Aqueous pigment dispersion for inkjet, and aqueous inkjet ink

Abstract

To provide aqueous pigment dispersion for inkjet by which aqueous inkjet ink having excellent discharge stability and printing image quality can be produced, and which also has excellent dispersion stability and re-dispersibility.SOLUTION: Aqueous pigment dispersion for inkjet ink contains pigment particles, alkali metal hydroxide, and organic solvent, where the pigment particles contain pigment and maleic acid-based resin having a crosslinking structure by the crosslinking agent, the crosslinking agent contains compound (B-1) having a plurality of functional group which reacts with carboxyl group in 1 molecule, the organic solvent contains glycol ether compound (A-1) represented by a specific general formula by 50 to 200 mass% for the content of the maleic acid-based resin, and furthermore, a molar amount of a functional group which reacts with the carboxyl group present in the compound (B-1) is defined.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous pigment dispersion for inkjet use, and an aqueous inkjet ink produced using the aqueous pigment dispersion for inkjet use. [Background technology]

[0002] Digital printing methods are becoming increasingly popular due to the trend toward smaller print runs and diversifying needs. Digital printing methods do not require plates, which allows for cost reductions and makes it possible to make printing equipment more compact.

[0003] Inkjet printing, a type of digital printing method, is a method in which tiny droplets of ink are ejected from an inkjet head and landed on a printing substrate to form an image and / or characters (hereinafter collectively referred to as "printed matter." Note that the above "image" also includes solid images and seamless images such as checkered patterns) on the printing substrate. Compared to other digital printing methods, inkjet printing is superior in terms of the size and cost of the printing device, running costs during printing, ease of full colorization, etc.

[0004] The types of inks used in inkjet printing methods are diverse, including oil-based, solvent-based, active energy ray curing, and water-based. Until now, solvent-based and active energy ray curing inks have been used for industrial printing applications. However, in recent years, there has been an increasing demand for water-based inks (inks that contain water as the main component) due to concerns and responses to harmful effects on the environment and people.

[0005] In this application, water-based inks used in ink-jet printing methods are referred to as "aqueous ink-jet inks."

[0006] In recent years, with the improvement of inkjet head performance, the use of inkjet printing has expanded not only for consumer use but also for industrial printing. In particular, in the commercial printing market and the packaging (label and package) printing market, there is active consideration of replacing plate-based printing methods such as offset printing and gravure printing with inkjet printing.

[0007] In order to realize the above replacement, it is essential to realize stable printing (improved ejection stability). In addition, from the viewpoint of improving the optical density of the printed matter and imparting properties such as water resistance and abrasion resistance, it is preferable to use a pigment as a colorant. However, in conventional aqueous inkjet inks, it is difficult to stably disperse the pigment, and there is a problem that the dispersion stability decreases over time. In addition, when a part of the water in the aqueous inkjet ink evaporates near the nozzle of the inkjet head, the pigment aggregates and the aqueous inkjet ink dries up, which may cause the nozzle to become clogged, resulting in a problem that the ejection stability may deteriorate.

[0008] In the present application, the properties relating to the redispersion of aggregated pigment particles and the restoration of dried aqueous inkjet ink are collectively referred to as "redispersibility."

[0009] In order to improve the above-mentioned dispersion stability and redispersibility, the design and use method of the pigment dispersion resin used to disperse the pigment in the aqueous inkjet ink are important. However, the pigment dispersion resin and the pigment dispersion method used in the past have a problem that the pigment dispersion resin has a weak adsorption property to the pigment, and the resin is easily detached from the pigment. Furthermore, when the aqueous inkjet ink contains a surfactant, the pigment dispersion resin that is not adsorbed to the pigment and is free in the aqueous inkjet ink inhibits the orientation of the surfactant to the air-liquid interface, resulting in a problem of deterioration of the print image quality.

[0010] Patent Documents 1 to 3 are examples of studies on the composition of pigment dispersion resins to improve the dispersion stability of pigments. Patent Document 1 discloses a polymer containing a colorant adsorptive group-containing monomer unit (α-olefin unit and / or ring structure-containing monomer unit), an (anhydride) maleic acid unit, and a (meth)allyl monomer unit; Patent Document 2 discloses a polymer containing a colorant adsorptive group-containing monomer unit (α-olefin unit and / or alkyl group-containing (meth)acrylic acid ester monomer unit), a maleic acid unit, and a maleimide unit or an N-substituted maleimide unit; and Patent Document 3 discloses a polymer containing a maleic acid unit and a maleic acid amide unit and having an acid value of 50 to 300 mgKOH / g; and colorant dispersions are produced using these polymers as pigment dispersion resins. In addition, paragraph 0066 of Patent Document 1, paragraph 0071 of Patent Document 2, and paragraph 0063 of Patent Document 3 also disclose that the dispersion stability of pigments can be improved by crosslinking the pigment dispersion resin with a crosslinking agent. In particular, in the examples of Patent Document 1, a resin having the above-mentioned structure is mixed with a pigment (CI Pigment Red 122, etc.), diethylene glycol, etc., and the like, and then the coated colorant is produced by kneading and removing the solvent, and a crosslinking agent is added to the coated colorant to perform a crosslinking treatment, thereby producing a colorant composition having a crosslinked structure. Furthermore, the colorant composition is also used to produce an aqueous inkjet ink (paragraphs 0142-0144, Tables 2-7). Patent Documents 2-3 do not disclose any specific examples of crosslinking the pigment dispersion resin.

[0011] On the other hand, when considering the use in the above industrial printing applications, the design and use of the pigment dispersion resins described in the above Patent Documents 1 to 3 cannot be said to simultaneously and suitably solve all of the problems of dispersion stability, ejection stability, redispersibility, and print image quality, and further improvements are required. In particular, even if the pigment dispersion resins described in the above Patent Documents 1 to 3 are used, it is difficult to say that they have a sufficient level of quality in terms of redispersibility and ejection stability (see also Comparative Example 10 in the examples of the present application, which will be described later). [Prior art documents] [Patent documents]

[0012] [Patent Document 1] JP 2020-203965 A [Patent Document 2] JP 2022-170110 A [Patent Document 3] JP 2023-092681 A Summary of the Invention [Problem to be solved by the invention]

[0013] As described above, when focusing on the composition of the pigment dispersion resin and the method of using it (pigment dispersion method), existing water-based inkjet inks have not been able to simultaneously and appropriately address all of the issues of dispersion stability, ejection stability, redispersibility, and print image quality.

[0014] Therefore, an object of the present invention is to provide an aqueous pigment dispersion for inkjet use, which can produce an aqueous inkjet ink having excellent ejection stability and print image quality, and which also has excellent dispersion stability and redispersibility. [Means for solving the problem]

[0015] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by an aqueous pigment dispersion for inkjet use having the following configuration, and an aqueous inkjet ink produced using the aqueous pigment dispersion.

[0016] That is, one embodiment of the present invention relates to an aqueous pigment dispersion for inkjet, as shown in [1] to [4] below, and an aqueous inkjet ink produced using the aqueous pigment dispersion for inkjet, as shown in [5] to [6] below. [1] An aqueous pigment dispersion for inkjet ink, comprising pigment particles, a basic compound, and an organic solvent, the pigment particles contain a pigment and a maleic acid-based resin, the maleic acid-based resin has a crosslinked structure formed by a crosslinking agent, the crosslinking agent contains a compound (B-1) having a plurality of functional groups reactive with carboxy groups in one molecule, the basic compound comprises an alkali metal hydroxide, the organic solvent contains a glycol ether compound (A-1) represented by the following general formula (1) in an amount of 50 to 200 mass % based on the content of the maleic acid-based resin, an aqueous pigment dispersion for inkjet ink, wherein the molar amount of the functional group reactive with the carboxy group present in the compound (B-1) is 80 to 99 mol % based on the total molar amount of the carboxy group and the carboxylate group present in the maleic acid-based resin before crosslinking. General formula (1): R 1 -(O-CH(CH3)-CH2) n -OH (In general formula (1), R 1 is an alkyl group having 1 to 4 carbon atoms, and n is 1 or 2. [2] The aqueous pigment dispersion for inkjet ink according to [1], wherein the molar amount of the basic compound is 10 to 200 mol % based on the total molar amount of carboxy groups and carboxylate groups present in the maleic acid-based resin before crosslinking. [3] The aqueous pigment dispersion for inkjet inks according to [1] or [2], wherein the water solubility of the crosslinking agent is 65 g / 100 gH2O or less. [4] The aqueous pigment dispersion for inkjet ink according to any one of [1] to [3], wherein the maleic acid-based resin further contains a structural unit derived from an alkene monomer. [5] An inkjet ink comprising the aqueous pigment dispersion for inkjet inks according to any one of [1] to [4]. [6] further comprising a surfactant, The inkjet ink according to [5], wherein the surfactant comprises an acetylene diol-based surfactant having an HLB value of 10 or less. Effect of the Invention

[0017] According to the present invention, it is possible to produce an aqueous inkjet ink having excellent ejection stability and print image quality, and it is also possible to obtain an aqueous pigment dispersion for inkjet use having excellent dispersion stability and redispersibility. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] First, the mechanism by which the above-mentioned effects are exhibited by the main components constituting the aqueous pigment dispersion for inkjet ink according to an embodiment of the present invention (hereinafter also simply referred to as "aqueous pigment dispersion (for inkjet ink) of this embodiment") will be described below. However, the above-mentioned mechanism is a conjecture by the present inventors, and the present invention is not limited by the following conjecture.

