Aqueous ink composition for writing instrument and writing instrument storing the same

Abstract

To provide an aqueous ink composition for writing instrument where, dispersion stability of a pigment or resin particles as a colorant, is excellent for a long term, blur or line skipping can be suppressed and by which preferable handwriting can be achieved, and a writing instrument storing the same.SOLUTION: There is provided an aqueous ink composition including: a colorant selected from a pigment and resin particles; a cationic polymer; a cellulose nanofiber; and water. The cationic polymer is a polymer including a specific constitutional unit, and a mass ratio of the cellulose nanofiber and the cationic polymer is 1:1 to 1:20. There is also provided a writing instrument storing, the aqueous ink composition for writing instrument.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 colorant 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 or resin particles as ink colorants are widely used because of their excellent light resistance and water resistance. Usually, pigments and resin particles have unstable dispersion stability in water, and unless these colorants are uniformly dispersed and kept in a stable state, aggregation and sedimentation occur, and the density of 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, ink compositions in which the dispersion stability of these colorants in water-based inks is improved by using various dispersants and additives have been disclosed (see, for example, Patent Documents 1 and 2).

[0003] Patent Document 1 discloses an aqueous ink containing water, a pigment, xanthan gum, and a non-crosslinked polyacrylic acid having a specific molecular weight or a salt thereof.

[0004] Patent Document 2 discloses an aqueous ink composition comprising a pigment, an aqueous medium, and a copolymer of an N-vinylpyrrolidone derivative and an acrylic acid derivative or a methacrylic acid derivative as a dispersant.

[0005] The aqueous inks (aqueous ink compositions) disclosed in Patent Documents 1 and 2 can stably disperse pigments in the ink by using dispersants and resins. However, the dispersion stability of the pigments in the ink is insufficient, and it is difficult to suppress the aggregation and sedimentation of the pigments for a long period of time, particularly when the particle size of the pigments is large or the specific gravity is high, which can result in a decrease in the writing density and poor writing.

[0006] Meanwhile, an aqueous ink composition for writing instruments containing 0.05 to 1.5 mass % of oxidized cellulose has been disclosed as a water-based ink composition for writing instruments that has a lower viscosity than conventional thickening / gelling agents such as xanthan gum, yet is excellent in particle storage stability, writing properties over time, and line drawing quality (see Patent Document 3). Although the above ink composition can suppress aggregation and sedimentation of the pigment in the ink to some extent, it is difficult to stably disperse the pigment over a long period of time, and there are cases where a decrease in handwriting density and poor writing results. Therefore, there is room for improvement in terms of pigment dispersion stability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-59877 A [Patent Document 2] Japanese Patent Application Publication No. 9-59554 [Patent Document 3] JP 2015-67722 A Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention provides a water-based ink composition for a writing instrument which is unlikely to cause aggregation or sedimentation of a colorant over time and which is capable of forming good handwriting, and a writing instrument containing the same. [Means for solving the problem]

[0009] The present invention relates to an aqueous ink composition for a writing instrument, which comprises a colorant selected from a pigment or resin particles, a cationic polymer, cellulose nanofibers, and water. The cationic polymer is also required to be a polymer having a structural unit represented by the following formula (I) or (II). [ka] (In the formula, R1 and R1' are each independently a hydrogen atom or 1-3 n1 represents an integer of 0 or 1, and X1 represents any one of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid. [ka] (In the formula, R2 is a hydrogen atom or 1-3 n2 represents an integer of 0 or 1, and X2 represents any one of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid. Further, the requirements are that R1 and R1' are both hydrogen atoms, R2 is a hydrogen atom, the mass ratio of the cellulose nanofiber to the cationic polymer is 1:1 to 1:20, and the average particle size of the colorant is 0.01 to 25 μm. 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

[0010] The present invention can provide a water-based ink composition for a writing instrument, which has excellent dispersion stability for a long period of time for pigments or resin particles as a colorant, and which is capable of forming good handwriting with suppressed smearing, skipped lines, etc., and a writing instrument containing the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The water-based ink composition for writing instruments according to the present invention (hereinafter sometimes referred to as "ink composition" or "ink") contains a colorant selected from pigments or resin particles, a cationic polymer, cellulose nanofibers, and water. Each component constituting the ink composition according to the present invention will be described below.

[0012] The ink composition according to the present invention contains a pigment or resin particles as a colorant. There are no particular limitations on the pigment or resin particles as long as they are dispersible in an aqueous medium such as water.

[0013] Examples of the pigment include inorganic pigments, organic pigments, glittering pigments, fluorescent pigments, phosphorescent pigments, etc. Furthermore, water-dispersed pigments can be used, which are pigments finely and stably dispersed in an aqueous medium in advance using a surfactant or resin.

[0014] If necessary, a pigment dispersant can be used, such as an anionic or nonionic surfactant, an anionic polymer such as polyacrylic acid or styrene-acrylic acid, or a nonionic polymer such as PVP or PVA.

[0015] The pigments applicable to the present invention also include microencapsulated pigments. Microcapsule pigments are pigments in which a core substance is encapsulated in a wall film formed by a wall film-forming material. By encapsulating the core substance in a microcapsule, the core substance is isolated and protected from the external environment, and the water resistance and light resistance of the core substance can be improved.

[0016] The core substance may be a coloring composition comprising a coloring material and a medium. For example, the coloring composition may be a coloring material obtained by dissolving or dispersing a dye or pigment in an aqueous medium or an oil-based medium.

[0017] The dyes include acid dyes, basic dyes, direct dyes, oil-soluble dyes, disperse dyes, and the like. Examples of the pigment include the pigments described above and water-dispersible pigments, and a pigment dispersant can be used as necessary.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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 predetermined 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 exhibited in the room temperature range when the application of heat or cold is removed.

[0025] As the reversible thermochromic composition, a heat-discolorable 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. can be used. 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 discoloration temperature range than when the temperature is decreased from a higher temperature side than the discoloration temperature range, and has color memory in a specific temperature range [the temperature range between the color development onset temperature t2 and the discoloration onset temperature t3 (temperature range in which two phases are essentially maintained)] in which the colored state is in a temperature range below the complete color development temperature t1, or the discolored state is in a high temperature range above the complete discoloration temperature t4.

[0026] 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 discoloration 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.

[0027] 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.

[0028] 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.

[0029] 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).

[0030] 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.

[0031] Examples of the wall film forming material, that is, the resin constituting the wall film, include urea resin, urethane resin, urea-urethane resin, epoxy resin, melamine resin, benzoguanamine resin, isocyanate resin, and the like.

[0032] 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.

[0033] The microencapsulated pigment can be produced by a microencapsulation method, which includes the conventionally known isocyanate-based interfacial polymerization method, the melamine-formaldehyde-based in situ polymerization method, the 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 depending on the application. Depending on the purpose, a secondary resin film may be provided on the surface of the microencapsulated pigment to impart durability or modify the surface properties for practical use.

[0034] The reversible thermochromic microcapsule pigment or the reversible photochromic microcapsule pigment preferably has a core substance:wall film mass ratio of 7:1 to 1:1, and by having the core substance:wall film mass ratio within the above range, it is possible to prevent a decrease in color density and clarity during color development. More preferably, the core substance:wall film mass ratio is 6:1 to 1:1.

