Aqueous inkjet ink and printed matter

Abstract

Provided is an aqueous inkjet ink containing a pigment, a water-soluble organic solvent, polyethylene glycol (A) having a number-average molecular weight of 200-10,000, and an acetylene diol-based surfactant having an HLB value of 3-14. The content of the pigment is 3-7.5 mass% with respect to the total amount of the aqueous inkjet ink. The water-soluble organic solvent contains 10-30 mass% of C2-6 alkanediols with respect to the total amount of the aqueous inkjet ink. The mass ratio of the content of the polyethylene glycol (A) and the content of the acetylene diol-based surfactant having an HLB value of 3-14 is set at 2-50.

Description

Water-based inkjet inks and printed materials

[0001] This disclosure relates to inkjet inks and printed materials produced using said aqueous inkjet inks.

[0002] Inkjet printing is a recording method that produces printed materials with text and / or images by ejecting droplets of ink from fine nozzles and adhering them to a printing substrate. Inkjet printing has the advantages of low noise and ease of operation. In addition, it has many other advantages, such as the ability to easily and inexpensively produce full-color prints and the ability to print on various printing substrates without contact. Due to these advantages, inkjet printing has become widespread. In particular, in recent years, the use of inkjet printing has expanded beyond office and home use (consumer printing applications) to include the commercial and industrial printing markets.

[0003] On the other hand, inks used in inkjet printing methods (referred to as "inkjet inks" in this disclosure) include water-based, solvent-based, and active energy ray-curing types. Among these, the demand for water-based inkjet inks is increasing rapidly, with the aim of reducing environmental impact and improving safety.

[0004] The aforementioned commercial and industrial printing markets demand high printing speeds and high print quality for the printed materials produced. Furthermore, the commercial printing market often utilizes easily permeable paper substrates such as fine paper, plain paper, and corrugated cardboard. Therefore, from the perspective of further expanding the commercial printing market with water-based inkjet inks, there is a need to be able to produce printed materials with excellent print quality and density on easily permeable paper substrates.

[0005] When printing on paper substrates that are difficult to penetrate, such as coated paper, water-based inkjet inks do not easily penetrate into the interior of the paper substrate. As a result, the colorants contained in the water-based inkjet ink tend to remain on the surface of the paper substrate, making it easier to obtain high print density. However, when printing on paper substrates that are easily penetrated, water-based inkjet inks easily penetrate into the interior of the paper substrate. Therefore, conventionally, when printing on easily penetrated paper substrates using water-based inkjet inks, there has been a problem in obtaining printed materials with high print density (high print density).

[0006] To address the challenge of increasing the density of printed materials, for example, Patent Document 1 discloses an aqueous inkjet ink containing self-dispersing carbon black, urethane resin, and polyethylene glycol having a number average molecular weight of 600 or more. According to Patent Document 1, an aggregate formed by the three components contained in the aqueous inkjet ink makes it difficult for the aqueous inkjet ink to penetrate the printing substrate, thereby improving the print density. It is also disclosed that the aggregate can suppress the evaporation of water from the aqueous inkjet ink, thereby improving the adhesion recovery property of the aqueous inkjet ink (the ability to remove the adhered material after the aqueous inkjet ink has adhered and to print again). However, the present inventors have found that, depending on the printing conditions, nozzle clogging (failure to discharge the aqueous inkjet ink from the nozzle) may occur when printing continuously, and that the aqueous inkjet ink has somewhat poor storage stability.

[0007] Furthermore, Patent Document 2 discloses an aqueous inkjet recording ink containing a self-dispersing pigment, methylcellulose, and specific glycol ethers. According to Patent Document 2, by using a combination of a self-dispersing pigment that readily interacts with paper fibers, methylcellulose that functions as an anti-bleed-through agent, and glycol ethers that function as a penetrating agent, it is possible to obtain printed materials with excellent drying properties while preventing a decrease in print density. On the other hand, for example, the methylcellulose used in the example of Patent Document 2 has a large mass-average molecular weight of 55,000 or more. It is presumed that aqueous inks containing methylcellulose with such a large mass-average molecular weight may have poor ejection stability when printing continuously, depending on the ejection conditions and printing speed.

[0008] Patent Document 3 describes an aqueous inkjet ink composition that simultaneously solves the problems of high concentration and intermittent printing (droplet placement misalignment that occurs when ejection is repeated at regular intervals), which contains a predetermined amount of pigment and propylene glycol, and specifies the kinematic viscosity at 20°C. On the other hand, Patent Document 3 employs the conventional method of increasing the pigment content as a way to achieve high concentration. Therefore, even if intermittent printing can be improved with the configuration of Patent Document 3, there is a possibility that the ejection stability when printing continuously, and furthermore, the storage stability, will be inferior.

[0009] Furthermore, Patent Document 4 discloses an inkjet ink that includes carbon black dispersed in a pigment dispersion resin of a specific configuration and self-dispersing carbon black, enabling high-density printing on thin, high-quality paper substrates for general printing. On the other hand, it has been found that depending on the paper substrate used, it may not be possible to completely suppress the penetration of the inkjet ink, resulting in printed materials with inferior density.

[0010] As described above, with conventional technology, it has been difficult to provide an aqueous inkjet ink that yields printed materials with excellent print density even when printing on printing substrates, especially easily permeable paper substrates, and that also exhibits excellent ejection stability, wettability, and storage stability.

[0011] Japanese Patent Publication No. 2018-083414, Japanese Patent Publication No. 2010-095710, Japanese Patent Publication No. 2018-177943, Japanese Patent Publication No. 2015-183027

[0012] To solve the aforementioned problems, one embodiment of the present invention provides an aqueous inkjet ink that can improve the density of printed materials obtained when printed on paper substrates, particularly high-quality paper, and also exhibits good ejection stability, storage stability, and wetting and spreading properties on the printing substrate.

[0013] Based on the above background, the inventors conducted diligent research and discovered an aqueous inkjet ink having the following configuration, thus completing the present invention.

[0014] That is, one embodiment of the present invention relates to an aqueous inkjet ink comprising a pigment, a water-soluble organic solvent, polyethylene glycol, and a surfactant, wherein the content of the pigment is 3 to 7.5% by mass of the total amount of the aqueous inkjet ink, the water-soluble organic solvent comprises alkanediols having 2 to 6 carbon atoms, the content of alkanediols having 2 to 6 carbon atoms is 10 to 30% by mass of the total amount of the aqueous inkjet ink, the polyethylene glycol comprises polyethylene glycol (A) having a number average molecular weight of 200 to 10,000, the surfactant comprises an acetylenediol-based surfactant having an HLB value of 3 to 14, and the ratio of the content of polyethylene glycol (A) to the content of the acetylenediol-based surfactant having an HLB value of 3 to 14 (polyethylene glycol (A) / acetylenediol-based surfactant) is 2 to 50 by mass. In the above embodiment, the water-soluble organic solvent contains C3-C6 alkanediols, and the ratio of the polyethylene glycol (A) content to the acetylenediol-based surfactant content having an HLB value of 3-14 (content of polyethylene glycol (A) / content of acetylenediol-based surfactant) is preferably 2-30 by mass. Another embodiment of the present invention relates to a printed article obtained by printing the aqueous inkjet ink of the above embodiment onto a printing substrate.

[0015] According to the present invention, it is possible to provide an aqueous inkjet ink that can improve the density of printed materials obtained when printed on paper substrates such as high-quality paper, and that has good ejection stability, storage stability, and wetting spread on the printing substrate.

[0016] The following describes an embodiment of a water-based inkjet ink (hereinafter simply referred to as "the water-based inkjet ink of this embodiment" or "the ink") which is one embodiment of the present invention.

[0017] The aqueous inkjet ink of this embodiment, having the configuration described above, enables high density printing of the resulting printed material when printing on a printing substrate, particularly paper substrates, and also exhibits effects such as improved ejection stability and wetting spread, as well as improved storage stability. Although the mechanism is not clear, the inventors speculate as follows. However, the present invention is not limited by the following speculation.

[0018] First, let's explain how to increase the density of printed materials when printing on paper substrates. Generally, increasing the pigment concentration can be achieved by increasing the density of printed materials. In particular, when printing on poorly permeable printing substrates such as coated paper and art paper, increasing the pigment concentration in the ink can achieve high density. However, with easily permeable paper substrates such as inkjet-specific paper and high-quality paper, the ink penetrates into the interior of the paper substrate during printing. Therefore, when printing on easily permeable paper substrates, it is difficult to achieve high density unless the pigment concentration is increased even further than that used for printing on the poorly permeable printing substrates. However, increasing the pigment concentration to achieve high density can easily disrupt the dispersion state of pigment particles in the ink, promoting aggregation of the pigment particles over time, which may worsen storage stability. As a result of diligent research by the inventors, we have found that by using polyethylene glycol, the polyethylene glycol and the cellulose in the paper substrate form an interaction, which can suppress the penetration of ink into the interior of the paper substrate. Here, it is preferable to set the pigment concentration of the ink to 3.0 to 7.5% by mass, in order to produce high-concentration printed materials on both poorly permeable and easily permeable paper substrates, and in addition to achieving excellent storage stability. Furthermore, it is preferable to use polyethylene glycol in combination with the pigment at such a pigment concentration.

[0019] Next, we will explain the wetting and spreading of water-based inkjet inks on a printing substrate. Generally, one way to sufficiently wet inkjet ink droplets is to lower the surface tension of the inkjet ink. Surfactants are commonly used as materials to lower surface tension. Among these, acetylenediol-based surfactants, which are nonionic surfactants containing an acetylene group, can be suitably used. Because acetylenediol-based surfactants have a triple bond in their molecular structure, they are less prone to molecular deformation and are more effective at lowering surface tension than surfactants composed only of single bonds. As a result, even a small amount of acetylenediol-based surfactant can sufficiently lower surface tension. This surface tension reduction effect is particularly pronounced with acetylenediol-based surfactants that have a low HLB value. However, generally, surfactants with low solubility in water, i.e., low HLB values, are difficult to mix with water-based inkjet inks, which are mainly composed of water, making it difficult to maintain storage stability and discharge stability. In contrast, in the ink of this embodiment, it is believed that polyethylene glycol makes the acetylenediol-based surfactant compatible, thereby maintaining the storage stability of the water-based inkjet ink. As can be seen from its structure, polyethylene glycol does not have ionic groups, and it can be inferred that it functions to protect the acetylenediol-based surfactant and also works to make the acetylenediol-based surfactant compatible.

[0020] If surfactants are not sufficiently miscible in water-based inkjet inks, not only can the storage stability of the ink be compromised, but the surfactants may also orient themselves at the interface of the water-based inkjet ink near the nozzles in the inkjet head, potentially worsening the ejection stability. Therefore, in this invention, the ratio of the content of acetylenediol-based surfactants with an HLB value of 3 to 14 to the content of polyethylene glycol (content of polyethylene glycol (A) / content of acetylenediol-based surfactant) is set to 2 to 50. This makes it possible to sufficiently miscible the acetylenediol-based surfactants with polyethylene glycol, thereby achieving both storage stability and ejection stability.

