combination of two or more. The amount of crosslinking agent used is preferably in the range of approximately 1 to 10 parts by mass per 100 parts by mass of the total solid content of the thermosensitive recording layer. This improves the water resistance of the thermosensitive recording layer.

[0040] If necessary, various auxiliary agents such as an oil repellent, an antifoaming agent, a viscosity adjusting agent, etc. may be added to the heat-sensitive recording layer within the range that does not impair the effects of the present invention.

[0041] The thermosensitive recording layer is generally formed on base paper by dispersing a leuco dye, developer, sensitizer, and optionally a storage stability improver, together or separately, in water as a dispersion medium, using various stirring and wet grinding machines such as ball mills, co-ball 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 salts, and other surfactants to form a dispersion, and then dispersing the dispersion to an average particle size of 2 μm or less, and mixing with adhesives, waxes, pigments, auxiliary agents, etc. as necessary, to prepare a coating liquid for the thermosensitive recording layer. There are no particular restrictions on the coating amount of the thermosensitive recording layer, and the dry mass is 1 to 12 g / m. 2 The preferred range is 2 to 10 g / m 2 More preferably, 2.5 to 8 g / m 2 More preferably, 3 to 5.5 g / m 2The heat-sensitive recording layer can be formed in two or more layers as needed, and the composition and coating amount of each layer may be the same or different.

[0042] [Undercoat layer] The thermosensitive recording medium of the present invention may also have an undercoat layer between the support and the thermosensitive recording layer, if necessary. The undercoat layer preferably contains an adhesive.

[0043] (glue) Examples of adhesives include polyvinyl alcohol and its derivatives, starch and its derivatives, cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose, water-soluble polymer materials such as sodium polyacrylate, polyvinylpyrrolidone, acrylamide-acrylic acid ester copolymers, acrylamide-acrylic acid ester-methacrylic acid ester copolymers, styrene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, casein, gelatin, and their derivatives, as well as emulsions of polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid esters, vinyl chloride-vinyl acetate copolymers, polybutyl methacrylate, and ethylene-vinyl acetate copolymers, and latexes of water-insoluble polymers such as styrene-butadiene copolymers and styrene-butadiene-acrylic copolymers. The adhesive content can be selected from a wide range, but is generally preferably about 10 to 70% by weight, and more preferably about 15 to 60% by weight, of the total solids content of the undercoat layer.

[0044] The adhesive preferably contains a binder resin with a glass transition temperature (Tg) of -10°C or lower. A glass transition temperature of -10°C or lower can improve image quality even in low energy regions. A glass transition temperature of -30°C or lower is more preferable because it can further improve image quality in low energy regions. On the other hand, a glass transition temperature of -50°C or lower is undesirable because stickiness occurs, so a glass transition temperature of -40°C or higher is preferable.

[0045] (hollow particles) The undercoat layer may contain hollow particles. The hollow particles are preferably made of an organic resin from the viewpoint of improving cushioning properties. The undercoat layer, which has high heat insulating properties due to the inclusion of hollow particles, can prevent the diffusion of heat applied to the thermosensitive recording layer and increase the sensitivity of the thermosensitive recording medium.

[0046] Hollow particles made of organic resins can be divided into expanded and non-expanded types depending on their manufacturing method. Of these two types, expanded hollow particles generally have a larger average particle diameter and a higher hollowness than non-expanded hollow particles. Therefore, expanded hollow particles can achieve better sensitivity and image quality than non-expanded hollow particles.

[0047] Non-expanded hollow particles can be produced by polymerizing seeds in a solution, polymerizing another resin so that the seeds are wrapped around the seeds, and then swelling and dissolving the seeds to remove them, thereby forming internal cavities. An alkaline aqueous solution or the like is used to swell and dissolve the seeds to remove them. Non-expanded hollow particles with a relatively large average particle size can also be obtained by subjecting core-shell particles, in which alkali-swellable core particles are coated with a shell layer that is not alkali-swellable, to an alkali swelling treatment.

[0048] Expanded hollow particles can be produced by preparing particles in which a volatile liquid is enclosed inside a resin, and then softening the resin by heating, thereby vaporizing and expanding the liquid inside the particles.

