Inkjet ink

Abstract

To provide an inkjet ink that enables formation of an image having excellent color developability and excellent storage stability.SOLUTION: An inkjet ink comprises a pigment, a dispersant, and water, wherein the dispersant is a copolymer of a monomer mixture comprising ω-carboxy-polycaprolactone monoacrylate, polyalkylene glycol (meth)acrylate, and a vinyl monomer comprising at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group.SELECTED DRAWING: None

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION Embodiments of the present invention relate to inkjet inks. [Background technology]

[0002] Inkjet recording, in which highly fluid inkjet ink is ejected as droplets from minute nozzles to record an image on a recording medium placed opposite the nozzles, has rapidly become popular in recent years due to its ability to print at high speeds and with low noise. Known inks used in inkjet recording include aqueous inks containing water as the primary solvent, ultraviolet-curable inks (UV inks) containing a high content of polymerizable monomers as the primary component, and hot-melt inks (solid inks) containing a high content of wax as the primary component, as well as so-called non-aqueous inks containing a non-aqueous solvent as the primary solvent.

[0003] Aqueous inkjet inks are used in a wide range of fields due to their safety and drying properties.

[0004] Patent Document 1 proposes an aqueous inkjet ink having excellent storage stability, which contains a dispersant having a crosslinked structure and which is a copolymer of a monomer mixture containing: a vinyl monomer (A) containing a carboxyl group; a vinyl monomer (B) containing at least one selected from the group consisting of a β-dicarbonyl group and a biphenyl group; a nonionic vinyl monomer (C) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more; and a vinyl monomer (D) containing a polyoxyalkylene chain having an added mole number of alkylene oxide of 15 or more and a sulfo group. Patent Document 2 proposes a dispersion liquid containing a dispersion resin as a dispersion resin for dispersing a colorant, the dispersion resin having a constituent unit A containing a hydrophobic monomer, a constituent unit B containing a polyalkylene glycol (meth)acrylate monomer, and a constituent unit C containing a hydrophilic vinyl monomer having a sulfonic acid group, and an ink composition for inkjet recording containing this dispersion liquid. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2022-135519 [Patent Document 2] Japanese Patent Publication No. 2022-052994 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of an embodiment of the present invention is to provide an inkjet ink that can form an image with excellent color development and has excellent storage stability. [Means for solving the problem]

[0007] One embodiment of the present invention relates to an inkjet ink comprising a pigment, a dispersant, and water, wherein the dispersant is a copolymer of a monomer mixture comprising ω-carboxy-polycaprolactone monoacrylate, polyalkylene glycol (meth)acrylate, and a vinyl monomer containing at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group. [Effects of the Invention]

[0008] According to an embodiment of the present invention, it is possible to provide an inkjet ink that can form an image with excellent color development and has excellent storage stability. DETAILED DESCRIPTION OF THE INVENTION

[0009] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0010] An inkjet ink according to one embodiment comprises a pigment, a dispersant, and water, and the dispersant is a copolymer of a monomer mixture including ω-carboxy-polycaprolactone monoacrylate, polyalkylene glycol (meth)acrylate, and a vinyl monomer having at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group. According to this embodiment, it is possible to provide an inkjet ink that can form an image with excellent color development and has excellent storage stability.

[0011] Hereinafter, the inkjet ink will be referred to simply as "ink," and the components that form the copolymer, ω-carboxy-polycaprolactone monoacrylate, polyalkylene glycol (meth)acrylate, and vinyl monomer having at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group, will be referred to as Monomer A, Monomer B, and Monomer C, respectively. In this disclosure, (meth)acrylic acid collectively refers to methacrylic acid, acrylic acid, or a combination thereof; (meth)acrylate collectively refers to methacrylate, acrylate, or a combination thereof; and (meth)acrylamide collectively refers to methacrylamide, acrylamide, or a combination thereof.

