Dispersion liquid of colored resin particles and aqueous ink composition

Abstract

To provide a dispersion liquid of colored resin particles encapsulating a pH indicator having excellent stability and an aqueous ink composition containing the same or the like.SOLUTION: There is provided a dispersion liquid of colored resin particles constituted by at least a vinyl-based monomer and a pH indicator, wherein the colored resin particles in which the content of the vinyl-based monomer is 30 mass% or more based on all polymer components constituting the colored resin particles and the content of the pH indicator is 1 mass% or more based on all polymer components are dispersed in water. The vinyl-based monomer preferably contains an aromatic vinyl monomer or a (meth)acrylic monomer and the pH indicator preferably exhibits colorless in a color change region.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] There has been a demand for written or painted parts that can subsequently change, develop or fade color under certain conditions, either arbitrarily or over time, and various proposals have been made to address this demand. Conventionally, ink compositions using pH indicators and particles encapsulating pH indicators have been already known. For example, 1) A color-changing inkjet ink composition containing a pH indicator that changes color to colorless or light color near neutral (see, for example, Patent Document 1); 2) A marking pen with a padding type containing an erasable ink composition containing water, a water-soluble organic solvent, a pH indicator, sodium hydroxide and an organic amine, the padding being made of polypropylene fiber (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 substances selected from the group consisting of 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 and a monomer component (B) containing a pH indicator moiety and a polymerizable moiety (see, for example, Patent Document 5); etc. are known.

[0004] However, when ink compositions using pH indicators such as those described in Patent Documents 1 to 3 are applied to daily life, the conditions for coloring and decoloring cannot be controlled well, and there are problems such as drawn lines not disappearing, or characters or applied parts being decolored at a speed that makes them unrecognizable, etc. Furthermore, when a phthalein dye is dissolved as a pH indicator and used in an ink composition, it causes significant bleeding and is poor in stability, making it unsuitable for practical use. The above-mentioned Patent Document 4 is a technology for microencapsulating a pH indicator with a water-absorbing polymer, but the problem of the invention is repeated use and adhesion, and the problem and technical idea of ​​the invention are different from those of the present invention. Also, the pH indicator copolymer of the above-mentioned Patent Document 5 is different from the structure of the invention in that it does not contain an ionic site (acrylic acid) as a monomer, and the problem and the action and effect of the invention are also different. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 63-286481 A (claims, examples, etc.) [Patent Document 2] JP 2005-329665 A (claims, examples, etc.) [Patent Document 3] JP 2007-204573 A (Claims, Examples, etc.) [Patent Document 4] JP-A-7-191013 (claims, examples, etc.) [Patent Document 5] JP 2012-163484 A (Claims, Examples, etc.) Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above-mentioned problems of the conventional technology and seeks to solve them, and aims to provide a dispersion of colored resin particles which, by encapsulating a pH indicator as a dye in specific particles, maintains its discoloration properties and has excellent stability when used as a dispersion of colored resin particles and in an aqueous ink composition, and which, in the case of an aqueous ink composition, has excellent bleeding prevention properties without impairing the writing performance, an aqueous ink composition which uses this as a colorant, and a writing instrument equipped with this. [Means for solving the problem]

[0007] In view of the above-mentioned problems in the conventional art, the present inventors conducted intensive research and discovered that a dispersion of colored resin particles composed of at least a specific monomer and a pH indicator, etc., can be obtained by setting the content of the specific monomer and the content of the pH indicator to a specific value or more, respectively, and thus completing the present invention.

[0008] That is, the dispersion of colored resin particles of the present invention is 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. The vinyl monomer preferably includes an aromatic vinyl monomer or a (meth)acrylic monomer. It is preferable that the pH indicator is colorless in the color change region. The pH indicator is preferably an oil-soluble substance having a phthalein skeleton. The content of the vinyl monomer is preferably 30% by mass to 95% by mass based on the total polymer components constituting the colored resin particles. The colored resin particles preferably have an average particle size of 20 to 800 nm. The water-based ink composition of the present invention is characterized by containing the dispersion liquid of colored resin particles having the above-mentioned constitution, a water-soluble organic solvent, and water. The writing instrument of the present invention is characterized by being equipped with the water-based ink composition having the above-mentioned structure. Effect of the Invention

