Aqueous ink composition and writing instrument and ballpoint containing the same

Abstract

To provide an aqueous ink composition that ensures superior dispersion stability for carbon black, capable of effectively inhibiting the sedimentation and aggregation of carbon black over time in the ink.SOLUTION: An aqueous ink composition comprises carbon black, an alkali-swellable acryl thickener, and self-emulsifiable polyurethane. The content of the self-emulsifiable polyurethane is 0.05-10 mass% relative to the total mass of the ink composition. There are also provided a writing instrument and a ballpoint, each containing the aqueous ink composition.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a water-based ink composition and a writing instrument, a ballpoint pen, containing the same. [Background technology]

[0002] Conventionally, black inks using carbon black as a coloring material have been widely used. While the inks exhibit high performance in terms of coloring power, weather resistance, light resistance, etc., carbon black is prone to settling over time, so studies have been conducted to improve the dispersion stability of carbon black (see, for example, Patent Document 1).

[0003] Patent Document 1 describes a water-based ink for ballpoint pens that contains carbon black and / or an organic pigment and a polysaccharide, in which the sedimentation of carbon black and the like is suppressed by the use of a polysaccharide having a thickening property such as xanthan gum.

[0004] However, although the inks of the above-mentioned conventional techniques can suppress the sedimentation of carbon black to a certain extent, the dispersion stability of carbon black is insufficient, and the carbon black aggregates over time, which may result in sedimentation or poor ink ejection. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-106004 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides an aqueous ink composition which exhibits excellent dispersion stability of carbon black and is capable of highly inhibiting settling and aggregation of carbon black over time in the ink. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides "1. A water-based ink composition containing carbon black, an alkali-swellable acrylic thickener, and a self-emulsifying polyurethane. 2. The ink composition according to item 1, wherein the content of the self-emulsifying polyurethane is 0.05 to 10 mass % based on the total mass of the ink composition. 3. At 20°C, shear rate is 3.84 sec -1 The viscosity is 60 to 2500 mPa s at a shear rate of 384 sec -1 3. The ink composition according to item 1 or 2, wherein the viscosity at 0.5° C. is 10 to 200 mPa·s. 4. The ink composition according to any one of items 1 to 3, which is an ink for a writing instrument. 5. A writing instrument containing the ink composition described in item 4. 6. A writing instrument as described in paragraph 5, which is a ballpoint pen." Effect of the Invention

[0008] According to the present invention, there are provided an aqueous ink composition which is an ink having excellent dispersion stability of carbon black and which can highly inhibit sedimentation and aggregation of carbon black over time in the ink, and a writing instrument and a ballpoint pen containing the same. Furthermore, the ink composition of the present invention can provide handwriting that has good writing properties and excellent water resistance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described in detail. In this specification, the units "parts", "%", "ratio" and the like indicating the composition are based on mass unless otherwise specified.

[0010] The aqueous ink composition according to the present invention (hereinafter, sometimes referred to as "ink composition" or "composition") contains carbon black, an alkali-swellable acrylic thickener, and a self-emulsifying polyurethane. Each component constituting the water-based ink composition according to the present invention will be described below.

[0011] (Carbon Black) There are no particular limitations on the carbon black that can be used in the ink composition, and any of the carbon blacks such as oil furnace black, gas furnace black, channel black, thermal black, acetylene black, and lamp black can be used. In addition, a carbon black dispersion in which the carbon black is dispersed in an aqueous solvent using a dispersant can also be used in the ink composition. Examples of commercially available products include Mitsubishi Carbon Black High Quality Color (HCF) series, Mid-range Color (MCF) series, Low Flowability (LFF) series, General Purpose Color (RCF) series, Special Color (SCF) series, General Purpose Conductive Series (all manufactured by Mitsubishi Chemical Corporation), BLACK PEARLS series, ELFTEX series, MOGUL series, MONARCH series, EMPEROR series, REGAL series (all manufactured by Cabot Corporation, USA), Colour Black series, Special Black series, Printex series, Lamp Black series, HIBLACK series (all manufactured by Degussa Japan Co., Ltd.), SUNBLACK series (all manufactured by Asahi Carbon Co., Ltd.), SP series (manufactured by Fuji Pigment Co., Ltd.), SANDYE SUPERCOLOUR series (manufactured by Sanyo Pigment Co., Ltd.), EMACOL series (manufactured by Sanyo Pigment Co., Ltd.), MICROPIGMO series (manufactured by Orient Chemical Industry Co., Ltd.), and WA series (manufactured by Dainichiseika Color & Chemicals Co., Ltd.).

