Inkjet ink and method for producing inkjet ink

Abstract

The invention provides an ink for ink jet and a method for producing the ink for ink jet. The ink for ink jet contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles include a pigment and a crosslinked resin. The cross-linked resin is a cross-linked product of a specific resin, a first cross-linking agent, and a second cross-linking agent. The first crosslinking agent has, in the molecule, an ethylene oxide structure in which the number of moles of addition is 1-4 or a propylene oxide structure in which the number of moles of addition is 1-4, and two or more epoxy groups. And a second crosslinking agent having an ethylene oxide structure in which the number of moles of addition is 5-15 and two or more epoxy groups in the molecule. In the cross-linked resin, the ratio (C2 / C1) of the cross-linking rate C2 by the second cross-linking agent to the cross-linking rate C1 by the first cross-linking agent is 18 / 82 or more and 82 / 18 or less.

Description

Technical Field

[0001] The present invention relates to an inkjet ink and a method for producing the inkjet ink. Background Art

[0002] Inkjet recording devices sometimes use aqueous inkjet inks containing pigments and aqueous media. Inkjet inks are required to be able to suppress clogging of the nozzles of the recording head of the inkjet recording device and to enable the formed image to have excellent abrasion resistance and a desired image density. In response to such requirements, for example, an inkjet ink comprising a pigment particle dispersion containing an azomethine metal complex pigment and a polymer dispersant has been proposed (Japanese Patent Publication No. 2019-116600). Summary of the invention

[0003] However, even when the inkjet ink described in Japanese Patent Application Laid-Open No. 2019-116600 is used, it is not possible to reliably form an image having excellent abrasion resistance and desired image density while sufficiently suppressing nozzle clogging.

[0004] In view of the above problems, an object of the present invention is to provide an inkjet ink capable of suppressing the occurrence of nozzle clogging and forming an image having excellent abrasion resistance and desired image density.

[0005] The inkjet ink of the present invention comprises an aqueous medium and pigment particles dispersed in the aqueous medium, wherein the pigment particles contain a pigment and a crosslinked resin, wherein the crosslinked resin is a crosslinked product of a specific resin, a first crosslinking agent, and a second crosslinking agent, wherein 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 alkanediol (meth)acrylate or a dialkanediol (meth)acrylate, wherein the content ratio of the first repeating unit is 1% by mass to 65% by mass, the content ratio of the second repeating unit is 1% by mass to 60% by mass, the content ratio of the third repeating unit is 10% by mass to 40% by mass, the content ratio of the fourth repeating unit is 1% by mass to 12% by mass, and the acid value of the specific copolymer is 50m The specific resin has a neutralization rate of 20% to 100%, the first cross-linking agent has an ethylene oxide structure with an addition mole number of 1 to 4 or a propylene oxide structure with an addition mole number of 1 to 4, and two or more epoxy groups in the molecule, the first cross-linking agent has a water solubility of 80% or more, the second cross-linking agent has an ethylene oxide structure with an addition mole number of 5 to 15, and two or more epoxy groups in the molecule, the second cross-linking agent has a water solubility of 80% or more, in the cross-linked resin, a ratio of a cross-linking rate C2 by the second cross-linking agent to a cross-linking rate C1 by the first cross-linking agent (C2 / C1) is 18 / 82 to 82 / 18, and in the cross-linked resin, a total cross-linking rate (C1+C2) by the first cross-linking agent and the second cross-linking agent is 25% to 95%.

[0006] The present invention provides a method for producing an inkjet ink, wherein the inkjet ink contains an aqueous medium and pigment particles dispersed in the aqueous medium, wherein the pigment particles contain a pigment and a crosslinked resin, and the method comprises: a neutralization step of neutralizing a specific copolymer to obtain a specific resin; a dispersing step of dispersing the specific resin and the pigment in water to obtain a pigment particle dispersion; and a crosslinking step of crosslinking the specific resin in the pigment particle dispersion using a first crosslinking agent and a second crosslinking agent to obtain the crosslinked resin, wherein 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 an alkanediol (meth)acrylate or a dialkanediol (meth)acrylate, and relative to 100% by mass of all repeating units contained in the specific copolymer, the content ratio of the first repeating unit is 1% by mass or more and 65% by mass or less, the content ratio of the second repeating unit is 1% by mass or more and 60% by mass or less, the content ratio of the third repeating unit is 10% by mass or more and 40% by mass or less, and the fourth repeating unit is 10% by mass or more and 40% by mass or less. The content ratio of the repeating unit is 1 mass % or more and 12 mass % or less, the acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less, the weight average molecular weight of the specific copolymer is 3000 or more and 18000 or less, the neutralization rate of the specific resin in the neutralization step is 20% or more and 100% or less, the first cross-linking agent has an ethylene oxide structure with an addition mole number of 1 or more and 4 or a propylene oxide structure with an addition mole number of 1 or more and 4 or less, and two or more epoxy groups in the molecule, and the first cross-linking agent has The water solubility is 80% or more, the second cross-linking agent has an ethylene oxide structure with an addition mole number of 5 or more and 15 or less, and 2 or more epoxy groups in the molecule, the water solubility of the second cross-linking agent is 80% or more, in the cross-linking step, the ratio of the cross-linking rate C2 through the second cross-linking agent to the cross-linking rate C1 through the first cross-linking agent (C2 / C1) is 18 / 82 or more and 82 / 18 or less, and in the cross-linked resin, the total cross-linking rate (C1+C2) through the first cross-linking agent and the second cross-linking agent is 25% or more and 95% or less.

[0007] The inkjet ink and the method for producing the inkjet ink of the present invention can provide an inkjet ink that can suppress the occurrence of nozzle clogging and can form an image having excellent abrasion resistance and desired image density. DETAILED DESCRIPTION

[0008] Hereinafter, the embodiments of the present invention will be described. 50 ) is a value measured using a dynamic light scattering particle size distribution measuring apparatus (“Zeta Sizernano ZS” manufactured by Maruvan Corporation).

[0009] In this specification, the acid value can be determined according to the method described in JIS (Japanese Industrial Standards) K0070:1992.

[0010] Unless otherwise specified, the measured value of the weight average molecular weight (Mw) is a value measured using a gel permeation chromatograph.

[0011] In this specification, acryl and methacryl may be collectively referred to as “(meth)acryl”. In this specification, acrylate and methacrylate may be collectively referred to as “(meth)acrylate”. Each component described in this specification may be used alone or in combination of two or more.

[0012] <First embodiment: inkjet ink>

[0013] The first embodiment of the present invention relates to an inkjet ink (hereinafter, sometimes referred to as ink). The ink of this embodiment contains an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles contain a pigment and a cross-linked resin. The cross-linked resin is a cross-linked product of a specific resin, a first cross-linking agent, and a second cross-linking agent. The specific resin is a neutralized product of a specific copolymer having a first repeating unit from α-methylstyrene, a second repeating unit from styrene, a third repeating unit from (meth)acrylic acid, and a fourth repeating unit from an alkanediol (meth)acrylate or a dialkanediol (meth)acrylate. Relative to 100% by mass of all repeating units possessed by the specific copolymer, the content ratio of the first repeating unit is 1% by mass or more and 65% by mass or less, the content ratio of the second repeating unit is 1% by mass or more and 60% by mass or less, the content ratio of the third repeating unit is 10% by mass or more and 40% by mass or less, and the content ratio 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 weight average molecular weight of the specific copolymer is greater than 3000 and less than 18000. The neutralization rate of the specific resin is greater than 20% and less than 100%. The first cross-linking agent has an ethylene oxide structure with an addition mole number of 1 to 4 or a propylene oxide structure with an addition mole number of 1 to 4, and two or more epoxy groups in the molecule. The water solubility of the first cross-linking agent is greater than 80%. The second cross-linking agent has an ethylene oxide structure with an addition mole number of 5 to 15 and less than 2 epoxy groups in the molecule. The water solubility of the second cross-linking agent is greater than 80%. In the cross-linked resin, the ratio of the cross-linking rate C2 via the second cross-linking agent to the cross-linking rate C1 via the first cross-linking agent (C2 / C1) is greater than 18 / 82 and less than 82 / 18. In the cross-linked resin, the total cross-linking rate (C1+C2) via the first cross-linking agent and the second cross-linking agent is greater than 25% and less than 95%.

[0014] In this specification, the so-called cross-linking rate means the ratio of the number of functional groups that actually form a cross-linked structure in the formed cross-linked product when the total number of functional groups (groups that can react with a cross-linking agent) contained in the raw materials used for cross-linking is set to 100%. The cross-linking agents (first cross-linking agent and second cross-linking agent) used in the ink of this embodiment mainly react with acid groups (especially -COOH). Therefore, in the ink of this embodiment, the "total number of functional groups contained in the raw materials used for cross-linking" is equivalent to the number of moles of carboxyl groups contained in the specific resin. In addition, the first cross-linking agent and the second cross-linking agent used in the ink of this embodiment have excellent reactivity with the specific resin. Therefore, the carboxyl groups contained in the specific resin and the epoxy groups contained in the first cross-linking agent and the second cross-linking agent will react at an extremely high reaction rate. That is, "the number of functional groups that actually form a cross-linked structure in the formed cross-linked product" is approximately the same as the total number of moles of epoxy groups contained in the first cross-linking agent and the second cross-linking agent. As described above, in the ink of this embodiment, the crosslinking ratio of the crosslinked resin is substantially the same as the percentage of the total number of moles of epoxy groups contained in the first crosslinking agent and the second crosslinking agent relative to the number of moles Y of carboxyl groups contained in the specific resin.

