Inkjet ink, ink set, and inkjet recording method

Abstract

Provided are: an inkjet ink that contains water and pigment particles containing a carbon black pigment and a polymer dispersant having a crosslinked structure, in which the volume average particle diameter of the pigment particles as measured by a dynamic light scattering method is 65 nm or more and less than 100 nm; and an inkjet recording method that uses said inkjet ink.

Description

Inkjet ink, ink set, and inkjet recording method

[0001] This disclosure relates to inkjet inks, ink sets, and inkjet recording methods.

[0002] Various studies have been conducted regarding inkjet inks and inkjet recording methods. For example, Patent Document 1 describes the following inkjet recording ink, with the objective of providing an inkjet recording ink that contains a pigment having an indanthrone skeleton and can suppress thickening and aggregation even in systems with a large amount of solvent. The inkjet recording ink described in Patent Document 1 is an aqueous inkjet recording ink containing crosslinked polymer particles containing a pigment having an indanthrone skeleton (for example, C.I. Pigment Blue 60), an organic solvent, and water, wherein the crosslinking rate of the crosslinked polymer is 10 mol% or more and 90 mol% or less, and the content of the organic solvent is 30% by mass or more and 65% by mass or less. Furthermore, Patent Document 2 describes the following inkjet recording ink composition, with the objective of providing an inkjet recording ink composition that, in inkjet printing, does not produce color bleeding or feathering in the recorded image, has little show-through, has high color development and high quality, and has high reliability against clogging. The inkjet recording ink composition described in Patent Document 2 is an inkjet recording pigment ink containing a pigment and water, and further contains a water-soluble dye, characterized in that when 10 parts by weight of an aqueous sodium chloride solution is added to 100 parts by weight of the ink, the pigment maintains its dispersion when a 1.0% by weight aqueous sodium chloride solution is added, and the pigment aggregates when a 2.0% by weight aqueous sodium chloride solution is added.

[0003] Patent Document 1: International Publication No. 2019 / 013173 Patent Document 2: Japanese Unexamined Patent Publication No. 2003-277655

[0004] However, there are cases where it is necessary to further improve the image density and inkjet ink ejection stability of images recorded by inkjet recording methods. For example, in high-speed inkjet recording methods (for example, inkjet recording methods that apply inkjet ink to a substrate being transported at a transport speed of 200 m / min or more; the same applies hereinafter), it is necessary to improve the ejection stability of inkjet ink from the inkjet head. Also, in high-speed inkjet recording methods, the drying time is shorter, which makes it easier for uneven image density to occur, and as a result, the image density tends to decrease, so it is necessary to further improve the image density.

[0005] This disclosure has been made in view of these circumstances. One embodiment of this disclosure aims to solve the problem of an inkjet ink that is excellent in the density of the recorded image and the ejection stability of the inkjet ink, an ink set containing this inkjet ink, and an inkjet recording method that applies the inkjet ink to a substrate being transported at a transport speed of 200 m / min or more, while being excellent in the density of the recorded image and the ejection stability of the inkjet ink.

[0006] This disclosure includes the following embodiments: <1> An inkjet ink containing water, a carbon black pigment, and pigment particles having a crosslinked polymer dispersant, wherein the volume-average particle diameter of the pigment particles, as measured by dynamic light scattering, is 65 nm or more and less than 100 nm. <2> Furthermore, the Hansen solubility parameter is 19 MPa １／２ ~22 MPa １／２The inkjet ink according to <1>, comprising an organic solvent and propylene glycol. <3> The inkjet ink according to <1> or <2>, wherein the acid value of the polymer dispersant having a crosslinked structure is 2.7 mmol / g or less. <4> The inkjet ink according to any one of <1> to <3>, wherein the carbon black pigment content relative to the total amount of inkjet ink is 6.5% by mass or more. <5> The inkjet ink according to any one of <1> to <4>, wherein the polymer dispersant having a crosslinked structure includes hydrophilic groups other than acid groups. <6> The inkjet ink according to any one of <1> to <5>, used as an ink in an inkjet recording method that includes applying ink to a substrate being transported at a transport speed of 200 m / min or more by an inkjet method to record an image. <7> An ink set comprising: black ink, which is an inkjet ink described in any one of <1> to <6>; magenta ink, which is an inkjet ink containing magenta pigment; and yellow ink, which is an inkjet ink containing yellow pigment, wherein the carbon black pigment content relative to the total amount of black ink is 6.5% by mass or more; the magenta pigment content relative to the total amount of magenta ink is 5.5% by mass or more; and the mass ratio of the magenta pigment content relative to the total amount of magenta ink, M, and the mass ratio of the yellow pigment content relative to the total amount of yellow ink, Y, satisfy the following relation (1): 1.30 ≤ M / Y ≤ 1.80 ... Relation (1) <8> An inkjet recording method comprising applying the inkjet ink described in any one of <1> to <6> to a substrate being transported at a transport speed of 200 m / min or more by an inkjet method to record an image.

[0007] According to one embodiment of the present disclosure, an inkjet ink that is excellent in the density of the recorded image and the ejection stability of the inkjet ink, an ink set containing the inkjet ink, and an inkjet recording method that applies the inkjet ink to a substrate being transported at a transport speed of 200 m / min or more are provided, while being excellent in the density of the recorded image and the ejection stability of the inkjet ink.

[0008] This figure conceptually illustrates an example of an inkjet recording apparatus used in the inkjet recording method of this disclosure.

[0009] In this specification, numerical ranges indicated using "~" mean a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described stepwise in this specification, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise. Also, in numerical ranges described in this specification, the upper or lower limit stated in one numerical range may be replaced with the value shown in the example. In this specification, the amount of each component in a composition means the total amount of multiple substances present in the composition if there are multiple substances corresponding to each component in the composition, unless otherwise specified. In this specification, a combination of two or more preferred embodiments is a more preferred embodiment. In this specification, the term "process" is included not only in the sense of an independent process, but also in the sense of a process that cannot be clearly distinguished from other processes, as long as the intended purpose of that process is achieved.

[0010] In this specification, “image” means any film formed by applying ink, and “image recording” means the formation of an image (i.e., a film). The concept of “image” in this specification also includes solid images. In this specification, “(meth)acryloyl group” is a concept that includes both acryloyl group and methacryloyl, “(meth)acrylate” is a concept that includes both acrylate and methacrylate, and “(meth)acrylic” is a concept that includes both acrylic and methacrylic. In this specification, the term “dispersant” simply means a polymeric dispersant, and the term “solvent” simply means an organic solvent.

[0011] [Inkjet Ink] The inkjet ink of this disclosure (hereinafter also simply referred to as "ink") contains water and pigment particles comprising a carbon black pigment and a polymer dispersant having a crosslinked structure, wherein the volume average particle diameter of the pigment particles measured by dynamic light scattering is 65 nm or more and less than 100 nm.

[0012] The inks of this disclosure exhibit excellent density of recorded images and ejection stability. The effect of improved image density is thought to be due to the volume-average particle diameter of the pigment particles being less than 100 nm. The effect of improved ink ejection stability is thought to be due to the volume-average particle diameter of the pigment particles being 65 nm or more, and the polymer dispersant in the pigment particles having a cross-linked structure.

[0013] Generally, high-speed inkjet recording methods (for example, inkjet recording methods that apply inkjet ink to a substrate being transported at a transport speed of 200 m / min or more) require improved inkjet ink ejection stability. In addition, high-speed inkjet recording methods tend to result in uneven image density due to shorter drying times, which in turn tends to reduce image density. Therefore, there is a need to further improve image density. The ink of this disclosure, when used in high-speed inkjet recording methods (for example, inkjet recording methods that apply inkjet ink to a substrate being transported at a transport speed of 200 m / min or more), exhibits more effective improvements in image density and ejection stability.

[0014] The inks described in this disclosure are explained in more detail below.

[0015] <Water> The ink of this disclosure contains water. The water content is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, based on the total amount of ink. The upper limit of the water content depends on the amounts of other components. The upper limits of the water content relative to the total amount of ink are, for example, 90% by mass and 80% by mass.

