Water-based ink composition for writing instruments

Abstract

To provide a water-based ink composition for a writing implement, which can provide handwriting that can withstand repeated rubbing of handwriting formed by writing and rubbing with stronger force, and a writing implement equipped with the same. [Solution] The aqueous ink composition for a writing instrument of the present invention comprises at least a pigment-containing colorant (A), olefin-based resin particles (B), and water, wherein the olefin-based resin particles (B) have a penetration hardness of 1 or more and an average particle size measured by a Coulter counter method of 15 μm or less, and the mass ratio of the solid content of the colorant (A) to the olefin-based resin particles (B) [(A) / (B)] is in the range of 1 to 100.

Description

Technical Field

[0001] The present invention relates to an aqueous ink composition for writing instruments, which can provide handwriting that can withstand repeated erasure of handwriting formed by writing or erasure with a stronger force, and moreover, has no streaking or appearance defects during writing due to bubble generation.

Background Art

[0002] Conventionally, as an aqueous ink composition for writing instruments having excellent fixing properties, wear suppression of the ball seat, provision of dark handwriting, and good erasability, for example, 1) An ink composition that can be easily erased even when writing with a high pen pressure and can exhibit excellent fixing properties. The ink composition contains at least a colorant and an adhesive resin. The average particle size of the colorant is 2 μm or more, the average particle size of the adhesive resin is 2 μm or more, and those having a particle size of 2 μm or less are contained in the adhesive resin at 30% by weight or more. An erasable ink composition characterized by this (for example, see Patent Document 1). 2) As an aqueous ballpoint pen that suppresses ink leakage, is excellent in suppressing wear of the ball seat, and has dark handwriting, a ballpoint pen tip that rotatably holds a ball at the tip of an ink storage cylinder is directly or via a tip holder. An aqueous ballpoint pen containing an aqueous ink composition for a ballpoint pen composed of at least water, a colorant, olefin resin particles and / or nitrogen-containing resin particles as organic resin particles, and a shear-thinning viscosity-imparting agent. The amount of axial movement of the ball of the ballpoint pen tip is 15 μm or more, and the average particle size of the organic resin particles is 15 μm or less (for example, see Patent Document 2).

[0003] 3) As an aqueous pigment ink composition for writing instruments in which lines written on a non-absorbent surface such as PET film exhibit superior abrasion resistance compared to lines written with a conventional felt-tip pen, an aqueous pigment ink composition for writing instruments is known, which contains a pigment, water, a dispersion resin, a silicone-based resin emulsion, and another resin emulsion, wherein the silicone-based resin emulsion and the other resin emulsion are used in combination in an amount such that the mass ratio of their solid contents is in the range of silicone-based resin emulsion / other resin emulsion = 0.1 to 0.8, and the viscosity measured with a conical plate rotational viscometer at 25°C and 50 rpm is greater than 2.0 mPa·s and less than or equal to 8 mPa·s (see, for example, Patent Document 3).

[0004] However, while the ink compositions for writing instruments described in Patent Documents 1 and 3, respectively, achieve good abrasion resistance by containing adhesive resins, silicone resin emulsions, and other resin emulsions, their abrasion resistance is still insufficient on surfaces that do not easily absorb water-based inks, such as inkjet postcards and lightly coated paper. There has been a strong demand for a water-based ink composition for writing instruments that can produce writing marks that can withstand repeated rubbing and stronger rubbing on these and other writing surfaces. The ink composition for writing instruments described in Patent Document 2 above shares some similarities with the present invention in that it uses olefin-based resin particles, but it differs from the present invention in terms of the problem to be solved and the technical concept (the structure of the invention and its effects).

[0005] On the other hand, conventional water-based ink compositions for writing instruments have had problems such as smudging during writing and poor appearance due to the generation of air bubbles. These problems arose, for example, from the impact of dropping the pen or, in the case of a retractable ballpoint pen, from the impact of clicking, which caused cavitation in the ink and promoted the generation of air bubbles. Furthermore, once tiny air bubbles were formed, they would merge and expand over time to become larger bubbles that remained in the ink and pen body, negatively affecting writing performance. These air bubbles caused problems such as smudging during writing due to bubbles getting trapped in the pen tip, and, in the case of ballpoint pens, the accumulation of enlarged air bubbles at the ink-follower interface, often resulting in cosmetic defects. Conventionally, water-based ink compositions for writing instruments have been routinely subjected to degassing treatment during manufacturing, but the problems caused by the aforementioned air bubbles have occurred, particularly when used in water-based ballpoint pens.

