Thermal recording composition and thermal recording body

Abstract

To provide a thermal recording composition that does not use leuco dyes, achieves both printer color development and heat resistance, and has excellent practical properties, and a thermal recording body that has excellent practical properties comprising a thermal recording layer formed using the thermal recording composition. [Solution] The above problem is solved by a thermal recording composition containing resin microparticles and water, wherein the resin microparticles are at least one selected from the group consisting of resin microparticles made of a single resin and core-shell type resin microparticles, and the single resin and the resin forming the core of the core-shell type resin microparticles each independently have a weight-average molecular weight of 2 to 2 million.

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] Thermal printing is a printing method that utilizes a heating element of a print head. For example, it is used in a wide range of fields such as facsimiles, registers, logistics and food barcode labels, ID cards, airline and railway tickets, X-ray films, and prescription pads, due to its simplicity and printing speed. Among them, the direct thermal printing method is a method in which the heating element of the print head is brought into direct contact with a heat-sensitive recording material and printed, and is widely used. The most widespread method for heat-sensitive recording materials at present is the leuco dye type, in which the heated leuco dye melts with a developer to develop color. Phenols represented by bisphenol A are often used as developers. In recent years, due to concerns about the endocrine disrupting properties of bisphenols, a direct thermal printing method different from the leuco dye type has been demanded.

[0003] Patent Document 1 discloses a melt-transparent type heat-sensitive recording material using resin fine particles in which a low molecular compound is swollen. The melt-transparent type has an opaque layer on a colored support or a support provided with a colored layer (hereinafter sometimes referred to as a colored base material). When heated, the opaque layer melts and becomes transparent, and a colored image is obtained by the appearance of the colored layer thereunder (hereinafter, this is referred to as color development).

[0004] [[ID=2I]] Patent Document 2 discloses a melt-transparent type heat-sensitive recording material using resin fine particles in which a low molecular compound is mixed as a clarifying agent.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] However, with the thermal recording material described in Patent Document 1, increasing the amount of low-molecular-weight compound to improve color development when printed with a printer worsens heat resistance, and raising the melting point of the low-molecular-weight compound to improve heat resistance worsens printer color development. In other words, there is a trade-off in achieving both printer color development and heat resistance, and it is not practical. Furthermore, in the invention described in Patent Document 2, the clearing agent is a low-molecular-weight compound, and the disclosed formulation has the problem of low heat resistance, making it impractical. In other words, conventional molten transparent thermal recording materials have difficulty simultaneously providing both thermal responsiveness and high-temperature storage stability, and a thermal recording material with excellent practical properties that balance printer color development and heat resistance has not yet been realized. Therefore, the problem that the present invention aims to solve is to provide a thermal recording composition that does not use leuco dyes, achieves both printer color development and heat resistance, and has excellent practical properties, and a thermal recording body that has the above-mentioned excellent practical properties, comprising a thermal recording layer formed using the thermal recording composition. [Means for solving the problem]

[0007] After diligent research by the inventors, it was discovered that the problems of the present invention can be solved in the following embodiment, and thus the present invention was completed.

[0008] [1] The present invention relates to a thermal recording composition containing resin fine particles and water, wherein the resin fine particles 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, and the single resin and the resin forming the core of the core-shell type resin fine particles each independently have a weight-average molecular weight of 2 to 2 million.

[0009] [2] The present invention relates to the thermal recording composition described in [1], wherein the resin fine particles have an average particle diameter of 50 nm to 2,000 nm.

[0010] [3] The present invention relates to the thermal recording composition described in [1] or [2], wherein the resin fine particles have a coefficient of variation (Cv value) of 15% or more.

[0011] [4] The present invention relates to a thermal recording composition according to any one of [1] to [3], wherein the resin fine particles are polymers of ethylenically unsaturated monomers.

[0012] [5] The present invention relates to the thermal recording composition described in [4], wherein the polymer of the ethylenically unsaturated monomer has a content of 1.0% by mass or less of structural units derived from a crosslinkable monomer, based on the total mass of structural units.

[0013] [6] The present invention relates to a thermal recording composition according to any one of [1] to [5], wherein the single resin and the resin forming the core of the core-shell type resin fine particles each independently have a glass transition temperature of 60°C or higher.

[0014] [7] The present invention relates to a thermal recording body comprising a thermal recording layer formed on a substrate using a thermal recording composition according to any one of [1] to [6]. [Effects of the Invention]

[0015] The present invention provides a thermal recording composition that does not use leuco dyes, achieves both printer color development and heat resistance, and exhibits excellent practical properties, as well as a thermal recording body having the above-mentioned excellent practical properties, comprising a thermal recording layer formed using the thermal recording composition. [Modes for carrying out the invention]

[0016] <<Thermal recording composition>> The thermal recording composition of the present invention contains resin microparticles and water, wherein the resin microparticles are at least one selected from the group consisting of resin microparticles made of a single resin and core-shell type resin microparticles, and the single resin and the resin forming the core of the core-shell type resin microparticles each independently have a weight-average molecular weight of 2 to 2 million. When the thermal recording composition of the present invention is coated onto a colored substrate, resin microparticles accumulate. At this time, voids are created between the resin microparticles, causing light scattering due to the particle shape, and a white thermal recording layer is formed. Next, when a certain amount of thermal energy is applied to this thermal recording layer, the resin forming the resin microparticles flows, filling the voids in the heated area. As a result, light scattering does not occur, a transparent resin layer is formed, and the color of the colored substrate becomes visible. In this invention, the weight-average molecular weight of the single resin constituting the resin microparticles and the resin forming the core of the core-shell type microparticles are low molecular weight resins ranging from 2 to 2 million. As a result, the resin melts and becomes transparent in the short time it comes into contact with the heat-generating element of the print head. Furthermore, because the weight-average molecular weight of the resin is within the above range, both the printer's color development and heat resistance are excellent. Embodiments of the present invention will be described in detail below.