[0016] <Pigment Particles> The ink of this disclosure contains at least one type of pigment particle. Here, the pigment particle comprises a carbon black pigment and a polymeric dispersant having a crosslinked structure. The crosslinked structure of the polymeric dispersant improves the ink's ejection stability. The reason for this is thought to be as follows: The pigment particle is thought to be formed by coating at least a portion of the carbon black pigment with the polymeric dispersant. Here, the crosslinked structure of the polymeric dispersant makes it difficult for the polymeric dispersant to detach from the carbon black pigment, thus improving the dispersion stability of the carbon black pigment. As a result, the ink's ejection stability is thought to be improved.

[0017] The pigment particles can be formed, for example, by mixing a carbon black pigment and an uncrosslinked polymer dispersant, and crosslinking the uncrosslinked polymer dispersant in the obtained mixture (for example, a carbon black pigment at least partially coated with the uncrosslinked polymer dispersant) with a crosslinking agent.

[0018] (Volume Average Particle Diameter) In the ink of the present disclosure, the volume average particle diameter of the pigment particles measured by dynamic light scattering method is 65 nm or more and less than 100 nm. The measurement of the volume average particle diameter of pigment particles by dynamic light scattering method is performed after fractionating the pigment particles by centrifugation or the like, for example, using MICROTRAC manufactured by Microtrac Bell. As described above, when the volume average particle diameter of the pigment particles is 65 nm or more, the ejection stability of the ink is improved, and when the volume average particle diameter of the pigment particles is less than 100 nm, the image density is improved. The volume average particle diameter of the pigment particles is preferably from 65 nm to 98 nm, more preferably from 70 nm to 95 nm, and still more preferably from 70 nm to 90 nm.

[0019] (Carbon Black Pigment) There are no particular restrictions on the carbon black pigment, and known carbon black pigments can be used without any particular restrictions. A commercially available product may be used as the carbon black pigment (see Examples described below).

[0020] From the viewpoint of further improving the image density, the content of the carbon black pigment relative to the total amount of the ink is preferably 4.0% by mass or more, more preferably 5.0% by mass or more, still more preferably 5.5% by mass or more, and even more preferably 6.5% by mass or more. The upper limit of the content of the carbon black pigment relative to the total amount of the ink includes, for example, 10.0% by mass and 9.0% by mass.

[0021] (Polymer Dispersant Having Crosslinked Structure) There is no particular limitation on the form of the polymer dispersant having a crosslinked structure, and it may be any of a random polymer, a block polymer, and a graft polymer.

[0022] In the present disclosure, both "polymer" and "resin" mean a compound having a weight average molecular weight (Mw) of 1000 or more.

[0023] In this disclosure, weight-average molecular weight (Mw) refers to the value measured by gel permeation chromatography (GPC). The GPC measurement is performed using an HLC®-8020GPC (manufactured by Tosoh Corporation) as the measuring instrument, with three TSKgel® Super Multipore HZ-H columns (4.6 mm ID × 15 cm, manufactured by Tosoh Corporation) and THF (tetrahydrofuran) as the eluent. The measurement is performed with a sample concentration of 0.45% by mass, a flow rate of 0.35 mL / min, a sample injection volume of 10 μL, and a measurement temperature of 40°C, using an RI detector. The calibration curve will be prepared from eight samples of "Standard Samples TSK standard, polystyrene" manufactured by Tosoh Corporation: "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", "A-1000", and "n-propylbenzene".

[0024] Whether or not the polymer dispersant contained in the ink has a cross-linked structure can be determined, for example, by the following method. First, the polymer dispersant is separated from the ink using a separation method such as solvent extraction. The separated polymer dispersant can then be analyzed using various analytical methods such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and thermal analysis to comprehensively determine whether or not it has a cross-linked structure.

[0025] A polymer dispersant having a crosslinked structure is formed, for example, by crosslinking an uncrosslinked polymer dispersant with a crosslinking agent. The uncrosslinked polymer dispersant is preferably a water-soluble polymer dispersant.

[0026] In this disclosure, "water-soluble" in "water-soluble polymer dispersant" means the property of dissolving 1 g or more in 100 g of water at 25°C. Preferably, "water-soluble" means dissolving 3 g or more (more preferably 10 g or more) in 100 g of water at 25°C.

[0027] Furthermore, even if an uncrosslinked polymer dispersant is water-soluble, a polymer dispersant with a crosslinked structure is not necessarily water-soluble.

[0028] Examples of uncrosslinked polymer dispersants include vinyl resins, acrylic resins, urethane resins, and polyester resins. The uncrosslinked polymer dispersant is preferably an acrylic resin. In this disclosure, resin and polymer are synonymous.

[0029] In this disclosure, acrylic resin means a resin comprising at least one of structural units derived from (meth)acrylic acid and structural units derived from (meth)acrylic acid esters.

[0030] The uncrosslinked polymer dispersant is preferably a polymer dispersant having a functional group that can be crosslinked by a crosslinking agent. Examples of crosslinkable functional groups include carboxyl groups or their salts, isocyanate groups, and epoxy groups. Among these, from the viewpoint of improving the dispersibility of pigments, the crosslinkable functional group is preferably a carboxyl group or its salt, and a carboxyl group is particularly preferred. In other words, the uncrosslinked polymer dispersant is preferably a resin containing carboxyl groups.

[0031] The uncrosslinked polymeric dispersant is preferably a copolymer containing structural units derived from monomers containing carboxyl groups (hereinafter referred to as "carboxyl group-containing monomers"). The copolymer may contain only one type of structural unit derived from carboxyl group-containing monomers, or two or more types. The copolymer may be a random copolymer or a block copolymer, but a random copolymer is preferred.

[0032] Examples of monomers containing a carboxyl group include (meth)acrylic acid, β-carboxyethyl acrylate, fumaric acid, itaconic acid, maleic acid, and crotonic acid.

[0033] The carboxyl group-containing monomer is preferably (meth)acrylic acid or β-carboxyethyl acrylate, and more preferably (meth)acrylic acid, from the viewpoint of crosslinkability and dispersibility.

[0034] The content of structural units derived from carboxyl group-containing monomers is preferably 5% to 40% by mass, more preferably 10% to 35% by mass, and even more preferably 10% to 30% by mass, based on the total amount of the uncrosslinked polymer dispersant.

[0035] The uncrosslinked polymer dispersant preferably contains structural units derived from hydrophobic monomers in addition to structural units derived from carboxyl group-containing monomers. The structural units derived from hydrophobic monomers contained in the uncrosslinked polymer dispersant may be one type or two or more types.

[0036] Examples of hydrophobic monomers include (meth)acrylates having an alkyl group with 1 to 20 carbon atoms, (meth)acrylates having an aromatic ring (e.g., benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, etc.), styrene, and styrene derivatives.

[0037] The content of structural units derived from hydrophobic monomers is preferably 60% to 95% by mass, more preferably 65% ​​to 90% by mass, and even more preferably 70% to 90% by mass, based on the total amount of the uncrosslinked polymer dispersant.

[0038] The uncrosslinked polymeric dispersant is preferably a random copolymer comprising structural units derived from a carboxyl group-containing monomer and at least one of structural units derived from a (meth)acrylate having an alkyl group having 1 to 20 carbon atoms and structural units derived from a (meth)acrylate having an aromatic ring; more preferably a random copolymer comprising structural units derived from (meth)acrylic acid and structural units derived from a (meth)acrylate having an aromatic ring; and even more preferably a copolymer comprising structural units derived from (meth)acrylic acid and structural units derived from benzyl (meth)acrylate.

[0039] The weight-average molecular weight (Mw) of the uncrosslinked polymer dispersant is not particularly limited, but from the viewpoint of pigment dispersibility, it is preferably 3,000 to 300,000, more preferably 5,000 to 200,000, and even more preferably 7,000 to 100,000.

[0040] The preferred range for the weight-average molecular weight of a polymer dispersant having a crosslinked structure is the same as the preferred range for the weight-average molecular weight of an uncrosslinked polymer dispersant.

[0041] The crosslinking agent used when crosslinking an uncrosslinked polymer dispersant is preferably a compound having two or more reaction sites with the uncrosslinked polymer dispersant (for example, a resin having a carboxyl group). One type of crosslinking agent may be used, or two or more types may be used.

