Thermal recording composition and thermal recording body

Abstract

To provide a thermal recording composition that exhibits uniform thermal sensitivity even when applied to a large area, and has excellent whiteness (opacity) before thermal printing and visibility after thermal printing, and a thermal recording body comprising a thermal recording layer formed using the composition. [Solution] The above problem is solved by a thermal recording composition comprising resin fine particles (A) having a coefficient of variation (CV value) of 15% or more, at least one surfactant (B) selected from the group consisting of a nonionic surfactant with an HLB value of 3 to 20 and anionic surfactants with an HLB value of 3 to 20, and water.

Description

Technical Field

[0001] The present invention relates to a composition for forming a heat-sensitive recording medium and a heat-sensitive recording medium using the composition.

Background Art

[0002] Conventionally, as a heat-sensitive recording material, a method of recording a color-developed image by utilizing a heat-coloring reaction between a colorless or light-colored dye precursor and a phenol or an organic acid has been widely put into practical use. This method has the merit that it does not require a large-scale apparatus and can record in a short time and at a low cost. For example, it is widely used in the POS field such as retail stores and supermarkets, the facsimile field, the printer field, the automatic ticket vending machine field, the heat-sensitive recording type label field, the on-demand printing field, and the like. On the other hand, in recent years, there have been increasing concerns about the environmental compatibility of dye precursors and developers, and heat-sensitive recording materials that do not contain a developer have been developed. For example, in Patent Document 1, it is reported that a coating liquid for a light-scattering layer that contains a composition for a coloring layer, a predetermined amount of polymer particles having no hollow and a clarifying agent, and substantially does not contain a dye precursor and a developer is applied onto a support and dried to sequentially form a coloring layer and a light-scattering layer, thereby obtaining a heat-sensitive recording material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the light scattering layer described in Patent Document 1 does not combine resin fine particles with a CV value of 15% or more with a nonionic or anionic surfactant with an HLB value of 3 to 20, making it difficult to form a uniform light scattering layer when the light scattering layer covers a large area. As a result, the thermal sensitivity is not uniform throughout the light scattering layer, leading to problems such as poor whiteness (opacity) before thermal printing and poor visibility after thermal printing. Therefore, the present invention aims to provide a thermal recording composition that exhibits uniform thermal sensitivity even when applied to a large area, and has excellent whiteness (opacity) before thermal printing and visibility after thermal printing, and a thermal recording body comprising a thermal recording layer formed using the composition. [Means for solving the problem]

[0005] <1> The present invention relates to a thermal recording composition comprising resin fine particles (A) having a coefficient of variation (CV value) of 15% or more, at least one surfactant (B) selected from the group consisting of a nonionic surfactant with an HLB value of 3 to 20 and an anionic surfactant with an HLB value of 3 to 20, and water.

[0006] <2> The present invention relates to a nonionic surfactant (B) having an HLB value of 3 to 20. <1> This relates to the thermal recording composition described.

[0007] <3> The present invention relates to a surfactant (B) comprising at least one selected from the group consisting of polyglycerin fatty acid ester surfactants, acetylene surfactants, polyoxyalkylene alkyl ether surfactants, and siloxane surfactants. <1> or <2> This relates to the thermal recording composition described.

[0008] <4> The present invention provides that the content of the surfactant (B) is 0.1% by mass or more and 10% by mass or less, based on the mass of the thermal recording composition. <1> ~ <3> This relates to any of the thermal recording compositions described below.

[0009] <5> The present invention provides that the resin fine particles (A) are at least one selected from the group consisting of resin fine particles made of a single resin and core-shell type resin fine particles. <1> ~ <4> This relates to any of the thermal recording compositions described below.

[0010] <6> The present invention relates to resin fine particles (A) which are made of a single resin, and the glass transition temperature of the single resin is 40°C or higher and 130°C or lower. <1> ~ <5> This relates to any of the thermal recording compositions described below.

[0011] <7> The present invention relates to a resin microparticle (A) which is a core-shell type resin microparticle, wherein the glass transition temperature of the core portion of the core-shell type resin microparticle is 40°C or higher and 130°C or lower, and the glass transition temperature of the shell portion is -20°C or higher and 30°C or lower. <1> ~ <6> This relates to any of the thermal recording compositions described below.

[0012] <8> The present invention relates to a resin fine particle (A) whose average particle diameter is 150 nm or more and 2,000 nm or less. <1> ~ <7> This relates to any of the thermal recording compositions described below.

[0013] <9> The present invention relates to a polymer of an ethylenically unsaturated monomer (a) in which the resin fine particles (A) are <1> ~ <8> This relates to any of the thermal recording compositions described below.

[0014] <10> The present invention relates to a polymer of the ethylenically unsaturated monomer (a) being an emulsion polymer using a reactive surfactant. <9> This relates to the thermal recording composition described.

[0015] <11> The present invention, on a substrate, <1> ~ <10> This relates to a thermal recording body comprising a thermal recording layer formed using any of the thermal recording compositions described above.

[0016] <12> The present invention further comprises a primer layer between the substrate and the thermal recording layer. <11> Regarding the thermal recording medium described. [Effects of the Invention]

[0017] According to the present invention, even when applied over a large area, a heat-sensitive recording composition having uniform heat sensitivity, excellent whiteness (opacity) before heat printing, and excellent visibility after heat printing, and a heat-sensitive recording medium including a heat-sensitive recording layer formed using the composition can be provided.

Embodiments for Carrying Out the Invention

[0018] <<Heat-Sensitive Recording Composition>> The heat-sensitive recording composition of the present invention is characterized by containing resin fine particles (A) having a coefficient of variation (CV value) of 15% or more, a nonionic surfactant having an HLB value of 3 to 20, and at least one surfactant (B) selected from the group consisting of anionic surfactants having an HLB value of 3 to 20, and water. By combining resin fine particles (A) having a predetermined CV value and a nonionic or anionic surfactant (B) having a predetermined HLB value, the heat sensitivity can be made uniform even when the coating area is increased. Thereby, a heat-sensitive recording medium excellent in whiteness (opacity) before heat printing and visibility after heat printing can be obtained regardless of the coating area.

[0019] <Resin Fine Particles (A)> The resin fine particles (A) only need to have a coefficient of variation (CV value) of 15% or more, and the type and production method are not particularly limited. The resin fine particles (A) may be at least one selected from the group consisting of resin fine particles composed of a single resin and core-shell type resin fine particles (core layer: inner layer, shell layer: outer layer), may be used alone, or may be used in combination of two or more. The resin fine particles (A) exist in the form of a dispersion in the heat-sensitive recording composition. And in the process of coating and drying on a substrate, they are laminated as water volatilizes to form a light-scattering layer with opacity (hereinafter also referred to as a heat-sensitive recording layer).