[0006] On the other hand, prior art that refers to the dissolved oxygen concentration in aqueous ink compositions for writing instruments includes, for example, 4) An aqueous ink comprising an ink composition containing at least a colorant, a water-soluble organic solvent, and water, wherein the amount of dissolved gas in the ink is in an unsaturated state (3 mg / l or less based on dissolved oxygen content, at 20°C) (see, for example, Patent Document 4), 5) A water-based ballpoint pen having an aqueous ink composition containing a colorant, water, and a monomer or oligomer of 2,5-dimercapto-1,3,4-thiadiazole, and an ink backflow prevention body that follows the consumption of the ink and is closely positioned at the trailing end of the ink, and a water-based ballpoint pen having a dissolved oxygen content of 0.1 to 10 mg / L of the aqueous ink composition at 20°C (see, for example, Patent Document 5), 6) Consists of at least water, an organic solvent, a colorant, a compound containing a sulfide group and / or a disulfide group, and sugars, at 25°C and a shear rate of 2500 sec. -1Ballpoint pen ink compositions having an ink viscosity of 500 mPa·s or less, and ballpoint pen ink compositions having a dissolved oxygen content of 4.0 mg / L or less (see, for example, Patent Document 6) are known.

[0007] However, in the ink compositions for writing instruments described in Patent Documents 4 to 6, simply specifying the amount of dissolved oxygen is sometimes insufficient to obtain sufficient effects over long periods of time under harsh conditions. Furthermore, while some compositions, such as those described in Patent Documents 5 and 6, incorporate monomers of 2,5-dimercapto-1,3,4-thiadiazole or sugars to suppress the generation of bubbles through chemical removal, these have not yet proven to be sufficiently effective, and further improvements are desired. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2004-26926 (Claims, Examples, etc.) [Patent Document 2] Japanese Patent Publication No. 2016-124179 (Claims, Examples, etc.) [Patent Document 3] Japanese Patent Publication No. 2019-14769 (Claims, Examples, etc.) [Patent Document 4] Japanese Patent Publication No. 8-183923 (Claims, Examples, etc.) [Patent Document 5] Japanese Patent Publication No. 2009-234093 (Claims, Examples, etc.) [Patent Document 6] Japanese Patent Publication No. 2017-201000 (Claims, Examples, etc.) [Overview of the project] [Problems that the invention aims to solve]

[0009] In view of the problems and current status of the above-mentioned prior art, the present invention aims to resolve these issues and provides an aqueous ink composition for writing instruments that can produce writing marks that can withstand repeated rubbing and rubbing with stronger force, not only on ordinary writing surfaces but also on writing surfaces that do not easily penetrate aqueous ink, such as inkjet postcard paper and lightly coated paper, and that do not cause smudging or cosmetic defects during writing due to the formation of air bubbles. [Means for solving the problem]

[0010] In view of the above-mentioned conventional problems, the present inventors conducted diligent research and found that by including olefin resin particles with specific physical properties in a compounding system containing at least a coloring agent containing a pigment and water, the above-mentioned aqueous ballpoint pen ink composition can be obtained, and thus the present invention was completed.

[0011] In other words, the aqueous ink composition for writing instruments of the present invention comprises at least a coloring agent (A) containing a pigment, olefin resin particles (B), and water, wherein the olefin resin particles (B) have a penetrating hardness of 1 or more, an average particle diameter measured by the Coulter counter method of 15 μm or less, and the mass ratio of the solid content of the coloring agent (A) to the olefin resin particles (B) [(A) / (B)] is in the range of 1 to 100. The content of the olefin resin particles (B) is preferably 0.01 to 20% by mass relative to the total amount of the ink composition. Shear rate of water-based ink composition for writing instruments: 383 sec -1 The viscosity (at 25°C) is preferably 80 mPa·s or less. It is preferable that the dissolved oxygen concentration in the aqueous ink composition for writing instruments is 5%O2 or higher. The surface tension of the aqueous ink composition for writing instruments is preferably 15 to 60 mN / m. The writing instrument of the present invention is characterized by incorporating the aqueous ink composition for writing instruments having the above-described configuration. [Effects of the Invention]

[0012] According to the present invention, in addition to an ordinary writing surface, for a writing surface including a paper surface where aqueous ink hardly penetrates, such as inkjet postcard paper or slightly coated paper, it is possible to provide handwriting that can withstand repeated rubbing or rubbing with a stronger force of the handwriting formed by writing, and there is also provided an aqueous ink composition for a writing instrument and a writing instrument equipped with the same, which have no streaking or appearance defects during writing due to bubble generation. The objects and effects of the present invention are recognized and obtained by using the components and combinations particularly pointed out in the claims. Both the above general description and the following detailed description are exemplary and explanatory, and do not limit the present invention described in the claims.

