Water-based ink composition for writing instruments and writing instruments

Abstract

To provide an aqueous ink composition for writing instruments that allows for smooth and uninterrupted writing on paper, and also allows for easy erasure of the writing line by dry wiping, even when writing on a non-absorbent surface. [Solution] An aqueous ink composition for writing instruments comprising at least a colorant, a film-forming resin, a water-soluble organic solvent, and water, wherein the boiling point of the water-soluble organic solvent is 170 to 290°C, the content of the water-soluble organic solvent is 1.0 to 50% by mass based on the total amount of the aqueous ink composition, the solid content of the film-forming resin is 1.0 to 20% by mass based on the total amount of the aqueous ink composition, and when the viscosity of the aqueous ink composition at 25°C initially is V0 [mPa·s] and the viscosity at 25°C after being left at 50°C for 4 hours is V1 [mPa·s], the ratio of V1 to V0 (V1 / V0) is less than 1.30.

Description

TECHNICAL FIELD

[0001] The present invention relates to an aqueous ink composition for writing instruments and a writing instrument. BACKGROUND ART

[0002] Conventionally, as aqueous ink compositions used for writing instruments such as aqueous ballpoint pens, compositions containing a colorant, water and the like are known (see, for example, Patent Document 1). Such aqueous ink compositions for writing instruments are required to have good writing performance that enables writing without blurring or breaking on paper surfaces. PRIOR ART DOCUMENT PATENT DOCUMENT

[0003] Patent Document 1 Japanese Unexamined Patent Application Publication No. 2023-78572 SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0004] Aqueous ink compositions for writing instruments may accidentally adhere to non-absorbent surfaces such as desks and plastic underlays when writing lines bleed out from paper surfaces. It is desirable that the ink composition adhering to a non-absorbent surface can be easily erased by dry wiping. However, aqueous ink compositions having good writing performance on paper surfaces tend to be difficult to erase by dry wiping when they adhere to non-absorbent surfaces. Therefore, there is a demand for an aqueous ink composition that achieves both good writing performance on paper surfaces and good erasability when adhering to non-absorbent surfaces.

[0005] The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide an aqueous ink composition for writing instruments which enables writing without blurring or breaking on paper surfaces and allows easy erasure of writing lines by dry wiping even when writing is performed on non-absorbent surfaces, and a writing instrument using the same. MEANS FOR SOLVING THE PROBLEM

[0006] To achieve the above objective, the present invention provides the following aqueous ink composition for writing instruments and writing instruments. [1] A water-based ink composition for writing instruments comprising at least a colorant, a film-forming resin, a water-soluble organic solvent, and water, wherein the boiling point of the water-soluble organic solvent is 170 to 290°C, the content of the water-soluble organic solvent is 1.0 to 50% by mass based on the total amount of the water-based ink composition, the solid content of the film-forming resin is 1.0 to 20% by mass based on the total amount of the water-based ink composition, and when the viscosity of the water-based ink composition at 25°C initially is V0 [mPa·s] and the viscosity at 25°C after being left at 50°C for 4 hours is V1 [mPa·s], the ratio of V1 to V0 (V1 / V0) is less than 1.30. [2] The aqueous ink composition for writing instruments according to [1] above, wherein the film-forming resin is an aqueous resin emulsion. [3] The aqueous ink composition for writing instruments according to [2] above, wherein the aqueous resin emulsion is an acrylic resin emulsion. [4] A water-soluble ink composition for writing instruments according to any of [1] to [3] above, wherein the content of the above water-soluble organic solvent in the water-soluble ink composition is A [mass%], the boiling point is a [°C], and the solid content of the film-forming resin is B [mass%], and the relationship of the following formula (I) is satisfied. 5.0≦(a×A) / (100×B)≦28.0…(I) [5] The aqueous ink composition for writing instruments according to any of [1] to [4] above, wherein the average particle size of the colorant is less than 2.0 μm. [6] The aqueous ink composition for writing instruments according to any one of [1] to [5] above, wherein the solid content of the above-mentioned colorant is 1.5 to 9.0% by mass based on the total amount of the aqueous ink composition. [7] A writing instrument comprising the ink composition described in any of [1] to [6] above. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an aqueous ink composition for writing instruments and a writing instrument using the same, which allows writing on paper without smudging or interruption, and also allows the written lines to be easily erased by dry wiping even when writing on a non-absorbent surface. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view showing one embodiment of the ballpoint pen according to the present invention. [Modes for carrying out the invention]

[0009] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as necessary.