[0042] A preferred combination of a crosslinking agent and an uncrosslinked polymer dispersant is a compound having two or more epoxy groups (i.e., a bifunctional or more epoxy compound) and a resin having a carboxyl group. In this combination, a crosslinked structure is formed by the reaction between the epoxy groups and the carboxyl groups. It is preferable that the formation of the crosslinked structure by the crosslinking agent is carried out after the pigment has been dispersed by the uncrosslinked polymer dispersant.

[0043] Examples of bifunctional or more epoxy compounds include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and trimethylolpropane triglycidyl ether.

[0044] Among these, the preferred epoxy compounds with two or more functions are polyethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, or trimethylolpropane triglycidyl ether.

[0045] The crosslinking agent may be a commercially available product. Examples of commercially available products include Denacol EX-321, EX-821, EX-830, EX-850, and EX-851 (manufactured by Nagase ChemteX Corporation).

[0046] The molar ratio of the reaction site in the crosslinking agent (e.g., epoxy group) to the reaction site in the uncrosslinked polymer dispersant (e.g., carboxyl group) is preferably 1:1.1 to 1:10, more preferably 1:1.1 to 1:5, and even more preferably 1:1.1 to 1:3, from the viewpoint of crosslinking reaction rate and dispersion stability after crosslinking.

[0047] In the inks of this disclosure, the ratio of the pigment content to the content of the polymer dispersant having a crosslinked structure is preferably 1:0.02 to 1:2 by mass, more preferably 1:0.03 to 1:1.5, and even more preferably 1:0.04 to 1:1.

[0048] In the ink manufacturing process (specifically, before crosslinking of the polymer dispersant), the ratio of the amount of pigment used to the amount of uncrosslinked polymer dispersant used is preferably 1:0.02 to 1:2 by mass, more preferably 1:0.03 to 1:1.5, and even more preferably 1:0.04 to 1:1.

[0049] -Acid Value- The acid value of the polymer dispersant having a crosslinked structure is preferably 3.0 mmol / g or less, more preferably 2.7 mmol / g or less, and even more preferably 2.5 mmol / g or less, from the viewpoint of further improving the ink discharge stability. Examples of lower limits for the acid value of the polymer dispersant having a crosslinked structure include 0.1 mmol / g, 0.5 mmol / g, and 1.0 mmol / g.

[0050] Here, the acid value (mol / g) of a cross-linked polymer dispersant refers to the amount of acid groups (mol / g) contained per gram of the cross-linked polymer dispersant. The acid value (mol / g) of a cross-linked polymer dispersant is measured by neutralization titration.

[0051] - Hydrophilic Groups - Polymer dispersants having a crosslinked structure preferably contain hydrophilic groups from the viewpoint of further improving the ink discharge stability.

[0052] Examples of hydrophilic groups include acidic groups and hydrophilic groups other than acidic groups. In this disclosure, the concept of "acidic group" includes both unneutralized and neutralized acidic groups.

[0053] The unneutralized acid group is preferably a carboxyl group (-COOH group). The neutralized acid group is preferably a -COOM group (where M represents an alkali metal ion, an ammonium ion, or an organic cation). The alkali metal is preferably a potassium ion or a sodium ion, and more preferably sodium. Examples of organic cations include alkylammonium cations having 1 to 10 carbon atoms, hydroxysubstituted alkylammonium cations having 1 to 10 carbon atoms, carboxysubstituted alkylammonium cations having 2 to 10 carbon atoms, and organic cations having 2 to 10 alkyleneimine units having 2 to 4 carbon atoms.

[0054] A polymer dispersant having a crosslinked structure is preferably made up of hydrophilic groups other than acidic groups, from the viewpoint of further improving the ink ejection stability. In this case, the polymer dispersant having a crosslinked structure may contain both hydrophilic groups other than acidic groups and hydrophilic groups that are acidic groups.

[0055] The hydrophilic group other than the acid group is preferably a polyalkylene oxy group, more preferably a polyethylene oxy group or a polypropylene oxy group. Here, in the polyalkylene oxy group (e.g., polyethylene oxy group or polypropylene oxy group), the number of repeating alkylene oxy groups (e.g., ethylene oxy group or propylene oxy group) is 2 or more, preferably 2 to 20, more preferably 2 to 10, even more preferably 2 to 6, and even more preferably 2 to 4.

[0056] <Organic Solvents> The inks of this disclosure preferably contain at least one organic solvent, from the viewpoint of further improving the ink ejection stability.

[0057] The organic solvent is preferably a water-soluble organic solvent.

[0058] In this disclosure, "water-soluble" in "water-soluble organic solvent" means the property of dissolving 1 g or more in 100 g of water at 25°C.

[0059] When the ink of the present disclosure contains an organic solvent, the proportion of the organic solvent in the total amount of the ink of the present disclosure is preferably 5% by mass to 50% by mass, more preferably 10% by mass to 40% by mass, and still more preferably 20% by mass to 35% by mass.

[0060] From the viewpoint of further improving the ejection stability of the ink, the ink of the present disclosure preferably contains propylene glycol, and more preferably has a Hansen solubility parameter (hereinafter also referred to as "HSP") of 19 MPa １／２ to 22 MPa １／２ , contains the organic solvent satisfying the above condition and propylene glycol.

[0061] In the present disclosure, the Hansen solubility parameter (HSP) means a value calculated by the software "Hansen Solubility Parameter in Practice" (HSPiP Ver. 6.0.04).

[0062] HSP is 19 MPa １／２ to 22 MPa １／２ Examples of the organic solvent satisfying the above condition include: diethylene glycol monoethyl ether (HSP = 21.6 MPa １／２ ), ethylene glycol monohexyl ether (HSP = 20.7 MPa １／２ ), propylene glycol monobutyl ether (HSP = 20 MPa １／２ ), ethylene glycol monoisopropyl ether (HSP = 21.1 MPa １／２ ), diethylene glycol monoethyl ether (HSP = 21.6 MPa １／２ ), propylene glycol monomethyl ether (HSP = 22.6 MPa １／２ ), methyl propylene triglycol (HSP = 19.5 MPa １／２ ), triethylene glycol monobutyl ether (HSP = 20.3 MPa １／２ ), propylene glycol monobutyl ether (HSP = 20 MPa １／２ ), diethylene glycol isopropyl ether (HSP = 20.3 MPa １／２), diethylene glycol isobutyl ether (HSP = 19.8 MPa １／２ ), diethylene glycol butyl ether (HSP = 21 MPa １／２ ), dipropylene glycol methyl ether (HSP = 20.5 MPa １／２ ), propylene glycol monopropyl ether (HSP = 20.5 MPa １／２ Examples include, ), and so on.

[0063] The ink disclosed herein has an HSP of 19 MPa １／２ ~22 MPa １／２ If the ink contains an organic solvent and propylene glycol, the ink of this disclosure may further contain other organic solvents (e.g., glycerin).

[0064] The ink disclosed herein has an HSP of 19 MPa １／２ ~22 MPa １／２ When the ink contains an organic solvent and propylene glycol, the HSP of the organic solvent contained in the ink of this disclosure is 19 MPa. １／２ ~22 MPa １／２ The total ratio of the organic solvent and propylene glycol is preferably 50% to 100% by mass, more preferably 60% to 100% by mass, and even more preferably 80% to 100% by mass.

[0065] The ink disclosed herein has an HSP of 19 MPa １／２ ~22 MPa １／２ When an organic solvent and propylene glycol are included, the HSP of the total amount of the ink of this disclosure is 19 MPa. １／２ ~22 MPa １／２ The total ratio of the organic solvent and propylene glycol is preferably 5% to 50% by mass, more preferably 10% to 50% by mass, and even more preferably 15% to 35% by mass.

[0066] <Resin Particles> The ink of this disclosure preferably contains at least one type of resin particle. The resin particle may contain resin and other core materials (excluding pigments), but it is preferable that the particle consists only of resin.

[0067] The resin particles are preferably made of acrylic resin, polyester resin, polyurethane resin, or polyolefin resin, and are more preferably made of acrylic resin.

