Aqueous ink composition for writing instrument, and writing instrument

Abstract

To provide an aqueous ink composition for writing instrument equipped with high opacifying properties and low glossiness which is hard to strike through a paper surface while maintaining high print density, by improving dispersibility of carbon black pigment.SOLUTION: An aqueous ink composition for writing instrument contains at least plant-derived carbon black pigment, dispersant, thickener, water-soluble organic solvent and water, where the plant-derived carbon black pigment is particles having the specific surface area of 10 to 1400 m2 / g and the median size of 0.9 to 6.0 μm, and the intensity ratio (G / D) between D band (1350 cm-1) and G band (1582 cm-1) in Raman spectrum is 0.9 to 2.0, and the dispersant is at least one kind selected from styrene acrylic acid resin-based water-soluble polymer, styrene maleic acid resin-based water-soluble polymer and nonionic surfactant, and the surface tension of the aqueous ink composition at 25°C is 30 to 60 mN / m.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous ink composition for a writing instrument that uses a plant-derived carbon black pigment. [Background technology]

[0002] In addition to being used as ink, ink compositions using carbon black pigments are also known for use in ballpoint pens (oil-based or water-based), fountain pens, and inkjet printer inks that contain carbon black or other graphite.

[0003] On the other hand, as a coloring material using environmentally friendly materials, an ink composition using bamboo charcoal obtained by burning bamboo has also been reported. For example, it has been reported that in a printing ink containing bamboo charcoal and a binder resin, bamboo has a honeycomb structure with fine holes lined up tightly compared to charcoal, and the binder resin penetrates near the entrance of the fine holes and into the inside, so that the ink has high hiding power, and that bamboo charcoal hardens into a glass-like state compared to charcoal, so that the binder resin coats the bamboo charcoal, resulting in good insulation properties (Patent Documents 1 and 2).

[0004] In the case of inkjet ink, if the particle size of the coloring material is small, the water or solvent will evaporate, causing thickening or gelation, resulting in a change in viscosity. On the other hand, if the particle size of the coloring material is large, the nozzle may be clogged when the ink is discharged. Patent Document 3 reports that by using water, a carbonized coloring material derived from plants, and a lignin resin, the lignin resin inhibits the aggregation of the carbonized coloring materials, thereby suppressing the occurrence of thickening or gelation.

[0005] As the water-based ink composition using carbon black as a colorant, there is used a compound represented by the general formula (I): [-CH2CR 1 (COO(CH2CH2O) n A water-insoluble resin-coated quartz crystal with a specific surface area of ​​200 m2 as measured by the BET method is shown in 2 / g or more 300m 2In an aqueous ink composition containing carbon black with a molecular weight of less than 1000 ppm by mass, an organic solvent, a neutralizing agent, and water, by setting the amount of hydrophilic structural units among the hydrophilic structural units and hydrophobic structural units constituting the water-insoluble resin to 15 mass% or less, various performance properties such as pigment dispersion are improved and good ink ejection properties are obtained during inkjet recording (Patent Document 4).

[0006] In addition, carbon black is used as a coloring material, polyvinyl butyral resin is used as a dispersant, and a cellulose acetate compound represented by the general formula (I): C6H5O(C2H4O) n H or general formula (II): C6H5CH2O(C2H4O) n An ink composition for an oil-based ballpoint pen containing at least one organic solvent represented by H has also been reported (Patent Document 5). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2018-193543 A [Patent Document 2] JP 2018-193414 A [Patent Document 3] JP 2022-167623 A [Patent Document 4] JP 2009-221326 A [Patent Document 5] JP 2002-275406 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, since carbon-based coloring materials such as carbon black tend to settle, it is necessary to improve the dispersibility of carbon black in the ink in order to maintain good print density and ink outflow properties during writing. The present invention aims to provide an aqueous ink composition for a writing instrument that has improved dispersibility of a carbon black pigment, maintains high print density, is less likely to strike through to the paper surface, has good hiding power, and has low gloss. [Means for solving the problem]

[0009] The aqueous ink composition for a writing instrument of the present invention contains at least a plant-derived carbon black pigment, a dispersant, a thickener, a water-soluble organic solvent, and water, and the plant-derived carbon black pigment has a specific surface area of ​​10 to 1400 m. 2 / g, particles with a median diameter of 0.9 to 6.0 μm and the D band (1350 cm -1 ) and G band (1582cm -1 ) has an intensity ratio (G / D) of 0.9 to 2.0, the dispersant is at least one selected from a styrene-acrylic acid resin-based water-soluble polymer, a styrene-maleic acid resin-based water-soluble polymer, and a nonionic surfactant, and the water-based ink composition has a surface tension of 30 to 60 mN / m at 25°C.

