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JP-2026075929-A - Water-based ink composition for writing instruments

JP2026075929AJP 2026075929 AJP2026075929 AJP 2026075929AJP-2026075929-A

Abstract

[Problem] 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. The viscosity at a shear rate of 383 s⁻¹ is 200 mPa·s or less, and the shear viscosity reduction index at shear rates of 9.6 to 76.6 s⁻¹ is 0.6 or less. [Selection Diagram] None

Inventors

  • 米谷 彩

Assignees

  • 三菱鉛筆株式会社

Dates

Publication Date
20260511
Application Date
20241023

Claims (8)

  1. It contains at least polymer microspheres and inorganic pigments. The viscosity at a shear rate of 383 s⁻¹ is 200 mPa·s or less, and the shear viscosity reduction index at shear rates of 9.6 to 76.6 s⁻¹ 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 movement distance of the ball within the holder of the ballpoint pen tip is 15 to 80 μm.
  8. The water-based ballpoint pen according to claim 6, wherein the ink consumption per unit area when writing on writing paper at a writing speed of 4.5/min is 0.3 mg/ cm² or more.

Description

This invention relates to an aqueous ink composition for writing instruments. Dyes and pigments are known as coloring components in water-based inks used in writing instruments such as water-based ballpoint pens. 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. Furthermore, the dyes themselves have poor lightfastness, leading to the letters and lines deteriorating over time. On the other hand, while pigment-based inks have no problems with water resistance or lightfastness, they present a problem because 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 can be compromised due to the differences in their properties. To solve this problem, ink compositions containing microcapsules encapsulating dyes have been proposed. Patent Document 1 discloses an aqueous ink composition for writing instruments, containing a matrix composed of water and polymer, and colored and uncolored microspheres having a water-insoluble dye. Furthermore, inorganic pigments are sometimes incorporated to obtain lines with characteristic appearances such as brilliance. 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. International Publication No. 2018/042818Japanese Patent Publication No. 2020-23615 Water-based ink composition for writing instruments The present invention provides an aqueous ink composition for writing instruments. It contains at least polymer microspheres and inorganic pigments. The viscosity at a shear rate of 383 s⁻¹ is 200 mPa·s or less, and the shear viscosity reduction index at shear rates of 9.6 to 76.6 s⁻¹ is 0.6 or less. Herein, in this specification, "aqueous ink composition" means an ink composition containing an aqueous medium as its liquid component. 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 viscosity can cause the pen tip to dry out, resulting in a negative impact on writing performance. In contrast, the inventors have found that by using a combination of polymer microspheres and the above-mentioned shear viscosity reduction index—that is, by using it in an aqueous ink composition with a high tendency for shear viscosity reduction of polymer microspheres—sedimentation of inorganic pigments can be suppressed even when inorganic pigments are contained in a low-viscosity aqueous ink. In other words, by having such a combination of polymer microspheres and shear viscosity reduction index despite its low viscosity, it is believed that even when stress other than shear occurs in the pen tip direction, such as centrifugal force, the inorganic pigment can withstand this stress, and sedimentation is suppressed. In particular, when two types of polymer microspheres with different specific gravities are used, the sedimentation and dry-up 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 creating a physical barrier to the inorganic pigment. The viscosity of the aqueous ink composition for writing instruments of the present invention at a shear rate of 383 s⁻¹ may be 200 mPa·s or less. This viscosity may 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 may 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 content of the polymer matrix and/or the content of the thickener. The shear viscosity reduction index of the aqueous ink composition for writing instruments of the present invention at shear rates of 9.6 to 76.6 s⁻¹ 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. Here, the shear viscosity index is a value that indicates the change in apparent viscosity in respons