Thermal recording medium

Abstract

Disclosed is a thermosensitive recording body having: an undercoat layer that contains a hollow particle at a hollow rate of 80-98%, and a thermosensitive recording layer that contains a leuco dye and a developer, in this order on one side of a paper support; and a back surface layer that contains a pigment, on the other side of the paper support. The Oken smoothness of the back surface layer is at least 500 seconds. The proportion of the hollow particle in the total solid of the undercoat layer is 5-30 mass%.

Description

[Technical Field]

[0001] The present invention relates to a thermal recording medium that utilizes a color-developing reaction between a leuco dye and a color developer. [Background technology]

[0002] Conventionally, thermosensitive recording media utilizing the thermal color-developing reaction between a leuco dye and a color developer have been well known. Such thermosensitive recording media are relatively inexpensive, and the recording equipment is compact and relatively easy to maintain. Therefore, they are widely used not only as recording media for facsimiles, various labels, and other output devices, but also as recording media for diagnostic imaging, such as those used in printers for non-destructive testing devices for ultrasound images, X-ray images, etc., or medical diagnostic devices.

[0003] Image printers that require uniformity and high resolution in recorded images use thermal recording media that have a support of synthetic paper with a multilayer structure and, if necessary, a biaxially oriented thermoplastic resin film containing an inorganic pigment. In recent years, due to growing environmental concerns, there has been an increasing demand for thermal recording media that use recyclable paper as a support, have excellent gradation reproducibility from low to high density, and can produce high-quality recorded images comparable to silver halide photography.

[0004] To prevent head residue adhesion and sticking, a method has been proposed in which a thermosensitive coloring layer is applied to base paper containing synthetic silica and / or synthetic aluminum silicate (see Patent Document 1). Other proposed thermosensitive recording media include a paper support containing an inorganic pigment with an oil absorption of 100 ml / 100 g or more (see Patent Document 2), and a paper support containing 15-30 wt. % calcium carbonate with an oil absorption of 70 ml / 100 g or more (see Patent Document 3). These thermosensitive recording media aim to improve dot reproducibility by suppressing or preventing head residue adhesion, a so-called residue removal effect. However, compared with media using synthetic paper or thermoplastic resin film as a support, they suffer from significant color unevenness, and the image quality required for recording media for diagnostic imaging is not necessarily satisfactory. Furthermore, cutters and slitters used in the lithographic processing and small-roll finishing processes during the manufacturing process, as well as cutters attached to printers, are prone to pigment powder falloff and paper fuzzing, causing image defects and posing hygiene problems in medical settings.

[0005] To solve this problem, we used a filler with a specific surface area of 180m 2 / g or more amorphous silica, and the density is 0.60 to 0.85 g / cm 2 A thermosensitive recording medium using a paper support has been proposed (see Patent Document 4).

[0006] In addition, image paper substrate thermosensitive recording paper is required to have high sensitivity and produce clear printed images in the low energy range, and there is a demand for thermosensitive recording media that can produce uniform, clear printed images without white gaps even at low energy levels. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 61-68291 [Patent Document 2] Japanese Patent Application Publication No. 61-98584 [Patent Document 3] Japanese Patent Application Publication No. 5-58027 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-101396 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 which has high sensitivity in a low energy range, and gives clear printed images with excellent image quality. [Means for solving the problem]

[0009] As a result of extensive research into achieving the above object, the present inventors have found that the above problem can be solved by incorporating hollow particles with a hollowness of 80 to 98% in the undercoat layer at a ratio of 5 to 30 mass % of the total solid content of the undercoat layer and by making the Oken smoothness of the back layer 500 seconds or more, thereby achieving the present invention. That is, the present invention relates to the following thermosensitive recording medium.

[0010] Item 1. A thermal recording medium having, on one side of a paper support, an undercoat layer containing hollow particles with a hollow ratio of 80 to 98%, and a thermal recording layer containing a leuco dye and a color developer, in this order, and on the other side of the paper support, a backing layer containing a pigment, The Oken smoothness of the back surface is 500 seconds or more, The content of the hollow particles is 5 to 30% by mass of the total solid content of the undercoat layer. Thermal recording medium. Item 2. The thermosensitive recording medium according to Item 1, wherein the back surface layer has an Oken smoothness of 1000 seconds or more. Item 3. The dry mass of the back layer is 3.0 g / m 2 Item 3. The thermosensitive recording medium according to Item 1 or 2. Item 4. The thermosensitive recording medium according to any one of Items 1 to 3, wherein the back surface layer contains kaolin. Item 5. The thermosensitive recording medium according to any one of Items 1 to 4, wherein the undercoat layer contains an adhesive having a glass transition temperature of −10° C. or lower. Item 6. The thermosensitive recording medium according to any one of Items 1 to 4, wherein the undercoat layer contains an adhesive having a glass transition temperature of −30° C. or lower. Item 7. The thermosensitive recording medium according to any one of Items 1 to 6, wherein the undercoat layer contains an adhesive, and the adhesive contains latex. Item 8. The thermosensitive recording medium according to any one of Items 1 to 7, wherein the surface having the thermosensitive recording layer has an Oken smoothness of 4000 seconds or more. Item 9. The thermosensitive recording material according to any one of Items 1 to 8, wherein the hollow particles have an average particle size (D50) of 3 to 15 μm, a maximum particle size (D100) of 10 to 30 μm, a ratio (D100 / D50) of the maximum particle size (D100) to the average particle size (D50) of the hollow particles is 1.8 to 3.0, and the volume percentage of the hollow particles having a particle size of 2.0 μm or less is 1% or less. [Effects of the Invention]