[0009] According to the present invention, by encapsulating a pH indicator as a dye within specific particles, it is possible to provide a dispersion of colored resin particles that maintains its color-changing properties and has excellent stability when used in an aqueous ink composition, and in the case of an aqueous ink composition, it is possible to provide a dispersion of colored resin particles that has excellent bleeding prevention properties without impairing the writing performance, an aqueous ink composition that uses this as a coloring material, and a writing instrument equipped with this. The objects and advantages of the invention will be realized and obtained by means of the elements and combinations particularly pointed out in the claims. Both the foregoing general description and the following detailed description are exemplary and explanatory but are not restrictive of the invention as claimed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The embodiments of the present invention are described in detail below. However, it should be noted that the technical scope of the present invention is not limited to the embodiments described in detail below, but extends to the inventions described in the claims and their equivalents. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field (including design matters and obvious matters). In this application, "discoloration" means a change from one color to another color, a loss of color from colored to colorless, or a color development from colorless to colored under certain conditions, and "discoloration property" means a property that causes the above-mentioned "discoloration."

[0011] <Dispersion of Colored Resin Particles> The dispersion of colored resin particles of the present invention is 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 based on the total polymer components constituting the colored resin particles, and the content of the pH indicator is 1 mass% or more based on the total polymer components, and are dispersed in water.

[0012] The vinyl monomer used in the present invention 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 properties of the pH indicator, and because the obtained colored resin particles have a sufficient line density to be used as a coloring material for writing instruments, etc., and because it also suppresses bleeding. The vinyl monomer used in the present invention may be, for example, an aromatic vinyl monomer or a (meth)acrylic monomer.

[0013] The aromatic vinyl monomer that can be used is a monomer containing an aromatic ring and a vinyl group in the molecule, and examples thereof include 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 from the viewpoints of reactivity and affinity with dyes.

[0014] Examples of the (meth)acrylic monomer that can be used include at least one 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 (each alone or a mixture of two or more). The term "(meth)acrylic acid" 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 from the standpoint of reactivity and affinity with dyes.

[0015] In the present invention, in addition to the above-mentioned aromatic vinyl monomers and (meth)acrylic monomers, from the viewpoint of obtaining colored resin particles having excellent discoloration properties, it is preferable to use aqueous monomers other than the above-mentioned aromatic vinyl monomers or (meth)acrylic monomers. Examples of aqueous monomers that can be used include at least one of glycerin monomethacrylate, sodium 2-sulfoethyl 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 invention include indigo carmine, 1,3,5-trinitrobenzene, nitramine, tropaeoline O, Poirier Blue C4B, Alizarin Yellow GG, Alizarin Yellow R, thymolphthalein complexone, thymol blue, α-naphthol benzein, 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 specific pH range and change or lose color to another color at least once in a specific 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, which is more practical and more preferable. Preferred pH indicators are those that are colorless in the color change region, and include bromothymol blue, phenol red, naphtholphthalein, cresol red, cresolphthalein, thymol blue, phenolphthalein, and thymolphthalein. Of these, those having a phthalein skeleton are most preferred. Naphtholphthalein, cresolphthalein, phenolphthalein, thymolphthalein, and other substances which have a phthalein skeleton are oil-soluble substances which develop color under medium to strong alkaline conditions and become colorless under weak alkaline conditions or lower and are poorly soluble in water. Furthermore, colored resin particles composed of a pH indicator having a phthalein skeleton and a vinyl monomer reduce the surface area of ​​the pH indicator that comes into contact with the external environment of the particle, thereby producing a particle dispersion which has excellent color change properties (coloring and decoloring) and stability.

