Phenol-free, heat-sensitive recording material

Abstract

Disclosed is a phenol-free, heat-sensitive recording material that comprises: (a) a substrate; and in order from a side close to the substrate, (b) an undercoat layer; and (c) a thermal-sensitive layer; wherein the thermal-sensitive layer comprises: a first developer comprising a sulfonamide compound represented by formula (1) below: wherein R1 and R2 may be the same or different, and each independently represents a hydrogen atom, an alkyl group having from 1 to 4 carbons, an alkoxy group having from 1 to 4 carbons, or a halogen atom, one or more second developers comprising at least one phenyl ureide compound represented by formula (2) below: wherein R3 and R4 may be the same or different, and each independently represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an aryl 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 toluenesulfonyl group, a toluenesulfonyloxyphenyl group, a monoalkylamino group, a dialkylamino group, or an arylamino group, and one or more stabilizers selected from the group consisting of compounds represented by formula (3) and (4) below, and a derivative thereof.

Description

PHENOL-FREE, HEAT-SENSITIVE RECORDING MATERIAL

[0001] The present invention relates generally to a phenol-free, heat-sensitive recording material.

[0002] Heat-sensitive recording materials for direct thermal printing are known. Such materials have a heat-sensitive, color-forming layer (thermal-reaction layer) applied to a carrier substrate. A color former and a color developer, which are typically present in the heat-sensitive, color-forming layer, react with one another under the influence of heat, thus leading to color development. Heat-sensitive recording paper is widely used as output sheets for a variety of printers, including fax machines, devices for industrial measuring instruments, medical devices, hand-held devices, POS systems, and ticketing systems.

[0003] Widely used color developers have included phenol-based bisphenol (BPA) and bisphenol S (BPS), which result in heat-sensitive recording materials having performance characteristics acceptable for many applications. Safety risks inherent in these phenol-based developers have, however, been identified.

[0004] Attempts have therefore been made to improve the safety of these heat-sensitive recording materials by replacing the phenolic developers with non-phenolic developers, while maintaining their performance properties, including their resistance to environmental influences (e.g., heat, moisture, and chemicals).

[0005] One such attempt was to use PERGAFAST 201 (N-(4-methylphenylsulphonyl)-N’-(3-(4-methylphenylsulfonyloxy) phenyl) urea), which has provided satisfactory results. This material, however, has now been identified as having an adverse environmental impact (i.e., PERGAFAST 201 has been classified under the Globally Harmonized System (GHS) with H code H411 (“Toxic to aquatic life with long-lasting effects”)).

[0006] Other attempts are evident from PLT 1 to PLT 6.

[0007] While the above references evidence attempts to achieve good performance properties, there is still a need to improve the level of environmental resistance and storage stability in these materials.

[0008] WO 2021 / 187097 A1US 9,034,790 B2US 9,034,790 B2JP 2021 / 100798 A1WO 2023 / 100902 A1JP 2019 / 130879 A1

[0009] A primary object of the present invention is to provide a heat-sensitive recording material with excellent oil resistance and plasticizer resistance.

[0010] The present invention therefore provides a phenol-free, heat-sensitive recording material that comprises: (a) a substrate; and in order from a side close to the substrate, (b) an undercoat layer; (c) a thermal-sensitive layer; and (d) optionally, a protective layer.

[0011] In a first exemplary embodiment, the thermal-sensitive layer comprises: i. a first developer comprising a sulfonamide compound represented by formula (1) below: wherein R1and R2may be the same or different, and each independently represents a hydrogen atom, an alkyl group having from 1 to 4 carbons, an alkoxy group having from 1 to 4 carbons, or a halogen atom, ii. one or more second developers comprising at least one phenyl ureide compound represented by formula (2) below: wherein R3and R4may be the same or different, and each independently represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an aryl 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 toluenesulfonyl group, a toluenesulfonyloxyphenyl group, a monoalkylamino group, a dialkylamino group, or an arylamino group, and one or more stabilizers selected from the group consisting of compounds represented by formula (3) and (4) below, and a derivative thereof.

[0012] In the first exemplary embodiment, when the second developer is one developer or a blend of more than one developer, the content ratio by mass between the first developer and the second developer ranges from about 1:1.2 or 2:1 to about 50:1, preferably from about 3:1 or 3.5:1 to about 20:1, and most preferably from about 4:1 or 4.5:1 to about 15:1. In other words, the ratio of the second developers to the first developer is preferably 0.02 to 1.2, more preferably 0.05 to 0.33, even more preferably 0.07 to 0.25. For example, a. when the second developer is a blend of two developers, the ratio between them is from about 10:1 to about 1:1.2 or 1:1, preferably from about 8:1 to about 1:1.2 or 1:1, and most preferably from about 6:1 to about 1:2 or 1:1; b. when the second developer is a blend of three developers, one developer is no more than 1 mass% based on the total solids content of the thermal-sensitive layer, while the other developers are blended in a content ratio of from about 10:1 to about 1:1, preferably from about 8:1 to about 1:1, and most preferably from about 6:1 to about 1:1; c. when the second developer is one developer or a blend of up to three developers, the first and second developer ratios are kept as described above, and the compound represented by formula (3) or (4) is added in a dry amount percentage of less than 75% or 50% of the first developer; and d. the total content of the compound represented by formula (3) and (4) is preferably 5 to 35 parts by mass, more preferably 10 to 30 parts by mass, per 100 parts by mass of the total of the first developer and second developer.

[0013] The stabilizer represented by formulas (3) and (4) is used alone or in combination. These compounds are used up to 12 mass%, preferably up to 9 mass%, and most preferably up to 6 mass% based on the total solids content of the thermal-sensitive layer.

[0014] In a preferred embodiment, the heat-sensitive layer comprises a first developer comprising a sulfonamide compound represented by formula (1), one or more second developers comprising at least one phenyl ureide compound represented by formula (2), and at least one stabilizer represented by formula (3) and (4).

[0015] In a second exemplary embodiment, the phenol-free, heat-sensitive recording material further comprises a protective layer.

[0016] In a preferred embodiment, the protective layer comprises one or more pigments and binders, and boric acid.

[0017] In a more preferred embodiment, the protective layer has a pigment / binder ratio of from about 0.6:1 to about 2.7:1 or 3.5:1, and a boric acid / binder ratio of from about 1:200 to about 1:10, or from about 1:3000 to about 1:5.

[0018] Other features and advantages of the invention would be apparent to one of ordinary skill from the following detailed description. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

[0019] The phenol-free, heat-sensitive recording material of the present invention, which is suitable for a wide range of applications, is characterized by its excellent oil resistance and plasticizer resistance.

[0020] The phenol-free, heat-sensitive recording material of the present invention, as described in detail below, is made up of the following layers: (a) a substrate; (b) an undercoat layer; (c) a thermal-sensitive layer; and (d) optionally, at least one of a thermal-protective layer and a back-coat layer.

[0021] Thermal-sensitive Layer The thermal-sensitive layer of the phenol-free, heat-sensitive recording material comprises: (a) one or more color formers (e.g., leuco dyes); (b) a phenol-free, color developer formulation comprising: i. a first developer; and ii. one or more second developers (c) one or more stabilizers; and (d) optionally one or more sensitizers.

