Aqueous inkjet ink and printed work

Abstract

The present disclosure relates to an aqueous inkjet ink including pigment particles, a surfactant, and an organic solvent, wherein: the pigment particles include a pigment and a pigment dispersion resin; the pigment dispersion resin has a crosslinked structure; the surfactant includes an acetylenediol-based surfactant (A-1) having an actually measured HLB value of 6-9 and a siloxane-based surfactant (A-2) having an actually measured HLB value of 8-14; and the organic solvent includes an alkanediol having 5-8 carbon atoms (but excluding 1,2-alkanediol having 5-8 carbon atoms) and / or a (di)propylene glycol monoalkyl ether represented by a specific structural formula.

Description

Water-based inkjet inks and printed materials

[0001] The present disclosure relates to an aqueous inkjet ink and a printed matter produced using the aqueous inkjet ink.

[0002] Inkjet printing is a recording method in which ink droplets are directly ejected from fine nozzles and deposited on a printing substrate to produce characters and / or images. Inkjet printing has numerous advantages, including low noise and ease of operation, ease of full-color printing, inexpensive printing equipment, and the ability to print on a variety of printing substrates without contact, leading to its widespread adoption. In particular, in recent years, inkjet printing has become increasingly popular not only for consumer applications in offices and homes, but also for commercial and industrial printing applications. In this context, there has been an increasing demand for inkjet inks containing water as a primary component (aqueous inkjet inks) with the aim of reducing the burden on the environment and workers.

[0003] The above "image" also includes seamless images such as solid images and checkered pattern images.

[0004] Aqueous inkjet inks have long been developed for use on plain paper and specialty paper as printing substrates. These applications are based on the premise that the liquid components of the aqueous inkjet ink are absorbed into the printing substrate. Therefore, when the aqueous inkjet ink is printed on poorly absorbent printing substrates, such as those used in the commercial and industrial printing applications, the image bleeds, making it impossible to produce printed materials suitable for practical use.

[0005] For example, coated paper, which is a poorly absorbent printing substrate, has low absorbency of liquid components. Therefore, during printing, if a droplet of aqueous inkjet ink lands adjacent to a previously landed droplet before the previous droplet has dried, adjacent droplets are likely to coalesce (a phenomenon known as beading). Beading can cause color bleeding in printed materials. Furthermore, some poorly absorbent printing substrates, including coated paper, have low surface free energy. When printing on such printing substrates, the aqueous inkjet ink has difficulty wetting and spreading on the surface of the printing substrate, which can easily lead to the occurrence of white spots (a phenomenon in which areas on the printing substrate where the aqueous inkjet ink does not adhere appear as spots and / or streaks).

[0006] One possible method for improving the beading phenomenon is to improve the drying properties of the aqueous inkjet ink and dry the previously landed droplet before the next droplet lands. However, aqueous inkjet inks with high drying properties tend to dry easily near the nozzle, which can result in a dried film of the aqueous inkjet ink adhering to the nozzle outlet. If this adhered dried film remains at the nozzle outlet, it can lead to a deterioration in the landing accuracy of the aqueous inkjet ink and nozzle clogging. Therefore, it is preferable that aqueous inkjet inks used in inkjet printing methods have "resolubility," which allows them to dissolve the dried film. However, imparting resolubility to aqueous inkjet inks is an extremely difficult task.

[0007] Another method for improving the beading phenomenon is to reduce the surface tension of the aqueous inkjet ink, and generally, a highly hydrophobic surfactant or organic solvent is used.

[0008] For example, Patent Document 1 discloses an ink composition for inkjet recording containing three types of acetylene diol surfactants with different structures. Patent Document 1 claims that it is possible to record images with excellent print quality (color unevenness, aggregation, bleed) and fixability (abrasion resistance) at high speed on a variety of printing substrates with different absorbencies. Patent Document 2 also discloses an aqueous ink composition containing a nonionic surfactant with an HLB value of 4 to 14. Patent Document 2 claims that it is possible to obtain printed matter that is excellent in wetting and spreading properties, definition, print density, water resistance, rubbing resistance (abrasion resistance), etc., and is free of aggregation, on offset media that may have been printed with offset ink.

[0009] Meanwhile, Patent Document 3 discloses a water-based ink for inkjet printing that aims to simultaneously solve the above-mentioned problems of resolubility (redispersibility) and color bleeding. Specifically, Patent Document 3 discloses a water-based ink containing a pigment, a polymer dispersant (preferably containing a polymer crosslinked with a crosslinking agent), and a specific water-soluble organic solvent. Furthermore, Example 2 of Patent Document 3 discloses a specific example of a water-based ink containing pigment particles containing a polymer dispersant having a crosslinked structure, the acetylene diol surfactant "Surfynol 440," the siloxane surfactant "Silface SAG005," and a water-soluble organic solvent such as 1,6-hexanediol (see paragraphs 0068, 0072, Table 2, etc. of Patent Document 3).

[0010] Japanese Patent Application Laid-Open No. 2015-124238 Japanese Patent Application Laid-Open No. 2004-510028 Japanese Patent Application Laid-Open No. 2020-105298

[0011] On the other hand, the aqueous inkjet inks disclosed in the above Patent Documents 1 and 2 have not been evaluated for ejection stability, which is an essential requirement for use in inkjet printing methods. As will be described in detail later, depending on the material used in combination, the effects of the highly hydrophobic material described above may not be fully exhibited, and the ejection stability of the aqueous inkjet ink may also deteriorate.

[0012] Furthermore, when the present inventors evaluated an aqueous inkjet ink having the configuration of Example 2 of Patent Document 3, they found that while the ink had good resolubility, the droplets of the aqueous inkjet ink were distorted (poor dot circularity) and blank areas occurred depending on the printing conditions.

[0013] As described above, until now, there has been no aqueous inkjet ink that simultaneously satisfies all of the requirements for high levels of dot circularity and ink wettability (reduction of white voids) in addition to ejection stability and resolubility.

[0014] Therefore, an object of one embodiment of the present invention is to provide an aqueous inkjet ink that has excellent resolubility and ejection stability and that produces excellent print quality on a variety of printing substrates.Another embodiment of the present invention is to provide a printed matter that produces excellent print quality on a variety of printing substrates.

[0015] In light of the above-mentioned background, the present inventors have conducted extensive research and have found the following aqueous inkjet ink, thereby completing the present invention.

[0016] That is, one embodiment of the present invention relates to an aqueous inkjet ink comprising pigment particles, a surfactant, and an organic solvent, wherein the pigment particles comprise a pigment and a pigment dispersing resin, the pigment dispersing resin has a crosslinked structure, the surfactant comprises an acetylene diol surfactant (A-1) having a measured HLB value of 6 to 9 and a siloxane surfactant (A-2) having a measured HLB value of 8 to 14, and the organic solvent comprises an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) and / or a glycol monoether represented by the following general formula 1. General formula 1: R １ -(O-CH(CH ３ )-CH ２ ) ｎ -OH (in general formula 1, R １ represents an alkyl group having 2 to 4 carbon atoms, and n is 1 or 2. Another embodiment of the present invention relates to a printed matter obtained by printing with the aqueous inkjet ink.

[0017] The aqueous inkjet ink according to one embodiment of the present invention has excellent resolubility and ejection stability, and also has the effect of providing excellent print quality on a variety of printing substrates, such as coated paper. In addition, the printed matter according to another embodiment of the present invention has the effect of providing excellent print quality on a variety of printing substrates.

[0018] An aqueous inkjet ink according to one embodiment of the present invention (hereinafter also simply referred to as "aqueous inkjet ink of this embodiment") will be described below. Note that the present invention is not limited to the embodiment described below, and includes embodiments that can be modified without changing the essential parts of the present invention.

[0019] The aqueous inkjet ink of this embodiment has excellent resolubility and ejection stability, and also produces excellent print quality on a variety of printing substrates, particularly coated paper. Although the mechanism behind this is not clear, the inventors speculate as follows. However, the present invention is not limited to this speculation.

[0020] First, resolubility will be described. As described above, if the aqueous inkjet ink present near the nozzle dries and the resulting dried film adheres to the nozzle outlet, this can cause a deterioration in the ejection stability of the aqueous inkjet ink. Therefore, it is preferable for the aqueous inkjet ink to have resolubility. The inventors have discovered that one method for improving the resolubility is to ensure that the pigment dispersion resin adsorbed to and / or coating the pigment does not detach even after the liquid components in the inkjet ink evaporate. In an embodiment of the present invention, the use of a pigment coated with a pigment dispersion resin having a crosslinked structure suppresses the detachment of the pigment dispersion resin, leading to improved resolubility. Furthermore, the aqueous inkjet ink of this embodiment easily returns the dried film adhering to the nozzle outlet to the aqueous inkjet ink, thereby achieving excellent ejection stability. Furthermore, this prevents the phenomenon in which a surfactant, described below, is adsorbed to the detached pigment dispersion resin and inhibits the interfacial orientation of the surfactant. Furthermore, since these surfactants can fully exhibit their functions, it is believed that the wettability of the aqueous inkjet ink is improved, resulting in improved print image quality.

[0021] Next, we will discuss print quality. Generally, surface tension is one of the factors that affect print quality. The surface tension of aqueous inkjet ink is mainly controlled by surfactants. For example, acetylene diol surfactants and siloxane surfactants are used as surfactants.

[0022] The present inventors have discovered that the wettability of an aqueous inkjet ink to a printing substrate can be dramatically improved by using an acetylenic diol-based surfactant (A-1) with a measured HLB value of 6 to 9 and a siloxane-based surfactant (A-2) with a measured HLB value of 8 to 14. The acetylenic diol-based surfactant (A-1) with a measured HLB value of 6 to 9 has low affinity for water and quickly orients at the interface. Therefore, when ink droplets of the aqueous inkjet ink land on a printing substrate, the wettability of the aqueous inkjet ink can be expected to be significantly improved. However, it is difficult to orient the acetylenic diol-based surfactant (A-1) alone uniformly at the interface, resulting in poor dot circularity and unevenness in solid areas, resulting in unsatisfactory print quality.

