JP2020104279A - Thermal recording medium - Google Patents

Abstract

To provide a thermal recording medium excellent in image quality and recording density and also excellent in coatability and strength of a coating layer during production.SOLUTION: A thermal recording medium has a thermal recording layer provided on a support via an undercoat layer, wherein the undercoat layer contains hollow particles, wherein the hollow particles are non-foaming type hollow particles having a core shell structure in which a core particle having alkali swellability is coated with a shell layer having no alkali swellability, wherein the hollow particles have an average particle diameter of 2.5-8 μm and a hollow ratio of 60% or more, and wherein the content of the hollow particles in the undercoat layer is 10-50 mass%.SELECTED DRAWING: None

Description

The present invention relates to a thermosensitive recording medium.

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

As the thermal recording material is developed for various applications, there is an increasing demand for improving the performance of the thermal recording material. That is, there is a demand for quality such that a colored image is deep and clear, and white spots (print defects) are less likely to occur. Further, there is also a manufacturing demand that the coating property is excellent in order to manufacture the thermal recording material at low cost.

Therefore, many improved techniques have been developed to meet such various demands. For example, it is said that the undercoat layer provided between the support of the thermosensitive recording medium and the thermosensitive recording layer contains hollow particles to enhance the heat insulating property of the undercoat layer, thereby improving the sensitivity of the thermosensitive recording medium. The method is known. Many improved techniques have been developed for the method of incorporating hollow particles in the undercoat layer.

For example, Patent Document 1 discloses a method of using an undercoat layer containing a foam type plastic spherical hollow filler. Further, Patent Document 2 discloses a method using non-foaming type hollow particles.

JP-A-5-238143 International Publication No. 2016/195076

However, in the method described in Patent Document 1, since the foaming type hollow particles are used as the hollow particles, the strength of the coating layer is weak, and there is room for improvement in coatability. Further, the method described in Patent Document 2 uses non-expanding type hollow particles made of styrene-acrylic resin as the hollow particles, but there is room for improvement in terms of image quality and recording density. ..

The present invention has been made in view of such a situation. That is, an object of the present invention is to provide a thermal recording material which is excellent in image quality and recording density, and also in coatability during production and strength of a coating layer.

In order to solve the above problems, the present inventors have proceeded with the study of hollow particles used in the undercoat layer. As a result, by using the non-foaming type hollow particles having a high hollow ratio and a relatively large particle diameter, a high-quality and high-sensitivity thermal recording material can be obtained even with a relatively small addition amount. It was found that it is possible to. The present invention has been completed based on these findings. That is, the present invention has the following configurations.

(1) A thermosensitive recording medium having a thermosensitive recording layer provided on a support through an undercoat layer, wherein the undercoat layer contains hollow particles, and the hollow particles are core particles having alkali swelling property. It is a non-foaming type hollow particle having a core-shell structure coated with a shell layer having no alkali swelling property, the average particle diameter of the hollow particle is 2.5 to 8 μm, and the hollow ratio of the hollow particle is 60% or more, and the content of the hollow particles in the undercoat layer is 10 to 50% by mass.

(2) The heat-sensitive recording material as described in (1) above, wherein the hollow particles are made of styrene-acrylic resin.

(3) The heat-sensitive recording material as described in (1) or (2) above, wherein the average particle diameter of the hollow particles is more than 3 μm and 6 μm or less.

(4) The thermal recording material according to any one of (1) to (3), wherein the amount of the undercoat layer formed is 2 to 6 g/m ² .

(5) The thermal recording material according to any one of (1) to (4), wherein the undercoat layer contains two or more kinds of hollow particles having different average particle diameters.

The thermosensitive recording medium of the present invention is excellent in image quality and recording density, and is also excellent in coatability during production and strength of the coating layer.

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 materials constituting the thermosensitive recording medium will be described below.

[Support]
The support is not particularly limited in kind, shape, size, etc., and includes, 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.

[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.

(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, and as a result, the sensitivity as a thermal recording medium can be enhanced.

Hollow particles made of an organic resin can be classified into a foaming type and a non-foaming type depending on the difference in the manufacturing method. As a method for producing the hollow particles, various production methods are known for both the foaming type and the non-foaming type. The following is a typical manufacturing method.