[0010] [Water-based ink composition for writing instruments] The aqueous ink composition for writing instruments of this embodiment (hereinafter also simply referred to as "aqueous ink composition") contains at least a colorant, a film-forming resin, a water-soluble organic solvent, and water, wherein the boiling point of the water-soluble organic solvent is 170 to 290°C, the content of the water-soluble organic solvent is 1.0 to 50% by mass based on the total amount of the aqueous ink composition, the solid content of the film-forming resin is 1.0 to 20% by mass based on the total amount of the aqueous ink composition, and when the initial viscosity of the aqueous ink composition at 25°C is V0 [mPa·s] and the viscosity at 25°C after being left at 50°C for 4 hours is V1 [mPa·s], the ratio of V1 to V0 (V1 / V0) is less than 1.30.

[0011] Conventional water-based ink compositions for writing instruments, when applied to a non-absorbent surface, would penetrate into the microscopic irregularities on the surface, making them difficult to erase. In contrast, the water-based ink composition for writing instruments according to this embodiment contains a film-forming resin, which allows it to form a film when applied to a non-absorbent surface, thereby improving erasability. Furthermore, by selecting and using a water-soluble organic solvent with a boiling point within a specific range, the film on the non-absorbent surface can be given appropriate softening properties, further improving erasability. By setting the content of the film-forming resin and water-soluble organic solvent in the water-based ink composition for writing instruments, as well as the V1 / V0 value, within the specified range, a good balance can be achieved between writing performance on paper and erasability on non-absorbent surfaces. As a result, the water-based ink composition for writing instruments according to this embodiment allows for smooth and uninterrupted writing on paper, and even when written on a non-absorbent surface, the writing can be easily erased with a dry wipe.

[0012] (Coloring agent) Examples of colorants include dyes and pigments. Dyes are preferably water-soluble dyes. Examples of water-soluble dyes include direct dyes, acid dyes, and basic dyes. Pigments are preferably water-dispersible pigments. Examples of pigments include organic pigments and inorganic pigments. From the viewpoint of further improving the erasability of writing lines by dry wiping when writing on a non-absorbent surface, colorants are preferably pigments. One type of colorant may be used alone, or two or more types may be used in combination.

[0013] Examples of direct dyes include the Color Index (hereinafter referred to as CI) Direct Black 17, 19, 38, 154; CIDirect Yellow 1, 4, 12, 29; CIDirect Orange 6, 8, 26, 29; CIDirect Red 1, 2, 4, 13; and CIDirect Blue 2, 6, 15, 78, 87.

[0014] Other examples of acid dyes include CIAcid Black 2, 31, CIAcid Yellow 3, 17, 23, 73, CIAcid Orange 10, CIAcid Red 13, 14, 18, 27, 52, 73, 87, 92, and CIAcid Blue 1, 9, 74, 90.

[0015] Examples of basic dyes include CIBasic Yellow 2, 3, CIBasic Red 1, 2, 8, 12, CIBasic Violet 1, 3, 10, and CIBasic Blue 5, 9, 26.

[0016] As pigments, for example, inorganic pigments such as carbon black and ultramarine, organic pigments such as copper phthalocyanine blue and benzidine yellow, and water-dispersible pigment products in which various pigments are pre-dispersed finely and stably in an aqueous medium can be used. Examples of water-dispersible pigment products include water-dispersible pigment products in which pigments are dispersed in an aqueous medium using surfactants or water-soluble resins, water-dispersible pigment products in which self-dispersible pigments are dispersed in an aqueous medium, and water-dispersible pigment products in which synthetic resin particles colored with various dyes are dispersed in an aqueous medium. Among these, from the viewpoint of storage stability, water-dispersible pigment products in which self-dispersible pigments are dispersed in an aqueous medium and water-dispersible pigment products in which synthetic resin particles colored with various dyes are dispersed in an aqueous medium are preferred. Here, examples of pigments in which self-dispersible pigments are imparted include pigments in which polar groups such as carboxyl groups and hydroxyl groups are retained on the surface by treating the surface with an oxidizing agent such as nitric acid, ozone, and hydrogen peroxide.

[0017] Examples of the aqueous dispersion pigment product obtained by dispersing a pigment in an aqueous medium using the above surfactant include Sandye Super Blue GLL (manufactured by Sanyo Color Works, Ltd., C.I. Pigment Blue 15:3B, pigment content: 24% by mass), Sandye Super Pink FBL (manufactured by Sanyo Color Works, Ltd., C.I. Pigment Red 146, pigment content: 21.5% by mass), TC Yellow FG (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., C.I. Pigment Yellow 81, pigment content: approx. 30% by mass), and TC Red FG (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., C.I. Pigment Red 220 / 166, pigment content: approx. 35% by mass).

[0018] Examples of the aqueous dispersion pigment product obtained by dispersing a pigment in an aqueous medium using the above water-soluble resin include WA color Black A 250 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., C.I. Pigment Black 7, pigment content: 15% by mass), WA-S color Green (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., C.I. Pigment Green 7, pigment content: 8% by mass), Micro Pigmo WMVT-5 (manufactured by Orient Chemical Industries Co., Ltd., C.I. Pigment Violet 23, pigment content: 20% by mass), and Emacol NS Yellow 4618 (manufactured by Sanyo Color Works, Ltd., C.I. Pigment Yellow 83, pigment content: 30% by mass).