[0049] Expanded hollow particles have a high hollow ratio and high thermal insulation properties due to the thermal expansion of the liquid inside during the manufacturing process, which can increase the sensitivity of the thermosensitive recording medium and improve recording density. Improved sensitivity is particularly important when coloring a mid-tone region where the thermal energy applied to the thermosensitive recording layer is small. Furthermore, forming the thermosensitive recording layer via a highly insulating undercoat layer can prevent the diffusion of heat applied to the thermosensitive recording layer, resulting in excellent image uniformity and improved image quality.

[0050] Resins that can be used for the expanded hollow particles include thermoplastic resins such as styrene-acrylic resin, polystyrene resin, acrylic resin, polyethylene resin, polypropylene resin, polyacetal resin, chlorinated polyether resin, polyvinyl chloride resin, polyvinylidene chloride resin, acrylic resin (e.g., acrylic resin containing acrylonitrile as a constituent component), styrene resin, vinylidene chloride resin, and copolymer resins mainly composed of polyvinylidene chloride and acrylonitrile. Typical gases contained inside the expanded hollow particles include propane, butane, isobutane, and air. Among the various resins listed above, acrylonitrile resin and copolymer resins mainly composed of polyvinylidene chloride and acrylonitrile are preferred for the resins used for the hollow particles in terms of the strength required to maintain the shape of the expanded particles.

[0051] The maximum particle diameter of the hollow particles in the present invention is preferably 10 to 30 μm, more preferably 10 to 25 μm. The maximum particle diameter is also referred to as D100. When the maximum particle diameter of the hollow particles is 10 μm or more, the cushioning properties of the undercoat layer are improved, thereby improving the adhesion of the thermosensitive recording medium to the thermal head during printing, resulting in a thermosensitive recording medium with high image quality. This high image quality can result in improved recording density in halftones that are developed with lower energy than that required to achieve the maximum recording density (Dmax). On the other hand, when the maximum particle diameter of the hollow particles is 30 μm or less, the smoothness of the undercoat layer is improved, allowing the thermosensitive recording layer provided via the undercoat layer to be uniform, resulting in a thermosensitive recording medium with fewer white gaps in the image.

[0052] The average particle diameter of the hollow particles in the present invention is preferably 4.0 to 15 μm, more preferably 4.5 to 15 μm. Here, the average particle diameter is the diameter at which the larger and smaller particles occupy equal volumes when divided into two groups based on particle diameter, i.e., the median particle diameter, also referred to as D50, which is the particle diameter at 50% volume frequency. When the average particle diameter of the hollow particles is 4.0 μm or greater, the cushioning properties of the undercoat layer are improved, improving adhesion of the thermal recording medium to the thermal head during printing and resulting in a thermal recording medium with high image quality. This high image quality can result in improved recording density in halftones that are developed with lower energy than that required to achieve the maximum recording density (Dmax). On the other hand, when the average particle diameter of the hollow particles is 15 μm or less, the smoothness of the undercoat layer is improved, allowing for a uniform thermal recording layer formed through the undercoat layer, resulting in a thermal recording medium with less white gaps in the image.

[0053] The maximum particle size (D100) and average particle size (D50) of hollow particles can be measured using a laser diffraction particle size analyzer. Alternatively, particle sizes can be measured from particle images (SEM images) using an electron microscope and expressed as the average value of 10 particles.

[0054] The ratio D100 / D50 of the maximum particle diameter (D100) to the average particle diameter (D50) of hollow particles is an index showing the degree of particle size distribution. This ratio D100 / D50 is preferably 1.8 to 3.0, more preferably 2.0 to 2.8. When the D100 / D50 of the hollow particles is 1.8 or more, the hollow particles are sufficiently expanded, the maximum particle diameter is sufficiently large, the hollow ratio is high, and the heat insulating properties of the primer layer can be improved. On the other hand, when the D100 / D50 of the hollow particles is 3.0 or less, the hollow particles are uniform in size, which improves the smoothness of the primer layer and reduces white spots in the image.

[0055] In the particle size distribution determined by a laser diffraction particle size analyzer, the volume percentage of hollow particles with a particle diameter of 2.0 μm or less is preferably 1% or less. Furthermore, the volume percentage of hollow particles with a particle diameter of 2.0 μm or less is preferably 0.5% or less, and more preferably none at all. Hollow particles with a particle diameter of 2 μm or less are too small to provide sufficient hollow regions, and therefore are thought to contribute very little to heat insulation. By keeping the volume percentage of hollow particles with a particle diameter of 2 μm or less in the undercoat layer to 1% or less, it is possible to improve recording density, image quality, etc.

