Water-based ink composition for writing instruments and writing instruments

Abstract

The present invention provides an aqueous ink composition for writing instruments and a writing instrument that exhibits excellent pigment settling properties, ink followability, and color development on paper, as well as excellent low gloss of drawn lines. [Solution] The aqueous ink composition for writing instruments of the present invention contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, and is characterized in that the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) is 0.5 to 20.

Description

Technical Field

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

Background Art

[0002] Conventionally, when imparting "concealability" that is less affected by the color of the background, aqueous ink compositions for writing instruments using high specific gravity pigments such as titanium oxide in the colorant are widely known. However, pigments with a high specific gravity and large particle size such as titanium oxide tend to settle, and cases where the ink viscosity is increased to solve this problem are known.

[0003] When the ink viscosity is increased, ink follow-up deteriorates during rapid writing, and streaks may occur, resulting in insufficient concealability. Also, when the particle size of the pigment is reduced as a means of suppressing sedimentation, the viscosity of the ink can be lowered to some extent, but there is a problem that it easily enters the mesh of the paper fibers, resulting in insufficient concealability. Although these points were exemplified with titanium oxide, it is known that the same problems occur not only with titanium oxide but also when using high specific gravity pigments such as glitter pigments like metallic flakes.

[0004] On the other hand, as an aqueous ink composition suitable for writing instruments and the like that has good pigment sedimentation properties, ink follow-up properties, etc. in the prior art, and as a related art of the present invention, for example, (1) Even when applying an aqueous ink using a pigment as a colorant to a marking pen, an aqueous ink composition for a marking pen that can maintain a stable dispersion state in the ink for a long time and continuously exhibit good writing performance without causing streaks or the like in the handwriting, which comprises water, a pigment, fermented cellulose, and a cross-linking agent that cross-links the pigment and the fermented cellulose (see, for example, Patent Document 1). (2) A water-based ink composition for writing instruments that has excellent dispersion stability and can suppress smudging and skipping lines, thereby forming good handwriting, comprising a microcapsule pigment consisting of a core material and a wall film enclosing the core material, a polyether phosphate ester as a dispersant, cellulose nanofibers, and water (see, for example, Patent Document 2), (3) A water-soluble ink composition for writing instruments characterized by comprising at least a colorant, a neutralized product of an isobutylene-maleic anhydride copolymer, a water-soluble resin and / or a resin emulsion, and water, wherein the acid value of the resin in the water-soluble resin and resin emulsion is 240 or less (see, for example, Patent Document 3). (4) At least one aqueous ink composition for writing instruments is known, characterized by comprising a colorant, a neutralized product of an isobutylene-maleic anhydride copolymer, a polyol having two or more OH groups, a water-soluble resin and / or a resin emulsion, and water (see, for example, Patent Document 4).

[0005] However, the aqueous ink compositions for writing instruments using fermented cellulose or cellulose nanofibers described in Patent Documents 1 and 2 above are sometimes affected by the underlying color when writing on black paper, resulting in insufficient color development on the paper surface. Furthermore, they have poor resistance to pigment sedimentation, and there are still issues with ink followability when writing quickly. The aqueous ink compositions for writing instruments described in Patent Documents 3 and 4 above are similar to the present invention in that they use a neutralized product of isobutylene-maleic anhydride copolymer, but they provide an aqueous ink composition for writing instruments that forms a uniform coating film and has excellent abrasion resistance and water adhesion resistance even when writing on non-absorbent surfaces. Therefore, they differ from the present invention in terms of the purpose and problems of the invention, as well as the technical concept (composition and its effects). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-10114 (Claims, Examples, etc.) [Patent Document 2] Japanese Patent Publication No. 2024-76078 (Claims, Examples, etc.) [Patent Document 3] Japanese Patent Publication No. 2023-78817 (Claims, Examples, etc.) [Patent Document 4] Japanese Patent Publication No. 2023-92768 (Claims, Examples, etc.) [Overview of the project] [Problems that the invention aims to solve]

[0007] In view of the problems and current state of the above-mentioned prior art, the present invention aims to solve these problems and provides an aqueous ink composition and writing instrument for writing instruments that can produce highly colored lines without being affected by the color of the background when writing on black paper, and that can highly simultaneously achieve the following characteristics: 1) resistance to pigment settling, 2) ink followability when writing quickly, 3) color development on the paper surface, and 4) low gloss of the lines. [Means for solving the problem]

[0008] In view of the above-mentioned conventional problems, the present inventors conducted diligent research and found that by containing at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, and by setting the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) within a specific range, an aqueous ink composition for writing instruments and a writing instrument for the above-mentioned purpose can be obtained, thus completing the present invention.