[0019] First, the aqueous pigment dispersion of the present embodiment includes pigment particles including a pigment and a maleic acid-based resin. The maleic acid-based resin has a crosslinked structure formed by a crosslinking agent. The crosslinking agent further includes a compound (B-1) having a plurality of functional groups in one molecule that react with a carboxy group. Unlike an acrylic resin that includes (meth)acrylic acid as a structural unit, the maleic acid-based resin has two adjacent carboxy groups and has a structure that easily repels electric charges, so that an aqueous pigment dispersion having excellent dispersion stability can be obtained.

[0020] The above-mentioned "(meth)acrylic acid" refers to acrylic acid and / or methacrylic acid. In addition, in the present application, "acrylic resin" refers to a resin using one or more polymerizable monomers selected from the group consisting of (meth)acrylic acid, acrylic acid esters, and methacrylic acid esters (however, a styrene-based monomer may be included as the polymerizable monomer).

[0021] The aqueous pigment dispersion of this embodiment contains 50 to 200 mass% of the glycol ether compound (A-1) represented by the above general formula (1) relative to the content of the maleic acid resin. In particular, when the pigment and the maleic acid resin are dispersed in the presence of the above amount of the glycol ether compound (A-1), the affinity between the pigment and the maleic acid resin is improved via the glycol ether compound (A-1), and the adsorption of the maleic acid resin to the pigment is improved. As a result, in addition to the dispersion stability and redispersibility of the aqueous pigment dispersion, the ejection stability of the aqueous inkjet ink produced using the aqueous pigment dispersion is improved.

[0022] The glycol ether compound (A-1) represented by the general formula (1) has a small surface tension. Therefore, the aqueous inkjet ink produced using the aqueous pigment dispersion of this embodiment can spread uniformly on the printing substrate. In other words, by using the aqueous pigment dispersion of this embodiment, it is easy to obtain an aqueous inkjet ink having a dot shape close to a circle and excellent print quality.

[0023] Furthermore, the aqueous pigment dispersion of this embodiment contains the compound (B-1) such that the molar amount of the functional group reacting with the carboxy group is 80 to 99 mol% relative to the total molar amount of the carboxy group and the carboxylate group (-COO-) present in the maleic acid-based resin before crosslinking. By using the compound (B-1) in the above amount, the crosslinking reactivity between the carboxy group and the crosslinking agent is improved. This effect is presumably due to the two adjacent carboxy groups present in the maleic acid-based resin described above.

[0024] As described above, by coating the pigment with a sufficient amount of maleic acid resin and crosslinking the maleic acid resin adsorbed on the pigment with high reactivity, the maleic acid resin can be maintained in a strong adsorption state and desorption can be suppressed. As a result, even when, for example, a part of the water in the aqueous inkjet ink evaporates near the nozzle of the inkjet head and the proportion of the organic solvent in the system increases, the dispersion state of the pigment particles is less likely to be broken, and the ejection stability is improved. Even if the pigment particles in the aqueous inkjet ink aggregate, they can be easily redispersed in the liquid medium by contacting the liquid medium, and even if the aqueous inkjet ink dries up, they can be easily restored to the aqueous inkjet ink before drying up by contacting the liquid medium with the film after drying up. In addition, when the aqueous inkjet ink using the aqueous pigment dispersion of this embodiment contains a surfactant, it is possible to prevent the inhibition of the orientation of the surfactant at the gas-liquid interface caused by the free maleic acid resin, which is less likely to occur due to the strong adsorption of the maleic acid resin by the above-mentioned crosslinking. As a result, the surfactant functions satisfactorily, improving the print quality.

[0025] Depending on the printing substrate used, maleic acid-based resins are also effective in improving the optical density of printed matter. When a printing substrate containing calcium comes into contact with an aqueous inkjet ink containing the aqueous pigment dispersion of this embodiment, calcium ions are eluted into the aqueous inkjet ink. When a maleic acid-based resin is used, two adjacent carboxyl groups react with the calcium ions, causing the charge repulsion to disappear and causing pigment aggregation. As a result, the pigment is deposited on the printing substrate, and a high optical density can be achieved.

[0026] On the other hand, it is believed that the aqueous pigment dispersion contains a small amount of maleic acid-based resin (also referred to as "free maleic acid-based resin" in the present application) that is not adsorbed to the pigment and is free in the aqueous pigment dispersion. In general, in an aqueous inkjet ink containing a resin that is dissolved in a liquid and is free (free resin), there is a risk of the ejection stability being reduced. Since even a small amount of free resin can affect the deterioration of ejection stability, it can be said that it is extremely important to deal with the free resin when producing an aqueous inkjet ink.

[0027] Therefore, the aqueous pigment dispersion of this embodiment contains an alkali metal hydroxide as a basic compound. When a basic compound having high resin dissolving ability, such as an alkanolamine, is used as the basic compound, the free maleic acid resin dissolves, and the molecular chains of the free maleic acid resin form an entangled state or an association state. As a result, as described above, this adversely affects the ejection stability of the aqueous inkjet ink. In contrast, when an alkali metal hydroxide having low resin dissolving ability is used, the dissolution of the free maleic acid resin can be suppressed, and the above-mentioned entangled state or association state can be made less likely to occur.

[0028] As described above, in order to solve the above-mentioned problems, an aqueous pigment dispersion for inkjet recording having the above-mentioned configuration is essential.

[0029] Next, each component constituting the aqueous pigment dispersion according to one embodiment of the present invention will be described in detail below.

[0030] <Pigment particles> Pigments The aqueous pigment dispersion for inkjet ink of this embodiment contains pigment particles. The pigment particles contain a pigment. As the pigment, any conventionally known organic or inorganic pigment can be used, and for example, pigments represented by the following color index names can be used. i.e., red pigments, CI Pigment Red 52, 5, 7, 9, 12, 17, 22, 23, 31, 48:1, 48:2, 48:3, 48:4, 49:1, 49:2, 57:1, 57:2, 112, 122, 123, 146, 147, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 184, 188, 202, 207, 209, 254, 255, 260, 264, 266, 269, 282; Violet pigments include CI Pigment Violet 19, 23, 29, 32, 36, 37, 42, 50; Orange pigments include CI Pigment Orange 1, 2, 3, 5, 7, 13, 14, 15, 16, 22, 34, 36, 38, 40, 43, 47, 48, 49, 51, 52, 53, 60, 61, 62, 64, 65, 66, 69, 71, 73; Blue pigments include CI Pigment Blue 15, 15:3, 15:4, 15:6, 16, 60, 64, and 79; As green pigments, CI Pigment Green 7, 10, 36, 48; Yellow pigments include CI Pigment Yellow 1, 2, 3, 5, 12, 13, 14, 16, 17, 24, 73, 74, 83, 87, 93, 94, 95, 97, 98, 109, 110, 111, 112, 120, 126, 127, 128, 129, 137, 138, 139, 147, 150, 151, 154, 155, 166, 167, 168, 170, 180, 185, 213; Black pigments include CI Pigment Black 1, 7, and 11; White pigments include CI Pigment White 4, 5, 6, 21, etc. These pigments may be used alone or in combination of two or more. A solid solution of two or more of the pigments listed above may also be used as a pigment.

[0031] The content of the pigment contained in the aqueous pigment dispersion for inkjet ink of this embodiment is adjusted depending on the use of the inkjet ink prepared using the aqueous pigment dispersion for inkjet ink, but for example, in the case of a white aqueous pigment dispersion (aqueous white pigment dispersion), the content is preferably 1.5 to 50 mass% of the total amount of the aqueous pigment dispersion for inkjet ink, more preferably 10 to 30 mass%, from the viewpoint of improving the dispersion stability and redispersibility and improving the ejection stability of the aqueous inkjet ink obtained. On the other hand, in the case of the aqueous white pigment dispersion for inkjet ink, the content of the pigment is preferably 15 to 75 mass% of the total amount of the aqueous white pigment dispersion, particularly preferably 30 to 60 mass%, from the viewpoint of improving the dispersion stability and redispersibility and obtaining a printed matter with high hiding power without deteriorating the ejection stability of the aqueous white inkjet ink produced using the aqueous white pigment dispersion.

[0032] <Maleic acid resin> The pigment particles contained in the aqueous pigment dispersion for inkjet ink of this embodiment contain a maleic acid-based resin. Generally, water-soluble and water-insoluble resins are known as the forms of resins used in aqueous inkjet inks, and the maleic acid-based resin may be either a water-soluble or water-insoluble resin. In the present application, a resin having a solubility of 1 g or more in 100 g of water at 25° C. is referred to as a "water-soluble resin," and a resin having a solubility of less than 1 g is referred to as a "water-insoluble resin."

[0033] From the viewpoints of increasing charge repulsion by two adjacent carboxy groups, improving dispersion stability and redispersibility, and also improving the ejection stability and print image quality of an aqueous inkjet ink produced using the aqueous pigment dispersion, the amount of structural units derived from maleic acid contained in the maleic acid-based resin is preferably 5 to 60 mol %, more preferably 10 to 50 mol %, even more preferably 15 to 40 mol %, and particularly preferably 15 to 30 mol %, of all monomers constituting the maleic acid-based resin.

[0034] The structural unit derived from maleic acid may be incorporated into the maleic acid-based resin by using maleic acid as a polymerizable monomer, or may be formed by using maleic anhydride as a polymerizable monomer, incorporating the structural unit derived from maleic anhydride into the maleic acid-based resin, and then ring-opening the structural unit derived from maleic anhydride to form the structural unit derived from maleic acid.

