Water-based ink, ink cartridge, and inkjet recording method

Abstract

To provide aqueous ink for inkjet which is excellent in storage stability and discharge stability.SOLUTION: Aqueous ink for inkjet has an organic pigment, and a resin having a polyethyleneimine structure. The resin having the polyethyleneimine structure has (i) an anionic group which is bonded to a nitrogen atom of the polyethyleneimine structure through a first bonding group, and (ii) a cyclic compound group which is bonded to the nitrogen atom of the polyethyleneimine structure through a second bonding group or forms cyclic imide together with the nitrogen atom of the polyethyleneimine structure, and both the molecular weight of the first bonding group and the molecular weight of the second bonding group are 10 or more and 300 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous ink, an ink cartridge, and an inkjet recording method. [Background technology]

[0002] In recent years, inkjet recording methods using pigment inks have been increasingly used to record images such as business documents clearly on recording media such as plain paper, and their frequency of use has increased dramatically. For such applications, a higher level of ink storage stability and ejection stability is required than ever before, so that images can be recorded stably even when inkjet recording devices are used for a long period of time. In particular, organic pigments often have lower storage stability and ejection stability than inorganic pigments such as carbon black, and there is a greater need to improve the above performance. Technologies to improve inkjet suitability, such as storage stability and ejection stability, by adding resins to pigment inks have been investigated. For example, a pigment formulation containing a pigment and a polyalkylene polyimine having a specific functional group has been proposed, which can also be used in the preparation of inkjet inks (see Patent Document 1). In addition, a polyalkylene imine polymer having a polyester group has been proposed for application to non-aqueous inks (see Patent Document 2). Furthermore, an ink containing two types of self-dispersing pigments, in which a specific functional group or polymer is bonded to the pigment, has been proposed (see Patent Document 3). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2005-206797 [Patent Document 2] Special Publication No. 2019-512037 [Patent Document 3] Japanese Patent Publication No. 2006-117938 [Overview of the project] [Problems that the invention aims to solve]

[0004] The present inventors investigated the storage stability of pigment inks using resins proposed in Patent Documents 1, 2, and 3. Patent Document 1 uses a resin in which specific functional groups are bonded to the nitrogen atom of a polyalkylene polyimine via long linking groups, but the storage stability and discharge stability of the ink were insufficient. Furthermore, the resin described in Patent Document 2 is intended for non-aqueous inks, and its performance when applied to aqueous inks is unknown. In addition, there is only one specific example of carbon black, and its performance with organic pigments is unknown. Moreover, in the example in Patent Document 3, the type of pigment used is unknown, and as mentioned above, the storage stability and discharge stability of organic pigments tend to be low, so it must be said that its performance is unknown. Therefore, it was found that it is necessary to improve the storage stability and discharge stability of aqueous inks containing organic pigments to meet the high levels required in recent years.

[0005] Therefore, the object of the present invention is to provide an aqueous inkjet ink with excellent storage stability and ejection stability, an ink cartridge using the aqueous ink, and an inkjet recording method. [Means for solving the problem]

[0006] The above objective is achieved by the present invention as described below. Specifically, the ink according to the present invention is an aqueous inkjet ink containing an organic pigment and a resin having a polyethyleneimine structure, wherein the resin having a polyethyleneimine structure has (i) an anionic group that is bonded to the nitrogen atom of the polyethyleneimine structure via a first linking group, and (ii) a cyclic compound group selected from the group consisting of cyclic hydrocarbon groups and heterocyclic compound groups that is bonded to the nitrogen atom of the polyethyleneimine structure via a second linking group, or forms a cyclic imide together with the nitrogen atom of the polyethyleneimine structure, and the molecular weights of both the first linking group and the second linking group are 10 or more and 300 or less. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an aqueous inkjet ink with excellent storage stability and ejection stability, an ink cartridge using the aqueous ink, and an inkjet recording method. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view showing one embodiment of the ink cartridge of the present invention. [Figure 2] This figure schematically shows an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. [Modes for carrying out the invention]

[0009] The present invention will be described in more detail below with reference to preferred embodiments. In the present invention, when a compound is a salt, the salt exists in the ink dissociated into ions, but for convenience, it will be expressed as "containing a salt." A "unit" of a resin refers to the smallest repeating unit that constitutes the resin, and means a structure formed by the (co)polymerization of one monomer. Also, water-based ink for inkjet printers may be simply referred to as "ink." Unless otherwise specified, physical properties are values ​​at room temperature (25°C).

[0010] The inventors have conducted various studies on improving storage stability and discharge stability by incorporating resins into inks. As a result, they have found that using an organic pigment and a resin in which anionic groups and cyclic compound groups are bonded to the nitrogen atom of a polyethyleneimine structure in a predetermined manner improves the storage stability and discharge stability of the ink. This resin having a polyethyleneimine structure (hereinafter sometimes simply referred to as "resin") has the following characteristics: The resin has (i) an anionic group that is bonded to the nitrogen atom of the polyethyleneimine structure via a first linking group. In addition, the resin has (ii) a cyclic compound group that is bonded to the nitrogen atom of the polyethyleneimine structure via a second linking group, or that forms a cyclic imide together with the nitrogen atom of the polyethyleneimine structure. The inventors hypothesize the following mechanism by which the storage stability and discharge stability of the ink are improved by using the organic pigment and resin with the above configuration.

[0011] Pigments are inherently hydrophobic (non-polar) substances, but compared to inorganic pigments such as carbon black, organic pigments have many polar groups, such as hydroxyl groups and carbonyl groups, that readily form hydrogen bonds on their particle surfaces. The polyethyleneimine structure, which consists of repeating ethyleneimine units represented by -CH2CH2-N<, becomes protonated due to the lone pair of electrons on the nitrogen atom, forming hydrogen bonds with the polar groups on the particle surface of the pigment. However, even if there are polar groups on the particle surface of an organic pigment, the overall structure is hydrophobic, and polyethyleneimine resin without substituents lacks sufficient interaction with organic pigments, resulting in poor storage stability.

[0012] In resins where anionic groups are bonded to the nitrogen atoms of a polyethyleneimine structure, the anionic groups and the nitrogen atoms of the resin, which exhibit cationic properties upon protonation, form ionic bonds, thus creating cross-linked structures within and between molecules. It was expected that when such resins coexist with organic pigments, a resin layer based on the cross-linked structure would form near the surface of the organic pigment particles, thereby improving the storage stability of the ink. However, such resins, like polyethyleneimine resins without substituents, lack sufficient interaction with organic pigments, which are inherently hydrophobic, and therefore do not achieve the desired storage stability.

