Water-based ink composition for ballpoint pens and ballpoint pens containing the same.

Abstract

The present invention provides a water-based ink composition for ballpoint pens and a ballpoint pen containing this composition, which can effectively suppress wear on the ball bearing seat, even in ballpoint pens with a structure prone to wear on the ball bearing seat, such as fine-tip ballpoint pens, and can also maintain the stability of ultrafine particle ink dispersion over a long period of time. [Solution] A water-based ink composition for ballpoint pens containing polyglycerin isostearate ester in an aqueous ink comprising at least one ultrafine particle with an average particle diameter of less than 0.1 μm, selected from alumina, titanium oxide, and silica, a pigment, and water, and a ballpoint pen containing this composition.

Description

Technical Field

[0001] The present invention relates to an aqueous ink composition for ballpoint pens. Furthermore, it relates to an aqueous ink composition for ballpoint pens and ballpoint pens that are excellent in the wear resistance of the ball receiving seat and the stability of the ink over time.

Background Art

[0002] A ballpoint pen has a mechanism in which the ball rotates during writing and the ink is transferred to the writing surface to form a handwriting. Therefore, as the ballpoint pen is used for long-distance writing, the part (ball receiving seat) that receives and contacts the ball inside the ballpoint pen tip wears as the rotation speed of the ball increases, causing problems such as clogging of the ink flow hole or smudging during writing or inability to write due to the crimping part holding the ball hitting the paper surface. Therefore, techniques have been proposed in which fine particles with a particle diameter of 0.1 μm or more and 30 μm or less, such as alumina, silicon carbide, and chromium oxide, having a new Mohs hardness of 4 or more, are added to an aqueous ink composition with a high viscosity, and ultrafine particles such as alumina, titanium oxide, silica, silicon carbide, and tungsten carbide with a particle diameter of less than 0.1 μm are added to the ink composition at 0.002 to 2% by weight (see, for example, Patent Documents 1 and 2).

[0003] In the technique of Patent Document 1, although a certain effect is exhibited in a normal ballpoint pen having a ball diameter of 0.48 mm or more, when used in a fine-line ballpoint pen using a ball with a ball diameter of 0.4 mm or less, since the area of the ball receiving seat is smaller than that of the normal ballpoint pen, the load per unit area of the receiving seat is large, the frictional resistance is high, and wear becomes significant. Furthermore, as the ball diameter decreases, the rotation speed of the ball increases when writing the same distance in the same time, so the load on the receiving seat increases, and thus a sufficient effect cannot be obtained for suppressing the wear of the receiving seat, and the above-mentioned writing defects or inability to write may occur. In contrast, the technology described in Patent Document 2 can suppress wear of the ball bearing seat even when used in the aforementioned fine-point ballpoint pen, thus eliminating the writing problems that occurred with the technology described in Patent Document 1. However, because ultrafine particles such as alumina and titanium oxide have a high specific gravity, Patent Document 2 describes that the ink viscosity is set high and thickeners and shear viscosity reducers are used in order to maintain the dispersion state in the ink. However, these fine particles tend to aggregate when they coexist with such thickeners and shear viscosity reducers for a long period of time, which impairs the dispersion stability in the ink and the wear suppression effect of the ball bearing seat. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2002-206066 [Patent Document 2] Special Publication No. 2007-518838 [Overview of the project] [Problems that the invention aims to solve]

[0005] The present invention provides an aqueous ink composition for ballpoint pens and a ballpoint pen containing it, which can effectively suppress wear (seat wear) of the ball-receiving seat of the ballpoint pen tip due to writing, even in fine-point ballpoint pens, and can maintain the dispersion stability of ultrafine particles over a long period of time, thereby enabling the continuous formation of good writing with excellent ink flow. [Means for solving the problem]

