Binder for thermal recording paper base layer, and composition for thermal recording paper base layer

Abstract

A binder for a thermal recording paper base layer, comprising a copolymer latex, wherein the copolymer latex contains 40% by mass or less of a structural unit derived from an aliphatic conjugated diene monomer and 60% by mass or more of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer, the average particle size of the copolymer latex based on a photon correlation method is 150 to 300 nm, and the swelling degree of the copolymer latex in toluene is 9 or more.

Description

Technical Field

[0001] The present invention relates to an adhesive for a thermal recording paper base layer and a composition for a thermal recording paper base layer containing the adhesive. Background Art

[0002] Thermal recording media are usually prepared by grinding and dispersing a colorless or light-colored colorless dye and a developer such as a phenolic compound into fine particles, mixing the two, adding a binder, filler, sensitivity enhancer, lubricant and other additives, and coating the resulting composition on a substrate such as paper, synthetic paper, film, plastic, etc., and are widely used as various recording media.

[0003] Thermal recording devices have the advantages of being relatively cheap, easy to handle, and compact, so thermal recording paper is used in a wide range of fields such as receipts, faxes, various tickets, and barcode labels. In particular, it is required to obtain high-concentration printing even with low energy, that is, excellent printing color rendering.

[0004] In this regard, the following methods are known: by providing a thermal recording paper bottom layer between the substrate and the thermal layer, the smoothness of the thermal layer coating surface is improved, and the thermal layer is easily in contact with a thermal head, thereby improving the heating efficiency; or the thermal recording paper bottom layer is made to function as a heat insulating layer, so that heat energy effectively acts on the melting of the thermal layer, thereby achieving high sensitivity of the thermal recording paper and improving the color rendering of the printing.

[0005] On the other hand, when using thermal recording paper with a thermal recording paper bottom layer for offset printing, the following problems may occur: the thermal recording paper bottom layer may peel off and easily cause paper picking. In addition, the thermal recording paper with a thermal recording paper bottom layer may easily cause scum to adhere to the thermal head, so it is required to improve the print head matching (sticking).

[0006] The subject of Patent Document 1 is to provide a binder for a thermal recording paper undercoat layer that can produce thermal recording paper with excellent printing color development, storage properties, print head compatibility, etc., and discloses a binder for a thermal recording paper undercoat layer formed from a copolymer latex having a composition, particle size, and gel component within a specific range.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 09-076636 Summary of the invention

[0010] Problems to be solved by the invention

[0011] However, the adhesive for thermal recording paper base layer described in Patent Document 1 cited above cannot be said to have reached a level that fully satisfies all the characteristics of thermal recording paper, such as printing color development, surface strength, and print head compatibility, and is still insufficient.

[0012] An object of the present invention is to provide a binder for a thermal recording paper backing layer and a composition for a thermal recording paper backing layer which can improve the printing color development, surface strength and print head compatibility of thermal recording paper.

[0013] Means for solving problems

[0014] The present invention provides an adhesive for a thermal recording paper base layer, which is an adhesive for a thermal recording paper base layer comprising a copolymer latex, wherein the copolymer latex contains 40% by mass or less of a structural unit derived from an aliphatic conjugated diene monomer and 60% by mass or more of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer, the average particle size of the copolymer latex based on a photon correlation method is 150 to 300 nm, and the swelling degree of the copolymer latex in toluene is 9 or more.

[0015] The adhesive for the thermal recording paper base layer having the above-mentioned structure has excellent smoothness of the thermal recording paper base layer, thereby improving the printing color rendering of the thermal recording paper. In addition, the surface strength of the thermal recording paper base layer is excellent, thereby improving the surface strength of the thermal recording paper. In addition, the print head compatibility of the thermal recording paper can be improved.

[0016] In addition, by making the copolymer latex contained in the binder for thermal recording paper base layer according to the present invention contain 11% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, the balance between the viscosity of the thermal recording paper base layer composition and the surface strength of the thermal recording paper base layer can be further improved.

