JP7544241B1 - Water-based inkjet inks and printed matter - Google Patents

Abstract

To provide an aqueous inkjet ink which, even when used for printing on poorly permeable and non-permeable substrates, produces printed matter free of white spots and color bleeding and which also has excellent water resistance, and further has excellent ejection stability immediately after the start of printing and after a long period of printing pause.
[Solution] An aqueous inkjet ink comprising a pigment, a surfactant (A), a water-soluble organic solvent, and a binder resin, wherein the surfactant (A) comprises a polyoxyalkylene monoalkyl ether compound (A-1) having a specific alkyl chain length and number of ethylene oxide groups, and a nonionic surfactant (A-2) having an HLB value of 1 to 10 (excluding the above compound (A-1)), and the water-soluble organic solvent comprises hexylene glycol.
[Selection diagram] None

Description

The present invention relates to an aqueous inkjet ink and a printed matter produced using the aqueous inkjet ink.

Digital printing methods, which do not require plate making, are rapidly becoming more popular as demand for short-run printing and reduced printing costs increases.

Inkjet printing, which is one type of digital printing method, ejects ink droplets from fine nozzles and applies them to a printing substrate (also referred to simply as "substrate" in this application) to print images and/or characters on the substrate. Inkjet printing has features such as easy operation of the printing device and low noise during printing, so printing devices (inkjet printers) that employ this inkjet printing method are in high demand among digital printing methods.

In this application, printing substrates on which images and/or characters are printed are collectively referred to as "printed matter." The above "image" also includes seamless images such as solid images and checkered pattern images.

Inks used in inkjet printing methods (referred to as "inkjet ink" in this application) are classified into solvent-based, water-based, UV-curable, etc., depending on their composition. On the other hand, in recent years, there has been an accelerating movement to restrict the use of raw materials that are harmful to humans and the environment. Accordingly, there is an increasing demand for water-based inkjet inks (also referred to simply as "water-based inkjet inks" in this application) rather than solvent-based inkjet inks and UV-curable inkjet inks that use these raw materials.

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

However, in the past, solid prints (prints with a coverage rate of 100%) made using aqueous inkjet ink on poorly permeable substrates such as coated paper and art paper, and non-permeable substrates such as film, suffered from a phenomenon called "whiteout," in which the aqueous inkjet ink did not cover the printed substrate, and a phenomenon called "bleeding," in which aqueous inkjet inks of different colors mixed together, making it difficult to obtain prints with the same print quality as plate-based printing methods. This is because water, the main solvent of aqueous inkjet ink, has a uniquely high surface tension, which results in poor wettability to the substrate, especially on the order of microseconds (μs).

In general, hydrophobic organic solvents and/or surfactants are often used to improve wettability of the substrate. However, these components have poor solubility in water, and therefore tend to foam and generate bubbles that can cause nozzle leakage. Furthermore, these components are oriented at the gas-liquid interface formed on the nozzle end surface of the inkjet head, destabilizing the gas-liquid interface and destabilizing the meniscus of the aqueous inkjet ink. These problems are particularly likely to lead to nozzle leakage (a phenomenon in which aqueous inkjet ink is not ejected from the nozzle) immediately after the start of printing. In addition, due to the structure of the inkjet head, the nozzle diameter is very small, at several tens of micrometers. In particular, when printing is suspended for a long period of time, the liquid components in the aqueous inkjet ink (water, water-soluble organic solvent, etc.) evaporate from the nozzle end surface, while fresh aqueous inkjet ink may not be sufficiently supplied to the inkjet head. When this happens, the aqueous inkjet ink near the nozzle end face experiences an increase in solids content and/or an increase in the proportion of water-soluble organic solvents with high boiling points, which can lead to an increase in viscosity due to aggregation and insolubilization of solid components (pigments, resins, etc.), and nozzle clogging can occur as a result of this increased viscosity.

In this application, the ejection stability immediately after printing starts is referred to as "initial ejection stability," and the ejection stability after a long period of printing pause is also referred to as "standby ejection stability."

On the other hand, especially in the packaging printing market, it is necessary that the ink film after drying does not peel off even when the printed matter with water on it is rubbed with a finger or the like. In other words, the ink film after drying is required to have a certain degree of water resistance. In order to make the ink film after drying water resistant in aqueous inkjet inks that contain a large amount of water-soluble raw materials, for example, it is necessary to use a binder resin with a large mass average molecular weight to make the ink film after drying a strong continuous film. On the other hand, if a certain amount or more of binder resin is contained in the aqueous inkjet ink before printing (liquid), the above-mentioned binder resins may interact with each other in the aqueous inkjet ink, or with the pigment dispersion resin, which may cause problems such as deterioration of ejection stability and/or partial inhibition of the orientation of the surfactant at the gas-liquid interface.

Thus, in order to expand the use of water-based inkjet inks in the packaging printing market, multiple issues need to be resolved simultaneously, including initial ejection stability, standby ejection, print image quality, and water resistance of the ink film. However, no water-based inkjet ink has been found to date that can simultaneously and effectively resolve all of these issues.

In this application, printed matter that is free of white areas and color mixing is also referred to as "printed matter with excellent print quality."

For example, Patent Document 1 discloses an aqueous ink that contains a water-insoluble polymer, a glycol ether-based organic solvent having a specific viscosity and vapor pressure, and 0.005 to 0.3 mass % of a silicone-based surfactant, with the content of the high-boiling organic solvent being a certain amount or less. According to Patent Document 1, the aqueous ink having the above composition is excellent in continuous ejection properties and suppression of color mixing bleeding in recorded matter (printed matter). Furthermore, the examples of Patent Document 1 disclose an example of an aqueous ink that uses, as surfactants, a polyether-modified silicone-based surfactant ("Silface SAG005"), an acetylene diol-based surfactant ("Surfynol 104PG50"), and polyoxyethylene lauryl ether ("Emulgen 120", in which the number of moles of ethylene oxide structure added is 12) (see paragraph 0090 and Tables 4 to 9 of Patent Document 1).

The ink used in the image recording method disclosed in Patent Document 2 contains an organic solvent having a specific structure and a specified CLogP value, an acetylene surfactant, a polyoxyethylene alkyl ether surfactant having an HLB value of 4 to 18, and a polyether-modified silicone surfactant. Patent Document 2 claims that the image recording method produces high-resolution prints with excellent abrasion resistance and no graininess or intercolor bleeding (mixed color bleeding). In the examples of Patent Document 2, "Surfynol 104," "Dynol 604," and "Surfynol DF110D" are used as the acetylene surfactants, and "Emulgen 103" and "Emulgen 108," which have 3 and 6 moles of added ethylene oxide structure, are used as the polyoxyethylene alkyl ether surfactants.

Furthermore, Patent Document 3 discloses an aqueous ink that contains an acetylene-based surfactant (preferably with an HLB value of 8 or less), other nonionic surfactants, and an aqueous medium, and specifies the HLB value and cloud point of the other nonionic surfactants. The aqueous ink is said to be less susceptible to ejection defects caused by drying and solidification in the nozzles of the inkjet head even when printing after a long period of printing suspension, and is less susceptible to deterioration in print quality caused by uneven drying when printed on a non-absorbent or poorly absorbent printing substrate. The aqueous ink specifically disclosed in Patent Document 3 uses, as surfactants, acetylene-based surfactants such as "Surfynol 420" and "Surfynol 104", as well as polyoxyalkylene monoalkyl ether surfactants ("Emulgen" series) with an added mole number of ethylene oxide structure of 10 or less.

JP 2017-8319 A International Publication No. 2023/171302 WO 2020/80121

However, the inventors have found through their research that the aqueous inkjet inks specifically disclosed in Patent Documents 1 to 3 may have problems with initial ejection stability and standby ejection depending on the printing conditions, and may also deteriorate the water resistance of the printed matter. The acetylene diol surfactants used in these specific examples are all equivalent to the hydrophobic surfactants described above, and are therefore considered to be effective in suppressing color mixing bleeding in printed matter, for example. However, these surfactants have problems such as the tendency for air bubbles to form or the meniscus of the aqueous inkjet ink to become unstable, as described above, and the aqueous inkjet inks described above have not completely solved these problems. Furthermore, as will be described in detail later, the use of a hydrophobic surfactant may adversely affect the water resistance of the printed matter depending on the materials used in combination, and from this perspective, it can be said that there is room for improvement in the aqueous inkjet inks disclosed in the above patent documents.

As described above, the techniques described in Patent Documents 1 to 3 have not yet resolved all of the above-mentioned problems at a high level.
Therefore, an object of one embodiment of the present invention is to provide an aqueous inkjet ink which can produce printed matter that is free of white voids and color bleeding and has excellent water resistance, even when used for printing on poorly permeable substrates and non-permeable substrates, and which also has excellent ejection stability immediately after the start of printing and after a long period of printing pause.

As a result of intensive research, the inventors have found that the aqueous inkjet ink having the following composition can simultaneously and to a high degree solve all of the above problems.

That is, one embodiment of the present invention relates to an aqueous inkjet ink as shown in [1] to [5] below, and a printed matter produced using the aqueous inkjet ink as shown in [6] below.
[1] An aqueous inkjet ink comprising a pigment, a surfactant (A), a water-soluble organic solvent, and a binder resin,
The surfactant (A) contains a compound (A-1) represented by the following general formula 1, and a nonionic surfactant (A-2) having an HLB value of 1 to 10 (excluding the compound (A-1)),
The water-based inkjet ink, wherein the water-soluble organic solvent comprises hexylene glycol.

R ¹ -(O-CH ₂ -CH ₂ ) _n -OH General formula 1
(In the general formula 1, ^R1 represents an alkyl group having 10 to 25 carbon atoms which may be branched, and n is an integer of 20 to 100.)

