WO2025126533A1 - Aqueous inkjet ink and printed work - Google Patents

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 matter

This disclosure relates to an aqueous inkjet ink and a printed matter produced using the aqueous inkjet ink.

Inkjet printing is a recording method in which ink droplets are ejected directly from fine nozzles and deposited on a printing substrate to produce characters and/or images. Inkjet printing has many advantages, including low equipment noise and good operability, ease of full color printing, inexpensive printing equipment, and the ability to print on a variety of printing substrates without contact, making it extremely popular. In particular, in recent years, inkjet printing has come to be used not only for consumer purposes in offices and homes, but also for commercial and industrial printing purposes. In this context, there is an ever-increasing demand for inkjet inks that contain water as their main component (aqueous inkjet inks) in order to reduce the burden on the environment and workers.

Note that the above "images" also include seamless images such as solid images and checkered images.

Water-based inkjet inks have long been developed for use on plain paper and specialty paper as printing substrates. In these applications, it is assumed that the liquid components of the water-based inkjet ink are absorbed into the printing substrate. Therefore, when the water-based inkjet ink is printed on poorly absorbent printing substrates such as those used in the commercial and industrial printing applications mentioned above, the image bleeds, making it impossible to produce printed matter that is suitable for practical use.

For example, coated paper, which is a poorly absorbent printing substrate, has low absorbency of liquid components, so when a droplet of water-based inkjet ink lands adjacent to the previous droplet before it has dried during printing, adjacent droplets are likely to merge (beading). Beading causes color mixing and bleeding in printed matter. In addition, some poorly absorbent printing substrates, including coated paper, have low surface free energy, and printing on such substrates makes it difficult for the water-based inkjet ink to wet and spread across the surface of the substrate, which can easily lead to the occurrence of white spots (a phenomenon in which spots on the substrate where the water-based inkjet ink is not applied appear as spots and/or streaks).

A possible method for improving the beading is to increase the drying property of the aqueous inkjet ink and dry the previously landed droplets before the next droplet lands. However, aqueous inkjet inks with high drying property 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 continues to exist at the nozzle outlet, it can cause a deterioration in the landing accuracy of the aqueous inkjet ink and cause nozzle clogging. For this reason, it is preferable for the aqueous inkjet ink used in the inkjet printing method to have "re-solubility" that allows it to dissolve the dried film. However, imparting re-solubility to aqueous inkjet ink is an extremely difficult task.

Another method to improve the beading phenomenon is to reduce the surface tension of the water-based inkjet ink, and generally, highly hydrophobic surfactants or organic solvents are used to achieve this.

For example, Patent Document 1 discloses an ink composition for inkjet recording that contains 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, bleeding) 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 that contains 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 wet spreadability, definition, print density, water resistance, rubbing resistance (abrasion resistance), etc. and does not aggregate on offset media that may be printed with offset ink.

On the other hand, 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 mixing bleeding. Specifically, Patent Document 3 discloses a water-based ink that contains a pigment, a polymer dispersant (preferably including a polymer cross-linked with a cross-linking agent), and a specific water-soluble organic solvent. Example 2 of Patent Document 3 shows a specific example of a water-based ink that contains pigment particles containing a polymer dispersant with a cross-linked structure, an acetylene diol surfactant "Surfynol 440", a 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).

JP 2015-124238 A Special Publication No. 2004-510028 JP 2020-105298 A

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 effect of the highly hydrophobic material described above may not be fully realized, and the ejection stability of the aqueous inkjet ink may also deteriorate.

In addition, when the present inventors evaluated an aqueous inkjet ink having the composition 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.

As described above, until now, there has been no water-based inkjet ink that simultaneously satisfies high levels of ejection stability, resolubility, dot circularity, and ink wettability (reduction of white areas).

In one embodiment of the present invention, therefore, it is an object of the present invention to provide an aqueous inkjet ink that has excellent resolubility and ejection stability, and that has excellent print quality on a variety of printing substrates. In another embodiment of the present invention, it is an object of the present invention to provide a printed matter that has excellent print quality on a variety of printing substrates.

In light of the above background, the inventors conducted intensive research and discovered the following aqueous inkjet ink, completing the present invention.

That is, one embodiment of the present invention is an aqueous inkjet ink comprising pigment particles, a surfactant, and an organic solvent,
The pigment particles include a pigment and a pigment dispersing resin,
The pigment dispersing resin has a crosslinked structure,
The surfactant comprises an acetylene 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;
The present invention relates to an aqueous inkjet ink, wherein the organic solvent contains 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 the general formula 1, R ¹ is 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 water-based inkjet ink.

The aqueous inkjet ink, which is one embodiment of the present invention, has the effect of being excellent in resolubility and ejection stability, and also excellent in print quality on a variety of printing substrates, such as coated paper. In addition, the printed matter, which is another embodiment of the present invention, has the effect of being excellent in print quality on a variety of printing substrates.

The following describes an aqueous inkjet ink according to one embodiment of the present invention (hereinafter, simply referred to as "aqueous inkjet ink of this embodiment"). 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.

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

First, resolubility will be described. As described above, if the aqueous inkjet ink near the nozzle dries and the dried film adheres to the outlet of the nozzle, this causes the ejection stability of the aqueous inkjet ink to deteriorate. For this reason, it is preferable that the aqueous inkjet ink has resolubility. In response to this, the present inventors have found that, as one method for improving the above-mentioned resolubility, it is important that the pigment dispersion resin adsorbed to and/or covering the pigment does not detach even after the liquid components in the inkjet ink evaporate. In the embodiment of the present invention, the pigment coated with a pigment dispersion resin having a crosslinked structure is used to suppress the detachment of the pigment dispersion resin, leading to improved resolubility. In addition, the aqueous inkjet ink of this embodiment has excellent ejection stability because the dried film adhered to the outlet of the nozzle can be easily returned to the aqueous inkjet ink. Furthermore, the phenomenon in which a surfactant described later is adsorbed to the detached pigment dispersion resin and the interfacial orientation of the surfactant is inhibited can be prevented. In addition, since these surfactants can fully exert their functions, it is considered that the wettability of the aqueous inkjet ink is improved and the print image quality is improved.

