JP7799307B2 - Dispersant, coating liquid composition and thermal recording material - Google Patents

Description

The present invention relates to a dispersant for a thermosensitive color-forming component, a coating liquid composition for forming a thermosensitive recording layer containing the dispersant and the thermosensitive color-forming component, and a thermosensitive recording medium having a thermosensitive recording layer formed from the coating liquid on a support.

Traditionally, thermosensitive recording media have been used as one type of information recording medium. Thermosensitive recording media include a sheet-like support such as paper or film, with a thermosensitive recording layer on one side. The thermosensitive recording layer is formed by dispersing thermosensitive color-forming components such as leuco dyes, color developers, and sensitizers, and then coating and drying the dispersion on the support.

To obtain a dispersion of thermosensitive color-forming components, water-soluble resins such as polyvinyl alcohol, which are also used as binders in protective layers and thermosensitive layers, are widely used as dispersants. However, water-soluble resins such as polyvinyl alcohol do not have sufficient dispersant capabilities, and there are issues with the dispersibility of thermosensitive color-forming components and the time required for dispersion.

For example, Patent Document 1 describes an invention that aims to provide a thermal recording medium that has good chemical resistance, oil resistance, recording sensitivity, background whiteness, and other properties, as well as excellent water resistance and water-resistant plasticizer resistance, and a method for producing the same. In the examples, it describes the use of an aqueous solution of styrene-acrylic acid-acrylamide copolymer ammonium salt, which functions as a binder and a dispersant for leuco dyes and color developers.

Furthermore, Patent Document 2 describes an invention aimed at providing a thermal recording medium with excellent water resistance, alcohol resistance, and corrosion resistance, and describes a styrene-maleic acid copolymerized ammonium salt as a dispersant.

Furthermore, Patent Document 3 describes an invention that uses a leuco dye dispersion that does not significantly increase the viscosity of the dispersion or decrease the stability over time, even when the volume average particle size is reduced to 0.3 μm or less during the dispersion process, and aims to provide a thermal recording material with a high level of background whiteness. This invention uses a carboxylic acid-modified polyvinyl alcohol with a mass-average degree of polymerization of 100 to 400 and a modification degree of 0.2 to 1.0 mol % as a dispersant for the leuco dye.

However, the carboxylic acid-modified polyvinyl alcohol described in Patent Document 3 is thought to have other issues, as there are no commercially available grades suitable for use as a dispersant for leuco dyes. In addition, while the dispersants described in Patent Documents 1 and 2 each aim to solve the above-mentioned issues, they still have the issue that it takes a long time to finely disperse a dispersion containing a thermosensitive color-forming component such as a leuco dye to the desired particle size.

Japanese Patent Application Publication No. 5-301461 Japanese Patent Application Laid-Open No. 2004-181781 Japanese Patent Application Laid-Open No. 2008-254436

The present invention aims to provide a dispersant that can finely disperse thermosensitive color-forming components such as leuco dyes, color developers, and sensitizers to the desired particle size in a shorter time than conventional methods; a coating liquid composition for forming a thermosensitive recording layer that contains the thermosensitive color-forming components and the dispersant; and a thermosensitive recording medium having a thermosensitive recording layer formed from the coating liquid on a support.

The inventors have discovered that by using a dispersant containing an alkali neutralized salt of a specific copolymer as a dispersant for heat-sensitive color-forming components such as leuco dyes, color developers, and sensitizers, it is possible to obtain a dispersion with the desired particle size in a shorter time than conventional methods.

That is, the present invention provides:
<1> A dispersant for at least one thermosensitive color-forming component selected from the group consisting of a leuco dye (A), a color developer (B), and a sensitizer (C), characterized in that the dispersant contains an alkali neutralized salt of a copolymer (D) that satisfies the following conditions:
(1) Copolymer of a monomer consisting of (meth)acrylic acid and a (meth)acrylic acid ester (D-1)
and/or a copolymer (D-2) of a monomer consisting of (meth)acrylic acid, a (meth)acrylic acid ester, and a styrene.
(2) an acid value of 40 to 400 mgKOH/g; (3) a weight average molecular weight of 3,000 to 30,000. <2> The dispersant according to <1>, wherein the alkali is at least one selected from the group consisting of ammonia, sodium hydroxide, and dimethylethanolamine.
<3> A coating liquid composition for forming a thermosensitive recording layer, comprising at least one thermosensitive color-developing component selected from the group consisting of a leuco dye (A), a color developer (B), and a sensitizer (C), and the dispersant described in <1> or <2>.
<4> A thermosensitive recording medium having a thermosensitive recording layer formed on a support using the coating liquid composition according to <3>.
is.