[0056] The hollow particles preferably have a void ratio of 80 to 98%, more preferably 90 to 98%. When the hollow ratio of hollow particles is 80% or more, high heat insulating properties can be imparted to the primer layer containing the hollow particles. On the other hand, when the hollow ratio of hollow particles is 98% or less, the strength of the film surrounding the hollow portion can be improved, thereby making the hollow particles less likely to be crushed during the formation of the primer layer.

[0057] The hollowness of hollow particles can be determined by measuring the true specific gravity using the IPA method and then calculating the hollowness from the true specific gravity value as follows. (1) Sample pretreatment Dry the sample at 60°C overnight to prepare the sample. (2) Reagents Isopropyl alcohol (IPA: first-grade reagent) (3) Measurement method ·Weigh the volumetric flask accurately (W1). ·Place approximately 0.5 g of the dried sample in a measuring flask and weigh it out accurately (W2). Add approximately 50 mg of IPA and shake thoroughly to completely remove any air from the capsule. Add IPA up to the mark and measure (W3). As a blank, add only IPA to the volumetric flask up to the mark and measure carefully (W4). (4) Calculation of true specific gravity True specific gravity = {(W2-W1)×((W4-W1) / 100)} / {(W4-W1)-(W3-W2)} (5) Calculation of hollow ratio Hollowness ratio (%)={1-1 / (1.1 / true specific gravity)}×100

[0058] The hollow ratio is calculated by the following formula (d 3 / D 3 ) × 100. In this formula, d represents the inner diameter of the hollow particle, and D represents the outer diameter of the hollow particle.

[0059] The content of hollow particles is preferably 5 to 40% by mass, and more preferably 5 to 35% by mass, of the total solid content of the undercoat layer. When the content of hollow particles is 5% by mass or more, the heat insulating properties of the undercoat layer can be improved. On the other hand, when the content of hollow particles is 40% by mass or less, problems in terms of coatability and the like are less likely to occur, a uniform undercoat layer can be easily formed, and recording density can be improved. In addition, the coating strength of the undercoat layer can be increased.

[0060] The undercoat layer may contain an oil-absorbing pigment having an oil absorption of 70 ml / 100 g or more, particularly about 80 to 150 ml / 100 g, and / or thermally expandable particles, where the oil absorption is determined according to the method of JIS K 5101.

[0061] Various oil-absorbing pigments can be used, and specific examples include inorganic pigments such as calcined kaolin, amorphous silica, precipitated calcium carbonate, and talc. The average primary particle size of these oil-absorbing pigments is preferably about 0.01 to 5 μm, and particularly about 0.02 to 3 μm. The amount of oil-absorbing pigment used can be selected from a wide range, but is generally preferably about 20 to 80 mass % of the total solids content of the undercoat layer, and more preferably about 25 to 75 mass %.

[0062] The undercoat layer is formed on the support by, for example, applying a coating liquid for the undercoat layer prepared by mixing an adhesive, hollow particles, a pigment, an auxiliary agent, etc., with water as a medium, and then drying the coating liquid. The amount of the coating liquid for the undercoat layer to be applied is not particularly limited, but is preferably 2 to 20 g / m2 in terms of dry mass. 2 The preferred range is 2 to 12 g / m 2 A degree is more preferable.

[0063] Examples of auxiliaries contained in the coating liquid for the undercoat layer include dispersants such as sodium dioctyl sulfosuccinate, sodium dodecylbenzenesulfonate, sodium lauryl alcohol sulfate, and fatty acid metal salts; waxes such as zinc stearate, calcium stearate, polyethylene wax, carnauba wax, paraffin wax, and ester wax; water-resistant agents such as hydrazide compounds, boric acid, dialdehyde starch, glyoxylates, and epoxy compounds; antifoaming agents; coloring dyes; and fluorescent dyes.

[0064] [Protective layer] The thermosensitive recording medium may also have a protective layer on the thermosensitive recording layer, if necessary. The protective layer preferably contains a pigment and an adhesive. Furthermore, the protective layer preferably contains a lubricant such as polyolefin wax, paraffin wax, or zinc stearate to prevent sticking to the thermal head, and may also contain an ultraviolet absorber. Furthermore, providing a glossy protective layer can also increase the added value of the product.