[0022] Color Formers Suitable color formers for use in the thermal-sensitive layer are selected from the group of colorless or pale-colored leuco dyes including, but not limited to, dyes capable of developing blue color, such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide, 3-(4-diethylamino-2-methylphenyl)-3-(4-dimethylaminophenyl)-6-dimethylaminophthalide, and fluoran; dyes capable of developing green color, such as 3-(N-ethyl-N-p-tolyl)amino-7-N-methylanilinofluoran, 3-diethyl-amino-7-anilinofluoran, and 3-diethylamino-7-dibenzylam-inofluoran; dyes capable of developing red color, such as 3,6-bis(diethylamino)fluoran-y-anilinolactam, 3-cyclohexy-lamino-6-chlorofluoran, 3-diethylamino-6-methyl-7-chloro-fluoran, and 3-diethylamino-7-chlorofluoran; dyes capable of developing black color, such as 3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran, 3-(N-methyl-N-cyclo-hexyl)amino-6-methyl-7-anilinofluoran,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-diethylamino-7-(o-chlorophenylamino)fluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-p-toluidino)-6-methyl-7-(p-toluidino)fluoran, 3-(N-ethyl-N-tetrahydrofurfurylamino)-6-methyl-7-anilino-fluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-dim-ethylamino-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 3-piperidino-6-methyl-7-anilinofluoran, 2,2-bis[4-[6'-(N-cyclohexyl-N-methylamino)-3'-methylspiro[phthalide-3,9'-xanthen]-2'-ylamino]phenyl]propane, and 3-diethylamino-7-(3'-trifluoromethylphenyl)aminofluoran; and dyes having absorption wavelengths in the near-infrared region, such as 3,3-bis[l-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-pyrrolidinophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylene-2-yl]-4,5,6,7-tetrachlorophthalide, 3-p-(p-dimethylaminoanilino)anilino-6-methyl-7-chlorofluoran, 3-p-(p-chloroanilino)anilino-6-methyl-7-chlorofluoran, and 3,6-bis(dimethylamino)fluorene-9-spiro-3'-(6'-dimethylamino)phthalide, and combinations thereof. In a preferred embodiment, the leuco dye is a dye capable of developing black color.

[0023] The one or more leuco dyes may be used in an amount of from about 5 to about 25 mass%, and preferably from about 7 to about 20 mass%, and most preferably from about 9 to about 17 mass%, based on the total solids content of the heat-sensitive layer. An amount of leuco dye below 5 mass% results in a reduction in color development ability, and thus a reduction in print density, while an amount of leuco dye that exceeds 25 mass% results in decreased heat resistance.

[0024] Formulation of Phenol-free, Color Developer As noted above, the phenol-free, color developer formulation is made up of: i. a first developer; and ii. one or more second developers.

[0025] The first developer comprises a sulfonamide compound represented by formula (1) below: wherein R1and R2may be the same or different, and each independently represents a hydrogen atom, an alkyl group having from 1 to 4 carbons, an alkoxy group having from 1 to 4 carbons, or a halogen atom.

[0026] Examples of the specific sulfonamide compound include N-[2-(3-phenylureido)phenyl]benzenesulfonamide, N-[2-(3-phenylureido)phenyl]-p-toluenesulfonamide, N-[2-(3-phenylureido)phenyl]-o-toluenesulfonamide, and N-[2-(3-(4-methylphenyl)ureido)phenyl]benzenesulfonamide. Among these, N-[2-(3-phenylureido)phenyl]benzenesulfonamide is preferable.

[0027] The second developer comprises: at least one phenyl ureide compound represented by formula (2) below, wherein, R3and R4may be the same or different, and each independently represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an aryl 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 toluenesulfonyl group, a toluenesulfonyloxyphenyl group, a monoalkylamino group, a dialkylamino group, or an arylamino group.

[0028] The halogen atom represented by R1to R4may be a fluorine atom, a chlorine atom, or a bromine atom, with a fluorine atom and a chlorine atom being preferred.

[0029] 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 C1-12alkyl group, preferably a C1-8alkyl group, more preferably a C1-6alkyl group, and still more preferably a C1-4alkyl group.

[0030] 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 C1-12alkoxy group, preferably a C2-8alkoxy group, more preferably a C2-6alkoxy group, and still more preferably a C2-4alkoxy group.

[0031] 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 C1-10alkylcarbonyloxy group.

[0032] 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 C1-10alkylcarbonylamino group.

[0033] 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 C1-10alkylsulfonylamino group.

[0034] 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.

[0035] The aryloxy group is preferably a C6-12aryloxy group. The arylcarbonyloxy group is preferably a C6-12arylcarbonyloxy group. The arylcarbonylamino group is preferably a C6-12arylcarbonylamino group. The arylsulfonylamino group is preferably a C6-12arylsulfonylamino group.

[0036] 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.

[0037] 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.

[0038] The arylamino group may be a monoarylamino group or diarylamino group, and is preferably a C6-12monoarylamino group.

[0039] R3is preferably an alkyl group, more preferably a C1-8linear alkyl group, still more preferably a C1-4linear alkyl group, and particularly preferably R3is a methyl group.

[0040] R4is preferably an aryl group, a toluenesulfonyloxyphenyl group, and a toluenesulfonyl group.

[0041] The position of the substituent bound to one benzene ring in the phenyl ureide structure in formula (2) may be the ortho-position, the meta-position, or the para-position with respect to the aminocarbonyl group on the benzene ring.

[0042] A suitable compound represented by formula (2) is 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate.

[0043] A suitable compound represented by formula (2) is N,N'-Di[3-(p-toluene sulfonyl)oxy]phenyl urea.

[0044] A suitable compound represented by formula (2) is N-(4-methylphenylsulphonyl)-N’-(3-(4-methylphenylsulfonyloxy) phenyl) urea.

[0045] In a preferred embodiment, the second developer is (2)-1 or (2)-2, preferably (2)-1. In another preferred embodiment, the second developers are (2)-3, and at least one selected from the group consisting of (2)-1 and (2)-2.

[0046] Stabilizers The stabilizer comprises: one or more stabilizers selected from the group consisting of compounds represented by formula (3) and (4) below, and a derivative thereof.

[0047] In the first exemplary embodiment, when the second developer is one developer or a blend of more than one developer, the content ratio by mass between the first developer and the second developer ranges from about 1:1.2 to about 50:1, preferably from about 3:1 to about 20:1, and most preferably from about 4:1 to about 15:1. For example, i. when the second developer is a blend of two developers, the ratio between them is from about 10:1 to about 1:1, preferably from about 8:1 to about 1:1, and most preferably from about 6:1 to about 1:1; ii. when the second developer is a blend of three developers, one developer is no more than 1 mass% based on the total solids content of the thermal-sensitive layer, while the other developers are blended in a content ratio of from about 10:1 to about 1:1, preferably from about 8:1 to about 1:1, and most preferably from about 6:1 to about 1:1.

[0048] The stabilizers represented by formulas (3) and (4) are used alone or in combination. These compounds are used up to 12 mass%, preferably up to 9 mass%, and most preferably up to 6 mass% based on the total solids content of the thermal-sensitive layer.

[0049] In a preferred embodiment, the thermal-sensitive layer comprises a first developer, one or more second developers comprising at least one compound selected from the group consisting of (2)-1, (2)-2, and (2)-3, and one or more stabilizers selected from the group consisting of (3) and (4).

[0050] In another preferred embodiment, the thermal-sensitive layer comprises a first developer, one or more second developers comprising (2)-1, and one or more stabilizers selected from the group consisting of (3) and (4).

[0051] In another embodiment, the first developer is N-[2-(3-phenylureido) phenyl] benzene sulfonamide, the second developer is 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate, and the stabilizer is 1,3 diphenyl urea. The total content of the compound represented by formula (3) and (4) is preferably 5 to 35 parts by mass, more preferably 10 to 30 parts by mass, per 100 parts by mass of the total of the first developer and second developer. This range can lower production costs and improve quality since an excess of the stabilizers tends to saturate or lower the preservation properties.

[0052] Sensitizers Suitable sensitizers for use in the thermal-sensitive layer include, but are not limited to, stearic acid amide, methoxycarbonyl-N-benz-stearic acid amide, N-benzoyl stearic acid amide, N-eicosanamide, ethylene-bis-stearic acid amide, behenamide, methylene-bis-stearic acid amide, N-methylol stearic acid amide, dibenzyl terephthalate, dimethyl terephthalate, dioctyl terephthalate, benzyl p-benzy-loxy benzoate, phenyl l-hydroxy-2-naphthoate, 2-naphthyl benzyl ether, m-terphenyl, p-benzyl biphenyl, di-p-chlorobenzyl oxalate, di-p-methylbenzyl oxalate, dibenzyl oxalate, p-tolylbiphenyl ether, di(p-methoxyphenoxyethyl) ether, 1,2-di(3-methylphenoxy)ethane, 1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane, 1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane, 1-(4- methoxyphenoxy)-2-(3-methylphenoxyl)ethane, p-methylthiophenylbenzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide, p-acetophenetidide, N-acetoacetyl-p-toluidine, di(-biphenylethoxy)benzene, p-di(vinyloxyethoxy)benzene, l-isopropylphenyl-2-phenylethane, diphenyl-sulfone, and combinations thereof. Preferred sensitizers are stearic acid amide, 2-naphthyl benzyl ether, di-p-chlorobenzyl oxalate, di-p-methylbenzyl oxalate, 1,2-di(3-methylphenoxy)ethane, 1,2-diphenoxyethane, diphenyl-sulfone, and combinations thereof.

