Oil-based ink composition for writing instrument and writing instrument using the same

Abstract

To provide an oil-based ink composition for writing instruments, which suppresses precipitation of metal salts resulting from reaction with ink ingredients, and to provide a writing instrument using the oil-based ink composition.SOLUTION: An oil-based ink composition for writing instruments comprises a colorant, an organic solvent, and polyacrylic acid having a weight-average molecular weight of 50000 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an oil-based ink composition for writing instruments and a writing instrument using the same. More specifically, the present invention relates to an oil-based ink composition for writing instruments that improves writing performance by suppressing metal salt deposits generated by reacting with ink components, and a writing instrument using the same.

Background Art

[0002] Conventionally, oil-based ink compositions for writing instruments containing a colorant, an organic solvent, a resin, etc. have been proposed.

[0003] Examples of such oil-based ink compositions for writing instruments include "Ink composition for oil-based ballpoint pen" disclosed in JP-A-6-212111, "Ink composition for oil-based ballpoint pen and oil-based ballpoint pen" disclosed in JP-A-8-41408, and "Oil-based ink for ballpoint pen" disclosed in JP-A-8-134393.

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when a writing instrument is made using the oil-based ballpoint pen ink compositions of Patent Documents 1 and 2, metal ions such as manganese ions, chromium ions, copper ions, zinc ions, nickel ions, etc. elute from a ballpoint pen tip body or an ink storage cylinder made of metals such as stainless steel, brass, and white metal as the pen tip part (tip material) or the ink storage cylinder, react with the ink components to generate metal salts, and there has been a problem of causing poor writing.

[0005] Furthermore, in order to improve writing performance, there are many techniques for improving lubricity by using surfactants such as N-acyl amino acids and N-acyl methyl taurine as in Patent Document 3. In this case, although it is possible to improve lubricity to some extent, it easily reacts with metal ions eluted from a metal ballpoint pen tip body, an ink storage cylinder, etc. in the ink and easily generates metal salts. Therefore, even when using a new surfactant to improve writing performance, there is a problem of suppressing metal salts in the ink.

[0006] Therefore, as in Patent Documents 1 to 3, depending on the selection of ink components such as a colorant, an organic solvent, a surfactant, and a resin, there is also compatibility with a metal ballpoint pen tip body and an ink storage cylinder, and a method for dealing with precipitates such as metal salts generated by reacting with metal ions present in the ink becomes very important.

[0007] An object of the present invention is to obtain an oil-based ink composition for writing instruments that suppresses metal salt precipitates generated by reacting with ink components.

[0008] In order to solve the above problems, the present invention "1. An oil-based ink composition for writing instruments, comprising a colorant, an organic solvent, and polyacrylic acid having a weight average molecular weight of 50,000 or less. 2. The oil-based ink composition for writing instruments according to claim 1, wherein the content of the polyacrylic acid is 0.1 to 15% by mass based on the total amount of the ink composition. 3. The oil-based ink composition for writing instruments according to claim 1 or 2, wherein the organic solvent is alkylene glycol alkyl ethers. 4. The oil-based ink composition for writing instruments according to claim 1 or 2, further comprising a surfactant. 5. The oil-based ink composition for writing instruments according to claim 4, wherein the surfactant is a phosphate ester-based surfactant or a fatty acid. 6. A writing instrument, comprising the oil-based ink composition for writing instruments according to claim 1 or 2."

Advantages of the Invention

[0009] The present invention can achieve the suppression of metal salt precipitates generated by the reaction of metal ions present in the ink with a polyacrylic acid having a weight-average molecular weight of 50,000 or less by forming a metal complex therewith.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described in detail. In this specification, "parts", "%", "ratio", etc. indicating formulation are based on mass unless otherwise specified, and the content is the mass% of the constituent components based on the mass of the ink composition.

[0011] The feature of the present invention is that the oil-based ink composition for writing instruments contains a colorant, an organic solvent, and a polyacrylic acid having a weight-average molecular weight of 50,000 or less.