[0021] There are other benefits to selecting polyethylene glycol. For example, if inkjet ink is ejected from the nozzle of an inkjet head and then ejection is stopped for a certain period of time, or if ejection continues continuously, the ejection of the inkjet ink may become unstable when the inkjet ink is ejected again from the same nozzle afterward. This is thought to be because some of the inkjet ink near the nozzle evaporates, increasing the viscosity due to an increase in the solid content concentration of the inkjet ink at the interface of the nozzle end face. However, in this embodiment, by including polyethylene glycol in the inkjet ink, the moisture retention of the inkjet ink is increased, making it possible to eject ink stably even after ejection has been stopped for a certain period of time, or even when continuous printing is continued without stopping.

[0022] On the other hand, in order to suitably exhibit the effects of polyethylene glycol as described above, it is preferable to consider the number-average molecular weight of the polyethylene glycol. Specifically, the number-average molecular weight of polyethylene glycol that can be suitably used in the aqueous inkjet ink of this embodiment is 200 to 10,000. By setting the number-average molecular weight to 200 or more, the interaction with cellulose as described above works effectively, improving the concentration of printed materials on easily permeable paper substrates, and also suitably exhibiting the function of protecting acetylenediol-based surfactants, making it easy to achieve both storage stability and discharge stability. On the other hand, by setting the number-average molecular weight to 10,000 or less, it is possible to prevent a decrease in discharge stability caused by polyethylene glycol with a large number-average molecular weight.

[0023] Furthermore, when polyethylene glycol exerts the effects described above, particularly when it is compatible with acetylenediol-based surfactants, the type and amount of organic solvent included with polyethylene glycol are also important. In the aqueous inkjet ink of this embodiment, 10 to 30% by mass of C2-C6 alkanediols are used. C2-C6 alkanediols are compatible with both polyethylene glycol and acetylenediol-based surfactants. As a result, polyethylene glycol plays a role in assisting the compatibility of acetylenediol-based surfactants, making it possible to further improve discharge stability and storage stability.

[0024] Furthermore, since alkanediols with 2 to 6 carbon atoms contain multiple hydroxyl groups and have a relatively small molecular weight, they are thought to play a role in moisturizing the area near the nozzle end face. This can contribute to further improvement in discharge stability.

[0025] As described above, the aqueous inkjet ink having the configuration of this embodiment makes it possible to solve the above-mentioned problems simultaneously and at a high level.

[0026] Furthermore, the aqueous inkjet ink specifically disclosed in the examples of Patent Document 1 differs from the aqueous inkjet ink of this embodiment in that it does not contain alkanediols having 2 to 6 carbon atoms. Paragraphs 0067 to 0068 of Patent Document 1 state that aqueous inkjet inks may contain any water-soluble organic solvent. However, Patent Document 1 neither describes nor suggests that it is preferable for the water-soluble organic solvent to contain alkanediols having a specific number of carbon atoms, nor does it describe the effect of alkanediols having 2 to 6 carbon atoms, which allows for sufficient compatibility between polyethylene glycol (A) and acetylenediol-based surfactants, thereby improving discharge stability and storage stability. In addition, Patent Documents 2 to 4 do not disclose specific examples containing polyethylene glycol (A) with a number-average molecular weight of 200 to 10,000. Furthermore, Patent Documents 2 and 3 do not describe that aqueous inkjet inks may contain polyethylene glycol (A). On the other hand, Patent Document 4 states that inkjet inks may contain polyethylene glycol as a wetting agent (paragraph 0024). However, the above description is only one of many compounds listed that can be used as a wetting agent. Patent Document 4 does not describe or suggest that polyethylene glycol having a specific number-average molecular weight can be selectively used to improve the density, storage stability, and discharge stability of printed materials.

[0027] The following describes in detail each component constituting the aqueous inkjet ink, which is one embodiment of the present invention.

[0028] <Pigments> The water-based inkjet ink of this embodiment contains pigments. By using water-based inkjet ink containing pigments, printed materials with high density and excellent lightfastness, water resistance, etc., can be obtained. In the case of white water-based inkjet ink containing white pigment, excellent opacity can also be obtained.

[0029] As the above pigments, any conventionally known organic and inorganic pigments can be used. For example, pigments represented by color index names can be used. Specific examples include: As red pigments, C.I. Pigment Red 2, 5, 7, 9, 12, 17, 22, 23, 31, 48:1, 48:2, 48:3, 48:4, 49:1, 49:2, 57:1, 57:2, 112, 122, 123, 146, 147, 149, 150, 166, 168, 170, 171, 175, 176, 177, 178, 184, 188, 202, 207, 209, 254, 255, 260, 264, 266, 269, 282; As violet pigments, C.I. Pigment Violet 19, 23, 29, 32, 36, 37, 42, 50; as orange pigment, 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 pigment, C.I. Pigment Blue 15, 15:3, 15:4, 15:6, 16, 60, 64, 79; as green pigment, C.I. Pigment Green 7, 10, 36, 48; as yellow pigment, C.I. Pigment yellows 1, 2, 3, 5, 12, 13, 14, 16, 17, 24, 73, 74, 83, 87, 93, 94, 95, 97, 98, 109, 110, 111, 112, 120, 126, 127, 128, 129, 137, 138, 139, 147, 150, 151, 154, 155, 166, 167, 168, 170, 180, 185, 213; as black pigments, C.I. Pigment Black 1, 7, 11; and as white pigments, C.I. Pigment White 4, 5, 6, 21, etc. These pigments may be used individually or in combination of two or more. A solid solution consisting of two or more of the above-listed pigments may also be used as a pigment.

[0030] The pigment content in the aqueous inkjet ink of this embodiment is preferably 3.0 to 7.5% by mass relative to the total amount of aqueous inkjet ink. If the pigment content is 3% by mass or more, high concentration can be achieved when printing on paper substrates, and if it is 7.5% by mass or less, storage stability can be easily improved. The pigment content is more preferably 3.5 to 7% by mass, and particularly preferably 4.0 to 6.5% by mass.

[0031] The pigment contained in the aqueous inkjet ink of this embodiment is preferably a black pigment, and more preferably carbon black (C.I. Pigment Black 7). Since carbon black has a small primary particle size of about 10 to 50 nm, when an aqueous inkjet ink containing carbon black is printed on a highly permeable paper substrate, the carbon black easily penetrates into the interior of the paper substrate, and the image density tends to decrease. Also, since aqueous inkjet ink containing carbon black is black, its content has a particularly large effect on the density of the printed material. In other words, if the concentration of carbon black in the aqueous inkjet ink is low, it is difficult to achieve high density in the printed material, especially when printing on a highly permeable paper substrate. On the other hand, if the concentration of carbon black is high, the dispersion state of carbon black, which has a small primary particle size, is easily disrupted, and aggregation over time is easily promoted. For the above reasons, the aqueous inkjet ink of this embodiment, which can achieve high density without significantly increasing the pigment concentration, can effectively exhibit its effects when carbon black is used as the pigment. In particular, carbon black is thought to have a relatively good affinity with polyethylene glycol, which effectively suppresses the aforementioned penetration into the paper substrate and aggregation within the aqueous inkjet ink. As a result, improvements in print density and ejection stability can be particularly effectively achieved.

[0032] (Dispersion form of pigment) The above pigments are preferably used in the form of resin-dispersed pigments or self-dispersing pigments. A resin-dispersed pigment is a pigment in which at least a portion of the pigment surface is coated with a pigment-dispersing resin, and the presence of the pigment-dispersing resin makes it possible to disperse the pigment in an aqueous medium (a medium consisting of a liquid containing at least water). On the other hand, a self-dispersing pigment is a pigment that can be dispersed in an aqueous medium without using a pigment-dispersing resin or a dispersant such as a surfactant for pigment dispersion. Specifically, it is a pigment that has been surface-treated to impart self-dispersibility. The above surface treatment may be, for example, oxidation treatment of the pigment surface with an oxidizing agent such as nitric acid, hypochlorous acid, hydrogen peroxide, or ozone; sulfonation treatment of the pigment surface with a sulfonating agent such as sulfuric acid, sulfur trioxide, or sulfamic acid; diazo coupling reaction treatment of the pigment surface with a compound having an acid group or a basic group and a diazonium base; or plasma treatment of the pigment surface. Note that resin-dispersed pigments and self-dispersing pigments may be used individually or in combination of multiple types. Furthermore, the aqueous inkjet ink of this embodiment may contain both resin-dispersed pigments and self-dispersing pigments. In the case of the aqueous inkjet ink of this embodiment, it is preferable that the pigment contains resin-dispersed pigments (and may also contain self-dispersing pigments) from the viewpoint of particularly improving the density of the printed material.

[0033] Whether the pigment is resin-dispersed or self-dispersing, the average secondary particle diameter of the pigment is preferably 30 to 300 nm, more preferably 50 to 250 nm, and particularly preferably 80 to 200 nm. The above average secondary particle diameter refers to the volume-based median diameter (D50) measured at 25°C using dynamic light scattering with a particle size distribution analyzer (for example, Microtrac-Bell's "Nanotrac UPA-EX150").

[0034] (Pigment Dispersion Resin) As described above, the aqueous inkjet ink of this embodiment can use a resin used for pigment dispersion (referred to simply as "pigment dispersion resin" in this disclosure). By selecting and examining the composition and molecular weight of the polymerizable monomers that constitute the pigment dispersion resin, the coating ability and charge of the pigment dispersion resin to the pigment can be easily adjusted, making it possible to impart dispersion stability and storage stability even to fine pigments, and furthermore, to obtain printed materials with excellent ejection stability, density, and color reproducibility.

[0035] Examples of pigment-dispersing resins included in the aqueous inkjet ink of this embodiment include resin microparticles containing pigment ("resin microparticles" will be described later), and resins in which the ratio of the amount of resin adsorbed on the pigment to the amount of resin added is 35% by mass or more.

[0036] As an example of a method for measuring the adsorption rate to the above-mentioned pigment, a pigment dispersion (or an aqueous inkjet ink containing the pigment and the resin to be investigated, but not containing any solid components other than the pigment and the resin to be investigated) diluted with water as necessary is subjected to centrifugation until the supernatant becomes clear (for example, 5 mL of sample at 80,000 rpm for 4 hours), and then the amount of resin contained in the recovered supernatant is measured (for example, the supernatant is left to stand in a 100°C environment to completely remove the liquid components, and then the mass of the evaporation residue is measured and taken as the amount of resin). Then, the amount of resin contained in the supernatant is subtracted from the amount of resin contained in the centrifuged pigment dispersion (or aqueous inkjet ink) (let's call it WR0 [g]), and the adsorption rate can be calculated by dividing the resulting value by WR0.

[0037] The type of pigment dispersion resin is not particularly limited (specific examples will be described later). In particular, it is preferable to use one or more selected from the group consisting of acrylic resins, maleic acid resins, urethane resins, polyester resins, and polyolefin resins, in terms of discharge stability, storage stability, material selectivity, and ease of synthesis. Furthermore, from the viewpoint of achieving high concentration, it is especially preferable to use one or more selected from the group consisting of acrylic resins and maleic acid resins.