[0053] In a preferred embodiment, the sensitizer is used in an amount of from about 1 to about 40 mass%, more preferably from about 2 to about 40 mass%, more preferably from about 5 to about 25 mass%, and most preferably from about 8 to about 20 mass%, based on the total solids content of the thermal-sensitive layer. An amount of sensitizer below 1 mass% results in a reduction in dynamic sensitivity, while an amount of sensitizer that exceeds 40 mass% results in decreased heat stability.

[0054] In another embodiment, the thermal-sensitive layer may not have any sensitizer, and even though dynamic sensitivity will likely be diminished, the heat stability will be further enhanced. In that case, the undercoat layer uniformity, and the type and dry percentage of hollow particles in the undercoat layer need to be carefully optimized to achieve the dynamic sensitivity requirements of the application. It would be clear to those of ordinary skill in the art about how to carry out these modifications.

[0055] Binders Suitable binders for use in the thermal-sensitive layer include, but are not limited to, starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, gum arabic, polyvinyl alcohols, carboxy-modified polyvinyl alcohols, acetoacetyl-modified polyvinyl alcohols, diacetone-modified polyvinyl alcohol, silicon-modified polyvinyl alcohols, carboxy-modified polyvinyl alcohol, sulfone-modified polyvinyl alcohol, diisobutylene-maleic anhydride copolymer salts, styrene-maleic anhydride copolymer salts, ethylene-acrylic acid copolymer salts, styrene-acrylic acid copolymer salts, styrene-butadiene copolymer, urea resins, melamine resins, amide resins, and polyurethane resins, and combinations thereof. When the medium of the thermal-sensitive-layer coating liquid is water, hydrophobic resins may be used in the form of latexes.

[0056] The thermal-sensitive layer preferably comprises at least one such binder in an amount ranging from about 5 to about 50 mass%, and more preferably from about 10 to about 40 mass%, based on a total solids content of the thermal-sensitive layer. A binder amount below 5 mass% results in a coating matrix with insufficient binder to wet the pigments in the thermal-sensitive-layer coating liquid, which is then prone to dusting issues upon drying, while a binder amount that exceeds 50 mass% is detrimental to imaging performance and / or dynamic sensitivity.

[0057] Pigments Suitable pigments for use in the thermal-sensitive layer include fine pigment particles having a high degree of whiteness and an average particle diameter of 10 μm or less, which serve to enhance the whiteness of the thermal-sensitive layer and improve the uniformity of images. Examples of such fine pigment particles include calcium carbonate, magnesium carbonate, kaolin, clay, talc, calcined kaolin, calcined clay, (amorphous) silica, diatomaceous earth, synthetic aluminum silicate, zinc oxide, titanium oxide, aluminum oxide, aluminum trihydroxide, barium sulfate, surface-treated calcium carbonate, silica, and like inorganic pigments, urea-formalin resin, styrene methacrylic acid copolymer resin, polystyrene resin, raw starch particles, and like organic pigments.

[0058] The pigment is preferably used in an amount ranging from about 50 mass% or less, and more preferably from about 30 mass% or less, based on the total solids content of the thermal-sensitive layer. An amount of pigment below 5 mass% results in a reduction in heat stability and effectively prevents residue from building up on the thermal head, while an amount of pigment that exceeds 50 mass% results in poorer imaging performance and / or dynamic sensitivity.

[0059] Additives Further, if necessary, additives can be used. Suitable additives for use in the thermal-sensitive layer are selected from the group including optical brighteners, lubricants, waxes, crosslinking agents, water retention aids, surfactant agents, dispersant agents, water-resistance-imparting agents, wetting agents, antifoaming agents, and the like. More specifically, additives include, but are not limited to, sodium dioctyl sulfosuccinate, sodium dodecylbenzene sulfonate, sodium polyacrylate, sodium lauryl alcohol sulfate, metal salts of fatty acids, and like dispersants; zinc stearate, calcium stearate, polyethylene wax, carnauba wax, paraffin wax, ester wax, and like waxes; adipic dihydrazide and like hydrazide compounds, glyoxal, boric acid, glyoxylic acid salt, dialdehyde starch, methylolurea, epoxy compounds, and like water-resistance-imparting agents; antifoaming agents (such as glycerol ester emulsion antifoaming agents, which are natural oil antifoaming agents), colorant dyes, and fluorescent dyes.

[0060] Method of Forming the Thermal-sensitive Layer The thermal-sensitive layer of the phenol-free, heat-sensitive recording material of the present invention, in accordance with one embodiment, may be formed by applying the heat-sensitive-layer coating liquid to the undercoat layer and drying. For example, a leuco dye, stabilizer, and specific developers are dispersed into finely divided particles with an average particle diameter ranging from about 2 μm or less using water as a dispersion medium by using mixing and pulverizing equipment, such as a ball mill, an attritor, or a sand mill, and blended. Then, one or more sensitizers, binders, and pigments, as well as one or more additives, are added to the finely dispersed dispersion, and the resulting formulation is applied to the undercoat layer to a dry coat weight of preferably from about 2 to about 12 grams per square meter (g / m2), and more preferably from about 3 to about 10 g / m2, and is then dried to form a thermal-sensitive layer.

[0061] Undercoat Layer The undercoat layer of the inventive phenol-free, heat-sensitive recording material, which is provided between the thermal-sensitive layer and the substrate, serves to improve recording sensitivity and recording runnability. Such a layer, which demonstrates very good adhesion to overlying and underlying layers, may be prepared from an aqueous coating formulation made up of a binder and at least one of the following: hollow particles, thermal expansion particles, and oil-absorbing pigments having an oil absorption of greater than or equal to 70 mL / 100 g (preferably, from about 80 mL / 100 g to about 150 mL / 100 g). The oil absorption is determined in accordance with JIS K 5101.

[0062] The hollow particles on the substrate allow for the formation of a uniform undercoat layer so that the thermal-sensitive layer formed on the undercoat layer has a uniform thickness, enhanced barrier properties, and heat insulation properties. This serves to prevent the developers from coming into contact with a plasticizer or an alkaline filler contained in neutral paper, thus inhibiting a decrease in color development ability as well as enhancing imaging performance and / or dynamic sensitivity.

[0063] In a preferred embodiment, the undercoat layer is prepared from an aqueous coating formulation comprising organic hollow particles, oil-absorbing pigments, and one or more binders and additives selected from the group of defoamers, fluorescent whitening agents, rheology modifiers, and wetting agents, surfactants, dispersing agents, optical brighteners, water retention aids, rheology modifiers, and the like, wherein the dry weight ratio of pigments to organic hollow particles ranges from about 10:1 to about 1:10.

[0064] Hollow Particles From the viewpoint of improving cushioning properties, it is preferable that the hollow particles be formed of an organic resin. An undercoat layer that has a high degree of thermal insulation properties by containing hollow particles suppresses diffusion of heat applied to the thermal-sensitive layer and can improve sensitivity as the heat-sensitive recording material. Hollow particles formed of an organic resin can be classified into foaming type and a non-foaming type, depending on the manufacturing methods. Regarding the two types, in general, the average particle size and the hollow ratio of the foaming hollow particles are greater than those of the non-foaming hollow particles. Therefore, with foaming hollow particles, higher sensitivity and image quality can be obtained as compared to the non-foaming hollow particles. However, the smoothness of the undercoat layer tends to decrease. Therefore, by containing a water retention agent, migration of the binder to the substrate side can be suppressed, and the smoothness can be improved while the thickness of the coating layer is maintained.

[0065] A representative method of manufacturing the foaming hollow particles will be described below.

[0066] First, particles in which volatile liquid is sealed in a resin are prepared, and the resin is softened during heating while vaporizing and expanding the liquid in the particles. As a result, hollow particles can be manufactured.

