Aqueous ink composition for writing instruments and writing instrument with aqueous ink composition accommodated therein

Abstract

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

Description

[Technical field]

[0001] The present invention relates to a water-based ink composition for a writing instrument and a writing instrument containing the same. More specifically, the present invention relates to a water-based ink composition for a writing instrument that has excellent dispersion stability of a microencapsulated pigment and can form good handwriting, and a writing instrument containing the same. [Background technology]

[0002] 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 large 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 occur, and the density of the handwriting formed by a writing instrument containing the ink decreases, or the ink discharge from the pen tip decreases, causing writing defects such as line skipping and blurring, making it difficult to obtain sufficient performance as a water-based ink for writing instruments. Therefore, an ink composition in which titanium oxide or carbon black is encapsulated in microcapsules to improve dispersion stability in water-based ink has been disclosed (see, for example, Patent Document 1).

[0003] Patent Document 1 discloses a water-based 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 temperatures 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.

[0004] 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). A 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 writing density compared to other writing instruments in which the coloring material is not encapsulated in microcapsules. In order to increase the writing density, it is possible to increase the blending ratio of the microcapsule pigment, but when the blending ratio is increased, the dispersion stability of the microcapsule pigment in the ink composition tends to decrease. In addition, it has been difficult to stably maintain the microcapsule pigment in the ink composition for a long period of time. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-122168 A [Patent Document 2] JP 2014-5422 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides a water-based ink composition for a writing instrument, which is capable of forming good handwritten marks without causing aggregation or sedimentation over time in the microencapsulated pigment, and a writing instrument containing the same. [Means for solving the problem]

[0007] 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 urea resin, urethane resin, and 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 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 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. Effect of the Invention

[0008] The present invention can provide a water-based ink composition for a writing instrument, which has excellent dispersion stability of a microencapsulated pigment over a long period of time and is capable of forming good handwriting with suppressed smearing, skipped lines, etc., and a writing instrument containing the same. [Brief description of the drawings]

[0009] [Figure 1] 1 is a graph illustrating hysteresis characteristics in a color density-temperature curve of a heat-discolorable, reversible thermochromic composition. [Diagram 2] 1 is a graph illustrating hysteresis characteristics in a color density-temperature curve of a heat-discolorable, reversible thermochromic composition having color memory properties. [Diagram 3] 1 is a graph illustrating hysteresis characteristics in a color density-temperature curve of a reversible thermochromic composition that develops color upon heating. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The water-based ink composition for writing instruments according to the present invention (hereinafter sometimes referred to as "ink composition" or "ink") comprises 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. Each component constituting the ink composition according to the present invention will be described below.

[0011] The ink composition according to the present invention contains a microencapsulated pigment as a colorant. A microcapsule pigment has a core substance encapsulated within a wall film formed from a wall-forming material.

[0012] The core substance may be a coloring composition comprising a coloring material and a medium. For example, the coloring composition may be 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.

[0013] The dyes include acid dyes, basic dyes, direct dyes, oil-soluble dyes, disperse dyes, and the like. Examples of pigments include inorganic pigments, organic pigments, glittering pigments, fluorescent pigments, and phosphorescent pigments. In addition, a pigment dispersant can be used as necessary. Examples of pigment dispersants include anionic and nonionic surfactants; anionic polymers such as polyacrylic acid and styrene-acrylic acid; and nonionic polymers such as PVP and PVA.

[0014] 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, and the like. The aqueous medium or oily medium can be used alone or in combination of two or more kinds.

[0015] 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 exhibit color changes 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.

[0016] Examples of photochromic compounds include conventionally known spirooxazine derivatives, spiropyran derivatives, naphthopyran derivatives, etc. that develop color when irradiated with sunlight, ultraviolet light, or blue light with a peak emission wavelength in the range of 400 to 495 nm, and lose color when the irradiation is stopped. For example, the compounds described in JP 2021-120493 A and WO 2020 / 137469 A can be cited as examples. 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.

