Marking pen ink composition

Abstract

To provide a new ink composition for writing instruments having good color development property and good sticking property.SOLUTION: An ink composition for writing instruments includes a liquid medium and core-shell type polymer particles that are dispersed in the liquid medium and have a core part and a shell part, in which an average particle diameter measured by a laser diffraction method of the core-shell type polymer particles is 50 nm to 700 nm, a glass transition temperature of the core part is higher than a glass transition temperature of the shell part, a coefficient of variation of the particle diameter of the core-shell type polymer particles is less than 20%, and the shell part can bond the core-shell type polymer particles together in drying.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an ink composition for a writing instrument. [Background technology]

[0002] 2. Description of the Related Art Conventionally, functional colored resin particles have been widely used as colorants in applications such as inks for writing instruments.

[0003] Patent Document 1 discloses functional acrylic polymer regular particles having functional groups localized on the particle surface, characterized in that the regular particles are a copolymer obtained by polymerizing, in a predetermined ratio, a (meth)acrylic monomer (A) and a polymerizable monomer (B) having at least one functional group in the molecule and being highly hydrophobic, and in that at least 70% or more of the total functional groups in the regular particles are localized so as to protrude from the surface of the regular particles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-29766 A Summary of the Invention [Problem to be solved by the invention]

[0005] There is a need to provide new ink compositions for writing instruments that combine good color development and adhesion. [Means for solving the problem]

[0006] As a result of intensive research, the present inventors have found that the above problems can be solved by the following means, and have completed the present invention. That is, the present invention is as follows: A liquid medium and a core-shell polymer particle having a core and a shell surrounding the core, the core being dispersed in the liquid medium. The core-shell polymer particles have an average particle size of 50 nm to 700 nm as measured by a dynamic light scattering method; the glass transition temperature of the core portion is higher than the glass transition temperature of the shell portion; The coefficient of variation of the particle size of the core-shell polymer particles is less than 20%, and The shell portion is capable of adhering the core-shell polymer particles to each other when dried. An ink composition for a writing instrument. <Aspect 2> The ink composition for a writing instrument according to Aspect 1, wherein the glass transition temperature of the shell portion of the core-shell polymer particles is 30°C or lower. <Aspect 3> The ink composition for a writing instrument according to Aspect 1 or 2, wherein the glass transition temperature of the core portion of the core-shell polymer particle is 25°C or higher. Aspect 4: The ink composition for a writing instrument according to any one of Aspects 1 to 3, wherein the shell portion of the core-shell polymer particle has a thickness of 1 to 30% of the diameter of the entire core-shell polymer particle. Aspect 5: The ink composition for a writing instrument according to any one of Aspects 1 to 4, wherein the difference in refractive index between the core portion and the shell portion is 0.01 or more. Effect of the Invention

[0007] According to the present invention, it is possible to provide a novel ink composition for a writing instrument that has both good color development and fixing properties. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] <Ink composition for writing instruments> The ink composition for a writing instrument of the present invention comprises a liquid medium, and core-shell polymer particles dispersed in the liquid medium and having a core portion and a shell portion, The average particle size of the core-shell polymer particles measured by a laser diffraction method is 50 nm to 700 nm, The glass transition temperature of the core is higher than the glass transition temperature of the shell, The coefficient of variation of the particle size of the core-shell polymer particles is less than 20%, and The shell portion is capable of bonding the core-shell polymer particles to each other when dried.

[0009] When a dispersion liquid containing monodispersed spherical particles is applied and dried, the particles are regularly aligned, resulting in a coating film that exhibits structural coloring. However, when such a coating film is simply produced, the adhesion is not good due to the lack of adhesion between the particles.

[0010] The problem of adhesion of the coating film is improved when a binder is further added to the dispersion liquid of Patent Document 1. However, in this case, the formation of an ordered structure of the particles is inhibited during drying, and the gaps between the particles are filled with the binder, reducing the difference in refractive index between the particles and the gaps, which can result in an extreme decrease in color development.

[0011] In response to this, the present inventors have found that the above-mentioned composition provides an ink composition for a writing instrument that has both good color development and adhesion properties.

