WO2020100501A1

WO2020100501A1 - Thermosensitive recording body

Info

Publication number: WO2020100501A1
Application number: PCT/JP2019/040630
Authority: WO
Inventors: 健太郎諸藤; 尚竹村; 坂本　和之
Original assignee: 王子ホールディングス株式会社
Priority date: 2018-11-14
Filing date: 2019-10-16
Publication date: 2020-05-22
Also published as: JPWO2020100501A1

Abstract

Provided is a thermosensitive recording body which has excellent recording density and image quality. The thermosensitive recording body is provided with an undercoat layer formed on one surface of a support and a thermosensitive recording layer formed on the undercoat layer, wherein: the thermosensitive recording layer contains a leuco dye and a coloring agent; the undercoat layer contains hollow particles and a binding resin; the maximum particle diameter (D100) of the hollow particles is 20-80 μm; and the ratio (D100/D50) of the maximum particle diameter (D100) of the hollow particles to the particle diameter (D50) at 50 volume% frequency of the hollow particles is 3.2-10.0.

Description

Thermal recording

The present invention relates to a thermosensitive recording medium.

A thermosensitive recording medium for recording a color image by utilizing a color development reaction of a colorless or pale leuco dye and a phenol or an organic acid has been widely put into practical use. Since such a thermal recording material forms a color image by simply heating it, the recording apparatus can be made compact, the maintenance of the recording apparatus is easy, and the noise is small. Therefore, thermal recording materials are widely used as various information recording materials in issuing machines such as label printers, automatic ticket vending machines, CD / ATM, order slip output machines such as restaurants, and data output machines of scientific research equipment. There is.

Demands for improving the performance of thermal recording media are increasing as the thermal recording media are used for various purposes. That is, there are demands in terms of quality such that an image is deep and vivid, and high quality with no white spots (print defects).

Therefore, in response to such demands, many improved technologies related to thermal recording media have been developed. For example, there is a method of improving the sensitivity of the thermosensitive recording medium by including hollow particles in the undercoating layer provided between the support of the thermosensitive recording medium and the thermosensitive recording layer to increase the heat insulating property of the undercoating layer. Are known. A technique in which the method of incorporating hollow particles in the undercoat layer is further improved has also been developed.

For example, in Patent Document 1, an undercoat layer containing hollow particles and a binder resin has a hollowness of 60 to 98% and a maximum particle diameter (D100) of 5.0 to 10.0 μm. A heat-sensitive recording material using hollow particles having a ratio D100 / D50 between the maximum particle diameter and the particle diameter of 50% by volume frequency (D50) of 1.5 to 3.0 is disclosed.
Further, in Patent Document 2, the heat-expandable resin particles used for the undercoat layer have an average particle size before expansion of preferably 1 to 25 μm, and the volume thereof expands 10 to 50 times by heating, resulting in a hollow ratio. It is disclosed that those having a ratio of 80% or more are preferable.

Japanese Patent No. 4108380 Japanese Patent No. 5781885

However, in the heat-sensitive recording material described in Patent Document 1, the maximum particle diameter (D100) of the hollow particles is as small as 5.0 to 10.0 μm, and the heat insulation is insufficient, so that the printing energy easily diffuses, The recording density had room for improvement.
Further, the heat-sensitive recording material described in Patent Document 2 does not have the viewpoint of making the particle diameter of the heat-expandable resin particles uniform, and the particle diameter after foaming has a large variation, so that the surface smoothness of the undercoat layer is Since there is a decrease, the image quality has room for improvement.

The present invention has been made in view of such a situation. That is, it is an object of the present invention to provide a thermal recording material which gives a high quality and clear printed image with little print defects and has high sensitivity and excellent halftone print density.

In order to solve the above-mentioned problems, the present inventors proceeded with the study of hollow particles used for the undercoat layer. As a result, they have found that the above problems can be solved by using expanded type hollow particles having a variation in particle diameter and a maximum particle diameter within a predetermined range. The present invention has been completed based on these findings. That is, the present invention has the following configurations.

(1) A thermosensitive recording medium comprising an undercoat layer formed on one surface of a support and a thermosensitive recording layer formed on the undercoat layer, wherein the thermosensitive recording layer is a leuco dye. A colorant, the undercoat layer contains hollow particles and a binder resin, the hollow particles have a maximum particle diameter (D100) of 20 to 100 μm, and the maximum particle diameter of the hollow particles (D100). And a particle diameter (D50) of 50% by volume, the ratio D100 / D50 is 3.0 to 10.0.

(2) The thermosensitive recording medium according to (1) above, wherein the undercoat layer further contains at least one of a cellulose derivative and a high molecular polysaccharide.

(3) The heat-sensitive recording material as described in (1) or (2) above, wherein the particle diameter (D50) of 50% by volume of the hollow particles is 10 to 25 μm.
(4) The thermosensitive recording medium according to any one of (1) to (3) above, wherein the hollow particles are contained in an amount of 5 to 90% by mass based on the total solid amount of the undercoat layer.

The thermosensitive recording medium of the present invention provides a high-quality and clear printed image with few print defects, has high sensitivity, and is excellent in halftone print density.