[0015] The number of moles of epoxy groups contained in the crosslinking agent (first crosslinking agent or second crosslinking agent) is calculated, for example, by dividing the amount of the crosslinking agent used for crosslinking the specific resin [g] by the epoxy equivalent of the crosslinking agent [g / eq.]. The number of moles Y of carboxyl groups contained in the specific resin is calculated by multiplying the amount of the specific resin used by the number of moles of carboxyl groups per 1 g of the specific resin. The number of moles of carboxyl groups per 1 g of the specific resin is calculated by dividing the acid value [mgKOH / g] of the specific copolymer as a raw material of the specific resin by the molecular weight of KOH (56.1).

[0016] The cross-linked resin is cross-linked by the first cross-linking agent and the second cross-linking agent. That is, there are sites cross-linked by the first cross-linking agent and sites cross-linked by the second cross-linking agent in the cross-linked resin molecule. Therefore, the cross-linking rate of the cross-linked resin is the total cross-linking rate (C1+C2) of the cross-linking rate C1 by the first cross-linking agent and the cross-linking rate C2 by the second cross-linking agent. The cross-linking rate C1 by the first cross-linking agent is obtained by dividing the number of moles X1 of epoxy groups contained in the first cross-linking agent by the number of moles Y of carboxyl groups contained in the specific resin. The cross-linking rate C2 by the second cross-linking agent is obtained by dividing the number of moles X2 of epoxy groups contained in the second cross-linking agent by the number of moles Y of carboxyl groups contained in the specific resin.

[0017] In short, in the specific resin, the crosslinking rate C1 by the first crosslinking agent, the crosslinking rate C2 by the second crosslinking agent, and the total crosslinking rate (C1+C2) are calculated by the following calculation formula.

[0018] Crosslinking rate C1 [%] = 100 × number of moles of epoxy groups contained in the first crosslinking agent X1 / number of moles of carboxyl groups contained in the specific resin Y

[0019] Crosslinking rate C2 [%] = 100 × number of moles of epoxy groups contained in the second crosslinking agent X2 / number of moles of carboxyl groups contained in the specific resin Y

[0020] Total crosslinking rate (C1+C2) [%] = C1+C2

[0021] The use of the ink of this embodiment is not particularly limited, and 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. The ink has excellent permeability on a permeable recording medium. Examples of permeable recording media include printing paper and a medium made of fiber (e.g., cloth). Examples of printing paper include plain paper, copy paper, recycled paper, thin paper, cardboard, and glossy paper.

[0022] Since the ink of the present embodiment has the above-mentioned structure, it is possible to suppress the occurrence of nozzle clogging and form an image with excellent abrasion resistance and desired image density. The reason is presumed to be as follows. In known inks, suppressing the occurrence of nozzle clogging, forming an image with a desired image density, and imparting excellent abrasion resistance to the formed image can only be satisfied individually, and it is difficult to satisfy all of them. For example, in known inks, in order to suppress the occurrence of nozzle clogging, it is effective to impart high dispersion stability to the pigment particles. However, the image formed by the known ink containing pigment particles with high dispersion stability, because the pigment particles will penetrate into the deep part of the recording medium together with the aqueous medium, most of them do not have sufficient image density. In addition, in the known ink, in order to impart the desired image density to the formed image, it is effective to improve the hydrophobicity of the pigment particles. Pigment particles with high hydrophobicity are difficult to penetrate into the deep part of the recording medium. Therefore, in the image formed by the known ink containing pigment particles with high hydrophobicity, a large number of pigment particles will remain on the surface of the recording medium, and the desired image density can be displayed. However, the image with a large number of pigment particles remaining on the recording medium tends to have low abrasion resistance. The reason is that part of the pigment particles are not directly attached to the recording medium, but remain on the surface of the recording medium in a state of being carried on other pigment particles attached to the recording medium. When a force is applied from the outside, such pigment particles are easily separated from the recording medium.

[0023] In this regard, the pigment particles contained in the ink of the present embodiment include a pigment and a cross-linked resin. The cross-linked resin is a cross-linked product of a specific resin and a first cross-linking agent and a second cross-linking agent. The specific resin is a neutralized product of a specific copolymer. Here, the cross-linked resin has a relatively large number of acid groups due to the relatively high acid value of the specific copolymer as a raw material. Therefore, the acid groups of the cross-linked resin are moderately neutralized. The hydrophilicity of the acid group (especially the neutralized acid group) is high. Therefore, the cross-linked resin has a moderately high hydrophilicity. In addition, the first cross-linking agent and the second cross-linking agent respectively have a relatively high hydrophilic structure, namely, an ethylene oxide structure or a propylene oxide structure. Among them, the hydrophilicity of the ethylene oxide structure with a relatively large number of addition moles of the second cross-linking agent is particularly high. On the other hand, the hydrophilicity of the propylene oxide structure or the ethylene oxide structure with a relatively small number of addition moles of the first cross-linking agent is not so high. The specific resin is cross-linked at a specified ratio by these first cross-linking agents and the second cross-linking agents with different hydrophilicities, so the hydrophilicity is adjusted to an appropriate range. Furthermore, the first crosslinking agent and the second crosslinking agent can react efficiently with the specific resin during the crosslinking reaction because of their high water solubility. Furthermore, the crosslinking resin can efficiently cover the pigment particles because of the appropriate molecular size of the specific copolymer as a raw material. Furthermore, the crosslinking resin is difficult to be separated from the pigment particles because of its crosslinked structure.

[0024] In addition, the specific copolymer as the raw material of the cross-linked resin has the first to fourth repeating units at a specified content ratio. The first repeating unit from α-methylstyrene will moderately increase the affinity between the cross-linked resin and the pigment particles. The second repeating unit from styrene will impart moderate hydrophobicity to the cross-linked resin and adjust the dispersion stability of the pigment particles. The third repeating unit from (meth) acrylic acid and the fourth repeating unit from alkanediol (meth) acrylate or dialkanediol (meth) acrylate will impart moderate hydrophilicity to the cross-linked resin and impart dispersion stability to the pigment particles. As a result, the pigment particles contained in the ink of this embodiment impart hydrophilicity and dispersion stability in a well-balanced manner. Therefore, the ink of this embodiment can suppress the occurrence of nozzle clogging and can form an image with excellent abrasion resistance and desired image density.

[0025] The ink of this embodiment will be described in more detail below. In addition, each component described below may be used alone or in combination of two or more.

[0026] [Content ratio of solid content in supernatant]

[0027] A portion of the cross-linked resin contained in the ink of the present embodiment may sometimes be separated from the pigment particles and released into the aqueous medium during the manufacture or storage of the ink of the present embodiment. Such cross-linked resin released into the aqueous medium is recorded as free resin. The solid content contained in the supernatant obtained by centrifuging the ink of the present embodiment at 1,050,000G for 3 hours is regarded as free resin. Free resin can cause nozzle clogging. Therefore, in the ink of the present embodiment, the content ratio of free resin is preferably low. However, in the ink of the present embodiment, free resin is inevitably generated and difficult to completely remove. In addition, the free resin also has the function of optimizing the abrasion resistance of the image formed by the ink of the present embodiment, so it does not have to be completely removed.

[0028] According to the above, in the ink of the present embodiment, the solid content in the supernatant obtained by centrifugation at 1,050,000G for 3 hours is preferably 2.0% by mass or less, more preferably 0.1% by mass or more and 1.5% by mass or less, and particularly preferably 0.5% by mass or more and 1.0% by mass or less. By setting the solid content in the supernatant to 0.1% by mass or more, it is possible to impart better abrasion resistance to the image formed by the ink of the present embodiment. By setting the solid content in the supernatant to 2.0% by mass or less, the ink of the present embodiment can more effectively suppress the occurrence of nozzle clogging. The solid content can be measured using the method described in the examples or a method based thereon.

[0029] [Pigment particles]

[0030] The pigment particles contain a pigment and a cross-linked resin. The pigment particles contain, for example, a core containing a pigment and a cross-linked resin covering the core. The total content of the pigment and the cross-linked resin in the pigment particles is preferably 90% by mass or more, and more preferably 100% by mass.

[0031] 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.

[0032] In the ink of the present embodiment, the content of the pigment particles is preferably 6.0 mass % or more and 20.0 mass % or less, and more preferably 10.0 mass % or more and 15.0 mass % or less. By setting the content of the pigment particles to 6.0 mass % or more, the ink of the present embodiment is more likely to form an image with a desired image density. In addition, by setting the content of the pigment particles to 20.0 mass % or less, the fluidity of the ink of the present embodiment can be optimized.

