Aqueous ink composition for writing instruments, and writing instrument housing same

Abstract

This aqueous ink composition for writing instruments contains water, a coloring agent, and sulfuric acid esterified cellulose fibers in which at least a portion of hydroxyl groups of cellulose fibers is converted into a sulfuric acid ester.

Description

Water-based ink composition for writing implements and writing implements containing the same

[0001] The present disclosure relates to a water-based ink composition for a writing instrument and a writing instrument containing the same.

[0002] Conventionally, inks using water as the primary solvent (water-based inks) have been widely used due to their low odor and high safety. Colorants such as pigments and resin particles typically have unstable dispersion stability in water. Unless the colorants are uniformly dispersed and stabilized, aggregation and / or sedimentation occur, resulting in poor writing quality, such as reduced density of handwriting formed by a writing instrument containing the ink, and poor ink discharge from the pen tip, resulting in skipped lines and blurred lines. Therefore, ink compositions have been disclosed that use various dispersants and / or additives to improve the dispersion stability of pigments or resin particles in water-based inks (see, for example, Patent Documents 1 to 3).

[0003] Patent Document 1 discloses an aqueous ink composition containing water, a pigment, and an alkali salt of a carboxy group-containing compound as a dispersant.

[0004] Patent Document 2 discloses a water-based ink containing water, a pigment, xanthan gum, and a non-crosslinked polyacrylic acid or a salt thereof having a specific molecular weight.

[0005] Patent Document 3 discloses an aqueous ink composition containing 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.

[0006] Furthermore, when the pen is stored with the pen tip facing downward, ink droplets tend to accumulate at the tip of the pen, a phenomenon known as drooping. Attempts have been made to prevent this drooping by adjusting the ink viscosity at rest to a high level using a shear thinning agent or by using various additives (see, for example, Patent Documents 4 and 5).

[0007] Japanese Patent Application Laid-Open No. 2004-018675 Japanese Patent Application Laid-Open No. 2004-059877 Japanese Patent Application Laid-Open No. 09-059554 Japanese Patent Application Laid-Open No. 2006-77074 Japanese Patent Application Laid-Open No. 2013-103986

[0008] The aqueous inks (aqueous ink compositions) disclosed in Patent Documents 1 to 3 are capable of stably dispersing a colorant in the ink by using a dispersant or resin. However, it has been found that the dispersion stability of the colorant in the ink is still insufficient in the prior art disclosed in Patent Documents 1 to 3, and particularly when the particle size of the pigment is large and / or the specific gravity is high, it is difficult to suppress aggregation and / or sedimentation of the pigment over a long period of time, which may result in a decrease in handwriting density and / or poor writing.

[0009] Furthermore, when the viscosity of the ink is increased using a shear thinning agent to increase the viscosity of the ink when left standing, it can suppress drooling from the nib to some extent, but there is a risk of deterioration in writing performance, such as a deterioration in dry-up performance and / or a tendency for the handwriting to become smudged due to the increased viscosity. To resolve the above-mentioned problems, Patent Document 4 uses xanthan gum or succinoglycan as a shear thinning agent, in combination with polyvinylpyrrolidone in a specific ratio. While the above-mentioned configuration effectively suppresses drooling and nib dry-up when left standing, if paper fibers or the like get caught between the ball and the fore-end during writing, if the pen is displayed with the ball pressed down, or if the tip is damaged or deformed by being dropped, etc., causing the ink flow path to inadvertently widen, the ink may drooling or even worsen, resulting in dripping, during storage at high temperatures or high humidity. Furthermore, when the shear load due to ball rotation is low, smearing is likely to occur at the beginning of writing, and the writing feel becomes heavy.In response to this, Patent Document 5 discloses a technology in which, without relying on viscosity adjustment using a shear thinning agent or the like, crystalline cellulose and resin particles are added in combination, and the resin particles suppress the sedimentation of crystalline cellulose in the ink over time and enhance the DC prevention effect, thereby achieving a high level of DC prevention effect and stability over time without reducing writing performance.However, even with the technology of Patent Document 5, it was found that sufficient effect cannot be obtained against the above-mentioned inadvertent expansion of the ink flow path or when stored under high temperature and humidity, and ink drooping may occur.Furthermore, since crystalline cellulose is a large particle of 10 μm or more obtained by hydrolyzing pulp to extract and purify the cellulose crystalline region, it is found that sedimentation may occur at the pen tip side due to the centrifugal load during the manufacturing of the writing instrument and / or long-term storage in the same position, which hinders the flow of ink during writing.

[0010] The present disclosure provides a water-based ink composition for a writing instrument and a writing instrument containing the same, which can solve at least one of the following problems 1 and 2. Problem 1: The colorant is less likely to aggregate and / or settle over time, allowing for the formation of good handwriting. Problem 2: When the writing instrument is stored for a long period of time with the pen tip facing downward, sagging does not occur, allowing for the formation of good handwriting without smearing or the like.

[0011] A first aspect of the present invention is a water-based ink composition for a writing instrument, comprising water, a colorant, and sulfated cellulose fibers in which at least a portion of the hydroxyl groups of the cellulose fibers have been sulfated.

[0012] Aspect 2 of the present invention is the aqueous ink composition for a writing instrument according to Aspect 1, wherein the sulfated cellulose fiber has a cellulose type I crystal structure, and at least a portion of the hydrogen atoms of the hydroxyl groups of the cellulose fiber are substituted with sulfo groups represented by the following formula (1): (-SO ３ － ) ｒ ・Z ｒ＋ (1) [where r is a natural number from 1 to 3, and Z ｒ＋ When r=1, it 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, and when r=2 or 3, it is at least one selected from the group consisting of an alkaline earth metal cation or a polyvalent metal cation.

[0013] A third aspect of the present invention is the aqueous ink composition for a writing instrument according to the first or second aspect, wherein the sulfated cellulose fibers have an average fiber width of 0.5 to 500 nm.

[0014] A fourth aspect of the present invention is the aqueous ink composition for a writing instrument according to the third aspect, wherein the sulfated cellulose fibers have an average fiber width of 30 nm or less.

[0015] A fifth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of the first to fourth aspects, wherein the amount of sulfur incorporated into the sulfated cellulose fiber is higher than 0.42 mmol / g.

[0016] A sixth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of the first to fifth aspects, wherein the degree of polymerization of the sulfated cellulose fibers is 30 or more.

[0017] A seventh aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of Aspects 1 to 6, wherein a dispersion obtained by dispersing the sulfated cellulose fibers in an aqueous solvent to a solids concentration of 0.5% by mass has a haze value of 20% or less.

[0018] Aspect 8 of the present invention is the water-based ink composition for a writing instrument according to any one of Aspects 1 to 7, wherein the sulfated cellulose fiber is blended in an amount of 0.01 to 3 mass % based on the total amount of the ink composition.

[0019] A ninth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of the first to eighth aspects, further comprising a shear thinning agent.

[0020] A tenth aspect of the present invention is the aqueous ink composition for a writing instrument according to Aspect 9, wherein the shear thinning agent is at least one selected from the group consisting of water-soluble polysaccharides, polymers having a molecular weight of 100,000 to 150,000, which contain as a main component an alkyl ester of methacrylic acid, crosslinked poly-N-vinylcarboxylic acid amides, benzylidene sorbitol and derivatives thereof, benzylidene xylitol and derivatives thereof, alkali-thickening acrylic resins, crosslinkable acrylic acid polymers, inorganic fine particles, nonionic surfactants having an HLB value of 8 to 12, and metal salts or amine salts of dialkyl sulfosuccinic acids.

[0021] An eleventh aspect of the present invention is the water-based ink composition for a writing instrument according to the ninth or tenth aspect, wherein the shear thinning agent is blended in an amount of 0.05 to 1% by mass based on the total amount of the ink composition.

[0022] A twelfth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of the first to eleventh aspects, wherein the colorant is dispersed in water.

[0023] A thirteenth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of Aspects 1 to 12, wherein the colorant comprises at least one selected from the group consisting of pigments and resin particles.

[0024] A fourteenth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of Aspects 1 to 13, wherein the colorant has an average particle size of 0.01 to 25 μm.

[0025] A fifteenth aspect of the present invention is the aqueous ink composition for a writing instrument according to any one of the first to fourteenth aspects, further comprising a colorant having a specific gravity of greater than 1.

[0026] A sixteenth aspect of the present invention is a writing instrument containing the aqueous ink composition for a writing instrument according to any one of the first to fifteenth aspects.

[0027] According to an embodiment of the present invention, it is possible to provide an aqueous ink composition for a writing instrument, which is less likely to cause aggregation and / or sedimentation of the colorant over time, allowing for the formation of good handwriting, and / or which is free from sagging and smearing when the writing instrument is stored for a long period of time with the pen tip facing downward, and a writing instrument containing the same.

[0028] The aqueous ink composition for a writing instrument according to an embodiment of the present invention (hereinafter, sometimes referred to as "ink composition" or "ink") contains water, a colorant, and sulfated cellulose fiber. Each component constituting the ink composition according to an embodiment of the present invention will be described below.

[0029] The ink composition according to an embodiment of the present invention contains water. The water is not particularly limited, but examples thereof include tap water, ion-exchanged water, ultrafiltered water, distilled water, etc. The water content relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 35 to 95 mass%, more preferably 40 to 90 mass%.

[0030] The ink composition according to an embodiment of the present invention contains a colorant. As the colorant, dyes, pigments, and resin particles that are soluble or dispersible in an aqueous medium can all be used. That is, the colorant may contain one or more selected from the group consisting of dyes, pigments, and resin particles. For example, the colorant may contain one or more selected from the group consisting of pigments and resin particles, or may consist of one or more selected from the group consisting of pigments and resin particles. Furthermore, the colorant may contain a dye or consist of a dye.

[0031] The dye may be an acid dye, a basic dye, a direct dye, etc. Examples of the acid dye include Naphthol Green B (C.I. 10020), Naphthol Yellow S (C.I. 10316), Acid Yellow 9 (C.I. 13015), Metanil Yellow (C.I. 13065), Orange I (C.I. 14600), Ponceau SX (C.I. 14700), Orange II (C.I. 15510), Sunset Yellow FCF (C.I. 15985), New Coccine (C.I. 16255), and Fast Acid Mazene. (C.I. 17200), Acid First Red 3G (C.I. 18050), Fast Light Yellow 3G (C.I. 18820), Tartrazine (C.I. 19140), Resorcinol Brown (C.I. 20170), Acid Blue 29 (C.I. 20460), Naphthol Blue Black (C.I. 20470), Acid Orange 56 (C.I. 22895), Acid Yellow 42 (C.I. 22910), Guinea Green B ( C.I. 42085), First Green FCF (C.I. 42053), Brilliant Blue FCF (C.I. 42090), Acid Violet 6B (C.I. 42640), Acid Blue 1 (C.I. 42045), Brilliant Blue G (C.I. 42655), Brilliant Blue P-1 (C.I. 42735), Water Blue (C.I. 42755), Eriogreen B (C.I. 44025), Acid Red (C.I. 4 5100), Violamin R (C.I. 45190), Uranine (C.I. 45350), Eosin (C.I. 45380), Phloxine B (C.I. 45410), Erythrosine (C.I. 45430), Nigrosine (C.I. 50420), Pyranine Conc (C.I. 59040), Alizarin Purple (C.I. 60730), Alizarin Cyanine Green F (C.I. 61570), Indigo Carmine (C.I. 73015), etc. can be used.Examples of basic dyes that can be used include chrysoidine (C.I. 11270), methyl violet FN (C.I. 42535), crystal violet (C.I. 42555), malachite green (C.I. 42000), Victoria blue FB (C.I. 44045), rhodamine B (C.I. 45170), acridine orange NS (C.I. 46005), and methylene blue B (C.I. 52015). Examples of direct dyes that can be used include Congo Red (C.I. 22120), Direct Sky Blue 5B (C.I. 24400), Violet BB (C.I. 27905), Direct Deep Black EX (C.I. 30235), Kayalas Black G Conc (C.I. 35225), Direct Fast Black G (C.I. 35255), and Phthalocyanine Blue (C.I. 74180).