[0020] [Properties of Resin Fine Particles (A)] The average particle diameter of the resin fine particles is preferably 150 nm or more, more preferably 250 nm or more, still more preferably 350 nm or more, from the viewpoints of the dispersion stability of the recording composition, the hiding property of the heat-sensitive recording layer, and the visibility of the heat-sensitive printing portion. Also, it is preferably 2000 nm or less, more preferably 1000 nm or less, still more preferably 600 nm or less, and for example, it may be 150 to 2000 nm, 250 to 1000 nm, or 350 to 600 nm. The average particle diameter can be measured as the average value of the equivalent circle diameters of each particle by introducing the surface image of the heat-sensitive recording layer observed at a magnification of 20,000 times using a scanning electron microscope (SEM) ("JSM-7800F" manufactured by JEOL Ltd.) into image processing software ("Winroof" manufactured by Mitani Corporation) and extracting the spherical structure. It is important that the coefficient of variation (CV value) of the resin fine particles (A) constituting the heat-sensitive recording layer is 15% or more. By having a coefficient of variation of 15% or more, the heat-sensitive recording layer has excellent hiding property. The coefficient of variation can be calculated by the following formula using the values of the equivalent circle diameters obtained by the above method for measuring the average particle diameter. Formula: Coefficient of variation (CV value) (%) = (σn / Dn) × 100 σ: Standard deviation of the equivalent circle diameter D: Average value of the equivalent circle diameter Here, the equivalent circle diameter is the diameter of a perfect circle corresponding to the area of the figure recognized as a particle, and the units of the standard deviation and the average value of the equivalent circle diameter are the same.

[0021] The refractive index of the resin fine particles (A) is preferably 1.3 or more, more preferably 1.45 or more. Also, it is preferably 3.0 or less, more preferably 2.0 or less, and for example, it may be 1.3 to 3.0, or may be 1.45 to 2.0. When the refractive index is 1.3 or more, it is preferable because the whiteness of the coating film becomes good.

[0022] When the resin fine particles (A) are made of a single resin, the glass transition temperature (Tg) is preferably 40°C or higher, more preferably 70°C or higher, from the viewpoint of the heat and humidity resistance of the thermal recording material. Also, from the viewpoint of thermal sensitivity, it is preferably 130°C or lower, more preferably 100°C or lower, and may be, for example, 40 to 130°C or 70 to 100°C. When the resin particles (A) are core-shell type particles, the glass transition temperature of the shell portion is preferably 30°C or lower, more preferably 10°C or lower, from the viewpoint of the thermal sensitivity and printability of the thermal recording material. It is also preferably -20°C or higher, more preferably -10°C or higher, and may be, for example, -20 to 30°C or -10 to 10°C. The glass transition temperature of the core portion is preferably 40°C or higher, more preferably 70°C or higher. It is also preferably 130°C or lower, more preferably 120°C or lower, and may be, for example, 40 to 130°C or 70 to 120°C. The glass transition temperature can be measured by DSC (Differential Scanning Calorimeter). Specifically, the glass transition temperature can be obtained by weighing approximately 2 mg of the dried sample on an aluminum pan, setting the aluminum pan in a DSC measuring holder, and reading the chart obtained under a heating condition of 5°C / min.

[0023] The resin fine particles (A) are preferably polymers of ethylenically unsaturated monomers (a), and more preferably acrylic resins or styrene-acrylic resins, from the viewpoint of easy control of the coefficient of variation and average particle size.

[0024] [Polymers of ethylenically unsaturated monomers (a)] Resin nanoparticles, which are polymers of ethylenically unsaturated monomer (a), can be produced by conventionally known methods, for example, by emulsion polymerization as described below. (Emulsification polymerization) First, an aqueous medium and a surfactant are placed in the reaction vessel and heated to a predetermined temperature. Meanwhile, water, an emulsifier, and ethylenically unsaturated monomer (a) are placed in the dropping vessel and stirred to prepare an emulsion of ethylenically unsaturated monomer (a). Then, under a nitrogen atmosphere, the prepared emulsion is added dropwise to the reaction vessel while a radical polymerization initiator is added. After the reaction starts, polymer particle nuclei are formed, and the particles gradually grow to form resin nanoparticles.

[0025] (Ethylene-unsaturated monomer (a)) Examples of the above ethylenically unsaturated monomer (a) include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, m-methylstyrene, vinylnaphthalene, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxytetraethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, and phenyl (meth)acrylate. Aromatic ethylenically unsaturated compounds such as lylates; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, heptyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate , linear or branched alkyl-containing ethylenically unsaturated monomers such as tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, behenyl (meth)acrylate; cyclohexyl (meth)acrylate, isovonyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentanyl (meth)acrylate , ethylene-unsaturated monomers containing alicyclic alkyl groups such as adamantyl (meth)acrylate; ethylene-unsaturated monomers containing fluorinated alkyl groups such as trifluoroethyl (meth)acrylate and heptadecafluorodecyl (meth)acrylate; ethylenically unsaturated monomers containing carboxyl groups such as (anhydrous) maleic acid, fumaric acid, itaconic acid, citraconic acid, or their alkyl or alkenyl monoesters, β-(meth)acryloxyethyl monoester succinate, acrylic acid, methacrylic acid, crotonic acid, and cinnamic acid;2-acrylamide, sodium methylpropanesulfonate, methallyl sulfonic acid, sodium methallyl sulfonic acid, sodium methallyl sulfonate, allyl sulfonic acid, sodium allyl sulfonate, ammonium allyl sulfonate, vinyl sulfonic acid, and other sulfo-containing ethylenically unsaturated monomers; (meth)acrylamide, N-methoxymethyl-(meth)acrylamide, N-ethoxymethyl-(meth)acrylamide, N-propoxymethyl-(meth)acrylamide, N-butoxymethyl-(meth)acrylamide, N-pentoxy Methyl-(meth)acrylamide, N,N-di(methoxymethyl)acrylamide, N-ethoxymethyl-N-methoxymethylmethacrylamide, N,N-di(ethoxymethyl)acrylamide, N-ethoxymethyl-N-propoxymethylmethacrylamide, N,N-di(propoxymethyl)acrylamide, N-butoxymethyl-N-(propoxymethyl)methacrylamide, N,N-di(butoxymethyl)acrylamide, N-butoxymethyl-N-(methoxymethyl)methacrylamide, N,N-di(pentoxymethyl ethylenically unsaturated monomers containing amide groups such as (Cyl) acrylamide, N-methoxymethyl-N-(pentoxymethyl)methacrylamide, N,N-dimethylaminopropyl acrylamide, N,N-diethylaminopropyl acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, and diacetone acrylamide; ethylenically unsaturated monomers containing hydroxyl groups such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerol mono(meth)acrylate, 4-hydroxyvinylbenzene, 1-ethynyl-1-cyclohexanol, and allyl alcohol; ethylenically unsaturated monomers containing polyoxyethylene groups such as methoxypolyethylene glycol (meth)acrylate and polyethylene glycol (meth)acrylate; ethylenically unsaturated monomers containing amino groups such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, methylethylaminoethyl (meth)acrylate, dimethylaminostyrene, and diethylaminostyrene;Epoxy group-containing ethylenically unsaturated monomers such as glycidyl (meth)acrylate and 3,4-epoxycyclohexyl (meth)acrylate; ketone group-containing ethylenically unsaturated monomers such as diacetone (meth)acrylamide, acetoacetoxy (meth)acrylate, and 2-acetoacetoxyethyl (meth)acrylate; allyl (meth)acrylate, 1-methylallyl (meth)acrylate, 2-methylallyl (meth)acrylate, 1-butenyl ( Meth)acrylate, 2-butenyl(meth)acrylate, 3-butenyl(meth)acrylate, 1,3-methyl-3-butenyl(meth)acrylate, 2-chlorallyl(meth)acrylate, 3-chlorallyl(meth)acrylate, o-allylphenyl(meth)acrylate, 2-(allyloxy)ethyl(meth)acrylate, allyllactyl(meth)acrylate, citronellyl(meth)acrylate, geranyl(meth)acrylate, Ethylene-unsaturated monomers having two or more ethylenically unsaturated groups, such as rhodinyl (meth)acrylate, cinnamyl (meth)acrylate, diallyl maleate, diaryl lutaconic acid, vinyl (meth)acrylate, vinyl crotate, vinyl oleate, vinyl linolenate, 2-(2'-vinyloxyethoxy)ethyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol (meth)acrylate, tetraethylene glycol (meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, 1,1,1-trishydroxymethylethane diacrylate, 1,1,1-trishydroxymethylethane triacrylate, 1,1,1-trishydroxymethylpropane triacrylate, divinylbenzene, divinyl adipate, diallyl isophthalate, diallyl phthalate, diallyl maleate, etc.;Examples include alkoxysilyl group-containing ethylenically unsaturated monomers such as γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-methacryloxypropyltributoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ-acryloxypropyltrimethoxysilane, γ-acryloxypropyltriethoxysilane, γ-acryloxypropylmethyldimethoxysilane, γ-methacryloxymethyltrimethoxysilane, γ-acryloxymethyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltributoxysilane, and vinylmethyldimethoxysilane; and methylol group-containing ethylenically unsaturated monomers such as N-methylol(meth)acrylamide, N,N-dimethylol(meth)acrylamide, and alkyl etherified N-methylol(meth)acrylamide.