[0012] The aqueous ink composition for a writing instrument may further contain a colorant in addition to the colorants that may be contained in the core and shell portions, particularly as mentioned below.

[0013] The aqueous ink composition for a writing instrument may further contain other components.

[0014] Each component of the present invention will now be described.

[0015] Liquid medium The liquid medium may be, for example, water, an organic solvent, etc. The liquid medium may be used alone or in combination.

[0016] As the water, for example, ion-exchanged water, distilled water, etc. can be used.

[0017] As the organic solvent, for example, polyhydric alcohols such as glycerin, diglycerin, ethylene glycol, diethylene glycol, propylene glycol, hexylene glycol, etc., glycol ethers such as propylene glycol monomethyl ether, 3-methoxy-3-methyl-1-butanol, etc. can be used.

[0018] <Core-shell type polymer particles> The core-shell polymer particles are particles which are dispersed in a liquid medium and have a core portion and a shell portion.

[0019] The core-shell polymer particles are substantially composed of a polymer. In this specification, "substantially composed" means that the content is 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, and 100% by mass or less, 98% by mass or less, or 95% by mass or less, based on the mass of the entire core-shell polymer particles.

[0020] The average particle diameter of the core-shell type polymer particles measured by dynamic light scattering is 50nm to 700nm. This average particle diameter may be 50nm or more, 60nm or more, 70nm or more, 80nm or more, 90nm or more, or 100nm or more, and may be 700nm or less, 500nm or less, 400nm or less, 370nm or less, 350nm or less, 330nm or less, 300nm or less, 270nm or less, 250nm or less, 230nm or less, 200nm or less, or 150nm or less. The ink composition for writing instruments of the present invention can bring about clear structural color development due to such an average particle diameter. The average particle diameter referred to here is the value of the histogram average particle diameter (D50) based on the scattering intensity distribution measured by dynamic light scattering.

[0021] The coefficient of variation of the particle size of the core-shell polymer particles is less than 20%. This coefficient of variation may be 18% or less, 15% or less, 13% or less, 10% or less, or 8% or less, and may be more than 0%, 1% or more, 2% or more, 3% or more, 4% or more, or 5% or more. The ink composition for writing instruments of the present invention can bring about clear structural coloring due to the uniformity of the particle size brought about by such a coefficient of variation. The coefficient of variation here is calculated based on the particle size distribution derived from the scattering intensity distribution measured by dynamic light scattering.

[0022] The difference in refractive index between the core and shell is preferably 0.01 or more from the viewpoint of providing a clear structural color. This difference in refractive index may be 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.07 or more, or 0.10 or more, and may be 0.20 or less, 0.18 or less, 0.15 or less, or 0.13 or less.

[0023] The refractive index of the core portion and the shell portion may be a refractive index calculated from the following formula (1) using the refractive index (ni) of the homopolymer of each structural unit constituting the core portion and the shell portion, and the mass ratio (wi) of each structural unit to the total amount of the structural units constituting the crosslinked resin. n=Σniwi (1)

[0024] The glass transition temperature of the homopolymer used in the above calculation can be a value described in the literature, for example, the value described in POLYMER HANDBOOK, FOURTH EDITION (Johannes Brandrup, Edmund Heinz Immergut, Eric A. Grulke Wiley, 2004) or the value in the manufacturer's catalog.

[0025] The content of the core-shell polymer particles, based on the mass of the ink composition for a writing instrument, in terms of solid mass, may be 50% by mass or less, 45% by mass or less, 40% by mass or less, or 35% by mass or less, and may be 1% by mass or more, 3% by mass or more, 5% by mass or more, 7% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, or 30% by mass or more.

[0026] (Core) The core portion is the portion surrounded by the shell portion.

[0027] The glass transition temperature of the core portion is higher than that of the shell portion. This allows the adhesion between particles by the shell portion to be achieved while maintaining the rigidity of the core portion. The glass transition temperature of the core portion may be higher than the glass transition temperature of the shell portion by 5°C or more, 10°C or more, 15°C or more, 20°C or more, 30°C or more, 40°C or more, 50°C or more, 60°C or more, 70°C or more, or 80°C or more.