An embodiment of the present invention will be described. However, the embodiments of the present invention are not limited to the following embodiments.

In the thermosensitive recording medium of the present embodiment, a thermosensitive recording layer is provided on a support through an undercoat layer. The heat-sensitive recording layer is a layer in which a portion thereof develops a color when heat is applied and a character, a design or the like is displayed. The undercoat layer is a layer having a role of improving fixing of the heat-sensitive recording layer, improving heat insulation so as not to diffuse the heat, and the like.
The materials constituting the thermosensitive recording medium will be described below.

[Support]
There is no particular limitation on the type, shape, size, etc. of the support. For example, high quality paper (acidic paper, neutral paper), medium quality paper, coated paper, art paper, cast coated paper, glassine paper, resin laminated paper In addition to polyolefin synthetic paper, synthetic fiber paper, non-woven fabric, synthetic resin film, etc., various transparent supports can be appropriately selected and used. The thickness of the support is not particularly limited and is usually about 20 to 200 μm. The density of the support is not particularly limited, and is preferably about 0.60 to 0.85 g / cm ³ .

[Undercoat layer]
The undercoat layer is provided between the support and the thermosensitive recording layer. The undercoat layer contains hollow particles and a binder resin. The undercoat layer preferably further contains a thickener.

(Hollow particles)
When the hollow particles made of an organic resin are contained in the undercoat layer, the heat insulation of the undercoat layer can be enhanced. The undercoat layer having a high heat insulating property can prevent diffusion of heat applied to the heat-sensitive recording layer and enhance the sensitivity as a heat-sensitive recording material.

Hollow particles made of organic resin can be classified into foam type and non-foam type, depending on the manufacturing method. Of these two types, expanded type hollow particles have properties suitable for improving the heat insulating property of the undercoat layer.

Hereinafter, a typical method for producing expanded type hollow particles will be described.
First, hollow particles can be produced by preparing particles in which a volatile liquid is contained inside a resin, softening the resin by heating, and vaporizing and expanding the liquid inside the particles.

The hollow ratio of expanded hollow particles is increased by heating and expanding the liquid inside during the manufacturing process. Since a large hollow ratio provides high heat insulation, the foamed hollow particles can enhance the sensitivity of the thermal paper and improve the recording density. The improvement of the sensitivity is important particularly in the case of developing a color in the halftone region where the thermal energy applied to the thermosensitive recording layer is small. Further, if the heat-sensitive recording layer is formed via an undercoat layer having a high heat insulating property, it is possible to prevent diffusion of heat applied to the heat-sensitive recording layer, thereby preventing image bleeding and improving image quality. From the above, in the present embodiment, the foaming type hollow particles excellent in the heat insulation of the undercoat layer are used.

Resins that can be used for the foam type hollow particles include styrene-acrylic resin, polystyrene resin, acrylic resin, polyethylene resin, polypropylene resin, polyacetal resin, chlorinated polyether resin, polyvinyl chloride resin, polyvinylidene chloride resin, Examples thereof include thermoplastic resins such as acrylic resins (for example, acrylic resins having acrylonitrile as a constituent component), styrene resins, vinylidene chloride resins, and copolymer resins mainly composed of polyvinylidene chloride and acrylonitrile. Propane, butane, isobutane, air and the like are generally used as the gas contained in the foamed hollow particles. Among the various resins described above, the resin used for the hollow particles is preferably an acrylonitrile resin or a copolymer resin mainly composed of polyvinylidene chloride and acrylonitrile, from the viewpoint of strength for maintaining the shape of the expanded particles.

The maximum particle size of the hollow particles is 20 to 100 μm, preferably 20 to 80 μm, more preferably 20 to 65 μm, and further preferably 20 to 40 μm. The maximum particle size is also referred to as D100. When the maximum particle size of the hollow particles is 20 μm or more, the cushioning property of the undercoat layer is improved, so that the adhesion of the thermal paper to the thermal head during printing is improved, and a high-quality thermal recording material can be obtained. This high image quality can bring about an improvement in the recording density in the halftone in which color is developed at a lower energy that gives the maximum recording density (Dmax). On the other hand, when the maximum particle diameter of the hollow particles is 100 μm or less, the smoothness of the undercoat layer is improved, so that the heat-sensitive recording layer provided through the undercoat layer can be made uniform and the white spots in the image hardly occur. The body is obtained. The maximum particle diameter of the hollow particles can be measured by a laser diffraction type particle size distribution measuring device. It is also possible to actually measure using an electron microscope.

When a powder is divided into two particles with a certain particle size, the diameter where the volume occupied by the particles on the large side and the particles on the small side are equal, that is, the particle size with a frequency of 50% by volume is called the median diameter. The median diameter is also referred to as D50. The ratio D100 / D50 between the maximum particle diameter (D100) and the median diameter (D50) is an index showing the degree of particle size distribution.

