Aqueous ink composition

Abstract

To provide an aqueous ink composition which has good coloring of characters and patterns written in dark paper, prevents blurring even when being re-written on a dried coating film, and has good temporal writing property.SOLUTION: An aqueous ink composition is composed of at least colored resin particles, a water-soluble acrylic resin, water and a pH adjusting agent, wherein the content of the colored resin particles is 20 wt.% or more and 35 wt.% or less, pigment volume concentration {(volume of colored resin particles+volume of particles other than colored resin particles) / (volume of colored resin particles+volume of particles other than colored resin particles+volume of water-soluble acrylic resin)}×100(%) is 86% or more and 99% or less, the pH adjusting agent is at least a hardly volatile water-soluble basic substance and / or a non-volatile water-soluble basic substance, and the pH of the ink is 6.0 or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a water-based ink composition which produces good color development in letters and designs written on dark paper, does not cause smudges even when rewritten on a dried coating film, and has good writability over time. [Background technology]

[0002] Traditionally, colored resin particles have been used as a colorant for water-based inks because they can be easily made to have a variety of colors, and because colored resin particles colored with different colorants can be considered as a single substance in the ink if the resin composition is the same, so there is little risk of color mixing or color change. In inks that contain a large amount of colored resin particles to ensure highly colored writing, there was a problem that when writing is rewritten on the dried writing or on the surface coating film (on the dried coating film), the residue of the dried coating film gets caught in the tip, causing severe writing smearing and making it impossible to write over time. For example, Patent Document 1 discloses an ink in Example 1 that uses 20% by weight of colored resin particles and does not contain a water-soluble acrylic resin. Patent Document 2 discloses an ink in Examples 3 and 4 that uses about 30% by weight of colored resin particles and does not contain a water-soluble acrylic resin. In such inks that contain a large amount of colored resin particles and no water-soluble acrylic resin, the colored resin particles adhere strongly to each other when the ink dries, and the film thickness becomes thick, so that when rewriting on the dried coating film, the residue of the dried coating film caught in the tip clogs the tip, causing writing smears. On the other hand, Patent Document 3 discloses an aqueous ink in Example 3 that uses 21% by weight of colored resin particles and 5% by weight of water-soluble acrylic resin as a binder (fixing agent for paper surface). However, because the amount of water-soluble acrylic resin added is large and the pigment volume concentration is low, the water-soluble acrylic resin adheres strongly to the dried coating film, and when rewriting on the dried coating film, the residue of the dried coating film caught in the tip clogs the tip, causing writing smears. In addition, the colored resin particles and titanium oxide aggregate due to the evaporation of water and the decrease in pH over time. Writing smears also occur due to the decrease in ink fluidity.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide an aqueous ink composition that has good color development of characters and patterns written on dark paper, does not smear when rewritten on a dried coating film, and does not develop smearing over time during writing.

Means for Solving the Problems

[0005] The present invention is an aqueous ink composition comprising at least colored resin particles, a water - soluble acrylic resin, water, and a pH adjuster, wherein the content of the colored resin particles is 20% by weight or more and 35% by weight or less, the pigment volume concentration {(volume of colored resin particles + volume of particles other than colored resin particles) / (volume of colored resin particles + volume of particles other than colored resin particles + volume of water - soluble acrylic resin)}×100 (%) is 86% or more and 99% or less, the pH adjuster is at least a non - volatile water - soluble basic substance and / or a non - volatile water - soluble basic substance, and the pH of the ink is 6.0 or more. This is the first gist. The second gist is to contain a water - soluble organic solvent having a boiling point of 180°C or higher, and the third gist is that the weight - average molecular weight of the water - soluble acrylic resin is 4000 or more and 200000 or less.

Effects of the Invention

[0006] In order to achieve good color development of handwriting, especially on dark-colored paper, using an ink containing colored resin particles, it is necessary to set the addition amount of the colored resin particles to 20% by weight or more. In an ink with the addition amount of the colored resin particles being 20% by weight or more, in order to prevent bleeding from occurring even when rewriting on the dried coating film, it is necessary to add a water-soluble acrylic resin. The water-soluble acrylic resin is dissolved in water by alkali neutralization. When a water-soluble acrylic resin is added to an ink containing 20% by weight or more of colored resin particles for the purpose of high color development, it adsorbs to particles such as the colored resin particles in the ink and exists between the particles. Even in the dried coating film of this ink, the acrylic resin exists between the colored resin particles. When rewriting on the dried coating film, even if the debris of the dried coating film gets mixed into the tip, the water-soluble acrylic resin immediately redissolves due to the alkaline aqueous solution present in the ink in the tip, the debris of the dried coating film is released, and bleeding does not occur. On the other hand, in a coating film that does not contain a water-soluble acrylic resin, the colored resin particles stick together. When the debris of the dried coating film gets mixed into the tip, it does not immediately dissolve in the moisture in the ink, and bleeding occurs. It is necessary that the pigment volume concentration of the coating film consisting of at least the colored resin particles and the water-soluble acrylic resin { (volume of colored resin particles + volume of particles other than colored resin particles) / (volume of colored resin particles + volume of particles other than colored resin particles + volume of water-soluble acrylic resin)} × 100 (%) is 86% or more and 99% or less. When the pigment volume concentration is less than 86%, the acrylic resin layer becomes thick and it takes time for redissolution, resulting in bleeding during writing. When it is greater than 99%, the water-soluble acrylic resin is insufficient, and a hard-to-dissolve part where the colored resin particles stick together is formed in the dried coating film, resulting in bleeding during writing. By the way, in a ballpoint pen filled with ink using a water-soluble acrylic resin, moisture evaporates over time, and the pH of the ink tends to decrease. This phenomenon is particularly prominent at the pen tip. When the pH of the ink decreases, the water-soluble acrylic resin becomes insoluble, and inks with a high pigment concentration such as the ink of the present invention cause aggregation of colored resin particles. As a result, the fluidity of the ink decreases and bleeding occurs. Therefore, by using an ink adjusted to a pH of 6.0 or higher with at least a hardly volatile water-soluble basic substance and / or a non-volatile water-soluble basic substance, insolubilization of the water-soluble acrylic resin can be prevented, and bleeding after aging can be prevented. Furthermore, regarding bleeding when writing on a coating film, when a water-soluble organic solvent with a boiling point of 180 °C or higher that is difficult to evaporate is added, this is maintained in a state of being contained in the apparent dry coating film for a long time. A coating film containing a water-soluble organic solvent with a boiling point of 180 °C or higher cannot exhibit its original coating film strength and is easily broken by impact. For this reason, even when the water-soluble acrylic resin is not sufficiently redissolved, the bleeding of the dry coating film mixed into the tip can be more quickly resolved by the impact during writing. Also, when the weight average molecular weight of the water-soluble acrylic resin is 4000 or more, it is preferably densely present between the colored resin particles to prevent adhesion between the colored resin particles. When it is 200000 or less, it is preferable because the water-soluble acrylic resin present as a film between the colored resin particles in the dry coating film is easily redissolved.