The method of manufacturing foam type hollow particles is to first create particles that contain a volatile liquid inside the resin, separate and dry the particles, then soften the resin by heating and vaporize and expand the liquid inside. To make hollow particles.

The method of manufacturing non-expandable hollow particles is to polymerize the seed in a solution, then polymerize other resin so as to wrap the seed, and then swell and dissolve the seed inside to remove the void inside. Is formed. When the seeds inside are swollen/dissolved and removed, an alkaline aqueous solution or the like is used.

The expanded type hollow particles generally have a larger average particle size and a higher hollow ratio than the non-expanded type hollow particles. Therefore, the expanded type hollow particles have better sensitivity and image quality than the non-expanded type hollow particles, but the strength of the coating layer is weak.

On the other hand, non-foaming type hollow particles generally have a small average particle size and a low hollow ratio. Therefore, in order to obtain good sensitivity and image quality, it is necessary to increase the content of hollow particles in the undercoat layer. However, when the content of the hollow particles is increased, the water retention property of the undercoat paint is lowered and the coatability is lowered (scratches and streaks are generated).

Therefore, in the present embodiment, it is decided to newly use non-foaming type hollow particles having a large average particle diameter in a low content. The hollow particles of the present embodiment can be obtained by subjecting core-shell particles obtained by coating core particles having an alkali swelling property with a shell layer having no alkali swelling property to an alkali swelling treatment. That is, the hollow particles of the present embodiment are non-expanding type hollow particles having a core-shell structure in which core particles having an alkali swelling property are coated with a shell layer having no alkali swelling property.

Monomers suitable for the method for producing non-expandable hollow particles include vinyl monomers such as styrene, acrylic, and acrylonitrile. Examples of the styrene-based monomer include styrene, methylstyrene, dimethylstyrene, trimethylstyrene, ethylstyrene, diethylstyrene, triethylstyrene, propylstyrene, butylstyrene, chlorostyrene, t-butylstyrene and the like. Examples of acrylic monomers include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, n-methacrylate. Butyl, isobutyl methacrylate, 2-ethylhexyl methacrylate, cyclohexyl methacrylate and the like can be mentioned. Examples of the acrylonitrile-based monomer include acrylonitrile and methacrylonitrile. Examples of other vinyl monomers include dimethyl maleate, dimethyl fumarate, maleic anhydride, N-methyl maleimide, N-phenyl maleimide and the like.

Among the various monomers described above, from the viewpoint of ease of production, a combination of a styrene-based monomer and an acrylic-based monomer is preferable, and a combination of a styrene-based monomer and a (meth)acrylic ester is More preferable. That is, the hollow particles are preferably made of styrene-acrylic resin, and more preferably made of styrene-(meth)acrylic acid ester copolymer resin.

The non-foaming type hollow particles have an average particle diameter of 2.5 to 8 μm. The average particle size of the hollow particles is preferably more than 3 μm and 6 μm or less, more preferably 3.5 to 5 μm, and further preferably 4 to 5 μm. When the non-foaming type hollow particles have an average particle diameter within the above-mentioned relatively large range, high image quality and high sensitivity can be achieved with a smaller addition amount. The average particle diameter of the hollow particles can be measured by a diffraction scattering method using a laser.

The non-foaming type hollow particles have a hollow ratio of 60% or more, preferably 70 to 85%. When the hollow ratio of the hollow particles is within the above-mentioned relatively large range, the heat insulating property can be improved with a smaller addition amount. The hollowness of the hollow particles can be determined by measuring the outer diameter and the inner diameter from an electron micrograph.

The non-foaming type hollow particles of the present embodiment have a large particle size and a high hollow ratio, and therefore are excellent in image quality and recording density even if the amount of the hollow particles added to the undercoat layer is small.

The content of the non-foaming type hollow particles in the undercoat layer is 10 to 50% by mass, and preferably 20 to 45% by mass. If it is less than 10% by mass, there is a problem from the viewpoint of improving the heat insulating property. On the other hand, if it is more than 50% by mass, the water retention property of the undercoat layer is lowered and the coatability becomes a problem. Here, the content of hollow particles in the undercoat layer is a numerical value obtained as a solid content.

