Ink jet inks

Abstract

To provide an inkjet ink which suppresses the occurrence of nozzle clogging and is excellent in storage stability.SOLUTION: In the inkjet ink, the pigment is a quinacridone pigment. The specific resins are neutralization products of specific copolymers containing an α - methylstyrene-derived unit, a styrene-derived unit, a (meth) acrylic acid-derived unit, and a (di) alkylene glycol (meth) acrylate-derived unit in specific proportions, and the specific copolymers have acid values of 50mgKOH / g or more and 300mgKOH / g or less and number-average molecular weights of 3000 or more and 18000 or less. The neutralization ratio of the specific resin is from 20% to 100%. The specific crosslinking agent contains a polyfunctional epoxy compound and has a water solubility rate of 80% or more. The crosslinking rate of the crosslinked resin is 25% or more and 90% or less. The alkali metal ion concentration in the aqueous media is 500ppm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to inkjet inks. [Background technology]

[0002] Various improvements have been made to inkjet inks to maintain stable quality over a long period of time. For example, Patent Document 1 discloses a technique for keeping the total concentration of sodium ions and potassium ions contained in the ink at 500 ppm or less, or the total concentration of sulfate ions, chloride ions, and nitrate ions at 500 ppm or less. Patent Document 2 discloses a technique for keeping the amount of cation ions in the ink at less than 200 mM. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-355966 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-13216 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the ink compositions disclosed in Patent Documents 1 and 2 may not be able to sufficiently prevent nozzle clogging, particularly when the ink contains a quinacridone pigment, and may not provide sufficient storage stability.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an inkjet ink that suppresses the occurrence of nozzle clogging and has excellent storage stability. [Means for solving the problem]

[0006] An inkjet ink according to one aspect of the present invention contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles include a pigment and a crosslinked resin. The pigment is a quinacridone pigment. The crosslinked resin is a crosslinked product of a specific resin and a specific crosslinking agent. The specific resin is a neutralized product of a specific copolymer having a first repeating unit derived from α-methylstyrene, a second repeating unit derived from styrene, a third repeating unit derived from (meth)acrylic acid, and a fourth repeating unit derived from an alkylene glycol (meth)acrylate or a dialkylene glycol (meth)acrylate. The content ratios of the first repeating unit to the fourth repeating unit in the specific copolymer are, relative to 100% by mass of all repeating units, 1% to 65% by mass for the first repeating unit, 1% to 60% by mass for the second repeating unit, 10% to 40% by mass for the third repeating unit, and 1% to 12% by mass for the fourth repeating unit. The acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less. The number average molecular weight of the specific copolymer is 3,000 or more and 18,000 or less. The neutralization rate of the specific resin is 20% or more and 100% or less. The specific crosslinking agent contains a multifunctional epoxy compound having two or more epoxy groups and one or more hydroxyl groups in the molecule, and has a water solubility rate of 80% or more. The crosslinking rate of the crosslinked resin is 25% or more and 90% or less. The alkali metal ion concentration in the aqueous medium is 500 ppm or less. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an inkjet ink that suppresses the occurrence of nozzle clogging and has excellent storage stability. DETAILED DESCRIPTION OF THE INVENTION

[0008] The inkjet ink (hereinafter simply referred to as "ink") according to an embodiment of the present invention will be described below.

[0009] In the following description, acrylic and methacrylic may be collectively referred to as "(meth)acrylic." Furthermore, acrylate and methacrylate may be collectively referred to as "(meth)acrylate."

[0010] In addition, the compound name may be followed by "system" to refer to the compound and its derivatives in a comprehensive manner. When the compound name is followed by "system" to refer to the name of a polymer, it means that the repeating unit of the polymer is derived from the compound or its derivative.

[0011] In the present embodiment, the acid value is a value determined according to the method described in JIS (Japanese Industrial Standards) K0070:1992.

[0012] The measured value of the number average molecular weight (Mn) is a value measured using gel permeation chromatography (GPC).

[0013] The volume median diameter (D50) is a value measured using a dynamic light scattering particle size distribution analyzer ("Zetasizer nano-ZS" manufactured by Malvern Instruments).

[0014] The neutralization rate is expressed as the percentage of the amount of basic compound used, MB, relative to the theoretical amount, MA, where MA is the theoretical amount of basic compound required to completely neutralize the resin, and MB is the actual amount of basic compound used. (100 × MB / MA)

[0015] The crosslinking rate indicates the ratio of the number of functional groups forming a crosslinked structure in the crosslinked resin, when the total number of functional groups (groups capable of reacting with a crosslinking agent) contained in the copolymer, which is the raw material for the crosslinked resin, is taken as 100%. The specific crosslinking agent used in the ink of this embodiment reacts primarily with acid groups (particularly carboxyl groups). Therefore, the "total number of functional groups contained in the copolymer" corresponds to the total number of acid groups contained in the copolymer. The crosslinking rate indicates a value calculated by the method described in the Examples below or a method based thereon.

[0016] The viscosity of the ink refers to the viscosity at 25° C. The measured viscosity is a value measured in accordance with the method described in "JIS Z8803:2011 (Method for measuring viscosity of liquids)."

[0017] The epoxy equivalent is a value determined according to the method described in JIS (Japanese Industrial Standards) K7236:2009.

[0018] Unless otherwise specified, the evaluation results for powder (values ​​indicating shape or physical properties, etc.) are the number average of values ​​measured for a considerable number of particles.

[0019] <Inkjet ink> The ink of this embodiment contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles include a pigment and a crosslinked resin. The pigment is a quinacridone pigment. The crosslinked resin is a crosslinked product of a specific resin and a specific crosslinking agent.

[0020] The specific resin is a neutralized product of a specific copolymer having a first repeating unit derived from α-methylstyrene, a second repeating unit derived from styrene, a third repeating unit derived from (meth)acrylic acid, and a fourth repeating unit derived from alkylene glycol (meth)acrylate or dialkylene glycol (meth)acrylate.

[0021] The content ratios of the first repeating unit to the fourth repeating unit in the specific copolymer are, relative to 100% by mass of all repeating units, 1% by mass or more and 65% by mass or less for the first repeating unit, 1% by mass or more and 60% by mass or less for the second repeating unit, 10% by mass or more and 40% by mass or less for the third repeating unit, and 1% by mass or more and 12% by mass or less for the fourth repeating unit.

[0022] The acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less. The number average molecular weight (Mn) of the specific copolymer is 3,000 or more and 18,000 or less. The neutralization rate of the specific resin is 20% or more and 100% or less. The specific crosslinking agent contains a multifunctional epoxy compound having two or more epoxy groups and one or more hydroxyl groups in the molecule, and has a water solubility rate of 80% or more. The crosslinking rate of the crosslinked resin is 25% or more and 90% or less. The alkali metal ion concentration in the aqueous medium is 500 ppm or less.

[0023] The ink of this embodiment, having the above-described configuration, suppresses the occurrence of nozzle clogging and has excellent storage stability. The reasons why the ink of this embodiment exhibits these effects are as follows.

[0024] (Optimization of cross-linked resin) In general, imparting high dispersion stability to pigment particles is effective in suppressing nozzle clogging. As described above, pigment particles contain a pigment and a crosslinked resin, but the pigment alone has low dispersibility in an aqueous medium. Therefore, in order to impart high dispersion stability to the pigment particles, it is necessary to improve the dispersion stability of the crosslinked resin.