[0067] Regarding the foaming hollow particles, the liquid in the particles is heated to expand in the process of manufacturing so that the hollow ratio increases and a high degree of thermal insulation properties can be obtained. Therefore, the sensitivity of the heat-sensitive recording material can be improved, and the recording density can be improved. The improvement in sensitivity is important when a medium-energy region where thermal energy applied to the thermal-sensitive layer is low is colored. In addition, when the thermal-sensitive layer is formed through an undercoat layer having a high degree of thermal insulation properties, diffusion of heat applied to the thermal-sensitive layer is suppressed, and thus, image uniformity becomes excellent, and image quality can also be improved. Therefore, in the embodiment, it is preferable to use a foaming hollow particle that is suitable for improvement in the thermal insulation properties of the undercoat layer.

[0068] Examples of the resin that can be used for the foaming hollow particles include a thermoplastic resin, such as a styrene-acrylic resin, a polystyrene resin, an acrylic resin, a polyethylene resin, a polypropylene resin, a polyacetal resin, a chlorinated polyether resin, a polyvinyl chloride resin, a polyvinylidene chloride resin, an acrylic resin (for example, an acrylic resin containing acrylonitrile as a component), a styrene resin, and a copolymer resin, such as a vinylidene chloride resin, mainly formed of polyvinylidene chloride and acrylonitrile. As gas in the foaming hollow particles, for example, propane, butane, isobutane, or air can be generally used.

[0069] As the resin used for the hollow particles, an acrylonitrile resin or a copolymer resin mainly formed of polyvinylidene chloride and acrylonitrile is preferable from the viewpoint of the strength for maintaining the shape of foamed particles, among the various resins described above.

[0070] On the other hand, with regard to the non-foaming hollow particles, in general, the average particle size is small, and the hollow ratio is also low. Therefore, in order to obtain excellent sensitivity and image quality, it is preferable to increase the content of the hollow particles in the undercoat layer.

[0071] In a method of manufacturing the non-foaming hollow particles, a seed is polymerized in a solution, another resin is polymerized to cover the seed, and the seed in the resin is removed by swelling and dissolution. As a result, voids are formed in the resin. In order to remove the seed in the resin by swelling and dissolution, an alkaline aqueous solution or the like is used. By performing an alkali swelling process on core-shell particles in which core particles having alkali swellability are covered with a shell layer having no alkali swellability, the non-foaming hollow particles having a relatively large average particle size can also be obtained.

[0072] Examples of a monomer that is suitable for the method of manufacturing the non-foaming hollow particles include a vinyl monomer, such as a styrene monomer, an acrylic monomer, or an acrylonitrile monomer. Examples of the styrene monomer include styrene, methylstyrene, dimethyl styrene, trimethylstyrene, ethylstyrene, diethylstyrene, triethylstyrene, propylstyrene, butylstyrene, chlorstyrene, and t-butylstyrene. Examples of the acrylic monomer include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, 2-ethylhexyl methacrylate, and cyclohexyl methacrylate. Examples of the acrylonitrile monomer include acrylonitrile and methacrylonitrile. Examples of other vinyl monomers include dimethyl maleate, dimethyl fumarate, maleic anhydride, N-methylmaleimide, and N-phenylmaleimide.

[0073] Among the various monomers described above, from the viewpoint of easy manufacturing, a combination of a styrene monomer and an acrylic monomer is preferable, and a combination of a styrene monomer and a (meth)acrylate is more preferable. That is, it is preferable that the hollow particles be formed of a styrene-acrylic resin, and it is more preferable that the hollow particles be formed of a styrene-(meth)acrylate copolymer resin.

[0074] The maximum particle size of the hollow particles in the embodiment is 10 μm to 30 μm and preferably 10 μm to 25 μm. The maximum particle size is also represented by D100.

[0075] When the maximum particle size of the hollow particles is 10 μm or more, the cushioning properties of the undercoat layer are improved. Therefore, adhesion of the heat-sensitive recording material with a thermal head during printing is improved, and a heat-sensitive recording material having high image quality can be obtained. This high image quality can bring about improvement in recording density in a medium-energy region that is colored with lower energy than the energy for providing the maximum recording density (Dmax).

[0076] On the other hand, when the maximum particle size of the hollow particles is 30 μm or less, the smoothness of the undercoat layer is improved. Therefore, the thermal-sensitive layer that is provided through the undercoat layer can be made uniform, and it is possible to obtain a heat-sensitive recording material in which the formation of white spots in an image is unlikely to occur.

[0077] The maximum particle size (D100) and the average particle size (D50) of the hollow particles can be measured using a laser diffraction particle size analyzer. In addition, the maximum particle size (D100) and the average particle size (D50) may be obtained by measuring the particle sizes from particle images (SEM images) using an electron microscope and obtaining the average values of 10 particle sizes.

[0078] The hollow ratio of the hollow particles is preferably 80% or more, more preferably 90% or more, and still more preferably 95% or more.

[0079] The hollow ratio of the hollow particles is obtained from the value of true specific gravity that is measured using an IPA method. (1) Pretreatment of Sample A sample is dried overnight at 60°C. (2) Reagent Isopropyl alcohol (IPA: Extra Pure Reagent) (3) Measurement Method - A volumetric flask is weighed (W1). - About 0.5 g of the dried sample is weighed in the volumetric flask (W2). - About 50 mg of IPA is added and is sufficiently shaken to completely remove air outside the capsule. - IPA is added up to a marked line and weighed (W3). - As a blank sample, only IPA is added up to a marked line and weighed in the volumetric flask (W4). (4) Calculation of True Specific Gravity True specific gravity = {(W2－W1)×(W4－W1)} / 100 / {(W4－W1)－(W3－W2)} (5) Calculation of Hollow Ratio Hollow ratio (%) = {1－1 / (1.1 / true specific gravity)}×100

[0080] In addition, the hollow ratio is a value obtained from the following expression: (d3 / D3) × 100. In the expression, d represents the inner diameter of the hollow particles, and D represents the outer diameter of the hollow particles. The average particle size of the hollow particles is preferably about 0.5 μm to 12 μm, and more preferably about 3 μm to 12 μm.

[0081] The content ratio of the hollow particles with respect to the total solid content of the undercoat layer is preferably 2 mass% to 90 mass%, more preferably 5 mass% to 70 mass%, still more preferably 5 mass% to 50 mass%, and still more preferably 10 mass% to 50 mass%. When the content ratio of the hollow particles is 2 mass% or more, the thermal insulation properties of the undercoat layer can be improved. On the other hand, when the content ratio of the hollow particles is 90 mass% or less, a problem is unlikely to occur in terms of coating properties or the like, a uniform undercoat layer is likely to be formed, and the recording density can be improved.

[0082] In this preferred embodiment, the aqueous coating formulation may further comprise an oil-absorbing pigment with oil absorption of about 70 ml / 100 g or more, preferably from about 80 to about 150 ml / 100 g, and / or thermal expansion particles. Using an oil-absorbing pigment can enhance the effect of inhibiting the adhesion of residue to a thermal head. The oil absorption referred to herein is a value determined in accordance with JIS K 5101. The oil-absorbing pigment may be any type of oil-absorbing pigment. Specific examples include inorganic pigments, such as calcined kaolin, amorphous silica, light calcium carbonate, and talc. Such oil-absorbing pigments preferably have an average primary particle diameter of about 0.01 to about 5 μm, and more preferably from about 0.02 to about 3 μm. The amount of the oil-absorbing pigment is preferably from about 2 to about 95 mass%, and more preferably from about 5 to about 90 mass%, based on the total solids content of the undercoat layer.

[0083] When an oil-absorbing inorganic pigment is used in combination with hollow plastic particles, the total amount of the pigment and particles is preferably from about 5 to about 90 mass%, more preferably from about 10 to about 90 mass%.