[0012] The content of carbon black is preferably from 0.1 to 30 mass %, more preferably from 1 to 20 mass %, and even more preferably from 3 to 20 mass %, based on the total mass of the ink composition. When the content is within the above range, good resistance to dry-up and coloring power of the ink in a ballpoint pen can be obtained, and the dispersion stability of the carbon black can be improved, making it possible to highly inhibit settling and aggregation of the carbon black over time in the ink. Carbon black can be used alone or in combination of two or more kinds.

[0013] (Alkali swelling type acrylic thickener) The alkali swelling type acrylic thickener is a substance that can impart viscosity to ink by neutralizing an acrylic polymer having an acid group such as a carboxy group with an alkaline substance such as an alkali metal hydroxide, an alkali metal salt of carbonate, an alkali metal salt of bicarbonate, ammonia, a monoalkanolamine, a dialkanolamine or a trialkanolamine, and swelling the polymer. Examples of the acrylic polymer having an acid group such as a carboxy group include copolymers of acrylic acid and copolymers of methacrylic acid, and more specifically, copolymers of methacrylic acid and acrylic acid ester, as well as polymers composed of three monomers of vinyl acetate, methyl methacrylate, and methacrylic acid. The polymer composed of the three monomers preferably has a weight ratio of each monomer in the range of 20 to 60% by weight of vinyl acetate, 10 to 15% by weight of methyl methacrylate, and 20 to 35% by weight of methacrylic acid, and an average molecular weight of the polymer in the range of 1.1 million to 6 million, and has the weight ratio and average molecular weight described above. Examples of commercially available alkali swelling acrylic thickeners include Primal ASE-60 (manufactured by Dow Chemical; a copolymer of methacrylic acid and acrylic acid ester), Leolate 1, and Leolate 101 (all manufactured by Elementis; polymers composed of three monomers, vinyl acetate, methyl methacrylate, and methacrylic acid, with each of the three monomers in the weight ratio range described above). The alkali swelling type acrylic thickener that can be used in the ink composition is not limited to these.

[0014] The content of the alkali swelling type acrylic thickener is preferably 0.05 to 3 mass %, more preferably 0.1 to 2 mass %, and even more preferably 0.3 to 1 mass %, based on the total mass of the ink composition. When the content of the alkali swelling type acrylic thickener is within the above range, good dry-up resistance and ink dischargeability in a ballpoint pen can be obtained, and the dispersion stability of the carbon black can be improved, making it possible to highly suppress precipitation and aggregation of the carbon black over time in the ink. The alkali swelling type acrylic thickeners can be used alone or in combination of two or more kinds.

[0015] (Self-emulsifying polyurethane) The self-emulsifying polyurethane is a polyurethane having an anionic or nonionic hydrophilic group such as a sulfonic acid group, a carboxy group, a hydroxy group, a polyoxyethylene group, etc., and is dispersed and stabilized in water even without the presence of an emulsifier. The self-emulsifying polyurethane may be neutralized with an alkaline substance such as an alkali metal hydroxide, an alkali metal carbonate, an alkali metal bicarbonate, ammonia, a monoalkanolamine, a dialkanolamine, or a trialkanolamine. The ink composition of the present invention may also contain a dispersion in which the self-emulsifiable polyurethane is dispersed in an aqueous solvent. The self-emulsifying polyurethane can also impart water resistance to paint marks or writing marks.