[0026] These monomers may be used individually or in combination of two or more. The ethylenically unsaturated monomer (a) preferably contains at least one selected from the group consisting of styrene, n-butyl acrylate, divinylbenzene, and methacrylic acid, from the viewpoint of controlling the glass transition temperature (Tg), which will be described later, to a suitable range.

[0027] The resin fine particles (A) may have reactive groups for forming crosslinks, and to obtain such resin fine particles (A), an ethylenically unsaturated monomer having reactive groups may be used as the ethylenically unsaturated monomer (a). The presence of reactive groups in the resin fine particles (A) introduces a crosslink structure within the thermal recording layer and between the thermal recording layer and the primer layer described later, thereby improving the printability of the thermal recording layer.

[0028] Crosslinking within the thermal recording layer can be introduced by reacting the reactive groups of resin microparticles (A) with each other, or by reacting the reactive groups of resin microparticles (A) via a polyfunctional crosslinking agent. Crosslinking between the thermal recording layer and the primer layer described later can be introduced by reacting the reactive groups of resin microparticles (A) with the reactive groups of the primer layer, or by crosslinking the reactive groups of resin microparticles (A) and the primer layer via a polyfunctional crosslinking agent, etc.

[0029] The reactive groups that the resin fine particles (A) may have are preferably epoxy groups, carboxyl groups, hydroxyl groups, ketone groups, and hydrazide groups, with ketone groups being more preferred. In particular, when the reactive group is a ketone group and the crosslinking agent is a hydrazide crosslinking agent, a ketone-hydrazide crosslink can be formed. Ketone-hydrazide crosslinking is suitable because it does not adversely affect the physical properties of the thermal recording layer and can form crosslinks at low temperatures and in a short time by the evaporation of water, and is effective when using a film substrate that is easily damaged by heating. Furthermore, because ketone groups have high hydrophilicity, when an ethylenically unsaturated monomer having a ketone group is used in the copolymer composition, the ketone group is introduced to the outside of the resin fine particles (A), i.e., near the interface with the aqueous medium, and it is thought that crosslinking with the hydrazide crosslinking agent can be efficiently formed.

[0030] When the resin fine particles (A) have ketone groups, the preferred content of ketone groups is in the range of 0.05 to 0.3 mmol / g, based on the mass of the resin fine particles (A). By introducing ketone groups in the range of 0.05 to 0.3 mmol / g, the film strength of the thermal recording layer is further improved, and the printability is further improved, without adversely affecting the opacity.

[0031] (Radical polymerization initiator) In the polymerization reaction of ethylenically unsaturated monomer (a), it is preferable to use a radical polymerization initiator. As the radical polymerization initiator, known oil-soluble polymerization initiators or water-soluble polymerization initiators can be used, and these may be used individually or in combination of two or more.

[0032] The oil-soluble polymerization initiator is not particularly limited and includes, for example, organic peroxides such as benzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy(2-ethylhexanoate), tert-butyl peroxy-3,5,5-trimethylhexanoate, and di-tert-butyl peroxide; and azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitride. In emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. Suitable water-soluble polymerization initiators include conventionally known ones such as ammonium persulfate (APS), potassium persulfate (KPS), hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride.

[0033] (emulsifier) When resin fine particles (A) are produced by emulsion polymerization, emulsifiers such as surfactants and polymer dispersants can be used. By using emulsifiers, the storage stability of the resin fine particles (A) and the heat and humidity resistance of the heat-sensitive recording layer can be improved. A reactive surfactant is preferred as the surfactant, and the polymer of the ethylenically unsaturated monomer (a) is preferably an emulsion polymer using a reactive surfactant. Reactive surfactants include anionic and nonionic surfactants, and more specifically, anionic reactive surfactants, anionic nonreactive surfactants, nonionic reactive surfactants, and nonionic nonreactive surfactants, with anionic reactive surfactants being preferred. These surfactants may be used individually or in combination of two or more types.

[0034] Here, a reactive surfactant refers to a surfactant that can polymerize with the ethylenically unsaturated monomer (a) described above. More specifically, it means a surfactant that has a reactive group capable of polymerizing with an ethylenically unsaturated bond. Examples of reactive groups include vinyl groups, allyl groups, alkenyl groups such as 1-propenyl groups, and (meth)acryloyl groups. The use of a reactive surfactant is preferable because it reduces the amount of free emulsifier components that adversely affect the heat and humidity resistance of the thermal recording layer, thereby further improving the long-term stability of the thermal recording layer.

[0035] (Anionic surfactant) Examples of anionic reactive surfactants include polyoxyethylene alkyl ether sulfates (commercial products include Aqualon KH-05, KH-10, KH-20 from Daiichi Kogyo Seiyaku Co., Ltd., Adekaria Soap SR-10N, SR-20N from ADEKA Corporation, and Latemul PD-104 from Kao Corporation); polyoxyalkylene styrene phenyl ether sulfates (commercial products include Aqualon AR-10, AR-20 from Daiichi Kogyo Seiyaku Co., Ltd.); sulfosuccinate esters (commercial products include Latemul S-120, S-120A, S-180P, S-180A from Kao Corporation, and Eleminol JS-2 from Sanyo Chemical Industries, Ltd.); and polyoxyethylene alkylphenyl ether sulfates. Examples include polyoxyethylene alkylphenyl sulfate systems (commercial products include Aqualon HS-10, HS-20, HS-30, BC-10, BC-20 manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and Adekarya Soap SDX-222, SDX-223, SDX-232, SDX-233, SDX-259, SE-10N, SE-20N manufactured by ADEKA Corporation); (meth)acrylate sulfate systems (commercial products include Antox MS-60, MS-2N manufactured by Nippon Surfactants Co., Ltd., and Eleminor RS-30 manufactured by Sanyo Chemical Industries, Ltd.); and phosphate ester systems (commercial products include H-3330PL manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and Adekarya Soap PP-70 manufactured by ADEKA Corporation).

