Aqueous ink composition and writing instrument

Abstract

To provide an aqueous ink composition capable of maintaining stable quality over a long period of time.SOLUTION: The ink composition contains at least a coloring material, water, and thiamine, a thiamine derivative or a salt thereof.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous ink composition and a writing instrument. [Background technology]

[0002] Writing instruments such as water-based ballpoint pens with water-based ink are widely used. The time it takes for such water-based ballpoint pens to be consumed by the user ranges from several months to a year after production. During this time, transportation is required, and the pens are required to maintain stable quality even after this period.

[0003] For example, Patent Document 1 discloses a ballpoint pen ink containing a pigment, specific fine particles, and a liquid medium, but when stored for a long period of time, the pigment and other components settle and accumulate, which can affect the writing feel if the pen is stored for a long period of time with the pen tip facing downwards. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-206066 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide an aqueous ink composition that can maintain stable quality over a long period of time, and a writing instrument that uses this aqueous ink composition. [Means for solving the problem]

[0006] As a result of extensive research, the present inventors have found that the above problems can be solved by using a specific component, and have thus completed the present invention.

[0007] That is, according to the present invention, (1) An aqueous ink composition containing at least a coloring material, water, and thiamine, a thiamine derivative, or a salt thereof; (2) The aqueous ink composition according to (1), wherein the aqueous ink composition contains 5.00% by mass or less of the thiamine, thiamine derivative, or salt thereof. (3) The aqueous ink composition according to (1) or (2), further comprising a water-soluble organic solvent, the water-soluble organic solvent being contained in an amount of 30% by mass or less in the aqueous ink composition. (4) A writing instrument equipped with the aqueous ink composition according to (1) or (2). is provided. [Effects of the Invention]

[0008] According to the present invention, there are provided an aqueous ink composition that can maintain stable quality over a long period of time, and a writing instrument that uses this aqueous ink composition. DETAILED DESCRIPTION OF THE INVENTION

[0009] The aqueous ink composition of the present invention is described below: The aqueous ink composition of the present invention is an aqueous ink composition containing at least a coloring material, water, and thiamine, a thiamine derivative, or a salt thereof.

[0010] (colorant) The coloring material used in the aqueous ink composition of the present invention may be a coloring material used for writing instruments, such as a dye, pigment, or colored resin particles that are soluble or dispersible in water.

[0011] As the dye, a dye that dissolves or disperses in water can be used, and examples thereof include acid dyes such as eosin, fuoxin, water yellow #6-C, acid red, water blue #105, brilliant blue FCF, nigrosine NB, and water black 256L; direct dyes such as direct black 154, direct sky blue 5B, and violet BB; and basic dyes such as rhodamine and methyl violet.

[0012] As the pigment, any of conventionally known inorganic and organic pigments such as titanium oxide, pseudopigments in which a resin emulsion is colored with a dye or pigment, white plastic pigments, luster pigments, pigments in which a silica or mica base is coated with a multilayer of iron oxide, titanium oxide, or the like on the surface, and the like can be used without any restrictions.

[0013] Examples of inorganic pigments that can be used include carbon black, titanium black, zinc white, red iron oxide, aluminum, chromium oxide, iron black, cobalt blue, iron oxide yellow, viridian, zinc sulfide, lithopone, cadmium yellow, vermilion, cadmium red, yellow lead, molybdate orange, zinc chromate, strontium chromate, white carbon, clay, talc, ultramarine, precipitated barium sulfate, baryte powder, calcium carbonate, white lead, dark blue, iron blue, manganese violet, aluminum powder, and brass powder.