[0012] When printing on a substrate that is prone to bleeding, a pretreatment agent is generally applied to the substrate in advance so that the ink does not penetrate too deeply into the substrate and remains on the surface, thereby enabling an image with good color development to be obtained. As the pretreatment agent, for example, one that contains an aggregating agent such as a polyvalent metal salt is often used, and examples of such an agent include an aqueous solution containing a divalent cation, etc. For example, when an ink that contains a pigment comes into contact with such a pretreatment agent on a substrate, the dispersion stability due to electrostatic repulsion on the substrate is disrupted, causing aggregation of the pigment and making the pigment more likely to remain on the substrate, which can suppress bleeding on the substrate and improve color development. Meanwhile, for example, the aqueous ink described in Patent Document 1 uses a vinyl monomer or the like containing a polyoxyalkylene chain with an added mole number of alkylene oxide of 15 or more as a dispersant, but when a polyalkylene glycol monomer is used, the color development of the image may be reduced. One reason for this is thought to be that the steric hindrance caused by the monomer inhibits the reaction between the acid (neutralizing group) and the pretreatment agent, which may make it easier for the pigment to penetrate into the fabric.

[0013] In one embodiment of the inkjet ink, the dispersant used is a copolymer of a monomer mixture containing ω-carboxy-polycaprolactone monoacrylate (monomer A), polyalkylene glycol (meth)acrylate (monomer B), and a vinyl-based monomer (monomer C) having at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group. The use of polyalkylene glycol (meth)acrylate (monomer B) containing a polyalkylene glycol group can effectively maintain dispersion stability in the ink. Furthermore, the use of ω-carboxy-polycaprolactone monoacrylate as monomer A together with monomer B can improve color development. One reason for this is presumably that the use of ω-carboxy-polycaprolactone monoacrylate as monomer A increases the degree of freedom of the acid (neutralizing group), making it less likely to inhibit the reaction between the ink and the pretreatment agent and making it easier for the pigment to remain on the substrate surface. In this way, when the ink contains a dispersant formed using ω-carboxy-polycaprolactone monoacrylate (monomer A), polyalkylene glycol (meth)acrylate (monomer B), and a vinyl monomer having at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group (monomer C), it is possible to achieve both good ink storage stability and good image color development.

[0014] Pigments can be used in the present invention, including organic pigments such as azo pigments, phthalocyanine pigments, polycyclic pigments, and dye lake pigments, as well as inorganic pigments such as carbon black and metal oxides. Examples of azo pigments include soluble azo lake pigments, insoluble azo pigments, and condensed azo pigments. Examples of phthalocyanine pigments include metal phthalocyanine pigments and metal-free phthalocyanine pigments. Examples of polycyclic pigments include quinacridone pigments, perylene pigments, perinone pigments, isoindoline pigments, isoindolinone pigments, dioxazine pigments, thioindigo pigments, anthraquinone pigments, quinophthalone pigments, metal complex pigments, and diketopyrrolopyrroles (DPPs). Examples of carbon black include furnace carbon black, lamp black, acetylene black, and channel black. Examples of metal oxides include titanium oxide and zinc oxide. These pigments can be used alone or in combination.

[0015] From the viewpoints of ejection stability and storage stability, the average particle size of the pigment is preferably 300 nm or less, more preferably 150 nm or less, and even more preferably 100 nm or less, as the volume-based average value in the particle size distribution measured by dynamic light scattering. From the viewpoints of print density and ink viscosity, the content of the pigment is preferably 0.1 to 20 mass % of the total amount of ink, more preferably 1 to 15 mass %, and even more preferably 5 to 10 mass %.

[0016] As the dispersant, a copolymer of a monomer mixture containing at least the following components can be used. ω-carboxy-polycaprolactone monoacrylate (monomer A) Polyalkylene glycol (meth)acrylate (monomer B) Vinyl monomer having at least one selected from the group consisting of an aromatic group and a β-diketone group (monomer C)

[0017] Monomer A is ω-carboxy-polycaprolactone monoacrylate. Examples of Monomer A include CH2=CHC(=O)O-(CH 10 C(=O)O)2-H, etc. Monomer A can be used alone or in combination of two or more.

[0018] The carboxy group derived from the monomer A may be neutralized. The carboxy group derived from the monomer (A) can be neutralized using a water-soluble basic compound as a neutralizing agent. Examples of the base include inorganic bases such as sodium hydroxide and organic bases such as triethanolamine. The degree of neutralization of the carboxy group derived from the monomer A may be, for example, 40 to 100 mol %. In the present disclosure, the "degree of neutralization of the carboxy group" refers to the ratio (mol %) of the number of molar equivalents of the water-soluble basic compound to the number of molar equivalents of the carboxy group before neutralization.