[0013] Furthermore, in resins in which cyclic compound groups are bonded to nitrogen atoms in a polyethyleneimine structure, the cyclic compound groups are adsorbed onto the particle surface of organic pigments through the action of van der Waals forces, π-π interactions, and hydrophobic interactions. This makes it easier to stabilize the dispersion state of the organic pigments, thereby improving the storage stability of the ink.

[0014] As described above, resins in which a cyclic compound group is bonded to the nitrogen atom of a polyethyleneimine structure can stabilize the dispersion state of organic pigments. However, the pH of typical water-based inkjet inks is neutral to alkaline (pH 7.0 or higher), and in neutral to alkaline conditions, the resin tends to deprotonate and its hydrophilicity decreases, so the ejection stability does not improve.

[0015] Therefore, a cyclic compound group was further introduced into a resin in which an anionic group was bonded to a nitrogen atom of a polyethyleneimine structure. Such a resin has improved storage stability due to the adsorption of the organic pigment onto the particle surface by the cyclic compound group and the ionic bond between the anionic group and the nitrogen atom that exhibits cationicity by protonation. Furthermore, since the cyclic compound group and the anionic group repel each other, the adsorption of the organic pigment onto the particle surface by the cyclic compound group is further enhanced, and the ionic bond between the anionic group and the nitrogen atom that exhibits cationicity by protonation is maintained more stably. As described above, since the organic pigment has more polar groups than the inorganic pigment, the polar groups form hydrogen bonds with the amino group, imino group, and anionic group of the resin, and the resin strongly adsorbs to the organic pigment. Therefore, by bonding an anionic group and a cyclic compound group to a polyethyleneimine resin having an ethyleneimine structure, the storage stability of the ink is further enhanced, and the high level of storage stability required in recent years can be satisfied.

[0016] With the above configuration, it has been found that not only can the resin easily adsorb to the organic pigment and improve the storage stability of the ink, but the ejection stability can also be improved. Details will be described below. When the ink is continuously ejected, minute ink droplets may adhere to the surface (ejection port surface) on which the ejection ports of the recording head are formed. When the liquid component evaporates from the ink droplets adhering to the ejection port surface, the organic pigment rapidly aggregates. At the ejection port surface, the ink adheres more easily to the portion where the aggregated organic pigment adheres than to the portion where the aggregated organic pigment does not adhere. When this process is repeated, the amount of ink adhesion increases, and normal ink ejection is hindered. The ink of the present invention can effectively suppress the aggregation of the organic pigment because the resin easily adsorbs to the organic pigment and can stably maintain the dispersed state of the organic pigment. Thereby, the ejection stability of the ink can be enhanced.

[0017] The anionic group and the cyclic compound group each need to be bonded to the nitrogen atom of the polyethyleneimine structure via a linking group. In this specification, the "linking group" means a divalent or higher partial structure between the nitrogen atom of the polyethyleneimine structure and the cyclic compound group or the anionic group which is a substituent. The molecular weight of the linking group needs to be 10 or more and 300 or less. The presence of a linking group having a predetermined molecular weight between the anionic group or the cyclic compound group and the nitrogen atom increases the degree of freedom of molecular motion of the resin, making it easier to adopt a conformation optimal for adsorption and ionic bonding to the particle surface of the organic pigment. On the other hand, when no linking group is present, or when the molecular weight is less than 10 or more than 300, a conformation optimal for adsorption or ionic bonding of the resin to the particle surface of the organic pigment cannot be taken, and storage stability and ejection stability cannot be obtained. The cyclic compound group may form a five-membered or six-membered cyclic imide together with the nitrogen atom of the polyethyleneimine structure. In this specification, "cyclic imide" means a compound having an imide group (-CO-NH-CO-) in the ring. The nitrogen atom portion of the imide group may be the nitrogen atom of the polyethyleneimine structure, and the hydrogen atom portion of the imide group may be the carbon atom of the polyethyleneimine structure. In this case, the cyclic imide portion adsorbs to the particle surface of the organic pigment, and the above effects can be obtained.

[0018] <Ink> The ink of the present invention is an inkjet ink containing an organic pigment and a resin having a specific structure. Hereinafter, the components constituting the ink of the present invention, the physical properties of the ink, and the like will be described in detail.

[0019] (Organic Pigment) The ink contains an organic pigment as a coloring material. The content (mass%) of the organic pigment in the ink is preferably 0.1 mass% or more and 15.0 mass% or less, more preferably 1.0 mass% or more and 10.0 mass% or less, based on the total mass of the ink.

[0020] Specific examples of organic pigments include azo, phthalocyanine, quinacridone, isoindolinone, imidazolon, diketopyrrolopyrrole, and dioxazine. Among these, organic pigments having a quinacridone skeleton or a diketopyrrolopyrrole skeleton are preferred. Organic pigments having these skeletons have a particularly large number of functional groups in their structure that readily form hydrogen bonds, such as carbonyl groups and imino groups, and form many hydrogen bonds with the amino groups, imino groups, and anionic groups of resins having a polyethyleneimine structure. As a result, the resin adsorbs the organic pigment more strongly, further improving storage stability and discharge stability.

[0021] Organic pigments having a diketopyrrolopyrrole skeleton include CI Pigment Orange: 71, 73, etc.; CI Pigment Red: 254, 255, 264, etc. Organic pigments having a quinacridone skeleton include CI Pigment Violet 19 (unsubstituted quinacridone), CI Pigment Red 122 (2,9-dimethylquinacridone), CI Pigment Red 202 (2,9-dichloroquinacridone), etc. As organic pigments having a quinacridone skeleton, quinacridone solid solution pigments formed from two or more quinacridone pigments can also be used. Among these, solid solution pigments of CI Pigment Red 202 and CI Pigment Violet 19; and solid solution pigments of CI Pigment Red 122 and CI Pigment Violet 19 are preferred.

[0022] As for pigment dispersion methods, resin-dispersed pigments using a resin (resin dispersant) as a dispersant, and self-dispersing pigments in which hydrophilic groups are bonded to the surface of the pigment particles can be used. In addition, resin-bonded pigments in which organic groups containing resin are chemically bonded to the surface of the pigment particles, and microcapsule pigments in which the surface of the pigment particles is coated with a resin or the like can be used.

