Water-based ink composition for writing instruments

Abstract

The present invention provides an aqueous ink composition for writing instruments that contains an inorganic pigment while suppressing the sedimentation of the inorganic pigment and drying of the pen tip. [Solution] The aqueous ink composition for writing instruments of the present invention, It contains at least polymer microspheres and inorganic pigments. shear rate 383s -1 The viscosity in is 200 mPa·s or less, and Shear rate 9.6~76.6s -1 The shear viscosity index is 0.6 or less.

Description

[Technical Field]

[0001] This invention relates to an aqueous ink composition for writing instruments. [Background technology]

[0002] Dyes and pigments are known as coloring components in water-based inks used in writing instruments such as water-based ballpoint pens.

[0003] Inks using dyes, especially those using water-soluble dyes, have a problem with poor water resistance. Specifically, the letters and lines can smudge or disappear when exposed to sweat or water, resulting in unclear writing. In addition, the dyes themselves have poor lightfastness, which can cause the letters and lines to deteriorate over time.

[0004] On the other hand, while inks using pigments have no problems in terms of water resistance and lightfastness, they have the problem that different pigments have different properties such as material, size, and specific gravity, requiring different dispersion treatments for each pigment. In particular, when mixing pigments of different hues to obtain a desired hue, the stability of the ink may be compromised due to the differences in their properties. To solve this problem, ink compositions having microcapsules containing dyes have been proposed.

[0005] Patent Document 1 discloses an aqueous ink composition for writing instruments, which contains a matrix composed of water and polymer, and colored microspheres and uncolored microspheres having a water-insoluble dye.

[0006] Furthermore, inorganic pigments are sometimes incorporated to obtain lines with characteristic appearances such as brilliance.

[0007] Patent Document 2 discloses an aqueous ballpoint pen ink composition characterized by containing at least an aluminum pigment and 8 to 25% by mass of dense urethane-based particles with an average particle size of 0.3 to 10 μm. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2018 / 042818 [Patent Document 2] Japanese Patent Publication No. 2020-23615 [Overview of the project] [Problems that the invention aims to solve]

[0009] In ink compositions containing inorganic pigments, it was desirable to suppress the sedimentation of the inorganic pigments and the drying out of the pen tip, i.e., the precipitation of solid matter due to the drying of the pen tip, during long-term storage. However, achieving both of these conditions simultaneously was not easy, and this could negatively affect writing performance.

[0010] The present invention provides an aqueous ink composition for writing instruments that contains an inorganic pigment while suppressing the sedimentation of the inorganic pigment and drying of the pen tip. [Means for solving the problem]

[0011] The inventors, after diligent research, discovered that the above problems could be solved by the following means, and thus completed the present invention. That is, the present invention is as follows: <Aspect 1> Containing at least polymer microspheres and inorganic pigments, shear rate 383s -1 The viscosity in is 200 mPa·s or less, and Shear rate 9.6~76.6s -1 The shear viscosity index in is 0.6 or less. Water-based ink composition for writing instruments. <Aspect 2> The polymer microsphere contains a first polymer microsphere and a second polymer microsphere, and The specific gravity of the first polymer microsphere is greater than the specific gravity of the second polymer microsphere. The aqueous ink composition for writing instruments according to Aspect 1. <Aspect 3> The aqueous ink composition for writing instruments according to Aspect 2, wherein the first polymer microsphere is a urea-urethane-based particle. <Aspect 4> The aqueous ink composition for writing instruments according to any one of Aspects 1 to 3, further containing a water-soluble organic solvent having a boiling point of 180°C or higher. <Aspect 5> The aqueous ink composition for writing instruments according to Aspect 4, wherein the water-soluble organic solvent contains at least one selected from the group consisting of ethylene glycol and glycerin. <Aspect 6> An aqueous ballpoint pen including an ink storage portion having the aqueous ink composition for writing instruments according to any one of Aspects 1 to 5. <Aspect 7> The aqueous ballpoint pen according to Aspect 6, wherein the axial movement distance of the ball within the holder of the ballpoint pen tip is 15 to 80 μm. <Aspect 8> When writing on writing paper at a writing speed of 4.5 / minute, the ink consumption per unit area is 0.3 mg / cm 2 or more. The aqueous ballpoint pen according to Aspect 6 or 7.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide an aqueous ink composition for writing instruments that contains an inorganic pigment while suppressing sedimentation of the inorganic pigment and drying up of the pen tip.

