JP7779884B2 - Thermal recording medium - Google Patents

Description

The present invention relates to a thermosensitive recording medium that utilizes a color-forming reaction between a colorless or pale-colored electron-donating leuco dye (hereinafter also referred to as "leuco dye") and an electron-accepting developer (hereinafter also referred to as "developer").

In general, thermal recording media are made by applying a coating liquid containing a colorless or light-colored leuco dye and a color developer to a support such as paper, synthetic paper, film, or plastic. When heated with a thermal head, hot stamp, thermal pen, or laser light, the color develops instantly through a chemical reaction, producing a recorded image. Thermal recording media are widely used as recording media for facsimiles, computer terminal printers, automatic ticket machines, measurement recorders, and receipts from supermarkets and convenience stores.

In recent years, thermal recording media have been used in a wide variety of applications, including various tickets, receipts, labels, bank ATMs, gas and electricity meter reading, and cash vouchers for car and horse racing tickets. As a result, a variety of performance features are required, such as water resistance, plasticizer resistance in the image area, heat resistance and oil resistance in the blank area, and storage stability of the image and blank areas under harsh conditions.

In such applications, stable printing on a thermal recording medium is required. However, frequent printing can cause recording paper to stick to the print head or head residue to accumulate in the print head, preventing proper printing. To solve these problems, a technique is known in which a phosphate ester compound is blended into the outermost layer, such as a protective layer (see, for example, Patent Document 1).
Also known is a thermosensitive recording medium in which the color development sensitivity is improved by simultaneously coating and drying a thermosensitive recording layer and a protective layer by a multi-layer curtain coating method (Patent Document 2, etc.).

JP 2012-56184 A Patent No. 5626218

However, even with such conventional techniques, it has been difficult to achieve both color development sensitivity and print runnability (resistance to head residue adhesion and sticking).
SUMMARY OF THE INVENTION An object of the present invention is to provide a thermosensitive recording medium which has good color development sensitivity, print running properties (particularly resistance to head residue adhesion), and abrasion resistance, and which also has good print resolution during high-speed printing.

After extensive research, the inventors discovered that color development sensitivity improves when the total content (solids content) of leuco dye, developer, and sensitizer in the thermosensitive recording layer is 70.0% by weight or more. However, a protective layer was provided to prevent head residue and sticking from occurring when the content of leuco dye, etc. in the thermosensitive recording layer is high. Furthermore, the provision of a protective layer improves scratch resistance. Furthermore, the inventors discovered that reducing the coating weight of the thermosensitive recording layer to 2.5 g/ ^m2 or less improves print resolution during high-speed printing.

That is, the present invention provides a thermosensitive recording medium comprising a support and, in this order, an undercoat layer, a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye, an electron-accepting developer, and a sensitizer, and a protective layer, wherein the total content (solid content) of the leuco dye, the developer, and the sensitizer in the thermosensitive recording layer is 76.0% by weight or more, the coating weight of the thermosensitive recording layer is 2.5 g/ ^m2 or less, the coating weight of the protective layer is 2.5 g/ ^m2 or less and 0.4 g/ ^m2 or more , the thermosensitive recording layer does not contain a lubricant, and the protective layer contains a lubricant .

The present invention makes it possible to obtain a thermal recording medium that has excellent color development sensitivity, print running properties (particularly resistance to head residue adhesion), and abrasion resistance, as well as excellent print resolution during high-speed printing.

The thermosensitive recording medium of the present invention is a thermosensitive recording medium comprising a support, an undercoat layer, a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye, an electron-accepting developer and a sensitizer, and a protective layer, in this order, wherein the total content (solid content) of the leuco dye, developer and sensitizer in the thermosensitive recording layer is 70.0% by weight or more, and the coating weight of the thermosensitive recording layer is 2.5 g/ ^m2 or less.

The following are examples of various materials that can be used in the thermal recording layer of the thermal recording medium of the present invention, but binders, crosslinking agents, pigments, etc. can also be used in each coating layer provided as needed, as long as they do not impede the desired effects on the above-mentioned issues.

<Undercoat layer>
The thermosensitive recording medium of the present invention has an undercoat layer on the support.
The primer layer consists primarily of a binder and a pigment.
The binder used in the undercoat layer may be any commonly used emulsion of a water-soluble polymer or a hydrophobic polymer. Specific examples include cellulose derivatives such as polyvinyl alcohol, polyvinyl acetal, hydroxyethyl cellulose, methyl cellulose, and carboxymethyl cellulose; starch and its derivatives; sodium polyacrylate; polyvinylpyrrolidone; acrylic acid amide/acrylic acid ester copolymers; acrylic acid amide/acrylic acid ester/methacrylic acid copolymers; styrene/maleic anhydride copolymer alkali salts; isobutylene/maleic anhydride copolymer alkali salts; polyacrylamide; sodium alginate; gelatin; and casein; and emulsions of hydrophobic polymers such as polyvinyl acetate, polyurethane, styrene/butadiene copolymers, polyacrylic acid, polyacrylic acid esters, vinyl chloride/vinyl acetate copolymers, polybutyl methacrylate, ethylene/vinyl acetate copolymers, and styrene/butadiene/acrylic copolymers. These binders may be used alone or in combination of two or more.