[0006] The present invention provides an aqueous ink composition for ballpoint pens that includes polyglycerin isostearate ester in an aqueous ink comprising at least one ultrafine primary particle with an average particle diameter of less than 0.1 μm, selected from alumina, titanium dioxide, and silica, a pigment, and water. Furthermore, the requirements are that the primary particles of the ultrafine particles have an average particle diameter of 0.001 μm or more, and that the ultrafine particles are used in an amount of 0.0005 to 0.5% by mass of the total ink composition. Furthermore, the requirements are that polyglycerin isostearate is used in an amount of 5 to 20% by mass of the total ink composition, and that the electrical conductivity is in the range of 10 to 500 μS / cm. Furthermore, the requirement is a ballpoint pen containing one of the aforementioned water-based ink compositions for ballpoint pens. [Effects of the Invention]

[0007] The present invention provides an aqueous ink composition for ballpoint pens and a ballpoint pen containing it, which can effectively suppress wear on the ball bearing seat even in ballpoint pens with a structure prone to wear on the ball bearing seat, such as fine-tip ballpoint pens, and maintain the ink dispersion stability of ultrafine particles over a long period of time, thereby enabling the continuous formation of good writing with excellent ink flow. [Modes for carrying out the invention]

[0008] In this invention, in order to resolve the problems in the technology described in Patent Document 2, we focused our research on the dispersion stability of ultrafine particles and found that these problems can be resolved by using polyglycerin isostearate ester. Polyglycerin isostearate, when added in small amounts, exhibits a high dispersion effect in water-based inks without increasing ink viscosity. Therefore, when used in combination with pigments and ultrafine particles, these particles can be dispersed in the ink, suppressing aggregation and sedimentation, and maintaining a uniform ink state over the long term. Furthermore, because it can also exert its effects as an activator, it has the advantage of providing a smooth writing feel without relying on general-purpose lubricants, and without requiring the inclusion of additives to improve the writing feel. Furthermore, by increasing the amount added, shear viscosity reduction properties can be imparted to the ink, making it possible to create a gel ink without using other shear viscosity reduction agents to match the desired ballpoint pen form. In this case, it is possible to construct a ballpoint pen using an ink with a higher dispersion effect on particles, eliminating the risks associated with using other shear viscosity reduction agents such as cellulose and polysaccharides, as well as the risk of malfunctions arising from an increase in the number of components.

[0009] As the polyglycerol isostearate ester, compounds represented by the following general formula (1) are preferred. [ka] [In the formula, n is an integer from 1 to 8, and R 1 ~R 4 Each is independent of R 5 These may be the same or different, and each represents an isostearoyl group or a hydrogen atom, with the number of isostearoyl groups being 1 to 4.

[0010] The aforementioned polyglycerin isostearate ester can impart shear viscosity reduction to the ink by increasing its blending amount. In this case, unlike gums such as gellan gum, variations in shear viscosity reduction depending on the material lot do not affect writing performance, and even when pigments are used, it does not degrade writing performance due to deterioration of ink stability. Therefore, it is useful not only as a dispersant but also as a shear viscosity reduction agent.

[0011] The amount added is typically in the range of 0.1 to 30% by mass, preferably 0.5 to 20% by mass, relative to the total amount of ink. Within this range, particularly high dispersion performance can be achieved, and the uniform dispersion of fine particles and pigments in the aqueous ink can be maintained for a long period of time. Furthermore, using it in an amount of 3 to 20% by mass relative to the total ink volume exhibits shear-thinning properties useful for ballpoint pen gel ink. Therefore, it is best suited for direct-ink or refillable ballpoint pens at concentrations below 3% by mass, and for gel ballpoint pens using an ink reservoir at concentrations of 3% by mass or more.

[0012] The aforementioned ultrafine particles are made from materials selected from alumina, titanium oxide, and silica, adjusted so that the average particle size of the primary particles is less than 0.1 μm. By holding these inorganic fine particles, which are less than 0.1 μm in size, in a uniformly dispersed state within the aqueous ink, as the ball rotates during writing, the ink flows onto the paper surface, and at the same time, the ball exerts significant pressure on the ball seat. As a result, the ultrafine particles in the ink between the ball and the ball seat become embedded in the ball seat. Consequently, the surface hardness of the ball seat increases, suppressing seat wear associated with ball rotation.