[0035] The maleic acid resin preferably contains a structural unit derived from an alkene monomer in addition to the maleic acid unit. By using a maleic acid resin containing a structural unit derived from an alkene monomer, not only is redispersibility improved, but also it is possible to obtain an aqueous inkjet ink having excellent ejection stability and print image quality. The alkene monomer preferably has a carbon number of 6 to 50, more preferably a carbon number of 8 to 30, and even more preferably a carbon number of 12 to 28. By using an alkene monomer having such a carbon number, the dispersion stability of the aqueous pigment dispersion is also improved. Examples of the alkene monomer that can be used include hexene (carbon number: 6), heptene (carbon number: 7), octene (carbon number: 8), nonene (carbon number: 9), decene (carbon number: 10), dodecene (carbon number: 12), tetradecene (carbon number: 14), hexadecene (carbon number: 16), octadecene (carbon number: 18), eicosene (carbon number: 20), docosene (carbon number: 22), tetracosene (carbon number: 24), octacosene (carbon number: 28), triacontene (carbon number: 30), dotriacontene (carbon number: 32), tetratriacontene (carbon number: 34), hexatriacontene (carbon number: 36), and octatriacontene (carbon number: 38).

[0036] The alkene monomer may be used alone or in combination of two or more. The position of the double bond in the alkene monomer is not limited, but may be present at the molecular terminal, that is, the double bond may be formed between the carbon atom at the 1st position and the carbon atom at the 2nd position.

[0037] When the maleic acid-based resin contains a structural unit derived from an alkene monomer, from the viewpoints of improving dispersion stability and redispersibility, as well as ejection stability when made into an aqueous inkjet ink, the amount of the structural unit derived from the alkene monomer contained in the maleic acid-based resin is preferably 40 to 95 mol %, more preferably 50 to 90 mol %, and even more preferably 60 to 85 mol %, of all monomers constituting the maleic acid-based resin.

[0038] The maleic acid resin may contain structural units other than the structural units derived from maleic acid and the structural units derived from alkene monomers. For example, the maleic acid resin contains a structural unit derived from a styrene monomer, which can improve dispersion stability, redispersibility, and ejection stability when made into an aqueous inkjet ink. As the styrene monomer, styrene, α-methylstyrene, 4-methylstyrene, 4,α-dimethylstyrene, α-cyclohexylstyrene, sodium 4-styrenesulfonate, etc. can be used. In addition, in terms of improving dispersion stability and ejection stability when made into an aqueous inkjet ink, when a styrene monomer is used, the amount of the structural units derived from the styrene monomer is preferably 40 to 95 mol %, more preferably 45 to 85 mol %, and even more preferably 50 to 75 mol % of the total monomers constituting the maleic acid resin.

[0039] The content of the maleic acid-based resin is preferably 5 to 70% by mass, more preferably 10 to 60% by mass, based on the content of the pigment. When the content of the maleic acid-based resin is within the above range, an aqueous pigment dispersion having excellent dispersion stability and redispersibility can be obtained.

[0040] <Basic compounds> The aqueous pigment dispersion for inkjet ink of this embodiment contains a basic compound. An alkali metal hydroxide is used as the basic compound. The basic compound neutralizes the carboxyl groups present in the maleic acid resin to form carboxylate groups. The anionic charge of the carboxylate groups prevents the maleic acid resins from approaching each other (electrostatic repulsion), and the pigment particles can be stably dispersed. Among them, alkali metal hydroxides have low resin dissolving ability, so they can suppress the dissolution of free maleic acid resins, and can make it difficult for molecular chains of the free maleic acid resins to become entangled or associated with each other. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide, and among them, potassium hydroxide is preferably used because of its low resin dissolving ability.

[0041] From the viewpoint of improving the dispersion stability and the ejection stability of the aqueous inkjet ink, the content of the basic compound in the aqueous pigment dispersion for inkjet ink of this embodiment is preferably 10 to 200 mol %, and more preferably 40 to 160 mol %, based on the total molar amount of carboxy groups and carboxylate groups present in the maleic acid-based resin before crosslinking.

[0042] The content of the basic compound relative to the total molar amount of the carboxy groups and carboxylate groups present in the maleic acid-based resin is calculated by dividing the molar amount of the basic group in the basic compound by the total molar amount of the carboxy groups and carboxylate groups in the maleic acid-based resin (including the carboxy groups and carboxylate groups involved in the crosslinking reaction described later), and can be calculated by the following formula (2).

[0043] Formula (2): [The molar amount of basic compound relative to the total molar amount of carboxyl groups and carboxylate groups present in maleic acid-based resin] = {(mass of basic compound (g) × valence of basic group / molecular weight of basic compound) / [(acid value of resin (KOH mg / g) × mass of resin (g)) / (56.1 × 1000)]} × 100

[0044] The acid value of the maleic acid resin (maleic acid resin precursor) before crosslinking is preferably 50 to 300 mgKOH / g, more preferably 65 to 200 mgKOH / g, and even more preferably 80 to 180 mgKOH / g. When the maleic acid resin has the above acid value, an excellent electrostatic repulsion effect is obtained, and the dispersion stability is improved even after crosslinking. Furthermore, in a printing substrate containing calcium, even a small amount of calcium ions sufficiently reacts, improving the optical density.

[0045] The "acid value of a resin" refers to the number of milligrams of potassium hydroxide (KOH) required to neutralize the acid groups contained in 1 g of the resin. The acid value is measured and calculated by the method described in the Examples section below.

[0046] <Glycol ether compound (A-1)> The aqueous pigment dispersion for inkjet ink of this embodiment contains a glycol ether compound (A-1) having a structure represented by the above general formula (1). By using the glycol ether compound (A-1) represented by the general formula (1), the pigment and the maleic acid resin have affinity through the glycol ether compound (A-1), and the dispersion stability and the ejection stability when made into an aqueous inkjet ink are improved. As the glycol ether compound (A-1) represented by the general formula (1), one or more solvents selected from the group consisting of propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol ethyl ether, dipropylene glycol ethyl ether, propylene glycol-n-propyl ether, dipropylene glycol-n-propyl ether, propylene glycol-n-butyl ether, and dipropylene glycol-n-butyl ether can be preferably used.

[0047] In particular, since not only the surface tension but also the molecular structure is small, the affinity between the pigment and the maleic acid-based resin is easily improved, thereby improving the dispersion stability, redispersibility, and ejection stability when made into an aqueous inkjet ink, and since the boiling point is low and the drying properties and print image quality when made into an aqueous inkjet ink are improved, it is preferable to select one or more compounds selected from the group consisting of propylene glycol methyl ether, propylene glycol ethyl ether, and dipropylene glycol methyl ether as the glycol ether compound (A-1) in the aqueous pigment dispersion of this embodiment, it is more preferable to select propylene glycol methyl ether and / or dipropylene glycol methyl ether, and it is particularly preferable to select propylene glycol methyl ether.

[0048] The content of the glycol ether compound (A-1) contained in the aqueous pigment dispersion for inkjet ink of this embodiment is preferably 50 to 200 mass% relative to the content of the maleic acid-based resin, and more preferably 75 to 150 mass%. By controlling the content of the glycol ether compound (A-1) within the above range, the dispersion stability, redispersibility, and ejection stability when made into an aqueous inkjet ink are favorable. In particular, by controlling the content of the glycol ether compound (A-1) to 75 to 150 mass% relative to the content of the maleic acid-based resin, the print quality when made into an aqueous inkjet ink can be improved.

[0049] Furthermore, from the viewpoints that the affinity between the pigment and the maleic acid-based resin becomes favorable, a pigment dispersion for inkjet ink having excellent dispersion stability and redispersibility can be obtained, and when made into an aqueous inkjet ink, the ejection stability and print image quality are favorable, the content of the glycol ether compound (A-1) contained in the pigment dispersion for inkjet ink is preferably 15 to 70 mass %, and particularly preferably 25 to 55 mass %, based on the mass of the pigment.

[0050] <Crosslinking agent> The aqueous pigment dispersion for inkjet ink of this embodiment contains a compound (B-1) having a plurality of functional groups in one molecule that react with a carboxy group as a crosslinking agent. The use of the crosslinking agent allows the carboxy group derived from the maleic acid resin to be crosslinked. As the functional group contained in the crosslinking agent that reacts with a carboxy group, for example, an isocyanate group, an aziridine group, a carbodiimide group, an oxetane group, an oxazoline group, or an epoxy group can be used. Among these, one or more selected from the group consisting of an aziridine group, a carbodiimide group, and an epoxy group are more preferred, and it is particularly preferred to select an aziridine group and / or an epoxy group. Examples of the crosslinking agent include an isocyanate compound, an aziridine compound, a carbodiimide compound, an oxetane compound, an oxazoline compound, and an epoxy compound.

[0051] Examples of the isocyanate compound include organic polyisocyanates and isocyanate-terminated prepolymers. Examples of the organic polyisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate and 2,2,4-trimethylhexamethylene diisocyanate; aromatic diisocyanates such as tolylene-2,4-diisocyanate and phenylene diisocyanate; alicyclic diisocyanates; aromatic triisocyanates; and urethane modified products of these compounds. Examples of the isocyanate-terminated prepolymer include compounds obtained by reacting the above-listed organic polyisocyanates and their modified products with low molecular weight polyols, etc.

[0052] Examples of the aziridine compound include N,N'-diphenylmethane-4,4'-bis(1-aziridinecarboxite), N,N'-toluene-2,4-bis(1-aziridinecarboxite), bisisophthaloyl-1-(2-methylaziridine), tri-1-aziridinylphosphine oxide, N,N'-hexamethylene-1,6-bis(1-aziridinecarboxite), 2,2'-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate], trimethylolpropane tri-β-aziridinylpropionate, tetramethylolmethane tri-β-aziridinylpropionate, tris-2,4,6-(1-aziridinyl)-1,3,5-triazine, and 4,4'-bis(ethyleneiminocarbonylamino)diphenylmethane.

