JP7670494B2 - Oil-based ink composition for writing implements and writing implements using the same - Google Patents

Description

The present invention relates to an oil-based ink composition for writing instruments and a writing instrument using the same.

Conventionally, oil-based ink compositions for writing instruments contain dyes as colorants because of their excellent color development properties, and many oil-based ink compositions for writing instruments have been proposed that use various dyes as colorants, such as basic dyes, acid dyes, and direct dyes, or dyes that are processed from these dyes.

Various dyes are used in such oil-based ink compositions for writing instruments. Examples of ink compositions using salt-forming dyes of triarylmethane-based basic dyes and azo-based yellow acid dyes are disclosed in JP-A-9-165542 ("Oil-based black ink") and JP-A-9-71745 ("Oil-based black ink"), and ink compositions using salt-forming dyes based on basic dyes are disclosed in JP-A-8-134393 ("Black ink composition for oil-based ballpoint pens").

"Unexamined Japanese Patent Publication No. 9-165542" "Unexamined Japanese Patent Publication No. 9-71745" "Unexamined Japanese Patent Publication No. 8-134393"

However, in the patent documents 1 to 3, unlike pigments, when the handwriting is wetted with water or alcohol, it dissolves again and the dye bleeds out. This causes problems such as writing characters becoming difficult to read and the dye transferring to unused writing media and staining them. Furthermore, when exposed to light (sunlight), the dye undergoes a chemical reaction and decomposes, which reduces the light resistance of the handwriting and causes fading.
Furthermore, the ink stability over time is easily affected by factors such as precipitation of the dye, and in particular, when a surfactant is contained, the surfactant may react with the dye to cause precipitation, raising concerns about the ink stability over time.

The object of the present invention is to obtain an oil-based ink composition for a writing instrument that has excellent water resistance, alcohol resistance, and light resistance (fastness) and excellent ink stability over time, and a writing instrument using the same.

In order to solve the above problems, the present invention
"1. An oil-based ink composition for a writing instrument, comprising a direct black, an amine salt-forming dye, and an organic solvent.
2. The oil-based ink composition for writing instruments according to item 1, characterized in that the content of the Direct Black and the amine salt forming dye is 0.1 to 50% by mass based on the total amount of the ink composition.
3. The oil-based ink composition for a writing instrument according to item 1 or 2, wherein the organic solvent has a solubility parameter (SP value) of 9 to 15 (cal/cm ³ ) ^1/2 .
4. The oil-based ink composition for a writing instrument according to any one of items 1 to 3, wherein the organic solvent has a boiling point of 170° C. or higher.
5. The oil-based ink composition for a writing instrument according to any one of items 1 to 4, characterized in that the oil-based ink composition for a writing instrument contains a surfactant.
6. A writing instrument containing the oil-based ink composition for a writing instrument according to any one of items 1 to 5.
7. An oil-based ballpoint pen, characterized in that the ink reservoir has a ballpoint pen tip that rotatably holds a ball at the tip of the ink reservoir, and the ink reservoir contains the oil-based ink composition for a writing instrument according to any one of items 1 to 6.

The present invention provides an oil-based ink composition for writing instruments that contains a salt-forming dye of direct black and an amine, and thus has excellent water resistance, alcohol resistance, and light resistance (fastness), and furthermore, by stably forming salts, the ink has excellent stability over time. The ink composition also has a writing instrument that uses the ink.

The following is a detailed description of an embodiment of the present invention. In this specification, unless otherwise specified, the terms "parts," "%," and "ratio" used to indicate the composition are based on mass, and the content is the mass% of the component based on the mass of the ink composition.

The present invention is characterized in that the oil-based ink composition for writing instruments contains direct black, an amine salt-forming dye, and an organic solvent.

The oil-based ink composition for writing instruments contains direct black, an amine salt-forming dye, and an organic solvent, so that handwriting remains visible even if it becomes wet with water or alcohol or is exposed to light (sunlight). In addition, the ink is stable due to salt formation, making it possible to achieve excellent stability over time.