Embodiments for Carrying Out the Invention

[0007] Specific examples of the colored resin particles include SW-111 (average particle size of 1.0 μm or less), SW-112 (average particle size of 1.0 μm or less), SW-113 (average particle size of 1.0 μm or less), SW-114 (average particle size of 1.0 μm or less), SW-115 (average particle size of 1.0 μm or less), SW-116 (average particle size of 1.0 μm or less), SW-107 (average particle size of 1.0 μm or less), SW-117 (average particle size of 1.0 μm or less), SW-127 (average particle size of 1.0 μm or less), SW-137 (average particle size of 1.0 μm or less), SW-147 (average particle size of 1.0 μm or less), SW-128 (average particle size of 1.0 μm or less) (all of the above are from Shinroi Kogyo Co., Ltd.), Lumicol NKW-2101E (average particle size of 0.4 μm), NKW-2102E (average particle size of 0.4 μm), NKW-2103E (average particle size of 0.4 μm), NKW-2104E (average particle size of 0.4 μm), NKW-2105E (average particle size of 0.4 μm), NKW-2106E (average particle size of 0.4 μm), NKW-2107E (average particle size of 0.4 μm), NKW-2117E (average particle size of 0.4 μm), NKW-2127E (average particle size of 0.4 μm), NKW-2137E (average particle size of 0.4 μm), NKW-2147E (average particle size of 0.4 μm), NKW-2167E (average particle size of 0.4 μm), NKW-2108E (average particle size of 0.4 μm), NKW-2109E (average particle size of 0.4 μm), NKW-C2102E (average particle size of 0.4 μm), NKW-C2103E (average particle size of 0.4 μm), NKW-C2104E (average particle size of 0.4 μm), NKW-C2105E (average particle size of 0.4 μm), NKW-C2117E (average particle size of 0.4 μm), NKW-C2147E (average particle size of 0.4 μm), NKW-C2167E (average particle size of 0.4 μm), NKW-C2108E (average particle size of 0.4 μm), NKW-6002E (average particle size of 0.1 μm), NKW-6013E (average particle size of 0.1 μm), NKW-6004E (average particle size of 0.1 μm), NKW-6005E (average particle size of 0.1 μm), NKW-6007E (average particle size of 0.1 μm), NKW-6047E (average particle size of 0.1 μm), NKW-6077E (average particle size of 0.1 μm), NKW-6008E (average particle size of 0.1 μm), NKW-6038E (average particle size of 0.1 μm), NKW-6202E (average particle size of 0.4 μm), NKW-6203E (average particle size of 0.4 μm), the same as NKW-6253E (average particle size 0.4 μm), the same as NKW-6204E (average particle size 0.4 μm), the same as NKW-6205E (average particle size 0.4 μm), the same as NKW-6207E (average particle size 0.4 μm), the same as NKW-6277E (average particle size 0.4 μm), the same as NKW-6208E (average particle size 0.4 μm), the same as NKW-6258E (average particle size 0.4 μm), the same as NKW-6200E (average particle size 0.4 μm), the same as NKW-3202E (average particle size 0.1 μm), the same as NKW-3203E (average particle size 0.1 μm), the same as NKW-3204E (average particle size 0.1 μm), the same as NKW-3205E (average particle size 0.1 μm), the same as NKW-3207E (average particle size 0.1 μm), the same as NKW-3277E (average particle size 0.1 μm), NKW-3208E (average particle size 0.1 μm) (above, manufactured by Nippon Fluorochemical Co., Ltd.) and the like. Also, non-colored resin particles can be colored with a dye and used. These colored resin particles may be used alone or in a mixture of two or more. The amount of the colored resin particles used needs to be 20% by weight or more and 35% by weight or less based on the total amount of the ink composition. If the amount of the colored resin particles used is less than 20% by weight, the color development is not sufficient. If it exceeds 35% by weight, clogging and poor ejection occur at the pen tip, resulting in writing streaks. When using a commercially available resin particle dispersion, the amount of the resin particles used can be calculated using the solid content in the resin particle dispersion. Also, when the average particle size of the colored resin particles is 0.1 μm or more and 1.0 μm or less, since the colored resin particles have a high light scattering ability as colored resin particles, they exhibit a higher hiding power compared to particle sizes outside this range. Therefore, it is preferable to contain 20% by weight or more and 35% by weight or less of the colored resin particles with an average particle size of 0.1 μm or more and 1.0 μm or less in the ink because high visibility can be obtained even on dark-colored paper without using titanium oxide or hollow particles. The average particle size in this specification is the average particle size (volume average diameter) based on the volume distribution measured by a nanoparticle size distribution measuring device, SALD-7100 (laser diffraction method), manufactured by Shimadzu Corporation.

[0008] The water-soluble acrylic resin is used to prevent scribing scratches even when chips are mixed into the dry coating film when re-scribing on the dry coating film. The water-soluble acrylic resin adsorbs onto the colored resin particles, and the water-soluble acrylic resin exists between the colored resin particles during drying. The chips of the dry coating film composed of at least the colored resin particles and the water-soluble acrylic resin are immediately redissolved and dispersed by the alkaline aqueous solution present in the ink in the chip. The water-soluble acrylic resin can be produced by polymerizing acrylic monomers with acrylic monomers or non-acrylic monomers. Examples of the monomers that can be used include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, n-decyl methacrylate, n-dodecyl methacrylate, lauryl methacrylate, stearyl methacrylate, styrene, vinyl acetate, acrylonitrile, acrylic acid, methacrylic acid, maleic acid, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, acrylamide, N-methylolacrylamide, glycidyl methacrylate, and the like.

[0009] A polymerization initiator is used when polymerizing the monomers. Specific examples thereof include azobisisobutyronitrile, ditertiary butyl peroxide, tertiary butyl hydroperoxide, benzoyl peroxide, parachlorobenzoyl peroxide, tertiary butyl isobutyrate, tertiary butyl per-2-ethylhexanoate, lauroyl peroxide, and the like.