[0023] Therefore, in an embodiment of the present invention, an acetylenic diol-based surfactant (A-1) having a measured HLB value of 6 to 9 and a siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 are used in combination to achieve improved dot circularity and unevenness. While the detailed mechanism is unknown, the siloxane-based surfactant (A-2) is a surfactant that, like the acetylenic diol-based surfactant, has excellent surface tension reducing ability. However, due to its high measured HLB value, it is thought to orient at the gas-liquid interface later than the acetylenic diol-based surfactant (A-1). As a result, the siloxane-based surfactant (A-2) is thought to orient in a manner that eliminates the non-uniformity of orientation caused by the acetylenic diol-based surfactant (A-1), thereby contributing to the uniform spreading of ink droplets of the aqueous inkjet ink and the reduction of unevenness in solid areas. As will be described in detail later, in a preferred embodiment, by using a gemini siloxane surfactant as the siloxane surfactant (A-2), the surface tension reducing ability can be significantly improved. As a result, the wettability of an aqueous inkjet ink containing a gemini siloxane surfactant can be significantly improved, resulting in a significant improvement in print image quality. In addition, in a preferred embodiment, a siloxane surfactant modified at both ends with polyether can also be used as the siloxane surfactant (A-2). In this case, although the detailed principle is unknown, it is possible to achieve uniform wetting and spreading on the printing substrate, as well as improved ejection stability.

[0024] On the other hand, when a pigment coated with the above-mentioned pigment dispersion resin having a crosslinked structure is used in combination with an acetylene diol surfactant and a siloxane surfactant, pinholes (hole-like voids that occur in areas where the aqueous inkjet ink should be printed) may occur in the printed material due to poor compatibility. Furthermore, aqueous inkjet inks using these components in combination may also have poor ejection stability. One of the reasons for the poor ejection stability is thought to be that when liquid components volatilize near the nozzle of the inkjet head, the compatibility balance is disrupted, resulting in non-uniform orientation of the surfactant in the aqueous inkjet ink present near the nozzle.

[0025] As a result of extensive research, the present inventors have discovered that the above-mentioned components can be further combined with an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) and / or a glycol monoether represented by the above general formula 1. It is believed that the combined use of these compounds with the acetylenic diol surfactant and the siloxane surfactant facilitates compatibility between the two. Furthermore, since the alkanediols having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) and / or the glycol monoethers represented by the above general formula 1 all have low surface tension, the acetylenic diol surfactant and the siloxane surfactant that are compatible with these compounds are likely to be uniformly oriented across the entire surface of the printed material. As a result, it is believed that pinholes are suppressed in the printed material. Furthermore, it is believed that the meniscus of the aqueous inkjet ink is stabilized within the inkjet head, thereby improving ejection stability.

[0026] As described above, the aqueous inkjet ink of this embodiment is essential to simultaneously and highly effectively solve the above-mentioned problems.

[0027] The aqueous inkjet inks specifically disclosed in the above-mentioned Patent Documents 1 and 2 differ from the present invention in that they do not use crosslinked polymer particles containing a pigment. Furthermore, Patent Documents 1 and 2 do not describe or suggest the use of a polymer having a crosslinked structure as a dispersing polymer, or that the polymer having a crosslinked structure improves resolubility and ejection stability, and furthermore, enables the effects of the surfactant to be fully exhibited. Meanwhile, the acetylene diol surfactant "Surfynol 440" specifically used in the examples (particularly Example 2) of Patent Document 3 has a measured HLB value of 10.5 (see also the examples described below). Therefore, the aqueous inkjet inks specifically disclosed in the above examples differ from the aqueous inkjet ink of the present embodiment in that they do not contain an acetylene diol surfactant (A-1) with a measured HLB value of 6 to 9. Furthermore, as mentioned above, Patent Document 3 aims to improve resolubility (redispersibility) and color bleeding, but does not describe dot circularity or white voids, which are issues of the present embodiment. In particular, Patent Document 3 does not describe or suggest that a printed matter with excellent dot circularity can be obtained by using in combination an acetylene diol surfactant (A-1) and a siloxane surfactant (A-2) having specific measured HLB values.

[0028] Next, each component that may be contained in the aqueous inkjet ink according to one embodiment of the present invention will be described in detail below.

[0029] Pigment Particles The aqueous inkjet ink of this embodiment contains pigment particles. The pigment particles contain a pigment and a pigment dispersing resin having a crosslinked structure.

[0030] In this embodiment, the phrase "the pigment dispersion resin has a crosslinked structure" means that a crosslinked structure is formed between the pigment dispersion resin molecules. By forming a crosslinked structure between the pigment dispersion resin molecules, the pigment is coated with the pigment dispersion resin having a crosslinked structure, which makes it easier to suppress detachment of the pigment dispersion resin and improve resolubility and ejection stability, as described above. The pigment dispersion resin may have a crosslinked structure within the pigment dispersion resin molecule.

[0031] In the present disclosure, a pigment dispersion resin that does not have an intermolecular crosslinked structure is also referred to as a "non-crosslinked dispersion resin," and a pigment dispersion resin that has an intermolecular crosslinked structure is also referred to as a "crosslinked dispersion resin."

[0032] <Pigment> The pigment particles contained in the aqueous inkjet ink of this embodiment contain a pigment. Printed matter produced using an aqueous inkjet ink containing a pigment has high density. Furthermore, by appropriately drying and / or thickening the ink after landing on the printing substrate, bleeding can be suppressed, and printed matter with excellent print quality can be obtained.

[0033] In the aqueous inkjet ink of this embodiment, the pigment may be an organic pigment, an inorganic pigment, or a combination of both.

[0034] When an inorganic pigment is used as the pigment, titanium oxide, zinc white, zinc sulfide, white lead, calcium carbonate, precipitated barium sulfate, white carbon, alumina white, kaolin clay, talc, bentonite, carbon black, black iron oxide, cadmium red, red iron oxide, molybdenum red, molybdate orange, chrome vermilion, yellow lead, cadmium yellow, yellow iron oxide, titanium yellow, chromium oxide, viridian, titanium cobalt green, cobalt green, cobalt chrome green, Victoria green, ultramarine, Prussian blue, cobalt blue, cerulean blue, cobalt silica blue, cobalt zinc silica blue, manganese violet, cobalt violet, and the like can be used.

[0035] For example, when carbon black is used as an inorganic pigment, carbon black produced by either the furnace method or the channel method can be suitably used. In addition, these carbon blacks, which have a primary particle diameter of 11 to 40 nm and a specific surface area measured by the BET method of 50 to 400 m, can be used. ２ Carbon black having properties such as a viscosity of 1000 s / g, a volatile content of 0.5 to 10%, and a pH value of 2 to 10 can be particularly preferably used.

[0036] Specific examples of organic pigments include azo pigments such as azo lake pigments, insoluble monoazo pigments, insoluble disazo pigments, and chelate azo pigments; and polycyclic pigments such as phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, and threne pigments. The hue is not particularly limited, and chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used.

[0037] Specific examples of pigments that can be used as the organic pigments, based on the color index, include cyan pigments such as C.I. Pigment Blue 1, 2, 3, 15:1, 15:3, 15:4, 15:6, 16, 21, 22, 60, and 64.

[0038] Examples of magenta pigments include C.I. Pigment Red 5, 7, 9, 12, 31, 48, 49, 52, 53, 57, 97, 112, 120, 122, 146, 147, 149, 150, 168, 170, 177, 178, 179, 184, 188, 202, 206, 207, 209, 238, 242, 254, 255, 264, 269, 282, C.I. Pigment Violet 19, 23, 29, 30, 32, 36, 37, 38, 40, and 50.

[0039] Furthermore, examples of yellow pigments include C.I. Pigment Yellow 1, 2, 3, 12, 13, 14, 16, 17, 20, 24, 74, 83, 86, 93, 94, 95, 109, 110, 117, 120, 125, 128, 129, 137, 138, 139, 147, 148, 150, 151, 154, 155, 166, 168, 180, 185, and 213.

[0040] Examples of black pigments include aniline black (C.I. Pigment Black 1), perylene black (C.I. Pigment Black 31, 32), azomethine azo black, etc. A black pigment can also be prepared by mixing a plurality of chromatic pigments such as the above-mentioned cyan pigments, magenta pigments, and yellow pigments, as well as the below-mentioned brown pigments and orange pigments.

[0041] In addition to the above pigments, C.I. Pigment Green 7, 10, 36, C.I. Pigment Brown 3, 5, 25, 26, C.I. Pigment Orange 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38, 40, 43, 62, 63, 64, 71, etc. can be used.

[0042] The pigments listed above can be used alone or in combination of two or more. The content of the pigment is preferably 0.1 to 20% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 7% by mass, based on the total mass of the aqueous inkjet ink.

[0043] <Pigment Dispersion Resin> The pigment dispersion resin is a resin that has the function of dispersing a pigment. The pigment dispersion resin, that is, both the uncrosslinked dispersion resin and the crosslinked dispersion resin, are preferably resins having an acid group in the molecular structure, and the acid group is preferably a carboxyl group and / or a carboxylate group (R—COO － Any resin can be used as the pigment dispersing resin as long as it has an acid group (preferably a carboxyl group and / or a carboxylate group) and has the function of dispersing a pigment.