[0035] 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.

[0036] The resin particles include those containing the above-mentioned dyes, pigments, or thermochromic or photochromic materials.

[0037] Examples of the resin particles containing a dye include colored resin particles in which a dye is homogeneously dissolved or dispersed in the resin particles, and colored resin particles in which a dye is dyed onto the resin particles.

[0038] Examples of the resin particles containing a pigment include colored resin particles in which the pigment is uniformly dispersed in the resin particles, colored resin particles in which the surfaces of the resin particles are coated with a pigment, etc. Here, the pigment may be surface-treated by various conventionally known methods for the purpose of improving dispersibility or adsorption to the resin constituting the resin particles.

[0039] Examples of resin particles containing a thermochromic material or a photochromic material include colored resin particles in which a reversible thermochromic composition is homogeneously dispersed in the resin particles (hereinafter sometimes referred to as "reversible thermochromic resin particles"), and colored resin particles in which a reversible photochromic composition is homogeneously dispersed in the resin particles (hereinafter sometimes referred to as "reversible photochromic resin particles").

[0040] The resin constituting the resin particles is not particularly limited as long as it is a thermoplastic resin or a thermosetting resin, and examples thereof include thermoplastic resins such as polystyrene, acrylic resin, polyester, polyvinyl chloride, polybutadiene, polymethyl methacrylate, acrylic-urethane copolymer resin, polyethylene, polypropylene, polyacrylonitrile, polyacetal, ethylene-propylene copolymer resin, ethylene-vinyl acetate copolymer resin, styrene-acrylic copolymer resin, styrene-butadiene copolymer resin, styrene-acrylonitrile copolymer resin, and acrylonitrile-butadiene copolymer resin; Thermosetting resins such as epoxy resins, epoxy acrylate resins, xylene resins, toluene resins, guanamine resins, benzoguanamine resins, melamine resins, urethane resins, phenolic resins, alkyd resins, polyamides, polyimides, polyamide esters, urea resins, silicone resins, and unsaturated polyesters. Examples of each can be given.

[0041] The resin particles according to the present invention include solid resin particles having no voids inside the particles, and hollow resin particles having voids inside the particles.

[0042] The resin particles can be produced by a pulverization method, a spray drying method, or a polymerization method in which polymerization is carried out in the presence of a dye, a pigment, or a thermochromic or photochromic material in an aqueous or oily medium, such as a suspension polymerization method, a suspension polycondensation method, a dispersion polymerization method, or an emulsion polymerization method.

[0043] The shape of the resin particles is not particularly limited, and resin particles having a spherical shape such as a perfect sphere, an oval sphere, or a substantially spherical shape, a polygonal shape, a flat shape, etc., can be used. Among these, spherical resin particles are preferable.

[0044] In addition, since the wall of a microcapsule pigment is made of a resin and contains a coloring composition using a dye, a pigment, or a thermochromic material or a photochromic material inside the microcapsule, the above-mentioned microcapsule pigment can also be used as a resin particle.

[0045] The colorants according to the present invention may be used alone or in combination of two or more.

[0046] The average particle size of the colorant is not particularly limited, but is preferably in the range of 0.01 to 25 μm, more preferably 0.05 to 20 μm. When the average particle size of the colorant is within the above range, the colorant has excellent dispersion stability in the ink composition, and furthermore, it becomes easier to obtain handwriting of a desired color.

[0047] 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.

[0048] 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.).

[0049] 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.

[0050] The blending ratio of the colorant is not particularly limited, but the colorant is blended in the range of preferably 1 to 15 mass %, more preferably 3 to 10 mass %, of the total amount of the ink composition. If the blending ratio exceeds 15 mass %, the ink dischargeability of the writing instrument containing the ink composition is likely to decrease, and writing defects such as smearing and skipping of 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.

[0051] When the colorant is a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, or a reversible thermochromic resin particle or a reversible photochromic resin particle, these colorants are preferably blended in the range of 5 to 40 mass%, more preferably 10 to 40 mass%, and even more preferably 10 to 30 mass% of the total amount of the ink composition. If the blending ratio exceeds 40 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 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.

[0052] The ink composition according to the present invention contains a cationic macromolecule (cationic polymer). A cationic polymer is a polymer obtained by polymerizing multiple monomers having cationic groups, and exhibits cationic properties in an aqueous medium such as water. The cationic polymer adsorbs to the colorant surface in the ink composition and positively charges the colorant surface, causing electrostatic repulsion between the colorants. This prevents contact between the colorants in the ink composition, suppresses aggregation of the colorants, and allows the colorants to be stably retained in the ink composition. In other words, the cationic polymer is effective as a dispersant for the colorant. Cationic polymers include not only polymers in which one type of monomer having a cationic group is polymerized in multiple units, but also block copolymers or random copolymers in which two or more types of monomers having a cationic group are polymerized in multiple units.

[0053] The cationic polymer is not particularly limited as long as it exhibits cationicity in an aqueous medium, and examples thereof include amine-based polymers such as polyethyleneimine, polyvinylpyridine, polyvinylamine, allylamine-based polymers, and diallylamine-based polymers. Among these, polymers having a constitutional unit represented by the following formula (I) or formula (II), i.e., allylamine-based polymers or diallylamine-based polymers, are preferred as cationic polymers, since the cationic group exhibits strong cationicity and is excellent in the effect of charging the colorant surface with a stronger positive charge. [ka] (In the formula, R1 and R1' are each independently a hydrogen atom or 1-3 n1 represents an integer of 0 or 1, and X1 represents any one of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid. [ka] (wherein R2 is a hydrogen atom or 1-3 n2 represents an integer of 0 or 1, and X2 represents any one of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid.

[0054] C 1-3 Examples of the linear or branched alkyl group include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. Examples of hydrogen halides include hydrogen chloride (hydrochloric acid), hydrogen bromide, and hydrogen iodide. Examples of the carboxylic acid include acetic acid, propionic acid, and butyric acid. Examples of sulfate esters include methyl sulfate and ethyl sulfate. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid, and 1-butanesulfonic acid. Examples of each can be given.

[0055] The cationic polymer according to the present invention is preferably a polymer having a structural unit in which R1 and R1' in formula (I) are both hydrogen atoms, and such a cationic polymer is represented by the following formula (Ia). [ka] (In the formula, n1a represents an integer of 0 or 1, and X 1a represents a hydrogen halide, a carboxylic acid, a sulfuric acid, a sulfate ester, a phosphoric acid, a sulfonic acid, or an amidosulfuric acid, and n represents a natural number.

[0056] The mass average molecular weight of the cationic polymer represented by formula (Ia) is in the range of 500 to 200,000, and n represents the degree of polymerization required to achieve a mass average molecular weight of 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 150,000, and more preferably 1,500 to 100,000.