(Salt-forming dyes)
In the present invention, a salt-forming dye of Direct Black and amine is used as a colorant, and Direct Black is not easily redissolved even when wet with water or alcohol, allowing handwriting to be seen, and is not easily chemically decomposed even when exposed to light (sunlight), allowing handwriting to be seen. Furthermore, by stabilizing Direct Black by forming a salt-forming bond with amine, it is possible to maintain the ink stability over time, and it is possible to stably maintain water resistance, alcohol resistance, and light resistance (fastness).
Furthermore, Direct Black contains a compound with an aromatic ring, which provides a lubricating effect at the writing tip, allowing the ink to be ejected smoothly. By increasing the amount of ink consumed, this works to the advantage of improving the light resistance of the handwriting and also prevents the handwriting from smudging, making it easier to improve writing performance, which is particularly effective when used in a ballpoint pen.

Direct black includes Direct Black 17, 19, 22, 32, 38, 51, 71, 154, 168, and 195, but when water resistance, alcohol resistance, and light resistance are taken into consideration, azo dyes (-N=N-) are preferred, and Direct Black 19 and 154 are more preferred. When salt formation stability with amines is taken into consideration, and water resistance, alcohol resistance, and light resistance are stably maintained, Direct Black 19, which has a large amount of (-N=N-), is preferred.

In addition, in consideration of the stability of the salt-forming bond with Direct Black, organic amines such as aromatic amines, aliphatic amines, alicyclic amines, alkoxyalkyl amines, and guanidine derivative amines are preferred as the amine.

The total content of colorants is preferably 5 to 60% by mass of the total ink composition. If it is less than 5% by mass, it tends to be difficult to obtain thick handwriting, and if it exceeds 60% by mass, it is likely to affect the solubility in the ink. From a more careful consideration, 7 to 50% by mass is more preferable, and even more so, 10 to 40% by mass.

(Organic Solvent)
The organic solvent preferably has a solubility parameter (SP value) of 9 to 15 (cal/cm ³ ) ^1/2 . This is to improve the dissolution stability of the salt forming dye of Direct Black and the amine and to obtain the effects of the present invention, and from further consideration, the solubility parameter (SP value) is preferably 9 to 13 (cal/cm ³ ) ^1/2 , and from further consideration, the solubility parameter (SP value) is preferably 9 to 12 (cal/cm ³ ) ^1/2 , and more preferably 10 to 12 (cal/cm ³ ) ^1/2 .
The solubility parameter (SP value) of the solvent in the present invention is a value expressed by the square root of the molecular cohesive energy, and is described in Chapter IV, Solubility Parameter Values, of the Polymer Handbook (Second Edition), and this value was used. The unit is (cal/ ^cm3 ) ^1/2 , and the value is at 25°C.
For those for which no data is given, the values can be calculated by the method described in R. F. Fedors, Polymer Engineering Science, 14, p. 147 (1967).

In addition, the boiling point of the organic solvent is preferably 170°C or higher. This is because if it is less than 170°C, the organic solvent is likely to evaporate, which can affect the ink stability over time and increase the ink viscosity, so from further consideration, it is preferable to have a boiling point of 220°C or higher. On the other hand, if the boiling point exceeds 300°C, it is likely to affect the drying properties of the handwriting, so it is preferable to have a boiling point of 300°C or lower, and from further consideration, it is preferable to have a boiling point of 280°C or lower.

In addition, in order to improve solubility, ink drying properties, bleeding, etc., the content of the organic solvent is preferably 10 to 70% by mass, and more preferably 20 to 60% by mass, based on the total amount of the ink composition.

Examples of the organic solvent include alkylene glycol alkyl ether solvents such as alkylene glycol monoalkyl ethers, alkylene glycol dialkyl ethers, and ethylene glycol monophenyl ether; polyhydric alcohol solvents such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, and glycerin; aromatic alcohols such as benzyl alcohol; and aliphatic alcohol solvents such as methanol, ethanol, 1-propanol, 2-propanol, isobutanol, and t-butanol.
In consideration of the dissolution stability with the salt-forming dye of Direct Black and an amine, and with the surfactant described below, it is preferable to use an alkylene glycol alkyl ether solvent or a polyhydric alcohol solvent, and in consideration of further consideration, an alkylene glycol alkyl ether solvent is preferable, and an alkylene glycol monoalkyl ether is preferable.