[0010] Commercially available water-soluble acrylic resins may be used. As aqueous solutions, Joncryl 52J (weight average molecular weight 1700, solid content 60.0%), PDX-6157 (weight average molecular weight 6000, solid content 34.0%), 57J (weight average molecular weight 4900, solid content 37.0%), 60J (weight average molecular weight 8500, solid content 34.0), 61J (weight average molecular weight 12000, solid content 30.5%), 62J (weight average molecular weight 8500, solid content 34.0%), 63J (weight average molecular weight 12500, solid content 30.0), 70J (weight average molecular weight 16500, solid content 30.0%), PDX-6180 (weight average molecular weight 14000, solid content 27.0%), HPD-196 (weight average molecular weight 9200, solid content 36.0%), HPD-71 (weight average molecular weight 17250, solid content 28.0%), HPD-96J (weight average molecular weight 16500, solid content 34.0%), PDX-6137A (weight average molecular weight 1600, solid content 28.0%), 501J (weight average molecular weight 12000, solid content 29.5%), 354J (weight average molecular weight 8500, solid content 33.5%), 6610 (weight average molecular weight 8500, solid content 33.5%), JDX-6500 (weight average molecular weight 10000, solid content 29.5%), PDX-6102B (weight average molecular weight 60000, solid content 24.5%), etc. may be mentioned. As solids, Joncryl 67 (weight average molecular weight 12500), 678 (weight average molecular weight 8500), 586 (weight average molecular weight 4600), 587 (weight average molecular weight 17000), 680 (weight average molecular weight 4900), 682 (weight average molecular weight 1700), 683 (weight average molecular weight 8000), 690 (weight average molecular weight 16500), 693 (weight average molecular weight 6000), 819 (weight average molecular weight 14500), HPD-671 (weight average molecular weight 17250), JDX-C3000A (weight average molecular weight 10000), JDX-C3080 (weight average molecular weight 10000) (the above are manufactured by BASF, Germany), etc. may be mentioned. The solid ones are neutralized with basic components such as aqueous ammonia or sodium hydroxide aqueous solution to neutralize the carboxyl groups of the water-soluble acrylic resin and are used after being solubilized.When a volatile basic component such as aqueous ammonia is used, a water-resistant handwriting can be obtained after the ink dries. These water-soluble acrylic resins can be used alone or in a mixture of two or more. The amount of the water-soluble acrylic resin added is preferably such that the pigment volume concentration is 86% or more and 99% or less. When the pigment volume concentration is less than 86%, the acrylic resin layer becomes thick and it takes time for redissolution, resulting in writing streaks. When it is greater than 99%, the water-soluble acrylic resin is insufficient, and a hard-to-disperse portion where the colored resin particles adhere to each other is formed in the dry coating film, resulting in writing streaks. Considering the redissolution rate of the acrylic resin, the pigment volume concentration is more preferably 88% or more, and even more preferably 92% or more. Also, considering sufficient prevention of adhesion between the colored resin particles, the pigment volume concentration is more preferably 98.5% or less, and even more preferably 97.5% or less. Further, when the weight average molecular weight of the water-soluble acrylic resin is 4000 or more, it is preferably densely present between the colored resin particles to prevent adhesion between the colored resin particles. When it is 200000 or less, it is preferable because the water-soluble acrylic resin present as a film between the colored resin particles in the dry coating film is easily redissolved. The volume of each material used in the calculation of the pigment volume concentration {(volume of colored resin particles + volume of particles other than colored resin particles) / (volume of colored resin particles + volume of particles other than colored resin particles + volume of water-soluble acrylic resin)}×100(%) can be calculated from the specific gravity of the material and the weight% in the ink. When using a commercially available aqueous solution of a water-soluble acrylic resin, the amount of the water-soluble acrylic resin used can be calculated using the solid content in the aqueous solution of the water-soluble acrylic resin. Regarding the solid content of the aqueous solution of the water-soluble acrylic resin, the amount of the non-volatile component remaining after volatilizing the volatile components in the resin aqueous solution was taken as the solid content.

[0011] Examples of the non-volatile water-soluble basic substance and / or non-volatile water-soluble basic substance that is a pH adjuster include sodium hydroxide, lithium hydroxide, triethanolamine, diethanolamine, monoethanolamine, aminomethylpropanediol, and the like. Considering the solubility of the water-soluble acrylic resin over time, it is preferable to adjust the initial pH so that the pH of the ink can be maintained at 6.0 or higher. When the pH drops below 6.0, the solubility of the water-soluble acrylic resin decreases, the colored resin particles aggregate, the fluidity decreases, and bleeding occurs. The pH during formulation is preferably 7.0 to 9.0. Incidentally, the pH adjuster of the hardly volatile water-soluble basic substance and / or the non-volatile water-soluble basic substance can also be used as a neutralizer for the water-soluble acrylic resin. Even when an aqueous solution of a water-soluble acrylic resin neutralized with volatile aqueous ammonia is used as an ink material, the initial pH is adjusted using a hardly volatile water-soluble basic substance and / or a non-volatile water-soluble basic substance for pH adjustment during ink formulation so that the pH of the ink can be maintained at 6.0 or higher, thereby maintaining the solubility of the water-soluble acrylic resin over time. When an aqueous solution of a water-soluble acrylic resin neutralized with volatile aqueous ammonia is used as an ink material and the initial pH is adjusted using a hardly volatile water-soluble basic substance and / or a non-volatile water-soluble basic substance for pH adjustment during ink formulation so that the pH of the ink can be maintained at 6.0 or higher, it is preferable because both water-resistant handwriting and maintaining the solubility of the water-soluble acrylic resin over time can be achieved.

[0012] Even for a seemingly dry coating film, it is preferable to add a water-soluble organic solvent with a boiling point of 180 °C or higher for the purpose of maintaining the state in which the coating film contains a solvent for a long time, making it weaker than the original coating film strength and more likely to crack under impact. For example, ethylene glycol (boiling point 198 °C), diethylene glycol (244 °C), dipropylene glycol (232 °C), 1,3-propanediol (213 °C), 1,2-butylene glycol (190 °C), 1,3-butylene glycol (207 °C), 1,4-butylene glycol (235 °C), 1,5-pentanediol (238 °C), diethylene glycol monomethyl ether (194 °C), diethylene glycol monoethyl ether (202 °C), triethylene glycol dimethyl ether (216 °C), 2-pyrrolidone (245 °C), N-methyl-2-pyrrolidone (202 °C), triethylene glycol (287 °C), tetraethylene glycol (327 °C), glycerin (290 °C), triethanolamine (360 °C), etc. Further, as those having no clear boiling point, there is polyethylene glycol, etc. These organic solvents may be used alone or in combination of two or more, and the amount used is preferably 2% by weight or more and 20% by weight or less based on the total amount of the ink composition. Also, a water-soluble organic solvent with a boiling point of 180 °C or lower may be used in combination with these solvents. All temperature notations (°C) in this specification are Celsius temperatures at 1 atmosphere (101325 Pa).