[0018] In addition, in the present invention, a light stabilizer and an ultraviolet absorber may be introduced into the particles together with the pH indicator within a range that does not impair the effects of the present invention. Examples of light stabilizers include HALS having a hindered amine skeleton, and examples of ultraviolet absorbers include benzophenone-based, benzotriazole-based, cyclic imino ester-based, cyanoacrylate-based, and polymer-type ultraviolet absorbers. Among these light stabilizers and ultraviolet absorbers, it is preferable 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 invention 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 can be, for example, by 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 then using ammonium persulfate, potassium persulfate, hydrogen peroxide, or the like as a polymerization initiator, or a polymerization initiator further combined with 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. It can be produced by emulsion polymerization using a crosslinking agent and, if necessary, a polymerizable surfactant (emulsifier) ​​such as polyoxyethylene-1-(allyloxymethyl)-alkyl ether 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. The dyeing was carried out simultaneously with the polymerization, but may be carried out by dissolving a pH indicator after polymerization. Use of a crosslinking agent such as the above-mentioned 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 invention, the emulsion polymerization may be carried out by further mixing an appropriate amount of dicyclopenta(ten)yl(meth)acrylate monomer with the vinyl monomer, particularly the (meth)acrylic monomer, to perform emulsion polymerization. When this dicyclopenta(ten)yl(meth)acrylate monomer is further mixed and then emulsion polymerization is carried out, the stability is not easily impaired even if the water in the dispersion evaporates, and a dispersion of colored resin particles with even better stability can be obtained. Dicyclopenta(ten)yl (meth)acrylate monomers which may be used include dicyclopentanyl acrylate monomer, dicyclopentenyl acrylate, dicyclopentanyl methacrylate monomer, dicyclopentenyl methacrylate. In the present invention, in addition to the dicyclopenta(t)nyl(meth)acrylate monomer, a suitable amount of 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 to effect crosslinking during the emulsion polymerization.

[0021] In the present invention, 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 invention, the term "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, in order to make the content of the vinyl monomer 30% by mass or more based on the total polymer components, it can be achieved by adjusting the type and amount of the polymer components used as well as the stirring conditions, such as the stirring speed, stirring time, and temperature. The effects of the present invention can be achieved by making the content of the vinyl monomer 30% by mass or more based on the total polymer components. On the other hand, if this content is less than 30% by mass, the stability over time will be poor, which is not preferable.

[0022] In addition, among the polymer components constituting the colored resin particles, the content of monomer components other than the vinyl monomer is the remainder of the total amount of the vinyl monomer and the crosslinking agent described below. The content of the other monomer components is preferably 5 to 85% by mass based on the total polymer components, from the viewpoints of further exerting the effects of the present invention, dispersibility, and reactivity.

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

[0024] The polymerizable surfactant that can be used as necessary is not particularly limited as long as it is a polymerizable surfactant that is usually used in the emulsion polymerization, but for example, the polymerizable surfactant is an anionic or nonionic polymerizable surfactant, such as Adeka Reasoap NE-10, NE-20, NE-30, NE-40, and SE-10N manufactured by Adeka Corporation, Latemul S-180, S-180A, and S-120A manufactured by Kao Corporation, Eleminol JS-20 manufactured by Sanyo Chemical Industries, Ltd., and 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 the monomers. The content of the crosslinking agent such as the triallyl isocyanurate is preferably 0 to 50% by mass, and more preferably 0.1 to 25% by mass, based on the total amount of the monomers.

[0025] In the present invention, in the above-mentioned preferred embodiment, specifically, by dissolving the above-mentioned pH indicator in at least the vinyl monomer and emulsion-polymerizing the same, or by dissolving a pH indicator in the mixed monomer containing at least the vinyl monomer and other monomer components and then polymerizing the monomer, a dispersion of colored resin particles in which 20 to 50 mass % of colored resin particles are dispersed in water as resin solids can be obtained. This dispersion of colored resin particles is a colorant that has more satisfactory color change properties (discoloration from one color to another, fading from colored to colorless, coloring from colorless to colored) and excellent 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 near neutral conditions (pH 6-8) and becomes colorless under medium to strong acidic conditions (pH 0-4), or becomes colorless under medium to strong alkaline conditions (pH 10-14), and the like. and colored resin particles that become colorless at low pH, color at medium to strong acidity (pH 0-4) and change to another color at weak acidity (pH 4-6), i.e., color-changing colored resin particles that can arbitrarily exhibit color change within a specified pH range can be produced. Therefore, the particles can be used in a variety of products, such as ink for writing instruments, materials that can be used to erase and develop color with a non-baked lead (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 supplies, kitchen supplies, tableware, drinking water piping parts, life hygiene products, home appliances, clothing, building materials, agricultural materials, automobile interior parts, stationery, ink compositions for inkjet printers, and determination of the degree of neutralization of concrete.

