Aqueous inkjet ink and printed matter

Abstract

An aqueous inkjet ink contains a coloring agent, a water-soluble organic solvent, a surfactant, and water, wherien the water-soluble organic solvent includes a C2-5 diol and hexylene glycol, and the surfactant includes an acetylenediol having a HLB value of 10 or less.

Description

Water-based inkjet inks and printed materials

[0001] An embodiment of the present invention relates to an aqueous inkjet ink and a printed matter formed using the aqueous inkjet ink.

[0002] With the increasing need for small-lot printing and reduced printing costs, digital printing methods that do not require plate making are rapidly becoming more popular.

[0003] An example of such a digital printing method is inkjet printing. In inkjet printing, ink droplets are ejected from fine nozzles and applied to a printing substrate (also referred to simply as "substrate" in this disclosure), thereby printing an image and / or text on the printing substrate. In this disclosure, the printing substrate and the image and / or text formed on the printing substrate by printing ink are collectively referred to as a "printed matter." The above-mentioned "image" also includes solid images and seamless images such as checkered images. Inkjet printing is characterized by easy operation of the printing device and low noise during printing. Therefore, among digital printing methods, there is high demand for printing devices (inkjet printers) that employ inkjet printing.

[0004] Inks used in inkjet printing methods (referred to as "inkjet inks" in this disclosure) are classified into solvent-based, water-based, ultraviolet (UV)-curable, and other types depending on their composition. Meanwhile, in recent years, there has been an accelerating trend toward restricting the use of raw materials that are harmful to humans and the environment. Accordingly, there has been an increasing demand for water-based inkjet inks (also simply referred to as "water-based inkjet inks" in this disclosure) rather than solvent-based inkjet inks and UV-curable inkjet inks that use the above-mentioned raw materials.

[0005] In recent years, with the improvement in inkjet head performance, 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 / package) printing market, there is active consideration of replacing plate-based printing methods such as offset printing and gravure printing with inkjet printing.

[0006] However, when printing aqueous inkjet inks on poorly permeable substrates such as coated paper and art paper, as well as non-permeable substrates such as film, it has traditionally been difficult to obtain printed materials with print quality equivalent to that achieved by plate-based printing. Specifically, inkjet printing is prone to a phenomenon known as "bleeding," in which different colors of aqueous inkjet inks mix together. Furthermore, printed materials using a single type of aqueous inkjet ink are prone to a phenomenon known as "beading," in which different shades of color appear as beads. Furthermore, solid prints (prints with a 100% coverage) produced using a single type of aqueous inkjet ink are prone to a phenomenon known as "whiteout," in which areas where the aqueous inkjet ink does not adhere to the printed substrate. These phenomena are attributed to the poor wettability of the aqueous inkjet ink to the substrate, particularly on the order of microseconds (μs), due to the uniquely high surface tension of water, the primary solvent of aqueous inkjet ink.

[0007] Generally, hydrophobic organic solvents and surfactants are used to improve the wettability of aqueous inks to substrates. However, these hydrophobic materials have poor solubility in water. Therefore, when such hydrophobic materials are used, there has been a problem of nozzle missing, which is thought to be caused by instability at the gas-liquid interface of the aqueous inkjet ink or the generation of bubbles. Here, the nozzle missing refers to a phenomenon in which the aqueous inkjet ink is not ejected from a nozzle. Thus, aqueous inkjet inks used for printing on poorly permeable and non-permeable substrates have traditionally had to simultaneously solve two problems: improving wettability to the substrate on the order of μs, and being able to eject ink from an inkjet head without nozzle missing even after long printing pauses or during continuous printing (good ejection stability regardless of usage conditions). However, an aqueous inkjet ink that can simultaneously and effectively solve all of these problems has not yet been discovered.

[0008] In the present disclosure, the ejection stability after a long printing pause is also referred to as “standby ejection stability.” Furthermore, the ejection stability during continuous printing is also referred to as “continuous ejection stability.”

[0009] For example, Patent Document 1 discloses an ink containing water as the main solvent but also three or more organic solvents with different water-octanol partition coefficients, and explains that use of this ink exhibits good wettability to non-absorbent media (non-permeable substrates). Patent Document 2 discloses an inkjet ink containing three or more compounds (organic solvents) each having a specific structure, with the content ratio of these compounds specified, and explains that use of this inkjet ink can achieve both good wettability and good drying properties on non-absorbent media (non-permeable substrates).

[0010] JP 2020-125382 A International Publication No. 2022 / 224786

[0011] However, both of the inks in Patent Documents 1 and 2 are insufficient in terms of wettability on the order of μs, and furthermore, there is a problem in that nozzle clogs occur due to instability of the gas-liquid interface at the nozzle end face of the inkjet head, and all of the above-mentioned problems have not yet been solved at a high level.

[0012] Therefore, an object of one embodiment of the present invention is to provide an aqueous inkjet ink that can produce printed matter with excellent print quality even when printing on poorly permeable substrates and non-permeable substrates, and that further has excellent ejection stability regardless of the conditions of use.

[0013] As a result of extensive research, the present inventors have found that all of the above-mentioned problems can be solved simultaneously and to a high degree by using an aqueous inkjet ink having the following composition.

[0014] That is, one embodiment of the present invention relates to an aqueous inkjet ink. Another embodiment of the present invention relates to a printed material obtained using the aqueous inkjet ink. More specifically, the embodiments of the present invention include the following. However, the present invention is not limited to the following embodiments and includes various embodiments. [1] An aqueous inkjet ink comprising a colorant, a water-soluble organic solvent, a surfactant, and water, wherein the water-soluble organic solvent comprises a diol having 2 to 5 carbon atoms and hexylene glycol, and the surfactant comprises an acetylenic diol having an HLB value of 10 or less. [2] The aqueous inkjet ink according to [1] above, wherein the content of the hexylene glycol is 0.1 to 30% by mass relative to the total amount of the aqueous inkjet ink. [3] The aqueous inkjet ink according to [1] or [2] above, wherein the content of the acetylenic diol having an HLB value of 10 or less is 0.1 to 2.0% by mass relative to the total amount of the aqueous inkjet ink. [4] A printed material obtained by printing the aqueous inkjet ink according to any one of [1] to [3] above. The disclosure of this application is related to the subject matter described in Japanese Patent Application No. 2023-089968, filed on May 31, 2023, the entire disclosure of which is incorporated herein by reference.

[0015] The aqueous inkjet ink according to one embodiment of the present invention can provide printed matter with excellent print quality even when printing on poorly permeable substrates and non-permeable substrates, and can also provide excellent ejection stability regardless of the conditions of use.

[0016] An aqueous inkjet ink according to one embodiment of the present invention (hereinafter simply referred to as "aqueous inkjet ink of this embodiment" or "ink") will be described below. Note that the present invention is not limited to the embodiment described below, and includes embodiments that can be modified without changing the essential parts of the present invention.

[0017] As described above, the aqueous inkjet ink of this embodiment having the above-described configuration has excellent wettability to substrates on the order of μs, and therefore can produce printed matter with excellent print quality even when printing on poorly permeable substrates and non-permeable substrates. Furthermore, the ink jet ink has excellent ejection stability even after a long printing break or during continuous printing. While the mechanism behind this is not clear, the inventors speculate as follows. However, the present invention is not limited by the speculation below.

