JP7706255B2 - Water-based ballpoint pen ink composition - Google Patents

Description

The present invention relates to an aqueous ink composition for ballpoint pens that has excellent stability over time, does not deteriorate writing performance, and, even with low-viscosity ink, does not cause the so-called "direct current phenomenon" in which the ink drips or leaks due to gravity from the gap between the holder and the ball.

Conventionally, water-based ballpoint pens have had issues such as ink dripping or leaking due to gravity from the gap between the holder and the ball, a phenomenon known as "DC phenomenon."

Known methods for solving this problem include an approach based on the structure of the ballpoint pen tip and an approach based on the ink composition. Of these, the latter approach, for example, is described in Patent Document 1, which is characterized by the inclusion of fine anhydrous silica particles to prevent ink leakage from the pen tip, and in Patent Document 2, which is described in Patent Document 2, it is known that a water-absorbing and swelling spherical polymer compound with a minimum film-forming temperature of 30° C. or less is added to prevent ink leakage.
Furthermore, the invention described in Patent Document 3 relates to an aqueous ballpoint pen in which a ballpoint pen tip that rotatably holds a ball is attached to the tip of an ink containing tube, either directly or via a tip holder, and an aqueous ballpoint pen ink composition containing at least water, a colorant, organic resin particles, and a shear thinning agent is contained in the ink containing tube, the aqueous ballpoint pen being characterized in that the amount of movement of the ball in the ballpoint pen tip in the axial direction is 15 μm or more, and the organic resin particles have an average particle diameter of 15 μm or less.An invention described in Patent Document 4 relates to an ink composition for a ballpoint pen characterized by containing at least crystalline cellulose and resin particles.

However, in the above-mentioned Patent Document 1, particles such as silica have a high specific gravity, making it difficult to maintain them uniformly in the ink over the long term, and in the above-mentioned Patent Document 2, there are several issues, such as the formation of a film at the pen tip, which can cause the ink to dry out.
The above-mentioned Patent Document 3 takes an approach from the viewpoint of the composition and structure of the ink, but in the case of low-viscosity ink, the current situation is that it has not yet been able to sufficiently solve the problem of the "DC phenomenon". The above-mentioned Patent Document 4 is an unconventional technology using crystalline cellulose, which suppresses the occurrence of the DC phenomenon, but there is a further need for an aqueous ballpoint pen ink composition that is excellent in stability over time, does not deteriorate writing performance, and does not cause the "DC phenomenon" even in the case of low-viscosity ink.

On the other hand, as a technology for using a resin emulsion or the like in an ink for a writing instrument, for example, Patent Document 5 discloses an aqueous ink composition for a fabric marking pen, which contains a colorant and a resin emulsion, the glass transition point of the resin contained in the resin emulsion being 20° C. or more and 100° C. or less, and the resin emulsion is a colored resin emulsion colored with the colorant. Patent Document 6 discloses an aqueous ink composition for a writing instrument, which contains a pigment, a guar gum derivative, a poorly water-soluble resin (resin emulsion), and water, characterized in that, when the total content of the pigment relative to the total amount of the ink composition is U and the total content of the guar gum derivative relative to the total amount of the ink composition is V, V/U satisfies 0.0005≦V/U≦0.05, and when the total content of the poorly water-soluble resin relative to the total amount of the ink composition is X, X/V satisfies 5≦X/V≦75, and a writing instrument using the same is also known.
However, the above-mentioned Patent Document 5 provides an aqueous ink composition that provides good resistance to bleeding and washing of handwriting or a coating film written on fabric, and the above-mentioned Patent Document 6 provides an aqueous ink composition that provides excellent color development and fixation of handwriting, which makes it possible to retain a colorant on the fiber surface of a writing surface even when the writing surface has large gaps between the fibers, such as paper or fabric, and thus differs from the present invention in the problem to be solved and the technical concept (the configuration and its action and effect) of the invention.

JP 2001-40266 A (Claims, Examples, etc.) JP 2006-63235 A (Claims, Examples, etc.) JP 2016-124179 A (Claims, Examples, etc.) JP 2013-103986 A (Claims, Examples, etc.) JP 2003-238879 A (claims, examples, etc.) JP 2019-11397 A (Claims, Examples, etc.)

