Oil-in-water ink composition for writing instrument

Abstract

Provided is an oil-in-water ink composition for a writing instrument, having: an aqueous phase and an oil phase dispersed in the aqueous phase in the form of an oil-in-water emulsion, wherein, the oil phase contains a shear-thinning additive, and the ratio of the viscosity of the oil phase at a shear rate of 383 sec-1 to the viscosity of the oil phase at a shear rate of 38.3 sec-1 is 0.8 or less.

Description

Oil-in-water ink composition for writing instruments

[0001] This invention relates to an oil-in-water droplet type ink composition for writing instruments.

[0002] One of the characteristics of water-based ballpoint pens is their light writing feel, but on the other hand, they often cause scratching. Also, while water-soluble dyes used as coloring agents have the advantage of high coloring power and freedom in color mixing, they have poor weather resistance, and the lines tend to bleed if water comes into contact with them.

[0003] Oil-based ballpoint pens have the advantage of not scratching when writing and minimizing ink bleeding, but they often have a heavy writing feel. Furthermore, the oil-soluble dyes used as coloring agents are water-resistant, preventing the bleeding that occurs when water comes into contact with the ink.

[0004] In recent years, oil-based ballpoint pens have been developed that improve writing feel by reducing the viscosity of the oil-based ink. However, low-viscosity oil-based ink flows out more when writing, which worsens the drying time of the lines and can cause ink to bleed through to the back of the paper and result in blotting.

[0005] To address these challenges, an aqueous ballpoint pen ink composition has been disclosed in which the ink properties are those of an oil-in-water emulsion (O / W emulsion), which can achieve both the smooth writing feel of the aqueous phase and the water resistance of oil-soluble dyes.

[0006] Patent Document 1 discloses an aqueous ballpoint pen ink composition in which an oil phase is contained in an aqueous phase in the form of an oil-in-water emulsion, wherein at least one of the oil phase or the aqueous phase contains a coloring agent, and the oil phase contains, among the components constituting the oil phase, an estolide which is a fatty acid oligomer formed by the condensation of fatty acids having hydroxyl groups or by the condensation of a fatty acid having hydroxyl groups and a fatty acid not having hydroxyl groups, or an ester of the estolide and an alcohol.

[0007] Patent Document 2 discloses an O / W type emulsion ink composition for ballpoint pens, which is an emulsion dispersion in water containing an oil-soluble dye as a coloring agent, an organic solvent in which the dye is dissolved and whose solubility in 100 g of water at 20°C is 5 g or less, a sucrose fatty acid ester with an HLB of 8 or less, and an oily component containing at least a sodium salt of acyl lactic acid having 8 or more carbon atoms.

[0008] Japanese Patent Publication No. 2013-221051 Japanese Patent Publication No. 2010-275402

[0009] In the field of oil-in-water droplet ink compositions for writing instruments, problems have arisen not only regarding writing feel, but also regarding the DC phenomenon that occurs when writing instruments are stored upside down; that is, the phenomenon in which ink leaks from the tip of the writing part when the writing instrument is stored with the writing part facing downwards. In particular, when the ink is used in a ballpoint pen, ink sometimes leaks from the tip of the writing part even when the ballpoint pen is not being used for writing.

[0010] The present invention provides a novel oil-in-water droplet type ink composition for writing instruments that can suppress the occurrence of the DC phenomenon when the writing instrument is stored upside down, while maintaining a good writing feel.

[0011] The present inventors, after diligent study, found that the above problems could be solved by the following means, and thus completed the present invention. That is, the present invention is as follows: <Aspect 1> Having an aqueous phase and an oil phase dispersed in the aqueous phase in the state of an oil-in-water emulsion, wherein the oil phase contains a shear viscosity reducing agent, and the shear rate of the oil phase is 383 sec. －１ The viscosity of the oil phase is 38.3 sec. －１ An oil-in-water droplet type ink composition in which the ratio of viscosity to the amount of the oil phase is 0.8 or less. <Aspect 2> Shear rate of the oil phase: 38.3 sec －１ The viscosity at 25°C is 1500 to 10000 mPa·sec, and the shear rate of the oil phase is 383 sec. －１ The oil-in-water droplet type ink composition according to Embodiment 1, wherein the viscosity at 25°C is 1150 to 5000 mPa·sec. Embodiment 3 The shear rate of the oil-in-water droplet type ink composition is 383 sec. －１The oil-in-water droplet ink composition according to Embodiment 1 or 2, wherein the viscosity at 25°C is 1 to 100 mPa·sec. Embodiment 4 The oil-in-water droplet ink composition for writing instruments according to any one of Embodiments 1 to 3, wherein the content of the oil phase is 1% by mass or more and 35% by mass or less relative to the oil-in-water droplet ink composition for writing instruments. Embodiment 5 The oil-in-water droplet ink composition according to any one of Embodiments 1 to 4, wherein the particle size of the oil droplets constituting the oil phase, as measured by dynamic light scattering, is 50 nm or more and 3000 nm or less. Embodiment 6 A writing instrument comprising an ink storage section having the oil-in-water droplet ink composition for writing instruments according to any one of Embodiments 1 to 5. Embodiment 7 The writing instrument according to Embodiment 6, which is a ballpoint pen.