[0036] Examples of anionic, nonreactive surfactants include higher fatty acid salts such as sodium oleate; alkylaryl sulfonates such as sodium dodecylbenzenesulfonate; alkyl sulfate esters such as sodium lauryl sulfate; and polyoxyethylene alkyl ether sulfates such as sodium polyoxyethylene lauryl ether sulfate (commercial products include Hightenol LA-10, LA-12, LA-16 manufactured by Daiichi Kogyo Seiyaku).

[0037] (Nonionic surfactant) Examples of nonionic reactive surfactants include polyoxyalkylene alkyl ethers (commercial products include Adekarya Soap ER-10 (HLB 12.3), ER-20 (HLB 15.1), ER-30 (HLB 16.4), ER-40 (HLB 17.1) manufactured by ADEKA Corporation, and Latemul PD-420 (HLB 12.6), PD-430 (HLB 14.4), PD-450 (HLB 16.2) manufactured by Kao Corporation; polyoxyalkylene styrene phenyl ethers (commercial products include Aqualon AN-10 (HLB 13.0), AN-20 (HLB 18.0) manufactured by Daiichi Kogyo Seiyaku Co., Ltd.; and polyoxyethylene alkylphenyl ethers (commercial products include Aqualon RN-20 (HLB 14.2), RN-30 (HLB 16.7) manufactured by Daiichi Kogyo Seiyaku Co., Ltd.).

[0038] Examples of nonionic, non-reactive surfactants include polyoxyalkylene alkyl ethers (commercially available products include: Noigen XL-50 (HLB 11.6), XL-100 (HLB 14.7), XL-1000 (HLB 19.3), TDS-50 (HLB 10.5), TDS-70 (HLB 12.1), TDS-80 (HLB 13.3), TDS-120 (HLB 14.8), Noigen TDX-80 (HLB 13.1), TDX-140 (HLB 14.4) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and Emulgen 106 (HLB 10.5), 108 (HLB 12.1), 1108 (HLB 13.5), 1135S-7 manufactured by Kao Corporation). 0 (HLB 17.9), etc.; Polyoxyalkylene styrene phenyl ethers (commercial products include Neugen EA-87 (HLB 87), EA-127 (HLB 11.7), EA-157 (HLB 14.3), Kao Corporation's Emulgen A-60 (HLB 12.8), A-90 (HLB 14.5), A-500 (HLB 18.0), etc.); Polyoxyethylene alkylphenyl ethers (commercial products include Aoki Oil & Fat Industry Co., Ltd.'s Branown N-505 (HLB 10.0), Branown NK-8055 (HLB 10.8), etc.); Polyglycerin fatty acid esters (commercial products include Nikko Chemicals Co., Ltd.'s NIKKOL Hexaglyn1-L (HLB 14.5), Decaglyn-1-L (HLB 15.5), Decaglyn-1-M (HLB 14.0), Decaglyn-1-LN (HLB 12.0), etc.; Polyoxyethylene glycerin fatty acid esters (commercial products include NIKKOL TMGS-5V (HLB 9.5), TMGS-15V (HLB 13.5), TMGO-5 (HLB 9.5), TMGO-15 (HLB 14.5), etc., manufactured by Nikko Chemicals Co., Ltd.); Polyoxyethylene sorbitan fatty acid esters (commercial products include NIKKOL, manufactured by Nikko Chemicals Co., Ltd.) TL-10 (HLB 16.9), TP-10EX (HLB 15.6), TS-10V (HLB 14.9), TS-30V (HLB 10.5), TO-10V (HLB 15.0), TO-106V (HLB 10.0), Kao Corporation's Leodor TW-L120 (HLB 16.7), TW-L106 (HLB 13.3), TW-P120 (HLB 15.6), TW-S120V (HLB 14.9), TW-S-106 (HLB 9.5), TW-O-106 (HLB 10.0) etc; Polyoxyethylene sorbitol fatty acid esters (Commercial products include NIKKOL GL-1 (HLB 15.5), GS-460 (HLB 13.0), GO-440V (HLB 12.5), GO-460V (HLB 14.0) manufactured by Nikko Chemicals Co., Ltd., and Leodol 430V (HLB 10.5), 440V (HLB 11.8), 460V (HLB 13.8) manufactured by Kao Corporation) Polyethylene glycol fatty acid esters (Commercial products include EMALEX-810 (HLB 11.0), 820 (HLB 14.0), 830 (HLB 15.0), 840 (HLB 16.0) manufactured by Nippon Emulsion Co., Ltd., and Emanone 1112 (HLB 13.7) and Emanone 31 99V (HLB 19.4), etc.; Sucrose fatty acid esters (commercial products include Mitsubishi Chemical Foods' S-970 (HLB 9.0), S-1170 (HLB 11.0), S-1570 (HLB 15.0), S-1670 (HLB 16.0), P-1570 (HLB 15.0), P-1670 (HLB 16.0), M-1695 (HLB 16.0), O-1570 (HLB 15.0), L-1695 (HLB 16.0), LWA-1570 (HLB 15.0), etc.); Polyoxyethylene lanolins (commercial products include Nikko Chemicals' NIKKOL TW-10 (HLB 12.0), TW-20 (HLB 13.0), BWA-5 (HLB 12.5), BWA-10 (HLB 15.5), BWA-20 (HLB 16.0), etc.; Polyoxyethylene hydrogenated castor oils (commercial products include NIKKOL HCO-20 (HLB 10.5), HCO-30 (HLB 11.0), HCO-40 (HLB 12.5), HCO-100 (HLB 16.5), etc., manufactured by Nikko Chemicals Co., Ltd.); Polyoxyethylene sterols (commercial products include NIKKOL HCO-20 (HLB 10.5), HCO-30 (HLB 11.0), HCO-40 (HLB 12.5), HCO-100 (HLB 16.5), etc., manufactured by Nikko Chemicals Co., Ltd.) Examples include BPS-5 (HLB 9.5), BPS-10 (HLB 12.5), BPS-20 (HLB 30), BPSH-25 (HLB 14.5), etc.; acetylenes (commercially available examples include Evonik's SURFYNOL 420 (HLB 4.0)); and siloxanes (commercially available examples include Shin-Etsu Silicone Co., Ltd.'s KF-351A (HLB 12.0)).

[0039] (Polymer dispersant) Examples of polymer dispersants include polyvinyl alcohol, polyvinylpyrrolidone, polyoxyethylene-polyoxypropylene block copolymer, (meth)acrylic acid-(meth)acrylate alkyl ester copolymer, styrene-(meth)acrylic acid-(meth)acrylate alkyl ester copolymer, styrene-(meth)acrylic acid copolymer, maleic acid-(meth)acrylate alkyl ester copolymer, styrene-maleic acid copolymer, styrene-maleic acid-(meth)acrylate alkyl ester copolymer, styrene-maleic acid half-ester copolymer, vinylnaphthalene-(meth)acrylic acid copolymer, vinylnaphthalene-maleic acid copolymer, vinylpyrrolidone-(meth)acrylate alkyl ester copolymer, vinylpyrrolidone-styrene copolymer, vinylpyrrolidone-vinyl acetate copolymer, vinyl acetate-crotonic acid copolymer, vinyl acetate-(meth)acrylic Examples include water-soluble vinyl copolymers such as acid copolymers, vinyl acetate-crotonic acid copolymers, polyvinyl sulfonic acid, sodium polyvinyl sulfonate, polystyrene sulfonic acid, sodium polystyrene sulfonate (e.g., Polinas PS-1 and Polinas PS-5 manufactured by Tosoh Corporation), styrene sulfonic acid-maleic acid copolymers, polyitaconic acid, polyhydroxyethyl (meth)acrylate, poly(meth)acrylamide, (meth)acrylamide-(meth)acrylic acid copolymers, polyvinyl methyl ether, methyl vinyl esters, and carboxyvinyl polymers; water-soluble polyurethane resins obtained by polyaddition reaction of polyisocyanate and polyol, in which the entire resin is water-soluble by the introduction of hydrophilic groups; and water-soluble polyester resins obtained by polycondensation reaction of polycarboxylic acid and polyol, in which the entire resin is water-soluble by the introduction of hydrophilic groups.