[0023] Among these, resin-dispersed pigments, in which the pigment is dispersed by a resin dispersant, are preferred. In particular, resin-dispersed pigments in which a resin having a polyethyleneimine structure is used as the resin dispersant for dispersing the pigment are preferred. However, when a resin other than a resin having a polyethyleneimine structure is used as the resin dispersant, it is preferable to use a resin having hydrophilic groups that disperse the pigment in the liquid medium through the action of hydrophilic groups such as anionic groups or ethylene oxide groups.

[0024] As long as the effects of the present invention are not impaired, the ink may contain other pigments in addition to the organic pigment. The content (mass%) of other pigments in the ink is preferably 0.1% by mass or more and 15.0% by mass or less, and more preferably 1.0% by mass or more and 10.0% by mass or less, based on the total mass of the ink. Examples of other pigments include inorganic pigments such as carbon black and titanium dioxide.

[0025] (Resin having a polyethyleneimine structure) The ink contains a resin having a polyethyleneimine structure to which anionic groups and specific cyclic compound groups are bonded (hereinafter sometimes referred to as "resin having a polyethyleneimine structure"). In this specification, "anionic group" refers to an acidic functional group that becomes anionic upon proton dissociation. The resin having a polyethyleneimine structure is a resin whose repeating unit is an ethyleneimine structure represented by -CH2CH2-N<. The resin having a polyethyleneimine structure may contain primary, secondary, or tertiary amino nitrogen.

[0026] Resins having a polyethyleneimine structure can be either branched or linear. Among these, it is preferable to use a resin having a branched polyethyleneimine structure. In other words, the polyethyleneimine structure is preferably branched. Branched polyethyleneimine contains ethyleneimine structures with primary, secondary, and tertiary amino nitrogens and has a randomly branched structure. Linear polyethyleneimine has ethyleneimine structures with secondary amino nitrogens except at both ends.

[0027] The anionic group introduced into the polyethyleneimine structure may be a free acid form to which a proton is bonded, or a salt form to which a cation such as a metal ion or ammonium ion is bonded. It may also be an anionic form after ion dissociation in the ink. Examples of anionic groups in their free acid form include carboxylic acid groups (-COOH), sulfonic acid groups (-SO3H), phosphate groups (-OPO3H2), and phosphonic acid groups (-PO3H2), and these may be the same or different. Considering stability against hydrolysis, it is preferable that the anionic group introduced into the polyethyleneimine structure is at least one of a carboxylic acid group and a sulfonic acid group.

[0028] The cyclic compound group bonded to the nitrogen atom of the polyethyleneimine structure is selected from the group consisting of cyclic hydrocarbon groups and heterocyclic compound groups. The cyclic compound group preferably has 1 to 10 carbon atoms and may have substituents such as alkyl groups and anionic groups having about 1 to 3 carbon atoms. Examples of cyclic hydrocarbon groups include alicyclic hydrocarbon groups and aromatic hydrocarbon groups. Examples of alicyclic hydrocarbon groups include monocyclic cycloalkyl groups such as cyclopropyl group, cyclobutyl group, cyclopentyl group, and cyclohexyl group, and polycyclic cycloalkyl groups such as isobornyl group, dicyclopentenyl group, dicyclopentanyl group, and adamantyl group. Examples of aromatic hydrocarbon groups include aryl groups such as phenyl group, tolyl group, naphthyl group, phenanthryl group, anthryl group, and fluorenyl group. As for heterocyclic compound groups, five-membered or six-membered rings are preferred due to their stability. Examples of five-membered heterocyclic compound groups include pyrrolidine group, pyrrole group, furan group, and imidazole group. Examples of six-membered heterocyclic compound groups include piperidine, pyridine, pyran, and piperazine groups.

[0029] (i) The anionic group is bonded to the nitrogen atom of the polyethyleneimine structure via a first linking group, and (ii) the cyclic compound group is bonded to the nitrogen atom of the polyethyleneimine structure via a second linking group. The cyclic compound group may form a five-membered or six-membered cyclic imide together with the nitrogen atom of the polyethyleneimine structure. The nitrogen atoms to which the cyclic compound group and the anionic group are bonded may be the same or different. From the viewpoint of ease of synthesis, it is preferable that the anionic group and the cyclic compound group are bonded to different nitrogen atoms. Hereinafter, the first linking group and the second linking group will be collectively referred to as the "linking group". The linking group is preferably divalent or pentavalent, and more preferably divalent from the viewpoint of ease of introduction when synthesizing the resin. The linking groups of the anionic group and the cyclic compound group may be the same structure or different structures. Examples of linking groups include atomic groups composed of carbon atoms, hydrogen atoms, oxygen atoms, nitrogen atoms, sulfur atoms, etc., and the hydrogen atom may be substituted with an alkyl group, a hydroxyl group, or a halogen atom. The molecular weight of the linking group must be between 10 and 300.

[0030] Examples of divalent linking groups include at least one group selected from the group consisting of alkylene groups, arylene groups, alkylene oxide groups, carbonyl groups (-CO-), ester groups (-COO-), and amide groups (-CONH-). Examples of alkylene groups include methylene groups and ethylene groups. Examples of arylene groups include phenylene groups and naphthylene groups. Examples of alkylene oxide groups include ethylene oxide groups and propylene oxide groups. Substituents may be groups that combine two or more of the above groups. In this invention, ester groups (-COO-) and amide groups (-CONH-) are functional groups different from carbonyl groups (-CO-). Among these, it is even more preferable that the first linking group and the second linking group are each independently at least one group selected from the group consisting of alkylene groups, arylene groups, carbonyl groups, ester groups, and amide groups. These linking groups allow for easy adjustment of the balance between hydrophilicity and hydrophobicity.

[0031] This section describes a method for bonding anionic groups or cyclic compound groups to the nitrogen atom of a polyethyleneimine structure via a linking group. For example, it is convenient to use a reagent having a reactive functional group at one end of the linking group and a cyclic compound group or anionic group at the other end. Hereinafter, such reagents will be referred to as anionic group introduction reagents and cyclic compound group introduction reagents, respectively. Specifically, examples include amidation using acid chlorides or acid anhydrides, the Michael reaction using acrylic acid, and alkylation of amines using alkyl halides or epoxy compounds.

