Aqueous ink composition for writing instrument and writing instrument

Abstract

To provide an aqueous ink composition for writing instruments that exhibits high preservative performance even without blending a preservative.SOLUTION: An aqueous ink composition for writing instruments comprises water, a water-soluble organic solvent, and a colorant, wherein the water activity of the aqueous ink composition for writing instruments is 0.80 or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Aqueous ink compositions for writing instruments (hereinafter also simply referred to as "ink compositions") contain a large amount of water as a solvent, and therefore are susceptible to the proliferation of microorganisms such as bacteria and mold. In particular, in writing instruments containing ink compositions, the ink ejection section comes into contact with the paper surface or the like during writing. Therefore, microorganisms from the paper surface or the like are introduced into the ink composition through the ink ejection section each time the writing instrument is used, and microorganisms are likely to proliferate in the ink composition. If microorganisms proliferate in an ink composition contained in a writing instrument, problems such as the emission of an unpleasant odor from the ink composition or changes in the physical properties of the ink composition, resulting in ink leakage, are likely to occur. For this reason, ink compositions are required to have high antiseptic properties.

[0003] Conventionally, a preservative has been blended into an ink composition to enhance the preservative performance of the ink composition. For example, Patent Document 1 below proposes an ink composition blended with an isothiazolinone antibacterial substance as a preservative to prevent the growth of microorganisms. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-98811 Summary of the Invention [Problem to be solved by the invention]

[0005] However, from the viewpoints of improving safety and reducing environmental impact, there is a demand for ink compositions that do not contain preservatives.

[0006] Therefore, an object of the present invention is to provide an aqueous ink composition for a writing instrument that has high antiseptic performance even without the inclusion of a preservative, and a writing instrument that uses the same. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides the following aqueous ink composition for a writing instrument and writing instrument. [1] An aqueous ink composition for a writing instrument, comprising water, a water-soluble organic solvent, and a colorant, wherein the water activity of the aqueous ink composition for a writing instrument is 0.80 or less. [2] The aqueous ink composition for a writing instrument according to [1], wherein the water activity is 0.35 or more and 0.72 or less. [3] The aqueous ink composition for a writing instrument according to [1] or [2], wherein the water-soluble organic solvent comprises at least one selected from the group consisting of monohydric alcohols, polyhydric alcohols, glycol ethers, and polyethylene glycols. [4] The aqueous ink composition for a writing instrument according to [3], wherein the molecular weight of the monohydric alcohol, the polyhydric alcohol, and the glycol ether is 400 or less. [5] The aqueous ink composition for a writing instrument according to [3], wherein the polyethylene glycol has a number average molecular weight of 400 or less. [6] The aqueous ink composition for a writing instrument according to any one of [3] to [5], wherein the total content of the monohydric alcohol, the polyhydric alcohol, the glycol ether, and the polyethylene glycol is 20 to 90 mass % based on the total amount of the aqueous ink composition for a writing instrument. [7] The aqueous ink composition for a writing instrument according to any one of [1] to [6], wherein the content of the water is 10 to 56 mass % based on the total amount of the aqueous ink composition for a writing instrument. [8] A writing instrument comprising the aqueous ink composition for a writing instrument according to any one of [1] to [7]. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an aqueous ink composition for a writing instrument that has high antiseptic performance even without the inclusion of a preservative, and a writing instrument using the same. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view showing one embodiment of a ballpoint pen according to the present invention. [Figure 2] 1 is a photograph showing the state of mold growth in the ink compositions of Examples 1 and 4 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as necessary.

[0011] [Water-based ink composition] The aqueous ink composition for writing instruments (hereinafter simply referred to as the "ink composition") of this embodiment is an aqueous ink composition for writing instruments, and contains at least water, a water-soluble organic solvent, and a colorant. The water activity of the ink composition is 0.80 or less. The ink composition contains the above components and has a water activity of 0.80 or less, thereby achieving high preservative performance even without the inclusion of a preservative. Furthermore, when the ink composition does not contain a preservative, it can achieve the following effects: high safety, low environmental impact, long-lasting preservative performance because the preservative is not consumed by microorganisms over time, no preservative inactivation due to metal ions, etc., a wide applicable pH range, and long-lasting preservative effect because the preservative is not adsorbed to the filling, tip, colorant, etc. Each component constituting the ink composition is described below.

[0012] (water) The ink composition of this embodiment contains water. The water may be either tap water or ion-exchanged water, but ion-exchanged water is preferred because it contains fewer impurities.