[0028] Here, the glass transition temperature (Tg) is a theoretically calculated value obtained by the FOX equation shown in the following formula (2). 1 / Tg=W1 / Tg1+W2 / Tg2++Wn / Tgn (2) (In the formula, Tg is the glass transition temperature of the acrylic resin, W1, W2, . . . , Wn are the weight fractions of each monomer, and Tg1, Tg2, . . . , Tgn are the glass transition temperatures of the homopolymers of each monomer.)

[0029] The glass transition temperature of the homopolymer used in the above calculation can be a value described in the literature, for example, the value described in POLYMER HANDBOOK, FOURTH EDITION (Johannes Brandrup, Edmund Heinz Immergut, Eric A. Grulke Wiley, 2004) or the value in the manufacturer's catalog.

[0030] The glass transition temperature of the core part is preferably, for example, 10° C. or more, 15° C. or more, 20° C. or more, 25° C. or more, 30° C. or more, 40° C. or more, 50° C. or more, 60° C. or more, 70° C. or more, 80° C. or more, 90° C. or more, or 100° C. or more, from the viewpoint of stably maintaining the shape of the core-shell particles and thereby obtaining good structural color development. This glass transition temperature may also be 200° C. or less, 180° C. or less, 150° C. or less, 130° C. or less, 120° C. or less, or 110° C. or less.

[0031] The core portion is substantially composed of a polymer. The polymer constituting the core portion may be composed of at least one monomer selected from the group consisting of (meth)acrylic monomers and styrene monomers.

[0032] Examples of (meth)acrylic monomers that can be used include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and 2-sulfoethyl (meth)acrylate, as well as metal salts thereof.

[0033] Examples of the styrene monomer that can be used include styrene and styrene derivatives. Examples of the styrene derivatives that can be used include m-chlorostyrene, p-chlorostyrene, m-fluorostyrene, p-fluorostyrene, p-methoxystyrene, m-tert-butoxystyrene, p-tert-butoxystyrene, p-vinylbenzoic acid, p-chloro-α-methylstyrene, and p-tert-butoxy-α-methylstyrene.

[0034] The core may further contain a coloring material. As the coloring material, various coloring materials that can be used in conventional inks, such as dyes, pigments, or mixtures of dyes and pigments, can be used. These coloring materials can be used alone or in combination.

[0035] As the dye, for example, water-soluble dyes, oil-soluble dyes, reactive dyes, etc., which are used in ordinary dye ink compositions, can be used. As the water-soluble dye, for example, direct dyes, acid dyes, basic dyes, food dyes, etc. can be used. As the water-insoluble dye, for example, mordant / acid mordant dyes, alcohol-soluble dyes, vat dyes, sulfur / sulfur vat dyes, metal complex dyes, disperse dyes, oil-soluble dyes, salt-forming dyes, azoic dyes, etc. can be used. These dyes may be used alone or in combination.

[0036] Examples of pigments that can be used include inorganic pigments such as carbon black and graphite, organic pigments such as talc, silica, alumina, mica and alumina silicate, azo pigments, condensed azo pigments, phthalocyanine pigments, anthraquinone pigments, quinacridone pigments, isoindolinone pigments, diketopyrrolopyrrole pigments, various lake pigments, fluorescent pigments, pearl pigments, and the like.

[0037] In particular, when the core portion is composed of a polymer, it is preferable from the viewpoint of production to use a water-insoluble dye as the coloring material.

[0038] When the core portion contains a colorant, the content of the colorant may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more, and may be 20% by mass or less, 15% by mass or less, 12% by mass or less, 10% by mass or less, 8% by mass or less, 5% by mass or less, 3% by mass or less, or 1% by mass or less, based on the mass of the core portion.

[0039] (Shell part) The shell portion is a portion surrounding the core portion. The shell portion is capable of adhering the core-shell polymer particles to each other when dry. Here, "adherable" means that the shell portion can be fixed to the shell portion of another core-shell polymer particle due to the ambient temperature during writing.