[0017] <Resin fine particles> The resin nanoparticles in this invention are at least one selected from the group consisting of resin nanoparticles made of a single resin and core-shell type resin nanoparticles (core layer: inner layer, shell layer: outer layer), and the single resin and the resin forming the core of the core-shell type resin nanoparticles each independently have a weight-average molecular weight of 2 to 2 million. The weight-average molecular weight can be measured by GPC (gel permeation chromatography). The weight-average molecular weight (Mw) is preferably 30,000 or more, more preferably 50,000 or more, from the viewpoint of polymerization stability. Furthermore, from the viewpoint of thermal responsiveness, it is preferably 1,500,000 or less, more preferably 1,000,000 or less, and may be, for example, between 30,000 and 1,500,000, or between 50,000 and 1,000,000.

[0018] The resin fine particles are not particularly limited, but are preferably polymers of ethylenically unsaturated monomers, more preferably acrylic resins (aqueous acrylic resins), and still more preferably styrene acrylic resins (aqueous styrene acrylic resins). When the resin fine particles are composed of a polymer of an ethylenically unsaturated monomer, the target resin can be obtained by emulsion polymerization of the ethylenically unsaturated monomer.

[0019] [Resin fine particles composed of a single resin] The method for producing resin fine particles composed of a single resin is not particularly limited. For example, when it is an acrylic resin, it can be produced by the following emulsion polymerization. First, an aqueous medium and a surfactant are charged into a reaction vessel and heated to a predetermined temperature. On the other hand, water, a surfactant, and an ethylenically unsaturated monomer containing a (meth)acrylic monomer are charged into a dropping vessel and stirred to prepare an emulsion of the ethylenically unsaturated monomer. Then, under a nitrogen atmosphere, a radical polymerization initiator is added while dropping the prepared emulsion into the reaction vessel. After the reaction starts, polymer particle nuclei are generated, and the particles gradually grow to form acrylic resin fine particles.

[0020] [Ethylenically unsaturated monomer] Examples of ethylenically unsaturated monomers that can be used in the production of the above aqueous acrylic resin 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 ( Aromatic ethylenically unsaturated monomers such as meth)acrylate and phenyl(meth)acrylate; 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, and undecyl(meth)acrylate. Linear or branched alkyl-containing ethylenically unsaturated monomers such as syl(meth)acrylate, lauryl(meth)acrylate, tridecyl(meth)acrylate, tetradecyl(meth)acrylate, pentadecyl(meth)acrylate, hexadecyl(meth)acrylate, heptadecyl(meth)acrylate, stearyl(meth)acrylate, isostearyl(meth)acrylate, behenyl(meth)acrylate, etc.; cyclohexyl(meth)acrylate, isobonyl(meth)acrylate, 1-adamantine Alicyclic alkyl group-containing ethylenically unsaturated monomers such as methyl(meth)acrylate; fluorinated alkyl group-containing ethylenically unsaturated monomers such as trifluoroethyl(meth)acrylate and heptadecafluorodecyl(meth)acrylate; carboxyl group-containing ethylenically unsaturated monomers such as maleic anhydride, 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;Sulfo-containing ethylenically unsaturated monomers such as 2-acrylamide, sodium 2-methylpropanesulfonate, methallyl sulfonic acid, methallyl sulfonic acid, sodium methallyl sulfonate, allyl sulfonic acid, sodium allyl sulfonate, ammonium allylsulfonate, vinyl sulfonic acid; (meth)acrylamide, N-methoxymethyl-(meth)acrylamide, N-ethoxymethyl-(meth)acrylamide, N-propoxymethyl-(meth)acrylamide, N-butoxymethyl-(meth)acrylamide, N-pentoxymethyl-(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 ethylenically unsaturated monomers containing amide groups such as (Cyl) acrylamide, N-butoxymethyl-N-(methoxymethyl)methacrylamide, N,N-di(pentoxymethyl)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; and ethylenically unsaturated monomers containing polyoxyethylene groups such as methoxypolyethylene glycol (meth)acrylate and polyethylene glycol (meth)acrylate.Examples include dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, methylethylaminoethyl (meth)acrylate, dimethylaminostyrene, diethylaminostyrene, etc., which are amino group-containing ethylenically unsaturated monomers such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, and methylethylaminoethyl (meth)acrylate; and 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 and acetoacetoxy(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, allyl lactyl (meth)acrylate, citronellyl (meth)acrylate, geranyl (meth)acrylate, rosinyl (meth)acrylate, cinnamyl (meth)acrylate, diallyl maleate, diaryl luteconate, vinyl (meth)acrylate, vinyl crotate, vinyl oleate, vinyl linolenate, 2-(2'-vinyloxyethoxy)ethyl (meth)acrylate, ethylene glycol di(meth)acrylate, triethylene glycol (meth)acrylate Ethylene-unsaturated monomers having two or more ethylenically unsaturated groups, such as 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.;Alkoxysilyl group-containing ethylenically unsaturated monomers such as γ-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-methacryloxypropyltributoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-acryloxypropyltrimethoxysilane, 3-acryloxypropyltriethoxysilane, 3-acryloxypropylmethyldimethoxysilane, 3-methacryloxymethyltrimethoxysilane, 3-acryloxymethyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltributoxysilane, vinylmethyldimethoxysilane; Methylol group-containing ethylenically unsaturated monomers such as N-methylol(meth)acrylamide, N,N-dimethylol(meth)acrylamide, alkyl etherified N-methylol(meth)acrylamide; are included. These monomers may be used alone or in combination of two or more.