[0016] Examples of commercially available self-emulsifying polyurethanes that can be used in the ink composition include Superflex 210 and 300 (both manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), DAOTAN VTW1265 / 36WA, VTW6460 / 35WA, TW6450 / 30WA, and TW6493 / 35WA (all manufactured by Daicel Allnex Corporation), Takelac W-6061T, W-6110, and WS-5000 (all manufactured by Mitsui Chemicals, Inc.), Adeka Bontiter HUX-370 and HUX-564 (all manufactured by ADEKA Corporation), and Hi-Tec S-9200 (manufactured by Toho Chemical Industry Co., Ltd.).

[0017] In addition, core / shell type resin particles having an acrylic resin as a core and the above-mentioned self-emulsifying polyurethane as a shell can also be applied to the ink composition of the present invention as a self-emulsifying polyurethane. Examples of commercially available products include Acrito WEM-200U and Acrito WEM-3000 (both manufactured by Manol Corporation). The self-emulsifying polyurethane that can be used in the ink composition is not limited to those mentioned above.

[0018] In order to improve the dispersion stability of carbon black and to highly suppress the precipitation and aggregation of carbon black over time in the ink, the content of the self-emulsifying polyurethane is preferably 0.05 to 10 mass %, more preferably 0.1 to 10 mass %, and even more preferably 0.5 to 5 mass %, based on the total mass of the ink composition. When the content of the self-emulsifying polyurethane is within the above range, excellent water resistance can be imparted to application marks and handwriting. The self-emulsifying polyurethane may be used alone or in combination of two or more.

[0019] The ink composition of the present invention exhibits excellent dispersion stability of carbon black by applying an alkali swelling type acrylic thickener and a self-emulsifying polyurethane together with carbon black, and can highly suppress the sedimentation and aggregation of carbon black over time. Although the details of this mechanism are unclear, it is speculated that in the ink composition of the present invention, first, the alkali swelling type acrylic thickener increases the ink viscosity, and the alkali swelling type acrylic thickener is adsorbed to the carbon black surface to suppress contact between carbon black particles, forming a dispersed state of carbon black, and further, the self-emulsifying polyurethane is adsorbed to the carbon black surface and the alkali swelling type acrylic thickener to further stabilize the dispersed state of carbon black, thereby suppressing the sedimentation and aggregation of carbon black.

[0020] The ink composition of the present invention can be used as printing ink, writing ink, coating ink, stamp ink, inkjet ink, coating material, ultraviolet-curable ink, paint, coloring liquid for fibers, and the like.

[0021] When the ink composition of the present invention is applied to an ink for a writing instrument, various additives such as dyes, pigments, water-soluble organic solvents, rust inhibitors, antioxidants, ultraviolet absorbers, defoamers, pH adjusters, lubricants, penetrants, and pigment dispersants may be used in the ink composition as necessary.

[0022] The dyes include acid dyes, basic dyes, direct dyes, reactive dyes, vat dyes, sulfur dyes, metal-containing dyes, cationic dyes, and disperse dyes.