[0017] The present invention also provides a composition for a thermal recording paper base layer comprising the above-mentioned binder for a thermal recording paper base layer and a pigment.

[0018] In addition, the composition for thermal recording paper base layer according to the present invention can improve the heat insulation property of the thermal recording paper base layer and further improve the printing color development property of the thermal recording paper by making the ratio of calcined kaolin in the pigment 80 mass % or more.

[0019] Effects of the Invention

[0020] According to the present invention, there are provided a thermal recording paper base layer adhesive and a thermal recording paper base layer composition capable of improving the balance among smoothness, surface strength and print head compatibility of the thermal recording paper base layer. DETAILED DESCRIPTION

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0022] The binder for a thermal recording paper base layer in the present embodiment includes a copolymer latex containing 40% by mass or less of a structural unit derived from an aliphatic conjugated diene monomer and 60% by mass or more of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer.

[0023] The structural unit derived from an aliphatic conjugated diene monomer refers to a structural unit formed by polymerization of an aliphatic conjugated diene monomer. As aliphatic conjugated diene monomers, monomers such as 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, substituted straight-chain conjugated pentadienes, and substituted and side-chain conjugated hexadienes can be cited. These monomers can be used alone or in combination of two or more. In the present embodiment, from the viewpoint of easy industrial manufacture, ease of acquisition and cost, 1,3-butadiene is preferably used. It should be noted that the copolymer latex can have one or more structural units derived from an aliphatic conjugated diene monomer.

[0024] The structural unit derived from the monomer copolymerizable with the aliphatic conjugated diene monomer refers to the structural unit formed by polymerization of the monomer copolymerizable with the aliphatic conjugated diene monomer. As the monomer copolymerizable with the aliphatic conjugated diene monomer, for example, monomers such as ethylene unsaturated carboxylic acid monomers, vinyl cyanide monomers, alkenyl aromatic monomers, unsaturated carboxylic acid alkyl ester monomers, unsaturated monomers containing hydroxyalkyl groups, and unsaturated carboxylic acid amide monomers can be cited. From the viewpoint of the balance between the viscosity of the composition for the thermal recording paper bottom layer and the surface strength of the thermal recording paper bottom layer, it is particularly preferred to contain the structural unit derived from the ethylene unsaturated carboxylic acid monomer. It should be noted that the copolymer latex can have one or more structural units derived from the monomer copolymerizable with the aliphatic conjugated diene monomer.

[0025] As the ethylene-based unsaturated carboxylic acid monomer, monocarboxylic acid or dicarboxylic acid monomers such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid may be mentioned. Alternatively, the anhydride thereof may be used. These monomers may be used alone or in combination of two or more.

[0026] As the vinyl cyanide monomer, acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile, etc. can be mentioned. These can be used alone or in combination of two or more. In the present embodiment, from the viewpoint of industrial ease of manufacture, ease of acquisition and cost, acrylonitrile or methacrylonitrile is particularly preferably used.

[0027] As alkenyl aromatic monomers, styrene, α-methylstyrene, methyl-α-methylstyrene, vinyltoluene and divinylbenzene etc. can be mentioned. These can be used alone or in combination of two or more. In the present embodiment, styrene is particularly preferably used from the viewpoint of industrial ease of manufacture, ease of acquisition and cost.

[0028] As the unsaturated carboxylic acid alkyl ester monomer, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, monomethyl fumarate, monoethyl fumarate and 2-ethylhexyl acrylate etc. can be enumerated. These can be used alone or in combination of two or more. In the present embodiment, from the viewpoint of industrially easy manufacture, the ease of obtaining and the cost, it is particularly preferred to use methyl methacrylate.

[0029] Examples of the unsaturated monomer containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, diethylene glycol maleate, diethylene glycol itaconate, 2-hydroxyethyl maleate, bis(2-hydroxyethyl) maleate, and 2-hydroxyethyl methyl fumarate. These may be used alone or in combination of two or more.

[0030] Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, and N,N-dimethyl acrylamide, etc. These may be used alone or in combination of two or more.