[0014] Examples of organic pigments include azo lakes, insoluble azo pigments, chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, nitroso pigments, etc. Examples of such organic pigments include CI Pigment Blue 17, CI Pigment Blue 15, CI Pigment Blue 17, CI Pigment Blue 27, CI Pigment Red 5, CI Pigment Red 22, CI Pigment Red 38, CI Pigment Red 48, CI Pigment Red 49, CI Pigment Red 53, CI Pigment Red 57, CI Pigment Red 81, CI Pigment Red 104, CI Pigment Red 146, CI Pigment Red 245, CI Pigment Yellow 1, CI Pigment Yellow 3, CI Pigment Yellow 12 ... Examples of pigments that may be used include CI Pigment Yellow 13, CI Pigment Yellow 14, CI Pigment Yellow 17, CI Pigment Yellow 34, CI Pigment Yellow 55, CI Pigment Yellow 74, CI Pigment Yellow 95, CI Pigment Yellow 166, CI Pigment Yellow 167, CI Pigment Orange 5, CI Pigment Orange 13, CI Pigment Orange 16, CI Pigment Violet 1, CI Pigment Violet 3, CI Pigment Violet 19, CI Pigment Violet 23, CI Pigment Violet 50, and CI Pigment Green 7.

[0015] The colored resin particles are not particularly limited as long as they are made of colored resin particles, and examples thereof include: 1) Colored resin particles in which a colorant consisting of a pigment such as an inorganic pigment (e.g., carbon black, titanium oxide) or an organic pigment (e.g., phthalocyanine pigment, azo pigment) is dispersed in the resin particles; 2) Colored resin particles, the surfaces of which are coated with a colorant comprising the pigment. 3) Colored resin particles dyed with a coloring agent such as a direct dye, an acid dye, a basic dye, a food dye, or a fluorescent dye. 4) Microspheres having a matrix composed of a polymer, a resin having OH groups, and a water-insoluble dye; 5) Colored resin particles that are thermochromic using leuco dyes, etc. (leuco dye-containing particles), 6) Photochromic dyes (compounds) that are photochromic dyes, colored resin particles that are made photochromic using fluorescent dyes, etc., are included.

[0016] The resin component of the colored resin particles 1) to 3) above may be at least one selected from, for example, polymers or copolymers of acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, acrylonitrile, butadiene, etc., benzoguanamine, phenolic resins, epoxy resins, urethane resins, etc., which may be subjected to crosslinking or other treatments as necessary. Conventionally known methods such as suspension polymerization and dispersion polymerization are used to color these resins.

[0017] The microspheres of 4) above have a matrix made of a polymer, a resin having an OH group, and a water-insoluble dye.

[0018] Examples of polymers constituting the matrix include epoxy polymers, melamine polymers, acrylic polymers, urethane polymers, and urea polymers. One of these may be used alone, or two or more may be used in combination.

[0019] The resin having an OH group is contained in the matrix. Examples of the resin having an OH group include terpene phenol resin, rosin phenol resin, alkylphenol resin, phenol novolac resin, cresol novolac resin, butyral resin, polyvinyl alcohol resin, polyol-modified xylene resin, ethylene oxide-modified xylene resin, maleic acid resin, hydroxyl group-modified acrylic resin, hydroxyl group-modified styrene acrylic resin, carboxyl-modified acrylic resin, and carboxyl-modified styrene acrylic resin.

[0020] The water-insoluble dye is a dye that is insoluble in water at room temperature, and examples of the water-insoluble dye include salt-forming dyes such as dyes having azo, metal complex azo, anthraquinone, and metal phthalocyanine chemical structures, disperse dyes, and oil-soluble dyes. From the viewpoint of color development, it is preferable to use salt-forming dyes.

[0021] Microspheres can be produced, for example, by the following production steps (emulsion polymerization method, phase separation method).

[0022] The process for preparing microspheres using emulsion polymerization involves preparing an oil phase, preparing an aqueous phase, and mixing the oil phase and aqueous phase to emulsify the components of the oil phase, followed by polymerization.

[0023] The oil phase contains an organic solvent such as phenyl glycol, benzyl alcohol, ethylene glycol monobenzyl ether, or ethyl acetate, the water-insoluble dye, the resin having an OH group, and a monomer or prepolymer. Multiple types of organic solvents may be contained.

[0024] This oil phase can be prepared by adding a water-insoluble dye and a resin having an OH group to an organic solvent while heating the organic solvent to a predetermined temperature and stirring the mixture, then adding a monomer or prepolymer that constitutes the polymer, such as a melamine monomer or prepolymer, an epoxy monomer or prepolymer, an acrylic monomer or prepolymer, or an isocyanate monomer or prepolymer, and optionally further adding another organic solvent.