[0023] Examples of pigments include inorganic pigments such as metal oxide pigments, such as titanium oxide, zinc oxide, iron oxide black, yellow iron oxide, red iron oxide, and composite oxide pigments, and ultramarine, while examples of organic pigments include azo pigments, indigo pigments, phthalocyanine pigments, quinacridone pigments, thioindigo pigments, threne pigments, perinone pigments, perylene pigments, phthalone pigments, dioxane pigments, isoindolinone pigments, metal complex pigments, methine azomethine pigments, diketopyrrolopyrrole pigments, etc. In addition to metal pigments made of aluminum, brass, or stainless steel, examples of pigments include pigments made by coating a substrate made of a transparent material with a metal oxide, such as pearl pigments and metal oxide-coated glass flake pigments, hologram pigments, pigments in which the surface of a thin metal flake is coated with a resin, and fluorescent pigments. The composition of the present invention may also use a pigment dispersion in which the pigment is a dispersoid. Examples of pigment dispersions that can be used in the composition include SP series (manufactured by Fuji Dye Co., Ltd.), SANDYESUPERCOLOUR series, EMACOL series (all manufactured by Sanyo Dye Co., Ltd.), Lumicol series (manufactured by Nippon Kinko Kagaku Co., Ltd.), MICROPIGMO AM series, MICROPIGMO (all manufactured by Orient Chemical Industry Co., Ltd.), WA series, NX color series (all manufactured by Dainichiseika Color & Chemicals Co., Ltd.), Toyocolor Multitech series, Toyocolor EMA-AF color series (all manufactured by Toyo Ink Co., Ltd.), etc.

[0024] The ink composition may also contain a thermochromic capsule pigment. As a thermochromic capsule pigment, a reversible thermochromic composition consisting of (i) an electron-donating color-forming organic compound, (ii) an electron-accepting compound, and (iii) a reaction medium that determines the temperature at which the color-forming reaction between the two occurs is suitable, and is encapsulated in microcapsules and used as a reversible thermochromic microcapsule pigment. As the reversible thermochromic composition, there can be used a heat-discoloring type microcapsule pigment in which a reversible thermochromic composition having a relatively small hysteresis width (ΔH=1 to 7° C.) is encapsulated in a microcapsule, the reversible thermochromic composition being described in JP-B-51-44706, JP-B-51-44707, JP-B-1-29398, etc., and discoloring around a predetermined temperature (discoloration point), exhibiting a discolored state in a temperature range above the high-temperature discoloration point and a colored state in a temperature range below the low-temperature discoloration point, and only one of the two states exists in the room temperature range, and the other state is maintained while the heat or cold required to express that state is applied, but returns to the state exhibited in the room temperature range when the application of the heat or cold is removed. Further, there are those exhibiting relatively large hysteresis characteristics (ΔH=8 to 50° C.) as described in JP-B-4-17154, JP-A-7-179777, JP-A-7-33997, JP-A-8-39936, etc., and those exhibiting large hysteresis characteristics as described in JP-A-2006-137886, JP-A-2006-188660, JP-A-2008-45062, JP-A-2008-280523, etc. In other words, the shape of the curve plotting the change in color density due to temperature change will change color along a path that is significantly different when the temperature is increased from a lower temperature side than the color change temperature range than when the temperature is decreased from a higher temperature side than the color change temperature range, and the colored state in a low temperature range below the complete color development temperature, or the decolored state in a high temperature range above the complete decolorization temperature, can also be applied to heat-discolorable microcapsule pigments that encapsulate a reversible thermochromic composition that has color memory properties in a specific temperature range. Specifically, the reversible thermochromic composition having color memory property applicable to the water-based ink has a complete color-developing temperature that is set to a temperature that can only be obtained in a freezer or in a cold region, such as -50 to 0°C, preferably -40 to -5°C, and more preferably -30 to -10°C, and a complete decolorization temperature that is set to a temperature that can be obtained from frictional heat produced by a friction body or a familiar heating body such as a hair dryer, such as 50 to 95°C, preferably 50 to 90°C, and more preferably 60 to 80°C, and a ΔH value of 40 to 100°C, thereby allowing the composition to effectively function in maintaining the color that it exhibits under normal conditions (temperature range in everyday life).

[0025] Examples of the water-soluble organic solvent include ethanol, propanol, butanol, glycerin, sorbitol, triethanolamine, ethylene glycol, diethylene glycol, thiodiethylene glycol, hexylene glycol, 1,3-butanediol, neoprene glycol, polyethylene glycol, propylene glycol, butylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol monomethyl ether acetate, propylene glycol monomethyl ether, propylene glycol monoethyl ether, 2-pyrrolidone, and N-methyl-2-pyrrolidone.