[0053] Examples of the carbodiimide compound include high molecular weight polycarbodiimides obtained by decarboxylation condensation of a diisocyanate compound in the presence of a carbodiimide catalyst. Such high molecular weight polycarbodiimides are commercially available as the "Carbodilite (registered trademark)" series manufactured by Nisshinbo Chemical Inc.

[0054] Examples of oxetane compounds include 4,4'-(3-ethyloxetan-3-ylmethyloxymethyl)biphenyl (OXBP), 3-ethyl-3-hydroxymethyloxetane (EHO), 1,4-bis[{(3-ethyl-3-oxetanyl)methoxy}methyl]benzene (XDO), di[1-ethyl(3-oxetanyl)]methyl ether (DOX), di[1-ethyl(3-oxetanyl)]methyl ether (DOE), 1,6-bis[(3-ethyl-3-oxetanyl)methoxy]hexane (HDB), 9,9-bis[2-methyl-4-{2-(3-oxetanyl)}butoxyphenyl]fluorene, and 9,9-bis[4-[2-{2-(3-oxetanyl)}butoxy]ethoxyphenyl]fluorene.

[0055] Examples of the oxazoline compound include compounds in which two or more (preferably 2 to 3) oxazoline groups are bonded to an aliphatic group or an aromatic group. More specifically, bisoxazoline compounds such as 2,2'-bis(2-oxazoline), 1,3-phenylenebisoxazoline, and 1,3-benzobisoxazoline, as well as compounds having a terminal oxazoline group obtained by reacting these compounds with a polybasic carboxylic acid, can be used.

[0056] Examples of epoxy compounds include polyglycidyl ethers such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycerin triglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, bisphenol A diglycidyl ether, and hydrogenated bisphenol A diglycidyl ether.

[0057] Among these, ethylene glycol diglycidyl ether and / or trimethylolpropane polyglycidyl ether are preferably used, and these compounds are commercially available from Nagase ChemteX Corporation under the names Denacol EX-810, EX-321, and EX-321L, or from ADEKA Corporation under the name Adeka Glycirol ED-505.

[0058] In the aqueous pigment dispersion for inkjet ink of this embodiment, the compound (B-1) is used so that the molar amount of the functional group reacting with the carboxy group and the carboxylate group is 80 to 99 mol % relative to the molar amount of the carboxy group and the carboxylate group present in the maleic acid resin before crosslinking. By using the compound (B-1) in the above amount, the crosslinking reactivity between the carboxy group and the carboxylate group and the crosslinking agent is improved, and the dispersion stability and redispersibility as well as the ejection stability of the aqueous inkjet ink are improved.

[0059] In addition, the crosslinking agent preferably has a suitable water solubility so that it can react efficiently with the carboxyl group and carboxylate group of the dispersion resin in water. For example, the water solubility of the crosslinking agent is preferably 0 to 65 g / 100 gH2O, and more preferably 5 to 30 g / 100 gH2O. In this application, the expression "water solubility is 0 g / 100 gH2O" means that the target material is substantially insoluble in water. In addition, the expression "water solubility is 0 to 65 g / 100 gH2O" and the expression "water solubility is 65 g / 100 gH2O or less" have the same meaning.

[0060] <Water> The water contained in the aqueous pigment dispersion for inkjet ink of this embodiment is preferably ion-exchanged water and / or reverse osmosis water. The content of water contained in the aqueous pigment dispersion for inkjet ink is preferably 30 to 90 mass %, and particularly preferably 45 to 90 mass %, of the total amount of the aqueous pigment dispersion for inkjet ink.

[0061] <Other ingredients> The aqueous pigment dispersion for inkjet ink of this embodiment may contain, in addition to the components described above, conventionally known surfactants, antioxidants, preservatives, viscosity modifiers, and the like, as necessary.

[0062] For example, the aqueous pigment dispersion for inkjet ink of this embodiment preferably contains an acetylenic diol surfactant having an HLB value of 2.5 to 4.5. By using an acetylenic diol surfactant having an HLB value of 2.5 to 4.5, for example, bubbles that may occur during dispersion treatment can be suppressed, and the maleic acid resin can be favorably adsorbed to the pigment, improving the dispersion stability and redispersibility of the obtained aqueous pigment dispersion. In addition, although the detailed principle is unclear, the acetylenic diol surfactant has a relatively high affinity with the maleic acid resin, so that, for example, when storing the aqueous pigment dispersion, it does not adversely affect the dispersion stability, and the ejection stability of the inkjet ink produced using the aqueous pigment dispersion is also good. Furthermore, it is also easy to improve the print quality of the aqueous inkjet ink. The method for calculating the HLB value will be described later.

[0063] <Method for producing aqueous pigment dispersion for inkjet ink> The aqueous pigment dispersion for inkjet ink of the present embodiment can be produced by a conventionally known method, and examples of the method include a method in which the steps of producing a mixed solution containing a pigment, a maleic acid resin, an alkali metal hydroxide, a glycol ether compound (A-1) represented by general formula (1), and water (preferably further containing an acetylene diol surfactant having an HLB value of 2.5 to 4.5); a step of dispersing the mixed solution; and a step of producing pigment particles by adding a crosslinking agent to the mixed solution after the dispersion treatment and performing a crosslinking treatment. Specific examples of the above-mentioned method include methods A and B shown below. In addition to the above-mentioned methods, the aqueous pigment dispersion for inkjet ink of the present embodiment can also be produced by method C shown below. However, the method for producing the aqueous pigment dispersion for inkjet ink of the present embodiment is not limited to these.

[0064] (Method A) A maleic acid resin, which is a water-insoluble resin, is dissolved in an organic solvent such as methyl ethyl ketone, and then the maleic acid resin is neutralized with an alkali metal hydroxide as necessary to prepare a maleic acid resin solution. The maleic acid resin solution may contain water. A pigment, water, a glycol ether compound (A-1) represented by general formula (1), and other water-soluble organic solvents, surfactants, etc. are added to this maleic acid resin solution, and if they are not added, an alkali metal hydroxide is further added to neutralize the maleic acid resin. After mixing and stirring (premixing) these mixtures, a dispersion treatment is performed using a conventionally known dispersing machine. After that, the organic solvent is distilled off by reduced pressure distillation, and then the crosslinking agent listed above is added to perform a crosslinking treatment. After the crosslinking treatment, centrifugal separation, filtration, etc. may be performed to remove coarse components.

[0065] (Method B) A maleic acid-based pigment dispersion resin aqueous solution is prepared in advance, containing a maleic acid-based resin, which is a water-soluble resin, water, a glycol ether compound (A-1), and, if necessary, other water-soluble organic solvents. A pigment and, if necessary, a water-soluble organic solvent, a surfactant, etc. are added to this maleic acid-based resin aqueous solution, and an alkali metal hydroxide is further added to neutralize the maleic acid-based resin. After mixing and stirring (premixing) the mixture, a dispersion treatment is performed using a conventionally known dispersing machine. Then, the above-listed crosslinking agent is added to perform a crosslinking treatment, thereby making the pigment dispersion resin water-insoluble. After the crosslinking treatment, centrifugal separation, filtration, etc. may be performed to remove coarse components.

[0066] (Method C) Using a planetary mixer, a Henschel mixer, a Banbury mixer, a kneader, or the like, a pigment, a maleic acid-based resin (which may be a water-soluble or water-insoluble resin), and, if necessary, water, a water-soluble organic solvent, a surfactant, and the like are mixed (kneaded) so that the solid content concentration is 50% by mass or more, and then, if necessary, melt-kneading is further performed using a twin-screw extrusion kneader or the like. Next, water, a glycol ether compound (A-1) represented by general formula (1), an alkali metal hydroxide, and, if necessary, other water-soluble organic solvents and / or surfactants, and the like are mixed with the obtained kneaded product, and a pulverization process is further performed using a conventionally known wet pulverizer or disperser. Then, a method in which the above-listed crosslinking agent is added to perform a crosslinking process. Note that the obtained kneaded product may be pulverized after the (melt) kneading process. In addition, after the pulverization process and / or the crosslinking process, centrifugal separation, filtration, and the like may be performed to remove coarse components.

[0067] A specific example of the crosslinking treatment method is a method of mixing and stirring a mixed liquid containing a pigment, a maleic acid resin, etc., and a compound (B-1) which is a crosslinking agent. In order to promote the crosslinking reaction, the mixture may be heated as necessary during stirring. The heating temperature is preferably 40°C or higher, more preferably 50°C or higher, even more preferably 60°C or higher, and even more preferably 70°C or higher, and is preferably 95°C or lower, more preferably 90°C or lower. The progress of the crosslinking reaction can be confirmed by measuring the change in pH of the mixed liquid during stirring. Specifically, the crosslinking reaction can be considered to have ended when the pH no longer changes. It is preferable to continue stirring until the crosslinking reaction is completed, that is, until the pH of the mixed liquid no longer changes.

[0068] As described above, stirring is preferably continued until no change occurs in the pH of the mixed solution, but this is preferably 0.5 hours or more, more preferably 1 hour or more, even more preferably 1.5 hours or more, still more preferably 3 hours or more, and is preferably 12 hours or less, more preferably 10 hours or less, even more preferably 8 hours or less, and still more preferably 5 hours or less.

[0069] <Characteristics of aqueous pigment dispersions for inkjet inks> From the viewpoint of achieving high levels of dispersion stability and redispersibility, ejection stability when used in an aqueous inkjet ink, and density or hiding power of printed matter, the volume-based median diameter (D50) of the pigment particles contained in the aqueous pigment dispersion of this embodiment is preferably 30 to 450 nm, more preferably 50 to 400 nm, and particularly preferably 70 to 350 nm. The above D50 is a value that can be measured in an environment of 25° C. using a dynamic light scattering particle size distribution measuring device such as Microtrack-Bell's Nanotrac UPA-EX150, using water as a dispersion medium.