[0012] According to the present invention, it is possible to provide a novel oil-in-water droplet type ink composition for writing instruments that can suppress the occurrence of the DC phenomenon when the writing instrument is stored upside down, while maintaining a good writing feel.

[0013] 《Oil-in-Water Ink Composition》 The oil-in-water ink composition of the present invention has an aqueous phase and an oil phase dispersed in the aqueous phase in the state of an oil-in-water emulsion, the oil phase contains a shear viscosity reducing agent, and the shear rate of the oil phase is 383 sec. －１ The viscosity of the oil phase is 38.3 sec. －１ The ratio to viscosity is 0.8 or less.

[0014] The inventors have found that the above configuration, particularly the ratio of the viscosity of the oil phase, can suppress the occurrence of the DC phenomenon when the writing instrument is stored upside down, while maintaining a good writing feel. More specifically, with the above ratio, the low viscosity of the oil phase under the shear force applied during writing provides smooth writing, while under static conditions, the viscosity of the oil phase is high, causing the oil phase (oil droplets) to act like resin particles, thereby suppressing the DC phenomenon. In particular, when the oil-in-water droplet type ink composition of the present invention is for a ballpoint pen, the DC phenomenon can be suppressed when the ballpoint pen is not being used for writing, as the oil phase (oil droplets) are held between the ball and the holder.

[0015] The shear rate of the oil-in-water droplet type ink composition for writing instruments of the present invention is 383 sec. －１ The viscosity at 25°C may be 1 to 100 mPa·sec. This viscosity may be 1 Pa·sec or more, 3 Pa·sec or more, 5 Pa·sec or more, 7 Pa·sec or more, 10 Pa·sec or more, 12 Pa·sec or more, or 15 Pa·sec or more, and may also be 100 mPa·sec or less, 90 mPa·sec or less, 80 mPa·sec or less, 75 mPa·sec or less, 70 mPa·sec or less, 60 mPa·sec or less, 55 mPa·sec or less, 50 mPa·sec or less, 40 mPa·sec or less, 30 mPa·sec or less, or 25 mPa·sec or less.

[0016] In this specification, viscosity can be measured, for example, by steady-state flow measurement at 25°C using a rheometer MCR-302 (manufactured by Anton Paar) with a cone plate (diameter: 25 mm, angle: 2°).

[0017] The oil-in-water ink composition of the present invention may further contain a coloring agent. This coloring agent may be contained in at least one of the oil droplets and the aqueous phase.

[0018] Furthermore, the oil-in-water droplet ink composition of the present invention may contain other components as optional.

[0019] The following describes each component of the present invention.

[0020] <Oil Phase> The oil phase is dispersed in the aqueous phase and together with the aqueous phase forms an oil-in-water emulsion. In the present invention, the oil phase contains a shear viscosity reducing agent.

[0021] The oil phase may contain oil phase components.

[0022] As the oil phase component, at least one selected from the group consisting of hydrocarbon oils, ester oils, higher alcohols, fatty acids, and silicone oils can be used.

[0023] Examples of hydrocarbon oils that can be used include polyolefins such as polybutene, olefin oligomers, aromatic hydrocarbons, and liquid paraffin.

[0024] As the ester oil, for example, the estolide or ester mentioned below can be used.

[0025] As the higher alcohol, generally, an alcohol having 6 or more carbon atoms, such as n - hexanol, methyl amyl alcohol, 2 - ethyl butanol, n - heptanol, 2 - heptanol, 3 - heptanol, n - octanol, 2 - octanol, 2 - ethyl hexanol, 3,5,5 - trimethyl hexanol, nonanol, n - decanol, undecanol, n - decanol, trimethyl nonyl alcohol, tetradecanol, heptadecanol, cyclohexanol, 2 - methyl cyclohexanol, etc. can be used.

[0026] As the fatty acid, for example, the fatty acids mentioned below can be used.

[0027] As the silicone oil, for example, dimethyl silicone oil, methyl phenyl silicone oil, etc. can be used.

[0028] In particular, as the oil - phase component, for example, an estolide which is a fatty acid oligomer in which fatty acids having a hydroxyl group are condensed or fatty acids having a hydroxyl group and fatty acids having no hydroxyl group are condensed, or an ester of the estolide and an alcohol can be mentioned. Further, the oil phase may contain the fatty acid itself having no hydroxyl group in this estolide or ester.

[0029] The shear rate of the oil phase is 383 sec －１ The ratio of the viscosity at the shear rate of the oil phase of 383 sec to the viscosity at the shear rate of the oil phase of 38.3 sec －１ may be 0.1 or more and 0.8 or less. This ratio may be 0.8 or less, 0.7 or less, or 0.6 or less, and may also be 0.1 or more, 0.2 or more, 0.3 or more, or 0.4 or more.

[0030] The shear rate of the oil phase is 38.3 sec －１The viscosity in is not particularly limited as long as it satisfies the above ratio, and may be, for example, 1500 to 10000 mPa·sec. This viscosity may be, for example, 1500 mPa·sec or more, 1800 mPa·sec or more, 2000 mPa·sec or more, or 2300 mPa·sec or more, and may also be 10000 mPa·sec or less, 9000 mPa·sec or less, 8000 mPa·sec or less, 7500 mPa·sec or less, 7000 mPa·sec or less, 6000 mPa·sec or less, 5000 mPa·sec or less, 4000 mPa·sec or less, or 3500 mPa·sec or less.