[0021] The above-mentioned ethylenically unsaturated monomer may have a reactive group for the purpose of crosslinking between the primer layer and the heat-sensitive recording layer described later. Examples of the reactive group include an epoxy group, a carboxy group, a hydroxy group, a ketone group, and a hydrazide group, and a ketone group is more preferable. 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. Also, when the aqueous acrylic resin is resin fine particles dispersible in an aqueous medium, if an ethylenically unsaturated monomer having a highly hydrophilic ketone group is used in the copolymer composition, the ketone group is introduced outside the resin fine particles, that is, near the interface with the aqueous medium, and it is considered that a crosslink can be efficiently formed with the hydrazide crosslinking agent.

[0022] When an aqueous acrylic resin contains 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 aqueous acrylic resin. By introducing ketone groups in the range of 0.05 to 0.3 mmol / g, crosslinking is formed without inhibiting the fusion of the aqueous acrylic resin, resulting in a stronger bond between the resin layer and the colloidal crystal layer. This improves the conformability of the colloidal crystal coating, allowing for the creation of a laminate with good adhesion and abrasion resistance.

[0023] {Radical polymerizable internally crosslinkable monomers (crosslinkable monomers)} Ethylene-unsaturated monomers may have two or more reactive groups capable of radical polymerization for the purpose of intermolecular crosslinking. From the viewpoint of printer color development, it is preferable that the constituent units derived from radically polymerizable internally crosslinkable monomers (crosslinkable monomers) of the polymer of ethylenically unsaturated monomers are 1.0% by mass or less, based on the total mass of the constituent units constituting the polymer of ethylenically unsaturated monomers.

[0024] {radical polymerization initiator} For use as radical polymerization initiators in the production of aqueous acrylic resins, known oil-soluble polymerization initiators and water-soluble polymerization initiators can be used. These may be used individually or in combination of two or more.

[0025] 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.

[0026] 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.

[0027] {surfactants} Surfactants are generally used in the manufacture of aqueous acrylic resins, and the use of surfactants can improve the stability of resin microparticles. Examples of surfactants include anionic and nonionic surfactants, with anionic surfactants being preferred. These may be used individually or in combination of two or more types.

[0028] Examples of surfactants include anionic reactive surfactants, anionic nonreactive surfactants, nonionic reactive surfactants, and nonionic nonreactive surfactants, with reactive surfactants being preferred. Here, a reactive surfactant refers to a surfactant that can polymerize with the ethylenically unsaturated monomers mentioned above. More specifically, it means a surfactant having a reactive group that can polymerize 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. Using a reactive surfactant is preferable because it reduces the amount of free emulsifier components that adversely affect the heat resistance and weather resistance (moisture resistance) of the thermal recording layer, thereby further improving the long-term stability of the thermal recording layer.

[0029] {chain movement agent} Molecular weight adjustment of aqueous acrylic resins is generally performed using chain transfer agents. Examples of chain transfer agents include compounds having thiol groups or hydroxyl groups. Examples of compounds having thiol groups include mercaptans such as lauryl mercaptan, 2-mercaptoethyl alcohol, dodecyl mercaptan, and mercaptosuccinic acid; alkyl mercaptopropions such as n-butyl mercaptopropionate and octyl mercaptopropionate; alkoxyalkyl mercaptopropions such as methoxybutyl mercaptopropionate; and alkyl thioglycolates such as octyl thioglycolate and methoxybutyl thioglycolate. In addition, alcohols such as methyl alcohol, n-propyl alcohol, isopropyl alcohol (IPA), t-butyl alcohol, and benzyl alcohol can also be used. These may be used individually or in combination of two or more types.

[0030] The amount of the chain transfer agent is preferably 0.1 to 2.0% by mass, more preferably 0.2 to 1.5% by mass, based on the mass of the ethylenically unsaturated monomer. Using 0.1% by mass or more results in good molecular weight reduction, while using 2.0% by mass or less allows for stable polymerization.

[0031] Furthermore, α-methylstyrene, α-methylstyrene dimer, etc., may be used as ethylenically unsaturated monomers with chain transfer ability. From the viewpoint of polymerization stability, it is preferable that these monomers be 10% by mass or less, based on the mass of the ethylenically unsaturated monomer.

[0032] {Other ingredients} In the manufacture of water-based acrylic resins, reducing agents, buffering agents, and neutralizing agents can be used as needed.

[0033] [Core-shell type resin microparticles] Core-shell type resin microparticles consist of a core and a shell made of water-insoluble polymers, and include a structure of a core (inner layer) and a shell (outer layer) that are incompatible with each other. The core maintains a spherical shape, while the shell has fluidity and functions as a binding site. The core-shell type resin microparticles in this invention may have a multilayer structure within the core and shell, and may have a gradient in composition. When a composition containing core-shell type resin microparticles is applied to a substrate, and the core-shell type resin microparticles accumulate, the shells of adjacent core-shell type resin microparticles and the shells of the core-shell type resin microparticles and the substrate layer readily bond to each other, resulting in a laminate that exhibits excellent adhesion.

[0034] The method for producing core-shell type resin fine particles is not particularly limited. Examples include polymerization of ethylenically unsaturated monomers in an aqueous medium, such as emulsion polymerization, and phase inversion emulsification, in which polymerization is performed in a non-aqueous system and then phase inversion is performed while desolventing. Emulsion polymerization is preferred because it allows for low viscosity and high solid content concentration. In emulsion polymerization, either two-stage polymerization, in which the monomer composition is changed between the first and second stages and added dropwise, or multi-stage polymerization, in which the monomer composition is changed in three or more stages and added dropwise, may be used. Core-shell type resin fine particles can be prepared by the above-described two-step polymerization, specifically by the procedure shown below. (1) First, an aqueous medium and a surfactant are placed in the reaction vessel and the temperature is raised. Then, under a nitrogen atmosphere, the first stage emulsion of ethylenically unsaturated monomers that forms the core is added dropwise while the radical polymerization initiator is added. After the reaction starts, the particles gradually grow according to the dropwise rate to form core particles. (2) Next, once the first stage of dropping is complete and the heat generation has subsided, the second stage of emulsion of ethylenically unsaturated monomers, which will form the shell, is added dropwise. Additional initiators may be added at this time. The added second stage of ethylenically unsaturated monomers is initially distributed to the core particles, but as polymerization progresses, it precipitates as polymers on the outer layer of the core particles, forming the shell layer.