[0032] The introduction rate (%) of anionic groups and cyclic compound groups bonded to nitrogen atoms in a polyethyleneimine structure via linking groups is preferably 0.5% to 50.0%, respectively, based on the number of nitrogen atoms in the polyethyleneimine structure. More preferably, the introduction rate (%) is 1.0% to 40.0%, and particularly preferably 2.0% to 30.0%.

[0033] When a dicarboxylic acid anhydride having a cyclic compound group is reacted with a primary amine having a polyethyleneimine structure at temperatures above 80°C, a five-membered or six-membered cyclic imide can be formed between the dicarboxylic acid and the nitrogen atom of the polyethyleneimine structure. Furthermore, when a dicarboxylic acid anhydride is reacted with a primary amine having a polyethyleneimine structure at temperatures below 50°C, an amide is formed between the primary amine and one carboxylic acid, while the other carboxylic acid remains unreacted to form an anionic group. Moreover, when a dicarboxylic acid anhydride is reacted with a secondary amine having a polyethyleneimine structure, one carboxylic acid forms an amide, while the other carboxylic acid remains unreacted to form an anionic group.

[0034] The weight-average molecular weight of the resin having a polyethyleneimine structure is preferably between 500 and 100,000. In particular, the weight-average molecular weight of the resin having a polyethyleneimine structure is more preferably between 1,500 and 70,000, as this can further improve storage stability and discharge stability.

[0035] The content (mass%) of the resin having a polyethyleneimine structure in the ink is preferably 0.05% by mass or more and 10.0% by mass or less, and more preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink. The content (mass%) of the resin having a polyethyleneimine structure in the ink is preferably 0.05 times or more and 0.50 times or less in mass ratio to the content (mass%) of the organic pigment. When the mass ratio is within this range, a good balance of the content of each component can be obtained, resulting in a better level of storage stability and discharge stability. The mass ratio is more preferably 0.10 times or more and 0.30 times or less.

[0036] The fact that anionic groups and cyclic compound groups are bonded to the nitrogen atoms of a resin having a polyethyleneimine structure via linking groups can be confirmed, for example, by calculating the introduction rate of these functional groups using the following method. First, a resin is synthesized by reacting polyethyleneimine with an introduction reagent to introduce anionic groups or cyclic compound groups. Then, the resin is purified and analyzed to measure the introduction rate based on the number of nitrogen atoms in the polyethyleneimine structure.

[0037] Dialysis can be performed to purify the resin. When performing dialysis, it is preferable to use a semipermeable membrane with a molecular weight cutoff of approximately 0.1 to 1 kDa as the dialysis membrane. For example, products such as "Biotech CE" (molecular weight cutoff 0.1 to 0.5 kDa type, manufactured by REPLIGEN) and "Spectra / Por" (molecular weight cutoff 1 kDa type, manufactured by REPLIGEN) can be used. The amount of deionized water used and the standing time can be adjusted as appropriate depending on the amount and type of impurities. Specifically, the amount of deionized water used can be approximately 10 to 100 times (by mass) the amount of the resin solution. The standing time can be 12 to 48 hours.

[0038] Known analytical methods can be used to measure the introduction rate of anionic groups and cyclic compound groups. For example, the following can be used. · 1 H or 13 Nuclear Magnetic Resonance (NMR) Spectroscopy for C (Reverse Gated Decoupling) • UV-Vis-NIR (ultraviolet-visible) spectroscopy analysis targeting aromatic groups • Infrared absorption (FT-IR) spectroscopy targeting carbonyl groups, etc. • Inductively coupled plasma emission (ICP) spectroscopy targeting sulfur atoms in SO3H groups

[0039] (Other resins) The ink may contain a resin other than the resin having a polyethyleneimine structure (another resin). The content (by mass) of the other resin in the ink is preferably 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink.

[0040] Other resins can be incorporated into the ink for purposes such as (i) stabilizing the dispersion of pigments, i.e., as resin dispersants or auxiliary agents for pigments, and (ii) improving various properties of the recorded image. Examples of resin forms include block copolymers, random copolymers, graft copolymers, and combinations thereof. The resin may be a water-soluble resin that can dissolve in an aqueous medium, or it may be resin particles dispersed in an aqueous medium. The resin particles do not need to contain colorants.

[0041] In this specification, "water-soluble resin" means that when the resin is neutralized with an alkali equivalent to its acid value, it exists in a liquid medium without forming particles whose particle size can be measured by dynamic light scattering. Whether or not a resin is water-soluble can be determined according to the following method. First, prepare a liquid containing the resin (resin solids content: 10% by mass) neutralized with an alkali equivalent to its acid value (sodium hydroxide, potassium hydroxide, etc.). Next, prepare a sample solution by diluting the prepared liquid 10 times (by volume) with pure water. Then, when the particle size of the resin in the sample solution is measured by dynamic light scattering, if no particles with a particle size are measured, the resin can be determined to be water-soluble. The measurement conditions in this case can be, for example, SetZero: 30 seconds, Number of measurements: 3, Measurement time: 180 seconds. As a particle size distribution analyzer, a particle size analyzer using dynamic light scattering (for example, product name "UPA-EX150", manufactured by Nikkiso) can be used. Of course, the particle size distribution analyzer and measurement conditions used are not limited to those described above.

[0042] The acid value of the other resin is preferably 0 mg KOH / g or more and 250 mg KOH / g or less. The "other resin" can also be used as a resin dispersant for dispersing organic pigments. In this case, in order to efficiently adsorb the resin having a polyethyleneimine structure onto the organic pigment, the acid value of the other resin is preferably 15 mg KOH / g or less, and more preferably 0 mg KOH / g. The weight-average molecular weight of the other resin is preferably 1,000 or more and 30,000 or less, and more preferably 5,000 or more and 15,000 or less. The weight-average molecular weight of the resin is a polystyrene equivalent value measured by gel permeation chromatography (GPC).

[0043] Other resins include acrylic resins, urethane resins, and urea resins. Among these, acrylic resins are preferred. Among acrylic resins, those having hydrophilic units and hydrophobic units as constituent units are preferred. In particular, acrylic resins having hydrophilic units derived from (meth)acrylic acid and hydrophobic units derived from monomers having aliphatic or aromatic groups are preferred.