[0019] Furthermore, examples of water-dispersible pigment products in which the above-mentioned self-dispersible pigments are dispersed in an aqueous medium include Fuji SP Black 8087 (manufactured by Fuji Pigment Co., Ltd., CIPigment Black 7, pigment content 15% by mass), Fuji Jet Black (manufactured by Fuji Pigment Co., Ltd., CIPigment Black 7, pigment content 15% by mass), Fuji Jet Black A-20 (manufactured by Fuji Pigment Co., Ltd., CIPigment Black 7, pigment content 20% by mass), Fuji Jet Black B-15 (manufactured by Fuji Pigment Co., Ltd., CIPigment Black 7, pigment content 15% by mass), Fuji Jet Black B-22 (manufactured by Fuji Pigment Co., Ltd., CIPigment Black 7, pigment content 22% by mass), BON JET BLACK CW-1 (manufactured by Orient Chemical Industry Co., Ltd., CIPigment Black 7, pigment content 20% by mass), and BON JET BLACK Examples include CW-2 (manufactured by Orient Chemical Industry Co., Ltd., CIPigment Black 7, pigment content 15% by mass), BON JET BLACK CW-3 (manufactured by Orient Chemical Industry Co., Ltd., CIPigment Black 7, pigment content 13% by mass), Aqua Black 162 (manufactured by Tokai Carbon Co., Ltd., CIPigment Black 7, pigment content 20% by mass), and Aqua Black 001 (manufactured by Tokai Carbon Co., Ltd., CIPigment Black 7, pigment content 20% by mass).

[0020] Further, examples of the aqueous dispersion pigment product obtained by dispersing the synthetic resin particles colored with the various dyes described above in an aqueous medium include NKW-2100E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 51% by mass), NKW-3200E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 37% by mass), NKW-C2100E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 50±3% by mass), NKW-6000E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 34±3% by mass), NKW-6200E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 51±3% by mass), NKW-7500E series (manufactured by Nippon Fluorescent Chemical Co., Ltd., non-volatile content: 51±3% by mass), SW-100 series (manufactured by Sinloihi Co., Ltd., solid content: 42±1% by mass), SF-3000N series (manufactured by Sinloihi Co., Ltd., solid content: 40±1.5% by mass), SF-5000 series (manufactured by Sinloihi Co., Ltd., solid content: 30±1.5% by mass), SF-8000 series (manufactured by Sinloihi Co., Ltd., solid content: 50±2% by mass), SP-10 series (manufactured by Sinloihi Co., Ltd., pigment content: about 40% by mass), SG-500 series (manufactured by Sinloihi Co., Ltd., pigment content: about 60% by mass), FW-8500KS series (manufactured by Sinloihi Co., Ltd., solid content: about 55% by mass), and the like.

[0021] The average particle diameter of the colorant is preferably 5.0 µm or less, more preferably less than 2.0 µm, still more preferably 1.0 µm or less, and particularly preferably 0.5 µm or less, from the viewpoints that it can further suppress blurring and breaking of writing lines caused by clogging when writing on paper surface and poor flow of ink, and can form darker writing lines on the paper surface. According to the aqueous ink composition for writing instruments according to the present embodiment, even when a colorant having a small average particle diameter is used, writing lines formed by writing on a non-absorbent surface can be easily erased by dry wiping. Further, the lower limit of the average particle diameter of the colorant is not particularly limited, and may be 100 nm or more, or 400 nm or more. The average particle diameter of the colorant can be measured by a particle size distribution analyzer (such as laser diffraction / scattering method). As the particle size distribution analyzer, SALD-7000 (trade name) manufactured by Shimadzu Corporation can be used, for example.

[0022] The solid content (non-volatile content) of the colorant is preferably 0.5 to 15% by mass, more preferably 1.5 to 9.0% by mass, even more preferably 3.0 to 8.5% by mass, and particularly preferably 4.0 to 8.0% by mass, based on the total amount of the aqueous ink composition. When this content is above the lower limit, it is possible to further suppress smudging and breakage of the writing line when writing on paper, and there is a tendency to be able to write darker lines on the paper. When this content is below the upper limit, it is possible to suppress a decrease in the discharge of the aqueous ink composition from the writing instrument, and there is a tendency to suppress the residue of the colorant on the non-absorbent surface when the writing line is wiped off from the non-absorbent surface.

[0023] (Film-forming resin) A film-forming resin is a resin that has the property of forming a film on an aqueous ink composition upon drying. When an aqueous ink composition contains a film-forming resin, the writing lines on a non-absorbent surface dry and form a film, improving erasability by dry wiping.