[0018] Acetylene diols are generally known as surfactants that do not easily foam, reducing the generation of bubbles, which is one cause of nozzle clogging. Furthermore, due to their extremely stable molecular structure and small molecular weight, they are easily oriented at the interface with the substrate, even over a short period of time, such as the order of microseconds. In particular, acetylene diols with an HLB value of 10 or less have good wettability with substrates with very low interfacial free energy, such as PP (polypropylene) film, and are expected to improve print quality. On the other hand, acetylene diols with an HLB value of 10 or less have poor solubility in water. Therefore, when acetylene diols with an HLB value of 10 or less are used, the gas-liquid interface at the nozzle end face of an inkjet head becomes unstable, which has traditionally been a problem, making nozzle clogging more likely. Therefore, the present inventors conducted extensive research and found that when diols having 2 to 5 carbon atoms and hexylene glycol are used as water-soluble organic solvents, acetylene diols with an HLB value of 10 or less do not become unstable at the gas-liquid interface, thereby suppressing nozzle clogging. Hexylene glycol (2-methyl-2,4-pentanediol) has multiple branched alkyl groups and is thought to have a high affinity with the above-mentioned acetylenic diols due to their structural similarity. Furthermore, because hexylene glycol is a diol, it naturally has a high affinity with diols having 2 to 5 carbon atoms. As a result, it is presumed that hexylene glycol and diols having 2 to 5 carbon atoms protect the acetylenic diols with an HLB value of 10 or less, thereby suppressing the above-mentioned destabilization. Furthermore, because these water-soluble organic solvents have low molecular weights and the above-mentioned high affinity, they rapidly orient at the interface with the substrate, along with the acetylenic diols with an HLB value of 10 or less, achieving wetting and spreading on the order of μs. As a result, printed materials with print quality equivalent to plate-based printing can be obtained regardless of the printing substrate and printing speed.

[0019] As described above, the aqueous inkjet ink having the configuration of this embodiment can solve all of the above-mentioned problems to a high degree.

[0020] Each component constituting the aqueous inkjet ink according to one embodiment of the present invention will be described in detail below.

[0021] <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 a diol having 2 to 5 carbon atoms and hexylene glycol.

[0022] (Diols Having 2 to 5 Carbon Atoms) 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, 2,2-dimethyl-1,3-propanediol, 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, it is preferable to use alkanediols having 2 to 5 carbon atoms as the diols having 2 to 5 carbon atoms, because the above-mentioned mechanism functions effectively and ejection stability is good, even after a long period of printing suspension, for example. More preferably, one or more selected from the group consisting of 1,2-ethanediol, 1,2-propanediol, and 1,3-butanediol can be used. Furthermore, it is particularly preferable to use 1,2-propanediol, because ejection stability is good even during continuous printing.

[0023] In order to achieve excellent ejection stability regardless of the conditions of use, the content of the diols having 2 to 5 carbon atoms is preferably from 0.5 to 30% by mass, more preferably from 2 to 25% by mass, and particularly preferably from 6 to 22% by mass, relative to the total amount of the aqueous inkjet ink.

[0024] (Hexylene Glycol) In order to obtain good ejection stability and good print quality of printed matter, the content of hexylene glycol in the aqueous inkjet ink of this embodiment is preferably 0.1 to 30 mass %, more preferably 0.6 to 24 mass %, and particularly preferably 1 to 20 mass %, relative to the total amount of the aqueous inkjet ink.

[0025] Furthermore, the content of the hexylene glycol is preferably 3 to 70% by mass, more preferably 5 to 50% by mass, and particularly preferably 10 to 30% by mass, relative to the total amount of water-soluble organic solvent contained in the aqueous inkjet ink. By ensuring that the content of hexylene glycol relative to the total amount of water-soluble organic solvent falls within the above range, when used in combination with a diol having 2 to 5 carbon atoms, it becomes easier to stabilize acetylenic diols having an HLB value of 10 or less at the gas-liquid interface, thereby achieving good ejection stability, for example, even after a long period of printing suspension. Furthermore, the orientation of the acetylenic diols at the interface with the substrate is optimized, and wettability to the substrate on the order of μs is also improved.

[0026] The ratio of the content of the diols having 2 to 5 carbon atoms to the content of the hexylene glycol (content of diols having 2 to 5 carbon atoms / content of hexylene glycol) contained in the aqueous inkjet ink is preferably 0.1 to 20. This content ratio is more preferably 0.15 to 8, and particularly preferably 0.2 to 5. By keeping this content ratio within the above range, when used in combination with the diols having 2 to 5 carbon atoms, the acetylenic diols having an HLB value of 10 or less can be stabilized at the gas-liquid interface, thereby achieving good ejection stability, even during high-speed continuous printing, for example. Furthermore, the orientation of the acetylenic diols is optimized, and wettability to the substrate is improved on the order of μs.

[0027] To achieve both ejection stability and print image quality, it is also preferable to consider the drying behavior of the water-soluble organic solvent. Specifically, the ratio of the energy (VE2) required to evaporate hexylene glycol to the energy (VE1) required to evaporate the diol having 2 to 5 carbon atoms, "(VE2) / (VE1)," is preferably 0.05 to 1.5, more preferably 0.1 to 1.1, and particularly preferably 0.2 to 0.9. When hexylene glycol and a diol having 2 to 5 carbon atoms are used in combination so that the energy ratio falls within the above range, it becomes easy to obtain both excellent ejection stability and a printed matter with excellent print image quality. While the detailed mechanism is unknown, adjusting the energy ratio can prevent the water-soluble organic solvent from volatilizing before it can achieve its effects, such as affinity with acetylenic diols having an HLB value of 10 or less and wetting and spreading on the printing substrate. As a result, it is possible to easily improve both the ejection stability and the print quality of the printed matter, regardless of the type of printing substrate, printing conditions, and printing speed.

[0028] The energy required to evaporate the water-soluble organic solvent can be calculated by the following formula (1-1).

[0029] Formula (1-1): VE=(BP-25)×MP×SH

[0030] In the above formula (1-1), VE is the energy (cal) required to evaporate the target water-soluble organic solvent contained in the aqueous inkjet ink. BP is the boiling point (°C) of the water-soluble organic solvent at 1 atmosphere. MP is the content (mass%) of the water-soluble organic solvent in the aqueous inkjet ink. SH is the specific heat (cal / g°C) of the water-soluble organic solvent at 25°C. The specific heat can be calculated using the Misnar estimation method described below.

[0031] The Missenard estimation method uses the sum of values ​​determined depending on the molecular structure and temperature of a target compound as the specific heat (cal / mol °C) at that temperature. However, since the unit of specific heat calculated by the Missenard estimation method is "cal / mol °C," in the above formula (1-1), the specific heat value of the target compound obtained by the Missenard estimation method is used by dividing it by the molecular weight of the compound. Note that examples of values ​​at 25 °C determined depending on the molecular structure as described above are shown in Table 1 below.

[0032]

[0033] (Other Water-Soluble Organic Solvents) The aqueous inkjet ink of this embodiment may contain water-soluble organic solvents other than the above-mentioned diols having 2 to 5 carbon atoms and hexylene glycol (referred to as "other water-soluble organic solvents" in the present disclosure).