[0011] The thermosensitive recording medium of the present invention has high sensitivity in the low energy range, is excellent in image quality, and gives clear printed images without uneven color development. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, the expressions "comprise" and "contain" include the concepts of "comprise," "consist essentially of," and "consist only of."

[0013] 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.

[0014] The latex in the present invention includes a gel or a dried film formed by drying a dispersion medium.

[0015] In the present invention, the "average particle size" refers to the volume-based median size measured by laser diffraction. More simply, particle sizes may be measured from particle images (SEM images) using an electron microscope, and the average value of 10 particles may be used.

[0016] The present invention provides a thermosensitive recording medium having, on one side of a paper support, an undercoat layer containing hollow particles with a hollow ratio of 80 to 98%, and a thermosensitive recording layer containing a leuco dye and a color developer, in this order, and having, on the other side of the paper support, a backing layer containing a pigment, The Oken smoothness of the back surface is 500 seconds or more, The content of the hollow particles is 5 to 30% by mass of the total solid content of the undercoat layer. It is characterized by:

[0017] [Paper support] The paper 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 (acid 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, etc. The thickness of the paper support is not particularly limited, and is usually about 20 to 200 μm. The density of the paper support is also not particularly limited, and is 0.60 to 1.00 g / cm. 3 The preferred range is 0.60 to 0.85 g / cm 3 A degree is more preferable.

[0018] [Undercoat layer] The thermosensitive recording medium of the present invention has an undercoat layer between the paper support and the thermosensitive recording layer, containing hollow particles with a void ratio of 80 to 98%. This prevents the penetration of the coating liquid for the thermosensitive recording layer into the paper support, improving image quality. Furthermore, the presence of hollow particles with a void ratio of 80 to 98% prevents blank areas in printed areas even with low energy, thereby enhancing the density of halftone prints. Examples of hollow particles include conventionally known particles, such as particles whose film material is made of acrylic resin, styrene resin, vinylidene chloride resin, etc. Here, the void ratio is calculated by (d / D) × 100. In this formula, d represents the inner diameter of the hollow particle, and D represents the outer diameter of the hollow particle. From the viewpoint of improving image quality, the void ratio is preferably 90 to 98%. The average particle diameter of the hollow particles is preferably approximately 3 to 15 μm, more preferably approximately 4 to 12 μm. Here, the average particle diameter is the particle diameter at 50% volume frequency, also known as the median diameter or D50. Particle size and particle size distribution can be measured using a laser diffraction particle size analyzer. They can also be measured using an electron microscope. The hollow particle content is 5 to 30% by mass, preferably 7 to 28% by mass, and more preferably 9 to 26% by mass, of the total solids content of the undercoat layer. By setting the content at 5% by mass or more, white spots in printed areas can be suppressed even at low energy levels, improving image quality. By setting the content at 30% by mass or less, color unevenness can be suppressed, resulting in clearer printed images. It is particularly noteworthy that the use of specific hollow particles in the undercoat layer and the addition of the backing layer (described below) significantly improve the smoothness of the surface bearing the thermosensitive recording layer. At the same time, the hollow particles in the undercoat layer increase the elasticity of the coating layer, suppressing white spots and color unevenness in low-energy areas and improving thermosensitive quality. By adjusting the average particle size, maximum particle size, and the ratio (D100 / D50) of the maximum particle size (D100) to the average particle size (D50) of the hollow particles, it is possible to effectively suppress white spots and uneven color development in low-energy areas.

[0019] The maximum particle size of the hollow particles is preferably 10 to 30 μm, more preferably 10 to 25 μm, and even more preferably 10 to 20 μm. The maximum particle size is also called D100.

[0020] The ratio (D100 / D50) of the maximum particle size (D100) to the average particle size (D50) is an index showing the degree of particle size distribution. D100 / D50 is preferably 1.8 to 3.0, and more preferably 1.8 to 2.8.