[0025] The aqueous phase can be prepared by mixing water and a dispersant, such as, but not limited to, polyvinyl alcohol.

[0026] The emulsification and polymerization process can be carried out by first emulsifying the components of the oil phase, and then polymerizing the components by adding the oil phase to the aqueous phase and emulsifying and mixing them while heating to a predetermined temperature using a homogenizer or the like.

[0027] The microsphere preparation process may include other steps, such as classifying the microspheres.

[0028] The process for preparing microspheres by the phase separation method comprises preparing a dye-containing solution, preparing a protective colloid-containing solution, and polymerizing a monomer or prepolymer.

[0029] The dye-containing solution can be prepared by dissolving a water-insoluble dye and a resin having an OH group in an organic solvent under heating. The water-insoluble dye, the resin having an OH group, and the organic solvent can be the same as those mentioned in the microsphere preparation process by emulsion polymerization.

[0030] The protective colloid-containing solution can be prepared by dissolving a protective colloid in water. Examples of the protective colloid include methyl vinyl ether-maleic anhydride copolymer.

[0031] Polymerization of the monomer or prepolymer can be carried out by adding a dye-containing solution to a protective colloid-containing solution heated to a predetermined temperature to disperse the solution in the form of oil droplets, adding the monomer or prepolymer thereto, and stirring while maintaining the temperature. In this way, the water-insoluble dye and the resin having an OH group are encapsulated in the polymer obtained by polymerizing the monomer or prepolymer.

[0032] In the obtained microspheres, the content of the resin having an OH group in the total amount of the microspheres is preferably 1% by mass or more and 40% by mass or less, and the content of the water-insoluble dye is preferably 10% by mass or more and 45% by mass or less.The average particle size of the microspheres is preferably 0.2 μm or more and 3.0 μm or less.

[0033] Here, the average particle size of the microspheres is the D50 value measured using a particle size analyzer (Microtrac HRA9320-X100 (manufactured by Nikkiso Co., Ltd.)) or the average particle size value calculated by cumulant method analysis in the scattering intensity distribution using a concentrated particle size analyzer FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.).

[0034] Examples of the thermochromic colored resin particles of 5) above include thermochromic colored resin particles (leuco dye-encapsulated particles) produced by microencapsulating a thermochromic composition containing at least a leuco dye that functions as a color former, a color developer that is a component capable of causing the leuco dye to develop color, and a color change temperature regulator that can control the color change temperature when the leuco dye and color developer develop color, so that the resulting particles have a predetermined average particle size (e.g., 0.1 to 6 μm).

[0035] Examples of microencapsulation methods include interfacial polymerization, interfacial polycondensation, in situ polymerization, liquid hardening coating, phase separation from an aqueous solution, phase separation from an organic solvent, melting dispersion cooling, air suspension coating, and spray drying, and can be appropriately selected depending on the application.

[0036] For example, in the phase separation method from an aqueous solution, thermochromic colored resin particles can be produced by a method including the following steps, particularly by a method including the following steps in this order: (1) Heating and melting the leuco dye, the color developer, and the color change temperature adjuster; (2) Adding the heated and melted leuco dye, developer, and discoloration temperature regulator to an emulsifier solution, and dispersing the mixture in the form of oil droplets by heating and stirring to prepare a dispersion; (3) A resin raw material capable of forming a wall film, such as a urethane resin, an epoxy resin, or an amino resin, is gradually added to the dispersion as a capsule film agent, specifically, an amino resin solution such as a methylolmelamine aqueous solution, a urea solution, or a benzoguanamine solution, and the resin raw material is reacted to form a capsule film, thereby obtaining a thermochromic microcapsule pigment; and (4) filtering the dispersion containing the thermochromic microencapsulated pigment;

[0037] In these thermochromic colored resin particles, the color-developing temperature and decolorizing temperature of each color can be set to an appropriate temperature by appropriately combining the types and amounts of the leuco dye, color developer, and color-change temperature adjuster.