[0032] Examples of pigments include inorganic pigments, organic pigments, luster pigments, fluorescent pigments, phosphorescent pigments, etc. Furthermore, water-dispersed pigments can also be used, which are pigments that have been finely and stably dispersed in an aqueous medium in advance using a surfactant and / or a resin.

[0033] Furthermore, a pigment dispersant can be used as needed. Examples of pigment dispersants include anionic and nonionic surfactants, anionic polymers such as polyacrylic acid and styrene-acrylic acid, and nonionic polymers such as PVP and PVA.

[0034] Pigments applicable to embodiments of the present invention include dyes such as acid dyes, basic dyes, direct dyes, oil-soluble dyes, and disperse dyes, as well as microencapsulated pigments in which the above-mentioned pigments are encapsulated in microcapsules. By encapsulating the dye or pigment in a microcapsule, the dye or pigment is isolated and protected from the external environment, thereby improving the water resistance and light fastness of the encapsulated material. An aqueous ink composition for a writing instrument containing a microencapsulated pigment in which a dye is encapsulated is included in the "aqueous ink composition for a writing instrument containing a pigment" aspect, and is also included in the "aqueous ink composition for a writing instrument containing a dye" aspect.

[0035] A microcapsule pigment encapsulating a dye or pigment can be formed by encapsulating a colored body, in which the dye or pigment is dissolved or dispersed in an oily medium, in a microcapsule.

[0036] Pigments applicable to embodiments of the present invention include microencapsulated pigments in which functional materials such as thermochromic materials that change color with temperature changes and photochromic materials that change color with light irradiation are encapsulated in microcapsules. These color changes may be reversible or irreversible. Because they can repeatedly exhibit color changes with temperature changes or light irradiation, reversible thermochromic materials or reversible photochromic materials are suitable as functional materials.

[0037] Examples of reversible thermochromic materials include a reversible thermochromic composition comprising at least (A) an electron-donating color-forming organic compound, (B) an electron-accepting compound, and (C) a reaction medium that determines the temperature at which the color-forming reaction of components (A) and (B) occurs. By encapsulating the reversible thermochromic composition in microcapsules, a reversible thermochromic microcapsule pigment can be formed.

[0038] Examples of reversible thermochromic compositions that can be used include heat-discolorable reversible thermochromic compositions having a relatively small hysteresis width (ΔH) (ΔH = 1 to 7°C), as described in Japanese Patent Publication Nos. 51-44706, 51-44707, and 1-29398. "Heat-discolorable" refers to a composition that discolors upon heating and develops color upon cooling. These reversible thermochromic compositions undergo color changes across a predetermined temperature (color change point), exhibiting a discolored state in a temperature range above the high-temperature color change point and a colored state in a temperature range below the low-temperature color change point. Only one of these two states exists at room temperature, and the other state is maintained while the heat or cold required to achieve that state is applied, but returns to the state it exhibits at room temperature once the application of heat or cold is removed.

[0039] The reversible thermochromic composition may be a heat-discolorable reversible thermochromic composition having a large hysteresis width (ΔH = 8 to 80°C) as described in, for example, JP-B No. 4-17154, JP-A No. 7-179777, JP-A No. 7-33997, JP-A No. 8-39936, or JP-A No. 2005-1369. "Heat-discolorable" means that the composition discolors upon heating and develops color upon cooling. This reversible thermochromic composition exhibits a change in color density with temperature that follows a significantly different path when the temperature is increased from a temperature lower than the color-change temperature range, and when the temperature is decreased from a temperature higher than the color-change temperature range. The temperature-discolorable reversible thermochromic composition reaches a temperature at which the composition completely develops color (t １ Coloring state in the following temperature range, or complete decolorization temperature t ４ The color disappears in the above high temperature range. ２ ~Discoloration start temperature t ３ It has color memory in the temperature range between (substantially two-phase maintaining temperature range).

[0040] When the reversible thermochromic composition having the color memory property is applied to the embodiment of the present invention, the reversible thermochromic composition is specifically a composition having a complete color development temperature t １ The temperature that can only be achieved in a freezer or in a cold region, and the complete decolorization temperature t ４ By specifying the temperature range to be that obtained from frictional heat generated 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, it can be made to function effectively in maintaining the color that is exhibited under normal conditions (the temperature range in everyday life).

[0041] The temperature that can only be obtained in a freezer or in a cold region is −50 to 0° C., preferably −40 to −5° C., and more preferably −30 to −10° C. The temperature that can be obtained from a familiar heating device such as a hair dryer is 50 to 95° C., preferably 50 to 90° C., and more preferably 60 to 80° C.

[0042] The reversible thermochromic composition may be a heat-coloring type reversible thermochromic composition using a gallic acid ester, as described in JP-B No. 51-44706, JP-A No. 2003-253149, etc. The heat-coloring type means that the color develops when heated and disappears when cooled.

[0043] The reversible thermochromic composition is a compatible solution containing the above-mentioned components (A), (B), and (C) as essential components, and the proportion of each component depends on the concentration, color change temperature, color change form, and / or type of each component. Generally, the component ratio that achieves the desired properties is 1 part of component (A) to 0.1 to 100, preferably 0.1 to 50, more preferably 0.5 to 20, part of component (B), and 1 to 800, preferably 5 to 200, more preferably 5 to 100, and even more preferably 10 to 100, part of component (C) (all of the above proportions are in parts by mass).

[0044] Examples of reversible photochromic materials include conventionally known photochromic compounds such as spirooxazine derivatives, spiropyran derivatives, and naphthopyran derivatives that develop color when irradiated with sunlight, ultraviolet light, or blue light having a peak emission wavelength in the range of 400 to 495 nm, and lose color when the irradiation is stopped. Examples include compounds described in JP 2021-120493 A and WO 2020 / 137469 A.

[0045] 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.

[0046] As the reversible photochromic material, a reversible photochromic composition obtained by dissolving the above-mentioned photochromic compound in an oligomer such as a styrene-based oligomer, an acrylic oligomer, a terpene-based oligomer, or a terpene-phenol-based oligomer can also be used. By encapsulating the reversible photochromic composition in microcapsules, a reversible photochromic microencapsulated pigment can be formed. By dissolving the photochromic compound in various oligomers, it is possible to improve the color density as well as the light resistance, and further to adjust the color change sensitivity.

[0047] Microencapsulation of microcapsule pigments can be performed by conventionally known methods such as isocyanate-based interfacial polymerization, in situ polymerization of melamine-formalin systems, liquid curing coating, phase separation from an aqueous solution, phase separation from an organic solvent, melt-dispersion cooling, air suspension coating, and spray drying, and is appropriately selected depending on the intended use. Examples of capsule materials include epoxy resins, urea resins, urethane resins, and isocyanate resins. Furthermore, a secondary resin coating can be further provided on the surface of the microcapsules depending on the intended purpose to impart durability or modify the surface properties for practical use.

[0048] The reversible thermochromic microcapsule pigment or reversible photochromic microcapsule pigment preferably has a mass ratio of inclusions to wall film of 7:1 to 1:1, and by having the mass ratio of inclusions to wall film within the above range, it is possible to prevent a decrease in color density and vividness during color development. More preferably, the mass ratio of inclusions to wall film is 6:1 to 1:1.

[0049] The reversible thermochromic microcapsule pigment or the reversible photochromic microcapsule pigment can also be made into a microcapsule 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.

[0050] Examples of the resin particles include resin particles containing the above-mentioned dye, pigment, thermochromic material, and / or photochromic material. Note that resin particles containing a dye are included in the embodiment of the "aqueous ink composition for a writing instrument containing resin particles" and also in the embodiment of the "aqueous ink composition for a writing instrument containing a dye." Furthermore, resin particles containing a pigment are included in the embodiment of the "aqueous ink composition for a writing instrument containing resin particles" and also in the embodiment of the "aqueous ink composition for a writing instrument containing a pigment."

[0051] Examples of 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.

[0052] Examples of resin particles containing a pigment include colored resin particles in which the pigment is uniformly dispersed in the resin particles, and colored resin particles in which the surfaces of the resin particles are coated with the pigment. Here, the pigment may be surface-treated by various conventionally known methods in order to improve dispersibility and / or adsorption to the resin that constitutes the resin particles.

[0053] Examples of resin particles containing a thermochromic material or a photochromic material include colored resin particles in which a reversible thermochromic composition is uniformly 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 uniformly dispersed in the resin particles (hereinafter sometimes referred to as "reversible photochromic resin particles").

[0054] Resin particles applicable to the embodiment of the present invention include solid resin particles with no voids inside the particle, and hollow resin particles with voids inside the particle.

[0055] 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 an aqueous or oily medium in the presence of a dye, a pigment, a thermochromic material, and / or a photochromic material, such as a suspension polymerization method, a suspension polycondensation method, a dispersion polymerization method, or an emulsion polymerization method.

[0056] 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 an approximately spherical shape, a polygonal shape, a flat shape, etc. Among these, spherical resin particles are preferred.

[0057] The colorants can be used alone or in combination of two or more.

[0058] The reversible thermochromic composition or reversible photochromic composition is preferably encapsulated in microcapsules and used as a microencapsulated pigment, because encapsulation in microcapsules makes it possible to form a chemically and physically stable pigment, and furthermore, the reversible thermochromic composition or reversible photochromic composition can maintain the same composition and exhibit the same effects under various use conditions.

[0059] The blending ratio of the colorant is not particularly limited, but in order to prevent the colorant from flocculating and / or settling over time and to produce good handwriting, the colorant is blended in a range of preferably 1 to 25% by mass, more preferably 3 to 20% by mass, based on the total amount of the ink composition. If the blending ratio exceeds 25% by mass, the ink discharge properties of a writing instrument containing the ink composition are likely to deteriorate, and writing defects such as smearing and skipped lines are likely to occur. On the other hand, if the blending ratio is less than 1% by mass, it becomes difficult to obtain a writing density suitable for the writing instrument. On the other hand, in order to prevent sagging during long-term storage of the writing instrument with the pen tip facing downward and to produce good handwriting without smearing, the colorant is blended in a range of preferably 1 to 35% by mass, more preferably 2 to 30% by mass, based on the total amount of the ink composition.