[0070] <Inkjet ink> An inkjet ink according to one embodiment of this invention (also referred to simply as "inkjet ink according to this invention" in this application) contains the aqueous pigment dispersion for inkjet ink according to this invention. The inkjet ink according to this invention uses water as the main solvent (medium), but it is preferable to use a water-soluble solvent in combination to prevent the inkjet ink from drying. Furthermore, since the water-soluble solvent has a lower surface tension than water, the inkjet ink containing the water-soluble solvent has improved permeability and wettability and spreadability on the printing substrate after printing.

[0071] Examples of the water-soluble solvent include polyhydric alcohols, polyhydric alcohol alkyl ethers, polyhydric alcohol aryl ethers, nitrogen-containing heterocyclic compounds, amides, amines, sulfur-containing compounds, propylene carbonate, ethylene carbonate, and other water-soluble solvents.

[0072] Examples of the polyhydric alcohols include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,2-propanediol, 1,3-propanediol, dipropylene glycol, tripropylene glycol, polypropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, Examples of the hexanediol include 1,5-hexanediol, 1,6-hexanediol, 3-methyl-1,3-butanediol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol, petriol, 3,3-dimethyl-1,2-butanediol, 2,2-diethyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, 2,4-dimethyl-2,4-pentanediol, 2,5-dimethyl-2,5-hexanediol, 5-hexene-1,2-diol, and 2-ethyl-1,3-hexanediol. Among these, 2-ethyl-1,3-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol, and 1,2-heptanediol are preferred.

[0073] Further, examples of the polyhydric alcohol alkyl ethers include, in addition to the compound represented by the general formula (1), ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, propylene glycol monobutyl ether, and the like.

[0074] In one embodiment, the aqueous inkjet ink of the present embodiment preferably contains a compound represented by general formula (1) (however, a compound different from that contained in the aqueous pigment dispersion) as a water-soluble solvent. As described above, the glycol ether compound (A-1) represented by general formula (1) has a small surface tension, and therefore can provide an aqueous inkjet ink having a dot shape close to a circle and excellent print quality. In addition, since it is unlikely to affect the dispersion state of the pigment particles, it does not deteriorate the ejection stability. In particular, from the viewpoint of obtaining a printed matter having an excellent dot shape while maintaining a suitable ejection stability, it is preferable to use a compound having a boiling point of 170 to 230°C under 1 atmosphere, more preferably to select a compound having a boiling point of 195 to 230°C, and particularly preferably to select a compound having a boiling point of 210 to 230°C. Examples of the compound represented by general formula (1) and having a boiling point of 210 to 230°C under 1 atmosphere include dipropylene glycol-n-propyl ether and dipropylene glycol-n-butyl ether, both of which can be preferably used. That is, in a preferred embodiment, the aqueous inkjet ink of this embodiment contains one or more compounds selected from the group consisting of propylene glycol methyl ether, propylene glycol ethyl ether, and dipropylene glycol methyl ether (more preferably propylene glycol methyl ether and / or dipropylene glycol methyl ether, and particularly preferably propylene glycol methyl ether), and dipropylene glycol-n-propyl ether and / or dipropylene glycol-n-butyl ether.

[0075] Examples of the polyhydric alcohol aryl ethers include ethylene glycol monophenyl ether, diethylene glycol monophenyl ether, tetraethylene glycol chlorophenyl ether, and ethylene glycol monobenzyl ether.

[0076] Examples of the nitrogen-containing heterocyclic compounds include 2-pyrrolidone, N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 1,3-dimethylimidazolidinone, ε-caprolactam, γ-butyrolactone, etc. Examples of the amides include formamide, N-methylformamide, N,N-dimethylformamide, etc. Examples of the amines include monoethanolamine, diethanolamine, triethanolamine, monoethylamine, diethylamine, triethylamine, etc. Examples of the sulfur-containing compounds include dimethylsulfoxide, sulfolane, thiodiethanol, etc.

[0077] The water-soluble solvents listed above may be used alone or in combination of two or more kinds.

[0078] The content of the water-soluble solvent in the aqueous inkjet ink is preferably 3 to 60% by mass, and more preferably 3 to 50% by mass. Meanwhile, the content of water in the aqueous inkjet ink is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. When the contents of the water-soluble solvent and water are within the above ranges, the ejection stability of the aqueous inkjet ink from the inkjet head is improved.

[0079] The inkjet ink of this embodiment preferably contains a surfactant. The use of a surfactant facilitates the improvement of the wetting and spreading properties of the inkjet ink and the prevention of coalescence of droplets, and the like, thereby significantly improving the print quality of printed matter. In particular, when used in combination with the aqueous pigment dispersion of this embodiment, the maleic acid-based resin present in the aqueous pigment dispersion is firmly adsorbed to the pigment by crosslinking or the like, and this can suppress the inhibition of the orientation of the surfactant at the gas-liquid interface. As a result, the surfactant sensitivity functions sufficiently, and print image quality is likely to improve.

[0080] As the surfactant, a conventionally known compound can be used. Among them, from the viewpoint of improving the print quality of the printed matter when used in combination with the aqueous pigment dispersion of the present embodiment, and from the viewpoint of not deteriorating the ejection stability because it has a relatively high affinity with the maleic acid-based resin and does not adversely affect the dispersion state of the pigment particles, although the detailed principle is unclear, it is preferable to use an acetylene diol-based surfactant having an HLB value of 10 or less as the surfactant. From the viewpoint of improving the print quality by preventing the droplets of the aqueous inkjet ink from coalescing with each other through a high orientation speed to the droplet surface, the HLB value is preferably 3 to 9.5, more preferably 4 to 9, and particularly preferably 7 to 8.5.

[0081] The HLB (Hydrophile-Lipophile Balance) value in this application is one of the parameters that indicate the hydrophilicity and hydrophobicity of a material. There are various methods for calculating the HLB value, such as the Griffin method, the Davis method, and the Kawakami method, but in this application, the value calculated using the Griffin method is used as the HLB value of the acetylene diol surfactant.

[0082] The Griffin method uses the molecular weight of the target compound to calculate the HLB value according to the following formula (3). Note that the smaller the HLB value, the more hydrophobic the target compound is, and the larger the HLB value, the more hydrophilic the compound is.

[0083] Formula (3): HLB value = 20 x (sum of molecular weights of hydrophilic parts) ÷ (molecular weight of material)

[0084] Examples of acetylene diol surfactants having an HLB value of 10 or less include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, hexadec-8-yne-7,10-diol, 4,7-dipropyl-dec-5-yne-4,7-diol, 6,9-dimethyl-tetradec-7-yne-6,9-diol, and 3,6-diisopropyl-2,7-dimethyloct-4-yne-3,6-diol. , octadec-9-yne-8,11-diol, 7,10-dimethylhexadec-8-yne-7,10-diol, 5,8-dibutyldodec-6-yne-5,8-diol, 4,7-diisobutyl-2,9-dimethyl-dec-5-yne-4,7-diol, 5,14-diethyl-8,11-dimethyloctadec-9-yne-8,11-diol, and ethylene oxide and / or propylene oxide modified products of the above listed compounds. However, the above listed compounds may be used alone or in combination of two or more. Furthermore, these compounds may be synthesized by a conventionally known synthesis method or may be commercially available products. Examples of the commercially available products include Surfynol 61, 104E, 104H, 104A, 104BC, 104DPM, 104PA, 104PG-50, 40, 440, 2502, DF110D, SE, SE-F, Dynol 604, 607 (manufactured by Evonik), Olfine E1004, PD-001, PD-002W, PD-004 (manufactured by Nissin Chemical Industry Co., Ltd.), Acetylenol E00, E13T, E40 (manufactured by Kawaken Fine Chemical Co., Ltd.), etc. Among these, it is preferable to select one or more compounds selected from the group consisting of Surfynol 420, 2502, SE, SE-F, Dynol 604, 607, Olfine E1004, Acetylenol E13T, and E40, which have an HLB value of 4 to 9. In particular, it is preferable to select one or more compounds selected from the group consisting of Surfynol 440, Surfynol 2502, and Dynol 604, which have an HLB value of 7 to 8.5. Surfynol 440 is an ethylene oxide modified product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (number of moles of ethylene oxide added: 3.5), Surfynol 2502 is an ethylene oxide and propylene oxide modified product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (number of moles of ethylene oxide added: 5, number of moles of propylene oxide added: 2), and Dynol 604 is an ethylene oxide modified product of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (number of moles of ethylene oxide added: approximately 4).

[0085] The inkjet ink of this embodiment preferably contains a binder resin. This improves the water resistance, solvent resistance, abrasion resistance, gloss, etc. of the printed matter. Examples of the binder resin include acrylic resin, urethane resin, styrene-butadiene resin, vinyl chloride resin, polyolefin resin, polyester resin, alkyd resin, etc. Furthermore, the binder resin can be used alone or in combination of two or more kinds.

[0086] The content of the binder resin is preferably 1 to 30 mass %, and more preferably 2 to 20 mass %, calculated as non-volatile matter, in the total amount of the inkjet ink.

[0087] The inkjet ink of this embodiment may contain additives as necessary. Examples of additives include wax, preservatives, etc. The content of the additives in the inkjet ink is preferably 0.05 to 10% by mass, and more preferably 0.2 to 5% by mass.

[0088] <Method of manufacturing water-based inkjet ink> The aqueous inkjet ink of the present embodiment can be produced by a conventionally known method. One example is a method in which the aqueous pigment dispersion of the present embodiment is mixed with water, a water-soluble solvent, a surfactant, a binder resin, and the like, and the mixture is thoroughly stirred and mixed, and then coarse particles are removed by a technique such as filtration or centrifugation. However, the method for producing the aqueous inkjet ink of the present embodiment is not limited to the above-mentioned method.