Brief Description of the Drawings

[0013] [Figure 1] It is an explanatory diagram for explaining the structure of a ballpoint pen tip and the like when applied to a ballpoint pen which is an example of the writing instrument of the present invention.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail. However, note that the technical scope of the present invention is not limited to the embodiments described in detail below, and extends to the invention described in the claims and its equivalents. The aqueous ink composition for a writing instrument of the present invention contains at least a colorant (A) containing a pigment, olefin resin particles (B), and water, the penetration hardness of the olefin resin particles (B) is 1 or more, and the average particle diameter measured by the Coulter counter method is 15 μm or less, and the mass ratio [(A) / (B)] of the solid content of the colorant (A) and the olefin resin particles (B) is in the range of 1 to 100.

[0015] The coloring agent (A) used in the present invention is a coloring agent containing a pigment, and can be used without limitation any conventionally known inorganic and organic pigments such as titanium dioxide, resin particle pigments containing pigments, pseudo-pigments obtained by coloring resin emulsions with dyes, white plastic pigments, pigments with silica or mica as a base material and a multi-layer coating of iron oxide or titanium dioxide on the surface, aluminum pigments, thermochromic pigments, photochromic particles, etc., as well as all dyes that dissolve or disperse in water.

[0016] Examples of inorganic pigments that can be used include azo lakes, insoluble azo pigments, chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, and nitroso pigments. More specifically, inorganic pigments such as carbon black, titanium black, zinc oxide, red iron oxide, aluminum, chromium oxide, iron black, cobalt blue, iron yellow, viridian, zinc sulfide, lithopone, cadmium yellow, vermilion, cadmium red, lead yellow, molybdide orange, zinc chromate, strontium chromate, white carbon, clay, talc, ultramarine, precipitated barium sulfate, barite powder, calcium carbonate, lead white, navy blue, navy blue, manganese violet, aluminum powder, brass powder, etc., CI Pigment Blue 17, CI Pigment Blue 3-15, CI Pigment Blue 17, CI Pigment Blue 27, CI Pigment Red 5, CI Pigment Red 22, CI Pigment Red 38, CI Pigment Red 48, CI Pigment Red 49, CI Pigment Red 53, CI Pigment Red 57, C Examples include I Pigment Red 81, CI Pigment Red 104, CI Pigment Red 146, CI Pigment Red 245, CI Pigment Yellow 1, CI Pigment Yellow 3, CI Pigment Yellow 12, CI Pigment Yellow 13, CI Pigment Yellow 14, CI Pigment Yellow 17, CI Pigment Yellow 34, CI Pigment Yellow 55, CI Pigment Yellow 74, CI Pigment Yellow 95, CI Pigment Yellow 166, CI Pigment Yellow 167, CI Pigment Orange 5, CI Pigment Orange 13, CI Pigment Orange 16, CI Pigment Violet 1, CI Pigment Violet 3, CI Pigment Violet 19, CI Pigment Violet 23, CI Pigment Violet 50, CI Pigment Green 7, etc.

[0017] Aluminum pigments that can be used are broadly classified into leafing type and non-leafing type depending on the type of grinding aid used, and these can be used individually or in combination. Examples of commercially available products include the non-leafing type Alpaste 1100MA, CS420, CS460, the WXM series in which the aluminum surface is rust-preventively treated with a phosphorus-based compound, the WL series in which the aluminum surface is rust-preventively treated with a molybdenum compound, the EMR series in which the surface of the aluminum flakes is coated with high-density silica (all manufactured by Toyo Aluminum Co., Ltd.), and SW-120PM (both manufactured by Asahi Kasei Chemicals Co., Ltd.).

[0018] Examples of thermochromic pigments that can be used include thermochromic pigments produced by microencapsulating a thermochromic composition containing at least a leuco dye that functions as a color developer, a developer that is a component capable of causing the leuco dye to develop color, and a color change temperature adjuster that can control the color change temperature in the color development of the leuco dye and the developer, so as to have a predetermined average particle size. Examples of photochromic particles that can be used include photochromic particles composed of at least one selected from photochromic dyes (compounds), fluorescent dyes, etc., and a resin such as a terpene phenol resin, or photochromic particles produced by microencapsulating a photochromic composition containing at least one selected from photochromic dyes (compounds), fluorescent dyes, etc., an organic solvent, and additives such as antioxidants, light stabilizers, and sensitizers, to a predetermined average particle size. In the present invention (including examples, etc.), the "average particle size of the coloring agent" is the D50 value measured with a particle size analyzer [Microtrac HRA9320-X100 (manufactured by Nikkiso Co., Ltd.)] or the average particle size value calculated using a concentrated particle size analyzer FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.) based on cumulant analysis of the scattering intensity distribution.