[0013] The water content is preferably 8 to 56% by mass, more preferably 10 to 56% by mass, even more preferably 10 to 40% by mass, particularly preferably 10 to 36% by mass, and extremely preferably 10 to 30% by mass, based on the total amount of the ink composition. The water in the ink composition refers to the water contained in the entire ink composition, and includes not only the water used as a solvent for the ink composition, but also the water used as a dispersion medium for each component.

[0014] (Water-soluble organic solvent) Examples of water-soluble organic solvents include monohydric alcohols, polyhydric alcohols, glycol ethers, polyethylene glycols, molecules having a cyclic structure in the molecule, and other molecules. These can be used alone or in combination of two or more. When the ink composition contains the water-soluble organic solvent, the water activity of the ink composition tends to decrease.

[0015] Examples of the monohydric alcohol include alcohols having 1 to 4 carbon atoms, such as ethanol, methanol, butanol, propanol, and isopropanol.

[0016] Examples of polyhydric alcohols include glycerin, diglycerin, triglycerin, tetraglycerin, hexylene glycol, thiodiethylene glycol, 1,2,6-hexanetriol, trimethylolpropane, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, dipropylene glycol, tripropylene glycol, 2-butene-1,4-diol, 2-ethyl-1,3 hexanediol, 2-methyl-2,4-pentanediol, 1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol, 4-methyl-1,2-pentanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 3-methyl-1,3-butanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2,4-butanetriol, etc. Among these, preferred polyhydric alcohols are glycerin, propylene glycol, ethylene glycol, and polyethylene glycol.

[0017] Examples of glycol ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, ethylene glycol mono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, and diethylene glycol mono-t-butyl ether. , 1-methyl-1-methoxybutanol, diethylene glycol dimethyl glycol, diethylene glycol ethyl methyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-iso-propyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol mono-iso-propyl ether, dipropylene glycol dimethyl ether, and the like.

[0018] Polyethylene glycol (PEG) may be a mixture of polyethylene glycols with different molecular weights. Examples of polyethylene glycol include PEG200 and PEG400. PEG200 and PEG400 are mixtures of polyethylene glycols with different molecular weights, and the number after PEG indicates the number average molecular weight of the polyethylene glycol.

[0019] Examples of molecules having a cyclic structure within the molecule include 1,3-dioxolane, 1,4-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2-pyrrolidone, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-propyl-2-pyrrolidone, N-butyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, sulfolane, γ-butyrolactone, 1,3-dimethyl-2-imidazolidinone, ethylene carbonate, and propylene carbonate.

[0020] Other molecules include formamide, acetamide, dimethyl sulfoxide, acetin, diacetin, triacetin, and the like.

[0021] From the viewpoint of sufficiently reducing the water activity of the ink composition, the molecular weight of the monohydric alcohol, polyhydric alcohol, glycol ether, molecule having a cyclic structure in the molecule, and other molecules is preferably 400 or less, and the number average molecular weight of the polyethylene glycol is preferably 400 or less.

[0022] When the writing instrument is a padded type, the viscosity of the ink composition is reduced, and from the viewpoint of providing a writing instrument equipped with the ink composition with good writing properties, it is preferable that the molecular weight of the above-mentioned monohydric alcohols, polyhydric alcohols, glycol ethers, molecules having a cyclic structure within the molecule, and other molecules is 400 or less, and it is preferable that the number average molecular weight of the above-mentioned polyethylene glycol is 400 or less.

[0023] In this specification, the number average molecular weight can be measured by a known method, and refers to a value calculated as a standard polystyrene standard, measured using, for example, GPC (gel permeation chromatography).

[0024] The water-soluble organic solvent preferably contains at least one selected from the group consisting of monohydric alcohols, polyhydric alcohols, glycol ethers, and polyethylene glycols. From the viewpoint of sufficiently reducing water activity, the water-soluble organic solvent may contain at least one selected from the group consisting of monohydric alcohols having a molecular weight of 400 or less, polyhydric alcohols having a molecular weight of 400 or less, glycol ethers having a molecular weight of 400 or less, and polyethylene glycols having a number-average molecular weight of 400 or less.

[0025] From the viewpoint of solubility in water, the water-soluble organic solvent may contain at least one selected from the group consisting of glycerin, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, and polyethylene glycol having a number average molecular weight of 400 or less.