Next, we will explain print image quality. Generally, one of the factors that affects print image quality is surface tension. The surface tension of water-based inkjet ink is mainly controlled by surfactants. For example, acetylene diol surfactants and siloxane surfactants are used as surfactants.

The inventors have discovered that the wettability of the aqueous inkjet ink to the printing substrate is dramatically improved by using an acetylene 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 acetylene diol-based surfactant (A-1) with a measured HLB value of 6 to 9 has low affinity with water and quickly orients at the interface. Therefore, when ink droplets of the aqueous inkjet ink land on the printing substrate, it is expected to have the effect of significantly improving the wettability of the aqueous inkjet ink. However, it is difficult to orient the aqueous inkjet ink uniformly at the interface using only the acetylene diol-based surfactant (A-1), which leads to deterioration of dot roundness and unevenness in solid areas, resulting in insufficient print quality.

Therefore, in an embodiment of the present invention, an acetylenic diol surfactant (A-1) with a measured HLB value of 6 to 9 and a siloxane surfactant (A-2) with a measured HLB value of 8 to 14 are used in combination to improve dot circularity and unevenness. Although the detailed principle is unknown, the siloxane surfactant (A-2) is a surfactant that has excellent surface tension reducing ability like the acetylenic diol surfactant, but due to its high measured HLB value, it is thought to orient at the air-liquid interface later than the acetylenic diol surfactant (A-1). As a result, the siloxane surfactant (A-2) is thought to orient so as to eliminate the uneven orientation caused by the acetylenic diol surfactant (A-1), and to exert an effect of contributing to the uniform spreading of ink droplets of the aqueous inkjet ink and the reduction of unevenness in solid areas. Although the details will be described 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 the aqueous inkjet ink containing the gemini siloxane surfactant can be significantly improved, and the print image quality is greatly improved. In addition, in a preferred embodiment, a siloxane surfactant modified at both ends with polyether can be used as the siloxane surfactant (A-2). In this case, although the detailed principle is unknown, in addition to uniform wetting and spreading on the printing substrate, improved ejection stability can also be achieved.

On the other hand, when a pigment coated with the above-mentioned pigment dispersion resin having a cross-linked structure is used in combination with an acetylene diol surfactant and a siloxane surfactant, there is a risk of pinholes (hole-like holes in areas where the aqueous inkjet ink should be printed) occurring in the printed matter due to poor compatibility. Furthermore, aqueous inkjet inks that use the above-mentioned components in combination may also have poor ejection stability. One of the reasons for poor ejection stability is thought to be that when liquid components evaporate near the nozzle of the inkjet head, the balance of the above-mentioned compatibility is lost, causing the surfactant to become non-uniformly oriented in the aqueous inkjet ink present near the nozzle.

The inventors have therefore conducted extensive research and found that the above-mentioned components can be used in combination 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 combination of these compounds with an acetylene diol surfactant and the siloxane surfactant makes them more compatible with each other. In addition, 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 are all compounds with low surface tension, the acetylene diol surfactant and the siloxane surfactant that are compatible with these compounds tend to be uniformly oriented over 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 in the inkjet head, and the ejection stability is also improved.

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

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 state 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, can fully exhibit the effects of the surfactant. On the other hand, "Surfynol 440", an acetylene diol surfactant 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 this embodiment in that they do not contain an acetylene diol surfactant (A-1) with a measured HLB value of 6 to 9. Furthermore, as described above, Patent Document 3 aims to improve resolubility (redispersibility) and color mixing bleeding, but does not state dot roundness or whiteout, which are issues in the embodiment of the present invention. In particular, Patent Document 3 does not state or suggest that a printed material with excellent dot circularity can be obtained by using an acetylene diol surfactant (A-1) and a siloxane surfactant (A-2) in combination, both of which have specific measured HLB values.

Next, we will provide a detailed explanation of each component that may be contained in the aqueous inkjet ink according to one embodiment of the present invention.

<Pigment particles>
The aqueous inkjet ink of this embodiment contains pigment particles. The pigment particles contain a pigment and a pigment dispersion resin having a crosslinked structure.

In this embodiment, "the pigment dispersion resin has a cross-linked structure" means that a cross-linked structure is formed between the pigment dispersion resin molecules. By forming a cross-linked structure between the pigment dispersion resin molecules, the pigment becomes covered with the pigment dispersion resin having a cross-linked structure, which makes it easier to suppress detachment of the pigment dispersion resin and improve re-solubility and ejection stability, as described above. The pigment dispersion resin may have a cross-linked structure within the pigment dispersion resin molecule.

In this disclosure, pigment dispersion resins that do not have a cross-linked structure between molecules are also referred to as "uncross-linked dispersion resins," and pigment dispersion resins that have a cross-linked structure between molecules are also referred to as "cross-linked dispersion resins."

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

In the aqueous inkjet ink of this embodiment, the pigment may be an organic pigment or an inorganic pigment. In addition, both may be used in combination.

When inorganic pigments are used as pigments, titanium oxide, zinc oxide, 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, etc. can be used.

For example, when carbon black is used as the inorganic pigment, carbon black produced by either the furnace method or the channel method can be suitably used. Among these carbon blacks, carbon blacks having properties such as a primary particle size of 11 to 40 nm, a specific surface area measured by the BET method of 50 to 400 m ² /g, a volatile content of 0.5 to 10%, a pH value of 2 to 10, etc. can be particularly suitably used.

On the other hand, 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. There are no particular limitations on the hue, and chromatic pigments such as yellow, magenta, cyan, blue, red, orange, and green can be used.

Specific examples of pigments that can be used as the organic pigments are listed in terms of color index, such as cyan pigments C.I. Pigment Blue 1, 2, 3, 15:1, 15:3, 15:4, 15:6, 16, 21, 22, 60, and 64.

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, 50, etc.

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, 213, etc.