[0044] As will be described later, the resins generally used in inkjet inks come in the form of water-soluble resins and resin microparticles. In this embodiment, either a water-soluble resin or resin microparticles may be used as the uncrosslinked dispersed resin. Furthermore, both water-soluble resins and resin microparticles can be used as the crosslinked dispersed resin. For example, when pigment-containing resin microparticles are present, the resin contained in the resin microparticles is determined to have the function of dispersing the pigment (pigment dispersed resin). In this case, if there is no crosslinked structure between the resin molecules, the resin contained in the resin microparticles is an uncrosslinked dispersed resin; if there is a crosslinked structure, the resin contained in the resin microparticles is a crosslinked dispersed resin. On the other hand, when a water-soluble resin is used as the uncrosslinked dispersed resin, for example, whether the water-soluble resin has the function of dispersing the pigment can be confirmed by a method conforming to JIS K 5101-1-4:2004.

[0045] Specifically, the primary particle diameter is 15 to 25 nm, and the nitrogen adsorption specific surface area is 120 to 260 m ２ / g, DBP absorption (granular) 40-80 cm ３ 600 g of carbon black (100 g / 100 g), 300 g of the target water-soluble resin, and 2,100 g of water are thoroughly mixed (premixed), and then dispersed for 4 hours using a 0.6 L bead mill (e.g., a "Dyno Mill" manufactured by Shinmaru Enterprises) filled with 1,800 g of grinding beads (e.g., zirconia beads with a diameter of 0.5 mm). After dispersion, the viscosity of the resulting carbon black dispersion at 25°C is measured using an E-type viscometer (e.g., a "TVE25L" type viscometer" manufactured by Toki Sangyo Co., Ltd.). The carbon black dispersion is then stored in a constant temperature incubator set at 70°C for one week, and the viscosity is measured again. If the viscosity of the dispersion immediately after dispersion is 100 mPa s or less and the absolute value of the viscosity change rate of the carbon black dispersion before and after storage is 10% or less, the water-soluble resin is deemed to have the function of dispersing a pigment.

[0046] Examples of resins that can be used as pigment dispersing resins include acrylic, maleic acid, urethane, polyester, etc. Among these, it is preferable to use a resin having an aromatic ring in its structure, as this resin can be firmly adsorbed to the pigment and stabilize the dispersed state of the pigment.

[0047] In the present disclosure, the term "acrylic resin" refers to a resin using one or more polymerizable monomers selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters. In addition to the polymerizable monomers listed above, a styrene-based monomer may also be used as the polymerizable monomer forming the acrylic resin. On the other hand, resins containing maleic acid (anhydride) (at least one selected from "maleic acid" and "maleic acid anhydride") as a polymerizable monomer are excluded from the "acrylic resin" in the present disclosure. Furthermore, the term "maleic acid-based resin" refers to a resin using at least maleic acid (anhydride) as a polymerizable monomer. The maleic acid-based resin may further use an α-olefin, a styrene-based monomer, acrylic acid, methacrylic acid, an acrylic acid ester, a methacrylic acid ester, or the like as a polymerizable monomer.

[0048] Both before and after the crosslinking treatment described below, it is preferable that at least a portion of the acid groups (e.g., carboxyl groups) contained in the pigment dispersing resin be neutralized with a basic compound to form anionized functional groups (e.g., carboxylate groups). This is because the charge repulsion between the anionized functional groups allows the pigment to be stably dispersed. Examples of basic compounds include ammonia; organic amines such as dimethylaminoethanol, diethanolamine, and triethanolamine; and alkali metal compounds such as lithium hydroxide, sodium hydroxide, potassium hydroxide, cesium hydroxide, disodium carbonate, sodium bicarbonate, dipotassium carbonate, and sodium borate. From the viewpoints of achieving excellent pigment dispersion stability in aqueous inkjet inks and suppressing pigment aggregation, it is preferable to use an alkali metal compound, and it is particularly preferable to use sodium hydroxide and / or potassium hydroxide. The basic compounds listed above can be used alone or in combination of two or more.

[0049] The pigment dispersion resin is preferably neutralized so that the pH after adding the entire amount of a basic compound to a 15 to 25% by mass (for example, 20% by mass) aqueous solution of the pigment dispersion resin is 7 to 12.

[0050] In the present disclosure, the term "aqueous solution" refers to a liquid containing an aqueous solvent and components dispersed and / or dissolved in the aqueous solvent. The pH of the aqueous solution of the pigment dispersion resin is a value at 25°C and can be measured by a conventional method. For example, the pH can be measured using a tabletop pH meter "F-71" (manufactured by Horiba, Ltd.) that uses a pH electrode "6337-10D" (manufactured by Horiba, Ltd.).

[0051] Specifically, the preferred amount of basic compound used to neutralize the pigment dispersion resin can be expressed in terms of the neutralization rate. From the viewpoint of pigment dispersion stability, the neutralization rate is preferably 10 to 200 mol%, more preferably 40 to 160 mol%, and particularly preferably 60 to 120 mol%. The neutralization rate here is calculated by dividing the molar amount of basic groups in the added basic compound by the molar amount of acid groups in the pigment dispersion resin, and can be calculated using the following formula 2. When using a resin that is at least partially neutralized (for example, contains at least carboxylate groups) as the pigment dispersion resin, the neutralization rate is calculated assuming that all acid groups in the resin are unneutralized (specifically, assuming that the carboxylate groups are carboxyl groups).

[0052] Formula 2:

[0053] The weight-average molecular weight (Mw) of the pigment dispersing resin before the crosslinking treatment described below is preferably 5,000 to 100,000. A weight-average molecular weight of 5,000 or more provides favorable dispersion stability, while a weight-average molecular weight of 100,000 or less provides favorable ejection stability. The weight-average molecular weight is more preferably 8,000 to 50,000, and even more preferably 10,000 to 35,000.

[0054] The weight-average molecular weight of the pigment dispersion resin can be measured by a conventional method, for example, by using a GPC ("HLC-8120GPC" manufactured by Tosoh Corporation) equipped with a TSKgel column (manufactured by Tosoh Corporation) and an RI detector, and measuring the weight-average molecular weight in terms of polystyrene using THF as a developing solvent.

[0055] Furthermore, the acid value of the pigment dispersion resin before the crosslinking treatment described below is preferably 60 to 180 mgKOH / g, more preferably 70 to 160 mgKOH / g, and particularly preferably 80 to 150 mgKOH / g, from the viewpoints of ensuring the dispersion stability of the pigment and suitably suppressing detachment of the pigment dispersion resin even in the dispersion treatment performed before the crosslinking treatment (details of which will be described later). By controlling the acid value within the above range, detachment of the pigment dispersion resin from the pigment after the crosslinking treatment described below is significantly suppressed, making it possible to obtain excellent resolubility and ejection stability.

[0056] The acid value of the pigment dispersion resin can be measured by a conventional method. For example, approximately 1 g of pigment dispersion resin that has been subjected to acid precipitation treatment is precisely weighed and placed in an Erlenmeyer flask, and 50 ml of a 1:9 (mass ratio) mixture of distilled water and dioxane is added to dissolve the pigment dispersion resin. This sample solution is titrated with a 0.1 mol / L potassium hydroxide ethanol solution (potency F). A potentiometric measuring device (for example, the "Automatic Potentiometric Titrator AT-710M" manufactured by Kyoto Electronics Manufacturing Co., Ltd.) is used for the titration. The acid value (mg KOH / g) can then be calculated using the amount of potassium hydroxide ethanol solution (α (mL)) required to reach the titration endpoint using the following equation 3:

[0057] Formula 3: Acid value (mgKOH / g) = (5.611×α×F) / S

[0058] In Equation 3, S is the amount (g) of the pigment dispersion resin sample collected, α is the amount (ml) of the 0.1 mol / L potassium hydroxide ethanol solution added dropwise up to the end point of the titration, and F is the titer of the 0.1 mol / L potassium hydroxide ethanol solution.

[0059] The content of the pigment dispersing resin relative to the pigment content is preferably 1 to 100% by mass. By making the content of the pigment dispersing resin 1% by mass or more relative to the pigment content, the viscosity of the aqueous inkjet ink can be controlled to a level suitable for inkjet printing applications, and by making the content 100% by mass or less, the dispersion stability, as well as the storage stability and ejection stability after dispersion can be improved. The content of the pigment dispersing resin is more preferably 2 to 50% by mass.

[0060] <<Pigment Dispersion Resin Having a Crosslinked Structure>> The pigment dispersion resin contained in the aqueous inkjet ink of this embodiment has a crosslinked structure. The crosslinking treatment gives the pigment dispersion resin a three-dimensional structure, reducing the solubility of the pigment dispersion resin. As a result, it is possible to suppress detachment of the pigment dispersion resin from the pigment. This also makes it easier to redissolve the aqueous inkjet ink that has dried near the nozzle.

[0061] Methods for forming a crosslinked structure (crosslinking treatment methods) include a method in which a crosslinked structure is formed by reacting a crosslinking agent or the like with reactive sites (reactive functional groups such as carboxyl groups and carboxylate groups) of an uncrosslinked dispersion resin that has been adsorbed onto the pigment surface in advance; a method in which an uncrosslinked dispersion resin having self-crosslinking groups is adsorbed onto the pigment surface in advance, and then the self-crosslinking groups are reacted; etc. Among these, the method of reacting with a crosslinking agent or the like is preferably used because there is a wide range of uncrosslinked dispersion resins that can be used, and the weight-average molecular weight and acid value before the crosslinking treatment described above are easily adjustable, and the crosslinking rate can be easily controlled with high precision.

[0062] <Crosslinking Agent> Examples of the crosslinking agent include isocyanate compounds, aziridine compounds, carbodiimide compounds, oxetane compounds, oxazoline compounds, and epoxy compounds. Examples of functional groups (reactive functional groups) that react with the reactive sites of the uncrosslinked dispersed resin include isocyanate groups, aziridine groups, carbodiimide groups, oxetane groups, oxazoline groups, and epoxy groups. Among these, one or more functional groups selected from the group consisting of aziridine groups, carbodiimide groups, and epoxy groups are preferred, and at least an epoxy group is more preferred.