[0017] 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.

[0018] Styrene oligomers are compounds having a styrene skeleton or hydrogenated products thereof, and examples thereof include low molecular weight polystyrene, styrene-α-methylstyrene copolymers, α-methylstyrene polymers, and α-methylstyrene-vinyltoluene copolymers. 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. Terpene phenol oligomers are copolymers of cyclic terpene monomers and phenols, or hydrogenated copolymers thereof, such as α-pinene-phenol copolymers.

[0019] 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. Examples of thermochromic materials used as coloring compositions include coloring compositions that are at least composed of (a) an electron-donating organic color-forming compound as a coloring material and (b) an electron-accepting compound as a medium.Further examples include coloring compositions that are at least composed of a homogeneous compatible mixture of the (a) component as a coloring material, the (b) component as a medium, and (c) a reaction medium that determines the temperature at which the coloring reaction of the (a) component and the (b) component occurs, that is, reversible thermochromic compositions that are at least composed of the (a) electron-donating organic color-forming compound, the (b) electron-accepting compound, and the (c) reaction medium that determines the temperature at which the coloring reaction of the (a) component and the (b) component occurs.

[0020] As the reversible thermochromic composition, a reversible thermochromic composition of the heat-discoloring type 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. The heat-discoloring type means that the composition is decolored 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 the 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 that it exhibits in the room temperature range when the application of heat or cold is removed (see FIG. 1).

[0021] As the reversible thermochromic composition, a heat-discoloring type reversible thermochromic composition having a large hysteresis width (ΔH=8 to 80° C.) described in JP-B-4-17154, JP-A-7-179777, JP-A-7-33997, JP-A-8-39936, JP-A-2005-1369, etc. may be used. The heat-discoloring type means that the composition is discolored by heating and colored by cooling. The shape of the curve plotting the change in color density due to temperature change in this reversible thermochromic composition is significantly different between when the temperature is increased from a lower temperature side than the color change temperature range and when the temperature is decreased from a higher temperature side than the color change temperature range, and the color changes at the complete color development temperature t 1 Coloring state in the following temperature range, or complete decolorization temperature t 4 The color disappears at the high temperature range above the specific temperature range [coloring start temperature t 2 ~Discoloration start temperature t 3 (Temperature range in which two phases are essentially maintained) (see Figure 2).

[0022] In addition, when the reversible thermochromic composition having the above-mentioned color memory property is applied to the present invention, the reversible thermochromic composition is specifically a composition having a complete color development temperature t 1 The temperature that can only be obtained in a freezer or in a cold region, and the temperature at which the color completely fades out, t 4By specifying the temperature range to be obtained from frictional heat produced by a friction body or from familiar heating bodies such as a hair dryer, and specifying the ΔH value to be 40 to 100°C, it can be made to effectively maintain the color it exhibits under normal conditions (daily living temperature range).

[0023] The temperature that can only be obtained in freezers, cold regions, etc. 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.

[0024] As the reversible thermochromic composition, a heat-coloring type reversible thermochromic composition using a gallic acid ester, as described in JP-B-51-44706, JP-A-2003-253149, etc., can also be used. The heat-coloring type means that the color develops when heated and the color disappears when cooled (see FIG. 3).

[0025] 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, discoloration temperature, discoloration form, and type of each component, but the component ratio that generally provides the desired properties is in the range of 1 part by weight 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, more preferably 5 to 100, and even more preferably 10 to 100, of component (c) (all the above ratios are parts by weight).

[0026] 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 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 exhibit the same effects.

[0027] The microencapsulated pigment may also contain various additives such as antioxidants, ultraviolet absorbers, infrared absorbers, dissolution aids, preservatives, and antifungal agents, provided that their functions are not adversely affected.

[0028] The microencapsulated pigment according to the present invention can be produced by a microencapsulation method, which includes the conventionally known isocyanate-based interfacial polymerization method, the in situ polymerization method such as a melamine-formaldehyde-based method, the in-liquid curing coating method, the phase separation method from an aqueous solution, the phase separation method from an organic solvent, the melting dispersion cooling method, the air suspension coating method, the spray drying method, and the like, and is appropriately selected according to the application.