Embodiments for Carrying Out the Invention

[0013] 《Aqueous Ink Composition for Writing Instruments》 The aqueous ink composition for writing instruments of the present invention contains at least polymer microspheres and an inorganic pigment, has a viscosity at a shear rate of 383 s -1 of 200 mPa·s or less, and has a shear thinning index at a shear rate of 9.6 to 76.6 s -1 of 0.6 or less.

[0014] Herein, in this specification, "aqueous ink composition" means an ink composition that includes an aqueous medium as a liquid component.

[0015] Because inorganic pigments have a high specific gravity, when used with water-based inks, it was necessary to increase the viscosity of the water-based ink to suppress sedimentation. However, increasing the viscosity could cause the pen tip to dry out, resulting in a negative impact on writing performance.

[0016] In response to this, the present inventors have found that by using a combination of polymer microspheres and the above-mentioned shear viscosity reduction index, that is, in an aqueous ink composition with a strong tendency to reduce the viscosity of polymer microspheres by shear, the sedimentation of inorganic pigments can be suppressed even when inorganic pigments are contained in a low-viscosity aqueous ink. In other words, it is believed that by having such a combination of polymer microspheres and shear viscosity reduction index despite having low viscosity, the inorganic pigments can withstand stresses other than shear in the direction of the pen tip, such as centrifugal force, and sedimentation is suppressed.

[0017] In particular, when two types of polymer microspheres with different specific gravities are used, the settling and drying resistance of the inorganic pigment are further improved. While we do not wish to be constrained by theory, this is thought to be because these polymer microspheres form a complex dispersion state, thereby further reducing the shear viscosity index while forming a physical barrier to the inorganic pigment.

[0018] The aqueous ink composition for writing instruments of the present invention has a shear rate of 383 s. -1The viscosity can be 200 mPa·s or less. This viscosity can be 200 mPa·s or less, 170 mPa·s or less, 150 mPa·s or less, 130 mPa·s or less, 100 mPa·s or less, 80 mPa·s or less, 70 mPa·s or less, or 65 mPa·s or less, and can also be 10 mPa·s or more, 15 mPa·s or more, 20 mPa·s or more, 25 mPa·s or more, 30 mPa·s or more, 35 mPa·s or more, or 40 mPa·s or more. This viscosity can be measured using an EMD type viscometer (Toki Sangyo Co., Ltd.). Furthermore, this viscosity can be reduced, for example, by reducing the polymer matrix content and / or the thickening agent content.

[0019] The aqueous ink composition for writing instruments of the present invention has a shear rate of 9.6 to 76.6 s. -1 The shear viscosity reduction index is 0.60 or less. This n value can be 0.60 or less, 0.55 or less, 0.50 or less, 0.45 or less, 0.40 or less, 0.35 or less, or 0.30 or less, and can also be 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. This shear viscosity reduction index can be increased by the type of shear viscosity reducing agent used and by increasing the amount added.

[0020] Here, the shear viscosity index is a value that indicates the change in apparent viscosity in response to a change in shear rate, and represents n that satisfies the following relationship. This shear viscosity index can be measured, for example, using a rheometer or rotational viscometer: τ = ηD n (In the formula, τ represents shear stress, D represents shear rate, and η represents apparent viscosity.)

[0021] Furthermore, when the shear viscosity index n=1, it means that the fluid is a Newtonian fluid, that is, the shear stress is proportional to the shear deformation rate, and a small shear viscosity index n means that the fluid has a strong tendency to lose viscosity under shear.

[0022] The residue of the aqueous ink composition for writing instruments of the present invention after standing at 140°C for 15 minutes can be measured using an HC103 halogen moisture meter (manufactured by METTLER TOLEDO) with the standard drying program, temperature 140°C, and drying time 15 minutes. For measurement, set an aluminum moisture measuring dish (AS ONE part number 3-7574-01, diameter 100 x 7 mm) in the halogen moisture meter, take 1 g of ink into a dropper, and drop 1 cm diameter ink droplets into approximately 20 locations, ensuring that they do not overlap.

[0023] The residue after standing at 140°C for 15 minutes can be 15 to 30% by mass. From the viewpoint of line density, it is preferable that this residue be 15% or more by mass, or 18% or more by mass, and from the viewpoint of suppressing pen tip drying out, it is preferable that it be 30% or less by mass, 25% or less by mass, 22% or less by mass, or 20% or less by mass. This residue can be increased by increasing the total mass of solids, i.e., polymer microspheres and inorganic pigments, and can also be increased by adding a non-volatile organic solvent, specifically an organic solvent with a boiling point of 180°C or higher.

[0024] The aqueous ink composition for writing instruments of the present invention may contain optional thickeners and / or shear-thinning agents.