[0009] In other words, the aqueous ink composition for writing instruments of the present invention contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, and is characterized in that the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) is 0.5 to 20. The aqueous ink composition for writing instruments preferably further contains 0.03 to 2% by mass of an anionic or nonionic surfactant. The viscosity of the ink in the aforementioned aqueous ink composition for writing instruments is determined by a shear rate of 38.3 s. -1 In this case, it is preferable that the pressure be between 10 and 500 mPa·s. 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]

[0010] The present invention provides an aqueous ink composition and writing instrument for writing instruments that, when written on black paper or the like, produce highly colored lines without being affected by the underlying color, exhibit excellent resistance to pigment settling, ink followability when writing quickly, and color development on the paper surface, resulting in low gloss lines. The object and effect of the present invention are recognized and obtained, in particular, by using the components and combinations indicated in the claims. Both the general description above and the detailed description below are illustrative and descriptive, and do not limit the present invention as described in the claims. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described in detail below. However, it should be noted that the technical scope of the present invention is not limited to the embodiments detailed below, but extends to the invention described in the claims and its equivalents. Furthermore, the present invention can be implemented based on the contents disclosed herein and common technical knowledge in the art (including design matters and obvious matters).

[0012] <Water-based ink composition for writing instruments> The aqueous ink composition for writing instruments of the present invention contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, and is characterized in that the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) is 0.5 to 20.

[0013] <Colorants> The colorants used in this invention include conventionally known inorganic and organic pigments such as titanium dioxide, resin particle pigments containing pigments, pseudo-pigments obtained by coloring resin emulsions with dyes, thermochromic pigments, photochromic particles, white plastic pigments, pigments with silica or mica as a base material and multi-layer coating of iron oxide or titanium dioxide on the surface, foil-like pigments (glitter pigments) obtained by cutting luminous films or foils, luminous pigments such as aluminum pigments, and all dyes that dissolve or disperse in water, without limitation.

[0014] 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.

[0015] Examples of the dyes include acid dyes such as eosin, phloxin, water yellow #6-C, acid red, water blue #105, brilliant blue FCF, nigrosine NB, etc.; direct dyes such as direct black 154, direct sky blue 5B, violet BB, etc.; basic dyes such as rhodamine, methyl violet, etc.

[0016] These color materials can be used individually or in combination of two or more (hereinafter referred to as "at least one kind"). Among these color materials, from the viewpoints of color development property, weather resistance, etc., it is preferable to use all pigments, or mainly use pigments and use dyes as complementary colors (the pigment content is 50% by mass or more to 100% by mass in the total amount of color materials). Pigments such as pigments dispersed in water, resin particle pigments, pseudo pigments, white plastic pigments, multi-layer coated pigments, glitter pigments, aluminum pigments, thermochromic pigments, photochromic particles, etc. are preferably used. The average particle diameter of these pigments varies depending on the writing instrument type (ball pen, marking pen, etc.), ink composition, viscosity, etc., but from the viewpoints of color development on the paper surface and suppression of pigment sedimentation, those with an average particle diameter of 0.05 to 20 μm are desirable. The content of these color materials can be appropriately increased or decreased according to the drawing line density of the ink, and is preferably 0.1 to 30% by mass (hereinafter, "% by mass" is simply referred to as "%") with respect to the total amount of the ink composition.

[0017] The "average particle diameter" in the present invention is the value of D-50 measured by a laser diffraction / scattering type particle diameter measuring device (model: LA-960, manufactured by Horiba, Ltd.). The shape of the color materials blended in the ink composition of the present disclosure may be spherical or non-spherical.