[0034] In the aqueous inkjet ink of this embodiment, examples of the other water-soluble organic solvents include alkanediols (excluding those having 2 to 5 carbon atoms and hexylene glycol), alkanetriols (excluding those having 3 to 6 carbon atoms), polyoxyalkylene diols (excluding those in which the oxyalkylene group is an oxyethylene group and / or an oxypropylene group and the oxyalkylene group has 2 to 4 carbon atoms and the oxyalkylene group has 2 to 5 carbon atoms), (poly)oxyalkylene monoalkyl ethers (excluding those in which the oxyalkylene group is an oxyethylene group or an oxypropylene group and the oxyalkylene group has 1 to 4 carbon atoms and the terminal alkyl group has 1 to 4 carbon atoms, or those in which the oxyalkylene group is an oxybutylene group or an oxypentylene group and the oxyalkylene group has 1 oxyalkylene group and the terminal alkyl group has 1 to 4 carbon atoms), Examples of usable solvents include (poly)oxyethylene dialkyl ethers (wherein the number of oxyalkylene groups is 1 to 4 and the number of carbon atoms in the terminal alkyl groups is 1 to 4), lactams (wherein the number of atoms constituting the lactam ring is 5 to 7. 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), and alkanolamines (wherein 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). These other water-soluble organic solvents may be used alone or in combination of two or more. In the present disclosure, "(poly)oxyalkylene" refers to oxyalkylene and polyoxyalkylene.

[0035] In some embodiments, it is preferable to use a water-soluble organic solvent having a static surface tension of 29 to 50 mN / m at 25°C as the other water-soluble organic solvent contained in the aqueous inkjet ink. By using a water-soluble organic solvent having this surface tension, it is possible to maintain an appropriate surface tension for ejection as an inkjet ink. Furthermore, good ejection stability can be easily obtained, even after a long period of printing suspension, for example. Furthermore, from the viewpoint of improving both the print quality and standby ejection properties of printed matter, it is also preferable to use a water-soluble organic solvent having a boiling point of 150 to 220°C at 1 atmosphere. Therefore, although not particularly limited, among the above-listed examples of other water-soluble organic solvents, (poly)oxyalkylene monoalkyl ethers having a static surface tension of 29 to 50 mN / m at 25°C and a boiling point of 150 to 220°C at 1 atmosphere can be preferably used. Specific examples include diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, diethylene glycol monoisopropyl ether, 1,3-butylene glycol monomethyl ether, and 3-methyl-1,3-butylene glycol monomethyl ether.

[0036] Meanwhile, the total content of water-soluble organic solvents contained in the aqueous inkjet ink of this embodiment is preferably 5 to 50% by mass, more preferably 9 to 40% by mass, even more preferably 12 to 35% by mass, and particularly preferably 15 to 30% by mass, relative to the total amount of the aqueous inkjet ink. By ensuring that the total content of water-soluble organic solvents falls within the above range, the inkjet ink can maintain an appropriate viscosity for ejection. Furthermore, good ejection stability can be easily achieved, for example, even after a long period of printing suspension. In one embodiment, the total content of the water-soluble organic solvents is preferably the total amount of C2 to C5 diols and hexylene glycol. In another embodiment, the total content of the water-soluble organic solvents may be the total amount of C2 to C5 diols, hexylene glycol, and other water-soluble organic solvents. In this case, the content of the other water-soluble organic solvents is preferably 60% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, relative to the total content of water-soluble organic solvents. The content of the other water-soluble organic solvents may even be 0% by mass.

[0037] In some embodiments, the aqueous inkjet ink preferably contains 5% by mass or less (or may contain 0% by mass) of a water-soluble organic solvent having a boiling point of 250°C or higher at 1 atmosphere, and most preferably 1% by mass or less (or may contain 0% by mass) of the water-soluble organic solvent with a boiling point of 250°C or higher, relative to the total amount of the aqueous inkjet ink. By not including a water-soluble organic solvent with a boiling point of 250°C or higher, or by including a water-soluble organic solvent with a boiling point of 250°C or higher, and keeping the amount of the water-soluble organic solvent within the above range, it is easy to maintain good ejection stability, even after a long period of printing suspension. Furthermore, water-soluble organic solvents with high boiling points have a high affinity with acetylenic diols having an HLB value of 10 or lower, which may inhibit the orientation of the acetylenic diols at the gas-liquid interface. By limiting the content of the water-soluble organic solvent with a boiling point of 250°C or higher within the above range, the orientation of the acetylenic diols having an HLB value of 10 or lower is not inhibited, and printed matter with excellent print quality and no white voids can be easily obtained.

[0038] In the aqueous inkjet ink of this embodiment, the content of the water-soluble organic solvent that is a heterocyclic compound is preferably 3% by mass or less (or may be 0% by mass) and most preferably 1% by mass or less (or may be 0% by mass) relative to the total amount of the aqueous inkjet ink. Heterocyclic compounds exhibit extremely high solubility in both hydrophilic and hydrophobic materials, and therefore may dissolve acetylenic diols with an HLB value of 10 or less, potentially inhibiting their orientation at the gas-liquid interface. Therefore, by not including a water-soluble organic solvent that is a heterocyclic compound, or if included, keeping the amount within the above range, the orientation of acetylenic diols with an HLB value of 10 or less at the gas-liquid interface is not inhibited, resulting in good print quality. Furthermore, the dissolution of the above-mentioned components into the heterocyclic compound can be prevented at the nozzle end surface of the inkjet head, maintaining a balance between the presence of these components, thereby improving ejection stability.

[0039] <Surfactant> The aqueous inkjet ink of this embodiment contains a surfactant. As described above, the surfactant includes an acetylenic diol having an HLB value of 10 or less. The aqueous inkjet ink of this embodiment containing an acetylenic diol having an HLB value of 10 or less has excellent wettability on the order of μs, and can produce printed matter having print quality equivalent to that of plate-based printing, even during high-speed printing.

[0040] (Acetylene diols having an HLB value of 10 or less) The acetylene diols having an HLB value of 10 or less may be synthesized by a conventionally known synthesis method, or commercially available products may be used. Examples of such commercially available products include Surfynol (registered trademark) 61, 104E, 104H, 104A, 104BC, 104DPM, 104PA, 104PG-50, 420, 440, 2502, SE, SE-F, DF-110D, Dynol (registered trademark) 604, 607 (manufactured by Evonik Japan), Olfine (registered trademark) E1004, PD-001, PD-002W, PD-004 (manufactured by Nissin Chemical Industry Co., Ltd.). These compounds may be used alone or in combination of two or more.

[0041] Among the acetylenic diols having an HLB value of 10 or less, it is preferable to use acetylenic diols having an HLB value of 4 to 10, and it is particularly preferable to use acetylenic diols having an HLB value of 7 to 10. When acetylenic diols having an HLB value of 4 to 10 are used, the acetylenic diols themselves are endowed with a certain degree of affinity for water, and the affinity with hexylene glycol and diols having 2 to 5 carbon atoms can be easily improved. As a result, it is possible to easily improve both standby dischargeability and continuous dischargeability while maintaining good print image quality of the printed matter. In particular, because the above-mentioned mechanism functions effectively, resulting in good ejection stability even after a long period of printing suspension, and further because the wettability is good even for substrates with very low interfacial free energy such as PP film, and printed matter with excellent print quality can be obtained regardless of the substrate, it is preferable to use Surfynol 440 and / or Surfynol 2502 as the acetylenic diol having an HLB value of 10 or less, and it is particularly preferable to use Surfynol 2502. Both Surfynol 440 and Surfynol 2502 are compounds that fall under the category of acetylenic diols having an HLB value of 7 to 10.