[0031] In addition to the above-mentioned monomers, monomers commonly used in emulsion polymerization, such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, and vinylidene chloride, can also be used.

[0032] When the copolymer latex has two or more structural units derived from monomers copolymerizable with the aliphatic conjugated diene monomer, the monomer copolymerizable with the aliphatic conjugated diene monomer preferably contains at least one ethylenically unsaturated carboxylic acid monomer selected from acrylic acid, methacrylic acid, fumaric acid and itaconic acid, and at least one alkenyl aromatic monomer selected from styrene and α-methylstyrene. Furthermore, in addition to the above monomers, it preferably contains at least one of at least one vinyl cyanide monomer selected from acrylonitrile and methacrylonitrile, at least one unsaturated carboxylic acid alkyl ester monomer selected from methyl acrylate and methyl methacrylate, and at least one hydroxyalkyl-containing unsaturated monomer selected from β-hydroxyethyl acrylate and β-hydroxyethyl methacrylate.

[0033] The copolymer latex contained in the adhesive for the bottom layer of thermal recording paper in the present embodiment contains 40% by mass or less of structural units derived from aliphatic conjugated diene monomers, and 60% by mass or more of structural units derived from monomers copolymerizable with aliphatic conjugated diene monomers. The structural units derived from aliphatic conjugated diene monomers preferably contain 20% by mass or more, more preferably 30% by mass or more, preferably 37% by mass or less, and more preferably 35% by mass or less. It should be noted that the above-mentioned mass % can be calculated based on the mass of all monomers constituting the copolymer latex. By adjusting to the above range, it is possible to achieve an improvement in the balance between the surface strength of the bottom layer of thermal recording paper and the print head matching of thermal recording paper. The proportion of structural units derived from aliphatic conjugated diene monomers can be 20-40% by mass, 20-37% by mass, 20-35% by mass, 30-40% by mass, 30-37% by mass, or 30-35% by mass. The ratio of the structural unit derived from the monomer copolymerizable with the aliphatic conjugated diene monomer may be 60-80 mass %, 63-80 mass %, 65-80 mass %, 60-70 mass %, 63-70 mass %, or 65-70 mass %.

[0034] The structural unit derived from the ethylene-based unsaturated carboxylic acid monomer preferably contains 1% by mass or more, more preferably 2% by mass or more, preferably 11% by mass or less, and more preferably 8% by mass or less. It should be noted that the above-mentioned mass % can be calculated based on the mass of all monomers constituting the copolymer latex. By setting it to the above range, the balance between the viscosity of the composition for the thermal recording paper bottom layer and the surface strength of the thermal recording paper bottom layer can be further improved. The proportion of the structural unit derived from the ethylene-based unsaturated carboxylic acid monomer can be 1-11% by mass, 1-8% by mass, 2-11% by mass, or 2-8% by mass.

[0035] The structural unit derived from the cyanide vinyl monomer is preferably 30% by mass or less, and more preferably 25% by mass or less. It should be noted that the above mass % can be calculated based on the mass of all monomers constituting the copolymer latex. The proportion of the structural unit derived from the cyanide vinyl monomer can be 0-30% by mass, 0-25% by mass, 5-30% by mass, 5-25% by mass, 10-30% by mass, 10-25% by mass, 15-30% by mass, or 15-25% by mass.

[0036] The structural unit derived from the alkenyl aromatic monomer preferably contains 30% by mass or more, more preferably 35% by mass or more, preferably 70% by mass or less, and more preferably 65% ​​by mass or less. It should be noted that the above mass % can be calculated based on the mass of all monomers constituting the copolymer latex. The proportion of the structural unit derived from the alkenyl aromatic monomer can be 30-70% by mass, 30-65% by mass, 35-70% by mass, or 34-65% by mass.