[0068] For resin particles, you may refer to, for example, paragraphs 0038 to 0114 of International Publication No. 2021 / 192720, paragraphs 0109 to 0120 of Japanese Patent Publication No. 2015-25076, etc.

[0069] The content of resin particles relative to the total amount of the ink of this disclosure is preferably 0.1% to 7.0% by mass, more preferably 0.5% to 5.0% by mass, and even more preferably 1.0% to 4.0% by mass.

[0070] Ink containing resin particles may be prepared using commercially available resin emulsions (i.e., aqueous dispersions of resin particles). Examples of commercially available resin emulsions include A-810 (Sansui Co., Ltd.), A-995 (Sansui Co., Ltd.), Hyros-X NE-2186 (Seiko PMC Co., Ltd.), Hyros-X TE-1048 (Seiko PMC Co., Ltd.), Saibinol SK-202 (Saiden Chemical Co., Ltd.), TOCRYL W-1048 (Toyo Chem), WC-M-1217 (Arakawa Chemical Industries, Ltd.), WC-M-1219 (Arakawa Chemical Industries, Ltd.), N985(A)-1 (E-Tech Co., Ltd.), and Neocryl A-1105 (DSM coating). Examples include resin), Acrit SE-810A, Acrit SE-953A-2, Acrit SE-1658F, Acrit SE-2974F, Acrit SE-2978F (Taisei Fine Chemical), Luxstar 7132-C (DIC), ST200 (Nippon Shokubai), Movinyl 972 (Japan Coating Resin Co., Ltd.), etc.

[0071] <Surfactants> The inks of this disclosure preferably contain at least one surfactant.

[0072] Examples of surfactants include polyoxyalkylene alkyl ether compounds (i.e., polyoxyalkylene alkyl ether surfactants), acetylene compounds (i.e., acetylene surfactants), and silicone compounds (i.e., silicone surfactants).

[0073] For information on surfactants, refer to paragraphs 0112-0130 of International Publication No. 2024 / 241860 as appropriate.

[0074] As the silicone compound, polyether-modified silicone compounds are preferred, and compounds represented by the following formula (D1) are more preferred.

[0075]

[0076] In formula (D1), R １ Each of these independently represents an alkyl group or hydroxyl group having 1 to 3 carbon atoms, R ２ R represents an alkanediyl group with 2 to 5 carbon atoms. ３ ∫ represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyl group. PO represents a propylene oxy group, and EO represents an ethylene oxy group. a, b, m, and n represent the average number of moles added for each unit, where a is 0 to 10, b is 1 to 50, m is 1 to 500, and n is 1 to 50.

[0077] In formula (D1), the arrangement of PO and EO may be a block copolymer arrangement or a random copolymer arrangement. In formula (D1), the arrangement of the structural unit denoted by the subscript m and the structural unit denoted by the subscript n may be a block copolymer arrangement or a random copolymer arrangement.

[0078] R １ R is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group. ２ It is preferably an alkanediyl group having 3 or 4 carbon atoms, and more preferably a trimethylene group. ３ It is preferably an alkyl group having 1 to 3 carbon atoms, and more preferably a methyl group.

[0079] It is more preferable that a is 0, b is 1 to 15, m is 1 to 10, and n is 1 to 5. Furthermore, it is even more preferable that a is 0, b is 3 to 10, m is 1 to 3, and n is 1 to 3.

[0080] The silicone compound may be a commercially available product. Examples of commercially available silicone compounds include BYK-302, BYK-307, BYK-331, BYK-333, BYK-345, BYK-347, BYK-348, BYK-349, BYK-378, BYK-3400, BYK-3450, BYK-3451, BYK-3455, BYK-3760 (all manufactured by Bic Chemie Japan), KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF-640, KF-642, and KF-64 3. Examples include KF-644, KF-6011, KF-6012, KF-6015, KF-6017, KF-6020 (all manufactured by Shin-Etsu Chemical Co., Ltd.), Silface SAG002, Silface SAG005, Silface SAG008, Silface SAG503A (all manufactured by Nisshin Chemical Co., Ltd.), TEGOWet KL245, TEGOWet 240, TEGOWet 250, TEGOWet 260, TEGOWet 270, TEGOWet 280 (all manufactured by Evonik), etc.

[0081] The surfactant content is preferably 0.01% to 5.0% by mass, more preferably 0.02% to 4.0% by mass, and even more preferably 0.1% to 3.0% by mass, based on the total amount of ink.

[0082] (Acetylene-based surfactants) As acetylene-based surfactants, compounds represented by the following formula (A1) are preferred.

[0083]

[0084] In formula (A1), R １ and R ４ Each of these independently represents an alkyl group having 3 to 10 carbon atoms, R ２ and R ３Each of these independently represents either a methyl group or an ethyl group. a, b, c, and d represent the average number of moles added for each unit, ranging from 0 to 50.

[0085] Suitable examples of acetylene-based surfactants include, for example: acetylene glycol selected from the group consisting of 2,4,7,9-tetramethyl-5-decine-4,7-diol, 3,6-dimethyl-4-octin-3,6-diol, 2,5,6,11-tetramethyl-6-dodecine-5,8-diol, 2,5-dimethyl-3-hexyne-2,5-diol, and 2,5,8,11-tetramethyl-6-dodecine-5,8-diol; and ethylene oxide adducts of the above acetylene glycols. Acetylene-based surfactants can be synthesized, for example, by reacting acetylene with a ketone or aldehyde corresponding to the desired acetylene glycol. Acetylene-based surfactants can be obtained, for example, by the methods described on pages 94-107 of "New Introduction to Surfactants" (Revised Edition) by Takehiko Fujimoto (published by Sanyo Chemical Industries, Ltd., 1992).

[0086] The acetylene-based surfactant preferably includes an acetylene-based surfactant having an HLB value of 3.0 to 9.0 (preferably 3.5 to 9.0). When the acetylene-based surfactant includes an acetylene-based surfactant with an HLB value of 3.0 or higher, the image resolution is further improved. When the acetylene-based surfactant includes an acetylene-based surfactant with an HLB value of 9.0 or lower, the image granularity is further improved.

[0087] Commercially available acetylene-based surfactants may be used. Examples of commercially available acetylene-based surfactants include those manufactured by Nisshin Chemical Co., Ltd. Examples of acetylene-based surfactants manufactured by Nisshin Chemical include Surfinol 104 (HLB value 4.0), Surfinol 420 (HLB value 4.0), Surfinol 440 (HLB value 8.0), Surfinol SE (HLB value 6.0), Surfinol SE-F (HLB value 6.0), Surfinol 61 (HLB value 6.0), Surfinol 82 (HLB value 4.0), Surfinol DF110D (HLB value 3.0), Dynol 604 (HLB value 8.0), Dynol 607 (HLB value 8.0), Surfinol 2502 (HLB value 8.0), Surfinol TG (HLB value 9.0), Orphin E1004 (HLB value: 7.0-9.0), Orphin E1010 (HLB value: 13.5), and others.

[0088] From the viewpoint of further improving the image blocking resistance, it is preferable that the acetylene-based surfactant includes an acetylene-based surfactant having an HLB value of 6 or less (more preferably 3 to 6).

[0089] The content of acetylene-based surfactant (for example, acetylene-based surfactant with an HLB value of 6 or less) relative to the total amount of ink is preferably 0.05% to 2.5% by mass, more preferably 0.1% to 2.0% by mass, and even more preferably 0.2% to 1.5% by mass, from the viewpoint of improving the blocking resistance of the image.

[0090] (Other Components) The inks in this disclosure may contain other components besides those described above, as necessary. Examples of other components include colloidal silica, inorganic salts, solid wetting agents (such as urea), fade inhibitors, emulsifying stabilizers, penetration enhancers, ultraviolet absorbers, preservatives, fungicides, pH adjusters, defoamers, viscosity modifiers, dispersion stabilizers, rust inhibitors, chelating agents, water-soluble polymer compounds, and the like.