Black pigments include aniline black (C.I. Pigment Black 1), perylene black (C.I. Pigment Black 31, 32), azomethine azo black, etc. In addition, chromatic pigments such as the cyan pigments, magenta pigments, and yellow pigments described above, as well as the brown pigments and orange pigments described below, can be mixed together to produce a black pigment.

Other pigments that can be used include 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.

The pigments listed above can be used alone or in combination of two or more. The pigment content 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.

<Pigment Dispersion Resin>
The pigment dispersion resin is a resin having a function of dispersing a pigment. Both the pigment dispersion resin, i.e., 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 more preferably a carboxyl group and/or a carboxylate group (R- ^COO- ). Any resin can be used as the pigment dispersion resin as long as it has an acid group (preferably a carboxyl group and/or a carboxylate group) and has a function of dispersing a pigment.

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

Specifically, 600 g of carbon black having a primary particle size of 15-25 nm, a nitrogen adsorption specific surface area of 120-260 ^{m 2} /g, and a DBP absorption (granular) of 40-80 cm ³ /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 (for example, "Dyno Mill" manufactured by Shinmaru Enterprises) filled with 1,800 g of grinding beads (for example, zirconia beads having a diameter of 0.5 mm). After dispersion, the viscosity of the obtained carbon black dispersion at 25°C is measured using an E-type viscometer (for example, "TVE25L type viscometer" manufactured by Toki Sangyo Co., Ltd.), and the carbon black dispersion is stored in a constant temperature incubator with a blast fan set at 70°C for one week, and the viscosity is measured again. In this case, 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 determined to have the function of dispersing the pigment.

　Types of resin that can be used as pigment dispersion resins include acrylic, maleic acid, urethane, and polyester resins. Among these, it is preferable to use resins that have aromatic rings in their structure, as they can be firmly adsorbed to the pigment and stabilize the pigment's dispersed state.

In this 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 that forms the acrylic resin. On the other hand, a resin that contains maleic acid (anhydride) (at least one selected from "maleic acid" and "maleic acid anhydride") as a polymerizable monomer is excluded from the term "acrylic resin" in this disclosure. In addition, the term "maleic acid-based resin" refers to a resin using at least maleic acid (anhydride) as a polymerizable monomer. In addition, the maleic acid-based resin may use α-olefin, styrene-based monomer, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, etc. as a polymerizable monomer.

Both before and after the crosslinking treatment described below, it is preferable that at least a part of the acid groups (e.g., carboxyl groups) contained in the pigment dispersion resin is neutralized with a basic compound to become an anionized functional group (e.g., carboxylate group). This is because the pigment can be stably dispersed due to the charge repulsion between the anionized functional groups. 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 hydrogen carbonate, dipotassium carbonate, and sodium borate. From the viewpoint of achieving excellent pigment dispersion stability in the aqueous inkjet ink 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 types.

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

In this 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 standard method. For example, it can be measured using a tabletop pH meter "F-71" (manufactured by Horiba, Ltd.) using a pH electrode "6337-10D" (manufactured by Horiba, Ltd.).

The preferred amount of basic compound used to neutralize the pigment dispersion resin can be specifically 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 the molar amount of basic groups in the added basic compound divided by the molar amount of acid groups in the pigment dispersion resin, and can be calculated using the following formula 2. Note that when a resin that is at least partially neutralized (e.g., contains at least a carboxylate group) is used 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 group is a carboxyl group).

Formula 2:

The weight average molecular weight (Mw) of the pigment dispersion resin before the crosslinking treatment described below is preferably 5,000 to 100,000. If the weight average molecular weight is 5,000 or more, the dispersion stability is favorable, and if it is 100,000 or less, the ejection stability is good. The weight average molecular weight is more preferably 8,000 to 50,000, and even more preferably 10,000 to 35,000.

The weight average molecular weight of the pigment dispersion resin can be measured by a conventional method. For example, the weight average molecular weight measured using a TSKgel column (manufactured by Tosoh Corporation) and a GPC ("HLC-8120GPC" manufactured by Tosoh Corporation) equipped with an RI detector and THF as a developing solvent is used to obtain the value measured as the weight average molecular weight converted into polystyrene.

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, in order to ensure the dispersion stability of the pigment and to effectively prevent detachment of the pigment dispersion resin even in the dispersion treatment performed before the crosslinking treatment (details of which will be described later). It is particularly preferably 80 to 150 mgKOH/g. By controlling the acid value within the above range, detachment of the pigment dispersion resin from the pigment after the crosslinking treatment described later is significantly prevented, making it possible to obtain excellent resolubility and ejection stability.

The acid value of the pigment dispersion resin can be measured by a standard method. For example, about 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 mixture of distilled water and dioxane = 1/9 (mass ratio) is added to dissolve the pigment dispersion resin. This sample solution is titrated with a 0.1 mol/L potassium hydroxide-ethanol solution (titer F). A potentiometric measuring device (for example, Kyoto Electronics Manufacturing Co., Ltd.'s "Automatic Potentiometric Titrator AT-710M") is used for the titration. The acid value (mgKOH/g) can then be calculated from the following formula 3 using the amount of potassium hydroxide-ethanol solution (α (mL)) required to reach the titration endpoint.

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

In formula 3, S is the amount (g) of the pigment dispersion resin sample, α is the amount (ml) of 0.1 mol/L potassium hydroxide-ethanol solution dripped up to the end point of the titration, and F is the titer of the 0.1 mol/L potassium hydroxide-ethanol solution.

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

<Pigment dispersing 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 makes the pigment dispersion resin have a three-dimensional structure, and the solubility of the pigment dispersion resin decreases. As a result, it is possible to suppress the pigment dispersion resin from being detached from the pigment. This also makes it easier to redissolve the aqueous inkjet ink that has dried near the nozzle.

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

<Crosslinking agent>
Examples of the crosslinking agent include isocyanate compounds, aziridine compounds, carbodiimide compounds, oxetane compounds, oxazoline compounds, epoxy compounds, etc. Examples of the functional group (reactive functional group) that reacts with the reactive site of the uncrosslinked dispersed resin include isocyanate groups, aziridine groups, carbodiimide groups, oxetane groups, oxazoline groups, epoxy groups, etc. 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.