[0044] (aqueous medium) The ink is an aqueous ink containing water as an aqueous medium. The ink may contain an aqueous medium which is water, or a mixed solvent of water and a water-soluble organic solvent. Resins having a polyethyleneimine structure have nitrogen atoms that exhibit cationic properties due to protonation, and anionic groups, and therefore have amphoteric properties and high hydrophilicity. For this reason, even if the pH of the ink changes or the concentration of components changes due to evaporation of liquid components, the dispersion state of the organic pigment can be stably maintained.

[0045] It is preferable to use deionized water (ion-exchanged water) as the water. The water content (mass%) in the ink is preferably 50.0% by mass or more and 95.0% by mass or less, based on the total mass of the ink. There are no particular restrictions on the water-soluble organic solvent as long as it is water-soluble (preferably, it dissolves in water in any proportion at 25°C). Specifically, monohydric or polyhydric alcohols, alkylene glycols, glycol ethers, nitrogen-containing polar compounds, sulfur-containing polar compounds, etc., can be used. The water-soluble organic solvent content (mass%) in the ink is preferably 5.0% by mass or more and 90.0% by mass or less, and more preferably 10.0% by mass or more and 50.0% by mass or less, based on the total mass of the ink.

[0046] (Other additives) Ink may contain various additives as needed, such as surfactants, pH adjusters, preservatives, fungicides, antioxidants, reduction inhibitors, evaporation accelerators, and chelating agents. Among these, it is preferable that the ink contains a surfactant. The surfactant content (mass%) in the ink is preferably 0.1% to 5.0% by mass, and more preferably 0.1% to 2.0% by mass, based on the total mass of the ink. Examples of surfactants include anionic surfactants, cationic surfactants, and nonionic surfactants.

[0047] (Ink properties) Since the ink is for use in an inkjet system, it is preferable to appropriately control its physical properties. The surface tension of the ink at 25°C is preferably 10 mN / m to 60 mN / m, and more preferably 20 mN / m to 40 mN / m. The viscosity of the ink at 25°C is preferably 1.0 mPa·s to 10.0 mPa·s. The pH of the ink at 25°C is preferably 5.0 to 10.0, and more preferably 7.0 to 8.5.

[0048] <Ink Cartridge> The ink cartridge of the present invention comprises ink and an ink storage section for storing this ink. The ink stored in this ink storage section is the ink of the present invention as described above. Figure 1 is a schematic cross-sectional view showing one embodiment of the ink cartridge of the present invention. As shown in Figure 1, an ink supply port 12 for supplying ink to the recording head is provided on the bottom surface of the ink cartridge. The inside of the ink cartridge is an ink storage section for storing ink. The ink storage section consists of an ink storage chamber 14 and an absorbent storage chamber 16, which are in communication with each other via a communication port 18. The absorbent storage chamber 16 is also in communication with the ink supply port 12. Liquid ink 20 is stored in the ink storage chamber 14, and absorbent materials 22 and 24 that hold the ink in an impregnated state are stored in the absorbent storage chamber 16. The ink storage section may not have an ink storage chamber for storing liquid ink, and the entire amount of ink to be stored may be held by an absorbent. Alternatively, the ink storage section may not have an absorbent, and the entire amount of ink may be stored in a liquid state. Furthermore, the ink cartridge may be configured to include an ink storage section and a recording head.

[0049] <Inkjet recording method> The inkjet recording method of the present invention is a method of recording an image on a recording medium by ejecting the ink of the present invention described above from an inkjet recording head. Methods for ejecting the ink include methods that impart mechanical energy to the ink and methods that impart thermal energy to the ink. In the present invention, it is particularly preferable to employ a method that imparts thermal energy to the ink to eject it. Aside from using the ink of the present invention, the steps of the inkjet recording method may be those of known origin.

[0050] Figure 2 is a schematic diagram showing an example of an inkjet recording apparatus used in the inkjet recording method of the present invention, where (a) is a perspective view of the main part of the inkjet recording apparatus and (b) is a perspective view of the head cartridge. The inkjet recording apparatus is provided with a transport means (not shown) for transporting the recording medium 32 and a carriage shaft 34. A head cartridge 36 can be mounted on the carriage shaft 34. The head cartridge 36 comprises recording heads 38 and 40 and is configured to hold an ink cartridge 42. While the head cartridge 36 is transported along the carriage shaft 34 in the main scanning direction, ink (not shown) is ejected from the recording heads 38 and 40 toward the recording medium 32. Then, the recording medium 32 is transported in the sub-scanning direction by the transport means (not shown), and an image is recorded on the recording medium 32. Any type of recording medium can be used as the recording medium 32, but it is preferable to use a paper-based recording medium that has permeability, such as a recording medium without a coating layer, such as plain paper, or a recording medium with a coating layer, such as glossy paper, art paper, or matte paper. [Examples]

[0051] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples unless it exceeds the gist of the invention. Unless otherwise specified, amounts of components indicated in "parts" and "%" are based on mass.

[0052] <Analysis of resins containing polyethyleneimine structure> The fact that anionic groups and cyclic compound groups are bonded to the nitrogen atom of a resin having a polyethyleneimine structure via linking groups was confirmed by calculating the introduction rate of these functional groups using the following method.

[0053] First, a resin synthesized by reacting polyethyleneimine with an introduction reagent was dissolved in deionized water to prepare a solution. This solution was sealed in a dialysis membrane (product name "Spectra / Por", molecular weight cutoff 1 kDa, manufactured by REPLIGEN) to remove low molecular weight components and salts. The dialysis membrane containing the solution was placed in deionized water approximately 100 times the volume of the solution (by mass) and allowed to stand for about 48 hours. After that, the dialysis membrane was removed, the outside was washed with deionized water, and the contents of the dialysis membrane (purified solution) were extracted. Water was removed from the obtained purified solution by vacuum distillation, and then it was dried in a vacuum heating dryer at 100°C for 12 hours to obtain a sample (resin). The introduction rates of anionic groups and cyclic compound groups were determined using the obtained sample.