[0091] (Physical properties of the ink) - Viscosity - The viscosity of the ink is preferably 1.2 mPa·s to 15.0 mPa·s, more preferably 2.0 mPa·s to 13.0 mPa·s, and even more preferably 2.5 mPa·s to 10.0 mPa·s. The viscosity of the ink is measured at a temperature of 30°C using a rotational viscometer, for example, a product named "VISCOMETER TV-22" manufactured by Toki Sangyo Co., Ltd.

[0092] -pH- The pH of the ink is preferably 6.0 to 11.0, more preferably 7.0 to 10.0, and even more preferably 7.0 to 9.5, from the viewpoint of the storage stability of the ink. The pH of the ink is measured at a temperature of 25°C using a pH meter, for example, a product named "WM-50EG" manufactured by Toa DKK Co., Ltd.

[0093] [An Example of an Inkjet Ink Manufacturing Method (Method A)] There are no particular limitations on the method of manufacturing the inks of this disclosure. An example of an ink manufacturing method (Method A) is described below. Method A is an example of a method for manufacturing an ink containing resin particles.

[0094] Method A includes the steps of preparing a pigment dispersion containing water, a carbon black pigment, and pigment particles having a cross-linked polymer dispersant, wherein the volume-average particle diameter of the pigment particles, as measured by dynamic light scattering, is 65 nm or more and less than 100 nm, and at least the above pigment dispersion, resin particles, and a Hansen solubility parameter of 19 MPa １／２ ~22 MPa １／２ The process includes a step of obtaining ink by mixing an organic solvent and propylene glycol.

[0095] The preferred embodiment of pigment particles in the pigment dispersion is the same as the preferred embodiment of pigment particles contained in the ink of the present disclosure described above. Furthermore, in the process of obtaining the ink, the ink components described above can be added and mixed as appropriate.

[0096] [Inkjet Recording Method] The inkjet recording method of this disclosure includes recording an image by applying the ink described above to a substrate being transported at a transport speed of 200 m / min or more using an inkjet method.

[0097] As mentioned above, in general, high-speed inkjet recording methods (for example, inkjet recording methods that apply inkjet ink to a substrate being transported at a transport speed of 200 m / min or more) tend to result in uneven image density and a decrease in image density due to the shorter drying time. However, since the inkjet recording method of this disclosure uses the ink described above, it is possible to effectively suppress the decrease in image density even when using a high-speed inkjet recording method (for example, inkjet recording methods that apply inkjet ink to a substrate being transported at a transport speed of 200 m / min or more). Furthermore, it is possible to suppress the decrease in image lightfastness, which is a problem that tends to occur with inks containing dyes.

[0098] The inkjet recording method described herein will be explained in detail below.

[0099] <Substrate> The substrate used in the inkjet recording method of this disclosure is not particularly limited, and examples include so-called coated paper used in general offset printing. Coated paper is obtained by applying a coating material to the surface of a generally untreated high-quality paper, neutral paper, etc., which is mainly composed of cellulose, to provide a coating layer.

[0100] Coated paper may be one that is generally available on the market. For example, general-purpose coated paper for printing can be used as the coated paper. Specifically, examples include coated papers (A2, B2) such as "OK Topcoat+" from Oji Paper Co., Ltd., "Aurora Coat" and "U-Light" from Nippon Paper Industries Co., Ltd., and art paper (A1) such as "Tokuryo Art" from Mitsubishi Paper Mills Ltd.

[0101] <Substrate transport speed> In the inkjet recording method of this disclosure, the substrate transport speed is 200 m / min or more. This enables high-speed inkjet recording.

[0102] The conveying speed of the substrate should be 200 m / min or more, and may or may not be constant. The upper limit of the conveying speed of the substrate is, for example, 300 m / min.

[0103] <Ink Application by Inkjet Method> In the inkjet recording method of this disclosure, an image is recorded by applying the aforementioned ink of this disclosure to a transported substrate by an inkjet method. The ink application by the inkjet method is performed by ejecting ink from an inkjet head.

[0104] Ink ejection is preferably performed using an inkjet head having a resolution of 300 dpi or higher (more preferably 600 dpi or higher, and even more preferably 800 dpi or higher). Here, dpi is an abbreviation for dots per inch, and 1 inch is equal to 2.54 cm.

[0105] From the viewpoint of obtaining a high-resolution image, the amount of ink droplets is preferably 1 pL (picoliters) to 10 pL, and more preferably 1.5 pL to 6 pL.

[0106] <Other Operations> The inkjet recording method of this disclosure may include other operations besides applying ink. Other operations include, for example, preheating the substrate and drying the ink-coated substrate.

[0107] Hereinafter, an example of an inkjet recording apparatus used in the inkjet recording method of this disclosure will be described with reference to the drawings.

[0108] In the drawings and descriptions herein, substantially identical elements (e.g., parts or components) may be denoted by the same reference numeral, and redundant descriptions may be omitted.

[0109] Figure 1 is a conceptual diagram showing an inkjet recording apparatus 100, which is an example of an inkjet recording apparatus used in the inkjet recording method of this disclosure.

[0110] As shown in Figure 1, the inkjet recording device 100 includes an ink application unit 101 that includes a first inkjet head 111A for ejecting a first ink, a second inkjet head 111B for ejecting a second ink, a third inkjet head 111C for ejecting a third ink, and a fourth inkjet head 111D for ejecting a fourth ink, applied to continuous paper 110 as a substrate. Here, at least one of the first to fourth inks is the ink of this disclosure. Alternatively, at least two of the first to fourth inks may be inks from the ink set of this disclosure described later.

[0111] The ink application unit 101 is configured, for example, with four full-line inkjet heads 111A, 111B, 111C, and 111D arranged in order from the upstream side in the transport direction of the continuous paper 110. Each of the inkjet heads 111A, 111B, 111C, and 111D applies, for example, black (K) ink, cyan (C) ink, magenta (M) ink, and yellow (Y) ink to the continuous paper 110. However, the types and number of colors are not limited to these.

[0112] A preferred full-line inkjet head used here is a non-recirculating head. This type of inkjet head is available from Kyocera. A second preferred full-line inkjet head has an ink circulation channel in the ink supply system. This channel allows fresh ink to be ejected and can be part of the ink supply system or part of a specially developed channel that runs behind the nozzle plate. The ink supply system preferably runs behind the nozzle plate to allow for the use of more ink without impairing the restart / standby behavior. This type of inkjet head is available from Fujifilm Dimatix and Kyocera.

[0113] The ink ejected from each inkjet head contains a pigment and an organic solvent. The details of the first and second inks are as described above.

[0114] The continuous sheet of paper 110 is fed out from the main winding roller 102 and sent by the transport roller 112 of the transport unit 103 onto the transport guide member 113 which is positioned opposite the ink application unit 101, and is transported (moved) guided by the transport guide member 113.

[0115] The continuous sheet of paper 110, to which ink has been applied by the ink application unit 101, is sent by the discharge roller 118 through a heating and drying device (not shown) and wound onto the winding roller 105.

[0116] Furthermore, the inkjet recording device may be equipped with a preheating and drying device for preheating the substrate, and a substrate inversion device for applying ink to both sides of the substrate. In addition, a heating device may be provided for applying radiant heating to the ink-coated substrate. "Radiant heating" means the propagation of thermal energy through space or a medium, and an example of this is the flow of electromagnetic waves (electromagnetic radiation).

[0117] <Ink Set> The ink set of this disclosure includes multiple inkjet inks, including the ink of this disclosure as described above. Since the ink set of this disclosure includes the ink of this disclosure, it provides the effects of the ink of this disclosure (i.e., the effects of improved image density and improved ejection stability).

[0118] An ink set according to one embodiment of the present disclosure comprises: black ink, which is an inkjet ink of the present disclosure; magenta ink, which is an inkjet ink containing magenta pigment; and yellow ink, which is an inkjet ink containing yellow pigment, wherein M, which is the content of magenta pigment relative to the total amount of magenta ink, and Y, which is the content of yellow pigment relative to the total amount of yellow ink, satisfy the following relation (1): 1.30 ≤ M / Y ≤ 1.80 … Relation (1)

[0119] The carbon black pigment content relative to the total amount of black ink is preferably 5% by mass or more, more preferably 5.5% by mass, and even more preferably 6.5% by mass or more. The magenta pigment content relative to the total amount of magenta ink is preferably 5% by mass or more, more preferably 5.5% by mass, and even more preferably 6% by mass or more. The yellow pigment content relative to the total amount of yellow ink is preferably 3.5% by mass or more, and even more preferably 4% by mass or more.