The pigment used in the undercoat layer may be any known pigment that has been commonly used, such as calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, calcined kaolin, clay, talc, or other inorganic pigments. These pigments may be used alone or in combination.
The amount of pigment in the undercoat layer is usually 50 to 95 parts by weight, preferably 70 to 90 parts by weight, per 100 parts by weight of the total solid content.
The coating liquid for the undercoat layer may contain various auxiliary agents such as dispersants, plasticizers, pH adjusters, antifoaming agents, water-retaining agents, preservatives, coloring dyes, and ultraviolet protection agents, as needed.

<Thermal Recording Layer>
The heat-sensitive recording layer contains a colorless or pale-colored electron-donating leuco dye, an electron-accepting developer and a sensitizer.
The leuco dye used in the present invention can be any known leuco dye in the field of conventional pressure-sensitive or heat-sensitive recording paper, and is not particularly limited. However, triphenylmethane compounds, fluoran compounds, fluorene compounds, divinyl compounds, etc. are preferred. Specific examples of typical colorless or pale-colored dyes (dye precursors) are shown below. These dye precursors may be used alone or in combination.

<Triphenylmethane leuco dyes>
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (also known as crystal violet lactone), 3,3-bis(p-dimethylaminophenyl)phthalide (also known as malachite green lactone)

<Fluoran-based leuco dyes>
3-diethylamino-6-methylfluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-6-methyl-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-(o-fluoroanilino)fluoran, 3-diethylamino-6-methyl-7-(m-methylanilino)fluoran, 3-diethylamino-6-methyl-7 -n-octylanilinofluoran, 3-diethylamino-6-methyl-7-n-octylaminofluoran, 3-diethylamino-6-methyl-7-benzylaminofluoran, 3-diethylamino-6-methyl-7-dibenzylaminofluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-diethylamino-6-chloro-7-p-methylanilinofluoran, 3-diethylamino-6-ethoxyethyl-7-anilinofluoran, 3-diethylamino-7-methylfluoran, 3-diethylamino-7-chlorofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-6-chloro-7-methylfluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-diethylamino-7-(o-chloroanilino)fluoran, 3-diethylamino-7-(p-chloroanilino)fluoran, 3-diethylamino-7-(o-fluoroanilino)fluoran, 3-diethylamino-benzo[a]fluoran, 3-diethylamino-benzo[c]fluoran, 3-dibutylamino-6-methyl-fluoran, 3-dibutylamino-6-methyl-7-anilinofluoran, 3-dibutylamino-6-methyl-7-(o,p-dimethylanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-dibutylamino-6-methyl-7-(o-fluoroanilino)fluoran n-trifluoromethylanilino)fluoran, 3-dibutylamino-6-methyl-7-chlorofluoran, 3-dibutylamino-6-ethoxyethyl-7-anilinofluoran, 3-dibutylamino-6-chloro-7-anilinofluoran, 3-dibutylamino-6-methyl-7-p-methylanilinofluoran, 3-dibutylamino-7-(o-chloroanilino)fluoran, 3-dibutylamino-7-(o-fluoroanilino)fluoran, 3-di-n-pentylamino-6-methyl-7-anilinofluoran, 3-di-n-pentylamino-6-methyl-7-(p-chloroanilino)fluoran, 3-di-n-pentylamino- 7-(m-trifluoromethylanilino)fluoran, 3-di-n-pentylamino-6-chloro-7-anilinofluoran, 3-di-n-pentylamino-7-(p-chloroanilino)fluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-propylamino)-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-cyclohexylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-xylamino)-6-methyl-7-(p-chloroanilino)fluoran, 3-(N-ethyl (N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-chloro-7-anilinofluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isobutylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-ethoxypropylamino)-6-methyl-7-anilinofluoran, 3-cyclohexylamino-6-chlorofluoran, 2-(4-oxahexyl)-3-dimethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)- 3-diethylamino-6-methyl-7-anilinofluoran, 2-(4-oxahexyl)-3-dipropylamino-6-methyl-7-anilinofluoran, 2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-amino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-diethylamino No-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 2-phenyl-6-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-benzyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 2-hydroxy-6-p-(p-phenylaminophenyl)aminoanilinofluoran, 3-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-dibutylaminophenyl)aminoanilinofluoran, 2,4-dimethyl-6-[(4-dimethylamino)anilino]-fluoran

<Fluorene-based leuco dyes>
3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide], 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide]

<Divinyl leuco dyes>
3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrabromophthalide, 3,3-bis-[2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide, 3,3-bis-[1,1-bis(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrabromophthalide, 3,3-bis-[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide

<Others>
3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-octyl-2-methylindol-3-yl)-4-azaphthalide, 3-(4-cyclohexylethylamino-2-methoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 3,3-bis(1-ethyl-2-methylindol-3-yl)phthalide, 3,6-bis(diethylamino)fluoran-γ-(3'-nitro)anilinolactam, 3,6-bis(diethylamino)fluoran-γ-(3'-nitro)anilinolactam (amino)fluoran-γ-(4'-nitro)anilinolactam, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-dinitrileethane, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2-β-naphthoylethane, 1,1-bis-[2',2',2'',2''-tetrakis-(p-dimethylaminophenyl)-ethenyl]-2,2-diacetylethane, bis-[2,2,2',2'-tetrakis-(p-dimethylaminophenyl)-ethenyl]-methylmalonic acid dimethyl ester