[0026] The content of the water-soluble organic solvent may be 30% by mass or more, 40% by mass or more, 50% by mass or more, or 60% by mass or more, based on the total amount of the ink composition. By having the content of the water-soluble organic solvent within the above range, the antiseptic performance of the ink composition can be further improved. From the viewpoints of solubility in water, viscosity, writability, and strike-through, the content of the water-soluble organic solvent may be 90% by mass or less, 80% by mass or less, 75% by mass or less, or 70% by mass or less, based on the total amount of the ink composition. The content of the water-soluble organic solvent may be 30 to 90% by mass, 40 to 80% by mass, 50 to 75% by mass, or 60 to 70% by mass, based on the total amount of the ink composition.

[0027] When the water-soluble organic solvent contains a glycol, the content of the glycol may be 20 to 90 mass %, 25 to 50 mass %, or 30 to 40 mass % based on the total mass of the ink composition, from the viewpoint of sufficiently reducing the water activity of the ink composition.

[0028] When the water-soluble organic solvent contains glycerin, the content of glycerin may be 20 to 90 mass%, 44 to 90 mass%, 60 to 90 mass%, or 64 to 90 mass%, based on the total amount of the ink composition, from the viewpoint of sufficiently reducing the water activity of the ink composition.

[0029] When the ink composition contains at least one selected from the group consisting of monohydric alcohols, polyhydric alcohols, glycol ethers, and polyethylene glycol ethers, the total content of the monohydric alcohols, polyhydric alcohols, glycol ethers, and polyethylene glycol ethers may be 20 to 90 mass%, 40 to 80 mass%, 50 to 75 mass%, or 60 to 70 mass%, based on the total amount of the ink composition for a writing instrument.

[0030] (coloring agent) Examples of colorants include dyes and pigments. The dye is preferably a water-soluble dye. Examples of water-soluble dyes include food dyes, direct dyes, acid dyes, and basic dyes. The pigment is preferably a water-dispersible pigment. Examples of water-dispersible pigments include organic pigments, inorganic pigments, and fluorescent pigments. The above colorants may be used alone or in combination of two or more.

[0031] Food dyes include gardenia yellow, safflower yellow, turmeric pigment, monascus yellow, palm oil carotene, monascus pigment, gardenia red pigment, safflower red pigment, beet red, cochineal pigment, lac pigment, madder pigment, perilla pigment, red cabbage pigment, red radish pigment, purple sweet potato pigment, purple corn pigment, grape skin pigment, elderberry pigment, grape juice pigment, blueberry pigment, chili pepper pigment, annatto pigment, chlorophyll, gardenia blue pigment, spirulina pigment, cacao pigment, tamarind pigment, persimmon pigment, sorghum pigment, plant charcoal pigment, hemoglobin, and other natural dyes, as well as Sunset Yellow FCF, Tartrazine, Amaranth, Erythrosine, Allura Red AC, New Coccine, Phloxine, Rose Bengale, Acid Red, Brilliant Blue FCF, and Indigo. Examples include artificial dyes such as carmine.

[0032] Examples of direct dyes include Color Index (hereinafter referred to as "CI") Direct Black 17, 19, 38, and 154; CI Direct Yellow 1, 4, 12, and 29; CI Direct Orange 6, 8, 26, and 29; CI Direct Red 1, 2, 4, and 13; and CI Direct Blue 2, 6, 15, 78, and 87.

[0033] Examples of acid dyes include CI Acid Black 2, 31, CI Acid Yellow 3, 17, 23, and 73, CI Acid Orange 10, CI Acid Red 13, 14, 18, 27, 52, 73, 87, and 92, and CI Acid Blue 1, 9, 74, and 90.

[0034] Examples of basic dyes include CI Basic Yellow 2 and 3, CI Basic Red 1, 2, 8, and 12, CI Basic Violet 1, 3, and 10, and CI Basic Blue 5, 9, and 26.

[0035] Examples of pigments that can be used include inorganic pigments such as carbon black and ultramarine, and organic pigments such as copper phthalocyanine blue, benzidine yellow, and pigment red. In addition, water-dispersed pigment products in which various pigments are finely and stably dispersed in an aqueous medium can also be used. Examples of water-dispersed pigment products include water-dispersed pigment products in which a pigment is dispersed in an aqueous medium using a surfactant or a water-soluble resin, water-dispersed pigment products in which a pigment that has been imparted with self-dispersibility is dispersed in an aqueous medium, and water-dispersed pigment products in which synthetic resin particles colored with various dyes are dispersed in an aqueous medium. Among these, water-dispersed pigment products in which a pigment that has been imparted with self-dispersibility is dispersed in an aqueous medium, and water-dispersed pigment products in which synthetic resin particles colored with various dyes are dispersed in an aqueous medium are preferred from the standpoint of storage stability. Examples of self-dispersed pigments include pigments whose surfaces have been treated with oxidizing agents such as nitric acid, ozone, and hydrogen peroxide to retain polar groups such as carboxyl groups and hydroxyl groups on the surface.