[0038] The photochromic colored resin particles of 6) above can be, for example, photochromic colored resin particles composed of at least one photochromic substance selected from photochromic dyes (compounds), fluorescent dyes, and other photochromic substances, and a resin such as a terpene phenol resin. Further, photochromic colored resin particles can include photochromic colored resin particles produced by microencapsulating a photochromic composition containing at least one photochromic substance selected from photochromic dyes (compounds), fluorescent dyes, and other photochromic substances, an organic solvent, and additives such as antioxidants, light stabilizers, and sensitizers, to a predetermined average particle size (e.g., 0.1 to 6 μm). The microencapsulation method can be the same as that used to produce the thermochromic resin particles described above.

[0039] By suitably using the above-mentioned photochromic substance, the photochromic colored resin particles can be made to have the property of being colorless in an indoor lighting environment (indoor lighting equipment selected from incandescent lamps, fluorescent lamps, lamps, white LEDs, etc.) and becoming colored in an ultraviolet irradiation environment (irradiation with wavelengths of 200 to 400 nm, or irradiation with sunlight containing ultraviolet rays).

[0040] Each of the colored resin particles 1) to 6) above can be used as a fluorescent pigment, a microencapsulated pigment of a thermochromic pigment or a photochromic pigment, a microsphere, etc. (coloring material). Furthermore, each of the colored resin particles 1) to 6) above can be produced by a corresponding production method, and if a commercially available product is available, it may also be used.

[0041] These coloring materials can be used alone or in combination of two or more. The average particle size of the dyes, pigments, colored resin particles, etc. that are soluble or dispersible in water among these coloring materials is not particularly limited, but may be, for example, 0.02 to 6 μm.

[0042] The content of these coloring materials is not particularly limited, but is preferably 1.0% by mass or more, more preferably 5.0% by mass or more, even more preferably 7.0% by mass or more, in terms of solids mass % of the total amount of the aqueous ink composition, and is preferably 35.0% by mass or less, more preferably 20.0% by mass or less, even more preferably 18.0% by mass or less.

[0043] (solvent) The aqueous ink composition of the present invention contains water as a solvent. That is, the solvent used in the aqueous ink composition of the present invention is an aqueous solvent containing water. As the aqueous solvent, water (e.g., ion-exchanged water, distilled water, tap water, etc.) or a mixed solvent of water and a water-soluble organic solvent can be used, and a mixed solvent of water and a water-soluble organic solvent is preferably used.

[0044] Examples of the water-soluble organic solvent that can be used include aromatics, alcohols, polyhydric alcohols, glycol ethers, hydrocarbons, esters, etc. These solvents may be used alone or in combination.

[0045] Examples of aromatic compounds that can be used include benzyl alcohol, ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, propylene glycol monophenyl ether, diethylene glycol monophenyl ether, alkylsulfonic acid phenyl ester, butyl phthalate, ethylhexyl phthalate, tridecyl phthalate, ethylhexyl trimellitate, diethylene glycol dibenzoate, and dipropylene glycol dibenzoate.

[0046] Examples of alcohols that can be used include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, n-hexanol, methylamyl alcohol, 2-ethylbutanol, n-heptanol, 2-heptanol, 3-heptanol, n-octanol, 2-octanol, 2-ethylhexanol, 3,5,5-trimethylhexanol, nonanol, n-decanol, undecanol, n-decanol, trimethylnonyl alcohol, tetradecanol, heptadecanol, cyclohexanol, and 2-methylcyclohexanol.

[0047] Examples of polyhydric alcohols that can be used include ethylene glycol, diethylene glycol, propylene glycol, 3-methyl-1,3 butanediol, triethylene glycol, dipropylene glycol, 1,3 propanediol, 1,3 butanediol, 1,5 pentanediol, hexylene glycol, octylene glycol, and glycerin.

[0048] Examples of glycol ethers that can be used include methyl isopropyl ether, ethyl ether, ethyl propyl ether, ethyl butyl ether, isopropyl ether, butyl ether, hexyl ether, 2-ethylhexyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylbutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, 3-methyl-3-methoxy-1-butanol, 3-methoxy-1-butanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, propylene glycol tertiary butyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monobutyl ether, and tetrapropylene glycol monobutyl ether.

[0049] As the hydrocarbons, for example, straight-chain hydrocarbons such as hexane, isohexane, heptane, octane, nonane, and decane, and cyclic hydrocarbons such as cyclohexane, methylcyclohexane, and ethylcyclohexane can be used.