[0029] Examples of the resin constituting the wall film include urea resin, urethane resin, urea-urethane resin, epoxy resin, melamine resin, benzoguanamine resin, and isocyanate resin.

[0030] Depending on the purpose, a secondary resin film may be further provided on the surface of the microencapsulated pigment according to the present invention to impart durability or to modify the surface properties for practical use.

[0031] 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.

[0032] The reversible thermochromic microencapsulated pigment or the reversible photochromic microencapsulated pigment can also be made into a microencapsulated pigment that exhibits a 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.

[0033] 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 dischargeability 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 to obtain high-density color development in handwriting.

[0034] The average particle size 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 projected area equivalent circle diameter (Heywood diameter) from the area of ​​the particle region, and measuring the average particle size of particles equivalent to a sphere of equal volume using this value.

[0035] In addition, if the particle size of all or the majority 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 analyzer (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.).

[0036] Furthermore, the volumetric particle size and average particle size may be measured using a calibrated laser diffraction / scattering particle size distribution analyzer (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.

[0037] 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 %, based on the total amount of the ink composition. If the blending ratio exceeds 25 mass %, the ink dischargeability of the writing instrument containing the ink composition is 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 is difficult to obtain a suitable writing density for the writing instrument.

[0038] When the microencapsulated pigment is a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, the microencapsulated pigment is preferably blended in the range 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 dischargeability of the writing instrument containing the ink composition decreases, and writing defects such as smearing and skipping of lines tend to occur. On the other hand, if the blending ratio is less than 5% by mass, it is difficult to obtain suitable discoloration and writing density as a writing instrument, and it is difficult to fully satisfy the discoloration function.

[0039] 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.

[0040] 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-174 (manufactured by Kusumoto Chemical Industries, Ltd.), HIPLAAD Examples of such surfactants include 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), and DISPERBYK-180 (manufactured by BYK Japan K.K.).

[0041] In addition to being excellent as a dispersant, polyether phosphate esters are also excellent in that they are unlikely to penetrate into the microencapsulated pigment and are 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 substance of the reversible thermochromic microencapsulated pigment (reversible thermochromic composition) is a homogeneous compatible solution containing the above-mentioned components (a), (b), and (c) as essential components, and when a dispersant penetrates into the microencapsulated pigment, one of the components may dissolve or precipitate, causing the color density of the reversible thermochromic microencapsulated pigment in a colored state to decrease, or the reversible thermochromic function of entering a decolorized state in a temperature range above the high-temperature discoloration point (complete discoloration temperature) and entering a colored state in a temperature range below the low-temperature discoloration point (complete discoloration temperature) to be impaired, making it difficult to cause a reversible color change from a colored state to a decolorized state. However, by using a polyether phosphate ester as a dispersant for the reversible thermochromic microencapsulated pigment, the reversible thermochromic composition is easily maintained in a homogeneous state within the microcapsules, and elution or precipitation of any of the components (a), (b), and (c) is suppressed. Therefore, it is preferable to use the reversible thermochromic microencapsulated pigment as the colorant in the ink composition according to the present invention.

[0042] 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 any of the above-mentioned resins, the polyether phosphate is more easily adsorbed onto the surface of the microcapsule pigment, the polyether phosphate exerts an excellent effect as a dispersant, and the polyether phosphate is more difficult to penetrate into the microcapsule, so that the dispersion stability and storage stability of the microcapsule pigment can be highly compatible.

[0043] The polyether phosphate is blended in an amount of preferably 1 to 30 parts by mass, more preferably 2 to 26 parts by mass, relative to 100 parts by mass of the microencapsulated pigment. When the blending ratio of the polyether phosphate to the microencapsulated pigment is within the above range, the polyether phosphate is more likely to exert its effect as a dispersant for the microencapsulated pigment.