[0013] In order to suppress color fading and improve color development, non-colored resin particles with an average particle size of 1 μm or more can also be used. Non-colored resin particles with an average particle size of 1 μm or more have the effect of filling the grooves of paper fibers and suppressing the penetration of colored resin particles into the paper, which reduces visibility. In the case of particles with an average particle size of 1 μm or less, the same effect can be obtained by using them in an aggregated state. Specific examples of non-colored resin particles include Eposta MV1002 (average particle size 2 μm, acrylic cross-linked product), MV1004 (average particle size 4 μm, acrylic cross-linked product), MV1006 (average particle size 6 μm, acrylic cross-linked product), MV1010 (average particle size 10 μm, acrylic cross-linked product), MV2003 (average particle size 3 μm, acrylic-styrene cross-linked product), MS (average particle size 2 μm, benzoguanamine formaldehyde), M-30 (average particle size 3 μm, benzoguanamine formaldehyde condensate), SS (average particle size 0.1 μm, melamine formaldehyde condensate), S (average particle size 0.2 μm, melamine formaldehyde condensate), S-6 (average particle size 0.4 μm, melamine formaldehyde condensate), S-12 (average particle size 1.2 μm, melamine-formaldehyde condensate) (above, manufactured by Nippon Shokubai Co., Ltd.), Tech Polymer MBX-5 (average particle size 5 μm, crosslinked polymethyl methacrylate), MBX-8 (average particle size 8 μm, crosslinked polymethyl methacrylate), MBX-12 (average particle size 12 μm, crosslinked polymethyl methacrylate), MBX-20 (average particle size 20 μm, crosslinked polymethyl methacrylate), MBX-30 (average particle size 30 μm, crosslinked polymethyl methacrylate), MBX-40 (average particle size 40 μm, crosslinked polymethyl methacrylate), MBX-50 (average particle size 50 μm, crosslinked polymethyl methacrylate), MBX-60 (average particle size 60 μm, crosslinked polymethyl methacrylate), MBX-80 (average particle size 80 μm, crosslinked polymethyl methacrylate), MBX-100 (average particle size 100 μm, crosslinked polymethyl methacrylate), SBX-4 (average particle size 4 μm, crosslinked polystyrene), SBX-6 (average particle size 6 μm, crosslinked polystyrene), SBX-8 (average particle size 8 μm, crosslinked polystyrene), SBX-12 (average particle size 12 μm, crosslinked polystyrene), SBX-17 (average particle size 17 μm, crosslinked polystyrene), SBX-30 (average particle size 30 μm, crosslinked polystyrene), MB-4 (average particle size 4 μm, crosslinked polymethyl methacrylate), MB-8 (average particle size 8 μm, crosslinked polymethyl methacrylate), MB-20 (average particle size 20 μm, crosslinked polymethyl methacrylate), SSX-101 (average particle size 1.5 μm, crosslinked polymethyl methacrylate), SSX-102 (average particle size 2.5 μm, crosslinked polymethyl methacrylate), SSX-103 (average particle size 3 μm, crosslinked polymethyl methacrylate), SSX-105 (average particle size 5 μm, crosslinked polymethyl methacrylate), SSX-108 (average particle size 8 μm, crosslinked polymethyl methacrylate), SSX-110 (average particle size 10 μm, crosslinked polymethyl methacrylate), SSX-115HEXE (average particle size 15 μm, crosslinked polymethyl methacrylate), SSX-120 (average particle size 27 μm, crosslinked polymethyl methacrylate), SSX-127 (average particle size 20 μm, crosslinked polymethyl methacrylate) (above, manufactured by Sekisui Chemical Co., Ltd.), MX-150 (average particle size 1.Examples include MX100TA (average particle size: 5 μm), MX180TA (average particle size: 3 μm), MX-500 (average particle size: 5 μm) (manufactured by Soken Chemical & Engineering Co., Ltd., etc.). These can be used alone or in combination of two or more. When the average particle size of the non-colored resin particles is 1 μm or more, the effect of suppressing the penetration of the colored resin particles by the non-colored resin particles is good, and particles of 2 μm or more are preferred. Also, although it varies depending on the amount of movement of the ball in the ballpoint pen tip, as long as it can be discharged, non-colored resin particles with a relatively small particle size with an average particle size of 0.1 μm or more and 10 μm or less can also control the dispersion. When used in aggregates of about 10 to 100 μm, good color development can be obtained even when used for rough black drawing paper, etc. The amount of non-colored resin particles used is preferably such that the total volume of the colored resin particles and the non-colored resin particles is 18 ml to 35 ml in 100 g of the ink.

[0014] When writing on dark-colored paper, titanium oxide may be added to make it easier to visually identify the color of the handwriting even before it dries. Specific examples include TITONE SR-1, R-650, R-62N, R-42, R-7E, R-21, R-25, R-32, R-5N, R-45M, TCR-10 (manufactured by Sakai Chemical Industry Co., Ltd.), Cronos KR-310, KR-380, 480 (manufactured by Titanium Industry Co., Ltd.), Typia R-900, R-902, R-960, R-931 (manufactured by DuPont Japan Limited), TITANIX JR-301, JR-805, JR-806, JR-603, JR-800, JR-403, JR-701, JRNC, JR-605 (manufactured by Teika Co., Ltd.), TIPAQUE R-820, R-830, R-550, R-780, R-780-2 (manufactured by Ishihara Sangyo Kaisha, Ltd.), FUJI SP WHITE1193 which is a dispersion of titanium oxide (pigment 50.0% by weight), 1131 (pigment 32.5% by weight, (titanium oxide: extender pigment = 4:3)), 1142 (pigment 29.5% by weight, (titanium oxide: extender pigment = 4:3)), 1154 (pigment 55.0% by weight), 1184W (pigment 30.0% by weight, (titanium oxide: extender pigment = 15:2)), 1197 (pigment 42.0% by weight, (titanium oxide: extender pigment = 9:1)) (manufactured by Fuji Pigment Co., Ltd.), etc. These titanium oxides may be used alone or in combination of two or more. When the ratio of colored resin particles / titanium oxide is 10:1 to 30:1, it is preferable as it enables both dispersion stability and color visual recognition of the handwriting before drying on dark-colored paper.

[0015] Within the range that does not impair the effects of the present invention, a known colorant can be used in an appropriate amount in addition to the colored resin particles.

