EP4644129A1

EP4644129A1 - Heat-sensitive recording body

Info

Publication number: EP4644129A1
Application number: EP23912124.7A
Authority: EP
Inventors: Takashi Takemura; Naonobu Sugiyama
Original assignee: Oji Holdings Corp
Current assignee: Oji Holdings Corp
Priority date: 2022-12-27
Filing date: 2023-12-26
Publication date: 2025-11-05
Also published as: CN120344401A; EP4644129A4; WO2024143345A1; JP2024093225A

Abstract

Disclosed is a heat-sensitive recording material comprising a heat-sensitive recording layer on a support, the heat-sensitive recording layer containing a leuco dye, a developer, and binders, the heat-sensitive recording layer containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as the binders, and containing, as the developer, at least one non-phenolic developer selected from the group consisting of an N,N'-diarylurea-based compound represented by the following formula (1): wherein
R represents a C_1-12 alkyl group, a C_7-12 aralkyl group, or a C_6-12 aryl group, the aralkyl group and aryl group may be substituted with a C_1-12 alkyl group, a C_1-12 alkoxy group, a C_6-12 aryl group, or a halogen atom, and a plurality of Rs may be the same or different, and
A¹ represents a hydrogen atom or a C_1-4 alkyl group, and a plurality of A¹s may be the same or different;
a compound represented by the following formula (2): wherein
R¹ to R⁵ are the same or different, and each represents 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; and
a compound represented by the following formula (3):

Description

Technical Field

The present invention relates to a heat-sensitive recording material.

Background Art

Typically, heat-sensitive recording materials, which can produce a recorded image on at least one surface of their support made of paper, synthetic paper, a synthetic resin film, or the like by applying thermal energy to cause a color-development reaction between a leuco dye and a developer, have advantages in that, for example, recording devices for these are compact and inexpensive, and are easily maintained. Such heat-sensitive recording materials are widely used as recording media for fax machines, automatic ticket vending machines, and scientific measuring instruments, as well as output media for various printers and plotters for POS labels, CAD, CRT medical images, and the like. In particular, those that comprise synthetic paper or a synthetic resin film as the support exhibit excellent water resistance and excellent image quality, and thus have shown a significant increase in usage as output media for various printers for food labels, medical labels, medical images, etc.
However, unlike a paper-based support, a film-based support, such as synthetic paper, does not absorb a paint that constitutes a layer formed on the support, and the binding properties between a base material and a heat-sensitive recording layer, and between the heat-sensitive recording layer and a protective layer are more likely to be reduced, compared to a support made of paper. Such insufficient binding properties cause peeling, particularly peeling from the base material due to elution of water-soluble components when wet with water; furthermore, peeling can easily occur when an external mechanical force is applied. This results in peeling of the print-recorded portion, causing problems such as defects in barcode reading.
In order to improve the binding properties between a synthetic paper base material and a heat-sensitive recording layer, PTL 1 and PTL 2 propose a heat-sensitive recording material comprising a heat-sensitive recording layer containing a styrene-butadiene copolymer. Although this heat-sensitive recording material achieves excellent binding properties, the coating film peels off when it is immersed in water; thus, further improvement is in demand.

Citation List

Patent Literature

PTL 1: JPH08-175007A
PTL 2: JPH09-226246A

Summary of Invention

Technical Problem

A primary object of the present invention is to provide a heat-sensitive recording material having no risk of transfer of printing to a print surface and having excellent thermal background fogging resistance.

Solution to Problem

The present inventors conducted extensive research to achieve the above object, and found that the above object can be achieved by providing a heat-sensitive recording material comprising a heat-sensitive recording layer on a support, the heat-sensitive recording layer containing a leuco dye, a developer, and binders, the heat-sensitive recording layer containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as the binders and containing a specific non-phenolic developer as the developer. The present invention has thus been completed. That is, the present invention relates to the following heat-sensitive recording material.
Item 1: A heat-sensitive recording material comprising a heat-sensitive recording layer on a support,

the heat-sensitive recording layer containing a leuco dye, a developer, and binders,
the heat-sensitive recording layer containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as the binders, and containing, as the developer, at least one non-phenolic developer selected from the group consisting of an N,N'-diarylurea-based compound represented by the following formula (1): wherein
- R represents a C_1-12 alkyl group, a C_7-12 aralkyl group, or a C_6-12 aryl group, the aralkyl group and aryl group may be substituted with a C_1-12 alkyl group, a C_1-12 alkoxy group, a C_6-12 aryl group, or a halogen atom, and a plurality of Rs may be the same or different, and
- A¹ represents a hydrogen atom or a C_1-4 alkyl group, and a plurality of A¹s may be the same or different;
- a compound represented by the following formula (2): wherein
  - R¹ to R⁵ are the same or different, and each represents 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; and
  - a compound represented by the following formula (3):

Item 2: The heat-sensitive recording material according to Item 1, wherein the N,N'-diarylurea-based compound represented by formula (1) is at least one member selected from the group consisting of N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea.
Item 3: The heat-sensitive recording material according to Item 1 or 2, wherein the N,N'-diarylurea-based compound represented by formula (1) is N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea.
Item 4: The heat-sensitive recording material according to any one of Items 1 to 3, wherein the compound represented by formula (2) is 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate.
Item 5: The heat-sensitive recording material according to any one of Items 1 to 4, wherein the content of the olefin-acrylic acid copolymer salt is 4 to 25 mass%, and the content of the styrene-butadiene latex is 15 to 40 mass%, based on the total solids content of the heat-sensitive recording layer.
Item 6: The heat-sensitive recording material according to any one of Items 1 to 5, wherein the content of the olefin-acrylic acid copolymer salt is 10 to 150 parts by mass per 100 parts by mass of the styrene-butadiene latex.
Item 7: The heat-sensitive recording layer according to any one of Items 1 to 6, wherein the total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is 25 to 50 mass% based on the total solids content of the heat-sensitive recording layer.
Item 8: The heat-sensitive recording material according to any one of Items 1 to 7, wherein the support is synthetic paper or a synthetic resin film.
Item 9: The heat-sensitive recording material according to any one of Items 1 to 8, wherein the support comprises a polyolefin-based resin.
Item 10: The heat-sensitive recording material according to any one of Items 1 to 9, comprising a protective layer on the heat-sensitive recording layer, the protective layer containing acetoacetyl-modified polyvinyl alcohol as a binder.