[0012] (Polycarboxylic acid) In the present invention, by containing a polyacrylic acid having a weight-average molecular weight of 50,000 or less, it is possible to suppress metal salt precipitates generated by reacting with metal ions present in the ink.

[0013] By containing a polyacrylic acid having a weight-average molecular weight of 50,000 or less, when metals such as stainless steel, brass, and white metal are used for the pen tip part (ball pen tip body) of the writing instrument member, the ink storage cylinder, etc., metal ions such as manganese ions, chromium ions, copper ions, zinc ions, nickel ions elute from the ball pen tip body and the ink storage cylinder, and before a metal salt is generated by reacting with the ink components, it is presumed that the carboxylic acid of the polyacrylic acid forms a metal complex with the metal ions in the ink eluted from the ball pen tip body, the ink storage cylinder, etc. at an appropriate molar ratio, suppressing metal salt precipitates. In particular, in the case of an oil-based ball pen, considering the influence that metal ions are more likely to elute due to moisture absorption over time from the gap between the ball pen tip body and the ball, containing a polyacrylic acid having a weight-average molecular weight of 50,000 or less is more effective.

[0014] Regarding the weight-average molecular weight of the polyacrylic acid, considering the dissolution stability in the oil-based ink, it is preferably 30,000 or less, more preferably 20,000 or less, and still more preferably 15,000 or less. Further, in order to suppress the precipitation of metal salts, an appropriate amount of carboxylic acid is required, so it is preferably 3,000 or more, more preferably 5,000 or more.

[0015] In addition, the content of the polyacrylic acid is more preferably 0.1 to 15% by mass based on the total amount of the ink composition. This is because if it is less than 0.1% by mass, it tends to be difficult to obtain the desired metal salt suppression effect, and if it exceeds 15% by mass, it is likely to affect the ink solubility and the writing feel due to an increase in ink viscosity. More preferably, it is 1 to 12% by mass, and still more preferably 2 to 10% by mass.

[0016] (Organic solvent) Examples of the organic solvent include alkylene oxide derivatives of glycerin such as polyoxyalkylene glyceryl ether, polyoxyalkylene alkyl glucoside, and polyoxypropylene alkyl ether; alkylene glycol alkyl ethers such as alkylene glycol monoalkyl ether and alkylene glycol dialkyl ether; amide solvents of β-alkoxypropionamides such as 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, and 3-ethoxy-N,N-dimethylpropanamide; glycol solvents such as diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, butylene glycol, and ethylene glycol; and alcohol solvents such as benzyl alcohol, methanol, ethanol, 1-propanol, 2-propanol, isopropanol, isobutanol, t-butanol, propargyl alcohol, allyl alcohol, 3-methyl-1-butyn-3-ol, ethylene glycol monomethyl ether acetate, and other higher alcohols.

[0017] Among these organic solvents, considering that the effects of the present invention are likely to be exerted, it is preferable to select from alkylene oxide derivatives of glycerin, alkylene glycol alkyl ethers, and amide solvents. More preferably, it is easily soluble in polyacrylic acid having a weight molecular weight of 50,000 or less. Considering ink stability, alkylene glycol alkyl ethers are preferable. Further considering, alkylene glycol monoalkyl ethers are preferable.

[0018] Also, regarding alkylene glycol monoalkyl ethers, regarding the number of carbon atoms in the alkylene glycol moiety of alkylene glycol monoalkyl ethers, it is easily soluble in polyacrylic acid having a weight molecular weight of 50,000 or less. Considering ink stability, the number of carbon atoms is preferably 2 or more and 10 or less. Considering that it is more easily stabilized and the above effects are easily obtained, the number of carbon atoms is 3 or more and 8 or less, and more preferably 4 or more and 6 or less. Also, regarding the number of carbon atoms in the alkyl ether moiety of alkylene glycol monoalkyl ethers, it is easily soluble in polyacrylic acid having a weight molecular weight of 50,000 or less. Considering ink stability, it is preferable that the alkyl ether moiety is shorter. For this reason, the number of carbon atoms is preferably 1 or more and 6 or less. Considering that it is more easily stabilized and the effects are easily obtained, it is preferably 1 or more and 4 or less.