[0016] For the purpose of preventing the tip of the pen from drying, sugar alcohols can also be used. Specific examples include PO-10 (monosaccharide 0 - 3%, disaccharide 1 - 5%, trisaccharide 1 - 5%, tetrasaccharide or higher 90 - 95%), PO-20 (monosaccharide 2 - 5%, disaccharide 9 - 14%, trisaccharide 11 - 16%, tetrasaccharide or higher 67 - 76%, 70% aqueous solution), PO-30 (monosaccharide 3 - 6%, disaccharide 13 - 19%, trisaccharide 14 - 19%, tetrasaccharide or higher 58 - 66%, 70% aqueous solution), SO syrup (monosaccharide 3 - 10%, disaccharide 35 - 50%, trisaccharide 20 - 30%, tetrasaccharide or higher 15 - 30%, 70% aqueous solution), PO-40 (monosaccharide 1 - 6%, disaccharide 45 - 55%, trisaccharide 15 - 25%, tetrasaccharide or higher 23 - 30%, 70% aqueous solution), PO-60 (monosaccharide 2 - 7%, disaccharide 62 - 67%, trisaccharide 14 - 20%, tetrasaccharide or higher 10 - 18%, 70% aqueous solution), Amamyl (monosaccharide 46 - 49%, disaccharide 30 - 40%, trisaccharide 5 - 13%, tetrasaccharide or higher 4 - 10%, 70% aqueous solution), PO-300 (monosaccharide 17 - 25%, disaccharide 25 - 33%, trisaccharide 33 - 39%, tetrasaccharide or higher 10 - 19%, 70% aqueous solution), PO-500 (monosaccharide 34 - 45%, disaccharide 26 - 32%, trisaccharide 14 - 20%, tetrasaccharide or higher 11 - 18%, 70% aqueous solution), Amalty syrup (monosaccharide 1 - 4%, disaccharide 75 - 80%, trisaccharide 10 - 17%, tetrasaccharide or higher 6 - 12%, 75% aqueous solution), Amalty MR (monosaccharide 0 - 3%, disaccharide 88 - 98%, trisaccharide 2 - 9%, tetrasaccharide or higher 0 - 4%), Resis (disaccharide 98% or higher), Sorbit T-70 (monosaccharide 71 - 100%, disaccharide 5 - 10%, trisaccharide 0 - 5%, tetrasaccharide or higher 0 - 5%, 70% aqueous solution), Milhen (lactitol), Marine Crystal (D-mannitol), Xylylit (xylitol), Erythritol (above are all manufactured by Mitsubishi Corporation Life Sciences, Ltd.), etc. These can be used alone or in combination of two or more. The usage amount of sugar alcohol is preferably 0.5 wt% or more and 5 wt% or less based on the total amount of the ink composition. Also, a higher ratio of tetrasaccharide or higher has a better effect of preventing the tip of the pen from drying, and the ratio is preferably 0.5 wt% or more based on the total amount of the ink composition.

[0017] To adjust the ink viscosity, high molecular polysaccharides can be used. Specific examples include pullulan, xanthan gum, welan gum, gellan gum, rhamsan gum, starch, cationic starch, dextrin, sodium starch glycolate, etc. and their derivatives, gum arabic, tragacanth gum, locust bean gum, guar gum and its derivatives, agar, carrageenan, alginic acid, alginate, pectin, gelatin, casein, sodium caseinate, glucomannan, dextran, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, sodium starch glycolate, lanolin derivatives, chitosan derivatives, etc. These high molecular polysaccharides can be used alone or in a mixture of two or more. Among these high molecular polysaccharides, xanthan gum can be preferably used in particular. An aqueous xanthan gum solution has high shear thinning viscosity and can achieve both prevention of sedimentation of colored resin particles and titanium oxide and good writing properties. Specific examples of xanthan gum include Kelzan, Kelzan S, Kelzan F, Kelzan AR, Kelzan M, Kelzan D (manufactured by Sankyo Co., Ltd. above), Kojin, Kojin F, Kojin T, Kojin K (manufactured by Kogyo Co., Ltd. above), Nokcoat (manufactured by Nisshin Oillio Group Ltd.), Inagel V-7, Inagel V-7T (manufactured by Ina Food Industry Co., Ltd. above), etc. The usage amount of the high molecular polysaccharide is preferably 0.1% by weight or more and 1.0% by weight or less based on the total amount of the ink composition.

[0018] In order to suppress the generation of bubbles during ink production, various defoamers can also be used. BYK-01, BYK-012, BYK-014, BYK-015, BYK-017, BYK-018, BYK019, BYK-021, BYK-022, BYK-023, BYK-024, BYK-025, BYK-028, BYK-038, BYK-039, BYK-044, BYK-093, BYK-094, BYK-1610, BYK-1615, BYK-1640, BYK-1650, BYK-1710, BYK-1711, BYK-1730, BYK-1740, BYK-1770, BYK-1780, BYK-1785, BYK-1798 (manufactured by BYK-Chemie Japan Co., Ltd. as above), KM-73, KM-73A, M-73E, KM-7751, KM-70, KM-71, KM-75, KM-7750D, KM-85, KM72, KM-72F, KM-72S, KM-72FS, KM-72GS, KM-89, KM-90, KM-98, KM-7752, KS-530, KS-531, KS-537, KS-538, KS-540, X-50-1176, KF-96, KF-96ADF, KF6701, KS-7708, X-50-1100, X-50-1244, KS-66, KS-69, KS-602A, FA-600 (manufactured by Shin-Etsu Chemical Co., Ltd. as above), AQ-501, AQ-530S, AQ-7533, AQ-7552SE (manufactured by Kusumoto Chemicals, Ltd. as above), etc. can be mentioned. These can be used alone or in combination of two or more. The usage amount of the defoamer is preferably 0.1% by weight or more and 1.0% by weight based on the total amount of the ink composition.

[0019] As a surface tension adjuster, an acetylene glycol-based surfactant may be added. When an acetylene glycol-based surfactant is added, even when writing on a writing surface that is difficult to absorb ink, such as glossy paper or masking tape, where the surface tension is lowered, it can be written without repelling. Examples of acetylene glycol-based surfactants include Surfynol 104E, 420, 440, SE, SE-F, 604, 607, 2502, DF110D (manufactured by Nissin Chemical Industry Co., Ltd., etc.). These can be used alone or in combination of two or more. The amount of the surfactant used is preferably 0.1% by weight or more and 3.0% by weight or less based on the total amount of the ink composition. When these surfactants are solids, they may be dissolved and used in alcohol-based solvents such as ethanol and isopropyl alcohol, or glycol-based solvents such as ethylene glycol and propylene glycol.

[0020] Furthermore, the aqueous ink composition of the present invention may appropriately contain a lubricant, a rust inhibitor, a preservative, or an antibacterial agent, etc. within a range that does not impair the effects of the present invention, if necessary. Examples of the lubricant include fatty acid esters of polyhydric alcohols, higher fatty acid esters of sugars, polyoxyalkylene higher fatty acid esters, alkyl phosphoric acid esters, etc. Also, examples of the rust inhibitor include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, saponins, etc., and examples of the preservative or antibacterial agent include pheno l, sodium omadine, sodium benzoate, benzisothiazoline, benzimidazo le-based compounds, etc.

[0021] When manufacturing the aqueous ink composition of the present invention, various conventionally known methods can be adopted. For example, it can be easily obtained by mixing and stirring with a stirrer such as a Henschel mixer, a propeller stirrer, a homogenizer, a turbo mixer, a high-pressure homogenizer, etc. having a high shearing force.