[0019] Monomer B is a polyalkylene glycol (meth)acrylate. Examples of monomer B include monomers with an alkylene oxide added thereto, such as polyalkylene glycol mono(meth)acrylate. The alkylene oxide may be one type or two or more types. The number of moles of alkylene oxide (AO) added is not particularly limited. For example, it may be 1 to 30. Monomer B preferably has a polypropylene glycol chain, a polyethylene propylene glycol chain, or the like, and more preferably a polyethylene glycol chain. This can further increase the hydrophilicity of the dispersant in the aqueous ink and further improve dispersion stability. Monomer B can be used alone or in combination of two or more types.

[0020] Monomer C is a vinyl monomer containing at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group. Monomer C may be, for example, a vinyl monomer containing an aromatic group, a vinyl monomer containing a β-dicarbonyl group, or a vinyl monomer containing an aromatic group and a β-dicarbonyl group. The β-dicarbonyl group and / or the aromatic group interact with the pigment surface through π-π interactions, hydrogen bonds, or the like, thereby increasing the adsorption of the dispersant to the pigment.

[0021] Examples of the aromatic group include a phenyl group and a biphenyl group. Examples of the β-dicarbonyl group include a β-diketone group and a β-keto acid ester group. Examples of the β-diketone group include an acetoacetyl group and a propionoacetyl group, and examples of the β-keto acid ester group include an acetoacetoxy group and a propionoacetoxy group.

[0022] Examples of monomer C include vinyl monomers containing an aromatic ring or biphenyl, such as styrene, benzyl (meth)acrylate, and ethoxylated o-phenylphenol (meth)acrylate, and vinyl monomers containing a β-dicarbonyl group, such as (meth)acrylate; 2-(acetoacetyloxy)ethyl (meth)acrylate, and the like.

[0023] The dispersant described above can contain units derived from monomer A, units derived from monomer B, and units derived from monomer C. The dispersant may contain units derived from other monomers, as necessary.

[0024] The blending ratio of each component in this copolymer is preferably as follows: The unit derived from monomer A may be, for example, 20 to 60 mol % or 30 to 50 mol % based on the total amount of the copolymer. The unit derived from Monomer B may be, for example, 1 to 30 mol %, or 5 to 20 mol % based on the total amount of the copolymer. The unit derived from monomer C may be, for example, 30 to 80 mol % or 40 to 70 mol % based on the total amount of the copolymer. Here, the amount of each unit can be determined from the total amount of monomers added to the synthesis system.

[0025] The dispersant is preferably used in a mass ratio of 0.1 to 10, more preferably 0.1 to 5, and may be used in a mass ratio of 0.2 to 3, based on the solid content of the pigment. The solid content of the dispersant is preferably 0.5 to 20% by mass, more preferably 1 to 10% by mass, and even more preferably 1 to 5% by mass, relative to the total amount of the ink.

[0026] A method for synthesizing the dispersant will be described below. Note that the dispersant is not limited to those synthesized by the following synthesis method, and any dispersant having the above-mentioned characteristics can be used.

[0027] An example of a method for synthesizing a dispersant includes synthesizing a copolymer using a monomer mixture containing at least monomer A, monomer B, and monomer C. The monomer mixture may optionally contain another monomer D.

[0028] Specifically, the copolymer can be synthesized by mixing monomer A, monomer B, monomer C, and, if necessary, monomer D all at once or in portions, and allowing the reaction to proceed in a solvent as needed.

[0029] The blending ratio of each monomer to the monomer mixture for obtaining the copolymer may be adjusted appropriately to obtain a preferable molar ratio in the copolymer described above. The polymerization may be any of random copolymerization, block copolymerization, and graft copolymerization. For example, a random copolymer can be obtained by polymerizing a raw material mixture containing all the monomer components at once. Block copolymers can be obtained by first polymerizing one type of monomer and then polymerizing the resulting polymer with other monomers, by first polymerizing one type of monomer and then block-polymerizing the resulting multiple polymers, or by controlling the polymerization reaction by living polymerization. Graft copolymers can be obtained by polymerizing a main chain copolymer using one or more monomers, introducing reactive groups into the copolymer, and then polymerizing other monomers using the reactive groups as initiators, or by introducing a polymer polymerized using other monomers using the reactive groups as initiators.

[0030] The polymerization can be carried out according to a solution polymerization method, a bulk polymerization method, a suspension polymerization method, an emulsion polymerization method, etc., and the solution polymerization method is preferred. Examples of polymerization solvents used in solution polymerization include aliphatic alcohols having 1 to 3 carbon atoms, such as ethanol and propanol; ketones, such as acetone and methyl ethyl ketone; and esters, such as ethyl acetate. Considering that the polymerization solvent is removed and replaced with water after polymerization, a low-boiling solvent is preferred. When a water-soluble organic solvent is used, the water replacement step may be omitted. In this case, the solvent boiling point is preferably 150°C or higher.