[0026] Examples of the rust inhibitor include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, diisopropylammonium nitrite, and saponin.

[0027] Examples of the antioxidant include dibutylhydroxytoluene, nordihydroxytoluene, flavonoids, butylhydroxyanisole, ascorbic acid derivatives, α-tocopherol, and catechins.

[0028] Examples of ultraviolet absorbents include 2-(2'-hydroxy-3'-t-butyl-5'-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, and p-benzoic acid-2-hydroxybenzophenone.

[0029] As the defoaming agent, for example, polydimethylsiloxane, etc. can be used.

[0030] As the pH adjuster, known acidic substances and basic substances can be used. Specifically, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen phosphate, potassium hydrogen phosphate, monoethanolamine, diethanolamine and triethanolamine can be exemplified.

[0031] Examples of lubricants include phosphate surfactants, higher fatty acids such as oleic acid, nonionic surfactants having a long-chain alkyl group, polyether-modified silicone oils, and thiophosphite triesters such as thiophosphite tri(alkoxycarbonylmethyl ester) and thiophosphite tri(alkoxycarbonylethyl ester).

[0032] Examples of the phosphate surfactant include polyoxyethylene alkyl ether phosphate, polyoxyethylene alkylene ether phosphate, polyoxyethylene aryl ether phosphate, and polyoxyethylene polyoxypropylene alkyl ether phosphate. Examples of the polyoxyethylene aryl ether phosphate include polyoxyethylene phenyl ether phosphate, polyoxyethylene dodecyl phenyl ether phosphate, polyoxyethylene dinonyl phenyl ether phosphate, polyoxyethylene monostyryl phenyl ether phosphate, polyoxyethylene distyryl phenyl ether phosphate, polyoxyethylene tristyryl phenyl ether phosphate, polyoxyethylene cumyl phenyl ether phosphate, and polyoxyethylene naphthyl ether phosphate. The phosphate surfactant may be an alkali metal salt or an alkanolamine salt such as a monoalkanolamine salt, a dialkanolamine salt, or a trialkanolamine salt.

[0033] Examples of the penetrating agent include fluorine-based surfactants, acetylene glycol-based surfactants, succinic acid-based surfactants, and silicone-based surfactants.

[0034] In addition to the above, the ink composition may contain metal soaps, polyalkylene glycol fatty acid esters, ethylene oxide addition type cationic surfactants, N-acylamino acid surfactants, β-alanine type surfactants, 2,5-dimercapto-1,3,4-thiadiazole oligomers, thiocarbamic acid salts, dimethyldithiocarbamic acid salts, α-lipoic acid, condensates of N-acyl-L-glutamic acid and L-lysine, and salts thereof. In addition, thickening inhibitors such as oligomers of N-vinyl-2-pyrrolidone, oligomers of N-vinyl-2-piperidone, N-vinyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, ε-caprolactam, and oligomers of N-vinyl-ε-caprolactam, as well as preservatives or antifungal agents such as sodium salt of phenol, 1,2-benzthiazolin-3-one, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl paraoxybenzoate, and 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine may also be added.

[0035] Furthermore, one or more water-soluble resins such as alkyd resins, acrylic resins, styrene-maleic acid copolymers, cellulose derivatives, polyvinylpyrrolidone, polyvinyl alcohol, dextrin, etc. may be added, and one or more wetting agents such as urea, nonionic surfactants, sorbitol, mannitol, sucrose, glucose, reduced starch hydrolysates, sodium pyrophosphate, etc. may be added.

