Dispersion liquid of colored resin particles, and water-based ink composition

Abstract

Provided are a dispersion liquid of colored resin particles, which contain a pH indicator having excellent stability, a water-based ink composition containing the dispersion liquid, and the like.　The dispersion liquid of colored resin particles is characterized in that the colored resin particles composed of at least a vinyl monomer and a pH indicator are dispersed in water with the content of the vinyl monomer being 30 mass% or more relative to the total polymer components constituting the colored resin particles, and the content of the pH indicator being 1 mass% or more relative to the total polymer components.　The vinyl monomer preferably contains an aromatic vinyl monomer or a (meth)acrylic monomer, and the pH indicator is preferably colorless in a color change region.

Description

Dispersion of colored resin particles, aqueous ink composition

[0001] The present specification relates to a dispersion of colored resin particles encapsulating a pH indicator, and an aqueous ink composition containing the same.

[0002] There has been a demand for written characters or coated areas to subsequently change color, develop color, or fade under certain conditions, either arbitrarily or over time, and various proposals have been made to address this. Conventionally, ink compositions using pH indicators and particles encapsulating pH indicators have already been known, including, for example: 1) a color-changing inkjet ink composition containing a pH indicator that changes color to colorless or a light color near neutral (see, for example, Patent Document 1); 2) a marking pen with a padding that is made of polypropylene fiber and that is equipped with an erasable ink composition containing water, a water-soluble organic solvent, a pH indicator, sodium hydroxide, and an organic amine (see, for example, Patent Document 2); 3) an aqueous ink composition comprising at least a fluorescent brightening agent, a pH indicator having a phthalein skeleton, a basic substance, water, and one or more selected from N-acylamino acid and / or a salt thereof, and N-acylmethyltaurine and / or a salt thereof (see, for example, Patent Document 3);

[0003] 4) A dye particle composition having a sensor function characterized by combining polar water-absorbing polymer particles with a pH indicator (see, for example, Patent Document 4); 5) A pH indicator copolymer obtained by copolymerizing a monomer component (A) containing an ionic moiety and a polymerizable moiety with a monomer component (B) containing a pH indicator moiety and a polymerizable moiety (see, for example, Patent Document 5); and the like are known.

[0004] However, when attempting to apply ink compositions using pH indicators such as those described in Patent Documents 1 to 3 to everyday life, problems exist, such as the inability to properly control the coloring and decoloring conditions, resulting in indelible lines or decoloring at a rate that makes characters and applied areas unrecognizable. Furthermore, when a phthalein-based dye is dissolved and used in an ink composition as a pH indicator, significant bleeding occurs and the ink is poorly stable, making it unsuitable for practical use. Patent Document 4 describes a technology for microencapsulating a pH indicator in a water-absorbing polymer, but its objectives are repeated use and fixation, which differ from those of the present disclosure in terms of the objective and technical concept of the invention. Furthermore, the pH indicator copolymer described in Patent Document 5 differs from that of the present disclosure in that it does not contain an ionic moiety (acrylic acid) as a monomer, and the objective and effects of the invention are also different.

[0005] JP 63-286481 A (claims, examples, etc.) JP 2005-329665 A (claims, examples, etc.) JP 2007-204573 A (claims, examples, etc.) JP 7-191013 A (claims, examples, etc.) JP 2012-163484 A (claims, examples, etc.)

[0006] The present disclosure has been made in view of the above-mentioned problems of the prior art and seeks to solve them, and aims to provide a dispersion of colored resin particles that, by encapsulating a pH indicator as a dye in specific particles, maintains its color-changing properties and has excellent stability when used as a dispersion of colored resin particles or in an aqueous ink composition, and in the case of an aqueous ink composition, has excellent bleeding prevention properties without impairing writing performance, an aqueous ink composition that uses this as a coloring material, and a writing instrument equipped with this.

[0007] In view of the above-mentioned conventional problems, the present inventor has conducted extensive research and has found that the above-mentioned desired dispersion of colored resin particles, an aqueous ink composition using the same, and a writing instrument can be obtained by using colored resin particles that are composed of at least a specific monomer and a pH indicator, etc., and by setting the content of the specific monomer and the content of the pH indicator to a specific value or more, respectively, and has thereby completed the present disclosure.

[0008] That is, the colored resin particle dispersion of the present disclosure is characterized in that the colored resin particles are composed of at least a vinyl monomer and a pH indicator, the content of the vinyl monomer being 30% by mass or more relative to all polymer components constituting the colored resin particles, and the content of the pH indicator being 1% by mass or more relative to all polymer components, and the colored resin particles are dispersed in water. The vinyl monomer preferably includes an aromatic vinyl monomer or a (meth)acrylic monomer. The pH indicator preferably exhibits colorlessness in the color change range. The pH indicator preferably is an oil-soluble substance having a phthalein skeleton. The content of the vinyl monomer preferably is 30% by mass to 95% by mass relative to all polymer components constituting the colored resin particles. The average particle diameter of the colored resin particles preferably is 20 to 800 nm. The aqueous ink composition of the present disclosure is characterized in that it contains the colored resin particle dispersion of the above-described configuration, a water-soluble organic solvent, and water. The writing instrument of the present disclosure is characterized in that it is equipped with the aqueous ink composition of the above-described configuration.