[2] The aqueous inkjet ink according to [1], wherein the water-soluble organic solvent further contains a diol having 2 to 5 carbon atoms.
[3] The aqueous inkjet ink according to [1] or [2], wherein the water-soluble organic solvent further contains a compound represented by the following general formula 2:

R ² -(O-CH(CH ₃ )-CH ₂ ) _m -OH General formula 2
(In the general formula 2, ^R2 represents an alkyl group having 2 to 4 carbon atoms which may be branched, and m is 1 or 2.)

[4] The aqueous inkjet ink according to any one of [1] to [3], wherein the ratio of the mass content of the compound (A-1) to the mass content of the nonionic surfactant (A-2) [compound (A-1):surfactant (A-2)] is from 1:0.5 to 1:20.
[5] The aqueous inkjet ink according to any one of [1] to [4], wherein the ratio of the sum of the mass content of the hexylene glycol and the mass content of the compound (A-1) to the mass content of the nonionic surfactant (A-2) [(hexylene glycol + compound (A-1)):surfactant (A-2)] is 1:1 to 14:1.
[6] A printed matter obtained by printing the aqueous inkjet ink according to any one of [1] to [5] on a printing substrate.

The aqueous inkjet ink, which is one embodiment of the present invention, produces prints that are free of white spots and color bleeding and have excellent water resistance, even when printed on poorly permeable and non-permeable substrates, and also has the effect of providing excellent ejection stability immediately after the start of printing and after a long period of printing pause.

The following describes an aqueous inkjet ink according to one embodiment of the present invention (hereinafter, simply referred to as "aqueous inkjet ink according to 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 having the above-mentioned configuration has excellent wettability on the order of μs, and even when printing on poorly permeable and non-permeable substrates, it is possible to obtain printed matter with excellent print quality without whiteout or color bleeding. Furthermore, stable ejection is possible even immediately after printing has started, and even after a long period of printing has been suspended. In addition, the printed matter also has excellent water resistance. Although the details of the mechanism are not clear, the inventors speculate as follows. However, the present invention is not limited by the following speculation.

As described above, in general, aqueous inkjet inks do not wet and spread on poorly permeable and non-permeable substrates due to the extremely high surface tension of the main component water, making it difficult to form fine printed matter. In contrast, when a surfactant is used, the surfactant is oriented at the interface with the substrate in a short time, such as on the order of μs. As a result, it is possible to wet and spread well on poorly permeable and non-permeable substrates, particularly substrates with very low interfacial free energy such as polypropylene (PP) films, and improvement in print quality is expected. This effect is particularly effective with surfactants with low solubility in water. On the other hand, there are problems with surfactants, such as the tendency for bubbles to form, which can be a cause of nozzle omissions, and the use of surfactants with poor solubility in water can cause orientation at the air-liquid interface formed on the nozzle end surface of the inkjet head, which can destabilize the meniscus of the aqueous inkjet ink. The air bubbles lead to a deterioration in standby ejection properties, and the instability of the meniscus leads to a deterioration in initial ejection stability.

In order to produce a printed matter with excellent water resistance, it is necessary to use a binder resin with a large mass average molecular weight to form a strong continuous film that does not dissolve in water. On the other hand, if such a binder resin is contained in the aqueous inkjet ink before printing (liquid) in a certain amount or more, the binder resin may cause a problem that the surfactant, which has low solubility in water, is partially inhibited from being oriented at the interface. In order to fully express the effect of the surfactant, there is a method of increasing the amount of surfactant added so that the surfactant is still sufficiently oriented at the interface even if it is inhibited. However, an excessive amount of surfactant also has the problem of inhibiting the formation of the continuous film, and increasing the amount of surfactant added deteriorates the water resistance of the printed matter.

The aqueous inkjet ink of this embodiment first contains, as surfactants, a nonionic surfactant (A-2) having an HLB value in a suitable range, and a compound (A-1) represented by the above general formula (1). Of these, the nonionic surfactant (A-2) corresponds to the surfactant with low solubility in water described above, and can impart excellent permeability and wettability to the aqueous inkjet ink on poorly permeable substrates and non-permeable substrates. On the other hand, the compound (A-1) has an alkyl chain with a suitable number of carbon atoms as the hydrophobic portion, and on the other hand, has a polyethylene oxide structure with a suitable added mole number as the hydrophilic portion.

When the nonionic surfactant (A-2) is used alone, there is a risk of nozzle leakage due to the generation of air bubbles and the instability of the meniscus of the aqueous inkjet ink near the nozzle, as described above. In addition, while the nonionic surfactant (A-2) has a high speed of orientation toward the air-liquid interface, the uniformity of the orientation is low. In particular, if a binder resin is present in the aqueous inkjet ink, the orientation described above may be hindered, causing the aqueous inkjet ink to wet and spread unevenly on the printing substrate, and there is a risk of blank spots occurring.

In contrast, the aqueous inkjet ink of this embodiment further uses compound (A-1). In the aqueous inkjet ink, the sufficiently large polyethylene oxide structure present in compound (A-1) has an affinity for water, while the alkyl chain has an affinity for surfactant (A-2). As a result, in the aqueous inkjet ink, the nonionic surfactant (A-2) is stabilized by compound (A-1). In addition, the speed of orientation to the interface is suppressed, while uniform orientation at the interface is possible. As a result, in the aqueous inkjet ink present in the inkjet head, the generation of air bubbles and destabilization of the meniscus are suppressed, and the initial ejection stability and standby ejection performance are improved. Furthermore, even after the liquid components of the aqueous inkjet ink on the printing substrate have evaporated, the surfactant (A-2) and the binder resin have an affinity via the compound (A-1), which prevents the individual materials from becoming non-uniform within the aqueous inkjet ink film (ink film) after drying, forming a uniform and continuous ink film, which is believed to make it possible to obtain printed matter with excellent water resistance.

Furthermore, in the aqueous inkjet ink of this embodiment, hexylene glycol is used as the water-soluble organic solvent. Hexylene glycol (2-methyl-2,4-pentanediol) has multiple branched alkyl groups and is more hydrophobic than other alkanediols, while it has high solubility in water and a low boiling point at 1 atmosphere. Therefore, it has a higher orientation to the interface than other alkanediols. In addition, it has a high affinity with the above-mentioned surfactant with low solubility in water (nonionic surfactant (A-2)), while dissolving well in water and promoting the orientation of the surfactant to the interface. As a result, in the presence of hexylene glycol and the above-mentioned compound (A-1), the nonionic surfactant (A-2), which is a surfactant with low solubility in water, can be uniformly present in the aqueous inkjet ink. In addition, it is believed that hexylene glycol increases the orientation speed of the nonionic surfactant (A-2) to the interface, compensating for the decrease in orientation speed caused by the addition of the above-mentioned compound (A-1), and when the aqueous inkjet ink lands on the substrate, the nonionic surfactant (A-2) is oriented to the interface in a short time. As a result, the aqueous inkjet ink droplets can be sufficiently wetted and spread, making it possible to obtain printed matter without white spots or color bleeding.

As a result of the above, it is believed that hexylene glycol and the above compound (A-1) contribute to the stabilization and homogenization of the nonionic surfactant (A-2), thereby preventing foaming and nozzle dropping, improving ejection stability, and enabling the production of printed matter with excellent print quality and water resistance.

Hexylene glycol can also dissolve binder resins well. Therefore, it is believed that the binder resin spreads sufficiently and can flow somewhat freely within the aqueous inkjet ink even after the water has evaporated from the aqueous inkjet ink applied to the substrate during drying. As a result, hexylene glycol can promote the formation of a continuous ink film. Furthermore, since hexylene glycol itself does not remain within the continuous film but eventually evaporates, highly water-resistant printed matter can be produced with low energy.

As described above, in order to solve all of the above-mentioned problems at a high level, an aqueous inkjet ink having the above-mentioned composition is essential.

The aqueous inkjet inks specifically disclosed in the above Patent Documents 1 to 3 differ from the aqueous inkjet ink of the present embodiment in that they do not contain the compound (A-1) represented by the above general formula 1 (all compounds used in Patent Documents 1 to 3 are compounds in which n in the above general formula 1 is less than 20), and do not use hexylene glycol. Furthermore, Patent Documents 1 to 3 make no mention of the above-mentioned mechanism, i.e., that hexylene glycol and the above compound (A-1) contribute to the stabilization and homogenization of the nonionic surfactant (A-2), thereby realizing improvements in initial ejection stability and standby ejection properties, as well as the water resistance of printed matter.

Next, we will explain in detail each component that makes up the aqueous inkjet ink, which is one embodiment of the present invention.

<Surfactant (A)>
The aqueous inkjet ink of this embodiment contains, as the surfactant (A), a compound (A-1) represented by general formula 1, and a nonionic surfactant (A-2) having an HLB value of 1 to 10.

≪Compound (A-1)≫
As described above, the compound (A-1) stabilizes the nonionic surfactant (A-2), so that the generation of bubbles and the destabilization of the meniscus are suppressed, and the initial ejection stability and standby ejection stability are improved. In addition, the compound (A-1) has an affinity with the surfactant (A-2) and the binder resin, resulting in a uniform and continuous ink film, and improving the water resistance of the printed matter. In the presence of the compound (A-1) and hexylene glycol, the nonionic surfactant (A-2) can be uniformly present in the aqueous inkjet ink. Since the inkjet ink droplets can sufficiently wet and spread, it is possible to obtain printed matter that is free of white spots and color bleeding.

From the viewpoint of stabilizing the nonionic surfactant (A-2) and facilitating affinity between the surfactant (A-2) and the binder resin, thereby improving the initial ejection stability, standby ejection property, and water resistance of the printed matter, n in general formula 1 is an integer from 20 to 100, more preferably an integer from 25 to 50, and particularly preferably an integer from 25 to 50.