[0060] When the colorant is a reversible color-changing 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 an amount of 5 to 40% by mass, more preferably 10 to 40% by mass, and even more preferably 10 to 30% by mass, based on the total amount of the ink. If the blending ratio exceeds 40% by mass, the ink dischargeability of a writing instrument containing the ink composition decreases, and writing defects such as blurring and skipped lines are likely to occur. On the other hand, if the blending ratio is less than 5% by mass, it is difficult to achieve the color change and writing density suitable for a writing instrument, and it is difficult to fully fulfill the color-changing function.

[0061] In compositions using water-soluble colorants among the above-mentioned colorants, sagging tends to occur particularly during storage under high humidity, and therefore the configuration of the present invention can be effective. The most effective means for preventing sagging is to add sulfated cellulose fiber, which will be described later, to the dye-based aqueous ink composition.

[0062] The ink composition according to an embodiment of the present invention contains sulfated cellulose fibers. Sulfated cellulose fibers are various cellulose fibers in which at least a portion of the hydroxyl groups in the glucose units constituting the cellulose have been modified by sulfate esterification. Any type of cellulose may be used as the base cellulose, but those based on cellulose having a cellulose type I crystalline structure are widely used. Examples of sulfated cellulose fibers include sulfated cellulose fibers having a cellulose type I crystalline structure in which at least a portion of the hydrogen atoms of the hydroxyl groups of the cellulose fibers have been substituted with sulfo groups represented by the following formula (1):

[0063] (-SO ３ － ) ｒ ・Z ｒ＋ (1) [where r is a natural number from 1 to 3, and Z ｒ＋When r=1, it is at least one selected from the group consisting of hydrogen ions, alkali metal cations, ammonium ions, aliphatic ammonium ions, and aromatic ammonium ions. When r=2 or 3, it is at least one selected from the group consisting of alkaline earth metal cations and polyvalent metal cations.] Sulfate-esterified cellulose fibers substituted with sulfo groups represented by the above formula (1) are, in other words, cellulose fibers having a cellulose type I crystal structure in which at least a part of the hydroxyl groups of the cellulose fibers has been substituted with sulfate ester groups ((-O-SO ３ － ) ｒ ・Z ｒ＋ ) can also be called sulfated cellulose fibers substituted with cellulose ester.

[0064] Sulfated cellulose fibers have a sulfo group ((-SO ３ － ) and a sulfate ester group (—O—SO ３ － ) (including sulfo groups contained in the cellulose sulphate esterified fiber), hydrophilicity is improved, which can improve dispersibility when dispersed in an ink composition. Furthermore, electronic repulsion of the introduced sulfo groups makes it easier to maintain a dispersed state in the ink composition, and a network structure can be formed by interactions between the cellulose sulphate esterified fibers. As a result, the cellulose sulphate esterified fiber prevents contact between colorants in the ink composition and suppresses aggregation of the colorants, allowing the colorants to be stably retained in the ink composition. Furthermore, the cellulose sulphate esterified fiber also functions as a thickener or gelling agent depending on the fiber length, which can lead to even more stable retention of the colorants in the ink composition. In other words, the cellulose sulphate esterified fiber can achieve the effect of improving the dispersion stability of the colorant (hereinafter referred to as "Effect 1").

[0065] Furthermore, in the ink configuration according to an embodiment of the present invention, by using the above-described sulfated cellulose fibers, it is presumed that when ink flow stops, the sulfated cellulose fibers near the tip (void) instantly retain the ink through interactions between the individual cellulose fibers, and when rewriting, the ink regains its original fluidity with only a slight load. Therefore, even under harsh conditions, such as long-term storage, storage at high temperatures or humidity, or when the ink flow path at the pen tip is accidentally widened, the ink can be effectively prevented from dripping or dripping. Furthermore, because the dispersion state can be re-established with only a slight load, the ink can be re-written without blurring, resulting in the formation of good handwriting from the beginning (hereinafter, these effects are referred to as "Effect 2"). In an embodiment of the present invention, the sulfated cellulose fibers can achieve at least one of the above-described "Effect 1" and "Effect 2."

[0066] Sulfated cellulose fibers have a sulfo group ((-SO ３ － ) and a sulfate ester group (—O—SO ３ － The inclusion of sulfo groups (including those contained in the sulfo groups in the hydroxy groups) can improve the hydrophilicity of the fibers, thereby improving the dispersibility of the fibers when dispersed in aqueous ink. Furthermore, the electronic repulsion of the introduced sulfo groups prevents aggregation in the ink and makes it easier to maintain the dispersed state, thereby ensuring ink stability during centrifugation during production and / or over time under normal conditions, while blocking the ink flow path under conditions where ink drooping occurs.

[0067] At least some of the hydrogen atoms of the hydroxyl groups of cellulose having a cellulose I type crystalline structure are sulfo groups ((-SO ３ － ) and a sulfate ester group (—O—SO ３ －Examples of sulfated cellulose fibers substituted with sulfo groups (including sulfo groups contained in cellulose acetate), include those on the order of microns that have not been subjected to defibration treatment (refining treatment) and those on the order of nanons that have been defibrated. In either case, the average fiber width of the sulfated cellulose fibers is preferably in the range of 0.5 to 500 nm.

[0068] Since the defibrated nano-order sulfated cellulose fibers are fine cellulose fibers that have been defibrated, they can be called sulfated fibrillated cellulose fibers or sulfated fine cellulose fibers (sulfated cellulose nanofibers). From the viewpoint of producing fine sulfated cellulose fibers that retain a cellulose I type crystalline structure, the average fiber width of the sulfated cellulose fibers is preferably 0.5 nm or more, more preferably 1 nm or more, and even more preferably 2 nm or more. Furthermore, from the viewpoint of removing other components of the pulp (such as bordered pores and vessel elements) during the production of the sulfated cellulose fibers to obtain pure sulfated cellulose fibers, the average fiber width is preferably 500 nm or less, more preferably 100 nm or less.

[0069] In the ink composition according to an embodiment of the present invention, if the ink composition contains a large amount of sulfated cellulose fiber or if a light-colored colorant is used, the hue of the sulfated cellulose fiber may affect the hue of the ink composition. Therefore, to ensure transparency when the sulfated cellulose fiber is dispersed in water, the average fiber width of the sulfated cellulose fiber is preferably adjusted to 0.5 to 500 nm, with 1 to 500 nm, 2 to 100 nm, 2 to 30 nm, 2 to 20 nm, and 2 to 10 nm being more preferred in that order. By having the average fiber width within the above range, the hue of the sulfated cellulose fiber 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.

[0070]

[0003] Here, ink compositions that use a reversible thermochromic material or a reversible photochromic material as a colorant reversibly change color from a colored state to a decolorized state in response to a change in temperature or the presence or absence of light irradiation, and because 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 sulfated cellulose fiber 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 be obtained when the sulfated cellulose fiber is dispersed in water, and it is preferable that the average fiber width of the sulfated cellulose fiber be within the above-mentioned range.

[0071] When the average fiber width of the fine cellulose fibers is less than 0.5 nm, they are dissolved in water as cellulose molecules, making it difficult for them to exhibit the physical properties (strength, rigidity, or dimensional stability) of fine cellulose fibers. On the other hand, when the average fiber width exceeds 500 nm, they cannot be called fine cellulose fibers, but are merely fibers contained in ordinary pulp, making it difficult for them to exhibit the physical properties (transparency, strength, rigidity, or dimensional stability) of fine cellulose fibers, and they tend to settle in the ink composition. When sulfated fine cellulose fibers are applied to an ink composition, in addition to the above-mentioned transparency, they have very high dispersibility in water, which makes it easy for the fine cellulose fibers to interact with each other, and the sulfated cellulose fibers easily form a dense network structure, thereby further improving the dispersion stability of the colorant. Furthermore, when sulfated fine cellulose fibers are applied to an ink composition, they have very high dispersibility in an aqueous medium and excellent followability during ink flow, making it possible to more effectively exhibit the ink sagging suppression effect.

[0072] Conventionally, to improve the dispersion stability of colorants, inks have been made highly viscous using thickeners such as fine cellulose and / or xanthan gum. While this can suppress aggregation and / or sedimentation of the colorant, such high-viscosity inks are subject to limitations on the writing instruments to which they can be applied. However, as described above, ink compositions using sulfated fine cellulose fibers are prone to forming a dense network structure due to the interaction between fibers, and can stably retain colorants for long periods of time despite having lower viscosities than ink compositions using conventional thickeners alone. Therefore, sulfated fine cellulose fibers exhibit a rheology control effect different from that of conventional thickeners, thereby improving the dispersion stability of colorants over time. Furthermore, because the ink composition can be made less viscous than ink compositions using conventional thickeners alone, the ink discharge properties of writing instruments containing this ink composition are improved, writing defects such as smearing are suppressed, the writing feel is improved, and the color development of handwritten lines is also excellent. Furthermore, when the ink composition is used in a writing instrument (ballpoint pen) equipped with a ballpoint pen tip, line breakage in handwriting can be suppressed, and good writing performance can be achieved.

[0073] When the average fiber width of the sulfated fine cellulose fibers is greater than 30 nm, the aspect ratio decreases, making it difficult for fibers to interact with each other. Furthermore, when the average fiber width is greater than 30 nm, the width 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 sulfated fine cellulose fibers is preferably in the range of 2 to 30 nm, more preferably 2 to 20 nm, and even more preferably 2 to 10 nm. In particular, from the viewpoint of transparency, the average fiber width of the sulfated fine cellulose fibers is preferably 20 nm or less, more preferably 10 nm or less. Adjusting the average fiber width to 10 nm or less reduces scattering of visible light, resulting in sulfated fine cellulose fibers with high transparency, and can suppress inadvertent color changes in the ink composition due to the blending of sulfated fine cellulose fibers. Also, from the viewpoint of suppressing sagging, a higher effect is exhibited when the average fiber width is 30 nm or less.

[0074] The average fiber width of sulfated cellulose fibers can be measured by dispersing sulfated cellulose fibers in a solvent such as pure water to prepare a mixed solution to a predetermined concentration, spin-coating this mixed solution on a silica substrate coated with polyethyleneimine (PEI), and observing the sulfated cellulose fibers on the silica substrate. Observation can be performed using a scanning probe microscope (e.g., SPM-9700, manufactured by Shimadzu Corporation). The average fiber width of sulfated cellulose fibers can be determined by randomly selecting 20 sulfated cellulose fibers from the obtained observation image, measuring the fiber width of each fiber, and averaging the results.

[0075] The sulfo group ((-SO ３ － ) and a sulfate ester group (—O—SO ３ －The amount of sulfur introduced (including sulfo groups contained in the sulfated cellulose fiber) can be expressed as the amount of sulfur introduced, and is not particularly limited as long as transparency and dispersibility can be maintained to a certain extent. For example, the amount of sulfur introduced per gram (mass) of sulfated cellulose fiber is preferably higher than 0.42 mmol / g, more preferably higher than 0.42 mmol / g and not higher than 9.9 mmol / g, even more preferably in the range of 0.50 to 9.9 mmol / g, and particularly preferably in the range of 0.60 to 9.9 mmol / g.