Hollow particles having an average particle diameter of 2.5 to 8 μm and a hollow ratio of 60% or more are referred to as first hollow particles. In the present embodiment, in addition to the first hollow particles, second hollow particles having different average particle diameters can be contained in the undercoat layer. The average particle diameter of the second hollow particles is preferably 0.3 to 1.5 μm, more preferably 0.5 to 1.2 μm. The second hollow particles preferably have a hollowness of 30 to 60%, more preferably 40 to 55%.

Since the second hollow particles have a smaller average particle diameter than the first hollow particles, by using both together, the gaps between the first hollow particles in the undercoat layer can be filled with the second hollow particles. Then, the undercoat layer can be made more excellent in heat insulation. The mixing ratio of the first hollow particles and the second hollow particles is preferably 1/4 to 10/1 in terms of mass ratio.

(Binder resin)
As the binder resin used for the undercoat layer, for example, a water-soluble polymer is used. Examples of the water-soluble polymer include polyvinyl alcohol and its derivatives, starch and its derivatives, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, ethyl cellulose and other cellulose derivatives, sodium polyacrylate, polyvinyl pyrrolidone, and casein. , Gelatin and their derivatives. Among these, carboxymethyl cellulose is particularly preferable. In the present embodiment, the effects of the present invention can be fully exhibited by including hollow particles having a relatively large hollow ratio in the undercoat layer in the presence of carboxymethyl cellulose.

Examples of the binder resin other than the water-soluble polymer include acrylamide-acrylic acid ester copolymer, acrylamide-acrylic acid ester-methacrylic acid ester copolymer, styrene-maleic anhydride copolymer, isobutylene-maleic anhydride. Acid copolymer, polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic ester, vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, emulsion of ethylene-vinyl acetate copolymer, styrene-butadiene copolymer , Styrene-butadiene-acrylic copolymer and the like. These resins are often used as latex dispersed in water. As the binder resin, one or more kinds can be appropriately selected and used from the above resins.

By using a pigment having a high porosity such as silica or calcined kaolin for the undercoat layer, the recording sensitivity of the thermosensitive recording layer can be further increased. This is because when a pigment having a high porosity is used, the heat insulating property is increased and the sensitivity and the image quality are improved. A known crosslinking agent, fluorescent dye, or the like can be further added to the undercoat layer, if necessary.

The undercoat layer is formed by applying a coating solution for an undercoat layer containing hollow particles and a binder resin onto a support and drying it. From the viewpoint of the coverage of the support, the amount of the undercoat layer formed is preferably 1.5 g/m ² or more, more preferably 2 to 6 g/m ² . Here, the formation amount of the undercoat layer is a numerical value obtained as a solid content.

[Thermal recording layer]
The heat-sensitive recording layer generally contains a dye precursor and a developer. The heat-sensitive recording layer displays characters and figures on the heated portion by melting the dye precursor and the developer on the heated portion, and ring-opening the dye precursor with the developer to develop color. It is a layer.

The thermosensitive recording layer is formed by applying a coating liquid for the thermosensitive recording layer onto the undercoat layer and drying. 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 storage stability improver, a sensitizer, etc. are dispersed together or separately. Is prepared using Sensitive coating amount of the recording layer coating solution is preferably in dry weight 2~12g / ^{m 2,} more preferably 2 to 8 g / ^{m 2,} more preferably on a support so as to 2~7g / ^{m 2} Applied to.

(Dye precursor)
As a typical dye precursor contained in the heat-sensitive recording layer, a colorless or light-colored leuco dye can be mentioned. 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-N-p-tolyl)amino-7-N-methylanilinofluorane, 3-diethylamino-7-anilinoflu Green color-forming dyes such as olane and 3-diethylamino-7-dibenzylaminofluorane, 3,6-bis(diethylamino)fluorane-γ-anilinolactam, 3-cyclohexylamino-6-chlorofluorane, 3-diethylamino Red coloring dye 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- Methylamino)-3'-methylspiro[phthalide-3,9'-xanthene-2'-ylamino]phenyl}propane, 3-diethylamino-7-(3'-trifluoromethylphenyl)aminofluorane, etc. Dye, 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 Are listed. Of course, it is not limited to these, and if necessary, two or more kinds may be used in combination. 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 content of the heat-sensitive recording layer. When the content of the dye precursor is 5% by mass or more, the color density is improved, and when the content of the dye precursor is 30% by mass or less, the heat resistance is improved. The content of the dye precursor per unit area in the heat-sensitive 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)
The color developer reacts with the dye precursor by heating and develops a color. The color developer is preferably used as finely divided particles in order to improve the sensitivity of the thermosensitive recording medium.