Advantageous Effects of Invention

The heat-sensitive recording material of the present invention has no risk of transfer of printing to a print surface and has excellent thermal background fogging resistance.

Description of Embodiments

In the present specification, the expression "comprise" or "contain" includes the concepts of "comprising," "consisting essentially of," and "consisting of."
In the present specification, a numerical range indicated by "... to ..." means a range including the numerical values given before and after "to" as the lower limit and the upper limit.
"Latex" as used herein includes one in the form of a gel or dry film formed by drying a dispersion medium.

Support

The support used in the present invention is not particularly limited in type, shape, dimension, or the like, and can be appropriately selected from, for example, high-quality paper (acid paper, neutral paper), medium-quality paper, coated paper, art paper, cast-coated paper, glassine paper, resin laminate paper, polyolefin-based synthetic paper, synthetic fiber paper, nonwoven fabrics, synthetic resin films, and various transparent supports. The thickness of the support is not particularly limited and is typically about 20 to 200 µm.
The support in the present invention is preferably synthetic paper or a synthetic resin film. The synthetic paper or a synthetic resin film used as the support in the present invention is not particularly limited, and the base material may be, for example, a polyolefin film, such as polyethylene or polypropylene; a polystyrene film; a polyester film, such as polyethylene terephthalate or polybutylene terephthalate; or a cellulose derivative film, such as cellulose triacetate. The support may contain a pigment. For example, it is possible to use synthetic paper produced by a method comprising kneading a polyolefin-based resin and a white inorganic pigment while heating; extruding the kneaded product from a die; stretching the extruded product in the longitudinal direction; forming on each side of the stretched product one or two layers of a film comprising a polyolefin-based resin and a white inorganic pigment; and stretching the resulting product in the transverse direction to make it translucent or opaque. This synthetic paper contains a polyolefin-based resin, and thus still has poor adhesion to a heat-sensitive recording layer; however, due to the formation of voids by the pigment, both cushioning properties and heat-insulation properties are achieved, and excellent color developability and excellent image quality are achieved. Although the use of synthetic paper as the support can produce an anchoring effect of the heat-sensitive recording layer, water resistance is reduced due to the presence of water that penetrates into the voids; for this reason, the use of synthetic paper as a support has been limited. However, according to the present invention, synthetic paper can fully exert its effect and can be preferably used.