[0031] In the polymerization, various additives such as a radical polymerization initiator, a polymerization chain transfer agent, and a RAFT agent can be used.

[0032] The polymerization conditions can be appropriately adjusted depending on the monomers used, additives such as a radical polymerization initiator, polymerization solvent, and the like.

[0033] The weight average molecular weight (Mw) of the dispersant may be, for example, about 5,000 to 100,000, or about 5,000 to 50,000, etc. The number average molecular weight (Mn) of the dispersant may be, for example, about 3,000 to 100,000, or about 4,000 to 50,000. Here, the weight average molecular weight and number average molecular weight are both values ​​determined by the GPC method in terms of standard polystyrene.

[0034] The carboxyl group derived from the monomer A may be neutralized. The neutralization of the carboxyl group is preferably carried out, for example, by adding a neutralizing agent after obtaining a copolymer from a monomer mixture. Examples of the neutralizing agent include inorganic bases such as sodium hydroxide and organic bases such as triethanolamine.

[0035] The ink preferably contains water as a solvent, and the main solvent may be water. The water is not particularly limited, but it is preferable that it contains as few ionic components as possible. In particular, from the viewpoint of pigment dispersion stability of the ink, it is preferable that the content of polyvalent metal ions such as calcium is low. As the water, for example, ion-exchanged water, distilled water, ultrapure water, etc. can be used. From the viewpoint of adjusting the ink viscosity, the water content of the ink is preferably 20% to 90% by mass, more preferably 30% to 80% by mass, and even more preferably 40% to 70% by mass.

[0036] It is preferable that a water-soluble organic solvent is blended into the ink. As the water-soluble organic solvent, an organic compound that is liquid at room temperature and soluble in water can be used, and it is preferable to use a water-soluble organic solvent that is uniformly mixed with an equal volume of water at 20°C under 1 atmosphere. Examples of the water-soluble organic solvent include lower alcohols such as methanol, ethanol, 1-propanol, isopropanol, 1-butanol, 2-butanol, isobutanol, and 2-methyl-2-propanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, and 1,2-hexanediol; glycerins such as glycerin, diglycerin, triglycerin, and polyglycerin; acetins such as monoacetin and diacetin; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, and ethylene glycol monobutyl ether. Examples of suitable water-soluble organic solvents include glycol ethers such as tetraethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol dimethyl ether, and tetraethylene glycol diethyl ether; triethanolamine, 1-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, β-thiodiglycol, and sulfolane. The boiling point of the water-soluble organic solvent is preferably 100° C. or higher, and more preferably 150° C. or higher.

[0037] These water-soluble organic solvents may be used alone or in combination of two or more as long as they form a single phase with water. The content of the water-soluble organic solvent in the ink is preferably 5 to 50% by mass, and more preferably 10 to 35% by mass.

[0038] The ink may include a surfactant. The surfactant is not particularly limited, and examples thereof include acetylene glycol surfactants, silicone surfactants, polyoxyethylene alkylphenols, polyoxyethylene alkyl ethers, polyoxyethylene fatty acid esters, etc. Examples of acetylene glycol surfactants include the "Surfynol" series manufactured by Nissin Chemical Industry Co., Ltd. and the "Acetylenol" series manufactured by Kawaken Fine Chemical Co., Ltd. Examples of silicone surfactants include the "Silface SAG" series manufactured by Nissin Chemical Industry Co., Ltd. The surfactant content is preferably 0.1 to 10% by mass, more preferably 0.1 to 5% by mass, based on the total amount of the ink.

[0039] The suitable range of ink viscosity varies depending on the nozzle diameter of the ejection head of the inkjet recording system, the ejection environment, etc., but in general, it is preferably 1 to 30 mPa·s, and more preferably 3 to 15 mPa·s at 23° C. The ink viscosity can be measured using a rotational viscometer.