[0036] Examples of water-dispersed pigment products in which a pigment is dispersed in an aqueous medium using the above-mentioned surfactants include Sandye Super Blue GLL (manufactured by Sanyo Dye Co., Ltd., CI Pigment Blue 15:3B, pigment content 24% by mass), Sandye Super Pink FBL (manufactured by Sanyo Dye Co., Ltd., CI Pigment Red 146, pigment content 21.5% by mass), WA Color Red A-01 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., CI Pigment Red 258, pigment content 8% by mass), TC Yellow FG (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., CI Pigment Yellow 81, pigment content approximately 30% by mass), and TC Red FG (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., CI Pigment Red 220 / 166, pigment content approximately 35% by mass).

[0037] Examples of water-dispersed pigment products in which a pigment is dispersed in an aqueous medium using the above-mentioned water-soluble resin include WA color Black A 250 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., CI Pigment Black 7, pigment content 15% by mass), WA-S color Green (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., CI Pigment Green 7, pigment content 8% by mass), Micropigmo WMVT-5 (manufactured by Orient Chemical Industries Co., Ltd., CI Pigment Violet 23, pigment content 20% by mass), and Emmacol NS Yellow 4618 (manufactured by Sanyo Dye Co., Ltd., CI Pigment Yellow 83, pigment content 30% by mass).

[0038] Examples of water-dispersed pigment products in which the self-dispersible pigment is dispersed in an aqueous medium include Fuji Jet Black (manufactured by Fuji Pigment Co., Ltd., CI Pigment Black 7, pigment content 15% by mass), Fuji Jet Black A-20 (manufactured by Fuji Pigment Co., Ltd., CI Pigment Black 7, pigment content 20% by mass), Fuji Jet Black B-15 (manufactured by Fuji Pigment Co., Ltd., CI Pigment Black 7, pigment content 15% by mass), Fuji Jet Black B-22 (manufactured by Fuji Pigment Co., Ltd., CI Pigment Black 7, pigment content 22% by mass), BON JET BLACK CW-1 (manufactured by Orient Chemical Industries Co., Ltd., CI Pigment Black 7, pigment content 20% by mass), BON JET BLACK CW-2 (manufactured by Orient Chemical Industries Co., Ltd., CI Pigment Black 7, pigment content 15% by mass), BON JET BLACK Examples include CW-3 (manufactured by Orient Chemical Industries Co., Ltd., CI Pigment Black 7, pigment content 13% by mass), Aqua Black 162 (manufactured by Tokai Carbon Co., Ltd., CI Pigment Black 7, pigment content 20% by mass), and Aqua Black 001 (manufactured by Tokai Carbon Co., Ltd., CI Pigment Black 7, pigment content 20% by mass).

[0039] Furthermore, examples of water-dispersed pigment products in which synthetic resin particles colored with the above-mentioned various dyes are dispersed in an aqueous medium include the NKW-2100E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content 51% by mass), the NKW-3200E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content 37% by mass), the NKW-C2100E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content 50±3% by mass), the NKW-6000E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content 34±3% by mass), the NKW-6200E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content 48 to 54% by mass), and the SW-1 Examples of such a pigment include the SF-00 series (manufactured by Shin-Roi-Hi Co., Ltd., solid content 42±1% by mass), the SF-3000N series (manufactured by Shin-Roi-Hi Co., Ltd., solid content 40±1.5% by mass), the SF-5000 series (manufactured by Shin-Roi-Hi Co., Ltd., solid content 30±1.5% by mass), the SF-8000 series (manufactured by Shin-Roi-Hi Co., Ltd., solid content 50±2% by mass), the SP-10 series (manufactured by Shin-Roi-Hi Co., Ltd., pigment content approximately 40% by mass), the SG-500 series (manufactured by Shin-Roi-Hi Co., Ltd., pigment content approximately 60% by mass), and the FW-8500KS series (manufactured by Shin-Roi-Hi Co., Ltd., solid content approximately 55% by mass).

[0040] The colorant may be a dye alone or a pigment alone, or a combination of a dye and a pigment.