[0057] The cationic polymer according to the present invention is a cationic polymer represented by the formula (Ia), in which n1a is 0 or 1, and X 1a A cationic polymer in which the base is any one of hydrochloric acid, acetic acid, and amidosulfuric acid is preferred. Such a cationic polymer is excellent in the effect of positively charging the colorant to generate electrostatic repulsion between the colorants, and further forms a network that extends throughout the ink composition by hydrogen bonds due to the N-H bonds of the primary amine, thereby exerting the effect of further improving the dispersion stability of the colorant.

[0058] Specific examples of the cationic polymer represented by formula (Ia) include those manufactured by Nittobo Medical Co., Ltd., product names: PAA-01, PAA-03, PAA-05, PAA-08, PAA-15C, PAA-25, PAA-HCl-01, PAA-HCl-03, PAA-HCl-05, PAA-HCl-3L, PAA-HCl-10L, PAA-SA, etc.

[0059] The cationic polymer according to the present invention is preferably a polymer having a structural unit in which R2 in formula (II) is a hydrogen atom or a methyl group, and such a cationic polymer is represented by the following formula (II-a). [ka] (In the formula, R 2a represents a hydrogen atom or a methyl group, n2a represents an integer of 0 or 1, and X 2a represents a hydrogen halide, a carboxylic acid, a sulfuric acid, a sulfate ester, a phosphoric acid, a sulfonic acid, or an amidosulfuric acid, and n represents a natural number.

[0060] The mass average molecular weight of the cationic polymer represented by formula (II-a) is in the range of 500 to 200,000, and n represents the degree of polymerization required to achieve a mass average molecular weight of 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 150,000, more preferably 2,000 to 100,000, and further preferably 5,000 to 50,000.

[0061] The cationic polymer according to the present invention is a polymer represented by the formula (II-a) represented by R 2a is a hydrogen atom, n2a is 1, and X 2a A cationic polymer in which the base is any one of hydrochloric acid, acetic acid, and amidosulfuric acid is preferred. Such a cationic polymer is excellent in the effect of positively charging the colorant to generate electrostatic repulsion between the colorants, and further forms a network that extends throughout the ink composition by hydrogen bonds due to the N-H bonds of the secondary amine, thereby exerting the effect of further improving the dispersion stability of the colorant.

[0062] Specific examples of the cationic polymer represented by formula (II-a) include those manufactured by Nittobo Medical Co., Ltd., product names: PAS-21, PAS-21CL, PAS-M-1L, PAS-M-1, PAS-22SA-40, PAS-M-1A, etc.

[0063] The cationic polymer according to the present invention is preferably a polymer having two or more kinds of constitutional units represented by formula (I) or formula (II). As such a cationic polymer, for example, a polymer represented by the following formula (III) can be mentioned. [ka] (In the formula, n3 and n3′ each independently represent an integer of 0 or 1; X3 and X3′ each independently represent any one of a hydrogen halide, a carboxylic acid, a sulfuric acid, a sulfate ester, a phosphoric acid, a sulfonic acid, and an amidosulfuric acid; R3 represents a hydrogen atom or 1-3 wherein m and n each independently represent a natural number.

[0064] The mass average molecular weight of the cationic polymer represented by formula (III) is in the range of 500 to 200,000, and m and n represent the degrees of polymerization required for the mass average molecular weight to be 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 150,000, more preferably 5,000 to 150,000, and further preferably 10,000 to 100,000.

[0065] As the cationic polymer according to the present invention, a cationic polymer in which, in formula (III), n3 and n3' are both 0 and R3 is a hydrogen atom, or n3 and n3' are both 1, X3 and X3' are both any of hydrochloric acid, acetic acid, or amidosulfuric acid, and R3 is a hydrogen atom is preferred. Specific examples of the cationic polymer represented by formula (III) include those manufactured by Nittobo Medical Co., Ltd., product names: PAA-D11, PAA-D11-HCL, PAA-D41-HCL, PAA-D19-HCL, PAA-D19A, and the like.

[0066] As the cationic polymer according to the present invention, for example, a polymer represented by the following formula (IV) can be used. [ka] (In the formula, n4 represents an integer of 0 or 1, X4 represents any one of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid, and X4' - represents a halide ion, a carboxylate ion, a sulfate ion, a sulfate ester ion, a phosphate ion, or a sulfonate ion, and R4 and R4′ each independently represent C 1-3 wherein m and n each independently represent a natural number.

[0067] Examples of halide ions include chloride ions, bromide ions, and iodide ions. Examples of carboxylate ions include acetate ion, propionate ion, butyrate ion, etc. Examples of sulfate ester ions include methyl sulfate ion and ethyl sulfate ion. Examples of sulfonate ions include methanesulfonate ion, ethanesulfonate ion, 1-propanesulfonate ion, and 1-butanesulfonate ion. Examples of each can be given.

[0068] The mass average molecular weight of the cationic polymer represented by formula (IV) is in the range of 500 to 200,000, and m and n represent the degrees of polymerization required for the mass average molecular weight to be 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 100,000, more preferably 5,000 to 50,000, and even more preferably 10,000 to 20,000.

[0069] As the cationic polymer according to the present invention, a cationic polymer in which n4 is 0, R4 and R4' are each independently a methyl group or an ethyl group in formula (IV) is preferred, and n4 is 0, R4 and R4' are both methyl groups, and X4' is - is chloride ion or ethyl sulfate ion (C2H5SO4 - ) is more preferable. Specific examples of the cationic polymer represented by formula (IV) include PAA-1123 (manufactured by Nittobo Medical Co., Ltd.).

[0070] As the cationic polymer according to the present invention, a polymer in which a primary amine of a polymer having a structural unit in which R1 and R1' in formula (I) are hydrogen atoms and n1 is 0 is partially modified can also be used. An example of such a cationic polymer is a polymer represented by the following formula (Ib). [ka] (In the formula, R 1b is C 1-3 A straight or branched alkyl group of C 1-3 represents a straight-chain or branched alkoxy group or an amino group, and m and n each independently represent a natural number.

[0071] C 1-3 Examples of the linear or branched alkyl group include a methyl group, an ethyl group, an n-propyl group, and an isopropyl group. C 1-3 Examples of the linear or branched alkoxy group include a methoxy group, an ethoxy group, an n-propoxy group, and an isopropoxy group. Examples of each can be given.

[0072] The mass average molecular weight of the cationic polymer represented by formula (Ib) is in the range of 500 to 200,000, and m and n represent the degrees of polymerization required for the mass average molecular weight to be 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 150,000, more preferably 1,500 to 100,000, and further preferably 5,000 to 50,000.

[0073] Specific examples of the cationic polymer represented by formula (Ib) include those manufactured by Nittobo Medical Co., Ltd., product names: PAA-U5000, PAA-U7030, PAA-AC5050A, PAA-N5000, PAA-N5050CL, and the like.

[0074] As the cationic polymer according to the present invention, a polymer having a sulfonyl group (-SO2-) in the structural unit represented by formula (II) can also be used. Such a cationic polymer is represented by the following formula (II-b). [ka] (In the formula, R 2b represents a hydrogen atom or a methyl group, n2b represents an integer of 0 or 1, and X 2b represents a hydrogen halide, a carboxylic acid, a sulfuric acid, a sulfate ester, a phosphoric acid, a sulfonic acid, or an amidosulfuric acid, and n represents a natural number.