In view of the problems and current state of the prior art described above, the present invention seeks to solve these problems, and aims to provide an ink composition for water-based ballpoint pens that has excellent stability over time, does not reduce writing performance, and does not cause the "DC phenomenon" even in the case of low-viscosity inks, and an aqueous ballpoint pen equipped with the same.

In view of the above-mentioned problems, the present inventors conducted intensive research and discovered that the above-mentioned aqueous ballpoint pen ink composition can be obtained by including at least a coloring material and a specific resin emulsion, thereby completing the present invention.

That is, the water-based ballpoint pen ink composition of the present invention is characterized by containing at least a coloring material and a polyester resin emulsion.
The viscosity at a shear rate of 383 sec ^-1 is preferably 1 to 100 mPa·sec at 25°C.
The polyester resin emulsion preferably has a number average molecular weight (Mn) of 10,000 or more.
The content of the polyester resin emulsion is preferably 0.5 to 30% by mass in terms of solid content concentration relative to the total amount of the ink composition.
The water-based ballpoint pen of the present invention is characterized by being equipped with the water-based ballpoint pen ink composition having the above-mentioned configuration.
It is preferable that the amount of movement of the ball in the ball tip in the axial direction is 15 to 60 μm.

According to the present invention, there are provided an ink composition for an aqueous ballpoint pen, which has excellent stability over time, does not deteriorate writing performance, and does not cause the so-called "DC phenomenon" in which the ink drips or leaks due to gravity from the gap between the holder and the ball, even if the ink is low viscosity, and an aqueous ballpoint pen equipped with the ink composition.
The objects and advantages of the invention will be realized and obtained by means of the elements and combinations particularly pointed out in the claims. Both the foregoing general description and the following detailed description are exemplary and explanatory but are not restrictive of the invention as claimed.

FIG. 1 is a vertical cross-sectional view showing an example of an ink refill for an aqueous ballpoint pen containing the aqueous ballpoint pen ink composition of the present invention. FIG. 2 is an explanatory diagram showing a longitudinal cross-sectional view of a key portion of a ballpoint pen tip to explain the axial movement amount of the ball of the ball tip in the ink refill of FIG. 1 (clearance between the ball of the ballpoint pen tip and the ball receiving seat). 1 is a longitudinal sectional view showing an example of a water-based ballpoint pen of the present invention. FIG. 4 is a front view of the water-based ballpoint pen of FIG.

Hereinafter, the embodiments of the present invention will be described in detail. However, it should be noted that the technical scope of the present invention is not limited to the embodiments described in detail below, but covers the inventions described in the claims and their equivalents.
The aqueous ballpoint pen ink composition of the present invention is characterized by containing at least a colorant and a polyester resin emulsion.

The coloring materials that can be used in the present invention include, without limitation, all dyes that dissolve or disperse in water, conventionally known inorganic and organic pigments such as titanium oxide, resin particle pigments containing pigments, pseudo-pigments in which resin emulsions are colored with dyes, white plastic pigments, pigments having a silica or mica base material and a multilayer coating of iron oxide, titanium oxide, etc. on the surface, thermochromic pigments, photochromic particles, etc.
Examples of dyes include acid dyes such as eosin, fuoxin, 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.

Examples of inorganic pigments 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 oxide yellow, viridian, zinc sulfide, lithopone, cadmium yellow, vermilion, cadmium red, yellow lead, molybdate orange, zinc chromate, strontium chromate, white carbon, clay, talc, ultramarine, precipitated barium sulfate, baryte powder, calcium carbonate, white lead, dark blue, dark blue, manganese violet, aluminum powder, and brass powder, C.I. Pigment Blue 17, C.I. Pigment Blue 15, C.I. Pigment Blue 17, C.I. Pigment Blue 27, C.I. Pigment Red 5, C.I. Pigment Red 22, C.I. Pigment Red 38, C.I. Pigment Red 48, C.I. Pigment Red 49, C.I. Pigment Red 53, C.I. Pigment Red 57, C.I. Pigment Red 81, C.I. Pigment Red 104, C.I. Pigment Red 146, C.I. Pigment Red 245, C.I. Pigment Yellow 1, C.I. Pigment Yellow 3, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 17, C.I. Pigment Yellow 34, C.I. Pigment Yellow 55, C.I. C.I. Pigment Yellow 74, C.I. Pigment Yellow 95, C.I. Pigment Yellow 166, C.I. Pigment Yellow 167, C.I. Pigment Orange 5, C.I. Pigment Orange 13, C.I. Pigment Orange 16, C.I. Pigment Violet 1, C.I. Pigment Violet 3, C.I. Pigment Violet 19, C.I. Pigment Violet 23, C.I. Pigment Violet 50, C.I. Pigment Green 7, etc.