[0042] In the present disclosure, the "HLB (Hydrophilic-Lipophilic Balance) value" is one of the parameters that represent the degree of hydrophilicity and hydrophobicity of a material. The smaller the HLB value, the more hydrophobic the material is, and the larger the HLB value, the more hydrophilic the material is. Known methods for determining the HLB value include a method of measuring it experimentally and a method of calculating it from the molecular structure, and examples of methods for calculating it from the molecular structure include the Griffin method, the Davis method, and the Kawakami method. In the present application, the value calculated by the Griffin method, which is represented by the following formula (1), is used as the HLB value.

[0043] Formula (1): (HLB value) = 20 × (total molecular weight of hydrophilic portion) ÷ (molecular weight of material)

[0044] The content of acetylenic diols having an HLB value of 10 or less contained in the aqueous inkjet ink of this embodiment is preferably 0.1 to 2.0 mass %, and particularly preferably 0.2 to 1.0 mass %, relative to the total amount of the aqueous inkjet ink. By ensuring that the content of acetylenic diols having an HLB value of 10 or less falls within the above range, print image quality is improved and ejection stability is also good, even after a long printing break or during high-speed continuous printing.

[0045] (Other Surfactants) The aqueous inkjet ink of this embodiment may contain surfactants other than the acetylenic diols having an HLB value of 10 or less (referred to as "other surfactants" in the present disclosure).

[0046] In the aqueous inkjet ink of this embodiment, the other surfactants that can be used include acetylenic diol surfactants (excluding acetylenic diols having an HLB value of 10 or less), acetylene monool surfactants, siloxane surfactants, fluorine-containing surfactants, polyoxyalkylene monoalkyl ether surfactants (wherein the oxyalkylene group is an oxyethylene group and / or an oxypropylene group and the number of oxyalkylene groups is 5 to 100 and / or the number of carbon atoms in the terminal alkyl group is 5 to 22); etc. These other surfactants may be used alone or in combination of two or more.

[0047] In particular, in one embodiment, it is preferable to use one or more surfactants selected from an acetylenic diol surfactant (excluding acetylenic diols having an HLB value of 10 or less) and a siloxane surfactant, and it is particularly preferable to use a siloxane surfactant. This embodiment is preferable in that it can significantly reduce the surface tension of the aqueous inkjet ink in a very short time, thereby improving the print image quality of the printed matter, and in that it has good wettability with respect to substrates with relatively high interfacial free energy, such as PET (polyethylene terephthalate) film.

[0048] Examples of commercially available siloxane surfactants that can be suitably used include TEGO (registered trademark) Wet 270, TEGO Wet 280, TEGO Twin 4000, TEGO Twin 4100, TEGO Glide 410, TEGO Glide 432, TEGO Glide 435, TEGO Glide 440, and TEGO Glide 450 manufactured by Evonik Japan Co., Ltd.; and the Silface SAG series manufactured by Nissin Chemical Industry Co., Ltd. Of these, TEGO Wet 270, TEGO Wet 280, TEGO Twin 4000, and TEGO Twin 4100 are particularly preferably used.

[0049] The total content of surfactants contained in the aqueous inkjet ink of this embodiment is preferably 0.1 to 5 mass %, more preferably 0.2 to 4.5 mass %, even more preferably 0.3 to 4 mass %, and particularly preferably 0.4 to 1.5 mass %, relative to the total amount of the aqueous inkjet ink.

[0050] <Water> The aqueous inkjet ink of this embodiment contains water. The water contained in the ink of this embodiment is preferably ion-exchanged water (deionized water) rather than ordinary water containing various ions. The water content is preferably 45 to 75% by mass, and particularly preferably 50 to 70% by mass, based on the total amount of the aqueous inkjet ink. Because water has a low boiling point, it volatilizes preferentially from the nozzle end face of the inkjet head, tending to increase the solids concentration at the gas-liquid interface. This can lead to non-uniform distribution of surfactants (especially those with poor solubility in water, such as acetylenic diols with an HLB value of 10 or less), potentially leading to poor jetting stability. By limiting the water content within the above range, good jetting stability is achieved, even after long periods of printing suspension or during continuous printing.

[0051] <Colorant> The aqueous inkjet ink of this embodiment contains a colorant. Conventionally known dyes and pigments can be used as the colorant. Among these, it is preferable to use a pigment, because it can produce printed matter with high density or hiding power and excellent lightfastness, waterfastness, etc., and it also dries and thickens appropriately after landing on the substrate, suppressing color mixing between ink colors and resulting in good image quality.

[0052] As the pigment, any conventionally known organic or inorganic pigment can be used, and for example, pigments represented by the following color index names can be used: red pigments, such as 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, and 282; violet pigments, such as C.I. As orange pigments, C.I. Pigment Violet 19, 23, 29, 32, 36, 37, 42, 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. 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; black pigments include 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.

[0053] The content of the pigment contained in the aqueous inkjet ink of this embodiment may be adjusted depending on the intended use of the printed matter produced using the aqueous inkjet ink. For example, the content of the pigment is preferably 0.5 to 30% by mass relative to the total amount of the aqueous inkjet ink. Furthermore, except for white aqueous inkjet inks (aqueous white inks), 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 printed matters with high density without deteriorating the jetting stability of the aqueous inkjet ink. On the other hand, in the case of aqueous white inks, 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 printed matters with high hiding power without deteriorating the jetting stability of the aqueous white ink.

[0054] <Resin> The aqueous inkjet ink of this embodiment may contain a resin. The use of a resin is preferred in that the viscosity of the ink can be adjusted to a range appropriate for ejection from an inkjet head. Furthermore, when the aqueous inkjet ink is printed on a poorly permeable substrate or a non-permeable substrate and dried, the viscosity increases at a speed on the order of μs, which is preferred in that bleeding and beading can be suppressed and print image quality can be improved.

[0055] In the present disclosure, the term "resin" refers to a compound in which one or more types of monomers are linked together by covalent bonds to form a main chain, and which has a mass average molecular weight of 1,000 or more.

[0056] In the present disclosure, the mass average molecular weight of a compound is measured by a method conforming to JIS K 7252, and is expressed in terms of polystyrene. Specific examples of measurement conditions are shown below. Apparatus used: Tosoh Corporation's "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

[0057] Generally, water-soluble resins and resin microparticles are known as the forms of resins used in aqueous inkjet inks. The resin contained in the aqueous inkjet ink of this embodiment may be a water-soluble resin or resin microparticles. Furthermore, a combination of a water-soluble resin and resin microparticles may be used.

[0058] In this disclosure, a resin having a solubility of 1 g or more in 100 g of water at 25° C. is referred to as a "water-soluble resin," and a resin having a solubility of less than 1 g is referred to as a "water-insoluble resin." Furthermore, among the water-insoluble resins, a resin that is dispersed in water in the form of particles and has a volume-based median diameter (also referred to as "D50" in this disclosure) of 10 to 1,000 nm is referred to as a "resin particle."