[0041] The content of the colorant may be 0.1 to 10 mass %, 1.0 to 8.0 mass %, or 2.0 to 5.0 mass % based on the total mass of the ink composition.

[0042] (preservatives) The ink composition of this embodiment may contain a preservative as long as it does not adversely affect safety, the environmental impact, and the like. However, the content is preferably less than 1000 ppm by mass, and it is more preferable that the ink composition does not contain a preservative.

[0043] Examples of preservatives include liquid disinfectants and synthetic antibacterial agents. Liquid disinfectants include hydrogen peroxide, glutaraldehyde, and peracetic acid. Synthetic antibacterial agents include aliphatic alcohol antibacterial agents, aromatic alcohol (phenol) antibacterial agents, carboxylic acid antibacterial agents, ester antibacterial agents, halogen antibacterial agents, nitrile antibacterial agents, amine antibacterial agents, isothiazolinone antibacterial agents, carbamate antibacterial agents, benzimidazole antibacterial agents, benzothiazole antibacterial agents, biguanide antibacterial agents, quaternary ammonium salt antibacterial agents, pyridinium antibacterial agents, fluorodichloromethylthio antibacterial agents, hydantoin antibacterial agents, pyrithione antibacterial agents, pyridine antibacterial agents, morpholine antibacterial agents, diphenyl ether antibacterial agents, disulfide antibacterial agents, oxazoline antibacterial agents, quinoline antibacterial agents, and triazole antibacterial agents.

[0044] The content of the preservative in the ink composition may be less than 1000 ppm by mass, 500 ppm by mass or less, 100 ppm by mass or less, or 50 ppm by mass or less.

[0045] (Other ingredients) In addition to the components described above, the ink composition of this embodiment may or may not contain additives such as a surfactant, a pH adjuster, a thickener, a lubricant, a dye dissolving aid, a drying improver, and a fixing resin.

[0046] The surfactant is not particularly limited as long as it is one that can be used in ink compositions. Examples of surfactants include succinic acid surfactants, silicone surfactants, acetylene glycol surfactants, and sodium alkylnaphthalene sulfonate. However, from the perspective of environmental regulations, it is preferable that the surfactant does not contain a fluorine-based surfactant. The amount of surfactant blended is not particularly limited, and may be, for example, 0.05 to 3.0 mass% based on the total amount of the ink composition.

[0047] The pH adjuster is not particularly limited as long as it is used in aqueous ink compositions, particularly ink compositions. Examples of pH adjusters include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, or aqueous solutions thereof; salts such as sodium carbonate and sodium benzoate; amine compounds such as triethanolamine, aminomethylpropanol, diethanolamine, monoethanolamine, dimethylethanolamine, morpholine, and triethylamine; and ammonia (water). The pH adjusters may be used alone or in combination of two or more.

[0048] The content of the pH adjuster in the ink composition is not particularly limited, and may be adjusted so that the pH of the ink composition is, for example, 6.0 or more and 10.0 or less.

[0049] Examples of thickeners include polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, sodium polyacrylate, and carboxyvinyl polymer. The above thickeners may be used alone or in combination of two or more. The amount of thickener to be added is not particularly limited, and may be, for example, 0.1 to 3% by mass, or 0.2 to 1% by mass, based on the total amount of the ink composition.

[0050] Examples of lubricants include higher fatty acids such as oleic acid, nonionic surfactants having a long-chain alkyl group, polyether-modified silicone oils, thiophosphite triesters such as thiophosphite tri(alkoxycarbonylmethyl ester) and thiophosphite tri(alkoxycarbonylethyl ester), as well as their metal salts, ammonium salts, amine salts, and alkanolamine salts. The content of the lubricant is preferably 0.3 to 5% by mass, and more preferably 0.5 to 3% by mass, based on the total mass of the ink composition.

[0051] The ink composition of this embodiment can be obtained by mixing the above-described components using a stirrer such as a dissolver, a Henschel mixer, a homomixer, etc. The stirring conditions for the stirrer are not particularly limited, but for example, an ink composition in which the components are uniformly dispersed can be obtained by stirring for 30 to 180 minutes at a rotation speed of 100 to 1000 rpm using a dissolver stirrer.

[0052] (water activity) The water activity of the ink composition in this embodiment is a value that represents the proportion of free water, and can be calculated, for example, by the following formula. Water activity = Water vapor pressure of the object in a sealed container / Vapor pressure of pure water

[0053] From the viewpoint of imparting high antiseptic performance to the ink composition even without blending a preservative, the water activity of the ink composition must be 0.80 or less, preferably 0.72 or less, and more preferably 0.70 or less. Furthermore, from the viewpoints of solubility, viscosity, writability, and strike-through, the water activity of the ink composition may be 0.35 or more, 0.40 or more, 0.50 or more, or 0.60 or more.