Heat-sensitive Recording Layer

The heat-sensitive recording layer in the present invention contains an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as binders, and is formed on a support. Thus, excellent water resistance, water-blocking properties, and adhesion between the support and the heat-sensitive recording layer can be obtained. In addition to water-blocking properties, a synergistic effect with a specific non-phenolic developer (described later) is obtained, achieving no risk of transfer of printing to a print surface.
The olefin-acrylic acid copolymer salt used in the present invention can exhibit high water resistance since the olefin portion has high hydrophobicity. Furthermore, the carboxyl group portion of the olefin-acrylic acid copolymer salt has excellent adhesion to a film and can exert high binding properties.
Examples of the olefin component in the olefin-acrylic acid copolymer salt include ethylene, propylene, butylene, isobutylene, and the like. Among these, an ethylene-acrylic acid copolymer resin is preferably used from the viewpoint of water resistance and adhesion. The proportion of the olefin component in the olefin-acrylic acid copolymer salt is preferably about 50 to 90 mol%.
The weight average molecular weight of the olefin-acrylic acid copolymer salt is preferably 20,000 to 200,000, and more preferably 30,000 to 150,000. Setting the molecular weight to 20,000 or more can improve water resistance and adhesion. Setting the molecular weight to 200,000 or less achieves excellent film-forming properties and can improve adhesion. The weight average molecular weight is measured by gel permeation chromatography using polystyrene as a standard sample.
Examples of the salt of the olefin-acrylic acid copolymer salt include salts with alkali metals, such as lithium, potassium, and sodium, ammonium salts, and the like. Among these, ammonium salts are preferred from the viewpoint of improving water dispersibility and water resistance.
The content of the olefin-acrylic acid copolymer salt is preferably 4 to 25 mass%, more preferably 5 to 20 mass%, and even more preferably 5 to 15 mass%, based on the total solids content of the heat-sensitive recording layer. Setting the content to 4 mass% or more can improve water resistance. Setting the content to 25 mass% or less can improve color developability.
The olefin-acrylic acid copolymer salts may be commercially available as, for example, Chemipearl S100, S650, S75N, etc., produced by Mitsui Chemicals, Inc., and Hi-Tec S3121, S8512, etc., produced by Toho Chemical Industry Co., Ltd., and can be easily obtained and used in the form of an emulsion.
The styrene-butadiene latex used in the present invention is preferably a latex having a butadiene monomer content of 20 to 45 mass% based on the solids content of all monomers, and having a gel content of 70 to 85%. Setting the copolymerization rate of butadiene to 20 mass% or more can improve film-forming properties. Setting the content to 45 mass% or less can increase cohesive force, and improve dry-pick strength. Furthermore, setting the gel content to 70% or more can improve adhesive strength. The content of the styrene monomer that constitutes the styrene-butadiene latex is preferably about 40 to 70 mass% from the viewpoint of improving the stability of the latex.
The styrene-butadiene latex used in the present invention contains styrene and butadiene as main constituent monomers, and may further contain, in the monomer composition, at least one member selected from various unsaturated carboxylic acid monomers for the purpose of modification. Specific examples of such unsaturated carboxylic acid monomers include itaconic acid, maleic acid, acrylic acid, methacrylic acid, fumaric acid, dicarboxylic acid anhydrides, dicarboxylic acid monoalkyl esters, and the like. It is desirable that these monomers are contained in an amount of 0.5 to 10 mass% in the monomer composition. The monomer composition of the styrene-butadiene latex can further contain other monomers. Specific examples of such other monomers include aromatic vinyl monomers, such as α-methylstyrene, vinyltoluene, and dimethylstyrene; acrylate monomers, such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate; methacrylate monomers, such as methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, and 2-hydroxypropyl methacrylate; glycidyl group-containing monomers, such as glycidyl acrylate and glycidyl methacrylate; and the like. These monomers can be used in an amount of about 0 to 35 mass% as long as they do not impair the effects of the present invention.
The method for producing the styrene-butadiene latex is not particularly limited. For example, known emulsion polymerization methods, such as continuous emulsion polymerization and batch emulsion polymerization, can be used. At this time, various known additives that are used for typical emulsion polymerization can be appropriately used, such as emulsifiers, chain transfer agents, polymerization initiators, electrolytes, and chelating agents. Furthermore, the polymerization temperature can be selected as either high or low.
The content of the styrene-butadiene latex is preferably 15 to 40 mass%, and more preferably 15 to 30 mass%, based on the total solids content of the heat-sensitive recording layer. Setting the content to 15 mass% or more can improve adhesion. Setting the content to 40 mass% or less can improve color developability.
The total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is preferably 25 to 50 mass%, more preferably 25 to 40 mass%, and even more preferably 25 to 30 mass%, based on the total solids content of the heat-sensitive recording layer. Setting the total content to 25 mass% or more can improve water resistance, water-blocking properties, and adhesion. Setting the total content to 50 mass% or less can improve color developability.
In the heat-sensitive recording layer, the content of the olefin-acrylic acid copolymer salt relative to the styrene-butadiene latex is preferably 10 to 150 parts by mass, more preferably 20 to 100 parts by mass, and even more preferably 30 to 90 parts by mass, per 100 parts by mass of the styrene-butadiene latex. Setting the content of the olefin-acrylic acid copolymer salt to 10 parts by mass or more can improve water resistance and water-blocking properties. Setting the content to 150 parts by mass or less can improve color developability and can produce a synergistic effect in terms of adhesion.
The binders used for the heat-sensitive recording layer in the present invention are the olefin-acrylic acid copolymer salt and the styrene-butadiene latex. However, various other known binders may also be used as necessary, as long as the effects of the present invention are not impaired. Examples of the other binders include water-soluble binders, for example, starches, such as oxidized starch, acid-modified starch, phosphorylated starch, enzyme-modified starch, cation-modified starch, esterified starch, etherified starch, and vinyl acetate-modified grafted starch; cellulose derivatives, such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, methoxycellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose; polyvinyl alcohols, such as fully (or partially) saponified polyvinyl alcohol, silicon-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and acetoacetyl-modified polyvinyl alcohol; and sodium polyacrylate, polyacrylamide, 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, sodium alginate, gelatin, and casein; and water-dispersible binders of latexes, such as polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid ester, polybutyl methacrylate, vinyl chloride-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, and styrene-butadiene-acrylic copolymers.
In the present invention, the leuco dye contained in the heat-sensitive recording layer may be selected from various known leuco dyes. For example, the leuco dye for use may be at least one black-color-forming leuco dye selected from 3-diethylamino-6-methyl-7-anilinofluoran, 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 3-di(n-pentyl)amino-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-(N-isoamyl-N-ethylamino)-7-(o-chloroanilino) fluoran, 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-ethylamino]-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-methylamino]-6-methyl-7-anilinofluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-di(n-butyl)amino-7-(2-chloroanilino) fluoran, and the like. It is also possible to use leuco dyes that develop colors other than black, such as red, purplish red, orange, blue, and green, as necessary.
The leuco dye used in the present invention is not limited to these and may be a combination of two or more. The content of the leuco dye is preferably about 3 to 30 mass% based on the total solids content of the heat-sensitive recording layer.
In the present invention, when the leuco dye is used in the form of solid fine particles, the leuco dye may be pulverized with a sand grinder, an attritor mill, a ball mill, a co-ball mill, or other various wet pulverizers using water as a dispersion medium. The resulting product may be dispersed with the dispersion medium, together with a water-soluble synthetic polymer compound, such as polyacrylamide, polyvinylpyrrolidone, polyvinyl alcohol, sulfone-modified polyvinyl alcohol, another modified polyvinyl alcohol, methylcellulose, carboxymethyl cellulose, hydroxypropyl methylcellulose, styrene-maleic anhydride copolymer salt, or a derivative thereof, and optionally with a surfactant, a defoaming agent, etc., to obtain a dispersion. The thus-obtained dispersion can be used for preparing a coating composition for a heat-sensitive recording layer.