[0075] The mass average molecular weight of the cationic polymer represented by formula (II-b) is in the range of 500 to 200,000, and n represents the degree of polymerization required to achieve a mass average molecular weight of 500 to 200,000. The mass average molecular weight is preferably in the range of 1,000 to 50,000, more preferably 1,500 to 25,000, and further preferably 2,000 to 10,000.

[0076] The cationic polymer according to the present invention is a polymer represented by the formula (II-b) represented by R 2b is a hydrogen atom or a methyl group, n2b is 1, and X 2b is preferably a cationic polymer in which R is hydrochloric acid, acetic acid, or amidosulfuric acid; 2b is a hydrogen atom, n2b is 1, and X 2b More preferred is a cationic polymer in which the acid is hydrochloric acid or acetic acid. Specific examples of the cationic polymer represented by formula (II-b) include those manufactured by Nittobo Medical Co., Ltd., product names: PAS-92, PAS-92A, PAS-2201CL, and the like.

[0077] Among the above-mentioned cationic polymers, the cationic polymer according to the present invention is preferably a cationic polymer represented by any one of formulas (Ia), (II-a) and (II-b). A colorant positively charged using these cationic polymers has excellent interaction with cellulose nanofibers, which will be described later, and thus has the effect of improving the dispersion stability of the colorant over time.

[0078] The mass average molecular weight of the cationic polymer is a value calculated as polyethylene glycol by the GPC method (gel permeation chromatography method).

[0079] The cationic polymer having the constitutional unit represented by the above formula (I) or (II) is easily adsorbed to the surface of the microcapsule pigment whose wall is composed of an organic substance, and therefore it is suitable to use the microcapsule pigment as a colorant. By using the above-mentioned cationic polymer, the dispersion stability of the microcapsule pigment in the ink composition can be improved.

[0080] 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 the above-mentioned resin, the cationic polymer having the structural unit represented by the above formula (I) or formula (II) is more easily adsorbed on the surface of the microcapsule pigment, and the effect of further improving the dispersion stability of the microcapsule pigment is exerted.

[0081] The blending ratio of the cationic polymer is not particularly limited, but the cationic polymer is blended in the range of preferably 0.05 to 2 mass %, more preferably 0.1 to 1 mass %, and even more preferably 0.1 to 0.5 mass % of the total amount of the ink composition. When the blending ratio is within the above range, the cationic polymer is easily adsorbed to the colorant, and the effect of acting as a dispersant for the colorant is easily exhibited.

[0082] The ink composition according to the present invention contains cellulose nanofibers (hereinafter sometimes referred to as "CNF"). Cellulose nanofiber is a material made by uniformly micronizing cellulose-based fiber raw materials at the nano level, and is obtained by mechanically untangling (defibrating) the fiber raw materials. The cellulosic fiber raw material is not particularly limited as long as it is a material mainly composed of cellulose, and examples thereof include pulp, natural cellulose, regenerated cellulose, crystalline cellulose made from pulp, etc. Also, fine cellulose obtained by mechanically depolymerizing a cellulosic fiber raw material can be used. The cellulose nanofibers form a network structure through interactions in the ink composition, preventing the aforementioned colorants adsorbed by the cationic polymer from coming into contact with each other, thereby having the effect of stably retaining the colorant in the ink composition.

[0083] The average fiber width of the cellulose nanofiber is not particularly limited, but is preferably in the range of 1 to 1000 nm, more preferably 2 to 500 nm. When the average fiber width is within the above range, the cellulose nanofiber is excellent in dispersibility in the ink composition, and the dispersed state of the cellulose nanofiber is easily maintained. In addition, the transparency of the cellulose nanofiber can be excellent.

[0084] 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 colorant, the hue of the cellulose nanofibers may affect the hue of the ink composition. Therefore, it is also suitable to adjust the average fiber width of the cellulose nanofibers to a range of preferably 2 to 30 nm, more preferably 2 to 20 nm, and even more preferably 2 to 10 nm, so that transparency can be obtained when the cellulose nanofibers are dispersed in water. By having the average fiber width 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 colorant. If the average fiber width of the cellulose nanofiber is greater than 30 nm, it approaches 1 / 10 of the wavelength of visible light, and when other additives are blended into the ink composition, refraction and scattering of visible light are likely to occur at the interface, resulting in scattering of visible light and a tendency for transparency to decrease. Therefore, from the viewpoints of handleability and transparency, the average fiber width of the cellulose nanofiber is preferably in the range of 2 to 30 nm, more preferably 2 to 20 nm, and even more preferably 2 to 10 nm.

[0085] Here, an ink composition containing a reversible thermochromic material or a reversible photochromic material as a colorant reversibly changes color from a colored state to a decolorized state due to a change in temperature or irradiation with 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 colorant 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 width of the cellulose nanofiber is within the above range.

[0086] The average fiber width of cellulose nanofibers refers to the number-average fiber width, and the number-average fiber width can be measured using known techniques. For example, cellulose nanofibers are dispersed in a solvent such as pure water to prepare a mixed solution with a predetermined concentration. This mixed solution is then spin-coated onto a silica substrate coated with polyethyleneimine (PEI), and the number-average fiber width can be measured by observing the cellulose nanofibers on the silica substrate. As an observation method, for example, a scanning probe microscope (e.g., Shimadzu Corporation, product name: SPM-9700) can be used. The number average fiber width (average fiber width) of the cellulose nanofibers can be obtained by randomly selecting 20 cellulose nanofibers in the obtained observation image, measuring the fiber width of each fiber, and averaging the measured values.

[0087] The average fiber length of the cellulose nanofibers is not particularly limited, but is preferably in the range of 10 to 1000 nm, more preferably 100 to 800 nm. When the average fiber length is within the above range, the cellulose nanofibers have excellent dispersibility in the ink composition, and the network structure formed by the cellulose nanofibers themselves is stabilized, making it easier to maintain the dispersed state. If the average fiber length is greater than 1000 nm, the dispersibility of the cellulose nanofibers in the ink composition 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.

[0088] The average fiber length of cellulose nanofibers refers to the number-average fiber length, and the number-average fiber length can be measured using known techniques. For example, a fine fiber sheet is obtained by filtering and desolvating wet cellulose nanofibers using a scanning electron microscope (SEM), freeze-drying the sheet in liquid nitrogen, and observing the sheet with the SEM. Twenty cellulose nanofibers are randomly selected from the obtained observation image, and the fiber length of each fiber is measured and averaged to determine the number-average fiber length (average fiber length) of the cellulose nanofibers.

[0089] Cellulose nanofibers have cellulose microfibrils, which are bundles of cellulose molecular chains, as their constituent units. Because cellulose microfibrils are bundled by hydrogen bonds between the cellulose microfibrils, it is difficult to finely disperse them into cellulose microfibrils. For this reason, the cellulose nanofibers used in the present invention are those obtained by modifying some of the hydroxyl groups of the cellulosic fiber raw material to introduce ionic functional groups, weakening the hydrogen bonds between the cellulose microfibrils, and finely finely dispersing them into cellulose microfibrils by electrostatic repulsion due to the ionic functional groups. Examples of such cellulose nanofibers include oxidized cellulose nanofibers, sulfated cellulose nanofibers, phosphated cellulose nanofibers, etc. Hereinafter, these cellulose nanofibers may be collectively referred to as "anionic functional group-introduced cellulose nanofibers."