Regarding the alkylene glycol alkyl ether solvent, in consideration of the ease of dissolving and stabilizing the Direct Black and the amine salt forming dye and the ink stability over time, the carbon number is preferably 2 to 10, more preferably 2 to 8, and more preferably 2 to 6. Furthermore, in consideration of good compatibility with the surfactant described below and the ease of improving the writing start performance and writing feel, the carbon number is preferably 1 to 8, more preferably 1 to 6, and even more preferably 4 to 6.
Among these, taking into consideration the solubility of the direct black and the amine salt-forming dye, as well as the ink stability over time, writing start performance, and writing feel, it is preferable to select from among triethylene glycol monoalkyl ether solvents, ethylene glycol monoalkyl ether solvents, solvents having a butylene glycol monoalkyl ether structure, and dipropylene glycol monoalkyl ether, and from further consideration, it is preferable to select from among triethylene glycol monoalkyl ether solvents, ethylene glycol monoalkyl ether solvents, and solvents having a butylene glycol monoalkyl ether structure, with triethylene glycol monoalkyl ether solvents being more preferable.

Regarding the number of carbon atoms in the alkyl ether moiety of the alkylene glycol alkyl ether, in consideration of the ease of dissolution stability with the direct black and the amine salt forming dye and the ink stability over time, the number of carbon atoms is preferably 1 to 8, and more preferably 3 to 6, and more preferably 1 to 3. Furthermore, in consideration of good compatibility with the surfactant described below and the ease of improving the writing performance and writing feel, the number of carbon atoms is preferably 1 to 8, and more preferably 1 to 6, and even more preferably 1 to 3.
Among these, taking into consideration the solubility of the direct black and the amine salt-forming dye, as well as the ink stability over time, the start-up performance, and the writing feel, it is preferable to select from among alkylene glycol monomethyl ether solvents, alkylene glycol monobutyl ether solvents, and alkylene glycol monophenyl ether solvents, and from these considerations, alkylene glycol monomethyl ether solvents and alkylene glycol monophenyl ether solvents are preferred, with alkylene glycol monomethyl ether solvents being even more preferred.

Examples of alkylene glycol monoalkyl ethers 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, butylene glycol monomethyl ether structure, butylene glycol monoethyl ether structure, etc. These may be used alone or in combination of two or more.
Examples of 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, etc. These may be used alone or in combination of two or more.

The content of the alkylene glycol alkyl ether solvent is preferably 50% or more of the total solvent content in the oil-based ink composition for writing instruments, and is preferably used as the main solvent. This is because the effect of the present invention is more easily achieved by stabilizing the dissolution with the salt-forming dye of Direct Black and amine, and the surfactant described below. Taking the above into consideration, the content of alkylene glycol alkyl ethers is preferably 70% or more of the total solvent content, and more preferably 90% or more.

In addition, taking into consideration the improvement of solubility, ink drying properties, bleeding, etc., the content of the organic solvent is preferably 10 to 80% by mass, more preferably 30 to 80% by mass, based on the total amount of the ink composition.

In the present invention, as a colorant other than the direct black and the amine salt forming dye, dyes, pigments, etc. may be appropriately selected and used, and dyes and pigments may be used in combination.
The dyes include oil-soluble dyes, acid dyes, basic dyes, metal-containing dyes, and various salt-forming dyes thereof, 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. As the dye, it is preferable to use a salt-forming dye in consideration of the stability over time due to the compatibility between the direct black and the salt-forming dye with the amine, and it is preferable to use a salt-forming dye of a basic dye and an organic acid, a salt-forming dye of an acid dye and a basic dye, or a salt-forming dye of an acid dye and an organic amine in consideration of the stability over time due to the stable salt-forming bond. The above-mentioned salt forming dyes are preferred because they are easily dissolved and stable in organic solvents having a solubility parameter (SP value) of 9 to 15 (cal/cm ³ ) ^1/2 and easily improve the writing performance and writing feel. From a further consideration, the solubility parameter (SP value) is preferably 9 to 13 (cal/cm ³ ) ^1/2 , and from further consideration, the solubility parameter (SP value) is preferably 9 to 12 (cal/cm ³ ) ^1/2 , and more preferably 10 to 12 (cal/cm ³ ) ^1/2 .

As for pigments, examples include inorganic, organic, and processed pigments, and specific examples include carbon black, aniline black, ultramarine, yellow lead, 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, and phosphorescent pigments. These may be used alone or in combination of two or more.

It is preferable to use a pigment as the colorant. In the case of a ballpoint pen, the pigment particles can get into the gap between the ball and the tip body, making it easier for the pigment to act like a bearing, and by suppressing metal contact, it is possible to improve lubricity, improve the writing feel, and suppress wear on the ball seat.