[0013] As the aforementioned ultrafine particles, those with an average particle diameter of less than 0.1 μm can be used, but those with an average particle diameter of 0.001 μm or more are preferred due to the above-mentioned effects and higher dispersibility. Specifically, particles in the range of 0.001 to 0.099 μm, more preferably 0.005 to 0.09 μm, are used. In this invention, the average particle diameter is determined by selecting a suitable measurement method from the approximate particle diameter. For example, this can be determined by using a scanning electron microscope (SEM) to take SEM images of each ultrafine particle, selecting any 20 ultrafine particles from the obtained images, measuring the diameter of those ultrafine particles, and averaging the results. Furthermore, particle size distribution can be measured using dynamic light scattering, and the average particle diameter calculated from cumulant analysis can also be used. Specifically, this can be done using a particle size distribution analyzer that employs dynamic light scattering, such as the "Zetasizer Nano ZS ZEN3600" (manufactured by Malvern Instruments). Furthermore, it can be calculated from the specific surface area calculated by BET analysis of the nitrogen adsorption isotherm measured by the nitrogen gas adsorption method. As the calculation method, when the particle diameter, specific surface area, and density of the elements constituting the ultrafine particles are d (nm), A (m 2 / g), and ρ (g / cm 3 ), respectively, it is represented by d = 6000 / (A × ρ). Specifically, it can be measured by a specific surface area measuring device such as "BELSORP-maxII" (manufactured by MicrotracBEL Corp.). In addition, for the ultrafine particles and microcapsules used in the following examples and comparative examples, values obtained by measuring using a method suitable based on the approximate particle diameter among these measurement methods are described.

[0014] Also, the ultrafine particles are preferably used in the range of 0.0005 to 0.5% by mass in the total amount of the ink composition. If it is less than 0.0005% by mass, it may be difficult to obtain the desired effect depending on the conditions, and even if it exceeds 0.5% by mass, no improvement in the desired effect is seen, so the above range is suitable.

[0015] Examples of the pigment include inorganic pigments such as carbon black and ultramarine blue, organic pigments such as copper phthalocyanine blue and benzidine yellow, and water-dispersed pigment products that are stably dispersed in an aqueous medium in advance using a surfactant or resin. For example, C.I. Pigment Blue 15:3B [product name: Sandye Super Blue GLL, pigment content 24%, manufactured by Sanyo Color Works Co., Ltd.], C.I. Pigment Red 146 [product name: Sandye Super Pink FBL, pigment content 21.5%, manufactured by Sanyo Color Works Co., Ltd.], C.I. Pigment Yellow 81 [product name: TC Yellow FG, pigment content of about 30%, manufactured by Dainichi Seika Kogyo Co., Ltd.], C.I. Pigment Red 220 / 166 [product name: TC Red FG, pigment content of about 35%, manufactured by Dainichi Seika Kogyo Co., Ltd.] and the like can be mentioned. As the fluorescent pigment, fluorescent pigment in the form of synthetic resin fine particles in which various fluorescent dyes are solid-solubilized in a resin matrix can be used. In addition, examples include pearl pigments, metallic pigments such as gold and silver, phosphorescent pigments, white pigments such as titanium dioxide used in correction pens, metal powders such as aluminum, and further, capsule pigments obtained by directly encapsulating or microencapsulating thermochromic compositions, photochromic compositions, fragrances, etc.

[0016] As the thermochromic composition, a reversible thermochromic composition comprising (a) an electron-donating color-forming organic compound, (b) an electron-accepting compound, and (c) a reaction medium that determines the color change temperature of both is preferable, and it is applied as a reversible thermochromic microcapsule pigment by encapsulating it in microcapsules.