[0120] Of the ink sets according to the above-described embodiment, a particularly preferred embodiment is an ink set comprising: black ink, which is an inkjet ink of the present disclosure; magenta ink, which is an inkjet ink containing magenta pigment; and yellow ink, which is an inkjet ink containing yellow pigment, wherein the content of carbon black pigment relative to the total amount of black ink is 6.5% by mass or more; the content of magenta pigment relative to the total amount of magenta ink is 5.5% by mass or more; and the content of magenta pigment M relative to the total amount of magenta ink and the content of yellow pigment Y relative to the total amount of yellow ink satisfy the following relation (1): 1.30 ≤ M / Y ≤ 1.80 … Relation (1)

[0121] According to the ink set of the above embodiment, in addition to the effects of the ink of this disclosure (i.e., the effects of improved image density and ejection stability), the effects of improved image color development and improved ink storage stability can be obtained. Here, image color development is a performance that is evaluated using the density of the images of each color, black and magenta, and the color difference ΔEab between the magenta image and the yellow image as indicators.

[0122] The effect of the image's color development is obtained by the relationship "1.30 ≤ M / Y" in relation (1), the fact that the carbon black pigment content relative to the total amount of black ink is 6.5% by mass or more, and the magenta pigment content relative to the total amount of magenta ink is 5.5% by mass or more.

[0123] The ink ejection stability is primarily the ejection stability of magenta ink, and is an effect obtained by "M / Y ≤ 1.80" in relation (1).

[0124] The magenta ink in the ink set of this disclosure is an inkjet ink containing a magenta pigment. Examples of magenta pigments include quinacridone pigment, naphthol pigment, benzimidazolon pigment, and condensed azo pigment. The magenta ink may optionally possess the characteristics of the inks of this disclosure other than those relating to the carbon black pigment.

[0125] The yellow ink in the ink set of this disclosure is preferably an inkjet ink containing a yellow pigment. Examples of yellow pigments include monoazo pigments (e.g., P.Y. 74), disazo pigments (e.g., P.Y. 155), condensed azo pigments (e.g., P.Y. 128), isoindolinone pigments (e.g., P.Y. 109, P.Y. 185), and benzimidazolon pigments (e.g., P.Y. 151, P.Y. 180). The yellow ink may optionally possess the characteristics of the inks of this disclosure other than those relating to the carbon black pigment.

[0126] The ink set of this disclosure may include other inks besides black ink, magenta ink, and yellow ink. Examples of other inks include cyan ink.

[0127] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples unless it exceeds the spirit of the disclosure. Hereinafter, the term "dispersant" simply means a polymeric dispersant, and the term "solvent" simply means an organic solvent.

[0128] <Synthesis of Dispersant A> Add 6.1 g of perbutyl O (manufactured by NOF Corporation) to 186.0 g of DPG (dipropylene glycol) and stir to prepare an initiator solution. Separately, add 282.6 g of benzyl methacrylate, 77.5 g of methacrylic acid, and 6 g of butyl 3-mercaptopropionate as a chain transfer agent to 139.5 g of DPG and stir to prepare a monomer solution. Place 241.5 g of DPG in a 2 L three-necked flask and raise the internal temperature to 85°C under a nitrogen stream. Begin adding the initiator solution and monomer solution simultaneously, over 5 hours and 4 hours, respectively. After adding the initiator solution, the mixture was stirred at 85°C for 2 hours, then cooled to 70°C. 64.5 g of 50% potassium hydroxide aqueous solution, 43.8 g of water, and 27.4 g of DPG were added, and the mixture was stirred at 70°C for 2 hours to obtain a DPG solution of dispersant A. Water was then added to this solution in an amount that brought the solid content concentration of dispersant A to 30% by mass, and the mixture was stirred for another 2 hours to obtain a DPG-water mixed solution of dispersant A (dispersant solution 1). The weight-average molecular weight (Mw) of dispersant A is 18000.

[0129] <Synthesis of Dispersants B to F> In a reaction vessel equipped with a gas inlet tube, thermometer, condenser, and stirrer, 153.5 parts by mass of tripropylene glycol monomethyl ether (product name "MFTG", manufactured by Nippon Emulsifier Co., Ltd.) as the reaction solvent is charged, and then the inside of the reaction vessel is replaced with nitrogen gas. Next, the inside of the reaction vessel is heated to 85°C, and a mixture of MFTG (76.8 parts by mass) as the reaction solvent, raw material monomers with the composition shown in Table 1 below (totaling 100 parts by mass), and V-601 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (3.0 parts by mass) as a polymerization initiator is added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition is complete, the reaction is continued at 85°C for another 3 hours to complete the polymerization and obtain a solution containing one of the water-soluble polymer dispersants B to F. The weight-average molecular weight of uncrosslinked dispersant B is 15000. The weight-average molecular weights (Mw) of dispersants B to F are as shown in Table 1. In Table 1, the values ​​for each monomer represent parts by mass, and blank spaces indicate that the corresponding monomer was not used. "Hydrophilic groups other than acidic groups" refers to polyalkylene oxy groups.

[0130]

[0131] The abbreviations for monomers in Table 1 are as follows: MAA… Methacrylic acid; C18MA… Stearyl methacrylate; PDEGA… Phenoxydiethylene glycol acrylate; PTEGA… Phenoxytetraethylene glycol acrylate; MeDEGA… Methoxydiethylene glycol acrylate

[0132] [Example 1] <Neutralization of Dispersant A (Preparation of Uncrosslinked Dispersant A Solution)> After cooling the DPG-water mixed solution of Dispersant A obtained above to room temperature, an aqueous NaOH solution is added as an aqueous solution of the neutralizing base to neutralize 80 mol% of the carboxyl groups in Dispersant A. This yields a solution containing a dispersant with a degree of neutralization of 80% (i.e., the dispersant obtained by neutralizing Dispersant A; hereinafter also referred to as "Uncrosslinked Dispersant A"). The solid content concentration in the obtained solution is adjusted to 21% by mass to obtain an Uncrosslinked Dispersant A solution (21% by mass solid content concentration).

[0133] <Preparation of Uncrosslinked Dispersion> Using uncrosslinked dispersant A solution, the mixture of the following composition is pre-dispersed to a uniform state. Then, using a bead mill (LMZ-015 (manufactured by Ashizawa Fine Tech)) with zirconia beads of 0.3 mm diameter, dispersion is performed until the target particle size is achieved. This yields an uncrosslinked dispersion containing uncrosslinked dispersant A. -Composition- ・#980 (manufactured by Mitsubishi Chemical Corporation, carbon black pigment, primary particle size 16 nm) ... 20 parts by mass ・Uncrosslinked dispersant A solution ... 28.6 parts by mass (solid content 6 parts by mass) ・Dipropylene glycol ... 6 parts by mass ・Water ... remaining amount totaling 100 parts by mass