[0025] Furthermore, the aqueous ink composition for writing instruments of the present invention may contain other components as optional.

[0026] The following describes each component of the present invention.

[0027] <Aqueous medium> Generally, aqueous media contain water and optional water-soluble organic solvents.

[0028] In aqueous media, water has the highest liquid content, and in particular, the water content may be more than 50% by mass, 60% or more by mass, 70% or more by mass, 75% or more by mass, or 78% or more by mass relative to the mass of the aqueous media, and may also be 100% or less by mass, 95% or less by mass, or 90% or less by mass.

[0029] Furthermore, the water content may be 50% by mass or more, 55% by mass or more, 60% by mass or more, or 63% by mass or more, relative to the mass of the water-based ink composition for ballpoint pens, and may also be 90% by mass or less, 85% by mass or less, 80% by mass or less, or 75% by mass or less.

[0030] (water) For water, for example, tap water, purified water, distilled water, ion-exchanged water, pure water, etc., can be used.

[0031] (Water-soluble organic solvent) As water-soluble organic solvents, for example, aromatics, alcohols, polyhydric alcohols, ethers, esters, etc., can be used, and among these, polyhydric alcohols are preferred from the viewpoint of writing properties. These solvents may be used individually or in combination.

[0032] Examples of aromatic compounds that can be used include benzyl alcohol, ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, propylene glycol monophenyl ether, diethylene glycol monophenyl ether, and alkylsulfonate phenyl esters.

[0033] Examples of alcohols that can be used include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, n-hexanol, methylamyl alcohol, 2-ethylbutanol, n-heptanol, 2-heptanol, 3-heptanol, n-octanol, 2-octanol, 2-ethylhexanol, 3,5,5-trimethylhexanol, nonanol, n-decanol, undecanol, n-decanol, trimethylnonyl alcohol, tetradecanol, heptadecanol, cyclohexanol, and 2-methylcyclohexanol.

[0034] Examples of polyhydric alcohols that can be used include dihydric alcohols such as ethylene glycol, propylene glycol, 3-methyl-1,3-butanediol, dipropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, hexylene glycol, and octylene glycol, as well as trihydric alcohols such as glycerin.

[0035] Examples of ethers that can be used include methyl isopropyl ether, ethyl ether, ethyl propyl ether, ethyl butyl ether, isopropyl ether, butyl ether, hexyl ether, 2-ethylhexyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethyl butyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, 3-methyl-3-methoxy-1-butanol, 3-methoxy-1-butanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, propylene glycol-t-butyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monobutyl ether, and tetrapropylene glycol monobutyl ether.

[0036] Examples of esters include propylene glycol methyl ether acetate, propylene glycol diacetate, 3-methyl-3-methoxybutyl acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether acetate, butyl formate, isobutyl formate, isoamyl formate, propyl acetate, butyl acetate, isopropyl acetate, isobutyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, propyl propionate, isobutyl propionate, isoamyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, propyl isobutyrate, Methyl valerate, ethyl valerate, propyl valerate, methyl isovalerate, ethyl isovalerate, propyl isovalerate, methyl trimethylacetate, ethyl trimethylacetate, propyl trimethylacetate, methyl caproate, ethyl caproate, propyl caproate, methyl caprylate, ethyl caprylate, propyl caprylate, methyl laurate, ethyl laurate, methyl oleate, ethyl oleate, caprylic acid triglyceride, tributyl citrate, octyl oxystearate, propylene glycol monolicinolate, methyl 2-hydroxyisobutyrate, 3-methoxybutyl acetate, etc. can be used.

[0037] The content of water-soluble organic solvent may be 3% by mass or more, 5% by mass or more, 7% by mass or more, 10% by mass or more, or 12% by mass or more, based on the total mass of the aqueous ink composition for writing instruments, and may also be 30% by mass or less, 25% by mass or less, 20% by mass or less, or 17% by mass or less.

[0038] In particular, from the viewpoint of resistance to drying up, it is preferable to use an organic solvent with a boiling point of 180°C or higher as the water-soluble organic solvent. As such a water-soluble organic solvent, it is preferable to use a polyhydric alcohol, especially ethylene glycol and glycerin.

[0039] <Polymer microspheres> Polymer microspheres are contained in the aqueous ink composition for writing instruments of the present invention.

[0040] The polymer microspheres may be styrene-based particles, urethane-based particles, urea-based particles, urea-urethane-based particles, melamine-based particles, or olefin-based particles. That is, they may have a polymer matrix consisting of styrene polymer, urethane polymer, urea polymer, urea-urethane polymer, melamine polymer, acrylic polymer, or olefin polymer.