[0010] Measurement temperature: 25℃, shear rate: 383sec -1 It is preferable that the viscosity is 1 to 100 mPa·s and the viscosity index n value is 0.1 to 0.8. The writing instrument of the present invention is equipped with the above-mentioned aqueous ink composition for a writing instrument. Effect of the Invention

[0011] The aqueous ink composition for writing instruments of the present invention uses a highly dispersible carbon black pigment, and therefore can maintain a high print density. Furthermore, the aqueous ink composition is less likely to strike through the paper surface, has good hiding power, and has low gloss and high blackness, so that it has excellent visibility. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The aqueous ink composition for a writing instrument of the present invention will be described in detail below. The aqueous ink composition for a writing instrument of the present invention (hereinafter also simply referred to as "aqueous ink composition") contains at least a plant-derived carbon black pigment, a dispersant, a thickener, a water-soluble organic solvent, and water, and the plant-derived carbon black pigment has a specific surface area of ​​10 to 1400 m. 2 / g, particles with a median diameter of 0.9 to 6.0 μm and the D band (1350 cm -1 ) and G band (1582cm -1 ) has an intensity ratio (G / D) of 0.9 to 2.0, the dispersant is at least one selected from a styrene-acrylic acid resin-based water-soluble polymer, a styrene-maleic acid resin-based water-soluble polymer, and a nonionic surfactant, and the water-based ink composition has a surface tension of 30 to 60 mN / m at 25°C.

[0013] The plant-derived carbon black pigment is a carbonized coloring material obtained by charcoal burning of plants such as bamboo and wood at a temperature of 250° C. or higher. The water-based ink composition of the present invention using the plant-derived carbon black pigment has excellent color development properties and has the effect of suppressing viscosity changes even when other components such as additives are blended therein.

[0014] The plant-derived carbon black pigment is not particularly limited, but for example, charcoal such as black charcoal, white charcoal, sawdust charcoal, bamboo charcoal, and powdered charcoal is used. Of these, bamboo charcoal and white charcoal such as binchotan charcoal are preferred. Bamboo charcoal, which is made from bamboo, has more fine pores than other types of charcoal. In addition, bamboo charcoal contains more glass-like material than charcoal, so it wets better with binder resin than charcoal, and has the effect of improving the hiding power of water-based ink.

[0015] The plant-derived carbon black pigment has a specific surface area of ​​10 to 1400 m 2 / g, and a median diameter of 0.9 to 6.0 μm. Preferably, the specific surface area is 10 to 1400 m 2 The particles of the plant-derived carbon black pigment may have any shape, such as a plate shape, a sphere shape, a scale shape, a lump shape, an irregular shape, a rod shape, etc. 2A specific surface area of ​​25 m2 or more is preferable in that it diffuses light and exhibits low brightness and low gloss. 2 In order for the aqueous ink composition to have high dispersibility, the specific surface area of ​​the plant-derived carbon black pigment is 200 m 2 / g or less. 2 Excessively large particles exceeding 1.0 μm / g are undesirable as a pigment because they are poorly dispersible in an aqueous ink composition. On the other hand, a median diameter of 0.9 μm or more is preferable in that the pigment remains on the paper surface, preventing strike-through and exhibiting concealing properties, and a median diameter of 6.0 μm or less provides excellent ink outflow properties when the pigment is mounted on a writing instrument.

[0016] Here, the specific surface area is measured in correlation with the particle size using the BET multipoint method by nitrogen adsorption. The relationship between the specific surface area and the particle size is S=6 / ρd(S: specific surface area (m 2 / g), ρ: Density (g / cm 3 As shown by the above-mentioned specific surface area (μm), the particles are smaller as the specific surface area increases. 2 / g and a median diameter of 0.9 to 5.0 μm. The median diameter in this specification refers to the volume-based median diameter (D50) measured with a particle size distribution meter [Microtrac HRA9320-X100 (manufactured by Nikkiso Co., Ltd.)].