[0090] Oxidized cellulose nanofibers are cellulose-based fiber raw materials with type I crystal structure in which at least a portion of the hydroxyl groups (-OH) in the β-glucose that constitutes the cellulose have been modified to at least one functional group, an aldehyde group (-CHO) or a carboxyl group (-COOH).

[0091] The method for producing oxidized cellulose nanofibers is not particularly limited, and they can be produced by known production methods. For example, a method in which an N-oxyl compound as an oxidation catalyst and a co-oxidant are allowed to act on a cellulose-based fiber raw material can be mentioned. As the cellulose nanofibers according to the present invention, TEMPO-oxidized cellulose nanofibers using TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) as an oxidation catalyst can be used. Specifically, cellulosic fiber raw materials are dispersed in water, and TEMPO is added as an oxidation catalyst and hypochlorous acid or its salts (e.g., sodium hypochlorite) as a co-oxidant, and the mixture is stirred at room temperature to react, modifying the hydroxyl group (primary hydroxyl group) at the C6 position of the β-glucose that constitutes the cellulose, and introducing a carboxyl group. The product is then separated into solid and liquid by suction filtration, and the resulting filtered material is washed and purified. The filtered material is dispersed in pure water to form a slurry, and mechanical defibration is performed to obtain oxidized cellulose nanofibers (TEMPO oxidized cellulose nanofibers).

[0092] Oxidized cellulose nanofibers can also be produced by the production method described in Japanese Patent No. 6769550, which uses hypochlorous acid or a salt thereof. Specifically, the cellulose-based fiber raw material is added to a sodium hypochlorite aqueous solution with an effective chlorine concentration of 14 to 43% by mass, and reacted with stirring at 30°C for 30 minutes to modify the hydroxyl group (primary hydroxyl group) at C6 of the β-glucose that constitutes the cellulose, and introduce a carboxyl group. The product is then separated into solid and liquid by suction filtration, and the resulting filtered material is washed and purified. The filtered material is dispersed in pure water to form a slurry, and mechanical defibration is performed to obtain oxidized cellulose nanofibers.

[0093] Sulfated cellulose nanofibers are cellulose-based fiber raw materials having a type I crystal structure in which at least a portion of the hydroxyl groups (-OH) in the β-glucose that constitutes the cellulose have been modified by sulfate esterification. For example, sulfated cellulose nanofibers in which at least a portion of the hydroxyl groups of the cellulose-based fiber raw materials have been modified to sulfo groups represented by the following formula (1) can be given as an example.

[0094] (-SO3 - ) r Z r+ (1) (wherein, r is an independent natural number from 1 to 3; Z r+ is at least one selected from the group consisting of a hydrogen ion, an alkali metal cation, an ammonium ion, an aliphatic ammonium ion, and an aromatic ammonium ion when r=1. Also, is at least one selected from the group consisting of an alkaline earth metal cation and a polyvalent metal cation when r=2 or 3.

[0095] The method for producing sulfated cellulose nanofibers is not particularly limited, and they can be produced by the known production method described in Japanese Patent No. 6,582,111, for example. Specifically, the cellulose-based fiber raw material is added to a reaction solution in which a sulfonating agent such as sulfamic acid and urea or / and its derivatives are dissolved in water, and the reaction is adjusted to 100 to 180°C for 5 minutes or more to modify at least a portion of the hydroxyl groups in the cellulose and introduce sulfo groups. The product is then separated into solid and liquid by suction filtration, and the resulting filtered material is washed and purified. The filtered material is dispersed in pure water to form a slurry, and mechanical defibration is performed to obtain sulfated cellulose nanofibers.

[0096] Phosphated cellulose nanofibers are cellulose-based fiber raw materials having a type I crystal structure in which at least some of the hydroxyl groups (-OH) in the β-glucose that constitutes the cellulose have been modified by phosphate esterification. For example, phosphated cellulose nanofibers are exemplified by those in which phosphate groups have been introduced by dehydrating at least some of the hydroxyl groups of the cellulose fiber raw materials with a compound containing a phosphate group or a salt thereof.

[0097] The method for producing the phosphated cellulose nanofibers is not particularly limited, and they can be produced by the known production method described in Japanese Patent No. 5798504, for example. Specifically, the cellulose fiber raw material is immersed in an aqueous solution of disodium hydrogen phosphate and heated at 170°C for two and a half hours to denature at least some of the hydroxyl groups in the cellulose and introduce phosphate groups. The resulting product is then washed and purified. This product is dispersed in ion-exchanged water to form a slurry, and mechanical defibration is performed to obtain phosphated cellulose nanofibers.

[0098] The method of mechanical defibration is not particularly limited, and defibration can be performed using devices such as a screw type mixer, a dispersion type mixer, a turbine type mixer, a homomixer, a high pressure homogenizer, an ultra-high pressure homogenizer, a double cylinder type homogenizer, an ultrasonic homogenizer, a water flow opposing collision type disperser, a beater, a disk type refiner, a conical type refiner, a double disk type refiner, a grinder, or a single-shaft or multi-shaft kneader.

[0099] The cellulose nanofiber according to the present invention has anionic functional groups such as carboxyl groups, sulfo groups, and phosphate groups, and therefore has improved hydrophilicity and excellent dispersibility when dispersed in an ink composition. Furthermore, the electronic repulsion of these anionic functional groups makes it easier to maintain the dispersed state in the ink composition, and a network structure is formed by the interaction between the cellulose nanofibers. This prevents contact between the colorants in the ink composition and suppresses the aggregation of the colorants, so the cellulose nanofiber has the effect of stably holding the colorant in the ink composition. In addition, the cellulose nanofiber also has the effect of acting as a thickener or gelling agent depending on the fiber length, which leads to the colorant being held more stably in the ink composition. In other words, the cellulose nanofiber has the effect of improving the dispersion stability of the colorant.

[0100] The ink composition according to the present invention uses a cationic polymer and cellulose nanofibers in combination, and thereby exhibits the effect of improving the dispersion stability of a colorant in the ink composition. As described above, the cationic polymer positively charges the colorant, and the colorant is maintained in a dispersed state in the ink composition due to electrostatic repulsion. In addition, anionic functional groups are introduced into the cellulose nanofibers, and the cellulose nanofibers are easily maintained in a dispersed state in the ink composition due to electronic repulsion by the anionic functional groups, and a network structure is formed by the interaction. In addition, ionic interaction occurs between the cationic colorant and the anionic cellulose nanofibers. Due to these actions, the colorant is adsorbed to the cellulose nanofibers while maintaining a dispersed state due to electrostatic repulsion, and the colorant is incorporated into the network structure formed by the cellulose nanofibers, so that the aggregation of the colorant is further suppressed. Therefore, the dispersion stability of the colorant in the ink composition can be further improved by using the cationic polymer and the cellulose nanofibers in combination.