[0038] In this disclosure, "acrylic resin" refers to a resin using one or more polymerizable monomers selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters (styrene monomers may also be used). However, resins containing (maleic anhydride) (at least one selected from "maleic acid" and "maleic anhydride") as a polymerizable monomer are not included in "acrylic resin" in this disclosure. Furthermore, "maleic acid resin" refers to a resin using maleic acid and / or maleic anhydride as a polymerizable monomer. In addition, (maleic anhydride) resins may use α-olefins, styrene monomers, acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, etc. as polymerizable monomers.

[0039] When using an acrylic resin or a maleic acid resin as the pigment-dispersing resin, from the viewpoint of improving ejection stability and storage stability, it is preferable to use a monomer having an alkyl group with 12 to 22 carbon atoms. Specific examples of the monomer having an alkyl group with 12 to 22 carbon atoms include, for example, α-olefins such as 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 1-docosene, and 1-tetracosene, and (meth)acrylate monomers such as lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, etc. Note that the notations “(meth)acrylic acid” and “(meth)acrylate” represent “methacrylic acid and acrylic acid” and “methacrylate and acrylate”, respectively.

[0040] The pigment-dispersing resin may be synthesized by a known method or may be a commercially available product. There is no particular limitation on its structure, and for example, resins having a random structure, a block structure, a graft structure, a comb structure, an alternating structure, a star structure, etc. can be used. Further, as the pigment-dispersing resin, a water-soluble resin may be selected or a water-insoluble resin may be selected.

[0041] When using a water-soluble resin as the pigment-dispersing resin, its acid value is preferably 30mgKOH / g~450mgKOH / g, more preferably 50~400mgKOH / g, and particularly preferably 100~350mgKOH / g. By setting the acid value within the above range, it becomes easy to maintain the dispersion stability of the pigment. In addition, the solubility of the pigment-dispersing resin in water can be ensured, and the interaction between the pigment-dispersing resins becomes suitable, so that the viscosity of the pigment dispersion can be suppressed, and it is also preferable from the point that it can be stably ejected from the inkjet head regardless of the use conditions.

[0042] On the other hand, when a water-insoluble resin is used as the pigment-dispersing resin, its acid value is preferably 0 to 100 mgKOH / g, more preferably 5 to 90 mgKOH / g, and even more preferably 10 to 80 mgKOH / g. If the acid value is within the above range, a printed matter with excellent drying property and good printing image quality can be easily obtained.

[0043] The acid value of the resin can be measured by a known apparatus. In the present disclosure, the acid value of the resin is a value measured by a potentiometric titration method in accordance with JIS K 2501. As an example of a specific measurement method, after dissolving the resin in a toluene-ethanol mixed solvent using AT-610 manufactured by Kyoto Electronics Industry Co., Ltd., titrating with a potassium hydroxide solution, and calculating the acid value from the titration amount up to the end point.

[0044] The mass average molecular weight (MW) of the pigment-dispersing resin is preferably 5,000 to 100,000. If the mass average molecular weight is 5,000 or more, the dispersion stability is suitable, and if it is 100,000 or less, the ejection stability is good. The above mass average molecular weight is more preferably 8,000 to 50,000, and particularly preferably 10,000 to 35,000. In addition, the mass average molecular weight (MW) of the pigment-dispersing resin before the crosslinking treatment described later is also preferably within the above range. The method for measuring the mass average molecular weight will be described later.

[0045] (Polymer having a crosslinked structure) The above pigment-dispersing resin may have a crosslinked structure. In that case, as a method for introducing a crosslinked structure into the pigment-dispersing resin (referred to as "un-crosslinked polymer" in the present disclosure) before the crosslinked structure is introduced, a method using a crosslinking agent can be preferably used. Specifically, after dispersing the pigment using the un-crosslinked polymer, a crosslinking agent is added to perform a crosslinking treatment, thereby introducing a crosslinked structure into the un-crosslinked polymer to obtain a pigment-dispersing resin having a crosslinked structure. By using a pigment-dispersing resin having a crosslinked structure, it is possible to suppress the desorption of the pigment-dispersing resin from the pigment surface and improve the ejection stability and storage stability of the aqueous inkjet ink.

[0046] As the uncrosslinked polymer mentioned above, any resin that can be used as a pigment dispersion resin can be used. Alternatively, two or more uncrosslinked polymers may be used in combination, and these two or more uncrosslinked polymers may be crosslinked during the crosslinking process.

[0047] When using acrylic resin or maleic acid resin as the uncrosslinked polymer, as described above, it is preferable to use a polymerizable monomer having an alkyl group with 12 to 22 carbon atoms, from the viewpoint of improving discharge stability and storage stability. Furthermore, regardless of the type of resin used, it is preferable to introduce aromatic groups into the uncrosslinked polymer. By using an uncrosslinked polymer having aromatic groups, the uncrosslinked polymer can be sufficiently adsorbed onto the pigment even in its pre-crosslinked state. As a result, after the crosslinking treatment, a pigment dispersion resin having a crosslinked structure can be present at high density on the pigment surface, and discharge stability is particularly improved. Examples of the aromatic groups include, but are not limited to, phenyl groups, naphthyl groups, anthryl groups, tolyl groups, xylyl groups, mesityl groups, and anisyl groups. In particular, it is preferable to select phenyl groups, naphthyl groups, and tolyl groups from the viewpoint of ensuring the dispersion stability of the pigment even in its pre-crosslinked state, and particularly improving the discharge stability of the aqueous inkjet ink after the crosslinking treatment. Furthermore, in order to favorably exhibit the above-mentioned effects, the molar amount of polymerizable monomers having aromatic groups relative to the total molar amount of polymerizable monomers constituting the uncrosslinked polymer is preferably 10 to 50 mol%, and more preferably 20 to 40 mol%.

[0048] The acid value of the uncrosslinked polymer is preferably 50 to 300 mg KOH / g, more preferably 60 to 250 mg KOH / g, even more preferably 70 to 200 mg KOH / g, and particularly preferably 70 to 160 mg KOH / g. By setting the acid value within the above range, it becomes easier to maintain the dispersion stability of the pigment even in the pre-crosslinking state, and after the crosslinking treatment, a high density of pigment dispersion resin having a crosslinked structure can be present on the surface of the pigment, improving the discharge stability. Furthermore, when using acid groups in the uncrosslinked polymer as crosslinking reaction sites, as described later, it becomes easier to form a sufficient amount of crosslinked structure. As a result, the discharge stability of aqueous inkjet ink can be easily improved.

[0049] The above crosslinking agent is preferably a compound having multiple functional groups in a single molecule that react with crosslinking reaction sites (e.g., acidic groups such as carboxyl groups and carboxylate groups) present in the pigment dispersion resin. By using such a compound, a high-density pigment dispersion resin with a crosslinked structure can be present on the pigment surface, easily improving discharge stability. Examples of the above crosslinking agent include aziridine compounds, isocyanate compounds, epoxy compounds, oxetane compounds, carbodiimide compounds, and oxazoline compounds. Among these, epoxy compounds are preferred as the crosslinking agent because they allow the crosslinking reaction to proceed near the surface of the pigment while maintaining a stable dispersion state of the pigment by the uncrosslinked polymer, thereby improving discharge stability. The crosslinking agent may be water-soluble or water-insoluble, but from the viewpoint of improving discharge stability by allowing the crosslinking reaction to proceed more efficiently in an aqueous medium, the solubility of the above crosslinking agent in 100 g of water at 25°C should be 0.1 to 50 g / 100 gH. ２ It is preferable that it be 0, and 0.2 to 40 g / 100 gH ２ It is more preferable that it be 0. Even more preferable that it is 0.5 to 30 g / 100 gH ２ It is O.

[0050] As described above, compounds having multiple epoxy groups in one molecule are preferably used as crosslinking agents. More preferably, compounds having two or more glycidyl ether groups in one molecule (polyglycidyl ether compounds) are used, and even more preferably, polyglycidyl ether compounds of polyhydric alcohols having hydrocarbon groups with 3 to 8 carbon atoms can be used. When a compound having multiple epoxy groups in one molecule is used as a crosslinking agent, the crosslinking reaction with the crosslinking reaction site can be carried out more efficiently in an aqueous medium, and a pigment dispersion resin having a crosslinked structure can be present at high density on the pigment surface, thus easily improving discharge stability. From this viewpoint, the epoxy equivalent is preferably 90 to 300 g / eq., and more preferably 100 to 200 g / eq.

[0051] 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'-diglycidyloxy biphenol, bisphenol A type diglycidyl ether, hydrogenated bisphenol A type diglycidyl ether, phthalate diglycidyl ester, terephthalate diglycidyl ester, and hydrogenated phthalate diglycidyl ester.

[0052] Furthermore, it is preferable that the crosslinking agent is added so that the functional group content (mol%) represented by the following formula (1) is 40 to 180 mol%. More preferably, the functional group content is 60 to 160 mol%, and particularly preferably 80 to 140 mol%. By blending the crosslinking agent so that the functional group content is within the above range, the desorption of uncrosslinked polymers can be suppressed, improving the dispersion stability and ejection stability of the aqueous inkjet ink. Moreover, controlling the functional group content as described above is also effective in improving the density of printed materials.

[0053] Formula (1): Functional group content (mol%) = [Number of moles of functional groups in the crosslinking agent] × 100 / [Number of moles of crosslinking reaction sites in the uncrosslinked polymer]

[0054] The pigment dispersion resin content in the aqueous inkjet ink of this embodiment is preferably 5 to 70% by mass, and more preferably 10 to 55% by mass, relative to the pigment content of the aqueous inkjet ink. Having the pigment dispersion resin content within the above range results in good storage stability.

[0055] (Resin-Dispersed Carbon Black) As described above, the pigment contained in the aqueous inkjet ink of this embodiment is preferably carbon black. When carbon black is used as the pigment and in the form of a resin-dispersed pigment (i.e., in the form of resin-dispersed carbon black), a commercially available product may be used as the resin-dispersed carbon black, or a commercially available carbon black may be dispersed using the pigment dispersion resin described above and the method described in the examples below. In the latter case, after the dispersion treatment, a crosslinking treatment may be performed using the crosslinking agent described above. Examples of the commercially available products include MICROPIGMO® WMBK-71 manufactured by Orient Chemical Industries, Ltd., and PRO-JET® BLACK APD 1000, APD 4000, etc. manufactured by Fujifilm Corporation.

[0056] The carbon black used in resin-dispersed carbon black preferably has a primary particle size of 11 to 40 nm, and more preferably 11 to 30 nm. Resin-dispersed carbon black using carbon black having the above primary particle size exhibits excellent storage stability. Furthermore, the carbon black used in resin-dispersed carbon black has a specific surface area of ​​50 to 400 m² as determined by the BET method. ２ It is preferably 90 to 360 m / g. ２It is more preferable that the specific surface area is / g. Resin-dispersed carbon black using carbon black having the above specific surface area has excellent storage stability. Furthermore, it is preferable that the volatile content of the carbon black used in the resin-dispersed carbon black is 0.5 to 10% by weight.