[0009] According to the present disclosure, by encapsulating a pH indicator as a dye in specific particles, the dispersion maintains its color change property and is stable when used as a dispersion of colored resin particles or in an aqueous ink composition. Furthermore, in the aqueous ink composition, the dispersion exhibits excellent bleeding prevention without impairing writing performance. This disclosure also provides a dispersion of colored resin particles, an aqueous ink composition using the dispersion as a colorant, and a writing instrument incorporating the dispersion. The objects and advantages of the present disclosure will be realized and attained by using the elements and combinations particularly pointed out in the claims. Both the general description above and the detailed description below are exemplary and explanatory and do not limit the scope of the disclosure as defined by the claims.

[0010] Embodiments of the present disclosure are described in detail below. However, please note that the technical scope of the present disclosure is not limited to the embodiments described below, but extends to the inventions set forth in the claims and their equivalents. Furthermore, the present disclosure can be implemented based on the contents disclosed in this specification and common general technical knowledge in the relevant field (including design matters and obvious matters). In this application, "color change" refers to a change from one color to another, a colored to colorless color loss, or a colorless to color development under certain conditions, and "color change property" refers to the property of causing the "color change."

[0011] <Dispersion of Colored Resin Particles> The dispersion of colored resin particles of the present disclosure is characterized in that colored resin particles composed of at least a vinyl-based monomer and a pH indicator are dispersed in water, the content of the vinyl-based monomer being 30% by mass or more relative to all polymer components constituting the colored resin particles, and the content of the pH indicator being 1% by mass or more relative to all polymer components.

[0012] The vinyl-based monomer used in the present disclosure is a monomer that constitutes the particle skeleton, and is used because it allows stable colored resin particles to be obtained while maintaining the color change property of the pH indicator, and because the obtained colored resin particles have a sufficient line density as a color material for writing instruments, etc., and can also suppress bleeding, etc. As the vinyl-based monomer used in the present disclosure, for example, an aromatic vinyl monomer or a (meth)acrylic monomer can be used.

[0013] Aromatic vinyl monomers that can be used are monomers containing an aromatic ring and a vinyl group in the molecule, such as styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, α-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-t-butylstyrene, 5-t-butyl-2-methylstyrene, vinylethylbenzene, divinylbenzene, trivinylbenzene, divinylnaphthalene, t-butoxystyrene, vinylbenzyldimethylamine, (4-vinylbenzyl)dimethylaminoethyl ether, N,N-dimethylaminoethylstyrene, N,N-dimethylaminomethylstyrene, 2-ethylstyrene, 3-ethylstyrene, 4-ethylstyrene, 2-t-butylstyrene, 3-t-butylstyrene, 4-t-butylstyrene, vinylxylene, vinylnaphthalene, vinylpyridine, diphenylethylene, and tertiary amino group-containing diphenylethylene. The tertiary amino group-containing diphenylethylene is not particularly limited, and examples thereof include 1-(4-N,N-dimethylaminophenyl)-1-phenylethylene. These aromatic vinyl monomers may be used alone or in combination of two or more. Among the aromatic vinyl monomer units, styrene and α-methylstyrene units are preferred in terms of reactivity and affinity with dyes.

[0014] Examples of (meth)acrylic monomers that can be used include at least one (each alone or a mixture of two or more) of methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, palmityl (meth)acrylate, stearyl (meth)acrylate, and behenyl (meth)acrylate. The term "(meth)acrylic acid" used above refers to "acrylic acid and / or methacrylic acid." These (meth)acrylic monomers may be used alone or in combination of two or more. Among the above (meth)acrylic monomer units, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, cyclohexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate units are preferred in terms of reactivity and affinity with dyes.

[0015] In the present disclosure, in addition to the above-mentioned aromatic vinyl monomers and (meth)acrylic monomers, aqueous monomers other than the above-mentioned aromatic vinyl monomers or (meth)acrylic monomers can be preferably used from the viewpoint of obtaining colored resin particles having excellent discoloration properties, etc. Examples of aqueous monomers that can be used include at least one of glycerin monomethacrylate, 2-sulfoethyl sodium methacrylate, polyethylene glycol monomethacrylate, polypropylene glycol monomethacrylate, polyethylene glycol-propylene glycol monomethacrylate, polyethylene glycol-tetramethylene glycol-monomethacrylate, and propylene glycol-polybutylene glycol-monomethacrylate.