[0101] As the cellulose nanofiber according to the present invention, bacterial cellulose nanofiber (fermented cellulose nanofiber) can also be used. Bacterial cellulose nanofibers are cellulose nanofibers synthesized by bacteria, preferably acetic acid bacteria. By culturing bacteria such as acetic acid bacteria in an appropriate medium with aeration and agitation, cellulose nanofibers with fiber widths of approximately 50 to 100 nm are secreted outside the cells, and bacterial cellulose nanofibers are obtained by separating and collecting these.

[0102] The blending ratio of the cellulose nanofibers is not particularly limited, but the cellulose nanofibers are blended in the range of preferably 0.01 to 0.045 mass %, more preferably 0.02 to 0.04 mass %, of the total amount of the ink composition. If the blending ratio is less than 0.01% by mass, it is difficult to obtain the desired effect of stably maintaining the colorant in a dispersed state, whereas if the blending ratio exceeds 0.045% by mass, it is difficult to obtain an improvement in the dispersion stability effect.

[0103] In the ink composition according to the present invention, the mass ratio of the cellulose nanofibers to the cationic polymer is preferably in the range of 1:1 to 1:20, more preferably 1:3 to 1:19, and even more preferably 1:5 to 1: 18. When the mass ratio is within the above range, ionic interactions are more likely to occur between the colorant positively charged by the cationic polymer and the cellulose nanofibers, making it easier to improve the dispersion stability of the colorant in the ink composition.

[0104] Conventionally, in order to improve the dispersion stability of the colorant, 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 colorant, 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 can stably hold the colorant for a long period of time while having a lower viscosity than ink compositions using a conventional thickener alone due to the electrostatic repulsion between colorants caused by the cationic polymer and the network structure formed by the cellulose nanofibers. Therefore, by using a cationic polymer 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 colorant over time is achieved. In addition, since the ink composition can be made lower in viscosity than ink compositions using a conventional thickener alone, the ink dischargeability of the writing instrument containing this ink composition is improved, writing defects such as smearing are suppressed, the writing feel is improved, and the color development of the handwriting can be excellent. When the ink composition is used in a writing instrument (ballpoint pen) equipped with a ballpoint tip, line cracking in handwriting can be suppressed, and good writing performance can be achieved.

[0105] The ink composition according to the present invention can also use a colorant having a relatively large average particle size (hereinafter, sometimes referred to as a "large particle size colorant") or a colorant having a specific gravity of more than 1 (hereinafter, sometimes referred to as a "high specific gravity colorant"). Large particle size colorants and high specific gravity colorants tend 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 of large particle size colorants and high specific gravity colorants over time while keeping the ink composition low in viscosity by using the above-mentioned cationic polymer and cellulose nanofibers in combination, and can stably disperse such colorants for a long period of time. Examples of the large particle colorant include glitter pigments, reversible thermochromic microcapsule pigments, and reversible photochromic microcapsule pigments. Examples of high specific gravity colorants include inorganic pigments, organic pigments, glittering pigments, microencapsulated pigments or resin particles using these, and reversible thermochromic microencapsulated pigments with a large hysteresis width (ΔH).

[0106] The lustrous pigment is not particularly limited as long as it is a pigment that exhibits lustrous properties by reflecting light, and examples thereof include naturally derived pigments such as fish scale foil, pigments in which a transparent base material is coated with a metal oxide or metal, and metallic pigments.

[0107] Examples of pigments in which a transparent substrate is coated with a metal oxide include those in which the substrate is made of a material selected from natural mica, synthetic mica, flat glass (flake-like glass), silica flakes, flaky aluminum oxide, etc., and the surface of the substrate is coated with a metal oxide. Examples of metal oxides include oxides of titanium, zirconia, chromium, vanadium, iron, etc., with titanium oxide being preferred. Depending on the coverage rate of the metal oxide that coats the surface of the substrate and the thickness of the coating, the substrate may exhibit a gold or silver color, or a yellow, red, blue, or green color with metallic luster. Note that a layer of a metal oxide such as titanium oxide may be further coated with a non-discoloring colorant such as iron oxide or a general dye or pigment. Pigments in which a transparent substrate is coated with a metal oxide include pearl pigments and cholesteric liquid crystal pigments.

[0108] Examples of pigments in which a transparent substrate is coated with a metal include flake glass coated with silver, flake glass coated with gold, flake glass coated with nickel chromium molybdenum, flake glass coated with brass, flake glass coated with a silver alloy, and flake glass coated with titanium. A pigment in which a transparent substrate is coated with a metal can be obtained, for example, by coating glass flakes with a metal by electroless plating or sputtering.

[0109] An example of the metallic pigment is an aluminum powder pigment. An example of an aluminum powder pigment is one obtained by pulverizing and grinding aluminum flakes in a ball mill together with a petroleum-based solvent such as a higher fatty acid or mineral spirits. Such aluminum powder pigments are very thin, scaly aluminum particles, and can be used in the form of a paste. Alternatively, thin film aluminum obtained by vacuum deposition may be pulverized into fine powder.

[0110] The average particle size of the glitter pigment is preferably in the range of 3 to 25 μm, more preferably 5 to 20 μm. If the average particle size exceeds 25 μm, it becomes difficult to obtain good ink dischargeability when used in a writing instrument. On the other hand, if the average particle size is less than 3 μm, it becomes difficult for the handwriting to show sufficient glitter.

[0111] The average particle size of the reversible thermochromic microencapsulated pigment or the reversible photochromic microencapsulated pigment is preferably in the range of 0.1 to 5 μm, more preferably 0.5 to 4 μm, and even more 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 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.

[0112] 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.

[0113] 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.).

[0114] 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.

[0115] Examples of inorganic pigments include carbon black, ultramarine, and titanium oxide of rutile type or anatase type, etc. The specific gravity of carbon black and ultramarine is in the range of 1.4 to 5.5, and that of titanium oxide is in the range of 3.7 to 4.2.

[0116] Examples of organic pigments include benzidine yellow, permanent red, lake red, Victoria blue lake, phthalocyanine blue, phthalocyanine green, aniline black, etc. The specific gravity is in the range of 1.2 to 2.9.

[0117] Examples of the luster pigment include pigments in which the above-mentioned transparent substrate is coated with a metal oxide or metal, and metallic pigments. The specific gravity of pigments in which a transparent substrate is coated with a metal oxide is in the range of 2.8 to 3.2 for pearl pigments and 1.1 to 1.5 for cholesteric liquid crystal pigments. The specific gravity of a pigment in which a transparent substrate is coated with a metal is in the range of 3.0 to 3.4. The specific gravity of the metallic pigment is in the range of 2.5 to 9.0.

[0118] Reversible thermochromic microencapsulated pigments with a large hysteresis width often use a compound having two or more aromatic rings in the molecule as component (c), and therefore tend to have a large specific gravity. The specific gravity of the reversible thermochromic microencapsulated pigment depends on the particle size, the components encapsulated in the microcapsules and their contents, the components and film thickness of the capsule wall, the colored state of the microencapsulated pigment, and the temperature. From the viewpoint of dispersion stability in the ink composition, the specific gravity of the reversible thermochromic microencapsulated pigment is preferably in the range of 1.05 to 1.20, more preferably 1.10 to 1.20, and even more preferably 1.12 to 1.15 when the microencapsulated pigment is in a completely colored state and water is used as the reference substance in an environment of 20°C. The specific gravity of the reversible thermochromic microencapsulated pigment can be measured by the following method.