[0019] Examples of methods for microencapsulating the above-mentioned thermochromic pigments include interfacial polymerization, interfacial polycondensation, insitu polymerization, liquid curing coating, phase separation from aqueous solutions, phase separation from organic solvents, melt-dispersion-cooling, air suspension coating, and spray drying, which can be appropriately selected depending on the application. For example, in the phase separation from aqueous solutions, a leuco dye, developer, and color change temperature regulator are heated and melted, then added to an emulsifier solution, heated and stirred to disperse into oil droplets, and then, as the capsule membrane material, a resin raw material such as urethane resin, epoxy resin, or amino resin is used, for example, an amino resin solution, specifically an aqueous solution of methylolmelamine, a urea solution, or a benzoguanamine solution is gradually added, and after further reaction, the dispersion is filtered to produce a thermochromic microencapsulated pigment. With this thermochromic pigment, the color development temperature and decolorization temperature of each color can be set to suitable temperatures by suitably combining the types and amounts of the leuco dye, developer, and color change temperature regulator. Furthermore, the microencapsulation method for the above-mentioned photochromic particles can be the same as the method for producing the thermochromic resin particles described above. These photochromic particles can be made to be colorless in an indoor lighting environment (such as incandescent lamps, fluorescent lamps, lamps, or white LEDs) and to develop color in an ultraviolet irradiation environment (irradiation with wavelengths of 200-400 nm or sunlight including ultraviolet light) by suitably using photochromic dyes (compounds), fluorescent dyes, etc.

[0020] Examples of dyes that can be used include acid dyes such as eosin, foxine, water black R445, yellow #6-C, water yellow #6-C, acid red, water blue #105, brilliant blue FCF, and nigrosine NB; direct dyes such as direct black 154, direct sky blue 5B, and violet BB; and basic dyes such as rhodamine and methyl violet.

[0021] These colorants (A) can be used individually or in combination of two or more. Furthermore, all of these colorants (A) may be pigments. The average particle size of pigments such as water-dispersible pigments, resin particle pigments, pseudo-pigments, white plastic pigments, multi-layer coated pigments, aluminum pigments, thermochromic pigments, and photochromic particles varies depending on the ball diameter, ink composition, viscosity, etc., but an average particle size of 0.05 to 20 μm is desirable. The content of these colorants (A) can be appropriately increased or decreased depending on the ink line density. Furthermore, in the present invention, as will be described later, it is necessary to optimize the mass ratio of the solid content of the olefin resin particles (B) and the colorant (A) used [(A) / (B)], but a ratio of 0.1 to 30% by mass (hereinafter, "mass%" will simply be referred to as "%") relative to the total amount of the ink composition is desirable.

[0022] The olefin resin particles (B) used in this invention have a hardness of 1 or higher by the penetration method and an average particle diameter of 15 μm or less as measured by the Coulter counter method. By using olefin resin particles with these properties, the present invention can produce writing that can withstand repeated rubbing and rubbing with stronger force, and also eliminates smudging and appearance defects during writing due to the formation of air bubbles (these effects will be described further later). The effects of the present invention cannot be achieved with olefin resin particles that have a hardness of less than 1 as measured by the penetration method, or with an average particle diameter exceeding 15 μm as measured by the Coulter counter method. Preferably, to further improve the effects of the present invention, it is desirable to use olefin resin particles (B) having a hardness of 1 or more by the penetration method and an average particle diameter of 10 μm or less as measured by the Coulter counter method. In the present invention (including the examples described later), "penetration hardness" is a value measured in accordance with JIS K2207.

[0023] The olefin resin particles that can be used are those that meet the above characteristics, namely, a hardness of 1 or higher by the penetration test and an average particle diameter of 15 μm or less as measured by the Coulter counter method. Their shape and structure are not particularly limited. For example, commercially available products include ChemiPearl W100, W200, W400, and W500 manufactured by Mitsui Chemicals, but even among the same type of olefin resin particles (ChemiPearl products), ChemiPearl W300, W308, W310, W700, and W900 do not satisfy the above characteristics (hardness by the penetration test, etc.).

[0024] The content of these olefin resin particles (B) is preferably 0.01 to 20% by mass, more preferably 1 to 5% by mass, relative to the total amount of the ink composition. When the content of these olefin resin particles (B) is less than 0.01% by mass, the effects of the present invention cannot be fully realized, while when it exceeds 20% by mass, problems such as ink smudging may occur.

[0025] In the present invention, the mass ratio of the solid content of the coloring agent (A) to the olefin resin particles (B) [(A) / (B)] must be in the range of 1 to 100. If this mass ratio of solid content [(A) / (B)] does not fall within the above range (less than 1 or greater than 100), the effects of the present invention cannot be achieved. To further enhance the effects of the present invention, it is desirable that the mass ratio of the solid content [(A) / (B)] be between 1 and 10.