From the same viewpoint as that of the value of n in the above-mentioned general formula 1, the group represented by R ¹ in general formula 1 is an alkyl group having 10 to 25 carbon atoms, which may have a branch, and more preferably an alkyl group having 12 to 22 carbon atoms, which may have a branch. In one embodiment, the affinity with hexylene glycol, which is a compound having a branched structure, can be improved, and the nonionic surfactant (A-2) can be further homogenized, so that a printed matter having no white spots and no mixed color bleeding and particularly excellent water resistance can be obtained. From the viewpoint of this, the group represented by R ¹ in general formula 1 is preferably an alkyl group having 10 to 22 carbon atoms, which has a branch, and particularly preferably an alkyl group having 11 to 16 carbon atoms, which has a branch.

The compound (A-1) may be obtained by synthesis using a conventional method, or a commercially available product may be used. Examples of commercially available products of compound (A-1) include the Emulgen series manufactured by Kao Corporation, the Nonion series manufactured by NOF Corporation, the EMALEX series manufactured by Nippon Emulsion Co., Ltd., the NIKKOL series manufactured by Nikko Chemicals Co., Ltd., the Emulmin series and Sanonic series manufactured by Sanyo Chemical Industries Co., Ltd., and the Brownon series and Finesurf series manufactured by Aoki Oil Industries Co., Ltd., but the commercially available products that can be used as compound (A-1) are not limited to these.

The content of compound (A-1) in the aqueous inkjet ink is preferably 0.05 to 2% by mass, and more preferably 0.1 to 1% by mass. If the content is 0.05% by mass or more, the above-mentioned effects can be reliably achieved, and if the content is 2% by mass or less, it is easy to suppress white spots and color mixing in printed matter, and to improve initial ejection stability and standby ejection performance.

<Nonionic surfactant (A-2)>
The surfactant (A-2) has an HLB value of 1 to 10. The HLB (Hydrophile-Lipophile Balance) value is one of the parameters that indicate the hydrophilicity/hydrophobicity of a material. Various methods are known for calculating the HLB value, such as the Griffin method, the Davis method, and the Kawakami method, but in the present application, the HLB value is calculated using the Griffin method.

The Griffin method is generally used for non-ionic materials. In the Griffin method, the HLB value is calculated by the following formula 3 using the molecular weight of the target material. The smaller the HLB value, the more hydrophobic the material is, and the larger the HLB value, the more hydrophilic the material is.

HLB value = 20 x (sum of molecular weights of hydrophilic parts) ÷ (molecular weight of material) Equation 3

The nonionic surfactant (A-2) is not particularly limited as long as it is a surfactant with an HLB value of 1 to 10 and does not correspond to the compound (A-1). On the other hand, from the viewpoint of obtaining a printed matter without white voids and color mixing even on a printing substrate with poor permeability and excellent initial ejection stability, it is preferable that the surfactant (A-2) contains an acetylene diol surfactant and/or a silicone surfactant, and from the viewpoint of obtaining an aqueous inkjet ink with particularly excellent initial ejection stability and standby ejection properties, it is most preferable to use a silicone surfactant.

Acetylene diol surfactants are preferably used because they have excellent orientation speed to the interface, improve the wettability of the aqueous inkjet ink, and make it easier to obtain printed matter without white spots or color bleeding. When an acetylene diol surfactant is used as the nonionic surfactant (A-2), the HLB value is preferably 1 to 8, and more preferably 1 to 4. Specific examples of acetylene diol surfactants having an HLB value of 1 to 4 include, for example, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, hexadeca-8-yne-7,10-diol, 4,7-dipropyl-dec-5-yne-4,7-diol, 6,9-dimethyl-tetradec-7-yne-6,9-diol, 3,6-diisopropyl-2,4-diol, 5,7-di ... ,7-dimethyloct-4-yne-3,6-diol, octadec-9-yne-8,11-diol, 7,10-dimethylhexadec-8-yne-7,10-diol, 5,8-dibutyldodec-6-yne-5,8-diol, 4,7-diisobutyl-2,9-dimethyl-dec-5-yne-4,7-diol, 5,14-diethyl-8,11-dimethyloctadec-9-yne-8,11-diol, and the like. Among these, from the viewpoint of being able to impart sufficient wettability to the aqueous inkjet ink even on a printing substrate with poor permeability and obtaining a printed matter with excellent print quality, it is preferable to use one or more compounds selected from the group consisting of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol, hexadeca-8-yne-7,10-diol, and 2,4,7,9-tetramethyl-5-decyne-4,7-diol. The above compounds may be used alone or in combination of two or more. The above compounds may be synthesized by a conventionally known method or may be commercially available products.

The amount of acetylene diol surfactant added is preferably 0.1 to 3 mass% of the total amount of the aqueous inkjet ink, more preferably 0.5 to 2 mass%, and even more preferably 0.8 to 1.5 mass%.

On the other hand, silicone surfactants have a slower orientation speed to the interface than acetylene diol surfactants, but have a high ability to reduce surface tension and are uniformly oriented to the interface, so they can be used preferably from the viewpoint of preventing color bleeding in printed matter and improving initial ejection stability. In this application, the HLB value of the silicone surfactant is also calculated using the Griffin method.

When using a silicon-based surfactant as the nonionic surfactant (A-2), it is preferable to use a gemini type silicon-based surfactant and/or a polyether-modified silicon-based surfactant (excluding gemini type silicon-based surfactants). Furthermore, the amount of silicon-based surfactant added is preferably 0.1 to 5 mass% of the total amount of ink, more preferably 0.5 to 3 mass%, and even more preferably 0.8 to 2.5 mass%.

<Gemini type silicone 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 silicon surfactants, for example, a siloxane chain (hydrophobic structure represented by -[SiR ³ R ⁴ -O] _x -, where R ³ and R ⁴ are each an arbitrary organic group, and x is an integer of 2 or more) and a 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 ^R3 and/or ^R4 in the structural formula of the siloxane chain above is an organic group containing a siloxane chain).
A structure in which a plurality of silicone surfactants, each having a plurality of hydrophilic structures, share at least a portion of the hydrophilic structures.

Gemini type surfactants have superior surface tension reducing ability compared to general surfactants. Therefore, by using a gemini type silicone surfactant, it is possible to achieve a better reduction in surface tension than with general silicone surfactants. As a result, it is possible to significantly improve the wettability of aqueous inkjet inks containing gemini type silicone surfactants, making it easier to improve the above-mentioned white voids and color bleeding.

Commercially available examples of Gemini type silicone surfactants include TEGO Twin 4000, TEGO Twin 4100, and TEGO Twin 4200 manufactured by Evonik Degussa, and KF-6100, KF-6104, KF-6105, KF-6106, and KF-6115 manufactured by Shin-Etsu Chemical Co., Ltd.

<Polyether-modified silicone surfactants (excluding Gemini silicone surfactants)>
The polyether-modified silicone surfactant (excluding gemini type silicone surfactants) that can be used in the aqueous inkjet ink of this embodiment includes compounds having a structure represented by the following general formula 4.

In general formula 4, p is an integer from 0 to 99, and q is an integer from 1 to 100. However, p+q is an integer from 1 to 100. Furthermore, R ⁵ is a methyl group or a structure represented by the following general formula 5, and R ⁶ is an alkyl group having 1 to 6 carbon atoms or a structure represented by the following general formula 5. However, when R ⁵ is a methyl group, p is 0. Furthermore, at least one of R ⁵ and R ⁶ has a structure represented by the following general formula 5 (R ⁵ and R ⁶ may both have a structure represented by the following general formula 5).

In general formula 5, r is an integer from 1 to 6, s is an integer from 1 to 50, and t is an integer from 0 to 50, where s+t is an integer from 1 to 100. ^R7 is either a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an acrylic group, or a methacrylic group. The addition pattern of the ethylene oxide groups and propylene oxide groups in the brackets [ ] may be block or random.

The polyether-modified silicone surfactant represented by the above general formula 4 is particularly suitable for use in reducing color bleeding.

Examples of commercially available polyether-modified silicone surfactants in which R ⁶ in the above general formula 4 is a structure represented by general formula 5 and R ⁵ is not a structure represented by general formula 5 include BY16-201 and SF8427 manufactured by Dow Corning Toray Co., Ltd., BYK-331, BYK-333, BYK-UV3500, and BYK-3420 manufactured by BYK-Chemie Co., Ltd., TEGO Glide 410, TEGO Glide 432, TEGO Glide 435, TEGO Glide 440, and TEGO Glide 450 manufactured by Evonik Degussa Co., Ltd., and Silface SWP-001, Silface SAG003, and Silface SAG005 manufactured by Nissin Chemical Industry Co., Ltd.

In addition, examples of commercially available polyether-modified silicone surfactants in which R ⁵ in the above general formula 4 is a structure represented by general formula 5 and R ⁶ is not a structure represented by general formula 5 include SF8428, FZ-2162, 8032 ADDITIVE, SH3749, FZ-77, L-7001, L-7002, FZ-2104, FZ-2110, F-2123, SH8400, and SH3773M manufactured by Dow Corning Toray Co., Ltd., BYK-345, BYK-346, BYK-347, BYK-348, and BYK-349 manufactured by BYK-Chemie Co., Ltd., and TEGO Wet 240, TEGO Wet 250, and TEGO Wet 310 manufactured by Evonik Degussa Co., Ltd. 260, TEGO Wet 270, TEGO Wet 280, and KF-351A, KF-352A, KF-353, KF-354L, KF355A, KF-615A, KF-640, KF-642, and KF-643 manufactured by Shin-Etsu Chemical Co., Ltd.

The ratio of the content of the compound (A-1) represented by general formula 1 used in the aqueous inkjet ink of this embodiment to the content of the nonionic surfactant (A-2) having an HLB value of 1 to 10 (compound (A-1):surfactant (A-2)) is preferably 1:0.5 to 1:20 by mass. The above ratio is more preferably 1:1.5 to 1:15, and even more preferably 1:2 to 1:10. By blending them in the above-mentioned suitable ratio, the nonionic surfactant (A-2) can be sufficiently stabilized by the compound (A-1), and a printed matter having excellent water resistance can be obtained while suppressing whiteout and color bleeding in the printed matter.