[0044] 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 holding the microencapsulated pigment to which the dispersant is adsorbed, as described above, in the ink composition.

[0045] 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 1,000 nm, the dispersibility of the cellulose nanofibers in the ink tends to be low, making it difficult for the ink to be ejected 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.

[0046] The 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 (for example, 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.

[0047] In the ink composition according to the present invention, when the ink composition contains a large amount of cellulose nanofibers or uses a light-colored microencapsulated pigment, the hue of the cellulose nanofibers may affect the hue of the ink composition. Therefore, in order to obtain transparency when the cellulose nanofibers are dispersed in water, it is preferable to adjust the average fiber length of the cellulose nanofibers 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 nanofibers is less likely to affect the hue of the ink composition, and the ink composition is more likely to exhibit a hue derived from the microencapsulated pigment.

[0048] 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 by temperature change or light irradiation, 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.

[0049] 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 stable retention of the microencapsulated pigment. 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.

[0050] 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.

[0051] The average fiber diameter refers to the number average fiber diameter. The number average fiber diameter can be measured using a known technique, for example, in the same manner as the above-mentioned number average fiber length.

[0052] The aspect ratio of the cellulose nanofibers (ie, the ratio of the average fiber length to the average fiber diameter) is preferably in the range of 100-400, and more preferably 110-350.

[0053] When the cellulose nanofiber has a relatively small fiber diameter and a relatively large aspect ratio, the cellulose nanofibers are more likely to interact with each other, which has the effect of further improving the dispersion stability of the microencapsulated pigment.

[0054] 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 TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) catalyst to convert the primary hydroxyl groups of cellulose into carboxyl groups, and then mechanically defibrating the fibers.

[0055] The cellulose nanofiber is blended in an amount of preferably 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, relative to 100 parts by mass of the microencapsulated pigment. By blending the cellulose nanofiber in proportion to the microencapsulated pigment within the above range, the ink composition can have a low viscosity while stably retaining the microencapsulated pigment.

[0056] Conventionally, in order to improve the dispersion stability of the microencapsulated pigment, the ink is 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 instrument to which it can be applied. However, as described above, the ink composition according to the present invention uses a polyether phosphate ester and cellulose nanofiber in combination to form a dense network structure, and while having a lower viscosity than an ink composition using a conventional thickener alone, it can stably hold the microencapsulated pigment for a long period of time. Therefore, by using a polyether phosphate ester and cellulose nanofiber in combination, a rheology control effect different from that of a conventional thickener 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 an ink composition using a conventional thickener alone, the ink dischargeability of a writing instrument containing this ink composition is improved, and writing defects such as blurring can be suppressed and the writing feel can be improved.

[0057] 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 having a large hysteresis (ΔH) often uses a compound having two or more benzene rings in the molecule as component (c), and therefore tends to have a large specific gravity, tends 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 dispersion stability tends to be even more poor. However, the ink composition according to the present invention can suppress the settling over time of such a reversible thermochromic microencapsulated pigment having 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 even while the ink composition has a low viscosity.

[0058] 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.

[0059] 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).

[0060] An ink composition using a shear thinning agent has high viscosity and is difficult to flow when left at rest or under low stress, but easily becomes 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 ink ejection stability from the pen tip when writing.

[0061] 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.

[0062] Examples of shear thinning agents include water-soluble polysaccharides, polymers having a molecular weight of 100,000 to 150,000 that are primarily 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 dialkyl sulfosuccinic acid, and the like. The shear thinning agents may be used alone or in combination of two or more.

[0063] 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 having gelling properties extracted from seaweed, such as agar and carrageenan.

[0064] 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.

[0065] The ink composition according to the present invention may also contain various additives, such as water-soluble organic solvents, polymer flocculants, dispersants, water-soluble resins, specific gravity adjusters, surfactants, pH adjusters, rust inhibitors, preservatives or fungicides, air bubble absorbers, antifoamers, antioxidants and ultraviolet absorbers, if necessary.