[0019] Also, regarding alkylene glycol monoalkyl ethers, the solubility parameter (SP value) is preferably 8 or more and 13 or less (cal / cm 3 ) 1 / 2 This is because, due to the compatibility with surfactants having an acetylene bond in the structure, it is easier to exert the effects of the present invention. Further considering, the solubility parameter (SP value) is preferably 9 or more and 12 or less (cal / cm 3 ) 1 / 2 and more preferably, considering further, the solubility parameter (SP value) is preferably 10 or more and 11 or less (cal / cm 3 ) 1 / 2 This is preferable. The solubility parameter (SP value) of the solvent referred to in the present invention is a value represented by the square root of the molecular cohesive energy, which is described in Chapter IV, Solubility Parameter Values of Polymer Handbook (Second Edition), and this value was used. The unit is (cal / cm3)1 / 2, and it refers to the value at 25°C. For those without data description, it can be calculated by the method described in R.F. Fedors, Polymer Engineering Science, 14, p147 (1967).

[0020] Also, for alkylene glycol monoalkyl ethers, the boiling point is preferably 170°C or higher. This is because if it is less than 170°C, the alkylene glycol monoalkyl ether is likely to evaporate, the ink viscosity is likely to increase, it is difficult to suppress wire drawing and ink bleeding, and it is likely to affect the writing feel. More preferably, it is 220°C or higher. On the other hand, if the boiling point exceeds 300°C, it is likely to affect the drying property of the handwriting, so the boiling point is preferably 300°C or lower, and more preferably 280°C or lower.

[0021] Also, considering improving solubility, penetration, handwriting drying property, bleeding, etc., the content of the organic solvent is preferably 10% by mass or more and 90% by mass or less, and more preferably 25% by mass or more and 80% by mass or less based on the total amount of the ink composition.

[0022] Also, considering improving solubility, penetration, handwriting drying property, bleeding, etc., the content of the organic solvent is preferably 10% by mass or more and 90% by mass or less, and more preferably 25% by mass or more and 80% by mass or less based on the total amount of the ink composition.

[0023] Examples of the alkylene glycol monoalkyl ether include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monophenyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, those having a butylene glycol monomethyl ether structure, those having a butylene glycol monoethyl ether structure, and the like. These may be used alone or in admixture of two or more thereof.

[0024] Examples of the alkylene glycol dialkyl ethers include ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol dibutyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol dibutyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, those having a dibutylene glycol dimethyl ether structure, those having a dibutylene glycol diethyl ether structure, and the like. These may be used alone or in combination of two or more.

[0025] Among the above alkylene glycol alkyl ethers, considering that they are easily soluble in polyacrylic acid having a weight molecular weight of 50,000 or less and the effects of the present invention can be easily obtained, triethylene glycol alkyl ether and diethylene glycol alkyl ether are preferable. Further considering, it is preferable to comprise triethylene glycol monomethyl ether and diethylene glycol monobutyl ether, and triethylene glycol monomethyl ether is more preferable.

[0026] In the present invention, as the colorant, dyes, pigments, etc. may be appropriately selected and used, or dyes and pigments may be used in combination. Examples of the dye include oil-soluble dyes, acid dyes, basic dyes, gold-containing dyes, and various salt-forming types of dyes such as salt-forming dyes of acid dyes and basic dyes, salt-forming dyes of organic acids and basic dyes, and salt-forming dyes of acid dyes and organic amines. These dyes may be used alone or in combination of two or more. Considering the compatibility with organic solvents and polyacrylic acid, it is preferable to use salt-forming dyes. Further considering that the salt-forming bond is stable and the stability over time can be maintained, it is preferable to use salt-forming dyes of basic dyes and organic acids, salt-forming dyes of acid dyes and basic dyes, and salt-forming dyes of acid dyes and organic amines.