[0018] The neutralized product (A) of the isobutylene / maleic anhydride copolymer used in the present invention is a neutralized product of a copolymer of isobutylene and maleic anhydride, which forms weak aggregation with the neutralized product and the pigment in the ink composition and exhibits a function of suppressing the sedimentation of the pigment. In the present invention, by further using it in combination with the fibrous polysaccharide (B) described later and within a predetermined mass ratio range, the effects of the present invention can be highly exhibited. In the neutralized product of the isobutylene-maleic anhydride copolymer used, the basic structure of the isobutylene-maleic anhydride copolymer is represented by the following formula (I), and the following formula (II) is the ammonia-neutralized product (modified product) of the isobutylene-maleic anhydride copolymer that can preferably be used in the present invention.

[0019]

Chemical formula

[0020]

Chemical formula

[0021] In the present invention, in addition to the ammonia-neutralized product of the above formula (II), a sodium hydroxide-neutralized product or an amine-neutralized product may also be used. Further, numerical values such as n in the above formula (I), l and m in (II), and j and k in the following formula (III) vary depending on the weight average molecular weight and the like described later, and a suitable range is determined. Further, in the present invention, imidized isobutylene-maleic anhydride copolymers represented by the following formula (III) obtained by imidizing (imide-modifying) isobutylene-maleic anhydride copolymers, and neutralized products thereof neutralized with ammonia, sodium hydroxide, amine, etc. may also be used.

Chemical formula

[0022] The weight average molecular weight of each neutralized product obtained by neutralizing the isobutylene-maleic anhydride copolymer such as the above formula (II) and (III) is preferably 3,000 to 400,000, more preferably 5,000 to 200,000, still more preferably 30,000 to 100,000, and particularly preferably 50,000 to 70,00. This weight average molecular weight is a value in terms of polystyrene by aqueous gel permeation chromatography (GPC) analysis. The isobutylene-maleic anhydride copolymer and its neutralized product can be commercially available. For example, "Isoban" manufactured by Kuraray Co., Ltd. can be used. For instance, Isoban-104 and 110 are commercially available ammonia neutralized (modified) products of the isobutylene-maleic anhydride copolymer in formula (II) above. When preparing the neutralized product of the isobutylene-maleic anhydride copolymer (from formula (I) above, etc.), for example, if all carboxyl groups of the isobutylene-maleic anhydride copolymer are neutralized, the degree of neutralization is considered to be 1. The required amounts of ammonia, sodium hydroxide, amine, etc., used for neutralization are then calculated and prepared to obtain each neutralized product.

[0023] The content of these neutralized isobutylene-maleic anhydride copolymers (A) is preferably 0.01 to 10% by mass, and more preferably 0.05 to 5% by mass, in terms of solid content, relative to the total amount of the ink composition. If the content of the neutralized isobutylene-maleic anhydride copolymer (A) is less than 0.01% by mass, the effects of the present invention cannot be achieved. On the other hand, if it exceeds 10% by mass, the viscosity increases, reducing ink ejection and potentially adversely affecting writing performance.

[0024] Examples of fibrous polysaccharides (B) used in the present invention include at least one of fermented cellulose, cellulose nanofibers, chitin, and chitosan. Preferably, due to their low impurity content, fermented cellulose and cellulose nanofibers are preferred. While various non-fibrous polysaccharides exist, such as xanthan gum and carboxymethylcellulose (CMC), their use alone will not achieve the effects of the present invention. However, as long as fibrous polysaccharides are included as essential components in the ink composition, xanthan gum, CMC, and other non-fibrous polysaccharides can be used as appropriate, within limits that do not impair the effects of the present invention. In this invention, "fibrous" means insoluble in water and having an aspect ratio of 25 or more.

[0025] The fermented cellulose that can be used is cellulose produced by cellulose-producing bacteria, such as bacteria belonging to the genera Acetobacter, Pseudomonas, Agrobacterium, etc., and is not particularly limited to these types of cellulose. Typically, fermented cellulose is cellulose. It can be produced by culturing the production microorganisms according to known methods (culture medium and conditions, etc.) and purifying the resulting fermented cellulose as desired. This fermented cellulose has a much finer fiber diameter compared to typical cellulose fibers derived from plants. Furthermore, the chemical structure of the polysaccharides that make up the fermented cellulose is basically a linear high-molecular-weight polysaccharide composed of β1-4 linked glucose molecules.