[0059] In addition, in the present disclosure, "D50" is a value measured in an environment of 25°C using a dynamic light scattering particle size distribution measuring device such as "Nanotrac UPA-EX150" manufactured by Microtrac Bell.

[0060] Examples of resins that can be used in the aqueous inkjet ink of this embodiment include acrylic resins, styrene resins, maleic acid (anhydride)-based resins, styrene-maleic acid (anhydride) resins, olefin-maleic acid (anhydride) resins, urethane resins, polyester resins, vinyl chloride resins, vinyl chloride-vinyl acetate resins, polyolefin resins, vinyl alcohol resins, etc. These resins may be used alone or in combination of two or more.

[0061] In the present disclosure, the term "acrylic resin" refers to a resin using an acrylic acid ester and / or a methacrylic acid ester as a polymerizable monomer (a styrene-based monomer may also be used). Furthermore, the term "maleic acid (anhydride)" refers to at least one selected from "maleic acid" and "maleic anhydride."

[0062] The resin is used for any purpose, for example, for dispersing pigments, as a binder, as a wax, or as a thickener, etc. The resin may be used for a single purpose or may have multiple purposes.

[0063] The preferred content of the resin contained in the aqueous inkjet ink of this embodiment varies depending on the application of the resin. The total amount of resin contained in the aqueous inkjet ink is preferably 1 to 25% by mass, more preferably 2 to 20% by mass, even more preferably 4 to 19% by mass, and particularly preferably 5 to 18% by mass, based on the total amount of the aqueous inkjet ink. By ensuring that the total amount of resin is within the above range, after the aqueous inkjet ink lands on the printing substrate, the viscosity of the aqueous inkjet ink increases, mainly at the gas-liquid interface, as water evaporates. As a result, color mixing with adjacently landed aqueous inkjet inks is suppressed, resulting in improved print quality.

[0064] (Pigment Dispersion Resin) As described above, the aqueous inkjet ink of this embodiment can use a resin used for pigment dispersion (also simply referred to as a "pigment dispersion resin" in the present disclosure). By selecting and considering the composition and molecular weight of the polymerizable monomers that make up the pigment dispersion resin, the covering ability and charge of the pigment dispersion resin can be easily adjusted, making it possible to impart dispersion stability to even fine pigments, and further making it possible to obtain printed matter that is excellent in ejection stability, color development, and color reproducibility.

[0065] The type of pigment dispersing resin is not particularly limited, and the above-mentioned types of resins can be used. Among them, it is preferable to use one or more resins selected from the group consisting of acrylic resins, urethane resins, polyester resins, and polyolefin resins in terms of ejection stability, wide range of material selection, ease of synthesis, etc.

[0066] The pigment dispersing resin may be synthesized by a known method or may be a commercially available product. There are no particular limitations on its 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 dispersing resin.

[0067] When a water-soluble resin is used as the pigment dispersion resin, its acid value is preferably more than 100 mgKOH / g and not more than 450 mgKOH / g, more preferably from 120 to 400 mgKOH / g. It is particularly preferably from 150 to 350 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 an inkjet head is possible regardless of the use conditions. Furthermore, this is preferable in that the solubility of the pigment dispersion resin in water can be ensured and the interaction between the pigment dispersion resins is favorable, thereby reducing the viscosity of the pigment dispersion and achieving good ejection properties from an inkjet head.

[0068] On the other hand, when a water-insoluble resin is used as the pigment dispersing 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. When the acid value is within the above range, printed matter having excellent drying properties and good print quality can be obtained.

[0069] The acid value of a resin can be measured using a known device. In the present disclosure, the acid value of a resin is a value measured by potentiometric titration in accordance with JIS K 2501. A specific example of a measurement method is a method in which a resin is dissolved in a toluene-ethanol mixed solvent using an AT-610 manufactured by Kyoto Electronics Manufacturing Co., Ltd., and then titrated with a potassium hydroxide solution, and the acid value is calculated from the titration amount up to the endpoint.

[0070] In the aqueous inkjet ink of this embodiment, from the viewpoint of improving the adsorption ability to the pigment and ensuring dispersion stability, it is preferable to introduce an aromatic group into the pigment dispersing resin. Examples of aromatic groups include, but are not limited to, phenyl, naphthyl, anthryl, tolyl, xylyl, mesityl, and anisyl groups. Of these, phenyl, naphthyl, and tolyl groups are preferred from the viewpoint of ensuring sufficient dispersion stability.

[0071] 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 incorporated is preferably 5 to 75 mass %, more preferably 5 to 65 mass %, and even more preferably 10 to 50 mass %, relative to the total amount of monomers constituting the pigment dispersion resin.

[0072] <Other Components> The aqueous inkjet ink of this embodiment may contain, in addition to the components described above, a pH adjuster and other additives. Examples of the other additives include a crosslinking agent, a preservative, an ultraviolet absorber, and an infrared absorber. For each of these components, one or more conventionally known compounds may be used.

[0073] <Method for Producing Aqueous Inkjet Ink> The aqueous inkjet ink of this embodiment can be produced by a conventionally known method. For example, when a water-insoluble colorant (water-insoluble colorant) is used as the colorant, the water-insoluble colorant is dispersed in a medium containing at least water (aqueous medium) to produce a water-insoluble colorant dispersion. On the other hand, when a water-soluble colorant (water-soluble colorant) is used as the colorant, the water-soluble colorant is dissolved in a water-based medium to produce an aqueous colorant solution. Then, water, a water-soluble organic solvent, a surfactant, etc. are added to the water-insoluble colorant dispersion and / or the aqueous water-soluble colorant solution, followed by thorough stirring and mixing, and then removing coarse particles by techniques such as filtration and centrifugation. However, the method for producing the aqueous inkjet ink of this embodiment is not limited to the above-mentioned method.

[0074] <Characteristics of Aqueous Inkjet Ink> The aqueous inkjet ink of this embodiment preferably has a viscosity at 25°C of 3 to 15 mPa·s. Within this viscosity range, droplets of the aqueous inkjet ink can be stably ejected not only from inkjet heads with ejection frequencies of approximately 4 to 10 kHz, but also from inkjet heads with high ejection frequencies of approximately 20 to 70 kHz. In particular, when the viscosity of the aqueous inkjet ink of this embodiment at 25°C is 4 to 10 mPa·s, the aqueous inkjet ink can be stably ejected even when an inkjet head with a design resolution of 600 dpi or higher is used. In this disclosure, the viscosity is measured at 25°C using a cone-plate rotational viscometer (E-type viscometer, cone angle 1°34') such as the TVE25L viscometer manufactured by Toki Sangyo Co., Ltd.

[0075] Furthermore, in order to obtain an aqueous inkjet ink that is excellent in ejection stability and print quality of printed matter, the static surface tension of the aqueous inkjet ink of this embodiment at 25° C. is preferably 18 to 35 mN / m, and particularly preferably 21 to 32 mN / m. In the present disclosure, the static surface tension is a value measured in an environment of 25° C. using the Wilhelmy method (plate method) with an "Automatic Surface Tensiometer CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd.