Examples of color developers that can be used in the present invention include inorganic acidic substances such as activated clay, attapulgite, colloidal silica, and aluminum silicate, 4,4'-isopropylidenediphenol, 1,1-bis(4-hydroxyphenyl)cyclohexane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 4,4'-dihydroxydiphenyl sulfide, hydroquinone monobenzyl ether, benzyl 4-hydroxybenzoate, 4,4'-dihydroxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-n-propoxydiphenyl sulfone, and bis(3-allyl-4-hydrogen). 4-hydroxyphenyl)sulfone, 4-hydroxy-4'-methyldiphenyl sulfone, 4-hydroxyphenyl-4'-benzyloxyphenyl sulfone, 3,4-dihydroxyphenyl-4'-methylphenyl sulfone, 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane, phenol condensation compositions described in JP-A 2003-154760, aminobenzenesulfonamide derivatives described in JP-A 8-59603, bis(4-hydroxyphenylthioethoxy)methane, 1,5-di(4-hydroxyphenylthio)-3-oxapentane, bis(p-hydroxyphenyl)butyl acetate, bis( methyl 1,1-bis(4-hydroxyphenyl)acetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,4-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, 1,3-bis[α-methyl-α-(4'-hydroxyphenyl)ethyl]benzene, di(4-hydroxy-3-methylphenyl)sulfide, 2,2'-thiobis(3-tert-octylphenol), 2,2'-thiobis(4-tert-octylphenol), 4,4'-bis(3-(phenoxycarbonylamino)methylphenylureido)diphenyl sulfone, compounds described in WO 02/081229 or JP 2002-301873 A, and N,N'-di-m-chloro Examples include aromatic carboxylic acids such as thiourea compounds such as phenylthiourea, p-chlorobenzoic acid, stearyl gallate, zinc bis[4-(n-octyloxycarbonylamino)salicylate] dihydrate, 4-[2-(p-methoxyphenoxy)ethyloxy]salicylic acid, 4-[3-(p-tolylsulfonyl)propyloxy]salicylic acid, and 5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylic acid, as well as salts of these aromatic carboxylic acids with polyvalent metals such as zinc, magnesium, aluminum, calcium, titanium, manganese, tin, and nickel, as well as antipyrine complexes of zinc thiocyanate and complex zinc salts of terephthalaldehyde acid and other aromatic carboxylic acids.

These color developers can be used alone or in combination of two or more. 1-[4-(4-hydroxyphenylsulfonyl)phenoxy]-4-[4-(4-isopropoxyphenylsulfonyl)phenoxy]butane is available, for example, from API Corporation under the trade name JKY-214, and the phenol condensation composition described in JP 2003-154760 A is available, for example, from API Corporation under the trade name JKY-224.
The compounds described in WO02/081229 and the like are available under the trade names NKK-395 and D-100 manufactured by Nippon Soda Co., Ltd. In addition, metal chelate color-developing components such as higher fatty acid metal double salts and polyvalent hydroxy aromatic compounds described in JP-A-10-258577 can also be contained.

The developer may contain a urea compound represented by the following general formula (Chemical Formula 1): This urea compound does not contain bisphenol A or bisphenol S.
(In the formula, X represents —O— or —NH—; R ¹ represents a hydrogen atom or —SO ₂ —R ³ ; R ³ represents a substituted or unsubstituted alkyl group, aralkyl group, or aryl group; R ² represents a hydrogen atom or an alkyl group; and m represents 0 or 1.)

As a color developer, the above-mentioned color developer and this urea compound may be used in combination, or the urea compound may be used alone as a color developer.

The urea compound is preferably the following (1) or (2):
(1) A first urea compound represented by the following general formula (Chemical Formula 2):
(wherein R ¹ , R ² and R ³ are defined as above.)
(2) A second urea compound represented by the following formula (Chemical Formula 3):
(In the formula, ^R2 is defined as above, and ^R4 to ^R8 are defined below.)

The first urea compound used in the present invention is preferably represented by the following formula (Chemical Formula 4).

In the general formula (Chemical Formula 2) and the general formula (Chemical Formula 4), ^R3 represents an alkyl group, an aralkyl group, or an aryl group, which may be substituted or unsubstituted, and is preferably a substituted or unsubstituted aryl group.
When these are substituted, the substituent is preferably an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or a halogen atom. In addition, multiple ^R3s may be the same or different.
The positions of R ¹ -O- in the benzene ring of the general formula (Chemical Formula 2) may be the same or different, and are preferably the 3-, 4- or 5-position.
The positions of R ³ —SO ₂ —O— in the benzene rings of general formula (chemical formula 2) and general formula (chemical formula 4) may be the same or different, and are preferably the 3-position, 4-position or 5-position.