[0026] The fermented cellulose that can be used is insoluble in water and forms a three-dimensional network structure in the ink composition. Unlike thickening polysaccharides such as xanthan gum, fermented cellulose has low viscosity and is less sticky, and can be used stably in various inks with different coexisting components, temperatures, and pH levels. Examples of fermented cellulose that can be used include the fermented cellulose obtained in the above production process, commercially available SunArtist series products from San-Ei Gen F.F.I., such as SunArtist H-PN and SunArtist H-PG, CELLULON R-27 from CP Kelco, and Fibnano HE-NFBC from Kusano Sakko Co., Ltd.

[0027] The cellulose nanofibers (hereinafter sometimes referred to as "CNF") that can be used are materials obtained by defibrating pulp to a fiber width of 3-4 nm by treating it with phosphate esters or the like. The average fiber length (number-average fiber length) of the cellulose nanofibers that can be used is not particularly limited, but is preferably 100 to 1000 nm, more preferably 150 to 800 nm. The aspect ratio of the cellulose nanofibers (i.e., the ratio of the average fiber length to the average fiber diameter) is preferably in the range of 25 to 250, and more preferably 35 to 200. Examples of cellulose nanofibers that can be used include TEMPO-oxidized cellulose nanofibers. Commercially available options include, for example, Rheocrysta C-2SP (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., 2% solids content) and Selenpia TC-02X (manufactured by Nippon Paper Industries Co., Ltd., 5% solids content). The above-mentioned TEMPO-oxidized cellulose nanofibers can be obtained by reacting wood fibers with a TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) catalyst to convert the primary hydroxyl groups of cellulose into carboxyl groups, and then mechanically defibrating them.

[0028] The total content of these fibrous polysaccharides (B) is preferably 0.03 to 2.0% by mass, and more preferably 0.05 to 1.0% by mass, in terms of active ingredient amount, relative to the total amount of the ink composition. In the present invention, it is necessary to set the mass ratio (A) / (B) of the neutralized isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) to 0.5 to 20, and more preferably to 2.0 to 15. In the present invention, it is presumed that by setting the mass ratio (A) / (B) to 0.5 to 20, the neutralized product (A) creates loose aggregation, which is then captured by the network of fibrous polysaccharide (B), thereby achieving the effects of the present invention. If the mass ratio (A) / (B) is less than 0.5, loose aggregation cannot be formed, the pigment will slip through the fibrous polysaccharides, reducing its resistance to pigment sedimentation, and even on the paper surface, it will penetrate the paper fibers, reducing its color development and preventing the effects of the present invention from being exhibited. On the other hand, if the mass ratio (A) / (B) exceeds 20, the pigment cannot be supported by loose aggregation, resulting in reduced resistance to pigment sedimentation, which is undesirable.

[0029] In the present invention, it is desirable to further include at least one anionic or nonionic surfactant, in order to maintain the effects of the present invention to an even higher degree. Examples of nonionic surfactants that can be used include polyoxyalkylene higher fatty acid esters, higher fatty acid partial esters of polyhydric alcohols, higher fatty acid esters of sugars, monophosphate esters of polyoxyethylene alkyl ethers or polyoxyethylene alkylaryl ethers, diphosphate esters of polyoxyethylene alkyl ethers or polyoxyethylene alkylaryl ethers, triesters of polyoxyethylene alkyl ethers or polyoxyethylene alkylaryl ethers, alkyl phosphate esters, alkyl ether phosphate esters, or derivatives thereof. 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-derived phenyl ethers, polyoxyethylene disstyrene-derived phenyl ethers, polyoxyethylene hydrogenated castor oil, polyoxyethylene lanolin, polyoxyethylene lanolin alcohol, polyoxyethylene alkylamines, polyoxyethylene fatty acid amides, and polyoxyethylene alkylphenyl formaldehyde condensates.

[0030] Furthermore, examples of anionic surfactants that can be used include alkylated sulfonates of higher fatty acid amides, alkylallyl sulfonates, naphthalene sulfonic acid formaldehyde condensates, and more specifically, alkyl sulfates, polyoxyethylene alkyl ether sulfates, N-acyl amino acid salts, N-acyl methyl taurate salts, polyoxyethylene alkyl ether acetates, alkyl phosphates, polyoxyethylene alkyl ether phosphates, and polycarboxylates. The nonionic surfactants described above are preferably monophosphate esters of polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether, diphosphate esters of polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether, triesters of polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether, alkyl phosphate esters, alkyl ether phosphate esters or derivatives thereof, polyoxyethylene distyrenated phenyl ether, polyoxyethylene polyoxypropylene copolymer, and polyoxyethylene fatty acid ester. Commercially available phosphate esters include Phosphanol RD-510Y, RB-410, and RS-610 from the Phosphanol series (manufactured by Toho Chemical Industry Co., Ltd.). Commercially available polyoxyethylene distyrenated phenyl ethers include Neugen EA-177 manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Among the anionic surfactants mentioned above, commercially available lauryl sulfate products include Emal 2FG manufactured by Kao Corporation.

