JP2024076078A - Aqueous ink composition for writing instruments and writing instrument with aqueous ink composition accommodated therein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aqueous ink composition for writing instruments that maintains excellent dispersion stability of a microcapsule pigment for an extended period, and forms superior handwriting with reduced blurring and line skipping, and a writing instrument with the aqueous ink composition accommodated therein.

SOLUTION: An aqueous ink composition for writing instruments contains a microcapsule pigment composed of a core substance and a wall membrane enclosing the core substance, polyether phosphoester as a dispersant, cellulose nanofibers, and water. The resin constituting the wall membrane is urea resin, urethane resin, or urea urethane resin. The core substance is a coloring composition composed of a colorant and a medium. A writing instrument has the aqueous ink composition accommodated therein.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to an aqueous ink composition for a writing instrument and a writing instrument containing the same. More specifically, the present invention relates to an aqueous ink composition for a writing instrument that has excellent dispersion stability of a microencapsulated pigment and can produce good handwriting, and a writing instrument containing the same.

Conventionally, inks using water as the main solvent (water-based inks) have been known and are widely used because of their low odor and high safety. In addition, water-based inks using pigments such as titanium oxide and carbon black as ink colorants are widely used because of their excellent light resistance and water resistance. Usually, these pigments have a high specific gravity and are unstable in dispersion stability in water. If the pigments are not uniformly dispersed and kept in a stable state, aggregation and sedimentation will occur, and the density of the handwriting formed by the writing instrument containing the ink will decrease, and the ink ejection from the pen tip will decrease, causing writing defects such as skipped lines and blurred lines, making it difficult to obtain sufficient performance as a water-based ink for writing instruments. Therefore, an ink composition has been disclosed in which titanium oxide or carbon black is encapsulated in microcapsules to improve dispersion stability in water-based ink (see, for example, Patent Document 1).

Patent Document 1 discloses an aqueous ink composition for writing instruments that contains a pigment such as titanium oxide or carbon black, and a microencapsulated pigment that encapsulates a poorly water-soluble medium having a specific gravity of less than 1 at a temperature below 20°C.
However, the dispersion stability of the microencapsulated pigment in the above-mentioned aqueous ink composition is insufficient, and it has been difficult to suppress aggregation and sedimentation of the microencapsulated pigment over a long period of time.

Also, an ink composition containing a thermochromic microcapsule pigment encapsulating a thermochromic composition consisting of a leuco dye, a color developer, and a color change temperature regulator is known (for example, Patent Document 2). Handwriting formed using a writing instrument containing this ink composition can be discolored by heating or rubbing, but since the thermochromic composition, which is a coloring material, is encapsulated in microcapsules, it tends to be difficult to obtain a high handwriting density compared to other writing instruments in which the coloring material is not encapsulated in microcapsules. In order to increase the handwriting density, it is possible to increase the blending ratio of the microcapsule pigment, but increasing the blending ratio tends to reduce the dispersion stability of the microcapsule pigment in the ink composition. Furthermore, it has been difficult to stably maintain the microcapsule pigment in the ink composition for a long period of time.

JP 2017-122168 A JP 2014-5422 A

The present invention aims to provide a water-based ink composition for writing instruments that is less susceptible to aggregation or sedimentation over time and can produce good handwriting, and a writing instrument that contains the same.

The present invention relates to an aqueous ink composition for a writing instrument, which comprises a microencapsulated pigment comprising a core substance and a wall membrane encapsulating the core substance, a polyether phosphate ester as a dispersant, cellulose nanofibers, and water.
Further, the requirements are that the resin constituting the wall film is any one of a urea resin, a urethane resin, and a urea-urethane resin, that the core substance is a coloring composition consisting of a coloring material and a medium, that the cellulose nanofiber is blended in an amount within the range of 1.5 to 12 parts by mass per 100 parts by mass of the microcapsule pigment, and that the polyether phosphate ester is blended in an amount within the range of 1 to 30 parts by mass per 100 parts by mass of the microcapsule pigment.
A further feature is a writing instrument containing the above-mentioned water-based ink composition for a writing instrument.
Another requirement is that the writing instrument is a ballpoint pen.

The present invention provides a water-based ink composition for writing instruments that has excellent dispersion stability of microencapsulated pigments over a long period of time, suppresses smearing and skipped lines, and produces good handwriting, as well as a writing instrument that contains the same.

1 is a graph illustrating hysteresis characteristics in a color density-temperature curve of a heat-discolorable, reversible thermochromic composition. 1 is a graph illustrating the hysteresis characteristic in a color density-temperature curve of a heat-discolorable, reversible thermochromic composition having color memory properties. 1 is a graph illustrating the hysteresis characteristic in a color density-temperature curve of a reversible thermochromic composition that develops color upon heating.

The aqueous ink composition for writing instruments according to the present invention (hereinafter sometimes referred to as "ink composition" or "ink") comprises a microencapsulated pigment consisting of a core substance and a wall membrane encapsulating the core substance, a polyether phosphate ester as a dispersant, cellulose nanofibers, and water. Each component constituting the ink composition according to the present invention is described below.

The ink composition according to the present invention contains a microencapsulated pigment as a colorant.
A microcapsule pigment has a core substance encapsulated in a wall film formed from a wall-forming material.

The core substance may be a coloring composition consisting of a coloring material and a medium. An example of the coloring composition is a coloring composition in which a dye or pigment as a coloring material is dissolved or dispersed in an aqueous medium or an oil-based medium.

Examples of the dye include acid dyes, basic dyes, direct dyes, oil-soluble dyes, and disperse dyes.
Examples of the pigment include inorganic pigments, organic pigments, glittering pigments, fluorescent pigments, and phosphorescent pigments. If necessary, a pigment dispersant can be used. Examples of the pigment dispersant include anionic and nonionic surfactants; anionic polymers such as polyacrylic acid and styrene-acrylic acid; and nonionic polymers such as PVP and PVA.

Examples of the aqueous medium include water, such as tap water, ion-exchanged water, ultrafiltered water, and distilled water.
Examples of oil-based media include esters such as monobasic acid esters, dibasic acid monoesters, dibasic acid diesters, partial esters or complete esters of polyhydric alcohols, aromatic hydrocarbons such as alkylbenzenes and alkylnaphthalenes, higher alcohols, ketones, ethers, etc.
The aqueous medium or oily medium can be used alone or in combination of two or more kinds.

As the coloring composition, a photochromic material that changes color upon irradiation with light can also be used. This color change may be reversible or irreversible, but a reversible photochromic material is preferred because it can repeatedly change color upon irradiation with light.
An example of a photochromic material used as a coloring composition is a coloring composition in which a photochromic compound as a coloring material is dissolved in an oligomer as a medium, that is, a reversible photochromic composition consisting of at least a photochromic compound and an oligomer.

Examples of photochromic compounds include conventionally known spirooxazine derivatives, spiropyran derivatives, naphthopyran derivatives, and the like that develop color when irradiated with sunlight, ultraviolet light, or blue light having a peak emission wavelength in the range of 400 to 495 nm, and disappear when the irradiation is stopped. For example, the compounds described in JP 2021-120493 A and WO 2020/137469 A can be exemplified.
Furthermore, a photochromic compound having a photomemory property (color memory photochromic property) can also be used. Examples of such photochromic compounds include diarylethene derivatives, such as those described in JP-A-2021-120493.