[0063] When a compound having an epoxy group is used as the crosslinking agent, it is preferable to use a compound having multiple epoxy groups in one molecule, it is more preferable to use a compound having two or more glycidyl ether groups in one molecule, and it is even more preferable to use a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having 3 to 8 carbon atoms. Furthermore, the epoxy equivalent of the compound having multiple epoxy groups in one molecule is preferably 90 to 300 g / eq., more preferably 100 to 200 g / eq., from the viewpoint of being able to more efficiently undergo a crosslinking reaction with reactive sites in the pigment dispersion resin in a liquid medium mainly composed of water.

[0064] Specific examples of the compound having a plurality of epoxy groups in one molecule include cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, 4,4'-diglycidyloxybiphenol, bisphenol A diglycidyl ether, hydrogenated bisphenol A diglycidyl ether, phthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, and hydrogenated phthalic acid diglycidyl ester.

[0065] The crosslinking agent may be water-soluble or water-insoluble. However, from the viewpoint of being able to more efficiently crosslink with the reactive sites of the pigment dispersing resin in a medium containing water (aqueous medium), the solubility of the crosslinking agent in 100 g of water at 25°C is preferably 0.1 to 50 g / 100 gH. ２ 0 is preferable, and 0.2 to 40 g / 100 gH ２ It is more preferable that the concentration is 0.5 to 30 g / 100 gH. ２ It is O.

[0066] From the viewpoint of reducing detachment of the pigment dispersing resin from the pigment and improving resolubility and ejection stability, and from the viewpoint of suppressing inhibition of the function of the surfactant by free pigment dispersing resin and improving the wettability of the aqueous inkjet ink, it is preferable that the crosslinking agent be added so that the functional group content (mol %) represented by the following formula 4 is 50 to 150 mol %. The functional group content is more preferably 70 to 120 mol %, and particularly preferably 80 to 100 mol %.

[0067] Formula 4:

[0068] For example, when a crosslinking agent EW (g) having a plurality of epoxy groups in one molecule and an epoxy equivalent EE (g / eq.) is mixed with a pigment dispersion resin PW (g) before crosslinking having an acid value AV (mgKOH / g), the functional group content is expressed by the following formula 4-2.

[0069] Equation 4-2: Functional group content (mol %)=100×(EW / EE) / {PW×AV / (56.1×1000)}

[0070] In addition, "56.1" in the above formula 4-2 is the molecular weight of potassium hydroxide.

[0071] <Method for Producing Pigment Particles Containing a Pigment Dispersion Resin with a Crosslinked Structure> An example of a method for producing pigment particles containing a pigment dispersion resin with a crosslinked structure (crosslinked pigment particles) is a method in which the following dispersion step and crosslinking step are performed in this order. Optionally, the following neutralization step may be performed before the dispersion step. First, carboxyl groups present in the pigment dispersion resin are mixed with a basic compound in an aqueous medium to neutralize at least a portion of the carboxyl groups (neutralization step). As described above, the neutralization converts the carboxyl groups present in the pigment dispersion resin into carboxylate groups. This pigment dispersion resin containing at least carboxylate groups is used in subsequent steps. The pigment dispersion resin obtained after the neutralization step is in the form of an aqueous solution. Next, a pigment is added to the aqueous solution of the pigment dispersion resin, and the two are mixed, followed by further dispersion treatment (dispersion step). This dispersion step produces an aqueous dispersion of pigment particles (uncrosslinked pigment particles) with the pigment dispersion resin chemically adsorbed to at least a portion of their surfaces. The pigment dispersion resin contained in the uncrosslinked pigment particles is an uncrosslinked dispersion resin. A crosslinking agent is then added to the aqueous dispersion of the uncrosslinked pigment particles to perform a crosslinking treatment (crosslinking treatment step). The crosslinking treatment step allows the production of an aqueous dispersion of crosslinked pigment particles.

[0072] <Neutralization Treatment Step> As described above, the pigment dispersing resin used in the dispersion treatment step preferably has an anionized functional group such as a carboxylate group. When using a pigment dispersing resin that does not have an anionized functional group (for example, a resin that has only carboxyl groups as acid groups), it is preferable to neutralize at least a portion of the functional groups to convert them to anionized functional groups by, for example, adding a basic compound.

[0073] <<Dispersion Treatment Step>> The disperser used in the dispersion treatment may be any commonly used disperser, and examples thereof include a media-type wet disperser, a media-less wet disperser, and a kneader. Examples of media-type wet dispersers include a ball mill, a roll mill, a sand mill, and a bead mill. Examples of media-less wet dispersers include a high-pressure homogenizer, and examples of kneaders include a kneader. Among these, it is preferable to select a bead mill from the viewpoint of crushing and refining coarse pigment particles. Examples of bead mills include a super mill, a sand grinder, an agitator mill, a grain mill, a dyno mill, a pearl mill, and a cobol mill (all trade names).

[0074] Using the dispersers listed above, the pigment can be pulverized to a desired particle size by the collision force with the media or shear stress. From the viewpoint of obtaining a pigment dispersion with a uniform particle size, it is preferable to pre-disperse (premix) the pigment and the pigment dispersing resin, and then further disperse (main dispersion) using the dispersers listed above. A commonly used mixing and stirring device such as a Disper can be used as the pre-disperser used for pre-mixing.

[0075] <Crosslinking Treatment Step> As described above, by subjecting the pigment dispersing resin adsorbed to the pigment to a crosslinking treatment, pigment particles in which the pigment dispersing resin is crosslinked can be obtained. Furthermore, a method of forming a crosslinked structure using a crosslinking agent can be suitably used as the crosslinking treatment method. Specifically, a method of maintaining a mixture containing uncrosslinked pigment particles, a crosslinking agent, and water under heating while stirring can be used.

[0076] The temperature during the crosslinking treatment is preferably 50° C. to 95° C., more preferably 70° C. to 85° C., from the viewpoint of efficiently promoting the crosslinking reaction. The time for the crosslinking treatment is preferably 0.5 to 10 hours, more preferably 1 to 8 hours, and even more preferably 2 to 5 hours, from the same viewpoint as above.

[0077] From the viewpoint of enabling stable ejection from a nozzle, the average particle size of the crosslinked pigment particles is preferably 60 to 200 nm, more preferably 70 to 175 nm, and particularly preferably 80 to 150 nm.

[0078] The average particle size is a volume-based median diameter that can be measured by dynamic light scattering, for example, using a Nanotrac UPA-EX150 manufactured by Microtrac Bell.

[0079] <Pigment Dispersion> The crosslinked pigment particles may be dispersed in an aqueous medium, i.e., may be in the form of an aqueous dispersion of crosslinked pigment particles. Because this improves ejection stability, the aqueous inkjet ink of this embodiment is preferably produced by mixing a previously produced aqueous dispersion of crosslinked pigment particles with other raw materials described below.

[0080] In the present disclosure, a composition containing pigment particles and an aqueous medium is referred to as a "pigment dispersion." Therefore, the above-described aqueous dispersion of uncrosslinked pigment particles and the above-described aqueous dispersion of crosslinked pigment particles are both included in the term "pigment dispersion." As described below, a pigment dispersion is used as a raw material for aqueous inkjet ink and is different from the aqueous inkjet ink. Specifically, a pigment dispersion and an aqueous inkjet ink differ in their pigment content. For example, the amount of pigment contained in a pigment dispersion is preferably 10 to 60% by mass, more preferably 15 to 55% by mass, and even more preferably 20 to 50% by mass, based on the total amount of the pigment dispersion. Meanwhile, the preferred amount of pigment contained in an aqueous inkjet ink is as described below. Furthermore, when the amount of pigment contained in the pigment dispersion (% by mass) is defined as PP and the amount of pigment contained in the aqueous inkjet ink (% by mass) is defined as PI, the value expressed as PP / PI is preferably 1.5 to 10, and more preferably 2 to 8.

[0081] In the present disclosure, the aqueous dispersion of uncrosslinked pigment particles is particularly referred to as an "uncrosslinked pigment dispersion," and the aqueous dispersion of crosslinked pigment particles is particularly referred to as a "crosslinked pigment dispersion."

[0082] The pigment dispersion may be produced by previously producing the pigment particles in a dry state (solid) and then mixing them with an aqueous medium, or may be produced by the above-mentioned method for producing pigment particles containing a pigment dispersing resin having a crosslinked structure.

[0083] The aqueous medium contained in the pigment dispersion contains at least water. It may also contain an organic solvent (described in detail below) that can be used as a raw material for aqueous inkjet inks. For example, the pigment dispersion may contain ethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-octanediol, or the like.

[0084] The pH of the pigment dispersion is preferably 8 to 12. If the pH is 8 or higher, the anionized functional groups present in the pigment dispersion resin can be stably present, and dispersion stability due to charge repulsion can be improved. From this viewpoint, the pH is more preferably 9 to 11. The pH of the pigment dispersion can be measured in the same manner as for the aqueous solution of the pigment dispersion resin described above.

[0085] <Surfactant> The aqueous inkjet ink of this embodiment contains, as surfactants, an acetylene diol surfactant (A-1) having a measured HLB value of 6 to 9 and a siloxane surfactant (A-2) having a measured HLB value of 8 to 14.

[0086] In this disclosure, the "HLB (Hydrophilic-Lipophilic Balance) value" is one of the parameters that represent the degree of hydrophilicity and hydrophobicity of a material. The smaller the HLB value, the more hydrophobic the material, and the larger the HLB value, the more hydrophilic the material. HLB values ​​can be calculated by calculation from molecular structure or by experimental measurement. In this disclosure, the HLB value is calculated by the following method (measured HLB value). (1) Dissolve 0.5 g of the target surfactant in 5 mL of ethanol. (2) At 25°C, titrate the mixture from (1) with a 2% aqueous phenol solution while stirring. The endpoint is the point at which the mixture becomes cloudy and no longer returns to a transparent state when the 2% aqueous phenol solution is added dropwise. (3) When the amount of 2% aqueous phenol solution added up to the endpoint is A [mL], calculate the measured HLB value according to the following formula 5.