[0076] Furthermore, when the aqueous inkjet ink of this embodiment contains a water-insoluble colorant, in order to achieve both high levels of ejection stability and high levels of density or hiding power of the printed matter, the volume-based median diameter (D50) of the water-insoluble colorant is preferably 30 to 450 nm, more preferably 50 to 400 nm, and particularly preferably 70 to 350 nm.

[0077] <Aqueous Inkjet Ink Set> The aqueous inkjet ink of this embodiment may be used singly, or may be used as an aqueous inkjet ink set combining two or more aqueous inkjet inks. Examples of such aqueous inkjet ink sets include a four-color aqueous inkjet ink set (process color ink set) consisting of a cyan aqueous inkjet ink (aqueous cyan ink), a magenta aqueous inkjet ink (aqueous magenta ink), a yellow aqueous inkjet ink (aqueous yellow ink), and a black aqueous inkjet ink (aqueous black ink); or a five-color aqueous inkjet ink set obtained by adding an aqueous white ink to the process color ink set. It is preferable that all of the aqueous inkjet inks constituting the aqueous inkjet ink set satisfy the requirements of the embodiment of the present invention described above.

[0078] <Ink-Pretreatment Liquid Set> The aqueous inkjet ink of this embodiment and the aqueous inkjet ink set described above can also be used in a form in which it is 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 to a printing substrate before printing with the aqueous inkjet ink, it is possible to form a layer (ink aggregation layer) that intentionally aggregates the solid components contained in the aqueous inkjet ink. Next, by landing the aqueous inkjet ink on the ink aggregation layer, it is possible to prevent coalescence and color mixing of droplets of the aqueous inkjet ink, and to significantly improve the print quality of the printed matter.

[0079] As the flocculant, for example, a water-soluble inorganic or organic salt containing a polyvalent metal ion, and a resin having a cationic group in which the cationic group equivalent is greater than the anionic group equivalent can be used.

[0080] <Inkjet Printing Method> The aqueous inkjet ink of this embodiment is used in the inkjet printing method described above. A printing method using the inkjet printing method typically includes an ink ejection step and a drying step of the ejected ink. In the above printing method, the aqueous inkjet ink of this 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).

[0081] (Discharge Process) In the discharge process, one example of the operation method of the inkjet head is a shuttle (scan) method in which the inkjet head is scanned back and forth in a direction perpendicular to the transport direction of the printing substrate, while discharging and recording the aqueous inkjet ink. Another example of the operation method is a single-pass method in which the aqueous inkjet ink is discharged and recording is performed as the printing substrate passes under a fixedly disposed inkjet head. The inkjet head equipped with the aqueous inkjet ink of this embodiment may be either a shuttle method or a single-pass method. Of these, the single-pass method is preferably selected because it is less likely to cause deviation in the landing position of droplets of the aqueous inkjet ink, improving the print quality of the printed matter, and further because it enables high-speed printing and can demonstrate high productivity as an alternative to plate-based printing.

[0082] The method of ejection from the inkjet head can also be selected from any known methods, such as a piezoelectric method that utilizes the volume change of a piezoelectric element, a thermal method that ejects aqueous inkjet ink by bubbles generated by heating a heater, and a valve method that ejects pressurized aqueous inkjet ink by opening and closing a nozzle cover (valve) with a solenoid.

[0083] The droplet volume of the aqueous inkjet ink ejected from the inkjet head is preferably 0.5 to 20 picoliters, and particularly preferably 0.5 to 15 picoliters, from the viewpoints of reducing drying load and improving print quality. Furthermore, from the viewpoint of improving print quality, it is preferable to adjust the printing conditions (specifically, the drive frequency and number of inkjet heads, and the printing speed). In some embodiments, the printing conditions may be adjusted so that the recording resolution of the printed matter is preferably 600 dpi or higher, more preferably 1200 dpi or higher.

[0084] (Drying Step) Examples of drying methods employed in the drying mechanism used in the drying step include heat drying, hot air drying, infrared drying (for example, infrared with a wavelength of 700 to 2500 nm), microwave drying, and drum drying. In the drying step, one or more of these methods can be selected and used as desired. Furthermore, when two or more of the above drying methods are used, the drying methods may be used separately (for example, consecutively) or simultaneously. For example, by using heat drying and hot air drying in combination, the aqueous inkjet ink can be dried more quickly than when each method is used alone.

[0085] 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, when a heat drying method is employed, the drying temperature is preferably 35 to 100° C. When a hot air drying method is employed, the hot air temperature is preferably 50 to 250° C. From the same viewpoint, when an infrared drying method is employed, it is preferable that 50% or more of the integrated value of the total output of the irradiated infrared rays is in the wavelength region of 700 to 2200 nm.

[0086] <Printed Material> A printed material according to one embodiment of the present invention has a printing substrate and a printed layer containing an image or characters formed on the printing substrate by printing the aqueous inkjet ink of this embodiment. Printing can be suitably carried out according to an inkjet printing method. Details of the printing method and printing conditions are as described above. (Printed Material) The printing substrate onto which the aqueous inkjet ink of this embodiment is printed is not particularly limited. In some embodiments, the printing substrate may be a poorly permeable substrate or a non-permeable substrate. Generally, printing on poorly permeable substrates and non-permeable substrates is prone to bleeding or beading, resulting in poor print quality. In contrast, by using the aqueous inkjet ink of this embodiment, printed materials having print quality equivalent to that of plate-based printing can be obtained even on poorly permeable substrates and non-permeable substrates, and even at high speeds.

[0087] The permeability of a printing substrate can be determined by the amount of water absorption measured by a dynamic scanning absorptivity meter. In the present disclosure, it is defined as the amount of pure water absorbed in a contact time of 100 msec, measured by the following method. Specifically, the permeability is defined as the amount of water absorption of 1 g / m ２ A printing substrate with a water absorption of less than 1 g / m is called an "impermeable substrate." ２ 6g / m or more ２ A printing substrate with a water absorption of less than 6 g / m is called a "hard-to-penetrate substrate." ２ A printing substrate that satisfies the above criteria is referred to as a "permeable substrate." The water absorption of a printing substrate can be measured, for example, using a dynamic scanning absorptivity meter (for example, "KM500win" manufactured by Kumagai Riki Kogyo Co., Ltd.) set under the following conditions, using a printing substrate approximately 15 to 20 cm square as a sample, in an environment of 23°C and 50% RH: - Measurement method: Spiral scanning - Measurement start radius: 20 mm - Measurement end radius: 60 mm - Contact time: 10 to 1,000 msec - Number of sampling points: 19 (measured at approximately equal intervals relative to the square root of the contact time) - Scanning interval: 7 mm - Rotating table speed switching angle: 86.3 degrees - Headbox conditions: width 5 mm, slit width 1 mm

[0088] Examples of impermeable substrates 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; and glass.

[0089] The printing substrates listed above may have a smooth surface or may have an uneven surface. The printing substrates may be transparent, translucent, or opaque. The printing substrates may be in the form of a roll or sheets. Additionally, a laminate obtained by bonding two or more of the printing substrates listed above to each other may be used as the printing substrate. A release adhesive layer or the like may be provided on the side opposite the printing surface, or an adhesive layer or the like may be provided on the printing surface after printing.

[0090] The printing surface of the printing substrates listed above may be subjected to surface modification such as corona treatment or plasma treatment. Surface modification is preferable in that it improves the wettability of the aqueous inkjet ink of this embodiment, and makes it easy to obtain printed matter that is excellent in print quality and drying properties, and that also has good abrasion resistance and substrate adhesion due to the uniformity of the printed surface.