[0084] Binders As the binder, a water-dispersible adhesive formed of a water-insoluble resin is preferable. Examples of the water-dispersible adhesive include a latex, such as polyvinyl acetate, polyurethane, a styrene-butadiene copolymer, a styrene-butadiene-acrylonitrile copolymer, an acrylonitrile-butadiene copolymer, polyacrylic acid, a polyacrylate, a vinyl chloride-vinyl acetate copolymer, polybutyl methacrylate, an ethylene-vinyl acetate copolymer, silylated urethane, an acrylic silicon composite, an acrylic silicon urethane composite, an urea resin, a melamine resin, an amide resin, and a polyurethane resin. Among these, a styrene-butadiene copolymer (SBR) is preferable. The content ratio of the latex can be selected from a wide range and, in general, is preferably 5 mass% or more, and more preferably 10 mass% or more, with respect to the total solid content of the undercoat layer. On the other hand, the content ratio is preferably 40 mass% or less, more preferably 30 mass% or less, and still more preferably 20 mass% or less. By adjusting the content ratio of the latex to 10 mass% or more, the cushioning properties of the undercoat layer are further improved.

[0085] The glass transition (Tg) temperature of the binder (in particular, the latex) is not particularly limited and is preferably 10°C or lower, more preferably 5°C or lower, and still more preferably -10°C or lower. By using a binder having a glass transition temperature of 10°C or lower, the cushioning properties of the undercoat layer are further improved. On the other hand, when the glass transition temperature is -50°C or lower, sticking is likely to occur, which is not preferable. Therefore, the glass transition temperature is preferably -40°C or higher.

[0086] The average particle size of the binder (in particular, the latex) is not particularly limited and is preferably 150 nm or more, more preferably 165 nm or more, and still more preferably 190 nm or more. On the other hand, the average particle size is preferably 300 nm or less and more preferably 250 nm or less. By adjusting the average particle size to 150 nm or more, migration of the latex to the support side is effectively suppressed, and a uniform undercoat layer can be formed. By adjusting the average particle size to 300 nm or less, the formation of voids caused by fusion of the latex is suppressed, penetration of the thermal-sensitive-layer-forming coating material is suppressed, and a uniform thermal-sensitive layer can be formed.

[0087] The average particle size of the binder can be measured using a laser diffraction particle size analyzer.

[0088] Water Retention Agents The water retention agent is contained in the undercoat layer so that migration of the undercoat-layer-forming coating material to the support side, in particular, migration of the binder component, is suppressed, and an undercoat layer in which the hollow particles are uniformly distributed without uneven distribution can be formed. When uneven distribution of the hollow particles decreases, the smoothness of the undercoat layer is improved. Therefore, the heat-sensitive layer that is provided through the undercoat layer can be made uniform. As a result, hollow particles having a relatively large maximum particle size of 10 mm to 30 mm are uniformly distributed in the undercoat layer, the formation of white spots in an image or the like can be suppressed, and the maximum color optical density is also improved. It is preferable that the water retention agent be a water-soluble water retention agent formed of a water-soluble resin. In the embodiment, when the undercoat layer contains hollow particles having a maximum particle size (D100) of 10 mm to 30 mm, the water-soluble resin in the undercoat layer is the water retention agent and is not the binder.

[0089] As the water retention agent, for example, various well-known materials, such as cellulose or a derivative thereof, a polymer polysaccharide, a polyacrylic acid-modified product, sodium alginate, or a maleic anhydride copolymer can be appropriately used. In particular, it is preferable that the water-soluble resin be at least one kind selected from the group consisting of starch, polyvinyl alcohol, and carboxymethyl cellulose. Specific examples of the polyvinyl alcohol include a modified polyvinyl alcohol, such as completely saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, or silicon-modified polyvinyl alcohol. Specific examples of the starch include starch, oxidized starch, hydroxyethyl starch, and a derivative, such as starch acetate. The content ratio of the water retention agent is not particularly limited and is preferably 0.3 mass% to 5 mass%, and more preferably 0.5 mass% to 2 mass% with respect to the total solid content of the undercoat layer. By adjusting the content ratio to 0.3 mass% or more, migration can be more effectively suppressed. By adjusting the content ratio to 5 mass% or less, an increase in the viscosity of the coating material is suppressed, and coating suitability is excellent. In addition, there is no possibility that water resistance will deteriorate, expansion (blistering) caused by water penetration will occur, or the undercoat layer will peel off.

[0090] Method of Forming the Undercoat Layer The undercoat layer is formed on the support, in general, by mixing the hollow particles and the binder, and optionally the water retention agent, the oil-absorbing pigment, such as calcined kaolin, an auxiliary agent, and the like in water as a medium to prepare the undercoat-layer coating liquid, applying the undercoat-layer coating liquid to the support, and drying.

[0091] Auxiliary agents for use in the undercoat-layer coating liquid include surfactants, dispersing agents, optical brighteners, and rheology modifiers.

[0092] The undercoat-layer coating liquid described above may be applied to a surface of the substrate to achieve a preferred average coat weight that ranges from about 3 to about 20 g / m2, more preferably from about 5 to 12 g / m2, based on the total dry weight of the coating formulation. Drying, as described above, can be accomplished by any known method or technique, including room-temperature air drying, hot-air drying, heating-surface-contact drying or heat radiation drying.

[0093] Substrate The substrate used in the practice of the present invention has a strength sufficient to withstand the tension and forces characteristic of the coating operation used to apply the undercoat layer to the substrate. The substrate is an opaque, translucent, or clear substrate. Envisioned substrates include optionally coated paper or paper-like substrates prepared from any suitable natural or synthetic fiber. Also envisioned are plastic base substrates, including light-transmissive substrates prepared from moldable, thermoformable, and / or extrudable materials, such as acrylic, polyacrylonitrile, polycarbonate, polydicyclopentadiene, polyethylene, polyethylene terephthalate (PET), polypropylene, including biaxially oriented polypropylene, polystyrene, polyvinyl chloride, mixtures and copolymers thereof, and the like.

[0094] For paper or paper-like substrates, some degree of sizing is preferred. Internal and / or surface sizing can be incorporated without limitation to enhance the coating holdout during coating operations.

[0095] The substrate, which may be made of one layer or multiple layers of substrate materials, has a preferred thickness of from about 25 to about 300 μm (more preferred, from about 50 to about 250 μm), a preferred basis weight of from about 25 to about 250 grams per square meter (g / m2) (more preferred, from about 40 to about 210 or 170 g / m2), and a preferred surface roughness of less than 6.4 μm, preferably less than 5 μm, and more preferably less than 4 μm (Parker Print Surf, T-555).

[0096] Protective Layer In a preferred embodiment, the phenol-free, heat-sensitive recording material of the present invention includes a protective layer, which serves to provide additional resistance properties against chemicals, such as plasticizers and oils, for end-use applications, such as lottery, gaming, and entertainment tickets, and to improve recording quality.

[0097] The protective layer is prepared from an aqueous coating formulation made up of one or more binders, and one or more auxiliary agents (e.g., a water-resistance-imparting agent) and / or pigments.

[0098] In a first exemplary embodiment, the protective layer is prepared from an aqueous coating formulation comprising a binder and one or more auxiliary agents.

[0099] In a second exemplary embodiment, the formulation comprises a binder and a pigment. In this embodiment, the amount of the binder is preferably from about 1 to about 95 mass%, and more preferably from about 2 to about 80 mass%, based on the total solids content of the protective layer. The amount of the pigment is preferably from about 5 to 95 mass%, more preferably from about 15 to about 90 mass%, and most preferably from about 25 to about 80 mass%, based on the total solids content of the protective layer.

[0100] Binders Suitable binders are selected from the group of starch, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, gum arabic, polyvinyl alcohol, carboxyl-modified polyvinyl alcohol, acetacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, silicon-modified polyvinyl alcohol, acrylic polymers, and ionomeric urethane resin latex.

[0101] In a preferred embodiment, the binder is made of blends of polyvinyl alcohol (PVOH) with low-medium, medium, and high degrees of polymerization. In applications in which blocking resistance is also required, it is preferred that a majority of the binder composition comprise a low-medium fully hydrolyzed PVOH. Also, the pigment-to-binder ratio plays an important role. The higher the ratio, the better the blocking resistance, but high P / B ratios are detrimental to plasticizer resistance. It may be possible to improve plasticizer resistance at relatively higher overcoat coverage, but it is not a preferred approach due to the additional cost and the detrimental effect on dynamic sensitivity.