[0089] <Characteristics of water-based inkjet ink> The aqueous inkjet ink of the present embodiment preferably has a viscosity of 3 to 15 mPa·s at 25°C. In this viscosity range, droplets of the aqueous inkjet ink can be stably ejected not only from an inkjet head having an ejection frequency of about 4 to 10 KHz, but also from an inkjet head having a high ejection frequency of about 20 to 70 KHz. In particular, when the aqueous inkjet ink of the present embodiment has a viscosity of 4 to 10 mPa·s at 25°C, the aqueous inkjet ink can be stably ejected even when an inkjet head having a design resolution of 600 dpi or more is used. In the present application, the viscosity is measured using a cone-plate type rotational viscometer (E-type viscometer, cone angle 1°34′) manufactured by Toki Sangyo Co., Ltd. under a 25°C environment.

[0090] In order to obtain an aqueous inkjet ink excellent in ejection stability and print quality of printed matter, the aqueous inkjet ink of this embodiment preferably has a static surface tension of 18 to 35 mN / m, and particularly preferably 21 to 32 mN / m, at 25° C. In the present application, the static surface tension is a value measured in an environment of 25° C. using the Wilhelmy method (plate method) with an automatic surface tensiometer CBVP-Z manufactured by Kyowa Interface Science Co., Ltd.

[0091] <Water-based inkjet ink set> The aqueous inkjet ink of the present embodiment may be used alone, or may be used as an aqueous inkjet ink set in which two or more aqueous inkjet inks are combined. Examples of the aqueous inkjet ink set include a set of four aqueous inkjet inks (process color ink set) consisting of a cyan aqueous inkjet ink (aqueous cyan ink), a magenta aqueous inkjet ink (aqueous magenta ink), a yellow aqueous inkjet ink (aqueous yellow ink), and a black aqueous inkjet ink (aqueous black ink); a set of five aqueous inkjet inks obtained by adding an aqueous white ink to the process color ink set; and the like. It is preferable that all the aqueous inkjet inks constituting the aqueous inkjet ink set are manufactured using the aqueous pigment dispersion of the present embodiment.

[0092] <Ink-pretreatment liquid set> The aqueous inkjet ink of this embodiment and the aqueous inkjet ink set can also be used in a form combined with a pretreatment liquid containing an aggregating agent (in the form of an ink-pretreatment liquid set). By applying a pretreatment liquid containing an aggregating agent onto a printing substrate before printing with the aqueous inkjet ink, a layer (ink aggregating layer) can be formed that intentionally aggregates the solid components contained in the aqueous inkjet ink. By landing the aqueous inkjet ink on the ink aggregating layer, it is possible to prevent the droplets of the aqueous inkjet ink from coalescing and mixing, and to significantly improve the print quality of the printed matter.

[0093] As the flocculant, for example, water-soluble inorganic or organic salts containing polyvalent metal ions, and resins having cationic groups and in which the cationic group equivalent is greater than the anionic group equivalent, can be used.

[0094] <Inkjet recording method> The aqueous inkjet ink of the present embodiment is used in the inkjet printing method described above. That is, the aqueous inkjet ink of the present embodiment is ejected onto a printing substrate from an inkjet head having fine nozzles (ejection step). In addition, the aqueous inkjet ink ejected onto the printing substrate is preferably dried by a drying mechanism (drying step).

[0095] ≪Discharge process≫ In the ejection process, the inkjet head can be operated in two different ways: a shuttle (scan) method in which the inkjet head is scanned back and forth in a direction perpendicular to the transport direction of the printing substrate to eject and record the aqueous inkjet ink, and a single-pass method in which the inkjet ink is ejected and recorded when the printing substrate passes under a fixedly disposed inkjet head. Either the shuttle method or the single-pass method may be used for the inkjet head equipped with the aqueous inkjet ink of this embodiment. Of these, the single-pass method is preferably selected because it is less likely to cause deviation in the landing position of droplets of the aqueous inkjet ink, improving the print quality of the printed matter.

[0096] The method of ejection from the inkjet head can be selected from any known methods, such as a piezoelectric method that utilizes the volume change of a piezoelectric element, a thermal method that ejects water-based inkjet ink by bubbles generated by heating a heater, and a valve method that ejects pressurized water-based inkjet ink by opening and closing a nozzle cover (valve) with a solenoid.

[0097] The amount of droplets of the aqueous inkjet ink ejected from the inkjet head is preferably 0.5 to 20 picoliters, and particularly preferably 0.5 to 15 picoliters, from the viewpoints of reducing the drying load, improving print quality, etc. Also, from the viewpoint of improving print quality, it is preferable to adjust the printing conditions (specifically, the driving frequency and number of inkjet heads installed, and the printing speed) so that the recording resolution of the printed matter is 600 dpi or more, and particularly preferably 1200 dpi or more.

[0098] ≪Drying process≫ Examples of drying methods employed in the drying mechanism used in the drying step include heat drying, hot air drying, infrared (e.g., infrared with a wavelength of 700 to 2500 nm) drying, microwave drying, and drum drying. In the drying step, one or more of these methods can be selected and used as desired. When two or more of the above drying methods are used, the drying methods may be used separately (e.g., consecutively) or simultaneously. For example, by using the heat drying method and the hot air drying method in combination, the aqueous inkjet ink can be dried more quickly than when each method is used alone.

[0099] In particular, from the viewpoint of preventing bumping of the liquid components in the aqueous inkjet ink and obtaining a printed matter with excellent print quality, when a heat drying method is employed, it is preferable that the drying temperature be 35 to 100° C., and when a hot air drying method is employed, it is preferable that the hot air temperature be 50 to 250° C. From the same viewpoint, when an infrared drying method is employed, it is preferable that 50% or more of the integrated value of the total output of the irradiated infrared rays is in the wavelength region of 700 to 1500 nm.

[0100] <Printing base material> The printing substrate on which the aqueous inkjet ink of this embodiment is printed is not particularly limited, and any known substrate such as a permeable substrate, a poorly permeable substrate, or a non-permeable substrate can be used.

[0101] In this application, the permeability of a printing substrate is judged by the amount of water absorption measured by a dynamic scanning absorptiometer. Specifically, the amount of pure water absorption measured by the following method for a contact time of 100 msec is 1 g / m 2 Printing substrates with a density of less than 1 g / m are called "non-porous substrates" and 2 More than 6g / m 2 Printing substrates with a permeability of less than 6g / m are called "low-permeability substrates" and 2 The printing substrate described above is referred to as a "permeable substrate." The water absorption of the printing substrate can be measured using a dynamic scanning absorptiometer (e.g., "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.) set under the conditions shown below, using a sample of the printing substrate approximately 15 to 20 cm square, in an environment of 23°C and 50% RH. Measurement method: Spiral Method ·Measurement start radius: 20mm Measurement end radius: 60mm ·Contact time: 10~1,000msec Number of sampling points: 19 (measured at approximately equal intervals relative to the square root of contact time) Scanning distance: 7mm Rotating table speed switching angle: 86.3 degrees Head box conditions: width 5mm, slit width 1mm

[0102] Examples of permeable substrates include uncoated paper such as sawdust paper, medium-quality paper, fine paper, and recycled paper, fabrics such as cotton, synthetic fabrics, silk, hemp, and nonwoven fabrics, leather, etc. Among these, uncoated paper such as sawdust paper, medium-quality paper, fine paper, and recycled paper is preferably used because it can produce printed matter with excellent print quality.

[0103] Examples of non-permeable or poorly permeable substrates include plastic substrates such as polyvinyl chloride, polyethylene terephthalate (PET), polypropylene, polyethylene, nylon, polystyrene, and polyvinyl alcohol; coated paper such as coated paper, art paper, and cast paper; metals such as aluminum, iron, stainless steel, and titanium; and glass.

[0104] The printing substrates listed above may have a smooth surface or may have an uneven surface. The printing substrates may be transparent, semi-transparent, or opaque. The printing substrates may be in the form of rolls or sheets. In addition, two or more of the printing substrates listed above may be bonded together to be used as the printing substrate. A peelable adhesive layer or the like may be provided on the opposite side of the printing surface, or an adhesive layer or the like may be provided on the printing surface after printing.

[0105] In one embodiment, the aqueous pigment dispersion of this embodiment and the aqueous inkjet ink produced using the aqueous pigment dispersion can be suitably used for printing substrates containing calcium. That is, calcium ions dissolved in the aqueous inkjet ink react with carboxy groups in the maleic acid-based resin, causing pigment aggregation. As a result, the pigment is deposited on the printing substrate, and a printed matter having high optical density and high clarity can be produced. As described above, the aqueous inkjet ink of this embodiment does not coalesce into droplets, so that clarity can be further improved, thereby achieving further improvement in print quality.

[0106] An example of a printing substrate containing calcium is a paper substrate containing calcium carbonate. The calcium carbonate is generally used as a filler or a pigment for paper. A pretreatment liquid containing a water-soluble inorganic salt or organic salt containing calcium ions as a flocculant may be applied to the printing substrate listed above. EXAMPLES

[0107] The aqueous inkjet ink of the present embodiment will be described in more detail below with reference to examples and comparative examples. In the following description, "parts" and "%" refer to "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0108] <Acid value> Approximately 1 g of the aqueous solution of the sample to be measured was precisely weighed and placed in an Erlenmeyer flask, and 50 ml of a mixture of distilled water and dioxane (mass ratio: distilled water / dioxane = 1 / 9) was added to dissolve it. The above sample solution was titrated with 0.1 mol / L potassium hydroxide-ethanol solution (titer F) using a potentiometer (Kyoto Electronics Manufacturing Co., Ltd., "Potentiometric Automatic Titrator AT-710M"), and the amount of potassium hydroxide-ethanol solution dropped until the titration endpoint (α (mL)) was measured. The acid value (mgKOH / g) of the dry sample was calculated using the following formula (4).