The alkyl group may be, for example, a linear, branched, or alicyclic alkyl group, and preferably has 1 to 12 carbon atoms. Examples of the alkyl group include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, cyclopentyl, hexyl, cyclohexyl, 2-ethylhexyl, and lauryl.

The number of carbon atoms in this aralkyl group is preferably 7 to 12. Examples of this aralkyl group include benzyl, 1-phenylethyl, 2-phenylethyl, 3-phenylpropyl, p-methylbenzyl, m-methylbenzyl, m-ethylbenzyl, p-ethylbenzyl, p-i-propylbenzyl, p-t-butylbenzyl, p-methoxybenzyl, m-methoxybenzyl, o-methoxybenzyl, m,p-di-methoxybenzyl, p-ethoxy-m-methoxybenzyl, p-phenylmethylbenzyl, p-cumylbenzyl, p-phenylbenzyl, o-phenylbenzyl, m-phenylbenzyl, p-tolylbenzyl, m-tolylbenzyl, o-tolylbenzyl, and p-chlorobenzyl, and other unsubstituted or substituted aralkyl groups with an alkyl group, alkoxy group, aralkyl group, aryl group, or halogen atom.

The number of carbon atoms in this aryl group is preferably 6 to 12. Examples of this aryl group include unsubstituted or aryl groups substituted with an alkyl group, an alkoxy group, an aralkyl group, an aryl group, or a halogen atom, such as phenyl group, p-tolyl group, m-tolyl group, o-tolyl group, 2,5-dimethylphenyl group, 2,4-dimethylphenyl group, 3,5-dimethylphenyl group, 2,3-dimethylphenyl group, 3,4-dimethylphenyl group, mesitylene group, p-ethylphenyl group, p-i-propylphenyl group, p-t-butylphenyl group, p-methoxyphenyl group, 3,4-dimethoxyphenyl group, p-ethoxyphenyl group, p-chlorophenyl group, 1-naphthyl group, 2-naphthyl group, and t-butylated naphthyl group.

In the above general formulae (Chemical Formulas 1, 2, and 4), ^R3 is preferably a group represented by the following formula:
In the above formula, R ⁴ to R ⁸ may be the same or different and represent a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an alkoxy group, an aryloxy group, an alkylcarbonyloxy group, an arylcarbonyloxy group, an alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group.

^R2 represents a hydrogen atom or an alkyl group, preferably a hydrogen atom, and the alkyl group is preferably an alkyl group having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, or a t-butyl group.
The positions of ^R2 in the benzene ring of general formula (Chemical Formula 2) may be the same or different, and are preferably the 3-, 4- or 5-positions.

As the first urea compound of the present invention, a urea compound represented by the following general formula (Chemical Formula 6) is more preferred.
In the general formula (Chemical Formula 6), ^R9 is an alkyl group or an alkoxy group, preferably an alkyl group, and o is an integer of 0 to 3, preferably 0 to 2, more preferably 0 to 1. The number of carbon atoms in this alkyl group is, for example, 1 to 12, preferably 1 to 8, more preferably 1 to 4.
The positions of R ⁹ in the benzene ring of the general formula (Chemical Formula 6) may be the same or different, and are preferably the 3-position, 4-position or 5-position, more preferably the 4-position.

Furthermore, examples of the first urea compound of the present invention include N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-ethyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-5-methyl-phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)-4-propyl-phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di- [3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[3-(p-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-xylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(1-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-isopropylbenzenesulfonyloxy)phenyl]urea, N, N'-di-[3-(p-t-butylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(m,p-dimethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-ethoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-propoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-butoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-kumi Examples include, but are not limited to, N,N'-di-[3-(p-cumylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-phenylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-chlorobenzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea.

The second urea compound used in the present invention is preferably N-[2-(3-phenylureido)phenyl]benzenesulfonamide, which is represented by the following formula and is available, for example, from Nippon Soda Co., Ltd. under the trade name NKK1304.

The sensitizer used in the present invention may be a conventionally known sensitizer, such as fatty acid amides such as stearic acid amide and palmitic acid amide, ethylene bisamide, montanic acid wax, polyethylene wax, 1,2-bis-(3-methylphenoxy)ethane, p-benzylbiphenyl, β-benzyloxynaphthalene, 4-biphenyl-p-tolyl ether, m-terphenyl, 1,2-diphenoxyethane, dibenzyl oxalate, di(p-chlorobenzyl) oxalate, di(p-methylbenzyl) oxalate, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-p-tolyl carbonate, phenyl-α- Examples of such an alkyl ester include naphthyl carbonate, 1,4-diethoxynaphthalene, 1-hydroxy-2-naphthoic acid phenyl ester, o-xylene-bis-(phenyl ether), 4-(m-methylphenoxymethyl)biphenyl, 4,4'-ethylenedioxy-bis-benzoic acid dibenzyl ester, dibenzoyloxymethane, 1,2-di(3-methylphenoxy)ethylene, bis[2-(4-methoxy-phenoxy)ethyl]ether, methyl p-nitrobenzoate, phenyl p-toluenesulfonate, o-toluenesulfonamide, and p-toluenesulfonamide.
These sensitizers may be used alone or in combination of two or more.