[0016] Examples of pH indicators used in the present disclosure include indigo carmine, 1,3,5-trinitrobenzene, nitramine, tropaeolin O, Poirier Blue C4B, Alizarin Yellow GG, Alizarin Yellow R, thymolphthalein complexone, thymol blue, α-naphtholbenzein, α-cresolphthalein, p-xylenol blue, metacresol purple, α-naphtholphthalein, cyanine, rosolic acid, neutral red, phenolsulfonephthalein, bromocresol purple, methylthymol blue, α-nitrophenol, m-nitrophenol, chlorophenol red, methyl red, bromocresol green, bromophenol blue, methyl orange, metacresol purple, cresol red, bromophenol red, bromothymol blue, phenol red, naphtholphthalein, cresol red, cresolphthalein, thymol blue, phenolphthalein, thymolphthalein, and p-nitrophenol.

[0017] These pH indicators develop a color in a predetermined pH range and change or lose color to another color at least once within the predetermined pH range (color change range), and may be used alone or in combination. For example, when bromothymol blue is used in combination with another pH indicator and filled into particles, the color tone change can be increased and the color change range can be expanded, making it more practical and more preferable. Preferred pH indicators are those that are colorless within the color change range, and bromothymol blue, phenol red, naphtholphthalein, cresol red, cresolphthalein, thymol blue, phenolphthalein, and thymolphthalein are desirable, with those having a phthalein skeleton being most preferred. Naphtholphthalein, cresolphthalein, phenolphthalein, thymolphthalein, and the like, which have a phthalein skeleton, are oil-soluble substances that develop color under medium to strong alkaline conditions and become colorless under weak alkaline conditions or less, and are poorly soluble in water. Furthermore, colored resin particles composed of a pH indicator having this phthalein skeleton and a vinyl monomer reduce the surface area of ​​the pH indicator that comes into contact with the external environment of the particles, and therefore a particle dispersion liquid with excellent color-changing properties (coloring and decoloring) and stability can be obtained.

[0018] In addition, in the present disclosure, a light stabilizer or an ultraviolet absorber may be introduced into the particles together with the pH indicator, as long as the effects of the present disclosure are not impaired. Examples of the light stabilizer include HALS having a hindered amine skeleton, and examples of the ultraviolet absorber include benzophenone-based, benzotriazole-based, cyclic iminoester-based, cyanoacrylate-based, and polymer-type ultraviolet absorbers. Among these light stabilizers and ultraviolet absorbers, it is desirable to use polymerizable light stabilizers and ultraviolet absorbers that can polymerize with the vinyl monomer.

[0019] The dispersion of colored resin particles of the present disclosure is a dispersion of colored resin particles composed of at least a vinyl monomer and a pH indicator in water, and the method for producing the dispersion may include, for example, dissolving the pH indicator in a vinyl monomer such as the aromatic vinyl monomer or a (meth)acrylic monomer, or in a mixed monomer containing the vinyl monomer and another aqueous monomer, and using ammonium persulfate, potassium persulfate, hydrogen peroxide, or the like as a polymerization initiator, or a polymerization initiator further containing a reducing agent, and further using triallyl isocyanurate, triallyl isocyanurate, polyethylene glycol dimethacrylate, polypropylene glycol dimethacrylate, pentaerythritol acrylate, ditrimethylolpropane acrylate, dipentaerythritol acrylate, methoxylated bisphenol A methacrylate, pentaerythritol methacrylate, ditrimethylolpropane methacrylate, dipentaerythritol methacrylate, ethoxylated polyglycerin methacrylate, or the like. The composition can be produced by emulsion polymerization using a crosslinker and, if necessary, a polymerizable surfactant (emulsifier) ​​such as polyoxyethylene-1-(allyloxymethyl)-alkyl ether ammonium sulfate, ether sulfate, polyoxyethylene nonylpropenyl phenyl ether ammonium sulfate, polyoxyethylene nonylpropenyl phenyl ether, ammonium polyacrylate, styrene-maleic acid copolymer ammonium, polyoxyethylene alkyl ether, polyoxyethylene styrenated phenyl ether, polyoxyethylene polyoxypropylene glycol, polyoxyalkylene decyl ether, polyoxyethylene tridecyl ether, alkylbenzene sulfonate, dioctyl sulfosuccinate, sodium lauryl sulfate, polyoxyethylene alkyl ether phosphate, polyoxyethylene styrenated phenyl ether phosphate, polyoxyethylene styrenated phenyl ether sulfate, or polyoxyethylene alkyl ether sulfate. Although the dyeing was carried out simultaneously with the polymerization, it may also be carried out by dissolving a pH indicator after the polymerization.The use of a crosslinking agent such as triallyl isocyanurate is preferred because it can improve the heat resistance, mechanical properties, resistance to hydrolysis of light stabilizers and ultraviolet absorbers, and weather resistance of the colored resin particles.