[0087] Equation 5: Measured HLB value = 0.89 x A + 1.11

[0088] <Acetylene Diol-Based Surfactant (A-1)> The aqueous inkjet ink of this embodiment contains an acetylene diol-based surfactant (A-1). The acetylene diol-based surfactant (A-1) preferably has a measured HLB value of 6 to 9, because it has low affinity for water and quickly orients at the gas-liquid interface, thereby providing excellent wettability to the printing substrate.

[0089] Examples of the acetylenic diol surfactant (A-1) that can be used include 2,4,7,9-tetramethyl-5-decyne-4,7-diol, an ethylene oxide adduct of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (wherein the number of moles of ethylene oxide added is 1.5 moles or less), and 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol. Examples of commercially available products that can be used as the acetylenic diol surfactant (A-1) include Surfynol DF-110D, Surfynol 104E, 104H, 104A, 104BC, 104DPM, 104PA, 104PG-50, and 420 (manufactured by Evonik Japan Co., Ltd.), and Acetylenol E00 and E13T (manufactured by Kawaken Fine Chemicals Co., Ltd.). These compounds may be used alone or in combination of two or more.

[0090] From the viewpoint of ensuring excellent print quality, the content of the acetylene diol surfactant (A-1) in the aqueous inkjet ink is preferably 0.1 to 5% by mass, more preferably 0.3 to 3% by mass, and particularly preferably 0.5 to 2% by mass.

[0091] <<Siloxane-based surfactant (A-2)>> The aqueous inkjet ink of this embodiment contains a siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14. The siloxane-based surfactant (A-2) can orient the acetylene diol-based surfactant (A-1) so as to compensate for the non-uniform orientation thereof, and is therefore effective in improving dot circularity and reducing unevenness. From this perspective, the difference between the measured HLB value of the acetylene diol-based surfactant (A-1) and the measured HLB value of the siloxane-based surfactant (A-2) is preferably 2 to 7.

[0092] The siloxane surfactant (A-2) that can be used includes gemini siloxane surfactants, siloxane surfactants modified at both ends with polyether, side-chain polyether-modified siloxane surfactants, etc. Among these, although the details are unknown, it is preferable that the siloxane surfactant (A-2) contains a gemini siloxane surfactant and / or a siloxane surfactant modified at both ends with polyether, because a small amount of addition can reduce unevenness and produce printed matter with good dot circularity, and further the ejection stability of the aqueous inkjet ink is improved.

[0093]

[0033] Furthermore, from the viewpoint of further enhancing the effects of dot circularity, wettability, and reduced unevenness, thereby obtaining printed matter with exceptionally excellent print quality, and from the viewpoint of maintaining and improving ejection stability, the aqueous inkjet ink of this embodiment preferably contains two or more types of siloxane-based surfactants (A-2), more preferably one or more of which are gemini siloxane-based surfactants and / or siloxane-based surfactants modified at both ends with polyether (excluding gemini siloxane-based surfactants). Furthermore, from the viewpoint of obtaining an aqueous inkjet ink that is excellent in wettability, resolubility, and ejection stability on a printing substrate and that can obtain printed matter with good dot circularity, it is particularly preferable that the siloxane-based surfactant (A-2) contains one or more gemini siloxane-based surfactants and one or more siloxane-based surfactants modified at both ends with polyether (excluding gemini siloxane-based surfactants).

[0094] <Gemini siloxane surfactant> Generally, a gemini surfactant has a structure in which surfactants having a hydrophilic structure and a hydrophobic structure are linked by a linking group (spacer) or a covalent bond. In addition, in the case of a gemini siloxane surfactant, for example, a siloxane chain (—[O—SiR １ R ２ ] ｘ -, where R １ and R ２are each any organic group, and x is an integer of 2 or more.), and the hydrophilic structure (for example, a polyether chain) has the following structure: A structure in which the bonding points between the siloxane chain and the hydrophilic structure are located in the middle of the siloxane chain and in the middle of the hydrophilic structure, respectively. A structure in which multiple siloxane chains are bonded via linking groups or the like (for example, R in the structural formula of the siloxane chain １ and / or R ２ a structure in which at least a part of the above is an organic chain containing a siloxane chain; a structure in which a plurality of siloxane surfactants, each having a plurality of hydrophilic structures, share a part of the hydrophilic structures.

[0095] Gemini surfactants have superior surface tension reducing ability compared to general surfactants. Therefore, by using a gemini siloxane surfactant, it is possible to achieve a surface tension reduction superior to that achieved by a non-gemini siloxane surfactant. As a result, it is possible to significantly improve the wettability of aqueous inkjet inks containing a gemini siloxane surfactant, thereby enabling the above-mentioned dot roundness and unevenness to be improved.

[0096] Examples of commercially available gemini siloxane surfactants with a measured HLB value of 8 to 14 include, but are not limited to, TEGOTwin 4100 and TEGOTwin 4200 manufactured by Evonik Japan Co., Ltd.

[0097] <<Dual-Terminated Polyether-Modified Siloxane Surfactant (excluding Gemini Siloxane Surfactants)>> Dual-terminated polyether-modified siloxane surfactants have a structure in which polyether chains are bonded to both ends of a polysiloxane skeleton. Aqueous inkjet inks containing dual-terminated polyether-modified siloxane surfactants improve print quality by uniformly wetting and spreading on the printing substrate, and also have good ejection stability. Compounds represented by the following general formula 6 are preferably used as dual-terminated polyether-modified siloxane surfactants.

[0098] General formula 6:

[0099] In General Formula 6, l represents an integer of 1 to 100, R ３ represents a structure represented by the following general formula 7.

[0100] General formula 7:

[0101] In General Formula 7, p represents an integer of 1 to 6, q represents an integer of 1 to 100, and r represents an integer of 0 to 80. However, p+r is an integer of 1 or more. ４ represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a methacryloyl group, or an acryloyl group. However, the ethylene oxide group (OC ２ H ４ ) and propylene oxide group (OC ３ H ６ The addition pattern of the aryl groups may be block or random.

[0102] Commercially available compounds represented by the above general formula 6 and having a measured HLB value of 8 to 14 include BYK-333, BYK-UV3500, and BYK-3420 manufactured by BYK-Chemie, and TEGOGlide 440 and TEGOGlide 450 manufactured by Evonik Japan, but are not limited to these.

[0103] <Side-Chain Polyether-Modified Siloxane-Based Surfactant (excluding Gemini Siloxane-Based Surfactants)> As the siloxane-based surfactant (A-2), a side-chain polyether-modified siloxane-based surfactant (excluding Gemini siloxane-based surfactants) can be used. Even if the side-chain polyether-modified siloxane-based surfactant has a relatively low molecular weight, it exhibits high orientation in an aqueous medium and can achieve excellent wettability. As the side-chain polyether-modified siloxane-based surfactant, it is preferable to use a surfactant represented by the following general formula 8:

[0104] General formula 8:

[0105] In General Formula 8, m is an integer of 0 or more, n is an integer of 1 or more, and m+n is an integer of 1 to 100. ５ is a structure represented by the above general formula 7, and R ６ represents an alkyl group having 1 to 6 carbon atoms.

[0106] Examples of commercially available products of the compound represented by the above general formula 8 and having a measured HLB value of 8 to 14 include BYK-347, BYK-348, BYK-349, and BYK-3451 manufactured by BYK-Chemie, and TEGOWet240, TEGOWet250, and TEGOWet260 manufactured by Evonik Japan Co., Ltd., but are not limited to these.

[0107] From the viewpoint of ensuring excellent print quality, the content of the siloxane surfactant (A-2) in the aqueous inkjet ink is preferably 0.1 to 5% by mass, more preferably 0.3 to 3.5% by mass, and particularly preferably 0.5 to 2.5% by mass.

[0108] Furthermore, from the viewpoint of obtaining printed matter with excellent dot circularity and no white voids, when the mass content of the acetylene diol surfactant (A-1) contained in the aqueous inkjet ink (the total mass content when two or more types of acetylene diol surfactant (A-1) are contained) is taken as 1, the mass content of the siloxane surfactant (A-2) (the total mass content when two or more types of siloxane surfactant (A-2) are contained) is preferably 0.3 to 3, and particularly preferably 0.5 to 2.

[0109] <Organic Solvent> <Specific Organic Solvent> The aqueous inkjet ink of this embodiment contains an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) and / or a glycol monoether represented by the above general formula 1. In the present disclosure, these organic solvents are collectively referred to as "specific organic solvents." Because all specific organic solvents have low surface tension, the acetylene diol surfactant (A-1) and the siloxane surfactant (A-2), which are compatible with the specific organic solvent, diffuse as the specific organic solvent wets and spreads, and are easily oriented uniformly at the interface. As a result, pinholes are easily suppressed in printed materials. Furthermore, the meniscus of the aqueous inkjet ink in the inkjet head is stabilized, thereby improving ejection stability.