[0057] The primary particle size of carbon black, as described above, can be measured by, for example, the following method. That is, an image of carbon black powder is taken using a transmission electron microscope, and then the image is magnified 10,000 to 200,000 times. Next, 100 carbon black particles are randomly selected from the magnified image, and the average of the short axis diameter and long axis diameter is measured and calculated for each of them. The simple average of the calculated average values ​​of the 100 particles is then taken as the primary particle size of the carbon black. The specific surface area of ​​carbon black can be measured by, for example, the method described in JIS K 6217-2:2017. Furthermore, the volatile content of carbon black is the percentage of mass lost when the carbon black is heated at 950°C for 7 minutes, and can be measured by, for example, the method described in JIS K 6218-1:2005 (Method A).

[0058] (Self-dispersing carbon black) On the other hand, when carbon black is used as the pigment and in the form of a self-dispersing pigment (i.e., in the form of self-dispersing carbon black), a commercially available product may be used as the self-dispersing pigment, or commercially available carbon black may be subjected to the surface treatment described above by a conventionally known method. Examples of the above commercially available products include CAB-O-JET® 200K, 300K, and 400K manufactured by Cabot Corporation, the BONJET® BLACK series manufactured by Orient Chemical Industries, Ltd., the Aqua-Black® series manufactured by Tokai Carbon Co., Ltd., and the Fuji-JET Black series manufactured by Fuji Pigment Co., Ltd.

[0059] <Polyethylene Glycol> The aqueous inkjet ink of this embodiment contains polyethylene glycol. By forming an interaction with cellulose in the paper substrate, the polyethylene glycol can suppress the penetration of the aqueous inkjet ink into the paper substrate, improving the density of the printed material even when printed on easily permeable paper substrates. In addition, it becomes easier to suppress the evaporation of the inkjet ink near the nozzle, improving the ejection stability of the inkjet ink. Furthermore, by defining the ratio of the content of acetylenediol-based surfactants with an HLB value of 3 to 14 contained in the aqueous inkjet ink to the content of polyethylene glycol, it is possible to achieve both storage stability and ejection stability.

[0060] As described above, the number average molecular weight of polyethylene glycol is preferably 200 to 10,000, in terms of increasing the density when printed on easily permeable paper substrates, as well as having excellent storage stability and discharge stability. In some embodiments, the number average molecular weight may be 300 to 9,000 or 400 to 4,000. The number average molecular weight is more preferably 200 to 4,000, and particularly preferably 200 to 500. If the number average molecular weight is 200 or more, even when printing on easily permeable paper substrates, the penetration of aqueous inkjet ink is suppressed, and the density of the printed material is improved. In addition, the acetylenediol-based surfactant is suitably protected, and storage stability and discharge stability are improved. On the other hand, if the number average molecular weight is 10,000 or less, the thixotropy of aqueous inkjet ink is reduced, which is thought to be due to polyethylene glycol with a large number average molecular weight, and discharge stability is improved. In particular, if the number-average molecular weight is 500 or less, the thixotropy is further reduced, allowing for stable ejection even after an inkjet head equipped with aqueous inkjet ink has been left idle for a certain period of time without printing.

[0061] Because it is suitably compatible with acetylenediol-based surfactants and provides good storage stability and discharge stability, the polyethylene glycol content is preferably 1.0 to 15.0% by mass, more preferably 2.0 to 10.0% by mass, and particularly preferably 3.0 to 8.0% by mass, based on the total mass of the aqueous inkjet ink.

[0062] In the aqueous inkjet ink of this embodiment, the viscosity can be set to be suitable for ejection from the inkjet head, and it is also easier to prevent mist (tiny droplets generated during the ejection of inkjet ink) and satellites (sub-droplets separated from the main droplets of inkjet ink), thereby improving ejection stability. From this viewpoint, the product of the pigment content relative to the total amount of aqueous inkjet ink and the polyethylene glycol (A) content relative to the total amount of aqueous inkjet ink is preferably 6 to 80, more preferably 10 to 70, and even more preferably 15 to 55.

[0063] In particular, when only resin-dispersed carbon black is used as the pigment, the above-mentioned effect of improving ejection stability and further improving the accuracy of the projectiles is preferably expressed, so the product of the mass of resin-dispersed carbon black relative to the total amount of aqueous inkjet ink and the mass of polyethylene glycol (A) relative to the total amount of aqueous inkjet ink is preferably 6 to 80, more preferably 10 to 50, and even more preferably 15 to 40.

[0064] On the other hand, when using only self-dispersing carbon black as the pigment, the same effects as those of resin-dispersed carbon black are exhibited. Therefore, the product of the amount of self-dispersing carbon black relative to the total amount of aqueous inkjet ink and the amount of polyethylene glycol (A) relative to the total amount of aqueous inkjet ink is preferably 6 to 80, more preferably 10 to 70, even more preferably 20 to 58, and particularly preferably 30 to 55.

[0065] Furthermore, when resin-dispersed carbon black and self-dispersed carbon black are used in combination, the sum of the product values ​​of the carbon black content relative to the total amount of aqueous inkjet ink and the polyethylene glycol content, calculated for each type of carbon black, is preferably 6 to 80, more preferably 10 to 70, even more preferably 12 to 60, and particularly preferably 15 to 55.

[0066] <Water-soluble organic solvent> The aqueous inkjet ink of this embodiment contains a water-soluble organic solvent. Furthermore, the water-soluble organic solvent contains alkanediols having 2 to 6 carbon atoms. By using alkanediols having 2 to 6 carbon atoms, it becomes easier to further improve the ejection stability.

[0067] In this disclosure, "water-soluble organic solvent" refers to an organic compound that has a solubility of 1% by mass or more in water at 25°C and is a liquid at 25°C. In this disclosure, polyethylene glycol, as described above, is not included in the definition of a water-soluble organic solvent.

[0068] (Alkanediols with 2 to 6 carbon atoms) Examples of the above-mentioned alkanediols with 2 to 6 carbon atoms include ethylene glycol (1,2-ethanediol), 1,2-propanediol (propylene glycol), 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-1,3-propanediol, 1,2-hexanediol, 3-methyl-1,3-butanediol, 3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol, etc. These compounds may be used individually or in combination of two or more. In some embodiments, the C2-C6 alkanediols preferably include at least one selected from the group consisting of ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,5-pentanediol, and 2-methyl-2,4-pentanediol.

[0069] In particular, it is preferable to use C3-6 alkanediols as the C2-6 alkanediols because they have moderately low hydrophilicity and are especially compatible with acetylenediol-based surfactants, thus easily improving discharge stability and storage stability. Furthermore, the above alkanediols may have C3 or C4, and it is even more preferable to use one or more selected from the group consisting of 1,2-propanediol and 1,2-butanediol, because they have good discharge stability even after a long period of printing hiatus, and they have an excellent balance between penetration and wetting spread of water-based inkjet inks, which also improves the density of the printed material. And it is especially preferable to use 1,2-propanediol because it provides good discharge stability even during continuous printing.

[0070] On the other hand, from a different perspective, it is also preferable to use ethylene glycol as the C2-C6 alkanediol mentioned above. Ethylene glycol is more hydrophilic than C3-C6 alkanediols such as 1,2-propanediol, and is effective in improving discharge stability. Furthermore, ethylene glycol has a structure similar to polyethylene glycol (A), and is effective in improving the affinity between the two and further improving discharge stability. On the other hand, because ethylene glycol has a higher surface tension than C3-C6 alkanediols, aqueous inkjet inks containing ethylene glycol generally tend to have poor wetting properties. However, in the case of the aqueous inkjet ink of this embodiment, by using polyethylene glycol (A) and an acetylenediol-based surfactant with an HLB value of 3 to 14 in combination with the ethylene glycol, it is possible to easily improve wetting properties.

[0071] In order to obtain an aqueous inkjet ink that is excellent in density and ejection stability of printed materials regardless of printing conditions, the content of the above C2-C6 alkanediols is preferably 10-30% by mass, more preferably 13-28% by mass, and particularly preferably 17-25% by mass, based on the total amount of aqueous inkjet ink.

[0072] Furthermore, in the aqueous inkjet ink of this embodiment, the ratio of the content of C2-C6 alkanediols to the content of polyethylene glycol (A) (alkanediol / polyethylene glycol (A)) is preferably 1.2 to 20 by mass. The above ratio is more preferably 1.3 to 10, and particularly preferably 1.5 to 7.0. By keeping the value of the above ratio within the above range, the dispersion state of the pigment particles can be stably maintained even in the presence of C2-C6 alkanediols and polyethylene glycol, thereby improving storage stability.

[0073] On the other hand, when using one or more compounds selected from the group consisting of C2-C3 alkanediols, i.e., ethylene glycol, 1,2-propanediol, and 1,3-propanediol, as C2-C6 alkanediols, it is preferable that their content be determined based on the content of polyethylene glycol (A) and an acetylenediol-based surfactant with an HLB value of 3 to 14. Specifically, the percentage (DC) of C2-C3 alkanediols can be defined by the following formula: Formula: DC = MD ÷ (MP + MA) In the above formula, MD, MP, and MA are the molar amounts of the following components contained in 100 g of the aqueous inkjet ink of this embodiment, and are values ​​greater than 0. MD: Molar amount of alkanediols having 2 to 3 carbon atoms MP: Molar amount of polyethylene glycol (A) MA: Molar amount of acetylenediol-based surfactant with an HLB value of 3 to 14 The value of DC defined in the above formula is preferably 5 to 60, and particularly preferably 7 to 30. By setting the above value to 5 to 60 (preferably 7 to 30), alkanediols having 2 to 3 carbon atoms, which have a small molecular size, can be stably made compatible with polyethylene glycol (A) and acetylenediol-based surfactants with an HLB value of 3 to 14. This facilitates improvements in storage stability, discharge stability, and the concentration of printed materials.

[0074] As will be described later, it is preferable to use an acetylenediol-based surfactant with an HLB value of 3 to 9 as an acetylenediol-based surfactant with an HLB value of 3 to 14. Therefore, in the above formula, it is preferable that the value of DC (i.e., DC = MD ÷ (MP + MA2)), which is calculated by setting the molar amount (MA) of the acetylenediol-based surfactant with an HLB value of 3 to 14 to the molar amount (MA2) of the acetylenediol-based surfactant with an HLB value of 3 to 9, also be within the above range. When the value of DC is adjusted to the above range, it becomes easier to further improve storage stability, discharge stability, and the density of the printed material.

[0075] (Other water-soluble organic solvents) The aqueous inkjet ink of this embodiment may contain water-soluble organic solvents other than the above-mentioned C2-C6 alkanediols (referred to as "other water-soluble organic solvents" in this disclosure).