[0076] When the amount of sulfur introduced per 1 g (mass) of sulfated cellulose fiber is 0.42 mmol / g or less, the hydrogen bonds between fibers are strong, and dispersibility tends to decrease. Conversely, by increasing the amount of sulfur introduced above 0.42 mmol / g, dispersibility tends to improve, and by increasing the amount of sulfur introduced above 0.50 mmol / g, electronic repulsion can be strengthened, making it easier to maintain a stable dispersed state. On the other hand, as the amount of sulfur introduced approaches 9.9 mmol / g, there is a concern that crystallinity may decrease, and the cost of introducing sulfur also tends to increase. Therefore, it is preferable to adjust the amount of sulfur introduced into the sulfated cellulose fiber to be within the above range.

[0077] In particular, from the viewpoint of dispersibility of fine fibers constituting sulfated pulp fibers, the amount of sulfur introduced into sulfated cellulose fibers is preferably more than 0.42 mmol / g and not more than 3.0 mmol / g, more preferably in the range of 0.50 to 3.0 mmol / g, and even more preferably in the range of 0.50 to 2.0 mmol / g. Also from the viewpoint of transparency of the sulfated cellulose fibers, it is preferable to adjust the amount of sulfur to be within the same range as above.

[0078] The amount of sulfur incorporated into the sulfated cellulose fiber can be determined by burning a predetermined amount of the sulfated cellulose fiber and measuring the sulfur content in the combustion product using a combustion ion chromatograph in accordance with IEC 62321.

[0079] The average fiber length and average fiber width of the sulfated cellulose fibers are not particularly limited, as long as they are adjusted so that the fibers can easily interact with each other and can easily obtain transparency when dispersed in water.

[0080] The average fiber length of sulfated cellulose fibers can be indirectly expressed by the degree of polymerization, which is preferably 30 or more, more preferably 40 to 1,000, and even more preferably 40 to 700.

[0081] If the degree of polymerization of the sulfated cellulose fibers is lower than 30, the fiber length decreases, making it difficult for the fibers to interact with each other. On the other hand, if the degree of polymerization of the sulfated cellulose fibers exceeds 1,000, the dispersibility decreases, and the slurry viscosity when formed into a slurry tends to become too high, resulting in poor dispersion stability. Therefore, it is preferable to adjust the degree of polymerization of the sulfated cellulose fibers to be within the above range. In particular, from the viewpoint of the ease of fiber-fiber interaction, it is preferable to adjust the degree of polymerization to 40 or more, and from the viewpoints of dispersibility and handleability, it is preferable to adjust the degree of polymerization to 1,000 or less, and more preferably 700 or less.

[0082] The method for measuring the degree of polymerization is not particularly limited, but can be, for example, measured by the copper ethylenediamine method. Specifically, the degree of polymerization of the sulfated cellulose fiber can be measured by dissolving the sulfated cellulose fiber in a 0.5 M copper ethylenediamine solution and measuring the viscosity of the solution by the viscometer.

[0083] For fibers whose fiber length can be measured visually, the average fiber length can be determined by, for example, using a scanning electron microscope (SEM), filtering wet sulfated cellulose fibers and removing the solvent to obtain a fine fiber sheet, freeze-drying the sheet in liquid nitrogen, and observing the sheet with the SEM. The average fiber length of 50 observed fibers can then be calculated.

[0084] In particular, when evaluating the transparency of sulfated cellulose fibers, those that are not turbid when dissolved in water are preferably used, and the turbidity of the aqueous solution can be more appropriately evaluated by the haze value of an aqueous solution of a specific concentration. Specifically, the haze value of a dispersion in which sulfated cellulose fibers are dispersed to a predetermined concentration can be measured as follows. First, a dispersion is prepared in which sulfated cellulose fibers are dispersed in an aqueous solvent to a solids concentration of 0.1 to 20% by mass. If the haze value of this dispersion is 20% or less, it can be said that the sulfated cellulose fibers are in a state in which they are able to exhibit transparency with little turbidity. Conversely, if the haze value of a dispersion prepared so that the solids concentration is within the above range is higher than 20%, it can be said that the dispersion is in a state in which it is difficult to exhibit appropriate transparency.

[0085] For example, when a dispersion containing sulfated cellulose fibers is prepared to have a solids concentration of 0.2 to 0.5% by mass, a haze value of 20% or less can be said to be a state in which transparency with little turbidity can be appropriately exhibited. In particular, since turbidity of the dispersion when sulfated cellulose fibers are dispersed at a predetermined concentration can be appropriately suppressed, the haze value is preferably 15% or less, and even more preferably 10% or less. Therefore, in order to exhibit a transparent state with little turbidity, it is preferable to prepare a dispersion containing sulfated cellulose fibers so that the solids concentration is 0.1 to 20% by mass, and the haze value is 20% or less, more preferably 15% or less, and even more preferably 10% or less.

[0086] The water-soluble solvent is not particularly limited, and may be, for example, water alone, or alcohols, ketones, amines, carboxylic acids, ethers, amides, etc., which may be used alone or in combination of two or more.

[0087] The haze value can be measured, for example, using a spectroscopic haze meter or spectrophotometer conforming to JIS K 7105. Briefly, the haze value of the sulfated cellulose fiber can be determined by dispersing sulfated cellulose fiber in the aforementioned dispersion to a predetermined solids concentration and measuring the dispersion using a spectroscopic haze meter or spectrophotometer conforming to JIS K 7105. Specifically, an integrating sphere (manufactured by JASCO Corporation, product name: ISN-470) is attached to a spectrophotometer (manufactured by JASCO Corporation, product name: 0V-570). The haze value is measured according to JIS K 7105 by measuring the light transmittance of a 0.5% by mass sulfated cellulose fiber dispersion using a quartz cell filled with pure water as a blank measurement value. The measurement wavelength range was 380 to 780 nm.

[0088] The method for producing sulfated cellulose fibers is not particularly limited, and known production methods can be applied, for example, as described in JP 2019-11411 A, Japanese Patent No. 6582111, and Japanese Patent No. 6797215. In particular, as described in Japanese Patent No. 6582111, first, a pulp fiber raw material containing cellulose such as wood is contacted (impregnated) with a reaction solution in which a sulfonating agent such as sulfamic acid and urea and / or a derivative thereof are dissolved in water, and the wet pulp fibers are heated to 100 to 180°C for 5 minutes or longer to cause a reaction, thereby introducing sulfo groups into at least some of the hydrogen atoms of the hydroxyl groups of the cellulose fibers (i.e., sulfate esterification of at least some of the hydroxyl groups) (chemical treatment step). Thereafter, the resulting mixture is processed into fine fibers of a predetermined size (micronization process) using a low-pressure homogenizer, a high-pressure homogenizer, a grinder (stone mill), a ball mill, a cutter mill, a jet mill, a single-screw extruder, a twin-screw extruder, an ultrasonic agitator, a household mixer, or the like, thereby producing sulfated cellulose fibers. The above-mentioned method is particularly suitable because it allows efficient production of sulfated cellulose fibers having an average fiber width on the nano-order.

[0089] The blending ratio of sulfated cellulose fiber is not particularly limited, but the sulfated cellulose fiber is blended in the range of preferably 0.01 to 3 mass %, more preferably 0.05 to 1 mass %, based on the total amount of the ink composition. If the blending ratio is less than 0.01 mass %, it is difficult to achieve the desired effect of stably maintaining the colorant in a dispersed state, and it is also difficult to achieve the desired sagging prevention effect. On the other hand, even if the blending ratio exceeds 3 mass %, it is difficult to achieve an improvement in dispersion stability. Furthermore, even if the blending ratio exceeds 3 mass %, it is not possible to achieve an improvement in the sagging prevention effect, and therefore no further addition is necessary.

[0090] 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 the desired color.

[0091] The average particle diameter was measured by determining the particle region using image analysis particle size distribution measurement software (manufactured by Mountech Co., Ltd., product name: MacView), calculating the diameter of a circle equivalent to a projected area (Heywood diameter) from the area of ​​the particle region, and measuring the average particle diameter of particles equivalent to a sphere of equal volume using this value.

[0092] When the particle diameter of all or most of the particles exceeds 0.2 μm, it is also possible to measure the average particle diameter of particles equivalent to an equal volume sphere by the Coulter method using a particle size distribution measuring device (manufactured by Beckman Coulter, Inc., product name: Multisizer 4e).

[0093] Furthermore, the volumetric particle size and average particle size may be measured using a laser diffraction / scattering particle size distribution measuring device (manufactured by HORIBA, Ltd., product name: LA-300) that has been calibrated based on values ​​measured using the above-mentioned software or a measuring device using the Coulter method.

[0094] In the ink composition according to an embodiment of the present invention, a colorant having a relatively large average particle size (hereinafter, sometimes referred to as a "large particle size colorant"), such as a reversible thermochromic microencapsulated pigment and / or a reversible photochromic microencapsulated pigment, or a glitter pigment, can also be used as a colorant. Colorants with a large average particle size tend to settle over time in the ink composition, resulting in poor dispersion stability. In particular, when the ink composition has a low viscosity, the dispersion stability tends to be even more poor. However, by using the sulfated cellulose fiber described above, the ink composition according to an embodiment of the present invention can suppress the settling of the large particle size colorant over time, thereby achieving good dispersion stability for the large particle size colorant. Furthermore, when sulfated fine cellulose fiber is used, the ink composition can maintain a low viscosity while stably dispersing the large particle size colorant for a long period of time.

[0095] The lustrous pigment is not particularly limited as long as it exhibits lustrous properties by reflecting light, and examples thereof include naturally occurring pigments such as fish scale foil, pigments in which a transparent substrate is coated with a metal oxide or metal, and metallic pigments.

[0096] 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 thereof is coated with a metal oxide. Examples of metal oxides include oxides of titanium, zirconia, chromium, vanadium, iron, etc., with titanium oxide being preferred. Furthermore, depending on the coverage rate and thickness of the metal oxide coating on the substrate surface, the pigment exhibits a gold or silver color, or a yellow, red, blue, or green color with a metallic luster. A layer of a metal oxide such as titanium oxide may be further coated with a non-discoloring colorant such as iron oxide and / or a common dye or pigment. Pigments in which a transparent substrate is coated with a metal oxide also include pearl pigments and cholesteric liquid crystal pigments.

[0097] Examples of pigments in which a transparent substrate is coated with a metal include glass flakes coated with silver, glass flakes coated with gold, glass flakes coated with nickel-chromium-molybdenum, glass flakes coated with brass, glass flakes coated with a silver alloy, and glass flakes coated with titanium. Pigments 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, sputtering, or the like.

[0098] An example of a metal pigment is an aluminum powder pigment. An example of an aluminum powder pigment is one obtained by pulverizing and polishing 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 obtained in a paste form. Alternatively, aluminum thin films obtained by vacuum deposition can be finely pulverized.

[0099] The average particle size of the reversible thermochromic microencapsulated pigment or 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 ejection properties 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.