[0031] The content of these anionic or nonionic surfactants is preferably 0.03 to 2% by mass, and more preferably 0.2 to 1.0% by mass, relative to the total amount of the ink composition, in order to further moderately mitigate the mild aggregation caused by the neutralized product to achieve suitable mild aggregation and to maintain the effects of the present invention to an even higher degree. By setting the content of these surfactants to 0.03% by mass or more, the effects of the present invention can be further exhibited, while by setting it to 2% by mass or less, the effects of the present invention can be maintained at an even higher level.

[0032] The aqueous ink composition for writing instruments of the present invention contains at least the above-mentioned colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and the remainder being water as a solvent (tap water, purified water, distilled water, ion-exchanged water, pure water, etc.), with a mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) being 0.5 to 20. Other components that do not impair the effects of the present invention may appropriately include dispersants, lubricants, pH adjusters, rust inhibitors, preservatives or antibacterial agents, thickeners, etc.

[0033] Examples of dispersants that can be used include resin compounds such as styrene-acrylic acid copolymers and styrene-maleic acid copolymers, as well as surfactants such as polyoxyethylene alkyl ethers and sodium alkylbenzene sulfonate. Examples of lubricants include nonionic compounds such as fatty acid esters of polyhydric alcohols, higher fatty acid esters of sugars, higher polyoxyalkylene fatty acid esters, and alkyl phosphate esters, which are also used as surface treatment agents for pigments; anionic compounds such as alkyl sulfonates and alkyl allyl sulfonates of higher fatty acid amides; derivatives of polyalkylene glycols; fluorinated surfactants; and polyether-modified silicones.

[0034] 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, and benzimidazole compounds.

[0035] The aqueous ink composition for writing instruments of the present invention contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, and the other components are appropriately combined according to the application of the ink for writing instruments (for ballpoint pens, marking pens, etc.), and the above mass ratio (A) / (B) is adjusted to be within a predetermined range, and the mixture is stirred and mixed using a stirrer such as a homomixer, homogenizer, or disper, and if necessary, coarse particles in the ink composition are removed by filtration or centrifugation to obtain the aqueous ink composition for writing instruments of the present invention. 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.

[0036] Furthermore, the aqueous ink composition for writing instruments of the present invention has been modified to further improve pigment sedimentation resistance and fast writing followability, and to maintain color development and low gloss on the paper surface, by adjusting the viscosity of the ink at 25°C to a shear rate of 38.3 s. -1 In this case, it is preferable that the pressure be between 10 and 550 mPa·s. The viscosity of the ink can be adjusted by suitably combining the amounts of the neutralized substance (A), fibrous polysaccharide (B), water, and other components, the mixing method (ink preparation), and the dispersion equipment.

[0037] The aqueous ink composition for writing instruments of the present invention, configured in this manner, contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water. By setting the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) to 0.5 to 20, the colorant, which is appropriately aggregated by the neutralized product of isobutylene-maleic anhydride copolymer (A), rides on the network of fibrous polysaccharide (B). This results in a highly pigmented line that is unaffected by the underlying color when writing on black paper, and provides an aqueous ink composition for writing instruments that highly balances the following characteristics: 1) resistance to pigment sedimentation, 2) ink followability when writing quickly, 3) color development on the paper surface, and 4) low gloss of the line. Furthermore, the inclusion of anionic surfactants such as alkyl sulfates or nonionic surfactants such as phosphate esters allows for even more appropriate control of the degree of aggregation, thus enabling the further effects of the present invention to be achieved.

[0038] <Writing instruments> The writing instrument of the present invention is characterized by incorporating an aqueous ink composition for writing instruments having the above-described properties. As writing instruments, they are incorporated into ballpoint pens, marking pens, felt pens, etc., which have pen tips such as ballpoint pen tips, fiber tips, felt tips, plastic tips, fiber cores, and porous cores.