[0036] When the ink composition is applied to a ballpoint pen, the shear rate is set to 3.84 sec. -1 The viscosity of the ink composition at a shear rate of 384 sec is preferably 60 to 2500 mPa·s, more preferably 100 to 1500 mPa·s. -1 The viscosity of the ink composition measured by is preferably 10 to 200 mPa·s, and more preferably 20 to 100 mPa·s. The ink viscosity is measured when the ink composition is at 20° C., and the measuring device may be, for example, a rheometer “DHR-2” manufactured by TA Instruments Co., Ltd.

[0037] The surface tension of the ink composition is, for example, 20 to 50 mN / m, preferably 25 to 45 mN / m, and more preferably 25 to 40 mN / m at 20° C. When the surface tension is within the above range, bleeding of application marks or handwriting and strike-through onto the paper surface can be easily suppressed, and the wettability of the ink to the paper surface can be improved. The surface tension is measured by the vertical plate method using a platinum plate, and the measuring device may be, for example, a surface tension meter (manufactured by Kyowa Interface Science Co., Ltd., product name: DY-300).

[0038] The pH value of the ink composition is, for example, 7 or more, preferably 7 to 10, more preferably 7 to 9.5, and even more preferably 7.5 to 9. When the liquid property of the ink composition is in the neutral to alkaline region, the thickening effect of the alkali swelling type acrylic thickener can be improved, and the dispersion stability of the carbon black can be enhanced. Even if the pH value exceeds 10, the thickening effect of the alkali swelling type acrylic thickener is not significantly improved. The pH value is measured when the ink composition is at 20° C., and the measuring device may be, for example, a pH meter (manufactured by DKK-TOA Corporation, product name: IM-40S type).

[0039] The ink composition is prepared, for example, by adding carbon black, self-emulsifying polyurethane, and various additives, if necessary, to an aqueous medium containing water and, if necessary, a water-soluble organic solvent, and further adding a dispersion prepared separately from the above by dispersing an alkali swelling type acrylic thickener in the aqueous medium and then neutralizing the dispersion with an alkaline substance, or by directly adding the alkali swelling type acrylic thickener and the alkaline substance, and stirring the mixture.

[0040] The writing instrument containing the ink composition of the present invention will now be described.

[0041] Examples of writing implements include ballpoint pens and marking pens with a ballpoint pen tip or a marking pen tip attached to the writing tip, as well as brush pens, fountain pens, and calligraphy pens.

[0042] A ballpoint pen comprises a pen tip, an ink filling mechanism, an ink supply mechanism, etc.

[0043] The pen tip is made of a metal ballpoint pen tip, which rotatably holds a ball in a ball house (ball holding chamber) of the ballpoint pen tip body.

[0044] The ball house can be, for example, one in which a ball receiving seat and ink outlet are formed inside by cutting metal, or one in which a number of inward protrusions are formed on the inner surface near the tip of a metal pipe by pressure deformation from the outside, and ink outlet gaps are formed between the inward protrusions, extending radially outward from the center. In particular, a ball house in which inward protrusions are formed by pressure deformation has a relatively small contact area with the rear end of the ball, providing a smooth writing feel even with low writing pressure.

[0045] The balls held in the ball house are made of, for example, cemented carbide, stainless steel, ruby, ceramics, etc., and have an outer diameter of 0.1 to 2.0 mm. Furthermore, the ballpoint pen tip can be configured so that a resilient member is disposed within the tip to resiliently spring the rear end of the ball forward, so that when not writing, the ball is pressed against the inner edge of the tip tip to keep it in a tight contact state, and when writing, the ball is retracted by writing pressure to allow the ink to flow out, thereby preventing ink leakage when not in use. The resilient member can be, for example, a thin metal wire spring, or one end of the spring having a straight portion (rod portion), and is configured to be pressable by a resilient force of 5 to 40 g.

[0046] The ink filling mechanism may be configured to directly fill the water-based ink composition, or may be equipped with an ink reservoir or ink occlusion body that can be filled with the water-based ink composition. When the ballpoint pen is provided with an ink occlusion body capable of being filled with a water-based ink composition, the ink occlusion body is preferably a fiber bundle made of twisted fibers. The ink filling mechanism may be a cartridge into which the ink composition can be directly filled.