Alternatively, the leuco dye for use may be formed into solid fine particles by dissolving a leuco dye in a solvent, emulsifying and dispersing the obtained solution in water using the water-soluble polymer mentioned above as a stabilizer, followed by evaporation of the solvent from the resulting emulsion. In either case, the average particle diameter of the dispersed leuco dye particles used in the form of solid fine particles is preferably 0.2 to 3.0 µm, and more preferably 0.3 to 1.0 µm, to obtain desired color sensitivity.
In the present invention, other than in the form of solid fine particles described above, the leuco dye may be used as composite particles comprising an organic polymer and a leuco dye. Such composite particles may be prepared by a known method. For example, the following describes preparation methods for the composite particles in which the organic polymer is at least one member selected from polyurea and polyurea-polyurethane. Specifically, the composite particles may be prepared by dissolving and mixing, in a water-insoluble organic solvent having a boiling point of 100°C or less, a leuco dye and a polymer-forming raw material that forms at least one member selected from polyurea and polyurea-polyurethane by polymerization; emulsifying and dispersing the obtained organic solvent solution in a hydrophilic protective colloid solution, such as polyvinyl alcohol, so that the average particle diameter is about 0.5 to 3 µm; further mixing, if necessary, a reactive substance, such as polyamine, therewith; heating the obtained emulsified dispersion to allow the organic solvent to volatilize away; and then polymerizing the polymer-forming raw material. Alternatively, the composite particles may be prepared by dissolving a leuco dye in a polymer-forming raw material; emulsifying and dispersing the obtained solution in the same manner as above so that the average particle diameter is about 0.5 to 3 µm; and then polymerizing the polymer-forming raw material.
The heat-sensitive recording layer in the present invention contains, as a developer, at least one non-phenolic developer selected from the group consisting of an N,N'-diarylurea-based compound represented by formula (1) above, a compound represented by formula (2) above, and the compound represented by formula (3) above. Thus, there is no risk of transfer of printing to a print surface. As used here, the phenomenon of transfer of printing to a print surface refers to the phenomenon described as follows: specifically, when two print surfaces that are pattern-printed with, for example, a solid-black checkered pattern or a barcode are overlaid while wet with water, with printed sides facing each other, and a blocking test is performed, followed by drying and peeling off from each other, the printing on each surface is transferred to the other print surface, or more simply, to the unprinted blank portion on the other print surface, as a faint, inverted mirror image. The observation of the print surfaces after peeling reveals no surface swelling, dissolution, or other changes, and also no sign of the occurrence of cohesive failure or delamination of the heat-sensitive recording layer or the optional protective layer; thus, it can be presumed that the developer has undergone some kind of action via moisture. When the support is synthetic paper or a synthetic resin film, moisture does not penetrate the support side and is likely to remain on the side opposite to the support, which is likely to produce a noticeable effect that can be discerned visually. Heat-sensitive recording materials that cause this phenomenon are problematic because they impair the quality of the printed materials and make it difficult to read the printed information.
In formula (1), the C_1-12 alkyl group for R may be linear, branched, or alicyclic, and preferably a C_1-6 alkyl group, and more preferably a C_1-3 alkyl group. Examples of C_1-12 alkyl groups include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a t-butyl group, a cyclopentyl group, a hexyl group, a cyclohexyl group, a 2-ethylhexyl group, a lauryl group, and the like. The alkyl group as used here also includes the alkyl moiety of a C_1-12 alkoxy group.
The aralkyl group refers to an aryl alkyl group, and examples of C_7-12 aralkyl groups include a benzyl group, a 1-phenylethyl group, a 2-phenylethyl group, and a 3-phenylpropyl group.
The aryl group means a monocyclic or polycyclic group formed of one or more 5- or 6-membered aromatic hydrocarbon rings. Examples of C_6-12 aryl groups include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, and the like. The aryl group as used herein also includes the aryl moiety of aralkyl groups.
Examples of halogen atoms include fluorine, chlorine, bromine, and iodine.
In formula (1), the position of substitution of each R-SO₃- may be the same or different. The substitution position is preferably the 3-position, the 4-position, or the 5-position, and more preferably the 3-position. When the C_7-12 aralkyl group and the C_6-12 aryl group represented by R are substituted, the number of substituents is not particularly limited, and is for example, 1 to 4.
The C_1-4 alkyl group represented by A¹ may be linear or branched. Examples include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a sec-butyl group, t-butyl group, and the like.
The substitution position of each A¹ may be the same or different. The substitution position is preferably the 3-position, the 4-position, or the 5-position.
The N,N'-diarylurea-based compound represented by formula (1) is not particularly limited, and is preferably at least one member selected from the group consisting of N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea. Of these, N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea is preferred.
The content of the N,N'-diarylurea-based compound is not particularly limited and may be adjusted according to the leuco dye for use. Typically, the content is preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, still more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more, per part by mass of the leuco dye. The content of the N,N'-diarylurea-based compound is also preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less, per part by mass of the leuco dye. Setting the content to 0.5 parts by mass or more can improve recording performance. Setting the content to 10 parts by mass or less can effectively decrease background fogging in a high-temperature environment.
In formula (2) above, the halogen atom represented by R¹ to R⁵ may be a fluorine atom, a chlorine atom, or a bromine atom, with a fluorine atom and a chlorine atom being preferred.
The alkyl group may be linear, branched, or cyclic, and is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. Typically, the alkyl group is a C_1-12 alkyl group, preferably a C_1-8 alkyl group, more preferably a C_1-6 alkyl group, and even more preferably a C_1-4 alkyl group.
The alkoxy group may be linear, branched, or cyclic, and is preferably a linear or branched alkoxy group, and more preferably a linear alkoxy group. Typically, the alkoxy group is a C_1-12 alkoxy group, preferably a C_2-8 alkoxy group, more preferably a C_2-6 alkoxy group, and even more preferably a C_2-4 alkoxy group.
The alkylcarbonyloxy group may be linear, branched, or cyclic, and is preferably a linear or branched alkylcarbonyloxy group, and more preferably a linear alkylcarbonyloxy group. The alkylcarbonyloxy group is also preferably a C_1-10 alkylcarbonyloxy group.
The alkylcarbonylamino group may be linear, branched, or cyclic, and is preferably a linear or branched alkylcarbonylamino group, and more preferably a linear alkylcarbonylamino group. The alkylcarbonylamino group is also preferably a C_1-10 alkylcarbonylamino group.
The alkylsulfonylamino group may be linear, branched, or cyclic, and is preferably a linear or branched alkylsulfonylamino group, and more preferably a linear alkylsulfonylamino group. The alkylsulfonylamino group is also preferably a C_1-10 alkylsulfonylamino group.
The aryl group means a monocyclic or polycyclic group formed of one or more 5- or 6-membered aromatic hydrocarbon rings. Examples of aryl groups include a phenyl group, a naphthyl group, and a biphenyl group.
The aryloxy group is preferably a C_6-12 aryloxy group. The arylcarbonyloxy group is preferably a C_6-12 arylcarbonyloxy group. The arylcarbonylamino group is preferably a C_6-12 arylcarbonylamino group. The arylsulfonylamino group is preferably a C_6-12 arylsulfonylamino group.
The monoalkylamino group may be linear, branched, or cyclic, and is preferably a linear or branched monoalkylamino group, and more preferably a linear monoalkylamino group. A monoalkylamino group whose alkyl group has 1 to 10 carbon atoms is also preferable.
The dialkylamino group may be linear, branched, or cyclic, and is preferably a linear or branched dialkylamino group, and more preferably a linear dialkylamino group. A dialkylamino group whose alkyl group has 1 to 10 carbon atoms is also preferable.
The arylamino group may be a monoarylamino group or diarylamino group, and is preferably a C_6-12 monoarylamino group.