[0027] Examples of the pigment include inorganic, organic, and processed pigments. Specifically, carbon black, aniline black, ultramarine, lead yellow, titanium oxide, iron oxide, phthalocyanine-based, azo-based, quinacridone-based, diketopyrrolopyrrole-based, quinophthalone-based, threne-based, triphenylmethane-based, perinone-based, perylene-based, dioxazine-based, metallic pigments, pearl pigments, fluorescent pigments, phosphorescent pigments, etc. These may be used alone or in a mixture of two or more.

[0028] The total content of the colorant is preferably 5 to 45% by mass based on the total amount of the ink composition. If it is less than 5% by mass, it is difficult to obtain a dark handwriting. If it exceeds 45% by mass, it is likely to affect the solubility in the ink. More preferably, it is 7 to 35% by mass, and even more preferably, it is 10 to 30% by mass.

[0029] (Resin) Moreover, it is preferable to use a resin as an ink viscosity regulator. Examples of the resin include polyvinyl butyral resin, ketone resin, polyacetal resin, polyvinyl alcohol resin, cellulose resin, terpene resin, alkyd resin, phenoxy resin, polyvinyl acetate resin, etc. Among them, considering the stability with organic solvents and the solubility with polyacrylic acid, it is preferable to contain polyvinyl butyral resin or ketone resin.

[0030] If the content of the resin is less than 1% by mass based on the total amount of the ink composition, it is difficult to obtain the desired effect. If it exceeds 40% by mass, the solubility in the ink tends to be poor. Therefore, 1 to 40% by mass based on the total amount of the ink composition is preferable. Further considering, 3% by mass or more is preferable. If it exceeds 30% by mass, the ink viscosity becomes too high and tends to affect the writing feel. Therefore, 3 to 30% by mass is preferable.

[0031] (Surfactant) The ink composition according to the present invention preferably contains a surfactant. This is because the surfactant softens the film formed at the tip of the writing instrument by the components (colorant, resin, etc.) contained in the ink composition, easily improves the writing performance, further improves the lubricity, easily improves the writing feel, and further enables the ink to be discharged smoothly and increases the ink consumption, thereby advantageously acting on the fastness (water resistance, alcohol resistance, light resistance) of the handwriting. Examples of the surfactant include silicone-based surfactants, fluorine-based surfactants, phosphate ester-based surfactants, polyoxyalkylene alkyl ethers, fatty acids, fatty acid esters, and the like. Among them, considering the above effects, it is preferably one or more selected from phosphate ester-based surfactants, polyoxyalkylene alkyl ethers, fatty acids, and fatty acid esters. Among them, when using phosphate ester-based surfactants, fatty acids, etc., phosphate groups, fatty acid groups, etc. are likely to be released in the ink and easily react with the metals of the metal pen tip (ball pen tip body) and the ink storage cylinder, generating metal salts easily. Therefore, using polyacrylic acid as in the present invention is effective and preferable. In particular, it is more effective when using phosphate ester-based surfactants.

[0032] Regarding the HLB value of the phosphate ester surfactant, the phosphate ester surfactant with an HLB of 13 or less is lipophilic and improves the lubricity of the ball, thus improving the writing feel. More effective is the phosphate ester surfactant with an HLB of 10 or less. However, on the other hand, the phosphate ester surfactant with an HLB of 13 or less may react with metal ions such as manganese ions and chromium ions eluted from the tip body of the metal material or the ink storage cylinder to form metal salts. Therefore, it is effective to use polyacrylic acid as in the present invention. Also, the phosphate ester surfactant preferably has an HLB of 6 or more. If the HLB value is less than 6, the lipophilicity becomes too strong, which easily affects the compatibility with the organic solvent and makes it difficult to stabilize the ink over time. Therefore, considering the ink aging performance more, an HLB value of 7 or more is preferable, and an HLB value of 8 or more is more preferable. Incidentally, the HLB can be determined by the Griffin method, Kawakami method, etc. As an example, it can be determined from the general formula such as HLB = 7 + 11.7log(Mw / Mo) (Mw: molecular weight of the hydrophilic group, Mo: molecular weight of the lipophilic group). In particular, in retractable writing instruments such as knock-type writing instruments and rotary feed writing instruments, unlike cap-type writing instruments, the pen tip is always exposed to the outside, so it is more likely to affect the writing performance at the time of drying of the writing tip portion. Therefore, it is more preferable to use the surfactant with the above HLB value.