The total content of colorants is preferably 5 to 45% by mass based on the total amount of the ink composition. This is because if it is less than 5% by mass, it tends to be difficult to obtain thick handwriting, and if it exceeds 45% by mass, it tends to affect the solubility in the ink. Taking this tendency into consideration, 7 to 35% by mass is preferable, and even more so, 10 to 30% by mass.

(Surfactant)
The ink composition according to the present invention preferably contains a surfactant. This is because the surfactant softens the coating formed on the writing tip by the components (colorant, resin, etc.) contained in the ink composition, and is likely to improve the writing performance, and further improves the lubricity and writing feel, so that the ink can be discharged smoothly, and by increasing the ink consumption, the robustness (water resistance, alcohol resistance, light resistance) of the handwriting tends to be favorable. As the surfactant, there are nonionic surfactants, anionic surfactants, cationic surfactants, etc., and it is preferable to use a nonionic surfactant. This is because the surfactant is nonionic, and thus the components (salt-forming dye of direct black and amine, resin, etc.) contained in the ink composition are less likely to cause precipitation, and the stability over time can be improved.

For nonionic surfactants, when considering stability over time, the HLB value is preferably 16 or less, more preferably 1 to 14, and particularly preferably 4 to 10. When considering improving the writing performance and writing feel, the HLB value is preferably 10 or less, and more preferably 6 or less. When considering a good balance between stability over time and writing performance, the HLB value is preferably 1 to 10, and more preferably 4 to 10. The HLB value can be determined by the Griffin method, etc.

In retractable writing instruments such as knock-type writing instruments and rotary-type writing instruments, unlike cap-type writing instruments, the pen tip is always exposed to the outside. In such cases, the writing tip is prone to drying out. Surfactants with the above HLB values can improve such problems, so their use is more preferable.

In addition, as the nonionic surfactant, fatty acid esters, polyalkylene alkyl ethers, polyoxyethylene alkyl amines, alkyl imidazolines, alkyl alkanolamides, oxyethylene oxypropylene block copolymers, surfactants having acetylene bonds, etc. can be mentioned. Among them, in consideration of the compatibility with the salt-forming dye of direct black and amine, and the above-mentioned writing performance, writing feel, and stability over time, it is preferable to select one or more from fatty acid esters, polyalkylene alkyl ethers, polyoxyethylene alkyl amines, alkyl imidazolines, and alkyl alkanolamides. In particular, in consideration of improving the writing performance, it is preferable to use fatty acid esters. These may be used alone or in combination of two or more.
The above-mentioned nonionic surfactants are preferred because they are easily dissolved and stable in organic solvents with a solubility parameter (SP value) of 9 to 15 (cal/cm ³ ) ^1/2 and easily improve the writing performance and writing feel, and from further consideration, the solubility parameter (SP value) is preferably 9 to 13 (cal/cm ³ ) ^1/2 , and from further consideration, the solubility parameter (SP value) is preferably 9 to 12 (cal/cm ³ ) ^1/2 , and more preferably 10 to 12 (cal/cm ³ ) ^1/2 .

Fatty acid esters include sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, fatty acid esters, sucrose fatty acid esters, glycerin fatty acid esters, polyglycerin fatty acid esters, polyoxyethylene fatty acid esters, etc. Among these, in consideration of the ease of improving the writing performance, it is preferable to select one or more from sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, glycerin fatty acid esters, and sucrose fatty acid esters, and further, it is preferable to use sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and sucrose fatty acid esters, which have a structure having a cyclic skeleton, and further, it is preferable to select one or more from sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and sucrose fatty acid esters, since fatty acid esters having multiple hydroxyl groups can appropriately retain moisture in the coating formed on the writing tip.

In addition, when considering the writing performance, it is preferable that the number of carbon atoms contained in the alkyl group of fatty acid esters such as sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and sucrose fatty acid esters is 1 to 20, and from a more specific perspective, it is preferable that the number of carbon atoms contained in the alkyl group of sorbitan fatty acid esters is 10 to 20, and more preferably 12 to 18.

Specific examples of sorbitan fatty acid esters and polyoxyethylene sorbitan fatty acid esters include sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitan monooleate, sorbitan monococoate, sorbitan dilaurate, sorbitan distearate, sorbitan dioleate, sorbitan trioleate, sorbitan tristearate, and compounds thereof; and polyoxyethylene sorbitan fatty acid esters such as polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan trioleate, polyoxyethylene sorbitan tristearate, and compounds thereof. Specific examples of sucrose fatty acid esters include sucrose laurate, sucrose myristic acid ester, sucrose palmitate, sucrose stearate, sucrose oleate, sucrose linoleate, sucrose linolenic acid ester, sucrose behenic acid ester, and sucrose erucic acid ester. Polyglycerin fatty acid esters are esters of polyglycerin and fatty acids, and specific examples of the fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, eicosanoic acid, and oleic acid.