According to the present invention, a dispersion liquid with the desired particle size can be obtained in a shorter time than conventional methods.

The following describes in detail an embodiment of the present invention. Note that the following description is an example of an embodiment of the present invention and is not intended to limit the scope of the present invention.

The dispersant of the present invention is a dispersant for a thermosensitive color-forming component and contains an alkali neutralized salt of copolymer (D) that satisfies the conditions described below.

<Thermal color-developing component>
The thermosensitive color-forming components contained in the thermosensitive recording layer of the thermosensitive recording medium are mainly a leuco dye (A) and a color developer (B), and a sensitizer (C) may be used as needed to enhance the color-forming sensitivity of the thermosensitive recording layer. The dispersant of the present invention is used to obtain a dispersion liquid in which at least one of these thermosensitive color-forming components (A) to (C) is dispersed.

<Leuco dye>
The leuco dye (A) is not particularly limited and may be used alone or in combination of two or more. Examples of the leuco dye (A) include triphenylmethanes such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide; fluorans such as 3-(N-ethyl-N-isopentyl)amino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-anilinofluoran, and 3-di(n-butyl)amino-6-methyl-7-anilinofluoran; and fluorenes such as 3,6,6'-tris(dimethylamino)spiro[fluorene-9,3'-phthalide] and 3,6,6'-tris(diethylamino)spiro[fluorene-9,3'-phthalide].

<Developer>
The developer (B) may be any electron-accepting compound capable of causing the leuco dye to develop color, and may be used alone or in combination of two or more. Examples of the developer (B) include organic acids such as benzoic acid, metal salts of organic acids such as zinc salicylate, phenols such as 4,4'-sulfonylbisphenol, thiophenols such as 4,4'-thiobis(6-tert-butyl-2-methylphenol), thiourea derivatives such as N,N'-diphenylthiourea, and diphenylsulfones such as 3,3'-diallyl-4,4'-dihydroxydiphenylsulfone.

<Sensitizer>
The sensitizer (C) may be any one that can increase the color-developing sensitivity of the thermosensitive recording layer, and may be used alone or in combination of two or more. Examples of the sensitizer (C) include 1,2-bis(3,4-dimethylphenyl)ethane, 1,2-diphenoxyethane, 1,2-bis(3-methylphenoxy)ethane, p-benzylbiphenyl, m-terphenyl, dimethyl phthalate, benzyl terephthalate, 2-naphthol benzyl ether, benzyl p-benzyloxybenzoate, 1-hydroxy-2-naphthonic acid phenyl ester, and di(p-methylbenzyl)oxalate.

<Dispersant>
The dispersant of the present invention may contain an alkali neutralized salt of copolymer (D) that satisfies the following conditions:
(1) Copolymer of a monomer consisting of (meth)acrylic acid and a (meth)acrylic acid ester (D-1)
and/or a copolymer (D-2) of a monomer consisting of (meth)acrylic acid, a (meth)acrylic acid ester, and a styrene.
(2) The acid value is 40 to 400 mgKOH/g. (3) The weight average molecular weight is 3,000 to 30,000.

The dispersant of the present invention contains an alkali neutralized salt of copolymer (D) that satisfies all of the above conditions (1) to (3), and can therefore achieve dispersion to a specified particle size in a shorter time than conventional dispersants.

The (meth)acrylic acid used in copolymer (D) of the present invention refers to acrylic acid and/or methacrylic acid.

The (meth)acrylic acid esters used in copolymer (D) of the present invention may be any (meth)acrylic acid ester. Specific examples include linear, branched, or cyclic alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate; aryl esters such as benzyl (meth)acrylate; functional group-containing esters such as 2-hydroxyethyl (meth)acrylate and glycidyl (meth)acrylate; and esters having a glycol structure such as 2-methoxyethyl (meth)acrylate. These may be used alone or in combination of two or more. Esters of linear, branched, or cyclic alkyls are preferred, more preferably esters of linear, branched, or cyclic alkyls having 2 to 18 carbon atoms in the alkyl group, even more preferably esters of linear, branched, or cyclic alkyls having 2 to 8 carbon atoms in the alkyl group, and particularly preferably esters of linear or branched alkyls having 2 to 8 carbon atoms in the alkyl group. Of these, methyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred.