[0065] The pigment contained in the protective layer is not particularly limited, and examples thereof include inorganic pigments such as amorphous silica, hydrous 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.

[0066] 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. Among 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.

[0067] The protective layer is formed on the thermosensitive recording layer by, for example, applying a coating liquid for the protective layer, which is prepared by mixing a pigment, an adhesive, and, if necessary, an auxiliary agent, etc., in water as a dispersion medium, and then drying the coating liquid. The amount of the coating liquid for the protective layer is not particularly limited, and is preferably 0.3 to 15 g / m2 in terms of dry mass. 2 The preferred range is 0.3 to 10 g / m 2 The preferred range is 0.5 to 8 g / m 2 More preferably, 1 to 8 g / m 2 The preferred range is 1 to 5 g / m 2 The protective layer may be formed in two or more layers as needed, and the compositions and coating amounts of the layers may be the same or different.

[0068] [Other layers] In the present invention, it is preferable that the support has an adhesive layer on at least one side. This increases the added value of the thermosensitive recording medium. For example, the adhesive layer can be coated on one side with a pressure-sensitive adhesive, rewettable adhesive, delayed-tack type pressure-sensitive adhesive, or the like to produce adhesive paper, rewettable adhesive paper, delayed-tack paper, or the like. Furthermore, the surface of the support opposite the thermosensitive recording layer can be given the functionality of thermal transfer paper, inkjet recording paper, carbonless paper, electrostatic recording paper, xeography paper, or the like, to produce a recording paper capable of double-sided recording. Of course, a double-sided thermosensitive recording medium can also be produced. A backing layer can also be provided to inhibit penetration of oil and plasticizers from the backside of the thermosensitive recording medium, to control curl, and to prevent static electricity. A linerless label that does not require a release paper can be produced by coating a protective layer with a release layer containing silicone and then coating one side with an adhesive.

[0069] [Thermal recording medium] The thermosensitive recording medium can be produced by forming each of the above layers on a support. The method for forming each of the above layers may be any known coating method, 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, or extrusion method. Each layer may be formed by applying and drying each coating liquid one by one, or the same coating liquid may be applied in two or more layers. Simultaneous multilayer coating, in which two or more layers are applied simultaneously, may also be performed. After each layer has been formed, or after all layers have been formed, the surface may be smoothed using a known method, such as a supercalender or soft calender.

[0070] The method for recording an image on the thermal recording medium of the present invention is not particularly limited and can be appropriately selected depending on the purpose. For example, a thermal head printer, laser light (e.g., carbon dioxide laser, UV laser, semiconductor laser light, YAG laser light, fiber laser light, solid laser light, dye laser light, etc.), etc. can be used as the image recording method. When laser light is used, the wavelength of the laser light is not particularly limited and can be appropriately selected depending on the purpose. [Example]

[0071] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" and "%" represent "parts by mass" and "% by mass", respectively.

[0072] (1) Preparation of leuco dye dispersion (liquid A) 40 parts of 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 0.7 μm, thereby obtaining a leuco dye dispersion liquid (Liquid A).

[0073] (2) Preparation of developer dispersion liquid (liquid B) 40 parts of Np-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 1.0 μm, to obtain a developer dispersion (Liquid B).

[0074] (3) Preparation of developer dispersion liquid (liquid C) 40 parts of N-[2-(3-phenylureido)phenyl]benzenesulfonamide, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 1.0 μm, to obtain a developer dispersion liquid (Liquid C).

[0075] (4) Preparation of developer dispersion liquid (liquid D) 40 parts of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 0.8 μm, to obtain a developer dispersion liquid (Liquid D).

[0076] (5) Preparation of sensitizer dispersion (liquid E) 40 parts of 1,2-di(3-methylphenoxy)ethane, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 1.0 μm, thereby obtaining a sensitizer dispersion (Solution E).

[0077] (6) Preparation of sensitizer dispersion (liquid F) 40 parts of oxalic acid di-p-methylbenzyl ester, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 0.9 μm, thereby obtaining a sensitizer dispersion (Liquid F).

[0078] (7) Preparation of sensitizer dispersion (liquid G) 40 parts of diphenyl sulfone, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 1.2 μm, thereby obtaining a sensitizer dispersion (Solution G).

[0079] (8) Preparation of sensitizer dispersion (liquid H) 40 parts of 1,2-diphenoxyethane, 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 1.2 μm, thereby obtaining a sensitizer dispersion (Liquid H).

