Aqueous inkjet ink and printed matter

Abstract

An aqueous inkjet ink contains a pigment, a pigment dispersion resin, a surfactant, a water-soluble organic solvent, and a binder resin, wherein the pigment dispersion resin contains at least one selected from the group consisting of block polymers and polymers having a crosslinked structure, the water-soluble organic solvent contains hexylene glycol, and if WR(g) is the sum of the content of the pigment dispersion resin and the content of the binder resin in 100g of the aqueous inkjet ink, and WP(g) is the content of the pigment in 100g of the aqueous inkjet ink, then the value represented by WR / WP is 1-7.

Description

Water-based inkjet inks and printed materials

[0001] SUMMARY OF THE INVENTION An embodiment of the present invention relates to an aqueous inkjet ink and a printed matter produced 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 the digital printing method is the inkjet printing method. In the inkjet printing method, ink droplets are ejected from minute nozzles and applied to a printing substrate (also simply referred to as "substrate" in this disclosure), thereby forming an image and / or characters on the printing substrate. The inkjet printing method 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 the inkjet printing method.

[0004] In this disclosure, the printing substrate and the image and / or characters formed on the printing substrate by printing ink are collectively referred to as a "printed matter." The above "image" also includes solid images and seamless images such as checkered images.

[0005] Inks used in inkjet printing methods (referred to as "inkjet inks" in this disclosure) are classified into solvent-based, water-based, ultraviolet-curable, and other types depending on their composition. Meanwhile, 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 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 ultraviolet-curable inkjet inks that use the above-mentioned raw materials.

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

[0007] However, in the past, when producing solid prints (prints with a 100% coverage) using aqueous inkjet inks on poorly permeable substrates such as coated paper and art paper, as well as non-permeable substrates such as film, it was difficult to obtain prints with print quality equivalent to that of plate-based printing. Specifically, when producing solid prints using aqueous inkjet inks, a phenomenon known as "whiteout" occurs, in which areas where the aqueous inkjet ink does not adhere to the printed substrate. Furthermore, a phenomenon known as "bleeding," in which aqueous inkjet inks of different colors mix with each other, is likely to occur. These phenomena are due to the poor wettability of the aqueous inkjet ink to the substrate, particularly on the order of microseconds (μs), caused by the uniquely high surface tension of water, the main solvent of aqueous inkjet inks.

[0008] Typically, hydrophobic organic solvents and / or surfactants are used to improve wettability of printing substrates. However, these components have poor solubility in water and are prone to foaming, resulting in the generation of air bubbles that can contribute to nozzle clogging. Furthermore, these components orient at the gas-liquid interface formed at the nozzle end face 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 clogging (a phenomenon in which aqueous inkjet ink is not ejected from the nozzle) immediately after the start of printing. Furthermore, due to the structure of the inkjet head, the nozzle diameter is very small, measuring several tens of micrometers. In particular, when printing is paused for an extended period of time, liquid components (water, organic solvents, etc.) in the aqueous inkjet ink volatilize from the nozzle end face, while fresh aqueous inkjet ink may not be sufficiently supplied to the inkjet head. When this happens, the aqueous inkjet ink present in the vicinity of the nozzle end face will have an increased solid content and / or an increased proportion of organic solvents with high boiling points, causing an increase in viscosity due to aggregation and insolubilization of solid components (pigments, resins, etc.), and nozzle clogging due to this increased viscosity will become more likely to occur.

[0009] In this disclosure, the ejection stability immediately after the start of printing 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."

[0010] On the other hand, particularly in the packaging printing market, it is necessary to ensure that the dried ink film does not peel off even when the printed material to which water has adhered is rubbed with a finger or the like. In other words, the dried ink film is required to have a certain degree of water resistance. In order to impart water resistance to the dried ink film in aqueous inkjet inks containing a large amount of water-soluble ingredients, for example, a binder resin with a large mass-average molecular weight must be used to form a strong, continuous ink film after drying. On the other hand, if a certain amount or more of binder resin is contained in the aqueous inkjet ink before printing (liquid), problems are likely to occur. Specifically, in the aqueous inkjet ink, the binder resins may interact with each other or with other resins (e.g., free pigment dispersion resins, as described below), which may cause problems such as deterioration of ejection stability and / or partial inhibition of the orientation of surfactants at the gas-liquid interface.

[0011] Thus, in order to expand the use of aqueous inkjet inks in the packaging printing market, multiple issues, such as initial ejection stability, standby ejection performance, print image quality, and water resistance of the ink film, must be simultaneously resolved. However, no aqueous inkjet ink that can simultaneously and suitably resolve all of these issues has been discovered to date.

[0012] For example, Patent Document 1 discloses an ink containing three or more organic solvents with different water-octanol partition coefficients, and describes that use of this ink not only exhibits good wettability to non-absorbent media (non-permeable substrates in the present disclosure) but also produces highly glossy printed matter. Patent Document 2 discloses an inkjet ink containing three or more organic compounds (organic solvents) each having a specific structure, with the contents and ratios of these organic compounds specified, and describes that use of this inkjet ink can achieve both good wettability and good drying properties on non-absorbent media. Patent Document 3 also discloses an aqueous ink containing an acetylene-based surfactant (preferably having an HLB value of 8 or less), another nonionic surfactant, and an aqueous medium, with the HLB value, cloud point, etc. of the other nonionic surfactant specified. This water-based ink is said to be less susceptible to ejection problems caused by drying and solidification in the nozzles of the inkjet head, even when printing after a long period of printing rest, and is also less susceptible to deterioration in print quality due to uneven drying when printed on non-absorbent or poorly absorbent printing substrates.

[0013] JP 2020-125382 A International Publication No. 2022 / 224786 International Publication No. 2020 / 080121

[0014] However, the inks described in Patent Document 1 and Patent Document 2 have insufficient wetting properties on the order of μs, making it difficult to completely prevent the aforementioned color bleeding. For example, the ink specifically disclosed in Patent Document 1 contains dipropylene glycol monopropyl ether, which has low surface tension. Therefore, depending on the configuration of the aqueous inkjet ink, the meniscus of the aqueous inkjet ink present in the nozzle portion of the inkjet head may become unstable, resulting in poor standby ejection performance. Furthermore, triethylene glycol and butyl diglycol, which are used in combination with dipropylene glycol monopropyl ether, have high boiling points at 1 atmosphere, which may lead to poor drying performance and color bleeding depending on the printing substrate used. This tendency is also seen in the inkjet ink specifically disclosed in Patent Document 2, where the mainly used 1,5-pentanediol and 1,8-octanediol have high boiling points at 1 atmosphere. On the other hand, diethylene glycol monohexyl ether has low surface tension. For this reason, similar to the case of Patent Document 1, the case of Patent Document 2 may result in color bleeding in printed matter and / or deterioration of standby ejection properties. Furthermore, the aqueous inkjet inks having the configurations disclosed in Patent Documents 1 and 2 also require improvement in the water resistance of the ink film after drying.

[0015]

[0005] On the other hand, the aqueous ink specifically disclosed in Patent Document 3 uses, as surfactants, Surfynol 420, Surfynol 104, the Emulgen series (polyoxyalkylene monoalkyl ether surfactants), and the like. As will be described in detail below, Surfynol 420 and Surfynol 104 are effective raw materials from the perspective of improving print image quality because they can cause aqueous inkjet inks to wet and spread well even on printing substrates with very low interfacial free energy. However, as mentioned above, these surfactants have problems such as the tendency for bubbles to form or the meniscus of the aqueous inkjet ink to become unstable, and these problems have not been completely resolved by the aqueous ink.

[0016] As described above, the techniques described in Patent Documents 1 to 3 have not yet been able to solve all of the above-mentioned problems at a high level.

[0017] Therefore, one embodiment of the present invention provides an aqueous inkjet ink that produces printed matter that is free from white spots and color bleeding, has excellent water resistance, and also has excellent ejection stability, even when printed on poorly permeable and non-permeable substrates.

[0018] In this disclosure, a printed matter that is free from white spots and color mixing is also referred to as a "printed matter with excellent print quality."

[0019] 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.

[0020] 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 produced 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 pigment, a pigment dispersion resin, a surfactant, a water-soluble organic solvent, and a binder resin, wherein the pigment dispersion resin comprises one or more polymers selected from the group consisting of polymers having a crosslinked structure and block polymers, and the water-soluble organic solvent comprises hexylene glycol, and wherein the value expressed by WR / WP is 1 to 7, where WR (g) is the sum of the content (g) of the pigment dispersion resin and the content (g) of the binder resin contained in 100 g of the aqueous inkjet ink, and WP (g) is the content of the pigment contained in 100 g of the aqueous inkjet ink. [2] The aqueous inkjet ink according to [1] above, wherein the content of hexylene glycol is 5 to 90 mass% relative to the total content of the water-soluble organic solvent contained in the aqueous inkjet ink. [3] The aqueous inkjet ink according to [1] or [2] above, wherein the surfactant comprises an acetylene diol surfactant. [4] The aqueous inkjet ink according to any one of [1] to [3] above, wherein the water-soluble organic solvent further comprises a diol having 2 to 5 carbon atoms. [5] The aqueous inkjet ink according to any one of [1] to [4] above, further comprising a wax. [6] A printed matter obtained by printing the aqueous inkjet ink according to any one of [1] to [5] above on a printing substrate. The disclosure of the present application is related to the subject matter described in Japanese Patent Application No. 2023-198146, filed on November 22, 2023, the entire disclosure of which is incorporated herein by reference.

[0021] The aqueous inkjet ink according to one embodiment of the present invention can provide printed matter that is free from white spots and color bleeding, has excellent water resistance, and also provides excellent ejection stability, even when printing on poorly permeable and non-permeable substrates.

[0022] 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.

[0023] The aqueous inkjet ink of this embodiment having the above-described configuration has excellent wettability on the order of μs, and therefore, even when printing on poorly permeable and non-permeable substrates, it is possible to obtain printed matter of excellent print quality without white voids or color bleeding. Furthermore, stable ejection is possible even after a long printing pause. Furthermore, the printed matter has excellent water resistance. Although the mechanism behind this is not clear, the inventors speculate as follows. However, the present invention is not limited by the speculation below.

[0024] Generally, aqueous inkjet inks do not wet or spread on poorly permeable or non-permeable substrates due to the extremely high surface tension of their main component, water, making it difficult to form fine images. In contrast, when a surfactant is used, the surfactant orients at the interface with the printing substrate in a short time, on the order of μs. As a result, the ink can be smoothly wetted and spread on poorly permeable or non-permeable substrates, particularly printing substrates with very low interfacial free energy, such as polypropylene (PP) film, and improved print quality can be expected. This effect is particularly effective when a surfactant with low solubility in water is used. On the other hand, surfactants tend to foam, which can lead to the generation of air bubbles that can contribute to nozzle clogging. Furthermore, when a surfactant with poor solubility in water is used, orientation also occurs at the air-liquid interface formed on the nozzle end surface of the inkjet head, destabilizing the meniscus of the aqueous inkjet ink and making nozzle clogging more likely.