[0037] The average particle size of the copolymer latex contained in the adhesive for the thermal recording paper bottom layer in the present embodiment is 150 to 300 nm based on the photon correlation method, preferably 170 nm or more, more preferably 190 nm or more, preferably 270 nm or less, and more preferably 250 nm or less. The average particle size can be 150 to 270 nm, 150 to 250 nm, 170 to 300 nm, 170 to 270 nm, 170 to 250 nm, 190 to 300 nm, 190 to 270 nm, or 190 to 250 nm. By adjusting to the above range, the balance between the smoothness and surface strength of the thermal recording paper bottom layer can be improved. The average particle size of the copolymer latex can be adjusted, for example, by changing the amount of water and emulsifier when making the copolymer latex. Regarding the average particle size of the copolymer latex, the average particle size based on the photon correlation method can be measured by the dynamic light scattering method according to JIS Z8826.

[0038] The degree of swelling of the copolymer latex contained in the adhesive for the thermal recording paper base layer in this embodiment in toluene is 9 or more, preferably 9.5 or more, preferably 18 or less, and more preferably 14 or less. The swelling degree can be 9-18, 9-14, 9.5-18, or 9.5-14. By adjusting to the above range, it is possible to achieve an improvement in the balance between the surface strength of the thermal recording paper base layer and the print head matching of the thermal recording paper. The swelling degree of the copolymer latex in toluene can be adjusted by the amount and addition method of the aliphatic conjugated diene monomer when the copolymer latex is polymerized, the amount and addition method of the chain transfer agent and molecular weight regulator, and the change of the average particle size of the copolymer latex. The swelling degree of the copolymer latex in toluene can be measured by the method described in the examples.

[0039] The copolymer latex contained in the binder for the thermal recording paper base layer of the present embodiment is obtained by emulsion polymerization, etc. Hereinafter, the emulsion polymerization according to the embodiment will be described.

[0040] When the emulsion polymerization is carried out, in addition to the above-mentioned monomer components, an emulsifier (surfactant), a polymerization initiator, and, if necessary, a chain transfer agent, a reducing agent, etc. may be added.

[0041] As emulsifiers (surfactants), for example, anionic surfactants such as sulfate ester salts of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl oxide disulfonates, aliphatic sulfonates, aliphatic carboxylates, dehydroabietates, formaldehyde condensates of naphthalenesulfonic acid, sulfate ester salts of nonionic surfactants, alkyl ester type of polyethylene glycol, alkylphenyl ether type, and alkyl ether type nonionic surfactants, etc. can be cited. These can be used alone or in combination of two or more. The amount of the emulsifier can be appropriately adjusted in consideration of the combination of other additives, etc.

[0042] As the polymerization initiator, for example, water-soluble polymerization initiators such as lithium persulfate, potassium persulfate, sodium persulfate, ammonium persulfate, cumene hydroperoxide, benzoyl peroxide, tert-butyl hydroperoxide, acetyl peroxide, diisopropylbenzene hydroperoxide, and oil-soluble polymerization initiators such as 1,1,3,3-tetramethylbutyl hydroperoxide can be cited. These can be used alone or in combination of two or more. Among these, potassium persulfate, sodium persulfate, cumene hydroperoxide, or tert-butyl hydroperoxide is preferably used. The amount of the polymerization initiator is not particularly limited, and is appropriately adjusted in consideration of the monomer composition, the pH of the polymerization reaction system, the combination of other additives, etc.

[0043] Examples of the chain transfer agent include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, tert-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan, and n-octadecyl mercaptan; xanthate compounds such as dimethyl xanthate disulfide and diisopropyl xanthate disulfide; thiuram compounds such as tetramethyl thiuram disulfide, tetraethyl thiuram disulfide, and tetramethyl thiuram monosulfide; phenol compounds such as 2,6-di-tert-butyl-4-methylphenol and styrenated phenol; allyl compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide; vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile, and α-benzyloxyacrylamide; and chain transfer agents such as triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, terpinene, and α-methylstyrene dimer. These may be used alone or in combination of two or more. The amount of the chain transfer agent may be appropriately adjusted in consideration of the combination with other additives and the like.