[0033] (pigment)

[0034] Examples of pigments include yellow pigments, orange pigments, red pigments, blue pigments, violet pigments, and black pigments. Examples of yellow pigments include CI Pigment Yellow (74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, and 193). Examples of orange pigments include CI Pigment Orange (34, 36, 43, 61, 63, and 71). Examples of red pigments include CI Pigment Red (122 and 202). Examples of blue pigments include CI Pigment Blue (15, more specifically 15:3). Examples of violet pigments include CI Pigment Violet (19, 23, and 33). Examples of black pigments include CI Pigment Black (7).

[0035] In the ink of this embodiment, the content of the pigment is preferably 3.0 parts by mass or more and 15.0 parts by mass or less, more preferably 7.0 parts by mass or more and 12.0 parts by mass or less. The content of the pigment in the pigment particles is preferably 45% by mass or more and 80% by mass or less, more preferably 55% by mass or more and 70% by mass or less.

[0036] (cross-linked resin)

[0037] The cross-linked resin is used, for example, to encapsulate the pigment in the pigment particles. The cross-linked resin is a cross-linked product of a specific resin and a first cross-linking agent and a second cross-linking agent. The total cross-linking rate (C1+C2) of the cross-linked resin by the first cross-linking agent and the second cross-linking agent is greater than 25% and less than 90%, preferably greater than 35% and less than 65%, and more preferably greater than 35% and less than 45%. By setting the cross-linking rate of the cross-linked resin to greater than 25%, it is possible to suppress the cross-linked resin from detaching from the pigment particles. By setting the total cross-linking rate (C1+C2) of the cross-linked resin to less than 90%, it is possible to optimize the hydrophilicity and dispersion stability of the pigment particles. In addition, the cross-linking rate of the cross-linked resin can be calculated using the method described in the Examples or a method based thereon.

[0038] In the ink of this embodiment, the content of the cross-linked 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 cross-linked resin to 0.5% by mass or more, the hydrophilicity and dispersion stability of the pigment particles can be further optimized. By setting the content of the cross-linked resin to 10.0% by mass or less, the generation of free resin can be suppressed.

[0039] In the pigment particles, the content of the cross-linking resin 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, relative to 100 parts by mass of the pigment. By setting the content of the cross-linking resin to 25 parts by mass or more and 60 parts by mass or less relative to 100 parts by mass of the pigment, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0040] (Specific resin)

[0041] The specific resin is a neutralized product of the 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 cross-linked resin, and optimize the hydrophilicity and dispersion stability of the pigment particles.

[0042] The specific resin preferably contains alkali metal atoms. That is, the specific resin is preferably a neutralized product obtained by neutralizing the specific copolymer using a neutralizing agent containing alkali metal atoms. Even if the ink of the present embodiment is exposed to the sun to a dry state, the alkali metal atoms will not volatilize from the ink of the present embodiment. Therefore, by neutralizing the specific copolymer using a neutralizing agent containing alkali metal atoms, the neutralized state (hydrophilicity) of the cross-linked resin will be maintained even if the ink of the present embodiment is exposed to the sun 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 more preferably NaOH or KOH.

[0043] (Specific copolymer)

[0044] The weight average molecular weight of the specific copolymer is preferably 3000 or more and 18000 or less, more preferably 5000 or more and 12000 or less, and particularly preferably 8000 or more and 10000 or less. By setting the weight average molecular weight of the specific resin to 3000 or more and 18000 or less, the cross-linked resin can efficiently cover the pigment particles. Thus, the cross-linked resin can be prevented from being separated from the pigment particles.

[0045] 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 is easy to fully introduce the cross-linked structure into the cross-linked resin, and the hydrophilicity and dispersion stability of the pigment particles can be optimized. By setting the acid value of the specific copolymer to 300 mgKOH / g or less, the hydrophilicity and dispersion stability of the pigment particles can be suppressed from becoming too high. As a result, it is possible to impart excellent abrasion resistance and desired image density to the image formed by the ink of this embodiment.

[0046] 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 an alkanediol (meth)acrylate or a dialkanediol (meth)acrylate.

[0047] Examples of the alkanediol (meth)acrylate include ethylene glycol (meth)acrylate, propylene glycol (meth)acrylate, and butylene glycol (meth)acrylate, and ethylene glycol (meth)acrylate is preferred.

[0048] Examples of the dialkylene glycol (meth)acrylate include diethylene glycol (meth)acrylate, dipropylene glycol (meth)acrylate, and dibutylene glycol (meth)acrylate, and dipropylene glycol (meth)acrylate is preferred.

[0049] The content ratio of the first repeating unit derived from α-methylstyrene is 1% by mass or more and 65% by mass or less, preferably 15% by mass or more and 55% by mass or less, more preferably 25% by mass or more and 50% by mass or less, and particularly preferably 40% by mass or more and 48% by mass or less, relative to 100% by mass of all repeating units possessed by the specific copolymer. By setting the content ratio of the first repeating unit to 1% by mass or more and 65% by mass or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0050] The content of the second repeating unit derived from styrene is 1% by mass or more and 60% by mass or less, preferably 10% by mass or more and 40% by mass or less, and more preferably 15% by mass or more and 27% by mass or less, relative to 100% by mass of all repeating units of the specific copolymer. By setting the content of the second repeating unit to 1% by mass or more and 60% by mass or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0051] The content ratio of the third repeating unit derived from (meth)acrylic acid is 10% by mass or more and 40% by mass or less, preferably 20% by mass or more and 35% by mass or less, and more preferably 25% by mass or more and 32% by mass or less, relative to 100% by mass of all repeating units of the specific copolymer. By setting the content ratio of the third repeating unit to 10% by mass or more and 40% by mass or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0052] The content ratio of the fourth repeating unit derived from alkanediol (meth)acrylate or dialkanediol (meth)acrylate is 1% by mass or more and 12% by mass or less, preferably 4% by mass or more and 11% by mass or less, and more preferably 4% by mass or more and 8% by mass or less, relative to 100% by mass of all repeating units of the specific copolymer. By setting the content ratio of the fourth repeating unit to 1% by mass or more and 12% by mass or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0053] Relative to 100 mass % of all repeating units possessed by the specific copolymer, preferably, the content ratio of the first repeating unit is 15 mass % or more and 55 mass % or less, the content ratio of the second repeating unit is 10 mass % or more and 40 mass % or less, the content ratio of the third repeating unit is 20 mass % or more and 35 mass % or less, and the content ratio of the fourth repeating unit is 4 mass % or more and 11 mass % or less.

[0054] The specific copolymer may be a random polymer or a block polymer. The specific copolymer is preferably a random polymer.

[0055] The monomer component as the raw material of the specific copolymer is preferably any one of components 1 to 3 shown in Table 1 below. The numerical range described in Table 1 below represents the numerical range of the content ratio [mass %] of each monomer. For example, component 1 represents: styrene is contained in an amount of 20 mass % to 24 mass %, α-methylstyrene is contained in an amount of 42 mass % to 46 mass %, ethylene glycol acrylate is contained in an amount of 4 mass % to 8 mass %, and methacrylic acid is contained in an amount of 25 mass % to 30 mass %.

[0056] [Table 1]

[0057]

[0058] (First cross-linking agent)

[0059] The first crosslinking agent has an ethylene oxide structure with an added mole number of 1 to 4 or a propylene oxide structure with an added mole number of 1 to 4, and two or more epoxy groups in the molecule. The first crosslinking agent has a water solubility of 80% or more.

[0060] The water solubility of the first crosslinking agent is preferably 90% or more, and more preferably 98% or more. By setting the water solubility of the first crosslinking agent to 90% or more, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0061] The water solubility of the first crosslinking agent is the ratio of the mass A of the first crosslinking agent dissolved in water to the total amount of the first crosslinking agent (10 g) when 10 g of the first crosslinking agent and 90 g of water are mixed at 25° C. (100×A / 10 g). For example, when 10 g of the first crosslinking agent and 90 g of water are mixed at 25° C., 9 g of the first crosslinking agent is dissolved in water, and 1 g of the first crosslinking agent is insoluble in water and precipitated (mass A: 9 g), the water solubility is 90%. In addition, the water solubility of the second crosslinking agent is also measured using the same method.

[0062] In the first crosslinking agent, the number of epoxy groups in the molecule is preferably 2 to 8, more preferably 2 to 3. In the first crosslinking agent, the number of added moles of ethylene oxide structure or propylene oxide structure in the molecule is preferably 1 to 3, more preferably 2 to 3.

[0063] Examples of the first cross-linking agent include propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylene glycol diglycidyl ether, and polyethylene glycol diglycidyl ether.

[0064] The first crosslinking agent is preferably a compound represented by the general formula (1-A) to (1-B), and more preferably a compound represented by the chemical formula (1-A1), (1-B1), (1-B2) or (1-B3). In the general formula (1-A), n represents an integer of 1 to 4. In the general formula (1-B), m represents an integer of 1 to 4.

[0065] [Chemistry 1]

[0066]

[0067] [Chemistry 2]

[0068]

[0069] The epoxy equivalent of the first crosslinking agent is preferably 100 g / eq. to 230 g / eq., more preferably 120 g / eq. to 190 g / eq., and particularly preferably 150 g / eq. to 170 g / eq. The epoxy equivalent can be determined, for example, by the method described in JIS (Japanese Industrial Standards) K7236:2009.