Examples of thermochromic pigments include thermochromic pigments produced by microencapsulating a thermochromic composition containing at least a leuco dye that functions as a color former, a color developer that is a component capable of causing the leuco dye to develop color, and a color change temperature regulator that can control the color change temperature in the color development of the leuco dye and the color developer, so as to have a predetermined average particle size (e.g., 0.1 to 6 μm).
Examples of photochromic particles 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; and 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 an antioxidant, a light stabilizer, and a sensitizer, so as to have a predetermined average particle size (e.g., 0.1 to 6 μm).

In the present invention (including the examples), the "average particle size" is the D50 value measured using a particle size analyzer [Microtrac HRA9320-X100 (manufactured by Nikkiso Co., Ltd.)].
The above-mentioned microencapsulation method of thermochromic pigment can be, for example, interfacial polymerization method, interfacial polycondensation method, in situ polymerization method, liquid curing coating method, phase separation method from aqueous solution, phase separation method from organic solvent, melt dispersion cooling method, air suspension coating method, spray drying method, etc., and can be appropriately selected according to the purpose.For example, in the phase separation method from aqueous solution, after heating and melting, leuco dye, color developer, and color change temperature regulator are put into emulsifier solution, heated and stirred to disperse in oil droplets, and then, as capsule membrane agent, resin raw material whose wall membrane is urethane resin, epoxy resin, amino resin, etc. is used, for example, amino resin solution, specifically, each liquid such as methylol melamine aqueous solution, urea solution, benzoguanamine solution, etc. is gradually put in, and then reacted to prepare, and then this dispersion is filtered to produce thermochromic microencapsulated pigment.In this thermochromic pigment, the type, amount, etc. of leuco dye, color developer, and color change temperature regulator can be suitably combined to set the color development temperature and decolorization temperature of each color to a suitable temperature.
The photochromic particles can be microencapsulated in the same manner as in the production of the thermochromic resin particles.
By suitably using a photochromic dye (compound), a fluorescent dye, or the like, the photochromic particles can be made to have the property of being colorless in an indoor lighting environment (indoor lighting equipment selected from incandescent lamps, fluorescent lamps, lamps, white LEDs, and the like) and of developing color in an ultraviolet ray irradiation environment (irradiation with light having a wavelength of 200 to 400 nm, or irradiation with sunlight containing ultraviolet rays).

These coloring materials can be used alone or in combination of two or more. Among these coloring materials, the average particle diameter of water-dispersible pigments, resin particle pigments, pseudopigments, white plastic pigments, multi-layer coated pigments, thermochromic pigments, photochromic particles, etc. varies depending on the ball diameter, ink composition, viscosity, etc., but it is preferable that the average particle diameter is 0.02 to 6 μm.
The content of these coloring materials can be appropriately increased or decreased depending on the line density of the ink, but it is desirable for the content to be 0.1 to 40% by mass (hereinafter, "% by mass" will be abbreviated as "%"), and preferably 1 to 10% by mass, relative to the total amount of the ink composition.

The polyester resin emulsion used in the present invention is contained in order to obtain the effects of the present invention.
The polyester resin emulsion used can be obtained by using at least a polyhydric alcohol component and a polycarboxylic acid component such as a polycarboxylic acid, a polycarboxylic acid anhydride, or a polycarboxylic acid ester.
Examples of the polyhydric alcohol component include alkylene glycols having 2 to 36 carbon atoms (ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,6-hexanediol, etc.), alkylene ether glycols having 4 to 36 carbon atoms (diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polybutylene glycol, etc.), alicyclic diols having 6 to 36 carbon atoms (1,4-cycloaliphatic diols), Examples of the polyhydric alcohol component include polyhydric alcohols such as cyclohexanedimethanol and hydrogenated bisphenol A, adducts of the above alicyclic diols with alkylene oxides having 2 to 4 carbon atoms (ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO), butylene oxide (hereinafter abbreviated as BO)) (number of moles added: 1 to 30), and adducts of bisphenols (bisphenol A, bisphenol F, bisphenol S, etc.) with alkylene oxides having 2 to 4 carbon atoms (EO, PO, BO, etc.) (number of moles added: 2 to 30). These polyhydric alcohol components may be used alone or in combination of two or more.