[0020] In the present disclosure, during the emulsion polymerization, an appropriate amount of dicyclopenta(tenyl)(meth)acrylate monomer or the like may be further mixed with the vinyl monomer, particularly the (meth)acrylic monomer, to perform emulsion polymerization. When this dicyclopenta(tenyl)(meth)acrylate monomer is further mixed and then emulsion polymerized, stability is less likely to be lost even if the water in the dispersion evaporates, and a dispersion of colored resin particles with even greater stability can be obtained. Dicyclopenta(tenyl)(meth)acrylate monomers that can be used include dicyclopentanyl acrylate monomer, dicyclopentenyl acrylate, dicyclopentanyl methacrylate monomer, and dicyclopentenyl methacrylate. Furthermore, in the present disclosure, during the emulsion polymerization, in addition to the dicyclopenta(tenyl)(meth)acrylate monomer, for example, the vinyl monomer, a monomer having a reactive crosslinking group such as an epoxy group, a hydroxymethylamide group, or an isocyanate group, or a polyfunctional monomer having two or more vinyl groups may be blended in an appropriate amount to crosslink the resulting mixture.

[0021] In the present disclosure, the content of the vinyl monomer among the polymer components constituting the colored resin particles must be 30% by mass or more, preferably 30 to 95% by mass, more preferably 30 to 80% by mass, and particularly preferably 30 to 70% by mass, based on the total polymer components constituting the colored resin particles. In the present disclosure, "total polymer components" refers to the polymerizable components constituting the colored resin particles, specifically the total amount of the vinyl monomer used, the other monomer components used, and the crosslinking agent described below. Furthermore, achieving a vinyl monomer content of 30% by mass or more based on the total polymer components can be achieved by adjusting the type and amount of the polymer components used, as well as stirring conditions such as stirring speed, stirring time, and temperature. By achieving a vinyl monomer content of 30% by mass or more based on the total polymer components, the effects of the present disclosure can be achieved. On the other hand, a content of less than 30% by mass is undesirable because it results in poor stability over time.

[0022] Furthermore, among the polymer components constituting the colored resin particles, the content of other monomer components other than the vinyl-based monomer is the remainder of the total amount of the vinyl-based monomer and the crosslinking agent described below. Preferably, the content of other monomer components is 5 to 85% by mass with respect to the total polymer components, from the viewpoints of further exhibiting the effects of the present disclosure, dispersibility, and reactivity.

[0023] In the present disclosure, the content of the pH indicator should preferably be 1% by mass or more, and is preferably 3 to 30% by mass, more preferably 5 to 20% by mass, and particularly preferably 10 to 15% by mass, based on the total polymer components, from the viewpoints of color change property, obtaining sufficient line density, stability, etc. By setting the content of the pH indicator to 1% by mass or more, sufficient color change property and sufficient line density can be exhibited, while a content of the pH indicator of 30% by mass or less can exhibit high levels of stability, sufficient color change property, and sufficient line density.

[0024] The polymerizable surfactant that can be used as needed is not particularly limited as long as it is a polymerizable surfactant commonly used in the emulsion polymerization. For example, the polymerizable surfactant may be an anionic or nonionic polymerizable surfactant, such as Adeka Reasoap NE-10, NE-20, NE-30, NE-40, or SE-10N manufactured by Adeka Corporation; Latemul S-180, S-180A, or S-120A manufactured by Kao Corporation; Eleminol JS-20 manufactured by Sanyo Chemical Industries, Ltd.; or Aqualon KH-10 manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. The amount of these polymerizable surfactants used is 0 to 50% by mass, preferably 0.1 to 50% by mass, based on the total amount of monomers. The content of the crosslinking agent, such as triallyl isocyanurate, is 0 to 50% by mass, preferably 0.1 to 25% by mass, based on the total amount of monomers.

[0025] In the present disclosure, in the preferred embodiment described above, specifically, by dissolving the pH indicator in at least the vinyl monomer and emulsion-polymerizing the resulting mixture, or by dissolving a pH indicator in the resulting mixture after polymerization of at least the vinyl monomer and other monomer components, a dispersion of colored resin particles in which 20 to 50 mass % of colored resin particles are dispersed in water as a resin solid content can be obtained. This dispersion of colored resin particles is a colorant that has more satisfactory color change properties (change from one color to another, fading from colored to colorless, color development from colorless to colored) and superior stability over time than conventional colorants. For example, by selecting a suitable pH indicator to be used, it is possible to obtain colored resin particles encapsulating a pH indicator that develops color under medium to strong alkaline conditions (pH 10-14) and becomes colorless at weak alkaline conditions (pH 8-10) or lower, colored resin particles encapsulating a pH indicator that develops color under strong acidic conditions (pH 0-3) to medium alkaline conditions (pH 10-11) and becomes colorless under strong alkaline conditions (pH 11-14), colored resin particles encapsulating a pH indicator that develops color at near-neutral pH (pH 6-8) and becomes colorless under medium to strong acidic conditions (pH 0-4), or colorless under medium to strong alkaline conditions (pH 10-14), and colored resin particles encapsulating a pH indicator that develops color under near-neutral pH (pH 6-8) and becomes colorless under medium to strong acidic conditions (pH 0-4), or colorless under medium to strong alkaline conditions (pH 10-14). and colored resin particles that develop a color under medium to strong acidity (pH 0 to 4) and change to another color under weak acidity (pH 4 to 6), i.e., color-changing colored resin particles that can arbitrarily exhibit color change properties within a predetermined pH range can be produced. Therefore, the particles can be used in a variety of products, such as inks for writing instruments, materials with decolorizing and color-changing properties that do not require baking (solid writing instruments), cosmetics, sanitary and toiletry products (for humans and pets), personal care applications such as medical equipment, baby products, nursing care products, bath products, kitchenware, tableware, drinking water piping parts, hygiene products, home appliances, clothing, construction materials, agricultural materials, automobile interior parts, stationery, ink compositions for inkjet printers, and for determining the degree of carbonation of concrete.