[0066] 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 a color change behavior from color (1) to color (2) by blending a non-color-changing colorant such as a general dye or pigment.

[0067] 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.

[0068] When the ink composition according to the present invention is used in a ballpoint pen, its viscosity is measured at a rotation speed of 1 rpm (shear rate of 3.84 sec) in an environment of 20° C. -1 When measured under the conditions of 20° C., the viscosity is preferably in the range of 1 to 2000 mPa·s, more preferably 3 to 1500 mPa·s, and further preferably 100 to 1000 mPa·s. -1 When measured under the conditions of (a) and (b), 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 the viscosity is 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 rotation speed 100 rpm (shear rate 384 sec -1 ) values ​​measured under the conditions.

[0069] When the ink composition according to the present invention is used in a marking pen, its viscosity, measured at a rotation speed of 20 rpm in an environment of 20° C., 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 the viscosity is 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 was measured using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34′×R24)) by placing the ink composition in an environment of 20°C.

[0070] 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 having the pH within the above range, excessive viscosity increase and deterioration of the ink composition can be suppressed. 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).

[0071] Examples of writing instruments that can accommodate the ink composition of the present invention include various writing instruments such as ballpoint pens, marking pens, fountain pens, brush pens, and calligraphy pens.

[0072] 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 the ink composition may be used, for example, by filling a ballpoint pen refill or ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism.

[0073] A ballpoint pen tip is composed of a tip body and a ball provided at the front end of the tip body. Examples of ballpoint pen tips include a tip in which a ball is held by a ball holding portion formed by pressing and deforming the vicinity of the tip of a metal pipe body inward from the outer surface, a tip in which a ball is held by a ball holding portion formed by cutting a metal material with a drill or the like, a tip in which a resin ball receiving seat is 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.

[0074] The material of the tip body and the ball is not particularly limited, and examples thereof include cemented carbide (super hard), stainless steel, ruby, ceramic, resin, rubber, etc. Furthermore, the ball can be subjected to a surface treatment such as a DLC coating.

[0075] 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 further preferably 0.2 to 1 mm. Generally, when an ink composition containing a microencapsulated pigment is applied to a ballpoint pen 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 writing 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 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. In addition, a ballpoint pen with a large diameter generally discharges 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 the microcapsule pigment for a long period of time, is applied to a ballpoint pen with a large diameter ball, particularly a ball with a diameter of 0.5 to 1.0 mm, the ink discharge from the pen tip is improved, and the ballpoint pen can be made to be capable of writing with a smoother writing feel and forming clear handwriting with high handwriting density.

[0076] 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.

[0077] A ballpoint pen refill (hereinafter sometimes referred to as a "refill") can be formed by connecting a ballpoint pen tip directly or via a connecting member to an ink container and directly filling the ink container with ink. A ballpoint pen can be formed by storing this refill in a barrel.

[0078] The rear end of the ink reservoir is filled with an ink backflow prevention body, which may be a liquid plug or a solid plug.

[0079] 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-olefins, α-olefin oligomers or cooligomers, 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 can be used alone or in combination of two or more kinds.

[0080] 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 with a hydrophobic surface treatment, fine particle silica with a methylated surface, 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.

[0081] 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.

[0082] In addition, it is also possible to form a ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism by using the barrel itself as the ink filling mechanism, filling the barrel directly with ink, and attaching a ballpoint pen tip to the front end of the barrel.

[0083] When the ink filled in the ink filling mechanism has a low viscosity, a ballpoint pen having 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.

[0084] 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 rate regulator and supplies ink to the pen tip through this, (2) a mechanism that has a comb-shaped ink flow rate 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 consisting of a number of disks arranged in parallel with comb-shaped intervals, with slit-shaped ink guide grooves running vertically through the disks in the axial direction and wider air vent grooves than the grooves, and with an ink guide core located in the axial center to guide ink from the ink filling mechanism to the pen tip.