When a compound having an epoxy group is used as a crosslinking agent, it is preferable to use a compound having multiple epoxy groups in one molecule, more preferable to use a compound having two or more glycidyl ether groups in one molecule, and even more preferable to use a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group with 3 to 8 carbon atoms. In addition, 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 crosslink with the reactive points in the pigment dispersion resin in a liquid medium mainly composed of water.

Specific examples of compounds having multiple 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.

The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of being able to more efficiently undergo a crosslinking reaction with the reactive sites of the pigment dispersion 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 g H ₂ O, more preferably 0.2 to 40 g/100 g H ₂ O, and even more preferably 0.5 to 30 g/100 g H ₂ O.

From the viewpoint of reducing detachment of the pigment dispersion resin from the pigment and improving resolubility and ejection stability, and from the viewpoint of suppressing inhibition of the functional expression of the surfactant by free pigment dispersion resin and improving the wettability of the aqueous inkjet ink, it is preferable that the crosslinking agent is added so that the functional group content (mol %) shown in 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 %.

Formula 4:

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

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

Note that "56.1" in the above formula 4-2 is the molecular weight of potassium hydroxide.

<Method of producing pigment particles containing a pigment dispersing resin having a crosslinked structure>
An example of a method for producing pigment particles containing a pigment dispersion resin having a crosslinked structure (crosslinked pigment particles) is, for example, a method in which a dispersion process and a crosslinking process are carried out in this order, as shown below. Optionally, a neutralization process as shown below may be carried out before the dispersion process. First, a carboxyl group present in the pigment dispersion resin and a basic compound are mixed in an aqueous medium to neutralize at least a part of the carboxyl group (neutralization process). As described above, the carboxyl group present in the pigment dispersion resin becomes a carboxylate group through the neutralization. This pigment dispersion resin having at least a carboxylate group is used in the subsequent process. The pigment dispersion resin obtained after the neutralization process is in the state of an aqueous solution. Next, a pigment is added to the aqueous solution of the pigment dispersion resin, and the two are mixed, and then a dispersion process is further carried out (dispersion process). The dispersion process produces an aqueous dispersion of pigment particles (uncrosslinked pigment particles) in which the pigment dispersion resin is chemically adsorbed on at least a part of the surface. The pigment dispersion resin contained in the uncrosslinked pigment particles is an uncrosslinked dispersion resin. Thereafter, a crosslinking agent is added to the aqueous dispersion of the non-crosslinked pigment particles to carry out a crosslinking treatment (a crosslinking treatment step). Through the crosslinking treatment step, an aqueous dispersion of crosslinked pigment particles can be produced.

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

<Dispersion processing process>
The dispersing machine used in the dispersion process may be any commonly used dispersing machine, for example, a media-type wet dispersing machine, a media-less type wet dispersing machine, a kneading machine, etc. Examples of the media-type wet dispersing machine include a ball mill, a roll mill, a sand mill, and a bead mill, an example of the media-less type wet dispersing machine includes a high-pressure homogenizer, and an example of the kneading machine includes a kneader. Among these, it is preferable to select a bead mill from the viewpoint of crushing and refining coarse particles of the pigment. Examples of the bead mill 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).

By using the dispersing machines listed above, the pigment can be refined to the desired particle size by the collision force with the media and shear stress. In particular, from the viewpoint of obtaining a pigment dispersion liquid with a uniform particle size, it is preferable to pre-disperse (premix) the pigment and pigment dispersion resin, and then further disperse (main dispersion) using the dispersing machine listed above. As the pre-dispersing machine used for premixing, a commonly used mixing and stirring device such as a Disper can be used.

<Cross-linking process>
As described above, by subjecting the pigment dispersion resin adsorbed to the pigment to a crosslinking treatment, it is possible to obtain pigment particles in which the pigment dispersion resin is crosslinked. In addition, as a method for the crosslinking treatment, a method for forming a crosslinked structure using a crosslinking agent can be suitably used. Specifically, a method in which a mixture containing uncrosslinked pigment particles, a crosslinking agent, and water is stirred and held under heating can be mentioned.

The temperature during the crosslinking process is preferably 50°C to 95°C, more preferably 70°C to 85°C, from the viewpoint of efficiently proceeding with the crosslinking reaction. The time for the crosslinking process 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.

From the viewpoint of stable ejection from the 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.

The above average particle size is the median size on a volume basis that can be measured by dynamic light scattering. For example, it can be measured using Microtrac Bell's "Nanotrac UPA-EX150."

<Pigment Dispersion>
The crosslinked pigment particles may be in a state of being dispersed in an aqueous medium, i.e., in a state of an aqueous dispersion of the crosslinked pigment particles. Since this can improve the ejection stability, the aqueous inkjet ink of this embodiment is preferably produced by a method of mixing a previously produced aqueous dispersion of the crosslinked pigment particles with other raw materials described later.

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

In addition, in this disclosure, the aqueous dispersion of uncrosslinked pigment particles is specifically referred to as an "uncrosslinked pigment dispersion," and the aqueous dispersion of crosslinked pigment particles is specifically referred to as a "crosslinked pigment dispersion."

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

The aqueous medium contained in the pigment dispersion contains at least water. It may also contain an organic solvent (details of which will be described later) 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, etc.

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 stable, and it is possible to increase the dispersion stability due to charge repulsion. From this perspective, a more preferable pH is 9 to 11. The pH of the pigment dispersion can be measured in the same manner as the aqueous solution of the pigment dispersion resin described above.

<Surfactant>
The aqueous inkjet ink of this embodiment contains, as surfactants, an acetylene 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.