[0040] The ink described above is preferably applied to a substrate to which a pretreatment agent has been applied. The pretreatment agent preferably contains a flocculant, such as a polyvalent metal salt. Examples of polyvalent metal salts include magnesium chloride, calcium chloride, strontium chloride, barium chloride, zinc(II) chloride, iron(II) chloride, copper(II) chloride, nickel(II) chloride, manganese(II) chloride, aluminum chloride, iron(III) chloride, magnesium nitrate, calcium nitrate, strontium nitrate, barium nitrate, zinc(II) nitrate, iron(II) nitrate, copper(II) nitrate, nickel(II) nitrate, lead(II) nitrate, manganese(II) nitrate, aluminum nitrate, iron(III) nitrate, magnesium sulfate, zinc(II) sulfate, iron(II) sulfate, copper(II) sulfate, nickel(II) sulfate, manganese(II) sulfate, aluminum sulfate, iron(III), and calcium acetate. The polyvalent metal salt may be anhydrous or hydrated. The pretreatment agent preferably contains, for example, a flocculant and water. The pretreatment agent may also contain, for example, a surfactant and / or a water-soluble organic solvent. The pretreatment agent may also contain a binder resin to fix the flocculant to the substrate.

[0041] An example of a method for producing a printed matter will be described below. The method for producing a printed material may include, for example, applying an inkjet ink to a substrate. The inkjet ink may be any of the inkjet inks described above. The method may also include applying a pretreatment agent to the substrate before forming an image by inkjet printing.

[0042] The inkjet ink can be applied to a substrate by inkjet printing, which allows for easy and flexible image formation on demand without contacting the substrate. The inkjet printing method is not particularly limited and may be any method such as a piezoelectric method, an electrostatic method, or a thermal method. For example, ink may be ejected from an inkjet head based on a digital signal, and the ejected ink droplets may be deposited on a substrate.

[0043] In this embodiment, the substrate is not particularly limited, and examples thereof include printing paper such as plain paper, coated paper, and specialty paper; cloth; inorganic sheets; films; and overhead projector sheets. These substrates may also be used as adhesive sheets with an adhesive layer on the backside. Among these, cloth is preferred. Examples of cloth include woven fabrics, knitted fabrics, and nonwoven fabrics. Examples of fibers constituting the cloth include inorganic fibers such as metal fibers, glass fibers, rock fibers, and silt fibers; regenerated fibers such as cellulose-based and protein-based fibers; semi-synthetic fibers such as cellulose-based fibers; synthetic fibers such as polyamide, polyester, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyvinyl alcohol, polyurethane, polyethylene, polypropylene, polystyrene, and polyethylene fluoride; and natural fibers such as cotton, hemp, silk, and wool. In addition, from the viewpoint of ink permeability, printing paper such as plain paper and coated paper can be preferably used.

[0044] Here, plain paper refers to ordinary paper on which no ink-receiving layer or film layer is formed. Examples of plain paper include fine paper, medium-quality paper, PPC paper, wood paper, recycled paper, etc.

[0045] Furthermore, as the coated paper, inkjet coated paper such as matte paper, glossy paper, and semi-glossy paper, as well as so-called coated printing paper, can be preferably used. Here, coated printing paper refers to printing paper that has traditionally been used in letterpress printing, offset printing, gravure printing, and the like, and is printing paper in which a coating layer is provided on the surface of fine or medium-quality paper using a paint containing an inorganic pigment such as clay or calcium carbonate and a binder such as starch. Coated printing paper is classified into lightly coated paper, fine lightweight coated paper, medium lightweight coated paper, fine coated paper, medium coated paper, art paper, cast coated paper, and the like, depending on the amount of paint applied and the coating method.

[0046] An example of an ink set using the inks described above will be described. The ink set can include inkjet inks and a pretreatment agent. The inks described above can be used as the inkjet inks. The pretreatment agent can be used as the pretreatment agent. [Example]

[0047] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples. In each table, "-" indicates no addition.

[0048] <Dispersants 1 to 10> Dispersants 1 to 10 were produced as follows. Table 1 shows the monomers and molar composition ratios used in the production of dispersants 1 to 10. Copolymers were synthesized using the monomers and molar composition ratios shown in Table 1. Methyl ethyl ketone / ethanol (70 / 30) was used as the solvent. After synthesis, the copolymers were neutralized with triethanolamine to a degree of neutralization of 80%.

[0049] The materials listed in Table 1 are as follows:

[0050] Monomer a1: ω-carboxy-polycaprolactone (n≒2) monoacrylate (manufactured by Toagosei Co., Ltd.) Monomer a2: methacrylic acid (Fujifilm Wako Pure Chemical Industries, Ltd.)