[0102] Pigments Suitable pigments are selected from the group of kaolin, aluminum hydroxide, light calcium carbonate, and silica particles. Preferably, the pigments are kaolin and aluminum hydroxide, which have less of an impact upon decreases in barrier properties against plasticizers, oils, etc. and reduction in recording density.

[0103] Auxiliary Agents Suitable auxiliary agents are selected from the group of lubricants (e.g., zinc stearate, calcium stearate, polyethylene wax, carnauba wax, paraffin wax, ester wax, and the like), dispersing agents or wetting agents (e.g., sodium dioctyl sulfosuccinate and like surfactants), antifoaming agents, ultraviolet-light absorbers (e.g., 2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole), hardening agents (e.g., boric acid), and water-soluble polyvalent metal salts (e.g., potassium alum, aluminium acetate, and the like). To further enhance the water resistance of the protective layer, a water-resistance-imparting agent, such as glyoxal, glyoxylate, dialdehyde starch, a hydrazide compound, an epichlorohydrin resin (e.g., polyamide epichlorohydrin, polyamidoamine epichlorohydrin, and polyamine epichlorohydrin), and an epoxy compound, may be used.

[0104] In a further exemplary embodiment, the protective layer is prepared from an aqueous coating formulation made up of one or more binders (e.g., PVOH), one or more auxiliary agents, including a boric acid hardening agent and a water-resistance-imparting agent (e.g., epichlorohydrin resin), and optionally, potassium aluminum sulfate water-soluble polyvalent metal salt, one or more lubricants (e.g., zinc stearate), defoamer, wetting agent, and one or more pigments.

[0105] The present inventors have found that the addition of boric acid enhances plasticizer resistance. The amount of boric acid used in this embodiment is at least about 0.01 mass%, preferably from about 0.05 to about 4 or 7 mass%, and most preferably from about 0.1 to about 3.5 mass%, based on the total solids content of the protective layer.

[0106] Potassium aluminum sulfate salt (potassium alum) may be used to lower the pH. Minor amounts of potassium alum may be added at a rate of 10 parts, preferably less than 5 parts, and most preferably less than 2 parts, per 100 parts of polyvinyl alcohol (PVOH) on a dry basis. The aqueous coating formulation needs to be balanced carefully to avoid a detrimental effect on durability properties.

[0107] Such a protective-layer coating liquid comprising a PVOH binder (PVOH) composition having as a majority a low-medium fully hydrolyzed PVOH in combination with boric acid and very low to no added potassium alum sulfate salt and a pigment-to-binder ratio (P / B ratio) of about 0.6:1 to about 2.7:1 achieves good blocking and plasticizer resistance properties at a relatively low protective-layer coat weight of about 1.6 to about 2.0 grams per square meter (g / m2). In a more preferred embodiment, the P / B ratio is between about 0.8:1 to about 2:1, and most preferably between about 1:1 to about 1.8:1. The coating formulation also has a boric acid / binder ratio of from about 0.2:100 to about 1:5, preferably from about 1:100 to about 1:10, which serves to further enhance plasticizer resistance.

[0108] Optional Back-coat Layer In a further preferred embodiment, the phenol-free, heat-sensitive recording material of the present invention includes a back-coat layer, which is applied to the underside of the substrate away from the thermal-sensitive layer.

[0109] In one such embodiment, the back-coat layer is prepared from a formulation comprising a pigment and a binder, which serves to enhance preservability, curling suitability, and printer travel performance, as well as backside printability by flexographic or offset methods.

[0110] In another such embodiment, the back-side layer is selected from the group of adhesives for adhesive labels, magnetic recording layers, coating layers for printing, thermal transfer recording layers, ink jet recording layers, or the like.

[0111] Sample Preparation The coatings and dispersions used to prepare the test samples (i.e., Examples and Comparative Examples) are described below.

[0112] Example 1-1Undercoat-layer Coating Liquid The following were mixed and stirred to obtain a undercoat-layer coating liquid: 75 parts of calcined clay (ANSILEX 93 calcined clay, supplied by BASF SE); 18 parts of an SBR binder (Ligos C3433, solid concentration: 50%, supplied by Trinseo, LLC); 100 parts of hollow particle pigment A (ROPAQUE TH2000AF pigment, non-foaming, solid concentration: 20%, supplied by The Dow Chemical Company); 12 parts of a 25% aqueous solution of oxidized starch (trade name: OJIACE A, manufactured by Oji Cornstarch Co., Ltd.); and 200 parts of water

[0113] Thermal-sensitive Layer Coating LiquidDeveloper Dispersions (A) and (B) Developer dispersions were prepared by mixing and then wet-grinding the components listed below by using a sand mill to an average particle size of less than 1 μm, the resulting dispersions being about 50% solids. 35 parts of water; 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (solid concentration: 20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); and 50 parts of a developer. Dispersion (A): The developer is N-[2-(3-phenylureido) phenyl] benzene sulfonamide. Dispersion (B): The developer is 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate. (Note: Developers are supplied as powder.)

[0114] Stabilizer Dispersions (E) and (F) Stabilizer dispersions were prepared by mixing and then wet-grinding the components listed below by using a sand mill to an average particle size of less than 1 μm, the resulting dispersions being about 50% solids. 35 parts of water; 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (solid concentration: 20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); and 50 parts of a stabilizer. Dispersion (E): The stabilizer is 1,3 diphenyl urea. Dispersion (F): The stabilizer is 4,4’ diamino diphenyl sulfone. (Note: Stabilizers (E) and (F) are supplied as powder.)

[0115] Sensitizer Dispersion A sensitizer dispersion was prepared by mixing and then wet-grinding the components listed below by using a sand mill to an average particle size of less than 1 μm, the resulting dispersions being about 50% solids. 35 parts of water; 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (solid concentration: 20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); and 50 parts of one or more sensitizers. Sensitizer Dispersion (G): The sensitizer is diphenyl sulfone.

[0116] Leuco Dye Dispersion (J) A leuco dye dispersion was prepared by mixing and then grinding the components listed below by using a sand mill to an average particle size of less than about 0.8 μm, the resulting dispersion being about 50% solids. 34.5 parts of water; 50 parts of a leuco dye (ODB-2 color former (Black 400), supplied by Sofix Corp.); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (solid concentration: 20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); 0.5 parts of a wetting agent (AEROSOL TR-70 wetting dispersing agent, supplied by Hubbard-Hall Inc.); and 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company).

[0117] Preparation of Thermal-sensitive-layer Coating Liquid 23 parts of the dispersion A, 14 parts of the dispersion B, 4.6 parts of the dispersion E, 19 parts of dispersion G, 18.6 parts of dispersion J, 65 parts of a 20% aqueous solution of oxidized starch, 40 parts of a 10% aqueous solution of completely saponified polyvinyl alcohol (Elvanol 90-50, degree of saponification: 99 mol%), 28 parts of light calcium carbonate (Magnum Gloss XF), 14 parts of zinc stearate wax (Hydrin Z-9-36, commercially available from Chukyo Yushi Co., Ltd., solid concentration: 36%), and 120 parts of water were mixed and stirred to obtain a thermal-sensitive-layer coating liquid.

[0118] Preparation of Heat-sensitive Recording Material On one surface of high-quality paper having a basis weight of 60 g / m2, the undercoat-layer coating liquid and the thermal-sensitive-layer coating liquid were applied and dried so that the coating amounts after drying were 8.0 g / m2and 3.5 g / m2, respectively, an undercoat layer and a thermal-sensitive layer were sequentially formed, and the surface was then smoothened with a supercalender to obtain a heat-sensitive recording material.