[0070] The crosslinking rate C1 of the crosslinked resin by the first crosslinking agent is preferably 5% or more and 75% or less, more preferably 10% or more and 45% or less, and particularly preferably 14% or more and 25% or less. By setting the crosslinking rate C1 to 5% or more and 75% or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0071] (Second cross-linking agent)

[0072] The second crosslinking agent has an ethylene oxide structure with an addition mole number of 5 to 15 and two or more epoxy groups in the molecule. The water solubility of the second crosslinking agent is 80% or more.

[0073] The water solubility of the second crosslinking agent is preferably 90% or more, and more preferably 98% or more. By setting the water solubility of the second crosslinking agent to 90% or more, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0074] In the second crosslinking agent, the number of epoxy groups in the molecule is preferably 2 to 8, more preferably 2 to 3. In the second crosslinking agent, the number of added moles of ethylene oxide structure in the molecule is preferably 7 to 14, more preferably 9 or 13.

[0075] Examples of the second cross-linking agent include polyethylene glycol diglycidyl ether.

[0076] The second crosslinking agent is preferably a compound represented by the general formula (2-A), and more preferably a compound represented by the chemical formula (2-A1) or (2-A2). In the general formula (2-A), s represents an integer of 5 or more and 15 or less.

[0077] [Chemistry 3]

[0078]

[0079] [Chemistry 4]

[0080]

[0081] The epoxy equivalent of the second crosslinking agent is preferably 250 g / eq. to 450 g / eq., more preferably 250 g / eq. to 400 g / eq., particularly preferably 250 g / eq. to 300 g / eq.

[0082] The crosslinking rate C2 of the crosslinked resin by the second crosslinking agent is preferably 5% or more and 75% or less, more preferably 10% or more and 45% or less, and particularly preferably 14% or more and 25% or less. By setting the crosslinking rate C2 to 5% or more and 75% or less, the hydrophilicity and dispersion stability of the pigment particles can be further optimized.

[0083] In the crosslinked resin, the ratio of the crosslinking rate C2 using the second crosslinking agent to the crosslinking rate C1 using the first crosslinking agent (C2 / C1) is 18 / 82 or more and 82 / 18 or less, preferably 30 / 70 or more and 70 / 30 or less, and more preferably 45 / 55 or more and 55 / 45 or less. By setting the ratio (C2 / C1) to 18 / 82 or more and 82 / 18 or less, the hydrophilicity and dispersion stability of the pigment particles can be optimized.

[0084] If the amount is small, the crosslinked resin may also have a crosslinked structure derived from other crosslinked resins other than the first crosslinking agent and the second crosslinking agent. In the crosslinked resin, the total crosslinking ratio (C1+C2) of the first crosslinking agent and the second crosslinking agent relative to the total crosslinking ratio C1 of all crosslinking agents (the first crosslinking agent, the second crosslinking agent, and other crosslinking agents) is ALL The ratio (100×(C1+C2) / C ALL ), preferably 90% or more, more preferably 99% or more, and particularly preferably 100%.

[0085] [Aqueous medium]

[0086] The aqueous medium contained in the ink of the present embodiment is a medium containing water. The aqueous medium can function as a solvent or as a dispersion medium. Specific examples of the aqueous medium include aqueous media containing water and a water-soluble organic solvent.

[0087] (water)

[0088] In the ink of this embodiment, the content ratio of water is preferably 25.0 mass % or more and 80.0 mass % or less, and more preferably 35.0 mass % or more and 60.0 mass % or less.

[0089] (Water-soluble organic solvent)

[0090] Examples of the water-soluble organic solvent include diol compounds, triol compounds, glycol ether compounds, lactam compounds, nitrogen-containing compounds, acetate compounds, thiodiglycol, and dimethyl sulfoxide.

[0091] Examples of the diol compound include ethylene glycol, 1,3-propylene glycol, 1,2-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. The diol compound is preferably ethylene glycol, diethylene glycol, 2-ethyl-1,2-hexanediol, 3-methyl-1,5-pentanediol, 1,3-propylene glycol, 1,5-pentanediol, or 1,2-propylene glycol.

[0092] Examples of the triol compound include glycerol and 1,2,3-butanetriol.

[0093] Examples of the glycol ether compound include diethylene glycol diethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether (diethylene glycol diethyl ether), triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, and propylene glycol monomethyl ether. Triethylene glycol monobutyl ether is preferred as the glycol ether compound.

[0094] Examples of the lactam compound include 2-pyrrolidone and N-methyl-2-pyrrolidone. The lactam compound is preferably 2-pyrrolidone.

[0095] Examples of the nitrogen-containing compound include 1,3-dimethylimidazolidinone, formamide, and dimethylformamide.

[0096] Examples of the acetate compound include diethylene glycol monoethyl ether acetate.

[0097] The water-soluble organic solvent is preferably glycerol, a glycol compound, a glycol ether compound or a lactam compound, more preferably glycerol, 1,3-propylene glycol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, ethylene glycol, triethylene glycol monobutyl ether, diethylene glycol diethyl ether or 2-pyrrolidone.

[0098] The water-soluble organic solvent is preferably the following mixed organic solvents 1 to 3. Hereinafter, the numerical range shown in parentheses after each component represents a preferred content ratio of each component in the ink of the present embodiment.

[0099] Mixed organic solvent 1: ethylene glycol (18.0% by mass or more and 22.0% by mass or less), and diethylene glycol diethyl ether (13.0% by mass or more and 17.0% by mass or less)

[0100] Mixed organic solvent 2: glycerol (8.0% by mass or more and 11.0% by mass or less), triethylene glycol monobutyl ether (10.0% by mass or more and 14.0% by mass or less), 3-methyl-1,5-pentanediol (10.0% by mass or more and 14.0% by mass or less), and 2-pyrrolidone (0.5% by mass or more and 3.0% by mass or less)

[0101] Mixed organic solvent 3: triethylene glycol monobutyl ether (7.0 mass % or more and 9.0 mass % or less), 2-pyrrolidone (1.0 mass % or more and 3.0 mass % or less), 1,3-propylene glycol (11.0 mass % or more and 14.0 mass % or less), and 1,5-pentanediol (11.0 mass % or more and 14.0 mass % or less)

[0102] The content ratio of the water-soluble organic solvent in the ink of the present embodiment is preferably 10.0 mass % or more and 50.0 mass % or less, and more preferably 30.0 mass % or more and 40.0 mass % or less.

[0103] [Surfactant]

[0104] 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. The surfactant is preferably a nonionic surfactant.

[0105] Examples of the nonionic surfactant include polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan monooleate ether, monodecanoyl sucrose, and ethylene oxide adducts of acetylene glycol. The nonionic surfactant is preferably an ethylene oxide adduct of acetylene glycol.

[0106] The content ratio of the surfactant in the ink of the present 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.

[0107] [Other ingredients]

[0108] The ink of the present embodiment may further contain known additives (for example, a dissolution stabilizer, an anti-drying agent, an antioxidant, a viscosity adjuster, a pH adjuster, and an antifungal agent) as necessary.

[0109] <Second embodiment: Method for producing ink>

[0110] The second embodiment of the present invention relates to a method for producing an ink. The method for producing an ink of the present embodiment is a method for producing an ink containing pigment particles dispersed in an aqueous medium. The pigment particles contain a pigment and a cross-linked resin. The method for producing an ink of the present embodiment includes: a neutralization step of neutralizing a specific copolymer to obtain a specific resin; a dispersion step of dispersing the specific resin and the pigment in water to obtain a pigment particle dispersion; and a cross-linking step of cross-linking the specific resin in the pigment particle dispersion using a first cross-linking agent and a second cross-linking agent to obtain a cross-linked resin. 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 an alkanediol (meth)acrylate or a dialkanediol (meth)acrylate. With respect to 100% by mass of all repeating units possessed by the specific copolymer, the content ratio of the first repeating unit is 1% by mass or more and 65% by mass or less, the content ratio of the second repeating unit is 1% by mass or more and 60% by mass or less, the content ratio of the third repeating unit is 10% by mass or more and 40% by mass or less, and the content ratio 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 greater than 50 mgKOH / g and less than 300 mgKOH / g. The weight average molecular weight of the specific copolymer is greater than 3000 and less than 18000. The neutralization rate of the specific resin in the neutralization step is greater than 20% and less than 100%. The first cross-linking agent has an ethylene oxide structure with an addition mole number of greater than 1 and less than 4 or a propylene oxide structure with an addition mole number of greater than 1 and less than 4, and two or more epoxy groups in the molecule. The water solubility of the first cross-linking agent is greater than 80%. The second cross-linking agent has an ethylene oxide structure with an addition mole number of greater than 5 and less than 15, and two or more epoxy groups in the molecule. The water solubility of the second cross-linking agent is greater than 80%. In the cross-linking step, the ratio of the cross-linking rate C2 using the second cross-linking agent to the cross-linking rate C1 using the first cross-linking agent (C2 / C1) is greater than 18 / 82 and less than 82 / 18. The total crosslinking rate (C1+C2) of the crosslinked resin using the first crosslinking agent and the second crosslinking agent is 25% or more and 95% or less.