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′-methyldiphenylsulfone, 4- Phenolic compounds such as 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 , 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, aromatic 4-carboxylic acid such as zinc 4-{3-(p-tolylsulfonyl)propyloxy]salicylate, and phenolic compounds thereof, aromatic Salts of carboxylic acids with polyvalent metals such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, nickel, etc., and antipyrine complexes of zinc thiocyanate, terephthalaldehyde acids and other aromatic carboxylic acid complexes 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.

（式中、ｎは１〜６の整数を表す。） Furthermore, 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenyl sulfone represented by the following general formula (1) and 4,4'-bis[(2-methyl-5- Phenoxycarbonylaminophenyl)ureido]diphenyl sulfone, 4-(2-methyl-3-phenoxycarbonylaminophenyl)ureido-4′-(4-methyl-5-phenoxycarbonylaminophenyl)ureidodiphenyl sulfone and other urea urethane derivatives, Examples thereof include diphenyl sulfone derivatives represented by the following general formula (2). Of course, it is not limited to these, and if necessary, two or more kinds of compounds can be used in combination.

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

The content of such a color developer is not particularly limited and may be adjusted according to the leuco dye used, and is generally preferably 0.5 parts by mass or more, and 0.8 parts by mass with respect to 1 part by mass of the leuco dye. The above is more preferable, 1 part by mass or more is further preferable, 1.2 parts by mass or more is still 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 with respect 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 embodiment, the heat-sensitive recording layer may further contain a storability improving agent, mainly for further enhancing the storability of the color image. Examples of such a shelf life improver 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 a storability improver is used, the amount used may be an amount effective for improving the storability, and is usually preferably about 1 to 30% by mass based on the total solid content of the thermosensitive recording layer. It is more preferably about 20% by mass.

A sensitizer may be contained in the heat-sensitive recording layer in the present embodiment. 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, and 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 ratio of the sensitizer may be an amount effective for sensitization, and is usually preferably about 2 to 40% by mass, and more preferably about 5 to 25% by mass in the total solid content of the thermosensitive recording layer. preferable.

The heat-sensitive recording layer may contain a binder. As the binder, the same resin as the binder resin used for the undercoat layer can be used.

The heat-sensitive recording layer may contain a crosslinking agent that hardens the binder. Examples of the crosslinking 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 with respect to 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, defoaming agents, viscosity modifiers, etc. may be added as long as the effects of the invention are not impaired. Can be included. 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 ethylenebisstearic acid amide; higher fatty acid esters and their derivatives. 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 agent, a defoaming agent and a viscosity modifier can be added to the heat-sensitive recording layer within a range that does not impair the effects of the present invention.

[Protective layer]
A protective layer for protecting the heat-sensitive layer from heat and various external factors can be further provided on the heat-sensitive recording layer. The protective layer is formed by applying the protective layer coating solution on the thermosensitive recording layer so that the dry weight is preferably 0.1 to 15 g/m ² , more preferably 0.5 to 8 g/m ^2. It is formed. The protective layer coating liquid is prepared by mixing, for example, a binder, a pigment, a cross-linking agent, a wax, and other auxiliaries with water as a dispersion medium. 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.

[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. Furthermore, simultaneous multi-layer coating may be performed in which two or more layers are simultaneously coated.

From the viewpoint of increasing the recording sensitivity and improving the image uniformity, a known method such as a super calender or a soft calender is used in any process after forming each layer or after forming all layers. It is preferable to use it for smoothing treatment.