The styrenes optionally used in copolymer (D) of the present invention refer to aromatic compounds capable of undergoing addition polymerization. Specific examples include styrene; styrene derivatives such as 4-methylstyrene, p-hydroxymethylstyrene, and divinylbenzene; and α-methylstyrene. These can be used alone or in combination of two or more types, with styrene being preferred.

Copolymer (D) can be obtained by any known production method without any particular limitations. It is usually obtained by solution polymerization in a solvent such as alcohol that can dissolve the monomers and their polymers, or by emulsion polymerization of the monomers in a state where the monomers are emulsified in water using an emulsifier or the like. In the case of solution polymerization, the solvent used during solution polymerization may be distilled off, if necessary, before or after the step of converting copolymer (D) into an alkali neutral salt.

Copolymer (D) must be a copolymer (D-1) of monomers consisting of at least (meth)acrylic acid and (meth)acrylic acid esters, or a copolymer (D-2) of monomers consisting of (meth)acrylic acid, (meth)acrylic acid esters, and styrenes, but may also be a mixture of copolymer (D-1) and copolymer (D-2).

The acid value of copolymer (D) must be 40 to 400 mgKOH/g from the perspective of dispersibility of the thermosensitive color-forming component. If the acid value is less than 40 mgKOH/g or more than 400 mgKOH/g, the ability to disperse the thermosensitive color-forming component will be insufficient. It is preferably 50 to 350 mgKOH/g, and more preferably 60 to 320 mgKOH/g. In this invention, the acid value refers to the value measured based on the neutralization titration method of JIS K0070-19K0070-1992.

The weight-average molecular weight of copolymer (D) must be 3,000 to 30,000. If the weight-average molecular weight is less than 3,000, the dispersion ability may be insufficient and the stability of the coating liquid composition after dispersion may be poor. Furthermore, if the weight-average molecular weight exceeds 30,000, it may be inconvenient to handle it in the process of neutralizing it with alkali to form an aqueous solution, to handle it during the preparation of the coating liquid composition, or to adjust the coating suitability and coating amount when forming a thermosensitive recording layer. It is preferably 4,000 to 28,000, and more preferably 6,000 to 25,000. In the present invention, the weight-average molecular weight is measured in terms of polystyrene using the GPC method under the following conditions:
Apparatus: HLC-8320GPC (manufactured by Tosoh Corporation)
Column: TSK-gel SUPER MULTIPORE HZ-H and SUPER MULTIPORE HZ-M, manufactured by Tosoh Corporation, used in combination. Eluent: tetrahydrofuran, 0.35 mL/min. Standard: TSKgel standard polystyrene (manufactured by Tosoh Corporation).

<Preparation of Alkali Neutralized Salt of Copolymer (D)>
The alkali neutralized salt of copolymer (D) of the present invention can be obtained by preparing an aqueous solution using an alkali in an amount equivalent to 50 to 110% of the acid value of copolymer (D). Here, the aqueous solution refers to a solution that can be uniformly diluted with water. While the solution is typically an aqueous solution, it may contain a solvent other than water, such as a water-compatible alcohol, as long as it forms a uniform aqueous solution. The concentration of the alkali neutralized salt of copolymer (D) contained in the dispersant can be adjusted appropriately taking into account the optimal concentration and viscosity of the coating liquid composition for forming a thermosensitive recording layer, but is preferably 20 to 40% by mass.

The alkali used to obtain the alkali neutralized salt of copolymer (D) includes, for example, ammonia; alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; and organic amines such as dimethylethanolamine, triethanolamine, and morpholine. However, at least one selected from the group consisting of ammonia, sodium hydroxide, and dimethylethanolamine is preferred. Among these, dimethylethanolamine is more preferred from the viewpoint of dispersibility of the thermosensitive color-developing component.