The total content (solids content) of the leuco dye, developer, and sensitizer in the thermosensitive recording layer of the present invention is 70.0% by weight or more. A total content (solids content) of 70.0% by weight or more improves color development sensitivity. The total content (solids content) of the leuco dye, developer, and sensitizer in the thermosensitive recording layer is preferably 72.0% by weight or more, and more preferably 79.0% by weight. The upper limit of the total content (solids content) is 90% by weight or less.

The coating weight of the thermosensitive recording layer of the present invention can be 2.5 g/ ^m2 or less, preferably 2.4 g/ ^m2 or less, and more preferably 2.1 g/ ^m2 or less. As described above, by setting the total content (solid content) of the leuco dye, developer, and sensitizer in the thermosensitive recording layer to 70.0 wt % or more, it is possible to maintain sensitivity that is acceptable for practical use even if the coating weight of the thermosensitive recording layer is reduced to 2.5 g/ ^m2 or less.
The lower limit of the coating amount of the thermosensitive recording layer is, for example, 1.2 g/m ^2. It is more preferable that the coating amount of the thermosensitive recording layer is 1.4 to 2.4 g/m ² .

It is preferable that the thermosensitive recording layer of the present invention does not contain an inorganic pigment. If the thermosensitive recording layer contains an inorganic pigment, the total content (solid content) of the leuco dye, developer, and sensitizer described above will be reduced accordingly, and color development sensitivity may be reduced.
Examples of inorganic pigments include kaolin, calcined kaolin, calcium carbonate, aluminum oxide, titanium oxide, magnesium carbonate, aluminum silicate, magnesium silicate, calcium silicate, aluminum hydroxide, and silica.

It is preferable that the thermosensitive recording layer of the present invention does not contain a lubricant. If the thermosensitive recording layer contains a lubricant, the total content (solid content) of the leuco dye, developer, and sensitizer described above will be reduced accordingly, and color development sensitivity may be reduced.
Examples of the lubricant include fatty acid metal salts such as zinc stearate and calcium stearate, waxes, and silicone resins.

Binders that can be used in the thermal recording layer include polyvinyl alcohols such as fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, acetoacetylated polyvinyl alcohol, carboxy-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, olefin-modified polyvinyl alcohol, nitrile-modified polyvinyl alcohol, pyrrolidone-modified polyvinyl alcohol, silicone-modified polyvinyl alcohol, and other modified polyvinyl alcohols; silane-modified acrylic resins; and high-Tg acrylic resins (preferably non-core-shell acrylic resins, whose glass transition point (Tg) is greater than 50°C and less than 95°C). This Tg can be determined by differential scanning calorimetry (DSC). C). Examples of suitable polymers include (meth)acrylic acid and other acrylic resins composed of monomer components copolymerizable with (meth)acrylic acid (excluding olefins); cellulose derivatives such as hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose, ethyl cellulose, and acetyl cellulose; starches such as oxidized starch, etherified starch, and esterified starch; styrene-maleic anhydride copolymers, styrene-butadiene copolymers, casein, gum arabic, polyvinyl chloride, polyvinyl acetate, polyacrylamide, polyacrylic esters, polyvinyl butyral, polystyrose, and their copolymers; polyamide resins, silicone resins, petroleum resins, terpene resins, ketone resins, and coumarone resins. These polymeric substances can be dissolved in solvents such as water, alcohols, ketones, esters, and hydrocarbons, or emulsified or dispersed in water or other media to form a paste. These polymeric substances can also be used in combination depending on the required quality.

In the present invention, stabilizers such as 4,4'-butylidene (6-t-butyl-3-methylphenol), 2,2'-di-t-butyl-5,5'-dimethyl-4,4'-sulfonyldiphenol, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, and 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane can also be added to improve the oil resistance of image areas, provided that the desired effects for the above-mentioned problems are not impaired. In addition, benzophenone-based and triazole-based ultraviolet absorbers, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc. can also be used in the thermal recording layer.

The types and amounts of the leuco dye, developer, sensitizer, and other components used in the thermosensitive recording layer of the present invention are determined according to the required performance and recording suitability and are not particularly limited, but typically, 0.5 to 10 parts by weight of developer, 0.1 to 10 parts by weight of sensitizer, 0.01 to 10 parts by weight of stabilizer, and 0.01 to 10 parts by weight of other components are used per part by weight of leuco dye. The binder is suitably about 5 to 25% by weight of the solid content of the thermosensitive recording layer.
In the present invention, the total content (solid content) of the leuco dye, developer, and sensitizer is 70.0% by weight or more, so it is desirable to reduce the binder content. However, if the binder content is less than 5% by weight, the coating properties, particularly the curtain film suitability, will be reduced, so the lower limit is preferably 5% by weight or more.

In the present invention, the leuco dye, developer, and any other optional additives are pulverized to particle sizes of several microns or less using a grinder such as a ball mill, attritor, or sand grinder, or an appropriate emulsifying device, and a binder and various additives depending on the purpose are added to form a coating liquid. The solvent used in this coating liquid can be water or alcohol, and the solids content is approximately 20 to 40% by weight.