Examples of the oligomer include styrene-based oligomers, acrylic-based oligomers, terpene-based oligomers, and terpene phenol-based oligomers.
By dissolving the photochromic compound in various oligomers, it is possible to improve both the light resistance and color density, and further to adjust the color change sensitivity.
The oligomers can be used alone or in combination of two or more.

The styrene oligomer is a compound having a styrene skeleton or a hydrogenated product thereof, and examples thereof include low molecular weight polystyrene, styrene-α-methylstyrene copolymer, α-methylstyrene polymer, and α-methylstyrene-vinyltoluene copolymer.
The acrylic oligomer may, for example, be an acrylic acid ester copolymer.
The terpene oligomer is a compound having a terpene skeleton, and examples thereof include α-pinene polymer, β-pinene polymer, and d-limonene polymer.
The terpene phenol oligomer is a compound obtained by copolymerizing a cyclic terpene monomer with a phenol or a hydrogenated product thereof, and examples thereof include α-pinene-phenol copolymers.

As the coloring composition, a thermochromic material that changes color due to a change in temperature can also be used. This color change may be reversible or irreversible, but a reversible thermochromic material is preferred because it can repeatedly exhibit color changes due to temperature changes.
The thermochromic material used as the coloring composition may be, for example, a coloring composition comprising at least (A) an electron-donating organic color-forming compound as the coloring material and (B) an electron-accepting compound as the medium.Furthermore, a coloring composition comprising at least the (A) component as the coloring material, the (B) component as the medium, and (C) a homogeneous compatible mixture of the reaction medium that determines the temperature at which the coloring reaction of the (A) component and the (B) component occurs, i.e., a reversible thermochromic composition comprising at least (A) an electron-donating organic color-forming compound, (B) an electron-accepting compound, and (C) a reaction medium that determines the temperature at which the coloring reaction of the (A) component and the (B) component occurs, may be exemplified.

As the reversible thermochromic composition, a heat-discoloring type reversible thermochromic composition having a relatively small hysteresis width (ΔH) (ΔH = 1 to 7°C) described in JP-B-51-44706, JP-B-51-44707, JP-B-1-29398, etc., can be used. Heat-discoloring type means that the composition is discolored by heating and colored by cooling. This reversible thermochromic composition discolors around a certain temperature (discoloration point), and exhibits a discolored state in a temperature range above the high-temperature discoloration point and a colored state in a temperature range below the low-temperature discoloration point. Of these two states, only one specific state exists in the room temperature range, and the other state is maintained while the heat or cold required to manifest that state is applied, but returns to the state it exhibits in the room temperature range when the application of heat or cold is removed (see Figure 1).

As the reversible thermochromic composition, there can be used a heat-discolorable type reversible thermochromic composition having a large hysteresis width (ΔH=8 to 80° C.) as described in JP-B-4-17154, JP-A-7-179777, JP-A-7-33997, JP-A-8-39936, JP-A-2005-1369, etc. The heat-discolorable type means that the color disappears when heated and the color appears when cooled. This reversible thermochromic composition changes color along a path that is significantly different when the temperature is increased from a lower temperature side than the color-changing temperature range from when the temperature is decreased from a higher temperature side than the color-changing temperature range, and has color memory in a specific temperature range [the temperature range between the color-changing onset temperature _t2 and the color-discoloring onset temperature _t3 (the temperature range where two phases are essentially maintained)] in a colored state in a temperature range below the complete color-changing temperature _t1 or in a decolored state in a high temperature range above the complete decolorizing temperature _t4 (see FIG. 2).

In addition, when the reversible thermochromic composition having the above-mentioned color memory property is applied to the present invention, the reversible thermochromic composition can effectively function to retain the color exhibited under normal conditions (daily living temperature range) by specifying the complete color development temperature _t1 to a temperature that can only be obtained in a freezer or a cold region, and the complete decolorization temperature _t4 to a temperature range that can be obtained from frictional heat produced by a friction body or a familiar heating body such as a hair dryer, and specifying the ΔH value to be 40 to 100°C.

The temperature that can only be obtained in a freezer or in a cold region is in the range of -50 to 0°C, preferably -40 to -5°C, and more preferably -30 to -10°C.
The temperature obtainable from a common heating element such as a hair dryer is in the range of 50 to 95°C, preferably 50 to 90°C, and more preferably 60 to 80°C.

As a reversible thermochromic composition, a heat-coloring type reversible thermochromic composition using a gallic acid ester, as described in JP-B-51-44706 and JP-A-2003-253149, can also be used. Heat-coloring type means that the color develops when heated and disappears when cooled (see Figure 3).

The reversible thermochromic composition is a compatible solution containing the above components (A), (B), and (C) as essential components. The ratio of each component depends on the concentration, color change temperature, color change form, and type of each component, but the component ratio that generally provides the desired properties is 1 part of component (A) to 0.1 to 100, preferably 0.1 to 50, and more preferably 0.5 to 20, of component (B), and 1 to 800, preferably 5 to 200, and more preferably 5 to 100, and even more preferably 10 to 100, of component (C) (all the above ratios are in parts by mass).

By encapsulating the reversible thermochromic material or the reversible photochromic material in a microcapsule to form a reversible thermochromic microcapsule pigment or a reversible photochromic microcapsule pigment, the microcapsule pigment can be made chemically and physically stable. Furthermore, the reversible thermochromic material or the reversible photochromic material can maintain the same composition under various conditions of use and can exert the same effects.

Microencapsulated pigments can also contain various additives such as antioxidants, ultraviolet absorbers, infrared absorbers, dissolution aids, preservatives, and fungicides, as long as they do not affect their functionality.

The microencapsulated pigment according to the present invention can be produced by a microencapsulation method. Examples of the microencapsulation method include the conventionally known isocyanate-based interfacial polymerization method, the in situ polymerization method using a melamine-formaldehyde system, the liquid curing coating method, the phase separation method from an aqueous solution, the phase separation method from an organic solvent, the melt dispersion cooling method, the air suspension coating method, and the spray drying method, and can be appropriately selected according to the application.

Examples of resins that make up the wall film include urea resin, urethane resin, urea-urethane resin, epoxy resin, melamine resin, benzoguanamine resin, isocyanate resin, etc.

Depending on the purpose, a secondary resin film can be further provided on the surface of the microencapsulated pigment according to the present invention to impart durability or modify the surface properties for practical use.

When the microencapsulated pigment according to the present invention is a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, the mass ratio of inclusions to wall film is preferably 7:1 to 1:1, and by having the mass ratio of inclusions to wall film within the above range, it is possible to prevent a decrease in color density and clarity during color development. More preferably, the mass ratio of inclusions to wall film is 6:1 to 1:1.

Reversible thermochromic microencapsulated pigments or reversible photochromic microencapsulated pigments can also be made into microencapsulated pigments that exhibit color change behavior from color (1) to color (2) by incorporating a non-color-changing colorant such as a general dye or pigment into the microcapsules.

The average particle size of the microencapsulated pigment is not particularly limited, but is preferably in the range of 0.1 to 5 μm, more preferably 0.3 to 5 μm, even more preferably 0.3 to 4 μm, and particularly preferably 0.5 to 3 μm. If the average particle size exceeds 5 μm, it becomes difficult to obtain good ink ejection properties when used in an ink composition for a writing instrument. On the other hand, if the average particle size is less than 0.1 μm, it becomes difficult for the handwriting to show high-density color development.