In this disclosure, the "HLB (Hydrophilic-Lipophilic Balance) value" is one of the parameters that indicate the degree of hydrophilicity and hydrophobicity of a material. The smaller the HLB value, the more hydrophobic the material is, and the larger the HLB value, the more hydrophilic the material is. There are two methods for determining the HLB value: calculating it from the molecular structure and measuring it experimentally. In this disclosure, the HLB value is calculated by measuring it experimentally using the method shown below (measured HLB value). (1) Dissolve 0.5 g of the target surfactant in 5 mL of ethanol. (2) At 25°C, titrate the mixture of (1) with a 2% aqueous phenol solution while stirring. The end point is when the mixture becomes cloudy and does not return to transparency when the 2% aqueous phenol solution is dropped. (3) When the amount of 2% aqueous phenol solution dropped by the end point is A [mL], calculate the measured HLB value according to the following formula 5.

Formula 5:
Measured HLB value = 0.89 x A + 1.11

<Acetylene diol surfactant (A-1)>
The aqueous inkjet ink of this embodiment contains an acetylenic diol surfactant (A-1). The acetylenic diol surfactant (A-1) preferably has a measured HLB value of 6 to 9, because it has low affinity for water and is quickly oriented at the gas-liquid interface, resulting in excellent wettability with respect to the printing substrate.

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.

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, from the viewpoint of ensuring excellent print quality. It is particularly preferably 0.5 to 2% by mass.

<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 be oriented so as to compensate for the non-uniform orientation of the acetylene diol-based surfactant (A-1), 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.

As the siloxane-based surfactant (A-2), gemini-type siloxane-based surfactants, siloxane-based surfactants modified with polyether at both ends, side-chain polyether-modified siloxane-based surfactants, etc. can be used. Among them, although the details are unclear, it is preferable that the siloxane-based surfactant (A-2) contains gemini-type siloxane-based surfactants and/or siloxane-based surfactants modified with polyether at both ends, because a small amount of the surfactant reduces unevenness and produces printed matter with good dot roundness, and because the ejection stability of the aqueous inkjet ink is improved.

In addition, from the viewpoint of obtaining a printed matter having particularly excellent print quality by further enhancing the effects of dot circularity, wettability, and reduction of unevenness, and from the viewpoint of being able to maintain and improve ejection stability, it is preferable that the aqueous inkjet ink of this embodiment contains two or more types of siloxane-based surfactants (A-2), and more preferably, one or more of them are gemini-type siloxane-based surfactants and/or siloxane-based surfactants modified at both ends with polyether (excluding those that are gemini-type siloxane-based surfactants). In addition, from the viewpoint of obtaining an aqueous inkjet ink that has excellent wettability, resolubility, and ejection stability on a printing substrate, and that can obtain a printed matter having good dot circularity, it is particularly preferable that the siloxane-based surfactant (A-2) contains one or more types of gemini-type siloxane-based surfactants and one or more types of siloxane-based surfactants modified at both ends with polyether (excluding those that are gemini-type siloxane-based surfactants).

<Gemini type siloxane surfactant>
In general, gemini surfactants have a structure in which surfactants having hydrophilic and hydrophobic structures are linked by a linking group (spacer) or a covalent bond. In the case of gemini siloxane surfactants, for example, the siloxane chain (hydrophobic structure represented by -[O-SiR ¹ R ² ] _x -, where R ¹ and R ² are each an arbitrary organic group, and x is an integer of 2 or more) and the hydrophilic structure (for example, a polyether chain) have 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 a plurality of siloxane chains are bonded via linking groups or the like (for example, at least a portion of ^R1 and/or ^R2 in the structural formula of the siloxane chain is an organic chain containing a siloxane chain).
A structure in which a plurality of siloxane-based surfactants, each having a plurality of hydrophilic structures, share a portion of the hydrophilic structures.

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 better reduction in surface tension than with non-gemini siloxane surfactants. As a result, it is possible to significantly improve the wettability of aqueous inkjet inks containing gemini siloxane surfactants, making it possible to improve the dot roundness and unevenness described above.

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

<Siloxane surfactants modified with polyether at both ends (excluding gemini type siloxane surfactants)>
The siloxane surfactant modified with polyether at both ends has a structure in which polyether chains are bonded to both ends of a polysiloxane skeleton. An aqueous inkjet ink containing the siloxane surfactant modified with polyether at both ends improves print quality by uniformly spreading the ink on the printing substrate, and also has good ejection stability. As the siloxane surfactant modified with polyether at both ends, a compound represented by the following general formula 6 can be preferably used.

General formula 6:

In formula 6, l represents an integer of 1 to 100, and ^R3 represents a structure represented by formula 7 below.

General formula 7:

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, provided that p+r is an integer of 1 or greater. ^R4 represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, a methacryloyl group, or an acryloyl group. However, the addition pattern of the ethylene oxide group (OC ₂ H ₄ ) and the propylene oxide group (OC ₃ H ₆ ) in the brackets [ ] may be block or random.

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

<Side chain polyether modified siloxane surfactants (excluding gemini type siloxane surfactants)>
As the siloxane-based surfactant (A-2), a side-chain polyether-modified siloxane-based surfactant (excluding gemini type 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 realize 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.

General formula 8:

In the 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. ^R5 is a structure represented by the general formula 7, and ^R6 represents an alkyl group having 1 to 6 carbon atoms.

Commercially available compounds represented by the above general formula 8 and having a measured HLB value of 8 to 14 include, but are not limited to, BYK-347, BYK-348, BYK-349, and BYK-3451 manufactured by BYK-Chemie, and TEGOWet 240, TEGOWet 250, and TEGOWet 260 manufactured by Evonik Japan.

From the viewpoint of ensuring excellent print quality, the content of the siloxane-based surfactant (A-2) in the aqueous inkjet ink is preferably 0.1 to 5% by weight, more preferably 0.3 to 3.5% by weight. Particularly preferably, it is 0.5 to 2.5% by weight.

In addition, from the viewpoint of obtaining printed matter with excellent dot circularity and no blank spaces, the mass content of the siloxane-based surfactant (A-2) (the total mass content when two or more types of siloxane-based surfactant (A-2) are contained) is preferably 0.3 to 3, and particularly preferably 0.5 to 2, when the mass content of the acetylenic diol-based surfactant (A-1) contained in the aqueous inkjet ink (the total mass content when two or more types of acetylenic diol-based surfactant (A-1) are contained) is taken as 1.