[0031] Shear rate of the oil phase: 383 sec －１ The viscosity at 25°C may be between 1150 and 5000 mPa·sec, as long as the above ratio is satisfied. This viscosity may be 1150 mPa·sec or more, 1200 mPa·sec or more, or 1230 mPa·sec or more, and may also be 5000 mPa·sec or less, 4500 mPa·sec or less, 4000 mPa·sec or less, 3500 mPa·sec or less, 3000 mPa·sec or less, 2900 mPa·sec or less, 2800 mPa·sec or less, 2700 mPa·sec or less, or 2600 mPa·sec or less.

[0032] The oil phase content may be 1% by mass or more and 35% by mass or less, relative to the mass of the oil-in-water droplet type ink composition. This content may be 1% by mass or more, 3% by mass or more, 5% by mass or more, 7% by mass or more, 9% by mass or more, or 10% by mass or more, and may also be 35% by mass or less, 33% by mass or less, 31% by mass or less, 27% by mass or less, 25% by mass or less, 22% by mass or less, 20% by mass or less, 17% by mass or less, 15% by mass or less, or 12% by mass or less.

[0033] The particle diameter of the oil droplets constituting the oil phase may be 50 nm or more and 1000 nm or less. This particle diameter may be 50 nm or more, 70 nm or more, 100 nm or more, 130 nm or more, 150 nm or more, 180 nm or more, 200 nm or more, or 220 nm or more, and may also be 1000 nm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 380 nm or less, 350 nm or less, 320 nm or less, or 300 nm or less.

[0034] Here, in the present invention, the "particle diameter" is, in principle, the value of the median diameter (D50) calculated on a volume basis in the scattering intensity distribution measured by the dynamic light scattering method. Exceptionally, for particles whose particle diameter cannot be measured by the dynamic light scattering method, for example, particles with a D50 exceeding 5 μm, the median diameter (D50) calculated on a volume basis in the laser diffraction method can be adopted.

[0035] Further, the oil phase may further contain a resin and / or an organic solvent in order to adjust the viscosity. The resin and the organic solvent are mentioned with respect to the aqueous phase.

[0036] (Estride or ester) The estride or ester that can be used in the oil phase may be an estride that is a fatty acid oligomer in which fatty acids having a hydroxyl group are condensed or a fatty acid having a hydroxyl group and a fatty acid having no hydroxyl group are condensed, or an ester of the estride and an alcohol.

[0037] As the fatty acid having a hydroxyl group in the above estride or ester, various conventionally used fatty acids can be used. Among them, it is preferable to use castor oil fatty acid whose main component is ricinoleic acid, hydrogenated castor oil fatty acid whose main component is 12-hydroxystearic acid, and the like. These fatty acids may be used alone or in combination.

[0038] Further, examples of the fatty acid having no hydroxyl group include capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, melissic acid, oleic acid, linoleic acid, linolenic acid, etc. Further, coconut oil fatty acid, palm oil fatty acid, olive oil fatty acid, beef tallow fatty acid, hydrogenated beef tallow fatty acid, etc. containing these components can also be used.

[0039] In the present invention, a fatty acid oligomer obtained by condensing the above-described fatty acids having a hydroxyl group with each other, or a fatty acid oligomer (estolide) obtained by condensing a fatty acid having a hydroxyl group and a fatty acid having no hydroxyl group is used. Here, the "fatty acid oligomer" refers to a condensate of dimer or higher. Those of dimer to heptamer are preferable. Monomers may be mixed in the fatty acid oligomer, but from the viewpoint of solubility in the oil phase, it is preferable that the average of the whole fatty acid oligomer is 1.5-mer or more, preferably 2.0-mer or more.

[0040] The content of the above estolide or ester may be 50% by mass or more, 60% by mass or more, or 70% by mass or more, and may also be 90% by mass or less, or 85% by mass or less, based on the mass of the oil phase.

[0041] Further, the oil phase may further contain other oil phase components. As the other oil phase components, for example, the fatty acid itself having no hydroxyl group in the above estolide or ester may be further contained.

[0042] (Shear-thinning viscosity-imparting agent) The shear-thinning viscosity-imparting agent is contained in the oil phase.

[0043] Examples of the shear-thinning viscosity-imparting agent include crosslinked acrylic resins, N-vinyl carboxylic acid amide polymers or copolymers, sorbitol derivatives such as dibenzylidene sorbitol, dextrin fatty acid esters, inulin fatty acid esters, hydrogenated castor oil, 12-hydroxystearic acid, N-acyl amino acid amides or N-acyl amino acid esters, glutamic acid-based gelling agents, urea-based gelling agents, urethane-based gelling agents, silicone-based gelling agents, smectite-based weapon compounds, montmorillonite-based inorganic compounds, bentonite-based inorganic compounds, hectorite-based inorganic compounds, silica, etc.