[0035] {Ethylene-unsaturated monomers} For ethylenically unsaturated monomers that can be used in the production of core-shell type resin nanoparticles, the description in the section on {ethylenically unsaturated monomers} in the above-mentioned <resin nanoparticles consisting of a single resin> can be applied.

[0036] Furthermore, the ethylenically unsaturated monomer may have reactive groups for the purpose of forming crosslinks between the core-shell type resin microparticle layer and the layer in contact with the core-shell type resin microparticle layer. By forming crosslinks between the core-shell type resin microparticle layer and the layer in contact with the colloidal crystal layer, the coating film resistance of the resulting laminate is improved.

[0037] Crosslinking between the core-shell type resin microparticle layer and the layer in contact with the core-shell type resin microparticle layer can be introduced by reacting the reactive groups of the core-shell type resin microparticles with each other, reacting the reactive groups of the core-shell type resin microparticles with the reactive groups of the aforementioned primer layer, or crosslinking the reactive groups of the core-shell type resin microparticles via a polyfunctional crosslinking agent.

[0038] As for the reactive groups that ethylenically unsaturated monomers may have, the description in the section on {ethylenically unsaturated monomers} in the above-mentioned <resin fine particles made of a single resin> can be referenced. When core-shell type resin microparticles contain 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 core-shell type resin microparticles. By introducing ketone groups in the range of 0.05 to 0.3 mmol / g, crosslinking is formed without inhibiting shell fusion, resulting in stronger interparticle and interlayer bonding. This allows for the maintenance of good wavelength selectivity even after various resistance tests, such as abrasion resistance tests. Furthermore, a content of 0.3 mmol / g or less improves the polymerization stability of the core-shell type resin microparticles.

[0039] When introducing reactive groups into core-shell type resin fine particles, it is preferable to introduce the reactive groups into the shell portion, as this allows for a more effective expression of the synergistic effect of thermal fusion due to the entanglement of polymer chains and the formation of crosslinks.

[0040] {radical polymerization initiator} For the production of core-shell type resin nanoparticles, known oil-soluble polymerization initiators and water-soluble polymerization initiators can be used as radical polymerization initiators, and the description in the section on {radical polymerization initiators} in the above-mentioned <resin nanoparticles consisting of a single resin> can be applied.

[0041] {surfactants} Surfactants are generally used in the production of core-shell type resin microparticles, and the stability of the core-shell type resin microparticles can be improved by using surfactants. Examples of surfactants include anionic and nonionic surfactants, with anionic surfactants being preferred. The description of these surfactants can be referenced from the {surfactants} section in the above-mentioned <resin microparticles made of a single resin>.

[0042] {chain movement agent} A chain transfer agent can be used to adjust the molecular weight of the core resin in core-shell type resin microparticles. Examples of chain transfer agents include compounds having thiol groups or hydroxyl groups, or ethylenically unsaturated monomers with chain transfer ability. The description of these chain transfer agents can be referenced from the section on {chain transfer agents} in the above-mentioned <resin microparticles consisting of a single resin>.

[0043] {Other ingredients} In the production of core-shell type resin microparticles, reducing agents, buffers, and neutralizing agents can be used as needed.

[0044] [Properties of resin microparticles] The average particle diameter of the resin fine particles is preferably 50 nm or more, more preferably 100 nm or more, and even more preferably 200 nm or more. It is also preferably 2,000 nm or less, more preferably 1,500 nm or less, and even more preferably 1,000 nm or less. For example, it may be between 50 and 2,000 nm, between 100 and 1,500 nm, or between 200 and 1,000 nm. In this invention, the average particle diameter can be measured by dynamic light scattering, and the peak of the obtained volume-based particle diameter distribution data (histogram) is defined as the average particle diameter. Furthermore, the coefficient of variation (Cv value) of the average particle size of the resin fine particles is preferably 15% or higher. A coefficient of variation of 15% or higher improves the randomness of the particle arrangement, resulting in better whiteness of the laminate. The coefficient of variation is a numerical value that represents the uniformity of particle size and can be calculated using the following formula. Formula: Coefficient of variation Cv value (%) = Standard deviation of particle size / Average particle size × 100 [In the formula, the units of standard deviation and average particle size are the same.]

[0045] When the resin microparticles are made of a single resin, the glass transition temperature (Tg) is preferably 60°C or higher. A Tg of 60°C or higher suppresses deformation of the particle shape due to external heat and force. This allows for excellent color development even when the thermal recording material is stored at high temperatures for extended periods. Furthermore, from the viewpoint of thermal responsiveness, the Tg is preferably 180°C or lower, and may be, for example, between 60°C and 180°C. When the resin particles are core-shell type particles, from this viewpoint, the glass transition temperature of the core portion is preferably 60°C or higher. It is also preferably 180°C or lower, and may be, for example, 60 to 180°C. From the viewpoint of strengthening the bonding of the resin fine particles, the glass transition temperature of the shell portion is preferably 40°C or lower and -20°C or higher, and may be, for example, -20 to 40°C. The glass transition point can be measured using a DSC (Differential Scanning Calorimeter). Specifically, approximately 2 mg of the dried sample is weighed onto an aluminum pan, the aluminum pan is placed in a DSC measuring holder, and the glass transition point is obtained by reading the chart obtained under a heating condition of 5°C / min.

[0046] The content of resin fine particles is preferably 20% by mass or more, more preferably 30% by mass or more, based on the total mass of the thermal recording composition. It is also preferably 70% by mass or less, more preferably 60% by mass or less, and may be, for example, 20 to 70% by mass, or 30 to 60% by mass.