[0026] In addition, in the present disclosure, the average particle size of the colored resin particles in the resulting dispersion of colored resin particles will vary depending on the vinyl monomer, the type and content of other monomers used, the polymerization conditions during polymerization, and the intended use of the product, but is preferably 20 to 800 nm, more preferably 20 to 300 nm, particularly 40 to 150 nm, and especially 60 to 110 nm. By achieving the above preferred average particle size range, the colored resin particles can be used in a variety of products, including writing instruments, inkjet inks, non-baked lead erasing / developing materials (solid writing instruments), cosmetic inks, sanitary and toiletry products (for humans and pets), and personal care applications. For ink compositions such as felt-tip pens, marking pens, and ballpoint pens, a particle size of 1000 nm or less will prevent clogging in the pen core and provide excellent storage stability. In the present disclosure, "average particle size" refers to the particle size (D50) at 50% cumulative volume of the particle size distribution calculated on a volume basis measured by laser diffraction. In the present disclosure (including the examples described below), the value is the D50 value measured using a particle size distribution measuring device (FPAR1000 (manufactured by Otsuka Electronics Co., Ltd.)).

[0027] The aqueous ink composition of the present disclosure is characterized by containing at least the dispersion of colored resin particles having the above-described configuration, a water-soluble organic solvent, and water. Examples of the water-soluble organic solvent that can be used include ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,5-hexanediol, 3-methyl-1,3-butanediol, and 2-methylpentanediol. alkylene glycols such as 1,2,4-diol, 3-methylpentane-1,3,5-triol, and 1,2,3-hexanetriol; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; glycerols such as glycerol, diglycerol, and triglycerol; lower alkyl ethers of glycols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol mono-n-butyl ether; N-methyl-2-pyrrolidone; and 1,3-dimethyl-2-imidalidinone.

[0028] Other water-soluble solvents that can be mixed include alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, isobutyl alcohol, hexyl alcohol, octyl alcohol, nonyl alcohol, decyl alcohol, and benzyl alcohol, amides such as dimethylformamide and diethylacetamide, and ketones such as acetone. The content of these water-soluble organic solvents varies depending on the type of writing implement, such as a felt-tip pen, marking pen, or ballpoint pen, and is particularly effective in an ink composition with a content of 1 to 40% by mass, and 10% by mass or less, based on the total amount of the ink composition, from the viewpoint of further improving the drying properties of drawn lines, and more preferably 3 to 8% by mass.

[0029] The content of water (tap water, purified water, ion-exchanged water, distilled water, pure water, etc.) is preferably 30 to 90 mass % and more preferably 40 to 60 mass % of the total amount of the ink composition. The content of colored resin particles varies depending on the product type, outflow mechanism, etc., but is preferably 1 to 30 mass % in terms of solid content of the total amount of the aqueous ink composition.

[0030] In the aqueous ink composition of the present disclosure, thickeners, preservatives or antifungals, pH adjusters, antifoaming agents, etc. can be appropriately selected and used as needed, as long as they do not impair the effects of the present disclosure. In the aqueous ink composition of the present disclosure, the colored resin particles in the dispersion of colored resin particles configured as described above exhibit discoloration depending on the pH concentration in the ink, so a pH adjuster can be preferably used. Examples of pH adjusters include at least one of ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, alkali metal salts of carbonate or phosphate such as sodium carbonate, and alkali metal hydroxides such as sodium hydroxide. Examples of thickeners include alkali-swellable association emulsions, alkali-swellable emulsions, polyvinylpyrrolidone, crosslinked acrylic acid polymers, carboxymethylcellulose (CMC) or its salts, fermented cellulose, crystalline cellulose, polysaccharides, etc. Examples of polysaccharides that can be used include xanthan gum, guar gum, hydroxypropylated guar gum, casein, gum arabic, gelatin, amylose, agarose, agaropectin, arabinan, curdlan, callose, carboxymethyl starch, chitin, chitosan, quince seed, glucomannan, gellan gum, tamarind seed gum, dextran, nigeran, hyaluronic acid, pustulan, funoran, HM pectin, porphyran, laminaran, lichenan, carrageenan, alginic acid, tragacanth gum, alkaloid gum, succinoglycan, locust bean gum, and tara gum. These may be used alone or in combination of two or more. Furthermore, if commercially available, these products can be used. Examples of the antiseptic or antifungal agent include at least one of phenol, sodium omadine, sodium pentachlorophenol, 1,2-benzisothiazolin-3-one, 2,3,5,6-tetrachloro-4(methylphonyl)pyridine, alkali metal salts of benzoic acid, sorbic acid, and dehydroacetic acid, and benzimidazole compounds.Examples of the lubricant include at least one of phosphate esters, polyalkylene glycol derivatives such as polyoxyethylene lauryl ether, fatty acid alkali salts, nonionic surfactants, fluorine-based surfactants such as perfluoroalkyl phosphate esters, and polyether-modified silicones such as polyethylene glycol adducts of dimethylpolysiloxane.