The ratio D100 / D50 of the hollow particles is 3.0 to 10.0, preferably 3.2 to 8.0, and more preferably 3.2 to 7.0. When D100 / D50 of the hollow particles is smaller than 3.0, the particle size distribution becomes very sharp, and the production becomes difficult. On the other hand, when D100 / D50 of the hollow particles is larger than 10.0, the maximum particle diameter becomes too large, so that the smoothness of the undercoat layer is deteriorated and white spots in the image occur.

The median diameter (D50) of hollow particles can be measured by a laser analysis type particle size distribution measuring device. It is also possible to actually measure using an electron microscope. The D50 of the hollow particles is not particularly limited and may be appropriately selected so as to fall within the above range, but it is preferably adjusted in the range of 10 to 25 μm from the viewpoint of improving the image quality of the thermal paper.

The content of the hollow particles is 5 to 90% by mass, preferably 10 to 70% by mass, based on the total solid content of the undercoat layer. When the content of the hollow particles is 5% by mass or more, the heat insulating property of the undercoat layer can be improved. On the other hand, when the content of the hollow particles is 90% by mass or less, problems such as coatability hardly occur. It is also preferable to add an oil absorbing pigment to the undercoat layer. By adding an oil-absorbing pigment to the undercoat layer, it is possible to effectively suppress printing problems such as sticking and head residue. Examples of oil absorbing pigments include calcined kaolin. The content of the calcined kaolin is preferably 5 to 80% by mass based on the total solid content of the undercoat layer.

(Thickener)
When the thickener is contained in the undercoat layer, it is possible to suppress the bias of the hollow particles in the undercoat layer coating liquid. As the thickener, various known materials such as cellulose and its derivatives, high molecular polysaccharides, modified polyacrylic acid, sodium alginate, and maleic anhydride copolymer can be appropriately used. Among the above, cellulose derivatives such as carboxymethyl cellulose (CMC) and high molecular polysaccharides are preferable as the material used for the thickener.

Since the maximum particle size of hollow particles is large, they have a large buoyancy and tend to gather upward in a liquid with low viscosity. When a cellulose derivative or a high molecular polysaccharide is used as a thickener for the undercoat layer coating solution, the hollow particles become difficult to float in the undercoat layer coating solution, and the uneven distribution of the hollow particles in the undercoat layer decreases. preferable. If the uneven distribution of the hollow particles is reduced, the smoothness of the undercoat layer is improved, so that the heat-sensitive recording layer provided through the undercoat layer can be made uniform and white spots in the image can be suppressed, The color density is also improved. Although the content of the thickener is not particularly limited, it is preferably in the range of 1 to 5% by mass based on the total solid amount of the undercoat layer. When the content is 1% by mass or more, the floating of the hollow particles is suppressed and the maximum color density is improved. When the content is 5% by mass or less, the viscosity increase of the coating material is suppressed and the coating suitability is excellent.

The undercoat layer generally supports a coating solution for the undercoat layer prepared by mixing and stirring plastic hollow particles, a binder resin, an oil-absorptive pigment such as calcined kaolin, an auxiliary agent, etc. in water as a medium. It is formed by applying on the body and drying. The coating amount of the coating liquid for the undercoat layer is not particularly limited, but is preferably about 2 to 20 g / m ^{2 and} more preferably about 2 to 12 g / m ^{2 as} a dry weight.

The binder resin can be appropriately selected from those that can be used in the heat-sensitive recording layer. Examples thereof include oxidized starch, starch-vinyl acetate graft copolymer, carboxymethyl cellulose, polyvinyl alcohol, and latex. Among them, latex is preferable from the viewpoint of excellent surface strength.

The latex is not particularly limited, and examples thereof include polyvinyl acetate, polyurethane, styrene-butadiene copolymer, styrene-butadiene-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyacrylic acid, polyacrylic acid ester, chloride. Vinyl-vinyl acetate copolymer, polybutyl methacrylate, ethylene-vinyl acetate copolymer, silylated urethane, acryl-silicon composite, and acryl-silicon-urethane composite, urea resin, melamine resin, amide resin, polyurethane resin And water-insoluble polymers such as

The content of the binder resin in the undercoat layer is preferably 1 to 60% by mass, more preferably 5 to 40% by mass. When the content of the binder resin is within the above range, the coatability will be good when the undercoat layer is coated. The content of the water-soluble polymer in the binder resin is preferably 1 to 50% by mass, more preferably 5 to 30% by mass.

[Thermal recording layer]
(Dye precursor)
The thermal recording layer generally contains a dye precursor and a developer. A typical dye precursor is a colorless or light-colored leuco dye. Examples of the leuco dye include triphenylmethane-based compounds, fluoran-based compounds, and diphenylmethane-based compounds, which can be appropriately selected and used. Further, leuco dyes include dyes having coloring tones such as red, vermilion, magenta, blue, cyan, yellow, green and black, which can be appropriately selected and used.