The average particle diameter was measured by determining the particle region using image analysis particle size distribution measurement software (manufactured by Mountec Co., Ltd., product name: MacView), calculating the diameter equivalent to a projected area circle (Heywood diameter) from the area of the particle region, and measuring the average particle diameter of particles equivalent to a sphere of equal volume using this value.

In addition, if the particle size of all or most of the particles exceeds 0.2 μm, it is also possible to measure the average particle size of particles equivalent to an equal volume sphere by the Coulter method using a particle size distribution measuring device (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.).

Furthermore, the volumetric particle size and average particle size may be measured using a calibrated laser diffraction/scattering particle size distribution measuring device (manufactured by Horiba, Ltd., product name: LA-300) based on values measured using the above-mentioned software or a measuring device using the Coulter method.

The blending ratio of the microencapsulated pigment is not particularly limited, but the microencapsulated pigment is blended in the range of preferably 1 to 25 mass %, more preferably 3 to 20 mass %, of the total amount of the ink composition. If the blending ratio exceeds 25 mass %, the ink discharge properties of the writing instrument containing the ink composition are likely to decrease, and writing defects such as smearing and skipped lines are likely to occur. On the other hand, if the blending ratio is less than 1 mass %, it becomes difficult to obtain a suitable writing density for the writing instrument.

When the microencapsulated pigment is a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, the microencapsulated pigment is preferably blended in an amount of 5 to 40% by mass, more preferably 10 to 40% by mass, and even more preferably 10 to 30% by mass, based on the total amount of the ink. If the blending ratio exceeds 40% by mass, the ink ejection properties of the writing instrument containing the ink composition will decrease, and writing defects such as smearing and skipped lines will be more likely to occur. On the other hand, if the blending ratio is less than 5% by mass, it will be difficult to obtain the color change and writing density suitable for a writing instrument, and it will be difficult to fully fulfill the color change function.

The ink composition according to the present invention contains a polyether phosphate ester as a dispersant.
Polyether phosphate ester is a polyether phosphate ester, a compound having multiple phosphate ester groups in one molecule, and the phosphate ester groups act as functional groups that adsorb to the microencapsulated pigment. The polyether phosphate ester is adsorbed to the microencapsulated pigment at multiple points to form a network that extends throughout the ink composition, thereby exerting an effect as a dispersant for the microencapsulated pigment.

The polyether phosphate ester is not particularly limited as long as it is a polyether phosphate ester known to be usable as a dispersant, and commercially available products can also be used.
Specific examples of polyether phosphate esters include DISPARLON 3500 (manufactured by Kusumoto Chemical Industries, Ltd.), DISPARLON DA-375 (manufactured by Kusumoto Chemical Industries, Ltd.), DISPARLON DA-325 (manufactured by Kusumoto Chemical Industries, Ltd.), DISPARLON AQ-320 (manufactured by Kusumoto Chemical Industries, Ltd.), DISPARLON AQ-330 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-152 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-153 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-154 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-118 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-153 ...18 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-153 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD ED-153 (manufactured by Kusumoto Chemical Industries, Ltd.), H ED-174 (manufactured by Kusumoto Chemical Industries Co., Ltd.), HIPLAAD ED-251 (manufactured by Kusumoto Chemical Industries Co., Ltd.), Plysurf A215C (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), Neoscore CM57 (manufactured by Toho Chemical Industry Co., Ltd.), Phosphanol RA-600 (manufactured by Toho Chemical Industry Co., Ltd.), Phosphanol ML-240 (manufactured by Toho Chemical Industry Co., Ltd.), Phosphanol RS-610 (manufactured by Toho Chemical Industry Co., Ltd.), Phosphanol RS-710 (manufactured by Toho Chemical Industry Co., Ltd.), ADEKACOL TS series (manufactured by ADEKA Corporation), ADEKACOL CS series (manufactured by ADEKA Corporation), DISPERBYK-180 (manufactured by BYK Japan Co., Ltd.), and the like can be illustrated.

In addition to being excellent as a dispersant, polyether phosphate ester is also excellent in that it is unlikely to penetrate into the microencapsulated pigment and is unlikely to enter the microcapsules and cause the core substance to dissolve or precipitate. Therefore, by using a polyether phosphate ester as a dispersant for the microencapsulated pigment, it is possible to improve the dispersion stability of the microencapsulated pigment in the ink composition and the storage stability of the microencapsulated pigment.
Here, the core material (reversible thermochromic composition) of the reversible thermochromic microcapsule pigment is a homogeneous compatible body having the above-mentioned components (A), (B), and (C) as essential components, and when the dispersant penetrates into the microcapsule pigment, any of the components may dissolve or precipitate, causing the reversible thermochromic microcapsule pigment to have a lower color density in the colored state, or to have a discolored state in a temperature range above the high-temperature discoloration point (complete discoloration temperature) and a colored state in a temperature range below the low-temperature discoloration point (complete discoloration temperature), and the reversible thermochromic function of being in a colored state in a temperature range below the low-temperature discoloration point (complete discoloration temperature) may be impaired, making it difficult to cause a reversible color change from a colored state to a discolored state. However, by using a polyether phosphate ester as a dispersant for the reversible thermochromic microcapsule pigment, the reversible thermochromic composition is easily maintained in a homogeneous state in the microcapsule, and any of the components (A), (B), and (C) is suppressed from dissolving or precipitating, so that it is preferable to use the reversible thermochromic microcapsule pigment as a colorant in the ink composition according to the present invention.

The resin constituting the wall of the microcapsule pigment is preferably a resin selected from urea resin (polyurea), urethane resin (polyurethane), and urea-urethane resin (polyurea-urethane). When the resin constituting the wall is one of the above-mentioned resins, the polyether phosphate ester is more easily adsorbed onto the surface of the microcapsule pigment, and the polyether phosphate ester exerts an excellent effect as a dispersant, while the polyether phosphate ester is more difficult to penetrate into the microcapsule, and the dispersion stability and storage stability of the microcapsule pigment can be highly compatible.

The polyether phosphate ester is preferably blended in an amount of 1 to 30 parts by weight, more preferably 2 to 26 parts by weight, per 100 parts by weight of the microencapsulated pigment. By blending the polyether phosphate ester in an amount within the above range relative to the microencapsulated pigment, the polyether phosphate ester is more likely to exert its effect as a dispersant for the microencapsulated pigment.

The ink composition according to the present invention contains cellulose nanofibers (hereinafter sometimes referred to as "CNF").
Cellulose nanofiber is a material obtained by untangling (defibrating) plant fibers such as wood fibers (pulp) to the nano level.
The cellulose nanofibers form a network structure in the ink composition due to the interaction between the cellulose nanofibers, and have the effect of stably retaining the microencapsulated pigment to which the dispersant is adsorbed in the ink composition.

The average fiber length of the cellulose nanofibers is not particularly limited, but is preferably in the range of 100 to 1000 nm, more preferably 200 to 800 nm, and even more preferably 250 to 700 nm. When the average fiber length is within the above range, the cellulose nanofibers have excellent dispersibility in the ink composition and the dispersed state is easily maintained, and the network structure formed by the cellulose nanofibers is stabilized.
If the average fiber length is greater than 1000 nm, the dispersibility of the cellulose nanofibers in the ink tends to be low, making it difficult to eject the ink from the pen tip of a writing instrument. On the other hand, if the average fiber length is less than 100 nm, it becomes difficult for the cellulose nanofibers to form a network structure.