[0119] (Method for measuring specific gravity of reversible thermochromic microencapsulated pigment) 1. Put 30 ml of glycerin aqueous solution and 1 g of fully colored, reversible thermochromic microcapsule pigment are mixed into a screw cap bottle to prepare a microcapsule pigment dispersion. 2. 30 ml of the microcapsule pigment dispersion is adjusted to 20° C. and centrifuged at 1000 rpm for 30 seconds. A refrigerated tabletop centrifuge (manufactured by Kokusan Co., Ltd., product name: H103N) can be used as the centrifuge. 3. Observe the microcapsule pigment dispersion. If most of the microencapsulated pigment has settled to the bottom of the beaker, repeat steps 1 and 2 using an aqueous solution with a higher glycerin concentration than that used at this time, and observe the state of the dispersion. If it is confirmed that most of the microencapsulated pigment is floating on the liquid surface, repeat steps 1 and 2 using an aqueous solution with a lower glycerin concentration than the glycerin aqueous solution used earlier, and observe the state of the dispersion. The above series of operations is repeated until it is visually confirmed that the majority of the microencapsulated pigment is not floating on the surface or settling, but that the glycerin aqueous solution is uniformly colored except for the surface of the glycerin aqueous solution and the area near the bottom of the screw tube bottle. The specific gravity of the glycerin aqueous solution when this state is observed is measured and is regarded as the specific gravity of the reversible thermochromic microencapsulated pigment. The specific gravity of the glycerin aqueous solution can be measured by the hydrometer method described in JIS K0061, Section 7.1, using an aqueous solution adjusted to 20°C.

[0120] 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.

[0121] 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 lubricants include higher fatty acids such as oleic acid; nonionic surfactants having a long-chain alkyl group; polyether-modified silicone oils; thiophosphite triesters such as thiophosphite tri(alkoxycarbonylmethyl ester) and thiophosphite tri(alkoxycarbonylethyl ester); phosphate surfactants such as polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether phosphate monoesters, polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether phosphate diesters, or metal salts, ammonium salts, amine salts, and alkanolamine salts of these phosphate esters.

[0122] The ink composition according to the present invention may also contain various additives, such as water-soluble organic solvents, thickeners, shear thinning agents, polymer flocculants, water-soluble resins, specific gravity adjusters, surfactants, pH adjusters, rust inhibitors, preservatives or fungicides, air bubble absorbers, antifoamers, antioxidants and ultraviolet absorbers, if necessary.

[0123] When the colorant contains a reversible thermochromic microencapsulated pigment or a reversible photochromic microencapsulated pigment, or a reversible thermochromic resin particle or a reversible photochromic resin particle, it is also possible to make an ink composition that exhibits a color change behavior from colored (1) to colored (2) by blending a non-color-changing colorant such as a general dye or pigment.

[0124] 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.

[0125] 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.

[0126] The viscosity of the ink composition according to the present invention is preferably in the range of 1 to 50 mPa s, more preferably 1 to 40 mPa s, and even more preferably 1 to 35 mPa s in an environment of 20° C. 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 can be measured, for example, using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34′×R24)) at a rotation speed of 20 rpm or 50 rpm, with the ink composition placed in an environment of 20°C.

[0127] 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 can be measured by placing the ink in an environment of 20° C. using a pH meter (manufactured by DKK-TOA Corp., product name: IM-40S).

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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. In general, when an ink composition containing a large particle colorant or a high specific gravity colorant is applied to a ballpoint pen with a small diameter ball, the colorant aggregates or settles over time, causing clogging at the pen tip due to the aggregates of the colorant, and the ink dischargeability from the pen tip may decrease, resulting in impaired writing density and writing defects such as smearing and skipped lines. However, when the ink composition according to the present invention, which has excellent dispersion stability of the colorant, 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 colorant 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 writing defects such as smearing and skipped lines are suppressed. Generally, a ballpoint pen with a large diameter 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 a colorant 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 one that allows writing with a smoother writing feel and can form clear and dense handwriting.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] In addition, the barrel itself can serve as an ink filling mechanism. By filling the barrel directly with ink and attaching a ballpoint tip to the front end of the barrel, a ballpoint pen equipped with a ballpoint tip and an ink filling mechanism can be formed.

[0139] When the ink filled in the ink filling mechanism has a low viscosity, the ballpoint pen equipped with the ballpoint pen tip and the ink filling mechanism may further include an ink supply mechanism for supplying the ink filled in the ink filling mechanism to the pen tip.

[0140] 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.

[0141] 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.

[0142] 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 reservoir filled with ink within 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 reservoir; (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.

[0143] 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.

[0144] 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.

[0145] 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%.

[0146] A marking pen can be formed by housing an ink occlusion body impregnated with ink inside a 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.

[0147] Also, a marking pen refill (hereinafter sometimes referred to as a "refill") can be formed by housing an ink occlusion body impregnated with ink in an ink reservoir and connecting a marking pen tip to the ink reservoir directly or via a connecting member. A marking pen can be formed by housing this refill in a barrel.

[0148] 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.

[0149] The marking pen having the marking pen tip and the ink filling mechanism may further have an ink supply mechanism. The ink supply mechanism supplies the ink composition filled in the ink filling mechanism to the pen tip.

[0150] 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.

[0151] 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.

[0152] 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 is impregnated with ink and contained in a barrel, and a marking pen tip made of a fiber processed body or a resin molded body having 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 filled directly 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 made of a fiber bundle or the like as an ink flow regulator; (3) a marking pen in which the barrel is filled directly 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 filled directly into the ink container.

[0153] 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.

[0154] 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. 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.

[0155] The writing implement according to the present invention, such as a ballpoint pen or a marking pen, can be provided with a cap to form a capped writing implement. By attaching a cap so as to cover the pen tip (writing tip), it is possible to prevent the writing tip from being contaminated or damaged. Furthermore, a writing instrument such as a ballpoint pen or a marking pen that has a refill stored in the barrel can be provided with a retractable mechanism to make it a retractable writing instrument. The retractable mechanism is provided in the barrel and allows the writing tip to protrude and retract from the barrel, preventing the writing tip from being contaminated or damaged.

[0156] 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 in 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.

[0157] 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.

[0158] The form of the ballpoint pen or marking pen is not limited to the above configuration, and may be a composite writing instrument (double-ended, retractable tip, etc.). Examples of composite writing instruments include (1) writing instruments equipped with tips of different shapes, (2) writing instruments equipped with tips that deliver ink of different tones or hues, and (3) writing instruments equipped with tips of different shapes and in which the color tones or hues of ink delivered from each tip are different.

[0159] A writing instrument containing the ink composition according to the present invention is preferably a writing instrument (ballpoint pen) having a ballpoint pen tip as the pen tip. The ink composition according to the present invention is preferably used for a ballpoint pen because the colorant is stably held in the ink composition, the colorant is unlikely to clog at the tip end, and good handwriting can be formed while suppressing poor writing. Furthermore, the ink composition is preferably used for a ballpoint pen because it has excellent dispersion stability of the colorant while having a low viscosity, and therefore has good ink dischargeability and can form handwriting with excellent color development.