[0024] Examples of film-forming resins include aqueous resin emulsions. Examples of aqueous resin emulsions include acrylic resins, urethane resins, styrene-butadiene resins, styrene-acrylic resins, vinyl chloride resins, vinyl chloride-vinyl acetate resins, acrylonitrile-butadiene resins, and modified vinyl acetate resins. Among these, acrylic resin emulsions are preferred from the viewpoint of easily achieving both writing properties on paper and erasability on non-absorbent surfaces. Examples of acrylic resin emulsions include alkyl acrylate / acrylic copolymer emulsions and alkyl acrylate / styrene copolymer emulsions. Film-forming resins may be used individually or in combination of two or more types.

[0025] The glass transition temperature (Tg) of the resin particles contained in the aqueous resin emulsion is preferably -10 to 100°C, more preferably -5 to 70°C, even more preferably -5 to 50°C, and particularly preferably -5 to 20°C. When Tg is above the lower limit, the film formed after drying does not become too soft, which tends to suppress the stretching of writing lines and the difficulty in erasing them. When Tg is below the upper limit, the film formed after drying does not become too hard, making it easy to erase with light force, and it does not become too brittle, resulting in less eraser residue and easier to erase completely.

[0026] The solid content of the film-forming resin is 1.0 to 20% by mass, preferably 1.5 to 15% by mass, more preferably 2.0 to 12% by mass, and even more preferably 2.0 to 8.0% by mass, based on the total amount of the aqueous ink composition. When this content is above the lower limit, a sufficient film can be formed on the writing line on a non-absorbent surface, improving erasability by dry wiping. On the other hand, when the content is below the upper limit, smudging and breakage of the writing line when writing on paper can be suppressed. Furthermore, when the content is below the upper limit, a decrease in the discharge of the aqueous ink composition from the writing instrument and a decrease in the opacity of the writing line can be suppressed, allowing for darker writing on the paper.

[0027] (Water-soluble organic solvent) The aqueous ink composition for writing instruments of this embodiment contains a water-soluble organic solvent having a boiling point of 170 to 290°C. The inclusion of the above-mentioned water-soluble organic solvent in the aqueous ink composition provides appropriate moisture retention to the film of the aqueous ink composition when written on a non-absorbent surface, resulting in the formation of a softened film and improved erasability by dry wiping.

[0028] Examples of water-soluble organic solvents with a boiling point of 170-290°C include dihydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-butylene glycol, 1,4-butanediol, 1,2-hexanediol, 2-ethyl-1,3-hexanediol, 2,3-butylene glycol, neopentyl glycol, hexylene glycol, and thiodiglycol; polyhydric alcohols such as glycerin, trimethylolethane, 3-methylpentane-1,3,5-triol, and diglycerin; glycol ethers such as ethylene glycol monobutyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, and diethylene glycol monobutyl ether; and pyrrolidones such as 2-pyrrolidone and N-methyl-2-pyrrolidone. Among these, dihydric alcohols and polyhydric alcohols are preferred from the viewpoint of easily imparting moisturizing properties to the film of the aqueous ink composition when written on a non-absorbent surface with a small amount of addition, and also easily providing good writing properties on the paper surface. Ethylene glycol, propylene glycol, diethylene glycol, and glycerin are more preferred, and ethylene glycol, propylene glycol, and diethylene glycol are even more preferred. The water-soluble organic solvent may be used alone or in combination of two or more.

[0029] The boiling point of the water-soluble organic solvent is 170 to 290°C, but is preferably 180 to 290°C, more preferably 185 to 290°C, and even more preferably 185 to 250°C.

[0030] The content of the water-soluble organic solvent is 1.0 to 50% by mass, preferably 3.0 to 45% by mass, more preferably 5.0 to 30% by mass, and even more preferably 5.0 to 25% by mass, based on the total amount of the aqueous ink composition for writing instruments. When this content is above the lower limit, it is possible to impart appropriate moisture retention to the film of the aqueous ink composition when writing on a non-absorbent surface, forming a softened film and improving erasability by dry wiping. On the other hand, when the content is below the upper limit, it is possible to suppress the amount of solvent in the film of the aqueous ink composition when writing on a non-absorbent surface, which would cause the film to stretch and reduce wipeability when dry wiping, and also suppress smudging and breakage of the writing line when writing on paper.

[0031] The aqueous ink composition for writing instruments of this embodiment may contain a water-soluble organic solvent having a boiling point of less than 170°C or more than 290°C, to the extent that it does not impair the effects of the present invention. The content of the water-soluble organic solvent having a boiling point of less than 170°C or more than 290°C may be less than 12% by mass, less than 7% by mass, or 0% by mass, based on the total amount of the aqueous ink composition for writing instruments.

[0032] (water) The aqueous ink composition for writing instruments of this embodiment contains water as a solvent or dispersion medium for a colorant or film-forming resin, or separately from the above-mentioned components. The water may be tap water or deionized water, but deionized water is preferred because it contains fewer impurities.