[0054] The following are examples of analytical methods for measuring the introduction rate of anionic groups and cyclic compound groups. · 1 H or 13 Nuclear Magnetic Resonance (NMR) Spectroscopy for C (Reverse Gated Decoupling) • UV-Vis-NIR (ultraviolet-visible) spectroscopy analysis targeting aromatic groups • Infrared absorption (FT-IR) spectroscopy targeting carbonyl groups, etc. • Inductively coupled plasma emission (ICP) spectroscopy targeting sulfur atoms in SO3H groups

[0055] The introduction rate of phenyl groups (cyclic compound groups) bonded to the nitrogen atom of the polyethyleneimine structure was calculated as follows. A small amount of the synthesized resin was sampled and subjected to the aforementioned purification and 1 ¹H-NMR analysis (D2O, internal standard: sodium 3-(trimethylsilyl)-1-propanesulfonate) revealed peaks originating from phenyl group protons at 7.2–7.6 ppm. Furthermore, the integral values ​​of the phenyl group protons and the ethylene group protons in the polyethyleneimine structure were determined, and the phenyl group introduction rate (%) based on the number of nitrogen atoms in the polyethyleneimine structure was calculated using the following formula. The introduction rates of other functional groups were also calculated by appropriately selecting from the above analytical methods. (Introduction rate, %) = (4 / 5) × (I Ph / I PEI ) × 100 I Ph : Integration value of protons of phenyl group I PEI : Integration value of protons of polyethyleneimine structure

[0056] <Measurement conditions of gel permeation chromatography (GPC)> The weight-average molecular weight was measured by GPC according to the conditions shown below. GPC apparatus: Trade name "HLC-8320GPC EcoSEC" (manufactured by Tosoh Corporation) Detector: Differential refractive index detector Column: Trade names "OHpak SB-802.5 HQ", "SB-803 HQ", "SB-804 HQ" (all manufactured by Shodex) Column temperature: 40 °C Eluent: (Aqueous solution containing 0.5 mol / L acetic acid and 0.2 mol / L sodium nitrate) / acetonitrile = 50 / 50 (volume basis) Molecular weight standard substance: Polyethylene glycol

[0057] <Synthesis of resin> ]](Resins 1 - 31) Into a flask equipped with a stirrer, a reflux condenser, a thermometer, and a gas inlet tube shown in Tables 1 and 2, polyethyleneimine, a base, a polymerization inhibitor for suppressing side reactions during the introduction reaction of an anionic group, and a solvent were placed and stirred at 25 °C for dissolution. Then, an introduction reagent was added and further stirred under the reaction conditions shown in Table 1 to bond the anionic group to the nitrogen atom of the polyethyleneimine structure. At this point, the introduction rate of the anionic group was measured by the above method. If necessary, the inlet tube of this reaction vessel was replaced with those shown in Tables 3 and 4.

[0058] ​Next, the introduction reagents, base, polymerization inhibitor to suppress side reactions during the introduction of the cyclic compound group, and solvent shown in Tables 3 and 4 were added to the reaction vessel. The mixture was further stirred under the reaction conditions shown in Tables 3 and 4 to bond the cyclic compound group to the nitrogen atom of the polyethyleneimine structure. After cooling to 25°C as needed, 50.0 parts of deionized water were added to the reaction vessel to prepare a solution containing the resin. The obtained solution was placed in a permeable membrane (trade name "Spectra / Por", molecular weight cutoff 1 kDa, manufactured by REPLIGEN) and sealed. The permeable membrane containing the sealed solution was immersed in a beaker containing 2,000 parts of deionized water, and the contents of the beaker were stirred with a stirrer for 24 hours to remove impurities from the solution. After that, the dialysis membrane containing the sealed solution was removed, the outside was washed with deionized water, and the contents of the dialysis membrane were extracted. The liquid components of the contents of the permeable membrane were removed by vacuum distillation to obtain a solid resin. The weight-average molecular weight was determined by the above GPC analysis. Furthermore, the introduction rate of cyclic compound groups was measured using a portion of the synthesized resins by the method described above. In resins 8-10 and 15, the formation of the five-membered or six-membered cyclic imides described above was confirmed. In resin 23, it was confirmed that two carboxylic acid groups were bonded to the nitrogen atom of the polyethyleneimine structure via a trivalent linking group.

[0059] (Resin 32) As resin 32, it was synthesized according to Example 1 of Patent Document 1 (production of sulfated alkoxylated polyethyleneimine), and -(CH2CH2O) m (CH2CH(CH3)O) n A polyethyleneimine having sulfonic acid groups linked by - was obtained. The molecular weight of the linking group of the sulfonic acid group was over 300.

[0060] (Resin 33) Resin 33 was synthesized according to Example 6 of Patent Document 1 (reaction of alkoxylated polyethyleneimine with phenyl isocyanate and subsequent functionalization with succinic anhydride). As a result, -(CH2CH2O) m (CH2CH(CH3)O) n Phenyl groups linked by - and -(CH2CH2O) m(CH2CH(CH3)O) n A polyethyleneimine having carboxylic acid groups linked by -COCH2CH2- was obtained. The molecular weights of the phenyl group and the linking group of the carboxylic acid group were both over 300.

[0061] (Resin 34) Polymer 1 was synthesized as resin 34 in accordance with production examples 1 to 6 of Japanese Patent Publication No. 2017-160289. This polymer is a polyethyleneimine having 2-ethylhexyl groups linked by -CH2CH(OH)CH2O-. The molecular weight of the linking group of the 2-ethylhexyl group was 74.

[0062] (Resin 35) Resin 35 was synthesized according to the description of dispersant 1 in Patent Document 2. The obtained resin 35 mainly had groups of the following structural formula (1) bonded to polyethyleneimine via carboxamide and carboxymide groups, and had copolyester side chains of ε-caprolactone, γ-valerolactone, and lauric acid. In structural formula (1), the right-hand group formed a six-membered cyclic imide together with the nitrogen atom of the polyethyleneimine structure. The molecular weight of the carboxylic acid linking group was 154.

[0063] [ka]

[0064] (Resin 36) As resin 36, a commercially available branched polyethyleneimine (product name "Epomin SP-200", resin content 98% or more, manufactured by Nippon Shokubai Co., Ltd.) was used. This resin had a polyethyleneimine structure in which anionic groups and cyclic compound groups were not bonded.