[0031] In addition, the aqueous ink composition of the present disclosure exhibits a shear rate of 383 sec -1 The viscosity at 25° C. is preferably 80 mPa sec or less, more preferably 50 mPa sec or less, and particularly preferably in the range of 2 to 20 mPa sec. By setting the viscosity at the above shear rate to 80 mPa sec or less, appropriate ink flow rate and ink ejection properties can be obtained, and by setting the viscosity to 2 mPa sec or more, the effect of suppressing bleeding and strike-through during writing, printing, etc. can be more easily exhibited.

[0032] Furthermore, from the viewpoint of leveling properties, the aqueous ink composition of the present disclosure preferably has a surface tension of 15 to 60 mN / m, and more preferably 25 to 50 mN / m. In this disclosure (including the examples described later), the surface tension of the ink (25°C) refers to a value measured using a surface tension measuring device: CBVP-Z manufactured by Kyowa Interface Science Co., Ltd.

[0033] The aqueous ink composition of the present disclosure can be prepared by appropriately combining at least the dispersion of colored resin particles having the above-described configuration, a water-soluble solvent, and other components depending on the intended use, such as a writing instrument (for a ballpoint pen, a marking pen, or the like) or an inkjet printer, and stirring and mixing the resulting mixture with a stirrer such as a homomixer, a homogenizer, or a disper, and further, if necessary, removing coarse particles from the ink composition by filtration or centrifugation, thereby preparing an aqueous ink composition suitable for use with a writing instrument, etc.

[0034] The writing instrument of the present disclosure is equipped with an aqueous ink composition of the above composition, and is equipped, for example, in a ballpoint pen, marking pen, or the like equipped with a pen tip such as a ballpoint pen tip, fiber tip, felt tip, plastic tip, fiber core, or porous core. Examples of ballpoint pens include those in which an aqueous ink composition for a writing instrument of the above composition is contained in a ballpoint pen ink container (refill), and the ink container contains an ink follower that is incompatible with the aqueous ink composition for a writing instrument of the above composition and has a low specific gravity relative to the aqueous ink composition, such as polybutene, silicone oil, or mineral oil. The structure of the ballpoint pen or marking pen is not particularly limited, and may be, for example, a direct-fill ballpoint pen or marking pen equipped with a collector structure (ink retention mechanism) in which the barrel itself serves as the ink container and is filled with the aqueous ink composition for a writing instrument of the above composition.

[0035] The aqueous ink composition of the present disclosure, which is configured as described above, contains at least a dispersion of colored resin particles and a water-soluble solvent, and by incorporating a pH indicator as a dye into specific particles, the aqueous ink composition has excellent stability even when the dispersion of colored resin particles is used while maintaining color change properties, and further has excellent bleeding prevention properties without impairing writing performance. Furthermore, the aqueous ink composition of the present disclosure contains colored resin particles that can exhibit color change properties at will within the above-mentioned predetermined pH range, so that the color change occurs under everyday use environments by absorbing carbon dioxide in the air and changing the pH of the composition. In addition to the above-mentioned particle properties, by appropriately adjusting the viscosity and surface tension to suitable ranges, as described above, it is possible to control the length of time from application, such as coating or writing, until the ink composition as a whole changes color (discolors and develops).

[0036] As described above, the aqueous ink composition of the present disclosure has excellent discoloration properties due to the pH indicator species, sufficient line density, and excellent stability over time, and therefore can be used in a variety of products, including ink compositions for writing implements such as felt-tip pens, marking pens, and ballpoint pens, inkjet inks, color-changing materials with non-baked leads (solid writing implements), cosmetic inks, sanitary and toiletry products (for humans and pets), personal care applications such as medical equipment, baby products, nursing care products, bath products, kitchenware, tableware, drinking water piping parts, hygiene products, home appliances, clothing, construction materials, agricultural materials, automotive interior parts, stationery, and inkjet printers.