[0111] The method for producing ink of the present embodiment is suitable as the method for producing ink of the first embodiment. In the method for producing ink of the present embodiment, the aqueous medium, pigment particles, pigment, cross-linked resin, specific copolymer, first cross-linking agent, and second cross-linking agent are the same as those of the first embodiment, so the description thereof is omitted. The method for producing ink of the present embodiment preferably further comprises: a centrifugation step of centrifuging the pigment particle dispersion after the cross-linking step to obtain a precipitate; and a re-dispersion step of re-dispersing the precipitate in water. Each step is described below.

[0112] (Neutralization Step)

[0113] In this step, the specific copolymer is neutralized to obtain the specific resin. The specific copolymer can be neutralized by, for example, mixing the specific copolymer with a neutralizing agent (e.g., an alkaline aqueous solution). Examples of the alkaline aqueous solution include an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution.

[0114] (Dispersion Step)

[0115] In this step, the specific resin and the pigment are dispersed in water to obtain a pigment particle dispersion. Specifically, first, the specific resin, the pigment and water are mixed to prepare a mixed solution. Then, the mixed solution is subjected to a dispersion treatment. Examples of the dispersion device used for the dispersion treatment include a wet dispersion device such as a media type disperser.

[0116] The content ratio of the specific resin in the mixed solution is, for example, 5.0 mass % or more and 25.0 mass % or less. The content ratio of the pigment in the mixed solution is, for example, 1.0 mass % or more and 10.0 mass % or less. In this step, a defoamer is preferably added to the mixed solution. The content ratio of the defoamer in the mixed solution is, for example, 0.01 mass % or more and 0.1 mass % or less.

[0117] In this step, the obtained pigment particle dispersion is preferably filtered using a filter (for example, having a pore size of 5 μm) to remove coarse particles.

[0118] (Cross-linking step)

[0119] In this step, the specific resin in the pigment particle dispersion is cross-linked using the first cross-linking agent and the second cross-linking agent to obtain a cross-linked resin. Specifically, the carboxyl group contained in the specific resin and the epoxy group contained in the first cross-linking agent and the second cross-linking agent are cross-linked. As a result, a cross-linked resin is produced as a reaction product of the first cross-linking agent, the second cross-linking agent and the specific resin. In this step, for example, after adding the first cross-linking agent and the second cross-linking agent to the pigment particle dispersion, the pigment particle dispersion is heated while stirring. The heating temperature can be set to, for example, 70° C. or more and 95° C. or less. The heating time can be set to, for example, 2 hours or more and 6 hours or less.

[0120] In this step, the amount of the first crosslinking agent added is preferably 3.0 parts by mass or more and 40.0 parts by mass or less, more preferably 5.0 parts by mass or more and 20.0 parts by mass or less, and particularly preferably 10.0 parts by mass or more and 16.0 parts by mass or less, relative to 100.0 parts by mass of the specific resin.

[0121] In this step, the amount of the second crosslinking agent added is preferably 5.0 parts by mass or more and 80.0 parts by mass or less, more preferably 10.0 parts by mass or more and 40.0 parts by mass or less, and particularly preferably 15.0 parts by mass or more and 25.0 parts by mass or less, relative to 100.0 parts by mass of the specific resin.

[0122] The pigment particle dispersion after this step can be used in the centrifugation step described later, and can also be used as ink after adding other ingredients (more specifically at least one of water, organic solvents, surfactants, dissolution stabilizers, humectants, penetrants and viscosity regulators) as needed.

[0123] (Centrifugation step)

[0124] In this step, the pigment particle dispersion after the cross-linking step is centrifuged to obtain a precipitate. The precipitate contains pigment particles. The supernatant contains free resin. The centrifugation conditions can be set, for example, to a rotation speed of 10,000 rpm or more and 100,000 rpm or less, and a centrifugation time of 12 hours or more and 48 hours or less.

[0125] (Re-dispersion step)

[0126] In this step, the precipitate is dispersed in water again. Ink can be obtained. In this step, other components (more specifically, at least one of an organic solvent, a surfactant, a dissolution stabilizer, a humectant, a penetrant, and a viscosity modifier) ​​can be further added to the dispersion of the precipitate as needed.

[0127] Since the method for producing the ink of the present embodiment further includes a centrifugation step and a redispersion step, the amount of free resin in the produced ink can be reduced.

[0128] In the method for producing the ink of the present embodiment, the ink obtained through the crosslinking step or the re-dispersion step can be treated using a filter (eg, a filter with a pore size of 5 μm or less), thereby removing foreign matter and coarse particles in the ink.

[0129] [Example]

[0130] Hereinafter, examples of the present invention will be described, but the present invention is not limited to the following examples.

[0131] In the embodiment, various cross-linking rates of the cross-linked resin are calculated by the following method. First, the mole number X1 of the epoxy group contained in the first cross-linking agent is calculated. The mole number X1 of the epoxy group contained in the first cross-linking agent is calculated by dividing the usage amount [g] of the first cross-linking agent by the epoxy equivalent [g / eq.] of the first cross-linking agent. Similarly, the mole number X2 of the epoxy group contained in the second cross-linking agent is calculated by dividing the usage amount [g] of the second cross-linking agent by the epoxy equivalent [g / eq.] of the second cross-linking agent. Next, the mole number Y of the carboxyl group contained in the specific resin is calculated. The mole number Y of the carboxyl group contained in the specific resin is calculated by multiplying the usage amount of the specific resin by the mole number of carboxyl groups per 1g of the specific resin. The mole number of carboxyl groups per 1g of the specific resin is calculated by dividing the acid value of the specific resin by the molecular weight of KOH (56.1). Next, the above-calculated values ​​are substituted into the following formula to calculate various cross-linking rates of the cross-linked resin.

[0132] Crosslinking rate C1 [%] = 100 × number of moles of epoxy groups contained in the first crosslinking agent X1 / number of moles of carboxyl groups contained in the specific resin Y

[0133] Crosslinking rate C2 [%] = 100 × number of moles of epoxy groups contained in the second crosslinking agent X2 / number of moles of carboxyl groups contained in the specific resin Y

[0134] Total crosslinking rate (C1+C2) [%] = C1+C2

[0135] (Preparation of copolymer (r-1))

[0136] In a four-necked flask equipped with a stirrer, a nitrogen inlet tube, a condenser, and a dropping funnel, 100.0 parts by mass of isopropanol and 250.0 parts by mass of methyl ethyl ketone were added. In addition, 20.0 parts by mass of styrene, 40.0 parts by mass of α-methylstyrene, 5.0 parts by mass of ethylene glycol acrylate, 25.0 parts by mass of methacrylic acid, and 0.3 parts by mass of azobisisobutyronitrile (AIBN, polymerization initiator) were mixed to prepare a monomer solution. In addition, 150.0 parts by mass of methyl ethyl ketone and 0.1 parts by mass of AIBN were mixed to prepare a methyl ethyl ketone solution.

[0137] Next, a nitrogen atmosphere is formed by introducing nitrogen gas into the four-necked flask. Next, while heating the contents of the four-necked flask at 70°C for reflux, a dropping funnel is used to supply all of the monomer solution into the four-necked flask over 2 hours. After supplying the monomer solution, the contents of the four-necked flask are then heated at 70°C for 6 hours. Next, while heating the contents of the four-necked flask at 70°C for reflux, a dropping funnel is used to supply all of the methyl ethyl ketone solution into the four-necked flask over 15 minutes. After supplying the methyl ethyl ketone solution, the contents of the four-necked flask are then heated at 70°C for 5 hours. Thus, a copolymer aqueous solution containing a specific copolymer, namely, copolymer (r-1), is obtained. Copolymer (r-1) is separated by distilling off methyl ethyl ketone and isopropanol from the copolymer aqueous solution.

[0138] (Measurement of acid value)

[0139] The acid value of the copolymer (r-1) was 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 chemicals). The measurement results are shown in Table 2 below.

[0140] (Measurement of weight average molecular weight)

[0141] The weight average molecular weight (Mw) of the copolymer (r-1) was measured by gel permeation chromatography (GPC) under the following conditions. The measurement results are shown in Table 2 below.

[0142] (GPC conditions)

[0143] Measuring device: HLC-8020GPC manufactured by Tosoh Corporation

[0144] Column: Ultra-high performance semi-micro SEC column (TSKgel Super Multipore HZ-H manufactured by Tosoh Corporation, filler: styrene-divinylbenzene resin, column dimensions: inner diameter 4.6 mm × length 15 cm, filler particle size: 6 μm)

[0145] Number of columns: 3

[0146] Eluent: Tetrahydrofuran

[0147] Eluent flow rate: 0.35mL / min

[0148] Sample solution volume: 10 μL

[0149] Column temperature: 40°C

[0150] Detector: RI (refractive index) detector

[0151] 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

[0152] (Preparation of copolymers (r-2) to (r-14))

[0153] In the preparation of the monomer solution, copolymers (r-2) to (r-14) were prepared by the same method as that for preparing copolymer (r-1), except that the types and amounts of the monomers used were changed as shown in Table 2 below. In Table 2 below, "EG acrylate" means ethylene glycol acrylate. "DPGA" means dipropylene glycol acrylate.