[0025] Furthermore, in order to produce 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 is insoluble in water. On the other hand, if such a binder resin is contained in a certain amount or more in the aqueous inkjet ink before printing (liquid), the binder resin may interact with the free pigment dispersion resin, causing problems such as worsening ejection stability and / or partially inhibiting the orientation of surfactants with low water solubility at the interface. On the other hand, the free pigment dispersion resin in the aqueous inkjet ink also poses the problem of inhibiting the formation of the continuous film, which may result in a deterioration in the water resistance of the printed matter.

[0026] Therefore, in the aqueous inkjet ink of this embodiment, hexylene glycol is first used as the water-soluble organic solvent. Hexylene glycol (2-methyl-2,4-pentanediol) has multiple branched alkyl groups and, compared to other alkanediols, is characterized by its strong hydrophobicity, high solubility in water, and low boiling point at 1 atmosphere. Therefore, hexylene glycol has a high affinity with the surfactants with low solubility in water described above, while also dissolving well in water. As a result, in the presence of hexylene glycol, even surfactants with low solubility in water can be uniformly present in the aqueous inkjet ink, and the orientation rate to the interface is appropriately reduced. As a result, hexylene glycol contributes to stabilizing the interface, preventing foaming and nozzle clogging and improving ejection stability.

[0027] Hexylene glycol also dissolves the binder resin with a large mass average molecular weight. As a result, even after the water has evaporated from the aqueous inkjet ink applied to the printing substrate, the binder resin remains fluid, facilitating the formation of a continuous film. Furthermore, since hexylene glycol itself does not remain in the continuous film but eventually evaporates, highly water-resistant printed materials can be produced with low energy consumption.

[0028] Furthermore, the aqueous inkjet ink of this embodiment uses a polymer having a crosslinked structure and / or a block polymer as the pigment dispersing resin, which has a stronger adsorption force to the pigment than a pigment dispersion using a random polymer as the pigment dispersing resin.

[0029] Generally, when a highly hydrophobic, water-soluble organic solvent such as hexylene glycol is added to a system containing a pigment dispersed in a pigment dispersion resin, the desorption of the pigment dispersion resin is promoted due to the high resin solubility of hexylene glycol. As a result, when a highly hydrophobic, water-soluble organic solvent is used, problems such as deterioration of dispersion stability and ejection stability, adverse effects on print quality, and deterioration of the water resistance of printed matter due to free pigment dispersion resin may occur. In contrast, the aqueous inkjet ink of this embodiment is less likely to cause the above-mentioned phenomena due to the strong adsorption of the pigment dispersion resin to the pigment. Furthermore, interactions between the binder resin and free pigment dispersion resin can be prevented. As a result, the meniscus of the aqueous inkjet ink at the nozzle end face of the inkjet head is stabilized, thereby suppressing nozzle clogging. Furthermore, free pigment dispersion resin no longer inhibits the formation of a continuous film of the binder resin, further improving the water resistance of printed matter.

[0030] Additionally, in the aqueous inkjet ink of this embodiment, the ratio of the sum (WR [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., the value expressed as WR / WP, is preferably 1 to 7. By setting the value expressed as WR / WP within the range of 1 to 7, the amount of pigment that may become discontinuous points during the formation of the continuous film can be adjusted within a suitable range, making it possible to easily obtain printed matter with excellent water resistance. Furthermore, as described above, the aqueous inkjet ink of this embodiment contains hexylene glycol, which has high solubility for binder resins and pigment dispersion resins. When the aqueous inkjet ink is applied to a printing substrate, it is believed that the water in the ink evaporates first, resulting in an increased hexylene glycol content in the aqueous inkjet ink on the printing substrate. As a result, as the binder resin dissolves, the binder resin is believed to become more fluid within the aqueous inkjet ink even after most of the water has evaporated. Furthermore, the pigment dispersion resin is softened by hexylene glycol, which increases its affinity with the binder resin, thereby reducing the adverse effects on the formation of a continuous film. Furthermore, by specifying the sum of the amount of binder resin and the amount of pigment dispersion resin relative to the pigment, the pigment is homogenized together with the softened pigment dispersion resin in the presence of the binder resin dissolved in hexylene glycol. The pigment is then fixed to the printing substrate together with the pigment dispersion resin, resulting in a printed product in which the pigment is uniformly distributed, which is thought to suppress the occurrence of whiteout and color bleeding that can occur due to poor uniformity. As a result, printed products with print quality and water resistance equivalent to plate-based printing can be obtained regardless of the printing substrate or printing speed.

[0031] 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.

[0032] The aqueous inkjet inks specifically disclosed in the above-mentioned Patent Documents 1 to 3 differ from the aqueous inkjet ink of this embodiment in that they do not use hexylene glycol. The aqueous inkjet ink disclosed in Patent Document 1 also differs from the aqueous inkjet ink of this embodiment in that the value represented by the WR / WP ratio is less than 1. On the other hand, the aqueous inkjet inks disclosed in Patent Document 2 (Examples 25 to 27) and Patent Document 3 include inks in which the value represented by the WR / WP ratio is 1 to 7, but the pigment dispersing resins used in these aqueous inkjet inks are all random polymers without a crosslinked structure.

[0033] Each component constituting the aqueous inkjet ink according to one embodiment of the present invention will be described in detail below.

[0034] <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.

[0035] (Hexylene Glycol) In order to easily improve both the ejection stability and the print image 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 20 mass %, and particularly preferably 1 to 10 mass %, relative to the total amount of the aqueous inkjet ink.

[0036] The content of the hexylene glycol is preferably 5 to 90% by mass, more preferably 10 to 80% by mass, particularly preferably 15 to 50% by mass, and extremely preferably 20 to 40% by mass, relative to the total amount of water-soluble organic solvent contained in the aqueous inkjet ink. By setting the content of hexylene glycol relative to the total amount of water-soluble organic solvent within the above range, the time required for stabilization of the surfactant at the interface can be optimized. As a result, good ejection stability can be obtained, for example, immediately after the start of printing and even during high-speed continuous printing. At the same time, wettability to the printing substrate on the order of μs is improved, thereby improving print image quality.

[0037] (C2-C5 Diols) The aqueous inkjet ink of this embodiment may further contain a C2-C5 diol. When the ink contains a C2-C5 diol, the surfactant can be further stabilized, improving standby ejection properties. Furthermore, when the aqueous inkjet ink applied to a printing substrate dries, the C2-C5 diols dissolve the binder resin together with hexylene glycol. This increases the viscosity of the aqueous inkjet ink, making it possible to suppress color bleeding. Furthermore, the binder resin can more easily form a continuous film, improving the water resistance of the printed matter. 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.

[0038] Among these, it is preferable to use alkanediols having 2 to 5 carbon atoms as the diols having 2 to 5 carbon atoms, in view of the above-mentioned effects, i.e., good ejection stability after a long printing pause and the ability to suppress color bleeding in printed matter. 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, in view of good ejection stability immediately after the start of printing and even during continuous printing.

[0039] In order to easily obtain good ejection stability under all conditions, including immediately after the start of printing, after a long period of printing suspension, and during continuous printing, 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.

[0040] Furthermore, when the aqueous inkjet ink applied to a printing substrate dries, the C2-5 diols, together with hexylene glycol, contribute to dissolving the binder resin, softening the pigment dispersion resin, and stabilizing the surfactant. This makes it easier to provide aqueous inkjet inks that are free of white spots and color bleeding, have excellent water resistance, and also have good ejection stability. From this perspective, in some embodiments, when the hexylene glycol content is taken as 1, the content of the C2-5 diols (i.e., the mass ratio of C2-5 diols / hexylene glycol) is preferably 1 to 6, more preferably 1.5 to 5, and particularly preferably 2 to 4.

[0041] (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 disclosure).

[0042] In the aqueous inkjet ink of this embodiment, examples of the other water-soluble organic solvents include alkanediols having 6 carbon atoms (excluding hexylene glycol), alkanetriols (provided that the number of carbon atoms is 3 to 6), polyoxyalkylene diols (provided that the oxyalkylene group is an oxyethylene group and / or an oxypropylene group, and the number of oxyalkylene groups is 2 to 4, excluding those having 2 to 5 carbon atoms), (poly)oxyalkylene monoalkyl ethers (provided that the oxyalkylene group is an oxyethylene group or an oxypropylene group, the number of oxyalkylene groups is 1 to 4, and the terminal alkyl group has 1 to 4 carbon atoms, or the oxyalkylene group is an oxybutylene group or oxypentylene group, the number of oxyalkylene group is 1, and the terminal alkyl group has 1 to 4 carbon atoms), (poly)oxyethylene dialkyl ethers (provided that the number of oxyalkylene groups is 1 to 4, and the terminal alkyl group has 1 to 4 carbon atoms), Examples of suitable water-soluble organic solvents include lactams (wherein the lactam ring has 5 to 7 atoms and the nitrogen and / or carbon atoms constituting the lactam ring may be bonded to an alkyl group having 1 to 2 carbon atoms, a hydroxyalkyl group having 1 to 2 carbon atoms, or a vinyl group), 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), and the like. 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 / or polyoxyalkylene.

[0043] ((Poly)oxyalkylene monoalkyl ethers) In one embodiment, the aqueous inkjet ink of this embodiment preferably uses the above-mentioned (poly)oxyalkylene monoalkyl ethers as another water-soluble organic solvent. Since the surface tension of the above-mentioned (poly)oxyalkylene monoalkyl ethers themselves is appropriately low, the ejection stability immediately after the start of printing is improved and white voids in the printed matter are easily prevented. The above-mentioned (poly)oxyalkylene monoalkyl ethers are not particularly limited. Specific examples of compounds that can be preferably used include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monoisobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, and butylene glycol monomethyl ether.

[0044] Among these compounds, compounds that have a high affinity with hexylene glycol and have a moderate boiling point and surface tension at 1 atmosphere are preferred, as they improve ejection stability immediately after the start of printing and can easily prevent white voids and color bleeding in printed materials. In some embodiments, it is preferred to use a compound in which the oxyalkylene group is an oxyethylene group or an oxypropylene group, and the terminal alkyl group has 1 to 3 carbon atoms and 4 to 9 carbon atoms. Among the compounds listed above, examples of compounds that satisfy the above requirements include ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and dipropylene glycol monopropyl ether.