[0021] In the particle size distribution, the volume percentage of hollow particles having a particle diameter of 2.0 μm or less is preferably 1% or less, more preferably 0.5% or less, and even more preferably none at all.

[0022] The undercoat layer may contain an oil-absorbing pigment and / or thermally expandable particles having an oil absorption of 70 ml / 100 g or more, particularly about 80 to 150 ml / 100 g, where the oil absorption is determined according to the method of JIS K 5101.

[0023] 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 2 to 95 mass %, and more preferably about 5 to 90 mass %, of the total solids content of the undercoat layer.

[0024] The undercoat layer is generally prepared by mixing water as a dispersion medium with adhesive, hollow particles, oil-absorbing pigments, various auxiliaries, etc., and applying a coating liquid for the undercoat layer onto a support in a dry weight ratio of preferably 3 to 20 g / m. 2 Approximately, more preferably 5 to 12 g / m 2 The coating is applied and dried to a thickness of approximately 1000 μm.

[0025] Examples of adhesives include polyvinyl alcohol and its derivatives, starch and its derivatives, cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl 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. Among these, latex-containing adhesives are preferred. The adhesive content can be selected from a wide range, but is generally preferably about 5 to 30% by weight, and more preferably about 10 to 20% by weight, of the total solids content of the undercoat layer.

[0026] The glass transition temperature (Tg) of the adhesive is not particularly limited, but is preferably -10°C or lower. A glass transition temperature of -10°C or lower can improve image quality even in a low energy range. A glass transition temperature of -30°C or lower is more preferable because it can further improve image quality in a low energy range.

[0027] 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, glyoxal, boric acid, dialdehyde starch, glyoxylates, methylol urea, and epoxy compounds; antifoaming agents; coloring dyes; and fluorescent dyes.

[0028] [Thermal recording layer] 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.

[0029] 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.

[0030] 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 20% 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.

[0031] Specific examples of the color developer 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, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 4,4'-bis(p- Tolylsulfonylaminocarbonylamino)diphenylmethane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2'-bis[4-(4-hydroxyphenyl)phenoxy]diethyl ether, 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'-isopropoxydiphenyl sulfone, 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'- Methyl diphenyl sulfone, 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, or 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)propyl]salicylic acid, aromatic carboxylic acids such as zinc 4-[3-(p-tolylsulfonyl)propyloxy]salicylate, 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, and 4-[3-(p-tolylsulfonyl)propyloxy]salicylate, and these phenolic compounds; salts of aromatic carboxylic acids with polyvalent metals such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, and nickel; and organic acids such as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids. substances, urea compounds such as Np-toluenesulfonyl-N'-3-(p-toluenesulfonyloxy)phenylurea, Np-toluenesulfonyl-N'-p-butoxycarbonylphenylurea, Np-tolylsulfonyl-N'-phenylurea, 4,4'-bis(p-toluenesulfonylaminocarbonylamino)diphenylmethane, 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, N,N'-di-m-chlorophenylthio Examples of suitable organic compounds include thiourea compounds such as urea, organic compounds having an -SO2NH- bond in the molecule such as N-(p-toluenesulfonyl)carbamoyl acid p-cumylphenyl ester, N-(p-toluenesulfonyl)carbamoyl acid p-benzyloxyphenyl ester, N-[2-(3-phenylureido)phenyl]benzenesulfonamide, and N-(o-toluoyl)-p-toluenesulfamide, and inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate.

[0032] Further examples include urea urethane derivatives such as 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, 4,4'-bis[(2-methyl-5-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, and 4-(2-methyl-3-phenoxycarbonylaminophenyl)ureido-4'-(4-methyl-5-phenoxycarbonylaminophenyl)ureidodiphenyl sulfone, all of which are represented by the following general formula (1); and diphenyl sulfone derivatives represented by the following general formula (2).

[0033] [ka]

[0034] [ka] (In the formula, n represents an integer of 1 to 6.)

[0035] The color developer is not limited to these, and two or more compounds can be used in combination as needed.

[0036] The content of the developer is not particularly limited and may be adjusted depending on the leuco dye used. Generally, 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, per 1 part by mass of leuco dye. Furthermore, the content of the developer 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, per 1 part by mass of leuco dye. By using an amount of 0.5 parts by mass or more, recording performance can be improved. On the other hand, by using an amount of 10 parts by mass or less, background fogging in high-temperature environments can be effectively suppressed.

[0037] 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.

[0038] 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, more preferably about 5 to 20% by mass, of the total solid content of the thermosensitive recording layer.