[0154] (Preparation of Aqueous Resin Solutions (R-1) to (R-14))

[0155] A copolymer (specifically, any one of copolymers (r-1) to (r-14)) as shown in Table 2 below, potassium hydroxide and water are mixed to obtain specific resin aqueous solutions (R-1) to (R-14). The specific resin aqueous solutions (R-1) to (R-14) contain neutralized resins (corresponding to specific resins). The amount of potassium hydroxide added is set to an amount when the neutralization rate of the neutralized resin reaches the amount shown in Table 2 below. The amount of water added is set to an amount when the solid content concentration (content ratio of the neutralized resin) of the specific resin aqueous solutions (R-1) to (R-14) reaches 30% by mass.

[0156] [Table 2]

[0157]

[0158]

[0159] The content ratios [mass %] of the first to fourth repeating units of copolymers (r-1) to (r-14) are shown in Table 3 below.

[0160] [Table 3]

[0161] Repeating Unit r-1 r-2 r-3 r-4 r-5 r-6 r-7 First 44 22 29 11 63 78 - second 22 54 29 64 - 6 62 third 28 14 39 21 31 11 31 fourth 6 10 2 4 6 6 8 Repeating Unit r-8 r-9 r-10 r-11 r-12 r-13 r-14 First 30 42 27 36 22 46 44 second 30 21 23 43 54 23 22 third 26 37 42 7 14 23 28 fourth 13 - 8 14 10 8 6

[0162] <Ink Preparation>

[0163] The inks of Examples 1 to 8 and Comparative Examples 1 to 22 were prepared by the following method. First, the chemical names, chemical formulas, epoxy equivalents, and water solubilities of the crosslinking agents (EX-920), (EX-810), (EX-850), (EX-821), (EX-830), (EX-841), (EX-612), (EX-313), and (EX-145) (all "Denacol (Japanese registered trademark) Silks" manufactured by Nagase Chemicals Co., Ltd.) used in the Examples are shown in Table 4 below. In Table 4 below, the chemical formula "(4-1) + (4-2)" of the crosslinking agent (EX-313) represents a mixture of the compound represented by the chemical formula (4-1) and the compound represented by the chemical formula (4-2).

[0164] [Table 4]

[0165]

[0166]

[0167] [Chemistry 5]

[0168]

[0169] [Chemistry 6]

[0170]

[0171] [Chemistry 7]

[0172]

[0173] [Chemistry 8]

[0174]

[0175] Among the cross-linking agents, the cross-linking agents (EX-920), (EX-810), (EX-850) and (EX-821) are first cross-linking agents, and the cross-linking agents (EX-830) and (EX-841) are second cross-linking agents.

[0176] [Preparation of ink of Example 1]

[0177] (Distributed Processing)

[0178] A mixture was obtained by mixing 15.0 parts by mass of a pigment (Printex (Japanese registered trademark) 80 manufactured by Olion Engineering Chemicals Co., Ltd., carbon black), 15.0 parts by mass of the above-mentioned specific resin aqueous solution (R-1) (containing 4.5 parts by mass of the neutralized copolymer (r-1)), 0.1 parts by mass of a defoamer (SN Defoamer 1340 manufactured by Sannopoulos Co., Ltd., amide wax surfactant), and ion exchange water. The amount of ion exchange water added was set to an amount when the mixture reached 100.0 parts by mass.

[0179] The mixture was dispersed for 4 hours using a media disperser ("Dignomil" manufactured by WAB). During the dispersion, the medium used zirconium oxide beads with a diameter of 0.5 mm. The filling rate of the medium was set to 60% by volume of the container capacity. The treatment temperature (refrigerator temperature) during the dispersion was set to 10°C. After the dispersion, the medium was removed from the contents of the media disperser to obtain a pigment particle dispersion. Next, the pigment particle dispersion was filtered using a filter with a pore size of 5 μm to remove foreign matter and coarse particles.

[0180] (Cross-linking treatment)

[0181] A three-necked flask with a thermometer and a stirring blade is used as a reaction container. 100.0 parts by mass of the filtered pigment particle dispersion is put into the reaction container. The temperature of the contents of the reaction container is maintained at 30°C using a water bath. Next, 0.51 parts by mass of a crosslinking agent (EX-920) as a first crosslinking agent and 0.78 parts by mass of a crosslinking agent (EX-830) as a second crosslinking agent are put into the reaction container. Next, the contents of the reaction container are stirred at 250 rpm for 1 hour. Next, while stirring the contents of the reaction container at 250 rpm, the contents of the reaction container are heated to 80°C at a heating rate of 0.5°C / min. Next, the temperature of the contents of the reaction container is maintained at 80°C, and the contents of the reaction container are stirred at 250 rpm for 4 hours. Thus, the contents of the reaction container are reacted. Next, it is left to cool until the temperature of the contents of the reaction container reaches room temperature. Thus, a pigment particle dispersion after crosslinking treatment is obtained.

[0182] In the pigment particle dispersion of Example 1, the crosslinking ratio C1 of the crosslinked resin using the first crosslinking agent, the crosslinking ratio C2 of the crosslinked resin using the second crosslinking agent, and the total crosslinking ratio (C1+C2) of the crosslinked resin using the first crosslinking agent and the second crosslinking agent were calculated by the following method. In addition, for convenience of calculation, it is assumed that 1 part by mass = 1 g.

[0183] The molar number X1 of epoxy groups contained in the first crosslinking agent is calculated by dividing the amount of the first crosslinking agent used (0.51 g) by the epoxy equivalent of the first crosslinking agent (176 g / eq). In Example 1, the molar number X1 of epoxy groups contained in the first crosslinking agent is 2.90 mmol. The molar number X2 of epoxy groups contained in the second crosslinking agent is calculated by dividing the amount of the second crosslinking agent used (0.78 g) by the epoxy equivalent of the second crosslinking agent (268 g / eq). In Example 1, the molar number X2 of epoxy groups contained in the second crosslinking agent is 2.91 mmol.

[0184] Next, calculate the number of moles Y of carboxyl groups contained in the neutralized copolymer (r-1) (i.e., the specific resin). First, divide the acid value of the copolymer (r-1) (181 mgKOH / g) by the molecular weight of KOH (56.1) to calculate the number of moles Y of carboxyl groups per 1 g of the neutralized copolymer (r-1). The number of moles Y of carboxyl groups per 1 g of the neutralized copolymer (r-1) is 3.23 mmol / g. Next, multiply the amount of the neutralized copolymer (r-1) used (4.5 g) by the number of moles Y of carboxyl groups per 1 g of the neutralized copolymer (r-1) to calculate the number of moles Y of carboxyl groups contained in the neutralized copolymer (r-1). The number of moles Y of carboxyl groups contained in the neutralized copolymer (r-1) is 14.54 mmol.

[0185] Next, the values ​​obtained above are substituted into the following formula to calculate the crosslinking rate C1 of the crosslinked resin using the first crosslinking agent, the crosslinking rate C2 of the crosslinked resin using the second crosslinking agent, and the total crosslinking rate (C1+C2) of the crosslinked resin using the first crosslinking agent and the second crosslinking agent.

[0186] Crosslinking rate C1 [%] = 100 × number of moles of epoxy groups contained in the first crosslinking agent X1 / number of moles of carboxyl groups Y

[0187] Crosslinking rate C2 [%] = 100 × number of moles of epoxy groups contained in the second crosslinking agent X2 / number of moles of carboxyl groups Y

[0188] Total crosslinking rate [%] = C1 + C2

[0189] (Centrifugal treatment and re-dispersion treatment)

[0190] The pigment particle dispersion after cross-linking treatment is transferred to a container, and the container is set on a centrifugal adhesion measuring device ("NS-C100" manufactured by Nanosies Co., Ltd.). The pigment particle dispersion after cross-linking treatment is centrifuged for 24 hours at a rotation speed of 50,000 rpm using the above-mentioned centrifugal adhesion measuring device. After centrifugation, the supernatant is removed from the container, and then, the same volume of ion exchange water as the removed supernatant is added to the container. Then, the precipitate in the container is dispersed again in the ion exchange water. In this way, the free components are removed from the aqueous medium of the pigment particle dispersion after cross-linking treatment.

[0191] (Add processing)

[0192] According to the component 1 shown in Table 5 below, each component is added to the container. Specifically, in Example 1, 60.0 parts by mass of the redispersed pigment particle dispersion (about 9 parts by mass of the pigment, about 3 parts by mass of the cross-linking resin), 20.0 parts by mass of ethylene glycol, 15.0 parts by mass of diethylene glycol diethyl ether, 0.3 parts by mass of a nonionic surfactant ("Olfin (Japanese registered trademark) E 1004" manufactured by Nissin Chemical Industry Co., Ltd.) and 4.7 parts by mass of ion exchange water are added. Using a stirrer ("Slider Model BL-600" manufactured by Shinto Scientific Co., Ltd.), the contents of the above container are stirred at a rotation speed of 400 rpm to obtain a mixture. The obtained mixed solution is filtered using a filter (pore size: 5 μm). Thus, the ink of Example 1 is obtained.