[0025] The present inventors have conducted extensive research to improve the dispersion stability of the crosslinked resin, and as a result have found that by setting the content ratios of the first to fourth repeating units in the specific copolymer, the acid value of the specific copolymer, the number average molecular weight (Mn), the neutralization rate of the specific resin, and the crosslinking rate of the crosslinked resin to values ​​within the above-mentioned ranges, and further by providing the crosslinked resin with a crosslinked structure derived from the specific crosslinking agent, it is possible to achieve excellent dispersion stability of the pigment particles in an aqueous medium and suppress the occurrence of nozzle clogging.

[0026] Specifically, each repeating unit in the specific copolymer has the following functions: the first repeating unit moderately increases the affinity between the crosslinked resin and the pigment particles; the second repeating unit imparts moderate hydrophobicity to the crosslinked resin, thereby adjusting the dispersion stability of the pigment particles; the third repeating unit and the fourth repeating unit impart moderate hydrophilicity to the crosslinked resin, thereby imparting dispersion stability to the pigment particles. In the ink of this embodiment, the content ratio of each repeating unit in the specific copolymer is optimized so that the crosslinked resin exhibits excellent dispersion stability.

[0027] Furthermore, the specific resin has a relatively high acid value of the specific copolymer used as the raw material, and has a relatively large number of acid groups. The acid groups of the specific resin are moderately neutralized. The acid groups (especially neutralized acid groups) are highly hydrophilic. Therefore, the specific resin has moderately high hydrophilicity. Furthermore, the specific crosslinking agent has hydroxy groups and a high water solubility. Therefore, the specific crosslinking agent also has moderately high hydrophilicity. The crosslinked resin is a crosslinked product of a specific resin and a specific crosslinking agent, each of which has moderately high hydrophilicity, and therefore has moderately high hydrophilicity. As a result, the crosslinked resin has excellent dispersion stability in an aqueous medium.

[0028] The specific crosslinking agent contains epoxy groups, which are highly reactive crosslinkable groups, and efficiently forms an appropriate amount of crosslinked structure with the specific resin. Furthermore, the crosslinked resin can efficiently coat pigment particles because the molecular size of the specific copolymer used as the raw material is appropriate. This prevents the crosslinked resin from detaching from the pigment particles, thereby preventing aggregation of the exposed pigment. As a result, the pigment particles have excellent dispersion stability in aqueous media.

[0029] (Optimization of alkali metal ion concentration) It is generally known that when the amount of ions in an ink is large, the ions neutralize the surface charge of the pigment particles, reducing the dispersion stability of the pigment particles in the aqueous medium and making them more susceptible to aggregation. As a result of various studies, the present inventors have found that aqueous inks containing quinacridone pigments are particularly susceptible to the effects of the amount of ions.

[0030] As a result of extensive research into how to prevent a decrease in the dispersion stability of pigment particles, the inventors of the present application have found that by setting the alkali metal ion concentration in the aqueous medium to 500 ppm or less, the ejection stability and storage stability of an aqueous ink containing a quinacridone pigment are significantly improved.

[0031] The application of the ink of this embodiment is not particularly limited, but it can be used, for example, to form an image on a permeable recording medium or a non-permeable recording medium. The ink of this embodiment is suitable for forming an image on a permeable recording medium. Permeable recording media have excellent ink permeability. Examples of permeable recording media include printing paper and media made from fibers (e.g., fabric). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, cardboard, and glossy paper.

[0032] Each component constituting the ink of this embodiment will be described in more detail below. Note that each component described below may be used alone or in combination of two or more.

[0033] [Aqueous medium] The aqueous medium is a medium containing water. The aqueous medium may function as a solvent or as a dispersion medium. Specific examples of the aqueous medium include an aqueous medium containing water and a water-soluble organic solvent.

[0034] (water) In the ink of this embodiment, the water content is preferably 25.0% by mass or more and 80.0% by mass or less, and more preferably 35.0% by mass or more and 60.0% by mass or less.

[0035] (Water-soluble organic solvent) Examples of the water-soluble organic solvent include glycol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, and dimethyl sulfoxide.

[0036] Examples of glycol compounds include ethylene glycol, 1,3-propanediol, propylene glycol, 1,2-pentanediol, 1,5-pentanediol, 1,2-octanediol, 1,8-octanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, and 2-ethyl-1,2-hexanediol. Of the glycol compounds, ethylene glycol, diethylene glycol, 2-ethyl-1,2-hexanediol, 3-methyl-1,5-pentanediol, 1,3-propanediol, 1,5-pentanediol, and propylene glycol are particularly preferred.

[0037] Examples of triol compounds include glycerin and 1,2,3-butanetriol.

[0038] Examples of glycol ether compounds include diethyl diglycol, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether (diethyl diglycol), triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether. Of the glycol ether compounds, triethylene glycol monobutyl ether is particularly preferred.

[0039] Examples of the lactam compound include 2-pyrrolidone and N-methyl-2-pyrrolidone, with 2-pyrrolidone being particularly preferred.

[0040] Examples of the nitrogen-containing compound include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0041] An example of the acetate compound is diethylene glycol monoethyl ether acetate.

[0042] The water-soluble organic solvent is preferably a glycol compound or a glycol ether compound, more preferably ethylene glycol or diethyldiglycol.

[0043] The content of the water-soluble organic solvent in the ink of this embodiment is preferably 10.0% by mass or more and 50.0% by mass or less, and more preferably 30.0% by mass or more and 40.0% by mass or less.

[0044] The content of the glycol compound in the ink of this embodiment is preferably 5.0% by mass or more and 45.0% by mass or less, and more preferably 15.0% by mass or more and 25.0% by mass or less.

[0045] The content of the glycol ether compound in the ink of this embodiment is preferably 5.0% by mass or more and 30.0% by mass or less, and more preferably 10.0% by mass or more and 20.0% by mass or less.

[0046] [Pigment particles] As described above, the pigment particles contain a pigment and a crosslinked resin. The pigment particles are composed of, for example, a core containing the pigment and a crosslinked resin coating the core. The total content of the pigment and the crosslinked resin in the pigment particles is preferably 90% by mass or more, and more preferably 100% by mass.

[0047] From the viewpoint of optimizing the color density, hue, or stability of the ink of this embodiment, the volume median diameter of the pigment particles is preferably 30 nm or more and 200 nm or less, and more preferably 80 nm or more and 130 nm or less.

[0048] In the ink of this embodiment, the pigment particle content is preferably 5.0% by mass or more and 20.0% by mass or less, and more preferably 10.0% by mass or more and 15.0% by mass or less. By setting the pigment particle content to 5.0% by mass or more, it becomes easier to form an image with the desired image density using the ink of this embodiment. Furthermore, by setting the pigment particle content to 20.0% by mass or less, the fluidity of the ink can be optimized.

[0049] (pigment) As described above, the pigment is a quinacridone pigment. Examples of the quinacridone pigment include CI Pigment Violet (19 or 42), CI Pigment Red (122, 202, 206, 207, or 209), and CI Pigment Orange (48 or 49).

[0050] Examples of commercially available quinacridone pigments that can be used include "TRM-11" manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd., "Cinquasia (registered trademark) Magenta D4550" manufactured by BASF, "Cinquasia (registered trademark) Pink D4450" manufactured by BASF, "Inkjet Magenta ES" manufactured by Clariant, "HOSTAPERM PINK E 02" manufactured by Clariant, "HOSTAPERM RED E3B" manufactured by Clariant, and "HOSTAPERM RED E5B 02" manufactured by Clariant.

[0051] In the ink of this embodiment, the content of the quinacridone pigment is preferably 3.0 parts by mass or more and 20.0 parts by mass or less, and more preferably 7.0 parts by mass or more and 12.0 parts by mass or less.

[0052] The content of the quinacridone pigment in the pigment particles is preferably 50% by mass or more and 90% by mass or less, and more preferably 70% by mass or more and 80% by mass or less.