[0037] Next, the present disclosure will be described in more detail with reference to Production Examples, Examples, and Comparative Examples, but the present disclosure is not limited to the following Examples, etc.

[0038] [Production Examples 1 to 20: Production of Dispersions of Colored Resin Particles (Particles 1 to 20)] Dispersions of colored resin particles were produced according to the following Production Examples 1 to 20. Note that "parts" below represent parts by mass. The total amount (total amount) of each component in Production Examples 1 to 20 is 100 mass%.

[0039] (Production Example 1) A 1-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1,000-ml separatory funnel for monomer introduction, and placed in a warm water bath. 370 parts of ion-exchanged water (in Production Examples 2 to 19, the amount of ion-exchanged water was adjusted to the range of 250 to 400 parts), 30 parts of a polymerizable surfactant [ADEKA CORPORATION, ADEKA REASOAP SE-10N], and 0.3 parts of ammonium persulfate were charged, and the internal temperature was raised to 50°C while introducing nitrogen gas. Meanwhile, a solution was prepared by mixing 60 parts of a monomer consisting of methyl methacrylate (Acrylate Ester M, Mitsubishi Chemical Corporation), 20 parts of a pH indicator [cresolphthalein, Fujifilm Wako Pure Chemical Industries, Ltd.], and 20 parts of a crosslinking agent [triallyl isocyanurate, Mitsubishi Chemical Corporation, "TAIC"]. This prepared solution was added from the separatory funnel to the flask maintained at a temperature of approximately 50°C over a period of 3 hours with stirring, and emulsion polymerization was carried out. The mixture was further aged for 5 hours to terminate the polymerization, yielding a dispersion of colored resin particles (particles 1). The content of methyl methacrylate was 79.2% by mass, based on the total polymer components constituting the colored resin particles, and the content of the pH indicator was 16.7% by mass, based on the total polymer components. The average particle diameter of the colored resin particles was 70 nm.

[0040] (Production Examples 2 to 20) In accordance with Production Example 1 above, dispersions of colored resin particles (particles 2 to 20) were obtained in Production Examples 2 to 20 using the formulations shown in Table 1 below, specifically, except that phenolphthalein or thymolphthalein was used instead of cresolphthalein as the other pH indicator, cyclohexyl methacrylate, n-butyl (meth)acrylate, styrene, or the like was used instead of methyl methacrylate as the vinyl monomer, and 1,4-butanediol acrylate ("Viscoat #195" manufactured by Osaka Organic Chemical Industry Co., Ltd.) was used as the crosslinking agent. The content of each vinyl monomer used relative to the total polymer components constituting the colored resin particles, as well as the content of the pH indicator and the average particle size of the colored resin particles are shown in Table 1 below.

[0041] The color change of the dispersions of the colored resin particles obtained in Production Examples 1 to 20 was evaluated by the following evaluation methods (an acid or a base was added dropwise so as to straddle the color change range of a pH indicator) and the dispersion stability of the dispersions. The results are shown in Table 1 below.

[0042] (Method for evaluating discoloration of dispersion) For each dispersion of colored resin particles of Production Examples 1 to 20 obtained above, an acid or a base was added dropwise so as to straddle the color change range of the pH indicator, i.e., the color of the dispersion before the addition of the acid or base was confirmed visually. The acid or base was added dropwise to the dispersion, and the dispersion was evaluated for discoloration, decolorization, or color development according to the following evaluation criteria. Evaluation criteria: A: Discoloration, decolorization, or color development. B: No color change.

[0043] (Method for evaluating dispersion stability of dispersion) 50 ml of the dispersion of each of the colored resin particles of Production Examples 1 to 20 obtained above was filled into a 100 ml highly transparent PP plastic bottle with a lid, sealed, and then stored with the cap facing up at 40°C for 1 month, after which the dispersion stability was evaluated according to the following evaluation criteria. Evaluation criteria: A: No separation of the dispersion was observed, and there was no sediment at the bottom, so it was good. B: Some separation was observed in the dispersion, and a small amount of sediment could be seen at the bottom. C: Significant separation was observed in the dispersion, and a lot of sediment could be seen at the bottom.

[0044] Examples 1 to 25 and Comparative Examples 1 to 4: Preparation of Aqueous Ink Compositions for Writing Instruments Examples 1 to 20 Using the dispersions of colored resin particles (Particles 1 to 20) produced in Production Examples 1 to 20 above, aqueous ink compositions for each writing instrument were prepared by a conventional method with the formulation shown below (total amount 100% by mass). The solids content of each dispersion of colored resin particles (Particles 1 to 20) produced in Production Examples 1 to 20 was 35 to 40% by mass. Ink Composition: (Total amount 100% by mass) Dispersion of colored resin particles for aqueous ink 50% by mass pH adjuster (triethanolamine) 1% by mass Water-soluble organic solvent (ethylene glycol) 5% by mass Ion-exchanged water 44% by mass Examples 21 to 25 and Comparative Examples 1 to 4 Aqueous ink compositions were prepared by a conventional method with the formulation shown in Table 2 below for Examples 21 to 25 and Comparative Examples 1 to 4.