The content of nonionic surfactant is more preferably 0.1 to 15% by mass of the total ink composition. This is because if it is less than 0.1% by mass, it is difficult to obtain the desired writing performance and writing feel, and if it exceeds 15% by mass, the ink tends to become unstable over time. Taking this tendency into consideration, 0.5 to 10% by mass is preferable, and 0.5 to 5% by mass is most preferable.

In addition to nonionic surfactants, surfactants such as fatty acids, silicone surfactants, fluorine-based surfactants, phosphate ester surfactants, and surfactants having acetylene bonds may also be used in combination.

(resin)
In order to further improve the suppression of ink leakage, it is preferable to use a resin as an ink viscosity adjuster. 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., and among them, taking into consideration the stability with the salt-forming dye of direct black and amine and with organic solvents, it is preferable to use a polyvinyl butyral resin or a ketone resin. Furthermore, the resin film formed on the handwriting is preferable because it is easy to impart water resistance, alcohol resistance, and light resistance (fastness), and in particular, it is easy to block ultraviolet rays and impart light resistance.

Polyvinyl butyral resin is easy to form a lubricating layer that can provide a higher lubricating effect. This is because the polyvinyl butyral resin forms an elastic ink layer between the ball and the ball seat, making direct contact difficult, and therefore the writing feel is easily improved, allowing the ink to be discharged smoothly, and by increasing the ink consumption, the robustness of the handwriting is easily improved. Furthermore, the polyvinyl butyral resin is preferable because the coating formed can more easily improve ink leakage, and when a pigment is used as a colorant, a pigment dispersion effect can also be obtained, so the polyvinyl butyral resin is preferable.
Here, the polyvinyl butyral resin is obtained by reacting polyvinyl alcohol (PVA) with butyraldehyde (BA), and has a structure having butyral groups, acetyl groups, and hydroxyl groups.

In addition, the polyvinyl butyral resin is preferably one having a hydroxyl group amount of 25 mol% or more. This is because polyvinyl butyral resins having a hydroxyl group amount of less than 25 mol% do not have sufficient solubility in organic solvents, making it difficult to obtain the effects of the present invention, and furthermore, considering the writing performance due to moisture absorption, it is preferable to use polyvinyl butyral resins having a hydroxyl group amount of 25 mol% or more. From a more detailed consideration, polyvinyl butyral resins having a hydroxyl group amount of 30 mol% or more are preferable, and furthermore, since it is easy to improve the writing feel, the ink can be discharged smoothly, and by increasing the ink consumption, it is easy to work favorably on the robustness of the handwriting, it is preferable. In addition, if a polyvinyl butyral resin having a hydroxyl group amount of more than 40 mol% is used, the moisture absorption amount is likely to increase, which is likely to affect the stability over time with the ink components, so polyvinyl butyral resins having a hydroxyl group amount of 40 mol% or less are preferable. For this reason, a polyvinyl butyral resin having a hydroxyl group amount of 30 to 40 mol % is preferred, and a hydroxyl group amount of 30 to 36 mol % is more preferred.
The amount (mol %) of hydroxyl groups in the polyvinyl butyral resin refers to the content of hydroxyl groups (mol %) relative to the total molar amount of butyral groups (mol %), acetyl groups (mol %), and hydroxyl groups (mol %).

A spinnability imparting agent may also be used as the resin. In particular, when polyvinylpyrrolidone resin is included, it is compatible with direct black and amine salt forming dyes, and it is preferable to include polyvinylpyrrolidone resin, since this improves the ink binding properties and makes it easier to suppress the generation of excess ink at the tip end.

If the content of the resin is less than 1% by mass relative to the total amount of the ink composition, the desired lubricity and ink leakage suppression performance are likely to be inferior, and if it exceeds 40% by mass, the solubility in the ink is likely to be inferior, so it is preferably 1 to 40% by mass relative to the total amount of the ink composition. Furthermore, from consideration, 3% by mass or more is preferable, and if it exceeds 30% by mass, the ink viscosity tends to become too high and affect the writing feel, so 3 to 30% by mass is preferable, and from further consideration, 5 to 25% by mass is preferable.