[0110] Examples of alkanediols having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) include, but are not limited to, 1,3-pentanediol, 1,4-pentanediol, 2,3-pentanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,3-hexanediol, 1,4-hexanediol, 2,3-hexanediol, 3,4-hexanediol, 1,5-hexanediol, 2,2-dimethyl-1,3-pentanediol, 2-methyl-1,3-pentanediol, 3-methyl-1,3-butanediol, 2-methyl-2,4-pentanediol (hexylene glycol), 2-ethyl-1,3-hexanediol, and 1,3-octanediol. Among these, it is particularly preferable to use an alkanediol having a particularly low surface tension, i.e., a surface tension of 20 to 30 mN / m at 25°C, from the viewpoints that the above-mentioned mechanism functions effectively, the surfactant in the aqueous inkjet ink can be uniformly oriented, and the ejection stability is also improved. On the other hand, although the detailed mechanism is unknown, compounds having a quaternary carbon atom (a carbon atom bonded to four atoms other than hydrogen atoms) can be preferably used from the viewpoint of favorably compatibilizing the above-mentioned surfactant and significantly improving the ejection stability. Of the compounds listed above, 3-methyl-1,3-butanediol and 2-methyl-2,4-pentanediol (hexylene glycol) can be mentioned as compounds having a surface tension of 20 to 30 mN / m at 25°C and having a quaternary carbon atom.

[0111] On the other hand, examples of glycol monoethers represented by general formula 1 include, but are not limited to, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, and dipropylene glycol monobutyl ether. Among these, it is preferable to use a compound in which n in general formula 1 is 2 as the glycol monoether represented by general formula 1, since this can favorably make the acetylene diol surfactant (A-1) and the siloxane surfactant (A-2) compatible, thereby improving the ejection stability and resolubility. Specific examples of compounds in which n is 2 in general formula 1 include dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, and dipropylene glycol monobutyl ether. Of these compounds, it is particularly preferable for the aqueous inkjet ink to contain dipropylene glycol monopropyl ether, as this particularly favorably exhibits the effects described above.

[0112] The specific organic solvents may be used alone or in combination of two or more. The total content of the specific organic solvents is preferably 0.5 to 25% by mass, more preferably 1 to 20% by mass, and particularly preferably 2 to 15% by mass, of the total amount of the aqueous inkjet ink.

[0113] The inkjet ink of this embodiment can also suitably contain organic solvents other than the specific organic solvent. Examples of organic solvents other than the above include, but are not limited to, ethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,2-octanediol, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, γ-butyrolactone, and 2-pyrrolidone. These organic solvents may be used alone or in combination of two or more.

[0114] <Binder Resin> The aqueous inkjet ink of this embodiment preferably contains a resin used as a binder (also referred to as a "binder resin" in the present disclosure) in order to improve drying properties and impart abrasion resistance.

[0115] As described above, water-soluble resins and resin microparticles are generally known as the forms of resins used in aqueous inkjet inks. The binder resin contained in the aqueous inkjet ink of this embodiment may be a water-soluble resin or resin microparticles. Furthermore, a combination of a water-soluble resin and resin microparticles may be used.

[0116] In this disclosure, 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." Furthermore, among the water-insoluble resins, a resin that is dispersed in water in the form of particles and has a volume-based median diameter (also referred to as "D50" in this disclosure) of 10 to 1,000 nm is referred to as a "resin particle."

[0117] The D50 of the resin fine particles can be measured using the same device and method as in the case of measuring the average particle diameter of the crosslinked pigment particles described above.

[0118] Examples of types of binder resins that can be used in the aqueous inkjet ink of this embodiment include acrylic, styrene, maleic acid, urethane, polyester, vinyl chloride, vinyl chloride-vinyl acetate, polyolefin, vinyl alcohol, etc. These resins may be used alone or in combination of two or more.

[0119] When the aqueous inkjet ink of this embodiment contains a binder resin, the type of resin used as the binder resin is preferably one or more types selected from the group consisting of acrylic, urethane, and polyester resins among those listed above.

[0120] The content of the binder resin in the aqueous inkjet ink of this embodiment is preferably 0.1 to 20 mass %, more preferably 1 to 15 mass %, and particularly preferably 2 to 10 mass %, of the total amount of the aqueous inkjet ink.

[0121] <Other Components> The aqueous inkjet ink of this embodiment may contain, in addition to the components described above, a pH adjuster and other additives. Examples of the other additives include a crosslinking agent, a preservative, an ultraviolet absorber, and an infrared absorber. For each of these components, one or more conventionally known compounds may be used.

[0122] <Water> The water contained in the aqueous inkjet ink of this embodiment is preferably ion-exchanged water and / or reverse osmosis water. The content of water contained in the aqueous inkjet ink is preferably 30 to 90 mass % of the total amount of the aqueous inkjet ink.

[0123] <Method for Producing Aqueous Inkjet Ink> The aqueous inkjet ink of this embodiment can be produced by a conventionally known method. One example is a method in which water, an organic solvent, an acetylene-based surfactant (A-1), a siloxane-based surfactant (A-2), and the like are added to an aqueous dispersion of crosslinked pigment particles produced by the method described above, and the mixture is thoroughly stirred and mixed, and then coarse particles are removed by techniques such as filtration and centrifugation. However, the method for producing the aqueous inkjet ink of this embodiment is not limited to the method described above.

[0124] <Characteristics of Aqueous Inkjet Ink> The aqueous inkjet ink of this embodiment preferably has a viscosity at 25°C of 3 to 15 mPa·s. Within this viscosity range, droplets of the aqueous inkjet ink can be stably ejected not only from inkjet heads with ejection frequencies of approximately 4 to 10 kHz, but also from inkjet heads with high ejection frequencies of approximately 20 to 70 kHz. In particular, when the viscosity of the aqueous inkjet ink of this embodiment at 25°C is 4 to 10 mPa·s, the aqueous inkjet ink can be stably ejected even when an inkjet head with a design resolution of 600 dpi or higher is used. In this disclosure, the viscosity is measured at 25°C using a cone-plate rotational viscometer (E-type viscometer, cone angle 1°34') such as the TVE25L viscometer manufactured by Toki Sangyo Co., Ltd.

[0125] Furthermore, in order to obtain an aqueous inkjet ink that is 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 20 to 30 mN / m, at 25° C. In the present disclosure, 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.

[0126] <Aqueous Inkjet Ink Set> Although the aqueous inkjet ink of this embodiment may be used alone, two or more aqueous inkjet inks may be combined to form an aqueous inkjet ink set. Examples of the aqueous inkjet ink set include a four-color aqueous inkjet ink set (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).

[0127] <Inkjet Recording Method> The aqueous inkjet ink of this embodiment is used in an inkjet printing method. That is, the aqueous inkjet ink of this embodiment is ejected onto a printing substrate from an inkjet head having fine nozzles (ejecting step). In addition, the aqueous inkjet ink ejected onto the printing substrate is preferably dried by a drying mechanism (drying step).

[0128] <<Discharge Process>> The inkjet head operation methods in the discharge process include 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 while discharging and recording the aqueous inkjet ink, and a single-pass method in which the inkjet ink is discharged and recording is performed as the printing substrate passes below a fixedly disposed inkjet head. The inkjet head equipped with the aqueous inkjet ink of this embodiment may be either a shuttle method or a single-pass method. 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 and improves the print quality of the printed matter.

[0129] The method of ejection from the inkjet head can also 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 aqueous inkjet ink by bubbles generated by heating a heater, and a valve method that ejects pressurized aqueous inkjet ink by opening and closing a nozzle cover (valve) with a solenoid.

[0130] The volume 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. Furthermore, 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 higher, and it is particularly preferable to adjust the printing conditions so that the resolution is 1200 dpi or higher.

[0131] <Drying Step> Examples of drying methods employed in the drying mechanism used in the drying step include heat drying, hot air drying, infrared drying (for example, infrared with a wavelength of 700 to 2500 nm), microwave drying, and drum drying. One or more of these methods can be selected and used as desired in the drying step. Furthermore, when two or more of the above drying methods are used, they may be used separately (for example, consecutively) or simultaneously. For example, by using heat drying and hot air drying in combination, the aqueous inkjet ink can be dried more quickly than when each method is used alone.

[0132] <<Printing Substrate>> The printing substrate on which the aqueous inkjet ink of this embodiment is printed can be suitably used for permeable substrates and poorly permeable substrates. In particular, from the viewpoint of obtaining a printed matter with excellent print quality, it is preferable to use a paper substrate as the printing substrate.

[0133] Examples of permeable substrates include uncoated paper such as sawdust paper, medium-grade paper, fine paper, and recycled paper; fabrics such as cotton, synthetic fiber fabrics, silk, hemp, and nonwoven fabrics; leather, etc. Among these, uncoated paper such as sawdust paper, medium-grade paper, fine paper, and recycled paper is preferably used because it allows prints with excellent print quality to be obtained.

[0134] Examples of the impermeable substrate include coated paper such as coated paper, art paper, cast paper, etc. Among these, coated paper is preferably used because it can produce printed matter with excellent print quality.

[0135] The printing substrates listed above may have a smooth or uneven surface. The printing substrates may be in the form of rolls or sheets. Two or more of the printing substrates listed above may be bonded together to form the printing substrate. A release adhesive layer or the like may be provided on the side opposite the printed surface, or an adhesive layer or the like may be provided on the printed surface after printing.

[0136]

[0063] It is also preferable to subject the printing surface of the printing substrates listed above to surface modification such as corona treatment and plasma treatment, in order to improve the wettability of the aqueous inkjet ink of this embodiment and obtain printed matter with excellent print quality and drying properties.