Average fiber length refers to the number-average fiber length. The number-average fiber length can be measured using known techniques. For example, the fiber lengths of 150 or more fibers (e.g., 150 fibers) are measured from an atomic force microscope image (3000 nm x 3000 nm) of cellulose nanofibers fixed on a mica slice, and the number-average fiber length (average fiber length) is calculated. The fiber length is measured using image analysis software "WinROOF" (manufactured by Mitani Shoji Co., Ltd.) in the length range of 100 nm to 2000 nm.

In the ink composition according to the present invention, if the ink composition contains a large amount of cellulose nanofiber or if the ink composition uses a light-colored microencapsulated pigment, the hue of the cellulose nanofiber may affect the hue of the ink composition. Therefore, in order to obtain transparency when the cellulose nanofiber is dispersed in water, it is preferable to adjust the average fiber length of the cellulose nanofiber to a range of preferably 100 to 1000 nm, more preferably 200 to 800 nm, and even more preferably 250 to 700 nm. By having the average fiber length within the above range, the hue of the cellulose nanofiber is less likely to affect the hue of the ink composition, and the ink composition is more likely to exhibit the hue derived from the microencapsulated pigment.

Here, an ink composition containing a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment as a colorant reversibly changes color from a colored state to a decolorized state upon temperature change or exposure to light, and since the ink composition is colorless in the decolorized state, the color of the ink composition is difficult to see. However, if the hue of the cellulose nanofiber affects the hue of the ink composition, the residual color in the decolorized state may become large, and the color of the ink composition may be visible even in the decolorized state. Therefore, even when such a microencapsulated pigment is used, it is preferable that transparency is obtained when the cellulose nanofiber is dispersed in water, and it is preferable that the average fiber length of the cellulose nanofiber is within the above range.

The ink composition according to the present invention uses a polyether phosphate ester as a dispersant in combination with cellulose nanofibers, thereby achieving the effect of improving the dispersion stability of the microencapsulated pigment in the ink composition.
As described above, the polyether phosphate ester acts as a dispersant for the microencapsulated pigment by adsorbing to the microencapsulated pigment at multiple locations, and a network structure is formed by the microencapsulated pigment and the polyether phosphate ester in the ink composition. Furthermore, a network structure is also formed by the cellulose nanofibers dispersed in the ink composition. As a result, the network structure of the microencapsulated pigment and the polyether phosphate ester and the network structure of the cellulose nanofibers are mutually entangled to form a dense network structure, which prevents the microencapsulated pigments from contacting each other in the ink composition, leading to the microencapsulated pigment being stably held. In other words, the combined use of the polyether phosphate ester and the cellulose nanofibers has the effect of improving the dispersion stability of the microencapsulated pigment.

The average fiber diameter of the cellulose nanofibers is not particularly limited, but is preferably in the range of 1 to 10 nm, and more preferably 2 to 5 nm. When the average fiber diameter is within the above range, the cellulose nanofibers tend to interact with each other, and the transparency is excellent when dispersed in water.

The average fiber diameter refers to the number average fiber diameter. The number average fiber diameter can be measured using known techniques, for example, in a manner similar to the measurement of the number average fiber length described above.

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 100 to 400, more preferably 110 to 350.

When cellulose nanofibers have a relatively small fiber diameter and a relatively large aspect ratio, they tend to interact with each other more easily, which has the effect of further improving the dispersion stability of the microencapsulated pigment.

As the cellulose nanofiber, for example, TEMPO oxidized cellulose nanofiber can be used.
TEMPO-oxidized cellulose nanofibers can be obtained by treating wood fibers with a catalyst called TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) to convert the primary hydroxyl groups of cellulose to carboxyl groups, and then mechanically defibrating the fibers.

The cellulose nanofiber is preferably blended in an amount of 1.5 to 12 parts by mass, more preferably 2 to 11 parts by mass, and even more preferably 2 to 10 parts by mass, per 100 parts by mass of the microencapsulated pigment. By blending the cellulose nanofiber in an amount within the above range relative to the microencapsulated pigment, the ink composition can have a low viscosity while still stably retaining the microencapsulated pigment.

Conventionally, in order to improve the dispersion stability of the microencapsulated pigment, the ink has been made highly viscous by using a thickener such as fine cellulose or xanthan gum. This can suppress the aggregation and sedimentation of the microencapsulated pigment, but such a high viscosity ink is limited to the writing implement to which it can be applied. However, as described above, the ink composition according to the present invention uses polyether phosphate ester and cellulose nanofiber in combination to form a dense network structure, and while it has a lower viscosity than ink compositions using conventional thickeners alone, it can stably retain the microencapsulated pigment for a long period of time. Therefore, by using polyether phosphate ester and cellulose nanofiber in combination, a rheology control effect different from that of conventional thickeners is exerted, and the effect of improving the dispersion stability of the microencapsulated pigment over time is achieved. In addition, since the viscosity can be made lower than that of ink compositions using conventional thickeners alone, the ink dischargeability of the writing implement containing this ink composition is improved, and writing defects such as smearing can be suppressed and the writing feel can be improved.

The ink composition according to the present invention is also effective for microencapsulated pigments with a large specific gravity that are particularly prone to settling in the ink composition, and can stably hold the microencapsulated pigments with a large specific gravity while maintaining the viscosity of the ink composition low. In addition, since the polyether phosphate ester used as a dispersant is unlikely to penetrate into the microencapsulated pigment as described above, it is preferable to use a reversible thermochromic microencapsulated pigment with a large hysteresis (ΔH) as the microencapsulated pigment in the ink composition according to the present invention.
A reversible thermochromic microencapsulated pigment with a large hysteresis (ΔH) often uses a compound having two or more benzene rings in the molecule as the component (c), so that the specific gravity is likely to be large, and the pigment is likely to settle over time in the ink composition, and may have poor dispersion stability. In particular, when the ink composition has a low viscosity, the pigment tends to have poorer dispersion stability. However, the ink composition according to the present invention can suppress the settling over time of such a reversible thermochromic microencapsulated pigment with a large specific gravity by using the above-mentioned polyether phosphate ester and cellulose nanofiber in combination. Furthermore, the reversible thermochromic microencapsulated pigment can be stably dispersed for a long period of time while the ink composition has a low viscosity.

The ink composition according to the present invention contains water.
The water is not particularly limited, but examples thereof include tap water, ion-exchanged water, ultrafiltered water, distilled water, and the like.

The ink composition of the present invention can also contain a thickener. By using a polyether phosphate ester, cellulose nanofibers, and a thickener in combination, the ink composition can have a lower viscosity than ink compositions that use a conventional thickener alone, while still maintaining the microencapsulated pigment in a stably dispersed state for a long period of time.
As the thickener, it is preferable to use a substance capable of imparting shear thinning properties to the ink composition (shear thinning agent).

Ink compositions using shear thinning agents have high viscosity and are difficult to flow when left at rest or under low stress, but easily become less viscous when external stress is applied. This makes it possible to prevent ink leakage, separation, and backflow when not writing, and makes it easy to improve the stability of ink ejection from the pen tip when writing.