Specific examples of compounds represented by formula (2) include those wherein R¹ to R⁵ are each an alkyl group or a hydrogen atom, preferably R¹ to R⁵ are each a C_1-8 linear alkyl group or a hydrogen atom, more preferably R¹ to R⁵ are each a C_1-4 linear alkyl group or a hydrogen atom, and even more preferably R¹ to R⁵ are each a methyl group or a hydrogen atom.
Other specific examples of compounds represented by formula (2) include those wherein R¹, R², R⁴, and R⁵ are each a hydrogen atom and R³ is 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 alkylcarbonylamino group, an arylcarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a monoalkylamino group, a dialkylamino group, or an arylamino group (preferably a hydrogen atom or an alkyl group, more preferably a hydrogen atom or a C_1-8 alkyl group, even more preferably a hydrogen atom or a C_1-4 alkyl group, and particularly preferably a methyl group).
The position of the substituent bound to one benzene ring in the diphenylurea structure in formula (2) may be the ortho-position, the meta-position, or the para-position, preferably the ortho-position or the meta-position, and more preferably the meta-position with respect to the aminocarbonyl group on the benzene ring.
The compound represented by formula (2) is not particularly limited, and is preferably at least one member selected from the group consisting of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 2-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, and 4-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate. Of these, 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate is preferable.
The content of the compound represented by formula (2) is not particularly limited and can be adjusted according to the leuco dye for use. Typically, the content of the compound represented by formula (2) is preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, still even more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more, per part by mass of the leuco dye. The content of the compound represented by formula (2) is also preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less, per part by mass of the leuco dye. Setting the content of the compound represented by formula (2) to 0.5 parts by mass or more can enhance recording performance. Setting the content of the compound represented by formula (2) to 10 parts by mass or less can effectively decrease background fogging in a high-temperature environment.
The content of the compound represented by formula (3) is not particularly limited and can be adjusted according to the leuco dye for use. Typically, the content is preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, even more preferably 1 part by mass or more, still more preferably 1.2 parts by mass or more, and particularly preferably 1.5 parts by mass or more, per part by mass of the leuco dye. The content of the compound represented by formula (3) is also preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 4 parts by mass or less, and particularly preferably 3.5 parts by mass or less, per part by mass of the leuco dye. Setting the content to 0.5 parts by mass or more can improve recording performance. Setting the content to 10 parts by mass or less can effectively decrease background fogging in a high-temperature environment.
As long as the effects of the present invention are not impaired, developers other than the N,N'-diarylurea-based compound represented by formula (1) above, the compound represented by formula (2) above, and the compound represented by formula (3) above may also be contained.
In the present invention, the heat-sensitive recording layer may further contain a sensitizer. Use of the sensitizer enhances the recording sensitivity. Examples of usable sensitizers include stearic acid amide, methoxycarbonyl-N-stearic acid benzamide, N-benzoyl stearic acid amide, N-eicosanoic acid amide, ethylenebisstearic acid amide, behenic acid amide, methylenebisstearic acid amide, N-methylol stearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, diphenylsulfone, 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-tolyl biphenyl 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-methylthiophenylbenzylether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, 1,2-diphenoxymethylbenzene, di(β-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, 1-isopropylphenyl-2-phenylethane, di-o-chlorobenzyl adipate, 1,2-bis(3,4-dimethylphenyl)ethane, 1,3-bis(2-naphthoxy)propane, diphenyl, benzophenone, and the like. Of these, 1,2-di(3-methylphenoxy)ethane is preferred. These sensitizers can be used in combination as long as the combined use does not impair the effects of the present invention. The sensitizer content may be an effective amount for sensitization, and is typically preferably 5 to 30 mass%, more preferably 10 to 30 mass%, and even more preferably 15 to 25 mass%, based on the total solids content of the heat-sensitive recording layer.
In the present invention, the heat-sensitive recording layer may further contain a stabilizer mainly in order to further enhance the preservation of the developed color image. As such a stabilizer, it is possible to use, for example, at least one member selected from the group consisting of phenol compounds, such as 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bisphenol, and 4,4'-[1,3-phenylenebis(1-methylethylidene)]bisphenol; epoxy compounds, such as 4-benzyloxyphenyl-4'-(2-methyl-2,3-epoxypropyloxy)phenylsulfone, 4-(2-methyl-1,2-epoxyethyl)diphenylsulfone, and 4-(2-ethyl-1,2-epoxyethyl)diphenylsulfone; and isocyanuric acid compounds, such as 1,3,5-tris(2,6-dimethylbenzyl-3-hydroxy-4-tert-butyl)isocyanuric acid. Usable stabilizers are, of course, not limited to these compounds, and two or more of such compounds can be used in combination as necessary.
When the stabilizer is used, its amount may be an effective amount for improving image preservation. The stabilizer is typically preferably used in an amount of about 1 to 25 mass%, and more preferably about 5 to 20 mass%, based on the total solids content of the heat-sensitive recording layer.
As other components that constitute the heat-sensitive recording layer, if necessary, inorganic or organic pigments, crosslinking agents, waxes, metal soaps, water resistance improving agents, dispersants, colored dyes, fluorescent dyes, and other auxiliary agents can be used.
Various inorganic pigments can be used as the inorganic pigment. Specific examples include inorganic pigments, such as calcium carbonate, such as light calcium carbonate, aluminum hydroxide, clay, such as kaolin, and talc. Of these, the inorganic pigment is preferably at least one member selected from the group consisting of calcium carbonate, aluminum hydroxide, and clay. The content of the inorganic pigment can be selected from a wide range, and is preferably 2 to 40 mass%, more preferably 3 to 30 mass%, and even more preferably 4 to 20 mass%, based on the total solids content of the heat-sensitive recording layer.
When the heat-sensitive recording layer contains a crosslinking agent, the water resistance of the heat-sensitive recording layer can be improved. Examples of crosslinking agents include aldehyde compounds, such as glyoxal; polyamine compounds, such as polyethyleneimine; epoxy compounds, polyamide resins, melamine resins, glyoxylic acid salts, dimethylolurea compounds, aziridine compounds, block isocyanate compounds; inorganic compounds, such as ammonium persulfate, ferric chloride, magnesium chloride, soda tetraborate, and potassium tetraborate; boric acid, boric acid triesters, boron polymers, hydrazide compounds such as adipic acid dihydrazide, glyoxylic acid salts, and the like. These may be used singly, or in a combination of two or more. The amount of the crosslinking agent used is preferably about 0.2 to 5 mass%, and more preferably about 0.3 to 3 mass%, based on the total solids content of the heat-sensitive recording layer.
The heat-sensitive recording layer is formed on the support, for example, by dispersing a leuco dye and a developer, and if necessary, with or separately from a sensitizer and a stabilizer, using water as a dispersion medium and using at least one of various stirrers or wet pulverizers, such as a ball mill, a co-ball mill, an attritor, or a vertical or horizontal sand, mill together with a water-soluble synthetic polymer compound, such as polyacrylamide, polyvinyl pyrrolidone, polyvinyl alcohol, methylcellulose, or a styrene-maleic anhydride copolymer salt, and other additives such as a surfactant to form a dispersion; then mixing the dispersion obtained by reducing the average particle diameter so that the average particle diameter is 2 µm or less with binders, and optionally an auxiliary agent, and the like to prepare a coating composition for a heat-sensitive recording layer; applying the coating composition for a heat-sensitive recording layer to the support; and then drying. The coated amount of the heat-sensitive recording layer is not particularly limited and is preferably about 1 to 12 g/m², more preferably 2 to 10 g/m², even more preferably 2.5 to 8 g/m², and particularly preferably 3 to 5.5 g/m², in terms of dry mass. Note that the heat-sensitive recording layer may be formed as two or more separate layers if necessary, and the composition and coated amount of each layer may be the same or different.