[0041] The polymer microspheres may be colored or uncolored. If the polymer microspheres are colored, they may further contain, in particular contain, water-insoluble dyes. In this case, the content of the water-insoluble dye in the polymer microspheres may be 10% by mass or more, 15% by mass or more, 20% by mass or more, or 30% by mass or more, and may also be 45% by mass or less, 40% by mass or less, or 35% by mass or less. This content can be calculated by (parts by mass of water-insoluble dye) / (parts by mass of water-insoluble dye + parts by mass of polymer) × 100.

[0042] Furthermore, the polymer microspheres may further contain a resin having an OH group.

[0043] The average particle size of polymer microspheres measured by laser diffraction may be, for example, between 0.3 μm and 10.0 μm. This average particle size may be between 0.3 μm and 0.5 μm or 1.0 μm, and may also be between 10.0 μm and 9.0 μm or 8.0 μm or 7.0 μm or 6.0 μm or 5.0 μm or 4.0 μm or 3.0 μm or 2.5 μm or 2.0 μm. The average particle size referred to here is the D50 value calculated based on volume in the laser diffraction method.

[0044] Polymer microspheres can be obtained by known methods, for example, the method described in Patent Document 1.

[0045] Furthermore, commercially available polymer microspheres may be used in this invention. Commercially available products include MP-1000 (polymethyl methacrylate, Soken Chemical), Epostor S (melamine-formaldehyde condensate, Nippon Shokubai), Nylon SP (nylon, Toray), Vinyl Chloride #121 (vinyl chloride, Nippon Zeon), MH5055 (Nippon Zeon), SX863(A), SX864(B), SX865(B) (all from JSR), Lowpeak OP-62, OP-84J, OP-91, Mutille PP120, Mutille PP240D, VONCOAT sPP-2000S, VONCOAT PP-1000, VONCOAT PP-1001, VONCOAT PP-1100 (DIC), CERAFLOUR series, AQUACER series, and AQUAMAT series (polyolefin, manufactured by Big Chemie Co., Ltd.).

[0046] The polymer microsphere content may be, for example, 5% by mass or more and 30% by mass or less, relative to the mass of the aqueous ink composition for writing instruments. From the viewpoint of suppressing the settling of inorganic pigments, a content of 5% by mass or more, 7% by mass or more, 10% by mass or more, 12% by mass or more, or 15% by mass or more is preferred, and from the viewpoint of not excessively increasing the viscosity of the aqueous ink composition for writing instruments, a content of 30% by mass or less, 25% by mass or less, 22% by mass or less, 20% by mass or less, or 18% by mass or less is preferred.

[0047] (Water-insoluble dye) Water-insoluble dyes are dyes that are insoluble in water at room temperature. Examples include salt-forming dyes, disperse dyes, and oil-soluble dyes. However, from the viewpoint of color development, salt-forming dyes are preferred.

[0048] Examples of salt-forming dyes include dyes having chemical structures such as azo, metal complex azo, anthraquinone, and metal phthalocyanine. For example, Valifast® Black 1807, Valifast® Blue 2620, Valifast® Brown 2402, Valifast® Green 1501, Valifast® Orange 2210, Valifast® Pink 2310, Valifast® Red 1355, Valifast® VIOLET 1701, and Valifast® Yellow 1101 from Orient Chemical Industry Co., Ltd. can be used.

[0049] As a disperse dye, at least one dye selected from, for example, CDisperse Yellow 198, CDisperse Yellow 42, CDisperse Red 92, CDisperse Violet 26, CDisperse Violet 35, CDisperse Blue 60, and CDisperse Blue 87 can be used.

[0050] As oil-soluble dyes, for example, Oil Black 860, Oil Blue 613, Oil Brown BB, Oil Green 530, Oil Orange 201, Oil Pink 312, Oil Red 5B, Oil Scarlet 318, Oil Yellow 105 from Orient Chemical Industry Co., Ltd. can be used.

[0051] (resin) Resins containing OH groups may be included in the matrix.

[0052] Examples of the resin having an OH group include terpene phenol resin, rosin phenol resin, alkyl phenol resin, phenol novolak resin, cresol novolak resin, butyral resin, polyvinyl alcohol resin, polyol-modified xylene resin, ethylene oxide-modified xylene resin, maleic acid resin, hydroxyl group-modified acrylic resin, hydroxyl group-modified styrene acrylic resin, carboxyl group-modified acrylic resin, carboxyl group-modified styrene acrylic resin, and the like.