Examples of the dye precursor include 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide, 3- (4-diethylamino-2-methylphenyl) -3- (4-dimethylaminophenyl)- Blue-coloring dyes such as 6-dimethylaminophthalide and fluorane, 3- (N-ethyl-Np-tolyl) amino-7-N-methylanilinofluorane, 3-diethylamino-7-anilinofluorane , 3-diethylamino-7-dibenzylaminofluorane and other green color-forming dyes, 3,6-bis (diethylamino) fluorane-γ-anilinolactam, 3-cyclohexylamino-6-chlorofluorane, 3-diethylamino- Red-coloring dyes such as 6-methyl-7-chlorofluorane and 3-diethylamino-7-chlorofluorane, 3- (N-ethyl-N-isoamyl) amino-6-methyl-7-anilinofluorane, 3- (N-methyl-N-cyclohexyl) amino-6-methyl-7-anilinofluorane, 3-diethylamino-6-methyl-7-anilinofluorane, 3-di (n-butyl) amino-6 -Methyl-7-anilinofluorane, 3-di (n-pentyl) amino-6-methyl-7-anilinofluorane, 3-diethylamino-7- (o-chlorophenylamino) fluorane, 3- (N- Ethyl-p-toluidino) -6-methyl-7-anilinofluorane, 3- (N-ethyl-p-toluidino) -6-methyl-7- (p-toluidino) fluorane, 3- (N-ethyl- N-tetrahydrofurfurylamino) -6-methyl-7-anilinofluorane, 3-diethylamino-6-chloro-7-anilinofluorane, 3-dimethylamino-6-methyl-7-anilinofluorane, 3-pyrrolidino-6-methyl-7-anilinofluorane, 3-piperidino-6-methyl-7-anilinofluorane, 2,2-bis {4- [6 '-(N-cyclohexyl-N-methyl Amino) -3'-methylspiro [phthalide-3,9'-xanthene-2'-ylamino] phenyl} propane, 3-diethylamino-7- (3'-trifluoromethylphenyl) aminofluorane, etc. 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- Il] -4,5,6,7-tetrachlorophthalide, 3-p- (p-dimethylaminoanilino) anilino-6-methyl-7-chlorofluorane, 3-p- (p-chloroanilino) anilino Dyes having absorption wavelength in the near infrared region such as -6-methyl-7-chlorofluorane and 3,6-bis (dimethylamino) fluorene-9-spiro-3 '-(6'-dimethylamino) phthalide Is mentioned. Of course, it is not limited to these, and two or more kinds may be used in combination as required. Among them, 3-di (n-butyl) amino-6-methyl-7-anilinofluorane, 3-di (n-pentyl) amino-6-methyl-7-anilinofluorane, and 3- (N -Ethyl-N-isoamylamino) -6-methyl-7-anilinofluorane is preferably used because it has excellent recording sensitivity and print storability.

The content of the dye precursor is preferably 5 to 30% by mass, more preferably 7 to 30% by mass, and further preferably 7 to 25% by mass, based on the total solid amount of the heat-sensitive recording layer. When the content of the dye precursor is 5% by mass or more, the color density can be improved. On the other hand, when the content of the dye precursor is 30% by mass or less, heat resistance can be improved. The content of the dye precursor per unit area in the thermal recording layer is preferably 0.2 to 2.0 g / m ² , more preferably 0.4 to 1.5 g / m ² . The content of the dye precursor per unit area can be measured by a high performance liquid chromatography method or the like.

(Developer)
Specific examples of the developer include, for example, 4-tert-butylphenol, 4-acetylphenol, 4-tert-octylphenol, 4,4′-sec-butylidene diphenol, 4-phenylphenol, 4,4′-dihydroxy. Diphenylmethane, 4,4'-isopropylidene diphenol, 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, bis Butyl (p-hydroxyphenyl) acetate, methyl bis (p-hydroxyphenyl) acetate, hydroquinone monobenzyl ether, bis (3-allyl-4-hydroxyphenyl) sulfone, 4-hydroxy-4′-methyldiphenyl sulfone, 4- Phenolic compounds such as allyloxy-4'-hydroxydiphenyl sulfone and 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 4-hydroxybenzophenone, dimethyl 4-hydroxyphthalate, 4-hydroxybenzoic acid Methyl, propyl 4-hydroxybenzoate, sec-butyl 4-hydroxybenzoate, phenyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate, benzyl 4-hydroxybenzoate, tolyl 4-hydroxybenzoate, 4- Chlorophenyl hydroxybenzoate, 4,4'-dihydroxydiphenyl ether, etc. Or a 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 Aromatic carboxylic acids such as 5- [p- (2-p-methoxyphenoxyethoxy) cumyl] salicylic acid and zinc 4- {3- (p-tolylsulfonyl) propyloxy] salicylate, and phenolic compounds and aromatic compounds thereof Salts of carboxylic acids with polyvalent metals such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, nickel, etc., and antipyrine complexes of zinc thiocyanate, complexes of terephthalaldehyde acids with other aromatic carboxylic acids Organic acidic substances such as zinc salts, 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] diphenylsulfone Urea compounds such as N, N′-di-m-chlorophenylthiourea, thiourea compounds such as N- (p-toluenesulfonyl) carbamoyl acid p-cumylphenyl ester, N- (p-toluenesulfonyl) carbamoyl acid p -An organic compound having a -SO2NH- bond in the molecule, such as benzyloxyphenyl ester, N- [2- (3-phenylureido) phenyl] benzenesulfonamide, N- (o-toluoyl) -p-toluenesulfoamide, Examples thereof include activated clay, attapulgite, colloidal silica, and inorganic acid substances such as aluminum silicate.