[0026] The aqueous ink composition for writing instruments of the present invention preferably contains, in addition to the colorant (A) containing the above-mentioned pigment and olefin resin particles (B), a water-soluble solvent and, as a remainder, water (tap water, purified water, distilled water, ion-exchanged water, pure water, etc.) as the solvent. Furthermore, within a range that does not impair the effects of the present invention, dispersants, lubricants, thickeners, pH adjusters, rust inhibitors, preservatives, or antibacterial agents may be appropriately included depending on the application of the writing instrument (for ballpoint pens, marking pens, etc.).

[0027] Suitable water-soluble solvents include, for example, glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, 3-butylene glycol, thiodiethylene glycol, and glycerin, as well as ethylene glycol monomethyl ether and diethylene glycol monomethyl ether, which can be used alone or in combination. The content of this water-soluble solvent should preferably be 5 to 40% of the total amount of the ink composition.

[0028] The dispersant that can be used in the present invention needs to be appropriately selected and blended, such as a water-soluble polymer dispersant or a surfactant. Examples of water-soluble polymer dispersants include polyacrylic acid, acrylic acid copolymers, and maleic acid resins. Specifically, resins such as acrylic resins, styrene-acrylic acid resins, and styrene-maleic acid resins are used in the form of salts to make them water-soluble. Typical alkali metals that form salts include sodium and potassium. Typical amines include aliphatic primary to tertiary amines such as mono, di, or trimethylamines, alcohol amines such as mono, di, or tripolamines, methylethanolamine, methylpropanolamine, and dimethylethanolamine, as well as ammonia, morpholine, and N-methylphorine.

[0029] Examples of surfactants that act as dispersants include nonionic surfactants and anionic surfactants. Examples of nonionic surfactants include polyoxyalkylene fatty acid esters, polyhydric alcohol fatty acid partial esters, and sugar fatty acid esters. Specifically, examples include glycerin fatty acid esters, polyglycerin fatty acid esters, propylene glycol fatty acid esters, pentaerythritol fatty acid esters, polyoxyethylene sorbitan 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 styrene-phenyl ethers, polyoxyethylene disstyrene-phenyl ethers, polyoxyethylene castor oil, polyoxyethylene lanolin, polyoxyethylene lanolin alcohol, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, and polyoxyethylene alkylphenyl formaldehyde condensates. Examples of anionic surfactants include alkylated sulfonates of higher fatty acid amides, alkylallyl sulfonates, and naphthalene sulfonic acid formaldehyde condensates. Specifically, examples include alkyl sulfates, polyoxyethylene alkyl ether sulfates, N-acyl amino acid salts, N-acyl methyl taurate salts, polyoxyethylene alkyl ether acetates, alkyl phosphates, and polyoxyethylene alkyl ether phosphates. These dispersants are not particularly limited as long as they further improve the dispersibility of the pigment, and can be used individually or in combination of two or more.

[0030] Examples of lubricants that can be used include nonionic types such as fatty acid esters of polyhydric alcohols, higher fatty acid esters of sugars, higher fatty acid esters of polyoxyalkylenes, and alkyl phosphate esters, which are also used as surface treatment agents for pigments; anionic types such as phosphate esters, alkyl sulfonates of higher fatty acid amides, and alkyl allyl sulfonates; derivatives of polyalkylene glycols; and polyether-modified silicones. Suitable thickeners include, for example, at least one selected from the group consisting of synthetic polymers, cellulose, and polysaccharides. Specifically, examples include gum arabic, tragacanth gum, guar gum, locust bean gum, alginic acid, carrageenan, gelatin, xanthan gum, gelan gum, succinoglycan, dieutan gum, dextran, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, fermented cellulose, oxidized cellulose, crystalline cellulose, starch glycolic acid and its salts, polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylic acid and its salts, polyethylene hydroxide, copolymers of vinyl acetate and polyvinylpyrrolidone, styrene-acrylic acid copolymers and their salts.

[0031] Examples of pH adjusters include ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, alkali metal salts of carbonic acid and phosphoric acid such as sodium tripolyphosphate and sodium carbonate, and alkali metal hydrates such as sodium hydroxide. Examples of rust inhibitors include benzotriazole, tolyltriazole, dicyclohexylammonium nitride, and saponins, while examples of preservatives or antibacterial agents include phenol, sodium omazine, sodium benzoate, thiazoline compounds, and benzimidazole compounds. Each of the above-mentioned components, such as dispersants, lubricants, thickeners, pH adjusters, rust inhibitors, preservatives, or antibacterial agents, may be used individually or in combination of two or more. Furthermore, commercially available products containing these components may be used if available.