[0017] As the reversible thermochromic composition, those described in Japanese Patent Publication No. Sho 51-44706, Japanese Patent Publication No. Sho 51-44707, Japanese Patent Publication No. Hei 1-29398, etc., change color before and after a predetermined temperature (color change point), exhibit a decolorized state in a temperature range above the high-temperature side color change point, and a colored state in a temperature range below the low-temperature side color change point. Only one of the two states exists in the normal temperature range, and the other state is maintained while the heat or cold required for the appearance of the state is applied, but returns to the state presented in the normal temperature range when the application of the heat or cold stops. A heating-decoloring type microcapsule pigment in which a reversible thermochromic composition having a relatively small hysteresis width (ΔH = 1 to 7°C) is encapsulated in microcapsules can be applied. Furthermore, there are those exhibiting relatively large hysteresis characteristics (ΔH=8~50℃) as described in Japanese Patent Publication No. 4-17154, Japanese Patent Publication No. 7-179777, Japanese Patent Publication No. 7-33997, Japanese Patent Publication No. 8-39936, etc., and those exhibiting large hysteresis characteristics as described in Japanese Patent Publication No. 2006-137886, Japanese Patent Publication No. 2006-188660, Japanese Patent Publication No. 2008-45062, Japanese Patent Publication No. 2008-280523, etc. In other words, the shape of the curve plotting the change in color intensity due to temperature changes follows a significantly different path depending on whether the temperature is raised from a temperature below the color change temperature range or lowered from a temperature above the color change temperature range. This means that the color development state at low temperatures below the complete color development temperature, or the decolorization state at high temperatures above the complete decolorization temperature, can be determined by using a reversible thermochromic composition that has color memory properties in a specific temperature range, and heat-decolorizing microcapsule pigments can also be applied. Specifically, as the reversible thermochromic composition having color memory, by specifying the complete color development temperature to a temperature that can only be obtained in a freezer, a cold region, etc., i.e., -50 to 0°C, preferably -40 to -5°C, more preferably -30 to -10°C, and the complete decolorization temperature to a temperature that can be obtained from frictional heat from a friction body, a hair dryer, or other readily available heating element, i.e., in the range of 50 to 95°C, preferably 50 to 90°C, more preferably 60 to 80°C, and specifying the ΔH value to 40 to 100°C, it can be made to function effectively in maintaining the color exhibited under normal conditions (daily living temperature range).

[0018] Furthermore, if necessary, dyes that are soluble in aqueous media can be used in combination. As the aforementioned dyes, acid dyes, basic dyes, direct dyes, etc., can be used. Examples of acid dyes include New Coccine (CI16255), Tartrazine (CI19140), Acid Blue Black 10B (CI20470), Guinea Green (CI42085), Brilliant Blue FCF (CI42090), Acid Violet 6B (CI42640), Soluble Blue (CI42755), Naphthalene Green (CI44025), Eosin (CI45380), Phloxine (CI45410), Erythrosine (CI45430), Nigrosine (CI50420), Acid Flavin (CI56205), etc. Basic dyes used include chrysoidine (CI 11270), methyl violet FN (CI 42535), crystal violet (CI 42555), malachite green (CI 42000), Victoria blue FB (CI 44045), rhodamine B (CI 45170), acridine orange NS (CI 46005), and methylene blue B (CI 52015). Direct dyes used include Congo Red (CI22120), Direct Sky Blue 5B (CI24400), Violet BB (CI27905), Direct Deep Black EX (CI30235), Kayaras Black G Concentrate (CI35225), Direct Fast Black G (CI35255), and Phthalocyanine Blue (CI74180).

[0019] The colorants containing the aforementioned pigments can be used by mixing one or more of them as appropriate, and are used in an amount of 1 to 30% by mass, preferably 2 to 25% by mass, and particularly preferably 2 to 15% by mass, of the ink composition.