In addition, since the nonionic surfactant (A-2) can be homogenized in the aqueous inkjet ink and the droplets of the aqueous inkjet ink can be sufficiently wetted and spread, it is possible to obtain a printed matter free of white spots and color bleeding. Therefore, the ratio of the total content of the hexylene glycol and the content of the compound (A-1) to the content of the nonionic surfactant (A-2) [(hexylene glycol + compound (A-1)): surfactant (A-2)] is preferably 1:1 to 14:1 by mass, more preferably 2:1 to 12:1, and particularly preferably 3:1 to 11:1.

<Water-soluble organic solvent>
The aqueous inkjet ink of this embodiment contains a water-soluble organic solvent. As described above, the water-soluble organic solvent contains hexylene glycol.

<Hexylene glycol>
From the viewpoints of obtaining good ejection stability and good print image quality of the printed matter, the content of hexylene glycol in the aqueous inkjet ink of this embodiment is preferably from 0.1 to 25 mass %, more preferably from 0.6 to 18 mass %, and particularly preferably from 1 to 10 mass %, of the total amount of the aqueous inkjet ink.

The content of the hexylene glycol is preferably 5 to 90% by mass, more preferably 10 to 60% by mass, and particularly preferably 10 to 40% by mass, based on the total amount of the water-soluble organic solvent contained in the aqueous inkjet ink. By setting the content of hexylene glycol relative to the total amount of the water-soluble organic solvent within the above range, the time required for stabilization at the interface of the nonionic surfactant (A-2) can be optimized. As a result, ejection stability is improved immediately after the start of printing and during continuous printing at high speed, and at the same time, wettability to the printing substrate on the order of μs is improved, thereby improving the print image quality.

<Diols having 2 to 5 carbon atoms>
The aqueous inkjet ink of this embodiment may further contain a diol having 2 to 5 carbon atoms. By containing a diol having 2 to 5 carbon atoms, the nonionic surfactant (A-2) can be further stabilized, and standby discharge properties can be improved. In addition, when the aqueous inkjet ink applied to a printing substrate dries, the binder resin is dissolved together with hexylene glycol, causing an increase in the viscosity of the aqueous inkjet ink, thereby suppressing color mixing bleeding. Examples of the diols having 2 to 5 carbon atoms include alkanediols having 2 to 5 carbon atoms, such as 1,2-ethanediol (ethylene glycol), 1,2-propanediol (propylene glycol), 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2-methyl-1,3-propanediol, and 3-methyl-1,3-butanediol, as well as polyoxyalkylene diols having 2 to 5 carbon atoms, such as diethylene glycol and hydroxyethoxypropanol. These compounds may be used alone or in combination of two or more.

Among these, from the viewpoints of good ejection stability after a long period of printing suspension and suppression of color bleeding in printed matter, it is preferable to use alkanediols having 2 to 5 carbon atoms, and it is more preferable to use one or more selected from the group consisting of 1,2-ethanediol, 1,2-propanediol, and 1,3-butanediol.
On the other hand, from the viewpoint that the affinity with the compound (A-1) is improved, the compound (A-1) can assist in the uniformization of the nonionic surfactant (A-2) and the binder resin in the ink film, and the uniformity and continuity of the ink film are improved, thereby improving the water resistance of the printed matter, diols having 3 to 5 carbon atoms in which hydroxyl groups are added to adjacent carbon atoms are preferred. From this viewpoint, it is preferred to use one or more diols having 2 to 5 carbon atoms selected from the group consisting of 1,2-propanediol, 1,2-butanediol, 2,3-butanediol, and 1,2-pentanediol.
It is particularly preferable to use 1,2-propanediol, which satisfies both of the above-mentioned two requirements and further has good ejection stability immediately after the start of printing and even during continuous printing.

In order to ensure good ejection stability under all conditions, including immediately after printing begins, after a long period of printing pause, and during continuous printing, the content of the diols having 2 to 5 carbon atoms is preferably 0.5 to 30% by mass, more preferably 2 to 25% by mass, and particularly preferably 6 to 22% by mass, of the total amount of the aqueous inkjet ink.

In addition, when the aqueous inkjet ink applied to the printing substrate dries, both hexylene glycol and the diols having 2 to 5 carbon atoms contribute to dissolving the binder resin, softening the pigment dispersion resin, and stabilizing the nonionic surfactant (A-2), thereby obtaining printed matter that is free of white voids and color bleeding and has excellent water resistance, and furthermore, resulting in an aqueous inkjet ink with good ejection stability.From this viewpoint, when the content of the hexylene glycol is taken as 1, the content of the diols having 2 to 5 carbon atoms is preferably 1 to 6, more preferably 1.5 to 5.5, and particularly preferably 2 to 5.

<Specific (poly)oxypropylene monoalkyl ethers>
The aqueous inkjet ink of this embodiment may contain a compound represented by the above general formula 2 (referred to as "specific (poly)oxypropylene monoalkyl ethers" in this application). The specific (poly)oxypropylene monoalkyl ethers have a structure similar to that of the compound (A-1) and can contribute to the stabilization of the nonionic surfactant (A-2). In addition, the specific (poly)oxypropylene monoalkyl ethers themselves have a moderately low surface tension, which makes it easy to improve the initial ejection stability and prevent blank spots in printed matter. In addition, the specific (poly)oxypropylene monoalkyl ethers can assist the compound (A-1) in homogenizing the nonionic surfactant (A-2) and the binder resin in the ink film, thereby improving the uniformity and continuity of the ink film. Furthermore, the specific (poly)oxypropylene monoalkyl ethers also function as a film-forming assistant for the binder resin. From the above, the use of specific (poly)oxypropylene monoalkyl ethers significantly improves the water resistance of printed matter.

Specific (poly)oxypropylene monoalkyl ethers that can be used include, for example, propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, propylene glycol monoisopropyl ether, propylene glycol mono-n-butyl ether, propylene glycol monoisobutyl ether, propylene glycol mono-tert-butyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol monoisopropyl ether, dipropylene glycol mono-n-butyl ether, dipropylene glycol monoisobutyl ether, and dipropylene glycol mono-tert-butyl ether. These compounds may be used alone or in combination of two or more.

Among these compounds, it is preferable to use a compound in which R 2 in the general formula 2 is an unbranched alkyl group having ^{2 or 3} carbon atoms, because it has a high affinity with the compound (A-1) and has a suitable boiling point and surface tension at 1 atmospheric pressure, thereby improving the ejection stability immediately after the start of printing, improving the water resistance of the printed matter, and preventing white spots. Among the compounds listed above, examples that satisfy these requirements include propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, dipropylene glycol monoethyl ether, and dipropylene glycol mono-n-propyl ether. In addition, it is particularly preferable to use a compound in which R 2 in the general formula 2 is an n-propyl group, i.e., propylene glycol mono-n-propyl ether and/or dipropylene glycol mono-n-propyl ether, because the affinity with hexylene glycol is also improved, thereby promoting the uniformity and continuous formation of the ink film, thereby significantly improving the water resistance of the printed matter, and suppressing color bleeding in the printed ^matter . In particular, from the viewpoint of improving standby dischargeability, it is particularly preferable to use dipropylene glycol monopropyl ether as the specific (poly)oxypropylene monoalkyl ether.

In order to achieve both the effects of improving initial ejection stability, preventing blank spots in printed matter, and improving water resistance, and in order to provide good ejection stability under all conditions, including immediately after printing begins, after a long period of printing pause, and during continuous printing, the content of the specific (poly)oxypropylene monoalkyl ethers is preferably 0.2 to 7% by mass, and particularly preferably 0.5 to 5% by mass, of the total amount of the aqueous inkjet ink.

<Other water-soluble organic solvents>
The aqueous inkjet ink of this embodiment may contain water-soluble organic solvents other than the above-mentioned hexylene glycol and the above-mentioned diols having 2 to 5 carbon atoms (referred to as "other water-soluble organic solvents" in the present application).

In the aqueous inkjet ink of this embodiment, the other water-soluble organic solvents may be, for example, alkanediols having 6 carbon atoms (excluding hexylene glycol); alkanetriols (having 3 to 6 carbon atoms); polyoxyalkylene diols (having oxyalkylene groups which are oxyethylene groups and/or oxypropylene groups and which have 2 to 4 oxyalkylene groups and which have 2 to 5 carbon atoms); (poly)oxyalkylene monoalkyl ethers (having oxyalkylene groups which are oxyethylene groups or oxypropylene groups and which have 1 to 4 oxyalkylene groups and which have a terminal alkyl group with 1 to 4 carbon atoms (compounds corresponding to the above-mentioned specific (poly)oxypropylene monoalkyl ethers). (excluding those in which the oxyalkylene group is an oxybutylene group or an oxypentylene group, the number of said oxyalkylene group is 1, and the number of carbon atoms in the terminal alkyl group is 1 to 4); (poly)oxyethylene dialkyl ethers (provided that the number of oxyalkylene groups is 1 to 4, and the number of carbon atoms in each of the terminal alkyl groups is 1 to 4); lactams (provided that the number of atoms constituting the lactam ring is 5 to 7. In addition, an alkyl group having 1 to 2 carbon atoms, a hydroxyalkyl group having 1 to 2 carbon atoms, or a vinyl group may be bonded to the nitrogen atom and/or carbon atom constituting the lactam ring); alkanolamines (provided that the number of amino groups is 1, the number of hydroxyl groups is 1 to 3, and the number of carbon atoms is 3 to 9); and the like can be used. These other water-soluble organic solvents may be used alone or in combination of two or more kinds.
In the present application, the terms "(poly)oxyalkylene" and "(poly)oxypropylene" respectively mean "oxyalkylene and/or polyoxyalkylene" and "oxypropylene and/or polyoxypropylene".

The total content of the water-soluble organic solvents contained in the aqueous inkjet ink of this embodiment is preferably 3 to 33% by mass, more preferably 5 to 30% by mass, and particularly preferably 7 to 30% by mass, of the total amount of the aqueous inkjet ink. By keeping the total content of the water-soluble organic solvents within the above range, it is possible to maintain an appropriate viscosity for ejection as an inkjet ink, and it is possible to achieve good ejection stability even after a long period of printing suspension, and it is possible to dry with low energy and to produce a printed matter with good water resistance.