[0033] The water content is preferably 30 to 90% by mass, more preferably 40 to 85% by mass, even more preferably 50 to 80% by mass, and particularly preferably 60 to 80% by mass, based on the total amount of the aqueous ink composition for writing instruments. The above water content refers to the amount of water contained in the entire aqueous ink composition, and includes the amount of water used as a solvent or dispersion medium for colorants or film-forming resins.

[0034] The aqueous ink composition for writing instruments of this embodiment may contain, in addition to the above-mentioned components, additives such as pH adjusters, thickeners, preservatives (antifungal agents), lubricants, dye dissolving aids, drying agents, and fixing resins.

[0035] As pH adjusters, inorganic salts such as sodium carbonate, sodium phosphate, sodium hydroxide, and sodium acetate, as well as organic basic compounds such as water-soluble amine compounds such as triethanolamine and diethanolamine, can be used. The pH adjuster content is preferably 0.1 to 3% by mass based on the total amount of the aqueous ink composition for writing instruments.

[0036] Examples of thickeners include natural thickeners such as xanthan gum, guar gum, locust bean gum, carrageenan, gum arabic, tragacando gum, alginic acid, gelatin, agar, casein, psyllium seed gum, and tamarind seed gum, as well as synthetic thickeners such as methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, sodium carboxymethylcellulose, sodium alginate, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl methyl ether, sodium polyacrylate, and carboxyvinyl polymer. The content of the thickener is preferably 0.1 to 3% by mass, and more preferably 0.2 to 1% by mass, based on the total amount of the aqueous ink composition for writing instruments. When the content of the thickener is 3% by mass or less, it tends to suppress an excessive increase in the viscosity of the aqueous ink composition for writing instruments, and, for example, when used in a ballpoint pen, it tends to suppress clogging of the aqueous ink composition at the tip of the ballpoint pen tip. On the other hand, when the thickening agent content is 0.1% by mass or more, it tends to suppress an excessive decrease in the viscosity of the water-based ink composition for writing instruments, thereby suppressing ink leakage from the pen tip of the writing instrument.

[0037] Examples of preservatives (antifungal agents) include carbolic acid, sodium salt of 1,2-benzisothiazolin-3-one, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl parahydroxybenzoate, and 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine. The preservative content is preferably 0.1 to 5% by mass, and more preferably 0.1 to 1% by mass, based on the total amount of the aqueous ink composition for writing instruments.

[0038] Examples of lubricants include higher fatty acids such as oleic acid, nonionic surfactants having long-chain alkyl groups, polyether-modified silicone oils, thiophosphite triesters such as tri(alkoxycarbonylmethyl thiophosphite) and tri(alkoxycarbonylethyl thiophosphite), and their metal salts, ammonium salts, amine salts, and alkanolamine salts. The lubricant content is preferably 0.3 to 5% by mass, and more preferably 0.5 to 3% by mass, based on the total amount of the aqueous ink composition for writing instruments.

[0039] The aqueous ink composition for writing instruments of this embodiment is obtained by mixing the above-mentioned components using a stirrer such as a dissolver, a Henschel mixer, and a homomixer. The stirring conditions of the stirrer are not particularly limited, but for example, by stirring with a dissolver stirrer at a rotation speed of 100 to 1000 rpm for 30 to 180 minutes, an aqueous ink composition for writing instruments in which each component is uniformly dispersed can be obtained.

[0040] In this embodiment, the aqueous ink composition for writing instruments has a viscosity of V0 [mPa·s] at an initial temperature of 25°C, and a viscosity of V1 [mPa·s] at 25°C after being left at 50°C for 4 hours. The ratio of V1 to V0 (V1 / V0) is less than 1.30. The 4-hour period at 50°C can be performed, for example, using a safety-spec constant temperature incubator for solvent drying (manufactured by Kusumoto Kasei Co., Ltd., product name: HISPEC HG type HG220) with a set temperature of 50°C. A V1 / V0 ratio of less than 1.30 allows for appropriate moisture retention in the film of the aqueous ink composition when writing on a non-absorbent surface, resulting in the formation of a softened film and improved erasability by dry wiping. Furthermore, from the viewpoint of further suppressing smudging and breakage of writing lines when writing on paper, a V1 / V0 ratio of 1.01 or higher is preferable, and a ratio of 1.05 or higher is more preferable.

[0041] Viscosities V0 and V1 were measured using an E-type viscometer under the following conditions: temperature 25°C, cone radius 24 mm, cone angle 0.8°, and cone plate rotation speed 0.1 rpm. Initial viscosity V0 is the viscosity of the aqueous ink composition before water or water-soluble organic solvents evaporate, for example, the viscosity of the aqueous ink composition immediately after preparation or when it is contained in the ink reservoir of a writing instrument. Viscosity V1 after standing is the viscosity after heating the aqueous ink composition in an open system where water or water-soluble organic solvents can evaporate, for example, the viscosity after standing in an open system in a constantly heated oven at 50°C for 4 hours.