In the present invention, in consideration of the stability of the ink components in the ink, it is preferable to use an organic amine (excluding the amine of the salt-forming dye of direct black and amine). In consideration of the stability of the organic amine and the salt-forming dye of direct black and amine, it is preferable to use a secondary amine or a tertiary amine. This is because, in terms of reactivity in oil-based ink, primary amines are the strongest, followed by secondary amines and tertiary amines, which are weaker in reactivity. Primary amines are prone to react with the salt-forming dye of direct black and amine and other components, and are likely to affect the stability of the ink over time. For this reason, it is preferable to use a secondary amine or a tertiary amine, and from further consideration, it is preferable to use a tertiary amine.

Examples of organic amines include amines having ethylene oxide, such as oxyethylene alkylamines and polyoxyethylene alkylamines; alkyl amines, such as laurylamine and stearylamine; aliphatic amines, such as dimethyl alkyl amines, such as distearylamine, dimethyl laurylamine, dimethyl stearylamine and dimethyl octylamine; and alkanol amines, such as diethanol amine and triethanol amine. Among these, amines having ethylene oxide, dimethyl alkyl amines, and alkanol amines are preferred, taking into consideration the stability in the ink.

Furthermore, the total amine value of the organic amine is preferably 70 to 300 (mgKOH/g) in consideration of the stability with the salt forming dye of Direct Black and amine and other components. If it exceeds 300 (mgKOH/g), the reactivity is strong and it easily reacts with the above components, so the ink stability over time is likely to be poor. If the total amine value is less than 70 (mgKOH/g), the stability of the surfactant is likely to be affected. If the stability with the above components is more considered, the range of 100 to 300 (mgKOH/g) is preferable, and if stability is more important, the range of 150 to 300 (mgKOH/g) is preferable.
The total amine value indicates the total amount of primary, secondary and tertiary amines, and is expressed as the number of milligrams of potassium hydroxide equivalent to the amount of hydrochloric acid required to neutralize 1 g of sample.

In addition, as viscosity adjusters, pseudoplasticity imparting agents such as fatty acid amides and hydrogenated castor oil, colorant stabilizers, plasticizers, chelating agents, water, etc. may be used as appropriate. These may be used alone or in combination of two or more.

The ink viscosity of the oil-based ink composition for writing instruments of the present invention is not particularly limited, but if the viscosity is too high, the writing performance, writing feel, and ink followability tend to deteriorate, so the ink viscosity at 20°C and a shear rate of 5 sec ^-1 (at rest) is preferably 30,000 mPa·s or less. Also, if the viscosity is too low, it is difficult to suppress ink leakage, so the ink viscosity is preferably 500 mPa·s or more, and more preferably 1,000 mPa·s or more. In consideration of further improving ink leakage suppression, writing feel, ink followability, and writing performance, the viscosity of the ink composition is preferably 500 to 25,000 mPa·s, more preferably 1,000 to 25,000 mPa·s, and more preferably 800 to 25,000 mPa·s. Furthermore, from the viewpoints of writing performance and writing feel, the viscosity is more preferably 1,000 to 20,000 mPa·s, and even more preferably 2,000 to 20,000 mPa·s.

(Ballpoint pen)
As for the ballpoint pen tip, the movement amount of the ball in the vertical axis direction of the ballpoint pen tip is preferably 3 to 25 μm. This is because, within the above range, when a salt-forming dye of direct black and amine is used, it is easy to maintain good writing performance such as writing start performance, blurring of handwriting, and tearing, while obtaining a thick handwriting, and further, by ensuring the ink discharge amount and increasing the ink consumption, a thick handwriting can be obtained and the robustness of the handwriting can be favorably improved. From further consideration, the movement amount in the vertical axis direction is preferably 5 to 20 μm, and more preferably 7 to 18 μm.
In the present invention, the amount of movement of the ball of the ball-point pen tip in the vertical axis direction is the shape of the ball-point pen tip of the ball-point pen in its initial state before writing begins.