[0039] In this invention, "marking pen" means a pen having a mechanism that supplies ink stored in an ink reservoir to a resin writing section by capillary action, and also includes pens referred to as "sign pens." Furthermore, "ballpoint pen" means a pen having a mechanism that causes ink stored in an ink reservoir to seep out by the rotation of a ball provided in the writing section. As a ballpoint pen, the aqueous ink composition for writing instruments having the above composition is contained in an ink reservoir (refill) for ballpoint pens equipped with a ball with a diameter of 0.18 to 2.0 mm, and an ink-following body is added. The ink-following body is a liquid that is incompatible with the aqueous ink composition for writing instruments having the above characteristics contained in the ink reservoir, and has a lower specific gravity than the aqueous ink composition, such as polybutene, silicone oil, or mineral oil. The structure of ballpoint pens, marking pens, etc., is not particularly limited. For example, a direct-ink type ballpoint pen or marking pen may have a collector structure (ink holding mechanism) in which the barrel itself serves as the ink reservoir and the barrel is filled with the aqueous ink composition for writing instruments having the above configuration. The viscosity of the ink at 25°C is given by a shear rate of 38.3 s². -1 In this case, a preferred range is 10 to 550 mPa·s, and the viscosity range of the aqueous ink composition for writing instruments of the present invention can be set to an even more preferred range depending on the type of writing instrument (ballpoint pen, marking pen, etc.). For ballpoint pens, a range of 30 to 550 mPa·s is preferred, and for marking pens (including felt-tip pens), a range of 10 to 70 mPa·s is preferred in that a good writing flow rate can be obtained.

[0040] A writing instrument configured in this way will be equipped with a water-based ink composition for writing instruments having the above-mentioned characteristics. As a result, when writing on black paper or similar materials, a highly vibrant line can be obtained without being affected by the underlying color. This will result in a writing instrument (ballpoint pen, marking pen, felt-tip pen, etc.) that can highly balance the following characteristics: 1) resistance to pigment settling, 2) ink followability when writing quickly, 3) color development on the paper surface, and 4) low gloss lines. [Examples]

[0041] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0042] [Examples 1-24 and Comparative Examples 1-10] A water-based ink composition for ballpoint pens was prepared by conventional methods using the formulation shown in Table 1 below. Similarly, a water-based ink composition for marking pens was prepared by conventional methods using the formulation shown in Table 2 below. The pH of each water-based ink composition for ballpoint pens and marking pens was measured at room temperature (25°C) using a pH meter (manufactured by HORIBA Corporation) and was found to be within the range of 7.9 to 8.2. For each of the aqueous ink compositions for writing instruments (ballpoint pens and marking pens) obtained above, the viscosity value at a shear rate of 38.3 / s at 25°C was measured using the method described below. Furthermore, writing instruments (aqueous ballpoint pens and marking pens) were prepared using the method described below, and their pigment settling resistance, speed-following ability, color development on paper, and low gloss of the drawn lines were evaluated using the evaluation methods described below. These results are shown in Tables 1 and 2 below.

[0043] (Method for measuring viscosity) The viscosity value at a shear rate of 38.3 / s at 25°C was measured using an EMD type viscometer (manufactured by Tokyo Keiki Co., Ltd.).

[0044] (Making a water-based ballpoint pen) Water-based ballpoint pens were prepared using each of the ink compositions obtained above. Specifically, a ballpoint pen barrel (manufactured by Mitsubishi Pencil Co., Ltd., product name: Signo UM-100) was used, and the water-based ink compositions obtained in Examples 1 to 12 and Comparative Examples 1 to 5 were filled into a refill consisting of a polypropylene ink reservoir tube with an inner diameter of 4.0 mm and a length of 113 mm, a stainless steel tip (carbide ball, ball diameter 0.5 mm), and a connector connecting the reservoir tube and the tip. An ink-following body mainly composed of mineral oil was loaded at the rear end of the ink to produce a water-based ballpoint pen.

[0045] (Making felt-tip pens) Marking pens were prepared using each of the ink compositions obtained above. Specifically, marking pens were prepared by loading each of the aqueous ink compositions obtained in Examples 13-24 and Comparative Examples 6-10 into a marking pen [a marking pen manufactured by Mitsubishi Pencil Co., Ltd., product name: PC-5M shaft, shaft material: recycled PP resin, pen tip: PET fiber core].