[0053] (Crosslinked resin) As described above, the crosslinked resin is a crosslinked product of a specific resin and a specific crosslinking agent. For example, the crosslinked resin coats the pigment in pigment particles. The crosslinked resin has a crosslinking rate of 25% or more and 90% or less, preferably 35% or more and 65% or less, and more preferably 35% or more and 45% or less. By setting the crosslinking rate of the crosslinked resin to 25% or more, it is possible to suppress detachment of the crosslinked resin from the pigment particles. By setting the crosslinking rate of the crosslinked resin to 90% or less, it is possible to optimize the dispersion stability of the pigment particles.

[0054] In the ink of this embodiment, the content of the crosslinked resin is preferably 0.5% by mass or more and 10.0% by mass or less, and more preferably 2.0% by mass or more and 4.5% by mass or less. By setting the content of the crosslinked resin to 0.5% by mass or more, the dispersion stability of the pigment particles can be further optimized. By setting the content of the crosslinked resin to 10.0% by mass or more, the generation of free resin can be suppressed.

[0055] In the pigment particles, the content of the crosslinked resin relative to 100 parts by mass of the pigment is preferably 25 parts by mass or more and 60 parts by mass or less, and more preferably 30 parts by mass or more and 45 parts by mass or less. By setting the content of the crosslinked resin relative to 100 parts by mass of the pigment to 25 parts by mass or more and 60 parts by mass or less, the dispersion stability of the pigment particles can be further optimized.

[0056] (Specific resin) As described above, the specific resin is a neutralized product of a specific copolymer. The neutralization rate of the specific resin is 20% or more and 100% or less, preferably 40% or more and 95% or less, and more preferably 50% or more and 70% or less. By setting the neutralization rate of the specific resin to 20% or more and 100% or less, it is possible to impart moderately high hydrophilicity to the crosslinked resin and optimize the dispersion stability of the pigment particles.

[0057] The specific resin preferably contains an alkali metal atom. That is, the specific resin is preferably a neutralized product obtained by neutralizing a specific copolymer with a neutralizing agent containing an alkali metal atom. The alkali metal atom does not volatilize even when the ink of this embodiment is exposed to a dry state. Therefore, by neutralizing the specific copolymer with a neutralizing agent containing an alkali metal atom, the neutralized state (hydrophilicity) of the crosslinked resin is maintained even when the ink of this embodiment is exposed to a dry state. The alkali metal atom is preferably a potassium atom or a sodium atom. The neutralizing agent is preferably a hydroxide containing an alkali metal atom, and NaOH or KOH is more preferred.

[0058] (Specific copolymer) The specific copolymer has a first repeating unit derived from α-methylstyrene, a second repeating unit derived from styrene, a third repeating unit derived from (meth)acrylic acid, and a fourth repeating unit derived from alkylene glycol (meth)acrylate or dialkylene glycol (meth)acrylate.

[0059] Examples of alkylene glycol (meth)acrylates include ethylene glycol (meth)acrylate, propylene glycol (meth)acrylate, and butylene glycol (meth)acrylate. Of the alkylene glycol (meth)acrylates, ethylene glycol (meth)acrylate is particularly preferred.

[0060] Examples of dialkylene glycol (meth)acrylates include diethylene glycol (meth)acrylate, dipropylene glycol (meth)acrylate, and dibutylene glycol (meth)acrylate. Of the dialkylene glycol (meth)acrylates, dipropylene glycol (meth)acrylate is particularly preferred.

[0061] In the specific copolymer, the content of the first repeating unit relative to 100% by mass of all repeating units is from 1% to 65% by mass, preferably from 15% to 55% by mass, more preferably from 25% to 50% by mass, and even more preferably from 40% to 48% by mass. By setting the content of the first repeating unit to be from 1% to 65% by mass, the affinity between the crosslinked resin and the pigment particles can be appropriately increased, and the dispersion stability of the pigment particles can be further optimized.

[0062] In the specific copolymer, the content of the second repeating unit relative to 100% by mass of all repeating units is from 1% to 60% by mass, preferably from 10% to 40% by mass, and more preferably from 15% to 27% by mass. By setting the content of the second repeating unit to from 1% to 60% by mass, it is possible to impart appropriate hydrophobicity to the crosslinked resin and further optimize the dispersion stability of the pigment particles.

[0063] In the specific copolymer, the content of the third repeating unit relative to 100% by mass of all repeating units is 10% to 40% by mass, preferably 20% to 35% by mass, and more preferably 25% to 32% by mass. By setting the content of the third repeating unit to 10% to 40% by mass, it is possible to impart appropriate hydrophilicity to the crosslinked resin and further optimize the dispersion stability of the pigment particles.

[0064] In the specific copolymer, the content of the fourth repeating unit relative to 100% by mass of all repeating units is from 1% to 12% by mass, preferably from 4% to 11% by mass, and more preferably from 4% to 8% by mass. By setting the content of the fourth repeating unit to be from 1% to 12% by mass, it is possible to impart appropriate hydrophilicity to the crosslinked resin and further optimize the dispersion stability of the pigment particles.

[0065] It is preferable that the content of the first repeating unit is 15% by mass or more and 55% by mass or less, the content of the second repeating unit is 10% by mass or more and 40% by mass or less, the content of the third repeating unit is 20% by mass or more and 35% by mass or less, and the content of the fourth repeating unit is 4% by mass or more and 11% by mass or less, relative to 100% by mass of all repeating units in the specific copolymer.

[0066] The specific copolymer may be a random polymer or a block polymer, and it is particularly preferred that the specific copolymer is a random polymer.

[0067] As described above, the acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less, preferably 100 mgKOH / g or more and 200 mgKOH / g or less, and more preferably 150 mgKOH / g or more and 200 mgKOH / g or less. By setting the acid value of the specific copolymer to 50 mgKOH / g or more and 300 mgKOH / g or less, it becomes easier to sufficiently introduce a crosslinked structure into the crosslinked resin and the dispersion stability of the pigment particles can be optimized. By setting the acid value of the specific copolymer to 300 mgKOH / g or less, it is possible to prevent the dispersion stability of the pigment particles from becoming excessively high. As a result, the desired image density can be imparted to images formed using the ink of this embodiment.

[0068] As described above, the number average molecular weight (Mn) of the specific copolymer is from 3,000 to 18,000, more preferably from 5,000 to 12,000, and even more preferably from 8,000 to 10,000. By setting the number average molecular weight (Mn) of the specific copolymer to from 3,000 to 18,000, the crosslinked resin can efficiently coat the pigment particles, thereby preventing the crosslinked resin from being detached from the pigment particles.

[0069] The composition of the monomers used as raw materials for the specific copolymer is preferably any one of compositions 1 to 3 shown in Table 1 below. In Table 1, the numerical values ​​indicate the range of the preferred content ratio [mass %]. For example, "18-22" for styrene in composition 1 indicates that the styrene content is 18 mass % or more and 22 mass % or less.

[0070] [Table 1]

[0071] (Specific cross-linking agent) As described above, the specific crosslinking agent contains a polyfunctional epoxy compound (X) having two or more epoxy groups and one or more hydroxyl groups in the molecule. The specific crosslinking agent has two or more epoxy groups, which are crosslinkable groups with excellent reactivity, and can efficiently form a crosslinked structure with the specific resin. The specific crosslinking agent has one or more hydroxyl groups, and can maintain a moderately high hydrophilicity of the crosslinked resin.

[0072] The water solubility of the specific crosslinking agent is 80% or more, preferably 90% or more, and more preferably 98% or more. By making the water solubility of the specific crosslinking agent 80% or more, the dispersion stability of the pigment particles can be optimized.