In addition, in the aqueous inkjet ink of this embodiment, the content of the water-soluble organic solvent having a boiling point of 220°C or higher at 1 atmosphere is preferably 5% by mass or less (or may be 0% by mass) of the total amount of the aqueous inkjet ink. By not including a water-soluble organic solvent having a boiling point of 220°C or higher, or by keeping the amount of the water-soluble organic solvent within the above range if it is included, good color bleeding can be achieved, for example, even in high-speed printing.

Furthermore, it is preferable that 90% by mass or more of the total content of the water-soluble organic solvent contained in the aqueous inkjet ink of this embodiment is a water-soluble organic solvent having a static surface tension of 25 to 50 mN/m at 25° C. Furthermore, it is more preferable that 70% by mass or more of the total content of the water-soluble organic solvent is a water-soluble organic solvent having a static surface tension of 26 to 40 mN/m at 25° C., and it is particularly preferable that 90% by mass or more of the total content of the water-soluble organic solvent is a water-soluble organic solvent having a static surface tension of 26 to 40 mN/m at 25° C. By using a water-soluble organic solvent having the above surface tension, it is possible to maintain a suitable surface tension for ejection as an inkjet ink, and for example, it is possible to improve the ejection stability even after a long period of printing suspension. In addition, it is possible to assist the aqueous inkjet ink in spreading uniformly on the printing substrate, and a printed matter having no white spots and excellent water resistance is obtained.
The static surface tension of the water-soluble organic solvent at 25° C. can be measured in the same manner as the static surface tension of the aqueous inkjet ink described below.

<Pigments>
The water-based inkjet ink of this embodiment contains a pigment.

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

The content of the pigment contained in the aqueous inkjet ink of this embodiment is adjusted depending on the use of the printed matter produced using the aqueous inkjet ink, but is preferably, for example, 0.5 to 30% by mass of the total amount of the aqueous inkjet ink. In addition, except for the case of a white aqueous inkjet ink (aqueous white ink), the content of the pigment is more preferably 1 to 15% by mass, and particularly preferably 1.5 to 10% by mass, in order to obtain a printed matter with high density without deteriorating the discharge stability of the aqueous inkjet ink. On the other hand, in the case of an aqueous white ink, the content of the pigment is more preferably 5 to 25% by mass, and particularly preferably 10 to 20% by mass, in order to obtain a printed matter with high hiding power without deteriorating the discharge stability of the aqueous white ink.

<Pigment Dispersion Resin>
The aqueous inkjet ink of the present embodiment may contain a resin (pigment dispersion resin) used for pigment dispersion purposes. Compared with a dispersion containing a pigment dispersed without using a pigment dispersion resin (a dispersion of a self-dispersed pigment, a dispersion of a pigment dispersed with a surfactant, etc.), a pigment dispersed using a pigment dispersion resin has excellent dispersion stability, and therefore the aqueous inkjet ink has excellent initial ejection stability and standby ejection properties. In addition, from the viewpoint of improving the water resistance of the printed matter, it is preferable to select a pigment dispersion resin. Furthermore, from the viewpoint of suppressing the detachment of the pigment dispersion resin from the pigment, the liberated pigment dispersion resin is adsorbed with the compound (A-1) and the nonionic surfactant (A-2), and the initial ejection stability, standby ejection properties, and the print image quality of the printed matter can be suppressed from being deteriorated, the pigment dispersion resin is preferably a polymer having a crosslinked structure and/or a block polymer.

The type of pigment dispersion resin is not particularly limited, and for example, acrylic resin, styrene resin, maleic acid (anhydride) resin, urethane resin, polyester resin, polyolefin resin, etc. can be used as desired. These resins may be used alone or in combination of two or more. Among them, it is preferable to use one or more selected from the group consisting of acrylic resin, maleic acid (anhydride) resin, and urethane resin, in terms of the ability to improve initial ejection stability and standby ejection, the large material selectivity, and ease of resin synthesis. Furthermore, acrylic resin and/or maleic acid (anhydride) resin are particularly preferable because they have a high affinity with compound (A-1) and the pigment is less likely to become non-uniform in the aqueous inkjet ink, so that the uniformity and continuous film formation of the ink film is not hindered by the pigment, improving the water resistance of the printed matter, and the initial ejection stability and standby ejection properties of the aqueous inkjet ink can be improved.

In this application, "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 (styrene and/or styrene derivatives may also be used). However, resins containing maleic acid (anhydride) (at least one selected from "maleic acid" and "maleic anhydride") as a polymerizable monomer are excluded from "acrylic resin". Furthermore, "maleic acid (anhydride) resin" refers to a resin using at least maleic acid (anhydride) as a polymerizable monomer. Furthermore, maleic acid (anhydride) resin may use one or more polymerizable monomers selected from the group consisting of α-olefins, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, and styrene derivatives.

The pigment dispersion resin may be synthesized by a known method or may be a commercially available product. There are no particular limitations on the structure, and resins having, for example, a random structure, a block structure, a comb structure, a star structure, etc. may be used. Furthermore, either a water-soluble resin or a water-insoluble resin may be selected as the pigment dispersion resin.

In this application, 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."

When a water-soluble resin is used as the pigment dispersion resin, its acid value is preferably 50 to 300 mgKOH/g, more preferably 60 to 250 mgKOH/g. Particularly preferably 70 to 160 mgKOH/g. By setting the acid value within the above range, it is possible to maintain the dispersion stability of the pigment, and stable ejection from the inkjet head can be achieved regardless of the conditions of use. In addition, the solubility of the pigment dispersion resin in water can be ensured, and the interaction between the pigment dispersion resins is favorable, so that the viscosity of the pigment dispersion liquid can be suppressed, which is also preferable in terms of achieving good ejection properties from the inkjet head.

On the other hand, when a water-insoluble resin is used as the pigment dispersion resin, its acid value is preferably 0 to 100 mgKOH/g, more preferably 5 to 90 mgKOH/g, and even more preferably 10 to 80 mgKOH/g. If the acid value is within the above range, a printed matter with excellent water resistance can be obtained.

In this application, the "acid value of a resin" refers to the number of milligrams of potassium hydroxide (KOH) required to neutralize the acid groups contained in 1 g of the resin. In this application, the acid value is calculated by the following method. For example, if a resin has na acid groups with a value of va per molecule and contains Wa mass % of polymerizable monomers with a molecular weight of Ma among the polymerizable monomers that make up the resin, the acid value (mg KOH/g) can be calculated by the following formula 6.

(Acid value) = {(va x na x Wa) ÷ (100 x Ma)} x 56.11 x 1000 Formula 6

In the above formula 6, the number "56.11" is the molecular weight of potassium hydroxide.

In the aqueous inkjet ink of this embodiment, it is preferable to introduce an aromatic group into the pigment dispersion resin from the viewpoint of improving the adsorption ability to the pigment and ensuring the dispersion stability of the pigment and the ejection stability of the aqueous inkjet ink. Examples of aromatic groups include, but are not limited to, phenyl groups, naphthyl groups, anthryl groups, tolyl groups, xylyl groups, mesityl groups, and anisyl groups. Among these, phenyl groups, naphthyl groups, and tolyl groups are preferable from the viewpoint of ensuring sufficient dispersion stability and ejection stability.

From the viewpoint of improving the dispersion stability and ejection stability of the pigment, as well as the print quality and drying properties of the printed matter, the amount of the aromatic ring-containing monomer introduced is preferably 5 to 75% by mass, more preferably 10 to 65% by mass, and even more preferably 15 to 55% by mass, based on the total amount of monomers constituting the pigment dispersion resin.

<Binder resin>
The aqueous inkjet ink of the present embodiment contains a binder resin. After printing the aqueous inkjet ink containing the binder resin on a poorly permeable substrate or a non-permeable substrate, the viscosity increases at a speed of the order of μs, suppressing bleeding and improving print quality. In addition, when the aqueous inkjet ink dries, the binder resin forms a continuous film, improving the water resistance of the printed matter.

The binder resin contained in the aqueous inkjet ink of this embodiment may be a water-soluble resin or resin fine particles. A combination of a water-soluble resin and resin fine particles may also be used.

In this application, "resin microparticles" refers to the above-mentioned water-insoluble resins that are dispersed in particulate form in water and have a volume-based median diameter (also referred to as "D50" in this application) of 10 to 1,000 nm. In this application, D50 is a value measured in a 25°C environment using a dynamic light scattering particle size distribution measuring device such as Microtrac-Bell's Nanotrac UPA-EX150.

When a water-soluble resin is used as the binder resin, the weight average molecular weight of the binder resin is preferably 1,000 to 25,000, and more preferably 5,000 to 20,000. By using a binder resin having the above weight average molecular weight, a strong continuous film is formed even with a short drying time, and water resistance is improved.

In the present application, the mass average molecular weight of a compound is a polystyrene equivalent value that can be measured by a method in accordance with JIS K 7252. Examples of specific measurement conditions are shown below.
・Apparatus used: Tosoh Corporation "HLC-8320GPC"
Columns used: TSKgel (registered trademark) SuperMultiporeHZ-M (3 columns)
Column temperature: 40°C
Developing solvent: tetrahydrofuran Flow rate: 0.6 mL/min Sample solution concentration: 0.1% by mass
Sample solution injection volume: 10 μL

As the type of the binder resin, any of acrylic resins, styrene resins, maleic acid (anhydride) resins, urethane resins, polyester resins, vinyl chloride resins, vinyl chloride-vinyl acetate resins, etc. These resins may be used alone or in combination of two or more.
Among these, it is preferable to use one or more selected from the group consisting of acrylic resin, urethane resin, and polyester resin as the binder resin.Furthermore, from the viewpoint of achieving both adhesion and abrasion resistance to soft absorbent substrates and non-absorbent substrates (for example, film substrates such as PP and PET described later) and improving ejection stability, it is preferable to use acrylic resin as the binder resin.In this case, it is particularly preferable that the amount of the acrylic resin is 50% by mass or more based on the total amount of binder resin in the aqueous inkjet ink.