[0047] <Water> The thermal recording composition of the present invention contains water. The water may be included when obtaining the resin fine particles as a dispersion, or it may be added when preparing the composition. The water content is preferably 30 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 further 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 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 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 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, it is preferably in the range of 1.0 to 20 μm. By being within this range, excellent coating or printability and whiteness are sufficiently ensured, and printed materials with excellent substrate conformability, abrasion resistance, and solvent resistance 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. 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.

[0053] <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.

[0054] 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, chromatic dyes, and neutralizing amines. 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.

[0055] <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 microparticles in the adjacent thermal recording layer; 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. The image formation method using laser light is preferred because it allows for image formation without damaging the substrate, primer layer, and non-image-forming areas. Furthermore, it is preferable to use an infrared laser because it causes minimal damage to the substrate, the resin forming the primer layer, and the resin microparticles. Examples of infrared laser markers include CO2 laser markers (wavelength 10600nm or 9600nm), YVO4 laser markers (wavelength 1064nm), YAG laser markers (wavelength 1064nm), and fiber laser markers (wavelength 1030-1100nm). [Examples]

[0056] 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.

[0057] <Average particle size, Cv value> After diluting the particulate dispersion 500 times with water, approximately 5 ml of the diluted solution was measured using dynamic light scattering (measurement device: NanoTrack UPA Co., Ltd., MicroTrack Bell). The peak of the obtained volume-based particle size distribution data (histogram) was defined as the average particle size. Furthermore, the coefficient of variation Cv value, which represents the uniformity of particle size, was calculated using the following formula. Formula: Cv value (%) = Standard deviation of particle size / Average particle size × 100 [In the formula, the units of standard deviation and average particle size are the same.]

[0058] <Glass transition temperature> The glass transition point was measured using a DSC (Differential Scanning Calorimeter, TA Instruments). Specifically, approximately 2 mg of a dried resin microparticle dispersion sample was weighed onto an aluminum pan, the aluminum pan was placed in a DSC measuring holder, and the endothermic peak of the baseline shift (inflection point) chart of the DSC curve obtained under a heating condition of 5°C / min was read to obtain the glass transition point.

[0059] <Weight average molecular weight (Mw)> The weight-average molecular weight was determined as a polystyrene equivalent value by GPC (gel permeation chromatography) measurement under the following conditions. Specifically, dried resin was dissolved in tetrahydrofuran to prepare a 0.1% solution, which was then measured using the following apparatus and measurement conditions. Substances insoluble in tetrahydrofuran were deemed unmeasurable and were considered to have exceeded the detection limit due to high molecular weight, i.e., those with a weight-average molecular weight exceeding 2 million. Equipment: HLC-8320-GPC system (manufactured by Tosoh Corporation) Column; TSKgel-SuperMultiporeHZ-M00214884.6 mm I.D. × 15 cm × 3 (Molecular weight measurement range 2,000 to approximately 2,000,000) Eluting solvent; tetrahydrofuran standard; polystyrene (manufactured by Tosoh Corporation) Flow rate: 0.6 mL / min, Sample solution volume: 10 μL, Column temperature: 40°C

[0060] <Manufacturing of resin microparticles consisting of a single resin> [Manufacturing Example 1] An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 42.0 parts styrene, 43.0 parts methyl methacrylate, 7.0 parts n-butyl acrylate, 3.0 parts acrylic acid, 5.0 parts α-methylstyrene, 4.0 parts (1.0 part solids) of a 25% aqueous solution of Aqualon AR-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., an anionic reactive surfactant (polyoxyethylene styrene-phenyl ether sulfates)), and 40.4 parts ion-exchanged water. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 68.9 parts of deionized water and 1.0% of the emulsion were added. The internal temperature was raised to 80°C and the vessel was thoroughly purged with nitrogen. Then, 6.0 parts of a 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 of a 2.5% aqueous solution of potassium persulfate (0.15 parts solids) were added dropwise over 3 hours, and the reaction was continued for a further 4 hours to obtain an aqueous dispersion of styrene-acrylic resin. After the reaction was complete, 1 equivalent of aqueous ammonia was added to neutralize the carboxyl groups contained in the resin microparticles, and the solids content of the aqueous dispersion was adjusted to 45.0% with deionized water. The weight-average molecular weight of the obtained microparticles was 90,000, the average particle size was 345 nm, the Cv value was 19%, and the Tg was 95°C.

[0061] [Manufacturing Example 2] An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 53.8 parts of styrene, 40.0 parts of methyl methacrylate, 4.0 parts of 2-ethylhexyl acrylate, 2.2 parts of methacrylic acid, 4.0 parts of a 25% aqueous solution of Aqualon AR-10 (1.0 part solids), and 40.4 parts of deionized water. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux condenser, 68.9 parts of deionized water and 1.2% of the emulsion were added. The internal temperature was raised to 80°C and the vessel was thoroughly purged with nitrogen. Then, 6.0 parts of a 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, 0.5 parts of octyl thioglycolate as a chain transfer agent, and 6.0 parts of a 2.5% aqueous solution of potassium persulfate (0.15 parts solids) were added dropwise over 3 hours, and the reaction was continued for a further 4 hours to obtain an aqueous dispersion of styrene acrylic resin. After the reaction was complete, 1 equivalent of aqueous ammonia was added to neutralize the carboxyl groups contained in the resin microparticles, and the solids content of the aqueous dispersion was adjusted to 45.0% with deionized water. The weight-average molecular weight of the obtained microparticles was 320,000, the average particle size was 320 nm, the Cv value was 24.0%, and the Tg was 100°C.