Examples of the polycarboxylic acid component include alkane dicarboxylic acids having 4 to 36 carbon atoms (succinic acid, apidic acid, sebacic acid, etc.), alkenyl succinic acids (dodecenyl succinic acid, etc.), alicyclic dicarboxylic acids having 4 to 36 carbon atoms (dimer acids (dimerized linoleic acid), etc.), alkene dicarboxylic acids having 4 to 36 carbon atoms (maleic acid, fumaric acid, citraconic acid, mesaconic acid, etc.), and aromatic dicarboxylic acids having 8 to 36 carbon atoms (phthalic acid, isophthalic acid, terephthalic acid or derivatives thereof, naphthalenedicarboxylic acid, etc.). These polycarboxylic acid components may be used alone or in combination of two or more kinds.

The number average molecular weight (Mn) of the polyester resin is preferably 10,000 or more, and more preferably 10,000 to 100,000. When the number average molecular weight (Mn) of the polyester resin is 10,000 or more, the effect of preventing direct current phenomenon and long-term storage stability are excellent. The number average molecular weight (Mn) is a value measured by gel permeation chromatography (GPC), and can be determined from a calibration curve prepared with a polystyrene standard sample using, for example, Shimadzu Corporation's "RID-6A" (column: Tosoh Corporation's "TSK-GEL", solvent: tetrahydrofuran (THF), column temperature: 40°C).

As the polyester resin emulsion, a commercially available product may be used. Examples of commercially available polyester resin emulsions include ELITEL KA-5034, ELITEL KA-5071S, ELITEL KA-1449, ELITEL KA-0134, ELITEL KA-3556, ELITEL KA-6137, ELITEL KZA-6034, ELITEL KT-8803, ELITEL KT-8701, ELITEL KT-9204, ELITEL KT-8904, ELITEL KT-0507, and ELITEL KT-9511 (all manufactured by Unitika Ltd.). These may be used alone or in combination of two or more types.

The content of these polyester resin emulsions is preferably 0.5 to 30% in terms of solid content concentration, based on the total amount of the ink composition, and more preferably 1 to 10%.
If the solids content of this polyester resin emulsion is less than 0.5%, the effects of the present invention cannot be achieved. On the other hand, if it exceeds 30%, the ink stability over time may decrease and the ink is likely to solidify at the pen tip, reducing the initial writing performance, which is undesirable.

<Water-based ballpoint pen ink composition>
The aqueous ballpoint pen ink composition of the present invention is characterized by containing at least the above-mentioned colorant and polyester resin emulsion, and may further contain a water-soluble solvent, water (tap water, purified water, distilled water, ion-exchanged water, pure water, etc.) as the solvent as the remainder, and may also contain dispersants, lubricants, thickeners, pH adjusters, rust inhibitors, preservatives, antibacterial agents, and the like as appropriate within the scope of the invention.
Examples of the water-soluble solvent that can be used include 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 the water-soluble solvent is preferably 5 to 40% based on the total amount of the ink composition.

Usable dispersants include nonionic and anionic surfactants and water-soluble resins, and preferably water-soluble polymers.
Examples of lubricants that can be used include nonionic lubricants such as fatty acid esters of polyhydric alcohols, higher fatty acid esters of sugars, polyoxyalkylene higher fatty acid esters, and alkyl phosphates, which are also used as surface treatment agents for pigments; anionic lubricants such as phosphoric acid esters, alkyl sulfonates of higher fatty acid amides, and alkyl aryl sulfonates; derivatives of polyalkylene glycols; and polyether-modified silicones.
The usable thickener is preferably at least one selected from the group consisting of synthetic polymers, cellulose, and polysaccharides, and specifically includes gum arabic, tragacanth gum, guar gum, locust bean gum, alginic acid, carrageenan, gelatin, xanthan gum, welan gum, succinoglycan, diutan gum, dextran, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, starch glycolic acid and its salts, polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylic acid and its salts, polyethylene oxide, copolymers of vinyl acetate and polyvinylpyrrolidone, styrene-acrylic acid copolymers and its salts, and the like.