The glass transition temperature (Tg) of the binder resin is preferably 50 to 120°C, more preferably 60 to 110°C, and particularly preferably 70 to 100°C. By using a binder resin having the above glass transition temperature, even when printing on a printing substrate with low heat resistance and drying the aqueous inkjet ink on the printing substrate at a low temperature of about 70°C, the binder resin forms a solid film and/or the binder resin molecular chains become entangled with each other, improving the abrasion resistance and water resistance of the printed matter. Furthermore, since the binder resin molecular chains do not become entangled with each other during ejection, stable ejection is possible even after being left to stand for a long period of time.

The glass transition temperature of a binder resin can be measured by a method conforming to JIS K 7121. Specifically, about 10 mg of a sample of the binder resin in question is placed in an aluminum sample pan whose mass has been measured in advance, and after the mass is measured again, the pan is sealed with a lid. Next, this sample container and a sample pan made without the binder resin are set in a holder in a Shimadzu Corporation "DSC-60" (differential scanning calorimeter), and measurements are performed under conditions of a temperature rise of 10°C/min to obtain a DSC chart. The intersection of the low-temperature baseline and the tangent at the inflection point of the baseline is then determined, and the temperature of the intersection is taken as the glass transition temperature. Indium is used for temperature calibration.

For acrylic resins, the value calculated using the following formula 7 can be used as the glass transition temperature.

1/Tg = Σ(Wn/Tgn) Equation 7

In the above formula 7, Tg represents the glass transition temperature (K) of the resin, Wn represents the mass fraction of the structural unit consisting of the polymerizable monomer n constituting the above resin, and Tgn represents the glass transition temperature (K) of the homopolymer consisting of each of the polymerizable monomers n. For the above Tgn, the values described in, for example, "Polymer Handbook (4th Edition)" (Wiley, 1998) can be used.

The acid value of the binder resin is preferably 0 to 100 mgKOH/g, more preferably 0 to 80 mgKOH/g. It is particularly preferably 10 to 60 mgKOH/g. By setting the acid value within the above range, even if a portion of the aqueous inkjet ink dries near the nozzle of the inkjet head, it is possible to suppress a significant increase in viscosity of the aqueous inkjet ink, thereby improving the ejection stability.

The preferred content of the binder resin contained in the aqueous inkjet ink of this embodiment is preferably 1 to 25% by mass, more preferably 2 to 20% by mass, and particularly preferably 5 to 18% by mass, of the total amount of the aqueous inkjet ink, from the viewpoints of improving standby ejection properties and preventing color bleeding in printed matter.

In addition, the aqueous inkjet ink of this embodiment preferably has a ratio of the sum ([g]) of the pigment dispersion resin content and the binder resin content to the pigment content (WP [g]) contained in 100 g of the aqueous inkjet ink, i.e., a value expressed as WR/WP, of 1 to 4. By setting the value expressed as WR/WP within the range of 1 to 4, the amount of pigment that may become discontinuous points when forming a continuous film when the aqueous inkjet ink dries can be adjusted within a suitable range, making it possible to obtain printed matter with excellent water resistance. Furthermore, the ejection stability of the aqueous inkjet ink is improved.

<Wax>
The aqueous inkjet ink of this embodiment preferably contains a wax. In addition, it is preferable to use polyolefin resin fine particles as the wax. Although the detailed reason is unclear, the polyolefin resin fine particles can be stably dispersed in the aqueous inkjet ink even when used in combination with the above-mentioned pigment dispersion resin. In addition, it is preferably selected from the viewpoint of being able to significantly improve the abrasion resistance and water resistance of the printed matter.

As the polyolefin, one or more selected from the group consisting of polyethylene, polypropylene, and polybutene can be suitably used, but it is particularly preferable to select polyethylene, as this can significantly improve the water resistance of the printed matter.

When wax is used, its D50 is preferably 10 to 200 nm, and more preferably 20 to 180 nm. If the D50 is within the above range, it is possible to preferably achieve the above-mentioned functions. In addition, clogging of the inkjet head nozzles is prevented, and thus an aqueous inkjet ink with excellent initial ejection stability can be obtained.

When a wax is used, the amount of the wax relative to the total amount of all resins (pigment dispersion resin, binder resin, and wax) contained in the aqueous inkjet ink is preferably 3 to 30% by mass, and more preferably 6 to 20% by mass. By keeping the amount within the above range, the functions of the respective resins are not hindered by each other.
Furthermore, since printed matter having sufficient water resistance can be obtained even during high-speed printing, the blending amount of wax relative to the total amount of the aqueous inkjet ink is preferably 0.5 to 1.5% by mass.

<Water>
The aqueous inkjet ink of this embodiment contains water. It is preferable to use ion-exchanged water (deionized water) as the water, rather than general water containing various ions. The content of water is preferably 45 to 85% by mass, and particularly preferably 50 to 80% by mass, based on the total amount of the aqueous inkjet ink. Since water has a low boiling point, it is preferentially evaporated from the nozzle end face of the inkjet head, and the solid content at the gas-liquid interface tends to become high. In contrast, by setting the content of water within the above range, good ejection stability is achieved under all conditions, immediately after the start of printing, after a long period of printing suspension, and during continuous printing.

<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.

<Method of manufacturing water-based inkjet ink>
The aqueous inkjet ink of this embodiment can be produced by a conventionally known method. As an example, a pigment dispersion liquid is produced by dispersing a pigment in a medium containing at least water (aqueous medium). Then, water, a water-soluble organic solvent, compound (A-1), a nonionic surfactant (A-2), a binder resin, and the like are added to the pigment dispersion liquid, 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 application, 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 viscometer manufactured by Toki Sangyo Co., Ltd.

In order to obtain an aqueous inkjet ink with excellent ejection stability and print quality, the aqueous inkjet ink of this embodiment preferably has a static surface tension at 25°C of 18 to 35 mN/m, and more preferably 21 to 32 mN/m. In this application, the static surface tension is 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 an aqueous inkjet ink set by combining two or more aqueous inkjet inks. Examples of the aqueous inkjet ink set include a set of four aqueous inkjet inks (process color ink set) consisting of a cyan aqueous inkjet ink (aqueous cyan ink), a magenta aqueous inkjet ink (aqueous magenta ink), a yellow aqueous inkjet ink (aqueous yellow ink), and a black aqueous inkjet ink (aqueous black ink); a set of five aqueous inkjet inks obtained by further adding an aqueous white ink to the process color ink set; and the like. It is preferable that all the aqueous inkjet inks constituting the aqueous inkjet ink set satisfy the requirements of the above-mentioned embodiment of the present invention.

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

As the flocculant, for example, a water-soluble inorganic or organic salt containing a polyvalent metal ion, and a resin having a cationic group and a cationic group equivalent greater than an anionic group equivalent can be used.

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

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

The method of ejection from the inkjet head can be selected from any known method. Examples of such ejection methods include a piezoelectric method that uses the volume change of a piezoelectric element, a thermal method that ejects water-based inkjet ink using bubbles generated by heating a heater, and a valve method that 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 particularly preferable to adjust the printing conditions so that the resolution is 1200 dpi or more.

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

In particular, from the viewpoint of preventing bumping of the liquid components in the aqueous inkjet ink and obtaining printed matter with excellent print quality, it is preferable that the drying temperature be 35 to 100°C when using the heat drying method, and that the hot air temperature be 50 to 250°C when using the hot air drying method. From the same viewpoint, it is preferable that, from the viewpoint of the same, when using the infrared drying method, 50% or more of the integrated value of the total output of the irradiated infrared rays is in the wavelength region of 700 to 2200 nm.

<Printing substrate>
The printing substrate on which the aqueous inkjet ink of the present embodiment is printed is not particularly limited. However, by using the aqueous inkjet ink of the present embodiment on a poorly permeable substrate or a non-permeable substrate, which is prone to color bleeding and uneven shading and therefore tends to deteriorate the print quality, and even in high-speed printing, a printed matter having print quality equivalent to that of plate printing can be obtained.

In this application, the permeability of a printing substrate is determined by the amount of water absorption measured by a dynamic scanning absorptiometer. Specifically, a printing substrate having an amount of pure water absorption of less than 1 g/ ^m2 at a contact time of 100 msec, as measured by the following method, is called a "non-permeable substrate", a printing substrate having an amount of water absorption of 1 g/ ^m2 or more but less than 6 g/ ^m2 is called a "hardly permeable substrate", and a printing substrate having an amount of water absorption of 6 g/m2 or ^more is called a "permeable substrate".
The water absorption of the printing substrate can be measured using a dynamic scanning absorptiometer (for example, "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.) set under the conditions shown below, using a printing substrate sample of approximately 15 to 20 cm square, in an environment of 23°C and 50% RH.
Measurement method: Spiral Method
・Measurement start radius: 20mm
Measurement end radius: 60 mm
・Contact time: 10~1,000msec
Number of sampling points: 19 (measured at approximately equal intervals relative to the square root of the contact time)
Scanning interval: 7 mm
・Speed switching angle of the rotating table: 86.3 degrees ・Head box conditions: width 5 mm, slit width 1 mm

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

The printing substrates listed above may have a smooth surface or may have an uneven surface. The printing substrate may be transparent, translucent, or opaque. The printing substrate may be in the form of a roll or a sheet. In addition, 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.

It is also preferable to subject the printing surface of the printing substrates listed above to surface modification such as corona treatment and plasma treatment, since this improves the wettability of the aqueous inkjet ink of this embodiment, and results in a printed product with excellent print quality and drying properties, and with a uniform printed surface, good abrasion resistance and adhesion to the substrate.

<Printed materials>
The aqueous inkjet ink of this embodiment can be used to produce a printed matter. The above-mentioned inkjet recording method can be used as a method for producing the printed matter.