[0080] (9) Preparation of pigment dispersion liquid (liquid I) 100 parts of hydrous kaolin, 1.3 parts of a dispersant (trade name: Aron T-50, manufactured by Toa Gosei Co., Ltd., solids concentration 40%), and 66.3 parts of water were mixed and ground using a sand mill (sand grinder, manufactured by Imex Co., Ltd.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) was 0.9 μm, thereby obtaining a pigment dispersion (Liquid I).

[0081] (10) Preparation of pigment dispersion liquid (liquid J) A pigment dispersion liquid (Liquid J) was obtained by mixing and stirring 100 parts of calcined kaolin, 0.8 parts of a dispersant (trade name: Aron T-50, manufactured by Toagosei Co., Ltd., solid content concentration 40%), and 127 parts of water.

[0082] Example 1 (11) Preparation of coating liquid for thermosensitive recording layer A coating liquid for a thermal recording layer was obtained by mixing and stirring 9.5 parts of Solution A, 17.2 parts of Solution C, 17.2 parts of Solution D, 15.4 parts of a 13% aqueous solution of polyvinyl alcohol (trade name: JF-05, manufactured by Nippon Vinyl Acetate Co., Ltd.), 15.4 parts of a 13% aqueous solution of polyvinyl alcohol (trade name: JF-10, manufactured by Nippon Vinyl Acetate Co., Ltd.), 13.5 parts of styrene-butadiene latex (trade name: L-1571, manufactured by Asahi Kasei Corporation, solids concentration: 48%), 2 parts of amorphous silica (trade name: Nipsil E-743, manufactured by Tosoh Silica Corporation), 29.2 parts of stearic acid amide (trade name: Himicron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration: 25%), 8.3 parts of Solution H, and 50.5 parts of water.

[0083] (12) Preparation of protective layer coating liquid A coating liquid for the protective layer was obtained by mixing and stirring 50.7 parts of liquid I, 125 parts of a 12% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenex Z-200, manufactured by Mitsubishi Chemical Corporation), 6.9 parts of zinc stearate (trade name: Hydrin Z-9-36, manufactured by Chukyo Yushi Co., Ltd., solids concentration 36%), 4.2 parts of paraffin wax (trade name: Hydrin P-7, manufactured by Chukyo Yushi Co., Ltd., solids concentration 30%), and 87 parts of water.

[0084] (13) Preparation of thermal recording medium Basis weight 62g / m 2 The coating liquid for the thermal recording layer and the coating liquid for the protective layer were applied to one side of a synthetic paper (FPH80, manufactured by Yupo Corporation) in an amount of 3.0 g / m after drying. 2 , 3.3g / m 2 The coating was then dried to form a thermal recording layer and a protective layer in this order, and the surface was smoothed by a supercalender to obtain a thermal recording medium.

[0085] Example 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of liquid B was used instead of 17.2 parts of liquid C.

[0086] Example 3 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Solution B was used instead of 17.2 parts of Solution D.

[0087] Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of liquid C and 17.2 parts of liquid D were replaced with 25.8 parts of liquid B and 8.6 parts of liquid C.

[0088] Example 5 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of liquid C and 17.2 parts of liquid D were replaced with 16.4 parts of liquid B, 11.5 parts of liquid C, and 6.6 parts of liquid D.

[0089] Example 6 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Solution C and 17.2 parts of Solution D were replaced with 16.4 parts of Solution B, 11.5 parts of Solution C, and 6.6 parts of Solution D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was replaced with 43.8 parts instead of 29.2 parts, the amount of Solution H was replaced with 0 parts instead of 8.3 parts, and the amount of water was replaced with 44.5 parts instead of 50.5 parts.

[0090] Example 7 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Solution C and 17.2 parts of Solution D were replaced with 16.4 parts of Solution B, 11.5 parts of Solution C, and 6.6 parts of Solution D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was changed from 29.2 parts to 39.4 parts, the amount of Solution H was changed from 8.3 parts to 2.5 parts, and the amount of water was changed from 50.5 parts to 46 parts.

[0091] Example 8 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, and 8.3 parts of Liquid G were replaced with 8.3 parts of Liquid H.

[0092] Example 9 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, and 8.3 parts of Liquid E were replaced with 8.3 parts of Liquid H.

[0093] Example 10 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, and 8.3 parts of Liquid F were replaced with 8.3 parts of Liquid H.