[0032] Furthermore, the pH (at 25°C) of the aqueous ink composition for writing instruments of the present invention is preferably adjusted to 5 to 10 using a pH adjuster or the like, and more preferably to 6 to 9.5, from the viewpoint of usability, safety, stability of the ink itself, and compatibility with the ink container.

[0033] The aqueous ink composition for writing instruments of the present invention has a shear rate of 383 sec. -1 The viscosity at 25°C is preferably 80 mPa·sec or less, more preferably 50 mPa·sec or less, and particularly preferably in the range of 5 to 20 mPa·sec. When this viscosity exceeds 80 mPa·sec at the above shear rate, the ink flow rate decreases, the writing feel becomes heavier, and it becomes difficult to write smoothly. On the other hand, by keeping it above 5 mPa·sec, the effect of suppressing ink bleeding and show-through is more easily achieved.

[0034] Furthermore, the aqueous ink composition for writing instruments of the present invention preferably has a surface tension of 15 to 60 mN / m, and more preferably 20 to 40 mN / m, from the viewpoint of leveling properties. In this invention (including the examples described later), the surface tension of the ink (at 25°C) refers to the value measured using a surface tension measuring device: CBVP-Z, manufactured by Kyowa Interface Science Co., Ltd. Furthermore, the aqueous ink composition for writing instruments of the present invention is preferably capable of mitigating metal corrosion in metal parts such as pen tips and ink reservoirs. Specifically, the presence of oxygen at the interface where the metal is in contact with the ink mitigates the corrosion reaction of the metal substrate by the oxide film on the metal surface. Therefore, it is preferable that the dissolved oxygen concentration in the aqueous ink composition for writing instruments be 5% O2 or higher. Furthermore, the present invention is particularly promising for suppressing writing streaks caused by air bubbles after storage under low-temperature conditions. Under low-temperature conditions, air dissolves easily in the ink. When the pen is moved to a room-temperature environment and subjected to impacts such as dropping or knocking, the insoluble air can create tiny bubbles in the ink, which can accumulate inside the connector and other parts, obstructing the ink flow path and potentially leading to poor writing. The range of viscosity values ​​and surface tension at the above shear rate can be adjusted by suitably combining the pigment-containing colorant, olefin resin particles, water-soluble solvent, etc., and suitably adjusting the content of each component, as well as by selecting a suitable mixing method. Furthermore, to achieve a dissolved oxygen concentration of 5% O2 or higher, this can be adjusted by adding a bubble inhibitor or by adjusting the stirring conditions during ink production. Any conventionally used foam inhibitor can be used, including at least one of the following: cysteine ​​and its derivatives, glutathione and its derivatives, hydroxylamine, hydroxylamine derivatives, ascorbic acid, ascorbic acid derivatives, erythorbic acid, erythorbic acid derivatives, their salts, or N-vinyl-2-pyrrolidone oligomers. The foam inhibitor is preferably used in an amount of 0.01 to 5.0% by mass relative to the total amount of the ink composition to adjust the dissolved oxygen concentration.

[0035] The aqueous ink composition for writing instruments of the present invention can be manufactured in no particular way compared to the manufacturing methods of other aqueous ink compositions. In other words, the aqueous ink composition for writing instruments of the present invention can be produced by appropriately combining, in addition to a colorant (A) containing the above-mentioned pigment and olefin resin particles (B), a water-soluble solvent, the remainder being water as a solvent, and other components according to the application of the ink for writing instruments (for ballpoint pens, marking pens, etc.), and mixing and stirring using a mixer, or more specifically, a bead mill, homomixer, homogenizer, etc., capable of applying strong shear, under suitable stirring conditions, and further, if necessary, removing coarse particles from the ink composition by filtration or centrifugation.

[0036] The aqueous ink composition for writing instruments of the present invention is used in ballpoint pens, marking pens, felt pens, etc., which are equipped with pen tips such as ballpoint pen tips, fiber tips, felt tips, and plastic tips. The ballpoint pen in the present invention includes one in which an aqueous ink composition for writing instruments having the above composition is contained in a ballpoint pen ink container (refill), and a substance that is incompatible with the aqueous ink composition contained in the ink container and has a lower specific gravity than the aqueous ink composition, such as polybutene, silicone oil, or mineral oil, is contained as an ink follower. For example, the aqueous ink composition for writing instruments can be manufactured by filling an aqueous ballpoint pen body equipped with a ballpoint pen tip having a ball with a diameter of 0.18 to 2.0 mm with the aqueous ink composition for writing instruments.