The arithmetic mean roughness (Ra) of the ball surface of the ballpoint pen tip is preferably 0.1 to 12 nm. This is because, within the above range, it is easy to maintain good writing performance such as writing performance, writing feel, smearing of handwriting, and tearing, and further, the ink is easily applied to the ball surface, the ink can be discharged smoothly, and by increasing the ink consumption, a thick handwriting can be obtained, which is advantageous for the robustness of the handwriting. The arithmetic mean roughness (Ra) of 0.1 to 10 nm is preferable because the ink is more easily applied to the ball surface, and from further consideration, 2 to 8 nm is preferable. The surface roughness can be measured using (Seiko Epson Corporation model name SPI38, 00N).

The material used for the balls is not particularly limited, but examples include cemented carbide balls whose main component is tungsten carbide, metal balls such as stainless steel, ceramic balls such as silicon carbide, silicon nitride, alumina, silica, and zirconia, and ruby balls.

Materials for ballpoint pen tips include metals such as stainless steel, nickel silver, brass, aluminum bronze, and aluminum, and resins such as polycarbonate, polyacetal, and ABS, but when considering wear of the ball seat, stability over time, and cost, it is preferable to use a stainless steel tip body.

(Example)
The present invention will now be described with reference to examples.
The ink composition for oil-based ballpoint pens in Example 1 uses a salt-forming dye of direct black and amine, an alkylene glycol alkyl ether solvent, a nonionic surfactant, a polyvinyl butyral resin, and a polyvinylpyrrolidone resin, and these are weighed out in predetermined amounts, heated to 60°C, and then completely dissolved using a disper stirrer to obtain an ink composition for oil-based ballpoint pens. The specific blending amounts are as follows:
The ink viscosity of Example 1 was measured using a Brookfield Viscometer RVDVII+Pro CP-52 spindle at a temperature of 20° C. and a shear rate of 5 sec ⁻¹ , and was found to be 3,300 mPa·s.

Example 1 (Ink Formulation)
Colorant (Direct Black 19 and amine salt-forming dye) 30.0% by mass
Alkylene glycol alkyl ether solvent (solubility parameter (SP value): 10.5 (cal/cm ³ ) ^1/2 , boiling point: 249° C.) 65.2% by mass
Nonionic surfactant (sorbitan fatty acid ester) 2.0% by mass
Polyvinyl butyral resin 2.5% by mass
Polyvinylpyrrolidone resin 0.3% by mass

Examples 2 to 30
As shown in Table 1, oil-based ink compositions for writing instruments of Examples 2 to 30 were obtained in the same manner as in Example 1, except that the ink components were changed. The measurement and evaluation results are shown in the table.

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

Tests and Evaluations The oil-based ink compositions (0.4 g) for writing instruments prepared in Examples 1 to 30 and Comparative Examples 1 to 3 were filled into oil-based ballpoint pen refills equipped with a ballpoint pen tip (having a coil spring in the tip that presses the ball directly against the inner wall of the tip end edge, ball movement in the vertical axis direction: 10 μm, arithmetic mean roughness (Ra): 3 nm) that rotatably holds a ball with a ball diameter of φ0.7 mm in an ink reservoir tube, to prepare oil-based ballpoint pens. The following tests and evaluations were carried out using writing paper JIS P3201 as the writing test paper.

In addition, when the ink viscosity of Examples 28 to 30 was measured in an environment of 20° C. and a shear rate of 5 sec ⁻¹ (revolutions per minute: 2.5 rpm) in the same manner as in Example 1, the ink viscosity was 5,000 mPa·s for Example 28, 10,000 mPa·s for Example 29, and 11,000 mPa·s for Example 30.

Water resistance test: A spiral writing was made on a piece of writing paper, and after drying, the paper was immersed in water for 24 hours, and the condition of the writing was observed.
The handwriting is good...
The handwriting has faded but is still legible...○
Handwriting is not visible... ×

Alcohol resistance test: A spiral writing was made on a piece of writing paper, and after drying, the paper was immersed in ethanol (50% aqueous solution) for 10 minutes, and the condition of the writing was observed.
The handwriting is good...
The handwriting has faded but is still legible...○
Handwriting is not visible... ×

Lightfastness test: A spiral writing test was performed on writing paper at a writing speed of 4.5 m/min under conditions of a writing angle of 70° and a writing load of 150 g. After leaving the paper for 1 hour, the paper was irradiated with a xenon fade meter X15F (manufactured by Suga Test Instruments Co., Ltd.) until the blue scale faded to grade 3, and the handwriting was observed.
Does not fade...◎
Discoloration is visible, but still distinguishable...○
Items with noticeable fading... ×