[0017] The plant-derived carbon black pigment has a specific surface area and a median diameter within a predetermined range, and also has a D band (1350 cm) in a Raman spectrum. -1 ) and G band (1582cm -1 ) intensity ratio (G / D) is 0.9 to 2.0. Raman spectroscopy is used for the structural analysis of carbon materials. The D band is a diamond structure (sp 3 The G band is a peak derived from the graphite structure (sp 2The G / D ratio of graphite is 13. If the ratio of G band to D band intensity (G / D) is 2.0 or less, the degree of crystallinity is low, and the amorphous structure causes diffuse reflection of light, which is desirable in that it produces a black color with low brightness and gloss. However, if the G / D ratio is less than 0.9, the material is highly brittle and unsuitable for use as a pigment.

[0018] The plant-derived carbon black pigment may be used alone or in combination of two or more. The content of the plant-derived carbon black pigment is usually 1.0 to 15 mass%, preferably 3.0 to 10 mass%, more preferably 5.0 to 8.0 mass% based on the total amount of the aqueous ink composition. When the content of the plant-derived carbon black pigment is within the above range, the viscosity of the aqueous ink composition can be easily adjusted, and the dispersibility is good.

[0019] The dispersant is at least one selected from a water-soluble polymer of a styrene-acrylic acid resin, a water-soluble polymer of a styrene-maleic acid resin, and a nonionic surfactant. In the present invention, the dispersant may be either a water-soluble polymer of a styrene-acrylic acid resin or a water-soluble polymer of a styrene-maleic acid resin, or a nonionic surfactant, or may be a mixture of both.

[0020] The water-soluble polymer of the styrene-acrylic acid resin system is an alkali-soluble resin with a carboxyl group in the side chain. The alkali-soluble resin disperses the plant-derived carbon black pigment and also works to improve the adhesion of the drawn lines when they dry. The weight-average molecular weight (Mw) of the water-soluble polymer of the styrene-acrylic acid resin system is approximately 2000 to 100,000. Specific examples include styrene-acrylic acid copolymer emulsions such as Joncryl 52J, Joncryl 57J, Joncryl 60J, and Joncryl 63J (all manufactured by BASF Japan Co., Ltd.), and RS-1191, VS-1047, and YS-1274 (all manufactured by Seiko PMC Co., Ltd.).

[0021] The styrene-maleic acid resin-based water-soluble polymer functions to adjust the viscosity of the water-based ink composition and improves the dispersibility of the plant-derived carbon black pigment. The weight-average molecular weight (Mw) of the styrene-acrylic acid resin-based water-soluble polymer is generally 1,000 to 50,000. Specific examples include styrene-maleic acid copolymer emulsions such as Arastar 700, Arastar 703S (both manufactured by Arakawa Chemical Industries Co., Ltd.), SMA-1440, SMA-2625, and SMA-17352 (both manufactured by Kawahara Oil Chemical Co., Ltd.). In this specification, the weight average molecular weight (Mw) is a polystyrene-equivalent value determined by aqueous gel permeation chromatography (GPC).

[0022] 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 fatty acid esters of glycerin, propylene glycol fatty acid esters, polyglycerin fatty acid esters, pentaerythritol fatty acid esters, polyoxyethylene sorbitan 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, and polyoxyethylene fatty acid amides.Commercially available products such as Emulgen A500 (polyoxyethylene distyrenated phenyl ether, manufactured by Kao Corporation) may also be used.

[0023] The content of the dispersant is usually 0.2 to 10 mass %, preferably 0.3 to 4 mass %, in terms of solid content concentration relative to the total amount of the aqueous ink composition. If the content of the dispersant is less than 0.2 mass %, the aqueous ink composition may become unstable over time. On the other hand, if it exceeds 10 mass %, the viscosity of the aqueous ink composition may be high.