When the ink composition according to the present invention contains a thickener, the blending ratio of the thickener is not particularly limited, but the thickener is preferably blended in the range of 0.1 to 20 mass % based on the total amount of the ink composition.

Examples of shear thinning agents include water-soluble polysaccharides, polymers having a molecular weight of 100,000 to 150,000 that are mainly composed of alkyl esters of methacrylic acid, crosslinked poly-N-vinyl carboxylic acid amides, benzylidene sorbitol and its derivatives, benzylidene xylitol and its derivatives, alkali-thickening acrylic resins, crosslinked acrylic acid polymers, inorganic fine particles, nonionic surfactants with an HLB value of 8 to 12, metal salts and amine salts of dialkylsulfosuccinic acid, and the like.
The shear thinning agents may be used alone or in combination of two or more.

Examples of water-soluble polysaccharides include xanthan gum, welan gum, zeta sea gum, diutan gum, macrophomopsis gum, succinoglycan, guar gum, locust bean gum and its derivatives, hydroxyethyl cellulose, alkyl alginates, glucomannan, and carbohydrates with gelling properties extracted from seaweed such as agar and carrageenan.

When the ink composition according to the present invention is used in a writing instrument (ballpoint pen) equipped with a ballpoint pen tip, the ink composition may also be blended with a lubricant. The lubricant improves the lubricity between the ball seat provided inside the tip body and the ball provided at the front end of the tip body, making it possible to easily prevent wear of the ball seat and improve the writing feel.
Examples of the lubricant include higher fatty acids such as oleic acid, nonionic surfactants having a long-chain alkyl group, and polyether-modified silicone oil.

The ink composition according to the present invention may also contain various additives, such as water-soluble organic solvents, polymeric flocculants, dispersants, water-soluble resins, specific gravity adjusters, surfactants, pH adjusters, rust inhibitors, preservatives or fungicides, air bubble absorbers, defoamers, antioxidants, and ultraviolet absorbers, as required.

When the microencapsulated pigment contains a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, it is possible to make an ink composition that exhibits color change behavior from color (1) to color (2) by blending a non-color-changing colorant such as a general dye or pigment.

The method for producing the ink composition according to the present invention is not particularly limited, and any conventionally known method can be used.
Specifically, the ink composition can be produced by stirring a mixture of the above-mentioned components with various stirrers such as a propeller stirrer, a homodisper, or a homomixer, or by dispersing the mixture with various dispersers such as a bead mill.

When the ink composition according to the present invention is used in a ballpoint pen, the viscosity thereof is preferably in the range of 1 to 2000 mPa·s, more preferably 3 to 1500 mPa·s, and even more preferably 100 to 1000 mPa·s, when measured in an environment of 20° C. and at a rotation speed of 100 rpm (shear rate of ³⁸⁴ sec ⁻¹ ). Also, the viscosity is preferably in the range of 1 to 200 mPa·s, more preferably 10 to 100 mPa·s, and even more preferably 10 to 50 mPa·s, when measured in an environment of 20° C. and at a rotation speed of 100 rpm (shear rate of 384 sec −1 ). By having the viscosity within the above range, the stability of the ink composition and the free flowability of the ink composition within the mechanism of the ballpoint pen can be maintained at a high level.
The viscosity of the ink composition was measured using a rheometer (manufactured by TA Instruments, product name: Discovery HR-2, cone plate (diameter 40 mm, angle 1°)) at a rotation speed of 1 rpm (shear rate 3.84 sec ^-1 ) or a rotation speed of 100 rpm (shear rate 384 sec ^-1 ) in an environment of 20°C.

When the ink composition according to the present invention is used in a marking pen, the viscosity thereof is preferably in the range of 1 to 30 mPa·s, more preferably 1 to 20 mPa·s, and even more preferably 1 to 10 mPa·s, when measured under conditions of an environment of 20° C. and a rotation speed of 20 rpm. By having the viscosity within the above range, the stability and fluidity of the ink composition can be maintained at a high level.
The viscosity of the ink composition is a value measured by using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34'×R24)) with the ink composition placed in an environment of 20° C.

The pH of the ink composition according to the present invention is preferably in the range of 3 to 10, more preferably 4 to 9. By keeping the pH within the above range, it is possible to suppress excessive increase in viscosity and deterioration of the ink composition.
The pH of the ink composition was measured by placing the ink in an environment of 20° C. using a pH meter (manufactured by DKK-TOA Corp., product name: IM-40S).

Examples of writing instruments that can contain the ink composition of the present invention include ballpoint pens, marking pens, fountain pens, brush pens, calligraphy pens, and other writing instruments.

When the ink composition according to the present invention is used in a ballpoint pen, the structure and shape of the ballpoint pen itself are not particularly limited, and it can be used, for example, by filling a ballpoint pen refill or ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism.

A ballpoint pen tip consists of a tip body and a ball attached to the front end of the tip body. Examples of ballpoint pen tips include a tip in which the ball is held by a ball holding section formed by pressing inward from the outer surface near the tip of a tip body made of a metal pipe, a tip in which the ball is held by a ball holding section formed by cutting with a drill or the like on a tip body made of a metal material, a tip with a resin ball receiving seat provided inside a metal or plastic tip body, and a tip in which the ball held by the tip is biased forward by a spring body.

The material of the tip body and ball is not particularly limited, and examples include cemented carbide (super hard), stainless steel, ruby, ceramic, resin, rubber, etc. Furthermore, the ball can be subjected to a surface treatment such as DLC coating.

The diameter of the ball is generally in the range of 0.2 to 3 mm, preferably 0.2 to 2 mm, more preferably 0.2 to 1.5 mm, and even more preferably 0.2 to 1 mm.
Generally, when an ink composition containing a microencapsulated pigment is applied to a ballpoint pen equipped with a small diameter ball, the microencapsulated pigment aggregates or settles over time, causing clogging at the pen tip due to aggregates of the microencapsulated pigment, and the ink dischargeability from the pen tip may decrease, resulting in impaired handwriting density and poor writing such as smearing and skipped lines. However, when the ink composition according to the present invention, which has excellent dispersion stability of the microencapsulated pigment, is applied to a ballpoint pen equipped with a small diameter ball, particularly a ball with a diameter of 0.3 to 0.5 mm, the dispersibility of the microencapsulated pigment is stably maintained over a long period of time, making it possible to provide a ballpoint pen in which the ink dischargeability from the pen tip is unlikely to decrease and poor writing such as smearing and skipped lines is suppressed.
Generally, a ballpoint pen with a large diameter ejects a large amount of ink from the pen tip, and allows writing with a smooth writing feel. When the ink composition according to the present invention, which has a lower viscosity than ink compositions using a conventional thickener alone and can stably hold a microencapsulated pigment for a long period of time, is applied to a ballpoint pen equipped with a large diameter ball, particularly a ball with a diameter of 0.5 to 1.0 mm, the ink ejection property from the pen tip is improved, allowing writing with a smoother writing feel and forming clear handwriting with high handwriting density.

An example of the ink filling mechanism is an ink reservoir that can be directly filled with ink.
The ink reservoir may be a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a tubular body made of metal.