[0109] Formula (4): Acid value (mgKOH / g)={(5.611×α×F) / S} / (C / 100)

[0110] In the above formula (4), S is the amount of sample collected (g), and C is the non-volatile content (mass %) of the aqueous solution of the sample.

[0111] <Resin manufacturing> (Production of Resin 1) In a reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer, 66.7 parts (70 mol%) of 1-hexene, 33.3 parts (30 mol%) of maleic anhydride, and 100 parts of methyl ethyl ketone as a solvent were charged, and after nitrogen replacement, the contents were heated to 90°C while stirring. After the temperature of the contents reached 90°C, 1.0 part of t-butylperoxy-2-ethylhexanoate was added dropwise as a radical polymerization initiator over 2 hours while stirring the contents. After the addition, the temperature of the contents was kept at about 90°C and stirring was continued for another 1 hour to carry out a polymerization reaction. Thereafter, the non-volatile content concentration of the contents was measured, and the polymerization conversion rate of the contents was calculated by comparing it with the non-volatile content concentration when the contents were assumed to be completely polymerized. After confirming that the polymerization conversion rate of the contents was 95 mol% or more, the temperature of the contents was lowered to 60°C. Next, 50.4 parts of water (15 equivalents relative to the amount of maleic anhydride charged) and 0.01 parts of diazabicycloundecene as a catalyst were added, and the contents were heated to 80°C while stirring, and the stirring was continued while maintaining the temperature at about 80°C for 4 hours to open the maleic anhydride structure. Then, the solvent (methyl ethyl ketone) was completely distilled off under reduced pressure. The acid value of the reaction product (resin) obtained using this mixed solution and measured by the above-mentioned method was 326 mgKOH / g. Thereafter, ion-exchanged water was added so that the non-volatile content was 20%, and the mixture was further stirred at 90°C for 2 hours to obtain an aqueous solution of resin 1 (non-volatile content concentration 30% by mass).

[0112] (Production of Resins 2 to 18) Resins 2 to 18 were obtained in the same manner as Resin 1, except that the raw materials and amounts charged for Resin 1 were changed as shown in Table 1. The unit of blending amounts shown in Table 1 is mol %.

[0113] [Table 1]

[0114] [Table 1]

[0115] <Production of aqueous pigment dispersion for inkjet ink> The raw materials listed in each column of Table 2 were put into a mixing vessel equipped with a stirrer so as to obtain the blending formula listed in "Bundle amount (when dispersed)" in Table 2, and stirred (premixed) for 1 hour. Next, circulation dispersion of the mixture was started using Shinmaru Enterprises' "Dyno Mill" (volume 0.6 L) filled with 1,800 g of zirconia beads with a diameter of 0.5 mm. Then, the D50 of the mixture was measured at regular intervals using the above-mentioned device, and circulation dispersion was stopped when the D50 became equal to or less than a predetermined value. Next, at room temperature (25°C), the crosslinking agent shown in "Amount blended (for crosslinking)" in Table 2 was added to the mixture after the circulation dispersion. The mixture was stirred for about 3 hours while being heated in an oil bath at 70°C, and then the pigment content was adjusted to 15% with ion-exchanged water to obtain an aqueous pigment dispersion for inkjet ink.

[0116] [Table 2]

[0117] [Table 2]

[0118] [Table 2]

[0119] [Table 2]

[0120] [Table 2]

[0121] The four types of pigments shown below were used as the "pigments" listed in Table 2. That is, the aqueous pigment dispersions for inkjet inks listed in each column of Table 2 consist of four colors: cyan, magenta, yellow, and black (a set of aqueous pigment dispersions for inkjet inks). In addition, the "D50" listed after the product names below is the D50 value used to determine the end of the circulating dispersion. Cyan: Toyo Color "LIONOGEN BLUE 7919", D50=140nm Magenta: Tokyo Color Materials' "TOSHIKI RED 150TR" and DIC's "FASTOGEN SUPER MAGENTA RTS" mixed in a mass ratio of 7:3, D50=140nm Yellow: Sanyo Pigment "FAST YELLOW 7413", D50=140nm Black: Orion Engineered Carbons "PrinteX 85", D50=100nm

[0122] Table 2 also lists the molecular weight of the basic compound and the acid value of the resin, which are required to calculate the molar amount of the basic compound relative to the total molar amount of the carboxyl groups and carboxylate groups present in the maleic acid resin using the above formula (2). Other details of the product names listed in Table 2 are as shown below. Surfynol DF110D: Evonik acetylene diol surfactant, active ingredient 32% by mass (dipropylene glycol cut product), HLB value 2.7 Surfynol 104: Acetylene diol surfactant manufactured by Evonik, HLB value 3.0 Surfynol 420: Acetylene diol surfactant manufactured by Evonik, HLB value 4.0 Surfynol 440: Acetylene diol surfactant manufactured by Evonik, HLB value 8.1 Surfynol 465: Acetylene diol surfactant manufactured by Evonik, HLB value 13.2 AIRASE 5355: Evonik polyether-modified polysiloxane surfactant TEGO Foamex 812: Evonik polyether-modified polysiloxane surfactant TEGO Foamex 852: Evonik polyether-modified polysiloxane surfactant TEGO Foamex 830: Organic polymer surfactant manufactured by Evonik TEGO Foamex 8820: Organic polymer surfactant manufactured by Evonik Denacol EX-314: Glycerol polyglycidyl ether manufactured by Nagase ChemteX Corporation (an epoxy compound equivalent to compound (B-1), water solubility 64%) Denacol EX-512: Polyglycerol polyglycidyl ether manufactured by Nagase ChemteX Corporation (an epoxy compound equivalent to compound (B-1), water solubility 100%) Denacol EX-321: Trimethylolpropane polyglycidyl ether manufactured by Nagase ChemteX Corporation (an epoxy compound equivalent to compound (B-1), water solubility 27%) Denacol EX-211: Neopentyl glycol diglycidyl ether (an epoxy compound equivalent to compound (B-1), insoluble in water) manufactured by Nagase ChemteX Corporation Chemitite DZ-22E: Polyfunctional aziridine compound manufactured by Nippon Shokubai Co., Ltd. (non-volatile content 30% by mass, equivalent to compound (B-1), insoluble in water) Carbodilite V-02: A polyfunctional carbodiimide compound manufactured by Nisshinbo Chemical Co., Ltd. (non-volatile content concentration 40% by mass, equivalent to compound (B-1), water solubility 100%) Denacol EX-121: 2-ethylhexyl glycidyl ether manufactured by Nagase ChemteX Corporation (an epoxy compound not corresponding to compound (B-1), insoluble in water)

[0123] [Examples 1 to 63, Comparative Examples 1 to 18] The aqueous pigment dispersion for inkjet ink produced above was used to carry out the following evaluations 1 and 2. The evaluation results were as shown in Table 2 above.

[0124] <Evaluation 1: Evaluation of dispersion stability> Using a dynamic light scattering particle size distribution measuring device (Microtrac-Bell's "Nanotrac UPA-EX150"), the volume-based median diameter (D50) of the above aqueous pigment dispersion for inkjet ink immediately after production was measured in an environment of 25°C and with water as a diluting solvent. Next, the aqueous pigment dispersion after D50 measurement was sealed in a sealed container so that the filling rate was 80% and stored in a constant temperature incubator with airflow set at 70°C. After a predetermined period of time had passed, the aqueous pigment dispersion in the sealed container was taken out of the constant temperature incubator and the D50 was measured again, and the rate of change in D50 before and after storage was calculated to evaluate the stability over time of D50. The evaluation criteria were as follows, with ◎+, ◎, ○, and △ being practically usable. Table 2 shows the evaluation results of the aqueous pigment dispersion with the worst evaluation result among the aqueous pigment dispersions constituting the set of aqueous pigment dispersions for inkjet ink. ◎+: The rate of change in D50 after 6 weeks of storage was greater than -20% and less than 20%. ◎: The D50 change rate after 4 weeks of storage was more than -20% and less than 20%, but after 6 weeks of storage the D50 change rate was -20% or less or 20% or more. ○: The D50 change rate after 2 weeks of storage was more than -20% and less than 20%, but the D50 change rate after 4 weeks of storage was -20% or less or 20% or more. △: The rate of change in D50 after one week of storage was more than -20% and less than 20%, but the rate of change in D50 after two weeks of storage was -20% or less or 20% or more. ×: The rate of change in D50 after storage for one week was −20% or less or 20% or more.

[0125] <Evaluation 2: Evaluation of redispersibility> The four colors of aqueous pigment dispersions constituting the set of aqueous pigment dispersions for inkjet inks produced by the above-mentioned method were each added in an amount of 50 μL to a Menthum can and dried by standing in a constant temperature incubator set at 40° C. The standing time was set to three conditions: 15 minutes, 30 minutes, and 45 minutes. After that, 5 g of water was added to the Menthum can after standing to dissolve the dried aqueous pigment dispersion, and the presence or absence of solidified matter was visually observed to evaluate redispersibility. The evaluation criteria were as follows, with ◎+, ◎, and ○ being usable. ◎+: No solidification was observed under drying conditions of 40℃ for 60 minutes. ⊚: No solidification was observed under drying conditions of 40°C and 45 minutes, but solidification was observed under drying conditions of 40°C and 60 minutes. ◯: No solidified matter was observed under drying conditions of 40° C. for 30 minutes, but solidified matter was observed under drying conditions of 40° C. for 45 minutes. △: No solidified matter was observed under drying conditions of 40°C for 15 minutes, but solidified matter was observed under drying conditions of 40°C for 30 minutes. ×: Solidified material was observed under drying conditions of 40°C for 15 minutes.