[0085] The material for the pen core is not particularly limited as long as it is a synthetic resin that can be injection molded into a structure in which multiple disks are arranged in a comb groove shape. Acrylonitrile-butadiene-styrene copolymer (ABS resin) is preferably used because it has high moldability and is easy to obtain pen core performance.

[0086] Specific examples of the configuration of a ballpoint pen containing the ink composition of the present invention include: (1) a ballpoint pen having an ink container filled with ink within a barrel, a ballpoint pen tip connected to the ink container directly or via a connecting member, and an ink backflow prevention body filled at the end face of the ink container; (2) a ballpoint pen in which ink is directly filled within the barrel and a mechanism is provided for supplying ink to the pen tip via 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 within the barrel and a mechanism is provided for supplying ink to the pen tip via the above-mentioned pen core.

[0087] 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 the ink composition 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.

[0088] Examples of marking pen tips include conventional porous members with interconnected pores, such as resin-processed fibers, fused heat-fusible fibers, and felt, which have a porosity selected from a range of approximately 30 to 70%, or extrusion-molded synthetic resin bodies with multiple ink outlet holes extending in the axial direction, one end of which can be processed into a bullet shape, rectangular shape, chisel shape, or other shape suited to the purpose for which it is used.

[0089] 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 in which crimped fibers are bundled 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%.

[0090] A marking pen can be formed by housing an ink occlusion body impregnated with ink inside the barrel and connecting a marking pen tip to the barrel directly or via a connecting member so as to connect to the ink occlusion body.

[0091] Also, 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 as to be connected to the ink occlusion body. A marking pen can be formed by storing this refill in a barrel.

[0092] The ink reservoir may be, for example, a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a tubular body made of metal.

[0093] A marking pen equipped with 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.

[0094] 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.

[0095] When the marking pen is provided with an ink supply mechanism, the ink filling mechanism may be an ink reservoir that can be filled directly with ink, in addition to the ink occlusion body described above. Also, the barrel itself may serve as the ink filling mechanism, and ink may be filled directly.

[0096] 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 consisting of a fiber bundle impregnated with ink is contained within a barrel, and a marking pen tip consisting of a fiber processed body or a resin molded body with capillary gaps formed therein 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 the barrel is directly filled with ink, 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 consisting of a fiber bundle or the like as an ink flow regulator; (3) a marking pen in which the barrel is directly filled with ink, and a mechanism is provided for supplying ink to the pen tip via the above-mentioned pen core; and (4) a marking pen in which the tip and an ink container are connected via a valve mechanism that opens when the tip is pressed, and ink is directly filled into the ink container.

[0097] When the ballpoint pen or marking pen according to the present invention is one in which ink is directly filled, an agitator such as an agitating ball for agitating the ink can be built into the ink reservoir 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 spherical body and a rod-shaped body. The material of the agitator is not particularly limited, and examples thereof include metal, ceramic, resin, glass, etc.

[0098] The writing instrument according to the present invention, such as a ballpoint pen or a marking pen, may be in the form of an ink cartridge as 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.

[0099] As the ink cartridge, one that also serves as the barrel that constitutes the writing instrument by connecting it to the writing instrument body, or one that covers and protects the barrel (rear barrel) after connecting it to the writing instrument body, is used. In addition to being used alone, the latter may be either one in which the writing instrument body and the ink cartridge are connected in the writing instrument before use, or one that is stored in the barrel in a disconnected state so that the user of the writing instrument can connect the ink cartridge in the barrel when using it and start using it.

[0100] In the writing instrument such as the ballpoint pen or marking pen according to the present invention, a cap is provided to cover the pen tip (writing tip) to make it a capped writing instrument, thereby preventing the writing tip from being contaminated or damaged. In addition, in writing instruments such as ballpoint pens or marking pens in which a refill is stored 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 being contaminated or damaged.

[0101] 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.