[0062] [Manufacturing Examples 3-12] An aqueous dispersion of resin fine particles consisting of a single resin was obtained in the same manner as in Production Example 2, except that the compound composition shown in Table 1 was changed. The water in the reaction vessel was charged to 67% of the total amount of ethylenically unsaturated monomers. The emulsion of ethylenically unsaturated monomers was prepared by adding water so that the concentration of ethylenically unsaturated monomers in the emulsion was 69% and the concentration of surfactant was 0.69%. In addition, the resin fine particles were neutralized by adding 1 equivalent of aqueous ammonia to the carboxyl groups contained in the resin fine particles. Furthermore, the amount of emulsion added to the reaction vessel in the first stage was changed from 1.2% to 0.5% for production examples 3-7, 0.4% for production examples 8 and 10, 0.8% for production example 9, and 0.9% for production examples 11 and 12.

[0063] [Manufacturing Example 13] An emulsion was obtained in the same manner as in Production Example 2. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and refluxer, 68.9 parts of deionized water and 1.0 part of the aqueous dispersion of resin fine particles obtained in Production Example 2 were added. The internal temperature was raised to 80°C and the vessel was thoroughly purged with nitrogen. Then, 6.0 parts of a 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, 0.5 parts of octyl thioglycolate as a chain transfer agent, and 6.0 parts of a 2.5% aqueous solution of potassium persulfate (0.15 parts solids) were added dropwise over 3 hours, and the reaction was continued for a further 4 hours to obtain an aqueous dispersion of styrene acrylic resin. After the reaction was complete, 1 equivalent of aqueous ammonia was added to neutralize the carboxyl groups contained in the resin fine particles, and the solid content of the aqueous dispersion was adjusted to 45.0% with deionized water. The obtained fine particles had a weight-average molecular weight of 300,000, an average particle diameter of 850 nm, a Cv value of 31%, and a Tg of 100°C.

[0064] [Table 1]

[0065] <Manufacturing of core-shell type resin microparticles> [Manufacturing Example 14] 57.0 parts styrene, 42.0 parts methyl methacrylate, 1.0 part acrylic acid, 4.0 parts 25% aqueous solution of Aqualon AR-10 (1.0 part solids), and 39.0 parts deionized water were mixed and stirred to prepare the first-stage emulsion of ethylenically unsaturated monomers. 95.0 parts deionized water and 1.0% of the first-stage emulsion were added to a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus. After raising the internal temperature of the reaction vessel to 70°C and thoroughly purging it with nitrogen, polymerization was started by adding 6.0 parts 2.5% aqueous solution of potassium persulfate (0.15 parts solids) as an initiator. While maintaining the internal temperature at 80°C, the remaining emulsion was reacted with 1.4 parts octyl thioglycolate and 4.2 parts 2.5% aqueous solution of potassium persulfate (0.11 parts solids) as chain transfer agents, added dropwise over 2 hours to synthesize core particles. Next, 20.0 parts styrene, 17.0 parts n-butyl acrylate, 3.0 parts acrylic acid, 1.7 parts 25% aqueous solution of Aqualon AR-10 (0.4 parts solids), and 16.7 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 of 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 1.6 parts 2.5% aqueous solution of potassium persulfate (0.04 parts solids) dropwise over 2 hours to obtain an aqueous dispersion of core-shell type resin fine particles. After the reaction, water was added to adjust the solid content to 45.0%. In addition, 2.3 parts of 25% aqueous ammonia were added to neutralize the core-shell type resin microparticles. The amount of aqueous ammonia added corresponds to the amount that neutralizes all carboxyl groups contained in the shell (hereinafter referred to as 1 equivalent). The average particle size of the obtained microparticles was 450 nm, the Cv value was 22%, the weight-average molecular weight of the core was 270,000, and the Tg of the core was 102°C.

[0066] [Manufacturing Examples 15-19] An aqueous dispersion of core-shell type resin fine particles was obtained in the same manner as in Production Example 14, except that the formulation composition was changed as shown in Table 2. The water in the reaction vessel was charged to 67% of the total amount of ethylenically unsaturated monomers. The emulsion of ethylenically unsaturated monomers was prepared by adding water so that the concentration of ethylenically unsaturated monomers in the emulsion was 69% and the concentration of surfactant was 0.69%. In addition, the amount of Aqualon AR-10 in the second stage emulsion of Production Example 19 was changed from 6.0 parts to 20.0 parts. The proportions of the 2.5% aqueous potassium persulfate at the start of the reaction, when adding the first stage emulsion, and when adding the second stage emulsion were the same ratio as in Production Example 14. Furthermore, the core-shell type resin fine particles were neutralized by adding 1 equivalent of aqueous ammonia to the carboxyl groups contained in the shell.

[0067] [Table 2]

[0068] <Manufacturing of aqueous dispersion of resin microparticles for comparison> [Comparative Manufacturing Example 1] In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux apparatus, 0.1 parts sodium dodecylbenzenesulfonate and 0.9 parts ammonium persulfate were dissolved in 180 parts water. Then, 3.9 parts styrene, 1.8 parts divinylbenzene, and 0.3 parts methacrylic acid were added, and after thorough nitrogen purging, polymerization was carried out at 75°C for 1 hour. Next, 45.0 parts water, 0.3 parts sodium dodecylbenzenesulfonate, 67.0 parts styrene, 62.5 parts ethyl acrylate, 15.0 parts methyl methacrylate, 5.5 parts methacrylic acid, and 8.0 parts t-dodecyl mercaptan were mixed and stirred to prepare the second-stage emulsion of ethylenically unsaturated monomers. Twenty minutes after the completion of the first-stage polymerization, the dropwise addition of the second-stage emulsion was started. While maintaining the internal temperature at 80°C, the second-stage emulsion was added dropwise over 3 hours to obtain an aqueous dispersion of styrene-acrylic resin. The obtained resin fine particles had an average particle size of 500 nm, a Cv value of 29%, and a core Tg of 111°C. Since the resin fine particles did not dissolve in tetrahydrofuran, the weight-average molecular weight of the core was assumed to be over 2 million.