[0044] The content of the shear viscosity reducing agent is not particularly limited as long as it satisfies the above ratio with respect to the viscosity of the oil phase, and may be, for example, 0.1% by mass or more and 25.0% by mass or less with respect to the mass of the oil phase. This content may be 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 0.7% by mass or more, 1.0% by mass or more, 2.0% by mass or more, 3.0% by mass or more, 4.0% by mass or more, or 4.5% by mass or more, and may also be 25.0% by mass or less, 23.0% by mass or less, 20.0% by mass or less, 17.0% by mass or less, 15.0% by mass or less, 12.0% by mass or less, 10.0% by mass or less, 7.0% by mass or less, 6.0% by mass or less, or 5.0% by mass or less.

[0045] <Aqueous Phase> The aqueous phase contains at least water. The aqueous phase may also further contain emulsifier components. The presence of emulsifier components allows the aqueous phase and the oil phase dispersed in the aqueous phase to exist in the form of an oil-in-water emulsion.

[0046] The aqueous phase may further contain colorants, resins, organic solvents, etc.

[0047] Furthermore, the aqueous phase may further contain a shear viscosity-reducing agent, particularly a water-soluble shear viscosity-reducing agent. Examples of water-soluble shear viscosity-reducing agents include polysaccharides such as succinoglucan and xanthan gum.

[0048] (Water) Distilled water and deionized water can be used as the water source.

[0049] The water content may be more than 50% by mass, 55% or more by mass, 60% or more by mass, 65% or more by mass, or 70% or more by mass, relative to the mass of the oil-in-water ink composition for writing instruments, and may also be 90% or less by mass, 85% or less by mass, 80% or less by mass, or 75% or less by mass.

[0050] Furthermore, the water content may be more than 50% by mass, 55% or more by mass, 60% or more by mass, 65% or more by mass, 70% or more by mass, 75% or more by mass, or 80% or more by mass, relative to the mass of the aqueous phase, and may also be 90% or less by mass, or 85% or less by mass.

[0051] (Emulsifier component) Any emulsifier can be used as the emulsifier component, for example, an emulsifier having one or more aromatic rings in its molecular skeleton can be used.

[0052] The aromatic emulsifier that can be used in the ink composition of the present invention is not particularly limited as long as it has one or more aromatic rings, and for example, polycyclic phenyl-type nonionic surfactants such as polyoxyethylene distyrenated phenyl ether, polyoxyethylene monotyrenated phenyl ether, and polyoxyethylene cumylphenyl ether, and ionic surfactants such as their sulfates can be used.

[0053] From the viewpoint of suppressing the coalescence of oil phase particles by the long chain of ethylene oxide (EO), the number of moles of ethylene oxide (EO) added to the emulsifier described above is preferably 40 mol or more, 45 mol or more, or 50 mol or more.

[0054] In addition to the emulsifiers mentioned above, emulsifiers with a low number of ethylene oxide addition moles that are strongly oriented towards the oil phase, specifically those with a number of ethylene oxide addition moles of 3 mol or more, 4 mol or more, or 5 mol or more, and 15 mol or less, 12 mol or less, or 10 mol or less, may also be used. Combining an emulsifier strongly oriented towards the oil phase with one strongly oriented towards the aqueous phase can be preferable from the viewpoint of increasing the micelle concentration at the interface and improving the stability of the emulsion.

[0055] Regarding the HLB value (hydrophilic-lipophilic balance value), it is preferable to use at least one emulsifier with an HLB value of 15 or higher for nonionic surfactants, from the viewpoint of suppressing excessive mixing of emulsifiers into the oil phase.

[0056] In addition to surfactants having aromatic rings in their molecules, any other emulsifiers with different structures may be added as emulsifier components. Examples of such emulsifiers include polyoxyethylene hydrogenated castor oil, linear hydrocarbon-type nonionic surfactants such as polyoxyethylene alkyl (C10-C18) esters, and sorbitan derivatives.

[0057] The amount of emulsifier component may be 5 parts by mass or more, 10 parts by mass or more, 15 parts by mass or more, 20 parts by mass or more, 25 parts by mass or more, 30 parts by mass or more, 35 parts by mass or more, 40 parts by mass or more, or 45 parts by mass or more per 100 parts by mass of the oil phase, and may also be 150 parts by mass or less, 140 parts by mass or less, 130 parts by mass or less, 120 parts by mass or less, 110 parts by mass or less, 100 parts by mass or less, 90 parts by mass or less, 80 parts by mass or less, 70 parts by mass or less, 65 parts by mass or less, 60 parts by mass or less, or 55 parts by mass or less.

[0058] (Resin) Any resin can be used as the resin, especially resins used for fixing the coating film. For example, sulfamide resins, maleic acid resins, terpene resins, terpene phenol resins, ester gums, xylene resins, alkyd resins, phenol resins, rosin, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl alcohol, acrylic resins, melamine resins, nitrocellulose resins, urea resins, etc., and derivatives thereof can be used.

[0059] The resin content can be adjusted according to the desired viscosity of the aqueous phase. For example, it may be 1% by mass or more, 2% by mass or more, or 3% by mass or more relative to the mass of the aqueous phase, or it may be 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 10% by mass or less, 7% by mass or less, or 5% by mass or less.

[0060] (Organic Solvents) Examples of organic solvents that can be used include aromatics, alcohols, polyhydric alcohols, glycol ethers, hydrocarbons, esters, etc. These solvents may be used individually or in combination.