[0026] In the present invention, the average particle size of the colored resin particles in the obtained dispersion of colored resin particles varies depending on the vinyl monomer, the type and content of other monomers used, the polymerization conditions during polymerization, and the like, as well as the application of the product to be used, but is preferably 20 to 800 nm, further 20 to 300 nm, particularly 40 to 150 nm, and especially 60 to 110 nm. By ensuring that the average particle size falls within the above preferred range, the particles can be used in a variety of products, including writing instruments, inkjet pens, materials for erasing and developing color with non-baked leads (solid writing instruments), ink for cosmetics, sanitary and toiletry products (for humans and pets), and personal care products. In the case of ink compositions for felt tip pens, marking pens, ballpoint pens and the like, if the particle size is 1000 nm or less, clogging does not occur in the pen core, and further, the ink composition has excellent storage stability. In the present invention, the term "average particle size" refers to the particle size (D50) at 50% cumulative volume in the particle size distribution calculated based on the volume standard measured by a laser diffraction method. In the present invention (including the examples described later), the value D50 measured using a particle size distribution measuring device [FPAR1000 (manufactured by Otsuka Electronics Co., Ltd.] 50 The value is

[0027] The water-based ink composition of the present invention is characterized by containing at least the dispersion liquid of colored resin particles having the above-mentioned constitution, 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. 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] In addition, water-soluble solvents such as 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 can also be mixed. The content of these water-soluble organic solvents varies depending on the type of writing instrument, such as a felt-tip pen, a marking pen, or a ballpoint pen, but is preferably 1 to 40% by mass relative to the total amount of the ink composition. In order to further improve the drying properties of drawn lines, it is particularly effective in ink compositions with a content of 10% by mass or less, 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 % based on the total amount of the ink composition. The content of the colored resin particles varies depending on the type of product, outflow mechanism, etc., but is preferably 1 to 30% by mass in terms of solid content relative to the total amount of the aqueous ink composition.

[0030] In the aqueous ink composition of the present invention, thickeners, preservatives or antifungal agents, pH adjusters, antifoaming agents and the like can be appropriately selected and used as necessary within the scope that does not impair the effects of the present invention. In the aqueous ink composition of the present invention, the colored resin particles in the dispersion liquid of the colored resin particles having the above-mentioned constitution have a discoloration property depending on the pH concentration in the ink, so that a pH adjuster can be suitably used. The pH adjuster may be at least one of ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, alkali metal salts of carbonate or phosphoric acid such as sodium carbonate, and alkali metal hydroxides such as sodium hydroxide. Examples of the thickener include an alkali swelling association type emulsion, an alkali swelling type emulsion, polyvinylpyrrolidone, a crosslinked acrylic acid polymer, carboxymethylcellulose (CMC) or a salt thereof, fermented cellulose, crystalline cellulose, and polysaccharides. 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, alkaline gum, succinoglycan, locust bean gum, tara gum, etc., and these may be used alone or in combination of two or more.In addition, if these commercially available products are available, they can be used. Examples of preservatives or antifungal agents 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. As the lubricant, 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, polyether-modified silicones such as polyethylene glycol adducts of dimethylpolysiloxane, etc. may be used. .

[0031] In addition, the aqueous ink composition of the present invention has 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 keeping this viscosity at 80 mPa s or less at the above shear rate, appropriate ink flow rate and ink ejection properties can be obtained, and by keeping the viscosity at 2 mPa sec or more, it is easier to suppress bleeding and strike-through when writing, printing, etc.

[0032] Furthermore, from the viewpoint of leveling properties, the aqueous ink composition of the present invention preferably has a surface tension of 15 to 60 mN / m, and more preferably 25 to 50 mN / m. In the present invention (including the examples described later), the surface tension of the ink (25° C.) is 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 invention can be prepared by appropriately combining at least the dispersion of colored resin particles having the above-mentioned configuration, a water-soluble solvent, and other components depending on the application, such as a writing instrument (for a ballpoint pen, for a marking pen, etc.) or an inkjet, and stirring and mixing them with a stirrer such as a homomixer, homogenizer, or disper, and further, if necessary, removing coarse particles in 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 invention is equipped with an aqueous ink composition having the above-mentioned composition, and is equipped, for example, with a ballpoint pen, marking pen, or the like equipped with a pen tip such as a ballpoint tip, fiber tip, felt tip, plastic tip, fiber core, or porous core. The ballpoint pen may be one in which the aqueous ink composition for a writing instrument having the above-mentioned composition is contained in a ballpoint pen ink container (refill), and a substance that is incompatible with the aqueous ink composition for a writing instrument having the above-mentioned composition contained in the ink container and has a low specific gravity compared to the aqueous ink composition, such as polybutene, silicone oil, mineral oil, etc., is contained as an ink follower. The structure of the ballpoint pen or marking pen is not particularly limited, and may be, for example, a direct ink type ballpoint pen or marking pen having a collector structure (ink retention mechanism) in which the barrel itself serves as an ink container and is filled with the aqueous ink composition for writing instruments having the above-described configuration.