[0039] The heat-sensitive recording layer of the present invention may contain a sensitizer, thereby increasing the recording sensitivity. Examples of sensitizers include stearic acid amide, methoxycarbonyl-N-stearic acid benzamilide, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylene bisstearic acid amide, behenic acid amide, methylene bisstearic acid amide, N-methylol stearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, diphenyl sulfone, benzyl p-benzyloxybenzoate, phenyl 1-hydroxy-2-naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, oxalic acid di-p-chlorobenzyl ester, oxalic acid di-p-methylbenzyl ester, oxalic acid dibenzyl ester, p-tolyl biphenyl ether, di(p-methoxyphenoxyethyl) ether, 1,2-di(3-methylphenoxy) ether, and the like. 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. The content of the sensitizer may be an amount effective for sensitization, and is usually preferably about 2 to 40% by mass, more preferably about 5 to 25% by mass, of the total solid content of the thermosensitive recording layer.

[0040] To improve the whiteness of the thermosensitive recording layer and the uniformity of images, a fine particle pigment with high whiteness and an average particle size of 10 μm or less can be incorporated into the thermosensitive recording layer. Examples of pigments that can be used include inorganic pigments such as calcium carbonate, magnesium carbonate, kaolin, clay, talc, calcined clay, silica, diatomaceous earth, synthetic aluminum silicate, zinc oxide, titanium oxide, aluminum hydroxide, barium sulfate, surface-treated calcium carbonate, and silica, as well as organic pigments such as urea-formaldehyde resin, styrene-methacrylic acid copolymer resin, and polystyrene resin. The pigment content should be such that it does not reduce the color density, i.e., 50% by weight or less of the total solids content of the thermosensitive color-developing layer.

[0041] Other components constituting the heat-sensitive recording layer include adhesives, and if necessary, crosslinking agents, waxes, metal soaps, water-resistant agents, dispersants, colored dyes, fluorescent dyes, etc.

[0042] Adhesives used in the coating liquid for the thermosensitive recording layer include, for example, aqueous adhesives such as water-soluble adhesives and water-dispersible adhesives. 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, hydroxymethylcellulose, 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. These can be used alone or in combination of two or more. At least one of these is blended in an amount of preferably about 5 to 50% by mass, more preferably about 10 to 40% by mass, of the total solid content of the thermosensitive recording layer.

[0043] 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.

[0044] Examples of waxes include waxes such as paraffin wax, carnauba wax, microcrystalline wax, polyolefin wax, and polyethylene wax; higher fatty acid amides such as stearic acid amide and ethylene bisstearic acid amide, higher fatty acid esters, and derivatives thereof.

[0045] Examples of metal soaps include polyvalent metal salts of higher fatty acids, such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate. If necessary, various auxiliary agents such as oil repellents, antifoaming agents, and viscosity adjusters may be added to the thermosensitive recording layer within a range that does not impair the effects of the present invention.

[0046] The thermosensitive recording layer is generally formed by dispersing a leuco dye, a developer, and optionally a sensitizer and a storage stability improver, together or separately, in water as a dispersion medium, using various stirring and wet grinding machines such as a ball mill, a Co-ball mill, an attritor, or a vertical or horizontal sand mill, together with water-soluble synthetic polymer compounds such as polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, methyl cellulose, styrene-maleic anhydride copolymer salt, and other surfactants, and then dispersing the dispersion to an average particle size of 2 μm or less, and mixing it with pigments, adhesives, auxiliaries, and other ingredients as required. The coating amount of the thermosensitive recording layer is not particularly limited, and is typically 1 to 12 g / m2 in dry mass. 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 2 The 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.

[0047] [Protective layer] The thermosensitive recording medium of the present invention preferably has a protective layer on the thermosensitive recording layer to improve storage stability and running performance during recording. The protective layer can be obtained by applying a protective layer coating liquid onto the thermosensitive recording layer and drying the coating liquid, which is obtained by mixing water as a medium, an adhesive, a pigment, and various auxiliary agents as necessary.

[0048] Examples of pigments used in the protective layer include amorphous silica, kaolin, light calcium carbonate, heavy calcium carbonate, calcined kaolin, titanium oxide, magnesium carbonate, aluminum hydroxide, colloidal silica, synthetic layered mica, and plastic pigments such as urea-formaldehyde resin fillers.

[0049] The adhesive used in the coating liquid for the protective layer is not particularly limited, and examples thereof include aqueous adhesives such as water-soluble adhesives and water-dispersible adhesives. The adhesive can be appropriately selected from those usable for the thermal recording layer. Among them, polyvinyl alcohol or modified polyvinyl alcohol is preferred because of its excellent binder effect with the pigment and excellent preservation properties of the recorded portion against solvents such as plasticizers and oils. In particular, various modified polyvinyl alcohols such as acetoacetyl-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and diacetone-modified polyvinyl alcohol are more preferably used.

[0050] The content of the adhesive is preferably about 20 to 85 mass % of the total solid content of the protective layer, and more preferably about 35 to 80 mass %.