[0045] Each of the obtained aqueous ink compositions (total amount 100% by mass) was subjected to a shear rate of 383 sec -1 The viscosity (25°C) and surface tension (25°C) at 25°C were measured by the methods described below, and the color development 3 hours after preparation of the pen body, color change 24 hours after writing, stability over time of the ink composition, and bleeding resistance were evaluated by the evaluation methods described below. Table 2 below shows the evaluation results for each of the aqueous ink compositions of Examples 1 to 25 and Comparative Examples 1 to 4.

[0046] (Method of Measuring Viscosity) Each of the obtained aqueous ink compositions (25°C) was measured at a shear rate of 383 sec using an EMD type viscometer manufactured by Tokyo Keiki Co., Ltd. -1 (Method for measuring surface tension) Each of the obtained aqueous ink compositions (25° C.) was measured using a surface tension measuring device: CBVP-Z manufactured by Kyowa Interface Science Co., Ltd.

[0047] (Method for evaluating color development 3 hours after preparation of the pen body) The water-based ink composition for writing instruments obtained above was used with a water-based felt-tip pen [PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.)] to evaluate the color development of the drawn line 3 hours after preparation of the pen body according to the following evaluation criteria. Evaluation criteria: A: Highly concentrated color development B: Slightly low concentration color development or slight discoloration C: Low concentration or discoloration

[0048] (Method for evaluating discoloration 24 hours after writing) The aqueous ink composition for writing implements obtained above was stored with the fine writing side facing down at 40°C for 2 weeks using a PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.), and then the fine writing side was used to write in a circle on writing paper, and the discoloration 24 hours after writing was evaluated according to the following evaluation criteria. Evaluation criteria: A: Same level as initial writing B: Blurred when written quickly C: Slight blurring D: Poor writing (unable to write)

[0049] (Method for evaluating the temporal stability of aqueous ink compositions) Each of the ink compositions prepared in Examples 1 to 25 and Comparative Examples 1 to 4 obtained above was filled into a glass vial, the lid was closed, and the composition was stored in an environment of 50°C. After a certain period of time, the period during which no aggregation or sedimentation was observed in the ink in the vial was defined as the "period during which stability is maintained," and the period was evaluated according to the following evaluation criteria. Evaluation criteria: A: 6 months or more B: 3 months or more but less than 6 months C: Less than 3 months

[0050] (Method for evaluating bleeding resistance) The water-based ink composition for writing implements obtained above was used to write spiral lines on writing paper using a PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.) and the bleeding state of the lines was visually evaluated according to the following evaluation criteria. Evaluation criteria: A: Clear handwriting without bleeding. B: Slight bleeding. C: Blurring and unclear handwriting. D: Significant bleeding and very unclear handwriting.

[0051]

[0052]

[0053] Considering Table 1 above, it was confirmed that the dispersions of colored resin particles of Production Examples 1 to 20, which fall within the scope of the present disclosure, were excellent in color development, color change, and dispersion stability. Furthermore, it was confirmed that the aqueous ink compositions of Examples 1 to 25 were excellent in color development 3 hours after preparation of the pen body, color change 24 hours after writing, stability over time of the ink composition, and bleeding resistance, compared to Comparative Examples 1 to 4, which contained a pH indicator.

[0054] The resulting dispersion of colored resin particles can be suitably used for writing instruments, inkjet inks, color-erasing and color-developing materials with non-baked leads (solid writing instruments), cosmetic inks, sanitary and toiletry products (for humans and pets), and personal care applications.

Claims

1. A dispersion of colored resin particles, characterized in that the colored resin particles are composed of at least a vinyl monomer and a pH indicator, and the content of the vinyl monomer is 30 mass% or more relative to the total polymer components constituting the colored resin particles, and the content of the pH indicator is 1 mass% or more relative to the total polymer components, and are dispersed in water.

2. The dispersion of colored resin particles according to claim 1, wherein the vinyl monomer includes an aromatic vinyl monomer or a (meth)acrylic monomer.

3. The dispersion of colored resin particles according to claim 1 or 2, wherein the pH indicator is colorless in the color change region.

4. The dispersion of colored resin particles according to claim 1 or 2, wherein the pH indicator is an oil-soluble substance having a phthalein skeleton.

5. The dispersion of colored resin particles according to claim 1 or 2, wherein the content of the vinyl monomer is 30% by mass to 95% by mass based on the total polymer components constituting the colored resin particles.

6. The dispersion of colored resin particles according to claim 1 or 2, wherein the average particle size of the colored resin particles is 20 to 800 nm.

7. An aqueous ink composition comprising at least the dispersion of colored resin particles according to claim 1 or 2, a water-soluble organic solvent, and water.

8. A writing instrument comprising the aqueous ink composition according to claim 7.

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