[0045] Among these compounds, it is preferable to use one or more compounds selected from the group consisting of propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and dipropylene glycol monopropyl ether, i.e., (poly)oxypropylene monoalkyl ethers. (Poly)oxypropylene monoalkyl ethers have a highly hydrophobic oxypropylene group, and therefore have a particularly high affinity with hexylene glycol. Furthermore, compared with compounds having an oxyethylene group, they tend to have a lower boiling point at 1 atmosphere. As a result, ejection stability immediately after the start of printing and print image quality are easily improved. Furthermore, as described below, this effect is particularly enhanced when the amount of polymerizable monomer having an aromatic group in the crosslinked polymer and block polymer is within a specific range.

[0046] Furthermore, from the viewpoint of particularly suitably exhibiting the above-mentioned effects and significantly improving the ejection stability and print image quality immediately after the start of printing, among the (poly)oxypropylene monoalkyl ethers listed above, one or more compounds selected from the group consisting of propylene glycol monomethyl ether, propylene glycol monoethyl ether, and propylene glycol monopropyl ether are particularly preferably used.

[0047] In some embodiments, the water-soluble organic solvent constituting the aqueous inkjet ink preferably contains hexylene glycol and one or more diols selected from the group consisting of diols having 2 to 5 carbon atoms and (poly)oxyalkylene monoalkyl ethers. In some embodiments, the water-soluble organic solvent may contain hexylene glycol and one or more diols having 2 to 5 carbon atoms. Alternatively, the water-soluble organic solvent may contain hexylene glycol and one or more (poly)oxyalkylene monoalkyl ethers. Alternatively, the water-soluble organic solvent may contain hexylene glycol, one or more diols having 2 to 5 carbon atoms, and one or more (poly)oxyalkylene monoalkyl ethers. In particular, from the viewpoint of facilitating improvements in standby jetting performance, color bleeding, and water resistance of printed matter, it is more preferable for the water-soluble organic solvent to contain hexylene glycol and one or more diols having 2 to 5 carbon atoms. More preferably, the water-soluble organic solvent may contain hexylene glycol, one or more diols having 2 to 5 carbon atoms, and one or more (poly)oxyalkylene monoalkyl ethers. As the diols having 2 to 5 carbon atoms, for example, at least one of 1,2-propanediol and 1,5-pentanediol can be used. Of these, it is preferable to use at least 1,2-propanediol, as this further improves the initial ejection stability.

[0048] The other water-soluble organic solvent contained in the aqueous inkjet ink of this embodiment preferably has a static surface tension of 27 to 40 mN / m at 25°C. 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, for example, a long period of printing suspension.

[0049] The total content of water-soluble organic solvents contained in the aqueous inkjet ink of this embodiment may be 3% by mass or more and 36% by mass or less, based on the total amount of the aqueous inkjet ink. In some embodiments, the total content is preferably 3 to 33% by mass, more preferably 5 to 30% by mass, and particularly preferably 7 to 30% by mass. In some embodiments, the total content may be preferably 10 to 27% by mass, more preferably 12 to 24% by mass, and even more preferably 13 to 21% by mass. By keeping the total content of water-soluble organic solvents within the above range, it is possible to maintain an appropriate viscosity for ejection as an inkjet ink. Furthermore, for example, good ejection stability can be easily achieved even after a long printing pause, and printed matter can be easily obtained that can be dried with low energy and has good water resistance.

[0050] In some embodiments, the aqueous inkjet ink preferably contains 1% by mass or less (or may contain 0% by mass) of a water-soluble organic solvent having a boiling point of 220° C. or higher at 1 atmosphere, relative to the total amount of the aqueous inkjet ink. By not containing a water-soluble organic solvent having a boiling point of 220° C. or higher, or by containing a water-soluble organic solvent in an amount within the above range, it becomes easier to suppress color bleeding, even in high-speed printing, for example.

[0051] <Surfactant> The aqueous inkjet ink of this embodiment contains a surfactant.

[0052] Among surfactants, it is preferable to use one or more selected from acetylenic diol surfactants and siloxane surfactants. These surfactants are preferred in that they can significantly reduce the surface tension of the aqueous inkjet ink in a very short time and have good wettability with respect to printing substrates with relatively high surface free energy, such as polyethylene terephthalate (PET) films, making it easy to improve the print quality of printed materials. Furthermore, it is particularly preferable to use acetylenic diol surfactants because they can easily produce printed materials with no white spots and excellent water resistance, and also improve ejection stability.

[0053] Furthermore, from the viewpoints of improving the wettability to the printing substrate and the print quality of the printed matter, as well as improving the water resistance of the printed matter after drying, it is preferable to use a surfactant having an HLB value of 9 or less as the surfactant. Furthermore, from the viewpoints of preventing excessive orientation to the interface, preventing foaming and nozzle dropping when used in combination with hexylene glycol, and easily achieving good ejection stability even after a long period of printing suspension, the HLB value is more preferably 4 to 9, and particularly preferably 7 to 9.

[0054] The content of the surfactant having an HLB value of 9 or less (preferably 4 to 9, particularly preferably 7 to 9) contained in the aqueous inkjet ink of this embodiment is preferably 0.1 to 1.8% by mass, and particularly preferably 0.2 to 1.0% by mass, based on the total amount of the aqueous inkjet ink. When the content of the surfactant having an HLB value of 9 or less (preferably 4 to 9, particularly preferably 7 to 9) is adjusted to fall within the above range, the above-mentioned effects can be more easily obtained, thereby further improving print image quality. Furthermore, ejection stability is improved even after long periods of printing suspension or during high-speed continuous printing. Furthermore, the water resistance of the printed matter after drying is also improved.

[0055] 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 disclosure, the value calculated by the Griffin method, represented by the following formula (1), is used as the HLB value.

[0056] Formula (1): (HLB value) = 20 × (total molecular weight of hydrophilic portion) ÷ (molecular weight of material)

[0057] (Acetylene Diol-Based Surfactant) The acetylene diol-based surfactant suitable for use in the aqueous inkjet ink of this embodiment may be synthesized by a conventionally known method or may be a commercially available product. Examples of such commercially available products include Surfynol (registered trademark) 61, 104E, 104H, 104A, 104BC, 104DPM, 104PA, 104PG-50, 420, 440, 465, 485, 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.), and the like. These compounds 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 Surfynol 440, Surfynol 2502, and Dynol 604, and it is particularly preferable to use Surfynol 2502, since this allows the above-mentioned mechanism to function effectively and provides an aqueous inkjet ink that is excellent in ejection stability even after a long period of printing suspension and in print quality. These are included in acetylenic diols having an HLB value of 7 to 9. Surfynol 440 is an ethylene oxide modified product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (number of moles of ethylene oxide added: 3.5). Surfynol 2502 is an ethylene oxide and propylene oxide modified product of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (number of moles of ethylene oxide added: 5, number of moles of propylene oxide added: 2). Dynol 604 is an ethylene oxide modified product of 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol (the number of moles of ethylene oxide added is about 4).

[0058] (Siloxane-Based Surfactant) Meanwhile, the siloxane-based surfactant preferably used in the aqueous inkjet ink of this embodiment may be one synthesized by a conventionally known method, or a commercially available product may be used. Examples of such commercially available products include TEGO (registered trademark) Wet 270, TEGO Wet 280, TEGO Glide 410, TEGO Glide 432, TEGO Glide 435, TEGO Glide 440, and TEGO Glide 450 manufactured by Evonik Japan Co., Ltd.; Silface SAG002, Silface SAG005, Silface SAG503A, and Silface SAG008 manufactured by Nissin Chemical Industry Co., Ltd.; and BYK (registered trademark)-348 and BYK-349 manufactured by BYK Japan KK These compounds may be used alone or in combination of two or more. In some embodiments, among siloxane-based surfactants, it is preferable to use a side-chain polyether-modified siloxane-based surfactant having 4 to 9 silicon (Si) atoms. For example, of the commercially available products listed above, one or more selected from the group consisting of TEGO Wet 270, TEGO Wet 280, BYK-348, and BYK-349 can be preferably used. These are preferred in that they are siloxane-based surfactants having the above-mentioned suitable HLB values, they have good ejection stability even after long periods of printing suspension, and they have good wettability even with printing substrates having very low surface free energy, such as biaxially oriented polypropylene (OPP) films, and can produce printed matter with excellent print quality.

[0059] (Other Surfactants) The aqueous inkjet ink of this embodiment may further contain surfactants other than the above-mentioned acetylene diol surfactants and siloxane surfactants (referred to as "other surfactants" in the present disclosure). Examples of other surfactants that can be used include acetylene monool surfactants, fluorine-based surfactants, polyoxyalkylene monoalkyl ether surfactants (for example, surfactants in which 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 terminal alkyl group has 5 to 22 carbon atoms). These other surfactants may be used alone or in combination of two or more.

[0060] The total content of surfactants contained in the aqueous inkjet ink of this embodiment is preferably 0.1 to 3 mass %, more preferably 0.2 to 2.4 mass %, even more preferably 0.3 to 1.8 mass %, and particularly preferably 0.3 to 1.3 mass %, relative to the total amount of the aqueous inkjet ink. By keeping the total content of surfactants within the above range, print image quality is improved and the water resistance of the printed matter after drying is also improved.

[0061] Furthermore, when an acetylenic diol surfactant is used in combination with a surfactant other than the acetylenic diol surfactant as the surfactant, the content of the acetylenic diol surfactant relative to the total amount of the surfactants is preferably 30% by mass or more, and particularly preferably 45% by mass or more. When the content of the acetylenic diol surfactant relative to the total amount of surfactants is adjusted to fall within the above range, printed matter with excellent print quality, free from white voids and color bleeding, can be easily obtained.

[0062] Pigment The aqueous inkjet ink of this embodiment contains a pigment.

[0063] 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.

[0064] 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.

[0065] <Pigment Dispersion Resin> As described above, the aqueous inkjet ink of this embodiment contains a pigment dispersion resin. In pigments that do not contain a pigment dispersion resin (self-dispersed pigments) and pigments that use a surfactant as a pigment dispersant (surfactant-dispersed pigments), when the water in the ink preferentially evaporates on the nozzle surface of the inkjet head or on the printing substrate, increasing the proportion of water-soluble organic solvents, the pigment dispersion state is destroyed, and pigment aggregation is likely to occur. As a result, not only does this result in a decrease in ejection stability and print quality, but it also may result in a decrease in the water resistance of printed matter. Therefore, as described above, from the perspective of improving the adsorption to the pigment surface and suppressing the desorption of the pigment dispersion resin in the aqueous inkjet ink, it is preferable to use a polymer with a crosslinked structure and / or a block polymer as the pigment dispersion resin. The use of these polymers can impart dispersion stability to even fine pigments, thereby suppressing a decrease in ejection stability and the print quality and water resistance of printed matter. Furthermore, even when the pigment dispersion resin is softened by hexylene glycol or the like, it is possible to suppress the desorption of the pigment dispersion resin from the pigment. As a result, it is possible to easily obtain printed matter with exceptionally excellent print quality, color development, and color reproducibility.