Examples of pH adjusters include ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, alkali metal salts of carbonates and phosphoric acids such as sodium tripolyphosphate and sodium carbonate, and hydrates of alkali metals such as sodium hydroxide. Examples of rust inhibitors include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, saponins, and the like. Examples of preservatives or antibacterial agents include phenol, sodium omadine, sodium benzoate, thiazoline compounds, benzimidazole compounds, and the like.
The above-mentioned components such as dispersants, lubricants, thickeners, pH adjusters, rust inhibitors, preservatives, and antibacterial agents may be used alone or in combination of two or more. In addition, commercially available products of these may be used if available.

The water-based ink composition for ballpoint pens of the present invention can be produced in a manner similar to that of other water-based ink compositions.
That is, the aqueous ink composition for a ballpoint pen of the present invention can be produced by mixing and stirring at least a colorant, a polyester resin emulsion, a water-soluble solvent, and other components using a mixer or the like, and further, for example, a bead mill, a homomixer, a homogenizer, or the like that can apply strong shear, with the stirring conditions set to suitable conditions, and further, if necessary, removing coarse particles in the ink composition by filtration or centrifugation.
Furthermore, the pH (25° C.) of the aqueous ballpoint pen ink composition of the present invention is preferably adjusted to a range of 5 to 10 with a pH adjuster or the like from the viewpoints of usability, safety, the stability of the ink itself, and compatibility with the ink container, and is more preferably adjusted to a range of 6 to 9.5.

The aqueous ballpoint pen ink composition of the present invention is prepared so that the viscosity at a shear rate of 383 sec ^-1 is in the range of 1 to 100 mPa·sec at 25°C.
If the viscosity exceeds 100 mPa·s at the above shear rate, the ink flow rate decreases, the writing feel becomes heavy, and it becomes difficult to write smoothly. On the other hand, if the viscosity is less than 1 mPa·sec, the effect of suppressing bleeding and strike-through of handwriting will not be exerted, which is undesirable.
In the present invention, in order to adjust the viscosity to within the above range at the above shear rate, this can be achieved by suitably combining the resin emulsion, water as a solvent, colorant, etc. to be used and suitably adjusting the contents of each of these.

The aqueous ballpoint pen ink composition of the present invention is loaded into a ballpoint pen equipped with a pen tip portion such as a ballpoint pen tip.
The aqueous ballpoint pen of the present invention may, for example, be one in which the aqueous ballpoint pen ink composition having the above composition is contained in an ink container (refill) for a ballpoint pen equipped with a ball having a diameter of 0.18 to 2.0 mm, and a substance that is incompatible with the aqueous ink composition contained in the ink container and has a low specific gravity relative to the aqueous ink composition, such as polybutene, silicone oil, mineral oil, etc., is contained as an ink follower. The structure of the aqueous ballpoint pen used is not particularly limited as long as it is equipped with a ball having a diameter within the above range, and in particular, it is desirable to fill an ink container made of a polypropylene resin tube with the aqueous ink composition, and finish the refill aqueous ballpoint pen with a stainless steel tip (the ball is made of a super steel alloy) at the tip.
Furthermore, the present invention may be a direct ink type ballpoint pen having a collector structure (ink retention mechanism) in which the barrel itself serves as an ink container and is filled with the aqueous ballpoint pen ink composition having the above-mentioned configuration.

1 to 4 are drawings showing an example of an ink refill equipped with a ballpoint pen tip such as a metal tip or a resin tip, and an example of an aqueous ballpoint pen in which the aqueous ballpoint pen ink composition of the present invention is mounted on a ballpoint pen for use.
An ink refill 10 containing the water-based ballpoint pen ink composition of the present invention may, for example, be one composed of a ballpoint pen tip 15, a joint member 14, and an ink reservoir tube 11, as shown in Fig. 1. The ink reservoir tube 11 is a resin tube such as polypropylene, and contains therein the water-based ballpoint pen ink composition 12 of the present invention, and further contains an ink follower 13 at the rear end of this ink composition 12.
A ballpoint pen tip 15 is attached to the tip of the ink refill 10 via a joint member 14. As shown in Figures 1 and 2, the ballpoint pen tip 15 is composed of a holder 16 and a writing ball 17 made of cemented carbide that is held by the holder 16. The holder 16 is formed in a cylindrical shape and is composed of a tapered portion 16a at its tip end that serves as a ball holding portion having a tapered shape, a connecting portion 16b at its rear end that has a small outer diameter, and a back hole 16c that is formed as an internal space from the rear end of the holder to the middle of the tapered portion 16a. The tip portion of the holder 16 that receives the writing ball 17 is a ball house 16d as shown in Figure 2.
The writing ball 17 is supported by a crimped portion 16e, an ink guide hole 16f and an ink groove 16g which communicate with the back hole 16c, and a ball receiving seat 16h.