[0040] When producing resin fine particles (A) by emulsion polymerization, the emulsion polymerization may use a two-stage polymerization method in which the monomer composition is changed between the first and second stages and added dropwise, or a multi-stage polymerization method in which the monomer composition is changed between three or more stages and added dropwise.

[0041] The content of resin fine particles (A) is preferably 20% by mass or more, more preferably 35% by mass or more, based on the total mass of the thermal recording composition. It is also preferably 60% by mass or less, more preferably 50% by mass or less, and may be, for example, 20 to 60% by mass, or 35 to 50% by mass.

[0042] <Surfactant (B)> It is important that the thermal recording composition of the present invention contains at least one surfactant (B) selected from the group consisting of nonionic surfactants with an HLB value of 3 to 20 and anionic surfactants with an HLB value of 3 to 20. By including a surfactant (B) having such a specific HLB value and structure, when used in combination with resin fine particles (A), the leveling properties of the thermal recording composition to the substrate are enhanced, resulting in a uniform thermally sensitive coating film without unevenness or sensitivity differences throughout the entire film, even when large-area coating or high-speed coating is performed. As a result, a thermal recording material with high visibility after thermal printing is obtained. The HLB value of surfactant (B) is preferably 5 or more and 15 or less, for example, 5 to 15.

[0043] The HLB value is a numerical representation of the hydrophilicity / lipophilicity of a material, with a smaller HLB value indicating higher lipophilicity. In this specification, the HLB value is calculated using the Griffin method formula shown in the following formula (1). Formula (1) HLB value = 20 × (sum of formula weights of hydrophilic parts) ÷ (molecular weight of material)

[0044] As surfactant (B), those with an HLB value of 3.0 to 20.0 from among the above-mentioned (anionic surfactants) and (nonionic surfactants) can be used.

[0045] The surfactant (B) is preferably nonionic, from the viewpoint of ensuring uniform electrostatic repulsion between resin fine particles (A) and obtaining a coating film with less uneven coating and more uniform thermal sensitivity. Furthermore, the surfactant (B) preferably comprises at least one selected from the group consisting of polyglycerin fatty acid ester surfactants, acetylene surfactants, polyoxyalkylene alkyl ether surfactants, and siloxane surfactants, from the viewpoint of improving uneven coating of the thermal recording composition and enhancing visibility, as described in claim 1.

[0046] The content of surfactant (B) is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, based on the total mass of the thermal recording composition. It is also preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, for example, it may be 0.1 to 10% by mass, 0.3 to 5% by mass, or 0.5 to 3% by mass. By having the surfactant (B) content within the above range, uneven coating of resin fine particles (A) is suppressed and visibility is improved.

[0047] <Water> The thermal recording composition of the present invention contains water. The water may be included when obtaining the resin fine particles (A) as a dispersion, or it may be added when preparing the composition. The water content is preferably 40 to 70% by mass, and more preferably 50 to 60% by mass, based on the total mass of the thermal recording composition. The viscosity of the thermal recording composition is preferably 3.0 mPa·s to 200 mPa·s from the viewpoint of coating suitability.

[0048] <Other ingredients> [Crosslinking agent] As described above, the thermal recording composition of the present invention may contain a crosslinking agent in order to form crosslinks within the thermal recording layer and between the thermal recording layer and the primer layer described later. The crosslinking agent is not particularly limited and can be appropriately selected depending on the reactive groups present in the thermal recording layer and / or primer layer. Examples include hydrazide compounds (polyhydrazides) having two or more hydrazino groups that react with active carbonyl groups to form keto-hydrazide crosslinks; isocyanate compounds that react with hydroxyl groups or amino groups to form urethane or urea bonds; and epoxy compounds that react with carboxyl groups, amino groups, etc., which can be appropriately selected depending on the application. More specifically, for example, if the components constituting the resin microparticles (A) and / or the primer layer have carboxyl groups, crosslinking can be formed via an epoxy crosslinking agent. Also, for example, if the components constituting the resin microparticles (A) and / or the primer layer have hydroxyl groups, crosslinking can be formed via a polyisocyanate crosslinking agent. Furthermore, for example, if the components constituting the resin microparticles (A) and / or the primer layer have ketone groups, crosslinking can be formed via a hydrazide crosslinking agent. As mentioned above, it is preferable to use a hydrazide crosslinking agent to form ketone-hydrazide crosslinks. Examples of hydrazide crosslinking agents include adipic acid dihydrazide and water-soluble resins modified with polyfunctional hydrazide groups.

[0049] [Additives] The thermal recording composition may contain various additives as long as they do not impair the effects of the present invention. For example, it may contain hydrophilic organic solvents, binder resins, film-forming aids, lubricants, plasticizers, antifreezes, curing agents, buffers, neutralizing agents, rheology modifiers, humectants, wetting agents, defoamers, leveling agents, thickeners, preservatives, UV absorbers, fluorescent whitening agents, light or heat stabilizers, antioxidants, colorants, dispersants, conductive particles, inorganic pigments, hollow resin particles, and hollow inorganic particles.

[0050] <<Thermal recording material>> The thermal recording body of the present invention comprises a thermal recording layer formed on a substrate using the thermal recording composition described above. The thermal recording layer is formed, for example, by coating and drying the thermal recording composition on the substrate. The thermal recording body may further include a primer layer between the substrate and the thermal recording layer. If a primer layer is included, for example, a primer layer may be formed by coating and drying a primer layer-forming composition described later on the substrate, and the thermal recording layer may be formed on the primer layer. By including a primer layer, the thermal recording composition can be uniformly coated onto substrates with poor wettability.

[0051] The means by which the thermal recording layer and the primer layer are formed using the thermal recording composition and the primer layer forming composition are not particularly limited, and may include coating methods such as bar coating, dipping, and calendering; and printing methods such as gravure printing, flexographic printing, or inkjet printing. The thickness of the thermal recording layer is not particularly limited, but from the viewpoint of coating properties and cost, it is preferably in the range of 1.0 to 20 μm. By being within this range, excellent coating or printability and whiteness (opacity) are sufficiently ensured, and printed materials with excellent printability can be obtained.

[0052] <Base material> The substrate is not particularly limited and can be appropriately selected depending on the application. Examples include paper substrates such as coated paper; cloth substrates; thermoplastic resin substrates such as polyvinyl chloride sheets, polyethylene terephthalate (PET) film, polypropylene film, polyethylene film, nylon film, polystyrene film, and polyvinyl alcohol film; metal substrates such as aluminum foil; and glass substrates.