<Coating liquid composition for forming a thermosensitive recording layer and its preparation>
The coating liquid composition for forming a thermosensitive recording layer of the present invention (hereinafter, sometimes simply referred to as the coating composition) contains at least one thermosensitive color-forming component selected from the group consisting of a leuco dye (A), a color developer (B), and a sensitizer (C), and a dispersant. As other components, various conventional binder components for forming a thermosensitive recording layer may be appropriately mixed and used. Specific examples include water-soluble polymers such as polyvinyl alcohol, starch or its derivatives, cellulose derivatives such as hydroxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose, sodium polyacrylate, polyvinylpyrrolidone, acrylamide/acrylic acid ester copolymers, acrylamide/acrylic acid ester/methacrylic acid terpolymers, alkali salts of styrene/maleic anhydride copolymers, alkali salts of isobutylene/maleic anhydride copolymers, polyacrylamide, sodium alginate, gelatin, and casein, as well as emulsions such as polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid esters, vinyl chloride/vinyl acetate copolymers, polybutyl methacrylate, and ethylene/vinyl acetate copolymers, and latexes such as styrene/butadiene copolymers and styrene/butadiene/acrylic copolymers. Other components may include, as needed, inorganic or organic fillers such as silica, talc, calcium carbonate, and polystyrene; lubricants such as fatty acid metal salts and waxes; ultraviolet absorbers, storage stabilizers, colorants, and antifoaming agents. The coating composition can be obtained by mixing and dispersing a thermosensitive color-forming component and a dispersant. When preparing a coating composition containing multiple thermosensitive color-forming components, the dispersion method is not particularly limited; each component may be dispersed individually and then mixed, or all thermosensitive color-forming components may be dispersed together. Examples of dispersing devices that can be used include a bead mill, a ball mill, an attritor, a sand mill, and a high-pressure jet mill. A dispersion method using beads (also referred to as media) is preferred, and microparticulation can be achieved by using zirconia beads with a diameter of 0.5 mm or less, or by coarsely pulverizing the material using zirconia beads with a diameter of 0.5 mm to 1.0 mm, followed by dispersing the material using zirconia beads with a diameter of 0.5 mm or less.

The particle size of the coating composition refers to the 90% cumulative particle size (hereinafter simply referred to as particle size D90) measured using a particle size distribution analyzer (Microtrac UPAEX150, manufactured by Nikkiso Co., Ltd.) using dynamic light scattering/laser Doppler methods. The particle size D90 is preferably 2.0 μm or less. Furthermore, from the standpoint of coatability and handling, the viscosity of the coating composition is preferably 10 to 800 mPa·s as measured with a Brookfield viscometer at 25°C. In the examples of the present invention, in order to simply evaluate the reduction in dispersion time, 25 parts of 0.3 mm zirconia beads, dispersant (6 parts as copolymer (D)), 30 parts of heat-sensitive color-developing component, 50 parts of ion-exchanged water, and 0.1 part of Surfynol 104E (manufactured by Nissin Chemical Industry Co., Ltd.) were weighed into a 250 cc plastic bottle, and the bottle was shaken in a paint shaker (manufactured by Toyo Seiki Seisakusho Co., Ltd.) to obtain a coating liquid composition having a particle diameter D90 measured at regular intervals of dispersion. This was used as an index for evaluating the dispersion state.

<Thermal Recording Material and Its Preparation>
The thermosensitive recording medium of the present invention has a thermosensitive recording layer formed from the coating liquid composition on a support. The support is not particularly limited as long as it is one that is commonly used as a support for thermosensitive recording mediums, and examples thereof include paper, synthetic paper, and laminated paper.

A thermosensitive recording layer can be formed on a support by applying a coating liquid composition to the support and drying it. There are no particular restrictions on the coating method, and it can be selected appropriately depending on the purpose. Examples include blade coating, air knife coating, gravure coating, roll coating, spray coating, dip coating, bar coating, and extrusion coating.

<Layers other than the thermosensitive coloring layer>
The thermosensitive recording medium of the present invention may have at least the thermosensitive recording layer on a support, but may also have a protective layer for protecting the thermosensitive recording layer or an intermediate layer between the support and the thermosensitive recording layer. The protective layer or intermediate layer contains a binder and an inorganic or organic filler, and may further contain other components as necessary.