[0073] The water solubility of a specific crosslinking agent is the ratio (100 × A / 10 g) of the mass A of the specific crosslinking agent dissolved in water to the total amount (10 g) of the specific crosslinking agent when 10 g of the specific crosslinking agent is mixed with 90 g of water at 25° C. For example, if 9 g of the specific crosslinking agent dissolves in water and 1 g of the specific crosslinking agent precipitates without dissolving in water when 10 g of the specific crosslinking agent is mixed with 90 g of water at 25° C. (mass A: 9 g), the water solubility is 90%.

[0074] In the polyfunctional epoxy compound (X), the number of epoxy groups in the molecule is preferably 2 or more and 8 or less, and more preferably 2 or more and 5 or less. In the polyfunctional epoxy compound (X), the number of hydroxy groups in the molecule is preferably 1 or more and 5 or less, and more preferably 1 or more and 3 or less.

[0075] The polyfunctional epoxy compound (X) is preferably glycerol polyglycidyl ether, polyglycerol polyglycidyl ether (particularly, diglycerol polyglycidyl ether or triglycerol polyglycidyl ether), or sorbitol polyglycidyl ether, and more preferably glycerol polyglycidyl ether, diglycerol polyglycidyl ether, or triglycerol polyglycidyl ether.

[0076] The specific crosslinking agent may further contain another polyfunctional epoxy compound (Y) in addition to the polyfunctional epoxy compound (X). The other polyfunctional epoxy compound (Y) is preferably a compound represented by the following chemical formula (2) (glycerin triglycidyl ether). In the specific crosslinking agent, the total content of the polyfunctional epoxy compound (X) and glycerin triglycidyl ether is preferably 80% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass.

[0077] Examples of the polyfunctional epoxy compound (X) include compounds represented by the following chemical formula (1), (3), (4), or (5). The specific crosslinking agent is preferably a mixture of a compound represented by the following chemical formula (1) and a compound represented by the following chemical formula (2), a compound represented by the following chemical formula (3), a compound represented by the following chemical formula (4), or a compound represented by the following chemical formula (5). The compounds represented by the chemical formulas (6) and (7) are examples of epoxy compounds that are not polyfunctional epoxy compounds (X).

[0078] [ka]

[0079] The epoxy equivalent of the specific crosslinking agent is preferably 100 g / eq. or more and 250 g / eq. or less, more preferably 130 g / eq. or more and 200 g / eq. or less, and even more preferably 130 g / eq. or more and 170 g / eq. or less.

[0080] [Alkali metals] As described above, in the ink of this embodiment, the concentration of alkali metal ions in the aqueous medium is 500 ppm or less, and more preferably 100 ppm or less.

[0081] The alkali metals include six elements: lithium, sodium, potassium, rubidium, cesium, and francium. As the alkali metal, sodium or potassium is particularly preferred.

[0082] [Surfactants] The ink of this embodiment preferably further contains a surfactant. The surfactant can optimize the permeability (wettability) of the ink of this embodiment to the recording medium. Examples of the surfactant include anionic surfactants, cationic surfactants, and nonionic surfactants. Nonionic surfactants are particularly preferred as the surfactant.

[0083] Examples of nonionic surfactants include polyoxyethylene dodecyl ether, polyoxyethylene hexadecyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide adducts of acetylene glycol, with the ethylene oxide adducts of acetylene glycol being preferred.

[0084] The content of the surfactant in the ink of this embodiment is preferably 0.05% by mass or more and 3.0% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

[0085] [Other ingredients] The ink of this embodiment may further contain known additives (for example, at least one of a dissolution stabilizer, an anti-drying agent, an antioxidant, a viscosity adjuster, a pH adjuster, and an anti-mold agent) as needed.

[0086] [Solid content in the supernatant] In the ink of this embodiment, some of the crosslinked resin may detach from the pigment particles and become free in the aqueous medium during production or storage of the ink. Hereinafter, the crosslinked resin that is free in the aqueous medium in this way will be referred to as free resin. Free resin has the function of improving the scratch resistance of images formed with the ink of this embodiment, but if the amount is large, it may cause nozzle clogging. Therefore, in the ink of this embodiment, it is preferable that the content of free resin is low. However, free resin is inevitably generated in the ink of this embodiment, and it is difficult to completely remove it.

[0087] In this embodiment, the solid content contained in the supernatant obtained by centrifuging the ink of this embodiment at, for example, 1,050,000 G for 3 hours is considered to be free resin. In the ink of this embodiment, the solid content in the supernatant obtained by centrifugation is preferably 2.0% by mass or less, more preferably 0.1% by mass to 1.5% by mass or less, and even more preferably 0.5% by mass to 1.0% by mass or less. By keeping the solid content in the above-mentioned supernatant to 2.0% by mass or less, the occurrence of nozzle clogging due to the ink of this embodiment can be more effectively suppressed. The above-mentioned solid content can be measured by the method described in the Examples or a method based thereon.

[0088] <Ink manufacturing method> Next, an example of a method for producing ink according to this embodiment will be described. The method for producing ink according to this embodiment includes, for example, a neutralization step of neutralizing a specific copolymer to prepare a specific resin, a dispersion step of dispersing a pigment and a specific resin in water to prepare a pigment particle dispersion, a crosslinking step of adding a specific crosslinking agent to the pigment particle dispersion to produce a crosslinked resin, an ion removal step of removing alkali metal ions from the pigment particle dispersion after the crosslinking step (or after the centrifugation step described below), and an addition step of adding an aqueous medium to the pigment particle dispersion after the ion removal step to prepare ink. The method for producing ink according to this embodiment preferably further includes a centrifugation step of centrifuging the pigment particle dispersion after the crosslinking step and replacing the resulting supernatant with an aqueous medium.

[0089] (neutralization process) In the neutralization step, the specific copolymer is neutralized to prepare the specific resin. For example, a method of neutralizing the specific copolymer includes mixing the specific copolymer with an aqueous solution containing a basic compound (e.g., KOH or NaOH). In the neutralization step, it is preferable to partially neutralize the specific copolymer rather than completely neutralize it. Specifically, the neutralization rate of the specific resin is 20% or more and 100% or less, and preferably 30% or more and 60% or less.

[0090] (Dispersion process) In the dispersion step, a pigment particle dispersion is prepared by dispersing the pigment and the specific resin in water. Examples of the dispersion device used in the dispersion step include a wet dispersion device such as a media-type disperser.

[0091] In the dispersion step, the content of the specific resin in the solution subjected to dispersion treatment is, for example, 4.0% by mass or more and 25.0% by mass or less. The content of the pigment in the solution subjected to dispersion treatment is, for example, 1.0% by mass or more and 20.0% by mass or less. The solution subjected to dispersion treatment may contain an antifoaming agent. The content of the antifoaming agent in the solution subjected to dispersion treatment is, for example, 0.01% by mass or more and 0.1% by mass or less. In the dispersion step, it is preferable to remove foreign matter and coarse particles from the obtained pigment particle dispersion by filtration using a filter (e.g., pore size 5 μm).

[0092] (Crosslinking process) In the crosslinking step, a specific crosslinking agent is added to the pigment particle dispersion. This causes a reaction between the specific resin contained in the pigment particle dispersion and the specific crosslinking agent, crosslinking the specific resin. As a result, a crosslinked resin is produced as a reaction product of the specific resin and the specific crosslinking agent. In the crosslinking step, after the specific crosslinking agent is added, the pigment particle dispersion is preferably heated while being stirred. The heating temperature is preferably, for example, 50°C or higher and 95°C or lower. The heating time is preferably, for example, 30 minutes or higher and 8 hours or lower.