[0160] When the colorant contains a reversible thermochromic microencapsulated pigment or a reversible thermochromic resin particle, 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] 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.

[0167] 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

[0168] 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.

[0169] Example 1 Preparation of Ink Composition An ink composition was prepared by mixing 30 parts of a blue pigment water dispersion (pigment blue 15:3) (solid content: 20%, average particle size: 0.2 μm), 1 part of an allylamine polymer (cationic polymer) [manufactured by Nittobo Medical Co., Ltd., product name: PAA-15C (mass average molecular weight: 15,000, concentration: 15%)], 0.025 parts of TEMPO oxidized cellulose nanofiber, 0.5 parts of a surfactant [manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL], 0.1 parts of triethanolamine, 5 parts of glycerin, 10 parts of diethylene glycol, and 53.375 parts of water.

[0170] Creation of writing implements The ink composition 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 into the rear end of the ink container, and a tail plug was fitted to the rear of the pipe, followed by degassing by centrifugation to obtain a ballpoint pen refill. Next, the above refill was incorporated into a barrel to prepare a ballpoint pen (retractable ballpoint pen). The ballpoint pen described above has a tip attached to a ballpoint pen refill that is stored in 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 mechanism (slide mechanism) attached to the rear side wall of the barrel.

[0171] Examples 2 to 9 and Comparative Examples 1 to 6 Preparation of Ink Composition The ink compositions of Examples 2 to 9 and Comparative Examples 1 to 6 were prepared in the same manner as in Example 1, except that the types and amounts of the materials to be blended were changed to those shown in Tables 1 and 2 below.

[0172] Creation of writing implements The writing instruments of Examples 2 to 9 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1.

[0173] [Viscosity measurement] The viscosity of each of the ink compositions prepared in Examples 1 to 9 and Comparative Examples 1 to 6 was measured at room temperature (20°C) and a rotation speed of 20 rpm using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34′×R24)). The measurement results are shown in Table 1.

[0174] [Table 1]

[0175] [Table 2]

[0176] The contents of the materials in Tables 1 and 2 are explained according to the note numbers. (1) Blue pigment water dispersion (Pigment Blue 15:3) (solid content: 20%, average particle size: 0.2 μm) (2) Black pigment water dispersion (carbon black) [manufactured by Fuji Pigment Co., Ltd., product name: FUJI SP BLACK 8065 (solid content: 20%, average particle size: 0.017 μm)] (3) Reversible thermochromic microencapsulated pigment (4) Allylamine polymer [manufactured by Nittobo Medical Co., Ltd., product name: PAA-15C (mass average molecular weight: 15,000, concentration: 15%)] (5) Diallylamine hydrochloride polymer [manufactured by Nittobo Medical Co., Ltd., product name: PAS-21CL (mass average molecular weight: 50,000, concentration: 25%)] (6) Diallylamine acetate-sulfur dioxide copolymer [manufactured by Nittobo Medical Co., Ltd., product name: PAS-92A (mass average molecular weight: 5,000, concentration: 20%)] (7) Allylamine hydrochloride-diallylamine hydrochloride copolymer [manufactured by Nittobo Medical Co., Ltd., product name: PAA-D19-HCl (mass average molecular weight: 40,000, concentration: 21%)] (8) TEMPO-oxidized cellulose nanofiber (cellulose nanofiber with cellulose type I crystal structure in which the hydroxyl group at C6 of cellulose is modified to a carboxyl group) (9) Sulfated cellulose nanofiber (cellulose nanofiber with cellulose type I crystal structure in which some of the hydroxyl groups of the cellulose are modified with sodium sulfate) (10) Sodium polytungstate (manufactured by SOMETU, product name: SPT) (11) Daiichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL

[0177] The reversible thermochromic microencapsulated pigment was prepared as follows. 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 8% polyvinyl alcohol aqueous solution, and stirred while heating, and then 2.5 parts of water-soluble aliphatic modified amine was added, and further stirring was continued to prepare a microcapsule dispersion. A reversible thermochromic microcapsule pigment having an average particle size of 1.9 μm was obtained from the above microcapsule dispersion by centrifugation. The reversible thermochromic microencapsulated pigment had a complete color development temperature t1 of -20°C and a complete decolorization temperature t4 of 60°C, and reversibly changed from blue to colorless due to temperature change. Furthermore, the reversible thermochromic microencapsulated pigment in a completely colored state had a specific gravity of 1.08 to 1.09 at 20° C. based on water.

[0178] [Evaluation of initial writing performance] Using each of the writing implements prepared in Examples 1 to 9 and Comparative Examples 1 to 6, 12 elliptical circles with a major axis of about 15 mm and a minor axis of about 8 mm were handwritten in a spiral pattern in a direction parallel to the short side of an A4 size test paper (portrait orientation) in a room temperature (20°C) environment, so that the circles touch each other, and this was done for five 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 3 and 4 below, with an evaluation of "A" 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: Many smudges or missing lines were observed in the handwriting, or writing was impossible.

[0179] [Evaluation of writing performance over time] Each writing implement 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 upwards (upright position). After 30 days, the implement was removed from the thermostatic chamber and, in a room temperature (20°C) environment, 12 elliptical circles with a major axis of about 15 mm and a minor axis of about 8 mm were handwritten in a spiral pattern so that the circles touched each other, parallel to the short side of an A4-sized test paper (portrait). This was repeated for five 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 3 and 4 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, and the color of the handwriting was slightly lighter than the initial handwriting, but this was at a level that did not cause any problems in practical use. C: Writing was possible, but the color of the handwriting was lighter 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.

[0180]

Table 3

[0181]

Table 4

Claims

1. An aqueous ink composition for a writing instrument, comprising a colorant selected from pigments or resin particles, a cationic polymer, cellulose nanofibers, and water.

2. 2. The ink composition according to claim 1, wherein the cationic polymer is a polymer having a structural unit represented by the following formula (I) or formula (II): 【Chemistry 1】 (In the formula, R １ and R １ Each ' is independently a hydrogen atom or C １－３ n1 represents an integer of 0 or 1; X １ represents any of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid.) 【Chemistry 2】 (In the formula, R ２ is a hydrogen atom or C １－３ n2 represents an integer of 0 or 1; X ２ represents any of hydrogen halide, carboxylic acid, sulfuric acid, sulfate ester, phosphoric acid, sulfonic acid, and amidosulfuric acid.)

3. The R １ and R １ 3. The ink composition of claim 2, wherein both of ' are hydrogen atoms.

4. The R ２ The ink composition according to claim 2, wherein is a hydrogen atom.

5. The ink composition according to any one of claims 1 to 4, wherein a mass ratio of the cellulose nanofibers to the cationic polymer is 1:1 to 1:

20.

6. The ink composition according to any one of claims 1 to 4, wherein the colorant has an average particle size of 0.01 to 25 µm.

7. A writing implement containing the ink composition according to any one of claims 1 to 4.

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

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