[0044] As the reducing agent, for example, sulfites, bisulfites, pyrosulfites, dithionites, dithionites, thiosulfates, formaldehyde sulfonates, benzaldehyde sulfonates; carboxylic acids and salts thereof such as L-ascorbic acid, isoascorbic acid, tartaric acid, and citric acid; reducing sugars such as dextrose and sucrose; amines such as dimethylaniline and triethanolamine. These can be used alone or in combination of two or more. Among these, L-ascorbic acid and isoascorbic acid are preferred. The amount of the reducing agent can be appropriately adjusted in consideration of the combination of other additives, etc.

[0045] When the emulsion polymerization is carried out, in addition, for the purpose of controlling the molecular weight and cross-linking structure of the copolymer, saturated hydrocarbons such as pentane, hexane, heptane, octane, cyclohexane, and cycloheptane; unsaturated hydrocarbons such as pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, and 1-methylcyclohexene; hydrocarbon compounds such as aromatic hydrocarbons such as benzene, toluene, and xylene can be used. These can be used alone or in combination of two or more. From the viewpoint of ease of recovery, cyclohexene or toluene is particularly preferably used.

[0046] In addition, as required, electrolytes, oxygen capture agents, chelating agents, dispersants, defoamers, anti-aging agents, preservatives, antibacterial agents, flame retardants, ultraviolet absorbers and other additives may be added to the reaction system involved in the emulsion polymerization. The types and amounts of these additives are not limited and may be used in appropriate amounts.

[0047] In the range that does not damage the effect of the present invention, the adhesive for the bottom layer of the thermal recording paper can also be mixed with other adhesives and used. In addition, as required, additives such as preservatives, anti-aging agents, printing adaptability improvers, surfactants, etc. can be matched. The types and usage amounts of these additives can be used appropriately and in appropriate amounts.

[0048] Next, the composition for a thermal recording paper base layer according to the present embodiment will be described.

[0049] The composition for a thermal recording paper base layer according to the present embodiment includes the above-mentioned binder for a thermal recording paper base layer according to the present embodiment.

[0050] The composition for a thermal recording paper base layer of the present embodiment may further contain a pigment and other additives in addition to the binder for a thermal recording paper base layer according to the present embodiment.

[0051] The pigment contained in the thermal recording paper base layer composition of this embodiment includes inorganic pigments such as kaolin, calcium carbonate, titanium dioxide, barium sulfate, talc, calcined kaolin, aluminum hydroxide, amorphous silica, and crystalline silica; and organic pigments such as polystyrene resins, acrylic resins, and urea resins.

[0052] In the present embodiment, as the pigment, calcined kaolin is preferably used, and the proportion of calcined kaolin in the pigment is preferably 80% by mass or more. The proportion of calcined kaolin can be 80-100% by mass, 85-100% by mass, or 90-100% by mass. By making it within the above range, the heat insulation of the bottom layer of the thermal recording paper can be improved, and the printing color development of the thermal recording paper can be further improved. Therefore, by combining with the adhesive for the bottom layer of the thermal recording paper of the present embodiment, the printing color development of the thermal recording paper can be further improved.

[0053] The other additives are not particularly limited, and examples thereof include water-soluble binders such as polyvinyl alcohol, starch, and carboxymethyl cellulose, various commercially available dispersants, and defoaming agents.

[0054] The content of the binder for thermal recording paper base layer in the composition for thermal recording paper base layer is preferably 5 to 40 parts by mass (solid content) based on 100 parts by mass (solid content) of the pigment.

[0055] The composition for the thermal recording paper base layer is applied on the base paper and dried, and then the composition for the thermal recording layer is applied and dried to form a thermal recording layer with a thickness of about 2 to 20 μm on the thermal recording paper base layer, thereby obtaining thermal recording paper.

[0056] As a method for coating the composition for the thermal recording paper base layer on the base paper, a conventionally known method can be adopted, for example, coating can be carried out using a coating device such as a knife coater, a roll coater, an air knife coater, a short-dwell coater, a rod coater, a gate roll coater, a gravure coater, a curtain coater, a size press, a SymSizer, etc.