[0119] Example 1-2 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 4.6 parts of the dispersion B were used in place of 14 parts of dispersion B and 14 parts of dispersion E were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0120] Example 1-3 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 32.6 parts of the dispersion A were used in place of 23 parts of dispersion A, 4.4 parts of the dispersion B were used in place of 14 parts of dispersion B, and 4.4 parts of dispersion E were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0121] Example 1-4 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 18.6 parts of the dispersion A were used in place of 23 parts of dispersion A, 18.6 parts of the dispersion B were used in place of 14 parts of dispersion B, and 4 parts of dispersion F were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0122] Example 1-5 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 18.6 parts of the dispersion A were used in place of 23 parts of dispersion A, 18.6 parts of the dispersion B were used in place of 14 parts of dispersion B, and 4 parts of dispersion E were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0123] Comparative Example 1-1 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 37.2 parts of the dispersion A were used in place of 23 parts of dispersion A in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion B and the dispersion E were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0124] Comparative Example 1-2 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 28 parts of the dispersion A were used in place of 23 parts of dispersion A and 9.2 parts of the dispersion B were used in place of 14 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion E was deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0125] Comparative Example 1-3 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 18.6 parts of the dispersion A were used in place of 23 parts of dispersion A and 18.6 parts of the dispersion B were used in place of 14 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion E was deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0126] Comparative Example 1-4 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 9.2 parts of the dispersion A were used in place of 23 parts of dispersion A and 28 parts of the dispersion B were used in place of 14 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion E was deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0127] Comparative Example 1-5 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 37.2 parts of the dispersion B were used in place of 14 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion A and the dispersion E were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0128] Comparative Example 1-6 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 27.4 parts of the dispersion A were used in place of 23 parts of dispersion A and 14 parts of the dispersion E were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion B was deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0129] Comparative Example 1-7 A heat-sensitive recording material was obtained in the same manner as in Example 1-1 except that 27.4 parts of the dispersion B were used in place of 14 parts of dispersion B and 14 parts of the dispersion E were used in place of 4.6 parts of dispersion E in the preparation of the thermal-sensitive-layer coating liquid of Example 1-1. Furthermore, the dispersion A was deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 1-1.

[0130] Example 2-1Undercoat-layer Coating Liquid The following were mixed and stirred to obtain an undercoat-layer coating liquid: 50 parts of calcined clay (ANSILEX 93 calcined clay, supplied by BASF SE); 60 parts of SBR binder (Patelacol UCO-101, Tg = -35°C, solid concentration: 50%, supplied by DIC Incorporated); 50 parts of hollow particle pigment B (foaming, average particle size: 9 μm, maximum particle size: 20 μm, hollow ratio: 97%, solid concentration: 20%); 9 parts of a 25% aqueous solution of oxidized starch (trade name: OJIACE A, manufactured by Oji Cornstarch Co., Ltd.); 1.1 parts of carboxymethyl cellulose (product name: Cellogen AG GUM, commercially available from DKS Co., Ltd.); and 140 parts of water

[0131] Thermal-sensitive-layer Coating LiquidDeveloper Dispersions (C) and (D) Developer dispersions were prepared by mixing and then wet-grinding the components listed below by using a sand mill to an average particle size of less than 1 μm, the resulting dispersions being about 50% solids. 35 parts of water; 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (solid concentration: 20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); and 50 parts of a developer. Dispersion (C): The developer is N,N'-Di[3-(p-toluene sulfonyl)oxy]phenyl urea. Dispersion (D): The developer is N-(4-methylphenylsulphonyl)-N’-(3-(4-methylphenylsulfonyloxy) phenyl) urea. (Note: Developers are supplied as powder.)

[0132] Sensitizer Dispersions Sensitizer dispersions were prepared by mixing and then wet-grinding the components listed below by using a sand mill to an average particle size of less than 1 μm, the resulting dispersions being about 50% solids. 35 parts of water; 0.04 parts of a defoaming agent (FOAM BLAST 269 defoamer solution, supplied by Emerald Performance Materials Company); 25 parts of a partially hydrolyzed polyvinyl alcohol binder solution (20%) (POVAL 3-86SD polyvinyl alcohol, supplied by Kuraray Co., Ltd.); and 50 parts of one or more sensitizers. Sensitizer Dispersion (H): The sensitizer is 1,2-Bis(3′-methylphenoxy)ethane.

[0133] Preparation of Thermal-sensitive-layer Coating Liquid 35 parts of the dispersion A, 8 parts of the dispersion B, 10 parts of the dispersion F, 33.2 parts of dispersion H, 27 parts of dispersion J, 78 parts of a 10% aqueous solution of completely saponified polyvinyl alcohol (Elvanol 90-50, degree of saponification: 99 mol%), 29.6 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.), 12 parts of SBR binder (Ligos C3433, solid concentration: 50%, supplied by Trinseo, LLC), and 120 parts of water were mixed and stirred to obtain a thermal-sensitive-layer coating liquid.

[0134] Protective-layer Coating LiquidSlurry Dispersion (K) A slurry dispersion was prepared by mixing and then stirring the components listed below. 40 parts of water; 0.32 parts of a dispersing agent (DISPEX N40V dispersing agent, supplied by BASF SE); and 60 parts of kaolin clay (ASTRA COTE kaolin clay, supplied by Imerys S.A.). 117 parts of the Slurry Dispersion (K) were added to the 180 parts of a 15% aqueous solution of modified polyvinyl alcohol binder (GOHSENX Z-200 modified fully hydrolyzed PVOH, supplied by Marubeni Specialty Chemicals Inc.) while stirring for about sixty minutes while maintaining good surface agitation. The resulting blend was further formulated with 7.8 parts of zinc stearate wax (Hydrin Z-9-36, commercially available from Chukyo Yushi Co., Ltd., solid concentration: 36%), and 80 parts of water were mixed and stirred to obtain a protective-layer coating liquid.

[0135] Preparation of Heat-sensitive Recording Material On one surface of high-quality paper having a basis weight of 60 g / m2, the undercoat-layer coating liquid, the thermal-sensitive-layer coating liquid and the protective-layer coating liquid were applied and dried so that the coating amounts after drying were 6.0 g / m2, 3.5 g / m2, and 2.0 g / m2, respectively, and an undercoat layer, a thermal-sensitive-layer, and protective layer were sequentially formed, and the surface was then smoothened with a supercalender to obtain a heat-sensitive recording material.

[0136] Example 2-2 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 10 parts of the dispersion E were used in place of 10 parts of dispersion F in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0137] Example 2-3 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion C were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0138] Example 2-4 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion C were used in place of 8 parts of dispersion B, and 10 parts of the dispersion E were used in place of 10 parts of dispersion F in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0139] Example 2-5 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion D were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0140] Example 2-6 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion D were used in place of 8 parts of dispersion B, and 10 parts of the dispersion E were used in place of 10 parts of dispersion F in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0141] Example 2-7 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 1 part of the dispersion D was added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0142] Example 2-8 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion C and 1 part of the dispersion D were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0143] Example 2-9 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 3.2 parts of the dispersion B and 4.8 parts of dispersion C were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0144] Example 2-10 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 3.2 parts of dispersion B, 4.8 parts of dispersion C, and 1 part of dispersion D were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0145] Comparative Example 2-1 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 43 parts of the dispersion A were used in place of 35 parts of dispersion A in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 8 parts of dispersion B and 10 parts of dispersion F were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1, and 5.0 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.) were added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0146] Comparative Example 2-2 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 43 parts of the dispersion A were used in place of 35 parts of dispersion A in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 8 parts of the dispersion B were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0147] Comparative Example 2-3 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 43 parts of the dispersion A were used in place of 35 parts of dispersion A and 10 parts of dispersion E were used in place of 10 parts of dispersion F in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 8 parts of the dispersion B were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0148] Comparative Example 2-4 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 10 parts of the dispersion F were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 5.0 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.) were added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0149] Comparative Example 2-5 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 8 parts of the dispersion C were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 10 parts of dispersion F were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1, and 5.0 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.) were added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0150] Comparative Example 2-6 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 3.2 parts of dispersion B and 4.8 parts of dispersion C were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 10 parts of dispersion F were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1, and 5.0 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.) were added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0151] Comparative Example 2-7 A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that 3.2 parts of dispersion B, 4.8 parts of dispersion C, and 1 part of dispersion D were used in place of 8 parts of dispersion B in the preparation of the thermal-sensitive-layer coating liquid of Example 2-1. Furthermore, 10 parts of dispersion F were deleted from the preparation of the thermal-sensitive-layer coating liquid of Example 2-1, and 5.0 parts of aluminum trihydrate (HYDRAL 710 aluminum trihydroxide, supplied by J.M. Huber Corp.) were added to the preparation of the thermal-sensitive-layer coating liquid of Example 2-1.