[0065] Tables 5 and 6 show the properties of the synthesized resins. The reagents used in the synthesis of the resins (polyethyleneimine, introduction reagent) are as follows. SP-200, SP-003, SP-006 (all with a resin content of 98% or more), HM-2000 (resin content of 93% to 95%), P-1000 (30% resin aqueous solution): all sold under the brand name "Epomin," are branched polyethyleneimine products manufactured by Nippon Shokubai. • PEI 2500: Linear polyethyleneimine, manufactured by Polysciences. • Acrylate ester compound 1: Compound represented by the following structural formula (2)

[0066] [ka]

[0067] This compound was synthesized according to the following preparation example. In a round-bottom flask, 3.00 parts mono(2-acryloyloxyethyl) succinate, 2.28 parts N-(2-aminoethyl)benzamide synthesized according to International Publication No. 2002 / 051836, and 30 parts ethanol were added and stirred at 25°C to dissolve. Then, 3.84 parts 4-(4,6-dimethoxy-1,3,5-triazine-2-yl)-4-methylmorpholinium chloride was added as a coupling agent and stirred at 25°C for 2 hours. The ethanol was then removed by vacuum distillation, and the resulting residue was dissolved in chloroform. The organic phase was washed by two-phase extraction with 1 mol / L hydrochloric acid and then 1 mol / L aqueous sodium carbonate solution, and then dried over anhydrous magnesium sulfate. Subsequently, filtration was performed, and chloroform was removed from the obtained filtrate by vacuum distillation to obtain 3.02 parts (yield 60%) of acrylic acid ester compound 1. • Acrylate ester compound 2: Compound represented by the following structural formula (3)

[0068] [ka]

[0069] This compound was synthesized in the same manner as in the preparation example of acrylic acid ester compound 1, except that N-(2-aminoethyl)benzamide was replaced with 1-benzoylpiperazine. • EX-145: Product name "Denacol", epoxy compound, manufactured by Nagase ChemteX, compound represented by the following structural formula (4).

[0070] [ka]

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] [Table 4]

[0075] [Table 5]

[0076] [Table 6]

[0077] <Preparation of Pigment Dispersion> Mixtures were prepared by mixing the pigments, resins, and liquid media shown in Table 7. The resulting mixtures were placed in a batch-type vertical sand mill (manufactured by AIMEX), filled with 150.0 parts of 0.3 mm diameter zirconia beads, and dispersed for 5 hours while water-cooled. After removing coarse particles by centrifugation, appropriate amounts of the same type of liquid media as shown in Table 7 were added as needed to obtain each pigment dispersion. The amounts of pigments and resins listed in Table 7 are all in terms of solid content. Details of each component in Table 7 are shown below. • Cab-o-jet 260M: A commercially available aqueous pigment dispersion (product name "Cab-o-jet 260M", manufactured by Cabot, pigment content: 10.0%) containing a self-dispersing pigment in which benzenesulfonic acid groups are bonded to the surface of pigment particles (CI Pigment Red 122). The liquid component of this dispersion is evaporated to increase the pigment content to 20.0%. • Cab-o-jet 200: A commercially available aqueous pigment dispersion containing a self-dispersing pigment in which benzenesulfonic acid groups are bonded to the surface of pigment (carbon black) particles (product name "Cab-o-jet 200", manufactured by Cabot, pigment content: 20.0%). • PGMEA: Propylene glycol monomethyl ether acetate • Solid solution pigment 1: Solid solution pigment of CI Pigment Red 122 and CI Pigment Violet 19 • Solid solution pigment 2: Solid solution pigment of CI Pigment Red 202 and CI Pigment Violet 19 • Acrylic resin: The liquid component of product name "DISPERBYK-190" (manufactured by Big Chemie Japan, acid value: 10 mg KOH / g) has been dried and solidified.

[0078] [Table 7]

[0079] <Ink preparation> The components shown on the left side of Table 8 were mixed and thoroughly stirred, then the ink was prepared by pressure filtration through a 2.5 μm pore size microfilter (manufactured by Fujifilm). Acetylenel E100 (trade name) is a nonionic surfactant manufactured by Kawaken Fine Chemicals. In Table 8, the column "Content of specific resin B (%)" indicates the content of resins having a polyethyleneimine structure in the ink.

[0080] <Rating> Each ink obtained above was evaluated for the following items. In this invention, "A" and "B" were considered acceptable levels, and "C" was considered an unacceptable level, according to the evaluation criteria for each item shown below. The evaluation results are shown on the right side of Table 8.

[0081] (Storage stability) Each ink was placed in a sealed container and stored at 80°C for a predetermined period. The viscosity of the ink before and after storage was measured using an E-type viscometer (product name "RE-80L", manufactured by Toki Sangyo Co., Ltd.). The storage period (in days) during which the difference in viscosity after storage (viscosity change) remained below 0.1 mPa·s, relative to the viscosity before storage, was measured. The storage period (in days) for each pigment, as well as for comparative example inks containing a polyethyleneimine structure without bonded anionic and cyclic compound groups, was defined as the "reference storage period," and the storage stability of the inks was evaluated according to the evaluation criteria shown below. A: The storage period was 1.5 times or more the standard storage period. B: The storage period was 1.1 times or more but less than 1.5 times the standard storage period. C: The storage period was less than 1.1 times the standard storage period.

[0082] (Discharge stability) Each of the inks obtained above was filled into an ink cartridge and set in an inkjet recording device (product name "PIXUS 9500", manufactured by Canon) that ejects ink from the recording head using thermal energy. In this embodiment, a solid image recording duty cycle of 100% is defined as recording a solid image with a resolution of 600 dpi x 600 dpi and applying 8 ink droplets of 3.5 pL each to a unit area of ​​1 / 600 inch x 1 / 600 inch. Using the above inkjet recording device, 1,000 solid images of 19 cm x 26 cm with a recording duty cycle of 100% were recorded continuously on a recording medium (plain paper, product name "GF-500", manufactured by Canon). The ink ejection stability was evaluated according to the evaluation criteria shown below. A: There were no non-dispensing issues even on the 1000th sheet. B: There were instances of non-dispensing between 500 and 1000 sheets. C: There was a failure to dispense before reaching 500 sheets.

[0083] [Table 8]

[0084] This embodiment includes the following configurations and methods.

[0085] [Configuration 1] An aqueous inkjet ink containing an organic pigment and a resin having a polyethyleneimine structure, The resin having the polyethyleneimine structure has (i) an anionic group that is bonded to the nitrogen atom of the polyethyleneimine structure via a first linking group, and (ii) a cyclic compound group selected from the group consisting of cyclic hydrocarbon groups and heterocyclic compound groups that is bonded to the nitrogen atom of the polyethyleneimine structure via a second linking group, or forms a cyclic imide together with the nitrogen atom of the polyethyleneimine structure. An aqueous ink characterized in that the molecular weights of both the first linking group and the second linking group are 10 or more and 300 or less.