The present invention will be described in more detail by way of examples, but the present invention is not limited thereto. 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 A-380, manufactured by Sansui Co., Ltd., non-foaming type hollow particles made of styrene-acrylic resin, average particle diameter 3.5 μm, hollow ratio 78%, solid content concentration 13.0%.
(Ii) Hollow particles B: trade name A-580, manufactured by Sansui Co., Ltd., non-foaming type hollow particles made of styrene-acrylic resin, average particle diameter 5.0 μm, hollow ratio 81%, solid content concentration 13.0%.
(Iii) Hollow particles C: trade name Ropaque SN-1055, Dow Chemical Co., non-foaming type hollow particles made of styrene-acrylic resin, average particle diameter 1.0 μm, hollow ratio 55%, solid content concentration 26.5. %
(Iv) Hollow particles D: trade name Cybinol NS-2000, manufactured by Saiden Chemical Co., Ltd., non-foaming type hollow particles, average particle diameter 2 μm, hollow rate 80%, solid content concentration 20.0%
(V) Hollow particles E: Matsumoto Yushi Co., Ltd., expandable hollow particles, average particle diameter 3.5 μm, hollow ratio 84%, solid content concentration 34.0%
(Vi) Calcined kaolin: BASF Corporation, trade name Ansilex 93
(Vii) Carboxylic Acid Modified Starch: Nitto Kagaku Co., Ltd., trade name Petrocoat C-8
(Viii) Styrene/butadiene latex: Asahi Kasei Corporation, trade name: L-1571, solid content concentration 48%
(Ix) Carboxymethyl cellulose: manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name Serogen AG Gum

(Example 1)
(1) Preparation of Coating Liquid for Undercoat Layer In a dispersion obtained by dispersing 45 parts of calcined kaolin in 53 parts of water, 308 parts of hollow particles A, 17 parts of a 30% aqueous solution of carboxylic acid-modified starch, styrene. 21 parts of butadiene-based latex and 1.6 parts of carboxymethyl cellulose were mixed and stirred to obtain a coating liquid for undercoat layer.

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

(3) Preparation of Developer Dispersion Liquid (Liquid B) 40 parts of 4-benzyloxy-4′-hydroxydiphenyl sulfone, 40 parts of 10% aqueous solution of polyvinyl alcohol (polymerization degree 500, saponification degree 88%), and water 20 Parts are mixed and ground using a sand mill (made by AIMEX, sand grinder) until a median diameter of 0.7 μm is obtained by a laser diffraction particle size analyzer SALD2200 (made by Shimadzu Corp.), and a color developer is dispersed. A liquid (B liquid) was obtained.

(4) Preparation of Sensitizer Dispersion Liquid (C Liquid) 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) is 1.0 μm using a sand mill (sand grinder manufactured by IMEX Co., Ltd.). The mixture was pulverized until it became a sensitizer dispersion liquid (C liquid).

(5) Preparation of coating liquid for heat-sensitive recording layer Liquid A 29 parts, liquid B 59 parts, liquid C 45 parts, 10% of acetoacetyl-modified polyvinyl alcohol (trade name: Gosenex Z-205, manufactured by Nippon Synthetic Chemical Co., Ltd.) Aqueous solution 45 parts, styrene-butadiene latex 9.5 parts, light calcium carbonate (trade name: Brilliant-15, manufactured by Shiraishi Industry Co., Ltd.) 25 parts, paraffin wax (trade name: Hydrin L-700, manufactured by Chukyo Yushi Co., Ltd., solid) (Concentration 30%) 12 parts, glyoxylic acid sodium salt (trade name: SPM-01, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.) 3 parts, and water 50 parts were mixed and stirred to obtain a coating liquid for heat-sensitive recording layer.

(6) Preparation of Coating Liquid for Protective Layer 300 parts of 10% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenx Z-200, manufactured by Nippon Synthetic Chemical Industry Co., Ltd.), aluminum hydroxide (trade name: Hydilite H-) 42M, Showa Denko KK 63 parts, polyethylene wax (trade name: Chemipearl W-400, Mitsui Chemicals, Inc., solid content concentration 40%) 0.5 parts, and a composition comprising 60 parts of water are mixed and stirred. A coating liquid for protective layer was obtained.