[0053] Among the above resins having an OH group, it is preferable to use a resin having a phenolic OH group, such as terpene phenol resin, rosin phenol resin, alkyl phenol resin, phenol novolak resin, etc., from the viewpoint of enhancing the color density of the aqueous ink composition for writing instruments.

[0054] Among the resins having a phenolic OH group, it is preferable to use terpene phenol resin from the viewpoint of enhancing the color density of the aqueous ink composition for writing instruments. Here, the terpene phenol resin means a copolymer of terpene and phenol. Here, "terpene" refers to a group of compounds having a structure in which a plurality of isoprene units are bonded, such as monoterpene (C 10 ), sesquiterpene (C 15 ), diterpene (C 20 ), sesterterpene (C 25 ), triterpene (C 30 ), tetraterpene (C<000001​​​​​​​​​​The fact that the specific gravity of the first polymer microsphere is greater than that of the second polymer microsphere can be confirmed by adding equal amounts of the first and second polymer microspheres to water, stirring to uniformly disperse them, leaving the mixture at room temperature for three days, and visually inspecting its appearance. For example, if the first polymer microsphere is colored and the second polymer microsphere is uncolored, a difference in color intensity will occur.

[0057] Therefore, for example, if the first polymer microsphere is urea-urethane particles, the second polymer microsphere is preferably selected from the group consisting of olefin-based particles, acrylic-based particles, or styrene-acrylic particles.

[0058] The first and second polymer microspheres may be colored polymer microspheres or uncolored polymer microspheres. For example, the first polymer microsphere may be a colored polymer microsphere and the second polymer microsphere may be an uncolored polymer microsphere.

[0059] The ratio of the mass of the first polymer microsphere to the total mass of the first and second polymer microspheres may be, for example, 0.60 or more and 0.90 or less. This ratio may be 0.60 or more, 0.65 or more, 0.70 or more, 0.75 or more, or 0.80 or more, and may also be 0.90 or less, or 0.85 or less.

[0060] <Inorganic pigments> Inorganic pigments are contained in the aqueous ink composition for writing instruments of the present invention.

[0061] Examples of inorganic pigments that can be used include inorganic coloring pigments, inorganic extender pigments, and inorganic luminescence pigments.

[0062] Examples of inorganic coloring pigments that can be used include 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, and manganese violet.

[0063] Examples of inorganic pigments that can be used include talc, muscovite, phlogopite, red mica, biotite, synthetic mica, sericite, synthetic sericite, kaolin, silicon carbide, smectite, aluminum oxide, magnesium oxide, zirconium oxide, antimony oxide, diatomaceous earth, aluminum silicate, magnesium aluminum metasilicate, calcium silicate, barium silicate, magnesium silicate, calcium carbonate, magnesium carbonate, hydroxyapatite, boron nitride, and silicon dioxide.

[0064] Examples of inorganic luminous pigments that can be used include titanium dioxide-based pigments, iron oxide-coated mica, bismuth oxychloride, fish scale foil, aluminum pigments, and brass powder.

[0065] As titanium dioxide-based pigments, not only pigments containing titanium dioxide as the majority component can be used, but also pigments in which titanium dioxide is coated on the surface, and pigments in which titanium dioxide is used as the mother particle and resin or other colorants other than titanium dioxide are coated on the surface. Examples of such titanium dioxide-based pigments include commercially available titanium dioxide pigments, as well as titanium black, titanium mica, titanium dioxide-coated mica, titanium dioxide-coated synthetic fluorphlogopite, titanium dioxide-coated bismuth oxychloride, iron oxide-coated titanium mica, Prussian blue-treated titanium mica, carmine-treated titanium mica, and titanium dioxide-coated glass powder. These titanium dioxide pigments may be used individually or in mixtures.

[0066] Examples of commercially available titanium dioxide pigments include R-25, R-62N, and R-38L from Sakai Chemical Industry Co., Ltd., Ti-Pure TS-6300, Ti-Pure TS-900, Ti-Pure R-706, and Ti-Pure R-931 from Chemours, and R-630, R-550, and R-980 from Ishihara Sangyo Co., Ltd.

[0067] Commercially available aluminum pigments can be used, such as 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 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.). These can be used individually or in combination of two or more.

[0068] The major axis diameter of the inorganic pigment may be, for example, 5 μm or more and 20 μm or less. This major axis diameter may be 5 μm or more, 7 μm or more, 9 μm or more, or 10 μm or more, and may also be 20 μm or less, 18 μm or less, or 15 μm or less. This major axis diameter can be measured by image analysis of images obtained with a scanning electron microscope (SEM). In this invention, the major axis diameter is the arithmetic mean of any 10 particles obtained by measuring the point where the length of the particle is maximum when observed with a scanning electron microscope.