[0137] That is, the present invention relates to aqueous inkjet inks as shown in [1] to [5] below, and to printed matter produced using the aqueous inkjet ink as shown in [6] below. [1] An aqueous inkjet ink comprising pigment particles, a surfactant, and an organic solvent, wherein the pigment particles comprise a pigment and a pigment dispersing resin, the pigment dispersing resin has a crosslinked structure, the surfactant comprises an acetylene diol surfactant (A-1) having a measured HLB value of 6 to 9, and a siloxane surfactant (A-2) having a measured HLB value of 8 to 14, and the organic solvent comprises an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediols having 5 to 8 carbon atoms) and / or a glycol monoether represented by the following general formula 1. General formula 1: R １ -(O-CH(CH ３ )-CH ２ ) ｎ -OH (in general formula 1, R １ is an alkyl group having 2 to 4 carbon atoms, and n is 1 or 2. [2] The aqueous inkjet ink according to [1], wherein the siloxane surfactant (A-2) having a measured HLB value of 8 to 14 includes a gemini siloxane surfactant. [3] The aqueous inkjet ink according to [1] or [2], wherein the siloxane surfactant (A-2) having a measured HLB value of 8 to 14 includes a siloxane surfactant modified at both ends with polyether (excluding gemini siloxane surfactants). [4] The aqueous inkjet ink according to [2] or [3], wherein the siloxane surfactant (A-2) having a measured HLB value of 8 to 14 includes two or more types of siloxane surfactants (A-2). [5] The aqueous inkjet ink according to [4], wherein the siloxane surfactant (A-2) having a measured HLB value of 8 to 14 comprises a gemini siloxane surfactant and a siloxane surfactant modified at both ends with a polyether (excluding gemini siloxane surfactants). [6] A printed matter obtained by printing with the aqueous inkjet ink according to any one of [1] to [5].

[0138] The present invention is related to the subject matter described in Japanese Patent Application No. 2023-209882, filed December 13, 2023, the disclosure of which is incorporated herein by reference.

[0139] The aqueous inkjet ink of this embodiment will be described in more detail below with reference to examples and comparative examples. In the following description, "parts" and "%" mean "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0140] <Production Example of Pigment Dispersion Resin 1> A reaction vessel equipped with a gas inlet tube, thermometer, condenser, and stirrer was charged with 93.4 parts of methyl ethyl ketone and purged with nitrogen gas. After the temperature inside the reaction vessel was raised to 110°C, a mixture of 25 parts of styrene as polymerizable monomers, 25 parts of acrylic acid, 20 parts of methyl methacrylate, 30 parts of lauryl methacrylate, and 6 parts of V-601 (manufactured by Wako Pure Chemical Industries, Ltd.) as a polymerization initiator was added dropwise to the reaction vessel over 2 hours. After completion of the dropwise addition, the reaction was continued for 3 hours while maintaining the temperature inside the reaction vessel at 110°C, and then 0.6 parts of V-601 (manufactured by Wako Pure Chemical Industries, Ltd.) was added, and the reaction was continued for another 1 hour at 110°C, thereby synthesizing pigment dispersion resin 1. The weight average molecular weight of the resulting pigment dispersion resin 1, measured by the method described above, was 29,000, and the acid value was 196 mgKOH / g. The amount of potassium hydroxide required to achieve a neutralization rate of 100% was calculated from the acid value of the pigment dispersion resin 1, and a 48% by mass aqueous potassium hydroxide solution containing an amount of potassium hydroxide equal to the calculated amount was added. Ion-exchanged water was then added to the mixture to achieve a solids concentration of 20%, and the mixture was stirred at 50°C for 1 hour to obtain an aqueous solution of pigment dispersion resin 1 (solids concentration 20%).

[0141] <Production Example of Pigment Dispersion Resins 2 to 10> Aqueous solutions of pigment dispersion resins 2 to 10 (all with a solids concentration of 20%) were obtained using the same raw materials and procedures as for pigment dispersion resin 1, except that the monomers shown in Table 1 were used as the polymerizable monomers. Pigment dispersion resins 1 to 10 are non-crosslinked dispersion resins.

[0142]

[0143] The abbreviations listed in Table 1 are as follows. Table 1 also lists the weight average molecular weights and acid values ​​of pigment dispersion resins 1 to 10. St: styrene AA: acrylic acid MMA: methyl methacrylate LMA: lauryl methacrylate

[0144] <Preparation Example of Aqueous Dispersion 1 of Uncrosslinked Pigment Particles> 20 parts of LIONOLBLUE FG-7351 pigment (C.I. Pigment Blue 15:3 manufactured by Toyocolor Co., Ltd.), 25 parts of an aqueous solution of pigment dispersion resin 1 (solids concentration 20%), and 55 parts of ion-exchanged water were mixed and pre-dispersed using a disper. After pre-dispersion, a main dispersion was carried out using a 0.6 L Dyno-Mill filled with 1,800 g of zirconia beads with a diameter of 0.5 mm. After the main dispersion, 33.3 parts of ion-exchanged water was added to the resulting pigment dispersion. Further, while heating at 60°C, a portion of the water and the methyl ethyl ketone contained in the aqueous solution of pigment dispersion resin 1 were distilled off under reduced pressure, and then ion-exchanged water was added to adjust the pigment concentration to 15%, thereby obtaining an aqueous dispersion 1 of cyan uncrosslinked pigment particles (pigment concentration 15%).

[0145] <Production Examples of Aqueous Dispersions 2 to 10 of Uncrosslinked Pigment Particles> Aqueous dispersions 2 to 10 of cyan uncrosslinked pigment particles were obtained using the same raw materials and method as for the aqueous dispersion 1 of uncrosslinked pigment particles, except that the aqueous solutions of pigment dispersion resins 2 to 10 were used instead of the aqueous solution of pigment dispersion resin 1. The pigment concentrations were all 15%.

[0146] <Production Example of Aqueous Dispersion 1 of Crosslinked Pigment Particles> 93.3 parts of the aqueous dispersion 1 of uncrosslinked pigment particles obtained above, 1.5 parts (an amount such that the content of the functional group represented by the above formula 4 is 90 mol%) of the crosslinking agent Denacol EX-321 (an epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent weight 140 g / eq.), and 5.2 parts of ion-exchanged water were mixed, and the mixture was heated to 80°C with stirring, and then maintained at 80°C for 3 hours to carry out a crosslinking treatment. Subsequently, ion-exchanged water was added to adjust the pigment concentration to 14%, thereby obtaining an aqueous dispersion 1 of cyan crosslinked pigment particles (pigment concentration 14%).

[0147] <Production Examples of Aqueous Dispersions 2 to 14 of Crosslinked Pigment Particles> Aqueous dispersions 2 to 14 of cyan crosslinked pigment particles were obtained in the same manner as for aqueous dispersion 1 of crosslinked pigment particles, except that the type of aqueous dispersion of uncrosslinked pigment particles used and the amount of each raw material added were changed as shown in Table 2. The pigment concentration was 14% in all cases.

[0148]

[0149] <Production Example of Binder Resin 1> Binder Resin 1, an A-B block polymer, was produced using the method described in Example 21 of WO 2008 / 139980. Specifically, in the polymerization of the first block, methacrylic acid was used as the polymerizable monomer, and the reaction was carried out at 80°C for 2 hours, followed by reprecipitation, to obtain a first block copolymer to which iodine had been added. The weight-average molecular weight (Mw) of the first block copolymer was 6,000. Next, the first block copolymer and styrene, methyl methacrylate, and lauryl methacrylate were used as polymerizable monomers in a mass ratio of 15:65:15, and the reaction was carried out at 80°C for 2.5 hours, followed by reprecipitation, to obtain Binder Resin 1 in which the iodine-added site of the first block was substituted with a second block composed of styrene, methyl methacrylate, and lauryl methacrylate. The weight-average molecular weight (Mw) of Binder Resin 1 was 18,000, and the acid value was 40. The binder resin 1 obtained above was mixed with 1.5 times (by mass) the amount of ion-exchanged water and stirred to completely dissolve it, thereby obtaining an aqueous solution (solid content concentration: 40%) of binder resin 1, which is an A-B block polymer.

[0150] <Production of aqueous inkjet ink> The raw materials listed in Table 3 were mixed while stirring using a disper until the mixture was sufficiently uniform, and then filtered through a 0.8 μm membrane filter to remove coarse particles that may cause head clogging, thereby producing an aqueous inkjet ink.

[0151]

[0152]

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159]

[0160] The abbreviations listed in Table 3 are as follows: Surfynol DF110D (acetylene diol surfactant manufactured by Evonik Japan, measured HLB value = 6.4) Surfynol 104 (acetylene diol surfactant manufactured by Evonik Japan, measured HLB value = 7.9) Surfynol 420 (acetylene diol surfactant manufactured by Evonik Japan, measured HLB value = 8.3) Surfynol 440 (acetylene diol surfactant manufactured by Evonik Japan, measured HLB value = 10.5) TEGOTwin 4100 (gemini type siloxane surfactant manufactured by Evonik Japan, measured HLB value = 8.1) TEGOTwin 4200 (gemini type siloxane surfactant manufactured by Evonik Japan, measured HLB value = 8.2) TEGOTwin4000 (gemini type siloxane surfactant manufactured by Evonik Japan, measured HLB value = 2.0) TEGOGlide440 (polyether-modified siloxane surfactant manufactured by Evonik Japan, measured HLB value = 12.1) BYK-3420 (polyether-modified siloxane surfactant manufactured by BYK Japan, measured HLB value = 13.8) TEGOGlide100 (polyether-modified siloxane surfactant manufactured by Evonik Japan, measured HLB value = 7.1) BYK-348 (side-chain polyether-modified siloxane surfactant manufactured by BYK Japan, measured HLB value = 12.2) BYK-349 (BYK Japan side chain polyether modified siloxane surfactant, measured HLB value = 10.2) Propylene glycol (surface tension: 36.5 mN / m) 1,5-pentanediol (surface tension: 42.2 mN / m) 1,3-octanediol (surface tension: 35.0 mN / m) 3-methyl-1,5-pentanediol (surface tension: 38.1 mN / m) 3-methyl-1,3-butanediol (surface tension: 29.9 mN / m) Hexylene glycol (2-methyl-2,4-pentanediol, surface tension: 29.1 mN / m) 1,2-butanediol (surface tension: 31.6 mN / m) 1,2-Hexanediol (surface tension: 26.5 mN / m), dipropylene glycol monopropyl ether (surface tension: 25.6 mN / m), propylene glycol monoethyl ether (surface tension: 26.3 mN / m), propylene glycol monopropyl ether (surface tension: 25.9 mN / m), propylene glycol monobutyl ether (surface tension: 26.3 mN / m), propylene glycol monomethyl ether (surface tension: 26.7 mN / m), diethylene glycol monoisobutyl ether (surface tension: 24.7 mN / m), diethylene glycol monobutyl ether (surface tension: 27.9 mN / m),

[0161] [Examples 1 to 98, Comparative Examples 1 to 11] The following evaluations were carried out using the aqueous inkjet inks of Examples 1 to 98 and Comparative Examples 1 to 11 produced above. The evaluation methods for the prepared aqueous inkjet inks are as shown below. The evaluation results are shown in Table 3 above.