[0076] In the aqueous inkjet ink of this embodiment, the above-mentioned other water-soluble organic solvents include, for example, alkanediols with 7 or more carbon atoms such as 1,2-heptanediol, 1,7-heptanediol, 2-ethyl-1,3-hexanediol, and 2,4-diethyl-1,5-pentanediol; trihydric alcohols such as glycerin and 1,2,4-butanetriol; polypropylene glycols such as dipropylene glycol and tripylene glycol; ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene Glycol monoethers such as propylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monopropyl ether, propylene glycol monobutyl ether, and dipropylene glycol monobutyl ether can be used, but are not limited to these. Furthermore, only one of these organic solvents may be used, or two or more may be used in combination.

[0077] In particular, since the density, discharge stability, and wetting spread of the printed material are all improved simultaneously, it is preferable to use glycol monoethers having 4 to 10 carbon atoms as the other water-soluble organic solvent, and it is more preferable to use glycol monoethers having 6 to 8 carbon atoms. Specific examples of the above-mentioned glycol monoethers having 6 to 8 carbon atoms include ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, propylene glycol monopropyl ether, and propylene glycol monobutyl ether.

[0078] The total amount of water-soluble organic solvent is preferably 10 to 40% by mass, and more preferably 15 to 35% by mass, based on the total amount of inkjet ink. The content of other water-soluble organic solvents may be 0% by mass.

[0079] <Surfactants> The aqueous inkjet ink of this embodiment contains a surfactant. As described above, the surfactant includes an acetylenediol-based surfactant having an HLB value of 3 to 14. In the aqueous inkjet ink of this embodiment containing an acetylenediol-based surfactant having an HLB value of 3 to 14, the acetylenediol-based surfactant quickly orients at the gas-liquid interface, thereby improving the wetting spread on the printing substrate. In particular, it is more preferable to use an acetylenediol-based surfactant having an HLB value of 3 to 9, as this improves both high concentration and wetting spread.

[0080] (Acetylenediol-based surfactants with an HLB value of 3 to 14) The above-mentioned acetylenediol-based surfactants with an HLB value of 3 to 14 or less may be those synthesized by conventionally known synthesis methods or commercially available products. Examples of such commercially available products include Surfinol® 61, 104E, 104H, 104A, 104BC, 104DPM, 104PA, 104PG-50, 420, 440, 465, 2502, SE, SE-F, Dynol® 604, 607 (manufactured by Evonik Japan), Orfin® E1004, PD-001, PD-002W, PD-004 (manufactured by Nisshin Chemical Industry Co., Ltd.), Acetylenel® E00, E13T, E40, E60, E100 (manufactured by Kawaken Fine Chemical Co., Ltd.), etc. These compounds may be used individually or in combination of two or more.

[0081] The HLB (Hydrophilic-Lipophobic Balance) value is one of the parameters that represent the degree of hydrophilicity and hydrophobicity of a material. A smaller HLB value indicates higher hydrophobicity of the material, while a larger HLB value indicates higher hydrophilicity. Methods for determining the HLB value include experimental measurement and calculation from molecular structure. Methods for calculation from molecular structure include the Griffin method, Davis method, and Kawakami method. In this disclosure, the HLB value used is the value calculated by the Griffin method, which is expressed by the following formula (2).

[0082] Formula (2): (HLB value) = 20 × (sum of molecular weights of hydrophilic portions in the material) ÷ ​​(molecular weight of the material)

[0083] The content of the acetylenediol-based surfactant having an HLB value of 3 to 14 is preferably 0.10 to 2.0% by mass, and particularly preferably 0.25 to 1.0% by mass, relative to the total amount of the aqueous inkjet ink. By setting the content of the acetylenediol-based surfactant having an HLB value of 3 to 14 within the above range, it becomes easier to improve the wettability of the inkjet ink without worsening the ejection stability.

[0084] Furthermore, as mentioned above, in order to easily achieve both storage stability and discharge stability, it is preferable that the mass ratio of polyethylene glycol (A) content (polyethylene glycol (A) / acetylenediol surfactant) to the total amount of the inkjet ink, based on the total amount of the inkjet ink, be 2 to 50 by mass, in relation to the mass of the acetylenediol surfactant with an HLB value of 3 to 14 in the total amount of the water-based inkjet ink.

[0085] In particular, if the aqueous inkjet ink of this embodiment contains C3-C6 alkanediols as the C2-C6 alkanediols and does not contain ethylene glycol, the ratio value is more preferably 2 to 40 by mass, even more preferably 2 to 30, particularly preferably 4 to 20, and most preferably 6 to 10.

[0086] On the other hand, when ethylene glycol is used alone as the C2-C6 alkanediol, or when ethylene glycol and C3-C6 alkanediols are used in combination as the C2-C6 alkanediol, the ratio is more preferably 3-48, and particularly preferably 6-45. As described above, ethylene glycol is highly hydrophilic and has a structure similar to polyethylene glycol (A), so polyethylene glycol (A) is considered to preferentially affinity with ethylene glycol. Therefore, by using a sufficient amount of polyethylene glycol (A) with an acetylenediol-based surfactant having an HLB value of 3-14, both storage stability and discharge stability are improved.

[0087] (Other surfactants) The aqueous inkjet ink of this embodiment may contain surfactants other than the acetylenediol-based surfactants having an HLB value of 3 to 14 (referred to as "other surfactants" in this disclosure).

[0088] In the aqueous inkjet ink of this embodiment, the above-mentioned other surfactants can include acetylenediol-based surfactants (excluding acetylenediol-based surfactants with an HLB value of 3 to 14), acetylene monool-based surfactants, siloxane-based surfactants, fluorine-based surfactants, polyoxyalkylene monoalkyl ether-based surfactants, etc. Examples of the above-mentioned polyoxyalkylene monoalkyl ether-based surfactants include compounds in which the oxyalkylene group is an ethylene oxide group and / or a propylene oxide group, and the number of such oxyalkylene groups is 5 to 100, and / or the number of carbon atoms of the terminal alkyl group is 5 to 22. These other surfactants may be used individually or in combination of two or more.

[0089] The total amount of surfactant contained in the aqueous inkjet ink of this embodiment is preferably 0.10 to 5.0% by mass, more preferably 0.20 to 4.0% by mass, and particularly preferably 0.25 to 3.0% by mass, relative to the total amount of the aqueous inkjet ink.

[0090] <Water> The aqueous inkjet ink of this embodiment contains water. The water contained in the aqueous inkjet ink of this embodiment is preferably ion-exchanged water (deionized water) rather than ordinary water containing various ions. Furthermore, the water content is preferably 30 to 90% by mass, and particularly preferably 45 to 85% by mass, relative to the total amount of aqueous inkjet ink. Because water has a low boiling point, it preferentially evaporates from the nozzle end face of the inkjet head, and the solid content concentration at the gas-liquid interface tends to increase. As a result, if a surfactant with poor solubility in water is used, the ink may become non-uniform, which may lead to a deterioration in ejection stability. Acetylenediol-based surfactants with an HLB value of 3 to 14 also have poor solubility in water, so ejection stability tends to decrease. In contrast, if the water content is within the above range and the aqueous inkjet ink has the above configuration, good ejection stability can be obtained even after a long period of printing hiatus or during continuous printing.

[0091] <Resin> The water-based inkjet ink of this embodiment may contain a resin. The resin can be used, for example, for pigment dispersion, binder, wax, etc. The resin used for pigment dispersion (pigment dispersion resin) is as described above. The resin included in the inkjet ink of this embodiment may be used for one purpose or may serve multiple purposes.

[0092] In this disclosure, "resin" refers to a compound in which one or more polymerizable monomers are covalently linked to form a main chain, and which has a mass-average molecular weight of 1,000 or more (excluding polyethylene glycol).

[0093] Furthermore, in this disclosure, the mass-average molecular weight of the compound is measured using a polystyrene equivalent value that can be measured by a method compliant with JIS K 7252. An example of specific measurement conditions is shown below: • Equipment used: Tosoh Corporation "HLC-8320GPC" • Column used: TSKgel® 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

[0094] The above-mentioned "binder application" refers to an application that imparts strength (durability) and / or adhesion to the printing substrate to the film of water-based inkjet ink after drying. For example, the resin that constitutes the main component of the above film is the resin used in the binder application (binder resin). On the other hand, the above-mentioned "wax application" refers to an application that becomes liquid when heated and / or pressurized, and imparts abrasion resistance to the film of water-based inkjet ink after drying. From this perspective, for example, a resin with a melting point of 50 to 160°C that melts without decomposing in a temperature environment above that melting point is the resin used in the wax application (wax resin).

[0095] As described above, the resin contained in the aqueous inkjet ink of this embodiment may serve multiple purposes. Therefore, for example, if the aqueous inkjet ink contains only one type of resin, and that resin coats the pigment and exhibits the function of a binder resin as described above, then this resin serves as both a pigment dispersion resin and a binder resin. On the other hand, from the viewpoint of suitably and simultaneously achieving all of the effects of this embodiment, such as improved density of printed materials on paper substrates, improved ejection stability and storage stability, and improved wetting spread on the printing substrate, it is preferable that each resin in the aqueous inkjet ink of this embodiment is used for a single purpose. For example, in the aqueous inkjet ink of this embodiment, it is preferable to include pigment dispersion resin and binder resin separately rather than using only one type of resin that serves as both a pigment dispersion resin and a binder resin.

[0096] Generally, water-soluble resins and resin microparticles are known as resin forms used in water-based inkjet inks. The resin contained in the water-based inkjet ink of this embodiment may be a water-soluble resin or resin microparticles. Alternatively, a combination of water-soluble resin and resin microparticles may be used.

[0097] In this disclosure, resins having a solubility of 1 g or more in 100 g of water at 25°C are referred to as "water-soluble resins," and resins having a solubility of less than 1 g are referred to as "non-water-soluble resins." Furthermore, among the non-water-soluble resins, resins dispersed in particulate form in water and having a median diameter (D50) on a volume basis of 10 to 1,000 nm are referred to as "resin fine particles." The above "median diameter (D50) on a volume basis" can be measured using the same apparatus, conditions, and methods as for the average secondary particle diameter of resin-dispersed pigments and self-dispersed pigments.

[0098] Examples of resins that can be used in the aqueous inkjet ink of this embodiment include acrylic resin, styrene resin, maleic acid resin, urethane resin, polyester resin, vinyl chloride resin, vinyl chloride-vinyl acetate resin, polyolefin resin, and polyvinyl alcohol resin. These resins may be used individually or in combination of two or more.

[0099] <Other Components> In addition to the components described above, the aqueous inkjet ink of this embodiment may also contain pH adjusters, other additives, etc. Examples of the above-mentioned other additives include crosslinking agents, preservatives, ultraviolet absorbers, and infrared absorbers. For each of these components, one or more conventionally known compounds can be used.