[0093] (Centrifugation process) In the centrifugation step, the pigment particle dispersion after the crosslinking treatment is centrifuged, and the resulting supernatant liquid is replaced with an aqueous medium. This allows the removal of free components contained in the aqueous medium of the pigment particle dispersion after the crosslinking treatment. The conditions for the centrifugation treatment are, for example, preferably a rotation speed of 10,000 rpm or more and 100,000 rpm or less. Furthermore, the centrifugation time is preferably 12 hours or more and 48 hours or less.

[0094] (Ion removal process) In the ion removal step, alkali metal ions are removed from the pigment particle dispersion after the crosslinking treatment (or from the pigment particle dispersion after the centrifugation treatment, if a centrifugation treatment has been performed), thereby adjusting the alkali metal ion concentration in the aqueous medium. Examples of methods for removing alkali metal ions include ion chromatography and ultrafiltration.

[0095] (addition process) In the addition step, an aqueous medium is added to the pigment particle dispersion liquid after the ion removal treatment to prepare an ink. In the addition step, other components (more specifically, at least one of a surfactant, a dissolution stabilizer, a humectant, a penetrating agent, and a viscosity modifier) ​​may be added as needed. In the addition step, after the aqueous medium is added, the resulting mixture is preferably stirred with a stirrer. Furthermore, it is preferable to remove foreign matter and coarse particles from the resulting ink using a filter (for example, a filter with a pore size of 5 μm or less).

[0096] The effects of the ink of this embodiment will be described below based on examples, although the present invention is not limited to the following examples.

[0097] [Preparation of Resin (R-1)] A four-neck flask equipped with a stirrer, nitrogen inlet tube, condenser, and dropping funnel was charged with 100.0 parts by weight of isopropyl alcohol and 250.0 parts by weight of methyl ethyl ketone. Separately, a monomer solution was prepared by mixing 20.0 parts by weight of styrene, 40.0 parts by weight of α-methylstyrene, 5.0 parts by weight of ethylene glycol acrylate, 25.0 parts by weight of methacrylic acid, and 0.3 parts by weight of azobisisobutyronitrile (AIBN, polymerization initiator). A methyl ethyl ketone solution was also prepared by mixing 150.0 parts by weight of methyl ethyl ketone and 0.1 parts by weight of AIBN.

[0098] Next, nitrogen gas was introduced into the four-neck flask to create a nitrogen atmosphere. Next, while the contents of the four-neck flask were heated to reflux at 70°C, the entire amount of the monomer solution was supplied to the four-neck flask using a dropping funnel over a period of 2 hours. After the monomer solution was supplied, the contents of the four-neck flask were heated to reflux at 70°C for an additional 6 hours. Next, while the contents of the four-neck flask were heated to reflux at 70°C, the entire amount of the methyl ethyl ketone solution was supplied to the four-neck flask using a dropping funnel over a period of 15 minutes. After the methyl ethyl ketone solution was supplied, the contents of the four-neck flask were heated to reflux at 70°C for an additional 5 hours. This resulted in an aqueous resin solution containing the copolymer resin (R-1). Next, the methyl ethyl ketone and isopropyl alcohol were distilled off from the aqueous resin solution to isolate the resin (R-1).

[0099] [Preparation of Resins (R-2) to (R-19)] Resins (R-2) to (R-19) were prepared in the same manner as for Resin (R-1), except that the types and amounts of monomers used in preparing the monomer solutions were changed as shown in Table 2 below. In Table 2 below, "EG acrylate" refers to ethylene glycol acrylate, and "DPGA" refers to dipropylene glycol acrylate.

[0100] [Acid value measurement] The acid values ​​of the synthesized resins (Resins (R-1) to (R-19)) were measured according to the method described in JIS (Japanese Industrial Standards) K0070:1992 (Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value, and unsaponifiable matter of chemical products). The measurement results are shown in Table 2 below.

[0101] [Measurement of number average molecular weight (Mn)] The number average molecular weight (Mn) of each of the synthesized resins (Resins (R-1) to (R-19)) was measured using gel permeation chromatography (GPC) under the following conditions. The measurement results are shown in Table 2 below.

[0102] (GPC conditions) Measuring device: Tosoh Corporation "HLC-8020GPC" Column: Ultra-high performance semi-micro SEC column ("TSKgel SuperMultipore HZ-H" manufactured by Tosoh Corporation, packing material: styrene-divinylbenzene resin, column size: inner diameter 4.6 mm x length 15 cm, packing particle size: 6 μm) Number of columns: 3 Eluent: tetrahydrofuran Eluent flow rate: 0.35mL / min Sample solution volume: 10 μL Column temperature: 40℃ Detector: RI (refractive index) detector Calibration curve: A calibration curve prepared using monodisperse polystyrene standard samples (F-40, F-20, F-4, F-1, A-5000, A-2500, and A-1000) manufactured by Tosoh Corporation and n-propylbenzene.

[0103] [Preparation of Resin Aqueous Solutions (r-1) to (r-19)] (Neutralization treatment) Resins (R-1) to (R-19) shown in Table 2 below (each of Resins (R-1) to (R-19)), potassium hydroxide, and water were mixed to prepare aqueous resin solutions (r-1) to (r-19). Neutralized resins were contained in the aqueous resin solutions (r-1) to (r-19). The amount of potassium hydroxide added was such that the neutralization rate of the neutralized resin was as shown in Table 2 below. The amount of water added was such that the solids concentration (neutralized resin content) of the aqueous resin solutions (r-1) to (r-19) was 30% by mass.

[0104] [Table 2]

[0105] Table 3 below shows the content (mass %) of each of the first to fourth repeating units in resins (R-1) to (R-19).

[0106] [Table 3]

[0107] As shown in Tables 2 and 3, resins (R-1) to (R-8) were copolymers having a first repeat unit content of 1% to 65% by mass, a second repeat unit content of 1% to 60% by mass, a third repeat unit content of 10% to 40% by mass, and a fourth repeat unit content of 1% to 12% by mass, based on 100% by mass of all repeat units, an acid value of 50 mg KOH / g to 300 mg KOH / g, and a number average molecular weight (Mn) of 3,000 to 18,000. Furthermore, the neutralization ratios of the neutralized resins (R-1) to (R-8) were 20% to 100%. In other words, the neutralized resins (R-1) to (R-8) were specific resins.

[0108] On the other hand, resins (R-9) to (R-14) were copolymers in which the content ratio of each repeating unit did not fall within the above-mentioned range. Resins (R-15) and (R-16) were copolymers in which the content ratio of each repeating unit and the acid value did not fall within the above-mentioned range. The neutralization ratio of neutralized resin (R-17) was a value outside the above-mentioned range. Resins (R-18) and (R-19) were copolymers in which the number average molecular weight (Mn) did not fall within the above-mentioned range. In other words, neutralized resins (R-9) to (R-19) were resins that were not specific resins.

[0109] <Ink Preparation> The inks of Examples 1 to 11 and Comparative Examples 1 to 28 were prepared by the following method. The manufacturers and trade names of the crosslinking agents used in the crosslinking treatment are shown below. The chemical names, chemical formulas, epoxy equivalents, and water solubility of the crosslinking agents are shown in Table 4 below. In Table 4 below, the chemical formula "(1)+(2)" of the crosslinking agent (EX-313) indicates that it contains a compound represented by chemical formula (1) and a compound represented by chemical formula (2).