<Protective layer>
The thermosensitive recording medium of the present invention has a protective layer on the surface of the thermosensitive recording layer. The thermosensitive recording layer of the present invention has a total content (solids content) of leuco dye, developer, and sensitizer of 70.0% by weight or more. As a result, the absolute amount of color former that melts during printing increases, increasing the area of direct contact between the head and the color former and relatively reducing the material that absorbs the melted color former, which tends to deteriorate print runnability and stick resistance. Therefore, in the present invention, it is essential to provide a protective layer in order to obtain a thermosensitive recording medium with good print runnability (especially resistance to head residue adhesion) and stick resistance.
This protective layer mainly comprises a binder and a pigment, and may further contain a lubricant.
As the binder, any of the binders that can be used in the above-mentioned thermosensitive recording layer can be used appropriately.

Examples of pigments include inorganic pigments such as kaolin, calcined kaolin, calcium carbonate, aluminum oxide, titanium oxide, magnesium carbonate, aluminum silicate, magnesium silicate, calcium silicate, aluminum hydroxide, and silica.
In particular, when the protective layer contains an inorganic pigment having an aspect ratio of 30 or more, the surface smoothness of the protective layer is improved, and the print running properties (particularly, resistance to head residue adhesion) are improved. An example of an inorganic pigment having an aspect ratio of 30 or more is kaolin.
The aspect ratio is expressed as the value obtained by dividing the maximum diameter of the pigment by its thickness (minimum length).

Examples of the lubricant include fatty acid metal salts such as zinc stearate and calcium stearate, waxes, and silicone resins.
When the protective layer contains a lubricant, the surface smoothness of the protective layer is improved, and the print running properties (particularly, resistance to head residue adhesion) are improved.
As described above, in the present invention, in order to increase the total content (solid content) of the leuco dye, developer and sensitizer in the thermosensitive recording layer to 70% by weight or more, it is preferable not to include a lubricant in the thermosensitive recording layer, but to include a lubricant in the protective layer in order to ensure smoothness as a thermosensitive recording medium.

The amount of binder or the total amount of binder and pigment in the protective layer is usually 80.0 to 100.0% by weight, preferably 90.0 to 100.0% by weight, in terms of solid content, and the amount of binder is preferably about 30.0 to 300.0 parts by weight per 100 parts by weight of pigment.
The coating solution for the protective layer may contain various auxiliary agents such as stabilizers, ultraviolet absorbers, dispersants, antifoaming agents, antioxidants, fluorescent dyes, etc., which can be used in the above-mentioned thermosensitive recording layer, as needed.

The coating amount of the protective layer is preferably 2.5 g/m ² or less, more preferably 2.3 g/m ² or less, and most preferably 2.0 g/m ² or less.
The lower limit of the coating amount of the protective layer is, for example, 0.4 g/m ^2. It is more preferable that the coating amount of the protective layer is 0.7 to 2.5 g/m ² .

<Support>
Examples of the support include paper, synthetic paper, film, and plastic sheet.

In the present invention, the means for applying the thermosensitive recording layer and the coating layers other than the thermosensitive recording layer, i.e., the protective layer, undercoat layer, etc., is not particularly limited, and they can be applied according to well-known conventional techniques. For example, an off-machine coater or an on-machine coater equipped with various coaters such as an air knife coater, a rod blade coater, a bent blade coater, a bevel blade coater, a roll coater, or a curtain coater can be appropriately selected and used.
Furthermore, various known techniques in the field of thermosensitive recording media can be added as necessary and appropriate, such as smoothing treatment such as supercalendering after coating each coating layer.

In particular, when the above-mentioned thermosensitive recording layer and protective layer are formed by applying and drying a coating liquid for the thermosensitive recording layer and a coating liquid for the protective layer by a multi-layer curtain coating method, the coverage of the thermosensitive recording layer and the protective layer is good even with a low coating amount of 2.5 g/ ^m2 or less, so that the surface smoothness of the thermosensitive recording layer and the protective layer is increased and the print running property (particularly, resistance to head residue adhesion) is further improved. As the coating method, simultaneous multi-layer curtain coating is preferred.

The following examples illustrate the present invention, but are not intended to limit it. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified.

[Example 1]
The following ingredients were stirred and dispersed to prepare a coating liquid for the undercoat layer.
Undercoat layer coating liquid 1
Calcined kaolin (trade name: Ansilex 90, manufactured by Engelhard Co., Ltd.)
<oil absorption 90 cc/100 g>) 100 parts styrene-butadiene copolymer latex (solid content 48%) 35.4 parts 10% aqueous solution of polyvinyl alcohol 45 parts Water 157.1 parts Next, the undercoat layer coating liquid was blade coated on one side of the support (54 g/ ^m2 base paper) and then dried to obtain an undercoat layer coated paper with a dry coating weight of 4.0 g/ ^m2 .