[0035] The aqueous ink composition of the present invention thus configured contains at least a dispersion of colored resin particles as described above and a water-soluble solvent. By encapsulating a pH indicator as a dye within specific particles, the aqueous ink composition has excellent stability while maintaining its color-changing properties, even when the dispersion of colored resin particles is used, and further has excellent bleeding prevention properties without impairing writing performance. Furthermore, the aqueous ink composition of the present invention contains colored resin particles that can arbitrarily exhibit color change within the above-mentioned specified pH range, and thereby, under normal usage conditions, the composition absorbs carbon dioxide in the air, causing a change in pH, and thereby enabling the composition to change color. In addition to the above-mentioned particle properties, by appropriately adjusting the viscosity and surface tension within suitable ranges, as described above, it becomes possible to control the length of time from application by coating, writing, or the like until the ink composition as a whole changes color (fading and re-coloring).

[0036] In this way, the aqueous ink composition of the present invention has excellent discoloration properties due to the pH indicator species, sufficient line density, and excellent stability over time, and can be used in a variety of products, including ink compositions for writing implements such as felt-tip pens, marking pens, and ballpoint pens, ink jet inks, materials for erasing and developing colors 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 supplies, kitchen utensils, tableware, drinking water piping parts, daily sanitary products, home appliances, clothing, building materials, agricultural materials, automotive interior parts, stationery, and ink jet printers. EXAMPLES

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

[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. In the following, "parts" refers to parts by mass. The total amount (total amount) of each component in Production Examples 1 to 20 was 100 parts by 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 separating 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. Separately, a liquid was prepared by mixing 60 parts of a monomer of methyl methacrylate (Acrylic Ester M, manufactured by Mitsubishi Chemical Corporation), 20 parts of a pH indicator (cresolphthalein, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 20 parts of a crosslinking agent (triallyl isocyanurate, manufactured by Mitsubishi Chemical Corporation, "TAIC"). This prepared liquid was added from the separating funnel to the flask kept at about 50° C. over a period of 3 hours with stirring to carry out emulsion polymerization. The polymerization was terminated by aging for a further 5 hours to obtain a dispersion liquid of colored resin particles (particles 1). The content of the methyl methacrylate was 50% by mass relative to the total polymer components constituting the colored resin particles, and the content of the pH indicator was 25% by mass relative to the total polymer components. The average particle size of the colored resin particles was 45 nm.

[0040] (Examples 2 to 20) In accordance with the above Production Example 1, dispersions of colored resin particles (particles 2 to 20) of Production Examples 2 to 20 were obtained using the formulation shown in Table 1 below, specifically, instead of cresolphthalein, phenolphthalein and thymolphthalein were used as other pH indicators, and instead of methyl methacrylate, cyclohexyl methacrylate, n-butyl (meth)acrylate, styrene, etc. were used as vinyl monomers, and 1,4-butanediol acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., "Viscoat #195") was used as a crosslinking agent. The content of each vinyl monomer used relative to the total polymer components constituting the colored resin particles, 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 discoloration of each of the colored resin particle dispersions obtained in the above Manufacturing Examples 1 to 20 was evaluated by the following evaluation method (acid or base was dropped so as to straddle the color discoloration range of the pH indicator) and the dispersion stability of the dispersion. The results are shown in Table 1 below.

[0042] (Method for evaluating discoloration of dispersion liquid) For each of the dispersions of colored resin particles obtained in Production Examples 1 to 20, an acid or a base was dropped so as to straddle the color change region of the pH indicator, i.e., the color of the dispersion before dropping the acid or the base was visually confirmed. The acid or the base was dropped into the dispersion, and the color change, disappearance, or color development was evaluated according to the following evaluation criteria. Evaluation criteria: ○: Discoloration, fading, or color development occurs. ×: No color change.