[0050] Examples of esters include propylene glycol methyl ether acetate, propylene glycol diacetate, 3-methyl-3-methoxybutyl acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether acetate, butyl formate, isobutyl formate, isoamyl formate, propyl acetate, butyl acetate, isopropyl acetate, isobutyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, propyl propionate, isobutyl propionate, isoamyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, propyl isobutyrate, Examples of usable surfactants include methyl valerate, ethyl valerate, propyl valerate, methyl isovalerate, ethyl isovalerate, propyl isovalerate, methyl trimethylacetate, ethyl trimethylacetate, propyl trimethylacetate, methyl caproate, ethyl caproate, propyl caproate, methyl caprylate, ethyl caprylate, propyl caprylate, methyl laurate, ethyl laurate, methyl oleate, ethyl oleate, caprylic acid triglyceride, tributyl acetate citrate, octyl oxystearate, propylene glycol monoricinoleate, methyl 2-hydroxyisobutyrate, and 3-methoxybutyl acetate.

[0051] Among these, polyhydric alcohols are preferred, and propylene glycol and glycerin are more preferred.The water-soluble organic solvents may be used alone or in combination of two or more.

[0052] The total amount of water-soluble organic solvents added is preferably 5% by mass or more, more preferably 7.5% by mass or more, even more preferably 10% by mass or more, based on the total mass of the aqueous ink composition, and is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less.

[0053] (thiamine, thiamine derivatives, or salts thereof) The aqueous ink composition of the present invention contains thiamine, a thiamine derivative, or a salt thereof. Specific examples of thiamine, a thiamine derivative, or a salt thereof include thiamine (chemical formula (1) below) or a salt thereof; fursultiamine (chemical formula (2) below) or a salt thereof; benfotiamine or a salt thereof; dibenzoylthiamine or a salt thereof; bisbentiamine or a salt thereof; and thiamine lauryl sulfate (chemical formula (3) below) or a salt thereof. Examples of salts include hydrochlorides, nitrates, and sulfates.

[0054] [ka]

[0055] Thiamine, thiamine derivatives, or salts thereof may be used singly or in combination of two or more.

[0056] By incorporating thiamine, a thiamine derivative, or a salt thereof, it is possible to suppress pH changes, freezing, and differences in concentration of the colorant between the top and bottom of the aqueous ink composition of the present invention, and it is possible to maintain a uniform state for a long period of time, thereby maintaining stable quality.

[0057] The amount of thiamine, thiamine derivatives, or salts thereof is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.5% by mass or more, based on the total mass of the aqueous ink composition, and is preferably 5.00% by mass or less, more preferably 4.0% by mass or less, even more preferably 3.0% by mass or less.

[0058] By setting the blending amount of thiamine, thiamine derivatives, or salts thereof to not more than the upper limit of the above range, it is possible to reduce the risk of poor writing due to an excessive blending amount of thiamine, thiamine derivatives, or salts thereof resulting in an increased solids content in the ink.

[0059] (Other ingredients) In addition to the above components, the aqueous ink composition of the present invention may further contain other components, such as dispersants, rust inhibitors, pH adjusters, thickeners, and surfactants.

[0060] Examples of dispersants include polyacrylic acid, acrylic acid copolymers, and maleic acid resins. Specifically, resins such as acrylic resins, styrene-acrylic resins, and styrene-maleic acid resins can be used in the form of salts to make them water-soluble. For example, salts can be formed with alkali metals such as sodium and potassium. Salts can also be formed with aliphatic primary to tertiary amines such as mono-, di-, or trimethylamine, alcohol amines such as mono-, di-, or tripropanolamine, methylethanolamine, methylpropanolamine, and dimethylethanolamine, as well as amines such as ammonia, morpholine, and N-methylpholine. Among these, styrene-acrylic resins are preferably used.

[0061] Examples of the rust inhibitor that can be used include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, saponins, etc. Among these, benzotriazole is preferably used.

[0062] Examples of pH adjusters include ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, alkali metal salts of carbonate or phosphate such as sodium tripolyphosphate and sodium carbonate, alkali metal hydrates such as sodium hydroxide, and N,N-bis(2-hydroxyethyl)glycine.