[0100] The average particle size of the luster 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 ejection properties when used in a writing instrument. On the other hand, if the average particle size is less than 3 μm, it becomes difficult to obtain sufficient luster in handwriting.

[0101] The specific gravity of the colorant is not particularly limited, but a colorant having a specific gravity of greater than 1 (hereinafter, sometimes referred to as a "high-specific-gravity colorant") can be used as a colorant in an ink composition according to an embodiment of the present invention. That is, an embodiment of the present invention may include a colorant having a specific gravity greater than 1 (at least a portion of the colorant may have a specific gravity greater than 1). A colorant having a specific gravity greater than 1 is prone to settling 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, by using the above-described sulfated cellulose fiber in the ink composition according to an embodiment of the present invention, the settling of the high-specific-gravity colorant over time can be suppressed, and the high-specific-gravity colorant exhibits good dispersion stability. Furthermore, when sulfated fine cellulose fiber is used, the high-specific-gravity colorant can be stably dispersed over a long period of time while maintaining a low viscosity of the ink composition.

[0102] Examples of high specific gravity colorants include inorganic pigments, organic pigments, luster pigments, microcapsule pigments or resin particles using these, and reversible thermochromic microcapsule pigments with a large hysteresis width (ΔH).

[0103] 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.

[0104] Examples of inorganic pigments include carbon black, ultramarine, and titanium oxide of rutile or anatase type. 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.

[0105] Examples of luster pigments include pigments in which the transparent substrate is coated with a metal oxide or metal, and metallic pigments. The specific gravity of pigments in which the transparent substrate is coated with a metal oxide is in the range of 2.8 to 3.2 for pearlescent pigments and in the range of 1.1 to 1.5 for cholesteric liquid crystal pigments. The specific gravity of pigments in which the transparent substrate is coated with a metal is in the range of 3.0 to 3.4. The specific gravity of metallic pigments is in the range of 2.5 to 9.0.

[0106] 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 high specific gravity. The specific gravity of a reversible thermochromic microencapsulated pigment is influenced by the particle size, the components and their contents encapsulated in the microcapsules, the components and film thickness of the capsule wall membrane, the colored state of the microencapsulated pigment, and the temperature. From the viewpoint of dispersion stability in an ink composition, the specific gravity of a 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 fully colored state and water is used as the reference substance in an environment of 20°C. The specific gravity of a reversible thermochromic microencapsulated pigment can be measured by the following method.

[0107] (Method for measuring the specific gravity of a reversible thermochromic microencapsulated pigment) 1. 30 ml of a glycerin aqueous solution and 1 g of a fully colored reversible thermochromic microencapsulated pigment are placed in a screw cap bottle and mixed to prepare a microencapsulated pigment dispersion. 2. 30 ml of the microencapsulated pigment dispersion is centrifuged at 20°C, 1000 rpm, and 30 seconds. A refrigerated tabletop centrifuge (manufactured by Kokusan Co., Ltd., product name: H103N) can be used as the centrifuge. 3. Observe the microencapsulated pigment dispersion. If the majority 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 the original glycerin aqueous solution, and observe the state of the dispersion. If the majority of the microencapsulated pigment is confirmed to be floating on the surface, repeat steps 1 and 2 using an aqueous solution with a lower glycerin concentration than the original glycerin aqueous solution, 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 and the area near the bottom of the screw-top tube. When this state is observed, the specific gravity of the glycerin aqueous solution is measured and used 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.

[0108] The specific gravity of the high-specific-gravity colorant is preferably in the range of 1.01 to 5.00, more preferably 1.05 to 4.50.

[0109]

[0033] A thickener may also be blended into the ink composition according to an embodiment of the present invention. By using sulfated cellulose fiber in combination with a thickener, the viscosity can be lowered compared to ink compositions using a conventional thickener alone, while still allowing the colorant to be stably maintained in a dispersed state for a long period of time. As the thickener, it is preferable to use a substance that can impart shear thinning properties to the ink composition (shear thinning agent).

[0110] An ink composition using a shear thinning agent has high viscosity and is difficult to flow when left at rest or under low stress, but easily reduces viscosity when external stress is applied. As a result, ink leakage and ink separation and backflow can be prevented when not writing, and ink ejection stability from the pen tip can be easily improved when writing.

[0111] When the ink composition according to an embodiment of the present invention contains a thickener, the blending ratio of the thickener is not particularly limited, but the thickener is blended in an amount preferably in the range of 0.1 to 20 mass % of the total amount of the ink composition.

[0112] Examples of shear thinning agents include water-soluble polysaccharides, polymers having a molecular weight of 100,000 to 150,000 and primarily composed of alkyl esters of methacrylic acid, crosslinked poly-N-vinylcarboxylic acid amides, benzylidene sorbitol and its derivatives, benzylidene xylitol and its derivatives, alkali-thickening acrylic resins, crosslinkable acrylic acid polymers, inorganic fine particles, nonionic surfactants having an HLB value of 8 to 12, and metal salts or amine salts of dialkyl sulfosuccinic acid. The shear thinning property of an ink refers to a rheological property in which the ink has a high viscosity and is difficult to flow when at rest or under low stress, but decreases in viscosity and exhibits good flowability as stress increases, and refers to a liquid property also known as thixotropy or pseudoplasticity.

[0113] When the ink composition according to an embodiment of the present invention contains a shear thinning agent, the blending ratio of the shear thinning agent is not particularly limited, but the shear thinning agent is blended in an amount of preferably 0.05 to 1 mass % of the total amount of the ink composition.

[0114] Examples of water-soluble polysaccharides include xanthan gum, welan gum, zeta sea gum, diutan gum, macrophomopsis gum, succinoglycan (e.g., succinoglycan, an organic acid-modified heteropolysaccharide whose constituent monosaccharides are glucose and galactose (average molecular weight: approximately 1,000,000 to 8,000,000)), guar gum, locust bean gum and derivatives thereof, alginate alkyl esters, glucomannan, agar, carrageenin, and other carbohydrates with gelling ability extracted from seaweed. Note that water-soluble cellulose derivatives containing hydroxyethyl cellulose may also be classified as water-soluble polysaccharides, but in the present disclosure, water-soluble cellulose derivatives containing hydroxyethyl cellulose do not belong to water-soluble polysaccharides (i.e., shear-thinning agents) but belong to the polymer flocculants described below.

[0115] Xanthan gum or succinoglycan is preferred as the thickener (shear thinning agent) to be used in combination with sulfated cellulose fiber, since it can improve the dispersion stability of the colorant.

[0116] The shear thinning agents may be used alone or in combination of two or more.

[0117] A polymer flocculant can also be blended into the ink composition according to an embodiment of the present invention. By blending a polymer flocculant, the colorant forms loose aggregates via the polymer flocculant, and colorant particles are prevented from coming into contact with each other and aggregating, thereby improving the dispersibility of the colorant. Furthermore, by using a polymer flocculant in combination with sulfated cellulose fiber, the colorant can be stably maintained in the ink composition, allowing the amount of sulfated cellulose fiber to be reduced in the ink composition.

[0118] When the ink composition according to an embodiment of the present invention contains a polymer flocculant, the blending ratio of the polymer flocculant is not particularly limited, but the polymer flocculant is preferably blended in the range of 0.05 to 1 mass % of the total amount of the ink composition.

[0119] Examples of polymer flocculants include polyvinylpyrrolidone, polyethylene oxide, and water-soluble cellulose derivatives.

[0120] Examples of water-soluble cellulose derivatives include carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose.

[0121] When the ink composition according to an embodiment of the present invention is used in a writing instrument (ballpoint pen) equipped with a ballpoint pen tip, the ink composition may 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, thereby easily preventing wear of the ball seat and improving the writing feel.

[0122] Examples of lubricants include higher fatty acids such as oleic acid; nonionic surfactants having a long-chain alkyl group; polyether-modified silicone oil; thiophosphite triesters such as thiophosphite tri(alkoxycarbonylmethyl ester) and thiophosphite tri(alkoxycarbonylethyl ester); phosphate ester surfactants such as phosphate monoesters of polyoxyethylene alkyl ethers or polyoxyethylene alkylaryl ethers; phosphate diesters of polyoxyethylene alkyl ethers or polyoxyethylene alkylaryl ethers; and metal salts, ammonium salts, amine salts, and alkanolamine salts of these phosphate esters. Additionally, metal soaps, polyalkylene glycol fatty acid esters, ethylene oxide-addition type cationic surfactants, β-alanine type surfactants, N-acylamino acids, N-acylmethyltaurine, 2,5-dimercapto-1,3,4-thiadiazole and its salts or oligomers, 3-amino-5-mercapto-1,2,4-triazole, thiocarbamate, dimethyldithiocarbamate, α-lipoic acid, condensates of N-acyl-L-glutamic acid and L-lysine and salts thereof, and the like may also be used.

[0123] The ink composition according to an embodiment of the present invention may also contain, as necessary, various additives such as water-soluble organic solvents, dispersants, water-soluble resins, surfactants, pH adjusters, rust inhibitors, preservatives or antifungal agents, wetting agents, bubble absorbers, antifoaming agents, specific gravity adjusters, antioxidants, ultraviolet absorbers, etc. These additives can be appropriately selected from those conventionally used in water-based inks.

[0124] When the colorant is a reversible thermochromic microcapsule pigment or a reversible photochromic microcapsule 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 color (1) to color (2) by blending a non-color-changing colorant such as a general dye or pigment.

[0125] The method for producing the ink composition according to an embodiment of 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 containing the above-mentioned components with a variety of stirrers such as a propeller stirrer, a homodisper, or a homomixer, or by dispersing the mixture with a variety of dispersers such as a bead mill.

[0126] When the ink composition according to the embodiment of the present invention is used in a ballpoint pen, its viscosity is measured at a rotation speed of 1 rpm (shear rate of 3.84 sec) in an environment of 20°C. －１ When measured under the conditions of 100 rpm (shear rate 384 sec), the viscosity is preferably in the range of 1 to 2000 mPa·s, more preferably 3 to 1500 mPa·s, and even more preferably 5 to 1000 mPa·s. －１When measured under the conditions of (1) to (3), the viscosity is preferably in the range of 1 to 200 mPa·s, more preferably 3 to 100 mPa·s, and even more preferably 5 to 50 mPa·s. By having the viscosity within the above range, the stability of the ink composition and the free flowability of the ink composition within the mechanism of the ballpoint pen can be maintained at a high level. The viscosity of the ink composition is measured using a rheometer (manufactured by TA Instruments, product name: Discovery HR-2, cone plate (diameter 40 mm, angle 1°)) with the ink placed in an environment of 20°C at a rotation speed of 1 rpm (shear rate 3.84 sec －１ ), or rotation speed 100 rpm (shear rate 384 sec －１ ) are values ​​measured under the conditions.

[0127] When the ink composition according to an embodiment of the present invention is used in a marking pen, its viscosity, measured at a rotation speed of 20 rpm in an environment of 20°C, is preferably in the range of 1 to 30 mPa·s, more preferably 1 to 20 mPa·s, and even more preferably 1 to 10 mPa·s. By having the viscosity within the above range, the stability and fluidity of the ink composition can be maintained at a high level. The viscosity of the ink composition is a value measured by placing the ink composition in an environment of 20°C using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34' x R24)).