<Organic Solvent>
<Specific organic solvents>
The aqueous inkjet ink of this embodiment contains 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. In the present disclosure, these organic solvents are collectively referred to as "specific organic solvents". Since all of the specific organic solvents have a low surface tension, the acetylene diol surfactant (A-1) and the siloxane surfactant (A-2), which have an affinity with the specific organic solvent, diffuse with the wetting and spreading of the specific organic solvent, and tend to be uniformly oriented at the interface. As a result, pinholes are easily suppressed in the printed matter. In addition, the meniscus of the aqueous inkjet ink in the inkjet head is stabilized, and ejection stability is also improved.

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, 1,3-octanediol, etc. Among them, it is particularly preferable to use an alkanediol having a particularly small surface tension, that is, 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 improved. On the other hand, although the detailed mechanism is unknown, a compound 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 particularly improving the ejection stability. Among the compounds listed above, 3-methyl-1,3-butanediol and 2-methyl-2,4-pentanediol (hexylene glycol) are examples of compounds having a surface tension of 20 to 30 mN/m at 25°C and having a quaternary carbon atom.

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 it can favorably make the acetylene diol surfactant (A-1) and the siloxane surfactant (A-2) compatible with each other, and thus improves the ejection stability and resolubility. Specific examples of compounds in which n in general formula 1 is 2 include dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, and dipropylene glycol monobutyl ether. Among these compounds, it is particularly preferable for the aqueous inkjet ink to contain dipropylene glycol monopropyl ether, since the above-mentioned effects are particularly favorably exhibited.

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, of the total amount of the aqueous inkjet ink. It is particularly preferably 2 to 15% by mass.

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. Furthermore, these organic solvents may be used alone or in combination of two or more.

<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.

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

In this disclosure, a resin with 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 with a solubility of less than 1 g is referred to as a "water-insoluble resin." Furthermore, among the above water-insoluble resins, resins that are dispersed in water in particulate form and have a volume-based median diameter (also referred to as "D50" in this disclosure) of 10 to 1,000 nm are referred to as "resin microparticles."

The D50 of the resin microparticles can be measured using the same device and method as for the average particle size of the crosslinked pigment particles described above.

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 types.

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.

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

<Other ingredients>
The aqueous inkjet ink of this embodiment may contain a pH adjuster and other additives in addition to the above-mentioned components. 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.

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

<Method of manufacturing water-based 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 above-mentioned method, and the mixture is thoroughly stirred and mixed, after which coarse particles are removed by a technique such as filtration or centrifugation. However, the method for producing the aqueous inkjet ink of this embodiment is not limited to the above-mentioned method.

<Characteristics of water-based inkjet ink>
The aqueous inkjet ink of this embodiment preferably has a viscosity of 3 to 15 mPa·s at 25° C. In this viscosity range, droplets of the aqueous inkjet ink can be stably ejected not only from an inkjet head having an ejection frequency of about 4 to 10 KHz, but also from an inkjet head having a high ejection frequency of about 20 to 70 KHz. In particular, when the aqueous inkjet ink of this embodiment has a viscosity of 4 to 10 mPa·s at 25° C., the aqueous inkjet ink can be stably ejected even when an inkjet head having a design resolution of 600 dpi or more is used. In the present disclosure, the viscosity is a value measured in a 25° C. environment using a cone-plate type rotational viscometer (E-type viscometer, cone angle 1°34′) such as the “TVE25L type viscometer” manufactured by Toki Sangyo Co., Ltd.

Furthermore, in order to obtain an aqueous inkjet ink with excellent ejection stability and print quality of printed matter, the aqueous inkjet ink of this embodiment preferably has a static surface tension at 25°C of 18 to 35 mN/m, and particularly preferably 20 to 30 mN/m. Note that in this disclosure, the static surface tension is a value measured in a 25°C environment using the Wilhelmy method (plate method) with an automatic surface tensiometer CBVP-Z manufactured by Kyowa Interface Science Co., Ltd.

<Water-based inkjet ink set>
The aqueous inkjet ink of the present embodiment may be used alone, or may be used as a set of aqueous inkjet inks in which two or more aqueous inkjet inks are combined. Examples of the aqueous inkjet ink set include a set of four aqueous inkjet inks (process color ink set) consisting of a cyan aqueous inkjet ink (aqueous cyan ink), a magenta aqueous inkjet ink (aqueous magenta ink), a yellow aqueous inkjet ink (aqueous yellow ink), and a black aqueous inkjet ink (aqueous black ink).

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

≪Discharge process≫
In the above-mentioned ejection step, the operation method of the inkjet head includes a shuttle (scan) method in which the inkjet head is scanned back and forth in a direction perpendicular to the transport direction of the printing substrate to eject and record the aqueous inkjet ink, and a single-pass method in which the aqueous inkjet ink is ejected and recorded when the printing substrate passes under a fixedly arranged inkjet head. Either the shuttle method or the single-pass method may be adopted for the inkjet head equipped with the aqueous inkjet ink of this embodiment. Among them, the single-pass method is preferably selected because the landing position of the droplets of the aqueous inkjet ink is less likely to shift and the print quality of the printed matter is improved.

The method of ejection from the inkjet head can also be selected from any known method. Examples of such ejection methods include the piezoelectric method, which uses the volume change of a piezoelectric element, the thermal method, which ejects water-based inkjet ink using bubbles generated by heating a heater, and the valve method, which ejects pressurized water-based inkjet ink by opening and closing the nozzle lid (valve) with a solenoid.

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

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

≪Printing base material≫
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.

Examples of permeable substrates include uncoated paper such as sawdust, medium-quality paper, fine paper, and recycled paper; fabrics such as cotton, synthetic fiber fabrics, silk, hemp, and nonwoven fabric; and leather. Among these, uncoated paper such as sawdust, medium-quality paper, fine paper, and recycled paper is preferably used because it produces printed matter with excellent print quality.