[0020] The present invention is an ink that uses water as its main medium, and the type of water used is not particularly limited; for example, tap water, deionized water, ultrafiltered water, and distilled water are examples. The water content relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 45 to 95% by mass, and more preferably 50 to 90% by mass.

[0021] Furthermore, if necessary, conventional water-soluble organic solvents that are compatible with water can be used. Specifically, these include ethanol, propanol, butanol, glycerin, sorbitol, triethanolamine, diethanolamine, monoethanolamine, ethylene glycol, diethylene glycol, thiodiethylene glycol, polyethylene glycol, propylene glycol, butylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol monomethyl ether acetate, sulforane, 2-pyrrolidone, N-methyl-2-pyrrolidone, and the like. Furthermore, the aforementioned water-soluble organic solvent can be used alone or in combination of two or more types, and is used in an amount of 2 to 60% by mass, preferably 5 to 35% by mass.

[0022] Furthermore, water-soluble resins can be added to impart adhesion and viscosity to the paper surface. Examples of such water-soluble resins include alkyd resins, acrylic resins, styrene-maleic acid copolymers, cellulose derivatives, polyvinylpyrrolidone, polyvinyl alcohol, and dextrin. One or more of these water-soluble resins can be used in combination, and they are used in an amount ranging from 1% to 30% by mass in the ink composition.

[0023] In addition, if necessary, pH adjusters such as inorganic salts like sodium carbonate, sodium phosphate, and sodium acetate, and organic basic compounds such as water-soluble amine compounds, rust inhibitors such as benzotriazole, tolyltriazole, and saponins, preservatives or fungicides such as carbolic acid, sodium salt of 1,2-benzthiazolin 3-one, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl parahydroxybenzoate, and 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine, defoamers, and fluorine-based or nonionic surfactants to improve ink penetration may be used. Wetting agents such as urea and sodium pyrophosphate may also be used. Furthermore, lubricants can also be added. Examples of lubricants include phosphate ester surfactants, metal soaps, polyalkylene glycol fatty acid esters, ethylene oxide-additive cationic surfactants, dicarboxylic acid-type surfactants, β-alanine-type surfactants, 2,5-dimercapto-1,3,4-thiadiazole and its salts or oligomers, 3-amino-5-mercapto-1,2,4-triazole, thiocarbamate, dimethyldithiocarbamate, condensates of N-acyl-L-glutamic acid and L-lysine and their salts. Additionally, bubbles can be chemically removed by adding ascorbic acid, erythorbic acid, α-tocopherol, catechins, synthetic polyphenols, kojic acid, alkylhydroxylamines, oxime derivatives, α-glucosylrutin, α-lipoic acid, phosphonates, phosphinates, sulfites, sulfoxylates, dithionites, thiosulfates, thiourea dioxide, etc. Furthermore, the function in the retractable form can be enhanced by adding thickening inhibitors such as N-vinyl-2-pyrrolidone oligomer, N-vinyl-2-piperidone oligomer, N-vinyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, ε-caprolactam, and N-vinyl-ε-caprolactam oligomer.

[0024] Furthermore, if necessary, a general-purpose dispersant can be used in combination to obtain dispersion stability of the pigment, provided that it does not impair the ink properties. Nonionic dispersants are particularly preferred because they can exhibit a dispersion effect while maintaining stable ink viscosity when polyglycerin isostearate is used as a shear viscosity reducing agent. Examples of nonionic dispersants include ethylene oxide adducts of alkyl ethers, ethylene oxide adducts of glycerol fatty acid esters, polyvinylpyrrolidone and its derivatives, and vinylpyrrolidone copolymers.