[0061] Examples of aromatic compounds that can be used include benzyl alcohol, ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, propylene glycol monophenyl ether, diethylene glycol monophenyl ether, alkylsulfonate phenyl ester, butyl phthalate, ethylhexyl phthalate, tridecyl phthalate, ethylhexyl trimellitate, diethylene glycol dibenzoate, and dipropylene glycol dibenzoate.

[0062] Examples of alcohols that can be used include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, n-hexanol, methylamyl alcohol, 2-ethylbutanol, n-heptanol, 2-heptanol, 3-heptanol, n-octanol, nonanol, n-decanol, undecanol, n-decanol, tetradecanol, heptadecanol, cyclohexanol, 2-methylcyclohexanol, etc.

[0063] Examples of polyhydric alcohols that can be used include ethylene glycol, diethylene glycol, 3-methyl-1,3-butanediol, triethylene glycol, dipropylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, hexylene glycol, octylene glycol, and the like.

[0064] Examples of glycol ethers that can be used include methyl isopropyl ether, ethyl ether, ethyl propyl ether, ethyl butyl ether, isopropyl ether, butyl ether, hexyl ether, 2-ethylhexyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethyl butyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, 3-methoxy-3-methyl-1-butanol, 3-methoxy-1-butanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, propylene glycol tertiary butyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monobutyl ether, tetrapropylene glycol monobutyl ether, and the like.

[0065] Examples of hydrocarbons that can be used include linear hydrocarbons such as hexane, isohexane, heptane, octane, nonane, and decane, and cyclic hydrocarbons such as cyclohexane, methylcyclohexane, and ethylcyclohexane.

[0066] Examples of esters include propylene glycol methyl ether acetate, propylene glycol diacetate, 3-methyl-3-methoxybutyl acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether acetate, butyl formate, isobutyl formate, isoamyl formate, propyl acetate, butyl acetate, isopropyl acetate, isobutyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, propyl propionate, isobutyl propionate, isoamyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, propyl isobutyrate, Methyl valerate, ethyl valerate, propyl valerate, methyl isovalerate, ethyl isovalerate, propyl isovalerate, methyl trimethylacetate, ethyl trimethylacetate, propyl trimethylacetate, methyl caproate, ethyl caproate, propyl caproate, methyl caprylate, ethyl caprylate, propyl caprylate, methyl laurate, ethyl laurate, methyl oleate, ethyl oleate, caprylic triglyceride, tributyl citrate, octyl oxystearate, propylene glycol monolicinolate, methyl 2-hydroxyisobutyrate, 3-methoxybutyl acetate, etc. can be used.

[0067] The content of the organic solvent can be adjusted according to the desired viscosity of the oil phase. For example, it may be 1% by mass or more, 2% by mass or more, or 3% by mass or more relative to the mass of the oil phase, or it may be 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 10% by mass or less, 7% by mass or less, or 5% by mass or less.

[0068] <Coloring agents> Various coloring agents that can be used in conventional inks can be used as coloring agents, such as dyes, pigments, or mixtures of dyes and pigments. These coloring agents may be used individually or in mixtures.

[0069] Water-soluble dyes can be used as dyes. All dyes that dissolve or disperse in water can be used as water-soluble dyes, and examples include acid dyes such as eosin, foxine, water yellow #6-C, acid red, water blue #105, brilliant blue FCF, and negrosine NB; direct dyes such as direct black 154, direct sky blue 5B, and violet BB; and basic dyes such as rhodamine and methyl violet.

[0070] Water-insoluble dyes are dyes that are insoluble in water at room temperature. Examples of such water-insoluble dyes include salt-forming dyes, disperse dyes, and oil-soluble dyes, and among these, salt-forming dyes are preferred from the viewpoint of color development.

[0071] Examples of salt-forming dyes include dyes having chemical structures such as azo, metal complex azo, anthraquinone, and metal phthalocyanine. For example, Valifast® Black 1807, Valifast® Blue 2620, Valifast® Brown 2402, Valifast® Green 1501, Valifast® Orange 2210, Valifast® Pink 2310, Valifast® Red 1355, Valifast® Violet 1701, and Valifast® Yellow 1101 from Orient Chemical Industry Co., Ltd. can be used.

[0072] As the disperse dye, at least one dye selected from, for example, C.I. Disperse Yellow 198, C.I. Disperse Yellow 42, C.I. Disperse Red 92, C.I. Disperse Violet 26, C.I. Disperse Violet 35, C.I. Disperse Blue 60, and C.I. Disperse Blue 87 can be used.

[0073] Examples of oil-soluble dyes that can be used include Oil Black 860, Oil Blue 613, Oil Brown BB, Oil Green 530, Oil Orange 201, Oil Pink 312, Oil Red 5B, Oil Scarlet 318, and Oil Yellow 105 from Orient Chemical Industry Co., Ltd.

[0074] As pigments, conventionally known inorganic and organic pigments such as titanium dioxide, resin particle pigments containing pigments or dyes, pseudo-pigments obtained by coloring resin emulsions with dyes or pigments, white plastic pigments, luminous pigments, pigments with silica or mica as a base material and a multi-layer coating of iron oxide or titanium dioxide on the surface, thermochromic pigments, photochromic particles, etc. can be used without limitation.