[0102] Examples of the retraction mechanism include: (1) a side-slide type retraction mechanism in which an operating part (clip) that can move in the front-rear direction protrudes 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 opening of the barrel; (2) a rear-end knock type retraction mechanism in which an operating part provided at the rear end of the barrel is pressed forward to cause the writing tip to appear and disappear from the front opening of the barrel; (3) a side-knock type retraction mechanism in which an operating part protruding from the outer surface of the side wall of the barrel is pressed radially inward to cause the writing tip to appear and disappear from the front opening of the barrel; and (4) a rotating type retraction mechanism in which an operating part at the rear of the barrel is rotated to cause the writing tip to appear and disappear from the front opening of the barrel.

[0103] The configuration of ballpoint pens and marking pens is not limited to the configurations described above, and they may be equipped with tips of different shapes, or with tips that dispense ink of different tones or hues, or they may be composite writing instruments (double-headed, retractable tip, etc.) that are equipped with tips of different shapes and dispense ink of different tones or hues.

[0104] A writing instrument containing the ink composition according to the present invention is preferably a writing instrument (ballpoint pen) having a ballpoint tip as a 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 at the tip end due to aggregation or sedimentation, causing writing defects such as blurring and line skipping. However, the ink composition according to the present invention is suitable for use in a ballpoint pen because the microencapsulated pigment is stably held in the ink composition, the microencapsulated pigment is unlikely to clog at the tip end, and writing defects can be suppressed to form good handwriting. Furthermore, since the ink composition has a low viscosity and excellent dispersion stability of the microencapsulated pigment, it is also suitable because it can form handwriting with good ink dischargeability and excellent color development.

[0105] When the microencapsulated pigment contains a reversible thermochromic microencapsulated pigment, handwriting formed on a 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 a heating or cooling tool.

[0106] Examples of the heating device include an electrically-heated discoloring device equipped with a resistive heating element such as a PTC element, a heat discoloring device filled with a medium such as hot water, a heat discoloring device 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.

[0107] Examples of cooling devices include electrochemically-induced color-changing devices using a Peltier element, color-changing devices filled with a refrigerant such as cold water or ice chips, cooling agents, refrigerators, freezers, and the like.

[0108] As the friction member and friction body, elastic bodies such as elastomers and plastic foams that are rich in elasticity and can generate appropriate friction and generate frictional heat when rubbed are preferred, but plastic molded bodies, stone materials, wood, metals, fabrics, etc. can also be used. Note that, although a general eraser used for erasing pencil marks may be used to rub the marks, eraser shavings are generated during rubbing, the above-mentioned friction members and friction bodies that generate almost no eraser shavings are preferably used.

[0109] Examples of the material of the friction member and the friction body include silicone resin, styrene-ethylene-butadiene-styrene block copolymer (SEBS resin), etc. Silicone resin is likely to adhere to the part erased by rubbing, and handwriting tends to be repelled when writing is repeated, so SEBS resin is more preferably used.

[0110] 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, the writing instrument 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.

[0111] 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 may be formed from a friction member, the barrel itself may be formed from a friction member, and in the case where a clip is provided, the clip itself may be formed from a friction member, or the friction member or friction body may 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.

[0112] In the case of a writing instrument equipped with a retractable mechanism, the location where the friction member or friction body is provided is not particularly limited, and for example, the barrel itself may be formed from a friction member, and if a clip is further provided, the clip itself may be formed from a friction member, or the friction member or friction body may be provided near the opening of the barrel, at the rear end of the barrel (the portion where the writing tip is not provided), or at the knock portion. EXAMPLES

[0113] The following examples are given, but the present invention is not limited thereto. In the examples, "parts" refers to "parts by mass" unless otherwise specified.