[0128] The pH of the ink composition according to an embodiment of the present invention is preferably in the range of 6 to 10, more preferably 7 to 9. By having the pH within the above range, it is possible to prevent the ink composition from becoming excessively viscous and undergoing deterioration. The pH of the ink composition is a value measured by placing the ink in an environment of 20°C using a pH meter (manufactured by DKK-TOA Corporation, product name: IM-40S).

[0129] Examples of writing instruments that can contain the ink composition according to the embodiment of the present invention include ballpoint pens, marking pens, fountain pens, brush pens, calligraphy pens, and other writing instruments.

[0130] When the ink composition according to an embodiment of 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 being filled into a ballpoint pen refill or ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism.

[0131] A ballpoint pen tip consists of a tip body and a ball attached to the front end of the tip body. Examples of ballpoint pen tips include a tip formed by deforming a metal pipe tip body near the tip end by pressing the ball inward from the outer surface in a ball-holding portion, a tip formed by cutting a metal tip body with a drill or the like to hold the ball, a tip with a resin ball receiving seat provided inside a metal or plastic tip body, and a tip in which the ball held by the tip is biased forward by a spring.

[0132] 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.

[0133] The diameter of the ball is generally 0.1 to 3 mm or 0.2 to 3 mm, with 0.1 to 2 mm, 0.2 to 2 mm, 0.2 to 1.5 mm, 0.2 to 1.2 mm, 0.2 to 1 mm, and 0.28 to 1 mm being preferred in that order. The ballpoint pen tip may be configured with a resilient member disposed within the tip that resiliently urges the rear end of the ball forward, so that when not writing, the ball is pressed against the inner edge of the tip tip to create a tight contact, and when writing, the ball is retracted by writing pressure to allow ink to flow out, thereby preventing ink leakage when not in use. Examples of the resilient member include a thin metal wire spring, a spring with a straight portion (rod portion) at one end, and a linear plastic processed body, and are configured to be resilient with a resilience of 5 to 40 g.

[0134] An example of the ink filling mechanism is an ink reservoir that can be directly filled with ink. The ink reservoir can be a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a metal tubular body.

[0135] A ballpoint pen refill (hereinafter sometimes referred to as "refill") can be formed by connecting a ballpoint pen tip to an ink reservoir directly or via a connecting member and directly filling the ink reservoir with ink. A ballpoint pen can be formed by storing this refill in a barrel.

[0136] The ink reservoir is filled with an ink backflow preventer at the rear end thereof, which may be a liquid stopper or a solid stopper.

[0137] The liquid plug is made of a non-volatile liquid and / or a hardly-volatile liquid, examples of which include petrolatum, spindle oil, castor oil, olive oil, refined mineral oil, liquid paraffin, polybutene, α-olefin, α-olefin oligomer or co-oligomer, dimethyl silicone oil, methylphenyl silicone oil, amino-modified silicone oil, polyether-modified silicone oil, fatty acid-modified silicone oil, etc. The non-volatile liquid and / or the hardly-volatile liquid can be used alone or in combination of two or more kinds.

[0138] It is preferable to thicken the non-volatile liquid and / or the hardly-volatile liquid to a suitable viscosity by adding a thickener. Examples of the thickener include clay-based thickeners such as silica whose surface has been hydrophobically treated, fine silica particles whose surface has been methylated, aluminum silicate, swellable mica, and hydrophobically treated bentonite and 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 dextrins; and cellulose-based compounds.

[0139] Examples of the solid plug include a solid plug made of polyethylene, polypropylene, polymethylpentene, etc. As the ink backflow preventer, a solid plug and the above-mentioned liquid plug can be used in combination.

[0140] In addition, by using the barrel itself as the ink filling mechanism, filling ink directly into the barrel, and attaching a ballpoint pen tip to the front end of the barrel, it is possible to form a ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism.

[0141] If the ink filled in the ink filling mechanism has a low viscosity, a ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink filled in the ink filling mechanism to the pen tip.

[0142] 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 axially through the disks and wider ventilation grooves than the grooves, and an ink guide core arranged in the axis center to guide ink from the ink filling mechanism to the pen tip.

[0143] There are no particular restrictions on the material for the pen core, as long as it is a synthetic resin that can be injection molded into a structure in which multiple discs are arranged in a comb-like groove pattern. Acrylonitrile-butadiene-styrene copolymer (ABS resin) is preferably used because it has high moldability and is easy to obtain pen core performance.

[0144] Specific examples of the configuration of a ballpoint pen containing an ink composition according to an embodiment of the present invention include: (1) a ballpoint pen having an ink reservoir filled with ink in a barrel, to which a ballpoint pen tip is connected either directly or via a connecting member, and in which an ink backflow preventer is filled at the end face of the ink; (2) a ballpoint pen in which ink is directly filled in the barrel and which is provided with a mechanism 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 in the barrel and which is provided with a mechanism for supplying ink to the pen tip via the above-mentioned pen core.

[0145] When the ink composition according to an embodiment of 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 it into a marking pen refill or a marking pen equipped with a marking pen tip and an ink filling mechanism.

[0146] Examples of marking pen tips include conventionally general-purpose porous members with interconnected pores, such as resin-processed fibers, fused-processed heat-melting fibers, and felt, with a porosity selected from a range of approximately 30 to 70%, or extrusion-molded synthetic resin bodies with a plurality of ink outlet holes extending in the axial direction, and one end of the tip can be processed into a shape appropriate for the purpose, such as a bullet shape, a rectangle, or a chisel shape, for practical use.

[0147] An example of the ink filling mechanism is an ink reservoir that can be filled with ink. The ink reservoir is a fiber bundle formed by bundling crimped fibers in the longitudinal direction, and is contained within a plastic cylinder or a covering such as a film, with a porosity adjusted to a range of approximately 40 to 90%.

[0148] A marking pen can be formed by housing an ink-impregnated ink reservoir inside the barrel and connecting the marking pen tip to the barrel directly or via a connecting member so that it is connected to the ink reservoir.

[0149] Furthermore, 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.

[0150] 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 metal tubular body.

[0151] A marking pen equipped with a marking pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink composition filled in the ink filling mechanism to the pen tip.

[0152] The ink supply mechanism is not particularly limited, and examples thereof include the ink supply mechanism provided in the ballpoint pen described above, as well as (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, etc. The valve mechanism can be a 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 released by writing pressure.

[0153] When the marking pen is provided with an ink supply mechanism, the ink filling mechanism may be an ink reservoir that can be directly filled with ink, in addition to the ink occlusion body described above. Also, the barrel itself may serve as the ink filling mechanism, allowing ink to be directly filled.

[0154] Specific examples of the configuration of a marking pen containing an ink composition according to an embodiment of the present invention include: (1) a marking pen in which an ink reservoir made of a fiber bundle impregnated with ink is contained within a barrel, and a marking pen tip made of a fiber processed body or a resin molded body with capillary gaps formed therein is connected to the barrel directly or via a connecting member so that the ink reservoir and the tip are connected; (2) a marking pen in which ink is directly filled into the barrel, and a mechanism is provided for supplying ink to the pen tip via an ink flow regulator that is a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like; (3) a marking pen in which ink is directly filled into the barrel, and a mechanism is provided for supplying ink to the pen tip via the pen core; and (4) a marking pen in which the tip and an ink reservoir are connected via a valve mechanism that opens when the tip is pressed, and ink is directly filled into the ink reservoir.

[0155] When the ballpoint pen or marking pen according to the embodiment of 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 into which the ink is filled in order to facilitate re-dispersion of the colorant. Examples of the shape of the agitator include a spherical body and a rod-like body. The material of the agitator is not particularly limited, and examples thereof include metal, ceramic, resin, and glass.

[0156] A writing instrument such as a ballpoint pen or a marking pen according to an embodiment of the present invention may have a detachable ink cartridge 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.

[0157] The ink cartridge may be one that doubles as the barrel that constitutes the writing instrument when connected to the writing instrument body, or one that covers and protects the barrel (rear barrel) after being connected to the writing instrument body. In the latter case, the ink cartridge may be used alone, or may be one in which the writing instrument body and 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 to start use.

[0158] A writing instrument such as a ballpoint pen or marking pen according to an embodiment of the present invention can be made into a cap-type writing instrument by providing a cap that is attached to cover the nib (writing tip), thereby preventing contamination or damage to the writing tip. Furthermore, a writing instrument such as a ballpoint pen or marking pen that contains a refill in the barrel can be made into a retractable writing instrument by providing a retraction mechanism in the barrel that allows the writing tip to protrude and retract from the barrel, thereby preventing contamination or damage to the writing tip. Since the nib of a retractable writing instrument (for example, a retractable ballpoint pen) is always exposed to the outside air, the ink composition according to an embodiment of the present invention is particularly effective.

[0159] Any retractable writing instrument can be used as long as the writing tip is housed in a barrel exposed to the outside air and the writing tip protrudes from the barrel opening when the retractable mechanism is activated. It can also be a composite retractable writing instrument that houses multiple refills in the barrel and causes the writing tip of one of the refills to protrude and retract from the barrel opening when the retractable mechanism is activated.

[0160] Examples of the retraction mechanism include: (1) a side-slide type retraction mechanism in which an operating part (clip) movable in the front-rear direction protrudes radially outward from the rear side wall of the barrel, and the writing tip is retracted from the front end opening of the barrel by sliding the operating part forward; (2) a rear-end knock type retraction mechanism in which the writing tip is retracted from the front end opening of the barrel by pressing forward the operating part provided at the rear end of the barrel; (3) a side-knock type retraction mechanism in which the writing tip is retracted from the front end opening of the barrel by pressing radially inward the operating part protruding from the outer surface of the side wall of the barrel; and (4) a rotary type retraction mechanism in which the writing tip is retracted from the front end opening of the barrel by rotating the operating part at the rear of the barrel.

[0161] The shapes of ballpoint pens and marking pens are not limited to the configurations described above, and they may be equipped with tips of different shapes, or with tips that dispense ink of different tones or hues, or they may be composite writing instruments (double-headed, retractable tip, etc.) that are equipped with tips of different shapes and dispense ink of different tones or hues.

[0162] A preferred writing instrument containing an ink composition according to an embodiment of the present invention is a writing instrument (ballpoint pen) equipped with a ballpoint tip as the pen tip. The ink composition according to an embodiment of the present invention has a high viscosity when left standing without shear stress, thereby stably retaining the ink composition within the ballpoint pen and providing good colorant dispersion stability. Furthermore, strong shear stress is applied to the ink composition by the rotation of the ball during writing, which tends to further reduce the viscosity of the ink composition near the ball, resulting in good ink discharge stability. In other words, the ink composition according to an embodiment of the present invention is suitable for use in ballpoint pens because it suppresses writing defects such as smearing and / or skipped lines, provides a good writing feel, and provides excellent color development in handwriting. Furthermore, the ink composition according to an embodiment of the present invention is suitable for use in ballpoint pens because ballpoint pen tips have poor ink retention at the pen tip and are prone to drooling.