[0094] Example 11 Preparation of coating liquid for undercoat layer A coating liquid for an undercoat layer was obtained by mixing and stirring 26.3 parts of hollow particles (trade name: average particle diameter 5.5 μm, hollowness 98%, manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd., solid content 33.0%), 35 parts of styrene butadiene latex (trade name: L-1571, manufactured by Asahi Kasei Corporation, solid content 48%), 18.6 parts of a 20% solution of oxidized starch (trade name: Petrocoat C-8, manufactured by Nippon Starch Chemical Co., Ltd.), 0.2 parts of carboxymethyl cellulose (trade name: SG Cellogen WSC, manufactured by Daiichi Kogyo Seiyaku Co., Ltd., solid content 95%), 39.7 parts of Liquid J, and 67.5 parts of water.

[0095] Basis weight 60g / m 2 The coating amount of the primer layer was 6.0 g / m2 after drying on one side of the high-quality paper. 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that the same thermosensitive recording layer paint as in Example 5 was applied to the undercoat layer surface of the support.

[0096] (Comparative Example 1) A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was changed from 29.2 parts to 0 parts, the amount of Liquid H was changed from 8.3 parts to 24.9 parts, and the amount of water was changed from 50.5 parts to 63 parts.

[0097] (Comparative Example 2) A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was changed from 29.2 parts to 0 parts, 8.3 parts of Liquid H was changed to 24.9 parts of Liquid G, and the amount of water was changed from 50.5 parts to 63 parts.

[0098] (Comparative Example 3) A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Liquid C and 17.2 parts of Liquid D were replaced with 16.4 parts of Liquid B, 11.5 parts of Liquid C, and 6.6 parts of Liquid D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was changed from 29.2 parts to 0 parts, 8.3 parts of Liquid H was replaced with 24.9 parts of Liquid E, and the amount of water was changed from 50.5 parts to 63 parts.

[0099] Comparative Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the thermosensitive recording layer in Example 1, 17.2 parts of Solution C and 17.2 parts of Solution D were replaced with 16.4 parts of Solution B, 11.5 parts of Solution C, and 6.6 parts of Solution D, the amount of stearic acid amide (trade name: High Micron L-271, manufactured by Chukyo Yushi Co., Ltd., solids concentration 25%) was changed from 29.2 to 0 parts, 8.3 parts of Solution H was changed to 24.9 parts of Solution F, and the amount of water was changed from 50.5 parts to 63 parts.

[0100] The above examples and comparative examples were evaluated by the following methods, and the results are shown in Table 1.

[0101] [Color stability] A sample of each thermal recording medium was printed using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.) at an applied energy of 0.20 mJ / dot (high gradation color density) under a 45°C, 50% RH environment, and the printed sample was left standing for 2 hours under a 45°C, 50% RH environment. The optical density of the printed area of ​​the sample immediately after printing and after leaving it for 2 hours was measured as reflection density using a spectrodensitometer (X-Rite 504, manufactured by X-Rite). In addition, the density change rate was calculated from the optical density immediately after printing and the optical density after leaving it for 2 hours, and used as an evaluation index (formula below). Optical density change rate (%) = (optical density after standing for 2 hours - optical density immediately after printing) / optical density immediately after printing x 100 The evaluation criteria were as follows: Density change rate (%): Less than 10%: Excellent color stability [Rating: ◎] Density change rate (%) 10% or more: Problematic for practical use [Judgment: ×]

[0102] [Low gradation sensitivity] A sample of each thermal recording medium was printed using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.) with an applied energy of 0.12 mJ / dot (low gradation color density), and the reflection density was measured using a spectrodensitometer (X-Rite504, manufactured by X-Rite Co., Ltd.). The obtained optical density was used as the evaluation index. The evaluation criteria were as follows: Optical density (-) 0.65 or higher: Excessive sensitivity. Highlights in the image are crushed to black, creating a practical problem. [Rating: ×] Optical density (-) 0.60 or more and less than 0.65: Slightly excessive sensitivity. No practical problems. [Rating: Good] Optical density (-) 0.50 or more but less than 0.60: Excellent sensitivity. Excellent gradation reproduction in the highlight areas of the image. [Rating: ◎] Optical density (-) 0.45 or more and less than 0.50: Slightly insufficient sensitivity. No practical problems. [Rating: Good] Optical density (-) 0.30 or more and less than 0.45: The sensitivity is slightly insufficient, but there is no practical problem with the reproduction of gradation in the highlight areas of the image. [Rating: △] Optical density (-) less than 0.30: Insufficient sensitivity. Highlight areas of the image are blown out, causing problems for practical use. [Rating: ×]