[0066] (Polymer Having a Crosslinked Structure) The polymer having a crosslinked structure may be a polymer obtained by introducing a crosslinked structure into a polymer usable as a pigment dispersing resin. A preferred method for introducing a crosslinked structure into a polymer serving as a pigment dispersing resin is a method using a crosslinking agent to form a crosslinked structure. More specifically, a preferred form of the polymer having a crosslinked structure in the aqueous inkjet ink of this embodiment may be a polymer in which a crosslinked structure is introduced into the molecule by dispersing a pigment using a polymer without a crosslinked structure (an uncrosslinked polymer) and then performing a crosslinking treatment by adding a crosslinking agent. Another form of the polymer having a crosslinked structure may be a polymer in which a crosslinked structure is introduced without using a crosslinking agent. Examples of methods for forming such a polymer include a method in which a pigment is dispersed using a polymer having polymerizable functional groups (e.g., an acrylic resin synthesized using a polyfunctional vinyl monomer) and then the polymerizable functional groups are reacted to bond to each other to introduce a crosslinked structure. Compared to methods that do not use a crosslinking agent, a method that uses a crosslinking agent, i.e., a method in which a pigment is dispersed using an uncrosslinked polymer and then a crosslinking treatment is performed by adding a crosslinking agent, is more suitable for use with the aqueous inkjet ink of this embodiment. When a method using a crosslinking agent is applied, the polymer is densified on the pigment surface, making it possible to easily suppress desorption of the polymer. In addition, the method using a crosslinking agent is preferable in that the crosslinking rate (described later) can be easily controlled by controlling the type and amount of the flocculant used and the crosslinking treatment conditions, and that the polymerizable functional groups can be prevented from reacting during the dispersion treatment, allowing the dispersion treatment to proceed stably.

[0067] In the present disclosure, the terms "a polymer obtained by a method of subjecting an uncrosslinked polymer to a crosslinking treatment by adding a crosslinking agent" and "a crosslinked product of an uncrosslinked polymer" are used interchangeably. That is, the above-mentioned "a polymer obtained by a method of dispersing a pigment using an uncrosslinked polymer and then subjecting the pigment to a crosslinking treatment by adding a crosslinking agent" and "a crosslinked product of an uncrosslinked polymer after dispersing the pigment" are the same.

[0068] The uncrosslinked polymer may be any of a homopolymer, a random polymer, a block polymer (details of which will be described later), a graft polymer, an alternating polymer, etc. Two or more uncrosslinked polymers may be used in combination, and the two or more uncrosslinked polymers may be crosslinked during crosslinking.

[0069] In some embodiments, the uncrosslinked polymer preferably has an aromatic group. Introducing an aromatic group into the uncrosslinked polymer allows the uncrosslinked polymer to be sufficiently adsorbed to the pigment even before crosslinking, and after crosslinking, the polymer can be present at a high density on the pigment surface, which is preferable in terms of easily improving ejection stability. 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 preferable because they can sufficiently ensure the dispersion stability and ejection stability of the pigment before crosslinking. Furthermore, the molar content of the polymerizable monomer having an aromatic group relative to the total molar content of the polymerizable monomers constituting the uncrosslinked polymer is preferably 10 to 50 mol %, more preferably 20 to 40 mol %. In particular, when the molar content is 10 to 50 mol%, the dispersion stability of the pigment and the ejection stability of the aqueous inkjet ink can be easily improved, even in the presence of hexylene glycol, and the print quality of the printed matter can also be easily improved. Furthermore, when a crosslinked product of an uncrosslinked polymer having the molar content of 10 to 50 mol% is used in combination with the above-mentioned (poly)oxypropylene monoalkyl ethers, the ejection stability immediately after the start of printing and the print quality are significantly improved.

[0070] In some embodiments, the weight average molecular weight of the uncrosslinked polymer is preferably 3,000 to 30,000, and more preferably 5,000 to 30,000. By setting the weight average molecular weight within this range, adsorption of the uncrosslinked polymer to the pigment surface is stabilized, making it easier to improve ejection stability.

[0071] The acid value of the uncrosslinked polymer is preferably 50 to 300 mgKOH / g, more preferably 60 to 250 mgKOH / g, even more preferably 70 to 200 mgKOH / g, and 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 even before crosslinking, and to ensure that the polymer is present at a high density on the pigment surface after crosslinking. Furthermore, when acid groups are used as crosslinking reaction sites (described below), a sufficient amount of crosslinked structure can be formed. For these reasons, when an uncrosslinked polymer having an acid value within the above range is used, the ejection stability of the aqueous inkjet ink can be easily improved. Furthermore, when the uncrosslinked polymer is used, the water solubility of the polymer itself is appropriately low, so the water resistance of the ink film after drying is less likely to decrease, resulting in good water resistance.

[0072] The acid value of the pigment dispersion resin can be measured using a known device. In the present disclosure, the acid value of the pigment dispersion resin is a value measured by potentiometric titration in accordance with JIS K 2501. A specific example of a measurement method is to use an AT-610 manufactured by Kyoto Electronics Manufacturing Co., Ltd. to dissolve the pigment dispersion resin in a toluene-ethanol mixed solvent, and then titrate with a potassium hydroxide solution, and calculate the acid value from the amount of titration up to the endpoint.

[0073] The crosslinking agent is preferably a compound having, in one molecule, multiple functional groups reactive with crosslinking reaction sites (e.g., carboxy groups and / or carboxylate groups) present in the pigment dispersion resin. Use of such a compound allows the polymer to be present at a high density on the pigment surface, improving ejection stability. Examples of crosslinking agents that can be used include aziridine compounds, isocyanate compounds, epoxy compounds, oxetane compounds, carbodiimide compounds, and oxazoline compounds. Of these, it is preferable to use an epoxy compound as the crosslinking agent, since this allows the crosslinking reaction to proceed in the vicinity of the pigment while maintaining a stable dispersion state of the pigment due to the uncrosslinked polymer.

[0074] The crosslinking agent may be water-soluble or water-insoluble, but may have a solubility of at least a certain level from the viewpoint of more efficiently proceeding with the crosslinking reaction in a liquid medium mainly composed of water. In some embodiments, the solubility of the crosslinking agent in 100 g of water at 25° C. is 0.1 to 50 g / 100 gH ２ 0 is preferable, and 0.2 to 40 g / 100 gH ２ The solubility of the crosslinking agent is more preferably 0.5 to 30 g / 100 gH ２ When a crosslinking agent having a solubility of at least a certain level is used, the crosslinking reaction can proceed in a liquid medium mainly composed of water, and the above-mentioned effects can be easily realized, that is, the dispersion stability, ejection stability, print quality, and water resistance of the printed matter can be easily improved. ２ By using a crosslinking agent with a molecular weight of 0 or less, the crosslinking agent can be prevented from dispersing in the liquid medium, and can be reliably reacted even when added in a small amount. As a result, it is possible to easily prevent, for example, deterioration in the ejection stability and print image quality of the aqueous inkjet ink, as well as the water resistance of the ink film, which would be caused by the addition of an excessive amount of crosslinking agent.

[0075] As described above, a compound having multiple epoxy groups in one molecule can be preferably used as the crosslinking agent. A crosslinking agent that can be suitably used is more preferably a compound having two or more glycidyl ether groups in one molecule, and even more preferably a polyglycidyl ether compound of a polyhydric alcohol having a hydrocarbon group having from 3 to 8 carbon atoms. When a compound having multiple epoxy groups in one molecule is used as the crosslinking agent, the epoxy equivalent of the compound may be preferably 90 to 300 g / eq., more preferably 100 to 200 g / eq., from the viewpoint of more efficiently crosslinking with crosslinking reaction sites (e.g., carboxy groups and / or carboxylate groups) present in the pigment dispersion resin in a liquid medium mainly composed of water.

[0076] Specific examples of compounds having multiple epoxy groups in one molecule include cyclohexanedimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, resorcinol diglycidyl ether, neopentyl glycol diglycidyl ether, 4,4'-diglycidyloxybiphenol, bisphenol A diglycidyl ether, hydrogenated bisphenol A diglycidyl ether, phthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, hydrogenated phthalic acid diglycidyl ester, etc. These compounds may be used alone or in combination of two or more.

[0077] Furthermore, it is preferable to add a crosslinking agent so that the crosslinking rate represented by the following formula (2) is 40 to 100 mol %. The crosslinking rate is more preferably 60 to 100 mol %, and particularly preferably 80 to 100 mol %. By setting the crosslinking rate to 40 mol % or more (preferably 60 mol % or more, and particularly preferably 80 mol % or more), and by firmly crosslinking the uncrosslinked polymer on the pigment surface, it becomes easy to improve dispersion stability, ejection stability, print image quality, and water resistance of printed matter.

[0078] Equation (2): Crosslinking rate (mol %)=[number of moles of reactive groups of crosslinking agent×100 / number of moles of crosslinking reaction sites (e.g., carboxy groups and carboxylate groups) possessed by the pigment dispersing resin (uncrosslinked polymer) before crosslinking]

[0079] (Block polymer) A block polymer is a polymer in which two or more types of polymers (blocks) with different structures are bonded together. As described above, block polymers can be more firmly adsorbed onto the pigment surface than general random polymers, making it possible to suppress desorption of the block polymer in the aqueous inkjet ink.

[0080] The block polymer may be, for example, a polymer having a linear molecular chain. When the blocks contained in the block polymer are A, B, C, etc., it may be, for example, a diblock polymer such as A-B, a triblock polymer such as A-B-A or A-B-C, or a 2xx block polymer represented by (A-B)x (where x is an integer of 2 or more).

[0081] When a block polymer is used, it is preferable to differentiate the functions of each block in order to improve the dispersion stability and ejection stability of the pigment. That is, when a block polymer is used as a pigment dispersing resin, it is preferable to use a polymer having one or more blocks for preventing pigment aggregation due to charge repulsion or the like (hereinafter referred to as "P blocks") and one or more blocks for adsorbing to the pigment (hereinafter referred to as "Q blocks").

[0082] In order to improve the adsorption to the pigment, the ink preferably contains a polymer having an aromatic group. From the above viewpoint, the difference between the molar amount of the polymerizable monomer having an aromatic group relative to the total molar amount of the polymerizable monomers constituting the P block and the molar amount of the polymerizable monomer having an aromatic group relative to the total molar amount of the polymerizable monomers constituting the Q block is preferably 5 to 100 mol %. In one embodiment, the difference is more preferably 5 to 50 mol %, and particularly preferably 10 to 40 mol %. In some embodiments, when the difference is 5 to 50 mol %, not only is it easy to improve the dispersion stability of the pigment and the jetting stability of the aqueous inkjet ink, but it also facilitates the improvement of the print quality of the printed matter. In other embodiments, the difference is preferably 40 to 100 mol %, and particularly preferably 60 to 100 mol %. In some embodiments, when the difference is 40 to 100 mol %, the adsorption of the block polymer to the pigment is particularly improved. As a result, even when hexylene glycol, a highly hydrophobic solvent, is used in combination, it becomes easy to improve the dispersion stability of the pigment, as well as the storage stability and ejection stability of the aqueous inkjet ink.