[0054] The water activity of the ink composition can be measured, for example, using a capacitance-type water activity measuring device (manufactured by Shibata Scientific Co., Ltd., trade name: Water Activity Measuring Device Model AW-1).

[0055] (viscosity) The viscosity of the ink composition of this embodiment at 25°C may be, for example, 2 to 6000 mPa·s. When the writing instrument containing the ink composition is a pad-filled pen, the viscosity of the ink composition at 25°C may be 2.0 to 25.0 mPa·s, 2.3 to 8.0 mPa·s, or 2.5 to 7.0 mPa·s. When the marking pen is a pad-filled pen, a viscosity of 2.0 mPa·s or more can prevent ink leakage from the marking pen after filling with the ink composition, and a viscosity of 25.0 mPa·s or less can reduce blurring of written lines when writing on paper. When the writing instrument containing the ink composition is a direct-filled pen, the viscosity of the ink composition at 25°C may be 25 to 50 mPa·s, 30 to 45 mPa·s, or 30 to 40 mPa·s. If the marking pen is a direct ink type, a viscosity of 25 Pa·s will prevent ink leakage from the pen tip and reduce bleeding of drawn lines, while a viscosity of 50 Pa·s or less will enable the ink composition to be discharged smoothly.

[0056] The viscosity of the ink composition at 25°C can be measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd., product name: TV-33) at 25°C, using a 0.8° x R24 cone plate, and at a rotation speed of 6 to 30 rpm. The ink composition may be a Newtonian fluid, in which case the viscosity value changes very little depending on the rotation speed of the cone plate, so the rotation speed conditions are not particularly limited. For ink compositions with relatively low viscosity (for example, 10 mPa s or less), measurements may be made at a rotation speed of 30 rpm, and for ink compositions with higher viscosity (for example, greater than 10 mPa s), measurements may be made at a rotation speed of 6 rpm.

[0057] The ink composition may be, for example, an ink composition for a writing instrument such as a marking pen, a ballpoint pen, or a dip pen ink.

[0058] [Marking pen] The marking pen of this embodiment comprises a pen tip made of a fiber core or a plastic core, and an ink reservoir, in which the ink composition is stored.

[0059] FIG. 1 is a cross-sectional view of a marking pen according to one embodiment of the present invention. The marking pen 10 in FIG. 1 is a spool-type marking pen. Although the marking pen 10 in FIG. 1 has nibs on both ends, it may have a nib on only one end. The marking pen 10 in FIG. 1 includes an ink reservoir 1, nibs 2a and 2b, a barrel 3, a spool 4, caps 5a and 5b, and retaining members 6a and 6b. In the marking pen 10, the space inside the barrel 3 serves as the ink reservoir 1, and this ink reservoir 1 contains a spool 4 containing the ink composition of this embodiment. Note that a cylindrical central shaft may be provided inside the barrel 3 in addition to the barrel 3, and the space inside the central shaft may serve as the ink reservoir 1.

[0060] The pen tips 2a and 2b are typical marking pen tips and are formed of fiber or plastic cores capable of supplying an ink composition. In the marking pen 10, the tip shapes of the pen tips 2a and 2b are bullet-shaped. Note that the tip shapes of the pen tips 2a and 2b in the marking pen 10 are not limited to bullet shapes, and may be chisel-shaped, brush-pen-shaped, or the like.

[0061] Furthermore, caps 5a and 5b are arranged to cover the pen tips 2a and 2b, respectively.

[0062] The holding members 6a and 6b are connected to the ends of the barrel 3, respectively, and fix the pen tips 2a and 2b to the padding 4.

[0063] The fiber thickness of the padding 4 is preferably 2 to 5 denier. By having the fiber thickness within this range, when the ink composition of this embodiment is filled into an ink storage section, it is possible to prevent ink leakage from the pen tip of a marking pen. The thickness of the fibers constituting the padding 4 can be adjusted, for example, by selecting the diameter of the fibers (threads) used to make the padding 4.

[0064] The porosity of the padding 4 is preferably 90% or less, and more preferably 80 to 88%. By keeping the porosity within this range, when the ink composition of this embodiment is filled into an ink storage section, ink leakage from the pen tip of a marking pen can be suppressed. For example, by increasing the fiber weight per fiber of the padding, the porosity of the padding 4 can be reduced, and by reducing the fiber weight, the porosity of the padding 4 can be increased.