[0051] In addition, various known auxiliaries such as lubricants, antifoaming agents, wetting agents, preservatives, fluorescent brighteners, dispersants, thickeners, colorants, antistatic agents, and crosslinking agents may be added to the protective layer as appropriate.

[0052] The coating amount of the protective layer coating solution is 0.5 to 10 g / m2 in dry mass. 2 Approximately, preferably 1 to 5 g / m 2 The protective layer may be formed in two or more layers as needed, and the composition and coating amount of each layer may be the same or different.

[0053] [Back layer] The thermosensitive recording medium of the present invention has a pigment-containing backing layer on the back surface of the paper support (the surface opposite to the thermosensitive recording layer). The Oken smoothness of the backing layer is 500 seconds or more, preferably 1000 seconds or more, and more preferably 2000 seconds or more. This suppresses color unevenness and results in clearer printed images. In image printers with compact recording equipment, the use of a platen roll drive reduces the pressing pressure on the opposing thermal head, making color unevenness and white spots more likely to occur in halftones where gradation reproduction is required. However, the thermosensitive recording medium of the present invention effectively produces clear printed images without color unevenness even in the low-energy range (halftone energy range). This is because the increased smoothness of the backing layer allows for uniform pressure on the platen roll. On the other hand, the Oken smoothness of the backing layer is preferably 100,000 seconds or less to suppress sticking to the platen roll. On the other hand, the Oken smoothness of the surface having the thermosensitive recording layer is preferably 5,000 seconds or more, more preferably 6,000 seconds or more, from the viewpoint of effectively suppressing color unevenness, with the thermosensitive recording layer side being the measurement surface. The upper limit of the Oken smoothness of the surface having the thermosensitive recording layer is not particularly limited, and is preferably 100,000 seconds or less from the viewpoint of improving winding properties such as winding slippage and blocking. The surface having the thermosensitive recording layer refers to the outermost surface of the thermosensitive recording medium that has the thermosensitive recording layer. The Oken smoothness is measured according to the method specified in JIS P8155:2010, with the back layer side or the surface having the thermosensitive recording layer being the measurement surface.

[0054] Examples of pigments include calcium carbonate, magnesium carbonate, kaolin, calcined kaolin, clay, talc, calcined clay, silica, diatomaceous earth, synthetic aluminum silicate, zinc oxide, titanium oxide, aluminum hydroxide, and barium sulfate. Among these, kaolin is particularly preferred because it improves smoothness. The content of the pigment is not particularly limited, and is generally preferably about 20 to 90 mass % of the total solids content of the back layer, and more preferably about 30 to 85 mass %.

[0055] The back layer is generally prepared by mixing an adhesive, a pigment, etc. with water as a dispersion medium and applying a coating liquid for the back layer onto a support in an amount of preferably 3.0 g / m2 in dry weight. 2 or more, more preferably 3.0 to 10 g / m 2 The coating is applied and dried to a thickness of about 3.0 g / m. 2 By doing so, it is possible to increase the smoothness, and as a result, it is possible to suppress uneven color development and make the printed image clearer.

[0056] The adhesive contained in the back surface layer is not particularly limited, and examples thereof include aqueous adhesives such as water-soluble adhesives and water-dispersible adhesives. The adhesive can be appropriately selected from those that can be used in the undercoat layer. The content of the adhesive can be selected from a wide range, but generally, it is preferably about 2 to 50 mass % of the total solid content of the back surface layer, and more preferably about 4 to 35 mass %.

[0057] [Thermal recording medium] There are no particular limitations on the methods for forming the thermosensitive recording layer, undercoat layer, backside layer, and protective layer that is provided as needed. For example, they can be formed by a method in which a coating liquid for the backside layer is applied to the support by an appropriate coating method such as bar coating, air knife coating, barrier blade coating, pure blade coating, rod blade coating, short dwell coating, curtain coating, or die coating, and then a coating liquid for the undercoat layer is applied to the other side of the support and dried, and then a coating liquid for the thermosensitive recording layer and further a coating liquid for the protective layer are applied to the undercoat layer and dried.

[0058] The undercoat layer is preferably a layer formed by a blade coating method. This allows the formation of a thermal recording layer of uniform thickness without unevenness on the support, thereby increasing recording sensitivity and improving the barrier properties of a protective layer that is provided as needed. Blade coating methods are not limited to coating methods using blades such as bevel type and bent type, but also include rod blade methods, bill blade methods, etc.