Furthermore, 4,4'-bis [(4-methyl-3-phenoxycarbonylaminophenyl) ureido] diphenyl sulfone represented by the following general formula (1), 4,4'-bis [(2-methyl-5- Urea urethane derivatives such as phenoxycarbonylaminophenyl) ureido] diphenylsulfone, 4- (2-methyl-3-phenoxycarbonylaminophenyl) ureido-4 ′-(4-methyl-5-phenoxycarbonylaminophenyl) ureidodiphenylsulfone, Examples thereof include a diphenyl sulfone derivative represented by the following general formula (2). In the formula, n represents an integer of 1 to 6. Of course, the developer is not limited to these, and if necessary, two or more kinds of compounds can be used in combination.

The content of such a color developer is not particularly limited and may be adjusted according to the leuco dye used. The content of the color developer is generally preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, still more preferably 1 part by mass or more, and 1.2 parts by mass or more with respect to 1 part by mass of the leuco dye. Is more preferable, and 1.5 parts by mass or more is particularly preferable. The content of the color developer is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, still more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less, relative to 1 part by mass of the leuco dye. .. When the amount is 0.5 parts by mass or more, the recording performance can be improved. On the other hand, when the amount is 10 parts by mass or less, the background fog in a high temperature environment can be effectively suppressed.

In the present invention, the heat-sensitive recording layer may further contain a storability improving agent, mainly for further improving the storability of the color image. Examples of such storage 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) )] Phenolic compounds such as bisphenol and 4,4 ′-[1,3-phenylenebis (1-methylethylidene)] bisphenol; 4-benzyloxyphenyl-4 ′-(2-methyl-2,3-epoxypropyloxy) ) Epoxy compounds such as phenyl sulfone, 4- (2-methyl-1,2-epoxyethyl) diphenyl sulfone, 4- (2-ethyl-1,2-epoxyethyl) diphenyl sulfone; and 1,3,5-tris At least one selected from isocyanuric acid compounds such as (2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl) isocyanuric acid can be used. Of course, it is not limited to these, and if necessary, two or more kinds of compounds can be used in combination.
When the storage stability improver is used, the amount used may be an amount effective for improving the storage stability, and is usually preferably about 1 to 30% by mass in the total solid amount of the heat-sensitive recording layer. It is more preferably about 20% by mass.

A sensitizer may be contained in the heat-sensitive recording layer in the present invention. Thereby, the recording sensitivity can be increased. Examples of the sensitizer include stearic acid amide, methoxycarbonyl-N-stearic acid benzamylde, N-benzoylstearic acid amide, N-eicosanoic acid amide, ethylenebisstearic acid amide, behenic acid amide, methylenebisstearic 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-benzylbiphenyl, oxalic acid di-p-chlorobenzyl ester, oxalic acid di-p-methylbenzyl ester, oxalic acid dibenzyl ester, p-tolylbiphenyl ether, di (p-methoxyphenoxyethyl) ether, 1, 2-di (3-methylphenoxy) ethane, 1,2-di (4-methylphenoxy) ethane, 1,2-di (4-methoxyphenoxy) ethane, 1,2-di (4-chlorophenoxy) ethane, 1,2-diphenoxyethane, 1- (4-methoxyphenoxy) -2- (3-methylphenoxy) ethane, p-methylthiophenylbenzyl ether, 1,4-di (phenylthio) butane, p-acetotoluidide, p- Acetophenetide, N-acetoacetyl-p-toluidine, 1,2-diphenoxymethylbenzene, di (β-biphenylethoxy) benzene, p-di (vinyloxyethoxy) benzene, 1-isopropylphenyl-2-phenyl Examples thereof include ethane, di-o-chlorobenzyl adipate, 1,2-bis (3,4-dimethylphenyl) ethane, 1,3-bis (2-naphthoxy) propane, diphenyl and benzophenone. These can be used together as long as they do not interfere. The content of the sensitizer may be an amount effective for sensitization, and is usually about 2 to 40% by mass, preferably about 5 to 25% by mass, based on the total solid amount of the heat-sensitive recording layer. preferable.