[0114] Preparation of reversible thermochromic microencapsulated pigments A reversible thermochromic composition consisting of 3 parts of 3',6'-bis[phenyl(3-methylphenyl)amino]spiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one as component (A), 3 parts of 1,1-bis(4-hydroxyphenyl)-2-ethylhexane as component (B), 5 parts of 2,2-bis(4-hydroxyphenyl)hexafluoropropane as component (C), and 50 parts of 4-benzyloxyphenylethyl caprate as component (C) was added to a mixed solution consisting of 35 parts of aromatic isocyanate prepolymer as a wall film material and 40 parts of cosolvent, and then emulsified and dispersed in an aqueous polyvinyl alcohol solution. After continuing to stir while heating, 2.5 parts of water-soluble aliphatic modified amine were added 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 with an average particle size of 0.6 μm. The reversible thermochromic microencapsulated pigment has a complete color change temperature t 1 is -20℃, complete discoloration temperature t 4 The color changed reversibly from blue to colorless when the temperature was changed to 60°C.

[0115] Example 101 Preparation of Ink Composition 10 parts of reversible thermochromic microcapsule pigment, 0.3 parts of 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 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 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 microencapsulated pigment (hereinafter referred to as "dispersant / microencapsulated pigment") was 3.

[0116] Examples 102 to 110 and 201 to 213 Preparation of Ink Composition The ink compositions of Examples 102 to 110 and 201 to 213 were prepared in the same manner as Example 101, except that the types and amounts of the 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.

[0117] Comparative Examples 101 to 104 and 201 to 204 Preparation of Ink Composition The ink compositions of Comparative Examples 101 to 104 and 201 to 204 were prepared in the same manner as 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.

[0118] Creation 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 preventive body (liquid plug) mainly composed of polybutene was filled into the rear end of the ink container, and a tail plug was fitted to the rear of the pipe, followed by deaeration 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.

[0119] Creation of writing implement B The ink composition of Example 212 was impregnated into an ink reservoir 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 produce writing instrument B (marking pen). Moreover, using the ink composition of Example 213, a writing instrument B (marking pen) was prepared in the same manner.

[0120] [Table 1]

[0121] [Table 2]

[0122] [Table 3]

[0123] 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) Product name: Proxel XL-2(S), manufactured by Arcsada Japan Co., Ltd.

[0124] [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 parallel direction to the short side of an A4 size test paper (portrait) in a room temperature (20°C) environment, and 15 cm straight lines were handwritten in a spiral shape so that the circles touched each other in a room temperature (20°C) environment (5 lines). Note that writing paper A conforming to the old JIS P3201 was used as 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 it was at a level that did not cause any problems in practical use. C: Many smudges or missing lines were observed in the handwriting, or writing was impossible.

[0125] [Evaluation of writing performance over time] Each writing implement A used in 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 used in 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, 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, and the color of the handwriting was the same or at the same level as the initial handwriting, resulting in a good handwriting. B: Some smearing or skipping of lines was observed in the handwriting, but it was at a level that did not cause any problems in practical use. 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.

[0126] [Table 4]

[0127] [Table 5]

[0128] [Table 6] [Explanation of symbols]

[0129] t 1 full color temperature t 2 color starting temperature t 3 Color fading start temperature t 4 Complete decolorization temperature T 1 Complete decolorization temperature T 2 Color fading start temperature T 3 color starting temperature T 4 full color temperature ΔH Hヒステリシス

Claims

1. An aqueous ink composition for a writing instrument, comprising: a microcapsule pigment comprising a core substance and a wall membrane encapsulating the core substance; a polyether phosphate ester as a dispersant; cellulose nanofibers; and water.

2. 2. The ink composition according to claim 1, wherein the resin constituting the wall film is any one of a urea resin, a urethane resin, and a urea-urethane resin.

3. 2. The ink composition according to claim 1, wherein the core substance is a coloring composition comprising a coloring material and a medium.

4. 2. The ink composition according to claim 1, wherein the cellulose nanofibers are blended in an amount of 1.5 to 12 parts by mass per 100 parts by mass of the microcapsule pigment.

5. 2. The ink composition according to claim 1, 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 microcapsule pigment.

6. A writing implement containing the ink composition according to any one of claims 1 to 5.

7. 7. The writing instrument of claim 6, which is a ballpoint pen.

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