[0059] In the present invention, it is preferable that at least one layer formed on the support is formed by curtain coating. This allows the formation of a layer with uniform thickness, thereby improving recording sensitivity and barrier properties against oil, plasticizers, alcohol, etc. Curtain coating is a method in which a coating liquid is allowed to flow down and fall freely to coat the support without contact. Known methods such as slide curtain, couple curtain, and twin curtain can be used, and are not particularly limited. In curtain coating, simultaneous multilayer coating allows the formation of layers with more uniform thickness. In simultaneous multilayer coating, each coating liquid may be laminated, coated, and then dried to form each layer. Alternatively, the coating liquid forming the lower layer may be applied, and then the coating liquid forming the upper layer may be applied on the lower layer while the lower layer coating surface is still wet without drying, and then dried to form each layer. In the present invention, simultaneous multilayer coating of the thermosensitive recording layer and the protective layer is preferable from the viewpoint of improving barrier properties.

[0060] In the present invention, from the viewpoint of increasing the recording sensitivity and improving the image uniformity, it is preferable to carry out a smoothing treatment using a known method such as a supercalender or a soft calender at any stage after the completion of the formation of each layer or after the completion of the formation of all layers.

[0061] In the present invention, a multicolor thermosensitive recording medium can be produced to further increase the added value of the product. Generally, a multicolor thermosensitive recording medium is an attempt to utilize the difference in heating temperature or the difference in thermal energy, and is generally constructed by sequentially laminating a high-temperature color-forming layer and a low-temperature color-forming layer, which develop different color tones, on a support. These can be broadly classified into two types: a decolorizing type and an additive coloring type. There are methods using microcapsules and a method for producing a multicolor thermosensitive recording medium using composite particles composed of an organic polymer and a leuco dye. [Example]

[0062] 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.

[0063] Example 1 (1) Preparation of coating liquid for back layer 79 parts of calcined kaolin, 41.7 parts of styrene-butadiene copolymer (trade name: L-1571, manufactured by Asahi Kasei Chemicals Corporation, glass transition point -3°C, solids concentration 48%), 4 parts of a 25% aqueous solution of oxidized starch (trade name: Oji Ace A, manufactured by Oji Cornstarch Co., Ltd.), and 150 parts of water were mixed and stirred uniformly to obtain a coating liquid for the back layer.

[0064] (2) Preparation of coating liquid for undercoat layer A coating liquid for an undercoat layer was obtained by uniformly mixing and stirring 79 parts of calcined kaolin, 41.7 parts of styrene-butadiene copolymer A (glass transition temperature -10°C, particle size: 190 nm, solids concentration 48%), 4 parts of a 25% aqueous solution of oxidized starch (trade name: Oji Ace A, manufactured by Oji Cornstarch Co., Ltd.), 68.2 parts of hollow particles A (average particle size 9 μm, maximum particle size 23 μm, D100 / D50=2.6, volume % of particle sizes 2.0 μm or less is 0%, hollow ratio 92%, solids concentration 22%), and 75 parts of water.

[0065] (3) 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 (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 0.5 μm, thereby obtaining a leuco dye dispersion (Liquid A).

[0066] (4) Preparation of developer dispersion liquid (B-1 liquid) 40 parts of 4-hydroxy-4'-isopropoxydiphenyl sulfone (D8, manufactured by Nippon Soda 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.) until the median diameter measured with a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) reached 0.7 μm, to obtain a developer dispersion (Liquid B).

[0067] (5) Preparation of sensitizer dispersion (liquid C) 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 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 (Liquid C).

[0068] (6) Preparation of coating liquid for thermosensitive recording layer A coating liquid for a thermosensitive recording layer was obtained by mixing and stirring 29.5 parts of solution A, 59.1 parts of solution B, 45.4 parts of solution C, 20 parts of a 5% aqueous solution of hydroxymethylcellulose, 45 parts of a 10% aqueous solution of fully saponified polyvinyl alcohol (degree of polymerization: 1000, degree of saponification: 99 mol%), 9.4 parts of a styrene-butadiene copolymer (trade name: L-1571, manufactured by Asahi Kasei Corporation, glass transition point -3°C, solids concentration: 48%), 17.1 parts of light calcium carbonate (trade name: Brilliant-15, manufactured by Shiraishi Kogyo Co., Ltd.), 11.7 parts of paraffin wax (trade name: Hydrin L-700, manufactured by Chukyo Yushi Co., Ltd., solids concentration: 30%), 2 parts of adipic acid dihydrazide (manufactured by Otsuka Chemical Co., Ltd.), and 120 parts of water.

[0069] (7) Preparation of protective layer coating liquid A coating liquid for a protective layer was obtained by mixing and stirring a composition consisting of 300 parts of a 12% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenex Z-200, saponification degree: 99.4 mol%, average polymerization degree: 1000, modification degree: 5 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), 19 parts of kaolin (trade name: HYDRAGLOSS90, manufactured by KaMin LLC), 35 parts of aluminum hydroxide (trade name: Higilite H-42M, manufactured by Showa Denko K.K.), 4 parts of silica (trade name: Mizukasil P-527, manufactured by Mizusawa Industrial Chemicals), 2.5 parts of polyethylene wax (trade name: Chemipearl W-400, manufactured by Mitsui Chemicals, Inc., solids concentration 40%), and 114.5 parts of water.