[0075] Examples of inorganic pigments that can be used include carbon black, titanium black, zinc oxide, red iron oxide, aluminum, chromium oxide, iron black, cobalt blue, iron yellow, viridian, zinc sulfide, lithopone, cadmium yellow, vermilion, cadmium red, lead yellow, molybdide orange, zinc chromate, strontium chromate, white carbon, clay, talc, ultramarine, precipitated barium sulfate, barite powder, calcium carbonate, lead white, navy blue, navy blue, manganese violet, aluminum powder, brass powder, etc.

[0076] Examples of organic pigments include azo lakes, insoluble azo pigments, chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, and nitroso pigments. Examples of such organic pigments include C.I. Pigment Blue 17, C.I. Pigment Blue 15, C.I. Pigment Blue 17, C.I. Pigment Blue 27, C.I. Pigment Red 5, C.I. Pigment Red 22, C.I. Pigment Red 38, C.I. Pigment Red 48, C.I. Pigment Red 49, C.I. Pigment Red 53, C.I. Pigment Red 57, C.I. Pigment Red 81, C.I. Pigment Red 104, C.I. Pigment Red 146, C.I. Pigment Red 245, C.I. Pigment Yellow 1, C.I. Pigment Yellow 3, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 17, C.I. Pigment Yellow 34, C.I. Pigment Yellow 55, C.I. Pigment Yellow 74, C.I. Pigment Yellow 95, C.I. Pigment Yellow 166, C.I. Pigment Yellow 167, C.I. Pigment Orange 5, C.I. Pigment Orange 13, C.I. Pigment Orange 16, C.I. Pigment Violet 1, C.I. Pigment Violet 3, C.I. Pigment Violet 19, C. Examples include I. Pigment Violet 23, C.I. Pigment Violet 50, and C.I. Pigment Green 7.

[0077] Examples of thermochromic pigments include thermochromic pigments produced by microencapsulating a thermochromic composition containing at least a leuco dye that functions as a color developer, a color developer that is a component capable of causing the leuco dye to develop color, and a color change temperature adjuster that can control the color change temperature in the color development of the leuco dye and the color developer, so that the composition has a predetermined average particle size (for example, 0.1 to 6 μm). This particle size may be, for example, 0.1 μm or more, 0.2 μm or more, 0.3 μm or more, 0.5 μm or more, 0.7 μm or more, or 0.9 μm or more, and may also be 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less.

[0078] As photochromic particles, for example, photochromic particles composed of at least one or more photochromic substances selected from photochromic dyes (compounds), fluorescent dyes, etc., and a resin such as a terpene phenol resin can be used. Alternatively, photochromic particles can be produced by microencapsulating a photochromic composition containing at least one or more photochromic substances selected from photochromic dyes (compounds), fluorescent dyes, etc., an organic solvent, and additives such as antioxidants, light stabilizers, and sensitizers, to a predetermined particle size (for example, 0.1 to 6 μm).

[0079] These photochromic particles can be made to be colorless in an indoor lighting environment (lighting fixtures selected from incandescent lamps, fluorescent lamps, lamps, white LEDs, etc., indoors) and to develop color in an ultraviolet irradiation environment (irradiation with wavelengths of 200 to 400 nm, or an irradiation environment with sunlight including ultraviolet rays) by suitably using the above-mentioned photochromic substance.

[0080] Examples of microencapsulation methods for the above-mentioned thermochromic pigments and photochromic particles include interfacial polymerization, interfacial polycondensation, insitu polymerization, liquid curing coating, phase separation from aqueous solutions, phase separation from organic solvents, melt-dispersion cooling, air suspension coating, and spray drying, which can be appropriately selected depending on the application.

[0081] For example, in a phase separation method from an aqueous solution, a thermochromic microcapsule pigment can be produced by a method including the following steps, particularly a method including the following steps in this order: (1) heating and melting a leuco dye, a developer, and a color change temperature regulator; (2) adding the heated and melted leuco dye, developer, and color change temperature regulator to an emulsifier solution, heating and stirring to disperse them into oil droplets to prepare a dispersion; (3) gradually adding a resin raw material capable of forming a wall film, such as a urethane resin, epoxy resin, or amino resin, for example, an amino resin solution, specifically an amino resin solution such as an aqueous solution of methylolmelamine, a urea solution, or a benzoguanamine solution, to the above dispersion, reacting the resin raw material to form a capsule film, thereby obtaining a thermochromic microcapsule pigment; and (4) filtering the dispersion containing the thermochromic microcapsule pigment.

[0082] In this thermochromic pigment, the color development temperature and decolorization temperature of each color can be set to a suitable temperature by appropriately combining the types and amounts of leuco dyes, developer agents, and color change temperature adjusters.

[0083] These colorants can be used individually or in mixtures of two or more. Furthermore, the particle size of these colorants, such as water-dispersible pigments, resin particle pigments, pseudo-pigments, white plastic pigments, multi-layer coated pigments, thermochromic pigments, and photochromic particles, varies depending on the ball diameter, ink composition, viscosity, etc., but a particle size of 0.02 to 6 μm is desirable. This particle size may be, for example, 0.02 μm or more, 0.05 μm or more, 0.07 μm or more, 0.10 μm or more, 0.20 μm or more, 0.30 μm or more, 0.50 μm or more, 0.70 μm or more, or 0.90 μm or more, and may also be 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less.