[0053] From the viewpoint of thermal recording applications, it is preferable that at least the surface of the substrate that comes into contact with the thermal recording layer is colored. Examples of such substrates include substrates having a colored layer and colored substrates, and the surface to which the thermal recording composition is applied may be smooth or uneven, and may be transparent, translucent, or opaque. The substrate may be a single type, or it may be a laminate formed by bonding two or more types of substrates together.

[0054] <Primer layer> In order to further improve the adhesion of the thermal recording layer to the substrate, it is preferable that the thermal recording material includes a primer layer between the substrate and the thermal recording layer, in contact with the thermal recording layer. The primer layer can be formed, for example, by pre-applying a primer layer-forming composition to a substrate. The primer layer-forming composition is not particularly limited and includes, for example, acrylic resins, styrene-acrylic resins, urethane resins, olefin resins, polyester resins, and composite resins obtained by compounding these resins. These resins may be used individually or in combination of two or more. The primer layer-forming composition preferably contains one or more resins selected from the group consisting of acrylic resin, styrene-acrylic resin, and urethane resin, from the viewpoint of excellent adhesion to the substrate and thermal recording layer, and the resistance of the primer layer.

[0055] The primer layer may contain additives as long as they do not impair the effects of the present invention. Examples of such additives include surfactants, achromatic black fine particles, and neutralizing amines. Examples of surfactants can be found in the description of <Surfactant (B)> above. The primer layer may also contain an antiblocking agent from the viewpoint of suppressing blocking. Examples of antiblocking agents include fatty acid amides or silica particles.

[0056] <Thermal printing> Examples of thermal printing (heat treatment) methods include: using a thermal printer to heat the thermal recording layer by applying a thermal head; irradiating with laser light to cause the photothermal conversion material in the thermal recording body to absorb the light and heat the resin fine particles in the adjacent thermal recording layer; and oven heating, microwave heating, and boiling. Among these, the method using a thermal printer is preferred because it does not require large-scale equipment and allows for easy thermal recording. Image formation methods using laser light are preferred because they allow image formation without damaging the substrate, primer layer, and non-image-forming areas. Furthermore, infrared lasers are preferred because they cause minimal damage to the substrate, the resin forming the primer layer, and the resin microparticles (A). Examples of infrared laser markers include CO2 laser markers (wavelength 10600 nm), YVO4 laser markers (wavelength 1064 nm), YAG laser markers (wavelength 1064 nm), and fiber laser markers (wavelength 1090 nm). [Examples]

[0057] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the examples and comparative examples, "parts" and "%" mean "parts by mass" and "% by mass" respectively, unless otherwise specified.

[0058] <Average particle size and coefficient of variation (CV) value> The average particle diameter was measured using a scanning electron microscope (SEM) (JEOL "JSM-7800F") to observe the surface image of the thermal recording layer at a magnification of 20,000x. This image was then imported into image processing software (Mitani Corporation "Winroof") to extract the spherical structure, and the average value of the equivalent circle diameter of each particle was calculated. The coefficient of variation (CV value) of the equivalent circle diameter was calculated using the equivalent circle diameter values ​​obtained by the above measurement method, according to the following formula. Coefficient of variation of the equivalent diameter of a circle (%) = (σn / Dn) × 100 σ: Standard deviation of the equivalent diameter of the circle D: Average value of the equivalent diameter of a circle [The equivalent diameter of a circle is the diameter of a true circle that corresponds to the area of ​​the shape recognized as a particle.]

[0059] <Glass transition temperature (Tg)> The glass transition point was measured using a DSC (Differential Scanning Calorimeter, manufactured by TA Instruments). Specifically, approximately 2 mg of the dried sample was weighed onto an aluminum pan, the aluminum pan was placed in a DSC measuring holder, and the chart obtained under a heating condition of 5°C / min was read to obtain the glass transition point.

[0060] <Manufacturing of aqueous dispersion of resin fine particles composed of a single resin> [Production example 1-1] Resin fine particles (A1-1) An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 40 parts styrene, 38 parts methyl methacrylate, 20 parts n-butyl acrylate, 2 parts methacrylic acid, 4.8 parts 25% aqueous solution of Aqualon AR-10 (1.2 parts solids), and 43.2 parts deionized water. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 64.8 parts deionized water and 0.59 parts of the divided emulsion (b) separated from the above emulsion were added. After raising the internal temperature to 80°C and thoroughly purging with nitrogen, 2.5 parts 2.5% aqueous solution of potassium persulfate (KPS) (0.06 parts solids) was added as an initiator to start emulsion polymerization. While maintaining the internal temperature at 80°C, the remainder of the above emulsion and 7.5 parts 2.5% aqueous solution of potassium persulfate (0.19 parts solids) were added dropwise over 3 hours, and the mixture was reacted for a further 4 hours to obtain an aqueous dispersion of styrene acrylic resin. After the reaction was complete, 1.5 parts of 25% aqueous ammonia were added to neutralize the mixture, and the solid content of the aqueous dispersion was adjusted to 45.0% with deionized water to obtain an aqueous dispersion of resin microparticles (A1-1). The average particle size of the obtained resin microparticles (A1-1) was 351 nm, the CV value was 42.2%, and the glass transition temperature (Tg) was 57°C.

[0061] [Production example 1-2~1-10] Resin fine particles (A1-2~A1-10) An aqueous dispersion of resin fine particles (A1-2~10) was obtained in the same manner as in Production Example 1-1, except that the compound composition was changed as shown in Table 1.

[0062] <Preparation of core-shell type resin fine particle dispersion> [Production example 2-1] Resin fine particles (A2-1) A mixture of 29 parts styrene, 20 parts methyl methacrylate, 1 part methacrylic acid, 1.5 parts 25% aqueous solution of Aqualon AR-10 (0.37 parts solids), and 25.9 parts deionized water was mixed and stirred to prepare the first-stage emulsion of ethylenically unsaturated monomers. 75.8 parts deionized water and 0.23 parts of the first-stage emulsion were added to a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux valve. After raising the internal temperature of the reaction vessel to 70°C and thoroughly purging it with nitrogen, polymerization was started by adding 4.0 parts 2.5% aqueous solution of potassium persulfate (0.10 parts solids) as an initiator. While maintaining the internal temperature at 80°C, the remaining emulsion was reacted with 3.6 parts 2.5% aqueous solution of potassium persulfate (0.09 parts solids) dropwise over 2 hours to synthesize core particles. Next, 11 parts styrene, 18 parts methyl methacrylate, 20 parts n-butyl acrylate, 1 part methacrylic acid, 2.6 parts 25% aqueous solution of Aqualon AR-10 (0.65 parts solids), and 6.8 parts deionized water were mixed and stirred to prepare the second stage emulsion of ethylenically unsaturated monomers. Twenty minutes after the completion of the first stage addition, the addition of the second stage emulsion was started. While maintaining the internal temperature at 80°C, the reaction proceeded by adding the second stage emulsion and 1.2 parts 2.5% aqueous solution of potassium persulfate (0.03 parts solids) dropwise over 2 hours to obtain an aqueous dispersion of resin fine particles. After the reaction, water was added to adjust the solid content to 45.0%. Then, 0.79 parts of 25% aqueous ammonia were added to neutralize the resin microparticles and obtain an aqueous dispersion of resin microparticles (A2-1). The average particle size of the obtained resin microparticles (A2-1) was 362 nm, the CV value was 32.5%, and the glass transition temperatures (Tg) of the core and shell were 105°C and 23°C, respectively.