[0070] (8) Preparation of thermal recording medium Basis weight 60g / m 2 The back layer coating liquid was applied to one side of the high-quality paper with a dry amount of 5.0 g / m 2 After forming a backside layer by coating and drying the coating liquid for the undercoat layer, the coating liquid for the thermosensitive recording layer, and the coating liquid for the protective layer on the opposite side, the coating amounts after drying were each 6.0 g / m 2 , 4.0g / m 2 , 2.0 g / m 2 The coating was then applied and dried so as to form an undercoat layer, a thermosensitive recording layer, and a protective layer in this order, and the surface was then smoothed with a supercalender to obtain a thermosensitive recording medium.

[0071] Example 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the undercoat coating liquid of Example 1, 68.2 parts of hollow particles A (average particle diameter 9 μm, maximum particle diameter 23 μm, D100 / D50=2.6, volume % of particles with a diameter of 2.0 μm or less is 0%, hollow ratio 92%, solid content 22%) was used instead of 68.2 parts of hollow particles B (average particle diameter 8 μm, maximum particle diameter 18 μm, D100 / D50=2.3, volume % of particles with a diameter of 2.0 μm or less is 0.8%, hollow ratio 80%, solid content 22%).

[0072] Example 3 In the preparation of the thermosensitive recording medium of Example 1, the coating amount of the back layer coating liquid after drying was 5.0 g / m 2 to 3.0 g / m2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that the above was changed to the above.

[0073] Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the back layer coating liquid in Example 1, 79 parts of calcined kaolin was changed to 79 parts of kaolin (trade name: HYDRAGLOSS90, manufactured by KaMin LLC).

[0074] 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 back layer in Example 1, 79 parts of calcined kaolin was changed to 79 parts of light calcium carbonate (product name: Brilliant-15, manufactured by Shiraishi Kogyo Co., Ltd.).

[0075] Example 6 In the preparation of the thermosensitive recording medium of Example 5, the coating amount of the back layer coating liquid after drying was 5.0 g / m 2 to 3.0 g / m 2 A thermosensitive recording medium was obtained in the same manner as in Example 5, except that the above was changed to the above.

[0076] Example 7 In the preparation of the thermal recording medium of Example 1, the basis weight was 60 g / m 2 Instead of using a high-quality paper substrate, the OK top coat + (basis weight 84.9 g / m 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that the backing layer coating liquid was not applied.

[0077] Example 8 A thermal recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the undercoat layer of Example 1, 41.7 parts of styrene-butadiene copolymer A (glass transition temperature -10°C, particle size: 190 nm, solid content 48%) was replaced with 41.7 parts of styrene-butadiene copolymer B (trade name: L-1571, manufactured by Asahi Kasei Corporation, glass transition temperature -3°C, solid content 48%).

[0078] Example 9 A thermal recording medium was obtained in the same manner as in Example 1, except that in preparing the coating liquid for the undercoat layer of Example 1, 41.7 parts of styrene-butadiene copolymer A (glass transition temperature -10°C, particle diameter: 190 nm, solid concentration 48%) was replaced with 41.7 parts of styrene-butadiene copolymer C (glass transition temperature -30°C, particle diameter: 190 nm, solid concentration 48%).

[0079] Example 10 A thermal recording medium was obtained in the same manner as in Example 1, except that in the preparation of the undercoat coating liquid of Example 1, 68.2 parts of hollow particles A (average particle diameter 9 μm, maximum particle diameter 23 μm, D100 / D50=2.6, volume % of particles with a diameter of 2.0 μm or less is 0%, hollow ratio 92%, solid content 22%) were replaced with 45.5 parts of hollow particles C (average particle diameter 5 μm, maximum particle diameter 14 μm, D100 / D50=2.8, volume % of particles with a diameter of 2.0 μm or less is 0.2%, hollow ratio 91%, solid content 33%), 41.7 parts of styrene-butadiene copolymer A (glass transition temperature −10° C., particle diameter: 190 nm, solid content 48%) were replaced with 41.7 parts of styrene-butadiene copolymer D (glass transition temperature −35° C., particle diameter: 190 nm, solid content 48%), and 75 parts of water were replaced with 97.7 parts.

[0080] Example 11 A thermosensitive recording medium was obtained in the same manner as in Example 10, except that in the preparation of the undercoat coating liquid for Example 10, 45.5 parts of hollow particles C (average particle diameter 5 μm, maximum particle diameter 14 μm, D100 / D50=2.8, volume % of particles with a particle diameter of 2.0 μm or less is 0.2%, hollow ratio 91%, solid content 33%) was changed to 22.7 parts.