[0091] The aqueous inkjet ink of this embodiment will be described in more detail below with reference to examples and comparative examples. In the following description, "parts" and "%" mean "parts by mass" and "% by mass", respectively, unless otherwise specified.

[0092] Preparation of Water-Soluble Colorant (Pigment) Dispersion: 450 g of carbon black ("PrinteX85" manufactured by Orion Engineered Carbons), 90 g of acrylic resin, and 2,460 g of water were added to a mixing vessel (volume 10 L) equipped with a stirrer and stirred (premixed) for 1 hour to obtain a reaction mixture. The acrylic resin was a random polymer of styrene / acrylic acid / behenyl acrylate = 45 / 30 / 25 (mass ratio), in which all acid groups had been neutralized with dimethylaminoethanol, and had an acid value of approximately 230 mg KOH / g and a mass-average molecular weight of 20,000. Next, circulation dispersion of the reaction mixture was initiated using a "Dynomill" (volume 0.6 L) manufactured by Shinmaru Enterprises, Inc., filled with 1,800 g of zirconia beads with a diameter of 0.5 mm. The D50 of the reaction mixture was measured at regular intervals (for example, every hour) using the above-mentioned device, and the circulating dispersion was terminated when the D50 reached 120 nm or less. In this manner, a black pigment dispersion was produced.

[0093] A cyan pigment dispersion was also produced using the same raw materials and method as for the black pigment dispersion, except that LIONOL BLUE 7358G (C.I. Pigment Blue 15:3) manufactured by Toyocolor Co., Ltd. was used as the pigment and circulation dispersion was carried out until D50 reached 180 nm or less.

[0094] <Production of aqueous inkjet ink sets> Using the water-soluble colorant (pigment) dispersions produced by the method described above, each raw material was charged into a mixing vessel equipped with a stirrer so as to obtain the formulation shown in each column of Table 2. After charging, the mixture was heated to 50°C and mixed for an additional hour. Next, the mixture was filtered through a membrane filter with a pore size of 0.8 μm to produce aqueous inkjet ink sets 1 to 52. Each set consisted of an aqueous black ink (K) and an aqueous cyan ink (C). A black pigment dispersion and a cyan pigment dispersion were used to produce these inks, respectively.

[0095] In the production of the aqueous inkjet ink set, each raw material was added while stirring the mixture in the mixing vessel. The components listed in each column of Table 2 were added in the order listed from top to bottom. However, when producing an aqueous inkjet ink that did not contain one or more of these components, that component was not added, and the next component was added in the order listed. Furthermore, for components containing two or more raw materials, the order of addition of the components within that component was arbitrary.

[0096]

[0097]

[0098]

[0099]

[0100] The meanings of the abbreviations and details of the product names listed in Table 2 are as follows. In Table 2, "bp" represents the boiling point, and "Nv" represents the solids concentration. VE1 represents the energy required to evaporate the diols having 2 to 5 carbon atoms, VE2 represents the energy required to evaporate hexylene glycol, and (VE2) / (VE1) represents the ratio thereof. (Diols having 2 to 5 carbon atoms) ・EG: Ethylene glycol (specific heat: 0.556 cal / g・℃, boiling point: 198°C) ・1,2-PD: 1,2-propanediol (specific heat: 0.574 cal / g・℃, boiling point: 188°C) ・1,2-BD: 1,2-butanediol (specific heat: 0.559 cal / g・℃, boiling point: 191°C) ・1,3-BD: 1,3-butanediol (specific heat: 0.559 cal / g・℃, boiling point: 207°C) ・1,5-PeD: 1,5-pentanediol (specific heat: 0.526 cal / g・℃, boiling point: 239°C) (Hexylene glycol) Specific heat: 0.555 cal / g・℃, boiling point: 196°C (Other water-soluble organic solvents) 3-methyl-1,3-Butylene glycol monomethyl ether (boiling point: 173°C, surface tension: 30mN / m) Diethylene glycol monoethyl ether (boiling point: 202°C, surface tension: 32mN / m) Diethylene glycol methyl ethyl ether (boiling point: 179°C, surface tension: 24mN / m) Glycerin (boiling point: 290°C, surface tension: 62mN / m) 2-pyrrolidone (boiling point: 245°C, surface tension: 45mN / m) (Acetylene diol surfactants) Surfynol 485: acetylene diol manufactured by Evonik Japan, HLB value: 17 Surfynol 465: acetylene diol manufactured by Evonik Japan, HLB value: 13 Olfine E1006: acetylene diol manufactured by Nissin Chemical Industry Co., Ltd., HLB value: 11 Olfine E1004: Acetylene diol manufactured by Nissin Chemical Industry Co., Ltd., HLB value: 9 Surfynol 2502: Acetylene diol manufactured by Evonik Japan, HLB value: 8 Surfynol 440: Acetylene diol manufactured by Evonik Japan, HLB value: 8 Surfynol 420: Acetylene diol manufactured by Evonik Japan, HLB value: 4 Surfynol 104: Acetylene diol manufactured by Evonik Japan, HLB value: 3 (Siloxane-based surfactant) TEGO Wet 280: Siloxane-based surfactant manufactured by Evonik Japan TEGO Twin 4100: Siloxane-based surfactant manufactured by Evonik Japan (Other components) Proxel GXL: 1,2-benzisothiazol-3-one dipropylene glycol-aqueous solution (1,2-benzisothiazol-3-one: dipropylene glycol: water = 2:6:2, preservative manufactured by Arch Chemicals) (resin) Resin A (for binder use): A resin produced by reproducing the manufacturing example of binder resin 24 in JP 2020-180178 A (used in the form of a resin solution with a solid content of 30%). The acid value of the obtained resin A was 29.5 mg KOH / g, the hydroxyl value was 21.5 mg KOH / g, and the mass average molecular weight was 17,500. Resin B (for binder use): QE-1042 manufactured by Seiko PMC Co., Ltd. (acrylic resin fine particles, solid content: 40.5%, acid value: 33 mg KOH / g),

[0101] Examples 1 to 43, Comparative Examples 1 to 9 The aqueous inkjet ink sets produced by the methods described above were used to carry out the following evaluations. The evaluation results are shown in Table 2.