Ink aging test: After one month in a 50° C. environment, the ink inside the tip body was observed under a microscope.
No precipitates, good condition...◎
A small amount of precipitate has occurred...○
Precipitation occurred, but no practical problems occurred... △
Items that cause deposits and lead to blurring or poor writing results... ×

Writing performance test: After handwriting, the pen was left in an environment of 20°C and 65% RH for 24 hours with the tip end extended, and then written under the writing conditions described below in a running test, and the length of the smudged handwriting at the start of writing was measured.
<Writing conditions> A straight line was written on a running test machine under the conditions of a writing load of 200 gf, a writing angle of 70°, and a writing speed of 4 m/min, and evaluation was performed.
The length of the smudged handwriting is less than 1mm...◎◎
The length of the smudged handwriting is 1mm or more but less than 5mm...
The length of the smudged handwriting is 5 mm or more but less than 15 mm...
The length of the smudged handwriting is 15 mm or more but less than 30 mm... △
The length of the smudged handwriting is 30 mm or more... ×

In Examples 1 to 30, good performance was obtained in the tests of water resistance, alcohol resistance, light resistance, ink aging, and writing performance. Furthermore, when Examples 1 to 30 were handwritten, the writing feel was smooth and good.
An oil-based ballpoint pen was created using the oil-based ink of Example 1 in the same manner as in Example 1, except that the ballpoint pen tip specifications were changed to a ball movement distance in the vertical axis direction of the ball: 15 μm and arithmetic mean roughness (Ra): 6 nm of the ball surface. The same tests and evaluations were carried out, and the result was equivalent to that of Example 1, with all tests being favorable.

In Comparative Examples 1 to 3, the coloring agent was not a salt-forming dye of direct black and an organic amine, so the water resistance, alcohol resistance, light resistance, and ink aging tests were poor.

In addition, when using a retractable writing instrument (retractable ballpoint pen) such as a knock-type writing instrument or a rotary-type writing instrument, the writing performance is easily affected by smudged handwriting, so it is effective to use an oil-based ink composition for writing instruments such as that of the present invention.

In order to improve writing performance (handwriting smudges) and prevent ink leakage, it is preferable to provide a valve mechanism in which the ball, which is rotatably held at the tip of the ballpoint pen tip, is pressed against the inner wall of the tip edge by a coil spring directly or via a pressing body, creating a gap between the ball and the inner wall of the tip edge due to the pressing force when writing, allowing ink to flow out, and to close the minute gap at the tip when not in use.

In addition, in this embodiment, for convenience, an oil-based ballpoint pen is exemplified in which an oil-based ballpoint pen refill in which an ink composition for a writing instrument is directly contained within the barrel. However, the writing instrument of the present invention may also be a directly-filled ballpoint pen, marking pen, or felt-tip pen in which the barrel serves as an ink-containing barrel and the ink composition for a writing instrument is directly contained within the barrel.
Further, in this embodiment, for convenience, a ballpoint pen tip formed by cutting a wire material is exemplified, but a ballpoint pen tip formed by pressing a pipe material may also be used.

The present invention can be used as a writing instrument, and more specifically, can be widely used as a ballpoint pen, such as a capped or retractable writing instrument.

Claims (7)

An oil-based ink composition for a writing instrument, comprising a salt forming dye of direct black and an amine, an organic solvent, and a polyvinyl butyral resin , wherein the organic solvent is a triethylene glycol monoalkyl ether solvent . 2. The oil-based ink composition for writing instruments according to claim 1, characterized in that the organic solvent is a triethylene glycol monoalkyl ether solvent, and the alkyl ether moiety of the organic solvent has 1 to 3 carbon atoms. The oil-based ink composition for writing instruments according to claim 1 or 2, characterized in that the polyvinyl butyral resin has a hydroxyl group content of 25 mol% or more. The oil-based ink composition for a writing instrument according to any one of claims 1 to 3, characterized in that the oil-based ink composition for a writing instrument contains a nonionic surfactant having an HLB value of 1 to 14. 5. The oil-based ink composition for a writing instrument according to claim 1, wherein the organic solvent has a solubility parameter (SP value) of 9 to 15 (cal/cm ³ ) ^1/2 . A writing instrument containing the oil-based ink composition for writing instruments described in any one of claims 1 to 5. 7. An oil-based ballpoint pen comprising an ink holder having a ballpoint pen tip at the tip of the ink holder that rotatably holds a ball, and the ink holder contains the oil-based ink composition for a writing instrument according to claim 1.