A binder can be contained in the thermal recording layer. As the binder, the same resin as the binder resin used in the undercoat layer can be used. As the binder used in the coating liquid for the heat-sensitive recording layer, for example, any water-based adhesive such as a water-soluble adhesive or a water-dispersible adhesive can be used. Examples of the water-soluble adhesive include polyvinyl alcohol, carboxy-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, modified polyvinyl alcohol such as silicon-modified polyvinyl alcohol, starch and its derivatives, methoxycellulose, carboxymethylcellulose, and hydroxy. Cellulose derivatives such as ethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, and ethyl cellulose, sodium polyacrylate, polyvinylpyrrolidone, polyamide, diisobutylene-maleic anhydride copolymer salt, styrene-acrylic acid copolymer salt, styrene-maleic anhydride Copolymer salt, ethylene-maleic anhydride copolymer salt, acrylic acid amide-acrylic acid ester copolymer, acrylic acid amide-acrylic acid ester-methacrylic acid copolymer, polyacrylamide, sodium alginate, gelatin, casein, Examples include gum arabic and the like. Water-dispersible adhesives include polyvinyl acetate, polyurethane, styrene-butadiene copolymer, styrene-butadiene-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyacrylic acid, polyacrylic acid ester, vinyl chloride-acetic acid. Water such as vinyl copolymer, polybutyl methacrylate, ethylene-vinyl acetate copolymer, silylated urethane, acryl-silicon composite, acryl-silicon-urethane composite, urea resin, melamine resin, amide resin, polyurethane resin Examples include insoluble polymer latex. These can be used alone or in combination of two or more. At least one of these is contained in the total solid content of the thermosensitive recording layer in an amount of preferably about 5 to 50% by mass, more preferably about 10 to 40% by mass.

A cross-linking agent that cures the binder of the thermosensitive recording layer or other layers can be contained in the thermosensitive recording layer. By incorporating a crosslinking agent, the water resistance of the heat-sensitive recording layer can be improved. Examples of the cross-linking agent include aldehyde compounds such as glyoxal, polyamine compounds such as polyethyleneimine, epoxy compounds, polyamide resins, melamine resins, glyoxylate salts, dimethylolurea compounds, aziridine compounds, blocked isocyanate compounds; ammonium persulfate. Inorganic compounds such as ferric chloride, magnesium chloride, sodium tetraborate, and potassium tetraborate; boric acid, boric acid triesters, boron-based polymers, hydrazide compounds, glyoxylates, and the like. These may be used alone or in combination of two or more. The amount of the crosslinking agent used is preferably in the range of about 1 to 10 parts by mass based on 100 parts by mass of the total solid content of the thermosensitive recording layer. Thereby, the water resistance of the heat-sensitive recording layer can be improved.

In the heat-sensitive recording layer, if necessary, known waxes, metal soaps, colored dyes, colored pigments, fluorescent dyes, oil repellents, antifoaming agents, viscosity modifiers, etc. Can be included.

Examples of waxes include waxes such as paraffin wax, carnauba wax, microcrystalline wax, polyolefin wax, and polyethylene wax; for example, higher fatty acid amides such as stearic acid amide and ethylenebisstearic acid amide, higher fatty acid esters, and derivatives thereof. Can be mentioned.

Examples of the metal soap include higher fatty acid polyvalent metal salts such as zinc stearate, aluminum stearate, calcium stearate, and zinc oleate. Further, if necessary, various auxiliary agents such as an oil repellent, a defoaming agent, and a viscosity modifier may be added to the heat-sensitive recording layer within a range that does not impair the effects of the present invention. It is also possible to dissolve these auxiliary agents in a solvent and emulsify them in water using a water-soluble polymer as an emulsifier for use.

The coating liquid for the heat-sensitive recording layer is, for example, a dispersion liquid in which water is used as a dispersion medium and fine particles of a dye precursor and a developer, a binder, a preservative improving agent, a sensitizer, etc. are dispersed together or separately. Is prepared using The coating amount of the heat-sensitive recording layer coating solution, preferably in dry weight ² ~ 12g / ^m 2, more preferably ² ~ 8g / ^m 2, more preferably on a support such that ² ~ 7g / ^m 2 body Applied to.

[Protective layer]
A protective layer for protecting the heat-sensitive recording layer from heat and various external factors can be provided on the heat-sensitive recording layer. The protective layer is prepared, for example, by mixing a binder, if necessary, a pigment, a wax, a cross-linking agent, and other auxiliaries. As the binder and the pigment, the materials exemplified in the heat-sensitive recording layer can be used. By containing a pigment or a wax, it is possible to prevent sticking of dust to the thermal head and sticking. In addition, it is possible to impart water resistance to the protective layer by adding a crosslinking agent. The protective layer is a thermosensitive recording medium in which water is used as a dispersion medium so that the coating liquid for the protective layer has a dry weight of preferably 0.1 to 15 g / m ² , more preferably 0.5 to 8 g / m ^2. It is formed by coating on a layer.

In the present invention, in order to increase the added value of the thermosensitive recording medium, the thermosensitive recording medium can be further processed to provide a thermosensitive recording medium having higher functions. For example, an adhesive paper, a rewetting adhesive paper, or a delayed tack paper can be obtained by subjecting the back surface to a coating process using an adhesive, a rewetting adhesive, a delayed tack type adhesive, or the like. Further, by utilizing the back surface, a function as a thermal transfer paper, an ink jet recording paper, a carbonless paper, an electrostatic recording paper, or a xeography paper can be imparted to the recording paper to enable double-sided recording. Of course, a double-sided thermosensitive recording medium can also be used. Further, it is possible to suppress the permeation of oil or a plasticizer from the back surface of the thermosensitive recording medium, or to provide a back layer for curl control or antistatic.
It is also possible to make a linerless label that does not require release paper by applying a release layer containing silicone on the protective layer and applying an adhesive on the back surface.