Protective Layer

The heat-sensitive recording material can comprise a protective layer formed on the heat-sensitive recording layer as necessary. The protective layer preferably contains a pigment and a binder. The protective layer preferably further contains a lubricant, such as polyolefin wax or zinc stearate, for the purpose of preventing the layer from sticking to the thermal head. The protective layer can also contain a UV absorber. When a glossy protective layer is formed, the obtained product can have increased added value.
The pigment contained in the protective layer is not particularly limited. Examples include inorganic pigments, such as amorphous silica, kaolin, clay, light calcium carbonate, heavy calcium carbonate, calcined kaolin, titanium oxide, magnesium carbonate, aluminum hydroxide, colloidal silica, and synthetic layered mica; plastic pigments, such as urea-formalin resin fillers; and the like.
The binder contained in the protective layer is not particularly limited, and an aqueous binder selected from water-soluble binders and water-dispersible binders can be used. The binder can be appropriately selected from those that can be used for the heat-sensitive recording layer. Of these, various modified polyvinyl alcohols, such as acetoacetyl-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, and diacetone-modified polyvinyl alcohol, can be more preferably used. From the viewpoint of improving water-blocking properties, acetoacetyl-modified polyvinyl alcohol is more preferably used.
The protective layer is formed on the heat-sensitive recording layer, for example, by mixing a pigment and a binder optionally with an auxiliary agent and the like using water as a dispersion medium to prepare a coating composition for a protective layer, applying the coating composition to the heat-sensitive recording layer, and then drying. The coated amount of the coating composition for a protective layer is not particularly limited and is preferably about 0.3 to 15 g/m², more preferably about 0.3 to 10 g/m², even more preferably about 0.5 to 8 g/m², particularly preferably about 1 to 8 g/m², and further particularly preferably about 1 to 5 g/m², in terms of dry mass. The protective layer may be formed as two or more separate layers if necessary, and the composition and coated amount of each layer may be the same or different.

Other Layers

In the present invention, the heat-sensitive recording material preferably comprises an adhesive layer on at least one surface of the support. This can increase the added value of the heat-sensitive recording material. For example, adhesive paper, remoistening adhesive paper, or delayed tack paper can be formed as the adhesive layer by subjecting one surface of the support to coating with, for example, an adhesive, a remoistening adhesive, or a delayed tack adhesive. Recording paper capable of two-sided recording can also be formed by imparting to the surface of the support opposite to the heat-sensitive recording layer a function as heat transfer paper, ink jet recording paper, carbon-free paper, electrostatic recording paper, xerography paper, or the like. Of course, the heat-sensitive recording material can be formed into a two-side heat-sensitive recording material. A back layer can also be provided to inhibit oil and plasticizer permeation from the back side of the heat-sensitive recording material, or for curl control and antistatic purposes. The heat-sensitive recording material can also be formed into linerless labels that do not require release paper by forming a silicone-containing release layer on the protective layer and applying an adhesive to the one side.

Heat-sensitive Recording Material

The heat-sensitive recording material can be produced by forming each layer described above on the support. Any known coating method, such as 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, and an extrusion method, can be used as the method for forming each layer described above on the support. The individual coating compositions may be applied in such a manner that a first coating composition is applied and dried and then a second coating composition is applied and dried to form one layer after another, or the same coating composition may be applied separately to form two or more layers. Further, simultaneous multilayer coating may also be performed, in which individual coating compositions are applied all at once to form two or more layers simultaneously. In any stage after each layer is formed or after all layers are formed, the layer may be subjected to a smoothing treatment by a known method, such as supercalendering or soft calendering.

Examples

The present invention is described in more detail with reference to Examples. However, the present invention is not limited to these Examples. In the Examples, "parts" and "%" represent "parts by mass" and "mass%" unless otherwise specified. The average particle diameters were measured with a SALD2200 laser diffraction particle diameter distribution analyzer (produced by Shimadzu Corporation). "Average particle diameter" as used herein refers to a median diameter (D50).