[0037] With water-based ballpoint pens, air is easily drawn in from the pen tip during writing. This air (containing oxygen) is then dissolved in the ink, which can easily lead to further bubble formation. Therefore, the ballpoint pen tip should, for example, be in the range of (AB) / 2 = 5 to 30 μm, and more preferably in the range of 10 to 25 μm, when the diameter of the ball housing 20 is A and the diameter of the ball 10 is B, passing through the center of the ball 10 and perpendicular to the axial direction. This improves resistance to smudging during writing, resulting in a good line and also improving the scratch resistance of the line. The structure of ballpoint pens, marking pens, felt-tip pens, etc., is not particularly limited. For example, a direct-ink type ballpoint pen or marking pen may be equipped with a collector structure (ink holding mechanism) in which the barrel itself serves as the ink reservoir and the above-described aqueous ink composition for writing instruments is filled into the barrel.

[0038] The aqueous ink composition for writing instruments of the present invention, configured as described above, comprises at least a coloring agent (A) containing a pigment, olefin resin particles (B), and water, wherein the olefin resin particles (B) have a penetrating hardness of 1 or more, an average particle diameter measured by the Coulter counter method of 15 μm or less, and the mass ratio of the solid content of the coloring agent (A) to the olefin resin particles (B) [(A) / (B)] is in the range of 1 to 100. This makes it possible to produce writing that can withstand repeated rubbing and stronger rubbing of the resulting writing on writing surfaces, including inkjet postcard paper and lightly coated paper, which are not easily penetrated by aqueous inks, in addition to ordinary writing surfaces.

[0039] The reason why the aqueous ink composition for writing instruments of the present invention, configured in this manner, and the writing instruments equipped with it exhibit the above-mentioned specific effects on writing surfaces, including inkjet postcard paper and lightly coated paper, which are difficult for aqueous inks to penetrate, is presumed to be because the interposition of olefin particles on the surface of the line coating improves sliding properties and reduces the frictional resistance of the lines drawn. Furthermore, it is presumed that the following mechanism is at work to ensure that the aqueous ink composition for writing instruments of the present invention and the writing instruments equipped with it do not experience streaking or cosmetic defects during writing due to bubble formation. Specifically, in the present invention, the inclusion of olefin resin particles (B) with the above characteristics traps minute bubbles generated in the ink composition and suppresses their coalescence and enlargement. This mechanism is presumed to be due to the hydrophobic nature of the olefin resin particles with the above characteristics, which readily adsorb to the gas-liquid interface. Therefore, they adsorb to the interface between bubbles generated by dropping the writing instrument or impacts such as those from retractable ballpoint pens, and the ink, thereby suppressing the coalescence of bubbles through direct contact. In particular, self-dispersing olefin resin particles have no steric hindrance, making them more readily adsorbed to the gas-liquid interface and thus more likely to produce the desired effect. Furthermore, the polyolefin resin particles used in this invention have a hardness of 1 or higher by the penetrating hardness test, and even if they dry on the pen tip, they easily form a flexible coating film, and can effectively suppress the generation of air bubbles without affecting the friction when writing begins. The aqueous ink composition for writing instruments of the present invention and the writing instruments equipped with it exhibit extremely excellent sustained effects that bring about the above-mentioned effects of the present invention. Moreover, the period and duration of the effect are long, and furthermore, because it is aqueous, it also exhibits excellent stability over time. [Examples]

[0040] Next, the present invention will be described in more detail with reference to Examples 1 to 6 and Comparative Examples 1 to 4 of aqueous ink compositions for writing instruments and writing instruments equipped therewith, but the present invention is not limited to the following examples.

[0041] [Examples 1-6 and Comparative Examples 1-4] Each aqueous ink composition for writing instruments was prepared by conventional methods according to the formulations shown in Table 1 below, which include each colorant (A), olefin resin particles (B) with various properties (hardness by penetration method, average particle size measured by Coulter counter method), and water-soluble solvents. Each of the obtained aqueous ink compositions for writing instruments (100% by mass) was subjected to a shearing rate of 383 seconds by the following methods. -1 Viscosity (mPa·s) at 25°C, surface tension, dissolved oxygen concentration, and the abrasion resistance and smudge resistance were evaluated using the following evaluation method after fabricating a writing instrument with the structure described below. These results are shown in Table 1 below.

[0042] [Shear rate 383sec -1 (Method for measuring viscosity and surface tension at 25°C) Using an EMD type viscometer (manufactured by Tokyo Keiki Co., Ltd.), the measurement temperature was 25°C and the shear rate was 383 sec. -1 The viscosity of the ink (mPa·s) was measured. Furthermore, the surface tension (at 25°C) was measured using a surface tension measuring device: CBVP-Z, manufactured by Kyowa Interface Science Co., Ltd.

[0043] (Method for measuring dissolved oxygen concentration) Dissolved oxygen concentration (%O2) was measured at a measurement temperature of 25°C using a needle-type oxygen meter, MicroxTX3 (manufactured by PreSens). This "%O2" represents the oxygen concentration of the air dissolved in the ink.