[0100] <Method for Manufacturing Water-Based Inkjet Ink> The water-based inkjet ink of this embodiment can be manufactured by known methods. In particular, it is preferable to manufacture a dispersion of resin-dispersed pigment and / or a dispersion of self-dispersing pigment in advance, as this provides an water-based inkjet inkjet ink with excellent dispersion stability and discharge stability. A preferred manufacturing method is to then mix the above-mentioned dispersion of resin-dispersed pigment and / or dispersion of self-resin-dispersed pigment with alkanediols having 2 to 6 carbon atoms, polyethylene glycol (A), an acetylenediol-based surfactant with an HLB value of 3 to 14, and, if necessary, other water-soluble organic solvents, other surfactants, binder resins, wax resins, etc., and then remove coarse particles from the resulting mixture. An example of a method for manufacturing the water-based inkjet ink of this embodiment is described below, but the manufacturing method is not limited to the following.

[0101] A dispersion of resin-dispersed pigments can be produced by thoroughly mixing (premixing) pigment, pigment-dispersing resin, water, and optionally polyethylene glycol (A), water-soluble organic solvent, surfactant, pH adjuster, and other additives, and then performing a dispersion treatment using a disperser. Media-type dispersers such as paint shakers, bead mills, and attritors, as well as media-less dispersers such as pressure homogenizers and ultrasonic dispersers, can be used as the disperser. Alternatively, a dispersion of resin-dispersed pigments containing a cross-linked pigment-dispersing resin can be produced by dispersing the pigment using an uncrosslinked polymer, and then adding a crosslinking agent to perform a crosslinking treatment. On the other hand, a dispersion of self-dispersing pigments can be produced by adding a surface treatment agent to a suspension made by mixing pigment, water, and optionally polyethylene glycol (A), water-soluble organic solvent, surfactant, pH adjuster, and other additives, and then performing a surface treatment.

[0102] To the dispersion of resin-dispersed pigment and / or self-dispersing pigment obtained in this manner, add alkanediols having 2 to 6 carbon atoms, polyethylene glycol (A), an acetylenediol-based surfactant with an HLB value of 3 to 14, and optionally a binder resin, wax resin, water, pH adjuster, and other additives, and stir and mix. If necessary, the mixture may be stirred and mixed while heating each component forming the mixture in the range of 40 to 100°C.

[0103] Then, the coarse particles contained in the above mixture are removed by methods such as filtration and centrifugation to obtain an aqueous inkjet ink. Known methods can be used as appropriate for filtration separation. For example, when using a filter, the pore size is preferably 0.3 to 5 μm, more preferably 0.5 to 3 μm. When using a filter, multiple types of filters with different pore sizes may be used in combination.

[0104] <Characteristics of Water-Based Inkjet Ink> The water-based inkjet ink of this embodiment preferably has a viscosity of 3 to 15 mPa·s at 25°C, and more preferably 3 to 9 mPa·s. Within this viscosity range, droplets of the water-based inkjet ink can be stably ejected not only from inkjet heads with ejection frequencies of about 4 to 10 kHz, but also from inkjet heads with high ejection frequencies of about 20 to 70 kHz. In particular, when the viscosity of the water-based inkjet ink of this embodiment is 3 to 9 mPa·s at 25°C, the water-based inkjet ink can be stably ejected even when using an inkjet head with a design resolution of 600 dpi or higher. In this disclosure, the viscosity is measured using a cone-plate type rotational viscometer (E-type viscometer, cone angle 1°34') such as the "TVE25L viscometer" manufactured by Toki Sangyo Co., Ltd., under conditions of 25°C.

[0105] Furthermore, in order to obtain an aqueous inkjet ink with excellent ejection stability and print quality of printed materials, the static surface tension of the aqueous inkjet ink of this embodiment at 25°C is preferably 18 to 35 mN / m, and particularly preferably 21 to 33 mN / m. In this disclosure, the static surface tension is the value measured at a 25°C environment using the Wilhelmy method (plate method) with an automatic surface tension meter such as the "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd.

[0106] <Water-based inkjet ink set> The water-based inkjet ink of this embodiment may be used individually, or it may be used as a set of water-based inkjet inks, combining two or more types of water-based inkjet inks. An example of such a set of water-based inkjet inks is a set of four water-based inkjet inks (process color ink set) consisting of cyan water-based inkjet ink (water-based cyan ink), magenta water-based inkjet ink (water-based magenta ink), yellow water-based inkjet ink (water-based yellow ink), and black water-based inkjet ink (water-based black ink). It is preferable that all water-based inkjet inks constituting the set of water-based inkjet inks satisfy the requirements of this embodiment described above.

[0107] <Ink-Pretreatment Solution Set> Furthermore, the water-based inkjet ink of this embodiment, and the set of the water-based inkjet ink described above, can also be used in combination with a pretreatment solution containing a flocculant (in the form of an ink-pretreatment solution set). By applying the pretreatment solution containing a flocculant to the printing substrate before printing with the water-based inkjet ink, a layer (ink flocculation layer) can be formed in which the solid components contained in the water-based inkjet ink are intentionally flocculated. Then, by depositing the water-based inkjet ink onto this ink flocculation layer, the coalescence and mixing of the droplets of the water-based inkjet ink can be prevented, and the print quality of the printed material can be significantly improved.

[0108] Furthermore, as the above-mentioned flocculant, for example, a water-soluble inorganic or organic salt containing polyvalent metal ions, and a resin having cationic groups and in which the equivalent amount of cationic groups is greater than the equivalent amount of anionic groups can be used.

[0109] <Inkjet Printing Method> The aqueous inkjet ink of this embodiment is used in an inkjet printing method. A typical inkjet printing method includes a step of ejecting aqueous inkjet ink from an inkjet head having fine nozzles onto a printing substrate (dispense step), and a step of drying the aqueous inkjet ink on the printing substrate using a drying mechanism (drying step).

[0110] (Ejection Process) As an example of the operation method of the inkjet head in the ejection process described above, there 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 ejecting and recording the aqueous inkjet ink. Another example of the operation method described above is the single-pass method in which the aqueous inkjet ink is ejected and recorded as the printing substrate passes under the fixedly positioned inkjet head. In the inkjet head equipped with the aqueous inkjet ink of this embodiment, either the shuttle method or the single-pass method may be adopted. Among these, the single-pass method is preferably selected because it is less likely to cause misalignment in the landing position of the aqueous inkjet ink droplets, improving the print quality of the printed material, and furthermore, it enables high-speed printing and can demonstrate high productivity as an alternative to plated printing.

[0111] Regarding the ejection method from the inkjet head, any known method can be arbitrarily selected. Examples of such ejection methods include the piezoelectric method, which utilizes the volume change of a piezoelectric element; the thermal method, which ejects water-based inkjet ink using bubbles generated by heating with a heater; and the valve method, which ejects pressurized water-based inkjet ink while opening and closing the nozzle cover (valve) with a solenoid.

[0112] The amount of aqueous inkjet ink droplets ejected from the inkjet head is preferably 0.5 to 20 picoliters, and particularly preferably 0.5 to 15 picoliters, from the viewpoint 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 material is preferably 600 dpi × 600 dpi or higher, and more preferably 1,200 dpi × 600 dpi or higher.

[0113] (Drying Process) Examples of drying methods used in the drying mechanism in the drying process include heating drying, hot air drying, infrared (e.g., infrared with a wavelength of 700 to 2500 nm) drying, microwave drying, and drum drying. One or more of these methods can be arbitrarily selected and used in the above drying process. When two or more of the above drying methods are used, each drying method may be used separately (e.g., consecutively) or simultaneously in combination. For example, by using heating drying and hot air drying in combination, the water-based inkjet ink can be dried more quickly than when each method is used individually.

[0114] In particular, from the viewpoint of preventing the boiling of the liquid component in the aqueous inkjet ink and obtaining printed materials with excellent print quality, when using the heat drying method, it is preferable to set the drying temperature to 35 to 100°C. When using the hot air drying method, it is preferable to set the hot air temperature to 50 to 250°C. From a similar viewpoint, when using the infrared drying method, 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.

[0115] <Printed Matter> A printed matter according to an embodiment of the present invention includes a printing substrate and a printing layer present on the printing substrate. The printing layer is formed using the aqueous inkjet ink of the present embodiment and is composed of a film formed by drying the aqueous inkjet ink of the present embodiment printed in the shape of an image and / or characters. As a method of printing the aqueous inkjet ink in the shape of an image and / or characters and then drying to form a printing layer, the above-described inkjet printing method can be preferably employed. Note that the above “image” includes solid images (images printed at a print rate of 100% so as to cover the entire surface of the printing substrate) and seamless images such as checkerboard images.

[0116] (Printing Substrate) The printing substrate on which the aqueous inkjet ink of the present embodiment is printed is preferably a paper substrate. As described above, polyethylene glycol forms an interaction with cellulose, which is the main component of the paper substrate, and penetration of the aqueous inkjet ink into the paper surface is suppressed. Since this effect is preferably exhibited and the spreading property of the aqueous inkjet ink is also excellent, the paper substrate is preferably a substrate with easy penetration. A substrate with easy penetration is a printing substrate through which water penetrates at a high speed. Specifically, the absorption coefficient for water measured by the Bristol method (J. TAPPI Paper Pulp Test Method No. 51-87) is 0.4 ml / m ２ msec １／２ or more. The above absorption coefficient can be measured, for example, using an automatic scanning liquid absorber manufactured by Kumagai Riki Kogyo Co., Ltd. Specifically, using the above device and water, the liquid absorption amount of water (ml / m ２ ) and the square root of the contact time (msec １／２ ), the slope of the straight line obtained by the least squares method is taken as the absorption coefficient.

[0117] Specific examples of the substrate with easy penetration include, for example, cardboard, liner paper, blotting paper, medium-quality paper, high-quality paper, recycled paper, plain paper, inkjet paper, etc. Among them, it is particularly preferable to use high-quality paper and inkjet paper because the interaction between the constituent components of the paper substrate and the inkjet ink of the present embodiment facilitates high density of the obtained printed matter.

[0118] The printing substrates listed above may have smooth or uneven surfaces. Furthermore, the printing substrates may be transparent, translucent, or opaque. In addition, the printing substrates may be in the form of rolls or sheets. Moreover, a laminate formed by bonding two or more of the listed printing substrates together may be used as the printing substrate. A release adhesive layer may be provided on the opposite side of the printed surface, or an adhesive layer may be provided on the printed surface after printing.