[0057] The embodiments of the binder for thermal recording paper base layer and the composition for thermal recording paper base layer include the following.

[0058] [1] A binder for a thermal recording paper backing layer, comprising a copolymer latex,

[0059] The copolymer latex contains 40% by mass or less of a structural unit derived from an aliphatic conjugated diene monomer and 60% by mass or more of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer.

[0060] The average particle size of the copolymer latex based on the photon correlation method is 150 to 300 nm.

[0061] The degree of swelling of the copolymer latex in toluene is 9 or more.

[0062] [2] The binder for a thermal recording paper base layer according to [1], comprising 20 to 40% by mass of a structural unit derived from an aliphatic conjugated diene monomer and 60 to 80% by mass of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer.

[0063] [3] The binder for a thermal recording paper base layer according to [1] or [2], wherein the copolymer latex contains 11% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.

[0064] [4] The binder for a thermal recording paper base layer according to any one of [1] to [3], wherein the copolymer latex contains 1 to 11% by mass of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.

[0065] [5] The binder for thermal recording paper base layer as described in [1], wherein the monomer copolymerizable with the aliphatic conjugated diene monomer contains at least one ethylenically unsaturated carboxylic acid monomer selected from acrylic acid, methacrylic acid, fumaric acid and itaconic acid, and at least one alkenyl aromatic monomer selected from styrene and α-methylstyrene.

[0066] [6] The binder for a thermal recording paper base layer according to [5], wherein the proportion of the structural unit derived from the alkenyl aromatic monomer is 30 to 70% by mass.

[0067] [7] The binder for a thermal recording paper backing layer as described in [5], further comprising, as a monomer copolymerizable with the aliphatic conjugated diene monomer, at least one vinyl cyanide monomer selected from acrylonitrile and methacrylonitrile, at least one unsaturated carboxylic acid alkyl ester monomer selected from methyl acrylate and methyl methacrylate, and at least one hydroxyalkyl-containing unsaturated monomer selected from β-hydroxyethyl acrylate and β-hydroxyethyl methacrylate.

[0068] [8] The binder for a thermal recording paper backing layer according to [7], wherein the proportion of the structural unit derived from the vinyl cyanide monomer is 5 to 30% by mass.

[0069] [9] The binder for a thermal recording paper base layer according to [7] or [8], wherein the proportion of the structural unit derived from the unsaturated carboxylic acid alkyl ester monomer is 0.1 to 5% by mass.

[0070]

[10] The binder for a thermal recording paper base layer according to any one of [7] to [9], wherein the proportion of the structural unit derived from the hydroxyalkyl group-containing unsaturated monomer is 0.1 to 5% by mass.

[0071]

[11] A composition for a thermal recording paper base layer, comprising the binder for a thermal recording paper base layer according to [1] to

[10] and a pigment.

[0072]

[12] The composition for a thermal recording paper base layer according to

[11] , wherein the ratio of the calcined kaolin in the pigment is 80% by mass or more.

[0073] Example

[0074] The present invention will be described in more detail below by way of examples. However, the present invention is not limited to the following examples. In addition, % and part are based on mass unless otherwise specified.

[0075] <Manufacturing of copolymer latex>

[0076] (Example 1)

[0077] The polymerization water and the first-stage emulsifier and polymerization initiator shown in Table 1 were charged into a nitrogen-substituted pressure-resistant polymerization reactor, and after sufficient stirring, the first-stage monomers and other components shown in Table 1 were added. From the time point when the temperature in the reactor reached 70°C, the second-stage monomers and other components shown in Table 1 were continuously added for 200 minutes. Then, the temperature in the polymerization reactor was raised to 80°C, and the polymerization was terminated at the time point when the polymerization conversion rate exceeded 95%. Next, the pH of the obtained copolymer latex was adjusted to 7.0 using sodium hydroxide to obtain copolymer latex A.