[0025] In addition, a small amount of a general-purpose shear-thickening agent can be used in combination as needed. Examples include xanthan gum, gellan gum, zeta-C gum, dieutan gum, macrophopsis gum, succinoglycans (average molecular weight approximately 1 to 8 million) whose constituent monosaccharides are organic acid-modified heteropolysaccharides of glucose and galactose, guar gum, locust bean gum and its derivatives, hydroxyethylcellulose, alkyl alginates, polymers with a molecular weight of 100,000 to 150,000 mainly composed of alkyl esters of methacrylic acid, glucomannan, gelling carbohydrates extracted from seaweed such as agar and carrageenan, poly-N-vinyl carboxylic acid amide crosslinks, benzylidene sorbitol and benzylidene xylitol or their derivatives, crosslinkable acrylic acid polymers, alkali-thickening acrylic thickeners, inorganic fine particles, nonionic surfactants with an HLB value of 8 to 12, and metal salts and amine salts of dialkyl sulfosuccinate.

[0026] In the aqueous ink composition having the aforementioned material composition, when polyglycerin isostearate is used in the ink at a concentration of 3 to 20% by mass so that it acts as a shear viscosity reducing agent, it is preferable to set the electrical conductivity of the aqueous ink composition to 500 μS / cm or less, as this improves its effect as a dispersant and enhances its performance as a gel ink. Specifically, it is preferable to adjust the electrical conductivity to be in the range of 10 to 500 μS / cm. When the electrical conductivity exceeds 500 μS / cm, it tends to become difficult to obtain shear viscosity reduction performance; therefore, when applying it as a gel ink, it is preferable to keep it below 500 μS / cm.

[0027] The aqueous ink composition is filled into a ballpoint pen. The structure and shape of the ballpoint pen itself are not particularly limited, and conventional general-purpose designs can be applied. For example, a ballpoint pen can have an ink reservoir tube filled with an ink composition inside the barrel, the ink reservoir tube is connected to a tip with a ball attached to the end, and an ink backflow prevention body is closely attached to the end face of the ink to prevent backflow. If the ink composition is low viscosity, it is also possible to use a structure in which an ink retaining member is attached to the front of the barrel and the ink composition is directly contained inside the barrel, or a structure in which the ink composition is impregnated into a porous body or a fibrous material and contained within it.

[0028] More specifically, the ballpoint pen tip can be a tip in which a ball is held in a ball-holding portion formed by pressing the tip of a straight or stepped metal pipe inward from the outer surface, or a tip in which a ball is held in a ball-holding portion formed by cutting a metal material with a drill or the like, or a tip in which the ball held in a metal pipe or a metal material formed by cutting is biased forward (outward) by an elastic body. Furthermore, the ball can be made of cemented carbide, stainless steel, ruby, ceramic, etc., and is preferably made of a diameter in the range of 0.1 mm to 2.0 mm. Furthermore, for balls with an outer diameter of 0.4 mm or less, the amount of ink dispensed during writing is relatively small, making it difficult to feel lubrication during writing. In addition, the increased ball rotation speed during writing and the small contact surface area make it unfavorable for wear, which is why the ink composition of the present invention is more useful.

[0029] The ink container tube for holding the aqueous ink composition is preferably made of a thermoplastic resin such as polyethylene, polypropylene, or polyethylene terephthalate, in terms of low ink evaporation and productivity. Furthermore, by using a transparent, colored transparent, or translucent molded body as the ink container tube, the ink color and remaining ink amount can be checked. In addition to directly connecting the tip to the ink storage tube, the ink storage tube and the tip may also be connected via a connecting member. Furthermore, the ink reservoir may be in the form of a refill, with the refill housed inside the barrel, or the barrel itself, with a tip attached to its end, may be used as the ink reservoir, with ink directly filled into the barrel.

[0030] Furthermore, the ink backflow prevention body filled at the rear end of the ink composition packed in the ink storage tube can be either liquid or solid. Examples of liquid ink backflow prevention bodies (liquid stoppers) include non-volatile media such as polybutene, α-olefin co-oligomer, silicone oil, and refined mineral oil. Optionally, silica, aluminum silicate, swelling mica, fatty acid amides, etc., can be added to the media. Examples of solid ink backflow prevention bodies include resin molded products. The liquid and solid ink backflow prevention bodies can also be used in combination.