[0069] [Comparative Manufacturing Example 2] An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 88.0 parts styrene, 10.0 parts divinylbenzene, 2.0 parts methacrylic acid, 4.0 parts 25% aqueous solution of Aqualon AR-10 (1.0 part 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 2.3 parts aqueous dispersion of resin fine particles obtained in Production Example 2 were added. The internal temperature was raised to 80°C and the mixture was thoroughly purged with nitrogen. Then, 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 0.5 parts octyl thioglycolate and 6.0 parts 2.5% aqueous solution of potassium persulfate (0.15 parts solids) were added dropwise over 3 hours as a chain transfer agent, and the mixture was reacted for a further 4 hours to obtain an aqueous dispersion of styrene acrylic resin. After the reaction was complete, one equivalent of aqueous ammonia was added to neutralize the carboxyl groups contained in the resin microparticles, and the solid content of the aqueous dispersion was adjusted to 45.0% with deionized water. The average particle size of the obtained microparticles was 690 nm, the Cv value was 25%, and the Tg was 108°C. Since the resin microparticles did not dissolve in tetrahydrofuran, the weight-average molecular weight was assumed to be over 2 million.

[0070] [Comparative Manufacturing Example 3] 81.0 parts styrene and 9.0 parts butyl acrylate were dissolved in 4.5 parts phenyl benzoate, and 4.5 parts of a 3.0% aqueous solution of Hythenol N-08 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., an anionic surfactant (polyoxyethylene alkylphenyl ether sulfate)) were mixed and stirred to prepare an emulsion of ethylenically unsaturated monomers. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux condenser, 40.0 parts of deionized water and 0.2 parts of a 3.0% aqueous solution of Hythenol N-08 were charged. The internal temperature was raised to 71°C and the vessel was thoroughly purged with nitrogen. Then, 1% by weight of the emulsion was added to the reaction vessel. After stirring for 5 minutes, 3.0 parts of a 5% aqueous solution of ammonium persulfate was added all at once as an initiator to start emulsion polymerization. After stirring for 15 minutes, the remaining emulsion was added dropwise over 4 hours while maintaining the internal temperature at 71±2°C. After the addition of the emulsion was complete, the internal temperature was raised to 75°C and the mixture was aged for another hour to obtain an aqueous dispersion of styrene acrylic resin. The average particle size of the obtained fine particles was 200 nm, the Cv value was 10%, and the Tg was 75°C. Since the resin fine particles did not dissolve in tetrahydrofuran, the weight-average molecular weight was assumed to be over 2 million.

[0071] [Comparative Manufacturing Example 4] An aqueous dispersion of resin fine particles consisting of a single resin was obtained in the same manner as in Example 1, except that 48.8 parts of styrene, 40 parts of methyl methacrylate, 4 parts of 2-ethylhexyl acrylate, 2.2 parts of methacrylic acid, and 5.0 parts of divinylbenzene were used as ethylenically unsaturated monomers. The average particle size of the obtained fine particles was 350 nm, the Cv value was 21%, and the Tg was 95°C. Since the resin fine particles did not dissolve in tetrahydrofuran, the weight-average molecular weight was assumed to be over 2 million.

[0072] [Comparative Manufacturing Example 5] An aqueous dispersion of resin fine particles consisting of a single resin was obtained in the same manner as in Example 1, except that 44 parts methyl methacrylate, 5 parts cyclohexyl methacrylate, 20 parts dicyclopenetanyl methacrylate, 30 parts t-butyl methacrylate, and 1 part acrylic acid were used as ethylenically unsaturated monomers. The average particle size of the obtained fine particles was 400 nm, the Cv value was 16%, and the Tg was 115°C. Since the resin fine particles did not dissolve in tetrahydrofuran, the weight-average molecular weight was assumed to be over 2 million.

[0073] The obtained resin fine particles for comparative examples are shown below.

[0074] [Table 3]

[0075] <Preparation of thermal recording composition> [Example 1] To 100.0 parts of an aqueous dispersion of resin fine particles made of a single resin as in Production Example 1, 0.5 parts of Emulgen 1108 (manufactured by Kao Corporation, an ether-based nonionic surfactant) and 1.0 part of Surfinol 420 (manufactured by Nisshin Chemical Industry Co., Ltd., an acetylene-based nonionic surfactant) were added and stirred to obtain a thermal recording composition.

[0076] [Examples 2-20, Comparative Examples 1, 3-5] A thermal recording composition was prepared in the same manner as in Example 1, except that the aqueous dispersion of resin fine particles was changed to one of those listed in Table 4.

[0077] [Comparative Example 2] 12.5 parts stearic acid amide, 12.5 parts palmitic acid amide, 25.0 parts a 10% aqueous solution of Kuraray Poval 5-88 (Kuraray Co., Ltd., partially saponified polyvinyl alcohol), and 50.0 parts water were mixed and ground using a sand mill (AIMEX Co., Ltd., sand grinder) until the average particle size was 1.0 μm to obtain a clarifying agent dispersion. A thermal recording composition was prepared by mixing 89.0 parts of the aqueous dispersion of fine particles obtained in Comparative Production Example 2, 100.0 parts of the above-mentioned clarifying agent dispersion, 225.0 parts of a 10.0% aqueous solution of Kuraray Poval 11-98 (manufactured by Kuraray Co., Ltd., fully saponified polyvinyl alcohol), 27.8 parts of Hydrin Z-9-36 (manufactured by Chukyo Oil & Fat Co., Ltd., aqueous dispersion of zinc stearate), and 50.0 parts of water.

[0078] <Evaluation of thermal recording compositions and thermal recording media> The following evaluations were performed using the obtained thermal recording composition. Furthermore, thermal recording bodies were prepared using the thermal recording composition as shown below, and the following evaluations were performed. The results are shown in Table 4.