[0152] Example 3-1 The undercoat-layer coating liquid was the same as in Example 2-1. The thermal-sensitive-layer coating liquid was the same as in Example 2-1. 21.6 parts of a 5% aqueous solution of boric acid (supplied by Astro Chemicals Inc.) and 21.6 parts of a 5% aqueous solution of potassium aluminum sulphate (supplied by Holland Company) were added to the protective-layer coating liquid of Example 2-1. A heat-sensitive recording material was obtained in the same manner as in Example 2-1 except that the protective-layer coating liquid as above was used in place of the protective-layer coating liquid of Example 2-1.

[0153] Example 3-2 to Example 3-10 The undercoat-layer coating liquid was the same as in Example 3-1. The thermal-sensitive-layer coating liquid was the same as in order from Example 2-2 to 2-10. The protective-layer coating liquid was the same as Example 3-1. A heat-sensitive recording material was obtained in the same manner as in Example 3-1 except that each thermal-sensitive-layer coating liquid was used in place of the thermal-sensitive-layer coating liquid of Example 3-1.

[0154] The prepared test samples were tested in accordance with the following test methods.

[0155] Test Methods Test samples were imaged by using an Atlantek 400 printer from Global Media Instruments using a thermal head having a resolution of 300 dpi and an energy per unit area of 16 mJ / mm2(Atlantek Steps: 10) for Example 1-1 to Comparative Example 1-7. All samples were printed using Pattern 4.

[0156]

[0157] Oil Resistance For this test, the surface of each imaged test sample was evenly covered with commercially available vegetable oil (using a paper towel impregnated with oil) so that no streaks could be seen when the samples were viewed under reflective light. The samples were kept in the dark to protect them from light and left at room temperature (22°C) for 72 hours. Afterwards, the samples were immersed and swirled around in hexanes for about 15 seconds to eliminate the excess oil. Then, the samples were left again at room temperature to allow the hexanes to evaporate. The residual optical density and background density after oil exposure were then measured and recorded using an X-Rite Densitometer (Exact Xp Standard 2 mm aperture).

[0158] Test samples were imaged by using an Atlantek 400 printer from Global Media Instruments using a thermal head having a resolution of 300 dpi and an energy per unit area of 10.3 mJ / mm2(Atlantek Steps: 6) for Example 2-1 to Comparative Example 2-7 and Example 3-1 to 3-10. All samples were printed using Pattern 4.

[0159] Plasticizer Resistance For this test, a paper core measuring 7.62 cm in diameter by 25.4 cm in length was covered with one layer of PET film, and then covered with one layer of a film containing plasticizers (i.e., Resinite Meat Packing Film RMF-61AH, supplied by AEP Industries Inc.).

[0160] Imaged test samples were each placed on the film-wrapped paper core (thermal-sensitive layer side up), and an additional layer of the plasticizer film was applied over the sample. Each sample was then placed in between two layers of plasticizer film so that plasticizer could migrate from the top and bottom of the sample. The samples were placed in a dry oven at 40°C for 8 hours to accelerate plasticizer migration. Afterwards, the samples were immersed and swirled around in hexanes for about 15 seconds to eliminate the excess plasticizers. Then, the samples were left at room temperature to allow the hexanes to evaporate. Residual optical density and background density were then measured and recorded using an X-Rite Densitometer (Exact Xp Standard 2 mm aperture).

[0161] Image Retention Image retention percentage refers to the ratio between the initial average reading before testing and the image after the evaluation multiplied by 100 as follows: Image retention = (optical density after test / initial optical density) x 100Rating of Image Retention 0-59%: Poor 60-69%: Fair 70-79%: Good 80-89%: Very Good 90-100%: Excellent

[0162] Summary of Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-7 Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-7 were then tested for oil resistance, and the test results are shown in Table 2 below.

[0163]

[0164] As shown in Table 2, Examples 1-1 to 1-5, in which a first developer was used with a second developer and stabilizer, exhibited significantly better oil resistance than Comparative Examples 1-1 to 1-7, which do not use a first developer, a second developer, or a stabilizer.

[0165] Summary of Examples 2-1 to 2-10 and Comparative Examples 2-1 to 2-7 Examples 2-1 to 2-10 and Comparative Examples 2-1 to 2-7 were then tested for plasticizer resistance, and the test results are shown in Table 3 below.

[0166]

[0167] As shown in Table 3, Examples 2-1 to 2-10, in which a first developer was used with a second developer and stabilizer, exhibited significantly better plasticizer resistance than Comparative Examples 2-1 to 2-7, which do not use at least one of a second developer and a stabilizer.

[0168] Summary of Examples 3-1 to 3-10 Examples 3-1 to 3-10 were then tested for plasticizer resistance, and the test results are shown in Table 4 below.

[0169]

[0170] As shown in Table 4, Examples 3-1 to 3-10, in which the protective layer used boric acid, exhibited significantly better plasticizer resistance than Examples 2-1 to 2-10.

[0171] Although exemplary embodiments have been described, it would be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described can be made. All such changes, modifications, and alterations should therefore be seen as within the scope of the disclosure.

[0172] The present application is based on, and claims priority from, U.S. Provisional Patent Application No. 63 / 526,760, filed on July 14, 2023, the disclosure of which is hereby incorporated by reference herein in its entirety.

Claims

1. A phenol-free, heat-sensitive recording material that comprises: (a) a substrate; and in order from a side close to the substrate, (b) an undercoat layer; and (c) a thermal-sensitive layer; wherein the thermal-sensitive layer comprises: a first developer comprising a sulfonamide compound represented by formula (1) below: wherein R1and R2may be the same or different, and each independently represents a hydrogen atom, an alkyl group having from 1 to 4 carbons, an alkoxy group having from 1 to 4 carbons, or a halogen atom, one or more second developers comprising at least one phenyl ureide compound represented by formula (2) below: wherein R3and R4may be the same or different, and each independently represents a hydrogen atom, a halogen atom, a nitro group, an amino group, an alkyl group, an aryl 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 toluenesulfonyl group, a toluenesulfonyloxyphenyl group, a monoalkylamino group, a dialkylamino group, or an arylamino group, and one or more stabilizers selected from the group consisting of compounds represented by formula (3) and (4) below, and a derivative thereof.

2. The heat-sensitive recording material according to claim 1, wherein the second developers are at least one selected from the group consisting of 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate ((2)-1), N,N'-Di[3-(p-toluene sulfonyl)oxy]phenyl urea ((2)-2), and N-(4-methylphenylsulphonyl)-N’-(3-(4-methylphenylsulfonyloxy) phenyl) urea ((2)-3).

3. The heat-sensitive recording material according to claim 1 or 2, wherein the ratio of the second developers to the first developer is 0.02 to 1.2.

4. The heat-sensitive recording material according to claim 1 or 2, wherein the second developer is 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate or N,N'-Di[3-(p-toluene sulfonyl)oxy]phenyl urea.

5. The heat-sensitive recording material according to claim 1 or 2, wherein the second developers are N-(4-methylphenylsulphonyl)-N’-(3-(4-methylphenylsulfonyloxy) phenyl) urea, and at least one selected from the group consisting of 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate and N,N'-Di[3-(p-toluene sulfonyl)oxy]phenyl urea.

6. The heat-sensitive recording material according to claim 1 or 2, wherein the first developer is N-[2-(3-phenylureido) phenyl] benzene sulfonamide, the second developer is 3[(phenyl carbamoyl) amino] phenyl 4-methylbenzenesulfonate, the stabilizer is 1,3 diphenyl urea, and the content of the stabilizer is 5 to 35 parts by mass per 100 parts by mass of the total of the first developer and second developer.

7. The heat-sensitive recording material according to claim 1 or 2, wherein the heat-sensitive recording material comprises a protective layer including boric acid.

8. The heat-sensitive recording material according to claim 1 or 2, wherein the undercoat layer includes SBR latex that has a Tg temperature of -10°C or lower.

9. The heat-sensitive recording material according to claim 1 or 2, wherein the undercoat layer includes hollow particles that have a hollow ratio of 90% or more.

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