Example 1

(1) Preparation of Leuco Dye Dispersion (Dispersion A)

40 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 0.5 µm, thus obtaining a leuco dye dispersion (dispersion A).

(2) Preparation of Developer Dispersion (Dispersion B)

40 parts of N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion B).

(3) Preparation of Sensitizer Dispersion (Dispersion C)

40 parts of 1,2-di(3-methylphenoxy)ethane (trade name: KS-232, produced by Sankosha Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a sensitizer dispersion (dispersion C).

(4) Preparation of Coating Composition for Heat-sensitive Recording Layer

34.1 parts of dispersion A, 68.2 parts of dispersion B, 45.5 parts of dispersion C, 26 parts of an olefin-acrylic acid copolymer salt (trade name: Hi-Tec S-3121, an aqueous dispersion of an ethylene-acrylic acid copolymer ammonia salt, produced by Toho Chemical Industry Co., Ltd., solids content: 26.5%), 41.7 parts of a styrene-butadiene latex (trade name: Smartex PA-9281, produced by Nippon A&L Co., Ltd., solids content: 48%), 5 parts of calcium carbonate (trade name: Brilliant 15, produced by Shiraishi Kogyo Co., Ltd.), 0.5 parts of adipic acid dihydrazide (produced by Otsuka Chemical Co., Ltd.), and 200 parts of water were mixed and stirred, thus obtaining a coating composition for a heat-sensitive recording layer.

(5) Preparation of Coating Composition for Protective Layer

A composition comprising 208 parts of a 12% aqueous solution of acetoacetyl-modified polyvinyl alcohol (trade name: Gohsenx Z-200, produced by The Nippon Synthetic Chemical Industry Co., Ltd.), 55 parts of kaolin (trade name: Hydragloss 90, produced by KaMin LLC), 2.5 parts of polyethylene wax (trade name: Chemipearl W-400, produced by Mitsui Chemicals Inc.; solids content: 40%), 20.8 parts of zinc stearate (trade name: Hidorin Z-8-36, produced by Chukyo Yushi Co., Ltd.; solids content: 36%), and 150 parts of water was mixed and stirred, thus obtaining a coating composition for a protective layer.

(6) Preparation of Heat-sensitive Recording Material

Synthetic paper (trade name: YUPO FPG95, paper thickness: 95 µm, produced by Yupo Corporation), which is produced by kneading a polyolefin-based resin and calcium carbonate as a white inorganic pigment with heating, extruding the kneaded product through a die, stretching the extruded product in the longitudinal direction, laminating a film made of a polyolefin-based resin and calcium carbonate as a white inorganic pigment on both sides of the stretched product, and then stretching the resulting product in the transverse direction to make it opaque, was used. The coating composition for a heat-sensitive recording layer was applied to one side of the synthetic paper and dried so that the dry mass was 4.0 g/m² to form a heat-sensitive recording layer, and the coating composition for a protective layer was applied thereto and dried so that the dry mass was 2.5 g/m² to form a protective layer. Thereafter, the surface was smoothed with a super calendar to obtain a heat-sensitive recording material.

Example 2

(7) Preparation of Developer Dispersion (Dispersion D)

40 parts of 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate, 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion D).
A heat-sensitive recording material was obtained in the same manner as in Example 1, except that dispersion D was used in place of dispersion B, in the preparation of the coating composition for a heat-sensitive recording layer in Example 1.

Example 3

(8) Preparation of Developer Dispersion (Dispersion E)

40 parts of the compound represented by formula (3), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion E).
A heat-sensitive recording material was obtained in the same manner as in Example 1, except that dispersion E was used in place of dispersion B, in the preparation of the coating composition for a heat-sensitive recording layer in Example 1.

Comparative Example 1

(9) Preparation of Developer Dispersion (Dispersion F)

40 parts of N-p-tolylsulfonyl-N'-3-(p-tolylsulfonyloxy)phenylurea (trade name: PF-201, produced by Solenis), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion F).

(10) Preparation of developer dispersion (Dispersion G)

40 parts of 4,4'-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone (trade name: UU, produced by Chemipro Kasei Kaisha, Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion G).
A heat-sensitive recording material was obtained in the same manner as in Example 1, except that 56.8 parts of dispersion F and 11.4 parts of dispersion G were used in place of 68.2 parts of dispersion B, in the preparation of the coating composition for a heat-sensitive recording layer in Example 1.

Comparative Example 2

(11) Preparation of Developer Dispersion (Dispersion H)

40 parts of 4-allyloxy-4'-hydroxydiphenylsulfone (trade name: BPS-MAE, produced by Nicca Chemical Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion H).

(12) Preparation of Developer Dispersion (Dispersion I)

40 parts of a crosslinked diphenylsulfone compound represented by the following formula (4) (trade name: D-90, produced by Nippon Soda Co., Ltd.), 40 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization: 500, degree of saponification: 88%), and 20 parts of water were mixed. The resulting mixture was pulverized with a sand mill (produced by Aimex Co., Ltd., a sand grinder) to an average particle diameter of 1.0 µm, thus obtaining a developer dispersion (dispersion I). wherein n represents an integer of 1 to 6.
A heat-sensitive recording material was obtained in the same manner as in Example 1, except that 56.8 parts of dispersion H and 11.4 parts of dispersion I were used in place of 68.2 parts of dispersion B, in the preparation of the coating composition for a heat-sensitive recording layer in Example 1.
The heat-sensitive recording materials thus obtained were evaluated as follows. Table 1 shows the results.

Recording Density

An image was recorded on each heat-sensitive recording material at an applied energy of 0.25 mJ/dot, at which the highest color density is obtained, using a thermal recording tester (trade name: TH-PMD, produced by Ohkura Electric Co., Ltd.). The reflection density of the obtained recorded portion was measured with a spectrodensitometer (X-Rite 504, produced by X-Rite). The evaluation criteria for recording density were the following.

A density of 1.30 or more: printing was clear and excellent.
A density of 1.10 or more and less than 1.30: barcode readability was good, no problem in practical use.
A density of less than 1.10: barcode readability was slightly poor, problematic in practical use.