[0081] Comparative Example 1 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that the back surface layer coating liquid was not applied.

[0082] Comparative Example 2 In the preparation of the thermosensitive recording medium of Example 1, the coating amount of the back layer coating liquid after drying was 5.0 g / m 2 to 1.0 g / m 2A thermosensitive recording medium was obtained in the same manner as in Example 1, except that the above was changed to the above.

[0083] Comparative Example 3 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the undercoat coating liquid in Example 1, 68.2 parts of hollow particles A (average particle diameter 9 μm, hollowness 92%, solid content 22%) was changed to 13.6 parts.

[0084] Comparative Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the undercoat coating liquid of Example 1, 68.2 parts of hollow particles A (average particle diameter 9 μm, hollow rate 92%, solid content 22%) was changed to 181.8 parts.

[0085] The thermosensitive recording media prepared in Examples 1 to 11 and Comparative Examples 1 to 4 were subjected to the following evaluations, and the results are shown in Table 1.

[0086] [Oken type smoothness] Measurements were made in accordance with JIS P8155:2010.

[0087] [Thermal printing] Using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.), recording was performed on each thermal recording medium in the halftone energy range with an applied energy of 0.32 mJ / dot, and the resulting printed area was measured in the visual mode with a Macbeth densitometer (RD-914, manufactured by Macbeth Co., Ltd.). For practical purposes, the recording density must be 1.00 or higher.

[0088] 〔image quality〕 The quality of the thermally printed image was visually observed and evaluated according to the following criteria. ⊚: There is absolutely no white space in the image quality. ○: There is almost no white space in the image quality. △: White spots in the image quality are somewhat noticeable (within acceptable limits). x: White spots in the image are very noticeable and numerous.

[0089] [Uneven coloring of printed area] Using a thermal recording evaluation machine (product name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermal recording medium was recorded in the halftone energy range with an applied energy of 0.16 mJ / dot, and the resulting solid print was visually observed and evaluated according to the following criteria. ⊚: No uneven coloring at all. ○: Almost no uneven coloring. △: Color unevenness is slightly noticeable (within acceptable range). ×: Color unevenness is very noticeable and the printed area is non-uniform.

[0090] [Table 1]

[0091] As can be seen from Table 1, the thermosensitive recording materials of Examples 1 to 11 had high sensitivity and excellent image quality in the low energy range, and produced clear printed images without any printing unevenness.

[0092] However, in Comparative Example 1, there was no backing layer and the smoothness was low, so white spots and uneven color development were noticeable, resulting in uneven printing. In Comparative Example 2, the amount of backing layer applied was insufficient and the smoothness was low, so white spots and uneven color development were noticeable, resulting in uneven printing. In Comparative Example 3, the amount of hollow particles blended in the undercoat layer was low, so there were many white spots in the printed area and the image quality was significantly poor. In Comparative Example 4, the amount of hollow particles blended in the undercoat layer was too high, so the coating layer of the thermosensitive layer was uneven and uneven color development was noticeable.

Claims

1. A thermosensitive recording medium having, on one side of a paper support, an undercoat layer containing hollow particles having a hollowness of 80 to 98% and a thermosensitive recording layer containing a leuco dye and a color developer, in this order, and having, on the other side of the paper support, a backing layer containing a pigment, The Oken smoothness of the back surface layer is 500 seconds or more, the dry mass of the back surface layer is 3.0 g / m 2 or more; The content of the hollow particles is 5 to 30% by mass of the total solid content of the undercoat layer. Thermal recording medium.

2. 2. The thermosensitive recording medium according to claim 1, wherein the back surface layer has an Oken smoothness of 1000 seconds or more.

3. 3. The thermosensitive recording material according to claim 1, wherein the back surface layer contains kaolin.

4. 4. The thermosensitive recording medium according to claim 1, wherein the undercoat layer contains an adhesive having a glass transition temperature of −10° C. or lower.

5. 4. The thermosensitive recording medium according to claim 1, wherein the undercoat layer contains an adhesive having a glass transition temperature of −30° C. or lower.

6. 6. The thermosensitive recording material according to claim 1, wherein the undercoat layer contains an adhesive, and the adhesive contains latex.

7. 7. The thermosensitive recording medium according to claim 1, wherein the surface having the thermosensitive recording layer has an Oken smoothness of 4000 seconds or more.

8. 8. The thermosensitive recording material according to claim 1, wherein the hollow particles have an average particle size (D50) of 3 to 15 μm, a maximum particle size (D100) of 10 to 30 μm, a ratio (D100 / D50) of the maximum particle size (D100) to the average particle size (D50) of the hollow particles is 1.8 to 3.0, and the volume percentage of the hollow particles having a particle size of 2.0 μm or less is 1% or less.

Images