In this embodiment of the water-based ballpoint pen, the clearance between the writing ball 17 of the ballpoint pen tip 15 and the ball receiving seat 16h, i.e., the amount of movement (movement distance) L of the writing ball 17 in the axial direction (up and down direction), is preferably 15 to 60 μm from the standpoint of preventing direct ink flow, of properly ejecting ink containing large particle diameter resin particles, and of ensuring good initial writing performance.
It is also preferable that the writing ball 17 has a surface roughness of 1 to 5 nm, from the viewpoint of appropriately transferring ink containing large-particle-size resin particles and ensuring good initial writing performance, and the surface roughness refers to an "average surface roughness Ra value" of 1 to 5 nm measured by the method specified in JIS B 0601:2013.

The ink refill 10 is mounted on a knock-type water-based ballpoint pen 20 as shown in Figs. 3 and 4, for example, and is used. The knock-type water-based ballpoint pen 20 is a compound water-based ballpoint pen in which the writing tip of each of the three ink refills 10 for water-based ballpoint pens (black, red, and blue) can be selectively protruded and retracted from the end 24 of the barrel 23 in which the front barrel 21 and rear barrel 22 are detachably screwed together. The knock mechanism 30 of the water-based ballpoint pen 20 is equipped with knock bodies 32 through which the three water-based ballpoint pen tubes 10 are detachably fitted and through which the springs 31 serving as return springs are inserted, and the ink refills 10 can be selectively protruded and retracted by knocking one of the knock bodies 32 downward, for use. The reference numeral 25 in the figure denotes a grip portion made of a rubber member or an elastomer member, and 26 denotes a clip portion. The ink refill 10 preferably has an inner diameter of 1.5 to 3.0 mm, and the ink follower preferably has a filling length of 5 to 20 mm. The spring 31, which is the return spring that constitutes the knock mechanism, has a spring factor, load during use, and load during storage that fall within the above-mentioned preferred ranges. Furthermore, although a double-type water-based ballpoint pen is shown in Figures 3 and 4, a single-type water-based ballpoint pen may also be used.

The reason why the ink composition for an aqueous ballpoint pen of the present invention, which is configured in this manner, and the aqueous ballpoint pen equipped with the same have excellent stability over time, do not deteriorate in writing performance, and do not cause the so-called "DC phenomenon" even in the case of a low-viscosity ink, is presumably because the polyester emulsion is easily dispersed uniformly in the ink and forms a thin, uniform, and flexible film on the surface of the ink droplets accumulated at the pen tip, thereby effectively achieving both stability over time and writing performance.
The ink composition for an aqueous ballpoint pen of the present invention and the water-based painting ballpoint pen equipped with the same have an extremely excellent sustained effect that allows the effects of the present invention to be exhibited, and the period during which the effects are exhibited and the duration of the effects are long. Furthermore, because the ink composition is aqueous, it also has excellent stability over time.

Next, the present invention will be explained in more detail with reference to Examples 1 to 8 of an ink composition for an aqueous ballpoint pen and an aqueous ballpoint pen equipped with the ink composition, but the present invention is not limited to the following examples.

[Examples 1 to 8]
According to the formulation shown in Table 1 below, each water-based ink composition for a ballpoint pen was prepared by a conventional method.
For each of the obtained aqueous ballpoint pen ink compositions (total amount 100% by mass), the viscosity (mPa·s) at a shear rate of 383 sec ⁻¹ (25° C.) was measured by the following methods, and a direct current test, a writing test (fading, line cracking), and stability over time were evaluated.
The results are shown in Table 1 below.