A ballpoint pen refill (hereinafter sometimes referred to as "refill") can be formed by connecting a ballpoint pen tip directly or via a connecting member to the ink container and directly filling the ink container with ink. A ballpoint pen can be formed by storing this refill inside the barrel.

The rear end of the ink filled into the ink container is filled with an ink backflow prevention body. Examples of ink backflow prevention bodies include a liquid plug or a solid plug.

The liquid plug is made of a non-volatile liquid and/or a difficult-to-volatile liquid, examples of which include petrolatum, spindle oil, castor oil, olive oil, refined mineral oil, liquid paraffin, polybutene, α-olefin, α-olefin oligomer or cooligomer, dimethyl silicone oil, methylphenyl silicone oil, amino-modified silicone oil, polyether-modified silicone oil, fatty acid-modified silicone oil, and the like.
The non-volatile liquid and/or the hardly-volatile liquid may be used alone or in combination of two or more kinds.

It is preferable to add a thickener to the non-volatile liquid and/or the low-volatility liquid to thicken it to a suitable viscosity.
Examples of thickeners include clay-based thickeners such as silica whose surface has been hydrophobically treated, fine particle silica whose surface has been methylated, aluminum silicate, swellable mica, and hydrophobically treated bentonite or montmorillonite; fatty acid metal soaps such as magnesium stearate, calcium stearate, aluminum stearate, and zinc stearate; dextrin-based compounds such as tribenzylidene sorbitol, fatty acid amides, amide-modified polyethylene wax, hydrogenated castor oil, and fatty acid dextrin; and cellulose-based compounds.

Examples of solid plugs include solid plugs made of polyethylene, polypropylene, polymethylpentene, and the like.
As the ink backflow preventer, a solid plug and the above-mentioned liquid plug can be used in combination.

It is also possible to make a ballpoint pen equipped with a ballpoint tip and an ink filling mechanism by using the barrel itself as an ink filling mechanism, filling the barrel directly with ink, and attaching a ballpoint tip to the front end of the barrel.

When the ink filled in the ink filling mechanism has a low viscosity, a ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink filled in the ink filling mechanism to the pen tip.

The ink supply mechanism is not particularly limited, and examples include (1) a mechanism that has an ink guide core made of a fiber bundle or the like as an ink flow regulator and supplies ink to the pen tip through this, (2) a mechanism that has a comb-shaped ink flow regulator and supplies ink to the pen tip through this, and (3) a mechanism that supplies ink to the pen tip through a pen core in which multiple disks are arranged in parallel with comb-shaped intervals, slit-shaped ink guide grooves that run vertically through the disks in the axial direction and wider air vent grooves are provided, and an ink guide core is arranged in the axial center to guide ink from the ink filling mechanism to the pen tip.

There are no particular limitations on the material of the pen core, so long as it is a synthetic resin that can be injection molded into a comb-shaped structure of multiple disks. Acrylonitrile-butadiene-styrene copolymer (ABS resin) is preferably used because it has high moldability and is easy to obtain pen core performance.

Specific examples of the configuration of a ballpoint pen that contains the ink composition according to the present invention include: (1) a ballpoint pen having an ink reservoir filled with ink in a barrel, a ballpoint pen tip connected to the ink reservoir directly or via a connecting member, and an ink backflow prevention body filled at the end face of the ink; (2) a ballpoint pen in which ink is directly filled in the barrel and a mechanism is provided for supplying ink to the pen tip by using a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like as an ink flow regulator; and (3) a ballpoint pen in which ink is directly filled in the barrel and a mechanism is provided for supplying ink to the pen tip via the above-mentioned pen core.

When the ink composition according to the present invention is used in a marking pen, the structure and shape of the marking pen itself are not particularly limited, and it may be used, for example, by filling a marking pen refill or a marking pen equipped with a marking pen tip and an ink filling mechanism.

Examples of marking pen tips include conventional porous materials with interconnected pores, such as resin-processed fibers, fused heat-melting fibers, and felt, which have a porosity of approximately 30 to 70%, or extrusions of synthetic resins with multiple ink outlet holes extending in the axial direction, and one end of the tip can be processed into a bullet, rectangular, chisel, or other shape appropriate for the purpose for which it is used.

An example of the ink filling mechanism is an ink occlusion body that can be filled with ink.
The ink occlusion body is a fiber bundle formed by bundling crimped fibers in the longitudinal direction, and is placed inside a covering such as a plastic cylinder or film, with the porosity adjusted to the range of approximately 40 to 90%.

A marking pen can be formed by housing an ink occlusion body impregnated with ink inside the barrel and connecting the marking pen tip to the barrel directly or via a connecting member so that it is connected to the ink occlusion body.

A marking pen refill (hereinafter sometimes referred to as a "refill") can be formed by storing an ink occlusion body impregnated with ink in an ink container and connecting a marking pen tip to the ink container directly or via a connecting member so that it is connected to the ink occlusion body. A marking pen can be formed by storing this refill in a barrel.

The ink container may be, for example, a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a metal tubular body.

A marking pen having a marking pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink composition filled in the ink filling mechanism to the pen tip.

The ink supply mechanism is not particularly limited, and examples thereof include, in addition to the ink supply mechanism provided in the ballpoint pen described above, (4) a mechanism provided with an ink flow rate regulator using a valve mechanism, which supplies ink to the pen tip by opening the valve.
The valve mechanism can be of the conventional, general-purpose pumping type that opens when the tip is pressed, and is preferably set to a spring pressure that can be pressed and opened by the pressure of the writing pen.

When the marking pen is equipped with an ink supply mechanism, the ink filling mechanism can be an ink reservoir that can be filled directly with ink, in addition to the ink occlusion body described above. Also, the barrel itself can be used as the ink filling mechanism, and ink can be filled directly.

Specific examples of the configuration of a marking pen containing the ink composition according to the present invention include: (1) a marking pen in which an ink occlusion body made of a fiber bundle impregnated with ink is contained in a barrel, and a marking pen tip made of a fiber processed body or a resin molded body with capillary gaps is connected to the barrel directly or via a connecting member so that the ink occlusion body and the tip are connected; (2) a marking pen in which ink is directly filled into the barrel and a mechanism is provided for supplying ink to the pen tip by using a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like as an ink flow regulator; (3) a marking pen in which ink is directly filled into the barrel and a mechanism is provided for supplying ink to the pen tip through the pen core; and (4) a marking pen in which a tip and an ink container are provided via a valve mechanism that opens when the tip is pressed, and ink is directly filled into the ink container.

When the ballpoint pen or marking pen according to the present invention is one in which ink is directly filled, an agitator such as a stirring ball for stirring the ink can be built into the ink container or barrel in which the ink is filled in order to facilitate redispersion of the colorant. Examples of the shape of the agitator include a sphere and a rod. There are no particular limitations on the material of the agitator, and examples include metal, ceramic, resin, and glass.

The writing instrument according to the present invention, such as a ballpoint pen or a marking pen, can also be in the form of an ink cartridge, which is a removable structure. In this case, after the ink contained in the ink cartridge of the writing instrument is used up, the writing instrument can be used again by replacing it with a new ink cartridge.

As ink cartridges, those that double as the barrel that constitutes the writing instrument when connected to the main body of the writing instrument, and those that cover and protect the barrel (rear barrel) after being connected to the main body of the writing instrument are used. In the latter case, the ink cartridge may be used alone, or the writing instrument may have an ink cartridge connected to the main body of the writing instrument before use, or it may be stored in the barrel in an unconnected state so that the user of the writing instrument can connect the ink cartridge in the barrel when using it and start using it.