[0163] When a reversible thermochromic microcapsule pigment or a reversible thermochromic resin particle is used as a colorant, handwriting formed on a surface using a writing instrument containing an ink composition according to an embodiment of the present invention can be discolored by rubbing with a finger, or by using a heating tool and / or a cooling tool.

[0164] Examples of the heating tool include an electrically heated discoloring tool equipped with a resistance heating element such as a PTC element, a heat discoloring tool filled with a medium such as hot water, a heat discoloring tool using steam or laser light, and the application of a hair dryer. However, a friction member or friction body is preferred because it can change color by a simple method.

[0165] Examples of cooling devices include electrically operated thermochromic devices using a Peltier element, thermochromic devices filled with a refrigerant such as cold water or ice chips, refrigerants, refrigerators and freezers.

[0166] As the friction member and friction body, elastic bodies such as elastomers and plastic foams that are highly elastic and can generate appropriate friction and frictional heat when rubbed are preferred, but plastic molded bodies, stone, wood, metal, fabric, etc. can also be used. Note that while a general eraser used to erase pencil marks may be used to rub the marks, eraser dust is generated during rubbing, and therefore the above-mentioned friction member and friction body that generate almost no eraser dust are preferably used.

[0167] Examples of materials for the friction member and friction body include silicone resin, styrene-ethylene-butadiene-styrene block copolymer (SEBS resin), etc. Silicone resin tends to adhere to areas that have been erased by rubbing, and handwriting tends to be repelled when writing is repeated, so SEBS resin is more preferably used.

[0168] The friction member or friction body may be a separate component of any shape from the writing instrument, but by providing it on the writing instrument, the writing instrument can be made more portable. Also, a writing instrument set can be obtained by combining a writing instrument with a separate friction member or friction body of any shape from the writing instrument.

[0169] 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, or if 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.

[0170] 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. 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 barrel opening, at the rear end of the barrel (the part where the writing tip is not provided), or at the knock section.

[0171] Examples are shown below, but the embodiments of the present invention are not limited thereto. In the examples, "parts" refers to "parts by mass" unless otherwise specified.

[0172] Example 1 The following Example 1 demonstrates that the ink composition according to an embodiment of the present invention and a writing instrument containing the same are less likely to experience aggregation and / or sedimentation of the colorant over time, and are capable of forming good handwriting.

[0173] Preparation of Ink Composition (Example 1-1) An ink composition was prepared by mixing 30 parts of a blue pigment aqueous dispersion (Pigment Blue 15:3) (solid content: 20%, average particle size: 0.2 μm), 0.08 parts of sulfated cellulose fiber A, 0.12 parts of a shear thinning agent (xanthan gum) [manufactured by Sansho Co., Ltd., product name: Kelzan], 0.5 parts of a surfactant [manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL], 1 part of triethanolamine, 10 parts of diethylene glycol, 5 parts of glycerin, and 53.3 parts of water.

[0174] Preparation of Writing Instrument (Example 1-1) The above ink composition was suction-filled into an ink reservoir 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 preventer (liquid stopper) mainly composed of polybutene was filled into the rear end of the ink reservoir, 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 above ballpoint pen has a structure in which the tip provided in the ballpoint pen refill is stored in the barrel while exposed to the outside air, and the tip protrudes from the front end opening of the barrel by operating a clip-shaped retraction mechanism (slide mechanism) provided on the rear side wall of the barrel.

[0175] Preparation of Ink Compositions (Examples 1-2 to 1-17 and Comparative Examples 1-1 to 1-12) The ink compositions of Examples 1-2 to 1-17 and Comparative Examples 1-1 to 1-12 were prepared in the same manner as in Example 1-1, except that the types and amounts of the materials to be blended were changed to those shown in Tables 1 and 2 below.

[0176] Preparation of Writing Instruments (Examples 1-2 to 1-13 and Comparative Examples 1-1 to 1-9) The writing instruments of Examples 1-2 to 1-13 and Comparative Examples 1-1 to 1-9 were prepared in the same manner as in Example 1-1.

[0177] Preparation of Writing Instruments (Examples 1-14 to 1-15 and Comparative Example 1-10) The ink compositions of Examples 1-14 to 1-15 and Comparative Example 1-10 were impregnated into an ink occlusion body made of polyester sliver covered with a synthetic resin film, and housed in a barrel made of polypropylene resin. A resin-processed pen body (bullet-shaped) made of an extrusion-molded product of polyacetal resin having a plurality of ink outlet holes extending in the axial direction was connected to the tip of the barrel via a holder, and a cap was attached to prepare a marking pen (cap-type marking pen).

[0178] Preparation of Writing Instruments (Examples 1-16 to 1-17 and Comparative Examples 1-11 to 1-12) The ink compositions of Examples 1-16 to 1-17 and Comparative Examples 1-11 to 1-12 were impregnated into an ink occlusion body made of polyester sliver covered with a synthetic resin film, and housed in a barrel made of polypropylene resin. A resin-processed pen body (chisel type) made of polyester fiber was attached to the tip of the barrel, and assembled in a connected state via a resin holder. A cap was attached to prepare a marking pen (cap-type marking pen).

[0179]

[0180]

[0181] The contents of the materials in Tables 1 and 2 are explained according to the note numbers. (1-1) Blue pigment aqueous dispersion (Pigment Blue 15:3) (solid content: 20%, average particle size: 0.2 μm) (1-2) Black pigment (carbon black) [manufactured by Mitsubishi Chemical Corporation, product name: MA-100 (average particle size: 0.024 μm)] (1-3) Reversible thermochromic microcapsule pigment (previously cooled to -20°C or below to develop a blue color) (1-4) Plate-shaped luster pigment (paste-type aluminum powder pigment) [manufactured by Toyo Aluminum K.K., product name: Alpaste EMERAL EMR-D5660H (solid content: 60%, average particle size: 9 μm)] (1-5) Plate-shaped luster pigment (pearl pigment) [manufactured by Merck Japan Ltd., product name: Iriodin 221 (average particle size: 15 μm)] (1-6) White pigment (titanium oxide) [manufactured by Teika Corporation, product name: JR-707 (average particle size: 0.25 μm)] (1-7) Pink fluorescent pigment aqueous dispersion (solid content: 45%, average particle size: 0.4 μm) (1-8) Sulfated fine cellulose fiber (fiber width: 3 to 6 nm, amount of sulfur introduced: 1.47 mmol / g, degree of polymerization: 400 to 600, haze value in an aqueous solution with a solid content concentration of 0.5% by mass: 4.67%) having a cellulose I-type crystal structure and in which some of the hydroxyl groups of the cellulose fiber are sulfated (to sodium sulfate). (1-9) Sulfated fine cellulose fibers having a cellulose I-type crystalline structure, in which some of the hydroxyl groups of the cellulose fibers have been sulfated (to form sodium sulfate esters) (fiber width: 3 to 6 nm, amount of sulfur introduced: 1.57 mmol / g, degree of polymerization: 400 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 1.71%) (1-10) Sulfated fine cellulose fibers having a cellulose I-type crystalline structure, in which some of the hydroxyl groups of the cellulose fibers have been sulfated (to form sodium sulfate esters) (fiber width: 3 to 6 nm, amount of sulfur introduced: 1.81 mmol / g, degree of polymerization: 400 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 0.99%)(1-11) Sulfated cellulose fiber having a cellulose type I crystal structure, in which some of the hydroxyl groups of the cellulose fiber have been sulfated (to form sodium sulfate esters) (average fiber width: 52 nm, amount of sulfur incorporated: 2.1 mmol / g, average fiber length: 13 μm). (1-12) Sulfated cellulose fiber having a cellulose type I crystal structure, in which some of the hydroxyl groups of the cellulose fiber have been sulfated (to form sodium sulfate esters) (average fiber width: 1 to 6 nm, amount of sulfur incorporated: 1.72 mmol / g, degree of polymerization: 200 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 1.09%). (1-13) Sulfated cellulose fiber having a cellulose type I crystal structure, in which some of the hydroxyl groups of the cellulose fiber have been sulfated (to form sodium sulfate esters) (average fiber width: 1 to 6 nm, amount of sulfur incorporated: 2.00 mmol / g, degree of polymerization: 50 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 1.13%). (1-14) BYK Japan Co., Ltd., product name: DISPER BYK-190 (solid content: 40%) (1-15) Sansho Co., Ltd., product name: KELZAN (1-16) Dow Chemical Japan Co., Ltd., product name: CELLOSIZE WP-09L (1-17) Daiichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL It can be said that the sulfated fine cellulose fibers of (1-8) to (1-10) above have a fiber width of 3 to 6 nm, and the average fiber width is also in the range of 3 to 6 nm.

[0182] The reversible thermochromic microcapsule pigment was prepared as follows: (A) 3 parts of 3',6'-bis[phenyl(3-methylphenyl)amino]spiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one; (B) 3 parts of 1,1-bis(4-hydroxyphenyl)-2-ethylhexane and 5 parts of 2,2-bis(4-hydroxyphenyl)hexafluoropropane; (C) 50 parts of 4-benzyloxyphenylethyl caprate; a reversible thermochromic composition was added to a mixed solution of 35 parts of an aromatic isocyanate prepolymer as a wall film material and 40 parts of a cosolvent, followed by emulsification and dispersion in an 8% aqueous polyvinyl alcohol solution. After continued stirring with heating, 2.5 parts of a water-soluble aliphatic modified amine was added, and further stirring was continued to prepare a microcapsule dispersion. From the above microcapsule dispersion, a reversible thermochromic microcapsule pigment having an average particle size of 1.9 μm was obtained by centrifugal separation. １ is -20℃, complete discoloration temperature t ４ The color of the reversible thermochromic microcapsule pigment in the fully colored state was 60°C, and it reversibly changed from blue to colorless upon temperature change. The specific gravity of the reversible thermochromic microcapsule pigment in the fully colored state was 1.08 to 1.09 at 20°C relative to water.

[0183] [Evaluation of Initial Writing Performance] Using each of the writing implements prepared in Examples 1-1 to 1-17 and Comparative Examples 1-1 to 1-12, five 15 cm straight lines were handwritten in the direction parallel to the short edge of an A4-size test paper (portrait) at room temperature (20°C). When using the writing implements prepared in Examples 1-16 to 1-17 and Comparative Examples 1-11 to 1-12, writing was performed with the wide surface of the pen body in close contact with the paper surface. Note that writing paper A conforming to the old JIS P3201 was used as the test paper. The resulting handwriting was visually confirmed and evaluated according to the following criteria. The evaluation results are shown in Tables 3 and 4 below, with a rating of "A" being considered acceptable. A: Good handwriting with consistent density and line width was obtained without any smearing or missing lines. B: Numerous smearing and / or missing lines were observed in the handwriting. Alternatively, writing was impossible.