[0083] The molar content of the polymerizable monomer having an aromatic group in the Q block, which is the block that adsorbs to the pigment, is preferably greater than that in the P block. As in the case of the uncrosslinked polymer described above, the aromatic group preferably has a phenyl group, a naphthyl group, or a tolyl group.

[0084] In addition, to improve adsorption to the pigment, the molar content of the polymerizable monomer having an aromatic group relative to the total molar content of the polymerizable monomers constituting the Q block is preferably 10 to 100 mol %. In one embodiment, the molar content of the polymerizable monomer having an aromatic group is more preferably 10 to 75 mol %, and particularly preferably 20 to 60 mol %. In some embodiments, when the molar content is 10 to 75 mol %, not only are the dispersion stability of the pigment and the ejection stability of the aqueous inkjet ink improved, but the print quality of the printed material is also easily improved. In other embodiments, the molar content may be preferably 40 to 100 mol %, and particularly preferably 60 to 100 mol %. In some embodiments, when the molar content is 40 to 100 mol %, the adsorption of the block polymer to the pigment is particularly improved. As a result, even when hexylene glycol, a highly hydrophobic solvent, is used in combination, it is easy to improve the dispersion stability of the pigment and the storage stability and ejection stability of the aqueous inkjet ink. Furthermore, when a block polymer having the Q block with a molar content of 40 to 100 mol % is used in combination with the above-mentioned (poly)oxypropylene monoalkyl ethers, the ejection stability immediately after the start of printing and the print image quality are significantly improved.

[0085] In the block polymer, the molar content of the polymerizable monomer having an aromatic group relative to the total molar content of the polymerizable monomers constituting the P block is preferably 20 mol % or less, and particularly preferably 10 mol % or less. In one embodiment, the molar content of the polymerizable monomer having an aromatic group may be 0 mol %. Note that a molar content of the polymerizable monomer having an aromatic group of 0 mol % means that the block polymer does not contain the polymerizable monomer having an aromatic group. When the content of the polymerizable monomer having an aromatic group is within the above range, the properties of the block polymer are fully exhibited, and the adsorption of the block polymer to the pigment can be easily increased even in the presence of hexylene glycol, a highly hydrophobic solvent, which is preferable from the viewpoint of improving the dispersion stability of the pigment and the ejection stability of the aqueous inkjet ink.

[0086] Furthermore, from the viewpoint of improving the storage stability and ejection stability of the aqueous inkjet ink, it is preferable that the acid value of the P block be larger than the acid value of the Q block. Specifically, the difference between the acid values ​​of the P block and the Q block is preferably 10 to 450 mgKOH / g, more preferably 30 to 430 mgKOH / g, and particularly preferably 50 to 400 mgKOH / g.

[0087] In addition, if the Q block, which is the block that adsorbs to the pigment, has a low glass transition temperature, for example, when the aqueous inkjet ink is stored at room temperature, the block polymer tends to easily detach from the pigment surface, and the water resistance of the ink film also tends to decrease. While the detailed mechanism is unknown, it is believed that the softened block polymer dissolves in highly hydrophobic hexylene glycol, accelerating its detachment from the pigment. Furthermore, it is believed that the ink film is formed in a state in which the block polymer easily detaches from the pigment, making the ink film more susceptible to destruction by external forces in the presence of water. From these perspectives, i.e., from the perspective of improving the dispersion stability of the pigment, the ejection stability of the aqueous inkjet ink, and the water resistance of printed matter, the glass transition temperature of the Q block is preferably 50 to 120°C, and particularly preferably 50 to 100°C. The glass transition temperature is measured and calculated in the same manner as for the binder resin, as described below. However, when measuring the glass transition temperature of the Q block, a polymer having the same structure as the Q block is used as the sample, rather than the block polymer itself.

[0088] 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 molar content of the aromatic group-containing polymerizable monomer relative to the molar content of the polymerizable monomers constituting the block polymer is preferably 5 to 75 mol %, more preferably 10 to 60 mol %, even more preferably 15 to 50 mol %, and particularly preferably 20 to 50 mol %.

[0089] In some embodiments, the acid value of the block polymer used in the aqueous inkjet ink is preferably 60 to 150 mgKOH / g, and particularly preferably 60 to 130 mgKOH / g, from the viewpoint of improving the storage stability and ejection stability of the aqueous inkjet ink as well as improving the water resistance of the ink film.

[0090] Furthermore, even when used in combination with hexylene glycol, in order to improve the dispersion stability of the pigment, the ejection stability of the aqueous inkjet ink, and the water resistance of the ink film, the glass transition temperature of the block polymer is preferably 40 to 105° C., and particularly preferably 45 to 90° C. The glass transition temperature of the block polymer is measured and calculated in the same manner as in the case of the binder resin, which will be described later.

[0091] The mass average molecular weight of the block polymer is preferably 3,000 to 35,000, and more preferably 5,000 to 30,000. By adjusting the mass average molecular weight within this range, the adsorption of the block polymer to the pigment surface is stabilized, improving the dispersion stability of the pigment and the ejection stability of the aqueous inkjet ink.

[0092] The molecular weight dispersity of the block polymer is preferably from 1.0 to 2.0, and more preferably from 1.0 to 1.7. By setting the molecular weight dispersity within the above range, it is possible to reduce the content of high-molecular-weight polymers that may deteriorate the dispersibility and ejection stability of the pigment, and low-molecular-weight polymers that may reduce the density and print quality of the printed matter.

[0093] In some embodiments, an example of a block polymer that can be suitably used is a block polymer in which the molar content of the polymerizable monomer having an aromatic group in the P block is 10 mol % or less, the glass transition temperature of the Q block is 50 to 100° C., the difference in acid value between the P block and the Q block is 50 to 400 mg KOH / g, and the molar content of the polymerizable monomer having an aromatic group in the block polymer is 20 to 50 mol %. Such a block polymer is also used in some embodiments described in the examples below.

[0094] Although the synthesis method of the block polymer is not limited, living polymerization is preferred, and living radical polymerization is more preferred. Living radical polymerization methods include a sulfur-based reversible chain transfer method (RAFT method), a method using an organotellurium compound (TERP method), a method using a transition metal catalyst (ATRP method), and a nitroxide-mediated radical polymerization method (NMP method), depending on the method for stabilizing the polymer growing end. Among these, the RAFT method is preferred because it can control the polymerization of not only conjugated monomers (polymerizable monomers having a substituent with a resonance stabilizing effect) but also polymerizable monomers other than the conjugated monomers.

[0095] (Characteristics, etc. common to polymers having a crosslinked structure and block polymers) The type of pigment dispersing resin is not particularly limited, and examples include acrylic resins, styrene resins, maleic acid (anhydride)-based resins, styrene-maleic acid (anhydride) resins, olefin-maleic acid (anhydride) resins, urethane resins, polyester resins, polyolefin resins, and polyvinyl alcohol resins. These resins may be used alone or in combination of two or more. Among these, in terms of ejection stability, wide material selectivity, ease of synthesis, etc., it is preferable to use one or more resins selected from the group consisting of acrylic resins, polyester resins, styrene-maleic acid (anhydride) resins, and olefin-maleic acid (anhydride) resins.

[0096] 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 acid anhydride." In some embodiments, the polymer having a crosslinked structure is preferably a polymer obtained by reacting one or more polymerizable monomers selected from the group consisting of acrylic acid, methacrylic acid, and maleic acid (anhydride) with styrene or benzyl methacrylate and one or more polymerizable monomers selected from the group consisting of 1-octadecene, methyl methacrylate, lauryl methacrylate, and cyclohexylmaleimide, and then crosslinking the resulting polymer with a crosslinking agent such as an epoxy compound. In some embodiments, the P block in the block polymer is preferably formed using one or more polymerizable monomers selected from the group consisting of benzyl methacrylate, stearyl methacrylate, butyl methacrylate, and methacrylic acid. On the other hand, the Q block is preferably formed using one or more polymerizable monomers selected from the group consisting of benzyl methacrylate, styrene, cyclohexyl methacrylate, 2-ethylhexyl acrylate, butyl methacrylate, and methacrylic acid.

[0097] Generally, water-soluble resins and water-insoluble resins are known as the types of resins used in aqueous inkjet inks. The pigment dispersion resin contained in the aqueous inkjet ink of this embodiment may be a water-soluble resin or a water-insoluble resin. When the pigment dispersion resin is a polymer having a crosslinked structure, it means an uncrosslinked polymer (before crosslinking treatment).

[0098] 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."

[0099] Furthermore, "D50" in the present disclosure is a value measured in a 25°C environment using a dynamic light scattering particle size distribution measuring device such as "Nanotrac UPA-EX150" manufactured by Microtrac Bell.

[0100] The pigment dispersing resin may be one synthesized by a known method or may be a commercially available product.

[0101] From the viewpoint of improving the dispersion stability and ejection stability of the pigment, the content of the pigment dispersing resin is preferably 10 to 80% by mass, and particularly preferably 15 to 50% by mass, relative to the content of the pigment.

[0102] <Binder Resin> The aqueous inkjet ink of this 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 rapidly on the order of μs, thereby suppressing bleeding and improving print image quality. Furthermore, when the aqueous inkjet ink dries, the binder resin forms a continuous film, thereby improving the water resistance of the printed matter.

[0103] The binder resin contained in the aqueous inkjet ink of this embodiment may be a water-soluble resin or resin fine particles (water-insoluble resin). Also, a water-soluble resin and resin fine particles may be used in combination.

[0104] 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 can be formed even with a short drying time, and water resistance can be improved.

[0105] 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

[0106] The binder resin may be any of acrylic resins, styrene resins, maleic (anhydride)-based resins, styrene-maleic (anhydride) resins, olefin-maleic (anhydride) resins, urethane resins, polyester resins, vinyl chloride resins, vinyl chloride-vinyl acetate resins, polyolefin resins, and the like. These resins may be used singly or in combination. Among these, it is preferable to use one or more resins selected from the group consisting of acrylic resins, urethane resins, and polyester resins as the binder resin. Furthermore, from the viewpoints of achieving both adhesion to the film substrate and abrasion resistance, as well as improving ejection stability, it is preferable to use an acrylic resin as the binder resin. In this case, it is particularly preferable that the amount of the acrylic resin be 50% by mass or more of the total amount of binder resin in the aqueous inkjet ink.