[Thermal recording material]
As a method for forming each of the above layers on the support, an air knife method, a blade method, a gravure method, a roll coater method, a spray method, a dip method, a bar method, a curtain method, a slot die method, a slide die method, an extrusion method. Any known coating method such as the above may be used. Each coating solution may be applied and dried one layer at a time to form each layer, or the same coating solution may be applied in two or more layers separately. Furthermore, simultaneous multi-layer coating may be performed in which two or more layers are simultaneously coated.

From the viewpoint of increasing recording sensitivity and suppressing white spots, it is preferable to perform a smoothing treatment using a known method such as a super calender or a soft calender after formation of each layer or after formation of all layers.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these. Unless otherwise specified, “part” and “%” represent “part by mass” and “mass%”, respectively.

The materials used in Examples and Comparative Examples are as follows.
(I) Hollow particles A: trade name EXPANCEL 461WE20d36, hollow particles manufactured by Akzo Nobel, median diameter (D50) 20 μm, maximum particle diameter (D100) 80 μm, solid content concentration 15.0%
(Ii) Hollow particles B: median diameter (D50) 12 μm, maximum particle diameter (D100) 65 μm, solid content concentration 15.0%
(Iii) Hollow particles C: median diameter (D50) 12 μm, maximum particle diameter (D100) 40 μm, solid content concentration 15.0%
(Iv) Hollow particles D: trade name Matsumoto Microsphere F series, hollow particles manufactured by Matsumoto Yushi Co., Ltd., median diameter (D50) 4.0 μm, maximum particle diameter (D100) 10 μm, solid content concentration 15.0%
(V) Hollow particles E: median diameter (D50) 11.9 μm, maximum particle diameter (D100) 125 μm, solid content concentration 15.0%
(Vi) Styrene-butadiene latex: trade name: L-1571, manufactured by Asahi Kasei, solid content concentration 48%
(Vii) Carboxymethyl Cellulose (CMC) A: Trade Name Serogen 7A, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. (viii) Carboxymethyl Cellulose (CMC) B: serogen AG gum, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.

(Example 1)
(1) Preparation of Coating Liquid for Undercoat Layer Undercoat layer: 467 parts of hollow particles A, 52 parts of styrene-butadiene latex, 2.5 parts of carboxymethylcellulose A, 1 part of carboxymethylcellulose B, and 75.0 parts of water are mixed and stirred to form an undercoat layer. A coating liquid for use was obtained.

(2) Preparation of Leuco Dye Dispersion (Liquid A) 40 parts of 3-di- (n-butyl) amino-6-methyl-7-anilinofluorane, 10 parts of polyvinyl alcohol (polymerization degree: 500, saponification degree: 88%) % Aqueous solution 40 parts and water 20 parts are mixed, and a median diameter by a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) becomes 0.5 μm using a sand mill (manufactured by AIMEX, sand grinder). It was pulverized to obtain a leuco dye dispersion (Liquid A).

(3) Preparation of Developer Dispersion Liquid (B-1 Solution) 40 parts of 4-hydroxy-4'-isopropoxydiphenylsulfone (N8, D8), polyvinyl alcohol (polymerization degree 500, saponification degree 88%) 40 parts of 10% aqueous solution and 20 parts of water are mixed, and a median diameter by a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) becomes 1.0 μm using a sand mill (sand grinder manufactured by IMEX Co., Ltd.). To a colorant dispersion (B liquid).

(4) Preparation of Sensitizer Dispersion Solution (C Solution) 40 parts of oxalic acid di-p-methylbenzyl ester (trade name: HS-3520, manufactured by DIC), polyvinyl alcohol (polymerization degree: 500, saponification degree: 88%) 40 parts of 10% aqueous solution and 20 parts of water are mixed, and a median diameter by a laser diffraction particle size analyzer SALD2200 (manufactured by Shimadzu Corporation) becomes 1.0 μm using a sand mill (sand grinder manufactured by IMEX Co., Ltd.). To obtain a sensitizer dispersion liquid (C liquid).

(5) Preparation of coating liquid for heat-sensitive recording layer Solution A 29.5 parts, solution B 59.1 parts, solution C 45.4 parts, 5% aqueous solution of hydroxypropylmethylcellulose 20 parts, completely saponified polyvinyl alcohol (trade name) : PVA110, saponification degree: 99 mol%, average degree of polymerization: 1000, manufactured by Kuraray Co., Ltd., 46 parts of 10% aqueous solution, butadiene copolymer latex (trade name: L-1571, manufactured by Asahi Kasei Corporation, solid content concentration: 48% ) 9.4 parts, light calcium carbonate (trade name: Brilliant-15, manufactured by Shiraishi Industry Co., Ltd.) 25.4 parts, paraffin wax (trade name: Hydrin L-700, manufactured by Chukyo Yushi Co., Ltd., solid content concentration 30%) 11 0.7 parts, 2 parts of adipic acid dihydrazide (manufactured by Otsuka Chemical Co., Ltd.), and 120 parts of water were mixed and stirred to obtain a coating liquid for heat-sensitive recording layer.