[0184] [Evaluation of Writing Performance After Aging] Each writing instrument subjected to the above-described writing test was left in a thermostatic chamber set at 50°C for 30 days with the pen tip facing downward. After 30 days, the writing instrument was removed from the thermostatic chamber and, at room temperature (20°C), five 15 cm straight lines were handwritten parallel to the short edge of an A4-size test paper (portrait). This was repeated. When using the writing instruments prepared in Examples 1-16 to 1-17 and Comparative Examples 1-11 to 1-12, the wide surface of the pen body was brought into close contact with the paper surface. Writing paper A conforming to 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 considered acceptable. A: The handwriting was free of smearing or skipped lines, and the color of the handwriting was the same as or at the same level as the initial handwriting, resulting in good handwriting. B: Some fading and / or skipping of lines was observed in the handwriting, and the color of the handwriting was slightly darker 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 darker than the initial handwriting, resulting in differences in the color of the handwriting. D: Many fading and / or skipping of lines was observed in the handwriting, or writing was impossible.

[0185]

[0186]

[0187] Example 2 The following Example 2 demonstrates that the ink composition according to an embodiment of the present invention and the writing instrument containing the same do not droop when stored for a long period of time with the nib of the writing instrument facing downward, and can form good handwriting without smearing or the like.

[0188] The compositions of the ink compositions of Examples and Comparative Examples are shown in Table 5 and Table 6, respectively. The numerical values ​​for the compositions in the tables indicate parts by mass.

[0189]

[0190]

[0191] The contents of the raw materials in the table will be explained according to the note numbers. (2-1) Red dye (Acid Red 289) (2-2) Blue dye (Acid Blue 90) (2-3) Yellow dye (Acid Yellow 9) (2-4) Black pigment (carbon black) [manufactured by Mitsubishi Chemical Corporation, product name: MA-100 (average particle size: 0.024 μm)] (2-5) Thermochromic blue pigment, specifically, a microcapsule pigment (T) encapsulating a reversible thermochromic composition consisting of 2.0 parts of 3-(4-diethylamino-2-hexyloxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide as component (A), 8.0 parts of 2,2-bis(4'-hydroxyphenyl)hexafluoropropane as component (B), and 50.0 parts of 4-benzyloxyphenylethyl caprate as component (C). １ : -14℃, T ２ : -6 ° C, T ３ : 48 ° C, T ４(2-6) BYK Japan Co., Ltd., product name: DISPER BYK-190 (solid content: 40%) (2-7) Daiichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL (2-8) Sulfated fine cellulose fiber having a cellulose I-type crystal structure and in which some of the hydroxyl groups of the cellulose fiber are sulfated (to sodium sulfate) (fiber width: 3 to 6 nm, amount of sulfur introduced: 1.47 mmol / g, degree of polymerization: 400 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 4.67%) (2-9) Sulfated fine cellulose fibers having a cellulose I-type crystalline structure and in which some of the hydroxyl groups of the cellulose fibers have been sulfated (to form sodium sulfate esters) (fiber width 3 to 6 nm, sulfur introduction amount: 1.57 mmol / g, polymerization degree: 400 to 600, haze value in a 0.5% by mass aqueous solution: 1.71%) (2-10) Sulfated fine cellulose fibers having a cellulose I-type crystalline structure and in which some of the hydroxyl groups of the cellulose fibers have been sulfated (to form sodium sulfate esters) (fiber width: 3 to 6 nm, sulfur introduction amount: 1.81 mmol / g, polymerization degree: 400 to 600, haze value in a 0.5% by mass aqueous solution: 0.99%) (2-11) Sulfated fine cellulose fibers having a cellulose I-type crystalline structure and in which some of the hydroxyl groups of the cellulose fibers have been sulfated (to form sodium sulfate esters) (average fiber width: 52 nm, sulfur introduction amount: 2.1 mmol / g, average fiber length: 13 μm) (2-12) Sulfated cellulose fiber having a cellulose I-type crystal structure, in which some of the hydroxyl groups of the cellulose fiber are sulfated (sodium sulfate esters) (average fiber width: 1 to 6 nm, amount of sulfur introduced: 2.00 mmol / g, degree of polymerization: 50 to 600, haze value in an aqueous solution with a solid content of 0.5% by mass: 1.13%) (2-13) Manufactured by Sansho Co., Ltd., product name: KELZAN (2-14) Manufactured by Dow Chemical Japan Co., Ltd., product name: CELLOSIZE WP-09L (2-15) Manufactured by Asahi Kasei Corporation, product name: CEOLUS RC-N30 (active ingredient: 1.5%) (2-16) Manufactured by Mitsui Chemicals, Inc., product name: CHEMIPEARL W900 (active ingredient: 40%)(2-17) Mitsui Chemicals, Inc., trade name: Chemipearl W401 (active ingredient: 40%) (2-18) BASF, trade name: Sokalan K90 It can be said that the sulfated fine cellulose fibers of (2-8) to (2-10) above have a fiber width of 3 to 6 nm, and the average fiber width is also within the range of 3 to 6 nm.

[0192] Preparation of Inks (Examples 2-1 to 2-13 and Comparative Examples 2-1 to 2-10) The raw materials in each Example and Comparative Example, excluding the shear thinning agent, were mixed and stirred at 20°C at 1000 rpm in a disperser for 1 hour, and then, if a shear thinning agent was included, it was added and stirred for another 1 hour to obtain a ballpoint pen ink composition. The shear thinning agent was used in the form of a pre-vehicle, having been dispersed in water in advance.

[0193] Preparation of Writing Instruments (Examples 2-1 to 2-10 and Comparative Examples 2-1 to 2-8) The ballpoint pen refills consisted of a conically cut ballpoint pen tip (containing a ball-pushing spring that repels the ball forward) that rotatably holds a ball at its tip, a connecting member to the front of which the ballpoint pen tip was fixed, an ink reservoir tube to which the connecting member was fixed at its tip opening and which contained ink and an ink backflow preventive inside, and a tail plug fixed to the rear opening of the ink reservoir tube. The ink backflow preventive was an ink backflow preventive kneaded using polybutene as a base oil and fatty acid amide as a thickener. The ballpoint pen refills used balls with a diameter of 0.5 mm.

[0194] Each ballpoint pen refill was placed in a barrel equipped with a clip on the rear outer surface, and biased rearward by a spring (coil spring). The ballpoint pen was a retractable type in which the ballpoint pen tip protrudes outward from the tip hole of the barrel by knocking the rear end (knock operating part) of the barrel forward.

[0195] Preparation of Writing Instruments (Examples 2-11 to 2-13 and Comparative Examples 2-9 to 2-10) The above ink composition was impregnated into an ink occlusion body made of polyester sliver covered with a synthetic resin film, and housed in a barrel made of polypropylene resin. A resin-processed pen body (bullet-shaped) made of an extrusion-molded product of polyacetal resin having a plurality of ink outlet holes extending in the axial direction was connected to the tip of the barrel via a holder, and a cap was attached to prepare a marking pen (cap-type marking pen).

[0196] The following tests were carried out with each sample ballpoint pen and marking pen. Drooping test: Using each writing instrument, the tip was exposed from the barrel and held facing downward, and left for 20 hours in an atmosphere of 20°C temperature and 95% relative humidity or 30°C temperature and 80% relative humidity, and then the appearance of the tip tip was visually observed. Writing test: Each writing instrument that was confirmed to be writable was left standing horizontally with the tip exposed from the barrel in an atmosphere of 20°C temperature and 60% relative humidity for 24 hours, and then a straight line was handwritten on JIS P3201 writing paper A and the condition of the handwriting was visually observed.

[0197] The test results are shown in Table 7 below.

[0198]

[0199] The evaluation symbols for the test results are as follows: Drooping test (Evaluations "A" and "B" were considered to be pass) A: No ink dripping was observed. B: Ink oozes out from the tip end. C: Ink droplets were observed at the tip end. Writing test (Evaluation "◯" was considered to be pass) ◯: Handwriting of a consistent density and line width was obtained stably. ×: Handwriting became smudged.

[0200] This application claims priority from two Japanese patent applications, Japanese Patent Application Nos. 2022-133582 and 2022-133588, both of which were filed on August 24, 2022. Japanese Patent Application Nos. 2022-133582 and 2022-133588 are incorporated herein by reference.

Claims

1. A water-based ink composition for writing instruments comprising water, a colorant, and sulfated cellulose fibers in which at least a portion of the hydroxyl groups of the cellulose fibers are sulfated.

2. The aqueous ink composition for writing instruments according to claim 1, wherein the sulfated cellulose fibers have a cellulose type I crystal structure, and at least some of the hydrogen atoms of the hydroxyl groups of the cellulose fibers are substituted with sulfo groups represented by the following formula (1). (-SA ３ － ) ｒ ・Z ｒ＋ (1) [Here, r is a natural number from 1 to 3, and Z ｒ＋ When r = 1, it is at least one selected from the group consisting of hydrogen ions, alkali metal cations, ammonium ions, aliphatic ammonium ions, and aromatic ammonium ions. When r = 2 or 3, it is at least one selected from the group consisting of alkaline earth metal cations or polyvalent metal cations.

3. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the average fiber width of the sulfated cellulose fibers is 0.5 to 500 nm.

4. The aqueous ink composition for writing instruments according to claim 3, wherein the average fiber width of the sulfated cellulose fibers is 30 nm or less.

5. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the amount of sulfur introduced into the sulfated cellulose fiber is higher than 0.42 mmol / g.

6. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the degree of polymerization of the sulfated cellulose fibers is 30 or more.

7. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the dispersion obtained by dispersing the sulfated cellulose fibers in a water-soluble solvent so that the solid content concentration is 0.5% by mass has a haze value of 20% or less.

8. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the sulfated cellulose fibers are blended in an amount of 0.01 to 3% by mass in the total amount of the ink composition.

9. The aqueous ink composition for writing instruments according to claim 1 or 2, further comprising a shear viscosity reducing agent.

10. The aqueous ink composition for writing instruments according to claim 9, wherein the shear viscosity reducing agent is one or more selected from the group consisting of water-soluble polysaccharides, polymers with a molecular weight of 100,000 to 150,000 mainly composed of alkyl esters of methacrylic acid, poly-N-vinyl carboxylic acid amide crosslinks, benzylidene sorbitol and its derivatives, benzylidene xylitol and its derivatives, alkali-thickening acrylic resins, crosslinkable acrylic acid polymers, inorganic fine particles, nonionic surfactants with an HLB value of 8 to 12, and metal salts or amine salts of dialkyl sulfosuccinate.

11. The aqueous ink composition for writing instruments according to claim 9, wherein the shear viscosity reducing agent is blended in an amount of 0.05 to 1% by mass in the total amount of the ink composition.

12. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the coloring agent is dispersed in water.

13. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the coloring agent comprises one or more selected from the group consisting of pigments and resin particles.

14. The aqueous ink composition for writing instruments according to claim 1 or 2, wherein the average particle size of the colorant is 0.01 to 25 μm.

15. An aqueous ink composition for writing instruments according to claim 1 or 2, comprising a colorant having a specific gravity greater than 1.

16. A writing instrument comprising the aqueous ink composition for writing instruments described in claim 1 or 2.

17. A refill comprising the aqueous ink composition for writing instruments described in Claim 1 or 2.