[0031] The aforementioned ballpoint pen may be either a capped or retractable type. As a retractable ballpoint pen, any structure in which the writing tip provided on the ballpoint pen refill is housed inside the barrel (outer barrel) and exposed to the outside air, and the writing tip protrudes from the barrel opening when the retractable mechanism is activated, can be used. Methods of operating the retraction mechanism include knocking, rotating, and sliding mechanisms. The aforementioned retractable pen may have a retractable mechanism at the rear end or side of the barrel, and by pressing the retractable mechanism, the writing tip of the ballpoint pen refill extends and retracts from the opening at the front of the barrel. Alternatively, the writing tip of the ballpoint pen refill extends and retracts from the opening at the front of the barrel by pressing a clip provided on the barrel. The aforementioned rotary type may be exemplified by having a rotating part at the rear of the barrel, and by rotating the rotating part, the writing tip of the ballpoint pen refill can be extended and retracted from the opening at the front of the barrel. The aforementioned sliding mechanism may be exemplified by having a sliding part on the side of the barrel, and by operating the slide, the writing tip of the ballpoint pen refill can be extended and retracted from the opening at the front of the barrel, or by sliding a clip part provided on the barrel, the writing tip of the ballpoint pen refill can be extended and retracted from the opening at the front of the barrel. The aforementioned retractable ballpoint pen may be a composite type retractable ballpoint pen (refillable) that houses multiple ballpoint pen refills inside the barrel. [Examples]

[0032] Examples are described below, but the present invention is not limited to these examples. The following table shows the composition of the water-based ballpoint pen inks used in the examples and comparative examples. Note that the composition values ​​in the table represent parts by mass. Furthermore, electrical conductivity was measured at 20°C using an electrical conductivity meter (Yokogawa Electric Corporation, SC82) and a known measurement method.

[0033] [Table 1]

[0034] [Table 2]

[0035] The contents of the raw materials listed in the table are explained according to the footnote numbers. (1) Manufactured by Sanyo Pigment Co., Ltd., Product name: Sandai Super Black C (E), Pigment content 30% (contains nonionic dispersant) (2) A dispersion obtained by mixing 10 parts carbon black, 10 parts N-vinylpyrrolidone and 1-butene copolymer, and 80 parts water in a ball mill for 1 hour. (3) Manufactured by Sanyo Shikkei Co., Ltd., product name: Sandai Super Red BS(E), pigment content 32% (contains nonionic dispersant) (4) Manufactured by Sanyo Shikkei Co., Ltd., product name: Sandai Super Green LXB(E), pigment content 30% (contains nonionic dispersant) (5) A microcapsule pigment containing a reversible thermochromic composition consisting of (a) 4.5 parts of 2-(2-chloroanilino)-6-di-n-butylaminofluorane, (b) 4.5 parts of 1,1-bis(4′-hydroxyphenyl)n-decane and 7.5 parts of 2,2-bis(4′-hydroxyphenyl)hexafluoropropane, and (c) 50.0 parts of 4-benzyloxyphenylethyl caprate (complete color development temperature (t1): -20℃, complete decolorization temperature (t4): 57℃, average particle size: 2.5μm, changes color from black to colorless) (6) Alumina with an average particle size of 0.01μm (7) Titanium dioxide with an average particle size of 0.08 μm (8) Silica with an average particle size of 0.03 μm (9) Alumina with an average particle size of 0.03 μm (10) Alumina with an average particle size of 0.05 μm (11) Manufactured by Arcsarda Japan, Product name: Proxel XL-2 (12) Manufactured by Daiichi Kogyo Seiyaku Co., Ltd., Product name: Prysurf A219B (13) Hexaglycerin monoisostearate (14) Decaglycerin monoisostearate (15) Decaglycerin diisostearate (16) Manufactured by Big Chemie Japan Co., Ltd., Product name: DISPER BYK-192 (17) Alumina with an average particle size of 1.0 μm (18) Titanium dioxide with an average particle size of 0.8 μm (19) Silica with an average particle size of 2.0 μm