[0069] The inorganic pigment content may be, for example, 1% by mass or more and 20% by mass or less, relative to the mass of the aqueous ink composition for writing instruments. This content may be 1% by mass or more, or 2% by mass or more, and may also be 20% by mass or less, 15% by mass or less, 12% by mass or less, 10% by mass or less, 8% by mass or less, 6% by mass or less, 5% by mass or less, or 4% by mass or less.

[0070] <Shear viscosity reducing agent> As a shear viscosity reducing agent, at least one selected from known substances can be used, specifically, water-dispersible gums such as rheozan gum, gellan gum, xanthan gum, and succinoglycans; organic thickeners such as cross-linked acrylic acid polymers; and inorganic shear viscosity reducing agents such as organoclay, hectonite clay, bentonite clay, and montmorillonite clay.

[0071] The content of the shear viscosity reducing agent can be, for example, 0.1% by mass or more and 0.6% by mass or less, relative to the mass of the aqueous ink composition for writing instruments. This content can be, for example, 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.4% by mass or more, and can also be 0.6% by mass or less, or 0.5% by mass or less.

[0072] <Thickening agent> As a thickening agent, at least one selected from known sources, specifically, polysaccharides such as cellulose derivatives and crystalline cellulose, alkali-swelling-associating emulsions, alkali-swelling emulsions, polyvinylpyrrolidone, and organic thickening agents such as cross-linked acrylic acid polymers, can be used.

[0073] The content of the thickening agent can be, for example, 0.1% by mass or more and 0.6% by mass or less, relative to the mass of the aqueous ink composition for writing instruments. This content can be, for example, 0.1% by mass or more, 0.2% by mass or more, 0.3% by mass or more, or 0.4% by mass or more, and can also be 0.6% by mass or less, or 0.5% by mass or less.

[0074] <Other ingredients> Other components include, for example, dispersants, lubricants, pH adjusters, rust inhibitors, preservatives, and antibacterial agents.

[0075] A water-soluble resin is used as the dispersant. Preferably, a water-soluble polymer is used as the dispersant.

[0076] As a lubricant, lubricants used in surface treatment agents for pigments can be used, such as nonionic surfactants like polyhydric alcohol fatty acid esters, sugar fatty acid esters, polyoxyalkylene fatty acid esters, and alkyl phosphate esters; anionic surfactants like phosphate esters, alkyl sulfonates of higher fatty acid amides, and alkyl allyl sulfonates; derivatives of polyalkylene glycols; and polyether-modified silicones.

[0077] Examples of pH adjusters include ammonia, urea, amines such as monoethanolamine, diethanolamine, and triethanolamine, alkali metal salts of carbonic acid and phosphoric acid such as sodium tripolyphosphate and sodium carbonate, and hydrates of alkali metal salts such as sodium hydroxide.

[0078] Furthermore, benzotriazole, toltriazole, dicyclohexylammonium nitride, saponins, etc., can be used as rust inhibitors. Phenol, sodium omazine, sodium benzoate, thiazoline compounds, benzimidazole compounds, etc., can be used as preservatives or antibacterial agents.

[0079] The other ingredients listed above may be used individually or in combination of two or more. Commercially available products containing these ingredients may also be used.

[0080] Water-based ballpoint pen The ballpoint pen of the present invention comprises at least an ink storage section, a writing section having a ball, and a holding section. The above-mentioned aqueous ink composition for writing instruments is stored in the ink storage section. It's a ballpoint pen.

[0081] By using the above-described aqueous ink composition for writing instruments, it is possible to provide a ballpoint pen that is less prone to corrosion of the pen tip even after long-term storage, and that does not skip or smudge when writing begins.

[0082] The ink consumption per unit area when writing on writing paper at a writing speed of 4.5 m / min is 0.3 mg / cm³. 2 The above is possible. The ink consumption is 0.3 mg / cm². 2 More than 0.4mg / cm 2 Above or equal to 0.5 mg / cm³ 2 It can be any of the above.

[0083] <Ink storage section> The ink storage section contains the above-mentioned aqueous ink composition for writing instruments.

[0084] The ink reservoir can be any object that can store ink and supply ink to the writing section, and may be a direct-ink type with a collector structure (ink holding mechanism), or a cotton-filled ballpoint pen.

[0085] <Writing Department> The writing section may be a writing section having a ballpoint pen tip at its front.