[0024] The thickener may be a synthetic polymer, cellulose, or polysaccharide, and may be used alone or in combination. Specifically, it may be polyvinylpyrrolidone, polyvinyl methyl ether, crosslinked acrylic acid polymer, cellulose and its derivatives, xanthan gum, rheosan gum, welan gum, gellan gum, and succinoglycan. Instead of a thickener that shows high viscosity in a small amount, a gelling agent that solidifies a liquid into a jelly-like substance may be used. The content of the thickener is usually 0.05 to 1.0% by mass, and preferably 0.1 to 0.5% by mass, based on the total amount of the aqueous ink composition.

[0025] The aqueous ink composition of the present invention is in a form in which a plant-derived carbon black pigment, a dispersant and a thickener are dissolved or finely dispersed in a water-soluble organic solvent and water.

[0026] The water-soluble organic solvent is not limited as long as it is one that is normally used in aqueous ink compositions, but is preferably at least one selected from the group consisting of aromatics, alcohols, polyhydric alcohols, glycol ethers, hydrocarbons, and esters. Specifically, in terms of compatibility with water, glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, 3-butylene glycol, thiodiethylene glycol, and glycerin; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, and propylene glycol monomethyl ether; 2-pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, formamide, acetamide, and dimethyl sulfoxide can be mentioned. The content of the water-soluble organic solvent in the aqueous ink composition is usually 3 to 30% by mass.

[0027] The water may be tap water, purified water, distilled water, ion-exchanged water, pure water, or the like. The water content in the aqueous ink composition is usually 30 to 90% by mass.

[0028] The aqueous ink composition of the present invention may contain various additives within the scope of the present invention, such as binder resins, pH adjusters, lubricants, wetting agents, foam inhibitors, defoamers, rust inhibitors, and preservatives. Binder resins are added to fix plant-derived carbon black pigments to the paper surface. Specific examples include vinyl chloride-vinyl acetate copolymer resins, acrylic resins, polyester resins, epoxy resins, polycarbonate resins, polyolefin resins, maleic acid resins, phenolic resins, polyurethane resins, polyamide resins, polyimide resins, melamine resins, phenoxy resins, and active energy ray-curable resins, as well as modified versions of these.

[0029] Examples of pH adjusters include ammonia, monoethanolamine, diethanolamine, triethanolamine, urea, sodium tripolyphosphate, sodium carbonate, sodium hydroxide, etc. It is preferable to adjust the pH (25° C.) of the aqueous ink composition of the present invention to a range of 6 to 9.5 by appropriately adding a pH adjuster.

[0030] Examples of the lubricant include anionic surfactants such as phosphate esters, alkylsulfonates of higher fatty acid amides, and alkylarylsulfonates; derivatives of polyalkylene glycols, and polyether-modified silicones.

[0031] Examples of wetting agents include phosphate esters, fluorine-based surfactants, acetylene-based surfactants, silicone-based surfactants, etc. Wetting agents are used to suppress the repellency of the ink composition on the writing surface.

[0032] Foam suppressors include, for example, cysteine, glutathione, hydroxylamine, ascorbic acid, erythorbic acid and their salts, and oligomers of N-vinyl-2-pyrrolidone.

[0033] The defoaming agent may be, for example, a silicone-based emulsion type, an oil type, or an oil compound type, and it is preferable to use a silicone-based emulsion in combination with an oily solvent.

[0034] Examples of the rust inhibitor include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, and saponins. Examples of preservatives include sodium benzoate, sodium omadine, thiazoline compounds, and benzimidazole compounds.

[0035] The above-mentioned additives may be used alone or in combination of two or more.

[0036] The aqueous ink composition of the present invention can be prepared by stirring and mixing at least the plant-derived carbon black pigment, the dispersant, the thickener, the water-soluble organic solvent, water, and other optional components with a stirrer such as a homomixer, a homogenizer, or a disper, and further removing coarse particles by filtration or centrifugation, as necessary.