[0040] The glass transition temperature of the shell portion may be −100° C. or higher, −80° C. or higher, −60° C. or higher, −40° C. or higher, −20° C. or higher, −10° C. or higher, −5° C. or higher, or 0° C. or higher. From the viewpoint of achieving good adhesion between particles, the glass transition temperature is preferably 30° C. or lower, 25° C. or lower, 20° C. or lower, 15° C. or lower, 10° C. or lower, or 5° C. or lower.

[0041] The shell portion is substantially composed of a polymer. The polymer constituting the shell portion may be composed of the monomers listed for the core portion. The monomer constituting the polymer of the core portion and the monomer constituting the polymer of the shell portion may be the same monomer or different monomers. Here, "the same monomer" means that, for example, when the polymer of the core portion is composed of a (meth)acrylic monomer, the polymer of the shell portion is also composed of a (meth)acrylic monomer, and it is not necessarily required that these (meth)acrylic monomers are the same type of monomer, that is, the monomer constituting the polymer of the core portion and the monomer constituting the polymer of the shell portion are the same.

[0042] The shell part preferably has a thickness of 1 to 30% of the diameter of the entire core-shell polymer particle from the viewpoint of bringing about clear structural color development. This thickness may be 1% or more, 2% or more, 3% or more, 5% or more, 7% or more, or 10% or more, and may be 30% or less, 25% or less, 20% or less, or 15% or less. The thickness of the shell part is a value obtained by subtracting the histogram average particle diameter (D50) value based on the scattering intensity distribution of the core part measured by the dynamic light scattering method from the average particle diameter of the core-shell polymer particles.

[0043] The shell may contain a coloring material, for example, the coloring materials listed for the core may be used as the coloring material.

[0044] <Coloring material> As coloring materials other than those contained in the core and shell, for example, coloring materials dispersed or dissolved in the liquid medium, the coloring materials listed for the core can be used. In particular, when a dye is used, it is preferable to select the dye according to the liquid medium used. For example, when the liquid medium is water, it is preferable to use a water-soluble dye as the dye.

[0045] The content of the colorant may be 0.1% by mass or more, 0.3% by mass or more, or 0.5% by mass or more, and may be 20% by mass or less, 15% by mass or less, 12% by mass or less, 10% by mass or less, 8% by mass or less, 5% by mass or less, 3% by mass or less, or 1% by mass or less, based on the mass of the ink composition for a writing instrument. This content may not include the colorant contained in the core portion and / or shell portion.

[0046] <<Method for producing an ink composition for a writing instrument>> The ink composition for a writing instrument of the present invention can be produced by a method that includes preparing core-shell polymer particles and preparing an ink composition.

[0047] Between the preparation of the core-shell polymer particles and the preparation of the ink composition, the method may include isolating the core-shell polymer particles.

[0048] Preparation of Core-Shell Polymer Particles The core-shell polymer particles can be prepared by a method including preparing a dispersion of the core portion and forming the shell portion on the core portion.

[0049] (Creating the core) The dispersion of the core portion can be prepared, for example, by emulsion polymerization, which can be performed by preparing an oil phase, preparing an aqueous phase, mixing the oil phase and the aqueous phase to emulsify the components of the oil phase, and then polymerizing the mixture.

[0050] The ratio of the mass of the oil phase to the mass of the aqueous phase may be 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more, and may be 0.50 or less, 0.45 or less, 0.40 or less, 0.35 or less, or 0.30 or less. By adjusting this ratio, the average particle size of the core part can be adjusted.

[0051] The oil phase is preferably added gradually while stirring in order to keep the coefficient of variation within the above range. The coefficient of variation can be adjusted by adjusting the rate of addition. Here, "gradual addition" means that the oil phase is added to the aqueous phase at a substantially constant rate over 2.0 hours or more, 2.5 hours or more, or 3.0 hours or more, i.e., without artificially adjusting the rate of addition during the addition.

[0052] The coefficient of variation can also be adjusted by the shear force applied to the oil phase and the aqueous phase by stirring.