[0126] <Manufacture of water-based inkjet inks A to F> 25.0 parts of the aqueous pigment dispersion produced above, 20.0 parts of propylene glycol, 5.0 parts of 1,2-hexanediol, 10.0 parts of QE-1042 (acrylic resin manufactured by Seiko PMC Co., Ltd., non-volatile content concentration 40.5%) as a binder resin, 35.8 parts of ion-exchanged water, 3.1 parts of Surfynol DF110D (acetylene diol surfactant manufactured by Evonik Co., Ltd., active ingredient 32 mass% (dipropylene glycol cut product), HLB value 2.7) as a surfactant, 0.5 parts of BYK-349 (siloxane surfactant manufactured by BYK Japan Co., Ltd.), 0.5 parts of triethanolamine as a pH adjuster, and 0.1 parts of Proxel GXL (s) (manufactured by Arch Chemicals Co., Ltd.) as a preservative were mixed and stirred at room temperature (25 ° C.) until the whole was sufficiently uniform. Thereafter, the mixture was filtered through a membrane filter having a pore size of 0.65 μm to obtain a water-based inkjet ink A. In addition, by using four colors, cyan, magenta, yellow, and black, as the aqueous pigment dispersions, four colors of aqueous inkjet ink A were produced for each aqueous pigment dispersion (a set of aqueous inkjet inks A).

[0127] In addition, water-based inkjet ink B (set) was obtained in the same manner as the water-based inkjet ink A above, except that 37.9 parts of ion-exchanged water and 1.0 part of Surfynol 104 (acetylene diol-based surfactant, active ingredient 100% by mass, HLB value 3.0, manufactured by Evonik Corporation) were used instead of 35.8 parts of ion-exchanged water and 3.1 parts of Surfynol DF110D.

[0128] In addition, water-based inkjet ink C (set) was obtained in the same manner as the water-based inkjet ink B above, except that Surfynol 420 (acetylene diol-based surfactant manufactured by Evonik, active ingredient 100% by mass, HLB value 4.0) was used instead of Surfynol 104.

[0129] In addition, water-based inkjet ink D (set) was obtained in the same manner as the water-based inkjet ink B above, except that Surfynol 440 (acetylene diol surfactant manufactured by Evonik, active ingredient 100 mass%, HLB value 8.1) was used instead of Surfynol 104.

[0130] In addition, water-based inkjet ink E (set) was obtained in the same manner as the water-based inkjet ink B, except that Surfynol 465 (acetylene diol surfactant manufactured by Evonik, active ingredient 100 mass%, HLB value 13.2) was used instead of Surfynol 104.

[0131] Furthermore, water-based inkjet ink F (set) was obtained in the same manner as the water-based inkjet ink A, except that dipropylene glycol-n-propyl ether was used instead of 1,2-hexanediol.

[0132] [Examples 64 to 137, Comparative Examples 19 to 36] The aqueous inkjet inks (set) produced by the above-mentioned method were used to carry out the following evaluations 3 and 4. The evaluation results are shown in Table 3.

[0133] <Evaluation 3: Evaluation of ejection stability> The inks constituting the above-mentioned aqueous inkjet ink set were filled into an inkjet ejection device equipped with a Kyocera Corporation inkjet head "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm). A nozzle check pattern was printed, and after confirming that there were no nozzle clogs (phenomenon in which aqueous inkjet ink is not ejected from the nozzle), the inkjet ejection device was left on standby in a 25°C environment. After a predetermined time had elapsed, a nozzle check pattern was printed again, and the number of nozzle clogs was visually counted to evaluate the ejection stability. The evaluation criteria were as follows, with ◎+, ◎, and ○ being considered as usable. Table 3 shows the score of the ink of the worst color. ◎+: No missing nozzles even after 4 hours of standby time ◎: No missing nozzles occurred when printing after waiting for 3 hours, but one or more missing nozzles occurred when printing after waiting for 4 hours. ○: No missing nozzles occurred when printing after waiting for 2 hours, but one or more missing nozzles occurred when printing after waiting for 3 hours. △: No missing nozzles at all when printed after waiting for 1 hour, but one or more missing nozzles occurred when printed after waiting for 2 hours. ×: One or more nozzles were missing when printing after waiting for one hour.

[0134] <Creating printed matter> An inkjet printing device was prepared above the conveyor, with four Kyocera inkjet heads "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) lined up along the transport direction of the substrate. The set of water-based inkjet inks shown in Table 3 was filled in the following order from the upstream side of the transport direction: black ink, cyan ink, magenta ink, and yellow ink. In addition, A4 size (21 cm wide x 30 cm long) wood-free paper (Xerox 4024) was fixed on the conveyor as a printing substrate. After that, the conveyor was driven at a constant speed, and when the printing substrate passed under the installation part of the inkjet head, the above-mentioned set of water-based inkjet inks were discharged under the condition of a drop volume of 2.6 pL, and images and characters were printed. Then, immediately, the printing substrate after printing was placed in a constant temperature incubator with a blower set at 70°C, and dried for 3 minutes to produce a printed matter.

[0135] For the above image and characters, we used JIS X 9201 high-definition color digital standard image data (CMYK / SCID) natural image N5 (bicycle). In addition, we set four conveyor speeds: 40 m / min, 60 m / min, 80 m / min, and 100 m / min. Printing was carried out under each conveyor speed condition, and prints were produced.

[0136] <Evaluation 4: Evaluation of dot shape (print quality)> Using a microscope, the dot shapes of the 4C (CMYK) printed parts of the above prints were observed at a magnification of 200 times, and the dot shapes (print quality) were evaluated. The evaluation results were as follows, with ◎+, ◎, ○, and △ being usable. ◎+: At all four printing speeds, the dots in the 4C printed area were close to circular, and each dot was independent; no merged dots were observed. ◎: In the prints made at 40m / min, 60m / min and 80m / min, the dots in the 4C printed area were close to circular, and each dot was independent, with no merged dots observed. However, at 100m / min, the dots were no longer circular and / or merged. ○: In the prints made at 40m / min and 60m / min, the dots in the 4C print were nearly circular, and each dot was independent, with no merged dots observed. However, at 80m / min, the dots were distorted in circular shape and / or merged dots were observed. △: In the print at 40 m / min, the dots in the 4C printed area were nearly circular, and each dot was independent, with no merged dots observed. However, at 60 m / min, the dots became distorted in circular shape and / or merged dots were observed. ×: At all four printing speeds, the circular dots in the 4C printed area were distorted and / or the dots merged.

[0137] [Table 3]

[0138] [Table 3]

[0139] As shown in Tables 2 and 3 above, the aqueous pigment dispersions for inkjet inks of Examples 1 to 63 having the above-mentioned configurations were superior in dispersion stability and redispersibility compared to the pigment dispersions of Comparative Examples 1 to 18. Furthermore, the aqueous inkjet inks produced using these aqueous pigment dispersions had good print quality and excellent ejection stability.

[0140] The above-mentioned Patent Documents 1 to 3 and the present application differ in the presence or absence of the glycol ether compound (A-1) represented by general formula (1). In fact, in Comparative Example 10, in which dispersion treatment was performed without using the glycol ether compound (A-1) represented by general formula (1), the dispersion stability and dot shape (printing image quality) of the printed matter were at a practically usable level, but the redispersibility and the ejection stability when made into an aqueous inkjet ink were at a poor level. As described above, the glycol ether compound (A-1) represented by general formula (1) is an essential material for improving the adsorption of the maleic acid resin to the pigment and improving the dispersion stability, redispersibility, and the above-mentioned ejection stability, and the results of the above-mentioned Comparative Example 10 can be said to support this hypothesis.

Claims

1. An aqueous pigment dispersion for inkjet ink, comprising pigment particles, a basic compound, and an organic solvent, the pigment particles contain a pigment and a maleic acid-based resin, the maleic acid-based resin has a crosslinked structure formed by a crosslinking agent, the crosslinking agent contains a compound (B-1) having a plurality of functional groups reactive with carboxy groups in one molecule, the basic compound comprises an alkali metal hydroxide, the organic solvent contains a glycol ether compound (A-1) represented by the following general formula (1) in an amount of 50 to 200% by mass based on the content of the maleic acid-based resin, the molar amount of the functional group reactive with the carboxy group present in the compound (B-1) is 80 to 99 mol % based on the total molar amount of the carboxy group and the carboxylate group present in the maleic acid-based resin before crosslinking. General formula (1): R 1 -(O-CH(CH 3 )-CH 2 ) n -OH (In general formula (1), R 1 is an alkyl group having 1 to 4 carbon atoms, and n is 1 or 2.

2. 2. The aqueous pigment dispersion for ink-jet ink according to claim 1, wherein the molar amount of the basic compound is 10 to 200 mol % based on the total molar amount of carboxy groups and carboxylate groups present in the maleic acid-based resin before crosslinking.

3. The water solubility of the crosslinking agent is 65 g / 100 gH 2 3. The aqueous pigment dispersion for inkjet ink according to claim 1, wherein the viscosity of the aqueous pigment dispersion is 0 or less.

4. The aqueous pigment dispersion for inkjet ink according to claim 1 or 2, wherein the maleic acid-based resin further comprises a structural unit derived from an alkene monomer.

5. An inkjet ink comprising the aqueous pigment dispersion for inkjet ink according to claim 1 or 2.

6. Further, the composition contains a surfactant, The ink-jet ink of claim 5 , wherein the surfactant comprises an acetylenic diol surfactant having an HLB value of 10 or less.

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