[0046] (Method for evaluating resistance to pigment settling) Each ink composition obtained above was placed in a test tube, centrifuged at 5000 rpm for 10 minutes, then the ink in the test tube was separated into upper and lower parts, spread, and evaluated according to the following evaluation criteria. Evaluation criteria: A: There is absolutely no difference in concentration between the top and bottom. B: When placed side by side and tested, there are slight differences in color, but they are barely noticeable. C: There is a difference between the top and bottom, with the top being lighter and the bottom being darker.

[0047] [Method for evaluating speed writing responsiveness] Using the water-based ballpoint pens and marking pens mentioned above, freehand writing was performed on ISO-compliant writing paper at 3x speed, 2x speed, and normal speed, and each line was evaluated according to the following evaluation criteria. Evaluation criteria: A: Even when writing at 3x speed, there is absolutely no skipping or smudging, allowing for smooth and stable writing. B: At both normal and double speed writing speeds, there is absolutely no skipping or smudging, allowing for smooth and stable writing. C: Even when writing normally, the ink doesn't follow the writing surface, causing the lines to break.

[0048] (Method for evaluating color development) After writing five spirals on black paper using the water-based ballpoint pen and marking pen mentioned above, the color development of the written lines was visually evaluated based on the following evaluation criteria. Evaluation criteria: A: It is very dark and vibrant, unaffected by the base color. B: Dark color unaffected by the base color C: Affected by the base color, light

[0049] (Method for evaluating the low gloss of drawn lines) Using the water-based ballpoint pens and marking pens mentioned above, 10mm square areas were created freehand on writing paper conforming to ISO standards. The low gloss of the coatings was visually evaluated based on the following evaluation criteria. Evaluation criteria: A: The coating has no gloss and possesses sufficient low gloss. B: The paint film has a slight sheen, and the low gloss is somewhat weak. C: The paint film has a glossy finish.

[0050] [Table 1]

[0051] [Table 2]

[0052] As is clear from the results in Table 1 above, the aqueous ink compositions for ballpoint pens of Examples 1 to 12, which constitute the present invention, and the aqueous ballpoint pens equipped with them, were found to provide highly vibrant lines when writing on black paper or the like without being affected by the underlying color, compared to the aqueous ink compositions for ballpoint pens of Comparative Examples 1 to 5, which are outside the scope of the present invention. Furthermore, they were found to provide aqueous ink compositions for ballpoint pens and aqueous ballpoint pens that highly balance the following characteristics: 1) resistance to pigment sedimentation, 2) ink followability when writing quickly, 3) color development on the paper surface, and 4) low gloss lines. Furthermore, as is clear from the results in Table 2 above, the aqueous ink compositions for marking pens of Examples 13 to 24, which constitute the present invention, and the marking pens equipped with them, compared to the aqueous ink compositions for marking pens of Comparative Examples 6 to 10, which are outside the scope of the present invention, yield highly colored lines without being affected by the underlying color when writing on black paper, and similarly, the aqueous ink compositions for marking pens and marking pens that can be obtained can achieve a high degree of balance between 1) resistance to pigment sedimentation, 2) ink followability when writing quickly, 3) color development on the paper surface, and 4) low gloss lines. [Industrial applicability]

[0053] It can be suitably used in aqueous ink compositions for writing instruments such as ballpoint pens and marking pens.

Claims

1. An aqueous ink composition for writing instruments, characterized in that it contains at least a colorant, a neutralized product of isobutylene-maleic anhydride copolymer (A), a fibrous polysaccharide (B), and water, wherein the mass ratio (A) / (B) of the neutralized product of isobutylene-maleic anhydride copolymer (A) to the fibrous polysaccharide (B) is 0.5 to 20.

2. The aqueous ink composition for writing instruments according to claim 1, further comprising 0.03 to 2% by mass of an anionic or nonionic surfactant.

3. The viscosity of the ink in the aforementioned aqueous ink composition for writing instruments is such that the shear rate is 38.3 s. －１ The aqueous ink composition for writing instruments according to claim 1 or 2, characterized in that the pressure is 10 to 500 mPa·s at that time.

4. A writing instrument characterized by incorporating the aqueous ink composition for writing instruments described in claim 1 or 2.

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