[0084] The content of these colorants can be appropriately increased or decreased depending on the ink line density, but may be 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 0.7% by mass or more, 0.9% by mass or more, or 1.0% by mass or more, relative to the total amount of the oil-in-water droplet type ink composition, and preferably 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less.

[0085] <Other Components> Other components include, for example, dispersants, leveling agents, rust inhibitors, preservatives, lubricants, pH adjusters, and surface modifiers. As leveling agents, for example, fluorine-based surfactants and silicone oils can be used. As lubricants, for example, phosphate esters can be used. As pH adjusters, for example, amines such as triethanolamine can be used. As surface modifiers, silicone-based surface modifiers can be used.

[0086] 《Writing Instruments》 The writing instrument contains the above-described oil-in-water ink composition for writing instruments. The writing instrument may comprise an ink reservoir, a writing section, and a holding section, in which case the ink reservoir may store oil-in-water ink. The writing instrument may be a ballpoint pen.

[0087] <Ink Storage Section> The ink storage section stores the above-mentioned oil-in-water ink composition for writing instruments.

[0088] Any type of ink storage unit can be used as long as it can store ink and supply ink to the writing unit. It may be a direct-ink type with a collector structure (ink holding mechanism), or it may be a cotton-filled ink storage unit.

[0089] Furthermore, the ink reservoir may be integrated with the writing section. In particular, if the writing instrument is a ballpoint pen, the ink reservoir may be a refill for the writing instrument.

[0090] In particular, when the ink storage section is a ballpoint pen refill, the thickness of the outer wall of the ink storage section, i.e., the value of (outer diameter - inner diameter) / 2 of the ink storage section, is preferably 0.3 mm or more, 0.5 mm or more, or 0.7 mm or more, and also 2.0 mm or less, 1.8 mm or less, 1.5 mm or less, or 1.2 mm or less, from the viewpoint of suppressing the generation of air bubbles and obtaining a stable writing flow rate.

[0091] <Writing section> The writing section may be a writing section having a ballpoint pen tip at its front.

[0092] A ballpoint pen tip may consist of a ball and a holder that rotatably holds the ball. The ball may be made of any material used for ballpoint pen balls, such as stainless steel, cemented carbide, or ceramics. The shape of the ballpoint pen tip is not particularly limited and may be, for example, bullet-shaped or needle-shaped.

[0093] Furthermore, to improve the stability of the writing flow and the writing feel, it is more preferable that the surface roughness Ra of the ball of the ball pen tip be less than 15 nm, less than 12 nm, or less than 10 nm.

[0094] In this invention (including the embodiments described later), the "surface roughness Ra" was measured using a non-contact surface shape measuring instrument (NewView 7200, Zygo Corporation) with a lens magnification of 50x, an evaluation length of 100 μm, and a Gaussian filter of 25 μm. All other conditions were met in accordance with JIS B0601 (Geometric specifications of products - surface properties).

[0095] <Holding part> The holding part may be a part that allows the writing instrument of the present invention to be held by hand, and may have a hollow structure that can house the ink storage part. The holding part may have a shape such as cylindrical or polygonal tube.

[0096] The present invention will be specifically described by examples and comparative examples, but the present invention is not limited thereto.

[0097] <Preparation of Oil-Based Ink Composition for Writing Instruments> <Example 1> 7.5 parts by mass of condensed fatty acid ester (Minerasol LB-601, Ito Oil Co., Ltd.), 2 parts by mass of oleic acid as a fatty acid (Lunaq OV, Kao Corporation), 0.5 parts by mass of dl-α-tocopherol as an antioxidant (Tocopherol, Mitsubishi Chemical Corporation), and 0.5 parts by mass of dextrin fatty acid ester as a shear viscosity reducing agent (Polysaccharide ester, Leopal KL2, Chiba Flour Milling Co., Ltd.) were mixed, and these were heated to a temperature of 90 to 100°C while stirring to obtain an oily solution in which all of these were completely dissolved.

[0098] The shear rate of the obtained oil phase solution was 383 sec. －１ Viscosity and shear rate at 38.3 sec －１ The viscosity was measured. Viscosity was measured using a rheometer MCR-302 (manufactured by Anton Paar) with a cone plate (diameter: 25 mm, angle: 2°) by steady flow measurement at 25°C.

[0099] Using these viscosities, the shear rate of the oil phase was 383 sec. －１ Viscosity in the oil phase, shear rate 38.3 sec. －１ The ratio to viscosity was calculated. In Tables 1 and 2, this ratio is referred to as the "viscosity ratio".

[0100] Separately from the oily solution, an emulsifier solution was prepared by dissolving 5 parts by mass of polyoxyethylene styrene-phenyl ether (Newcol N780 (ethylene oxide 80 molar adduct), Nippon Emulsifier Co., Ltd.) as an emulsifier component in 35 parts by mass of purified water while stirring. Next, the emulsifier solution was gradually added to the oily solution to invert the phase from water in oil (w / o) to oil in water (o / w) to obtain an oil in water emulsion.

[0101] Subsequently, while stirring, a pigment dispersion was added to this oil-in-water emulsion to obtain 100 parts by mass of the writing instrument ink composition of Example 1. The pigment dispersion consisted of 7 parts by mass of carbon black as a pigment, 1 part by mass of styrene acrylic resin as a dispersant, 2 parts by mass of glycerin as an organic solvent, 0.6 parts by mass of phosphate ester as a lubricant, 0.6 parts by mass of triethanolamine as a pH adjuster, and 38.3 parts by mass of water.