[0119] The following are examples of embodiments of the present invention. However, the present invention is not limited to the following embodiments and includes various embodiments that are modified without changing the essential parts of the present invention. [1] An aqueous inkjet ink comprising a pigment, a water-soluble organic solvent, polyethylene glycol, and a surfactant, wherein the content of the pigment is 3 to 7.5% by mass with respect to the total amount of the aqueous inkjet ink composition, the water-soluble organic solvent comprises alkanediols having 2 to 6 carbon atoms, the content of alkanediols having 2 to 6 carbon atoms is 10 to 30% by mass with respect to the total amount of the aqueous inkjet ink composition, the polyethylene glycol comprises polyethylene glycol (A) having a number average molecular weight of 200 to 10,000, the surfactant comprises an acetylenediol-based surfactant having an HLB value of 3 to 14, and the ratio of the content of polyethylene glycol (A) to the content of the acetylenediol-based surfactant having an HLB value of 3 to 14 (polyethylene glycol (A) / acetylenediol-based surfactant) is 2 to 50 by mass. [2] The aqueous inkjet ink according to [1] above, wherein the water-soluble organic solvent contains alkanediols having 3 to 6 carbon atoms, and the ratio of the content of polyethylene glycol (A) to the content of an acetylenediol-based surfactant having an HLB value of 3 to 14 (polyethylene glycol (A) / acetylenediol-based surfactant) is 2 to 30 by mass. [3] The aqueous inkjet ink according to [1] or [2] above, wherein the water-soluble organic solvent further contains glycol monoethers having 4 to 10 carbon atoms. [4] The aqueous inkjet ink according to any one of [1] to [3] above, wherein the pigment contains a resin-dispersed pigment. [5] The aqueous inkjet ink according to any one of [1] to [4] above, for printing on a paper substrate. [6] A printed article obtained by printing the aqueous inkjet ink according to any one of [1] to [5] above on a printing substrate. [7] The printed article according to [6] above, wherein the printing substrate is a paper substrate.

[0120] This disclosure relates to the subject matter described in Japanese Patent Application No. 2024-209727, filed on 2 December 2024, all of which are incorporated herein by reference.

[0121] The aqueous inkjet ink of this embodiment will be described in more detail below with reference to examples and comparative examples. In the following description, "parts" and "%" refer to "parts by mass" and "% by mass," respectively, unless otherwise specified.

[0122] <Production of Pigment Dispersion> (Dispersion of resin-dispersed pigment containing pigment-dispersing resin without cross-linking structure) 600 g of carbon black (PrinteX85, manufactured by Orion Engineered Carbons), 120 g of acrylic resin, and 2,280 g of water were placed in a mixing container (volume 10 L) equipped with a stirrer, and stirred (premixed) for 1 hour to obtain a mixture. The above acrylic resin was a random polymer of styrene / acrylic acid / lauryl methacrylate = 35 / 30 / 35 (mass ratio) in which all acid groups were neutralized with dimethylaminoethanol, with an acid value of 234 mg KOH / g and a mass-average molecular weight of 20,000. Next, the above mixture was circulated and dispersed using a bead mill (Dinomill, manufactured by Shinmaru Enterprises, volume 0.6 L) filled with 1,800 g of zirconia beads with a diameter of 0.5 mm. Then, at regular intervals (for example, every hour), the average secondary particle diameter of the mixture was measured using the apparatus described above, and the circulating dispersion was terminated when the average secondary particle diameter fell to 120 nm or less. Black pigment dispersion 1 (pigment concentration 20% by mass) was produced by this method.

[0123] Furthermore, cyan pigment dispersion 1 (pigment concentration 20% by mass) was produced using the same raw materials and method as the black pigment dispersion above, except that LIONOL BLUE 7919 (C.I. Pigment Blue 15:3, manufactured by Toyo Color Co., Ltd.) was used as the pigment and circulating dispersion was carried out until the D50 was 180 nm or less. Furthermore, magenta pigment dispersion 1 (pigment concentration 20% by mass) was produced using the same raw materials and method as the cyan pigment dispersion above, except that Toshiki Red 150TR (C.I. Pigment Red 150, manufactured by Tokyo Shikizai Co., Ltd.) was used as the pigment. Furthermore, yellow pigment dispersion 1 (pigment concentration 20% by mass) was produced using the same raw materials and method as the cyan pigment dispersion above, except that FAST YELLOW 7413 (C.I. Pigment Yellow 74, manufactured by Sanyo Shikizai Co., Ltd.) was used as the pigment.

[0124] (Dispersion of resin-dispersed pigment containing a cross-linked pigment-dispersing resin) 75 parts of the black pigment dispersion 1 obtained by the method described above, 1.5 parts of Denacol EX-321 (an epoxy compound manufactured by Nagase ChemteX, epoxy equivalent: 140 g / eq.), which is a cross-linking agent (an amount such that the functional group content shown in formula (1) above is approximately 90 mol%), and 3.5 parts of deionized water were added to the reaction vessel. Next, the contents of the reaction vessel were heated to 80°C while stirring, and after reaching 80°C, stirring was continued for 3 hours while maintaining the temperature to carry out the cross-linking reaction. After that, the reaction vessel was cooled until the internal temperature was 30°C or lower, and then 20 parts of deionized water were added to adjust the solid content concentration to obtain a black pigment dispersion (black pigment dispersion 2) containing a cross-linked pigment-dispersing resin (pigment concentration 15% by mass).

[0125] Furthermore, except that the black pigment dispersion 1 was replaced with cyan pigment dispersion 1, magenta pigment dispersion 1, and yellow pigment dispersion 1 respectively, the same raw materials and methods as for the black pigment dispersion 2 were used to obtain cyan pigment dispersion 2, magenta pigment dispersion 2, and yellow pigment dispersion 2 (all with a pigment concentration of 15% by mass), which are dispersions of resin-dispersed pigments containing a cross-linked pigment dispersion resin.

[0126] (Dispersion of self-dispersing pigments) As black pigment dispersion 3, BONJET BLACK CW-3 (manufactured by Orient Co., Ltd., aqueous dispersion of self-dispersing carbon black, pigment concentration 13% by mass) was diluted with ion-exchanged water to a pigment concentration of 10% by mass and used. As cyan pigment dispersion 3, CAB-O-JET 250C (manufactured by Cabot, aqueous dispersion of self-dispersing C.I. pigment blue 15:4, pigment concentration 10% by mass) was used as is. As magenta pigment dispersion 3, CAB-O-JET 265M (manufactured by Cabot, aqueous dispersion of self-dispersing C.I. pigment red 122, pigment concentration 10% by mass) was used as is. As yellow pigment dispersion 3, CAB-O-JET 270Y (manufactured by Cabot, aqueous dispersion of self-dispersing C.I. pigment yellow 74, pigment concentration 10% by mass) was used as is.

[0127] <Manufacturing of Water-Based Inkjet Ink Set> Using the pigment dispersion liquid manufactured by the method described above, each raw material was added to a mixing container equipped with a stirrer to achieve the formulations listed in each column of Table 1-1 to 1-9. After adding all the raw materials, stirring and mixing were continued for 1 hour at room temperature (approximately 23°C). The mixture was then filtered through a membrane filter with a pore size of 0.8 μm to produce the water-based inkjet ink. Inkjet inks were manufactured using black dispersion liquid, cyan dispersion liquid, magenta dispersion liquid, and yellow dispersion liquid as pigment dispersion liquids, and the resulting combinations of water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink constituted the water-based inkjet ink set.

[0128] In the manufacturing of water-based inkjet ink sets, the raw materials were added while stirring the mixture in the mixing container at room temperature. Furthermore, the components listed in each column of Tables 1-1 to 1-9 were added in the order listed from top to bottom. However, when manufacturing water-based inkjet ink that did not contain one or more of these components, those components were omitted, and the next component was added in the correct order. For components containing two or more raw materials, the order of addition within that component was arbitrary.

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135]

[0136]

[0137]

[0138] The meanings of the abbreviations and product names listed in Tables 1-1 to 1-9 are as follows. In Tables 1-1 to 1-9, "Mn" represents the number-average molecular weight, and "Nv" represents the solid content concentration. (Alkanediols with 2 to 6 carbon atoms) ・EG: Ethylene glycol ・1,2-PD: 1,2-propanediol ・1,2-BD: 1,2-butanediol ・1,3-BD: 1,3-butanediol ・1,5-PentD: 1,5-pentanediol ・2m-2,4-PentD: 2-methyl-2,4-pentanediol (Polyethylene glycol) ・PEG-200: Polyethylene glycol-200 manufactured by Sanyo Chemical Industries (number average molecular weight: 200) ・PEG-400: Polyethylene glycol-400 manufactured by Sanyo Chemical Industries (number average molecular weight: 400) ・PEG-600: Polyethylene glycol-600 manufactured by Sanyo Chemical Industries (number average molecular weight: 600) ・PEG-1540: Polyethylene glycol-1540 manufactured by Sanyo Chemical Industries (number average molecular weight: 1,450)・PEG-2000: Polyethylene glycol-2000 manufactured by Sanyo Chemical Industries (number average molecular weight: 2,000) ・PEG-4000S: Polyethylene glycol-4000S manufactured by Sanyo Chemical Industries (number average molecular weight: 3,400) ・PEG-6000P: Polyethylene glycol-6000P manufactured by Sanyo Chemical Industries (number average molecular weight: 8,600) ・PEG-10000: Polyethylene glycol-10000 manufactured by Sanyo Chemical Industries (number average molecular weight: 11,000) (Other water-soluble organic solvents) ・PM: Propylene glycol monomethyl ether (4 carbon atoms) ・PnP: Propylene glycol monopropyl ether (6 carbon atoms) ・DPnB: Dipropylene glycol monobutyl ether (10 carbon atoms) ・EDG: Diethylene glycol monoethyl ether (6 carbon atoms) ・BDG: Diethylene glycol monobutyl ether (8 carbon atoms) ・MTG: Triethylene glycol monomethyl ether (7 carbon atoms) ・BTG: Triethylene glycol monobutyl ether (10 carbon atoms) (Acetylenediol-based surfactants) ・Surfinol 104: Acetylenediol-based surfactant manufactured by Evonik Japan, HLB value = 3.0 ・Surfinol 440: Acetylenediol-based surfactant manufactured by Evonik Japan, HLB value = 8.1・Acetylenel E40: Acetylenediol-based surfactant manufactured by Kawaken Fine Chemicals, HLB value = 8.8 ・Acetylenel E60: Acetylenediol-based surfactant manufactured by Kawaken Fine Chemicals, HLB value = 10.8 ・Surfinol 465: Acetylenediol-based surfactant manufactured by Evonik Japan, HLB value = 13.2 ・Surfinol 485: Acetylenediol-based surfactant manufactured by Evonik Japan, HLB value = 17.1 ・TW280: TEGO WET 280 (siloxane-based surfactant manufactured by Evonik Japan) (resin) ・NeoRez R600: Urethane resin microparticles manufactured by DSM Coating Resins, solid content concentration 33% ・NeoCryl A1091: DSM Coating Resins acrylic resin microparticles, solid content concentration 45% (Other ingredients) ・Proxel GXL: 1,2-benzoisothiazole-3-one dipropylene glycol aqueous solution (1,2-benzoisothiazole-3-one:dipropylene glycol:water = 2:6:2, preservative manufactured by Arch Chemicals) (Specifications) *1: Not calculated because it does not contain self-dispersing carbon black *2: Not calculated because it does not contain resin-dispersed carbon black *3: Not calculated because it contains resin-dispersed carbon black and self-dispersing carbon black

[0139] [Examples 1-98, Comparative Examples 1-11] The aqueous inkjet ink sets manufactured by the method described above were used for the evaluations shown below. The evaluation results are shown in Tables 3-1 to 3-11.