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 Water-Based Solution of Acrylic Pigment Dispersion Resin>
A reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer was charged with 90 parts of butanol, and the inside of the reaction vessel was replaced with nitrogen gas. Next, the inside of the reaction vessel was heated until the temperature inside the reaction vessel reached 110°C, and then a mixture of 30 parts of acrylic acid, 35 parts of behenyl acrylate, and 35 parts of styrene, which are polymerizable monomers; and 4 parts of V-601 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), which is a polymerization initiator, was dropped into the reaction vessel over 2 hours. After the dropwise addition was completed, the polymerization reaction was continued for 3 hours while maintaining the internal temperature at 110°C. Thereafter, 0.4 parts of V-601 was added, and the polymerization reaction was continued for 1 hour while maintaining the internal temperature at 110°C, to obtain a solution of an acrylic pigment dispersion resin.
Next, the contents in the reaction vessel were cooled to room temperature, 38 parts of dimethylaminoethanol were added to neutralize the acrylic pigment dispersion resin, and then 100 parts of ion-exchanged water were added.Then, the contents were heated to 100°C or higher, and butanol was distilled off by azeotroping with ion-exchanged water, and ion-exchanged water was added to adjust the solid content concentration to 20%, thereby obtaining an aqueous solution of the acrylic pigment dispersion resin.The molecular weight of the obtained acrylic pigment dispersion resin was 16,000, and the acid value was 234 mgKOH/g.

<Production Example of Magenta Pigment Dispersion>
A mixing vessel was charged with 20 parts of FASTOGEN SUPER MAGENTA RTS (C.I. Pigment Red 122 manufactured by DIC Corporation) as a pigment, 25 parts of an aqueous solution of an acrylic pigment dispersion resin, and 55 parts of water, and the mixture was pre-dispersed with a stirrer. After that, a main dispersion was carried out using a DYNO-MILL with a volume of 0.6 L filled with 1,800 g of zirconia beads having a diameter of 0.5 mm, to obtain a magenta pigment dispersion having a pigment concentration of 15%.

<Production Example of Yellow Pigment Dispersion>
A yellow pigment dispersion having a pigment concentration of 15% was obtained using the same materials and method as for the magenta pigment dispersion, except that Lysopac Yellow 5515C (C.I. Pigment Yellow 155 manufactured by Vibrants) was used as the pigment.

<Production Example of Water-Based Solutions of Acrylic Binder Resins 1 and 2>
A water-based solution of acrylic binder resin 1 was produced by reproducing the manufacturing example of fixing resin 1 in JP 2018-203802 A. However, the degree of adjustment of the solid content concentration by water after distilling off toluene was changed so that the solid content concentration of the water-based solution of acrylic binder resin 1 was 35%. The acid value of resin 1 was 49 mgKOH/g and the glass transition temperature was 81°C.
Further, the manufacturing example of fixing resin 34 in JP 2018-203802 A was retested, and an aqueous solution of acrylic binder resin 2 (solid concentration 35%) was manufactured in the same manner as the acrylic binder resin 1, except that the solid concentration was 35%. The acid value of acrylic binder resin 2 was 49 mgKOH/g, and the glass transition temperature was 42°C.

<Production of Water-Based Inkjet Ink Set>
Using the pigment dispersion liquid produced by the above-mentioned method, each raw material was charged into a mixing vessel equipped with a stirrer so as to obtain the compounding formulation described in each column of Table 1 below. After all materials were charged, the mixture was heated to 50°C while being stirred, and the mixture was further stirred and mixed for 1 hour while maintaining the temperature of the mixture at 50°C. Thereafter, the mixture was filtered through a membrane filter having a pore size of 0.8 μm to produce an aqueous inkjet ink. A set of aqueous inkjet inks consisting of an aqueous magenta ink (M) and an aqueous yellow ink (Y) was produced by producing aqueous inkjet inks using each of the above magenta pigment dispersion liquid and yellow pigment dispersion liquid.

When producing the aqueous inkjet ink, each raw material was added while stirring the mixture in the mixing vessel. In addition, in each column of Table 1, the ingredients were added in the order listed, starting from the top row. However, when producing an aqueous inkjet ink that does not contain one or more of these ingredients, that ingredient was not added, and the next ingredient was added in the order listed. In addition, for ingredients that contain two or more ingredients, the order of addition of the ingredients was arbitrary.

The meanings of the abbreviations and details of the product names listed in Table 1 above are as follows. In Table 1, "bp" represents the boiling point, "Nv" represents the solid content concentration, and "HLB" represents the HLB value. The boiling points shown in Table 1 are values at 1 atmospheric pressure.
Hexylene glycol: boiling point 196°C at 1 atmosphere, surface tension 27 mN/m at 25°C
1,2-PD: 1,2-propanediol (boiling point at 1 atmosphere: 188°C, surface tension at 25°C: 37mN/m)
1,5-PeD: 1,5-pentanediol (boiling point at 1 atmosphere: 239°C, surface tension at 25°C: 42mN/m)
PnP: Propylene glycol monopropyl ether (boiling point at 1 atmosphere: 150° C., surface tension at 25° C.: 26 mN/m)
DPnP: dipropylene glycol monopropyl ether (boiling point at 1 atmosphere: 212°C, surface tension at 25°C: 26 mN/m)
DPnB: Dipropylene glycol monobutyl ether (boiling point at 1 atmosphere: 230° C., surface tension at 25° C.: 24 mN/m)
iPDG: diethylene glycol monoisopropyl ether (boiling point at 1 atmosphere: 207°C, surface tension at 25°C: 30 mN/m)
Glycerin: boiling point 290°C at 1 atmosphere, surface tension 65mN/m at 25°C
Nonion K-220: A compound in which R ¹ is an alkyl group having 12 carbon atoms and n=20 in the general formula (1) (surfactant manufactured by NOF Corporation, solid content 100%)
Nonion K-230: A compound in which R ¹ is an alkyl group having 12 carbon atoms and n=30 in the general formula (1) (surfactant manufactured by NOF Corporation, solid content 100%)
Emulgen 1150S-60: A compound in which R ¹ is an alkyl group having 11 carbon atoms and n=50 in the general formula (1) (surfactant manufactured by Kao Corporation, solid content 60%)
Brownon BE-30: A compound in which R ¹ is an alkyl group having 22 carbon atoms and n=30 in the general formula (1) (surfactant manufactured by Aoki Oil Industries Co., Ltd., solid content 100%)
Nonion K-2100: a compound in which R ¹ is an alkyl group having 12 carbon atoms and n=100 in the general formula (1) (surfactant manufactured by NOF Corporation, solid content 50%)
Surfynol 104: acetylene diol surfactant manufactured by Evonik Japan, HLB value = 3.0
Surfynol 440: acetylene diol surfactant manufactured by Evonik Japan, HLB value = 8.1
TEGO Twin 4100: Gemini type silicone surfactant (HLB value = 0 to 2) manufactured by Evonik Japan
TEGO Wet 280: Silicone surfactant manufactured by Evonik Japan (a polyether-modified silicone surfactant in which ^R5 in the above general formula 4 is a structure represented by general formula 5 and ^R6 is not a structure represented by general formula 5, HLB value = 3 to 5)
TEGO Glide 440: Silicone surfactant manufactured by Evonik Japan (a polyether-modified silicone surfactant in which R ⁶ in the above general formula 4 is a structure represented by general formula 5 and R ⁵ is not a structure represented by general formula 5, HLB value = 3 to 5)
Surfynol 465: acetylene diol surfactant manufactured by Evonik Japan, HLB value = 13.2
A-615GE: PES RESIN A-615GE (polyester resin manufactured by Takamatsu Oil Co., Ltd., solid content 25%, glass transition temperature 47°C)
AQ515: AQUACER 515, polyethylene wax emulsion manufactured by BYK Japan, solid content 35%
Proxel GXL: 1,2-benzisothiazol-3-one in dipropylene glycol solution (1,2-benzisothiazol-3-one:dipropylene glycol:water=2:6:2, preservative manufactured by Arch Chemicals. Dipropylene glycol is a water-soluble organic solvent with a boiling point of 232°C at 1 atmospheric pressure and a surface tension of 36 mN/m at 25°C.)

[Examples 1 to 43, Comparative Examples 1 to 7]
The aqueous inkjet ink set produced by the above-mentioned method was used to carry out the following evaluations, and the evaluation results are shown in Table 1 above.

<Evaluation 1: Evaluation of ejection stability (initial ejection)>
An inkjet ejection device equipped with a Kyocera inkjet head "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) installed in an environment of 25° C. was filled with the water-based magenta ink or water-based yellow ink constituting the water-based inkjet ink set. Next, a nozzle check pattern was printed, and after confirming that the ink was ejected normally from all the nozzles, the ink was left for 1 minute. Thereafter, a solid print with a print rate of 100% was performed on OK topcoat paper under printing conditions of a frequency of 40 kHz and 1200×1200 dpi. Then, the obtained solid print was checked with a magnifying glass to see whether the water-based inkjet ink was applied to the part where the water-based inkjet ink was supposed to be printed first, and the ejection stability (initial ejection) was evaluated. The evaluation criteria were as follows, with ◎ and ○ being practically usable. The above evaluation was performed for each of the water-based magenta ink and water-based yellow ink constituting the water-based inkjet ink set. Table 1 also shows the results of the evaluation of the water-based magenta inks and water-based yellow inks that gave poor evaluation results.
⊚: In the solid print, no chipping was observed in the area that should have been printed first. ◯: In the solid print, a chipping of less than 1 cm was observed in the area that should have been printed first. ×: In the solid print, a chipping of 1 cm or more was observed in the area that should have been printed first.