[0107] The glass transition temperature (Tg) of the binder resin is preferably 50 to 120° C., particularly preferably 60 to 110° C. By using a binder resin having the above glass transition temperature, the scratch resistance and water resistance of the printed matter are improved, and since the binder resins do not become entangled with each other during ejection, stable ejection is possible even after leaving the ink to stand for a long period of time.

[0108] The glass transition temperature of the binder resin can be measured by a method conforming to JIS K 7121. Specifically, approximately 10 mg of a sample of the binder resin of interest 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 prepared without the binder resin are placed in a holder in a Shimadzu DSC-60 (differential scanning calorimeter), and measurements are performed at a temperature increase rate of 10°C / min to obtain a DSC chart. The intersection of the low-temperature baseline and the tangent to the inflection point of the baseline is then determined, and the temperature of this intersection is taken as the glass transition temperature. Indium is used for temperature calibration.

[0109] On the other hand, for acrylic resins, the value calculated by the following formula (3) can be used as the glass transition temperature.

[0110] Formula (3): 1 / Tg = Σ(Wn / Tgn)

[0111] In the above formula (3), Tg represents the glass transition temperature (K) of the resin, Wn represents the mass fraction of the structural unit consisting of polymerizable monomer n constituting the resin, and Tgn represents the glass transition temperature (K) of the homopolymer consisting of each polymerizable monomer n. For the Tgn, for example, values ​​described in "Polymer Handbook (4th Edition)" (Wiley, 1998) can be used.

[0112] 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 nozzles of the inkjet head, it is possible to suppress a significant increase in viscosity of the aqueous inkjet ink. This makes it possible to easily improve the ejection stability. Furthermore, the water resistance of the printed matter is also improved.

[0113] In this disclosure, 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 disclosure, the acid value used is a value 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 a polymerizable monomer having a molecular weight of Ma among the polymerizable monomers constituting the resin, the acid value (mg KOH / g) can be calculated using the following formula (4):

[0114] Equation (4): (Acid value) = {(va × na × Wa) ÷ (100 × Ma)} × 56.11 × 1000

[0115] In the above formula (4), the number "56.11" is the molecular weight of potassium hydroxide.

[0116] From the viewpoint of achieving both the print quality and water resistance of the printed matter, as well as the ejection stability, the content of the binder resin is preferably 1.5 to 20% by mass, and particularly preferably 3.5 to 18% by mass, relative to the total amount of the aqueous inkjet ink.

[0117] <Wax> The aqueous inkjet ink of this embodiment preferably contains a wax. Furthermore, it is preferable to use polyolefin resin microparticles as the wax. Although the detailed reason is unclear, polyolefin resin microparticles can be stably dispersed in the aqueous inkjet ink even when used in combination with the pigment dispersion resin described above. Furthermore, they are preferably selected because they can significantly improve the abrasion resistance and water resistance of printed matter.

[0118] The polyolefin may be at least one selected from the group consisting of polyethylene, polypropylene, and polybutene. Polyethylene is particularly preferred because it can significantly improve the water resistance of printed matter.

[0119] When a wax is used, its D50 is preferably 10 to 200 nm, and more preferably 20 to 180 nm. When the D50 is within the above range, the above-mentioned functions can be suitably exhibited. Furthermore, clogging of the inkjet head nozzles is prevented, resulting in an aqueous inkjet ink with excellent ejection stability.

[0120] 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 individual resins are not impaired. Furthermore, since printed matter having sufficient water resistance can be obtained even during high-speed printing, the amount of wax relative to the total amount of the aqueous inkjet ink is preferably 0.5 to 1.5% by mass.

[0121] <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 85 mass % of the total amount of the aqueous inkjet ink, and particularly preferably 50 to 80 mass %. Because water has a low boiling point, it volatilizes preferentially from the nozzle end face of the inkjet head, and the solids concentration at the gas-liquid interface tends to be high. In contrast, by setting the water content within the above range, good ejection stability is achieved under all conditions, including immediately after the start of printing, after a long printing break, and during continuous printing.

[0122] <WR / WP> In some embodiments, the aqueous inkjet ink preferably has a ratio of the sum (WR [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 represented by WR / WP, of 1 to 7. As described above, by adjusting the value represented by WR / WP to a value within the range of 1 to 7, the amount of pigment that may become discontinuous points during the formation of the continuous film can be adjusted within a suitable range, thereby enabling the production of printed matter with excellent water resistance. Furthermore, because the pigment can be uniformly present within the aqueous inkjet ink, the occurrence of whiteout and color bleeding is suppressed, making it possible to obtain print image quality equivalent to that of plate-based printing. Furthermore, the value represented by WR / WP is particularly preferably 1 to 4, since this suppresses an increase in viscosity at the nozzle interface even after a long period of printing suspension, thereby improving ejection stability.

[0123] <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.

[0124] <Method for Producing Aqueous Inkjet Ink> The aqueous inkjet ink of this embodiment can be produced by a conventionally known method. For example, a pigment dispersion is produced by dispersing a pigment in a medium containing at least water (aqueous medium) using a polymer and / or block polymer having a crosslinked structure. In some embodiments, when producing an aqueous inkjet ink containing a polymer having a crosslinked structure, it is preferable to obtain the pigment dispersion by dispersing the pigment and the pre-crosslinked polymer, and then adding a crosslinking agent to perform a crosslinking treatment. Water, hexylene glycol, a binder resin, a surfactant, and the like are then added to the pigment dispersion, 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.

[0125] <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.

[0126] Furthermore, in order to obtain an aqueous inkjet ink that is excellent in ejection stability and print quality of printed matter, the aqueous inkjet ink of this embodiment preferably has a static surface tension of 18 to 35 mN / m, and particularly preferably 21 to 32 mN / m, at 25° C. In the present disclosure, the static surface tension is a value measured in an environment of 25° C. using the Wilhelmy method (plate method) with an "Automatic Surface Tensiometer CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd.

[0127] <Aqueous Inkjet Ink Set> Although only one aqueous inkjet ink of this embodiment may be used alone, two or more aqueous inkjet inks may also be combined to form an aqueous inkjet ink set. 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); and 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.

[0128] <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.

[0129] 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.

[0130] <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).

[0131] (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.

[0132] 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.

[0133] 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.

[0134] (Drying Step) Examples of drying methods employed in the drying mechanism used in the drying step include heat drying, hot air drying, infrared drying (for example, infrared with a wavelength of 700 to 2500 nm), microwave drying, and drum drying. One or more of these methods can be selected and used as desired in the drying step. Furthermore, when two or more of the above drying methods are used, they may be used separately (for example, consecutively) or simultaneously. For example, by using heat drying and hot air drying in combination, the aqueous inkjet ink can be dried more quickly than when each method is used alone.

[0135] 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.

[0136] <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 color bleeding and uneven shading, 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.

[0137] 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 measuring 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

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] <Production Example of Pigment Dispersion Resin 1> A reaction vessel equipped with a gas inlet tube, thermometer, condenser, and stirrer was charged with 26 parts of 1-octadecene (polymerizable monomers), 13 parts of maleic anhydride, 30.5 parts of cyclohexylmaleimide, and 30.5 parts of styrene, and 100 parts of methyl ethyl ketone (organic solvent), followed by nitrogen substitution. Next, the contents of the reaction vessel were heated with stirring until the internal temperature reached 130°C. After reaching 130°C, 1.0 parts of t-butylperoxy-2-ethylhexanoate (radical polymerization initiator) was added dropwise over 2 hours while continuing stirring. After the dropwise addition, the polymerization reaction was carried out by continuing stirring for another 1 hour while maintaining the internal temperature at 130°C. Thereafter, after confirming by measuring the solids content that the polymerization conversion rate had reached 95% by mass or more, the reaction vessel was cooled until the internal temperature reached around 60°C. After cooling, 12 parts of ion-exchanged water and 0.01 parts of diazabicycloundecene (catalyst) were added to the reaction vessel, and the contents were heated with stirring until the internal temperature reached 80°C. After the internal temperature reached 80°C, the internal temperature was maintained at 80°C while continuing stirring for 4 hours, thereby causing a ring-opening reaction of maleic anhydride, and pigment dispersion resin 1 was synthesized. The mass average molecular weight of pigment dispersion resin 1, measured using the method described above, was 28,000, and the acid value was 149 mgKOH / g. Thereafter, the amount of potassium hydroxide required to achieve a neutralization rate of 100%, calculated from the acid value of the resulting pigment dispersion resin, was added, and ion-exchanged water was further added to achieve a non-volatile content of 20%. This solution was then heated to 50°C, stirred for 1 hour, and cooled again to obtain an aqueous solution of pigment dispersion resin 1 (non-volatile content of 20% by mass).

[0143] The above-mentioned "aqueous solution" refers to a solution containing an aqueous medium and components dispersed and / or dissolved in the aqueous medium.

[0144] Examples 1 to 42, Comparative Examples 1 to 5 Production Example of Magenta Pigment Dispersion 1 20 parts of FASTOGEN SUPER MAGENTA RTS (C.I. Pigment Red 122, manufactured by DIC Corporation) pigment, 25 parts of an aqueous solution of pigment dispersion resin 1 (non-volatile content concentration: 20% by mass), and 55 parts of ion-exchanged water were added to a mixing vessel. The contents were stirred for 1 hour (preliminary dispersion), and then main dispersion was carried out using a 0.6 L Dyno-Mill filled with 1,800 g of zirconia beads with a diameter of 0.5 mm. After main dispersion, 33.3 parts of ion-exchanged water was added to the resulting pigment dispersion. Further, while heating at 60°C, a portion of the ion-exchanged water and all of the methyl ethyl ketone were distilled off under reduced pressure. Ion-exchanged water was then added so that the pigment concentration became 15%, yielding Magenta Pigment Dispersion 1 with a pigment concentration of 15% by mass.

[0145] <Production Example of Magenta Pigment Dispersion 2> 93.3 parts of the above Magenta Pigment Dispersion 1, 1.2 parts of Denacol EX-321 (an epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 140 (g / eq.)) as a crosslinking agent, and 5.5 parts of ion-exchanged water were added to a mixing vessel. The contents were then heated to 80°C with stirring, and once the temperature reached 80°C, stirring was continued for 3 hours to carry out a crosslinking reaction. Ion-exchanged water was then added to adjust the pigment concentration to 14% by mass, thereby obtaining Magenta Pigment Dispersion 2 (pigment concentration 14% by mass) in which the pigment dispersing resin was crosslinked (crosslinking rate 90 mol%).