[0033] In the surfactant, examples of the fatty acid include oleic acid and linoleic acid, examples of the fatty acid ester include fatty acid esters obtained by subjecting a fatty acid and a monohydric alcohol to an esterification reaction, and fatty acid esters obtained by subjecting an alcohol such as a polyhydric alcohol to an esterification reaction. Examples of the silicone surfactant include dimethyl silicone, methylphenyl silicone, polyether-modified silicone, and higher fatty acid ester-modified silicone. Examples of the fluorine surfactant include perfluoro group-containing butyl sulfonate, perfluoro group-containing carboxylate, perfluoro group-containing phosphate ester, perfluoro group-containing phosphate ester type formulation, perfluoroalkyl betaine, and perfluoroalkylamine oxide compound. Examples of the phosphate ester surfactant include phosphoric acid monoesters of polyoxyethylene alkyl ether or polyoxyethylene alkyl aryl ether, phosphoric acid diesters of polyoxyethylene alkyl ether or polyoxyethylene alkyl aryl ether, phosphoric acid triesters of polyoxyethylene alkyl ether or polyoxyethylene alkyl aryl ether, alkyl phosphate ester, alkyl ether phosphate ester, or derivatives thereof. Examples of the polyoxyalkylene alkyl ether include polyoxyethylene alkyl ether and polyoxypropylene alkyl ether.

[0034] The content of the surfactant is preferably 0.1% by mass or more and 5.0% by mass or less based on the total amount of the ink composition. This is because if it is less than 0.1% by mass, it is difficult to obtain the desired lubricity, and if it exceeds 5.0% by mass, the ink tends to become unstable over time. More preferably, it is 0.3% by mass or more and 3.0% by mass or less based on the total amount of the ink composition, and even more preferably, it is 0.5% by mass or more and 3.0% by mass or less.

[0035] In addition, as other additives, pseudoplasticity imparting agents such as fatty acid amide and hydrogenated castor oil, organic amines for neutralizing surfactants, colorant stabilizers, drawability imparting agents, plasticizers, chelating agents, water, etc. may be appropriately used. These may be used alone or in combination of two or more kinds.

[0036] In addition, materials for the ballpoint pen tip body, ink storage cylinder, etc. include metal materials such as stainless steel, cupronickel, brass (copper alloy), aluminum bronze, and aluminum, and resin materials such as polycarbonate, polyacetal, and ABS. In the case of writing instruments using a ballpoint pen tip body, ink storage cylinder, etc. made of stainless steel, manganese ions and chromium ions such as manganese and chromium contained in the stainless steel are likely to react with the ink components to generate metal salts. Therefore, using polyacrylic acid as in the present invention is effective and preferable.

[0037] In addition, the ball of the ballpoint pen tip is not particularly limited, but a cemented carbide ball mainly composed of tungsten carbide uses cobalt, nickel, etc. as its binder. The polyacrylic acid is less likely to corrode the ball of the ballpoint pen tip than a chelating agent such as aminocarboxylic acid with respect to metals such as cobalt and nickel. Therefore, it is less likely to cause rotational resistance of the ball due to corrosion, and it is easy to obtain a good writing feeling, and it can be preferably used for cemented carbide balls. Furthermore, it is preferable because it is superior in safety to chelating agents such as aminocarboxylic acid and has a small environmental load. In addition, the diameter of the ball is not particularly limited, but is generally about 0.2 mm to 2.0 mm.

[0038] (Examples) Next, the present invention will be described by showing examples. The ink composition for an oil-based ballpoint pen of Example 1 employed a colorant, an organic solvent, a phosphate ester-based surfactant, and a ketone resin. A predetermined amount of these was weighed, heated to 60°C, and then completely dissolved using a dispersing stirrer to obtain an ink composition for an oil-based ballpoint pen. The specific blending amounts are as follows.