[0063] Examples of thickeners include polysaccharides such as cellulose derivatives, crystalline cellulose, rheozan gum, gellan gum, xanthan gum, and succinoglycan; organic thickeners such as alkali-swelling association emulsions, alkali-swelling emulsions, polyvinylpyrrolidone, and cross-linked acrylic acid polymers; and inorganic thickeners such as montmorillonite clay minerals.

[0064] The surfactant may be a nonionic surfactant or an anionic surfactant.

[0065] Examples of nonionic surfactants include polyoxyalkylene higher fatty acid esters, higher fatty acid partial esters of polyhydric alcohols, and higher fatty acid esters of sugars. Specific examples include acetylene glycol, glycerin fatty acid esters, polyglycerin fatty acid esters, propylene glycol fatty acid esters, pentaerythritol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbit fatty acid esters, polyoxyethylene glycerin fatty acid esters, polyethylene glycol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene phytosterols, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene styrenated phenyl ethers, polyoxyethylene distyrenated phenyl ethers, polyoxyethylene castor oil, polyoxyethylene lanolin, polyoxyethylene lanolin alcohols, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, and polyoxyethylene alkylphenyl formaldehyde condensates.

[0066] Examples of anionic surfactants include alkylated sulfonates of higher fatty acid amides, alkyl aryl sulfonates, and naphthalene sulfonate formaldehyde condensates. Specific examples include alkyl sulfates, polyoxyethylene alkyl ether sulfates, N-acyl amino acid salts, N-acyl methyl taurine salts, polyoxyethylene alkyl ether acetates, alkyl phosphates such as alkyl phosphate ester salts, and polyoxyethylene alkyl ether phosphates.

[0067] The dispersants, anti-rust agents, pH adjusters, thickeners, and surfactants may be used alone or in combination of two or more. Furthermore, if commercially available products are available, they may be used. Furthermore, to improve the writing properties of the aqueous ink composition of the present invention, a humectant or a fixing resin (including dispersions) such as a urethane-based, olefin-based, or acrylic-based resin may also be used in combination.

[0068] The blending amount of the other components is not particularly limited, but is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and is preferably 5% by mass or less, more preferably 4% by mass or less, even more preferably 3% by mass or less, per one other component relative to the total amount of the aqueous ink composition.

[0069] The aqueous ink composition of the present invention can be produced by a conventionally known method, and can be produced without any particular difference from the method for producing other aqueous ink compositions. That is, the aqueous ink composition of the present invention can be produced by blending at least a colorant, water, thiamine, a thiamine derivative or a salt thereof, and a water-soluble organic solvent and other components used as needed according to the intended use of the ink (for ballpoint pens, marking pens, felt-tip pens, etc.), and mixing and stirring them. The mixing and stirring can be carried out by setting suitable stirring conditions using a mixer or the like, or further, for example, a bead mill, homomixer, homogenizer, or the like that can apply strong shear. In addition to the mixing and stirring, if necessary, a step of removing coarse particles from the aqueous ink composition by filtration or centrifugation may be carried out.

[0070] The aqueous ink composition of the present invention can maintain stable quality over a long period of time, and can be suitably mounted in batting-type or direct-fill ballpoint pens, marking pens, felt-tip pens, etc., which are equipped with a pen tip such as a ballpoint pen tip, fiber tip, felt tip, or plastic tip and which occlude the aqueous ink composition of the above composition. [Example]

[0071] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following, "parts" are based on mass unless otherwise specified.

[0072] (Production of leuco dye-containing particles) The leuco dye-containing particles used in the examples and comparative examples were prepared as follows. One part of methyl-3',6'-bisdiphenylaminofluoran as a leuco dye, two parts of bis(4-hydroxyphenyl)phenylmethane as a color developer, and 24 parts of bis(4-hydroxyphenyl)phenylmethane dicaprylate as a color-changing temperature regulator were heated and melted at 100°C to obtain 27 parts of a homogeneous composition.