[0051] Monomer b1: hydroxyl-terminated polyalkylene glycol monomethacrylate (AO addition mole number: 8, molecular weight: 439, terminal: OH) (manufactured by NOF Corporation) Monomer b2: Polyalkylene glycol monomethacrylate (AO addition mole number: 9, molecular weight: 496, terminal: methoxy group) (manufactured by NOF Corporation) Monomer b3: Polyalkylene glycol monomethacrylate (AO addition mole number: 23, molecular weight: 1100, terminal: methoxy group) (manufactured by NOF Corporation) Monomer b4: Polyalkylene glycol monomethacrylate (AO addition mole number: 4, molecular weight: 276, terminal: methoxy group) (manufactured by NOF Corporation)

[0052] Monomer c1: benzyl methacrylate (benzyl methacrylate) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Monomer c2: 2-(acetoacetyloxy)ethyl methacrylate (manufactured by Taisei Kayaku Co., Ltd.)

[0053] [Table 1]

[0054] <Ink manufacturing> Inks were produced for Examples 1 to 11 and Comparative Examples 1 to 4. Table 2 shows the ink formulations for Examples 1 to 11 and Comparative Examples 1 to 4. The units for each material listed in Table 2 are parts by mass.

[0055] The materials listed in Table 2 are as follows:

[0056] Pigment 1: "MOGUL L" (product name) (manufactured by Cabot Corporation) Pigment 2: Copper phthalocyanine pigment Pigment 3: Quinacridone pigment Pigment 4: Condensed disazo pigment

[0057] Dispersants 1 to 10: Dispersants 1 to 10 prepared above Water-soluble organic solvent 1: Diethylene glycol Water-soluble organic solvent 2:1,2-hexanediol Surfactant 1: "Silface SAG002" (product name) (manufactured by Nissin Chemical Industry Co., Ltd.)

[0058] <Evaluation> The following evaluations were carried out on the inks of Examples 1 to 11 and Comparative Examples 1 to 4. The results are shown in Table 2.

[0059] 1.Storage stability The ink viscosity immediately after preparation was taken as the initial value, and the viscosity change rate after leaving the ink in a 70°C environment for 4 weeks was calculated using the following formula. The ink was stored in a sealed container. The ink viscosity was measured at 23°C using an Anton Paar MCR302 rheometer (cone angle 1°, diameter 50 mm). Viscosity change rate: [((viscosity after 4 weeks) - (initial viscosity)) / (initial viscosity)] x 100 (%) (Evaluation criteria) A: The absolute value of the viscosity change rate is more than 5% and less than 10%. B: The absolute value of the viscosity change rate exceeds 10%, or the ink has gelled and cannot be measured.

[0060] 2.Cohesiveness The ink coagulation was evaluated as an index of color development by spraying an aqueous solution containing calcium acetate onto white cotton, and then dropping the ink with a dropper. (Evaluation criteria) A: The ink clumps. B: There is evidence of ink clumping, but the ink is thin and spreads around the outline. C: There is no sign of ink clumping and the ink spreads.

[0061] [Table 2]

[0062] As shown in Table 2, all of the inks of Examples 1 to 11 showed excellent results in the evaluations of storage stability and aggregation resistance.

[0063] In Comparative Example 1, where methacrylic acid was used instead of ω-carboxy-polycaprolactone monoacrylate as the dispersant, the coagulation property decreased. This is presumably due to the short chain of the acid group. In Comparative Example 2, where methacrylic acid was used instead of ω-carboxy-polycaprolactone monoacrylate as the dispersant, the coagulation property was also low even though the acid value of the dispersant was high. In Comparative Example 3, ω-carboxy-polycaprolactone monoacrylate was not used as the dispersant, but methacrylic acid was used, but the chain of the hydrophilic group of the polyalkylene glycol monomethacrylate was shorter than in Comparative Examples 1 and 2. In Comparative Example 3, the coagulation property was slightly higher than in Comparative Examples 1 and 2, but lower than in Examples 1 to 10. Furthermore, storage stability was also reduced in Comparative Example 3. Comparative Example 4, which did not use polyalkylene glycol monomethacrylate, had low storage stability.

Claims

[Claim 1] comprising a pigment, a dispersant, and water; the dispersant is a copolymer of a monomer mixture containing ω-carboxy-polycaprolactone monoacrylate, polyalkylene glycol (meth)acrylate, and a vinyl-based monomer containing at least one selected from the group consisting of an aromatic group and a β-dicarbonyl group.

Images