[0065] The marking pen of this embodiment is suitable for use both in writing on non-permeable surfaces such as glass plates, resin plates and metal plates, and in writing on paper.

[0066] While a preferred embodiment of the writing instrument of the present invention has been described above using a marking pen as an example, the writing instrument is not limited to the above embodiment. For example, the form of the writing instrument is not limited to a cap-type like the marking pen shown in Figure 1, but may be a retractable type.

[0067] Furthermore, the form of the writing instrument is not limited to a batting type such as the marking pen shown in Fig. 1, but may also be a direct ink type. A direct ink type writing instrument may be a writing instrument having a valve mechanism in the barrel (barrel tube) that directs the ink composition in the barrel to the pen tip when the valve mechanism is opened, or a direct ink type writing instrument having a structure in which the ink composition is directly contained inside the barrel and an ink flow rate adjusting member in the form of a comb groove or an ink flow rate adjusting member made of a fiber bundle is interposed.

[0068] The writing instrument may also be a ballpoint pen, for example, a ballpoint pen equipped with a core having an ink reservoir that stores an ink composition and a ballpoint tip provided at one end of the ink reservoir. The ballpoint tip may be composed of a ball holder and a ball rotatably held by the ball holder, and may be fixed to the ink reservoir by a joint. A backflow preventer may be housed in the ink reservoir on the side opposite the ballpoint tip, adjacent to the ink composition. A resilient member that urges the ball forward may be housed inside the ballpoint tip and the joint. When the ballpoint pen has a resilient member, the resilient member presses the ball against the inner edge of the tip of the ball holder when not writing, causing the inner edge of the tip of the ball holder to closely contact the ball. When writing with the ball holder, a gap is created between the ball and the inner edge of the tip of the ball holder due to writing pressure, thereby preventing the outflow of the ink composition.

[0069] The ballpoint pen may be a single-core ballpoint pen with one core, or a multi-core ballpoint pen with multiple cores.

[0070] The ball may be made of a material such as cemented carbide, stainless steel, ruby, or ceramic.

[0071] The outer diameter of the ball may be, for example, 0.3 to 2.0 mm, preferably 0.4 to 1.5 mm, and more preferably 0.5 to 1.0 mm.

[0072] The ink reservoir in a ballpoint pen can be made of a resin such as polypropylene, polyethylene, polyethylene terephthalate, nylon, polyacetal, or polycarbonate, or a metal. The shape of the ink reservoir is not particularly limited, and can be, for example, cylindrical.

[0073] Although an example of an embodiment of a ballpoint pen has been described above, the ballpoint pen is not limited to the above embodiment. For example, the ballpoint pen may also have the structure of the direct ink type writing implement described above. [Example]

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0075] [Preparation of Ink Composition] (Examples 1 to 9 and Comparative Examples 1 to 4) The materials listed in Table 1 below were mixed to obtain ink compositions of Examples 1 to 9 and Comparative Examples 1 to 4. Unless otherwise specified, the units of values ​​in the table are parts by mass, and for materials containing a solvent, the amounts shown include the amount of the solvent.

[0076] (Viscosity measurement of ink composition) The viscosity of the ink compositions of the Examples and Comparative Examples was measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd., product name: TV-33) with a 0.8°×R24 cone plate at 25°C. The ink compositions of the Examples and Comparative Examples were all Newtonian fluids, and the rotation speed of the cone plate was 30 rpm for Examples 1, 4, 7, 9 and Comparative Examples 1 to 3, and 6 rpm for Examples 2 to 3, 5 to 6, 8 and Comparative Example 4.

[0077] Details of some of the materials in Table 1 are shown below.

[0078] (Water-soluble organic solvent) Polyethylene glycol (PEG200) (manufactured by Sanyo Chemical Industries, Ltd., trade name: PEG200, number average molecular weight: 200) Polyethylene glycol (PEG600) (manufactured by Sanyo Chemical Industries, Ltd., trade name: PEG600, number average molecular weight: 600) (coloring agent) Dye (Sunset Yellow FCF, manufactured by Daiwa Chemical Industry Co., Ltd.) Pigment (Dainippon Ink and Chemicals, Inc., Pigment Red 258, pigment content: 100% by mass) Carbon black dispersion (manufactured by Orient Chemical Industries, Ltd., carbon black content: 20% by mass, water content: 80% by mass) (pigment dispersant) Styrene-acrylic copolymer (manufactured by BASF, trade name: Joncryl 683)

[0079] [Evaluation of Ink Composition] The water activity of the ink composition was measured and the antiseptic performance was evaluated as follows.