100°C Heat Resistance

An image was recorded on each heat-sensitive recording material at an applied energy of 0.25 mJ/dot (maximum color density) using a thermal recording tester (trade name: TH-PMD, produced by Ohkura Electric Co., Ltd.). Each of the obtained pieces of paper was cut to a length of 9 cm and a width of 4 cm. The resulting piece of paper was subjected to treatment in which the piece of paper was allowed to stand in a dryer at 100°C for 1 hour. The reflection density of the background portion (unprinted blank portion) before and after the treatment, as well as the reflection density of the recorded portion after the treatment, were measured with a spectrodensitometer (X-Rite 504, produced by X-Rite). The evaluation criteria for the background portion were the following. The evaluation criteria for the recorded portion are based on the evaluation criteria for recording density mentioned above.

A background density of 0.10 or less: excellent heat resistance.
A background density of 0.20 or less and more than 0.10: no problem in practical use.
A background density exceeding 0.20: poor heat resistance with significant background fogging.

Transfer Test

An image was recorded on each heat-sensitive recording material at an applied energy of 0.25 mJ/dot (maximum color density) using a thermal recording tester (trade name: TH-PMD, produced by Ohkura Electric Co., Ltd.). Each of the obtained pieces of paper was cut to a length of 9 cm and a width of 4 cm to prepare two identical pieces of paper. Water was sprayed on the print surface side of each of the pieces of paper with a spray bottle, and the two pieces of paper were then overlaid, with the print surface sides facing each other. A weight of 1 kg was placed on the overlaid test pieces, and the test pieces and weight were allowed to stand for 3 days in an environment of 23°C and 65% RH. After being allowed to stand for 3 days, the test pieces and the weight were allowed to stand in a dryer at 40°C for 1 day to evaporate the moisture. After moisture was evaporated, the two test pieces were peeled off from each other and visually observed in terms of whether the printing of one piece was transferred to the print surface of the other. The evaluation criteria for transfer of printing were the following.

A: No transfer of printing at all when visually observed.
B: Transfer of printing visually observed.

Table 1

	Recording Density	Heat resistance at 100°C for 1 hour			Transfer test
		Before treatment	After treatment
		Background portion	Background portion	Printed portion
Example 1	1.33	0.03	0.04	1.33	A
Example 2	1.37	0.03	0.05	1.35	A
Example 3	1.16	0.03	0.03	1.11	A
Comparative Example 1	1.31	0.03	0.34	1.26	B
Comparative Example 2	1.35	0.07	0.23	1.35	B

Industrial Applicability

The heat-sensitive recording material of the present invention has no risk of transfer of printing to a print surface and has excellent thermal background fogging resistance, and therefore fully meets the demand for improved performance in applications, such as handheld terminals and delivery slips that require water-blocking properties and are used in harsh environments.

Claims

A heat-sensitive recording material comprising a heat-sensitive recording layer on a support,
the heat-sensitive recording layer containing a leuco dye, a developer, and binders,

the heat-sensitive recording layer containing an olefin-acrylic acid copolymer salt and a styrene-butadiene latex as the binders, and containing, as the developer, at least one non-phenolic developer selected from the group consisting of an N,N'-diarylurea-based compound represented by the following formula (1): wherein
R represents a C_1-12 alkyl group, a C_7-12 aralkyl group, or a C_6-12 aryl group, the aralkyl group and aryl group may be substituted with a C_1-12 alkyl group, a C_1-12 alkoxy group, a C_6-12 aryl group, or a halogen atom, and a plurality of Rs may be the same or different, and

A¹ represents a hydrogen atom or a C_1-4 alkyl group, and a plurality of A¹s may be the same or different;

a compound represented by the following formula (2): wherein
R¹ to R⁵ are the same or different, and each represents 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; and

a compound represented by the following formula (3):
The heat-sensitive recording material according to claim 1, wherein the N,N'-diarylurea-based compound represented by formula (1) is at least one member selected from the group consisting of N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(o-toluenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(mesitylenesulfonyloxy)phenyl]urea, N,N'-di-[3-(4-ethylbenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(2-naphthalenesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-methoxybenzenesulfonyloxy)phenyl]urea, N,N'-di-[3-(benzylsulfonyloxy)phenyl]urea, N,N'-di-[3-(ethanesulfonyloxy)phenyl]urea, N,N'-di-[3-(p-toluenesulfonyloxy)-4-methyl-phenyl]urea, N,N'-di-[4-(p-toluenesulfonyloxy)phenyl]urea, N,N'-di-[4-(benzenesulfonyloxy)phenyl]urea, N,N'-di-[4-(ethanesulfonyloxy)phenyl]urea, and N,N'-di-[2-(p-toluenesulfonyloxy)]phenylurea.
The heat-sensitive recording material according to claim 1, wherein the N,N'-diarylurea-based compound represented by formula (1) is N,N'-di-[3-(p-toluenesulfonyloxy)phenyl]urea.
The heat-sensitive recording material according to claim 1, wherein the compound represented by formula (2) is 3-[(phenylcarbamoyl)amino]phenyl-4-methylbenzenesulfonate.
The heat-sensitive recording material according to claim 1, wherein the content of the olefin-acrylic acid copolymer salt is 4 to 25 mass%, and the content of the styrene-butadiene latex is 15 to 40 mass%, based on the total solids content of the heat-sensitive recording layer.
The heat-sensitive recording material according to claim 1, wherein the content of the olefin-acrylic acid copolymer salt is 10 to 150 parts by mass per 100 parts by mass of the styrene-butadiene latex.
The heat-sensitive recording material according to claim 1, wherein the total content of the olefin-acrylic acid copolymer salt and the styrene-butadiene latex is 25 to 50 mass% based on the total solids content of the heat-sensitive recording layer.
The heat-sensitive recording material according to claim 1, wherein the support is synthetic paper or a synthetic resin film.
The heat-sensitive recording material according to claim 1, wherein the support comprises a polyolefin-based resin.
The heat-sensitive recording material according to claim 1, comprising a protective layer on the heat-sensitive recording layer, the protective layer containing acetoacetyl-modified polyvinyl alcohol as a binder.