[0037] The viscosity of the aqueous ink composition is preferably 1 to 100 mPa·s, and the viscosity index n value is preferably 0.1 to 0.8. The viscosity is measured using an EMD viscometer (manufactured by Tokyo Keiki Co., Ltd.) at a shear rate of 383 sec at a measurement temperature of 25° C. -1 The viscosity index (n value) specified in this invention (including the examples described later) is an index of pseudoplasticity, and is a value calculated from the relationship between shear rate and apparent viscosity in the Newton-Ostwald viscosity equation. The range of viscosity at each shear rate and the range of viscosity index n value can be adjusted by appropriately combining the thickener, water as a solvent, coloring material, etc. used, appropriately adjusting the contents of each of these, and further by selecting an appropriate kneading method, etc.

[0038] The surface tension of the aqueous ink composition is 30 to 60 mN / m, and preferably 35 to 55 mN / m, as measured at 25° C. using an automatic surface tensiometer CBVP-Z (manufactured by Kyowa Interface Science Co., Ltd.).

[0039] The writing implement of the present invention is equipped with the aqueous ink composition. The aqueous ink composition is equipped in a ballpoint pen or a marking pen equipped with a pen tip such as a ballpoint pen tip, a fiber tip, a felt tip, or a plastic tip. For example, in the case of a ballpoint pen, the aqueous ink composition is accommodated in an ink container for a ballpoint pen equipped with a ball having a diameter of 0.18 to 2.0 mm, and polybutene, silicone oil, mineral oil, or the like is further accommodated as an ink follower. The structure of the ballpoint pen and marking pen is not particularly limited, and may be, for example, a direct ink type ballpoint pen or marking pen equipped with a collector structure (ink retention mechanism) in which the barrel itself serves as an ink container and is filled with an aqueous ink composition. EXAMPLES

[0040] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to the following examples. [Preparation of Water-Based Ink Composition] The aqueous ink compositions of Examples 1 to 6 and the aqueous inks of Comparative Examples 1 to 8 were prepared and evaluated as follows. [Evaluation of Water-Based Ink Compositions] (1) Ink flow The aqueous ink composition was filled into the refill of a ballpoint pen (manufactured by Mitsubishi Pencil Co., Ltd.; product name (Uni-ball Signo UM-100)), and the cap was then placed with the pen tip horizontally at 25° C. and 50% RH. After leaving the pen for one month, a straight line was handwritten on a sheet of PPC paper, and the written line was evaluated according to the following criteria. A: I was able to write without any difficulty from the beginning. B: There were some defects such as slight blurring or bleeding in the lines from the beginning of writing. C: There were noticeable defects such as blurring or bleeding in the lines from the beginning of writing.

[0041] (2) Strike-through resistance After a spiral was handwritten on a sheet of PPC paper, the back side of the written surface was observed and evaluated for bleed-through according to the following criteria. A: No bleed-through B: Some bleed-through is observed. C: Most of the lines are showing through

[0042] (3) Concealment The hiding power when handwritten on PPC paper was evaluated according to the following criteria. A: It has sufficient hiding power. B: Slightly lacking in hiding power C: Almost no hiding power

[0043] (4) Lightness L* The aqueous ink composition was spread on uncoated paper using a bar coater, and the brightness (light / dark) was evaluated from the surface of the ink reservoir inside the barrel using a color meter SC-P (manufactured by Suga Test Instruments Co., Ltd.) under the following measurement conditions. [Optical conditions] Reflection measurement; diffuse light illumination, 8° receiving light Compliant with JIS Z 8722 condition c A:30 or less B: Over 30 and under 40 C:40 or more

[0044] (5) 60° gloss The specular gloss was measured in accordance with JIS Z 8741-1997. The 60° gloss was evaluated according to the following criteria. A: 2 or less B: More than 2 and less than 10 C:10 or more

[0045] [Example 1] Crushed bamboo charcoal (D50 (median diameter) 0.95μm, specific surface area 108.3m 2An aqueous ink composition was prepared by mixing 5.5% by mass of styrene-acrylic resin (trade name JONCRYL 60J, manufactured by BASF Ltd., polyacrylic acid ester, weight average molecular weight 9,000), 2.0% by mass of styrene-acrylic resin, 0.4% by mass of xanthan gum, 15% by mass of propylene glycol, 3.0% by mass of bubble inhibitor, 0.5% by mass of lubricant, 1.5% by mass of pH adjuster, 0.5% by mass of preservative, and 71.1% by mass of distilled water. The total of the plant-derived carbon black pigment, dispersant, thickener, water-soluble organic solvent, additives, and distilled water is 100% by mass. Next, the water-based ink composition was filled into an ink reservoir in a barrel for a marking pen (product name Posca PC-5M; pen tip material (PET fiber core; manufactured by Mitsubishi Pencil Co., Ltd.) to prepare a marking pen. The ink outflow, strike-through resistance, hiding power, lightness L*, and 60° gloss of the aqueous ink composition were evaluated according to the above-mentioned criteria. The results are shown in Table 1.