[0102] <Evaluation 1: Evaluation of Discharge Stability (Standby Discharge)> An inkjet discharge device equipped with a Kyocera Corporation inkjet head "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) was filled with the water-based black ink or water-based cyan ink constituting the above-mentioned water-based inkjet ink set. After filling, the water-based black ink or water-based cyan ink was pressurized until it oozed out of the nozzles of the inkjet head, the nozzle plate was wiped, and then a nozzle check pattern was immediately printed at a frequency of 40 kHz. Then, for those nozzles in the nozzle check pattern that showed no missing nozzles, a solid image (printing rate 100%) measuring 10 cm in width and 30 cm in length was immediately printed. OK topcoat paper was used as the printing substrate, and the printing conditions were a frequency of 40 kHz, a conveyor drive speed of 50 m / min, and a resolution of 1200 x 1200 dpi. After printing, the inkjet ejection device was left on standby for 1 hour in an environment of 25°C, and then a solid image was printed again using the same printing substrate and printing conditions as above. At this time, standby ejection performance was evaluated by visually checking whether the initial portion of the solid print was printed. The evaluation criteria were as follows, and a rating of "C+" or higher was deemed usable. The above evaluation was performed on each of the aqueous black ink and aqueous cyan ink that constituted the aqueous inkjet ink set. Table 1 also lists the results of the aqueous black ink and aqueous cyan ink that received poor evaluation results.(Evaluation criteria) A: No missing nozzles were confirmed in the nozzle check pattern, and no chipping was observed at the start of printing in the solid print printed immediately after printing the nozzle check pattern or after waiting for one hour. B: No missing nozzles were confirmed in the nozzle check pattern, and no chipping was observed at the start of printing in the solid print printed immediately after printing the nozzle check pattern, but chipping of less than 3 cm was observed at the start of printing in the solid print printed after waiting for one hour. C+: No missing nozzles were confirmed in the nozzle check pattern, and no chipping was observed at the start of printing in the solid print printed immediately after printing the nozzle check pattern, but chipping of 3 cm or more but less than 5 cm was observed at the start of printing in the solid print printed after waiting for one hour. C: No missing nozzles were confirmed in the nozzle check pattern, and in the solid print printed immediately after printing the nozzle check pattern, no chipping was observed at the start of printing, but in the solid print printed after waiting for one hour, a chipping of 5 cm or more was observed at the start of printing. D+: No missing nozzles were confirmed in the nozzle check pattern, but in the solid print printed immediately after printing the nozzle check pattern, a chipping was observed at the start of printing. D: One or more missing nozzles were confirmed in the nozzle check pattern.

[0103] Evaluation 2: Evaluation of Discharge Stability (Continuous Dischargeability) An inkjet discharge device having the same configuration as in Evaluation 1 above was filled with the aqueous black ink or aqueous cyan ink constituting the aqueous inkjet ink set. After filling, the aqueous black ink or aqueous cyan ink was pressurized until it oozed out of the nozzles of the inkjet head. The nozzle plate was then wiped, and immediately, a nozzle check pattern was printed at a frequency of 40 kHz. Then, for those nozzles in the nozzle check pattern for which no nozzle clogs were found, 10 solid images (100% printing rate) measuring 10 cm wide and 30 cm long were immediately printed in succession. OK topcoat paper was used as the printing substrate, and the printing conditions were a frequency of 40 kHz, a conveyor drive speed of 50 m / min, and a resolution of 1200 x 1200 dpi. Continuous dischargeability was evaluated by visually inspecting each of the 10 solid prints for completeness of printing. The evaluation criteria were as follows, and a rating of "B" or higher was deemed usable. The above evaluation was carried out for each of the water-based black ink and water-based cyan ink that made up the water-based inkjet ink set. Table 1 also lists the results of the water-based black ink and water-based cyan ink that received poor evaluation results.(Evaluation criteria) A: No missing nozzles were confirmed in the nozzle check pattern, and no chipping was observed in any of the 10 solid prints B+: No missing nozzles were confirmed in the nozzle check pattern, but chipping occurred on the 10th sheet of solid prints printed immediately after printing the nozzle check pattern B: No missing nozzles were confirmed in the nozzle check pattern, but chipping occurred on the 9th sheet of solid prints printed immediately after printing the nozzle check pattern C: No missing nozzles were confirmed in the nozzle check pattern, but chipping occurred on any of the 4th to 8th sheets of solid prints printed immediately after printing the nozzle check pattern D+: No missing nozzles were confirmed in the nozzle check pattern, but chipping occurred on any of the 1st to 3rd sheets of solid prints printed immediately after printing the nozzle check pattern D: One or more missing nozzles were confirmed in the nozzle check pattern.

[0104] <Preparation of Black / Cyan Gradient Printed Material> An inkjet ejection device was prepared, in which two Kyocera Corporation inkjet heads "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) were arranged side by side along the transport direction of the substrate. An aqueous inkjet ink set was filled in the following order from the upstream side in the transport direction: aqueous black ink, aqueous cyan ink. Furthermore, an A4-size (21 cm wide x 30 cm long) OPP film (Mitsui Chemicals Tohcello Co., Ltd. biaxially oriented polypropylene film "OPU-1": thickness 20 μm) was fixed on the conveyor as the printing substrate. The conveyor was then driven at 50 m / min, and as the printing substrate passed below the installation position of the inkjet heads, the aqueous inkjet ink set was ejected at a drop volume of 2.6 pL, respectively, to print a black / cyan gradation image. More specifically, a black gradation image (5 cm wide x 30 cm long) printed with an aqueous black ink and varying in 10% increments between 10 and 100% coverage was obtained, and a cyan gradation image (5 cm wide x 30 cm long) printed with an aqueous cyan ink and varying in 10% increments between 10 and 100% coverage was obtained, arranged side by side. The printed substrate was then immediately placed in a constant temperature incubator set at 70°C and dried for 3 minutes to produce a black / cyan gradation print.

[0105] <Evaluation 3: Evaluation of print quality (beading, whiteout)> The black / cyan gradation prints produced by the method described above were visually observed. The print coverage was checked at locations where unevenness in density (beading) began to appear, and for prints where no unevenness in density was observed at a print coverage of 80%, the presence or absence of whiteout at a print coverage of 100% was further checked to evaluate the print quality of the black / cyan gradation prints. The evaluation criteria were as follows, and a rating of "C" or higher was deemed usable. Table 1 lists the evaluation results of the ink that had the worst evaluation result among the inks constituting the aqueous inkjet ink set. (Evaluation criteria) A: No unevenness in density was observed at a printing rate of 80%, and no white spots were observed at a printing rate of 100%. B: No unevenness in density was observed at a printing rate of 80%, but white spots were observed at a printing rate of 100%. C: No unevenness in density was observed at a printing rate of 70%, but unevenness in density was observed at a printing rate of 80%. D+: No unevenness in density was observed at a printing rate of 60%, but unevenness in density was observed at a printing rate of 70%. D: Unevenness in density was observed at a printing rate of 60%.

[0106] As shown in Table 2 above, the aqueous inkjet inks of Examples 1 to 43 having the configuration of this embodiment were superior in ejection stability to the aqueous inkjet inks of Comparative Examples 1 to 9, and also produced excellent printed images with no beading or white voids observed. These results demonstrate that the aqueous inkjet inks having the configuration of this embodiment are excellent aqueous inkjet inks that combine good print quality of printed matter with good ejection stability.

Claims

1. An aqueous inkjet ink comprising a colorant, a water-soluble organic solvent, a surfactant, and water, wherein the water-soluble organic solvent comprises a diol having 2 to 5 carbon atoms and hexylene glycol, and the surfactant comprises an acetylenic diol having an HLB value of 10 or less.

2. The aqueous ink-jet ink according to claim 1, wherein the content of the hexylene glycol is 0.1 to 30% by mass based on the total amount of the aqueous ink-jet ink.

3. The aqueous inkjet ink according to claim 1 or 2, wherein the content of the acetylene diols having an HLB value of 10 or less is 0.1 to 2.0 mass % based on the total amount of the aqueous inkjet ink.

4. A printed matter obtained by printing with the aqueous inkjet ink according to claim 1 or 2.

Images