[0043] (Method of Evaluating Dispersion Stability of Dispersion Liquid) 50 ml of each of the dispersions of colored resin particles obtained in Production Examples 1 to 20 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 one month. The dispersion stability was then evaluated according to the following criteria. Evaluation criteria: A: No separation of the dispersion liquid was observed, and no sediment was found at the bottom. Good condition. B: Slight separation is observed in the dispersion, and a small amount of sediment can be seen at the bottom. C: Significant separation is observed in the dispersion, with a large amount of sediment 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 25) Using the dispersions of the colored resin particles (particles 1 to 20) produced in the above Production Examples 1 to 20, aqueous ink compositions for each writing instrument were prepared by a conventional method according to the blending composition shown below (total amount 100 mass%). The solid content of each of the dispersions of the colored resin particles (particles 1 to 20) produced in Production Examples 1 to 20 was 35 to 40 mass%. Ink composition: (total 100% by mass) Dispersion of colored resin particles for each water-based 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 20 to 25 and Comparative Examples 1 to 4) In Examples 20 to 25, the compositions were prepared as shown in Table 2 below. In Comparative Examples 1 to 4, the compositions were prepared as shown in Table 2 below. Water-based ink compositions were prepared in a conventional manner.

[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, the color change 24 hours after writing, the stability over time of the ink composition, and the bleeding resistance were evaluated by the evaluation methods described below. Table 2 below shows the evaluation results of 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 using a Tokyo Keiki Co., Ltd. EMD type viscometer at a shear rate of 383 sec -1 The measurements were performed under the following conditions. (Method of measuring surface tension) The surface tension of each of the resulting 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 pen creation) The writing implement aqueous ink composition obtained above was used with an aqueous felt-tip pen [PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.)] to evaluate the color development of lines drawn 3 hours after the preparation of the pen body according to the following evaluation criteria. Evaluation criteria: A: Highly concentrated color B: Slightly low color concentration or slight discoloration C: The concentration is low or discolored. Evaluation criteria:

[0048] (Method for evaluating discoloration 24 hours after writing) The aqueous ink composition for writing instruments obtained above was stored with the fine writing side facing down for two weeks at 40°C using a PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.), after which the fine writing side was used to write in a circle on a piece of writing paper, and discoloration 24 hours after writing was evaluated according to the following evaluation criteria. Evaluation criteria: A: Same level as initial writing B: Quick writing and smudges C: Somewhat blurred D: Poor handwriting (can't write)

[0049] (Method for evaluating stability over time of aqueous ink composition) 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 ink 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 was evaluated according to the following evaluation criteria. Evaluation criteria: A: More than 6 months B: 3 months or more but less than 6 months C: Less than 3 months

[0050] (Method of evaluating bleeding resistance) The writing implement aqueous ink composition obtained above was used to visually inspect the bleeding state of lines drawn in a "spiral writing" on writing paper using PM-120T (manufactured by Mitsubishi Pencil Co., Ltd.) according to the following evaluation criteria. Evaluation criteria: A: Clear handwriting without smudging. B: There is some bleeding. C: Blurred and unclear handwriting. D: Significant bleeding and very unclear handwriting.

[0051] [Table 1]

[0052] [Table 2]

[0053] Considering Table 1 above, it was confirmed that the dispersions of colored resin particles of Production Examples 1 to 20 within the scope of the present invention were excellent in color development, color change resistance, and dispersion stability. Furthermore, it was confirmed that the aqueous ink compositions of Examples 1 to 25 were superior 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. [Industrial Applicability]

[0054] The resulting dispersion of colored resin particles can be ideally used in writing instruments, inkjet inks, materials for erasing and developing color with non-baked leads (solid writing instruments), inks for cosmetics, 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 based on all polymer components constituting the colored resin particles, and the content of the pH indicator is 1 mass% or more based on all polymer components, and the colored resin particles are dispersed in water.

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

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

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

5. 3. The dispersion of colored resin particles according to claim 1, 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. 3. The dispersion of colored resin particles according to claim 1, wherein the average particle size of the colored resin particles is 20 to 800 nm.

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

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

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