[0193] [Preparation of inks of Examples 2 to 8 and Comparative Examples 1 to 22]

[0194] The inks of Examples 2 to 8 and Comparative Examples 1 to 22 were prepared using the same method as that for preparing the ink of Example 1, except that the type of the specific resin aqueous solution used in the dispersion treatment, the types and amounts of the first and second crosslinking agents used in the crosslinking treatment, and the composition of the components put into the container in the addition treatment (any one of components 1 to 3 in Table 5 below) were changed to those shown in Table 6 below.

[0195] [Table 5]

[0196]

[0197] [Table 6]

[0198]

[0199]

[0200] [Measurement of solid content of supernatant]

[0201] The inks of Examples 1 to 8 and Comparative Examples 1 to 22 were centrifuged using the following method. Next, the proportion of solid components contained in the obtained supernatant was measured. Specifically, first, the measurement object (any one of the inks of Examples 1 to 8 and Comparative Examples 1 to 22) was centrifuged at a speed of 140,000 rpm (1,050,000 G) for 3 hours using an ultracentrifuge ("himac (Japanese registered trademark) CS150FNX" manufactured by Eppendorf Highmac Technologies Co., Ltd., rotor: S140AT). Thus, the pigment particles contained in the measurement object were precipitated.

[0202] Next, transfer 30 μL of the supernatant contained in the object to be measured after centrifugation to an aluminum container for thermogravimetric measurement. Next, measure the mass (A) of 30 μL of the supernatant. Next, use a thermogravimetric analyzer ("TG / DTA7200" manufactured by Hitachi High-Tech Sciences, Ltd.) to perform thermogravimetric analysis according to the scheme shown in Table 7 below. Next, measure the weight loss (B) of the object to be measured at a temperature between 200°C and 500°C. The weight loss (B) is regarded as the mass of the solid component (mainly free resin) in the supernatant. The content ratio of the solid component in the supernatant is calculated according to the following formula. The measurement results are shown in Table 8 below.

[0203] Solid content ratio in supernatant [mass %] = 100 × B / A

[0204] [Table 7]

[0205] No. Start [℃] Limit [℃] Speed ​​[℃ / min] Keep [minutes] atmosphere 1 30 80 50 15 <![CDATA[N2]]> 2 80 200 50 15 <![CDATA[N2]]> 3 200 500 50 5 atmosphere

[0206] When the content ratio of the solid content in the supernatant liquid was 2.0 mass % or less, it was judged as "A (good)", and when it exceeded 2.0 mass %, it was judged as "B (bad)".

[0207] <Evaluation>

[0208] The following methods were used to evaluate nozzle clogging, image density, and abrasion resistance of the formed images for each ink of Examples 1 to 8 and Comparative Examples 1 to 22. The evaluation results are shown in Table 8 below.

[0209] [Evaluation Machine]

[0210] The evaluation machine used was an inkjet recording device equipped with a line head (a test machine manufactured by Kyocera Document Solutions Co., Ltd.) The inks of Examples 1 to 8 and Comparative Examples 1 to 22 to be evaluated were respectively filled in the black ink tank of the evaluation machine.

[0211] [Nozzle clogged]

[0212] The evaluation of nozzle clogging was carried out in an environment with a temperature of 25°C and a humidity of 60% RH. Matt paper for inkjet ("Superfine Paper" manufactured by Seiko Epson Co., Ltd.) was used as the recording medium. A solid image (150 mm × 200 mm) was continuously formed on 100 sheets of recording media using an evaluation machine. Next, a cleaning process was performed to remove the ink from the recording head of the evaluation machine. Next, a wiping process was performed, in which the ink ejection surface of the recording head of the evaluation machine was wiped with a clean wiping cloth (hereinafter, the process of wiping after the cleaning process is sometimes recorded as a cleaning process). Next, a nozzle check pattern was formed on the matte paper using an evaluation machine. The results confirmed that in any of the evaluation objects, ink was ejected from all nozzles (7968). That is, the number of nozzles that caused nozzle clogging was 0. Next, the recording head of the evaluation machine was cleaned. Next, the evaluation machine was left standing for 7 days without sealing the recording head of the evaluation machine. Next, the recording head of the evaluation machine was cleaned. Next, the evaluation machine was used to form a nozzle check pattern on the matte paper. Next, the formed nozzle check pattern was observed, and the number of nozzles with nozzle clogging was confirmed. The ratio of the number of nozzles with nozzle clogging in the recording head of the evaluation machine to the total number of nozzles was used as the evaluation value of nozzle clogging. Nozzle clogging was determined according to the following criteria.

[0213] (Nozzle clogging criteria)

[0214] A (good): The evaluation value is less than 10%.

[0215] B (bad): The evaluation value is 10% or more.

[0216] [Image Density]

[0217] The image density was evaluated in an environment of 25°C and 50%RH. Plain Paper Copy (PPC) ("C 2 ”). A solid image of 10 cm×10 cm is formed on a recording medium using an evaluation machine. In the formation of the solid image, the evaluation machine is set so that the volume of each drop of ink ejected from the recording head is 11 pL (11 pL of ink per 1 pixel). The image density of the formed solid image is measured using a reflection densitometer (“RD-19” manufactured by X-Rite). In the measurement of the image density, the image density of 10 randomly selected locations in the solid image is measured separately, and the arithmetic mean of the image density of the 10 locations is used as the evaluation value (ID) of the image density. The image density is determined according to the following criteria.

[0218] (Image density standard)

[0219] A (good): ID is 1.15 or above.

[0220] B (bad): ID is less than 1.15.

[0221] [Abrasion resistance]

[0222] In the evaluation of the abrasion resistance, A4-sized PPC paper ("C 2 ”). First, clean the recording head of the evaluation machine. Next, use the evaluation machine to form a 100mm×100mm solid image on the evaluation paper (printing rate is 100%). Stack unused evaluation paper (paper A) on the evaluation paper with the solid image formed. Next, place a 1kg rectangular weight on paper A. At this time, place the weight in such a way that the center of gravity of the weight is located directly above the center of the solid image. Next, use paper A to move in a certain direction with the above-mentioned weight placed. 5 times, thereby rubbing the solid image with paper A (load 1 kg). Using a reflection densitometer ("FD-9" manufactured by Konica Minolta Co., Ltd.), the image density of the dirty image transferred from the solid image to paper A was measured. Specifically, using the above-mentioned reflection densitometer, the image density of 10 randomly selected locations in the dirty image was measured, and the highest value of the image density of the 10 locations was used as the evaluation value of the abrasion resistance. The abrasion resistance was evaluated according to the following criteria.

[0223] (Standard for abrasion resistance)

[0224] A (good): The evaluation value is less than 0.020.

[0225] B (bad): The evaluation value is 0.020 or more.

[0226] [Table 8]

[0227]

[0228]

[0229] The inks of Examples 1 to 8 contain an aqueous medium and pigment particles dispersed in the aqueous medium. The pigment particles contain a pigment and a cross-linked resin. The cross-linked resin is a cross-linked product of a specific resin, a first cross-linking agent, and a second cross-linking agent. The specific resin is a neutralized product of a specific copolymer containing a specified repeating unit at a specified content ratio. The acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less. The weight average molecular weight of the specific copolymer is 3000 or more and 18000 or less. The neutralization rate of the specific resin is 20% or more and 100% or less. The first cross-linking agent has an ethylene oxide structure with an addition mole number of 1 or more and 4 or a propylene oxide structure with an addition mole number of 1 or more and 4 or less, and 2 or more epoxy groups in the molecule. The water solubility of the first cross-linking agent is 80% or more. The second cross-linking agent has an ethylene oxide structure with an addition mole number of 5 or more and 15 or less, and 2 or more epoxy groups in the molecule. The water solubility of the second cross-linking agent is 80% or more. In the cross-linked resin, the ratio (C2 / C1) is 18 / 82 or more and 82 / 18 or less. In the cross-linked resin, the total cross-linking rate (C1+C2) is 25% or more and 95% or less. The inks of Examples 1 to 8 can suppress the occurrence of nozzle clogging and can form images having a desired image density and excellent abrasion resistance.

[0230] On the other hand, since the water solubility of the first crosslinking agent in the ink of Comparative Example 1 is less than 80%, it is judged that the crosslinking resin using the first crosslinking agent is not sufficiently crosslinked. As a result, in the ink of Comparative Example 1, the crosslinking resin constituting the pigment particles is not sufficiently crosslinked, and the content ratio of the free resin increases. Therefore, the ink of Comparative Example 1 causes nozzle clogging. In addition, since the crosslinking resin in the ink of Comparative Example 1 is mainly crosslinked by the second crosslinking agent, the hydrophilicity of the crosslinking resin is too high. Therefore, the image density of the image formed by the ink of Comparative Example 1 is not good.

[0231] The ink of Comparative Example 2 uses a crosslinking agent that does not have an ethylene oxide structure with an addition molar number of 5 or more and 15 or less in the molecule as the second crosslinking agent. Therefore, it is judged that the crosslinking agent cannot impart sufficient hydrophilicity to the crosslinked resin. As a result, in the ink of Comparative Example 2, the hydrophilicity of the crosslinked resin is too low, and the abrasion resistance of the formed image is poor.