The binder used in the protective layer or intermediate layer is not particularly limited, and any known binder can be appropriately selected depending on the purpose. Examples include polyvinyl alcohol, cellulose derivatives, starch or its derivatives, carboxyl-modified polyvinyl alcohol, and polyacrylic acid or its derivatives.

The inorganic or organic fillers used in the protective layer or intermediate layer can be the same as those used in the thermosensitive recording layer, but aluminum hydroxide and silica are particularly useful. The amount of inorganic or organic filler is preferably 30% to 80% by mass of the entire protective layer, and more preferably 40% to 70% by mass. The coating weight of the protective layer is 3.0 g/ ^m2 or less; if it is more than this, it may hinder the transfer of heat to the thermosensitive recording layer below the protective layer.

The protective layer and intermediate layer are formed on the support in the same manner as the thermal recording layer.

Thermal recording media of the present invention are not particularly limited in shape and can be appropriately selected depending on the purpose, with suitable forms including labels, sheets, and rolls. Applications include cash register receipts, and can also be used as pressure-sensitive thermal recording labels by laminating an adhesive layer and a release liner on the back side, or as heat-activated thermal recording labels that do not require a release liner by providing a heat-activated adhesive layer on the back side that has no adhesive properties at room temperature (10°C to 30°C) but develops adhesive strength when heated.

The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. Note that "parts" and "%" without a unit indicated are all "parts by mass" and "% by mass."

(Production Example 1)
<Production of Copolymer (D)>
A four-neck separable flask equipped with a stirrer, a condenser, a thermometer, a nitrogen inlet tube, and a dropping funnel was charged with 1,000 parts of propylene glycol monomethyl ether acetate as a solvent, and the temperature was raised to 143 ° C. under a nitrogen atmosphere while stirring. The dropping funnel was charged with a monomer mixture obtained by uniformly mixing 50 parts of acrylic acid, 40 parts of methacrylic acid, 710 parts of methyl methacrylate, 200 parts of butyl acrylate, and 25 parts of di-tert-butyl peroxide as a polymerization initiator, and the mixture was added dropwise over 3 hours to a flask with an internal temperature of 143 ° C. After completion of the dropwise addition, the internal temperature was maintained at 140 ° C. for 2 hours, and then the temperature was raised to 190 ° C. to distill off the solvent and unreacted materials in the flask. After distillation, the mixture was removed from the flask and cooled to obtain a solid copolymer (D-1-1). The copolymer (D-1-1) had a weight average molecular weight (Mw) of 10,000 and an acid value of 60 mgKOH/g.
<Preparation of Dispersant 1>
A four-neck flask equipped with a stirrer, a condenser, and a thermometer was charged with 671 parts of ion-exchanged water, and 300 parts of powdered copolymer (D-1-1) was added while stirring. Then, 29 parts of dimethylethanolamine (acid value equivalent of copolymer (D-1-1)) was added as an alkali, and the internal temperature was raised to 80 ° C. Then, the internal temperature was kept at 80 ° C. for 2 hours to dissolve the copolymer (D-1-1), and a dispersant 1 containing 30% by mass of copolymer (D-1-1) was prepared.

(Production Examples 2 to 14, 16)
<Production of Copolymer (D)>
Solid copolymers (D-1-2), (D-2-1) to (D-2-12), and (RD-2) were obtained in the same manner as in Production Example 1, except that the type and amount of solvent, the type and amount of monomer, and the amount of polymerization initiator were changed as shown in Table 1. The weight average molecular weight (Mw) and acid value of each copolymer are shown in Table 2.
<Preparation of Dispersants 2 to 14 and 16>
Dispersants 2 to 14 and 16 containing 30 mass% of copolymers (D-1-2), (D-2-2) to (D-2-12) and (RD-2) were prepared in the same manner as in Production Example 1, except that the amount of water charged and the type and amount of alkali charged (all acid value equivalents of each copolymer (D)) were changed as shown in Table 3. 28% aqueous ammonia was used.