[0039] Example 1 (Ink Composition) Colorant (salt-forming dye of acid dye and basic dye) 10.0% by mass Alkylene glycol alkyl ethers (solubility parameter (SP value): 10.5 (cal / cm3) 1 / 2 , boiling point: 249°C) 53.0% by mass Polyacrylic acid (weight-average molecular weight: 10,000) 5.0% by mass Phosphate ester surfactant 2.0% by mass Ketone resin 30.0% by mass

[0040] Examples 2 to 16 As shown in Table 1, except for changing the ink components, oil-based ink compositions for writing instruments of Examples 2 to 16 were obtained in the same procedure as in Example 1. The evaluation results are shown in the table.

[0041] Comparative Examples 1 to 3 As shown in the table, except for changing the ink components, oil-based ink compositions for writing instruments of Comparative Examples 1 to 3 were obtained in the same procedure as in Example 1. The evaluation results are shown in the table.

Table 1

Table 2

[0042] Tests and Evaluations The oil-based ink compositions for writing instruments (0.4 g) prepared in Examples 1 to 16 and Comparative Examples 1 to 3 were filled into an oil-based ball pen refill equipped with a ball pen tip made of stainless steel material that rotatably holds a ball with a ball diameter of φ0.7 mm (the tip has a coil spring that presses the ball directly against the inner wall at the tip edge of the tip), and an oil-based ball pen was manufactured. The following tests and evaluations were conducted using writing paper JIS P3201 as the writing test paper.

[0043] Metal salt precipitate test in the ball pen tip: After 2 weeks at 60°C and 80% humidity, the ink inside the tip was observed under an optical microscope (manufactured by Olympus) at a magnification of 100 times. ◎ ··· No metal salt precipitate, good ○ ··· Slight generation of metal salt precipitate △ ··· Metal salt precipitate occurred, but no problem in practical use × ··· Metal salt precipitate occurred, causing scratching or writing problems

[0044] Writing feel: Evaluated by a sensory test using handwriting. ◎ ··· Very smooth ○ ··· Smooth △ ··· Slightly less smooth × ··· Heavy

[0045] Solubility test: The ink after 1 week at room temperature was observed under a microscope. ◎ ··· Stable dissolution, good ○ ··· Slight generation of insoluble matter × ··· Insoluble matter occurred, causing scratching or writing problems

[0046] In Examples 1 to 16, good performance was obtained in the metal salt precipitate test, writing feel, and solubility test in the ball pen tip.

[0047] In Comparative Examples 1 to 3, since polyacrylic acid with a weight molecular weight of 50,000 or less was not used, metal salt precipitates occurred, resulting in scratching and writing problems.

[0048] Also, in this example, for the sake of convenience, an oil-based ball pen containing an oil-based refill directly containing an oil-based ink composition for writing instruments in the shaft cylinder is exemplified. However, the writing instrument of the present invention may be a direct-fill type ball pen, marking pen, or signature pen having a shaft cylinder as an ink-containing cylinder and directly containing an ink composition for writing instruments in the shaft cylinder. In addition, in this embodiment, for the sake of convenience, a ballpoint pen tip formed by cutting a wire rod is exemplified, but a ballpoint pen tip formed by pressing a pipe material may also be used.

Industrial Applicability

[0049] The present invention can be used as a writing instrument, and more specifically, it can be widely used as a ballpoint pen as a writing instrument such as a cap type or a retractable type.

Claims

1. An oil-based ink composition for writing instruments, comprising a coloring agent, an organic solvent, and polyacrylic acid having a weight-average molecular weight of 50,000 or less.

2. The oil-based ink composition for writing instruments according to Claim 1, wherein the content of the polyacrylic acid is 0.1 to 15% by mass based on the total amount of the ink composition.

3. The oil-based ink composition for writing instruments according to Claim 1 or 2, wherein the organic solvent is alkylene glycol alkyl ethers.

4. The oil-based ink composition for writing instruments according to Claim 1 or 2, further comprising a surfactant.

5. The oil-based ink composition for writing instruments according to Claim 4, wherein the surfactant is a phosphate ester-based surfactant or a fatty acid.

6. A writing instrument, comprising the oil-based ink composition for writing instruments according to Claim 1 or 2.