[0078] (Examples 2, 3, 5, 6, Comparative Examples 1 to 5)

[0079] Copolymer latexes B, C, E to K were obtained in the same manner as in Example 1 except that the monomers and other components were charged in the proportions shown in Tables 1 and 2.

[0080] (Example 4, Comparative Example 6)

[0081] Preparation of seed polymer: Into a nitrogen-substituted pressure-resistant polymerization reactor were placed polymerization water, the monomers shown in Tables 1 and 2, and other components, and reacted at 60° C. for 1 hour with stirring to prepare a seed polymer.

[0082] The seed polymer was transferred into a nitrogen-substituted pressure-resistant polymerization reactor, and polymerization water and the first-stage emulsifier and polymerization initiator shown in Tables 1 and 2 were charged into the nitrogen-substituted pressure-resistant polymerization reactor. After sufficient stirring, the first-stage monomers and other components shown in Tables 1 and 2 were added. From the time point when the temperature in the reactor reached 70°C, the second-stage monomers and other components shown in Tables 1 and 2 were continuously added for 200 minutes. Then, the temperature in the polymerization reactor was raised to 80°C, and the polymerization was terminated at the time point when the polymerization conversion rate exceeded 95%. Next, the pH of the obtained copolymer latex was adjusted to 7.0 using sodium hydroxide to obtain copolymer latexes D and L.

[0083] The components in Tables 1 and 2 are represented by the following abbreviations.

[0084] (monomer)

[0085] BDE: 1,3-butadiene

[0086] STY: Styrene

[0087] ACN: Acrylonitrile

[0088] MMA: Methyl Methacrylate

[0089] HEA: Hydroxyethyl acrylate

[0090] FA: Fumaric acid

[0091] IA: itaconic acid

[0092] AA: Acrylic acid

[0093] MAA: Methacrylic acid

[0094] (Other ingredients)

[0095] αMSD: α-methylstyrene dimer

[0096] TDM: tert-dodecyl mercaptan

[0097] CHX: Cyclohexene

[0098] KPS: Potassium Persulfate

[0099] EML: Sodium dodecylbenzene sulfonate

[0100] In the synthesis step of the copolymer latex, the polymerization conversion rate of the copolymer latex can be calculated by weighing the reaction liquid collected from the reactor, drying it at 150°C for 1 hour, and weighing it again to measure the solid content.

[0101] Polymerization conversion rate (%) = [(solid content (g) - solid content other than monomers contained in the reaction solution (g)) / monomer content added to the reaction system (g)] × 100

[0102] <Measurement of Average Particle Size of Copolymer Latex by Photon Correlation Method>

[0103] The results are shown in Tables 1 and 2.

[0104] <Determination of Swelling Degree of Copolymer Latex in Toluene>

[0105] The copolymer latex adjusted to pH 8 using sodium hydroxide was dried at 23°C for 12 hours, and then dried under reduced pressure at 23°C for 24 hours to prepare a film, which was cut into 5 mm squares. The cut film was immersed in toluene for 48 hours, and then the film was taken out and the mass of the film in the state of swelling in toluene was measured (the mass at this time is referred to as the "swollen film mass"). Then, the film was dried under reduced pressure at 23°C for 48 hours, and the mass of the dried film was measured (the mass at this time is referred to as the "dry film mass"). The swelling degree was calculated by the following formula. The results are shown in Tables 1 and 2.

[0106] Swelling degree = swollen film mass / dry film mass

[0107] <Production of Thermal Recording Paper Base Coating>

[0108] The mixture consisting of the following composition was stirred and dispersed to prepare a composition for a thermal recording paper base layer.

[0109]

[0110] Next, the thermal recording paper base layer composition was applied to a surface of 8 g / m 2 Coated on a weight of 45g / m 2 The high-quality paper is coated and dried to obtain the bottom coating paper of thermal recording paper.