A writing instrument such as a ballpoint pen or marking pen according to the present invention can be made into a capped writing instrument by providing a cap that is attached to cover the pen tip (writing tip), thereby preventing the writing tip from being contaminated or damaged.
Furthermore, in writing instruments such as ballpoint pens or marking pens that house a refill inside the barrel, a retractable mechanism can be provided inside the barrel that allows the writing tip to protrude and retract from the barrel, making the writing instrument a retractable type, thereby preventing the writing tip from becoming contaminated or damaged.

Any retractable writing instrument can be used as long as the writing tip is housed within a barrel with the writing tip exposed to the outside air and the retractable mechanism is activated to cause the writing tip to protrude from the barrel opening.
It may also be a composite type retractable writing instrument in which a plurality of refills are housed within the barrel, and the writing tip of any one of the refills is caused to protrude and retract from the barrel opening by operation of a retraction mechanism.

Examples of the retraction mechanism include: (1) a side-slide retraction mechanism in which an operating part (clip) that can move in the front-rear direction is protruded radially outward from the rear side wall of the barrel, and the operating part is slid forward to cause the writing tip to appear and disappear from the front end opening of the barrel; (2) a rear-end knock retraction mechanism in which the operating part at the rear end of the barrel is pressed forward to cause the writing tip to appear and disappear from the front end opening of the barrel; (3) a side-knock retraction mechanism in which the operating part protruding from the outer surface of the barrel side wall is pressed radially inward to cause the writing tip to appear and disappear from the front end opening of the barrel; and (4) a rotating retraction mechanism in which the operating part at the rear of the barrel is rotated to cause the writing tip to appear and disappear from the front end opening of the barrel.

The configuration of ballpoint pens and marking pens is not limited to the above configurations, and they may be equipped with tips of different shapes, or with tips that emit ink of different tones or hues, or may be composite writing instruments (double-ended, retractable tip, etc.) that are equipped with tips of different shapes and emit ink of different tones or hues.

A writing instrument containing the ink composition according to the present invention is preferably a writing instrument (ballpoint pen) having a ballpoint tip as the pen tip. In an ink composition applied to a ballpoint pen, if the dispersion stability of the microencapsulated pigment in the ink composition is unstable, the microencapsulated pigment may easily clog the tip end due to aggregation or sedimentation, resulting in poor writing such as blurring or skipped lines. However, the ink composition according to the present invention is suitable for use in ballpoint pens because the microencapsulated pigment is stably held in the ink composition and the microencapsulated pigment is unlikely to clog the tip end, suppressing poor writing and forming good handwriting. Furthermore, since the ink composition has a low viscosity but excellent dispersion stability of the microencapsulated pigment, it is also suitable because it has good ink ejection properties and can form handwriting with excellent color development.

When the microencapsulated pigment contains a reversible thermochromic microencapsulated pigment, the writing made on the surface to be written on using a writing instrument containing the ink composition of the present invention can be discolored by rubbing with a finger or by using a heating or cooling tool.

Examples of heating devices include electrical heating and color-changing devices equipped with a resistive heating element such as a PTC element, heating and color-changing devices filled with a medium such as hot water, heating and color-changing devices using steam or laser light, and the application of a hair dryer. However, friction members and friction bodies are preferred because they can change color in a simple manner.

Examples of cooling devices include electrically-operated thermochromic devices using Peltier elements, thermochromic devices filled with a refrigerant such as cold water or ice chips, cold storage agents, refrigerators, freezers, etc.

As the friction member and friction body, elastic bodies such as elastomers and plastic foams that have a high elastic feel and can generate moderate friction and generate frictional heat when rubbed are preferred, but plastic molded bodies, stone, wood, metal, fabric, etc. can also be used. Note that a general eraser used to erase pencil marks can be used to rub the marks, but since eraser dust is generated during rubbing, the above-mentioned friction members and friction bodies that generate almost no eraser dust are preferably used.

Examples of materials for the friction member and friction body include silicone resin and styrene-ethylene-butadiene-styrene block copolymer (SEBS resin). Silicone resin tends to adhere to areas that have been erased by rubbing, and handwriting tends to be repelled when writing is repeated, so SEBS resin is more preferably used.

The friction member or friction body may be a member of any shape that is separate from the writing instrument, but by providing it on the writing instrument, it can be made highly portable. Also, a writing instrument set can be obtained by combining a writing instrument with a friction member or friction body of any shape that is separate from the writing instrument.

In the case of a writing instrument with a cap, the location where the friction member or friction body is provided is not particularly limited; for example, the cap itself can be made of a friction member, the barrel itself can be made of a friction member, and if a clip is provided, the clip itself can be made of a friction member, or the friction member or friction body can be provided at the tip (top) of the cap or the rear end of the barrel (the part where the writing tip is not provided), etc.

In the case of a writing instrument with a retractable mechanism, the location where the friction member or friction body is provided is not particularly limited. For example, the barrel itself can be made of a friction member, and if a clip is further provided, the clip itself can be made of a friction member, or the friction member or friction body can be provided near the barrel opening, at the rear end of the barrel (the part where the writing tip is not provided), or at the knock section.

The following examples are provided, but the present invention is not limited thereto. Unless otherwise specified, "parts" in the examples refer to "parts by mass."

Preparation of reversible thermochromic microcapsule pigment (A) component, 3 parts of 3',6'-bis[phenyl(3-methylphenyl)amino]spiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one, (B) component, 3 parts of 1,1-bis(4-hydroxyphenyl)-2-ethylhexane, 5 parts of 2,2-bis(4-hydroxyphenyl)hexafluoropropane, (C) component, 50 parts of 4-benzyloxyphenylethyl caprate, a reversible thermochromic composition consisting of 35 parts of aromatic isocyanate prepolymer as a wall film material and 40 parts of cosolvent, was added and emulsified in an aqueous polyvinyl alcohol solution, and stirring was continued while heating, followed by adding 2.5 parts of water-soluble aliphatic modified amine, and further stirring was continued to prepare a microcapsule dispersion. The above microcapsule dispersion was filtered with a filter press machine to obtain a reversible thermochromic microcapsule pigment having an average particle size of 0.6 μm.
The reversible thermochromic microcapsule pigment had a complete color-developing temperature _t1 of -20°C and a complete decolorizing temperature _t4 of 60°C, and reversibly changed from blue to colorless due to temperature change.

Example 101
Preparation of Ink Composition 10 parts of a reversible thermochromic microencapsulated pigment, 0.3 parts of a polyether phosphate ester (manufactured by Toho Chemical Industry Co., Ltd., product name: Phosphanol RS-710), 15 parts of glycerin, 1 part of triethanolamine, 0.2 parts of a preservative (manufactured by Arcsada Japan Co., Ltd., product name: Proxel XL-2(S)), and 73.17 parts of water were mixed and stirred. Then, 0.33 parts of TEMPO oxidized cellulose nanofiber (average fiber length: 600 nm, average fiber diameter: 3-4 nm, aspect ratio: 150-200) was added to this mixture, stirred, and filtered to prepare an ink composition.
The amount of cellulose nanofiber per 100 parts by mass of the microencapsulated pigment (hereinafter referred to as "CNF/microencapsulated pigment") was 3.3, and the amount of polyether phosphate ester per 100 parts by mass of the microencapsulated pigment (hereinafter referred to as "dispersant/microencapsulated pigment") was 3.