[0232] The ink of Comparative Example 3 uses a crosslinking agent having only one epoxy group in the molecule as the second crosslinking agent. In the ink of Comparative Example 3, since the second crosslinking agent is a type of crosslinking agent that cannot form an intermolecular crosslinking structure, the crosslinking resin is not fully crosslinked, and the content ratio of the free resin increases. Therefore, the ink of Comparative Example 3 causes nozzle clogging. In addition, the abrasion resistance of the image formed by the ink of Comparative Example 3 is also poor.

[0233] The ink of Comparative Example 4 does not use the second crosslinking agent. In the ink of Comparative Example 4, since the crosslinking resin is crosslinked only by the first crosslinking agent, the hydrophilicity of the crosslinking resin is too low. Therefore, the abrasion resistance of the image formed by the ink of Comparative Example 4 is not good. The ratio (C2 / C1) of the ink of Comparative Example 8 and Comparative Example 21 is too small, and the crosslinking resin is mainly crosslinked by the first crosslinking agent. As a result, the inks of Comparative Example 8 and Comparative Example 21 confirmed the same results as the ink of Comparative Example 4.

[0234] The ink of Comparative Example 5 does not use the first crosslinking agent. In the ink of Comparative Example 5, since the crosslinking resin is crosslinked only by the second crosslinking agent, the hydrophilicity of the crosslinking resin is too high. Therefore, the image density of the image formed by the ink of Comparative Example 5 is not good. The ratio (C2 / C1) of the ink of Comparative Example 9 and Comparative Example 22 is too large, and the crosslinking resin is mainly crosslinked by the second crosslinking agent. As a result, the inks of Comparative Example 9 and Comparative Example 22 confirmed the same results as the ink of Comparative Example 5.

[0235] In the ink of Comparative Example 6, the crosslinking rate of the crosslinked resin was too high. As a result, it was determined that the hydrophilicity of the crosslinked resin in the ink of Comparative Example 6 was low and the dispersion stability of the pigment particles was insufficient. As a result, the ink of Comparative Example 6 could not suppress the occurrence of nozzle clogging.

[0236] In the ink of Comparative Example 7, the crosslinking ratio of the crosslinked resin is too low. Since the crosslinked resin of the ink of Comparative Example 7 is not sufficiently crosslinked, the content ratio of the free resin is increased. Therefore, the ink of Comparative Example 7 causes nozzle clogging. In addition, the abrasion resistance of the image formed by the ink of Comparative Example 7 is also poor.

[0237] In the inks of Comparative Examples 10 to 17, the repeating units contained in the copolymers did not satisfy the prescribed composition. In addition, in the inks of Comparative Examples 16 and 17, the acid value of the specific copolymer was too low or too high. Therefore, it was determined that the dispersion stability of the pigment particles in the inks of Comparative Examples 10 to 17 was low, and the occurrence of nozzle clogging could not be suppressed. In addition, the images formed by the inks of Comparative Examples 14 and 15 had poor image density and abrasion resistance.

[0238] The neutralization rate of the specific resin in the ink of Comparative Example 18 was too low. Therefore, it was determined that the dispersion stability of the pigment particles in the ink of Comparative Example 18 was low and the occurrence of nozzle clogging could not be suppressed.

[0239] In the inks of Comparative Examples 19 and 20, the weight average molecular weight of the specific copolymer is too small or too large. Since the molecular size of the specific copolymer of the inks of Comparative Examples 19 and 20 is not appropriate, it is judged that the pigment cannot be efficiently coated by the specific copolymer. As a result, the inks of Comparative Examples 19 and 20 cannot suppress the occurrence of nozzle clogging.

Claims

1. An inkjet ink comprising an aqueous medium and pigment particles dispersed in the aqueous medium, The pigment particles include a pigment and a cross-linked resin, The cross-linked resin is a cross-linked product of a specific resin, a first cross-linking agent, and a second cross-linking 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 alkanediol (meth)acrylate or dialkanediol (meth)acrylate, With respect to 100 mass % of all repeating units possessed by the specific copolymer, the content ratio of the first repeating unit is 1 mass % to 65 mass %, the content ratio of the second repeating unit is 1 mass % to 60 mass %, the content ratio of the third repeating unit is 10 mass % to 40 mass %, and the content ratio of the fourth repeating unit is 1 mass % to 12 mass %, The acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less, The weight average molecular weight of the specific copolymer is 3000 or more and 18000 or less, The neutralization rate of the specific resin is 20% to 100%, The first cross-linking agent has an ethylene oxide structure with an added mole number of 1 to 4 or a propylene oxide structure with an added mole number of 1 to 4, and two or more epoxy groups in the molecule. The water solubility of the first cross-linking agent is more than 80%, The second crosslinking agent has an ethylene oxide structure with an addition mole number of 5 or more and 15 or less, and 2 or more epoxy groups in the molecule. The water solubility of the second cross-linking agent is more than 80%, In the cross-linked resin, a ratio (C2 / C1) of a cross-linking rate C2 by the second cross-linking agent to a cross-linking rate C1 by the first cross-linking agent is 18 / 82 or more and 82 / 18 or less, In the cross-linked resin, a total cross-linking rate (C1+C2) by the first cross-linking agent and the second cross-linking agent is 25% to 95%.

2. The inkjet ink according to claim 1, characterized in that The solid content in the supernatant obtained by centrifugation at 1,050,000 G for 3 hours was 2.0% by mass or less.

3. The inkjet ink according to claim 1 or 2, characterized in that: The first cross-linking agent comprises at least one of the compounds represented by the following general formulas (1-A) to (1-B), In the general formula (1-A), n represents an integer of 1 to 4, In the general formula (1-B), m represents an integer of 1 to 4.

4. The inkjet ink according to claim 3, characterized in that: The first cross-linking agent comprises at least one of the compounds represented by the following chemical formulas (1-A1), (1-B1), (1-B2) and (1-B3), 5. The inkjet ink according to claim 1 or 2, characterized in that: The second cross-linking agent comprises a compound represented by the following general formula (2-A): In the general formula (2-A), s represents an integer of 5 or more and 15 or less.

6. The inkjet ink according to claim 5, characterized in that: The second cross-linking agent comprises at least one of the compounds represented by the following chemical formulas (2-A1) to (2-A2), 7. The inkjet ink according to claim 1 or 2, characterized in that: With respect to 100 mass % of all repeating units in the specific copolymer, The content ratio of the first repeating unit is 15 mass % or more and 55 mass % or less, The content ratio of the second repeating unit is 10 mass % or more and 40 mass % or less, The content ratio of the third repeating unit is 20 mass % or more and 35 mass % or less, The content ratio of the fourth repeating unit is 4 mass % or more and 11 mass % or less.

8. The inkjet ink according to claim 1 or 2, characterized in that: The epoxy equivalent of the first cross-linking agent is 100 g / eq. or more and 230 g / eq. or less.

9. The inkjet ink according to claim 1 or 2, characterized in that: The epoxy equivalent of the second crosslinking agent is 250 g / eq. to 450 g / eq. both inclusive.

10. A method for producing an inkjet ink, the inkjet ink comprising an aqueous medium and pigment particles dispersed in the aqueous medium, The pigment particles include a pigment and a cross-linked resin, The manufacturing method comprises: A neutralization step, neutralizing the specific copolymer to obtain a specific resin; a dispersing step of dispersing the specific resin and the pigment in water to obtain a pigment particle dispersion; as well as a cross-linking step, using a first cross-linking agent and a second cross-linking agent to cross-link the specific resin in the pigment particle dispersion to obtain the cross-linked resin; 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 an alkanediol (meth)acrylate or a dialkanediol (meth)acrylate, With respect to 100 mass % of all repeating units possessed by the specific copolymer, the content ratio of the first repeating unit is 1 mass % to 65 mass %, the content ratio of the second repeating unit is 1 mass % to 60 mass %, the content ratio of the third repeating unit is 10 mass % to 40 mass %, and the content ratio of the fourth repeating unit is 1 mass % to 12 mass %, The acid value of the specific copolymer is 50 mgKOH / g or more and 300 mgKOH / g or less, The weight average molecular weight of the specific copolymer is 3000 or more and 18000 or less, The neutralization rate of the specific resin in the neutralization step is 20% to 100%, The first cross-linking agent has an ethylene oxide structure with an added mole number of 1 to 4 or a propylene oxide structure with an added mole number of 1 to 4, and two or more epoxy groups in the molecule. The water solubility of the first cross-linking agent is more than 80%, The second crosslinking agent has an ethylene oxide structure with an addition mole number of 5 or more and 15 or less, and 2 or more epoxy groups in the molecule. The water solubility of the second cross-linking agent is more than 80%, In the cross-linking step, a ratio (C2 / C1) of a cross-linking rate C2 by the second cross-linking agent to a cross-linking rate C1 by the first cross-linking agent is 18 / 82 or more and 82 / 18 or less, In the cross-linked resin, a total cross-linking rate (C1+C2) by the first cross-linking agent and the second cross-linking agent is 25% to 95%.

Images