[0073] To a homogeneous hot solution of 27 parts of the composition obtained above, 10 parts of isocyanate and 10 parts of polyol were added as capsule film agents and mixed with stirring. Next, 60 parts of a 12% aqueous polyvinyl alcohol solution was used as a protective colloid to emulsify the mixture at 25°C to prepare a dispersion. Next, 5 parts of a 5% polyamine were used and treated at 80°C for 60 minutes to obtain microcapsules. The microencapsulated aqueous dispersion obtained by the above procedure was spray-dried to produce powder, thereby producing leuco dye-encapsulating particles.

[0074] (Production of colored resin particles) The colored resin particles used in the examples and comparative examples were prepared as follows. An oil phase solution was prepared by adding 2.4 parts by weight of a water-insoluble dye (Valifast Blue 2620, Orient Chemical Industries Co., Ltd.) to 9.6 parts by weight of benzyl alcohol as an organic solvent and 7 parts by weight of an allophanate-modified hexamethylene diisocyanate (D-178NL, manufactured by Mitsui Chemicals, Inc.) as a prepolymer. Meanwhile, 200 parts by weight of distilled water was heated to 60°C, and 15 parts by weight of polyvinyl alcohol (PVA-205, manufactured by Kuraray Co., Ltd.) as a dispersant was dissolved therein to prepare an aqueous phase solution. The oil phase solution was added to the aqueous phase solution at 60°C, and the mixture was emulsified and mixed by stirring with a homogenizer for 6 hours to complete the polymerization. The resulting dispersion was centrifuged to obtain urethane-based particles (blue particles) as colored resin particles.

[0075] Example 1 An aqueous ink composition was prepared by mixing and stirring 40.00 parts of leuco dye-encapsulating particles, 5.00 parts of styrene acrylic resin (Joncryl 63J), 47.05 parts of ion-exchanged water, 5.00 parts of propylene glycol, 0.30 parts of benzotriazole, 0.70 parts of triethanolamine, 0.30 parts of xanthan gum, 0.40 parts of phosphate ester (polyoxyethylene lauryl ether phosphate ester (phosphate ester-type anionic surfactant)), 0.50 parts of acetylene glycol, and 0.75 parts of thiamine lauryl sulfate in a conventional manner.

[0076] Example 2 An aqueous ink composition was prepared in the same manner as in Example 1, except that the amounts of each component were changed to those shown in Table 1.

[0077] Example 3 An aqueous ink composition was prepared by mixing and stirring 45.00 parts of colored resin particles, 3.00 parts of styrene acrylic resin (Joncryl 63J), 43.90 parts of ion-exchanged water, 5.00 parts of glycerin, 0.30 parts of benzotriazole, 0.60 parts of triethanolamine, 0.30 parts of xanthan gum, 0.40 parts of phosphate ester (polyoxyethylene lauryl ether phosphate ester (phosphate ester-type anionic surfactant)), 0.50 parts of acetylene glycol, and 1.00 parts of thiamine lauryl sulfate in a conventional manner.

[0078] Example 4 An aqueous ink composition was prepared in the same manner as in Example 3, except that trehalose was used instead of glycerin and the amounts of each component used were changed to those shown in Table 1.

[0079] Example 5 An aqueous ink composition was prepared by mixing and stirring 7.00 parts of carbon black, 3.90 parts of styrene acrylic resin (Joncryl 63J), 64.70 parts of ion-exchanged water, 20.00 parts of propylene glycol, 0.30 parts of benzotriazole, 0.70 parts of triethanolamine, 0.3 parts of xanthan gum, 0.60 parts of phosphate ester (polyoxyethylene lauryl ether phosphate ester (phosphate ester-type anionic surfactant)), 0.50 parts of acetylene glycol, and 2.00 parts of thiamine lauryl sulfate in a conventional manner.

[0080] Example 6 An aqueous ink composition was prepared in the same manner as in Example 5, except that the carbon black and styrene acrylic resin were not added, the dye Water Black 256L was added, and the amounts of each component were changed to the amounts shown in Table 1.