[0140] <Evaluation 1: Density Evaluation> An inkjet ejection unit equipped with a Kyocera KJ4B-1200 inkjet head (design resolution 1200 dpi, nozzle diameter 20 μm) installed in a 25°C environment was filled with water-based inkjet ink. After filling, the water-based inkjet ink inside the inkjet head was pressurized until it seeped out of the nozzles. Then the nozzle plate was wiped, and a nozzle check pattern was immediately printed at a frequency of 40 kHz. After confirming that there were no nozzle clogs in the nozzle check pattern, a solid image with 100% print density was printed on inkjet-specific paper (NPiForm NEXT-IJ manufactured by Nippon Paper Industries), high-quality paper (NPiForm 55 manufactured by Nippon Paper Industries), or brown corrugated cardboard (Rengo corrugated cardboard (A4 size), thickness approximately 3 mm). Immediately after printing, each printing substrate was placed in an 80°C air oven and dried for 2 minutes to obtain solid prints. The printing conditions for the solid images were a frequency of 40 kHz, a conveyor drive speed of 50 m / min, and a print resolution of 1,200 × 1,200 dpi. Printing was performed using the combinations of water-based inkjet ink sets and printing substrates described in Tables 3-1 to 3-2, and one solid print was produced using each of the water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink that make up the water-based inkjet ink set. The density of the obtained solid prints was then measured using a spectrophotometer (eXact Advance, manufactured by X-rite). The density was then evaluated by comparing it to the density of the solid print produced using the water-based inkjet ink set of Comparative Example 6 as a reference. The density measurement conditions were as follows: ISO status T was used as the density standard, the viewing angle was 2°, and the light source was D50. The evaluation criteria were as shown in Table 2 below, with an evaluation of "2" or higher considered to be in the usable range. However, Tables 3-1 to 3-11 show the results for the water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink that received poor evaluations. (Evaluation Criteria)

[0141] <Evaluation 2: Evaluation of Wetting Spread> The degree of streaking on the solid print material with 100% print coverage, created in Evaluation 1 above, was checked visually and with a magnifying glass. Furthermore, if no streaks were observed in any of the water-based inkjet inks constituting the water-based inkjet ink set, a half-solid print material with 50% print coverage was created using the same inkjet ejection device and printing substrate as in Evaluation 1. The degree of streaking on this half-solid print material was then checked visually and with a magnifying glass to evaluate the wetting spread. The evaluation criteria were as follows, with an evaluation of "3" or higher considered to be in the usable range. However, the above evaluation was performed for each of the water-based inkjet inks constituting the water-based inkjet ink set: water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink. Tables 3-1 to 3-11 show the results for the ink with the worst evaluation. (Evaluation Criteria) 4 ＋ : No streaks were observed visually or with a magnifying glass in the half-solid print. 4: No streaks were observed visually or with a magnifying glass in the solid print, but streaks were observed visually and / or with a magnifying glass in the half-solid print. 3: In the solid print, slight streaks were visible with a magnifying glass, but not visible to the naked eye. 2: In the solid print, streaks were clearly visible with a magnifying glass, and also slightly visible to the naked eye. 1: In the solid print, streaks were clearly visible to the naked eye.

[0142] <Evaluation 3: Evaluation of Discharge Stability> Using the inkjet printing device used in Evaluation 1 above, a nozzle check pattern was printed at a frequency of 40 kHz to confirm that there were no nozzle failures. Then, 100 sheets of a solid image with a print width of 10 cm and a length of 30 cm and 100% print coverage were printed continuously on coated paper that is not easily permeable (OK Topcoat+ manufactured by Oji Paper Co., Ltd.). After printing 100 sheets continuously, the nozzle check pattern was printed again, and the number of nozzle failures was visually counted to evaluate the discharge stability. The printing conditions for the solid image were the same as in Evaluation 1. The evaluation criteria were as follows, and an evaluation of "3" or higher was considered to be in the practically usable range. However, the above evaluation was performed for each of the water-based inkjet ink sets: water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink. Tables 3-1 to 3-11 show the results for the ink with the worst evaluation. Furthermore, for Examples 5B and 5C, 83B and 83C, and 95B and 95C, this evaluation was not performed because the composition of the aqueous inkjet ink was the same as that of Examples 5, 83, and 95, respectively. (Evaluation Criteria) 4 ＋ : No nozzle clogging was observed after continuous printing. 4: 1 to 3 nozzle clogging was observed after continuous printing. 3: 4 to 9 nozzle clogging was observed after continuous printing. 2: 10 to 29 nozzle clogging was observed after continuous printing. 1: 30 or more nozzle clogging was observed after continuous printing.

[0143] <Evaluation 4: Evaluation of Storage Stability> Using a dynamic light scattering particle size distribution analyzer (Microtrac-Bell "Nanotrac UPA-EX150"), the volume-based median diameter (D50) of each ink constituting the above aqueous inkjet ink set was measured immediately after manufacturing at a 25°C environment with water as the diluent. Next, the aqueous inkjet inks after D50 measurement were sealed in a sealed container to a filling rate of 80% and left to stand in a forced-air constant-temperature incubator set to 70°C. After a predetermined period, the D50 of the aqueous inkjet inks in the sealed container removed from the forced-air constant-temperature incubator was measured again, and the D50 time-dependent stability was evaluated by calculating the rate of change of D50 before and after storage. The evaluation criteria are as follows, with a score of "2" or higher being in the practically usable range, and a score of "3" or higher being in the practically suitable range. Tables 3-1 to 3-11 show the results for the water-based black ink, water-based cyan ink, water-based magenta ink, and water-based yellow ink that received poor evaluations. Furthermore, for Examples 5B and 5C, 83B and 83C, and 95B and 95C, this evaluation was not performed because the composition of the water-based inkjet ink was the same as in Examples 5, 83, and 95, respectively. (Evaluation Criteria) 4: Particle size change rate after four weeks of storage is less than ±20% 3: Particle size change rate after two weeks of storage is less than ±20% 2: Particle size change rate after one week of storage is less than ±20% 1: Particle size change rate after one week of storage is greater than ±20%

[0144] <Evaluation 5: Evaluation of Discharge Stability (Initial)> Except for using coated paper (OK Topcoat+) manufactured by Oji Paper Co., Ltd., which is not an easily permeable substrate, a solid print with 100% print coverage was prepared under the same conditions as Evaluation 1 above (inkjet discharging device and printing conditions). Then, the discharge stability (initial) was evaluated by checking with a magnifying glass whether each water-based inkjet ink was applied to the area of ​​the obtained solid print where the water-based inkjet ink should have been applied first. The evaluation criteria were as follows, and an evaluation of "2" or higher was considered to be in the usable range. Note that the evaluation of discharge stability (initial) described above was performed only in Examples 5, 35-47, and 71-81. Therefore, in Tables 3-1 to 3-11, if the entry for Evaluation 5 is "-", it means that the evaluation was not performed. (Evaluation Criteria) 5: The water-based inkjet ink was applied to the area where it should have been applied first. 4: The water-based inkjet ink was not applied to the area where it should have been applied first, and was applied from a position less than 1 mm away from that area. 3: The water-based inkjet ink was not applied to the area where it should have been applied first, and was applied from a position between 1 mm and less than 3 mm away from that area. 2: The water-based inkjet ink was not applied to the area where it should have been applied first, and was applied from a position between 3 mm and less than 6 mm away from that area. 1: The water-based inkjet ink was not applied to the area where it should have been applied first, and was applied from a position 6 mm or more away from that area.

[0145] Tables 3-1 to 3-11 reproduce some of the specifications listed in Tables 1-1 to 1-9. The notes (*a), (*b), (*c), and (*d) in 3-1 to 3-11 are as follows: (*a): Since the composition of the water-based inkjet ink is the same as in Example 5, evaluations 3, 4, and 5 were not performed. (*b): Since the composition of the water-based inkjet ink is the same as in Example 75, evaluations 3, 4, and 5 were not performed. (*c): Since the composition of the water-based inkjet ink is the same as in Example 83, evaluations 3, 4, and 5 were not performed. (*d): Since the composition of the water-based inkjet ink is the same as in Example 95, evaluations 3, 4, and 5 were not performed.

[0146]

[0147]

[0148]

[0149]

[0150]

[0151]

[0152]

[0153]

[0154]

[0155]

[0156]

[0157] As shown in Tables 3-1 to 3-11 above, the aqueous inkjet inks of Examples 1 to 98 having the configuration of this embodiment were found to be superior to the aqueous inkjet inks of Comparative Examples 1 to 11 in all aspects, including high concentration on paper substrates, ejection stability, solid coverage, and storage stability, as they were within the practical range.

[0158] Comparative Example 10 reproduces the configuration of Example 3 of Patent Document 1 described above (however, similar products were used for the self-dispersing carbon black and resin). As a result of the evaluation, it was confirmed that the ejection stability was scored "2", which is not at a practical level. Furthermore, the storage stability was also scored "2", which is practical but not at a suitable level. It is thought that the aqueous inkjet ink of Comparative Example 10 did not contain alkanediols with 2 to 6 carbon atoms, so the acetylenediol-based surfactant could not be sufficiently miscible, resulting in poor ejection stability and storage stability.

Claims

1. An aqueous inkjet ink comprising a pigment, a water-soluble organic solvent, polyethylene glycol, and a surfactant, wherein the content of the pigment is 3 to 7.5% by mass of the total amount of the aqueous inkjet ink, the water-soluble organic solvent comprises alkanediols having 2 to 6 carbon atoms, the content of alkanediols having 2 to 6 carbon atoms is 10 to 30% by mass of the total amount of the aqueous inkjet ink, the polyethylene glycol comprises polyethylene glycol (A) having a number average molecular weight of 200 to 10,000, the surfactant comprises an acetylenediol-based surfactant having an HLB value of 3 to 14, and the ratio of the content of polyethylene glycol (A) to the content of the acetylenediol-based surfactant having an HLB value of 3 to 14 (polyethylene glycol (A) / acetylenediol-based surfactant) is 2 to 50 by mass.

2. The aqueous inkjet ink according to claim 1, wherein the water-soluble organic solvent contains alkanediols having 3 to 6 carbon atoms, and the ratio of the content of polyethylene glycol (A) to the content of an acetylenediol-based surfactant having an HLB value of 3 to 14 (polyethylene glycol (A) / acetylenediol-based surfactant) is 2 to 30 by mass.

3. The aqueous inkjet ink according to claim 1 or 2, wherein the water-soluble organic solvent further comprises a glycol monoether having 4 to 10 carbon atoms.

4. The aqueous inkjet ink according to any one of claims 1 to 3, wherein the pigment comprises a resin-dispersed pigment.

5. An aqueous inkjet ink according to any one of claims 1 to 4, used for printing on a paper substrate.

6. A printed article obtained by printing an aqueous inkjet ink according to any one of claims 1 to 5 onto a printing substrate.

7. The printed material according to claim 6, wherein the printing substrate is a paper substrate.

Images