(7) Preparation of thermosensitive recording medium Coating amount after drying of coating liquid for undercoat layer, coating liquid for thermosensitive recording layer and coating liquid for protective layer on one side of woodfree paper having a basis weight of 60 g/m ^2. There are 4.0 g ^{/ m} 2, was applied and dried so that 4.0 g / ^{m 2,} and 2.0 g / ^{m 2,} an undercoat layer, the thermosensitive recording layer, and after the protective layer are sequentially formed, The surface was smoothed with a super calendar to obtain a thermosensitive recording medium 1.

(Example 2)
A thermosensitive recording medium 2 was prepared in the same manner as in Example 1 except that the amount of the hollow particles A used was changed to 192 parts and the hollow particles C were used in the preparation of the undercoat layer coating material of Example 1. Obtained.

(Example 3)
In the preparation of the undercoat layer coating composition of Example 1, the calcined kaolin dispersion was changed to a dispersion obtained by dispersing 60 parts of calcined kaolin in 70 parts of water, and the amount of the hollow particles A used was 192 parts. A thermosensitive recording medium 3 was obtained in the same manner as in Example 1 except that the above was changed.

(Example 4)
A thermosensitive recording medium 4 was obtained in the same manner as in Example 1 except that 308 parts of the hollow particles B were used in place of the hollow particles A in the preparation of the coating material for the undercoat layer of Example 1.

(Example 5)
A thermosensitive recording medium 5 was obtained in the same manner as in Example 2 except that 192 parts of the hollow particles B were used in place of the hollow particles A in the preparation of the undercoat layer coating material of Example 2.

(Example 6)
A thermosensitive recording medium 6 was obtained in the same manner as in Example 3 except that 192 parts of the hollow particles B were used in place of the hollow particles A in the preparation of the undercoat layer coating material of Example 3.

(Comparative Example 1)
A thermosensitive recording medium 7 was obtained in the same manner as in Example 1 except that the hollow particles C were changed to 151 parts in the preparation of the undercoat layer coating material of Example 1.

(Comparative example 2)
In the preparation of the undercoat layer coating composition of Example 1, the calcined kaolin dispersion was changed to a dispersion obtained by dispersing 25 parts of calcined kaolin in 29 parts of water, and hollow particles were used instead of the hollow particles A. A thermosensitive recording medium 8 was obtained in the same manner as in Example 1 except that 226 parts of C was used.

(Comparative example 3)
A thermosensitive recording medium 9 was obtained in the same manner as in Example 1 except that 200 parts of the hollow particles D were used in place of the hollow particles A in the preparation of the undercoat layer coating material of Example 1.

(Comparative Example 4)
In the preparation of the undercoat layer coating composition of Example 1, the calcined kaolin dispersion was changed to a dispersion obtained by dispersing 25 parts of calcined kaolin in 29 parts of water, and hollow particles were used instead of the hollow particles A. A thermosensitive recording medium 10 was obtained in the same manner as in Example 1 except that 300 parts of D was used.

(Comparative example 5)
A thermosensitive recording medium 11 was obtained in the same manner as in Example 1 except that 117 parts of hollow particles E were used in place of the hollow particles A in the preparation of the undercoat layer coating material of Example 1.

(Comparative example 6)
In the preparation of the coating material for the undercoat layer of Example 1, the dispersion of calcined kaolin was changed to a dispersion obtained by dispersing 75 parts of calcined kaolin in 88 parts of water, and hollow particles were used instead of the hollow particles A. A thermosensitive recording medium 12 was obtained in the same manner as in Example 1 except that 29 parts of E was used.

(Comparative Example 7)
In the preparation of the coating material for the undercoat layer of Example 1, the dispersion of calcined kaolin was changed to a dispersion obtained by dispersing 75 parts of calcined kaolin in 88 parts of water, and hollow particles were used instead of the hollow particles A. A thermosensitive recording medium 13 was obtained in the same manner as in Example 1 except that 77 parts of B was used.

[Evaluation of thermal recording material]
The thermosensitive recording medium thus obtained was allowed to stand for 72 hours under the conditions of 40° C. and 50% RH, and the following items were evaluated. The results are shown in Table 1.