<Evaluation 2: Evaluation of ejection stability (standby ejection property)>
An inkjet ejection device equipped with a Kyocera inkjet head "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) installed in an environment of 25° C. was filled with the water-based magenta ink or water-based yellow ink constituting the water-based inkjet ink set. After filling, the water-based magenta ink or water-based yellow ink was pressurized until it oozed out from the nozzle of the inkjet head. Next, the nozzle plate to which the oozed water-based inkjet ink had adhered was wiped, and the inkjet ejection device was left on standby for one hour. Thereafter, a solid image was printed on OK topcoat paper under printing conditions of a frequency of 40 kHz, a conveyor driving speed of 50 m/min, and a resolution of 1200×1200 dpi. Then, for the obtained solid print, whether the water-based inkjet ink was applied to the part where the water-based inkjet ink was supposed to be printed first was visually confirmed, and standby ejection properties were evaluated. The evaluation criteria were as follows, with ◎, ○, and △ being considered usable. The above evaluation was performed for each of the water-based magenta ink and water-based yellow ink that constituted the water-based inkjet ink set. Table 1 also shows the results of the water-based magenta ink and water-based yellow ink that were evaluated and received poor evaluation results.
◎: In the solid print printed after waiting for 1 hour, no chipping was observed in the part that should have been printed first. ○: In the solid print printed after waiting for 1 hour, a chipping of less than 3 cm was observed in the part that should have been printed first. △: In the solid print printed after waiting for 1 hour, a chipping of 3 cm or more but less than 5 cm was observed in the part that should have been printed first. ×: In the solid print printed after waiting for 1 hour, a chipping of 5 cm or more was observed in the part that should have been printed first.

<Preparation of magenta/yellow gradation print>
An inkjet ejection device was prepared in which two Kyocera inkjet heads "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) were arranged along the transport direction of the substrate, and the aqueous magenta ink and the aqueous yellow ink were filled in this order from the upstream side of the transport direction. In addition, an A4 size (width 21 cm x length 30 cm) OPP film (Mitsui Chemicals Tohcello "OPU-1", thickness 20 μm) was fixed on the conveyor as a printing substrate. Thereafter, the conveyor was driven at 50 m/min, and when the printing substrate passed under the installation part of the inkjet head, the aqueous inkjet ink set was ejected under the condition of a drop volume of 2.6 pL, and a magenta/yellow gradation image was printed. Immediately after printing, the printed substrate was placed in a constant temperature incubator with a blower set at 70° C. and dried for 3 minutes to produce a magenta/yellow gradation print.
The above-mentioned "magenta/yellow gradation image" refers to an image in which a magenta gradation image (an image in which the printing rate is changed in 10% increments between 10% and 100%) printed using water-based magenta ink and measuring 5 cm in width and 30 cm in length, and a yellow gradation image (a 5 cm in width and 30 cm in length) printed using water-based yellow ink are arranged side by side with their long sides touching each other.
Also, a magenta/yellow gradation print was produced in the same manner as above, using an OPP film (FOR-AQ, thickness 20 μm) manufactured by Futamura Chemical Co., Ltd. as the printing substrate.

<Evaluation 3: Evaluation of print quality (white areas)>
The magenta/yellow gradation print produced by the above-mentioned method was visually observed. The print quality of the magenta/yellow gradation print was evaluated by checking the presence or absence of blank spaces at a printing rate of 100%. The evaluation criteria were as follows, with ◎, ○, ○△, and △ being usable. The above evaluation was performed for each of the two types of printing substrates on which the magenta/yellow gradation print was printed.
◎: No white spots were observed in both the printed areas of the magenta gradation image and the yellow gradation image for both of the two types of printed substrates. ○: Slight white spots were observed in the printed areas of the magenta gradation image or the yellow gradation image for only one of the two types of printed substrates. ◯△: Slight white spots were observed in both the printed areas of the magenta gradation image and the yellow gradation image for only one of the two types of printed substrates. △: Clear white spots were observed in both the printed areas of the magenta gradation image and the yellow gradation image for at least one of the two types of printed substrates. ×: Clear white spots were observed in both the printed areas of the magenta gradation image and the yellow gradation image for both of the two types of printed substrates.

<Evaluation 4: Evaluation of print quality (mixed color bleeding)>
The magenta/yellow gradation print produced by the above-mentioned method was visually observed. The print quality of the magenta/yellow gradation print was evaluated by checking the print coverage rate at the boundary between the printed part of the magenta gradation image and the printed part of the yellow gradation image where color bleeding began to occur. The evaluation criteria were as follows, with ◎, ○, ○△, and △ being considered as usable. Table 1 also lists the results of the two types of substrates that were evaluated, with poor evaluation results.
◎: No color bleeding was observed on both substrates even at a printing rate of 80%. ○: Color bleeding was observed on at least one substrate at a printing rate of 80%. ○△: Color bleeding was observed on at least one substrate at a printing rate of 70%. △: Color bleeding was observed on at least one substrate at a printing rate of 60%. ×: Color bleeding was observed on both substrates at a printing rate of 60%.

<Evaluation 5: Evaluation of water resistance>
An inkjet ejection device equipped with two Kyocera Corporation inkjet heads KJ4B-1200 (design resolution 1200 dpi, nozzle diameter 20 μm) arranged in the conveying direction of the printing substrate was installed under a 25 ° C. environment, and water-based magenta ink and water-based yellow ink were filled in this order from the upstream inkjet head. In addition, an OPP film (FOR-AQ, thickness 20 μm) manufactured by Futamura Chemical Co., Ltd. was fixed on the conveyor. Thereafter, the conveyor was driven at 50 m / min, and when the printing substrate passed under the installation part of the inkjet head, one of the above water-based inkjet inks was ejected under the condition of a drop volume of 2.6 pL, and a monochromatic solid image (printing rate 100%) having a width of 20 cm and a length of 20 cm was printed. Then, immediately after printing, the printed printing substrate was placed in a constant temperature incubator set at 70 ° C. and dried for 3 minutes to produce a monochromatic solid print.
Next, a test piece was cut out from the obtained monochrome solid print and set in a Gakushin type AB-301 friction fastness tester manufactured by Tester Sangyo Co., Ltd. Then, a test attachment white cotton cloth (Kanakin No. 3) sufficiently moistened with ion-exchanged water was attached to a friction element (weight: 200 g), and the friction element was subjected to various loads and subjected to multiple vibrations a predetermined number of times, after which the state of the surface of the print and the degree of coloring of the cotton cloth were visually confirmed to evaluate the abrasion resistance. The evaluation criteria were as follows, with ◎, ○, ○△, and △ being considered as usable. The above evaluation was performed for each of the water-based magenta ink and the water-based yellow ink, and Table 1 shows the results of those with poor evaluation results.
◎: Even after a 300g weight was placed on the frictional element (500g in total) and the element was shaken 20 times, there were no abrasion marks on the printed surface and no discoloration on the cotton cloth. ○: Even after a 300g weight was placed on the frictional element (500g in total) and the element was shaken 10 times, there were no abrasion marks on the printed surface and no discoloration on the cotton cloth, but after shaking 20 times under the same load conditions, there were abrasion marks on the printed surface and/or discoloration on the cotton cloth. ○△: Even after a 300g weight was placed on the frictional element (500g in total) and the element was shaken 5 times, there were no abrasion marks on the printed surface and no discoloration on the cotton cloth, but after shaking 10 times under the same load conditions, there were abrasion marks on the printed surface and/or discoloration on the cotton cloth. △: After shaking five times without placing a weight on the frictional element (load 200 g), there were no scratches on the printed surface and no discoloration of the cotton cloth was observed. However, after shaking five times with a 300 g weight placed on the frictional element (total 500 g), scratches on the printed surface and/or discoloration of the cotton cloth were observed. ×: After shaking five times without placing a weight on the frictional element (load 200 g), scratches on the printed surface and/or discoloration of the cotton cloth were observed.

As shown in Table 1 above, the aqueous inkjet inks (sets) of Examples 1 to 43 having the configuration of the present invention had superior ejection stability compared to the aqueous inkjet inks of Comparative Examples 1 to 7, and also had good solid coverage and little color mixing. Furthermore, the water resistance of the ink film was also good. From these results, it was confirmed that the aqueous inkjet inks having the configuration of the present invention are excellent aqueous inkjet inks that combine ejection stability, print quality of printed matter, and water resistance.

Claims (6)

An aqueous inkjet ink comprising a pigment, a surfactant (A), a water-soluble organic solvent, and a binder resin,
The surfactant (A) contains a compound (A-1) represented by the following general formula 1, and a nonionic surfactant (A-2) having an HLB value of 1 to 10 (excluding the compound (A-1)),
The water-based inkjet ink, wherein the water-soluble organic solvent comprises 2-methyl-2,4-pentanediol .

R ¹ -(O-CH ₂ -CH ₂ ) _n -OH General formula 1
(In the general formula 1, ^R1 represents an alkyl group having 10 to 25 carbon atoms which may be branched, and n is an integer of 20 to 100.)
The aqueous inkjet ink according to claim 1, wherein the water-soluble organic solvent further comprises a diol having 2 to 5 carbon atoms. The aqueous inkjet ink according to claim 1 , wherein the water-soluble organic solvent further comprises a compound represented by the following general formula 2:

R ² -(O-CH(CH ₃ )-CH ₂ ) _m -OH General formula 2
(In the general formula 2, ^R2 represents an alkyl group having 2 to 4 carbon atoms which may be branched, and m is 1 or 2.)
The aqueous inkjet ink according to any one of claims 1 to 3, wherein the ratio of the mass content of the compound (A-1) to the mass content of the nonionic surfactant (A-2) [compound (A-1):surfactant (A-2)] is 1:0.5 to 1:20. The aqueous inkjet ink according to any one of claims 1 to 3, wherein a ratio of the sum of the mass content of the 2-methyl-2,4-pentanediol and the mass content of the compound (A-1) to the mass content of the nonionic surfactant (A-2) [( 2-methyl-2,4-pentanediol + compound (A-1)):surfactant (A-2)] is 1:1 to 14:1. A printed matter obtained by printing the aqueous inkjet ink according to any one of claims 1 to 3 on a printing substrate.