[0042] The initial viscosity V0 of the aqueous ink composition for writing instruments at 25°C is preferably 500 to 4500 mPa·s, and more preferably 1000 to 3000 mPa·s, from the viewpoint of ensuring that the writing line is not faint or interrupted when writing on paper, and that a darker line can be written on the paper.

[0043] In an aqueous ink composition for writing instruments, the properties of the film formed by the aqueous ink composition when written on a non-absorbent surface, and its ability to be erased by dry wiping, are affected by the boiling point and content of the water-soluble organic solvent. For example, if the boiling point of the water-soluble organic solvent is high, the moisture retention is high, so a small amount is sufficient, and if the boiling point is low, a larger amount is preferable. Therefore, from the viewpoint of further improving the erasability of the written line by dry wiping when written on a non-absorbent surface, it is preferable that the aqueous ink composition satisfies the following relationship (I), where A [mass%] is the content of the water-soluble organic solvent in the aqueous ink composition, a [°C] is the boiling point, and B [mass%] is the solid content of the film-forming resin. 5.0≦(a×A) / (100×B)≦28.0…(I)

[0044] Here, if the water-based ink composition for writing instruments contains multiple water-soluble organic solvents with different boiling points, the value of (a × A) in formula (I) is the sum of the products of the boiling point and content of each water-soluble organic solvent. For example, if the composition contains two types of water-soluble organic solvents, one with a boiling point of a1 [°C] and a content of A1 [mass%], and the other with a boiling point of a2 [°C] and a content of A2 [mass%], the value of (a × A) will be (a1 × A1 + a2 × A2). Also, if the film-forming resin contains multiple film-forming resins of different types, the value of B in formula (I) is the total solid content of the multiple film-forming resins.

[0045] The value of (a×A) / (100×B) above is more preferably between 7.5 and 25.0, from the viewpoint of further improving the erasability of the written line by dry wiping when writing on a non-absorbent surface.

[0046] The pH of the water-based ink composition for writing instruments is preferably 6 to 12, and more preferably 7 to 9.

[0047] Water-based ink compositions for writing instruments can be used in, for example, ballpoint pens, marking pens, fountain pens, etc., and are particularly suitable for use in ballpoint pens.

[0048] Writing lines produced using a water-based ink composition for writing instruments are erasable on non-absorbent surfaces, but do not need to be erasable on paper.

[0049] [Ballpoint pen] Figure 1 is a schematic cross-sectional view showing a ballpoint pen according to one embodiment of the present invention. In the ballpoint pen 100 shown in Figure 1, the aqueous ink composition 12 is filled (contained) in an ink storage member 14. A ballpoint pen tip 20 is attached to one end of the ink storage member 14. This ballpoint pen tip 20 consists of a ball holder 24 and a ball 26 that is rotatably held by the ball holder 24, and is fixed to the ink storage member 14 by a joint 22. Also, inside the ink storage member 14, on the side opposite to the ballpoint pen tip 20, a backflow prevention body 16 is housed adjacent to the aqueous ink composition 12. Here, the backflow prevention body 16 is positioned so that there is no gap between it and the aqueous ink composition 12.

[0050] Furthermore, in the ballpoint pen 100, the core 10 is composed of an ink storage member 14, a joint 22, a ballpoint pen tip 20, a water-based ink composition 12, and a backflow prevention body 16. This core 10 is attached to the main body shaft 18, and a tail plug 28 having a ventilation hole is attached to the rear end of the main body shaft 18 (the end opposite to the ballpoint pen tip 20).

[0051] According to the ballpoint pen 100, the water-based ink composition for writing instruments contained in the ink storage member 14 is the ink composition for writing instruments according to the above embodiment, so that writing can be done on paper without smudging or interruptions, and even when writing on a non-absorbent surface, the writing line can be easily erased by dry wiping.

[0052] The components of the ballpoint pen 100 will be described below, but for components other than the water-based ink composition 12, general components used in ordinary ballpoint pens can be applied. For the water-based ink composition 12, the water-based ink composition for writing instruments according to the above embodiment is used.

[0053] The ink storage member 14 can be made of a resin such as polypropylene, polyethylene, polyethylene terephthalate, nylon, polyacetal, or polycarbonate, or of metal. Furthermore, the shape of the ink storage member 14 is not particularly limited and can be cylindrical, for example.

[0054] The backflow prevention body 16 has functions such as preventing the water-based ink composition from flowing out (flow prevention) and preventing the water-based ink composition from drying out (sealing). Known backflow prevention bodies having such functions can be used without particular limitation. Such a backflow prevention body 16 is composed of, for example, a base oil and a thickener. Examples of base oils include mineral oil, polybutene, silicone oil, glycerin, and polyalkylene glycol. Examples of thickeners include metal soap-based thickeners, organic thickeners, and inorganic thickeners.