(image quality)
Using a thermosensitive recording evaluation machine (trade name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermosensitive recording material was recorded at an applied energy of 0.16 mJ/dot, and the obtained printed portion was visually observed. The image quality to be determined here is an evaluation regarding unevenness of image density in a portion of the thermal recording paper having no coating defect, and the determination criteria are as follows.
⊚: Almost no print defects and uniform recording density ○: Slight print defects Δ: Print defects, print density varies but no problem in actual use ×: Many print defects, actual use There is a problem on

(Recording density)
Using a thermosensitive recording evaluation machine (trade name: TH-PMD, manufactured by Okura Electric Co., Ltd.), each thermosensitive recording material was recorded with applied energy: 0.16 mJ/dot, 0.24 mJ/dot, and the obtained printing portion was recorded. The measurement was performed in a visual mode of a Macbeth densitometer (RD-914, manufactured by Macbeth Co.). The higher the value, the darker the print density.
0.16 mJ/dot: 0.90 or more is desirable 0.24 mJ/dot: 1.30 or more is desirable

(Coatability)
The undercoat layer obtained by applying and drying the undercoat layer coating liquid on one surface of a high-quality paper having a basis weight of 60 g/m ² so that the applied amount after drying is 4.0 g/m ^2. The surface of was observed. The judgment criteria are as follows.
⊚: Almost no coating defects, uniform coating surface can be obtained ◯: Micro scratches are seen △: Micro scratches and streaks are seen, but no problem in practical use ×: Many scratches and streaks are seen, There is a problem in actual use

(Coating layer strength)
The coated surface strength of each thermosensitive recording medium was evaluated depending on whether or not a dry pick was generated in the RI printing tester.
◎: No peeling of coating layer is observed ○: Peeling of coating layer may be seen slightly △: Peeling of coating layer is seen slightly, but no problem in practical use ×: Peeling of coating layer is large Seen, there is a problem in actual use

As can be seen from Table 1, the thermal recording media of Examples 1 to 6 were excellent in image quality, recording density, coating property and coating layer strength.
The thermosensitive recording medium of Comparative Example 1 is a non-foaming type hollow particle, but since the hollow particle C having a small average particle diameter and a small hollow ratio is used, the image quality and the recording density (0.16 mJ/dot) are high. It was inferior.
The thermosensitive recording medium of Comparative Example 2 had a high content of hollow particles, and thus was inferior in coatability.
The thermosensitive recording medium of Comparative Example 3 is a non-foaming type hollow particle, but since the hollow particle D having a small average particle diameter is used, it is inferior in image quality and recording density (0.16 mJ/dot). ..
The thermosensitive recording medium of Comparative Example 4 was a non-foaming type hollow particle, but the hollow particle D having a small average particle diameter was used, and the content of the hollow particle was high. Was also inferior.
Since the thermosensitive recording medium of Comparative Example 5 uses the foaming type hollow particles E, the recording density (0.24 mJ/dot), coatability and coating layer strength were also poor.
The thermal recording materials of Comparative Example 6 and Comparative Example 7 were inferior in image quality and recording density (0.16 mJ/dot) because the content of hollow particles was small.

Claims (5)

A thermal recording medium having a thermal recording layer provided on a support through an undercoat layer,
The undercoat layer contains hollow particles,
The hollow particles are non-foaming type hollow particles having a core-shell structure in which a core particle having an alkali swelling property is coated with a shell layer having no alkali swelling property,
The average particle diameter of the hollow particles is 2.5 to 8 μm,
The hollow ratio of the hollow particles is 60% or more,
The thermosensitive recording medium, wherein the content of the hollow particles in the undercoat layer is 10 to 50% by mass. The thermal recording medium according to claim 1, wherein the hollow particles are made of styrene-acrylic resin. The thermosensitive recording medium according to claim 1 or 2, wherein the average particle diameter of the hollow particles is more than 3 µm and 6 µm or less. The under formation amount of the coating layer is heat-sensitive recording material according to claim 1, characterized in that a 2 to 6 g / m ^2. 5. The thermosensitive recording medium according to claim 1, wherein the undercoat layer contains two or more kinds of hollow particles having different average particle diameters.