[0044] (writing implements) Using the barrel of a retractable ballpoint pen (manufactured by Mitsubishi Pencil Co., Ltd., product name: Uni-ball Signo RT), a refill consisting of a polypropylene ink container with an inner diameter of 3.8 mm and a length of 113 mm, where the diameter of each ink-following element corresponds to the inner diameter, a ballpoint pen tip (holder: stainless steel, ball: cemented carbide ball, ball diameter 0.5 mm), and a connector connecting the container tube and the tip, was filled with the aqueous ink compositions for writing instruments obtained above, and an ink-following element made of polybutene was filled at the rear end of the ink to produce writing instruments (5 of each). Furthermore, the [(AB) / 2] ratio in this retractable ballpoint pen, according to Figure 1, was 15 μm.

[0045] <Method for evaluating abrasion resistance> Using the above writing instrument, circular writing was done on the surface of coated paper (New Yupo 80, Yupo Corporation), and the writing was allowed to dry. The writing was then rubbed by moving a Kimwipe with a 500g weight on it five times, and the abrasion resistance of the writing was evaluated according to the following evaluation criteria. Evaluation criteria: A: Upon observation after the test, no peeling or fading of the handwriting was observed. B: Upon observation after the test, some peeling had occurred in the handwriting, but the handwriting itself remained mostly intact. C: Upon observation after the test, the paint had peeled off to the point where no trace of the writing remained.

[0046] <Evaluation method for bubble generation> The above-mentioned retractable water-based ballpoint pen was left undisturbed for one month at a temperature of 5°C, then left undisturbed for one day at a temperature of 25°C, and then the retractable action was performed 20 times with the pen tip facing downwards. The presence or absence of bubbles after being left undisturbed for one week at 25°C was evaluated based on the following criteria. Evaluation criteria: A: No air bubbles are observed. B: One bubble less than 0.5 mm in diameter is observed. C: Bubbles with a diameter of 0.5 mm or more but less than 1 mm are observed. D: Bubbles equivalent to 1 mm in diameter or larger are observed.

[0047] <Method for evaluating resistance to fading> The above writing instrument was left undisturbed for one month at a temperature of 5°C, then left undisturbed for one day at a temperature of 25°C, and then the pen was clicked 20 times with the tip facing downwards. A spiral with a diameter of 2 cm was written on writing paper, and the presence or absence of abrasion resistance due to air bubble formation was evaluated according to the following criteria. Evaluation criteria: A: The writing lines are good. B: There is some slight smudging as you continue writing, but the smudging improves as you continue writing. C: As I continued writing, there was noticeable smudging, and the smudging did not improve even with further writing.

[0048] [Table 1]

[0049] As is clear from the results in Table 1 above, the writing instruments equipped with the ink compositions for writing instruments of Examples 1 to 6, which fall within the scope of the present invention, were found to produce writing that can withstand repeated rubbing and rubbing with stronger force, compared to Comparative Examples 1 to 4, which fall outside the scope of the present invention. Furthermore, they were found to be water-based ink compositions for writing instruments and writing instruments equipped with them, without skipping during writing due to the generation of air bubbles. [Industrial applicability]

[0050] A water-based ink composition suitable for writing instruments such as water-based ballpoint pens and marking pens, and writing instruments equipped with this composition, can be obtained.

Claims

1. The material comprises at least 0.1 to 30% by mass of a coloring agent (A) containing a pigment with an average particle size of 0.55 to 20 μm, 0.01 to 20% by mass of olefin resin particles (B), and water, wherein the penetrating hardness of the olefin resin particles (B) is 1 to 10, the average particle size measured by the Coulter counter method is 2.5 to 10 μm, the mass ratio of the solid content of the coloring agent (A) to the olefin resin particles (B) [(A) / (B)] is in the range of 1 to 10, and the shear rate is 383 sec. －１ A water-based ink composition for ballpoint pens, characterized by having a viscosity (at 25°C) of 5 to 50 mPa·s.

2. The aqueous ink composition for ballpoint pens according to claim 1, characterized in that the content of the olefin resin particles (B) is 0.01 to 5% by mass with respect to the total amount of the ink composition.

3. The aqueous ink composition for ballpoint pens according to claim 1 or 2, characterized by containing 0.01 to 5% by mass of a foam inhibitor.

4. Shear rate 383sec －１ A water-based ink composition for ballpoint pens according to any one of claims 1 to 3, characterized in that the viscosity (at 25°C) is 5 to 20 mPa·s.

5. A water-based ink composition for ballpoint pens according to any one of claims 1 to 4, characterized in that the surface tension is 15 to 60 mN / m (25°C).

6. A water-based ballpoint pen characterized by being equipped with the water-based ink composition for ballpoint pens described in any one of claims 1 to 5.

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