[0110] Crosslinking agent (EX-313): "Denacol (registered trademark) EX-313" manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-512): Denacol (registered trademark) EX-512 manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-521): "Denacol (registered trademark) EX-521" manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-614B): "Denacol (registered trademark) EX-614B" manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-612): "Denacol (registered trademark) EX-612" manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-810): "Denacol (registered trademark) EX-810" manufactured by Nagase ChemteX Corporation Crosslinking agent (EX-145): "Denacol (registered trademark) EX-145" manufactured by Nagase ChemteX Corporation

[0111] [Table 4]

[0112] Among the crosslinking agents, crosslinking agents (EX-313), (EX-512), (EX-521), and (EX-614B) were specific crosslinking agents that were polyfunctional epoxy compounds having two or more epoxy groups and one or more hydroxyl groups in the molecule and had a water solubility of 80% or more.

[0113] [Preparation of Ink in Example 1] (Distributed processing) A mixture was obtained by mixing 15.0 parts by mass of a quinacridone pigment (BASF's "Cinquasia® Magenta D4550"), 15.0 parts by mass of a resin aqueous solution (r-1) (containing 4.5 parts by mass of neutralized resin (R-1)), 0.1 parts by mass of a defoamer (San Nopco Ltd.'s "SN Deformer 1340", an amide wax surfactant), and ion-exchanged water. The amount of ion-exchanged water added was such that the mixture would become 100 parts by mass.

[0114] The resulting mixture was dispersed for 4 hours using a media-type disperser (Dynomill, manufactured by Willy & Bachofen GmbH (WAB)). Zirconia beads with a diameter of 0.5 mm were used as the media for the dispersion process. The media filling rate was 60% by volume relative to the vessel capacity. The processing temperature (chiller temperature) was set to 10°C. After the dispersion process, the media was removed from the contents of the media-type disperser to obtain a pigment particle dispersion. The pigment particle dispersion was then filtered through a filter with a pore size of 5 μm to remove foreign matter and coarse particles.

[0115] (Crosslinking treatment) A three-neck flask equipped with a thermometer and a stirring blade was used as a reaction vessel. 100.0 parts by mass of the filtered pigment particle dispersion was added to the reaction vessel. The temperature of the contents of the reaction vessel was maintained at 30°C using a water bath. Next, 0.82 parts by mass of crosslinking agent (EX-313) was added to the reaction vessel. Next, the contents of the reaction vessel were stirred at 250 rpm for 1 hour. Next, while stirring the contents of the reaction vessel at 250 rpm, the temperature of the contents of the reaction vessel was increased to 80°C at a rate of 0.5°C / min. Next, while maintaining the temperature of the contents of the reaction vessel at 80°C, the contents of the reaction vessel were stirred at 250 rpm for 4 hours. As a result, the neutralized resin (R-1) was crosslinked with the crosslinking agent (EX-313), producing a crosslinked resin. Next, the contents of the reaction vessel were allowed to cool to room temperature. This resulted in a crosslinked pigment particle dispersion.

[0116] (Centrifugation) The pigment particle dispersion after crosslinking treatment was transferred to a container, and this container was placed in a centrifugal adhesion measuring device ("NS-C100" manufactured by Nano Seeds Co., Ltd.). Using the centrifugal adhesion measuring device described above, the pigment particle dispersion after crosslinking treatment was centrifuged at a rotation speed of 50,000 rpm for 24 hours. After centrifugation, the supernatant was removed from the container, and an equal volume of ion-exchanged water as the removed supernatant was added to the container. In this way, free components were removed from the aqueous medium of the pigment particle dispersion after crosslinking treatment.

[0117] (Ion removal treatment) 100 g of the pigment particle dispersion after the centrifugal separation was subjected to circulating ultrafiltration using an ultrafiltration membrane (UF pencil type module AIP-0013D manufactured by Asahi Kasei Corporation). Ultrafiltration was continued until the pigment concentration reached 25% by mass, and the same amount of ion-exchanged water as the filtrate was added to the pigment particle dispersion after the filtration. At this time, 25% of the liquid volume of the pigment particle dispersion had been replaced with ion-exchanged water, and for convenience, this state is referred to as a replacement rate of 25%.

[0118] (Additive treatment) A container was charged with 60.0 parts by mass of the pigment particle dispersion after ion removal treatment (approximately 9 parts by mass of pigment, approximately 3 parts by mass of resin (R-1)), 20.0 parts by mass of ethylene glycol, 15.0 parts by mass of diethyldiglycol, 0.3 parts by mass of a nonionic surfactant ("Olfine (registered trademark) E1004" manufactured by Nissin Chemical Industry Co., Ltd.), and 4.7 parts by mass of ion-exchanged water. The contents of the container were stirred at a rotation speed of 400 rpm using a mixer ("Three-One Motor BL-600" manufactured by Shinto Scientific Co., Ltd.) to obtain a mixed solution. The resulting mixed solution was filtered using a filter (pore size: 5 μm). This yielded the ink of Example 1.

[0119] [Preparation of Inks in Examples 2 to 11 and Comparative Examples 1 to 28] Inks of Examples 2 to 11 and Comparative Examples 1 to 28 were prepared in the same manner as in Example 1, except that the type of pigment, the type of resin dispersion (resin), and the type and amount of crosslinker used were changed as shown in Table 5 below. In Table 5 below, "ES" represents "Inkjet Magenta ES" manufactured by Clariant. "E3B" represents "HOSTAPERM RED E3B" manufactured by Clariant. "D4550," "ES," and "E3B" are all quinacridone pigments.

[0120] [Calculation of cross-linking rate] The crosslinking rate of the crosslinked resin for each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28 was calculated using the following method. For ease of calculation, 1 part by mass was assumed to equal 1 g. First, the number of moles of crosslinking functional groups (epoxy groups) contained in the crosslinking agent was calculated by dividing the amount (g) of crosslinking agent used by the epoxy equivalent (g / eq.) of the crosslinking agent. Next, the number of moles of carboxy groups per gram of the resin after neutralization was calculated by dividing the acid value of each of the resins (Resins (R-1) to (R-19)) by the molecular weight (56.1) of potassium hydroxide (KOH). The number of moles of carboxy groups contained in the resin after neutralization, Y, was calculated by multiplying the amount of resin used after neutralization in the crosslinking treatment by the number of moles of carboxy groups per gram of the resin after neutralization.

[0121] The crosslinking rate of the crosslinked resin was calculated using the calculated number of moles of crosslinkable functional groups X and number of moles of carboxyl groups Y according to the following formula (1). The calculation results are shown in Table 5 below.

[0122] [Number 1] Crosslinking rate (%) = 100 × number of moles of crosslinking functional groups (X) / number of moles of carboxyl groups (Y) (1)

[0123] For example, for the ink of Example 1, the amount of crosslinking agent (EX-313) used (0.82 g) was divided by the epoxy equivalent (141 g / eq) to calculate the number of moles of crosslinking functional groups in the crosslinking agent (X: 5.82 mmol). Next, the acid value of resin (R-1) (181 mg KOH / g) was divided by the molecular weight of KOH (56.1) to calculate the number of moles of carboxy groups per gram of resin (R-1) after neutralization (3.23 mmol / g). The amount of resin (R-1) used after neutralization in the crosslinking treatment (4.5 g) was multiplied by the number of moles of carboxy groups per gram of resin (R-1) after neutralization (3.23 mmol / g) to calculate the number of moles of carboxy groups in resin (R-1) after neutralization (Y: 14.54 mmol). The crosslinking rate of 40% of the resin (R-1) after neutralization was calculated according to the following formula (1).

[0124] [Measurement of alkali metal ion concentration] The inks to be measured (each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28) were centrifuged by the following method, and the alkali metal ion concentrations of the resulting supernatants were measured.