[0086] A ballpoint pen tip may consist of a ball and a holder that rotatably holds the ball. The ball may be made of any material used for ballpoint pen balls, such as stainless steel, cemented carbide, or ceramics. The shape of the ballpoint pen tip is not particularly limited and may be, for example, bullet-shaped or needle-shaped.

[0087] The axial travel distance of the ball within the holder may be 15 to 80 μm. This travel distance may be 15 μm or more, 20 μm or more, 25 μm or more, or 30 μm or more, and may also be 80 μm or less, 75 μm or less, 70 μm or less, 65 μm or less, 60 μm or less, 55 μm or less, or 50 μm or less.

[0088] Furthermore, from the viewpoint of writing quality, it is desirable that the surface roughness Ra of the ball be less than 10 nm, and particularly preferably, the writing ball is characterized by having a surface roughness Ra of 4 nm or less.

[0089] In this invention (including the embodiments described later), the "surface roughness Ra" was measured using a non-contact surface shape measuring instrument (NewView7200, Zygo) with a lens magnification of 50x, an evaluation length of 100 μm, and a Gaussian filter of 25 μm. All other measurements were performed in accordance with JIS B0601 (Geometrical Characteristics Specifications of Products - Surface Properties).

[0090] The water-based ballpoint pen of the present invention is less prone to tip corrosion during long-term storage, and as a result, skipping at the start of writing can be suppressed. Therefore, the water-based ballpoint pen of the present invention is particularly beneficial when using a ball with a low surface roughness. [Examples]

[0091] The present invention will be specifically described by examples and comparative examples, but the present invention is not limited thereto.

[0092] Preparation of water-based ink compositions for writing instruments The substances shown in Table 1 were mixed in the proportions shown in Table 1 to prepare the aqueous ink compositions for writing instruments of the examples and comparative examples.

[0093] The "colored urea-urethane particles" shown in Table 1 are particles obtained by the method described in Patent Document 1, using a urea-urethane polymer as the polymer matrix and containing a magenta dye. The average particle size of the colored urea-urethane particles was 1.0 μm.

[0094] Furthermore, details of the other substances shown in Table 1 are as follows: Colorless urea-urethane particles: Polymer microspheres from which the dye has been removed from colored urea-urethane particles, with an average particle size of 1.0 μm. Colorless styrene-acrylic particles: ROPAQUE MP-1040 (DOW Corporation) Colorless olefin particles: AQUACER498 (manufactured by BIC Chemie Japan Co., Ltd.) Luminous pigment (10 μm): Alpaste EMRD5660 (manufactured by Toyo Aluminum Co., Ltd.) Luminous pigment (7μm): Alpaste WXM-0630 (manufactured by Toyo Aluminum Co., Ltd.) Titanium dioxide pigment: CR-95 (manufactured by Ishihara Sangyo Co., Ltd.) Polyglyceryl ether: Polyoxyethylene (13) polyglyceryl ether (SC-E750, Sakamoto Pharmaceutical Co., Ltd.) pH adjuster: aminomethylpropanol

[0095] Physical properties of water-based ink compositions for writing instruments <viscosity> The viscosity of the prepared ink was measured using an EMD type viscometer (Toki Sangyo Co., Ltd.) at a temperature of 25°C and a shear rate of 383 s⁻¹. -1 The measurements were taken under the following conditions.

[0096] <Shear thinning index> Using a Modular Compact Rheometer MCR302 (Anton Paar), shear rates of 9.6 to 76.6 -1 The shear viscosity index was measured.

[0097] <Remainder> The prepared ink was left to stand at 140°C for 15 minutes, and the residual amount was calculated from the mass before and after standing.

[0098] The residual moisture content was measured using an HC103 halogen moisture meter (METTLER TOLEDO) with a standard drying program, a temperature of 140°C, and a drying time of 15 minutes. For the measurement, an aluminum moisture measuring dish (AS ONE part number 3-7574-01, diameter 100 x 7 mm) was placed in the halogen moisture meter, 1 g of ink was taken with a dropper, and 1 cm diameter ink droplets were dropped into approximately 20 locations, ensuring that the droplets did not overlap.

[0099] <Is separation possible?> Equal amounts of the two types of microspheres used in Examples 1-6 and Comparative Examples 3-5 were placed in water and stirred to uniformly disperse them. These were then left to stand at room temperature for 3 days, and the presence or absence of color variations was visually confirmed. In Table 1, samples where color variations were observed are marked "Acceptable," and samples where no color variations were observed are marked "Unacceptable."