[0102] The shear rate of the prepared oil-based ink composition for writing instruments was 383 sec. －１ The viscosity in the oil phase was measured in the same manner as in the oil phase.

[0103] Furthermore, the particle size of the oil droplets in the obtained oil-in-water ink composition for writing instruments was measured by dynamic light scattering. The measurement was performed using a particle size analyzer (concentrated particle size analyzer FPAR-1000, manufactured by Otsuka Electronics Co., Ltd.) and cumulant analysis of the scattering intensity distribution.

[0104] <Examples 2-8 and Comparative Examples 1-5> Except for changing the type and content of each component as shown in Tables 1-4, oil-in-water ink compositions for writing instruments were prepared in the same manner as in Example 1 for Examples 2-8 and Comparative Examples 1-5. Details of the components shown in Tables 1 and 2 are as follows: Glutamine gel: Glutamic acid-based gelling agent (GP-1, manufactured by Ajinomoto Healthy Supply) Urethane gel: Urethane-based gelling agent (RHEOBYK-410, manufactured by BYK) Polysaccharide A: Succinoglucan (Leozan, manufactured by Sansho) Polysaccharide B: Xanthan gum (Nomucoat Z, manufactured by Nisshin Oillio)

[0105] 《Evaluation》 Ballpoint pens were manufactured by filling each of the prepared oil-in-water ink compositions for writing instruments into a ballpoint pen. Specifically, the barrel of a ballpoint pen (Signo UM-100, Mitsubishi Pencil Co., Ltd.) was used, and the above-mentioned ink composition and an ink-following body were loaded into a refill consisting of a polypropylene ink reservoir tube with an outer diameter of 60 mm, an inner diameter of 3.8 mm, and a length of 113 mm, a stainless steel tip (carbide ball, ball diameter 0.38 mm, surface roughness less than 10 nm), and a connector connecting the reservoir tube and the tip. The following evaluation tests were conducted using these.

[0106] <Writing feel> The ballpoint pens were used for writing, and the writing feel was evaluated sensorily. The evaluation criteria were as follows: A: Very smooth. B: Smooth. C: Heavy.

[0107] <DC Resistance> Under conditions of 23±2°C and 65±10% relative humidity, the pen tip of each example or comparative example, with the ink refill attached, was extended from the tip of the barrel and dropped downwards from a height of 1m onto a receiving plate inclined at 50° to the horizontal plane. Afterward, a circle of approximately 20-25cm was written freehand five times, and the pen was immediately fixed downwards and left for 60 minutes. The size of the ink droplets accumulated at the tip of the ballpoint pen was then measured. The evaluation criteria were as follows: A: 0mm (no ink droplets) B: Greater than 0mm and less than 2mm C: Greater than 2mm

[0108] The configurations and evaluation results of the examples and comparative examples are shown in Tables 1 and 2.

[0109]

[0110]

[0111] From Tables 1 and 2, the shear rate of the oil phase is 383 sec. －１ Viscosity in the oil phase, shear rate 38.3 sec. －１ It can be understood that the oil-in-water ink composition for writing instruments of the example, in which the ratio to viscosity is 0.8 or less, can suppress the occurrence of the DC phenomenon when the writing instrument is stored upside down while maintaining a good writing feel.

[0112] On the other hand, as in Comparative Example 5, when xylene resin is added to simply increase viscosity, it can be seen that while DC resistance improves, the writing quality is compromised.

Claims

1. The aqueous phase and the oil phase dispersed in the aqueous phase are in the form of an oil-in-water emulsion, the oil phase contains a shear viscosity reducing agent, and the shear rate of the oil phase is 383 sec. －１ The viscosity of the oil phase is 38.3 sec. －１ An oil-in-water droplet type ink composition for writing instruments, wherein the ratio of the viscosity to the amount of the liquid is 0.8 or less.

2. Shear rate of the oil phase: 38.3 sec －１ The viscosity at 25°C is 1500 to 10000 mPa·sec, and the shear rate of the oil phase is 383 sec. －１ The oil-in-water droplet type ink composition for writing instruments according to claim 1, wherein the viscosity at 25°C is 1150 to 5000 mPa·sec.

3. Shear rate of the oil-in-water ink composition for writing instruments: 383 sec －１ The oil-in-water droplet type ink composition for writing instruments according to claim 1 or 2, wherein the viscosity at 25°C is 1 to 100 mPa·sec.

4. The oil-in-water ink composition for writing instruments according to any one of claims 1 to 3, wherein the content of the oil phase is 1% by mass or more and 35% by mass or less with respect to the oil-in-water ink composition for writing instruments.

5. An oil-in-water ink composition for writing instruments according to any one of claims 1 to 4, wherein the particle size of the oil droplets constituting the oil phase, as measured by dynamic light scattering, is 50 nm or more and 3000 nm or less.

6. A writing instrument comprising an ink storage section having the oil-in-water droplet type ink composition for writing instruments described in any one of claims 1 to 5.

7. The writing instrument according to claim 6, which is a ballpoint pen.

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