[0080] (Water activity measurement) The water activity of the ink compositions of the examples and comparative examples to be measured was measured using a capacitance-type water activity measuring device (manufactured by Shibata Scientific Co., Ltd., product name: Water Activity Measuring Device Model AW-1) at a measurement temperature of 25° C. The measured water activity values ​​are shown in Table 1.

[0081] (Evaluation of antiseptic performance) The antiseptic performance of the ink compositions of the Examples and Comparative Examples was evaluated (preservative effectiveness test) according to the following procedure.

[0082] For 2 mL of ink composition, mold spore solution (number of spores: 1 x 10 4 ~×10 6 Each test solution was poured onto a plate medium (agar medium), and the day the test solution was poured onto the agar medium was designated as day 0, followed by incubation at 26°C until day 28.

[0083] On days 0, 14, and 28 after the start of the culture, mold growth on the agar medium was visually confirmed, and the antiseptic performance of each ink composition was evaluated according to the following evaluation criteria. A: No mold was found as of the 14th day. B: Mold was confirmed on the 14th day, but no mold was confirmed on the 28th day. C: Mold was confirmed on the 14th and 28th days.

[0084] The evaluation results of the antiseptic performance are shown in Table 1. For reference, photographs of the state of mold growth in the test liquids containing the ink compositions of Examples 1 and 4 and Comparative Example 1 are shown in FIG.

[0085] [Making a marking pen] Marking pens of Examples 1 to 9 and Comparative Examples 1 to 4 containing the ink compositions of Examples 1 to 9 and Comparative Examples 1 to 4 were prepared according to the following procedure.

[0086] The ink composition was impregnated into the padding 4 (porosity 90%) and housed in the barrel 3. The polyester fiber nib 2a (porosity 55%) was attached to the tip of the barrel via the retaining member 6a, and the cap 5a was attached to obtain a marking pen having the structure shown in Figure 1. The nib 2b (porosity 55%) and cap 5b had been attached to the barrel 3 beforehand.

[0087] The marking pens thus prepared were used to draw straight lines on paper, and the state of ejection of the ink composition was checked to confirm writing performance. The marking pens of Examples 1 to 9 and Comparative Examples 1 to 3 ejected the ink composition without any problems, and no problems with writing performance were confirmed. On the other hand, the marking pen of Comparative Example 4 ejected the ink composition more slowly than the marking pens of Examples 1 to 9 and Comparative Examples 1 to 3, and the ink composition of Comparative Example 4 was not suitable for use in a stencil-filled marking pen.

[0088] [Table 1]

[0089] 1...ink storage section, 2a, 2b...pen tip, 3...shaft body, 4...filling (ink composition), 5a, 5b...cap, 6a, 6b...holding member, 10...marking pen.

Claims

1. An aqueous ink composition for a writing instrument, comprising water, a water-soluble organic solvent, and a colorant, wherein the water activity of the aqueous ink composition for a writing instrument is 0.80 or less.

2. 2. The aqueous ink composition for a writing instrument according to claim 1, wherein the water activity is 0.35 or more and 0.72 or less.

3. 2. The aqueous ink composition for a writing instrument according to claim 1, wherein the water-soluble organic solvent comprises at least one selected from the group consisting of monohydric alcohols, polyhydric alcohols, glycol ethers, and polyethylene glycols.

4. 4. The aqueous ink composition for a writing instrument according to claim 3, wherein the molecular weight of the monohydric alcohol, the polyhydric alcohol, and the glycol ether is 400 or less.

5. 4. The aqueous ink composition for a writing instrument according to claim 3, wherein the polyethylene glycol has a number average molecular weight of 400 or less.

6. 4. The aqueous ink composition for a writing instrument according to claim 3, wherein the total content of the monohydric alcohol, the polyhydric alcohol, the glycol ether, and the polyethylene glycol is 20 to 90 mass% based on the total amount of the aqueous ink composition for a writing instrument.

7. 2. The aqueous ink composition for a writing instrument according to claim 1, wherein the content of the water is 10 to 56 mass % based on the total amount of the aqueous ink composition for a writing instrument.

8. A writing instrument comprising the aqueous ink composition for a writing instrument according to any one of claims 1 to 7.

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