Furthermore, examples of poorly permeable substrates include coated papers such as coated paper, art paper, and cast paper. Among these, coated paper is preferably used because it produces prints with excellent print quality.

The printing substrates listed above may have a smooth surface or may have an uneven surface. The printing substrate may be in the form of a roll or a sheet. Two or more of the printing substrates listed above may be bonded together and used as the printing substrate. A peelable 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.

In order to improve the wettability of the aqueous inkjet ink of this embodiment and obtain a printed matter with excellent print quality and drying properties, it is also preferable to perform surface modification such as corona treatment and plasma treatment on the printing surface of the printing substrates listed above.

That is, the present invention relates to aqueous inkjet inks as shown in [1] to [5] below, and to a 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,
The pigment particles include a pigment and a pigment dispersing resin,
The pigment dispersing resin has a crosslinked structure,
The surfactant comprises an acetylene 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;
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 the 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-based surfactant (A-2) having a measured HLB value of 8 to 14 includes a gemini siloxane-based surfactant.
[3] The aqueous inkjet ink according to [1] or [2], 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 a polyether (excluding gemini siloxane-based surfactants).
[4] The aqueous inkjet ink according to [2] or [3], 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 [4], wherein the siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 includes 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 [1] to [5].

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

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 "%" refer to "parts by mass" and "% by mass", respectively, unless otherwise specified.

<Production Example of Pigment Dispersion Resin 1>
A reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer was charged with 93.4 parts of methyl ethyl ketone and substituted with nitrogen gas. After the temperature inside the reaction vessel was raised to 110 ° C., a mixture of 25 parts of styrene as a polymerizable monomer, 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 dropped into the reaction vessel over 2 hours. After the dropwise addition was completed, the reaction was continued for 3 hours while maintaining 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. to synthesize pigment dispersion resin 1. The weight average molecular weight of the obtained pigment dispersion resin 1 measured by the above-mentioned method was 29,000, and the acid value was 196 mg KOH / g. Thereafter, 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 an aqueous potassium hydroxide solution with a concentration of 48% by mass was added, containing an amount of potassium hydroxide equal to the calculated amount. Further, ion-exchanged water was added so that the solid content concentration was 20%, and the mixture was stirred at 50° C. for 1 hour to obtain an aqueous solution of pigment dispersion resin 1 (solid content concentration 20%).

<Production Examples of Pigment Dispersion Resins 2 to 10>
Except for using the monomers shown in Table 1 as the polymerizable monomers, aqueous solutions of pigment dispersion resins 2 to 10 (all having a solid content of 20%) were obtained using the same raw materials and operations as for pigment dispersion resin 1. Note that pigment dispersion resins 1 to 10 are non-crosslinked dispersion resins.

The abbreviations used in Table 1 are as follows: Table 1 also shows the weight average molecular weights and acid values of pigment dispersing resins 1 to 10.
St: styrene AA: acrylic acid MMA: methyl methacrylate LMA: lauryl methacrylate

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

<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 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%.

<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 functional group content represented by the above formula 4 is 90 mol%) of Denacol EX-321 (an epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 140 g/eq.) as a crosslinking agent, and 5.2 parts of ion-exchanged water were mixed, and the mixture was heated to 80° C. while stirring, and then held at 80° C. for 3 hours to carry out a crosslinking treatment. Thereafter, 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%).

<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 all 14%.

<Production Example of Binder Resin 1>
Binder resin 1, which is 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 a polymerizable monomer, and the mixture was reacted at 80° C. for 2 hours and reprecipitated to obtain a first block copolymer to which iodine was 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 mixture was reacted at 80° C. for 2.5 hours and reprecipitated to obtain binder resin 1 in which the iodine-added site of the first block was substituted with a second block consisting of styrene, methyl methacrylate, and lauryl methacrylate. The weight average molecular weight (Mw) of the 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, stirred, and completely dissolved, thereby obtaining an aqueous solution (solid content concentration 40%) of the binder resin 1, which is an A-B block polymer.

<Production of Water-Based Inkjet Inks>
The raw materials listed in Table 3 were mixed while stirring using a disperser until 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.

The abbreviations listed in Table 3 are as follows:
Surfynol DF110D (acetylene diol surfactant manufactured by Evonik Japan, actual HLB value = 6.4)
Surfynol 104 (acetylene diol surfactant manufactured by Evonik Japan, actual HLB value = 7.9)
Surfynol 420 (acetylene diol surfactant manufactured by Evonik Japan, actual HLB value = 8.3)
Surfynol 440 (acetylene diol surfactant manufactured by Evonik Japan, actual HLB value = 10.5)
TEGOTwin 4100 (gemini type siloxane surfactant manufactured by Evonik Japan, actual HLB value = 8.1)
TEGOTwin 4200 (gemini type siloxane surfactant manufactured by Evonik Japan, actual HLB value = 8.2)
TEGOTwin 4000 (gemini type siloxane surfactant manufactured by Evonik Japan, actual HLB value = 2.0)
TEGOGlide 440 (a siloxane surfactant modified with polyether at both ends, manufactured by Evonik Japan, measured HLB value = 12.1)
BYK-3420 (BYK Japan Co., Ltd., both ends of polyether-modified siloxane surfactant, actual HLB value = 13.8)
TEGOGlide 100 (a siloxane surfactant modified with polyether at both ends, manufactured by Evonik Japan, measured HLB value = 7.1)
BYK-348 (side chain polyether modified siloxane surfactant manufactured by BYK Japan, actual HLB value = 12.2)
BYK-349 (BYK Japan Co., Ltd. side chain polyether modified siloxane surfactant, actual 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)

[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.