[0022] The aqueous ink composition of the present invention is filled and used in a container in which a chip is set in an ink storage tube. As the ink storage tube, those made of metal or synthetic resin can be used. If it is made of a transparent or translucent synthetic resin, the remaining amount of ink can be clearly indicated. For example, polyethylene, polypropylene, polyurethane, polyethylene terephthalate, polyamide, polyacrylonitrile-based resin, polyarylate, ethylene-vinyl alcohol copolymer, fluororesin, etc. are available, and polypropylene can be preferably used in terms of price and visibility. It is also possible to use an ink storage tube kneaded with a dye, a pigment, a coloring material having polarization, etc. As the inner diameter, those having an inner diameter of 1.5 mm to 20.0 mm can be used. For the purpose of preventing ink drying and backflow, an ink backflow prevention body may be filled at the rear end of the ink. As the ink backflow prevention body, various known ones such as a high-viscosity non-volatile liquid, a gelled non-volatile liquid, and a sponge-like one can be used, and a float may be used in combination.

[0023] As the structure of the ballpoint pen tip, a conventionally general structure can be used. For example, a ballpoint pen tip in which a ball is rotatably held in a ball holding portion formed by pressing and deforming the vicinity of the tip of a metal pipe inward from the outer surface, with the ball partially protruding, or a ballpoint pen tip in which a ball is rotatably held in a ball holder formed with a through hole by cutting a metal material with a drill or the like, with the ball partially protruding. In addition, for the purpose of preventing ink bleeding during non-writing, a structure can be adopted in which a spring is arranged behind the ball to press the ball against the inner surface of the ball holder. As the material of the ball, cemented carbide, stainless steel, ceramic, resin, rubber, etc. can be used. In particular, in order to obtain sufficient ink discharge while stabilizing the rotation during writing by providing sufficient corrosion resistance, a ball made of a ceramic material such as cemented carbide or silicon carbide / silicon nitride is preferable. In order to sufficiently discharge the ink of the present invention and, when rewriting on a dry coating film, efficiently redissolve the debris of the dry coating film caught in the tip of the ballpoint pen by loosening it by the movement of the ball, it is necessary to ensure a sufficient gap between the ball and the ball holder. As a dimension indicating the size of the gap, there is a ball forward and backward movement amount which is the range in which the ball moves in the axial direction, and it is preferable to set the amount to 25 μm or more and 180 μm or less. When the ball forward and backward movement amount is 25 μm or more, the debris of the dry coating film caught in the tip when rewriting on the dry coating film is easily loosened by the movement of the ball during writing, and the redissolution of the dry coating film and the redispersion of the colored resin particles can be promoted. However, when the ball forward and backward movement amount is 180 μm or more, the amount of debris of the dry coating film caught in the tip when rewriting on the dry coating film becomes too large, it takes time for redissolution, and clogging is likely to occur. Incidentally, although the ball diameter is not limited as long as the above ball forward and backward movement amount can be set, considering the problem that the ball may come off and the ease of processing, the ball diameter is preferably 0.5 mm or more and 1.2 mm or less. As a particularly preferable example, a ballpoint pen tip having a ball diameter of 1.0 mm and a ball forward and backward movement amount of 70 μm or more and 150 μm or less, or a ball diameter of 0.8 mm and a ball forward and backward movement amount of 60 μm or more and 100 μm or less is suitable for using the ink of the present invention. Also, when the discharge amount per 10 m of the ink is 0.04 g or more and 0.15 g or less, it is preferable because the visibility when writing on dark paper such as black high-quality paper is improved.

[0024] The form of the ballpoint pen of the present invention may be either a cap type or a retractable type, and is not particularly limited. In the case of a retractable ballpoint pen, a single ballpoint pen refill may be housed in the shaft cylinder, and the tip may be configured to be retractable from the tip hole of the shaft cylinder, or it may be a composite type retractable writing instrument in which a plurality of ballpoint pen refills are housed in the shaft cylinder. Also, a ballpoint pen that applies pressure to the ink, such as by enclosing compressed air in the ink storage tube, to assist the discharge of the ink during writing may be used.

[0025] Hereinafter, the present invention will be described in detail with reference to examples.

[0026] (Preparation of Aqueous Ink Composition) Aqueous ink compositions of Examples 1 to 21 and Comparative Examples 1 to 6 having the compositions shown in Tables 2 to 5 below were prepared. The unit of the amount of each component described in Tables 2 to 5 is parts by weight. Specifically, the following materials were used as the materials for the compositions shown in Tables 2 to 5.

[0027] <Colored Resin Particles> Colored Resin Particle 1: Lumicol NKW-6038E (average particle size 0.1 μm, solid content 34.0%, styrene·acrylonitrile copolymer, manufactured by Nippon Fluorochemical Co., Ltd.) Colored Resin Particle 2: Lumicol NKW-6208E (average particle size 0.4 μm, solid content 50%, styrene·acrylonitrile copolymer, manufactured by Nippon Fluorochemical Co., Ltd.) Colored Resin Particle 3: Lumicol NKW-2108E (average particle size 0.4 μm, solid content 51%, styrene·acrylonitrile copolymer, manufactured by Nippon Fluorochemical Co., Ltd.) Colored Resin Particle 4: MX-80H3wT (average particle size 0.8 μm, crosslinked acrylic monodisperse particles, manufactured by Soken Chemical & Engineering Co., Ltd.) 50.0 parts by weight Propylene Glycol 16.0 parts by weight Triethanolamine Lauryl Sulfate 2.0 parts by weight C.I. Basic Blue 7 1.0 part by weight Ion-exchanged Water 30.0 parts by weight The above components were mixed and stirred at 80°C for 6 hours to obtain a dispersion of Colored Resin Particle 4.

[0028] <Surfactant> Surfactant 1: Surfynol 420 (acetylene glycol, manufactured by Nissin Chemical Industry Co., Ltd.) Surfactant 2: NIKKOL SO-10V (sorbitan oleate, manufactured by Nikko Chemicals Co., Ltd.)