The developer dispersions (Liquids A1 to A5), leuco dye dispersion (Liquid B), and sensitizer dispersions (Liquids C1 to C3) having the following formulations were each wet-ground in a sand grinder until the average particle size reached 0.5 microns.
A1 liquid (developer dispersion liquid)
4,4'-hydroxydiphenyl sulfone 5.7 parts Polyvinyl alcohol 10% aqueous solution 5.1 parts Water 9.9 parts A2 solution (developer dispersion)
4-hydroxy-4'-benzyloxydiphenyl sulfone (manufactured by Nicca Chemical Co., Ltd.)
Product name: BPS-MA3) 6.0 parts 10% aqueous solution of polyvinyl alcohol 11.3 parts Water 6.5 parts A3 solution (developer dispersion)
4,4'-bis(3-(phenoxycarbonylamino)methylphenylureido)diphenyl sulfone (UU manufactured by Fine Ace Co., Ltd.)
3.5 parts 10% aqueous solution of polyvinyl alcohol 7.4 parts water 3.3 parts A4 liquid (developer dispersion)
N-[2-(3-phenylureido)phenyl]benzenesulfonamide
(Product name NKK1304) 6.0 parts 10% aqueous solution of polyvinyl alcohol 7.5 parts Water 9.0 parts A5 liquid (developer dispersion)
N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea (product name S-176) 3.5 parts Polyvinyl alcohol 10% aqueous solution 4.4 parts Water 5.3 parts Solution B (leuco dye dispersion)
3-Dibutylamino-6-methyl-7-anilinofluoran (ODB-2)
2.85 parts 10% aqueous solution of polyvinyl alcohol 4.8 parts Water 3.5 parts C1 solution (sensitizer dispersion)
Diphenyl sulfone (manufactured by Sankosha, trade name: DPS) 5.7 parts 10% aqueous solution of polyvinyl alcohol 7.1 parts Water 8.6 parts C2 solution (sensitizer dispersion)
Stearamide (trade name: Finawax, manufactured by Fine Ace Co., Ltd.) 5.7 parts, 10% aqueous solution of polyvinyl alcohol 14.3 parts, water 3.8 parts, C3 solution (sensitizer dispersion)
1,2-bis-(3-methylphenoxy)ethane (manufactured by Sankosha, trade name: KS232)
0.5 parts Polyvinyl alcohol 10% aqueous solution 0.6 parts Water 0.8 parts

Next, the dispersions were mixed in the following proportions to prepare a coating liquid for the heat-sensitive recording layer.
Coating liquid for thermal recording layer A1 liquid (developer dispersion) 20.7 parts B liquid (leuco dye dispersion) 11.1 parts C1 liquid (sensitizer dispersion) 21.4 parts C2 liquid (sensitizer dispersion) 23.8 parts Polyvinyl alcohol 10% aqueous solution 32 parts

Then, the following mixture was mixed in the following ratio to prepare a coating liquid for the protective layer.
Coating liquid for protective layer: Flat kaolin (50% dispersion, manufactured by IMERYS, product name: Contour 1500, aspect ratio: 30)
7.0 parts 10% aqueous solution of polyvinyl alcohol 30 parts polyamide epichlorohydrin resin (manufactured by Seiko PMC Corporation, trade name: WS4030,
Solid content: 25%) 4.0 parts modified polyamine resin (manufactured by Taoka Chemical Co., Ltd., trade name: Sumirez Resin SPI-102A,
45% solids), 2.2 parts zinc stearate (trade name: Hydrin Z-7-30, 30% solids,
Particle size: 5.5 microns, Chukyo Yushi) 1.7 parts Next, using a slide curtain coater, a coating film was formed on the undercoat layer of the undercoat layer-coated paper from the lower layer side using the thermosensitive recording layer coating liquid and the protective layer coating liquid, and the coating film was simultaneously multi-layer curtain coated at a coating speed of 600 m/min. After drying, a supercalendering treatment was performed to obtain a thermosensitive recording material having a thermosensitive recording layer with a dry coating weight of 1.8 g/ ^m2 and a protective layer with a dry coating weight of 0.7 g/ ^m2 .

[Color development sensitivity]
Printing was performed on the prepared thermal recording medium with an applied energy of 0.35 mJ/dot or 0.42 mJ/dot using a TH-PMD manufactured by Okura Electric Co., Ltd. The image density after printing was measured with a Macbeth densitometer (using an amber filter).
<Solvent resistance>
Ethanol (50.0%) was applied to the blank portion of the prepared thermal recording medium with a cotton swab, and the medium was then left to stand at 23°C x
After leaving the sample to stand for 24 hours under an environmental condition of 50% RH, the sample was visually evaluated according to the following criteria.
○: No coloring at all △: Slight coloring, but not a problem in practical use ×: Strong coloring

<Printing runnability (head residue resistance)>
A 10 cm long grid pattern was printed on the prepared thermal recording medium using a label printer (Resply R-8) manufactured by Sato Corporation, and the residue (head residue) adhering to the thermal head after printing was visually inspected and evaluated according to the following criteria.
◯: Almost no adhesion of head residue was observed.
Δ: A small amount of head residue was observed, but the formed image was not void or blurred, and was of a level that was not problematic for practical use.
x: A large amount of head residue was observed, and voids and blurring of the formed image were observed.