[0125] In an environment of 23°C, 2 g of the ink to be measured was placed in a sealed container and centrifuged for 3 hours at 140,000 rpm (equivalent to a centrifugal force of 1,050,000 G) using an ultracentrifuge (Eppendorf Himac Technologies Co., Ltd., "himac(R) CS150FNX", rotor: S140AT). This allowed the pigment particles contained in the ink to settle. After centrifugation, 1 mL of the supernatant liquid contained in the ink to be measured was collected using a syringe. The collected supernatant liquid was diluted 10 times with water to prepare the measurement sample.

[0126] The alkali metal ion concentration of the measurement sample was measured using an ICP (inductively coupled plasma) mass spectrometer ("iCAP PRO ICP-OES Duo" manufactured by Thermo Fisher Scientific). Based on the obtained value, the alkali metal ion concentration (ppm) in the supernatant was calculated. The measurement results are shown in Table 5 below. Note that, when calculating the alkali metal ion concentration, a calibration curve created using samples with known alkali metal ion concentrations was used. In Table 5 below, when the alkali metal ion concentration in the supernatant was 500 ppm or less, the result was judged as A (good), and when it exceeded 500 ppm, the result was judged as B (poor).

[0127] [Table 5]

[0128] <Evaluation> The solid content of the supernatant, nozzle clogging, and storage stability were evaluated for each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28 by the following methods. The evaluation results are shown in Table 7 below.

[0129] [Solids in supernatant] The inks to be evaluated (each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28) were centrifuged using the following method, and the solid content of the resulting supernatant was measured. Specifically, the inks to be evaluated were first centrifuged for 3 hours at a rotation speed of 140,000 rpm (1,050,000 G) using an ultracentrifuge (Eppendorf-Himac Technologies Co., Ltd., "himac (registered trademark) CS150FNX", rotor: S140AT). This allowed the pigment particles contained in the inks to be evaluated to settle.

[0130] Next, 30 μL of the supernatant liquid contained in the ink after the centrifugal separation process was transferred to an aluminum container for thermogravimetric measurement. The mass A of the 30 μL of the supernatant liquid was then measured. Next, using a thermogravimetric analyzer ("TG / DTA7200" manufactured by Hitachi High-Tech Science Corporation), thermogravimetric analysis was performed according to the scheme shown in Table 6 below, and the mass loss B of the ink to be evaluated at temperatures from 200°C to 500°C was measured.

[0131] [Table 6]

[0132] The weight loss (B) was considered to be the mass of solids (mainly free resin) in the supernatant. The measured weight loss (A) and weight loss (B) were used to calculate the solid content in the supernatant according to the following formula (2). The calculated content was used as the evaluation value of the solid content of the supernatant.

[0133] [Number 2] Solid content in the supernatant (mass%) = 100 × B / A (2)

[0134] (Evaluation criteria for supernatant solids) The solid content of the supernatant was evaluated according to the following criteria. A (preferable): The solid content in the supernatant is 2.0% by mass or less B (unfavorable): The solid content in the supernatant exceeds 2.0% by mass

[0135] [Nozzle clogging] In the evaluation of nozzle clogging, A4-sized matte inkjet paper ("Super Fine Paper" manufactured by Seiko Epson Corporation) was used as the evaluation paper. An inkjet recording device equipped with a line head (a test machine manufactured by Kyocera Document Solutions) was used as the evaluation machine. The inks to be evaluated (each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28) were filled into the magenta ink tank of the evaluation machine.

[0136] First, using the evaluation machine, 150 mm x 200 mm solid images were continuously printed on 100 sheets of evaluation paper. Next, a purging process was performed to purge the ink to be evaluated from the recording head of the evaluation machine. Next, a wiping process was performed to clean the recording head by wiping the ink ejection surface of the recording head of the evaluation machine with a cleaning wiper. Hereinafter, the operation of cleaning the recording head by the purging process and wiping process will be referred to as the cleaning process.

[0137] Next, a nozzle check pattern image was formed on the evaluation paper using the evaluation machine. As a result, regardless of which ink was used, the ink to be evaluated was ejected from all nozzles (7,968 nozzles). In other words, the number of nozzles where nozzle clogging occurred (hereinafter referred to as "non-ejecting nozzles") was zero. Next, the cleaning process was performed again on the recording head of the evaluation machine. Next, the evaluation machine was left stationary for 7 days without a cap attached to the recording head. Next, the cleaning process was performed again on the recording head of the evaluation machine.

[0138] Next, a nozzle check pattern image was formed on the evaluation paper using the evaluation machine as an evaluation image. The evaluation image was checked, and the ratio of the number of non-ejecting nozzles to the total number of nozzles (7,968) was calculated. The calculated ratio of the number of non-ejecting nozzles was used as the evaluation value for nozzle clogging.

[0139] (Nozzle clogging evaluation criteria) Nozzle clogging was evaluated according to the following criteria. A (Good): Evaluation value is less than 10% B (bad): Evaluation value is 10% or more

[0140] [Storage stability] The viscosity (initial viscosity V1) of the inks to be evaluated (each of the inks of Examples 1 to 11 and Comparative Examples 1 to 28) at 25°C was measured using a vibration viscometer (VM-200T manufactured by Nittetsu Hokkaido Control Systems Co., Ltd.). Next, approximately 30 g of the ink to be evaluated was placed in a 50 mL container and sealed. The container was placed in an incubator with the internal temperature set to 60°C and kept warm for one month.

[0141] Next, the container was removed from the incubator and allowed to stand at room temperature for 3 hours. Next, the ink to be evaluated was removed from the container, and the viscosity at 25°C (post-treatment viscosity V2) was measured using the vibration viscometer. Using the measured initial viscosity V1 and post-treatment viscosity V2, the viscosity change rate (%) was calculated according to the following formula (3). The calculated viscosity change rate was used as an evaluation value for storage stability.

[0142] [Number 3] Viscosity change rate (%)=100×(V1-V2) / V1…(3)

[0143] (Evaluation criteria for storage stability) The storage stability was evaluated according to the following criteria. A (Good): Absolute value of viscosity change rate is less than 5% B (bad): The absolute value of the viscosity change rate is 5% or more

[0144] [Table 7]

[0145] (Examples 1 to 11) In the inks of Examples 1 to 11, the pigment was a quinacridone pigment (any of D4550, ES, and E3B). The crosslinked resin was a crosslinked product of a specific resin (a neutralized product of any of resins (R-1) to (R-8)) and a specific crosslinking agent (any of EX-313, EX-512, EX-521, and EX-614B), and the crosslinking rate was 25% or more and 90% or less. The alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0 mass% or less. As a result, the inks of Examples 1 to 11 were judged to have excellent initial dispersibility and dispersion stability, and to have good nozzle clogging prevention and storage stability.

[0146] (Comparative Examples 1 to 5, 12 to 15, 27, and 28) In the inks of Comparative Examples 1 to 5, 12 to 15, 27, and 28, the pigment was a quinacridone pigment (any of D4550, ES, and E3B). The crosslinked resin was a crosslinked product of a specific resin (a neutralized product of any of resins (R-1) to (R-6), and (R-8)) and a specific crosslinker (any of EX-313, EX-512, EX-521, and EX-614B), and the crosslinking rate was 25% to 90%. Furthermore, the solid content of the supernatant liquid was 2.0% by mass or less. However, the alkali metal ion concentration in the aqueous medium exceeded 500 ppm. As a result, the inks of Comparative Examples 1 to 5, 12 to 15, 27, and 28 were judged to be poor in terms of nozzle clogging and storage stability. It is believed that the high alkali metal ion concentration made it difficult for electrostatic repulsion to occur between pigment particles, resulting in poor dispersion stability of the pigment particles.