(6) Preparation of coating liquid for protective layer Acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenx Z-200, saponification degree: 99.4 mol%, average polymerization degree: 1000, modification degree: 5 mol%, Nippon Synthetic Chemical Industry) 300 parts of a 12% aqueous solution of Kogyo Co., Ltd., kaolin (trade name: HYDRAGLOSS90, KaMin LLC) 19 parts, aluminum hydroxide (trade name: Hydilite H-42M, Showa Denko KK) 35 parts, silica (commodity) Name: Mizukasil P-527, manufactured by Mizusawa Chemical Co., Ltd. 4 parts, polyethylene wax (trade name: Chemipearl W-400, manufactured by Mitsui Chemicals, solid content concentration 40%) 2.5 parts, and water 114.5 parts The composition was mixed and stirred to obtain a coating liquid for protective layer.

(7) Preparation of heat-sensitive recording material An undercoat layer coating solution, a heat-sensitive recording layer coating solution, and a protective layer recording coating solution are applied onto one surface of a high-quality paper having a basis weight of 60 g / m ² after drying. the amount each ^{^{3.0g / m 2, 4.0g / m}} 2, was applied and dried so that 2.0 g / ^{m 2,} the undercoating layer, thermosensitive recording layer, and after sequentially forming a protective layer, Super The surface was smoothed with a calendar to obtain a heat-sensitive recording material.

(Example 2)
A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the hollow particles A were changed to the hollow particles B in the preparation of the coating liquid for undercoat layer of Example 1.

(Example 3)
A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the hollow particles A were changed to the hollow particles C in the preparation of the coating liquid for undercoat layer of Example 1.

(Example 4)
In the preparation of the coating liquid for the undercoat layer of Example 1, except that 467 parts of the hollow particles A, 140 parts of a dispersion obtained by dispersing 60 parts of calcined kaolin in 80 parts of water and 67 parts of the hollow particles C were used. A thermosensitive recording medium was obtained in the same manner as in Example 3.

(Example 5)
A thermosensitive recording medium was obtained in the same manner as in Example 3 except that 2.5 parts of carboxymethylcellulose A and 1.0 part of carboxymethylcellulose B were not used in the preparation of the coating liquid for undercoat layer of Example 3.

(Comparative Example 1)
A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the hollow particles A were changed to the hollow particles D in the preparation of the coating liquid for undercoat layer of Example 1.

(Comparative example 2)
A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the hollow particles A were changed to the hollow particles E in the preparation of the undercoat layer coating liquid of the first example.

[Halftone recording density]
Using a thermosensitive recording evaluation machine (trade name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermosensitive recording material was recorded in a halftone energy region of applied energy: 0.16 mJ / dot, and the obtained printing portion was recorded on Macbeth. It was measured in a visual mode of a densitometer (RD-914, manufactured by Macbeth Co.). The larger the value is, the darker the print density is. The practical print density is required to be 1.00 or more.

[image quality]
A bar code was recorded using a label printer (trade name: L-2000, manufactured by Ishida Co., Ltd.), and the recorded image quality was visually observed and evaluated according to the following criteria.
⊚: The image quality is excellent and there is no white spots in the image or the bar code is thick.
◯: No white spots in image quality and thick bar code, no problem.
Δ: Almost no white spots in image quality and thick bar code, and there is no problem in practical use.
Poor: There are white spots in the image and thick bar code, which is a practical problem.

As can be seen from Table 1, the thermal recording materials of Examples 1 to 5 were excellent in both recording density and print image quality.
In the thermosensitive recording medium of Comparative Example 1, both the maximum particle diameter (D100) of hollow particles and D100 / D50 were too small, and therefore the heat insulating property was insufficient, and the recording density and the print image quality were poor.
In the thermosensitive recording medium of Comparative Example 2, printing defects occur because the maximum particle diameter (D100) of the hollow particles is too large. Further, when D100 / D50 is too large, it means that the average particle diameter (D50) is smaller than the maximum particle diameter (D100), that is, it contains many fine particles as hollow particles. Is represented. Therefore, the heat insulating property is insufficient, and the recording density and the print image quality are poor.

Claims

A thermosensitive recording medium comprising an undercoat layer formed on one surface of a support and a thermosensitive recording layer formed on the undercoat layer,
The heat-sensitive recording layer contains a leuco dye and a color former,
The undercoat layer contains hollow particles and a binder resin,
The hollow particles have a maximum particle size (D100) of 20 to 100 μm,
A thermosensitive recording medium characterized in that the ratio D100 / D50 of the maximum particle diameter (D100) of the hollow particles to the particle diameter of 50% by volume frequency (D50) is 3.0 to 10.0.
The thermosensitive recording medium according to claim 1, wherein the undercoat layer further contains at least one of a cellulose derivative and a high molecular polysaccharide.
The thermosensitive recording medium according to claim 1 or 2, wherein a particle size (D50) of 50% by volume of the hollow particles is 10 to 25 µm.
The thermosensitive recording medium according to any one of claims 1 to 3, wherein the hollow particles are contained in an amount of 5 to 90% by mass based on the total solid amount of the undercoat layer.