[0079] [Preparation of thermal recording media] A resin composition for the primer layer shown below was applied to a polyester film (Toray Industries, Ltd. Lumirror X30, hereinafter referred to as PET) using a bar coater to a thickness of 3.0 μm after drying, and then dried in an oven at 50°C for 3 minutes to form a primer layer. Next, the thermal recording composition shown in Table 4 was applied to the primer layer at a coating amount of 5 g / m² after drying. 2 The material was coated using a bar coater and heated at 50°C for 3 minutes to obtain a thermal recording material with the configuration of PET / primer layer / thermal recording layer. In addition, in Example 15, which used the thermal recording composition of Manufacturing Example 14, the basis weight was 75 g / m². 2 A thermal recording layer was directly formed on one side of black high-quality paper to obtain a thermal recording body with a high-quality paper / thermal recording layer configuration. (Resin composition for primer layer) An emulsion of ethylenically unsaturated monomers was prepared by pre-mixing and stirring 30.7 parts of styrene, 64.2 parts of n-butyl acrylate, 5.0 parts of methacrylic acid, 5.6 parts of a 25% aqueous solution of Aqualon AR-10 (1.4 parts solids), and 40.4 parts of deionized water. In a reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux condenser, 68.9 parts of deionized water and 3% of the emulsion were added. The internal temperature was raised to 80°C and the mixture was thoroughly purged with nitrogen. Then, 6.0 parts of a 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 of a 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 solids content of the aqueous dispersion was adjusted to 45.0% with deionized water. To 100.0 parts of the obtained aqueous dispersion of styrene-acrylic resin, 1.0 part of Emulgen 1108 was added as a surfactant and stirred to prepare a resin composition for the primer layer.

[0080] [Practical characteristics] The practical characteristics were determined based on an overall assessment of the printer's color reproduction and heat resistance. (Printer color reproduction) A thermal recording medium was subjected to thermal printing with a solid black area using a direct thermal printer TD-4420DN (Brother Corporation), and the optical density (OD value) of the thermally printed area was measured. The optical density was measured using a TR-927 (Macbeth Corporation). Based on the obtained optical density, it was quantified according to the following criteria. 100: Optical density is less than 0.2 75: Optical density is 0.2 or higher and less than 0.3 50: Optical density is 0.3 or higher and less than 0.4 25: Optical density is 0.4 or higher

[0081] (Heat resistance) After leaving the thermal recording material at 60°C for 3 months, the optical density (OD value) was measured in the same manner as in the printer color development test described above. The optical density before and after the heat resistance test was compared, and the rate of decrease in optical density was calculated. A larger rate of decrease indicates a deterioration in the color development of the thermal recording material and poorer heat resistance. The obtained rate of decrease was quantified according to the following criteria. 100: The rate of decrease in optical density is less than 10%. 80: The rate of decrease in optical density is 10% or more, but less than 20%. 60: The rate of decrease in optical density is 20% or more, but less than 30%. 40: The rate of decrease in optical density is 30% or more, but less than 40%. 20: The rate of decrease in optical density is 40% or more.

[0082] (Criteria for judging practical characteristics) The combined values ​​for printer color reproduction and heat resistance were calculated, and the practical characteristics were evaluated based on the following criteria using the total value. S: Total value is 180 or more A: The total value is 160 or greater, but less than 180. B: The total value is 130 or greater, but less than 160. C: The total value is less than 130.

[0083] [Whiteness] The optical density (OD value) of the thermal recording material was measured. The following criteria were used for evaluation based on the obtained optical density. S: Optical density is less than 0.2 A: Optical density is 0.2 or higher and less than 0.3 B: Optical density is 0.3 or higher and less than 0.4 C: Optical density is 0.4 or higher.

[0084] [Transparency] The optical density (OD value) of the thermal recording material was measured when a heat seal test was performed on the thermal recording material using a TP-701B heat seal tester (manufactured by Tester Sangyo Co., Ltd.) at 180°C for 1.0 second. The obtained optical density was evaluated according to the following criteria. S: Optical density is 1.2 or higher. A: Optical density is 0.9 or higher and less than 1.2 B: Optical density is less than 0.9

[0085] [Adhesion] Using a direct thermal printer TD-4420DN (Brother Corporation), a solid black print was applied to a thermal recording medium in a 10cm x 20cm area. The residue adhering to the heating element of the printer head was visually observed and evaluated according to the following criteria. A larger amount of residue indicates poorer adhesion of the thermal recording layer to the substrate. S: No residue, or very little residue. A: A moderate amount of residue is produced. B: A large amount of residue is generated, or printing problems occur.

[0086] [Table 4]

[0087] According to Table 4, thermal recording media using resin fine particles with a weight-average molecular weight of 2 to 2 million and a thermal recording composition containing water exhibit good printer color development and heat resistance, demonstrating excellent practical characteristics. On the other hand, thermal recording media using the thermal recording composition of the comparative manufacturing example showed poor practical characteristics, as it was difficult to achieve both printer color development and heat resistance.

Claims

1. A thermal recording composition containing resin fine particles and water, The resin fine particles 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. A thermal recording composition wherein the single resin and the resin forming the core of the core-shell type resin fine particles each independently have a weight-average molecular weight of 2 to 2 million.

2. The thermal recording composition according to claim 1, wherein the resin fine particles have an average particle diameter of 50 nm to 2,000 nm.

3. The thermal recording composition according to claim 1, wherein the resin fine particles have a coefficient of variation (Cv value) of 15% or more.

4. The thermal recording composition according to claim 1, wherein the resin fine particles are a polymer of ethylenically unsaturated monomers.

5. The thermal recording composition according to claim 4, wherein the polymer of the ethylenically unsaturated monomer has a content of 1.0% by mass or less of structural units derived from the crosslinkable monomer, based on the total mass of the structural units.

6. The thermal recording composition according to claim 1, wherein the single resin and the resin forming the core of the core-shell type resin fine particles each independently have a glass transition temperature of 60°C or higher.

7. 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 6.

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