[Method for measuring viscosity at a shear rate of 383 sec ⁻¹ (25° C.)]
Using an E-type rotational viscometer [VISCOMETER RE215 (manufactured by Toki Sangyo Co., Ltd.)], the ink viscosity (mPa·s) was measured at a cone of 1°34'*R24, a measurement time of 60 seconds, a measurement temperature of 25° C., and a shear rate of 383 sec ⁻¹ .

(DC test evaluation method)
(Water-based ballpoint pen)
Using the shaft of a ballpoint pen [manufactured by Mitsubishi Pencil Co., Ltd., product name: Signo UM-151], a refill consisting of a polypropylene ink reservoir tube with an inner diameter of 3.8 mm and a length of 113 mm conforming to Figure 1, a ballpoint pen tip (holder: stainless steel, ball: cemented carbide ball, ball diameter 0.7 mm), and a joint connecting the reservoir tube and the tip was filled with each of the aqueous ink compositions for ballpoint pens obtained above, and an ink follower made of polybutene was filled into the rear end of the ink to prepare aqueous ballpoint pens (five of each). The amount of movement (movement distance) L of the writing ball 17 in the axial direction (vertical direction) was 30 μm, and the surface roughness (Ra value) was 2 nm.

<DC test>
The above-mentioned water-based ballpoint pens filled with any of the inks of the Examples or Comparative Examples were left uncapped with the writing tip facing down for one day in an environment of 25°C and 60% relative humidity, and were evaluated according to the degree of leakage from the writing tip, using the following evaluation criteria.
Evaluation criteria:
A: No leakage at all.
A': No leakage at all or only slight staining of the ballpoint pen tip.
B: Droplets form around the ballpoint pen tip but do not progress any further.
C: The droplets are getting larger and are still progressing, but they are not dripping.
D: Liquid drips from the ballpoint pen tip.

<Written Exam>
Using the water-based ballpoint pen prepared in the same manner as in the DC test, a spiral line was drawn by hand and evaluated in the following two categories (fading and line cracking) according to the respective evaluation criteria.
(Evaluation Criteria for Blurred Images)
A: There is no smudging at all.
B: There are some slight smudges, but they are not noticeable.
C: There are some smudged areas.
D: The smearing is quite noticeable.

(Evaluation Criteria for Line Cracks)
A: No line cracks at all.
B: There are some slight cracks, but they are not noticeable.
C: Lines are cracked but characters are recognizable.
D: Line cracks are very noticeable.

(Method of evaluating stability over time)
Each of the water-based ballpoint pens obtained above was stored for one month in an environment of 60° C. and 30% relative humidity with the pen tip facing downwards in a capped state. After that, it was left at room temperature for six hours, and after the ballpoint pen had reached the same temperature as room temperature, spiral writing was performed by hand and the writing properties were evaluated according to the following evaluation criteria.
(Evaluation criteria for stability over time)
A: There is no problem with writing.
B: There is no problem with writing, but the writing feel is somewhat heavy.
C: Writability is inferior to that of a scribe.
D: Cannot be written.

As is clear from the results in Table 1 above, it was confirmed that the aqueous ballpoint pens equipped with the aqueous ballpoint pen ink compositions of Examples 1 to 8 within the scope of the present invention have excellent stability over time, do not deteriorate in writing performance (fading, line cracking), and do not cause the so-called "direct current phenomenon" even in the case of low-viscosity inks. In particular, it was confirmed that Examples 1 to 3, which contain polyester resin emulsions with a number average molecular weight (Mn) of 10,000 or more, are significantly superior to Examples 4 to 8.

An ink composition suitable for water-based ballpoint pens is obtained.

Claims (5)

An aqueous ballpoint pen ink composition comprising at least a colorant and a polyester resin emulsion having a number average molecular weight (Mn) of 10,000 or more . 2. The aqueous ballpoint ink composition according to claim 1, which has a viscosity of 1 to 100 mPa·sec at a shear rate of 383 ^{sec −1} at 25° C. 3. The aqueous ink composition for a ballpoint pen according to claim 1 , wherein the content of the polyester resin emulsion is 0.5 to 30% by mass based on the total amount of the ink composition. An aqueous ballpoint pen comprising the ink composition for aqueous ballpoint pens according to any one of claims 1 to 3 . 5. The water-based ballpoint pen according to claim 4 , wherein the amount of movement of the ball in the ball tip in the axial direction is 15 to 60 μm.

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