[0086] [Configuration 2] The aqueous ink according to configuration 1, wherein the first linking group and the second linking group are each independently at least one group selected from the group consisting of alkylene groups, arylene groups, carbonyl groups, ester groups, and amide groups.

[0087] [Configuration 3] The aqueous ink according to configuration 1 or 2, wherein the anionic group is at least one of a carboxylic acid group and a sulfonic acid group.

[0088] [Structure 4] The aqueous ink according to any one of the configurations 1 to 3, wherein the introduction rate (%) of the anionic group bonded to the nitrogen atoms of the resin having the polyethyleneimine structure via the first linking group is 1.0% or more and 40.0% or less, based on the number of nitrogen atoms in the polyethyleneimine structure.

[0089] [Composition 5] The aqueous ink according to any one of the configurations 1 to 4, wherein the introduction rate (%) of the cyclic compound group bonded to the nitrogen atom of the resin having the polyethyleneimine structure via the second linking group is 1.0% or more and 40.0% or less, based on the number of nitrogen atoms in the polyethyleneimine structure.

[0090] [Composition 6] The aqueous ink according to any one of the configurations 1 to 5, wherein the weight-average molecular weight of the resin having the polyethyleneimine structure is 1,500 or more and 70,000 or less.

[0091] [Composition 7] The aqueous ink according to any one of configurations 1 to 6, wherein the polyethyleneimine structure is branched chain-like.

[0092] [Structure 8] The aqueous ink according to any one of configurations 1 to 7, wherein the resin having the polyethyleneimine structure is a resin dispersant for dispersing the organic pigment.

[0093] [Composition 9] The aqueous ink according to any one of claims 1 to 8, wherein the organic pigment is an organic pigment having a quinacridone skeleton or a diketopyrrolopyrrole skeleton.

[0094] [Configuration 10] The aqueous ink according to any one of the configurations 1 to 9, wherein the content (mass%) of the resin having the polyethyleneimine structure is 0.05 times or more and 0.50 times or less by mass ratio to the content (mass%) of the organic pigment.

[0095] [Composition 11] The aqueous ink according to any one of the configurations 1 to 10, wherein the content (mass%) of the resin having the polyethyleneimine structure is 0.10 times or more and 0.30 times or less in mass ratio to the content (mass%) of the organic pigment.

[0096] [Composition 12] The aqueous ink according to any one of the configurations 1 to 11, wherein the content (by mass) of the resin having the polyethyleneimine structure is 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink.

[0097] [Composition 13] The aqueous ink according to any one of the configurations 1 to 12, wherein the content (by mass) of the organic pigment is 0.1% by mass or more and 15.0% by mass or less, based on the total mass of the ink.

[0098] [Composition 14] An ink cartridge comprising ink and an ink storage section for storing the ink, An ink cartridge characterized in that the ink is the water-based ink described in any one of the items 1 to 13.

[0099] [Composition 15] An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink according to any one of the items 1 to 13 of the configuration.

Claims

1. An aqueous inkjet ink containing an organic pigment and a resin having a polyethyleneimine structure, The resin having the polyethyleneimine structure has (i) an anionic group bonded to the nitrogen atom of the polyethyleneimine structure via a first linking group, and (ii) a cyclic compound group selected from the group consisting of cyclic hydrocarbon groups and heterocyclic compound groups that bond to the nitrogen atom of the polyethyleneimine structure via a second linking group, or form a cyclic imide together with the nitrogen atom of the polyethyleneimine structure. An aqueous ink characterized in that the molecular weights of both the first linking group and the second linking group are 10 or more and 300 or less.

2. The aqueous ink according to claim 1, wherein the first linking group and the second linking group are each independently at least one group selected from the group consisting of alkylene groups, arylene groups, carbonyl groups, ester groups, and amide groups.

3. The aqueous ink according to claim 1, wherein the anionic group is at least one of a carboxylic acid group and a sulfonic acid group.

4. The aqueous ink according to claim 1, wherein the introduction rate (%) of the anionic group bonded to the nitrogen atoms of the resin having the polyethyleneimine structure via the first linking group is 1.0% or more and 40.0% or less, based on the number of nitrogen atoms in the polyethyleneimine structure.

5. The aqueous ink according to claim 1, wherein the introduction rate (%) of the cyclic compound group bonded to the nitrogen atoms of the resin having the polyethyleneimine structure via the second linking group is 1.0% or more and 40.0% or less, based on the number of nitrogen atoms in the polyethyleneimine structure.

6. The aqueous ink according to claim 1, wherein the weight-average molecular weight of the resin having the polyethyleneimine structure is 1,500 or more and 70,000 or less.

7. The aqueous ink according to claim 1, wherein the polyethyleneimine structure is branched.

8. The ink according to claim 1, wherein the resin having the polyethyleneimine structure is a resin dispersant for dispersing the organic pigment.

9. The aqueous ink according to claim 1, wherein the organic pigment is an organic pigment having a quinacridone skeleton or a diketopyrrolopyrrole skeleton.

10. The aqueous ink according to claim 1, wherein the content (mass%) of the resin having the polyethyleneimine structure is 0.05 times or more and 0.50 times or less in mass ratio to the content (mass%) of the organic pigment.

11. The aqueous ink according to claim 1, wherein the content (mass%) of the resin having the polyethyleneimine structure is 0.10 times or more and 0.30 times or less in mass ratio to the content (mass%) of the organic pigment.

12. The aqueous ink according to claim 1, wherein the content (by mass) of the resin having the polyethyleneimine structure is 0.1% by mass or more and 5.0% by mass or less, based on the total mass of the ink.

13. The aqueous ink according to claim 1, wherein the content (by mass) of the organic pigment is 0.1% by mass or more and 15.0% by mass or less, based on the total mass of the ink.

14. An ink cartridge comprising ink and an ink storage section for storing the ink, An ink cartridge characterized in that the ink is the water-based ink described in any one of claims 1 to 13.

15. An inkjet recording method that records an image on a recording medium by ejecting ink from an inkjet recording head, An inkjet recording method characterized in that the ink is an aqueous ink according to any one of claims 1 to 13.

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