Examples 102 to 110 and 201 to 213
Preparation of ink compositions The ink compositions of Examples 102 to 110 and 201 to 213 were prepared in the same manner as in Example 101, except that the types and amounts of materials to be blended were changed to those shown in Tables 1 and 2 below. The "CNF/microencapsulated pigment" and "dispersant/microencapsulated pigment" in each ink composition are as shown in Tables 1 and 2.

Comparative Examples 101 to 104 and 201 to 204
Preparation of Ink Compositions The ink compositions of Comparative Examples 101 to 104 and 201 to 204 were prepared in the same manner as in Example 101, except that the types and amounts of the materials to be blended were changed to those shown in Table 3 below. The "CNF/microencapsulated pigment" and the dispersant "microencapsulated pigment" in each ink composition are as shown in Table 3.

Preparation of writing implement A The ink composition of Example 101 was filled by suction into an ink container made of a polypropylene pipe, and then connected to a ballpoint pen tip having a 0.5 mm diameter carbide ball at its tip via a resin holder. Next, a viscoelastic ink backflow prevention body (liquid plug) mainly composed of polybutene was filled from the rear end of the ink container, and a tail plug was fitted to the rear of the pipe, and degassing was performed by centrifugation to obtain a ballpoint pen refill.
Next, the above refill was incorporated into a barrel to prepare a writing instrument A (ballpoint pen).
In the above-mentioned ballpoint pen, the tip provided on the ballpoint pen refill is stored inside the barrel while being exposed to the outside air, and the tip protrudes from the front end opening of the barrel by operating a clip-shaped retractable mechanism (slide mechanism) provided on the rear side wall of the barrel.
Moreover, using the ink compositions of Examples 102 to 110, 201 to 211 and Comparative Examples 101 to 104, 201 to 204, writing implements A (ballpoint pens) were produced in the same manner.

Preparation of Writing Instrument B The ink composition of Example 212 was impregnated into an ink occlusion body made of polyester sliver covered with a synthetic resin film and housed in a barrel made of polypropylene. A resin-processed pen body (chisel type) made of polyester fiber was attached to the tip of the barrel via a resin holder, and a cap was attached to prepare Writing Instrument B (marking pen).
Further, using the ink composition of Example 213, a writing instrument B (marking pen) was prepared in the same manner.

The contents of the materials in Tables 1 to 3 will be explained according to the note numbers.
(1) TEMPO oxidized cellulose nanofiber (average fiber length: 600 nm, average fiber diameter: 3-4 nm, aspect ratio: 150-200)
(2) TEMPO oxidized cellulose nanofiber (average fiber length: 300 nm, average fiber diameter: 3-4 nm, aspect ratio: 75-100)
(3) Polyether phosphate ester (manufactured by Toho Chemical Industry Co., Ltd., product name: Phosphanol RS-710)
(4) Polyvinylpyrrolidone (5) Polyether-modified silicone oil (manufactured by Momentive Performance Materials Japan, product name: TSF-4452)
(6) Arcsada Japan Co., Ltd., Product name: Proxel XL-2 (S)

[Evaluation of initial writing performance]
Using each writing implement A (ballpoint pen) prepared in Examples 101-110, 201-211, and Comparative Examples 101-104, 201-204, 12 elliptical circles (major axis: 15 mm, minor axis: about 8 mm) per line were handwritten in a spiral shape so that the circles touched each other in a direction parallel to the short side of an A4 size test paper (portrait) at room temperature (20°C). Also, using each writing implement B (marking pen) prepared in Examples 212 and 213, a 15 cm straight line was handwritten in a direction parallel to the short side of an A4 size test paper (portrait) at room temperature (20°C), with the wide surface of the pen body in close contact with the paper surface, and this was done for 5 lines. Note that writing paper A conforming to the old JIS P3201 was used for the test paper.
The resulting handwriting was visually inspected and evaluated according to the following criteria. The evaluation results are shown in Tables 4 to 6 below, with ratings of "A" and "B" being acceptable.
A: There was no smearing or skipping of lines in the handwriting, and good handwriting with consistent density and line width was obtained.
B: Some smearing or skipping of lines was observed in the handwriting, but was at a level that did not cause any practical problems.
C: Many smudges or missing lines were observed in the handwriting, or writing was impossible.

[Evaluation of writing performance after aging]
Each writing implement A subjected to the above-mentioned writing test was left in a thermostatic chamber set at 50° C. for 30 days with the pen tip facing down (upright state). After 30 days, it was removed from the thermostatic chamber, and in a room temperature (20° C.) environment, 12 elliptical circles (major axis: 15 mm, minor axis: about 8 mm) per line were handwritten in a spiral shape so that the circles touch each other, in a direction parallel to the short side of an A4-sized test paper (portrait), for three consecutive lines. Similarly, each writing implement B subjected to the above-mentioned writing test was left in a thermostatic chamber set at 50° C. for 30 days with the pen tip facing down (upright state). After 30 days, it was removed from the thermostatic chamber, and in a room temperature (20° C.) environment, a 15 cm straight line was handwritten in a direction parallel to the short side of an A4-sized test paper (portrait), with the wide surface of the pen body in close contact with the paper surface, for five lines. The test paper used was writing paper A conforming to the old JIS P3201.
The resulting handwriting was visually inspected and evaluated according to the following criteria. The evaluation results are shown in Tables 4 to 6 below, with ratings of "A" and "B" being acceptable.
A: There was no smearing or skipping of lines in the handwriting, the color of the handwriting was the same as or of the same level as the initial handwriting, and a good handwriting was obtained.
B: Some smearing or skipping of lines was observed in the handwriting, but was at a level that did not cause any practical problems.
C: Writing was possible, but the color of the handwriting was darker than the initial handwriting, and there was a difference in the color of the handwriting.
D: Many smudges or missing lines were observed in the handwriting, or writing was impossible.

_t1 Complete coloring temperature _t2 Coloring start temperature _t3 Discoloration start temperature _t4 Complete discoloration temperature _T1 Complete discoloration temperature _T2 Discoloration start temperature _T3 Coloring start temperature _T4 Complete coloring temperature ΔH Hysteresis width

Claims (7)

An aqueous ink composition for writing instruments comprising a microcapsule pigment consisting of a core substance and a wall membrane encapsulating the core substance, a polyether phosphate ester as a dispersant, cellulose nanofibers, and water. The ink composition according to claim 1, wherein the resin constituting the wall film is a urea resin, a urethane resin, or a urea-urethane resin. The ink composition according to claim 1 or 2, wherein the core substance is a coloring composition comprising a coloring material and a medium. The ink composition according to any one of claims 1 to 3, in which the cellulose nanofibers are blended in an amount ranging from 1.5 to 12 parts by mass per 100 parts by mass of the microencapsulated pigment. The ink composition according to any one of claims 1 to 4, wherein the polyether phosphate ester is blended in an amount ranging from 1 to 30 parts by mass per 100 parts by mass of the microencapsulated pigment. A writing instrument containing the ink composition according to any one of claims 1 to 5. The writing instrument according to claim 6, which is a ballpoint pen.

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