[0146] <Production Example of Yellow Pigment Dispersions 1 and 2> Except for using Lysopac Yellow 5515C (C.I. Pigment Yellow 155 manufactured by Vibrants) as the pigment, Yellow Pigment Dispersion 1 having a pigment concentration of 15% by mass was obtained using the same materials and method as for Magenta Pigment Dispersion 1. Furthermore, Except for using Yellow Pigment Dispersion 1 instead of Magenta Pigment Dispersion 1, Yellow Pigment Dispersion 2 (pigment concentration 14% by mass) in which the pigment dispersing resin was crosslinked (crosslinking rate 90 mol%) was obtained using the same materials and method as for Magenta Pigment Dispersion 2.

[0147] <Production Example of Pigment Dispersion Resin 2> 56 parts of 2-butanone were charged into a reaction vessel equipped with a gas inlet tube, thermometer, condenser, and stirrer. Next, 56 parts of benzyl methacrylate (polymerizable monomer), 0.3 parts of 2,2'-azobisisobutyronitrile (polymerization initiator), and 2.2 parts of 2-(dodecylthiocarbonothioylthio)-isobutyric acid were charged. After the atmosphere inside the reaction vessel was purged with nitrogen gas, the contents inside the reaction vessel were heated to 75°C. Next, a polymerization reaction was carried out for 3 hours while maintaining the internal temperature at 75°C, thereby obtaining a benzyl methacrylate polymer (A block). After completion of the polymerization reaction, the contents were cooled to room temperature, and then 44 parts of 2-butanone, 28 parts of butyl methacrylate (polymerizable monomer), and 16 parts of methacrylic acid were charged into the reaction vessel. The atmosphere inside the reaction vessel was again purged with nitrogen gas, and the contents of the reaction vessel were heated until the temperature reached 75°C. Next, a polymerization reaction was carried out for 3 hours while maintaining the internal temperature at 75°C, thereby obtaining a pigment dispersion resin 2 having an A-B block structure in which a copolymer (block B) composed of butyl methacrylate and methacrylic acid was added to the A block. The contents of the reaction vessel were then cooled to room temperature, and 17 parts of dimethylaminoethanol were added to neutralize the pigment dispersion resin 2. Next, 150 parts of ion-exchanged water was further added. The mixture was then heated to azeotrope 2-butanone with the ion-exchanged water, thereby distilling off the 2-butanone. Ion-exchanged water was then added to adjust the non-volatile content to 20% by mass, thereby obtaining an aqueous solution of pigment dispersion resin 2 (non-volatile content concentration: 20% by mass).

[0148] The resulting pigment dispersion resin 2 had a mass average molecular weight of 23,000, an acid value of 104 mgKOH / g, and a molecular weight dispersity of 1.4.

[0149] <Production Examples of Magenta Pigment Dispersion 3 and Yellow Pigment Dispersion 3> Magenta pigment dispersion 3 having a pigment concentration of 15% by mass was obtained using the same materials and method as for the above-described Magenta Pigment Dispersion 1, except that an aqueous solution of Pigment Dispersion Resin 2 was used instead of the aqueous solution of Pigment Dispersion Resin 1. Furthermore, yellow pigment dispersion 3 having a pigment concentration of 15% by mass was obtained using the same materials and method as for the above-described Magenta Pigment Dispersion 3, except that Lysopac Yellow 5515C (C.I. Pigment Yellow 155 manufactured by Vibrants) was used as the pigment.

[0150] <Preparation of Aqueous Inkjet Ink Set 1> Using the pigment dispersion prepared by the method described above, each raw material was added to a mixing vessel equipped with a stirrer so as to obtain the formulation shown in each column of Table 1. After all materials were added, the mixture was heated to 50°C while stirring, and then the mixture was stirred and mixed for an additional hour while maintaining the temperature at 50°C. The mixture was then filtered through a membrane filter with a pore size of 0.8 μm to prepare an aqueous inkjet ink. The above-mentioned magenta pigment dispersion and yellow pigment dispersion were used to prepare the above-mentioned aqueous inkjet ink sets consisting of an aqueous magenta ink (M) and an aqueous yellow ink (Y).

[0151] In the production of aqueous inkjet inks, each raw material was added while stirring the mixture in the mixing vessel. The components listed in each column of Table 1 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.

[0152]

[0153]

[0154]

[0155] The meanings of the abbreviations and details of the product names listed in Table 1 are as follows: In Table 1, "bp" represents the boiling point, "HLB" represents the HLB value, and "Nv" represents the solid content concentration. (Diols having 2 to 5 carbon atoms) 1,2-PD: 1,2-propanediol (boiling point: 188°C) 1,5-PeD: 1,5-pentanediol (boiling point: 239°C) (Other water-soluble organic solvents) PGM: Propylene glycol monomethyl ether (boiling point: 121°C) PGP: Propylene glycol monopropyl ether (boiling point: 150°C) DPGP: Dipropylene glycol monopropyl ether (boiling point: 210°C) EDG: Diethylene glycol monoethyl ether (boiling point: 196°C) BDG: Diethylene glycol monobutyl ether (boiling point: 231°C) (Acetylenol-based surfactants) Surfynol 104: Acetylenol-based surfactant manufactured by Evonik Japan, HLB value: 3.0 Surfynol 420: acetylene diol-based surfactant manufactured by Evonik Japan, HLB value: 4.0 Surfynol 440: acetylene diol-based surfactant manufactured by Evonik Japan, HLB value: 8.1 Acetylenol E40: acetylene diol-based surfactant manufactured by Kawaken Fine Chemicals, HLB value: 8.8 Acetylenol E60: acetylene diol-based surfactant manufactured by Kawaken Fine Chemicals, HLB value: 10.8 Surfynol 2502: acetylene diol-based surfactant manufactured by Evonik Japan, HLB value: 7.8 (siloxane-based surfactant) TEGO Wet 280: siloxane-based surfactant manufactured by Evonik Japan, HLB value: 3 to 5 (other surfactants) FS-300: fluorine-based surfactant manufactured by DuPont, solids content 40% (resin) Resin A: A resin produced by reproducing the production example of binder resin 1 in JP 2018-203802 A (used in the form of a resin solution with a solid content of 35%). The acid value of resin A was 49 mg KOH / g, and the glass transition temperature was 81°C. Resin B: A resin produced by reproducing the production example of binder resin 34 in JP 2018-203802 A (used in the form of a resin solution with a solid content of 35%).The acid value of Resin B was 49 mgKOH / g and the glass transition temperature was 42°C. PES Resin A-615GE: Polyester resin manufactured by Takamatsu Oil & Fat Co., Ltd., solid content 25%, glass transition temperature 47°C (Pigment dispersions) Dispersion 1: The magenta pigment dispersion 1 and yellow pigment dispersion 1 prepared previously were used. Dispersion 2: The magenta pigment dispersion 2 and yellow pigment dispersion 2 prepared previously were used. Dispersion 3: The magenta pigment dispersion 3 and yellow pigment dispersion 3 prepared previously were used. (Other Components) AQ515: AQUACER 515, polyethylene wax emulsion manufactured by BYK Japan, solids content 35% Proxel GXL: dipropylene glycol-water solution of 1,2-benzisothiazol-3-one (1,2-benzisothiazol-3-one: dipropylene glycol: water = 2:6:2, preservative manufactured by Arch Chemicals) (Specifications) In Table 1, WR represents the sum of the mass of the binder resin and the mass of the pigment dispersion resin. The mass of the pigment dispersion resin is a value calculated from the ratio of the pigment dispersion resin in the pigment dispersion liquid to the blend amount of the pigment dispersion liquid. Also in Table 1, the total amount of water-soluble organic solvents represents the ratio of the total amount of water-soluble organic solvents to the total mass of the ink.

[0156] The aqueous inkjet ink set produced by the above-described method was used to carry out the following evaluations. The evaluation results are shown in Table 1.

[0157] <Evaluation 1: Evaluation of Discharge Stability (Initial Printing)> An inkjet ejection device equipped with a Kyocera Corporation inkjet head "KJ4B-1200" (design resolution 1200 dpi, nozzle diameter 20 μm) installed in an environment of 25°C was filled with the aqueous magenta ink or aqueous yellow ink constituting the above aqueous inkjet ink set. Next, a nozzle check pattern was printed, and after confirming that ink was being ejected normally from all nozzles, the device 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. The resulting solid print was then evaluated for discharge stability (initial print) by checking with a magnifying glass whether the aqueous inkjet ink was applied to the area where it was originally supposed to be printed. The evaluation criteria were as follows, with ratings of A, B, and C+ being deemed usable. The above evaluations were performed on the aqueous magenta ink and the aqueous yellow ink that make up the aqueous inkjet ink set. Table 1 also lists the results of the aqueous magenta ink and the aqueous yellow ink that received poor evaluations. (Evaluation Criteria) A: No chipping was observed in the area that should have been printed first in the solid print. B: Chips of less than 1 mm were observed in the area that should have been printed first in the solid print. C+: Chips of 1 mm or more but less than 5 mm were observed in the area that should have been printed first in the solid print. C: Chips of 5 mm or more but less than 1 cm were observed in the area that should have been printed first in the solid print. D: Chips of 1 cm or more were observed in the area that should have been printed first in the solid print.

[0158] <Evaluation 2: 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) installed in an environment of 25°C was filled with the water-based magenta ink or water-based yellow ink constituting the above-mentioned 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 nozzles of the inkjet head. Next, the nozzle plate to which the oozed water-based inkjet ink had adhered was wiped, and the inkjet discharge device was then left to stand by for 1 hour. Thereafter, a solid image was printed on OK topcoat paper under printing conditions of a frequency of 40 kHz, a conveyor drive speed of 50 m / min, and a resolution of 1200 × 1200 dpi. The obtained solid prints were then visually inspected to determine whether the aqueous inkjet ink had been applied to the areas where the aqueous inkjet ink was to be printed first, thereby evaluating standby ejection performance. The evaluation criteria were as follows, with ratings of A, B, C+, and C being considered usable. The above evaluation was performed on each of the aqueous magenta ink and aqueous yellow ink that constituted the aqueous inkjet ink set. Table 1 also lists the results of the aqueous magenta ink and aqueous yellow ink that received poor evaluation results. (Evaluation criteria) A: In the solid print printed after waiting for 1 hour, no chipping was observed in the part that was supposed to be printed first. B: In the solid print printed after waiting for 1 hour, a chipping of less than 1 cm was observed in the part that was supposed to be printed first. C+: In the solid print printed after waiting for 1 hour, a chipping of 1 cm or more but less than 3 cm was observed in the part that was supposed to be printed first. C: 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 was supposed to be printed first. D: In the solid print printed after waiting for 1 hour, a chipping of 5 cm or more was observed in the part that was supposed to be printed first.