(Production Example 15)
<Production of Acrylamide-Containing Monomer Copolymer (RD-1) and Preparation of Dispersant 15>
A four-neck flask equipped with a stirrer, a cooler, a thermometer, and a nitrogen inlet tube was charged with 429 parts of isopropyl alcohol, 300 parts of acrylic acid, 30 parts of methacrylic acid, 470 parts of styrene, 100 parts of butyl acrylate, 100 parts of acrylamide, 15 parts of normal dodecyl mercaptan, and 20 parts of azoisobutyronitrile, and the mixture was heated to an internal temperature of 85 ° C. over 30 minutes while stirring under a nitrogen atmosphere. After holding at 85 ° C. for 1 hour, 1 part of azoisobutyronitrile was added. After holding for a further 1 hour, 274 parts of 28% aqueous ammonia and 2160 parts of ion-exchanged water were charged and stirred for 10 minutes. The internal temperature was then raised to 95 ° C. and distilled under atmospheric pressure to remove the isopropyl alcohol, and a dispersant 15 containing 30% by weight of styrene acrylic acid acrylamide copolymer (RD-1) was obtained. The copolymer (RD-1) had a weight average molecular weight (Mw) of 15,000 and an acid value of 250 mgKOH/g.

(Production Example 17)
<Preparation of Anionic Polyvinyl Alcohol (RD-3) Dispersant>
A four-neck flask equipped with a stirrer, a condenser, and a thermometer was charged with 800 parts of ion-exchanged water, and while stirring, 200 parts of anionic polyvinyl alcohol having sulfonic acid groups (Gohsenex L3266, manufactured by Mitsubishi Chemical Holdings Corporation), which is a common polyvinyl alcohol for dispersing dyes and color developers, was then charged. The internal temperature was then raised to 80°C and maintained at that temperature for 2 hours to obtain Dispersant 17 containing 20% by mass of anionic polyvinyl alcohol (RD-3).

Explanation of abbreviations in Tables 1 and 2: PGM-Ac: propylene glycol monomethyl ether acetate (reflux temperature: 143 to 146°C)
Xy: xylene (reflux temperature: 138 to 141°C)
AA: Acrylic acid MAA: Methacrylic acid St: Styrene MMA: Methyl methacrylate BA: Butyl acrylate AAm: Acrylamide MAn: Maleic anhydride MA-nBu: Maleic acid mono-n-butyl ester (half ester of maleic anhydride and n-butanol)
DTBP: di-tert-butyl peroxide

Explanation of abbreviations in Table 3 DMEA: dimethylethanolamine NaOH: sodium hydroxide KOH: potassium hydroxide TEA: triethanolamine

<Evaluation of leuco dye dispersibility in dispersants>
Example 1
25 parts of 0.3 mm zirconia beads, 16 parts of Dispersant 1 described in Production Example 1 as a dispersant (4.8 parts as Copolymer (D-1-1)), 30 parts of leuco dye (3-di-butylamino-6-methyl-7-anilinofluoran (ODB-2)), 54 parts of ion-exchanged water, and 0.1 parts of Surfynol 104E (manufactured by Nissin Chemical Industry Co., Ltd.) were weighed into a 250 cc plastic bottle and shaken for 120 minutes with a paint shaker, after which 70 parts of ion-exchanged water was added and mixed to become uniform. Thereafter, the zirconia beads were removed, and coating liquid composition L1 was obtained. 25 parts of the coating composition were sampled 60 minutes and 90 minutes after the start of shaking, and the particle size (D90) of the coating composition, along with that of the coating composition after 120 minutes of shaking, was measured using a particle size distribution measuring device (Microtrac UPAEX150, manufactured by Nikkiso Co., Ltd.) using a dynamic light scattering/laser Doppler method. The measurement results are shown in Table 4. The faster the D90 value reaches 2.0 μm or less, the better the dispersibility.

(Examples 2 to 14, Comparative Examples 1 and 2)
Coating liquid compositions L2 to L16 were obtained in the same manner as in Example 1, except that the type of dispersant was changed as shown in Table 4. The particle diameters of coating liquid compositions L2 to L16 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 4.

(Comparative Example 3)
25 parts of 0.3 mm zirconia beads, 24 parts of Dispersant 17 described in Production Example 17 as a dispersant (4.8 parts as Copolymer (RD-3)), 30 parts of leuco dye (3-di-butylamino-6-methyl-7-anilinofluoran (ODB-2)), 46 parts of ion-exchanged water, and 0.1 parts of Surfynol 104E (manufactured by Nissin Chemical Industry Co., Ltd.) were weighed into a 250 cc plastic bottle and shaken for 120 minutes using a paint shaker. 70 parts of ion-exchanged water was then added and mixed to a uniform consistency. The zirconia beads were then removed to obtain Coating Liquid Composition L17. The particle diameter of Coating Liquid Composition L17 at each time point was measured in the same manner as in Example 1. The measurement results are shown in Table 4.