[0047] The ink supply mechanism is not particularly limited, and examples thereof include: (1) a mechanism having an ink guide core made of a fiber bundle or the like as an ink flow rate adjusting member, which guides the aqueous ink composition to the pen tip; (2) a mechanism in which a large number of disks are arranged in parallel with a comb-like space between them, and a slit-shaped ink guide groove that runs vertically through the disks in the axial direction and a wider ventilation groove is provided than the groove, and the ink composition is supplied to the pen tip through the ink guide groove; (3) a mechanism in which an ink composition is guided to the pen tip via a pen core, and the mechanism is configured such that a large number of disks are arranged in parallel with a comb-like space between them, and a slit-shaped ink guide groove that runs vertically through the disks in the axial direction and a wider ventilation groove is provided than the groove, and an ink guide core is arranged in the axis center to guide ink from an ink reservoir to the pen tip; and (4) a mechanism in which the aqueous ink composition is supplied directly to the pen tip from an ink container or barrel equipped with a pen tip. The ink reservoir equipped with a pen tip may be an ink refill in which the ballpoint pen tip is attached to the tip of an ink reservoir tube directly or via a holder, and the rear end of the ink is filled with an ink backflow prevention material made of a viscous liquid such as grease.

[0048] The ballpoint pen may be provided with a pen tip extension / retraction mechanism. Examples of the pen tip extension / retraction mechanism include a knock type, a rotation type, and a slide type. The ballpoint pen may also be a retractable type in which the pen tip can be stored in the barrel.

[0049] A ballpoint pen containing the composition of the present invention can be constructed by appropriately selecting mechanisms from the above-mentioned pen tip, ink filling mechanism, ink supply mechanism, and pen tip extending / retracting mechanism.

[0050] (Example) Inks having the compositions shown in Tables 1 and 2 below were prepared.

[0051] The viscosity of the ink was measured at 20° C. using a rheometer “DHR-2” manufactured by TA Instruments.

[0052] The pH value of the ink was measured at 20° C. using a pH meter (manufactured by DKK-TOA Corporation, product name: IM-40S type).

[0053] [Table 1]

[0054] [Table 2]

[0055] Each component in the table is explained in detail below. (1) Carbon black water dispersion (carbon black content 25% by mass, manufactured by Fuji Pigment Co., Ltd., product name: Fuji SP Black 8065) (2) Carbon black water dispersion (carbon black content 15% by mass, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., product name: WA Black A250) (3) Elementis, product name: Leolate 1 (active ingredient content: 30% by mass) (4) Dow Chemical Company, trade name: Primal ASE-60 (active ingredient content: 28% by mass) (5) Sumitomo Pharma Food & Chemical Co., Ltd., product name: Monart Gum GS) (6) Manufactured by Sansho Corporation, product name: Leozan (7) Superflex 210 (active ingredient content: 35% by mass), manufactured by Daiichi Kogyo Seiyaku Co., Ltd. (8) Daicel Allnex Corporation, product name: DAOTON VTW 1265 / 36WA (active ingredient content: 36% by mass) (9) Daicel Allnex Corporation, product name: DAOTON TW 6450 / 30WA (active ingredient content: 30% by mass) (10) Daicel Allnex Corporation, product name: DAOTON VTW 6460 / 35WA (active ingredient content: 35% by mass) (11) Product name: Superflex 300 (active ingredient content: 30% by mass), manufactured by Daiichi Kogyo Seiyaku Co., Ltd. (12) Mitsui Chemicals, Inc., product name: Takelac W-6061T (active ingredient content: 30% by mass) (13) Mitsui Chemicals, Inc., product name: Takelac WS-5000 (active ingredient content: 30% by mass) (14) Product name: NeoCrylBT-62 (active ingredient content: 40% by mass), manufactured by Kusumoto Chemical Co., Ltd. (15) Superflex E-2000 (polyurethane water dispersion: surfactant used as polyurethane dispersant, active ingredient content: 50% by mass), manufactured by Daiichi Kogyo Seiyaku Co., Ltd. (16) Polyoxyethylene styryl phenyl ether phosphate ester (product name: Plysurf AL, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.)