[0053] (Core: oil phase) The oil phase may contain monomers, colorants, and crosslinkers, and may be prepared by mixing these materials.

[0054] As the monomer, the monomers listed for the core portion can be used.

[0055] As the crosslinking agent, for example, triallyl isocyanurate or the like can be used.

[0056] (Core: aqueous phase) The aqueous phase may contain water, a surfactant, and a polymerization initiator, and can be prepared by mixing these materials.

[0057] As the surfactant, it is preferable to use ether sulfate from the viewpoint of not excessively increasing the average particle size of the core part. The content of the surfactant in the aqueous phase may be more than 0 mass%, 0.5 mass% or more, or 1.0 mass% or more based on the total mass of the aqueous phase, and may be 5.0 mass% or less, 4.0 mass% or less, 3.0 mass% or less, 2.0 mass% or less, or 1.5 mass% or less. The average particle size of the core part can be adjusted by adjusting the type and amount of the surfactant.

[0058] As the polymerization initiator, for example, ammonium persulfate or the like can be used.

[0059] (Preparation of the shell part) The shell portion can be prepared by adding water and a surfactant to an aqueous dispersion of the core portion to prepare a second aqueous phase, and then adding a monomer thereto to carry out seed polymerization using the core portion as a seed.

[0060] The monomers mentioned for the shell part can be used as the monomer. The ratio of the mass of the monomer used in the shell part to the mass of the monomer used in the core part can be 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more, and can be 1.5 or less, 1.4 or less, 1.3 or less, 1.2 or less, or 1.1 or less. By adjusting this ratio, the thickness of the shell relative to the average particle size can be adjusted.

[0061] As the surfactant, it is preferable to use ether sulfate from the viewpoint of not excessively increasing the thickness of the shell portion. The content of the surfactant in the second aqueous phase may be more than 0 mass%, 0.5 mass% or more, or 1.0 mass% or more based on the total mass of the second aqueous phase, and may be 5.0 mass% or less, 4.0 mass% or less, 3.0 mass% or less, 2.0 mass% or less, or 1.5 mass% or less. The average particle size of the core portion can be adjusted by adjusting the type and amount of the surfactant.

[0062] Preparation of Ink Composition The ink composition can be prepared by a conventional method while mixing the prepared core-shell type polymer particles and other components constituting the ink composition for a writing instrument using a stirring device such as a disperser. EXAMPLES

[0063] The present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to these.

[0064] Preparation of Ink Composition Example 1 (Creating the core) A 2-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1000-ml separatory funnel for monomer injection, and set in a hot water bath. 359 parts of distilled water, 5.5 parts of 2-sulfoethyl sodium methacrylate (acrylic ester SEM-Na, Mitsubishi Chemical Corporation, Monomer A), 22 parts of ether sulfate as a surfactant (ADEKA REASOAP SE-10N, ADEKA Corporation), and 0.5 parts of ammonium persulfate were charged into the flask to prepare an aqueous phase. The internal temperature of the aqueous phase was raised to 50° C. while introducing nitrogen gas.

[0065] On the other hand, an oil phase was prepared by mixing 98.5 parts of methyl methacrylate (monomer B), 3.5 parts of a salt forming dye (Valifast Black 3830, Orient Chemical Co., Ltd.), and 11 parts of triallyl isocyanurate (TAIC, Nippon Kasei Co., Ltd.) as a crosslinking agent.

[0066] This oil phase was added from the separating funnel to the aqueous phase maintained at about 50° C. with stirring over a period of 3 hours to carry out emulsion polymerization. The mixture was further aged for 5 hours to complete the polymerization, yielding 500 parts of a primary dispersion of colored core particles.

[0067] Using this dispersion, the following measurements were carried out: -The glass transition temperature of the core was measured using the above FOX equation (Equation (2)); -The average particle size of the core portion was measured by dynamic light scattering; The refractive index of the core portion was measured using the above formula (1).