Table 4 shows that the particle diameters of coating liquid compositions L1 to L14 in Examples 1 to 14 all had D90 values of less than 2.0 μm after 90 minutes of shaking, indicating that the dispersants used had excellent leuco dye dispersibility. On the other hand, the particle diameters of coating liquid compositions L15 to L17 in Comparative Examples 1 to 3 all had D90 values of 2.0 μm or greater even after 90 minutes of shaking, indicating that they all had inferior dispersibility compared to the Examples.

<Evaluation of developer dispersibility of dispersant>
Example 15
Coating liquid composition M1 was obtained by replacing the leuco dye in Example 1 with a developer (4,4'-sulfonylbisphenol). The particle diameters of coating liquid composition M1 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 5.

(Examples 16 to 28, Comparative Examples 4 and 5)
Coating liquid compositions M2 to M16 were obtained in the same manner as in Example 15, except that the type of dispersant was changed as shown in Table 5. The particle diameters of coating liquid compositions M2 to M16 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 5.

(Comparative Example 6)
Coating liquid composition M17 was obtained by replacing the leuco dye of Comparative Example 3 with a developer (4,4'-sulfonylbisphenol). The particle diameters of coating liquid composition M17 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 5.

Table 5 shows that the particle diameters of coating liquid compositions M1 to M14 in Examples 15 to 28 all had D90 values of less than 2.0 μm after 90 minutes of shaking, indicating that the dispersants used had excellent leuco dye dispersibility. On the other hand, the particle diameters of coating liquid compositions M15 to M17 in Comparative Examples 4 to 6 all had D90 values of 2.0 μm or greater even after 90 minutes of shaking, indicating that all of them had inferior dispersibility compared to the Examples.

<Evaluation of Color Former Dispersibility in Dispersants>
(Example 29)
Coating liquid composition N1 was obtained by replacing the leuco dye in Example 1 with a color former (1,2-diphenoxyethane). The particle diameters of coating liquid composition N1 at each time point were measured in the same manner as in Example 1. The measurement results are shown in Table 6.

(Examples 30 to 42, Comparative Examples 7 and 8)
Coating liquid compositions N2 to N16 were obtained in the same manner as in Example 29, except that the type of dispersant was changed as shown in Table 6. The particle diameters of coating liquid compositions N2 to N16 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 6.

(Comparative Example 9)
Coating liquid composition N17 was obtained by replacing the leuco dye of Comparative Example 3 with a color former (1,2-diphenoxyethane). The particle diameters of coating liquid composition N17 at each time were measured in the same manner as in Example 1. The measurement results are shown in Table 6.

Table 6 shows that the particle diameters of coating liquid compositions N1 to N14 of Examples 29 to 42 all had D90 values of less than 2.0 μm after 90 minutes of shaking, indicating that the dispersants used had excellent leuco dye dispersibility. On the other hand, the particle diameters of coating liquid compositions N15 to N17 of Comparative Examples 7 to 9 all had D90 values of 2.0 μm or greater even after 90 minutes of shaking, indicating that they all had inferior dispersibility compared to the Examples.

Claims (3)

A dispersant for at least one thermosensitive color-forming component selected from the group consisting of a leuco dye (A) and a sensitizer (C), characterized in that the dispersant contains an alkali neutralized salt of a copolymer (D) that satisfies all of the following conditions, and the alkali is dimethylethanolamine :
(1) Copolymer of a monomer consisting of (meth)acrylic acid and a (meth)acrylic acid ester (D-1)
and/or a copolymer (D-2) of a monomer consisting of (meth)acrylic acid, a (meth)acrylic acid ester, and a styrene.
(2) The acid value is 40 to 400 mgKOH/g. (3) The weight average molecular weight is 3,000 to 30,000. A coating liquid composition for forming a thermosensitive recording layer, comprising at least one thermosensitive color-developing component selected from the group consisting of a leuco dye (A) and a sensitizer (C), and the dispersant according to claim 1 . A thermosensitive recording medium having a thermosensitive recording layer formed from the coating liquid composition according to claim 2 on a support.