[0103] [Heat-resistant storage stability] A sample of each thermal recording medium was printed using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.) with an applied energy of 0.12 mJ / dot (low gradation color density), and the printed sample was left to stand in a chamber under dry conditions at 50°C for 48 hours, after which the optical density of the printed portion of the processed sample was measured as reflection density using a spectrodensitometer (X-Rite504, manufactured by X-Rite Co., Ltd.). In addition, the residual rate was calculated from the optical density of the untreated sample and the optical density after heat resistance treatment, and used as an evaluation index (the following formula). Heat resistance remaining rate (%) = optical density after heat resistance treatment / dynamic color development characteristic optical density x 100

[0104] [Humidity resistance storage] A sample of each thermal recording medium was printed using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.) with an applied energy of 0.16 mJ / dot (color density from mid-tone to high-tone), and the printed sample was left to stand in a chamber at 40°C and 90% RH for 48 hours, after which the optical density of the printed portion of the processed sample was measured as reflection density using a spectrodensitometer (X-Rite504, manufactured by X-Rite Co., Ltd.). In addition, the residual rate was calculated from the optical density of the untreated sample and the optical density after humidity resistance treatment, and used as an evaluation index (formula below). Humidity resistance remaining rate (%) = optical density after humidity resistance treatment / dynamic color development characteristic optical density x 100

[0105] [Table 1]

[0106] As can be seen from Table 1, Examples 1 to 11 were judged to have no practical problems with respect to color stability, while Comparative Examples 1 to 4 were judged to have practical problems, indicating that the use of stearic acid amide as a sensitizer leads to improved color stability. Furthermore, Examples 6 to 10 showed that the type of sensitizer used in combination with stearic acid amide caused differences in low-gradation sensitivity.

[0107] With regard to the heat-resistant storage stability at low gradation, Example 4, which used two types of color developers, and Example 5, which used three types of color developers, maintained a retention rate of 80%, demonstrating excellent results.

[0108] With regard to humidity-resistant storage stability in the intermediate to high gradations, Example 5, which used three types of color developers, showed better results than Examples 1 to 4, which used two types of color developers.

Claims

1. A thermosensitive recording medium having a thermosensitive recording layer containing at least a leuco dye, a color developer, and a sensitizer on a support, The developer contains at least two non-phenolic developers, The sensitizer contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer, or contains a saturated fatty acid amide represented by the following general formula (1) as a first sensitizer and a second sensitizer: A thermal recording medium characterized by: 【Chemical 1】 (wherein R represents an alkyl group having 15 to 21 carbon atoms).

2. 2. The thermosensitive recording material according to claim 1, wherein the color developers comprise at least two non-phenolic color developers selected from the group consisting of N-p-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate.

3. 2. The thermosensitive recording material according to claim 1, wherein the color developer comprises N-p-tolylsulfonyl-N'-3-(p-trisulfonyloxy)phenylurea and at least one selected from N-[2-(3-phenylureido)phenyl]benzenesulfonamide and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate.

4. 2. The thermosensitive recording material according to claim 1, wherein the color developers are N-p-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate.

5. 5. The thermosensitive recording material according to claim 1, wherein the saturated fatty acid amide is stearic acid amide.

6. 5. The thermosensitive recording material according to claim 1, wherein the second sensitizer is at least one selected from the group consisting of diphenoxyethane compounds and diphenyl sulfones.

7. 5. The thermosensitive recording material according to claim 1, wherein the second sensitizer is 1,2-diphenoxyethane or diphenyl sulfone.

8. 5. The thermosensitive recording medium according to claim 1, wherein the saturated fatty acid amide is contained in an amount of 60 to 80% by mass of the total amount of the sensitizer.

9. 5. The thermosensitive recording medium according to claim 1, wherein the second sensitizer is contained in a ratio of 1 part by mass or less to 1 part by mass of the first sensitizer.

10. 5. The thermosensitive recording medium according to claim 1, wherein the support is synthetic paper.

11. 5. The thermosensitive recording medium according to claim 1, further comprising an undercoat layer between the support and the thermosensitive recording layer.

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