[0068] (Preparation of the shell part) A 2-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1000-ml separatory funnel for monomer injection, and placed in a hot water bath. The flask was equipped with a nitrogen gas inlet tube and a 1000-ml separatory funnel for monomer injection, and placed in a hot water bath. 150 parts of the obtained primary dispersion, 366 parts of distilled water, 2 parts of ether sulfate (ADEKA REASOAP SE-10N, ADEKA) as a surfactant, and 0.4 parts of ammonium persulfate were charged, and the internal temperature was raised to 50° C. while introducing nitrogen gas.

[0069] On the other hand, a mixture of 15.6 parts of methyl methacrylate monomer (monomer B) and 15.6 parts of n-butyl acrylate monomer (monomer E) was added from the separatory funnel to the flask kept at about 50° C. for 3 hours under stirring to carry out seed polymerization. The mixture was further aged for 5 hours to terminate the polymerization, and 549.6 parts of a dispersion of core-shell type polymer particles were obtained. The content (solid content) of the core-shell type polymer particles in this dispersion was 36% by mass.

[0070] Using this dispersion, the following measurements were carried out: -The glass transition temperature of the shell was measured using the above FOX equation (Equation (2)); The average particle size and the coefficient of variation of the core-shell polymer particles were measured by a dynamic light scattering method, and the ratio of the shell thickness to the average particle size was calculated using the measurement result of the average particle size; The refractive index of the shell was measured using the above formula (1).

[0071] (Preparation of Ink Composition) 100 parts by mass of an ink composition for writing instruments was prepared using 90 parts by mass of the dispersion of the prepared core-shell polymer particles, 3 parts by mass of glycerin as a moisturizer, 0.3 parts by mass of a preservative (Bioden S, Yamato Chemical Industry Co., Ltd.), and 6.7 parts by mass of ion-exchanged water.

[0072] Examples 2 to 5 and Comparative Examples 2 to 3 The ink compositions for writing instruments of Examples 2 to 5 and Comparative Examples 2 to 3 were prepared in the same manner as in Example 1, except that the type and amount of each component was adjusted so that the content of each substance in Example 1 was the content shown in Table 1.

[0073] Comparative Example 1 An ink composition for a writing instrument of Comparative Example 1 was prepared in the same manner as in Example 1, except that the core portion was prepared as follows.

[0074] (Creating the core) A 2-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1000-ml separatory funnel for monomer injection, and set in a warm water bath. 359 parts of distilled water, 5.5 parts of 2-sulfoethyl sodium methacrylate (acrylic ester SEM-Na, Mitsubishi Chemical Corporation, Monomer A), 22 parts of ether sulfate as a surfactant (ADEKA REASOAP SE-10N, ADEKA Corporation, Surfactant A), and 0.5 parts of ammonium persulfate were charged into the flask to prepare an aqueous phase. Nitrogen gas was introduced into the aqueous phase.

[0075] On the other hand, an oil phase was prepared by mixing 98.5 parts of methyl methacrylate (monomer B), 3.5 parts of a salt forming dye (Valifast Black 3830, Orient Chemical Co., Ltd.), and 11 parts of triallyl isocyanurate (TAIC, Nippon Kasei Co., Ltd.) as a crosslinking agent.

[0076] This oil phase was added to the above-mentioned water phase, and after shearing treatment with a homomixer, the internal temperature was raised to 50° C., and emulsion polymerization was carried out. The mixture was further aged for 5 hours to terminate the polymerization, and 500 parts of a primary dispersion of colored core particles was obtained.

[0077] Comparative Example 4 An ink composition for a writing instrument of Comparative Example 1 was prepared in the same manner as in Example 1, except that the core portion was prepared as follows.

[0078] (Creating the core) A 2-liter flask was equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube, and a 1000-ml separatory funnel for monomer injection, and set in a hot water bath. 359 parts of distilled water, 5.5 parts of 2-sulfoethyl sodium methacrylate (acrylic ester SEM-Na, Mitsubishi Chemical Corporation, Monomer A), 22 parts of sodium dodecylbenzenesulfonate (Neopelex G-15, Kao Corporation, Surfactant B) as a surfactant, and 0.5 parts of ammonium persulfate were charged into the flask to prepare an aqueous phase. The internal temperature was raised to 50° C. while introducing nitrogen gas into the aqueous phase.