[0036] Ink preparation Water, ultrafine particles (fine particles), and a dispersant were mixed and pre-dispersed using a bead mill. Then, all components except polyglycerin isostearate were added to the water, mixed and stirred, and then polyglycerin isostearate was added. The mixture was stirred at 1000 rpm for 30 minutes at 60°C in a disperser, and then filtered to prepare each ink.

[0037] Preparation of an ink backflow prevention device An ink backflow prevention body was obtained by adding 1.5 parts of fatty acid amide as a thickening agent to 98.5 parts of polybutene as a base oil, and then kneading the mixture using a three-roll machine.

[0038] Making ballpoint pen A 1.0 g each of the ink compositions of Examples 1-5 and Comparative Examples 1-5 was filled into two types of ballpoint pen refills, each having a stainless steel tip fitted to one end of a polypropylene pipe, which holds a cemented carbide ball with a diameter of 0.4 mm and 0.5 mm, respectively. After placing the ink backflow prevention body at the rear end of the ink, the ballpoint pen refill was assembled into the outer barrel, and the cap was attached to produce sample ballpoint pen A.

[0039] Making Ballpoint Pen B Sample ballpoint pen B was prepared by filling a pen-type ballpoint pen (manufactured by Pilot Corporation, LVE-10EF) with 2.0 g of the ink compositions of Examples 6-8 and Comparative Examples 6-8 into a pen tip that holds a cemented carbide ball with a diameter of 0.5 mm.

[0040] The following tests were performed using the aforementioned sample ballpoint pen. Written exam Each ballpoint pen sample, confirmed to be writable, was tested using an automatic writing test machine to continuously write spiral circles on JIS P3201 writing paper A. The test checked whether the ink was completely consumed and the condition of the pen tip after consumption. The test machine was used under the following conditions: writing load of 100g, writing angle of 70°, and writing speed of 4m / min. Ink stability test Each ballpoint pen sample was left in a 50°C environment for 30 days with the pen tip facing downwards. After that, the condition of the handwriting was visually inspected when a spiral circle was continuously written by hand on JIS P3201 writing paper A at room temperature. The results of each of the above tests are shown in the table below.

[0041] [Table 3]

[0042] The evaluation of the test results is as follows: Written exam ○: I was able to write completely, and no wear was observed on the ball seat. △: I was able to finish writing, but there was significant wear on the ball seat. ×: The ball bearing was severely worn, and writing stopped working midway through. Ink stability test ○: Good handwriting can be obtained. ×: Shows smudging, skipped lines, or is unusable for writing.

Claims

1. A water-based ink composition for ballpoint pens, characterized by containing polyglycerin isostearate ester in an aqueous ink comprising at least one ultrafine primary particle with an average particle diameter of less than 0.1 μm, selected from alumina, titanium dioxide, and silica, a pigment, and water.

2. The aqueous ink composition for ballpoint pens according to claim 1, wherein the primary particles of the ultrafine particles have an average particle diameter of 0.001 μm or more.

3. The aqueous ink composition for ballpoint pens according to claim 1, wherein ultrafine particles are used in an amount of 0.0005 to 0.5% by mass of the total amount of the ink composition.

4. The aqueous ink composition for ballpoint pens according to claim 1, wherein polyglycerin isostearate is used in an amount of 5 to 20% by mass of the total ink composition.

5. The aqueous ink composition for ballpoint pens according to claim 1, wherein the electrical conductivity is in the range of 10 to 500 μS / cm.

6. A ballpoint pen containing the aqueous ink composition for ballpoint pens described in any one of claims 1 to 5.

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