<Stick resistance>
The prepared thermosensitive recording medium was subjected to 100 mm solid printing in an environment of 0° C. using a thermosensitive handy terminal printer (HT180 manufactured by Canon Inc.), and the printed matter was visually evaluated.
○: No streaky white spots in the printed area △: A few streaky white spots in the printed area ×: Many streaky white spots in the printed area [Printing defect rate]
The prepared thermal recording medium was used to print the Arabic numerals '2' and '3' in solid form using a Zebra label thermal printer (product name: 140XiIIIPlus) at a printing speed of 8 inch/s and a printing speed of 2 inch/s, and the printing defect rate was calculated using the following formula.
Printing defect rate = 100 - (coloring area at printing speed of 8 inches/s / coloring area at printing speed of 2 inches/s) (%)
The color-developed area was measured by capturing the printed image as digital data, determining the pixels corresponding to grayscale levels 0 to 97 as the color-developed area, and integrating the total number of color-developed pixels to determine the color-developed area.
The smaller the rate of print defects, the finer the printing during high-speed printing, and a print defect rate of 35% or less is considered good.

[Example 2]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the dry coating amount of the thermosensitive recording layer was changed to 2.1 g/m ² .
[Example 3]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the total content (solid content) of the leuco dye, developer and sensitizer in the thermosensitive recording layer was changed to 70.0% by weight.
[Example 4]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the total content (solid content) of the leuco dye, developer and sensitizer in the thermosensitive recording layer was changed to 85.0% by weight.

[Example 5]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the dry coating amount of the protective layer was changed to 1.0 g/m ² .
[Example 6]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that no lubricant was added to the protective layer.
[Example 7]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the inorganic pigment in the protective layer was replaced with aluminum hydroxide.
[Example 8]
A thermal recording medium was prepared in the same manner as in Example 1, except that the developer dispersion in the thermal recording layer was changed to 23.8 parts of A2 liquid and 14.2 parts of A3 liquid, the sensitizer dispersion was changed to 1.9 parts of C3 liquid, and the leuco dye blending ratio was changed to 5.0 parts.
[Example 9]
A thermosensitive recording medium was prepared in the same manner as in Example 8, except that the developer dispersion in the thermosensitive recording layer was changed to 22.5 parts of A4 liquid and 13.2 parts of A5 liquid.

[Comparative Example 1]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that the total content (solid content) of the leuco dye, developer and sensitizer was changed to 46.0% by weight by incorporating an inorganic pigment into the thermosensitive recording layer.
[Comparative Example 2]
A thermosensitive recording medium was prepared in the same manner as in Example 1, except that no protective layer was provided.

The results are shown in Tables 1 and 2.

As shown in Table 1, the thermosensitive recording medium of the present invention was excellent in color development sensitivity and print running property. Furthermore, it was also excellent in print definition (print defect rate) during high-speed printing.
In each example in which kaolin having an aspect ratio of 30 or more was used as the inorganic pigment in the protective layer, the color development sensitivity was further improved compared to Example 7 in which aluminum hydroxide was used as the inorganic pigment.
In the case of Example 9, in which the developer contained the urea compound represented by the general formula (1) (NKK1304 and S176), the solvent resistance was improved compared to the other Examples.

On the other hand, in Comparative Example 1, in which the total content (solid content) of the leuco dye, developer, and sensitizer in the thermosensitive recording layer was less than 70.0 wt %, the color development sensitivity was poor, coupled with the fact that the coating amount of the thermosensitive recording layer was 2.5 g/m ² or less. In addition, the print resolution (print defect rate) during high-speed printing was poor, which is thought to be due to the reduced sensitivity.
In the case of Comparative Example 2 in which no protective layer was provided, the color development sensitivity was good, but the print running properties were poor.

Claims (6)

A thermosensitive recording medium comprising a support, an undercoat layer, a thermosensitive recording layer containing a colorless or light-colored electron-donating leuco dye, an electron-accepting developer and a sensitizer, and a protective layer provided in this order on the support,
the total content (solid content) of the leuco dye, the color developer, and the sensitizer in the thermosensitive recording layer is 76.0% by weight or more;
the coating amount of the thermosensitive recording layer is 2.5 g/ ^m2 or less,
the coating amount of the protective layer is 2.5 g/ ^m2 or less and 0.4 g/ ^m2 or more ,
A thermosensitive recording medium , wherein the thermosensitive recording layer does not contain a lubricant, and the protective layer contains a lubricant . 2. The thermosensitive recording material according to claim 1, wherein the coating weight of the protective layer is 2.0 g/m ^<2> or less. The thermosensitive recording medium according to claim 1 or 2, wherein the thermosensitive recording layer does not contain an inorganic pigment. 3. The thermosensitive recording material according to claim 1, wherein the developer contains a urea compound represented by the following general formula (1):
(In the formula, X represents —O— or —NH—; R ¹ represents a hydrogen atom or —SO ₂ —R ³ ; R ³ represents a substituted or unsubstituted alkyl group, aralkyl group, or aryl group; R ² represents a hydrogen atom or an alkyl group; and m represents 0 or 1.) The thermosensitive recording medium according to claim 1 or 2, wherein the protective layer contains an inorganic pigment having an aspect ratio of 30 or more. 3. The thermosensitive recording material according to claim 1, wherein the print defect rate, as expressed by the following formula, is 35% or less.
100 - (Coloring area at a printing speed of 8 inches/s / Coloring area at a printing speed of 2 inches/s) (%)