[0079] On the other hand, an oil phase was prepared by mixing 98.5 parts of methyl methacrylate (monomer B), 3.5 parts of a salt forming dye (Valifast Black 3830, Orient Chemical Co., Ltd.), and 11 parts of triallyl isocyanurate (TAIC, Nippon Kasei Co., Ltd.) as a crosslinking agent.

[0080] This oil phase was added from the separating funnel to the aqueous phase maintained at about 50° C. with stirring over a period of 3 hours to carry out emulsion polymerization. The mixture was further aged for 5 hours to complete the polymerization, yielding 500 parts of a primary dispersion of colored core particles.

[0081] "evaluation" Color development Each ink composition was filled and a marking pen was used to hand-draw a line about 25 cm long on writing paper. After drying, the line was viewed from directly above at an angle of 90 degrees, and the change in color when viewed from an angle of 90 degrees and 45 degrees was evaluated according to the following criteria. A: The color tone changes clearly. B: The color change is slightly unclear. C: No change

[0082] <Adhesiveness> Each ink composition was loaded and a marking pen was used to hand-draw a line approximately 25 cm long on writing paper. After drying, the written line was lightly rubbed perpendicular to the line with a finger wearing a finger cot (Kokuyo Co., Ltd., Meku-2 Office Finger Cot) and evaluated according to the following criteria to see if the written line was rubbed off or extended. A: No change B: Partial peeling observed C: Most of the material peels off

[0083] Table 1 shows the configurations and evaluation results of the examples and comparative examples.

[0084] [Table 1]

[0085] It can be seen from Table 1 that the ink compositions for writing instruments of Examples 1 to 5, which have an average particle size of 50 nm to 700 nm, a coefficient of variation of particle size of less than 20%, and a shell portion capable of bonding the core-shell polymer particles to each other when dry, provide good color development and fixation properties.

[0086] In contrast, it can be seen that the ink composition for a writing instrument of Comparative Example 1, which had a coefficient of variation of more than 20%, and the ink composition for a writing instrument of Comparative Example 4, which had an average particle size of more than 700 nm, both had poor color development, and that the ink composition for a writing instrument of Comparative Example 2, which had no shell portion, had poor adhesion. It can also be seen that the ink composition for a writing instrument of Comparative Example 3, in which the shell portion is composed of the same type of polymer as the core portion, also had poor adhesion, similar to Comparative Example 2.

Claims

1. The present invention comprises a liquid medium, and core-shell type polymer particles dispersed in the liquid medium, the core particles having a core and a shell portion surrounding the core, The core-shell polymer particles have an average particle size of more than 100 nm and not more than 700 nm as measured by a dynamic light scattering method, The glass transition temperature of the core portion is 25° C. or higher and is 40° C. or higher than the glass transition temperature of the shell portion; The glass transition temperature of the shell portion is 30° C. or lower, The coefficient of variation of the particle size of the core-shell polymer particles is less than 20%, The shell portion is capable of bonding the core-shell polymer particles to each other when dried, and The difference in refractive index between the core portion and the shell portion is 0.01 or more and 0.15 or less. An ink composition for a marking pen.

2. The ink composition for a marking pen according to claim 1 , wherein the glass transition temperature of the core portion of the core-shell polymer particle is 50° C. or higher.

3. 3. The ink composition for a marking pen according to claim 1, wherein the shell portion of the core-shell polymer particle has a thickness of 1 to 30% of the diameter of the entire core-shell polymer particle.

4. An ink composition for a marking pen described in any one of claims 1 to 3, wherein the coefficient of variation is less than 10%.

5. An ink composition for a marking pen described in any one of claims 1 to 4, wherein the glass transition temperature of the shell portion is -20°C or higher.

6. An ink composition for a marking pen described in any one of claims 1 to 5, wherein the core portion contains a colorant.

7. The ink composition for a marking pen as described in claim 6, wherein the colorant is a dye.

8. A marking pen having an ink composition for a marking pen described in any one of claims 1 to 7.

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