[0055] Furthermore, when the ballpoint pen tip 20 is pointed downwards, it is preferable to adjust the viscosity of the backflow prevention body 16 and the difference in specific gravity between the water-based ink composition 12 and the backflow prevention body 16 so that the backflow prevention body 16 does not settle in the water-based ink composition 12. In addition, it is preferable that the backflow prevention body 16 has a composition that is incompatible with the water-based ink composition 12.

[0056] The main shaft 18 and the end cap 28 can be made of, for example, a plastic material such as polypropylene.

[0057] For example, the joint 22 can be made of polypropylene, polyethylene, polyethylene terephthalate, nylon, polyacetal, and polycarbonate.

[0058] Examples of materials that make up the ball holder 24 in the ballpoint pen tip 20 include metals and alloys.

[0059] The ball 26 usually contains metal. The ball 26 may be a cemented carbide ball. A cemented carbide is an alloy of a main component and a binder, for example, the main component may include silicon carbide, zirconium oxide, and tungsten carbide, and the binder may include metals such as cobalt, chromium, titanium, and nickel. The diameter of the ball 26 is preferably 0.3 to 1.2 mm.

[0060] The ballpoint pen 100 of the embodiment having the above-described configuration can be manufactured by a conventional ballpoint pen manufacturing method, except for the aqueous ink composition.

[0061] Although preferred embodiments of the aqueous ink composition for writing instruments and the writing instrument filled with the aqueous ink composition of the present invention have been described above, the aqueous ink composition for writing instruments and the writing instrument are not limited to the above embodiments. For example, the writing instrument may be a writing instrument other than the ballpoint pen of the above embodiment, such as a marking pen or a fountain pen. Furthermore, the ballpoint pen of the above embodiment does not have to have a main body shaft 18, and the ink storage member 14 may serve as the main body shaft. Moreover, the ballpoint pen of the above embodiment may have a pressurizing mechanism such that the aqueous ink composition and the backflow prevention body in the ink storage member 14 are pressurized from the rear end (the end opposite to the ballpoint pen tip 20). Furthermore, the ballpoint pen of the above embodiment does not have to have a backflow prevention body 16. [Examples]

[0062] The present invention will be specifically described below with reference to examples, but the scope of the present invention is not limited to these examples.

[0063] [Preparation of water-based ink composition] (Examples 1-17 and Comparative Examples 1-5) The materials listed in Tables 1 to 3 below were mixed to obtain the aqueous ink compositions of Examples 1 to 17 and Comparative Examples 1 to 5. In the tables, the amount of each component is shown as the content (unit: mass%) relative to the total amount of the aqueous ink composition. Note that the amount of colorant in the table is the total amount of the aqueous dispersion (including water-soluble organic solvents and water, etc.) described later. The amount of film-forming resin in the table is the total amount of the emulsion (including water, etc.) described later. The total content of water-soluble organic solvent in the table is the sum of the amount of glycerin or propylene glycol contained in the colorant and the amount of water-soluble organic solvent added separately. The total content of water in the table is the sum of the amount of water contained in the colorant, the amount of water contained in the film-forming resin, and the amount of water added separately.

[0064] Details of each material in Tables 1 to 3 are as follows. (Coloring agent) Carbon black aqueous dispersion (manufactured by Fuji Pigment Co., Ltd., product name: Fuji SP Black 8087, carbon black content: 15% by mass, average particle size of carbon black: 0.1 μm, glycerin content: 10% by mass, water content: 70% by mass) Aqueous dispersion of colored resin particles (manufactured by Nippon Fluorescent Chemicals Co., Ltd., product name: Lumikol NKW-7517E, colored resin particle content: 51% by mass, average particle size of colored resin particles: 0.4 μm, color: pink, propylene glycol content: 5% by mass, water content: 44% by mass) (Water-soluble organic solvent) Propylene glycol (boiling point: 188°C) Ethylene glycol (boiling point: 197°C) Diethylene glycol (boiling point: 245°C) Glycerin (boiling point: 290°C) (Film-forming resin) Water-based resin emulsion A (manufactured by Daido Chemical Industries, Ltd., product name: Vinysol 1086WP, alkyl acrylate / acrylic copolymer emulsion, solids content: 40% by mass, Tg: 10℃) Water-based resin emulsion B (BASF, product name: Joncryl 7780, styrene-acrylic emulsion, solids content: 48% by mass, Tg: 92℃) Water-based resin emulsion C (manufactured by Nisshin Chemical Industry Co., Ltd., product name: Vinibran 603, vinyl chloride / vinyl acetate emulsion, solids content: 50% by mass, Tg: 64℃) Water-based resin emulsion D (manufactured by Nisshin Chemical Industry Co., Ltd., product name: Vinibran 271, vinyl chloride emulsion, solids content: 43% by mass, Tg: -3℃) (Thickening agent) Xanthan gum (water) Ion-exchanged water