[0134] <Preparation of Pigment Dispersion 1 Containing a Crosslinked Dispersant> Next, water is added to the uncrosslinked dispersion obtained above to dilute it to a pigment concentration of 11.5 parts by mass, obtaining a diluted solution of the uncrosslinked dispersion (100 parts by mass). Next, the diluted solution of the uncrosslinked dispersion (100 parts by mass) is mixed with 0.242 parts by mass of Denacol EX-321 (manufactured by Nagase ChemteX, trimethylolpropane polyglycidyl ether: crosslinking agent) and 2.7 parts by mass of a 4% boric acid solution. The resulting liquid is reacted at 70°C for 5 hours and then cooled to 25°C, thereby crosslinking the uncrosslinked dispersant A in the uncrosslinked dispersion with the crosslinking agent to obtain a crosslinked dispersion containing a crosslinked dispersant. The crosslinked dispersion obtained above (i.e., pigment dispersion) is centrifuged at 7000 G for 20 minutes using a centrifuge to remove coarse particles. Next, the centrifugation-treated crosslinked dispersion is filtered through a LABO-PURE filter (0.5 μm) manufactured by Rokitechno to further remove coarse particles. Then, the filtered crosslinked dispersion is ultrafiltered through an ultrafiltration apparatus (cross-flow type ultrafilter (UF), manufactured by Sartorius) equipped with a polyethersulfone (PESU) membrane (micropore size: 0.1 μm) at a flow rate of 600 mL per minute. At this time, the liquid temperature is adjusted to 25°C, and ultrafiltration is performed 10 times, with each ultrafiltration being one time for every 1x volume increase of the charged liquid. After that, ion-exchanged water is added to bring the pigment concentration to 15% by mass to obtain pigment dispersion 1 containing a crosslinked dispersant. The acid value of the crosslinked dispersant in pigment dispersion 1 is 2 mmol / g. The acid value is determined by neutralization titration. Note that in crosslinking, the unneutralized acid group (-COOH group) and the neutralized acid group (-COOH) in the polymer dispersant are used. － A cross-linked structure is formed in the polymer dispersant by the reaction between at least one of the acid groups and the epoxy group of the crosslinking agent (specifically, the epoxy group in the structure of the glycidyl group). In this way, at least one of the unneutralized acid groups and the neutralized acid groups is consumed to form the cross-linked structure, so the acid value of the polymer dispersant decreases due to crosslinking. Therefore, the acid value of the dispersant contained in the cross-linked dispersion can be adjusted by the degree of neutralization of the acid groups before crosslinking and the amount of boric acid aqueous solution and crosslinking agent added thereafter.

[0135] <Ink Preparation> Using the pigment dispersion 1 described above and the components shown in Table 2, an ink having the composition shown in Table 2 is obtained. The volume-average particle size (nm) of the pigment particles is measured by dynamic light scattering by separating a measurement sample from the pigment dispersion 1 and using the separated measurement sample with a MICROTRAC manufactured by Microtrac Bell. The obtained volume-average particle size (nm) is shown in Table 2.

[0136] In the "Ink Composition" column of Table 2, the numerical values ​​corresponding to each component represent the content relative to the total ink volume, and a blank space means that the corresponding component is not present. For water, "residue" refers to the amount remaining when the total is 100% by mass.

[0137] Details of each component in Table 2 are as follows: -Carbon Black Pigments- ・#980 … Manufactured by Mitsubishi Chemical Corporation, primary particle size 16 nm ・#960 … Manufactured by Mitsubishi Chemical Corporation, primary particle size 16 nm ・#1200 … Manufactured by Mitsubishi Chemical Corporation, primary particle size 13 nm ・Nipex 170IQ … Manufactured by Orion, primary particle size 17 nm ・Printex 80 … Manufactured by Orion, primary particle size 16 nm -Dispersants- ・Joncryl 6180 … Styrene-acrylic copolymer manufactured by BASF ・Dispersants A to F … As described above -Solvents- ・GL … Glycerin (HSP = 37.4 MPa) １／２ ) ・PG…Propylene glycol (HSP = 31.0 MPa) １／２ ) ・DEGmEE … Diethylene glycol monoethyl ether (HSP = 21.6 MPa) １／２ ) EGmHE … Ethylene glycol monohexyl ether (HSP = 20.7 MPa) １／２ ) - Surfactants - BYK-345 ... Silicone-based surfactant (manufactured by BYK-Chemie) - Resin particles - Neocryl A-1105 ... Acrylic resin particles (manufactured by DSM coating resin)

[0138] <Inkjet Recording> Prepare an inkjet head (Samba G3L) manufactured by Fujifilm Dimatix, and load the above ink into the storage tank connected to the inkjet head. Set the inkjet head and storage tank in the inkjet recording device. Use gloss coated paper "OK ​​Topcoat 104" (manufactured by Oji Paper Co., Ltd.) as the substrate, and while transporting the substrate, ink is applied from the inkjet head to the transported substrate to perform image recording (i.e., inkjet recording). The detailed conditions for inkjet recording are as follows.

[0139] <Inkjet Recording Conditions> ・Substrate: Glossy coated paper "OK ​​Topcoat+" (paper thickness 82 μm) (manufactured by Oji Paper Co., Ltd.) ・Substrate transport mechanism: Roll to roll ・Inkjet head temperature: 32℃ ・Inkjet head resolution: 1200 dpi x 600 dpi ・Ink droplet size: 3.5 pL ・Environment around inkjet head: Temperature 25℃ ± 1℃, relative humidity 25℃ ± 5% ・Substrate transport speed: 200 m / min ・Substrate tension during transport: 60 N

[0140] <Evaluation> The inkjet recording described above was performed, and the following evaluation was conducted. The results are shown in Table 2.

[0141] (Image Density (OD (Optical Density))) Perform the inkjet recording described above, and record a solid image with an ink droplet size of 3.5 pL and a resolution of 1200 dpi x 600 dpi. Measure the OD of the solid image using a Konica Minolta FD-7 fluorescence spectrophotometer. Based on the measurement results, evaluate the image density (OD) according to the following criteria.

[0142] - Criteria for evaluating image density (OD) - A: OD is 2.3 or higher B: OD is between 2.1 and less than 2.3 C: OD is between 1.9 and less than 2.1 D: OD is between 1.7 and less than 1.9 E: OD is less than 1.7

[0143] <Ink ejection stability> After ejecting ink from the inkjet head, the ejection is stopped for 10 minutes without capping the inkjet head, and then the ink ejection is resumed. At this time of resumption, the impact position shift due to the ejection delay is measured, and the ink ejection stability is evaluated according to the following criteria.

[0144] - Criteria for evaluating ink ejection stability - A: The impact point deviation due to ejection delay is 100 μm or less on average 1 cm before recording starts. B: The impact point deviation due to ejection delay is greater than 100 μm on average and 200 μm or less on average 1 cm before recording starts. C: The impact point deviation due to ejection delay is greater than 200 μm on average 1 cm before recording starts, but the impact point deviation after 1 cm of recording is 200 μm or less on average 1 cm before recording starts. D: The impact point deviation due to ejection delay is 200 μm or more on average both 1 cm before and 1 cm after recording starts.

[0145] [Examples 2-28, Comparative Examples 1-4] The same procedure as in Example 1 was followed, except that the ink composition (including the acid value of the dispersant containing the crosslinked structure) was changed as shown in Tables 2-5. The results are shown in Tables 2-5. The acid value of the dispersant containing the crosslinked structure was adjusted by adjusting the amount of Denacol EX-321 and 4% by mass boric acid solution added to adjust the amount of crosslinking.

[0146] Comparative Example 1 is an example in which the dispersion treatment time was increased during the preparation of the uncrosslinked dispersion in Example 1 so that the volume average particle size of the pigment particles was less than 65 nm. Comparative Example 2 is an example in which the dispersion treatment time was shortened during the preparation of the uncrosslinked dispersion in Example 1 so that the volume average particle size of the pigment particles was 100 nm or more. Comparative Examples 3 and 4 are examples in Examples 1 and 2 in which the dispersant was not crosslinked, that is, examples in which the dispersant does not contain a crosslinked structure.

[0147]

[0148]

[0149]

[0150]

[0151] As shown in Tables 2 to 5, in each example using an ink containing water, a carbon black pigment, and pigment particles having a cross-linked polymer dispersant, and in which the volume-average particle diameter of the pigment particles measured by dynamic light scattering is 65 nm or more and less than 100 nm, the image density and ink ejection stability are excellent. In contrast, in Comparative Example 1, in which the volume-average particle diameter of the pigment particles is less than 65 nm, the ink ejection stability decreases. In Comparative Example 2, in which the volume-average particle diameter of the pigment particles is 100 nm or more, the image density decreases. Furthermore, in Comparative Examples 3 and 4, in which the polymer dispersant in the pigment particles does not have a cross-linked structure, the ink ejection stability decreases.

[0152] As shown in Examples 1 and 3, the ink has a Hansen solubility parameter (HSP) of 19 MPa. １／２ ~22 MPa １／２ When an organic solvent and propylene glycol (PG) are included (Example 3), the image density is further improved.