[0111] <Evaluation of Smoothness of Thermal Recording Paper Base Coated Paper>

[0112] The measurement was performed using an Oken smoothness meter (manufactured by Asahi Seiko Co., Ltd.) using the air leakage method according to JAPAN TAPPI Pulp Test Method No. 5-2: 2000. The time required for 10 mL of air to leak from the gap between the measuring head and the thermal recording paper base coating paper as the test piece was evaluated in three grades: A, B, and C. The results are shown in Tables 1 and 2.

[0113] (Excellent) A>B>C (Inferior)

[0114] <Evaluation of Surface Strength of Thermal Recording Paper Base Coated Paper>

[0115] The degree of paper peeling when printing the thermal recording paper base coated paper simultaneously using the RI printer was visually determined, and the case where the amount of paper peeling was the smallest was set as 5.0, and the visual evaluation was performed relatively in units of 0.1 from 5.0 (excellent) to 1.0 (poor), and the evaluation was performed as follows, with three levels of A, B, and C. The results are shown in Tables 1 and 2.

[0116] A: 5.0~4.6

[0117] B: 4.5~4.0

[0118] C: 3.9~1.0

[0119] (Excellent) A>B>C (Inferior)

[0120] <Production of Thermal Recording Paper>

[0121] A developer dispersion, a dye dispersion, and a sensitizer dispersion having the following formulations were prepared.

[0122] [Developer dispersion]

[0123] 2,2'-bis(p-hydroxyphenyl)propane 6.0 parts

[0124] 10% polyvinyl alcohol aqueous solution 18.8 parts

[0125] 11.2 parts water

[0126] [Dye dispersion]

[0127] 2.0 parts of 3-diethylamino-6-methyl-7-anilinofluoran

[0128] 4.6 parts of 10% polyvinyl alcohol aqueous solution

[0129] 2.6 parts water

[0130] [Sensitizer dispersion]

[0131] 4.0 parts of β-naphthyl benzyl ether

[0132] Polyvinyl alcohol 5.0 parts

[0133] 3.0 parts water

[0134] Next, the developer dispersion, dye dispersion, sensitizer dispersion, and kaolin clay dispersion were mixed together at the following ratio to prepare a composition for a heat-sensitive recording layer.

[0135] [Thermosensitive recording layer composition]

[0136]

[0137] The heat-sensitive recording layer composition was applied in an amount of 7 g / m 2 The coating was applied on the primer-coated paper in a manner of , dried, and processed with a supercalender to produce thermal recording paper.

[0138] <Evaluation of Print Head Matching (Adhesive Paper) of Thermal Recording Paper>

[0139] Using a thermal printer (TH-PMD manufactured by Okura Electric), a printing test based on a 5 km operation test at a temperature of 120°C was performed, and the sticking of the paper to the thermal head was visually determined. The amount of adhesion of the sample to the thermal head was set to 5.0, and the visual evaluation was performed relatively in units of 0.1 from 5.0 (excellent) to 1.0 (poor). The evaluation was performed as follows, with three levels of A, B, and C. The results are shown in Tables 1 and 2.

[0140] A: 5.0~4.0

[0141] B: 3.9~3.0

[0142] C: 2.9~1.0

[0143] (Excellent) A>B>C (Inferior)

[0144] [Table 1]

[0145]

[0146] [Table 2]

[0147]

Claims

1. A binder for a thermal recording paper base layer, comprising a copolymer latex, The copolymer latex contains 30 to 37% by mass of a structural unit derived from an aliphatic conjugated diene monomer and 60% by mass or more of a structural unit derived from a monomer copolymerizable with the aliphatic conjugated diene monomer. The average particle size of the copolymer latex based on the photon correlation method is 150 to 300 nm. The degree of swelling of the copolymer latex in toluene is 9 or more and 14 or less.

2. The adhesive for thermal recording paper base layer according to claim 1, wherein The copolymer latex contains 11% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.

3. A composition for a thermal recording paper base layer, comprising the binder for a thermal recording paper base layer according to claim 1 or 2 and a pigment.

4. The composition for a thermal recording paper base layer according to claim 3, wherein The ratio of the calcined kaolin in the pigment is 80% by mass or more.

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