<Evaluation of ejection stability>
Each aqueous inkjet ink was filled into an inkjet ejection device installed in a 25°C environment and equipped with a Kyocera head (KJ4B-1200). A nozzle check pattern was printed to confirm that the aqueous inkjet ink was ejected normally from all nozzles, and then 100 sheets of solid printing with a printing rate of 100% was performed continuously on OK topcoat + paper manufactured by Oji Paper Co., Ltd. under printing conditions of 1200 x 1200 dpi and drop volume of 3 pl. Thereafter, a nozzle check pattern was printed again, and the ejection stability was evaluated by visually counting the number of nozzles that had dropped out. The drive frequency of the inkjet head was set to two conditions of 40 kHz and 64 kHz, and evaluation was performed at each drive frequency. The evaluation criteria were as follows, with D or higher being considered to be in 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

<Evaluation of redissolution property>
A drop of each ink was placed on a glass plate, and then placed in a thermostatic chamber 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 thermostatic chamber, and pure water was dropped onto the (dried) film to visually check whether it returned to the aqueous inkjet ink. The resolubility was evaluated by carrying out the above evaluation while changing the time of leaving the ink in the thermostatic chamber. The evaluation criteria were as follows, with C or higher being considered to be in the practical range.
A+: Even after drying for 40 minutes, the ink returned to a uniform aqueous inkjet ink when observed with the naked eye, and no foreign matter such as a dried film or pigment agglomerates was found. A: Even after drying for 30 minutes, the ink returned to a uniform aqueous inkjet ink when observed with the naked eye, and no foreign matter such as a dried film or pigment agglomerates was found. However, after drying for 40 minutes, the above foreign matter was observed with the naked eye after dripping pure water. B: Even after drying for 20 minutes, the ink returned to a uniform aqueous inkjet ink when observed with the naked eye, and no foreign matter such as a dried film or pigment agglomerates was found. However, after drying for 30 minutes, the above foreign matter was observed with the naked eye after dripping pure water. C: Even after drying for 10 minutes, the ink returned to a uniform aqueous inkjet ink when observed with the naked eye, and no foreign matter such as a dried film or pigment agglomerates was found. However, after drying for 20 minutes, the above foreign matter was observed with the naked eye after dripping pure water. E: After drying for 10 minutes, foreign matter such as a dried film or pigment agglomerates was observed with the naked eye.

<Evaluation of dot roundness>
The inkjet ejection device used in the 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 ejection stability evaluation. The "gradation image" is an image in which the printing rate is continuously changed from 5 to 60% within a predetermined area. After printing the gradation image, the OK topcoat + paper on which the aqueous inkjet ink was 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 with an image evaluation device ("PIAS-II" manufactured by Quality Engineering Associates) to measure the circularity of the dots. The closer the circularity is to 1, the more circular it is, meaning that it shows a good dot shape. The evaluation criteria are as follows, and C or higher is considered to be in 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.

<Evaluation of wettability>
The inkjet ejection device used in the above ejection stability evaluation was filled with each aqueous inkjet ink. Next, a solid image with a printing rate of 100% was printed under the same printing conditions and using the same type of printing substrate as in the above ejection stability evaluation. After printing the solid image, the print was dried for 1 minute using an air oven at 70°C. Then, the degree of white voids in the obtained solid print was checked visually and with a magnifying glass to evaluate wettability. The evaluation criteria were as follows, with a rating of C or higher being considered to be in the practical range.
A: No white spots were observed with the naked eye or with a magnifying glass. B: A slight white spot was observed with a magnifying glass, but no white spots were observed with the naked eye. C: A slight white spot was observed with the naked eye. E: A clear white spot was observed with the naked eye.

As can be seen from Table 3, the aqueous inkjet inks evaluated in Examples 1 to 98 had a practical level of quality in all of the areas 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 ejection stability when using an inkjet head with a drive frequency of 40 kHz, and were rated as level "A" or "A+" in resolubility, dot circularity, and wettability. In solving the problems of the present invention described above, the results show that it is extremely suitable to use a gemini siloxane surfactant and a siloxane surfactant modified at both ends with polyether as the siloxane surfactant (A-2), 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 %) shown in the above formula 4 to 70 to 120 mol %.

Furthermore, among the examples listed above, the aqueous inkjet inks of Examples 23, 26, 37, 52, 66, 93, and 96 were rated as "A" or "A+" 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 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 as specific organic solvents, and it has been confirmed that these compounds are particularly effective materials for solving the problems of the present invention described above.

On the other hand, Comparative Example 1 is a system in which the pigment dispersion resin is not cross-linked. Because the pigment dispersion resin does not sufficiently adsorb to the pigment, the print image quality and resolubility are impaired by the detached pigment dispersion resin, and it was confirmed that the system is not suitable for practical use in all aspects of quality.

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

In contrast, 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.

In addition, the aqueous inkjet inks of Comparative Examples 4 to 7 and 11 do 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. As a result of the evaluation, it was found that the surfactant could not be sufficiently compatible, and the surfactant was non-uniformly oriented at the nozzle interface, which impaired the ejection stability and also deteriorated the print image quality. Furthermore, the aqueous inkjet ink of Comparative Example 11 does not contain an acetylene diol surfactant (A-1), and it was confirmed that, like Comparative Examples 3 and 8, the dot circularity and wettability did not reach a practical level.

Claims (6)

1. An aqueous inkjet ink comprising pigment particles, a surfactant, and an organic solvent,
The pigment particles include a pigment and a pigment dispersing resin,
The pigment dispersing resin has a crosslinked structure,
The surfactant comprises an acetylene 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;
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 the general formula 1, R ¹ is an alkyl group having 2 to 4 carbon atoms, and n is 1 or 2.)
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 gemini siloxane-based surfactant. 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). The aqueous inkjet ink according to claim 1, which contains two or more types of siloxane-based surfactants (A-2) having a measured HLB value of 8 to 14. The aqueous inkjet ink according to claim 4, wherein the siloxane-based surfactant (A-2) having a measured HLB value of 8 to 14 includes a gemini siloxane-based surfactant and a siloxane-based surfactant modified at both ends with polyether (excluding gemini siloxane-based surfactants). A printed matter printed with the aqueous inkjet ink according to any one of claims 1 to 5.