[0147] (Comparative Example 6) In the ink of Comparative Example 6, the pigment was a quinacridone pigment (D4550). The crosslinked resin was a crosslinked product of a specific resin (neutralized resin (R-1)) and a specific crosslinking agent (EX-614B). Furthermore, the solid content of the supernatant liquid was 2.0% by mass or less. However, the crosslinking rate of the crosslinked resin was less than 25%, and the alkali metal ion concentration in the aqueous medium exceeded 500 ppm. As a result, the ink of Comparative Example 6 was judged to be poor in terms of nozzle clogging and storage stability. It is presumed that the high alkali metal ion concentration made it difficult for electrostatic repulsion to occur between pigment particles, and that the crosslinking rate was too low, making it easy for the specific resin to detach from the pigment particles, resulting in low dispersion stability of the pigment particles.

[0148] (Comparative Example 7) In the ink of Comparative Example 7, the pigment was a quinacridone pigment (D4550). The crosslinked resin was a crosslinked product of a specific resin (neutralized product of resin (R-1)) and a specific crosslinking agent (EX-614B). The alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0 mass % or less. However, the crosslinking rate of the crosslinked resin was less than 25%. As a result, the ink of Comparative Example 7 was judged to be poor in terms of nozzle clogging and storage stability. It is presumed that the crosslinking rate was too low, which caused the crosslinked resin to easily detach from the pigment particles, resulting in low dispersion stability of the pigment particles.

[0149] (Comparative Example 8) In the ink of Comparative Example 8, the pigment was a quinacridone pigment (D4550). The crosslinked resin was a crosslinked product of a specific resin (neutralized product of resin (R-1)) and a specific crosslinking agent (EX-521). The alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0 mass % or less. However, the crosslinking rate of the crosslinked resin exceeded 90%. As a result, the ink of Comparative Example 8 was judged to be poor in terms of nozzle clogging and storage stability. It is presumed that the crosslinking rate was too high, which reduced the hydrophilicity of the crosslinked resin and resulted in low dispersion stability of the pigment particles.

[0150] (Comparative Example 9) In Comparative Example 9, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% or more and 90% or less, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. However, the crosslinked resin was a crosslinked product of a specific resin (a neutralized product of resin (R-1)) and a crosslinking agent (EX-612) with a water solubility of less than 80%. The solid content of the supernatant liquid exceeded 2.0% by mass. As a result, the ink of Comparative Example 9 was judged to be poor in terms of nozzle clogging and storage stability. It is believed that the dispersion stability of the pigment particles was low because the crosslinking agent was not a specific crosslinking agent. It is also believed that the high amount of solid content in the supernatant liquid made nozzle clogging more likely to occur.

[0151] (Comparative Examples 10 and 11) In the inks of Comparative Examples 10 and 11, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% or more and 90% or less, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0 mass% or less. However, the crosslinked resin was a crosslinked product of a specific resin (neutralized product of resin (R-2) or (R-3)) and a crosslinker (EX-810 or EX-145) that did not have a hydroxy group. As a result, the inks of Comparative Examples 10 and 11 were judged to be poor in terms of nozzle clogging and storage stability. It is presumed that the dispersion stability of the pigment particles was low because the crosslinking agent was not a specific crosslinking agent.

[0152] (Comparative Examples 16 to 21) In the inks of Comparative Examples 16 to 21, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% or more and 90% or less, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0 mass% or less. However, the crosslinked resin was a crosslinked product of a non-specific resin (a neutralized product of any of resins (R-9) to (R-14)) and a specific crosslinking agent (any of EX-313, EX-512, EX-521, and EX-614B). As a result, the inks of Comparative Examples 16 to 21 were judged to be poor in terms of nozzle clogging and storage stability. It is presumed that the dispersion stability of the pigment particles in resins (R-9) to (R-14) was low because the content ratios of each repeating unit were outside the above-mentioned ranges.

[0153] (Comparative Examples 22 and 23) In the inks of Comparative Examples 22 and 23, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% or more and 90% or less, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0% by mass or less. However, the crosslinked resin was a crosslinked product of a non-specific resin (a neutralized product of resin (R-15) or (R-16)) and a specific crosslinking agent (EX-521 or EX-614B). As a result, the inks of Comparative Examples 22 and 23 were judged to be poor in terms of nozzle clogging and storage stability. Resins (R-15) and (R-16) had repeating unit contents outside the above-mentioned ranges, and their acid values ​​were outside the range of 50 mgKOH / g or more and 300 mgKOH / g or less, suggesting that the dispersion stability of the pigment particles was poor.

[0154] (Comparative Example 24) In the ink of Comparative Example 24, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% or more and 90% or less, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0% by mass or less. However, the crosslinked resin was a crosslinked product of a non-specific resin (neutralized product of resin (R-17)) and a specific crosslinking agent (EX-313). As a result, the ink of Comparative Example 24 was judged to be poor in terms of nozzle clogging and storage stability. Because the neutralization rate of the neutralized resin (R-17) was outside the range of 20% or more and 100% or less, it is presumed that the hydrophilicity of the crosslinked resin was low, resulting in poor dispersion stability of the pigment particles.

[0155] (Comparative Examples 25 and 26) In the inks of Comparative Examples 25 and 26, the pigment was a quinacridone pigment (D4550). The crosslinking rate of the crosslinked resin was 25% to 90%, and the alkali metal ion concentration in the aqueous medium was 500 ppm or less. Furthermore, the solid content of the supernatant liquid was 2.0% by mass or less. However, the crosslinked resin was a crosslinked product of a non-specific resin (neutralized resin (R-18) or (R-19)) and a specific crosslinking agent (EX-512 or EX-521). As a result, the inks of Comparative Examples 25 and 26 were judged to be poor in terms of nozzle clogging and storage stability. The number-average molecular weights (Mn) of resins (R-18) and (R-19) were outside the range of 3,000 to 18,000, and it is presumed that the molecular size of the crosslinked resin was inappropriate, preventing efficient coating of the pigment. [Industrial Applicability]

[0156] The ink of this embodiment can be used to form an image.

Claims

1. The ink contains an aqueous medium and pigment particles dispersed in the aqueous medium, the pigment particles include a pigment and a crosslinked resin; the pigment is a quinacridone pigment, the crosslinked resin is a crosslinked product of a specific resin and a specific crosslinking agent, the specific resin is a neutralized product of a specific copolymer having a first repeating unit derived from α-methylstyrene, a second repeating unit derived from styrene, a third repeating unit derived from (meth)acrylic acid, and a fourth repeating unit derived from an alkylene glycol (meth)acrylate or a dialkylene glycol (meth)acrylate; The content ratios of the first repeating unit to the fourth repeating unit in the specific copolymer are, relative to 100% by mass of all the repeating units, the content of the first repeating unit is 1% by mass or more and 65% by mass or less, the content of the second repeating unit is 1% by mass or more and 60% by mass or less, the content of the third repeating unit is 10% by mass or more and 40% by mass or less, the content of the fourth repeating unit is 1% by mass or more and 12% by mass or less, the acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less; the number average molecular weight of the specific copolymer is 3,000 or more and 18,000 or less; the neutralization rate of the specific resin is 20% or more and 100% or less, the specific crosslinking agent contains a polyfunctional epoxy compound having two or more epoxy groups and one or more hydroxyl groups in the molecule, and has a water solubility of 80% or more; The crosslinking rate of the crosslinked resin is 25% or more and 90% or less, The inkjet ink, wherein the aqueous medium has an alkali metal ion concentration of 500 ppm or less.

2. 2. The ink-jet ink according to claim 1, wherein the solid content in the supernatant obtained by the centrifugation treatment is 2.0% by mass or less.

3. The ink-jet ink according to claim 1 , wherein the polyfunctional epoxy compound comprises at least one of glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, and sorbitol polyglycidyl ether.

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