[0159] <Preparation of Magenta / Yellow 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 printing substrate. Each set of aqueous inkjet inks was loaded in the order of aqueous magenta ink and aqueous yellow ink from the upstream side in the transport direction. Furthermore, an A4-size (21 cm wide x 30 cm long) OPP film ("OPU-1" manufactured by Mitsui Chemicals Tohcello, Inc., thickness 20 μm) was fixed onto the conveyor as the printing substrate. The conveyor was then driven at 50 m / min, and as the printing substrate passed below the installation section of the inkjet heads, the aqueous inkjet ink set was ejected at a drop volume of 2.6 pL, printing a magenta / yellow gradation image. 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 magenta / yellow gradation print. The "magenta / yellow gradation image" refers to a 5 cm wide x 30 cm long magenta gradation image printed using aqueous magenta ink (with a printing rate varied in 10% increments between 10 and 100%) and a 5 cm wide x 30 cm long yellow gradation image printed using aqueous yellow ink, arranged adjacent to each other with their long sides in contact. Furthermore, a magenta / yellow gradation print was produced using a Futamura Chemical OPP film (FOR-AQ, thickness 20 μm) as the printing substrate, using the same method as above.

[0160] <Evaluation 3: Evaluation of print quality (whiteout)> The magenta / yellow gradation print produced by the method described above was visually observed. The print quality of the magenta / yellow gradation print was evaluated by checking for the presence or absence of whiteout at a printing rate of 100%. The evaluation criteria were as follows, with ratings A, B, C+, and C being considered usable. The above evaluation was performed on each of the two types of printing substrates on which the magenta / yellow gradation print was printed. (Evaluation criteria) A: No white spots were observed on either of the two types of printing substrates, and on both the printed areas of the magenta gradation image and the yellow gradation image. B: Slight white spots were observed on either the printed areas of the magenta gradation image or the yellow gradation image on only one of the two types of printing substrates. C+: Slight white spots were observed on either the printed areas of the magenta gradation image and the yellow gradation image on only one of the two types of printing substrates. C: Clear white spots were observed on either the printed areas of the magenta gradation image and the yellow gradation image on at least one of the two types of printing substrates. D: Clear white spots were observed on either the printed areas of the magenta gradation image and the yellow gradation image on both of the two types of printing substrates.

[0161] <Evaluation 4: Evaluation of print quality (bleeding)> The magenta / yellow gradation print produced by the method described above was visually observed. The print quality of the magenta / yellow gradation print was evaluated by checking the coverage rate at the boundary between the printed portion of the magenta gradation image and the printed portion of the yellow gradation image where bleeding began to occur. The evaluation criteria were as follows, with ratings of A, B, and C+ being considered usable. Table 1 also lists the results of the two types of printing substrates that were evaluated, with the poorest evaluation results. (Evaluation criteria) A: No color bleeding was observed on both printing substrates even at a printing rate of 80%. B: Color bleeding was observed on at least one printing substrate at a printing rate of 80%. C+: Color bleeding was observed on at least one printing substrate at a printing rate of 70%. C: Color bleeding was observed on at least one printing substrate at a printing rate of 60%. D: Color bleeding was observed on both printing substrates at a printing rate of 60%.

[0162] <Evaluation 5: Evaluation of Water Resistance> 20 μL of the above aqueous inkjet ink was dropped onto an OPP film (FOR-AQ, 20 μm thick) manufactured by Futamura Chemical Co., Ltd., and coated using an automatic coater ("PI-1210" manufactured by Tester Sangyo Co., Ltd.) equipped with an SA-203 bar coater (ROD No. 3). The OPP film coated with the aqueous inkjet ink was then placed in a constant temperature incubator set at 70°C, where the printed substrate was allowed to dry for 3 minutes. The ink film was then rubbed back and forth over a 1 cm width with a cotton swab moistened with ion-exchange water, and the number of times the cotton swab was rubbed back and forth until the ink film at the rubbed area was completely peeled off was counted. This evaluation was performed at five locations on the same ink film, and the water resistance was evaluated by calculating the average number of reciprocating motions. The evaluation criteria were as follows, with ratings of A+, A, B, C+, and C deemed usable. Evaluation was also carried out for each of the water-based magenta ink and water-based yellow ink, and the results of those with poor evaluations are listed in Table 1. (Evaluation criteria) A+: The coating film did not peel off even after rubbing 40 times A: The coating film peeled off after 30 to 39 times B: The coating film peeled off after 21 to 29 times C+: The coating film peeled off after 11 to 20 times C: The coating film peeled off after 6 to 10 times D: The coating film peeled off after 5 times or less

[0163] Examples 43 to 85 Production Examples of Pigment Dispersion Resins 3 to 15 Synthesis was carried out in the same manner as for the pigment dispersion resin 1, except that the types and amounts of the polymerizable monomers used were changed as shown in Table 2, to obtain aqueous solutions of pigment dispersion resins 3 to 15 (each having a non-volatile content of 20% by mass).

[0164]

[0165]

[0166] Table 2 also lists the composition of the pigment dispersion resins described above, as well as the mass average molecular weight, acid value, and molar amount of the polymerizable monomer having an aromatic group contained in each pigment dispersion resin. Details of the crosslinking agents listed in Table 2 are as follows: Denacol EX-321 (epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 140 (g / eq.)) Denacol EX-313 (epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 141 (g / eq.)) Denacol EX-612 (epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 166 (g / eq.)) Denacol EX-614B (epoxy compound manufactured by Nagase ChemteX Corporation, epoxy equivalent 173 (g / eq.))

[0167] <Production Example of Magenta Pigment Dispersions 4 to 23> Magenta pigment dispersions 5 to 7, 11, and 15 to 23 (before crosslinking treatment) and yellow pigment dispersions 5 to 7, 11, and 15 to 23 (before crosslinking treatment, each with a pigment concentration of 15% by mass) were produced using the same raw materials and method as those used for Magenta Pigment Dispersion 1 and Yellow Pigment Dispersion 1, except that the pigment dispersing resins 3 to 15 described above were used. Then, magenta pigment dispersions 4 to 23 and yellow pigment dispersions 4 to 23 (each with a pigment concentration of 14% by mass), in which the pigment dispersion resin was crosslinked, were produced using the same materials and by the same method as for magenta pigment dispersion 2 and yellow pigment dispersion 2, except that magenta pigment dispersion 1 or yellow pigment dispersion 1, or magenta pigment dispersions 5 to 7, 11, 15 to 23 (before crosslinking treatment) or yellow pigment dispersions 5 to 7, 11, 15 to 23 (before crosslinking treatment); a crosslinking agent shown in Table 2; and ion-exchanged water were used in the blending amounts shown in the row of "Crosslinking treatment" in Table 2.

[0168] <Production Examples of Pigment Dispersion Resins 16 to 38> Synthesis was performed in the same manner as for the Pigment Dispersion Resin 2, except that the types and amounts of the polymerizable monomers used were changed as shown in Table 3, to obtain aqueous solutions of Pigment Dispersion Resins 16 to 38 (each having a non-volatile content of 20% by mass).

[0169]

[0170]

[0171] Table 3 also lists the configuration of the pigment dispersion resin 2 described above. Table 3 also lists the mass average molecular weight, molecular weight dispersity, molar amount of polymerizable monomer having an aromatic group, acid value, and glass transition temperature of each pigment dispersion resin. Note that for the molar amount of polymerizable monomer having an aromatic group and acid value, the values ​​for each block, the difference in values ​​between blocks, and the value for the entire pigment dispersion resin are also listed. Furthermore, for the glass transition temperature, the values ​​for each block and the value for the entire pigment dispersion resin are also listed. Details of the abbreviations listed in Table 3 above are as follows: BzMA: benzyl methacrylate St: styrene CHMA: cyclohexyl methacrylate 2HEA: 2-ethylhexyl acrylate BMA: butyl methacrylate SMA: stearyl methacrylate MAA: methacrylic acid

[0172] <Production Examples of Magenta Pigment Dispersions 24 to 46 and Yellow Pigment Dispersions 24 to 46> Magenta pigment dispersions 24 to 46 and yellow pigment dispersions 24 to 46 (each with a pigment concentration of 15% by mass) were produced using the same materials and methods as for Magenta Pigment Dispersion 3 and Yellow Pigment Dispersion 3, except that the aqueous solutions of Pigment Dispersion Resins 16 to 38 were used instead of the aqueous solution of Pigment Dispersion Resin 2, respectively.

[0173] <Production of aqueous inkjet ink set 2> A set of aqueous inkjet inks was produced using the same raw materials and in the same amounts as in Example 3, except that the magenta pigment dispersion and yellow pigment dispersion used were changed to those shown in Table 4.

[0174] <Production of aqueous inkjet ink set 3> A set of aqueous inkjet inks was produced using the same raw materials and in the same amounts as in Example 8, except that the magenta pigment dispersion and yellow pigment dispersion used were changed to those shown in Table 5.

[0175] The resulting aqueous inkjet ink set was then used to carry out the above-mentioned evaluations 1 to 5. The evaluation results are shown in Tables 4 and 5 below.

[0176]

[0177]

[0178] As shown in Tables 1, 4, and 5 above, the aqueous inkjet inks (ink sets) of Examples 1 to 85 having the configuration of the present invention were superior in ejection stability compared to the aqueous inkjet inks of Comparative Examples 1 to 5, and also had good solid coverage and little color mixing. Furthermore, the water resistance of the ink film was also good. These results confirmed that the aqueous inkjet inks having the configuration of this embodiment are excellent aqueous inkjet inks that combine ejection stability, print quality of printed matter, and water resistance.

Claims

1. An aqueous inkjet ink comprising a pigment, a pigment dispersion resin, a surfactant, a water-soluble organic solvent, and a binder resin, wherein the pigment dispersion resin comprises one or more selected from the group consisting of polymers having a crosslinked structure and block polymers, and the water-soluble organic solvent comprises hexylene glycol, and wherein the sum of the content (g) of the pigment dispersion resin and the content (g) of the binder resin contained in 100 g of the aqueous inkjet ink is WR (g), and the content of the pigment contained in 100 g of the aqueous inkjet ink is WP (g), the value expressed by WR / WP is 1 to 7.

2. The aqueous inkjet ink according to claim 1, wherein the content of said hexylene glycol is 5 to 90 mass % based on the total content of said water-soluble organic solvent contained in said aqueous inkjet ink.

3. The aqueous ink-jet ink according to claim 1 or 2, wherein the surfactant comprises an acetylene diol-based surfactant.

4. The water-based inkjet ink according to claim 1 or 2, wherein the water-soluble organic solvent further comprises a diol having 2 to 5 carbon atoms.

5. The aqueous inkjet ink according to claim 1 or 2, further comprising a wax.

6. A printed matter obtained by printing the aqueous inkjet ink according to claim 1 or 2 onto a printing substrate.

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