[0056] The prepared ink compositions were used to carry out the following evaluations. The writability and water resistance of handwriting were evaluated by writing with a ballpoint pen containing the ink. The results obtained are shown in Tables 3 and 4.

[0057] Making ballpoint pen refills Each ink was filled into an ink reservoir made of polypropylene resin and equipped with a ballpoint pen tip at the front end that rotatably holds a 0.4 mm diameter ball made of cemented carbide, and the rear end was filled with an ink backflow prevention agent made of a viscous liquid. A tail plug was then fitted to the rear of the pipe to obtain a ballpoint pen refill.

[0058] Making a ballpoint pen The obtained ballpoint pen refill was attached to the exterior of a retractable ballpoint pen to prepare a ballpoint pen. The test paper used was writing paper A conforming to the old JIS P3201.

[0059] (1) Evaluation of dispersion stability of carbon black A sample bottle containing 10 ml of ink was left to stand at 70°C for 7 days, after which the overall appearance of the ink and the dispersion of the carbon black in the ink were visually observed. The evaluation criteria were A, B, and C, which were considered to be acceptable, and D and E, which were considered to be unacceptable. The dispersion of the carbon black in the ink was observed under a microscope at 100x magnification. A: No settling or aggregation of carbon black was observed, and the carbon black was uniformly dispersed in the ink. B: Although very slight agglomeration of carbon black was observed, no settling of carbon black was observed. C: No sedimentation of carbon black was observed, but carbon black agglomeration was observed in places. D: Carbon black aggregation is observed, and areas of high and low carbon black concentration in the ink are visible. Settling of carbon black is a concern. E: Significant carbon black aggregation is observed. It is clear that the carbon black is not uniformly dispersed in the ink. Sedimentation of carbon black is of great concern. (2) Evaluation of writability A spiral was written on the test paper and the writing was visually observed. The evaluation criteria were A and B as passing, and C as failing. A: No scratches or interruptions are observed. B: Slight scratches are observed. C: Significant smearing is observed, making writing difficult. (3) Evaluation of water resistance of handwriting Writing was performed with each ballpoint pen containing the ink of Examples 1 to 9, and after drying for 10 minutes, water was dropped onto the handwriting, and the handwriting after drying was visually observed. As a result, no traces of carbon black flowing out were confirmed in the handwriting formed with the ballpoint pens containing the inks of Examples 1 to 2 and Examples 8 to 9, and although traces of carbon black flowing out were confirmed in the handwriting formed with the ballpoint pens containing the inks of Examples 3 to 7, they were slight, and it was confirmed that all of the handwritings had good water resistance.

[0060] The test results are shown in Tables 3 and 4 below.

[0061] [Table 3]

[0062] [Table 4]

Claims

1. An aqueous ink composition comprising carbon black, an alkali-swellable acrylic thickener, and a self-emulsifying polyurethane.

2. 2. The ink composition according to claim 1, wherein the content of the self-emulsifying polyurethane is 0.05 to 10% by mass based on the total mass of the ink composition.

3. 2. The ink composition according to claim 1, wherein the viscosity at 20° C. and a shear rate of 3.84 sec-1 is 60 to 2500 mPa·s, and the viscosity at a shear rate of 384 sec-1 is 10 to 200 mPa·s.

4. The ink composition according to any one of claims 1 to 3, which is an ink for a writing instrument.

5. A writing implement containing the ink composition according to claim 4.

6. 6. The writing instrument of claim 5, which is a ballpoint pen.