[0162] <Evaluation of Discharge Stability> Each aqueous inkjet ink was filled into an inkjet discharge device installed in a 25°C environment and equipped with a Kyocera Corporation head (KJ4B-1200). A nozzle check pattern was printed to confirm that the aqueous inkjet ink was being discharged normally from all nozzles. Then, 100 consecutive solid prints were performed at 100% coverage on OK Topcoat + paper manufactured by Oji Paper Co., Ltd. under printing conditions of 1200 x 1200 dpi and a drop volume of 3 pl. A nozzle check pattern was then printed again, and the number of nozzle clogs was visually counted to evaluate discharge stability. The drive frequency of the inkjet head was set to two conditions: 40 kHz and 64 kHz, and evaluation was performed at each drive frequency. The evaluation criteria were as follows, with a grade of D or higher considered to be within the practical range. A: No missing nozzles at all B: 1 to 3 missing nozzles C: 4 to 6 missing nozzles D: 7 to 9 missing nozzles E: 10 or more missing nozzles

[0163] <Evaluation of Resolubility> One drop of each ink was placed on a glass plate and placed in a thermo-hygrostat set at a temperature of 50°C and a humidity of 40% to dry. After a predetermined time had passed, the glass plate was removed from the thermo-hygrostat, and pure water was dropped onto the (dried) film to visually check whether it had returned to the aqueous inkjet ink. The resolubility was then evaluated by varying the time the ink was left standing in the thermo-hygrostat and performing the above evaluation. The evaluation criteria were as follows, with C or higher being considered to be in the practical range. A+: Even after 40 minutes of drying, the ink returned to a uniform aqueous inkjet ink to the naked eye, and no foreign matter such as a dried film or pigment aggregates was found. A: Even after 30 minutes of drying, the ink returned to a uniform aqueous inkjet ink to the naked eye, and no foreign matter such as a dried film or pigment aggregates was found. However, after 40 minutes of drying, the above foreign matter was observed visually after dripping of pure water. B: Even after 20 minutes of drying, the ink returned to a uniform aqueous inkjet ink to the naked eye, and no foreign matter such as a dried film or pigment aggregates was found. However, after 30 minutes of drying, the above foreign matter was observed visually after dripping of pure water. C: Even after 10 minutes of drying, the ink returned to a uniform aqueous inkjet ink to the naked eye, and no foreign matter such as a dried film or pigment aggregates was found. However, after 20 minutes of drying, the above foreign matter was observed visually after dripping of pure water. E: After 10 minutes of drying, foreign matter such as a dried film or pigment aggregates was observed visually.

[0164] <Evaluation of Dot Circularity> The inkjet ejection device used in the above-mentioned ejection stability evaluation was filled with each aqueous inkjet ink. Next, a gradation image was printed using the same printing conditions and the same type of printing substrate as in the above-mentioned ejection stability evaluation. The "gradation image" is an image in which the printing rate continuously changes from 5 to 60% within a predetermined area. After printing the gradation image, the OK top coat + paper on which the aqueous inkjet ink had been printed was placed in an air oven at 70°C and dried for 1 minute to obtain a gradation print. Then, a portion of the gradation print with a printing rate of 10% was observed using an image evaluation device ("PIAS-II" manufactured by Quality Engineering Associates, Inc.), and the dot circularity was measured. The closer the circularity is to 1, the more circular the dot is, indicating a good dot shape. The evaluation criteria were as follows, with a grade of C or higher being considered to be within the practical range. A: The circularity was 1 or more and 2 or less. B: The circularity was more than 2 and 3 or less. C: The circularity was more than 3 and 3.5 or less. E: The circularity was greater than 3.5.

[0165] <Evaluation of Wettability> Each aqueous inkjet ink was filled into the inkjet ejection device used in the above-mentioned ejection stability evaluation. Next, a solid image was printed at a coverage rate of 100% using the same printing conditions and the same type of printing substrate as in the above-mentioned ejection stability evaluation. After printing the solid image, the print was dried for 1 minute in an air oven at 70°C. The degree of white bleed in the obtained solid print was then checked visually and with a magnifying glass to evaluate the wettability. The evaluation criteria were as follows, with a rating of C or higher being considered to be in the usable range. A: No white bleed was observed visually or with a magnifying glass. B: Slight white bleed was observed with a magnifying glass, but no white bleed was observed visually. C: Slight white bleed was observed visually. E: White bleed was clearly observed visually.

[0166] As can be seen from Table 3, the aqueous inkjet inks evaluated in Examples 1 to 98 all had a practical level of quality in terms of resolubility, ejection stability, dot circularity, and wettability. In particular, the aqueous inkjet inks of Examples 23, 26, 37, 38, 40, 52, 53, 55, 66 to 68, 89 to 93, and 96 were rated as level "A" in the evaluation of ejection stability when using an inkjet head driven at a frequency of 40 kHz, and were rated as level "A" or "A+" in the evaluation of resolubility, dot circularity, and wettability. In order to solve the above-mentioned problems of the present invention, the results show that it is extremely suitable to use, as the siloxane surfactant (A-2), a combination of a gemini siloxane surfactant and a siloxane surfactant modified at both ends with polyether, to set the mass content (total amount) of the siloxane surfactant (A-2) to 0.5 to 2 when the mass content (total amount) of the acetylene diol surfactant (A-1) contained in the aqueous inkjet ink is taken as 1, and to set the functional group content (mol %) represented by the above formula 4 to 70 to 120 mol %.

[0167] Furthermore, among the examples listed above, the aqueous inkjet inks of Examples 23, 26, 37, 52, 66, 93, and 96 were rated at the "A" or "A+" level in all evaluations, including the evaluation of ejection stability in an inkjet head with a drive frequency of 64 kHz. In addition to the above-mentioned elements, these aqueous inkjet inks further contain, as the specific organic solvent, one or more compounds selected from the group consisting of 3-methyl-1,3-butanediol, 2-methyl-2,4-pentanediol (hexylene glycol), and dipropylene glycol monopropyl ether, and it has been confirmed that these compounds are particularly effective materials for solving the above-mentioned problems of the present invention.

[0168] On the other hand, Comparative Example 1 is a system in which the pigment dispersing resin is not crosslinked. Because the pigment dispersing resin is not sufficiently adsorbed to the pigment, the desorbed pigment dispersing resin impairs the print quality and resolubility, and it was confirmed that the system is not suitable for practical use in all aspects of quality.

[0169] Furthermore, in Comparative Examples 2, 9, and 10, which did not contain the siloxane-based surfactant (A-2), the acetylene diol-based surfactant could not be uniformly oriented at the gas-liquid interface, and the wettability and dot roundness on coated paper were unsatisfactory. Furthermore, a deterioration in ejection stability was also confirmed, which is presumably due to the non-uniform orientation of the acetylene diol-based surfactant near the nozzle.

[0170] Conversely, in Comparative Examples 3 and 8, which did not contain the acetylene diol surfactant (A-1), the wettability was insufficient and the printed matter had poor dot circularity.

[0171] In addition, the aqueous inkjet inks of Comparative Examples 4 to 7 and 11 did not contain an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediol having 5 to 8 carbon atoms) and / or a glycol monoether represented by the above general formula 1 as an organic solvent. Evaluation results showed that the surfactants could not be sufficiently compatibilized, and the surfactants were non-uniformly oriented at the nozzle interface, impairing ejection stability and also deteriorating print image quality. Furthermore, the aqueous inkjet ink of Comparative Example 11 did not contain the acetylene diol surfactant (A-1), and, like Comparative Examples 3 and 8, it was confirmed that the dot circularity and wettability did not reach practical levels.

Claims

1. An aqueous inkjet ink comprising pigment particles, a surfactant, and an organic solvent, wherein the pigment particles comprise a pigment and a pigment dispersion resin, the pigment dispersion resin has a crosslinked structure, the surfactant comprises an acetylene diol surfactant (A-1) having a measured HLB value of 6 to 9, and a siloxane surfactant (A-2) having a measured HLB value of 8 to 14, and the organic solvent comprises an alkanediol having 5 to 8 carbon atoms (excluding 1,2-alkanediol having 5 to 8 carbon atoms) and / or a glycol monoether represented by the following general formula 1. General formula 1: R １ -(O-CH(CH ３ )-CH ２ ) ｎ -OH (in general formula 1, R １ is an alkyl group having 2 to 4 carbon atoms, and n is 1 or 2.

2. The aqueous ink-jet ink according to claim 1, wherein the siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 includes a gemini type siloxane-based surfactant.

3. The aqueous inkjet ink according to claim 1, wherein the siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 includes a siloxane-based surfactant modified at both ends with polyether (excluding gemini siloxane-based surfactants).

4. The aqueous inkjet ink according to claim 1, comprising two or more types of siloxane-based surfactants (A-2) having a measured HLB value of 8 to 14.

5. The aqueous inkjet ink according to claim 4, wherein the siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 comprises a gemini siloxane-based surfactant and a siloxane-based surfactant both ends of which are modified with polyether (excluding gemini siloxane-based surfactants).

6. A printed matter obtained by printing with the aqueous inkjet ink according to any one of claims 1 to 5.

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