[0081] Example 7 7.00 parts of carbon black, 3.90 parts of styrene-acrylic resin (Joncryl 63J), 66.70 parts of ion-exchanged water, 5.00 parts of glycerin, 15.00 parts of propylene glycol, 0.30 parts of benzotriazole, 0.70 parts of triethanolamine, 0.40 parts of phosphate ester (polyoxyethylene lauryl ether phosphate ester (phosphate ester type anionic surfactant)), 0.50 parts of acetylene glycol, and 0.50 parts of thiamine lauryl sulfate were mixed and stirred by a conventional method to prepare an aqueous ink composition.

[0082] (Example 8) An aqueous ink composition was prepared in the same manner as in Example 7, except that carbon black and styrene-acrylic resin were not blended, and Water Black 256L, a dye, was blended, and the amounts of each component used were changed to the amounts shown in Table 1.

[0083] (Comparative Example 1) An aqueous ink composition was prepared in the same manner as in Example 1, except that thiamine lauryl sulfate was not blended, and the amounts of each component used were changed to the amounts shown in Table 1.

[0084] (Comparative Example 2) An aqueous ink composition was prepared in the same manner as in Example 3, except that thiamine lauryl sulfate was not blended, and the amounts of each component used were changed to the amounts shown in Table 1.

[0085] (Comparative Example 3) An aqueous ink composition was prepared in the same manner as in Example 7, except that thiamine lauryl sulfate was not blended, and the amounts of each component used were changed to the amounts shown in Table 1.

[0086] (Evaluation Method) The measurements and evaluations in the examples and comparative examples were carried out as follows.

[0087] <pH Stability> The aqueous ink compositions prepared in the Examples and Comparative Examples were filled into brown plastic bottles with a capacity of 500 g, and the pH was measured using a pH meter. The bottles were then stored at room temperature for 6 months, and the pH after 6 months of storage was measured using a pH meter. The pH at the time of filling and the pH after 6 months of storage were compared, and the results were evaluated according to the following criteria. 〇: pH change is 0.5 or less △: pH change is greater than 0.5 and less than 1 ×: pH change is greater than 1

[0088] <Freezing resistance> 10 mL of the aqueous ink compositions prepared in the Examples and Comparative Examples were filled into a 15 mL vial and allowed to stand at -10°C for 24 hours, and then evaluated according to the following criteria. 〇: Does not freeze when left at -10℃ for 24 hours ×: Frozen at -10°C for 24 hours

[0089] <Difference in density between upper and lower colorants> 18 kg of the aqueous ink compositions prepared in the Examples and Comparative Examples were filled into a 20 L pail and stored at room temperature for 6 months. After 6 months of storage, the inks in the upper and lower portions of the pail were used to draw colors using a bar coater #6, and the colors were measured using a colorimeter and evaluated according to the following criteria. ○: The difference between the upper ink and the lower ink is ΔE00≦4.5 ×: The difference between the upper ink and the lower ink is ΔE00>4.5

[0090] The evaluation results are shown in Table 1 below. Note that in the evaluation of freeze resistance in Table 1, "-" indicates that the pH stability was low and the ink state was judged to be uneven upon visual inspection (aggregation, separation, etc. was observed), and therefore evaluation was not possible. Similarly, in the evaluation of the difference in concentration of the colorant between the top and bottom, "-" indicates that the ink state was uneven and was judged to be unsuitable for development, and therefore evaluation was not possible.

[0091] [Table 1]

[0092] The results in Table 1 show that aqueous ink compositions containing at least a colorant, water, and thiamine, a thiamine derivative, or a salt thereof have excellent pH stability and freeze resistance, and have little difference in concentration between the top and bottom of the colorant. [Industrial Applicability]

[0093] The aqueous ink composition of the present invention can be suitably used in writing instruments such as ballpoint pens, marking pens, and felt-tip pens.

Claims

1. An aqueous ink composition comprising at least a coloring material, water, and thiamine, a thiamine derivative, or a salt thereof.

2. 2. The aqueous ink composition according to claim 1, wherein the aqueous ink composition contains 5.00% by mass or less of the thiamine, thiamine derivative, or salt thereof.

3. The aqueous ink composition according to claim 1 or 2, further comprising a water-soluble organic solvent, the water-soluble organic solvent being contained in an amount of 30% by mass or less in the aqueous ink composition.

4. A writing instrument equipped with the aqueous ink composition according to claim 1 or 2.

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