[0063] [Production Examples 2-2~2-4] Resin fine particles (A2-2~A2-4) An aqueous dispersion of resin fine particles (A2-2 to A2-4) was obtained in the same manner as in Production Example 2-1, except that the compound composition was changed as shown in Table 2.

[0064] [Table 1]

[0065] [Table 2]

[0066] The abbreviations used in Tables 1 and 2 are shown below. St: Styrene BzMA: Benzyl methacrylate MMA: Methyl methacrylate BA: n-butyl acrylate 2EHA:2-Ethylhexylacrylate CHMA: Cyclohexyl methacrylate AA: Acrylic acid MAA: Methacrylic acid AAm: Acrylamide 2HEMA:2-hydroxyethyl methacrylate DM: Dimethylaminoethyl methacrylate AAEM: 2-Acetoacetoxyethyl methacrylate DAAM: Diacetone acrylamide EGDMA: Ethylene glycol dimethacrylate DVB: Divinylbenzene MPTES:γ-methacryloxypropyltriethoxysilane Aqualon AR-10: A reactive surfactant based on polyoxyalkylene styrene styrene-phenyl ether sulfate, manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Aqualon KH-10: A reactive surfactant based on polyoxyethylene alkyl ether sulfate, manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Hytenol NF-08: Manufactured by Daiichi Kogyo Seiyaku Co., Ltd., polyoxyethylene styrene-phenyl ether sulfate ammonium salt (95% methanol solution)

[0067] [Manufacturing Example 3-1] In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and refluxer, 100 parts styrene, 1 part AIBN, 40 parts polyvinylpyrrolidone K30 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 950 parts methanol, and 180 parts deionized water were added and mixed. The mixture was then reacted under a nitrogen atmosphere while being heated to 40°C for 24 hours. The resulting dispersion of resin microparticles was separated into solid and liquid by centrifugation, and deionized water was added to adjust the solid content to 45.0% to obtain an aqueous dispersion of resin microparticles (A3-1). The average particle size of resin microparticles (A3-1) was 750 nm, the CV value was 25.1%, and the glass transition temperature (Tg) was 100°C.

[0068] [Manufacturing Example 3-2] Polymerization was carried out in the same manner as in Production Example 3-1, except that the amount of polyvinylpyrrolidone K30 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) used in the polymerization of resin microparticles (A3-1) was changed from 40 parts to 20 parts, and an aqueous dispersion of resin microparticles (A3-2) was obtained. The average particle size of resin microparticles (A3-2) was 2030 nm, the CV value was 28.3%, and the glass transition temperature (Tg) was 101°C.

[0069] <Manufacturing of aqueous dispersion of resin microparticles (A4-1)> [Manufacturing Example 4-1] A polymerization reaction was carried out in the same manner as in Example 1-1, except that 99 parts styrene, 1 part acrylic acid, and 1.6 parts 25% aqueous solution of Aquaron AR-10 (0.4 parts solids) were used instead of 40 parts styrene, 38 parts methyl methacrylate, 20 parts n-butyl acrylate, 2 parts methacrylic acid, and 4.8 parts (1.2 parts solids) of a 25% aqueous solution of Aquaron AR-10, to obtain an aqueous dispersion of resin fine particles (A4-1). The average particle size of the obtained resin fine particles (A4-1) was 225 nm, the CV value was 9.2%, and the glass transition temperature (Tg) was 101°C.

[0070] <Manufacturing of primer layer composition> [Manufacturing Example 5-1] Primer layer composition (PP-1) An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 30.7 parts styrene, 64.2 parts n-butyl acrylate, 5.0 parts methacrylic acid, 5.6 parts 25% aqueous solution of Aqualon AR-10 (1.4 parts solids), and 40.4 parts deionized water. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 68.9 parts deionized water and 3% of the emulsion were added. After raising the internal temperature to 80°C and thoroughly purging with nitrogen, 6.0 parts 2.5% aqueous solution of potassium persulfate (0.15 parts solids) was added as an initiator to start emulsion polymerization. While maintaining the internal temperature at 80°C, the remaining emulsion and 6.0 parts 2.5% aqueous solution of potassium persulfate (0.15 parts solids) were added dropwise over 3 hours, and the mixture was reacted for a further 4 hours to obtain an aqueous dispersion of styrene acrylic resin. After the reaction was complete, 3.9 parts of 25% aqueous ammonia were added to neutralize the mixture, and the solid content of the aqueous dispersion was adjusted to 45.0% with deionized water to obtain an aqueous dispersion of primer layer resin (P-1). Next, 1.0 part of Emulgen 1108 as a surfactant was added to 99.0 parts of the aqueous dispersion of the obtained primer layer resin (P-1) and stirred to prepare the primer layer composition (PP-1).

[0071] [Manufacturing Example 5-2] Primer layer composition (PP-2) Polymerization was carried out in the same manner as in Production Example 5-1, except that 5 parts of the 64.2 parts of n-butyl acrylate used in the polymerization of the primer layer resin (P-1) were replaced with acetoxyacetyl methacrylate, to obtain an aqueous dispersion of the primer layer resin (P-2). Next, 99 parts of the obtained aqueous dispersion of primer layer resin (P-2) were mixed with 1 part of Emulgen 1108 as a surfactant and stirred to prepare primer layer composition (PP-2).

[0072] <Manufacturing of thermal recording compositions> [Examples 1-1 to 1-27, Comparative Examples 1-1 to 1-3] A thermal recording composition (C-1) was prepared by adding 1 part of Emulgen 1108 manufactured by Kao Corporation to 99 parts of an aqueous dispersion of resin fine particles (A1-1) and stirring with a disperser.

[0073] [Examples 1-2 to 27, Comparative Examples 1-1 to 1-3] Except for the changes in the formulation shown in Tables 3 and 4, the thermal recording compositions (C-2 to 27, CX-1 to 3) were prepared in the same manner as in Example 1-1.

[0074] [Examples 1-28] To 100 parts of the recording composition (C-7) prepared in Examples 1-7, 0.45 parts of dihydrazide adipic acid (ADH) were added to prepare the thermal recording composition (C-28).

[0075] <Evaluation of thermal recording compositions> [viscosity] The viscosity (mPa·s) of the obtained thermal recording composition was measured at 60 rpm using a Type B viscometer under 25°C conditions.

[0076] [Dispersion stability] The obtained thermal recording composition was left for one month under 25°C conditions, then coated onto one side of colored fine paper (black), and the presence or absence of aggregates was visually checked and evaluated according to the following criteria. ◎: No aggregates were observed (good). ○: Aggregates are slightly visible, but the appearance is not affected (practical). ×: Aggregates are found that impair the appearance (not suitable for practical use).