[0046] [Examples 2 to 6] A water-based ink composition was prepared in the same manner as in Example 1 by mixing a plant-derived carbon black pigment, a dispersant, a thickener, a water-soluble organic solvent, a bubble suppressant, a lubricant, a pH adjuster, a preservative and distilled water in the ratios shown in Table 1, and a marking pen was produced. The ink outflow, strike-through resistance, hiding power, lightness L*, and 60° gloss of the aqueous ink composition were evaluated according to the above-mentioned criteria. The results are shown in Table 1.

[0047] [Comparative Examples 1 to 8] Carbon black pigment, dispersant, thickener, water-soluble organic solvent, bubble suppressant, lubricant, pH adjuster, preservative and distilled water were mixed in the ratios shown in Table 1 to prepare water-based inks in the same manner as in Example 1, and marking pens were produced. The ink outflow, strike-through resistance, hiding power, lightness L*, and 60° gloss of the aqueous ink composition were evaluated according to the above-mentioned criteria. The results are shown in Table 1.

[0048] In Comparative Example 1, an aqueous ink was prepared in the same manner as in Example 1, except that spherical carbon black with a small D50 of 0.31 μm was used as the pigment instead of crushed bamboo charcoal, but the ink showed poor strike-through and hiding power. In addition, due to the poor hiding power, the written lines were transparent and affected by the gloss of the underlying paper surface, resulting in a bright, glossy black color for the written lines. In Example 5, Comparative Example 2, in which flake graphite was used instead of crushed bamboo charcoal, and in Example 6, Comparative Example 3, in which earthy graphite was used instead of bamboo charcoal powder, the pigment had a bulky, scaly or clumpy shape, so that the strike-through and hiding power were excellent, but the writing lines were a bright, glossy black color due to the high G / D. The aqueous ink of Comparative Example 4, which used lignin as a dispersant, had poor dispersibility, and when it was mounted on a writing instrument, the ink flow was poor and it was unsuitable for use (represented by C* in the evaluation results). The water-based ink of Comparative Example 5, which contained a low propylene glycol content of 3 mass % compared to Example 1, had a high surface tension of 62 mN / m (measured at 25° C.) and was poor in ink outflow properties. In Comparative Example 6, when the amount of dispersant was increased, the surface tension decreased to 24 mN / m, and bleeding occurred in the drawn lines. The aqueous inks of Comparative Examples 7 and 8, which did not use a water-soluble organic solvent, had a slightly high viscosity or n value, and the ink outflow performance was also rated B.

[0049] [Table 1]

Claims

1. An aqueous ink composition comprising at least a plant-derived carbon black pigment, a dispersant, a thickener, a water-soluble organic solvent, and water, The plant-derived carbon black pigment has a specific surface area of ​​10 to 1,400 m 2 / g, particles with a median diameter of 0.9 to 6.0 μm and a D band (1350 cm -1 ) and G band (1582 cm -1 ) the intensity ratio (G / D) is 0.9 to 2.0; the dispersant is at least one selected from the group consisting of a styrene-acrylic acid resin-based water-soluble polymer, a styrene-maleic acid resin-based water-soluble polymer, and a nonionic surfactant; The aqueous ink composition for a writing instrument has a surface tension of 30 to 60 mN / m at 25°C.

2. 383 sec at 25°C -1 The viscosity of the composition is 1 to 100 mPa s and the viscosity index n value is 0.1 to 0.8, 2. The aqueous ink composition for writing instruments according to claim 1, wherein the water-soluble organic solvent is at least one selected from the group consisting of aromatic compounds, alcohols, polyhydric alcohols, glycol ethers, hydrocarbons and esters.

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

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