[0029] <Water> Ion-exchanged Water

[0030] <Water-soluble Acrylic Resin> Preparation of 1 to 3 water-soluble acrylic resins

[0031] [Table 1] The substances shown in water-soluble acrylic resins 1 to 3 in Table 1 were charged into a 500 ml reaction vessel equipped with a stirrer, a nitrogen gas inlet, a thermometer, and a reflux condenser, and polymerized with stirring at 80 °C for 7 hours in a nitrogen gas stream to obtain a transparent and viscous polymer component, which was dried to obtain water-soluble acrylic resins 1 to 3. The unit of the numbers in the column of each component and the total in Table 1 is g (gram). The weight-average molecular weight of the synthesized acrylic resin was measured by gel permeation chromatography (GPC) using a tetrahydrofuran solution as the developing solvent, and converted using polystyrene as the standard substance. The column used for measurement was Shodex OHpakSB-804HQ (manufactured by Showa Denko K.K.). Water-soluble acrylic resin aqueous solution 1: 20 parts by weight of water-soluble acrylic resin 1 10 parts by weight of 25% aqueous ammonia 3 parts by weight of isopropyl alcohol 67 parts by weight of ion-exchanged water The above components were stirred at 25 °C for 5 hours with a propeller stirrer to obtain an acrylic resin aqueous solution 1 with a weight-average molecular weight of 18,000 0 Water-soluble acrylic resin aqueous solution 2: Acrylic resin 2 was used instead of acrylic resin 1 in the acrylic resin aqueous solution 1, and an acrylic resin aqueous solution 2 with a weight-average molecular weight of 120,000 was obtained in the same manner. Water-soluble acrylic resin aqueous solution 3: Acrylic resin 3 was used instead of acrylic resin 1 in the acrylic resin aqueous solution 1, and an acrylic resin aqueous solution 3 with a weight-average molecular weight of 215,000 was obtained in the same manner. Water-soluble acrylic resin aqueous solution 4: Joncryl 52J (weight-average molecular weight 1700, solid content 60.0%, manufactured by BASF, Germany) Water-soluble acrylic resin aqueous solution 5: Joncryl PDX-6157 (weight average molecular weight 6000, solid content 34.0%, manufactured by BASF, Germany) Water-soluble acrylic resin aqueous solution 6: Joncryl PDX-6102B (weight average molecular weight 60000, solid content 24.5%, manufactured by BASF, Germany) Water-soluble acrylic resin aqueous solution 7: JDX-6500 (weight average molecular weight 10000, solid content 29.5%, manufactured by BASF, Germany) <Acrylic resin emulsion> Acrylic resin emulsion 1: Joncryl PDX-7696 (weight average molecular weight 100000 - 200000, solid content 40.0%, manufactured by BASF, Germany)

[0032] <Non-colored resin particles> Non-colored resin particles 1: Epotar S12 (average particle size 1.2 μm, melamine formaldehyde condensate, manufactured by Nippon Shokubai Co., Ltd.) Non-colored resin particles 2: Epotar MV1002 (average particle size 2.0 μm, manufactured by Nippon Shokubai Co., Ltd.) Non-colored resin particles 3: Epotar MV1004 (average particle size 4.0 μm, manufactured by Nippon Shokubai Co., Ltd.) Non-colored resin particles 4: Epotar S6 (average particle size 0.4 μm, manufactured by Nippon Shokubai Co., Ltd.) Non-colored resin particles 5: Epotar MV1006 (average particle size 6.0 μm, manufactured by Nippon Shokubai Co., Ltd.)

[0033] <Titanium oxide> Titanium oxide 1: FUJI SP WHITE1193 (50 wt% titanium oxide, manufactured by Fuji Pigment Co., Ltd.)

[0034] <Water-soluble organic solvent> Water-soluble organic solvent 1: Ethylene glycol (boiling point 198 °C) Water-soluble organic solvent 2: Glycerin (boiling point 290 °C) Water-soluble organic solvent 3: Polyethylene glycol 200 (no boiling point data) Water-soluble organic solvent 4: Propylene glycol (boiling point 188 °C) Water-soluble organic solvent 5: Butyl cellosolve (boiling point 171 °C)

[0035] <Sugar alcohol> Sugar alcohol 1: PO-10 (monosaccharide 0 - 3%, disaccharide 1 - 5%, trisaccharide 1 - 5%, tetrasaccharide or higher 90 - 95%, manufactured by Mitsubishi Corporation Life Sciences, Ltd.) Sugar alcohol 2: PO-20 (monosaccharide 2 - 5%, disaccharide 9 - 14%, trisaccharide 11 - 16%, tetrasaccharide or higher 67 - 76%, 70% aqueous solution, manufactured by Mitsubishi Corporation Life Sciences, Ltd.)

[0036] <Polysaccharide> Polysaccharide 1: Kelzan AR (xanthan gum, manufactured by Sankyo Co., Ltd.)

[0037] <Defoaming agent> Defoaming agent 1: BYK-094 (manufactured by BYK-Chemie Japan Co., Ltd.) Defoaming agent 2: AQ-501 (manufactured by Kusumoto Chemicals, Ltd.)

[0038] <Preservative> Preservative 1: Proxel GXL(S) (manufactured by Lonza Japan Co., Ltd.) Preservative 2: Sun Eye Back Sodium Omadine (manufactured by San-Ai Oil Co., Ltd.)

[0039] <pH adjuster> pH adjuster 1: 20% aqueous sodium hydroxide solution pH adjuster 2: Triethanolamine

[0040] <Ink preparation> After stirring each component of each example and comparative example with a propeller stirrer for 3 hours, it was adjusted to a predetermined pH with a pH adjuster to obtain the aqueous ink compositions of each example and comparative example. When adjusting to a predetermined pH with a pH adjuster, the pH was adjusted while measuring the pH using a pH meter (manufactured by Horiba, Ltd., RAQUAtwin).

[0041] A ballpoint pen tip (ball diameter: 1.0 mm, ball forward and backward movement amount: 90 μm) was connected to an ink container used in Dual Metallic K110 (Pentel Co., Ltd.) via a ballpoint pen tip holder. The ink container was filled with about 0.8 g of the aqueous ink compositions obtained in Examples 1 to 22 and Comparative Examples 1 to 6. After arranging an ink backflow preventer in layers at the ink interface, centrifugal defoaming was performed so that the pen tip faced outward to produce a test ballpoint pen. The ink compositions of Examples 1 to 22 and Comparative Examples 1 to 6 used for the test sample ballpoint pens for evaluation are shown in Tables 2 to 5. In the column of pH adjuster in Tables 2 to 5, when adjusted to a predetermined pH using pH adjuster 1, it was denoted as NaOH; when adjusted to a predetermined pH using pH adjuster 2, it was denoted as TEA; when no pH adjuster was used, it was denoted as unadjusted. The predetermined pH adjusted using a pH adjuster was described in the column of target initial pH in Tables 2 to 5.

[0042] Using the test ballpoint pen described above, the following retesting on a dry coating film, visibility test on dark paper, measurement of ink pH over time, and writing test over time were conducted.