[0065] [Viscosity measurement] The initial viscosity V0 (unit: mPa·s) of the aqueous ink compositions obtained in Examples 1-17 and Comparative Examples 1-5 was measured using an E-type viscometer (cone-plate viscometer) under the conditions of a temperature of 25°C, a cone radius of 24 mm, a cone angle of 0.8°, and a cone plate rotation speed of 0.1 rpm. Next, the aqueous ink compositions were left in an open system for 4 hours in a safety-type constant temperature incubator for solvent drying (manufactured by Kusumoto Kasei Co., Ltd., product name: HISPEC HG type HG220) set to a temperature of 50°C. The viscosity V1 (unit: mPa·s) of the aqueous ink compositions after standing was measured in the same manner as the measurement of V0 above. From the obtained values ​​of V0 and V1, the ratio of V1 to V0 (V1 / V0) was determined. The results are shown in Tables 1-3.

[0066] [Characteristic evaluation] Ballpoint pens were manufactured by filling the ink reservoir of a ballpoint pen having the structure shown in Figure 1 with the aqueous ink compositions obtained in Examples 1 to 17 and Comparative Examples 1 to 5, respectively. A stainless steel ball holder 24 was used, and a cemented carbide ball (diameter: 0.5 mm) containing tungsten carbide as the main component and cobalt as a binder was used for the ball 26. The manufactured ballpoint pens were evaluated as follows.

[0067] (Writability on paper) Using the ballpoint pens we created, we wrote letters and spirals on report paper and visually observed the condition of the writing lines. Based on the observation results, we evaluated the writing performance according to the following criteria. The evaluation results are shown in Tables 1 to 3. A: The writing line was dark and free of smudging or breaks. B: There were no smudges or breaks in the writing lines. C: The writing lines were either smudged or broken. D: The writing line was either faint or broken, or it was impossible to write.

[0068] (Erasability from non-absorbent surfaces) Using the prepared ballpoint pen, letters and spirals were written on a PET film. A tissue paper was placed over the written lines, and a 300g weight was placed on top. The tissue paper was then pulled by hand to dry-wipe the lines. This process was repeated 1 to 3 times, and the condition of the written lines was observed visually. Based on the observation results, the dry-wipe erasure ability was evaluated according to the following criteria. The evaluation results are shown in Tables 1 to 3. A: I was able to completely erase the writing lines with just one dry wipe. B: The writing lines could be completely erased with two or three dry wipes. C: Even after wiping it dry three times, the writing lines could not be completely erased. D: Even after wiping it dry three times, the writing lines smudged and could hardly be erased.

[0069] [Table 1]

[0070] [Table 2]

[0071] [Table 3] [Explanation of symbols]

[0072] 10... Core, 12... Water-based ink composition, 14... Ink storage component, 16... Backflow prevention body, 18... Main shaft, 20... Ballpoint pen tip, 22... Joint, 24... Ball holder, 26... Ball, 28... End cap, 100... Ballpoint pen.

Claims

1. A water-based ink composition for writing instruments containing at least a colorant, a film-forming resin, a water-soluble organic solvent, and water, The boiling point of the aforementioned water-soluble organic solvent is 170 to 290°C. The content of the aforementioned water-soluble organic solvent is 1.0 to 50% by mass based on the total amount of the aqueous ink composition. The solid content of the film-forming resin is 1.0 to 20% by mass based on the total amount of the aqueous ink composition. The initial viscosity of the aqueous ink composition at 25°C is V. ０ [mPa·s], viscosity at 25°C after being left at 50°C for 4 hours, V １ When [mPa·s], the above V ０ V １ The ratio (V １ / V ０ A water-based ink composition for writing instruments, wherein the ratio of ) is less than 1.

30.

2. The aqueous ink composition for writing instruments according to claim 1, wherein the film-forming resin is an aqueous resin emulsion.

3. The aqueous ink composition for writing instruments according to claim 2, wherein the aqueous resin emulsion is an acrylic resin emulsion.

4. The aqueous ink composition for writing instruments according to claim 1, wherein the content of the water-soluble organic solvent in the aqueous ink composition is A [mass%], the boiling point is a [°C], and the solid content of the film-forming resin is B [mass%], and the relationship shown in the following formula (I) is satisfied. 5.0≦(a×A) / (100×B)≦28.0…(I)

5. The aqueous ink composition for writing instruments according to claim 1, wherein the average particle size of the colorant is less than 2.0 μm.

6. The aqueous ink composition for writing instruments according to claim 1, wherein the solid content of the colorant is 1.5 to 9.0% by mass based on the total amount of the aqueous ink composition.

7. A writing instrument comprising the ink composition according to any one of claims 1 to 6.

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