[0077] [Table 3]

[0078] [Table 4]

[0079] The abbreviations used in Tables 3 and 4 are shown below. Emulgen 1108: Polyoxyethylene alkyl ether-based surfactant manufactured by Kao Corporation. Emulgen 1118S-70: Polyoxyalkylene alkyl ether surfactant manufactured by Kao Corporation. Neugen XL-1000: Polyoxyalkylene alkyl ether surfactant manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Neugen EA-87: Polyoxyethylene styrene-phenyl ether-based surfactant manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Emulgen A-60: Polyoxyethylene styrene-phenyl ether-based surfactant manufactured by Kao Corporation. Hexaglyn1-L: Polyglycerin fatty acid ester-based surfactant manufactured by Nikko Chemicals Co., Ltd. SURFYNOL420: Acetylene-based surfactant manufactured by Evonik. KF-351A: Siloxane-based surfactant manufactured by Shin-Etsu Silicone Co., Ltd. Hytenol NF-08: Manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Ammonium polyoxyethylene styrene-phenyl ether sulfate (95% methanol solution)

[0080] <Preparation of thermal recording media> [Example 2-1] A primer layer composition (PP-1) was applied to one side of A0 size (841 mm x 1,189 mm) colored fine paper (black) using a bar coater to a dry thickness of 2 μm, and then dried to form a primer layer. Next, a thermal recording composition (C-1) was applied onto the primer layer and dried to obtain a thermal recording body (D-1) having a thermal recording layer. The thickness of the thermal recording layer was 5 μm.

[0081] [Examples 2-2 to 2-30, Comparative Examples 2-1 to 2-3] Thermal recording bodies (D-2 to 30, DX-1 to 3) were obtained in the same manner as in Example 2-1, except that the thermal recording composition, the thickness of the thermal recording layer, and the primer layer composition were changed to those shown in Table 5.

[0082] <Evaluation of thermal recording media> The obtained thermal recording media were evaluated as follows. The evaluation results are shown in Table 5. [Whiteness] Ten locations were randomly selected from the coated surface of the thermal recording layer, and the ISO whiteness was measured using a portable integrating sphere spectrophotometer Ci60 (X-rite). The average value (W1) of the 10 measurements was calculated and evaluated according to the following criteria. ◎: W1 is 60 or higher (extremely good) ○: W1 is 50 or higher, but less than 60 (good) △: W1 is 40 or more, but less than 50 (practical) ×: W1 is less than 40 (not practical)

[0083] [Uniform opacity] Of the 10 measurements taken in the above evaluation of [whiteness], the minimum value (W1 min ) and maximum value (W1 max The ratio (ΔW1) to ) was calculated and evaluated according to the following evaluation criteria. ΔW1 is calculated by Equation 1 below, and the closer the value of ΔW1 is to 100%, the more uniformly the thermal recording layer is formed. Equation 1:ΔW1=W1 min / W1 max ◎: ΔW1 is 85% or higher (good) ○: ΔW1 is 75% or more and less than 85% (practical) ×: ΔW1 is less than 75% (not practical)

[0084] [Thermal Sensitive] The surface of the thermal recording material coated with the thermal recording layer was heated using a thermal printer equipped with a thermal head (Brother Industries PocketJet PJ-673) with a density setting of "5" to form an image. Ten 10cm x 10cm squares were formed at appropriate intervals. In addition, the ISO whiteness was measured for 10 image areas in the same manner as the evaluation of [whiteness], and the minimum value (W2 min ) and maximum value (W2 max The ratio (ΔW2) to ) was calculated and evaluated according to the following evaluation criteria. ΔW2 is calculated using Equation 2 below, and the closer the value of ΔW2 is to 100%, the more uniformly the image is formed. Equation 2: ΔW2 = W2 min / W2max ◎: ΔW2 is 85% or higher (good) ○: ΔW2 is 75% or more and less than 85% (practical) ×: ΔW2 is less than 75% (not practical)

[0085] [Visibility] For the image-formed thermal recording material, the average ISO whiteness (W2) of 10 heated areas (image areas) was measured, and the ratio of W1 to W2 (ΔW3) was calculated and evaluated according to the following criteria. ΔW3 is calculated using Equation 3 below, and a larger value of ΔW3 indicates that the image and non-image areas are clearly distinguishable, resulting in better visibility. Equation 3:ΔW3=W1 / W2 ◎: ΔW3 is 7 or higher (good) ○: ΔW3 is 4 or greater and less than 7 (practical) ×: ΔW3 is less than 4 (not practical)

[0086] [Print quality] Image formation was performed using the same method as for the [thermal sensitivity] evaluation described above, and the amount of head residue adhering to the thermal head was visually observed and evaluated according to the following criteria. ◎: Very little head residue ○: Some residue is generated on the head, but it is at a level that does not pose a practical problem. ×: A large amount of printhead residue was generated, causing a blockage inside the printer.

[0087] [Heat and moisture resistance] The ISO whiteness change rate (ΔW4) of the non-imaged areas of a thermal recording material was evaluated according to the following criteria after being left in a constant humidity chamber at 40°C and 90% relative humidity for 24 hours. A smaller ΔW4 value indicates better stability of the thermal recording layer and superior long-term reliability of the thermal recording material. ◎: ΔW4 is 5% or less ○: ΔW4 is greater than 5% and less than or equal to 7% △: ΔW4 is greater than 7% and less than or equal to 10%. ×: ΔW4 exceeds 10%

[0088] [Table 5]

[0089] According to Table 5, a thermal recording material using the thermal recording composition of the present invention, which comprises resin fine particles (A) with a CV value of 15% or more, a surfactant (B) within an appropriate range, and water, exhibits good uniform opacity and thermal sensitivity on the substrate and good visibility even when forming images over a large area. On the other hand, thermal recording materials using the compositions of comparative examples have uneven opacity and thermal sensitivity and poor visibility when forming images.

Claims

1. A thermal recording composition comprising resin fine particles (A) having a coefficient of variation (CV value) of 15% or more, at least one surfactant (B) selected from the group consisting of a nonionic surfactant with an HLB value of 3 to 20 and an anionic surfactant with an HLB value of 3 to 20, and water.

2. The thermal recording composition according to claim 1, wherein the surfactant (B) is a nonionic surfactant having an HLB value of 3 to 20.

3. The thermal recording composition according to claim 1, wherein the surfactant (B) comprises at least one selected from the group consisting of polyglycerin fatty acid ester surfactants, acetylene surfactants, polyoxyalkylene alkyl ether surfactants, and siloxane surfactants.

4. The thermal recording composition according to claim 1, wherein the content of the surfactant (B) is 0.1% by mass or more and 10% by mass or less, based on the mass of the thermal recording composition.

5. The thermal recording composition according to claim 1, wherein the resin fine particles (A) are at least one selected from the group consisting of resin fine particles made of a single resin and core-shell type resin fine particles.

6. The thermal recording composition according to claim 1, wherein the resin fine particles (A) are resin fine particles made of a single resin, and the glass transition temperature of the single resin is 40°C or higher and 130°C or lower.

7. The thermal recording composition according to claim 1, wherein the resin fine particles (A) are core-shell type resin fine particles, the glass transition temperature of the core portion of the core-shell type resin fine particles is 40°C or higher and 130°C or lower, and the glass transition temperature of the shell portion is -20°C or higher and 30°C or lower.

8. The thermal recording composition according to claim 1, wherein the resin fine particles (A) have an average particle diameter of 150 nm or more and 2,000 nm or less.

9. The thermal recording composition according to claim 1, wherein the resin fine particles (A) are a polymer of an ethylenically unsaturated monomer (a).

10. The thermal recording composition according to claim 9, wherein the polymer of the ethylenically unsaturated monomer (a) is an emulsion polymer using a reactive surfactant.

11. A thermal recording body comprising a thermal recording layer formed on a substrate using the thermal recording composition according to any one of claims 1 to 10.

12. The thermal recording body according to claim 11, further comprising a primer layer between the substrate and the thermal recording layer.

Images