[0043] Retesting on a dry coating film The aqueous ink compositions obtained in the examples and comparative examples were used to form a coating film on high-quality paper ("Kinmari SW" manufactured by Kitakoshi Corporation, paper thickness (JIS P 8118): 95 ± 3 μm) on the F surface (felt side (surface)) using a select roller (OSP-100 manufactured by Matsuo Sangyo Co., Ltd.) and dried in an environment of 25°C and 65% for 24 hours. After writing two 10-fold vortices with a maximum diameter of about 1 cm by hand on the dry coating film using the test ballpoint pen filled with the aqueous ink compositions obtained in the examples and comparative examples, spiral circular writings with a diameter of about 1.5 cm were written in 4 circles for 10 steps (40 circles in total) on another high-quality paper, and the number of circles until writing without streaking was counted. The test results are shown in the column of retesting on a dry coating film in Tables 2 to 5. The unit of the numerical values of the test results is [circle], and when all 40 circles were streaked, it was described as unable to write.

[0044] Visibility test on dark paper Using the test ballpoint pens filled with the aqueous ink compositions obtained in the examples and comparative examples, a 10-cm straight line was written on black paper (New Color R Black, manufactured by Rintech Co., Ltd.) at a writing angle of 70°, a writing speed of 7 cm / second, and a writing load of 100 g, and the visibility of the handwriting was visually evaluated in three levels. The evaluation results are shown in the dark paper visibility columns of Tables 2 to 5. Evaluation criteria: × Visible with difficulty upon close inspection 〇 Visible ◎ Clearly visible

[0045] Measurement of ink pH over time The ballpoint pens for writing filled with the aqueous ink compositions obtained in the examples and comparative examples were placed horizontally in an environment of 50°C and 30% humidity. After being taken out three months later and left at room temperature for 24 hours, the pH of the ink in the ink storage tube was measured using a pH meter (RAQUAtwin, manufactured by Horiba, Ltd.). The measurement results are shown in the ink pH over time columns of Tables 2 to 5.

[0046] Writing test over time The ballpoint pens for writing filled with the aqueous ink compositions obtained in the examples and comparative examples were placed horizontally in an environment of 50°C and 30% humidity. After being taken out three months later and left at room temperature for 24 hours, a spiral-shaped 20-m writing was performed at a writing angle of 70°, a writing speed of 7 cm / second, and a writing load of 100 g, and the discharge amount was measured. The measurement results are shown in the writing test over time columns of Tables 2 to 5. The unit of the numerical values of the test results is [g].

[0047]

Table 2

[0048]

Table 3

[0049]

Table 4

[0050]

Table 5

[0051] The aqueous ink compositions of Examples 1 to 22 have a content of colored resin particles of 20% by weight or more and 35% by weight or less, a pigment volume concentration {(volume of colored resin particles + volume of particles other than colored resin particles) / (volume of colored resin particles + volume of particles other than colored resin particles + volume of water-soluble acrylic resin)}×100(%) of 86% or more and 99% or less, and a pH of 6.0 or more after 3 months at 50°C. Therefore, even when written on black paper, the visibility is good, and even when rewritten on the dried coating film, it can be restored to normal handwriting within 20 circles, and there is sufficient discharge amount even after 3 months at 50°C and it can be written without streaking. Comparing Example 1 and Example 2, the streaking is reduced in the rewrite on the dried coating film, and the discharge amount after 3 months at 50°C is improved. The comparison between Example 1 and Example 3 also gave the same result. In addition, Example 4 using glycerin and acrylic resin 2 with a weight average molecular weight of 120,000 showed further improvement in both the rewrite test on the dried coating film and the discharge amount after 3 months at 50°C. Examples 4 to 6 differ in the amount of colored resin particles added. In Example 5, which is the lower limit, the visibility on dark paper is slightly inferior, but it is still at a level where it can be sufficiently visually recognized. Also, in Example 6, which is the upper limit, the discharge amount after 3 months at 50 °C decreases, but it can be written without streaking. Examples 4 and 7 to 9 differ in pigment volume concentration. In rewriting on the dry coating film, all of them can be rewritten, but there are slightly more streaks at the upper and lower limits. There is no difference in visibility on dark paper. Examples 4 and 10 to 12 differ in pH. When the pH is 8 or higher, the streaks in rewriting on the dry coating film slightly increase, but when the pH is 6.0 at the lower limit, the discharge amount after 3 months at 50 °C decreases. However, it can be written without streaking. Examples 4 and 13 to 16 differ in water-soluble organic solvents. For glycerin and polyethylene glycol 200 with high boiling points, the streaks in rewriting on the dry coating film are good, and the discharge amount after 3 months at 50 °C is the best for glycerin. Examples 2, 4, and 17 to 20 differ in the weight average molecular weight of the water-soluble acrylic resin. In rewriting on the dry coating film of Example 2 with 1700 and Example 18 with 21500, the streaks slightly increase, and in Example 18, the discharge amount after 3 months at 50 °C also decreases. In Example 5 and Example 22, the addition amount of colored resin particles is at the lower limit, but the visibility on dark paper can be improved by the addition amount of non-colored resin particles. However, there is a tendency for the streaks in rewriting on the dry coating film to increase.

[0052] In contrast, Comparative Examples 1 to 2 are outside the range of colored resin particles. In Comparative Example 1, which is below the lower limit, the visibility on black paper is poor, and in Comparative Example 2, which exceeds the upper limit, the discharge amount after 3 months at 50 °C is low and the ink followability is poor. In Comparative Example 3, an acrylic emulsion is used instead of the water-soluble acrylic resin. Since the acrylic emulsion does not redissolve once it dries, the clogs in the ball pen tip are difficult to dissolve, and when rewritten on the dry coating film, it becomes impossible to write. Also, writing after 3 months at 50 °C becomes impossible due to the drying of the pen tip. Examples 4 to 5 are outside the range of pigment volume concentration, but they became impossible to write or had many streaks when rewritten on the dry coating film. In Example 6, the pH after 3 months at 50 °C is 5.8, which is below 6.0, so the solubility of the water-soluble acrylic resin decreases, the fluidity decreases due to the aggregation of the colored resin particles, and it becomes impossible to write.

Claims

1. An aqueous ink composition comprising at least colored resin particles, a water-soluble acrylic resin, water, and a pH adjuster, wherein the content of the colored resin particles is 20% by weight or more and 35% by weight or less, and the pigment volume concentration { (volume of colored resin particles + volume of particles other than colored resin particles) / (volume of colored resin particles + volume of particles other than colored resin particles + volume of water-soluble acrylic resin)} × 100 (%) is 86% or more and 99% or less, and the pH adjuster is at least a hardly volatile water-soluble basic substance and / or a non-volatile water-soluble basic substance, and the pH of the ink is 6.0 or more.

2. The aqueous ink composition according to Claim 1, comprising a water-soluble organic solvent having a boiling point of 180°C or higher.

3. The aqueous ink composition according to Claim 1 or Claim 2, wherein the weight average molecular weight of the water-soluble acrylic resin is 4000 or more and 200000 or less.

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