[0153] As shown in Examples 3 and 5, when the acid value of the polymer dispersant having a crosslinked structure is 1.5 mmol / g or less (Example 5), the ink ejection stability is further improved.

[0154] As shown in Examples 1 and 6, when the carbon black pigment content relative to the total amount of ink is 6.5% by mass or more (Example 6), the image density is further improved.

[0155] As shown in Examples 22 and 23, when the polymer dispersant having a crosslinked structure contains hydrophilic groups other than acidic groups (Example 22), the ink ejection stability is further improved.

[0156] [Example 101] The ink from Example 7 described above is prepared as black ink K1 (hereinafter simply referred to as "ink K1"). The carbon black pigment in the ink from Example 7 described above is changed to pigment red 122, which is a magenta pigment, and dispersed so that the volume average particle diameter of the pigment particles is 110 nm, and the content of magenta pigment in the total amount of ink is adjusted to 5.5% by mass. This prepares magenta ink M1 (hereinafter simply referred to as "ink M1"). The carbon black pigment in the ink from Example 5 described above is changed to pigment yellow 74, which is a yellow pigment, and dispersed so that the volume average particle diameter of the pigment particles is 125 nm, and the content of yellow pigment in the total amount of ink is adjusted to 4.0% by mass. This prepares yellow ink Y1 (hereinafter simply referred to as "ink Y1"). In this way, an ink set consisting of ink K1, ink M1, and ink Y1 is prepared.

[0157] In Table 6, the numbers in each ink column represent the pigment content (mass %) relative to the total ink volume.

[0158] <Inkjet Recording> Prepare an inkjet recording device having the same configuration as the inkjet recording device shown in Figure 1. Ink K1 is ejected from the first inkjet head of the inkjet recording device, ink M1 is ejected from the third inkjet head of the inkjet recording device, and ink Y1 is ejected from the fourth inkjet head of the inkjet recording device. In this embodiment, the second inkjet head of the inkjet recording device is not used. As a result, solid images of black and magenta (i.e., images with 100% halftone dots) and an image of magenta with 100% halftone dots and yellow with 40% halftone dots superimposed are recorded. The recording conditions for each image are the same as the recording conditions for the solid image in Example 1.

[0159] <Evaluation> The inkjet recording described above was performed, and the following evaluation was conducted. The results are shown in Table 6.

[0160] (Evaluation of image color reproduction) The OD of solid images of black and magenta is measured using a Konica Minolta FD-7 fluorescence spectrometer. The a* and b* of an image created by overlaying 100% magenta dots and 40% yellow dots are measured using a Konica Minolta FD-7 fluorescence spectrometer, and the value E(E = ((a*)) is calculated from the measured a* and b*. ２ + (b*) ２ ) １／２ ) is determined. Based on these measurement results, the color reproduction of the image is evaluated according to the following evaluation criteria. In the following evaluation criteria, rank A is the best for image color reproduction.

[0161] - Criteria for evaluating color reproduction - A: The OD of the black solid image is 2 or greater, the OD of the magenta solid image is 1.4 or greater, and the value E obtained from the image obtained by overlaying 100% magenta halftones and 40% yellow halftones is greater than 75. B: The OD of the black image is 2 or greater, the OD of the magenta image is 1.4 or greater, and the value E obtained from the image obtained by overlaying 100% magenta halftones and 40% yellow halftones is 75 or less. C: Satisfies only one of the following conditions: the OD of the black image is 2 or greater, and the OD of the magenta image is 1.4 or greater. D: Does not satisfy either the OD of the black image being 2 or greater, or the OD of the magenta image being 1.4 or greater.

[0162] (Ink ejection stability) For each of the inks in the ink set, ink K1, ink M1, and ink Y1, the ejection stability will be evaluated using the same conditions and evaluation criteria as for the ink ejection stability in Example 1. The result of the ink with the worst evaluation result among ink K1, ink M1, and ink Y1 will be taken as the ink ejection stability for that ink set.

[0163] (Storage Stability of Ink) For each of the inks in the ink set, ink K1, ink M1, and ink Y1, the viscosity is measured, then stored at 60°C for 14 days, and then the viscosity is measured again. These viscosities are measured at a temperature of 30°C using a rotational viscometer (specifically, a product name "VISCOMETER TV-22" manufactured by Toki Sangyo Co., Ltd.). Based on these results, the percentage change in viscosity due to the above storage is determined for each of the inks K1, M1, and Y1. Based on the results obtained, the storage stability of the inks is evaluated according to the evaluation criteria below.

[0164] - Criteria for evaluating the storage stability of inks - A: The viscosity change rate is 10% or less for all inks. B: The viscosity change rate is 10% or more and less than 15% for at least one ink. C: The viscosity change rate is 15% or more for at least one ink.

[0165] [Examples 102-108] The same procedure as in Example 101 was followed, except that the combination of pigment content (mass%) relative to the total amount of ink was changed as shown in Table 6. The results are shown in Table 6.

[0166]

[0167] In Table 6, the "value of M / Y" refers to "M / Y" in relation to equation (1), that is, the ratio of the content of magenta pigment (by mass) in the total amount of magenta ink to Y, which is the content of yellow pigment (by mass) in the total amount of yellow ink. In the column "1.30 ≤ M / Y ≤ 1.80 ... relation (1)", "A" and "N" mean that relation (1) is satisfied ("A") and not satisfied ("N"), respectively. In the column "6.5 ≤ K, 5.5 ≤ M", "A" and "N" mean that both of the following conditions are met ("A"): the carbon black pigment content in the total amount of black ink is 6.5% by mass or more and the magenta pigment content in the total amount of magenta ink is 5.5% by mass or more; and at least one of these conditions is not met ("N").

[0168] As shown in Table 6, in Examples 101 to 103, which use an ink set in which the carbon black pigment content relative to the total amount of black ink is 6.5% by mass or more, the magenta pigment content relative to the total amount of magenta ink is 5.5% by mass or more, and the magenta pigment content M relative to the total amount of magenta ink and the yellow pigment content Y relative to the total amount of yellow ink satisfy the relational expression (1) (1.30 ≤ M / Y ≤ 1.80), particularly excellent results are obtained in all aspects of image color development, ink ejection stability, and ink storage stability.

[0169] The disclosure of Japanese Patent Application No. 2025-054587, filed on 27 March 2025, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. An inkjet ink comprising water, a carbon black pigment, and pigment particles having a cross-linked polymer dispersant, wherein the volume-average particle diameter of the pigment particles, as measured by dynamic light scattering, is 65 nm or more and less than 100 nm.

2. Furthermore, the Hansen solubility parameter is 19 MPa １／２ ~22 MPa １／２ The inkjet ink according to claim 1, comprising an organic solvent and propylene glycol.

3. The inkjet ink according to claim 1, wherein the acid value of the polymer dispersant having the crosslinked structure is 2.7 mmol / g or less.

4. The inkjet ink according to claim 1, wherein the content of the carbon black pigment relative to the total amount of the inkjet ink is 6.5% by mass or more.

5. The inkjet ink according to claim 1, wherein the polymer dispersant having the crosslinked structure includes hydrophilic groups other than acidic groups.

6. The inkjet ink according to claim 1, used as the ink in an inkjet recording method that includes applying ink to a substrate being transported at a transport speed of 200 m / min or more by an inkjet method to record an image.

7. An ink set comprising: black ink, which is an inkjet ink according to claim 1; magenta ink, which is an inkjet ink containing magenta pigment; and yellow ink, which is an inkjet ink containing yellow pigment, wherein the content of the carbon black pigment relative to the total amount of the black ink is 6.5% by mass or more; the content of the magenta pigment relative to the total amount of the magenta ink is 5.5% by mass or more; and the mass ratio of the magenta pigment content relative to the total amount of the magenta ink, M, and the mass ratio of the yellow pigment content relative to the total amount of the yellow ink, Y, satisfy the following relation (1): 1.30 ≤ M / Y ≤ 1.80 … Relation (1) 8. An inkjet recording method comprising applying the inkjet ink described in claim 1 to a substrate being transported at a transport speed of 200 m / min or more by an inkjet method to record an image.

Images