[0100] Making a ballpoint pen Using the barrel of a ballpoint pen (Signo UM-151, Mitsubishi Pencil Co.), a refill consisting of a polypropylene ink reservoir tube with an inner diameter of 3.8 mm and a length of 113 mm, a ballpoint pen tip (holder: stainless steel, ball: cemented carbide ball, ball diameter 0.38 mm), and a connector connecting the reservoir tube and the tip was filled with the above-mentioned inks. An ink-following body made of polybutene was filled into the rear end of the ink, and the refill was degassed by centrifugation (500 G, 5 minutes) to produce five water-based ballpoint pens.

[0101] The axial travel distance of the ball within the holder was 35 μm.

[0102] <Settling Resistance> After the centrifugal treatment described above, water-based ballpoint pens were left at 25°C with the pen tip facing vertically downwards for 6 months. The sedimentation resistance was then evaluated by conducting mechanical writing tests using these water-based ballpoint pens. Specifically, a writing test machine conforming to JIS S6039-2001 was used, and the mechanical writing test was performed by spiral writing on the test paper under the following conditions: writing speed of 4.5 m / min, writing angle of 60°, and writing load of 0.98 N. The test paper was high-quality paper (basis weight range 40-157 g / m²) made from 100% chemical pulp, conforming to JIS P3201. 2 (with a whiteness of 75.0% or higher) was used.

[0103] The sedimentation resistance was evaluated by visually observing the density of the lines drawn at any point between 0 and 100 m in the machine writing test described above. The evaluation criteria are as follows: A: No smudging was observed in the lines. B: Slight smudging was observed in the lines. C: Significant smudging was observed in the lines drawn.

[0104] <Initial writing characteristics after high temperature and prolonged time> The prepared ballpoint pens were left to stand for 1.5 months at 50°C and 0% relative humidity. Then, a line approximately 25 cm long was drawn by hand on the aforementioned writing paper, and the writing feel was evaluated subjectively. The evaluation criteria were as follows: A: No smudging occurred at all. B: Although some smudging occurred, the lines were still clearly recognizable. C: Smudging occurred to such an extent that it significantly affected the recognition of the drawn lines.

[0105] <Ink Consumption> The ink consumption per unit area was calculated from the lines obtained from the machine writing test described above. Specifically, the mass of the ballpoint pen before writing (W1) was measured, and the mass of the ballpoint pen after writing 100m on writing paper using a ballpoint pen writing test machine under the following conditions (W2) was measured, and the ink outflow amount (W1-W2) was calculated (evaluated). Furthermore, the "ink consumption per unit area" was calculated by dividing the ink consumption per unit length by the line width.

[0106] Table 1 shows the configurations and evaluation results of the examples and comparative examples.

[0107] [Table 1]

[0108] Table 1 shows that the aqueous ink compositions for writing instruments of the examples, which contain at least polymer microspheres and inorganic pigments, have a viscosity of 200 mPa·s or less, and a shear reduction index of 0.6 or less, can suppress the sedimentation of inorganic pigments and the drying up of the pen tip.

[0109] In particular, it can be seen that when the polymer microspheres contain a first polymer microsphere and a second polymer microsphere, and these are separable by centrifugation, the sedimentation of the inorganic pigment and the drying up of the pen tip are especially good.

Claims

1. It contains at least polymer microspheres and inorganic pigments. Shear rate 383s －１ The viscosity in is 200 mPa·s or less, and Shear rate 9.6-76.6s －１ The shear viscosity index in is 0.6 or less. Water-based ink composition for writing instruments.

2. The polymer microsphere contains a first polymer microsphere and a second polymer microsphere, and The specific gravity of the first polymer microsphere is greater than the specific gravity of the second polymer microsphere. The aqueous ink composition for writing instruments according to claim 1.

3. The aqueous ink composition for writing instruments according to claim 2, wherein the first polymer microspheres are urea-urethane particles.

4. The aqueous ink composition for writing instruments according to claim 1 or 2, further containing a water-soluble organic solvent with a boiling point of 180°C or higher.

5. The aqueous ink composition for writing instruments according to claim 4, wherein the water-soluble organic solvent contains at least one selected from the group consisting of ethylene glycol and glycerin.

6. A water-based ballpoint pen comprising an ink storage section having the water-based ink composition for writing instruments described in claim 1 or 2.

7. The water-based ballpoint pen according to claim 6, wherein the axial travel distance of the ball within the holder of the ballpoint pen tip is 15 to 80 μm.

8. The ink consumption per unit area when writing on writing paper at a writing speed of 4.5 minutes is 0.3 mg / cm². ２ The water-based ballpoint pen described in claim 6 is as described above.

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